Knoevenagel condensation, membrane

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

Lai SM, Ng CP, Martin-Aranda R, and Yeung KL. Knoevenagel condensation reaction in zeohte membrane microreactor. Micropor Mesopor Mater 2003 66(2-3) 239-252. 

Lai SM, Martin-Aranda R, Yeung KL (2003) Knoevenagel condensation reaction in a membrane microreactor. Chem Commun 22 218-219... 

PVRs equipped with hydrophilic membranes can be used for any liquid reaction where the water produced limits the equilibrium conversion or acts as an inhibiting agent. For example, PVRs have been studied for the dehydration reaction of butenediol to form tetrahydrofuran (Liu and Li, 2002), the synthesis of methylisobutylketone (Staudt-Bickel and Lichtenthaler, 1996), the Knoevenagel condensation reaction between benzaldehyde and ethyl cyanoacetate or ethyl acetoacetate or diethyl malonate (Zhang et al, 2004). Additional information can be found in reviews by Sanchez Marcano and Tsotsis (2002) and by Van der Bruggen (2010). 

A microfluidic reaction system has also been used for the production of prodrugs. A multichannel membrane microreactor was fabricated and tested for Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate to produce a-cyanocinnamic acid ethyl ester, a known intermediate for the production of an antihypertensive drug [9]. Knoevenagel condensations of carbonylic coiiqtounds and malonic esters yield several important key products such as nitriles used in anionic polymerization, and the a,p-unsaturated ester intermediates employed in the synthesis of several therapeutic drugs that include niphendip-ine and nitrendipine. Unlike most condensation reactions. 

Knoevenagel condensation between carbonyl compounds and methylene malo-nic esters on a CsNaX zeolitic coating in a microreactor demonstrated an order of magnitude higher productivity as compared with a traditional packed-bed reactor while the selectivity remained the same in both reactors [107,108]. A nearly fourfold increase in reaction conversion was obtained for the microreactor when NH2 modified CsNaX zeolitic coatings were apphed [109]. The conversion was further improved when zeolitic coatings were grown onto a stainless steel membrane (0.2 pm pores) inserted in a microreactor [110]. 

Lai, S.M., Ng, C.R, Martin-Aranda, R. and Yeung, K.L. (2003) Knoevenagel condensation reaction in zeolite membrane microreactor. Microporous and Mesoporous Materials, 66,239-252. 

Zeolite membranes have been applied for gas permeation and separation, and liquid pervaporation. A clear advantage of microscale zeolite membranes is the higher probability of obtaining a defect-free interface, since this probability increases for smaller membrane areas [41]. In zeolite MMRs, the zeolites are incorporated as a catalyst for reaction and a membrane for separation, as well as structural material of the reactors. Reactions conducted in MMRs include mainly Knoevenagel condensation [3, 42,43] and selective oxidation reactions [39]. Supra-equilibrium conversion may be obtained in the former, while the latter displays improved performance against catalyst deactivation. 

Lai S M, Ng C P, Martin-Aranda R and Yeung K L (2003), Knoevenagel condensation reaction in zeolite membrane microreactor , Micropor Mesopor Mat, 66, 239-252. 

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