WOx domain size, acid properties and mechanistic aspects of glycerol hydrogenolysis over Pt/WOx/ZrO2

Abstract

Supported WOx catalysts are widely investigated in glycerol hydrogenolysis for their high selectivity to 1,3-propanediol (1,3-PDO). The high performance is often related to surface Brönsted acid site. However, the intrinsic structure of Brönsted acid is unclear and its controllable preparation has not been investigated in detail. In addition, many reaction mechanisms have been proposed up to now, but with few direct evidences in spectroscopy studies. In this work, Pt/WOx/ZrO2 catalysts containing various amounts of WOx were studied in glycerol hydrogenolysis. The reaction is found to be structurally sensitive to WOx domain size, with medium polymerized WOx shown to benefit the formation of 1,3-PDO. By doping a suitable amount of Mn into monolayer covered WOx/ZrO2, large amounts of WOx with medium polymerization degree were created. Thus, the turnover frequency of 1,3-PDO (TOF1,3-PDO/W) increased 2.6 times in comparison to the best result of none Mn-doping Pt/WOx/ZrO2 catalysts. Characterizations of WOx structure and acid properties indicate that super strong Brönsted acid site is created by the interaction between medium polymerized WOx and Pt particle. This type of acid is linearly correlated with the formation rate of 1,3-PDO. The adsorption state of glycerol was studied using infrared spectroscopy, and the secondary −OH is found to be strongly adsorbed to Brönsted acid site on WOx containing catalyst, while its interaction with Pt/ZrO2 is much weaker. The natural structure of the active site is proposed to be Pt-(WOx)n-H, integrating super strong Brönsted acid site and metallic Pt site together. Combined with FTIR investigations of different surface hydrogen species and in situ 2-propanol conversion, the reaction mechanism was also determined.