Yuriy Román-Leshkov

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.

Tin-containing zeolites are highly active catalysts for the isomerization of glucose in water

The isomerization of glucose into fructose is a large-scale reaction for the production of high-fructose corn syrup (HFCS; reaction performed by enzyme catalysts) and recently is being considered as an intermediate step in the possible route of biomass to fuels and chemicals. Here, it is shown that a large-pore zeolite that contains tin (Sn-Beta) is able to isomerize glucose to fructose in aqueous media with high activity and selectivity. Specifically, a 10% (wt/wt) glucose solution containing a catalytic amount of Sn-Beta (1∶50 Sn:glucose molar ratio) gives product yields of approximately 46% (wt/wt) glucose, 31% (wt/wt) fructose, and 9% (wt/wt) mannose after 30 min and 12 min of reaction at 383 K and 413 K, respectively. This reactivity is achieved also when a 45 wt% glucose solution is used. The properties of the large-pore zeolite greatly influence the reaction behavior because the reaction does not proceed with a medium-pore zeolite, and the isomerization activity is considerably lower when the metal centers are incorporated in ordered mesoporous silica (MCM-41). The Sn-Beta catalyst can be used for multiple cycles, and the reaction stops when the solid is removed, clearly indicating that the catalysis is occurring heterogeneously. Most importantly, the Sn-Beta catalyst is able to perform the isomerization reaction in highly acidic, aqueous environments with equivalent activity and product distribution as in media without added acid. This enables Sn-Beta to couple isomerizations with other acid-catalyzed reactions, including hydrolysis/isomerization or isomerization/dehydration reaction sequences [starch to fructose and glucose to 5-hydroxymethylfurfural (HMF) demonstrated here].

Mechanism of glucose isomerization using a solid Lewis acid catalyst in water.

^1H and ^(13)C NMR spectroscopy on isotopically labeled glucose reveals that in the presence of tin-containing zeolite Sn-Beta, the isomerization reaction of glucose in water proceeds by way of an intramolecular hydride shift (see scheme) rather than proton transfer. This is the first mechanistic demonstration of Sn-Beta acting as a Lewis acid in a purely aqueous environment.

Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites

Isomerization of sugars is used in a variety of industrially relevant processes and in glycolysis. Here, we show that hydrophobic zeolite beta with framework tin or titanium Lewis acid centers isomerizes sugars, e.g., glucose, via reaction pathways that are analogous to those of metalloenzymes. Specifically, experimental and theoretical investigations reveal that glucose partitions into the zeolite in the pyranose form, ring opens to the acyclic form in the presence of the Lewis acid center, isomerizes into the acyclic form of fructose, and finally ring closes to yield the furanose product. The zeolite catalysts provide processing advantages over metalloenzymes such as an ability to work at higher temperatures and in acidic conditions that allow for the isomerization reaction to be coupled with other important conversions.

Domino Reaction Catalyzed by Zeolites with Brønsted and Lewis Acid Sites for the Production of γ-Valerolactone from Furfural†

The development of more carbon efficient and economically viable lignocellulosic biomass conversion technologies is critical for the sustainable production of liquid transportation fuels and chemicals. The molecule g-valerolactone (GVL) has gained attention as a versatile platform chemical for the production of liquid alkenes, as a solvent for biomass processing, as an approved fuel additive, and as a precursor for renewable polymers. Biomass-derived GVL is currently produced by the multistep processing of the carbohydrate fractions of lignocelluloses, wherein acid catalysts transform sugars into levulinic acid (LA), and noble-metal catalysts reduce LA to GVL with molecular hydrogen (H2). [5,6] This strategy suffers from several limitations that hinder the large-scale manufacture of GVL. In particular, the LA-reduction step necessitates precious-metal catalysts (e.g., Ru or Pt) or high H2 pressures (> 30 bar), which have been shown to negatively impact the economics of GVL-derived transportation fuels. Formic acid has emerged as an alternative to molecular H2, but noble metals and/or harsh conditions are still required to carry out the hydrogenation step. Inexpensive supported transition metals (e.g., Cu/ Al2O3) are active but suffer from leaching and/or sintering during the reaction. For the large-scale production of GVL, catalytic schemes are required that maximize product yields without the use of precious metals, high H2 pressure, or an excessive number of unit operations. Transfer-hydrogenation (TH) reactions, such as the Meerwein–Ponndorf–Verley (MPV) reaction, offer an attractive alternative to molecular H2 for the reduction of targeted functional groups. The interaction between the catalyst, the hydrogen donor, and the acceptor molecule can be modulated to impact activity and selectivity. Many catalysts are active for TH reactions, including organometallic compounds, transition metals, and metal oxides featuring acid/base properties. Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination sites (e.g., Zr or Sn) have also been used as solid Lewis acids to promote TH reactions. Wise and Williams used homogeneous Ru complexes and Chia and Dumesic used heterogeneous metal oxides to produce GVL from levulinate derivatives by TH reactions with high yields. Corma and co-workers demonstrated the efficacy of Sn-Beta and Zr-Beta zeolites for the intermolecular MPV reaction between alcohols and ketones in organic solvents. Sn-Beta and other tin-containing silicates have also been shown to transform hexoses, pentoses, and trioses through intramolecular hydride and carbon-atom shifts in both organic and aqueous media. Herein, we report an integrated catalytic process for the efficient production of GVL from furfural (Fur) through sequential TH and hydrolysis reactions catalyzed by zeolites with Bronsted and Lewis acid sites (Scheme 1). In our

