Atmospheric-Pressure Conversion of CO2 to Cyclic Carbonates over Constrained Dinuclear Iron Catalysts.

The conversion of CO2 and epoxides to cyclic carbonates over a silica-supported di-iron(III) complex having a reduced Robson macrocycle ligand system is shown to proceed at 1 atm and 80 °C, exclusively producing the cis-cyclohexene carbonate from cyclohexene oxide. We examine the effect of immobilization configuration to show that the complex grafted in a semirigid configuration catalytically outperforms the rigid, flexible configurations and even the homogeneous counterparts. Using the semirigid catalyst, we are able to obtain a TON of up to 800 and a TOF of up to 37 h-1 under 1 atm CO2. The catalyst is shown to be recyclable with only minor leaching and no change to product selectivity. We further examine a range of epoxides with varying electron-withdrawing/donating properties. This work highlights the benefit arising from the constraining effect of a solid surface, akin to the role of hydrogen bonds in enzyme catalysts, and the importance of correctly balancing it.

First-Principle Colloidal Gate for Controlling Liquid and Molecule Flow Using 2D Claylike Nanoparticles.

Herein, we exploit the natural tendency of two-dimensional (2D) clay nanoparticles to self-assemble and restrict water permeability in soils to fabricate a first of its kind synthetic, pH-activated, reversible, and tunable colloidal flow gate. To realize this, we studied the effect of the pH level of a suspension of claylike layered double hydroxide (LDH) nanoparticles on the LDH coagulation process. We then packed the LDH into a fixed-bed column and examined the effect of pH on mass transport through the column. We found that the 2D platelike LDH particles coagulate in an edge-to-edge configuration, which renders highly nonisotropic aggregates, pivotal for obstructing the transport of liquid and molecules therein. We showed that the coagulation and flow through the column may be regulated by imposing various pH levels as an external stimulus to affect LDH zeta potential. Hence, this work shows that the flow through a column comprising a 2D particle bed can be regulated in a reversible manner by simply alternating the pH of the wash solution, equilibration time, or gate dimensions. Furthermore, we show that, subject to pH treatment, we may open and close the colloidal gate for the transport of large molecules and provide selective transport thereof.

Fe-N-C electrocatalysts in the oxygen and nitrogen cycles in alkaline media: the role of iron carbide.

Fe-N-C electrocatalysts hold a great promise for Pt-free energy conversion, driving the electrocatalysis of oxygen reduction and evolution, oxidation of nitrogen fuels, and reduction of N2, CO2, and NOx. Nevertheless, the catalytic role of iron carbide, a component of nearly every pyrolytic Fe-N-C material, is at the focus of a heated controversy. We now resolve the debate by examining a broad range of Fe3C sites, spanning across many typical size distributions and carbon environments. Removing Fe3C selectively by a non-oxidizing acid reveals its inactivity towards two representative reactions in alkaline media, oxygen reduction and hydrazine oxidation. The activity is assigned to other pre-existing sites, most probably Fe-Nx. DFT calculations prove that the Fe3C surface binds O and N intermediates too strongly to be catalytic. By settling the argument on the catalytic role of Fe3C in alkaline electrocatalysis, we hope to spur innovation in this critical field.

A rare 4-fold interpenetrated metal-organic framework constructed from an anionic indium-based node and a cationic dicopper linker.

A unique 4-fold interpenetrated metal-organic framework, TIF-1, was synthesized by combining an anionic indium node with a cationic linker. This framework shows a rare type of 4-fold interpenetrated dia network, constructed from tessellation of biangular and tetragonal type metal-organic micropores. The porosity of TIF-1 is moderate due to four-fold interpenetration and charge-balancing anions. The cationic feature of this MOF may give good efficiency for selective small anion exchange or separation. In addition, the thermal stability and moderate CO2 adsorption property of the complex were studied.

Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn-Na-W/SiO2 Catalyst.

