P-xylene Research Articles

Regulating anisotropic diffusion in zeolite for reinforced para-xylene synthesis via CO2 hydrogenation in the presence of toluene

The synthesis of valuable aromatics via CO2 transformation, especially para-xylene (PX), is of paramount significance but still remains greatly challenging due to the low efficiency and poor selectivity. The present work utilized a composite catalyst based on ZnZrOx with a modified H-ZSM-5 exhibiting nano-prism stacking to realize the selective synthesis of PX. Methoxy species generated from CO2-derived formate produced as an intermediate over the ZnZrOx were readily incorporated into the xylene products for effective alkylation, contributed to an enhanced aromatics-based cycle that provided a selectivity for xylenes among all C5+ hydrocarbons of 91.1 %. The lengthened straight channels in the H-ZSM-5 along the b-axis, derived from the crystal stacking pattern, increased differences in the diffusion properties of the xylene isomers, and thereby steering the movement of molecules toward a product shape-selective pathway, leading to 90.1 % selectivity for PX among all xylenes, and provided exceptional CO2 utilization efficiency of ∼60 %.

Source differentiation of BTEX compounds in groundwater contaminated due to refinery activities

This study aims to identify sources of groundwater contamination in a refinery area using integrated compound-specific stable isotope analysis (CSIA), oil fingerprinting techniques, hydrogeological data, and distillation analysis. The investigations focused on determination of the origin of benzene, toluene, ethylbenzene, and xylenes (BTEX), and aliphatic hydrocarbons as well. Groundwater and floating oil samples were collected from extraction wells for analysis. Results indicate presence of active leaks in both the northern and southern zones. In the northern zone, toluene was found to primarily originate from oil products like aviation turbine kerosene (ATK or aviation fuel), kerosene, regular gasoline, and diesel fuel. Additionally, stable isotope ratios of carbon and hydrogen for ethylbenzene, o-xylene (ortho xylene) and p-xylene (para xylene) in zone A suggested the pollution originated from gasoline within the northern zone. The origin of super gasoline (with higher octane) identified in southern zone using δ13C and δ2H values of toluene in the floating oil and groundwater samples. Further, biodegradation of toluene likely occurred in southern zone according to δ13C and δ2H. The findings underscore the critical importance of integrating CSIA and fingerprinting techniques to effectively address the challenges of source identification and relying solely on each method independently is insufficient. Accordingly, comparing the GC-MS results of floating oil samples with ATK and jet fuel (JP4) standards can be effectively utilized for source differentiation. However, this method showed no practical application to distinguish different types of diesel or gasoline. The accuracy and reliability of source identification of BTEX compounds may significantly improve when hydrogeological data incorporates with stable isotopes analysis. Additionally, the results of this study will elevate the procedures for fuel-related contaminants source identification of the polluted groundwater that is crucial to develop effective remediation strategies.

Production of biobased polyethylene terephthalate and its precursors for diversification in the global sugarcane industry

The technical and economic viability of producing fully bio-based polyethylene terephthalate (PET) or its monomers was investigated. First generation (1 G) biorefineries, utilizing A-molasses, and combined first and second generation (1G2G) biorefineries, utilizing molasses and sugarcane bagasse and brown leaves, were considered. Simulations were developed for self-sufficient biorefineries, where the energy demands were met by utilizing the existing sugar mill boiler, accompanied by a new medium-pressure boiler and biogas generator. 1G2G scenarios required the replacement of the sugar mill boiler with a new, high-efficiency combined heat and power (CHP) plant, where electricity and useful heat were generated from a portion of 2 G biomass and other residues. 1 G PET was unprofitable at a minimum selling price (MSP) of $2946.t−1. 1 G production of Ethylene ($1847.t−1), monoethylene glycol (MEG) ($1648.t−1), 5-Hydroxymethylfurfural (HMF) ($1576.t−1) and iso-butanol (iButOH) ($1292.t−1) were more economically viable than PET, terephthalic acid (TPA) ($2689.t−1) and p-xylene (PX) ($2983.t−1), due to less complex processing and lower energy demands. 1G2G biorefineries were less profitable than 1 G equivalents, due to the cost of pretreatment and new CHP plants. The most profitable scenario was 1 G iButOH, with an MSP 19.0 % lower than its market price. New technologies to produce p-xylene and TPA from biomass in fewer steps, such as Virent Inc.’s Bioforming® technology, could potentially improve the economic viability of PET in the future.

