M-xylene Isomerization Research Articles

Unconventional seed-assisted strategy for Al-rich hierarchical ZSM-48 zeolite

Inferior diffusion capacity and insufficient acid density hinder the practical application of ZSM-48 zeolite. Finding a simple and practical strategy to simultaneously address these two defects remains a challenge. In response to this dilemma, we developed an unconventional seed-assisted synthesis strategy for Al-rich hierarchical ZSM-48 zeolite. This approach allows for achieving a broader range of silica to alumina ratio and accelerates the entire crystallization process through the selection of unconventional seeds. The synergy between the seed and organic template was demonstrated to play a pivotal role in facilitating nucleation. Direct evidence from 1H-29Si CP MAS NMR, TG, and IR results demonstrates that hexamethonium ions (HM2+) electrostatically adsorb at the defect sites on the seed, thereby promoting nucleation sites formation. Smaller seed crystals undergo more etching during the induction period, resulting in additional defects and enhanced nucleation ability. The obtained catalyst exhibits a diffusion time constant (Deff/L2) nine times that of conventional ZSM-48 zeolite when using p-xylene as a probe molecule. In m-xylene isomerization reaction, Al-rich hierarchical ZSM-48 demonstrates excellent stability along with higher selectivity and yield for p-xylene compared to typical ZSM-5 catalysts. Remarkably, long-term testing of 1000 h yields over 22.5 % of p-xylene, indicating the potential of this catalyst as an alternative for xylene isomerization reaction. This work not only advances the practical application process of ZSM-48 catalyst but also provides valuable insights for optimizing other zeolites.

Construction of Submicron Spherical ZSM-48 Zeolite: Crystallization Mechanism and Catalytic Application

Morphology control of zeolite with a one-dimensional channel, especially for ZSM-48 zeolite, is a great challenge due to its inherent characteristics of growth along the main channels. How to shorten the diffusion path of molecules is crucial to alleviating the diffusion limitation. Here, a direct synthesis route for submicron (500 nm) spherical ZSM-48 zeolite was developed with the assistance of F– ions at low crystallization temperature. Through detailed investigation of the role of F– ions and the crystallization mechanism using various characterization techniques, a reliable formation mechanism of spherical ZSM-48 zeolite was proposed. First, F– ions addition shortens the induction period by promoting the dissociation rearrangement of aluminosilicate precursors, which promotes the preferential formation of secondary structural units. Moreover, F– ions induce the formation of ZSM-48 spheres by promoting surface silanol group condensation. ZSM-48 spheres with sizes of 500–2500 nm can be successfully obtained by regulating the crystallization temperature. In m-xylene isomerization, submicron spherical ZSM-48 zeolite exhibits significantly improved stability compared with the conventional one. Moreover, the selectivity for p-xylene is much higher than that of a typical ZSM-5 zeolite (68% vs 48%). The excellent catalytic performance of submicron spherical ZSM-48 highlights not only the positive impact of shortened channels for mass transfer but also its potential as a new catalyst for m-xylene isomerization.

Construction of a One-Dimensional Al-Rich ZSM-48 Zeolite with a Hollow Structure.

Diffusion limitation and acid deficiency are two main challenges that the ZSM-48 zeolite faces in practical application. To date, there have been few effective strategies to solve both problems, simultaneously. Also, it is also a challenge to construct a hollow structure in a one-dimensional (1D) zeolite. Herein, an Al-rich ZSM-48 zeolite with a hollow structure is constructed through an alumination-recrystallization strategy, thereby solving the problems related to diffusion and acidity simultaneously. The hollowness and enrichment of aluminum can be controlled by judiciously matching the desilication and recrystallization. The silica to alumina ratio (SAR) of the ZSM-48 zeolite can be tuned from 130 to 45, which breaks the SAR limitation of conventional synthesis. On the basis of the different characterization results, the whole crystallization can be divided into two stages: rapid desilication-alumination and time-consuming recrystallization. In the selective desilication-recrystallization process, the preferential special distribution of the organic template leads to the formation of a hollow structure and the healing of mesopores at the outer shell, as evidenced by structured illumination microscopy images. Due to the enhancement in diffusion ability and acid density, the obtained hollow Al-rich ZSM-48 zeolite exhibits excellent catalytic stability and high p-xylene yield (∼26%) in the m-xylene isomerization reaction (WHSV = 18 h-1), indicating its strong industrial application potential.

