Subscriber access provided by University of Winnipeg Library
Biofuels and Biomass
Hydrotreating of Model Mixtures and Catalytic Fast Pyrolysis Oils over Pd/C Richard J. French, Kellene A. Orton, and Kristiina Iisa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03106 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Hydrotreating of Model Mixtures and Catalytic Fast Pyrolysis Oils over Pd/C Richard J. French*, Kellene A. Orton, Kristiina Iisa National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401, USA
ABSTRACT Noble metal catalysts may be attractive for hydrotreating of catalytic fast pyrolysis (CFP) oils. Model mixtures of raw non-catalytic fast pyrolysis and CFP oils representative of upgrading over HZSM-5 were hydrotreated in a batch reactor at 250 to 360°C with palladium/carbon catalyst and 100 bar (cold) of hydrogen. The CFP oils gave high carbon yields of >95%, consumed less hydrogen per gram of oil produced, and had lower final oxygen concentrations than the raw oils did. GC/MS results were consistent with hydrogenation of alkenes and some oxygen-functionalized aromatic rings (e.g. phenolics) followed by hydrodeoxygenation at 360°C. Complete deoxygenation was not obtained, and higher temperatures are recommended. The model oils’ chemical transformations, carbon yields, and deoxygenation were similar to those of oils produced in a bench-scale reactor with HZSM-5 catalyst.
INTRODUCTION Biomass refers to any biologically-derived material that can be available in large quantities, and it is a potential source of liquid fuels (biofuels). The terrestrial biomass produced in the United States has the potential to be converted sustainably to biofuels that will meet 30% of the country’s 1 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
gasoline needs.1
Page 2 of 38
A number of combustible liquids can be produced from biomass, but
hydrocarbons can take advantage of current refining, blending, and distribution infrastructure and so have potential economic advantages. One way to produce hydrocarbon biofuels is hydrotreating of fast pyrolysis oils.2,
3
Fast
pyrolysis oil is prepared by heating biomass to approximately 500°C in about one second, then condensing the vapors in 1-2 seconds.4 The pyrolysis oil has many undesirable fuel properties imparted by its high oxygen content.5 Hydrotreating removes oxygen from pyrolysis oils by hydrodeoxygenation to produce hydrocarbons, but at the cost of a high consumption of hydrogen. Therefore, it may be better to catalytically modify the pyrolysis vapors before condensing them to produce a lower-oxygen liquid (catalytic fast pyrolysis or CFP oil) which would require less extensive hydrotreating. The CFP oil is also more stable for storage and shipping than the raw (uncatalyzed) pyrolysis oil. Uncatalyzed fast pyrolysis oil contains reactive oxygenates, e.g. aldehydes and sugars, which cause extensive coke formation and catalyst bed plugging during hydrotreating and require several stages of hydrotreating at increasing severity.2, 3, 6 CFP produces oils with reduced complexity and may eliminate the most reactive species enabling a simpler onestage hydrotreating process.7-9 Raw pyrolysis oil hydrodeoxygenation has been studied extensively, but there are more limited studies on the hydrotreating of CFP oils7,
9-12.
Sulfided NiMo and CoMo catalysts have been
applied to upgrading of CFP oils in continuous7, 10 and batch reactor9 systems. While there is no clear consensus on the acceptable organic oxygen concentration in the hydrotreated product, 500 m2/g, Table S3). The elemental composition was 85.2% carbon, 0.56% hydrogen, 0.37% nitrogen, 0.16% sulfur, 5.1% oxygen and 9.8% ash by weight on a dry basis. Hydrotreating. The hydrotreating was performed in a 100 mL stirred batch reactor (Parr). 6.2 g of raw model oil, 8.1 g of CatHiO model, 9.1 g of CatMedO model, or 9.5 g of CatLoO model oil was added to the reactor so that the moles of hydrogen added to the headspace would equal the moles needed to completely hydrogenate and hydrodeoxygenate the model oil to alkanes. In the case of the real oils, 6.3 g of raw oil or 8.2 g of CFP bottom oil was used. The oil was diluted to 25 mL with dodecane to allow for adequate stirring and temperature control. 0.6 g of Pd/C catalyst was added, and the reactor was pressurized with 3 bar of helium and 100 bar of hydrogen. The reactor was then sealed and heated at maximum power to the setpoint in about 20 minutes. The observed maximum pressures ranged from 130 to 310 bar, being highest at the highest temperature with the raw oil and lowest with the CatLoO at the lowest temperature. The temperature was held 5 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 38
for one hour, then the electric heater was removed and the vessel allowed to cool with stirring to < 30°C (~1 hour). Gas from the reactor was recovered in a 25 L gas bag. Solids were recovered by vacuum filtering the liquids poured from the reactor. The reactor was rinsed with pyridine to recover any remaining oil; then the nitrogen concentration in the recovered rinsate and known composition of pyridine were used to calculate the oil in the rinsate. The liquid, rinse, and solid products were analyzed individually for C, H, N, O (direct), Karl Fischer water, and ash at Huffman Hazen Laboratories (Golden, CO). The raw oils, both real and model, produced a recoverable (< 2.7 g) aqueous phase, and the aqueous and oil phases were analyzed separately for these experiments. Gases from hydrogen through C4 hydrocarbons were analyzed by sampling from the gas bag into a Micro-GC (Varian CP 4910, with MS5A, PoraBond Q and CPSil columns). Gas chromatography/mass spectrometry (GC/MS) of liquids.
