Selective Dehydrogenation Research Articles

Toward Selective Dehydrogenation of Glycerol to Lactic Acid over Bimetallic Pt–Co/CeOx Catalysts

Selective synthesis of lactic acid from bioderived substrates still remains a grand challenge. We reported a series of unique bimetallic Pt–Co/CeOx catalysts, prepared by fractional precipitation m...

Tuning catalytic selectivity of propane oxidative dehydrogenation via surface polymeric phosphate modification on nickel oxide nanoparticles

Thermal stability has long been recognized as a major limitation for the application of ligand modification in high-temperature reactions. Herein, we demonstrate polymeric phosphate as an efficient and stable ligand to tune the selectivity of propane oxidative dehydrogenation. Beneficial from the weakened affinity of propene, NiO modified with polymeric phosphate shows a selectivity 2–3 times higher than NiO towards the production of propene. The success of this regulation verifies the feasibility of ligand modification in high-temperature gas-phase reactions and shines a light on its applications in other important industrial reactions.

Selective gas conversion of isopropyl alcohol over silver nanoparticles (Ag-NPs) supported on new mesoporous silica precipitated from natural resources

A series of silver nanoparticles (Ag-NPs) loaded onto new mesoporous silica (MS), produced from natural cheap pumice was prepared via wet impregnation and characterized by means of several analytical techniques. Catalysts Ag–x/MS with x = 3, 10 and 20 wt% have been investigated in the gas conversion of isopropyl alcohol (IPA) under N2 and air atmospheres and exhibit a particular catalytic behavior different from conventional crystalline and supported silver-based catalysts. In N2 atmosphere, Ag–x/MS catalysts provide propene by dehydration process with selectivity greater than 95% whereas under the air atmosphere at low temperature (≤ 423 K), the reaction only produces acetone by oxidative dehydrogenation. This bifunctionality was due to special textural properties of MS support, characterized by high specific surface area (423 m2 g−1) and Bronsted acidic sites which are formed by bridged siloxanes (≡Si–O–Si≡) and silanols groups (≡Si–OH) leading to the formation of propene and on the other hand, to the high dispersion and redox properties of Ag-NPs under air atmosphere giving rise to “Ag-Ox” species which are selectively active for acetone production. The selective oxidative dehydrogenation toward acetone was confirmed by the non-detection of total oxidation products (CO/CO2) suggesting that the MS support preserves Ag-NPs against agglomeration and the redox properties of surface active “Ag-Ox” during the long-time service. The exceptional surface characteristics of MS produced from pumice and its promoting effect in the dispersion of active metal elements in the conversion of IPA exceeds broadly those obtained on various silica supports reported in the literature.

Single-Sites and Nanoparticles at Tailored Interfaces Prepared via Surface Organometallic Chemistry from Thermolytic Molecular Precursors.

Heterogeneous catalysts are complex by nature, making particularly difficult to assess the structure of their active sites. Such complexity is inherited in part from their mode of preparation, which typically involves coprecipitation or impregnation of metal salts in aqueous solution, and the associated complex surface chemistries. In this context, surface organometallic chemistry (SOMC) has emerged as a powerful approach to generate well-defined surface species, where the metal sites are introduced by grafting tailored molecular precursors. When combined with thermolytic molecular precursors (TMPs) that can lose their organic moieties quite readily upon thermal treatment, SOMC provides access to supported isolated metal sites with defined oxidation state and nuclearity inherited from the precursor. The resulting surface species bear unusual coordination imposed by the surface that provides them high reactivity in comparison with their molecular precursor. In addition, these molecularly defined species bare strong resemblance with the active sites of supported metal oxides. However, they typically contain a higher proportion of active sitesmaking structure-activity relationship possible. They thus constitute ideal models for this important class of industrial catalysts that are used in numerous applications such as olefin epoxidation (Shell process), olefin metathesis (triolefin process), ethylene polymerization (Phillips catalysts), or propane dehydrogenation (Catofin and related processes). This SOMC/TMP approach can thus provide detailed information about the structure of active sites in industrial catalysts, their mode of initiation and deactivation, as well as the role of the support and specific thermal treatment on the final activity of the catalysts. Nonetheless, these structurally characterized surface sites still exhibit heterogeneous environments borrowed from the support itself, that explain the intrinsic complexity of heterogeneous catalysis. Furthermore, SOMC/TMP can also be used to generate and investigate supported metal nanoparticles. Starting from the well-defined isolated sites, that also contain adjacent surface OH groups, one can graft a second metal and then generate after treatment under hydrogen small and narrowly dispersed alloys or nanoparticles with tailored interfaces that can show improved catalytic performances and are amiable todetailed structure-activity relationships. This approach is illustrated by two case studies: (1) formation of supported copper nanoparticles at tailored interfaces that contain isolated metal sites for the selective hydrogenation of carbon dioxide to methanol, allowing for a detailed understanding of the role of dopants and supports in heterogeneous catalysis, and (2) preparation of highly selective and productive propane dehydrogenation catalystsbased on silica-supported Pt xGa y alloy. Overall, this Account shows how the combination of SOMC and TMP helps to generate catalysts, particularly suited for elucidating structural characterization of active sites at a molecular-level which in turn enables structure-activity relationship to be drawn. Such detailed information obtained on well-defined catalysts can then be used to understand complex effects observed in industrial catalysts (effects of supports, additives, dopants, etc.), and to extractinformation that can then be used to improve them in a more rational way.

