Propane Dehydrogenation over Alumina-Supported Iron/Phosphorus Catalysts: Structural Evolution of Iron Species Leading to High Activity and Propylene Selectivity

Ferric Single-Site Catalyst Confined in a Zeolite Framework for Propane Dehydrogenation

Non-oxidative dehydrogenation of propane is a highly efficient approach for industrial preparation of propene that is commonly catalyzed by noble Pt or toxic Cr catalysts and suffers from coking. In this work, ferric catalyst confined in a zeolite framework was synthesized by a hydrothermal procedure. The isolated Fe in the framework formed distorted tetrahedra, which were beneficial for the selective dehydrogenation of propane and reached over 95 % propene selectivity and over 99 % total olefins selectivity. This catalyst had a silanol-free structure and was oxygen tolerant, hydrothermally stable, and coke free, with a deactivation constant of 0.01 h-1 . This study provided guidance for the synthesis of structural heteroatomic zeolite and efficient propane non-oxidative dehydrogenation over early transition metals.

Site Diversity and Mechanism of Metal-Exchanged Zeolite Catalyzed Non-Oxidative Propane Dehydrogenation

Metal-exchanged zeolites are well-known propane dehydrogenation (PDH) catalysts; however, the structure of the active species remains unresolved. In this review, existing PDH catalysts are first surveyed, and then the current understanding of metal-exchanged zeolite catalysts is described in detail. The case of Ga/H-ZSM-5 is employed to showcase that advances in the understanding of structure-activity relations are often accompanied by technological or conceptional breakthroughs. The understanding of Ga speciation at PDH conditions has evolved owing to the advent of in situ/operando characterizations and to the realization that the local coordination environment of Ga species afforded by the zeolite support has a decisive impact on the active site structure. In situ/operando quantitative characterization of catalysts, rigorous determination of intrinsic reaction rates, and predictive computational modeling are all significant in identifying the most active structure in these complex systems. The reaction mechanism could be both intricately related to and nearly independent of the details of the assumed active structure, as in the two main proposed PDH mechanisms on Ga/H-ZSM-5, that is, the carbenium mechanism and the alkyl mechanism. Perspectives on potential approaches to further elucidate the active structure of metal-exchanged zeolite catalysts and reaction mechanisms are discussed in the final section.

Integrating adsorption and in situ advanced oxidation for the treatment of organic wastewater by 3D carbon aerogel embedded with Fe-doped carbonitrides

Wastewater containing organic pollutants with high toxicity and poor biodegradability poses a considerable threat to human health and the ecosystem. Although adsorption and advanced oxidation processes (AOPs) are currently the most widely used technologies for wastewater treatment, limitations of these two independent processes make the treatment effect unsatisfactory. Herein, a system of integrating adsorption and subsequent in situ AOPs is established by a 3D carbon aerogel embedded with Fe-doped carbonitrides (Fe-NC/CAG). The SEM and BET analysis demonstrate that Fe-NC/CAG possesses porous structures with a specific surface area of 518.7 m²/g. The XRD result indicates the formation of Fe⁰ and Fe₃O₄ in Fe-NC/CAG. The impacts of operational parameters such as Fe-NC/CAG dosage, pollutants concentration, temperature, initial pH, and inorganic ions on the adsorption efficiency are investigated. The adsorption kinetics is predominantly based on the pseudo-second-order model. After adsorbing organic pollutants, the Fe-NC/CAG is immersed in peroxymonosulfate (PMS) solution. The adsorbed pollutants are in situ degraded by PMS-based AOPs, leading to the regeneration of Fe-NC/CAG. At optimum conditions, the integrating process established by Fe-NC/CAG achieves over 90% removal of antibiotics, phenolics, and dyes as well as keeps stable performance even after 6 cycles. This integrating adsorption and AOPs system is expected to open up a rich field for wastewater treatment.

Ga+-Chabazite Zeolite: A Highly Selective Catalyst for Nonoxidative Propane Dehydrogenation

Ga-chabazite zeolites (Ga-CHA) have been found to efficiently catalyze propane dehydrogenation with high propylene selectivity (96%). In situ Fourier transform infrared spectroscopy and pulse titrations are employed to determine that upon reduction, surface Ga₂O₃ is reduced and diffuses into the zeolite pores, displacing the Brønsted acid sites and forming extra-framework Ga⁺ sites. This isolated Ga⁺ site reacts reversibly with H₂ to form GaH_x (2034 cm^-1) with an enthalpy of formation of ∼-51.2 kJ·mol^-1, a result supported by density functional theory calculations. The initial C₃H₈ dehydrogenation rates decrease rapidly (40%) during the first 100 min and then decline slowly afterward, while the C₃H₆ selectivity is stable at ∼96%. The reduction in the reaction rate is correlated with the formation of polycyclic aromatics inside the zeolite (using UV-vis spectroscopy) indicating that the accumulation of polycyclic aromatics is the main cause of the deactivation. The carbon species formed can be easily oxidized at 600 °C with complete recovery of the PDH catalytic properties. The correlations between GaH_x vs Ga/Al ratio and PDH rates vs Ga/Al ratio show that extra-framework Ga⁺ is the active center catalyzing propane dehydrogenation. The higher reaction rate on Ga⁺ than In⁺ in CHA zeolites, by a factor of 43, is the result of differences in the stabilization of the transition state due to the higher stability of Ga³⁺ vs In³⁺. The uniformity of the Ga⁺ sites in this material makes it an excellent model for the molecular understanding of metal cation-exchanged hydrocarbon interactions in zeolites.

