Perspective pubs.acs.org/journal/ascecg
Green Solvents in Biomass Processing Lindsay Soh*,† and Matthew J. Eckelman‡ †
Department of Chemical and Biomolecular Engineering, Lafayette College, 740 High Street, Easton, Pennsylvania 18042, United States ‡ Department of Civil and Environmental Engineering, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115, United States ABSTRACT: As new applications expand in the field of biomass refining and valorization, appropriate solvent development and usage will aid in viable implementation. This work presents an overview of green solvent applications within biomass production. Particular solvent needs for biomass fractions to produce fuels and value-added products are presented. Green solvent metrics with respect to functionality and environmental, safety, and health impacts from a process and life cycle view are also addressed and applied to conventional and neoteric solvents. Current and potential applications of various solvents for extractions and conversions are provided for consideration in a biorefinery setting with discussion of future needs. KEYWORDS: Green solvents, Biomass processing, Green metrics, Solvent selection guide, Biomass valorization, Biorefinery, Biobased chemicals
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INTRODUCTION The US Department of Energy characterizes a biorefinery by the ability to efficiently convert “a broad range of biomass feedstocks into commercially viable biofuels and other bioproducts.”1 Biorefineries have the potential to provide a broad range of products from fuels to chemicals and materials while optimizing feedstock utilization, economics, and overall sustainability.2 Biological material consists of a diverse mixture of molecules whose inherent complexity can be utilized for multiple functions. Feedstocks may be carbohydrate-rich (e.g., corn, sugar cane), lignocellulosic (e.g., corn stover, switchgrass), or lipid-rich (e.g., palm, microalgae) and can be processed into a variety of different fuel and chemical products, depending on their composition.3 Just as petroleum refineries produce numerous fractions with distinct end-uses across a range of molecular weights, from heavy tars and waxes to light alkanes, biorefineries will ideally derive multiple products from the different fractions of input biomass, but must also support the added costs associated with growth and transport of an inherently lower energy density feedstock.4 As such biorefinery operations seek to supplement high volume, low value fuel streams with high value, low volume products that provide additional financial returns.5 Moreover, the coproduction of molecular feedstocks and chemicals enhances the overall sustainability of fuel production by decreasing energy use, waste, and life cycle impacts.3,6 The choice of solvents used for biomass processing will in part determine both the economic viability and environmental sustainability of a biorefinery. Biomass processes include extraction, fractionation, and conversions such as fermentation © 2016 American Chemical Society
or transesterification, followed by further separations to achieve target products.3,7 Solvents are used during many of these steps in order to provide high extraction yields, mediate conversions, and partake in product formulations.8 The use of solvents may lead to higher quality products, reduce byproduct formation, enable product/catalyst separations, reduce the necessary number of process steps, and even serve as reactants themselves.9 Solvent use in biomass processing also has multiple trade-offs. Higher product yields must be balanced against solvent costs from supply, operations/separations, and disposal. For example during oil extraction from soybeans, hexane is often used as a solvent leaving only small amounts of residual oil, which increases yields dramatically compared to dry expeller press extractions; unfortunately, environmental and health issues can result from hexane use via fugitive emissions, toxicity, and for separation.10 From a life cycle perspective, upstream impacts associated with solvent production and transport are also relevant. Furthermore, solvents are used in large volumes throughout the chemicals industry, and assessments of chemical and pharmaceutical products have found that they can greatly contribute to waste production and life cycle impacts.11 In the pharmaceuticals industry, process waste can be upward of 80% Special Issue: Building on 25 Years of Green Chemistry and Engineering for a Sustainable Future Received: July 14, 2016 Revised: September 14, 2016 Published: September 19, 2016 5821
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ACS Sustainable Chemistry & Engineering of solvents employed for conversions and separations.11 Solvents used for biomass processing are of particular concern because the quantities of biofuels and biobased products being targeted are so large.12,13 Thus, it is imperative that appropriate solvents be available and chosen to not only provide high functionality but also minimize life cycle energy use, cost, and human/environmental impacts. Several green solvent guides have evolved that attempt to weigh some of these metrics and highlight greener solvent options.14−16 These guides include many conventional solvents (ranking “least bad” alternatives) and recently have been updated with newer potentially sustainable and/or bioderived alternatives.15,16 Some guides also mention the use of neoteric solvents such as supercritical fluids, ionic liquids, and switchable solvents; however these have not been comprehensively evaluated due to their slow industry adoption and varied properties. Several reviews have broadly surveyed the field of green solvents pointing to the gaps in alternatives and research needed for the field.17−19 The few that point to specific biomass applications of green solvents7,20−22 are either not specific to solvents or are geared toward particular solvents or biomass fractions. Therefore, this work aims to provide a comprehensive analysis of key metrics for green solvents relevant to biomass processing. First, we review current solvent use in biomass processing and present metrics and rating systems used for solvent selection. Subsequently, promising green solvent candidates with applications for biomass processing are selectively reviewed and directions for future work in the field are highlighted. Solvent Use in Biomass Processing. Industrial products derived from biomass can be divided into three categories− energy, molecules, and materials.23 The processing steps required to obtain chemical and material products include extraction (mechanical or physicochemical), conversion reactions, and separations which may be costly with current technologies.24 For example, energy products generated via thermochemical conversions to produce biocrude must then be fractionated to various hydrocarbons and syngas (as with petroleum refineries).25,26 In chemical conversion processes, selected biomass fractions may first be extracted for further reaction. These steps require solvents, such as hexane, DMSO, and methanol, which have inherent hazards associated with toxicity and/or flammability. Advancements in technology and process modeling will aid in biorefinery design for particular feedstocks and desired products.27 Biomass Composition and Feedstock Types. The major chemical constituents of biomass include carbohydrates/sugars, lipid, lignin, and proteins,27 each of which vary in proportion depending on the particular feedstock and growth conditions. For example, woody biomass mainly consists of cellulose, hemicellulose, and lignin,25 whereas microalgae lack lignin altogether.28 Additionally, the lipid, starch, and protein content of microalgae can vary temporally, depending on the phase of growth and growth medium nutrient concentrations. 29 Simplistically, biomass can be divided into subcategories of lipid-rich, carbohydrate-rich, or lignocellulosics, each of which require different processes to create varying fuel and product profiles. A major fuel product investigated for lipid-rich feedstocks is biodiesel, or fatty acid alkyl esters (FAAE), derived from triglycerides in the lipid fraction. Extraction of the lipid fraction typically uses hexane (or for more analytical purposes,
