Researchers in China are proposing new pathways for the CO2 catalytic conversion of oleic acid into C8-C15 alkanes. The yield of C8–C15 products reaches 73.10 mol% in a CO2 atmosphere—much higher than the 49.67 mol% yield obtained in an H2 atmosphere.In the the absence of an external H2 source, the researchers report generating aviation fuel-like products through the aromatization of C3H6, oxidative dehydrogenation involving CO2 and C3H8, and hydrogen transfer reactions account for hydrogen liberation in oleic acid and achieve its re-arrangement in the final alkane products. A paper on their work appears in the RSC journal Green Chemistry.
In this study, we employ HZSM-5-supported nano-Ni metal particles as the catalyst to deoxygenate oleic acid, the most common ingredient of fatty acids, in both CO2 and H2 atmospheres. Distinct reaction pathways for deoxygenation of oleic acid in CO2 and H2 atmospheres are also fully explored on the basis of reaction intermediates, the C=C bond effect on catalytic cracking, H2 spillover, and hydrogen transfer for alkane production.
The catalytic roles of nano-Ni and the acidic sites of the zeolite are determined through separated experiments using single Ni, single HZSM-5, and nano-Ni/HZSM-5 as catalysts. Most importantly, the cause of the intrinsic hydrogen re-arrangement in oleic acid to generate the resultant liquid products in CO2 atmosphere with no external H2 source is resolved.—Xing et al.
The catalyst used in the study consisted of 10 wt% Ni/HZSM-5 with highly dispersed nano-Ni metal particles. Oleic acid deoxygenation experiment was conducted in a high pressure autoclave.
The reaction pathway in a CO2 atmosphere was significantly different from that in H2 atmosphere, as evidenced by the presence of 8-heptadecene, γ-stearolactone, and 3-heptadecene as reaction intermediates, as well as CO formation pathway.
Owing to the highly dispersed Ni metal center on the zeolite support, hydrogen spillover is observed in H2 atmosphere, which inhibits the production of short-chain alkanes and reveals the inherent disadvantage of H2 in this point.
The results presented herein demonstrate an innovative approach to CO2 utilization chemistry and prove unequivocally that CO2-processing of oleic acid over nano-Ni/zeolite catalyst can produce sustainable and clean aviation fuel-like alkanes in an efficient and effective manner.—Xing et al.
Shiyou Xing, Pengmei Lv, Haoran Yuan, Lingmei Yang, Z. M. Wang, Zhenhong Yuan and Yong Chen (2017) “Insight into forced hydrogen re-arrangement and altered reaction pathways in a protocol for CO2 catalytic processing of oleic acid into C8-C15 alkanes” Green Chemistry doi: 10.1039/C7GC01853C