Soft Oxidant Research Articles

Microkinetic analysis of the CO2 effect on OCM over a La‐Sr/CaO catalyst

AbstractGiven its role as a primary side product and a potential soft oxidant in the oxidative coupling of methane (OCM), understanding the effect of CO2 co‐feeding on OCM emerges as a key milestone to optimize the process. To grasp the molecular impact of CO2, a mechanistic investigation over a La‐Sr/CaO catalyst was carried out via microkinetic modeling. Seven catalyst descriptors with a precise physico‐chemical meaning were regressed for both pure O2 and CO2 co‐feeding in order to assess eventual structural changes induced in the catalyst by the presence of CO2 in the feed. Global significance was achieved in both regressions and experimental trends were successfully reproduced by the specifically determined catalyst descriptors. CO2 co‐feeding is deemed responsible for generating a new active phase, for example, by converting metal oxides into (oxy‐)carbonates, among others, resulting in a decrease in active site density (D16) from 10 × 10−5 mol/m2 to 7 × 10−5 mol/m2. In the presence of the CO2‐induced phase, the catalyst exhibits higher attraction for unsaturated hydrocarbons as indicated by the higher initial sticking probabilities of CH3• (D11) and C2H4 (D15), which increase from 4.9 × 10−4 to 8 × 10−2 and from 2.1 × 10−2 to 3 × 10−2, respectively. Additionally, there are also lower the overall energy barriers for the activation of hydrocarbons on the catalyst, stemming from the decrease in the H abstraction enthalpy from CH4 (D1) from 14 to 6 kJ/mol. The operating conditions, in particular the O2 content, are critical in distinguishing the effect of CO2 co‐feeding. While at typical operating conditions, CO2 promotes the total oxidation of methane, in the prerequisite of reduced amount of O2, it may also act as an additional oxygen donor. This work provides molecular details on the CO2 induced changes in catalyst properties but also provides unprecedent quantified insights of the reaction mechanism underlying experimental observations.

Unraveling the influence of CO2 and Ce loading over ZSM-5 for oxidative dehydrogenation of propane

Utilization of CO2 as a soft oxidant for oxidative dehydrogenation of propane to propene (ODHP) is of great importance to attain net-zero carbon emissions and to reduce greenhouse gases contributing to global warming. Here, Ce was impregnated into ZSM-5 (denoted as CZ-5 catalyst) to enhance the C3H8 conversion and C3H6 selectivity due to the redox properties of Ce to activate CO2 into CO and O* and induce propane bond cleavage (C-H). After characterization of the catalyst’s physicochemical properties, NH3-TPD and pyridine IR analysis revealed that introducing Ce regulates the acidic properties of ZSM-5 catalysts by increasing the number of Lewis acid sites while simultaneously decreasing the amount of Brönsted acid sites. The increment in Lewis’s acid sites considerably improved the C3H6 selectivity. The oxygen transfer from Ce4+ to Ce3+ during the reaction also contributed to the outstanding performance of the 12 %CZ-5 catalyst. Regarding optimum conditions, 2 % CO2 concentration and 12 % Ce loading were the optimum amount of oxidant and Ce content that produce the highest C3H6 yield of 34.08 %. The catalytic effectiveness and stability were preserved despite several hours of propane conversions at 550 °C. Therefore, modifying ZSM-5 with Ce could be a feasible way for improving propene selectivity and catalyst stability for ODHP.

