Samir H. Mushrif

Insights into the Interplay of Lewis and Brønsted Acid Catalysts in Glucose and Fructose Conversion to 5-(Hydroxymethyl)furfural and Levulinic Acid in Aqueous Media

5-(Hydroxymethyl)furfural (HMF) and levulinic acid production from glucose in a cascade of reactions using a Lewis acid (CrCl3) catalyst together with a Brønsted acid (HCl) catalyst in aqueous media is investigated. It is shown that CrCl3 is an active Lewis acid catalyst in glucose isomerization to fructose, and the combined Lewis and Brønsted acid catalysts perform the isomerization and dehydration/rehydration reactions. A CrCl3 speciation model in conjunction with kinetics results indicates that the hydrolyzed Cr(III) complex [Cr(H2O)5OH](2+) is the most active Cr species in glucose isomerization and probably acts as a Lewis acid-Brønsted base bifunctional site. Extended X-ray absorption fine structure spectroscopy and Car-Parrinello molecular dynamics simulations indicate a strong interaction between the Cr cation and the glucose molecule whereby some water molecules are displaced from the first coordination sphere of Cr by the glucose to enable ring-opening and isomerization of glucose. Additionally, complex interactions between the two catalysts are revealed: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of [Cr(H2O)5OH](2+). In contrast, Lewis acidity increases the overall rate of consumption of fructose and HMF compared to Brønsted acid catalysis by promoting side reactions. Even in the absence of HCl, hydrolysis of Cr(III) decreases the solution pH, and this intrinsic Brønsted acidity drives the dehydration and rehydration reactions. Yields of 46% levulinic acid in a single phase and 59% HMF in a biphasic system have been achieved at moderate temperatures by combining CrCl3 and HCl.

Revealing pyrolysis chemistry for biofuels production: conversion of cellulose to furans and small oxygenates.

Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify α-cyclodextrin as an appropriate small-molecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.

Catalytic oxidation of cellobiose over TiO2 supported gold-based bimetallic nanoparticles

A series of Au–M (M = Cu, Co, Ru and Pd) bimetallic catalysts were supported on TiO2via a deposition–precipitation (DP) method, using urea as a precipitating agent. The resulting catalysts were employed in the catalytic oxidation of cellobiose to gluconic acid and the properties of these catalysts were carefully examined using various characterization techniques. Cu–Au/TiO2 and Ru–Au/TiO2 catalysts demonstrated excellent catalytic activities in the oxidation of cellobiose to gluconic acid, though with contrasting reaction mechanisms. Complete conversion of cellobiose (100%) with a gluconic acid selectivity of 88.5% at 145 °C within 3 h was observed for reactions performed over Cu–Au/TiO2; whereas, a conversion of 98.3% with a gluconic acid selectivity of 86. 9% at 145 °C within 9 h was observed for reactions performed over Ru–Au/TiO2. A reaction pathway was proposed based on the distribution of reaction products and kinetic data. It is suggested that cellobiose is converted to cellobionic acid (4-O-beta-D-glucopyranosyl-D-gluconic acid) and then gluconic acid is formed through the cleavage of the β-1,4 glycosidic bond in cellobionic acid over Cu–Au/TiO2 catalysts. On the other hand, for reactions over the Ru–Au/TiO2 catalyst, glucose was observed as the reaction intermediate and gluconic acid was formed as a result of glucose oxidation. For reactions over Co–Au/TiO2 and Pd–Au/TiO2 catalysts, fructose was observed as the reaction intermediate, along with small amounts of glucose. Co and Pd remarkably promoted the successive retro-aldol condensation reactions of fructose to glycolic acid, instead of the selective oxidation to gluconic acid.

Fructose-water-dimethylsulfoxide interactions by vibrational spectroscopy and molecular dynamics simulations.

