Soft Hydrothermal Synthesis of New Microporous Materials Based on Phosphate-Like Species
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successful synthesis of some of these compounds is not always well established because the role of one or several reactants is not understood. In other cases, the yield is poor or the resulting solid is polyphasic. In some sense, it might seem that the results came so quickly and are so interesting that do not give opportunity of spending time on thinking about. In this paper we intend to show how the use of basic chemical information, with the help of simple models as the PCM or the VMP, provide us the basis of general preparative strategies conducting to single-phased reaction products, with reasonable yields, and susceptible to be extended in designing new compounds. With this aim we will restrict ourselves to the study of two topics: the hydrothermal synthetic chemistry of the phosphate and phosphate-like oxovanadium (IV) microporous compounds (including layered structures), and the chemistry of a specific class of tubular materials built up from phosphate-like oxoanions and transition metal divalent cations. CHEMISTRY OF OXO VANADIUM (IV) PHOSPHATE MICROPOROUS COMPOUNDS In contrast with V(V), the aqueous solution chemistry of V(IV) is characterized by a relative hydrolytic stability. At low pH values vanadyl is present as VO(H 20) 5 2+ (I). Hydrolysis and condensation occur at less acidic pH values, what leads to VO(OH)(H 2O) 4 + (11) and (VO) 2 (OH) 2 (H2O) 6 2 + (111), which are soluble species [11 ]. Olation is fast and the predominant species in the absence of coordinating anions is the stable dimmer III, as predicted from the PCM model (the partial charge on the OH group is very low and the condensation in acidic medium stops at this species) [4]. In basic media (pH from 9 to 13) the Schlemper fully oxolated polyanion (VO) 18 0 24 12 - (IV) exists and, at low vanadium concentrations, the monomeric anion VO(OH) 3 -has been detected (presumably in equilibrium with the polyvanadate)[12,14]. Simple compounds of I are known (fe. V0S0 4 6H 2 0 [15]), and some salts of IV also have been isolated (M12 VigO 42 x H 2 0, M=Na, K, [12] M9 (H4 V 18 0 42 X y H 2 0, M=Cs, K, X=Cl, Br, I [16]. However, the species 11 and III exist only as recognizable parts of complex and cage-like organophosphonates [17]. Other polymetalate cores [(VO)j(OH)j(O)k ]n are stabilized through coordination with alkoxydes (ft. the tripod alkoxo group (OCH 2 )3 (CNO2 )3 - gives several compounds with i~j,k= 6,0,1; 6,6,1; 10,0,3; 10,0,6; 10,2,4)[14]. The hydrolytic and condensation processes may be thus substantially altered by the presence in solution of another ions able to coordinate the metal cations, as occurs for phosphate or phosphate-like oxoanions. The resulting soluble molecular species (precursors of the solids) usually have a chemical activity towards hydrolysis and condensation very different from that expected for the aquocations only. In fact, the ability of these polydentate ligands to connect the -formally- preexisting ionic entities would imply the possibility of forming polymeric species. Basis on these simple ideas, we have
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