Catalytic Lifetime Research Articles

Microkinetic Model to Rationalize the Lifetime of Electrocatalysis: Trade-off between Activity and Stability.

Electrocatalysts which can operate for several years are required to produce hydrogen and commodity chemicals in an environmentally friendly manner. However, designing materials with long operational lifetimes is challenging, due to the lack of a conceptual framework to predict catalytic lifetimes quantitatively. Here, we report a microkinetic equation which quantifies the lifetime of an electrocatalyst undergoing dissolution. This equation was obtained by taking advantage of the fact that catalysis is much faster than deactivation, which allows the ordinary differential equations to be solved via the quasi steady-state approximation. All chemical reactions were modeled as irreversible, first-order elementary reactions. Under this assumption, the catalytic rate correlates linearly with the deactivation rate, leading to a trade-off relationship between activity and stability. Our model was supported by the correlation between theoretical and experimental lifetimes (r2 = 0.86) of a manganese oxide electrocatalyst during the oxygen evolution reaction.

Unveiling the enhanced catalytic performance of low-Si/Al-ratio beta-zeolite on C4 alkylation combing with experiments and molecular simulations

The acidity and pore structure of zeolite catalysts play a crucial role in the selectivity and stability of C4 alkylation due to the synergistic effect of reaction activity and diffusion. Herein, the effect of three acidic beta zeolites with different Si/Al ratios on adsorption, diffusion, and catalytic performance was studied using experiments and molecular simulations. The beta zeolite with the lower Si/Al ratio leads to the faster relative diffusion of isobutane to 1-butene and the smaller adsorption amount of alkylate products. Furthermore, the diffusion of molecules in the zigzag channel along the z direction of beta zeolites is much slower than other two directions. The experimental results indicate that lower Si/Al ratio cause the higher content of strong BAS in the zeolite, resulting in the higher hydride activity and the longer catalytic lifetime. This work provides a theoretical basis for the development of high catalytic activity zeolite catalysts.

ZSM-5@Zn/ZSM-5 core@shell nanocrystals as integrated catalysts for efficient methanol conversion into aromatics

The use of ZSM-5@Zn/ZSM-5 core@shell nanocrystals as integrated catalyst for efficient aromatic production from methanol aromatization is demonstrated. By combining the attributes associated with core@shell structural and acidic features, ZSM-5@Zn/ZSM-5 exhibited synergistic and improved catalytic performance toward methanol aromatization. The effect of Zn/ZSM-5 shell thickness and the acid density of the ZSM-5 core on synergistic performance was systematically investigated. Results showed that aromatic selectivity could be increased with shell thickness and the acid density of the ZSM-5 core, while catalytic stability was subject to the optimization on both shell thickness and core acid density. The strengthening of dehydroaromatization on the shell catalyst after Zn introduction and the hydrogen transfer for aromatization with increasing core acid density was confirmed. A double boost on catalytic stability from methanol conversion along with 40.7% increase on aromatic selectivity could be achieved for core@shell catalyst with a coating ratio of 0.3 compared with that on single core catalyst. The fabrication of ZSM-5 core with low SiO2/Al2O3 ratio for Zn/ZSM-5 not only provided more acid sites to promote the aromatization, but also promoted the dealkylation of the formed aromatics, which could enhance the selectivity of light aromatics and catalytic lifetime. The findings in the work not only enrich the family of core@shell composite zeolite catalysts, but also provide the fundamental understanding on the aromatization process over core@shell structural features.

Modulation of Al Distribution in High-Silica ZSM-5 Zeolites for Enhancing Catalytic Performance.

The spatial distribution of framework Al (AlF) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special AlF distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique AlF distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA+ and tetrahedral [AlO4]- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA+. This effect leads to AlF atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate AlF distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

Constructing thermally stable nickel sub-nanoparticles via in situ hydroxyl trapping for methane dry reforming

Nickel based catalysts are widely utilized in methane dry reforming (DMR) reaction, and their catalytic properties are mainly determined by the dispersibility and anti-sintering ability of Ni species on catalyst supports. However, developing small size and stable nickel catalysts with strong resistance to particle sintering still remains challenge. Herein, vapor-phase-trapping technology is innovatively applicated at elevated temperature to prepare ultra-small size and highly dispersed Ni sub-nanoparticles on the SBA15 within rehydroxylation modification. The nickel vapor originated from precursors of acetylacetonate nickel is in situ captured by terminal hydroxyl groups on the surface of support at high temperature, realizing the uniform dispersion of nickel species as well as moderate interactions between nickel species and support. The highly dispersed and extra-small size Ni sub-nanoparticles of 1.94 nm with appropriate metal-support interaction contribute to excellent catalytic activity and lifetime of 150 h for DMR reaction. This work provides a rational design and precise modulation of efficient nickel-based catalyst.

