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Csefalvay E.,City University of Hong Kong | Csefalvay E.,Budapest University of Technology and Economics | Akien G.R.,City University of Hong Kong | Akien G.R.,Center for Environmentally Beneficial Catalysis | And 3 more authors.
Catalysis Today | Year: 2014

Ethanol equivalent (EE) is defined as the mass of ethanol needed to deliver the equivalent amount ofenergy from a given feedstock using energy equivalency or produce the equivalent amount of mass of acarbon-based chemical using molar equivalency. The production of ethanol from biomass requires energy,which in a sustainable world could be produced from biomass. Therefore, we also define a real ethanolequivalent (EEx) indicating that the ethanol equivalent also includes the use of 1 unit of bioethanol toproduce x units of bioethanol. Thus, the abbreviation EE2.3used in this paper shows a 2.3 output/inputbioethanol ratio or efficiency. Calculations of the corresponding mass of corn and size of landwere basedon the first generation corn-based bioethanol technology as commercially practiced in the US in 2008.Since the total energy and essential materials requirements of a given process can be calculated, theEE2.3of a production process or even a total technology can be estimated. We show that the EE2.3couldbe used as a translational tool between fossil- and biomass-based feedstocks, products, processes, andtechnologies. Since the EE2.3can be readily determined for any given biomass-based technology, therequired mass of biomass feedstock, the size of land, and even the volume of water can be calculated.Scenario analyses based on EE2.3could better visualize the demands of competing technologies on theenvironment both for the experts and to the general public. While differentiating between 1, 1000, and100,000 BTUs for different options is rather difficult for most people, comparing the amount of the landneeded to produce the same amount of energy or mass via different technologies is more straightforward. © 2014 Elsevier B.V.

Kumar M.,Center for Environmentally Beneficial Catalysis | Chaudhari R.V.,Center for Environmentally Beneficial Catalysis | Subramaniam B.,Center for Environmentally Beneficial Catalysis | Jackson T.A.,Center for Environmentally Beneficial Catalysis | Jackson T.A.,University of Kansas
Organometallics | Year: 2015

M06-L-based quantum chemical calculations were performed to examine two key elementary steps in rhodium (Rh)-xantphos-catalyzed hydroformylation: carbonyl ligand (CO) dissociation and the olefin insertion into the Rh-H bond. For the resting state of the Rh-xantphos catalyst, HRh(xantphos)(CO)2, our M06-L calculations were able to qualitatively reproduce the correct ordering of the equatorial-equatorial (ee) and equatorial-axial (ea) conformers of the phosphorus ligands for 16 derivatives of the xantphos ligand, implying that the method is sufficiently accurate for capturing the subtle energy differences associated with various conformers involved in Rh-catalyzed hydroformylation. The calculated CO dissociation energy from the ea conformer (ΔE = 21-25 kcal/mol) was 10-12 kcal/mol lower than that from the ee conformer (ΔE = 31-34 kcal/mol), which is consistent with prior experimental and theoretical studies. The calculated regioselectivities for propene insertion into the Rh-H bond of the ee-HRh(xantphos)(propene)(CO) complexes were in good agreement with the experimental l:b ratios. The comparative analysis of the regioselectivities for the pathways originating from the ee-HRh(xantphos)(propene)(CO) complexes with and without diphenyl substituents yielded useful mechanistic insight into the interactions that play a key role in regioselectivity. Complementary computations featuring xantphos ligands lacking diphenyl substituents implied that the long-range noncovalent ligand-ligand and ligand-substrate interactions, but not the bite angles per se, control the regioselectivity of Rh-diphosphine-catalyzed hydroformylation of simple terminal olefins for the ee isomer. Additional calculations with longer chain olefins and the simplified structural models, in which the phenyl rings of the xantphos ligands were selectively removed to eliminate either substrate-ligand or ligand-ligand noncovalent interactions, suggested that ligand-substrate π-HC interactions play a more dominant role in the regioselectivity of Rh-catalyzed hydroformylation than ligand-ligand π-π interactions. The present calculations may provide foundational knowledge for the rational design of ligands aimed at optimizing hydroformylation regioselectivity. © 2015 American Chemical Society.

