Fast Catalyst Deactivation Research Articles

Regeneration of methane splitting catalysts by interfacial hydrogenation

Methane splitting, also known as decomposition or pyrolysis, has a unique potential to accelerate the transition from the current carbon-based economy towards the foreseen hydrogen economy. Low-temperature catalytic methane splitting systems are unavailable due to fast catalyst deactivation, caused by solid carbon that encapsulates the catalyst. Catalyst regeneration must be performed to reactivate the catalyst and achieve a long-term operational lifetime. This can be accomplished by cyclically refeeding a portion of the produced hydrogen back to the catalyst, which promotes the hydrogenation of carbon atoms, preferentially at the interface between carbon deposits and catalytic nanoparticles. Interfacial hydrogenation ideally breaks the bonds that connect carbon allotrope products to the metal catalyst, causing the former to detach, and freeing the catalytic metal surface for further reaction. In this work, a proof-of-concept of this technology is provided, showing the full regeneration of a bulk-type Ni catalyst during 22 cycles of methane splitting, at 550 °C and 1 bar. Furthermore, a commercial SiO2-Al2O3-supported Ni catalyst has been used to study the regenerability of a highly active nanostructured material by interfacial hydrogenation, using different reactor designs. It has been demonstrated that regeneration is beneficial to improve the stability of Ni-based systems, at the applied working conditions. Nevertheless, tip-grown carbon nanotubes were considered as a cause of permanent deactivation, which could not be solved by refeeding hydrogen into the system.

Decoupling Aromatic Yield and Coke Formation in Methanol‐to‐Aromatic Reaction on Zn(II) Exchanged H‐ZSM‐5 Zeolite

AbstractMethanol to aromatic reaction is an alternative pathway to produce aromatics from non‐petroleum resources. Zn‐modified ZSM‐5 zeolite is the widespread catalyst for MTA due to its high activity and selectivity to aromatics. However, a desired high yield of aromatics is usually accompanied with undesired coke formation and fast deactivation of catalyst, because aromatic molecules also serve as the precursor for coke formation. To decouple the aromatic yield and coke formation rate, in this work we use Zn‐ZSM‐5 to investigate coke formation and catalyst deactivation. The coke distribution along the catalyst bed was analyzed and its correlation with the concentration gradient of methanol, olefins and aromatics along the bed was deciphered. Coke was found to form very fast at catalyst bed in the copresence of methanol and aromatics. On the contrary, much less coke was observed at the later layers of the catalyst bed where methanol had been fully converted and aromatics were present in the highest concentration. Based on this finding, we adjusted the reaction conditions to successfully improve the yield of aromatics without reducing catalyst lifetime in compromise, by applying a higher methanol partial pressure that enables a faster consumption of methanol and reduces the period of copresence of methanol and aromatics.

Efficient Ozone Elimination Over MnO2 via Double Moisture-Resistance Protection of Active Carbon and CeO2.

The widespread ozone (O3) pollution is extremely hazardous to human health and ecosystems. Catalytic decomposition into O2 is the most promising method to eliminate ambient O3, while the fast deactivation of catalysts under humid conditions remains the primary challenge for their application. Herein, we elaborately developed a splendidly active and stable Mn-based catalyst with double hydrophobic protection of active carbon (AC) and CeO2 (CeMn@AC), which possessed abundant interfacial oxygen vacancies and excellent desorption of peroxide intermediates (O22-). Under extremely humid (RH = 90%) conditions and a high space velocity of 1200 L h-1 g-1, the optimized CeMn@AC achieved nearly 100% O3 conversion (140 h) at 5 ppm, showing unprecedented catalytic activity and moisture resistance toward O3 decomposition. In situ DRIFTS and theory calculations confirmed that the exceptional moisture resistance of CeMn@AC was ascribed to the double protection effect of AC and CeO2, which cooperatively prevented the competitive adsorption of H2O molecules and their accumulation on the active sites of MnO2. AC provided a hydrophobic reaction environment, and CeO2 further alleviated moisture deterioration of the MnO2 particles exposed on the catalyst surface via the moisture-resistant oxygen vacancies of MnO2-CeO2 crystal boundaries. This work offers a simple and efficient strategy for designing moisture-resistant materials and facilitates the practical application of the O3 decomposition catalysts in various environments.