Effect of kaolin calcined temperature on the preparation and crystallization mechanism of SAPO-34 molecular sieve for methanol-to-olefins performance

SAPO-34 molecular sieves synthesized using calcined kaolin as partial raw material under three-stage of high-low-high temperature hydrothermal condition are explored systematically. Especially, the effect of kaolin calcination temperature on the crystallization mechanism of SAPO-34 and the methanol to olefin performance are investigated thoroughly. It is found that the sample (SP-500) synthesized by using kaolin calcined at 500 °C (MK-500) demonstrates clean cubic structure, large specific surface area and mild acidity, providing the initially activated Si and Al source for the synthesis of SAPO-34 molecular sieve due to the well scattered structure of MK-500. From the crystallization mechanism consideration, it can be proposed that the SAPO-34 molecular sieves will be controlled by the low degree of Si aggregation in metakaolin and quantity of Si (3Al,1Si) species. As a result, SP-500 displays excellent catalytic lifetime during methanol to olefin reaction for its special Si coordination environment, showing the highest stable light olefin yield of 80.6 % and a methanol conversion above 90 % for more than 720 min. In contrast, SP-800 sample with the most strong acid sites is only 390 min, even if showcasing the highest selectivity of light olefins (84.5%).

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Titanosilicates are widely applied in the alkene epoxidation reactions with high reaction rate and selectivity to desired products. Their catalytic performance depends on the structure topology, the micro-environment of Ti active sites, and the hydrophobicity/hydrophilicity of zeolite framework. Herein, we focus on a hydrophilic substrate of allyl alcohol (AAL) and investigated catalytic performance of four titanosilicates (TS-1, Ti-MOR, Ti-MWW, and Re-Ti-MWW) in the AAL epoxidation reaction with hydrogen peroxide as the oxidant. Among them, Re-Ti-MWW, synthesized via the post-modification of Ti-MWW with piperidine, exhibited the highest activity. Moreover, the preferred solvent changed from MeCN for Ti-MWW to H2O for Re-Ti-MWW. The relative diffusion rate of AAL over Re-Ti-MWW was up to 466 × 10-7 s-1, much larger than those of other zeolites. The higher diffusion rate of Re-Ti-MWW was probably derived from the higher framework hydrophilicity as revealed by the smaller water contact angle of Re-Ti-MWW compared to the other zeolites, which contributed to the high activity in AAL epoxidation. In the continuous slurry reactor, Re-Ti-MWW achieved a high catalytic lifetime of 163 h, with the selectivity of desirable glycidol product maintained at > 97% in the H2O solvent system, showing high potential as an industrial catalyst for the AAL epoxidation reaction.

Recent Progress in the Application of Transition‐Metal Containing MFI topologies for NH3‐SCR‐DeNOx and NH3 oxidation

AbstractTransition metal‐containing MFI‐based catalysts are widely investigated in the selective catalytic reduction of NOx with ammonia (NH3‐SCR‐DeNOx), and the selective catalytic oxidation of ammonia (NH3‐SCO) into nitrogen and water vapor. While MFI‐based catalysts are less intensively studied than smaller pore zeolites (i. e., chabazite, CHA) they are still used commercially for these processes and are of great interest for future study in particular to better understand structure‐activity relationships. Hierarchically porous MFI catalysts (containing both micropores and mesopores) often show enhanced catalytic properties compared to conventional (microporous) materials in both NH3‐SCR‐DeNOx and NH3‐SCO. Thus, a critical overview of the current understanding of the salient physico‐chemical properties that influence the performance of these catalysts is examined. Furthermore, strategies for the development of ZSM‐5 based catalysts with enhanced catalytic lifetime, supported by the investigations of reaction mechanisms are reviewed and discussed.

Construction of the mosaic aluminophosphate zeotypes based on nonclassical crystallization and in situ selective etching strategies

Owing to short diffusion lengths and more accessible active sites, hierarchical zeolitic materials can substantially improve catalytic activities and lifetimes compared to the conventional counterparts. Here we develop a straightforward and mesopore-template-free strategy to prepare aluminophosphate-based LTA zeotypes with mosaic architectures and hierarchical porosities. The influence of synthesis parameters and time-dependent study on the crystallization process were studied comprehensively using multiple techniques. The results demonstrate that the mosaic architectures were formed based on nonclassical crystallization, i.e., phase transformation of a lamellar phase to aligned crystalline AlPO4-42 nanoparticles followed by crystal fusion, and in situ selective etching of grain boundaries and defects. Meanwhile, heteroatom-substituted AlPO4-42 zeotypes with the mosaic morphologies can also be synthesized, which show enhanced ideal adsorbed solution theory (IAST) selectivities and efficient dynamic separation performances in the CO2/N2 mixture. This work not only provides a novel mesoporogen-free strategy for the preparation of hierarchical zeotypes, but also sheds new insight into the crystallization pathway.

