Influence of Pore Structure and Framework Al Sites on the State of Benzene (C6H6 and C6D6) Molecules Entrapped in the Zeolites of BEA Type: In Situ IR Spectroscopy Study

Abstract

In situ FTIR spectroscopy has been employed in this study to monitor the binding states of C6H6 and C6D6 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 Al3+ 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 Al3+ 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.