Hydrothermal Reactions of Biomolecules Relevant for Microalgae

Review pubs.acs.org/IECR

Hydrothermal Reactions of Biomolecules Relevant for Microalgae Liquefaction Shujauddin M. Changi,⊥,†,‡ Julia L. Faeth,⊥,† Na Mo,⊥,† and Phillip E. Savage*,†,§ †

Department of Chemical Engineering, University of Michigan, 3074 H. H. Dow Building, Ann Arbor, Michigan 48109, United States Eli Lilly & Co., Lilly Corporate Center, Indianapolis, Indiana 46285, United States § Department of Chemical Engineering, Pennsylvania State University, 158 Fenske Lab, University Park, Pennsylvania 16802, United States ‡

ABSTRACT: Hydrothermal liquefaction (HTL) of microalgae, a process that uses water at high temperature and high pressure to make a renewable crude bio-oil, is receiving increased attention. Understanding the governing reaction pathways for the biomolecules in the microalgae cell could lead to improved conversion processes. This review collects information pertinent to the behavior of microalgae biomolecules (e.g., proteins, polysaccharides, lipids, chlorophyll) and their hydrothermal decomposition products (e.g., amino acids, sugars, fatty acids) in high temperature water (HTW). We report on studies involving individual compounds and their mixtures. The mixture systems are particularly important as they move closer to mimicking the true chemistry of HTL of microalgae by providing opportunities for interactions between different molecules that would be present during HTL. Throughout this review, we highlight gaps in the understanding of different chemical reactions that may take place during HTL of microalgae. Given the explosion of interest and activity in the field of algal biofuels and hydrothermal processing, it is not surprising that several review articles have emerged in recent years.3−16 These reviews all provide important perspectives on various aspects of hydrothermal processing (e.g., engineering challenges, catalysis, process options, results from work with algal biomass) or algal biofuels (e.g., barriers to commercialization, life-cycle assessments, process options). None of the reviews to date, however, have taken a molecular perspective and focused on the specific reaction paths that may be operative during hydrothermal conversion of algal biomass. This review fills that gap by summarizing what is known about the products, pathways, and kinetics for the hydrothermal reactions of different biomolecules representative of those in microalgae and different products thereof, in the absence of added catalysts. Our scope is confined to considerations about the hydrothermal chemistry of model compounds, where molecular-level information is available. Many of the model biomolecules discussed herein originate from terrestrial plants. Though not from microalgae, such compounds are within scope for this review because one expects proteins, lipids, and polysaccharides from terrestrial biomass to undergo the same types of reactions as proteins, lipids, and polysaccharides from aquatic biomass. The chemistry is determined by the types of functional groups and linkages in a biomolecule, not by whether the biomolecule originated in a plant grown on land or in water. We do not discuss prior work with algal biomass as that body of work does not admit a molecular perspective and it has already been reviewed previously.4,5,9,11−13,16 Moreover, we do not discuss

1. INTRODUCTION One approach to manufacturing chemicals and fuels from renewable resources is the conversion of biomass. This approach includes many different technological pathways (e.g., fast pyrolysis, fermentation of sugars, gasification, aqueous phase reforming). A very large majority of the work done to date has had terrestrial biomass or its components (e.g., corn starch, vegetable oil, cellulose, switch grass, corn stover) in view. More recently, however, aquatic biomass in general and microalgae in particular have begun to attract more attention.1 Attractive features of microalgae include its fast growth rate, its photosynthetic efficiency exceeding that of terrestrial biomass, its potential for high lipid content, and its cultivation being feasible on nonarable land and with wastewater or brackish water. Whereas terrestrial biomass is composed primarily of the biopolymers lignin, hemicellulose, and cellulose, microalgae comprise primarily lipids, proteins, and polysaccharides. The absence of lignin, the most recalcitrant component in terrestrial biomass, makes microalgae susceptible to thermochemical conversion at lower temperatures than those needed for terrestrial biomass. Because microalgae are aquatic biomass and energy savings can be realized by processing the biomass in its wet state (rather than expending energy to dry the biomass), hydrothermal processes have been particularly popular for treating algal biomass.2 Hydrothermal processing uses the combined action of thermal energy, elevated pressure, and hydrolytic attack of water molecules at susceptible bonds in the biomolecules to break down the biomass into smaller molecules that are closer in size to those needed for fuels or chemicals. Potential products from hydrothermal treatment of algal biomass include carbonized solids (i.e., biocoal), crude biooil, fuel gases, H2, biodiesel, alkanes, aromatic hydrocarbons, nutraceuticals, and animal feed. © XXXX American Chemical Society

Received: July 28, 2015 Revised: October 27, 2015 Accepted: October 29, 2015

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temperature to 34.79 at 200 °C, 20.39 at 300 °C, and 14.07 at 350 °C.19 These values are similar to those of polar organic solvents at room temperature. Not surprisingly, then, organic compounds enjoy significantly increased solubility in HTW as the temperature increases. Small organic compounds become completely miscible in water when the mixture exceeds its critical temperature. Thus, organic chemistry can be conducted in HTW in a single homogeneous fluid phase. Yet another property that undergoes significant change as liquid water is heated is its ion product, Kw. This value is 10−14 (mol/kg)2 at room temperature, and it increases to a maximum value of 10−11.2 at about 250 °C.20 This increase corresponds to an increased concentration of H+ ions, which will then increase the rates of any acid-catalyzed reactions. Many of the bonds in algal biomolecules (ester linkages in triglycerides, peptide bonds in proteins, ether linkages in polysaccharides) are susceptible to acid-catalyzed, hydrolytic cleavage, so the elevated ion product can be expected to facilitate the desired decomposition reactions. As temperature is increased beyond 300 °C, the ion product decreases, and it takes on values much lower than 10−14 at supercritical temperatures. Supercritical water is not a favorable medium for ion formation and ionic chemistry unless the density (and pressure) is very high. Freeradical reactions tend to be more important than ionic reactions in supercritical water near the critical pressure. 2.3. Microalgae Biomolecules. The main biochemical components in the cells of microalgae are proteins, lipids, and carbohydrates.21 These components account for the majority of the mass (ash-free, dry basis) in the microalgae cell. The cells also contain chlorophyll, carotenoids, sterols, and nucleic acids, along with inorganic material (e.g., ash).22 The relative amounts of these different components depend on the microalgae species, the specific growth conditions (e.g., illumination, nutrients) and the growth phase (e.g., exponential). Protein is often more abundant in microalgae than either lipids or carbohydrates.23,24 In one study, the protein content ranged from as low as 6 wt % to as high as 71 wt % (on a dry microalgae basis).21 The protein content exceeds 50 wt % for many different species, and work has been done to identify the amino acids present within the various proteins in microalgae. A study of freshwater microalgae showed little variation in the relative abundances of different amino acids for different species and for different growth phases.25 The most abundant amino acids were glutamic acid, aspartic acid, leucine, alanine, arginine, valine, lysine, threonine, serine, phenylalanine, and isoleucine. Free amino acids may also exist in the microalgae cell, and these free molecules can account for up to 40% of the total amino acid content in the cell.23 The lipid content can be very high (e.g., >50 wt %) when microalgae are cultivated under stressed conditions (e.g., nutrient limiting). More typically, lipid contents range from 5−20 wt %. Algal lipids include fatty acid chains esterified to glycerol, sugars, or bases. The lipids can be categorized as polar lipids (such as phospholipids and glycolipids), which are the major components of the cell membranes, or as neutral lipids (triacylglycerides), which the cell stores as intracellular lipid bodies when under stress. These lipids comprise C12−C22 fatty acids with various degrees of unsaturation and in various proportions. C16 and C18 fatty acids are typically the most abundant. The carbohydrates in microalgae exist in the form of starch, cellulose, and other polysaccharides. Mono- and oligosaccharides can also be present. Polysaccharides typically dominate,

engineering, environmental, or economic issues related to microalgae biofuels, nor do we discuss the variety of finished fuel and chemical products available from algal biomass, as those topics are outside our scope. We focus on hydrothermal treatment temperatures of 250−350 °C, as these are typically used to convert algal biomass to a crude bio-oil. These conditions are below the critical temperature of water (374 °C). Before discussing the hydrothermal reactions of proteins, polysaccharides, and lipids, we first provide a background section that gives an overview of hydrothermal processing, the unique properties of high temperature water (HTW), the types of molecules present in microalgae, and the types of organic compounds that are relevant model compounds for studying this system.

2. BACKGROUND This section provides the background knowledge required to appreciate the details about hydrothermal reactions given in subsequent sections. We briefly explain the appeal of hydrothermal treatment for algal biomass and how the properties of water facilitate the desired chemistry. We then discuss the chemical constituents of microalgae, which provide the context for the balance of this review. 2.1. Hydrothermal Processing. Hydrothermal treatment is generally understood to involve processing in a dense aqueous phase at elevated temperature and pressure. When the temperature is below the critical temperature of water (374 °C), the system pressure is at or above the saturation pressure so that the water exists as a liquid phase at the processing conditions. When the temperature is supercritical, the pressure generally exceeds the critical pressure (220.6 bar) and the processing is said to take place in supercritical water. Hydrothermal processing requires less energy than drying and then processing the dried biomass whenever the biomass feedstock contains more than about 30 wt % moisture. This advantage arises from the thermodynamic properties of water. For example, the enthalpy change required to heat saturated liquid water from 25 to 300 °C (∼1240 kJ/kg) is less than half the enthalpy change required to dry biomass at 70 °C (∼2520 kJ/kg H2O). Moreover, the thermal energy in the 300 °C stream can be recovered in a well-engineered process using heat integration. The thermal energy in 70 °C water vapor has little value and is essentially waste heat. Hydrothermal treatment of algal biomass can be done at low temperatures (∼200 °C) to make carbonized solids, intermediate temperatures (∼300 °C) to make crude bio-oil, and supercritical temperatures (∼400 °C) to make CH4 or H2. Savage2 provides a perspective about these different routes, and reviews are available on hydrothermal treatment of biomass, in general,3,6−8,10,14,15 and microalgae in particular.4,5,9,11−13,16 When hydrothermal treatment is used to produce crude bio-oil, the process is termed hydrothermal liquefaction (HTL), which is the main focus of this review. 2.2. Properties of Water. In addition to providing a more energy efficient path for wet biomass than drying, hydrothermal processing also takes advantage of the properties of HTW and supercritical water. As liquid water is heated along its saturation curve, the added thermal energy causes more molecular motion, which disrupts the hydrogen bonding network in the bulk fluid. The hydrogen bonds are fewer and less persistent.17,18 This change in the hydrogen bonding causes significant changes in the bulk properties of the water. The dielectric constant decreases from around 78.49 at room B