Selective recovery of para-xylene from polyethylene terephthalate plastic

The conversion of polyethylene terephthalate (PET) plastic into low-boiling monomers like para-xylene (PX) that can be purified easier with lower energy consumption represents a promising strategy for circular economy that seamlessly integrating into current prevailing high-quality PET production chain. Herein, a selective bimetallic RuFe catalyst was successfully fabricated for hydrodeoxygenation (HDO) of dimethyl terephthalate (DMT) (a model compound of PET) and PET plastics to PX with yield as high as 88.9 % and 82.6 %, respectively, comparable to state-of-the-art examples. H2-TPR and EXAFS combined with benchmark test over model catalysts confirmed that small Ru clusters and Ru-Nx coordination are responsible for the high aromatics selectivity, on which vertical adsorption of carbonyl group (CO) is preferred. This study not only shows a promising PX-based PET recycling route, the highly selective C–O bond cleavage on Ru catalyst and corresponding structure-selectivity relationship also guide the catalyst design for other value-added biomass-based HDO reactions.

Design and optimization for the separation of xylene isomers with a novel double extractants-based extractive distillation

Xylene is a crucial chemical raw material, serving as a synthetic monomer and solvent extensively employed in coating, medicine, rubber and other industries. It contains of three isomers: o-xylene (OX), m-xylene (MX), and p-xylene (PX), their separation is considered a worldwide challenge due to their extremely close boiling points. A novel extractive distillation based on double extractants is first proposed to separate these isomers in this paper, while it was considered impractical to separate these isomers by distillation technology alone in the past. Through the analysis of residual curve and extractant screening, two potential solvents, i.e., N-Methylpyrrolidone (NMP) and Tetramethylene sulfone (Sul) were used as extractants, and then the separation sequences were designed and optimized. The extractive distillation processes were optimized by sequential iterative method according to the minimum total annual cost (TAC), and the best separation sequence and process parameters were determined. For comparison, it was found that the optimized double extractant-based extractive distillation (DEED) process has the best economic performance with TAC of 5.72×106$, and the energy consumption was greatly reduced by 41.2% compared to the single extractant-based extractive distillation (SEED). This article provides a new perspective on energy-efficient distillation technology for industrial xylene separation and purification production.

Simulating Gas-Solid Phase Isomerization of C8 Aromatics Affected by Catalyst Shapes in Fixed Bed Reactors with Particle-Resolved CFD Approach

Paraxylene (PX) is essential for producing polyethylene terephthalate fibers. We conduct particle-resolved CFD simulations, coupled with our reaction kinetics, on a fixed bed reactor randomly filled with catalyst shapes including sphere, cylinder, trilobe and quadrilobe to enlarge PX productivity in the catalytic isomerization of C8 aromatics. Our findings reveal the effects of catalyst shapes on velocity, concentration, temperature fields, and pressure drop. Although all catalyst shapes present particle utilization above 70 %, trilobes and quadrilobes show superior PX yield and ethylbenzene conversion due to their larger surface area and reduced mass transfer distance. Yet, among these catalysts, trilobes have twice the pressure drop of the others for complex geometric surface. Therefore, quadrilobes are more practical as their higher bed voidage, smaller pressure drop, better reactivity for desired reactions, and lower adiabatic temperature rise. This work provides guidance for catalyst design and process optimization to enhance PX productivity in aromatic isomerization.

Epitaxial Growth of Two-Dimensional MWW Zeolite.

