Wall-coated catalysts

The superiority of microreactors for reduced deactivation due to less thermal load on the catalyst has been proven several times (e.g. [47]), even benchmarked to a fixed bed [18] or to a foam [19]. However, one should be careful when operating the catalyst in a microreactor at its kinetic limit. The deactivation behavior of a specific catalyst could be more visible than in a conventional fixed bed or even coated foam. The mass of thin wall-coated catalysts operated free from mass transfer limitations could be much less for reaching 100% conversion. However, there is no backup catalyst mass. Hence additional catalyst mass has to be considered for process layout [20]. Also, leaks and the parallelizing of microstructures (concerning their feed distribution and heat distribution) are a challenge that can influence the catalyst stability and thus the operation of a microreactor [40]. 

After activation by heating, the catalyst was dusted over the surface of a thin polydimethylsiloxane (PDMS) layer, being coated on the PDMS top plate of the micro reactor [19]. Such a modified plate was baked for 1 h at 100 °C. A high surface area and firm immobilization of the catalyst resulted. Then, the micro reactor was assembled from the top and another bottom plate, having at one micro-channel wall the catalyst layer. Stable operation with the PDMS micro reactor up to 175 °C could be confirmed. 

G. P., Datye, A., Wall coating of a CuO/ Zn0/Al203 methanol steam reforming catalyst for micro channel reformers, Chem. Eng.J. 2004, 101, 113-121. 

In addition to employing packed beds, a small group of researchers have demonstrated the use of wall coating as a means of incorporating heterogeneous catalysts into continuous flow systems. One such example... 

In addition to flow maldistribution, small deviations in the channel diameter (which typically originate from imperfect manufacturing) cause a broadening of the RTD. The deviations may also be the result of nonuniform coating of the channel walls with catalyst material (Section 5). If the number of parallel channels is large (i.e., N > 30), a normal distribution of the channel diameters with a standard deviation a can be assumed. The effect of the diameter variation on the pressure drop in the microreactor can be estimated on the basis of the relative standard deviation, G = ffd/ t [81 ] ... 

Along with the inclusion of heterogeneous metal catalysts in continuous flow reactors, numerous authors have evaluated the advantages associated with the incorporation of acid and base catalysts in these reaction systems, using a range of packed beds, monoliths, and wall-coated reactors. 

Employing a silicon micro reactor [channel dimensions = 500 or 1000 pm (width) x 250 pm (depth)], wall-coated with the acidic zeolite titanium silicate-1 (TS-1, Si Ti ratio = 17) (83) (3 pm), Gavrilidis and co-workers [52] demonstrated a facile method for the epoxidation of 1-pentene (84) (Scheme 6.23). Using H202 (85) (0.18 M, 30wt%) as the oxidant and 84 (0.90 M) in MeOH, the effect of reactant residence time on the formation of epoxypentane (86) was evaluated at room temperature. The authors observed increased productivity within the 500 pm reaction channel compared with the 1000 pm channel, a feature that is attributed to an increase in the surface-to-volume ratio and thus a higher effective catalyst loading. 

Both reactor types R3 and R4 use the segmented flow (Taylor) principle. They are divided into two categories R3 has very small channels (<1 mm) and R4 are monolith reactors (honeycomb), well developed on the laboratory scale with at least one example of industrial application. Category R3 includes single-channel and multi-ple-channel reactors [10], etched in silicon [10] or glass [10,11], with wall-coated or immobilized catalysts in the case of gas-liquid-solid additions [12], and capillary microreactors for gas-liquid-liquid systems [13]. 

Of course, not all multiphase microstructured reactors are presented in Table 9.1. Either because they have attracted (too ) little interest, because they may have been qualified as microreactors in spite of their overall size but caimot be considered as microstmctured , or because they combine several contacting principles. Examples are a reactor developed by Jensen s group featuring a chaimel equipped with posts or pillars, thus resembling more a packed bed but with a wall-coated layer of catalyst [20], and a string catalytic reactor proposed by Kiwi-Minsker and Renken [21], that may applied to multiphase reactions. 

Recently, the hydrogenation a mixture of toluene, styrene and 1-octene, representing a model feed for hydrotreating in the refining industry, was performed in monolith reactors [37]. One is a y-alumina monolith of diameter 1 cm and 15 or 30 cm long and the other is a more conventional cordierite monolith with a wall-coated layer of y-alumina. In both monoliths, the channels size is 1-2 mm and the catalyst is based on Ni. Substantial alkene conversions of more than 50% were observed in the small-channel reactors, which was attributed to the intensified mass-transfer rate generally measured in monolith reactors [16]. 

Wall-coated microchannel reactor [48, 51] A reactor which incorporates either a conventional packed bed or a stack of microstructured wafers with Pd catalysts is used... 

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

Due to short diffusion pathways in the microsystem, the overall mass transport in the phases or the transfer via phase boundaries is often magnitudes higher than in conventional reactor systems. However, with regard to the desired high loadings with catalyst and low cost for fluid compression or pumping, the mass transfer to the catalyst and the mass transport within porous catalyst still has to be effective. As for the heat transport the differentiation between packed bed and wall-coated microreactor is necessary for mass transport considerations. The mass transport in packed bed microreactors is not significantly different to normal tubular packed bed reactors, so that equations like the Mears criteria (Eq. 6) can be used. 

In the case of the wall-coated microchannel system Eq. 6 rewrites in the form of Eq. 8 (Gorke et al. (2009)) due to the accessible geometric surface of the catalyst is different and the mass balance between transport to the surface and the reaction in the catalyst has to be fulfilled. 

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