Silica supported HPA

In 2001 a 220,000 tonnes per annum plant for the production of ethyl acetate using a silica-supported HPA was successfully commissioned on the BP Chemicals site in Hull, UK. A schematic of the Avada plant is shown in Figure 2 (7). The Avada process is superior to other traditional processes in terms of... 

Species Unsupported HPAs Silica-supported HPAs ... 

HPA- and HPA-salts-catalyzed Fries rearrangement of phenyl esters have been deeply investigated. Three kinds of catalysts, namely, bulk HPA, silica-supported HPA (HPA/SiOj) (including its cesium salts), and sol-gel silica-supported HPA (HPA-Si02), °- were compared. The insoluble... 

The acylation of aromatics with crotonic acid can be carried out with pure and silica gel-supported HPA (Scheme The proto-... 

In contrast to silica-supported H3[PWi204o], the sol-gel H3[PWi204o] catalysts prepared by the hydrolysis of tetraethyl orthosilicate showed only a negligible activity in the Fries reaction of phenyl acetate, yielding mainly phenol with 92 - 100% selectivity. This may be explained by a weaker acid strength of the sol-gel catalysts due to strong interaction of the HPA protons with the silica matrix and the presence of relatively high amount of water in sol-gel catalysts. ... 

An Efficient and Convenient Method for Synthesis of 1,2-dihydro-1 -aiyl-37/-naphth[ 1,2-e] [ 1,3]oxazin-3-one Derivatives Using Silica-Supported Preyssler HPAs, Hj fNaPjWj iOjjJ/SiOj, as a... 

An Efficient Catalytic Synthesis of l,2-dihydro-l-aryl-3//-naphth[l,2-e][l,3] oxazin-3-one Derivatives Using Silica-Supported Preyssler HPA, Hj [NaPjW3(,0 (,]/... 

One-pot multicomponent condensation of benzyl and/or benzoin, aldehydes, ammonium acetate, and primary amines were used for synthesis of 2,4,5-trisubstituted and l,2,4,5-tetrasubstituted-l//-imidazole derivatives under reflux conditions using silica-supported Preyssler nanoparticles HPA as a catalysts. This catalyst has several advantages (simple workup, inexpensive, and reusability). These catalysts were also successfully employed in the synthesis of triaryloxazoles (Schemes 3.6 and 3.7) [49]. 

A-phenylquinazolin-4-amines derivatives were obtained in high yields with excellent purity from the reaction of 2-aminobenzamide, orthoesters, and substituted anilines in the presence of silica-supported Preyssler nanoparticles and various HPAs (Scheme 3.12) [53]. 

An efficient synthesis of 2/f-indazolo[2,l-(>]phthalazine-l,6,ll(13//)-trione derivatives has been achieved in one-pot reaction at room temperature from the three-component condensation reaction of phthalhydrazide, dimedone, and aromatic aldehydes under solvent-free conditions in good to excellent yields and short reaction times using reusable silica-supported Preyssler HPAs as a heterogeneous acid catalyst has been investigated (Scheme 3.14) [55]. 

In order to increase the stability, HPAs were incorporated in sols of alumina [11] and silica [12]. HPAs immobilized on supports like silica, although stable in the presence of water, could not be used as electrolytes in fuel cells in the powdered form. For this reason, composite membranes [13, 14] have been developed, using a polymer as binder for the formation of thermally and mechanically resistant membranes with low gas permeability and silicotungstic acid or phosphotungstic acid immobilized on silica as the inorganic proton conductor. 

For the best conversion of lactic acid into AA, Katryniok et al. (2010) used silica-supported heteropolyacids (HPAs) as catalysts at 275°C in a fixed-bed gas phase reactor. More improvement of lactic acid (91%) conversion was observed when using the highly acidic silicotungstic acid. The proposed mechanism for conversion was decarbonylation/dehydration. However, by using less acidic molybdenum-based HPAs, propanoic acid was the dominant product, pointing to a more pronoxmced decarboxylation pathway producing H2 (and CO2). The resulting H2 can be used in situ to reduce lactic acid to propanoic acid (Katryniok et al., 2010). 

POMs/HPA themselves are usually non-porous solids, with surface area less than 10 m2/g and low decomposition temperatures. Therefore, they have limited surface sites for surface-catalysed reactions. A number of attempts have been made to disperse POMs on inert supports, with the intention of effectively increasing the number of accessible active catalytic sites. A number of materials have been used as solid supports for the dispersion of HPA, for example silica, carbon, zirconia, alumina, and porous silica. 

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