Reoxidation Of Catalyst Research Articles

Surface Chemical Effects on Fischer–Tropsch Iron Oxide Catalysts Caused by Alkali Ion (Li, Na, K, Cs) Doping

Among the alkali metals, potassium is known to significantly shift selectivity toward value-added, heavier alkanes and olefins in iron-based Fischer–Tropsch synthesis catalysts. The aim of the present contribution is to shed light on the mechanism of action of alkaline promoters through a systematic study of the structure–reactivity relationships of a series of Fe oxide FTS catalysts promoted with Group I (Li, Na, K, Cs) alkali elements. Reactivity data are compared to structural data based on in situ, synchrotron-based XRD and XPS, as well as temperature-programmed studies (TPR-H2, TPC-CO, TPD-CO2, and TPD-H). It has been observed that the alkali elements induced higher carburization rates, higher basicities, and lower adsorbed hydrogen coverages. Catalyst stability followed the trend Na-Fe > unpromoted > Li-Fe > K-Fe > Cs-Fe, being consistent with the ability of the alkali (Na) to prevent active site loss by catalyst reoxidation. Potassium was the most active in promoting high α hydrocarbon formation. It is active enough to promote CO dissociative adsorption (and the formation of FeCx active phases) and decrease the surface coverage of H-adsorbed species, but it is not so active as to cause premature catalyst deactivation by the formation of a carbon layer resulting in the blocking active sites.

Surface anchoring requirements for vanadia clusters on titanium oxide surfaces and their impact on activity for oxidative dehydrogenation of ethanol

Titanium oxide site requirements for the anchoring of catalytically active forms of dispersed vanadia species are investigated. Using a series of oxygen-modified titanium nitride materials, we systematically modify the available vanadia-anchoring moieties on the titanium-bearing support surface. This strategy in turn results in a very narrow size distribution of well-dispersed vanadia clusters anchored on the support. This synthesis technique ensures that only highly dispersed vanadia is present on the catalyst surface even at nominal monolayer coverages. A deeper insight into the catalytic behavior for ethanol partial oxidation of the active vanadia species is obtained by comparing intrinsic kinetic and thermodynamic parameters of kinetically relevant reaction steps (heat of ethanol adsorption, hydrogen abstraction activation energy, and the energetics of catalyst reoxidation). It is found that these parameters are dependent on the relative amount of oxygen in the titanium-bearing support and the resulting distribution of vanadia species, which validates the premise of a potential participation of lattice oxygen atoms of the titania support into the catalytic activity of active vanadia species.

Sorption-enhanced Fischer-Tropsch synthesis – Effect of water removal

The Sorption-Enhanced Fischer-Tropsch synthesis (SEFTS), with water removal by means of a solid sorbent, has been demonstrated for the first time experimentally. Commercial water sorbents (Zeolites type 13X and 4 A) were thoroughly characterized to determine water sorption capacity at relevant temperatures (100–250°C) as well as multicycle stability after 100 cycles at 210°C. The adsorption capacity of both zeolties decreased with increasing temperature. The adsorption capacity of 4 A remained almost constant after 100 adsorption/desorption cycles, while 13X lost almost 50% of its capacity. Post characterization of the 13X sample showed distorted crystallinity after 100 cycles which explains the drop in stability. The SEFTS experiments were performed as a cyclic operation, first FT reaction and then water sorbent regeneration, at 210 °C and 5 bar. The system with zeolites showed 10% higher CO conversion after 65 hours on stream compared to the system without any sorbents. The selectivities to C5+ was higher for the system with sorbents compared to the system without zeolites. A much steeper deactivation curve was also observed for the system without zeolites at steady state conditions. This might be due to the fact that zeolites adsorbs the water and remove it from the catalyst active centers, thus increase catalyst stability by preventing the catalyst re-oxidation. Overall, this work opens an opportunity for enhancing the catalyst activity in FTS by in situ water sorption.

