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, b oth unmodified and modified via controlled deposition of coke, and adsorpti on isotherms were determined. The mechanism of adsorption most probably inc ludes multilayer adsorption at lower relative pressures, followed by capill ary condensation. It is similar for all the adsorbates studied. The sorptio n capacity of the parent samples for benzene and nitrogen is relatively hig h and independent of the Al content. Thus, the Al centers do not substantia lly influence the lyophilic character of MCM-41 in relation to these adsorb ates. In the case of water, however, a decrease in sorption capacity with a n increase in the Al amount implies that clusters of the adsorbate molecule s formed around the Al centers cause a partial clogging of the pores. The A l content is a factor controlling adsorption of water both directly, as a n umber of adsorption centers, and indirectly via influencing the content of coke. Coke strongly decreases both the surface area and the sorption capaci ty of the materials. It also causes a random formation of additional macrop ores. From the adsorption energy distributions, it is inferred that the car bonaceous deposits create centers capable of bonding the benzene molecules.