Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

Improving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one‐dimensional zeolites (ZSM‐22 and mordenite) and a γ‐alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n‐heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low‐molecular‐weight reactants.     

Visualizing the crystal structure and locating the catalytic activity of micro- and mesoporous ZSM-5 zeolite crystals by using in situ optical and fluorescence microscopy

A combination of optical and fluorescence microscopy was used to study the morphology of micro‐ and mesoporous H‐ZSM‐5 zeolite crystals (17×4×4 μm) and to evaluate, in a spatially resolved manner, the effect of mesoporosity, introduced via desilication, on catalytic performance. For this purpose, the oligomerization of various styrene molecules was used as a model reaction, in which the carbocation intermediates formed in the zeolite pores act as reporter molecules. In situ confocal fluorescence measurements after the template removal process showed that the crystals generally consist of three different subunits that have pyramidal boundaries with each other. Examination of these crystals during styrene oligomerization revealed differences in the catalytic activity between the purely microporous and the combined micro‐ and mesoporous crystals. The introduction of intracrystalline mesoporosity limits the formation to dimeric carbocation intermediates and facilitates the transport of styrene molecules inside the zeolite volume. This leads to a more uniform coloration and fluorescence pattern of the crystals. Moreover, the oligomerization of various styrene compounds, which differ in their reactivity, provides a good way of estimating the Brønsted acid strength in a spatially resolved manner, showing a nonhomogeneously distributed Brønsted acidity over the volume of the crystals. More detailed information on the structure of the ZSM‐5 crystals was revealed for mesoporous crystals during the oligomerization of 4‐methoxystyrene. This reaction induced an “explosion” of the crystal leading to the formation of a complex system with at least eight different subunits. Finally, polarized‐light microscopy was used to unravel the pore geometry in these individual building blocks. The observed differences in catalytic behavior between micro‐ and mesoporous ZSM‐5 crystals are strengthened by the microspectroscopic techniques employed, which show that upon desilication the crystal morphology is affected, the product distribution is changed towards less conjugated carbocation intermediates, and that a gradient in Brønsted acid strength appears to be present.     

Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations

Experimental evidence for the presence of tert‐butyl cations, which are important intermediates in acid‐catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with ¹H/¹³C magic‐angle‐spinning NMR spectroscopy, the tert‐butyl cation was successfully identified on zeolite H‐ZSM‐5 upon conversion of isobutene by capturing this intermediate with ammonia.     

