James A. Dumesic

Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water

Concerns about the depletion of fossil fuel reserves and the pollution caused by continuously increasing energy demands make hydrogen an attractive alternative energy source. Hydrogen is currently derived from nonrenewable natural gas and petroleum, but could in principle be generated from renewable resources such as biomass or water. However, efficient hydrogen production from water remains difficult and technologies for generating hydrogen from biomass, such as enzymatic decomposition of sugars, steam-reforming of bio-oils and gasification, suffer from low hydrogen production rates and/or complex processing requirements. Here we demonstrate that hydrogen can be produced from sugars and alcohols at temperatures near 500 K in a single-reactor aqueous-phase reforming process using a platinum-based catalyst. We are able to convert glucose—which makes up the major energy reserves in plants and animals—to hydrogen and gaseous alkanes, with hydrogen constituting 50% of the products. We find that the selectivity for hydrogen production increases when we use molecules that are more reduced than sugars, with ethylene glycol and methanol being almost completely converted into hydrogen and carbon dioxide. These findings suggest that catalytic aqueous-phase reforming might prove useful for the generation of hydrogen-rich fuel gas from carbohydrates extracted from renewable biomass and biomass waste streams.

Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates

Diminishing fossil fuel reserves and growing concerns about global warming indicate that sustainable sources of energy are needed in the near future. For fuels to be useful in the transportation sector, they must have specific physical properties that allow for efficient distribution, storage and combustion; these properties are currently fulfilled by non-renewable petroleum-derived liquid fuels. Ethanol, the only renewable liquid fuel currently produced in large quantities, suffers from several limitations, including low energy density, high volatility, and contamination by the absorption of water from the atmosphere. Here we present a catalytic strategy for the production of 2,5-dimethylfuran from fructose (a carbohydrate obtained directly from biomass or by the isomerization of glucose) for use as a liquid transportation fuel. Compared to ethanol, 2,5-dimethylfuran has a higher energy density (by 40 per cent), a higher boiling point (by 20 K), and is not soluble in water. This catalytic strategy creates a route for transforming abundant renewable biomass resources into a liquid fuel suitable for the transportation sector, and may diminish our reliance on petroleum.

Phase modifiers promote efficient production of hydroxymethylfurfural from fructose.

Furan derivatives obtained from renewable biomass resources have the potential to serve as substitutes for the petroleum-based building blocks that are currently used in the production of plastics and fine chemicals. We developed a process for the selective dehydration of fructose to 5-hydroxymethylfurfural (HMF) that operates at high fructose concentrations (10 to 50 weight %), achieves high yields (80% HMF selectivity at 90% fructose conversion), and delivers HMF in a separation-friendly solvent. In a two-phase reactor system, fructose is dehydrated in the aqueous phase with the use of an acid catalyst (hydrochloric acid or an acidic ion-exchange resin) with dimethylsulfoxide and/or poly(1-vinyl-2-pyrrolidinone) added to suppress undesired side reactions. The HMF product is continuously extracted into an organic phase (methylisobutylketone) modified with 2-butanol to enhance partitioning from the reactive aqueous solution.

Production of 5-hydroxymethylfurfural and furfural by dehydration of biomass-derived mono- and poly-saccharides

Furan derivatives, such as 5-hydroxymethylfurfural (HMF) and furfural, obtained from renewable biomass-derived carbohydrates have potential to be sustainable substitutes for petroleum-based building blocks used in production of fine chemicals and plastics. We have studied the production of HMF and furfural by dehydration of fructose, glucose and xylose using a biphasic reactor system, comprised of reactive aqueous phase modified with DMSO, combined with an organic extracting phase consisting of a 7 : 3 (w/w) MIBK–2-butanol mixture or dichloromethane (DCM). Experiments with the MIBK–2-butanol mixture were conducted at a temperature of 443 K using mineral acid catalysts (HCl, H2SO4 and H3PO4) at a pH from 1.0 to 2.0, whereas experiments with DCM as the extracting solvent were conducted at 413 K and did not require the use of an acid catalyst. The modifiable nature of the biphasic system allowed us to identify preferred DMSO and pH levels for each sugar to maximize the HMF selectivity at high sugar conversions, leading to selectivities of 89%, 91%, and 53% for dehydration of fructose, xylose, and glucose, respectively. Using these reaction conditions for each monosaccharide unit, we can process the corresponding polysaccharides, such as sucrose (a disaccharide of glucose and fructose), inulin (a polyfructan), starch (a polyglucan), cellobiose (a glucose dimer) and xylan (a xylose polysaccharide), with equally good selectivities at high conversions. In addition, we show that the biphasic reactor system can process high feed concentrations (10 to 30 wt%) along with excellent recycling ability. By processing these highly functionalized polysaccharides, that are inexpensive and abundantly available, we eliminate the need to obtain simple carbohydrate molecules by acid hydrolysis as a separate processing step.

Integrated Catalytic Conversion of γ-Valerolactone to Liquid Alkenes for Transportation Fuels

Lactic Fuels In the quest to find sustainable alternatives to petrochemicals, a small cyclic ester, γ-valerolactone, derived from cellulose offers promising raw material. Bond et al. (p. 1110) show that carbon dioxide can be catalytically excised from the lactone efficiently at high pressure, leaving a mixture of butanes. In a second-stage reactor, the butanes can be strung together to form heavier hydrocarbons similar to those found in automotive and jet fuels. The method simultaneously yields fuel and a relatively pure stream of pressurized carbon dioxide amenable to sequestration or further chemical modification. A biomass-derived compound is transformed into hydrocarbon fuels and a CO2 stream amenable to sequestration. Efficient synthesis of renewable fuels remains a challenging and important line of research. We report a strategy by which aqueous solutions of γ-valerolactone (GVL), produced from biomass-derived carbohydrates, can be converted to liquid alkenes in the molecular weight range appropriate for transportation fuels by an integrated catalytic system that does not require an external source of hydrogen. The GVL feed undergoes decarboxylation at elevated pressures (e.g., 36 bar) over a silica/alumina catalyst to produce a gas stream composed of equimolar amounts of butene and carbon dioxide. This stream is fed directly to an oligomerization reactor containing an acid catalyst (e.g., H ZSM-5, Amberlyst-70), which couples butene monomers to form condensable alkenes with molecular weights that can be targeted for gasoline and/or jet fuel applications. The effluent gaseous stream of CO2 at elevated pressure can potentially be captured and then treated or sequestered to mitigate greenhouse gas emissions from the process.