Fundamentals of enhanced oxygen releasability of Mn-Na2WO4/SiO2 through cofed water for efficient oxidative coupling of methane in a chemical looping mode

The oxidative coupling of methane (OCM) to C2H6 and C2H4 (C2-hydrocarbons) is an industrially attractive reaction, which has not yet been commercialized. Apart from the low selectivity to C2-hydrocarbons at high degrees of CH4 conversion, another substantial drawback is the necessity to use pure O2 when this oxidant and methane are cofed. Original OCM studies used an approach called chemical looping OCM (CL-OCM) that consists of alternating feeding of methane and air for product formation and catalyst reoxidation, respectively. The developed catalysts, however, suffer from their low ability to provide lattice oxygen for the desired reaction. Herein, we demonstrate that this catalyst property can be significantly improved when performing CL-OCM over Mn-Na2WO4/SiO2 catalysts in the presence of water. In comparison to water-free CL-OCM, both methane conversion and C2-hydrocarbons selectivity increase resulting in at least doubling of the yield of these products. The selectivity to ethylene of about 53 % was obtained at about 24 % CH4 conversion. The total selectivity to C2-hydrocarbons was about 76 %. The analysis of selectivity-conversion relationships for C2H4, C2H6, CO and CO2 proved that water does not alter their general formation pathways but influences the kinetics of their formation.In situ X-ray diffraction, in situ UV–vis spectroscopic, and temperature-programmed tests (H2-TPR and O2-TPD with or without cofed H2O) showed that Mn2O3 acts as the oxygen carrier in CL-OCM. In-situ DRIFTS proved the presence of O2– and O22–. They can interact with water yielding OH radicals that accelerate CH4 conversion.

Catalytic aerobic oxidation of carbohydrates to formic acid over H5PV2Mo10O40: Rate relationships among catalyst reduction, catalyst re-oxidation and acid-catalyzed reactions and evidence for the Mars-van Krevelen mechanism

Catalytic aerobic oxidation of biomass-derived carbohydrates is a promising way to sustainably produce formic acid. Understanding the rate relationships among catalyst reduction, catalyst re-oxidation and acid-catalyzed reactions occurring in catalytic systems is significant for the revelation of catalytic mechanism and the selection of reaction conditions. Herein, the rate relationships among the multiple reactions over an excellent homogeneous catalyst, H5PV2Mo10O40 (HPA-2) were investigated. A rate matching relationship between catalyst reduction and catalyst re-oxidation was unveiled by the separate redox kinetics of catalyst, indicating that higher O2 pressures are required to maintain the activity of HPA-2 at higher temperatures. The consistency between the separate redox kinetics and the catalytic oxidation provided the first kinetic evidence of the Mars-van Krevelen mechanism for catalytic oxidation of carbohydrates. Furthermore, rate relationships among the multiple reactions at different conditions were discussed to clarify the correlation between product selectivity and reaction conditions.

Catalytic selective oxidation of isobutane in a decoupled redox-process

The catalytic selective oxidation of isobutane to methacrolein (MAC) and methacrylic acid (MAA) is known to proceed via a Mars-Van Krevelen mechanism. When the reaction is performed in a common reactor with co-feeding of the reactant and oxygen, reactive oxygen species are present on the catalyst surface whereby non-selective oxidation reactions occur. In this work, we studied the possibility to decouple the redox mechanism meaning to separate the reactant oxidation step and the catalyst re-oxidation step for the selective oxidation of isobutane (IBAN) to MAC and MAA. For this purpose, two reactor configurations were employed: a periodic reactor and a two-zone fluidized bed reactor (TZFBR). Whereas the periodic reactor allows the temporal separation of the two steps, the TZFBR enables the spatial separation.A Cs-based Keggin type heteropolycompound was employed as catalyst and characterized before and after reaction by XRD, Raman and XPS to better understand the structural and chemical changes during the periodic operation. The effect of the oxygen/IBAN concentration on the performance was studied. For the periodic reactor it was found that the conversion of isobutane and the formation of products decreased with the consumption of available oxygen species in the catalyst during a period. For the whole periodic tests, low concentration of oxygen injected into the reoxidation part of the cycles led to a high selectivity to the desired products (MAC+MAA).

Effect of promoters on o-xylene oxidation pathways reveals nature of selective sites on TiO2 supported vanadia

The o-xylene oxidation pathways on TiO2 supported vanadium oxide catalysts are affected by Cs+ and Sb4+ promoters. Reaction kinetics change remarkably at 360 °C: while at lower temperatures the rate determining step is the C-H bond cleavage in xylene methyl groups, the catalyst reoxidation becomes kinetically limiting at higher temperatures. Promotion by Sb4+ is related to an increase in the availability of oxygen at high temperatures for the reoxidation of V sites. The presence of Cs+ enhances the reoxidation of V sites and increases the ratio of nucleophilic to electrophilic oxygen on the surface. Catalytic tests of benzene oxidation confirm the lack of electrophilic sites on Cs-promoted VOx/TiO2 catalyst. Combined evidence points to a key role of the bridging lattice oxygen in V-O-V species as active site for the activation of o-xylene and its selective transformation to phthalic anhydride.

