Synthesis of Cobalt-Zinc Phosphates Templated by Polyamines

Open-framework structures of metal phosphates, especially those zinc and cobalt phosphates, have exhibited many fascinating structural features and potential applications in catalysis, separation processes, and as photoluminantphosphors (Rao et al., 2000; Chang et al., 2004). The feasibility of zinc and cobalt to tetrahedrally coordinate their phosphates makes them to be an important group of transition metal phosphates (Mandal & Natarajan, 2002; Harrison et al., 1991; Neeraj et al., 1999; Fan et al., 2000). Usually, these materials have been synthesized by employing hydrothermal or solvothermal conditions in the presence of organic amines as the structure-directing agents (SDA). Studies have shown that the important role of host–guest charge matching between the inorganic frameworks and the organic amines makes linear polyamines the preferred candidates for the structure-directing agents to form the higher negative charge framework of zinc and cobalt phosphates (Bu et al., 1997, 1998; Feng et al., 1997). Moreover, diethylenetriamine (DETA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) become typical amines and have directed the formation of a large number of open-framework structures of zinc (or cobalt) phosphates by varying their concentrations accompanying with the changes in inorganic compositions (Choudhury et al., 2000). Recently, a new synthesis route involving alkylformamide as a template precursors has been reported (Lakiss et al., 2005; Vidal et al., 2000). It seems that organic amines generated in situ by the decomposition of alkylformamide show a special effect on the formation of metal phosphates with a novel structure. In our studies aimed to synthesize transition metal phosphates, more than one cobalt-zinc phosphates with different framework structures have been obtained from a single gel using diethylenetriamine as the organic basis (Lu et al., 2008; Li et al., 2009). It is noticed that the guest species encapsulated in their structures are not diethylenetriamine, but some smaller amines decomposed from it. Controlling the in situ decomposition of these linear polyamines plays the key role for directing the synthesis of pure-phase cobalt-zinc phosphates. According to these matters, large amines can also be used as a source for smaller amines as they will provide hydrolysis decomposition in the reaction mixture during crystallization. In the synthesis a temperature of crystallization is an important factor for controlling the hydrolysis of linear polyamines, and higher temperature makes decomposition of polyamines easier. On the other hand, adding small amines, such as propylamine, butylamine, can control the decomposition of the chain-type polyamines.


Introduction
Open-framework structures of metal phosphates, especially those zinc and cobalt phosphates, have exhibited many fascinating structural features and potential applications in catalysis, separation processes, and as photoluminantphosphors Chang et al., 2004). The feasibility of zinc and cobalt to tetrahedrally coordinate their phosphates makes them to be an important group of transition metal phosphates (Mandal & Natarajan, 2002;Harrison et al., 1991;Neeraj et al., 1999;Fan et al., 2000). Usually, these materials have been synthesized by employing hydrothermal or solvothermal conditions in the presence of organic amines as the structure-directing agents (SDA). Studies have shown that the important role of host-guest charge matching between the inorganic frameworks and the organic amines makes linear polyamines the preferred candidates for the structure-directing agents to form the higher negative charge framework of zinc and cobalt phosphates (Bu et al., 1997(Bu et al., , 1998Feng et al., 1997). Moreover, diethylenetriamine (DETA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) become typical amines and have directed the formation of a large number of open-framework structures of zinc (or cobalt) phosphates by varying their concentrations accompanying with the changes in inorganic compositions (Choudhury et al., 2000). Recently, a new synthesis route involving alkylformamide as a template precursors has been reported (Lakiss et al., 2005;Vidal et al., 2000). It seems that organic amines generated in situ by the decomposition of alkylformamide show a special effect on the formation of metal phosphates with a novel structure. In our studies aimed to synthesize transition metal phosphates, more than one cobalt-zinc phosphates with different framework structures have been obtained from a single gel using diethylenetriamine as the organic basis (Lu et al., 2008;Li et al., 2009). It is noticed that the guest species encapsulated in their structures are not diethylenetriamine, but some smaller amines decomposed from it. Controlling the in situ decomposition of these linear polyamines plays the key role for directing the synthesis of pure-phase cobalt-zinc phosphates. According to these matters, large amines can also be used as a source for smaller amines as they will provide hydrolysis decomposition in the reaction mixture during crystallization. In the synthesis a temperature of crystallization is an important factor for controlling the hydrolysis of linear polyamines, and higher temperature makes decomposition of polyamines easier. On the other hand, adding small amines, such as propylamine, butylamine, can control the decomposition of the chain-type polyamines.

