As a typical atomically dispersed catalyst, single -atom catalysts (SACs) aroused enormous research interest in heterogeneous catalysis. In recent years, by incorporating one or several metal atom(s) into the first and/or outer coordination shells of single atoms, the structure of metal has evolved to densely populated single atoms, dual -metal pairs, and multi-atom ensembles. The inter-site or inter-atom synergetic interactions between adjacent metal atoms in these structures boost the adsorption and activation of reactants, potentially leading to improved catalytic performances as compared with corresponding SACs. This perspective highlights the structural advantages of densely populated single atoms, dual -metal pairs, and multi-atom ensembles using many of the successful examples, aiming to shed light on the design of novel catalysts at atomic level.

It is of great significance for upgrading single-atom catalysts (SACs) to further improve their intrinsic catalytic activities while maintaining the merits of maximum atom utilization and high selectivity. Here, we report a simple and practical strategy to construct phosphorus (P) atom bridged ruthenium (Ru) ensemble (Ru-P-Ru) in carbon skeleton, which is achieved by redispersing Ru clusters with C-P species and in-situ generated PH3. The turnover frequency of the Ru-P-Ru catalyst is 9-fold higher than that of the RuP4 SAC in the selective hydrodeoxygenation of o-phthalic anhydride, as well as other diverse hydrogenations of C=X bonds (X = O, C, N) with good recyclability. Experimental and computational studies reveal that the d-band centers of Ru in the Ru-P-Ru are closer to the Fermi level than that of isolated Ru SAC, significantly promoting the absorption and activation of substrates. This fabrication strategy is also applicable to other M-P-M catalysts, enriching the knowledge of atomically dispersed catalysts.

Single-atom catalysts (SACs) have well-defined active sites, making them of potential interest for organic synthesis 1 – 4 . However, the architecture of these mononuclear metal species stabilized on solid supports may not be optimal for catalysing complex molecular transformations owing to restricted spatial environment and electronic quantum states 5 , 6 . Here we report a class of heterogeneous geminal-atom catalysts (GACs), which pair single-atom sites in specific coordination and spatial proximity. Regularly separated nitrogen anchoring groups with delocalized π-bonding nature in a polymeric carbon nitride (PCN) host 7 permit the coordination of Cu geminal sites with a ground-state separation of about 4 Å at high metal density 8 . The adaptable coordination of individual Cu sites in GACs enables a cooperative bridge-coupling pathway through dynamic Cu–Cu bonding for diverse C–X (X = C, N, O, S) cross-couplings with a low activation barrier. In situ characterization and quantum-theoretical studies show that such a dynamic process for cross-coupling is triggered by the adsorption of two different reactants at geminal metal sites, rendering homo-coupling unfeasible. These intrinsic advantages of GACs enable the assembly of heterocycles with several coordination sites, sterically congested scaffolds and pharmaceuticals with highly specific and stable activity. Scale-up experiments and translation to continuous flow suggest broad applicability for the manufacturing of fine chemicals.

Direct oxidative amination of the sp(3) C-H bond is an attractive synthesis route to obtain amides. Conventional catalytic systems for this transformation are based on transition metals and complicated synthesis processes. Herein, direct and efficient oxidative amination of the methyl C-H bond in a wide range of N-heterocycles to access the corresponding amides over metal-free porous carbon is successfully developed. To understand the fundamental structure-activity relationships of carbon catalysts, the surface functional groups and the graphitization degree of porous carbon have been purposefully tailored through doping with nitrogen or phosphorus. The results of characterization, kinetic studies, liquid-phase adsorption experiments, and theoretical calculations indicate that the high activity of the carbon catalyst is attributed to the synergistic effect of surface acidic functional groups (hydroxyl/carboxylic acid/phosphate) and more graphene edge structures exposed on the surface of carbon materials with a high graphitization degree, in which the role of acidic functional groups is to adsorb the substrate molecule and the role of the graphene edge structure is to activate O-2.

