Carbon Dioxide Reforming of Methane over Nickel-Supported Zeolites: A Screening Study

: As the utilization of zeolites has become more frequent in the dry reforming of methane (DRM) reaction, more systematic studies are required to evaluate properly the inﬂuence of zeolites’ composition and framework type on the performance. Therefore, in this work, a step-by-step study was performed with the aim of analyzing the effects of Ni loading (5, 10 or 15 wt.% over USY(3) zeolite), Si/Al ratio (3, 15 or 38 on USY zeolites with 15 wt.% Ni) and framework type (USY, BEA, ZSM-5 or MOR for 15 wt.% Ni and Si/Al ratios of ≈ 40) on catalysts’ properties and performances. Increasing Ni loadings enhanced CH 4 and CO 2 conversions even though the catalysts’ stability was decreasing over the time. The variation of the Si/Al ratio on USY and the use of different zeolites had also a remarkable impact on the catalytic performance. For instance, at 500–600 ◦ C reaction temperatures, the catalysts with higher basicity and reducibility exhibited the best results. However, when the temperature was further increased, catalysts presenting stronger metal–support interactions (nickel nanoparticles located in mesoporous cavities) displayed the highest conversions and stability over time. In brief, the use of 15 wt.% Ni and a USY zeolite, with both micro- and mesopores and high surface area, led to the best performances, mainly attributed to a favorable number of Ni 0 active sites and the establishment of stronger metal–support interactions (due to nanoparticles conﬁnement inside the mesopores).

Introduction
Currently, an important effort is required for increasing the share of renewables in the global energy system in order to accomplish the international climate goals [1]. That being said, a potential alternative could be the use of wastes such as sewage sludge, agricultural and crop residues, animal dung, industrial organic wastes, or wastewaters for the production of biogas through anaerobic digestion [1]. Biogas, composed mainly of CH 4 and CO 2 , can be then used for the production of biomethane (through upgrading and purification [2]) or even syngas (CO and H 2 ; through reforming strategies). Consequently, biogas constitutes an attractive renewable carbon source, and its exploitation is advantageous from both financial and environmental points of view.
Among the different alternatives for syngas production, steam reforming is the one already implemented on an industrial scale and leads to H 2 /CO ratios of ≈3. However, the simultaneous utilization of carbon dioxide and methane for the production of syngas (dry reforming of methane, DRM: CO 2 + CH 4 → 2CO + 2H 2 ) represents a promising activity due to their acidity, hindering the elimination of carbon deposits [20]. Finally, Frontera et al. [29] compared ≈5 wt.% Ni-supported Silicalite-1, MCM-41 and delaminated zeolite ITQ-6 catalysts performances toward DRM. Even if Ni-ITQ-6 presented the best results, Ni-Silicalite-1 exhibited the highest H 2 /CO ratio and the lowest carbon deposition. Authors concluded that increasing the number of defects (e.g., silanol groups) on the supports' surface promotes the formation of smaller and well-dispersed metal particles, boosting performances.
As mentioned earlier, tuning the Al content on zeolites' framework (and hence, the Si/Al ratio) and changing the type of structure used led to important changes in the physicochemical properties and performances toward DRM. However, the evaluation of these effects requires a systematic study under the same experimental conditions. Consequently, in the present work, a systematic series of Ni/Zeolites with different Ni loadings (5, 10 and 15 wt.%), Si/Al ratios (3, 15 and 38), and zeolite framework type (USY, BEA, MOR, ZSM-5) were analyzed to assess the influence of these parameters on the physicochemical properties and performances toward DRM reaction. Samples were prepared by ion-exchange of commercial zeolites followed by in cipient wetness impregnation with nickel and characterized by chemical analysis, N 2 sorption, CO 2 sorption, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), diffuse reflectance spectroscopy UV-Vis (DRS UV-Vis), H 2 temperature programmed reduction (H 2 -TPR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Finally, samples were catalytically tested in dry reforming of methane reaction.

Catalysts Preparation
USY, BEA, ZSM-5, and MOR commercial zeolites, without a significant number of extra-framework aluminium (EFAL) species, Si/Al ratios ranging from 3 to 47 and provided by Grace Davison and Zeolyst, were used as starting materials for the preparation of a series of Ni-containing Na-Zeolites (Table 1). In order to reduce the potential impact of the compensating cation nature in the performances and physicochemical properties, all zeolites (initially containing Na loadings ranging from <0.1 to 2.1 wt.% according to ICP analysis) were ion-exchanged with a NaNO 3 solution using a volume/mass ratio of 4 mL g zeolite −1 , room temperature and 4 h of stirring [30,31]. Samples were filtered using distilled water, dried overnight at 100 • C and, after repeating the process three times, calcined at 500 • C under air flow for 6 h and using a heating rate of 2 • C min −1 . Finally, the incorporation of Ni (5, 10, and 15 wt.% for USY (3) and 15 wt.% for the rest of supports) over the final Na-Zeolite supports was accomplished by incipient wetness impregnation, using nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O, Sigma Aldrich, >99%) as metal precursor. After drying overnight at 80 • C, catalysts were calcined at 500 • C for 6 h under air flow, being finally named as xNi/y(z) with x referring to the Ni loading (confirmed by ICP analysis in all cases), y to the zeolite framework type (USY, BEA, ZSM-5, or MOR), and z to the Si/Al global ratio (also determined by ICP analysis).

Characterization Techniques
Catalysts elemental analysis was performed by inductively coupled plasma (ICP) to determine the composition of the prepared samples in terms of Ni, Si, Al, and Na loadings. N 2 and CO 2 sorption measurements were carried out at −196 • C and 0 • C, respectively, on an Autosorb iQ equipment from Quantachrome. Before adsorption, samples were degassed under vacuum at 90 • C for 1 h and then at 350 • C for 4 h. The total pore volume (V total ) was calculated from the adsorbed volume of N 2 at a relative pressure (p/p 0 ) of 0.95, whereas the micropores volumes (V micro ) and external surface areas (S ext ) were determined using the t-plot method. Mesopores (and/or intercrystalline spaces) volumes (V meso ) were given by the difference V total − V micro . Powder XRD patterns were obtained in a Bruker AXS Advance D8 diffractometer, using Cu Kα radiation and operating at 40 kV and 40 mA. The scanning range was set from 5 to 80 • (2θ), with a step size of 0.03 • /2 s. Thermogravimetric analyses (TGA-DSC) were performed on a Setsys Evolution TGA from Setaram instruments. Samples (35-40 mg) were first saturated in water and then heated between 20 and 400 • C (10 • C min −1 ) under air flow (30 mL min −1 ). Then, the interaction of water with the samples in terms of strength was characterized as done in our former studies [31] by determining an index (h) reported by Anderson et al. [32], calculated as the ratio between the mass loss at 150 and 400 • C. TGA was also used to characterize spent catalysts, by performing analysis from room temperature to 800 • C (10 • C min −1 ). In addition, DRS spectra in the UV-Vis region were obtained in a Varian Cary 5000 UV-Vis-NIR spectrophotometer equipped with a diffuse reflectance accessory in the 200-800 nm range. Reflectance spectra were converted into the Schuster-Kubelka-Munk (SKM) function (F(R)) and presented versus wavelength to evaluate the presence of the different nickel species in the samples. Additionally, NiO band gaps were calculated by plotting F(R) 2 as a function of the energy (E = hc/λ) and extrapolating the linear region to F(R) 2 = 0 [33][34][35]. The final band gap energies can depend on the combination of two factors: the average size of the metal particles and the strength of the interaction between the metal and the support [36][37][38][39][40]. An increase of the band gap energy is usually observed with the decrease of the size of the Ni particles [36,37]. H 2 -TPR experiments were performed in a Micromeritics AutoChem II equipment, where catalysts were first pretreated at 250 • C under argon flow and then cooled down to the room temperature. After that, the reduction of the catalysts was carried out under a 5%H 2 /Argon flow, increasing the temperature from room temperature to 900 • C (10 • C min −1 ) and monitoring the hydrogen consumption using a TCD detector. SEM analysis was performed for parent zeolites using a Hitachi-S2400 microscope. Finally, TEM analysis was performed for the pre-reduced samples using a HRTEM 2010 JEOL LaB6 microscope (200 kV). Average Ni 0 particle sizes were determined by using ImageJ ® software (>100 particles counted for each catalyst).

Catalytic Tests
Catalysts were tested in a Hastelloy-X tubular reactor with an internal diameter of 9 mm, using a Microactivity reference catalytic reactor (EFFI, PID Eng, and Tech Spain). Catalysts (powder form, particle size < 63 µm) were loaded into the reactor mixed with quartz (inert) and reduced in situ at 470 • C for 2 h in a 5% H 2 /Ar flow (30 mL min −1 ). The catalytic bed length/catalyst particle diameter ratio was >150, while the reactor diameter/catalyst particle diameter ratio was >143. After pretreatment, dry reforming of methane was conducted under atmospheric pressure using a CH 4 /CO 2 ratio of 1:1 and a total GHSV of 72 L g −1 h −1 (0.050 g of catalyst per test). Good reproducibility of catalytic results under these conditions was obtained. For activity measurements, the reaction temperature was increased from 470 • C (5 • C min −1 ) up to 650 • C and kept at 650 • C for 12 h. The gaseous products were then analysed online by a micro-GC equipped with a TCD detector and two columns (Plot U and Molecular sieves) in parallel. CH 4 and CO 2 conversions as well as the H 2 /CO ratios were calculated using the molar flows (F) of the reactants and products, as shown in Equations (1)- (3):

