Acidic support

Acidic Supports. Three methods will be considered for enzyme immobilizations onto carboxylic acid-containing supports. All result in the formation of amide bonds to the primary amines of lysine residues on the protein, but the selectivity for lysine varies with the method used. In the acyl azide method,8 the carboxylic acid group is sequentially converted to its methyl ester, acyl hydrazide and acyl azide (Eq. 4.10)  

The acyl azide is subject to nucleophilic attack, and is most reactive toward primary amino groups (lysine), but will also react with tyrosine hydroxyl, cysteine sulfhy-dryl, and serine hydroxyl groups (Eq. 4.11)  

Woodward s reagent K9 is a water-soluble dehydrating reagent that activates carboxylic acid groups toward nucleophilic attack. It may be used under mild, aqueous conditions, and sequential activation and coupling steps prevent enzyme cross-linking (Eqs. 4.12 and 4.13)  

If cross-linking is not expected to occur, then Woodward s reagent K may be included in an enzyme solution, and a single-step immobilization reaction may be performed. 

Carbodiimide dehydrating reagents10 activate carboxylic acid groups toward nucleophilic attack by the formation of an (9-acylisourea intermediate (Eq. 4.14)  

CVD of Me2Au(acac) onto activated carbon produced small gold particles, but the activity of the resulting catalyst was very low (T50 500 K).41 

Modifying activated carbon fibres (ACF) by ferric oxide improved the gold dispersion and their activity at room temperature, but they deactivated quickly.111 

It is very difficult and indeed perhaps impossible to derive any general conclusions as to how the method of preparation affects the structure and activity of supported gold catalysts, as so much depends on the gold precursor and the support each system needs to be considered on its own. Only a few strong correlations are apparent, and there is much speculation it is easier to list the unresolved questions than the definitive conclusions. 

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The Isolene II process was commercialized in 1971 by Toray Industries (120—122). The catalyst is Pt on an acidic support. Operating conditions ate reported to be 250—500°C and 1—3 MPa. The first Isolene II plant was built at Toray s Kawasaki complex. 

The catalyst for the second stage is also a bifimctional catalyst containing hydrogenating and acidic components. Metals such as nickel, molybdenum, tungsten, or palladium are used in various combinations and dispersed on sofid acidic supports such as synthetic amorphous or crystalline sihca—alumina, eg, zeofites. These supports contain strongly acidic sites and sometimes are enhanced by the incorporation of a small amount of fluorine. 

Esterification. Extensive commercial use is made of primary amyl acetate, a mixture of 1-pentyl acetate [28-63-7] and 2-metliylbutyl acetate [53496-15-4]. Esterifications with acetic acid are generally conducted in the Hquid phase in the presence of a strong acid catalyst such as sulfuric acid (34). Increased reaction rates are reported when esterifications are carried out in the presence of heteropoly acids supported on macroreticular cation-exchange resins (35) and 2eohte (36) catalysts in a heterogeneous process. Judging from the many patents issued in recent years, there appears to be considerable effort underway to find an appropriate soHd catalyst for a reactive distillation esterification process to avoid the product removal difficulties of the conventional process. 

Hydrocracking is catalyzed by substances that promote cracking and hydrogenation together. In commercial use are Ni, Co, Cr, W, and V or their oxides, presulfided before use, on acid supports. Zeolites loaded with palladium also have been used. 

Polyphosphoric acid supported on diatomaceous earth (p. 342) is a petrochemicals catalyst for the polymerization, alkylation, dehydrogenation, and low-temperature isomerization of hydrocarbons. Phosphoric acid is also used in the production of activated carbon (p. 274). In addition to its massive use in the fertilizer industry (p. 524) free phosphoric acid can be used as a stabilizer for clay soils small additions of H3PO4 under moist conditions gradually leach out A1 and Fe from the clay and these form polymeric phosphates which bind the clay particles together. An allied though more refined use is in the setting of dental cements. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

Interestingly the electrochemical promotional effect was found only in the case of perchloric acid supporting electrolyte. No promotion effect was found in presence of strongly adsorbed anions (HS04 Cl ). 

