Stronger Acid Strength Research Articles

Selective and Controllable Cracking of Polyethylene Waste by Beta Zeolites with Different Mesoporosity and Crystallinity.

Waste plastics bring about increasingly serious environmental challenges, which can be partly addressed by their interconversion into valuable compounds. It is hypothesized that the porosity and acidity of a zeolite-based catalyst will affect the selectivity and effectiveness, enabling a controllable and selective conversion of polyethylene (PE) into gas-diesel or lubricating base oil. A series of embryonic, partial- and well-crystalline zeolites beta with adjustable porosity and acidity are prepared from mesoporous SBA-15. The catalysts and catalytic systems are studied with nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and adsorption kinetics and catalytic reactions. The adjustable porosity and acidity of zeolite-beta-based catalysts achieve a controllable selectivity toward gas-diesel or lubricating base oil for PE cracking. With a catalyst with mesopores and appropriate acid sites, a fast escape and reduced production of cracking of intermediates are observed, leading to a significant fraction (88.7%) of lubricating base oil. With more micropores, a high acid density, and strong acid strength, PE is multiply cracked into low carbon number hydrocarbons. The strong acid center of the zeolite is confirmed to facilitate significantly the activation of hydrogen (H2),and, an in situ ammonia poisoning strategy can significantly inhibit hydrogen transfer and effectively regulate the product distribution.

Regulating acid properties of ZSM-5 nanosheets by Fe incorporation towards high-performance alkylation of benzene with methanol

A clear understanding of the acidity-activity relationship is an important prerequisite for designing functional materials in alkylation of benzene with methanol. Herein, a series of b-axis-oriented [Fe/Al]-MFI nanosheets with similar thickness but varied acidities were prepared by tuning Fe incorporation to systematically study the effect of acidity on their alkylation performance. Comprehensive physiochemical studies demonstrate that tetrahedral framework type Fe species are dominated. The incorporation of Fe significantly weakens the strong acid strength of zeolite thus endowing the samples with proper acidity, consequently showing a strong inhibition effect on the by-product ethylbenzene. Moreover, a long-term stable operation (>300 h) is further achieved over the [Fe]-S-1 nanosheets by optimizing the content of framework Fe. This work elucidates the important relationship between acidity and alkylation activity.

Novel Environmentally Friendly Nanomaterials for Drag Reduction of the Emulsified Acid System.

Matrix acidizing is a technique that is widely used in the petroleum industry to remove scales and create channels in the rock. Removal of scales and creation of channels (wormhole) enhance productivity. Conventional acidizing fluids, such as hydrochloric acid (HCl) for carbonate and a mixture of hydrofluoric acid (HF) and HCl acid, are used for the matrix acidizing process. However, these fluids have some drawbacks, including strong acid strength, corrosion at high temperatures, and quick reactions with scale and particles. Emulsified acid systems (EASs) are used to address these drawbacks. EASs can create deeper and narrower wormholes by reducing the reaction rate of the acid due to the external oil phase. However, EASs have a much higher viscosity compared to conventional acidizing fluids. The high viscosity of EASs leads to a high drag that restricts pumping rates and consumes energy. This study aims to utilize environmentally friendly and widely available nanomaterials as drag-reducing agents (DRAs) of the EAS. The nanomaterials used in this study are carbon nanodots (CNDs). CNDs have unique properties and are used in diverse applications in different industries. The size of these CNDs is usually smaller than 10 nm. CNDs are characterized by their biocompatibility and chemical stability. This study investigates the use of CNDs as DRAs for EAS. Several experiments have been conducted to investigate the CNDs as a DRA for the EAS. The developed EAS was initially tested for conductivity and drop-test analysis to ensure the formation of an inverted emulsion. Thereafter, the thermal stability for the range of temperatures and the rheological properties of the EAS were evaluated to meet the criteria of field operation. Then flow experiments with EASs were conducted before and after adding the CNDs to investigate the efficacy of drag reduction of EASs. The results revealed that CNDs can be used as viscosity reducers for the EAS, where adding the CNDs to the EAS reduces the viscosity at two different HCl concentrations (15 and 20%). It reduces the viscosity of the EAS in the presence of corrosion inhibitors as well as other additives to the EAS, showing its compatibility with the field formulation. The drag reduction was observed at the range of temperatures investigated in the study. The conductivity, stability, and rheology experiments for the sample taken after the flow experiment are consistent, ensuring CNDs work as a DRA. The developed EAS with CNDs is robust in terms of field mixing procedures and thermally stable. The CNDs can be used as a DRA with EAS, which will reduce drag in pipes, increasing pumping rates and saving energy.

