Mesoporous metal oxide solid acids

Mesoporous Metal Oxide Solid Acids Three-dimensional porous metal oxides have been recently synthesized and applied to acid-catalyzed reactions. The use of mesoporous metal oxides is an interesting approach to develop a solid acid catalyst with enhanced activity. The mesopores in the oxide allow the reactants to access additional active acid sites in the pores, resulting in improved rates of acid catalysis. Mesoporous niobium oxides and tantalum oxides treated with phosphoric acid or sulfuric acid have been examined as solid acid catalysts [57-59]. These mesoporous oxides exhibited remarkable activity in Friedel-Crafts alkylation and 1-hexene isomerization in the liquid phase. For sulfated mesoporous tantalum oxides /m-TsL O ), the effect of pore size has been investigated using... 

This paper describes a new synthesis strategy of preparing thermally stable mesostructured transition metal oxides, namely, two-step synthesis (TSS). Basically, the synthesis course involves two steps (1) formation of a mesostructured transition metal oxide solid mediated by surfactant in a basic aqueous solution and (2) treatment of the solid product in an acidic organic solvent containing the respective precursor from which the solid product was produced. The final material synthesized according to such a method is thermally stable and structurally mesoporous with high surface area and uniform pores arranged disorderedly. 

Liquid crystalline mesophases can also be prepared in non-aqueous solution. Using ethanolic solutions of non-ionic block copolymers as a medium, Zhao et for example, have developed a route by which mesoporous metal phosphates and borates can readily be prepared. A mixture of the metal alkoxide and the acidic chloride is used to give a complex in ethanolic solution. This acid-base pair reacts in one component of the liquid crystalline assembly to give a mesophase solid of uniform composition. Evaporation of the ethanol leaves the mesostructure. This appears to be one of the most promising routes to the formation of non-silica mesoporous solids. Many reports of these solids have appeared, particularly of metal oxides such as titanium dioxide, but loss of ordering on the mesoscale frequently occurs upon template removal. The route of Sanchez, which involves the use of titanate precursor species in the sol-gel, is a promising approach to stable mesostructured titania that retains its porosity. 

This part surveys the development of new solid acid catalysts such as metal oxides, including nanosheets, nanotubes, mesoporous materials, carbon-based catalysts, and ion-exchanged resins. Recent achievements in carbon-carbon bond formation, one-pot sequential reactions by sobd acids and bases, and acid-base bifunctional catalysts are also included. 

Fructose, one of the most common ketohexoses, readily dehydrates to afford HMF in the presence of Br0nsted acids in polar solvents. A variety of aprotic polar solvents, including DMSO, DMF, N,N-dimethylacetamide (DMA), and sulfolane, are used for these liquid-phase reaction because of the solubility of carbohydrates. A variety of solid acids, such as ion-exchange resins [156], zeolites [157, 158], metal oxides, and heteropoly acid salts, have been examined for HMF production from fructose [159,160]. Niobic acid, niobium phosphate, vanadium phosphate, sulfated zirconia, Amberlyst-15, and acid-functionalized mesoporous silicas are also found to exhibit high catalytic activity for fructose dehydration [161-167]. Moreover, soHd acid catalysts have also been examined in ionic liquids [168-175]. 

The disadvantages of using mineral acids such as concentrated HCl or H2SO4 to hydrolyse biomass is that they are toxic, corrosive, hazardous and difficult to recycle. The use of heterogeneous solid acids can ease product separation and provide better catalyst recyclability. For example, mesoporous transition metal oxides have been used in biomass transformations. " Polymer-based acids have been employed for the hydrolysis of various organic substrates. " In particular, carbon-based solid acids made by sulfonation of carbonized polymers, such as the solid acid shown in Figure 7.7, have shown promise. Sulfonated bio-char has been similarly used. ... 

Hagaman et al. (2012) studied interaction of benzoic acid with metal oxides using solid-state O NMR spectroscopy. Complexes formed by dry benzoic acid with mesoporous silica and nonporous titania and alumina were analyzed. Chemical reactions with silica were not observed, but the behavior of benzoic acid on silica was a function of the water content. The acid was characterized by high mobility as evidenced by a liquid-like, Lorentzian NMR resonance. Excess benzoic acid remained as the crystalline hydrogen-bonded dimer. Benzoic acid reacted with titania and alumina surfaces in equilibrium with air to form the corresponding titanium and aluminum benzoates. In both materials, the oxygen of the O-labeled acid was bound to the metal, showing the bond... 

Tanev et al. have reported the synthesis of mesoporous materials via a route which involves self-assembly between neutral primary amines and neutral inorganic framework precursors.12 The regularity of the pore structure in these materials has been illustrated by lattice images which show a honeycomb like structure. The system of channels of these molecular sieves produces solids with very high internal surface area and pore volume. This fact combined with the possibility of generating active sites within the channels produces a very unique type of acid catalyst. In the case of transition metal substituted M41S, the principal interest lies in their potential as oxidation catalysts, especially Ti and V substituted MCM and HMS type materials, and more recently synthesised large pore materials.13... 

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