Gary Brice Mcvicker

Conversion of isobutane over solid acids—A sensitive mechanistic probe reaction

Abstract Amorphous solid acids, such as SiO 2 Al 2 O 3 and halided-Al 2 O 3 , convert isobutane at elevated temperatures (700–920 K) to a limited number of products. These solid acids crack isobutane 30 to 400 times faster than a blank reactor, and yield a product consisting primarily of methane, propene, butenes, and hydrogen. This product distribution is similar to that obtained thermally. Conversion rates over amorphous solid acids display a first-order isobutane pressure dependency. In contrast to amorphous solid acids, a highly active, ultrastable Y, faujasite exhibits a paraffinic rather than an olefinic product. Hydrogen is not produced in significant quantities over faujasite but is incorporated into the conversion products. Isomerization to n -butane is a major path, rather than a trace reaction, alkylation reactions yielding C 5 (+) hydrocarbons readily occur and methane production is reduced. The rates of formation of cracking fragments, n -butane, and C 5 (+) products are second-order in isobutane. The product distribution and kinetics lead to the conclusion that amorphous solid acids convert isobutane by a different mechanism than faujasite. Over amorphous solid acids, the data are not consistent with the presence of “free” carbonium ion intermediates whereas with faujasite it is. Radical cations are suggested as initial intermediates over all solid acid systems and differences in their subsequent reactions account for the observed variation in products and kinetic orders.

Chemisorption properties of iridium on alumina catalysts

Abstract Highly dispersed 0.3 to 1.0% iridium on alumina catalysts exhibit H Ir and CO Ir ratios near two for the strongly bound fraction of these adsorbates. Iridium particles in such catalysts are not detectable by high-resolution TEM which places an upper limit of 0.6 nm on their size. Increasing the metal concentration or oxidative calcination resulted in an increase in the average iridium particle size and a corresponding decrease in adsorbate/metal ratios. Average crystallite sizes calculated from chemisorption data and observed directly by TEM for partially agglomerated catalysts were found to be in good agreement. Infrared spectra of adsorbed carbon monoxide were observed to be dependent upon both iridium crystallite size and surface coverage. Under saturation coverage conditions ( CO Ir > 1 ), highly dispersed catalysts displayed a major carbonyl band at 2060 cm −1 . Agglomerated catalysts ( CO Ir ), in contrast, exhibit a band maxima centered in the 2020–2025 cm −1 region. Taken together the chemisorption, TEM, and infrared data indicate that isolated iridium atoms can adsorb up to two adatoms while iridium clusters (>0.6 nm) adsorb a single adatom per exposed metal site.

The preparation, characterization, and use of supported potassium-Group VIII metal complexes as catalysts for CO hydrogenation

Abstract A major disadvantage of supported Group VIII metal Fischer-Tropsch catalysts, when compared to bulk catalysts, is the difficulty in effectively promoting such catalysts with potassium. A novel approach for preparing well-dispersed, highly promoted potassium-Group VIII metal catalysts has been developed. Catalysts were prepared by impregnating Al 2 O 3 or SiO 2 with well-characterized potassium-Group VIII metal carbonyl complexes and then thermally decomposing the supported complexes under hydrogen. Since potassium and the Group VIII metals are associated in the precursor complexes, the deposition and subsequent decomposition of such complexes maximize potassium-Group VIII metal contacting on the support surface. Several complex derived catalysts were found to be more active and to exhibit higher selectivities for C 2 -C 5 olefins than conventionally prepared potassium-Group VIII metal catalysts of the same metals stoichiometry. These results suggest that the use of preformed carbonyl complexes as supported catalyst precursors can increase any promotional effect potassium can have upon supported Group VIII metals.

Comparison of the acidities of WO3/Al2O3 and ultrastable faujasite catalysts

The acidity of WO3 on γ-alumina is compared with that of ultrastable faujasite using both base adsorption techniques and model compound conversion studies. The addition of WO3 to γ-alumina introduces Bronsted acidity, and the density of Bronsted sites is increased by high-temperature calcination. The acid sites displayed by the supported tungsten oxide catalyst are considerably weaker than those found in ultrastable faujasite.

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.

Surface area stabilization of IrAl2O3 catalysts by CaO, SrO, and BaO under oxygen atmospheres: Implications on the mechanism of catalyst sintering and redispersion

Abstract An approach for preventing sintering and maintaining high metal dispersions of Ir Al 2 O 3 catalysts in the presence of oxygen at elevated temperatures has been developed. Well-dispersed Ir Al 2 O 3 catalysts were readily sintered under oxygen at temperatures above 450 °C. If, however, Ba, Ca, or Sr oxides were impregnated onto the A1 2 O 3 support along with the Ir, then complete inhibition of sintering was observed up to 650 °C. This effect only occurred when the concentration of Ba, Ca, or Sr oxides was in excess of the concentration of acid sites on the Al 2 O 3 support. Studies of presintered Ir Al 2 O 3 catalysts to which BaO was subsequently added demonstrated that Ir could be redispersed by treating the system with oxygen at 600 °C. The oxidative stabilization and redispersion of Ir Al 2 O 3 catalysts are consistent with the capture of mobile, molecular, iridium oxide species by well-dispersed Group IIA-oxide sites. The trapping mechanism is assumed to proceed via the formation of stable, immobile surface iridates.

