Gas-phase epoxidation of propylene over gold/TS-1 catalysts

Bradley Martin Taylor, Purdue University

Abstract

Propylene oxide (PO) is one of the most synthesized chemicals because of its use in the production of everything from polyurethanes to block co-polymers. While the gas-phase epoxidation of propylene has been shown to proceed efficiently over Au-Ti based catalysts in the presence of hydrogen and oxygen, this work utilizes gold supported on titanium silicalite-1 (TS-1) in an attempt to better understand the nanoscale active gold species and its synergism with titanium. While selective elimination of titanium sites within the support crystallite was unsuccessful, it was shown that very low metal loadings are capable of significant PO production rates. In a series of Au/TS-1 catalysts with varying gold and titanium contents prepared by deposition precipitation, gold loading was closely related to titanium loading, implying low loadings result in an inherently small number of very active sites. High gold loadings resulted in poor activity and stability. Catalysts prepared with lower metal contents were very active when rates were normalized to the total gold content (350 gPO/hr/gAu at 200°C for 0.01 wt % Au/TS-1 (Si/Ti = 500)), implying that active metals were more efficiently utilized for the epoxidation reaction. The low gold loadings coupled with the absence of gold particles in TEM micrographs make it likely that, in these materials, significant activity is attributable to gold entities much smaller than 2 nm. In an effort to examine the role of crystal morphology on activity, mesoporous scale defects were introduced into TS-1 support materials through the addition of carbon pearls during the synthesis procedure. Catalysts prepared using these support materials were consistently active and stable, with a 0.33 wt % Au catalyst producing 132 gPO/hr/kgcat at 200°C, the highest PO rate thus reported. The superior activity and stability of Au/TS-1 catalysts allowed for the first comprehensive kinetic analysis of the propylene epoxidation system in the absence of significant deactivation. A hybridized factorial design of experiments was utilized to efficiently collect kinetic information. Fractional reactant orders required a minimum of two active sites participating in the rate determining step. A reaction mechanism based on these experiments, DFT calculations and other results from the literature is presented.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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