Gas To Liquid Research Articles

Reactive simulation of gas-catalyst thermochemical properties in a pilot-plant catalytic MTO fluidized bed

The methanol-to-olefin (MTO) process enables economical production of light olefins thus decreases the reliance on petroleum resources. The reactive multiphase flow patterns and reaction behaviors inside pilot-plant MTO fluidized reactor are numerically simulated with multiphase particle-in-cell model. Based on the model validation, the thermochemical properties of catalytic particle (e.g., heat transfer coefficient (HTC), temperature) and gas phase thermal characteristics are comprehensively investigated with the discussion on several crucial operating parameters. The results reveal that large HTC of catalyst particle is located in the crucial reaction zone and freeboard zone. The HTC of catalyst material covers a range of 50 to 150 W/(m2·K). Gas-solid hydrodynamics and gas concentrations exhibit spatial non-uniform distribution. Moreover, the reactant gas flow rate is more positively sensitive to the product distributions and gas thermal properties. Enlarging the reaction pressure significantly enhances the heat transfer. The vertical dispersion coefficient of catalyst is two orders of magnitude larger than the horizontal ones.

Mechanisms and Kinetics of the Dehydrogenation of C6–C8 Cycloalkanes, Cycloalkenes, and Cyclodienes to Aromatics in H-MFI Zeolite Framework

This work employs periodic density functional theory to elucidate dehydrogenation mechanisms of C6–C8 cycloalkanes, cycloalkenes, and cyclodienes into aromatics during methanol-to-olefin (MTO) chemistries in H-MFI zeolites. Aromatic compounds act as co-catalysts that predominantly form ethylene over propylene products and lead to site-blocking polyaromatic compounds; thus, understanding the formation of aromatic compounds during MTO is critical to understanding its selectivity and catalyst stability. Ring dehydrogenation reactions occur via sequential hydride transfer followed by deprotonation. The rate-controlling hydride transfer reactions were investigated with surface-bound protons (Z–H) and alkyls (Z–CnH2n+1) as hydride-accepting species. Hydride transfers via proton-mediated routes occur with intrinsic free energy barriers (623 K) of 215 kJ mol–1 for C6H12, 164 kJ mol–1 for C6H10, and 141 kJ mol–1 for C6H8, whereas methyl-mediated counterparts occur with intrinsic free barriers of 101, 95, and 81 kJ mol–1, respectively. Transition states for hydride transfer reactions prefer channel intersections within MFI networks, and their activation barriers suggest that methyl-mediated routes are favored over proton-mediated counterparts during MTO. Methyl substituents on hydrocarbon rings generally increase activation barriers for hydride transfers that occur proximal to the −CH3, partly because of steric hindrances between the hydride acceptor and ring-bound −CH3. Rapid double-bond isomerization within cyclohexenes and cyclohexadienes allows hydride transfer reactions to occur away from sterically hindering methyl substituents in methylated and dimethylated C6 ring hydrocarbons. The impact of carbocation substitution in hydride-accepting species was explored by contrasting barriers of methyl (Z–CH3), ethyl (Z–C2H5), 2-propyl (Z–C3H7), and tert-butyl (Z–C4H9) surface-bound alkyls. Intrinsic activation barriers decrease with increasing substitution of the alkyl hydride acceptor, consistent with those alkyls forming more stable carbocations. However, when accounting for steric hindrances associated with the co-adsorption of hydrocarbon rings near surface-bound alkyls, apparent free barriers are larger for more-substituted alkyls. Taking these apparent barriers into account, we predict that methyl-mediated hydride transfer reactions are responsible for the aromatization of hydrocarbon rings during MTO, leading to the CH4 formed during MTO reactions as the aromatic pool is enriched.

