Membrane reactors catalyst incorporation

Abstract Membrane reactors with a catalyst bed are designed to be used in various reactions, such as hydrogenation, dehydrogenation, oxidation and reforming reactions. The catalyst can be introduced into the reactor as a bed in several ways in the form, for example, of pellets, extrudates or tablets or it can be incorporated in the reactor as a catalytic membrane wall. However, in many cases, the studies concentrate on the membrane itself, the development of catalysts is ignored, and commercial catalysts are used in the experiments. Most of the catalysts tested are aluminium oxide (alumina, AI2O3) based, as alumina is a mature support and already well proven in convectional reactors. However, some new catalyst materials such as carbon nanotubes (CNTs), carbon black, gels and anodic aluminium oxide (AAO) are developed as innovative catalyst supports and catalysts, since there is also a need for new catalysts for membrane reactors. 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

In the second mode of incorporating a catalyst into a membrane reactor, the catalyst is attached to the membrane surface on the feed or permeate side or to the surfaces of the membrane pores. This case and the third mode where the membrane is inherently catalytic are often called catalytic membrane reactor (CMR). 

The requirement for catalytic surface area may determine in what form the catalyst should be incorporated in the membrane reactor. If the underlying reaction calls for a very high catalytic surface area, the catalyst may need to be packed as pellets and contained inside the membrane tubes or channels rather than impregnated on the membrane surface or inside the membrane pores due to the limited available area or volume in the lauer case. 

A composite membrane was prepared by incorporating Ti-MCM-41 in PDMS (polydimethylsiloxane). Using this catalyst, the epoxidation of 1-octene was studied with focus on the influence of the polymer environment on the actual catalytic performance. Three different oxidants were investigated, as well as the influence of the solvent. It was found that the newly developed catalyst is especially interesting under solvent free reaction conditions where it might suppress side reactions. Furthermore, the removal of reagents and catalyst from the reaction mixture after reaction is facilitated and epoxidation in a continuous counter current membrane reactor becomes feasible. 

Reports are also available on CO2 selective membrane reactors for WGS reaction. Zou et al. [40] first time synthesized polymeric C02-selective membrane by incorporating fixed and mobile carriers in cross-linked poly vinyl alcohol. Micro-porous Teflon was used as support. They used Cu0/Zn0/Al203 catalyst for low temperature WGS reaction. They investigated the effect of water content on the CO2 selectivity and CO2/H2 selectivity. As the water concentration in the sweep gas increased, both CO2 permeability and CO2/H2 selectivity increased significantly. Figure 6.18 shows the influence of temperature on CO2 permeability and CO2/H2 selectivity. Both CO2 permeability and CO2/ H2 selectivity decrease with increasing reactimi temperature. After the catalyst activation, the synthesis gas feed containing 1% CO, 17% CO2, 45% H2 and 37% N2 was pumped into the membrane reactor. They are able to achieve almost 100% CO conversion. They also developed a one-dimensional non-isothermal model to simulate the simultaneous reaction and transport process and verified the model experimentally under an isothermal condition. 

The term membrane means a permeable phase acting as a selective barrier and controlled by mass transport. A membrane can be porous or dense material, and separation takes place due to a difference in chemical potential gradients (Dittmeyer et al, 2001). There are two materials involved in an MR a membrane and a catalyst. A membrane can have catalytic and separation functions by itself, or each material can function independently depending upon how the catalyst and membrane are incorporated in an MR. In a tubular MR, the catalyst bed is packed in the annulus or inside the tube, in which case the MR is termed a packed bed membrane reactor. 

Ammonia oxidation for the manufacture of NO (an intermediate in nitric acid production) is carried out in an oxidation reactor incorporating a Ca- and Sr-substituted lanthanum ferrite perovskite membrane. NO selectivity of the order of 98% is achieved by the membrane, while N2 is rejected completely. The membrane reactor obviates the need for expensive noble metal catalysts and does not produce environmentally harmful N2O. 

Catalytic membranes brought new and attractive applications of metal-incorporated mesoporous materials. Mesoporous nickel-silicate membranes were used as efficient catalysts in the selective oxidation of styrene to epoxy ethyl benzene and benzene to phenol. The use of membranes also offered a very good possibility to control the hydrogen peroxide feed and the selectivity in oxidation of styrene to styrene oxide and to increase the reaction rate. The effect of the H2O2 permeance on the conversion of styrene and benzene was also evidenced [83]. The conversion of styrene with membrane reactor has been compared with that realized in a conventional batch reactor with powdery catalyst indicating superior results. 

In order to improve the catalytic activity and selectivity of the membrane reactor, a catalyst has to be used. A simple way is to place the catalyst pellets on/next to the membrane, as shown in Fig. 7.9a. The membrane mainly functions as either a product extractor or a reactant distributor, although it also plays some role in the reaction.The reaction selectivity is mainly determined by the catalyst. This incorporation mode is most popular in practical use and is easily operated. Since the catalyst is physically separated from the membrane, only the separation function of the membrane needs to be controlled. The high selectivity of the dense ceramic membranes leads to highly attractive results (pure Hj extraction in dehydrogenation reactions and direct use of air in partial oxidation reaction). But the permeability of the membrane has to be improved as high as possible. 

The selection of a module shape depends on a number of factors, including cost, heat management, manufacturability, maintainability, operability, efficiency and membrane replacement. Membrane modules and thus membrane reactors can be combined into number of stages. The options for membrane system layout are virtually endless, as stages can be combined in various ways incorporating both compressors and recycle streams. Furthermore, membrane reactors inevitably contain catalysts and there are several ways in which the catalyst can be incorporated (Tsotsis et al., 1993). The classification of membrane reactors that incorporate catalysts is mainly based on the location of the catalyst with respect to the membrane as shown in Table 9.2. 

There have been very few experimental studies examining membrane reactors for ATR, and all of them have dealt with Pd-based membranes. Recent collaborations, between the Films and Inorganic Membrane Laboratory at The University of Queensland in Australia and Saudi Aramco s Research and Development Center, on the use of sihca-based membrane reactors for the ATR of liquid fuels has yielded some promising experimental results. Cobalt-doped silica membranes (Uhhnann et ah, 2009) were incorporated with a commercial catalyst in an ATR catalytic membrane reactor to process a gasoline feed. In comparison to the fixed bed reactor (employing the same... 

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