Energy Fuels 2010, 24, 2747–2757 Published on Web 03/29/2010
: DOI:10.1021/ef1000375
Thermal Decomposition of Acetic and Formic Acid Catalyzed by Red Mud;Implications for the Potential Use of Red Mud as a Pyrolysis Bio-Oil Upgrading Catalyst§ Elham Karimi,† Ariel Gomez,‡ Stefan W. Kycia,‡ and Marcel Schlaf*,† † Department of Chemistry, The Guelph-Waterloo Centre for Graduate Work in Chemistry (GWC)2, University of Guelph, Guelph, Ontario, N1E 4 V5 Canada, and ‡Department of Physics, The Guelph-Waterloo Physics Institute (GWPI), University of Guelph, Guelph, Ontario, N1E 4 V5 Canada
Received January 12, 2010. Revised Manuscript Received March 3, 2010
Acetic and formic acid impart a high acidity on pyrolysis bio-oil (obtained by fast pyrolysis of lignocellulosic biomass), which is one of the factors preventing its direct use as a fuel. At temperatures g 330 °C, Red Mud, a waste byproduct of the aluminum industry produced at >70 million tons p.a., is a good catalyst for thermal decomposition of these acids. Formic acid can serve as an internal source of hydrogen through the formation of synthesis gas and the water gas shift reaction. The formation of C6-C10 hydrocarbons in the nonpolar phase of the resulting product mixture and the identification of C3 and C4 hydrocarbons and CO2 in the gas phase and acetone in the polar liquid phases can be rationalized through mechanisms involving ketene as the intermediate formed by acetic acid dehydration, with subsequent formation of acetone. Higher hydrocarbons, mostly alkanes and alkenes, are then formed through iterative aldol condensation, hydrogenation, hydrogenolysis, and deoxygenation reactions of the primary products. During the reaction, the Red Mud used in these reactions undergoes a distinct color change to gray, yielding a nonalkaline magnetic material containing Fe3O4 and metallic iron rather than Fe2O3.
the water, oxygen, and acid content of the oil, while;by necessity;increasing its energy density as a fuel.6,7 A chemically logical approach to improve the characteristics of bio-oil, therefore, is to either directly convert the carboxylic acids into nonacidic species or to hydrogenate the carbonyl functions of the aldehydes and ketones present into their corresponding alcohols, which can then react in situ with the carboxylic acids, phenols, and catechols present, yielding the corresponding esters and aryl ethers as well as water as the necessary byproduct. In particular, the decomposition of the major components HOAc and HCOOH into nonacidic components should substantially improve the chemical and physical characteristics of the bio-oil. The anticipated net effects of an internal (i.e., using syn or hydrogen gas generated in situ) or external (i.e., using an additional feed of hydrogen gas) hydrogenation/hydrogenolysis upgrade are a lower oxygen content, lower acidity, lower viscosity (due to lower polarity or even reductive cleavage of the constituents to lower molecular weight compounds), and higher water content. In a best case scenario, the water will however spontaneously phase-separate from the lower polarity organics resulting from the hydrogenation reactions. With a sufficient reduction in oxygen content and acidity, the organic phase may become directly usable as an oxygenated clean-burning engine fuel. This approach is presently under intense investigation using heterogeneous hydrogenation/hydrogenolysis catalysts commonly employed by the petrochemical industry, e.g., CuO 3 CrO3, Ni, Pd, Pt, or Ru on various supports and promoters such as SiO2, Al2O3, ZnO, TiO2, or zeolites.8 It has however met with significant challenges due to the high