Potential of GTL-Derived Biosolids for Water Treatment: Fractionization, Leachate, and Environmental Risk Analysis

This study aims to understand the potential of using biosolids produced from the world’s largest gas-to-liquid (GTL) plant for water treatment applications. The metal fractionization of the two samples: raw biosolid (BS) and the pyrolyzed biosolid-BS char (BSC) (temperature: 450 °C, heating rate: 5 °C/min, residence time: 30 min) into exchangeables (F1), reducible (F2), oxidizable (F3), and residual (F4) were carried out following the Community Bureau of Reference (BCR) procedure. Characterization showed an increased carbon content and reduced oxygen content in the biochar sample. Additionally, the presence of calcium, magnesium, and iron were detected in smaller quantities in both samples. Based on the extraction results for metals, the environmental risk analysis was determined based on RAC (Risk Assessment Code) and PERI (Potential Ecological Risk Index) indices. Furthermore, leaching studies following the TCLP (Toxicity Characteristic Leaching Procedure) were conducted. The results prove that pyrolyzing stabilizes the metals present in the raw material as BS sample had high F1 fractions, and the BS char had a greater F4 fraction. While the RAC and PERI indices show that the pyrolyzed BS has a ‘low risk’, much reduced compared to the original BS sample, this is confirmed by the leaching studies that displayed minimal leaching from the pyrolyzed sample. Overall, this study proves that the GTL biosolids can best be applied for water treatment after pyrolysis.

Optimization of the Biotreatment of GTL Process Water Using Pseudomonas aeruginosa Immobilized in PVA Hydrogel

The COD reduction in gas to liquid (GTL) process water was optimized using response surface methodology (RSM). The biodegradation process was carried out in a spouted bed bioreactor (SBBR) using Pseudomonas aeruginosa immobilized in polyvinyl alcohol (PVA) gel. Different factors affecting the biological treatment of GTL process water (PW) were investigated. Three variables including PVA volume fraction, initial COD, and pH were investigated in the batch experiments. The biodegradation experiments were carried out by varying the initial COD values from 1000 to 3000 mg/L, pH from 5 to 8, and PVA v% from 20 to 30%. The maximum COD reduction was estimated to occur at an initial COD of 2595 mg/L, PVA v% of 27%, and pH of 7.3. At optimum conditions, the bioreactor system was able to achieve a maximum COD reduction of 89%, which is quite close to the RSM prediction value of 90%. The optimum operating conditions were used to carry out continuous biodegradation, and the results indicated that the COD reduction increased from 60% to 62% with an increase in the air flow rate from 2 to 3.3 La/Lr.min. However, by increasing the liquid flow rate from 2.1 to 4.2 mL/min and back to 2.1 mL/min, the COD reduction decreased from 66% to 39%. The system responded quickly to the change in liquid flow rate and returned to the initial COD level. This indicates that the system is highly stable and can easily recover.

Selective production of light olefins over zeolite catalysts: Impacts of topology, acidity, and particle size

Compared to conventional steam cracking, catalytic cracking of naphtha and methanol-to-olefins (MTO) over zeolite catalysts have been proved as the most successful processes for producing light olefins. To achieve the maximum light olefins yield and the longest lifetime, zeolite catalysts with excellent catalytic performance should be designed, by choosing suitable structure and regulating their physicochemical properties. In this review, the impacts of topology, acidity, and particle size of zeolites on the catalytic performance of naphtha cracking and MTO reactions, especially the light olefins selectivity and lifetime are systematically discussed. We hope that this review can be helpful not only to deeply understand the structure–property-performance relationship of various zeolite catalysts, but also to design more excellent zeolite catalysts for these two processes.

An energy graph eigendecomposition approach to fault detection and isolation applied to a gas-to-liquids process

Fault detection and isolation (FDI), which make up a large part of a process monitoring protocol, is a refined scheme which aims to detect and isolate anomalies that occur within an industrial plant. For the past 50+ years, much work has been done on developing FDI schemes for a vast array of different applications. In recent years, novel energy-based FDI techniques were proposed, as energy is seen as a unifying parameter of different domains. Additionally, the proposed energy-based approaches attempt to capture causal (or structural) information of the considered physical system. Keeping with this theme, this study will determine, after some alterations, the applicability and performance of some of the previously proposed energy-based approaches, especially compared to one another, when applied to a single, larger-scale gas-to-liquid (GTL) process. The approaches covered in this study include one qualitative eigendecomposition approach, one quantitative eigendecomposition approach, and a graph matching approach utilising a distance parameter (DC-value). Even though the quantitative eigendecomposition approach revealed improved detection sensitivity over the DC-value approach for faulty conditions, both techniques revealed a detection accuracy of 89%. The DC-value approach could correctly isolate 71% of the fault cases while the quantitative and qualitative eigendecomposition approaches could respectively only match 37.69% and 11% correctly. Even though the eigendecomposition approaches could not outperform the DC-value approach, the resolution benefit of 6 parameters that it offers warrants further research.

