Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Catalytic alkane dehydrogenation is a reaction with tremendous potential for application. We describe a highly active PSCOP‐pincer iridium catalyst for transfer dehydrogenation of cyclic and linear alkanes. The dehydrogenation of linear alkanes occurs under relatively mild conditions with high regioselectivity for α‐olefin formation. In addition, the catalyst system is very effective in the dehydrogenation of heterocycles to form heteroarenes and olefinic products.

Catalytic dehydrogenation is an effective route to convert light alkanes to monoolefins with the same carbon number and H2. In this paper the research progress in the reaction mechanism, catalyst and reactor for propane/butane dehydrogenation in recent years has been introduced. Catalytic dehydrogenation of alkanes is a strong endothermic reaction and its conversion for single pass is limited by the thermodynamic equilibrium. Both the reaction and the product separation consume a large amount of energy. Improving the conversion of alkanes for single pass by optimizing the operating conditions, on the basis of guaranteeing the high selectivity to olefins and the long-term safety and stability of the unit operation, is the key to reduce energy consumption for the whole process. During the dehydrogenation of alkanes, two H atoms bonded with two adjacent C atoms adsorb on the same active site, and the active site draws the two H atoms closer to attract each other, leading to the scission of C–H bonds to form H2 and olefins. The olefins enter to the gas phase directly without adsorption. The widely used supported Pt and CrO x catalysts have been introduced systematically, including the preparation methods, existing state of active components, carriers, additives, deactivation and regeneration, as well as problems encountered in application. At the same time, the newly reported catalysts, such as supported metal catalysts (Ni, NiSn and Sn), metal oxide catalysts (Ga2O3/ZnO and ZnO/Nb2O5), other mixed metal oxides and/or composite metal oxides catalysts, as well as metal sulfide catalysts have also been discussed briefly. Furthermore, the performances of packed-bed, moving-bed and circulating fluidized-bed reactors have been analyzed comparatively. The circulating fluidized-bed reactor is optimal due to its continuous reaction and catalyst regeneration and the efficient heat supplying for the endothermic dehydrogenation by high-temperature regenerated catalyst. The ADHO process, based on environment-friendly metal oxide catalyst coupled with cocurrent circulating fluidized-bed reactor, offers a novel high-efficiency and low-consumption dehydrogenation technology for the chemical industry.

Alkane dehydrogenation is an enabling route to make alkenes useful as chemical intermediates. This study demonstrates the high reactivity of Lewis acid-base (LAB) site pairs at ZrO2 powders for dehydrogenation of C2-C4 alkanes and the essential requirement for chemical treatments to remove strongly bound H2O and CO2 titrants to avoid the high temperatures required for their desorption and the concomitant loss of active sites through sintering and annealing of ZrO2 crystallites. The energies and free energies of bound intermediates and transition states from density functional theory (DFT), taken together with kinetic analysis and isotopic methods, demonstrated the kinetic relevance and heterolytic character of the first C-H activation at terminal C-atoms for all alkanes with a modest activation barrier (84 kJ mol-1) at essentially bare Zr-O LAB site pairs. β-Hydride elimination from the formed alkyl carbanions lead to their desorption as alkene products in steps that are favored over their parallel C-C cleavage reactions (by 100 kJ mol-1), leading to high dehydrogenation selectivities (>98%) at the temperatures required for practical yields in such endothermic dehydrogenation reactions (700-900 K). The facile recombination of bound proton-hydride pairs then completes a dehydrogenation turnover. These findings provide compelling evidence for the remarkable reactivity and selectivity of LAB sites on earth-abundant oxides and for the need to uncover them through chemical treatments, which combine to give gravimetric dehydrogenation rates that exceed those on the toxic (Cr) or costly (Pt) catalysts used in practice.

The catalytic dehydrogenation of light alkanes over Pt catalysts is generally accepted to follow a reverse Horiuti–Polanyi mechanism. Using a microkinetic analysis in combination with results from density functional theory calculations, we show that although propane dehydrogenation (PDH) occurs by two successive dehydrogenation steps on terraces, an unexpected non-reverse Horiuti–Polanyi mechanism accounts for more than half the propylene production at the under-coordinated active sites that dominate the kinetics of PDH. The main reaction is composed of three dehydrogenation steps that have two β-H atoms and one α-H atom removed from propane, followed by the hydrogenation of CH3CCH2; starting from this species, the formation of propylene and byproducts proceed by way of two parallel competing reactions. The proposed mechanism has been verified by exploring several key and general aspects of the kinetic behavior observed in the dehydrogenation of light alkanes, and it is found that only when adsorbate–adsorbate interactions are taken into consideration can the experimentally determined kinetics be properly reproduced. Increasing the H2 partial pressure from low values favors an increase in the coverage of free sites due to the gasification of adsorbed coke precursors, which in turn gives rise to lowered energy barriers for C–H bond breaking, thereby achieving an increased propane consumption rate. As the H2/C3H8 ratio increases, the rate of propylene production first goes up and then declines, and a maximum is observed at a H2/C3H8 ratio of 1.33, which occurs when the negative effect of the increased free 4-fold hollow sites that bring about deep dehydrogenation begins to dominate the positive effect of the increased free step sites that are responsible for activating propane. The mechanism formulated here proves to be valid even if the temperature, pressure, or the H2/C3H8 ratio is varied and hence provides a foundation for the rational design of metal and alloy catalysts for light alkane dehydrogenation.

