P-xylene Selectivity Research Articles

Xylene Vapor Permeation in MFI Zeolite Membranes Made by Templated and Template-Free Secondary Growth of Randomly Oriented Seeds: Effects of Xylene Activity and Microstructure

Many studies have reported high p-xylene/o-xylene permselectivities with MFI zeolite membranes; however, these results were obtained at low xylene activity (<0.01). The present work investigates the vapor permeation of xylene isomers through MFI zeolite membranes at various xylene activities up to 0.033. Using MFI zeolite membranes synthesized with and without organic templates, the effects of xylene activity and membrane microstructure on separation performance in xylene vapor permeation were systematically studied. Both p-xylene and o-xylene permeances decrease with increasing xylene activity, but the decrease is more pronounced for p-xylene resulting in a reduction in p-xylene/o-xylene selectivity. The MFI zeolite membrane fabricated without template exhibits higher p-xylene selectivity than the templated membrane due to the presence of less intercrystalline gaps in the template-free zeolite layer, and the lesser decrease in p-xylene selectivity for the template-free membrane was confirmed with increas...

Synergistic effect of acidity and extraframework position in faujasite on renewable p-xylene production.

p-Xylene is a commodity chemical used for the manufacture of plastic bottles and textiles. For the biomass-based route from 2,5-dimethylfuran (DMF) and ethylene, the properties of the catalyst such as acidity effect, product selectivity and catalyst activity play an important role. To determine the effect of acidity and extraframework position in faujasite zeolite on p-xylene selectivity, type Y (Si/Al = 40 and Si/Al = 2.55) and X (Si/Al = 1.25) zeolites containing the extraframework Lewis acids Na+, K+, Li+, Ag+ and Cu+, and a Brønsted acid-containing zeolite, HY (Si/Al = 40), were prepared and tested for p-xylene production under solvent-free conditions and at low conversions (less than 35%). Here it is reported that NaX zeolite catalyses DMF and ethylene conversion to p-xylene with 91% selectivity at 30% conversion, which is better than the 25% p-xylene selectivity obtained when using HY at similar conversion. ANOVA was used to show that there is a synergistic effect between acidity and extraframework position on the rate of p-xylene production. At 7% DMF conversion, Lewis acids were more selective than the Brønsted acid tested (50 versus 30% p-xylene selectivity). p-Xylene selectivity is optimal when using Lewis acids with moderate acidity and extraframework positions located in the faujasite supercage (sites II and III).

One-Step Conversion of Biomass-Derived Furanics into Aromatics by Brønsted Acid Ionic Liquids at Room Temperature

Aromatics play essential and unique roles in the areas ranging from synthetic chemistry to the manufacturing industry. Production of aromatics from biomass is of great fundamental interest and practical importance to ease the burden of fossil resources. This work delineates a one-step route for the synthesis of renewable aromatics from various biobased furanics and dienophiles by acidic ionic liquids at mild conditions. [Bmim]HSO4 was used as a catalyst and solvent for the direct conversion of 2,5-dimethylfuran and acrylic acid into p-xylene and 2,5-dimethylbenzoic acid; up to 89% aromatic selectivity was achieved at 87% conversion of 2,5-dimethylfuran at room temperature and atmospheric pressure, and totally 84% selectivity of p-xylene can be obtained with a subsequent decarboxylation reaction. The reaction mechanism study supplemented with isotopic tracing and DFT calculations revealed the lowest-energy pathway for the two main products. Various starting materials were studied for further extensions of ...

