Methanol Aromatization Research Articles

ZSM-5@Zn/ZSM-5 core@shell nanocrystals as integrated catalysts for efficient methanol conversion into aromatics

The use of ZSM-5@Zn/ZSM-5 core@shell nanocrystals as integrated catalyst for efficient aromatic production from methanol aromatization is demonstrated. By combining the attributes associated with core@shell structural and acidic features, ZSM-5@Zn/ZSM-5 exhibited synergistic and improved catalytic performance toward methanol aromatization. The effect of Zn/ZSM-5 shell thickness and the acid density of the ZSM-5 core on synergistic performance was systematically investigated. Results showed that aromatic selectivity could be increased with shell thickness and the acid density of the ZSM-5 core, while catalytic stability was subject to the optimization on both shell thickness and core acid density. The strengthening of dehydroaromatization on the shell catalyst after Zn introduction and the hydrogen transfer for aromatization with increasing core acid density was confirmed. A double boost on catalytic stability from methanol conversion along with 40.7% increase on aromatic selectivity could be achieved for core@shell catalyst with a coating ratio of 0.3 compared with that on single core catalyst. The fabrication of ZSM-5 core with low SiO2/Al2O3 ratio for Zn/ZSM-5 not only provided more acid sites to promote the aromatization, but also promoted the dealkylation of the formed aromatics, which could enhance the selectivity of light aromatics and catalytic lifetime. The findings in the work not only enrich the family of core@shell composite zeolite catalysts, but also provide the fundamental understanding on the aromatization process over core@shell structural features.

Hollow Zeolite Nanoreactor with Double Shells for Methanol Aromatization: Explicit Recognition on Catalytic Function of Inverse Elemental Zone and Shell-Cavity.

Core@shell catalyst composited of dual aluminosilicate zeolite can effectively regulate the distribution of acid sites to control hydrocarbon conversion process for the stable formation of target product. However, the diffusion restriction reduces the accessibility of inner active sites and affects synergy between core and shell. Herein, hollow ZSM-5 zeolite nanoreactor with inverse aluminum distribution and double shells are prepared and employed for methanol aromatization. It is demonstrated that the intershell cavity alleviated the steric hindrance from zeolites channel and provided more paths and pore entrance for guest molecule. Correspondingly, olefin intermediates generated from methanol over the external shell are easier to adsorb at internal acid sites for further reactions. Importantly, the diffusion of generated aromatic macromolecules to the external surface is also promoted, which slows down the formation of internal coke, and ensures the use of internal acid sites for aromatization. The aromatics selectivity of the nanoreactor remained at 8% after 154h, while that of solid core@shell catalyst decreased to 2% after 75h. This finding promises broader insight to improve internal active site utilization of core@shell catalyst at the diffusion level and can be great aid in the flexible design of multifunctional nanoreactors to enhance the relay efficiency.

Self-assembly of hierarchical ZSM-5 particles on rice husk template as hybrid catalysts for boosting methanol aromatization

Herein, natural rice husks with hierarchically porous structures were used as a platform to assemble ZSM-5 particles (denoted as hierarchical ZSM-5) to improve diffusion properties in the methanol-to-aromatics (MTA) reactions. Rice husk could work as the platform to immobilize zeolite grains and provide additional silicon sources for the growth of zeolite particles. Sodium hydroxide amount and SiO2/Al2O3 ratio strongly determined the acid properties, and the rotational crystallization condition favored the zeolite formation. Improved diffusion properties of hierarchical ZSM-5 particles were confirmed by comprehensive pore analysis and benzene diffusion experiments, showing an excellent diffusion property towards aromatic molecules. Compared with commercial microporous zeolite in the catalytic MTA reaction, hierarchical ZSM-5 exhibited superior aromatics selectivity (40.8 vs. 35.8 %), higher liquid hydrocarbon yield (22.3 vs. 14.7 wt%), and longer lifetime (60 vs. 40 h). These results offer new insights into the controllable synthesis of Hier-ZSM-5 with a hierarchically porous structure using biomass as templates.

