Continuous Upgrading of Fast Pyrolysis Oil by Simultaneous

Sep 8, 2016 - Karimi , E.; Teixeira , I. F.; Ribeiro , L. P.; Gomez , A.; Lago , R. M.; Penner , G.; Kycia , S. W.; Schlaf , M. Catal. Today 2012, 190...
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Continuous Upgrading of Fast Pyrolysis Oil by Simultaneous Esterification and Hydrogenation Roger Hilten, Justin Weber, and James R. Kastner* Biochemical Engineering, College of Engineering, The University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: A mixture of fast pyrolysis oil (FPO) and methanol (1/1 v/v) was continuously converted to methyl levulinate (ML), methyl acetate (MA), and C3 or greater methyl esters using metal-acid functionalized zeolites (Ni and Ru/HZSM-5) and an iron oxide catalyst with both acid and base sites (250 °C, 600 psig). Fractional conversion of FPO components was 60% or greater using the iron oxide catalyst, and space time yields approached 150 and 30−50 g/L cat/h for MA and C3 methyl esters, respectively, at 250 °C (W/F = 0.4 h, liquid hourly space velocity = 5−11.2 h−1). Product yield and concentration using the iron oxide catalyst were comparable to those of the Ni and Ru/HZM-5 catalysts and achieved performance levels higher than those of SiO2‑Al2O3 and HZSM-5. Two potential pathways for acetic acid conversion (ketonization and esterification) and ML formation from levoglucosan were observed. Using the bifunctional catalysts in the presence of hydrogen resulted in significant coke reduction (60−80%) and the production of esters of carboxylic acids C3 or greater (e.g., pentanoic and hexanoic acid methyl esters) and MA from the mixture. More interestingly, contrary to the other catalysts, an increase in phenolic levels (e.g., 2methoxy phenol) was observed using the iron oxide catalyst with H2 and isopropanol (replacing H2), indicating the presence of undetected lignin oligomers in the feed and their subsequent hydrogenolysis. Simultaneous esterification and hydrogenation resulted in percent reduction in total acid numbers ranging from 66 to 76%.

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INTRODUCTION Biomass fast pyrolysis rapidly generates an oxygenated liquid hydrocarbon stream called fast pyrolysis oil (FPO) in high yields (60−70%, single phase bio-oil). FPO is difficult to convert to biofuels or a liquid that can be blended with current fuels due to the presence of incompletely depolymerized lignin, phenolics, hydroxyacetaldehydes, hydroxyketones, water (30− 40%), reactive acids (e.g., 30−100 g/L acetic acid), and anhydrosugars (50−150 g/L). FPO is acidic (corrosive) and upon heating polymerizes, causing an increased viscosity.1−4 The carboxylic acids and anhydrosugars (e.g., levoglucosan, LG) are difficult to remove or transform to fuel intermediates; cross reactions occur, causing polymerization, and tars and other compounds cause coking.5 Efforts to stabilize the oil for storage and shipment to biorefineries has focused on esterification by adding alcohol without catalysts, catalytic hydrodeoxygenation (HDO), and catalytic cracking.2−4,6,8,9 Esterification without catalysts does not convert key compounds such as acetol and levoglucosan in the oil to upgradable intermediates,9 and the pathways for conversion of these compounds is rarely studied. Removal of the acids by decarboxylation and decarbonylation produces lower molecular weight products and lowers fuel yields. Upgrading pyrolysis oils using acidic zeolites alone, such as HZSM-5, is difficult due to coke formation and catalyst deactivation.7,8 Catalytic hydrodeoxygenation alone results in limited conversion of the acids.5 Because FPO has high levels of carboxylic acids, hydroxyketones, hydroxyaldehydes, phenolics, and levoglucosan, conversion of these compounds to upgradeable intermediates or fuels appears to be a viable synergistic process to couple with catalytic HDO and integrate with fast pyrolysis. Levoglucosan © 2016 American Chemical Society

and acetic acid in the oil can be separately esterified to form alkyl levulinates (fuel blending agents and a potential platform chemical) and esters using solid acid catalysts,10−12 yet little information is available on the continuous transformation of these two components in FPO, although they represent the largest fraction. Conversion of LG in FPO to valeric biofuels is a reaction pathway which has rarely been considered. This pathway requires an acid-metal catalyst to generate alkyl levulinates from LG with subsequent hydrogenation to valerate esters, which are fully compatible with transportation fuels.13,14 Most research on this pathway has focused on catalytic hydrogenation of liquid acid derived levulinic acid13,14 with no reports in the literature on direct conversion of LG in fast pyrolysis oils to valerate esters. Heterogeneous bifunctional catalysts with acid and metal sites represent a significant advantage over homogeneous liquid phase acids because they eliminate corrosion problems, can potentially be reused, and can perform multiple reactions. Levoglucosan can be converted to alkyl levulinates when alcohol/water mass ratios exceed 5−10% alcohol via direct esterification rearrangement to the alkyl levulinate using acidic resins.10 A Pt (metal) on HZSM-5 (acid) catalyst has been shown to convert levulinic acid to valerate esters in a multistep process, yet direct conversion of levoglucosan in FPO to valerate esters has not been investigated.13 Much work on catalytic esterification of FPO has been focused on the use of sulfonated polystyrene resins as catalysts in batch systems (3− Received: August 1, 2016 Revised: September 5, 2016 Published: September 8, 2016 8357

