Showing papers on "Dehydrogenation published in 2018"

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

Abstract: Catalytic hydrogenation and dehydrogenation reactions form the core of the modern chemical industry. This vast class of reactions is found in any part of chemical synthesis starting from the milligram-scale exploratory organic chemistry to the multi-ton base chemicals production. Noble metal catalysis has long been the key driving force in enabling these transformations with carbonyl substrates and their nitrogen-containing counterparts. This review is aimed at introducing the reader to the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts

Abstract: The sustainable use of the resources on our planet is essential. Noble metals are very rare and are diversely used in key technologies, such as catalysis. Manganese is the third most abundant transition metal of the Earth's crust and based on the recently discovered impressive reactivity in hydrogenation and dehydrogenation reactions, is a potentially useful noble-metal "replacement". The hope of novel selectivity profiles, not possible with noble metals, is also an aim of such a "replacement". The reactivity of manganese complexes in (de)hydrogenation reactions was demonstrated for the first time in 2016. Herein, we summarize the work that has been published since then and especially discuss the importance of homogeneous manganese catalysts in comparison to cobalt and iron catalysts.

Breaking the scaling relationship via thermally stable Pt/Cu single atom alloys for catalytic dehydrogenation.

Abstract: Noble-metal alloys are widely used as heterogeneous catalysts. However, due to the existence of scaling properties of adsorption energies on transition metal surfaces, the enhancement of catalytic activity is frequently accompanied by side reactions leading to a reduction in selectivity for the target product. Herein, we describe an approach to breaking the scaling relationship for propane dehydrogenation, an industrially important reaction, by assembling single atom alloys (SAAs), to achieve simultaneous enhancement of propylene selectivity and propane conversion. We synthesize γ-alumina-supported platinum/copper SAA catalysts by incipient wetness co-impregnation method with a high copper to platinum ratio. Single platinum atoms dispersed on copper nanoparticles dramatically enhance the desorption of surface-bounded propylene and prohibit its further dehydrogenation, resulting in high propylene selectivity (~90%). Unlike previous reported SAA applications at low temperatures (<400 °C), Pt/Cu SAA shows excellent stability of more than 120 h of operation under atmospheric pressure at 520 °C.

Oxidative dehydrogenation of propane to propylene with carbon dioxide

Abstract: Oxidative dehydrogenation of propane in the presence of carbon dioxide (ODPC) is a sustainable approach and an attractive catalytic route for propylene production with less environmental footprint than the conventional oxidative dehydrogenation path with oxygen. Researchers have considered CO2 as a mild oxidant that can overcome the problems of over-oxidation and low propylene selectivity, that are typically associated with the current synthesis routes. This article provides a critical review on the current mechanistic understanding of three different catalyst types used in the ODPC reaction based on experimental studies; (i) zeolites with different frameworks, (ii) porous materials-supported metal oxides, and (iii) transition metal oxides and other metal catalysts. A detailed review of the literature compares the framework, pore structure, nature of active sites, reducibility, and the role of promoters and supports for each type of catalytic materials in the absence and presence of CO2, and is followed by a thorough discussion on the promotional effects of CO2 as a soft oxidant on C H bond scission. Future directions with respect to materials design, synthesis and reaction conditions are also discussed.

Isoelectronic Manganese and Iron Hydrogenation/Dehydrogenation Catalysts: Similarities and Divergences

Abstract: ConspectusSustainable processes that utilize nontoxic, readily available, and inexpensive starting materials for organic synthesis constitute a major objective in modern chemical research. In this context, it is highly important to perform reactions under catalytic conditions and to replace precious metal catalysts by earth-abundant nonprecious metal catalysts. In particular, iron and manganese are promising candidates, as these are among the most abundant metals in the earth’s crust, are inexpensive, and exhibit a low environmental impact. As far as chemical processes are concerned, hydrogenations and acceptorless alcohol dehydrogenation (AAD), sometimes in conjunction with hydrogen autotransfer reactions, are becoming important areas of research. While the first is a very important synthetic process representing a highly atom-efficient and clean methodology, AAD is an oxidant-free, environmentally benign reaction where carbonyl compounds together with dihydrogen as a valuable product and/or reactant (au...

Lewis-Brønsted Acid Pairs in Ga/H-ZSM-5 To Catalyze Dehydrogenation of Light Alkanes.

