Energy Fuels 2010, 24, 6586–6600 Published on Web 11/09/2010
: DOI:10.1021/ef101154d
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Red Mud as a Catalyst for the Upgrading of Hemp-Seed Pyrolysis Bio-oil )
Elham Karimi,† Cedric Briens,‡,§ Franco Berruti,‡,§ Sina Moloodi, Tommy Tzanetakis, Murray J. Thomson, and Marcel Schlaf*,† †
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Department of Chemistry, The Guelph-Waterloo Centre for Graduate Work in Chemistry (GWC)2, University of Guelph, Guelph, Ontario N1G 2W1, Canada, ‡Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada, §Institute for Chemicals and Fuels from Alternative Resources (ICFAR), 22312 Wonderland Road North, RR#3, Ilderton, Ontario N0M 2A0, Canada, and Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada Received August 26, 2010. Revised Manuscript Received October 20, 2010
Hemp-seed pyrolysis bio-oil was upgraded in a batch laboratory-scale pressure reactor under 800 psi (cold) hydrogen gas at 350-365 °C using a non-alkaline, nontoxic FexOy/SiO2/TiO2 catalyst [reduced red mud (RRM)] obtained by the reduction of red mud with HOAc/HCCOH. The upgraded liquid obtained was separated into stable organic and aqueous phases. Comparative analyses between the crude oil and the organic and aqueous phases of upgraded products showed that the RRM-upgraded bio-oil is composed of fewer carbonyl-containing and polar oxygenated compounds but more saturated hydrocarbons. The upgraded oil phases are less viscous than the native oil and stable against resin formation for at least 60 days. The catalytic activity of RRM is related to its ability to catalyze both deoxygenation and cracking reactions that convert reactive components (aldehydes, ketones, and carboxylic acids), which make the oil unstable over time, into less reactive deoxygenated products.
catalyst for the reactions of Scheme 1, (b) hydrogenates the reactive aldehyde and ketone components into alcohols that can no longer undergo aldol condensations while enabling esterification/etherification of any excess acids/phenols present, thus further lowering the acidity and overall reactivity of the bio-oil, and (c) lowers the overall oxygen and water content of the oil by eliminating CO2 and water, which ideally would phase separate from an upgraded organic oil phase of higher energy density that is then also stabilized against resin formation. Considering the prevalence of formic and acetic acids in a typical bio-oil, Scheme 25 summarizes the main reaction pathways required to meet the objectives (a-c) laid out above, in which the decomposition of formic acid, if present in sufficient amounts, can serve as an internal source of hydrogen and the ketonization of acetic acid (or other carboxylic acids) serves as an efficient decarboxylation pathway. The decomposition of formic acid is intrinsically viable at high temperatures (.400 °C)6 but benefits from the presence of a catalyst,7,8 while the also required acid ketonizations and acid and carbonyl (and possibly furan, pyran, and alkene) hydrogenations require a catalyst to proceed.9-15
Introduction Bio-oil obtained by fast pyrolysis of lignocellulosic or other biomass is a complex mixture of alcohols, furan and pyran derivatives, aldehydes, ketones, esters, lactones, free carboxylic acids, phenols, and catechols.1-3 It is typically characterized by high water (∼15-30%, w/w) and oxygen (∼40%, w/w) contents and low pH (∼2.5 on the aqueous scale) caused by the presence of carboxylic acids (mainly formic and acetic acids), as well as phenols and catechols originating from the lignin portion of the feedstock used. The combination of the reactive carbonyl components with phenols, acid, and water make the oil corrosive, difficult to ignite and burn cleanly, and unstable against condensation and carbon-carbon bond-forming cross-linking reactions. These reactions can result in resin formation and phase separation of an aqueous phase triggered by the generation of additional water. Scheme 1 illustrates some of the conceivable reaction pathways of this process (notably phenol resin formation4 and aldol condensations), which renders typical bio-oils unsuitable for direct use as fuel and makes their longer term storage problematic. The actual use and storage of pyrolysis bio-oil as a fuel or petrochemical feed therefore requires its stabilization through a reductive catalytic upgrading process that (a) raises the pH of the oil by converting the carboxylic acids present into nonacidic and noncorrosive products that can no longer act as a
(5) Karimi, E.; Gomez, A.; Kycia, S. W.; Schlaf, M. Energy Fuels 2010, 24, 2747. (6) Yu, J.; Savage, P. E. Ind. Eng. Chem. Res. 1998, 37, 2. (7) Trillo, J. M.; Munuera, G.; Criado, J. M. Catal. Rev.;Sci. Eng. 1972, 7, 51. (8) Borowiak, M. A.; Jamroz, M. H.; Larsson, R. J. Mol. Catal. A: Chem. 1999, 139, 97. (9) Glinski, M.; Kijenski, J.; Jakubowski, A. Appl. Catal., A 1995, 128, 209. (10) Pestman, R.; van Duijne, A.; Pieterse, J. A. Z.; Ponec, V. J. Mol. Catal. A: Chem. 1995, 103, 175. (11) Pestman, R.; Koster, R. M.; Pieterse, J. A. Z.; Ponec, V. J. Catal. 1997, 168, 255. (12) Pestman, R.; Koster, R. M.; Boellaard, E.; van der Kraan, A. M.; Ponec, V. J. Catal. 1998, 174, 142.
