Silica heteropoly acid

New acid catalyst comprising Keggin-type heteropoly acid supported on mesoporous silica for dehydration of acetic acid... 

The first attempt to synthesize and characterize Kegj -type heteropoly acid supported on various mesoporous silicas and its application to add catalysis in the formation of acetic anhydride via dehydration of acetic acid were described in this study. A variety of characterization techniques such as Na adsorption, TEM and XRD were applied... 

The selectivity for acetic anhydride in the catalytic dehydration of acetic acid could be controlled by the pore size of pure mesoporous silica SBA-15. New acid catalyst comprising Keggin-type heteropoly acid supported on SBA-15 enhanced the activity etfectively when tungstophosphoric acid was highly dispersed on the silica substrate. 

Heteropoly acids such as H3PW12O40 (PW) are good catalysts for the hydration of limonene and other monoterpenes. PWs can be used as homogeneous catalysts in solution or supported on, for example, silica or MCM-41 materials. In aqueous acetic acid limonene gives, in the presence of PW, mainly a-terpineol (7) and a-terpinyl acetate (8) [17]. 

Typically, Nation ionomer is the predominant additive in the catalyst layer. However, other types of CLs with various hygroscopic or proton conductor additives have also been developed for fuel cells operafed xmder low relative humidity (RH) and/or at elevated temperatures. Many studies have reported the use of hygroscopic y-Al203 [52] and silica [53,54] in the CE to improve the water retention capacity and make such CEs viable for operation af lower relative humidity and/or elevated temperature. Alternatively, proton conducting materials such as ZrP [55] or heteropoly acid HEA [56] have also been added... 

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

Studies with sulfated zirconia also show similar fast catalyst deactivation in the alkylation of isobutane with butenes. It was found, however, that original activities were easily restored by thermal treatment under air without the loss of selectivity to trimethylpentanes. Promoting metals such as Fe, Mn, and Pt did not have a marked effect on the reaction.362,363 Heteropoly acids supported on various oxides have the same characteristics as sulfated zirconia.364 Wells-Dawson heteropoly acids supported on silica show high selectivity for the formation of trimethylpentanes and can be regenerated with 03 at low temperature (125°C).365... 

Heterogeneous catalysts that exhibit good characteristics are silica-supported mixed Mo-V heteropoly acids and their Pd salts,1317 Pd on titania,1318 supported H3PMO12O40, and heteropoly acids and salts with Pd(OAc)21320 or PdCl2.1321... 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

Under acid condition (at pH 1) silica reacts with ammonium molybdate to form a yellow colored heteropoly acid, silicomolybdic acid. The reactions are shown below ... 

Thus, heteropoly acids may not be stable under reaction conditions (Mestl et al., 2001). This statement is in line with the results of Raman investigations of supported /J-silicomolybdic acid that unambiguously demonstrated its decomposition to surface molybdena during methane oxidation (Banares et al., 1995). It was also shown that silica-supported silicomolybdic acid and silica-supported molybdenum surface oxide species with the same molybdenum loadings performed identically (Banares et al., 1995). Thus, the presence of any heteropoly acid structure during high-temperature oxidation can be ruled out. 

These materials are thermally stable so they find use as solid acid catalysts for a variety of vapor phase reactions. They can be used as the bulk oxyacid but are more efficient when they are supported on materials such as alumina, silica or carbon. The supported heteropoly acids are usually prepared by either incipient wemess or equilibrium impregnation procedures. 3 Carbon supported heteropoly... 

If a title is self-explanatory and adequate for indexing purposes, there is no abstract or annotation in Chemical Abstracts. Such cases are rare for analytical methods. In general one wants to know what was done, how, and the results obtainable for given substances or conditions. The information desired may be much the same as that suggested for a satisfactory summary. For example, the method may be for small amounts of silica. The reader wants to know if the procedure is colorimetric. If so, was the colored species developed a heteropoly acid If so, what was the color-forming reagent Too often these and similar questions are not answered. 

The coking during propene oligomerization over silica-supported heteropoly acid H3PW12O40 and its palladium-doped form and subsequent catalyst regeneration have been studied using and P MAS NMR. " ... 

It is generally believed that tungsten is bom in mineralizing fluids chiefly as tungstate ion, tungstic acid, sodium tungstate, or as heteropoly acid. The relative amount of these compoimds depends on temperature, pH, and silica concentration of the solution. Tungsten... 

Silicon (Si, at. mass 28.09) occurs in its compounds in the IV oxidation state, e.g., in silica (Si02) or in silicic acids. A characteristic feature of silicon(rV) is its ability to form the tetrafluoride (Sip4) and heteropoly acids, e.g., with molybdic acid. 

Several zeolites in the II-form, two activated clays, a silica-alumina, a sulfonic acid resin and a silica-occluded heteropoly acid were tested in the reaction of cyclohcxcne and toluene (excess) at 110 °C [64]. The ortho / meta / para ratio of the mixtures strongly depends on the structure of the catalysts involved. With zeolite H-USY and Filtrol-24 as active catalysts the meta / para ratio is found to be about 2 1, in agreement with the thermodynamic equilibrium, and the ortho-isomer is essentially absent.By contrast 11-Bcta and H-mordenite gave a meta /para ratio of 1 4.5. As H-USY appeared to be a good isomerization catalyst for the cyclohexyltoluenes, the mechanism may involve ortho / para-alkylation followed by isomerization. Researchers of UOP (Dcs Plaines, USA) found a separation method for meta / para cyclohexyltoluenc (undisclosed technique). Altogether the results open a new low-waste route to 3-methylbiphenyl. 

A wide variety of solid-acid catalysts is available [17] acidic clays, zeolites, silica-occluded heteropoly acids, sulfonated polysiloxanes, Nafion (a sulfonated perfluoroalkyl resin) and Nafion-silica composites, and a variety of hybrid sulfonated mesoporous systems (see Chapter 3). 

Methyl terf-butyl ether (MTBE) is an important industrial product used as oxygenate additive in reformulated gasoline. Environmental concern makes its future uncertain, however. Although mainly manufactured by reaction of isobutylene with methanol, it is also produced commercially from methanol and fcrr-butyl alcohol, a by-product of propylene oxide manufacture. Numerous observations from the use of heteropoly acids have been reported. These compounds were used either as neat acids [74], or supported on oxides [75], silica or K-10 montmorillonite [76]. They were also used in silica-included form [77] and as acidic cesium salts [74,77]. Other catalysts studied were sulfated ZrOj [76], Amberlyst 15 ion-exchange resin [76], HZSM-5 [76], HF-treated montmorillonite, and commercial mineral acid-activated clays [75]. Hydrogen fluoride-treatment of montmorillonite has been shown to furnish particularly active and stable acid sites thereby ensuring high MTBE selectivity (up to 94% at 413 K) [75]. 

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