Abstract: The active sites for propane dehydrogenation in Ga/H-ZSM-5 with moderate concentrations of tetrahedral aluminum in the lattice were identified to be Lewis–Bronsted acid pairs. With increasing availability, Ga+ and Bronsted acid site concentrations changed inversely, as protons of Bronsted acid sites were exchanged with Ga+. At a Ga/Al ratio of 1/2, the rate of propane dehydrogenation was 2 orders of magnitude higher than with the parent H-ZSM-5, highlighting the extraordinary activity of the Lewis–Bronsted acid pairs. Density functional theory calculations relate the high activity to a bifunctional mechanism that proceeds via heterolytic activation of the propane C–H bond followed by a monomolecular elimination of H2 and desorption of propene.

Recent Advances in Catalysis with Transition‐Metal Pincer Compounds

Abstract: Pincer complexes are useful tools for organic synthesis. Their high stability and easy functionalization have allowed the development of novel catalytic systems that have had a tremendous impact in different areas of chemistry. Thus, catalytic reactions are nowadays a fundamental part of several synthetic routes, as they allow “greener” procedures with high atom efficiency. In this context, pincer complexes have contributed to the establishment of novel and efficient catalytic reactions. Thus, herein we summarize the most recent relevant advances involving pincer complexes as catalysts.

High-Purity Hydrogen Generation via Dehydrogenation of Organic Carriers: A Review on the Catalytic Process

Abstract: High-purity hydrogen delivery for stationary and mobile applications using fuel cells is a subject of rapidly growing interest. As a consequence, the development of efficient storage technologies and processes for hydrogen supply is of primary importance. Promising hydrogen storage techniques rely on the reversibility and high selectivity of liquid organic hydrogen carriers (LOHCs), for example, methylcyclohexane, decalin, dibenzyltoluene, or dodecahydrocabazole. LOCHs have high gravimetric and volumetric hydrogen density, and they involve low risk and capital investment because they are largely compatible with the current transport infrastructure used for fossil fuels. A further advantage comes from the high purity (close to 100%) of the hydrogen generated by dehydrogenation, suitable to directly feed fuel cells without the need for bulky purification modules. Partial dehydrogenation (PDH) of liquid fuels has recently emerged as a transition technology for hydrogen delivery purposes. The principle is to ...

Ordered Porous Nitrogen-Doped Carbon Matrix with Atomically Dispersed Cobalt Sites as an Efficient Catalyst for Dehydrogenation and Transfer Hydrogenation of N-Heterocycles.

Abstract: Single-atom catalysts (SACs) have been explored widely as potential substitutes for homogeneous catalysts. Isolated cobalt single-atom sites were stabilized on an ordered porous nitrogen-doped carbon matrix (ISAS-Co/OPNC). ISAS-Co/OPNC is a highly efficient catalyst for acceptorless dehydrogenation of N-heterocycles to release H2 . ISAS-Co/OPNC also exhibits excellent catalytic activity for the reverse transfer hydrogenation (or hydrogenation) of N-heterocycles to store H2 , using formic acid or external hydrogen as a hydrogen source. The catalytic performance of ISAS-Co/OPNC in both reactions surpasses previously reported homogeneous and heterogeneous precious-metal catalysts. The reaction mechanisms are systematically investigated using first-principles calculations and it is suggested that the Eley-Rideal mechanism is dominant.

Metal-Free Dehydrogenation of N-Heterocycles by Ternary h-BCN Nanosheets with Visible Light

Abstract: An efficient metal-free catalytic system has been developed using hexagon boron carbon nitride (h-BCN) nanosheets for the dehydrogenations of N-heterocycles with visible light, while releasing hydrogen gas (H2), and thus no proton acceptor is needed. This acceptorless dehydrogenation of hydroquinolines, hydroisoquinolines or indolines to the corresponding aromatic N-heterocycles has been achieved with excellent yield by visible light irradiation under the ambient temperature. With h-BCN as the photocatalyst and water as the solvent, this protocol shows broad substitution tolerance, environment benign and high efficiency. This metal-free photoredox catalytic system for organic synthesis expands our knowledge of chemical reactivity and enables new environmentally friendly synthetic protocols.

Photocatalytic Hydrogen Peroxide Production by Anthraquinone-Augmented Polymeric Carbon Nitride

