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Heterogeneously Catalyzed Alkylation Reactions

Friedel-Crafts reactions are commonly performed both at the laboratory and industrial scale by using acid catalysts and acyl chlorides or alcohols as acylating or alkylating agents. The advantages of SCFs for these types of reactions are clear. There is less need for reactive acid catalysts such as AICI3, which are difficult to separate from the reaction mixture and are, on the whole, environmentally unfriendly [45]. Instead, solid acid catalysts can be used efficiently [46, 47]. [Pg.379]

Traditional Friedel-Crafts synthesis uses low temperatures and long reaction times to control the reaction. In many cases, reactions are performed over 24-72 h, which nevertheless produces a mixture of products that need separation to recover the desired product with appropriate purity. In the case of continuous flow reactors, however, higher temperatures are required than in a batch reactor because the residence time within small-volume reactors is comparatively low even so, high selectivities and conversions have still been achieved at these temperatures [48]. [Pg.379]

The formation of water as a by-product from the reactions does not pose a sig-niflcant problem in SCF flow reactors, as aqueous phases are easily separated from the organic phase. Water appears to be flushed from the active catalytic sites by the flow of SCCO2. Because the residence time within the reactor is short, the products are transported away from the catalyst surface before further reaction can proceed to any great extent. This gives control over the reaction conditions can be manipulated in a SCF flow reactor to give a high selectivity for mono substitution, which is usually the desired reaction. [Pg.379]

Other examples of alkylation reactions have been shown to occur with high efficiency using SCCO2 as a solvent system. Clark and co-workers have successfully [Pg.379]

Acylation reactions, however, have been less successful. This limitation is probably not due to the chemistry of the SCFs, but lies with the halide species generated in situ within the reactor when traditional acid chlorides are used as reactants. Immobilized enzymes from Candida antarctica have been shown, however, to exhibit high activity in SCCO2 at temperatures less than 70 °C this enzyme is capable of acylating glucose, with a suitable acyl-donor [51]. [Pg.380]


Addition of water (36) or alcohols (37—39) direcdy to butadiene at 40—100°C produces the corresponding unsaturated alcohols or ethers. Acidic ion exchangers have been used to catalyze these reactions. The yields for these latter reactions are generally very low because of unfavorable thermodynamics. At 50°C addition of acetic acid to butadiene produces the expected butenyl acetate with 60—100% selectivity at butadiene conversions of 50%. The catalysts are ion-exchange resins modified with quaternary ammonium, quaternary phosphonium, and ammonium substituted ferrocenyl ions (40). Addition of amines yields unsaturated alkyl amines. The reaction can be catalyzed by homogeneous catalysts such as Rh[P(C(5H5)3]3Q (41) or heterogeneous catalysts such as MgO and other solid bases (42). [Pg.342]

The zeolite is rigid and ordered, and lacks conformational adaptability, in contrast to an enzyme, which can coil, uncoil, and twist around. Yet the zeolite can incorporate transition metal functions—these are of prime importance in enzyme catalysis—and it can effect redox reactions reactions over zeolites can be inhibited by competitive adsorption of reactants, products, solvents, or poisons—a phenomenon observed in biological and some other inorganic heterogeneous catalytic systems Rideal kinetics have been identified in some zeolite-catalyzed alkylations, a pattern which has its parallels in the enzyme field a few cases of stereospecificity (such as orfho-alkylation effects, unusual olefin isomer ratios), where a transition state not otherwise attainable intervenes, may exist. What better group of catalysts than zeolites might there have been to activate the evolutionary process in the dark, fermenting Pre-Cambrian seas some 1,000,000,000 years ago ... [Pg.281]

Results are summarized in Fig. 5. Over SOaCrOz and SA, the reaction took place smoothly at 373 K, though the disproportionation of E2 proceeded higher temperature. Modified clay minerals also catalyzed the reaction at an appreciable rate. Alumina and HY showed low activity and other catalysts were almost inactive for the reaction. This clearly shows that strong solid acids can catalyze the alkylation reaction. The heterogeneous alkylation of E2 is an acid catalyzed reaction. [Pg.621]

Recent Advances in Heterogeneous TM-Catalyzed Af-Alkylation Reactions... [Pg.326]

Owing to the high activities of the homogeneous Ru and Ir catalysts in N-alkylation reactions, many heterogeneous Ru and Ir catalysts have been developed. In 2009, Mizuno and co-workers reported an effective A-alkylation of (hetero)ary-lamines with benzylic and aliphatic alcohols catalyzed by Ru(0H)x/Al203 complex without any co-catalysts or additives (Eq. 34) [135]. The catalyst is recoverable and reusable without obvious loss of activity (1st, 89 % 2nd, 87 %). Later in the same year, the authors reported another additive-free method for synthesis of secondary and tertiary amines from alcohols and urea catalyzed by Ru(OH)x/Ti02 (Eq. 35)... [Pg.327]

It is in the very nature of the catalytic process that the intermediate compound formed between catalyst and reactant is of extreme lability therefore not many cases are on record where the isolation by chemical means, or identification by physical methods, of intermediate compounds has been achieved concomitant with the evidence that these compounds are true intermediaries and not products of side reactions or artifacts. The formation of ethyl sulfuric acid in ether formation, catalyzed by HjSO , and of alkyl phosphates in olefin polymerization, catalyzed by liquid phosphoric acid, are examples of established intermediate compound formation in homogeneous catalysis. With regard to heterogeneous catalysis, where catalyst and reactant are not in the same... [Pg.65]

Nonracemic tra s-2-[aryl(alkyl)thio]cyclohexanols with l/ ,2/ -configuration are prepared with modest to good enantioselectivity in high yield by reaction of 1,2-epoxycyclohexane with various thiols in dichloromethane at 25 °C catalyzed by zinc L-tartrate in a heterogeneous reaction103. [Pg.629]

Alkylation of 2-naphthoxide ion (Eq. (6)) occurs mainly on carbon in aqueous solvents and mainly on oxygen in aprotic solvents. The product distribution is often used as a probe of the solvent environment in heterogeneous reactions. Brown and Jenkins 54) found that 40-100 % RS spacer chain catalysts 15 and 16 gave up to 98 % O-benzylation of 2-naphthoxide ion with benzyl bromide. The shorter spacer chain catalyst 16 gave 85% O-alkylation, and a conventional benzyltrimethylammonium ion resin 2 gave about 70 % O-alkylation. Because of low activity, product distribution data were obtained with varied amounts of catalyst and were extrapolated to equimolar amounts of catalyst and substrate to obtain the catalyzed O/C product ratios. Interpretation of the data also was complicated by independent evidence that catalysts 15 adsorbed 2-naphthoxide ion, in addition to that bound by ion exchange54). Essentially the same results were obtained with catalysts 24 which lack the ester link in the spacer chain 106). [Pg.74]


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Heterogeneous reaction

Heterogeneously catalyzed

Heterogeneously catalyzed reaction

Reaction heterogeneous reactions

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