Article

Establishing Discrete Ising Model for Zeolite Deactivation: Inspiration from the Game of Go

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  • Sinopec Engineering Incorporation
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Abstract

Herein, we have built a discrete Ising model for deactivation considering the cage connectivity, inspired by the game of Go. An analytical solution for a simplistic 1D model is found, and it shows good consistency with the experimental results over ZSM-12. Some anomalous pseudo-phase transition phenomena in the deactivation process and in the acid density are presented by modeling the deactivation of SAPO-34. This model may bring new methodologies to research on the zeolite deactivation mechanism.

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... 、 环境治理 [6][7][8] 、 气体分离 [9][10][11] Fig.3 Sub-nanometer observation of zeolite framework and hydrocarbon pool species with iDPC-STEM technique [58] 图 4 可接触酸性位数量的相变现象 [61] Fig.4 Pseudo-phase transition phenomenon of the number of accessible acid sites [61] · ·3854 Fig.5 Illustration of Watts-Strogatz Model [74] 图 6 平均扩散距离与点缺陷密度的关系 [75] Fig.6 Relationship between average diffusion distance and vacancy density [75] Fig.7 Effect of linear vacancies [75] · ·3857 Fig.8 Superior performance and lifetime noticed at zeolites with special surface structure [22] · ·3858 Fig.9 Surface-selective modification of ZSM-5 zeolites [60] 图 10 多级结构 SAPO-34 分子筛 [20] Fig.10 Hierarchical SAPO-34 zeolite crystals [20] Fig.14 Mechanism with three reaction cycles [59] · ·3862 www.hgxb.com.cn 第 9 期 ...
... 、 环境治理 [6][7][8] 、 气体分离 [9][10][11] Fig.3 Sub-nanometer observation of zeolite framework and hydrocarbon pool species with iDPC-STEM technique [58] 图 4 可接触酸性位数量的相变现象 [61] Fig.4 Pseudo-phase transition phenomenon of the number of accessible acid sites [61] · ·3854 Fig.5 Illustration of Watts-Strogatz Model [74] 图 6 平均扩散距离与点缺陷密度的关系 [75] Fig.6 Relationship between average diffusion distance and vacancy density [75] Fig.7 Effect of linear vacancies [75] · ·3857 Fig.8 Superior performance and lifetime noticed at zeolites with special surface structure [22] · ·3858 Fig.9 Surface-selective modification of ZSM-5 zeolites [60] 图 10 多级结构 SAPO-34 分子筛 [20] Fig.10 Hierarchical SAPO-34 zeolite crystals [20] Fig.14 Mechanism with three reaction cycles [59] · ·3862 www.hgxb.com.cn 第 9 期 ...
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Based on the manipulation of chemical reactions and chemical bonds, the chemical industry solves transport problems on a spatial and temporal scale that exceeds 12 orders of magnitude, creating and producing new substances for human society. The discovery and use of crude oil have brought the chemical industry into a new era and supported the operation of modern society. However, for China, the resource endowment determines that the development of the coal chemical industry has unique strategic significance. Unlike the hydrocarbon cracking process, the modern new coal chemical industry relies on the self-assembly of small molecular from raw materials and the shape-selective effect of zeolites to produce fuels and chemicals with high selectivity. Therefore, at the sub-nanometer scale, that is, the reaction and transfer behavior of small molecules on shape-selective zeolites can no longer be regarded as continuous, but discrete, which leads to a series of phenomena such as deactivation with pseudo-phase transitions on the macroscale. Based on the discrete transfer phenomena in zeolites at the nanoscale, this paper reviews the research and discovery of a series of new phenomena using graph theory, small-world networks, and advanced characterization methods to analyze the deactivation and transfer phenomena. The atomic-level precise structural modification of the zeolite and the innovation it brings to the macro coal chemical process is introduced. Towards the future, the paradigm for analyzing the discrete behavior in zeolites established in this article will provide a new way of thinking for precisely adjusting the zeolite structure and developing the next generation coal chemical process.
... However, ZSM-5 failed to convert some bulky compounds with a larger kinetic diameter effectively [16], such as macromolecular lignin derivatives in MSW and large lipids in CV [3,17]. These bulky reactants aggregate on the surface of zeolite to form coke, which, as a result, will block the channels and accelerate the deactivation of the zeolite [18]. In general, mesoporous zeolites have significant advantages in effective mass transfer for bulky reactants, but some previous studies on mesoporous zeolites Al-MCM-41 and Al-SBA-15 have detected that they only possessed relatively weak deoxidation and aromatization ability [19,20]. ...
