木质素模型分子苯氧基苯乙酮通过烯醇互变解聚氢解成苯乙酮和苯酚

Lignin depolymerized phenolic compounds and biofuel precursors are ideal value-added products for lignin residues generated in biorefineries and modern paper pulp facilities. Hydrogenolysis of lignin is an efficient depolymerization method for the production of carbon-neutral sustainable fuels and platform chemicals. Lignin is underutilized due to its complex structure, mainly because of its complex interunit linkage crosslinks such as α-O-4, β-O-4, 4-O-5, and β-5. This paper centers on the hydrolysis reaction of three major ether bonds (α-O-4, β-O-4, 4-O-5) in lignin and lignin model compounds based on different catalysts for hydrogenative degradation and catalytic systems. The methods and strategies to inhibit the condensation reactions are summarized. In particular, density functional theory calculation of the reaction pathways are combined with isotopically labeled reaction pathways to deeply analyze the hydrogenation degradation mechanism of biomass and further improve the yield of monophenols during the hydrogenation degradation of lignin. Finally, a brief summary of the challenges and prospects of lignin hydrogenation degradation is proposed.

Lignin is a by‐product of biorefineries and pulp and paper manufacturers. Lignin is a renewable source of phenolic precursors for fuels and chemicals. Hydrogenolysis of lignin cleaves the abundant β‐O‐4 bonds and releases phenolics. However, selective hydrogenolysis of lignin's β‐O‐4 bonds is challenging because it requires high‐pressure H2. Here we show efficient hydrogenolysis of lignin model compounds and technical lignin by Ru/C catalyst and internal hydrogen. The aliphatic hydroxyl groups (Cα−OH) in lignin enabled Ru‐catalyzed dehydrogenation of internal hydrogen and the formation of reactive keto intermediate, which facilitated the β‐O‐4 cleavage into phenolic monomers. Furthermore, solvents that had a high donor number (Lewis basicity) enhanced the yield of phenolic monomers, equal to 27.9 wt.% from technical lignin. These findings offer a novel approach for biorefineries to design lignin isolation processes and/or solvent systems to maximize phenolic monomers and to control product selectivity/stability.

… transfer hydrogenolysis (… acetophenone and phenol at 220 ℃ under 1 MPa N 2 for 2h. The SHTH reaction proceeded through a dehydrogenation reaction followed by a hydrogenolysis …

Abstract It has been suggested in the literature that keto‐to‐enol tautomerization plays a vital role for lignin fragmentation under mild conditions. On the other hand, previous modelling has shown that the adsorbed keto form is more stable than enol on the Pd(111) catalyst. The current density functional theory study of lignin model molecules shows that, in the gas‐phase, keto is more stable than enol, but on the Pd surface, we find enol conformers that are at least as stable as keto. This supports the experimental result that the favourable reaction pathway for lignin depolymerization involves keto‐enol tautomerization. An energy decomposition analysis gives insights concerning the origin of the fine energy balance between the keto and enol forms, where the molecule–surface interaction (−7 eV) and the molecular strain energy (+3 eV) are the main contributors to the adsorption energy.

… hydrogenation/… –enol tautomerization and that we observe quantitative conversion (100%) (Table 4, entry 2) in the presence of K 2 CO 3 , we conclude that the keto–enol tautomerization …

… /C-catalysed transfer hydrogenolysis of lignin model compounds and organosolv lignin that uses … The insertion of Pd to the enol tautomer followed by the reductive cleavage of the ether …

Abstract Although Pd/Fe bimetallic catalysts have been extensively studied in vapor-phase hydrodeoxygenation of phenolics (i.e., guaiacol, cresol), little is yet known about their performance in liquid-phase reactions. In this work, we present a mechanistic study on the Pd/Fe bimetallic catalysts in liquid-phase hydrodeoxygenation of phenolics. The role of tautomerization in hydrodeoxygenation of the lignin-derived phenolics, particularly for ring saturation, is systematically studied by employing two representative modeling compounds: phenol (a molecule that is keto-enol tautomeric) and diphenyl ether (a molecule that does not allow ketol-enol tautomerization). It was found that although the addition of oxyphilic Fe inhibits the direct aromatic ring saturation reaction typically occurring on Pd, tautomerization opens another reaction pathway toward ring saturation products (i.e. cyclohexanone, cyclohexanol, cyclohexane et al.), where both tautomerization and the hydrogenation of keto isomers are significantly enhanced to produce cyclohexanol followed by direct hydrogenolysis of the cyclohexanol to cyclohexane.