*To whom correspondence should be addressed. Fax: 519-766-1499. E-mail:
[email protected]. (1) Mullen, C. A.; Boateng, A. A. Energy Fuels 2008, 22, 2104. (2) Garcia-Perez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Roy, C. Biomass Bioenergy 2007, 31, 222. (3) Mohan, D.; Pittman, C. U.; Steele, P. H. Energy Fuels 2006, 20, 848. (4) Hesse, W. Ullmann’s Encyclopedia of Industrial Chemistry; Online Electronic Reference Work, 7th ed.; John Wiley and Sons: New York, 2000. r 2010 American Chemical Society
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Scheme 1. Possible Reaction Sequences Leading to Resin Formation in Pyrolysis Bio-oil
Scheme 2. Fundamental Reactions Required for a Reductive Upgrading of Pyrolysis Bio-oil5
The corrosiveness, water content, and high polarity of biooil makes the identification of a stable and promiscuous catalyst for the reactions of Scheme 2 challenging. Commonly used catalysts composed of a hydrogenating heavy metal (e.g., Re, Ru, Rh, Ir, Ni, Pd, and Pt) and a metal oxide support (e.g., SiO2, Al2O3, ZnO, and TiO2) are susceptible to coking and fouling of the catalyst surface by the highly polar substrate and can, under the necessarily aqueous acidic conditions, also suffer from the destruction of the support medium and leaching of the toxic heavy metal into the product, making it either unusable or requiring further distillation prior to combustion in a boiler, turbine, or piston engine.
Red mud is the waste byproduct of the Bayer process for the refining of bauxite ore into pure Al2O3, the first step in the production of aluminum metal. It consist of a highly alkaline (pH 14) mixture of Fe2O3, Al2O3, SiO2, TiO2, CaO, and Na2O and is produced as an aqueous slurry on a very large scale (>70 106 ton/year). At present, this nontoxic material is routinely deposited in vast landfills. Alternative uses of red mud are therefore of great economic and ecologic interest.16 Red mud has previously been investigated as a catalyst for the hydroliquefaction of biomass17 and, at a higher temperatures (∼800 °C), for the production of hydrogen from methane18 and a variety of other processes, notably coal
(13) Das, J.; Parida, K. React. Kinet. Catal. Lett. 2000, 69, 223. (14) Yokoyama, T.; Yamagata, N. Appl. Catal., A 2001, 221, 227. (15) Deng, L.; Fu, Y.; Guo, Q. X. Energy Fuels 2009, 23, 564.
(16) http://www.redmud.org. (17) Klopries, B.; Hodek, W.; Bandermann, F. Fuel 1990, 69, 448. (18) Balakrishnan, M.; Batra, V. S.; Hargreaves, J. S. J.; Monaghan, A.; Pulford, I. D.; Rico, J. L.; Sushil, S. Green Chem. 2009, 11, 42.
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19
liquefaction. Recognizing that the composition of red mud is essentially that of a very cheap20 multifunctional acid/base, hydrogenation, water-gas shift reaction (WGSR), and Fischer-Tropsch catalyst (FexOy, TiO2, and SiO2) for the reactions listed in Scheme 2,19 we recently demonstrated its use as a catalyst for the decomposition of formic and acetic acids and mixtures thereof into ketones, alcohols, and hydrocarbons at 350 °C and hydrogen pressures ranging from 0 to 800 psi.5 In this process, the red mud is reduced to a gray magnetic, non-alkaline material [reduced red mud (RRM)] that maintains its catalytic activity upon reuse. Also, because of the elemental composition (Na, Ca, Al, Si, Ti, and Fe) of red mud, any leaching of the catalyst into the upgraded products is not of environmental concern, because they are nontoxic and their oxides are ubiquitous in the bio- and lithospheres. Here, we present the first study of the use of this material as an upgrading catalyst for an actual bio-oil under the same reaction conditions. Figure 1. Schematics of the ICFAR bubbling fluidized-bed pilot plant.