Structural and Textural Properties
Catalysts' XRD patterns are illustrated in Figure 1. As seen, most of the catalysts exhibited diffraction peaks ascribed to NiO phases at ≈37, ≈43, and ≈63 • [41], being NiO crystallite sizes of 18-26 nm determined by Scherrer equation for all 15 wt.% Ni-containing samples. These peaks were not clear in the case of 5Ni/USY(3) and 10Ni/USY(3), probably due to the lower metal content. Additionally, it can be verified that the impregnation of Ni over the studied zeolite supports did not lead to structural changes in the zeolites, as the characteristic diffraction peaks of USY, BEA, ZSM-5, and MOR zeolites were still observed at 5-40 • [42] in the metal-loaded samples' patterns.
Regarding the textural properties assessed by N 2 sorption (Table 2; isotherms in Figure S1), it could be seen that USY(3) was mainly a microporous zeolite with a considerably low mesoporous volume (V meso = 0.04 cm 3 g −1 ). The incorporation of different Ni loadings over this support led to a reduction in both microporous volumes (V micro ) and external surface areas (S ext ). While the first effect could be related to the lower accessibility of N 2 to the zeolite pores due to the location of NiO particles in the pore entries blocking the access, the second could suggest a deposition of the NiO species on the external surface of the zeolite. It is clear that samples with higher Si/Al ratios, USY (15) and USY (38), presented significantly higher V meso and S ext values, which could be ascribed to the postsynthesis dealumination treatments carried out for the preparation of these commercial zeolites [43]. Indeed, Levecque et al. [43] reported that USY (15) and USY (38) zeolites (commercially named as CBV 720 and CBV 780, respectively), were obtained from a lower Si/Al ratio zeolite (named as CBV 300) by a combination of leaching and steam dealumination treatments. This was reported to result in the formation of disrupted crystal morphologies, cracks and voids (mesoporosity) [43]. Indeed, the mesoporous cavities/holes present in these samples could be observed in complementary TEM micrographs of 15Ni/USY (15) and 15Ni/USY(38) presented in Figure S2. Again, Ni impregnation decreased the V micro , V meso , and S ext values in these samples, which could be attributed to the location of NiO species on the external surface and/or inside the created mesopores [31]. Regarding the framework type, BEA(38) zeolite presented more relevant V meso and S ext values due to the intercrystalline spaces present in this zeolite [44]. The considerably smaller crystallite size of BEA (38) zeolite when comparing to USY (38), ZSM-5(40), or MOR (47) can be confirmed by the SEM micrographs presented in Figure S3. The aggregates of such small crystallites are known to generate intercrystalline spaces that could act as mesoporous cavities able to locate metal particles [44]. The reduction of the V meso in this sample after Ni incorporation suggested the location of NiO species inside these intercrystalline spaces acting as mesopores, as it could be seen in Figure S2. In addition, the slight variation in terms of V micro after nickel incorporation over BEA (28) zeolite (from 0.15 to 0.11 cm 3 g −1 ) suggested that the slight reduction in BEA diffraction peaks' intensity after Ni addition ( Figure 1) could not be ascribed to a structural damage. Finally, in ZSM-5 (40) and MOR (47), the impact of Ni incorporation in the textural properties was similar to that observed in USY(3), as these were essentially microporous zeolites.
Processes 2022, 10, x FOR PEER REVIEW 6 of 20 intercrystalline spaces acting as mesopores, as it could be seen in Figure S2. In addition, the slight variation in terms of Vmicro after nickel incorporation over BEA (28) zeolite (from 0.15 to 0.11 cm 3 g −1 ) suggested that the slight reduction in BEA diffraction peaks' intensity after Ni addition ( Figure 1) could not be ascribed to a structural damage. Finally, in ZSM-5 (40) and MOR (47), the impact of Ni incorporation in the textural properties was similar to that observed in USY(3), as these were essentially microporous zeolites.

Reducibility
Catalysts' reducibility is depicted in the H 2 -TPR profiles presented in Figure 2. To be pointed out is that, according to previous XRD data, nickel was present as NiO in all catalysts. Consequently, its reduction is expected to occur through the following reaction: NiO + H 2 → Ni 0 + H 2 O . As seen, the higher the metal loading the most relevant the fraction of NiO species located on the external surface of the USY(3) zeolite (reduction processes at 300 to 400 • C, as reported in literature [41,45]), in accordance with the previously discussed impact of Ni addition in the textural properties. It can be observed that the higher the USY zeolites' Si/Al ratio the greater the fraction of NiO species located inside the mesopores of the zeolites (reduction processes at 500 to 650 • C [31]), as previously noticed through the analysis of the textural properties evolution and in agreement with literature [46][47][48][49]. Finally, in terms of framework type, a more important fraction of NiO reduced above 500 • C could be observed for 15Ni/BEA (38) zeolite when comparing with the corresponding ZSM-5 and MOR samples. This confirmed the presence of NiO particles in the intercrystalline spaces of this zeolite acting as mesopores [46][47][48]. Furthermore, in the case of ZSM-5 and MOR samples, NiO species were mainly located on their external surface, probably due to the absence of large cavities for the accommodation of NiO particles in these structures. Indeed, the presence of a small reduction peak at higher temperatures in these last zeolites could be attributed to a heterogeneous distribution of the NiO particles in terms of size [50][51][52]. In short, H 2 -TPR data clearly showed that Ni loading and zeolites' composition and structure type are clearly responsible for modifications in the metal-support interactions, manifested in the different reducibility behaviors. Table 2. Textural properties obtained using N 2 sorption for all the samples from the present work and the corresponding supports.

Reducibility
Catalysts' reducibility is depicted in the H2-TPR profiles presented in Figure 2. To be pointed out is that, according to previous XRD data, nickel was present as NiO in all catalysts. Consequently, its reduction is expected to occur through the following reaction: NiO + H → Ni + H O. As seen, the higher the metal loading the most relevant the fraction of NiO species located on the external surface of the USY(3) zeolite (reduction processes at 300 to 400 °C, as reported in literature [41,45]), in accordance with the previously discussed impact of Ni addition in the textural properties. It can be observed that the higher the USY zeolites' Si/Al ratio the greater the fraction of NiO species located inside the mesopores of the zeolites (reduction processes at 500 to 650 °C [31]), as previously noticed through the analysis of the textural properties evolution and in agreement with literature [46][47][48][49]. Finally, in terms of framework type, a more important fraction of NiO reduced above 500 °C could be observed for 15Ni/BEA (38) zeolite when comparing with the corresponding ZSM-5 and MOR samples. This confirmed the presence of NiO particles in the intercrystalline spaces of this zeolite acting as mesopores [46][47][48]. Furthermore, in the case of ZSM-5 and MOR samples, NiO species were mainly located on their external surface, probably due to the absence of large cavities for the accommodation of NiO particles in these structures. Indeed, the presence of a small reduction peak at higher temperatures in these last zeolites could be attributed to a heterogeneous distribution of the NiO particles in terms of size [50][51][52]. In short, H2-TPR data clearly showed that Ni loading and zeolites' composition and structure type are clearly responsible for modifications in the metal-support interactions, manifested in the different reducibility behaviors.

Surface Properties
Catalysts were also characterized in terms of their affinity with water due to the possible inhibitory effect of H2O in DRM, related with the occurrence of RWGS (CO2 + H2 → CO + H2O) as a side reaction [4]. As done in previous works [31,53], TGA-DSC experiments were carried out for determining the h indexes, indicators of catalysts'

Surface Properties
Catalysts were also characterized in terms of their affinity with water due to the possible inhibitory effect of H 2 O in DRM, related with the occurrence of RWGS (CO 2 + H 2 → CO + H 2 O) as a side reaction [4]. As done in previous works [31,53], TGA-DSC experiments were carried out for determining the h indexes, indicators of catalysts' hydrophobicity. Higher Ni loadings slightly increased the interaction strength with water, while higher Si/Al ratios led to more hydrophobic materials (Table 3). However, no clear tendency could be found in terms of zeolite framework type, with relatively similar h indexes obtained for BEA (38), ZSM-5 (40) and MOR (47) samples when comparing with 15Ni/USY (38). DRS UV-Vis spectra were also collected ( Figure 3). As observed, all samples presented the characteristic bands of NiO at ≈300, ≈380, ≈410, and ≈720 nm [41,45]. In addition, NiO band gap energies were determined (Table 3), with comparable obtained values for all samples. hydrophobicity. Higher Ni loadings slightly increased the interaction strength with water, while higher Si/Al ratios led to more hydrophobic materials (Table 3). However, no clear tendency could be found in terms of zeolite framework type, with relatively similar h indexes obtained for BEA (38), ZSM-5 (40) and MOR (47) samples when comparing with 15Ni/USY (38). DRS UV-Vis spectra were also collected ( Figure 3). As observed, all samples presented the characteristic bands of NiO at ≈300, ≈380, ≈410, and ≈720 nm [41,45]. In addition, NiO band gap energies were determined (Table 3), with comparable obtained values for all samples. Finally, the affinity of the samples toward CO2 was characterized by the collection of CO2 isotherms (Figure 4), as done in literature [54][55][56][57][58]. While similar results were obtained for 5 and 15 wt.% Ni containing USY(3) zeolite samples, suggesting that CO2 affinity is not strongly influenced by the Ni loading, samples with different Si/Al ratios presented significantly different behaviors. Indeed, zeolites basicity is related to the Si/Al ratio since Finally, the affinity of the samples toward CO 2 was characterized by the collection of CO 2 isotherms (Figure 4), as done in literature [54][55][56][57][58]. While similar results were obtained for 5 and 15 wt.% Ni containing USY(3) zeolite samples, suggesting that CO 2 affinity is not strongly influenced by the Ni loading, samples with different Si/Al ratios presented significantly different behaviors. Indeed, zeolites basicity is related to the Si/Al ratio since CO 2 interacts with zeolites' compensating cations forming carbonate-like species [30,59], thus depending on its nature and content [30]. The nature of the cations is the same in all samples from this work (Na + ), but the content depends on the number of framework Al species leading to negative charges. Thus, zeolites with higher Si/Al ratios (lower number of framework Al atoms) are expected to be less basic [56,60,61]. This effect was confirmed in the obtained isotherms, where CO 2 uptakes decreased for all the relative pressures when increasing the Si/Al ratio (Table 1). In terms of the framework type effect, CO 2 uptakes for a chosen relative pressure followed the order: 15Ni/MOR(47) > 15Ni/ZSM-5(40) > 15Ni/BEA(38) >> 15Ni/USY (38). These results could be again attributed to the amount of Na + in the samples (ion exchange degrees of 38, 43, 86, and 94% for USY (38), BEA(38), ZSM-5(40) and MOR (47) samples, respectively, with H + occupying the remaining cationic positions). Furthermore, the type of structure could induce different interactions with sodium cations, which could modify the nature of the interactions with CO 2 molecules. Indeed, even if the Si/Al ratios chosen were similar in these four samples, the cations exchange capacity is strongly influenced by the framework type, probably due to accessibility issues.
Processes 2022, 10, x FOR PEER REVIEW 9 of 20 CO2 interacts with zeolites' compensating cations forming carbonate-like species [30,59], thus depending on its nature and content [30]. The nature of the cations is the same in all samples from this work (Na + ), but the content depends on the number of framework Al species leading to negative charges. Thus, zeolites with higher Si/Al ratios (lower number of framework Al atoms) are expected to be less basic [56,60,61]. This effect was confirmed in the obtained isotherms, where CO2 uptakes decreased for all the relative pressures when increasing the Si/Al ratio (Table 1). In terms of the framework type effect, CO2 uptakes for a chosen relative pressure followed the order: 15Ni/MOR(47) > 15Ni/ZSM-5(40) > 15Ni/BEA(38) >> 15Ni/USY (38). These results could be again attributed to the amount of Na + in the samples (ion exchange degrees of 38, 43, 86, and 94% for USY (38), BEA(38), ZSM-5(40) and MOR (47) samples, respectively, with H + occupying the remaining cationic positions). Furthermore, the type of structure could induce different interactions with sodium cations, which could modify the nature of the interactions with CO2 molecules. Indeed, even if the Si/Al ratios chosen were similar in these four samples, the cations exchange capacity is strongly influenced by the framework type, probably due to accessibility issues.  (40), and (b‴) 15Ni/MOR (47). Note that the y-axis is different in both graphs, to clearly see differences between the isotherms obtained for samples presented in (B).