Chapter 18 - Pincer Iridium and Ruthenium Complexes for Alkane Dehydrogenation

Light alkenes are the key building blocks in chemical industry. Dehydrogenation of light alkanes to light olefins offers an important route for the value-added utilization of alkanes and the diversification in olefin supplies. Featured with the free coking and no thermodynamic restriction, oxidative dehydrogenation of alkanes to olefins has attracted much attention. The early reported metal oxide catalysts usually have good activities, but tend to deeply oxidize olefins to CO2, leading to poor yield of olefin products. Later on, metal-free catalysts start to catch the eye. As a new estabilished catalytic system, metal-free boron-based catalysts show distinctive reaction characteristics of the high selectivity of olefins and the inhibition of deep oxidation reactions, as compared to metal-based catalysts. Hexagonal boron nitride ( h -BN) was first proved to be a highly active catalyst in the oxidative dehydrogenation reaction. Afterwards, some borides (e.g., SiB6, CB4) and supported boron catalysts have also been reported. The boron-based catalysts exhibit high activities and selevtivities to olefins with negligible formations of CO2, which provides a new sight in the selective cleavage of C–H bonds. In the combination of spectroscopic and kinetic analysis, the BO x species on the catalyst surface was determined as the active sites. The unique catalysis of boron-based catalysts updates the traditional perception on the alkane activation and has becoming a new hotspot worldwide. Besides, other metal-free catalysts, nanostructured carbon-based materials including carbon nanotubes, carbon nanofibers and nano-diamonds have also been shown a great potential in the dehydrogenation of light alkanes. The carbonyl/quinone groups are verified to be the active sites for dehydrogenation of light alkanes; the carboxyl, carboxylic anhydride and lactone groups could act as deep oxidative sites and cause the reduction of olefin selectivity. Moreover, doping of heteroatoms (B, P and N) into carbon matrix is effective for the improvement of olefin selectivity. This review timely highlights the recent developments of metal-free catalysts in the oxidative dehydrogenation of light alkanes, particulary focusing on the newly established boron-based catalytic system. At the end, we propose the perspective research directions of the dehydrogenation of light alkanes.

Catalytic dehydrogenation of light alkanes can effectively produce olefins and hydrogen. Even though Pt and CrOx -based catalysts are widely applied in industry, research to improve the activity and stability of these catalysts continued. This review summarizes important achievements obtained in recent years, focusing on the development of supports, promoters and preparation methods of Pt and CrOx -based catalysts, which mainly aimed to improve the dispersion of the active species and to enhance coke resistance. Furthermore, the high cost of Pt-based catalysts and environmental problems encountered with CrOx -based catalysts have spurred the development of alternative catalysts. The dehydrogenation performances and characteristics of promising alternative VOx -, modified Ni- and Sn-based catalysts are also reviewed. Comparison with the catalytic reforming process of naphtha further probes the necessity of catalyst acidity in these two different processes. The choice of the dehydrogenation reactor is discussed, and future perspectives and research directions are indicated.