chloroform/methanol30), though more selective extractions may be done with other solvents such as switchable solvents31 or carbon dioxide.32,33 The lipid fraction may also contain other valuable products including hydrocarbons, carotenoids, tocopherols, fatty acids, diglycerides, and sterols34 useful for production of fuels, chemical products, and/or platform chemicals.35−37 Residual biomass may be rich in protein and carbohydrate materials38 and could be further extracted or converted to value added products. Carbohydrate-rich feedstocks represent a wide array of both edible and nonedible crops that are considered abundant and relatively inexpensive.20 While bioethanol is currently the most abundant fuel produced from carbohydrates, hydrogen and biogas are other possible fuel products.39 The carbohydrate fractions require both extraction and conversion processes to make fuels and chemicals, requiring solvents for both solubilization and reaction compatibility.40 These applications require chemical and enzymatic synthesis of chemicals in polar solvents such as water but can also be extended to other potentially green solvents such as supercritical fluids, ionic liquids, fluorinated solvents, deep eutectic solvents, and bioderived solvents.20 A comprehensive review of these solvents for carbohydrate-rich biomass applications has been provided by Farrán et al.;20 however, the hazardous properties of some of these solvents may preclude them from being considered green (e.g., ionic liquids, fluorinated solvents, certain bioderived alternatives). Lignocellulosic feedstocks are generally considered the most difficult to process as they consist of tough fibers resistant to decomposition. Lignocellulosic materials must first be depolymerized and partially deoxygenated in order to be converted to fuels or chemicals, 41 requiring both pretreatment and conversion steps.42 As with carbohydrates, bioethanol is the main fuel currently produced from lignocellulosics43 while biooil and syngas44 are also potential fuel products that require high temperature/pressure conversions. There are multiple options for pretreatment, several of which involve significant solvent use−alcohols for organosolv treatment, phosphoric acid and acetone for fractionation, ionic liquids, and most commonly water.45 Following pretreatment, solvent application varies depending on the solute and intended application−e.g., cellulose dissolution requires disruption of the cellulose chain hydrogen bond system46 whereas sugar conversions may require stabilization of active species.47 Effective solvents range from organic solvent mixtures, acids, bases, and ionic liquids on a research scale.43,48 Numerous solvents have been tested and proposed to substitute in existing conversion pathways or enable new ones, but challenges for these solvents include cost, chemical and environmental characterization, yields, selectivity, and effective recycling.49,50 Nonfuel Biorefinery Products. Besides fuels, potential biorefinery products vary widely, depending on feedstock type. Such products include large scale chemicals, materials, edibles and nutraceuticals, and pharmaceuticals that have the added complexity of retaining active component functionality.20,51 A number of potential platform compounds have been identified by Bozell and Peterson as lignocellulosic biorefinery products.5 These compounds include ethanol, furans, glycerol and its derivatives, hydrocarbons, carboxylic acids (lactic acid and succinic acid), aldehydes/ketones (hydroxypropionic aldehyde and levulinic acid), and polyols (sorbitol and xylitol), chosen due to their abundance in the literature, multifunctionality, ability to replace existing petro5822
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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Solubility Parameters for Selected Solvents Hildebranda
Kamlet−Taftb
Hansen63
compound
molecular class
δT
δd
δp
δh
α
β
π*
green rankd
water glycerol 1,3 propanediol methanol dimethyl sulfoxide ethanol acetonitrile pyridine acetone tetrahydrofuran chloroform methyl acetate 1,2-dichloroethane toluene ethyl acetate 2-methyl thf heptane hexane
alcohol alcohol alcohol alcohol polar aprotic alcohol polar aprotic amine ketone (polar aprotic) ether halogenated ester halogenated aromatic ester ether hydrocarbon hydrocarbon
47.8 36.2 31.7 29.6 26.7 26.5 24.4 21.8 19.9 19.5 18.9 18.7 18.5 18.2 18.2 18.1 15.3 14.9
15.5 17.4 16.8 15.1 18.4 15.8 15.3 19 15.5 16.8 17.8 15.5 16.5 18.0 15.8 16.9 15.3 14.9
16.0 12.1 13.5 12.3 16.4 8.8 18 8.8 10.4 5.7 3.1 7.2 7.8 1.4 5.3 5.0 0.0 0.0
42.3 29.3 23.2 22.3 10.2 19.4 6.1 5.9 7.0 8.0 5.7 7.6 3.0 2.0 7.2 4.3 0.0 0.0
1.17 0.93c 0.80c 0.93 0.00 0.83 0.19 0.00 0.01 0.00 0.44 0.00 0.00 0.00 0.00 0.00c 0.00 0.00
0.14c 0.67c 0.77c 0.66c 0.76 0.75c 0.31 0.64 0.48 0.55 0.00 0.42 0.00 0.11 0.45 0.58c 0.00 0.00
1.09 1.04c 0.84c 0.58c 1.00 0.51c 0.75 0.87 0.71 0.58 0.58 0.60 0.81 0.54 0.55 0.53c −0.08 −0.08
R R P R P R P H R P HH P H16 P R P P H
Calculated from δT = (δd2 + δp2 + δh2)1/2. bData from ref 66 unless otherwise noted. cData from ref 67. dRankings from ref 15 unless noted. R = recommended, P = problematic, H = hazardous, HH = highly hazardous. a
solvent helps to retain high yield selectivity, though an appropriate alternative should be found for this carcinogenic solvent.60 Metrics and Selection Guides: What Is a Green Solvent? Solubility Parameters. Solubility parameters serve as practical tools for determining the appropriateness of a solvent for specific applications. Solubility is an important selection criterion as poor solvent function can lead to low yields and increased process waste. Other important factors include selectivity in the complex biomass matrices and reaction compatibility, which can be influenced by the solubility parameters as well as other physicochemical properties.18,61 The Hildebrand solubility parameter (δT) is a theory-based metric that was developed based on the energy of vaporization for the liquid.62 Defined as the square root of the cohesive energy density, it thermodynamically postulates that a solute will likely dissolve in a solvent if they have proximate solubility parameters. The Hansen solubility parameters are based on δT but account for differing molecular interactions from dispersion (δd), dipole−dipole (polar; δp), and hydrogen bonding forces (δh) such that δd2 + δp2 + δh2 = δT2.63 Practically, δh has come to represent the energies of interactions not included in the other parameters. These parameters have been widely adopted as they are grounded in theory but provide granularity for molecular interactions and are generally valid and easy to obtain.63,64 Minimizing the difference between the solute− solvent solubility parameters increases the chance of mutual solubility. More formally, an experimentally determined solubility parameter sphere radius, Ro, can be compared with the solubility parameter difference, Ra, where Ra2 = 4(δd2 − δd1)2 + (δp2 − δp1)2 + (δh2 − δh1)2. The ratio, RED = Ra/Ro reflects the relative energy difference between the two componentsvalues of RED less than 1 indicate likely mutual solubility. While the use of RED is useful, Ro values are not always reported and require a substantial amount of experimental data for their calculation.64 As a biomass-relevant example, the solubility sphere for cellulose acetate was determined experimentally by measuring
