Characterization of gold-titania catalysts for propylene epoxidation

Eric Eugene Stangland, Purdue University

Abstract

Propylene oxidation to propylene epoxide (PO) was conducted over a variety of Au/TiO2 catalysts and model compounds in an attempt to determine the origins of their unique epoxidation capabilities in C 3H6/O2/H2 mixtures. While almost any gold-titania catalyst preparation gives some level of PO activity, there are clearly optimal preparation methods. PO is usually formed with >99% selectivity at temperatures of 373 K and below, but there catalysts often show low C 3H6 conversions and deactivation due to PO oligomerization on the catalyst surface. Attempts to increase conversion, either by raising the temperature or decreasing WHSV, result for most catalysts in a decrease of PO selectivity, primarily due to the further oxidation of PO to ethanal and CO2. Catalyst deactivation, however, is slowed at temperature near 473 K due to the oxidation and removal of the dynamic PO-deposited carbon film. XPS studies confirm strong interactions between PO and most TiO 2 supports, but alkali ions deposited at the time of preparation reduce PO isomerization and surface interactions, thus increasing the observed PO selectivity as compared to cases where the alkali is not present. The results show that both further oxidation reactions of PO, and subsequent catalyst deactivation via PO oligomerization, can be controlled by using a support containing isolated Ti-O moieties, thus increasing the observable PO yield. The complex PO-catalyst interaction, combined with the probable temperature dependent stability of the active intermediate involved in epoxidation, results in a maximum for the PO turnover frequency and yield for all but the most optimum catalysts between 323–473 K. Reaction order studies combined with an observed kinetic isotope effect upon D2 substitution for H2 produces kinetics consistent with a mechanism involving surface hydroperoxy species, despite a lack of spectroscopic proof. Experiments using alcohol co-feeds as H2 substitutes are also successful in forming active epoxidation intermediates. The observed kinetic induction period, along with kinetic and XPS experiments with TiO 2-Au(OH)3 systems, may indicate a role for TiO2-induced stabilization of “oxidized (hydrated)” Au states as the active site that forms the active hydroperoxy species.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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