Investigations of the Adsorption of n-Pentane in Several Representative Zeolites

Abstract

We have examined the adsorption of n-pentane in several representative zeolites such as silicalite (MFI), ferrierite (FER), zeolite L (LTL), and faujasite zeolites with FAU structure including siliceous Y (Si-Y) and Na-Y by using FT-Raman spectroscopy in combination with thermogravimetric analysis (TGA) with particular attention being paid to the conformational and dynamic behavior of the guest molecule. The results indicate that the framework topology mainly dictates the conformation of n-pentane in a zeolite. For the zeolites with channel systems such as silicalite, ferrierite, and zeolite L, the population of the all-trans conformer increases upon loading, given that the geometry of the isomer fits better in the channel. When n-pentane is adsorbed in zeolites with a large cavity, such as Si-Y and Na-Y, the distribution of the all-trans (TT) and trans-gauche (TG) conformers is similar to that of pure liquid, suggesting that the large supercage in the framework imposes minimal effect on the conformational equilibrium. The dynamics of the guest molecule is, however, influenced significantly by the existence of cations. Adsorption of n-pentane in a siliceous framework such as silicalite and Si-Y results in extensive molecular motion at room temperature, the degree of which decreases with decreasing temperature. In zeolites ferrierite, L, and Na-Y, the presence of cations in the framework markedly hinders the overall molecular motion. The cations clearly play a role in the observed static disorder of the guest molecule in zeolite L. Important information regarding the location of the n-pentane molecules within silicalite and ferrierite is also obtained.

H−ZSM-5 Modified Zeolite: Quantum Chemical Models of Acidic Sites

“…11 Acidity and reactivity of zeolite materials have also been thoroughly investigated by computational [12][13][14][15][16][17][18][19][20][21] and/or spectroscopic approaches. 3,12,15,[22][23][24][25] Themicrocavitiesandframeworkofthezeolitesystems, 12-14,26-28 together with the chemical composition 13,15,17 of the material, determine a large variety of structural and morphological properties, which play a crucial role in the adsorption, [29][30][31][32][33] and hence in the catalytic activity and selectivity. [34][35][36][37] Of course, the exchange of Si 4+ ions by couples of Al 3+ and H + ions increases the acidic and electrophilic local character of the zeolite framework.…”

Long-Time Non-Debye Kinetics of Molecular Desorption from Substrates with Frozen Disorder

“…These circumstances, in essence, mean that fluctuations of the NaX structure can hardly be considered frozen in experiments [ 2 ] on the adsorption-desorption of n -pentane molecules. The latter ones belong to the type of n -alkanes and are chain-like formations with pronounced anisotropy (see, e.g., [ 23 ]). As a result, due to the presence of rotational and bending degrees of freedom, the adsorbate molecules when interacting with the walls of the micropores can affect the internal structure of the adsorbent.…”

Modern Spectroscopic Methods Applied to Nanoscale Porous Materials

“…37 The framework topology determines the conformation of the sorbed n-pentane molecules. In small-pore zeolites conformers that fit best into the channel are preferential, whereas in large-pore zeolites various conformers exist, indicated that the framework imposes only slightly on the conformational equilibrium.…”

“…10,13,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In addition, FTIR and Raman spectroscopy are very sensitive to atomic bonding and thus gives the opportunity to study active sides and interactions between embedded functional molecules and framework atoms in porous materials. 5,12,[33][34][35][36][37][38][39][40][41][42] Raman spectroscopy compares favourably to FTIR spectroscopy because of the better signal-to-noise ratio in the low-energy spectral range and the better spatial resolution when a microscope is used. Besides, Raman spectroscopy does not require any special sample preparation, which makes this technique a truly non-invasive and non-destructive method for structural analysis.…”