Propane dehydrogenation over extra-framework In(i) in chabazite zeolites

Indium on silica, alumina and zeolite chabazite (CHA), with a range of In/Al ratios and Si/Al ratios, have been investigated to understand the effect of the support on indium speciation and its corresponding influence on propane dehydrogenation (PDH). It is found that In2O3 is formed on the external surface of the zeolite crystal after the addition of In(NO3)3 to H-CHA by incipient wetness impregnation and calcination. Upon reduction in H2 gas (550 °C), indium displaces the proton in Brønsted acid sites (BASs), forming extra-framework In+ species (In-CHA). A stoichiometric ratio of 1.5 of formed H2O to consumed H2 during H2 pulsed reduction experiments confirms the indium oxidation state of +1. The reduced indium is different from the indium species observed on samples of 10In/SiO2, 10In/Al2O3 (i.e., 10 wt% indium) and bulk In2O3, in which In2O3 was reduced to In(0), as determined from the X-ray diffraction patterns of the product, H2 temperature-programmed reduction (H2-TPR) profiles, pulse reactor investigations and in situ transmission FTIR spectroscopy. The BASs in H-CHA facilitate the formation and stabilization of In+ cations in extra-framework positions, and prevent the deep reduction of In2O3 to In(0). In+ cations in the CHA zeolite can be oxidized with O2 to form indium oxide species and can be reduced again with H2 quantitatively. At comparable conversion, In-CHA shows better stability and C3H6 selectivity (∼85%) than In2O3, 10In/SiO2 and 10In/Al2O3, consistent with a low C3H8 dehydrogenation activation energy (94.3 kJ mol-1) and high C3H8 cracking activation energy (206 kJ mol-1) in the In-CHA catalyst. A high Si/Al ratio in CHA seems beneficial for PDH by decreasing the fraction of CHA cages containing multiple In+ cations. Other small-pore zeolite-stabilized metal cation sites could form highly stable and selective catalysts for this and facilitate other alkane dehydrogenation reactions.

Theoretical insights into non-oxidative propane dehydrogenation over Fe3C

Identifying catalysts for non-oxidative propane dehydrogenation has become increasingly important due to the increasing demand for propylene coupled to decreasing propylene production from steam cracking as we shift to lighter hydrocarbon feedstocks. Commercialized propane dehydrogenation (PDH) catalysts are based on Pt or Cr, which are expensive or toxic, respectively. Recent experimental work has demonstrated that earth-abundant and environmentally-benign metals, such as iron, form in situ carbide phases that exhibit good activity and high selectivity for PDH. In this work, we used density functional theory (DFT) to better understand why the PDH reaction is highly selective on Fe3C surfaces. We use ab initio thermodynamics to identify stable Fe3C surface terminations as a function of reaction conditions, which then serve as our models for investigating rate-determining and selectivity-determining kinetic barriers during PDH. We find that carbon-rich surfaces show much higher selectivity for propylene production over competing cracking reactions compared to iron-rich surfaces, which is determined by comparing the propylene desorption barrier to the C-H scission barrier for dehydrogenation steps beyond propylene. Electronic structure analyses of the d-band center and the crystal orbital Hamilton population (COHP) of the carbides demonstrate that the high selectivity of carbon-rich surfaces originates from the disruption of surface Fe ensembles via carbon. Finally, we investigated the role of phosphate in suppressing coke formation and found that the electron-withdrawing character of phosphate destabilizes surface carbon.

Propane dehydrogenation: catalyst development, new chemistry, and emerging technologies

This review describes recent advances in the propane dehydrogenation process in terms of emerging technologies, catalyst development and new chemistry.

Recent progress in heterogeneous metal and metal oxide catalysts for direct dehydrogenation of ethane and propane

Metal and metal oxide catalysts for non-oxidative ethane/propane dehydrogenation are outlined with respect to catalyst synthesis, structure–property relationship and catalytic mechanism.