Two-dimensional (2D) zeolite, with a high aspect ratio, has more open skeletons and accessible active sites than its three-dimensional (3D) counterpart. However, traditional methods of obtaining 2D zeolites often cause structural damage and widespread skeleton defects, hindering efficient selectivity in molecular separation. In this study, we present, for the first time, a direct epitaxial synthesis of 2D zeolite (Epi-MWW) guided by hexagonal boron nitride (h-BN) with a coincidence matching of site lattices to MWW zeolite. The as-grown Epi-MWW zeolite possesses a high crystallinity and intact hexagonal 2D morphology, with an average thickness of 10 nm and an aspect ratio of over 50. Thanks to its excellent molecular accessibility, the diffusion time constants of o-xylene (OX) and p-xylene (PX) are as 12 and 133 times higher than those of conventional MCM-22, respectively; the PX/OX selectivity of Epi-MWW is 7.4 times better than MCM-22 as calculated by the ideal adsorbed solution theory.

“Redox switches” of Fe species on zeolite catalysts: Modulating the acidity and the para-xylene yield from methanol

Molecular switches are widely studied in optical devices, computer science, DNA sensor systems, and chiral synthesis; however, their use in heterogeneous catalytic processes is rarely reported. Herein, we report a Fe-based redox switch for tuning the acidity of a ZSM-5-based catalyst in the methanol-to-aromatics reaction. In this reaction, the yield of the target product, para-xylene (PX), is low because various types of acids on the catalyst activate side reactions. Fe oxides and zeolite generate medium-strength Lewis acids, which activate the aromatization of methanol but suppress the dealkylation of xylene. Gradual reduction of Fe oxides during the reaction simultaneously decreases the conversion of methanol, the yield of aromatics, and the yield of PX. The oxidation state of the Fe species and the associated catalytic performance can be regenerated in the air at 550 °C. The redox switches caused regular fluctuation in the catalytic performance and remained stable throughout 16 regeneration cycles (up to 80 h). The employed strategy enabled a PX yield of up to 60% (carbon base) using a SiO₂-coated Zn/P/Fe/ZSM-5 catalyst, which is 3–6 times higher than previously reported values. The result showed a new mode of acidity modulation of the catalyst.

An efficient one-pot synthesis of p-xylene from bio-based 2,5-hexanedione and ethanol

A brand-new p-xylene (PX) production route via 2,5-hexanedione (HDO) and ethanol was put forward. And the reaction was successfully catalyzed by organic base - tetrapropylammonium hydroxide modified HZSM-5 zeolite. Under optimum condition, the conversion of 2,5-HDO and selectivity to PX was 89.6 % and 35.2 %, respectively, much higher than the patent HZSM-5 zeolite. For comparison, PX was also produced from 2,5-dimethyfuran (2,5-DMF) and ethanol or 2,5-HDO and ethylene under the same conditions. However, the PX selectivity and yield obtained from 2,5-HDO and ethanol was much poorer than that when 2,5-DMF or ethylene as reactant due to the series competitive dehydration reactions. To investigate the relationship between catalyst properties and performance, the physicochemical properties of catalysts were comprehensively characterized by XRD, N2 adsorption-desorption isotherms, ICP, NH3/CO2-TPD, pyridine-IR, SEM, TEM and MAS NMR. Conclusion can be drawn that the appropriate acidity and hierarchical pore structure in T0.5-HZ catalyst (HZSM-5 modified by 0.5 M tetrapropylammonium hydroxide) were beneficial to high PX selectivity and yield. Among them, the acid site with weak acid site would catalyze 2,5-HDO intramolecular aldol condensation to 3-methylcyclopentenone (MCO), while strong acid site was beneficial for 2,5-DMF hydrolysis to 2,5-HDO. Thus, acid site with moderate strength was advantageous to the conversion of 2,5-HDO to 2,5-DMF. Besides, the enough amount of acid sites and suitable B/L ratio of 0.98 can satisfy the requirements of various dehydration reactions and Diels-Alder reaction between 2,5-DMF and dienophiles, enhancing PX selectivity and yield. Additionally, the hierarchical pore structure enhanced mass transfer efficiency and improved the reaction rate. The promising production for bio-based PX via 2,5-HDO and ethanol lay solid foundation for industrialized production route from lignicellulose.