Low-temperature catalytic dechlorination of model plastic pyrolytic oil over zeolite catalysts

Catalytic dechlorination of model plastic pyrolytic oil was investigated over different types of zeolites and ion-exchanged 13X zeolites at the low temperatures of 20–120 °C. Among different types of parent zeolites (13X, β, MCM-41), 13X had the highest catalytic dechlorination performance. After ion exchange of 13X with Ag+, its catalytic dechlorination performance was significantly enhanced, and the chlorine removal over Ag-13X reached 97.2 % after reaction for 5 h at 120 °C. The characterization results indicated that the acidity of the zeolite governed its catalytic dechlorination performance, and higher acid amount and stronger acid strength are preferred. Different types of chlorinated compounds followed different dechlorination mechanisms. The primary chlorinated alkanes (e.g., 1-chlorooctane) underwent nucleophilic substitution by the surface –OH groups or H2O to form alcohols and HCl, or dehydrochlorination reaction to produce olefins and HCl, while higher chlorinated alkanes with adjacent hydrogen (e.g., chlorocyclohexane) mainly underwent dehydrochlorination reaction to produce olefins and HCl. Chlorinated aromatics (e.g., chlorobenzene) was removed mainly by physical adsorption. Ag-13X showed good stability, and the chlorine removal still retained 72.1 % after 102 h on stream. After four dechlorination-regeneration cycles, the chlorine removal had only a slight decrease due to the partial collapse of its structure and slight loss of its acid amount.

Enhanced esterification performance by SO42−/ZrO2 anchored Nb2O5 nanotube

SO42-/ZrO2 anchored Nb2O5 nanotubes (SO42-/ZrO2@Nb2O5-NT) are prepared via the high temperature calcination, proton exchange, exfoliation, recombination, and impregnation processes. The structures and anchoring model can be confirmed by XRD, HRTEM, and FT-IR technologies. The acidic properties and specific surface area are characterized by NH3-TPD and N2 adsorption–desorption isotherms. The catalytic performances are evaluated via an esterification reaction of acetic acid and n-butanol. The results show that SO42-/ZrO2@Nb2O5-NT have stronger acid strength and higher acid content than that of Nb2O5-NT. The optimal esterification scheme is as follows: catalyst dosage is 11 wt.%, molar ratio of alcohol to acid is 2.7:1, reaction temperature is 118 ℃, and reaction time is 7 h. The esterification rate can be reached up to 92.31% over SO42-/ZrO2@Nb2O5-NT. We have rationalized esterification performance of these selected catalysts in light of structure–property relationships, which gives useful insight for the development and application of next generation esterification catalyst.

Green synthesis of shaped nano-ZSM-5 toward more efficient methanol-to-hydrocarbon conversion

Commonly, in the synthesis of zeolites, calcination is necessary prior to ion exchange to remove organic templates. Herein, shaped nano-ZSM-5 was successfully prepared with the assistance of template-free nano-aggregate seed without the calcination step before ion exchange. This synthesis method dramatically increased mesopores volume compared with the one calcined prior to ion exchange. Besides, employing nano-aggregate as seed gave the synthesized ZSM-5 a markedly smaller particle size and higher yield than the one prepared with conventional silicate-1 as seed, and simultaneously accelerated the crystallization process by providing more nuclei. A possible formation mechanism of the shaped nano-ZSM-5 zeolites was proposed. The shaped nano-ZSM-5 possessed the smallest crystal size, largest mesopore volume, most acid sites and strongest acid strength, which endowed it highest catalytic activity and longest lifetime in methanol to hydrocarbons reaction. This work could attract more attention on the application of nanoscale ZSM-5 catalyst to industry.