… (25) addressing keto–enol tautomerization of lignin on Pd(111) predicted higher stability of … groups on low-barrier hydrogenation involving a keto–enol tautomerization step. We have …

… found that the transfer hydrogenolysis only operated under … of the keto-to-enol tautomerization, we herein use density … a more simple lignin derivative model with the interlinkage …

… Lignin emerges as a promising source for bio-based aromatics. However, existing catalytic … keto-tautomer reaction mechanism. According to this mechanism, the initial hydrogenation of …

… 1,3-shift mechanism involving hydrido-rr-allylic intermediates; the … the £-enol is produced and dissipated faster than the Z-enol under … We present the data in terms of the amount of enol …

… the formation of an enolic intermediate could be obtained by … attained from the Pd-catalyzed hydrogenolysis of 2-methyl-1-… mechanism remains unknown; it was speculated “that enol …

Abstract Palladium on carbon catalyzes C−O bond cleavage of aryl ethers (diphenyl ether and cyclohexyl phenyl ether) by alcohols (R−OH) in H 2 . The aromatic C−O bond is cleaved by reductive solvolysis, which is initiated by Pd‐catalyzed partial hydrogenation of one phenyl ring to form an enol ether. The enol ether reacts rapidly with alcohols to form a ketal, which generates 1‐cyclohexenyl−O−R by eliminating phenol or an alkanol. Subsequent hydrogenation leads to cyclohexyl−O−R.

… benchmark, its photocatalytic mechanism and applications in … produce phenol and enol intermediates. Experimental data … 3 ), successfully achieving efficient hydrogenolysis of the lignin …

The highly‐efficient utilization of lignin and its derivatives is of great importance. Phenylacetaldehyde, as an important industrial raw material, is rarely reported to be directly obtained from biomass sources via thermal catalytic transformation. In this work, we found that NiOOH catalyst could efficiently catalyze the cleavage of β‐O‐4 lignin model compound to phenylacetaldehyde and phenol in water without need of any exogenous hydrogen. The catalytic mechanism study indicates that NiOOH could catalyze the elimination of Cα−OH to form an enol ether intermediate, which is then hydrated to phenylacetaldehyde and phenol.

Keto–enol tautomerization of carbonyl compounds to their enol form is theoretically predicted to enable a low-barrier pathway for hydrogenation of normally very stable C═O bond. In the scope of this anticipated mechanism, the reaction can proceed via two consecutive steps, including the formation of enol followed by an H insertion into the enolic C═C bond, and exhibits a lower activation barrier than the direct H insertion into the carbonyl group. Here, we present an experimental study on atomistic level details of hydrogenation of a simple carbonyl compound acetophenone over Pt(111) providing experimental evidence that keto–enol tautomerization plays a crucial role in this reaction. By employing a combination of spectroscopic and imaging techniques, we show that acetophenone forms ketone–enol dimers, in which the normally unstable form of enol is stabilized by H-bonding with the carbonyl group of the neighboring acetophenone molecule. These ketone–enol dimers can attach an H atom to form a reaction inter...

We present a mechanistic study addressing formation, adsorption configuration, and stability of acetophenone oligomers on a catalytically active Pt(111) surface by a combination of scanning tunneli...

Abstract Tautomerisation of simple carbonyl compounds to their enol counterparts on metal surfaces is envisaged to enable an easier route for hydrogenation of the C=O bond in heterogeneously catalyzed reactions. To understand the mechanisms of enol formation and stabilization over catalytically active metal surfaces, we performed a mechanistic study on keto–enol tautomerization of a monocarbonyl compound acetophenon over Pt(111) surface. By employing infrared reflection adsorption spectroscopy in combination with scanning tunneling microscopy, we found that enol can be formed by building a ketone–enol dimer, in which one molecule in the enol form is stabilized through hydrogen bonding to the carbonyl group of the second ketone molecule. Based on the investigations of the co‐adsorption behavior of acetophenone and hydrogen, we conclude that keto–enol tautomerization occurs in the intramolecular process and does not involve hydrogen transfer through the surface hypothesized previously.

