Dehydrogenation Of Light Alkanes Research Articles

Electronic interaction between Cr3+ ions in chromia-alumina catalysts for light alkane dehydrogenation

In this work chromia-alumina catalysts have been studied by the electron paramagnetic resonance (EPR) spectroscopy in the temperature range -120–200 °C. It was observed, that the activity of the catalysts in the dehydrogenation of n-butane and i-butane correlates with the amount of strongly interacting Cr3+ ions of the amorphous chromia phase. A disproportionate increase in the antiferromagnetic coupling energy ΔE between Cr3+ ions was shown in the concentration range 4.7–9.8%wt.Cr (from 210 to 420 cm−1) and 9.8–13.3%wt.Cr (from 420 to 450 cm−1). It was established, that an increase in the exchange interaction between Cr3+ ions is more favourable for the dehydrogenation of i-butane as compared to n-butane.

Theoretical and experimental investigations into light alkane dehydrogenation over chromium-containing catalyst

To avoid potential economic and technical risks in a hybrid C3/C4 alkane dehydrogenation plant, it is of great importance to better understand the similarities and differences of propane and isobutane dehydrogenation. However, few investigations on this issue have been conducted. To fulfill the research gap, a combined Density Functional Theory (DFT) and experimental investigation has been carried out in this work. Our findings indicate that, in the adsorption phase, both alkanes are apt to adsorb onto the tetra-coordinate Cr(4) site of the Cr2O3(0001) catalytic surface. But isobutane has a stronger adsorption capability than propane. In the dehydrogenation phase, both alkanes follow the stepwise C–H bond activation mechanism rather than the concerted mechanism. However, the activation energy required for isobutane dehydrogenation (i.e., 2.10 eV) is a bit lower than that for propane (i.e., 2.19 eV). The findings are also confirmed by experiments. It is believed that the present work discloses the similarities and differences of propane dehydrogenation (PDH) and isobutane dehydrogenation (BDH) on the same Cr-catalyst and could be used to guide the rational design, optimization and control of hybrid alkane dehydrogenation plants.

Catalytic Oxidative Dehydrogenation of Light Alkanes over Oxygen Functionalized Hexagonal Boron Nitride

AbstractThe catalytic activity of oxygen functionalized hexagonal boron nitride (h‐BN) with >B−O−O−B< and >B−O−B< active sites at the zigzag edges for oxidative dehydrogenation (ODH) of light alkanes, specifically ethane (C2H6), propane (C3H8), butane (C4H10), and isobutane (HC(CH3)3) is explored. It has been found that the reaction pathway involves two H atom transfer steps with small activation energies. We demonstrate that the synergy of two active sites, >B−O−O−B< and >B−O−B<, is crucial for the first and second H‐transfer, respectively. With the increase in molecular mass of the considered light alkanes, the ODH reaction temperature decreases. In the case of butane and isobutane, the ODH reaction occurs almost at the same temperature indicating that the reaction is independent of the shape of the isomer. The rate‐limiting nature of the first H‐transfer step is predicted. The charge redistribution during H‐transfers and localized oxygen atomic states in the conduction band are explored to suggest possible descriptors for the rational design of new catalysts. The universal action of the >B−O−O−B< and >B−O−B< active sites for ODH of the light alkanes paves the way for metal‐free BN‐based materials for future catalytic applications.

Sensitivity Analysis and Multi‐Objective Optimization of Oxidative Dehydrogenation of Propane in a Fixed‐bed Reactor over Vanadium/Graphene for Propylene Production

AbstractOxidative dehydrogenation of light alkanes is an attractive route for the production of olefins. This method does not need much energy, while common processes such as propane dehydrogenation and cracking require external energy resources and cause rapid catalyst deactivation. In this research, an isothermal one‐dimensional model of a fixed‐bed reactor was considered for oxidative dehydrogenation of propane over a V2O5/graphene catalyst. Operational parameter sensitivity analysis showed that high temperature (450–500 °C), high pressure (7–8 atm), a moderate molar propane to air feed ratio (about 0.7), and a low feed flow rate enhanced the propane conversion and propylene productivity. However, for the production of propylene as desirable product, low temperature, low pressure, a high molar ratio, and a high feed flow rate are preferred. Thus, to determine the exact interval of operational parameter values, a multi‐objective optimization was performed for the yields of propylene and COx, revealing a maximum propylene yield of ∼40 %. For safety reasons, a feed molar ratio of propane to air of > 0.6 was preferred, which is not in the flammable region.