Ultrafast Synthesis and Regulating Al Status of Mesoporous ZSM-48 Zeolite via a Pretreated-Seed-Solution-Assisted Strategy

Ultrafast synthesis of nanocrystal assembled mesoporous ZSM-48 zeolite spheres of highly crystalline was achieved within 1.5 h by using pretreated-seed-solution-assisted strategy. Interestingly, by altering the introducing sequence of aluminum source (in the pretreated seed solution or in the synthesis gel), the obtained ZSM-48 with similar particle size and total acid sites exhibited quite different crystallization behavior and acid property in terms of Al distribution and acid strength, because of the distinct local microenvironment (Al-rich or Al-free) in the pretreated seed solution. The HZ48-Al-pre (introduce an Al source to the pretreated seed solution) had more framework Al(IV) content and stronger acid strength than that of the HZ48-Al-post (synthesized by the Al-free pretreated seed solution). The corresponding Pt supported catalysts showed a great difference in the n-C12 hydroisomerization reation. The Pt/HZ48-Al-pre catalyst had a very low selectivity (41.1%), and the maximum iso-yield was only 35.5%. However, the ultrafast synthesizedPt/HZ48-Al-post-1.5h catalyst exhibited a superior selectivity (90.1%) with a total i-C12 yield of 82.3% at 330 °C, which was a high level in the exist literature reports.

Deep eutectic solvent assists H-β in aldol condensation of acetovanillone and p-hydroxybenzaldehyde to prepare a high density aviation oil precursor

An efficient deep eutectic solvent (DES) catalytic system was developed for the aldol condensation of lignin derivatives acetovanillone (AV) with p-hydroxybenzaldehyde (PHB) to prepare C16 high energy fuel precursors to solve the problems of low coverage of lignin materials, low catalyst selectivity, and compatibility with solvents. With the optimized reaction conditions of a substrate molar ratio of 1:2, 5 g ChCl/Fa (1:6), 15 wt% H-β, and 100 ℃ for 120 min, the conversion rate of AV reached 88.68%, while the selectivity and yield of C16 reached 78.5% and 69.61%, respectively. The stability of H-β in DES was found to be better than that in conventional solvents based on SEM, TEM, XRD, and TG analysis. Moreover, NH3-TPD proved that DES could effectively increase the strong acid strength of H-β, thereby enhancing reactivity. Moreover, DES increased the specific surface area of the H-β catalyst from 109.59 m2/g to 404.118 m2/g and had a significant pore-enlarging effect (from 1.529 nm to 2.157 nm), which accelerated the reaction progress. DES was also found to be less toxic to H-β, enabling it to be regenerated. Furthermore, the shape-selective catalytic mechanism of the DES-assisted H-β system was demonstrated in the feedstock pool constructed by different lignin-derived platform compounds.

Activated Bentonite Nanocomposite for the Synthesis of Solketal from Glycerol in the Liquid Phase

Activated bentonites are low-cost acid catalysts used in several reactions. However, their application at an industrial scale is affected by the formation of colloidal suspensions when these bentonites are in aqueous solutions. In order to overcome these limitations, this work proposes obtaining a catalyst based on a composite containing natural bentonite within a silica–resin structure, which allows separating and re-utilizing the catalyst more easily and without centrifugal filtration requirements. By means of characterization techniques, the present study determined that the activated bentonite composite presented a total specific surface area of ~360 m2 g−1, ~4 mmol of acid sites per gram of bentonite, and sites with strong acid strength, all of which bestowed activity and selectivity in the solketal synthesis reaction from glycerol and acetone, reaching equilibrium conversion within a short reaction time. Furthermore, the present work developed a Langmuir–Hinshelwood–Hougen–Watson kinetic model, achieving an activation energy of 50.3 ± 3.6 kJ mol−1 and a pre-exponential factor of 6.4 × 106 mol g−1 L−1 s−1, which are necessary for reactor design.

