Selective hydrogenation of levulinic acid to γ-valerolactone using bimetallic Pd-Fe catalyst supported on titanium oxide

The synthesis of γ-valerolactone (GVL), a versatile precursor in the manufacture of high-value chemicals such as polymer plasticizer, solvent, jet fuel, and, agrochemicals, has been targeted by many research groups. In this study, we report the catalytic performance of iron-modified palladium supported on titanium oxide (denoted as Pd(5.0)-Fe(5.0)/TiO2; 5.0 was the loading amount of Pd and Fe, respectively) in the selective hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) in stainless-steel batch reactor system. Pd(5.0)-Fe(5.0)/TiO2 catalyst was synthesized by a simple hydrothermal method at 150°C for 24 h, then followed by reduction with H2 at 500°C for 3 h. The X-ray diffraction (XRD) patterns of Pd(5.0)-Fe(5.0)/TiO2 before and after reduction showed the formation of metallic Pd at 2θ = 40.34° and 47.2° corresponding to Pd(111) and Pd(200), respectively. The hydrogenation of LA to GVL effectively occurred in the H2O solvent, whereas in 2-propanol the formation of ester was the main side product. The highest yield of GVL (52.5%) was obtained over Pd(5.0)-Fe(5.0)/TiO2 catalyst at a temperature of 170 °C, initial H2 pressure of 4.0 MPa, solvent H2O 3 ml at a reaction time of 7 h. The yield of GVL slightly increased to 63.3% when the reaction time was prolonged to 15 h.


Introduction
Biomass feedstock valorizations are currently being explored for the production of bio-based platform chemicals, fuels, and various commodity products by using bimetallic-based heterogeneous catalysts. The use of bimetallic catalysts is a promising option for biomass feedstock upgrading, as the interaction between metals can modify the surface properties of the catalyst that meet with the complex surface [1][2][3]. Furfural (FFald) and levulinic acid (LA) have been identified to be the most promising platform molecules that can turn into a variety of value-added derivative molecules including polymer, plasticizer, solvent, jet fuel precursor, and agrochemicals [4]. FFald and LA were produced effectively from C-5 sugars in hemicellulosic biomasses, such as xylan and arabinose or C-6 sugars in form of glucose or fructose via acidic hydrolysis [5].
In the hydrogenation of LA into g-valerolactone, heterogeneous catalysts of the monometallic bimetallic precious metal transition have been widely used to achieve high yields of GVL. This hydrogenation reaction is important because the resulting products are used in the chemical industry in the production of high-yield chemicals. Additionally, the study is interesting because there is a combination of two metals with just the right reaction condition. Bourne et al. [6] [7]. Recently, Luo et al. reported bimetallic Au-Pd/TiO2 and Ru-Pd/TiO2 catalysts for hydrogenation of LA into GVL. The formation of bimetallic alloy or dilution of Au or Ru by Pd improved the selectivity and stability of the catalysts [8]. We have reported the catalytic performance of the bulk structure of intermetallic Ni-Sn catalysts in LA hydrogenation and showed high selectivity in aqueous phase hydrogenation of LA into GVL at 433 K, 4.0 MPa for 6 h and produced GVL yield of >99%. The activity and selectivity of used bulk Ni-Sn catalysts slightly decreased after several reaction runs that might be due to the leaching of metals (Ni and Sn) into the reaction solution [9]. In our most recent extended work, Ni-Sn alloy supported on aluminum hydroxide (denoted as prepared Ni-Sn(x)/AlOH; x = Ni/Sn molar ratio, 7.9 to 1.0) demonstrated highly active and stable catalysts for aqueous-phase hydrogenation of LA into exclusive GVL product. The used solvent reaction and the high dispersion of Ni-Sn alloy on aluminum hydroxide (bayerite and gibbsite) facilitated reaction rate at a much lower temperature than that of bulk ones. Moreover, the Ni-Sn(1.4)/AlOH catalyst was reusable for at least six consecutive runs without any significant loss of activity and selectivity [10].
In the present paper, we report the catalytic performance of iron-modified palladium supported on titanium oxide (denoted as Pd(5.0)-Fe(5.0)/TiO2; 5.0 was the loading amount of Pd and Fe, respectively) in the selective hydrogenation of levulinic acid to g-valerolactone in stainless-steel batch reactor system in various solvents. The purpose of this work is to study the effect of Fe loading amount to Pd (%wt), the solvent used, and initial H2 pressure on the conversion of LA and the yield of GVL. The structure-activity relationship of Pd(5.0)-Fe(5.0)/TiO2 catalyst in LA hydrogenation to GVL is discussed.

