Catalysts For Aromatization Research Articles

Influence of zinc state on the catalyst properties of Zn/HZSM-5 zeolite in 1-hexene aromatization and cyclohexane dehydrogenation

• A two-step wetness impregnation method is beneficial to increase ZnOH + species. • Increasing the amount of ZnOH + species is beneficial to 1-hexene aromatization. • Hydrogen-transfer capability increases linearly with the amount of ZnOH + species. • Dehydrogenation capability increases linearly with the amount of ZnOH + species. Rational design of Zn-containing HZSM-5 zeolite (Zn/HZSM-5) with high reactivity and excellent aromatization performance for olefin aromatization is crucially desired. We develop a new and uncomplicated method to synthesize Zn/HZSM-5 (IMX/Z5) with superior aromatization performance in the paper. Compared to incipient wetness impregnation (IMP/Z5) and mechanical mixing (MIX/Z5), the as-prepared IMX/Z5 presents a higher amount of surface ZnOH + species (2.87%) while keeping identical bulk zinc content. As a result, more surface ZnOH + favor both the aromatization of 1-hexene and cyclohexane dehydrogenation. For the two olefin aromatization pathways (hydrogen transfer and dehydrogenation), it is the first time found both the hydrogen transfer ability and the dehydrogenation ability increase linearly with the amount of surface ZnOH + species while keeping identical bulk zinc content. We believe that the linear relationships are essential to design next generation olefin aromatization catalysts.

High Stability of Methanol to Aromatic Conversion over Bimetallic Ca,Ga-Modified ZSM-5.

The production of valuable aromatics and the rapid catalyst deactivation due to coking are intimately related in the zeolite-catalyzed aromatization reactions. Here, we demonstrate that these two processes can be decoupled by promoting the Ga/HZSM-5 aromatization catalyst with Ca. The resulting bimetallic catalysts combine high selectivity to light aromatics with extended catalyst lifetime in the methanol-to-aromatics process. Evaluation of the catalytic performance combined with detailed catalyst characterization suggests that the added Ca interacts with the Ga-LAS, with a strong effect on the aromatization processes. A genetic algorithm approach complemented by ab initio thermodynamic analysis is used to elucidate the possible structures of bimetallic extraframework species formed under reaction conditions. The promotion effect of minute amounts of Ca is attributed to the stabilization of the intra-zeolite extraframework gallium oxide clusters with moderated dehydrogenation activity.

Effective factors on performance of zeolite based metal catalysts in light hydrocarbon aromatization

Abstract Aromatic hydrocarbons are essential compounds, that the presence of which in fuels can improve the octane number. The conversion of the light alkanes to high value aromatics is vital from theoretical and industrial standpoints. Zeolites such as ZSM-5 play an essential role in the aromatization of light alkanes. This paper highlights the mechanism of aromatization of light alkanes such as methane, ethane, propane, butane, and its isomers. Furthermore, effective factors on the aromatization of light alkanes including metal type, crystallinity, acidity, space velocity, pretreatment of zeolites, co-feeding of light hydrocarbon, and operating factors such as temperature have been investigated to determine how a system of zeolite with metals can be useful to reach aromatization with high conversion.

Insight into the performance of different Pt/KL catalysts for n-alkane (C6–C8) aromatization: catalytic role of zeolite channels

The weakened catalytic role of the zeolite channels might be a considerable reason for the low superiority of Pt/KL catalysts in n-octane aromatization.

Synthesis and consequence of Zn modified ZSM-5 zeolite supported Ni catalyst for catalytic aromatization of olefin/paraffin