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different amino acids as degradation products.26 The yields of tyrosine, arginine, alanine, isoleucine, leucine, histidine, and phenylalanine initially increased but then decreased as temperature or residence time was increased. This behavior indicates their participation in secondary decomposition reactions. The maximum yield for each amino acid occurred at different temperatures. A simplified kinetics model for decomposition of fish protein to amino acids (lumped together) had a reaction order of 1.615 for the protein, an activation energy of 145.1 kJ/ mol, and a pre-exponential factor of 9.476 × 109 (mg/g)0.615 s−1. Rogalinski et al.27 showed that bovine serum albumin (as a model protein) underwent hydrolytic depolymerization to release peptide fragments and amino acids, which then decomposed further in high-temperature water (HTW) to form different gaseous compounds (e.g., carbon dioxide, carbon monoxide, hydrogen, methane), low alkanes and alkenes, alcohols (up to C5), amides, aldehydes, and carboxylic acids. For all temperatures examined, a maximum in amino acid yield was observed as time increased. This maximum shifted to shorter residence times as the temperature increased. At 310 and 330 °C, all amino acids were completely decomposed after 140 s. Zhu et al.28,29 reported similar results for hydrolysis of bean dregs in subcritical water. The hydrolysis products primarily comprised arginine, lysine, and alanine. At 200 °C and 20 min, the total amino acid yield was 52.9%. Recently, Teri et al.40 investigated soy protein and albumin as model compounds of microalgae under mild (300 °C and batch holding times up to 90 min) and severe conditions (350 °C and batch holding times up to 90 min) and found that most of the reaction was complete between 10 and 20 min for both these cases. They also characterized the biocrude obtained from these proteins for elemental composition and higher heating value (HHV) along with component examination using GC−MS. The authors found phenol, 2-pyrrolidone, piperidine, indole, and amides (hexadecanamide, oleamide) as products from soy protein hydrolysis under these conditions. However, no reaction pathways were proposed to explain their formation. To summarize, most of the previous studies on hydrothermal treatment of proteins were driven with an aim to maximize amino acid production from protein-rich feedstock. See the reviews by Zhu et al.32 and Peterson et al.14 for additional details. These maxima for amino acid yields occurred at conditions that are milder than those typically used for HTL of microalgae, which indicates that amino acid decomposition occurs readily at hydrothermal conditions relevant for microalgae liquefaction. Even when studies exist at the more severe conditions of interest for microalgae liquefaction, detailed reaction paths explaining the possible product formation from protein hydrolysis do not exist in literature. Thus, the hydrothermal reactions of different amino acids to form additional products is important to eventually understand the fate of proteins in HTW. In this light, the next section presents a literature review on the behavior of amino acids in hydrothermal systems. 3.2. Amino Acids. Several amino acids have been studied in water at high temperatures and pressures to understand their reactivity and stability under these conditions. This section summarizes current understanding of the hydrothermal behavior and reactivity of amino acids. 3.2.1. Decarboxylation and Deamination Pathways. Amino acids contain amine and carboxyl functional groups that undergo chemical reactions in HTW. The two main

with >90 wt % of the carbohydrate fraction being polysaccharide in many species of microalgae and 60−90% in several diatoms.23 Glucose is the main sugar present in the polysaccharides, but ribose, xylose, rhamnose, fucose, arabinose, mannose, and galactose can also be present. The relative abundance of these sugars differs for different species and it is also a function of the growth conditions and growth phase during which the microalgae are harvested. 2.4. Model Compounds Relevant for Hydrothermal Processing. Experiments with whole algal biomass can provide information that is essential for process development and process optimization, and they may even permit speculation regarding a few of the operative reaction pathways. The complexity of the biomass feedstock, however, makes resolution of reaction fundamentals quite difficult. Therefore, model compounds are often used to elucidate the reaction paths, kinetics, and potential mechanisms. The philosophy regarding model compound selection is that the compound should contain some important linkage that also exists in the microalgae biomass feedstock, or it should represent one of the many intermediate products that are produced during hydrothermal treatment. Triglycerides and phospholipids are examples of the first type of model compound, and amino acids, and mono- and disaccharides are examples of the second type. Accordingly, compounds relevant for understanding the hydrothermal treatment of microalgae include proteins and amino acids, poly-, oligo-, di-, and monosaccharides, triglycerides, glycolipids, phospholipids, and fatty acid esters, chlorophyll and phytol, plant sterols such as cholesterol, and carotenoids such as β-carotene. The following sections summarize the current state of knowledge regarding the hydrothermal reactions of compounds that represent the protein, lipid, and carbohydrate fractions of microalgae. The section that follows this material focuses on the hydrothermal reactions of compounds that represent other components (e.g., chlorophyll, carotenoids) in the microalgae cell, and the final section of the review focuses on interactions between compounds, as occur during the reactions of multicomponent systems.

3. PROTEINS AND AMINO ACIDS As mentioned earlier, proteins are a major constituent of microalgae and can theoretically form several products during the HTL of microalgae. Some of these products can react further with other biomacromolecules forming secondary products. Additionally, proteins are a primary source that can add the undesired nitrogen element to the biocrude product formed from HTL of microalgae. Thus, it is important to understand the behavior of proteins individually and in mixtures using model compounds. This section first discusses hydrothermal reactions of proteins and polypeptides and then the discussion turns to the reactions of individual amino acids, either alone or as part of an amino acid mixture. 3.1. Proteins. Several researchers have studied the hydrolysis of proteins in sub- and supercritical water, quantified the kinetics, and described the product distribution.14,26−39 All proteins examined in these studies eventually break down into simpler amino acids when subjected to the hydrothermal environment. The distribution of amino acid products varies with the conditions employed and the protein source used. We highlight just a few selected studies in this section. Hydrothermolysis of fish protein from 180−320 °C, 5−26 MPa, and 5−60 min residence times produced mixtures of 17 C

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Figure 1. Reaction network of amino acid decomposition in HTW. Reproduced from Sato et al. (2004),50 ACS Publications.

This study was the first to elaborate extensively on the hydrothermal behavior of different types of amino acids. Abdelmoez et al.42,43 extensively studied the kinetics of 17 amino acids in saturated subcritical water (230−290 °C) and batch holding times ranging from 2.5−40 min, both individually and as mixtures. They too observed deamination and decarboxylation as competing pathways for alanine and glycine decomposition. Glycine, alanine, valine, and proline formed as intermediate products from complex amino acids, which is consistent with the results reported by Vallentyne.41 For other amino acids, such as phenylalanine, the authors proposed a deamination pathway due to the formation of formic acid, carbonic acid, and ammonia. However, these authors may have overlooked the possibility of carbonic acid also being formed by CO2 from a decarboxylation pathway. The authors also studied amino acids together in mixtures and reported that leucine, isoleucine, phenylalanine, and histidine showed almost a 2- to 3-fold increase in decomposition rate constants when reacted in a mixture. By contrast, tyrosine, serine, and arginine behaved oppositely, with lower rate constants when reacted in mixtures. Methionine and lysine showed no significant differences when reacted alone or in mixtures. Serine had the highest activation energy (152 kJ/mol), whereas lysine had the lowest value (52 kJ/mol) in a mixture. Lastly, the authors found leucine, isoleucine, phenylalanine, serine, threonine, and histidine to be labile at acidic and near-neutral pH but more stable at a basic

primary hydrothermal decomposition paths are decarboxylation and deamination. Vallentyne41 studied the decomposition of acidic amino acids (aspartic acid and glutamic acid), aliphatic amino acids (glycine, alanine, valine, leucine, and isoleucine), hydroxyl amino acids (serine and threonine), sulfur-containing amino acids (cystine and methionine), aromatic amino acids (phenylalanine and tyrosine), heterocyclic amino acids (proline, hydroxyproline, and histidine), and basic amino acids (lysine and arginine) under subcritical water conditions (T < 255 °C), individually and in equimolar mixtures. The different amino acids reacted differently to give a wide range of products. For example, glycine, alanine, phenylalanine, and glutamic acid underwent decarboxylation to form the corresponding amines. Aspartic acid, however, underwent deamination to form malic acid. Other complex amino acids, such as serine, methionine, threonine, histidine, and arginine, formed simpler amino acids like glycine, alanine, and proline. Proline and hydroxyproline did not react in this environment. Vallentyne41 modeled the reaction rates for glutamic acid, phenylalanine, threonine, and serine using first-order kinetics. He also showed that equimolar mixtures of amino acids in aqueous solution gave reaction rates similar to the rates for individual amino acids, with glycine, alanine, and phenylalanine being exceptions. These amino acids decomposed more rapidly in mixtures than when alone. Alanine was also found to decompose faster in the presence of glucose. D

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Figure 2. Hydrothermal reaction pathways of α-alanine, β-alanine, and glycine in HTW. Reproduced with permission from Cox and Seward (2007),55 Elsevier B.V.

or at least the same rate-determining step regardless of the side chain. Li and Brill45 also studied the kinetics of decarboxylation of aliphatic amino acids (glycine, alanine, aminobutyric acid, valine, leucine, and isoleucine) and β-aminobutyric acid in aqueous solution from 310−330 °C and 275 bar over the pH range of 1.5−8.5 using an in situ FT-IR spectroscopy flow reactor. The decarboxylation rates follow the order glycine > leucine ∼ isoleucine ∼ valine > alanine > α-aminobutyric acid > β-aminobutyric acid. Additionally, they also investigated the decarboxylation rate with respect to the different positions of the amino acid group and found the rates for α > β ≫ γ. Islam et al.46 studied the stability of a mixture of ten amino acids (aspartic acid, threonine, serine, sarcosine, glutamic acid, αaminobutyric acid, β-alanine, γ-aminobutyric acid, 5-aminovaleric acid, and 6-aminohexanoic acid; 10 mM each) at

pH of 10. In contrast, methionine, tyrosine, lysine, and arginine were more stable in acidic media. Li and Brill44 studied the effects of different side chain constituents and pH on the hydrothermal stability of phenylalanine, serine, threonine, proline, histidine, and methionine at 270−340 °C, and pH = 1.5−8.5, using an FTIR spectroscopy flow reactor. They found that the decarboxylation rate of phenylalanine, serine, threonine, proline, and methionine was independent of pH in the range 3−8.5. However, the decarboxylation rates increased in a pH range 1.5−3 with a maximum rate occurring at about pH = 2.5. On the other hand, the decarboxylation rate for histidine increased under acidic and basic conditions, with a minimum rate occurring at its natural pH of 7.44. A strong correlation was observed between Ea and ln(A) (the kinetic compensation effect), implying that amino acids shared the same mechanism E