The work here presents a thorough evaluation of the effect of Mn-Na-W/SiO2 catalyst surface parameters on its performance in the oxidative coupling of methane (OCM). To do so, we used microporous dealuminated ß-zeolite (Zeo), or mesoporous SBA-15 (SBA), or macroporous fumed silica (Fum) as precursors for catalyst preparation, together with Mn nitrate, Mn acetate and Na2WO4. Characterizing the catalysts by inductively coupled plasma-optical emission spectroscopy, N2 physisorption, X-ray diffraction, high-resolution scanning electron microscopy-energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and catalytic testing enabled us to identify critical surface parameters that govern the activity and C2 selectivity of the Mn-Na-W/SiO2 catalyst. Although the current paradigm views the phase transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right surface properties. Specifically, the SBA-based catalyst, characterized by relatively closed surface porosity, demonstrated low activity and low C2 selectivity. By contrast, for the same composition, the Zeo-based catalyst showed an open surface pore structure, which translated up to fourfold higher activity and enhanced selectivity. By varying the overall composition of the Zeo catalysts, we show that reducing the overall W concentration reduces the size of the Na2WO4 species and increases the catalytic activity linearly as much as fivefold higher than the SBA catalyst. This linear dependence correlates well to the number of interfaces between the Na2WO4 and Mn2O3 species. Our results combined with prior studies lead us to single out the interface between Na2WO4 and Mn2O3 as the most probable active site for OCM using this catalyst. Synergistic interactions between the various precursors used and the phase transition are discussed in detail, and the conclusions are correlated to surface properties and catalysis.

Sensors: A Highly Sensitive Diketopyrrolopyrrole-Based Ambipolar Transistor for Selective Detection and Discrimination of Xylene Isomers (Adv. Mater. 21/2016).

An ambipolar organic field-effect transistor (OFET) based on poly(diketopyrrolopyrrole-terthiophene) (PDPPHD-T3) is shown by P. Sonar, H. Haick, and co-workers on page 4012 to sensitively detect xylene isomers at low to 40 ppm level in multiple sensing features. Combined with pattern-recognition algorithms, a sole ambipolar FET sensor, rather than arrays of sensors, is able to discriminate highly similar xylene structural isomers from each other.

A Highly Sensitive Diketopyrrolopyrrole-Based Ambipolar Transistor for Selective Detection and Discrimination of Xylene Isomers.

An ambipolar poly(diketopyrrolopyrrole-terthiophene)-based field-effect transistor (FET) sensitively detects xylene isomers at low ppm levels with multiple sensing features. Combined with pattern-recognition algorithms, a sole ambipolar FET sensor, rather than arrays of sensors, can discriminate highly similar xylene structural isomers from one another.

Understanding the role of defect sites in glucan hydrolysis on surfaces.

Though unfunctionalized mesoporous carbon consisting of weakly Brønsted acidic OH-defect sites depolymerizes cellulose under mild conditions, the nature of the active site and how this affects hydrolysis kinetics--the rate-limiting step of this process--has remained a puzzle. Here, in this manuscript, we quantify the effect of surface OH-defect site density during hydrolysis catalysis on the rate of reaction. Our comparative approach relies on synthesis and characterization of grafted poly(1→4-ß-glucan) (ß-glu) strands on alumina. Grafted ß-glu strands on alumina have a 9-fold higher hydrolysis rate per glucan relative to the highest rate measured for ß-glu strands on silica. This amounts to a hydrolysis rate per grafted center on alumina that is 2.7-fold more active than on silica. These data are supported by the lower measured activation energy for hydrolysis of grafted ß-glu strands on alumina being 70 kJ/mol relative to 87 kJ/mol on silica. The observed linear increase of hydrolysis rate with increasing OH-defect site density during catalysis suggests that the formation of hydrogen bonds between weakly Brønsted acidic OH-defect sites and constrained glycosidic oxygens (i.e., those juxtaposed adjacent to the surface) activates the latter for hydrolysis catalysis. Altogether, these data elucidate crucial structural requirements for glucan hydrolysis on surfaces and, when coupled with our recent demonstration of long-chain glucan binding to mesoporous carbon, present a unified picture, for the first time, of adsorbed glucan hydrolysis on OH-defect site-containing surfaces, such as unfunctionalized mesoporous carbon.

Glucan adsorption on mesoporous carbon nanoparticles: effect of chain length and internal surface.