Recent modifications of MCM-22 and MOR zeolite in MTO reaction: A review

Since the discovery of the Methanol-to-Olefins (MTO) process as a sustainable and nonpetroleum method for generating light olefins, there has been a growing interest in the utilization of acidic zeolite catalysts. In this review, we highlighted the application and modification of MCM-22 and MOR zeolite catalysts, shedding light on their distinctive properties and the ongoing endeavors to optimize their catalytic performance. Notably, the choice of catalyst and specific modifications significantly influence the outcomes of light olefin selectivity, propylene-to-ethylene (P/E) ratios, and catalytic lifetime. This research offers insights into the current status of research on MCM-22 and MOR zeolites and imparts a valuable understanding of the developments of both catalysts in this crucial catalytic field.

Ionothermal synthesis of ZSM-5/AlPO4-5 composite zeolites and their catalytic performances in the methanol-to-olefin reaction

Composite zeolites, specifically ZSM-5/AlPO4-5 ([E]-Z5/Al5, [B]-Z5/Al5 and [H]-Z5/Al5), were prepared using tri-substituted imidazolium bromide ionic liquids (ILs). The ILs used were 1-ethyl-2,3-dimethylimidazolium bromide ([EMMIM]Br), 1-butyl-2,3-dimethylimidazolium bromide ([BMMIM]Br) and 1-hexyl-2,3-dimethylimidazolium bromide ([HMMIM]Br). The structure, morphology and acidity of the composite zeolites were analyzed using various techniques, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FT-IR), N2 isothermal adsorption-desorption, thermogravimetric analysis (TGA), temperature-programmed desorption of ammonia (NH3-TPD) and nuclear magnetic resonance spectroscopy (NMR). The catalysts obtained demonstrate outstanding performance in the methanol-to-olefin (MTO) reaction, with a catalytic lifetime extended by approximately tenfold and an approximately 10 % increase in the yield of light carbon olefins compared to the traditional hydrothermal Z5/Al5 catalyst. There is a strong relationship between the selectivity and stability of the MTO reaction with the optimized grain size and catalyst acidity.

Co-aromatization of light alkanes over Mo/ZSM-5: How the presences of CH4 and C2H6 & C3H8 influence the aromatization of each other

The chemical inertness of methane is a significant bottleneck for its direct conversion into value-added light aromatics. However, co-feeding higher hydrocarbons with methane produces intermediates that facilitate the participation of methane in aromatization. This synergistic effect has typically been observed under conditions that do not allow the conversion of methane without any co-reactants (over non-Mo-based catalysts at temperatures below 650 ℃). Therefore, the interaction between methane and higher hydrocarbons remains unexplored under conditions that allow methane activation. In this study, we explored the behavior of co-reactants in the direct aromatization of simulated shale gas (a mixture of methane, ethane, and propane) over Mo/ZSM-5. Our aim was to comprehend the complex co-aromatization system under conditions optimized for rigorous activation of methane, which leads to a considerable production of aromatics. The majority of reaction indices in the shale gas aromatization system were governed by the presence of ethane and propane, which were represented by a rapid catalytic lifetime cycle. Mathematical calculations, based on experimentally derived C and H balances, revealed an unexpected anti-synergy within the components of shale gas where the conversion of methane to aromatics was partially suppressed by the presence of ethane and propane. These findings expand the understanding of the complex yet desirable shale gas aromatization system, and also highlight the need for the development of new catalysts capable of further enhancing the activity of methane among the reactants.

Controllable synthesis of gallium-containing MFI zeolite for bifunctionally catalyzing methanol aromatization

Ga-MFI zeolites with controllable crystal sizes were hydrothermally synthesized by using colloidal silicalite-1 as seed crystals and their catalytic performances for methanol-to-aromatics (MTA) were evaluated. It was found that the crystal size of as-made Ga-MFI zeolite became small with the adding amounts of seed crystals. Small-sized Ga-MFI zeolite with short channels shows much better coking-resistance and thus longer catalytic lifetime owning to its more favorable outward diffusion of aromatic products. In addition, the stability could be further improved by impregnating gallium species into Ga-MFI zeolite, but which slightly sacrificed its aromatic selectivity. The Ga species with different locations in zeolite play distinct roles in MTA. The weak intrinsic protonic acidity from tetrahedral framework Ga3+ could suppress hydrogen transfer reactions for light alkane formation while non-framework gallium species especially Ga2O3 nanoparticles promoted the dehydrocyclization process. The synergy between the strong Brønsted and weak Lewis acid sites contributes to the excellent aromatic yield.

