Insights into the Activity and Deactivation of the Methanol-to-Olefins Process over Different Small-Pore Zeolites As Studied with Operando UV-vis Spectroscopy

Methanol-to-Olefins Studied by UV Raman Spectroscopy as Compared to Visible Wavelength: Capitalization on Resonance Enhancement

Resonance Raman spectroscopy can provide insights into complex reaction mechanisms by selectively enhancing the signals of specific molecular species. In this work, we demonstrate that, by changing the excitation wavelength, Raman bands of different intermediates in the methanol-to-hydrocarbons reactions can be identified. We show in particular how UV excitation enhances signals from short-chain olefins and cyclopentadienyl cations during the induction period, while visible excitation better detects later-stage aromatics. However, visible excitation is prone to fluorescence that can obscure Raman signals, and hence, we show how fast fluorescence rejection techniques like Kerr gating are necessary for extracting useful information from visible excitation measurements.

Quantitative Kinetic Insights from Operando-UV/Vis Spectroscopy: An Application to NH3-SCR of NOx on Cu-CHA Catalysts

We employ UV/Vis Diffuse Reflectance spectroscopy directly coupled with a packed bed flow reactor to extract quantitative kinetic information. We use as a show-case the CuII/CuI redox dynamics during the reduction half cycle of the NH3-Selective Catalytic Reduction (SCR) on Cu-CHA catalysts. Our measurements enable quantification of the fraction of oxidized Cu, reconstructed by Multivariate Curve Resolution (MCR) together with monitoring of the gas-phase evolution during the reaction. These data both on the dynamics of the gas-phase and of the active site oxidation state have been used to assess the reduction half cycle rate equation and estimate the rate constant. Our results in terms of reaction orders and kinetic constant are in line with previous findings in the literature. Overall, our results demonstrate that the combined analysis of the UV spectra and of the gas-phase dynamics provides converging and unparalleled kinetic insight: this approach effectively resolves ambiguities concerning RHC kinetics and mechanism. More in general, this work provides evidence that operando spectroscopy can be used to extract quantitative kinetic information on catalytic cycles.

Investigating the Sole Olefin-Based Cycle in Small-Cage MCM-35-Catalyzed Methanol-to-Olefins Reactions

Small-pore zeolites catalyze the methanol-to-olefins (MTO) reaction via a dual-cycle mechanism, encompassing both olefin- and aromatic-based cycles. Zeolite topology is crucial in determining both the catalytic pathway and the product selectivity of the MTO reaction. Herein, we investigate the mechanistic influence of MCM-35 zeolite on the MTO process. The structural properties of the as-synthesized MCM-35 catalyst, including its confined cages (6.19 Å), were characterized, confirming them as the catalytic centers. Then, the MTO reactions were systematically performed and investigated over a MCM-35 catalyst. Feeding pure methanol to the reactor yielded minimal MTO activity despite the formation of some aromatic species within the zeolite. The results suggest that the aromatic-based cycle is entirely suppressed in MCM-35, preventing the simultaneous occurrence of the olefin-based cycle. However, cofeeding a small amount of propene in methanol can obviously enhance the methanol conversion under the same studied reaction conditions. Thus, the exclusive operation of the olefin-based cycle in the MTO reaction, independent of the aromatic-based cycle, was demonstrated in MCM-35 zeolite.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Enabling the Methanol Economy: Opportunities and Challenges for Heterogeneous Catalysis in the Production of Liquid Fuels via Methanol

ConspectusThirty years ago, George A. Olah proposed the concept of the methanol economy, where methanol replaces fossil fuels as a means of energy storage, ground transportation fuel, and raw material for the manufacture of other carbon-based products. Over the years, with rising global warming concerns, the concept has evolved. A special interest is devoted to the development of catalytic processes that allow the transformation of carbon dioxide, via methanol, into CO2 neutral liquid hydrocarbons. These products could replace the oil-based fuels currently used by combustion engines. The rapid depletion of such fuels would avoid a considerable amount of CO2 emissions during the current energy transition.Over the past decade, we have focused on different key processes that should allow for maximal atom efficiency and, therefore, minimal energy consumption in a field, CO2 valorization, that can easily become a zero-sum game. In this Account, we highlight the importance of catalyst design to overcome the process challenges in the production of liquid fuels from methanol. Additionally, progress in multifunctional catalysts able to directly convert, in one single reactor, CO2 to liquid fuels is also discussed in detail. This integrated option is of particular interest since it allows an important decrease in operational units while increasing throughput by converting, in situ, a thermodynamically limited intermediate.

