Synthesis of sulphated transition metal oxides supported on mesoporous silica using direct impregnation method and their catalytic activities
The purpose of the present study is to further develop a method of direct impregnation in the preparation of sulfated titania, zirconia and ferric oxides supported on the mesoporous silica molecular sieve (MCM-41) as acid catalyst composites (C. Chen et al., 2001; Y. Wang et al., 2009) and to characterize their structures and acid nature. Sulfated transition metal oxides such as S-TiO2, S-ZrO2, and S-Fe2O3 as solid superacids have been received much attention due to their significant catalytic activities in hydrocarbon conversions such as esterification, isomerization, alkylation, etc. The remarkable catalytic activities for these sulfated transition metal oxides are mainly attributed to their properties as superacids. The generation and structures of Brönsted and Lewis acid sites are responsible for these activities (Corma et al., 1994; Yadav & Nair, 1999). Unfortunately, the relatively small surface area for the sulfated transition metal oxides may limit their usefulness in catalytic activities (J. Wang & Mou, 2008; Y. Wang et al., 2009). Nano-scale particle mesoporous molecular sieves have advantages in catalysis due to their large pore size, pore volume, uniform pore structures and high specific surface area (Giraldo et al., 2007; Kresge et al., 1992). The large pore size and volume provide adequate diffusion of molecules through the catalyst pores. Fine solid powders with high surface areas ensure probable collisions among the reaction substrates. However, these mesoporous silica materials such as MCM-41 and SBA-15 have their limitation in the low acid strength without the addition of other metal ions (Giraldo et al., 2007). A direct impregnation method has been developed to explore the positive characteristic of both materials discussed above. The sulfated metal oxides continue manifest their remarkable catalytic activities while the high surface area of the mesoporous silica structure ensures a large number of acid sites. The use of direct exchange of metal containing precursors in the as-synthesized MCM-41 substrate produced catalyst composites which were evaluated for the esterification of acetic acid and n-butanol. S-TiO2/MCM-41 and S-ZrO2/MCM-41 catalyst composites exhibit the higher esterification rates than S-Fe2O3/MCM-41. After calcination the MCM-41 and metal compounds were found to agglomerate and the metals dispersed onto as well as inside the mesopore structure. S-TiO2/MCM-41 with up to 80wt% metal loading still retained the MCM-41 mesoporous structure after high temperature calcination. However for S-ZrO2/MCM-41 and S-Fe2O3/MCM-41 composites, the mesopores became blocked with up to 70wt% and 50wt% content loadings respectively. Both Brönsted and Lewis acid sites were detected on the three kinds of catalyst composites in spite of their diminishing in numbers with increasing temperature. For both the acid sites and the structures were responsible for the catalytic performances of S-TiO2/MCM-41, S-ZrO2/MCM-41, and S-Fe2O3/MCM-41 catalyst composites as the solid superacids catalysts in the reactions.