Interplay of On- and Off-Surface Processes in the B2O3-Catalyzed Oxidative Dehydrogenation of Propane: A DFT Study

Metal-free boron-based catalytic systems are growing to be promising choices in the oxidative dehydrogenation (ODH) of light alkanes to olefins. However, the ambiguity in the understanding of the m...

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.

Nitrogen-Doped Graphene Monolith Catalysts for Oxidative Dehydrogenation of Propane.

It’s of paramount importance to develop renewable nanocarbon materials to replace conventional precious metal catalysts in alkane dehydrogenation reactions. Graphene-based materials with high surface area have great potential for light alkane dehydrogenation. However, the powder-like state of the graphene-based materials seriously limits their potential industrial applications. In the present work, a new synthetic route is designed to fabricate nitrogen-doped graphene-based monolith catalysts for oxidative dehydrogenation of propane. The synthetic strategy combines the hydrothermal-aerogel and the post thermo-treatment procedures with urea and graphene as precursors. The structural characterization and kinetic analysis show that the monolithic catalyst well maintains the structural advantages of graphene with relatively high surface area and excellent thermal stability. The homogeneous distributed nitrogen species can effectively improve the yield of propylene (5.3% vs. 1.9%) and lower the activation energy (62.6 kJ mol−1 vs. 80.1 kJ mol−1) in oxidative dehydrogenation of propane reaction comparing with un-doped graphene monolith. An optimized doping amount at 1:1 weight content of the graphene to urea precursors could exhibit the best catalytic performance. The present work paves the way for developing novel and efficient nitrogen-doped graphene monolithic catalysts for oxidative dehydrogenation reactions of propane.

Understanding the Unique Antioxidation Property of Boron-Based Catalysts during Oxidative Dehydrogenation of Alkanes.

Boron-based catalysts show excellent performance in oxidative dehydrogenation (ODH) of light alkanes to alkenes with high selectivity and extremely good antioxidation properties. However, the anti-deep-oxidation mechanism remains unclear. Herein, we chose h-BN and B2O3 as representative boron-based catalysts to investigate their reactions with two important intermediates in the light alkane ODH, Et· (evolving to ethene) and EtO· (evolving to ethene or COx), to elucidate the origin of the antioxidation of alkanes. The density functional theory calculations reveal that surface boron sites could eliminate alkoxy in their vicinity, resulting in exceptional inhibition of alkane deep-oxidation. The analysis of the electronic and geometric structures of key stationary points showed that the oxophilicity of B determined the low deep-oxidation of alkanes, and the homoleptic coordination of B with all three ligating atoms being O moderately enhanced its oxophilicity. This work represents a novel conceptual advance in the mechanistic understanding of alkane ODH.

Elucidation of active species and reaction mechanism of sulfide V-K/Al2O3 catalyst for isobutane dehydrogenation

Supported sulfide vanadium-based catalyst in dehydrogenation of light alkanes is becoming a promise to replace the commercial used Pt- and Cr-based catalysts, due to the lower cost and toxicity. Nonetheless, the nature of active sites and reaction mechanism for dehydrogenation of isobutane over sulfide vanadium-based catalysts are still ambiguous. In this study, a series of sulfide V-K/γ-Al2O3 catalysts were prepared, presenting diverse distribution of isolated, oligomeric and polymeric vanadium species on catalyst surface. The oligomeric species were demonstrated to be the active dehydrogenation sites, and the TOFoligo showed a closely correlation with the isobutane conversion. Besides, the dehydrogenation reaction mechanism was studied by in-situ DRIFTS. The results showed that isobutane would first dissociative adsorbed on catalyst surface, and the stable intermediates of vanadium-isobutenyl were detected. Furthermore, the deposited coke rather than sulfur loss was found to play the crucial role in deactivation. The vast majority of deposited coke existed as polyaromatic species and the other slight ones presented as aliphatic form. This study enriches the fundamental explorations in active sites and reaction mechanism of sulfide V-K/γ-Al2O3 catalysts during isobutane dehydrogenation, and provides an in-depth strategy to improve the activity and stability of vanadium-based dehydrogenation catalysts.