Evaluating the efficacy of zeolites synthesized from natural clay for the methanol-to-hydrocarbon process

Introducing sustainability into advanced catalytic material design is essential to address growing environmental concerns. Among them, synthesizing inorganic zeolite materials from non-traditional sources (like natural clay) offers several advantages, contributing to sustainability and environmental stewardship. With this objective, we used kaolin to synthesize zeolites with different topologies: SSZ-13 (8-MR with CHA topology), ZSM-5 (10-MR with MFI topology), and Beta (12-MR with BEA topology) (MR: member ring), where a simple and flexible synthetic protocol was adopted without any significant changes. All these zeolites were subjected to catalytic performance evaluation concerning the industrially relevant methanol-to-hydrocarbon (MTH) process. Herein, the kaolin-derived zeolites, especially ZSM-5, led to superior performance and demonstrated enhanced catalyst deactivation-resistant behavior compared to their zeolite counterparts prepared from traditional synthetic routes. Various characterization tools (including under operando conditions) were employed to understand their reactions and deactivation mechanisms. Overall, making zeolites from non-traditional sources presents a pathway for sustainable and environmentally friendly material production, offering benefits such as reduced resource dependence, lower energy consumption, and tailored physicochemical properties beneficial to catalysis. In a broader context, such a research approach contributes to the transition toward a more sustainable and circular economy.

Cavity-controlled methanol conversion over zeolite catalysts

The successful development and application in industry of methanol-to-olefins (MTO) process brought about an innovative and efficient route for olefin production via non-petrochemical resources and also attracted attention of C1 chemistry and zeolite catalysis. Molecular sieve catalysts with diversified microenvironments embedding unique channel/cavity structure and acid properties, exhibit demonstrable features and advantages in the shape-selective catalysis of MTO. Especially, shape-selective catalysis over 8-MR and cavity-type zeolites with acidic supercage environment and narrow pore opening manifested special host-guest interaction between the zeolite catalyst and guest reactants, intermediates and products. This caused great differences in product distribution, catalyst deactivation and molecular diffusion, revealing the cavity-controlled methanol conversion over 8-MR and cavity-type zeolite catalyst. Furthermore, the dynamic and complicated cross-talk behaviors of catalyst material (coke)-reaction-diffusion over these types of zeolites determines the catalytic performance of the methanol conversion. In this review, we shed light on the cavity-controlled principle in the MTO reaction including cavity-controlled active intermediates formation, cavity-controlled reaction routes with the involvement of these intermediates in the complex reaction network, cavity-controlled catalyst deactivation and cavity-controlled diffusion. All these were exhibited by the MTO reaction performances and product selectivity over 8-MR and cavity-type zeolite catalysts. Advanced strategies inspired by the cavity-controlled principle were developed, providing great promise for the optimization and precise control of MTO process.

Addition of Pore-Forming Agents and Their Effect on the Pore Architecture and Catalytic Behavior of Shaped Zeolite-Based Catalyst Bodies

Porous materials, such as solid catalysts, are used in various chemical reactions in industry to produce chemicals, materials, and fuels. Understanding the interplay between pore architecture and catalytic behavior is of great importance for synthesizing a better industrial-grade catalyst material. In this study, we have investigated the modification of the pore architecture of zeolite-based alumina-bound shaped catalyst bodies via the addition of different starches as pore-forming agents. A combination of microscopy techniques allowed us to visualize the morphological changes induced and make a link between pore architecture, molecular transport, and catalytic performance. As for the catalytic performance in the methanol-to-hydrocarbons (MTH) reaction, pore-forming agents resulted in up to ∼12% higher conversion, an increase of 74% and 77% in yield (14% and 13% compared to 8.6% and 7.7% of the reference sample in absolute yields) toward ethylene and propylene, respectively, and an improved lifetime of the catalyst materials.

Understanding W/H-ZSM-5 catalysts for the dehydroaromatization of methane

Tungsten is the most interesting and promising metal to replace molybdenum in methane dehydroaromatization (MDA) catalysis.

