Heterogeneous Catalysis in Microreactors

How are NMR measurements of adsorbates and reactive species on catalysts made This has been the subject of several recent reviews 6-8). In brief, there are a number of approaches that differ in the extent to which they are intended to reproduce the conditions of an actual catalysis experiment. Heterogeneous catalysis in a microreactor is typically carried out in a carrier gas flow stream at temperatures of 673 K or even higher. Products emerging from the microreactor are most commonly analyzed with an on-line gas chromatograph (GC). The contact time of the reactant on the catalyst typically ranges between several tenths of a second to several tens of seconds. In contrast, the acquisition of a 13C spectrum of an adsorbate on a typical catalyst at 298 K takes several minutes or tens of minutes, even with enriched compounds. As the sample temperature increases to the range commonly used in catalysis, NMR sensitivity rapidly decreases, and the discrepancy in time scale becomes greater. 

Besser, R. S., Ouyang, X., Surangalikar, H., Fundamental characterization studies of a microreactor for gas-solid heterogeneous catalysis, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6 (11-14 March 2002), AIChE Pub. No. 164, New Orleans, 2002, 254-261. 

Microwave technology has now matured into an established technique in laboratory-scale organic synthesis. In addition, the application of microwave heating in microreactors is currently being investigated in organic synthesis reactions [9-11] and heterogeneous catalysis [12, 13]. However, most examples of microwave-assisted chemistry published until now have been performed on a... 

This imposes some restriction on potential applications of microreactors, particularly with regard to their use in heterogeneous catalysis (see below). Another interesting aspect of MRT is enhanced mass transfer due to dramatically lowered mixing times ( 1 s compared to some 10 s for conventional reactors) and short diffusion times to such an extent that the influence of mass transfer on the reaction kinetics can be almost neglected. 

Most often, in the heterogeneous catalysis the catalysts are soUds — usually metals, very often precious metals — while the reactants are liquid or gaseous. The catalysts are usually composed of a number of (metal) components in various combinations. The tests are performed in parallel under the otherwise equal temperature and pressure conditicms in a large number of microreactors. fii each microreactor the combination of the tested catalyst components is different and this results in a different yield or selectivity of the test reactirm. This is usually evaluated by composition analysis of the reaction products. 

Figure 10.1 (a) SS falling-film microreactor for gas-liquid reactions [14], (b) catalyst-coated SS microchannel plates [14] and (c) catalyst-coated titanium microchannel plates, stacked together for gas-phase reactions in heterogeneous catalysis [17]. 

Table 1. Categories for decision on p>acked bed microreactor versus various options of catalyst wall coatings in microreactors for heterogeneous catalysis...

Veser G, Friedrich G, Freygang M, Zengerle R, A Simple and flexible microreactor for investigation of heterogeneous catalytic gas phase reactions, in Reaction kinetics and development of catalytic processes, Froment GF, Waugh KC, Eds., Studies in Surface Science and Catalysis, 122 237-246, Elsevier, Amsterdam, 1999. 

Microreactor for heterogeneous and enzymatic catalysis reactions Nanostmctured peaks are created in Si substrate by deep reactive ion etching (black silicon) Increased reaction rates in the microreactor due to increased surface areas Roumanie et al. 2008... 

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