Ethane dehydrogenation over the single-atom alloy catalysts: Screening out the excellent catalyst with the dual descriptors

Single atom alloys (SAAs) catalysts become potential to provide excellent activity, selectivity, and stability toward the selective dehydrogenation of light alkanes by activating the desired C–H bond, however, the ideal metallic combination to best catalyze C2H6 dehydrogenation is unclear yet. In this study, the activity and selectivity of ethane dehydrogenation on fifteen types of SAA catalysts (Co, Ir, Ni, Pd and Pt doped-Cu, Ag and Au) were fully investigated using DFT calculations and microkinetic modeling, and compared with that on the widely reported Pd, Pt, Cr and Pt3Sn. The results show that the activity of C2H6 dehydrogenation to gas phase C2H4 on all considered SAA catalysts has a relationship with C2H4 desorption energy, the easier the desorption of C2H4 is, the lower the activity of C2H6 dehydrogenation to gas phase C2H4 is. Similarly, C2H4 selectivity has a relationship with C2H5 adsorption energy, the weaker C2H5 adsorption energy is, the higher C2H4 selectivity is. Essential reason was explained based on the analysis of electronic properties. Thus, the dual descriptors, C2H4 desorption energy and C2H5 adsorption energy, were proposed to evaluate the activity and selectivity of C2H6 dehydrogenation to gas phase C2H4, respectively. Among these SAA catalysts, the low cost NiCu catalyst with the best activity and selectivity toward gas phase C2H4 formation is screen out, which is superior to the noble metals Pd and Pt widely reported. This study is expected to provide a simple and valuable method to screen out high performance SAA catalysts in alkane dehydrogenation to alkene.

Stabilizing Oxygen Vacancies in ZrO2 by Ga2O3 Boosts the Direct Dehydrogenation of Light Alkanes

Stabilizing Oxygen Vacancies in ZrO₂ by Ga₂O₃ Boosts the Direct Dehydrogenation of Light Alkanes

Dynamics and Site Isolation: Keys to High Propane Dehydrogenation Performance of Silica-Supported PtGa Nanoparticles.

Nonoxidative dehydrogenation of light alkanes has seen a renewed interest in recent years. While PtGa systems appear among the most efficient catalyst for this reaction and are now implemented in production plants, the origin of the high catalytic performance in terms of activity, selectivity, and stability in PtGa-based catalysts is largely unknown. Here we use molecular modeling at the DFT level on three different models: (i) periodic surfaces, (ii) clusters using static calculations, and (iii) realistic size silica-supported nanoparticles (1 nm) using molecular dynamics and metadynamics. The combination of the models with experimental data (XAS, TEM) allowed the refinement of the structure of silica-supported PtGa nanoparticles synthesized via surface organometallic chemistry and provided a structure–activity relationship at the molecular level. Using this approach, the key interaction between Pt and Ga was evidenced and analyzed: the presence of Ga increases (i) the interaction between the oxide surface and the nanoparticles, which reduces sintering, (ii) the Pt site isolation, and (iii) the mobility of surface atoms which promotes the high activity, selectivity, and stability of this catalyst. Considering the complete system for modeling that includes the silica support as well as the dynamics of the PtGa nanoparticle is essential to understand the catalytic performances.