Kumar M.,University of Kansas | Kumar M.,Center for Environmentally Beneficial Catalysis | Chaudhari R.V.,University of Kansas | Subramaniam B.,University of Kansas | And 2 more authors.
Organometallics | Year: 2014

Density functional theory calculations have been performed to gain insight into the origin of ligand effects in rhodium (Rh)-catalyzed hydroformylation of olefins. In particular, the olefin insertion step of the Wilkinson catalytic cycle, which is commonly invoked as the regioselectivity-determining step, has been examined by considering a large variety of density functionals (e.g., B3LYP, M06-L); a range of substrates, including simple terminal (e.g., hexene, octene), heteroatom-containing (e.g., vinyl acetate), and aromatic-substituted (e.g., styrene) alkenes, and different ligand structures (e.g., monodentate PPh3 ligands and bidentate ligands such as DIOP, DIPHOS). The calculations indicate that the M06-L functional reproduces the experimental regioselectivities with a reasonable degree of accuracy, while the commonly employed B3LYP functional fails to do so when the equatorial-equatorial arrangement of phosphine ligands around the Rh center is considered. The different behavior of the two functionals is attributed to the fact that the transition states leading to the Rh-alkyl intermediates along the pathways to isomeric aldehydes are stabilized by the medium-range correlation containing π-π (ligand-ligand) and π-CH (ligand-substrate) interactions that cannot be handled properly by the B3LYP functional due to its inability to describe nonlocal interactions. This conclusion is further validated using the B3LYP functional with Grimme's empirical dispersion correction term: i.e., B3LYP-D3. The calculations also suggest that transition states leading to the linear Rh-alkyl intermediates are selectively stabilized by these noncovalent interactions, which gives rise to the high regioselectivities. In the cases of heteroatom- or aromatic-substituted olefins, substrate electronic effects determine the regioselectivity; however, these calculations suggest that the π-π and π-CH interactions also make an appreciable contribution. Overall, these computations show that the steric crowding-induced ligand-ligand and ligand-substrate interactions, but not intraligand interactions, influence the regioselectivity in Rh-catalyzed hydroformylation when the phosphine ligands are present in an equatorial-equatorial configuration in the Rh catalyst. © 2014 American Chemical Society.

Kumar M.,University of Kansas | Kumar M.,Center for Environmentally Beneficial Catalysis | Busch D.H.,University of Kansas | Busch D.H.,Center for Environmentally Beneficial Catalysis | And 4 more authors.
Physical Chemistry Chemical Physics | Year: 2014

The tautomerization of Criegee intermediates via a 1,4 β-hydrogen atom transfer to yield a vinyl hydroperoxide has been examined in the absence and presence of carboxylic acids. Electronic structure calculations indicate that the organic acids catalyze the tautomerization reaction to such an extent that it becomes a barrierless process. In contrast, water produces only a nominal catalytic effect. Since organic acids are present in parts-per-billion concentrations in the troposphere, the present results suggest that the acid-catalyzed tautomerization, which can also result in formation of hydroxyl radicals, may be a significant pathway for Criegee intermediates. This journal is © the Partner Organisations 2014.

Kumar M.,University of Kansas | Kumar M.,Center for Environmentally Beneficial Catalysis | Busch D.H.,University of Kansas | Busch D.H.,Center for Environmentally Beneficial Catalysis | And 4 more authors.
Journal of Physical Chemistry A | Year: 2014

Density functional theory calculations predict that the gas-phase decomposition of carbonic acid, a high-energy, 1,3-hydrogen atom transfer reaction, can be catalyzed by a monocarboxylic acid or a dicarboxylic acid, including carbonic acid itself. Carboxylic acids are found to be more effective catalysts than water. Among the carboxylic acids, the monocarboxylic acids outperform the dicarboxylic ones wherein the presence of an intramolecular hydrogen bond hampers the hydrogen transfer. Further, the calculations reveal a direct correlation between the catalytic activity of a monocarboxylic acid and its pKa, in contrast to prior assumptions about carboxylic-acid- catalyzed hydrogen-transfer reactions. The catalytic efficacy of a dicarboxylic acid, on the other hand, is significantly affected by the strength of an intramolecular hydrogen bond. Transition-state theory estimates indicate that effective rate constants for the acid-catalyzed decomposition are four orders-of-magnitude larger than those for the water-catalyzed reaction. These results offer new insights into the determinants of general acid catalysis with potentially broad implications. © 2014 American Chemical Society.

Kumar M.,University of Kansas | Kumar M.,Center for Environmentally Beneficial Catalysis | Busch D.H.,University of Kansas | Busch D.H.,Center for Environmentally Beneficial Catalysis | And 4 more authors.
Journal of Physical Chemistry A | Year: 2014

Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecular reaction of the Criegee intermediate (CI) with carbon monoxide (CO) that is proposed to be important in both atmospheric and industrial chemistry. A new mechanism is suggested in which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting in the formation of a carbonyl compound (aldehyde or ketone depending upon the CI) and carbon dioxide. Fourteen different CIs, including ones resulting from biogenic ozonolysis, are considered. Consistent with previous reports for other CI bimolecular reactions, the anti conformers are found to react faster than the syn conformers. However, this can be attributed to steric effects and not hyperconjugation as generally invoked. The oxidation reaction is slow, with barrier heights between 6.3 and 14.7 kcal/mol and estimated reaction rate constants 6-12 orders-of-magnitude smaller than previously reported literature estimates. The reaction is thus expected to be unimportant in the context of tropospheric oxidation chemistry. However, the reaction mechanism suggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds. © 2014 American Chemical Society.

