Thomas R. Hughes

X-RAY PHOTOELECTRON SPECTROSCOPY: A TOOL FOR RESEARCH IN CATALYSIS

Abstract Over the last 10 years, a new technique for measuring electron binding energies has been developed by Siegbahn and co-workers at Uppsala [l] .

Aromatization of Hydrocarbons over Platinum Alkaline Earth Zeolites

A new platinum/zeolite catalyst is more active and selective for the aromatization of paraffins, especially hexanes and heptanes, than are the platinum/alumina naphtha reforming catalysts presently used in petroleum refining. The new catalyst, which contains highly dispersed platinum clusters in barium-exchanged potassium Zeolite L, is also effective for alkylcyclopentane aromatization. The preparation and characterization of the Pt/BaKL-zeolite catalyst are described. Its catalytic performance and the reactions it catalyzes are compared with those of other platinum reforming catalysts. Unlike the conventional reforming catalysts, which utilize acidic sites of the support as well as platinum sites, the Pt/BaKL catalyst is nonacidic and catalyzes aromatization using only the catalytic properties of the platinum clusters. The catalyst is extremely sensitive to poisoning by sulfur, but its stability for reforming of thoroughly desulfurized feed is illustrated by an uninterrupted one-year run with a refinery light naphtha.

Mechanism and poisoning of the molecular redistribution reaction of alkanes with a dual-functional catalyst system

Abstract The disproportionation of alkanes was carried out over a dual-functional catalyst comprising platinum-on-alumina mixed with tungsten oxide-on-silica. The reaction involves formation of low concentrations of olefinic intermediates over a dehydrogenation catalyst followed by disproportionation of these olefins over a disproportionation catalyst. Product olefins are rehydrogenated to the corresponding paraffins. A series of layered bed experiments involving the individual catalyst components was performed to confirm this sequence of reactions. Because a complex mixture of reacting species and products is generated from a single reactant and cross reactions occur between different classes of hydrocarbons, the overall process is termed molecular redistribution rather than simply disproportionation (1). In this system, straight-chain alkanes are more reactive than branched, with pure isobutane being only slightly reactive. With mixtures of n -butane and isobutane, the former reacts not only with itself but also with isobutane, in which case isopentane is a major product. The molecular redistribution catalyst system is extremely sensitive to poisoning by impurities, and the catalyst must be protected by a suitable guard chamber in the feed line. An excess of either olefins or hydrogen over the low concentrations formed as reaction intermediates inhibits the reaction. Catalyst stability increases with decreasing temperature and with increasing pressure and is also influenced by the ratio of the two catalyst components.

Brönsted and Lewis acid site concentrations in fluorided alumina from the infrared spectra of adsorbed pyridine species

Abstract Previous studies have shown that the acidity of alumina is altered by fluoriding but have not established whether the acidic sites formed are of the Lewis or Bronsted type. In the present work, infrared spectra of adsorbed pyridine species proved that both types of sites are present in alumina fluorided with hydrogen fluoride vapor or aqueous solutions of hydrogen fluoride or ammonium fluoride. Quantitative measurements showed that many of the Bronsted sites were removed at elevated temperatures. However, an appreciable concentration remained even after dehydration at 538 °. Exposure to water vapor increased the concentration of Bronsted sites and decreased that of Lewis sites. Thus the acidic sites of fluorided alumina appear to undergo a reversible dehydration-rehydration analogous to the interconversion of Bronsted and Lewis sites in silica-alumina. The catalytic roles of the two types of acidic sites remain to be established.

Catalytic Processes For Octane Enhancement By Increasing the Aromatics Content of Gasoline

Abstract Catalytic processes for enhancing the octane of gasoline by increasing its aromatic hydrocarbon content are reviewed. Major improvements have been made in the most important of these processes, the catalytic reforming of naphtha over bifunctional catalysts. However, liquid yield still declines severely in going to high octane, because of poor selectivity for light paraffin aromatization. New, monofunctional platinum aromatization catalysts produce much higher yields of high octane reformate from light paraffins. Reformate aromatics content and octane can also be increased by postreforming processes that remove low octane paraffins by shape-selective cracking over acidic zeolites. New processes can be used to increase both the supply of gasoline and its octane rating by converting very light hydrocarbons into aromatics-rich liquids.