Bimetallic Pt-Sn nanocluster from the hydrogenolysis of a well-defined surface compound consisting of [([tbnd]AlO[sbnd])Pt(COD)Me] and [([tbnd]AlO[sbnd])SnPh3] fragments for propane dehydrogenation

The bimetallic Pt-Sn catalysts are known to be the most efficient ones for propane dehydrogenation reaction; however, the synthesis of bimetallic Pt-Sn nanocluster (below 1 nm) with an alloy structure still remains a formidable challenge. An approach for the controllable synthesis of Pt-Sn nanocluster based on Surface Organometallic Chemistry on oxide concept is developed here, which involves two basic steps: the sequential grafting of Pt(COD)Me2 and HSnPh3 at the surface of dehydroxylated θ-Al2O leading to the formation of well-defined bimetallic surface compound consisting of [(AlO)Pt(COD)Me] and [(AlO)SnPh3] fragments, and the hydrogenolysis of this surface compound under a mild condition giving rise to the ultra-small bimetallic Pt-Sn cluster around 0.75 nm. The achieved Pt-Sn/θ-Al2O3 catalysts exhibit high activity, selectivity and stability in propane dehydrogenation to propylene.

Promotional role of Ceria in CeO2/MgAl2O4 spinel catalysts in CO2 assisted selective oxidative dehydrogenation of ethylbenzene to styrene

With the aim of understanding the influence of CeO2 on MgO, Al2O3 and MgAl2O4 in CO2 assisted oxidative dehydrogenation of ethylbenzene to styrene was attempted and the results conveyed that the MgAl2O4 spinel supported CeO2 (CeO2/MgAl2O4) catalyst exhibited superior activity with 80% yield of styrene and in apart activity consistency up to 50h of time-on-stream, revealing the establishment of synergism in CeO2/MgAl2O4, ascertained from XRD, BET Surface area, FT-IR, H2-TPR, UV–vis DRS, XPS, TPD and HRTEM techniques, acid-base characteristics of MgAl2O4, smaller particle size of CeO2 and its high dispersion are responsible for elevated activity, stability and regenarable capability.

Solvent effects on high-pressure hydrogen gas generation by dehydrogenation of formic acid using ruthenium complexes

High-pressure H2 was produced by the selective dehydrogenation of formic acid (DFA) using ruthenium complexes at mild temperatures in various organic solvents and water. Among the solvents studied, 1,4-dioxane was the best candidate for this reaction to generate high gas pressure of 20 MPa at 80 °C using the Ru complex having a dearomatized pyridine-based pincer PN3P* ligand. This complex shows reusability for the high-pressure DFA in 1,4-dioxiane while maintaining the catalytic performance, however, deactivation occurred in other solvents. In dimethyl sulfoxide, its decomposition products may cause catalytic deactivation. The gas pressure generated in 1,4-dioxane was lower than that in water due to the high dissolution of 1,4-dioxane into CO2 according the vapor-liquid equilibrium calculations. The role of solvent is crucial since it affected the catalytic performance and also the generated gas pressure (H2 and CO2) from FA.