Highly Selective Production of p-Xylene from 2,5-Dimethylfuran over Hierarchical NbOx-Based Catalyst

A hierarchical NbOx-based catalyst with both Bronsted acid and Lewis acid sites was synthesized in the absence of corrosive hydrofluoric acid, exhibiting high catalytic activity for biobased p-xylene (PX) production from 2,5-dimethylfuran (DMF). The as-prepared composite was composed of Nb2O5 and NbOPO4 crystals, and the densities of Bronsted acid and Lewis acid were determined to be 232.9 and 80.4 μmol/g, respectively. The well-balanced Bronsted/Lewis acidity and the hierarchical structure with small mesopores (3 nm) and large mesopores (48 nm) contributed to the high activity and stability: a conversion of 87.2% with the PX selectivity of 92.7%, and a carbon balance of 94.6% was achieved after 6 h of reaction at 523 K. In comparison with Sn-Beta, NbOx-based catalyst prepared in this work showed obvious advantages in suppressing carbon deposition: 90.9 and 54.7 mmol of PX were obtained over the NbOx-based catalyst and the Sn-Beta, respectively, after 24 h. Spent catalysts were regenerated through calcina...

Shape selective catalysis in methylation of toluene: Development, challenges and perspectives

Toluene methylation with methanol offers an alternative method to produce p-xylene by gathering methyl group directly from C1 chemical sources. It supplies a “molecular engineering” process to realize directional conversion of toluene/methanol molecules by selective catalysis in complicated methylation system. In this review, we introduce the synthesis method of p-xylene, the development history of methylation catalysts and reaction mechanism, and the effect of reaction condition in para-selective technical process. If constructing p-xylene as the single target product, the major challenge to develop para-selective toluene methylation is to improve the p-xylene selectivity without, or as little as possible, losing the fraction of methanol for methylation. To reach higher yield of p-xylene and more methanol usage in methylation, zeolite catalyst design should consider improving mass transfer and afterwards covering external acid sites by surface modification to get short “micro-tunnels” with shape selectivity. A solid understanding of mass transfer will benefit realizing the aim of converting more methanol feedstock into para-methyl group.

P-Xylene selectivity enhancement in methanol toluene alkylation by separation of catalysis function and shape-selective function

Catalyst reactivity and selectivity always compromise with each other in most reactions. HZSM-5 catalyst for the methanol toluene alkylation reaction suffers from active site loss when modified by elements such as Si, P, Mg and B to attain high p-xylene selectivity, incurring problems of low toluene conversion and rapid catalyst deactivation. In the present study, the separation of catalysis function and shape-selective function was achieved by epitaxial growing a Silicate-1 layer over HZSM-5 crystal to create an HZSM-5@S-1 catalyst. HZSM-5@S-1 was shown to possess less weak acid sites but more strong acid sites than the parent HZSM-5. With reduced contact time, the catalyst showed increased p-xylene selectivity up to 88% but no obvious degradation in toluene conversion. The enhancement of Silicate-1 shell to PX selectivity was less effective when applied to low Si/Al ratio ZSM-5 samples, due to the re-incorporation of aluminium leached from low Si/Al ratio parental ZSM-5 samples into the newly formed Silicate-1 framework. Benefiting from retaining most of the strong acid sites of HZSM-5, the catalyst showed high toluene conversion with increasing WHSV without apparent deactivation over a 130hours service life test.

Catalytic Upgrading of Fast Pyrolysis Products with Fe-, Zr-, and Co-Modified Zeolites Based on Pyrolyzer–GC/MS Analysis

To explore the influence of Fe-, Zr-, and Co-modified zeolites on bio-oil characteristics, the catalytic fast pyrolysis of cellulose combined with sawdust and cotton stalk was investigated using a pyrolyzer reactor. The results indicate that Fe-loaded zeolite favors naphthalene and 1-methylnaphthalene formation through catalytic deoxygenation and hydrocarbon formation. Zr-modified zeolite enhances the contents of ketones and aromatics with higher benzene and p-xylene selectivity. Co leads to higher production of anhydrous sugars but suppresses the progress of aromatics evolution, whereas the selectivity of Co for toluene and p-xylene is the highest. A comprehensive reaction network is also proposed to describe the cellulose catalytic pyrolysis process. Considering the catalytic adaptability, catalytic pyrolysis of typical biomass samples with Fe/HZSM-5 was investigated. The total contents of aromatic hydrocarbons are in the following order: sawdust > cellulose > cotton stalk. The selectivities of naphthal...

