Vinyl Species Research Articles

Multiscale modeling of hydrogenolysis of ethane and propane on Ru(0001): Implications for plastics recycling

Plastic waste presents an environmental threat. Chemical recycling via hydrogenolysis can convert plastic waste into waxes, lubricants, and fuels. Among catalysts, Ru stands out for its superior activity and selectivity. The chemistry of light alkane hydrogenolysis can help understanding plastics deconstruction. We perform first-principles calculations, develop descriptor-based relations, and conduct microkinetic modeling and analysis on ethane and propane. Predictions are in excellent agreement with experimental data. We identify a similar cracking pattern for both hydrocarbons entailing a deeply dehydrogenated species with the removal of four hydrogen atoms: CHCH* +* → 2CH* for ethane and CH3CCH* +2 * → CH3C* + CH* for propane. We find that the rate-determining step is the C-C cracking for ethane and the first dehydrogenation from the terminal carbon (CH3CH2CH3*+* → CH3CH2CH2*+H*) for propane. The vinyl species CH2CH* produced from propane cracking is responsible for whether a single or multiple cracking events occur and effectively controls the selectivity. Specifically, ethane formation in propane hydrogenolysis is suppressed at elevated temperatures due to over-cracking via multiple (two here) cracking events being preferred over hydrogenation and desorption of ethane from the catalyst. Our workflow and models provide a baseline for future studies on heavier hydrocarbons. Insights into recent experimental studies of polyethylene over Ru-based catalysts are discussed.

Nanotubular OMS-2 Supported Single-Atom Platinum Catalysts Highly Active for Benzene Oxidation

The cryptomelane-type octahedral molecular sieve (OMS-2) exhibits excellent catalytic activity for the oxidation of volatile organic compounds (VOCs) due to its unique structure, and single-atom catalysts (SACs) have recently attracted much attention in heterogeneous catalysis. Herein, we adopted the hydrothermal redox reaction of KMnO4 and HCl aqueous solution at 120 °C for 12 h to first synthesize the nanotubular OMS-2 support and used the vitamin C and NaBH4 reduction methods to prepare the OMS-2-supported single-atom Pt and PtNP catalysts for benzene oxidation, respectively. It was found that the OMS-2-supported single-atom Pt catalyst with a Pt loading of 0.0383 wt % (0.0383Pt1/OMS-2) exhibited the best catalytic performance for benzene oxidation (a 90% benzene conversion was achieved at 189 °C and 20 000 mL g–1 h–1 (space velocity)), which was associated with its high surface oxygen vacancy density, good low-temperature reducibility, and strong benzene adsorption ability. It was also shown that benzene could be dissociated more readily over the supported single-atom Pt1 catalyst than over the supported PtNP catalyst, and the phenolates or benzoquinone as well as methyl groups, lipid, and vinyl species were the main intermediates in the oxidation of benzene over 0.0383Pt1/OMS-2. The results of this study are useful in developing the high-performance single-atom catalysts that can be applied for the oxidative removal of VOCs.

Radical Mechanism of Ir III /Ni II -Metallaphotoredox-Catalyzed C(sp 3 )–H Functionalization Triggered by Proton-Coupled Electron Transfer: Theoretical Insight

Photoredox catalysis can be induced to activate organic substrates or to modulate the oxidation state of transition-metal catalysts via unique single-electron transfer processes, so as to achieve c...

Potassium tert‐Butoxide‐Catalyzed Synthesis of α‐Methylene‐β‐Lactams from Propiolamides

AbstractWe reported herein the regio‐ and stereoselective synthesis of α‐methylene‐β‐lactams via a 4‐exo‐dig‐cyclization of N‐benzyl‐N‐methyl‐propiolamides catalyzed by a base. The experimental evidence suggests that this transformation proceeds via a concerted ionic cyclization with a carbon nucleophilic addition to the carbon‐carbon triple bond, followed by protonation of the vinyl species promoted by tert‐butanol. The obtained α‐methylene‐β‐lactams were suitable substrates for the Heck coupling conditions affording the corresponding products via carbon‐carbon bond formation.

Gold(I)-Catalyzed Aromatization: Expeditious Synthesis of Polyfunctionalized Naphthalenes.

SummaryA gold-catalyzed 6-endo-dig carbocyclization of alkyne with the pendent diazo group is reported. It provides an expeditious approach for the synthesis of multi-functionalized naphthalene derivatives under mild conditions. Mechanistic studies suggest that a vinyl gold carbene is generated as the key intermediate in this cascade transformation that smoothly delivers naphthalene products through an unprecedented stepwise aromatization or an intermolecular aromatic substitution process. The unique endocyclic vinyl species is inaccessible with other precursors; thus, novel carbene cascade transformations could be envisioned with the current catalytic model. Functional groups, such as alkenyl, hydroxyl, amino, and carboxyl groups, remain untouched under these conditions. In addition, the utility of these generated 2-carboxyl naphthalenes is illustrated by the synthesis of chiral 1,2′-binaphthalene ligands and π-conjugated polycyclic hydrocarbons (CPHs).