Highly solvent-dependent naphthalene diimide-based fluorescent polymer sensors for detecting p-xylene

P-xylene (PX), as one of xylenes, is a kind of very important chemical raw material. It is generally fabricated using methanol (MeOH) and toluene (TOL), and serves as a useful intermediary. However, it is difficult to use a simple and convenient tool to distinguish it from TOL and MeOH. Herein, we reported a simple naphthalene diimide-based polycaprolactone polymer (NDI-PCL) as a high-sensitive fluorescent sensor for xylene detection from MeOH and TOL. NDI-PCL was synthesized via ring-opening polymerization (ROP) of ε-caprolactone. NDI-PCL showed a remarkable fluorescence change from blue to green and yellow because of the formation of charge transfer (CT) interaction between naphthalene diimide (NDI) and aromatic solvents. NDI-PCL was successfully applied as the high-sensitive sensor for discriminating binary solvent mixtures such as PX in TOL, MeOH in PX, and MeOH in TOL, and their limit of detection (LOD) were as low as 19 ppm, 16 ppm and 9 ppm. These excellent fluorescence properties ensured that NDI-PCL indicated great potential application in detecting aromatic solvents.

The role of Ni-containing species on methanol-toluene methylation to p-xylene over ZSM-5 zeolite

The methylation of toluene with methanol is an important process for the preparation of p-xylene (PX). The modification of Ni-containing species in HZSM-5 can improve the yield of PX. However, the active site existence form of Ni is unclear. The intrinsic reaction mechanisms for methylation of toluene with methanol over HZSM-5 with NiOH+ and Ni2+ as Lewis acid have been investigated. The results indicate that the introduction of Ni2+ decreases the activity for the methanol dissociation, whereas NiOH+ enhances the activity with a reduction in activation energy of 100.7 kJ/mol. Furthermore, the generation of three xylenes, as well as the conversion of PX to 1,2,4-Trimethylbenzene (1,2,4-TMB) have also been studied over NiOH+/HZSM-5. It is shown that PX is most easily formed with the lowest energy barrier of 84.8 kJ/mol in four products, implying the introduction of NiOH+ in HZSM-5 favors the formation of PX. In addition, molecular dynamics simulations show that toluene reacts with methanol to form xylenes mainly at the intersection of channels. PX exhibits a much higher self-diffusion coefficient than MX and OX when the temperature rises from 498 to 698 K. It can be concluded that NiOH+ is the active Ni-containing species and NiOH+/HZSM-5 is an excellent catalyst for the production of PX via the methylation of toluene with methanol.

Insight into the enhanced catalytic performance of phosphate-modified ZrO2/SBA-15 for the conversion of biobased 2,5-dimethylfuran and ethylene into p-xylene

Catalytic conversion of biomass-derived platform chemicals into bulk chemicals p-xylene (PX) is appealing yet challenging due to low catalytic efficiency and severe reaction conditions. In this work, we fabricated a series of phosphate-modified ZrO2/SBA-15 (xP-Zr/SBA-15) via adjusting the addition of phosphoric acid, and used them for the catalytic conversion biomass-derived 2,5-dimethylfuran (DMF) and ethylene to PX. Because of the enhanced adsorption capability for the reactant molecules (DMF and ethylene) and the good synergistic effect between Brønsted acid sites (P-OH) and Lewis acid sites (Zr4+), 30P-Zr/SBA-15 revealed an excellent catalytic performance in the selective conversion of DMF into PX. The DMF conversion was 98.5%, PX yield was 92.0% at 250 °C for 12 h, and the corresponding PX productivity was upto 50.5 mmolPX·gcat−1·h−1. The results are far superior to those obtained on most previously reported catalysts in the literature under similar reaction conditions. The kinetic study revealed that 30P-Zr/SBA-15 has a lower apparent activation energy for the tandem reaction including the Diels-Alder reaction (57.0 kJ·mo−1) and dehydration reaction (37.7 kJ·mo−1). Moreover, the 30P-Zr/SBA-15 catalyst exhibited the excellent recyclability and could be recycled at least six times without a significant loss of activity. On the basis of the temperature-programmed surface reaction of DMF and ethylene, a reasonable reaction mechanism was proposed.