Synthesis and initial characterization
The main reactants are Zn(CH 3 COO) 2 (denoted as ZnAc 2 ), Co(CH 3 COO) 2 (denoted as CoAc 2 ), H 3 PO 4 , linear polyamines, and ethylene glycol. In a typical synthetic procedure, solution A was prepared by mixing ZnAc 2 ·2H 2 O with 85% H 3 PO 4 and ethylene glycol (EG), the mixture was stirred at room temperature until ZnAc 2 ·2H 2 O was dissolved. Solution B was prepared by mixing CoAc 2 ·4H 2 O, distilled water, and 85% H 3 PO 4 . Solution A and a kind of linear polyamine, such as diethylenetriamine were added to solution B. Then the pH value of the mixture was adjusted to 8.0 by adding an assistant amine. Finally the mixture was transferred to a stainless steel autoclave and heated at 140˚C~160˚C for 3-10 days. Specific gel compositions and crystallization conditions of three cobalt-zinc phosphates are shown in Table. 1). XRD patterns of the three compounds also show their phases being distinct from each other (Fig.1). www.intechopen.com

Crystal structure of CoZnPO 4 -V
Single crystal analysis and Inductively Coupled Plasma atomic emission spectroscopy (ICP) analysis reveal that CoZnPO 4 -V is composed of C 12 N 9 H 52 Zn 4.5 Co 1.5 P 8 O 32 ; it crystallizes in the monoclinic system, space group P2 1 /c (No. 14), with a = 31.936(3) Å, b = 8.3775(7) Å, c = 15.7874(13) Å, = = 90°, = 97.0530(10)°, V = 4191.8(6) Å 3 , and z = 4. Three edge-shared 4-ring units of Zn-O and P-O tetrahedral groups could be the building unit, which was named as 4 3 unit (Fig. 3a). The neighboring units link with each other through 4-ring to form an infinite zigzag chain along the b-axis direction (Fig. 3b). 2D (4, 10) nets are conformably formed of the chains of 4-ring units from the corner-shared 4 3 units ( Fig. 3b). Viewed along the plane parallel to b-axis, the neighboring 4 3 units link with each other in a corner-shared way to form another infinite chain (Fig. 3c). The neighboring chains link together from 4-ring every two 4 3 units to form a layer. Therefore, 16-ring channels are constructed (Fig. 3c). On the other hand, the wall of each 16-ring channel can be described as the coiling of a net composed of 10-rings and infinite 4-ring zigzag chain (Fig. 3b).

Crystal structure of CoZnPO 4 -VI
Single crystal analysis and Inductively Coupled Plasma atomic emission spectroscopy (ICP) analysis reveal that CoZnPO 4 -VI is composed of C 6 N 4 H 22 Co 0.16 Zn 5.84 (PO 4 ) 4 (HPO 4 ) 2 ; it crystallizes in the triclinic system, space group P 1 (No. 2), with a = 5.1919(10) Å, b = 8.7263 (17)  The three-dimensional framework of CoZnPO 4 -VI can be described by with two secondary structural units (SBU-a and SBU-b). SBU-a (Fig.4a) is a special structure composed of two 6rings ladder linked face to face; SBU-b (Fig.4b) is a common 4-ring ladder in the molecular sieves. The linkage of SBU-a conducted by sharing 4-rings form the layers along ab-plane. SBU-a and SBU-b connect with one sharing oxygen atom in the structure, forming the threedimensional framework of CoZnPO 4 -VI and resulting in 16-ring large channels along the direction of c-axis (Fig.4c). The arms of channels are constituted mainly by SBU-a and SBU-b.