An Agilent 7890A gas
chromatograph coupled with an Agilent 5975C mass selective detector was used for analysis of the model liquid products and pyridine rinses. The chromatographic column was a Restek Rtx1701 (14% cyanopropylphenyl/86% dimethylsiloxane) of dimensions 60 m x 0.25 mm x 0.25 µm GC parameters were set as follows: injection port was set at 250 °C with a split ratio of 30:1 and column flow was set at 1 mL/min and injection volume was 1 µL. The oven was set to 45°C for 10 min followed by a temperature ramp of 3°C/min to a final temperature of 250°C held for 5 min. The MS transfer line temperature was set at 280°C, MS source and quad temperatures were set at 230°C and 150°C, respectively. The MS was operated in continuous scan mode in the range of m/z 29-600. Samples of hydrotreated oil, aqueous phases (if available), and pyridine rinses were prepared gravimetrically in acetonitrile containing internal standards for GC/MS analysis.
6 ACS Paragon Plus Environment
Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Compounds included in the model compound mixtures were used to prepare standards for instrument calibration, and additional compounds selected as products of hydrotreatment of the model compounds were also included for instrument calibration. Several compounds included in the model compound mixtures (methanol, glyoxal, acetaldehyde, and acetone in the raw oil and 2methylfuran in the CFP mixture) were not detected due to co-elution with acetonitrile or elution prior to the MS solvent delay, and these compounds are excluded from the data presented here due to lack of detection. Since peaks other than those calibrated were detected, all peaks more than 0.5% of the largest peak area for the organic or aqueous liquids and 0.1% of the largest peak (which was pyridine) for the pyridine rinses were reported, identified by match to an NIST library. Reference samples were prepared by diluting the model mixtures in dodecane in the same proportion as used during hydrotreating and then in acetone to ensure mutual solubility. This was then diluted in acetonitrile in the same manner as the other samples. Analysis of the reference samples gave recoveries of about 60-100% for various peaks. Therefore, the GC/MS results should be considered qualitative. A comparison between the real and model oils was performed in a separate GC/MS analysis on initial oils (unhydrotreated) and oils that had been hydrotreated at 360°C. Samples were diluted in acetone 1:10 gravimetrically. A volume of 1 microliter was injected onto an Agilent G1530A GC - HP 5973 MS at a 50:1 split ratio. The column used for separation of compounds was a 30 m x 0.25 mm x 0.25 µm Agilent DB-5 (5% -Phenyl)-methylpolysiloxane 190915-433 capillary column. The GC oven temperature was held at 40°C for 3 minutes, ramped to 240°C at 10°C/min, then to 300°C at 6°C/min and held for 5 minutes. Compound concentrations were qualitatively compared based on the percentage of the total ion current (TIC) peak area represented by each
7 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
individual compound. Compounds were grouped by functional groups. Compounds with NIST library matches less than 70% were assigned as “Other”. RESULTS AND DISCUSSION Compositions of Model Oils. The model oil compositions were decided based on analytical pyrolysis (Py-GC/MS) experiments of pine ex situ CFP in which progressive amounts of biomass pyrolysis vapors were passed over a 500°C fixed bed of HZSM-5 catalyst. The results of the experiments are summarized in Figure S1. Initially at low cumulative biomass-to-catalyst mass ratio, the measured liquid-range products were all aromatic hydrocarbons, predominantly 1-ring compounds, toluene, xylenes, and benzene with smaller amounts of 2-ring compounds, such as indenes, indanes, and naphthalenes. As the cumulative biomass- to-catalyst mass ratio increased, oxygenates were formed, with the first oxygenates being furans and phenols, as reported elsewhere.41 As the biomass-to-catalyst mass ratio further increased, methoxyphenols (guaiacols) and acids appeared. The composition of each group of compounds changed as the catalyst became more deactivated, for example the proportion of alkylated aromatics (e.g. xylenes) increased and that of non-alkylated aromatics (e.g. benzene) decreased as the biomass-to-catalyst mass ratio increased. Initially the furanics consisted of furan and methyl furans but at higher biomass-tocatalyst ratios, furfurals and other more oxygenated furanics were formed. Similarly, methoxyphenols were originally relatively simple compounds, such as 2-methoxyphenol, but more oxygenated compounds, such as 4-hydroxy-3-methoxycinnamaldehyde and vanillin, (4-hydroxy3-methoxybenzaldehyde) were formed later, at higher biomass-to-catalyst ratio. The model CFP oils were prepared based on the most prominent compounds measured in these experiments. The compositions of the model oil mixtures are given in detail in Table S1 and a summary is shown in Figure 1. 8 ACS Paragon Plus Environment
