Adsorption and activation of small molecules on boron nitride catalysts.

Hexagonal boron nitride possesses a unique layered structure, high specific surface area and similar electronic properties as graphene, which makes it not only a promising catalyst support, but also a highly effective metal-free catalyst in the booming field of green chemistry. Reactions involving small molecules (e.g., oxygen, low carbon alkanes, nitrogen and carbon dioxide) have always been a hot topic in catalytic research, especially associated with the adsorption and activation regime of different forms of small molecules on catalysts. In this review, we have investigated the adsorption of different small molecules and the relevant activation mechanisms of four typical chemical bonds (OO, C-H, NN, CO) on hexagonal boron nitride. Recent progress on approaches adopted to enhance the activation capacity such as doping, defect engineering and heterostructuring are summarized, highlighting the potential applications of nonmetallic hexagonal boron nitride catalysts in various reactions. This comprehensive investigation offers a reference point for the enhanced mechanistic understanding and future design of effective and sustainable catalytic systems based on boron nitride.

Copper-Catalyzed and Proton-Directed Selective Hydroxymethylation of Alkynes with CO2.

An intriguing strategy for copper-catalyzed hydroxymethylation of alkynes with CO2 and hydrosilane was developed. Switched on/off a proton source, for example, t BuOH, direct hydroxymethylation and reductive hydroxymethylation could be triggered selectively, delivering a series of allylic alcohols and homobenzylic alcohols, respectively, with high levels of Z/E, regio- and enantioselectivity. Such a selective synthesis is attributed to the differences in response of vinylcopper intermediate to proton and CO2 . The protonation of vinylcopper species is demonstrated to be prior to hydroxymethylation, thus allowing a diversion from direct alkyne hydroxymethylation to reductive hydroxymethylation in the presence of suitable proton.

Template-induced Al distribution in MOR and enhanced activity in dimethyl ether carbonylation.

As the activity of dimethyl ether (DME) carbonylation over mordenite proportionally correlates with the Brønsted acid sites (BAS) in 8-membered ring (8-MR), enhancing the concentration of BAS in the 8-MR of MOR is important to improve the efficiency of the reaction. Herein, we report that the distribution of the BAS in the zeolite catalyst H-MOR can be altered by the synthesis of H-MOR with different cyclic amine structure-directing templates, several of which have not been reported previously for MOR synthesis. By combining FTIR, ICP, TG analysis and DFT calculations, it is verified that the strength of the interaction between amine or sodium cations and [AlO4]- in the zeolite framework plays a decisive role in Al distribution, owing to the competitive effect between Na+ and the cyclic amine compensating negative charges from the framework [AlO4]-. Quantitative analysis of the BAS in the 12-MR and 8-MR identifies the optimum template for maximizing the BAS in the 8-MR. It is shown that the enhanced activity of the H-MOR for the DME carbonylation to methyl acetate correlates with the increase in the BAS in the 8-MR. Our finding thus provides a facile strategy to direct Al location within different channels of the zeolite, which must benefit spatially confined reaction systems.

Recent advances in dialkyl carbonates synthesis and applications.

Dialkyl carbonates are important organic compounds and chemical intermediates with the label of "green chemicals" due to their moderate toxicity, biodegradability for human health and environment. Indeed, owing to their unique physicochemical properties and versatility as reagents, a variety of phosgene-free processes derived from CO or CO2 have been explored for the synthesis of dialkyl carbonates. In this critical review, we highlight the recent achievements (since 1997) in the synthesis of dialkyl carbonates based on CO and CO2 utilization, particularly focusing on the catalyst design and fabrication, structure-function relationship, catalytic mechanisms and process intensification. We also provide an overview regarding the applications of dialkyl carbonates as fuel additives, solvents and reaction intermediates (i.e. alkylating and carbonylating agents). Additionally, this review puts forward the substantial challenges and opportunities for future research associated with dialkyl carbonates.

DFT investigations for the reaction mechanism of dimethyl carbonate synthesis on Pd(II)/ß zeolites.

Density functional theory (DFT) calculations have been used to investigate the oxidative carbonylation of methanol on Pd(II)/ß zeolite. Activation energies for all the elementary steps involved in the commonly accepted mechanism, including the formation of dimethyl carbonate, methyl formate and dimethoxymethane, are presented. Upon conducting the calculations, we identify that the Pd(2+) cation bonded with four O atoms of the zeolite framework acts as the active site of the catalyst. Molecularly adsorbed methanol starts to react with oxygen molecules to produce a methanediol intermediate (CH2(OH)2) and O atom. Then, another methanol can react with the O atom to produce the (CH3O)(OH)-Pd(II)/ß zeolite species. (CH3O)(OH)-Pd(II)/ß zeolite can further react with carbon monoxide or methanol to give monomethyl carbonate or di-methoxide species ((CH3O)2-Pd(II)/ß zeolite). Dimethyl carbonate can form via two distinct reaction pathways: (I) methanol reacts with monomethyl carbonate or (II) carbon monoxide inserts into di-methoxide. Our calculation results show the activation energy of reaction (I) is too high to be achieved. The methanediol intermediate is unstable and can decompose to formaldehyde and H2O immediately. Formaldehyde can either react with an O atom or methanol to form formic acid or a CH3OCH2OH intermediate. Both of them can react with methanol to form the secondary products (methyl formate or dimethoxymethane). Upon conducting calculations, we confirmed that the activation energies for the formation of methyl formate and dimethoxymethane are higher than that of dimethyl carbonate. All these conformations were characterized at the same calculation level.