Controlling the in situ decomposition of linear polyamines to synthesize open-framework cobalt-zinc phosphates
CoZnPO 4 -I, CoZnPO 4 -II, and CoZnPO 4 -III, have been synthesized from a single gel at 200 °C. Organic amine in the starting mixture is diethylenetriamine. Structural analysis reveals that species encapsulated in the structures of CoZnPO 4 -I, CoZnPO 4 -II, and CoZnPO 4 -III are ammonium, ethylenediaminum, and diethylenetriaminum, respectively, which can be attributed to the partly in situ decomposition of diethylenetriamine molecules during the crystallization procedure ( Fig.13a, 13b, and 13c). When a second small organic amine, such as propylamine, or butylamine, was added to reacting mixture, single phase of CoZnPO 4 -I, CoZnPO 4 -II, or CoZnPO 4 -III can be obtained respectively, according to the kind or concentration of small organic amine, which plays an important role on controlling the in situ decomposition of linear polyamines.  Three open-framework cobalt-zinc phosphates have been synthesized hydrothermally from a single gel at 200°C by using DETA as the SDA with n-propylamine as the assistant organic amine. They are all large vivid blue crystals with sizes more than 400 μm. Scanning electron micrograph (SEM) images show their morphologies being distinct from each other, shaped as elongated hexagonal-dipyramid, square bifrustum, and cubical, respectively (Fig.5). XRD patterns of CoZnPO 4 -I, -II, and -III (Fig.6) have shown that they are three distinguishing phases with different diffraction peaks.

Synthesis and initial characterization
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Crystal structures of three compounds
It is clear that there are only diethylenetriamine and n-propylamine as the SDAs present in the starting materials. However, organic species encapsulated in the structure of CoZnPO 4 -I, -II, and -III are ammonium, ethylenediaminum, and diethylenetriaminum cations, respectively (Fig.13a, 13b, 13c). The appearance of ethylenediaminum and ammonium may be attributed to the in situ decomposition of diethylenetriamine during the crystallization. Single crystal analysis reveals that CoZnPO 4 -I is a novel cobalt-zinc phosphate. It is composed of (NH 4 ) 2 Co 0.34 Zn 1.66 (PO 4 ) 2, which was modeled as (NH 4 ) 2 Co 2 (PO 4 ) 2 in the single crystal data and crystallizes in the hexagonal system, space group P6 3 (No. 173), with a =b =10.7207(5)Å, c =8.7241(8) Å, = =90°, λ =120°, V=868.36(10) Å 3 , and z =4. Viewing along the plane parallel to c-axis, a net consisting of 6-ring channels has been found. The 6ring channels with regularly alternating Zn(Co)-and P-centered tetrahedral can be considered as the building units of CoZnPO 4 -I (Fig. 7a). J. V. Smith determined a simple hexagonal net, which has only one type of 3-connected node and is described as the Schläfli symbol 6 3 because each node lies between three circuits of 6 nodes (Smith, J. V. Am. Mineral. 1977).For any node in a simple hexagonal net in horizational position, an additional perpendicular linkage pointing either upward (U) or downward (D) may result in the linkage joined to a node of another simple hexagonal net lying either above or below the first hexagonal net. Eight ways of the sequence of U and D linkages around each 6-ring has been used to enumerate 4-connected, 3-dimensional nets and classification of framework of silicates and aluminophosphates. The linking sequences of UDUDUD around each 6-ring produces a dense tridymite framework, whereas the UUUDDD results in an open framework ABW containing 8-ring channels. However, infinity of frameworks can be produced if more than one sequence occurs in the hexagons of a 6 3 net. This has just happened in the structure of CoZnPO 4 -I. In its framework, the 6-rings of tetrahedral are present in two different conformations based on the more than one type circuit as the following sequence of up and down tetrahedral: UDUDUD and UUUDDD in the proportion 1:3. A second building unit, which is called a 6 9 4 6 cage, has been formed (Fig. 13a).