Synthesis of mesoporous high-silica zeolite Y and their catalytic cracking performance

Mesoporous high-silica zeolite Y with advantages of improved accessibility of acid sites and mass transport properties is highly desired catalytic materials for oil refinery, fine chemistry and emerging biorefinery. Here, we report the direct synthesis of mesoporous high-silica zeolite Y (named MSY, SiO 2 /Al 2 O 3 ≥ 9.8) and their excellent catalytic cracking performance. The obtained MSY materials are mesoporous single crystals with octahedral morphology, abundant mesoporosity and excellent (hydro)thermal stability. Both the acid concentration and acid strength of H-form MSY are obviously higher than those of commercial ultra-stable Y (USY), which should be attributed to the uniform Al distribution of MSY zeolite. The H-MSY displays an obviously reduced deactivation rate and improved catalytic activity in the cracking reaction of bulky 1,3,5-triisopropylbenzene (TIPB), as compared with its mesoporogen-free counterpart and USY. In addition, H-MSY was investigated as catalyst for the cracking of industrial heavy oil. The MSY-based catalyst (after aging at 800 o C in 100% steam for 17 h) exhibits superior conversion (7.64% increase) and gasoline yield (16.37% increase) than industrial fluid catalytic cracking (FCC) catalyst under the investigated conditions. Mesoporous high-silica zeolite Y with high (hydro)thermal stability has been successfully synthesized, which possesses larger acid concentration and stronger acid strength than USY, and exhibits superior catalytic performance in the cracking of hydrocarbons and industrial heavy oil.

Synergistic effect of low valence manganese ions in manganese-based catalysts for non-oxidative dehydrogenation of isobutane

A series of manganese-based catalysts supported on γ-Al2O3 with low loading amount of manganese from 0.5% to 2.0% were prepared by incipient impregnating method. All catalysts were tested on non-oxidative dehydrogenation of isobutane at 600 °C. In order to explore physicochemical properties of all catalysts, various characterizations including XRD, H2-TPR, XPS, UV-vis, NH3-TPD and TG were carried out on manganese-based catalysts. The experimental results indicated that MnOx species were highly dispersed on γ-Al2O3 support, and both Mn2+ and Mn3+ ions are main active species for isobutane dehydrogenation. The reduced catalyst 1.5%Mn/γ-Al2O3 exhibited the most optimal dehydrogenation performance among all catalysts, reaching at isobutane conversion of 36% and isobutene selectivity of 79%. The main reasons for the excellent activity and selectivity of 1.5%Mn/γ-Al2O3 were related to the high proportion of low valence manganese ions including Mn2+ and Mn3+ as well as the high concentration of medium strong acid sites and weak strong acid strength.

Step-change performance enhancement of ethylene aromatization on Ga-ZSM-5 through steaming treatment

In continuation of our two-step process development for the conversion of ethane to aromatics, this work focuses on catalyst performance improvement for ethylene aromatization. The approach is to apply steaming dealumination method to tailor ZSM-5 acidity to optimize its catalytic performance. Our results indicate that applying steaming treatment for the base Ga-ZSM-5 catalyst at specified conditions could reduce the total acid amount from 0.46 to 0.15–0.25 mmol/g, with 46–67% acidity loss, accompanied with the decrease of the acid strength for the strong acid sites. As a result, the catalyst prepared in this way exhibited a step-change performance enhancement in terms of cycle lifetime, target product yield/capacity, byproduct, coke formation and so on. Through examining the reaction mechanism and acid strength requirements for individual steps involved in ethylene aromatization, we conclude that steps that require strong acid strength – such as cracking and aromatics alkylation (both leading to catalyst deactivation) are effectively suppressed due to the lower acid amount/strength obtained after steaming treatment. The findings and this zeolite acidity tailoring technique will provide insights and solutions for zeolite-based acidic catalyst design.

