Effect of pore characteristics on the dynamics of cyclohexane molecules confined in ZSM-5 and MCM-41 molecular sieves: FTIR and QENS study

Abstract

Fourier-transform infrared and quasi-elastic neutron scattering investigations have revealed that the cyclohexane molecules undergo phase transformation on adsorption at room temperature in HZSM-5 and MCM-41 molecular sieves. A comparison with similar studies on polycrystalline silica and zeolite-Y confirmed that the physical state of occluded molecules depended upon the size and the structure of the pores in host material. The confinement in HZSM-5 resulted in the development of non-equilibrium and highly condensed phase of cyclohexane, which remained trapped in the zeolitic pores even after partial desorption. This compression of cyclohexane as a result of adsorption in HZSM-5 is in agreement with the reported phenomena of super-cooling on physical confinement of fluids inside the narrow pores. On the other hand, relatively smaller amounts of cyclohexane were held in the pores of MCM-41 during the adsorption under identical conditions; and for all the loadings below 5 mmol g⁻¹ the guest molecules acquired a transition state the density of which lay between that of the vapor and the liquid phases of cyclohexane. The results of this study are consistent with our recent findings on the formation of a clustered state of benzene during its adsorption into microporous materials (A. Sahasrabudhe, V. S. Kamble, A. K. Tripathi and N. M. Gupta, J. Phys. Chem. B, 2001, 105, 4374; A. K. Tripathi, A. Sahasrabudhe, S. Mitra, R. Mukhopadhyay, N. M. Gupta and V. B. Kartha, Phys. Chem. Chem. Phys., 2001, 3, 4449), and are also in line with the current theories on capillary condensation of fluids in narrow pores.