Metal Particles Size
TEM micrographs were collected for all samples after reduction under H2 flow at 470 °C ( Figure 5; histograms presented in Figure S4). By analyzing the micrographs, the average Ni 0 particle sizes and metallic dispersions were obtained (Table 3). For USY(3) support and Ni contents from 5 to 15 wt.%, the estimated average Ni 0 sizes were 18-19 nm and metallic dispersions 5%, being concluded that the studied metal loadings did not strongly influence these parameters. Regarding the samples with increasing Si/Al ratios, no clear correlation was observed, contrary to what was reported in literature for Ni/BEA samples with Ni contents lower than 4 wt.% where the increase of the Si/Al ratio favored the metallic dispersion [38]. Moreover, one can note that, for higher Si/Al ratios, the standard deviation was slightly higher and the dispersion lower (3%), indicating a more heterogeneous distribution of the Ni 0 particles in these samples. This could be due to the presence of nickel particles both on the external surface and inside the mesopores of the zeolites  (47). Note that the y-axis is different in both graphs, to clearly see differences between the isotherms obtained for samples presented in (B).

Metal Particles Size
TEM micrographs were collected for all samples after reduction under H 2 flow at 470 • C ( Figure 5; histograms presented in Figure S4). By analyzing the micrographs, the average Ni 0 particle sizes and metallic dispersions were obtained (Table 3). For USY(3) support and Ni contents from 5 to 15 wt.%, the estimated average Ni 0 sizes were 18-19 nm and metallic dispersions 5%, being concluded that the studied metal loadings did not strongly influence these parameters. Regarding the samples with increasing Si/Al ratios, no clear correlation was observed, contrary to what was reported in literature for Ni/BEA samples with Ni contents lower than 4 wt.% where the increase of the Si/Al ratio favored the metallic dispersion [38]. Moreover, one can note that, for higher Si/Al ratios, the standard deviation was slightly higher and the dispersion lower (3%), indicating a more heterogeneous distribution of the Ni 0 particles in these samples. This could be due to the presence of nickel particles both on the external surface and inside the mesopores of the zeolites (see Figure S2). Finally, regarding the framework type effect, 15Ni/USY(38), 15Ni/ZSM-5(40) and 15Ni/MOR(47) samples showed similar average Ni 0 particle sizes while 15Ni/BEA (38) presented the smallest Ni 0 particles and the highest dispersion (5%).

Nickel Loading Effect
In terms of catalytic performances towards DRM, Figure 6 illustrates CH4 ( Figure 6A) and CO2 ( Figure 6B) conversions for all Ni/USY(3) zeolites with increasing Ni loadings as a function of the reaction temperature. As verified, higher temperatures led to higher conversions in all cases because of the endothermic nature of this reaction. Higher Ni loadings clearly promoted the catalytic performances, which means that the additional amount of Ni (leading to higher metallic surface areas as the average Ni 0 particle sizes were 18-19 nm in all samples) boosted the number of active sites enhancing the results, in accordance with literature [62][63][64]. Stability tests were also done at 650 °C and the catalytic results are shown in Figure 6C as a function of time-on-stream. Complementary results are also found in Figure S5 (CH4 and CO2 conversions at 0, 200, 450, and 700 min) and Table S1 (conversion losses, %, after 200, 450 and 700 min). As observed, 15Ni/USY(3) still possessed higher CO2 and CH4 conversions than 5Ni/USY(3) and 10Ni/USY(3) catalysts after 13 h. All catalysts demonstrated CO2 conversions higher than CH4 conversions, which is due to the simultaneous occurrence of RWGS, consuming CO2. This behavior was more obvious in the case of 15Ni/USY(3), which exhibited a H2:CO ratio below 1 (≈0.85), stressing on the production of more CO than H2. In addition, samples with 10 and 15 wt.% Ni showed a decrease of 15-16% in the conversions during the long-term tests (Table S1), while 5Ni/USY(3) presented higher stability (7-8% decrease). This behavior could be related to the weaker metal-support interactions present in catalysts with higher Ni loadings, being the occurrence of sintering processes and even coke deposition favored.

Nickel Loading Effect
In terms of catalytic performances towards DRM, Figure 6 illustrates CH 4 ( Figure 6A) and CO 2 ( Figure 6B) conversions for all Ni/USY(3) zeolites with increasing Ni loadings as a function of the reaction temperature. As verified, higher temperatures led to higher conversions in all cases because of the endothermic nature of this reaction. Higher Ni loadings clearly promoted the catalytic performances, which means that the additional amount of Ni (leading to higher metallic surface areas as the average Ni 0 particle sizes were 18-19 nm in all samples) boosted the number of active sites enhancing the results, in accordance with literature [62][63][64]. Stability tests were also done at 650 • C and the catalytic results are shown in Figure 6C as a function of time-on-stream. Complementary results are also found in Figure S5 (CH 4 and CO 2 conversions at 0, 200, 450, and 700 min) and Table S1 (conversion losses, %, after 200, 450 and 700 min). As observed, 15Ni/USY(3) still possessed higher CO 2 and CH 4 conversions than 5Ni/USY(3) and 10Ni/USY(3) catalysts after 13 h. All catalysts demonstrated CO 2 conversions higher than CH 4 conversions, which is due to the simultaneous occurrence of RWGS, consuming CO 2 . This behavior was more obvious in the case of 15Ni/USY(3), which exhibited a H 2 :CO ratio below 1 (≈0.85), stressing on the production of more CO than H 2 . In addition, samples with 10 and 15 wt.% Ni showed a decrease of 15-16% in the conversions during the long-term tests (Table S1), while 5Ni/USY(3) presented higher stability (7-8% decrease). This behavior could be related to the weaker metal-support interactions present in catalysts with higher Ni loadings, being the occurrence of sintering processes and even coke deposition favored. Catalysts were analyzed by XRD ( Figure S6) and TGA ( Figure S7) after tests. The three spent samples exhibited diffraction peaks ascribed to Ni 0 phases at 44.5, 51.9 and 76° ( Figure S6). In addition, the presence of graphitic carbon (2θ = 27 and 54.4°) was clear in 10 and 15 wt.% Ni-containing samples. Furthermore, a structural damage of the zeolite could be suggested for 15Ni/USY(3), as faujasite zeolite diffraction peaks could not be clearly identified in 2θ = 5-40° range, given that this phenomenon was already reported in literature by Inoue et al. [23]. In terms of TGA profiles ( Figure S7), two main mass loss processes were observed below 400 °C, attributed to water desorption [65], and above 450 °C, ascribed to coke volatilization/decomposition [65,66] and/or zeolites dehydroxylation [65,67,68]. Thus, by comparing the profiles above 450 °C in the three USY(3)-based catalysts, it could be concluded that the use of higher Ni loadings led to formation of more coke, in accordance with the literature [62].

Si/Al Ratio Effect
Complementary, when using a constant Ni loading (15 wt.%), the same zeolite structure (USY) and increasing Si/Al ratios (3, 15 or 38), 15Ni/USY(3) exhibited the best conversions at 500-600 °C ( Figure 7A,B). This could be related to the fact that 15Ni/USY(3) presented higher basicity than the 15Ni/USY (15) and 15Ni/USY (38) catalysts, as proved also in the CO2 adsorption curves seen in Figure 4. In fact, mechanistic considerations Catalysts were analyzed by XRD ( Figure S6) and TGA ( Figure S7) after tests. The three spent samples exhibited diffraction peaks ascribed to Ni 0 phases at 44.5, 51.9 and 76 • ( Figure S6). In addition, the presence of graphitic carbon (2θ = 27 and 54.4 • ) was clear in 10 and 15 wt.% Ni-containing samples. Furthermore, a structural damage of the zeolite could be suggested for 15Ni/USY(3), as faujasite zeolite diffraction peaks could not be clearly identified in 2θ = 5-40 • range, given that this phenomenon was already reported in literature by Inoue et al. [23]. In terms of TGA profiles ( Figure S7), two main mass loss processes were observed below 400 • C, attributed to water desorption [65], and above 450 • C, ascribed to coke volatilization/decomposition [65,66] and/or zeolites dehydroxylation [65,67,68]. Thus, by comparing the profiles above 450 • C in the three USY(3)-based catalysts, it could be concluded that the use of higher Ni loadings led to formation of more coke, in accordance with the literature [62].

