Xylene Selectivity Research Articles

Utilization of Metal-Functionalized ZSM-5 for Methanol and Low-Carbon Hydrocarbon Coupling Aromatization

Aromatics assume a paramount role as indispensable organic chemical feedstock within diverse industrial domains. Simultaneously, the global aromatics market is scarce, particularly with the exorbitant demand for high-value aromatics. Generating aromatics via coal-based methanol and low-carbon hydrocarbon coupling reactions has become a novel green and sustainable development trajectory. In this study, HZSM-5 catalysts featuring different Si/Al ratios and active metal-functionalized modifications were utilized to explore the aromatization effect in light of the Si/Al ratio, types of active components, and metal-loading content in a fixed-bed reactor. The outcomes were that the conversion ratios for methanol and n-pentane attained 99.9% and 83.1%, respectively. Remarkably, an oil phase yield of 32.1% was accomplished, along with an aromatic content of approximately 74.2%, while xylene selectivity reached approximately 37.6% for the 1.0%-ZnO/ZSM-5 (50) catalyst. Ultimately, a reaction mechanism for the coupling of methanol and n-pentane to yield aromatics using a 1.0%-ZnO/ZSM-5(50) catalyst is postulated.

Production of aromatic hydrocarbons from catalytic fast pyrolysis of microalgae over Fe-modified HZSM-5 catalysts.

In this study, catalytic fast pyrolysis of the microalga C. pyrenoidosa over Fe-modified HZSM-5 catalysts was performed to produce aromatic hydrocarbons using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Fe-modified HZSM-5 catalysts were prepared using wet impregnation and characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption-desorption and temperature-programmed desorption of ammonia (NH3-TPD) analysis. The effect of Fe loading (3, 5, 8, and 12 wt%) and pyrolysis temperature (450, 500, 550, 600 °C) on the content of pyrolysis products and the content and selectivity of aromatics was examined. Results showed that the parent HZSM-5 catalyst structure remained intact upon increasing Fe loading, although its specific surface area, micropore volume and average pore diameter decreased. In addition, its external specific surface area and total pore volume increased. The modification of Fe metal increased the total acid amount, especially strong acid sites, which were highest when Fe loading was 8 wt%. The prepared Fe-modified HZSM-5 catalysts exhibited pronounced deoxygenation and denitrogenation effects, which significantly decreased the acid, aldehyde, ketone, furan and nitrogenous compounds, thus improving the production of monocyclic aromatic hydrocarbons (MAHs) while effectively suppressing the formation of polycyclic aromatic hydrocarbons (PAHs). The highest content of aromatic hydrocarbons was obtained with an Fe loading of 8 wt% and pyrolysis temperature of 500 °C, which was 42.5%. As Fe loading increased, benzene selectivity decreased, while the selectivity of toluene and xylene first increased and then decreased. With an increase in pyrolysis temperature, benzene selectivity increased, whereas toluene and xylene selectivity increased at first and then decreased. Reaction mechanisms for the formation of aromatic hydrocarbons during the catalytic pyrolysis of microalgae are put forward. This research revealed that catalytic fast pyrolysis of microalgae over Fe-modified HZSM-5 catalysts is an effective method to produce MAHs.

Catalytic upgrading of lignite pyrolysis volatiles to light aromatics under methanol atmosphere over Zr and/or Fe modified hollow ZSM-5 zeolites

Fe and Zr modified hollow ZSM-5 zeolites were prepared and their applications in upgrading of lignite pyrolysis volatiles coupling with methanol to light aromatics were investigated. The results demonstrated that hollow zeolites with larger voids and shorter diffusion length favored the production of light aromatics. Adding Fe and Zr into ZSM-5 zeolites further enhanced BTX yields due to the synergism between metals and acid sites for promoting hydrodeoxygenation reactions. Meanwhile, Zr-Fe/HZ-5 catalyst with increasing Lewis acid sites facilitated methylation leading to high selectivity of xylene under methanol atmosphere. Moreover, diffusion behaviors of xylene were quantified. Higher self-diffusion coefficient of molecules in hollow zeolites led to the formation of less coke, while the incorporation of Fe and Zr promoted the formation of catalytic coke due to the increase of alkylation resulting in partial conversion of BTX into PAHs on external surfaces although the total coke yields were further decreased.