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In this work, catalytic co-pyrolysis characteristics and kinetics of chlorella vulgaris (CV), municipal solid waste (MSW), and their blends over hierarchical ZSM-5 zeolite were studied by thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Moreover, three zeolite additives, namely, ZSM-5, Al-MCM-41, and Al-SBA-15, were selected to compare their effects on catalytic co-pyrolysis and coking characteristics with hierarchical ZSM-5 zeolite. Results showed that co-pyrolysis of CV and MSW demonstrated significant synergistic effects at 260–330 °C, especially at the ratio of 5:5. Two model-free methods, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), were used to calculate the kinetic parameters. Product distribution results demonstrated that co-pyrolysis could improve pyrolysis products by increasing monocyclic aromatic hydrocarbons and aliphatic hydrocarbons as well as reducing polycyclic aromatic hydrocarbons and nitrogen compounds. Compared with other three kinds of zeolite additives, hierarchical ZSM-5 with both micropores and mesopores achieved superior monocyclic aromatic hydrocarbon selectivity (34.14%) and inferior acid selectivity (9.54%) for co-pyrolysis, thereby satisfying the preferred criteria. In addition, the difference from the thermogravimetric curve between after pyrolysis and fresh zeolites indicated that hierarchical ZSM-5 also had the best coking resistance. In brief, co-pyrolysis of chlorella vulgaris and municipal solid waste with hierarchical ZSM-5 was definitely a feasible way for high-quality bio-oil generation. Graphical Abstract
... These chemical species experience severe diffusion limitations and can easily block the zeolite pores. When the coking reaction reaches a point where whole parts of the active sites are completely blocked by the coke, these active sites are considered to be de facto "dead" [50]. ...
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In the present study, two strategies for improving the catalyst efficiency either by (1) raising the effectivediffusion coefficient or (2) reducing the diffusion length of zeolite catalysts are presented. These two strategiesare exemplified by the preparation of (i) ordered mesoporous material with zeotype secondary building unitsand (ii) self-assembled nanosized zeolites. The textural properties, acidity, and the results from the cumenecracking show that the strength and amount of Brønsted acid sites of the mesoporous materials are inferior, dueto their amorphous pore walls. Even though the lifespan of ordered mesoporous material with zeotype secondarybuilding units during cumene cracking can be improved due to the effective diffusion coefficient enhancement,the strength and amount of Brønsted acid sites related to the reaction are not akin to conventional microporouszeolites. While the self-assembled nanosized zeolites with short diffusion length shows enhanced catalyticperformance by diffusion controlled reactions.
... In addition, these coke speciesm ainly form in the intracrystalline hierarchical spaces insteado fo nt he outer surface of the SAPO-34 zeolites, which has no significant effect on the hydrocarbon generation. [59][60][61] Moreover,f or the spent catalysts after CO 2 hydrogenation for 48 h, the amount of coke decreases with the reduced crystal size of the zeolites, and acid-treated SAPO-34 zeolitese xhibit lower amountso fc oke( Figure S7 a-c in the Supporting Information), indicating that the smaller crystal size and/or the meso-/macropore structure favor diffusion of the coke precursor.A saresult of the coke, the BET surfacea rea and pore volumeo ft he zeolite decrease significantly after reaction. [62,63] As shown in the resultso ft he N 2 physisorption experiment ( Figure S8 and Table S5 in the Supporting Information), the pore structures of thermally regenerated SAPO-34-C and SAPO-34-H-a samples are similart ot hat of the fresh zeolite even after reaction for 92 h. ...
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... stic information. The authors identified the script as the combinational notion of a sequence and its meanings, described by linguistic labels. For adding, scripting, and analyzing in a Go game, systems are shown. December is also viewed in various scientific fields, but not so much as in the field of computer programming. In the field of physics, Cai, Ma, Hou, Cui, Jia, Zhang, . . . Wei (2017) built a discrete model for decontamination, taking into account the possibility of connecting to a cell, inspired by the game Go. An analytical solution was found for a simplified 1D model and shows good consistency with the experimental results for ZSM-12. Some abnormal pseudo-phase transition phenomena in the deactivation process and ...
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... Ton V. W. Janssens [34] proposed a simple model contains the first-order rate constant to measure catalyst activity and a deactivation coefficient to characterize the deactivation behavior. Wei et al. [35] established a discrete Ising model for the deactivation of SAPO-34 zeolite inspired from the traditional game Go and predicted an acid percentage of 57% to make full use of each acid cage and an optimal acid percent of 66% to obtain maximum access to the active sites. The existence of mesopores/macropores introduces defects and voids into the crystals which can effectively increase the accessibility of the catalytic active sites and the utilization of the catalyst, i.e., retards the coke deposition rate and improve the lifetime performance [36]. ...
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... It is speculated that small SAPO-34 crystal size can slow down the formation rate of coke compounds [5,14,19], and lead to an increasing quantity of coke, especially the poly-cyclic aromatic compounds, inside cages after complete deactivation [5,14,19]. Cai et al. [20] showed that the formation of poly-cyclic aromatics may cause serve blocking of the pores and strong hinderance of the diffusion of reactant and product molecules inside SAPO-34 zeolites, which lead to the increase of inaccessible active sites and species for reactant and eventually the deactivation of catalyst. ...
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... We tested the propane yield over zeolite with different Si/Al ratio, and noticed that higher acid density will lead to a higher rate of hydrogen transfer reaction, resulting in a higher peak at 300°C (Fig. 4B). But this doesn't have much influence on our results above and will be discussed in future study [36]. ...
