A Straightforward Descriptor for the Deactivation of Zeolite Catalyst H-ZSM-5

ZSM-5@ceramic foam composite catalyst derived from spent bleaching clay for continuous pyrolysis of waste oil to produce monocyclic aromatic hydrocarbons

Spent bleaching clay, a solid waste generated during the refining process of vegetable oils, lacks an efficient treatment solution. In this study, spent bleaching clay was innovatively employed to fabricate ceramic foams. The thermal stability analysis, microstructure, and crystal phase composition of the ceramic foams were characterized by TG-DSC, SEM, and XRD. An investigation into the influence of Al2O3 content on the ceramic foams was conducted. Results showed that, as the Al2O3 content increased from 15 wt% to 30 wt%, there was a noticeable decrease in bulk density and linear shrinkage, accompanied by an increase in compressive strength. Additionally, the ceramic foams were used as catalyst supports, to synthesize ZSM-5@ceramic foam composite catalysts for pyrolysis of waste oil. The open pores of the ZSCF catalyst not only reduced diffusion path length but also facilitated the exposure of more acid sites, thereby increasing the utilization efficiency of ZSM-5 zeolite. This, in turn, engendered a significant enhancement in monocyclic aromatic hydrocarbons content from 39.15 % (ZSM-5 powder catalyst) to 78.96 %. Besides, a larger support pore size and a thicker ZSM-5 zeolite coating layer led to an increase in monocyclic aromatic hydrocarbons content. As the time on stream was extended to 56 min, the monocyclic aromatic hydrocarbon content obtained with the composite catalyst remained 12.41 % higher than that of the ZSM-5 powder catalyst. These findings validate the potential of the composite catalyst. In essence, this study advances the utilization of spent bleaching clay and introduces a novel concept for ceramic foam fabrication. Furthermore, it contributes to the scaling up of catalytic pyrolysis technology.

Elucidating the deactivation mechanism of beta zeolite catalyzed linear alkylbenzene production with oxygenated organic compound contaminated feedstocks

Zeolite catalyzed alkylation of benzene with long-chain α-olefins is a promising method for the detergent industry. Considering the long-chain α-olefins from Fischer-Tropsch synthesis always contain some oxygenated organic compounds, the effect of which on the alkylation of benzene with 1-dodecene was comprehensively investigated over beta zeolite herein. n-heptanol, n-heptaldehyde and n-heptanoic acid were selected as the model oxygenated organic compounds, and it was revealed that an obvious decrease of lifetime occurred when only trace amount of oxygenated organic compounds were added into the feedstocks. The deactivated catalyst was difficult to regenerate by extraction with hot benzene or coke-burning. A series of characterization tests complementary with DFT calculations revealed that the deactivation was mainly caused by the firm adsorption of oxygenated organic compounds on the acid sites. Further, comparison with the open-framework MWW zeolite revealed a similar effect of oxygenated organic compounds and deactivation mechanism for both beta and MWW, but beta is less sensitive to the oxygenated organic compounds. The main reason lies in the three-dimensional framework of beta, wherein the much higher adsorption energy of 1-dodecene makes it difficult to be replaced by oxygenated organic compounds. Additionally, beta could be regenerated more easily by extraction with hot benzene compared with MWW. But coke-burning caused a sharp decrease of its lifetime, which is mainly due to the decreased acid sites after calcination.

Ca Cations Impact the Local Environment inside HZSM-5 Pores during the Methanol-to-Hydrocarbons Reaction

The methanol-to-hydrocarbons (MTH) process is an industrially relevant method to produce valuable light olefins such as propylene. One of the ways to enhance propylene selectivity is to modify zeolite catalysts with alkaline earth cations. The underlying mechanistic aspects of this type of promotion are not well understood. Here, we study the interaction of Ca²⁺ with reaction intermediates and products formed during the MTH reaction. Using transient kinetic and spectroscopic tools, we find strong indications that the selectivity differences between Ca/ZSM-5 and HZSM-5 are related to the different local environment inside the pores due to the presence of Ca²⁺. In particular, Ca/ZSM-5 strongly retains water, hydrocarbons, and oxygenates, which occupy as much as 10% of the micropores during the ongoing MTH reaction. This change in the effective pore geometry affects the formation of hydrocarbon pool components and in this way directs the MTH reaction toward the olefin cycle.