Low-pressure CO2 hydrogenation coupled with toluene methylation to para-xylene using atomic Pd-doped ZnZrOx–HZSM-5

Selective methylation of aromatics using CO2/H2 presents a promising avenue for producing high-value-added chemicals with high selectivity. Herein, we introduced atomically dispersed Pd species into ZnZrOx solid solution and combined with HZSM-5 to create a bifunctional catalyst, applying to selectively synthesize para-xylene through toluene methylation using CO2/H2 at low pressure. Remarkably, 0.1 wt% Pd in PdZnZrOx–HZSM-5 afforded the selectivity of xylene in CO-free products and para-xylene in xylene to 90.0% and 85.6% at 0.5 MPa, respectively. Spectroscopic characterizations revealed that atomically dispersed Pd species enhance the dissociation of adsorbed hydrogen and facilitate the creation of oxygen vacancies, benefiting the CO2 hydrogenation to CHxO intermediates. Density functional theory calculations suggested that Pd-doped ZnZrOx reduces the energy barrier for hydrogenating H2COOH* to H2CO*, aiding to form CH3O* intermediate for toluene methylation. This work provides insights into the design of catalysts for the selective methylation of aromatics using CO2/H2 at low pressure.

A MoS2 quantum dot functionalized TiO2 nanotube array for selective detection of xylene at low temperature

Electrochemically synthesized TiO2 nanotube array was functionalized with MoS2 quantum dots and transformed into metal–insulator–metal type sensor which exhibited xylene selectivity with high response magnitude at low operating temperature.

Effect of mesopore spatial distribution of HZSM-5 catalyst on zinc state and product distribution in 1-hexene aromatization

1-hexene aromatization is a promising technology to convert excess olefin in fluid catalytic cracking (FCC) gasoline to high-value benzene (B), toluene (T), and xylene. Besides, the increasing market demand of xylene has put forward higher requirements for new generation of catalyst. For increasing xylene yield in 1-hexene aromatization, the effect of mesopore structure and spatial distribution on product distribution and Zn loading was studied. Catalysts with different mesopore spatial distribution were prepared by post-treatment of parent HZSM-5 zeolite, including NaOH treatment, tetra-propylammonium hydroxide (TPAOH) treatment, and recrystallization. It was found the evenly distributed mesopore mainly prolongs the catalyst lifetime by enhancing diffusion properties but reduces the aromatics selectivity, as a result of damage of micropores close to the catalyst surface. While the selectivity of high-value xylene can be highly promoted when the mesopore is mainly distributed interior the catalyst. Besides, the state of loaded Zn was also affected by mesopores spatial distribution. On the optimized catalyst, the xylene selectivity was enhanced by 12.4% compared with conventional Zn-loaded parent HZSM-5 catalyst at conversion over 99%. It was attributed to the synergy effect of mesopores spatial distribution and optimized acid properties. This work reveals the role of mesopores in different spatial positions of 1-hexene aromatization catalysts in the reaction process and the influence on metal distribution, as well as their synergistic effect two on the improvement of xylene selectivity, which can improve our understanding of catalyst pore structure and be helpful for the rational design of high-efficient catalyst.

Aromatic hydrocarbons rich bio-oil production from Miscanthus pyrolysis by coupling torrefaction and MoO3/ZSM-5 dual catalysis process

The valorization of Miscanthus pyrolysis by coupling torrefaction and MoO3/ZSM-5 dual catalysis process was studied to generate the aromatic hydrocarbons rich bio-oil, biochar, and combustible gas. With the optimal conditions of torrefied Miscanthus at 280 °C, MoO3/ZSM-5 catalysis process converted 24.17 % bio-oil, 30.27 % biochar, and 45.56 % gas. The coupled system accessed bio-oil phase deoxygenation by reducing the oxygen-containing compound content to 6.27 %. The aromatic yield reached 91.38 %, with an outstanding benzene, toluene and xylene (BTX) selectivity of 52.24 %. Isolating investigations revealed that torrefaction declined the oxygen content of Miscanthus to 5.24 %, increasing their calorific value and promoting subsequent catalytic pyrolysis through deoxygenation. When only using the ZSM-5 catalyst, the increase in torrefaction temperature enhanced aromatic hydrocarbon compounds conversion in the bio-oil, leading to a boost in their content from 55.01 % to 75.97 %. Correspondingly, MoO3 promoted deoxygenation, decreased the ketone content from 40.4 % to 21.92 %, and inhibited the generation of aldehydes. The combination of MoO3 and ZSM-5 with torrefaction delivered a superimposed effect on yielding aromatic hydrocarbons and BTX, which allowed a promising product upgrading method for obtaining high-quality bio-oil.

