Preparation of nanoporous polyoxometalate from rice husk silica for gas adsorption and catalysis applications

Rice husk ash (RHA) generated by the direct-fried combustion process of the biomass power plants in Thailand is produced in large qualities every year. In the present study, silica (SiO2) was extracted from RHA using an alkali extraction (provided by Kamalasai Bio Power 2010 Co., Ltd), and was used as an alternative silica source for the development of the multifunctional polyoxometalate materials. Three new silica-based polyoxometalates hybrid materials, [SiW12O40]2∙6ImH∙2H3O∙6H2O (1), [(Eu(H2O)5)2(ox)(Eu(H2O)3SiW11O39)]∙4ImH∙20H2O (2), [Eu2(H2O)2(Im)2(ox)(SiW11O39)2] ∙ImH∙H2O (3) (ox = oxalate, Im = imidazole, ImH = imidazolium), were isolated and structurally characterized by using single crystal X-ray diffraction analysis. Compound 1 consists of [SiW12O40] cluster, highly disordered imidazolium and oxonium cations as well as the highly disordered lattice water molecules. The components are assembled through extensive hydrogen bonding interactions into a three-dimensional cubic face centered supramolecular structure. The structure of 2 has a two-dimensional network constructed from a [Eu(H2O)3SiW11O39) cluster and [(Eu(H2O)5(ox)]. While 3 is a discrete polyoxometalate lanthanide complex consisting of oxalate bridged [Eu(H2O)(Im)SiW11O39] clusters. The activated 1 shows negligible CO2 uptake (4.49 and 4.86 cm3g-1 at 0 and 25 °C, respectively, under pressure of 1 bar). While the activated 2 also exhibits CO2 uptake under the same experimental conditions (2.63 and 2.43 cm3g-1). It is noteworthy that both 1 and 2 can also catalyzed synergistically the cycloaddition reaction of CO2 with epichlorohydrin under mild conditions. Moreover, the solid-state photoluminescence studies of 2 and 3 exhibited a bright red luminescence emission at room temperature under UV light (λexciatation = 394 nm) with the decay lifetime of 0.76 and 0.22 ms, respectively. In addition, a new sodium vanadate solid of (Na6F(H2O)18)[(VO4)2]∙2H3O∙HF (4) was obtained as a by-product during attempted to explore the silicate-vanadate system. The structure of 4 comprises of four components (i.e. (Na6F(H2O)18), (VO4), H3O and HF) interact with each other through extensive O–H∙∙∙O, O–H∙∙∙F hydrogen bonding interactions to give rise to a three-dimensional supramolecular architecture. Notably, the discrete structure of 4 can be transformed to an infinite three-dimensional network of β-NaVO3 phase via thermally induced solid-state reactivity.