leum-based platform and commodity chemicals, as well as potential for high volume production and scale up.5 Besides these organic chemicals, amino acids from waste biomass may also be valorized as a source of nitrogen-containing commodity chemicals.21 In order to produce and utilize these biorefinery products, selective extractions and conversions are needed that involve solvents for dissolution and fractionation. For example, successive extraction of lignocellulose using alkaline organosolv extraction of lignin is a potential means for component separation. However, the output stream requires solvent purification, commonly with dichloromethane,52 which is highly volatile and an ozone-depleting substance.53 Currently, active research seeks other methods for separation and isolation of pure lignin in one step, such as filtration54 and alternative solvent extraction.55,56 Applications to conversions include platform molecule reactions to produce other value-added products, where solvents are needed for enhanced yields and separations. For example, glycerol can undergo many reactions−oxidation, hydrogenolysis, dehydration, pyrolysis, gasification, esterification, etherification, oligomerization, polymerization, carboyxlationproducing a suite of value added chemicals with widespanning solubility parameters.57,58 Solvents used for these reactions can range from water to dioxane or sulpholane depending on the system and catalyst.103 Determining appropriate solvent alternatives requires consideration of solvent functionality for the specific biomass compositions and desired products as well as environmental, health, and safety (EHS) impacts from process and life cycle perspectives. In order to produce and utilize these compounds, extractions and conversion are needed that involve solvents for dissolution and fractionation. For instance, fractionation of spent sulphite liquor has been performed with isopropanol to produce lignosulfonates and succinic acid.59 Also, dichloroethane is used as a solvent during the reaction of starch based starting materials to produce chloromethyl furfural (the starting material for HMF and levulinate compounds). Here, the 5823
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ACS Sustainable Chemistry & Engineering Table 2. Metrics Considered for Industry Green Solvent Selection Guides
medicinal chemistry applications and also may be useful in evaluating solvents used in biomass processing.14−16 The major metrics reported in the most recently published Pfizer,14 Sanofi,15 and GlaxoSmithKline (GSK)16 solvent selection guides are shown in Table 2. The metrics are generally broken up into (process) safety, human health, and environmental categories. GSK has also added categories for both waste generation and life cycle assessment (discussed below). In addition to these industry efforts, groups from ETH and Rowan University have also created rankings of common solvents that incorporate many of the same metrics.72,73 The publication of relevant chemical databases such as the Global Harmonized System (GHS) has helped to provide a rich source of health and safety data regarding many chemicals including solvents.74 In the process safety category, the key metrics regard flammability and explosion potential for the solvent. GSK’s tool also utilizes several metrics related to chemical reactivity. Human health metrics include those related both to hazard and exposure. Acute and chronic toxicities as well as reprotoxicity, mutagenicity, and carcinogenicity are considered by all three guides, typically with data from GHS or International Agency for Research on Cancer’s (IARC) cancer class system.75 For newer solvents where data are incomplete or unavailable,76 predictive methods could be useful to screen for low impact alternatives based on chemical properties (e.g., log P, water solubility)77,78 though ultimately experimental validation would be needed. For solvents a major exposure route is via inhalation, and thus boiling point has been included in this category by the Sanofi guide as a proxy for volatility. GSK accounts for the exposure potentials by considering occupational exposure limits and saturation concentrations to create a vapor hazard ratio. The industry solvent selection guides also consider environmental impacts with varying levels of sophistication. All three guides account for ecotoxicity and ozone depletion potential,
the dissolution of the solute in various solvents and calculating the solubility parameters based on solubility differences.65 The resulting parameters (δd = 18.3, δp = 11.0, δh = 9.8, Ra = 6.2 Ma1/2) can be compared to parameters of new solvents to identify potential solvent alternatives. Another common set of widely used solubility parameters are the Kamlet−Taft parameters, which comprise hydrogen bond donating acidities (α), hydrogen bond accepting basicities (β), and polarizability (π*).66 These solvatochromatic parameters are relatively easy to measure, and the unique hydrogen bonding donation and acceptor parameters allow for differentiation of these interactions. As such, the Kamlet−Taft parameters are well surveyed and are even available for a number of potentially green solvents, however they provide varied results depending on the dyes used for quantification.67 Solubility parameters for common solvents used in biomass processing are shown in Table 1 along with their classification in a green solvent ranking tool (discussed below). Since the parameters are influenced by molecular properties, classes of molecules tend be grouped. Solubility parameters can also be used to vet for solvent selectivity in multiple solute mixtures by careful choice of nonoverlapping portions of solubility spheres.61 However, a trade-off may occur from lower capacity requiring more solvent and related operating cost and low selectivity requiring more downstream processing.68,69 While solubility parameters are useful screening tools, other physicochemical properties (e.g., solute molecular size, viscosity, boiling point)18,70 will also affect solvent performance. In addition, trade-offs between performance and cost, EHS, and life cycle impacts must also be considered.18,69,71 EHS Metrics and Selection Guides. There are a variety of EHS metrics that are used to identify green solvents and screen out undesirable candidates. Several solvent selection guides using EHS metrics have been published by industry for 5824