Microwave susceptor positional configuration for palm empty fruit bunch and waste tire co-pyrolytic oil yield and selectivity of value-added chemicals

The co-pyrolysis of the palm empty fruit bunch (EFB) and waste tire (WT) was conducted at a constant microwave power of 600 W, a reaction temperature of 500 °C, and EFB-to-WT ratio of 65:35 using activated carbon as a carbonaceous microwave susceptor (CMS). The study investigated the effect of CMS positional configuration on heating rates, pyrolytic oil yield, and chemical selectivity. Nine CMS positions were tested, and the results showed that the CMS located at the bottom of the feedstock (P1) produced the highest average heating rate (80.45 °C.min−1) with the fewest hotspots. In contrast, the feedstock sandwiching the CMS (P3) had the lowest average heating rate (45.86 °C.min−1), leading to an inconsistent heating profile due to hotspot formation. The study also found that faster heating rates (>70 °C.min−1) significantly affected pyrolytic oil yield, with position P1 producing a higher yield (39.53 wt%). The properties of the pyrolytic oil were characterised using GCMS, which revealed that higher average heating rates favoured the formation of d-limonene, with position P1 containing a higher fraction of this compound (35.24%). Slower average heating rates (<60 °C.min−1) favoured phenolic selectivity. Surprisingly, the CMS position with significant hotspots promoted the formation of monoaromatic hydrocarbons, resulting in a higher relative content of these compounds. Furthermore, moderate average heating rates (60–70 °C.min−1) favoured benzene, toluene, ethylene, and xylene (BTEX) selectivity, with toluene and xylene being the most prevalent compounds. Notably, CMS positions had no discernible effect on the heating value (HHV) of the pyrolytic oil produced, which had an average value of 42.00 MJ.kg−1 ± 1.0.

Xylene Synthesis Through Tandem CO2 Hydrogenation and Toluene Methylation Over a Composite ZnZrO Zeolite Catalyst.

Selective synthesis of specific value-added aromatics from CO2 hydrogenation is of paramount interest for mitigating energy and climate problems caused by CO2 emission. Herein, we report a highly active composite catalyst of ZnZrO and HZSM-5 (ZZO/Z5-SG) for xylene synthesis from CO2 hydrogenation via a coupling reaction in the presence of toluene, achieving a xylene selectivity of 86.5 % with CO2 conversion of 10.5 %. A remarkably high space time yield of xylene could reach 215 mg gcat -1 h-1 , surpassing most reported catalysts for CO2 hydrogenation. The enhanced performance of ZZO/Z5-SG could be due to high dispersion and abundant oxygen vacancies of the ZZO component for CO2 adsorption, more feasible hydrogen activation and transfer due to the close interaction between the two components, and enhanced stability of the formate intermediate. The consumption of methoxy and methanol from the deep hydrogenation of formate by introduced toluene also propels an oriented conversion of CO2 .

Xylene Synthesis Through Tandem CO2 Hydrogenation and Toluene Methylation Over a Composite ZnZrO Zeolite Catalyst

AbstractSelective synthesis of specific value‐added aromatics from CO2 hydrogenation is of paramount interest for mitigating energy and climate problems caused by CO2 emission. Herein, we report a highly active composite catalyst of ZnZrO and HZSM‐5 (ZZO/Z5‐SG) for xylene synthesis from CO2 hydrogenation via a coupling reaction in the presence of toluene, achieving a xylene selectivity of 86.5 % with CO2 conversion of 10.5 %. A remarkably high space time yield of xylene could reach 215 mg gcat−1 h−1, surpassing most reported catalysts for CO2 hydrogenation. The enhanced performance of ZZO/Z5‐SG could be due to high dispersion and abundant oxygen vacancies of the ZZO component for CO2 adsorption, more feasible hydrogen activation and transfer due to the close interaction between the two components, and enhanced stability of the formate intermediate. The consumption of methoxy and methanol from the deep hydrogenation of formate by introduced toluene also propels an oriented conversion of CO2.

Temperature-dependent formaldehyde and xylene dual selectivity in ZnCo2O4 sphere-like architectures

The gas selectivity of gas sensing materials based on oxide semiconductors is an important parameter for evaluating their functions. Revealing the dominant factor of sensing selectivity is a prerequisite for optimizing gas sensing performance. Herein, the temperature-dependent gas sensing selectivity is achieved in ZnCo2O4 sphere-like architectures via simple solvothermal-annealing treatments instead of noble metal loading or heterojunction construction. The ZnCo2O4 sphere-like architectures after two solvothermal-annealing treatments show superior selectivity to formaldehyde at 100 ℃ and simultaneously present high xylene selectivity at 180 ℃. However the as-prepared ZnCo2O4 sphere-like architectures without treatments merely exhibit high selectivity to formaldehyde at 100 ℃. Such gas sensing behaviors prominently stem from the catalytic promotion discrepancy induced by oxygen vacancies at different working temperatures. The feasible gas sensing selectivity regulation strategy holds great promise for other oxide semiconductor materials.