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Transport phenomena play an essential role in catalysis. While zeolite catalysis is widely applied in industrial chemical processes, its efficiency is often limited by the transport rate in the micropores of the zeolite. Experimental and theoretical methods are useful for understanding the transport phenomena on multiscale levels. Traditional diffusion models usually use a linear driving force and an isotropic continuum medium, such that transport in a hierarchical catalyst structure and the occurrence of nonlinear deactivation cannot be well understood. Due to the presence of spatial confinement and an ordered structure, some aspects of the transport in a zeolite cannot be regarded as continuum phenomena and discrete models are being developed to explain these. Graph theory and small‐world networks are powerful tools that have allowed pseudo‐phase transition phenomena and other nontrivial relationships to be clearly revealed. Discrete models that include graph theory can build a bridge between microscopic quantum physics and macroscopic catalyst engineering in both the space and time scales. For a fuller understanding of transport phenomena in diverse fields, several theoretical methods need to be combined for a comprehensive multiscale analysis.
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Hollow SAPO-34 with thin-wall were successfully synthesized by mixed templates and varying the silicon content in the gel. The silicon enrichment at the surface of crystals occurs on both low silicon and high silicon samples. The cubic crystals of the molecular sieve transformed from hollow to dense phase with increase of silicon content. The surface of molecular sieve crystals turned from being abundant of macroporous openings to be integrated without visible macroporous cracks with the increase of silicon content in the gel. The thin-wall SAPO-34 with low acid density and Si(4Al) framework structure, synthesized from the low silicon content (nSiO2/nAl2O3 = 0.1) of the gel, presented the highest ethylene plus propylene selectivity. The hollow structure SAPO-34 with medium acid density obtained with medium silicon content (nSiO2/nAl2O3 = 0.26) in the gel possessed the longest lifetime.
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As an important catalyst of methanol-to-propylene (MTP), ZSM-5 zeolite has an anisotropic diffusion path and large pore size, resulting in the formation of undesirable heavy aromatic by-products Herein, we developed a surface-specific silica deposition method to block straight channels of nanosized ZSM-5 crystals selectively. By such coating method, we can selectively suppress the yield of aromatics from the original 13% to 2.4% at 100% conversion of methanol. Trapped hydrocarbon pool species are directly confirmed by Aberration-Corrected S/TEM for the first time. Such a method of trapping and restricting hydrocarbon pool species in multiscale zeolite with 10-membered rings would significantly benefit its catalytic efficiency and olefin diffusion. Moreover, this is providing new methodologies for zeolite structure construction and will be greatly beneficial to industrial MTP process.
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A series of hierarchically structured ZSM‐5 zeolites (ZSM‐5 HSZs) with a Si/Al ratio of about 200 have been successfully synthesized in the absence of additional mesoporous agents. XRD, N2 sorption isotherms, SEM, TEM and NH3‐TPD results showed that the obtained HSZs materials possess high crystallinity, interconnected micro/mesoporous structures and similar total acid amounts. Meanwhile, in the methanol‐to‐propylene (MTP) reaction, compared to the microporous counterpart, all synthesized ZSM‐5 HSZs demonstrated superior performance benefitting from their hierarchical porous structures. Interestingly, ²⁷Al MAS NMR and Py‐FTIR results confirmed the inter‐transformation of framework and extra‐framework Al species with the extension of materials crystallization duration and consequently the variation of materials surface acidity. As a result, the optimized catalyst achieved the highest initial propylene selectivity of 54.4 % and the longest catalyst lifetime (t90) of 175 h under the weight hourly space velocity (WHSV) of 1.0 g g–1 h–1. Moreover, a new “in situ alkali etching‐hydrothermal restoring“ mechanism has been proposed to elucidate the structural evolution of here reported high‐silica ZSM‐5 HSZs during the synthesis.
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Kinetic Monte Carlo (kMC) is an essential tool in heterogeneous catalysis enabling the understanding of dominant reaction mechanisms and kinetic bottlenecks. Here we present MonteCoffee, which is a general-purpose object-oriented and programmable kMC application written in python. We outline the implementation and provide examples on how to perform simulations of reactions on surfaces and nanoparticles and how to simulate sorption isotherms in zeolites. By permitting flexible and fast code development, MonteCoffee is a valuable alternative to previous kMC implementations.
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A great interest has emerged in the development of nano‐sized and hierarchical SAPO‐34 zeolites because of their enhanced accessibility and improved catalytic activity in methanol‐to‐olefin (MTO) conversions. Here a series of nano‐sized SAPO‐34 catalysts with tunable hierarchical structures are synthesized in the system of Al₂O₃‐H₃PO₄‐SiO₂‐triethylamine(TEA)‐H₂O by using a mesoporogen‐free nanoseed‐assistant method. The nano‐sized hierarchical SH‐3.0 catalyst (TEA/Al₂O₃=3.0) possesses the highest crystallinity, abundant intracrystalline meso/macropores, and suitable acidity among all of the obtained catalysts, showing the highest selectivity of ethylene and propylene up to 85.4%. This value reaches the top level in MTO reactions ever reported under the similar conditions. Detailed coke analyses reveal that the small‐sized methyl‐substituted benzene and bulky methyl‐substituted pyrene are mainly located inside the crystals instead of on the surface of crystals, which provides further insight into the understanding of the deactivation of the SAPO‐34 catalyst during MTO reaction. Significantly, the simple and cost‐effective synthetic process and superb catalytic performance of the nano‐sized hierarchical SAPO‐34 promise their practical large‐scale application in future MTO units.
