Preparation and thermal decomposition of zinc/copper trimellitates and pyromellitates

Abstract

The aim of this research was to prepare and characterise zinc, copper and mixed zinc/copper trimellitates (1,2,4-benzene tricarboxylates) and pyromellitates (1,2,4,5- benzene tetracarboxylates). The characterisation included a study of the thermal decomposition for selected salts. The literature is reviewed, discussing the synthesis, structural properties, and thermal decompostions of carboxylate salts. This review highlights the growing interest in benzene carboxylates, and a summary of the applications of such salts is given at the end of the review. The preparative route investigated in the work reported here involved reacting zinc/copper hydroxycarbonate, (Zn(_x)Cu(_1-x))(_n)(CO(_2))(_p)(OH)q, with the carboxylic acid. This preparation was chosen in an attempt to overcome problems encountered during previous projects, which are discussed in the introduction. The preparation and characterisation of zinc/copper hydroxycarbonates were studied, concentrating on the preparation of single phase products. The hydroxycarbonate route was successful in producing single metal and mbced metal salts of trimellitic and pyromellitic acid. Limits of the solid solution series were deduced for the mixed metal carboxylates. Three trimelitate crystal structures, Zn(_2)OH(C(_6)H(_3)(COO)(_3)) . 2H(_2)O, ZnCuOH(C(_6)H(_3)(COO)(_3)) . 2H(_2)O, CuH(C(_6)H(_3)(COO)(_3)) . 2.5H(_2)O, and one pyromellitate structure, ZnH(_2)(C(_6)H(_2)(COO)(_4)) . 6H(_2)O, were determined. The trimellitate structures were all polymeric layers, with the anion bridging between the metal sites. The pyromellitate had an ionic structure with [Zn(H(_2)O)(_6)](^2+) cations and [H(_2)(C(_6)H(_2)(COO)(_4))](^2)- anions. The dianion had very strong, intramolecular H-bonding. Using these and other crystal structures, it was possible to deduce a correlation between IR absorptions for carboxylate groups and the type of carboxylate coordination. The thermal decomposition of the metal salts was studied. An investigation was undertaken using DSC and temperature programmed decomposition to deduce the mechanism for the thermal decomposition of selected salts. The effect of the cation ratio (for the mixed metal salts) upon the decomposition was also studied. The work reported here has shown the success of the hydroxycarbonate route for the formation of polycarboxylate salts. Characterisation for a number of new zinc/copper salts is reported. A study of the thermal decompositions for selected salts has deduced the mechanism and factors affecting the decomposition.