Yan W.,Center for Environmentally Beneficial Catalysis | Yan W.,University of Kansas | Ramanathan A.,Center for Environmentally Beneficial Catalysis | Ghanta M.,Center for Environmentally Beneficial Catalysis | And 4 more authors.
Catalysis Science and Technology | Year: 2014

Significant ethylene epoxidation activity was observed over Nb- and W-incorporated KIT-6 materials with aqueous hydrogen peroxide (H2O2) as the oxidant and methanol as solvent under mild operating conditions (35 °C and 50 bar) where CO2 formation is avoided. The Nb-KIT-6 materials generally show greater epoxidation activity compared to the W-KIT-6 materials. Further, the ethylene oxide (EO) productivity observed with these materials [30-800 mg EO h-1 (g metal)-1] is of the same order of magnitude as that of the conventional silver (Ag)-based gas phase ethylene epoxidation process. Our results reveal that the framework-incorporated metal species, rather than the extra-framework metal oxide species, are mainly responsible for the observed epoxidation activity. However, the tetrahedrally coordinated framework metal species also introduce Lewis acidity that promotes their solvolysis (which in turn results in their gradual leaching) as well as H2O2 decomposition. These results and mechanistic insights provide rational guidance for developing catalysts with improved leaching resistance and minimal H2O2 decomposition. © 2014 the Partner Organisations.

Subramaniam B.,University of Kansas | Subramaniam B.,Center for Environmentally Beneficial Catalysis | Akien G.R.,Center for Environmentally Beneficial Catalysis
Current Opinion in Chemical Engineering | Year: 2012

Gas-expanded liquids (GXLs) are a continuum of tunable solvents generated by mixing liquid solvents and compressed near-critical gases such as CO 2 and light olefins. The compressed gas provides tunability of the physical and transport properties of GXLs making them ideal for performing sustainable catalysis characterized by process intensification at mild conditions, high product selectivity and facile separation of catalyst and products. Sustainable technology alternatives to industrial hydroformylations and epoxidations that employ GXLs as enabling solvents are provided. In these examples, the GXLs involve conventional organic as well as non-traditional solvents such as ionic liquids (ILs) and compressible gases such as CO 2 (as inert) or light olefins (as substrates). Such technologies are essential for facilitating sustainable growth of the fledgling biorefining industry. © 2012 Elsevier Ltd. All rights reserved.

Qi L.,City University of Hong Kong | Qi L.,University of California at Santa Barbara | Mui Y.F.,City University of Hong Kong | Lo S.W.,City University of Hong Kong | And 4 more authors.
ACS Catalysis | Year: 2014

The conversion of fructose, glucose, and sucrose to 5-(hydroxymethyl) furfural (HMF) and levulinic acid (LA)/formic acid (FA) was investigated in detail using sulfuric acid as the catalyst and γ-valerolactone (GVL) as a green solvent. The H2SO4/GVL/H2O system can be tuned to produce either HMF or LA/FA by changing the acid concentration and thus allowing selective switching between the products. Although the best yields of HMF were around 75%, the LA/FA yields ranged from 50% to 70%, depending on the structure of the carbohydrates and the reaction parameters, including temperature, acid, and carbohydrate concentrations. While the conversion of fructose is much faster than glucose, sucrose behaves like a 1:1 mixture of fructose and glucose, indicating facile hydrolysis of the glycosidic bond in sucrose. The mechanism of the conversion of glucose to HMF or LA/FA in GVL involves three intermediates: 1,6-anhydro-β-d-glucofuranose, 1,6-anhydro-β-d-glucopyranose, and levoglucosenone. © 2014 American Chemical Society.

Akien G.R.,City University of Hong Kong | Akien G.R.,Center for Environmentally Beneficial Catalysis | Qi L.,City University of Hong Kong | Horvath I.T.,City University of Hong Kong
Chemical Communications | Year: 2012

Several intermediates and different reaction paths were identified for the acid catalysed conversion of fructose to 5-(hydroxymethyl)-2-furaldehyde (HMF) in different solvents. The structural information combined with results of isotopic-labelling experiments allowed the determination of the irreversibility of the three steps from the fructofuranosyl oxocarbenium ion to HMF as well as the analogous pyranose route. © 2012 The Royal Society of Chemistry.

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