Xingbang Hu

CO oxidation on metal-free nitrogen-doped carbon nanotubes and the related structure–reactivity relationships

The reduction or elimination of noble metal used in carbon monoxide (CO) oxidation remains a challenge. Based on systematic research with methods using density functional theory, metal-free nitrogen-doped carbon nanotubes (NCNTs) are found to be effective catalysts for CO oxidation. The reaction route involves the direct oxidization of CO to CO2 by O2 adsorbed on the surface of NCNTs, as in CO + O2 → CO2 + O. The remaining O atom can further oxidize CO to CO2. The barrier heights of the rate-determining step range from 0.477 to 0.619 eV for different catalysts investigated in the present study. These values are quite close to those processes using noble metal catalysts. The catalytic ability of NCNTs is influenced by the tube length, diameter and number of doped N atoms. The barrier height for the rate-determining step slightly oscillates with increased tube length, and progressively increases with expanded tube diameter. The structure–reactivity relationships of NCNTs in CO oxidation are revealed. A lower negative charge of the doped N atom corresponds to more reactive NCNTs. NCNT–O2 with large spin densities on the O atom also show more powerful reactivity for CO oxidation. The present paper provides a way for the development of metal-free catalysts for CO oxidation.

Dicarboxylic acid salts as task-specific ionic liquids for reversible absorption of SO2 with a low enthalpy change

Six acid salt ionic liquids (ASILs), triethylbutylammonium dicarboxylates, have been synthesized to act as green materials for SO2 capture. The experimental results reveal that the ASILs can trap SO2 reversibly and chemically with a large capacity of 0.112 up to 0.232 (mass ratio) at 15.5 kPa and of 0.374 to 0.456 (mass ratio) at 100 kPa and 40 °C. Two of these ASILs are interestingly found to have a low viscosity that enables the fast mass transfer of SO2. A reaction mechanism is proposed to explain the chemical absorption based on FTIR spectra and structural calculations using the density functional theory. Thermodynamic analysis indicates the enthalpy of the reaction of SO2 with the ASILs is low (−29.9 and −42.2 kJ mol−1 for [N2224][dimaleate] and [N2224][dimalonate], respectively). Additionally, the ASILs have a high thermal stability, which favors their potential application in flue gas desulfurization.

Hydration of alkynes at room temperature catalyzed by gold(I) isocyanide compounds

An effective method using gold(I) isocyanide complexes as catalysts for the transformation of various alkynes to the corresponding ketones is successfully developed. The hydration process proceeds smoothly at room temperature with quite high yield (up to 99%). The catalytic center is the isocyanide-Au(I)+ cation. Further theoretical research reveals a direct hydration mechanism by H2O, and the rate-determining step has an energy barrier of 23.7 kcal mol−1. These results show a good example to reduce unnecessary steps and achieve milder reaction conditions at the same time for the hydration of alkynes.

Impact of α-D-glucose pentaacetate on the selective separation of CO2 and SO2 in supported ionic liquid membranes

A biocompatible, nontoxic, and nonvolatile compound, α-D-glucose pentaacetate (GPA), was impregnated in supported ionic liquid membranes (SILMs) and used for the selective separation of CO2 and SO2. It was found that GPA could cooperate with [Bmim][BF4] to influence the permeability of CO2 and enhance the CO2/N2 selectivity. Further permeation experiments demonstrated that the addition of GPA could reduce the permeability of N2 in the SILMs and thereby improve the CO2/N2 and SO2/N2 selectivities. The highest CO2/N2 selectivity obtained in the paper is 79 ± 0.9 for dry gas and 86 ± 1.6 for humidified gas, and the highest SO2/N2 selectivity is 686 ± 19.8 for dry gas. This environmentally benign separation process with high gas permeability and good selectivity may be expected to have a potential application for the separation of CO2 and SO2 from a flue gas stream.

The ionic liquid-mediated Claus reaction: a highly efficient capture and conversion of hydrogen sulfide

Ionic liquids (ILs) were demonstrated to be highly efficient media for the liquid-phase Claus reaction. The reaction of H2S with SO2 in ILs proceeds very fast and almost completely to result in solid sulfur (S8) under mild conditions without the addition of any catalysts. Various ILs with different cations and anions were investigated and a simple IL 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]) was found to be the most effective for the capture and conversion of H2S. It enables the transformation of H2S to S8 with a conversion ratio as high as >96% within 3 min. This finding opens up a promising method for the capture and conversion of H2S from gas streams.