Experimental Section General. An authentic operational sample of red mud was supplied by Rio Tinto Alcan’s Jonquiere, Quebec operation. RRM was prepared as previously described.5 Hemp-seed bio-oil was supplied by the fast-pyrolysis plant of the Institute for Chemicals and Fuels from Alternative Resources (ICFAR) located at the University of Western Ontario, London, Ontario, Canada.21 Gas chromatography (GC) analysis was performed on a Varian 3800 GC using a 30 m DB-1701 column operating at a constant flow rate of 1 mL/min. GC-mass spectrometry (MS) analysis were performed either on a Varian Saturn 2000 GC-MS running in default electron ionization (EI) mode (4 V axial bias and 1400 V multiplier) or at the Advanced Analysis Centre of the University of Guelph (see the Supporting Information). GC-MS fragmentation patterns of unknowns were matched to National Institute of Standards and Technology (NIST) 2005 and/or Saturn databases supplied with the instrument. Water contents were determined by Karl Fischer titration using a Metrohm 870 KF Titritino Plus titroprocessor. pH values (relative to starting solutions) were determined by diluting the starting solutions as well as the polar and nonpolar phases (1:9) with either water (polar/aqueous phase) or methanol (nonpolar phase), respectively, and measuring the pH using a Metrohm 827 pH lab glass electrode calibrated against authentic buffer solutions (Metrohm 6.2307.110, pH 7 at 25 °C phosphate-based aqueous buffer solution). The pH values cited are thus not directly equivalent to the aqueous scale but refer to the relative acidities before and after upgrading in their respective methanol sample solutions; i.e., they are only meaningful in comparison to each other. Nuclear magnetic resonance (NMR) analysis of liquid products was performed using a NMR Bruker Cryoplatform 600 MHz. Nicolet 380 Fourier transform infrared (FTIR) spectroscopy was used for infrared (IR) analysis (CaF2 cells/plates). Production of Hemp-Seed Bio-oil. All pyrolysis experiments were carried out using a fluidized-bed pilot plant (Figure 1). The heart of the plant was an atmospheric fluid-bed reactor, 0.078 m in diameter, with a 0.52 m long cylindrical section, equipped with an expanded section made up of a 0.065 m long truncated cone with an upper diameter of 0.168 m, topped by a second, 0.124 m long, cylindrical section. The total volume of
this configuration was 6.09 10-3 m3, some of which was occupied by the sand particles of the fluidized bed. This assembly provided a nominal vapor residence time of 2 s (the nominal vapor residence time was calculated by dividing the gaseous volume of the reactor by the total nitrogen flow rate). A hot filter was installed at the gas exit. The fluidizing nitrogen was injected through a perforated distributor plate at the base of the fluidized bed, which had a static height of 0.15 m. Silica sand with a Sauter mean diameter of 200 μm was used to form the fluidized bed. The reactor was equipped with 17 thermowells for temperature measurements and control. A horizontal pulsating feeder was used to inject ground hemp into the reactor at a height of 0.1 m above the distributor plate.22 This feeder ensured an excellent and very rapid dispersion of the injected hemp seed into the fluidized bed of hot sand particles. Band heaters on the outside column wall, with a three-zone temperature controller, kept the bed and freeboard at the specified temperature. Hemp seed, when injected into the reactor, produced vapors that exited at the top of the reactor through the hot filter section and flowed into two condensers in series (one of which is shown in Figure 1). Persistent aerosols were then separated in an electrostatic demister. A more detailed description of the pilot plant and its experimental procedures was previously provided.23 The exact yield of bio-oil was obtained from the mass of bio-oil collected in the condensers and demister. The condensers and demister were weighed before and after the experiment. Pyrolysis was carried out at different temperatures and with a vapor residence time of 2 s. Each test was conducted with 300 g of hemp-seed powder. The yields of bio-oil obtained varied between 45 and 60% (w/w) per run. Fluidizing and carrier nitrogen volumetric flow rates were adjusted to keep the nominal vapor residence time constant at all temperatures. The bio-oil was stored in a flame-proof refrigerator at -4 °C. Upgrading Reactions. All reactions were carried out in a 300 mL Parr reactor (316 SS) fitted with a pressure dial, gold-coated burst disk (pmax = 5000 psi) and a vent valve. In a typical experiment, bio-oil (25 g) and RRM (5 g) were mixed in the stainless-steel autoclave. The unit was sealed, flushed, and pressurized with hydrogen gas to 800 psi at ambient temperature. Reactions were stirred magnetically using a glass-coated stir bar and brought to the reaction temperature of 350 °C (measured internally through a 316 SS thermocouple well) at a
(19) Sushil, S.; Batra, V. S. Appl. Catal., B 2008, 81, 64. (20) Dependent upon geographical location, we estimate the cost of red mud to a potential user to be equal or less to that of 1 ton of iron ore (∼U.S. $100 at time of writing); i.e., the material might be available for the cost of transportation by ship, barge, or railcar, allowing for sacrificial use. (21) http://www.eng.uwo.ca/icfar/.
(22) Berruti, F. M.; Ferrante, L.; Briens, C.; Berruti, F. Proceedings of the Green Process Engineering Conference; Venice, Italy, 2009. (23) Xu, R.; Ferrante, L.; Briens, C.; Berruti, F. J. Anal. Appl. Pyrolysis 2009, 86, 58.
