Analysis of equilibrium and dynamic adsorption of benzene vapor over unimodal and bimodal silica-based mixed-metal oxides

In this study, purification of a benzene-lean gas stream over unimodal and bimodal silica-based mixed-metal oxides was investigated for the purpose of understanding the effects of secondary metal oxide and support pore structure on the adsorption capacities and kinetics. The unimodal silica fell in the borderline of micropores and mesopores, i.e., they consisted of large micropores and small mesopores with large surface area, while the bimodal silica consisted of large micropores, small mesopores and large mesopores with large mesopore volume. The titania and zirconia mixed-metal oxides showed reduced surface area and pore volume as a result of partial blockage of the pores, however, they exhibited improved adsorption behavior relative to the bare silica. Equilibrium adsorption measurements revealed unimodal adsorbents are superior to their bimodal analogues by exhibiting higher benzene vapor uptake, with capacities reaching 10.05, 11.71 and 11.25 mmol/g for SiO2, TiO2/SiO2 and ZrO2/SiO2 samples at 25 degrees C and 101 kPa. In contrast, dynamic breakthrough tests with 465 ppm benzene vapor concentration indicated faster adsorption kinetics for bimodal adsorbents relative to unimodal materials as a result of their larger pores which produced lesser intraparticle diffusion resistance. It was also found that, while titania or zirconia incorporation enhances the equilibrium adsorption capacity of the bare silica, it deteriorates the adsorption rate. Similarly, all the adsorbents depicted partition coefficient in the range of 0.31-1.75 mmol/g/mu M. The mass transfer coefficients estimated from concentration profiles of bimodal SiO2, TiO2/SiO2 and ZrO2/SiO2 samples were 0.58 and 0.45, and 0.41 s(-1), respectively. Overall, the findings of this investigation indicated that the introduction of large mesopores in the structure of microporous-mesoporous silica is a facile approach in developing mixed-metal oxide adsorbents with improved adsorption capacity and kinetics for the abatement of benzene vapor emissions.