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solvents, except certain chlorinated solvents (where incineration requires large amounts of sodium hydroxide to eliminate chlorine from flue gas).88 Additional studies from the same group focused on energy use compared incineration with solvent reuse via distillation for 45 solvents using their Ecosolvent tool.72,89 Between incineration and distillation, incineration is generally preferred if the solvent has high energy density whereas distillation is favored for solvents that have high recovery potential and relatively large life cycle impacts during production.90 These results are consistent with the work of Amelio et al.91 who also provide a guide to determine whether it is better to distill or incinerate particular binary mixtures. As separation via distillation requires over 30% of the energy used in the US chemical manufacturing sector,92 other researchers have recommended avoiding distillation as a separation technique to cut down on life cycle impacts associated with the high energy use.17 Careful solvent choice, reductions in use, and end-of-life considerations are clearly important for minimizing energy use and environmental impacts associated with solvents across the life cycle for biomass processing. The use of multiple metrics within the LCA framework often gives rise to trade-offs that can inform solvent choice. For example, the production of dichloromethane has relatively low greenhouse gas (GHG) emissions but also emits ozone depleting substances. Considered on a life cycle basis, efficiencies in solvent utilization or energy required for separations have benefits that compound all the way through the product life cycle, decreasing emissions and subsequent impacts from upstream processes. Thus, recyclability is a key parameter in evaluating life cycle impacts of solvents. The assumption that separations will be done via distillation may not necessarily be valid for emerging biorefinery applications with new separations techniques (e.g., chromatography, membranes, switchable solvents) and for thermally labile solutes.93 Other solvent properties that may limit the recyclability of a solvent include azeotrope formation, stability, peroxide formation, and potential for self-reaction.94 Accordingly, alternative separations options and/or the use of neoteric solvents with reduced separations requirements can lead to better performance with significant life cycle energy savings and decreased impacts.17,93,95 While LCA covers many EHS metrics such as toxicity, it does so solely from the perspective of routine process emissions, while hazards associated with potential accidents or releases and implications for safety are not considered. Eckelman96 proposed a new metric called life cycle inherent toxicity i*, which quantifies the total toxicity of all chemicals used in a supply chain, regardless of actual exposure. i* values for inhalation/ingestion exposure routes and cancer/noncancer end points were estimated for 181 organic chemicals from the ecoinvent LCI database. Values were uncorrelated with both conventional LCA toxicity results and the toxicity of the target chemicals, indicating that upstream use of hazardous materials is not currently being captured by either LCA or EHS metrics. Figure 1 shows i* values for solvents against the entire set of organic chemicals. Unsurprisingly solvents as a whole did not differ in i* from traditional solvents as their production routes do not generally vary greatly. However, green solvent recommendations did not necessarily correlate with low i* values, meaning that the production of green solvents may still involve significant use of toxic materials. Considering bioderived solvents, glycerol as a coproduct from biodiesel
though other metrics are inconsistent (Table 2). For Sanofi, the major environmental assessment is done via boiling point correlation where high boiling point components are penalized as they require high energy for solvent recycling.15 Thus, this simple approach begins to address some life cycle impacts associated with solvent reuse (also noted by GSK). However, the use of boiling point as a selection parameter may have trade-offs, for instance, while low boiling point solvents require less distillation energy for reuse, they may have higher risk in terms of air emissions and exposure.15 Life Cycle Metrics. By definition, life cycle assessment (LCA) based metrics consider not just the use of a solvent but also its sourcing, production, and end-of-life treatment or disposal.9,79 LCA metrics can reflect resource use (e.g., energy and water), environmental impacts (e.g., stratospheric ozone depletion, global warming, acidification of soils and waterways), and resulting human health impacts (e.g., human toxicity, respiratory disease).80 Energy use is a central consideration in green solvent selection, and LCA modeling allows for estimation not only of process energy where solvents are used but also considers the energy required to produce a solvent, including feedstock energy, heating, and electricity use. These can all be combined into a measure of primary energy use termed cumulative energy demand (CED). Energy use is also important in LCA as the dominant contributor of many types of harmful air emissions. For biobased solvents (discussed below), agriculture-related impact categories such as land use and water consumption as well as fertilizer, herbicide, and pesticide impact to eco-toxicity and water quality must also be considered.81,82 Life cycle inventory (LCI) data in repositories such as ecoinvent, GaBi, or the U LCI database can provide cradle-to-gate information on solvent production, though these databases are often limited to the most common solvents.83 Unlike the other solvent guides, GSK adopted a life cycle approach by considering solvent waste as well as LCA metrics (net mass of a materials consumed, gross energy usage, total water consumption, total organic carbon, fossil fuel feedstocks, photochemical ozone creation potential, global warming potential, acidification, eutrophication) in a composite score. These metrics also take into account physicochemical properties such as water solubility, vapor pressure, and boiling point, as a proxy for the energy required for distillation.16 Several LCA studies have focused specifically on solvents. Capello et al.84 estimated relative environmental impacts for 50 common solvents via a single solvent production route, finding that in most cases, much of the impact (across multiple impact categories) is based in the energy needed for extraction and processing. Notably energy consumptive processes during solvent production included esterification and alkylation. In addition to the impacts from solvent production (cradle-togate), choices regarding solvent use, recycling, and disposal will have significant impacts on the life cycle including types of separation methods used and the required product purities. For example, solvent substitution in the synthesis of Galantamine· HBr decreased cradle-to-gate resource requirements by imparting improved recyclability and reaction yields as well as decreased losses.85 Waste solvents may be discarded or valorized via recycling, biological treatment, or incineration,16,86,87 the choice of which affects life cycle impacts. For example, Hellweg et al. compared EHS, persistence and exposure, and LCA results for 13 solvents assuming solvent incineration post use. They found a resultant energy credit (net energy benefit) from incineration for most 5825