Article
Hierarchical structure brings efficiency in many processes including metabolism, plant growth, and even in social networks. In society, connectivity between people forms a hierarchical network, and superior efficiency for information transfer is noticed. Such network is called small-world network. Although the number of people known by a single person is insignificant compared to the total number of people in the society, a very small number of long-range connections can bring extremely high information transfer efficiency to the social network. This is the key property of the small-world network. By modeling zeolite structure with small world network and regarding vacancies and cracks as long-range connections, we managed to quantify the efficiency in hierarchical zeolite structure. We shows the influence of cracks and vacancies to transfer phenomenon in zeolite structure. By adding 6% of vacancies in perfect 3D zeolite structure, we can get a 30% equivalent volume reduction for zeolite crystals. Such studies may bring new methodology for quantifying zeolite transfer and broaden horizons for zeolite structure design.
Article
The nucleation and growth of SAPO‐34 crystal with TEA as single template was monitored with ex situ time‐resolved characterization methods. The investigation focused on the evolution of the intermediate phases at different crystallization stages of SAPO‐34. The morphology transformation of the intermediate phases at different crystallization time revealed the unique self‐assembly process of the sub‐crystals. The cubic SAPO‐34 crystals can be constructed by eight pyramidal sub‐units. Additionally, the construction order of cha cages and d6r units in the initial crystallization stage was determined. The prior appearance of cha cages than d6r units can be attributed to structure‐directing effect of protonated TEA, which charge balanced with the framework negative charge introduced by Si incorporation. A further analysis shows Si species was incorporated into framework by direct participation in the initial crystallization stage and substitution for framework P during the later stage.
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Nano ZSM-12 zeolite, with a huge external surface area and lower ratio of BrönstedBr¨Brönsted to Lewis acids, exhibited high activity and selectivity for the direct transformation of methanol into multi-methylbenzenes with molecule sizes that were larger than the 1-D pore of ZSM-12, via the production of xylene inside the pore and complete external surface methylation.
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Understanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub-nm length scale in a single zeolite ZSM-5 crystal, which has been partially deactivated by the methanol-to-hydrocarbons reaction using 13C-labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30–60 13C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.
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The game of Go has long been viewed as the most challenging of classic games for artificial intelligence owing to its enormous search space and the difficulty of evaluating board positions and moves. Here we introduce a new approach to computer Go that uses ‘value networks’ to evaluate board positions and ‘policy networks’ to select moves. These deep neural networks are trained by a novel combination of supervised learning from human expert games, and reinforcement learning from games of self-play. Without any lookahead search, the neural networks play Go at the level of state-of-the-art Monte Carlo tree search programs that simulate thousands of random games of self-play. We also introduce a new search algorithm that combines Monte Carlo simulation with value and policy networks. Using this search algorithm, our program AlphaGo achieved a 99.8% winning rate against other Go programs, and defeated the human European Go champion by 5 games to 0. This is the first time that a computer program has defeated a human professional player in the full-sized game of Go, a feat previously thought to be at least a decade away.
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Fluid catalytic cracking (FCC) is a major process in oil refineries to produce gasoline and base chemicals from crude oil fractions. The spatial distribution and acidity of zeolite aggregates embedded within the 50–150 μm-sized FCC spheres heavily influence their catalytic performance. Single-molecule fluorescence-based imaging methods, namely nanometer accuracy by stochastic chemical reactions (NASCA) and super-resolution optical fluctuation imaging (SOFI) were used to study the catalytic activity of sub-micrometer zeolite ZSM-5 domains within real-life FCC catalyst particles. The formation of fluorescent product molecules taking place at Brønsted acid sites was monitored with single turnover sensitivity and high spatiotemporal resolution, providing detailed insight in dispersion and catalytic activity of zeolite ZSM-5 aggregates. The results point towards substantial differences in turnover frequencies between the zeolite aggregates, revealing significant intraparticle heterogeneities in Brønsted reactivity.
Article
A clear understanding of the methane formation mechanism in the initial MTO process is beneficial for the illustration of the initial C–H bond activation mechanism and the first C–C bond formation route. Thus, attempts are made here to unravel the methane formation pathway in the initial MTO process by elaborately designing experiments. It is shown that methane is generated together with formaldehyde or methoxymethyl cation by attacking the C–H bond of methanol or dimethyl ether (DME) with surface methoxy species (SMS). The reaction of DME and SMS provides strong evidence for the occurrence of C–H bond cleavage and the “methoxymethyl cation mechanism” in the initial MTO process.
