(592h) Modeling the Adsorption of Isobutane/Butene on Hydrophobicity/Hydrophilicity Modified Solid Acid Surface

Piao Cao, Weizhong Zheng, Weizhen Sun, Ling Zhao

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

The alkylate produced by the C4 alkylation reaction with a high-octane number and cleaning burning, is an ideal blending component for gas upgrading. Compared with the traditional concentrated sulfuric acid process, the solid acid catalyst has the advantages of environmental friendliness and easy separation, but the easy deactivation characteristics is the dilemma faced by the industrialization of solid acid process. The key to inhibiting the inactivation of solid acid is to increase the ratio of isobutane/olefins (I/O) in the microenvironment around the active site of solid acid, and to avoid oligomerization of the initially formed carbocation with butene molecules. Researches have shown that the surface modification by grafting functional groups is an effective way to modify the textural, chemical and morohological properties of solid acids, thus influence the adsorption selectivity and catalytic properties. However, the interaction mechanism between surface groups and C4 molecules and the adsorption and diffusion behavior of C4 molecules are still unclear.

In this work, the adsorption behaviors of isobutane and 2-butene on silica surface with a series of functionalized surface groups, i.e. [-OH], [-C₃H₇], and [-C₃H₆-SO₃H], were studied using molecular dynamics (MD) simulation. The transport diffusion coefficients of C4 molecules were obtained in order to assess the effect of surface functionalization with organic groups.

MD simulations reveal a strong interaction of isobutane molecules and butene molecules with the silica surfaces, which induces significant concentration gradients between two silica walls in all systems. In addition, it was found that the hydrophobic modification of the surface by grafting propyl groups could inhibit the adsorption of butane on the surface, thus improving the ratio of I/O which is beneficial to inhibit the polymerization reaction of butene and obtain a higher quality of alkylate. However, the hydrophilic modification of the surface by grafting propanesulfonic acid groups results in a higher ratio of I/O compared to unmodified surface with silanol groups, which can be attributed to the stronger interaction between propanesulfonic acid groups with butene than isobutane. In addition, the butene molecules diffuse faster than isobutane molecules in all simulation systems while the difference in diffusion coefficient between isobutane and butene is minimal in propyl functional system, which indicates the ability of the hydrophobic groups to tune the ratio of I/O in the interface between C4 reactant phase and solid acid catalyst. This simulation study provides a microscopic insight into the adsorption and diffusion behavior of C4 molecules on hydrophobicity or hydrophilicity modified solid acid surface, which can further guide the design and improvement of the solid acid catalyst of isobutane alkylation.