Synergistic effect of macroporosity and crystallinity on catalyst deactivation behavior over macroporous Ni/CexZr1-xO2–Al2O3 for dry reforming of methane

Dry reforming of methane (DRM) is an outstanding process for transforming methane and carbon dioxide into synthesis gases (H2 and CO). In this study, a special geometrical modification on Ni/CexZr1-xO2–Al2O3 catalyst was applied by introducing a macroporous structure into the support to accelerate the metal support interaction that optimized the distribution of the active sites over the catalyst. Furthermore, we investigated a synergistic effect of the macroporosity and CexZr1-xO2 (CZ) crystallinity over the Ni/CexZr1-xO2–Al2O3 catalyst on its catalytic performance in DRM. Macroporous Ni/CexZr1-xO2–Al2O3 catalysts were prepared via washing (Ni-MaCZA-W) and hydrothermal (Ni-MaCZA-H) methods. Non-macroporous Ni-CZA-H with crystalline CZ was prepared for comparison. The crystalline CZ over macroporous Ni-MaCZA-H had a strong interaction with Ni due to easy accessibility of Ni species to CZ in the macroporous support, resulting in highly dispersed Ni nanoparticles and abundant oxygen vacancies. The unique properties of Ni-MaCZA-H increased catalytic activity and stability in DRM, suppressing coke formation via efficient coke gasification. The Ni species and amorphous CZ over Ni-MaCZA-W resulted in a bimodal Ni size distribution and fast catalyst deactivation. However, even though the crystalline CZ over Ni-CZA-H generated a unimodal distribution of Ni nanoparticles, the non-macroporous Ni-CZA-H resulted in gradual deactivation in the catalytic activity and the production of a different type of coke.

Synthesis of Azoxy Compounds: from Copper Compounds to Mesoporous Silica-Encaged Ultrasmall Copper Catalysts.

Azoxy compounds have aroused extensive attention due to their unique biological activities, but the chemical synthesis of these compounds often suffers from limitations due to their requirement for stoichiometric oxidants, high costs, and restricted substrate range. Herein, a series of azoxy compounds were constructed via facile coupling reactions by using cost-effective N-methoxyformamide and nitroso compounds over Cu-based catalysts, affording high product yields with excellent tolerance of functional groups. Significantly, the mesoporous silica nanosphere-encapsulated ultrasmall Cu (Cu@MSN) catalyst was developed via a one-pot synthetic method and first used for the synthesis of azoxy compounds. As compared with copper salt catalysts, the Cu@MSN catalyst exhibited remarkably enhanced catalytic activity and superior recycling stability. Such a Cu@MSN catalyst overcame the inherent drawbacks of low activity, fast deactivation, and difficult recycling of traditional metal salt catalysts in organic reactions. This work provides a green and efficient method for the construction of azoxy compounds and also creates new prospects for the application of nanoporous materials confined metal catalysts in organic synthesis.

Effects of zeolite frameworks and hierarchical structures on catalytic bioethanol dehydration: In-situ DRIFTS and DFT studies

Herein, the combined in-situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory studies were employed to investigate the effects of different zeolite frameworks and hierarchical structures on catalytic bioethanol dehydration. The findings reveal that different zeolite frameworks enable the formation of distinct intermediates, hence promoting different mechanistic pathways. Interestingly, the FER is highly selective to ethylene and inhibits the formation of by-products thanks to the confined porous structure of the FER. Although the pristine small pore FER often suffers from fast catalyst deactivation, the incorporation of hierarchical structures in the FER framework can mitigate this significantly. Accordingly, the catalytic stability of the hierarchical FER was improved remarkably with a high ethylene yield (∼95%), whereas the pristine FER suffers from fast deactivation. Additionally, coke formation over the hierarchical FER catalyst was also reduced significantly compared to that of the pristine FER. Importantly, the in-situ DRIFTS studies reveal that the different reaction pathways over hierarchical and commercial FER have been observed in which the hierarchical one promotes the monomeric pathway due to the facile desorption of corresponding products and intermediates, whilst the pristine one promotes bioethanol conversion via both pathways of monomeric and dimeric pathways to produce ethylene and diethyl ether, respectively.