Page 8 of 38
Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
The composition of the raw pyrolysis oil (Table S2) was chosen based on a mixture in the literature39 with some modifications to make the mixture more representative of pyrolysis oils (replacing hydroxyacetaldehyde by acetaldehyde and glucose by levoglucosan). The raw model oil is a mixture of nine oxygenated compounds, only two of which are the same as the compounds in the catalytic models. It included alcohols (methanol) and carbohydrates (levoglucosan) in addition to the oxygenate groups present in the CFP oils. The fraction of aromatic hydrocarbons decreased from the CatLoO oil to the CatHiO and was zero in the Raw model oil (Figure 1). Correspondingly, the oxygenates increased in percentage and so did the variety of oxygenated compounds, with the CatLoO oil only containing furans and simple hydroxyaromatics (phenols and naphthols) while the CatHiO also included carbonyls, acid, and methoxyphenols. Due to the lower solubility of water in the less polar compounds formed during CFP, separate aqueous and organic phases are formed during CFP and, therefore, the water contents in the CFP oils are lower than in the raw pyrolysis oil. Hydrotreating Results. Table S4 in the Supporting Information gives details on the mass and component balances in the experiments. Total mass balances from weighing of reactor contents before and after reaction with the addition of gas formation gave good closures (97% on average). Balances from the separated components (oil phase, aqueous phase, solids, gas, oil retained in pyridine rinse) were somewhat lower (93% on average). The carbon balances were 88-108% after subtraction of dodecane and pyridine (used for oil dilution and rinsing, respectively). This suggests relatively high variability in carbon closure due to the uncertainties in liquid recovery and dodecane/pyridine subtraction. The almost uniformly low oxygen balances (75% on average) are attributed to an inability to recover quantitatively small droplets or films of aqueous phase, which were formed during hydrotreating. 9 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The key results from the experiments are included in Table 2. The percentage of carbon retained in the liquids, i.e. carbon efficiency, was estimated by difference by subtracting carbon in product gas and solids (coke) so as to compensate for the difficulty in recovering the liquids and the errors associated with subtracting dodecane and pyridine from oil products. The carbon efficiency was high (>95%) for both the model and real CFP oils, showing the potential for a high yield of hydrocarbon oil from CFP oils. In contrast, the carbon yields for the raw oils were significantly lower, 69-86% at the highest temperature, in line with literature results6, 13, 14. The low liquid yields from the raw pyrolysis model are caused by high production of carbon oxides (Table 3); at 360°C, over 30% of the input oil mass was converted to carbon oxides for the raw model. The carbon oxides probably originate from the acid, carbonyls, and carbohydrate present at high levels in the raw model oil. The real raw oil produced the second-largest amount of carbon oxides (Table 3); however, the yield of carbon oxides from the real raw oil was significantly lower than from the model raw oil (12% vs. 31-33% at 360°C). This suggests that the more complicated, larger molecules in the real pyrolysis oil, e.g. pyrolytic lignin and sugar oligomers, are less likely to form carbon oxides than the simple molecules in the model raw oil, e.g. methanol, glyoxal, hydroxyacetaldehyde, and hydroxyacetone. The carbon-oxide formation is significantly lower for the CFP oils than for the raw oils (< 5% vs. 12-33%), explaining the higher carbon efficiencies for the CFP oils. In addition to carbon oxides, up to 2% methane and 1% C2-C4 hydrocarbons were formed from the raw model oil, and lower fractions from the CFP oils, except from the CatLoO model for which no gaseous hydrocarbons were detected (Table 3). A likely source for methane is methoxy groups in the methoxyphenols, which were present in all other feed oils except the CatLoO oil (Figure 1). Coke formation was low for all CFP oils (95%).The desired oxygen level of 1% was not achieved, but the conditions were quite mild in these experiments with limited availability of hydrogen (hydrogen in reactor equivalent to calculated total consumption for hydrodeoxygenation and saturation), which likely limited deoxygenation. 12 ACS Paragon Plus Environment
