Vapor Phase Beckmann Rearrangment Over Modified Al-MCM-41 Molecular Sieves

Vapor phase Beckmann rearrangement reaction of cyclohexanone oxime to caprolactam was studied over modified Al-MCM-41 catalysts in this thesis. A series of phosphorus (P) and boron (B) modified Al-MCM-41 molecular sieves were prepared using impregnation method. The activity of these catalysts was studied by a continuous flow fixed-bed reactor. All the catalysts were characterized by XRD, SEM, N₂ adsorption, FT-IR, NH3-TPD, MAS NMR, UV-Vis, XPS, TGA, and GC-MS. The coke and catalytic activity sites of the catalyst were also studied. The possible molecular structure of P modified Al-MCM-41 catalyst and the possible reaction mechanism was investigated by computational chemistry method.P modified Al-MCM-41 catalyst can improve the selectivity for caprolactam. It increased from 64% to 80%. Nitriles and ketones are main side products. With increased temperature, more nitriles and less ketones were generated. No conversion decline was observed during the 40 hours process time, indicating the high stability of PAM-3 catalyst. More weak acid sites are favorable for caprolactam selectivity.B modified Al-MCM-41 catalyst can also improve the selectivity for caprolactam. It can reach 78% in this work. Boron can improve caprolactam selectivity. However, boron content does not linearly influence caprolactam selectivity. The acitivity of boron modified Al-MCM-41 catalyst was influenced by boron distribution in the catalyst. The conglomeration of boron on the catalyst decreased the selectivity of caprolactam and finally led to catalyst deactivation. There is a close relationship between caprolactam and weak acid sites, especially weak Br?nsted acid sites. And these acid centers are acitive sites.Coke formation on the catalyst is the main reason for its deactivation. The coke was mainly amorphous structure. Both soft coke and hard coke were deposited on the used boron modified Al-MCM-41 catalyst. And the coke was deposited preferentially on acid sites, especially the strong acid sites. The coke deposited on the catalyst is mainly aliphatic-type carbonaceous compounds and no aromatic-type coke formed. The catalyst deacitivition was attributed to the pore and active sites being blocked by the excessive dehydrogenated carbon species.The Computational Chemistry results reveal that the Geometry B is the possible structure of P modified Al-MCM-41 catalyst. The vapor phase Beckmann rearrangement reaction mechanism proposed by Nguyen et al is reasonable. A transfer of R1 group is a rate determining step, wihle 1,2-H shift reaction is not.