纳米 ZSM-22分子筛的水热合成及在甲醇转化反应中的性能

分子筛结构的独特性和多样性使其在催化、吸附分离和离子交换等领域有着广泛应用.近年来,纳米分子筛制备和应用受到极大关注.与传统微米分子筛相比,纳米分子筛具有较小的晶粒尺寸、较大的外表面积和较高的表面活性,能显著提高其分离和催化性能.制备纳米晶体的常用方法有过量模板法、空间限定法、晶种法、离子热合成法及微反应器合成法等.目前,已合成出多种拓扑结构的纳米分子筛,包括 FAU, MFI, MEL和CHA等. ZSM-22是一种具有 TON拓扑结构的一维十元环直孔道分子筛(孔口尺寸为0.45 nm ×0.55 nm),在长链烷烃异构化和烯烃异构化等反应中表现出优异的催化活性.水热合成法是制备 ZSM-22分子筛最常用的方法,所得样品晶粒尺寸为2–15μm,但由于 ZSM-22分子筛是一种亚稳态结构,为了防止杂晶生成,合成通常是在剧烈搅拌(通常大于400 r/min)下进行.目前已有报道在较低转速下合成 ZSM-22分子筛,但产物仍为微米晶体;或在微波辅助水热合成条件下合成亚微米 ZSM-22分子筛,但晶体尺寸不可调且合成过程需要较高功率的微波反应器.因此,在水热条件下合成纯纳米 ZSM-22分子筛仍然是一个巨大挑战.本文在上述研究基础上采用改进的水热合成法成功合成出纳米 ZSM-22分子筛,考察了转速﹑硅铝比及乙醇共溶剂对晶粒尺寸的影响,比较了纳米和常规微米 ZSM-22分子筛的甲醇转化反应性能.结果表明,采用改进的水热合成法能够在较低转速下合成出纳米 ZSM-22分子筛,晶体尺寸在150–800 nm范围可调.通过考察转速对晶粒尺寸的影响,发现静态合成条件下无法形成 ZSM-22分子筛,表明 ZSM-22分子筛合成需要一定的转速.转速在10–50 r/min变化时,可以合成出不同晶体尺寸的 ZSM-22分子筛,且随转速提高, ZSM-22分子筛晶体尺寸先减小后增大,表明纳米 ZSM-22分子筛合成存在最佳转速.另外,配料硅铝比能显著影响 ZSM-22分子筛晶体尺寸,随配料硅铝比增加, ZSM-22分子筛晶体尺寸先减小后增大.通过在合成体系中添加乙醇作为共溶剂,考察了有机溶剂对 ZSM-22分子筛晶粒尺寸的影响,发现有机溶剂能显著增大 ZSM-22的晶体尺寸.将本文合成的纳米和常规微米 ZSM-22分子筛用于甲醇转化反应,考察了晶体尺寸对 ZSM-22分子筛甲醇转化反应性能的影响.发现与常规微米 ZSM-22分子筛相比,纳米 ZSM-22分子筛催化剂寿命显著提高,说明晶粒尺寸减小能有效减缓积碳导致的分子筛失活;同时,反应产物中乙烯和芳烃选择性有所提高,这是由于外表面积增大所致.此外,还考察了不同硅铝比 ZSM-22分子筛的甲醇转化反应性能.结果表明,分子筛硅铝比会影响催化剂寿命,但晶体尺寸对催化剂寿命影响更大. ZSM-22分子筛硅铝比增大有助于提高低碳烯烃选择性,减少芳烃生成.     

Modeling of monolayer adsorption of hydrogen on ZSM‐5 zeolite by lattice density functional theory

The adsorption isotherms of hydrogen on microporous zeolite ZSM‐5, at supercritical conditions, have been modeled using the monolayer lattice density functional theory (LDFT) models, where the simple cubic lattice, face‐centered cubic lattice, body‐centered cubic lattice and tetragonal lattice structures are assumed for the arrangements of the adsorption sites inside pores based on the size and shape of the zeolite. The results indicate that the monolayer LDFT models appear to be effective in describing hydrogen adsorption on zeolite ZSM‐5 at supercritical conditions, and the calculated adsorption isotherms agree well with the experimental isotherms measured previously. The layer density of adsorbed phase is presented versus the bulk density and temperature. It is found that the densities of adsorbed phase on adsorbent surface are much higher than the bulk density for temperature range under study. However, in the core region, the layer densities are close to the bulk density. The monolayer adsorption is suitable for hydrogen on ZSM‐5 zeolite. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of Coke Deposits on the Catalytic Performance of Large Zeolite H‐ZSM‐5 Crystals during Alcohol‐to‐Hydrocarbon Reactions as Investigated by a Combination of Optical Spectroscopy and Microscopy

The catalytic activity of large zeolite H‐ZSM‐5 crystals in methanol (MTO) and ethanol‐to‐olefins (ETO) conversions was investigated and, using operando UV/Vis measurements, the catalytic activity and deactivation was correlated with the formation of coke. These findings were related to in situ single crystal UV/Vis and confocal fluorescence micro‐spectroscopy, allowing the observation of the spatiotemporal formation of intermediates and coke species during the MTO and ETO conversions. It was observed that rapid deactivation at elevated temperatures was due to the fast formation of aromatics at the periphery of the H‐ZSM‐5 crystals, which are transformed into more poly‐aromatic coke species at the external surface, preventing the diffusion of reactants and products into and out of the H‐ZSM‐5 crystal. Furthermore, we were able to correlate the operando UV/Vis spectroscopy results observed during catalytic testing with the single crystal in situ results.     