Tracking Structural Deactivation of H-Ferrierite Zeolite Catalyst During MTH with XRD

We used the methanol-to-hydrocarbon (MTH) reaction as a shape-selective model reaction to investigate coke formation in zeolite H-Ferrierite. Despite being a 2D topology in terms of channel propagation, the FER framework displays a lattice expansion in all three dimensions of space upon deactivation. Therefore, the volume of the unit cell is an excellent X-Ray diffraction (XRD) descriptor for the catalyst deactivation. A model with dummy atoms added, also proved to be an accurate approach to measure the amount of internal coke and/or water inside the pore network correlated with thermogravimetric analysis results. While the catalyst deactivation of the H-Ferrerite during the MTH was fast, a comparably long induction period was observed. We were able to track such fast deactivation with the aforementioned descriptors by means of an operando XRD study by a standard laboratory diffractometer.

Catalytic reforming of polyethylene pyrolysis vapors to naphtha range hydrocarbons with low aromatic content over a high silica ZSM-5 zeolite

In this study, the microwave-assisted pyrolysis coupled with ex-situ catalytic reforming of polyethylene for naphtha range hydrocarbons, with low aromatic content, was investigated. Experimental results revealed that ZSM-5 zeolites with low SiO₂/Al₂O₃ ratios led to high aromatic selectivity, while an extremely high SiO₂/Al₂O₃ ratio significantly reduced the aromatic selectivity. The high selectivity of C5-C12 hydrocarbons (98.9 %) with low selectivity of C5-C12 aromatics (28.5 %) was obtained over a high silica ZSM-5 zeolite at a pyrolysis temperature of 500 °C, catalytic cracking temperature of 460 °C, and a weight hourly space velocity of 7 h^-1. The liquid oil produced was mainly composed of C5-C12 olefins that can be easily converted into paraffin-rich naphtha by hydrogenation or hydrogen transfer reactions as the feedstock for new plastic manufacturing. 8 cycles of regeneration-reaction cycles were carried out successfully with little change on the product distribution, showing the great potential for continuous production of low-aromatic liquid oil. Catalyst characterization showed that the catalyst deactivation was primarily caused by coke deposition (approximately 16.0 wt%) on the surface of the catalysts, and oxidative regeneration was able to recover most of the pore structure and acidity of the zeolite by effectively removing coke. This study provides a better understanding for the plastic-to-naphtha process and even for scale-up studies.

Dynamic confinement of SAPO-17 cage on the selectivity control of syngas conversion

The OXZEO (oxide−zeolite) bifunctional catalyst concept has enabled selective syngas conversion to a series of value-added chemicals and fuels such as light olefins, aromatics and gasoline. Herein we report for the first time a dynamic confinement of SAPO-17 cages on the selectivity control of syngas conversion observed during an induction period. Structured illumination microscopy, intelligent gravimetric analysis, UV-Raman, X-ray diffraction, thermogravimetry and gas chromatography-mass spectrometer analysis indicate that this is attributed to the evolution of carbonaceous species as the reaction proceeds, which gradually reduces the effective space inside the cage. Consequently, the diffusion of molecules is hindered and the hindering is much more prominent for larger molecules such as C₄₊. As a result, the selectivity of ethylene is enhanced whereas that of C₄₊ is suppressed. Beyond the induction period, the product selectivity levels off. For instance, ethylene selectivity levels off at 44% and propylene selectivity at 31%, as well as CO conversion at 27%. The findings here bring a new fundamental understanding that will guide further development of selective catalysts for olefin synthesis based on the OXZEO concept.

