Insighting into the Effect of Acid Sites on the Catalytic Performance of HZSM-5 During the One-Step Preparation of Isoprene by Formaldehyde and Isobutene

Abstract

In this paper, by changing the Si/Al ratio of the catalyst, the reaction temperature and the weight hourly space velocity, the optimal reaction conditions for the preparation of isoprene in one-step condensation of isobutene and formaldehyde on HZSM-5 molecular sieve catalyst was investigated. The catalysts were characterized by N2 adsorption–desorption, X-ray diffraction (XRD), NH3-temperature programmed desorption (NH3-TPD), thermogravimetric-differential scanning calorimetry (TG-DSC) analysis and Fourier transform infrared spectra of the adsorbed pyridine (Py-IR). The catalytic performance of this reaction was studied and the possible reaction network diagram was proposed. It is concluded that the active sites of gas phase condensation between formaldehyde and isobutene were medium strong Lewis acid sites. Meanwhile the strong acid sites would cause side reactions and carbon deposition. It is speculated that the active sites of the reaction of formaldehyde and isobutene are Lewis acid sites on the HZSM-5 catalyst. The mechanism is presumed to activate the formaldehyde molecule at the Lewis acid sites, and then the formaldehyde molecule is condensed with isobutene to form isoprene.