Controllable synthesis of gallium-containing MFI zeolite for bifunctionally catalyzing methanol aromatization

Ga-MFI zeolites with controllable crystal sizes were hydrothermally synthesized by using colloidal silicalite-1 as seed crystals and their catalytic performances for methanol-to-aromatics (MTA) were evaluated. It was found that the crystal size of as-made Ga-MFI zeolite became small with the adding amounts of seed crystals. Small-sized Ga-MFI zeolite with short channels shows much better coking-resistance and thus longer catalytic lifetime owning to its more favorable outward diffusion of aromatic products. In addition, the stability could be further improved by impregnating gallium species into Ga-MFI zeolite, but which slightly sacrificed its aromatic selectivity. The Ga species with different locations in zeolite play distinct roles in MTA. The weak intrinsic protonic acidity from tetrahedral framework Ga3+ could suppress hydrogen transfer reactions for light alkane formation while non-framework gallium species especially Ga2O3 nanoparticles promoted the dehydrocyclization process. The synergy between the strong Brønsted and weak Lewis acid sites contributes to the excellent aromatic yield.

Suitable location of Lewis acid over ZnOH+/HZSM-5 catalysts effectively enhance dehydrogenation activity and catalyze the aromatization process of C6 olefins in MTA

The aromatization of long-chain olefins is a critical step in the methanol to aromatics (MTA) process, which determines the formation of aromatic products. The role of Lewis (L) acid and the key intermediate are needed to be explored in aromatization process. The dehydrogenation is investigated at different L acid location of ZnOH+/HZSM-5 by the density functional theory (DFT) method. It can be concluded that ZnOH+/HZSM-5 exhibits the best activity with a distance of 3.754 Å between Lewis and Brønsted acids. And the dehydrogenation activity of the catalyst becomes increasingly superior with increasing Bader charges (|e|) of Zn on ZnOH+/HZSM-5 at different location of L acid. 2-methyl-1,4-pentadiene most readily generates benzene among the wide range of C6 olefins, suggesting that it is a vital intermediate in MTA. Therefore, C6 olefins prefer to be dehydrogenated to form dienes and then cyclized to form 5MR, followed by ring expansion and dehydrogenation to form benzene, whereas C6 olefins with branched chains are more active as reactants than that with linear chain. These results can provide valuable ideas for the design and prediction of Zn-modified HZSM-5 catalysts to enhance methanol aromatization activity.

Relay and Feedback Conversion Effect of Hydrocarbons between ZSM-5 Grains on Product Selectivity and Coke Formation during Methanol Aromatization

Relay and Feedback Conversion Effect of Hydrocarbons between ZSM-5 Grains on Product Selectivity and Coke Formation during Methanol Aromatization

Simultaneously enhanced aromatics selectivity and catalyst lifetime in methanol aromatization over [Zn, Al]-ZSM-5 via isomorphous substitution with optimized acidic properties and pore structure

Zinc-modified HZSM-5 zeolites as efficient catalysts have been widely applied in methanol to aromatics (MTA) reactions, however, the relationship between aromatics selectivity and catalytic stability in traditionally loaded catalysts seems to be a stubborn trade-off. To improve both catalytic indicators simultaneously and reveal the relation of the states of introduced Zn species to the consequential influence in physiochemical properties, a series of [Zn, Al]-Z5 zeolites with different Zn contents were prepared via isomorphous substitution under a fixed Si/Me (Me = Al or Al and Zn) ratio. Results revealed that there were three possible states of the introduced Zn species, namely the highly-dispersed ZnO phase, the Zn(OH)+ occupying the ion-exchange sites, and the tetra-coordinated ZnO42- integrated into the zeolite framework. A portion of Zn was successfully integrated into the five-membered rings of zeolite in the form of Si-O(H)-Zn, which played the role of strong Brönsted acid sites and led to the formation of hierarchical structures aside from the intrinsic micropores. Besides, extra-framework Zn(OH)+ was formed by the interaction between dissociate Zn species and the Si-O(H)-Al structure, giving rise to larger proportions of Lewis acid sites. Benefitting from the optimized acidic properties and pore structure, 0.5[Zn, Al]-Z5 exhibited higher aromatics selectivity and longer catalyst lifetime (26.0% and 132 h) than [Al]-Z5 without Zn addition (19.6% and 52 h) in methanol aromatization. Overall, our findings opened a new way to improve the efficiency of MTA reaction and broadened the method to obtain metal-modified zeolite catalysts.