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Table 1. Properties of Hemp-Seed Bio-oil before and after Upgrading pHa b
water content (mg/mL)
entry
reaction
Woil after (g)
organic phase
aqueous phase
organic phase
aqueous phase
aqueous phase (g) (color)
1 2 3
crude oil RRM catalystc control
n/a 18.5 18.8
6.4 7.0 6.8
n/a 7.4 n/a
40 130 220
n/a 760 n/a
none 5.5 (pale yellow) none
a Procedure: 1 g of reaction mixture and 9 g of water were shaken manually, then the mixture was sonicated for 15 min and stored at room temperature for 24 h; also see the Experimental Section. b Reaction conditions: bio-oil (25 g) and RRM (5 g), 350 °C, 800 psi H2(g) (cold); also see the Experimental Section. Entry 1, bio-oil properties; entry 2, upgraded with RRM; entry 3, control reaction without catalyst. c Weight of re-isolated RRM after reaction, 4.6-4.8 g.
Figure 2. Physical appearance of solvent extracts of hemp-seed bio-oil before and after upgrading.
heating rate of 3 °C/min using an electric band heater. After 5 h, a gas sample was taken from the headspace of the reactor and analyzed by GC-MS. The reactor was cooled to ambient temperature and opened, and its contents (products þ catalyst) were removed and weighed. From this mixture, bio-oil samples for GC, GC-MS, IR, and NMR were prepared as described below without catalyst separation, because this would influence the product composition. To establish the fate of the catalyst, identical reactions were carried out, in which the bio-oil products were ignored and the catalyst solids were recovered by suspending the reaction mixture in MeOH, filtration, washing with more MeOH, and drying the remaining solids in an oven at 120 °C overnight. The recovered catalyst weight was 4.6-4.8 g; i.e., 92-96% of the catalyst mass remains, with the mass loss being attributed to the loss of chemically bound water and imperfect recovery of solids during workup. The catalyst recovered by this procedure maintains its catalytic activity in a repeat reaction. Solvent Extraction. A total of 200 mg of oil decanted from the product/catalyst mixture was dissolved in 3 mL of solvent of the polarity series (water, methanol, acetone, acetonitrile, ethyl acetate, diethyl ether, and n-hexane) and sonicated for 3 min. The clear part of each sample was separated using a glass wool column and subjected to analysis as described below. NMR Sample Preparation. The extraction solvents used in Solvent Extraction were removed in vacuo, and 100 mg of the
remaining bio-oil extract were dissolved in 1 mL of deuterated chloroform for 1H and 13C NMR analyses (400 or 600 MHz). GC Sample Preparation. A total of 1 mL of methanol and 0.5 mL of each filtered solution produced by the method described in Solvent Extraction were combined in 1.5 mL GC vials. GC traces were obtained as described above using a 30 m DB-1701 column. GC-MS Sample Preparation. Samples of 200 mg of oil decanted from the product/catalyst mixture were extracted with 3 mL of solvent of the polarity series (water, methanol, acetone, acetonitrile, ethyl acetate, diethyether, and n-hexane), filtered by a glass wool column. The solvent was removed in vacuo, and the samples used for GC-MS analysis are as described in GC Sample Preparation). Chemical Class Composition Identification Using Activated Silica Gel24. A total of 6 g of native bio-oil or product/catalyst mixture was dissolved/suspended in 50 mL of n-pentane. The mixture was stirred magnetically using a stir bar for 5 min and then separated into two fractions as n-pentane-soluble and -insoluble compounds (asphaltanes) by decantation. The n-pentane-insoluble part was weighed after complete removal of n-pentane in vacuo. The weight of this material is reported as asphaltane and, for the upgraded bio-oil, includes ∼1.0 g of recovered catalyst (6 g/30 g 5 g = 1.0 g), i.e., overestimates the amount of insoluble organic material. (24) Acikgoz, C.; Kockar, O. M. J. Anal. Appl. Pyrolysis 2009, 85, 151.
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Table 2. Different Regions by the Proton Integration Percentage of Deuterated Chloroform 1H NMR Spectra for Different Solvent Extractions of Fresh Untreated and RRM-Upgraded Bio-oil after Removing the Extraction Solvents parafinic (0-3 ppm) (%)
CHR(OH) (3-5.5 ppm) (%)
olefinic/aromatic (5.5-8 ppm) (%)
aldehyde/carboxylic acid (8-12 ppm) (%)
extraction solvent
untreated
RRM upgraded
untreated
RRM upgraded
untreated
RRM upgraded
untreated
RRM upgraded
methanol acetone acetonitrile ethyl acetate diethyl ether
88.15 91.30 87.34 90.79 88.39
95.60 94.02 91.96 92.59 94.05
5.98 5.10 8.73 5.80 6.92
0.31 1.28 1.48 2.78 1.93
5.77 0.92 2.00 1.80 2.76
4.05 4.70 6.45 4.63 4.03
0.11 2.68 1.93 1.61 1.93
0.08 0.00 0.11 0.00 0.00
Figure 3. 1H NMR (CDCl3) spectra of methanol extraction for (top) untreated and (bottom) upgraded bio-oil.