Page 12 of 38
Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
The results do suggest that higher temperatures, perhaps 400°C as found with sulfided base metal catalysts7 may be required for complete deoxygenation of the CFP oils. GC/MS Analysis of the Model Oils. Gas chromatography and GC/MS help to identify specific products.
In the relatively simple model liquids, this makes it feasible to identify
hydrodeoxygenation reaction pathways.
Graphs showing the concentrations of the input
compounds in the hydrotreated liquids are included in Figure S2 in the SI. Several oxygenates from the CatHiO model and their possible products are shown in Figure 4. This oil is chosen because it contained a larger variety of compounds and is therefore of greater interest. Cyclopentenone (Figure 4a) shows saturation of the double bond in the ring at 250°C, and paravinyl guaiacol (Figure 4b) shows complete hydrogenation of the vinyl group at the same temperature. For both, the saturated products react further at higher temperatures, but further reaction products were below the identification threshold in this analysis. However, when the 360°C oil was analyzed together with the real oils in a different analysis, cyclopentanol and cyclopentane as well as para-ethylphenol, para-ethylcyclohexanone, and ethylcyclohexane were identified as reaction products, suggesting hydrogenation and deoxygenation of cyclopentanone as well as demethoxylation, ring hydrogenation and eventual deoxygenation of ethylguaiacol. 2-Methylphenol (Figure 4c) undergoes some ring hydrogenation to form the ketone and, at more severe conditions, the alcohol and the saturated hydrocarbon. The methylcyclohexane detected is not likely to originate from toluene because the toluene concentration is constant at varying hydrotreating conditions, similar to para-xylene in Figure 5. Thus, the reaction pathway appears to be ring saturation to form the ketone, ketone hydrogenation to the alcohol and then alcohol hydrodeoxygenation to the cycloalkane.
13 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 38
Naphthol (Figure 4d) forms the 5,6,7,8-tetrahydro derivative exclusively under low temperature hydrotreating, indicating hydrogenation of the ring without the hydroxy group.
The
tetrahydronaphthol then decreases sharply at 360°C coincident with a sharp increase in decalin, showing saturation of both rings and deoxygenation.
Neither naphthalene nor the 1,2,3,4-
tetrahydro derivative are formed in measurable amounts. Hydrocarbon hydrogenation is shown in Figure 5. Xylene does not hydrogenate appreciably at any condition (Figure 5a), similar to toluene (not shown). In contrast, indene shows complete conversion to indane (Figure 5b), reflecting hydrogenation of the non-aromatic ring, comparable to the hydrogenation of the double bond in cyclopentenone and p-vinyl guaiacol. Methylnaphthalene (Figure 5c) shows complete conversion to methyltetralins already at 250°C, but no methyl decalin is measured at any temperature. The reappearance of a small amount of methyl naphthalene at 360°C can be attributed to the higher thermodynamic stability of aromatics compared to aliphatics with increasing temperature. Both 2- and 6-methyltetralins were detected. The latter in which the ring without a methyl group is hydrogenated had a higher area count than the former in which the ring with the methyl group is hydrogenated. Even though the tetralins are measured by area counts alone, the responses of the two compounds should be similar, and this shows preference for hydrogenation away from the methyl group. No ring-growth to anthracene or phenanthrene is detected. This suggests that there is little tendency to form coke and perhaps limited formation of C-C bonds in general. The overall picture from the GC/MS of the models is a tendency toward hydrogenation of alkenes and one of the rings of naphthalene but not of single ring aromatics. Hydrogenation of aromatic rings in 2-ring compounds occurs preferentially in the ring without substituents. 2methylphenol differs from the other examples with respect to hydrogenation of a single ring, 14 ACS Paragon Plus Environment
Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
suggesting that the combination of ortho di-substituents promotes hydrogenation of the aromatic ring. This suggests that guaiacol should go through a similar pathway. Logical breakdown products of guaiacol were not detected in this analysis, but the second GC analysis, described below, did show some products (cyclohexanediol, cylohexanone, and cyclohexane) consistent with this.