Introduction and Motivation Pyrolysis bio-oil as obtained by the fast pyrolysis of lignocellulosic biomass is characterized by a high oxygen (∼4050 w/w %) and moisture (∼15-30 w/w %) content and the presence of acetic acid (HOAc, 5-25 w/w %), formic acid (HCOOH, 3-10 w/w %) and other carboxylic acids, ketones, aldehydes, (poly-)alcohols, and esters as well as phenols and catechols originating from the lignin component of the biomass used.1-5 The resulting high polarity, corrosiveness, viscosity, and chemical instability of the bio-oil (a direct consequence of acid catalyzed condensation reactions leading to resin formation and phase separation) makes its direct use as fuel in boilers difficult, requiring specifically designed corrosion-resistant apparatuses, often with coinjection of methanol or ethanol. The situation is even more challenging for the highly desirable use of bio-oil as a fuel for turbines or piston engines, which demand a much higher fuel quality and are highly susceptible to corrosion-induced failure of their fuel injection systems and moving parts. For either application, the use of bio-oil as a fuel will therefore require a catalytic upgrading process aimed at simultaneously reducing §
Dedicated to Prof. Ulf Schuchardt on the occasion of his retirement. *To whom correspondence should be addressed. Fax: 519-766-1499. E-mail:
[email protected]. (1) Mohan, D.; Pittman, C. U.; Steele, P. H. Energy Fuels 2006, 20, 848. (2) Garcia-Perez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Roy, C. Biomass Bioenergy 2007, 31, 222. (3) Mullen, C. A.; Boateng, A. A. Energy Fuels 2008, 22, 2104. (4) Gayubo, A. G.; Valle, B.; Aguayo, A. T.; Olazar, M.; Bilbao, J. Energy Fuels 2009, 23, 4129. (5) Lohitharn, N.; Shanks, B. H. Catal. Commun. 2009, 11, 96. (6) Fisk, C. A.; Morgan, T.; Ji, Y. Y.; Crocker, M.; Crofcheck, C.; Lewis, S. A. Appl. Catal., A 2009, 358, 150. r 2010 American Chemical Society
(7) Zhao, C.; Kou, Y.; Lemonidou, A. A.; Li, X. B.; Lercher, J. A. Angew.Chem. Int. Ed. 2009, 48, 3987. (8) Personal communication, Lisa Myers, Director R&D Biofuels, and Kristi Fjare, ConocoPhillips Company , 2007.
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Table 1. Typical Elemental and Phase Composition of Dried Red Mud by X-Ray Fluorescence Spectroscopy (XRF) oxide
relative amount [w/w %]
phase present
Fe2O3
30-40
Al2O3
15-25
SiO2
10-20
TiO2 CaO Na2O H2O
3-8 1-6 5-10 10
Fe2O3 [Hematite] FeO(OH) [Goethite] Al2O3 3 3H2O [Gibbsite] Al2O3 3 H2O [Boehmite] SiO2 [Quartz] NaaAlbSicOd [Bayer sodalite] (sodium aluminum silicate) TiO2 [Rutile, Anatase] CaaAlbSicOd (calcium aluminum silicate) NaOH chemically bound water
comprehensive review by Batra and Sushil.14 The same authors also recently demonstrated the use of Red Mud as a catalyst for the cracking of methane to hydrogen.15 As stated above, two major (and arguably most troublesome) components of pyrolysis bio-oil are HOAc and HCOOH.16,17 Previous studies have established that, at T > 350 °C, R-Fe2O3 is an effective catalyst for the hydrogenation of HOAc to acetaldehyde18,19 and that both Fe3O4 and TiO2 catalyze the dehydration and dehydrogenation of HCOOH to CO þ H2O and CO2 þ H2, respectively (eqs 1 and 2), with the latter being potentially more active.20,21 A further alternative pathway for the decomposition of HOAc present in bio-oil would be its thermally induced ketonization to acetone, CO2, and water,22-24 followed by secondary aldol condensation, hydrogenation, and dehydration reactions ultimately leading to alkanes under reducing conditions, i.e., a hydrogen atmosphere (eq 3).