Zn modified ZSM-5 zeolite supported Ni bifunctional catalysts were prepared by in-situ hydrothermal synthesis (Zn-Ni-ZSM-5), co-impregnation method (Zn-Ni/ZSM-5) as well as combining in-situ hydrothermal synthesis of Zn-ZSM-5 and sequential impregnation of Ni (Ni/Zn-ZSM-5) and used for the selective converting olefin to aromatics applicable for FCC gasoline hydro-upgrading. The Lewis (L) acid sites generated by ZnOH+ species and hydrogenation active sites from Ni0 species were varied with preparation method, in which their locations at the external surface or micropores of zeolite changed the acid properties of catalysts. Comparing to the Zn-Ni/ZSM-5 from the co-impregnation method, direct in-situ hydrothermal method could incorporate Zn and Ni species within zeolite, which makes the strong interaction between the metal species and the framework Al to form medium-strong L acids (ZnOH+/NiOH+). This leads to the decrease of ratio of Brønsted acid sites to L acid sites (B/L) from 0.40 to 0.12 and the difficulty in reducing of Ni cations to Ni0 from 57% to 30% Ni0 percent. Ni/Zn-ZSM-5 sample prepared by incorporating Zn species into micropores and sequential depositing Ni species at the external surface of zeolite possesses a low B/L ratio similar to Zn-Ni-ZSM-5 (0.20 versus 0.12) but a much higher Ni0 percent (52% versus 30%). Zn-Ni-ZSM-5 shows closer intimacy between Zn and Ni species comparing to Zn-Ni/ZSM-5 and Ni/Zn-ZSM-5. For the 1-hexene aromatization, the formed iso-alkene at acid sites was quickly saturated to iso-alkanes by the adjacent Ni0 particles over Ni-Zn-ZSM-5 catalyst in hydrogen steam. However, the location of aromatization active sites (ZnOH+, L acid sites) in micropores and hydrogenation active sites (Ni0 species) on external surface of Ni/Zn-ZSM-5 makes the tandem conversion of n-alkene → iso-alkene → aromatics, rather than the n-alkene → iso-alkene → iso-alkane. Thus, Zn-Ni-ZSM-5 with close intimacy of ZnOH+ and Ni0 particles shows a high iso-alkane selectivity (27.9% iC4+), while Ni/Zn-ZSM-5 exhibits a high aromatics selectivity (39.6 %) in the conversion of 1-hexene in hydrogen steam. Similar results have also found in the conversion of n-heptane. Here, the control of the intimacy of Zn and Ni in ZSM-5 zeolites has been realized to be a good protocol to adjust the selective conversion of olefins/alkanes to aromatics versus iso-alkanes.

Improved protocol for optimizing Mo/ZSM-5 catalyst for methane aromatization

A systematic DoE approach is developed to optimize Mo-based ZSM-5 catalyst recipe for methane dehydro-aromatization process. Aim is to achieve maximum benzene productivity with improve catalyst stability. Three major factors - Si/Al ratio of ZSM-5, pH of Mo incorporation and catalyst activation along with two different responses - benzene yield and final/peak methane conversion (or in other words the extent of catalyst stability) were considered for constructing the DoE. The benzene yield and final/peak methane conversion vary significantly depending on the catalyst recipe including the catalyst activation protocol/conditions. The combination of Si/Al ratio of 15, pH of Mo incorporation 10 and control catalyst activation by pre-carburization identified through DoE helped to achieve maximum/optimum benzene yield and the catalyst stability. Moderate acidity of zeolite, high Mo dispersion and presence of higher quantity of catalytically active Mo-carbide/oxycarbide species seems to be the reason for high benzene productivity and catalyst stability.

Highly Ordered Mesoporous Cobalt Oxide as Heterogeneous Catalyst for Aerobic Oxidative Aromatization of N‐Heterocycles

AbstractN‐heterocycles are key structures for many pharmaceutical intermediates. The synthesis of such units normally is conducted under homogeneous catalytic conditions. Among all methods, aerobic oxidative aromatization is one of the most effective. However, in homogeneous conditions, catalysts are difficult to be recycled. Herein, we report a heterogeneous catalytic strategy with a mesoporous cobalt oxide as catalyst. The developed protocol shows a broad applicability for the synthesis of N‐heterocycles (32 examples, up to 99 % yield), and the catalyst presents high turnover numbers (7.41) in the absence of any additives. Such a heterogenous approach can be easily scaled up. Furthermore, the catalyst can be recycled by simply filtration and be reused for at least six times without obvious deactivation. Comparative studies reveal that the high surface area of mesoporous cobalt oxide plays an important role on the catalytic reactivity. The outstanding recycling capacity makes the catalyst industrially practical and sustainable for the synthesis of diverse N‐heterocycles.