Abstract: We describe the exploitation of the selective catalytic property of anthraquinone (AQ) for solar photocatalytic synthesis of hydrogen peroxide (H2O2) as a green, sustainable alternative to organic-solvent-based and energy-intensive industry-benchmark processes that also rely on AQ catalysis. We accomplished this by anchoring AQ onto polymeric carbon nitride (C3N4), a metal-free visible light photocatalyst (band gap energy = 2.7 eV), that has been previously demonstrated for selective H2O2 synthesis. A net H2O2 production rate of 361 μmol g−1 h−1 and an apparent quantum yield (AQY) of 19.5% at 380 nm excitation were achieved using AQ-augmented C3N4 under simulated 1-sun illumination in the presence of an organic electron donor (2-propanol); these results were 4.4-fold and 8.3-fold higher than those reported for bare C3N4, respectively. A suite of experimental analyses confirmed the unique roles of AQ co-catalysis in (i) capturing electrons from the conduction band of C3N4, thereby reducing futile exciton recombination, which is otherwise prevalent in bare C3N4; (ii) effectively mediating electron transfer to drive hydrogenation reaction to form anthrahydroquinone (AQH2) from AQ; and (iii) catalyzing oxygen reduction to H2O2 through the dehydrogenation of AQH2 back to AQ, resulting in the facile and selective formation of H2O2. In addition, the reduced decomposition of produced H2O2 by the C3N4/AQ composite photocatalysts, when compared to bare C3N4 or C3N4 composited with common metallic co-catalysts such as Pt and Ag, was found to contribute to the significant enhancement in H2O2 production through the oxidation of both organic and water.

Anchoring and Upgrading Ultrafine NiPd on Room‐Temperature‐Synthesized Bifunctional NH2‐N‐rGO toward Low‐Cost and Highly Efficient Catalysts for Selective Formic Acid Dehydrogenation

Abstract: Hydrogen is widely considered to be a sustainable and clean energy alternative to the use of fossil fuels in the future. Its high hydrogen content, nontoxicity, and liquid state at room temperature make formic acid a promising hydrogen carrier. Designing highly efficient and low-cost heterogeneous catalysts is a major challenge for realizing the practical application of formic acid in the fuel-cell-based hydrogen economy. Herein, a simple but effective and rapid strategy is proposed, which demonstrates the synthesis of NiPd bimetallic ultrafine particles (UPs) supported on NH2 -functionalized and N-doped reduced graphene oxide (NH2 -N-rGO) at room temperature. The introduction of the NH2 N group to rGO is the key reason for the formation of the ultrafine and well-dispersed Ni0.4 Pd0.6 UPs (1.8 nm) with relatively large surface area and more active sites. Surprisingly, the as-prepared low-cost NiPd/NH2 -N-rGO dsiplays excellent hydrophilicity, 100% H2 selectivity, 100% conversion, and remarkable catalytic activity (up to 954.3 mol H2 (mol catalyst)-1 h-1 ) for FA decomposition at room temperature even with no additive, which is much higher than that of the best catalysts so far reported.

Electrochemical Oxidative C-H Amination of Phenols: Access to Triarylamine Derivatives.

Abstract: Dehydrogenative C-H/N-H cross-coupling serves as one of the most straightforward and atom-economical approaches for C-N bond formation. In this work, an electrochemical reaction protocol has been developed for the oxidative C-H amination of unprotected phenols under undivided electrolytic conditions. Neither metal catalysts nor chemical oxidants are needed to facilitate the dehydrogenation process. A series of triarylamine derivatives could be obtained with good functional-group tolerance. The electrolysis is scalable and can be performed at ambient conditions.

Fast Dehydrogenation of Formic Acid over Palladium Nanoparticles Immobilized in Nitrogen-Doped Hierarchically Porous Carbon

Abstract: In this work, a hierarchically porous carbon was prepared from carbonization of a nitrogen-containing metal–organic framework, followed by activation under ultrasonication in aqueous potassium hydroxide (aq KOH). The activated carbon was applied as a support for immobilizing ultrafine palladium (Pd) nanoparticles (1.1 ± 0.2 nm). As a result, the as-prepared Pd nanoparticles on N-doped porous carbon with both micro- and mesoporosity exhibit an excellent activity for the dehydrogenation of formic acid, showing a high turnover frequency (TOF, 14 400 h–1) at 60 °C. This activation approach of carbon opens an avenue for the syntheses of highly active supported ultrafine metal NPs for catalysis.

Highly Productive Propane Dehydrogenation Catalyst Using Silica-Supported Ga–Pt Nanoparticles Generated from Single-Sites

Abstract: The development of more effective alkane dehydrogenation catalysts is a key technological challenge for the production of olefins from shale gas, an abundant source of light hydrocarbons. Surface organometallic chemistry provides an original approach to generate nanometric Ga–Pt bimetallic particles supported on partially dehydroxylated silica containing gallium single-sites, which displays high activity, selectivity, and stability in propane dehydrogenation. This catalyst was prepared via sequential grafting of a platinum precursor onto silica possessing site-isolated gallium sites followed by H2 reduction. Monitoring generation of the reduced species, Gaδ+Pt0/SiO2, via in situ X-ray absorption spectroscopy reveals formation of a GaxPt (0.5 < x < 0.9) alloy with a fraction of gallium remaining as isolated sites. This bimetallic material exhibits catalytic performance that far surpasses each of the individual components and other reported Ga–Pt based catalysts; this is attributed to the highly dispersed G...