Electrospinning synthesis of porous boron-doped silica nanofibers for oxidative dehydrogenation of light alkanes

The discovery of the high activity and selectivity of boron-based catalysts for oxidative dehydrogenation (ODH) of alkanes to olefins has attracted significant attention in the exploration of a new method for the synthesis of highly active and selective catalysts. Herein, we describe the synthesis of porous boron-doped silica nanofibers (PBSNs) 100–150 nm in diameter by electrospinning and the study of their catalytic performance. The electrospinning synthesis of the catalyst ensures the uniform dispersion and stability of the boron species on the open silica fiber framework. The one-dimensional nanofibers with open pore structures not only prevented diffusion limitation but also guaranteed high catalytic activity at high weight hourly space velocity (WHSV) in the ODH of alkanes. Compared to other supported boron oxide catalysts, PBSN catalysts showed higher olefin selectivity and stability. The presence of Si−OH groups in silica-supported boron catalysts may cause low propylene selectivity during the ODH of propane. When the ODH conversion of ethane reached 44.3%, the selectivity and productivity of ethylene were 84% and 44.2% gcat−1 s−1, respectively. In the case of propane ODH, the conversion, selectivity of olefins, and productivity of propylene are 19.2%, 90%, and 76.6 μmol gcat−1 s−1, respectively. No significant variations in the conversion and product selectivity occurred during 20 h of operation at a high WHSV of 84.6 h−1. Transient analysis and kinetic experiments indicated that the activation of O2 was influenced by alkanes during the ODH reaction.

Stabilizing effect of α-Cr2O3 on highly active phases and catalytic performance of a chromium alumina catalyst in the process of isobutane dehydrogenation

• Stabilization of active phases of amorphous Cr 2 O 3 and polychromates by α-Cr 2 O 3 • Cr 2 O 3 -phases prevent agglomeration of active phase particles • Cr 2 O 3 -phases contribute of high catalytic performance The demand for C 3 -C 5 olefins is constantly growing, that is why it is important to improve the performance of catalysts for dehydrogenation of light alkanes. The study regards the influence of the supports porous system, the contribution of α-Cr 2 O 3 particles to the state of the active phase and the catalytic performance of the chromia-alumina catalysts in the isobutane dehydrogenation reaction. The catalysts were synthesized by impregnating the support with chromic acid. The supports and the catalysts were studied by the following techniques: low-temperature nitrogen adsorption, temperature-programmed desorption and reduction; UV-Vis- and Raman-spectroscopy, X-ray phase and X-ray fluorescence analyses. It was found that crystals of α-Cr 2 O 3 are formed on a support with 56 m 2 /g specific surface area and 7.1% (m/m) chromium content, that contributes to stabilization of the particles of highly active phases of amorphous Cr 2 O 3 and polychromates and catalytic performance. Meanwhile, no α-Cr 2 O 3 particles are formed on a support with a 103 m 2 /g specific surface area and 7.4% (m/m) chromium content. Therefore, in the course of 54 regeneration reactions cycles, the rates of isobutylene formation and isobutylene selectivity are significantly reduced due to agglomeration of amorphous Cr 2 O 3 particles, formation of di-, tri- chromates, and migration of part of chromium into the support structure.

Isobutane dehydrogenation over high-performanced sulfide V-K/γ-Al2O3 catalyst: Modulation of vanadium species and intrinsic effect of potassium

Compared with industrial used Pt- and Cr-based catalyst in dehydrogenation (DH) of light alkanes, the sulfide V-K/γ-Al2O3 catalyst reported in this study shows lower cost and toxicity, and significant DH performance. The yield to isobutene reached as high as 52.9%, which is among the highest reported to date. We attribute such high isobutene yield to the precise modulation of polymerization degree for vanadium species via doping of potassium and indicating that the synergy between vanadium species and acid sites is critical to enhance the DH performance. Our previous work showed sulfidation promoted the increase of DH performance for vanadium-based catalyst, and we go further in this study to explore the correlation between increased range of DH performance and the added potassium. The different loaded potassium leads to variation in sulfidation degree, affecting the properties of vanadium species and acid properties consequently. The potassium was distributed uniformly on surface of the sulfide vanadium-based catalyst and was predominantly bonded with the vanadium species rather than with the γ-Al2O3 support. With increasing the potassium amount from 0 to 3 wt%, the acid amount kept decreasing, and some specific strong acid sites appeared once adequate sulfur was introduced in the V-K/γ-Al2O3 catalyst. The characterization and DFT results both revealed that the doped potassium contributes to regulating the vanadium species in the oligomeric state. The synergy between vanadium species and acid properties was regulated by the added potassium simultaneously, and thus the DH performance was enhanced. This study provides promising strategy for preparation of environment-friendly model industrial DH catalyst.