Flow and mass transfer for the adsorption separation of para-xylene in a packed bed by particle-resolved CFD simulations

As the leading polyester product chain, para-xylene (PX) is the most important xylene isomer. In industry, high purity paraxylene is mainly produced by adsorption separation technology. In this study, the PX adsorption process has been investigated by a 3D particle-resolved CFD model. For spherical particle packed beds, the channeling effect, flow stagnation and backflow regions are the main factors leading to non-ideal flow. According to the residence time distribution (RTD) and adsorbent efficiency, the more uniform the flow in the packed bed, the higher the adsorption separation efficiency. Therefore, low feed flow rate and large bed-to-particle diameter ratio (N) are beneficial to the adsorption process. In addition, the effect of particle geometries (trilobe, five-lobe, Raschig ring, four-spoke ring, and six-hole cylindrical) on the flow and adsorption characteristics of the packed bed has been investigated in terms of pressure drop, adsorbed amount and adsorbent efficiency. Internal void particles perpendicular to the flow direction are unfavorable for the adsorption process. The four-spoke ring adsorbent with more surface areas has the largest bed adsorbed amount and exhibits the best adsorption performance. The trilobe adsorbent has the largest adsorbed amount per unit bed pressure drop and is the optimally shaped adsorbent for industrial PX adsorption separation.

Zirconium-tin phosphate as an efficient catalyst for renewable p-xylene synthesis from biobased 2,5-dimethylfuran and ethylene

Efficient production of renewable p-xylene (PX) from bio-based 2,5-dimethylfuran (DMF) and ethylene is challenging. Herein, novel nanoflake-like zirconium-tin phosphate catalysts were synthesized, and their catalytic performance was evaluated in the conversion of DMF and ethylene to PX. The DMF conversion, PX selectivity and PX yield reached up to 99%, 95.2% and 94.2%, respectively over Zr0.6Sn0.4P under relatively mild conditions. The PX productivity was 61.1 mmolPX·gcat−1·h−1, which is better than that of previously reported catalysts. By combining characterization with activity test, it was found that enhanced DMF and ethylene adsorption is beneficial to promoting the reaction between DMF and ethylene. Precise regulation of acid sites is crucial for inhibiting the occurrence of side reactions and improving the selectivity of PX. The kinetic analysis revealed that Zr0.6Sn0.4P had a relatively low apparent activation energy (39 kJ·mol−1) for this reaction than previously reported catalysts. In addition, the Zr0.6Sn0.4P catalyst exhibited an excellent reusability.

Surface reconstruction enabling MoO2/MoP hybrid for efficient electrocatalytic oxidation of p-xylene to terephthalic acid

Selective oxidation of p-xylene (PX) to terephthalic acid (TA) remains exceptionally challenging since it easily undergoes deep oxidation. Herein, a nickel foam-supported molybdenum dioxide/molybdenum phosphide hybrid electrocatalyst (MoO2/MoP/NF) is reported for highly selective generation of TA via electrocatalytic oxidation (ECO) of PX in alkaline medium. The efficient MoO2/MoP/NF anode material displays a unique cluster-like nanocone architecture, showing abundant active sites and rapid charge transfer kinetics. Benefit from the synergy between MoO2 and MoP, the MoO2/MoP/NF provides a high TA selectivity of 94.8% and an outstanding faradaic efficiency of 76.9% at the conversion of 71.6%. Additionally, the anodic oxidation of PX over MoO2/MoP/NF promotes the cathodic hydrogen production. The potential-induced surface reconstruction of the as-synthesized MoO2/MoP/NF yields new phases of phosphomolybdate and potassium molybdate. The top P site on the phosphomolybdate surface facilitates the adsorption of reaction intermediates but weakens the adsorption of TA, thereby yielding high selectivity toward TA.