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Figure 3. Reaction pathways for phenylalanine in HTW. Reproduced with permission from Changi et al. (2012),57 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

elevated temperatures (250−400 °C heated for 2 min). They too found that α- and β-amino acids, such as aspartic acid, threonine, serine, and sarcosine, showed low stability against elevated temperature, but γ-amino acids, such as glutamic acid, γ-aminobutyric acid, 5-aminovaleric acid, and 6-aminohexanoic acid, were very stable even under supercritical conditions. They proposed that the γ-amino acids formed closed ring-like structures (lactams) that made them stable even in the supercritical water environment. However, their study was limited to short residence times of up to 2 min. Other authors have also mentioned decarboxylation as the main reaction of alanine and glycine in HTW and reported the first-order rate constants for these reactions.47−49 Sato et al.50 measured the decomposition of five amino acids (alanine, leucine, phenylalanine, serine, and aspartic acid) at 200−340 °C, 20 MPa, and 20−180 s, using a continuous-flow tubular reactor. They reported that deamination to organic acids and ammonia and decarboxylation to amines and CO2 were the two main pathways under these conditions. Alanine decomposed to lactic acid and pyruvic acid and then finally liberated carbon dioxide. For aspartic acid, deamination was the prominent reaction, whereas serine decomposed to glycine and alanine (see Figure 1). They modeled the degradation rates of amino acids using first-order kinetics and the rates were in the following order, aspartic acid > serine > phenylalanine > leucine > alanine. These authors suggested that alanine and glycine are produced as intermediates from serine. However, very short residence times were employed in this work, preventing its direct application to microalgae liquefaction, which typically takes place for longer times. Klinger et al.51 studied alanine and glycine in hydrothermal conditions at 250−450 °C, 34 and 24 MPa, with residence times of 2.5−3.5 s and found a competition between the rates of decarboxylation and deamination. Alanine formed both lactic acid (via deamination) and ethylamine (via decarboxylation). Lactic acid subsequently decomposed to form acetaldehyde or reacted with water to form propionic acid. Glycine, however, primarily underwent decarboxylation to form methylalmine as the major product and only small amounts of glycolic acid were formed due to deamination. Activation energies for alanine and glycine decomposition were reported to be 160 and 156 kJ/ mol, respectively. Glycine decomposition was faster than alanine decomposition, similar to the observations made by Sato et al.49 Otake et al.52 reported that under their experimental conditions (250 °C, 2.5 GPa, and 12 h),

deamination of amino acids was favored over decarboxylation as confirmed by using IR. They attributed this difference in the pathways relative to others41,46−48 to the high pressures used in their study (1.0−5.5 GPa). Interestingly, Klinger et al.51 reported diketopiperazine, formed due to the dimerization of glycine, but did not report a dipeptide of alanine under their conditions. 3.2.2. Oligomerization Pathways. Similar to Klinger et al.,51 several other researchers have reported dimer formation along with trimers and tetramers of glycine and alanine at 200−350 °C, 15−40 MPa, and 120 s.46,53,54 The yields of trimers and tetramers were much less than the yields of dimers. To detect oligomers at higher temperatures, Islam et al.46 constructed a novel supercritical water-flow reactor. At 400 °C, glycine did not show evidence of any oligomers, indicating that glycine reactions in supercritical water differ from those at temperatures below 350 °C. Cox and Seward,55 who used a custombuilt spectrophotometric cell and in situ observation, reported dimerization and subsequent cyclization of α-alanine, β-alanine, and glycine (see Figure 2). They fit a model of amino acid oligomerization kinetics to their experimental data and concluded that oligomerization followed second-order kinetics. This study was conducted at temperatures from 120−165 °C. Otake et al.52 reported formation of oligomers up to pentamers from alanine and glycine at temperatures from 180−400 °C, reaction times from 2 to 24 h, and at high pressure (1.0−5.5 GPa). Sakata et al.56 showed that the rate of glycine dimerization under hydrothermal conditions increases with pH, with a maximum rate occurring at pH of 9.8 and 150 °C. 3.2.3. Reaction Pathways at more Severe Conditions. Despite the number of studies having been performed on the behavior of amino acids in HTW, there is a paucity of information about detailed characterization of products, pathways, and kinetics at the higher temperatures and longer batch holding times relevant for HTL of microalgae. That is, none of the studies cited above have considered the reaction of amino acids at temperatures above 340 °C for batch holding times longer than 3 min. The microalgae HTL literature, on the other hand, contains many studies at temperatures above 340 °C and times much longer than 3 min. To the best of our knowledge, only the work of Changi et al.57 and Chen et al.58 have examined these more severe hydrothermal conditions. Changi et al.57 studied phenylalanine in HTW from 220−350 °C from 0−240 min and identified thoroughly all the products, F

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Industrial & Engineering Chemistry Research Table 1. Summary of Hydrothermal Treatment of Proteins and Amino Acids kinetics biomolecule proteins

substrate fish proteins bovine serum albumin bean dregs soy protein

amino acids

glycine

alanine

arginine

phenylalanine

threonine

serine

pyroglutamic acid valine

leucine

isoleucine

reference Zhu et al. (2008)26 Rogalinski et al. (2005)27 Zhu et al. (2010)29 Teri et al. (2014)40 Rogalinski et al. (2005)27 Abdelmoez et al. (2007)42 Li and Brill (2003)44 Sato et al. (2004)50 Klinger et al. (2007)51 Rogalinski et al. (2005)27 Zhu et al. (2010)29 Vallentyne (1964)41 Abdelmoez et al. (2007)42 Li and Brill (2003)44 Sato et al. (2004)50 Klinger et al. (2007)51 Zhu et al. (2010)29 Vallentyne (1964)41 Abdelmoez et al. (2007)42 Li and Brill (2003)44 Changi et al. (2012)57 Chen et al. (2012)58 Vallentyne (1964)41 Li and Brill (2003)44 Vallentyne (1964)41 Abdelmoez et al. (2007)42 Li and Brill (2003)44 Sato et al. (2004)50 Vallentyne (1964)41 Abdelmoez et al. (2007)42 Li and Brill (2003)44 Abdelmoez et al. (2007)42 Li and Brill (2003)44 Abdelmoez et al. (2007)42

temperature range (°C)

pressure range (MPa)

reaction time (min)

180−320

5−26

5−60

250−330

15−27

200−240

Ea (kJ/mol)

order

Ao (s−1)

batch

1.615

145.1

0.5−1.5

continuous flow

1

9.46 × 109 (mg/g)0.615 s−1 2.32 × 108

1.8−3.4

5−30

batch (SS)

1

0.0516

14.6

300 and 350

8.6 and 16.5

10−90

batch (SS)

NRa

NRa

NRa

250−330

15−27

0.5−1.5

continuous flow

1

1.3 × 1010

125.9

230−290

Psat

2.5−40

batch

1

1.3 × 1013

169.8

270−340

27.5

0.5

1

9.3 × 1010

138.4

200−340

20

0.3−3

continuous flow with FT-IR (titanium) continuous flow

1

3.5 × 1013

166

250−450

24−34

0.04−0.6

continuous flow

1

1.4 × 1012

156

250−330

15−27

0.5−1.5

continuous flow

1

6.9 × 1010

134.1

200−240

1.8−3.4

5−30

batch (SS)

1

3.3 × 107

94

152−216

Psat

5.5 (days)

batch (Pyrex tubes)

1

3 × 1013

184

230−290

Psat

2.5−40

batch

1

4.5 × 105

93.2

270−340

27.5

0.5

1

5.8 × 1014

190.6

200−340

20

0.3−3

continuous flow with FT-IR (titanium) continuous flow

1

2.7 × 1012

154

250−450

24−34

0.04−0.6

continuous flow

1

3.6 × 1011

160

reactor system

114.8

200−240

1.8−3.4

5−30

batch (SS)

1

4.7 × 10

59.9

177−295

Psat

5 (days)

batch (Pyrex tubes)

1

1.7 × 108

128.9

230−290

Psat

2.5−40

batch

1

4.9 × 108

125.3

270−340

27.5

0.5

1

1.9 × 1013

171

220−350

Psat

0−240

continuous flow with FT-IR (titanium) batch (SS)

1

2.5 × 1012

144

130−190

Psat

5−240

batch (quartz)

1

1.2 × 1015

170

113−200

Psat

370 (days)

batch (Pyrex tubes)

1

2 × 1012

141.4

270−340

27.5

0.5

1

8.3 × 1011

142.1

152−216

Psat

5.5 (days)

continuous flow with FT-IR (titanium) batch (Pyrex tubes)

1

4 × 109

122.8

230−290

Psat

2.5−40

batch

1

6.1 × 108

112.2

270−340

27.5

0.5

1

1.4 × 109

110.9

200−340

20

0.3−3

continuous flow with FT-IR (titanium) continuous flow

1

9.9 × 1012

149

216−280

Psat

NRa

batch (Pyrex tubes)

1

2 × 109

149.8

230−290

Psat

2.5−40

batch

1

3.7 × 1011

157

270−340

27.5

0.5

1

3.8 × 1014

185.6

230−290

Psat

2.5−40

continuous flow with FT-IR (titanium) batch

1

1.5 × 1010

138.5

270−340

27.5

0.5

1

6.1 × 1010

141.9

230−290

Psat

2.5−40

1

1.3 × 105

83.1

G

continuous flow with FT-IR (titanium) batch

3

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substrate

lysine a

reference Li and Brill (2003)44 Abdelmoez et al. (2007)42

temperature range (°C)

pressure range (MPa)

reaction time (min)

270−340

27.5

0.5

230−290

Psat

2.5−40

reactor system continuous flow with FT-IR (titanium) batch

order

Ao (s−1)