Recent Advances on Gallium-Modified ZSM-5 for Conversion of Light Hydrocarbons.

Light olefins are key components of modern chemical industry and are feedstocks for the production of many commodity chemicals widely used in our daily life. It would be of great economic significance to convert light alkanes, produced during the refining of crude oil or extracted during the processing of natural gas selectively to value-added products, such as light alkenes, aromatic hydrocarbons, etc., through catalytic dehydrogenation. Among various catalysts developed, Ga-modified ZSM-5-based catalysts exhibit superior catalytic performance and stability in dehydrogenation of light alkanes. In this mini review, we summarize the progress on synthesis and application of Ga-modified ZSM-5 as catalysts in dehydrogenation of light alkanes to olefins, and the dehydroaromatization to aromatics in the past two decades, as well as the discussions on in-situ formation and evolution of reactive Ga species as catalytic centers and the reaction mechanisms.

Catalytic dehydrogenation of isobutane over supported MoOx/K-Al2O3

Potassium-promoted, alumina-supported molybdenum oxide is investigated for non-oxidative isobutane dehydrogenation, with an emphasis on sub-monolayer coverages. After pre-reduction in H2, initial isobutene turnover frequencies are largely independent of Mo loading, but deactivation rate constants increase by >100-fold as loadings increase to monolayer, leading to a >13-fold difference in reaction rates at extended time on stream. Mo oxidation state by in situ X-ray absorption spectroscopy is stable with time on stream, arguing against continued catalyst restructuring as the origin of deactivation. Across the set of loadings, isobutane dehydrogenation and coke formation are correlated with partially reduced Mo4+ and deeply reduced Moδ+ sites, respectively. Given the stable dehydrogenation activity of low-loaded Mo, in contrast to the rapid deactivation of high-loaded Mo, sub-monolayer metal oxides may warrant further investigation as light alkane dehydrogenation even when their bulk counterparts may not.

Identification of earth-abundant materials for selective dehydrogenation of light alkanes to olefins

Selective ethane dehydrogenation (EDH) is an attractive on-purpose strategy for industrial ethylene production. Design of an effective, stable, and earth-abundant catalyst to replace noble metal Pt is the main obstacle for its large-scale application. Herein, we report an experimentally validated theoretical framework to discover promising catalysts for EDH, which combines descriptor-based microkinetic modeling, high-throughput computations, machine-learning concepts, and experiments. Our approach efficiently evaluates 1,998 bimetallic alloys by using accurately calculated C and CH3 adsorption energies and identifies a small number of new promising noble-metal-free catalysts for selective EDH. A Ni3Mo alloy predicted to be promising is successfully synthesized, and experimentally proven to outperform Pt in selective ethylene production from EDH, representing an important contribution to the improvement of light alkane dehydrogenation to olefins. These results will provide essential additions in the discovery and application of earth-abundant materials in catalysis.