Cobalt catalyzed ethane dehydrogenation to ethylene with CO2: Relationships between cobalt species and reaction pathways

TiO2, ZrO2 and a series of TiO2-ZrO2 (TxZ1, x means the atomic ratio of Ti/Zr = 10, 5, 1, 0.2 and 0.1) composite oxide supports were prepared through co-precipitation, and then 3 wt% Co was loaded through wetness impregnation methods. The obtained 3 wt% Co/TiO2 (3CT), 3 wt% Co/ZrO2 (3CZ) and 3 wt% Co/TxZ1 (3CTxZ1) catalysts were evaluated for the oxidative ethane dehydrogenation reaction with CO2 (CO2-ODHE) as a soft oxidant. 3CT1Z1 catalyst exhibits excellent catalytic properties, with C2H4 yield, C2H6 conversion and CO2 conversion about 24.5 %, 33.8 % and 18.0 % at 650 °C, respectively. X-Ray Diffraction (XRD), in-situ Raman, UV–vis diffuse reflectance spectra (UV–vis DRS), H2 temperature-programmed reduction (H2-TPR), Electron paramagnetic resonance (EPR) and quasi in-situ X-ray Photoelectron Spectroscopy (XPS) have been utilized to thoroughly characterize the investigated catalysts. The results revealed that 3CT1Z1 produced TiZrO4 solid solution with more metal defect sites and oxygen vacancies (Ov), promoting the formation of Co2+-TiZrO4 structure. Furthermore, the presence of Ov and Ti3+can facilitate the high dispersion and stabilization of Co2+, as well as suppressing the severe reduction of Co2+, leading to superior ethane oxidative dehydrogenation activity. Besides, less Co0 is beneficial to ODHE reaction, because of its promotion effects for reverse water gas shift reaction; however, more Co0 results in dry reforming reaction (DRE). This work will shed new lights for the design and preparation of highly efficient catalysts for ethylene production.

Photocatalytic Oxidative Coupling of Ethane to n-Butane Using CO2 as a Soft Oxidant over NiTi-Layered Double Hydroxide.

Selective conversion of ethane (C2 H6 ) to high-value-added chemicals is a very important chemical process, yet it remains challenging owing to the difficulty of ethane activation. Here, a NiTi-layered double hydroxide (NiTi-LDH) photocatalyst is reported for oxidative coupling of ethane to n-butane (n-C4 H10 ) by using CO2 as an oxidant. Remarkably, the as-prepared NiTi-LDH exhibits a high selectivity for n-C4 H10 (92.35%) with a production rate of 62.06 µmol g-1 h-1 when the feed gas (CO2 /C2 H6 ) ratio is 2:8. The X-ray absorption fine structure (XAFS) and photoelectron characterizations demonstrate that NiTi-LDH possesses rich vacancies and high electron-hole separation efficiency, which can promote the coupling of C2 H6 to n-C4 H10 . More importantly, density functional theory (DFT) calculations reveal that ethane is first activated on the oxygen vacancies of the catalyst surface, and the C─C coupling pathway is more favorable than the C─H cleavage to C2 H4 or CH4 , resulting in the high production rate and selectivity for n-C4 H10 .

Ethane Oxidative Dehydrogenation over Pt-Rare Earth Intermetallic Compounds with CO2 as Soft Oxidant

Ethane Oxidative Dehydrogenation over Pt-Rare Earth Intermetallic Compounds with CO₂ as Soft Oxidant

Highly Selective Photocatalytic Methane Oxidation to Methanol Using CO2 as a Soft Oxidant

Light-driven methane conversion to methanol with high selectivity is considered a challenging and promising reaction for sustainable development. Herein, Cu/CeO2 catalysts were prepared for photocatalytic methane conversion with CO2 as a soft oxidant. Methanol was the sole liquid product with a high selectivity of 96.2% and a high yield of 88.9 μmol g–1 h–1 for the 2Cu/CeO2 catalyst. Investigation of the structure–performance relationship indicated that oxygen vacancies facilitated the trapping of photogenerated electrons and avoided carrier recombination. In addition, as a soft oxidant, CO2 can also capture photogenerated electrons to be reduced to CO and further accelerate the formation of CH3OH. A catalytic mechanism study demonstrated that methane and hydroxyl were oxidized by photogenerated holes to •CH3 and •OH, respectively, and thus, methanol was generated through the combination of •CH3 and •OH.