Insights into bimetallic synergistic effect towards γ-valerolactone production under Co doped Zr-TiO2

Co doped into 5% Zr-TiO2 (Co@5% Zr/-TiO2) was prepared by a simple, facile sol-gel method, and employing in the catalytic transfer hydrogen (CTH) process starting from ethyl levulinate (EL) to γ-valerolactone (GVL). Lewis/Brønsted acid ratio and BET surface area increased with the incorporation of Co into Ti-Zr-O support. In addition, the synergistic effect between Co-Zr in TiO2 enhanced the strong acid strength sites. Co@5% Zr-TiO2 exhibited the highest catalytic performance (EL conversion 95%, GVL yield 88%) under optimum condition (0.2 M EL, 15 mL 2-PrOH, 190 °C, 11 h). Noticeably, Co@5% Zr-TiO2 exhibited higher stability in 4th recycling experiments as compared to 5% Zr-TiO2 under identical condition. From computational calculation, EL adsorption process was more spontaneous with ΔEad= -8.240 eV. The protonation process made the reactants get close to the surface with a bonded O12-Ti2.179 Å and O6-Zr of 2.376 Å. Finally, GVL showed a strong leaving trend after the formation.

Understanding the effects of acid strength of active center and local confinement environment on the conversion of methanol to olefins in H-RUB-50

Brønsted acid (BA) strength and confinement effect of the cavity-type zeolites play an important role in the formation of products during methanol to olefins (MTO) conversion. In the present study, olefin-based cycle using 2,3-dimethyl-2-butene (iso-C6) as the hydrocarbon-pool (HCP) species was systematically investigated in H-RUB-50 zeolite with 8-membered ring (MR) pore opening and small LEV cavity by using periodic density functional theory calculations combined with microkinetic modeling, so as to evaluate the effects of different local confined space, active acid sites and acid strength caused by different Al sites on the catalytic performance of MTO reaction. The free energy profiles at four different BA sites (Al1O2, Al1O4, Al2O2 and Al2O5) generated by Al sitting at T1 and T2 sites in 8-MR window show that the olefin-based cycle is conducive to the formation of ethene at the four BA sites in H-RUB-50 zeolite, and Al1O2 BA site with the strongest acid strength is the most active site for MTO reaction. The change trend of TOF of ethene and propene shows that the acid strength is closely related to the formation of ethene, but for the formation of propene, not only the acid strength, but also the local confined space of the active center plays a more important role. In H-RUB-50 zeolite, the selectivity to ethene based on the olefin-based cycle is higher than to propene, which is consistent with the previous theoretical and experimental results. It is shown that the overall catalytic activity and ethene selectivity of H-RUB-50 at T1 site is much better than that at T2 site. This study provides a useful guidance for the experimental synthesis and adjustment of Al sites to improve the catalytic performance of MTO.

Acetalization of glycerol over sulfated UiO-66 under mild condition

Sulfated metal–organic framework has attracted much attention as a versatile heterogeneous acid for its strong acid strength, large surface area and regular pore channels. In this work, sulfated UiO-66 (UiO-SO3H-x) was prepared via the direct oxidation of 2-mercaptobenzoic acid substituted UiO-66. The structure and property of the synthesized UiO-SO3H-x were characterized via XRD, FTIR, static water contact angles, XPS, TG-DTA, N2 adsorption–desorption, SEM and TEM in detail. It was found that sulfonic acid groups dispersed homogenously in the framework of UiO-SO3H-x. And UiO-SO3H-0.2 exhibited excellent performance and stability for the synthesis of solketal from surplus glycerol among the tested solid acids (H3PW12O40, Amberlyst-45, HY, HBEA, HZSM-5 and ZrHP). The detected conversion of glycerol reached 70.3% with a 99.8% selectivity toward solketal at 60 °C, and it could maintain its initial activity in five recycles. The highest TOF of each acid site in UiO-SO3H-0.2 reached 676.0 h−1. Moreover, it was versatile for the acetalization of glycerol with other ketones/aldehydes.

