Jiaxu Liu

Effect of alkali metal ion modification on the catalytic performance of nano-HZSM-5 zeolite in butene cracking

Nano-HZSM-5 was modified by different alkali metal ions; the effect of the modification on its acidity and catalytic performance in butene cracking was investigated by using NH3-TPD characterization and catalytic evaluation in a small fixed-bed reactor. The results indicated that although the catalysts modified with various ions of Li, Na and K are different in their metal loading required to achieve the highest selectivity to ethylene and propylene, the best selectivity value obtained over all these modified catalysts is located in 50–60%. The activity of the alkali modified catalysts decreases slowly with the extension of the reaction time on stream; however, the selectivity to ethylene and propylene does not increase accordingly. The conversions of different butene isomers over the zeolite catalysts are decreased in the sequence of butene-1 > trans-butene-2 > cis-butene-2 > iso-butene, which may reflect the order of their reactivity towards cracking.

Catalytic conversion of n-butane over Au-Zn-modified nano-sized HZSM-5

Abstract An Au-Zn-modified HZSM-5 catalyst was prepared using deposition-precipitation and wet impregnation methods. To obtain a better understanding of the relationship between catalyst characteristics and catalytic performance, a thorough study of various catalyst samples (HZSM-5, Au/HZSM-5, Zn/HZSM-5, and Au-Zn/HZSM-5) was performed. The interactions between Au and Zn species in Au-Zn/HZSM-5 were determined using ultraviolet-visible and X-ray photoelectron spectroscopies. Compared with Zn/HZSM-5, the introduction of Au to give Au-Zn/HZSM-5 effectively promotes the dehydrogenation and aromatization of n-butane, and also suppresses hydrogenolysis over Zn species. The n-butane conversion (70.8%) and selectivity for olefins and aromatics (61.98%) increased significantly. The dry gas selectivity also decreased significantly to 28.4%. Au-Zn-containing HZSM-5 is a useful catalyst for the conversion of light alkanes.

Enhancing hydrothermal stability of nano-sized HZSM-5 zeolite by phosphorus modification for olefin catalytic cracking of full-range FCC gasoline

In this study, phosphorus modification by trimethyl phosphate impregnation was employed to enhance the hydrothermal stability of nano-sized HZSM-5 zeolites. A parallel modification was studied by ammonium dihydrogen phosphate impregnation. The modified zeolites were subjected to steam treatment at 800 °C for 4 h (100% steam) and employed as catalysts for olefin catalytic cracking (OCC) of full-range fluid catalytic cracking (FCC) gasoline. X-ray diffraction, N2 physical adsorption and NH3 temperature-programmed desorption analysis indicated that, although significant improvements to the hydrothermal stability of nano-sized HZSM-5 zeolites can be observed when adopting both phosphorus modification strategies, impregnation with trimethyl phosphate displays further enhancement of the hydrothermal stability. This is because higher structural crystallinity is retained, larger specific surface areas/micropore volumes form, and there are greater numbers of surface acid sites. Reaction experiments conducted using a fixed-bed micro-reactor (catalyst/oil ratio = 4, time on stream = 4 s) showed OCC of full-range FCC gasoline—under a fluidized-bed reaction mode configuration—to be a viable solution for the olefin problem of FCC gasoline. This reaction significantly decreased the olefin content in the full-range FCC gasoline feed, and specifically heavy-end olefins, by converting the olefins into value-added C2–C4 olefins and aromatics. At the same time, sulfide content of the gasoline decreased via a non-hydrodesulfurization process. Nano-sized HZSM-5 zeolites modified with trimethyl phosphate exhibited enhanced catalytic performance for OCC of full-range FCC gasoline.

Construction of an operando dual-beam fourier transform infrared spectrometer and its application in the observation of isobutene reactions over nano-sized HZSM-5 zeolite

An operando dual-beam Fourier transform infrared (DB-FTIR) spectrometer was successfully de-veloped using a facile method. The DB-FTIR spectrometer is suitable for the real-time study of the dynamic surface processes involved in gas/solid heterogeneous catalysis under real reaction condi-tions because it can simultaneously collect reference and sample spectra. The influence of gas-phase molecular vibration and heat irradiation at real reaction temperatures can therefore be eliminated. The DB-FTIR spectrometer was successfully used to follow the transformation of isobutene over nano-sized HZSM-5 zeolite under real reaction conditions.