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geometric mean of normalized scores for each category with cutoffs such that a majority of positive attributes do not overshadow serious concerns in another category.16 The GSK guide is most useful when choosing among known alternatives, but for specific applications (such as application to biomass processing), the functional appropriateness of a solute−solvent pair as well as cost, availability, and legislative restrictions of the solvent must also be considered.16 In the Sanofi solvent selection guide, a clear rank (recommended, problematic, hazardous, or highly hazardous) is given to each solvent which includes a number of bioderived and less common solvents.15 As part of the ACS Green Chemistry Institute Pharmaceutical Round Table and CHEM21 initiatives, this guide represents multiple stakeholders and final scores have taken expert recommendations into consideration. Prat et al. reconciled multiple solvent guides and found a 67% convergence of recommended solvents.76 The most egregious differences existed for acetonitrile, methyl tetrahydrofuran, chlorobenzene, and N-methyl pyrrolidone due to methodological differences such as weightings of exposure metrics and use of different toxicity classifications.15 Figure 2 illustrates solvent recommendations over Hansen solubility parameter space to highlight types of solvents that are available as green alternatives. All guides agreed that most alcohols and esters (acetates) were recommended leading to the clustering of green options around δd around 15−16 and δp between 4 and 7. The GSK and Sanofi recommendations unsurprisingly have close correlations; however, they do not comprehensively cover the same Hansen space as problematic and hazardous solvents (particularly those with low δh and δp) warranting the need for careful consideration of solvent use and/or the generation of new alternatives. These guides do not include neoteric solvents such as ionic liquids, switchable solvents, or supercritical fluids, which would expand the range of existing options but are currently not broadly used. In the wide range of applications for biomass processing, having green solvent options for the diverse range products will aid the development of more sustainable biorefineries. Benazzouz et al. used Hansen parameters and toxicity/hazard metrics to evaluate a database of 220 solvents for alternatives that span Hansen space more evenly than the solvent selection guides.65 Forty potential green solvents were identified with this approach (Figure 2); however hazard was screened via only toxicity and boiling point. Furthermore, the authors considered a solvent green if it met criteria for either the human health or ecotoxicity category and thus could have negative ramifications in the other. Accordingly some recommended alternatives (e.g., heptane and formic acid) are considered problematic by other solvent selection guides. In addition to the commercial guides
Figure 1. Life cycle inherent toxicity (i*) values for solvents compared to chemicals surveyed by Eckelman.96 Several solvent examples are highlighted across the range of i* values including analysis of petroleum versus biologically derived glycerol.
production had a significant reduction in i* from glycerol synthesis via epichlorohydrin. However, a similar analysis for corn ethanol versus petroleum ethanol revealed comparable i* values for inhalation carcinogenicity, where upstream toxicity for corn ethanol stems from agriculture uses of fertilizers, herbicides, and pesticides. Because glycerol is a relatively low value coproduct from biodiesel production, only a small portion of upstream soy cultivation impacts are allocated to the glycerol coproduct.97,98 Recommended Solvent Alternatives. The industry solvent selection guides discussed above each combine multiple metrics in order to rank and recommend green solvent alternatives. The industry guides have been published for general consideration but have also been found to decrease impacts for internal operations.14,16 The Pfizer guide provides a simple comparison of common solvents, deeming them “preferred,” “usable,” or “undesirable.” They also offered a solvent replacement table for easy quick alternatives such as substituting heptane for hexane/pentane or toluene for benzene. (The authors do note that while the alternative solvent is not necessarily green in all respects, it provides a relatively better choice.) While the Pfizer guide was simple and covered a modest selection of solvents, it was well received with a purported 50% reduction in chlorinated solvent use and 97% reduction in ether use across Pfizer’s research and development division though how this translated to actual manufacturing processes is not clear. The GSK guide is a product of several iterations16,94,99−101 and focuses on small molecule, organic solvents in the pharmaceutical industry. This guide uses the
Figure 2. Distribution of solvents over Hansen space based on collated hazard rankingshighly hazardous (dark red circle), hazardous (red circle), problematic (yellow circle), or recommended by the Sanofi15 (green circle) and GSK16 (triangle) guides as well as by Benazzouz et al.15 (square). 5826
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Figure 3. Distribution of select biologically based solvents in Hansen space overlaid with GSK19 (open circle) and Sanofi18 (filled circle) suggested green solvents. Data sourced from the work of Hansen63 and Chernyak.58
Tobiszewski et al.18 compared 151 potential solvents including some biobased solvents18 using multicriteria decision analysis to weight solvents based on toxicological and hazard metrics. Their rankings generally align with other solvent guides though provided more granularity as to numerical rankings between solvents but did not include a life cycle ranking of different solvents. In order to provide more versatility and solubility parameter coverage, several articles have attempted to use computational techniques to find alternatives.19,102,103 A large number of potential green solvents have been screened via the COnductor-like Screening MOdel for Real Solvents (COSMO-RS), which takes into account electronic and thermodynamic effects to quantify molecular interactions.19 Solvents can be clustered based on their molecular properties (strong/weak electron donor, protic/aprotic, polarity), and promising solvent alternatives can be further screened based on EHS data. Using this approach, several notable groups lack adequate substitutes, i.e., strong electron pair donor bases and aprotic slightly dipolar (asymmetric halogenated hydrocarbon) solvents.102 The authors also note that Hansen solubility parameters are helpful for screening solvents and as a preliminary means for selection. Despite the availability of solvent selection guides and assessment options with varying levels of complexity, a recent survey of chemical companies (mostly pharmaceutical based) found that simple metrics (process mass intensity and E-factor) were being used by most of the surveyed companies but that other metrics were less ubiquitous. The survey found that while all companies avoided chlorinated solventsminimizing the use of dichloromethane (a regulated solvent in the EU) and effectively eliminating the use of chloroform and carbon tetrachloridethat the adoption of new green solvents was not widespread. Many companies have explored the use of water for extractions and reactions but are aware that it will not be able to cover all solvent needs.104 Byrne et al. recently published a comprehensive review on the current state of the art regarding solvent selection guides, compiling recommended solvents and strategies taken by various industry and academic research groups.86 They note that more quality data is needed for less common solvents, since the results have high sensitivity to the existing data. Despite individual differences, they found that the guides have started to reach a consensus but that the inconsistent application of sustainability criteria restricts completeness.