Catalyst preparation
A typical procedure for the synthesis of Pd(5.0)-Fe(5.0)/TiO2 (5.0 was the loading amount of Pd and Fe, respectively) described as follows (11): PdCl2 (0.0638 g) was dissolved in deionized water (denoted as solution A) and FeCl3×6H2O (0.0605 g) was dissolved in ethanol/ethylene glycol (2.0/1.0 volume ratio) (denoted as solution B) at room temperature. Solutions A, B, and TiO2 (as support material; 1.0 g) were mixed at room temperature; the temperature was subsequently raised to 50 o C under gentle stirring for 12 h. The pH of the mixture was adjusted to 12 through the dropwise addition of an aqueous solution of NaOH (3.1 M). The mixture was then placed into a sealed-Teflon autoclave for the hydrothermal process at 150 o C K for 24 h. The resulting black precipitate was filtered, washed with distilled water, and then dried under vacuum overnight. Prior to the catalytic reaction, the obtained black powder was treated under hydrogen at 500 o C for 3 h.

Catalyst characterization
The X-ray diffraction (XRD) analysis was performed on a Miniflex 600 Rigaku instrument with Cu as a monochromatic source of CuKα radiation (l = 0.15444 nm). The XRD was operated at 40 kV and 15 mA with a step width of 0.02 o , a scan speed of 4 o min -1 (α1 = 0.154057 nm, α2 = 0.154433 nm), solar slit 1.25 o , and using a Ni Kb filter. The BET surface area (SBET) and pore volume (Vp) were measured using N2 physisorption at -196 °C on a Belsorp Max (BEL Japan). The samples were degassed at 200 o C for 2 h to remove physisorbed gases prior to the measurement. The amount of nitrogen adsorbed onto the samples was used to calculate the Brunauer-Emmett-Teller (BET) surface area via the BET equation. The pore volume was estimated to be the liquid volume of nitrogen at a relative pressure of approximately 0.995 according to the Barrett-Joyner-Halenda (BJH) approach based on desorption data (12). The NH3-TPD was carried out on a Belsorp Max (BEL Japan). The samples were degassed at an elevated temperature of 100-200 o C for 2 h to remove physisorbed gases prior to the measurement. The temperature was then kept at 200 o C for 2 h while flushed with He gas. NH3 gas (balanced NH3, 80% and He, 20%) was introduced at 100 o C for 30 min, then evacuated by helium gas to remove the physisorbed also for 30 min. Finally, temperature-programmed desorption was carried out at a temperature of 100-800 o C and the desorbed NH3 was monitored by TCD.

Catalytic reaction
The typical procedure for hydrogenation of lauric acid is described as follows: Catalysts (0.04 g), levulinic acid (0.23 g), and H2O (5 ml) as the solvent, and 1,6-hexanediol (0.025 g) were placed into a glass reaction tube in an autoclave reactor system of TAIATSU Techno reactor (a Pyrex tube was fitted inside of a sus316 jacket to protect the vessel from corrosion in acidic media. The reactor was flushed with 1.0 MPa H2 gas 10 times before removing undesired gases. After H2 was introduced into the reactor with an initial H2 pressure of 4.0 MPa at room temperature, the temperature of the reactor was increased to 170 o C. After 7 h, the conversion of levulinic acid and the yield of g-valerolactone were determined by gas chromatography (GC) analysis.

Product analysis
GC analysis of the reactant (levulinic acid) and product (g-valerolactone) was performed on a Perkin Elmer Auto System XL equipped with a flame ionization detector and Restek Rtx® BAC Plus capillary column. GC analysis was operated at the detector and injector temperatures of 250 o C, N2 as a carrier gas, rates of air, and H2 were 450 ml/min and H2 45 ml/min, respectively. The used 5.0Pd-5.0Fe/TiO2 catalyst was easily separated using either simple centrifugation or filtration, then finally dried overnight under vacuum at room temperature prior to the reusability testing.