Thermal and catalytic pyrolysis of a real mixture of post-consumer plastic waste: An analysis of the gasoline-range product

In this work, the thermal and catalytic pyrolysis of different types of plastic waste and a real mixture were investigated in a fixed-bed reactor over different catalysts (CaO, MgO, HY, HZSM-5). Important differences in gas, liquid, and solid yields were found as a function of polymer type. The highest gas yield was obtained with expanded polystyrene (52.3%), and the maximum oil production with high-impact polystyrene (55.5%), while polypropylene film led to the highest char release (17.5%). Regarding the composition of the liquid oil, high-impact polystyrene showed the highest yield of gasoline-range product (426 g per kg of pyrolyzed plastic), mainly composed of aromatics compounds (90%). The addition of catalysts increased the gas yield to the detriment of the oil produced. The effect was more evident for zeolite-type catalysts, i.e., the gas yield raised from 43.3 (non-catalytic) to 51.5% (HZSM-5). Low influence on the oil composition, i.e., gasoline-range product, was detected. This can be explained by the fast deactivation of catalysts because of coke deposition. Only an increase in the fraction of gasoline in liquid oil was observed when low-cost catalysts (CaO and MgO) were used, without significant changes in the composition of this product.

Aqueous-phase reforming of hydroxyacetone solution to bio-based H2 over supported Pt catalysts

Aqueous-phase reforming (APR) is an attractive process to produce bio-based hydrogen from waste biomass streams, during which the catalyst stability is often challenged due to the harsh reaction conditions. In this work, three Pt-based catalysts supported on C, AlO(OH), and ZrO2 were investigated for the APR of hydroxyacetone solution in a fixed bed reactor at 225 °C and 35 bar. Among them, the Pt/C catalyst showed the highest turnover frequency for H2 production (TOF of 8.9 molH2 molPt−1 min−1) and the longest catalyst stability. Over the AlO(OH) and ZrO2 supported Pt catalysts, the side reactions consuming H2, formation of coke, and Pt sintering result in a low H2 production and the fast catalyst deactivation. The proposed reaction pathways suggest that a promising APR catalyst should reform all oxygenates in the aqueous phase, minimize the hydrogenation of the oxygenates, maximize the WGS reaction, and inhibit the condensation and coking reactions for maximizing the hydrogen yield and a stable catalytic performance.

Alkene Epoxidation and Thioether Oxidation with Hydrogen Peroxide Catalyzed by Mesoporous Zirconium-Silicates

Mesoporous zirconium-silicates have been prepared using two different methodologies, evaporation-induced self-assembly and solventless organometallic precursor dry impregnation of commercial SiO2. The samples were characterized by elemental analysis, XRD, N2 adsorption, TEM, DRS UV–vis and Raman spectroscopic techniques. The catalytic performance of the Zr-Si catalysts was assessed in the epoxidation of three representative alkenes, cyclohexene, cyclooctene and caryophyllene, as well as in the oxidation of methyl phenyl sulfide using aqueous hydrogen peroxide as a green oxidant, with special attention drawn to the structure/activity relationship and catalyst stability issues. The key factors which affect substrate conversion and epoxide selectivity have been defined. The catalysts with larger contents of oligomeric ZrO2 species revealed higher activity. The nature of alkene and, in particular, its molecular hindrance is crucial, since the adsorption of the epoxide product is the main factor leading to fast catalyst deactivation. In fact, bulky epoxides do not show this effect. After optimization, the oxidation of caryophyllene gave endocyclic monoepoxide with 77% selectivity at 87% alkene conversion. Methyl phenyl sulfoxide afforded 37% of sulfoxide and 63% of sulfone at 57% sulfide conversion. The nature of catalysis was truly heterogeneous and no Zr leaching was observed.

