Molecular origin of decreased diffusivity with loading for benzene/HY

Adsorption Mechanism of Benzene Alkylation System Catalyzed by an Acid Catalyst: Effect of Pressure

Alkylbenzene is an important chemical intermediate, and its industrial production is mainly through the alkylation reaction of benzene and olefin under the action of an acid catalyst. In this article, the adsorption mechanism of benzene/propylene binary in HY zeolite was first revealed by Monte Carlo molecular simulation. It was found that the adsorption mechanism of benzene and propylene changes at the adsorbate loading of 36 molecules/UC, and benzene plays a dominant role. Below this loading, the adsorption sites of benzene and propylene mainly occupy the "ideal" adsorption sites, and benzene has a "first-hand advantage" toward those sites. Above 36 molecules/UC, benzene and propylene coform "molecular clusters" in the supercage of HY, resulting in the enhanced localization of adsorbates, suggesting that at low and intermediate adsorbate loadings the adsorption property is expected to effectively improve by introducing more superior adsorption sites. However, above 36 molecules/UC, the interaction between adsorbents in the clusters becomes dominant. At this point, it is critical to change the cage-like structure of the micropore and introduce more mesopore to facilitate the disintegration of adsorbates clusters and reduce the steric resistance so that the advantage of adsorption sites can be brought into play. These results lay the foundation for optimizing the operating conditions of alkylation reactions and can be further used to explain the effect of loading on similar adsorption separation or catalytic systems, such as catalytic cracking.

Unraveling the adsorption mechanism of mono- and diaromatics in faujasite zeolite

Monte Carlo simulations are performed to study the adsorption of aromatic molecules (toluene, styrene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, and naphthalene) in all-silica faujasite (FAU) zeolite. For monoaromatics, a two-stage "ideal adsorption" and "insertion adsorption" mechanism is found by careful inspection of locations and distributions of the adsorbed toluene molecules. The validity of this mechanism is confirmed for all monoaromatics considered in the current study. Remarkably, the number of C atoms per unit cell corresponding to the inflection point of adsorbate loading (CI-P) is defined as a valid and convenient characterizing factor in the packing efficiency of monoaromatics in the FAU zeolite. For the case of naphthalene, a type of diaromatic, the three-stage mechanism is proposed, which consists of the first two stages and a third stage of "overideal adsorption". The so-called overideal adsorption is labeled because the naphthalene molecules start to occupy the S site nonideally at loadings that approach saturation, leading to a more localized feature of the adsorbates. The explicit adsorption mechanism can be used to understand the loading dependence of isosteric adsorption heat for the aromatics concerned.