Metal‐Catalyzed Remote Functionalization of ω‐Ene Unsaturated Ethers: Towards Functionalized Vinyl Species

AbstractThe combined ruthenium‐catalyzed chain walking with the nickel‐catalyzed cross‐coupling reaction of ω‐alkenyl ethers provide a unique entry to functionalized vinyl species. This transformation illustrates the power and flexibility of remote functionalization by demonstrating the compatibility of two independent reactions involving unrelated sites.

Metal-Catalyzed Remote Functionalization of ω-Ene Unsaturated Ethers: Towards Functionalized Vinyl Species.

The combined ruthenium-catalyzed chain walking with the nickel-catalyzed cross-coupling reaction of ω-alkenyl ethers provide a unique entry to functionalized vinyl species. This transformation illustrates the power and flexibility of remote functionalization by demonstrating the compatibility of two independent reactions involving unrelated sites.

Study on catalytic properties and carbon deposition of Ni-Cu/SBA-15 for cyclohexane dehydrogenation

A series of Ni-Cu supported on SBA-15 were prepared by impregnation, and used as catalysts in cyclohexane dehydrogenation for hydrogen production. The results indicated that the addition of Cu into Ni changes the crystal structure of metal Ni, and forms Ni-Cu alloy. Thus, Cu improves the reduction properties of Ni. Conversely, Ni stables and disperses metal Cu. With the space limitation of the ordered channels and high surface area of SBA-15, the bimetallic Ni-Cu/SBA-15 catalysts expose large amounts of selective active sites composed uniformly with Ni and Cu. Therefore, they present not only excellent catalytic performance for cyclohexane dehydrogenation, but also low coke formation. The in-situ DRIFT studies have shown the vinyl species, indicating the existence of alkenes in the reactive intermediates. Additionally, the strong absorption of benzene on the metal could induce benzene was dehydrogenated further to carbon.

The adsorption of ethylene on Au/Pd(100) alloy surfaces

The surface chemistry of ethylene is explored on model Au/Pd(100) alloy surfaces using a combination of temperature-programmed desorption and reflection–absorption infrared spectroscopy. The heat of adsorption of ethylene on the model alloy surface is found to increase monotonically with increasing palladium coverage in the alloy, from ~33kJ/mol for a completely gold-covered surface to ~80kJ/mol as the gold coverage decreases to zero. A large change in heat of adsorption is found for palladium coverages between 0 and ~0.35 monolayers, where previous studies have shown that the surface comprises exclusively isolated palladium sites. The heat of adsorption changes more slowly for higher palladium coverages, when palladium–palladium bridge sites appear. Vinyl species are identified for palladium coverages above ~0.8 MLfrom a vibrational mode at ~1120cm−1, which disappears when the sample is heated to ~250K, due to vinyl decomposition.

Fe(iv) alkylidenes via protonation of Fe(ii) vinyl chelates and a comparative Mössbauer spectroscopic study.

Treatment of cis-Me2Fe(PMe3)4 with di-1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H2), a tetradentate ligand precursor, afforded (bdvp)Fe(PMe3)2 (1-PMe3) and 2 equiv. CH4, via C-H bond activation. Similar treatments with tridentate ligand precursors PhCH[double bond, length as m-dash]NCH2(E-CH[double bond, length as m-dash]CHPh) ((pipp)H2) and PhCH[double bond, length as m-dash]N(2-CCMe-Ph) ((pipa)H) under dinitrogen provided trans-(pipp)Fe(PMe3)2N2 (2) and trans-(pipvd)Fe(PMe3)2N2 (3), respectively; the latter via one C-H bond activation, and a subsequent insertion of the alkyne into the remaining Fe-Me bond. All three Fe(ii) vinyl species were protonated with H[BArF 4] to form the corresponding Fe(iv) alkylidene cations, [(bavp)Fe(PMe3)2][BArF 4] (4-PMe3), [(piap)Fe(PMe3)3][BArF 4] (5), and [(pipad)Fe(PMe3)3][BArF 4] (6). Mössbauer spectroscopic measurements on the formally Fe(ii) and Fe(iv) derivatives revealed isomer shifts within 0.1 mm s-1, reflecting the similarity in their bond distances.

