Elif Gurbuz - Academia.edu (original) (raw)

Papers by Elif Gurbuz

Journal of the American Chemical Society, Sep 10, 2015

This study combines theory and experiment to determine the kinetically relevant steps and site re... more This study combines theory and experiment to determine the kinetically relevant steps and site requirements for deoxygenation of alkanols and alkanals. These reactants deoxygenate predominantly via decarbonylation (C−C cleavage) instead of C−O hydrogenolysis on Ir, Pt, and Ru, leading to strong inhibition effects by chemisorbed CO (CO*). C−C cleavage occurs via unsaturated species formed in sequential quasi-equilibrated dehydrogenation steps, which replace C−H with C−metal bonds, resulting in strong inhibition by H 2 , also observed in alkane hydrogenolysis. C−C cleavage occurs in oxygenates only at locations vicinal to the CO group in RCCO* intermediates, because such adjacency weakens C−C bonds, which also leads to much lower activation enthalpies for oxygenates than hydrocarbons. C−O hydrogenolysis rates are independent of H 2 pressure and limited by H*-assisted C−O cleavage in RCHOH* intermediates on surfaces with significant coverages of CO* formed in decarbonylation events. The ratio of C−O hydrogenolysis to decarbonylation rates increased almost 100-fold as the Ir cluster size increased from 0.7 to 7 nm; these trends reflect C−O hydrogenolysis reactions favored on terrace sites, while C−C hydrogenolysis prefers sites with lower coordination, because of the relative size of their transition states and the crowded nature of CO*-covered surfaces.

Journal of the American Chemical Society, 2015

An integrated biofuels strategy: Catalytic conversion of lignocellulosic biomass to liquid hydroc... more An integrated biofuels strategy: Catalytic conversion of lignocellulosic biomass to liquid hydrocarbon fuels Replacement of fossil fuels with new sustainable resources becomes crucial due to depletion of petroleum reserves, increasing global energy demand and arising environmental concerns. Lignocellulosic biomass provides sustainable and environmentally friendly ways of producing chemicals and fuels as an alternative for fossil fuels. One critical step is the conversion of lignocellulosic biomass to versatile intermediates such as levulinic acid (LA), which can be transformed into liquid fuels, fuel additives and even other specialty chemicals. In this respect, we studied a LA-based catalytic process to convert lignocellulosic biomass into liquid hydrocarbon fuels for use in the transportation sector. Using experimental results for all associated reactions, we synthesized an integrated biomass-to-fuels strategy that has a number of advantages over existing strategies. The first ste...

Replacing petroleum using biomass as source of carbon requires the effective conversion of both t... more Replacing petroleum using biomass as source of carbon requires the effective conversion of both the hemicellulose (C5 sugars) and cellulose (C6 sugars) fractions of the lignocellulosic biomass. However, these fractions have different physical and chemical properties making it difficult to employ a unified processing strategy for conversion to fuels and chemicals. Accordingly, typical processing strategies employ a pretreatment step in which the C5 sugars are removed from the C6 sugars, allowing these two classes of sugars to be processed by separate routes. The hemicellulose fraction of biomass is more reactive than the cellulose fraction and thus, using short reaction times and low acid concentrations it is possible to achieve high conversions of C5 sugars to furfural, while preserving the cellulose for other applications. Increasing the reaction time and/or acid concentration leads to the conversion of C6 sugars to levulinic acid, but at the expense of furfural degradation. Herein...

Lignocellulosic biomass, an alternative for petroleum, is a sustainable source of carbon for prod... more Lignocellulosic biomass, an alternative for petroleum, is a sustainable source of carbon for producing chemicals and fuels in an environmentally friendly way. Conversion of lignocellulosic biomass to valuable intermediates is a critical step to develop effective biomass-to-biofuel strategies. Levulinic acid (LA) is one of these platform chemicals that can be produced from lignocellulosic biomass and transformed into liquid fuels, fuel additives and even other specialty chemicals. In this respect, we developed an LA-based catalytic strategy, in which an alkylphenol solvent is used for extraction of intermediates from the sulfuric acid solution, to convert lignocellulosic biomass into liquid hydrocarbon fuels. Then, we compared the alkylphenol strategy with a previously reported butyl acetate strategy in terms of cost-effectiveness. In the butyl acetate strategy, first, the hemicellulose fraction of the biomass is solubilized and removed using dilute acid pretreatment. Pretreated biom...