Ea (kJ/mol)

1

1.3 × 1014

180.8

1

2.5 × 10

46.4

1

NR = Not reported.

influence the behavior of one another under milder conditions, extending studies to more severe and relevant HTL conditions could be beneficial. Such a study could provide a clearer understanding of the fate of proteins and amino acids during the HTL of microalgae.

which led to a carbon balance of essentially 100%. Decarboxylation was the primary pathway for phenylalanine decomposition. However, this work also identified styrene and phenylethanol as additional products under these previously unexplored conditions. They proposed that styrene formed via deamination of phenylethylamine, the former undergoing hydration to form phenylethanol. Changi et al.57 also observed oligomer formation. They developed a reaction network (Figure 3) and a kinetics model built on this foundation that fit their experimental data. Chen et al. broadened the work of Changi et al. by using quartz reactors to test whether the stainless steel reactors used by Changi et al. caused metal-catalyzed reactions to occur. They also explored much lower temperatures from 130−190 °C with a batch holding time up to 240 min. Chen et al. reported low conversions of phenylalanine (only about 3%) from 130−190 °C, thereby suggesting the possibility of obtaining value-added products such as amino acids from HTL of microalgae at mild conditions. At higher temperatures, these authors, like Changi et al., observed decomposition of phenylalanine to phenylethylamine and the formation of secondary decomposition products, styrene and phenylethanol. However, they reported a slightly higher rate constant and lower yields of phenylethylamine and higher yields of styrene compared to Changi et al. They attributed these quantitative differences to the different reactor surfaces used in the two works. To summarize, proteins hydrolyze rapidly to form a mixture of amino acids. Amino acids, depending on their structure and/ or the solution pH, can undergo additional reactions (decarboxylation, deamination, dehydration, and oligomerization) to form secondary products. Much is known about the kinetics, mechanisms, and pathways of individual reactions for proteins and various amino acids in water but under comparatively mild hydrothermal conditions, i.e., less than 340 °C. However, only phenylalanine has been studied at temperatures greater than 340 °C and batch holding times greater than 3 min, conditions that are more relevant to HTL of microalgae. Table 1 summarizes the reaction conditions and kinetics for different proteins and amino acids in HTW as found in various literature sources. Although phenylalanine is one of the constituents of algal cells, it primarily constitutes only the hydrophobic class of amino acids. Additional work at more severe conditions on the hydrothermal reactions of other classes of amino acids (e.g., aspartic acid (hydrophilic), serine (neutral)) found in microalgae may shed new light on the chemistry that occurs during HTL of microalgae. Some of these amino acids could follow different pathways at the different reaction conditions. In closing this section, we note that little or no work has been done studying mixtures of amino acids (representing the true cocktail found in algal cells) at temperatures above 350 °C and batch holding times greater than 30 min, though such conditions have often been used in microalgae HTL studies. Since amino acids have been shown to

4. LIPIDS One of the many reasons microalgae are of interest as a feedstock for biofuel production is due to the potential for high oil content in algal biomass. Lipid contents ranging from 20 to 50% on a dry weight basis are common,59 and lipid contents up to 90% dry weight may be observed, under certain conditions.60 High oil contents are preferable for the production of biocrude via HTL, since experimental evidence indicates that biocrude yields follow the trend lipids > proteins > carbohydrates.61 Several researchers have investigated the hydrothermal reaction of model compounds including acylglycerides, fatty acids, and fatty acid esters in both sub- and supercritical water. Some of these efforts have been summarized in earlier reviews.10,14,15,62 However, these reviews offer little information on the reaction kinetics of lipid model compounds on a molecular level and focus predominantly on whole biomass. The following section summarizes key information about the hydrothermal reaction pathways of lipids and lipid model compounds and focuses primarily on work that has not been previously reviewed. 4.1. Neutral Lipids. A significant fraction of the lipids present in microalgae are neutral lipids, which include acylglycerides (triglycerides, diglycerides, and monoglycerides), free fatty acids, and sterols.63 As a result, acylglycerides make good model compounds for understanding various reactions of algal lipids. Of course, many terrestrial plant oils are also rich in acylglycerides, of which triglycerides typically make up the majority. Hydrothermal processing of plant oils, and therefore acylglycerides, has been studied for decades. In fact, a patent dating to 1854 describes a method of splitting fats to produce fatty acids and glycerol using HTW in a sealed vessel.64 Even today, steam splitting is the process used for industrial manufacture of commodity fatty acids. This section discusses recent developments in the hydrothermal treatment of acylglycerides, fatty acids, and sterols. 4.1.1. Acylglycerides. Holliday et al. examined the hydrolysis of soybean, linseed, and coconut oils in subcritical water at temperatures of 260−280 °C, and in supercritical water at 375 °C.65 They observed 97% conversion of triglycerides after reaction times of 15−20 min under subcritical reaction conditions. Treatment under supercritical conditions caused degradation of the fatty acid products, which the authors attributed to decomposition, pyrolysis, and/or polymerization. Li et al. also reported hydrothermal treatment of triglycerides at supercritical conditions, but little information about the chemistry was reported.66 King et al. further investigated soybean oil hydrolysis using a flow reactor with a view cell at temperatures of 250−340 °C.67 At 339 °C and a pressure of H

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Figure 4. Reaction network for acylglyceride hydrolysis in HTW.

reactions of soybean oil and water in a 1:4 ratio (on a weight basis) at 250, 275, and 300 °C. The hydrothermal reaction was modeled as a set of three reversible hydrolysis reactions of tri-, di-, and monoglycerides with their respective fatty acids, as described by Alenezi et al.76 Assuming Arrhenius temperature dependence, the authors calculated kinetic parameters using a system of ordinary differential equations. The model accurately predicts fatty acid yields for experiments in which fatty acids were added prior to reaction, verifying the autocatalytic behavior of fatty acids in acylglyceride hydrolysis. Johnson and Tester78 developed an empirical model for the hydrolysis of triglycerides in HTW to produce fatty acids, using their experimental data along with that from Sturzenegger and Sturm.79 The model also includes glycerol partitioning to the aqueous phase, as well as degradation of the fatty acid products. The model predicts different steady state concentrations of free fatty acids for different temperatures, a phenomenon that is also observed in the experimental data, even though the fatty acids were produced from whole microalgae cells, not pure triglycerides or plant oils. The research described thus far in this section focuses primarily on triglycerides. The remainder of the section investigates reactions of monoglycerides in HTW. Fujii et al. examined the hydrolysis of monoglycerides and decomposition of fatty acids in subcritical water, using both isothermal and temperature-programmed reactions.80 The isothermal experiments, ranging in temperature from 210−270 °C and reaction times from 0−60 min, verified that the hydrolysis of monoglycerides and the decomposition of substituent fatty acids obey first-order kinetics. The temperature-programmed experiments, which used three different linear heating rates, indicated that the hydrolysis of monoglycerides becomes possible at a temperature of approximately 200 °C, regardless of the heating rate. At the highest heating rate of 13.6 °C/min, the temperature increased from 200 to 300 °C in about 7 min, and the conversion of monoglyceride was nearly complete. The maximum fatty acid concentrations were also observed at this temperature and time, for the same heating rate. At temperatures greater than 300 °C, the fatty acids produced via hydrolysis further reacted via decomposition or conversion to other compounds. The activation energies for reactions of several different monoglycerides were calculated, resulting in decreasing activation energies with increasing acyl chain lengths. This inverse relationship between activation energies and acyl chain lengths contradicts the findings of a study by Khuwijitjaru et al. investigating the activation energies for hydrolysis of several fatty acid alkyl esters (discussed in greater detail in the section on fatty acid esters, 4.3.1).81 However, Fujii et al. did not cite possible explanations for this difference. We postulate that this contradiction may be due to differences between the alkyl groups and glyceride groups. However, to the best of our knowledge, no additional literature examining hydrolysis in HTW is available to substantiate this claim.

about 15 MPa, the soybean oil was fully miscible with the subcritical water, resulting in complete hydrolysis of the oil during a 14 min residence time. The authors reported some isomerization from cis to trans isomers, as well as double bond migration in some of the fatty acid products. Kusdiana and Saka studied the hydrolysis of rapeseed oil and observed 100% conversion of the triglycerides after a 3 min reaction time at 350 °C.68 Lowering the reaction temperature increased the amount of time necessary to achieve 100% conversion. However, for a reaction temperature of 255 °C, a maximum fatty acid yield of 80% was observed and did not increase, even after extended reaction times. Along similar lines, Teri et al.40 hydrolyzed sunflower oil and castor oil at mild conditions (300 °C and batch holding times up to 90 min) and at severe conditions (350 °C with batch holding times up to 90 min) and observed that within the first 10 min, fatty acid yields of more than 90% were obtained in both cases. Kocsisová et al. hydrolyzed triacylglycerides (TAGs) and fatty acid esters in subcritical water at temperatures of 280−340 °C.69 These authors observed that hydrolysis of both TAGs and fatty acid esters followed the first order rate equation. Isomerization and migration of double bonds were observed in the unsaturated fatty acid products, consistent with earlier research.67 Minami and Saka hydrolyzed rapeseed oil at temperatures of 250−320 °C, producing fatty acids.70 The fatty acid yields formed sigmoidal temporal profiles, consistent with fatty acids autocatalyzing the hydrolysis reaction. The authors developed a kinetic model including the autocatalytic behavior of fatty acids, which correlated the fatty acid yields of the hydrolysis reactions. Pinto and Lanças examined the hydrolysis of corn oil in subcritical water at 150−280 °C and achieved complete conversion of the oil to the respective fatty acids at 280 °C.71 Several additional studies reinforced the conclusion that acylglycerides decompose to their constituent fatty acids during hydrothermal treatment.61,72−75 Alenezi et al. examined the hydrolysis of sunflower oil in subcritical water. The sunflower oil they used contained 77 wt % triglycerides, 20 wt % diglycerides, and 2.5 wt % monoglycerides.76 They calculated kinetic parameters for the hydrolysis reactions of the aforementioned acylglycerides, namely the reversible hydrolysis reactions of triglycerides to form diglycerides, diglycerides to form monoglycerides, and monoglycerides to form free fatty acids and glycerol. Figure 4 below displays a simplified diagram of this reaction network. The reverse reaction rates were reported to be quite close to zero, which supports the irreversibility of acylglyceride hydrolysis under the conditions investigated. The authors reported that the activation energy of the first hydrolysis reaction, from triglyceride to diglyceride, was the highest at 98 kJ/mol. Additionally, an “induction period” was observed in the production of fatty acids. The authors concluded that the hydrolysis of the acylglycerols to produce fatty acids is autocatalyzed by the free fatty acids present in the system. Milliren et al. further investigated the hydrothermal treatment of lipid model compounds.77 They performed batch I