Single-atom metal-nitrogen-carbon (M-N-C) catalysts have sparked intensive interests, however, the development of an atomically dispersed metal-phosphorus-carbon (M-P-C) catalyst has not been achieved, although molecular metal-phosphine complexes have found tremendous applications in homogeneous catalysis. Herein, we successfully construct graphitic phosphorus species coordinated single-atom Fe on P-doped carbon, which display outstanding catalytic performance and reaction generality in the heterogeneous hydrogenation of N-heterocycles, functionalized nitroarenes, and reductive amination reactions, while the corresponding atomically dispersed Fe atoms embedded on N-doped carbon are almost inactive under the same reaction conditions. Furthermore, we find that the catalytic activity of graphitic phosphorus coordinated single-atom Fe sharply decreased when Fe atoms were transformed to Fe clusters/nanoparticles by post-impregnation Fe species. This work can be of fundamental interest for the design of single-atom catalysts by utilizing P atoms as coordination sites as well as of practical use for the application of M-P-C catalysts in heterogeneous catalysis.

Bimetallic catalysts based on nonprecious transition metals have attracted increasing attention because of their unique synergistic effects in catalytic reactions, but the understanding of the nature of synergistic effects and their roles in a specific hydrogenation reaction remains lacking. Herein, a series of bimetallic CuxCoy/Al2O3 (x/y = 5:1, 2:1, 1:1, 1:2, 1:5) nanocomposite catalysts were fabricated via the successive calcination and reductive activation process of layered double hydroxide precursors. Their catalytic performance in the selective hydrogenation of bioderived ethyl levulinate to 1,4-pentanediol (1,4-PeD) depended sensitively on the chemical composition of bimetallic CuCo catalysts. The optimal bimetallic Cu(2)Coi/Al2O3 catalyst exhibited markedly improved catalytic activity and selectivity compared to monometallic Cu/Al2O3, as confirmed by its lower apparent activation energy barrier of 65.1 kJ mol-t of the rate-determining step and its high selectivity of 93% to 1,4-PeD. Detailed characterization analyses and intrinsic catalytic studies revealed that the presence of CoOx species in the bimetallic CuxCoy/Al2O3 catalysts enhanced the metallic Cu dispersion and H-2 activation ability. More importantly, the strong electronic interaction at the interface of Cu and adjacent CoOx species modified the chemical states of Cu species to create proper surface Cu-0/Cu+ distributions and, particularly, provided synergic catalysis sites of Cu and electron-deficient CoOx species, which was primarily responsible for the excellent catalytic performance of bimetallic CuCo catalysts. The bimetallic CuCo catalysts exhibited good stability in both batch and fixed-bed continuous flow reactions. Furthermore, present CuCo nanocomposite catalyst could be applied to the highly selective hydrogenation of other carboxylic esters and lactones to synthesize valuable C4, CS, and C6 diols.

Low-cost, active and stable catalysts, with a bifunctional capability if possible, are required to achieve the chemical transformations between saturated and unsaturated N-heterocycles. In this work, Co@N-doped graphene shells (Co@NGS) was used as a bifunctional catalyst with high activity and stability for the oxidative dehydrogenation (ODH) and hydrogenation (HYD) of quinolines. The excellent performance can be attributed to the synergetic effect of N-doped graphene, underlying Co nanoparticles, and the encapsulation structure in which carbon shells protect Co from leaching and aggregation. Poisoning tests with KSCN and spectroscopic analysis clearly unveil that the active sites for ODH and HYD are quite different: N-doped graphene shells modified by Co NPs via electron transfer serve as active sites for the O-2 activation in ODH, while the underlying Co NPs promoted by N dopants favor the H-2 activation in HYD. This finding challenges the previous concept of N-doped carbon sites as active sites for both ODH and HYD. The bifunctional property is due to the access of both N-doped graphene and Co sites to small molecules in our one-pot pyrolyzed Co@NGS catalysts. (C) 2017 Elsevier Inc. All rights reserved.

