Joseph E. Baumgartner

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 .

Reactions of neopentane, methylcyclohexane, and 3,3-dimethylpentane on tungsten carbides: The effect of surface oxygen on reaction pathways

Abstract High surface area tungsten carbides with WC and β-W 2 C structure were prepared by direct carburization of W0 3 in CH 4 H 2 mixtures. Their surfaces appear devoid of excess polymeric carbon and adsorb between 0.2 and 0.4 monolayers of CO and H. These materials are very active in neopentane hydrogenolysis. Chemisorbed oxygen inhibits hydrogenolysis reactions and leads to the appearance of isopentane among the reaction products. Neopentane isomerization to isopentane occurs only on Pt, Ir, and An surfaces. Thus, oxygen-exposed tungsten carbides catalyze reactions characteristic of noble metal catalysts. 3,3-Dimethylpentane isomerizes much faster than neopentane on oxygen-exposed carbides; the isomer distribution suggests that isomerization proceeds via a methyl shift mechanism rather than through the C 5 -ring hydrogenolysis pathways characteristic of highly dispersed Pt. The apparent involvement of 3,3-dimethyl-l-pentene reactive intermediates is consistent with carbenium-type methyl shift pathways. Secondary carbon atoms, capable of forming stable carbenium ions, are present in 3,3-dimethylpentane but not in neopentane; they account for the high 3,3-dimethylpentane isomerization rate and selectivity on oxygen-exposed tungsten carbide powders. Both dehydrogenation and isomerization reactions of methylcyclohexane occur on these carbide powders. These results suggest the presence of a bifunctional surface that catalyzes dehydrogenation and carbenium ion reactions typically occurring on reforming catalysts.

Kinetic coupling and hydrogen surface fugacities in heterogeneous catalysis. I: Alkane reactions on Te/NaX, H-ZSM5, and Ga/H-ZSM5

Hydrogen removal occurs by recombinative desorption and by hydrogen transfer during dehydrogenation steps required for alkane and cycloalkane conversion on Te/NaX, H-ZSM5, and Ga/H-ZSM5 catalysts. Recombinative desorption limits the rate of n-heptane and methylcyclohexane aromatization on Te/NaX and prevents equilibration between gas-phase H2 and H-adatoms formed in intermediate dehydrogenation steps. The resulting high surface hydrogen fugacities lead to low steady-state concentrations of required unsaturated intermediates. Te ions catalyze rate-limiting hydrogen desorption steps during alkane reactions on Te/NaX. On H-ZSM5, hydrogen removal limits the rate of propane conversion to aromatics. Hydrogen adatoms are removed predominantly by reactions with coadsorbed hydrocarbon fragments, leading to high cracking selectivity. Ga ions introduce a recombinative desorption function that partially relieves the resulting high hydrogen surface fugacities and allows dehydrogenation steps to occur without concurrent cracking. Thus, Ga ions increase aromatics selectivity by providing a “porthole” for the removal of hydrogen adatoms as dihydrogen. We propose that rate-limiting hydrogen desorption steps, and the high surface hydrogen fugacities that result, control the rate and selectivity of dehydrogenation and related reactions on many nonmetal surfaces.

Bifunctional reactions of alkanes on tungsten carbides modified by chemisorbed oxygen

Abstract Tungsten carbides modified by chemisorbed oxygen catalyze n-heptane isomerization with high selectivity. Kinetic, isotopic tracer, and deuterium-exchange measurements show that the reaction proceeds via sequential n-heptane dehydrogenation and heptene isomerization steps. At low temperatures, isomerization rates are limited by heptene rearrangements but dehydrogenation steps become increasingly rate-limiting as temperature increases. The isomer distribution in n-heptane and 3,3-dimethylpentane reaction products and the 13C distribution in isoheptanes formed from n-heptane-1-13C show that isomerization occurs predominantly by methyl migration steps typical of carbenium-ion rearrangements on acid sites. WOx species on carbide surfaces appear to introduce acid sites similar to those present in supported tungsten oxides. n-Heptane dehydrocyclization and hydrogenolysis reactions also require heptene intermediates. Dehydrocyclization occurs predominantly by (1, 6) ring closure while hydrogenolysis leads to random cleavage of carbon-carbon bonds in n-heptane.

Synthesis, characterization, and catalytic properties of clean and oxygen-modified tungsten carbides

Abstract High surface area WC and β-W 2 C powders (30–100 m 2 g −1 ) were prepared by direct isothermal carburization of WO 3 and W 2 N in CH 4 -H 2 mixtures. After surface cleaning with H 2 , their surfaces are equilibrated with bulk stoichiometric carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on tungsten carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x ) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x ). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.