A subtle balancing act for quinones

Catalysis Palladium catalysis is widely used in drug synthesis to form carbon-carbon bonds, but typically both carbon centers need to be activated ahead of time. Although introducing oxygen as an oxidant diminishes the need for preactivation, the catalysis then tends to be less efficient, especially in the case of arene coupling. Salazar et al. pinpointed the role of quinone co-catalysts in these processes and determined that after accelerating carbon-carbon bond formation, the quinone slows down catalyst reoxidation by oxygen. Appending bulky substituents to the quinone struck a better balance and substantially enhanced catalytic efficiency. Science , this issue p. [1454][1] [1]: /lookup/doi/10.1126/science.abd1085

Pd-Catalyzed Aerobic Oxidative Coupling of Thiophenes: Synergistic Benefits of Phenanthroline Dione and a Cu Cocatalyst.

Substituted bithiophenes are prominent fragments in functional organic materials, and they are ideally prepared via direct oxidative C-H/C-H coupling. Here, we report a novel PdII catalyst system, employing 1,10-phenanthroline-5,6-dione (phd) as the ancillary ligand, that enables aerobic oxidative homocoupling of 2-bromothiophenes and other related heterocycles. These observations represent the first use of phd to support Pd-catalyzed aerobic oxidation. The reaction also benefits from a Cu(OAc)2 cocatalyst, and mechanistic studies show that Cu promotes C-C coupling, implicating a role for CuII different from its conventional contribution to reoxidation of the Pd catalyst.

Palladium-Catalyzed Oxidative Dehydrosilylation for Contra-Thermodynamic Olefin Isomerization

We report a newly developed, palladium-catalyzed dehydrosilylation of terminal alkylsilanes that combines with chain-walking hydrosilylation to create a one-pot isomerization of internal olefins to terminal olefins. This catalytic dehydrosilylation is one of the few examples of thermal catalytic functionalizations of unactivated alkylsilanes. The reaction involves transmetalation of an alkylsilane, β-hydride elimination, release of the terminal olefin, and reoxidation of the palladium catalyst. A variety of linear internal olefins underwent the overall isomerization to terminal olefins in good yields and in good regioselectivities. Particularly noteworthy, isomerizations occurring over seven carbon units proceeded in yields that are comparable to those of isomerizations occurring over one carbon unit.

Continuous Production of Formic Acid from Biomass in a Three-Phase Liquid–Liquid–Gas Reaction Process

In this contribution, we present for the first time a continuous production of formic acid under industrially relevant conditions using real biomass as a feedstock. A scale up to 2 L of reaction volume including product separation and catalyst recycling in an automated continuous miniplant has been realized and demonstrated for 670 h time-on-stream using sucrose as a feedstock with space-time yields of 1.1 gFA L–1 h–1, a turnover number (TON) of 840, and an average turnover frequency (TOF) of 1.3 h–1. Very remarkably, by performing a sensitivity analysis, a maximum space-time yield of 1.7 gFA L–1 h–1 was achieved. Moreover, we could demonstrate that oxygen pressures of only 0.5 bar are sufficient for complete catalyst reoxidation in our miniplant. This shows great potential for simply using air as the oxidant in a large-scale application. Very importantly, an experiment with molasses as the substrate showed the use of a technically relevant biomass for a long-term economic application of the continuous OxFA process. By using optimized conditions from the sensitivity analysis, a total amount of 1.2 g h–1 of formic acid could be continuously produced by our miniplant.

Sustainable utilization of chlorine via converting HCl to Cl2 over a robust copper catalyst

Offering a substantial method to convert HCl to Cl2 over CuCl2/pumice, Deacon process is of great importance in chemical industry. In the process, the difficulty of Cl2 formation/desorption and catalyst re-oxidation have been unanimously recognized as the origin of the instability of CuO-based catalysts. In this work, the stability of the CuO/Al2O3 catalyst was significantly improved by adding Co. The fabricated CuO(12)–CoOx(13)/Al2O3 catalyst showed unprecedented stability with HCl conversion (xHCl) above 80%, lasting for at least 1000 h. The X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and H2-temperature programmed reduction (H2-TPR) characterizations suggested that the addition of Co greatly enhanced the dispersion of CuO on Al2O3 and hindered the reduction of CuO due to their strong interaction. The results of reaction kinetics suggested that the lattice oxygen participated in the reaction and the reaction order on oxygen was estimated to be 0.15, suggesting that the catalyst obeyed the Mars-van Krevelen (M-K) mechanism, while only small amount of oxygen was required to maintain the activity of catalysts compared to CuO/Al2O3. Density functional theory (DFT) calculations proved that the CuO–CoOx/Al2O3 ternary system was more stable than CuO/Al2O3 binary system and easier to be oxidized by O2, which greatly reduced the difficulty of Cl2 associative desorption.