Screening of solvent
In the first experiments, we evaluate the use of various solvents as solvent media for the hydrogenation of levulinic acid (LA) to g-valerolactone (GVL) using Pd(5.0)-Fe(5.0)/TiO2 catalyst as shown in figure 1 and the results are summarised in table 1. Three types of solvents such as 1,4dioxane, 2-propanol, and water which are corresponding to polar aprotic, polar semi-protic, and polar protic, respectively, were employed. In polar aprotic solvent (1,4-dioxane), 29.3% yield of gvalerolacton at 29.3% conversion of levulinic acid was obtained (entry 1). In the polar semi-protic solvent of 2-propanol, the products were GVL (27.4%) and isopropyl levulinate (39.8%) at the relatively high conversion of levulinic acid (c.a. 67.2%) under the same reaction conditions (entry 2). In H2O, 52.4% yield of GVL and 10.4% yield of pseudo-LA were obtained at 62.7% conversion of LA (entry 3). After reaction temperature was prolonged to 15 h, conversion of LA as well as the yield of GVL slightly increased to 68.6% and 63.3%, respectively, suggesting the extended reaction time caused a further reaction of pseudo-LA to the final product of GVL (entry 4). In the H2O/2-propanol and H2O/1,4-dioxane mixture solvents, the hydrogenation of levulinic acid occurred with unsatisfactory results (entries 5 and 6) as well as in absence of solvent (entry 7). The relatively high yield of g-valerolactone in water solvent could be attributed to the presence of H-bonded water molecule or single chemisorbed water on metallic surfaces that could dramatically reduce the energy span of the reaction pathway, hence enhancing the catalytic activity [13]. Alternatively, the increase of surface concentration in hydrogen atom due to dissociated water and strong interaction between water with the substrate by hydrogen bonding lowered the activation energy barrier and led to high hydrogenation rates [14,15].

Effect of Fe loading amount
The influence of Fe intercalation on the catalytic performance of the Pd/TiO2 catalyst in levulinic acid hydrogenation was evaluated and the Fe amounts were 1.0, 3.0, and 5.0 %wt to the amount of Pd and the results are summarised in table 2.

Effect of initial H2 pressure
The effect of initial H2 pressure on the conversion and yield in the levulinic acid hydrogenation was evaluated using Pd(5.0)-Fe(5.0)/TiO2 catalyst and the results are summarised in table 3. At initial H2 pressure of 1.0 MPa, an 18.8% yield of GVL was obtained (entry 1). Both conversions of levulinic acid and yield of GVL increased gradually when the initial H2 pressure was increased up to 3.0 MPa (entries 2 and 3) then tended to remain unchanged at initial H2 of 4.0 MPa which decreased in yield of pseudo-LA (entry 4).

Structure-activity relationship
Based on XRD analysis and reaction profiles, we discuss the structure of active Pd and the presence of FeOx species in bimetallic Pd(5.0)-Fe(x)/TiO2 catalysts after reduction with H2 at 500 o C for 3 h. The as-prepared Pd(5.0)-Fe(5.0)/TiO2 (without reduction with H2) was as referenced catalysts and inactive for hydrogenation of lauric acid (table 1, entry 6). The XRD patterns of Pd(5.0)-Fe(x)/TiO2 catalysts before and after reduction with H2 at 500 o C for 3 h (figures 2 (a) and (b), respectively) suggest that the formation of metallic Pd(0) was observed at 2q = 40.34 o and 47.2 o corresponding to Pd(111) and Pd(200), respectively [16]. On the other hand, the metallic or oxide Fe species were unable to detect due to its crystallite sizes were lower than that the detection limit of XRD.

Conclusion
A series iron-modified palladium supported on titanium oxide (Pd(5.0)-Fe(x)/TiO2 (5.0 and x are loading amount of Pd and Fe, respectively) catalysts were successfully synthesized using a simple hydrothermal method at 150 o C for 24 h followed by reduction with H2 at 500 o C for 3 h. Results of XRD analysis of Pd(5.0)-Fe(x)/TiO2 catalysts suggest that the formation of metallic Pd(0) was observed at 2q = 40.34 o and 47.2 o corresponding to Pd(111) and Pd(200), respectively. The hydrogenation of LA to GVL effectively occurred in the H2O solvent, whereas in 2-propanol the formation of ester (iso-propyl levulinate (39.8%) was the main side product at the relatively high conversion of levulinic acid (67.2%). The highest yield of GVL (52.5%) was obtained over Pd(5.0)-Fe(5.0)/TiO2 catalyst at a temperature of 170 ℃, initial H2 pressure of 4.0 MPa, solvent H2O 3 ml at a reaction time of 7 h. The yield of GVL slightly increased to 63.3% when the reaction time was prolonged to 15 h.