Influences of ordered mesoporous silica on product distribution over Nb-promoted cobalt catalyst for Fischer-Tropsch synthesis

Abstract

Nb-promoted cobalt catalysts supported on ordered mesoporous silica were prepared by sequential impregnation method and characterized by N2 physisorption, XRD, TEM, EDS-Mapping, H2-TPR, and CO-TPD. The performance of samples and the product distribution were investigated on a fixed bed reactor for Fischer-Tropsch synthesis (FTS). In respect of porous properties, the modification of supports with Nb can reduce the BET surface areas, pore volume, and pore size, possibly owing to the fact that the pore of ordered mesoporous silica was covered partially by NbOx. After the modification of Nb, both the structures of SBA-15 and KIT-6 are very stable as a catalyst support, but that of Co/Nb-MCM-41 was obviously distorted. The formation of smaller cobalt particles occurs in the narrow pore size of support. The presence of NbOx on catalyst surface can significantly weaken the cobalt-support, and accelerate the reduction of cobalt oxides. Meanwhile, NbOx can significantly enhance the adsorption stability and amount of CO adsorbed at FTS reaction condition for Co/Nb-KIT-6 sample. The CO conversion and C5+ selectivity for Co/Nb-KIT-6 sample are 92.1% and 85.1%, which are all higher than other samples supported on SBA-15 and MCM-41, and Co/Nb-KIT-6 sample has a lower CH4 selectivity up to 7.7%. Therefore, the supports with larger pore size are in favour of the formation of larger Co particles, resulting in higher FTS reaction activity. In addition, the introduction of Nb into cobalt catalysts can accelerate FTS activity and the growth of hydrocarbon chains. However, Co/Nb-MCM-41 sample with smaller pores is beneficial to the formation of oxy-compound and its selectivity to alcohols is 4.5%. Wherein, Co/Nb-KIT-6 sample has a higher selectivity to C2+-OH up to 75%. Thus, KIT-6 modified by Nb can be applied as a promising support for FTS catalysts in terms of enriching the range of FTS products.