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the reaction kinetics of unsaturated fatty acids under conditions relevant to microalgae HTL. To summarize, the literature indicates that some smaller (≤C12) saturated fatty acids are subject to hydrothermal decomposition at temperatures around 300 °C. Some of the larger fatty acids (≥C16) are more stable. To better understand why long chain fatty acids are more stable in conditions relevant to HTL, additional investigation into reactions of fatty acids with chain lengths ≥C12 is needed. Reaction kinetics of unsaturated fatty acids have not yet been investigated in HTW without a catalyst, to the best of our knowledge. There is a need for work in this area to better understand the fate of several unsaturated fatty acids present in microalgae. 4.1.3. Sterols. Very little information is available about the hydrothermal treatment of sterols, another type of neutral lipid. Hietala and Savage89 recently reported on the hydrothermal reactions of cholesterol under conditions relevant for microalgae HTL. Cholesterol is a plant sterol present in microalgae.90 Moreover, cholesterol and its derivatives have been identified as components of microalgae biocrude produced via HTL.91 The initial rate of disappearance of cholesterol was consistent with first-order kinetics. The Arrhenius parameters are Ea = 127 ± 12 kJ/mol and log A (s−1) = 8.35 ± 2.41. The primary reaction path leads to cholestadienes, formed via dehydration of the parent molecule. Other products included cholestenes, cholestatrienes, and cholestenones. 4.2. Polar Lipids. In addition to neutral lipids like triglycerides and fatty acids, polar lipids are also found in microalgae cells. Polar lipids are more complex than their neutral counterparts and include phospholipids, glycolipids and sulfolipids.92 Algal phospholipids may include phosphatidic acid, diphosphatidyl glycerol, phosphatidylethanolamine, phosphatidyl glycerol, phosphatidyl choline, and phosphatidyl inositol, among others. Glycolipids in microalgae may include monogalactosyl diglyceride and digalactosyl diglyceride. The literature provides few articles describing the hydrothermal reactions of polar lipids, but the available literature is summarized below. Illijas et al. examined the lipids in both fresh and frozen− thawed samples of Exophyllum wentii in great detail, quantifying glycolipids and phospholipids.93 There was a notable decrease in the total polar lipids (glycolipids and phospholipids) after the freeze−thaw cycle, accompanied by an increase in free fatty acids including palmitic acid and arachidonic acid. The authors concluded that this shift was likely due to enzymatic hydrolysis of the polar lipids. Although no hydrothermal treatment was involved, hydrothermal reaction of such lipids may have similar hydrolytic effects. Changi et al. examined the reaction of 1,2-dioleoyl-snglycero-3-phosphocholine (DOPC) in HTW.94 Reaction products included lyso-phosphatidylcholines, phosphoric acid, oleic acid, and esters containing oleic acid. Autocatalysis by oleic acid and phosphoric acid was observed. From the experimental results, the authors proposed reaction pathways for the hydrothermal reaction of DOPC and subsequent products and then used these pathways to construct a kinetic model. Arrhenius parameters were calculated for each reaction, and the model agreed with experimental results within ±20% for major products. Hydrothermal reactions of polar lipids have not been investigated as thoroughly as their neutral counterparts. Nevertheless, kinetic parameters have been calculated for at least one type of polar lipid known to be present in microalgae,

The extensive research on acylglycerides described above has led to the development of several models describing acylglyceride hydrolysis. From simple yield observations to the calculation of kinetic parameters, reactions of acylglycerides in HTW have been a topic of great interest in the research community investigating chemical reactions in subcritical water. However, very few studies have examined the reactions of acylglycerides in supercritical water. To the best of our knowledge, only Holliday et al.65 and Li et al.66 report hydrolysis of acylglycerides in supercritical water. Future work can focus on exploring hydrolysis of acylglycerides in supercritical water to better understand how neutral lipid molecules may influence the HTL of microalgae at temperatures above 350 °C. 4.1.2. Fatty Acids. Fatty acids are the immediate hydrolysis products from acylglycerides, which are commonly found in microalgae biomass. For this reason, the behavior of free fatty acids in HTW is of great significance to HTL of algal biomass. Khuwijitjaru et al. determined the solubility of even-numbered fatty acids from C8 to C18 at temperatures of 60−230 °C and pressures of 5 and 15 MPa.82 Pressure had little effect on solubility, but increasing temperatures led to an increased solubility of fatty acids in water. The authors also estimated the enthalpy of solution for each of the fatty acids studied. Fujii et al. examined the temperature-programmed decomposition of caprylic, capric, and lauric acids (8-, 10-, and 12carbon saturated fatty acids) in water at different linear heating rates ranging from 6−14 °C/min.80 On the basis of kinetic analysis of the results, the decomposition of each fatty acid examined followed first-order reaction kinetics. Increasing fatty acid chain length resulted in decreased values of the activation energy and frequency factor (pre-exponential factor), and an increase in reaction rate. Reaction of the corresponding monoglycerides revealed a maximum concentration of fatty acid at a reaction temperature of 300 °C, after which the fatty acid concentration decreased due to thermal decomposition of the fatty acid. This behavior seems to indicate that saturated fatty acids begin to decompose at temperatures of ≥300 °C. Saturated fatty acids with chain lengths greater than those investigated by Fujii et al. appear to be more stable in HTW. Fu et al. observed pentadecane molar yields of less than 1% from palmitic acid (C16) after 17 h in water at 370 °C.83 Mo and Savage reported that palmitic acid was stable for 3 h in water at 200 and 300 °C, but at 400 °C generated a total product yield (including both gas and liquid products) of 16 wt %.84 Watanabe et al. reported just 2% conversion of stearic acid (C18) after a 30 min reaction in SCW at 400 °C and a density of 0.17 g/cm3.85 The authors proposed energetic stabilization of the stearic acid by SCW as an explanation for low stearic acid conversion. The major product in SCW was a C16 alkene, whereas the major product of the reaction in an argon atmosphere without water was a C17 alkane. The authors hypothesized that the stearic acid undergoes decarboxylation in SCW and that the carboxylic group dissociated in an inert atmosphere. From these results and additional catalytic SCW experiments, the authors proposed a reaction network for the SCW water decomposition of stearic acid. Reactions of unsaturated fatty acids have also been examined in water, but to a lesser extent than saturated fatty acids. Furthermore, much of the research on reactions of unsaturated fatty acids in water focuses on catalyzed reactions or gasification reactions in SCW,86−88 topics that are beyond the scope of this review. To the best of our knowledge, no prior work explores J

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hydrothermal reaction conditions, some studies have examined the production of lipids from one- or two-carbon carboxylic acids. McCollom et al. examined Fischer−Tropsch-type (FTT) reactions of formic and oxalic acids under hydrothermal conditions, at temperatures of 100−250 °C and a reaction time of 2−3 days.99 Although yields of organic products (including alkanes, alkenes, and alkyl formate esters, among other compounds) with a chain length greater than C10 were less than 0.1%, the authors verified that the observed products were not contaminants, but in fact products of FTT reactions in water. The authors observed catalytic effects from the stainless steel reactors, based on additional experiments in glass vessels in which no organic products were observed. Similar work by Rushdi and Simoneit at temperatures of 100−400 °C indicates that a temperature ≥150 °C is necessary for the initiation of Fischer−Tropsch hydrocarbon synthesis reactions, and that cracking and reforming reactions begin to be competitive at temperatures ≥300 °C.100 These results indicate that hydrothermal reactions can not only decompose fatty acids and lipid-like compounds, but also synthesize them from smaller molecules, albeit at a much slower rate. In summary, reactions of several different types of lipids, including (but not limited to) acylglycerides, fatty acids, fatty acid esters, and polar lipid compounds have been investigated in high temperature, high pressure water. Table 2 summarizes the reaction conditions and kinetics for different lipids in HTW. Acylglycerides hydrolyze to form fatty acids and glycerol. Saturated fatty acids tend to be stable in subcritical water and may begin to decompose in supercritical water, although conversion is low without the use of a catalyst. Polar lipids, like phospholipids, have not been thoroughly examined. Some research also examines the hydrothermal treatment of glycerol, and the formation of lipids under hydrothermal conditions. Moving forward, reactions of acylglycerides in supercritical water, polar lipids in sub- and supercritical water, glycerol in sub- and supercritical water, and unsaturated fatty acids in suband supercritical water are not very well understood and could benefit from additional research.

which provides a comparison of the reaction rate for polar lipids compared to neutral lipids found in microalgae. Additional research in this area is necessary to better characterize the behavior of polar lipids in high temperature, high-pressure water and the influence of the parent constituent chain and degree of unsaturation. 4.3. Other Lipid-Related Compounds. The hydrothermal reactions of several other compounds related to algal lipids have also been investigated. These include fatty acid esters and glycerol, both of which are liberated from acylglycerides via hydrolysis. This section briefly summarizes work with these model compounds. 4.3.1. Fatty Acid Esters. Khuwijitjaru et al. explored the kinetics of hydrolyzing fatty acid esters in water.81 Fatty acid esters with different acyl or alkyl chains were hydrolyzed in a batch reactor at temperatures of 210−270 °C, and all ester hydrolysis reactions followed first-order reaction kinetics. The authors found that activation energies for ester hydrolysis increased with increasing chain length, likely the result of steric hindrance. Kocsisová et al. also examined the hydrolysis of fatty acid esters and reported first-order reaction kinetics.69 Changi et al. studied the hydrothermal reaction of ethyl oleate (fatty acid ester) at temperatures of 240−300 °C, along with the esterification of oleic acid (the reverse reaction) in sub- and supercritical ethanol.95 The authors found the hydrolysis of ethyl oleate to be autocatalytic, in which the presence of the corresponding carboxylic acid catalyzes further hydrolysis of the fatty acid ester. Using experimental data,95,96 the authors developed a unified model describing the hydrolysis and esterification of ethyl oleate and oleic acid, respectively. Further work by this group led to the development of a mechanistic model describing hydrolysis and esterification. Though first-order reaction kinetics can seemingly describe the hydrolysis of fatty acid esters under some conditions, this reaction is autocatalyzed by the presence of the constituent fatty acids and the true kinetics are more complex. 4.3.2. Glycerol (From Acylglycerides). As acylglycerides are hydrolyzed, they release fatty acids. Once all the fatty acids have been released, the glycerol backbone of the acylglyceride remains. Glycerol can also react under hydrothermal conditions, and some research has been dedicated to examining its hydrothermal reaction pathways and kinetics. Bühler et al. used a continuous flow reactor system and found that the composition of the product mixture formed via degradation of glycerol depended on the pressure of the reaction mixture, which they explained to be a likely indicator of competing reaction pathways.97 The authors proposed that competition between ionic and free radical reactions leads to the nonArrhenius behavior that was observed. Major products of the degradation of glycerol included methanol, ethanol, various aldehydes, as well as carbon monoxide, carbon dioxide, and hydrogen. Qadariyah et al. examined glycerol degradation in sub- and supercritical water in a batch reactor system. Acrolein was a major reaction product in both sub- and supercritical environments, acetaldehyde was observed only in subcritical water, and allyl alcohol was only observed in supercritical water.98 Kinetic analysis of the reaction system revealed that the overall reaction followed pseudo-first order reaction kinetics, with an activation energy of 39.6 kJ/mol in the subcritical range. 4.4. Formation of Lipid in Hydrothermal Conditions. In addition to investigating the decomposition of lipids in