Synergistic effect of Brønsted/Lewis acid in olefin aromatization during MTO over Zn modified H-SAPO-34 zeolite: A periodic DFT study

H-SAPO-34 modified with extra-framework Zn2+ cations improved the selectivity to ethene and ratio of ethene to propene in the early stage of the methanol-to-olefin (MTO) reaction. In order to reveal the promotion mechanism, density functional theory calculations were performed to study the aromatization of 3-methyl-1,3-pentadiene to m-xylene. On three Zn-modified H-SAPO-34, the methylation and dehydrogenation processes, which are the key reactions of olefin aromatization over undoped H-SAPO-34, were studied in detail. It is found that the methylation reaction is more likely to occur at Brønsted acid sites (BAS), and the spatial proximity of BAS and Lewis acid sites (LAS) in zeolite enhances the acidic strength of BAS, thereby improving the methylation activity. The dehydrogenation reaction occurring at the LAS provided by Zn2+ proceeds via the "carbenium" mechanism, with significantly reduced energy barriers and improved dehydrogenation activity. This dehydrogenation mechanism was further testified by the metadynamics (MetaD) simulations. The synergistic effect of Brønsted acid and Lewis acid enables Zn2+ modified H-SAPO-34 to have high olefin aromatization activity, which in turn regulates the selectivity of lower olefins. The present results well interpret the experimental observation that the ethene selectivity is improved at the initial stage of MTO.

Insights into the methanol to olefins (MTO) performance of SAPO-34 under the stripper conditions of fluid catalytic cracking (FCC)

FCC is an important process for light olefins production, but it's a challenge to enhance the ethylene selectivity. In view of the high selectivity of ethylene and propylene (SEP) of MTO and the similar process of FCC and MTO, methanol may be co-fed from the stripper units to improve the selectivity of light olefins (SLO), especially ethylene, of a conventional FCC process. Herein, the MTO performance of SAPO-34 under the stripper conditions was studied, and the influences of the coke from heavy oil cracking were analyzed. When MTO was performed following the conditions (500 °C, 7.5–35 h−1) in the FCC stripper, the SEP and SLO achieved 83–86% and 91–96%, respectively, the weight ratio of ethylene to propylene (mEe:mPe) was about 1.2–1.6, and dimethyl ether was not detected. Although more complicated coke was introduced by heavy oil, the active polymethylbenzenes can help retaining the SEP (82.2%) and SLO (89.3%), and the methanol utilization efficiency and mEe:mPe were enhanced from 92.5% to 1.21 to 95.3–95.9% and 1.34, respectively. More importantly, the SEP, SLO and mEe:mPe can further be enhanced to 85.6%, 91.6%, and 1.52, respectively when SAPO-34 with less strong acid sites was applied. Consequently, SAPO-34 may be used as an FCC catalysts promoter for the development of a modified FCC process with significantly enhanced SLO.

Insight into induction period of methanol conversion reaction: Reactivity of ethene-precursors over H-ZSM-5 zeolite is independent of Brϕnsted acid site density

