Slurry-phase Catalysts

As mentioned above, the catalysts employed in multipurpose reactors are usually suspended as small solid particles in the liquid containing the compounds to be processed. The above-mentioned weight fractions of catalyst from 0.07 to about 2.5 % (w/w) are common in slurry reactors. Together with the reaction products most of the catalyst is removed from the reactor and the catalyst is subsequently separated from the reaction products by filtration or centrifugation in a special unit. Heavy catalyst particles are often attractive, because the catalyst can be separated from the reaction products merely by settling and decanting the liquid. 

The minimum size of the catalyst particles employed in slurry phase reactors is ca 3 pm, the size required for separation of the catalyst from the liquid reaction product(s) by filtration or centrifugation. As covered above, the surface area of catalyst particles of at least 3 pro is too small to lead to economically acceptable rates of production. Because the size of the catalyst particles cannot be reduced, employment of (highly) porous catalyst particles is required. Because of the presence of pores within catalyst particles of at least 3 pm, the total surface area is much larger than the external surface area. Employment of porous catalyst particles, however, has two drawbacks-the rate of transport through the generally narrow pores in the catalyst particles can determine the rate of the reaction [5,6], and the mechanical strength of porous catalyst particles is often difficult to maintain at the level required to avoid attrition. 

The lengths of the pores in the catalyst particles can severely affect the selectivity of catalytic reactions. For a fairly rapid reaction leading to a product that is liable to further reaction to undesirable products, equally rapid removal of the desired initial reaction product from the pore system of a catalyst particle is necessary. Extensive research (Unilever) has been performed on the properties of nickel catalysts for the hardening (partial hydrogenation) of edible oils [7]. Catalyst pres- 

With the batch reactors used in the fine-chemical industry, the rate of the catalytic reaction is generally not decisively important. The number of catalyst particles per unit volume of the liquid to be treated is one of the experimental factors determining the apparent activity of the catalyst. Because the size of the catalyst particles usually affects the apparent activity of the catalyst only, the size is not critical, provided the particles are no smaller than ca 3 pm. When the size of the particles is below this, separation of the catalyst from the reaction product(s) is difficult, and with still smaller sizes even impossible. The requirement to avoid particles smaller than ca 3 pm imposes fairly severe requirements on the mechanical strength of catalyst particles employed in slurry-phase reactors. When the catalyst particles are liable to attrition, which leads to particles smaller than 3 pm, it is difficult to purify the reaction product(s) completely from the catalyst. Especially with fine-chemicals to be used in the food or pharmaceutical industry, contamination of the reaction product with the catalyst is usually not acceptable. Either mechanically strong catalyst particles must therefore be employed with slurry-phase catalysts or the reactor must be adapted to minimize attrition. With a bubble-column reactor the attrition of suspended catalyst particles is much smaller than with a reactor equipped with a stirrer that vigorously agitates the suspension. 

Catalysts in which a metal is the active component are, however, pyrophoric after thermal treatment of the metal precursor in a reducing gas flow. Grinding of a reduced catalyst in an inert atmosphere without intermediate exposure to atmospheric air has been performed with nickel fat-hardening catalysts. After the grinding procedure the small catalyst particles are taken up in hardened fat, which protects the nickel against oxidation. The procedure is, however, tedious and cannot be readily executed with the small batches of catalyst usually used in the fine-chemical industry. 

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A wide variety of catalytic materials are used as slurry-phase catalysts, most being metals supported on high surface area alumina, carbon, and silica (Fig. 2, label 3). Physical properties such as density are important since these catalysts must be suspended in the reaction mix. Since rapid agitation could lead to abrasion and attrition of the catalyst particles, strength is important. 

As covered in Section 2.4 by Van Diepen and Moulijn, multipurpose reactors are usually used in the fine-chemical industry-the relatively small quantities of products of the fine-chemical industry do not justify setting up a separate reactor for each chemical conversion, as is customary in the bulk chemical industry. A range of products is therefore produced in the same reactor (Figure 1), which is consecutively loaded with a number of different catalysts. Solid catalysts are usually suspended in liquid in the multipurpose reactor. Slurry-phase catalysts are, therefore, mostly employed in the fine-chemical industry. 