Theoretical Insights on Methylbenzene Side‐Chain Growth in ZSM‐5 Zeolites for Methanol‐to‐Olefin Conversion

The key step in the conversion of methane to polyolefins is the catalytic conversion of methanol to light olefins. The most recent formulations of a reaction mechanism for this process are based on the idea of a complex hydrocarbon‐pool network, in which certain organic species in the zeolite pores are methylated and from which light olefins are eliminated. Two major mechanisms have been proposed to date—the paring mechanism and the side‐chain mechanism—recently joined by a third, the alkene mechanism. Recently we succeeded in simulating a full catalytic cycle for the first of these in ZSM‐5, with inclusion of the zeolite framework and contents. In this paper, we will investigate crucial reaction steps of the second proposal (the side‐chain route) using both small and large zeolite cluster models of ZSM‐5. The deprotonation step, which forms an exocyclic double bond, depends crucially on the number and positioning of the other methyl groups but also on steric effects that are typical for the zeolite lattice. Because of steric considerations, we find exocyclic bond formation in the ortho position to the geminal methyl group to be more favourable than exocyclic bond formation in the para position. The side‐chain growth proceeds relatively easily but the major bottleneck is identified as subsequent de‐alkylation to produce ethene. These results suggest that the current formulation of the side‐chain route in ZSM‐5 may actually be a deactivating route to coke precursors rather than an active ethene‐producing hydrocarbon‐pool route. Other routes may be operating in alternative zeotype materials like the silico‐aluminophosphate SAPO‐34.     

Catalytic Para‐Xylene Maximization. V. Toluene Methylation with Methanol and Disproportionation of Toluene Using Pt/ZSM‐5 and Pt/Mordenite Catalysts

Toluene was methylated with methanol and disproportionated using catalysts containing different Pt contents (0.2, 0.4 and 0.6%) supported on H‐ZSM‐5 or H‐mordenite (H‐M) zeolites in a fixed‐bed flow‐reactor operated atmospherically at temperatures of 300–500 °C in a flow of hydrogen. Platinum dispersion in the zeolite supports and acid sites strength distribution were evaluated using hydrogen chemisorption (1:1 stoichiometry) and ammonia temperature programmed desorption (TPD) in a differential scanning calorimeter (DSC). Toluene methylation was much faster on all catalysts than toluene disproportionation (DISP). Both reactions were more accelerated using H‐ZSM‐5 containing catalysts than H‐M containing catalysts. The yield of xylenes, and in particular para‐xylene, was significantly influenced by the yield of trimethylbenzenes (TMBs) in product. The selectivities for para‐, ortho‐ and meta‐xylenes production were found largely dependent on the Pt content in the catalysts, particularly when supported on H‐ZSM5‐zeolite. However, using Pt/H‐M catalysts, these selectivities were not strictly controlled by Pt content in the catalysts.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Hexane Cracking over Steamed Phosphated Zeolite H‐ZSM‐5: Promotional Effect on Catalyst Performance and Stability

The nature behind the promotional effect of phosphorus on the catalytic performance and hydrothermal stability of zeolite H‐ZSM‐5 has been studied using a combination of ²⁷Al and ³¹P MAS NMR spectroscopy, soft X‐ray absorption tomography and n‐hexane catalytic cracking, complemented with NH₃ temperature‐programmed desorption and N₂ physisorption. Phosphated H‐ZSM‐5 retains more acid sites and catalytic cracking activity after steam treatment than its non‐phosphated counterpart, while the selectivity towards propylene is improved. It was established that the stabilization effect is twofold. First, the local framework silico‐aluminophosphate (SAPO) interfaces, which form after phosphatation, are not affected by steam and hold aluminum atoms fixed in the zeolite lattice, preserving the pore structure of zeolite H‐ZSM‐5. Second, the four‐coordinate framework aluminum can be forced into a reversible sixfold coordination by phosphate. These species remain stationary in the framework under hydrothermal conditions as well. Removal of physically coordinated phosphate after steam‐treatment leads to an increase in the number of strong acid sites and increased catalytic activity. We propose that the improved selectivity towards propylene during catalytic cracking can be attributed to local SAPO interfaces located at channel intersections, where they act as impediments in the formation of bulky carbenium ions and therefore suppress the bimolecular cracking mechanism.     

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C₁–n‐C₄ alkanes on Zn‐modified high‐silica zeolites ZSM‐5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C? H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n‐butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C₁–n‐C₄ alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.     

