Hydrogen membrane reactor catalyst

Key words catalytic membrane, packed-bed membrane reactor, catalyst distribution, deactivation, hydrogen spill-over, sustainability. 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

Ishihara, T. et al., Decomposition of methane over Ni/Si02 catalysts with membrane reactor for the production of hydrogen, Chem. Lett., 93, 1995. 

Using unmodified Ru-BINAP and Rh-Et-DUPHOS catalysts Jacobs et al. performed hydrogenation reactions of dimethylitaconate (DMI) and methyl-2-acetamidoacrylate (MAA), respectively. [11,47] The continuous hydrogenation reaction was performed in a 100 mL stirred autoclave containing an MPF-60 membrane at the bottom, which also acts as a dead-end membrane reactor. The hydrogenation reactions will be discussed in paragraph 4.6.1. 

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L"1 d"1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d"1 with an ee of >99%. The catalyst half-life times were... 

Liese el al. attached a transfer-hydrogenation catalyst to a soluble polymer and applied this system in a continuously operated membrane reactor.[60] A Gao-Noyori catalyst was bound to a soluble polysiloxane polymer via a hydrosilylation reaction (Figure 4.41). 

For a packed-bed membrane reactor (PBMR) the membrane is permselective and removes the product as it is formed, forcing the reaction to the right. In this case, the membrane is not active and a conventional catalyst is used. Tavolaro et al. [45] demonstrated this concept in their work on CO2 hydrogenation to methanol using a LTA zeolite membrane. The tubular membrane was packed with bimetallic Cu/ZnO where CO2 and H2 react to form EtOH and H2O. These condensable products were removed by LTA membrane which increased the reaction yield when compared to a conventional packed bed reactor operating under the same conditions [45]. 

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

Simulations based on kinetic modelling of the reduction of acetophenone with propan-2-ol, using polymer-enlarged and the unmodified catalysts, revealed that comparable performance cannot be obtained by batch operation. Polymer enlargement allowed a continuous operation of transfer hydrogenation in a chemical membrane reactor.353... 

A commercial Cu based catalyst supplied by Haldor-Topsoe was applied to the water-gas shift reaction. At 210 °C, a permeating flux of 4.5 Ndm3 nT2 s 1 was determined for pure hydrogen at a very low pressure drop of 0.2 bar. Then the membrane reactor was coupled with a conventional water-gas shift reactor. At 260-300 °C reaction temperature and a GHSV of 2 085 h 1, the maximum conversion achievable due to the thermodynamic equilibrium could be exceeded by this new technology by 5-10%. 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

The Vycor glass tube used in the membrane reactor experiments was filled with 3.2 g of catalyst. As the accessible area was ca. 30 cm2, the ratio of membrane area to catalyst mass is in the range specified by Eq. (35). Consequently, there should be sufficient membrane area available to remove significant amounts of hydrogen and, therefore, to have an effect on the reaction process. 

The use of a membrane reactor in steam reforming has several advantages. Because of the lower temperature operation, the energy consumption of the process is reduced which results in lower emission of C02. The lower temperature also requires less expensive catalyst, tubing and other reactor materials. Since hydrogen of sufficient purity is produced directly from the reformer, the downstream shift conversion can be omitted. Moreover, the dimensions of the C02 removal and final purification units can be reduced. Hence, significant savings in equipment costs can be expected. 

Of course, this reactive adsorption is favoured by removal of hydrogen from the reaction zone. When 80% of the hydrogen is removed in the membrane reactor, the H2S tolerance of the catalyst is about halve the tolerance when no hydrogen is removed from the reaction zone. A higher degree of sulphur removal from the feed stream should be accomplished when operating a membrane steam reformer. 

K. Hou, M. Fowles and R. Hughes, Potential Catalyst Deactivation Due to Hydrogen Removal in a Membrane Reactor Used for Methane Steam Reforming , Chem. Eng. Sci., 54 3783-91 (1999). 

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