Title : Effect of transition metal promoter on cobalt based syngas to olefin synthesis
Abstract:
Fischer-Tropsch to olefin synthesis has attracted much attention, especially to production of olefin feed stock mainly ethylene, propylene and butylene (C2=-C4=).
For selective formation of light olefin, iron based catalysts exhibits high selectivity and activity. However, iron based catalysts suffer from high selectivity to CO2 and much faster deactivation due to carbide formation on catalyst, cobalt-manganese based catalyst has received much attention due to lower methane and CO2 selectivity, especially using natural gas based feed stock.
In present study, prepared cobalt and manganese catalyst was promoted by various transition metals to enhance activity and selectivity to olefins and further reduction of CO2 selectivity. Prepared catalysts were characterized by XRD, N2 adsorption, TPR and ICP and tested for catalytic activity.
Materials and Methods:
Cobalt and Manganese catalyst was prepared using method published elsewhere using sodium carbonate as precipitating agent. Promoter in specified amount was impregnated with aqueous solution of promoter. Catalyst was dried and calcined. Catalyst are denoted as Co/MnOx-A (where A is Cr, Fe, Zn, Cu, and Ga).
Catalysts were shaped in 40-60 mesh size and tested in fixed bed high-throughput reactor at 513 K, 0.5 MPa, 2000 ml/g H2/CO=2. Prior to reaction, catalyst was reduced with 50% hydrogen in nitrogen at 623 K for 8 hours at atmospheric pressure.
Results and Discussion:
XRD pattern shows presence of highly crystalline material with almost identical XRD of promoted and unprompted catalyst with presence of Co-Mn spinel phases (Co2MnO4 and CoMn2O4) with minor presence of CoMnO3 phase, no free oxide phase detected.
Surface area changes with promoter addition. Except Zn, all promoter decrease surface area of catalyst compare to control. Promoters has significant effects on pore volume. Fe and Zn tends to decrease pore volume, while Cu increases pore volume. Cr and Ga has almost no effect on pore volume.
TPR shows reducibility of catalyst changes with promoter.
Catalytic performance at steady state (Table 2) shows Cu and Fe is decreasing catalyst activity, while Cr and Ga has almost no effect on its activity. Zn has positive effect on catalyst activity and olefin selectivity may be co-related to increased surface area and decreased pore size. Fe and Cu increase alcohol selectivity. Zn and Cu also decreases CO2 selectivity. Methane formation remains unaffected with promoter.
Significance:
Zn promoted Co/MnOx catalyst is prominent catalyst for direct production of olefin from syngas. It increases olefin selectivity with reduced CO2 selectivity without affecting catalyst performance.
