The Modifications Of Mo/HZSM-5 Catalysts And Methane Dehydro-Aromatization In The Absence Of Oxygen

Considerable attention has been focused on methane dehydro-aromatization (MDA) over Mo/HZSM-5 catalysts in the absence of oxygen since the first report from Wang et al. It is now generally accepted that the activation of C-H bond of methane and the formation of the initial C-C bond in intermediates occur on the molybdenum carbide species that are formed in the induction period through the reduction of molybdenum oxides either associated or nonassociated with the Bronsted acid sites of HZSM-5 zeolite, and the residual Bronsted acid sites are responsible for catalyzing the oligomerization and cyclization of C2 intermediates to form stable aromatics. In such a series of reactions, a proper matching between the reactive molybdenum carbide species and the free Bronsted acid sites is crucial for the smooth transformation of C: intermediates to target aromatics, otherwise, Mo/HZSM-5 catalyst will deactivate rapidly due to the heavy formation of carbonaceous deposits under high temperature and nonoxidative conditions.In this thesis, two modifications, steaming-treatment and HCl-treatment, to the distribution of Bronsted acid sites on HZSM-5 zeolite was employed in order to investigate the catalytic performance and catalytic stability of the as-prepared Mo/HZSM-5 catalyst. Firstly, steam-treatment to HZSM-5 zeolite and Mo/HZSM-5 with a steaming time range of 0.5-1 h on the catalytic performance of MDA over Mo/HZSM-5 catalyst prepared with impregnation has been studied in detail, and the deactivation rate constant (kd) and the Bronsted acid sites per unit cell were calculated to quantitatively assess the stability of Mo/HZSM-5 catalysts treated with steam at 813 K before and after impregnation of molybdenum species and the corresponding variation of their Bronsted acid sites. Secondly, HCl-treatment to HZSM-5 zeolite by varing temperature and concentrations of HCl has been performed, investigating the effect of HCl-treatment to HZSM-5 zeolite on the catalytic performance of Mo/HZSM-5 catalysts in MDA reaction.In the meantime, 1H MAS NMR,NH3-TPD,XRD,BET measurements were performed. The results reveal that:(1) There is a V-shape relationship between the deactivation rate constant, kd and the number of free Bronsted acid sites per unit cell, moreover, the closer to 1 the number of B1 acid sites per unit cell, the smaller value the kd gets, representing a catalyst with good stability. But when the number of B1 acid sites per unit cell goes away from 1, the kd value will increase gradually. This type of V-shape relationship could be understood based on the dual functions of free Bronsted acid sites in MDA reaction. (2) After being treated with HCl, the amount of free Bronsted acid sites was modified, and those modified Mo/HZSM-5 catalysts have better catalytic performance for MDA reaction compared with the unmodified Mo/HZSM-5 catalyst. Moreover, there is a good consistency between the amount of Bronsted acid sites and the deactivation rate constant, kd, over Mo/HZSM-5 catalyst obtained by treating HZSM-5 zeolite with 0.1 M HCl for 2h at 303 K, a small kd holds a small amount of B acid sites.