Experimental and Simulation Studies of the Stepped Adsorption of Toluene on Pure-Silica MEL Zeolite

Abstract

In this work, the adsorption of toluene on puresilica MEL zeolite is investigated using a variety of experimental and simulation methods. First, we measured the volumetric and calorimetric isotherms at 301 K and at 315 K. The volumetric adsorption isotherm presents a substep at a loading of roughly 4 molecules per unit cell that shifts to higher pressures at higher temperatures and that coincides with a sudden increase in the isosteric heat of adsorption. Grand canonical Monte Carlo simulations reveal that the substep at half load is caused by the adsorption of toluene molecules at different energetic sites within the porous network. According to this, toluene molecules occupy first the intersections, and once all intersections are filled, additional toluene molecules place themselves within the channels. The structure of the adsorbate/adsorbent system was further investigated by performing X-ray powder diffraction experiments of the zeolite at three different loads: empty, at half load (before the substep), and at high load (after the substep). Numerous new low intensity peaks and splittings of existing peaks at the empty and half-loaded diffractograms appear in the diffraction pattern of the high load sample. Atomic structural models compatible with the experimental spectra were obtained by performing N-reverse Monte Carlo simulations. Whereas at half load a good fit of the experimental diffraction pattern can be obtained using the rigid zeolite approximation, at high load, this is only possible when the flexibility of the zeolite is incorporated. In this structural model, the channel cross sections are deformed from a nearly circular shape in the empty zeolite to a more elliptical shape in the case of the high load zeolite.

Study of Short-Chain Alcohol and Alcohol–Water Adsorption in MEL and MFI Zeolites

“…The alkyl chain of ethanol is large enough to favor an adsorption that seems to be more energetic when zigzag channels are present (see Figure 4 and Table 1 below to compare the heats of adsorption). Since intersections are first occupied in the adsorption processes in this type of zeolites, 35 this seems an indication that the zigzag channel intersections should be more energetic than those of linear channels present in MEL. These effects are not visible at all in 1-butanol, whose adsorption is completely dominated by steric effects given its larger molecular size as compared with both MEL and MFI channels.…”

Effect of Silica Alumina Ratio and Thermal Treatment of Beta Zeolites on the Adsorption of Toluene from Aqueous Solutions

“…The X-ray powder diffraction patterns of both as-received and calcined Beta25, Beta38 and Beta360, before and after saturation with TOL are reported in Figure 4. By comparing the X-ray powder diffraction pattern of both the as-received and the calcined materials before and after TOL adsorption (see Figure 4) it can be observed that the peaks intensities in the low 2θ region change thus confirming the incorporation of molecules in the framework due to adsorption, moreover the differences in the patterns in the intermediate and high 2θ region indicates that the process is associated with the framework flexibility (expansion or contraction of the cell volume) [42,43]. Similar behaviour is also shown when the three zeolite samples before and after thermal treatment are compared.…”

Competitive Adsorption of Xylenes at Chemical Equilibrium in Zeolites

“… 37 , 38 Many industrial applications strongly rely on the selective hosting capabilities of zeolites. 39 − 44 …”

“… 73 − 75 Several studies where MCs are used to study adsorption of aromatics in zeolites can be found in the literature. 44 , 76 − 86 Most molecular simulation studies of adsorption of hydrocarbons in zeolites have focused on adsorption from a vapor phase. 78 This is due to the difficult insertion and deletion of molecules at conditions close to saturation, leading to inefficient simulations.…”