Insights into the catalytic activity and surface modification of MoO3 during the hydrodeoxygenation of lignin-derived model compounds into aromatic hydrocarbons under low hydrogen pressures

MoO3 is an effective catalyst for the hydrodeoxygenation (HDO) of lignin-derived oxygenates to generate high yields of aromatic hydrocarbons without ring-saturated products. The catalyst is selective for the C–O bond cleavage under low H2 pressures (≤1 bar) and temperatures ranging from 593 to 623 K. A bond-dissociation energy analysis of relevant phenolic C–O bonds indicates that the bond strengths follow an order of Ph–OH > Ph–OMe > Ph–O–Ph > Ph–O–Me. However, for all model compounds investigated, the MoO3 catalyst preferentially cleaves phenolic Ph–OMe bonds over weaker aliphatic Ph–O–Me bonds. Characterisation studies reveal that the catalyst surface undergoes partial carburisation as evidenced by the presence of oxycarbide- and oxycarbohydride-containing phases (i.e., MoOxCyHz). The transformation of bulk phases and the surface modification of MoO3 by carbon–H2 are investigated to understand the role of surface carbon in the stabilisation and enhanced activity of the partially reduced MoO3 surface.

Self-assembly of noble metal monolayers on transition metal carbide nanoparticle catalysts

Tough core-shell catalysts One approach for increasing the activity of precious metals in catalysis is to coat them onto less expensive earth-abundant transition metal cores such as nickel, but often these structures alloy and deactivate during reactions. Hunt et al. synthesized several types of transition metal carbide nanoparticles coated with atomically thin precious-metal shells. Titanium-doped tungsten carbide nanoparticles with platinum-ruthenium shells were highly active for methanol electrooxidation, stable over 10,000 cycles, and resistant to CO deactivation. Science, this issue p. 974 Transition metal carbide nanoparticles coated with noble metal monolayers resist CO poisoning during catalysis. We demonstrated the self-assembly of transition metal carbide nanoparticles coated with atomically thin noble metal monolayers by carburizing mixtures of noble metal salts and transition metal oxides encapsulated in removable silica templates. This approach allows for control of the final core-shell architecture, including particle size, monolayer coverage, and heterometallic composition. Carbon-supported Ti0.1W0.9C nanoparticles coated with Pt or bimetallic PtRu monolayers exhibited enhanced resistance to sintering and CO poisoning, achieving an order of magnitude increase in specific activity over commercial catalysts for methanol electrooxidation after 10,000 cycles. These core-shell materials provide a new direction to reduce the loading, enhance the activity, and increase the stability of noble metal catalysts.

Emerging catalytic processes for the production of adipic acid

Research efforts to find more sustainable pathways for the synthesis of adipic acid have led to the introduction of new catalytic processes for producing this commodity chemical from alternative resources. With a focus on the performance of oxygen and hydrogen peroxide as preferred oxidants, this minireview summarizes recent advances made in the selective oxidation of cyclohexene, cyclohexane, cyclohexanone and n-hexane to adipic acid. Special attention is paid to the exploration of catalytic pathways involving lignocellulosic biomass-derived chemicals such as 5-hydroxymethylfurfural, D-glucose, γ-valerolactone and compounds representative of lignin and lignin-derived bio-oils.