ARTICLE pubs.acs.org/IECR
Production of C3 Hydrocarbons from Biomass via Hydrothermal Carboxylate Reforming Curt R. Fischer,† Andrew A. Peterson,*,‡ and Jefferson W. Tester§ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ABSTRACT: We demonstrate a route for the production of C3 hydrocarbons from renewable biomass by the hydrothermal conversion of well-known fermentation end-products. Specifically, the major commercial C3 hydrocarbons, propane and propylene, can be obtained from butyric acid and 3-hydroxybutyrate (3HB) in substantial yields and industrially relevant productivities by hydrothermal decarboxylation. Butyric acid decarboxylates in supercritical water to give propane as the major product at 454 °C and 25 MPa. 3HB undergoes joint dehydration and decarboxylation in subcritical water to yield propylene at 371 °C and 25 MPa with yields of up to 48 mol %. Although catalysts may be found that increase yields and selectivities, these processes were demonstrated without any added heterogeneous catalysts, and have the further advantage of requiring no external H2 source.
1. INTRODUCTION Many renewably produced fuels and chemicals are chemically distinct from existing fossil fuels. As such, their widespread use would require substantial changes to existing infrastructure. Additionally, current commercial biomass-to-biofuel conversion processes are criticized for low energy efficiencies,1-4 and many proposed future routes require a source of H2 in reforming.5-7 In the current work, we show that both of the major commercial C3 hydrocarbons, propane and propylene, can be made via hydrothermal reforming of well-known fermentation products, butyric acid and 3-hydroxybutyrate, in a process that potentially can have high thermal efficiencies. Since this process relies on fermentation, the front-end infrastructure for hexose or pentose production is similar to that for ethanol production. However, unlike ethanol, propane and propylene production does not require energy-intensive distillation steps to separate the products from the predominantly aqueous product stream; instead, the hydrophobic products will phase separate from the water. (Conventional refinery separations may be necessary to further purify products, however.) Additionally, neither process introduced here requires an external H2 source. Propane (C3H8) and propylene (C3H6) both have existing large markets within the petrochemicals industry. As the primary component of liquefied petroleum gases (LPG), markets for propane are in excess of 107 tonnes per year (t/a) in the United States and 108 t/a worldwide.8 Propane can burn more cleanly than higher hydrocarbons,9 while having a volumetric energy density 3-8 times greater than compressed gaseous fuels (i.e., compressed natural gas or hydrogen). Propylene is the monounsaturated form of propane and is a basic industrial chemical building block. World production per year is in excess of 107 t/a.10 Direct renewable routes to propane and propylene that do not rely on exogenous H2 feedstocks would be of considerable practical interest, because such routes could provide carbonneutral or carbon-negative products that could feed into existing industrial infrastructures. We employed hydrothermal conversion as the key step in a route for the production of propylene and propane from biomass without exogenous hydrogen requirements. Figure 1 summarizes our two-step approach. In our pathway, r 2011 American Chemical Society
fermentations are employed to produce partially deoxygenated intermediate chemicals. These intermediate chemicals are subsequently decarboxylated and/or dehydrated in hydrothermal media to produce the C3 hydrocarbons. Previous research has shown the likelihood of the hydrothermal reactions described here,11-22 and decarboxylation and dehydration reactions have been discussed in a recent review of biomass processing in hydrothermal environments.23 Hydrothermal processing can be a highly efficient means of processing wet feedstocks;23 since the hydrocarbon products spontaneously separate from the aqueous phase, no costly product-water distillation step is necessary. The chemical routes are summarized in Figure 1 and described below.
