Activation and isomerization of n-butane on sulfated zirconia model systems—an integrated study across the materials and pressure gaps

Abstract

Butane activation has been studied using three types of sulfated zirconia materials, single crystalline epitaxial films, nanocrystalline films, and powders. A surface phase diagram of zirconia in interaction with SO(3) and water was established by DFT calculations, which was verified by LEED investigations on single-crystalline films and by IR spectroscopy on powders. At high sulfate surface densities a pyrosulfate species is the prevailing structure in the dehydrated state; if such species are absent, the materials are inactive. Theory and experiment show that the pyrosulfate can react with butane to give butene, H(2)O and SO(2), hence butane can be activated via oxidative dehydrogenation. This reaction occurred on all investigated materials; however, isomerization could only be proven for powders. Transient and equilibrium adsorption measurements in a wide pressure and temperature range (isobars measured via UPS on nanocrystalline films, microcalorimetry and temporal analysis of products measurements on powders) show weak and reversible interaction of butane with a majority of sites but reactive interaction with <5 micromol g(-1) sites. Consistently, the catalysts could be poisoned by adding sodium to the surface in a ratio S/Na = 35. Future research will have to clarify what distinguishes these few sites.

Adsorption–desorption equilibrium investigations of n-butane on nanocrystalline sulfated zirconia thin films

“…The thickness of the films was found to play a crucial role in the formation of the tetragonal phase; if the films are too thin the layer will not crystallise during thermal treatment. Calcined films were found to have the essential features, including equivalent sulfur content and crystalline phase, of active powder catalysts, [43] thus validating them as a model system. These films enable surface science techniques to be applied while not compromising the chemical complexity of the catalyst.…”

“…Thermal treatment of the films in synthetic air retards the loss of sulfur and thus elemental compositions of the films are comparable to a typical active powder catalyst (Table 1). [43] In addition to the loss of sulfur, the C 1s signal is diminished and shifts to 284.6 eV, consistent with the decomposition of the SAM and the main source of carbon being either atmospheric contamination (adventitious) or SAM decomposition products. The silicon substrate peaks are more prominent after thermal treatment, indicating a decrease in layer thickness.…”

Oxo‐Anion Modified Oxides

“…1 S atom nm -2 and then remains constant [35]. ) is relevant for good performance in n-butane isomerization, and this condensation product is only formed at high sulfate surface density upon dehydration [38,39,40,41,42].…”

An Integrated Catalytic and Transient Study of Sulfated Zirconias: Investigation of the Reaction Mechanism and the Role of Acidic Sites in n‐Butane Isomerization

“…[28] The materials gap was extensively studied for this SZ system, leading to the formation of identical active surface centers on epitaxial films as well as on powders; results which were additionally confirmed by theoretical calculations. [29] The present contribution aims to bridge the pressure gap considerations by carrying out experiments on optimized powder catalysts under vacuum and at atmospheric pressure, using a temporal analysis of products (TAP) reactor system to allow a direct comparison between transient responses under vacuum with flow experiments conducted at atmospheric pressure. TAP therefore enables n-butane partial pressures ranging from 10 Pa (vacuum) to 5 kPa (atmospheric pressure) to be covered.…”

“…[30][31][32] Herein is described a systematic study covering experiments on powder systems which had previously been optimized in terms of their catalytic activity. [29,30,33] The materials were extensively characterized by standard methods, such as XRD, XPS, and DRIFTS, and an estimation of the number of surface centers was obtained from the results of ammonia adsorption. [6,34] Furthermore, the determination of kinetic parameters, such as heats of adsorption for the reactant n-butane and the product isobutane, by TAP modeling is also described.…”