Precisely regulating acid density and types to promote the stable two-step conversion of methanol to aromatics via light hydrocarbons

Acidic properties of ZSM-5 zeolites for the aromatization of light hydrocarbons strongly determined the final catalytic selectivity and stability for two-step methanol conversions of aromatics via light hydrocarbons. Herein, the density and types of acid sites of ZSM-5 were precisely adjusted to explore their influences on the final catalytic performance. Different from the indistinctive effect of SiO2/Al2O3 ratios on traditional one-step methanol conversion to aromatics, decreasing acid density by increasing SiO2/Al2O3 ratios from 36 to 70 decelerated coke formation and significantly extended the catalytic lifetime from 46.5 to 190.8 h. Further modification with low Zn loading decreased B/L ratio from 19.0 to 1.7 and established a good catalytic synergy between Zn-Lewis and Brønsted acid sites for aromatization of light hydrocarbons, which increased the aromatics selectivity from 27.5% to 31.4%. The decreased catalytic stability after introducing Zn species was attributed to the coke coverage on acid sites but not the final blockage of pore channels as mentioned in previous studies, which was indicated by the similar coke rate of the deactivated catalyst and their gradually increased surface area with increasing Zn content. This work provides more insights into the regulation of acid properties for aromatization catalyst to boost the conversion of methanol to aromatics.

Heterogeneous Diels–Alder tandem catalysis for converting cellulose and polyethylene into BTX

Producing biomass-derived aromatic hydrocarbons via controllable Diels–Alder reactions is a promising approach to recover energy and chemicals from waste streams. A tandem Diels–Alder catalysis consisting of SAPO-34 and Fe/HZSM-5 (stacked catalysis or mixed catalysis) was evaluated for thermochemical conversion of cellulose and polyethylene blends into benzene, toluene, and xylenes (BTX). Aromatization catalyst type significantly affected the activity of tandem catalysis, and the BTX obtained from the HZSM-5 stacked catalysis was ~2.3 times higher than that of the USY stacked one. An introduction of Fe active promoters into HZSM-5 increased the Lewis to Brønsted acid sites molar ratio (L/B) from 0.4 to 4.1. The comparison between Fe/HZSM-5 stacked catalysis and parent HZSM-5 single catalysis indicated that the former was more effective for BTX production, obtaining a nearly two-fold increase in yield with a high selectivity of 82.8%. A close proximity between Fe/HZSM-5 and SAPO-34 in the mixed catalysis increased the BTX enhancement to 1.8. A synergistic effect was provided by the coordination of Lewis and Brønsted acid sites in the Fe/HZSM-5 mixed catalysts for facilitating BTX generation, achieving a maximum of 25.9% at a Fe/HZSM-5 to SAPO-34 mass ratio of 1:1 with a theoretical L/B of 7.2. This work provides a sustainable strategy to produce biomass-derived aromatic hydrocarbons.

A Comparative Study of Aromatization Catalysts: The Advantage of Hybrid Oxy/Carbides and Platinum-Catalysts Based on Carbon Gels

This manuscript is focused on the relationship between sol-gel synthesis processes and the development of new active phases with fitted morphology, porosity and surface chemistry. The influence of the above parameters on the catalytic performance of the prepared materials for the aromatization of n-hexane to benzene is also evaluated. Different series of catalysts were prepared, either using noble metals (i.e., Pt) or metal oxides (i.e., Mo, W), as active phases. In both cases, the catalytic performance and stability of classical aromatization catalysts was significantly improved. Interesting one-pot carboreduction process of the metal oxide during carbonization is suggested as a real alternative for the preparation of high-performance aromatization catalysts, leading to the formation of less acidic and non-stoichiometric oxides and carbides.

Synthesis of Ni(ii)-phosphotungstic acid/nanocrystalline HZSM-5 catalyst for ultra clean gasoline in a single-stage reactor

An excellent catalyst for hydrodesulfurization, aromatization and olefin hydrogenation of FCC gasoline, is designed in this paper.

