Kinetic Model for Hydrodeoxygenation Reaction of M-Cresol On Alumina-Supported Pt Catalyst
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Variety of oxygenated compounds are detected in the pyrolysis oils, which leads to a number of associated problems such as low stability and reduced heating values. As a consequence, complete deoxygenation of bio oils with minimum hydrogen uptake is one of the proposed strategies toward improving the bio‐oil quality. The oxygenated compounds come with various functionalities and consequently the deoxygenation chemistry is highly sensitive to these functional groups. In order to design effective catalysts to eliminate oxygen from all oxygenated functionalities, the first step is to gain complete understanding of the deoxygenation mechanisms of individual functional groups. This study has focused on the phenolic functional groups, which abundantly appear in bio oils. 3-methylphenol or m-cresol was chosen as a representative molecule for methylated phenol compounds. The reactions were carried out under plug-flow mode and in gas-phase condition, which will allow more effective reaction kinetic modeling compared to liquid-phase reactions. Conversions of m-cresol occurred over 1.7 wt% Pt/Al2O3 catalyst at atmospheric pressure and temperatures ranging from 513 K to 563 K. In presence of hydrogen, the deoxygenation of m‐cresol into hydrocarbons proceeds through two consecutive steps: hydrogenation followed by dehydration reactions. On Pt, 3‐methylphenol is first hydrogenated to 3‐methylcyclohexanol, which is consequently deoxygenated to unsaturated cyclic methylcyclohexenes via dehydration reaction on the support acid site. Once methylcyclohexenes were formed, they were quickly transformed into methylcyclohexane and toluene via hydrogenation and dehydrogenation reactions on metal sites. From the hydrogen consumption standpoint, toluene is a more favorable hydrocarbon product than methylcyclohexane.
In order to achieve a thorough understanding of effects of the bifunctional catalyst on the overall conversion and product selectivity of m‐cresol reactions, Langmuir‐Hinshelwood kinetic model involving dual sites (i.e. metal and support) was employed to fit the reaction data. Besides four surface reactions listed above, desorption of toluene from the Pt metal was considered as one of the rate-limiting steps. In order to achieve a reasonable regression, equilibirium adsorption constants (Ki) of adsorbed species on metal and support estimated from density functional theory (DFT) calculations and equilibrium reaction constants (KM) were constrained during the fitting. The important kinetic parameters such as elementary rate constants (ki) and activation barriers (Δ Ea) were extracted from the model. Turn-over rate (TOR) were normalized by the fitted rate constants with the number of metal and acid active sites provided by CO chemisorption and temperature-programmed desorption (TPD) of isopropylamine experiments. Identification of rate-limiting step and acknowledgement of individual roles of metal and acid sites in overall conversion and product selectivity of m-cresol reactions will assist us in design the most effective catalysts to convert phenolic compounds into desirable aromatic hydrocarbons.