Detecting Cage Crossing and Filling Clusters of Magnesium and Carbon Atoms in Zeolite SSZ-13 with Atom Probe Tomography

The conversion of methanol to valuable hydrocarbon molecules is of great commercial interest, as the process serves as a sustainable alternative for the production of, for instance, the base chemicals for plastics. The reaction is catalyzed by zeolite materials. By the introduction of magnesium as a cationic metal, the properties of the zeolite, and thereby the catalytic performance, are changed. With atom probe tomography (APT), nanoscale relations within zeolite materials can be revealed: i.e., crucial information for a fundamental mechanistic understanding. We show that magnesium forms clusters within the cages of zeolite SSZ-13, while the framework elements are homogeneously distributed. These clusters of just a few nanometers were analyzed and visualized in 3-D. Magnesium atoms seem to initially be directed to the aluminum sites, after which they aggregate and fill one or two cages in the zeolite SSZ-13 structure. The presence of magnesium in zeolite SSZ-13 increases the lifetime as well as the propylene selectivity. By using operando UV-vis spectroscopy and X-ray diffraction techniques, we are able to show that these findings are related to the suppression of aromatic intermediate products, while maintaining the formation of polyaromatic compounds. Further nanoscale analysis of the spent catalysts showed indications of magnesium redistribution after catalysis. Unlike zeolite H-SSZ-13, for which only a homogeneous distribution of carbon was found, carbon can be either homogeneously or heterogeneously distributed within zeolite Mg-SSZ-13 crystals as the magnesium decreases the coking rate. Carbon clusters were isolated, visualized, and analyzed and were assumed to be polyaromatic compounds. Small one-cage-filling polyaromatic compounds were identified; furthermore, large-cage-crossing aromatic molecules were found by isolating large coke clusters, demonstrating the unique coking mechanism in zeolite SSZ-13. Short-length-scale evidence for the formation of polyaromatic compounds at acid sites is discovered, as clear nanoscale relations between aluminum and carbon atoms exist.

Elucidation of radical- and oxygenate-driven paths in zeolite-catalysed conversion of methanol and methyl chloride to hydrocarbons

Understanding hydrocarbon generation in the zeolite-catalysed conversions of methanol and methyl chloride requires advanced spectroscopic approaches to distinguish the complex mechanisms governing C-C bond formation, chain growth and the deposition of carbonaceous species. Here operando photoelectron photoion coincidence (PEPICO) spectroscopy enables the isomer-selective identification of pathways to hydrocarbons of up to C₁₄ in size, providing direct experimental evidence of methyl radicals in both reactions and ketene in the methanol-to-hydrocarbons reaction. Both routes converge to C₅ molecules that transform into aromatics. Operando PEPICO highlights distinctions in the prevalence of coke precursors, which is supported by electron paramagnetic resonance measurements, providing evidence of differences in the representative molecular structure, density and distribution of accumulated carbonaceous species. Radical-driven pathways in the methyl chloride-to-hydrocarbons reaction(s) accelerate the formation of extended aromatic systems, leading to fast deactivation. By contrast, the generation of alkylated species through oxygenate-driven pathways in the methanol-to-hydrocarbons reaction extends the catalyst lifetime. The findings demonstrate the potential of the presented methods to provide valuable mechanistic insights into complex reaction networks.

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

Acid-catalyzed reactions inside zeolites are one type of broadly applied industrial reactions, where carbocations are the most common intermediates of these reaction processes, including methanol to olefins, alkene/aromatic alkylation, and hydrocarbon cracking/isomerization. The fundamental research on these acid-catalyzed reactions is focused on the stability, evolution, and lifetime of carbocations under the zeolite confinement effect, which greatly affects the efficiency, selectivity and deactivation of zeolite catalysts. Therefore, a profound understanding of the carbocations confined in zeolites is not only beneficial to explain the reaction mechanism but also drive the design of new zeolite catalysts with ideal acidity and cages/channels. In this review, we provide both an in-depth understanding of the stabilization of carbocations by the pore confinement effect and summary of the advanced characterization methods to capture carbocations in zeolites, including UV-vis spectroscopy, solid-state NMR, fluorescence microscopy, IR spectroscopy and Raman spectroscopy. Also, we clarify the relationship between the activity and stability of carbocations in zeolite-catalyzed reactions, and further highlight the role of carbocations in various hydrocarbon conversion reactions inside zeolites with diverse frameworks and varying acidic properties.

