Depolymerisation of lignin to aromatics is a challenging task. We herein report that a Cu(OAc)2/BF3·OEt2 catalyst is eﬀective in simultaneously cleaving C–C bonds in β-1 and β-O-4 ketones, yielding esters and phenols. In-depth studies show that C–H bond activation is the rate determining step for C–C bond cleavage. BF3·OEt2 promotes the reaction via activating the β-C–H bond. This study oﬀers the potential to obtain aromatic esters from lignin.
Conversion of low-carbon olefins to higher alcohols or olefins via the formation of C–C bonds is an increasingly important topic. We herein report an example of converting isobutene and formaldehyde (38 wt % aqueous solution) to 3-methyl-1,3-butanediol (MBD), a precursor for isoprene. The reaction occurs through a Prins condensation–hydrolysis reaction over a praseodymium (Pr)-doped CeO2 catalyst. The best MBD yield (70%) is achieved over the Pr-doped CeO2 catalyst. Catalyst characterizations with high-angle annular dark field transmission electron microscopy (HAADF-TEM), pyridine adsorption infrared (IR) and Raman spectroscopy, and density functional theory (DFT) calculations show that the doped Pr is uniformly and highly dispersed in the CeO2 crystalline phase. In addition, the Pr doping creates more oxygen vacancy sites on CeO2 and thus enhances the Lewis acidity of the catalyst, which is responsible for the catalytic performance of the Pr-CeO2 catalyst.
We herein report a new strategy of directly converting amines and CO to formamides with 100% atom utilization efficiency. It is suitable for up to 25 amine substrates with no additives. Ru/ceria is found to be an excellent catalyst for this reaction due the efficient co-activation of CO and amine on Ru species.
One of the challenges of depolymerizing lignin to valuable aromatics lies in the selective cleavage of the abundant C–O bonds of β-O-4 linkages. Herein we report a photocatalytic oxidation–hydrogenolysis tandem method for cleaving C–O bonds of β-O-4 alcohols. The Pd/ZnIn2S4 catalyst is used in the aerobic oxidation of α-C–OH of β-O-4 alcohols to α-C═O with 455 nm light, and then a TiO2–NaOAc system is employed for cleaving C–O bonds neighboring the α-C═O bonds through a hydrogenolysis reaction by switching to 365 nm light. Interestingly, the oxidation–hydrogenolysis tandem reaction can be conducted in one pot to offer ketones and phenols (up to 90% selectivity) via a dual light wavelength switching (DLWS) strategy. EPR and metal loading experiments elucidate that Ti3+ in TiO2 is formed in situ and is responsible for the photocatalytic hydrogenolysis through electron transfer from Ti3+ to the β-O-4 ketones.
Efficient cleavage of lignin β-O-4 ether bonds to produce aromatics is a challenging and attractive topic. Recently a growing number of studies reveal the initial oxidation of CαHOH to Cα=O can decrease the β-O-4 bond dissociation energy (BDE) from 274.0 kJ•mol-1 to 227.8 kJ•mol-1, and thus the β-O-4 bond is more readily cleaved in the subsequent transfer hydrogenation, or acidolysis. Here we show that the first reaction step, except in the above-mentioned pre-oxidation methods, can be a Cα-OH bond dehydroxylation to form a radical intermediate on the acid-redox site of a NiMo sulfide catalyst. The formation of a Cα radical greatly decreases the Cβ-OPh BDE from 274.0 kJ•mol-1 to 66.9 kJ•mol-1 thereby facilitating its cleavage to styrene, phenols and ethers with H2 and alcohol solvent. This is supported by control experiments using several reaction intermediates as reactants, analysis of product generation and by radical trap with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as well as by density functional theory (DFT) calculations. The dehydroxylation-hydrogenation reaction is conducted under non-oxidative condition, which is beneficial for stabilizing phenols products.
We herein report a two-step strategy for oxidative cleavage of lignin C–C bond to aromatic acids and phenols with molecular oxygen as oxidant. In the first step, lignin β-O-4 alcohol was oxidized to β-O-4 ketone over a VOSO4/TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)] catalyst. In the second step, the C–C bond of β-O-4 linkages was selectively cleaved to acids and phenols by oxidation over a Cu/1,10-phenanthroline catalyst. Computational investigations suggested a copper-oxo-bridged dimer was the catalytically active site for hydrogen-abstraction from Cβ–H bond, which was the rate-determining step for the C–C bond cleavage.
Lignin in lignocellulosic biomass is the only renewable source for aromatic compounds, and effective valorization of lignin remains a significant challenge in biomass conversion processes. We have performed density functional theory calculations and experiments to investigate the cleavage mechanism of the C–O ether bond in the lignin model compound 2-phenoxy-1-phenylethanol with a β-O-4 linkage over a Pd(111) catalyst surface model. We propose the favorable reaction pathway to proceed as follows: the dilignol reactant gets dehydrogenated first on the α-carbon and then on the −OH group to generate its corresponding ketone 2-phenoxy-1-phenylethanone; the ketone continues to get dehydrogenated on the β-carbon by first a equilibrated keto–enol tautomerization to its enol form and then −OH dehydrogenation; the C–O ether bond cleavage happens afterward, leading to one-aromatic-ring surface intermediates followed by hydrogenation to yield acetophenone and phenol.
We present an experimental and computational study of the elementary steps of hydrazine hydrogen transfer on crystalline MoO2, and demonstrate its unique bifunctional metallic-basic properties in a catalytic hydrogenation reaction. Density functional theory (DFT) calculations suggest that the stepwise hydrogen transfer via the prior cleavage of the N–H bond rather than the N–N bond, is the key step to create the dissociated hydride and proton species on the dual Mo and O sites, marking its difference with common oxides. Crystalline MoO2 shows exceptionally high chemoselectivity toward the nitro reduction over C=C, C≡C, and C≡N groups at room temperature and lower, down to 0 °C, rendering it as a promising catalytic material for hydrogenation reactions.
Selective oxidative cleavage of a C-C bond offers a straightforward method to functionalize organic skeletons. Reported herein is the oxidative C-C bond cleavage of ketone for C-N bond formation over a cuprous oxide catalyst with molecular oxygen as the oxidant. A wide range of ketones and amines are converted into cyclic imides with moderate to excellent yields. In-depth studies show that both α-C-H and β-C-H bonds adjacent to the carbonyl groups are indispensable for the C-C bond cleavage. DFT calculations indicate the reaction is initiated with the oxidation of the α-C-H bond. Amines lower the activation energy of the C-C bond cleavage, and thus promote the reaction. New insight into the C-C bond cleavage mechanism is presented.
Creation of substrate-accessible interfacial defect sites will bring about new catalytic discoveries because substrate binding and activation on these sites are pivotal for controlling reaction intermediate and product selectivity. The partial oxidation of pristine Cu2O can lead to an excellent selective oxidation catalyst (CuO/Cu2O). The CuO/Cu2O, containing embedded CuO nanodomains on the surface and possessing abundant coordinatively unsaturated copper sites at the CuO-Cu2O interface, shows very high activity toward C-C bond cleavage and excellent selectivity toward formamides in trialkylamines oxidation. This result is exceptional because the previous works mainly offer dealkylated amines via C-N bond cleavage. The unusual catalysis by CuO/Cu2O is attributed to the co-activation of oxygen and amines in close proximity at the CuO-Cu2O interface. The present study contributes a new concept of delicate controlling substrate-accessible interfacial active sites on pristine oxide surfaces, and also offers a novel formamide synthesis method by trialkylamine oxidation.
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