Olefin methylation over iron zeolites and the methanol to hydrocarbons reaction

The effect of olefin addition to a stream of dimethyl ether on the methanol homologation reaction is investigated using iron-substituted zeolites Fe-beta and Fe-ZSM-5. The reaction was investigated using plug-flow microreactors in the temperature range of 240–400 °C, at a total pressure of 0.239 MPa and a WHSV of 6.12 (g DME/gcat-hr). For Fe-beta (Si/Fe= 9.2) catalysts, isobutene co-feeding almost doubles dimethyl ether (DME) consumption rate and shifts selectivity towards larger olefins with carbon numbers from 5 to 7. Addition of isobutene above 6.3%, however, resulted in a reduction of DME consumption rates, an effect assigned to the replacement of surface methoxy groups for adsorbed olefins in the zeolite pores. Below a temperature of 340 °C hydride-transfer rates are negligible; reaction rates are stable for over 5.5 h and the products consist almost exclusively of olefins and a small amount of methane. Above 360 °C the onset of catalytic hydride transfer processes is observed leading to fast catalyst deactivation rates and an increase in the concentration of aromatic species. Iron ZSM-5 (Si/Fe = 21.4) catalysts under similar reaction conditions consumes methanol faster than Fe-beta at approximately three times the TOF (on a per iron basis). The Fe-ZSM-5 catalyst was selective to a distribution of products (C5 to C8) as compared to Fe-beta which was selective to primarily C5 and C7. Co-feeding larger olefins (2-methyl-2-butene, 2,3-dimethyl-2-butene, 2,3,3-trimethyl-1-butene, and 2,4,4-trimethyl-2-pentene) at a 3.9% olefin concentration over Fe-beta changed selectivity towards cracking products (C4 compounds such as isobutene). As the size of the olefin increases, a reduction of DME consumption rate is also observed. These results show that co-feeding olefins with DME over Fe-zeolites is a promising route to increase methylation rates at relatively low temperatures producing larger branched olefins and that the product distribution is highly dependent on the zeolite pore size and structure of the olefin.

Hydrogenolysis of Glycerol to Propylene Glycol: Energy, Tech-Economic, and Environmental Studies.

Hydrogenolysis of glycerol to propylene glycol represents one of the most promising technologies for biomass conversion to chemicals. However, conventional hydrogenolysis processes are often carried out under harsh H2 pressures and temperatures, leading to intensive energy demands, fast catalyst deactivation, and potential safety risks during H2 handling. Catalytic transfer hydrogenolysis (CTH) displays high energy and atom efficiency. We have studied a series novel solid catalysts for CTH of glycerol. In this work, detailed studies have been conducted on energy optimization, tech-economic analysis, and environmental impact for both processes. The key finding is that relatively less energy demands and capital investment are required for CTH process. CO2 emission per production of propylene glycol is much lower in the case of transfer hydrogenolysis. The outcome of this study could provide useful information for process design and implementation of novel hydrogenolysis technologies for other energy and environmental applications.

Conversion of Biomass-Derived Methyl Levulinate to Methyl Vinyl Ketone

A high-throughput screening exercise testing 60 different catalysts resulted in 5 wt % Pt on sulfided carbon as the best catalyst in the conversion of bio-based methyl levulinate (ML) to methyl vinyl ketone (MVK) in a gas-phase continuous process. Up to 18% yield of MVK was obtained, but fast catalyst deactivation was observed. For a better understanding of the reaction mechanism, the potential reaction intermediates [α-angelica lactone (α-AL), γ-valerolactone, methyl ethyl ketone (MEK), and levulinic acid (LA)] were also fed as starting materials under the same reaction conditions as those used for ML. Of the different pathways possible, the route via AL seems to be the most likely route. Since the side product methanol led to the hydrogenation of MVK to MEK, LA is a better substrate in this reaction toward MVK at a medium reaction temperature. Herein, we report the highest yield of MVK (>50%) from LA at 350 °C. However, this knowledge of the reaction pathway via AL also opened up the possibility of a high-temperature conversion process of ML to MVK. It was found that ML could be converted to MVK in 71% selectivity at 600 °C using 40% CaO on γ-Al2O3 as the catalyst. Here, the catalyst merely serves to accelerate the ring closure of ML to AL, which undergoes an electrocyclic reaction under extrusion of CO to form MVK.