Room‐Temperature Hydration of Alkynes Catalyzed by Different Carbene Gold Complexes and their Precursors

The room‐temperature hydration of alkynes catalyzed by NHC‐gold(I) (NHC=N‐heterocyclic carbene), NAC‐gold(I) (NAC=nitrogen acyclic carbene), and isocyanide gold(I) complexes was investigated carefully in the presence of different weakly coordinating anions. NHC(IPr)‐AuCl/KB(C6F5)4 (NHC(IPr)=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) was found to be the most active catalyst at room temperature, and the room‐temperature hydration of different alkynes could be completed in 7 h using only 0.5 mol % NHC(IPr)‐AuCl/KB(C6F5)4. It was further demonstrated that the catalyst system could be simply reused at least six times without a noticeable loss of catalytic activity.

Oxidation of olefins using molecular oxygen catalyzed by a part per million level of recyclable copper catalyst under mild conditions

Copper catalysts with an imidazole salt tag ([Cu-Imace-R-H][X], X− = F−, Cl−, Br−, I−, CF3CO2−, HSO4−, NO3−, PF6− or BF4−; R = H or CH3) show quite high reactivity for the oxidation of non-aromatic olefins with good selectivity for epoxides. The reactions perform well with a part per million (ppm) catalyst loading at mild temperature and ambient pressure. The highest turnover frequency (TOF) reaches up to 900 000 h−1. The catalytic activity is easy to control by changing the anion of [Cu-Imace-R-H][X]. This catalyst is effective for a series of substrates, including internal and terminal olefins, tri- and tetra-substituted olefins and aromatic olefins. In addition, the copper catalyst can be conveniently separated from the reaction system and reused for at least six cycles without any obvious loss of catalytic activity.

The Effect of Nano Confinement on the C–H Activation and its Corresponding Structure-Activity Relationship

The C–H activation of methane, ethane, and t-butane on inner and outer surfaces of nitrogen-doped carbon nanotube (NCNTs) are investigated using density functional theory. It includes NCNTs with different diameters, different N and O concentrations, and different types (armchair and zigzag). A universal structure-reactivity relationship is proposed to characterize the C–H activation occurring both on the inner and outer surfaces of the nano channel. The C–O bond distance, spin density and charge carried by active oxygen are found to be highly related to the C–H activation barriers. Based on these theoretical results, some useful strategies are suggested to guide the rational design of more effective catalysts by nano channel confinement.

Tandem copper hydride–Lewis pair catalysed reduction of carbon dioxide into formate with dihydrogen

AbstractThe reduction of CO2 into formic acid or its conjugate base, using dihydrogen, is an attractive process. While catalysts based on noble metals have shown high turnover numbers, the use of abundant first-row metals is underdeveloped. The key steps of the reaction are CO2 insertion into a metal hydride and regeneration of the metal hydride with H2, along with the concomitant production of formate. For the first step, copper is known as one of the most efficient metals, as shown by the numerous copper-catalysed carboxylation reactions, but this metal has difficulties activating H2 to achieve the second step. Here, we report a catalytic system involving a stable copper hydride that activates CO2, working in tandem with a Lewis pair that heterolytically splits H2. In this system, unprecedented turnover numbers for copper are obtained. Surprisingly, through a combination of stoichiometric and catalytic reactions, we show that classical Lewis pairs outperform frustrated Lewis pairs in this process.Due to its ready availability and low cost, copper is an attractive metal for the homogeneous reduction of CO2 to formate. However, although CO2 can readily insert into copper hydrides to produce metal-bound formate, subsequent regeneration of the catalytic species with H2 is more challenging. Here a dual strategy is used, whereby a copper hydride activates CO2 and a Lewis pair heterolytically splits H2, leading to dramatically improved performance.

Structure and asymmetric epoxidation reactivity of chiral Mn(III) salen catalysts modified by different axial anions

A series of chiral Mn(III) catalysts [salen–Mn(III)][X] (X− = Cl−, OAc−, NO3−, BF4−, CF3SO3−, OCH2CH3−) were synthesized by ion exchange. The influence of the axial anion on both the electronic structure and steric configuration of [salen–Mn(III)][X] were carefully investigated. Besides, the reactivity and enantioselectivity of these catalysts were explored in the asymmetric epoxidation of olefins. The obtained results indicate that the axial anions have influences on both electronic structure and steric configuration of the chiral Mn(III) salen complexes. Controlling the reactivity and enantioselectivity of these chiral Mn(III) salen complexes can be achieved by changing the axial anions.