Photoluminescence of surface chromium centers in the Cr/Al2O3 system that is active in isobutane dehydrogenation

A comprehensive study of optical and catalytic properties of highly dispersed single-phase η-, γ- and γ*-Al2O3 ([Cr3+] ∼ 10–4 wt.%) powders and the related model chromia-alumina catalysts with different chromium content ([Cr3+] = 0.25, 0.5 and 1 wt%) was carried out. The catalytic characteristics (product yield, conversion and selectivity) in isobutane dehydrogenation to isobutylene were measured for all the samples under consideration. The surface Cr3+S luminescence centers were detected by photoluminescence spectroscopy in all the samples. The content of Cr3+S centers in the samples was estimated before and after isobutane dehydrogenation. It was shown that the amount of surface Cr3+S centers correlates with isobutylene yields of the reaction. Among the studied samples, the maximum amount of Cr3+S luminescence centers was found for the model 1% Cr/γ-Al2O3 catalyst. This sample has the highest values of isobutane conversion and isobutylene yield. The comprehensive study of the Cr/Al2O3 systems made it possible to propose a method for predicting the catalytic activity of nanostructured materials using the photoluminescence detection of surface luminescence centers and their quantitative evaluation.

Effect of Ba addition to Ga-α-Al2O3 catalyst on structure and catalytic selectivity for dehydrogenation of ethane

Modified Ga-α-Al2O3 catalyst with Ba showed high catalytic activity, selectivity and low carbon formation on catalytic ethane dehydrogenation to ethylene even in the presence of steam. The resultant Ba-Ga-α-Al2O3 (Ba/Ga molar ratio = 0.10) catalyst showed high ethylene selectivity (98%), high activity, and stability. Temperature-programmed oxidation measurements revealed that Ba addition suppressed coke formation at Ga sites. XRD, XAFS and TEM results showed the existence of highly dispersed β-Ga2O3 on the α-Al2O3 support, irrespective of Ba addition. Observation of hydrogen adsorption using FT-IR spectroscopy revealed a decrease in the surface tetrahedrally coordinated Ga (designated as Ga(T)) sites by Ba modification, indicating that the surface Ga(T) sites are covered with Ba. Density functional theory calculation revealed that coke formation through ethylene decomposition is likely to occur at surface Ga(T) sites, and revealed that the addition of an optimal amount of Ba to Ga-α-Al2O3 inhibits coke formation at the surface Ga(T) sites, leading to high ethylene selectivity.

First principles micro-kinetic model of catalytic non-oxidative dehydrogenation of ethane over close-packed metallic facets

Catalytic dehydrogenation of light alkanes may offer more efficient routes to selectively producing light olefins, which are some of the most important chemical building blocks in the industry, in terms of scale. We present a descriptor based micro-kinetic model of the trends in selectivity and activity of non-oxidative dehydrogenation of ethane over close-packed metal facets and through varied reaction conditions. Our model predicts and explains the experimentally observed promotion effect on turnover rate from co-feeding hydrogen as an effect of the shifting equilibria in steady state. At low conversion reaction conditions over Pt, the path to ethene goes through ethane dehydrogenation to ethyl, CH3CH2∗, then to ethene while the non-selective pathway to methane and deeply dehydrogenated species is predicted to go through dehydrogenation via CH3CH∗. This implies that the desorption step of ethene is not the limiting step for selectivity and that geometric effects that stabilize CH2CH2∗ compared to CH3CH∗ are desirable properties of a better catalyst. Removing reactive bridge and 3-fold sites facilitates this, which may be achievable by sufficient concentrations of tin in platinum. The included model code furthermore provides a base for easy tuning and for expanding the study to other thermodynamic conditions, other facets, alloys or the reaction network to longer hydrocarbons or to oxidative pathways.

