Microporous silica membranes reaction

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

Clearly, in such a reaction the removal of the produced water will lead to an enhanced conversion. Commercially available polymer membranes cannot withstand the severe operation and cleaning conditions for this process (150-300°C) and microporous silica membranes again come into the picture. 

Research on other types of materials for H2 separation has been motivated by relatively high cost of Pd and possible membrane degradation by acidic gases and carbon as summarized in Tsuru et al.76 These authors examined microporous silica membranes together with an Ni catalyst layer for SMR reaction. However, this type of membrane allows the permeation of hydrogen as well as other gases in reactants and products, which markedly reduces hydrogen selectivity and limits methane... 

C.L. Lin et al. [71] reported deposition of silica layers (plugs) with a thickness of about 1.5 pm within the pores of commercial, mesoporous y-alumina films (pore diameter 4 nm, thickness 1-3 pm) on a-alumina supports (US filter). The deposits were obtained by reaction of TEOS-oxygen (10-20%) mixtures in He as carrier gas applied in the OSG mode to the mesoporous layer. No further details (e.g., temperature or pressure) were given. Depending on these unknown conditions, dense as well as microporous silica membranes with pores down to estimated values of 0.4-0.6 nm were obtained. These membranes have interesting combinations of permselectivity and flux values for several gas combinations (see Chapter 9 on gas transport properties). 

The conclusion is that for relatively small molecules (H2, CO2, etc.), permeation in microporous (silica) membranes is not limited by surface reactions and direct penetration in the pores is the dominant mechanism in a wide range of temperature and pressure conditions [63]. This conclusion does not hold for large non-spherical molecules. Here sorption is necessary, the sticking coefficient becomes very important and surface reactions probably will limit the permeation as soon as bulk permeation becomes appreciable. To the knowledge of the present author, no investigations of this phenomenon in microporous membranes have yet been reported. 

A number of studies have been reported on the MRs for hydrogen production by various reforming reactions, which are summarized in Table 2.8. Most research uses microporous silica membranes and a PBMR configuration. PSS substrate is usually used for high mechanical strength. A key point is to use optimum reaction conditions for a comparable hydrogen removal rate to production rate. The membrane characteristics for H2 permeance and selectivity over time are a vital consideration in the commercial feasibility of this technology. 

Key words microporous silica, membrane reactors, water-gas shift reaction, autothermal reforming, steam reforming. 

Microporous silica membranes operate on a molecular sieving principle and are usually tailored to separate H2 from other, larger, gases. Hence their incorporation into membrane reactor technology is most suited to those reactions that either consume or produce H2 and especially, given their thermal stability, for those reactions involving H2 that occur at temperatures up to 800°C. This section explores the membrane reactor performance of several of the catalytic reactions where silica-based membranes have been effectively demonstrated at the research level. 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

It is worth noting that both Pd-aUoy and sUica-based membranes present some problem about material instability in the WGS environment. The Pd-aUoy membranes can be negatively affected by surface carbonization, sulfur poisoning, and hydrothermal embrittlement, whereas the amorphous silica-based membranes can show some degradation caused by the condensation reaction of sUanol in hydrothermal conditions (Tang et al., 2010). In particular, the siliceous MFI-type zeolite membranes, constituted by a crystalline microporous zeolite membrane, in recent years have been seen as attractive candidates for the WGS reaction because of the high-temperature hydrogen separation and for their intrinsic sulfur tolerance and hydrothermal stability. 

Table 2.7 summarizes some of the dehydrogenation reactions studied in the literature using porous MRs. As can be seen, the membranes for dehydrogenation reactions are primarily microporous silica due to it high hydrogen perm-selectivity. Meso- or macroporous membranes are not suitable for dehydrogenation reactions because their low perm-selectivity may lead to much loss of the reactants. The catalysts for dehydrogenation may be packed in the reactor or loaded inside the membrane wall. All the studies demonstrate enhanced conversions or even beyond-equilibrium values. 

Pure silica microporous solids show no such strong affinity for polar molecules, and are of interest to separate hydrocarbon molecules of different shapes. In particular, the separation of xylene isomers is of great industrial significance, and since the relative diffusivities of the para isomer are known to be much higher than those of the ortho (and meta) isomers within silicalite, the preparation of silicalite membranes is an attractive target. If the aluminium content can be made vanishingly small, the membranes can be used for hydrocarbon separation at elevated temperatures without the effects of coke formation due to catalytic reaction. 

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