Effects of MTW Zeolite Crystallite Morphology on Product Formation in Isomerization of m-Xylene

Effects of MTW Zeolite Crystallite Morphology on Product Formation in Isomerization of m-Xylene

Preparation of graphitic carbon nitride g-C3N4-HMCM-22 composite catalysts and enhanced para-selectivity in m-xylene isomerization

Preparation of graphitic carbon nitride g-C3N4-HMCM-22 composite catalysts and enhanced para-selectivity in m-xylene isomerization

Gas-phase isomerisation of m-xylene on isoreticular zeolites with tuneable porosity

Nanoporous crystalline aluminosilicates, zeolites, are synthesised by solvothermal method producing three-dimensional (3D) crystals. An alternative, more controllable approach of zeolite synthesis - ADOR – can be used for preparation of isoreticular zeolites with tuneable porosity. Zeolites porosity allows their wide use as isomerisation catalysts due to their shape selectivity effects. Selective isomerisation of m-xylene towards p-xylene is an industrially important reaction due to a high demand for the latter as a substrate for terephthalic acid production. In this work, we investigated the influence of pore size (shape selectivity effect) on the isomerization of m-xylene using a system of isoreticular Al-containing zeolites. These materials, prepared by ADOR approach (UTL, IPC-7, IPC-2, IPC-6, and IPC-4) had different layers connectivity, and therefore various channel systems. We tracked the influence of the pore systems of ADOR zeolites (8- up to 14-ring channels) on the catalytic performance in gas-phase m-xylene isomerisation. We investigated the crystallinity and interlayer distances, phase purity, textural properties of prepared materials, their crystals morphology, and aluminium content. M-xylene isomerisation was carried out in a fixed-bed reactor at 350 °C. ADOR catalysts were compared with standard ZSM-5 zeolite. We show the dependence of zeolite porosity on the performance in isomerisation of m- to p-xylene. Smaller pore zeolites: IPC-4 (8- and 10-ring channels) and IPC-6 (8-, 10- and 12-ring channels) exhibited the lowest conversions, while the highest conversion and p-xylene yields were observed for IPC-2 (10- and 12-ring channels). Presence of extra-large, 14-ring channels (IPC-7 and UTL) resulted in the drop of selectivity due to the xylene disproportionation.

Comparison of Physicochemical Properties and Catalytic Activity in the m-Xylene Isomerization of Catalysts Based on ZSM-12 Zeolites Prepared at Hydrothermal Conditions and under the Action of Microwave Radiation

Comparison of Physicochemical Properties and Catalytic Activity in the m-Xylene Isomerization of Catalysts Based on ZSM-12 Zeolites Prepared at Hydrothermal Conditions and under the Action of Microwave Radiation

Direct Preparation of *MRE Zeolites with Ultralarge Mesoporosity: Strategy and Working Mechanism.

Introduction of mesopore is critical for applications where mass-transport limitations within microporous networks, especially for zeolite with one-dimensional microporous network, hinder their performance. Generally, the creation of mesopore in zeolite through a direct synthesis route is strongly dependent on complex and expensive organic molecules, which limits their commercial application. Here, we successfully developed a facile synthesis route for preparing ZSM-48 zeolite (*MRE topology) with ultralarge mesoporosity in which typical 1,6-hexylenediamine worked as an organic structure-directing agent, innovatively assisted by a simple crystal growth modifier (tetraethylammonium bromide, TEABr). The working mechanism of TEABr during crystallization was revealed and proposed on the basis of TEM, thermal gravimetric mass spectrum, and 13C cross-polarization magic angle spinning NMR characterization results. In the process, TEA+ ions preferentially interacted with the solid during the induction period, which effectively suppressed the aggregation of ZSM-48 primary nanorods. As a result, ultralarge mesoporosity of 0.97 cm3·g-1 was constructed through the stacking of the nanorods. Interestingly, TEA+ ions only took part in the crystallization process and did not occlude in the pores of the final zeolites indicating its potential in recyclability. Moreover, similar synthesis strategy could be applied for the preparation of hierarchical ferrierite zeolites, implying the universality of this strategy. Compared with a conventional sample, ZSM-48 zeolite with ultralarge mesoporosity showed superior catalytic stability in the m-xylene isomerization reaction due to its significantly enhanced diffusion and mass transfer capability, which will greatly promote the practical application of ZSM-48 zeolite.