The adsorption of cellulose-derived long-chain (longer than ten glucose repeat units on size) glucans onto carbon-based acid catalysts for hydrolysis has long been hypothesized; however, to date, there is no information on whether such adsorption can occur and how glucan chain length influences adsorption. Herein, in this manuscript, we first describe how glucan chain length influences adsorption energetics, and use this to understand the adsorption of long-chain glucans onto mesoporous carbon nanoparticles (MCN) from a concentrated acid solution, and the effect of mesoporosity on this process. Our results conclusively demonstrate that mesoporous carbon nanoparticle (MCN) materials adsorb long-chain glucans from concentrated acid hydrolyzate in amounts of up to 30% by mass (303 mg/g of MCN), in a manner that causes preferential adsorption of longer-chain glucans of up to 40 glucose repeat units and, quite unexpectedly, fast adsorption equilibration times of less than 4 min. In contrast, graphite-type carbon nanopowders (CNP) that lack internal mesoporosity adsorb glucans in amounts less than 1% by mass (7.7 mg/g of CNP), under similar conditions. This inefficiency of glucan adsorption on CNP might be attributed to the lack of internal mesoporosity, since the CNP actually possesses greater external surface area relative to MCN. A systematic study of adsorption of glucans in the series glucose to cellotetraose on MCN shows a monotonically decreasing free energy of adsorption upon increasing the glucan chain length. The free energy of adsorption decreases by at least 0.4 kcal/mol with each additional glucose unit in this series, and these energetics are consistent with CH-π interactions providing a significant energetic contribution for adsorption, similar to previous observations in glycoproteins. HPLC of hydrolyzed fragments in solution, (13)C Bloch decay NMR spectroscopy, and GPC provide material balance closure of adsorbed glucan coverages on MCN materials. The latter and MALDI-TOF-MS provide direct evidence for adsorption of long-chain glucans on the MCN surface, which have a radius of gyration larger than the pore radius of the MCN material.

Dialkylimidazolium ionic liquids hydrolyze cellulose under mild conditions.

The average molecular weight of cellulose derived from filter paper, poplar, and Avicel decreases by up to two orders of magnitude during typical mild dissolution protocols using ionic liquids (ILs). About an order of magnitude greater cellulose depolymerization rate during ionic liquid dissolution occurs in 1-butyl-3-methylimidazolium chloride (BmimCl) and 1-ethyl-3-methylimidazolium chloride (EmimCl) compared to 1-ethyl-3-methylimidazolium acetate (EmimOAc), and, unintuitively, greater IL purity results in greater cellulose depolymerization. The following data support the mechanism of cellulose hydrolysis to be acid-catalyzed: (i) increase in number of reducing ends following cellulose dissolution in IL; (ii) addition of N-methylimidazolium base suppresses cellulose depolymerization during dissolution in IL; (iii) small amounts of glucose and traces of hydroxymethyl furfural are present following cellulose dissolution in IL. The acid is presumably synthesized via IL decomposition to generate a carbene and proton, consistent with hypothesis derived from molecular modeling. Titration experiments conducted here measure the amount of acid synthesized to be in the 4000 ppm range for high-purity BmimCl IL during mild processing conditions for cellulose dissolution. This data is relevant for understanding the extent of IL decomposition during biomass dissolution.

Grafted poly(1→4-ß-glucan) strands on silica: a comparative study of surface reactivity as a function of grafting density.

Grafted poly(ß-glucan) (ß-glu) strands on the surface of silica are synthesized with varying degrees of grafting density, and display an amorphous-like environment via (13)C CP/MAS NMR spectroscopy. Thermal gravimetric analysis of these materials under oxidative conditions shows increased ß-glu thermal stability with higher degrees of grafting density. The range of temperature stability between the most and least hydrogen-bound grafted ß-glu strands spans 321 to 260 °C. This range is bound by the combustion temperature previously measured for crystalline and amorphous cellulose, with the former having greater oxidative stability, and is likely controlled by the extent of hydrogen bonding of a grafted ß-glu strand with the underlying silica surface. When using these materials as reactants for glycosidic bond hydrolysis, the total number of reducing ends formed during reaction is quantified using a BCA colorimetric assay. Results demonstrate that the material with greatest interaction with silica surface silanols undergoes hydrolysis at an initial rate that is 6-fold higher than the material with the lowest degree of such interaction. The role of the surface as a reactive interface that can endow oxidative stability and promote hydrolysis activity has broad implications for surface-catalyzed processes dealing with biomass-derived polymers.

Grafted cellulose strands on the surface of silica: effect of environment on reactivity.

The design, synthesis and characterization of materials consisting of grafted poly(1 → 4-ß-glucan) strands on silica is reported. The silanol-rich environment provided in these materials activates the glycosidic bond for hydrolysis under mild conditions.