Computational Study of Carbostannylation Implicating Bimetallic Catalysis Involving “AuI–Vinyl–PdII” Species

We have investigated computationally the gold and palladium cocatalyzed reaction of alkynes with vinylstannane. Our work has involved a careful and thorough exploration of different mechanistic possibilities. We find that palladium acting alone as a catalyst leads to a very high reaction barrier, consistent with the experimental observation that there is no reaction in the presence of just palladium. However, the involvement of a gold(I) complex lowers the reaction barrier considerably, and the vinylstannylation reaction can proceed with a modest activation energy of about 10 kcal/mol. Our key finding is that the introduction of the gold complex avoids the formation of high-energy structures involving vinyl species in a trans arrangement on palladium. Our work confirms the role of intermediates containing both palladium and gold as suggested by Blum. For the gold–palladium cocatalyzed reaction, we also investigated an alternative mechanism suggested by Blum. With some modifications, this mechanism has a slightly higher reaction barrier, but if it does occur, then we predict a strong dependency on the counterion, in agreement with related experimental findings.

Density functional theory study of the adsorption and reaction of C2H4 on Fe3C(100)

Spin-polarized density functional theory (DFT) and a periodic slab model were employed to investigate the adsorption of C 2 H 4 on Fe 3 C(100), which is an active phase of an Fe-based catalyst for Fischer-Tropsch synthesis. The competition between dehydrogenation and cleavage of C 2 H 4 was analyzed. The μ-bridging adsorption mode is more stable than the π or di-σ adsorption modes. Partial rehybridization of the C atoms of C 2 H 4 ( sp 2 → sp 3 ) caused by the interaction of C 2 H 4 with the Fe 3 C(100) surface resulted in the C atoms in C 2 H 4 having a quasi-tetrahedron geometry. On Fe 3 C(100) dehydrogenation of C 2 H 4 occurs, while C-C bond cleavage is not competitive. The calculations indicated that vinylidene (CCH 2 ) and vinyl (CHCH 2 ) species are the most abundant C 2 species, which may be the major monomeric forms of C 2 H 4 in the chain growth in Fischer-Tropsch synthesis DFT calculations indicate that on Fe 3 C(100), C 2 H 4 favors dehydrogenation reaction, while the C-C cleavage is not competitive. Vinylidene (CCH 2 ) and vinyl (CHCH 2 ) are the most abundant.

Microstructural Evolution in Linear Low Density Polyethylene During Peroxide Modification: A Monte Carlo Simulation Study

A Monte Carlo (MC) simulation is performed on peroxide induced branching reactions in a linear low density polyethylene (LLDPE) melt processed with different dicumyl peroxide (DCP) concentrations at various processing times and temperatures. The MC simulation can successfully monitor the microstructural changes due to the induced long chain branching (LCB), decreased terminal double bonds, and increased trans vinyl bonds in LLDPE during peroxide modification. The induced structural changes are in good agreement with the experimental results obtained by rheometery and SEC analysis. The changes in the gel content, , and MWD as a result of the peroxide modification are also determined. The simulation results also reveal that the chemical mechanism by which the modification proceeded is closely related to the changes in concentrations of reactive vinyl species of the PE molecules. Moreover, the results indicate that the degree of induced LCB increases almost linearly with the DCP concentrations. The findings of this paper indicate that the MC simulation method is able to predict the suitable processing conditions needed for inducing desirable structural changes in a LLDPE.

Building a Parent Iridabenzene Structure from Acetylene and Dichloromethane on an Iridium Center

Parenthood: The reaction of [TpIr(C2H4)2] (1) (Tp=hydrotris(pyrazolyl)borate) with acetylene in CH2 Cl2 affords a 1:1 mixture of the "parent" metallabenzene 2 (that is, all the ring carbon centers are CH units) and the β-Cl substituted vinyl species 3. Generation of 2 is by the coupling of an iridacyclopentadiene (formed from two acetylene molecules at the Ir center) with the dichloromethane-derived chlorocarbene ":C(H)Cl" and a subsequent α-Cl elimination event.

Acetate coverage effect on the reactivity of vinyl acetate synthesis on Pd/Au alloy surfaces

Vinyl acetate (VA) synthesis on Pd/Au(111) and Pd/Au(100) surfaces has been systematically investigated through first-principles density functional theory (DFT) calculations. The DFT results showed that for VA synthesis, the ‘Samanos’ reaction mechanism (i.e., direct coupling of coadsorbed ethylene and acetate species and subsequent β-hydride elimination to form VA) is more favorable than the ‘Moiseev’ mechanism (i.e., ethylene first dehydrogenates to form vinyl species which then couple with the coadsorbed acetate species to form VA). More importantly, it was found the surface coverage of acetate has a significant effect on the reactivity of VA synthesis, and the activation energy of the rate-controlling step on Pd/Au(100) surface is smaller than that on Pd/Au(111) surface (0.88 vs. 0.95 eV), indicating the former is more active than the latter.

