AN IN-SITU C-13 MAS NMR-STUDY OF BENZENE ISOPROPYLATION OVER H-ZSM-11 - CUMENE FORMATION AND SIDE-REACTIONS

C-13-MAS NMR in the adsorbed phase was performed in situ to investigate the mechanism of the main and side-reaction pathways of benzene isopropylation over H-ZSM-11. Propene 1-C-13, propene 2-C-13 and benzene (1-6)-C-13 were used as labelled reactants in different runs. Cumenes selectively labelled with C-13-isotopes either on alpha- or on beta-positions of the alkyl chain or in the aromatic ring were synthesized in situ at room temperature with 100% yield when the excess of benzene was used. In contrast, when the molar ratio of the reacting propene and benzene was equal to 1, alkylation was accompanied by propene oligomerization, followed by isomerization and cracking of oligomers and sec-butylbenzene formation. The results on tracing the fate of 1-C-13 and 2-C-13 carbon atoms of propene in course of benzene isopropylation are in line with classical carbenium ion alkylation mechanism. Desorption and/or diffusion of cumene is supposed to be a rate limiting step. The side-reactions become significant at 473 K due to cumene conversion. The main product of cumene conversion is n-propylbenzene, formed via intermolecular reaction of cumene and benzene. At long reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of the alkyl chain carbon atoms of both cumene and n-propylbenzene. Formation of cracking products, i.e., toluene, ethylbenzene and butylbenzenes is also observed. The rate of isomerization is higher than the rates of scrambling and fragmentation. External surface passivation by silica deposition was performed to differentiate processes taking place at the external surface or inside the channels. All reaction pathways were found to proceed on both external and internal zeolite surface.