Machine learning-driven prediction and optimization of monoaromatic oil production from catalytic co-pyrolysis of biomass and plastic wastes

Catalytic co-pyrolysis of biomass and plastic wastes is an efficient way for monoaromatic-rich oil production, while it is difficult to conclude oil evolution rule due to the complex interaction of multiple factors during co-pyrolysis. Hence, multiple machine learning algorithms were devised to aid process optimization and zeolite catalyst screening. The results showed that gradient boosting decision tree model performed the best prediction accuracy and generalization ability, as its coefficient of determination (R2) and root-mean-square error were 0.90 and 5.04, compared to random forest (R2 ∼ 0.84), extreme gradient boosting (R2 ∼ 0.90), and light gradient boosting machine (R2 ∼ 0.86). Both feature importance and partial dependence analysis showed that operating parameters, catalyst properties, and feedstock composition were in the descending order to affect oil yield with monoaromatic selectivity. The reaction temperature (∼500 °C) and feedstock/catalyst ratio (<5:1) had preferable influence on higher oil yield. The optimal selectivity of benzene, toluene, ethylbenzene and xylene (BTEXs) in oil would be obtained at plastic proportion of around 60 wt% and Si/Al ratio of 20–30. The two-way PDA about interaction effect between multiple factors for oil products were also discussed in detail. The work favors to rationalize large-scale oil production from pyrolysis of solid wastes.

Anti-carbon deposition performance of twinned HZSM-5 encapsulated Ru in the toluene alkylation with methanol

Toluene methylation with methanol to produce para-xylene has been extensively and intensively studied. However, the methanol-to-hydrocarbons (MTH) side reaction in this reaction is difficult to be inhibited, which will cause a mass of carbon deposition and cover the catalyst surface, resulting in catalyst deactivation. Here, a dual-functional Ru@HZSM-5 catalyst with high para-selectivity and low carbon deposition was prepared by encapsulating Ru metal with HZSM-5. According to catalytic performance studies, the Ru@HZSM-5 catalyst produced xylene selectivity of 98% and para-xylene selectivity of 96%. Meanwhile, we find that carbon precursors (e.g. ethylene) were very little when Ru catalyst was used, but the results of HZSM-5 catalyst were completely opposite. Ru@HZSM-5 catalyst achieves a lower carbon deposition rate of only 6% of HZSM-5. The main possible reason for this is that the initial C–C bond between methanol and the olefin is difficult to form.

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.

B-Axis-Oriented ZSM-5 Nanosheets for Efficient Alkylation of Benzene with Methanol: Synergy of Acid Sites and Diffusion

ZSM-5 nanosheets are promising catalysts for the catalytic reactions controlled by diffusion limitations. This study reveals its significant application in the alkylation of benzene with methanol. The b-axis-oriented ZSM-5 nanosheets with similar acid property but varied thicknesses of about 30, 90, and 300 nm were prepared to investigate the effect of thickness on their catalytic properties for alkylation reactions. Comparative results demonstrate that the sample with a thickness of 30 nm exhibits higher benzene conversion, xylene selectivity, and methyl selectivity (up to 97%), accompanied by an ultralong lifetime (up to 1000 h, 10 times longer than that of the sample with a thickness of 300 nm) and lower byproduct ethylbenzene selectivity. This is ascribed to the shortened straight channel length, increased specific surface area, and enlarged mesopore volume that significantly facilitate the diffusion of reactants and products, increase the accessibility of acid sites, and decrease the coke formation. Moreover, compared with conventional ZSM-5 nanocrystals, ZSM-5 nanosheets deliver a substantially extended lifetime due to fewer framework defects. Most significantly, this study unravels the diffusion effect on ethylbenzene selectivity over ZSM-5 nanosheets with different thicknesses and illustrates the role of strong Brønsted acid sites in the dynamic changes of ethylbenzene selectivity. In light of the above analysis, we developed a precoking strategy and an introducing-heteroatom strategy to precisely tailor the catalyst acidity, further suppressing the ethylbenzene formation (<0.3%) while maintaining long-term stable operation (>300 h).

