Rolf E. Jentoft

Hydrophobic Zeolites for Biofuel Upgrading Reactions at the Liquid–Liquid Interface in Water/Oil Emulsions

HY zeolites hydrophobized by functionalization with organosilanes are much more stable in hot liquid water than the corresponding untreated zeolites. Silylation of the zeolite increases hydrophobicity without significantly reducing the density of acid sites. This hydrophobization with organosilanes makes the zeolites able to stabilize water/oil emulsions and catalyze reactions of importance in biofuel upgrading, i.e., alcohol dehydration and alkylation of m-cresol and 2-propanol in the liquid phase, at high temperatures. While at 200 °C the crystalline structure of an untreated HY zeolite collapses in a few hours in contact with a liquid medium, the functionalized hydrophobic zeolites keep their structure practically unaltered. Detailed XRD, SEM, HRTEM, and BET analyses indicate that even after reaction under severe conditions, the hydrophobic zeolites retain their crystallinity, surface area, microporosity, and acid density. It is proposed that by preferentially anchoring hydrophobic functionalities on the external surface, the direct contact of bulk liquid water and the zeolite is hindered, thus preventing the collapse of the framework during the reaction in liquid hot water.

Low‐cost, heated, and/or cooled flow‐through cell for transmission x‐ray absorption spectroscopy

A transmission x‐ray absorption spectroscopy cell that can be used for air‐sensitive samples with in situ treatment is described. The cell is designed with a relatively small size for use with air‐sensitive powdered catalyst samples that must be loaded in a glove box. Samples can be treated in situ with gas flow or vacuum and temperature control up to 500 °C. The cell is constructed of stainless steel and designed for durability as well as ease of repair. The cells are vacuum tight and equipped with beryllium windows sealed with vacuum O‐ring flanges for easy loading. Each cell, with all parts excluding the windows, costs about

Sulfated zirconia with ordered mesopores as an active catalyst for n-butane isomerization

2700.

In situ analysis of metal-oxide systems used for selective oxidation catalysis: how essential is chemical complexity?

Zirconia/surfactant composites were hydrothermally synthesized in aqueous sulfuric acid at 373 K using Zr(O-nPr)4 as oxide precursor and hexadecyl-trimethyl-ammonium bromide as template. Mesostructural features similar to those of MCM-41 were detected by X-ray diffractometry, with d=4.6 nm. A sample obtained from a starting mixture with Zr:S:CTAB = 2:2:1 was stable enough for removal of occluded organics. After calcination at 813 K, the d-value was 3.6 nm, the surface area 200 m2/g, and the mean pore diameter estimated by the BJH method 2.2 nm. Extended X-ray absorption fine structure analysis suggests Zr to be in a short-range structure (<4 Å) similar to that of Zr in monoclinic ZrO2. Scanning electron microscopy including energy dispersive X-ray analysis showed 1-5 μm sulfur-containing ZrO2 spheres. The material catalyzes the isomerization of n-butane to i-butane at 378 K with a steady activity in the order of magnitude of commercial sulfated ZrO2.

Formation of Bronzes during Temperature-programmed Reduction of MoO3 with Hydrogen - An In situ XRD and XAFS Study

The mode of operation of selective oxidation reactions is described by a series of chemical rules defining the catalyst and some reaction intermediates. In contrast to catalytic processes over metallic elements, little is known, however, about the atomistic details of selective oxidation. In particular, the participation of the subsurface region of the catalyst in the kinetically relevant elementary steps (Mars–van Krevelen mechanism) is not positively verified. Using in situ X-ray absorption techniques to study binary and ternary molybdenum oxides the present contribution shows that it is possible to tackle some of the problems in selective oxidation by direct experimental observation. The modification of the Mo–O local bonding interaction upon thermal reduction of MoO3to MoO3-xis illustrated. This was also found for mixed Mo–V oxides in which the chemical state of the vanadium seemed unaffected by the reaction but the surface Mo : V ratio varied substantially with the gas phase composition. It is further shown that the solid-state phase transformation between reduced and oxidised forms of molybdenum oxides occur so rapidly, that possibly relevant suboxide cannot be identified by ex situ phase analysis. Observation of the time-law of redox transformations showed that lattice oxygen is only available for selective oxidation if the associated solid-state transformation occurs in the kinetic regime of reaction control and not in that of diffusion control.

n-pentane isomerization and disproportionation catalyzed by promoted and unpromoted sulfated zirconia