Si/Al Ratio Effect
Complementary, when using a constant Ni loading (15 wt.%), the same zeolite structure (USY) and increasing Si/Al ratios (3, 15 or 38), 15Ni/USY(3) exhibited the best conversions at 500-600 • C ( Figure 7A,B). This could be related to the fact that 15Ni/USY(3) presented higher basicity than the 15Ni/USY (15) and 15Ni/USY (38) catalysts, as proved also in the CO 2 adsorption curves seen in Figure 4. In fact, mechanistic considerations regarding DRM reaction have pointed out that the increase in basicity is an important factor to limit carbon deposition [69,70]. However, the best performances and higher stability over the time were exhibited by catalysts with higher Si/Al ratios at 650 • C. This behavior could be related to the stronger interaction between nickel nanoparticles and USY support in these samples. For instance, the presence of mesopores in zeolites with higher Si/Al ratios led, as seen by the previous analysis of the H 2 -TPR profiles, to the location of NiO nanoparticles in these cavities (confinement effect), inducing stronger metal-support interactions and preventing sintering and even coke deposition, as seen in Ni-based mesoporous silica catalysts in literature [69]. Complementary, and due to the occurrence of RWGS reaction (CO 2 + H 2 → CO + H 2 O), the interaction of the samples with water could present an eventually remarkable effect in the performances. Indeed, when RWGS is a side-reaction, favored by higher reaction temperatures [71], the formation of water is expected during the reaction, leading to deactivation and sintering, besides the potential negative effect in the zeolite structure (steaming). Consequently, a weaker interaction between these molecules and the zeolite-based catalysts could avoid H 2 O to be adsorbed over the zeolite sites responsible for CO 2 activation (e.g., compensating cations [72]), at the same time that eventual deactivation processes due to water could be hindered. The hydrophobic character of higher Si/Al ratio zeolites ( Table 2) could thus reduce the inhibitory effect of water in the reaction.
Processes 2022, 10, x FOR PEER REVIEW 12 of 20 regarding DRM reaction have pointed out that the increase in basicity is an important factor to limit carbon deposition [69,70]. However, the best performances and higher stability over the time were exhibited by catalysts with higher Si/Al ratios at 650 °C. This behavior could be related to the stronger interaction between nickel nanoparticles and USY support in these samples. For instance, the presence of mesopores in zeolites with higher Si/Al ratios led, as seen by the previous analysis of the H2-TPR profiles, to the location of NiO nanoparticles in these cavities (confinement effect), inducing stronger metal-support interactions and preventing sintering and even coke deposition, as seen in Ni-based mesoporous silica catalysts in literature [69]. Complementary, and due to the occurrence of RWGS reaction (CO2 + H2 → CO + H2O), the interaction of the samples with water could present an eventually remarkable effect in the performances. Indeed, when RWGS is a side-reaction, favored by higher reaction temperatures [71], the formation of water is expected during the reaction, leading to deactivation and sintering, besides the potential negative effect in the zeolite structure (steaming). Consequently, a weaker interaction between these molecules and the zeolite-based catalysts could avoid H2O to be adsorbed over the zeolite sites responsible for CO2 activation (e.g., compensating cations [72]), at the same time that eventual deactivation processes due to water could be hindered. The hydrophobic character of higher Si/Al ratio zeolites ( Table 2) could thus reduce the inhibitory effect of water in the reaction.  Catalysts were also analyzed using XRD and TGA after reaction tests ( Figure S6 and Figure S7, respectively). Results indicated that higher Si/Al ratio zeolites exhibited a lower resistance towards coke deposition (diffraction peaks ascribed to carbon in XRD patterns and important mass loss above 450 • C in TGA profiles), in agreement with the previously discussed impact of the basicity in carbon formation prevention. Furthermore, the clearer identification of faujasite peaks in the 5-40 • range in spent 15Ni/USY(15) and 15Ni/USY (38) patterns could indicate that the weaker interaction between water and these zeolites hindered the occurrence of steaming processes able to damage or partially destruct USY structure. On the contrary, the higher hydrophilicity of 15Ni/USY(3) could be responsible for the occurrence of more relevant steaming processes, damaging the structure, as suggested by the less intense zeolite diffraction peaks in this sample after test.

Framework Type Effect
In the last study, and considering the higher activity and stability exhibited by 15Ni/USY (38) at 650 • C, the effect of the zeolite framework type in the performances was assessed over BEA, ZSM-5, and MOR zeolites with Si/Al ratios of ≈40 and impregnated with 15 wt.% Ni, with the results presented in Figure 8. Catalysts were also analyzed using XRD and TGA after reaction tests ( Figures S6 and S7, respectively). Results indicated that higher Si/Al ratio zeolites exhibited a lower resistance towards coke deposition (diffraction peaks ascribed to carbon in XRD patterns and important mass loss above 450 °C in TGA profiles), in agreement with the previously discussed impact of the basicity in carbon formation prevention. Furthermore, the clearer identification of faujasite peaks in the 5-40° range in spent 15Ni/USY(15) and 15Ni/USY (38) patterns could indicate that the weaker interaction between water and these zeolites hindered the occurrence of steaming processes able to damage or partially destruct USY structure. On the contrary, the higher hydrophilicity of 15Ni/USY(3) could be responsible for the occurrence of more relevant steaming processes, damaging the structure, as suggested by the less intense zeolite diffraction peaks in this sample after test.

Framework Type Effect
In the last study, and considering the higher activity and stability exhibited by 15Ni/USY (38) at 650 °C, the effect of the zeolite framework type in the performances was assessed over BEA, ZSM-5, and MOR zeolites with Si/Al ratios of ≈40 and impregnated with 15 wt.% Ni, with the results presented in Figure 8.  As previously observed for Si/Al ratio series, two tendencies were verified in terms of CO 2 and CH 4 conversions at different temperatures ( Figure 8A,B). Indeed, while at 500-600 • C, the order was the following: 15Ni/MOR(47) > 15Ni/ZSM-5(40) > 15Ni/USY (38) ≈ 15Ni/BEA(38); at higher temperatures, similar conversions were exhibited by 15Ni/USY (38) and 15Ni/MOR (47), with 15Ni/BEA (38) being the less performing catalyst. As previously analyzed by the collection of CO 2 adsorption isotherms, the order of basicity (related to a greater exchange degree of Na + in the structure) was the following: 15Ni/MOR(47) > 15Ni/ZSM-5(40) > 15Ni/BEA (38) >> 15Ni/USY (38). This trend is indeed equivalent to the ones found for the activities below 600 • C, agreeing with previous considerations on the effect of basicity in the Si/Al ratio zeolites series. In terms of stability over time at 650 • C ( Figure 8C), USY (38) and MOR(47)-based catalysts presented the best results (relative and absolute conversion losses of 12-23% and 8-15%, respectively; Table S1), followed by 15Ni/ZSM-5(40) and 15Ni/BEA (38), with the last two significantly reducing their activity after 12 h (relative and absolute conversion losses of 36-55% and 18-32%, respectively; Table S1). As previously pointed out, the favored metal-support interactions found in 15Ni/USY (38) and its higher hydrophobicity could be the reason of the improved performances (activity and stability) exhibited at higher temperatures by this sample.
Spent catalysts were again characterized by XRD ( Figure S6) and TGA ( Figure S7). As seen both in XRD patterns and TGA profiles, carbon deposits were present in all catalysts, especially on MOR(40) and ZSM-5(40)-supported ones. These two catalysts presented the highest basicity which, as previously seen, should hinder coke decomposition. However, based on literature [62], larger Ni 0 crystallites enhance the formation of carbon deposits and these two samples presented less dispersed nickel nanoparticles, mostly located on their outer surface.

Comparison with Literature
Finally, the most stable and active catalysts from this work, 15Ni/USY(38) and 15Ni/MOR (47), were compared to other Ni-supported catalytic systems from literature ( Table 4). As verified, conversion losses below 15% were verified for both catalysts from this work after 12 h under reaction conditions. These results were similar or better than other found in literature for Ni-based catalysts. However, the significant differences in terms of operating conditions limit the establishment of direct comparisons.

Discussion
Based on the previous results and in order to establish structure-reactivity correlations, the relative variation of the main catalysts' properties reported for DRM reaction [5] in the three studies from this work was analyzed and shown in Figure 9.