We present here detailed mechanistic studies of electrocatalytic hydrogenation (ECH) in aqueous solution over skeletal nickel cathodes to probe the various paths of reductive catalytic C-O bond cleavage among functionalized aryl ethers relevant to energy science. Heterogeneous catalytic hydrogenolysis of aryl ethers is important both in hydrodeoxygenation of fossil fuels and in upgrading of lignin from biomass. The presence or absence of simple functionalities such as carbonyl, hydroxyl, methyl or methoxyl groups is known to cause dramatic shifts in reactivity and cleavage selectivity between sp3 C-O and sp2 C-O bonds. Specifically, reported hydrogenolysis studies with Ni and other catalysts have hinted at different cleavage mechanisms for the C-O ether bonds in α-keto and α-hydroxy β-O-4 type aryl ether linkages of lignin. Our new rate, selectivity, and isotopic labeling results from ECH reactions confirm that these aryl ethers undergo C-O cleavage via distinct paths. For the simple 2-phenoxy-1-phenylethane or its alcohol congener, 2-phenoxy-1-phenylethanol, the benzylic site is activated via Ni C-H insertion, followed by beta elimination of the phenoxide leaving group. But in the case of the ketone, 2-phenoxyacetophenone, the polarized carbonyl π system apparently binds directly with the electron rich Ni cathode surface without breaking the aromaticity of the neighboring phenyl ring, leading to rapid cleavage. Substituent steric and electronic perturbations across a broad range of β-O-4 type ethers create a hierarchy of cleavage rates that supports these mechanistic ideas while offering guidance to al-low rational design of the catalytic method. On the basis of the new insights, the usage of co-solvent acetone is shown to enable control of product selectivity.

… could result from keto–enol tautomerization of intermediate 4… , 8, acetophenone, 6, and benzaldehyde, 5, could be formed … with 8 formed as a subsequent of keto–enol tautomerization of …

… formation of acetophenone. The authors convincingly showed that guaiacol was formed with concurrent formation … In addition to facilitate keto enol tautomerism, KOH might also involve …

… by H + , followed by keto–enol tautomerization furnishes the hydration products 3 and 4. … to acetophenone. Our DFT calculations indicate that the conversion to acetophenone is …

Catalytic hydrogenolysis is an efficient method for converting lignin-derived aryl ethers into monoaromatic products by cleaving the C–O bond and maintaining the phenyl rings. However, it is difficult to obtain aromatics with high selectivity because ring hydrogenation proceeds in parallel. Herein, we showed that a Cl-modified Pt/γ-Al2O3 catalyst exhibited higher selectivity for aromatics (91.6%) compared to unmodified Pt/γ-Al2O3 (3.9%) in C–O bond cleavage of diphenyl ether (DPE, 4-O-5 linkage in lignin). Characterization of the catalysts and density functional theory (DFT) calculations indicated that the Cl species preferred to localize at the terrace sites of Pt nanoparticles (NPs). Due to the decrease in the number of active sites, the Cl-modified Pt/γ-Al2O3 catalyst exhibited low activity for 2-propanol dehydrogenation, and thus the catalytic activity for the hydrogenation of aromatic products was suppressed. On the other hand, DPE adsorbed at the low coordination site, which caused the phenyl rings to move away from the metal surface; this configuration was unfavorable for the hydrogenation reaction. According to DFT calculations, the high selectivity for benzene was mainly attributed to the lower energy barrier for C–O bond cleavage of phenol at the low-coordination sites.

… hydrogenolysis. The H 2 evolution reaction of 2-propanol and the density functional theory (DFT) calculations … we provided a detailed mechanism for the hydrogenolysis of DPE. Of more …

… selectivity during the hydrogenolysis of diphenyl ether (DPE), a … Ni sites for benzene ring hydrogenation. In addition, the electron… the hydrogenolysis of C–O aryl ether bonds. During the …

… Our calculations revealed that diphenyl ether is not only … density functional theory to study the mechanism of the Ni-SIPr catalyst for the hydrogenolysis of the C–O bond of diphenyl ether …

Abstract The hydrogenolysis of the aromatic C−O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (k H/k D=5.7) for the reactions of diphenyl ether under H2 and D2 atmosphere and a positive dependence of the rate on H2 chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate‐limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C−O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C−O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).

… activity towards the hydrogenolysis of diphenyl ether (DPE), a … Density functional theory (DFT) calculations further showed … lignin-related hydrogenolysis reactions under mild conditions. …

… ethers has been explored over Ni/SiO 2 catalyst at very mild conditions (393 K, 0.6 MPa). … O bond of diphenyl ether is cleaved by parallel hydrogenolysis and hydrolysis (hydrogenolysis …

… for the conversion of diphenyl ether was investigated over Rh/… analysis using density functional theory (DFT) along with the … the working conditions, DFT calculation was performed. The …

Abstract Deep insight of reaction mechanism in lignin model compounds is helpful to achieve the directed depolymerization of lignin or biomass to chemicals or fuels. In this study, the density functional theory (DFT) calculation was employed to investigate the cleavage mechanism of the C–O bonds in lignin dimers. Additionally, the intrinsic chemical reactivity of molecular in term of the Fukui function was applied to predict the most probable sites which react with hydrogen free radicals (H·). It was found that the O atoms in lignin dimers are the most reaction site involving H· because of the large f (0). By this method, the most rational path from a series of reaction paths was screen out. Apart from the Fukui function, the average local ionization energy (ALIE) was analyzed to prove the reliability of Fukui function. The kinetic analysis of the reaction path was performed to further understand the impact of temperature on the reaction rate constant (KTST). It is observed that benzyl phenyl ether (BPE) with higher KTST could be easily cleaved because of the relatively low energy barrier.

