Search

The chain growth mechanisms of Fischer–Tropsch synthesis (FTS) remain a controversial topic, especially considering the much stronger adsorption ability of CO over H2, resulting in CO pre-adsorption or saturation.

The adsorption and dissociation of O2on Ir(100) has been studied with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy together with density functional theory calculations. At low coverage, XPS shows a single O 1s peak at 530.6 eV, associated with bridged O atoms in an Ir(100)-(3 × 1)-O structure. At saturation coverage, an additional peak at 529.5 eV appears, accompanied by an Ir(100)-(2 × 1)-O structure. The O 1s peak at 530.6 eV is assigned to oxygen atoms adsorbed on bridge sites of Ir atoms, and the peak at 529.5 eV to oxygen atoms on fourfold hollow sites of Ir. Based on our detailed experimental and computational study, oxygen diffusion into the subsurface region can be ruled out. The present study provides new findings on an additional oxygen state of the O/Ir(100) system. It also provides evidence for the absence of subsurface oxygen in Ir, implying that some discussions on subsurface oxygen in the literature may be misleading.

The famous aromaticity index NICS (nucleus independent chemical shift) was introduced 25 years ago. The authors use this anniversary for a short and to some degree personal retrospective.

A Co(II) PNNH pincer catalyst system for the highly E-selective transfer-semihydrogenation of internal alkynes using ammonia borane and MeOH as the hydrogen source is described. The reaction is proposed to occur via the Co(ii) oxidation state.

(In situ) liquid-phase spectroscopic investigations on state-of-the-art Pd catalysts modified with pyridyl-substituted diphosphine ligands for alkene alkoxycarbonylations have been performed for characterizing resting state complexes in solution.

Manganese-catalyzed hydrogenation reactions have aroused widespread interest in recent years. Among the catalytic systems described, especially PNP- and NNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, ketones, nitriles, aldimines and esters. Furthermore, NNP-Mn pincer compounds are efficient catalysts for the hydrogenolysis of less reactive amides, ureas, carbonates, and carbamates. Herein, the synthesis and application of specific imidazolylaminophosphine ligands and the corresponding Mn pincer complexes are described. These new catalysts have been characterized and studied by a combination of experimental and theoretical investigations, and their catalytic activities have been tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of N-heterocycles can be performed under very mild conditions.

Metal leaching is an important obstacle for the development of CO-containing heterogeneous catalysis systems. The modulation of the metal surface could substantially influence the structural stability under a CO atmosphere. In this work, the inductively coupled plasma (ICP) results indicate that the leaching amount of cobalt in the CoGa intermetallic compound (CoGa IMC/SiO2) catalyst is notably less than that of the Co/SiO2 catalyst in hydroformylation reaction. The in situ Fourier Transform Infrared Spectroscopy (FTIR) results prove that aldehydes can be generated on CoGa IMC/SiO 2 by heterogeneous hydroformylation reaction without the formation of Co carbonyl species. Moreover, the analysis of metal bond interaction proves that although the Co-Ga bond has a weaker metal interaction strength in the bulk phase, it has higher structural stability than the Co-Co bond with the adsorption of CO molecules. Above all, this work provides a strategy to suppress metal leaching for heterogeneous catalysis.

Nature 599(7884), 234 - 238 (2021). doi:10.1038/s41586-021-03923-3, Propane dehydrogenation (PDH) to propene is an important alternative to oil-based cracking processes, to produce this industrially important platform chemical1,2. The commercial PDH technologies utilizing Cr-containing (refs. 3,4) or Pt-containing (refs. 5,6,7,8) catalysts suffer from the toxicity of Cr(vi) compounds or the need to use ecologically harmful chlorine for catalyst regeneration9. Here, we introduce a method for preparation of environmentally compatible supported catalysts based on commercial ZnO. This metal oxide and a support (zeolite or common metal oxide) are used as a physical mixture or in the form of two layers with ZnO as the upstream layer. Supported ZnO$_x$ species are in situ formed through a reaction of support OH groups with Zn atoms generated from ZnO upon reductive treatment above 550 ��C. Using different complementary characterization methods, we identify the decisive role of defective OH groups for the formation of active ZnO$_x$ species. For benchmarking purposes, the developed ZnO���silicalite-1 and an analogue of commercial K���CrO$_x$/Al$_2$O$_3$ were tested in the same setup under industrially relevant conditions at close propane conversion over about 400 h on propane stream. The developed catalyst reveals about three times higher propene productivity at similar propene selectivity., Published by Nature Publ. Group, London [u.a.]

