Date of Award

8-9-2014

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Christopher T. Williams

Abstract

Recent advances for the deoxygenation of biomass, have demonstrated that hydrodeoxygenation (HDO) is one of the most promising route for the upgrading pyrolysis bio-oils. Catalytic hydrodeoxygenation of pyrolysis bio-oil have shown to be an efficient and economical process, since the raw materials consist mainly of waste. Propanoic Acid (PAc) is considered as one of the main constituents of pyrolysis bio-oils. However, these carboxylic acids are extremely corrosive and difficult to deoxygenate. Therefore, much effort have been given to the development of novel catalytic techniques, to improve the activity, stability and selectivity for the HDO of carboxylic acids.

This dissertation explores the use of supported group VIII noble metals for the HDO of PAc. The reactions were evaluated in a conventional continuous plug-flow reactor operated between 200-400ºC under atmospheric pressure with concentrations of 1.2% PAc/20%H2/balance He. The first part of this work consisted on the synthesis, characterization and evaluation of SiO2 supported catalysts (M = Pd, Pt, Rh, Ru, Ni). The activity and kinetics involving the reaction rate orders with respect to PAc and H2 and activation energies were discussed in detail. The reaction activity based on the TOF follows the sequence: Pd > Ru > Pt > Rh > Ni. The reaction over Pd, Pt and Rh catalysts proceeds mainly via decarbonylation (DCN) and decarboxylation (DCX) pathways at each reaction temperature. For Ru and Ni catalysts, while decarbonylation and decarboxylation pathway were predominant at lower temperatures (e.g., 200-250ºC), at higher temperatures (>300 ºC) the formation of diethyl ketone was observed. Additionally, the kinetics of Pd over different supports (carbon, SiO2 and TiO2) were examined. The activity based on the TOF decreases in the following order: Pd/SiO2 > Pd/TiO2 > Pd/C. The reaction orders in acid and H2 were found to be approximately 0.5 and zero, respectively, regardless of the support. The apparent activation energies studied in a temperature range of 200-240 ºC, were 16.7 ± 0.6, 19.3 ± 1.6 and 11.7 ± 0.7 kcal/mole for Pd/C, Pd/TiO2 and Pd/SiO2 catalysts, respectively.

Secondly, the effects of metal nanoparticle size ranging between 1.9 to 12.4 nm for over Pd/SiO2 under differential conversion catalysts was investigated. The particle sizes were determined by chemisorption (O2-H2 titration), XRD and STEM. While the catalytic TOF remained constant between 3.0-12.4 nm it decreased by a factor of 2-3 with decreasing particle size down to 1.9 nm. The reaction rate is therefore considered to be largely structure-insensitive over the range studied. The reaction rate orders with respect to PAc (~0.5) and H2 (~0), and the apparent activation energy (~12 Kcal/mole), were found to be the same for both 2.0 and 12.4 nm particle sizes. In contrast, the reaction rate order with respect to PAc (~1.0) and H2 (~0.3) was different for hydrogenation to produce EtCHO. These differences are explained by a change in the rate-determining step for the HDO of propanoic acid.

Furthermore, a deuterium isotopic substitution of PAc to study a kinetic isotope effect (KIE) and elucidate the reaction mechanism was explored. A combined experimental and computational kinetic isotope effect (KIE) study was performed for the catalytic hydrodeoxygenation (HDO) of deuterium-labeled propanoic acid (PAc-2, 2-D2) over Pd catalyst. For the experimental study, the kinetics were measured in a plug flow reactor over a 5 wt% Pd/C catalyst (as described in the first part) under differential conversion. Different experimental KIE values for the high (kH/kD = 1.16 ± 0.07) and low (kH/kD = 1.62 ± 0.05) partial pressures of hydrogen were observed. Density functional theory calculations were performed to obtain the reaction parameters of the elementary steps involved in the HDO of PAc on Pd (111), and a microkinetic model was developed to estimate the KIE for the low hydrogen partial pressure case from first principles. The computed result (kH/kD = 1.49) is in good agreement with the experiment. In addition, the product distribution showed to be C2H6 and CO suggesting decarbonylation (DCN) is the main reaction pathway. Strong evidence is provided for the proposed mechanism for the formation of C2H6 on Pd(111).

The catalytic activity and selectivity trend over carbon supported over group VIII noble metals (M = Pt, Rh, Ru, Ir, Ni, Ag, Au and Cu) was also explored. The catalysts exhibited mainly selectivity toward methane and C2 hydrocarbons, showing strong overall preference for decarbonylation (DCN) versus hydrogenation. The catalytic activity at 200ºC in terms of TOF decreased in the sequence Rh≥ Pt > Ir > ~Ru ~Ni, with no measurable activity found for Au, Ag, and Cu. A reaction rate order of ~0.5 and 0 with respect to PAc and H2, respectively, was found for all catalyst, except Ni/C. The latter exhibited a reaction rate order roughly of 0.2 and -0.2 with respect to PAc and H2, respectively. Comparison with previous studies on the HDO of PAc over 5wt% Pd/C is reported.

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