The temperature-programmed reduction of MoO3 from 300 K to 773 K in 50 vol-% hydrogen in He (10 5 Pa) at different heating rates (0.1, 0.2, and 5 K/min) was investigated by in situ XRD and XAFS. At heating rates of ~ 0.1 and ~ 0.2 K/min the formation of the molybdenum bronze H0.34 MoO3 was observed by in situ XRD in the early stage of the reduction of MoO3. At a heating rate of 5 K/min the formation of a more disordered bronze (HxMoO3 with x ~ 0.07) prior to the detection of the product phase MoO2 was observed by in situ XAFS. In both studies the consumption of the bronze was found prior to the complete reduction of MoO3. A simplified mechanism for the temperature-programmed reduction of MoO3 in hydrogen is proposed that includes (i) in corporation of H2 in the MoO3 bulk and formation of a more or less ordered bronze, (ii) consumption of the bronze and formation of nucleation site for MoO2, (iii) nucleation of MoO2 and nuclei growth.

Kinetics of solid-state reactions in heterogeneous catalysis from time-resolved X-ray absorption spectroscopy

Isomerization and disproportionation of n-pentane were catalyzed by sulfated zirconia, Fe- and Mn-promoted sulfated zirconia, and Pt-, Fe-, and Mn-promoted sulfated zirconia in a flow reactor at temperatures of −25 to 50°C and n-pentane partial pressures of 0.005–0.01 atm. Incorporation of the Fe and Mn promoters increased the activity of the sulfated zirconia by two orders of magnitude at 50°C; addition of Pt to the latter catalyst increased the activity only slightly. The primary reactions, disproportionation (to give butanes and hexanes) and isomerization, occurred in parallel; secondary disproportionation reactions gave heptanes, propane, butanes, and pentanes. The data are consistent with acid-base catalysis and carbenium ion intermediates, and the isomerization is inferred to proceed both by unimolecular and bimolecular mechanisms. H2 in the feed stream and Pt in the catalyst both led to reductions in the rate of catalyst deactivation.

Decoupling HZSM‐5 Catalyst Activity from Deactivation during Upgrading of Pyrolysis Oil Vapors

The potentials of X-ray absorption spectroscopy (XAS) (quantitative phase composition and average valence together with a short-range order structure analysis) combined with a time-resolution in the second range make time-resolved (TR-) XAS a powerful tool for investigating the reactivity of solids in catalysis and solid-state chemistry. General aspects of TR-XAS investigations are discussed (i.e., instrumentation, data analysis). In addition, some experiments illustrate how the kinetics of solid-state reactions in heterogeneous catalysis can be elucidated from TR-XAS studies.

H–D exchange between CD4 and solid acids: AlCl3/sulfonic acid resin, promoted and unpromoted sulfated zirconia, and zeolite HZSM-5

The independent evaluation of catalyst activity and stability during the catalytic pyrolysis of biomass is challenging because of the nature of the reaction system and rapid catalyst deactivation that force the use of excess catalyst. In this contribution we use a modified pyroprobe system in which pulses of pyrolysis vapors are converted over a series of HZSM-5 catalysts in a separate fixed-bed reactor controlled independently. Both the reactor-bed temperature and the Si/Al ratio of the zeolite are varied to evaluate catalyst activity and deactivation rates independently both on a constant surface area and constant acid site basis. Results show that there is an optimum catalyst-bed temperature for the production of aromatics, above which the production of light gases increases and that of aromatics decrease. Zeolites with lower Si/Al ratios give comparable initial rates for aromatics production, but far more rapid catalyst deactivation rates than those with higher Si/Al ratios.

Structure–activity relationships of heterogeneous catalysts from time-resolved X-ray absorption spectroscopy

The H–D exchange reaction between CD4 and each of a family of solid acids (the zeolite HZSM-5, sulfated zirconia, iron- and manganese-promoted sulfated zirconia, and AlCl3/sulfonic acid resin) was investigated with a batch recirculation flow reactor; the data determine initial rates of the exchange reaction giving CD3H at temperatures ranging from 440 K for AlCl3/sulfonic acid resin to 688 and 703 K for the zeolite and promoted sulfated zirconia, respectively. Extrapolated results show that the reaction is three orders of magnitude faster with the AlCl3/sulfonated resin (an analogue of the very strongly acidic combination of AlCl3 and H2SO4) than with HZSM-5 or promoted sulfated zirconia and two orders of magnitude faster with the latter than with sulfated zirconia.