5. CARBOHYDRATES Many researchers have investigated the hydrothermal conversion of polysaccharides (e.g., cellulose, hemicellulose, starch, guar gum, and polygalacturonic acid) into mono- and oligosaccharides and their derivatives. This section begins with a discussion of the reactions of different polysaccharides and then moves to a review of the reactions of disaccharides and monosaccharides. 5.1. Polysaccharides. This section presents an overview of the hydrothermal reactions of cellulose, hemicellulose, and starch. The interested reader is referred to the review articles mentioned in the introduction for more detailed information. 5.1.1. Cellulose. Cellulose is one of the most common organic materials on earth and a structural component of the primary cell wall of microalgae. It consists of glucose units, linked by β-1,4-glycosidic bonds.15 Cellulose has a high degree of crystallinity, which makes it insoluble in water at room temperature. In water at near-critical conditions, however, cellulose can be hydrolyzed to low-molecular-weight oligomers that are soluble. These fragments can then be hydrolyzed further to generate smaller water-soluble products such as glucose and fructose.75 Cellulose degradation in subcritical water at temperatures from 240−310 °C and pressures from 20−25 MPa can K

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L

fatty acid esters

fatty acids

monoglycerides

triglycerides

biomolecule

ethyl laurate

methyl palmitate

methyl myristate

methyl laurate

methyl decanoate

stearic acid methyl octanoate

monocaprylin monocaprin monolaurin caprylic acid capric acid lauric acid palmitic acid palmitic acid

algae oil

castor oil corn oil

sunflower oil

rapeseed oil

linseed oil coconut oil

soybean oil

substrate

Teri et al. (2014)40 Pinto and Lancas (2006)71 Johnson and Tester (2013)78 Fujii et al. (2006)80 Fujii et al. (2006)80 Fujii et al. (2006)80 Fujii et al. (2006)80 Fujii et al. (2006)80 Fujii et al. (2006)80 Fu et al. (2010)83 Mo and Savage (2014)84 Watanbe (2006)85 Khuwijitjaru et al. (2004)81 Khuwijitjaru et al. (2004)81 Khuwijitjaru et al. (2004)81 Khuwijitjaru et al. (2004)81 Khuwijitjaru et al. (2004)81 Khuwijitjaru et al. (2004)81

4.7−6.4 4.7−6.4 5−20

15−120 0−60

NRa 35−134

35−134 35−134

210−270

35−134

35−134

0−60

0−60

0−60

0−60

0−60

0−60 0−60 0−60 0−60 0−60 0−60 17 h 25−180

NRa NRa NRa NRa NRa NRa NRa 1−24

35−134

0−200

10−90 40

10−90 5−20

10−60

5−20

8−70 8−70 3−20

20.5

8.6−16.5 NRa

8.6−16.5 20

210−270

210−270

210−270

210−270

400 210−270

25−350 25−350 25−350 25−350 25−350 25−350 370 200−400

250−350

300−350 150−280

300−350 270−350

10−20

260−280 260−280 255−350

Holliday et al. (1997)65 Holliday et al. (1997)65 Kusdiana and Saka (2004)68 Kocsisova et al. (2006)69 Minami and Saka (2006)70 Teri et al. (2014)40 Alenezi et al. (2009)76

0−100

NRa

250−320

250−300

Milliren et al. (2013)77

8−70 7−15 0−100

NRa 13.1−40.4 NRa

12

260−375 270−340 250−300

Holliday et al. (1997)65 King et al. (1999)67 Milliren et al. (2013)77

reaction time (min)

pressure range (MPa)

280−340

temperature range (°C)

reference

Table 2. Summary of Hydrothermal Treatment of Lipids

batch

batch

batch

batch

batch

batch batch

batch batch batch batch batch batch batch batch

batch

batch batch

batch flow

flow

batch batch batch and flow flow

batch

batch flow batch

reactor system

1

1

1

1

1

NRa 1

1 1 1 1 1 1 NRa 1

multiple

autocatalyzed by fatty acids NRa 1 in triglyceride, 1 in water NRa NRa

1

NRa NRa 1 in acylglyceride, 1 in water autocatalyzed by fatty acids NRa NRa NRa

order

NRa NRa NRa NRa NRa NRa 98

NRa NRa NRa NRa NRa NRa 5 × 106 (mol/mol of oil)−1 (min)−1 NRa NRa

NRa ∼56 ∼55 ∼61 ∼65 ∼71 ∼65

∼3 × 102 ∼7 × 102 ∼3 × 103 ∼7 × 103 ∼2 × 103

∼105 ∼95 ∼75 ∼68 ∼65 ∼50 NRa NRa

∼5 × ∼8 × ∼8 × ∼1 × ∼8 × ∼5 × NRa NRa

NRa ∼5 × 102

NRa

NRa 107 106 104 103 102 101

27

1 × 100.2 (mol/L)−2 (min)−1

NRa NRa

NRa NRa 90

Ea (kJ/mol)

NRa NRa 1 × 105.37

Ao (s−1)

kinetics

Industrial & Engineering Chemistry Research Review

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∼68

∼72

NRa

NRa 39.6

77

127

∼5 × 103

∼7 × 103

NRa

NRa NRa

∼4 × 104

1 × 108.35 1 batch 10−180 8.6−16.5 300−350

NR = Not reported. a

1,2-dioleoyl-sn-glycero-3phosphocholine (DOPC) cholesterol

glycerol other

fatty acid methyl ester blend

butyl laurate

propyl laurate

Hietala and Savage (2015)89

1 (uncatalyzed only) batch 15−150 1−17 175−350

various pseudo-first flow batch 0.5−3 20−60 25−45 29.6 349−475 200−400

1 flow 5−20 12 280−340

1 batch 35−134 210−270

0−60

1 batch 0−60 35−134 210−270

Khuwijitjaru et al. (2004)81 Khuwijitjaru et al. (2004)81 Kocsisova et al. (2006)69 Buhler et al. (2002)97 Qadariyah et al. (2011)98 Chang et al. (2012)94

reactor system reaction time (min) pressure range (MPa) temperature range (°C) reference substrate biomolecule

Table 2. continued

satisfactorily be described by a first-order rate law.101 Cellulose decomposed quickly in HTW above 350 °C, and at 400 °C and a pressure of 25 MPa, the hydrolysis product yield was 77% and the cellulose conversion was almost 100% at residence times as short as 0.05 s.102 The rate constant for cellulose hydrolysis also showed a sharp bend at near-critical conditions in an Arrhenius plot. The authors attributed this behavior to an increase in cellulose solubility under near-critical conditions. Cantero et al.103 proposed a reaction network in Figure 5 and also developed a mathematical model to fit the experimental data for cellulose and its derivatives during hydrothermal decomposition. The activation energy (Ea) and pre-exponential factor (ln A) for cellulose hydrolysis also showed a large change around the critical point of water. Ea was 154.4 ± 9.5 kJ/mol below the critical point, but 430.3 ± 6.3 kJ/mol above the critical point, whereas ln(A, s−1) increased from 29.6 ± 1.9 to 80.8 ± 1.2 as conditions were changed to supercritical. Once again, the authors postulated that the mass-transfer limitations were removed under supercritical conditions, thereby leading to higher hydrolysis rate constants. The extensive studies on the hydrothermal reactions of cellulose75,101−103 typically aimed to achieve the highest yield of total products or a target product, such as glucose. The previous research on reaction mechanisms and pathways elucidated mainly the formation of different types of products and offered a possibility of controlling selectivity to certain products. Secondary pathways of hydrolysis of monosaccharides to other products are also very well studied and will be discussed in detail in section 5.3. 5.1.2. Hemicellulose. Hemicellulose is a heteropolysaccharide, generally comprising five different sugar monomers (Dxylose, L-arabinose, D-galactose, D-mannose, and D-glucose), with xylose being the most abundant.104 It can be found in the cell wall lining, and dried freshwater algal biomass (slime) was reported to contain 16.3 wt % hemicellulose.105 It is less resistant to hydrolysis than cellulose. When treated with hot, compressed water at 200−230 °C, the amorphous structure of hemicellulose is fragmented and materials in the cell wall dissolve.106 Hemicellulose is solubilized in water at temperatures above 180 °C. At 220 °C, water alone can dissolve hemicellulose completely within 2 min.107,108 Hemicellulose yields xylose as the major hydrolysis product at mild temperatures. Xylose is a five-carbon sugar that can be dehydrated to furfural and further converted into other products (i.e., formic acid) at mild conditions (∼200 °C, 10 MPa).10,109 Detailed reaction pathways of xylose in HTW are discussed in section 5.3. Hydrothermal reactions of hemicellulose have been very well studied at low, subcritical temperatures (∼180 °C),106−108 as have the decomposition the primary products of hemicellulose hydrolysis at higher supercritical temperatures (∼420 °C). Detailed pathways and kinetics have been established, according to the aforementioned research.102,110 5.1.3. Starch. Starch is a polysaccharide mainly formed by β1,6-glycosidic linkages. Starch undergoes depolymerization to produce oligosaccharides and monosaccharides when treated in HTW (180−250 °C, 15 min).111 The products included maltose, glucose, fructose, 5-hydroxymethylfurfural (5-HMF), and furfural. Organic acids (e.g., lactic, acetic, formic, butyric) were formed in negligible amounts even when the initial starch concentration was increased from 40 to 200 g/L at 200 °C, indicating negligible decomposition of 5-HMF and furfural under these conditions.

