Membrane microreactors product yield

The field of chemical process miniaturization is growing at a rapid pace with promising improvements in process control, product quality, and safety, (1,2). Microreactors typically have fluidic conduits or channels on the order of tens to hundreds of micrometers. With large surface area-to-volume ratios, rapid heat and mass transfer can be accomplished with accompanying improvements in yield and selectivity in reactive systems. Microscale devices are also being examined as a platform for traditional unit operations such as membrane reactors in which a rapid removal of reaction-inhibiting products can significantly boost product yields (3-6). 

Product yield as a function of residence time for a fixed-bed reactor (triangles), a multi-channel microreactor (circles) and a multi-channel membrane microreactor (squares) (Lai etal., 2003) (Copyright permission 2006 Royal Society of Chemistry). 

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. 

Wan et al. [39] investigated the selective oxidation of aniline by hydrogen peroxide to azoxybenzene in a multi-channel MMR, with or without water removal, employing TS-1 nanozeolite as catalyst. The reaction was conducted at different residence times and temperatures. A hydrophilic ZSM-5 membrane was used to remove water selectively from the reaction mixture by membrane pervaporation. The results indicate that catalyst deactivation was reduced during the reaction. An improvement in the product yield and selectivity toward azoxybenzene was also observed. Increasing temperature was beneficial for both yield and selectivity, but beyond 340 K, microreactor operation was ineffective due to bubble formation and hydrogen peroxide decomposition. 

Another example for microreactor-controlled selectivity in the reaction of olefin with x02 can be found in the photosensitized oxidation of DPB in ZSM zeolite [160], Nafion membranes [161], and vesicles [163], In homogeneous solution, ZSM zeolite, and Nafion membranes, the oxidation of DPB with 02 yielded the [2 + 4] reaction product, endoperoxide 6, as the unique product (Figs. 15 and 18). In sharp contrast, in vesicle medium, the oxidation produced the aldehydes 1 and 2 ([2 + 2] reaction) in quantitative yield as described in Section III.B.l (Fig. 

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. 

Nanofibrous membrane-supported Ti02 was used as a catalyst for the oxidation of benzene to phenol in a microreactor at ambient conditions through in situ production of hydroxyl radicals as oxidant [99]. Reaction conditions were optimized and the performance of the microreactor was compared with the conventional laboratory scale reaction, in which hydrogen peroxide was used as oxidant. The microreactor gave a better yield of 14% for phenol compared to 0.14% in the conventional laboratory scale reaction. Reaction conditions such as reaction time, reaction pH, and applied potential were optimized. With optimized reaction conditions, selectivity of >37% and >88% conversion of benzene were obtained. 

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