2. RESULTS AND DISCUSSION In the first scheme, butyric acid can be produced via fermentation from 5- and 6-carbon sugars,24 which include those produced from starches, sugar cane, and (hemi)cellulose. For glucose, the result is the removal of four of the six oxygen atoms as carbon dioxide, resulting in butyric acid and a byproduct of two moles of hydrogen: C6 H12 O6 f CH3 ðCH2 Þ2 COOH þ 2CO2 þ 2H2
ð1Þ
In a second, hydrothermal step, we show that butyric acid can be decarboxylated in an aqueous phase to form propane, requiring no separation of the butyric acid from its fermentation broth: CH3 ðCH2 Þ2 COOH f CH3 CH2 CH3 þ CO2
ð2Þ
Thus, a complete deoxygenation of the glucose can be achieved without the need for exogenous H2. To demonstrate the feasibility of producing propane from butyric acid, potassium butyrate at 2% w/w in water was reacted in a continuous high-pressure tank (HPT) reactor at 454 °C and 25 MPa operating with an average residence time of Received: November 19, 2010 Accepted: February 8, 2011 Revised: January 28, 2011 Published: March 10, 2011 4420
dx.doi.org/10.1021/ie1023386 | Ind. Eng. Chem. Res. 2011, 50, 4420–4424
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Schematic of the reaction pathways to produce propane or propylene from biological feedstocks that have been pretreated to form C6 (glucose) and C5 (xylose) sugars.
approximately 13 min. The potassium salt of butyric acid was used; this may enhance decarboxylation reactions17 and resulted in easier laboratory handling because of lower olfactory output of butyric acid. In these feasibility experiments, approximately 10% of the feed butyric acid was converted in a single pass as measured by HPLC of the input and output streams, shown in Figure 2. The composition of the gas stream is shown in the bottom panes of Figure 2. (Compositions are normalized on a reducing basis, as described in the Materials and Methods section.) The C3 hydrocarbons propane and propylene together make up approximately 70% of the gaseous products, and propane is the dominant gaseous product in this uncatalyzed environment. The selectivity toward propane drops with run time, but always remains high. For a fuel-producing application, the selectivity toward propane is a key metric because of propane’s greater energy density. The bottom pane of Figure 2 shows the ratio of propane to propylene in the same experiment. The process is quite selective toward propane over propylene: the ratio starts out above 3:1, but drops to around 2:1 with increasing run time. In a separate scheme, a sugar such as glucose is fermented to produce a polymer of 3-hydroxybutyric acid (3HB), which can accumulate inside microorganisms to levels in excess of 0.9 g polymer/g dry cell weight. For glucose, 1.5 equiv of O2 are consumed, resulting in the reaction below: 3 C6 H12 O6 þ O2 f 9½ OCHðCH3 ÞCH2 CðOÞ9 2 þ 2CO2 þ 3H2 O
ð3Þ
In the second step, we show that the polymer can be reacted in the aqueous suspension to form propylene, in a hydrolytic depolymerization22 followed by combined decarboxylation/ dehydration.20 9½ OCHðCH3 ÞCH2 CðOÞ9 þ H2 O f CH3 CHðOHÞCH2 COOH
ð4Þ
CH3 CHðOHÞCH2 COOH f CH3 CHdCH2 þ CO2 þ H2 O
ð5Þ
Again, a completely deoxygenated hydrocarbon can be produced without exogenous H2. To demonstrate the production of propylene from biologically derived 3HB polymer (PHB), we reacted 0.08 g of solid, biologically-derived PHB granules (Sigma-Aldrich, St. Louis, MO) with 7.65 g of water at a maximum temperature of 400 °C for 20 min. (Since the cold reactor was introduced into a sandbath, the reactive mixture experienced a continuum of temperature and pressure conditions over the heat-up and cool-down time.) This screening reaction resulted in a 48% yield (mol/mol) of propylene, with unreacted PHB granules on the wall of the reactor that were not quantifiable. This demonstrated the production of propylene, but did not sort out whether the reaction was occurring in sub- or supercritical-water conditions. The water-soluble monomer 3-hydroxybutyric acid (3HB) is also a