Catalytic Cracking of Light-naphtha Fraction over Zeolite-based Composites for On-purpose Propylene Production

Catalytic cracking of light-naphtha fraction over zeolite-based composites in fixed-bed mode was investigated to establish an efficient method for on-purpose propylene production. The composite catalysts, consisting of MFI-type zeolites containing iron, gallium and aluminum species (Fe–Ga–Al-MFI) and metal-oxide binder (e.g., silicon oxide, aluminum oxide), were employed for cracking of light-naphtha fraction. Fe–Ga–Al-MFI zeolites as matrix, containing each heteroatom at adequate ratio in the zeolite framework, exhibited both overall acid strength suitable for selective formation of light olefins and enhanced activity for dehydrogenation of light alkanes to alkenes, so that high overall yields of light olefins (ethylene, propylene and butenes) were attained by suppressing aromatics formation compared to cracking of light-naphtha fraction using conventional Al-MFI zeolite (ZSM-5). The unique acidity of the Fe–Ga–Al-MFI zeolite was maintained in the extruded form by using neutral and inactive silicon–oxide binder, which was selected to enhance mechanical strength and/or reduce pressure drop during reaction. The zeolite-based composite (Fe–Ga–Al-MFI/SiO2) selectively converted light-naphtha fraction (n-hexane) into light olefins including propylene with catalyst lifetime longer than 2000 h, suitable for fixed-bed operation, due to its excellent resistance to coke formation. Furthermore, the cracking reactions proceeded in the absence of steam at moderate temperatures below 650 °C, so catalytic cracking using the present zeolite-based composite saved considerable thermal energy required in the reaction unit, and the total amount of hydrocarbon feedstock was reduced by ca. 15 %, compared to conventional thermal cracking at 850 °C. The present review discusses the excellent properties of these zeolite-based catalysts and catalytic cracking of light-naphtha fraction emphasizing the catalytic chemistry and reaction engineering of the catalytic process.

Mechanism and Kinetics of Light Alkane Dehydrogenation and Cracking over Isolated Ga Species in Ga/H-MFI

Author(s): Phadke, NM; Mansoor, E; Head-Gordon, M; Bell, AT | Abstract: The objective of this study is to examine the mechanisms and kinetics of C2H6 dehydrogenation and n-C4H10 dehydrogenation and cracking over isolated Ga species in Ga/HMFI and to compare these results to those reported previously for C3H8 dehydrogenation and cracking. C2H6 dehydrogenation is found to be catalyzed by both [GaH]2+ and [GaH2]+ cations at similar turnover frequencies. Rate measurements over Ga/H-MFI containing predominantly [GaH2]+ cations reveal that C2H6 dehydrogenation rates exhibit a Langmuir.Hinshelwood dependence on C2H6 partial pressure at elevated temperatures (g730 K), consistent with the involvement of chemisorbed [C2H5.GaH]+ species. The reaction kinetics suggest that C2H6 dehydrogenation proceeds via heterolytic C-H cleavage of adsorbed C2H6 by [GaH2]+ cations to form H2 and [C2H5-GaH]+ species, which further decompose via β-hydride elimination to form C2H4. By contrast, C4H10 dehydrogenation and both terminal and central cracking are catalyzed exclusively by [GaH]2+ cations. All three reactions exhibit a Langmuir-Hinshelwood dependence on C4H10 partial pressure and are inhibited by H2. Ratios of dehydrogenation to cracking (total) and terminal to central cracking are approximately independent of C4H10 partial pressure consistent with the involvement of a common C4H10-derived surface intermediate. The observed reaction kinetics are consistent with an alkyl-mediated mechanism occurring over [GaH]2+, analogous to that reported previously for C3H8 dehydrogenation/cracking over Ga/H-MFI (Phadke, N. M.; et al. J. Am. Chem. Soc. 2019, 141, 1614-1627). The mechanism proceeds via facile, heterolytic dissociation of adsorbed C4H10 to form [C4H9-GaH]+-H+ cation pairs via methyl C-H-activated pathways. Dehydrogenation then proceeds via β-hydride elimination, respectively, forming butene, while terminal and central cracking proceed via C-H-activated H+ attack. Methylene activation was also considered but found to occur at a significantly lower rate. Theoretical analysis of the proposed reaction pathways leads to apparent activation enthalpies in good agreement with values extracted from the measured kinetics, thereby supporting the proposed pathways and the roles of [GaH]2+ and [GaH2]+ cations in the dehydrogenation and cracking of light alkanes on Ga/H-MFI.