DOI: 10.1021/acs.energyfuels.6b01906 Energy Fuels 2016, 30, 8357−8368

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calcined in air at 500 °C for 5 h. Prior to runs, the catalyst was activated by 10 g being placed in a tubular reactor (Parr) under a flow of 100 mL/min of 5% H2/95% N2, heated at 10 °C/min to 500 °C, and held for 2 h. The expected metal loading was 5% for both catalysts. Surface Area/Pore Size Distribution. Surface areas of the solid acid catalysts (0.1 g sample size) were measured by N2 adsorption over a relative pressure range (P/P0) of 0.05−0.35 using a seven-point BET analysis equation (Quantachrome AUTOSORB-1C; Boynton Beach, FL). Pore size distribution, average pore radius, and total pore volume were estimated from N2 desorption curves using BJH analysis. All samples were degassed in the range from 250 to 300 °C for 3−4 h before analysis. Temperature-Programmed Desorption (TPD). SiO2-Al2O3, HZSM-5, Ni-HZSM-5, Ru-HZSM-5, and the reduced red mud were analyzed by NH3 and CO2 TPD. TPD was used to estimate acid and base site strength of the catalysts. All samples were degassed ranging from 250 to 300 °C for 3−4 h before NH3 or CO2 TPD analysis. Samples (0.2 g) were loaded in a quartz U-tube and packed between two quartz-wool layers, degassed at 185 °C (450 °C for RRM and CO2 TPD) for 30 min in helium, saturated with ammonia (pure electronic grade) at 40 °C (50 °C for CO2) for 15 min, flushed with helium at 40 °C for 15 min, and then desorbed with helium from 40 to 800 °C (50 to 650 °C for CO2) at 10 °C/min (all flows at 80 mL/min). Desorbed NH3 or CO2 was detected using a thermal conductivity detector (TCD, 16× attenuation), and measurements were made using a Quantachrome AUTOSORB-1C instrument. Acid or base site density (μmoles NH3 or CO2/g catalyst) was estimated using an NH3 or CO2 TCD standard curve and calculating the peak areas for NH3/CO2 desorption via numerical integration (a baseline subtracted chromatogram was used for acid site density estimation). A four-point standard was generated via triplicate pulse injection of known volumes of NH3 or CO2. Tar/Coke Analysis. Recovered catalysts were washed with an equal volume mixture of toluene, acetone, and methanol to remove tar. Approximately 2 g of reacted catalyst was placed on a Whatman filter (Qualitative #1 Filter Paper, 70 mm) and rinsed under vacuum with the solvent mixture. The rinsed catalyst was then dried at 105 °C for 1 h, cooled to room temperature, and weighed to determine the mass of tar removed. Catalyst coke formation was determined by heating the washed catalyst in a thermogravimetric analyzer (TGA) at 10 °C/min to 650 °C under air flow. The change in the mass of catalyst was assumed to be due to the complete combustion of coke. The weight of reactor char was then determined as the difference between reactor weight change and combined coke and tar weight. GC/MS/FID Analysis. Conversion and product yields for esterification of the FPO/methanol mixture was determined using GC/FID/MS and HPLC (detailed below). Methyl levulinate, methyl acetate, acetic acid (confirmation with HPLC), cyclopentanone, and furfural were quantified via five-point standard curves using a GC/FID. Acetic acid, levoglucosan, glucose, fructose, furfural (confirmation with GC/FID), and 5-hydroxymethylfurfural were quantified using HPLC and five-point standard curves. Longer chain methyl esters (C3−C8), acetals, and cyclic ketone concentrations were estimated using quantitative GC/MS, as detailed below. GC/FID analysis (1 μL sample size, 100/1 split ratio) was performed using an HP 5890 and HP Innowax capillary column (0.25 mm × 0.25 μm × 30 m) with a temperature program of 45° (hold for 2.5 min) to 160 °C (ramp at 10 °C/min). Helium was used as the mobile phase (1 mL/min) with the inlet held at 240 °C and the detector at 250 °C. Reaction products were verified via GC/MS. The chemical composition of the reacted samples was also determined by GC/MS analysis (HP-6890, HP-5973 mass selective detector), and GC/FID and HPLC analyses. The GC/MS contained an HP-5 MS column (30 m, 0.25 mm ID, 0.25 μm film thickness). The method used was as follows: inlet temperature 240 °C, detector temperature 280 °C (mass spectrometer interface temperature), flow at 1 mL min−1 He, and heating in an oven at 40 °C for 3 min followed by a ramp at 8 °C min−1 to 250 °C (held for 5 min). The mass spectrometer scan range was from 20 to 350 mass units.

22 h) and has not determined the effect on other key components in the oil (e.g., levoglucosan and phenolics).6,9−16 Acid resins have low surface area and thermal stability, potentially limiting their space time yields (STYs, g product/L catalyst/h) and the ability to be used in continuous reaction systems at higher temperatures. Batch reactor conditions change with time, making it difficult to measure reaction rates, product yields, selectivity, and space time yields. The development of continuous catalytic FPO upgrading routes which can be integrated with fast pyrolysis (e.g., using the thermal energy in FPO to drive chemical reactions), especially if operational at lower residence times, would be of great interest. In-line esterification via reactive condensation with ethanol has been integrated with fast pyrolysis,9 yet this work was conducted without catalysts, did not quantify compositional changes in the oil (other than acetic acid and ethyl acetate), and did not explore coupling with low temperature hydrogenation. To date, there has been very little research investigating continuous catalytic esterification/hydrogenation of fast pyrolysis oils using solid acid-metal catalysts. Such studies would enable accurate measurement of reaction rates, space time yields, selectivity, and catalyst longevity. Alternative to acidic resins, acidic zeolites and metal functionalized zeolites may provide a better alternative because their acidity and hydrogenation activity can be manipulated to alter reaction pathways, they typically have higher surface areas, are thermally stable, and can be used to convert the recovered oxygenates to gasoline (e.g., methanol or ethanol to gasoline processes, MTG or ETG).17,18 The goal of this work was to determine the feasibility of continuously generating alkyl levulinates and valerate esters directly from components in fast pyrolysis oil using solid acid/metal catalysts and to gain a better understanding of the reaction pathways.