Ao (s−1) order

kinetics

Ea (kJ/mol)

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M

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Figure 5. Reaction network for cellulose and glucose decomposition in HTW. Reproduced with permission from Cantero et al. (2012),103 Elsevier B.V.

report that the hydrolysis of cornstarch at 350 °C produces low yields of biocrude oil (material soluble in dichloromethane) and yields of solids (insoluble in both dichloromethane and in water) as high as 30 wt %. There was little variation in the product yields with the batch holding time. The biocrude contained heptane, furan, oxepane, and cyclopentenone. The authors offer no reaction pathways to explain the formation of these products. The authors also reported that at 300 °C the yield of biocrude was about half its value under the more severe conditions, suggesting that the hydrothermal reactions of this polysaccharide are sensitive to temperature. The more severe conditions convert more of the material to compounds that partition into dichloromethane rather than water. Overall, the hydrothermal reaction kinetics for starch in subcritical water from 180−310 °C are well established, but there has been less work on its behavior in near- and supercritical water, especially with regards to elucidating pathways and kinetics. This information is important, given that the HTL of microalgae is typically conducted at 300−350

Glucose was the most abundant product from hydrolysis of starch at concentrations from 0.2 to 10 wt % in the aqueous feed slurry, pressures of 60−240 bar, and temperatures of 170− 380 °C in a tubular reactor with residence times of about 180 s.62 Glucose yields can be markedly enhanced by using dissolved and dissociated carbon dioxide as an acid catalyst. The kinetics of production of glucose from corn starch was studied by Rogalinski et al.,101 and they reported an activation energy of 147.9 kJ/mol with a pre-exponential factor of 5.3 × 1012 s−1 over the temperature range from 210 to 310 °C. According to Rogalinski et al., the rate of starch degradation is faster than that of cellulose degradation. The degradation of starch takes place more rapidly in hot water (210−310 °C) because the β-1,6-glycosidic linkages in starch are easier to hydrolyze than the β-1,4-glycosidic linkages in cellulose. To the best of our knowledge, the recent report by Teri et al.40 is the sole account describing the behavior of starch in water at T > 300 °C and t > 10 min. These more severe conditions are more relevant to microalgae HTL. Teri et al. N

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Figure 6. Reaction pathway of D-xylose in HTW. Reproduced with permission from Aida et al. (2010),110 Elsevier B.V.

Figure 7. Reaction network for glucose in HTW. Reproduced from Kabyemela et al. (1997),118 ACS Publications. O

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Industrial & Engineering Chemistry Research °C. Additional work at conditions that better represent those of interest for microalgae processing via HTL could provide important new insights about the reaction pathways and kinetics. 5.2. Disaccharides. The susceptibility of a disaccharide to hydrolysis in subcritical water largely depends on the constituent monosaccharides and the type of glucosidic bond. Oomori et al.112 examined 11 disaccharides at 180−260 °C and 10 MPa using a tubular reactor, which gave residence times from 0−300 s. Trehalose, α-1,1-glucosylglucoside, was the most resistant to hydrolysis; whereas sucrose, β-2,1-glucosylfructoside, was the most easily hydrolyzed.112 The Weibull equation adequately related the fraction of remaining disaccharide and the residence time. Sucrose is a disaccharide containing the monosaccharides glucose and fructose. Hydrolysis of sucrose yields equivalent amounts of glucose and fructose as primary products. These products can then decompose to form HMF and acidic compounds. In water at 180, 190, and 200 °C, sucrose conversion was nearly complete at 240, 120, and 60 s, respectively.112 The decomposition rate of sucrose in subcritical water (160−200 °C, 10 MPa) was proportional to the concentrations of both the remaining and reacted sucrose. The reaction did not obey first-order kinetics. Khajavi et al. adopted a modified autocatalytic rate expression for sucrose decomposition in subcritical water. The activation energy was reported to be 98.0 kJ/mol,113 which was in the range of 92 to 134 kJ/mol given by Rhim et al.114 Apart from these studies, none of which examined temperatures above 260 °C or reaction times longer than 6 min, the literature provides no additional information on hydrolysis reactions of disaccharides. There is a need for more information on the kinetics and influence of process variables, especially at the temperatures and times relevant for HTL of microalgae. 5.3. Monosaccharides. The hydrothermal reactions of glucose, fructose, and xylose have received tremendous scrutiny. In this part of this review article we highlight knowledge regarding the reaction products, pathways, and kinetics. Hydrothermal treatment of D-xylose at 250−420 °C and pressures from 40−100 MPa produced furfural, D-xylulose, glyceraldehyde, glycolaldehyde, dihydroxyacetone, pyruvaldehyde, lactic acid, and formaldehyde.110,115−117 Figure 6 shows that the operative reactions included dehydration, a retro-aldol reaction, and a Lobry de Bruyn-Alberta van Ekenstein (LBET) pathway.110 As the water density increased, the rate constant for the forward LBET pathway decreased, while the rate constants for the dehydration of D-xylulose to furfural and the retro-aldol reaction of D-xylose increased. Kabyemela et al.118 provide the set of reaction pathways in Figure 7 for glucose and fructose decomposition in near critical water (300−400 °C, 25−40 MPa). Additional pathways not shown in Figure 7 are the conversion of pyruvaldehyde into acetaldehyde and formic acid., and conversion of pyruvaldehyde into formaldehyde and acetic acid.119 Isomerization of glucose to fructose occurs readily, especially in a mildly acidic environment. Isomerization occurs mainly through the Lobry de Bruyn-Alberda van Ekenstein transformation.120 In the subcritical region, glucose decomposition rates did not vary significantly with pressure.118 In the supercritical region, however, the glucose decomposition rate decreased with increasing pressure. This decrease with pressure was due primarily to the decrease in the epimerization rate of glucose to

fructose. The rate shifting with pressure in the supercritical region may be used as a possible method of controlling the selectivity in the hydrothermal formation and decomposition of glucose. The initial concentration is another process variable that plays a key role in the pathways for glucose decomposition in HTW. Yu and Wu121 found that the reaction rate constant of glucose decomposition increases substantially with decreasing initial glucose concentration. This observation was attributed to the effect of the ion product of HTW, as the decomposition of glucose is likely to be influenced by the relative amounts of glucose and OH− ions. Indeed, the apparent reaction rate constant was a function of the molar ratio of [OH−] and glucose. The glucose decomposition rate constant is highly sensitive to initial glucose concentration, especially when the initial glucose concentration is less than 100 mg/L. The selectivity to 5-HMF increased as the initial concentration of glucose increased. Additional effects of the initial concentration on the relative rates of different paths are indicated below in Figure 8.

Figure 8. Effect of initial glucose concentration on glucose reaction pathways in HTW. Reproduced from Yu and Wu (2011),121 ACS Publications.

Lactic acid is a valuable product from sugar (glucose, fructose), and the mechanism for this conversion in alkaline solution is well established.122 The mechanism proposed for converting cellulose to lactic acid in subcritical water without the addition of any alkaline catalyst contained the same pathways as the alkaline mechanism, which suggests that water in the subcritical region may act as an effective acid−base catalyst. Glycolaldehyde is an intermediate product in the pathway for converting sugar (e.g., glucose) into acids as shown in Figure 7. A conversion pathway for glycolaldehyde into lactic acid at 300 °C has been proposed by Kishida et al. Figure 9 shows glycolaldehyde (indicated as 1) may undergo aldol condensation to give erythrose (indicated as 2). Lactic acid may be produced from erythrose by one of the three pathways (I, II, and III).123 This study suggested that this pathway is very important to glucose decomposition when the desired final P

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Figure 9. Proposed pathways for the conversion of glycolaldehyde into lactic acid. R1, aldol condensation; R2, elimination of H2O; R3, keto−enol tautomerization; R4, reverse aldol condensation; R5, benzilic acid rearrangement; R6, Lobry de Bruyn−Albeda van Ekenstein transformation (LBAE). Reproduced with permission from Kishida et al. (2006),123 Elsevier B.V.

remain to be explored, as the influence of process variables and the reaction kinetics and products are not yet clear. Monosaccharides have received the most attention in the literature. Although the pathways and kinetics of hydrothermal reactions of glucose, fructose, and xylose have been elucidated, there remain opportunities to improve yields of desired products via catalysis and reaction engineering.

product is lactic acid. Other products, such as humin, 2,5-dioxo6-hydroxyhexanal (DHH),124 and 1,2,4-benzentriol, also form from hydrothermal reactions of monosaccharides.125 Cantero et al.126 reported the Arrhenius parameters for all the reactions marked in Figure 5. For the glucose to fructose reaction, the activation energy and pre-exponential factor ln (A, s−1) are 111.5 ± 9.1 kJ/mol and 26.4 ± 2.1, respectively. The kinetics of 5-HMF formation from glucose was calculated considering the water concentration in the reaction. At temperatures below 330 °C, the activation energy is 291.8 ± 34 kJ/mol and ln (A, s−1) is 62.3 ± 6.5. Above 330 °C, the activation energy was 34.1 ± 3.7 kJ/mol and ln (A, s−1) was 10.4 ± 4.3. This shift was attributed to the change in ion product and shift in mechanism from ionic to free radical.126 Product selectivity from glucose conversion in near critical water is a function of temperature and water density. At low temperatures (∼300 °C), the production of 5-HMF via fructose dehydration is favored, whereas at high temperatures (∼400 °C), the formation of pyruvaldehyde via retro-aldol condensation of glucose to fructose is favored.126 At low water density (∼100 kg/m3), lactic acid formation is favored, whereas at high water density (∼600 kg/m3), 5-HMF formation reactions are favored. As described above, monosaccharides such as glucose, fructose, and xylose have received great attention because they are the hydrolysis products of polysaccharides. The main reaction pathways are established, and additional details continue to emerge. To summarize, this section provided a brief review of the hydrothermal pathways, products, and kinetics for polysaccharides (such as cellulose, hemicellulose, and starch), disaccharides (such as sucrose), and monosaccharides (D-xylose, glucose, fructose). Table 3 summarizes the processing conditions and kinetics of different carbohydrates in HTW. Hydrothermal reactions of cellulose and hemicellulose have been very well studied from low, subcritical temperatures (∼200 °C) to higher supercritical temperatures (∼400 °C), and the major pathways and kinetics have been established. Starch and disaccharides, however, have received less attention to date. Hydrothermal reactions of starch and disaccharides in supercritical water

