Probing Heterodimer and Multiadsorbate Hydrocarbon Adsorption Trends in the MFI Framework

Acidic zeolites are highly versatile and industrially relevant catalysts and molecular sieves with numerous applications. Zeolites can be functionalized with a range of possible active sites, including Bronsted acid functionalities. Three-dimensional zeolite frameworks such as MFI have been a popular choice in petrochemical applications, such as isomerization, cracking, and oligomerization chemistries. Adsorption is generally considered to be a coverage-dependent phenomenon. Additional complications arise when the adsorbate mixture is not homogeneous and contains multiple molecular identities, resulting in preferential adsorption of certain adsorbate classes at the expense of others. The extent of preference is important in hydrocarbon processing and separations, typically characterized by heterogeneous mixtures of varying molecular identity, size, and shape. Following our previous investigation into the trends of monomer and homodimer adsorption, we quantify heterodimer and multiadsorbate adsorption energies of various combinations of hydrocarbons (alkenes and alkanes) in HZSM-5. We examine heterodimer adsorption trends at three tetrahedral positions in the MFI framework, contrast homodimer and heterodimer adsorption behavior, examine the effect of branching, and investigate the adsorption of trimers, tetramers, and higher. Linear scaling relations between binding energies and molecular size are identified and reported. Alkenes capable of pi-bonding generally adsorb stronger than alkane-only hydrocarbon mixtures. Dependence on the degree of branching as well as the carbon number can provide better estimates for adsorption energies. Additionally, the linear scaling trends reported can likely be extrapolated to larger adsorbates or many more adsorbates in proximity to a single acid site (high loadings). These correlations provide higher accuracy and better understanding of adsorption in mixed-feed separations and bimolecular catalytic reactions.