Discussion
Based on the previous results and in order to establish structure-reactivity correlations, the relative variation of the main catalysts' properties reported for DRM reaction [5] in the three studies from this work was analyzed and shown in Figure 9. Starting by the Ni loading series ( Figure 9A), as previously observed in Figure 6, the activity was enhanced by the increase of metal content while a higher stability was shown by 5Ni/USY (3). The increase in the CO2 and CH4 conversions could be clearly ascribed to the increment in the number of nickel active sites when increasing Ni loading, as Starting by the Ni loading series ( Figure 9A), as previously observed in Figure 6, the activity was enhanced by the increase of metal content while a higher stability was shown by 5Ni/USY (3). The increase in the CO 2 and CH 4 conversions could be clearly ascribed to the increment in the number of nickel active sites when increasing Ni loading, as reducibility at lower temperatures was enhanced while no significant modifications in the dispersion/average Ni 0 size were verified. In addition to this, the higher stability of 5Ni/USY(3) over the time could be ascribed to the stronger metal-support interactions in this sample. Indeed, the incorporation of a lower metal loading induced a more homogeneous distribution of the nanoparticles both on the external surface and inside zeolite' cavities, improving the interaction with the zeolite sites and avoiding agglomeration processes responsible for catalyst deactivation.
Regarding the USY zeolites with variable Si/Al ratios ( Figure 9B), it could be concluded that the modification of this parameter affected catalysts' properties in opposite directions. Indeed, while the metallic dispersion was not strongly affected, the textural properties and metal-support interactions were favored by the decrease of the Al content. Indeed, the dealumination processes carried out commercially to produce these zeolites were responsible for the formation of cavities acting as mesopores, increasing the mesoporous volumes and the external surface areas, and allowing the encapsulation of nickel nanoparticles inside the structure. On the contrary, the basicity, dependent on the number of aluminum atoms in the zeolite, as well as NiO species' reducibility at lower temperatures, were favored by lower Si/Al ratio zeolites. When analyzing the catalytic performances (Figure 7), the improved basicity and reducibility of lower Si/Al ratio zeolites could explain the higher performances below 600 • C. On the contrary, the higher textural properties and its consequent effect on the metal-support interactions through nanoparticles confinement inside mesopores could explain the higher conversions attained by higher Si/Al ratio zeolites above 600 • C. In addition, another property not reported in literature as relevant for this reaction, the hydrophobicity (improved for higher Si/Al ratios), could play a prominent role at higher temperatures in the prevention of zeolites' structure collapse due to steaming processes as well as in the reaction mechanism, as water (produced via RWGS reaction) is adsorbed on the same sites as CO 2 in the case of zeolites, leading to an inhibitory effect.
Finally, regarding the framework type ( Figure 9C), 15Ni/MOR(47) and 15Ni/ZSM-5(40) exhibited the best basicity (due to the higher Na + loading) and reducibility (due to the purely microporous structure inducing a major location of NiO species on zeolites' external surface). In addition, the highest textural properties and strongest metal-support interactions were found in 15Ni/USY (38), while 15Ni/BEA (38) presented only the best features in terms of metallic dispersion. For instance, catalytic performances (Figure 8), were more promising below 600 • C for 15Ni/MOR (47) and even 15Ni/ZSM-5(40) catalysts, which, as seen in the study of the Si/Al ratio effect, could be ascribed to the better basicity and reducibility of NiO species in these materials. On the contrary, at higher reaction temperatures, the conversions and stability over the time were improved for 15Ni/USY (38), probably due to the nanoparticles encapsulation inside its mesopores (stronger metalsupport interactions) and, eventually, the weaker interaction with water molecules.

Conclusions
In this work, the influence of the Ni loading, the Si/Al ratio, and the framework type on the properties and performances of Ni-based zeolites towards DRM reaction was analyzed. For this purpose, three catalysts series were systematically prepared, characterized, and finally tested. In terms of Ni loading effect, studied over 5, 10 and 15 wt.% Ni catalysts supported over a USY zeolite with a Si/Al ratio of 3 (USY(3)), the increase of this parameter was found to improve both CO 2 and CH 4 conversions at different reaction temperatures. This effect was ascribed to the increase in the number of nickel active sites, as no significant differences in terms of metallic dispersion were found among the samples. Using 15 wt.% Ni, the effect of the Si/Al ratio was studied over three USY supports (USY(3), USY (15) and USY (38)). Based on the results, using lower Si/Al ratio zeolites was beneficial for the activities below 600 • C, while the tendency was the opposite at higher temperatures. Similarly, when analyzing the influence of the framework type by comparing USY (38) to BEA (38), , and MOR(47) as supports, the last two zeolites showed the best activities below 600 • C, while using USY (38) was found as more favorable at higher reaction temperatures. Thus, it was observed that the more basic catalysts with easily reducible NiO species exhibited higher performances below 600 • C, while, at harsher conditions, the existence of stronger metal-support interactions due to nanoparticles confinement in mesoporous cavities and even the weaker interaction with water (product of RWGS reaction able to adsorb in the same sites as CO 2 ) were identified as key for understanding the better performances.


s2fieldsofstudy: Chemistry Biology

text:
Synthesis of alkoxy-isoflavones as potential α-glucosidase inhibitors

The aim of the present study was to synthesize isoflavone-enaminones 3a-c and 7-alkoxy-isoflavones 4a-c, evaluate their inhibition of α-glucosidase, and analyze the bioisosteric effect of the presence versus absence of aromatic moieties in these benzopyran derivatives. All the test compounds exhibited greater inhibition of α-glucosidase than the positive control acarbose. The series of isoflavones 3a-c and 4a-c showed higher inhibitory activity (IC50 = 6.3–87.6 µM) than the parental 7-hydroxyisoflavones 2a-c (IC50 = 109.4–173.2 µM), suggesting that the attachment of a 4’-chloroacetophenone moiety to the 7-hydroxyl group of 2a-c is an efficient way to increase the inhibition of α-glucosidase. Furthermore, the aromatic moieties of the series of compounds 3 and 4 enhance inhibitory activity by hydrophobic effects, according to docking calculations. Graphical abstract Graphical abstract

Introduction α-Glucosidase is an enzyme that catalyzes the hydrolysis of glycosidic bonds from the non-reducing portion of the oligosaccharide substrates. It plays a key role in the digestion of dietary carbohydrates and the processing of glycoproteins. By inhibiting this enzyme, the digestion of starch and other dietary sugars decreases, which helps to avoid hyperglycemia by maintaining blood glucose at normal levels [1]. The reduction of sugar digestion makes α-glucosidase an attractive therapeutic target for the treatment of type 2 diabetes [2], cardiovascular diseases [3], and cancer [4,5]. Regarding diabetes, about 422 million people worldwide are affected by this metabolic disease, characterized by high levels of blood glucose (hyperglycemia) derived from a deficiency in the secretion and/or action of insulin [6]. Therefore, α-glucosidase inhibitors may represent an effective therapeutic strategy for reducing postprandial hyperglycemia.
Recently, our group reported that enaminone-chromone derivatives can potentially act as α-glucosidase inhibitors [15]. Several interactions were found between the chromone derivatives and α-glucosidase, including π-stacking and hydrophobic effects. Such interactions are key for enhancing inhibitory activity. For instance, enaminone-chromone 1c bears a phenyl group at the C-3 position of the chromone ring system (isoflavone skeleton) and a 4-chlorophenyl group at the enaminone moiety. It displays considerably greater inhibition of α-glucosidase than compounds 1a-b (Fig. 1A).
In accordance with the aforementioned, the 7-hydroxy group and its 7-alkoxylated isoflavone derivatives as well as 6-methoxy-containing or 6-methoxy-free analogs may generate a significant inhibition of α-glucosidase. This largely depends on the nature of the C-7 alkoxy groups and the C-6 substituents in such compounds. For the choice of compounds to be tested presently, it was taken into account that functionalized benzopyrans with a 2-(4-chlorophenyl)-2-oxoethoxy group at the C-7 position (i.e., compound 1c) increased the inhibition of the enzyme, focusing particular attention on the role played by the enaminone moiety. Hence, the aim of the current contribution was to synthesize a series of enaminone-isoflavones 3a-c and 7-(2-(4-chlorophenyl)-2-oxoethoxy)isoflavones 4a-c, evaluate them as potential inhibitors of α-glucosidase (Fig. 1B), and test them for their antioxidant effect. The analysis of the results on the inhibition of α-glucosidase provided insights into the structure-activity relationship of the isoflavone-based derivatives. Additionally, docking studies were carried out to better understand the mechanism of binding of the 7alkoxy-isoflavone derivatives to the crystal structure of isomaltase, an α-glucosidase of Saccharomyces cerevisiae.

Results and discussion
Chemistry Compounds 7a-c were prepared in good yields by the acylation of resorcinols 5a-c with 2-(4-methoxyphenyl) acetic acid (6), in the presence of boron trifluoride On the other hand, the treatment of 2,4-dihydroxyacetophenones 7a-c with DMFDMA gave the series of isoflavones 2a-c in high yields, along with 7-methoxyisoflavones 10a-b as by-products in low yields [32]. Then O-alkylation of isoflavones 2a-c with 8 provided the target compounds 4a-c (Scheme 2).
The structure of all test compounds was elucidated by 1D and 2D NMR techniques and HRMS. The intramolecular cyclization and condensation of the α-arylacetophenones 9a-c led to the enaminone-isoflavones 3a-c as single stereoisomers. The Z geometry of the latter compounds was established by NOE experiments. The irradiation of the signal assigned to the methyl protons of the dimethylamino group produced an enhancement of the signal corresponding to the H-6 (for 3a) and H-8 protons of the benzene ring of the chromen-4-one scaffold. This stereoselectivity has also been observed in similar systems, which is probably due to the higher stability achieved by the planar π-conjugated acrylate system when the bulky dimethylamino group is located at the opposite side of the carbonyl group [15].
Compared to their precursors 7a-c, compounds 2a-c (with a benzopyran ring moiety) showed greater inhibition of α-glucosidase. In order to explore the possibility that the enaminone moiety in 3a-c favors inhibition, derivatives 4ac (without this fragment) were evaluated. The latter compounds exhibited better inhibitory activity than compounds 3a and 3b. The best inhibition was found with 4a, the IC 50 value (6.3 µM) of which indicated a 49-fold greater inhibition of α-glucosidase than 12.
Regarding the inhibitory effect of compounds 3a-b and 4a-b, the analysis of the structure-activity relationship revealed that the polar enaminone group is not as relevant as the (4-chlorophenyl)-2-oxoethoxy group. In contrast, the enaminone group and the chlorine atoms in 3c favored improved inhibition with respect to 4c. Furthermore, the Scheme 1 Synthesis of enaminone-isoflavones 3a-c π-stacking or hydrophobic effect of the additional aromatic rings at C-3 and C-7 of the isoflavone ring (series 3 and 4) may have played a more important role than the H-bond donor effect of 7-OH (series 2) in the interaction with the enzyme. At the C-6 position, the absence of a substituent in the isoflavone and benzene rings in 3a, 4a, and 9a led to better inhibitory activity than the presence of a methoxy group in 3b, 4b, and 9b or a chlorine atom in 4c and 9c.

