Adsorption of n-heptane, toluene and o-xylene on mesoporous UL-ZSM5 materials

Abstract

The mesoporous UL-ZSM5 materials (Si/Al = 100) with different micro- and meso-porosity were synthesized and characterized with respect to their adsorption equilibrium properties. These materials have zeolitic structure in the form of nanoparticles, which are inter-grown in the walls of the wormhole-like mesopores of Al-Meso-100, which was used as a mesoporous precursor in the synthesis. They are considered to be of a great potential for applications in catalysis and separation. Adsorption isotherms of n-heptane, toluene and o-xylene on three samples of UL-ZSM5, i.e., UL-ZSM5-100-2, UL-ZSM5-100-6 and UL-ZSM5-100-8 (100 and 2, 6 and 8 in the sample codes refer to Si/Al ratio and different lengths of crystallization time in days, respectively), as well as the Al-Meso-100 precursor, were obtained using a digital CAHN microbalance at different temperatures. Measurements were also made on ZSM5-100 zeolite, which was used as a reference material. The experimental data were analyzed using the Langmuir–Freundlich (Sips) adsorption isotherm model. Henry’s constants for the three sorbates on Al-Meso-100, UL-ZSM5-100-2 and UL-ZSM5-100-6 were found to be in the increasing sequence of n-heptane < toluene < o-xylene. Furthermore, affinities for toluene and o-xylene on the same samples are as follows: Al-Meso-100 < UL-ZSM5-100-2 < UL-ZSM5-100-6 < UL-ZSM5-100-8. Such adsorption affinities were found to be dependent on the framework microporosity and were consistent with those reported recently for light hydrocarbons adsorption on mesoporous silica SBA-15 materials. The isosteric heats of adsorption estimated by the Clausius–Clapeyron equation are significantly higher for toluene and o-xylene in comparison with n-heptane, and they are strongly correlated to the sorbate loadings. The sharp initial decrease of isosteric heats of adsorption with respect to coverage, which shows different behavior for the different sorbate/adsorbent systems, is discussed in some detail.