The reductive catalytic upcycling of polyolefin plastic waste

Plastics, and polyolefins (POs) in particular, revolutionized the everyday life, and after use the waste accumulation is raising severe environmental concerns fostering an intense the research towards more sustainable and profitable recycling routes.In this contribution, we focus the attention concerning recent advances on the reductive upgrading of POs promoted by heterogeneous catalysts, that is emerging as a valid alternative to pyrolysis since POs are transformed into valuable chemicals and fuels, allowing overcome the issue of elevated temperatures, as well as low selectivity and fast catalyst deactivation. As a matter of fact, the introduction of H2 gas in the reactor allows to carry out PO degradation in milder conditions coupled with a higher control on the produced hydrocarbons, depending on the employed catalyst and the reaction parameters. Two main processes have been so far developed: hydrogenolysis and hydrocracking, respectively leading to formation of diesel and gasoline. At a molecular level, their difference relies on the different typologies of catalysts, which are monofunctional metal catalysts in the former case (where both surface hydrides and surface alkyl species are formed and recombine each other), and bifunctional acid/metal catalysts in the latter (where, after a carbocation is formed by protonation, the reaction proceeds by isomerization and scission). Despite the conceptually simple and univocal definition of the two mechanisms, several catalytic processes have been developed in the last years, varying for both reaction conditions and obtained products, thus generating a quite complex scenario.Hereafter, the most significant results are reported to provide a consistent base for further progress.

Coaromatization of methane and propane over Ga supported on HZSM-5 catalysts: The effect of mesoporosity on deactivation behavior

Ga supported on mesoporous HZSM-5 (GaOy/meso-HZSM-5) and conventional HZSM-5 (GaOy/micro-HZSM-5) catalysts were compared to investigate the effect of mesoporosity on the deactivation behavior in the coaromatization of methane and propane. The trend of reaction intermediate distribution between the catalysts became gradually different with time-on-stream, implying a distinction in deactivation behavior. These are attributed to the behavior of acid site blockage, because the production of BTX are driven by acid sites. The reaction with 13CH4 and further analyses revealed that the interaction between coke and Ga species that serve as Lewis acid sites is much stronger in GaOy/micro-HZSM-5 than in GaOy/meso-HZSM-5 post-reaction catalyst. Therefore, large amount of highly condensed coke was deposited in GaOy/micro-HZSM-5 during the reaction, leading to fast catalyst deactivation. Consequently, the structural difference arising from mesoporosity affects physicochemical properties of the catalysts thereby contributing to different deactivation behavior between the catalysts.

Influence of HZSM-5-based catalyst deactivation on the performance of different reactor configurations for the conversion of bioethanol into hydrocarbons

The conversion of bioethanol into hydrocarbons, BTH process, has been simulated in reactors with different catalyst regimes (packed bed, non-circulating fluidized bed and circulating fluidized bed). The production of value-added products, as light olefins (ethylene, propylene and butenes) and gasoline (C4+ hydrocarbons), has been targeted. Different activity profiles have been predicted for packed and fluidized bed reactors, and a circulating fluidized bed reactor has allowed a steady-state operation. The obtained product distribution in this reactor can be easily tuned by modifying the reaction conditions and the mean residence time of the catalyst in the reactor. Maximum yields of olefins (ca. 85%) are predicted to be achieved at low temperature (300 °C), low space time (0.1 gcat h g−1) or high content of water in the feed (75%), whereas the yield of gasoline is otherwise maximized (ca. 70%) at high temperature (400 °C), high space time (1.0 gcat h g−1) or low content of water in the feed (5%, azeotrope mixture). The results highlight the interest of having a useful and versatile simulation model to be used as a tool for the prediction of different reactor performances in processes with complex reaction networks and fast catalyst deactivation as the BTH process.

Ketonization of Propionic Acid on Lewis Acidic Zr-Beta Zeolite with Improved Stability and Selectivity

Ketonization reactions provide a feasible approach to remove oxygen and increase carbon chain length for conversion of biomass derived carboxylic acids. However, the reaction suffers from fast catalyst deactivation and low ketone selectivity. In this work, Lewis acidic heteroatom Ti-, Zr-, Ce-, and Sn-Beta zeolites were prepared using a two-step post-synthesis method and applied in vapor phase ketonization of propionic acid at 350 °C. Among these zeolites, Zr-Beta shows both the highest activity and selectivity. Characterizations indicate that Zr prefers the vacant tetrahedral site when the Zr content is 96% at a space time of 2 h. The amount of coke deposition on Zr-Beta is about 1/3 of that on H-Beta, and the structure of Zr-Beta is preserved after 60 h of reaction. The results of this work indicate that Zr-Beta zeolite is a promising catalyst for ketonization with good stability and high selectivity toward ketone.