Fabrication and molecular transport studies of highly c-Oriented AFI membranes

The AFI membrane with one-dimensional straight channels is an ideal platform for various applications. In this work, we report the fabrication of a highly c-oriented, compact and stable AFI membrane by epitaxial growth from an almost close-packed and c-oriented monolayer of plate-like seeds that is manually assembled on a porous alumina support. The straight channels in the membrane are not only aligned vertically along the membrane depth, but are also continuous without disruption. The transport resistance is thus minimized and as a result, the membrane shows almost two orders of magnitude greater permeance in pervaporation of hydrocarbons compared to reported values in the literature. The selectivity of p-xylene to 1,3,5-triisopropylbenzene (TIPB) is approximately 850. In addition, through gas permeation studies on a number of gas and liquid molecules, different transport mechanisms including activated Knudsen diffusion, surface diffusion and molecular sieving were discovered for different diffusion species. The ratio of kinetic diameter to channel diameter, dm/dc, and the ratio of the Lennard-Jones length constant to channel diameter, σm/dc, are found very useful in explaining the different transport behaviors. These results should be useful not only for potential industrial applications of the AFI membranes but also for the fundamental understanding of transport in nanoporous structures.

Direct dual-template synthesis of HZSM-5 zeolite for enhanced p-xylene selectivity in toluene methylation with CH3Br

HZSM-5 catalysts of different crystal sizes directly synthesized by a single-step method using TPAOH and TEA as templates were characterized by XRD, SEM, N2 adsorption–desorption and NH3-TPD techniques.

Heteropolyacid catalysts for Diels-Alder cycloaddition of 2,5-dimethylfuran and ethylene to renewable p-xylene

The Diels-Alder cycloaddition of biomass-derived furans and subsequent dehydration are promising routes for the sustainable production of commodity chemicals such as p-xylene (PX). In this paper, we have investigated the catalytic performances of a range of phosphotungstic acid (HPW) and silicotungstic acid (HSiW) catalysts supported on various oxides, i.e., SiO2, Al2O3, TiO2 and ZrO2 and their structure-activity correlation in the conversion of 2,5-dimethylfuran (DMF) and ethylene to PX. The characterization studies of the catalysts using XRD, BET, Raman and 31P MAS-NMR spectroscopy reveal that all of the supported heteropolyacid (HPA) catalysts (except HPW/ZrO2) retain their Keggin structure on the surface of oxide supports. Results from ammonia- and n-propylamine-TPD studies show that all of the supported HPA catalysts possess well-defined Brønsted acid sites with the total acidity decreasing in the following order: HPA/SiO2>HPA/Al2O3>HPA/ZrO2>HPA/TiO2. The conversion of DMF and the initial rate of PX production generally increase with an increase in the total acidity, with HPA/SiO2 being the most active catalyst. The turnover frequency of PX production for HPA/SiO2 is also considerably greater than those for the HPAs supported on Al2O3, ZrO2, and TiO2, which suggests that the higher activity of HPA/SiO2 is at least partly due to the enhanced strength of Brønsted acid sites. Both the silica-supported HSiW and HPW catalysts demonstrate remarkably high PX selectivity (82–85%) at high DMF conversion (91–94%) at 250°C after 6h reaction. The effects of reaction conditions such as acid loading, reaction temperature, and reaction time have also been investigated with the most active silica-supported HSiW catalysts to optimize the PX yield.

Aromatic Hydrocarbon Production from Eucalyptus urophylla Pyrolysis over Several Metal‐Modified ZSM‐5 Catalysts

