Introduction catalysts

Recently, the asymmetric variants of the Stetter [114-118], crossed-benzoin [114, 117-120], and transeslerification [121] reactions have attracted great interest as asymmetric nucleophilic acylation processes. A prerequisite for asynunetric catalysis is the availability of a chiral catalyst. Introduction of chirahty into the thiamin framewoik follows the same principles as that for the related imidazoUum systems, mainly the introduction of a chiral centre next to the nitrogen atom of the thiazole ring [117]. 

Synthesis of butynediol from acetylene and formaldehyde reactor height 18 m, diameter 1.5 m, 100 °C, 3 bar, copper acetyhde catalyst, introduction of cold acetylene at various points in the reactor. 

Modification of catalyst introduction into reaction zone in particular realization of multi-step catalyst introduction into tubular turbulent apparatus ("zone" model) is the convenient way of polymer MM and MMD control [62]. For "zone" model realization in turbulent regime the following conditions should be fulfilled [1, 27, 66] ... 

Catalyst (Introduction) A substance that increases the rate of a chemical reaction, but is not consumed by it. 

It was shown that solid-state ion exchange is also a suitable route to preparation of active acidic or bifunctional catalysts. Introduction of Ca or Mg into mordenite [21] or La " into Y-type zeolite, mordenite or ZSM-5 [22] by solid-state reaction yielded, after brief contact with small amounts of water, acidic zeolite catalysts which were, for instance, active in disproportionation and/or dealkylation of ethylbenzene or in cracking of n-decane [43]. The contact with water was essential to generate, after solid-state ion exchange, acidic Brpnsted centres (compare, for instance. Figure 2). In the case of solid-state exchange between LaClj and NH -Y an almost 100% exchange was achieved in a one-step procedure, and the hydrated La-Y reaction product exhibited a catalytic performance (selectivity in ethylbenzene disporportionation, time-onstream behaviour) comparable to or even better than that of a conventionally produced La-Y (96) catalyst [22,23]. In fact, compared to the case of NH -Y the introduction of La " " by solid-state reaction proceeded less easily and was frequently lower than 100% with H-ZSM-5 or H-MOR. 

Part V Studies of Immobilized Catalysts - Introduction 403 Table 1 Enantioselective sulfoxidation of benzylphenylsulfide. 

In 2004, Fossey and Richards [21] reported the synthesis of 2,6-bis(2-oxazolinyl) phenylplatinum(I I) NCN-Pincer complexes (63a-d) (Scheme 16.20). These cationic complexes were employed as Lewis acid catalysts for the Michael reaction between methyl vinyl ketone (49c) and ethyl cyanoacetate (64) and Diels-Alder reaction between acrylonitrile (67) and cyclopentadiene (14). The highest rates of the Michael reactions were observed when Pt-oxazolines (63a) and (63b) were used as catalysts, and the lowest rates of the Diels-Alder reaction was observed when Pt-oxazoline (63c) was used as a catalyst. Introduction of a nitro substituent on para position as shown for (63c) resulted in higher Lewis acidity but reduced catalytic activity. 

Catalyst Introduction Approximately operating life years... 

Hendricks, T. R., Dams, E. E., Wensing, S. T., and Lee, I. 2007. Effects of catalyst introduction methods using PAMAM dendrimers on selective electroless nickel deposition on polyelectrolyte multilayers. Langmuir 23(13), 7404-7410. 

Thus, to reduce the density of Bronsted acid sites in the bifunctional zeolite catalysts, introduction of both Ca + and Pd was carried out, and this was done either simultaneously (method A) or successively (method B). Best results were obtained by method B. As an example. Fig. 62 shows the conversion of ethylbenzene and the yields of ethylcyclohexane, xylenes, dimethylcyclohexanes, alkanes, benzene and diethylbenzenes over a Pd,Ca,H-ZSM-5 catalyst prepared by a two-step solid-state ion exchange. In a first step, CaCl2 was incorporated via SSIE, followed by a second step, viz., SSIE of PdCl2 into Ca,H-ZSM-5 obtained in the first step. 

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