Highly selective and stable Zn\u2013Fe/ZSM-5 catalyst for aromatization of propane

Light alkane aromatization for aromatic compound production, used in petrochemical industries is an attractive area of research. The effect of second metal co-impregnation was investigated in stabilizing zinc on ZSM-5 in aromatization of propane. HZSM-5 was modified with zinc and iron metal by co wet-impregnation and characterized using XRF, XRD, BET, N2-adsorption, FTIR, FTIR-Pyridine, SEM, TEM, H2-TPR and XPS. The effect of different loadings of Iron on Zn/ZSM-5 was investigated on acidity, aromatic yield, product distribution and aromatization performance. Performance test was conducted in a fixed bed reactor at 540 °C, one atmosphere. GHSV of 1200 mL/g-h. Co-impregnation of Zn with Fe improved the catalytic activity and aromatic yield for 10 h time on stream as compared to parent HZSM-5 and Zn/ZSM-5 of very low aromatic yield and propane conversion. Impregnation of Zn as the dehydrogenating metal on HZSM-5 steadily increased aromatic yield from 5% on HZSM-5 to 25% and was steadily dropped to 20% after 10 h TOS. The co-impregnation of iron of 1–3 wt% loading as the second metal for zinc stability with 2 wt% Zn on ZSM-5 improved propane conversion and aromatic yield to 55% for the 10 h TOS. This further enhanced aromatic product distribution and minimized light gases.

Nickel/gallium modified HZSM-5 for ethane aromatization: Influence of metal function on reactivity and stability

Bimetallic catalysts often outperform their monometallic counterparts due to the synergistic effect. Here we reported that the Ni/Ga co-functionalized HZSM-5 catalyst, specifically with Ni1/Ga1 stoichiometric ratio, shows significantly improved selectivity (above 80 %) and stability than the Ni/HZSM-5 catalyst and higher activity (with identical selectivity) than the Ga/HZSM-5 catalyst in ethane aromatization. According to the transient experiment, the Ga and Ni1Ga1/HZSM-5 catalysts show an induction period of a few minutes. However, for Ni/HZSM-5 and Ni1Ga1/3/HZSM-5, the induction period increased to >10 min and >30 min, respectively, where up to 100 % of ethane was converted into methane and coke through cracking/hydrogenolysis. Based on the generalized power-law equation model, a modified model for deactivation kinetic was proposed. Consequently, the influence of the metal function on the kinetic parameters was discussed. We anticipate the formation of Ni3Ga intermetallic alloy and its synergy with the exchanged Gaδ+ to be responsible for optimal BTX formation.

High Selectivity to Aromatics by a Mg and Na Co-modified Catalyst in Direct Conversion of Syngas.

The demand for aromatics, especially benzene, toluene, and xylene, has been increased in recent years as the crucial feedstocks of coatings and pharmaceutical industry. In this work, a modified Fischer–Tropsch synthesis (FTS) catalyst FeNaMg was fabricated via a sol-precipitation method and integrated with an HZSM-5 aromatization catalyst for the aromatics synthesis from syngas by a one-step process. Syngas was first converted to lower olefins as intermediates on the active component of the FeNaMg catalyst followed by aromatization on zeolite. Different characterization approaches, such as BET, XRD, XPS, hydrogen temperature-programmed reduction, temperature-programmed desorption of CO, TG, and SEM, revealed that Mg efficiently optimized physicochemical properties of the Fe-based catalyst by generating a MgFe2O4 spinel structure. Further investigation demonstrated that the MgFe2O4 spinel structure could increase the syngas adsorption area, facilitating the reduction and carburization of the Fe phase, while Mg decreased CO2 selectivity (31.26 to21%) by restraining the water–gas shift reaction and improved the utilization efficiency of carbon. At the same time, alkali metal Na changed the surface electronic environment of the FTS catalyst to enhance CO adsorption as an electronic promoter, which suppressed methane formation by restraining over hydrogenation. Therefore, the synergism that existed between Mg and Na during the reaction escalated the CO conversion and aromatics selectivity to 96.19 and 51.38%, respectively.