Interfaces coupling of Co8FeS8-Fe5C2 with elevated d-band center for efficient water oxidation catalysis

Maximizing the oxygen evolution reaction (OER) catalytic activity of carbon coated core-shell electrocatalysts is significant for the application of water electrolyzers and rechargeable metal-air batteries, yet the modulation of the catalytic properties through interfaces coupling remains challenging. Here, we construct Fe5C2 phase interlayered between carbon shell and Co8FeS8 core (Co8FeS8-Fe5C2@C) for enhancing the alkaline OER catalytic performance. By altering the interlayer phase with Co1−xS as the control sample (Co8FeS8-Co1−xS@C), synchrotron X-ray absorption spectroscopy analysis integrated with density functional theory calculations indicate that the induced interfacial electron coupling of Co8FeS8-Co1−xS and Co8FeS8-Fe5C2 can upshift the d-band center toward Fermi level, optimalize Gibbs free energy for oxygen-containing intermediates, facilitate electron transfer between Co8FeS8 and carbon shell. Consequently, the target Co8FeS8-Fe5C2@C catalyst with stronger interface coupling and optimal electron modulation shows an significant overpotential (η10) decrease by 93 mV compared with Co8FeS8@C, along with a Tafel slop of 48.9 mV dec−1 and a long catalytic lifetime, outperforming commercial RuO2 and other reported analogous catalysts. This work opens up further opportunities of interlayer modification in carbon caoted core-shell catalyst to effictivly tailor the d-Band centers for effectively strengthen its catalytic performance.

Highly Efficient Decomposition of Perfluorocarbons for over 1000 Hours via Active Site Regeneration

AbstractTetrafluoromethane (CF4), the simplest perfluorocarbon (PFC), has the potential to exacerbate global warming. Catalytic hydrolysis is a viable method to degrade CF4, but fluorine poisoning severely restricts both the catalytic performance and catalyst lifetime. In this study, Ga is introduced to effectively assists the defluorination of poisoned Al active sites, leading to highly efficient CF4 decomposition at 600 °C with a catalytic lifetime exceeding 1,000 hours. 27Al and 71Ga magic‐angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the introduced Ga exists as tetracoordinated Ga sites (GaIV), which readily dissociate water to form Ga−OH. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density function theory (DFT) calculations confirmed that Ga−OH assists the defluorination of poisoned Al active sites via a dehydration‐like process. As a result, the Ga/Al2O3 catalyst achieved 100 % CF4 decomposition keeping an ultra‐long catalytic lifetime and outperforming reported results. This work proposes a new approach for efficient and long‐term CF4 decomposition by promoting the regeneration of active sites.

Highly Efficient Decomposition of Perfluorocarbons for over 1000 Hours via Active Site Regeneration.

Tetrafluoromethane (CF4), the simplest perfluorocarbon (PFC), has the potential to exacerbate global warming. Catalytic hydrolysis is a viable method to degrade CF4, but fluorine poisoning severely restricts both the catalytic performance and catalyst lifetime. In this study, Ga is introduced to effectively assists the defluorination of poisoned Al active sites, leading to highly efficient CF4 decomposition at 600°C with a catalytic lifetime exceeding 1,000 hours. 27Al and 71Ga magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the introduced Ga exists as tetracoordinated Ga sites (GaIV), which readily dissociate water to form Ga-OH. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density function theory (DFT) calculations confirmed that Ga-OH assists the defluorination of poisoned Al active sites via a dehydration-like process. As a result, the Ga/Al2O3 catalyst achieved 100% CF4 decomposition keeping an ultra-long catalytic lifetime and outperforming reported results. This work proposes a new approach for efficient and long-term CF4 decomposition by promoting the regeneration of active sites.