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Solid acid materials with tunable structural and acidic properties are promising heterogeneous catalysts for manipulating and/or emulating the activity and selectivity of industrially important catalytic reactions. On the other hand, the performances of acid-catalyzed reactions are mostly dictated by the acidic features, namely, type (Brønsted vs Lewis acidity), amount, strength, and local environment of acid sites. The latter is relevant to their location (intra- vs extracrystalline), and possible confinement and Brønsted-Lewis acid synergy effects that may strongly affect the host-guest interactions, reaction mechanism, and shape selectivity of the catalytic system. This account aims to highlight some important applications of state-of-the-art solid-state NMR (SSNMR) techniques for exploring the structural and acidic properties of solid acid catalysts as well as their catalytic performances and relevant reaction pathway invoked. In addition, density functional theory (DFT) calculations may be exploited in conjunction with experimental SSNMR studies to verify the structure-activity correlations of the catalytic system at a microscopic scale. We describe in this Account the developments and applications of advanced ex situ and/or in situ SSNMR techniques, such as two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) homonuclear correlation spectroscopy for structural investigation of solid acids as well as study of their acidic properties. Moreover, the energies and electronic structures of the catalysts and detailed catalytic reaction processes, including the identification of reaction species, elucidation of reaction mechanism, and verification of structure-activity correlations, made available by DFT theoretical calculations were also discussed. Relevant discussions will focus primarily on results obtained from our laboratories in the past decade, including (i) quantitative and qualitative acidity characterization utilizing assorted probe molecules, (ii) probing the spatial proximity and synergy effect of acid sites, and (iii) influence of acid features and pore confinement effect on catalytic activity, transition-state stability, reaction pathway, and product selectivity of solid acid catalysts such as zeolites, metal oxides, and heteropolyacids. It is conclusive that a synergy of acidity (local effect) and pore confinement (environmental effect) tend to strongly dictate the formations of intermediates and transition states, hence, the reaction pathways and catalytic performance of solid acid catalysts. We hope that these information can provide additional insights toward our understanding in heterogeneous catalysis, especially the roles of structural and acidic properties on catalytic performances and reaction mechanism of acid-catalyzed systems, which should be beneficial for rational design of solid acid catalysts.
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Chabazite zeolites with trimodal porosity (native micropores of CHA framework/additional larger micropores/mesopores) were synthesized by use of a diquarternary ammonium type surfactant C22-4-4 cations and fluoride anions in the synthesis of SSZ-13 zeolite. The fully crystalline hierarchical SSZ-13 zeolites are similar in acidity to bulk SSZ-13 zeolite. The increased diffusion rate in the hierarchical SSZ-13, proved by uptake experiments of bulky molecules and selective staining by thiophene oligomers formed in micropores larger than the CHA pore system, resulted in much slower catalyst deactivation in the methanol-to-olefins (MTO) reaction. Confocal fluorescent images of hierarchical SSZ-13 zeolite reveal a more homogeneous distribution of carbonaceous deposits, indicating that the micropore space has been completely utilized during the MTO reaction.
Article
We proposed a methylbenzenes intermediate equilibrium analysis for Methanol-to-olefins process based on Anderson-Schulz-Flory dostribution under commercial conditions. The key parameter was controlled by temperature. Such study will bring new insights into MTO and other chain growth processes.
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Guest editors Bert M. Weckhuysen and Jihong Yu introduce the Recent Advances in Zeolite Chemistry and Catalysis issue of Chemical Society Reviews
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The methanol-to-olefins (MTO) reaction is an interesting and important reaction for both fundamental research and industrial application. The Dalian Institute of Chemical Physics (DICP) has developed a MTO technology that led to the successful construction and operation of the world’s first coal to olefin plant in 2010. This historical perspective gives a brief summary on the key issues for the process development, including studies on the reaction mechanism, molecular sieve synthesis and crystallization mechanism, catalyst and its manufacturing scale-up, reactor selection and reactor scale-up, process demonstration, and commercialization. Further challenges on the fundamental research and the directions for future catalyst improvement are also suggested.
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We present a review of the description of disordered porous media according to the Dual Site-Bond model introduced by Vicente Mayagoitia. The model formulation, percolation properties and applications to problems of practical interest are discussed.
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SAPO-34 materials with comparable Brønsted acid site density but different crystal sizes were applied as methanol-to-olefin (MTO) catalysts to elucidate the effect of the crystal size on their deactivation behaviors. 13C HPDEC MAS NMR, FTIR, and UV/vis spectroscopy were employed to monitor the formation and nature of organic deposits, and the densities of accessible Brønsted acid sites and active hydrocarbon-pool species were studied as a function of time-on-stream (TOS) by 1H MAS NMR spectroscopy. The above-mentioned spectroscopic methods gave a very complex picture of the deactivation mechanism consisting of a number of different steps. The most important of these steps is the formation of alkyl aromatics with large alkyl chains improving at first the olefin selectivity, but hindering the reactant diffusion after longer TOS. The hindered reactant diffusion leads to a surplus of retarded olefinic reaction products in the SAPO-34 pores accompanied by their oligomerization and the formation of polycyclic aromatics. Finally, these polycyclic aromatics are responsible for a total blocking of the SAPO-34 pores, making all catalytically active sites inside the pores nonaccessible for further reactants.
Article
Coke formation in the ZSM-5 catalyst was simulated by the Monte Carlo method, in which the zeolite was represented by a two-dimensional lattice with two types of site. One was located at channel intersections, which were active for both reaction and adsorption, and the other was situated at the segments only for adsorption. The coke content X at different times t was found to obey the relationship X=Atα; moreover, the exponent α was found to be related to the limitation of diffusion on coke formation. The effect of varying the reactant pressure was also investigated. It was found that deactivation of the zeolite at high pressure is due to the external coke, whereas deactivation at low pressure is because of the internal coke.
