Comparison of sorption capacities on different samples of MCM-41

Aydoğdu, Birsu
MCM-41(Mobil Composition Matter-41) is one of the three members of M41S family and has a highly ordered hexagonal honeycomb like structure with a narrow pore size distribution in mesopore range, high surface area, high pore volume and high thermal stability. These features make MCM-41 proper to use for adsorption, catalysis, ion exchange and separation processes. . In this study sorption capacities of C8 aromatics (o-, m-, p-xylene and ethylbenzene at 30 °C, 50 °C and 65 °C) on a MCM-41 sample synthesized in our laboratory were determined gravimetrically by using a commercial automated electro balance system and compared with results obtained in a previous and similar MSc thesis study with a sample of different origin and characteristics; specifically low BET surface area (492 m2/g). MCM-41 sample was synthesized by hydrothermal synthesis method with cetyltrimethylammoniumbromide (CTAMBr as surfactant) and tetraethyl ortosilicate (TEOS as silica source) in basic conditions. This MCM-41 sample was calcined at 540 oC for 8 h and characterized by XRD, nitrogen adsorption at 77 K, TGA, TEM, SEM and SEM-EDX. According to XRD data, main characteristic peak for synthesized MCM-41 was obtained at 2θ=2.28°. Three small reflection peaks can be seen at 2θ values of 2.59, 4.27° and 4.5°. XRD pattern of the MCM-41, indicated that the desired structure of MCM-41 was successfully synthesized. Surface area, pore volume and average pore diameter were obtained from the nitrogen adsorption data at 77 K as 1154 m2/g, 1.306 cm3/g and 2.75 nm respectively. TGA analysis showed that the 540 oC is proper for the calcination. SEM -EDX analysis gave an oxygen atomic concentration 66.40% and silicon atomic concentration 33.60%. These results showed that the chemical composition of the synthesize material was in almost pure SiO2 form. The adsorbed amount for all isomers at the same pressure decreased as the temperature of the adsorption isotherms increases as expected for physical adsorption. Nitrogen adsorption of MCM-41 in this study showed type IV isotherm with H2 type hysteresis loop according the IUPAC classification. However, for o-,m-, and p-xylene an approximately linear increase in the adsorbed amount as a function of relative pressure was observed from the adsorption isotherms. Except for adsorption isotherms of m-xylene and p-xylene at 65 oC all isotherms of xylenes showed hysteresis loops. Hysteresis loops narrowed down with increasing temperature. p-xylene and m-xylene adsorption isotherms at 65 oC were reversible and did not show any hysteresis loop. Ethylbenzene adsorption isotherms at 30 oC, 50 °C and 65 oC also showed a linear increase in the adsorption amount as a function of relative pressure like xylenes. At 50 °C and 65 oC adsorption isotherms of ethylbenzene were reversible without a hysteresis loop. For all adsorbates volume of adsorbed amounts were calculated on the assumption that they exist as saturated liquids at the isotherm temperature and found to be significantly lower than pore volume obtained from nitrogen adsorption isotherm at 77K. Sorption capacities of these hydrocarbons on MCM-41 were also very low when compared to values found in a previous study which involved a MCM-41 sample of significantly lower surface area ( 492 m2/g ). This may be attributed to structure degradation which requires further investigation.
Citation Formats
B. Aydoğdu, “Comparison of sorption capacities on different samples of MCM-41,” M.S. - Master of Science, Middle East Technical University, 2013.