Catalytic Conversion Of I-butane Over Modified Nano-HZSM-5Zeolite

Short-chain alkanes aromatization is an effective way to utilize short-chain hydrocarbon resources. However, the activation of alkanes needs high reaction temperature resulting in large amount of dry-gas (methane and ethane) by-products. So far, the mechanism of short-chain alkanes activation is still controversy and the insight into aromatization mechanism is still limited. Therefore, in this paper, we carry out experimental and theoretical studies on i-butane conversion over ZnO modified nano-HZSM-5catalysts, and get the following results and conclusions:1. ZnO/HZSM-5catalysts with different ZnO loadings were characterized by XRD, UV-Vis, N2physisorb, NH3-TPD and FT-IR. The acidity and the amount of Br(?)nsted acid sites of catalyst decreased simultaneously after the modification of ZnO. At low ZnO loadings, Zn2+is the main Lewis acid site; as the increase of ZnO loading, small ZnO clusters formed " inside channel and large ZnO crystals formed at outer surface both of which possess (Zn-O-Zn)2+active center; at high ZnO loadings,(Zn-O-Zn)2+Lewis acid site is the main active center.i-Butane and propylene were strongly adsorbed on ZnO modified catalysts. The emergence of (-CH2-) group vibration in FT-IR spectra certifies the C-H bond dissociative adsorption of (-CH3) group. The deformation vibration of C-H bond on (-CH3) group of/-butane and propylene in IR spectrum shifts towards low wave number direction at high ZnO loadings, suggesting the existence of strong interaction.2. The result of i-butane conversion over Br(?)nsted-Lewis acid type ZnO2.97%/nano-HZSM-5catalyst at different reaction pressures indicates that C-H bond cracking and C-C bond cracking are primary i-butane activation paths.i-Butene and propylene generated by primary activation could undergo secondary reaction to form C2-C4olefin intermediates. Ethane, propane and aromatics are formed by the deep secondary reactions of olefin intermediates. Reaction pressure has greater influence on i-butane conversion paths than reaction temperature and space velocity. Increasing reaction pressure promotes C-C bond cracking and aromatization of olefin intermediates. However, decreasing reaction pressure promotes C-H bond cracking, inhibits olefin intermediate aromatization and coke deposition.3. The Lewis acid type ZnO10.55%/nano-HZSM-5catalyst could catalyze i-butane aromatization and generate large amount of dry-gas (methane and ethane). DFT calculation results show that [Zn-(C4H9-)]+and (ZnOH)+with weak acidity are formed by i-butane dissociative adsorption over (Zn-O-Zn)2+active center. The dehydrogenation and demethylation of [Zn-(C4H9-)]+generate i-butene and hydrogen, propylene and methane, respectively. These olefin intermediates could oligomerize over (Zn-O-Zn)2+active center via two paths:1). the olefin intermediate directly oligomerize with [Zn-(C4Hg-)]+carbanion to olefin dimer;2). the olefin intermediate could be protonated by (ZnOH)+and react with [Zn-(C4H9-)]+carbanion to form olefin dimer. Then, the formed olefin dimer cyclize with the participation of adjacent (ZnOH)+to aromatic precursor. The above mentioned aromatization process still need the participation of both Lewis acid sites and Br(?)nsted acid sites. However, the acidity of associated Br(?)nsted acid sites is very weak thus the reaction is dominant by Lewis acid sites, which exihibits the character of carbanion. DFT calculation also suggests that the demethylation activation of i-butane over Br(?)nsted acid sites with strong acidity and (Zn-O-Zn)2+is the main source of methane. Moreover, calculation results show that the dehydrogenation activation is favored by dynamics at low reaction pressure while the demethylation activation is favored by thermodynamics at high reaction pressure.4.i-Butane conversion over different metal modified nano-HZSM-5catalysts at low reaction pressure result indicates that the strong interaction between reactant, olefin intermediates and metal active centers is the essence of aromatization ability. Results of i-butane conversion over Pt, Zn, Ga and Mo modified catalysts show that the strong adsorption between Zn, Ga and olefin intermediates at low reaction pressures is the essence of their aromatization activity. DFT calculation results indicate the adsorption ability between active centers and olefin intermediates decreases at the sequence of (GaH)2+>(Zn-O-Zn)2+>Zn2+>H+. The sequence is in accordance with the intrinsic reaction energy barrier of i-butene desorption from these active centers. Compared with Ga, Mo, Pt, Zn modified nano HZSM-5catalyst obtains suitable chemical adsorption strength causing good activity, selectivity and reaction stability.