Adsorption of Alkanes in Zeolites LTA and FAU: Quasi-Equilibrated Thermodesorption Supported by Molecular Simulations

Abstract

Adsorption of n-alkanes from pentane to decane (C5−C10) in zeolites LTA (ITQ-29 and 5A) and faujasite (FAU, dealuminated high-silica Na-FAU, NaY, and NaX) as well as branched hexane isomers in FAU was investigated by means of experimental quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) and theoretical Monte Carlo molecular simulations. A novel QE-TPDA method of studying porosity provided highquality desorption−adsorption profiles and adsorption isobars of hydrocarbons. A unique two-step adsorption of long-chain alkanes in LTA zeolites was observed and explained in detail based on the results of the theoretical calculations. Although adsorption of n-alkanes in FAU is a one-step process, it also does not occur in a uniform manner because of strong lateral interactions at high loadings. We also investigated the influence of cation content on adsorption equilibrium and energetics for the studied adsorbent−adsorbate systems, finding a nonlinear relation between heat of adsorption and content of cations in FAU. Investigations on packing of the molecules of n-alkanes were also performed, revealing relations between chain length of the probe molecule and sorption capacity of the studied materials.

On the Use of Water and Methanol with Zeolites for Heat Transfer

“…HS-FAU is Na + exchanged dealuminated high-silica faujasite with Si/Al > 100, NaY is Na + exchanged faujasite with Si/Al ≈ 2.61, while NaX is Na + exchanged faujasite with Si/Al ≈ 1.06. The characteristics of these materials, i.e., low-temperature nitrogen adsorption and powder X-ray diffraction, were reported in our previous works. , …”

“…For methanol, we used a 4 °C min –1 ramp for all materials, while for water, 2 °C min –1 for NaY and NaX and 1 °C min –1 for HS-FAU were used. It is worth mentioning that adsorption measurements of water in NaX and HS-FAU were reported in our previous work. , Between each cycle, they were kept at RT for 2 h. More details on data reduction and methodology are available in the literature …”

“…These structures contain 88, 56, and 2 Al in the unit cell, respectively, and the same number of Na cations to compensate for the net negative charge of the system. The structural models were reported in previous works, ,,, which were created following the methodology developed by Balestra et al: (i) random distribution of Si atoms following Lowenstein’s rules, (ii) extra-framework cations initially located at their crystallographic positions, and (iii) structural minimization using Baker’s method with full-flexible core–shell potential. , More details about the assembly of the structures can be found in the Supporting Information (Section S1).…”

Adsorption of n-alkanes in ZIF-8: Influence of crystal size and framework dynamics

“…It was also shown that the molecules of n-alkanes are located only in ZIF-8 cages, similarly to LTA zeolites. 43 The QE-TPDA experimental technique was also used to study surface-related adsorption on the ZIF-8. Nanocrystalline ZIF-8 desorbs signicant amounts of pre-adsorbed n-nonane at relatively low temperature.…”

“…Adsorption of n-alkanes in ZIF-8 takes place in the cavities of this material, in a very similar way that we found for LTA-type zeolites. 43 The molecules prefer to locate close to the framework atoms, so the centres of cages are less occupied. Either way, no specic sites for adsorption of C5C9 n-alkanes can be found in the structures of ZIF-8 (Figure S10S14), which was conrmed by the analysis of radial distribution functions (Figure S17).…”