Supported Pt Enabled Proton-Driven NAD(P)+ Regeneration for Biocatalytic Oxidation.

The utilization of biocatalytic oxidations has evolved from the niche applications of the early 21st century to a widely recognized tool for general chemical synthesis. One of the major drawbacks that hinders commercialization is the dependence on expensive nicotinamide adenine dinucleotide (NAD(P)+) cofactors, and so, their regeneration is essential. Here, we report the design of carbon-supported Pt catalysts that can regenerate NAD(P)+ by proton-driven NAD(P)H oxidation with concurrent hydrogen formation. The carbon support was modified to tune the electronic nature of the Pt nanoparticles, and it was found that the best catalyst for NAD(P)+ regeneration (TOF = 581 h-1) was electron-rich Pt on carbon. Finally, the heterogeneous Pt catalyst was applied in the biocatalytic oxidation of a variety of alcohols catalyzed by different alcohol dehydrogenases. The Pt catalyst exhibited good compatibility with the biocatalytic system. Its NAD(P)+ regeneration function successfully supported biocatalytic conversion from alcohols to corresponding ketone or lactone products. This work provides a promising strategy for chemical synthesis via NAD(P)+-dependent pathways utilizing a cooperative inorganic-enzymatic catalytic system.

Chemodivergent Synthesis of One-Carbon-Extended Alcohols via Copper-Catalyzed Hydroxymethylation of Alkynes with Formic Acid.

The development of selective catalytic reactions that utilize easily available reagents for the efficient synthesis of alcohols is a long-standing goal of chemical research. Here an intriguing strategy for the chemodivergent copper-catalyzed hydroxymethylation of alkynes with formic acid and hydrosilane has been developed. By simply tuning the amount of formic acid and reaction temperature, distinct one-carbon-extended primary alcohols, that is, allylic alcohols and ß-branched alkyl alcohols, were produced with high levels of Z/E-, regio-, and enantioselectivity.

Morphology-Dependent Catalytic Performance of Mordenite in Carbonylation of Dimethyl Ether: Enhanced Activity with High c/b Ratio.

The morphology of zeolite often plays an important role in catalytic performance. Controllable synthesis of zeolite with special morphology and elucidating the structure-performance relationship of microporous zeolite are significant to its application. In this work, rod-assembled H-MOR with a controllable c/b ratio was successfully fabricated via template-free hydrothermal methods. NH3 adsorption IR and pyridine-adsorbed IR were employed to quantify the amount of acid sites in 8-MR and 12-MR of MOR. By excluding the influence of acidic properties on activity, the rate of methyl acetate formation via dimethyl ether carbonylation has been proved proportionally dependent on c/b ratio. Pressure-composition-temperature measurements revealed that the reactant, CO, readily enriches in the longer channels in the c-axis direction, resulting in an enhanced methyl acetate formation. Meanwhile, a higher concentration of CO in channels as well as severer diffusion limitation in the H-MOR with a higher c/b ratio results in a faster deactivation rate.

Global stability of a network-based SIS epidemic model with a general nonlinear incidence rate.

In this paper, we develop and analyze an SIS epidemic model with a general nonlinear incidence rate, as well as degree-dependent birth and natural death, on heterogeneous networks. We analytically derive the epidemic threshold R0 which completely governs the disease dynamics: when R0 < 1, the disease-free equilibrium is globally asymptotically stable, i.e., the disease will die out; when R0 > 1, the disease is permanent. It is interesting that the threshold value R0 bears no relation to the functional form of the nonlinear incidence rate and degree-dependent birth. Furthermore, by applying an iteration scheme and the theory of cooperative system respectively, we obtain sufficient conditions under which the endemic equilibrium is globally asymptotically stable. Our results improve and generalize some known results. To illustrate the theoretical results, the corresponding numerical simulations are also given.

Mussel-inspired one-step copolymerization to engineer hierarchically structured surface with superhydrophobic properties for removing oil from water.

In the present study, a superhydrophobic polyurethane (PU) sponge with hierarchically structured surface, which exhibits excellent performance in absorbing oils/organic solvents, was fabricated for the first time through mussel-inspired one-step copolymerization approach. Specifically, dopamine (a small molecular bioadhesive) and n-dodecylthiol were copolymerized in an alkaline aqueous solution to generate polydopamine (PDA) nanoaggregates with n-dodecylthiol motifs on the surface of the PU sponge skeletons. Then, the superhydrophobic sponge that comprised a hierarchical structured surface similar to the chemical/topological structures of lotus leaf was fabricated. The topological structures, surface wettability, and mechanical property of the sponge were characterized by scanning electron microscopy, contact angle experiments, and compression test. Just as a result of the highly porous structure, superhydrophobic property and strong mechanical stability, this sponge exhibited desirable absorption capability of oils/organic solvents (weight gains ranging from 2494% to 8670%), suggesting a promising sorbents for the removal of oily pollutants from water. Furthermore, thanks to the nonutilization of the complicated processes or sophisticated equipment, the fabrication of the superhydrophobic sponge seemed to be quite easy to scale up. All these merits make the sponge a competitive candidate when compared to the conventional absorbents, for example, nonwoven polypropylene fabric.