(18b) Tandem Dehydrogenation/Hydrogenolysis of Bio-Derived Polyols to Valuable Chemicals On Pt/C Catalyst: Catalysis and Kinetic Studies

During the last decade, extensive efforts have been made on hydrogenolysis of bio-derived polyols to fuels and high value added chemicals using heterogeneous catalysts. The catalysts and processes suffer from low selectivity, harsh reaction conditions and large consumption of expensive hydrogen.^[1] Recently a tandom catalysis approach was reported involving hydrogen generation (by reforming of polyols) and instantaneous in situ hydrogenolysis of those polyols to hydrodeoxygenated products on bi-functional supported metal catalysts such as Pt/NaY and Pt/Al₂O₃.^[2,3] This approach does not require addition of external hydrogen but produces useful products even at relatively mild reaction conditions (inert gas pressure < 2 MPa) compared to conventional hydrogenation (hydrogen pressure > 8~10 MPa). However, the selectivity to useful liquid products (S < 60%) is still a major challenge for further improvement. This is because significant amounts of waste gaseous products (S > 35%) are generated simultaneously. Significant amount of valuable carbon is lost to methane and CO₂ due to methanation and water gas shift reactions on noble metal catalysts under relatively high temperatures (T > 473 K). Furthermore, the overall activity of supported metal catalysts (e.g. Pt/C and Ru/C) is low (TOF < 30 h^-1),^[4]which poses challenges for the further development of catalysts for the tandem reaction.

In this paper, we report tandem dehydrogenation/hydrogenolysis of abundantly available polyols such as glycerol, xylitol and sorbitol to lactic acid and glycols with linear alcohols as co-products in the presence of Pt/C catalysts. It is found that Pt/C displays high activity (TOF: 257~484 h^-1) and atom efficiency (S > 95%) at temperatures as low as 403~433 K.^[5,6] Transmission electron microscopy of carbon (e.g. activated carbon, mesoporous carbon and graphene oxide) supported Pt catalysts shows that amorphous Pt nanoparticles show high activity for dehydrogenation while Pt {111} is believed to be the catalytically active surface for hydrogenolysis reactions using in situformed hydrogen. Furthermore, various parameters including substrate concentration, catalyst loading and reaction temperature are studied systematically. Based on the experimental data, a detailed analysis of mechanistic Langmuir-Hinshelwood type kinetic models following a rigorous model discrimination procedure will be presented along with statistical analysis of the model parameters. This study will be useful in understanding the mechanism of the reactions involved in dehydrogenation/hydrogenolysis of polyols, as well as reactor design and process development.

Reference

[1] Chheda, J.; Huber, G.; Dumesic, J. Angew. Chem, Int. Ed.2007, 46, 7164-7183.

[2] D’Hondt, E.; Vyver, S. V. d.; Sels, B. F.; Jacobs, P. A. Chem. Commun. 2008, 6011-6012.

[3] Roy, D.; Subramaniam, B.; Chaudhari, R. V. In 21st North American Catalysis Society Meeting, San Francisco, CA, 2009.

[4] Torres, A.; Roy, D.; Subramaniam, B.; Chaudhari, R. V. Internatonal Symposium on Chemical Reaction Engineering, Philadelphia, PA, 2010.

[5] Chaudhari*, R. V.; Torres, A.; Jin, X.; Subramaniam, B. Ind. & Eng. Chem. Res., 2013 (accepted).

[6] Jin, X.; Roy, D.; Subramaniam, B.; Chaudhari, R. V.* AIChE Annual Meeting, Pittsburgh, PA, 2012.