Catalysts alumina-halogen

Thus, at 372°C. and over the range of pressures and hydrogen to n-pentane ratios covered in the investigation, it appears that the proposed mechanism can account in large part for the observed kinetic data. However, Starnes and Zabor (S8) have proposed an alternative mechanism, based on their studies of n-pentane isomerization over platinum-alumina-halogen catalysts. They postulate that the paraffin is adsorbed on platinum sites with dissociation of a hydrogen atom, followed by polarization of the adsorbed species. 

Calculation of Conversion of 1-Methyl-2-Ethylcyclopentane to C Aromatic Isomers over Platinum-Alumina-Halogen Catalyst (K3) ... 

The reaction of u-butenes to give isobutylene is cataly zed by a wide variety of solid acids but requires relatively high temperature. Typical catalysts include alumina, halogenated alumina, amorphous silica-alumina, supported phosphoric acid, and supported tungsten or molybdemmi oxide. The most characteristic features of the skeletal isomerization of n-butenes... 

Another type of selectivity effect that can become important during the deactivation of oxidation catalysts for halogenated hydrocarbons arises from the inherent selectivity of the catalyst. Ramanathan and Spivey 0 studied the catalytic oxidation of dichloroethane and of trichloroethane over chromia on alumina catalyst. The selectivity observed is summarized in Figure 9, below ... 

UOP s Isomar process (56,117—119) was originally developed to use dual-functional catalysts. The first-generation catalyst contained Pt and halogen on alumina. Operating conditions using this catalyst were 399°C 1.25 MPa 2 LHSV and H2/hydrocarbon ratio of 6 1. A Cg naphthene concentration of... 

Ethylene oxide is produced in large, multitubular reactors cooled by pressurized boiling Hquids, eg, kerosene and water. Up to 100 metric tons of catalyst may be used in a plant. Typical feed stream contains about 30% ethylene, 7—9% oxygen, 5—7% carbon dioxide the balance is diluent plus 2—5 ppmw of a halogenated moderator. Typical reactor temperatures are in the range 230—300°C. Most producers use newer versions of the Shell cesium-promoted silver on alumina catalyst developed in the mid-1970s. 

Support-phase changes or loss of surface area are, of course, irreversible, and replacement of the catalyst may be appropriate. Catalyst damage may take the form of phase changes to the alumina support from gamma to theta or alpha phase. The last is catalyticaky inert because of insignificant surface area. Theta alumina has a low surface area (< 100 /g) relative to gamma alumina (180 m /g) and has poor halogen retention. 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

Promoters may influence selectivity by poisoning undesired reactions or by increasing the rates of desired intermediate reactions so as to increase the yield of the desired product. If they act in the first sense, they are sometimes referred to as inhibitors. An example of this type of action involves the addition of halogen compounds to the catalyst used for oxidizing ethylene to ethylene oxide (silver supported on alumina). The halogens prevent complete oxidation of the ethylene to carbon dioxide and water, thus permitting the use of this catalyst for industrial purposes. 

In addition to this work on charcoal- and silica-supported catalysts and on evaporated platinum films, a number of studies have been made on alumina-supported platinum catalysts (e.g., 111-114, 81,115) in which the aim has been the study of reactions at the platinum alone. In these cases, one cannot automatically dismiss the possibility of participation of the alumina support (i.e., of dual function behavior of the catalyst) because it is known that alumina may have acidic properties, particularly when retained halogen is present. In general terms, there is no immediate answer to this problem because the nature of this sort of catalyst wall be much dependent on the details of catalyst history, preparation, and use. However, there can be little doubt that in many experimental studies using plati-num/alumina, and in which the assumption has been made that the alumina support is inert, this assumption is essentially valid. For instance, one may note the inert alumina used by Davis and Venuto (111) and the justification provided by Gault et al. (116) for the inertness of the alumina used in a substantial body of previous work irrespective of whether the catalyst was... 

The catalyst is reported to be a true solid acid without halogen ion addition. In the patent describing the process (239), a Pt/USY zeolite with an alumina binder is employed. It was claimed that the catalyst is rather insensitive to feed impurities and feedstock composition, so that feed pretreatment can be less stringent than in conventional liquid acid-catalyzed processes. The process is operated at temperatures of 323-363 K, so that the cooling requirements are less than those of lower temperature processes. The molar isobutane/alkene feed ratio is kept between 8 and 10. Alkene space velocities are not reported. Akzo claims that the alkylate quality is identical to or higher than that attained with the liquid acid-catalyzed processes. 

Pt supported on an acidic support is a typical catalyst for the skeletal isomerization of light n-paraffins. The acidic supports can be acidic oxides, e.g., halogenated (Cl, F) alumina or sulfated zir-conia (Zr02/S04), or an appropriate zeolite, e.g., Mordenite. Pt-(C1, F)-alumina catalysts have a high performance at low temperatures and efficiently operate at temperatures between 115 and 150°C. Such low temperatures thermodynamically favor isomerization and thus, highly branched products are obtained. Zeolite supports are less active at lower temperatures and have to be operated at about... 

Historically, the earliest C8 aromatic isomerization catalysts tended to use amorphous supports with a halogen such as chloride or fluoride. Due to water sensitivity and corrosion issues, these were replaced by large-pore zeolites such as mordenite. The larger pore size was more favorable toward bimolecular transalkylation, whereas the chlorided alumina support tended to promote cracking. In both... 

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

Although hydrogenation of pyrrole over a rhodium/alumina catalyst gives some 1-pyrroline (Scheme 6.18a), a better method is to dehydro-halogenate A-chloropyrrolidine by heating it with alcoholic potassium hydroxide (Scheme 6.18b). 2,5-Dihydro-1//-pyrrole, containing 15% pyrrolidine, is obtained by the zinc/hydrochloric acid reduction of pyrrole. 

A more detailed interpretation of the chemistry of catalytic cracking was based on studies with pure hydrocarbons.121-123 A simplified summary put forward by Heinemann and coworkers123 (Fig. 2.1) shows how Cg open-chain and cyclic alkanes are transformed to benzene by the action of both the hydrogenating (metal) and acidic (halogenated alumina) functions of the catalyst. 

The work with l-bromo-2-chloroethane allowed the influence of the nature of the halogen on its reactivity to be observed as either vinyl bromide or vinyl chloride are formed. The ratio of the chloride to the bromide in the products changed with the nature of the catalyst, being around 0.1 for sulphates of Ni, Co, Mn, Cu, Zn and for silica—alumina, 0.6 for alumina and 5 for KOH—Si02 [179]. 

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