Phosphatation of Zeolite H‐ZSM‐5: A Combined Microscopy and Spectroscopy Study

A variety of phosphated zeolite H‐ZSM‐5 samples are investigated by using a combination of Fourier transfer infrared (FTIR) spectroscopy, single pulse ²⁷Al, ²⁹Si, ³¹P, ¹H‐³¹P cross polarization (CP), ²⁷Al‐³¹P CP, and ²⁷Al 3Q magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, scanning transmission X‐ray microscopy (STXM) and N₂ physisorption. This approach leads to insights into the physicochemical processes that take place during phosphatation. Direct phosphatation of H‐ZSM‐5 promotes zeolite aggregation, as phosphorus does not penetrate deep into the zeolite material and is mostly found on and close to the outer surface of the zeolite, acting as a glue. Phosphatation of pre‐steamed H‐ZSM‐5 gives rise to the formation of a crystalline tridymite AlPO₄ phase, which is found in the mesopores of dealuminated H‐ZSM‐5. Framework aluminum species interacting with phosphorus are not affected by hydrothermal treatment. Dealuminated H‐ZSM‐5, containing AlPO₄, retains relatively more framework Al atoms and acid sites during hydrothermal treatment than directly phosphated H‐ZSM‐5.     

Dispersion and Orientation of Zeolite ZSM‐5 Crystallites within a Fluid Catalytic Cracking Catalyst Particle

Confocal fluorescence microscopy was employed to selectively visualize the dispersion and orientation of zeolite ZSM‐5 domains inside a single industrially applied fluid catalytic cracking (FCC) catalyst particle. Large ZSM‐5 crystals served as a model system together with the acid‐catalyzed fluorostyrene oligomerization reaction to study the interaction of plane‐polarized light with these anisotropic zeolite crystals. The distinction between zeolite and binder material, such as alumina, silica, and clay, within an individual FCC particle was achieved by utilizing the anisotropic nature of emitted fluorescence light arising from the entrapped fluorostyrene‐derived carbocations inside the zeolite channels. This characterization approach provides a non‐invasive way for post‐synthesis characterization of an individual FCC catalyst particle in which the size, distribution, orientation, and amount of zeolite ZSM‐5 aggregates can be determined. It was found that the amount of detected fluorescence light originating from the stained ZSM‐5 aggregates corresponds to about 15 wt %. Furthermore, a statistical analysis of the emitted fluorescence light indicated that a large number of the ZSM‐5 domains appeared in small sizes of about 0.015–0.25 μm², representing single zeolite crystallites or small aggregates thereof. This observation illustrated a fairly high degree of zeolite dispersion within the FCC binder material. However, the highest amount of crystalline material was aggregated into larger domains (ca. 1–5 μm²) with more or less similarly oriented zeolite crystallites. It is clear that this visualization approach may serve as a post‐synthesis quality control on the dispersion of zeolite ZSM‐5 crystallites within FCC particles.     

Coupling of Methanol and Carbon Monoxide over H‐ZSM‐5 to Form Aromatics

The conversion of methanol into aromatics over unmodified H‐ZSM‐5 zeolite is generally not high because the hydrogen transfer reaction results in alkane formation. Now circa 80 % aromatics selectivity for the coupling reaction of methanol and carbon monoxide over H‐ZSM‐5 is reported. Carbonyl compounds and methyl‐2‐cyclopenten‐1‐ones (MCPOs), which were detected in the products and catalysts, respectively, are considered as intermediates. The latter species can be synthesized from the former species and olefins. ¹³C isotope tracing and ¹³C liquid‐state NMR results confirmed that the carbon atoms of CO molecules were incorporated into MCPOs and aromatic rings. A new aromatization mechanism that involves the formation of the above intermediates and co‐occurs with a dramatically decreased hydrogen transfer reaction is proposed. A portion of the carbons in CO molecules are incorporated into aromatic, which is of great significance for industrial applications.     

