277663 Selective Liquid Phase Oxidation of Glycerol Using (Group I-B)-Pd Bimetallic Catalysts Prepared by Electroless Deposition Method

Tuesday, October 30, 2012: 1:50 PM
315 (Convention Center )
Abraham A. Rodriguez, Christopher T. Williams and John R. Monnier, Chemical Engineering, University of South Carolina, Columbia, SC

Glycerol (1,2,3-propanetriol) is the byproduct of biodiesel production from the transesterification of vegetable oils or animal fats, produced at a rate of 10 pounds of glycerol for every 100 pounds of biodiesel produced.  Increase in production of Biodiesel has led to a surplus of crude glycerol which has destabilized the glycerol market. Thus, new ways of utilizing this excess feedstock are needed in order to cope with the current and future production trends. The heterogeneously catalyzed selective liquid phase oxidation of glycerol has been studied by many research groups using mono and bimetallic Pd, Pt, and Au catalyst on different supports.  Literature shows that bimetallic catalysts generally yield higher activities and selectivities when compared to their monometallic counterparts. Using the Electroless Deposition (ED) method, Group I-B metals have been deposited on a commercial Pd/C catalyst. ED permits controlled deposition of a second metal on a primary metal surface, which allows for better correlation of catalyst composition and surface composition with performance. A 5 wt% Pd/C catalyst provided by BASF was used as base catalyst for this study and a series of bimetallic catalysts were prepared, with coverage of Group-IB metal on Pd/C (θI-B) ranging from 0 to 100%. Batch reactor kinetic results for glycerol oxidation over these Au-Pd/C bimetallic catalysts under basic conditions showed enhanced activity for bimetallic compositions with θAu>0.30, exhibiting a maximum activity at θAu=0.50, with a rate constant ~20 times larger than for palladium and ~6 times larger than for gold.  The volcano shaped curve suggests the formation of bimetallic sites composed of Au-Pd in close contact. XPS data shows a shift to lower binding energies in the Au 4f region for all bimetallic compositions when compared to a 1% Au/C catalyst, suggesting electron transfer from Pd→Au. The bimetallic effect could be explained by this electron transfer forming a bimetallic site where the terminal OH group of the glycerol molecule is more easily activated.  Selectivity to glyceric acid for the bimetallic catalysts was 80% (±2%) for catalysts below θAu<0.90, dropping to 60% and lower for catalyst with θAu>0.98.  This can be explained by the ability of Au under this reaction conditions to form H2O2, which is an unselective oxidant, combined with the low availability at high coverages of Pd sites which can decompose H2O2, causing further oxidation of glyceric acid to 2 and 1 carbon products.  Further work with different bimetallic compositions and different reaction conditions is in progress to better understand these effects, and will also be discussed in this presentation.

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