The applicability of a heteroaromatic photoredox catalyst in an additive-free photo-mediated hydroamination of stilbenes is described. Initiated by the excitation of a highly potent organic pyrimid...

Structures, energies, and stability of aliphatic PNP ligated CoIII/CoI amine and amido complexes in different spin (singlet, triplet, and open-shell singlet) states and coordination spheres have be...

The mechanisms of iridium-catalyzed nonoxidative conversion of light hydrocarbons have been computed on flat Ir(111) and stepped Ir(211) surfaces under given conditions. On Ir(111), CH4 dissociatio...

The dicarbonylation of 1,3‐butadiene to adipic acid derivatives offers the potential for a more cost‐efficient and environmentally benign industrial process. However, the complex reaction network of regioisomeric carbonylation and isomerization pathways, make a selective and direct transformation particularly difficult. Here, we report surprising solvent effects on this palladium‐catalysed process in the presence of 1,2‐bis‐di‐tert‐butylphosphin‐oxylene (dtbpx) ligands, which allow adipate diester formation from 1,3‐butadiene, carbon monoxide, and methanol with 97 % selectivity and 100 % atom‐economy under scalable conditions. Under optimal conditions a variety of di‐ and triesters from 1,2‐ and 1,3‐dienes can be obtained in good to excellent yields., High value‐added conversion: From gas to nylon precursor. This 100 % atom economic and industrially relevant process is strongly influenced by the system polarity.

For the catalytic reductive amination of carbonyl compounds, the kind of active metal used is the most important factor determining the catalytic selectivity in heterogeneous catalysis systems. However, a detailed understanding of the intrinsic mechanism is still lacking. In this work, by evaluating the reductive amination of butyraldehyde and the hydrogenation reaction of secondary imine on various metal catalysts, the competitive hydrogenation reactions of primary imine and secondary imine are proven to be the key factors determining primary amine selectivity. DFT calculations verify that Co, Ni, and Ru, rather than Fe, Rh, Pd, and Pt, tend to show high primary amine selectivity in terms of adsorption energy and activation energy. It should be noted that the different stable adsorption modes of secondary imine on metal surfaces (C&N adsorption mode on Fe, Co, Ni, Ru, and Rh, and N adsorption mode on Pd and Pt) are key factors affecting these reaction characteristics. Moreover, microkinetic simulation proves that the coadsorption of NH3 improves primary amine selectivity on Co, Ni, and Ru surfaces. Combining the theoretical and experimental results, it is clearly verified that the active metal determines the catalytic selectivity of the primary amine by affecting the competitive hydrogenation reactions of the primary imine and secondary imine.

The cycloaddition mechanisms of CO2 and epichlorohydrin catalyzed by salophen as an organocatalyst were investigated using B3LYP density functional theory computations. The kinetically most favored neutral pathway involves a concerted and synergistic mechanism of phenolate as a nucleophile and phenol as a H-bonding donor in epoxide ring opening and CO2 addition. The computed apparent barrier (33 kcal mol−1) is in line with the reaction conditions (120 °C and 10 bar CO2). A similar and anionic route with a much lower apparent barrier (17 kcal mol−1) to react under milder conditions has been proposed and this has to be proven experimentally.

To understand the metal–support interaction of oxide supported transition metal catalysts, we computed the reaction mechanisms of dry and steam reforming of methane on a tetragonal ZrO2(101) supported Ni catalyst. Based on the limited number of active sites on the surface, an irregular and non-ideal Ni13 cluster on ZrO2(101) is identified as a catalyst. A simple reaction mechanism is proposed, and the first direct dissociation step of CO2, CH4 and H2O is the most difficult based on the computed Gibbs free energies and no surface CHXO and CHXOH intermediates are involved, different from that on the flat Ni(111) surface. Analysis of other supported nickel catalysts shows that not only the support but also the size and shape of the metal clusters play an important role in the reaction mechanisms and kinetics.

The bis(thiophosphinite) pincer complexes [(RPSCSPR)Rh(py)(H)(Cl)] (RPSCSPR = C6H4–2,6-(SPR2)2 with R = iPr, 2a and R = Ph, 2b) are prepared by metalation of the ligand precursor with [Rh(cod)Cl]2 in the presence of pyridine. Both complexes are highly active for the dehydropolymerisation of H3B·NMeH2, producing poly(aminoborane)s (H2BNMeH)n (e.g. 1 mol% 2a, Mn = 20.200 g mol−1) in very short reaction times at room temperature. Detailed NMR studies on the iPr substituted complex 2a reveal B–N bond cleavage as a dominant process, resulting in the initial formation of a BH3 adduct, which inhibits catalysis, as evidenced by a pronounced induction period. This can be removed by addition of amine to ultimately bring the catalyst on-cycle for dehydrogenation and polymer formation. A mechanistic scenario is discussed based on the volumetric, spectroscopic and size-exclusion chromatography data as well as M06L-SCRF density functional theory computations.