n-Pentane was also removed in vacuo from the soluble fractions, and the residue was separated on activated silica gel (70-230 mesh, pretreated at 110 °C for 2 h) using a 25 1 cm inner diameter column. The column was eluted successively with 200 mL of n-pentane, 200 mL of toluene, and 200 mL of methanol to produce aliphatic (nonpolar), aromatic, and polar fractions, respectively. The eluting solvent of each fraction was removed in vacuo, and the fractions were weighed and subjected to other characterizations and analyses by IR, NMR, and GC. Phase Separation of the Oil Using Aqueous Salt Solution25. A total of 6 g of a 30% (w/w) aqueous solution of K2CO3 or CaCl2
was added to 2 g of crude bio-oil or product/catalyst mixture. The mixtures were shaken vigorously for 15 min and then sonicated for 30 min. The resulting solutions were sealed and stored at room temperature for 24 h, resulting in phase separation. The top phase consists of an aqueous salt solution extract of the bio-oil or catalyst/bio-oil mixture. The bottom phase consists of the heavy organic components plus recovered catalyst. The two phases were separated by decantation and weighed. The top phase was extracted with 3 40 mL of diethyl ether. The top ether layer from this extraction contains the heavy oxygenated components. It was dried over Na2SO4, evaporated in vacuo at T < 30 °C, and weighed. The bottom phase still contains the light oxygenated components, the weight of which is indirectly determined by subtracting the directly
(25) Song, Q.-H.; Nie, J.-Q.; Ren, M.-G.; Guo, Q.-X. Energy Fuels 2009, 23, 3307.
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Figure 4. 13C J-MOD NMR (CDCl3) spectra of methanol extraction for (top) untreated and (bottom) upgraded bio-oil (quartenary and CH2, up; CH and CH3, down).
considered as tar (insoluble in methylene chloride), which was dried in vacuo at T < 40 °C and weighed. The filtrate was dried over Na2SO4, filtered, and evaporated in vacuo at 30 days at room temperature. Upgrading reactions were carried out at 350 °C (heating rate, 3 °C/min; total reaction time, 5 h) and 800 psi H2(g) pressure (cold), i.e., the same conditions as established by our previous study on red mud as a catalyst for the conversion of 6594
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Figure 6. IR spectra of (top) untreated and (bottom) organic phase of upgraded bio-oil (neat film, CaF2).
HCOOH/HOAc. In these reactions, the red mud is reduced to a gray magnetic material shown by powder X-ray diffraction (XRD) to contain Fe3O4 and Fe metal.5 The same behavior is observed with the hemp oil rather than HCOOH/HOAc as the substrate, but initial qualitative screening of the phase-separated product mixtures of some test reactions established that using gray RRM obtained by the reaction with HCOOH/HOAc yielded almost neutral (Table 1) as opposed to still strongly alkaline product phases (pH > 9.5) obtained with native red mud. Given the limited amount of bio-oil available to us at the time (∼1.5 L total for the whole study) and anticipated problems with strongly alkaline product phases during the complex analysis sequences carried out on the products (see below), we therefore limited this study to the RRM. Once reduced to RRM, the catalyst does not change its appearance and can be reused without the loss of activity; i.e., recycled RRM maintains its magnetism and qualitatively yields the same phase-separated
Figure 7. IR spectrum of the aqueous phase formed during upgrading bio-oil (neat film, CaF2 cell).