Efficient-selective-catalytic aromatization of methanol over Zn-HZSM-5/SAPO-34: Action mechanism of Zn

To improve the performance and stability of catalytic methanol aromatization, the Zn-modified HZSM-5/SAPO-34(Zn-ZS) composite molecular sieve was synthesized by hydrothermal synthesis, and the modified composite molecular sieve was characterized using XRD, SEM, N2 isothermal adsorption-desorption, NH3-TPD, and XPS, and the catalysts were evaluated in catalytic methanol aromatization reactions. The results showed that the introduction of in-situ Zn greatly changes the morphology of the molecular sieve, presenting a polycrystalline stacked bloom-like structure, which brings abundant micro-pore channels to the molecular sieve, but at the same time, the introduction of metallic Zn also will cause more meso-pore channels losses. The introduction of secondary external Zn brought a large number of weak strong acidic sites, and the amount of molecular sieve acid was nearly doubled; weak acidic sites were strengthened, and the ratio of weak to strong acid amount became smaller. The BTX selectivity of the Zn-doped molecular sieve was significantly enhanced and the anti-coking performance was improved. The composite molecular sieve doped with 2% Zn achieves a 41% selectivity for the BTX within the 47 h lifetime, showing good reaction stability.

Role of Catalyst Domain Size in the Hydrogenation of CO2 to Aromatics over ZnZrOx/ZSM-5 Catalysts

The direct conversion of carbon dioxide into aromatic compounds may provide an environmentally friendly resource for a family of chemical building blocks with high global demand─aromatic chemicals. An array of bifunctional catalysts composed of ZnZrOx for the conversion of CO2 to methanol and ZSM-5 for methanol aromatization is evaluated considering multiple operational and catalytic parameters, with a focus on the particle size of each catalyst functionality. Aromatic yields generally increase as the ZSM-5 domain size decreases for different configurations of catalyst domains─mixed powders, mixed pellets, and separate beds. However, for a physical mixture of the catalyst components prior to pelletization (mixed powder case), varying the ZSM-5 crystal size inversely impacts the particle size of ZnZrOx, leading to enlarged metal oxide domains and enhanced intermediate diffusion barriers for very small crystals of ZSM-5. Larger crystals of ZSM-5 can be employed to obtain a higher concentration of benzene, toluene, and xylene (BTX) among aromatics. Upon employing a physical mixture of the ZnZrOx/ZSM-5 catalyst with a Si/Al ratio of 300 and a ZSM-5 crystal size of 300 nm, a total aromatics selectivity of 44% at CO2 conversion of 6% was obtained at 320 °C, WHSV of 7200 mL gcatalyst–1 h–1, and H2/CO2 ratio of 3.

Intensified shape selectivity and alkylation reaction for the two-step conversion of methanol aromatization to p-xylene

Two-step conversion of methanol to aromatics via light hydrocarbons can significantly improve the conversion stability compared with direct aromatization of methanol, but it remains a challenge to achieve a high p-xylene (PX) selectivity. Herein, silica coating was firstly used to passivate external acid sites of ZSM-5 catalyst for the aromatization of light hydrocarbons by the chemical liquid deposition method. With the increase of SiO2 deposition, the density of the external acid sites of the catalyst was decreased from 0.1 to 0.03 mmol·g−1, which inhibited the surface secondary reactions and increased the PX/X from 34.6% to 60.0%. In view of the fact that the aromatization process in the second step was partly inhibited as methanol was consumed in advance in the upper methanol-to-light hydrocarbons catalyst layer, part of methanol was directly introduced into the lower aromatization catalyst layer to promote the alkylation process during the aromatization, which decreased the toluene selectivity from 34.5% to 14.3% but increased the xylene selectivity from 40.0% to 55.3%. It was also found that an appropriate external acid density was needed for aromatization catalyst to strengthen the alkylation process and improve the selectivity of xylene under the conditions of methanol introduction.