AbstractMetal‐modified HZSM‐5 catalysts were prepared by the ion exchange of NH4ZSM‐5 (SiO2/Al2O3=23) using Zn, Ga, Ni, and combinations thereof. The prepared catalysts were used to evaluate catalytic pyrolysis for the conversion of Eucalyptus urophylla to fuels and chemicals, specifically aromatic hydrocarbons, by using a micro‐scale pyrolysis reactor coupled with GC–MS. Two different biomass‐to‐catalyst ratios (1:5 and 1:10 w/w) were studied. The catalyst prepared with Ga by total ion exchange (Ga‐HZSM‐5 B, ≈7 wt % Ga) yielded the highest amount of aromatic hydrocarbons, whereas the catalysts modified with Ni produced the lowest yield of aromatic hydrocarbons. A correlation between the methane yield and benzene and p‐xylene selectivity was found for the catalysts, which was observed mainly for the Ni‐, Zn‐, and Ga‐Ni‐modified catalysts. The Ga‐modified catalysts were the most selective for xylenes, whereas the Ni‐based catalysts were the most selective for benzene production with a concurrent increase in methane production. The Zn‐modified catalysts were the most selective for toluene production.

Catalytic Conversion on Zeolites: Synthesis and Characterization

In this present work, catalytic cracking of durene over HY and Hβ zeolites, and their silica modified forms was examined at the temperature of 450 C. The reaction was carried out in gas phase using a fix bed reactor. HY and Hβ zeolites showed high conversion because of their large pore size comparable to the kinetic diameter of durene. Silica modified HY zeolite showed a slight increase in selectivity, which is attributed to blocking of external acid sites. It is conceived that gradual increase in silica content could gradually enhance p-xylene selectivity. Similar effect could also be expected for Hβ zeolites. One can maximize toluene selectivity compared to p-xylene, but the latter is commercially more important than the former. This study depicts that other large pore zeolites could also be good to crack durene to p-xylene.

Catalytic fast pyrolysis of mushroom waste to upgraded bio-oil products via pre-coked modified HZSM-5 catalyst

In this paper, HZSM-5 catalyst was modified by pre-coked to cover the strong external acid sites by methanol to olefins reaction, and the modified catalysts were then applied to conduct the catalyst fast pyrolysis of mushroom waste for upgraded bio-fuel production. Experiment results showed that the strong external acid sites and specific surface area decreased with pre-coked percentage increasing from 0% to 5.4%. Carbon yields of hydrocarbons increased at first and then decreased with a maximum value of 53.47%. While the obtained oxygenates presented an opposite variation tendency, and the minimum values could be reached when pre-coked percentage was 2.7%. Among the achieved hydrocarbons, toluene and p-xylene were found to be the main products, and the selectivity of p-xylene increased at first and then decreased with a maximum value of 34.22% when the pre-coked percentage was 1.3%, and the selectivity of toluene showed the opposite tendency with a minimum value of 25.47%.

Synergetic effect of H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst to enhance the selective hydrogenation of p-xylene

A H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst that combined para-directed isomerization and shape-selective hydrogenation was synthesized by coating MFI membranes onto the surface of Pt/Al2O3 pellet. The binary p/m xylene mixture was performed as the feed to evaluate the enhanced selective hydrogenation property of this bi-layered core–shell catalyst. In the hydrogenation of xylene isomers, the hydrogenation efficiency and selectivity of p-xylene over the H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst exceeded those over the monolayer H-ZSM-5@Pt/Al2O3 and Silicalite-1@Pt/Al2O3 core–shell catalysts, while maintaining the tiny formation of the unexpected hydrogenation products of m- or o-xylene. The effect of reaction temperature was also investigated on the H-ZSM-5/Silicalite-1@Pt/Al2O3 catalyst. The p-xylene hydrogenation conversion and 1,4-dimethylcyclohexane selectivity of as high as 90.8% and 97.2%, respectively, were achieved at 300°C. The isomerization on the H-ZSM-5 shell and the selective hydrogenation on the Silicalite-1@Pt/Al2O3 cooperated concertedly and promoted mutually. This bi-layered core–shell catalyst, with the synergetic effect of equilibrium shift and shape-selectivity, might have great potential to develop into a novel approach for xylenes separation.

