Abstract
Quasi equilibrated temperature programmed desorption and adsorption (QE-TPDA) of hexane and cyclohexane was applied for characterization of zeolites 5A, ZSM-5, 13X, Y, NaMOR and ordered mesoporous silicas MCM-41, MCM-41/TMB, SBA-15 and HMS. Similar QE-TPDA profiles of hexane and cyclohexane with a single desorption maximum were observed for the wide pore zeolites. No adsorption of cyclohexane for zeolite 5A and a single desorption maximum for ZSM-5 were found, while two-step desorption profiles of hexane were observed for these zeolites. Similar values of the adsorption enthalpy and entropy of hexane and cyclohexane were obtained by fitting the Langmuir model functions for the zeolites X and Y. For NaMOR and ZSM-5 larger differences in these parameters were found. A single desorption peak found at low temperatures in the QE-TPDA profiles of hexane and cyclohexane for the studied silicas was attributed to the multilayered adsorption on their mesopore surface. The adsorption isobars calculated from the thermodesorption profiles were fitted with the BET function. This way values of the specific surface area and the adsorption heat were calculated. Additionally values of the initial heat of adsorption were found by fitting the Henry’s law to the high-temperature sections of the linearized isobars. The largest deviations from the BET and Henry functions and the largest values of the adsorption heats found for SBA-15 indicated the greatest heterogeneity of the adsorption sites on its surface.
Highlights
Porosity of solids plays an important role in many technological applications, mainly in separation and heterogeneous catalysis
The two step profiles of hexane observed for 5A and ZSM-5 zeolites are in agreement with the earlier findings (Sivasankar and Vasudevan 2005) and may be explained in terms of the ‘‘commensurate freezing’’ effect, i.e. an ordering of the molecules adsorbed in the three dimensional micropore system, resulting from matching of its periodicity with the length of the molecule (Smit and Maesen 1995)
For the wide pore zeolites the profiles observed for both hydrocarbons are quite similar, only in the case of NaMOR intensity of the thermodesorption peaks was smaller for cyclohexane
Summary
Porosity of solids plays an important role in many technological applications, mainly in separation and heterogeneous catalysis. Main tools for characterization of porosity are based on measurements of physisorption isotherms. Low temperature N2 adsorption is standard method for determination of the specific surface area, micro- and mesopore volume, and the pore size distribution. Extensive reference data and numerous theoretical models and computational methods are available for various porous materials, e.g. activated carbons and micelle templated mesoporous silicas. For low silica zeolites, N2 adsorption is not appropriate for porosity characterization, as quadrupole interactions of N2 molecules with the extraframework cations and the framework Al atoms result in prolonged equilibration times and pore filling patterns that do not correlate with the micropore sizes (Thommes 2010). Adsorption of Ar at 87 K gives much better results, but it is still not widely used
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