Efficient synthesis of chain-like ZSM-5 zeolite for the m-xylene isomerization reaction

Chain-like ZSM-5 zeolite with long b-axis length and mesoporosity has been synthesized rapidly and efficiently in supercritical CO2. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images demonstrate that chain-like ZSM-5 zeolite has good crystallinity and chain-like morphology. N2 sorption isotherm indicates that chain-like ZSM-5 has mesopores. The formation of p-xylene from m-xylene isomerization has been tested in catalytic paltform, showing that chain-like ZSM-5 has higher conversion of m-xylene and selectivity of p-xylene, due to the combination of the long b-axis length and the mesoporosity in the chain-like ZSM-5 sample, compared with conventional ZSM-5 zeolite.

Catalytic nanosponges of acidic aluminosilicates for plastic degradation and CO2 to fuel conversion

The synthesis of solid acids with strong zeolite-like acidity and textural properties like amorphous aluminosilicates (ASAs) is still a challenge. In this work, we report the synthesis of amorphous “acidic aluminosilicates (AAS)”, which possesses Brønsted acidic sites like in zeolites and textural properties like ASAs. AAS catalyzes different reactions (styrene oxide ring-opening, vesidryl synthesis, Friedel−Crafts alkylation, jasminaldehyde synthesis, m-xylene isomerization, and cumene cracking) with better performance than state-of-the-art zeolites and amorphous aluminosilicates. Notably, AAS efficiently converts a range of waste plastics to hydrocarbons at significantly lower temperatures. A Cu-Zn-Al/AAS hybrid shows excellent performance for CO2 to fuel conversion with 79% selectivity for dimethyl ether. Conventional and DNP-enhanced solid-state NMR provides a molecular-level understanding of the distinctive Brønsted acidic sites of these materials. Due to their unique combination of strong acidity and accessibility, AAS will be a potential alternative to zeolites.

Facile and seed-direct synthesis of pure EUO zeolite with enhanced catalytic performance

The preparation of pure EUO zeolite under conventional conditions usually takes more than three days, limiting the application of the EUO zeolite. It is widely acquainted that the seed-assisted synthesis of zeolites without organotemplates is inexpensive, environmentally-friendly and lead to Al-rich products. However, synthesis with EUO seeds in the absence of organic-templates always leads to the formation of impurity phases. In this work, EUO zeolite (s-EUO) is successfully obtained by seed-directed synthesis without organotemplates. This is the first seed- directed synthesis of pure EUO zeolite with special Al source. Physicochemical characterization results show that the synthesized s-EUO zeolite exhibits high crystallinity and high specific surface area, and contains uniform crystals and plenty acidic sites. The s-EUO crystallization (by this method) occurs in a considerably shorter time (18 h) and in a more mild condition (160 °C, static state), compared to that of the conventional EUO zeolites (c-EUO, 72 h at 180 °C, at rotation) in the presence of organotemplate. Moreover, the catalytic performance of the H-form of the s-EUO zeolite is enhanced to that of the c-EUO zeolite for m-xylene isomerization, confirming the NH3-TPD results.

Synthesis of mesoporous-zeolite materials using Beta zeolite nanoparticles as precursors and their catalytic performance in m-xylene isomerization and disproportionation

Hierarchical Beta nanozeolites with the properties of both Beta nanozeolite and mesoporous material were prepared by two different methods: (1) Direct Hydrothermal Method (DHM) by the assembly of nanocrystals of preformed Beta seeds in the presence of the surfactant P123 under hydrothermal conditions, (2) Acidification of Beta seeds suspension (without the use of the mesopore directing agent). For the two methods, the effect of crystallization time of Beta nanozeolite precursor (seeds) was studied: 6, 12 and 24 h. The obtained materials were characterized using nitrogen sorption measurements, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), ammonia Temperature Programmed Desorption (NH3-TPD) and adsorption/desorption of pyridine monitored by FTIR. The characterization results show that acidification of Beta seeds suspension leads to best structured nano-Beta (Beta nanoparticles). However, with the DHM method physical mixtures of Beta nanoparticles and mesoporous SBA-15 phases were obtained.The m-xylene isomerization/disproportionation reaction in gas-phase was carried out over all the synthesized solids in order to explore their catalytic properties. It was found that the m-xylene conversion and isomerization/disproportionation ratio were closely related to the crystallization time of Beta zeolite precursors, mesopores creation in hierarchical Beta zeolite nanoparticles and the method used to produce the extra meso-porosity.