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On the other hand, it can be found along the (010) plane that double 4-rings and one 6-ring linked in turn by sharing edges to form an infinite chain (Fig. 7b). These chains are crosslinked along the c-axis direction to the similar chains from the linkage Zn-O-P or P-O-Zn to form the sheet structure with edgesharing infinite 10-ring chains (Fig. 7b). Adjacent sheets are cross-linked with each other along the vertical direction of (010) plane via the Zn-O-P bonds displaced by +1/2 in the [001] direction to form the three-dimensional framework of CoZnPO 4 -I (Fig.7c). Each 10-ring is blocked by double 4-and 6-rings around it. So the tendency to construct 10-ring channels is interdicted with only clathrate 6 9 4 6 cages to be formed.   6 ]. Its structure is made of CoO 6 , ZnO 4 (or CoO 4 ), and PO 4 polyhedra. The cobalt octahedron is surrounded by six zinc tetrahedra to form the CoZn(or Co) 6 O 6 cluster which are linked via six PO 4 tetrahedra to other clusters to form its framework. The clusters are arranged i n s u c h a m a n n e r t h a t e a c h c l u s t e r i s displaced by half its a-distance from its neighbors, forming a honeycomb layer. The next layer of clusters is identical to the first but is displaced along the a-axis by half the unit cell so that the honeycomb channels are capped. The framework of AB-type stacking results in the formation of 8-ring channels at an angle to a-axis (Fig. 9).

The in situ framework transformation of cobalt-zinc phosphates in the presence of linear polyamines
Although linear polyamines cannot decompose at low crystallization temperature, experiments have shown the crystallization time also affects the crystal structure transformation. Another typical result that we have found is the transformation of two cobalt-zinc phosphates, CoZnPO 4 -VI and CoZnPO 4 -VII. CoZnPO 4 -VI was synthesized by using TETA as the template, which can be transformed into CoZnPO 4 -VII with the increase of crystallization time in 140°C.  .10) shows the morphology of CoZnPO 4 -VII is as a relatively large irregular block, it is distinct from CoZnPO 4 -VI (Fig.1c). XRD patterns of CoZnPO 4 -VI and CoZnPO 4 -VII have also shown they are two distinguishing phases with different diffraction peaks ( Fig.10 and Fig.1c).

Crystal structure of CoZnPO 4 -VII
Single crystal analysis and Inductively Coupled Plasma atomic emission spectroscopy (ICP) analysis reveal that CoZnPO 4 -VII is composed of C 6 H 22 Co 0 N 4 O 16 P 4 Zn 4 ; it crystallizes in the triclinic system, space group P 1 (No. 2), with a = 8.0871 (16)   The biggest feature in the structure of CoZnPO 4 -VII is that there are two cross-linked 8-ring channels in the structure. The three-dimensional framework of CoZnPO 4 -VII can be also described as the stack of a secondary structural unit (SBU-c) (Fig.11a). In the direction of aaxis, the connection of 8-rings by three 4-rings forms a ladder (SBU-c) (Fig.11b); in the direction of b-axis ladders are joined by sharing edges, spreading the layers running abplane. Then adjacent layers are connected with 4-rings. All described above spread the three-dimensional framework of CoZnPO 4 -VII and exhibit a 8-ring channel system running along the direction of a-axis (Fig.11c). The arms of channels are constituted mainly by SBU-c. At the same time they form another 8-ring channel system running the direction of c-axis (Fig.11d).

Relationship between CoZnPO 4 -VI and CoZnPO 4 -VII
The similarities between CoZnPO 4 -VI and CoZnPO 4 -VII provide the possibility of transformation. On one hand, both of them are composed of Co,Zn,P,O elements, and all Zn (Co) atoms and P atoms in their structures are tetrahedrally coordinated by oxygens, which have been seen from their asymmetric unit structures (Fig.12). On the other hand, the templates filled in their inorganic frameworks are TETA, and both of them crystallize in the triclinic system, space group P 1 (No. 2). All of these result in the transformation of CoZnPO 4 -VI and CoZnPO 4 -VII with the increase of crystallization time.

Investigation of the in situ decomposition of linear polyamines during hydrothermal synthesis of cobalt-zinc phosphates
In this section, polyamines such as diethylenetriamine, triethylenetetramine, and tetraethylenepentamine were used as structure directing agents to investigate the in situ decomposition of linear polyamines during the crystallization of microporous zinc-cobalt phosphate materials. Mixtures of water and ethylene glycol were used as solvents.