The development of efficient and environmental benign non-noble bimetallic nanocatalysts is highly desirable and attractive in the upgrading of biomass-derived platform compounds to high-valued chemicals. A series of highly dispersed and versatile CuxNiy (x/y= 7:1, 3:1, 1:1, 1:3, 1:7) alloy supported nanocatalysts derived from layer double hydroxides (LDHs) precursors were fabricated and used for the selective hydrogenation of furfural to tetrahydrofurfuryl alcohol (THFA) and furfuryl alcohol (FOL). It was found that the chemical composition, preparation method and especially the reduction temperature of LDHs precursors greatly affected the properties of the resultant CuxNiy/MgAlO catalysts. Systematic characterizations revealed that the reduction temperature of catalyst precursor was closely related to the dispersion and homogeneous composition of CuNi alloy nanoparticle as well as the surface basicity of catalysts, which played crucial roles in achieving excellent catalytic performances. The optimized CuNi/MgAlO and Cu1Ni3/MgAlO nanocatalysts showed high activity and selectivity for the hydrogenation of furfural to THFA in ethanol compared with the monometallic Ni and the CuNi supported catalysts prepared with other methods, such enhanced catalytic performance was investigated to be enabled by the synergistic effect within the CuNi alloy nanoparticles. Interestingly, our bimetallic nanocatalysts could also realize efficient production of FOL from the selective hydrogenation of furfural at its aldehyde group by simply changing the solvent to methanol. Moreover, the bimetallic nanocatalysts showed good recyclability in the liquid phase hydrogenation. Our efficient and versatile CuNi alloy nanocatalysts not only provide promising candidates for effective upgrading of furfural but also broaden the application of non-noble bimetallic nanocatalysts for hydrogenative transformations. (C) 2016 Elsevier B.V. All rights reserved.

The efficient hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) over a hydrotalcite-derived non-precious metal Co/Al2O3 catalyst was achieved. Its core-shell structure and a strong interaction between Co and Al species stabilized the Co particles against leaching and sintering. This magnetic non-precious metal catalyst showed a comparable catalytic performance to a commercial Ru/C for the liquid hydrogenation of LA. It was easily separated from the reaction medium with an external magnet. The catalyst exhibited excellent recyclability, complete LA conversion and >99% GVL selectivity, and would be useful in a large scale biorefinery.

植物通过光合作用将CO2和水转化为生物质，利用生物质我们可以得到能源原料、化工中间体等有用的产品．从生物质获得可再生能源与资源方面研究得较多的是纤维素和木质素的转化［1-5］．纤维素由D-葡萄糖单体通过β-糖苷键连接而形成，水解打开β-糖苷键可以得到寡聚葡萄糖和葡萄糖单体，寡聚葡萄糖可以进一步水解为葡萄糖单体．葡萄糖单体能作为合成众多能源、化工成品的前体，例如5-羟甲基糠醛（5-HMF ）［6］，乙二醇［7-9］，丙二醇［1，11］，己二酸［8］，乙酰丙酸（LA）［12-15］，γ-戊内酯（GVL）等［16-19］．其中γ-戊内酯由于其独特的物理化学性质，在工业中有很大的用途：由于其蒸汽压（80℃时3．5 kpa）低、稳定，可以在全球范围内大量运输；其具有果香味且无毒，可以作为食品添加剂；其还可以作为高品位且生态友好的溶剂［20-22］，燃料添加剂，或者进一步合成戊酸酯［23］、5-壬酮［24］、丁烯以及长链烯烃［25-26］等，这些化合物具有更高的热值，可以作为高品味燃料的候选；γ-戊内酯还可以通过羧基化反应得到己二酸，而己二酸又是合成尼龙的前体，这就实现了从纤维素到尼龙的合成路线，及所谓的“生物质基尼龙”［10］。

Considerable effort has been applied to the development of new processes and catalysts for cellulose conversion to valuable platform chemicals. Isosorbide is among the most interesting products as it can be applied as a monomer and building block for the future replacement of fossil resource-based products. A sustainable method of isosorbide production from cellulose is presented in this work. The strategy relies on a bifunctional Ru catalyst supported on mesoporous niobium phosphate in a H-2 atmosphere under pressure without further addition of any soluble acid. Over 50% yield of isosorbide with almost 100% cellulose conversion can be obtained in 1h. The large surface area, pore size, and strong acidity of mesoporous niobium phosphate promote the hydrolysis of cellulose and dehydration of sorbitol; additionally, the appropriate size of the supported Ru nanoparticles avoids unnecessary hydrogenolysis of sorbitol. Under a cellulose/catalyst mass ratio of 43.3, the present bifunctional catalyst could be stably used up to six times, with its mesoporous structure well preserved and without detectable Ru leaching into the reaction solution.