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Figure 8. 1H NMR (DMSO-d6) spectrum of the aqueous phase formed during upgrading of bio-oil. Table 4. Chemical Class Composition Identification of Untreated and Upgraded Bio-oil by Chromatographic Fractionation on an Activated Silica-Gel Column Using n-Pentane, Toluene, and Methanol as Consecutive Eluents
integration data reveal a substantial decrease in the relative amounts of alcohols, aldehydes, and acids (i.e., polar and reactive components) and increase in the relative amounts of alkanes, alkenes, and aromatics (i.e., nonpolar components). As an example, the 1H and 13C NMR spectra of the methanol extractions in CDCl3 are shown in Figures 3 and 4. In the MeOH extracts of the upgraded oil, almost no peaks associated with the functional groups R-CH(OH)-R0 , R-(CdO)-R0 , R-CHO, and R-COOH are present, indicating that they have been converted into nonpolar MeOH-insoluble products, i.e., alkanes and alkenes. Qualitative GC analysis of the various extracts mirrors the results of the NMR study, with the GC traces of extracts obtained from the upgraded oil and showing a substantially larger number of peaks, i.e., volatile components. As an example, Figure 5 compares the GC traces of the hexane extracts of the crude and upgraded oil using quantitatively identical injection conditions (auto-sampler robot) and flame ionization detector (FID) response scales (0-12.5 mV). To identify and compare the components of crude and upgraded oil, hexane and acetone extractions were analyzed by GC-MS. The identified components and their retention times are listed in Table 3, with the upgraded oil again showing fewer oxygenated compounds.31 Both the crude and upgraded oils show a large number of long alkyl chain molecules, free fatty acids, or their esters, which are derived from the high triglyceride content of the feedstock. GC-MS analysis of a gas sample obtained from the headspace of the pressure reactor also shows the presence of C3-C5 alkanes/alkenes, as well as CO2 generated from the ketonization or direct decarboxylation of carboxylic acids present. The maximum pressure observed during the RRM-catalyzed reaction was 2200 psi, while the control reaction in the absence of RRM resulted in the lower maximum pressure of 1800 psi (entry 3 in Table 1). The GC-MS of a gas-phase sample of the control was essentially identical to that of the RRM reaction, but as stated above, no aqueous phase was generated and the product was an intractable resin that was therefore not further analyzed. Figure 6 compares the IR spectra (neat, CaF2 cell) of the crude oil to that of the oily organic phase obtained from the upgrade reaction (entry 2 in Table 1). The upgraded fraction
weight fraction analysis
bio-oil untreated RRM upgraded
asphaltane (%)
nonpolar hydrocarbons (%)
50.0 11.7
11.7 53.3
polar hydrocarbons aromatics (%) (%) 21.7 10.0
16.6 25.0
product distributions by the extractions and analyses described below. The magnetism of the reduced catalyst may also aid in its cost-effective recovery and separation from the product mixture, if a larger scale process would ever be implemented. Table 1 compares the pH and water content by Karl Fischer titration of the crude oil, an upgraded sample, and a control reaction without catalyst. The upgraded oil separates into an oily organic phase and an aqueous phase, both of which had a higher pH than the crude oil. The control reaction did not generate an aqueous phase but gave an intractable resin, which showed a substantial increase in water content caused by thermally induced condensation reactions. The following sections describe more detailed comparative analyses of crude versus upgraded biooil by a variety of techniques. Comparative Analysis of Crude versus Upgraded Bio-oil by NMR, GC, GC-MS, and IR. The crude and upgraded biooil were extracted with a series of solvents of increasing polarity, comprising hexane, diethyl ether, ethyl acetate, acetone, acetonitrile, methanol, and water. The physical appearance and color comparison of these solvent extractions is shown in Figure 2. To determine the effect of upgrading on the type and relative concentration of functional groups in the bio-oil, the extraction solvents, diethyl ether, ethyl acetate, acetone, acetonitrile, and methanol, were removed in vacuo and the residue was dissolved in CDCl3 and analyzed by NMR. The relative percentages of the 1H NMR proton integrations of different peak regions, i.e., paraffinic (0-3 ppm), CH-O (3-5.5 ppm), olefinic/ aromatic (5.5-8 ppm), and aldehyde/carboxylic acid (8-12 ppm), of bio-oil before and after upgrading for different solvent extractions are listed in Table 2. In all cases, the
(31) See the Supporting Information for actual GC-MS spectra and detailed analysis conditions.
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Table 5. Summary of Results of Aqueous Salt Extraction of Untreated and Upgraded Bio-oil salt
untreated
upper layer (heavy and light oxygenated) (%) 48.5 K2CO3 (aqueous, heavy oxygenated light 30 wt %) (%) oxygenated (%) 35.5 13 upper layer (heavy and light oxygenated) (%) K2CO3 (aqueous, 60 30 wt %) heavy oxygenated light (after 60 days) (%) oxygenated (%) 55 5 upper layer (heavy and light oxygenated) (%) 55 CaCl2 (aqueous, heavy oxygenated light 30 wt %) (%) oxygenated (%) 43 12
upgraded
bottom layer (tar and hydrocarbons) (%) 51.5 tar (%) hydrocarbons (%) 30 21.5 bottom layer (tar and hydrocarbons) (%) 40 tar (%) hydrocarbons (%) 40 0 bottom layer (tar and hydrocarbon) (%) 45 tar (%) hydrocarbons (%) 33 12
upper layer (heavy and light oxygenated) (%) 4.9 heavy oxygenated light (%) oxygenated (%) 3 1.9 upper layer (heavy and light oxygenated) (%) 5.1 heavy oxygenated light (%) oxygenated (%) 3.4 1.7 upper layer (heavy and light oxygenated) (%) 6 heavy oxygenated light (%) oxygenated (%) 4.2 1.8
bottom layer (tar and hydrocarbons) (%) 95.1 tar (%) hydrocarbons (%) 14.0 81.1 bottom layer (tar and hydrocarbons) (%) 94.9 tar (%) hydrocarbons (%) 14.0 80.9 bottom layer (tar and hydrocarbon) (%) 94 tar (%) hydrocarbons (%) 17 77
Figure 9. Effect of RRM-catalyzed upgrading of bio-oil on the amount of polar and nonpolar hydrocarbons, aromatics, and asphaltane components of the oil, as determined by chemical class separation through chromatographic fractionation on an activated silica-gel column using n-pentane, toluene, and methanol as consecutive eluents.