On the question of carbonium ions as intermediates over silica-alumina and acidic zeolites

Abstract Activation energies for the isomerization of an equilibrated mixture of 2-methylpent-1-ene and 2-methylpent-2-ene to several isomers with an ultrastable Y catalyst have been found to correspond closely with the barriers to the rearrangement of the same “postulated” carbonium ions in super-acids. This provides evidence for relatively stable carbonium ions as reaction intermediates on this solid acid. The same rearrangements with a conventional amorphous SiO 2 Al 2 O 3 catalyst involve distinctly different barriers and are more indicative of the presence of surface alkoxy compounds as intermediates. With these catalysts it is proposed that rearrangements may occur through polar (cationic-like) transition states.

The interaction of palladium with alumina and titanium oxide supports

In a series of recent electron microscopy studies attempts were made to relate the phenomenological consequences of strong metal-support interactions (SMSI), such as suppressed chemisorption capacity, with morphological changes in either the support or supported metal particles. In the present investigation, a combination of high-resolution transmission electron microscopy and hydrogen chemisorption measurements were employed to address the sintering behavior of Pd supported on Al2O3 and TiO2 following reduction at temperatures up to 800 °C. Palladium was chosen, since it has the unique ability to both adsorb and absorb (β-hydride formation) hydrogen. This behavior was anticipated to provide an added dimension for characterizing SMSI systems. Palladium on alumina was selected as a control system. At reduction temperatures in excess of 700 °C, Pd particles sinter on both Al2O3 and TiO2 supports. The extent of sintering is, however, more extensive in the case of the TiO2 support. The quantities of hydrogen adsorbed and absorbed by PdTiO2 decreased in parallel with increasing reduction temperature, reaching essentially zero uptakes at reduction temperatures above 500–600 °C. The amount of H2 adsorbed by PdAl2O3 decreased about 60% upon increasing the reduction temperature from 500 to 700 °C, in agreement with the extent of sintering detected by TEM which occurs within this temperature range. The quantity of H2 absorbed by PdAl2O3, however, did not significantly decrease after reduction at 700 °C. The anomalous behavior of PdTiO2 is rationalized in terms of a model whereby reduction of TiO2 to Ti4O7 leads to the simultaneous formation of mobile titanium suboxides, which are free to migrate onto the metal particle surface. The process ultimately results in the decoration of the Pd particles by titanium suboxide species. In this coated state, Pd is no longer capable of either adsorbing or absorbing hydrogen.

Chemisorption properties of platinum and iridium supported on TiO2Al2O3 mixed-oxide carriers: Evidence for strong metal-support interaction formation

Fresh Pt and Ir on A12O3 catalysts reduced at 773 K exhibit high HM ratios (1 to 2.5) indicative of highly dispersed metal phases. In contrast, TiO2-supported Pt and Ir catalysts display low HM ratios (<0.04) following hydrogen reduction at 773 K. The suppressed hydrogen uptakes typically exhibited by TiO2-based Group VIII metal catalysts result from a strong metal-support interaction (SMSI) and not from a poor metals dispersion. Following reduction at 773 K, the HM ratios displayed by Pt and Ir catalysts supported on TiO2Al2O3 mixed-oxide carriers are intermediate in value between those observed on TiO2 and Al2O3 supports. The extent of H2 uptake suppression is dependent upon the relative TiO2 and Group VIII metal concentrations on the Al2O3 surface. The magnitude of H2 uptake suppression (and thus SMSI formation) increases at higher TiO2/Group VIII metal ratios. Higher reduction temperatures and extended reduction times at a given elevated temperature also enhance SMSI formation. Taken together, these results imply that the generation of reduced Group VIII metal-TiO2−x species on the Al2O3 surface may be responsible for the SMSI effect.

Properties of aluminum-fluoride catalysts prepared by the fluoridation of aluminum oxide with trifluoromethane

Abstract High-purity AlF3 has been prepared by allowing γ-Al2O3 to react with gaseous trifluoromethane at 670–770 K under 101 kPa total pressure. The use of gaseous trifluoromethane is a new, general method for preparing metal fluorides from metal oxides. AlF3 prepared using this procedure retained the physical form of the starting γ-Al2O3. A 1 16 -in. γ-Al2O3 extrudate, for example, yielded an AlF3 extrudate with comparable physical dimensions and crush strengths. X-Ray diffraction, BET surface area, pore volume, and surface acidity measurements were employed to characterize various AlF3 samples. Significant decreases in surface area and pore volume as well as surface acidity occurred upon increasing the concentration of AlF3 from 90 to 100%. This behavior presumably results from the fluoridation of residual γ-Al2O3. AlF3 extrudates were utilized as supports for Pt and Pd catalysts. Specific benzene hydrogenation activities of these catalysts are comparable to those of Pt and Pd on γ-Al2O3. In a unique application, Pd AlF 3 was used to hydrogenate m-diethylbenzene in superacid ( HF TaF 5 ) solution.