Silicalite-1 zeolite acidification by zinc modification and its catalytic properties for isobutane conversion

A series of Zn-modified Silicalite-1 (S-1) zeolites (Znx/S-1) were prepared by the wetness-impregnation method and applied in the catalytic conversion of isobutane. The structure and location of Zn species in Znx/S-1 were investigated using UV-Vis and N2 physical adsorption. The acidity and origin of the acid sites in Znx/S-1 were studied by NH3-temperature programmed desorption and Fourier-transform infrared analysis. The catalytic performance of Znx/S-1 for isobutane conversion was investigated in a fixed-bed microreactor. In the experiments, the acidity of S-1 zeolite was dramatically increased by modification with Zn, with both Lewis and Bronsted sites identified in Znx/S-1. The relationship between Bronsted acid sites and Zn–OH groups on ZnO clusters of Znx/S-1 was also revealed for the first time. Furthermore, Znx/S-1 catalysts exhibited excellent catalytic performances in both isobutane dehydrogenation and butene isomerization reactions. A high selectivity of total butene products ranging from 84.6 to 97.2 was achieved on the catalysts with different Zn loadings. Moreover, the linear correlation between isobutane conversion and the acid amount (determined by NH3-TPD) confirmed that the weak-to-medium acid sites in Znx/S-1 should play a key role in isobutane conversion.

Isobutane aromatization over a complete Lewis acid Zn/HZSM-5 zeolite catalyst: performance and mechanism

In short-chain alkane aromatization catalyzed by metal-modified HZSM-5, the metal cations acting as Lewis acid sites are considered to be the active centres for the dehydrogenation. However, other possible effects of the metal cations during the reaction have been neglected. In this study, a combination of experiments and DFT calculations has been carried out to investigate the complete effects of the Zn on ZSM-5 in isobutane aromatization. Zn8.47/HZSM-5 (Zn/Al > 1) with weak acidic properties was prepared as a Lewis acid sites-dominated catalyst. It was found that the (Zn–O–Zn)2+ Lewis acid sites acted as the main active sites and no bridged hydroxyl groups (Bronsted acid sites) were present. In isobutane conversion, it was surprisingly found that Zn8.47/HZSM-5 showed better catalytic performance at low temperature than HZSM-5 (Bronsted acid sites-dominated catalyst) and Zn2.34/HZSM-5 (Bronsted and Zn-Lewis acid sites co-existing catalyst). DFT calculations and operando dual-beam Fourier transform infrared spectrometry (DB-FTIR) characterization were employed to understand how the isobutane aromatization could be catalyzed on the Lewis acid site (Zn–O–Zn)2+ exclusively without the participation of bridged hydroxyl groups. It was directly observed over DB-FTIR that the Lewis acid-type Zn/HZSM-5 enjoyed a faster aromatization rate and higher stability than HZSM-5 under real aromatization conditions. This firstly testified that the complete reaction of isobutane aromatization could be exclusively catalyzed by (Zn–O–Zn)2+ Lewis acid sites without the participation of bridged hydroxyl groups. The [Zn-(OH)−]+ species could be produced in situ and acted as the Bronsted acid sites. Owing to the weak acidity of [Zn-(OH)−]+, the Lewis acid sites played a key role during the aromatization reactions via the carbanionic mechanism.

The Crucial Role of Skeleton Structure and Carbon Number on Short-Chain Alkane Activation over Zn/HZSM-5 Catalyst: An Experimental and Computational Study

For the initial activation of short-chain alkanes over Zn/HZSM-5 catalyst, the impact of branching degree and carbon numbers of reactants on the competition between dehydrogenation and cracking was systematically studied by experiment and calculation. The experiments were carried out on fixed-bed flow micro-reactor over HZSM-5 and Zn/HZSM-5 catalysts, by using n-butane/i-butane and propane/n-hexane as reactants with different branching degree and carbon numbers. Compared with HZSM-5, Zn/HZSM-5 obviously accelerated the cracking of C–H bond of short-chain alkanes and increased the selectivity of BTX aromatics. The selectivity to hydrogen produced from n-butane and n-hexane was higher than i-butane and propane, respectively. On the contrary, the selectivity to methane was correspondingly lower, i-butane and propane with high percentage of terminal carbons effectively suppressed the dehydrogenation. The key point to decide the reaction process through dehydrogenation or cracking is the initially activated sites of reactant. To verify this conclusion, theoretical calculations were carried out. The results showed that the (Zn–O–Zn)2+ Lewis acid sites of Zn/HZSM-5 accelerated the cracking of C–H and C–C bond simultaneously. Namely, if the initial activation occurred on the terminal carbons of reactant, the subsequent reaction would be kinetically competitive between cracking and dehydrogenation. Cracking would inevitably occur (thermodynamically favorable), and then the by-products of methane and ethane were produced in large amount. If the initial activation occurred on the internal carbons, the dehydrogenation reaction is kinetically favorable, which is beneficial to reducing the dry gas production. Therefore, cracking is more favorable than dehydrogenation for smaller alkanes and branched alkanes with high percentage of terminal carbons.Graphical AbstractThe percentage of terminal carbons of short-chain alkanes determines the probability of dehydrogenation route over Zn/HZSM-5. The activation of the internal carbon via the dehydrogenation route is more favorable for the normal alkane with higher carbon number during the first stage of conversion suppressing the formation of the by-products of methane and ethane.