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Three-dimensional site-bond-site network model was used for simulating the deactivation of catalysts due to poisoning and blockage of pores by coke formation. We have simulated the deactivation of partly amorphous chrome-alumina catalyst during the dehydrogenation of butene to butadiene as well as the deposition of coke in a structured zeolite catalyst ZSM-5 during methanol to gasoline conversion. A simple process rate model is used to describe the deactivation of chrome-alumina catalyst while a detailed Monte-Carlo reaction model is used for methanol conversion giving importance to different aspects like adsorption, desorption, reaction and diffusion. The study shows the importance of morphological and topological characteristics of the pore-structure of the catalyst represented by measurable parameters like: connectivity of network, size distribution of cavities and channels, and correlation of sizes of cavities and channels. The present model correlates more effectively the available experimental deactivation data for the cases studied, both for short and long times and serves as an useful tool for determining the kinetic constants of the deactivation process.
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The diffusion and deactivation during methanol conversion over SAPO-34 were studied on differently sized crystals using an oscillating microbalance reactor. The reactive diffusivity of 1.1–10-8m2/s measured from methanol conversion on differently sized crystals was comparable with the methanol steady-state diffusivity of 3.3×10-9m2/s as measured by the uptake method. A percolation model based on a realistic three-dimensional pore network and taking into account the site coverage effect and the correlated percolation phenomenon, was developed to describe the diffusion and deactivation due to coke deposition. Simulated changes in relative effective diffusivity with the coke content or the fraction of occupied sites in the network were found to agree well with experimental results. The diffusivity decreased significantly with increasing coke content in the cages. Water adsorption was found to be a simple and reliable method to measure the pore volume changes with the coke content, relating the coke content with the fraction of occupied sites in the network.
Article
The acid strength distribution of different samples such as HZSM-5, modified HZSM-5, TS-1 and SiO2 was investigated by modified Hammett indicator and ammonia temperature programmed desorption (NH3-TPD). It has been shown that only acid sites with H0⩽+2.27 contribute to the conversion of toluene in toluene disproportionation, which is only small percent in the total acid amount. The acid sites with strength +4.8⩽H0⩽+6.8 may be ascribed to the acid sites that were produced by silanol group in HZSM-5. In study of HZSM-5 and modified HZSM-5 by NH3-TPD, the lower temperature peak (LT) and higher temperature peak (HT) are ascribed to the acid sites with strength −3.0
Article
Deactivation of a ZSM-5 catalyst by coke formation is modeled in the presence of diffusional limitations. Coke formation starts with the deposition of a coke precursor, proceeds through growth reactions and stops under the influence of termination reactions. Parallel and consecutive coking mechanisms are considered. The influence of the composition of the reaction mixture is investigated. The concentrations of reactants and reaction products and the coke content are obtained as a function of the position inside the catalyst. The overall rate of the main reaction decreases monotonically with increasing coke content of the catalyst. This is not necessarily so for the coking reaction, when the rate of coke growth is large with respect to the rate of deposition of coke precursor.
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a b s t r a c t Hierarchically nanoporous zeolites possessing mesopores as well as the inherent micropores of zeolite structures are garnering attention as a solution to the diffusion limitation problem of solely microporous classical zeolites. The hierarchical structure enables zeolites to have maximum structural functions in a limited space and volume owing to the high diffusion efficiency. Hierarchical zeolites are in high demand for the development of advanced materials for applications in adsorption, separation and catalysis. Herein, recent advances in synthesis routes to hierarchically nanoporous zeolites are reviewed with their catalytic contributions. Particular emphasis is given to the recently developed synthesis method which uses surfactants that are functionalized with a zeolite-structure-directing group. This type of surfactants can direct porous structures in micro-and meso-length scales simultaneously.
Article
Networks possess closed paths or cycles connecting their elements. Until now indeed, modeling of porous media and surface structures has been performed disregarding their intricate nature as networks. A qualitative and quantitative adequate simulation of heterogeneous media by Monte Carlo methods is proposed. This allows a deeper understanding of the nature of such media and the prediction of a variety of processes taking part in them.
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A three-dimensional random walk through a network of two intersecting channel systems is shown to be characterized by a diffusion tensor with interdependent principal elements. By application of this model to ZSM-5 type zeolites, the predicted relations are found to be in agreement with the results of both MD calculations and initial, PFG NMR diffusion experiments with oriented zeolite crystallites.