Influence of boron promotion on the physico-chemical properties and catalytic behavior of Zn/ZSM-5 in the aromatization of n-hexane

The characterization and catalytic performance of Zn/ZSM-5 and Zn-B/ZSM-5 catalysts in the n-hexane aromatization was reported. Catalysts were prepared by post-synthesis ion exchange and thoroughly characterized by means of XRD, Py-FTIR, N2 adsorption, 27Al and 11B MAS NMR spectroscopy and electron microscopy techniques. The incorporation of zinc reduces the Brønsted acid site density and increases the Lewis acidic sites of the final ZSM-5 based catalysts. The presence of boron slightly reduces the initial activity of the catalyst but slows down its deactivation rate.

Studies on molybdenum carbide supported HZSM-5 (Si/Al = 23, 30, 50 and 80) catalysts for aromatization of methane

Here in, for the first time we are reporting molybdenum carbide reduction into metallic molybdenum during methane aromatization on HZSM-5 (Si/Al ratio = 23, 30, 50 and 80) at methane space velocity of 1800 mL.gcat−.h−. Benzene yield was influenced by the surface metallic molybdenum through the non-aromatic carbon deposits formation via linear hydrocarbons degradation on HZSM-5 with fewer acidity (Si/Al ratio = 30, 50 and 80). Our XPS analysis results demonstrated improved surface metallic molybdenum in spent Mo2C/HZSM-5 = 80 (0.71 atom. %) and 50 (0.54 atom. %) samples over Mo2C/HZSM-5 = 30 (0.33 atom. %) and 23 (0.20 atom. %) samples. Furthermore, HR-TEM and FFT analysis images clearly established fine distribution of distorted spherical shaped Mo2C particles with 6–14 nm size in spent Mo2C/HZSM-5 = 23. On the other hand, Mo2C particle size was increased upto 22 nm in Mo2C/HZSM-5 = 80. The ease reduction of Mo2C into metallic molybdenum and aggregation of Mo2C particles in spent higher Si to Al ratio (50 and 80) samples was associated with weak interactions between Mo2C and the HZSM-5 with fewer acidity. At 700 °C, the order of benzene yield as follows: Mo2C/HZSM-5 = 80 (2.2%) < Mo2C/HZSM-5 = 50 (3.25%) < Mo2C/HZSM-5 = 30 (5.2%) < Mo2C/HZSM-5 = 23 (8.0%).

Nano-ZrO2 as hydrogenation phase in bi-functional catalyst for syngas aromatization

Oxide-zeolite bi-functional catalyst has shown promising potential in syngas aromatization. As hydrogenation phase in bi-functional catalyst, the preparation method of metal oxide will influence bi-functional catalyst properties. Herein, a series of nano-ZrO2 is successfully synthesized by using UiO-66 pyrolysis, hydrothermal, water/oil interface and sol-gel method. After thoroughly characterized through X-ray diffraction, transmission electron microscopy, N2 adsorption, pyridine-adsorbed infrared spectroscopy, temperature programmed desorption of NH3/CO/H2, X-ray photoelectron spectroscopy, electron paramagnetic resonance and temperature programmed reduction of H2. We find ZrO2 preparation method directly influences its surface oxygen vacancy content, inducing the variation of acid amount and acid distribution over ZrO2 surface. Combine the result of density functional theory calculation with catalyst evaluation, the oxygen vacancy determines ZrO2 catalytic properties. As the catalytic site, oxygen vacancy is responsible for CO activation, and beneficial for the intermediates CH3OH formation. In this case, using hydro-ZrO2 as hydrogenation phase, adjusting the zeolite acidity and the ratio between metal oxide and zeolite, the derived bi-functional catalyst shows 21.59% CO conversion under 1200 ml/(gcat·h), the aromatic selectivity reaches 95.13% in C5+. Furthermore, the catalyst shows excellent stability during a 120 h stability test.

Highly Dispersed Single-Atom Pt and Pt Clusters in the Fe-Modified KL Zeolite with Enhanced Selectivity for n-Heptane Aromatization.