Bio-based hydrocarbons and oxygenates from catalytic bio-ethanol dehydration: comparison between gallium and germanium oxides as promoters on HBeta zeolites with various silica to alumina ratios

Hydrocarbons and oxygenates can be produced from methanol or ethanol via dehydration and other reactions as green alternative routes. One of the most important factors, which govern the activity and product distribution, is the silica/alumina ratio (related to acid strength and acid density) of a zeolite support used as a catalyst in ethanol dehydration. H-ZSM-5 zeolite was often used as a catalyst for methanol or ethanol dehydration because of its shape selectivity, but its pore size limits the production of large hydrocarbons. In this work, the effects of acid density and acid strength in the product distribution were investigated using various silica/alumina ratios of H-Beta zeolites with a larger pore size, aiming to produce larger hydrocarbons than gasoline range. Gallium and germanium oxides were also doped on the H-Beta zeolites, and their effects on the product distribution were examined and compared. As a result, it was found that increasing silica/alumina ratio resulted in the reduction of acid density and acid strength of H-Beta zeolites, and the highest oil yield was obtained from using a moderate silica/alumina ratio of H-Beta (silica/alumina ratio = 37), which highly consisted of large hydrocarbons (aromatics with carbon numbers higher than 9). The support with the lowest acid density and strength tended to produce oxygenates, non-aromatics, and benzene rather than large hydrocarbons. When loaded on the zeolites with the lowest and highest acidity, gallium and germanium oxides suppressed the oil yield, promoted the production of oxygenates and distillate-range hydrocarbons, and did not change the selectivity of p-xylene in mixed xylenes. However, the differences made by the two promoters were observed to be enhanced by the use of the support with moderate acidity, especially on the increases in non-aromatic formation and p-xylene selectivity.

Characteristics of catalysts for enhanced green production of distillates and chemicals in bio-oil from catalytic dehydration of bio-ethanol

The dehydration of ethanol to gasoline and C6–C8 hydrocarbons has been made possible by using a HZSM-5 zeolite due to its proper acid property and shape selectivity. HZSM-5 had been also used as a support of phosphorous- (P), antimony- (Sb), and bismuth- (Bi) oxides for ethanol dehydration, which mainly gave high gasoline in oil. Since the moderate pore size of HZSM-5 limits the production of heavier biofuels in the distillate ranges (namely, kerosene and gas oil), therefore, in this work, the zeolites, HY and HBeta, with a larger pore size were employed, aiming to produce larger hydrocarbons. HZSM-5 was also used for comparison. Moreover, Group 5A oxides (P-, Sb-, and Bi oxides) were also doped onto the zeolites at 5 wt.% of elemental loading, expectedly to further enhance the production of distillate-range products. The results from the experiments carried out at 500 °C showed that, regardless of the type of loaded oxides, the distillate production was primarily governed by the channel opening size of zeolites, but the yield also depended on other parameters such as acid strength, and the complexity of the pore channel. HZSM-5 tended to produce high contents of large hydrocarbons ( $${\text{C}}_{9}^{ + }$$ ) whereas HBeta and HY tended to produce high amounts of C6–C8 hydrocarbons, especially benzene, and toluene. HBeta and HY gave high selectivity of p-xylene in mixed xylenes. Furthermore, oxygenates were produced in a trace amount using the parent zeolites, but their formation was significantly enhanced with using oxide-promoted catalysts, especially when HY was used as the support. In addition, Group 5A oxides doped on HBeta and HY can increase the oil yield. The content of distillates in bio-oil was also more greatly enhanced by Sb x O y and Bi x O y than by P x O y , promoted on HBeta and HY, possibly due to the promoter loading that created new stronger Bronsted acid sites. Generally speaking, high production of distillates in bio-oil preferably requires a support that possesses high strength of Bronsted acid sites in a large channel opening with low tortuosity of channel structure. Furthermore, the selected promoter should offer a new type of acid sites on the catalyst.

