Catalysts mobilization

The important issues are the generality of the concept, the robustness of the catalyst system and the influence of the thickness of the water layer [65], This water layer has an enormous impact on the catalytic activity (Figure 3.15). It was observed that if the layer is too thin the activity of the catalyst is much lower due to a decrease of the catalyst mobility. The catalyst is bound to the silica resulting in a heterogeneous... 

This also holds for the other mechanism—known as the Langmuir-Hinshelwood mechanism—where the two reactant molecules have to be chemisorbed side by side. Such pairs can be formed statistically by the molecules hitting the surface and being bound by chemisorption forces at the very spots where they hit (localized adsorption) or they may result from collisions of the molecules moving along the surface of the catalyst-(mobile adsorption). It may again be stated that the formation of pairs in the case of immobile localized adsorption is more favorable for reaction than in the case of mobile adsorption. 

Catalyst Mobile small particles 1-200 pm Mobile particles 0.1-5 mm Spheres, extrudates, etc. 1.5-6 mm Blocks with channels covered with catalyst layer 10-150 pm... 

A number of ways was employed to make contact between soot and catalyst. It was shown tliat the contact between soot and catalyst is of prime importance for tlie reactivity of the catalyzed soot oxidation. Several catalysts were found to be unable enhancing the soot oxidation rate when the contact between soot and catalyst is poor. This explains why catalyst coated particulate filters have been found quite inactive under practical conditions. For catalytic systems wliich seem to be able to decrease soot combustion temperatures a possible explanation can be probably found in catalyst mobility. 

Zeolite Catalyst - Mobil Versus Esso (Exxon)... 

Spray dried zeolite bearing catalyst (Mobil). USY REY catalysts (Davison Division of W.R. Grace). 

Numerous soot oxidation catalysts have been reported since the 1980s, because soot oxidation is fundamentally a simple complete oxidation reaction (carbonaceous compounds CO2 + H2O), so that sophisticated catalysts with high selectivity are not required. However, there is a critical problem in establishing contact and interaction, directly or indirectly, between the reactant (soot) and the catalyst, both of which are solid materials. Therefore, soot oxidation catalysts reported to date can be classified according to the assumed working mechanism that solves this problem. In this review the authors classify the catalysts into the four types shown in Fig. 2.5, based on the mediator for the oxidation reaction that connects the active sites of catalyst and soot surfaces mobile catalysts, mobile oxygen catalysts, NO2 mediating... 

When more hydrophobic monomers are used, the hydro-phobicity of the CCT reagents can be altered and when tetraphenylcobaloxime boron fluoride (COPhBF) is used, it is found to reside predominantly in the organic phase. The best results are often achieved under monomer-flooded conditions that keep the glass transition temperature (Tg) of the monomer-filled particles below the reactor temperature. CCT does actually reduce the rate of polymerization such that the monomer can be fed at a rate that maintains the instantaneous conversion low enough for the particle to be swollen with monomer, allowing diffusion of the catalysts between the aqueous and monomer phases. If the amount of the catalyst is reduced and the rate increases, this can lead to viscous, glassy particles that prevent catalyst mobility, which is observed as a... 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

The three major commercial Hcensors of xylenes isomerization processes are Engelhard, UOP, and Mobil. Several other companies have developed and used their own catalysts. These companies include Mitsubishi Gas—Chemical, Toray, ICI, Amoco, and Shell. AH of these processes are discussed herein. 

A second Mobil process is the Mobil s Vapor Phase Isomerization Process (MVPI) (125,126). This process was introduced in 1973. Based on information in the patent Hterature (125), the catalyst used in this process is beHeved to be composed of NiHZSM-5 with an alumina binder. The primary mechanism of EB conversion is the disproportionation of two molecules of EB to one molecule of benzene and one molecule of diethylbenzene. EB conversion is about 25—40%, with xylene losses of 2.5—4%. PX is produced at concentration levels of 102—104% of equiHbrium. Temperatures are in the range of 315—370°C, pressure is generally 1480 kPa, the H2/hydrocatbon molar ratio is about 6 1, and WHSV is dependent on temperature, but is in the range of 2—50, although normally it is 5—10. 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

A thkd method utilizes cooxidation of an organic promoter with manganese or cobalt-ion catalysis. A process using methyl ethyl ketone (248,252,265—270) was commercialized by Mobil but discontinued in 1973 (263,264). Other promoters include acetaldehyde (248,271—273), paraldehyde (248,274), various hydrocarbons such as butane (270,275), and others. Other types of reported activators include peracetic acid (276) and ozone (277), and very high concentrations of cobalt catalyst (2,248,278). 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

Two modifications of the duidized-bed reactor technology have been developed. In the first, two gas-phase duidized-bed reactors coimected to one another have been used by Mobil Chemical Co. and Union Carbide to manufacture HDPE resins with broad MWD (74,75). In the second development, a combination of two different reactor types, a small slurry loop reactor followed by one or two gas-phase duidized-bed reactors (Sphetilene process), was used by Montedision to accommodate a Ziegler catalyst with a special particle morphology (76,77). This catalyst is able to produce PE resins in the form of dense spheres with a diameter of up to 4—5 mm such resins are ready for shipping without pelletization. 

Acetaldehyde can be used as an oxidation-promoter in place of bromine. The absence of bromine means that titanium metallurgy is not required. Eastman Chemical Co. has used such a process, with cobalt as the only catalyst metal. In that process, acetaldehyde is converted to acetic acid at the rate of 0.55—1.1 kg/kg of terephthahc acid produced. The acetic acid is recycled as the solvent and can be isolated as a by-product. Reaction temperatures can be low, 120—140°C, and residence times tend to be high, with values of two hours or more (55). Recovery of dry terephthahc acid follows steps similar to those in the Amoco process. Eastman has abandoned this process in favor of a bromine promoter (56). Another oxidation promoter which has been used is paraldehyde (57), employed by Toray Industries. This leads to the coproduction of acetic acid. 2-Butanone has been used by Mobil Chemical Co. (58). 

Vapor-Phase Processes. Although vapor-phase alkylation has been practiced since the early 1940s, it could not compete with Hquid-phase processes until the 1970s when the Mobil—Badger vapor-phase ethylbenzene process was introduced (Eig. 4). The process is based on Mobil s ZSM-5 zeohte catalyst (38,52,53). The nonpoUuting and noncorrosive nature of the process is one of its major advantages over the AlCl hquid-phase system. 

The Xylene Plus process of ARGO Technology, Inc. (95,96) and the FINA T2BX process (97) also use a fixed-bed catalyst in the vapor phase for transalkylation of toluene to produce xylenes and benzene. The Mobil low temperature disproportionation (LTD) process employs a zeoHte catalyst for transalkylation of toluene in the Hquid phase at 260—315°C in the absence of hydrogen (98). 

Two catalysts have emerged as commercially viable. The Mobil—Badger ethylbenzene process, which has been in commercial use since 1980, employs a ZeoHte catalyst and operates in the gas phase. A Hquid-phase ethylbenzene process joindy Hcensed by Lummus and UOP uses a Y-type ZeoHte catalyst developed by Unocal. This Hquid-phase process was commercialized in 1990. The same Y-type ZeoHte catalyst used for the production of ethylbenzene is being offered for the production of cumene but has not yet been commercialized. 

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