acidity and polarity of the bio-oil substrate, which can lead to rapid catalyst coking, deactivation, and fouling or even complete destruction due to the dissolution of the active metal/metal oxide or the catalyst support by the aqueous acid present.9 This would also lead to the presence of trace amounts of the typically toxic active heavy metal used as the catalyst in the resulting products, substantially complicating or even preventing its use as a fuel without a secondary distillation. Both these problems could potentially be overcome, if a cheap, expendable/sacrificial catalyst material could be found that contains no or only trace amounts of toxic heavy metals. This latter requirement limits the choice to those metals in the periodic table, which in either their most stable oxidation state (þI to þIV) or in the form of their oxides can be considered to be nontoxic and therefore could be released into the biosphere without environmental concerns. These cations/oxides are Naþ, Kþ, and Ca2þ (as chlorides, carbonates, or hydroxides); TiO2 (the standard white pigment and masking agent in most paints and coatings), SiO2, Fe2O3, and Fe3O4; Al2O3; and possibly ZnO/ZnCO3. With the exception of Kþ and ZnO/ ZnCO3, this list of metal oxides is essentially the composition of Red Mud,10 which is generated on a very large scale of >70 million tons/year as the waste byproduct of the Bayer process for the refinement of bauxite into pure Al2O3. A secondary and potentially beneficial effect of using Red Mud as the catalyst for bio-oil upgrading is that any Fe3þ leached into the upgraded product may in fact act as a combustion catalyst.11,12 Table 1 gives a typical elemental and phase composition of Red Mud,13 which is typically available as a 50% (w/w) slurry of pH 14 and mean particle size of ∼5 μm (70% < 10 μm). The large volume of Red Mud produced (each ton of Al2O3 produced requires ∼2-3 tons of bauxite ore, generating 1-2 tons of Red Mud as the byproduct) makes the material extremely cheap (, $100 U.S./ton), as its alkalinity and CaO, SiO2, and TiO2 contents presently render its use as iron ore uneconomical. Red Mud is therefore disposed of in landfills, and while the nontoxic nature of Red Mud does not classify it as hazardous waste, the identification of alternative uses of Red Mud constitute at the same time an environmental challenge and an economic opportunity. This has long been recognized, and a summary of the known applications of Red Mud as a heterogeneous catalyst is given in a recent
HCOOH f H2 O þ CO ΔG° ¼ -30:1 kJ=mol HCOOH f H2 þ CO2 ΔG° ¼ -58:6 kJ=mol
ð1Þ ð2Þ
17
nHOOCCH3 þ mH2 f oCH3 CðOÞCH3 þ pCH3 CH2 OOCCH3 þ qCH3 CHOH þ rCn H2n þ sCn H2n þ 2 þ :::
ð3Þ
Considering the known activity of iron catalysts for both Fischer-Tropsch chemistry and the water gas shift reaction (WGSR) and the potential role of NaO, SiO2, and Al2O3 as promoters for these reactions, we hypothesized that Red Mud may be a viable catalyst for the decomposition of HOAc as well as mixtures of HOAc and HCOOH as present in pyrolysis bio-oil to nonacidic products with HCOOH acting as an internal hydrogen source. Scheme 1 summarizes a conceivable Red Mud catalyzed reaction cascade leading from the acids to alcohols, alkenes, alkanes, esters, and (when considering the
(15) Balakrishnan, M.; Batra, V. S.; Hargreaves, J. S. J.; Monaghan, A.; Pulford, I. D.; Rico, J. L.; Sushil, S. Green Chem. 2009, 11, 42. (16) Wagner, F. S. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 2001. (17) Drury, D. J. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 2001. (18) Grootendorst, E. J.; Pestman, R.; Koster, R. M.; Ponec, V. J. Catal. 1994, 148, 261. (19) Pestman, R.; Koster, R. M.; Boellaard, E.; van der Kraan, A. M.; Ponec, V. J. Catal. 1998, 174, 142. (20) Trillo, J. M.; Munuera, G.; Criado, J. M. Cat. Rev., Sci. Eng. 1972, 7, 51. (21) Borowiak, M. A.; Jamroz, M. H.; Larsson, R. J. Mol. Catal. A 1999, 139, 97. (22) Deng, L.; Fu, Y.; Guo, Q. X. Energy Fuels 2009, 23, 564. (23) Das, J.; Parida, K. React. Kinet. Catal. Lett. 2000, 69, 223. (24) Glinski, M.; Kijenski, J.; Jakubowski, A. Appl. Catal., A 1995, 128, 209.
