Stephanie G. Wettstein

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.

Conversion of Hemicellulose into Furfural Using Solid Acid Catalysts in γ‐Valerolactone

The effective conversion of lignocellulosic biomass into fuels and chemicals requires the utilization of both hemicellulose and cellulose, consisting primarily of C5 and C6 sugars, respectively. Catalytic conversion strategies for hemicellulose are of particular importance because biological conversion of C5 sugars is not as efficient as the conversion of C6 sugars. In addition, C5 sugars/oligomers are produced as a side stream in the pulp and paper industry, which provides an opportunity to create value-added products. Among the products that can be obtained from C5 sugars, furfural is a particularly promising option, as it can replace crude-oil-based organics for the production of resins, lubricants, adhesives, and plastics, as well as valuable chemicals, such as furfuryl alcohol and tetrahydrofurfuryl alcohol. Current methods for production of furfural from hemicellulose use mineral acid catalysts which are corrosive, difficult to recover from the reaction mixture, and pose environmental and health risks. Importantly, current yields for the production of furfural in water are low (e.g., < 60%). Biphasic systems improve the yield of furfural and its separation from the mineral acid, and can be employed for lignocellulosic biomass which has been pretreated with mineral acids. Ideally, it is desirable to replace mineral acids with solid acids in lignocellulose processing. However, the use of solid acid catalysts in an aqueous environment is challenging in view of catalyst degradation and/or leaching in aqueous solution at elevated temperatures (e.g., 430 K). Moreover, biphasic systems typically require the use of salts to achieve good separation of the phases and to improve the efficiency of the extracting organic layer, and solid catalysts cannot be used in this case because the exchange of protons on the catalyst with cations in solution leads to deactivation of the heterogeneous catalyst. The aforementioned difficulties associated with the conversion of xylose into furfural can be alleviated by using gvalerolactone (GVL) as a solvent in a monophasic system with solid acid catalysts. Importantly, GVL is a solvent which can be produced from lignocellulose, and Horvath and coworkers have been strong proponents for the use of GVL as a solvent in biomass processing. Using GVL as the solvent increases the rate of xylose conversion and decreases the rates of furfural degradation reactions. In addition, furfural has a higher volatility than GVL and can thus be obtained as a top product in a distillation step. Alternatively, GVL, a valuable chemical with multiple uses, can be synthesized as the end product of the process, thereby eliminating product purification steps. Furthermore, the use of a monophasic reaction system eliminates the loss of the product in the aqueous phase, the need for a liquid–liquid separation step, and reduces mixing requirements. Additionally, by minimizing the concentration of water present in the reactor, it is possible to use solid catalysts for the conversion of xylose (and xylose oligomers) into furfural with minimal degradation of the catalyst and without leaching of acid sites into solution. Figure 1 shows the furfural yields achieved, after complete xylose conversion, for different solid acid catalysts. The catalysts contained Bronsted and/or Lewis acid sites, and just GVL was used as the solvent. Even though water was not added in the reaction mixture, it is a by-product of dehydration, and its concentration can reach up to 0.7 wt% with quantitative yields of furfural. Catalysts, such as g-Al2O3 (galumina), Sn-SBA-15, and Sn-beta, which contain only Lewis acid sites, resulted in the lowest yields of furfural (see Figure S1 in the Supporting Information for FTIR measure-

Integrated conversion of hemicellulose and cellulose from lignocellulosic biomass

Using gamma-valerolactone (GVL) as solvent, the cellulosic fraction of lignocellulosic biomass can be converted into levulinic acid (LA), while at the same conditions the hemicellulose fraction can be converted into furfural. This process allows for the conversion of hemicellulose and cellulose simultaneously in a single reactor, thus eliminating pre-treatment steps to fractionate biomass and simplifying product separation.

