Gustavo A. Fuentes

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 .

Towards A Better Understanding of Sintering Phenomena in Catalysis

Summary A Generalized Power Law Equation to describe the kinetics of sintering phenomena is presented. Explicit account is made of the asymptotic value of dispersion at long times. Its use suggests that most variations in sintering order with time and/or temperature are a mathematical artifact. Analysis of the atomic migration model shows that a limiting value for dispersion is predicted. Application of the Generalized Power-Law equation to reported sintering data for different supported catalysts shows that sintering can be represented as a first order process. Observed activation energies for sintering processes are below 40 Kcal/mol. The general equation can be readily obtained by arguments of free energy minimization.

Molecular selection of MoOx species during migration on Al2O3 and zeolites Y and ZSM-5

Surface migration is the basis of the solid state preparation of various materials. In the case of heterogeneous catalysts, it involves the spreading of metal oxides with low Tammann temperature, such as MoO3, on various refractory supports like γ-Al2O3 and zeolites. In spite of its importance, the speciation of oxide surface migration has not been extensively studied. In this work Mo/γ-Al2O3, Mo/ZY and Mo/ZSM-5 were prepared from mixtures of MoO3 and γ-Al2O3, or zeolites Y and ZSM-5, subjected to thermal activation. X-Ray diffraction (XRD), Raman and UV-vis diffuse reflectance spectroscopies were used to study the spread of MoOx species on refractory metal oxide supports. It was found that during thermal activation the formation and migration of different MoOx species depends strongly on the structure of the support.

Effect of tungsten on supported platinum catalysts

Addition of W to 0.3 wt % Pt/γ-Al 2 O 3 catalysts has been found to stabilize and even increase Pt dispersion after high temperature reduction (up to 1073 K). This effect appears to be caused by formation of Pt species bound to WO x during drying and calcination and by strong interaction between Pt crystallites and the mixed oxide surface after H 2 reduction. UV-vis spectra show the formation of WO x and PtO y species on the surface of alumina in the calcined state. Interaction between Pt 4+ - and W 6+ -bearing structures seems to enhance UV absorption with minor variations in the location of the band maxima. Pt/WO 3 -Al 2 O 3 * were in general more stable towards deactivation than monometallic Pt catalysts (reduced at 773 K) during hydrogenation of benzene. Reduction at 1073 K produced catalysts with lower initial activity but with increased resistance towards deactivation. Impregnation of Pt on WO x -(γ-Al 2 O 3 ) gave the best overall catalysts apparently because of an improved interaction of crystallites with the modified surface of Al 2 O 3 . This result is important in order to control deactivation and sintering of the metallic phase if high operation temperatures are involved.

Characterization of MoOx species on γ-Al2O3, Y and ZSM-5 zeolites during thermally activated solid–solid synthesis

We characterized the Mo surface species present during thermally activated solid–solid synthesis of Mo/γ-Al2O3, Mo/H-ZY and Mo/H-ZSM-5 starting from mixtures of MoO3 with γ-Al2O3 or zeolites H-Y and H-ZSM-5. We identified different surface molybdate species by using diffuse reflectance measurements in the UV–vis region and a bandgap analysis of the deconvoluted spectra. Species assignments were confirmed via theoretical DFT calculations of simplified zeolite clusters containing MoOx species as well as by analysis of the spectra of pure standards. Migration of MoOx onto H-Y and H-ZSM-5 zeolites resulted in an important fraction of tetrahedral Mo species (monomers and dimers). On the other hand, migration on γ-Al2O3 led to the presence of a larger fraction of octahedral Mo oligomeric species. The variation in surface structures seems to be a function of the Si/Al ratio and of the pore structure of supports. Our results are consistent with a mechanism involving formation and migration of monomers and dimers (equivalent to [MoO4]2− and [Mo2O7]2−) leading to the formation of oligomers (resembling [Mo7O24]6−). We present a simplified kinetic scheme for the formation of the different Mo compounds consistent with the characterization results.

NITROUS OXIDE FORMATION DURING LIGHT-OFF OVER A COMMERCIAL PD-CONTAINING THREE-WAY CATALYTIC CONVERTER: THE EFFECT OF LOW-SULFUR GASOLINE

In this work we report how the use of low-sulfur gasoline decreases formation of N2O as a by-product during light-off of commercial Pd-only three-way catalytic converters (TWC). Our results also show that even in the absence of sulfur a large quantity of N2O (reaching instantaneous concentrations up to 700 ppm) is produced at low temperature, between 200° and 400°C. At high temperature (400° to 600°C) and lean conditions we found an unexpected emission of N2O (200 ppm), which is apparently produced by reduction of NO with C3H8 on the TWC. The emission of N2O during light-off is practically independent of the air-to-fuel ratio but depends strongly of the sulfur content in gasoline. Under rich and stoichiometric conditions, the presence of SO2 promotes N2O formation (concentrations from 700 to 900 ppm) via NO reduction by CO. The presence of SO2 in a lean environment has no effect upon formation of N2O.