Diels-Alder conversion of biomass-derived furans and ethylene to renewable aromatics over mesoporous titanium phosphate

Metal phosphates such as P-Beta, Zr-P and Sn-P have drawn lots of attention as emerging catalysts for biomass conversion due to its mild acidity which results in high selectivity to the targeted products while minimizing the unwanted side reactions such as coking that are prevalent in reactive biomass-derived compounds. In this study, we have investigated the production of sustainable aromatics such as p-xylene (PX) and toluene (TOL) via Diels-Alder cycloaddition (DAC) of biomass-derived 2,5-dimethylfuran (DMF) and 2-methylfuran (MF) with ethylene over titanium phosphate catalysts. Specifically, the effects of synthesis method (e.g., sol-gel vs hydrothermal), activation method and P/Ti molar ratio on the structural and acid properties of titanium phosphate catalysts were systematically studied to understand the catalytic consequences of the titanium phosphate materials for the DAC of furans. In addition, amphiphilic surfactant was applied to all catalysts for the formation of mesoporous structure. The characterization studies of the prepared catalysts using XRD, BET, FT-IR, XPS, NMR spectroscopy and TPD of ammonia indicated that the degree of Ti-O-P bond formation, catalysts surface area and surface acidity are largely affected by the catalysts synthesis conditions. Overall, the mesoporous titanium phosphate catalyst, synthesized by hydrothermal method at 180 ℃ for 12 h followed by ethanol refluxing at 60 ℃ for 24 h at a molar P/Ti ratio of 1 showed the highest surface area and the highest acid site density which arise from its distinct ordered Ti-O-P bonding structure as evidenced by XPS and NMR. The catalyst demonstrated the remarkably high PX selectivity of > 90 % at the high DMF conversion of > 90 % and the reusability after two successive reaction-regeneration cycles, showing high phosphorus leaching tolerance.

Highly Selective Transformation of CO2 + H2 into Para-Xylene via a Bifunctional Catalyst Composed of Cr2O3 and Twin-Structured ZSM-5 Zeolite

The abundant C1 source CO2 can be utilized to produce value-added chemicals through hydrogenation technology. A bifunctional catalyst consisting of reducible metal oxide Cr2O3 and acidic zeolite ZSM-5 was designed for the direct conversion of CO2 + H2 into valuable aromatics, especially para-xylene (PX), via the methanol-mediated pathway. The twin structure of ZSM-5 (ZSM-5T), with sinusoidal channels that are predominantly exposed to the external surface, enhances the possibility of the transformation of methanol into PX due to the favorable diffusion dynamic of PX in the sinusoidal channels. Via the bifunctional catalyst Cr2O3&ZSM-5T, a PX selectivity of 28.7% and PX space-time yield (STY) of 2.5 gCH2 h−1 kgcat−1 are achieved at a CO2 conversion rate of 16.5%. Furthermore, we rationally modify the ZSM-5T zeolite via Cu species doping and amorphous SiO2 shell coating (Cu-ZSM-5T@SiO2). After combining with the Cr2O3 catalytic component, the CO2 conversion (18.4%) and PX selectivity (33.8%) are increased to some extent, which systematically increases the STY of PX to 3.0 gCH2 h−1 kgcat−1. The physicochemical property of the acidic zeolite and the corresponding structure-function relationship in enhancing the PX productivity are discovered. Our work provides a novel catalyst design idea to boost PX synthesis performance from CO2 hydrogenation.