Hydrogenolysis has emerged as one of the most effective means of converting polymeric lignin into monoaromatic fragments of value. Reported yields may be higher than for other methods and can exceed the theoretical yields estimated from measures of the content of lignin's most readily cleaved alkyl-aryl ether bonds in β-ether units. The high yields suggest that other units in lignin are being cleaved. Diaryl ether units are important units in lignin, and their cleavage has been examined previously using simple model compounds, such as diphenyl ether. Herein, we analyze the hydrogenolysis of model compounds that closely resemble the native lignin 4-O-5 diaryl ether units. The results provide unexpected insights into the reactivity and partial cleavage of these compounds. The models and lignin polymer produce not only monomers, but also unusual 1,3,5-meta-substituted aromatics that appear to be diagnostic for the presence and the cleavage of the 4-O-5 diaryl ether unit in lignin.

… Ni and Co, exhibited excellent hydrogenolysis activity towards diphenyl ether (DPE). … hydrogenolysis and hydrodeoxygenation capabilities. Density Functional Theory (DFT) calculations …

… activity in the hydrogenolysis of lignin … diphenyl ether (DPE, lignin 4-O-5 linkage) as a probe molecule to explore the effect of Pt size on the DPE hydrogenolysis using DFT calculations. …

A highly efficient Ni/N–C catalyst was synthesized by facile pyrolysis of a Ni-containing metal–organic framework (Ni-MOF), and its catalytic hydrogenolysis performance towards C–O bonds in lignin was evaluated in detail using diphenyl ether (DPE) as a model compound. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses show that the flower-like nanosheets of the Ni-MOF shrink, forming a loose and ordered spherical structure during pyrolysis. Under the optimal conditions, DPE was completely converted and the selectivity to monomers (benzene, cyclohexanol and cyclohexane) reached 99.1%. During the catalytic hydrogenolysis conversion (CHC) of DPE, the direct cleavage of the Caromatic–O bond affording benzene and phenol is the major reaction pathway, and a low H2 pressure is crucial to increase the monomer selectivity. Furthermore, Ni/N–C-450 shows high hydrogenolysis activity for other lignin-derived aryl ethers, such as benzyl phenyl ether, dibenzyl ether, dinaphthalene ether, benzyl 2-naphthyl ether and 3-methoxyphenol.

… NMR spectrum of [ 13 C 2 ]-7 shows resonances at 158.4 and 107.4 ppm ( 1 J CC =80 Hz), which are consistent with the chemical shifts of an enol … bound ketyl radical intermediate. To …

Abstract The catalytic conversion of diphenyl ether (DPE), a dimeric model compound representing 4-O-5 lignin linkages, has been investigated using a hydrophobized bifunctional Pd/HY catalyst since the hydrophobicity of the catalyst significantly improved conversion and carbon balance. Partial hydrogenation of DPE was found to be an essential step before C O bond cleavage and C C bond formation, which are the target reactions in this study. The main products resulting from the C O cleavage are phenol and cyclohexanone, which subsequently can undergo C C coupling via alkylation and aldol condensation. The balance between hydrogenation activity of the metal and the acidic function of zeolite was found to play an important role for maximizing the yield of C C coupling products, which are desirable in the upgrading of lignin-derived compounds to fuel components.

… , diphenyl ether (DPE) was selected as the model compound of the 4-O-5 bond to explore the catalytic process of C–O bond cleavage … the most active catalyst for the cleavage reaction. …

The cleavage of C–O linkages of aryl ethers into aromatic platform compounds is a challenging reaction but of great importance for the sustainable future. Herein, we reported the efficient H2-assis...

… the catalyst, solvent and molecular hydrogen on the mechanism and kinetics of C,-0 bond cleavage reactions using diary1 ether … 2,2’-Dinaphthyl ether and diphenyl ether were used to …

Renewable lignocellulosic biomass holds the potential to cater to the escalating energy requirements of the expanding global population. The intricate task of creating a durable catalyst capable of selectively breaking...

… reaction pathway for diphenyl ether conversion (Figure 5b) follows two major routes: (i) hydrogenolysis of diphenyl ether … and (ii) hydrolysis of diphenyl ether to two molecules of phenol, …