Tailoring p-xylene selectivity in toluene methylation on medium pore-size zeolites

The p-xylene selectivity in toluene methylation was investigated over medium pore-size zeolites (H-ZSM5, H-ZSM11 and H-NU10) as well as after deposition of tetraethyl orthosilicate on the surface, isomorphous substitution of aluminum by iron and variation the crystal size. Higher reaction temperatures consistently resulted in higher p-xylene selectivity in the three reaction pathways for the formation of p-xylene during toluene methylation, i.e., methylation of toluene, isomerization of xylenes, further methylation and subsequent formation of xylenes after dealkylation of light hydrocarbons. The increased selectivity is attributed to the accumulation of slowly diffusing reaction products in the zeolite pores. Thus, the isomerization of m- and o-xylenes, as well as dealkylation of higher alkylated products influenced the xylene selectivity most strongly at high temperatures, while the intrinsic selectivity of the toluene alkylation dominates at lower reaction temperatures due to the absence of transport restrictions.

Facile synthesis of HZSM-5 with controlled crystal morphology and size as efficient catalysts for chlorinated hydrocarbons oxidation and xylene isomerization

ZSM-5 zeolites with controllable crystal morphologies (microsphere self-assembled from nanorod crystals, round-boat and cross shape) were synthesized in extremely diluted solutions (H2O/SiO2 = 350) by only adjusting the tetrapropylammonium hydroxide concentration under dynamic or static condition. The synthesized ZSM-5 were characterized by X-ray diffraction, infrared spectra, scanning electron microscopy, transmission electron microscopy, N2 sorption and NH3 temperature programmed desorption, and compared with the commercial ZSM-5 zeolite. Catalytic tests for the formation of p-xylene from o-xylene isomerization and the catalytic oxidation of chlorinated hydrocarbons showed that ZSM-5 microsphere composed of nanorod crystals exhibited both high conversion and improved p-xylene selectivity, and high catalytic activities for 1,2-dichloroethane and chlorobenzene oxidation and low selectivity to chlorinated by-products, which were attributed to the existence of nanosized ZSM-5 crystals and mesopores, high internal surface area and total acidity.

Solvent-tuned hydrophobicity for faujasite-catalyzed cycloaddition of biomass-derived dimethylfuran for renewable p-xylene

Phase equilibria of the high temperature/high pressure heterogeneous catalytic reaction of 2,5-dimethylfuran (DMF), a biomass-derived furan, with ethylene to produce p-xylene was studied as a function of DMF conversion in a constant-pressure reactor. Adsorption of reactants and products onto a zeolite catalyst was computed using a configurational-bias Grand Canonical Monte Carlo (CB-GCMC) simulation, and the vapor–liquid behavior was computed using ASPEN Plus with an appropriate equation of state. It was found that the amount of water adsorbed in H–Y (Si/Al = 2.6) increases significantly as DMF is consumed, but is small in the presence of an aliphatic solvent, here n-heptane, even at high DMF conversion. The presence of the solvent reduces side reactions with water, such as hydrolysis followed by oxyalkylation, and increases p-xylene selectivity, in agreement with experiment. The decrease in the amount of water adsorbed is due to the increased hydrophobic environment in the zeolite as a result of the addition of n-heptane. The increase of ethylene inside the cage by the addition of n-heptane results in a slight increase of p-xylene alkylation. This work presents the first use of molecular simulation to understand mechanistic effects of solvents on the catalytic production of p-xylene in an H–Y zeolite.

Separating Xylene Isomers by Commensurate Stacking of p‐Xylene within Channels of MAF‐X8

The development of energy-efficient processes for selective separation of p-xylene from mixtures with its isomers is of vital importance in the petrochemical industries. Current industrial practice uses BaX zeolite that has high adsorption selectivity for p-xylene. Finding para-selective structures is challenging. With state-of-the-art simulation methodologies we systematically screened a wide variety of zeolites and metal-organic frameworks (MOFs). Our investigations highlight the crucial importance of the channel dimension on the separation. MAF-X8 is particularly noteworthy because the channel dimensions and geometry allow "commensurate stacking" which we exploit as a separation mechanism at saturation conditions. Due to a significantly improved capacity compared to BaX, the cycle times for p-xylene with MAF-X8 are found to be about a factor of 4.5 longer. This is expected to result in significant process improvements.