Mechanistic Interpretations and Insights for the Oxidative Dehydrogenation of Propane via CO2 over Cr2O3/Al2O3 Catalysts

Oxidative dehydrogenation (ODH) of alkanes using carbon dioxide as a soft oxidant has recently emerged as a potentially attractive alternative to steam cracking for the production of light olefins. To elucidate reaction pathways and their dependence on the operating conditions, CO2-assisted propane dehydrogenation over a redox-active Cr2O3/Al2O3 catalyst was examined in a packed bed reactor as a function of temperature, Cr2O3/CO2 feed ratio, and residence time. Previous ODH studies have largely focused on CO2-rich conditions with the aim of preventing coke formation. However, at T = 600 °C the present study finds that the use of propane-rich conditions (1 ≤ C3H8/CO2 ≤ 2.5) maximizes propylene production and selectivity while maintaining catalyst stability. It is postulated that the selective Mars van Krevelen dehydrogenation process is optimized at these ratios. Excess CO2 apparently promotes nonselective dehydrogenation and dry reforming pathways that generate additional CO, adversely impacting catalyst stability via the Bouduard reaction. This hypothesis is supported by complementary investigations of the reverse water gas shift reaction and thermodynamic analysis. The findings and methodology presented here are likely applicable to related ODH processes with other alkanes and redox-active catalysts.

CO2-Assisted Oxidative Dehydrogenation of Propane over VOx/In2O3 Catalysts: Interplay between Redox Property and Acid–Base Interactions

In this work, a series of VOx-loaded In2O3 catalysts were prepared, and their catalytic performance was evaluated for CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) and compared with In2O3 alone. The optimal composition is obtained on 3.4V/In2O3 (surface V density of 3.4V nm–2), which exhibited not only a higher C3H6 selectivity than other V/In catalysts and In2O3 under isoconversion conditions but also an improved reaction stability. To elucidate the catalyst structure–activity relationship, the VOx/In2O3 catalysts were characterized by chemisorption [NH3-temperature-programmed desorption (TPD), NH3-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), CO2-TPD, and CO2-DRIFTS], H2-temperature-programmed reduction (TPR), in situ Raman spectroscopy, UV–vis diffuse reflectance spectroscopy, near-ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and further examined using density functional theory. The In–O–V structure and the extent of oligomerization, which play a crucial role in improving selectivity and stability, were identified in the VOx/In2O3 catalysts. In particular, the presence of surface VOx (i) inhibits the deep reduction of In2O3, thereby preserving the activity, (ii) neutralizes the excess basicity on In2O3, thus suppressing propane dry reforming and achieving a higher propylene selectivity, and (iii) introduces additional redox sites that participate in the dehydrogenation reaction by utilizing CO2 as a soft oxidant. The present work provides insights into developing selective, stable, and robust metal-oxide catalysts for CO2-ODHP by controlling the conversion of reagents via desired pathways through the interplay between acid–base interactions and redox properties.

Catalytic oxidative dehydrogenation of ethane using carbon dioxide as a soft oxidant over Co‐HMS catalysts to ethylene

AbstractIn this work, a series of cobalt doped hexagonal mesoporous silica (Co‐HMS) with the different Co content were prepared by direct hydrothermal method and applied to oxidative dehydrogenation (ODH) reaction of ethane with carbon dioxide. Structural and chemical properties of the catalysts were characterized by nitrogen adsorption, X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), electron dispersive spectroscopy (EDS), X‐ray photoelectron spectroscopy (XPS), and Raman and ultraviolet–visible (UV‐vis) spectroscopy. These characterizations showed that cobalt not only entered the framework of Co‐HMS catalysts but also was evenly dispersed in the bulk. The valence state and coordination mode of cobalt played an important role in the whole ODH reaction system of ethane with carbon dioxide. 2% Co‐HMS with dominant Co2+ had the highest conversion of ethane (26.0%). With the different cobalt content in the catalyst, the proportion of Co2+(Td) and Co2+(Oh) is also different. 1% Co‐HMS with more Co2+(Td) performed the highest ethylene selectivity of 86.7%, which was stable during the 10 h reaction time.