Conversion of Hemicellulose to Furfural and Levulinic Acid using Biphasic Reactors with Alkylphenol Solvents

Diminishing fossil fuel resources and the increasing impact of global climate change have driven research towards the utilization of lignocellulosic biomass resources as renewable feedstocks for the production of energy, fuels, and chemicals. The conversion of lignocellulosic biomass into fuels and chemicals requires effective utilization of the C5 and C6 sugars present in hemicellulose and cellulose, respectively, by either processing these fractions together or separating and processing them separately. While simultaneous processing, such as in gasification or pyrolysis, offers the potential for simplicity of operation, the fractionation of hemicellulose and cellulose allows the processing of each fraction to be tailored to take advantage of the different chemical and physical properties of these fractions, and provides increased flexibility of operation. For example, chemical processing methods can be employed to convert C5 sugars into fuels/chemicals in hemicellulose, while employing recent advances in biological conversions allows to convert the C6 sugars in cellulose into fuels and/or chemicals. [1, 2] One can also take advantage of the physical properties of cellulose for pulp and paper applications. Herein, we show that the hemicellulose fraction of lignocellulosic biomass can be converted into furfural and levulinic acid by using biphasic reactors with alkylphenol solvents that selectively partition furanic compounds from acidic aqueous solutions. These furfural and levulinic acid products are valu !->able compounds for a variety of chemical applications, 4] and they serve as precursors for the synthesis of liquid transportation fuels. The conversion of cellulose to chemicals and liquid fuels has been demonstrated through the formation of several platform molecules, such as glucose, 5-hydroxymethylfurfural, and levulinic acid (LA), utilizing chemical routes; however, fewer studies address the conversion of hemicellulose into chemicals and fuels. 11] Previous studies for the production of furfural (FuAL) from C5 sugars (i.e. , xylose) suffer from the low concentrations of FuAL in the product stream due to low xylose concentrations (1–2 wt %) obtained from hemicellulose deconstruction. 11] In addition, even though the production of LA from furfuryl alcohol (FuOH) has been reported with good yields over ion-exchange resin catalysts (e.g. , Amberlyst), 13] the regeneration of these catalysts following deactivation by deposition of solid humins during reaction is problematic. In addition, while zeolite catalysts (i.e. , ZSM-5) can be used to replace resin catalysts and can be regenerated with a calcination treatment following deactivation, employing these catalysts results in significantly lower LA yields, especially when increased LA concentrations are desired in the product stream. Considering the aforementioned challenges for processing hemicellulose, we present a new biorefining strategy for converting the hemicellulose portion of lignocellulosic biomass to FuAL and LA by utilizing biphasic systems that consist of an extractive organic layer and an aqueous layer that contains a mineral acid. These biphasic systems achieve high concentrations of FuAL and LA, enabling the recovery of both products at the top of distillation columns, and eliminating issues related to deactivation and regeneration of solid acid catalysts. Three organic solvents, 2-sec-butylphenol (SBP), 4-n-hexylphenol (NHP) and 4-propyl guaiacol (PG), are demonstrated to be effective extracting agents for the production of FuAL and LA in these biphasic systems. Information on the toxicity and availability of these alkylphenol solvents is given in the Supporting Information. The use of these solvents is particularly advantageous because they (i) have high partition coefficients for extraction of FuAL, FuOH, and LA; (ii) do not extract significant amounts of mineral acids from aqueous solutions; (iii) have higher boiling points than the final product ; and (iv) could potentially be synthesized directly from biomass (i.e. , lignin), such that these solvents would not have to be transported to the site of the biomass conversion steps. For the first step of this biorefining strategy (Figure 1), solid biomass (i.e. , corn stover) is subjected to mild pretreatment in a dilute-acid, aqueous solution to solubilize the hemicellulose as xylose. After filtering the solution from the solid cellulose and lignin, an organic solvent (i.e. , SBP) is added to the aqueous solution, and these liquids are heated in a biphasic reactor to achieve dehydration of xylose to FuAL, which is a valuable chemical intermediate. FuAL can be distilled from SBP and sold as a chemical or, as depicted in Figure 1, converted to LA by first hydrogenating FuAL to FuOH over a metal-based catalyst (e.g. , copper) 15] and then reacting the FuOH with water in a biphasic reactor to form LA. Similar to FuAL, the LA product can be distilled from the organic solvent and sold as a chemical. Xylose dehydration to FuAL has been demonstrated with high yields (ca. 90 %) in several previous studies using mineral acids and salts in biphasic systems with organic solvents, such as methyl isobutyl ketone (MIBK), 2-butanol, and tetrahydrofuran (THF). 11, 16] However, the low partition coefficients for extraction of FuAL in these systems (i.e. , the ratio of the FuAL concentration in the organic solvent to the FuAL concentration in aqueous solution) required the use of large amounts of organic solvent relative to the aqueous xylose solution, resulting [a] E. I. G rb z, Dr. S. G. Wettstein, Prof. J. A. Dumesic Chemical and Biological Engineering Department University of Wisconsin Madison, WI 53706 (USA) Fax: (+ 1) 608-262-5434 E-mail : dumesic@engr.wisc.edu Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100608.