Hierarchical porous MCM-68 zeolites: Synthesis, characterization and catalytic performance in m-xylene isomerization

Novel hierarchical MCM-68 zeolite materials were successfully prepared by two simple, effective and environmentally friendly methods: H2O2 microexplosion under microwave and NH3(aq) treatment. The characterization of the materials with powder XRD, SEM, TEM, N2-sorption, and NH3-TPD proved that abundant mesopores across zeolite crystals were created while the microporous MSE framework was well preserved. The mesopore volume of hierarchical MCM-68 zeolites is up to 0.19 cm3 g−1. The crystallinities of hierarchical ones and microporous MCM-68 are nearly the same according to the XRD result. The acidities of hierarchical MCM-68 are generally lower than their raw materials. Gas-phase isomerization reaction of m-xylene was carried out over these hierarchical MCM-68 zeolites as well as HMCM-68 zeolite under different conditions to characterize the catalytic property of newly prepared hierarchical zeolites. It was found that the m-xylene conversion and disproportionation/isomerization rate ratio were closely related to the created mesopores in hierarchical MCM-68 zeolites under high reaction temperature and low weight hourly space velocity (WHSV). Moreover, these hierarchical MCM-68 zeolites exhibited improved resistance to coke formation compared with HMCM-68.

Dealumination of mordenite zeolite and its catalytic performance evaluation in m-xylene isomerization reaction

Mordenite zeolite with Si/Al = 18 was successfully synthesized by the hydrothermal method. This zeolite was modified by partially dealuminated in HNO3 at 70 °C. Our analysis showed that the surface area of modified mordenite was increased more than that of the mordenite zeolite. Acid leaching removes impurities and aluminium from the zeolite network, improving Si/Al ratio of the framework and increasing the external surface area of the zeolite.The catalytic performances of these synthesized catalysts were investigated for meta-xylene isomerization reaction in a fixed bed reactor. The performances were then benchmarked with the performance of SAPO-11, a commercially available catalyst.The results showed that p-xylene/o-xylene ratio over the catalysts was found to be in the order: dealuminated mordenite > mordenite > SAPO-11, however dealuminated mordenite and SAPO-11 gave higher m-xylene conversion than that of the mordenite. KEY WORDS: Dealuminated mordenite, Catalyst, m-Xylene isomerization, SAPO-11 Bull. Chem. Soc. Ethiop. 2017, 31(2), 281-289. DOI: http://dx.doi.org/10.4314/bcse.v31i2.9

Comparison of catalytic performance of synthesized EU-1 zeolite with dealuminated EU-1 zeolite for m-xylene isomerization reaction

EU-1 zeolite was synthesized with high purity by a hydrothermal method and under optimum conditions: synthesis time 72 h, temperature 200°C, and aging time 12 h. Then, the synthesized EU-1 zeolite was modified by dealumination with nitric acid and the changes of the properties such as surface area, pore volume, and Si/Al ratio were investigated. The catalytic performance of these two catalysts was studied and compared with a commercial mordenite catalyst for meta-xylene isomerization reaction in a fixed bed reactor. The results showed that the modification of catalyst with acid increases Si/Al ratio from 25 to 50 due to the removal of a number of aluminium atoms from the framework of zeolite. Also the catalyst surface area increased from 300.237 m2 g‒1 for EU-1 to 333.639 m2 g‒1 for modified EU-1. According to results, the modified Eu-1 had higher para/ortho ratio, meta-xylene conversion, and para-xylene yield than EU-1 and commercial mordenite in the meta-xylene isomerization reaction.

Effect of synthesis parameters on the crystallinity of EU-1 zeolite for the m-xylene isomerization reaction

The EU-1 zeolite with Si/Al = 25 was synthesized by hexamethonium bromide as a template. The effects of crystallization time, synthesis temperature, and the pH of the synthesis gel on the structure and crystallinity of EU-1 zeolite were investigated. The synthesized EU-1 zeolite was characterized by X-ray powder diffraction, BET, Fourier transforms infrared spectroscopy, scanning electron microscope, and inductively coupled plasma. The most effective conditions for synthesis of EU-1 were identified as follow: synthesis time 72 h, temperature 200°C, pH of gel ≃ 13, and aging time of 12 h. The catalytic performance of this catalyst was investigated for meta-xylene isomerization reaction in a fixed bed reactor under the following conditions: pressure of 10 bars, the temperature of 380°C, feed space velocity of 3.35 h–1 and 4 g of the EU-1 catalyst. In comparison to other reported catalysts our synthetic EU-1 showed good m-xylene conversion and high PX/OX ratio.