Article
Nonlinear transport processes in disordered systems such as porous media and heterogeneous solids are studied, which are represented by two- or three-dimensional networks of interconnected bonds, by a Bethe network (a branching network with no closed loops) of a given coordination number, or by a continuum in which circular or spherical inclusions have been inserted at random. The bonds represent the pores of the pore space, or the conducting and insulating regions of a disordered solid, to which we assign effective properties (radii or conductances) selected at random from a probability density function. Three types of nonlinear transport processes are considered. (1) The relation between the current q and the potential gradient v is of power-law type (as in, for example, flow of power-law fluids or the electric current in doped polycrystalline semiconductors). (2) The relation between q and v is piecewise linear, characterized by a threshold (as in flow of Bingham fluids or in mechanical or dielectric breakdown of composite solids). (3) A large v is imposed on the system, so that a linear transport theory is not valid. The behavioral study of the effective transport and topological properties of the system, such as the permeability, conductivity, diffusivity, and the shape of the samplespanning cluster of conducting paths shows that in all cases the concepts of percolation theory play a prominent role, even if the system is well connected and percolation may seem not to play any role. For most cases, new effective-medium approximations (EMAs) are derived for estimating effective transport properties. Compared to the case of linear transport, new EMAs are considerably more accurate in predicting the scaling properties of the transport coefficients near a critical point such as the percolation threshold. For a power-law transport process, an exact solution is also derived for the Bethe networks. Using the concepts of percolation theory, scaling laws relating the effective properties to various regimes of transport and to topological properties of the system are also given. A relation between the volumetric flow rate of a power-law fluid in porous media and the macroscopic pressure drop is derived, which contains no adjustable parameter and is valid at any porosity. To test the accuracy of our analytical predictions, Monte Carlo simulations are carried out for several cases. In most cases, good agreement is found between the simulation results and predictions. The extension of the results to other types of nonlinearities is also discussed.
Article
The deactivation of a zeolite catalyst in the conversion of methanol to hydrocarbons is described as a reduction of the effective amount of catalyst with time on stream. With the assumptions that the conversion of methanol is a first-order reaction, and that the loss of active catalyst is proportional to the conversion, an expression for the conversion with time on stream is obtained, which describes the experimental data well. This expression contains the rate constant, that characterizes the activity, and a deactivation coefficient that describes the deactivation behavior as parameters. It is shown that active catalysts show a more sudden decrease in conversion, and that the deactivation rate determines the time at which the decrease in conversion is observed. If the initial conversion is close to 100%, the lifetime to 50% conversion does not depend on the activity, and the deactivation coefficient is directly derived from the experimental data, by dividing the measured lifetime to 50% conversion by the applied contact time. The lifetime to all other conversion levels is dependent on both deactivation and activity, which implies that a catalyst lifetime to breakthrough of methanol does not scale with the deactivation rate. Likewise, it is shown that the conversion capacity is a good characterization of the deactivation, and this can be readily calculated as the product of the space velocity of methanol (WHSV) and the lifetime to 50% conversion. The amount of converted methanol at other conversion levels depends on the deactivation, the activity, and applied contact time (space velocity), and is therefore less appropriate to use as a characterization of the deactivation behavior.
Article
In this review, we discuss past theoretical works on fluid-solid reactions in a porous medium. Such reactions are often accompanied by a continuous alteration of the pore structure of the medium, and at high conversions they exhibit percolation-type behavior, i.e. the solid matrix of the medium and/or the fluid phase lose their macroscopic connectivity. These phenomena are, therefore, characterized by a percolation threshold which is the volume or area fraction of a phase (solid or fluid) below which that phase exists only in isolated clusters or islands. Important classes of such processes are acid dissolution of a porous medium and gas—solid reactions with pore volume growth, e.g. coal gasification, and with pore closure, e.g. lime sulfation, and catalyst deactivation. These processes are characterized by continuous changes in the pore space as a result of a chemical reaction. We also consider here other processes such as the flow of fines, stable emulsions and solid particles in a porous medium which also alter the structure of the pore space, but by physical interaction of the particles and the solid surface of the pores. In this review we compare two different modelling approaches to reactions accompanied by structural changes. First we review the continuum approach, which is based on the classical equations of transport and reaction supplemented with constitutive equations describing the effect of structural changes on reaction and transport parameters. We then outline the relevant concepts, ideas and techniques of percolation theory and the statistical physics of disordered media, and review their application to the phenomena mentioned above. In particular, we emphasize the fundamental role of connectivity of the porous medium in such phenomena. Since in both approaches one needs to estimate the effective transport properties of the porous medium that is undergoing continuous change, we also review continuum and statistical methods of estimating the effective transport properties of disordered porous media.
Article
Ultraviolet (UV) Raman spectroscopy has been used to characterize coke formation in ZSM-5 and USY zeolites under propene at temperatures from 300K to 773K. The strong fluorescence background always present with normal Raman spectra is completely avoided in UV Raman spectra. Three groups of UV Raman bands near ≈ 1390, ≈ 1600 and ≈ 3000 cm−1 regions were detected for the two zeolites, and these bands varied significantly at different stages of coke formation. At room temperature, adsorbed propene was formed in the two zeolites and showed similar spectra. At elevated temperatures, the coke formation behavior in the two zeolites is quite different. For example, at 773K the coke species in ZSM-5 are mainly polyolefinic and aromatic species, but polyaromatic and pregraphite species are predominant in USY. The major portion of coke species formed in ZSM-5 can be removed even by He purging at 773K while the coke species in USY are very stable and can only be removed in O2 flow at temperatures above 773K. The difference in coke formation in ZSM-5 and USY is likely due to the different pore structure and acidity of the two zeolites.