6. OTHER BIOMOLECULES Besides the major biochemical components of protein, polysaccharide, and lipid, which we have discussed in earlier sections, microalgae also contain other materials such as nucleic acid, peptidoglycan, biotin, and pigments. Nucleic acids can account for up to 3−6 wt % of microalgae on a dry basis. Nucleic acids include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which are made of nucleotides. Hydrolytic stability of nucleic acids was studied at 250 °C.127 Nucleic acids undergo hydrolytic breakdown of primary structures followed by the decomposition of each of the components. No attempt was made to measure the hydrolysis rate because hydrolysis was rapid at 250 °C. White suggested an extremely short half-life for DNA in water at 250 °C, and that the N-glycosyl bond in adenosine was the least stable portion of the DNA molecule. The released purine and pyrimidine bases also readily decomposed at 250 °C with halflife values of less than 1 h. Therefore, few reaction mechanisms of the hydrolysis reaction at high temperature were proposed. Near room temperature, however, the reaction has been examined in greater detail.128,129 Likewise, other biomolecules such as peptidoglycan130 and biotin131,132 have been examined in water at temperatures below 100 °C. These studies can provide some helpful information, but since the temperature used in the experiment was much lower than those used in microalgae HTL, we do not discuss this prior work here. Pigments in microalgae include chlorophyll a, chlorophyll b, lutein, Mg 2,4 D, carotene, and antheraxanthin. Chlorophyll is one of the most common pigments in all types of microalgae.133 Phytol is a diterpenic alcohol that is readily hydrolyzed Q

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Industrial & Engineering Chemistry Research Table 3. Summary of Hydrothermal Treatment of Carbohydrates kinetics

biomolecule polysaccharides

substrate cellulose

starch

disachharides

sucrose

maltose lactose cellobiose isomaltose palatinose gentiobiose leucrose turanose melibiose monosaccharides

glucose

glucose to fructose glucose to 5-HMF (5-HMF formation showed a drastic change at 330 °C) 5-HMF: intermediate product of glucose xylose a

temperature range (°C)

pressure range (MPa)

reaction time (min)

reactor system

order

Rogalinski et al. (2008)101 Sasaki et al. (2000)102 Cantero et al. (2013)103 Cantero et al. (2013)103 Teri et al. (2014)40 Orozco et al. (2012)111 Teri et al. (2014)40 Rogalinski et al. (2008)101 Khajavi et al. (2005)113 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Oomori et al. (2004)112 Kabyemela et al. (1997)118 Yu and Wu (2011)121 Jin et al. (2004)122 Kishida et al. (2006)123 Cantero et al. (2013)103 Cantero et al. (2013)103 Cantero et al. (2013)103

240−310

20−25

0−3.3

flow

1

7.7 × 1013

163.9

320−400

25

0.0083−0.167

flow

1

NRa

NRa

320−370

25

0−0.5

flow

1

7.2 × 1012

154.4

370−400

25

0−0.5

flow

1

1.23 × 1035

430.3

300,350

8.6,16.5

10−90

batch

NRa

NRa

NRa

180−235

3

15

batch

NRa

NRa

NRa

300,350

8.6,16.5

10−90

batch

NRa

NRa

NRa

210−310

20−25

0−3.3

flow

1

5.3 × 1012

147.9

160−200

10

0−5

flow

NRa

6.26 × 109

98

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

180−260

10

0−5

flow

NRa

NRa

NRa

300,350,400

25−40

0.0003−0.033

flow

NRa

NRa

96

175−275

10

0−1

flow

1

NRa

109−90

300

8.9

0.5

batch

NRa

NRa

NRa

300

Psat

10

batch

NRa

NRa

NRa

300−400

25

0−0.5

flow

1

2.9 × 1011

111.5

300−330

25

0−0.5

flow

1

1.1 × 1027

291.8

330−400

25

0−0.5

flow

1

3.3 × 104

34.1

Luijkx et al. (1993)125 Aida et al. (2010)110

290−400

27.5

0−15

flow

1

NRa

NRa

350, 400

40−100

0.004−0.023

flow

NRa

NRa

NRa

reference

Ao (s−1)

Ea (kJ/mol)

NR = Not reported.

from the porphyrin portion of chlorophyll, which was considered as the most likely precursor of pristane that might occur in petroleum from natural source materials.134 The major products from hydrothermal reactions of phytol (240−350 °C)

include neophytadiene, isophytol, and phytone, which are also found in the crude bio-oil obtained from microalgae during its HTL.91 The minor products include pristine, phytene, phytane, and dihydrophytol. Phytol disappearance in HTW was R

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Industrial & Engineering Chemistry Research Table 4. Summary of Hydrothermal Treatment of Minor Biomolecules in Microalgae kinetics biomolecule nucleic acid

substrate deoxyribonucleic acid

biotin pigments a

phytol

reference

temperature range (°C)

pressure range (MPa)

reaction time (min)

reactor system

order

Ao (s−1)

Ea (kJ/mol)

batch

NRa

NRa

NRa

0.1

NRa (very fast) 0−1500

batch

NRa

NRa

NRa

20−80

0.1

1−300

batch

NRa

NRa

NRa

240,270,300,350

Psat

60

batch

1

8.7 × 109

145

White (1984)127

250

3.97

Lindahl and Nyberg (1972)129 Holmberg et al. (2005)131 Changi et al. (2012)135

25−80

NR = Not reported.

Figure 10. Schematic representation of Maillard reaction between glucose and glycine. Reproduced from Peterson et al. (2010),136 ACS Publications.

confirmed to follow first-order kinetics. The activation energy and pre-exponential factor were calculated to be 145 ± 20 kJ/ mol and 8.9 × 109 s−1.135 Overall, research on the minor components in microalgae mostly aims at understanding the reaction pathways rather than detailed product yields under varying conditions. Our search of the literature led to no previous work on nucleic acid, peptidoglycan, and biotin at temperatures relevant for HTL. Additionally, there is a paucity of work focusing on reaction kinetics of the different pigments of microalgae in hydrothermal conditions. Table 4 summarizes the only literature available reporting the processing conditions and kinetics for minor biomolecules in microalgae.

section reviews the limited work that examines these issues to date. 7.1. Polysaccharide and Amino Acid. Peterson et al.136 examined a mixture of glycine (amino acid) and glucose (saccharide) at 250 °C and 10 MPa. They found that the simultaneous presence of both glucose and glycine resulted in production of a dark brown mass with a nutty odor and strong absorbance at 420 nm, which are characteristic of products from the Maillard reaction.137,138 Figure 10 shows the scheme for the Maillard reaction.136 The Maillard reaction occurs between the amine group of the amino acid and carbonyl group of a sugar. The reaction network includes the formation of an Amadori compound. This product then undergoes a number of reactions to form polymeric compounds referred to as melanoidins. Degradation products of glucose that contain carbonyl groups may also take part in the first step of the Maillard reaction. For example, Peterson et al.136 demonstrated that pyruvic aldehyde, a glucose degradation product, reacted more strongly with glycine than did glucose, and had the same qualitative effects of producing brown colored products with an odor. As a result of these interactions, one can reasonably expect the presence of

7. MULTICOMPONENT SYSTEMS The previous four sections discussed in detail the reactions of individual biomolecules in HTW. It is conceivable that reactions of individual compounds do not proceed independently when present in a mixture. Additionally, it is conceivable that some of the components in microalgae growth media might influence reaction rates and pathways during HTL. This S

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Figure 11. Reaction network for binary mixture of ethyl oleate and phenylalanine in HTW. Reproduced with permission from Changi et al. (2012),57 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

ethyl oleate and phenylethylamine/ammonia. They are an undesirable component in an algal bio-oil because they lead to a decrease in the fatty acid yields and add nitrogen to the product. Brown et al.91 and Valdez et al.139 have identified and quantified fatty acid amides as products from HTL of microalgae. Valdez et al.141 reported that the yields of palmitic acid amide varied from 0.5 to 3 (mg/g of dry microalgae), depending on whether hexane or hexadecane was used for the separation and recovery of bio-oil. However, these authors have only proposed the reaction of fatty acids with ammonia to account for the amide formation, neglecting the possibilities of amines reacting with fatty acids. Recently, Chiaberge et al.140 have confirmed the presence of several amides in bio-oil derived from HTL of microalgae. They further investigated whether these amides formed via reactions of fatty acids with amino acids. They reacted 20 amino acids individually with hexadecanoic acids, at high temperature and pressure. The amides that formed covered the complete spectrum of the amides recognized in the bio-oil chromatogram. A reaction pathway for the binary mixture of amino acid and fatty acid ester was proposed by Changi et al.57 as shown in Figure 11. 7.3. Fatty Acid and Ammonium Bicarbonate. Rushdi and Simoneit examined the hydrothermal reactions of nnonadecanoic acid, n-hexadecanedioic acid, and n-hexadecanamide in a reducing environment (created using ammonium bicarbonate).141 The batch reactions were carried out over 72 h at 300 °C, and the products from n-nonadecanoic acid and nhexadecanedioic acid included several amides and alkyl nitriles. The reaction of n-hexadecanamide produced hydrolysis and dehydration products, such as n-hexadecanoic acid and hexadecanenitrile, respectively. These findings indicate that

proteins/amino acids and sugars in microalgae to lead to the formation of high-molecular-weight material. This material might result in processing difficulties, including fouling of process equipment. Peterson et al. 136 also reported that the rates of disappearance of glucose and glycine were strongly influenced by the presence of each other. The presence of glucose always resulted in higher glycine conversion, while the presence of glycine either increased or decreased the glucose conversion, depending on the initial concentrations, at 250 °C and 10 MPa. For example, when the initial glucose concentration was low (