Catalysts for Gas-to-Liquids Conversion – Effects of Support Type and Gas Space Velocity

 

Due to high activity of cobalt metal in syngas conversion to higher hydrocarbons, supported cobalt catalysts have been widely used for gas-to-liquids (GTL) conversion. Cobalt is supported on refractory metal oxide, among which Al₂O₃, SiO₂ and TiO₂ have been used most extensively by major companies engaged in development and commercial practice of this technology.

In this study we investigated effects support type and gas space velocity on catalytic performance of 15 wt% Co on three supports (g-Al₂O₃, SiO₂ and TiO₂). Catalysts were prepared by incipient wetness impregnation of supports with aqueous solution of Co(NO₃)₂∙6H₂O. Catalysts were characterized by different methods such as BET, XRD, TPR, and H₂-TPD. Catalytic tests were performed in a fixed-bed reactor at the following conditions: 220 °C, 300 psig, H₂/CO = 2 and gas space velocities of 1 and 3.85 NL/g-cat/h. The catalysts were activated in situ with hydrogen at 375 °C for 12 h.

Physical properties of calcined catalysts are shown in the following Table.

 

Catalyst	BET SA (m2/g)	Pore volume (cm3/g)	Pore diameter (nm)
15 Co/Al2O3	153	0.42	9.8
15 Co/SiO2	221	1.24	20.1
15 Co/TiO2	13.9	0.092	11.8

TPR results show existence of two step reduction of Co₃O₄. Degree of reduction was about 93% (H₂-TPD followed by pulse re-oxidation) and dispersion 8.6% (Co/Al₂O₃) catalyst, whereas cobalt was completely reduced on silica and titania supports. Dispersions on these two supports were 2.4% and 3.2%, respectively.

During testing at SV = 3.85 NL/g-cat/h the alumina supported catalyst achieved the highest CO conversion, 59%, whereas the silica supported catalyst was the least active 8.8% CO conversion. During testing at SV = 1 NL/g-cat/h CO conversion increased and varied from 15.5% (SiO₂ support) to 94% (Al₂O₃ support). Turnover frequencies (TOFs) were consistent with CO conversion trends and ranged from 2.8*10^-3 (s^-1) on silica support to 12*10^-3 (s^-1) on the alumina support. Lower TOFs on the silica and titania supports are ascribed to metal support interactions resulting in coverage of cobalt crystallites with support moieties.

On alumina supported catalyst we observed decrease of methane and increase of C₅⁺ selectivity with increase in CO conversion, however this trend was not observed on the silica and titania supported catalysts. Titania supported catalyst had the best selectivity (6.5% of CH₄ and 89% of C₅⁺ hydrocarbons) at SV = 3.85 NL/g-cat/h (CO conversion of 21.2%).

Acknowledgement

This research was supported by a grant from the Qatar National Research Fund (NPRP 28-6-7-32).