suitable candidate for hydrothermal reforming, and recombinant Escherichia coli have been developed that can produce 3HB directly without requiring the formation of PHB.25 To better understand the conversion of PHB to propylene, we studied the conversion of its water-soluble monomer 3-hydroxybutyric acid in the HPT reactor under both subcritical- and supercritical-water conditions. Figure 3 summarizes this experiment. The reactor was kept at subcritical conditions (T = 371 °C) at the beginning of the run, which resulted in a gas stream that was primarily composed of propylene, as shown in Figure 3. Under these conditions the yield of propylene (relative to 3HB fed) was 34% on a molar basis. After 200 min of subcritical operation, the temperature in the HPT reactor was increased to a supercritical temperature of 403 °C, indicated by the gray area in Figure 3. This increase resulted in a decrease in both total gas flow as well as propylene and CO2 concentrations in the gas exit stream. The lower gas yield may be due in whole or in part to the lower fluid density and thus lower residence times under supercritical conditions (159 kg/m3 at 403 °C and 250 bar) than under subcritical conditions (water density 521 kg/m3 at 373 °C and 250 bar); however, the gas composition clearly shifted away from propylene and toward H2, indicating that selectivity toward propylene is favored at subcritical conditions. 4421
dx.doi.org/10.1021/ie1023386 |Ind. Eng. Chem. Res. 2011, 50, 4420–4424
Industrial & Engineering Chemistry Research
Figure 2. Run summary for propane production from butyric acid. The top plot shows the temperature versus run time, defined as the elapsed time from which potassium butyrate was first fed to the reactor. The second and third plots show time histories of the molar flow rates of the feed and the products, and the gas composition of C3 (propane and propylene), C2 (ethane and ethylene), C1 (methane), and C0 (hydrogen) hydrocarbons measured in the gas phase, respectively. These compositions have been normalized on a reducing basis as described in the Materials and Methods section. The bottom curve shows the ratio of alkane/alkene in the C3 gas (the propane to propylene ratio). At approximately 250 min a dip in the pump flow rate was observed, which is apparent in the second plot. The reactor pressure was maintained at 24.5 MPa.
Results with PHB and 3HB feeds demonstrate the feasibility of a glucose to propylene pathway under subcritical conditions, and show that propylene is formed with a 30-fold molar excess relative to all other gaseous hydrocarbons and with a volumetric productivity greater than 0.1 mol m-3 min-1. The optimization of reaction conditions and the use of catalysts may further enhance both selectivity and rates. These observations compare well to current industrial practices: for propylene production by steam cracking of hydrocarbons,26 yields of propylene on converted feedstocks are as little as 3-5% and volumetric productivities are 0.6-30 mol m-3 min-1. The ability to selectively create high-purity propylene streams may obviate the need for expensive downstream separation steps such as cryo-distillation. More polar solvents, such as subcritical water, stabilize intermediates of ionic reaction mechanisms. In less polar solvents such as supercritical water, radical-based reaction mechanisms may be more favorable.27 Thus, an ionic intermediate may exist in the conversion of 3HB to propylene, while free-radical
ARTICLE
Figure 3. Run summary for propylene production from 3-hydroxybutyric acid. The top plot shows the measured reactor temperature versus run time; after 200 min, the reactor temperature was increased from 371 to 403 °C, which corresponds to the gray shading. The center plot shows the total gas production. The 0 s are the raw measurements, and the circles are the gas flow rates corrected for helium, which is not a product but the gas used to sparge the water. The bottom curve shows the gas composition as a function of run time. The reactor pressure was kept at 25 MPa.
