Heterogeneously catalyzed hydrogen-free deoxygenation of saturated C8, C12 and C18 carboxylic acids

Heterogeneously catalyzed hydrogen-free deoxygenation
of saturated C8, C12 and C18 carboxylic acids

S. Mohite, U. Armbruster, M. Richter, D.L. Hoang, A. Martin
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Außenstelle Berlin,
Richard-Willstätter-Str. 12, 12489 Berlin

Biodiesel comprises fatty acid methyl esters with various chain lengths. It has clear environmental advantages compared to fossil fuels, nevertheless, some drawbacks rise from its limited shelf life, corrosion in vehicle engines, and lower energy value. These are mainly related to oxygen content of biodiesel, which should be removed. Possible ways for upgrading are pyrolysis [1] or hydrogenation [2,3]. In presence of H2 and noble metal catalysts (Pd, Ru, Ni) the latter route selectively forms hydrocarbons. Though H2 addition leads to high yields of desired paraffins, the use of external H2 sources lowers process economy. A hydrogen-free route may benefit from decomposition of by-product glycerol that generates H2 in situ. This work focused on model studies to investigate the hydrogen-free deoxygenation of C8, C12, and C18 acids exclusively.

Supported Ni and Pd catalysts (1-10 wt-%) were prepared by impregnation of ZrO2, active carbon, zeolites, and hydrotalcites. Autoclaves (25 ml) served for catalytic tests. Catalysts were reduced in situ with H2 (300 °C, 2 h) and the vessel was flushed with N2. Then, solutions of carboxylic acids in dodecane or tetralin were added and the reaction was started.

Initial catalyst survey was done with lauric acid (C12) at 300 °C. With supported Ni catalysts, highest conversion was found on 10%Ni/ZrO2 (59 %) and 5%Ni/ZrO2 (57 %), but alkane selectivity was little. In a second series, Pd catalysts supported on active carbon were tested. The best performing catalyst was 10%Pd/C with 37 % undecane yield at 68 % conversion of lauric acid. Such Pd catalysts then were used for deoxygenation of caprylic acid (C8) and stearic acid (C18). At similar reaction conditions (300 °C, 6 h), chain length of carboxylic acid has a strong impact on conversion as well as alkane selectivity (caprylic acid: X = 35 %, S = 54 %; lauric acid: X = 55 %, S = 68 %, stearic acid: X = 93 %, S = 13 %). The observed maximum in selectivity may be due to increased cracking as a side reaction, the probability of which increases with number of carbon atoms and bonds. At 300 °C, conversion without H2 addition may not be favorable for long chain carboxylic acids anymore.

Further work aimed at optimization of reaction conditions with 10%Pd/C catalyst. Raising temperature also was found to have a strong promotional effect on carboxylic acid conversion as well as alkane selectivity. Increasing pressure lowered conversion, but improved selectivity towards alkane. Characterization of spent catalyst 10%Pd/C showed a slight loss in Pd content due to leaching, but no agglomeration of metal particles.

[1] D.G. Lima et al., J. Anal. Appl. Pyrolysis 71 (2004) 987.
[2] I. Kubickova et al., Catal. Today 106 (2005) 197.
[3] http://www.nesteoil.com/default.asp?path=1,41,539,7516,7522