Deoxygenation of palmitic and stearic acid over supported Pd catalysts: Effect of metal dispersion

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Abstract

Catalytic deoxygenation of palmitic and stearic acids mixture was studied over four synthesized Pd catalysts supported on synthetic carbon (Sibunit) in a semibatch reactor and dodecane as a solvent at 260–300 °C. The catalysts were prepared by precipitation deposition method using Pd chlorides as metal precursors. All catalysts contained 1 wt.% Pd, however, the metal dispersion was systematically varied. An optimum metal dispersion giving the highest reaction rate was observed. The main liquid phase products were n-heptadecane and n-pentadecane, which were formed in parallel. In addition to the particle size effect the impact of mass transfer was elucidated and a detail discussion on temperature programmed desorption of CO from the fresh and spent samples was provided.

Graphical abstract

Catalytic deoxygenation of palmitic and stearic acids mixture was studied over four synthesized Pd catalysts supported on synthetic carbon (Sibunit) in a semibatch reactor and dodecane as a solvent at 260–300 °C. The catalysts were prepared by precipitation deposition method using Pd chlorides as metal precursors. All catalysts contained 1 wt.% Pd, however, the metal dispersion was systematically varied. An optimum metal dispersion giving the highest reaction rate was observed. The main liquid phase products were n-heptadecane and n-pentadecane, which were formed parallel.

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Introduction

Fatty acids and their esters are potential raw materials for producing long chain, diesel-like hydrocarbons, which are industrially manufactured in Finland and are known as second generation biodiesel [1], [2], [3], [4], [5], [6]. One of the possibilities to produce hydrocarbons is to apply catalytic deoxygenation of fatty acids and their esters. Several active metals, such as Pd, Ni, Ru, Ir, Os, Rh supported on silica, alumina, active carbon as well as some alloys and bimetallic catalysts have been studied [4] in this reaction. The most promising catalyst reported in the literature has been Pd/C in stearic acid deoxygenation at 300 °C under 17 bar under helium in a semibatch reactor [4] with selectivity of 95% to heptodecane at complete conversion. Besides stearic acid also other potential feedstocks, such as esters and unsaturated acids were investigated in catalytic deoxygenation. The main reaction pathway in deoxygenation of fatty acids results in formation of one carbon shorter hydrocarbons compared to the acid. Other products are gaseous carbon monoxide and carbon dioxide. In case of esters the first reaction step is the formation of a corresponding fatty acid, thereafter the reaction proceeds analogously as for acids.

In the catalytic deoxygenation of fatty acids the effect of metal particle size on activity, i.e. structure sensitivity, has not been previously studied. In the current work the main aim was to systematically study the effect of metal particle size and dispersion on catalytic deoxygenation of a mixture of palmitic and stearic acids. Four different Pd catalysts exhibiting different metal dispersions were investigated in this work. A synthetic mesoporous carbon, Sibunit, was applied as a support material, which is characterized by high mechanical strength, chemical and thermal stability, high purity and a controllable narrow pore size distribution [7].

Section snippets

Experimental setup

The catalytic deoxygenation of stearic acid was performed in 300 ml semibatch reactor. The reaction mixture containing typically 0.1 M of a mixture of palmitic and stearic acids (Fluka, 59 mol% palmitic and 40 mol% stearic acid), in dodecane was injected into the reactor containing 1 g of prereduced Pd/C catalyst (powder; size < 50 μm to avoid internal diffusion limitations). The liquid phase volume was 50 ml. The reaction temperature was varied within 260–300 °C at pressure 17.5 bar. The flow of carrier

Catalyst characterization results

The BET specific surface area of the used support, Sibunit, was 411 m2/gcat.. The BET specific surface area of the fresh B catalyst was 379 (m2/g). The specific surface areas of three spent catalysts, i.e. catalyst A, B and D were also determined by nitrogen adsorption in order to evaluate if coking is one of the possible reasons for catalyst deactivation (Table 1). The BET specific surface area of spent catalysts A and D decreased by 23 and 10%, respectively, indicating the catalytic activity

Conclusions

Four different 1 wt.% Pd/C catalysts supported on synthetic mesoporous carbon (Sibunit) with different metal dispersion were prepared, characterized and tested in deoxygenation of stearic acid and palmitic acid mixture at 260–300 °C under overall pressure of 17.5 bar of 5 vol% of hydrogen in helium. The main liquid phase products were heptadecane and pentadecane being formed in a parallel mode. Additionally the fresh and spent catalysts were characterized by nitrogen adsorption, hydrogen TPR and CO

Acknowledgements

This work is part of activities at the Åbo Akademi Process Chemistry Centre of Excellence Programmes (2000–2011) financed by the Academy of Finland. The work was partly supported by DGAPA-PAPIIT (UNAM, Mexico) through grant N 120706-3. The part of research work was supported by RFBR Grant No. 07-03-12159.

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