Comparing the effects of hollow structure and mesoporous structure of ZSM-5 zeolites on catalytic performances in methanol aromatization

Comparing the effects of hollow and mesoporous structures over ZSM-5 zeolites on catalytic behaviors in methanol aromatization was done. Hollow zeolites possessing larger voids, increasing Bronsted acid sites and more framework Al in channel crossings promoted higher benzene, toluene and xylene (BTX) selectivity. Zero-length column (ZLC) desorption of cyclohexane stated that higher diffusion rate was observed over mesopores zeolite and hollow zeolite. However, lower effective diffusion constants (Deff) and higher activation energy of hollow zeolite demonstrated that mass transfer was not only limited by microporosity but by surface barriers resulting from surface defects and pore blockage. The highest diffusion rate of cyclohexane over hollow zeolite was presented indicating that surface barrier was not dominant in diffusion limitation. The stability of hollow ZSM-5 was lower compared to mesoporous ZSM-5 as a result of more strong acid sites and higher external silanol groups, which favored to carbon deposition. Additionally, the collapse of hollow structures with thinner shells also contributed to higher deactivation rate.

Synergistic Catalysis of Brønsted Acid, Al-Lewis Acid, and Zn-Lewis Acid on Steam-Treated Zn/ZSM-5 for Highly Stable Conversion of Methanol to Aromatics

The development of stable and selective ZSM-5 catalysts for methanol aromatization remains an ongoing challenge. Here, we demonstrate highly stable aromatization on ZSM-5 by preliminary steam treatment before Zn introduction. Steam treatment destroyed part of frameworks, which decreased acid density from 0.5 to 0.25 mmol g–1 and increased Al-Lewis acid sites (Al-LAS) at the expense of Brønsted acid sites (BAS). However, aromatization activity was further increased and the lifetime was still kept at 100 h. A comparative experiment found that increasing the SiO2/Al2O3 ratio from 60 to 240 also decreased acid density to 0.25 mmol g–1, but BAS-based acid sites aggravated coke formation and reduced lifetime to 50 h. This reflected that Al-LAS and BAS on steam-treated ZSM-5 provided synergistic catalysis for stable aromatization. Introducing Zn into steam-treated ZSM-5 further transformed part of BAS to Zn-LAS but not changed acid density, which determined increased aromatics selectivity and a quite different well-maintained stability.

Effect of CO2 on the catalytic performance of Zn/ZSM-5 towards the conversion of methanol to aromatics

The selectivity of methanol conversion to aromatics on Zn/ZSM-5 is often limited, mainly because the hydrogen removed during the process combines with low-carbon olefin intermediates to regenerate alkanes and reduce the effective utilization of carbon atoms. Herein, CO2 is added to the methanol aromatization reaction as a receptor that consumes hydrogen in situ, which reduces the hydrogen transfer reaction and improves the selectivity of aromatics. It is proposed that aromatic selectivity under CO2 atmosphere is determined by the chemical state of Zn species. Specifically, ZnO converts CO2 and consumes hydrogen, and ZnOH+ promotes methanol aromatization. Under the same conditions, aromatic selectivity of 3ZnZ18-IM under CO2 atmosphere can reach 85.2 %, which is much higher than that of 71.4 % under N2 atmosphere. This study provides a new strategy for methanol to aromatics (MTA) reaction, which has a positive impact on solving the energy and environmental problems in the production of aromatics.

