Benzene adsorption and desorption in mordenite
The zeolite-catalyzed synthesis of cumene from benzene and propene is an industrially important reaction. We used small mordenite crystals to study benzene adsorption and desorption behaviour for sodium, proton and nitric acid treated mordenite. Adsorption of benzene was for all samples fast and completed within 25 seconds at a benzene partial pressure of 0.12 bar in nitrogen at 423 K. The largest benzene uptake was found for the acid treated mordenite 4.5 wt.% followed by the sodium mordenite 4.0 wt.% and the proton mordenite 3.5wt.%. Lower uptake for the proton mordenite could be explained by the presence of minor blockades formed during the ion-exchange and calcination process. The higher uptake for the acid treated mordenite was explained by the partial removal of these pore blockades and more efficient stacking of benzene molecules due to the absence of cations. Desorption rates were very different for the three samples; with Na-MOR 60% of the benzene desorbed in 24 hours, H-MOR in 1 hour and for the acid treated mordenite within 10 minutes. Benzene adsorption isotherms were measured for proton and acid treated mordenite. A simple Langmuir model fit yielded a maximal benzene loading for proton mordenite of 3.0-3.4 wt.%, with isosteric heats of adsorption between 43 and 52 (±5 kJ/mol). For the acid treated mordenite a maximum loading of 5.5 wt.% was found at 423 K, and slightly lower heats of adsorption. The origin of the marked differences in desorption rate is not clear, as it cannot be related to large differences in adsorption strength. However, it is clear that post-synthesis modification is a strong tool to influence the desorption and diffusion behaviour in this system.