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EXPERIMENTAL SECTION

Catalyst Preparation. A total of seven catalysts were used in this study. Two inexpensive materials were tested as acid catalysts: SiO2Al2O3 (6.5% Al, > 90% 100 mesh; Sigma) and H2 reduced red mud (RRM, 24% Fe, 17% Al, 9% Si, 16% Na, 5% Ca, 5% Ti). The SiO2Al2O3 powder was calcined at 500 °C in air for 5 h. To minimize pressure drop across the catalyst bed, the catalyst was granulated by mixing with water, drying, crumbling, and sieving to the desired size (0.5−1 mm). The RRM was prepared as described previously.23 The acidic zeolite HZSM-5 was used as the solid acid catalyst and as a support for the generation of bifunctional catalysts (Ru and Ni/ HZSM-5). HZSM-5 was produced by calcining NH4-ZSM-5 (Zeolyst International, CBV 5524 G) at 550 °C for 4 h in air to produce the hydrogen form, HZSM-5. The pH was measured by mixing catalyst in water at a 1/1 ratio and then measuring the pH of the water using a standard pH probe. After being calcined, the pH declined from 4.98 to 3.06. The NH4-ZSM-5 catalyst was received from the manufacturer as a fine powder. To minimize the pressure drop across the catalyst bed, the catalyst was granulated by thoroughly being mixed with water in a beaker, dried at 100 °C, physically crumbled, and sieved to the desired size (∼2−4 mm). Preparation of Ru and Ni-HZSM-5 catalyst used the incipient wetness impregnation technique and the following method. An aqueous solution of water and ethanol (9/1 v/v) and the Ru or Ni salt (RuCl3H2O or Ni(NO3)2 6H2O, Sigma) was first prepared. The pH of the solution was adjusted to solubilize the salt, if needed, with 2 mL of HNO3 (68%). Approximately 108 g of granulated HZSM-5 was contacted with a volume of the salt solution equal to the pore volume of HZSM-5 (0.185 mL/g based on BET analysis). After contact, the solution was covered with parafilm and stored in the dark at room temperature for 12 h. Subsequently, the material was dried at 105 °C for 12 h in air and stored in a sealed container. Next, the material was 8358

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Table 1. Physical Properties of Zeolites, Metal Impregnated Zeolites, and Sulfonated Acidic Resin with Different Catalystsa SiO2-Al2O3 surface area (m2/g) pore volume (cm3/g) average pore size (radius Å) bulk density (g/cm3) 200−300 °C, μmoles NH3/g (Type 1) 300−550 °C, μmoles NH3/g (Type 2) Type 1/Type 2 100−150 °C, μmoles CO2/g 450−600 °C, μmoles CO2/g a

405 0.25 12 0.46 260 145 1/0.56 NP NP

RRM properties 30.7 0.024 16 0.93 acid sites (NH3-TPD) 300 220 1/0.67 base sites (CO2-TPD) 21 270

HZSM-5

Ru-HZSM-5

Ni-HZSM-5

316 0.17 11 0.41

313 0.17 11 0.41

264 0.15 11 0.41

230 317 1/1.38

170 360 1/2.12

3200 360 1/0.11

NP NP

NP NP

NP NP

NP: not performed.

In quantitative GC/MS analysis of the collected liquid sample (1 μL injection volume), hexanol was used as the internal standard at 1.0 g/ L, and the sample was analyzed using the previously described methods. Neat compounds of methyl acetate, ethyl acetate, furfural, 2butanone, tetrahydrofuran, cyclopentanone, and 2-cyclopenten-1-one (99.9%, Sigma) were mixed with hexanol (1.0 g/L) in an acetone and methanol (50/50 v/v) solvent to generate standard mixtures (three- to four-point standard curves based on TIC, 0.8−11 g/L, plot of peak area ratio versus concentration) and analyzed on the GC/MS using identical methods. Concentrations of ethyl esters of C3 or greater (e.g., pentanoic acid and ethyl ester) were estimated using the standard curve for ethyl acetate. Identification of products in the condensed liquid outlet was based on GC/MS matching with a NIST database and then retention time matching using purchased standards on the GC/MS using the HP-5 column and on the GC/FID using an Innowax column. The compounds identified in this manner were levoglucosan, methyl acetate, ethyl acetate, methyl levulinate, ethyl levulinate (EL), acetic acid, furfural, acetol (1-hydroxy2-propanone), and cyclopentanone. For other intermediates and products identified by GC/MS that were not or could not be purchased, we report match factors with the NIST database for these compounds (see next section). Mass Spectral Analysis. Identification of products in the condensed liquid outlet was based on GC/MS matching with a NIST database and then retention time matching using purchased standards on the GC/MS using the HP-5 column and on an Innowax column using the GC/FID. The compounds identified in this manner were acetic acid, ethyl acetate, ethyl levulinate, acetone, and furfural. Levoglucosan and ethanol were quantified using an HPLC. For other intermediates and products identified by GC/MS that could not be purchased, we report match factors with the NIST database for these compounds. Two match factors are reported: a reverse search, ignoring peaks in the unknown not in the library spectrum (RMatch), and a probability value (Prob.%). The probability value assumes the unknown is represented by a spectrum in the library and employs the differences between adjacent hits in the hit list to get the relative probability that any hit in the hit lists is correct. In general, according to NIST, 900 or greater is an excellent match; 800−900 is a good match, and 700−800 is a fair match. Less than 600 is a very poor match. HPLC Analysis. The water-soluble components in the oil were analyzed by a high performance liquid chromatograph (LC-20 AT, Shimadzu Corp., United States) equipped with a RID-10A refractive index detector and a 7.8 × 300 mm Coregel 64-H Transgenomic analytical column for sugars (e.g., levoglucosan) and carboxylic acids (e.g., acetate). About 2 mL of sample (diluted with DI water if needed) was filtered through a 0.45 μm filter into 2 mL autosampling vials. A sample size of 5 μL was injected into the column using the LC20 AT Shimadzu autoinjector. The samples were analyzed at 6.89 MPa (1000 psi) and 60 °C with an eluent (4 mN H2SO4) flow rate of 0.6 mL min−1 for a 55 min run time. Acetic acid, formic acid, levoglucosan,