Structural Transformation-Involved Synthesis of Nanosized ERI-Type Zeolite and Its Catalytic Property in the MTO Reaction

Nanosized ERI-type aluminophosphate was prepared by the calcination of a precursor material (denoted as ECNU-38P) synthesized using 1,1,6,6-tetramethyl-1,6-diazacyclododecane-1,6-diium hydroxide (TDDH) as a structure-directing agent. The structure of ECNU-38P is related to ERI topology but exhibits a highly disordered manner and contains both four- and six-coordinated Al atoms. In situ XRD patterns revealed a rarely reported temperature-induced three-dimensional (3D)-to-3D structural transformation from ECNU-38P to the ordered ERI-type ECNU-38 zeolite at 573-623 K. Nanosized ERI-type silicoaluminophosphate Si-ECNU-38 was also obtained by introducing Si atoms into the synthetic system of ECNU-38P. The catalytic performance of ERI-type silicoaluminophosphates in the methanol-to-olefin (MTO) reaction was revealed to be highly related to the crystal sizes.

Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

MAPO-18 Catalysts for the Methanol to Olefins Process: Influence of Catalyst Acidity in a High-Pressure Syngas (CO and H2) Environment

The transition from integrated petrochemical complexes toward decentralized chemical plants utilizing distributed feedstocks calls for simpler downstream unit operations. Less separation steps are attractive for future scenarios and provide an opportunity to design the next-generation catalysts, which function efficiently with effluent reactant mixtures. The methanol to olefins (MTO) reaction constitutes the second step in the conversion of CO2, CO, and H2 to light olefins. We present a series of isomorphically substituted zeotype catalysts with the AEI topology (MAPO-18s, M = Si, Mg, Co, or Zn) and demonstrate the superior performance of the M(II)-substituted MAPO-18s in the conversion of MTO when tested at 350 °C and 20 bar with reactive feed mixtures consisting of CH3OH/CO/CO2/H2. Co-feeding high pressure H2 with methanol improved the catalyst activity over time, but simultaneously led to the hydrogenation of olefins (olefin/paraffin ratio < 0.5). Co-feeding H2/CO/CO2/N2 mixtures with methanol revealed an important, hitherto undisclosed effect of CO in hindering the hydrogenation of olefins over the Brønsted acid sites (BAS). This effect was confirmed by dedicated ethene hydrogenation studies in the absence and presence of CO co-feed. Assisted by spectroscopic investigations, we ascribe the favorable performance of M(II)APO-18 under co-feed conditions to the importance of the M(II) heteroatom in altering the polarity of the M-O bond, leading to stronger BAS. Comparing SAPO-18 and MgAPO-18 with BAS concentrations ranging between 0.2 and 0.4 mmol/gcat, the strength of the acidic site and not the density was found to be the main activity descriptor. MgAPO-18 yielded the highest activity and stability upon syngas co-feeding with methanol, demonstrating its potential to be a next-generation MTO catalyst.

The concept of active site in heterogeneous catalysis

Catalysis is at the core of chemistry and has been essential to make all the goods surrounding us, including fuels, coatings, plastics and other functional materials. In the near future, catalysis will also be an essential tool in making the shift from a fossil-fuel-based to a more renewable and circular society. To make this reality, we have to better understand the fundamental concept of the active site in catalysis. Here, we discuss the physical meaning - and deduce the validity and, therefore, usefulness - of some common approaches in heterogeneous catalysis, such as linking catalyst activity to a 'turnover frequency' and explaining catalytic performance in terms of 'structure sensitivity' or 'structure insensitivity'. Catalytic concepts from the fields of enzymatic and homogeneous catalysis are compared, ultimately realizing that the struggle that one encounters in defining the active site in most solid catalysts is likely the one we must overcome to reach our end goal: tailoring the precise functioning of the active sites with respect to many different parameters to satisfy our ever-growing needs. This article ends with an outlook of what may become feasible within the not-too-distant future with modern experimental and theoretical tools at hand.