Hydroxyl-Mediated Non-oxidative Propane Dehydrogenation over VOx /γ-Al2 O3 Catalysts with Improved Stability.

Abstract: Supported vanadium oxides are one of the most promising alternative catalysts for propane dehydrogenation (PDH) and efforts have been made to improve its catalytic performance. However, unlike Pt-based catalysts, the nature of the active site and surface structure of the supported vanadium catalysts under reductive reaction conditions still remain elusive. This paper describes the surface structure and the important role of surface-bound hydroxyl groups on VOx / γ-Al2 O3 catalysts under reaction conditions employing in situ DRIFTS experiments and DFT calculations. It is shown that hydroxyl groups on the VOx /Al2 O3 catalyst (V-OH) are produced under H2 pre-reduction, and the catalytic performance for PDH is closely connected to the concentration of V-OH species on the catalyst. The hydroxyl groups are found to improve the catalyst that leads to better stability by suppressing the coke deposition.

Catalytic amino acid production from biomass-derived intermediates

Abstract: Amino acids are the building blocks for protein biosynthesis and find use in myriad industrial applications including in food for humans, in animal feed, and as precursors for bio-based plastics, among others. However, the development of efficient chemical methods to convert abundant and renewable feedstocks into amino acids has been largely unsuccessful to date. To that end, here we report a heterogeneous catalyst that directly transforms lignocellulosic biomass-derived α-hydroxyl acids into α-amino acids, including alanine, leucine, valine, aspartic acid, and phenylalanine in high yields. The reaction follows a dehydrogenation-reductive amination pathway, with dehydrogenation as the rate-determining step. Ruthenium nanoparticles supported on carbon nanotubes (Ru/CNT) exhibit exceptional efficiency compared with catalysts based on other metals, due to the unique, reversible enhancement effect of NH3 on Ru in dehydrogenation. Based on the catalytic system, a two-step chemical process was designed to convert glucose into alanine in 43% yield, comparable with the well-established microbial cultivation process, and therefore, the present strategy enables a route for the production of amino acids from renewable feedstocks. Moreover, a conceptual process design employing membrane distillation to facilitate product purification is proposed and validated. Overall, this study offers a rapid and potentially more efficient chemical method to produce amino acids from woody biomass components.

Control of coordinatively unsaturated Zr sites in ZrO 2 for efficient C–H bond activation

Abstract: Due to the complexity of heterogeneous catalysts, identification of active sites and the ways for their experimental design are not inherently straightforward but important for tailored catalyst preparation. The present study reveals the active sites for efficient C–H bond activation in C1–C4 alkanes over ZrO2 free of any metals or metal oxides usually catalysing this reaction. Quantum chemical calculations suggest that two Zr cations located at an oxygen vacancy are responsible for the homolytic C–H bond dissociation. This pathway differs from that reported for other metal oxides used for alkane activation, where metal cation and neighbouring lattice oxygen form the active site. The concentration of anion vacancies in ZrO2 can be controlled through adjusting the crystallite size. Accordingly designed ZrO2 shows industrially relevant activity and durability in non-oxidative propane dehydrogenation and performs superior to state-of-the-art catalysts possessing Pt, CrOx, GaOx or VOx species. Identifying active sites and designing rationally heterogeneous catalysts are not inherently straightforward due to their complexity. Here, the authors reveal the nature of active sites for efficient C–H bond activation in C1-C4 alkanes over bare ZrO2 and provide fundamentals for controlling their concentration.

NiCu single atom alloys catalyze the CH bond activation in the selective non- oxidative ethanol dehydrogenation reaction

Abstract: NiCu single atom alloy (SAA) nanoparticles supported on silica are reported to catalyze the non-oxidative dehydrogenation of ethanol, selectively to acetaldehyde and hydrogen products by facilitating the C H bond cleavage. The activity and selectivity of the NiCu SAA catalysts were compared to monometallic copper and to PtCu and PdCu single atom alloys, in a flow reactor at moderate temperatures. In-situ DRIFTS showed that the silica support facilitates the O H bond cleavage of ethanol to form ethoxy intermediates over all the supported alloy catalysts. However, these remain unreactive up to 250 °C for the Cu/SiO2 monometallic nanoparticles, while in the NiCu SAA, acetaldehyde is formed at much lower temperatures, below 150 °C. In situ DRIFTS was also used to identify the C H activation step as the rate determining step of this reaction on all the copper catalysts we examined. The presence of atomically dispersed Ni in Cu significantly lowers the C H bond activation barrier, whereas Pt and Pd atoms were found less effective. This work provides direct evidence that the C H bond cleavage is the rate determining step in ethanol dehydrogenation over this type catalyst.