Catalytic Conversion of Biomass to Monofunctional Hydrocarbons and Targeted Liquid-Fuel Classes

It is imperative to develop more efficient processes for conversion of biomass to liquid fuels, such that the cost of these fuels would be competitive with the cost of fuels derived from petroleum. We report a catalytic approach for the conversion of carbohydrates to specific classes of hydrocarbons for use as liquid transportation fuels, based on the integration of several flow reactors operated in a cascade mode, where the effluent from the one reactor is simply fed to the next reactor. This approach can be tuned for production of branched hydrocarbons and aromatic compounds in gasoline, or longer-chain, less highly branched hydrocarbons in diesel and jet fuels. The liquid organic effluent from the first flow reactor contains monofunctional compounds, such as alcohols, ketones, carboxylic acids, and heterocycles, that can also be used to provide reactive intermediates for fine chemicals and polymers markets.

Catalytic routes for the conversion of biomass into liquid hydrocarbon transportation fuels

Concerns about diminishing fossil fuel reserves along with global warming effects caused by increasing levels of CO2 in the atmosphere are driving society toward the search for new renewable sources of energy that can substitute for coal, natural gas and petroleum in the current energy system. Lignocellulosic biomass is abundant, and it has the potential to significantly displace petroleum in the production of fuels for the transportation sector. Ethanol, the main biomass-derived fuel used today, has benefited from production by a well-established technology and by partial compatibility with the current transportation infrastructure, leading to the domination of the world biofuel market. However, ethanol suffers from important limitations as a fuel (e.g., low energy density, high solubility in water) than can be overcome by designing strategies to convert non-edible lignocellulosic biomass into liquid hydrocarbon fuels (LHF) chemically similar to those currently used in internal combustion engines. The present review describes the main routes available to carry out such deep chemical transformation (e.g., gasification, pyrolysis, and aqueous-phase catalytic processing), with particular emphasis on those pathways involving aqueous-phase catalytic reactions. These latter catalytic routes achieve the required transformations in biomass-derived molecules with controlled chemistry and high yields, but require pretreatment/hydrolysis steps to overcome the recalcitrance of lignocellulose. To be economically viable, these aqueous-phase routes should be carried out with a small number of reactors and with minimum utilization of external fossil fuel-based hydrogen sources, as illustrated in the examples presented here.

Gamma-valerolactone, a sustainable platform molecule derived from lignocellulosic biomass

Lignocellulosic biomass typically contains more than 50 wt% sugars that can be upgraded to valuable platform molecules, such as levulinic acid (LA) and gamma-valerolactone (GVL). This article focuses on upgrading GVL produced from lignocellulosic biomass to various chemicals and fuels, such as polymers, fuel additives, and jet fuel. We also review the use of GVL as a solvent for biomass processing, which led to significant improvements in product yields and a more simplified process for producing biomass-derived chemicals such as LA, furfural, and hydroxymethylfurfural.

Nonenzymatic Sugar Production from Biomass Using Biomass-Derived γ-Valerolactone

Renewable Breakdown Routine In order to transform cellulose-containing biomass into liquid fuels such as ethanol, it is first necessary to break down the cellulose into its constituent sugars. Efforts toward this end have focused on chemical protocols using concentrated acid or ionic liquid solvents, and on biochemical protocols using cellulase enzymes. Luterbacher et al. (p. 277) now show that γ-valerolactone, a small molecule solvent that can itself be sourced renewably from biomass, promotes efficient and selective thermal breakdown of cellulose in the presence of dilute aqueous acid. A solvent sourced from biomass may offer a cost-effective means of breaking down cellulose for biofuels production. Widespread production of biomass-derived fuels and chemicals will require cost-effective processes for breaking down cellulose and hemicellulose into their constituent sugars. Here, we report laboratory-scale production of soluble carbohydrates from corn stover, hardwood, and softwood at high yields (70 to 90%) in a solvent mixture of biomass-derived γ-valerolactone (GVL), water, and dilute acid (0.05 weight percent H2SO4). GVL promotes thermocatalytic saccharification through complete solubilization of the biomass, including the lignin fraction. The carbohydrates can be recovered and concentrated (up to 127 grams per liter) by extraction from GVL into an aqueous phase by addition of NaCl or liquid CO2. This strategy is well suited for catalytic upgrading to furans or fermentative upgrading to ethanol at high titers and near theoretical yield. We estimate through preliminary techno-economic modeling that the overall process could be cost-competitive for ethanol production, with biomass pretreatment followed by enzymatic hydrolysis.

Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium–Rhenium Catalysts

A ReO(x)-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C-O bonds for a broad range of cyclic ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experimentally observed reactivity trends, NH(3) temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydrogenolysis of C-O bonds by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % Rh-ReO(x)/C (1:0.5) was confirmed by NH(3) TPD, and the estimated acid site density and standard enthalpy of NH(3) adsorption were 40 μmol g(-1) and -100 kJ mol(-1), respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups that reside α to carbon in the C-O bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the selective C-O hydrogenolysis of biomass-derived feedstocks.