does not show the large νOH and νCO peaks present in the crude oil but well-defined νC-H and νCdC bands, again indicating that any alcohol, aldehyde, ketone, and carboxylic functions had been converted to alkanes, alkenes, and aromatic compounds. Figure 7 shows the IR of the corresponding aqueous phase, and Figure 8 shows the corresponding 1 H NMR spectrum. Both spectra are dominated by water (∼75% by KF titration) and show only small amounts of unidentified organics. Comparative Stability Analysis of Crude versus Upgraded Bio-oil by Solvent Extraction and Salt Solution Partition. An important criterion for the effectiveness of the bio-oilupgrading reaction is the relative stability of the organic product fraction against further condensation and resin formation and the total amount of insoluble asphaltanes/tar present in the oil.
To determine more quantitatively the effect of the upgrading reaction on these two parameters, starting 30 days after receipt of the bio-oil, we carried out a larger scale solvent extraction study following the protocol developed by Acikgoz and Kockar,24 which separates the oil into the four different chemical classes, asphaltane, nonpolar hydrocarbon, polar hydrocarbon, and aromatics, according to their relative (in)solubility in n-pentane, toluene, and MeOH and a salt solution partition into hydrocarbons, heavy and light oxygenates, and tar (equivalent to asphaltanes), followed by solvent extraction of the phases obtained by extraction with diethyl ether and methylene chloride following the protocol developed by Song and co-workers.25 In both cases, the relative amounts of the fractions obtained were determined gravimetrically, as described in detail in the Experimental Section. 6597
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Figure 10. Evaluation of the relative stability of crude and upgraded bio-oil fractions obtained by chemical class separation through chromatographic fractionation on an activated silica-gel column by measuring the relative amounts of tar and polar and less polar hydrocarbons at 30, 60, and 90 days: (left) untreated and (right) upgraded.
Figure 11. Effect of RRM-catalyzed upgrading of bio-oil on the amount of polar and nonpolar hydrocarbons, aromatics, and asphaltane components of the oil, as determined by chemical class separation by salt solution extraction (values shown are the averages of the CaCl2 and K2CO3 extractions shown in Table 5).
Tables 4 and 5 give the results of these studies in numerical format. The fractions obtained by either protocol consistently show substantially reduced amounts of asphaltanes/tar and oxygenated components and a substantially increased amount of nonpolar hydrocarbons and aromatics (Figures 9 and 11). To compare the relative stability of the upgraded versus crude, bio-oil samples of both aged for 30, 60, and 90 days were reanalyzed by the solvent extraction protocol. Figure 10 shows that the fraction distribution in the upgraded oil remains essentially unchanged, while the amount of insoluble asphaltane increases to ∼90% (w/w) in the crude oil, which is accompanied by a visually apparent increasing resinification of the material. A similar trend is observed when reanalyzing the oil after 30 and 60 days using the salt solution partition protocol (Figure 12). Notably, with both methods, the amount of recoverable nonpolar hydrocarbons drops to ∼0% (w/w) after 60 and 90 days, respectively.32
Comparative Analysis of Crude versus Upgraded Bio-oil by TGA. An ideal upgrading result would be the conversion of bio-oil into a fully distillable fuel meeting an American Society for Testing and Materials (ASTM) or similar standard. This can most easily be evaluated by carrying out a TGA in comparison to a fuel that meets this criterion. We therefore subjected the crude and upgraded oils to TGA and compared them to a commercial diesel oil sample that meets the CAN/CGSB-3.517 automotive low-sulfur diesel fuel standard. Figures 13 and 14 and Table 6 give the results of the TGA experiments. The TGA curve can be divided into three regions according to temperature. The first region is vaporization of the light components. As can be seen in Figure 13, at temperatures lower than 300 °C, most of the light components of the fuels vaporize. The upgraded fuel has more volatiles because its TGA curve is lower until 250 °C. Between 200 and 300 °C, the crude oil shows a peak in the weight derivative shown in Figure 14; i.e., the crude oil seems to have more components boiling off, as shown in weight derivative, which we attribute to a thermally induced formation of water through condensation reactions, which
(32) The 60 day result for the salt partition protocol promoted us not to attempt an extraction after 90 days.
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Figure 12. Evaluation of the relative stability of crude and upgraded bio-oil fractions obtained by chemical class separation by salt solution extraction (K2CO3; see Table 5) by measuring the relative amounts of tar and polar and less polar hydrocarbons at 30 and 60 days: (left) untreated and (right) upgraded.