Experimental methods
The important basis for determination of polyamines' decomposition is consideration of the template molecules in the framework of all crystals. From the summarized above, we know that the template molecules in the framework of CoZnPO 4 -I, CoZnPO 4 -II, CoZnPO 4 -III, CoZnPO 4 -IV, CoZnPO 4 -V and CoZnPO 4 -VI are NH 4 + , en, DETA, DETA, DETA and TETA, respectively (Fig.13).
Previous studies have shown preliminary that temperature is an important factor for controlling the hydrolysis of linear polyamines. Other, the addition of different assistant templates has different effects on decomposition of propylamines while using the straightchain polyamines as a precursor of the template in the synthesis of cobalt-zinc phosphates. In order to investigate the decomposition of polyamines, the same ratio of gel should react for a certain time at different temperatures, and the pH value of gel should be adjusted to pH (7.0-9.0) by the addition of different assistant templates. The selected molar ratio of gel is 2ZnAc 2 •2H 2 O:1CoAc 2 •4H 2 O:9.6EG:10.5H 3 PO 4 :95.4H 2 O:1.8R (R is DETA, TETA, TEPA respectively).

Discussions of factors affecting decomposition of polyamine
The results are shown in Table. 2 when crystallizing at different temperatures for 6 days.  However, when an assistant amine, such as n-propylamine, was added to the mixture to increase the pH value to 7-9, even if the mixture is crystallized at 200 °C, three metal phosphates (CoZnPO 4 -I, -II, -III) have been obtained with diethylenetriamine still encapsulated in the pores of CoZnPO 4 -III. It indicates that the presence of n-propylamine can restrain the hydrolysis of diethylenetriamine during the crystallization. It can be found from Table 2 that, apart from n-propylamine, other assistant amines, such as nbutylamine, dipropylamine, and tripropylamine, can also restrain the decomposition of diethylenetriamine to some extent. The influence of second organic amine may be attributed to the effect of the pH on hydrolysis of the linear polyamines. In the case of triethylenetetramine or tetraethylenepentamine, the controlling effects of assistant amine are less prominent than for diethylenetriamine. CoZnPO 4 -II and -III have only been obtained as the pure phases by varying assistant amine or crystallizing temperature. The reacting mixture with the molar ratio of 2:1:9.6:10.5:95.4:1.8 ZnAc2:CoAc2:EG:H3PO4:H2O:R was crystallized for 6 days; y = yes, compound has been obtained; n = no,compound has not been obtained. Table 2. Products obtained by adding various assistant amines to adjust the pH value of the mixture to 7-9 a

Conclusions
Synthesis of zinc-cobalt phosphate materials have been investigated by using linear polyamines, such as tetraethylene-pentamine, triethylenetetramine or diethylenetriamine as SDA. During the investigation of decomposition of polyamines seven cobalt-substituted zinc phosphates were obtained: CoZnPO 4 -I-VII. CoZnPO 4 -I, CoZnPO 4 -II, and CoZnPO 4 -III, have been synthesized from a single gel at 200 °C in the presence of diethylenetriamine as the structure-directing agent. Structural analysis reveals that species encapsulated in the structures of CoZnPO 4 -I, CoZnPO 4 -II, and CoZnPO 4 -III are ammonium, ethylenediaminum, and diethylenetriaminum, respectively, which can be attributed to the partly in situ decomposition of diethylenetriamine molecules during the crystallization procedure. CoZnPO 4 -IV，CoZnPO 4 -V and CoZnPO 4 -VI with differently unique 3D framework topologies have been synthesized under hydrothermal or solvothermal conditions in the presence of linear polyamines as structure directing agents (SDA). CoZnPO 4 -VI and CoZnPO 4 -VII, which were synthesized by using TETA as the template, are symbiotic and CoZnPO 4 -VI can be transformed into CoZnPO 4 -VII with the increase of crystallization time in 140°C.
In the synthesis of zinc-cobalt phosphates, temperature is an important factor, favoring the hydrolysis of linear polyamines at higher temperature. Investigations have also found that the in situ decomposition of tetraethylene-pentamine, triethylenetetramine, or diethylenetriamine can be controlled by using second organic amines, such as propylamine as the assistant organic additive. The presence of an assistant amine can restrain the hydrolysis of diethylenetriamine during the crystallization. If synthesis conditions can be controlled, different structures can be transformed into each other.

Acknowledgment
This work was funded by the Natural Science Foundation of China (20873069) and State Basic Research Project (2009CB623502).