glucose, furfural, acetol, and 5-hydroxymethylfurfural in the liquid samples were identified by comparing retention times with standards and were quantified using a five-point standard curve. TAN. The total acid number (TAN) of the product was measured for catalytic esterification experiments using fast pyrolysis oil mixed with methanol. TAN (mg KOH/g) was determined by dissolving the sample in a mixture of toluene and isopropyl alcohol and titrating at room temperature with KOH to an end point indicated by pnaphtholbenzein (ASTM D974). Catalytic Esterification and Hydrogenation of Oil. The fast pyrolysis oil (FPO) was generated from Southern Pine as previously described and stored at 4 °C until use.9 Prior to catalytic upgrading, the FPO was mixed with methanol at 1/0.98 ratio (v/v). To test the effect of a liquid hydrogen donor on upgrading, FPO was mixed with methanol and isopropanol (H2 donor) at a 1/0.305/0.67 volume ratio (FPO/methanol/isopropanol). The FPO and methanol (or isopropanol) mixture was mixed with N2 or H2 gas (100 mL/min) and injected continuously (downward) using an HPLC pump across a packed bed reactor or PBR (Parr Moline, IL) maintained at 250 °C in a tube furnace (Thermocraft Lab-Temp 1760-W tube furnace) at 600 psig. The PBR consisted of a 2.4 cm inner diameter reactor with a 38 cm length. A 15 cm preheater section was incorporated into the reactor to ensure that bio-oil was in vapor phase prior to crossing the catalyst bed (5 g catalyst or a packing height ranging from 1.2 to 2.7 cm, depending on catalyst bulk density) that was held in place by stainless steel screens and quartz wool above and below the bed. Catalyst bulk density ranged from 0.413 (HZSM-5, Ni-HZSM-5, and Ru-HZSM-5) and 0.46 (SiO2-Al2O3) to 0.93 g mL−1 (RRM). Liquid feed was typically 1.0 cm3 min−1, corresponding to a catalyst mass to feed ratio (W/F) of 0.401 h (g cat/g feed −1 h−1). Note that the feed rate was calculated based on the total inlet volumetric flow rate and the measured concentration of components in the inlet stream. The liquid feed was mixed with an inert gas (N2) or H2 (100% H2) which had a flow rate of 100 cm3 min−1 (controlled using mass flow controllers, Brooks Delta II Smart). Weight hourly space velocity (WHSV) was calculated as the mass flow rate (g h−1) of liquid feed divided by the catalyst mass (g) and was 2.5 g feed gcat−1 h−1. These reaction conditions corresponded to a liquid hourly space velocity [LHSV = reactant liquid flow rate (cm3 h−1)/reactor volume (cm3)] ranging from 5 to 11.2 h−1. Given a carrier gas flow rate of 100 cm3 min−1, gasphase residence time in the catalytic zone (H = 1−4 cm) ranged from approximately 3−7 s. The outlet gas passed through custom designed condenser vessel (Parr) and was chilled using a Brookfield TC-602 water bath.

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RESULTS AND DISCUSSION

Catalyst Characterization. Except for RRM, all of the prepared catalysts had high surface areas ranging from 260 to 400 m2/g for the zeolites and SiO2-Al2O3 (Table 1). The incorporation of metal (Ru and Ni) into HZSM-5 had little 8359

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mixed with methanol, HPLC analysis indicated high levels of levoglucosan and acetic acid in the FPO, and methanol was selected for esterification because it is currently used to stabilize FPO.3,4 Two inexpensive materials were tested as acid catalysts, SiO2-Al 2O3 and H2 RRM, due to the propensity for coke formation when upgrading FPO. Ni/HZSM-5 was selected as an alternative metal-acid functional catalyst due to the expense of Ru and the demonstrated ability of this catalyst to effectively perform hydrodeoxygenation of FPO.24 Thus, FPO and methanol were mixed and used as feedstock for catalytic upgrading (initially at 600 psig and 250 °C). Analysis of the mixture indicated the presence of acetic acid (17.5 g L−1), formic acid (47 g L−1), acetol (18.1 g L−1), furfural (2.4 g L−1), 5-hydroxymethyfurfural (2.85 g L−1), and levoglucosan (102.4 g L−1). Feedstock analysis also indicated the presence of methoxyated phenolics (9 g L−1 total, including 2-methoxyphenol, 2-methoxy-4-methyl-phenol, 4-ethyl-2-methoxy-phenol, 2-methoxy-4-propyl-phenol, 2-methoxy-5-(1-propenyl)(E)-phenol, and 2-methoxy-4-(1-propenyl)-( E)-phenol). Also, large peaks of glycolaldehyde dimethyl acetal (GDA, approximated at 8.3 g L−1 via GC/MS) and dimethoxytetrahydrofuran (not quantified due to lack of standard or similar compound) were observed. The formation of the acetal (GDA, Figure 4S) upon methanol addition does suggest the presence of glycoaldehyde in the original oil, which we were not able to quantify with our analytical methods. Interestingly, from the standpoint of methyl ester production and yields, the H2 RRM resulted in values similar to or higher than the other catalysts (Figures 2 and 3). RRM generated a methyl acetate yield of 11.5 mol % compared to 4−10 mol % for the other catalysts. In addition, when compared to other catalysts, the RRM treated FPO/methanol mixture had higher levels of long chain fatty acid methyl esters (Figure 5S). The higher acetic acid conversions using SiO2-Al2O3 and RRM corresponded to lower acetic acid levels, higher methyl ester levels, and higher TAN levels compared to those of the other catalysts (Figure 6S). This may have been due to higher levels of formic acid and phenolics in oil generated using these catalysts. Although methyl levulinate was identified in the product stream for all catalysts except SiO2-Al2O3, it was a small percentage of the esters with methyl acetate forming the largest percentage of the esters (62−95%). For all catalysts tested, there was a significant reduction in phenolic concentrations, ranging from 76 to 100% conversion (RRM had the lowest value), and there was no measurable level of glycolaldehyde dimethyl acetal in the collected liquid product. In addition to methyl ester production, a range of ketones, including acetone and cyclic ketones, were produced, suggesting that a ketonization pathway was active (Figures 3 and 5S).21 Although methyl ester formation was confirmed using FPO, significant coke formation was measured via TGA analysis of the spent, recovered catalysts. For all catalysts, the percent coke ranged from 50 to 73% ( Figure 9S), indicating that significant deactivation would occur with prolonged use. Effect of Pressure. Given the positive results using RRM, we next performed additional testing with this material. It has been suggested that RRM is capable of reforming compounds in FPO (e.g., formic acid) to form H2.25 Thus, we wanted to determine if pressure (and possibly the partial H2 pressure) would affect the product mixture, yields, and coke formation. As reaction pressure was increased, there was a significant increase in conversion of all reactants (Figure 4). Correspondingly, TAN levels declined from 38 at 14.7 psig to 15 at 1000

effect on surface area and pore size distribution (Table 1 and Figure 1S). N2 adsorption isotherms for the zeolite catalysts were of Type 1 with hysteresis occurring between the adsorption and desorption paths (again, there were no significant differences upon metal incorporation into the zeolites), while isotherms for SiO2-Al2O3 and RRM were of the Type 2 shape, indicating the presence of macropores (Figures 2S). NH3-TPD analysis of the catalysts clearly noted differences between Type 1 and 2 sites upon metal incorporation (Figure 3S). Ruthenium and nickel incorporation into HZSM-5 apparently created more Type 2 sites and increased the strength of Type 1 sites (i.e., raised the NH3 desorption temperature, Figure 3S and Table 1). For NiHZSM-5, there was a significant increase in Type 1 acid sites and thus a corresponding decrease in the Type 1/Type 2 ratio. Interestingly, TPD analysis indicated that RRM had an acid site concentration and strength similar to those of SiO2-Al2O3 and HZSM-5 yet a significantly lower surface area. Moreover, CO2TPD indicated high concentration of a strong base site with CO2 desorption occurring from 450 to 600 °C in the RRM catalyst. This base site was conditional upon the H2 reduction temperature because this site disappeared at higher reduction temperatures (Figure 1).

Figure 1. CO2 TPD analysis of freshly prepared H2 RRM at 300 and 400 °C compared to a control which did not receive CO2 (300 °C RRM).

Our characterization results for HZSM-5 are similar to previous results which reported a Type 1/2 ratio of 1/1.14 and Type 1 and 2 acid site densities of 211 and 240 μmol/g, respectively.19,20 Higher Type 2 acid site density for Ru-HZSM5 relative to that of Ru/H-β zeolite has been reported, which increased activity for hydrogenation of levulinic acid and γvalerolactone to pentatonic acid and pentanoic acid ethyl ester.14 Ni-HZSM-5 promoted with K-minimized coking and prolonged catalytic esterification, dehydration, and hydrogenation of levulinic acid to valeric biofuel.19 Similar to our Ni-HZSM-5 results, NH3 desorption peaks were observed from 150 to 450 °C and 450 to 600 °C.21 Nickel incorporation into HZSM-5 has also been reported to increase Type 2 (400−600 °C) acid site concentrations.22 The acid and base sites in RRM have previously been analyzed, and the base site is attributed to the alkali metal in RRM (Ca, Na, and K).23 Catalytic Esterification/Hydrogenation of Fast Pyrolysis Oils. Experiments were conducted to determine if methyl levulinate, methyl esters, and valeric biofuels could be produced from fast pyrolysis oil given the high levels of LG and carboxylic acids and recent evidence for conversion of LG to methyl levulinate using solid acid catalysts.10 After the materials were 8360

DOI: 10.1021/acs.energyfuels.6b01906 Energy Fuels 2016, 30, 8357−8368

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Figure 2. Effect of catalyst type on conversions (A), yield (B), and STY (C) from catalytic esterification of fast pyrolysis oil (W/F = 0.4 h; 250 °C; 600 psi).

psig (Figure 7S), and methyl acetate and ≥C3 methyl esters increased (Figure 5). Most noticeably, there was an increase in acetic acid and phenolic concentrations when the reaction was performed at atmospheric pressure (negative conversion in Figure 4) due to the formation of acetate from levoglucosan23 and potentially phenolics from lignin oligomers. Other observable trends with increasing pressure were (1) an increased conversion of glycolaldehyde dimethyl acetal, (2) the formation of methyl levulinate, and (3) the formation of ≥C3 acid methyl esters (e.g., methylpropionate and hexanoic acid methyl ester, Figure 5). Increasing pressure did reduce coke formation, but levels were still high, ranging from 35 to 43% over 300 to 1000 psig compared to 56% at 150 psig (Figure 8S).

Results from the esterification of bio-oil with methanol (without H2) using the metal/metal oxide/acid/base (only the RRM) catalysts suggests that simultaneous ketonization, esterification, and transfer hydrogenation pathways are operational under these conditions. Previous research using mixed metal oxides with high iron levels (Fe2O3, magnetite) indicated acetone and 2-butanone yields of 8−10% and 3−5%, respectively (W/F = 1.4, 400 °C, STY = 40 g/L cat/h) using water extracted FPO.23 In these experiments with an iron oxide catalyst, levoglucosan was converted to acetic acid, acetol, and formic acid, which subsequently entered a ketonization reaction pathway.23 CeZr metal oxide was shown to generate higher acetone yields (∼40%) using pure acetic acid and acetol, and ketonization reactions to acetone were significantly inhibited by 8361

DOI: 10.1021/acs.energyfuels.6b01906 Energy Fuels 2016, 30, 8357−8368

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catalyst based on liquid flow rate, catalyst bulk density, and mass). Similar work reported the formation of methyl esters (methyl formate, acetate, and propionate with methanol) from low molecular weight carboxylic acids, yet product concentrations, yields, and the formation of higher molecular weight methyl or ethyl esters (e.g., methyl levulinic acid or pentanoic acid methyl ester) were not reported.15,16 A more detailed yield and pathway analysis of FPO catalytic esterification was provided by Hu et al. in 2012.10 Using acid resin (Amberlyst 70) and methanol (0.9/1 MeOH/oil mass ratio), these authors reported levels of ∼3.8% methyl acetate and ∼0.25% methyl levulinate from bio-oil with ∼5% acetic acid and ∼4% levoglucosan at 170 °C and 120 min.10 Using these values and assuming MA is solely derived from acetic acid (and a fraction of levoglucosan is not converted to acetic acid) and ML is derived from levoglucosan, one can estimate yields (mass produced/mass charged) of 77 and 6.25% for MA and ML, respectively. Using an identical analysis, our results indicated similar MA yields (32−93% mol MA/mol acetic fed, 17−30 g/ L MA) yet significantly lower ML yields (0.4−1.6% mol ML/ mol levoglucosan fed). Hu et al. indicated that levoglucosan was converted to ML via hydrolysis and esterification, forming methyl glucopyranoside, which is subsequently transformed to ML. In recent work, it has been demonstrated that levoglucosan can be continuously converted to EL using HZSM-5 [120−230 °C, 600 psig, 80% ethanol (v/v), 12 min] with two potential pathways for EL formation from levoglucosan: one with glucose and ethyl α- D-glucopyranoside as intermediates and the other with furfural.29 Although we do not present direct evidence, past results using pure levoglucosan and reactions at lower temperatures10,29 suggest ML was formed from levoglucosan via a series of hydrolysis and esterification steps. Simultaneous Esterification and Hydrogenation. Finally, in an attempt to reduce coke levels and determine if simultaneous continuous esterification and hydrogenation could be achieved using bifunctional catalysts (metal and acid sites), experiments were conducted with added hydrogen. Multiple changes were observed when hydrogen was added: (1) an increase in phenolics concentration using RRM, (2) a reduction in coke formation for RRM, Ni-HZSM5, and RuHZSM5, (3) an increase in cyclic ketones (e.g., cyclopentanone and 2-methyl-cyclopentanone) for Ni-HZSM5 and Ru-HZSM5, and (4) an increase in methyl esters of C3 or greater (e.g., propanoic acid methyl ester, pentanoic acid methyl ester, hexanoic acid methyl ester, heptanoic acid methyl ester, and octanoic acid methyl ester). Only when using RRM did we observe an increase in phenolics (∼40%) relative to the total estimated inlet concentration of 9 g/L (Figures 6, 7, and 11S). Although we did not observe an increase in phenolics using Ni and Ru-HZSM5, the significant reduction in coke and increase in cyclic ketones (e.g., cyclopentanone and 2-methyl-cyclopentanone) suggest that a fraction of the phenolics were converted to cyclic ketones. For RRM, Ni-HZSM5, and RuHZSM5, the reduction in coke formation ranged from 62, 88, and 67%, respectively (Figure 9S). As in the experiments without hydrogen, low levels of methyl levulinate were observed (0.75−4 g/L), yet the formation of pentanoic acid methyl esters and other longer chained esters suggests that hydrogenation of methyl levulinate or levulinic acid via LG occurred. Except for SiO2-Al2O3, the addition of hydrogen did not reduce TAN levels (relative to experiments without H2), which ranged from 23 to 33 mgKOH/g for all catalysts (Figure

Figure 3. Outlet concentrations for catalytic esterification of FPO/ methanol mixture at 250 °C and 600 psig (W/F = 0.4 h).

furfural and p-cresol (270−350 °C).26 Recent research also indicates that Brønsted acid sites in HZSM-5 catalyze ketonization of acetic acid to form acetone.27 Amorphous silica, silicate, and HZSM-5 were also shown to ketonize propanoic acid, forming 3-pentanone at relatively high selectivity (400−500 °C, 50−87%, 20−50% yield); HZSM5′s activity for 3-pentanone formation declined significantly below 300 °C. Detailed pathways, including transfer hydrogenation, dehydration, and decarboxylation steps, leading to the formation of linear and cyclic ketones from components in FPO have been presented.23,26 In this work, linear ketone yields ranged from 1.2 to 1.6% for SiO2-Al2O3 and the zeolites and from 1.65 to 5% for RRM. The lower yields relative to literature results were potentially due the presence of water, furfural, and methoxy phenolics (i.e., due to the transformation of FPO relative to pure compounds or mixtures of neat compounds), inhibiting ketonization and competition for acid sites by esterification reactions. The formation of methyl acetate, methyl propionate, and methyl levulinate indicate esterification reactions were active for all of the catalysts due to the presence of acid sites. Our results are similar to those of previous work using acid resins and low molecular weight alcohols (methanol and ethanol) to esterify FPO in batch systems; final TAN values ranging from 6 to 22 mg/g at 60 to 70 °C in 1 to 22 h residence times have been reported.15,16,28 Excluding SiO2-Al2O3, TAN values for upgrading bio-oil (without H2) in this work ranged from 36 to 25 for RRM, HZSM-5, and Ni or Ru/HZSM-5. However, this reduction in acid level (∼64−75%) was achieved in 5.5−12 min at a higher reaction temperature (250 °C) via a continuous process (an estimated liquid residence time in contact with 8362

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Figure 4. Effect of pressure (15−1000 psi) on catalytic esterification of fast pyrolysis oil using reduced iron oxides in red mud on conversions (A), yields (B), and space time yields (C) (W/F = 0.4 h; 250 °C).

because it was the only treatment indicating increased formation of phenolics (Figures 6, 7, and 11S). Iron silica and Fe/activated carbon catalysts have been shown to convert methoxy phenolics (e.g., 2-methoxy phenol or guaiacol) to phenol, cresol, and aromatic hydrocarbons (toluene and benzene) via a series of hydrogenolysis and dehydration steps.32,33 Iron catalysts (Fe2O3/alumina-S) have also been shown to crack lignin related model dimers (e.g., 4hydroxydiphenylether).34 Interestingly, in our work, when

6S) and is indicative of the incomplete acetic acid conversion (Figures 6 and 7). The increase in phenolic levels (e.g., 2-methoxy phenol) indicates the presence of undetected lignin oligomers in the feed and their subsequent hydrogenolysis, potentially via active sites in the red mud (in the presence of H2). Lignin oligomers can be as high as 30 wt % in fast pyrolysis bio-oil and are formed during pyrolysis via repolymerization mechanisms.30,31 This hydrogenolysis step was apparently unique to the RRM 8363

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Figure 5. Effect of pressure (15−1000 psig) on outlet concentrations for catalytic esterification of an FPO/methanol mixture using RRM (250 °C). C3 > esters indicates methyl esters of C3 and greater, including methyl levulinate.

hydrogen was replaced with isopropanol (i.e., FPO/methanol/ isopropanol, 1/0.3/0.7 volume ratio; 250 °C, 600 psig) using RRM, phenolics increased, and a range of hydrogenated lignin products were observed, including phenol, o-xylene, p-xylene, and methylated aromatics (Figure 12S). These results suggest that RRM may have promoted a transfer hydrogenation mechanism using isopropanol as the hydrogen donor for hydrogenolysis of lignin oligomers. We acknowledge that additional data are required to confirm the mechanism for hydrogenolysis using RRM, but it is believed that our results combined with literature analysis suggest such a mechanism could account for the increased phenolic levels and hydrogenated lignin products in the upgraded FPO outlet. Recently, Ni/Cu on carbon and Al2O3 catalysts have been shown to promote hydrogenolysis of lignin derived components in FPO when using isopropanol as a hydrogen donor. Similar to our work with RRM in the presence of H2 or isopropanol, increased levels of phenol, o- and p-cresol, and other phenolics were observed when treating FPO in batch reactors with isopropanol (300 °C, 4 h).35 In addition to the low levels of methyl levulinate, the formation of longer chain methyl esters of carboxylic acids (e.g., butanoic acid, dimethyl ester, pentanoic acid, and methyl ester) were observed when FPO was processed with hydrogen. Similar results were recently obtained when using a bifunctional (acid-metal sites, Ru/HZSM-5) catalyst and performing

continuous simultaneous esterification/hydrogenation of levoglucosan in the presence of acetate.29 Upon hydrogen addition, the ethyl levulinate yield (ethanol was used in this work) decreased significantly, and the formation of longer chain ethyl esters such as pentanoic (ethyl valerate) and hexanoic (caproic acid) acid ethyl esters were observed.29 These results suggest that levoglucosan is rapidly esterified, forming ethyl levulinate, and then hydrogenated, forming ethyl valerate.14,18,25 We believe a similar mechanism occurred in this work with methanol and hydrogen and is potentially responsible for the formation of the higher molecular weight methyl esters, especially pentanoic acid and methyl ester, when upgrading FPO in the presence of methanol and H2. However, it is also possible that long chain fatty acids or lipids were present in the FPO (which we were not able to detect) and then subsequently esterified, resulting in the formation of C7−C10 fatty acid methyl esters. Finally, the increased levels of cyclic ketones upon hydrogenation addition suggest that acetic acid can undergo ketonization to form acetone, which can condense with pyruvaldehyde (generated from acetol via transfer hydrogenation conversion to pyruvaldehyde and 1,2-propylene glycol) and then undergo hydrogenation to form a diketone (e.g., 2,5-hexadione).23,26 The diketones can then undergo intramolecular cyclization to form cyclic ketones (e.g., 3-methyl cyclopent-2-en-1-one). The increased formation of cyclo8364

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Figure 6. Effect of H2 on catalytic esterification of fast pyrolysis oil mixed with methanol (A), yields (B), and space time yields (C). (W/F = 0.4 h, 250 °C).

place after a cyclone/hot gas filter of a fluidized bed FPO system. Alcohol or alcohol plus H2 addition could take place before the quench step after the cyclone/hot gas filter.37,38 Instead of rapidly cooling to form a liquid (bio-oil), this step could use a catalyst to perform esterification and ketonization (and partial hydrogenolysis/hydrogenation if H2 or a hydrogen donor is present) of the FPO using the energy from the fast pyrolysis step (e.g., 400−500 °C for fast pyrolysis to 250 °C for catalytic upgrading). Subsequently, further hydrotreating and hydrocracking at higher temperatures would be performed on the upgraded oil. The red mud could possibly be used as a

pentanone suggests that furfural formed from levoglucosan undergoes sequential hydrogenation and dehydration in the presence of the bifunctional catalysts with acid and metal sites (specifically Ni and Ru/HZSM-5).36 Recent work with pure LG indicates a portion of LG can be reformed into acetic acid, 1hydroxy-2-propanone, and furfural.23 Overall, our results suggest it might be possible to convert a partial slip stream of fast pyrolysis oil to methyl or ethyl esters and other upgraded components or treat the entire stream as the first stage in a two-stage process. We envision that alcohol (either methanol, ethanol, or isopropanol) addition would take 8365

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Figure 7. Effect of H2 on outlet concentrations for catalytic esterification of an FPO/methanol mixture using RRM (250 °C, 600 psig). C3 > esters indicates methyl esters of C3 and greater, including methyl levulinate. GDA was not present in SiO2-Al2O3 and Ru-HZSM5, and GDA levels were very low in the other catalyst treatments (92−96% conversion).

sacrificial catalyst (an idea previously put forth by Schlaff’s group)39 and guard column for a more expensive catalyst in a two-stage esterification/hydrotreating step, given that it is a solid waste and thus inexpensive and has the ability to simultaneously catalyze ketonization, esterification, and hydrogenolysis steps. If isopropanol is used, then H2 would not have to be mixed with the liquid phase, given the results from our work and the literature indicating that catalytic hydrogenolysis/ hydrogenation occurs via hydrogen transfer using appropriate catalysts (reduced red mud in our case, and Ni catalysts as reported in the literature).40,41 Because commercial production of ethanol from lignocellulosics or methanol from captured CO2 is not currently economical (i.e., commercial biorefineries or carbon capture and CO2 hydrogenation are not in operation), methanol would be sourced from natural gas or methane.42 However, recent reports on the use of an indium oxide catalyst capable of converting CO2 to methanol (100% selectivity, 0.23 s, 1000 h time on stream without deactivation at 300 °C) suggest CO2 could be a possible source of methanol in the future.43

Similarly, isopropanol would be sourced from the petroleum derived feedstock propylene.44 Ethanol would be sourced from corn in the United States or sugar cane in Brazil because these countries and the associated biomass are the sources for ethanol production.45 The significantly lower price of methanol (∼$0.58/gal)42 suggests it would be more practical to use compared to ethanol (∼$1.5−1.7/gal) or isopropanol.46,47 If use of isopropanol is proven feasible on a large scale (i.e., as effective as H2), then its recovery and regeneration (i.e., hydrogenation of acetone back to isopropanol in a separate reactor)41 would be critical because it is significantly more expensive than methanol (∼$2.30/gal).48 Future work to address scale-up issues would include (1) understanding the effect of contact time and the oil/alcohol ratio and H2 partial pressure on conversion and yields, (2) catalyst longevity studies and determining possible catalyst regeneration methods, (3) determining if red mud can be reduced in situ using components in bio-oil (e.g., formic acid) and still be as effective as H2 reduced red mud, (4) determining the consistency of red mud catalyst from batch to batch to 8366

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Energy & Fuels achieve upgrading results, (5) a more detailed characterization of the spent red mud, if red mud is used as a catalyst, for possible reuse applications and to determine if it is considered remediated after its use, and (6) a detailed kinetic analysis of the esterification and hydrogenolysis/hydrogenation reactions, which is required if isopropanol is used as an internal hydrogen and alcohol source, and a process must be developed to regenerate isopropanol for reuse as a hydrogen donor.

CONCLUSIONS Simultaneous continuous esterification, ketonization, and hydrogenation of fast pyrolysis oil was demonstrated when an external alcohol was mixed with bio-oil and multifunctional catalysts having metal, acid, and base sites, which were used in the transformation. Although RRM had the lowest surface area and pore volume, this catalyst proved to be the most effective, generating STYs and product concentrations similar to those of the other catalysts (HZSM-5 and Ni and Ru/HZSM-5). RRM was also active in hydrogenolysis, producing additional phenolics from lignin oligomers in the FPO and generating the lowest coke levels in the presence of external H2. The presence of base sites and low levels of strong acid sites coupled with the mixed oxide nature of RRM may have contributed to its observed activity. The formation of ML, MA, and C3 or greater methyl esters were indicative of esterification/hydrogenation activity. Two potential pathways for acetic acid conversion (ketonization and esterification) and ML formation from levoglucosan were observed. Using the bifunctional catalysts in the presence of hydrogen resulted in significant coke reduction (60−80%) and the production of esters of carboxylic acids C3 or greater (e.g., pentanoic and hexanoic acid methyl esters) and MA from the mixture. More interestingly, contrary to the other catalysts, an increase in phenolic levels (e.g., 2-methoxy phenol) was observed using the iron oxide catalyst with H2 or isopropanol, indicating the presence of undetected lignin oligomers in the feed and their subsequent hydrogenolysis. Simultaneous esterification and hydrogenation resulted in percent reduction in TAN ranging from 66 to 76%. The results indicate that alcohol addition, potentially using an alcohol capable of esterification and acting as a hydrogen donor, to fast pyrolysis oil vapor (after fast pyrolysis and hot gas filtration) coupled with ex situ catalysis could be incorporated into current continuous FPO processes to upgrade the oil.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01906. Catalyst characterization results (pore size distribution, isotherms, NH3 TPD), GC/MS chromatograms, and catalyst coke analysis (PDF)

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The authors graciously thank Joby Miller, Richard Spier, and Andrew Smola for their invaluable contributions of time and effort in analyzing materials and gaseous compositions. Support for this research was provided in part by a grant from the Southeastern Sun Grant Center with funds provided by the United States Department of Transportation Research and Innovative Technology Administration (Grant DTOS59-07-G00050) and by DOE (Grant DEFG3608GO8814: Biorefining and Carbon Cycling Program).

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AUTHOR INFORMATION

Corresponding Author

*Phone: 706-583-0155; Fax: 706-542-8806, E-mail: jkastner@ engr.uga.edu. Notes

The authors declare no competing financial interest. 8367

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