Formulated Mn-promoted SAPO-34/kaolin/alumina sol micro-size catalyst with a superior performance for methanol to light olefins conversion in a fluidized bed reactor

Formulation of zeolite molecular sieves powder is necessary to prepare an excellent micro-size catalyst for methanol conversion to light olefins (MTO) in a fluidized bed reactor. In this work, new fluidizable micro-meso pore catalysts were successfully fabricated by spray drying a slurry containing Mn(x)SAPO-34 powder, kaolin filler, alumina sol binder, and HCl additive. A mixed template of TEAOH/morpholine with different amounts of Mn ion was used in primary gel during hydrothermally synthesis of the crystals. The catalytic samples were characterized utilizing different methods. A moderate acidity reached by catalyst formulation using kaolin and alumina sol as well as through Mn incorporation into the chabazite cages was helpful for MTO performance of the final shape-selective catalyst in terms of selectivity to light olefins and lifetime. The enhancement effect of Mn promoter on ethylene production was much more than propylene because of entering Mn to the micro-pores and imposing restriction on heavier hydrocarbons diffusion. The uniformity and crystalinity were also appeared to be enhanced once Mn(0.05) was introduced into the gel composition. The suggested spray dried catalyst was SD-Mn(0.05)SAPO-34 (70–120 µm) in which provided a superior selectivity up to 88.4% to light olefins and catalytic lifetime of 330 min in a fluidized bed reactor at 450℃, WHSV = 4 h−1, and XMeOH = 0.75 in the feed.

Insight into the effect of F− on acidity of H-SAPO-18 and its catalytic performance for conversion of methanol to olefins

In this study, F− was added to the synthesis gel to regulate the acid property of SAPO-18 molecular sieve and enhance its catalytic performance for the methanol-to-olefin (MTO) reactions. The results indicate that adding an appropriate amount of F− during synthesis can increase the crystallinity of SAPO-18 molecular sieve and reduce the overall strong acid density and external surface acid content. The H-SAPO-18-I-20-0.0025 molecular sieve exhibits a much higher catalytic lifetime (1000 min) and light olefins selectivity (45.2% to propene and 96.1% to C2-5=) in MTO at 400 °C than those of the traditional H-SAPO-18-1-0. The results of the isotope labeling experiments, GC-MS, and 13C MAS NMR results provided direct evidence that the aromatic-based cycle contributes more to the conversion of methanol into light olefins, and hexamethylbenzene serves as the most abundant hydrocarbon species that produces propene via the paring route.

Modified seed-induced synthesis of ZSM-5 aggregates and their catalytic performance in methanol to propylene conversion: Co-effect of ball milling and alkaline treatment of seed

Industrial ZSM-5 catalysts suffer from coke deposition and loss of activity in methanol to propylene conversion. Developing zeolites with a better diffusion path, which is essential for the design of high-efficiency catalysts, is still a challenge to be achieved. The synthesis of seed-induced ZSM-5 zeolite with a short diffusion path could be a solution to reduce coke accommodation and have a higher catalytic lifetime. However, it would not be beneficial with current methods as far as the industrial aspect. Herein, we have proposed a successful and industrially-feasible approach for decreasing the costs of seed-induced ZSM-5 synthesis as well as improving the lifetime and catalytic performance of ZSM-5 zeolites in the methanol to propylene (MTP) reaction. This modified seeding route includes a physical factor (ball milling) and a chemical factor (alkaline treatment). Compared to a conventional seed-induced route, this new technique allowed us to increase catalyst lifetime (from 159 h to 265 h), propylene/ethylene selectivity (from 5.23 to 8.19), and amount of produced propylene per gram of catalyst (from 182 gC3H6/gcat to 443 gC3H6/gcat). This modified seed has a high potential for use on an industrial scale, based on the results.

Hydrogenation of CO2 to higher alcohols on an efficient Cr-modified CuFe catalyst

CO2 hydrogenation to higher alcohols (C2+OH) is a promising approach to achieve carbon recycling, but it is a challenge due to complex reaction network. Herein, various Cr-modified CuFe (Cr(x)-CuFe) catalysts were prepared, and their catalytic performance and reaction mechanism in CO2 hydrogenation to C2+OH were investigated. Introduction of small amounts of Cr enhances the interaction between Cu and Fe species, which alleviates CO over-dissociation and inhibits generation of iron carbides. In contrast, more acetate and acetaldehyde intermediates are produced via promoting the reaction of CHx with non-dissociated CO. Cr(1%)-CuFe showed CO2 conversion, C2+OH selectivity and space-time yield (STY) as high as 38.4%, and 29.2% and 104.1 mg gcat−1 h−1, at 320 °C, 4.0 MPa and GHSV of 6000 mL gcat−1 h−1. Interestingly, C2+OH STY was further elevated to 268.5 mg gcat−1 h−1, with a catalytic lifetime of at least 120 h, when GHSV was increased to 48000 mL gcat−1 h−1.