Kinetics of long chain n-paraffin dehydrogenation over a commercial Pt-Sn-K-Mg/γ-Al2O3 catalyst: Model studies using n-dodecane

A kinetic modeling study on long chain n-paraffin dehydrogenation using a commercial Pt-Sn-K-Mg/γ-Al2O3 catalyst was carried out in a continuous flow set-up using n-dodecane as a model component at various temperatures (450-470 °C), pressures (0.17-0.30 MPa), H2/paraffin mole ratios (3:1-6:1) and space times (0.22-1.57 g h mol−1). The commercial catalyst was characterized by XRD, BET, MIP, SEM and CO chemisorption. An empirical exponential equation was found to predict the mono- and di-olefin yields very well. In addition, 6 mechanistic models based on the LHMW mechanism were derived and tested by non-linear least squares fitting of the experimental data. The model which assumes that surface reactions and particularly the dehydrogenation of the metal-alkyl chain to the adsorbed mono-olefin and di-olefin as the rate determining steps was found to give the best fit with the experimental data. In addition, activation energies and adsorption enthalpies for each elementary reaction were obtained. The kinetic testing and modeling have shown that the high mono-olefins selectivity for long chain paraffin dehydrogenation can be obtained by operating at low space time (when P, T and m are same), high pressure (when τ, T and m are same) and high H2/paraffin ratio (when τ, P and T are same), as well as low reaction temperature (when τ, P and m are same) but with little effect.

Oxidative Cross-Esterification and Related Pathways of Co-Adsorbed Oxygen and Ethanol on Pd–Au

Ethanol is one of the most important industrial chemicals considering its wide range of uses. From ultrahigh vacuum (UHV) experiments, we were able to effectively produce acetaldehyde and ethyl acetate on a Pd–Au model catalyst. The presence of oxygen at the Pd–Au interface sites was found to be highly reactive for ethanol dehydrogenation, significantly increasing acetaldehyde production and promoting cross-esterification to produce ethyl acetate. Density functional theory calculations show that compared to the bare PdAu(111) surface, the presence of preadsorbed oxygen and the subsequently formed hydroxyl group leads to highly active and selective initial ethanol dehydrogenation at the O–H and α–C-H position, which was corroborated by isotope-labeling experiments. Specifically, using temperature-programmed desorption with the isotope CD3CH2OH, we identified C–C bond breakage through methane production. Furthermore, the simultaneous desorption of D2 with ethyl acetate from the oxidation of CD3CD2OD suggest...

Computationally guided design of a new Rh catalyst for selective formic acid dehydrogenation: Validation with caution

Formic acid possesses many desirable properties as a promising hydrogen energy carrier. Density functional theory (DFT) was utilized to design new Rh complexes with 2,2′-biimidazoline ligands to explore their potential as catalysts for formic acid dehydrogenation. These designed complexes were prepared and examined as catalysts under various temperatures and ratios of formic acid to sodium formate. It was found that although our complexes gave a high TOF of 20,000 h−1 at 90 °C under certain formic acid/sodium formate ratios, the dehydrogenation reaction was prematurely deactivated once 30% of the formic acid was consumed. Possible deactivation mechanisms were investigated by NMR and HRMS. Our observations suggest that while guided rational design of catalysts by DFT has much potential, the method does have limitations as evidenced by our experimental comparisons and thus pure DFT design should be conducted with caution.

Cover Feature: Tuning the Selectivity of AuPd Nanoalloys towards Selective Dehydrogenative Alkyne Silylation (Chem. Eur. J. 23/2019)

It takes two to tandem! Tailor-made AuPd nanoalloys were developed for the selective cross-dehydrogenative coupling of terminal alkynes and hydrosilanes. Although monometallic Au nanoparticles do not catalyze that transformation, monometallic Pd nanoparticles show low selectivity for the desired alkynylsilane. The catalyst with cooperatively interacting Au and Pd metals tolerates a large number of functional groups and works at room temperature under air atmosphere. More information can be found in the Communication by M. Wissing and A. Studer on page 5870.

Tuning the Selectivity of AuPd Nanoalloys towards Selective Dehydrogenative Alkyne Silylation

The cross-dehydrogenative coupling of terminal alkynes and hydrosilanes catalyzed by AuPd nanoalloys is described. Metal nanoparticles are readily prepared in 15 minutes from commercially available and cheap starting materials by using a photochemical approach. The ratio of Au and Pd in the alloys heavily influences their reactivity. These cooperative nanoalloy catalysts tolerate a large number of functional groups (e.g., free amines and acids), operate at room temperature under air atmosphere at low loading (2 mol %), and the cross-dehydrogenative coupling can easily be scaled up.

Influence of Metal Oxide Support Acid Sites on Cu-Catalyzed Nonoxidative Dehydrogenation of Ethanol to Acetaldehyde

Selective ethanol dehydrogenation to acetaldehyde is an important reaction because acetaldehyde is a useful commodity chemical and can serve as a building block to produce other high-value products...