A series of H-ZSM-5 (TZ-m, m represents different Si/Al ratios) has been prepared and their Methanol to Olefin (MTO) performance are investigated. By adjusting the catalyst weight of H-ZSM-5 loaded in the pulse micro-reactor, the total Brϕnsted acid sites (BAS) amount of TZ-m catalysts is unified. The resultant similar methanol conversion (∼24 %) over different TZ-m zeolites confirms that the MTO reaction is dominantly catalyzed by BAS. Nevertheless, the BAS density considerably influences the reaction pathways; more condensed BAS enhances the relative propagation of aromatic-based cycle to alkene-based cycle, and vice versa. However, isotopic co-feeding and switching experiment reveals that the reactivity of aromatic hydrocarbon pool (HCP) species, which has been considered as main ethene precursors, is almost independent of acid site density. Although large amounts of aromatics are generated via hydrogen transfer reactions on the sample with high acid site density, most of the confined aromatics function as coke precursors instead of active HCPs. The accumulation of such confined aromatics in zeolite channels further limits olefins mobility and their access to BAS, leading to decreased reactivity of alkene HCPs on high acid site-density samples.

Methanol‐to‐Light Olefins Reaction using Zn‐Ag Modified SSZ‐13 Catalyst

AbstractThe 2 wt.% of Zn and Ag (Zn1‐x‐Agx; x=0.2, 0.4, 0.6 %) modified SSZ‐13 (SZ‐13) zeolite was prepared by wet‐impregnation method, and its catalytic activity was demonstrated for methanol‐to‐olefin (MTO). The Zn0.6‐Ag0.4/SZ‐13 catalyst exhibited high catalytic activity due to change in properties of catalyst. The Zn particles were highly dispersed inside the SZ‐13; while Ag particles were dispersed on the surface of SZ‐13, confirmed by HRTEM and surface area. The small % of Ag over SZ‐13 promotes ethylene production via aromatization of reaction, and highly dispersed Zn on SZ‐13 help to produce stable ethylene and propylene. The Zn0.6‐Ag0.4/SZ‐13 catalyst showed (∼10 %) higher light olefin (87 %) selectivity than parent SZ‐13 as well as mono metal incorporated SZ‐13. The higher catalytic activity was attributed due to effect of both metals (Zn and Ag), where Ag promotes partial aromatization on the surface, and Zn increases the Lewis acidity in the zeolite pore. This catalyst can help to improve the selectivity of light olefin using MTO reaction.

A highly efficient SAPO-34 catalyst for improving light olefins in methanol conversion: Insight into the role of hierarchical porosities and tailoring acid properties based on in situ NH3-poisoning

Hierarchical SAPO-34 zeolites with tunable porosity were hydrothermally synthesized by modulating the Si/Al molar ratios of the gel precursors, and the morphologies of SAPO-34 crystals gradually evolve from well-dispersed ultra-small nanocrystals to compactly polycrystalline aggregates, and to analogous polycrystallinites but adhered with minute amorphous debris by increasing the Si/Al ratio ranging from 0.20 to 0.67. The catalytic tests during methanol-to-olefins (MTO) reaction indicate that the Hier-SAPO-34-0.20 (Si/Al = 0.20) catalyst constructed by loosely nano-sized crystals centered at 50 nm presents prominent catalytic performances with about 2.5 times prolonged catalytic longevity and around 10 % improvement for the selectivity toward light olefins (ethylene and propylene) as compared to a reference catalyst. More interestingly, when Hier-SAPO-34-0.20 catalyst was in situ pretreated with NH3 flowing, the selectivity of light olefins can be greatly increased by 18.1 % at the initial stage and the corresponding even selectivity with time-on-stream can be elevated by 6.9 %. It founds that in situ NH3-poisoned catalysts with improved light olefins selectivity are also suitable for ZSM-5 zeolite. These results reveal that the weakened strong acid sites modified by NH3-poisoning play a pivotal role in suppressing the undesired hydrogen transfer reactions and thus giving a high selectivity for light olefins. This work highlights the importance of acidity fine-tailoring by in-situ NH3-poisoning strategy on hierarchical zeolites for the improvement of light olefin selectivity, which would be valuable guidance for industrial applications.