Modification of Al2O3 by activated carbon-supported vanadia catalyst studies for ODH of ethylbenzene in presence of CO2 as a soft oxidant

Modification of Al2O3 by activated carbon-supported vanadia catalyst studies for ODH of ethylbenzene in presence of CO2 as a soft oxidant

N2O Utilization as a Soft Oxidant for the Catalytic Synthesis of Styrene from Ethylbenzene over Ce–Co/CNTs Catalyst

N2O Utilization as a Soft Oxidant for the Catalytic Synthesis of Styrene from Ethylbenzene over Ce–Co/CNTs Catalyst

Research progress of CO2 oxidative dehydrogenation of propane to propylene over Cr-free metal catalysts.

CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) is an attractive strategy to offset the demand gap of propylene due to its potentiality of reducing CO2 emissions, especially under the demands of peaking CO2 emissions and carbon neutrality. The introduction of CO2 as a soft oxidant into the reaction not only averts the over-oxidation of products, but also maintains the high oxidation state of the redox-active sites. Furthermore, the presence of CO2 increases the conversion of propane by coupling the dehydrogenation of propane (DHP) with the reverse water gas reaction (RWGS) and inhibits the coking formation to prolong the lifetime of catalysts via the reverse Boudouard reaction. An effective catalyst should selectively activate the C–H bond but suppress the C–C cleavage. However, to prepare such a catalyst remains challenging. Chromium-based catalysts are always applied in industrial application of DHP; however, their toxic properties are harmful to the environment. In this aspect, exploring environment-friendly and sustainable catalytic systems with Cr-free is an important issue. In this review, we outline the development of the CO2-ODHP especially in the last ten years, including the structural information, catalytic performances, and mechanisms of chromium-free metal-based catalyst systems, and the role of CO2 in the reaction. We also present perspectives for future progress in the CO2-ODHP.Graphical abstract

Support effect in Cu‐based catalysts for vapor phase dehydrogenation of 1‐decanol to decanal using CO 2 as a soft oxidant

Support effect in Cu‐based catalysts for vapor phase dehydrogenation of 1‐decanol to decanal using <scp> CO ₂ </scp> as a soft oxidant

A critical evaluation of the catalytic role of CO2 in propane dehydrogenation catalyzed by chromium oxide from a DFT-based microkinetic simulation.

Propane dehydrogenation under CO2 is an important catalytic route to obtain propene with a good balance between selectivity and stability. However, a precise description of the catalytic role of CO2 in propane dehydrogenation is still absent. In this work, we focus on the elucidation of the role of CO2 by using DFT-based microkinetic simulation. The influence of CO2 is categorized as direct and indirect effects. It was found that the chemisorbed CO2 can directly abstract hydrogen from propane and propyl with a comparable barrier to the counterpart at the surface oxygen site. On the other hand, the dissociation of CO2 yields active surface species of CO* and O* which are actively involved in the removal of surface hydroxyls. It is found that the TOFs of both propane conversion and propene formation are significantly increased with the presence of CO2, which is explained by the reduced apparent activation energy. The primary hydrogen abstraction is identified to be the most influential step from the DRC analysis. The main effects of CO2 are concluded to be removing hydrogen and restoring oxygen vacancies from reaction pathway analysis.

Dual photocatalysis for CO2 reduction along with the oxidative coupling of benzylamines promoted by Cu/Cu2O@g-C3N4 under visible irradiation

A novel twin functional photocatalytic approach in which CO2 acted as a soft oxidant and facilitated oxidation of amines to imines along with the simultaneous reduction to methanol is described.