Assessment on the Effect of Sulfuric Acid Concentration on Physicochemical Properties of Sulfated-Titania Catalyst and Glycerol Acetylation Performance

In this research, a solid acid catalyst was synthesized to catalyse glycerol acetylation into acetins. The sulphated-titania catalysts were prepared via the wet impregnation method at different sulfuric acid concentrations (5%, 10%, 15%, and 20%) and denoted as 5SA, 10SA, 15SA, and 20SA, respectively. The synthesized catalysts were characterized using FTIR, XRD, TGA, BET, NH3-TPD, XRF, and SEM-EDX. The synthesized catalysts were tested on glycerol acetylation reaction at conditions: 0.5 g catalyst loading, 100–120 °C temperature, 1:6 glycerol/acetic acid molar ratios, and 2–4 h reaction time. The final product obtained was analysed using GC-FID. An increment in sulfuric acid concentration reduces the surface area, pore volume, and particles size. However, the increment has increased the number of active sites (Lewis acid) and strong acid strength. 15SA catalyst exhibited excellent glycerol conversion (>90%) and the highest selectivity of triacetin (42%). Besides sufficient surface area (1.9 m2 g−1) and good porosity structure, the great performance of the 15SA catalyst was attributed to its high acid site density (342.6 µmol g−1) and the high active site of metal oxide (95%).

High-temperature selective catalytic reduction of NO with NH3: Optimization of ZrO2 and WO3 complex oxides

The catalyst for selective catalytic reduction of NOx at high temperatures is significantly demanded by current gas-fired exhaust purification. In this work, the complex oxides composed of two different structures of ZrO2-WO3 and WO3/ZrO2 with zirconium as the main component were prepared for NH3-SCR of NO at high temperatures by blending method and equal volume impregnation method, respectively. Combined with XRD, NH3-TPD, FE-SEM, BET and in situ DRIFTS characterization, the effects of component optimization of two kinds of ZrO2 and WO3 complex oxides on the performance for NH3-SCR of NO were mainly studied. The optimal compatibility of ZrO2 and WO3 complex oxides was determined, and the corresponding N2 selectivity and anti-interference ability of water vapor and SO2 were further investigated. Results showed that the performance of ZrO2-WO3 prepared by blending method for NH3-SCR of NO was significantly better than that of WO3/ZrO2 prepared by impregnation method. The ZrO2-WO3 containing 20% WO3 has the best performance in NH3-SCR of NO with NO conversion higher than 90% and N2 selectivity higher than 90% at 450–650 °C, and the performance was not affected by SO2, slightly affected by competitive adsorption of water vapor at 450 °C and not disturbed by water vapor above 500 °C. Further investigation showed that the NH3-SCR of NO over the ZrO2-WO3 reaction followed both L-H and E-R mechanisms. Characterization analysis revealed that ZrO2-WO3 has finer microscopic particles, larger specific surface area, higher acid amount and stronger acid strength than WO3/ZrO2.

The optimization of hydrothermal synthesis of MnxCo3‐xO4/GC catalyst for low temperature NH3‐SCR /using design of experiments