Evaluation of the antioxidant activity
Antioxidants are responsible for protecting the body against oxidative stress and many disorders (e.g., diabetes). To determine the potential of the presently synthesized compounds as antioxidants, their free radical scavenging was examined by the DPPH radical assay, with butylated hydroxytoluene as the positive control. The reduction in color intensity represented the percentage of inhibition of DPPH (Table 1), finding that the 2-acylphenols 7b-c, αarylacetophenones 9a-c, and isoflavones 2a-c, 3a-c, 4a-c, and 10a-b did not have any significant antiradical activity up to the maximum concentration tested. Only compound 7a showed some inhibition (61% at 2.5 mM), suggesting that this effect is mainly related to the hydroxyl groups substituted in the benzene ring.

Docking results
To identify the binding mode of the compounds with the greatest inhibition of α-glucosidase, molecular docking studies were carried out. The crystal structure of isomaltase from S. cerevisiae served as the target protein because it is a type of α-glucosidase. The positive controls were maltose (11) and acarbose (12), two competitive inhibitors of isomaltase. The results of the interactions are illustrated in 2D and 3D (Fig. 2). Compounds 2a, 3a, 4a, and 9c as well as the controls 11 and 12 recognized many of the amino acid residues at the active site. Among such residues were Asp64, His112, Arg213, Asp215, Glu277, His351, Asp352, and Arg442, as reported in other studies on isomaltose inhibitors [33][34][35][36]. For all the compounds, the two residues Scheme 2 Synthesis of 7arylalkoxy-isoflavones 4a-c nt not tested a IC 50 = 0.84 ± 0.08 mM Fig. 2 Portrayal of the interactions within the active site of isomaltase by isoflavone derivatives 2a, 3a, and 4a, aryloxycarbonyl compound 9c, and controls 11 and 12. The 3D models illustrate the hydrogen bonds and the amino acid residues at the active site of the enzyme. The non-hydrogen bonds were omitted for clarity. In the 2D model, the following bonds are displayed with dotted lines: conventional hydrogen (green), carbon-hydrogen (brown), π-sigma (purple), π-π T-shaped (fuchsia), π-alkyl (pink), and halogens (cyan). The amino acids are depicted with circles: basic (pink), acid (orange), polar (dark blue), and non-polar (yellow) involved in catalysis (Glu277 and Asp352) are active in the binding process, suggesting that the derivatives of 7-alkoxyand 7-hydroxyisoflavones bind to the active site of the enzyme to inhibit α-glucosidase [33].
The binding energy values (ΔG) and the type of interactions are shown in Table 2. The best ΔG values were observed for 2a (−7.51 Kcal/mol), 3a (−9.82 Kcal/mol), 4a (−9.58 Kcal/mol), and 9c (−9.83 Kcal/mol), each being better than the corresponding value for maltose (11) (−5.55 Kcal/mol), a competitive inhibitor. The best binding to the active site of isomaltase was found with derivative 9c. There were hydrophilic and hydrophobic interactions in isoflavones 2a, 3a, 4a, and 9c, but not in the two controls. The predominant hydrophilic interactions in all the derivatives and maltose (11) are those between some substituent of the compounds and the Asp352 or Arg442 residues. There is a hydrophobic interaction with Phe178 for these four derivatives, as reported previously [34][35][36].
The results of the docking study coincide with those of the enzyme inhibition assays. That is, among the four aforementioned compounds with the binding energy, 3a, 4a, and 9c produced the greatest inhibition of the α-glucosidase enzyme. The corresponding IC 50 values were 18.8 ± 0.13, 6.3 ± 0.11, and 34.1 ± 0.013 µM, respectively, much lower than the IC 50 of 12 (309.2 ± 0.9 µM). The analysis of the docking results for compounds 3a, 4a, and 9c showed that the 4-methoxyphenyl group at C-3 of the benzopyran ring (C-1 for derivative 9c) exhibits a hydrophilic interaction with a hydrogen atom of group methoxy between carboxamide group of Asn415. The hydrophobic interactions π-π stacked and π-alkyl were observed with Arg315, Phe314, and Lys156. The presence Table 2 Docking results of isoflavones 2a, 3a, and 4a, aryloxycarbonyl compound 9c, maltose (11), and acarbose (12) Asp69, Tyr72, His112, Tyr158, Phe159, Phe178, Arg213, Asp215, Val216, Glu277, Gln279, His 280, Phe303, Asp307, Arg315, Tyr316, His351, Asp352, Gln353, Glu411, Arg442, Arg446 Halogen bonding Asp69 Halogen bonding Asp69 of the (4-chlorophenyl)-2-oxoethoxy fragment at C-7 of the isoflavone ring presents hydrophobic interactions with the active site of α-glucosidase such as π-sigma interactions with Val216, π-alkyl with Tyr72, His112 and Phe178, and halogen bonding with Asp69. Hydrophilic interactions of conventional hydrogen bond type and carbon-hydrogen bond were observed with Gln279 with Asp352, respectively. Considering these findings, the presence of polar methoxy or hydroxy groups on the aromatic ring at C-3 of the benzopyran system could increase the affinity with α-glucosidase. Furthermore, analogs with a chlorine substituent at C-6 of the benzopyran ring increase their affinity for the enzyme. On the other hand, bioisosteric replacement of the (4-chlorophenyl)-2-oxoethoxy group by a (4-chlorophenyl)-2-oxoacetamide group at C-7 of the isoflavone ring could be explored, since that oxoacetamide moiety may show hydrophilic interactions that stabilize binding energies at the active site of the enzyme.
These results suggest that isoflavones 2a and 4a (without a substituent at C-6), as well as compounds 7a and 9a (without a substituent on the catechol ring), exhibited a greater inhibitory effect than their chlorine-substituted analogs (2c, 4c, 7c, and 9c) and methoxy-substituted analogs (2b, 4b, 7b, and 9b). In addition, compounds 2c and 3c with the chlorine substituent at C-6 of the benzopyran ring (C-5 for compound 9c) exhibited better enzyme inhibition than compounds 2b and 3b with the methoxy substituent (C-5 for compound 9b), which indicates that the negative inductive effect of the chlorine atom increases its affinity with the enzyme compared to the mesomeric effect of the methoxy group. Therefore, these scaffolds provide promise for the development of new drugs to treat diabetes mellitus.

Physicochemical properties
The physicochemical properties used to evaluate the druglikeness of the synthesized compounds are shown in Table 3. The octanol-water partition coefficient Log P (expressed as Log Po/w) is defined as the partition coefficient of an un-ionized compound in two immiscible phases (octanol and water/buffer) at equilibrium.
Lipophilicity is an important parameter in medicinal chemistry due to its correlation with molecular hydrophobicity, which affects the bioavailability, permeability, and metabolism of drugs as well as their toxicity [37,38]. Permeability is considered high with Log P values close to 5 and low with negative values. All compounds showed acceptable Log P values within a range of 2.17-4.95. For the series 3a-c, 4a-c, and 9a-c, the presence of the (4chlorophenyl)-2-oxoethoxy group and chlorine atoms increased the Log P values, particularly in 3c and 4c (Log P 4.95 and 4.69, respectively).
Log S is a descriptor of the octanol-water distribution coefficient for the partitioning of ionizable species in biphasic media. It reflects the true behavior of ionizable compounds in solution at a given pH value or range [39]. Compounds 2a-c, 3a-c, 4a-c, 7a-c, 9a-c, and 10a-b showed a slight to moderate solubility in water with their Log S ranging from −2.86 to −6.79. The controls, 11 and 12, exhibited values of 0.25 and 0.58, respectively (their polar groups form hydrogen bonds with the water).
The polar surface area (PSA) defines the sum of the surfaces of the polar atoms (N and O) and slightly polar atoms (S and P), and the hydrogen atoms attached to them. This parameter predicts intestinal absorption and the capacity to cross the blood-brain barrier [40]. A PSA below 90 Å 2 is indicative of the capacity of a drug to cross the blood-brain barrier. All of the present derivatives meet this requirement. Interestingly, the compounds with a methoxy group at the C-6 position of the benzopyran ring showed higher PSA values than those with a chlorine or hydrogen atom. For acarbose (12), the PSA value obtained indicates poor penetration of the central nervous system.
Finally, the bioavailability of these compounds was evaluated using Lipinski's rule of 5, which states that a  (4) <10 hydrogen bond acceptors [38]. As can be appreciated in Table 3, the synthesized compounds are in agreement with these guidelines (except 3b and 3c with a molecular weight >500 g/mol).

Conclusions
The series of isoflavones 2-4 were easily synthesized in good yields. In vitro assays showed a greater inhibition of α-glucosidase by these derivatives than the positive control acarbose (12). Furthermore, alkoxy-isoflavone derivatives 4a-b displayed better inhibition than enaminoneisoflavones 3a-b. Docking studies revealed that the πstacking and hydrophobic effects of the aromatic moieties at C-3 and C-7 of the chromone ring play a major role in the interaction with the active site of the enzyme. Compound 7a showed a low degree of antioxidant activity. Hence, the series of α-arylacetophenones and isoflavones herein prepared can be considered promising scaffolds for the development of new compounds with enhanced pharmacological activity.

General experimental procedures
Raw data measurements of melting points were taken on an Electrothermal apparatus and are uncorrected. NMR spectra of 1 H (300, 500, 600 or 750 MHz) and 13  General method for the synthesis of 2-4dihydroxyacetophenones (7a-c) Under a nitrogen atmosphere and at 0°C, BF 3 • OEt 2 (2.0 mol equiv.) was added to a solution of phenols 5a-c (1.0 mol equiv.) and 2-(4-methoxyphenyl)acetic acid (6) (1.2 mol equiv.), and the resulting mixture was stirred at 80°C for 3 h. The reaction crude was poured into cold water (10 ml) and adjusted to neutral pH with an aqueous saturated solution of NaHCO 3 (10%), followed by extraction with EtOAc (3 × 30 ml). The organic layer was dried (Na 2 SO 4 ) and concentrated under vacuum. The residue was purified by flash chromatography over silica gel (hexane/EtOAc, 8:2 or 98:2) to provide the corresponding products 7a-c.