The difficulty of thermal treatment of slurry-phase catalysts has resulted in the predominant use of Raney metals and precious metals in the fine-chemical industry. Raney metals are produced from an aluminum alloy of the metal to be used as a catalytically active component. Treatment with alkali leaches aluminum from the alloy and leaves a very finely divided metal [10] as ca 10 to 20 nm metal particles clustered into conglomerates of several microns. Aluminum remaining in the metal after treatment with alkali protects the metal against oxidation. The aluminum reacts very slowly with the water in which the Raney metals are stored and... 

Finally, a potential application of molecular metal clusters which is virtually unexplored is the intentional generation of colloidal metal particles and the production of slurry phase catalysts (Table I). This may offer scope for future work. 

Survey of the patent Hterature reveals companies with processes for 1,4-butanediol from maleic anhydride include BASF (94), British Petroleum (95,96), Davy McKee (93,97), Hoechst (98), Huels (99), and Tonen (100,101). Processes for the production of y-butyrolactone have been described for operation in both the gas (102—104) and Hquid (105—108) phases. In the gas phase, direct hydrogenation of maleic anhydride in hydrogen at 245°C and 1.03 MPa gives an 88% yield of y-butyrolactone (104). Du Pont has developed a process for the production of tetrahydrofuran back-integrated to a butane feedstock (109). Slurry reactor catalysts containing palladium and rhenium are used to hydrogenate aqueous maleic acid to tetrahydrofuran (110,111). 

Another recent new application of a microporous materials in oil refining is the use of zeolite beta as a solid acid system for paraffin alkylation [3]. This zeolite based catalyst, which is operated in a slurry phase reactor, also contains small amounts of Pt or Pd to facilitate catalyst regeneration. Although promising, this novel solid acid catalyst system, has not as yet been applied commercially. 

In continuation of a previous work (1), catalytic hydrogenation of cinnamaldehyde has been studied in slurry phase using a high-pressure autoclave. A series of carbon powder (CP)-supported Pt catalysts with widely varying Pt dispersion and Pt location on the support has been used in the study. The purpose has been to find out how the location of the metal on the support and its dispersion affect the two parallel reaction paths, namely the hydrogenation of the C=0 and C=C bonds. 

Three carbon powders from three different commercial manufacturers were used to make six catalysts. For each carbon, two types of catalysts, namely HEC and HDC, were prepared using slurry-phase preparation methods. For the HDC catalysts a chloroplatinic acid solution containing the requisite amount of Pt (to generate a nominal 1.5 wt% Pt/CP) was added to an alkaline CP slurry. This was... 

Butanol over Naphtha Reforming Type Catalysts in Conventional and High Throughput Slurry Phase Reactors... 

In this study butyl acetate (AcOBu) was hydrogenolysed to butanol over alumina supported Pt, Re, RePt and Re modified SnPt naphtha reforming catalysts both in a conventional autoclave and a high throughput (HT) slurry phase reactor system (AMTEC SPR 16). The oxide precursors of catalysts were characterized by Temperature-Programmed Reduction (TPR). The aim of this work was to study the role and efficiency of Sn and Re in the activation of the carbonyl group of esters. 

We have demonstrated that supported Pd and Cu catalysts are effective in catalyzing the oxidative carbonylation at low pressure reaction condition and the supported metal catalysts can be easily separated from the product mixture in both fixed bed and slurry phase reactors (12,17). The objective of this study is to investigate the feasibility of using Al203-supported Pd catalysts for catalyzing the reductive carbonylation of nitrobenzene with ethanol. 

Withers, H.P., Eliezer, K.F., and Mitchell, J.W. 1990. Slurry-phase Fischer-Tropsch synthesis and kinetic studies over supported cobalt carbonyl derived catalysts. Ind. Eng. Chem. Res. 29 1807-14. 

Rao, V. U. S., Stiegel, G. J., Cinquegrane, G. J., and Srivastava, R. D. 1992. Iron-based catalysts for slurry-phase Fischer-Tropsch process Technology review. Fuel Process. Technol. 30 83-107. 

Sun, S., Tsubaki, N., and Fujimoto, K. 2000. Characteristic feature of Co/Si02 catalysts for slurry phase Fischer-Tropsch synthesis. J. Chem. Eng. Jpn. 33 232-38. 

FIGURE 14.2 TPR profiles of the Co/A1203 catalysts prepared by the slurry phase impregnation method. 

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

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