Hydrogen production via selective dehydrogenation of hydrazine borane and hydrous hydrazine over MoOx-promoted Rh catalyst

Hydrazine borane (N2H4BH3, 15.4 wt%) and hydrous hydrazine (N2H4·H2O, 8 wt%) are promising chemical hydrogen storage materials that have attracted much attention in recent years due to their high gravimetric hydrogen capacities. The selective and rapid catalytic dehydrogenation of N2H4BH3 and N2H4·H2O is the key to the potential use of the abovementioned materials in hydrogen applications. In this study, a series of Rh-MoOx nanoparticles (NPs) with various metal compositions have been successfully prepared via a simple one-step chemical reduction approach in the absence of a surfactant/support at room temperature. The obtained surfactant/support-free Rh-MoOx NPs exhibit good dispersibility and small particle size. Among all of the as-prepared Rh-MoOx NPs, the optimized Rh-MoOx catalysts with a molar ratio of 1:1 show the highest catalytic properties with 100% H2 selectivity for hydrogen evolution from aqueous N2H4BH3 and N2H4·H2O solutions at 323 K under alkaline conditions. The turnover frequency (TOF) values of Rh0.5(MoOx)0.5 NPs are 2000 and 750 h−1 for hydrogen evolution from N2H4BH3 and N2H4·H2O, respectively, which are among the highest values ever reported for Rh-based catalysts. The superior catalytic performances could be attributed to the small particles size, low crystallinity, modified electronic structure, and strong basic sites of Rh NPs induced by the MoOx dopant. The highly rapid and selective Rh-MoOx catalyst may further encourage the deployment and application of N2H4BH3 and N2H4·H2O as safe and convenient sources of hydrogen for fuel cells.

Catalytic Dehydrogenation of Formic Acid with Ruthenium‐PNP‐Pincer Complexes: Comparing N‐Methylated and NH‐Ligands

AbstractComplexes Ru(H)(Cl)(CO)(HN{CH2CH2P(iPr)2}2) 7 and Ru(H)(Cl)(CO)(CH3N{CH2CH2P(iPr)2}2) 8 were compared for the selective dehydrogenation (DH) of formic acid (FA) at different pH values. Remarkably, highest activities were observed in acidic environment (pH 4.5). Under all investigated conditions, the N‐methylated complex 8 showed improved performance compared to 7. These observations can be mechanistically explained with protonation of 4H/3Me being the key step in formic acid DH for both complexes.

Subnanometer cobalt oxide clusters as selective low temperature oxidative dehydrogenation catalysts

The discovery of more efficient, economical, and selective catalysts for oxidative dehydrogenation is of immense economic importance. However, the temperatures required for this reaction are typically high, often exceeding 400 °C. Herein, we report the discovery of subnanometer sized cobalt oxide clusters for oxidative dehydrogenation of cyclohexane that are active at lower temperatures than reported catalysts, while they can also eliminate the combustion channel. These results found for the two cluster sizes suggest other subnanometer size (CoO)x clusters will also be active at low temperatures. The high activity of the cobalt clusters can be understood on the basis of density functional studies that reveal highly active under-coordinated cobalt atoms in the clusters and show that the oxidized nature of the clusters substantially decreases the binding energy of the cyclohexene species which desorb from the cluster at low temperature.

A Simple and Effective Principle for a Rational Design of Heterogeneous Catalysts for Dehydrogenation of Formic Acid.

Efficient and selective dehydrogenation of formic acid is a key challenge for a fuel-cell-based hydrogen economy. Though the development of heterogeneous catalysts has received much progress, their catalytic activity remains insufficient. Moreover, the design principle of such catalysts are still unclear. Here, experimental and theoretical studies on a series of mono-/bi-metallic nanoparticles supported on a NH2 -N-rGO substrate are combined for formic acid dehydrogenation where the surface energy of a metal is taken as a relevant indicator for the adsorption ability of the catalyst for guiding catalyst design. The AuPd/NH2 -N-rGO catalyst shows record catalytic activity by reducing the energy barrier of rate controlling steps of formate adsorption and hydrogen desorption. The obtained excellent results both in experiments and simulations could be extended to other important systems, providing a general guideline to design more efficient catalysts.