No abstract

No abstract

In situ FTIR spectroscopy has been employed in this study to monitor the binding states of C 6 H 6 and C 6 D 6 molecules in the zeolites of beta family, for the loadings of 0.1-0.6 mmol g -1 at room temperature. Different samples of zeolite beta, varying from each other in Si/Al ratio and comprising of polymorphs B and A in different proportions, were used as adsorbent in these experiments. In all cases, the frequency of the in-plane C-C/C-H stretch vibrations of adsorbate molecules matched with the vibrational spectrum of liquid phase benzene, irrespective of the loading. The bands arising from out-of-plane bending vibrations, on the other hand, were found to undergo splitting, and the frequencies and the relative intensities depended strongly on the presence of Al 3+ cations and the polymorphic composition. The loading-dependent changes in the width and the relative intensity of these two kinds of absorbance bands suggest that the benzene molecules trapped in the zeolitic channels existed in a highly condensed (clustered) state. The electrostatic field associated with the Al 3+ cations and the pore characteristics of the host matrix are found to have considerable influence on the physical state of occluded benzene. Thus, the increase in the polymorph B content in a sample from 55% (BEA, zeolite beta) to 90% (NCL-5) resulted in the constrained transport and adsorption of benzene molecules in the channels of NCL-5, attributed to the more tortuous pore characteristics of the B polymorph. The results of this study also provide strong evidence for the physical perturbation to OH/OD groups by occluded benzene, leading possibly to their tilt toward the zeolite wall. The reoriented hydroxy groups in turn experience a weak hydrogen bonding between themselves, resulting in the broadening of ν(OH) bands and in the lowering of their frequency. A fraction of the hydroxy groups may also convert to water molecules in this process. Furthermore, the entrapped benzene and OH groups are found to undergo reversible H/D isotopic exchange, and the process is promoted on rise in temperature. In conformity with our earlier studies on this subject, no specific interaction is envisaged between the π-bonds of the benzene molecules and the surface hydroxy groups/zeolitic framework sites.

No abstract

No abstract