Catalysis hydrocarbon conversions

One area in which such surface science strategies have proved to be remarkably successful in following rather complex surface catalytic reactions has been in the area of hydrocarbon conversion catalysis [1-3], initially focusing on reforming [4-10] and hydrogenation [11,12] reactions. This work demonstrated, for example, that hydrocarbons could undergo drastic transformations on noble metal surfaces, such as the conversion of ethylene into strongly bound, but relatively uiu-eactive ethylidyne species on Pt(lll) [13-23]. 

The activation energy for proton transfer can be viewed as a lattice oxygen Lewis-base and proton Br0nsted-acid synergetic event [3]. One generally finds that activation energies of proton-activated reactions arc rather high between 100 and 200 kJ/mol for proton-activated elementary reaction steps in hydrocarbon conversion catalysis. ITiis is the main reason for the relatively low TOP per proton ( 102 s ) for this type of reaction. Similarly to enzymes [31], the weak van der Waals-type interaction determines the size- and shape-dependent behavior. 

Until now, the most powerful contribution of neutron diffraction, even on powders, has been the guest location, especially for deuterated hydrocarbon molecules. As a matter of fact, one of the most important industrial applications of zeolites is hydrocarbon conversion catalysis and separation. The advantage of neutrons over X-rays comes from the fact that deuterium and carbon have similar scattering lengths (be = 6.65 fm, b = 6.67 fm) which involves participation of all the nuclei of the guest molecule in the structure factor. 

H. Pines, The Chemistry of Catalysis Hydrocarbon Conversions, Academic Press, Inc., New York, 1981, pp. 173—174. 

Control of the multitude of pathways which feed molecules can take is the primary objective of aU catalyst and process developments. The work covered in this chapter focuses primarily on describing the approaches in material and catalysis development which have led to major advances in zeolite application in hydrocarbon conversion. The breaking and formation of carbon-carbon and carbon-hydrogen bonds constitute the majority of the chemical transformations involved here with the less prevalent, but very important, breaking of carbon bonds with sulfur, nitrogen and oxygen taking place in parallel. 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

The acid-catalyzed conversions of hydrocarbons have been extensively studied and are widely reported in chemical literature. Many important petroleum and petrochemical processes involve catalysis by acids. In contrast to this, the use of bases to catalyze hydrocarbon conversions has received little attention except for polymerization of conjugated dienes and styrene to high polymers. 

In an automobile s catalytic converter, CO and hydrocarbons present in the exhaust gases are oxidized. Unfortunately the effectiveness of these units decreases with use. The phenomenon was studied by Summers and Hegedus in /. Catalysis, 51, 185 (1978) by means of an accelerated aging test on a palladium impregnated porous pellet packed bed converter. From the reported data on hydrocarbon conversion shown below, develop an expression to represent the deactivation rate of this catalyst. 

The preparation of new solid acids, their characterization, mechanistic studies, and theoretical approaches to understand the fundamental aspects of acid-catalyzed hydrocarbon conversion constitute a very large fraction of the topics discussed in the last decade in all journals related to catalysis and physical chemistry. However, in contrast with liquid-acid-catalyzed activation processes, many fundamental questions concerning the initial step, the true nature of the reaction intermediates, and the number of active sites remain open for discussion. For this reason, the results obtained in liquid-superacid-catalyzed chemistry, which can be rationalized by classical reaction mechanisms, supported by the usual analytical tools of organic chemists, represent the fundamental basis to which scientist in the field refer. 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

Coking, widely experienced in the catalysis of hydrocarbon conversion (7), can deactivate both metallic and acid catalytic sites for hydrocarbon reactions (2). Accumulation of such carbonaceous deposits affects selectivity in hydrocarbon conversion (5). Adsorbed ethene even inhibits facile o-p-Hj conversion over Ni or Pt (4 ), the surface of which it appears is very nearly covered at lower temperatures in such deposits. H spillover may enhance hydrocarbonaceous residue formation (6). Accumulated carbonaceous residues can be removed by temperature programmed oxidation, reduction and hydrogenation TPO, TPR, TPH, etc (7) as part of catalyst regeneration. 

A are detected by Jt-ray diffraction. This behavior contrasts with that of SiOi-supported Pd, for which Fajula et al. report a decrease in particle size under syngas conversion conditions 88). In the presence of strong acid sites, methanol and dimethyl ether are converted further to branched higher hydrocarbons. This catalysis is reminiscent of that of HZSM-5 in Mobil s MTG process. A tentative reaction scheme, relating sites with products, has been given 313). 

The accepted papers cover every aspect of catalysis on microporous materials. A significant number of contributions describe the synthesis, modification, instrumental and chemical characterisation of zeolites and other micro- and mesoporous materials. Catalytic reactions involve hydrocarbon cracking, nucleophilic aromatic substitution, methanol to hydrocarbon conversion, hydration of acetylene, various alkylation reactions, redox transformations, Claisen rearrangement, etc. A whole range of appealing chemistry can be enjoyed by reading the contributions. 

Supported metal catalysis are employed in a variety of commercially important hydrocarbon conversion processes. Such catalysts consist, in general, of small metal crystallites (0.S to 5 nm diameter) dispersed on non-metallic oxide supports. One of the major ways in which a catalyst becomes deactivated is due to accumulation of carbonaceous deposits on its surface. Catalyst regeneration, or decoking, is normally achieved by gasification of the deposit in air at about 500°C. However, during this process a further problem is frequently encountered, which contributes to catalyst deactivation, namely particle sintering. Other factors which can contribute to catalyst deactivation include the influence of poisons such as sulfur, phosphorus, arsenic and... 

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

A Brait, K Seshan, H Weinstabl, A Ecker, J A Lercher. Evaluation of commercial FCC catalysts for hydrocarbon conversion II. Time-on-stream behavior of n-hexane conversion and comparison of n-hexane conversion to MAT. J Applied Catalysis A General 169, 315-329, 1998. 

The state and accomplishments of catalysis science are demonstrated through discussions of the ammonia synthesis, carbon monoxide hydrogenation, and hydrocarbon conversion over platinum. 

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