Chiral Lewis acids supported on silica gel and alumina, and their use as catalysts in Diels-Alder reactions of methacrolein and bromoacrolein [103]... 

The synthesis of imidazoles is another reaction where the assistance of microwaves has been intensely investigated. Apart from the first synthesis described since 1995 [40-42], recently a combinatorial synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles has been described on inorganic solid support imder solvent-free conditions [43]. Different aldehydes and 1,2 dicarbonyl compounds 42 (mainly benzil and analogues) were reacted in the presence of ammonium acetate to give the trisubstituted ring 43. When a primary amine was added to the mixture, the tetrasubstituted imidazoles were obtained (Scheme 13). The reaction was done by adsorption of the reagent on a solid support, such as silica gel, alumina, montmorillonite KIO, bentonite or alumina followed by microwave irradiation for 20 min in an open vial (multimode reactor). The authors observed that when a non-acid support was used, addition of acetic acid was necessary to obtain good yields of the products. 

Tricyclic pyrazolo-quinolines 239 were prepared from /1-chloro arylalde-hydes and hydrazine derivatives under microwave irradiation with an acid support [156]. The method, appHed to a series of tricyclic compounds (Scheme 89), can be used, in principle, also for the synthesis of bicycHc and even monocycHc pyrazoles. 

With 6-alkenoic acids the intermediate radical partially cyclizes to a cyclopentyl-methyl radical in a 5-exo-trig cycHzation [139] (Eq. 6) [138 a, 140] (see also chap. 6). To prevent double bond migration with enoic acids the electrolyte has to be hindered to become alkaline by using a mercury cathode. Z-4-Enoic acids partially isomerize to -configurated products. Results from methyl and deuterium labelled carboxylic acids support an isomerization by way of a reversible ring closure to cyclopropyl-carbinyl radicals. The double bonds of Z-N-enoic acids with N > 5 fully retain their configuration [140]. 

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. 

As already described (1) the inhibition of the NO reduction by CO due to carbon deposits in mixtures containing hydrocarbons depends on the support, with the most acidic supports leading to higher amounts of carbon deposits. 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

A series of anchored Wilkinson s catalysts were prepared by reacting the homogeneous Wilkinson catalyst with several alumina/heteropoly acid support materials. These catalysts were used to promote the hydrogenation of 1-hexene. The results were compared with those obtained using the homogeneous Wilkinson and a l%Rh/Al203 catalyst with respect to catalyst activity and stabihty as well as the reaction selectivity as measured by the amount of double bond isomerization observed. The effect which the nature of the heteropoly acid exerted on the reaction was also examined. 

Immobilized Bronsted Acids Supported Fluorosulphuric Acid (FSOsH/SiOA and Trifluoroacetic Acid (CFsCOOH/SiOA- 1.6 mmoles of (3-aminopropyl) trimethoxysilane was dissolved in 100 mL of chloroform, and 1 g of silica gel was added. The slurry was heated under reflux for 24 h. The slurry... 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

Simple bidentate ligands involving dipyridyl- or dipyrimidylamino fragment (L23) form Pd catalysts of moderate activity for the cross-coupling of terminal acetylenes (copper-free reaction) or arylboronic acids. Supported versions of such ligands were also reported (see Chapter 9.9 for more details about supported catalysts).449,450... 

Hawkins and Brash investigated some aspects of the biosynthesis of 11R-and 12R-HETE in eggs of S. purpuratus [192,194]. Based on experiments using octadeuterio-AA they showed that the 11- or 12-keto intermediate is not involved. Isolation and characterization of both HR- and 12R- hydro-peroxyeicosatetraenoic acids from incubations of desalted ammonium sulfate fractions of the egg homogenate with arachidonic acid support involvement of a lipoxygenase. 

G. D. Kishore Kumar and S. Baskaran, A facile, catalytic, and environmentally benign method for selective deprotection of ferf-butyldimethylsilyl ether mediated by phosphomolybdic acid supported on silica gel, /. Org. Chem., 70 (2005) 4520-4523. 

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