To understand the reaction mechanisms of the dehydrogenation and retrosynthesis of alkanes, the consecutive dissociation of methane, ethane, ethene and ethyne, as well as propane, propene and propyne, on the fcc Ru(111) surface has been investigated using periodic density functional theory computations (rPBE). Methane dissociation has the energy minimum path of → → → CH* → C*. Although ethane dissociation does not have ethene and ethyne as intermediates, they have the same final surface species with the minimum energy paths for ethane [ → → CH3CH* → CH3C* → → HC*C* → HC* + C*], ethene [ → → → HC*C* → HC* + C*] and ethyne [CH*CH* → HC*C* → HC* + C*]. Propane dissociation has the competitive routes of n-propyl [ → CH3CH2CH* → CH3CH2C* → CH3CH*C* → CH3C*C* → CH3C* + C* →→ HC* + C*] and isopropyl with propyne as an intermediate [CH3CH*CH3 → CH3C*CH3 → → CH3C*CH* → CH3C*C* →→ HC* + C*], and the n-propyl route has propene as an intermediate for dissociation [ → CH3C*CH2*/CH3CH*CH* → CH3CH*C*/CH3C*CH* → CH3C*C* →→ HC* + C*]. In these reactions, the most stable surface intermediates are HC*, CH3C* and CH3CH2C* as homologs, as found experimentally on other metal surfaces. Our results rationalized the experimentally observed interconversion between + H* and CH3C* as well as surface HC*C* and CH3C*C* as key intermediates for the first C–C bond dissociation [HC*C* → HC* + C*; CH3C*C* → CH3C* + C*]. On the basis of surface C* and H2 gas, the retrosynthesis of methane, ethane and propane has increasing apparent barriers of 1.08, 1.51 and 1.66 eV, respectively, at 490 K and 1 atm H2 and 0.83, 1.14 and 1.15 eV, respectively, at 19.7 atm H2. Surface carbon coverage changes the formation of alkanes from endergonic to exergonic. This pressure- and coverage-dependency is very important for understanding the reaction mechanism and selectivity. Surface alkynyl groups should be the intermediates for C–C coupling. The computed vibrational frequencies of CH*, CH3C*, CH2C*, HC*C* and agree with the experiments. The comparison in adsorption energies and reaction barriers and energies shows that the fcc Ru(111) surface is more active than the hcp Ru(0001) surface despite their very similar surface structures.

A series of hydroxy-functionalized phosphonium salts were studied as bifunctional catalysts for the conversion of CO2 with epoxides under mild and solvent-free conditions. The reaction in the presence of a phenol-based phosphonium iodide proceeded via a first order rection kinetic with respect to the substrate. Notably, in contrast to the aliphatic analogue, the phenol-based catalyst showed no product inhibition. The temperature dependence of the reaction rate was investigated, and the activation energy for the model reaction was determined from an Arrhenius-plot (Ea =39.6 kJ mol-1 ). The substrate scope was also evaluated. Under the optimized reaction conditions, 20 terminal epoxides were converted at room temperature to the corresponding cyclic carbonates, which were isolated in yields up to 99 %. The reaction is easily scalable and was performed on a scale up to 50 g substrate. Moreover, this method was applied in the synthesis of the antitussive agent dropropizine starting from epichlorohydrin and phenylpiperazine. Furthermore, DFT calculations were performed to rationalize the mechanism and the high efficiency of the phenol-based phosphonium iodide catalyst. The calculation confirmed the activation of the epoxide via hydrogen bonding for the iodide salt, which facilitates the ring-opening step. Notably, the effective Gibbs energy barrier regarding this step is 97 kJ mol-1 for the bromide and 72 kJ mol-1 for the iodide salt, which explains the difference in activity.

Periodic density functional theory calculations were employed to study the mechanisms of H2O dissociative adsorption on the Ni(211) surface at different coverages. Due to stronger H-bonding, H2O cl...

