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 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.

Pelletization of Immobilized Amine Carbon Dioxide Sorbents with Fly Ash and Poly(vinyl chloride)

“…Additionally, aromatic benzene interacts with the Si−OH groups of amine/silica sorbents and also the Al−OH groups of zeolite . These results indicate that PS could serve as a pellet binder.…”

“…The spectra of the FA/sorbent powder and ground pellet in Figure a exhibit identical background shapes; therefore, their IR features can be compared directly. Both spectra show a characteristic broad M−OH stretching band for the hydrogen‐bonded M−OH groups (M=Si or Al) of silica and FA between =3670 and 3420 cm −1 , −NH 2 /−NH bands for immobilized EI 423 and TEPA at =3420–3100 (asymmetric/symmetric stretching doublet) and 1606 cm −1 (bending singlet), and −CH 2 /−CH bands for the alkyl groups of the amines and the PVC 62 polymer binder at =3000–2700 (stretching) and 1500–1400 cm −1 (bending). The full spectra of PVC 62 and FA are shown in Figure S3.…”

“…Additionally,a romatic benzene interacts with the SiÀOH groups of amine/silica sorbents [13] and also the AlÀOH groups of zeolite. [21] Theser esults indicate that PS could serve as apellet binder. All of the pellets with the alternative polymerb inderse xhibited reductions of more than 60 %i n their mechanical strengths compared with that of the pellets with comparable M w PVC.T he lower strengtho ft hese pellets is attributed to fewer or weaker interactions of the ÀOH, ÀOC=OÀ,a nd aromatic groups of the polymer binder with the hydroxy and amine groups of the FA and sorbent.…”

“…TheD RIFTS spectra of the FA/EI 423 -S (20/80) powder and ground FA/EI 423 -S (20/80)_PVC 62 pellet, shown in Figure 5, reveal the molecular-scale chemical interactions of the binder with the FA and sorbent. Importantly,t he spectra were collected from samples with small particle sizes and with the same IR parameters to eliminate differences in their overall background shape.T he spectra of the FA/sorbent powder and ground pellet in Figure 5a exhibit identical background shapes;t herefore,t heir IR features can be compared directly.B oth spectra show ac haracteristic broad MÀOH stretchingb and for the hydrogen-bonded MÀOH groups (M = Si or Al) of silica and FA between ñ = 3670 and 3420 cm À1 , [13,21,22] ÀNH 2 /ÀNH bandsf or immobilized EI 423 and TEPAa tñ = 3420-3100 (asymmetric/symmetric stretching doublet) and 1606 cm À1 (bending singlet), [1i, 23] and ÀCH 2 / ÀCH bands for the alkyl groups of the amines [1i] and the PVC 62 polymer binder [24] at ñ = 3000-2700 (stretching) and 1500-1400 cm À1 (bending). Thef ull spectra of PVC 62 and FA are shown in Figure S3.…”

Steam-Stable Basic Immobilized Amine Sorbent Pellets for CO2 Capture Under Practical Conditions

“…Si−OH•••pielectron/toluene associations nearly eliminated the remaining 3735 cm −1 band intensities of all supports. 25,42 Using the 3735/808 IR ratios for toluene-impregnated and fresh FA/MS-10/90 powder, we estimate that 13% of the powder surface area is inaccessible to toluene and presumably bulkier binder molecules. This 13% is near the 12% found for the 22 wt % loaded support.…”