Thermal Stability of Pt Nanoparticles Supported on WO x /Al 2 O 3 for n-Heptane Hydroconversion

The thermal stabilization of γ-Al 2 O 3 using W +6 ions has been found useful to the synthesis of Pt/Al 2 O 3 catalysts. The sequential impregnation method was used to study the effect of W 6+ upon Pt/ γ-Al 2 O 3 reducibility, Pt dispersion, Raman spectroscopy and n-heptane hydroconversion. The W/Pt atomic ratios varied from 3.28 to 75. We found that the W 6+ ions delayed reduction of a fraction of Pt +4 atoms beyond 773 K. At the same time, W 6+ inhibited sintering of the metallic crystallites once they were formed on the surface. For the sample with a W/Pt atomic ratio of 3.28, W 6+ did not inhibit the H 2 reduction of Pt oxides even below of 773 K, the Pt oxides were reduced completely, however, the Pt dispersion decreased for this sample with respect to the Pt/γ-Al 2 O 3 catalyst. After reduction at 1073 K, sequential samples impregnating Pt on WOx/γAl 2 O 3 were more active and stable during n-heptane hydroconversion than monometallic Pt/γAl 2 O 3 catalyst. Selectivities for dehydrocyclization, isomerization and Hydrocracking changed significantly when the W/Pt atomic ratio and reduction temperature increased. Initial and final reaction rates were more sensitive to reduction temperature. W 6+ ions promoted high thermal stability of Pt crystallites when sequential catalysts were reduced at 1073 K and deactivation of bimetallic catalysts reduced at 773 K and 1073 K was less than the deactivation of Pt/Al 2 O 3 catalyst.

SBA-15 mesoporous molecular sieves doped with ZrO2 or TiO2 as supports for Mo HDS catalysts

SBA-15 materials with different TiO 2 or ZrO 2 loadings were prepared by chemical grafting procedure and used as supports for Mo hydrodesulfurization (HDS) catalysts. Ti and Zr oxide species were found to be well-dispersed on SBA-15 surface (DRS, XRD). In the catalysts supported on the TiO 2 or ZrO 2 modified materials, the dispersion of Mo oxide species increased with TiO 2 or ZrO 2 loading in the SBA-15 support. Catalytic activity tests in hydrodesulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) showed that the modification of SBA-15 supports with Ti and Zr species significantly improves the performance of unpromoted Mo catalysts in HDS of refractory dibenzothiophenes.

Asymptotic behavior during sintering of supported catalysts

By using scaling analysis it is shown that the presence of a steady-state particle distribution is compatible with standard mechanisms of crystallite sintering in catalysts if fragmentation kernels are added. Analytical solutions are found when aggregation and fragmentation kernels are homogeneous, but the results can be extended to kernels with different properties. This provides a theoretical justification for the use of steady-state dispersions during kinetic analysis of sintering using the generalized power law equation.

Synthesis and MAS-NMR analysis of highly stable pillared clays

Summary 27 A1 MAS-NMR analysis of montmorillonite pillared with single and mixed Keggin polycations and treated up to 1173 K reveal that pillaring causes a major change in the octahedral to tetrahedral transition observed in the natural clay. At 1173 K a dual population of sites is present in pillared samples, and the signal corresponding to octahedral symmetry is larger than that of tetrahedral sites. The difference clearly corresponds to the pillaring process. 29 Si MAS-NMR shows that the tetrahedral layers(SiO 4 ) are not significantly affected by pillaring, because both montmorillonite and the pillared samples experience a similar transition of the Q 3 structure observed at room temperature onto a Q 4 structure at high temperature. DRIFTS shows an appreciable change in the OH region during pillaring as well as during high temperature treatment. Upon pillaring and heating to 673 K the OH signal at 3543 cm −1 disappears, perhaps owing to the formation of pillar-layer bonds. It is possible that anchoring occurs by interaction between the pillars and free OH groups on the lamina. Heating to 973 K also causes signals at 3695(pillar OH) and 3750 cm −1 (Si-OH) to appear.