(9) Elliott, D. C.; Hart, T. R. Energy Fuels 2008, 23, 631. (10) See http://www.redmud.org/home.html for an excellent introduction to this material (accessed Mar 2010). (11) Farrar, D. H. Velino Ventures Inc.: Thornhill, CA, 1990, US 4908045. (12) Farrar, D. H. Velino Ventures Inc.: Thornhill, CA, 1990, EP 359390. (13) As determined by X-ray Fluorescence Spectroscopy at Rio Tinto Alcan (Jonquiere, QC, Canada). (14) Sushil, S.; Batra, V. S. Appl. Catal., B 2008, 81, 64.
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Scheme 1. Conceivable Reaction Cascade for the Upgrading of Pyrolysis Bio-Oil through Catalyzed Decomposition of HOAc and HCOOH Followed by Secondary Condensation and Hydrogenation Reactions
lignin-derived phenol and catechol fraction of bio-oil) aryl ethers. As a consequence, we are here presenting an initial study on the use of Red Mud as a decomposition catalyst for HOAc and mixtures of HOAc and HCOOH as a simple model system for bio-oil.
methanol (nonpolar phase), respectively, and measuring the pH using a Metrohm 827 pH lab glass electrode calibrated against authentic buffer solutions. At conversions > 90%, the remaining HOAc content was determined by quantitative GC calibrated at 100, 500, and 1000 mmol/L against dimethyl sulfone (100 mmol/L) as an internal standard (ISTD). To avoid any reactivity of the ISTD under the harsh reaction conditions applied, it was added to samples of the polar/nonpolar phases of known volume and weight after the reaction in the same concentration. The X-ray powder diffraction data were collected in a STOE two circle goniometer using the Cu KR radiation (λ=1.54178 A˚) produced by an ENRAF NONIUS FR571 rotating anode X-ray generator. The pattern was measured in the interval from 5 to 60° in 2θ using a 0.02 step size and 40 s of counting time. Decomposition Reactions. Red Mud supplied as a slurry by Rio Tinto Alcan and containing ∼50% w/w iron oxide was dried to a constant weight at 120 °C.25 Washed Red Mud was prepared by extracting the material three times with water (200 g of Red Mud in 300 mL of water) through mechanical agitation (stirring for 2 h each) and then filtering and drying the material as above. This lowered the pH of the resulting mother liquor from an initial 14 to 11.3.26 a. Typical Procedure for the Hydrogen-Pressurized Decomposition of HOAc Acid Using Red Mud As the Catalyst. HOAc (glacial, 60.01 mL, 1049 mmoL) and Red Mud (10 g ≈ 5 g Fe2O3=31 mmol,=3 mol % to acetic acid) were mixed in the stainless steel autoclave. The unit was sealed, flushed, and pressurized with hydrogen gas to 800 psi at ambient temperature and then heated with stirring. The reaction mixture was worked up and analyzed as described above. b. Typical Procedure for the Decomposition of HOAc/ HCOOH Acid Mixtures with Similar Ratios to Actual Bio-Oil Using Red Mud As the Catalyst under Unpressurized Sealed
Experimental Section General. All decomposition reactions were carried out in a 300 mL Parr reactor (316 SS) fitted with a pressure dial, goldcoated burst disk (pmax=5000 psi), and a vent valve. Reactions were stirred magnetically using a glass-coated stir bar and brought to the reaction temperature of 350 ( 2 °C (measured internally through a 316 SS thermocouple well) at a heating rate of 3 °C/min using an electric band heater. At the end of the reaction time (4-8 h), the autoclave was cooled to room temperature. GC and GC-MS analysis were performed on a Varian 3800 GC and/or Varian Saturn 2000 GC-MS running in default EI mode (4 V axial bias, 1400 V multiplier) using a 30 m DB-1701 column operating at a constant flow rate of 1 mL/min. MS fragmentation patterns of unknowns were matched to that of either authentic compounds injected under identical analysis conditions or the NIST 98 and/or Saturn databases supplied with the instrument. Gas samples were withdrawn via the vent valve for qualitative GC-MS analysis of the reaction gas headspace; the autoclave was then opened, and its contents were carefully removed and weighed. Solid and liquid phases were separated by filtration. The solid catalyst residue was extracted with methanol, dried overnight at 383 K, and weighed back. Aqueous and organic liquid phases were separated by decantation. The composition of liquid products was analyzed by GC and GC-MS of the organic (nonpolar) and aqueous (polar) phases of products. 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 phase) or
(25) A full analysis sheet as supplied by Rio Tinto Alcan is given in the Supporting Information. (26) An 10-time extraction of 10 g of red mud with 100 mL of water for 15 min each gave a pH of ∼9.
