Article pubs.acs.org/EF
Catalytic Upgrading of Switchgrass-Derived Pyrolysis Oil Using Supported Ruthenium and Rhodium Catalysts W. Nan,† C. R. Krishna,‡ T-J. Kim,† L. J. Wang,§ and D. Mahajan*,†,‡ †
Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794, United States Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Natural Resources and Environmental Design, North Carolina Agricultural and Technical State University, Greensboro, North Carolina 27411, United States ‡
ABSTRACT: Upgrading of fast pyrolysis oils produced from swtichgrass was carried out using 5 wt % Ru and 5 wt % Rh on a carbon support as catalysts slurried in a polyethylene glycol solvent in a 300 mL Parr batch reactor in the presence of hydrogen. A hydrodeoxygenation (HDO) reaction was evaluated in the temperature range of 200−280 °C under hydrogen pressure of 300−1000 psig. The raw pyrolysis oil and the upgraded products were characterized by gas chromatography (GC), gas chromatography/mass spectrometry (GC/MS), and Fourier transform infrared spectroscopy (FTIR) techniques to establish the effectiveness of the hydrogenation process. With Ru/C at 280 °C and 1000 psig, the GC/MS data showed the absence of acetic acid and the principal liquid product slate included alcohols, hydrocarbons, cyclic compounds, and phenolics at a relative concentration of 5.2, 21.2, 3.8, and 35.7%, respectively.
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INTRODUCTION Because of increasing petroleum prices and a need to reduce unsustainable emissions of greenhouse gases generated from burning fossil fuels, replacement of fossil fuels with renewables has drawn worldwide attention.1 Although renewable energy sources, such as wind, solar, biomass, and hydropower, can be turned into heat or electric power, a variety of biomass is the only carbon source that can be conveniently converted into liquid fuels for transportation.2 Switchgrass as a biomass source has several advantages: (1) it is a perennial bunchgrass that requires low agricultural inputs and does not need annual reseeding; (2) the yield of dry biomass obtained from switchgrass can reach up to 5 tons per acre; and (3) the high yield of dry biomass can meet the requirement of a secure feedstock supply, making production of commercial biofuels from switchgrass a feasible proposition.3 Biomass can be converted into a liquid fuel by pyrolysis, in which the biomass is thermally decomposed in the absence of oxygen at an ambient pressure and a controlled temperature of 220 °C or higher.4 Under these conditions, gases, condensable volatile vapors, and biochar products are produced and the volatile vapors can be condensed into pyrolysis oil (or bio-oil). To obtain a high yield of pyrolysis oil, a fast pyrolysis method is preferred. Fast pyrolysis involves a high heating rate and a short residence time (99%, molecular weight of 380−420) were supplied by Sigma-Aldrich. Hydrogen [high-purity (HP) grade], helium [ultra-high-purity (UHP) grade], and compressed air (dry grade) were obtained from Praxair. 2.2. Hydrogenation Experiments. In a typical experiment, 10 mL of pyrolysis oil was mixed with 90 mL of PEG solvent in a 300 mL autoclave reactor equipped with a magnetically driven stirrer. The PEG was used as a solvent because it was miscible with pyrolysis oil. After stirring the mixture of pyrolysis oil and PEG, the two liquid phases became a uniform solution. This process greatly decreased the viscosity of pyrolysis oil. After the addition of 0.1 g of catalyst, the reactor was sealed and then purged with 50 psi H2 for 3 min to remove air and then pressurized with H2 to the intended pressure (300, 535, or 1000 psig). The reactor was heated to the desired temperature 4589
dx.doi.org/10.1021/ef500826k | Energy Fuels 2014, 28, 4588−4595
Energy & Fuels
Article
Figure 1. Total ion chromatogram of a swtichgrasss-derived sample of pyrolysis oil used in the study. sample was syringed into a quartz tube (2.5 cm × 3.0 mm inner diameter) and pyrolyzed at 300 °C for 2 min. The GC column used was PerkinElmer Elite-5MS (30 m × 0.25 mm × 1.0 μm). The GC temperature profile was as follows: injector T, 250 °C; oven T, 45 °C/ 4 min; ramp rate, 3 °C/min; and final T, 280 °C held for 20 min. Helium was used as a carrier gas at a flow rate of 1 mL/min. The split ratio was set to 30:1. The MS scan range was m/z 33−600 at a rate of 0.1 s per scan with an interscan delay of 0.1 s. The mass spectrometer was calibrated using heptacosafluorotributylamine and tuned prior to analysis. Both interface and source temperatures of MS were 250 °C. The m/z values of the fragments of compounds were recorded. The identification of fragments was achieved by matching m/z values to those in the National Institute of Standards and Technology (NIST) MS Library Search 2.0. A treated pyrolysis oil sample prepared at 280 °C and 1000 psi was extracted with n-hexane at a volume ratio of 1:1 for overnight to separate the pyrolysis oil and PEG solvent. Two phases were formed: a brown liquid top phase (n-hexane solubles) and a viscous black− brown bottom phase (n-hexane insolubles) after extraction. The n-hexane-soluble fraction was then analyzed by GC/MS. The PerkinElmer Frontier Fourier transform infrared spectroscopy (FTIR) was used to identify products and establish differences in the molecular composition of liquid samples. For solvent PEG, no peaks were observed in the 1500−1800 cm−1 wavenumber region. However, the FTIR instrument subtracts solvent peaks to yield the infrared (IR) spectrum of the products.
Swtichgrass-derived pyrolysis oil contains many oxygenated polar components. The majority of them were phenolic compounds, which was also previously reported by others for fast pyrolysis.17−24 Carboxylic acids, mostly formic and acetic acids, were also detected. They are produced from the degradation of cellulose in biomass and make pyrolysis oil corrosive. Other components in the crude pyrolysis oil included were ketones, aldehydes, and small amounts of alcohols. Only pyridine was found as a nitrogen-containing compound, which showed that swtichgrass-derived pyrolysis oil had a lower nitrogen content than other varieties of biomass-derived oils. This finding is consistent with a previously reported study.23 3.2. Effect of Catalysts on the Distribution of Gaseous Products. The majority (90−99%) of the gas collected at the end of the upgrading experiments was unreacted H2, indicating that the reactions were performed under a hydrogen-excess condition. Figure 2 shows the composition of the noncondensable byproduct gas generated during upgrading. CO2 was a dominant component in the gaseous product, which was 70−90% of the product gas. It is known that CO2 is mainly produced from the decarboxylation of organic acids, while CO is produced from decarbonylation and water−gas shift reaction. CH4 was only found when the operating temperature was above 250 °C. Because CH4 formation contributes to the reduction of the C/H ratio, it is an undesired gas byproduct. Other gaseous species were ethane, propene, and propane, but their contents were small (