Instead of focusing on new guides, the authors suggest focusing on specific applications86 (as with chromatography105,106 and certain chemical syntheses9), biomass processing being a prime target for application in an emerging field with high expectations of process sustainability. In developing solvent selection guides for biomass, metrics and weighting schemes should be chosen that reflect both technology and policy goals for bioenergy and biobased chemicals, such as reducing fossil energy use and GHG emissions.107 Prospective Green Solvents for Biomass Applications. Conventional solvents used for biomass processing are listed in Table 1 along with their recommendation according to the Sanofi guide.15 Many of these solvents are listed as problematic or hazardous and would benefit from greener substitutes. Solvent selection guides point to potential alternatives that may be useful across a variety of extraction and conversion platforms. For example, substituting chloroform with DMSO or hexane with heptane already provides significant advantages for extractions, though both of these substitutes are still problematic.14 While efficiencies of use and substitutions within conventional solvents can decrease solvent impacts, neoteric solvents are not comprehensively included in the guides and may provide sustainable alternatives to current organic solvents. While promising, the commercial use of neoteric solvents is still nascent. The rest of this section highlights several leading neoteric solvents for biomass processing and draws attention to research opportunities that may aid in their development. Water itself may also be considered an ideal green solvent due to its nontoxicity and abundance, with added benefits due to the typically high water content of biomass; however its applicability is limited to specific chemistries and attention must still be paid toward its separation and treatment.108 Water’s role as a solvent, reagent, and catalyst particularly in hydrothermal processes for a number of biomass processing applications and feedstocks is well captured in numerous recent reviews108−111 and will not be covered in this work. Biobased Solvents. Not only are solvents needed for biomass processing, they can also be produced from biomass itself. Biobased solvents are potentially renewable alternatives that can allow for more sustainable processing of biomass while serving as drop-in replacements for nonrenewable organic solvents. The most prevalent example is the production of ethanol from carbohydrate and lignocellulosic feedstocks, which typically is used for fuel but is also a common solvent. Solvents such as acetone or methanol can be produced from 5827
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ACS Sustainable Chemistry & Engineering lignocellulosic feedstocks, but their life cycle impacts may not always be lower than their fossil-based counterparts because of both the early stage of the technology as well as upstream impacts from agricultural production.42 Use of agricultural waste feedstocks, along with maturation of processing techniques should lead to lower environmental impacts, which can already be seen for many biobased chemicals in terms of reductions in nonrenewable energy use and GHG emissions.112 However, some biobased solvents are toxic despite their renewable origin and/or have other deleterious EHS properties that must be considered. Here several promising biobased solvents, derived from various feedstocks, are highlighted; the production and applications of biobased solvents are more exhaustively reviewed elsewhere.113−117 Furfural from Lignocellulosics. Furfural shows promise as a platform chemical for many products.118 Currently, higher yields of furfural can be achieved from extracted sugars than directly from lignocellulosic biomass, however the direct approach would greatly decrease processing costs and impacts.118 Early uses of furfural included plastics preparations and paint stripping though it can also be used for the dissolution of cellulose (cellulose acetate, nitrocellulose)119 and has also been used for removal of aromatics and other contaminants from rosin.120 Two other promising solvents derived from furfural are 2-methyl tetrahydrofuran (2-MeTHF) and γ-valerolactone (GVL).121 2-MeTHF, GVL, and furfural (in order of increasing δd) are included in Figure 3. GVL can be derived from carbohydrates via HMF.20 It is classified as a polar aprotic solvent and has a relatively high boiling point, high viscosity, and generally good stability to acidic and high basic reactions.122 It has been found to suitably replace other polar aprotic solvents such as dimethylformamide, N-methylpyrrolidin-2-one, dimethylacetamine, and acetonitrile as a reaction medium with benefits stemming from low leaching of palladium based catalysts and improved recyclability by limiting of heavy metal contamination. Of a survey of solvents for use in the Sonogashira reaction, GVL was the only one identified as a green solvent.122 GVL was also used as a solvent for the conversion of sugars to HMF or levulinic acid and formic acid with performance similar to other polar aprotics like DMSO, THF, acetonitrile, or acetone.123 2-Methyl THF124 can be derived from furfural or levulinic acid and is a promising solvent with low water miscibility, low boiling point (80.2 °C), and high stability, as well as biodegradability. It is potentially useful for syntheses involving organometallics, biotransformations, or processing of lignocellulosic materials, but drawbacks, including ready peroxide formation, azeotrope formation with water, and low flash point, must be considered in process design.115 This ether is a promising alternative for n-hexane in triglyceride extractions and has also been used for lignocellulosic fractionation.125 Purportedly, it has also been extended to several syntheses at GSK during route development and has been used in 16% of all pilot plant campaigns from 2007 to 2009.126 Fatty Acid Alkyl Esters and Glycerol-Based Solvents from Lipid-Based Feedstocks. Extracted oils primarily consist of triglycerides of various chain lengths and degrees of unsaturation. Triglycerides are often transesterified with an alcohol to produce biodiesel in the form of fatty acid alkyl esters (FAAE) and glycerol as a coproduct, both of which also show promise as solvents.127,128 Additionally, glycerol can be used as a platform for other chemicals and solvents, valorizing this increasingly prevalent waste product.127
Transesterification is typically done with methanol (currently the most economical alcohol) to produce fatty acid methyl esters (FAME). Due to their low toxicity, biodegradability, high flash point, low vapor pressure, and high solvent power, FAME can also be considered as green, biobased solvents.115 FAME have been used as a solvent alternative for chlorinated and oxygenated solvents in various applications including polymerization of styrene,129 cleaning,130 and even as a cosolvent in biodiesel production itself in order to produce a single phase between polar and nonpolar substrates.131 Because FAME properties (solubility, viscosity, cloud point, oxidative stability) vary by alkyl chain structure, pure FAME or blends (which occur naturally depending on the feedstock) can be used for various applications. In Figure 3, FAME δd values increase with chain length (from 6 to 18 carbons) while δp and δh decrease. Only slight increases in δd are observed with increasing degrees of unsaturation (oleic, linoleic, and linolenic methyl esters). The solvent power as measured by kauri-butanol values indicate that shorter chain length of both the alcohol or alkyl chain and unsaturated FAAE tend to have greater solvent power.128,130 Glycerol is extremely viscous and has a very high boiling