Experimental Evidence on the Formation of Ethene through Carbocations in Methanol Conversion over H‐ZSM‐5 Zeolite

The methanol to olefins conversion over zeolite catalysts is a commercialized process to produce light olefins like ethene and propene but its mechanism is not well understood. We herein investigated the formation of ethene in the methanol to olefins reaction over the H‐ZSM‐5 zeolite. Three types of ethylcyclopentenyl carbocations, that is, the 1‐methyl‐3‐ethylcyclopentenyl, the 1,4‐dimethyl‐3‐ethylcyclopentenyl, and the 1,5‐dimethyl‐3‐ethylcyclopentenyl cation were unambiguously identified under working conditions by both solid‐state and liquid‐state NMR spectroscopy as well as GC‐MS analysis. These carbocations were found to be well correlated to ethene and lower methylbenzenes (xylene and trimethylbenzene). An aromatics‐based paring route provides rationale for the transformation of lower methylbenzenes to ethene through ethylcyclopentenyl cations as the key hydrocarbon‐pool intermediates.     

Tailoring Zeolite ZSM‐5 Crystal Morphology/Porosity through Flexible Utilization of Silicalite‐1 Seeds as Templates: Unusual Crystallization Pathways in a Heterogeneous System

Diffusion limitation in micropores of zeolites leads to a demand for optimization of zeolite morphology and/or porosity. However, tailoring crystallization processes to realize targeted morphology/porosity is a major challenge in zeolite synthesis. On the basis of previous work on the salt‐aided, seed‐induced route, the template effect of seeds on the formation of micropores, mesopores and even macropores was further explored to selectively achieve desired hierarchical architectures. By carefully investigating the crystallization processes of two typical samples with distinct crystal morphologies, namely, 1) nanocrystallite‐oriented self‐assembled ZSM‐5 zeolite and 2) enriched intracrystal mesoporous ZSM‐5 zeolite, a detailed mechanism is proposed to clarify the role of silicalite‐1 seeds in the formation of diverse morphologies in a salt‐rich heterogeneous system, combined with the transformation of seed‐embedded aluminosilicate gel. On the basis of these conclusions, the morphologies/porosities of products were precisely tailored by deliberately adjusting the synthesis parameters (KF/Si, tetrapropylammonium bromide/Si and H₂O/Si ratios and type of organic template) to regulate the kinetics of seed dissolution and seed‐induced recrystallization. This work may not only provide a practical route to control zeolite crystallization for tailoring crystal morphology, but also expands the knowledge of crystal growth mechanisms in a heterogeneous system.     

NH3-SCR反应中Fe-ZSM-5的Fe物种分布和酸性位对其水热稳定性及抗硫和抗碳氢性能的影响

以Na+型和H+型ZSM-5为载体制备了Fe-ZSM-5催化剂并用于氨选择性催化还原(NH3-SCR)氮氧化物. Fe-H-ZSM-5在新鲜时和750 oC含10%水的空气中老化后,其SCR活性均优于Fe-Na-ZSM-5.表征结果显示, Fe-H-ZSM-5和Fe-Na-ZSM-5的Fe物种分布和酸性位有所不同.高温水热老化后Fe-H-ZSM-5分子筛骨架的脱铝较Fe-Na-ZSM-5严重.水和硫的存在对Fe-H-ZSM-5和Fe-Na-ZSM-5的SCR活性的影响相似,即降低了低温活性,略提高了高温活性. Fe-Na-ZSM-5表现出比Fe-H-ZSM-5更好的抗碳氢中毒性能.这两种催化剂的SO2和碳氢中毒是可逆的.     

Thermodynamics of Carbon Dioxide Adsorption on the Protonic Zeolite H‐ZSM‐5

Adsorption of carbon dioxide on H‐ZSM‐5 zeolite (Si:Al=11.5:1) was studied by means of variable‐temperature FT‐IR spectroscopy, in the temperature range of 310–365 K. The adsorbed CO₂ molecules interact with the zeolite Brønsted‐acid OH groups bringing about a characteristic red‐shift of the O? H stretching band from 3610 cm^?1 to 3480 cm^?1. Simultaneously, the ν₃ mode of adsorbed CO₂ is observed at 2345 cm^?1. From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm^?1, upon changing temperature (and CO₂ equilibrium pressure), the standard adsorption enthalpy of CO₂ on H‐ZSM‐5 is ΔH⁰=?31.2(±1) kJ mol^?1 and the corresponding entropy change is ΔS⁰=?140(±10) J mol^?1 K^?1. These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali‐metal exchanged, zeolites.