Combining autoclave and LCWM reactor studies to shed light on the kinetics of glucose oxidation catalyzed by doped molybdenum-based heteropoly acids.

In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor. Hereby, we investigated the influence of Nb- and Ta-doping on Mo-based Keggin-polyoxometalates for both reaction steps independently. Most importantly, we could demonstrate a significant increase of glucose oxidation kinetics by Ta- and especially Nb-doping by factors of 1.1 and 1.5 compared to the classical HPA-Mo. Moreover, activation energies for the substrate oxidation step could be significantly reduced from around 80 kJ mol−1 for the classical HPA-Mo to 61 kJ mol−1 for the Ta- and 55 kJ mol−1 for the Nb-doped species, respectively. Regarding catalyst reoxidation kinetics, the doping did not show significant differences between the different catalysts.

A comparison of two-phase and three-phase CO2 methanation reaction kinetics

In a previous publication related to three-phase CO2 methanation (3PM) reaction kinetics (Lefebvre et al., 2018) it was postulated that (i) the liquid phase influences the effective reaction rate but not the intrinsic chemical reaction rate and (ii) gas concentration in the liquid phase, not gas partial pressure, is the relevant parameter to describe 3PM reaction kinetics. In this earlier publication, it was also reported that (iii) measurement uncertainties related to gas concentration in the liquid phase are high and (iv) catalyst reoxidation during the starting procedure of the three-phase experiments may not have been fully excluded.The aim of the present publication is to prove the postulates (i) and (ii). To achieve this, the two-phase CO2 methanation (2PM) reaction kinetics is investigated in a plug flow laboratory reactor. Using the data of 213 validated experiments, a power law kinetic rate equation is developed, which describes 2PM reaction kinetics on a commercially available catalyst for inlet CO2 partial pressures of 1 bar and temperatures between 200 °C and 300 °C.This two-phase kinetic rate equation is applied to calculate 3PM reaction rates using temperatures and gas concentrations in the liquid phase from previous 3PM experiments. It is shown that the two-phase kinetic rate equation can describe 3PM experiments with good agreement, i.e. a liquid phase does not influence the intrinsic reaction rate but the concentration of reacting species on the catalyst surface and gas concentration, not gas partial pressure, is the relevant parameter to describe the CO2 methanation reaction kinetics.

Low-temperature activity and mechanism of WO3-modified CeO2-TiO2 catalyst under NH3-NO/NO2 SCR conditions

The CeO2-TiO2 (CeTi) and CeO2/WO3-TiO2 (CeWTi) catalysts were prepared by a sol-gel precipitation method and their NH3-NO/NO2 selective catalytic reduction (SCR) performance was studied. N2O formation and effect of oxygen concentration on SCR performance over CeWTi catalyst were also investigated while varying the NO2/NOx ratio. Results indicate that fast SCR behavior of CeWTi catalyst has the best NH3-NO/NO2 SCR performance due to the catalyst reoxidation rate by NO2 higher than by O2. Compared with CeTi catalyst, CeWTi catalyst exhibits higher de-NOx performance under NH3-NO/NO2 SCR conditions. As the CeTi and CeWTi catalysts exhibit similar redox property, addition of WO3 provides more acid sites which accelerate the reaction between NH4NO3 and NO to get a superior low-temperature activity. Amount of N2O formation shows a peak at 250 °C mainly derived from NH4NO3 decomposition.