Tailoring the framework aluminum arrangement in ZSM-5 zeolite to regulate reaction route for alkylation of benzene with methanol

Aluminum (Al) arrangement, as a description of distance between the framework Al atoms, can be categorized as the proximity (Alp) and isolation (Ali) states in the zeolite cage, which may change the local hydrophilicity and determine the adsorption capacity in the pores of zeolite. Herein, a phase-transfer method was employed to tailor the framework Al arrangement, which were beneficial to enhance the catalytic performance in alkylation of benzene and methanol. The samples were characterized to verify their Al arrangement and evaluated on the alkylation of benzene with methanol. The conversion of benzene and the selectivity of xylene was significantly improved in the zeolite with more Ali. Furthermore, in-situ IR spectroscopy demonstrated that the adsorption capacity of benzene and methanol influenced by the Al arrangement was the main reason for the enhancement of alkylation. Also, 2D IR correlation maps further illustrated that the improvement of the alkylation is the result of the preference in different reaction routes.

One-pot synthesis of lignin biochar supported Ni for catalytic pyrolysis of Chlorella vulgaris and its model compounds: The formation mechanism of aromatic hydrocarbons

A novel catalyst of lignin biochar supported Ni was prepared through one-pot pyrolysis for the catalytic pyrolysis of Chlorella vulgaris. The catalysts with various Ni loading ratios were rich in functional groups with surface areas of 290.87–458.97 m2/g and average micropore sizes of 0.47–0.56 nm. The in-situ reduction during catalyst preparation yielded 33.13 % of Ni0 with average particle size of 37.57 nm. The selectivity of benzene, toluene, and xylene (BTX) from the catalytic pyrolysis of the microalgae reached up to 75.62 % with the quantitative yield of benzene of 58.01 mg/g. The catalytic pyrolysis of the microalgae model compounds (rapeseed oil, glucose, soybean protein, and several amino acids) revealed high catalytic activity for the production of aromatic hydrocarbons based on glucose and soybean protein with 95.72 % and 90.75 % in selectivity, respectively. The study on model compounds conversion elucidated the formation mechanism of aromatic hydrocarbons by removal of heteroatoms (N and O).

Cooperative External Acidity and Surface Barriers of HZSM-5 in the Coupling Reaction of CH3Cl and CO to Aromatics

The acidity and diffusion barriers of zeolites, especially the external surface, are prevailing parameters in the zeolite-catalyzed synthesis of aromatics. Nonetheless, how to well coordinate the external acidity and surface barriers to obtain high benzene–toluene–xylene (BTX) selectivity and exceptional stability remains a great challenge. Herein, we experimentally demonstrate the synergistic effect of external acidity and surface barriers in the coupling of CH3Cl and CO to aromatics via a surface modification strategy. The results manifest that product selectivity is controlled by external acidity, while catalyst lifetime is controlled by surface barriers. Accordingly, when simultaneously passivating the external acidic sites and reducing the surface barriers, the BTX selectivity was enhanced by 133%, and the catalyst lifetime was prolonged. Our findings provide a new perspective for rational designing of catalysts with superior performance based on controlling the acidity and mass transfer on purpose.

Preferential Synthesis of Toluene and Xylene from CO2 Hydrogenation in the Presence of Benzene through an Enhanced Coupling Reaction

The conversion of CO2 into high-value-added chemicals, especially aromatics, is appealing for the alleviation of carbon emissions. However, preferential production of valuable aromatics including benzene, toluene, and xylene (BTX) from CO2 is still a great challenge through direct hydrogenation routes due to the complicated side reactions. Herein, we apply a composite catalyst containing metal oxides (ZnZrOx) and Zn-modified zeolites (Zn/ZSM-5) to selectively synthesize toluene and xylene through CO2 hydrogenation in the presence of benzene. A total selectivity of toluene plus xylene as high as 92.8% is achieved with C1–5 alkane selectivity of less than 4%, as the CO2 conversion reaches 21% and the benzene conversion reaches 22.2%. We demonstrate that the appropriate acid properties of zeolites play a pivotal role in the coupling reaction, which can be readily adjusted by the introduction of Zn. The increase of moderate Zn-induced Lewis acid sites with consumption of strong Brønsted acid sites not only promotes the formation of intermediate alkylation species but also facilitates the recovery of occupied active sites by regulating the successive transformation of formate intermediates and desorption of aromatic products. The enhanced coupling effect results in the elevation of both reactant conversion and specific aromatics products’ (toluene and xylene) selectivity. This work may provide an efficient strategy for the synthesis of high-value aromatics through coupling reactions from CO2 hydrogenation.