Biological evaluation
Inhibition of α-glucosidase α-Glucosidase inhibition was determined according to a method described by Salehi et al. with some modifications [47]. For this assay, α-glucosidase from S. cerevisiae was employed. A reaction was prepared by mixing 20 µl of αglucosidase solution (0.5 unit/ml), 120 µl of 0.1 M phosphate buffer (pH 6.9), and 10 µl of the sample at several concentrations. The solution was incubated in a 96-well microplate at 37°C for 15 min. Subsequently, the enzymatic reaction was initiated by adding 20 µl of 5 mM PNP solution in 0.1 M phosphate buffer (pH 6.9), followed by incubation at 37°C for 15 min. The reaction was stopped by adding 80 µl of 0.2 M sodium carbonate solution and absorbance was read at 405 nm in a microplate reader (Epoch, BioTek ® ). The reaction system without any test compounds was used as the control, and the system without α-glucosidase served as the blank for correcting the background absorbance. The rate of inhibition on α-glucosidase by the sample was calculated with Eq. (1): DPPH radical scavenging assay The scavenging of free radicals by the synthesized compounds was assessed with the previously reported 2,2diphenyl-1-picrylhydrazyl radical (DPPH) assay, with some modifications [48]. After a solution of each compound was elaborated at a concentration of 2.5 mM in DMSO, a solution of DPPH (133.33 µM in absolute ethanol) was added at a ratio of 1:3 (v/v). The mixture was incubated at 37°C for 30 min before reading the absorbance in a microplate reader (Epoch, BioTek®) at 517 nm. Butylhydroxytoluene served as the positive control. Scavenging capacity was expressed as the percent decrease in DPPH at 2.5 mM: where A control is the absorbance of the DPPH solution (control) and A test is the absorbance of the DPPH solution plus a compound.

Molecular docking studies
Molecular docking simulations were carried out on the AutoDock 4 program [49]. The α-glucosidase homology model was constructed by using the crystal structure of isomaltase from Saccharomyces cerevisiae in complex with its competitive inhibitor maltose (Protein Data Bank (PDB): 3A4A), which was retrieved from the PDB (http://www. rcsb.org/). Docking was validated with maltose to identify the main side chains present at the active site of the enzyme. Water molecules were removed, hydrogen atoms were added to the polar atoms (considering pH at 7.4), and Kollman charges were assigned with AutoDock Tools 1.5.6. The 3D structures of maltose and acarbose were downloaded from the ZINC 15 database [50]. The alkoxyisoflavone derivatives were sketched in two dimensions with ChemSketch (https://www.acdlabs.com/resources/ freeware/chemsketch/) and converted into 3D mol2 format in the Open Babel GUI program [51]. The maltose and acarbose ligands were optimized with PM6 on Gaussian 9875 software to obtain the minimum energy conformation for the docking studies. All the possible rotatable bonds, torsion angles, atomic partial charges and non-polar hydrogens were determined for each ligand. The grid dimensions in AutoDockTools were 60 × 72 × 66 Å 3 with points separated by 0.375 Å, centered at: X = 24.0, Y = −8.0 and Z = 23.028. The hybrid Lamarckian genetic algorithm was applied for minimization, utilizing default parameters. A total of 100 docking runs were conducted, adopting the conformation with the lowest binding energy (kcal/mol) for all further simulations. AutoDockTools was used to prepare the script and files as well as to visualize the docking results, which were edited in Discovery 4.0 Client.


One-pot alkanolamines-assisted synthesis of magnetic mesoporous silica for synthetic dye adsorption

Magnetic mesoporous silica (MMS) was synthesized in a one-pot system using various alkanolamines (triethanolamine, diethanolamine, tris (hydroxymethyl)aminomethane) as a basic catalyst. The characterization of the composites was conducted using scanning electron microscope, transmission electron microscope, X-ray diffractometer, surface area analyzer, and X-ray photoelectros spectroscopy. The MMS synthesized with tris(hydroxymethyl)aminomethane (MMSTRIS) showed the highest specific surface area, pore volume, and average pore diameter. However, when the composites were applied as adsorbents for brilliant green (BG) dye, MMS synthesized with diethanolamine (MMSDEA) showed the highest maximum adsorption capacity of 339.7 mg g−1. The fastest adsorption rate constant of 1.57 × 10−2 g mg−1 min−1 was obtained for MMSTRIS, which has the largest average pore size among all composites. The adsorption kinetic study suggested that the adsorption of BG onto the prepared MMS composites was mainly chemisorption process, which most likely involves electrostatic interaction and hydrogen bonding between BG molecule and the surface of the composites.

Introduction
Synthetic dyes have been extensively used for the dyeing process in various industries due to their low cost and easy production, wide spectrum of color, and stability of the color [1]. During the dyeing process, some parts of dyes are lost and discharged as wastewater effluent. The amount of this loss depends on the dye used and the type of dyed materials. Globally, approximately 700,000 metric tons of 100,000 commercial dyes are produced and up to 70,000 metric tons of these dyes are discharged into wastewater [2]. Most of the wastewater is previously treated to meet the suitable criteria for discharge water. However, the dye residues in wastewater still pose potential threats to the aquatic environment. The presence of dye in surface water, even in a very low concentration, may cause esthetic problems due to the change of water color. Furthermore, the light penetration into the water will be inhibited, preventing aquatic plant photosynthesis and eventually reducing the oxygen production in water [3,4]. Some synthetic dyes are also capable of decreasing the photosynthesis rate of aquatic plants by inhibiting the growth rate, chlorophyll pigment, and metabolic activity [5,6].
Various treatment technologies such as coagulation, chemical oxidation, biodegradation, membrane filtration, adsorption, photocatalysis, and others have been applied for removing dyes from wastewater [7][8][9][10][11][12]. Each technology has its advantages and limitations. Among those technologies, adsorption is still considered the most effective method due to being simple, inexpensive, and effective [13,14]. Thus, 1 3 the effort in developing effective adsorbents is a continuing process.
Mesoporous silica, i.e., amorphous silica materials with pore size ranging from 2 to 50 nm is considered one of the best candidates as an effective adsorbent of dyes, since it has many remarkable properties such as high specific surface area, good stability, chemical inertness, as well as porous and tunable structure [15,16]. However, the use of mesoporous silica as an adsorbent requires a tedious separation process such as filtration and centrifugation to separate the used adsorbent from the treated solution.
The incorporation of magnetic iron oxide nanoparticles on the mesoporous silica material has been attracting much attention for adsorption process, due to its high specific surface area, pore volume, tunable structure, functionalizable surface and easy separation [17][18][19]. Magnetic iron nanoparticles immobilized on mesoporous silica is expected to create a synergistic effect in removing pollutants from aqueous solution by combining the best properties of both materials [20]. In addition, the presence of magnetic nanoparticles on mesoporous silica will eliminate the filtration or centrifugation process needed after the adsorption process. The used adsorbent can be easily separated from the aqueous phase using an external magnetic force, providing a cleaner and fast method [18,21].
Magnetic mesoporous silica (MMS) is frequently synthesized using two steps: magnetic iron nanoparticle synthesis, followed by surface modification of the nanoparticles. In this study, MMS was synthesized in a one-pot system using various alkanolamine compounds as a basic catalyst. The addition of various alkanolamine compounds is expected to affect the MMS properties, since the properties of mesoporous silica are dictated by many factors, including the choice of reagents. The use of the one-pot system is preferred, since it eliminates the multi-step process and reduces the chemical waste and synthesis time. The efficiency of MMS composites in this work as an adsorbent then was evaluated using brilliant green (BG, Fig. S1) as a synthetic dye model pollutant.

Synthesis of MMS
The fabrication of MMS composites was carried out using CTAC surfactant as a template and alkanolamine as a basic catalyst in the one-pot system. The synthesis procedure is illustrated in Fig. S2. MMS was prepared using the following procedure: 100 ml of H 2 O was purged with nitrogen gas for 30 min and 410 mg of FeCl 3 .6H 2 O and 232.6 mg of FeCl 2 .4H 2 O were dissolved into it. After all of the powders had dissolved, 1 mL of 28% ammonia was added into the solution dropwise and the mixture was stirred for 2 h. After that, 320 mg of CTAC was added and the mixture was stirred for another 10 min. 2.24 mmol of alkanolamine compound (TEA, DEA or TRIS) was added and the mixture was sonicated for 30 min. The molecular structure of each alkanolamine is given in Fig. S3. The mixture was heated to 80 °C and stirred during heating. During this step, 0.5 mL of TEOS as silica precursor was added into the solution mixture dropwise. The solution was stirred for 3 h afterward. After the reaction was complete, the products were washed with ethanol and water subsequently. The produced MMS materials were calcined at 500 °C for 5 h to remove the templates. The composites synthesized using TEA, DEA and TRIS were called MMS TEA , MMS DEA and MMS TRIS , respectively.

Characterization of MMS
A scanning electron microscope (SEM, Hitachi, S-4300) combined with an EDX (Horiba, EMAX) analyzer was used to capture the surface morphology and roughly estimate the composition of the materials. The average size and morphology of the materials were observed with a transmission electron microscope (TEM, JEOL, JEM-2100). The specific surface area, total pore volume, and pore size of the materials were determined by N 2 adsorption-desorption isotherm at 77 K using a micrometer (Tristar II 3020, Micrometrics). A Fourier-transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum 400) instrument was used to characterize the functional groups present on the materials. The phase structure of the materials was characterized using an X-ray powder diffraction (XRD, RigakuUltima II/PC) instrument with CuK α radiation. The surface chemistry of the materials was analyzed using an X-ray photoelectron spectroscopy (XPS, ULVAC, Quantera XSM) instrument.

