Mechanism of Adsorption of Water, Benzene, and Nitrogen on Al-MCM-41 and Effect of Coking on the Adsorption

Water, benzene, and nitrogen were adsorbed on A1-MCM-41 molecular sieves, both unmodified and modified via controlled deposition of coke, and adsorption isotherms were determined. The mechanism of adsorption most probably includes multilayer adsorption at lower relative pressures, followed by capillary condensation. It is similar for all the adsorbates studied. The sorption capacity of the parent samples for benzene and nitrogen is relatively high and independent of the Al content. Thus, the Al centers do not substantially influence the lyophilic character of MCM-41 in relation to these adsorbates. In the case of water, however, a decrease in sorption capacity with an increase in the Al amount implies that clusters of the adsorbate molecules formed around the Al centers cause a partial clogging of the pores. The Al content is a factor controlling adsorption of water both directly, as a number of adsorption centers, and indirectly via influencing the content of coke. Coke strongly decreases both the surface area and the sorption capacity of the materials. It also causes a random formation of additional macropores. From the adsorption energy distributions, it is inferred that the carbonaceous deposits create centers capable of bonding the benzene molecules.