Coking and decoking chemistry for resource utilization of polycyclic aromatic hydrocarbons (PAHs) and low-carbon process

Low-carbon process for resource utilization of polycyclic aromatic hydrocarbons (PAHs) in zeolite-catalyzed processes, geared to carbon neutrality-a prominent trend throughout human activities, has been bottlenecked by the lack of a complete mechanistic understanding of coking and decoking chemistry, involving the speciation and molecular evolution of PAHs, the plethora of which causes catalyst deactivation and forces regeneration, rendering significant CO2 emission. Herein, by exploiting the high-resolution matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry (MALDI FT-ICR MS), we unveil the missing fingerprints of the mechanistic pathways for both formation and decomposition of cross-linked cage-passing PAHs for SAPO-34-catalyzed, industrially relevant methanol-to-olefins (MTO) as a model reaction. Notable is the molecule-resolved symmetrical signature: their speciation originates exclusively from the direct coupling of in-cage hydrocarbon pool (HCP) species, whereas water-promoted decomposition of cage-passing PAHs initiates with selective cracking of inter-cage local structures at 8-rings followed by deep aromatic steam reforming. Molecular deciphering the reversibly dynamic evolution trajectory (fate) of full-spectrum aromatic hydrocarbons and fulfilling the real-time quantitative carbon resource footprints advance the fundamental knowledge of deactivation and regeneration phenomena (decay and recovery motifs of autocatalysis) and disclose the underlying mechanisms of especially the chemistry of coking and decoking in zeolite catalysis. The positive yet divergent roles of water in these two processes are disentangled. These unprecedented insights ultimately lead us to a steam regeneration strategy with valuable CO and H2 as main products, negligible CO2 emission in steam reforming and full catalyst activity recovery, which further proves feasible in other important chemical processes, promising to be a sustainable and potent approach that contributes to carbon-neutral chemical industry.

Rapid and low-cost synthesis of ZSM-5 zeolite nanosheet assemblies for conversion of methanol to gasoline

Nanoscale zeolites with short diffusion paths show significantly enhanced mass transportation and easier accessibility for active sites in catalysis compared to microsized zeolites. However, the high cost and complex process involved in existing synthetic methods limit their application, so the development of a rapid and low-cost synthesis technology is of great significance. Herein, ZSM-5 zeolite nanosheet assemblies (Z5-NS-t) are rapidly synthesized by a microwave irradiation (MW) assisted seed-induced method using low-cost polyhexamethylene biguanide hydrochloride (PHMB) as a crystal growth inhibitor of the b-axis. The crystallization kinetics of the resultant materials and the formation mechanism of ZSM-5 zeolite nanosheets, as well as the influence of physico-chemical properties on their catalytic performance for the methanol-to-gasoline (MTG) reaction, are systematically investigated. Compared with the ZSM-5 nanoparticle assemblies (Z5-NP-60), which are synthesized by using the same method except for the addition of PHMB, Z5-NS-180 nanosheets with an approximate density of strong Brønsted acid sites exhibit excellent catalytic performance in the MTG reaction, giving rise to a high gasoline selectivity of 73.2% at a methanol conversion of 99.9% and a large amount of external carbon deposits. This is attributed to the synergy of accessibility of acid sites located in the pore intersections and the shorter diffusion paths for coke precursors out of the zeolite channel. This study provides an effective strategy for the synthesis of ZSM-5 nanosheets as catalysts for the MTG reaction, and it is expected to be applied for the synthesis of other zeolite nanosheets for important catalytic processes.

Catalyst-scale investigation of polydispersity effect on thermophysical properties in a commercial-scale catalytic MTO fluidized bed reactor

The methanol-to-olefin (MTO) process is commercialized worldwide, yet the understanding of in-furnace phenomena is still lacking. Accordingly, this work explores the multiphase flow and thermochemical characteristics in a commercial-scale MTO fluidized bed reactor using a three-dimensional multiphase particle-in-cell model. After model validations, the intrinsic relationship of solid transportation and thermochemical properties is elucidated together with the study of the polydispersity effect on reactor performance. The non-uniform gas-solid distribution in the reactor demonstrates the turbulent fluidization regime. Heat transfer coefficients (HTCs) of about 300 and 200 W/(m2·K) are observed in the medium and upper regions, respectively. The small velocity, particle Reynolds number, temperature, and HTC correspond to those large-size catalysts in the dense region where particles are nearly close packing with a large solid volume fraction. Increasing particle size distribution (PSD) width increases the catalyst HTCs and the mass fraction of C2H4, promoting reactor performance. Besides, increasing the PSD width enlarges gas viscosity, gas specific heat capacity, and gas thermal conductivity but decreases gas density.