AbstractBACKGROUNDIn this study, a new catalyst for low‐temperature selective catalytic reduction of NOx with NH3 (NH3‐SCR) were prepared by using glucose and discussed optimization of the synthetic conditions of MnxCo3−xO4/GC (glucose carbon) catalysts by using the design of the experiments. A response surface methodology (RSM) combined with the central composite design (CCD) was applied to model and optimize the synthetic conditions.RESULTSAs a result, the optimum NOx conversion of 95.6% and N2 selectivity of 81.8% were obtained at Mn/Co 2.1, urea/(Mn + Co) mole ratio 11.5, glucose/(Mn + Co) mole ratio 8.9, preparation temperature 175.4 , and calcination temperature 376.3 . The SEM (Scanning electron microscope) image and EDS (Energy Dispersive Spectroscopy) analysis showed that metal oxides of Mn and Co were uniformly accumulated on the carbon microsphere surface. XRD (X‐ray diffraction) patterns showed that CoCo2O4 content in the catalyst increased with the increase of the glucose/(Mn + Co) mole ratio, which reduced the interaction between Mn and Co, and played a negative impact on catalytic performance. FTIR (Fourier‐transform infrared) spectra showed that the catalyst surface in the optimum preparation conditions had more oxygen‐containing functional groups and that the excessive glucose carbon was unfavourable to catalytic performance. For urea/(Mn + Co), as urea/(Mn + Co) decreased, in addition to the decrease in Mn and Co content in the catalyst due to the decrease in precipitant, CoCo2O4 content in the catalyst also increased and oxygen‐containing functional groups decreased, both of which resulted in the decrease of the catalytic performance.CONCLUSIONThe H2‐TPR and NH3‐TPD results demonstrated that the catalyst prepared in the optimum conditions possessed a stronger reducing ability, more acid sites, and stronger acid strength than the other. © 2021 Society of Chemical Industry

Effect of Lewis/Brønsted acid sites in HZSM−5 zeolite on the selectivity of para-xylene during methylation of toluene with methanol

• The participation of Lewis acid site is good for the dissociation of methanol. • The activity of methanol dissociation has positive correlation with the acid strengths of HZSM-5 zeolites. • ZnOH + /BAS-HZSM−5 with the moderate acid strength is conducive to the formation of PX. The methylation reaction of methanol and toluene to xylenes has been carried out in Brønsted Acid Sites (BAS)−HZSM−5, AlOH 2+ /BAS−HZSM−5 and ZnOH + /BAS−HZSM−5 catalysts. The stronger acid strengths are produced in AlOH 2+ /BAS−HZSM−5 and ZnOH + /BAS−HZSM−5 due to the synergistic effect of Lewis Acid Sites (LAS) and BAS compared to BAS−HZSM−5, which is beneficial to the dissociation of methanol. AlOH 2+ /BAS−HZSM−5 shows the highest activity for methanol dissociation with the strongest acid strength. The Gibbs free energy barrier is only 68.2 kJ·mol −1 , but the formation of Para -xylene (PX) needs to overcome a high barrier of 129.2 kJ·mol −1 . However, ZnOH + /BAS−HZSM−5 exhibits relative high activity for the formation of methoxy with the Gibbs free energy barrier of 113.9 kJ·mol −1 , and 92.9 kJ·mol −1 for PX. Meanwhile, the formation of PX is easiest among three xylenes. Therefore, ZnOH + /BAS−HZSM−5 can be as the promising catalyst for the preparation of PX during methylation of toluene with methanol.

Systematic Catalyst Screening of Zeolites with Various Frameworks and Si/Al Ratios to Identify Optimum Acid Strength in OME Synthesis

AbstractOxymethylene dimethyl ethers (OMEx oligomers with the chain length x) present a promising emission reducing diesel additive, which can potentially be produced based on renewable resources. In this work, the OMEx synthesis catalysed by zeolites was investigated starting from paraformaldehyde and methanol. The catalytic performance of zeolites with different framework types (BEA, FAU, MFI, MOR) and various Si/Al ratios was comparatively studied. The zeolites were characterised by means of NH3‐TPD, ICP‐OES, XRD and N2 physisorption. A relationship between catalytic activity and Si/Al ratio was established demonstrating the highest turnover frequency for zeolites with high Si/Al ratios. HZSM‐5 114 with a TOF of 3375 molOME3–5 molAcidSites−1 h−1 was determined as the best‐performing zeolite which indicates that the OMEx synthesis requires a moderately strong acid strength.