Nano-scale insights regarding coke formation in zeolite SSZ-13 subject to the methanol-to-hydrocarbons reaction

The methanol-to-hydrocarbons (MTH) process, commonly catalyzed by zeolites, is of great commercial interest and therefore widely studied both in industry and academia. However, zeolite-based catalyst materials are notoriously hard to study at the nano-scale. Atom probe tomography (APT) is uniquely positioned among the suite of characterization techniques, as it can provide 3D chemical information with sub-nm resolution. In this work, we have used APT to study the nano-scale coking behavior of zeolite SSZ-13 and its relation to bulk coke formation on the macro-/micro-scale studied with operando and in situ UV-vis spectroscopy and microscopy. Radial distribution function analysis (RDF) of the APT data revealed short carbon–carbon length scale affinities, consistent with the formation of larger aromatic molecules (coke species). Using nearest neighbor distribution (NND) analysis, an increase in the homogeneity of carbon was found with increasing time-on-stream. However, carbon clusters could not be isolated due to spatial noise and limited clustering. Therefore, it was found that the coke formation in zeolite SSZ-13 (CHA) is reasonably homogeneous on the nano-scale, and is rather similar to the silicoaluminophosphate analogue SAPO-34 (CHA) but different in nano-scale coking behavior compared to previously studied zeolite ZSM-5 (MFI).

A correlation between the micro- and nano-scale coking behavior of SSZ-13 was discovered with in situ/operando spectroscopy and atom probe tomography (APT), which allows for spatial reconstruction and analysis of relations between framework elements and carbon atoms.

Probing coke formation during the methanol-to-hydrocarbon reaction on zeolite ZSM-5 catalyst at the nanoscale using tip-enhanced fluorescence microscopy

The deactivation mechanism of the widely used zeolite ZSM-5 catalysts remains unclear to date due to the lack of analytical techniques with sufficient sensitivity and/or spatial resolution. Herein, a combination of hyperspectral confocal fluorescence microscopy (CFM) and tip-enhanced fluorescence (TEFL) microscopy is used to study the formation of different coke (precursor) species involved in the deactivation of zeolite ZSM-5 during the methanol-to-hydrocarbon (MTH) reaction. CFM submicron-scale imaging shows a preferential formation of graphite-like coke species at the edges of zeolite ZSM-5 crystals within 10 min of the MTH reaction (i.e., working catalyst), whilst the amount of graphite-like coke species uniformly increased over the entire zeolite ZSM-5 surface after 90 min (i.e., deactivated catalyst). Furthermore, TEFL nanoscale imaging with ∼35 nm spatial resolution revealed that formation of coke species on the zeolite ZSM-5 surface is non-uniform and a relatively larger amount of coke is formed at the crystal steps, indicating a higher initial catalytic activity.

Inhomogeneities in coke formation during methanol-to-hydrocarbon reaction on the zeolite ZSM-5 catalyst are imaged with ∼35 nm spatial resolution using tip-enhanced fluorescence microscopy.

How Many Molecules Can Fit in a Zeolite Pore? Implications for the Hydrocarbon Pool Mechanism of the Methanol-to-Hydrocarbons Process

The methanol-to-hydrocarbons (MTH) process is a very advantageous way to upgrade methanol to more valuable commodity chemicals such as light alkenes and gasoline. There is general agreement that, at steady state, the process operates via a dual cycle “hydrocarbon pool” mechanism. This mechanism defines a minimum number of reactants, intermediates, and products that must be present for the reaction to occur. In this paper, we calculate (by three independent methods) the volume required for a range of compounds that must be present in a working catalyst. These are compared to the available volume in a range of zeolites that have been used, or tested, for MTH. We show that this straightforward comparison provides a means to rationalize the product slate and the deactivation pathways in zeotype materials used for the MTH reaction.

Molecular Routes of Dynamic Autocatalysis for Methanol-to-Hydrocarbons Reaction

The industrially important methanol-to-hydrocarbons (MTH) reaction is driven and sustained by autocatalysis in a dynamic and complex manner. Hitherto, the entire molecular routes and chemical nature of the autocatalytic network have not been well understood. Herein, with a multitechnique approach and multiscale analysis, we have obtained a full theoretical picture of the domino cascade of autocatalytic reaction network taking place on HZSM-5 zeolite. The autocatalytic reaction is demonstrated to be plausibly initiated by reacting dimethyl ether (DME) with the surface methoxy species (SMS) to generate the initial olefins, as evidenced by combining the kinetic analysis, in situ DRIFT spectroscopy, 2D ¹³C-¹³C MAS NMR, electronic states, and projected density of state (PDOS) analysis. This process is operando tracked and visualized at the picosecond time scale by advanced ab initio molecular dynamics (AIMD) simulations. The initial olefins ignite autocatalysis by building the first autocatalytic cycle-olefins-based cycle-followed by the speciation of methylcyclopentenyl (MCP) and aromatic cyclic active species. In doing so, the active sites accomplish the dynamic evolution from proton acid sites to supramolecular active centers that are experimentally identified with an ever-evolving and fluid feature. The olefins-guided and cyclic-species-guided catalytic cycles are interdependently linked to forge a previously unidentified hypercycle, being composed of one "selfish" autocatalytic cycle (i.e., olefins-based cycle with lighter olefins as autocatalysts for catalyzing the formation of olefins) and three cross-catalysis cycles (with olefinic, MCP, and aromatic species as autocatalysts for catalyzing each other's formation). The unraveled dynamic autocatalytic cycles/network would facilitate the catalyst design and process control for MTH technology.