Conversion of straight-chain paraffins into aromatics is particularly attractive but extremely challenging in the oil refining industry. Constructing the Pt-supported catalysts with high aromatic selectivity is vital. Here, we report a strategy to use Fe-modified KL zeolites to improve the Pt atom utilization efficiency and anchor them inside KL zeolite channels via atomic-layer deposition technique. A combination of highly dispersed single-atom Pt and electron-rich Pt clusters is fabricated on the KL zeolite through the creation of proper nucleation sites. The resulted catalyst (PtFe-1/KL) exhibits excellent performance for the n-heptane aromatization (90.1% aromatic selectivity) with an apparent activation energy of 131 kJ/mol and much enhanced stability at a relatively lower temperature (420 °C). Experimental analysis and density functional theory calculation demonstrate that the single-atom Pt might play a key role in the initial dehydrogenation of n-heptane to 1-heptene, and the superior stable Pt clusters encapsulated inside Fe-decorated KL zeolite channels accelerate the 1-heptene dehydrocyclization to aromatics. The synergetic interaction between single-atom Pt and Pt clusters enables the PtFe-1/KL catalyst to be one of the most effective n-heptane aromatization catalysts reported to date.

Enhancing Propane Aromatization Performance of Zn/H-ZSM-5 Zeolite Catalyst with Pt Promotion: Effect of the Third Metal Additive-Sn

In this study, an attempt was made to modify Zn/HZSM-5 aromatization catalyst by successively impregnating Sn and Pt. Among prepared catalysts, the Zn6.0Sn0.2Pt0.1/HZSM-5 catalyst exhibited excellent catalytic performances in both activation and aromatics selectivity. Multi-techniques including XRD, nitrogen physisorption, NH3-IR, TEM, HAADF-STEM/EDX and operando dual beam Fourier transform infrared (DB-FT-IR) spectroscopy were applied to study Zn6.0Sn0.2Pt0.1/HZSM-5 catalyst to gain insight into its active sites and collaboration of them. It is concluded that, in the Zn6.0Sn0.2Pt0.1/HZSM-5 catalyst, the aromatization of propane was catalyzed by the cooperation between Bronsted acid sites (bridging hydroxyls)–Lewis acid sites [zinc species and metallic alloy (SnPt nano clusters)]. Sn had crucial mediation effect which remarkably improved metal dispersion and finely tuned metal–metal interactions. Furthermore, the 3000-h fixed-bed propane aromatization stability test indicated that the Zn6.0Sn0.2Pt0.1/HZSM-5 catalyst also had very prominent anti-coking deactivation ability.

Non-oxidative aromatization and ethylene formation over Ga/HZSM-5 catalysts using a mixed feed of methane and ethane

The effects of the total number of acid site and gallium loading on Ga/HZSM-5 catalysts for non-oxidative aromatization and ethylene formation were investigated using a mixed feed of methane and ethane, analogous to shale gas. When the acid sites of the catalysts increased with the change in the Si/Al ratio, the total conversion of hydrocarbons and the total yield of BTX and ethylene increased, but the selectivity of ethylene tended to decrease. This suggests that the Brønsted acid sites of catalysts is responsible for the dispersion of gallium species and the aromatization of intermediate. On the other hand, when the gallium loading of HZSM-5(15) catalysts increased from 1.0 to 7.0 wt%, the ethylene yield increased, but the BTX yield decreased. The excessive gallium loading resulted in the drastic change of textural properties and the dealumination leading to the decrease of Brønsted acid sites, which caused the decrease of BTX selectivity and the increase of ethylene selectivity. In addition, with the increase of the gallium loading of the HZSM-5(15) catalysts, while the ethane conversion did not change, the methane conversion and ethylene yield gradually increased. This suggests that the dehydrogenative coupling of methane was promoted by the gallium species formed at high gallium loading catalysts such as gallyl ion. Characterizations by N2 adsorption/desorption, ICP-AES, XRD, XPS, TEM, NH3-TPD, pyridine FT-IR, H2-TPR, AAS, and 27Al NMR were used to investigate the relationship between the catalytic performance for aromatization and ethylene formation.