reactions may be responsible for propane conversion to butyrate. The HPT reactor used was a continuous stirred tank reactor, which can allow free radical species to build up over the course of a run.28 This free-radical pool may push the selectivity toward more fragmented compounds, such as CH4 and H2, providing an explanation of the decreasing selectivity toward propane throughout the run. In net, these processes as designed have an upper stoichiometric limit on the yield given by their overall reactions (assuming a glucose feedstock): C6 H12 O6 f CH3 CH2 CH3 þ 2H2 þ 3CO2 3 C6 H12 O6 þ O2 f CH3 CHCH2 þ 3H2 O þ 3CO2 2
ð6Þ ð7Þ
giving one mole of either propane or propylene from each starting mole of glucose, with an additional 2 mols of H2 in the case of propane. Optimizations of and expansions to the reaction schemes introduced in the current work are possible and desirable. For example, the recent results of Fu et al.18 suggest that Pt and Pd catalysts may be advantageous for related decarboxylation processes, which may have applications in optimizing yields in the current work. Additionally, as advances in metabolic 4422
dx.doi.org/10.1021/ie1023386 |Ind. Eng. Chem. Res. 2011, 50, 4420–4424
Industrial & Engineering Chemistry Research engineering expand the range of organic compounds producible in high titer, many other variations of this process may be envisioned on other fermentation products. The results of this study demonstrate the feasibility of a direct route to biologically derived propane or propylene, and show that each is the dominant product formed in its own reaction scheme. To our knowledge, this is the first report of a practicable, biologically derived route to these compounds without the use of an external H2 source. The schemes presented have the added advantage of using the same cellulose, starch, and sugar feedstocks as ethanol, in a process that has the potential to be efficient and to produce drop-in replacements for conventional fuels.
ARTICLE
reducing basis. Since one mole of butyric acid (C3H7COOH) can equivalently produce 10 mols of H2 but only one mole of propane (C3H8), strict molar comparisons would not tell a representative story of the fate of the feedstock. To facilitate comparisons between products, all species are normalized based on their hydrogen equivalence (HE). We define the fully oxidized species CO2 and H2O to have an HE of 0, and all other species have an HE equal to the number of H2 molecules it would take to form them from only CO2 and H2O, for example, the formation reactions 3CO2 þ 10H2 f C3 H8 þ 6H2 O
ð8Þ
CO2 þ 4H2 f CH4 þ 2H2 O
ð9Þ
and
3. MATERIALS AND METHODS The continuous reactions of potassium butyrate and 3-hydroxybutyric acid were carried out in a 637-mL continuous stirred tank reactor constructed of Inconel-625 (UNS N06625) which was described in detail by Marrone29 and is referred to in this communication as the HPT reactor. Reactant solutions were prepared by dissolving reactants in water purified to 18.2 MΩ cm and were subsequently sparged with helium to remove atmospheric O2. The feed was kept under helium head pressure and was delivered to the reactor at a flow rate of 5 g/min by an Acuflow Series II HPLC pump capable of pulseless flow. The temperature in the reactor was monitored with five immersed thermocouples, and the reactor temperature was controlled based on these temperatures. The reactor impeller was maintained at 500 rpm. Upon exiting the reactor, the flow was immediately quenched as it passed through coiled tubing submersed in a cold water bath, after which point it went through a 2 μm filter and a back-pressure regulator which maintained the specified system pressure. The pressure was monitored both upstream and downstream of the reactor. Gas flow rates were measured using an inverted buret