The Role of Catalysis for the Sustainable Production of Bio-fuels and Bio-chemicals, 2013

Acid catalysts are ubiquitous in biomass conversion because of their ability to deoxygenate molec... more Acid catalysts are ubiquitous in biomass conversion because of their ability to deoxygenate molecules by way of multiple chemical pathways. In this chapter, the importance and current state of acid catalysis for the conversion of lignocellulose into chemicals and fuels is outlined in the context of aqueous-phase processing. Selected examples are used to highlight the use of catalytic materials featuring Bronsted or Lewis acid sites in lignocellulosic biomass conversion processes, and to showcase the role of acidity in catalytic coupling and process intensification. The chapter presents some of the outstanding challenges to acid catalysis and includes a perspective on its future outlook in an integrated biorefining strategy.

A Global Challenge, Second Edition, 2011

Current Opinion in Chemical Engineering, 2012

Fractionation of lignocellulosic biomass increases process flexibility and allows for integrated ... more Fractionation of lignocellulosic biomass increases process flexibility and allows for integrated processing of C 5 and C 6 sugars. Recent advances using acidic treatments to deconstruct biomass in combination with organic solvents to create biphasic systems have allowed for increased yields of platform chemicals such as furfural, hydroxymethylfurfural, and levulinic acid. Management of the mineral acids used in pretreatment steps remains a challenge, but proper organic solvent selection, such as 2-sec-butylphenol, allows for complete recovery and recycle of mineral acid. Using solvents with high partition coefficients for extraction of products in biphasic unit operations allows the concentrations of products to be increased and improves the efficiency of downstream processing options, such as distillation or further upgrading reactions. Overall, fractionation of lignocellulosic biomass allows for a flexible, integrated processing approach that we hope will advance biorefining operations, allowing commercial biomass processing to become a reality.

Journal of Catalysis, 2009

Studies of aldol condensation/hydrogenation reactions of 2-hexanone were carried out over Pd/CeZr... more Studies of aldol condensation/hydrogenation reactions of 2-hexanone were carried out over Pd/CeZrO x and CeZrO x catalysts at temperatures between 573 and 673 K, and pressures of 5-26 bar. These studies were formulated to address the catalytic upgrading to transportation fuels of the mono-functional oxygenated compounds (consisting primarily of C 4-C 6 ketones, alcohols, carboxylic acids and heterocyclics) formed by the catalytic conversion of polyols over a Pt-Re/C catalyst. Characterization by XRD, TPR and NH 3 /CO 2-TPD showed that Pd/CeZrO x catalyst consists of a partially reducible solid solution of cerium and zirconium oxides, and possesses both acidic and basic functionalities. Reaction kinetics studies show that in addition to the expected C 12 condensation product (7-methyl-5-undecanone), the CeZrO x-based catalysts produce C 18 and C 9 secondary species, along with light alkanes (6C 7). Low loadings of Pd (e.g., 0.25 wt%) lead to optimal activity and selectivity for the production of C 12 species. The high apparent activation energy of the formation of C 9 (140 kJ/mol) compared to the formation of C 12 and C 18 species (15 and 28 kJ/mol, respectively) indicates that these species may be formed as a result of the decomposition of heavier condensation products. The self-coupling of 2-hexanone was found to be positive order in both 2-hexanone and hydrogen. The addition of primary alcohols and carboxylic acids as well as water and CO 2 to the feed was found to reversibly inhibit the self-coupling activity of 2-hexanone. This inhibition is strongest in the presence of CO 2 , and TPSR studies indicate that CO 2 is removed from the surface by conversion to CO in the presence of reduced ceria species.

Journal of Catalysis, 2012

Abstract Formic acid and butyl formate conversion were studied in the liquid phase over Pd and Ru... more Abstract Formic acid and butyl formate conversion were studied in the liquid phase over Pd and Ru catalysts. Pd/C was more active, selective, and stable for CO 2 /H 2 production in the liquid phase. Kinetic studies were performed over Pd/C at temperatures from 403 to 443 K, at space velocities from 3.8 to 970 h −1 , in the presence of CO and H 2 at partial pressures from 0 to 0.4 and 12.6 atm, respectively, and liquid water. Space velocity studies probed the importance of primary decomposition pathways to CO 2 and CO compared to the secondary water–gas shift reaction. Generally, the rate of the primary pathway was an order of magnitude higher than the rate of the secondary pathway. Over Pd/C, formic acid decomposed primarily via decarboxylation (to CO 2 /H 2 ), whereas butyl formate primarily decomposed via decarbonylation (to CO/butanol). When water was present, the formate ester hydrolyzed, which increased the selectivity toward CO 2 and H 2 .