Ag nanoparticles immobilized over highly porous crystalline organosilica for epoxidation of styrene using CO2 as oxidant

Selective epoxidation of olefins is a very important reaction as epoxides are widely been used as platform chemical in polymer and pharmaceutical industries. Unlike conventional oxidants like molecular O2 or peroxides, the use of CO2 as a soft oxidant for the oxidation of olefins is very challenging as it offers the utilization of waste and plentiful CO2 together with the potential for the mitigation of its harmful environmental effect. Thus, this process is cost effective and environmentally challenging. Herein, we report the synthesis of a new crystalline porous organosilica material TSOS-1 with orthorhombic crystal structure by using a tailor made bridging organoslilane precursor prepared through the Schiff base condensation of p-terphenyl-4,4′'-dialdehyde and 3-aminopropyl-trimethoxysilane. This novel crystalline material TSOS-1 has been synthesized hydrothermally in the absence of any structure-directing agent and it showed high BET surface area (220 m2 g−1) and nanoscale porosity. TSOS-1 is used as a support for stabilizing tiny AgNPs to obtain a robust nanocatalyst Ag@TSOS-1, which efficiently catalyses the conversion of styrene into predominantly styrene oxide (SO) using CO2 as a soft oxidant in an autoclave reactor under mild reaction conditions.

Recent progress in Cr-based catalysts for oxidative dehydrogenation of light alkanes by employing CO2 as a soft oxidant

Abstract CO2 can be used as a soft oxidant for oxidative dehydrogenation of light alkanes (CO2-ODH), which is beneficial to realize the reuse of CO2 and meet the demand for olefins. The core of this reaction is the catalyst. Cr-based catalysts have attracted much attention for their excellent catalytic performance in CO2-ODH reactions due to their various oxidation states and local electronic structures. In this paper, the synthesis and modification methods of Cr-based catalysts for CO2-ODH are reviewed. The structure–activity relationship and reaction mechanism are also summarized. Moreover, the reasons for the deactivation of Cr-based catalysts are analysed and the main challenges faced by Cr-based catalysts in the CO2-ODH process, as well as the future development trend and prospect, are discussed.

Supported Vanadium Catalysts for Selective Sulfur‐Oxidative Dehydrogenation of Propane

AbstractThe catalytic oxidative dehydrogenation of propane (ODHP) is a challenging reaction due to facile competing overoxidation to COx. The gaseous disulfur molecule, S2, is isoelectronic with O2 and has been shown to act as an alternative, “soft oxidant” for the analogous process (SODHP) over bulk metal sulfide catalysts. However, these bulk catalysts suffer from low surface areas and ill‐defined active sites – issues that might be addressed with a supported catalyst. Here we investigate supported V/Al2O3 materials for SODHP. We show that these catalysts are highly selective for propylene, far surpassing the yields of the prior bulk systems. Isolated sulfided vanadium species are found to be more active and selective than crystalline vanadium sulfide. Additionally, we compare the S2 and O2 oxidants over sulfided and calcined V/Al2O3 materials, respectively, and find that the propylene selectivity is enhanced using S2 as the oxidant. These results suggest that sulfur is a promising soft oxidant that can be used to achieve high propylene selectivities over supported metal sulfides.

Promoting propane dehydrogenation with CO2 over Ga2O3/SiO2 by eliminating Ga-hydrides

Due to the shortage supply of propylene and the development of shale gas, there is increased interest in on-purpose propane dehydrogenation (PDH) technology for propylene production. Ga-based catalysts have great potential in PDH, due to the high activity, low carbon deposit and deactivation. Ga-hydrides formed during PDH reduce the rate, selectivity and yield of propylene. In this contribution, CO2 is introduced into PDH as a soft oxidant to eliminate the unfavorable intermediate species Gaδ+-Hx re-generating Ga3+-O pairs, and also minimize coke deposition thereby improving the catalytic performance. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy experiments show that CO2 can effectively eliminate Gaδ+-Hx. At different temperatures, co-feeding CO2 during PDH over Ga2O3/SiO2 catalysts with different loadings significantly improves the stability of the conversion and selectivity, especially the latter, and provide a new dimension for improving the performance of PDH process.

Methane oxidation to methanol over copper-containing zeolite

Methane oxidation to methanol over copper-containing zeolite