Article
The modes of coking and of deactivation of zeolites during n-heptane cracking at 723 K were established on the basis of (i) the composition of the carbonaceous compounds responsible for deactivation (coke), (ii) the deactivating effect of the coke molecules and (iii) the reduction by coke of the volume accessible to nitrogen and to n-hexane (kinetic diameter similar to n-heptane). The zeolites [USHY, H Mordenite (HMOR), HZSM5 and H Erionite (HERI)] were chosen to determine the effect of different parameters of the pore structure: (i) pore size, (ii) existence (USHY, HERI) or non-existence of cavities (HMOR, HZSM5), (iii) the possibility for the reactant to diffuse unidirectionally (HMOR) or tridirectionally. The retention of coke molecules is due to trapping in the cavities (or at channel intersections). Their size is intermediate between that of the apertures and that of the cavities (or channel intersections). The coking rate is all the faster when the space available near the acid sites is large and when the coke precursors desorb slowly. On all the zeolites, coke formation occurs through oligomerization of the olefinic cracking products followed by cyclization of the oligomers, transformation through hydrogen transfer into monoaromatics, alkylation of these monoaromatics, then cyclization and hydrogen transfer to give bi-aromatics, tri-aromatics, etc. There is no site poisoning by coke; deactivation occurs through the three following modes: (i) limitation of the access of n-heptane to the active sites, (ii) blockage of the access to the sites of the cavities (or channel intersections) in which the coke molecules are situated and (iii) blockage of the access to the sites of the pores in which there are no coke molecules.
Article
Two ZSM-5 catalysts were prepared with bulk ratios of 39 and 74. After conversion to the hydrogen form they were partially deactivated at 370 °C in a methanol/nitrogen stream. Coke contents were determined by combustion in a microbalance. Measurement of the surface and near-surface ratio by X-ray photoelectron spectroscopy showed internal and external coke formation. Internal coke formation predominated during methanol conversion over ZSM-5 until the catalyst was essentially deactivated. External coke was then formed, probably by the thermal cracking of methanol.
Article
A zeolite dealumination mechanism is proposed on the basis of sensitivity-enhanced 27Al DQ-MAS NMR spectra (see picture), which revealed for the first time the detailed evolution of extra-framework aluminum (EFAL) species and the spatial proximities of various aluminum species in dealuminated HY zeolites. Three types of EFAL species in close proximity to framework aluminum were identified.
Article
Formation of coke in large H-ZSM-5 and H-SAPO-34 crystals during the methanol-to-olefin (MTO) reaction has been studied in a space- and time-resolved manner. This has been made possible by applying a high-temperature in-situ cell in combination with micro-spectroscopic techniques. The buildup of optically active carbonaceous species allows detection with UV/Vis microscopy, while a confocal fluorescence microscope in an upright configuration visualises the formation of coke molecules and their precursors inside the catalyst grains. In H-ZSM-5, coke is initially formed at the triangular crystal edges, in which straight channel openings reach directly the external crystal surface. At reaction temperatures ranging from 530 to 745 K, two absorption bands at around 415 and 550 nm were detected due to coke or its precursors. Confocal fluorescence microscopy reveals fluorescent carbonaceous species that initially form in the near-surface area and gradually diffuse inwards the crystal in which internal intergrowth boundaries hinder a facile penetration for the more bulky aromatic compounds. In the case of H-SAPO-34 crystals, an absorption band at around 400 nm arises during the reaction. This band grows in intensity with time and then decreases if the reaction is carried out between 530 and 575 K, whereas at higher temperatures its intensity remains steady with time on stream. Formation of the fluorescent species during the course of the reaction is limited to the near-surface region of the H-SAPO-34 crystals, thereby creating diffusion limitations for the coke front moving towards the middle of the crystal during the MTO reaction. The two applied micro-spectroscopic techniques introduced allow us to distinguish between graphite-like coke deposited on the external crystal surface and aromatic species formed inside the zeolite channels. The use of the methods can be extended to a wide variety of catalytic reactions and materials in which carbonaceous deposits are formed.
Article
The process of converting methanol to hydrocarbons on the aluminosilicate zeolite HZSM-5 was originally developed as a route from natural gas to synthetic gasoline. Using other microporous catalysts that are selective for light olefins, methanol-to-olefin (MTO) catalysis may soon become central to the conversion of natural gas to polyolefins. The mechanism of methanol conversion proved to be an intellectually challenging problem; 25 years of fundamental study produced at least 20 distinct mechanisms, but most did not account for either the primary products or a kinetic induction period. Recent experimental and theoretical work has firmly established that methanol and dimethyl ether react on cyclic organic species contained in the cages or channels of the inorganic host. These organic reaction centers act as scaffolds for the assembly of light olefins so as to avoid the high high-energy intermediates required by all "direct" mechanisms. The rate of formation of the initial reaction centers, and hence the duration of the kinetic induction period, can be governed by impurity species. Secondary reactions of primary olefin products strongly reflect the topology and acid strength of the microporous catalyst. Reaction centers form continuously through some secondary pathways, and they age into polycyclic aromatic hydrocarbons, eventually deactivating the catalyst. It proves useful to consider each cage (or channel) with its included organic and inorganic species as a supramolecule that can react to form various species. This view allows us to identify structure-activity and structure selectivity relationships and to modify the catalyst with degrees of freedom that are more reminiscent of homogeneous catalysis than heterogeneous catalysis.
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