Hydrodeoxygenation of Biomass Derivatives on Metal-Modified Molybdenum Carbides

Hydrodeoxygenation of Biomass Derivatives on Metal-Modified Molybdenum Carbides

 

Weiming Wan, Jingguang G. Chen

Department of Chemical Engineering, Columbia University, New York, NY, 10027

 

Biomass has attracted significant attentions as an alternative resource for fuel and chemicals. However, biomass derivatives have a high O/C ratio and low energy density that make them unsuitable for use in the fuel and chemical sectors.^[1] Thus, the hydrodeoxygen (HDO) reaction is necessary to upgrade these biomass derivatives to value-added products. For example, furfural and glycerol are two typical biomass derivatives. Through HDO reactions, furfural can be converted to 2-methylfuran, a promising fuel additive, and glycerol can be converted to propylene or propanediol, an important monomer.

Transition metal carbides are novella class of interesting materials that often demonstrate unique catalytic properties. Due to the high oxygen binding energy, which helps to break the C-O bond, many metal carbides have shown activities towards HDO reactions. Mo₂C is a typical transition metal carbide and it shows high HDO activity for oxygenates such as furfural and propanol.^[2],[3] However, the high oxygen binding energy makes it hard to remove the surface oxygen to complete the catalytic cycle. Metal modifiers can be used to adjust the oxygen binding energy. The strong oxophilic metal modifiers, such as Co and Fe, have slightly lower oxygen binding energies than Mo₂C, and the modified Mo₂C surface could have a similar HDO selectivity with better stability. In contrast, the weak oxophilic metal modifiers, such as Cu and Pt, have much lower oxygen binding energies than Mo₂C, and the modified Mo₂C surface should have a different reaction pathway and selectivity.

The current work combines density functional theory (DFT) calculations and surface science experiments using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) under ultrahigh vacuum (UHV) conditions. The main objective is to study the effect of metal modifiers on the adsorption geometry and selective HDO of furfural and glycerol on the Mo₂C surface. The Mo₂C surface was prepared by cycles of ethylene treatment on the Mo(110) single crystal, and the metal layer was prepared by physical vapor deposition. 

In the furfural HDO reaction, compared with the Mo₂C surface, furfural has a similar adsorption geometry on Co-modified Mo₂C, suggesting the same reaction pathway on both surfaces. The lower oxygen binding energy of the Co/Mo₂C surface improves the catalytic stability in the TPD experiments. On the other hand, Pt/Mo₂C has lower oxygen binding energy than Co/Mo₂C and the furan ring is flat on the Pt/Mo₂C surface. The flat furan ring and the low oxygen binding energy help produce more furan through decarbonylation.

In the glycerol HDO reaction, the Mo₂C surface can break all the C-O bonds to make propylene. Similarly, the Fe/Mo₂C surface can produce propylene from glycerol with a better stability. On the other hand, the Cu/Mo₂C surface can selectively break the C-O bonds. With a high Cu coverage, the Cu/Mo₂C surface can break one C-O bond to form acetol; with a low Cu coverage, the Cu/Mo₂C surface can break two C-O bonds to form ally alcohol.

This work provides guidance for designing metal-modified carbide HDO catalysts and a detailed discussion of the DFT calculations and the surface science experiments will be presented.

References:

[1]    K. Xiong, W. Yu, D. G. Vlachos, J. G. Chen. ChemCatChem 2015, 7 (9), 1402.

[2]    K. Xiong, W. Lee, A. Bhan, J. G. Chen. ChemSusChem 2014, 7 (8), 2146.

[3]    H. Ren, W. Yu, M. Salciccioli, Y. Chen, Y. Huang, K. Xiong, D. G. Vlachos, J. G. Chen. ChemSusChem 2013, 6 (5), 798.