point (290 °C). These unique properties along with the fact that it is nontoxic and compatible with many organic and inorganic compounds has led to its use in many food applications.132 As a polar organic solvent, glycerol can replace solvents such as DMSO and DMF for the dissolution of salts, enzymes, and organics. Because of its high boiling point, separation from glycerol via distillation is possible although other separations are encouraged from an energy-use perspective. Glycerol can also be used as a hydrogen donor in transfer hydrogenation reactions.132 While the high viscosity of glycerol may be a disadvantage from a mass transfer perspective, it may be overcome by operating at temperatures above 60 °C or using cosolvents. Additionally, glycerol has been added to choline chloride to form a deep eutectic solvent with reduced viscosity.133 As previously mentioned, glycerol can be used as a platform chemical to produce a number of derivatives that are less viscous and have a variety of solvent attributes. For example, glycerol carbonate had the highest δd and δh (Figure 3) of the products surveyed, with the polarity of a cyclic alkylene carbonate and the proticity of glycerol.134 It is known to have high solvent power for general use in reaction media, and in paint/polymer cleaning. Additionally, it has low volatility (boiling point 110−115 °C), low toxicity, low flammability, and is biodegradable. Glycerol carbonate is already industrially produced and used in a variety of applications including cosmetics and even medicinal purposes.132 Other promising glycerol-derived solvents include 1,3- propanediol, triacetin, and diacetin15,16 based on their green properties and potential applications. Glycerol-derived oligomers have also been developed for several purposes including as a solvent in fragrance formulations and surfactants for emulsifying oil− water solutions such as those with triglycerides and water.127 Terpenes from Waste Biomass. The use of terpenes, an isoprene-based class of molecules, from the extraction of natural products has been of interest as replacements for the pervasive use of hexane in biomass extractions due to their low polarity and hydrogen bonding.115 A major component of citrus peel wastes, d-limonene, has been deemed as safe by the USFDA, and thus have been used pervasively in flavors and fragrances illustrating a market in home and personal care products, nutraceuticals, and natural product extractions.135 5828
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ACS Sustainable Chemistry & Engineering
cosolvent or using amphiphilic materials) and catalysis.145 A review of biodiesel production using IL illustrates their use for a number of biomass feedstocks for extraction and conversion processes but points out that costs must be reduced by developing lower cost salts, decreasing viscosities which impair mass transfer for both extractions and reactions, and decreasing impacts associate with acute toxicity.146 While some efforts have been made toward toxicity testing, further efforts should be made toward more comprehensive assessment of hazard and life cycle impacts for both IL production (even simple metrics such as E-factor or atom economy) as well as recycling. Recent IL made from readily available/renewable materials147 with low impact syntheses, and those with high recyclability and cost, are expected to provide more sustainable alternatives.148 For example, cholinium-based IL have been combined with renewable groups such as such as argininate149 or perfluoroalkanoates140 for extraction and pretreatment applications. During the use of aqueous solutions of cholinium choride for the extraction of phycobiliproteins from algae, higher yields were found compared to conventional methods with the added benefit of protein functionality retention postextraction.150 George et al. have developed low cost IL for lignocellulose pretreatment which, according to their technoeconomic analysis, is competitive with current low cost pretreatment chemicals. The study also suggests that the recycle rate of the IL deeply impacts the processing cost; even for a high recycle rate of 99.6%, IL costs are estimated at 39% of the minimum ethanol selling price.151 Deep eutectic solvents have also been developed by combining levulinic acid and polyols with choline chloride gaining tunable properties based on substituent group type and size as well as the addition of other solvent components. The use of the renewable hydrogen bond donor groups is a good step for these ionic solvents and the tunability will aid in their targeted development for specific applications.133 Switchable Solvents. Switchable solvents (SS) possess a property that can easily be converted such as polarity or hydrophilicity using an external trigger such as CO2 addition or temperature changes.152 Using SS can potentially reduce the energy needed for separations and recovery/recycle of solvent and catalysts.152 SS have been used for several biomass applications including extraction of phenols from lignocellulosic biomass with N,N-dimethylcyclohexylamine,153 fractionation of lignocellulosics from spruce wood using switchable IL,154 and even the use of temperature “switchable” butadiene sulfone (thermolytic, producing butadiene and SO2) for lignocellulosic pretreatment.155 While these are promising applications, solvent-biomass ratios as well as solvent recovery and product purity must be further optimized.31 In terms of LCA, energy use for extractions from algal biomass was compared among several extractions including hexane, supercritical carbon dioxide (scCO2), and the switchable solvent, N-ethyl butylamine. SS had the lowest energy requirements when considering algal drying and extraction since it was able to use wet biomass and solvent recovery had minimal energy requirements.156 While the study is limited in scope−it did not assess solvent production or reuse of scCO2 it indicates that the use of switchable solvents is promising for extractions from biomass with high water content. The properties of SS can greatly influence their utility. For example switchable hydrophilicity solvents (SHS) need to have log Kow values between 1.2 and 2.5 and pKaH > 9.5 in order to be have appropriate hydrophilic/hydrophobic tendencies and
Further use in cleaners and degreasers has opened the market for green, biodegradable cleaning formulations. d-Limonene is a prime example of the valorization of this waste product and exemplifies how a biorefinery would reduce waste and improve value chains.135 Other terpenes, α-pinene and p-cymene (tree leaf oils) have been investigated for their solvent properties and found appropriate for extractions, formulations, and even dissolving polystyrene.193,194 Recently these three terpenes were evaluated for their ability to extract triglycerides from microalgae with yields of final fatty acid methyl esters comparable to or even better than that of hexane.136 The extracted lipid composition with α-pinene was most similar to that of hexane.136 As seen in Figure 3, terpenes have very low δp and δh with increasing δd from α-pinene, d-limonene, then pcymene, though all within a close range. Other notable candidates include dimethyl ether,137 lactic acid,138 and ethyl lactate139 from lignocellulosic biomass. This expanding field can produce both drop-in replacements for current petrochemicals or lead to new value chains via the generation of an expanding chemical platform.116,117 As proven applications of biobased solvents amass and the industry expands, green solvent metrics should be considered in order to ensure that the alternatives and processes actually meet sustainable criteria. Many prospective solvents have been tested for properties such as toxicity with proven applications