Photocatalytic Oxidative Dehydrogenation of Ethane Using CO2 as a Soft Oxidant over Pd/TiO2 Catalysts to C2H4 and Syngas

Abstract: Using CO2 as the soft oxidant for the oxidative dehydrogenation of ethane (ODE) is a potential advancement for ethylene production from ethane. However, the current ODE reaction is primarily operated at high temperatures (e.g., 873 K), and the development of alternative approaches for the ODE reaction under the mild conditions is still a challenge. Herein, we report a photocatalytic ODE using CO2 as the oxidant over Pd/TiO2 catalysts at room temperature. The presence of CO2 significantly promoted the production of C2H4 and syngas, and the 1% Pd/TiO2 catalyst exhibited a C2H4 production rate of 230.5 μmol/gcat·h and syngas of 282.6 μmol/gcat·h. Density functional theory (DFT) calculation verified that the intermediate energy level provided by Pd and the covalent bond in Pd–O stimulated the electron transfer, excitation, and separation. The photoinduced electron, hole, and isolate OH on the surface of TiO2 played essential roles during the whole process. In addition, the possible reaction pathways of photoc...

Facile synthesis of graphene-supported Ni-CeO x nanocomposites as highly efficient catalysts for hydrolytic dehydrogenation of ammonia borane

Abstract: Development of low-cost and high-performance catalysts for hydrogen generation via hydrolysis of ammonia borane (NH3BH3, AB) is a highly desirable pathway for future hydrogen utilization. In this work, Ni nanocatalysts doped with CeOx and supported on graphene (Ni-CeOx/graphene) were synthesized via a facile chemical reduction route and applied as robust catalysts for the hydrolysis of AB in aqueous solution at room temperature. The as-synthesized Ni-CeOx/graphene nanocomposites (NCs) exhibited excellent catalytic activity with a turnover frequency (TOF) as high as 68.2 min−1, which is 49-fold higher than that for a simple Ni nanoparticle catalyst and is among the highest values reported for non-noble metal catalysts in AB hydrolysis. The development of efficient and low-cost Ni-CeOx/graphene catalysts enhances the feasibility of using ammonia borane as a chemical hydrogen storage material, which may find application ina hydrogen fuel-cell based economy.

Synergistic effects in atomic-layer-deposited PtCox/CNTs catalysts enhancing hydrolytic dehydrogenation of ammonia borane

Abstract: Highly dispersed and narrowly size-distributed Pt-Co bimetallic nanoparticles supported on CNTs are fabricated by atomic layer deposition (ALD) of Pt followed by CoO ALD for hydrolytic dehydrogenation of ammonia borane. The controlled decoration of CoO significantly enhances the hydrogen evolution activity of the Pt-based catalysts due to the remarkable Pt-Co synergy, i.e., the TOF value is up to 675.1 molH2 molPt−1 min−1 for bimetallic PtCo20/CNTs catalyst, which is 1.8 times higher than that of monometallic Pt/CNTs catalyst. The underlying nature of the Pt-Co synergy is systematically investigated by combining multiple characterizations with kinetic and isotopic analyses. It is revealed that the decoration of CoO onto Pt provides unique Pt-Co interfaces and targeted Pt electronic properties, thereby exhibiting the lower activation energy and favorable O H bond cleavage being the rate-determining step for the hydrolysis reaction. The insights revealed here provide inspiration and guide for the rational design of other advanced nanocatalysts.

A Highly Stable Copper-Based Catalyst for Clarifying the Catalytic Roles of Cu0 and Cu+ Species in Methanol Dehydrogenation.

Abstract: Identification of the active copper species, and further illustration of the catalytic mechanism of Cu-based catalysts is still a challenge because of the mobility and evolution of Cu0 and Cu+ species in the reaction process. Thus, an unprecedentedly stable Cu-based catalyst was prepared by uniformly embedding Cu nanoparticles in a mesoporous silica shell allowing clarification of the catalytic roles of Cu0 and Cu+ in the dehydrogenation of methanol to methyl formate by combining isotope-labeling experiment, in situ spectroscopy, and DFT calculations. It is shown that Cu0 sites promote the cleavage of the O-H bond in methanol and of the C-H bond in the reaction intermediates CH3 O and H2 COOCH3 which is formed from CH3 O and HCHO, whereas Cu+ sites cause rapid decomposition of formaldehyde generated on the Cu0 sites into CO and H2 .