William Ellis Gates

Selective isomerization of alkanes on supported tungsten oxide acids

Abstract Tungsten oxide species form strong acid sites on ZrO 2 supports. After calcination at 1000–1100 K and promotion with Pt, these solids catalyze C 7+ alkane isomerization at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. Alkane isomerization proceeds via biomolecular reactions involving hydrogen transfer from alkanes or H 2 , which cause the desorption of isomeric carbocations before β-scission occurs. On Pt/SO x -ZrO 2 , carbocation desorption is slow, leading to long surface residence times and extensive cracking. On Pt/WO x -ZrO 2 , carbocation desorption is rapid and surface isomerization steps limit n-heptane isomerization turnover rates. Saturation coverage by WO x surface species inhibits ZrO 2 sintering and its tetragonal to monoclinic structural transformation. High isomerization turnover rates appear to require the presence of WO x clusters on ZrO 2 surfaces. X-ray absorption at the W-L 1 and W-L III edges suggests the predominant presence of distorted octahedral species, even after dehydration at 673 K, in all WO x -ZrO 2 samples calcined at 1073 K. Tetrahedral species, which lead to a strong pre-edge feature in the W-L 1 absorption edge, are not detectable in these samples. UV-visible spectra suggest an increase in WO x domain size with increasing loading. These distorted octahedral WO x domains on ZrO 2 differ markedly in structure, reduction rates, and alkane isomerization turnover rates and selectivities from tetrahedral WO x species on Al 2 O 3 .

Preparation of bulk and supported heteropolyacid salts

Abstract We describe the preparation of bulk and supported cesium and ammonium heteropolyacid and acid salts of 12-tungstophosphoric acid. Novel synthetic schemes to prepare supported forms of insoluble cesium- and ammonium-acid salts are discussed, and their behavior in model compound isomerization and alkylation reactions is described. In particular, we discuss the egg-white morphology associated with Cs-acid salts supported on silica extrudates prepared by a novel in situ-reaction/precipitation route. We also describe hydrothermal techniques developed for directly preparing the ammonium-acid salts of 12-tungstophosphoric acid and the extension of that technique to other heteropolyacid salts.

PtIrAl2O3 catalysts—The effect of PtIr interaction on Ir agglomeration and catalytic performance

Interaction between Pt and Ir has been found to occur on freshly impregnated PtIrAl2O3 subjected only to drying at 393 K. Iridium in the freshly prepared bimetallic precursor material retains the same reduction behavior as monometallic Ir. Iridium promotes the reduction of Pt in the bimetallic precursor material. This promotion results in a 20 K lower temperature-programmed reduction peak temperature than that observed for monometallic Pt. The incorporation of Pt into Ir clusters retards the oxidative agglomeration of Ir. Reduction followed by calcination causes a higher degree of Ir oxidative agglomeration than direct calcination at the same temperature. The incorporation of Pt into Ir clusters decreases the cracking activity of the Ir component as well as the coking tendency of the Pt component, and therefore enhances benzene selectivity. The catalytic performance of a PtIr catalyst approaches that of a Pt-only catalyst as the degree of Ir agglomeration increases. The cracking of n-hexane molecules takes place more frequently at the terminal carbon atom position as the degree of Ir agglomeration increases.

Preparation and catalytic properties of supported heteropolyacid salts

Abstract Heteropoly compounds are receiving increasing interest as environmentally compatible solid acid catalysts. Partial Cs + exchange into 12-tungstophosphoric acid converts the water soluble, low surface area ( 2 g −1 ) parent acid into submicron water-insoluble Bronsted acids with surface areas exceeding 100 m 2 g −1 . The insolubility of the Cs acid salt makes preparation of large particle supported catalysts by conventional impregnation procedures impractical, although applications in packed bed or slurry reactors require millimeter-sized pellets to avoid pressure drop or filtration problems. Here, we report on a new but simple technique that produces an eggwhite distribution of active Cs-acid salt of 12-tungstophosphoric acid within a large silica extrudate by carrying out a reaction deposition directly inside the extrudate. We examine these catalysts in aromatic alkylation reactions.

Effect of Sulfur on the Performance of Pt/KL Hexane Aromatization Catalyst

Abstract The sulfur sensitivity of a monofunctional Pt/KL catalyst has been investigated. Sulfur, in the ppb range, introduced in the form of thiophene functions as a stiochiometric poison and lowers, thereby, the number of available Pt sites. Additionally, feed sulfur greatly accelerates Pt agglomeration. Pt/KL deactivation resulting from metal agglomeration primarily occurs by blockage of KL channels by 10–50 A Pt and/or PtS particles. The sulfur sensitivity of the Pt/KL catalyst is highly dependent upon the molecular structure of the sulfur source. The order of sensitivity, at constant sulfur level, is thiophene ≫ 1,2-benzodiphenylene sulfide > propanethiol > sulfolane.

Rare earth modified silica-aluminas as supports for bifunctional catalysis

Abstract We have explored rare earth oxide-modified amorphous silica-aluminas as “permanent” intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides “titrates” the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions.