ARTICLE pubs.acs.org/EF
Analysis of Oxygenated Compounds in Hydrotreated Biomass Fast Pyrolysis Oil Distillate Fractions Earl D. Christensen,† Gina M. Chupka,† Jon Luecke,† Tricia Smurthwaite,‡ Teresa L. Alleman,† Kristiina Iisa,† James A. Franz,‡ Douglas C. Elliott,‡ and Robert L. McCormick*,† † ‡
National Renewable Energy Laboratory, Golden, Colorado 80401, Pacific Northwest National Laboratory, Richland, Washington 99352
bS Supporting Information ABSTRACT: Three hydrotreated bio-oils with different oxygen contents (8.2, 4.9, and 0.4 w/w) were distilled to produce light, naphtha, jet, diesel, and gas oil boiling range fractions that were characterized for oxygen-containing species by a variety of analytical methods. The bio-oils were originally generated from lignocellulosic biomass in an entrained-flow fast pyrolysis reactor. Analyses included elemental composition, carbon type distribution by 13C nuclear magnetic resonance, acid number, gas chromatography/ mass spectroscopy, volatile organic acids by liquid chromatography, and carbonyl compounds by 2,4-dinitrophenylhydrazine derivatization and liquid chromatography. Acid number titrations employed an improved titrant electrode combination with faster response that allowed the detection of multiple end points in many samples and allowed for acid values attributable to carboxylic acids and to phenols to be distinguished. The results of these analyses showed that the highest oxygen content bio-oil fractions contained oxygen as carboxylic acids, carbonyls, aryl ethers, phenols, and alcohols. Carboxylic acids and carbonyl compounds detected in this sample were concentrated in the light, naphtha, and jet fractions (25%) oils generated in this process were then upgraded using a fixedbed catalytic hydrotreating process operated over a range of processing conditions (flow rate and temperature) to produce samples with varying levels of oxygen.12 The samples produced contained total oxygen of nominally 8.2, 4.9, and 0.4% w/w, on the basis of a weight average of several combined samples, and are referred to as high oxygen content (HOC), medium oxygen content (MOC), and low oxygen content (LOC) oils, respectively. The oils produced were distilled to isolate fractions with the following boiling range targets: C5 71 °C (lights), 71 182 °C (naphtha), 182 260 °C (jet), 260 338 °C (diesel), and 338 566 °C (gas oil). A solid residue was collected from the distillation of the HOC and MOC oils at a yield of about 10% by weight. The actual boiling point distribution for the distillate fractions was determined by simulated distillation (ASTM D2887). For the total acid number (TAN) method development, a raw pyrolysis oil was analyzed. Elemental Analysis. Carbon, hydrogen, and nitrogen contents were determined following ASTM D5291. Sulfur was measured according to ASTM D2622. The oxygen content was measured following the Merz oxygen method.13 Elemental analysis results are reported on an asreceived basis; however, because these are distillate fractions, these samples are not expected to contain water at significant levels. Total Acid Number. The TAN is the amount of potassium hydroxide (KOH), in milligrams, needed to neutralize the acid in 1 g of oil. Initial TAN analysis was conducted in a manner similar to ASTM D664-09a Test Method B for acidity of petroleum products by potentiometric titration. Samples were tested using a Metrohm 809 Titrando automatic titrator equipped with a double junction pH solvatrode. The solvent used was 2-propanol (Sigma-Aldrich), and the instrument was set to dispense 50 mL of solvent. The titrant used was 0.1 N KOH in 2-propanol (BDH) standardized with potassium hydrogen phthalate (Sigma-Aldrich). However, this method was modified significantly for improved results, as described in the Results section. It has been noted in past research that acid number curves for pyrolysis oils differ significantly from those for mineral oils. The pyrolysis oil products produce less distinct curves, making it difficult for automatic detection to properly assign an end point.4 Gas Chromatography/Mass Spectroscopy (GC-MS). GCMS analysis of the samples was performed using a Hewlett-Packard (HP) 5890 gas chromatograph equipped with an HP 5972 massselective detector (MSD). The column used for compound separation was a Restek Rtx-1701, with dimensions of 60 m 0.25 mm and 0.25 μm internal coating. Table S-1 in the Supporting Information lists the GC parameters used for analysis. The MSD was set to operate in continuous scan mode from m/z 35 to 500. Each sample was dissolved in chloroform (EMD) at a dilution ratio of approximately 1:20 for injection onto the GC column. The diesel and gas oil fractions were found to produce considerably smaller peaks on the chromatogram, which eluted much later than the compounds in the lighter fractions. To improve the resolution of the compounds in these fractions, they were analyzed at a dilution ratio of 1:5 in chloroform and analyzed on a nonpolar column, an Agilent Technologies DB-1 with dimensions of 60 m 0.32 mm and 1.0 μm internal coating. The GC parameters used for this column are listed in Table S-2 in the Supporting Information. Volatile Organic Acids by High-Performance Liquid Chromatography (HPLC). Each sample was extracted with water containing 0.2% trifluoroacetic acid (TFA) at approximately a 10:1 mass ratio of water to sample. The aqueous layer was removed and filtered through a Whatman 1.0 μm polytetrafluoroethylene (PTFE) filter (cat. no. 6784-2510)
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for HPLC analysis. To estimate the extraction efficiency, one sample was extracted twice using the above procedure. The second extraction contained peaks that were approximately 10% of the area of the peaks in the first extraction, indicating that most of the components were removed in the first step. HPLC analysis was performed using an Agilent Technologies 1200 series HPLC equipped with a multiwavelength ultraviolet (UV) detector set at 210 nm. A Shodex volatile acids column with dimensions of 8 mm 300 mm (cat. no. WAT034298) was used for compound separation. The mobile phase was 0.1% phosphoric acid isocratic at a flow rate of 1.0 mL/min. The sample injection volume was 20 μL. An acid standard mixture purchased from Supelco (cat. no. 46975-U) was used for acid peak identification. Phenolic compounds were expected to be extracted with the volatile acids, and several phenolic compounds were added to the volatile acid standard for retention time confirmation by HPLC: phenol, o-cresol, m-cresol, p-cresol, 2-ethyl phenol, 2,6-dimethoxy phenol, and catechol. Table S-3 in the Supporting Information lists the acids and phenols used in the standard solutions. Standard solutions were injected in triplicate, and the response factors of the standard peaks were used to estimate the concentration of acids or phenols in the samples. Carbonyl Compounds by HPLC. Analysis of the aldehyde and ketone components was performed by first derivatizing the compounds of interest using 2,4-dinitrophenylhydrazine (DNPH), which selectively reacts with aldehydes and ketones, resulting in hydrazone DNPH derivatives.14 16 Waters Sep-Pak DNPH-Silica cartridges (cat. no. WAT037500) with a capacity of approximately 75 000-ng formaldehyde were used for DNPH derivatization. HPLC analysis was performed using an Agilent Technologies 1200 Series HPLC equipped with UV detection set to detect at 360 and 300 nm. A Restek Allure AK column with dimensions of 200 mm 4.6 mm and 5-μm particle size (cat. no. 9159525-700) was used for compound separation. The mobile phase was a mixture of 18 MΩ deionized water and HPLC-grade acetonitrile, set to a gradient shown in Table S-4 in the Supporting Information at a flow rate of 1.5 mL/min. The injection volume was 10 μL. Two carbonyl DNPH derivative standard mixtures (cat. no. 47671-U and 47285-U) purchased from Supelco were used for compound peak identification and quantification. Two different sample preparations were performed, as the amounts of aldehyde/ketone present in the samples varied greatly. For samples containing 1% w/w aldehyde/ ketones, a 0.2% TFA/water solution was used to extract the aldehydes/ ketones from the distillate fraction. It was determined that a ratio of 10:1 w/w acidified water/sample shaken for 30 min equated to approximately 50% extraction efficiency. A ratio of 1000:1 w/w for 16 h was used for the final analysis. After extraction, 80 μL of the acidified water was derivatized on a DNPH cartridge using the same process used for the pure samples. A five-point calibration curve was made using a combination of the two carbonyl DNPH standard solutions. Eighteen compounds were contained in the combined mix; these compounds and their retention times are listed in Table S-5 in the Supporting Information. For each compound, a linear area response curve with forced origin and four data points was generated with r2 exceeding 0.9995. 13 C Nuclear Magnetic Resonance. Quantitative 13C nuclear magnetic resonance (NMR) spectra were acquired at 499.67 MHz, using a 45° observed pulse; acquisition and delay times of 3 s with 1H Waltz 5463
dx.doi.org/10.1021/ef201357h |Energy Fuels 2011, 25, 5462–5471
Energy & Fuels
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Figure 1. Actual fraction distillation range in comparison to targets (by simulated distillation). decoupling off during the acquisition delay period for nuclear Overhauser enhancement (NOE) suppression; and 0.05 M Cr(acac)3 for T1 reduction and quenching of any residual NOE. These conditions led to an average integral error of about (2% (in carbon aromaticity). Carbon13 spectra are referenced to internal CDCl3 (77.24 ppm), tetramethylsilane (0 ppm), or the α-carbon of linear long-chain saturated hydrocarbons (14.16 ppm). Samples consisted of 0.15 0.2 mL of fuel diluted to 1.0 mL in CDCl3 in 5-mm outer diameter NMR tubes. Spectra resulted from 400 1600 scans. The spectra were integrated to obtain carbon mole fractions of the following functional groups: carbonyl, carboxylate, phenolic, ether, aromatic C H, and paraffinic C H, C H2, and C H3. The specific range of chemical shift integrated for each group is shown with the results in Table 2. The subregions chosen to quantify specific structural groups are considered applicable by the authors, but no standard format has been established for distillate fuel analysis. Several of the chemical shift assignments contain structural groups that normally are either not found in hydrotreated fuels (e.g., ketone, carboxylic, phenolic, and ether carbons) or found in low concentrations.
’ RESULTS AND DISCUSSION Simulated Distillation and Elemental Analysis. Actual fraction distillation ranges determined by simulated distillation are shown in Figure 1 and compared to the target range. Because of the small scale of the distillation procedure used, the distillation temperature ranges determined by GC are considerably wider than the target range in all of the fractions. With the exception of gas oil, the end points fall well above the target end point temperature. The light and naphtha fractions cover a very similar distillation temperature range. The simulated distillation results indicate that, for the naphtha fractions of all three oils, T10, T50, and end point values fall above the maximum temperatures set in the gasoline specification, ASTM D4814, of 70, 121, and 225 °C, respectively. The jet fraction T10 temperatures fall below the 205 °C maximum specified in ASTM D1655; however, the end points fall above the 300 °C maximum. The T90 for the diesel fraction of all three samples falls above the maximum of 338 °C given in ASTM D975. These materials might still be useful as refinery feedstocks or blend components in finished fuels; however, care should be taken in using the distillate fraction
designations because they do not correspond to what could be obtained at a larger scale. The results of the elemental analysis for each distillate fraction are listed in Table 1. For all three sets of distillate samples, the hydrogen-to-carbon ratio (H/C) decreases with increasing boiling point. The HOC naphtha and light fractions contained the highest oxygen content, and the oxygen-to-carbon ratio (O/C) decreases very rapidly with increasing boiling point. For the MOC fractions, the trend is different, with the light fraction having the lowest oxygen content and O/C. Oxygen content and O/C then increase on going from lights to naphtha to a maximum for jet, and decrease in the higher boiling fractions. For the fractions derived from LOC oil, we see very low O/C for all fractions. Some studies indicate that if the oxygen content is reduced to below 10 wt %, then an upgraded oil will be adequately nonpolar to be soluble in hydrocarbon,17 although this was not confirmed in this study. The samples had significant sulfur content. Naphtha fractions were within the allowable range for gasoline in the United States (