The solvation of fructose in dimethyl sulfoxide (DMSO) and DMSO-H(2)O (or DMSO-D(2)O) mixtures was investigated using vibrational spectroscopy (Raman, ATR/FTIR) and molecular dynamics (MD) simulations. The analysis of the fructose hydroxyl hydrogen-DMSO oxygen radial distribution function showed that the coordination number of DMSO around the furanose form of fructose is ~3.5. This number is smaller than the number of hydroxyl groups of fructose because one DMSO molecule is shared between two hydroxyl groups and because intramolecular hydrogen bonds are formed. In the case of fructose-DMSO mixtures, a red shift of the Raman S═O asymmetric stretch is observed, which indicates that fructose breaks the DMSO clusters through strong hydrogen bonding between the hydrogen atoms of its hydroxyl groups and the oxygen atom of DMSO. The Raman scattering cross sections of the DMSO S═O stretch when a DMSO molecule interacts with another DMSO molecule, a fructose molecule, or a water molecule were estimated from the spectra of the binary mixtures using the coordination numbers from MD simulations. It was also possible to use these values together with the MD-estimated coordination numbers to satisfactorily predict the effect of the water fraction on the Raman scattering intensity of the S═O stretching band in ternary mixtures. MD simulations also showed that, with increasing water content, the DMSO orientation around fructose changed, with the sulfur atom moving away from the carbohydrate. The deconvolution of the fructose IR OH stretching region revealed that the hydroxyls of fructose can be separated into two groups that participate in hydrogen bonds of different strengths. MD simulations showed that the three hydroxyls of the fructose ring form stronger hydrogen bonds with the solvent than the remaining hydroxyls, providing an explanation for the experimental observations. Finally, analysis of ATR/FTIR spectra revealed that, with increasing water content, the average hydrogen-bond enthalpy of the fructose hydroxyls decreases by ~2.5 kJ/mol.

Biomass Oxidation: Formyl CH Bond Activation by the Surface Lattice Oxygen of Regenerative CuO Nanoleaves†

An integrated experimental and computational investigation reveals that surface lattice oxygen of copper oxide (CuO) nanoleaves activates the formyl C-H bond in glucose and incorporates itself into the glucose molecule to oxidize it to gluconic acid. The reduced CuO catalyst regains its structure, morphology, and activity upon reoxidation. The activity of lattice oxygen is shown to be superior to that of the chemisorbed oxygen on the metal surface and the hydrogen abstraction ability of the catalyst is correlated with the adsorption energy. Based on the present investigation, it is suggested that surface lattice oxygen is critical for the oxidation of glucose to gluconic acid, without further breaking down the glucose molecule into smaller fragments, because of C-C cleavage. Using CuO nanoleaves as catalyst, an excellent yield of gluconic acid is also obtained for the direct oxidation of cellobiose and polymeric cellulose, as biomass substrates.

Direct arylation polymerization towards narrow bandgap conjugated microporous polymers with hierarchical porosity

Conjugated microporous polymers are synthesized through facile one-step direct arylation polymerization of a single monomer unit, 8,11-dibromodithieno[3,2-a:2′,3′-c]phenazine, without preactivation of C–H bonds using organometallic reagents. The resulting polymers exhibit hierarchical porous structures and a narrow bandgap of 1.5 eV.

Insights into the synergistic role of metal–lattice oxygen site pairs in four-centered C–H bond activation of methane: the case of CuO

The activation of methane by transition metal/metal oxide catalysts is pertinent for developing/optimizing processes which help to convert this abundantly available resource to value-added chemicals. First principles calculations reveal that the under-coordinated lattice Cu–O pair on different CuO surfaces synergistically activates methane with barriers as low as 60.5 kJ mol−1 on the high-energy CuO(010) surface and 76.6 kJ mol−1 on the most stable CuO(111) surface. The significantly low activation barrier is due to (1) the stabilization of the transition state (TS) and the reduced strain on the dissociating methane molecule and (2) the stabilization of the co-adsorbed products of dissociation, resulting in favorable thermodynamics. The mechanism, which is also applicable to the chemisorbed oxygen-containing Cu(111) surface, involves simultaneous copper addition and hydrogen abstraction by the chemisorbed/lattice oxygen via a 4-centered (CH3-(Cu)–H-(O)) TS, stabilized by the Cu–CH3 and O–H dipole–dipole interaction. The activation barriers for the subsequent dissociation of surface CH3 moieties and coupling of CH3 with CH2 on the CuO(111) surface are both much higher than the barrier of the first C–H bond dissociation in methane. The mechanistic insights elucidated in this article could be applicable to methane activation by other metal–oxygen (M–O) site-pairs and thus can serve to screen potential oxide surfaces for the purpose.