Sustainable synthesis of AEI/CHA intergrowth zeolites for methanol-to-olefins conversion

The conventional hydrothermal synthesis of zeolites is inevitably accompanied by the production of mother liquors containing plentiful unreacted raw materials and organic templates. The direct emission or improper use of these mother liquors will cause not only the environmental contamination but also the waste of resources. Here we report a promising synthetic strategy for the effective recycle of mother liquors produced from the industrial production of SAPO-34 zeolites. Mother liquors with the addition of extra Si, P and Al were spray-dried and subsequently converted to the crystal seeds by vapor-phase-transport (VPT) method. Zeolite products with improved porosity and altered acidity were eventually acquired by VPT-Seeds-assisted hydrothermal crystallization, in which the topologic structures (e.g., CHA, AEI or AEI/CHA intergrowth) of zeolites depended significantly on the crystalline phase of VPT-Seeds. We demonstrated further via FT-IR and kinetics study that VPT-Seeds accelerated the crystallization of zeolites dramatically without the severe dissolution during the whole stage of synthesis. The as-synthesized AEI/CHA intergrowth zeolites showed the enhanced catalytic lifetime (increased at least by 27.6%) and higher selectivity in (C2H4+C3H6) (increased by 2.5%) during methanol-to-olefins (MTO) reactions than the pure CHA zeolites synthesized by the conventional hydrothermal method. Zeolites synthesized using the secondary recycled mother liquors in the same way also showed the comparable catalytic performances in MTO process, again verifying the feasibility of our methods to recycle mother liquors. The sustainable synthesis route developed in this work should be applicable to the industrial production of a wide variety of zeolites.

Ketenes in the Induction of the Methanol‐to‐Olefins Process

AbstractKetene (CH2=C=O) has been postulated as a key intermediate for the first olefin production in the zeolite‐catalyzed chemistry of methanol‐to‐olefins (MTO) and syngas‐to‐olefins (STO) processes. The reaction mechanism remains elusive, because the short‐lived ethenone ketene and its derivatives are difficult to detect, which is further complicated by the low expected ketene concentration. We report on the experimental detection of methylketene (CH3−CH=C=O) formed by the methylation of ketene on HZSM‐5 via operando synchrotron photoelectron photoion coincidence (PEPICO) spectroscopy. Ketene is produced in situ from methyl acetate. The observation of methylketene as the ethylene precursor evidences a computationally predicted ketene‐to‐ethylene route proceeding via a methylketene intermediate followed by decarbonylation.

Ketenes in the Induction of the Methanol-to-Olefins Process.

Ketene (CH2 =C=O) has been postulated as a key intermediate for the first olefin production in the zeolite-catalyzed chemistry of methanol-to-olefins (MTO) and syngas-to-olefins (STO) processes. The reaction mechanism remains elusive, because the short-lived ethenone ketene and its derivatives are difficult to detect, which is further complicated by the low expected ketene concentration. We report on the experimental detection of methylketene (CH3 -CH=C=O) formed by the methylation of ketene on HZSM-5 via operando synchrotron photoelectron photoion coincidence (PEPICO) spectroscopy. Ketene is produced in situ from methyl acetate. The observation of methylketene as the ethylene precursor evidences a computationally predicted ketene-to-ethylene route proceeding via a methylketene intermediate followed by decarbonylation.

