Adsorption structures of non-aromatic hydrocarbons on silicalite-1 using the single-crystal X-ray diffraction method

Abstract

The actual adsorption structures of non-aromatic hydrocarbons on the MFI-type zeolites have not yet been determined. This is due to the presence of twinning, which makes crystallographic analysis difficult. We recently overcame this problem, and now report the various adsorption structures of n-butane, n-pentane, n-hexane, 1-butene, cis and trans-2-butene, 2-butyne and isopentane on silicalite-1 (MFI-type zeolite) as determined via single-crystal X-ray diffraction. The structures were elucidated for both low and high loadings of each guest molecule in order to clarify the adsorption process. The low-loaded structures provide valuable insight into guest-framework interactions and initial adsorption behavior. The n-alkanes are initially adsorbed in the sinusoidal channel, while 2-butyne is adsorbed in the straight channel. In the case of the normal hydrocarbons, the molecular configuration (bent or linear) of the compound determines which channel is the preferred adsorption site. Bent molecules prefer the sinusoidal channel and linear molecules prefer the straight channel. In contrast, isopentane is initially adsorbed at the intersection, since the channels are too narrow to maintain the preferred distance between the framework and the bulky isopentane molecule. In the high-loaded structures, the guest molecules occupy additional sites, such that the normal hydrocarbons are located in both channels and isopentane is found at the intersection and the sinusoidal channel.

Adsorption structure of dimethyl ether on silicalite-1 zeolite determined using single-crystal X-ray diffraction

“…Recently, we have reported the adsorption structures of C 4 -C 6 hydrocarbons (Fujiyama, Seino et al, 2014). Linear 2butyne prefers the straight channel, and bent n-butane prefers the sinusoidal channel as expected.…”

Highly efficient separation of linear and branched C4 isomers with a tailor‐made metal–organic framework

“…Conventional porous adsorbents, such as zeolites and activated carbons, have been used extensively in pressure and/or temperature cyclic configurations. Nevertheless, the disadvantages of low linear/branched isomer separation selectivity and the small uptake capacity of conventional adsorbents limit their practical applications 7,11,19,20 . Among the porous materials, metal–organic frameworks (MOFs) consisting of metal ions/clusters and organic ligands, have triggered enormous research interests due to their great potential to address the ongoing challenges pertaining to physisorption‐based applications 21‐24 .…”

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

“…In order to simplify the analysis, only zeolites that possess straight channels are considered in this work. Although previous studies have demonstrated that the shape traced by the channel path (e.g., straight, sinusoidal) influences alkane adsorption behavior, [19][20][21][42][43][44] zeolites with channels that do not trace straight paths have been omitted because of their small number in the database. However, even among zeolites having only straight channels, the atoms that comprise the channel openings may connect to form circles, ovals, or other shapes, and these shapes are expected to influence the adsorption thermodynamics.…”