Electron Diffraction Enables the Mapping of Coke in ZSM-5 Micropores Formed during Methanol-to-Hydrocarbons Conversion

Unveiling the coke formation in zeolites is an essential prerequisite for tackling the deactivation of these catalysts in the transformations of hydrocarbons. Herein, we present the direct mapping of coke in the micropores of ZSM‐5 catalysts used in methanol‐to‐hydrocarbons conversion by single‐crystal electron diffraction analysis. The latter technique revealed a polycyclic aromatic structure along the straight channel, wherein the high‐quality data permit refinement of its occupancy to about 40 %. These findings were exploited to analyze the evolution of micropore coke during the reaction. Herein, coke‐associated signals, which correlate with the activity loss, indicate that the nucleation of coke commences in the intersections of sinusoidal and straight channels, while the formation of coke in the straight pores occurs in the late stages of deactivation. The findings uncover an attractive method for analyzing coke deposition in the micropore domain.

Real-time regeneration of a working zeolite monitored via operando X-ray diffraction and crystallographic imaging: how coke flees the MFI framework

X-ray diffraction is used to investigate regeneration of an H-ZSM-5 zeolite catalyst used in the conversion of methanol to hydrocarbons.

Understanding the Preparation and Reactivity of Mo/ZSM-5 Methane Dehydroaromatization Catalysts

Methane dehydroaromatization is a promising reaction for the direct conversion of methane to liquid hydrocarbons. The active sites and the mechanism of this reaction remain controversial. This work is focused on the operando X‐ray absorption near edge structure spectroscopy analysis of conventional Mo/ZSM‐5 catalysts during their whole lifetime. Complemented by other characterization techniques, we derived spectroscopic descriptors of molybdenum precursor decomposition and its exchange with zeolite Brønsted acid sites. We found that the reduction of Mo‐species proceeds in two steps and the active sites are of similar nature, regardless of the Mo content. Furthermore, the ZSM‐5 unit cell contracts at the beginning of the reaction, which coincides with benzene formation and it is likely related to the formation of hydrocarbon pool intermediates. Finally, although reductive regeneration of used catalysts via methanation is less effective as compared to combustion of coke, it does not affect the structure of the catalysts.

Selectivity descriptors for the direct hydrogenation of CO2 to hydrocarbons during zeolite-mediated bifunctional catalysis

Cascade processes are gaining momentum in heterogeneous catalysis. The combination of several catalytic solids within one reactor has shown great promise for the one-step valorization of C1-feedstocks. The combination of metal-based catalysts and zeolites in the gas phase hydrogenation of CO2 leads to a large degree of product selectivity control, defined mainly by zeolites. However, a great deal of mechanistic understanding remains unclear: metal-based catalysts usually lead to complex product compositions that may result in unexpected zeolite reactivity. Here we present an in-depth multivariate analysis of the chemistry involved in eight different zeolite topologies when combined with a highly active Fe-based catalyst in the hydrogenation of CO2 to olefins, aromatics, and paraffins. Solid-state NMR spectroscopy and computational analysis demonstrate that the hybrid nature of the active zeolite catalyst and its preferred CO2-derived reaction intermediates (CO/ester/ketone/hydrocarbons, i.e., inorganic-organic supramolecular reactive centers), along with 10 MR-zeolite topology, act as descriptors governing the ultimate product selectivity.

Stabilizing the framework of SAPO-34 zeolite toward long-term methanol-to-olefins conversion

As a commercial MTO catalyst, SAPO-34 zeolite exhibits excellent recyclability probably due to its intrinsic good hydrothermal stability. However, the structural dynamic changes of SAPO-34 catalyst induced by hydrocarbon pool (HP) species and the water formed during the MTO conversion as well as its long-term stability after continuous regenerations are rarely investigated and poorly understood. Herein, the dynamic changes of SAPO-34 framework during the MTO conversion were identified by 1D ²⁷Al, ³¹P MAS NMR, and 2D ³¹P-²⁷Al HETCOR NMR spectroscopy. The breakage of T-O-T bonds in SAPO-34 catalyst during long-term continuous regenerations in the MTO conversion could be efficiently suppressed by pre-coking. The combination of catalyst pre-coking and water co-feeding is established to be an efficient strategy to promote the catalytic efficiency and long-term stability of SAPO-34 catalysts in the commercial MTO processes, also sheds light on the development of other high stable zeolite catalyst in the commercial catalysis.