Benzofuran as deactivation precursor molecule: Improving the stability of acid zeolites in biomass pyrolysis by co-feeding propylene

Benzofuran (BZF) is produced during the biomass and waste pyrolysis, playing a key role in the catalytic deactivation in the processing of the resulting fractions, as well as in catalytic pyrolysis reactions. The transformation of this chemical over a typical pyrolysis catalyst (ZSM-5) is studied in this article. When this compound is processed alone, it presents a very high reactivity. At low temperature, the formation of large amount of coke precursors leads to a fast catalyst deactivation. As temperature increases, the weight of cracking reactions (yielding benzenes or naphtalenes) becomes more important, increasing the catalyst’s stability, although new polyaromatic carbon species are detected.A new strategy to minimize the deactivation caused by this compound, consisting of co-feeding propylene, is proposed. The presence of an alkene leads to the formation of alkylated derivatives of Diels-Alder condensation products, hindering their oligomerization capacity and minimizing the formation of polyaromatic coke deposits. Different temperatures and alkene/BZF ratios were tested, concluding that increasing the alkene concentration and the reaction temperature monotonously increase the catalyst’s stability in presence of BZF.

Deactivation of V2O5−WO3/TiO2 DeNOx Catalyst under Commercial Conditions in Power Production Plant

Nitrogen dioxide is one of the most dangerous air pollutants, because its high concentration in air can be directly harmful to human health. It is also responsible for photochemical smog and acid rains. One of the most commonly used techniques to tackle this problem in large combustion plants is selective catalytic reduction (SCR). Commercial SCR installations are often equipped with a V2O5−WO3/TiO2 catalyst. In power plants which utilize a solid fuel boiler, catalysts are exposed to unfavorable conditions. In the paper, factors responsible for deactivation of such a catalyst are comprehensively reviewed where different types of deactivation mechanism, like mechanical, chemical or thermal mechanisms, are separately described. The paper presents the impact of sulfur trioxide and ammonia slip on the catalyst deactivation as well as the problem of ammonium bisulfate formation. The latter is one of the crucial factors influencing the loss of catalytic activity. The majority of issues with fast catalyst deactivation occur when the catalyst work in off-design conditions, in particular in too high or too low temperatures.

Non-Thermal Plasma for Process and Energy Intensification in Dry Reforming of Methane

Plasma-assisted dry reforming of methane (DRM) is considered as a potential way to convert natural gas into fuels and chemicals under near ambient temperature and pressure; particularly for distributed processes based on renewable energy. Both catalytic and photocatalytic technologies have been applied for DRM to investigate the CH4 conversion and the energy efficiency of the process. For conventional catalysis; metaldoped Ni-based catalysts are proposed as a leading vector for further development. However; coke deposition leads to fast deactivation of catalysts which limits the catalyst lifetime. Photocatalysis in combination with non-thermal plasma (NTP), on the other hand; is an enabling technology to convert CH4 to more reactive intermediates. Placing the catalyst directly in the plasma zone or using post-plasma photocatalysis could generate a synergistic effect to increase the formation of the desired products. In this review; the recent progress in the area of NTP-(photo)catalysis applications for DRM has been described; with an in-depth discussion of novel plasma reactor types and operational conditions including employment of ferroelectric materials and nanosecond-pulse discharges. Finally, recent developments in the area of optical diagnostic tools for NTP, such as optical emission spectroscopy (OES), in-situ FTIR, and tunable diode laser absorption spectroscopy (TDLAS), are reviewed.

Stable High‐Pressure Methane Dry Reforming Under Excess of CO2

AbstractDry reforming of methane (DRM), the conversion of carbon dioxide and methane into syngas, offers great promise for the recycling of CO2. However, fast catalyst deactivation, especially at the industrially required high pressure, still hampers this process. Here we present a comprehensive study of DRM operation at high pressure (7–28 bars). Our results demonstrate that, under equimolar CH4 : CO2 mixtures, coke formation is unavoidable at high pressures for all catalysts under study. However, under substoichiometric CH4 : CO2 ratios (1 : 3), a stable high pressure operation can be achieved for most catalysts with no sign of deactivation for at least 60 hours at 14 bars, 800 °C and 7500 h−1. In addition to the enhanced stability, under these conditions, the amount of CO2 abated per mol of CH4 fed increases by a 50 %.