gas-separator; gas samples were taken from this same inverted buret. For the batch screening reaction of PHB described, a batch reactor was assembled of 316SS (UNS 31600) fittings of 2.54 cm internal diameter, with a measured internal volume of approximately 25.5 mL. After introduction of the reactant mixture, the reactor was exposed to a 400 °C sandbath for 20 min. Several minutes after removal from the sandbath heat, the reactor was quenched in an ambient water bath. (The delay before quenching was for safety reasons.) A known volume chamber was attached to the top of the vessel to measure gas production; the gas sample for analytical purposes was taken between the two chambers. Gaseous products were measured by gas chromatography. Hydrogen and helium were separated by a 60/80 Carboxen 1000 and a 60/80 Molecular Sieve 5A column (in series) using N2 as the carrier gas and a thermal conductivity detector. Hydrocarbons were separated on a Gas Pro GSC column using a temperature ramp to 180 °C with He as a carrier gas and a flame ionization detector. CO2, O2, N2, and CO were separated by a 60/80 Carboxen 1000 in series with a 60/80 Molecular Sieve 5A with a He carrier gas and detection by thermal conductivity. All GCs were manufactured by HP. Liquid products were analyzed by HPLC with UV detection and separated on an organic acid column using a 0.01 N sulfuric acid mobile phase delivered at 0.7 mL/min. In both the GC and HPLC methods, concentrations were calibrated to known standards daily. In this communication, when the word “normalized” precedes molar compositions, these concentrations were reported on a
show that propane has an HE of 10 and methane has an HE of 4. Since HE is stoichiometrically related to the heat of combustion of the reaction products, its use makes for less-distorted view of the product spectrum than using mole ratios directly. This is conceptually equivalent to the use of Faradaic yields in electrochemistry.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Ginkgo Bioworks, Boston, MA. Stanford University, Palo Alto, CA. § Cornell University, Ithaca, NY. ‡
’ ACKNOWLEDGMENT The authors acknowledge Tracy Mathews, William H. Green, Michael Antal, Daniel Klein-Marcushamer, and Gregory Stephanopoulos for valuable technical discussions. Funding was provided by an ignition grant by the Deshpande Center for Technological Innovation. ’ REFERENCES (1) Pimentel, D.; Patzek, T. W. Ethanol production using corn, switchgrass, and wood; biodiesel production using soybean and sunflower. Nat. Resour. Res. 2005, 14, 65. (2) Kwiatkowski, J.; McAloon, A.; Taylor, F.; Johnston, D. Modeling the process and costs of fuel ethanol production by the corn dry-grind process. Ind. Crop. Prod. 2006, 23, 288. (3) Kim, S.; Dale, B. Environmental aspects of ethanol derived from no-tilled corn grain: nonrenewable energy consumption and greenhouse gas emissions. Biomass Bioenergy 2005, 28, 475. (4) Campbell, J.; Lobell, D.; Field, C. Greater transportation energy and GHG offsets from bioelectricity than ethanol. Science 2009, 324, 1055. (5) Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature 2007, 447, 982. (6) Taher, D.; Thibault, M.; Di Mondo, D.; Jennings, M.; Schlaf, M. Acid-, water- and high-temperature-stable ruthenium complexes for the total catalytic deoxygenation of glycerol to propane. Chem.—Eur. J. 2009, 15, 10132. (7) Huber, G.; Chheda, J.; Barrett, C.; Dumesic, J. Production of liquid alkanes by aqueousphase processing of biomass-derived carbohydrates. Science 2005, 308, 1446. 4423
dx.doi.org/10.1021/ie1023386 |Ind. Eng. Chem. Res. 2011, 50, 4420–4424
Industrial & Engineering Chemistry Research
ARTICLE