Group-III Nitrides Catalyzed Transformations of Organic Molecules

The selective conversion of simple alkane feedstocks to high-value organic compounds constitutes one of major challenges in organic synthesis. There are three general catalytic methods for alkane functionalizations: reactions via radical intermediates and through carbene/nitrene insertion pathways typically occur preferentially at the secondary and tertiary C−H bonds, whereas transition metal-mediated C−H bond activation holds promise for the functionalization of primary C−H bonds. From a practical point of view, the installation of a functional group at the terminal position of alkanes is of particular attractive for large-scale synthesis of specialty or commodity chemicals. This review focuses on the area of molecular transition-metal-catalyzed alkane transformations through catalytic alkane dehydrogenation. In the first section, the development of iridium pincer and related complexes for alkane dehydrogenation is summarized. Among various catalysts, the PCP type Ir pincer complexes have proven to be most active toward transfer dehydrogenation of alkanes. α-Olefins are the kinetic products at the early stage of the dehydrogenation process, but they can be rapidly isomerized to internal alkenes. Recently, other non-phosphorus-ligated Ir and non-Ir metal dehydrogenation catalysts have also been developed, but they are generally less efficient than the PCP Ir catalysts. The second section first describes two dehydrogenation-based alkane transformations without the incorporation of heteroatom-containing functionality. The dehydroaromatization reaction involves multiple steps of dehydrogenation of linear alkanes to form conjugate trienes, which undergo electrocyclization and another step of dehydrogenation to generate aromatic products. The alkane-alkene coupling reaction employs an Ir pincer catalyst for dehydrogenation and a tantalum catalyst for alkene/alkene coupling, thus providing a method for upgrading light alkanes to higher alkanes that may be suitable to transportation fuel. Following that, the selective installation of functional groups at the terminal positions of alkanes through a dehydrogenation-alkene isomerization-hydrofunctionalization strategy using a dual catalyst system is described. Several combinations between a PSCOP Ir pincer dehydrogenation catalyst and molecular Fe or Rh catalysts for tandem isomerization and hydrofunctionalization of terminal alkenes have been developed for terminal selective alkane silylation, borylation, carbonylation, and aminomethylation. The third section covers alkane metathesis and its application to polyethylene (PE) degradation. The alkane metathesis reaction described here consists of two catalysts, one Ir dehydrogenation catalyst and one olefin metathesis catalyst. The metathesis process can find potential application in upgrading low carbon number n -alkanes to higher-molecular-weight fuel alkanes. A cross-alkane metathesis strategy has been developed for PE degradation. Using excess of low-value light alkanes as the reagent/solvent, PE with molecular weight up to 1.7 million undergoes multiple times of cross-alkane metathesis at 175℃ to produce liquid fuels or high-quality waxes. The catalysts can tolerate the commercial HDPE, LDPE and LLDPE, and enable the efficient degradation of postconsumer PE plastic wastes. Finally, the review discusses the limitations of the known catalytic approaches and future opportunities in this field. Catalyst development will be the central theme of research, and insights into the factors controlling the activity and selectivity gained in mechanistic studies will guide the design of more efficient dehydrogenation catalysts. The cooperative catalysis involving enantioselective catalysts will provide a protocol for synthesis of valuable fine chemicals from simple saturated hydrocarbons. Lastly, we should not overlook the potential of cooperation between photo- or electro-catalysis and transition-metal catalysis, which may allow the exploitation of alkane feedstocks more cleanly and efficiently.

Hydrogen as an energy carrier promises a sustainable energy revolution. However, one of the greatest challenges for any future hydrogen economy is the necessity for large scale hydrogen production not involving concurrent CO2 production. The high intrinsic hydrogen content of liquid‐range alkane hydrocarbons (including diesel) offers a potential route to CO2‐free hydrogen production through their catalytic deep dehydrogenation. We report here a means of rapidly liberating high‐purity hydrogen by microwave‐promoted catalytic dehydrogenation of liquid alkanes using Fe and Ni particles supported on silicon carbide. A H2 production selectivity from all evolved gases of some 98 %, is achieved with less than a fraction of a percent of adventitious CO and CO2. The major co‐product is solid, elemental carbon.

Hydrogen as an energy carrier promises a sustainable energy revolution. However, one of the greatest challenges for any future hydrogen economy is the necessity for large scale hydrogen production not involving concurrent CO2 production. The high intrinsic hydrogen content of liquid-range alkane hydrocarbons (including diesel) offers a potential route to CO2 -free hydrogen production through their catalytic deep dehydrogenation. We report here a means of rapidly liberating high-purity hydrogen by microwave-promoted catalytic dehydrogenation of liquid alkanes using Fe and Ni particles supported on silicon carbide. A H2 production selectivity from all evolved gases of some 98 %, is achieved with less than a fraction of a percent of adventitious CO and CO2 . The major co-product is solid, elemental carbon.

Alkane dehydrogenation catalyzed by the Ir(III) complexes (PCP‘)Ir(H)2 (1) [PCP‘ = η3-C6H3(CH2PH2)2-1,3] and CpIr(PH3)(H)+ (10) [Cp = η5-C5H5] is investigated with density functional theory (DFT). For both systems the theoretical results show that catalytic alkane dehydrogenation to alkene proceeds through (i) alkane oxidative addition, (ii) dihydride reductive elimination, (iii) β-H transfer from alkyl ligand to metal, and finally (iv) elimination of the olefin. Barriers for steps (i), (ii), and (iv) are critical for the catalytic cycle. The (PCP‘)Ir(H)2 system is special because these three barriers are balanced (16, 15, and 22 kcal/mol, respectively), whereas in the CpIr(PH3)(H)+ system these three barriers are unbalanced (9, 24, and 41 kcal/mol, respectively). Thus, in the catalytic cycle for alkane dehydrogenation by (PCP‘)Ir(H)2 the reaction endothermicity is achieved gradually. The higher stability of the formally Ir(V) complexes and the η2-alkene complex, which has some Ir(V)-like character, in th...

In this critical review, the recent advances of the CO2-assisted catalytic dehydrogenation of light alkanes are summarized.

The 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 (C 2 H 6 ), propane (C 3 H 8 ), butane (C 4 H 10 ), and isobutane (HC(CH 3 ) 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.

Small Pt clusters in Na-[Fe]ZSM5 give high alkene selectivities, near-equilibrium alkene yields, and unprecedented stability in the catalytic dehydrogenation of light alkanes.