Plastic Waste Conversion over a Refinery Waste Catalyst

Polypropylene (PP) makes up a large share of our plastic waste. We investigated the conversion of PP over the industrial Fluid Catalytic Cracking catalyst (FCC-cat) used to produce gasoline from crude oil fractions. We studied transport limitations arising from the larger size of polymers compared to the crude oil-based feedstock by testing the components of this catalyst separately. Infrared spectroscopy and confocal fluorescence microscopy revealed the role of the FCC matrix in aromatization, and the zeolite Y domains in coking. An equilibrium catalyst (ECAT), discarded during FCC operation as waste, produced the same aromatics content as a fresh FCC-cat, while coking decreased significantly, likely due to the reduced accessibility and activity of the zeolite domains and an enhanced cracking activity of the matrix due to metal deposits present in ECAT. This mechanistic understanding provides handles for further improving the catalyst composition towards higher aromatics selectivity.

Identification of extremely hard coke generation by low-temperature reaction on tungsten catalysts via Operando and in situ techniques

The coke formation in the catalytic system mainly cause to the catalyst deactivate resulting the dramatic decreasing of the catalyst performance then the catalyst regeneration was required. In this study, adding MgO physically mixed with WO₃/SiO₂ catalysts were prepared and compared with the ones prepared by physically mixing with SiO₂. Adding MgO affected the generation of new species of coke deposited on WO₃/SiO₂ and MgO itself. Comparing the reaction temperature when adding MgO between at 300 and 450 °C, the different pathway of reaction and the coke formation were found. At 450 °C, the metathesis reaction was more pronounced and the lower temperature of coke deposited on WO_x/SiO₂ was found. Surprisingly, the extremely hard coke occurred during reaction at 300 °C that the maxima of coke formation was found over 635 °C. This due to the fact that the reduction of reaction temperature from 450 to 300 °C affected the decreasing of the metathesis activity. Conversely, the increasing of dimerization and isomerization of butenes-isomer was observed especially 1-butene and iso-butene. Thus, it could suggest that those quantity of them play the important role to generate the charged monoenyl or cyclopentenyl species by participating with ethene through the dimerization, resulting in the formation of extremely hard coke.

Conversion of Oxygenates on H-ZSM-5 Zeolites—Effects of Feed Structure and Si/Al Ratio on the Product Quality

The conversion of different biogenic feedstocks to hydrocarbons is a major challenge when ensuring hydrocarbon and fuel supply in spite of the heterogeneity of this feed. Flexible adaptation to changing compositions is mandatory for the respective processes. In this study, different oxygenate model feeds, such as alcohols, aldehydes, carboxylic acids and esters, were converted at 500 °C and 5 barg H2 using H-ZSM-5 zeolite catalysts with various Si/Al ratios to identify the relationship between the feed structure and the final product distribution. As the main outcome, the product distribution becomes increasingly independent of the feed structure for Al-rich H-ZSM-5 catalyst samples at low Time on Stream (ToS). Some minor exceptions are the increased formation of aromatics during ToS for carbonyl oxygenates compared to primary alcohols and the dominance of initial deoxygenation products for Si-rich H-ZSM-5 samples. This is interpreted by a multi-stage reaction sequence, which involves the initial deoxygenation of the feed and the subsequent integration of the olefin intermediates into a reaction network. The results pave the way towards the achievement of a desired product distribution in the conversion of different oxygenates simply by the adaption of the Al content of H-ZSM-5.

