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.