(264c) Kinetics of High Pressure Catalytic Reaction Pathways for Dihydroeugenol Over Pt/ZrO2

The novel H₂Bioil [1] process involving high pressure biomass fast-hydropyrolysis followed by vapor phase fixed-bed catalytic hydrodeoxygenation (HDO) has been suggested for the conversion of biomass to liquid fuels. The first stage fast-hydropyrolysis generates hundreds of oxygenated compounds, complicating the study and development of new catalysts for the second stage HDO reactor. Therefore, model compound studies provide a means to understand the reactivity of specific oxygen functional groups and to gain fundamental understanding of the link to catalyst functionality.

Kinetic data was obtained for the reaction of the lignin-derived compound dihydroeugenol (2-methoxy-4-propylphenol) in hydrogen over a 2%Pt/ZrO₂ catalyst in a vapor-phase fixed-bed reactor at 24 atm total pressure. Weight hourly space velocities were varied from 100-1000 g hr^-1 g cat^-1 to obtain reaction pathways. Reaction orders and activation energies were obtained over a temperature range of 200-400^oC, hydrogen partial pressure range of 0-23 atm and dihydroeugenol partial pressure range of 0.4-1.2 atm under differential conditions.

Three main reaction pathways were identified: HDO, transalkylation, and hydrogenation of the aromatic ring. The WHSV studies suggest that the HDO pathway occurs via a series reaction, with methoxy group removal occurring first followed by phenol group removal. Transalkylation occurs via methyl attachment to the aromatic ring. The four isomers of the pure hydrogenation product 2-methoxy-4-propylcyclohexanol have: 1) positive apparent activation energies in the range of 25 to 40 kJ mol^-1 in the temperature range 200-240^oC and negative apparent activation energies of -10 to -40 kJ mol^-1 for temperatures of 280-320^oC 2) dihydroeugenol orders of 0.5-0.7 and 3) hydrogen orders of 1.9-2.5 at 300^oC. The similarity of these kinetic results to those for benzene hydrogenation suggest that the mechanism of ring hydrogenation for dihydroeugenol is similar to that of other aromatic compounds [2]. At 300^oC the aromatic HDO products 4-propyl phenol and propylbenzene have apparent activation energies of 88 and 128 kJ mol^-1, dihydroeugenol orders of 0.6 and 0.6, and hydrogen orders of 0.36 and 0.38, respectively. The analogous hydrogenated HDO products 4-propylcyclohexanol and propylcyclohexane have apparent activation energies of 27 and 67 kJ mol^-1, dihydroeugenol orders of 0.33 and 0.43, and hydrogen orders of 3.2 and 2.4 respectively. The reaction sequences and rate expressions will be discussed for the three reaction pathways.  

References:

1)      R. Agrawal, N. R. Singh, Synergistic Routes to Liquid Fuel for a Petroleum Deprived Future, AIChE Journal,55,7,1898-1905, 2009.

2)      S. D. Lin, M. A. Vannice, Hydrogenation of Aromatic Hydrocarbons over Supported Pt Catalysts I. Benzene Hydrogenation, Journal of Catalysis, 143, 539-553, 1993.