Mechanistic and kinetic studies on biodiesel production catalyzed by an efficient pyridinium based ionic liquid

Biodiesels produced from renewable sources exhibit superior fuel properties and renewability and they are more environmentally friendly than petroleum-based fuels. In this paper, a three-step transesterification, catalyzed by a pyridinium-based Bronsted acidic ionic liquid (BAIL), for biodiesel production was investigated using density functional theory (DFT) calculations at the B3LYP/6-311++G(d) level. The DFT results elucidate the detailed catalytic cycle, which involves the formation of a covalent reactant–BAIL–(methanol)n (n = 1/3) intermediate and two transition states. Hydrogen bond interactions were found to exist throughout the process of the catalytic cycle, which are of special importance for stabilizing the intermediate and transition states. Thus, a mechanism involving cooperative hydrogen bonding for BAIL-catalyzed biodiesel production was established. The Gibbs free energy profile based on the above mechanism was validated by the subsequent kinetic study. The trend of activation energy from kinetic mathematical models was reasonably consistent with that obtained from the DFT calculations.

Energetics and dynamics of hydrogen adsorption, desorption and migration on a carbon-supported palladium cluster

The functionality of metal-doped carbon materials in catalytic and hydrogen storage applications is governed by the characteristics of their interaction with hydrogen. The dynamics of hydrogen chemisorption, its migration and desorption are investigated by first-principles molecular dynamics simulations of a model Pd4 cluster supported on a coronene molecule. Longer time scale events are accelerated using the metadynamics technique. This article is a comprehensive report of the energetics and dynamics of (i) dissociative chemisorption of molecular hydrogen on a carbon-supported Pd cluster, (ii) transport of atomic hydrogen on the cluster, towards the carbon support and (iii) the associative desorption of atomic hydrogens in a molecular form. It is found that the dissociative chemisorption of hydrogen on the cluster involves a negligible energy barrier and takes place readily at room temperature. The migration of atomic hydrogen from the tip of the tetrahedral Pd4 cluster to its sides, on the other hand, involves an energy barrier of ∼6 KJ/mole. This energy barrier, however, is reduced when the cluster is partially saturated with pre-existing hydrogens adsorbed on its sides. The energy barrier for the subsequent migration of atomic hydrogen, from the sides of the cluster towards the carbon support, is however not affected significantly by the presence of pre-existing hydrogens. Overall, the transport of atomic hydrogen from the tip of the cluster towards the carbon support is energetically favourable and only involves low energy barriers. The associative desorption of two hydrogen atoms from the cluster to form molecular hydrogen is an endothermic process and also involves a small energy barrier. This work sheds light onto the mechanism of the hydrogen interaction with Pd-doped-carbon-supported catalytic materials and, more generally, onto the mechanism of hydrogen spillover on such materials.

Extraction of (9,8) single-walled carbon nanotubes by fluorene-based polymers.

Selective polymer wrapping is a promising approach to obtain high-chiral-purity single-walled carbon nanotubes (SWCNTs) needed in technical applications and scientific studies. We showed that among three fluorene-based polymers with different side-chain lengths and backbones, poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,10-anthracene)] (PFH-A) can selectively extract SWCNTs synthesized from the CoSO4 /SiO2 catalyst, which results in enrichment of 78.3 % (9,8) and 12.2 % (9,7) nanotubes among all semiconducting species. These high-chiral-purity SWCNTs may find potential applications in electronics, optoelectronics, and photovoltaics. Furthermore, molecular dynamics simulations suggest that the extraction selectivity of PFH-A relates to the bending and alignment of its alkyl chains and the twisting of its two aromatic backbone units (biphenyl and anthracene) relative to SWCNTs. The strong π-π interaction between polymers and SWCNTs would increase the extraction yield, but it is not beneficial for chiral selectivity. Our findings suggest that the matching between the curvature of SWCNTs and the flexibility of the polymer side chains and the aromatic backbone units is essential in designing novel polymers for selective extraction of (n,m) species.