Synthesis of novel heteroditopic carbene–pyridine palladium(II) chloro vinyl complexes. Comparative reactivity of different palladium vinyl derivatives toward transmetalation with alkynyl stannane

Abstract DeC-activated alkynes such as di-t-butyl-ethyne-dicarboxylate (DTA) or the more reactive dimethyl-ethyne-dicarboxylate (DMA) do not react with palladium(II) chloro methyl complexes bearing heteroditopic carbene–pyridine (C–N) as spectator ligands to give the corresponding vinyl derivatives. In order to prepare this type of derivatives we resorted to a dedicated synthetic protocol based on the displacement of labile pyridine-thioether ligands of vinyl palladium pyridine-thioeter species by the carbene–pyridine ligand of carbene–pyridine silver chloride complexes. However, the novel carbene–pyridine vinyl complexes obtained by such synthetic protocol, if compared with other palladium vinyl species with similar steric requirements, display a markedly reduced reactivity toward transmetalation with ethynyl-stannane.

Fast pyrolysis of coconut biomass – An FTIR study

The reaction pathway of coconut shell fast pyrolysis was studied by analysis of the transient evolution product profiles as a function of temperature, measured directly in the sample bed. Fast pyrolysis of coconut shell produced (i) pyrolysis liquid containing CH, CO, and COC bands, (ii) char with the absence of COH and CO suggesting that ether and carbonyl compounds were decomposed below 600°C, and (iii) gaseous product majorly consisting of CO2. Increase the heating rate has a greater effect on increasing the yield of aliphatic and carbonyl compounds compared to that of aromatic compounds. The selectivity of the fast pyrolysis liquid was shifted from vinyl species to carbonyl species at temperatures between 500 and 615°C. In situ IR study suggests that CO and CO2 were mainly produced from cracking of CH and CO bonds. Fast pyrolysis of D2O-saturated coconut shell showed that moisture in the biomass does not increase the H2 yield at high temperature and H2 is mainly produced from breaking of CH bonds.

Identification at the Single Molecule Level of C2Hx Moieties Derived from Acetylene on the Pt(111) Surface

A low-temperature scanning tunneling microscope operated at 4.7 K was used to observe individual molecules produced from the thermal decomposition and hydrogenation reactions of acetylene on the Pt(111) surface. Acetylene molecules observed on the surface following adsorption at 50 K are seen to persist up to room temperature at the same time as two other moieties are observed to form. One moiety greatly increases in amount when the acetylene is coadsorbed with hydrogen and is attributed to the vinyl species, HCCH2, in agreement with a recent study using reflection absorption infrared spectroscopy. A third species observed at room temperature is identified as vinylidene, CCH2. The identification of these species is aided by using voltage pulses from the STM to further decompose them into species containing fewer atoms. Ethylidyne, CCH3, is identified after heating the surface to 400 K and was confirmed by comparison to results obtained following ethylene adsorption. Exposure of the surface held at 800 K to acetylene produced C2 molecules on the surface, which could be subsequently hydrogenated to ethylidyne. The hydrogenation of residual surface carbon also leads to the formation of ethylidyne, suggesting that the residual carbon was in the form of C2 molecules rather than carbon atoms.

Mechanistic Analysis of Gold(I)-Catalyzed Intramolecular Allene Hydroalkoxylation Reveals an Off-Cycle Bis(gold) Vinyl Species and Reversible C–O Bond Formation

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C-O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex.

The adsorption and reaction of vinyl acetate on Au/Pd(100) alloy surfaces

The surface chemistry of vinyl acetate monomer (VAM) is studied on Au/Pd(100) alloys as a function of alloy composition using temperature-programmed desorption and reflection–adsorption infrared spectroscopy. VAM adsorbs weakly on isolated palladium sites on the alloy with a heat of adsorption of ~55kJ/mol, with the plane of the VAM adsorbed close to parallel to the surface. The majority of the VAM adsorbed on isolated sites desorbs molecularly with only a small portion decomposing. At lower gold coverages (below ~0.5ML of gold), where palladium–palladium bridge sites are present, VAM binds to the surface in a distorted geometry via a rehybridized vinyl group. A larger proportion of this VAM decomposes and this reaction is initiated by CO bond scission in the VAM to form adsorbed acetate and vinyl species. The implication of this surface chemistry for VAM synthesis on Au/Pd(100) alloys is discussed.