Molecular insights into the catalytic oxidation of methanol-to-olefins wastewater with phosphoric acid modified sludge biochar

With the development of methanol-to-olefin (MTO) process, the effective disposal of wastewater was one key factor for the long-period and benign development of this technology. Herein, a sludge-based biochar catalyst (GSC-P) was synthesized and used in photo-Fenton reaction for the degradation of MTO wastewater from the outlet of a biological aerated filter. More iron was distributed on the surface of GSC-P catalyst, facilitating the photo-Fenton oxidation of MTO wastewater, with chemical oxygen demand (COD) removal rate of 75.4% and total organic carbon (TOC) removal rate of 62.5%. The 2223 unique molecular formulas assigned by a Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in the original MTO wastewater showed that CHO compounds shared the lowest peak numbers (20.2%) but the highest peak abundance (51.6%) among the four groups. Besides, lipids, unsaturated hydrocarbons, lignins and proteins were the main structural types. After photo-Fenton treatment of 60 min, there were 56.7%–74.0% of compounds removed by the analysis of van Krevelen diagram, indicating that the MTO wastewater was degraded efficiently. Three possible evolution processes of dissolved organic matters during the photo-Fenton reaction were disclosed at the molecular-level.

Methanol-to-olefins catalysis on window-cage type zeolites/zeotypes with syngas co-feeds: Understanding syngas-to-olefins chemistry

Catalyst lifetime and product selectivity of methanol-to-olefins (MTO) catalysis on window-cage type zeolites and zeotypes are examined and interpreted to elucidate the critical role of high-pressure H2 and CO in MTO catalysis at conditions relevant for syngas-to-olefins (STO) conversion (H2/CH3OH >100, CO/CH3OH >100). Propylene co-feed experiments elucidate that acid-catalyzed hydrogenation reactions transpire and enhance olefins-cycle propagation by intercepting formaldehyde-mediated condensation and dehydrocyclization reaction cascades to result in an increase in catalyst lifetime (>7×) and decrease ethylene-to-propylene (∼1.5×) and ethylene-to-butenes ratios (∼1.6×) during MTO with high-pressure H2 co-feeds. CO is mechanistically relevant in increasing ethylene-to-propylene (∼1.5–3.0×) and ethylene-to-butenes ratio (∼1.7×) during MTO catalysis only at high CO (3–8 bar) and syngas pressures (24 bar, H2/CO∼2–16); at these high pressures, CO participates in carbonylation reactions to enhance aromatics-cycle propagation and enable a pathway for ethylene production via methyl acetate formation. These results suggest that high-pressure syngas introduces new catalytic pathways to distinguish MTO catalysis from STO catalysis and MTO catalysis in the presence of high-pressure syngas reagents.

Regulating the distribution of acid sites in ZSM-11 zeolite with different halogen anions to enhance its catalytic performance in the conversion of methanol to olefins

Purposive regulation of the spatial location and distribution of acid sites in a zeolite catalyst, though still rather challenging, has proved to be an effective measure to enhance its performance in the conversion of methanol to olefins (MTO). Herein, a series of ZSM-11 zeolites were synthesized using different sodium halides (with tetrabutylammonium hydroxide (TBAOH) as the structure-directing agent); the effect of halogen anions (Cl-, Br- and I-) in the synthesis gel on the Al siting and thereon the spatial location and distribution of acid sites in the ZSM-11 zeolites as well as their relation to the catalytic performance in MTO was elaborately investigated. The results indicate that for the synthesis of high Si/Al ratio ZSM-11 zeolites (Si/Al > 60), the Cl- anions present in the synthesis gel can induce more Al atoms siting on the external surface of ZSM-11 crystals, thereby leading to a short catalytic lifetime in MTO due to the rapid coke deposition. In contrast, larger Br- or I- anions in the synthesis gel, owing to their stronger covalent interaction with the TBA+ cations, may moderate the formation of crystal silicate nuclei and lead to a uniform distribution of Al species in the inner straight channels, which can effectively suppress the surface coke deposition and inhibit the aromatic-based cycle for MTO. As a result, the high Si/Al ratio ZSM-11 zeolites obtained with Br- or I- in the synthesis gel exhibit much better catalytic stability in MTO and higher selectivity to propene and butene, in comparison with the ZSM-11 counterpart synthesized using Cl-. These results help to clarify the relation between the catalytic performance of ZSM-11 in MTO and the location of acid sites and thus bring forward a facile strategy to enhance the catalytic performance of zeolites through purposively regulating the spatial distribution of acid sites.