but other properties such as biodegradability, which may be assumed from their bioderived nature, may not hold after conversion to new compounds. In silico design strategies have also been applied to biobased solvents by generating feasible molecular candidates from agriculturally based building blocks and then predicting functional properties in order to determine the best fit for particular applications.103 Using this approach and EHS metrics, 15 potential solvent alternatives were identified for nitrocellulose dissolution using glycerol as a starting molecule, isolating diacetin as a new green solvent candidate. As with this groups previous work, these algorithms could be used to widen the search for green solvents and find appropriate solvent applications for current and growing biorefinery applications. Ionic Liquids and Deep Eutectic Solvents. Ionic liquids (IL) are often considered green solvents due to their low vapor pressure (minimizing inhalation exposure and facilitating product distillation) but their greenness is sometimes disputed based on the fact that these solvents typically have high toxicity140 and require a large number of synthesis steps leading to environmental impacts.17,141 More recently deep eutectic solvents have become of interest in the green solvents community and are included in this section.142 The identity of the anion and cation dictates IL properties potentially allowing for optimization and/or tuning for different applications.143 For instance in the dissolution of lignin, Hart et al. found that a minimum anion hydrogen bond basicity was necessary to dissolve lignin, while cation properties were less influential on solubility.144 In addition to dissolution, the role of IL in lignocellulosic processing can include pretreatment, mediating separations for close-boiling or azeotropic mixtures (e.g., water and ethanol), regeneration of cellulose into specific macro- and microstructures, and fractionation.143 IL can also assist in the hydrolysis of polysaccharides with recent advances toward IL assisted enzymatic hydrolysis including the development of IL that do not inhibit enzyme performance and enzymes that have heightened activity in IL.143 IL use in lipidrich feedstocks is mostly concentrated on extraction (as a 5829
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ACS Sustainable Chemistry & Engineering be effectively triggered by CO2.157 Typically SHS also have low volatility for low exposure and reuse potentiala difference compared to many organic solvents where low volatility is an impediment for distillation. In regards to green metrics, SHS generally have lower toxicity, volatility, and flammability with increased functionalization. Furthermore, experimental and predicted data indicate that SS are generally safer than toluene although more EHS data is needed to find the greenest options and provide more comprehensive impact assessments.157 Carbon Dioxide. Pressurized CO2 in its liquid and supercritical forms has favorable solvent properties (e.g., tunability and low mass transfer resistance). Due to its abundance and nontoxicity, much interest in CO2 as a green solvent has grown though few commercial applications of high pressure CO2 utilization currently exist potentially due to capital high costs and energy use. The application of CO2 within a biorefinery setting is promising due to the selective solubility of low-polarity compounds17,158 and its ability to meditate conversion reactions such as transesterification for biodiesel production.36 The high diffusivity and low viscosity of supercritical and dense CO2 compared to other liquid solvents offer advantages for penetration of solid biomass matrices.159,160 This feature has been used for the efficient and selective extraction of oils from a variety of feedstocks including microalgae,32,161−163 fish,164,165 and oil seeds (e.g., Jatropha,166 palm,167 soy168). These oils contain low-polarity compounds including triglycerides,32 tocopherols,169 and carotenoids.162 While the use of pure CO2 is selective against polar lipids such as phospholipids,32 cosolvents may be used to select for these more polar components.170 This selectivity is also useful for fractionation and refinement of extracts into enriched products. For example, FAAE have been fractionated based on their alkyl chain length and degree of unsaturation by varying pressure, temperature, and CO2 flow rate.33,171 To extend applicability of CO2 toward more polar compounds, it may be added to other solvents to create CO2 liquids (CXL) that demonstrate intermediate properties (e.g., polarity, viscosity, density) between CO2 and the liquid. Furthermore, CO2 can be used as an antisolvent and cause precipitation of products from solutions, allowing for facile product separations as has been demonstrated by valueadded compounds from lignin,22 carotenoids from microalgae,172 and more recently, for the production of particles with tunable size and porosities.173−177 In addition to extractions, reactions in CO2 are advantageous due to the potential for CO2 to enhance reaction kinetics and selectivity.178 The increased rate of mass transfer may allow for greater interaction between immiscible phases included reagents and heterogeneous catalysts179 as well as increased solubility of gaseous reagents in both scCO2 and CXLs.180,181 The CO2 can also aid in product and catalyst recovery as with organic aqueous tunable solvents (OATS).182 Regarding safety metrics, CO2 cannot be oxidized and is generally immune to free radical chemistry.183 Even if conventional solvents are necessary, the use of CO2 may decrease the inherent hazard of these systems by decreasing the flammability envelope of a system, improving the usage of a particular feedstock and reducing energy usage through process intensification. In addition to kinetics, the tunability of CO2 may lead to improved reaction selectivity allowing for better product distribution and more efficient overall processing.180 Particular applications of CO2 for priority biorefinery applications have been detailed previously160,184 and continue to be developed
for a number of different purposes including cellulose processing185 and conversions of bioderived chemicals.186 To overcome current difficulties associated with scale-up and the high capital and retrofit costs for high pressure equipment,183 more data for solute-in-CO2 and CO2-in-solute solubilities would enable for better modeling and process design.187 In terms of scale up, variables such as vessel size, CO2 flow rates, and kinetic curves will effect convection and diffusion based mass transfer limitations and must be accounted for in design. In moving toward a biorefinery approach, the development of continuous extraction and fractionation processes would provide an economic means by which to apply CO2 as a solvent.187 Further integration of CO2 for use with multiple products and processes could help to decrease overall costs.188 Regarding CO2 recycling, avoiding large pressure drops could lead to significant energy savings due to lower compression needs.187 As such, the operating pressure and the choice of supercritical, subcritical, or CXL systems heavily influences the operating conditions and requisite costs associated with high pressure capital equipment.187 While pressure is an important factor for energy consumption, pilot scale experiments have shown that pumping only accounted for 17% of an oil-seed extraction experiment at 35 °C and 55 MPa whereas chilling and heating had a much larger contribution; strategies such as heat integration could therefore lead to large energy savings.189 However, this study reports an oil recovery of 97.4 g of oil per 1 kWh energy input which represents a very marginal energy return on investment (EROI) (