Multimodal Imaging of Autofluorescent Sites Reveals Varied Chemical Speciation in SSZ-13 Crystals

A multimodal imaging study of chabazite is used to show the distribution of and discriminate between different emissive deposits arising as a result of the detemplation process. Confocal imaging, 3D fluorescence lifetime imaging, 3D multispectral fluorescence imaging, and Raman mapping are used to show three different types of emissive behaviours each characterised by different spatial distributions, trends in lifetime, spectral signals, and Raman signatures. A notable difference is seen in the morphology of agglomerated surface deposits and larger subsurface deposits, which experience lifetime augmentation due to spatial confinement. The distribution of organic residue throughout the crystal volume is comparable to XRF mapping that shows Si enrichment on the outer edges and higher Al content through the centre, demonstrating that a fluorescence-based technique can also be used to indirectly comment on the compositional chemistry of the inorganic framework.

Insight into the effects of confined hydrocarbon species on the lifetime of methanol conversion catalysts

The methanol-to-hydrocarbons reaction refers collectively to a series of important industrial catalytic processes to produce either olefins or gasoline. Mechanistically, methanol conversion proceeds through a 'pool' of hydrocarbon species. For the methanol-to-olefins process, these species can be delineated broadly into 'desired' lighter olefins and 'undesired' heavier fractions that cause deactivation in a matter of hours. The crux in further catalyst optimization is the ability to follow the formation of carbonaceous species during operation. Here, we report the combined results of an operando Kerr-gated Raman spectroscopic study with state-of-the-art operando molecular simulations, which allowed us to follow the formation of hydrocarbon species at various stages of methanol conversion. Polyenes are identified as crucial intermediates towards formation of polycyclic aromatic hydrocarbons, with their fate determined largely by the zeolite topology. Notably, we provide the missing link between active and deactivating species, which allows us to propose potential design rules for future-generation catalysts.

Synthesis of Submicron SSZ-13 with Tunable Acidity by the Seed-Assisted Method and Its Performance and Coking Behavior in the MTO Reaction

Submicron SSZ-13 with different acidities was synthesized successfully with the assistance of nanosized SSZ-13 seeds. The methanol-to-olefins (MTO) properties of submicron SSZ-13 were evaluated. The lifetime of submicron SSZ-13 was enhanced because of the crystal size reduction. The selectivity of light olefins was improved evidently at the early stage of the MTO reaction as the acidity density decreased. TG, GC-MS, and in situ UV/vis spectra were utilized to investigate coking behavior during the MTO reaction. It was found that the acidity density influences the nature and rate of coke formation. The majority of the hydrocarbon pool species over SSZ-13 with a low acidity density (125.2 μmol/g) were methylated benzene carbocations, while that over SSZ-13 with a high acidity density (330.2 μmol/g) were methylated naphthalene carbocations. The low acidity density of SSZ-13 can suppress the hydrogen transfer reaction and polyaromatic generation.

Disentangling Reaction Processes of Zeolites within Single-Oriented Channels

Establishing structure-reactivity relationships for specific channel orientations of zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, a well-defined model system was developed to build structure-reactivity relationships for specific zeolite-channel orientations during various catalytic reaction processes, for example, the methanol- and ethanol-to-hydrocarbons (MTH and ETH) process as well as oligomerization reactions. The entrapped and effluent hydrocarbons from single-oriented zeolite ZSM-5 channels during the MTH process were monitored by using operando UV/Vis diffuse reflectance spectroscopy (DRS) and on-line mass spectrometry (MS), respectively. The results reveal that the straight channels favor the formation of internal coke, promoting the aromatic cycle. Furthermore, the sinusoidal channels produce aromatics, (e.g., toluene) that further grow into larger polyaromatics (e.g., graphitic coke) leading to deactivation of the zeolites. This underscores the importance of careful engineering of materials to suppress coke formation and tune product distribution by rational control of the location of zeolite acid sites and crystallographic orientations.

Role of Al in Na-ZSM-5 zeolite structure on catalyst stability in butene cracking reaction

The Na-ZSM-5 catalysts (SiO₂/Al₂O₃ molar ratio = 20, 35, and 50) were prepared by rapid crystallization method to investigate their performance in butene cracking reaction. The XRD, XRF, NH₃-TPD, FT-IR, TPO, UV-Vis, and ¹H, ²⁷Al, ²⁹Si MAS NMR techniques were used to identify the physical and chemical properties of Na-ZSM-5 catalysts. The silanol group (Si-OH) was the main acid site of Na-ZSM-5, and it was proposed to be the active site for the butene cracking reaction. The butene conversion and coke formation were associated with the abundance of silanol groups over the Na-ZSM-5 catalyst. The dealumination, resulting in the deformation of tetrahedral framework aluminum species was a key factor for Na-ZSM-5 catalyst deactivation, because of the Si-O-Al bond breaking and formation of Si-O-Si bond. The stability of the Si-O-Al bond was linked to the molar number of sodium since the Na atom interacts with the Si-O-Al bond to form Si-ONa-Al structure, which enhances the stability of the silanol group. Therefore, the Si-ONa-Al in zeolite framework was an essential structure to retain the catalyst stability during the reaction. The Na-ZSM-5 with the lowest SiO₂/Al₂O₃ molar ratio showed the best performance in this study resulting the highest propylene yield and catalyst stability.