Production of levulinic acid and gamma-valerolactone (GVL) from cellulose using GVL as a solvent in biphasic systems

Cellulose deconstruction at 428 K was studied in biphasic reaction systems consisting of GVL and aqueous solutions containing HCl (0.1–1.25 M) and a solute, such as salt or sugar. This biphasic system achieves high yields of levulinic and formic acids (e.g., 70%), and leads to complete solubilization of cellulose. The GVL solvent extracts the majority of the levulinic acid (e.g., greater than 75%), which can subsequently be converted to GVL over a carbon-supported Ru–Sn catalyst. This approach for cellulose conversion eliminates the need to separate the final product from the solvent, because the GVL product is the solvent. In addition, this approach eliminates the deposition of solid humin species in the cellulose deconstruction reactor, allowing these species to be collected and used for other processing options.

Production of Biofuels from Cellulose and Corn Stover Using Alkylphenol Solvents

Technically viable and economic conversion of lignocellulosic biomass into chemicals and fuels is an important challenge. One effective strategy is to first partially remove oxygen to produce reactive intermediates, denoted as platform molecules, followed by conversion of these molecules into desired products. These platform molecules have fewer functional groups than carbohydrates in biomass (e.g. , xylose, glucose), offering the potential for selective catalytic upgrading processes. One such platform molecule, which is receiving considerable attention in the literature, is levulinic acid (LA). From LA a variety of fuels and chemicals can be made, such as valeric acid esters, methyltetrahydrofuran (MTHF), and LA esters and ketals. Another valuable product is g-valerolactone (GVL), obtained from the reduction of LA. GVL can be used directly as a fuel additive, or as a precursor for fuels and chemicals. It is possible to achieve LA yields greater than 50 % by using aqueous solutions of sulfuric acid for cellulose deconstruction; 12] however, the commercial production of LA and its derivatives presents serious challenges. First, LA must be separated from the mineral acid to recycle the acid catalyst and avoid negative effects in downstream processes. Second, LA is produced in a low concentration, and its purification/recovery is expensive. Finally, the solvents normally used (e.g. , water) have lower boiling points than LA, which means that to recover the product by distillation the solvent needs to be evaporated. In this Communication, we show that alkylphenol solvents (i.e. , substituted benzene compounds containing a hydroxyl group and alkyl groups, R ; see Figure 1) can be used to selectively extract LA from aqueous solutions of sulfuric acid, thereby eliminating the previously mentioned drawbacks. In addition, alkylphenol solvents extract GVL from water with a higher partition coefficient compared to LA. Accordingly, the GVL concentration in the alkylphenol solvent can be increased by the conversion of LA to GVL combined with the recycle of this stream for successive extractions, thus facilitating recovery of GVL from the solvent by distillation. Importantly, we show that a carbon-supported Ru–Sn catalyst can be used for the selective reduction of LA to GVL by H2 in the presence of alkylphenols, without hydrogenation of the solvent. Finally, the aqueous phase containing sulfuric acid after extraction of LA by the alkylphenol solvent can be recycled for subsequent cycles of cellulose deconstruction, providing an effective strategy for sulfuric acid management in the process. An advantage of using alkylphenol solvents is that the alkyl groups strongly modify the physical properties of the solvent. For example, while phenol is relatively soluble in water, the presence of a longer alkyl chain increases the hydrophobicity of the molecule, decreasing the solubility of the alkylphenol in polar media (e.g. , water) and increasing the boiling point, which facilitates purification of the product at the top of a distillation column without need to evaporate the solvent. The alkyl chain length can be chosen to be sufficiently long to have a high boiling point, while keeping a good balance between hydrophilic and hydrophobic groups to extract LA and not sulfuric acid. Table 1 shows that with 2-sec-butylphenol, the partition coefficient for LA extraction remains at a value of approximately 2 (entries 1–3), while the partition coefficient for extraction of formic acid (FA) increases when the concentration of the products is increased. When the length of the alkyl chain of the alkylphenol increases, the polarity of the alkylphenol is reduced and the partition coefficient for LA decreased to 1.2 when using 4-n-pentylphenol (entry 4) and to 0.8 when using 4-n-hexylphenol (entry 5). The FA partition coefficient does not change significantly. GVL is more hydrophobic than LA and thus the partition coefficients are higher (entries 6–8), which allows the GVL concentration to be increased by successive recycling steps after hydrogenation of LA (Figure 1). However, as the amount of GVL in the 2-sec-butylphenol organic phase increases (entries 9–12), the LA partition coefficient decreases. Thus, the extent of solvent recycling prior to distillation represents a compromise between achieving high concentrations of GVL while also maintaining a high partition coefficient for LA extraction. Another advantage of the alkylphenol solvents is that the extraction can be carried out at elevated temperatures (entry 13), suggesting that the process could be carried out at the temperatures employed for cellulose deconstruction, thereby minimizing the need for heat exchangers, leading to energy and equipment savings. In addition, sulfuric acid was not detected in the organic phase for any of the entries in Table 1. Thus, it is possible to use the aqueous phase for multiple steps of cellulose deconstruction. After extraction of LA, the next step in the process is hydrogenation of LA to GVL. Previous literature reports that ruthenium on carbon (Ru/C) could be an effective catalyst. However, Ru/C hydrogenated the C=C bonds in 2-sec-butylphenol, forming butylcyclohexanol and butylcyclohexanone (corresponding to 0.3 % conversion of 2-sec-butylphenol at conditions listed in Table 2, entry 1). In addition, the Ru/C catalyst undergoes deactivation with time-on-stream in the presence of FA, even at [a] Dr. D. M. Alonso, Dr. S. G. Wettstein, Dr. J. Q. Bond, Prof. T. W. Root, Prof. J. A. Dumesic Chemical and Biological Engineering Department University of Wisconsin Madison, WI 53706 (USA) Fax: (+ 1) 608-262-5434 E-mail : dumesic@engr.wisc.edu Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100256.