Finely regulating methanol concentration to control the alkylation depth in methanol aromatization for optimizing product distribution

Uncontrollable alkylation in methanol to aromatics severely restricts the light aromatic selectivity and catalytic stability. Herein, we proposed a new strategy to control the alkylation depth by precisely regulating methanol concentration in alkylation environment. Firstly, the high Si/Al ratio ZSM-5 (HZ) was packed over Zn doped low Si/Al ratio ZSM-5 (Zn/LZ) to form dual bed to inhibit deep alkylation for high catalytic stability and BTX selectivity. Secondly, part of methanol was introduced into lower Zn/LZ bed to promote methyl-cyclopentene alkylation for aromatic production and product optimization. After such regulation, toluene selectivity in aromatics decreased from 27.9% to 20.8% accompanied by an increase in xylene selectivity from 37.1% to 44.0%. BTX selectivity in aromatics also improved to 66.7% with 97.5 h of catalyst lifetime, much higher than 50.0% and 20 h of the single Zn/LZ bed. This work realized stable light aromatic production from methanol by controlling the microscopic reaction routes.

Methanol Aromatization over Zn and Ce Modified Hierarchical ZSM-5 Catalysts: Effect of Ce as a Promoter

Zinc (Zn) and cerium (Ce) modified hierarchical ZSM-5 (HZ) catalysts were prepared by incipient wetness impregnation, and their catalytic performances in methanol to aromatics reaction were evaluated in a fixed-bed reactor operating at 400°C under 0.1 MPa and WHSV of 1.0 h−1. The structure, acidity, and state of Zn species of the Zn/Ce modified HZ catalysts were investigated via several characterization techniques, including XRD, NH3-TPD, H2-TPR, nitrogen adsorption, and XPS. The results showed that the aromatization activity of 2 wt% Zn-modified HZ (2% Zn/HZ) could be significantly enhanced by loading 0.6 wt% Ce, which was mainly attributed to the increase in surface ZnOH+ species. Moreover, it was found that Ce inhibited pore blockage caused by Zn. Consequently, compared to 2% Zn/HZ, 2% Zn-0.6% Ce/HZ exhibited higher aromatic selectivity and longer catalyst lifetime.

Strengthening catalytic synergy of two function-complementary ZSM-5 by optimizing their spatial organizations in fixed-bed reactor to boost methanol aromatization

The effect of proximity of two different SiO 2 /Al 2 O 3 ratio ZSM-5 catalysts with complementary acidic properties on their catalytic synergy over methanol stepwise aromatization was specifically studied based on a series of different packing modes. The results showed that the powder mixing mode with high proximity excessively enhanced the synergy by intensifying mass transfer between two catalysts. Although it presented a high aromatics selectivity of 57.2%, but the catalyst lifetime was only 10 h due to the intense reverse of polymethylbenzene. The granule mixing mode with larger particle size only slightly improved the catalyst lifetime. Packing two ZSM-5 catalysts in separated dual beds mode could prolong the lifetime to 157 h but the aromatics selectivity was reduced to 21.0%, indicating that low proximity is not favored the catalytic synergy. Interestingly, the suitable proximity of designed alternative sequence beds mode not only intensified intermediates transport to some extent but also relieved the negative influence of aromatic reversion. The moderate synergy remarkably increased the aromatics selectivity to 28.9% with a satisfactory lifetime of 117 h. This work preliminarily elucidates the effect of proximity determined by the spatial organization of double ZSM-5 catalysts on their catalytic synergy for methanol aromatization. Packing Zn/Z-60 and NZ-500 in alternating sequence beds (packing mode) in the tubular fixed bed observably prolonged catalytic lifetime with a satisfied aromatics selectivity. • The effect of proximity on catalytic synergy of double ZSM-5 for MTA was studied. • The catalyst proximity was precisely regulated based on eight packing modes. • Enhanced mass transfer for aromatization was achieved under high proximity. • Catalytic stability was increased by decreasing catalyst proximity. • Mild catalyst proximity over alternate beds enhanced catalytic synergy for MTA.

Size controllable synthesis of ZSM-5 zeolite and its catalytic performance in the reaction of methanol conversion to aromatics.

