Vanillin hydrodeoxygenation reaction kinetics over supported monometallic and bimetallic Pd-based catalysts in different solvents and binary environments.

Aliu, Elias (2020). Vanillin hydrodeoxygenation reaction kinetics over supported monometallic and bimetallic Pd-based catalysts in different solvents and binary environments. University of Birmingham. Ph.D.

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Abstract

In this thesis, the kinetics of hydrodeoxygenation (HDO) reaction over commercial monometallic and synthesised bimetallic Pd-based catalysts was investigated using vanillin as the representative model of oxygenates present in bio-oils. Prior to the kinetic studies, preliminary experiments were carried out to investigate non-catalytic effect, solvent effect, role of catalytic supports and active elements in HDO reaction of vanillin. It was found that non-catalytic effect contributed less than 1% in conversion, while contribution from catalytic supports was no more than 4% in conversion. In contrast, the amount of vanillin converted varies remarkably from 99% under Pd-based catalysts to 21% under Rh-based catalyst. Likewise, the amount of vanillin converted changes significantly from 100% under 2-propanol to 70% with toluene as the reaction solvent. Consequently, it was concluded that the reaction is severely affected by the choice of solvent and catalytic active element. In addition, from the preliminary studies completed it was found that the selectivity toward the deoxygenated product creosol varies significantly with the processing condition. Within the range of processing conditions investigated, the combination of processing parameters that maximises selectivity toward creosol and conversion include 338 K temperature, 3.0 MPa hydrogen gas partial pressure, 0.5 kg/m3 catalyst loading, and 500 rpm agitation speed. From vanillin HDO reactions over Pd/C, Pd/Al2O3, and PdRh/Al2O3 catalyst using ethylacetate as the solvent, it was found that nonfirst-order kinetics reliably describes the initial reaction rate dependence on starting vanillin concentration and hydrogen gas partial pressure. More importantly, a derived Langmuir – Hinshelwood – Hougen – Watson (LHHW) expression was successfully used to reliably model the kinetics of the reaction over the various catalysts. Interestingly, at 95% confidence level the estimated intrinsic activation energy for the reaction varies from 24.10 kJ/mol under PdRh/Al2O3 to 64.08 kJ/mol under Pd/Al2O3 catalyst. Additional experiments performed using 2-propanol, tetrahydrofuran, and toluene as the reaction solvent with bimetallic PdRh/Al2O3 particles as the catalyst reaffirms the nonfirst-order dependence of the initial reaction rate on starting vanillin concentration and hydrogen gas partial pressure. Nonetheless, at 95% confidence level the estimated intrinsic activation energy for the reaction varies from 23.60 kJ/mol in 2-propanol to 56.30 kJ/mol in toluene. The observed variation in the intrinsic activation energies for the reaction thus possibly explains the changes in performance (i.e. conversion and selectivity toward creosol) under different catalysts and solvents. In order to examine the impact of other prominent compounds present in bio-oil, vanillin HDO reaction was conducted in binary environments using the prepared bimetallic PdRh/Al2O3 particles as the catalyst and ethylacetate as the solvent. The secondary components in the binary environments were acetic acid and guaiacol. In both vanillin – acetic acid and vanillin – guaiacol environments, nonfirst-order kinetics still best describes the initial reaction rate dependence on starting vanillin concentration and hydrogen gas partial pressure. However, the order of reaction with respect to hydrogen gas partial pressure changes from 0.4 in vanillin-only environment to 0.6 in the binary environments. Notably, the presence of guaiacol or acetic acid in the starting mixture caused significant change in intrinsic activation energy of the reaction. At 95% confidence level, the estimated intrinsic activation energy of the reaction changes from 24.10 kJ/mol in vanillin-only environment to 51.00 kJ/mol in vanillin – acetic acid and 34.10 kJ/mol in vanillin – guaiacol environment. Hence, the studies completed in this thesis suggest the energy barrier to vanillin HDO reaction is strongly affected by the presence of other bio-oil model compounds, solvent and catalyst type.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):