Hydrogen transfer and activation of propane and methane on ZSM5-based catalysts

Hydrogen exchange between undeuterated and perdeuterated light alkanes (CD4-C3H8, C3D8-C3H8) occurs on H-ZSM5 and on Ga- and Zn-exchanged H-ZSM5 at 773 K. Alkane conversion to aromatics occurs much more slowly because it is limited by rate of disposal of H-atoms formed in C-H scission steps and not by C-H bond activation. Kinetic coupling of these C-H activation steps with hydrogen transfer to acceptor sites (Gan+, Znm+) and ultimately to stoichiometric hydrogen acceptors (H+, CO2,O2, CO) often increases alkane activation rates and the selectivity to unsaturated products. Reactions of13 CH4/C3H8 mixtures at 773 K lead only to unlabelled alkane, alkene, and aromatic products, even though exchange between CD4 and C3H8 occurs at these reaction conditions. This suggests that the non-oxidative conversion of CH4 to higher hydrocarbons on solid acids is limited by elementary steps that occur after the initial activation of C-H bonds.

Alkane rearrangement pathways on tellurium-based catalysts

Isotopic tracer studies and reaction pathway analyses suggest that olefins and diene and triene species are reactive intermediates in n-heptane dehydrocyclization on Te/NaX. Their concentration is limited by surface hydrogen overpressures caused by a rate-limiting hydrogen desorption step in the dehydrogenation sequence. The distribution of toluene isotopomers formed from dehydrocyclization of n-heptane-1-13C is consistent with a reaction sequence involving thermal cyclization of an equilibrated mixture of conjugated and nonconjugated heptatrienes. Methylhexanes are formed predominantly by methyl and ethyl shift reactions of heptenes, and not by hydrogenolysis of CS ring species. Alkane hydrogenolysis on Te/NaX occurs predominantly by thermal cracking pathways of n-heptane and heptenes. No catalytic function for either (1,5) or (1,6) ring closure was observed on Te/NaX; a catalytic dehydrogenation function, however, suffices for dehydrocyclization to occur with high selectivity.

Synthesis and catalytic properties of eggshell cobalt catalysts for the Fischer-Tropsch synthesis

CO diffusional restrictions decrease C5+ synthesis rates and selectivity within large (1–3 mm) catalyst pellets often required in Fischer-Tropsch (FT) synthesis reactors. Eggshell catalysts, where Co is located preferentially near outer pellet surfaces, reduce the severity of these transport restrictions and lead to higher synthesis rates and C5+ selectivity. Maximum C5+ selectivities occur on catalysts with intermediate shell thickness, within which transport restrictions limit the removal of reactive olefins but not the arrival of reactants at catalytic sites. A new synthetic technique leads to sharp distributions of active sites near outer pellet surfaces by controlling the rate of imbibition of cobalt nitrate melts. Also, slow reduction of the impregnated salt leads to moderate Co dispersions (0.05–0.10) even at high local Co loadings present within shell regions.

The Role of Surface Fugacities and of Hydrogen Desorption Sites in Catalytic Reactions of Alkanes

Abstract Hydrogen removal occurs by recombinative desorption and by hydrogen transfer during dehydrogenation steps required in the conversion of alkanes on H-ZSM5 and on H-ZSM5 modified by Ga or Zn. On H-ZSM5, hydrogen removal limits the rate of propane conversion to aromatics. The resulting high surface hydrogen fugacities lead to cracking, a step that removes both adsorbed hydrogen and deactivating residues. Ga and Zn ions catalyze the recombinative desorption of H-adatoms, lower hydrogen surface fugacities, and allow acid sites to turnover without cracking. Metal ions act as “portholes” for the desorption of H-atoms as H 2 . In-situ EXAFS neasurements show that Ga +3 ions reduce during reactions of propane or pretreatment by H 2 . Hydrogen desorption and propane aromatization rates depend on the density of reduced Ga species in H-ZSM5, which catalyze rate-limiting hydrogen removal steps. The overall catalytic sequence is bifunctional; Bronsted acid chemistry is apparently enhanced by the presence of sites that stabilize hydride ions and catalyze H + -H − recombination reactions.

INHIBITED DEACTIVATION OF Pt SITES AND SELECTIVE DEHYDROCYCLIZATION OF N-HEPTANE WITHIN L-ZEOLITE CHANNELS

ABSTRACT Selective dehydrocyclization and terminal hydrogenolysis are intrinsic catalytic properties of clean Pt ensembles. On mesoporous materials, carbon deposits poison these ensembles rapidly during catalysis. Zeolite channels inhibit deactivation processes and preserve this unique selectivity of Pt ensembles at reaction conditions that favor dehydrocyclization thermodynamics. Dehydrocyclization turnover rate and terminal hydrogenolysis selectivity are similar on Pt/K-L and fresh Pt/Si0 2 , but decrease rapidly as the Si0 2 -supported catalysts deactivate. Catalytic ring closure is a structure-sensitive reaction on Group VIII metals; it occurs selectively only on Pt and requires large unblocked ensembles. On other metals (Rh and Ir) and on deactivated Pt, dehydrocyclization occurs predominantly via slower pathways with less stringent ensemble requirements, possibly involving surface dehydrogenation and thermal cyclization steps.