Journal of Catalysis, 2014

Ethane hydrogenolysis involves CC bond rupture in unsaturated species in quasi-equilibrium with g... more Ethane hydrogenolysis involves CC bond rupture in unsaturated species in quasi-equilibrium with gaseous reactants and H 2 on metal clusters, because CC bonds weaken as C-atoms replace hydrogen with exposed metal atoms from catalyst surfaces. The nature and reactivity of such adsorbed species are probed here using kinetic data and density functional theory (DFT) for the case of Ir surfaces, but with conclusions that appear to be general to hydrogenolysis on noble metals. On surfaces saturated with chemisorbed H-atoms (H Ã), theory and experiments indicate that CC cleavage occurs predominantly via an a,b-bound Ã CHCH Ã species that forms via sequential dehydrogenation of adsorbed ethane; all other intermediates cleave CC bonds at much lower rates (>10 7-fold). Measured activation energies (213 kJ mol À1) and free energies (130 kJ mol À1) reflect the combined values for quasi-equilibrated steps that desorb H Ã , adsorb C 2 H 6 , form C 2-intermediates by dehydrogenation, and form the transition state from Ã CHCH Ã species. DFT-derived activation energies (218 kJ mol À1) and free energies estimated from these values and statistical mechanics treatments of reaction and activation entropies (137 kJ mol À1) are in excellent agreement with measured values. The removal of four H-atoms in forming the kinetically-relevant Ã CHCH Ã intermediates, taken together with measured effects of H 2 pressure on hydrogenolysis rates, show that 2-3 H Ã must be removed to bind this intermediate and the transition state, as expected from the structure of the proposed adsorbed species and H Ã adsorption stoichiometries on Ir surface atoms that vary slightly with surface coordination on the non-uniform surfaces of metal clusters. Theory and experiments combine here to provide mechanistic insights inaccessible to direct observation and provide compelling evidence for reaction pathways long considered to be plausible for hydrogenolysis on noble metals. The extent of unsaturation in the single relevant intermediate and its CC cleavage rates will depend on the identity of the metal, but the elementary steps and their kinetic relevance appear to be a general feature of metal-catalyzed hydrogenolysis.

Green Chemistry, 2012

A 300 ml autoclave (Parr Instruments) with magnetic stirring was used for lignin extraction and d... more A 300 ml autoclave (Parr Instruments) with magnetic stirring was used for lignin extraction and depolymerization. In a typical lignin conversion reaction, 10 g of wood powder and 0.5 g of catalyst were mixed in 100 ml water. The reactor was purged with hydrogen and pressurized to 500 psi. The reactor temperature was increased to 473 K using an external electrical heater and controlled with a type J thermocouple connected to a PID controller. After holding the temperature for the desired reaction time, the reactor was cooled to room temperature and depressurized. The reaction mixture was filtered to separate the liquid products from the wood residue and catalyst. The liquid phase was analyzed by GC (Shimadzu GC-2010 equipped with FID) and GC-MS (Shimadzu GCMS-QP2010S). The phenolic monomers (shown in Figure S.1) in the aqueous phase were extracted by contacting with 50 ml of diethyl ether (DEE) (Sigma Aldrich) and purified in a rotary evaporator. Information on the phenolic monomers obtained from aqueous depolymerization of wood chips using Pd, Pt and Rh based catalysts are shown in Table S.2. These liquid mixtures were then contacted with water to extract the guaiacyl propanol and syringyl propanol to the aqueous phase to isolate propyl syringol and propyl guaiacol. The LDS used in this study was prepared according to the conditions of entry 1 in Table S.2, followed by extraction with DEE. After removal of DEE in a rotary evaporator, the organic phase was contacted with water to remove guaiacyl propanol and syringyl propanol. The remaining organic phase contained 20 wt% propyl guaiacol and 80 wt% propyl syringol. The boiling points of propyl guaiacol and propyl syringol are shown in Table S.1. The phenylpropane monomers in lignin (i.e. C 9 , C 10 and C 11) are connected to each other by C x-O-C y (e.g. β-O-4, 4-O-5) and C x-C y bonds (e.g. 5-5, β-5, β-1). Among the various lignin linkages, the β-O-4 linkage is the most dominant both in hardwood