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Conditions. HOAc and HCOOH with a ratio ∼3:1 (by weight) was used as the bio-oil model. HOAc (glacial, 42 mL, 734 mmol), HCOOH (88% wt aq. solution, 12 mL, 273 mmol), and Red Mud (7 g ≈ 2.8 g Fe2O3 =17.5 mmoL, 2.4 mol % to acetic acid) were mixed in the stainless steel autoclave. The unit was sealed, flushed with hydrogen/argon gas at ambient temperature, and then heated with stirring. The reaction mixture was worked up and analyzed as described above.
a Entry numbers in Tables 1 and 2 match and describe the same experiments. b R, Red Mud, 16% w/w wrt HOAc; WR, washed Red Mud, 16% w/w wrt HOAc; UR, reused Red Mud, 12% w/w wrt HOAc; none, control reaction without catalyst. c H2/Ar flushed reactions start at ambient pressure. d Actual amounts/weights excluding 12% water contained in the commercially available HCOOH solution. e By quantitative GC using dimethylsulfone (100 mmol/L) as an internal standard. f Approximate by weight and water content established by Karl Fischer titration (see Table 3). g No phase separation observed. h Denoted as nonpolar due to oily appearance, well separated from solid polar catalyst. i HOAc with traces of acetone (by GC/GC-MS).
1.3 0.1 3.1 10.5h 2.1 1.2 0.2 1.3 2.1 n/ag n/ag n/ag n/ag 14.4 28.0 5.8 n/ag 8.3 10.4 18.0 11.7 6.6 7.8 56.0i 43.3i 39.9i 96 92 99 98 98 97 92 94 98 99 90% water. No nonpolar phase is generated in reaction 10, and the increase in pH is the highest (ΔpH=2.96) for any of the reactions. This means that in this reaction the HOAc substrate must have been primarily converted into gaseous reduced products. A qualitative analysis of the gas phase headspace of this reaction by GC-MS shows a dominant peak identified as propane by comparison against an authentic sample.31 The same peak along with CO2 is also observed in gas phase samples of all other reactions;including the control reactions without the Red Mud catalyst;which also show secondary, much smaller peaks tentatively assigned to propene and isomers of butane or butane, with the unsaturated products being more prevalent in the absence of the Red Mud catalyst.32 The specific results of control reactions carried out in the absence of Red Mud are listed in entries 11, 12, and 13. In no case was a nonpolar phase generated, and the conversions were 300 °C (indicated by the plateauing of the pressure curve for pure HCOOH, see Figure 1) and the thermal ketonization of HOAc at T > 325 °C. In keeping with the much lower conversions ( 300 °C, Red Mud catalyzes a complete catalytic decomposition of both HOAc and HCOOH and that HCOOH can indeed serve as an internal source of reducing equivalents.
Washing or reusing of Red Mud (both of which lower its alkalinity) impacts its catalytic activity and also changes the product distribution and composition but does not impact the viability of HCOOH as the internally supplied reducing agent. 2755
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Figure 6. Powder XRD pattern of a sample of Red Mud before (top, nonmagnetic red solid, composition as listed in Table 1) and after (bottom, magnetic gray solid) its use as a HOAc/HCOOH decomposition catalyst. & = hematite (Fe2O3), / = magnetite (Fe3O4), # = iron metal (Fe), þ = rutile (TiO2), $ = Fe7.9Al2.6Mg1.5 silicate.