Comparative study on different strategies for synthesizing all-silica DD3R zeolite crystals with a uniform morphology and size

In the last three decades, the all-silica deca-dodecasil 3R (DD3R) zeolite has been extensively studied as a significant potential class of porous materials in adsorptive separations. However, the use of most existing synthesis methods is unable to produce pure DD3R crystals with a uniform morphology and size. The present research, is therefore intended to provide a facile protocol to synthesize pure DD3R crystals with a controllable morphology and size and with a high reproducibility and productivity. Special attention was focused on investigating the effects of the type of seeds and the mineralizing reagent on the phase-purity, morphology, and crystal size of the resultant DD3R crystals. Various techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption-desorption at 77 K, and thermogravimetric analysis (TGA) were then used to characterize the synthesized samples. The results show that by adding a small amount of "amorphous" DD3R or "amorphous" ZSM-58 seeds, the pure DD3R crystals with a uniform morphology and size can be synthesized using 1-adamantanamine (1-ADA) as a structure-directing agent (SDA), KF was used as a mineralizing reagent, and LUDOX AS-30 as a silicon source at 443 K for 1 d. In addition, the pure, large and uniform hexahedron DD3R crystals can be prepared using fumed silica as seeds, although the crystallization time takes a longer period of 3 d. The present work could stimulate fundamental research and industrial applications of the all-silica DD3R zeolite.

Imaging spatiotemporal evolution of molecules and active sites in zeolite catalyst during methanol-to-olefins reaction

Direct visualization of spatiotemporal evolution of molecules and active sites during chemical transformation in individual catalyst crystal will accelerate the intuitive understanding of heterogeneous catalysis. So far, widespread imaging techniques can only provide limited information either with large probe molecules or in model catalyst of large size, which are beyond the interests of industrial catalysis. Herein, we demonstrate a feasible deep data approach via synergy of multiscale reaction-diffusion simulation and super-resolution structured illumination microscopy to  illustrate the dynamical evolution of spatiotemporal distributions of gas molecules, carbonaceous species and acid sites in SAPO-34 zeolite crystals of several micrometers that are typically used in industrial methanol-to-olefins process. The profound insights into the inadequate utilization of activated acid sites and rapid deactivation are unveiled. The notable elucidation of molecular reaction-diffusion process  at the scale of single catalyst crystal via this approach opens an interesting method for mechanism study in materials synthesis and catalysis.

Synthesis of Highly Selective and Stable Co-Cr/SAPO-34 Catalyst for the Catalytic Dehydration of Ethanol to Ethylene

In this study, silicoaluminophosphate (SAPO)-34 and Me (Me = Cr, Co)-modified SAPO-34 were synthesized and used as catalysts to investigate the catalytic performance by means of a probe reaction from ethanol to ethylene. The metal oxides were loaded on the SAPO-34 support via an impregnation method. The synthesized catalysts were characterized using XRD, SEM, EDX, FT-IR, NH3-TPD, BET, and TGA techniques. Compared to SAPO-34, SAPO-34 doped with metal oxides showed the same chabazite (CHA) topology. The structure and properties of the catalyst were further optimized by varying the amount of Me. The experimental results showed that Co-Cr/SAPO-34 exhibited the best catalytic performance when the reaction temperature reached 400 °C at a weight hourly space velocity (WHSV) of 3.5 h−1, for which the single-pass conversion of ethanol was determined as 99.15%, and the selectivity of ethylene was 99.4% at an optimum catalytic performance in the reaction of up to 600 min. In addition, Co-Cr/SAPO-34 exhibited better catalytic activity and anti-coking ability than pure SAPO-34, which was attributed to its enhanced pore structure and moderate acidity. It can also be concluded from the results of this experiment that the performance of the Co-Cr bimetal-supported catalyst is better than that of the Cr mono-metal catalyst.