Figure 13. TGA curve for two bio-oil samples and diesel.
Figure 14. Derivative of the TGA curve (DTGA) for two bio-oil samples and diesel.
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Conclusion and Outlook
Table 6. TGA Residue Weight for All Cases sample identificationa
residue after 600 °C (%)
initial weight of the fuel drop (mg)
untreated (I) untreated (II) upgraded (I) upgraded (II)
6.51 8.13 12.32 11.35
20.5 10.67 12.12 24.44
a
RRM, obtained by the reaction of mining waste red mud with HOAc/HCOOH, is a nontoxic and extremely cheap catalyst for the upgrading of pyrolysis bio-oils that, with an external supply of reducing equivalents in the form of hydrogen gas, achieves a substantial reduction of the concentration of oxygen-based functional groups in the oil by generating water and CO2 as the oxygen-carrying byproducts, effectively lowering the overall oxygen content of the oil and rendering it stable against resin formation. Future studies will focus on the use of red-mud-based catalysts on more typical and highly acidic bio-oils, as obtained from more common agri- and silvi-cultural byproducts, such as wheat straw, corn stover, and wood or bark chips. With a higher HCOOH content, these feedstocks should allow for an effective direct use of red mud with the in situ generation of hydrogen and the gray RRM employed in this study.5 A further intriguing possibility under consideration by the authors is the use of red mud as the bed material in a fluidized-bed reactor used to produce the oil, which may effectively lead to a simultaneous bio-oil production and upgrading process directly yielding a low acid and low oxygen content bio-oil. The low cost and high availability of the catalyst also suggests that a co-processing of biomass/red mud with a continuous feed-through of both the carbon- and metal-oxide-based feedstocks may be viable, leading to a simultaneous value addition to both the biomass and mining wastes used; i.e., in addition to the upgraded bio-oil, large amounts of a non-alkaline magnetic material similar to the RRM employed here could be obtained. Because of its higher iron content by weight, it may find direct use as an iron ore or become a component of aluminoferrite belite cement.33
Corresponds to the sample identification in Figure 13.
are no longer possible in the upgraded fuel. A second smaller peak in the weight derivative of the original fuel occurs between 300 and 400 °C and at 400-500 °C for the upgraded fuel. This change of behavior could be attributed to the polymerization of alkenes present in the upgraded fuel versus pyrolysis of heavy fractions in the untreated fuel in the range of 300-500 °C, as previously observed by others.27,28 Discussion The sum of the analytical results obtained suggests that, at 350 °C and 800 psi hydrogen pressure, RRM is in fact acting as an effective catalyst for the upgrading of the hemp-seed pyrolysis bio-oil by a set of deoxygenation reactions that are consistent with or at least comprise those laid out in Scheme 2. Phase separation of an aqueous phase [∼75% (w/w) water and 25% (w/w) oxygenated organics by Karl Fischer titration] and generation of CO2 lead to substantial mass loss (∼30%, w/w), yielding ∼70% (w/w) of an organic product phase that is primarily composed of non-oxygenated, more volatile and less reactive hydrocarbons that, as a consequence of the nature of the feedstock used, appear primarily derived from longchain fatty acids. The stability of the upgraded oil against resin formation also points to a much reduced concentration of reactive oxygen-based functional groups, allowing for its storage for extended periods of time. While not evaluated here (because of the limited amount of material available), both of these factors should result in a higher energy density and better burn characteristics of the oil. The TGA shows that the upgrading did not result in a highly desirable fully distillable fuel but did increase the concentration of the lighter components of the fuel. However, it also increased polymerization and, therefore, the amount of solid residue formed at 600 °C. On the basis of the 1H NMR and GC/GC-MS results discussed above (Tables 2 and 3), this is caused by a higher concentration of polymerizable alkene components in the upgraded oil, which, because of their fatty-acid-derived long chains, are relatively difficult to completely hydrogenate on the heterogeneous iron catalyst.
Acknowledgment. The authors thank Drs. Guy Forte and Guy Peloquin, Rio Tinto Alcan (Jonquiere, Quebec Operation), for supplying an authentic operational process sample of red mud, Dr. Dyanne Brewer of the Advanced Analysis Centre (AAC) at the University of Guelph for assistance with the mass spectrometric analysis, and the Agricultural Biorefinery Innovation Network (ABIN) as funded by Agriculture and Food Canada as well as FPInnovations Canada for supporting this research. Supporting Information Available: Detailed analysis of the composition of red mud by X-ray fluorescence (XRF; Rio Tinto Alcan) and details of the GC-MS analysis of the compounds listed in Table 3. This material is available free of charge via the Internet at http://pubs.acs.org. (33) Vangelatos, I.; Pontikes, Y.; Angelopoulos, G. N. Proceedings of the International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress; Eskis-ehir, Turkey, 2009.
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