A closer examination of the pressure vs temperature curves shown in Figure 1 is instructive. As expected simply on the basis of vapor pressure increases, for all four reactions, a slow monotonous increase of pressure with the temperature is observed. However, with the actual reaction mixture, i.e., HOAC, HCOOH, and Red Mud, there is a small but significant step in pressure observed at ∼240 °C that does not occur in the control reactions, hinting at a synergistic effect when all three components are present. Since significant decomposition of HCOOH already occurs at these temperatures, it may be caused by an initial reductive activation of the Red Mud by CO(g) or H2(g). HCOOH decomposition is complete at T > 300 °C, and the rapid increase of pressure above 330 °C cannot be explained by vapor pressure increases alone, as water is liquid under the reaction conditions.34 The distinct second step in pressure increase observed in all reactions where HOAc is present must therefore be due to HOAc decomposition. The uncatalyzed thermolysis of HOAc occurs at 442 °C and 1 atm of pressure by parallel pathways to CH4 and CO2 as well as ketene and water.16 Here, the onset of rapid HOAc decomposition and pressure increase occurs at the lowest temperature when Red Mud is present (∼330 °C); i.e., Red Mud does lower the activation barrier for ketonization, but which catalyst component is actually responsible for this is not clear. The alternative pathway to CO2 and CH4 involves direct C-C bond cleavage and thus should be less likely under these conditions. The decomposition still occurs at a ∼20 °C higher temperature without Red Mud present, but still lower than 442 °C. The reaction in this case is possibly catalyzed by the 316SS reactor wall consisting of Cr2O3, which forms the passivated surface of stainless steel, and Ni. Both metals are known HCOOH decomposition catalysts.20
While resulting in comparable high conversions of HOAc, the use of less alkaline washed or nonalkaline reused Red Mud (Tables 2 and 3, entries 1 vs 2, 4 vs 5, and 6 vs 7) consistently leads to a less nonpolar and more polar phase being produced. Postulating that the first reaction step for the formation of any nonpolar products is the ketonization of HOAc to acetone (see Scheme 1), this suggests that with the less alkaline catalyst more direct hydrogenation to acetaldehyde and ethanol occurs,35 while with the more alkaline catalyst, base-catalyzed aldol condensation of acetone and/or acetaldehyde to extended carbon chain lengths;at least initially;dominates, which is then followed by hydrogenation, leading to the formation of the C6-C10 alkenes and alkanes identified in the nonpolar phases of oily appearance (Table 4). Conclusion Red Mud, either in its virgin alkaline or nonalkaline reused (and reduced) form, is an effective catalyst for the thermal decomposition and hydrogenation of HOAc, HCOOH, and mixtures of the two to nonacidic products, into mainly acetone, alkenes, alkanes, and secondary reaction products derived from acetone, acetaldehyde, and ethanol. Mixtures of HOAc and HCOOH serve as a simple model system for pyrolysis bio-oil, in which the latter can act as an internal source of hydrogen. Whether or not Red Mud can be adapted as a catalyst for the upgrading of the chemically much more complex actual pyrolysis bio-oil obtained from a variety of ligno-cellulosic sources is currently under investigation in our laboratory. Acknowledgment. The authors thank Drs. Guy Forte and Guy Peloquin, Rio Tinto Alcan (Jonquiere, Quebec Operation)
(34) The vapor pressure of water at 330 °C is 1866 and 2398 psi at 350 °C, i.e., lower than the observed pressures of >3000 psi (source: CRC Handbook of Chemistry and Physics).
(35) Followed by further reactivity. See Table S1, Supporting Information.
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Supporting Information Available: Detailed analysis of the composition of Red Mud by XRF (Rio Tinto Alcan). Color image of the appearance of Red Mud before and after the reaction with HOAc/HCOOH at 350 °C. List of possible other reaction products in the polar phase identified by GC-MS (4 pages). This information is available free of charge via the Internet at http://pubs.acs.org/.
for supplying an authentic operational process sample of Red Mud and Profs. Franco Berruti, Cedric Briens, and Murray Thomson for fruitful discussions. Funding for this work was provided by the Agricultural Biorefinery Innovation Network Canada (ABIN) as sponsored by Agriculture and AgriFood Canada and the Paprican Division of FPInnovations, Canada.
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