Adsorption and Structure of Benzene on Silica Surfaces and in Nanopores

Abstract

Grand canonical Monte Carlo simulations are used to study the adsorption of benzene on atomistic silica surfaces and in cylindrical nanopores. The effect of temperature and surface chemistry is addressed by studying the adsorption of benzene at 293 and 323 K on both fully and partially hydroxylated silica surfaces or nanopores. We also consider the adsorption of benzene in a cylindrical nanopore of diameter D=3.6 nm and compare our results with those obtained for planar surfaces. The structure of benzene in the vicinity of the planar surface and confined in the cylindrical nanopore is determined by calculating orientational order parameters and examining positional pair correlation functions. The density profiles of adsorbed benzene reveal the strong layering of the adsorbate, which decays with the distance from the silica surface. At a given temperature and at low pressures, the film adsorbed at the fully hydroxylated silica surface is larger than that for the partially hydroxylated silica surface. This result is due to an increase in the density of silanol groups that induces an increase in the polarity of the silica surface, which becomes more attractive for the adsorbate. Our results also suggest that the benzene molecules prefer an orientation in which their ring is nearly perpendicular to the surface when fully hydroxylated surfaces are considered. When partially hydroxylated surfaces are considered, a second preferential orientation is observed where the benzene ring forms an angle of approximately 50 degrees with the silica surface. In this case, the average orientation of the benzene molecules appears disordered as in the bulk phase. These results suggest that determining the experimental orientation of benzene in the vicinity of a silica surface is a difficult task even when the surface chemistry is known. Capillary condensation in the nanopores involves a transition from a partially filled pore (a thin film adsorbed at the pore surface) to a completely filled pore configuration where the confined liquid coexists at equilibrium with the external gas phase. The disordered orientation of the adsorbed benzene molecules in the case of the partially hydroxylated surface favors the condensation of benzene molecules (the condensation pressure for this substrate is lower than that for the fully hydroxylated surface). Finally, these results are consistent with the structural analysis, showing that (1) benzene tends to relax its liquid structure a little in order to optimize its molecular arrangement near the pore wall and (2) the disordering of the liquid structure induced by the surface becomes stronger as the interaction with the pore wall increases.

Monte Carlo Simulation on Adsorption Properties of Benzene, Toluene, and p-Xylene in MCM-41

“…Figures 4 and 5 show that for a given adsorbate molecule and a given D, the ρ r peaks of the molecules located close to the pore wall just below P c are almost similar to those above P c , suggesting that the adsorption on adsorptive sites close to the pore wall has already been finished below P c . A similar phenomenon for benzene adsorption in MCM-41 was reported by Coasne et al 23 This phenomenon can probably be attributed to the strong interaction between the adsorbate molecule and adsorptive site. The average densities of the adsorbates in the two different pores are calculated to be 0.75, 0.72, and 0.707 g/ cm 3 for benzene, toluene, and p-xylene, respectively (see Figure 2).…”

Molecular simulation of the adsorption and structure of benzene confined in mesoporous silicas

“…In future work, we plan to study the adsorption of benzene on silica surfaces and nanopores (Coasne et al 2007) and compare with available experimental data Ribeiro Carrott et al 2001). We will investigate the effect of the surface chemistry by varying the density of OH groups of the silica substrate.…”

Mesoporous silica nanoparticle film as sorbent for in situ and real-time monitoring of volatile BTX (benzene, toluene and xylenes)

“…Indeed, BTX adsorption may occur in mesoporous silica nanoparticle film at different energy levels since the sites of adsorption are displaying different curvatures and accessibilities. This interpretation is reinforced by a recent molecular simulation study of the adsorption of benzene on atomistic silica surfaces and nanopores [34].…”