Direct conversion of cellulose to levulinic acid and gamma-valerolactone using solid acid catalysts

Cellulose was converted with high yield (69%) to levulinic acid (LA) using Amberlyst 70 as the catalyst and using a solution of 90 wt% gamma-valerolactone (GVL) and 10 wt% water as the solvent, compared to the low yield of 20% obtained in water. The LA was upgraded to GVL without any neutralization or purification steps due to the solubilization of humins by the GVL solvent. High LA yields (54%) were also obtained from real biomass (corn stover).

Production of butene oligomers as transportation fuels using butene for esterification of levulinic acid from lignocellulosic biomass: process synthesis and technoeconomic evaluation

Levulinic acid (LA) is a valuable platform chemical upon which biorefining strategies for the production of chemicals, fuels and power can be established. Herein, we report the results of process synthesis and technoeconomic analysis studies for the conversion of lignocellulose derived LA to liquid fuels through the intermediate formation of levulinate esters. In this strategy, esterification of levulinic and formic acids with alkenes (i.e., butene) produces hydrophobic esters, which extract the unconverted LA from the aqueous sulfuric acid solution. Following the γ-valerolactone (GVL) production from LA and levulinate esters, GVL is converted to butene, hence providing the butene required for esterification and butene oligomers. The minimum selling price of butene oligomers from a 1365 dry tons per day of loblolly pine processing facility is calculated to be

Conversion of Sugars and Biomass to Furans Using Heterogeneous Catalysts in Biphasic Solvent Systems

4.92 per gallon of gasoline equivalent. Our analysis shows that the biomass feedstock price is the main cost driver.

Bimetallic catalysts for upgrading of biomass to fuels and chemicals

Within the last decade, interest in using biphasic systems for producing furans from biomass has grown significantly. Biphasic systems continuously extract furans into the organic phase, which prevents degradation reactions and potentially allows for easier separations of the products. Several heterogeneous catalyst types, including zeolites, ion exchange resins, niobium‐based, and others, have been used with various organic solvents to increase furan yields from sugar dehydration reactions. In this minireview, we summarized the use of heterogeneous catalysts in biphasic systems for furfural and 5‐hydroxymethylfurfural production from the past five years, highlighting trends in chemical and physical properties that effect catalytic activity. Additionally, the selection of an organic solvent for a biphasic system is extremely important and we review and discuss properties of the most commonly used organic solvents.