Co-decorated carbon nanotube-supported Co-Mo-K sulfide catalyst for higher alcohol synthesis

Abstract

Using chemical reduction-deposition method, a type of metallic cobalt-decorated multi-walled carbon nanotubes, noted as y%(mass percentage)Co/MWCNTs, was prepared. TEM, SEM and XRD measurements demonstrated that the metallic cobalt was evenly coated on the MWCNT substrate, with granule-diameter of the Co (0)(x)-crystallites of 5-8 nm. Using the y%Co/MWCNTs as support, a type of supported Co-Mo-K sulfide catalysts, noted as x%(Co (i) Mo (j) K (k) )/(y%Co/MWCNTs), for higher alcohol synthesis (HAS) was developed. It was experimentally shown that using the Co-modified MWCNTs in place of simple MWCNTs or activated carbon (AC) as the catalyst support led to a significant increase in activity of CO hydrogenation conversion and improvement in the selective formation of C2+-alcohols. Under the reaction condition of 5.0 MPa, 613 K, CO/H-2/N-2 = 45/45/10 (v/v) and GHSV = 3600 ml(STP)h(-1) g(-cat.)(-1), the observed STY of C1-4-alcohols reached 154.1 mgh(-1)g(-cat.)(-1) at 12.6% conversion of CO over the 11.6%(Co1Mo1K0.6)/(6.4%Co/MWCNTs) catalyst, which was 1.76 and 2.33 times as high as that (87.7 and 66.1 mgh(-1)g(-cat.) (-1)) of the reference systems supported by simple MWCNTs and AC respectively. Ethanol became the predominant product of the CO hydrogenation, with carbon-based selectivity ratio of C2-4-alcohols to CH3OH reaching 3.6 in the products. It was experimentally found that using the Co-modified MWCNTs in place of simple MWCNTs or AC as the catalyst support caused little change in the apparent activation energy for the conversion of CO, but led to a slight increase in the molar percentage of catalytically active Mo-species (Mo4+) in the total Mo-amount at the surface of the functioning catalyst. Based upon the results of TPD investigation, it could be inferred that, under the reaction condition of HAS, there existed a considerably larger amount of adsorbed H-species and CO-species on the functioning catalyst, thus in favour of increasing the rate of a series of surface hydrogenation reactions in HAS.