Reversible Segregation of Ni in LaFe0.8Ni0.2O3±δ During Coke Removal

AbstractThe deactivation of supported nickel catalysts by coking is an important technological subject for many chemical processes, especially when high concentrations of unsaturated hydrocarbons are present in the feed gas. Here, the reversible segregation of Ni from a LaFeO3±δ perovskite‐type host lattice was exploited to completely recover a LaFe0.8Ni0.2O3±δ catalyst after it had been deliberately subjected to severe carbon deposition during CO2 methanation in ethylene rich feed gas for several hours. Temperature programmed reduction, X‐ray diffraction, electron microscopy, X‐ray absorption spectroscopy and catalytic activity tests were used to follow the catalyst structure along the various steps of reduction, reaction, coking and subsequent regeneration, while Raman spectroscopy and electron microscopy were used to characterize the nature of the carbon deposits. It is shown that upon reduction Ni atoms segregate to the surface of the perovskite to form catalytic active Ni particles. Oxidation stimulates Ni atoms to readopt the coordination environment of Fe in the perovskite matrix. This property persisted after severe catalyst deactivation by filamentous, partially graphitic carbon. It is demonstrated that simple catalyst reoxidation can be applied to oxidize all carbon deposits while additionally reverting segregated Ni back into the host lattice, thus protecting Ni from particle growth and resultant long‐term loss of catalyst activity over multiple regeneration cycles.

Promotional effect of antimony on the selective catalytic reduction NO with NH3 over V-Sb/Ti catalyst

ABSTRACTThe effect of antimony on the selective catalytic reduction (SCR) performance and SO2 durability of V-Sb/Ti was investigated. The physicochemical characteristics of catalyst were characterized by various techniques, including Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), NH3/SO2-temperature programmed desorption (TPD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs), X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR). The V-Sb/Ti catalyst showed excellent activity in the range 200–300°C (compared with V/Ti), with an optimum achieved for 2 wt.% antimony. The total amount of acidic sites and NH3 adsorption characteristics did not affect the catalytic efficiency. The Sb5+ fraction was highest for V-2.0Sb/Ti and exhibited a positive correlation with the V4+ fraction. This phenomenon is related to the effect of synergistic between vanadium and antimony, promoting the conversion of V5+ to V4+ by Sb5+. Increasing the V4+ fraction in V-Sb/Ti increased the catalytic activity, which was mainly attributed to enhanced catalyst re-oxidation capability due to the addition of antimony. Furthermore, the addition of antimony delayed the adsorption of SO2 onto the V-Sb/Ti catalyst surface, improving the resistance to this gas. Therefore, the addition of antimony to V/Ti improved NOx conversion and SO2 durability.

Ethanol Partial Oxidation over VOx/TiO2 Catalysts: The Role of Titania Surface Oxygen on Vanadia Reoxidation in the Mars–van Krevelen Mechanism

The mechanism for the reoxidation step in the Mars–van Krevelen mechanism for ethanol partial oxidation over vanadia anchored on titanium oxide is examined. Kinetic parameters such as ethanol heat of adsorption, the activation energy for the rate-limiting step (α-hydrogen abstraction on the adsorbed ethoxide) were obtained while the energetics of the catalyst reoxidation step were explored. A comparison of the parameters obtained from kinetic analysis and the apparent activation energies reported in the literature indicated that a kinetic model that incorporates a catalyst reoxidation step, where molecular oxygen adsorbs into a titania vacancy, accurately predicted the kinetic parameters. In contrast, a model where molecular oxygen directly adsorbs on the reduced vanadia resulted in an underestimation of the ethanol heat of adsorption and activation energy for the α-hydrogen abstraction step. A computational analysis was implemented to elucidate a mechanistic pathway for reduced vanadia that incorporates ...

Cooperative Light-Activated Iodine and Photoredox Catalysis for the Amination of Csp3 -H Bonds.

An unprecedented method that makes use of the cooperative interplay between molecular iodine and photoredox catalysis has been developed for dual light‐activated intramolecular benzylic C−H amination. Iodine serves as the catalyst for the formation of a new C−N bond by activating a remote Csp3 −H bond (1,5‐HAT process) under visible‐light irradiation while the organic photoredox catalyst TPT effects the reoxidation of the molecular iodine catalyst. To explain the compatibility of the two involved photochemical steps, the key N−I bond activation was elucidated by computational methods. The new cooperative catalysis has important implications for the combination of non‐metallic main‐group catalysis with photocatalysis.

Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst.

Copper salts find widespread use in Pd-catalyzed oxidation reactions, and they are typically used as oxidants or redox-active cocatalysts. Here, we probe the origin of a dramatic acceleration effect of Cu(OTf)2 in the C-H/C-H aerobic oxidative coupling of o-xylene. NMR spectroscopic analysis of the PdII catalyst in the presence of Cu(OTf)2, together with other experimental and DFT computational studies of the catalytic reaction, show that Cu(OTf)2 activates the PdII catalyst for C-H activation via a non-redox pathway and has negligible impact on catalyst reoxidation. These observations led to the testing of other metal triflate salts as cocatalysts, the results of which show that Fe(OTf)3 is even more effective than Cu(OTf)2.