(8) Consumption of liquefied petroleum gases. Energy Information Administration, United States Department of Energy, 2010. Available online, http://www.eia.doe.gov/emeu/international/ (accessed 13 June 2010). (9) Murray, J., Lane, B., Lillie, K., McCallum, J. The report of the alternative fuels group of the cleaner vehicles task force: An assessment of the emissions performance of alternative and conventional fuels. Technical report, The Cleaner Vehicles Task Force, Department of the Environment, Transport and the Regions, United Kingdom, 2000. (10) McCoy, M.; Reisch, M.; Tullo, A.; Tremblay, J.-F.; Voith, M. Facts & figures of the chemical industry. Chem. Eng. News 2009, 87, 29. (11) Klingler, D.; Berg, J.; Vogel, H. Hydrothermal reactions of alanine and glycine in suband supercritical water. J. Supercrit. Fluids 2007, 43, 112. (12) Yu, J.; Savage, P. Decomposition of formic acid under hydrothermal conditions. Ind. Eng. Chem. Res. 1998, 37, 2. (13) Maiella, P.; Brill, T. Spectroscopy of hydrothermal reactions. 10. Evidence of wall effects in decarboxylation kinetics of 1.00 m HCO2X (X = H, Na) at 280-330_C and 275 bar. J. Phys. Chem. A 1998, 102, 5886. (14) Bell, J. L. S.; Palmera, D. A.; Barnesb, H. L.; Drummond, S. E. Thermal decomposition of acetate: III. Catalysis by mineral surfaces. Geochim. Cosmochim. Acta 1994, 58, 4155. (15) Meyer, J. C.; Marrone, P. A.; Tester, J. W. Acetic acid oxidation and hydrolysis in supercritical water. AIChE J. 1995, 41, 2108. (16) Brill, B.; Belsky, A.; Maiella, P. Hydrothermal decarboxylation of RCO2H studied by real-time vibrational spectroscopy. Prepr. Symp. Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 340–343. (17) Watanabe, M.; Iida, T.; Inomata, H. Decomposition of a long chain saturated fatty acid with some additives in hot compressed water. Energy Convers. Manage. 2006, 47, 3344. (18) Fu, J.; Lu, X.; Savage, P. E. Catalytic hydrothermal deoxygenation of palmitic acid. Energy Environ. Sci. 2010, 3, 311. (19) Ramayya, S.; Brittain, A.; DeAlmeida, C.; Mok, W.; Antal, M. J. Acid-catalysed dehydration of alcohols in supercritical water. Fuel 1987, 66, 1364. (20) Mok, W. S. L.; Antal, M. J.; Jones, M. Formation of acrylic acid from lactic acid in supercritical water. J. Org. Chem. 1989, 54, 4596. (21) Dunn, J. B.; Burns, M. L.; Hunter, S. E.; Savage, P. E. Hydrothermal stability of aromatic carboxylic acids. J. Supercrit. Fluid. 2003, 27, 263. (22) Saeki, T.; Tsukegi, T.; Tsuji, H.; Daimon, H.; Fujie, K. Hydrolytic degradation of poly[(R)-3-hydroxybutyric acid] in the melt. Polymer 2005, 46, 2157. (23) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Fr€oling, M.; Antal, M. J., Jr.; Tester, J. W. Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies. Energy Environ. Sci. 2008, 1, 32. (24) Papoutsakis, E. Equations and calculations for fermentations of butyric acid bacteria. Biotechnol. Bioeng. 1984, 26, 174. (25) Gao, H.-J.; Wu, Q.; Chen, G.-Q. Enhanced production of D-(-)3-hydroxybutyric acid by recombinant Escherichia coli. FEMS Microbiol. Lett. 2002, 213, 59. (26) Calamur, N., Carrera, M. Propylene. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2005. (27) Antal, M. J. J.; Brittain, A.; DeAlmeida, C.; Ramayya, S.; Roy, J. C. Heterolysis and homolysis in supercritical water. In Supercritical Fluids; Squires, T. G., Paulaitis, M. E., Eds.; ACS Symposium Series 329; American Chemical Society: Washington, DC, 1987; Chapter 7, pp 77-86. (28) Vogel, F.; Blanchard, J. L. D.; Marrone, P. A.; Rice, S. F.; Webley, P. A.; Peters, W. A.; Smith, K. A.; Tester, J. W. Critical review of kinetic data for the oxidation of methanol in supercritical water. J. Supercrit. Fluids 2005, 34, 249. (29) Marrone, P. Hydrolysis and oxidation of model organic compounds in sub- and supercritical water: Reactor design, kinetics measurements, and modeling. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1998 4424
dx.doi.org/10.1021/ie1023386 |Ind. Eng. Chem. Res. 2011, 50, 4420–4424
