Hydrocarbons surface acidity

Surface science studies have generated much insight into how hydrocarbons react on the surfaces of platinum single crystals. We refer to Somorjai [G.A. Somor-jai. Introduction to Surface Chemistry and Catalysis (1994), Wiley, New York] for a detailed overview. Also, the reactions of hydrocarbons on acidic sites of alumina or on zeolites have been studied in great detail [H. van Bekkum, E.M. Flanigan and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice (1991), Elsevier, Amsterdam],... 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

In these sections of our chapter, we emphasize research advances in the area of surface acidity of specific solids that have occurred during the period from 1970 to the fall of 1976. As stated earlier, the class of solids with which we are chiefly concerned are metal oxides that catalyze skeletal rearrangements of hydrocarbons via carbonium ion intermediates. However, we have included reviews of silica gel and alumina, which are relatively inactive, because the properties of these solids form a useful frame of reference. The initial sections (Sections III.A-III.D) deal predominantly with amorphous catalysts the final sections (Sections III.E and III.F), with crystalline catalysts. 

Other types of organic compounds that have been determined in ground and surface waters include polyaromatic hydrocarbons, amino acids,... 

The Fe203 superacid was found to be quite effective for oxidation of hydrocarbons to CO and C02 when the reaction was performed at temperatures above 100°C. The catalyst gave a 29% conversion for the reaction of butane at 300°C to form CO and C02 in the ratio 4 6 under the conditions in which none of the reactions occurred at 300°C over Fe203, without the sulfate treatment (177). The decrease in oxygen of the catalyst surface was observed together with the complete recovery of activity by supply of 02. The catalyst was entirely poisoned by the addition of pyridine, the oxidation being related to the surface acidity. The activity enhancement of oxidation by the sulfate addition was also observed with the Sn02 superacid (135, 145). Iron and tin oxides are known to be oxidation catalysts thus those superacids would be the oxidation catalysts with superacidity. 

In making all operating and design decisions. It Is Important to keep in mind the definition of the true reaction zone. Fundamentally, this Is the Interfaclal area between the immiscible hydrocarbon and acid catalyst liquid phases in the reactor. Reactants and products flow across this boundary. The olefins In the feed stream react Instantaneously with the sulfuric acid catalyst and combine with the relatively small amount of isobutane present In solution In the acid catalyst to form alkylate. Alkylate passes out through the Interfaclal surface reaction boundary into the hydrocarbon phase while Isobutane passes in to resaturate the catalyst. To suppress undesirable polymerization and other reactions It Is necessary to ... 

Perhaps the most difficult of the requirements is the emulsification and contacting. For efficient operations the emulsion characteristics must be uniform throughout the reactor. An excess of acid is preferred, since this results in a hydrocarbon-in-acid emulsion. In these mixtures the viscosity and surface tension of the continuous acid phase are effective in minimizing the tendency of the dispersed hydrocarbon droplets to coalesce and separate, acting under the influence of the very large specific gravity differential between the light and heavy phases. 

Synthetic zeolites have gained importance as industrial catalysts for cracking and isomerization processes, because of their unique pore structures, which allow the shape-selective conversion of hydrocarbons, combined with their surface acidity, which makes them active for acid-catalyzed reactions. Many attempts have been made to introduce redox-active TMI into zeolite structures to create catalytic activity for the selective oxidation and ammoxidation of hydrocarbons as well as for SCR of nitrogen oxides in effluent gases (69-71). In particular, ZSM-5 doped with Fe ions has attracted attention since the surprising discovery of Panov et al. (72) that these materials catalyze the one-step selective oxidation of benzene to phenol... 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

In the process of catalytic cracking, characteristic reactions such as chain scission, hydrogen transfer and condensation take place under certain temperature and pressure conditions and when an appropriate catalyst is utilized, products with certain range of molecular weights and structures are obtained. Catalysts with surface acid sites and with the ability of hydrogen ion donation such as silica-alumina and molecular sieve catalyst have been already widely utilized. These catalysts can also enhance the isomerization of products and increase the yield of isomeric hydrocarbons. However, large amounts of coke will deposit on the surface of catalysts and consequently lead to their deactivation. Therefore, the recycling of catalysts is difficult to achieve. 

Previous attempts to estimate Drago parameters for solid surfaces met with limited success. Fowkes and co-workers (198-201) calculated Q and Ex values for SiOj, TiOj, and Fe Oj using a combination of UV and IR spectroscopies and a flow calorimeter. They determined heats of adsorption of pyridine, triethylamine, ethyl acetate, acetone, and polymethylmethacrylate (PMMA) in neutral hydrocarbon solutions. However, their results did not provide consistent Q/Ea parameters for the surface acid sites. It should be noted that the heats determined were for high surface coverages, and these values provide a lower bound for the actual acid strength distribution. 

The results for other conditions for polystyrene pyrolysis were reported. For example, pyrolysis on different catalysts was shown to lead to modifications of the yield of specific components in the pyrolysate. During the pyrolysis of PS on solid acid catalysts, the increase of contact time and surface acidity enhanced the production of ethylbenzene. Pyrolysis in the presence of water increases the yield of volatile products and that of monomer [30]. Studies on the generation of polycyclic aromatic hydrocarbons (PAHs) in polystyrene pyrolysates also were reported [36]. It was demonstrated that the content in PAHs in polystyrene pyrolysates increases as the pyrolysis temperature increases. The analysis of the end groups in polystyrenes with polymerizable end groups (macromonomers) was reported using stepwise pyrolysis and on-line methylation [46]. 

In order to elucidate the reasons for the dependence of the catalytic properties of these samples on their preparation method, we studied the acid surface properties of cobalt- and chromium-modified Zr02 catalysts by ammonia thermoprogrammed desorption and IR-spectroscopy. Our results again indicated that the activity of these catalysts in the SCR of NOx by hydrocarbons is a function both of the surface acidity and content of the active metal. The acid site concentration of the starting Zr02 samples prepared by various methods is significant (0.13 and 0.23 mmol/g) but these samples are inactive, while 10% CriOilZtOi prepared by the sol-gel method displays considerable activity in the reaction studied with lower surface acidity. The acid site concentration of the sample with the same composition prepared by the precipitation method is reduced by a factor of 2.5 and, thus, this catalyst has much lower activity in the selective catalytic reduction. 

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