Highly efficient catalytic conversion of biomass-derived 2,5-dimethylfuran into renewable p-xylene over zirconium phosphate catalysts

A series of zirconium phosphates (ZrP-x) with various P/Zr ratios were prepared. Among the catalysts, ZrP-2.5 with a high specific surface area of 410.9 m2·g−1 and abundant mesopores exhibited outstanding catalytic performance in the synthesis of p-xylene (PX) from 2,5-dimethylfuran (DMF) and ethylene. A high PX yield of 89.2% and a productivity of 72.3 mmolPX·gcat−1·h−1 were achieved at 250 °C using a small amount of catalyst. The excellent catalytic performance of ZrP-2.5 was attributed to the synergy between the large quantities of acidic sites with appropriate acid strength distribution, balanced Brønsted and Lewis acidity, and abundant mesopores. In addition, the catalyst had excellent recyclability and stability. In-situ FT-IR spectra of adsorbed DMF and ethylene revealed the crucial roles of P-OH and the coordinately unsaturated Zr(IV) in the adsorption and reaction process, which is beneficial to understanding the reaction mechanism and designing highly efficient catalysts.

Conversion of Methanol to Para-Xylene over ZSM-5 Zeolites Modified by Zinc and Phosphorus.

In this work, the influence of different phosphorus sources and the modification of zinc and phosphorus on the performance of the conversion of methanol to aromatics (MTA) was investigated. The results showed that the phosphorus source had a significant impact on the selectivity of para-xylene (PX) in xylene and catalyst stability. The introduction of P resulted in the covering of the active acid sites and the narrowing of the pore of the ZSM-5 zeolite, which improved the shape-selectivity for PX in the methanol conversion reaction. Compared with the modifiers of H3PO4 and (NH4)3PO4, the ZSM-5 zeolite modified by (NH4)2HPO4 exhibited better catalyst stability and PX-selectivity due to its larger specific surface area, pore volume and suitable acidity. When the ZSM-5 zeolite was modified by Zn and P, the effect of Zn and P on the selectivity to aromatics and PX in xylene was almost opposite. With the increase in P-loading, the selectivity of PX in xylene gradually increased but at the cost of decreasing the aromatic-selectivity. On the other hand, the loading of Zn introduced Zn-Lewis acid sites to provide aromatization active centers and improved the aromatic-selectivity. However, excessive Zn reduced the selectivity of PX in xylene. The catalyst activity and aromatic-selectivity could be improved to some extent with an appropriate ratio of Zn and P, while maintaining or increasing the para-selectivity of xylene. Compared with 5% P/ZSM-5 catalyst modified with only (NH4)2HPO4, the PX selectivity in xylene over the Zn-P/ZSM-5 catalyst modified with 5% Zn and 1% P improved from 86.6% to 90.1%, and the PX yield increased by 59%.

Thermodynamic and Mechanistic Analyses of Direct CO2 Methylation with Toluene to para-Xylene.

Direct CO2 methylation with toluene, as one of the CO2 hydrogenation technologies, exhibits great potential for the CO2 utilization to produce the valuable para-xylene (PX), but the tandem catalysis remains a challenge for low conversion and selectivity due to the competitive side reactions. The thermodynamic analyses and the comparation with two series of catalytic results of direct CO2 methylation are conducted to investigate the product distribution and possible mechanism in adjusting the feasibility of higher conversion and selectivity. Based on the Gibbs energy minimization method, the optimal thermodynamic conditions for direct CO2 methylation are 360-420 °C, 3 MPa with middle CO2/C7H8 ratio (1:1 to 1:4) and high H2 feed (CO2/H2 = 1:3 to 1:6). As a tandem process, the toluene feed would break the thermodynamic limit and has the higher potential of >60% CO2 conversion than that of CO2 hydrogenation without toluene. The direct CO2 methylation route also has advantages over the methanol route with a good prospect for >90% PX selectivity in its isomers due to the dynamic effect of selective catalysis. These thermodynamic and mechanistic analyses would promote the optimal design of bifunctional catalysts for CO2 conversion and product selectivity from the view of reaction pathways of the complex system.