ZSM-5 zeolites were hydrothermally synthesized with commercial silica sol, and the crystal size was controlled by adding silicalite-1 seed in the synthetic system. The crystal size of ZSM-5 was affected by the crystallization time of seed, seed content and seed size. ZSM-5 zeolites with controllable particle size in the range of 200–2200 nm could be obtained. The prepared samples with different particle sizes were used for the reaction of methanol conversion to aromatics (MTA). The results suggested that the HZSM-5 catalyst with small crystal size showed much longer catalyst lifetime and higher selectivity for C5+ hydrocarbons and aromatics, especially C9+ aromatics in the MTA reaction. Moreover, the introduction of zinc (Zn) into the HZSM-5 zeolites can considerably promote the selectivity to aromatics in the products. Zn modified HZSM-5 zeolites with different Zn loading amounts were prepared by the incipient wetness impregnation method, and the highest aromatics selectivity was obtained when the Zn loading was 1.0%. The improvement of methanol aromatization was ascribed to the synergistic effect of Brønsted acid sites and the newly formed Zn-Lewis acid sites.

Kinetics on the Integration of Methanol Aromatization with Raffinate Oil over ZSM-5/ZSM-11 Zeolite

Kinetics on the Integration of Methanol Aromatization with Raffinate Oil over ZSM-5/ZSM-11 Zeolite

Coupling of methoxy group with organic matter during methanolysis of heavy hydrocarbon using oil shale as an example

Alcoholysis has been extensively studied for the conversion of heavy hydrocarbons such as coal and biomass, but the roles of alcohols have not been well elucidated. This work aims to investigate the roles of methanol during methanolysis of heavy hydrocarbon with oil shale as an example. It is found that the oil yield of methanolysis is always higher than that of pyrolysis regardless of the reaction temperature but the effect of methanol on the solid product yield is temperature-dependent. At temperatures lower than 290 °C, methanol promotes the oil shale conversion and the solid product yield of methanolysis is lower than that of pyrolysis, which is mainly attributed to the strong supercritical extraction capability of methanol on the oil shale’s pyrolysis products as well as the promoting effect of methanol on reactions of ether oxygen and carboxyl/carbonyl bonds. At temperatures higher than 320 °C, coupling of methoxy moiety in methanol with the organic matter of oil shale and its pyrolysis products as well as the methanol aromatization lead to the formation of oil components such as aromatics, phenols and high boiling point compounds, which result in the increased oil yield. The complex methanol reactions also make methanol become parts of the solid product, leading to a higher solid yield in the methanolysis in comparison with the pyrolysis.

Methanol aromatization over Zn-modified HZSM-5 catalysts derived from ZIF-8

Methanol to aromatics (MTA) is an effective way to produce aromatics by non-petroleum route. The acidity and structure of zeolite catalysts are the key to aromatics yield and stability of catalysts in MTA reaction. Here, Zn-modified HZSM-5 catalysts were prepared by physical mixing method (PM), in-situ hydrothermal synthesis method (SH) and steam-assisted crystallization method (SAC) using ZIF-8 as sacrificial template and Zn source. XRD, SEM, TEM, N2 adsorption-desorption, ICP-OES, NH3-TPD, Py-IR, XPS, H2-TPR, TG characterization methods were used to investigate the physical and chemical properties of the catalysts. The influence of preparation methods of catalysts, the addition amount of ZIF-8, different Zn sources and reaction conditions on the products distribution in MTA reaction were investigated. The results showed that the crystallinity of ZnZ8(SAC)/Z5 catalyst was improved and the hierarchical pore structure was obtained successfully after introduced ZIF-8 by steam-assisted crystallization method. Among them, ZnZ8 mainly exists as ZnOH+ species, which improved the utilization rate of Zn atoms and more L acid sites and moderate acid concentration were formed. The addition amount of ZIF-8 can affect the crystalline size, pore structure and the concentration of ZnOH+ species of ZnZ8(SAC)/Z5. An appropriate ratio of B/L acid concentration was formed when adding an appropriate amount of ZIF-8. Compared with the traditional Z5 catalyst modified with Zn(NO3)2 as Zn source (1% ZnZN(SAC)/Z5), there are smaller particle size, larger specific surface area and mesopore volume as well as suitable acid distribution in 1% ZnZ8(SAC)/Z5 catalyst. The preparation method of catalyst, the addition amount of ZIF-8, different Zn sources and reaction conditions have a significant effect on the selectivity of BTX.