A chemo‐, regio‐, and stereoselective mono‐hydroamidation of (un)symmetrical 1,3‐diynes is described. Key for the success of this novel transformation is the utilization of an advanced palladium catalyst system with the specific ligand Neolephos. The synthetic value of this general approach to synthetically useful α‐alkynyl‐α, β‐unsaturated amides is showcased by diversification of several structurally complex molecules and marketed drugs. Control experiments and density‐functional theory (M06L‐SMD) computations also suggest the crucial role of the substrate in controlling the regioselectivity of unsymmetrical 1,3‐diynes., The advanced Pd catalyst system with Neolephos as a ligand enables highly selective hydroamidation of unbiased (un)symmetrical 1,3‐diynes. In this way, a wide range of synthetically useful α‐alkynyl‐α,β‐unsaturated amides are afforded in good to high yields with excellent chemo‐, regio‐, and stereoselectivities.

Here, we describe a selective palladium catalyst system for chemodivergent functionalization of alkynes with syngas. In the presence of the advanced ligand L2 bearing 2-pyridyl substituent as a bui...

The first general and regioselective Pd‐catalyzed cyclocarbonylation to give α‐methylene‐β‐lactones is reported. Key to the success for this process is the use of a specific sterically demanding phosphine ligand based on N‐arylated imidazole (L11) in the presence of Pd(MeCN)2Cl2 as pre‐catalyst. A variety of easily available alkynols provide under additive‐free conditions the corresponding α‐methylene‐β‐lactones in moderate to good yields with excellent regio‐ and diastereoselectivity. The applicability of this novel methodology is showcased by the direct carbonylation of biologically active molecules including natural products., The advanced Pd/L11 system enables highly regioselective carbonylation of alkynols for synthesis of a wide range of 4‐membered α‐methylene‐β‐lactones.

To investigate the hydrocracking mechanisms of polycyclic aromatic hydrocarbons, we used HZSM-5 with one Al substitution (T1, T3, T5, T7, T11 and T12) as solid acid catalyst and 9,10-dihydroanthrac...

Reduction of carbonyl moieties to the corresponding alcohol using simply hydrazine hydrate has been considerably unfeasible until now due to the well-known condensation reaction. However, herein, we report that using an excess of 20-fold equivalents, the reduction proceeds in excellent yields.

A general and atom‐economical synthesis of 1,1‐diborylalkanes from alkenes and a borane without the need for an additional H2 acceptor is reported for the first time. The key to our success is the use of an earth‐abundant zirconium‐based catalyst, which allows a balance of self‐contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H2 acceptor, which was required in other reported systems. Furthermore, substrates such as simple long‐chain aliphatic alkenes that did not react before also underwent 1,1‐diboration in our system. Significantly, the unprecedented 1,1‐diboration of internal alkenes enabled the preparation of 1,1‐diborylalkanes., An earth‐abundant zirconium‐based catalyst has been developed for the selective 1,1‐diboration of bulk and inexpensive alkenes with a borane without the need for an additional H2 acceptor. Many aryl alkenes as well as long‐chain aliphatic alkenes convert smoothly into their corresponding 1,1‐diborylalkanes and the method also allows the unprecedented remote 1,1‐diboration of internal alkenes.

A few years ago, we introduced alternative-type bulk ZrO2-based catalysts for nonoxidative propane dehydrogenation (PDH). Currently, they belong to the state of the art catalysts owing to their env...

The surface and metal-dependent morphologies and energies of molybdenum carbide supported metal catalysts (Mn/MoxC; M = Co, Ni, Cu, Pd, Pt) have been systematically investigated on the basis of periodic density functional theory computations. On the hexagonal Mo2C phase, metallic Mo2C(001) prefers dispersion at very low loading for Co and Ni, up to high loading for Pd and Pt; but only aggregation for Cu. On Mo/C-mixed Mo2C(101), all metals prefer dispersion up to high loading and aggregation at very high loading. On cubic Mo/C-mixed δ-MoC(001), all metals prefer aggregation up to high loading and three-dimensional clusters at very high loading. On Mo2C(001), all adsorbed metals are negatively charged, while on Mo2C(101) and MoC(001), Co, Ni and Cu are positively charged, and Pt is negatively charged, while Pd is nearly neutral. On all three surfaces, the adsorption of these metals is favored thermodynamically. Although correlation between electron transfer and adsorption energy has been found, no correlations between the adsorption energy (and electron transfer) and electronegativity (and ionization potential and electron affinity) of the atoms can be estimated. These results provide a basis for the study and design of size- and shape-dependent (single-atom, nanocluster and nanoparticle) catalysts and their respective associated catalytic processes.