Stability of zeolite catalysts is a highly desirable property for commercial methanol to olefins conversion but extremely challenging to achieve. Here, the authors combine the catalyst pre-coking and water co-feeding to develop an efficient strategy to enhance the long-term stability of SAPO-34 catalyst.

The synthesis of pure and uniform nanosized TS-1 crystals with a high titanium content and a high space–time yield

Concentrated basic MFI building unit species played a key role in the crystal synthesis of uniform nanosized anatase-free titanium-rich TS-1.

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.

The Effect of Co-feeding Methyl Acetate on the H-ZSM5 Catalysed Methanol-to-Hydrocarbons Reaction

The reactivity of methanol and methyl acetate mixtures over a HZSM-5 catalyst is studied over a period of 6 h at 350 °C, with small molecular weight olefins and aromatic compounds observed as reaction products. Post-reaction analysis of the catalyst shows the coke content to increase with methyl acetate content. Vibrational spectra (DRIFTS and inelastic neutron scattering, INS) indicate the major hydrocarbon species present in the coked catalysts to be methylated aromatic molecules, with INS spectra indicating a greater degree of methylation in the catalysts used with higher methyl acetate content. The greater extent of deactivation at higher methyl acetate concentrations is tentatively attributed to a diminishment of water in the zeolite cavity, which would otherwise facilitate re-generation of the active sites.

Ethane aromatization and evolution of carbon deposits over nanosized and microsized Zn/ZSM-5 catalysts

Nanosized Zn/HZSM-5 facilitated the faster escape of light aromatics from zeolite channels and exhibited higher product yield and improved stability.

Topology-dependent hydrocarbon transformations in the methanol-to-hydrocarbons reaction studied by operando UV-Raman spectroscopy

The methanol-to-hydrocarbons (MTH) reaction represents a versatile, industrially viable alternative to crude-oil based processes for the production of chemicals and fuels. In the MTH reaction, the shape selectivity of acidic zeolites is exploited to direct the synthesis towards the desired product. However, due to unavoidable side reactions occurring under processing conditions, all MTH catalysts suffer deactivation due to coke formation. Though it is likely that some common characteristics for carbon formation exist for all zeolite topologies, it has been proposed that the differences in shape selectivity among the different catalysts will also influence the individual deactivation mechanisms. As deactivating species are mostly aromatic compounds, highly methylated benzenes and/or polycyclic aromatic hydrocarbons (PAHs) have been discussed. In some cases, these can further grow to extended carbon structures. Here, we have investigated the hydrocarbon reactivities and carbon formation for five topologically different zeolite catalysts through an operando UV-Raman approach, taking advantage of the high sensitivity of this technique towards aromatic and other carbonaceous species. The combination of the spectroscopic tool with activity measurements allowed us to obtain valuable details and some general trends on the deactivation paths during MTH. This approach made accessible unique insight on the complex chemistry of MTH by allowing the real-time observation of hydrocarbon transformations typical for the peculiar topology of each catalyst, usually inaccessible by ex situ techniques.

Deactivation of Zeolite Catalyst H-ZSM-5 during Conversion of Methanol to Gasoline: Operando Time- and Space-Resolved X-ray Diffraction

The deactivation of zeolite catalyst H-ZSM-5 by coking during the conversion of methanol to hydrocarbons was monitored by high-energy space- and time-resolved operando X-ray diffraction (XRD) . Space resolution was achieved by continuous scanning along the axial length of a capillary fixed bed reactor with a time resolution of 10 s per scan. Using real structural parameters obtained from XRD, we can track the development of coke at different points in the reactor and link this to a kinetic model to correlate catalyst deactivation with structural changes occurring in the material. The "burning cigar" model of catalyst bed deactivation is directly observed in real time.