Production of Propene from n-Butanol: A Three-Step Cascade Utilizing the Cytochrome P450 Fatty Acid Decarboxylase OleTJE

Propene is one of the most important starting materials in the chemical industry. Herein, we report an enzymatic cascade reaction for the biocatalytic production of propene starting from n-butanol, thus offering a biobased production from glucose. In order to create an efficient system, we faced the issue of an optimal cofactor supply for the fatty acid decarboxylase OleTJE , which is said to be driven by either NAD(P)H or H2 O2 . In the first system, we used an alcohol and aldehyde dehydrogenase coupled to OleTJE by the electron-transfer complex putidaredoxin reductase/putidaredoxin, allowing regeneration of the NAD+ cofactor. With the second system, we intended full oxidation of n-butanol to butyric acid, generating one equivalent of H2 O2 that can be used for the oxidative decarboxylation. As the optimal substrate is a long-chain fatty acid, we also tried to create an improved variant for the decarboxylation of butyric acid by using rational protein design. Within a mutational study with 57 designed mutants, we generated the mutant OleTV292I , which showed a 2.4-fold improvement in propene production in our H2 O2 -driven cascade system and reached total turnover numbers >1000.

Iron doping boosts the reactivity and stability of the γ-Al2O3 nanosheet supported cobalt catalyst for propane dehydrogenation

This study describes a new iron-doping strategy to improve both the reactivity and stability of a cobalt catalyst in propane dehydrogenation, meanwhile, the defective γ-Al₂O₃ nanosheet synergistically boosted the dehydrogenating activity of that.

Effects of phosphorus addition on selectivity and stability of Pd model catalysts during cyclohexene dehydrogenation

P addition to Pd increases dehydrogenation selectivity and increases catalyst stability.

Dehydrogenation of Propane to Propylene Using Promoter-Free Hierarchical Pt/Silicalite-1 Nanosheets

Propane dehydrogenation (PDH) is the extensive pathway to produce propylene, which is as a very important chemical building block for the chemical industry. Various catalysts have been developed to increase the propylene yield over recent decades; however, an active site of monometallic Pt nanoparticles prevents them from achieving this, due to the interferences of side-reactions. In this context, we describe the use of promoter-free hierarchical Pt/silicalite-1 nanosheets in the PDH application. The Pt dispersion on weakly acidic supports can be improved due to an increase in the metal-support interaction of ultra-small metal nanoparticles and silanol defect sites of hierarchical structures. This behavior leads to highly selective propylene production, with more than 95% of propylene selectivity, due to the complete suppression of the side catalytic cracking. Moreover, the oligomerization as a side reaction is prevented in the presence of hierarchical structures due to the shortening of the diffusion path length.

Dehydrogenation of Propane to Propylene with Highly Stable Catalysts of Pt-Sn Supported Over Mesoporous Silica KIT-6

During several reactions, similar to dehydrogenation of propane to propylene, coke is one of the main reasons for the catalyst deactivation. The coke formation and further deactivation of the catalyst are strongly dependent to the active site in the catalyst and/or the properties of the support. KIT-6 with interconnected porous and high surface area can handle with the coke formation, and can disperse easily the deposited Pt nanoparticles. In this sense, a series of Pt-Sn/KIT-6 catalysts were synthesized with distinct Sn loadings and used in the dehydrogenation of propane. The performance of these catalysts during reaction varied with the Sn loading. The specific activities for propylene formation obtained with the catalysts were comparable to the best result reported in the literature. The nanoparticles present in the catalyst through pretreatment and reaction condition was the Pt-Sn alloy (1:1 atomic ratio), and that alloy is suggested to be the active phase. This Pt-Sn alloy was stable during the entire reaction time, that even in two catalysts containing a considerable amount of coke, deactivation was not observed. Also, the support (KIT-6) with high connectivity helped to avoid deactivation by coke.

3D Interconnected Mesoporous Alumina with Loaded Hemoglobin as a Highly Active Electrochemical Biosensor for H2 O2

Alumina is one of the most common and stable metal oxides in nature, which has been developed as a novel adsorbent in enrichment of biomolecules due to its excellent affinity to phosphor or amino groups. In this study, ordered mesoporous alumina (OMA) with interconnected mesopores and surface acidic property is synthesized through a solvent evaporation induced co-assembly process using poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer as a template and aluminium acetylacetonate (Al(acac)₃ ) as the aluminium source. The pore size (12.1-19.7 nm), pore window size (3.5-9.0 nm) and surface acidity (0.092-0.165 mmol g^-1 ) can be precisely adjusted. The highly porous structure endows the OMA materials with high hemoglobin (Hb) immobilization capacity (170 mg g^-1 ). The obtained Hb@OMA composite is used as an electrocatalyst of biosensor for convienet and fast detection of hydrogen peroxide (H₂ O₂ ) with a low H₂ O₂ detection limit of 1.7 × 10^-8 m and a wide linear range of 2.5 × 10^-8 to 5.0 × 10^-5 m. Moreover, the Hb@OMA sensors show a good performance in real time detection of H₂ O₂ released from Homo sapiens bone osteosarcoma, indicating their potential application in complex biological processes.