Mesoporous EU-1 zeolite synthesized in the presence of cationic polymer

Mesoporous EU-1 zeolite (M-EU-1) is rationally synthesized from a starting aluminosilicate gel in the presence of small organic template (hexamethonium bromide, HMBr2) and mesoscale cationic polymer (polydiallyldimethylammonium chloride, PDADMAC). After characterizations of the EU-1 with by XRD, N2 sorption, SEM, TEM, NMR, and TG techniques, it is shown that the sample has good crystallinity, meso/microporosity, high BET surface area (442 m2/g), and large pore volume (0.28 cm3/g). Particularly, the mesoporosity in the M-EU-1 could be adjusted by the amount of mesoscale cationic polymer added in the starting gels. Catalytic tests in m-xylene isomerization show that the M-EU-1 is more stable than the conventional EU-1 synthesized in the absence of the mesoscale template.

Synthesis, characteristics of hierarchical EU-1 zeolite for xylene isomerization probe reaction

Introduced a method of synthesizing hierarchical EU-1 zeolite with organosilanes as additive, and studied the influences of following different kinds of organosilanes on the synthesis of hierarchical EU-1 zeolite: γ-glycidoxy propyl trimethoxy silane (GPTMS), N-β-(aminoethyl)-γ-aminopropyl methyl dimethoxyl silane (APAEDMS), and N-(β-aminoethyl)-γ-aminopropyl dimethoxyl (ethyoxyl) silane (TMPED). The hierarchical EU-1 samples were characterized by XRD, SEM, N2 adsorption, FT-IR and NH3-TPD to analyze the crystallinity, morphology, surface area, pore size distribution and acidity. The results showed that hierarchical EU-1 zeolites were successfully synthesized; organosilanes have great influence on crystal morphology of EU-1 zeolites; the exterior surface area of hierarchical EU-1 zeolite, which synthesized with organosilanes (APAEDMS) adding into synthesis system, increased by 62.1% and mesopore volume increased by 129.1% compared with conventional EU-1 zeolites, thus can reduce the diffusional restriction markedly in catalytic reaction. The catalytic performance of hierarchical EU-1 zeolites were evaluated in m-xylene isomerization on fixed bed reactor. The catalytic data showed that the isomerization activity PX/X of the hierarchical EU-1 zeolites reached around 24.09% in theoretical thermodynamic equilibrium from 23.83%, and the selectivity of C8 aromatic hydrocarbon increased from 75.16% to 84.87%. The conversion of p-xylene increased from 16.30% to 18.41%.

Organotemplate‐Free Synthesis of a High‐Silica Zeolite with a TON Structure in the Absence of Zeolite Seeds

A high-silica zeolite with a TON structure designated as ZJM-4 was successfully synthesized in the absence of any organic templates and zeolite seeds. Many factors such as the SiO2/K2O ratio, Si/Al ratio, SiO2/H2O in the starting aluminosilicate gels, crystallization temperature, and crystallization time strongly influence the formation of the ZJM-4 zeolite. Interestingly, this synthesis shows very efficient silica utilization in the starting gels (>80 %). Catalytic tests on the isomerization of m-xylene to p-xylene show that the ZJM-4 catalyst has high selectivity to p-xylene (ca. 80 %), which could be potentially important for industrial applications for the ZJM-4 zeolite as a candidate catalyst in the future.

Theoretical Investigation of the Isomerization and Disproportionation of m-Xylene over Medium-Pore Zeolites with Different Framework Topologies

The strain energies of three trimethylated diphenylmethane (3mDPM) and six tetramethylated diphenylmethane (4mDPM) isomers serving as the main reaction intermediates of m-xylene isomerization and disproportionation over eight medium-pore zeolites with different framework topologies have been determined theoretically in order to elucidate the effects of zeolite pore structure on this aromatic transformation. Although the strain energies of 3mDPM and 4mDPM derivatives in MCM-22, TNU-9, and NU-87, all of which have large 12-ring cavities/channels, are always lower than 40 kJ mol–1, some of them in cavity-free ZSM-5, ZSM-57, and TNU-10 are characterized by the strain energies higher than 40 kJ mol–1. In particular, all the species in ZSM-22 and ZSM-23 with narrower one-dimensional 10-ring channels have the strain energies much higher than 40 kJ mol–1. On the other hand, the energy difference (<30 kJ mol–1) between the (dimethylphenyl)methylium ion and the transition state for formation of the tetramethylated benzenium-type carbenium ions was calculated to be much lower than the energy barrier (183 kJ mol–1) to the hydride transfer from the reactant molecule. The overall results of this study clearly show that transition-state shape selectivity is responsible for the formation of 3mDPM derivatives, as well as of slightly larger 4mDPM ones, in medium-pore zeolites.