Adsorption experiments
Adsorption experiments were carried out in a batch system. Adsorption isotherm of BG onto MMS composites was studied at 25 °C with the initial concentration of BG ranging from 10 to 90 mg L −1 . A fixed amount of adsorbent was added into the BG solution and stirred for 60 min. Adsorption kinetics of BG onto MMS composites was investigated by mixing 30 mg of adsorbent with 60 mL of 200 mg L −1 BG solution. The residual concentration of BG in the mixture was measured at certain time intervals (0-120 min). All adsorption experiments were conducted in the pH of the original BG solution unless stated otherwise. The adsorption experiment was repeated three times and the result was obtained from the average value of the triplicate. The residual concentration of BG in the aqueous solution was measured by using UV-Vis spectrophotometer (Hitachi, U-4100 Spectrophotometer) at a wavelength of 624 nm. The adsorption efficiency and adsorbed amount of dye (q, mg g −1 ) were calculated using Eqs. 1 and 2: where C 0 (mg L −1 ) is the initial concentration of BG and C t (mg L −1 ) is the concentration of BG in aqueous solution at time t; C e (mg L −1 ) is the concentration of BG in aqueous solution at equilibrium, V is the volume of solution (L), and m is the adsorbent mass (g).
The recyclability of MMS composites as adsorbents for BG dye was evaluated by employing acidified ethanol to desorb BG from MMS composites. After sequential washing using distilled water and oven-drying, the adsorbent was reused for adsorption of BG in solution. This adsorption-desorption process was carried out for four cycles.

Characterization of MMS
SEM and TEM images of all composites in Fig. 1 showed that all of the MMS composites have a spherical shape and agglomeration was observed in all composites; MMS TRIS showed the least agglomeration. From TEM images, the average particle size of the composites was around 10 nm. The rough estimation of the percentage (w/w) of O, Fe and Si atoms in the composites from EDX measurement showed that MMS TEA had the highest Fe content among all composites, while MMS TRIS showed the lowest Fe content (Table S1). The X-ray diffractogram of all MMS composites is presented in Fig. 2a. The presences of characteristics peaks at 30°, 35°, 43°, 54°, 57° and 62° were ascribed to (220), (311), (400), (422), (511), and (440) planes of Fe 3 O 4 , respectively (JCPDS:75-0033), indicating the presence of magnetic iron oxide in all composites. This result showed that the magnetic iron oxide immobilized in silica material maintains its crystalline phase. All composites showed a relatively similar pattern of X-ray diffraction without any obvious peaks of silica material.
The FTIR instrument further confirmed the presence of functional groups in the synthesized composites. The FTIR spectra of MMS composites are presented in Fig. 2b. Each composite showed similar absorbance peak with different intensity, indicating the presence of similar functional groups on the composites. The absorption band at 1090 cm −1 was due to the asymmetric stretching vibration of Si-O-Si bonding. The peak at 800 cm −1 was attributed to symmetric stretching of Si-O-Si. The appearance of Si-O-Si vibration peaks on the materials indicated that the condensation of silicon alkoxide has occurred [22]. The absorbance peak at 570 cm −1 which corresponded to the vibration of the Fe-O bonding also suggested iron oxide's presence in the materials. The FTIR analysis results demonstrated the presence of SiO 2 and Fe 3 O 4 in all of the synthesized MMS.
The specific surface area and porosity of the composites were determined by liquid N 2 adsorption (at 77 K). The N 2 adsorption-desorption isotherms obtained for the composite materials are shown in Fig. 2c. The N 2 adsorption-desorption isotherm curves of all composites exhibited a type IV isotherm and an H1-type hysteresis loop which corresponded to mesoporous materials with a narrow range of mesopores [23]. Table 1 summarizes the textural properties of the synthesized MMS. The surface area of the composites not only depends on the particle size of the material, but also the amount of iron nanoparticles immobilized in the materials. The more the iron contents in the composite, the lower is the surface area due to the blockage of mesopores by the iron nanoparticles. This data agreed with the EDX data in Table S1. MMS TRIS with the largest specific surface area showed the lowest iron content. All of the synthesized composites showed an average pore size in the range of mesopore size. The MMS TEA and MMS DEA showed relatively similar BET surface area, pore volume, and BJH average pore size, while the MMS TRIS showed the highest values among all. These results suggested that the use of alkanolamines with different basicities and structure could influence the properties of the synthesized MMS composites. Alkanolamines could influence the hydrolysis and polycondensation reaction of silica parent material [22]. They could act both as the basic reactant and hydrolysis-retarding agent (complexing agent), which keeps the seeds separated during the condensation process [24].
XPS spectra of MMS composites (Fig. 3a) showed the peaks of Si, C, O and Fe elements. The binding energies of Fe(III)2p 3/2 and Fe(III)2p 1/2 can be seen at 710.5 eV and 724 eV, respectively. This indicated that the iron oxide present in all composites is in the form of magnetite [25]. The presence of silica was shown by Si 2p peak at 102 eV [26]. The XPS spectra of MMS after the adsorption of BG showed a slight increase of the binding energies of Fe(III) in the iron oxide, as shown in Fig. 3b. The binding energies of Fe(III)2p 3/2 and Fe(III)2p 1/2 were observed at 715 eV and 730 eV, respectively. This increase was due to the decrease of electron density of iron after binding with N + of BG molecule [27].

Adsorption studies
The adsorption performance of the MMS composites was studied using BG as a model pollutant. The effect of solution pH on the adsorption efficiency was studied under the initial solution pH of 3 to 6. At pH lower than 3, the BG solution started to decolorize while at pH higher than 6, and the turbidity and precipitation started to occur. Initial pH of the solution plays an important role in the adsorption efficiency of BG onto MMS composites, since the stability and structure of BG molecule and the dissociation of functional groups on the composites are predominantly controlled by the solution pH. It was found that the optimum pH of BG adsorption onto all composites was 3; however, since the initial concentration of BG on pH 3 was significantly reduced (Fig. S4), the adsorption studies were conducted under the original pH of BG solution which was around 4.
The adsorption kinetics study provides information about the efficiency of adsorption process and is an important aspect of an adsorption process related to the industrial batch application [28]. In this work, the kinetic data were evaluated using pseudo-first-order and pseudo-second-order kinetic models, given in Eqs. 3 and 4, respectively.
Parameter t is time (min), q e and q t (mg g −1 ) are adsorption capacities at equilibrium and time t, respectively, and k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are rate constants.
Experiment data were fitted to both kinetic models (Fig. 4) and all obtained kinetic parameters are given in Table 2. It is seen that the adsorption of BG onto all composites followed the pseudo-second-order kinetic model with R 2 values greater than 0.99. Moreover, the adsorption  capacities obtained from the experiment (q exp ) were similar to the values calculated (q cal ) from the linear graph equation of the pseudo-second-order model. This implies that BG's adsorption onto the surface of MMS composites was mainly chemisorption or surface complexation [29]. The pseudo-second-order rate constant of BG adsorption on MMS TRIS is the highest among other composites, followed by the rate constant of BG adsorption on MMS TEA and MMS DEA . It is notable that the larger the average pore size of the composite, the higher is the adsorption rate constant of BG on the MMS composite. The Langmuir and Freundlich isotherm models were used to describe the adsorption process of BG onto the MMS composites. The Langmuir isotherm model assumes that the adsorption process is monolayer, while the Freundlich isotherm model applies to the multilayer adsorption  process. The non-linear equations of Langmuir and Freundlich model are given in Eqs. 5 and 6, respectively.
Parameter C e (mg L −1 ) is the concentration of the dye at equilibrium, q e (mg g −1 ) is the adsorption capacity at equilibrium, q max (mg g −1 ) is the maximum adsorption capacity, K L (L mg −1 ) is the Langmuir constant, and K F and n are the Freundlich adsorption constants.
The adsorption experiment data of BG onto all MMS composites were fitted using both isotherm models. The fitting curves are shown in Fig. 5. and the adsorption isotherm parameters are given in Table 3. The adsorption of BG onto MMS composites fitted well both Langmuir and Freundlich isotherm models (R 2 > 0.95) in the studied adsorptive concentration range. The 1/n value obtained from Freundlich isotherm was greater than 0 for all composites, indicating that the adsorption of BG on all prepared MMS composites was favorable [30]. The maximum adsorption capacity of BG onto all MMS composites in this work was comparable to some previously reported works (Table S2).
Adsorption kinetic study suggested that the adsorption of BG onto MMS composites was mainly through the chemisorption process. Adsorption of BG onto MMS composites could involve electrostatic interaction between the protonated ammonium of BG molecule and deprotonated hydroxyl group available on the surface of the composites. In addition, hydrogen bonding between hydroxyl groups of the adsorbent and amine group of BG might also proceed [31] as illustrated in Fig. S5. Further study is necessary to elucidate a detailed adsorption mechanism of BG onto the prepared MMS composites. The regeneration ability of the MMS composites was studied, since regeneration and recyclability properties of an adsorbent are crucial for the implementation in the real wastewater. The result of recyclability studies of all MMS composites is given in Fig. 6. The adsorption capacity of all MMS composites decreased quite significantly from one cycle to another. After the fourth cycle, the adsorption capacity of MMS decreased by 73%, 59%, and 82% for MMS TEA , MMS DEA , and MMS TRIS , respectively. However, after the third cycle, all composites still showed a good adsorption capacity of around 90-100 mg g −1 . A complete recovery of BG from MMS composite was hard to achieve, most likely due to the strong affinity of MMS for BG molecule. In addition, after sequential adsorption-desorption and washing processes, the active sites of MMS might be blocked or changed, causing the decrease of the adsorption capacity.

Conclusions
Magnetic mesoporous silica (MMS) composites have been synthesized using various alkanaolamines as a basic catalyst in a one-pot system. The use of different alkanolamines resulted in MMS with different surface area, pore volume, and average pore size. The characterization analysis showed the presence of magnetic nanoparticles in all composites. The application of the prepared composites for the removal of brilliant green (BG) from solution showed satisfactory results with excellent maximum adsorption capacities and adsorption rate. The main plausible mechanisms of adsorption of BG onto MMS were the chemical interaction such as hydrogen bonding and electrostatic interaction between BG and the hydroxyl groups of the composites. All of the prepared composites still showed high adsorption capacity toward BG after the third adsorption-desorption cycle. The results showed the potential of MMS as an adsorbent for the removal of BG dye from wastewater.