At higher temperatures, hydrodeoxygenation occurs thus supporting the
hydrogenation/dehydration mechanism noted in the literature26,
29, 30, 32,
but some direct
deoxygenation may also be occurring. Additional mass spectral data are presented in Tables S5 and S6. GCMS comparison of the real and model oils. The results of the GC/MS analysis comparing the model and real oils (raw model, raw real, CatHiO and CFP2 oils before and after hydrotreating at 360°C) is shown in Figures 6 and 7 and additional mass spectral data are presented in Table S7. Compounds containing one six-membered ring (benzenes, phenolics, and/or cyclohexane derivatives) are shown in green, furans in purple, multiple rings in blue, while non-ring compounds are shown in orange. Methoxyphenols, phenols, and other aromatic oxygenates are distinguished by pattern in Figure 6 while varying levels of ring saturation are indicated by pattern in Figure 7. One feature of Figure is that the models and real oils have similar total amounts of identified oxygenates. Both the models and real oils contain large fractions of methoxy aromatics, which react in hydrotreating conditions to form phenolics and saturated rings with methoxy, ketone, and alcohol groups (one-ring non-aromatic oxygenates in Figure . This supports the notion of aromatic saturation followed by deoxygenation more than direct deoxygenation. The detected furans were mostly furfural before hydrotreating and tetrahydrofurans afterward, signifying saturation of the double bonds in the furan ring. Acids (acetic acid) and 0-ring aldehydes (hydroxyacetaldehyde) were detected only in the real raw oil. However, these were present in the raw model oil (Figure 1 15 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
and Table S1), but due to slight differences in solvent delays, they were not detected in the raw model oil. The real oils have higher percentages of unidentified compounds (designated other in Figure -7) due to the more complex composition of the real oils. Figures 6-7 also reveal only slight increases in the fractions of detected fully deoxygenated compounds for the model and real CFP oils before and after hydrotreating. A small fraction of the phenols and methoxyphenols were converted to cyclohexanes (one-ring no aromatic hydrocarbons in Figure . The oil oxygen content decreased more than suggested by the figure due to the loss of methoxy groups from methoxyphenols and conversion of e.g. furfural to simpler furans with only the ring-oxygen remaining. The high fraction of remaining phenols and furans under the conditions studied demonstrates the recalcitrant nature of the oxygen in these compounds. The raw oils, both model and real, showed a larger increase of completely deoxygenated compounds (20-30 percentage points) than the CFP oils did, including the formation of linear and cyclic hydrocarbons. A striking feature of Figure is how much more multi-ring aromatics the real oils contain than the model oils as well as the extensive hydrogenation of these compounds. The majority of this difference is due to 17-21% GC/MS area contribution from 3-ring structures in the real oils (1721 % in CFP 2 and 6 % in Raw HT) while no 3-ring structures are present in the model liquids. The higher content of 3-ring aromatics in the real CFP oil collected from the fluidized bed reactor, compared to the model oil, whose composition was based on micro-pyrolyzer experiment, may be due to the different contact times in the two systems. The fluidized bed had a nominal reaction time on the order of 0.1 s and the microreactor of 0.01 s. Also, during CFP two fractions of oil were collected, a bottom (heavier) fraction and a top (lighter) fraction,40 but only the larger bottom fraction was hydrotreated here. Lower molecular weight compounds, such as one-ring aromatic hydrocarbons, are concentrated in the top fraction40, so this provides an additional explanation for 16 ACS Paragon Plus Environment
Page 16 of 38
Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
the low levels of 1-ring hydrocarbons in the real CFP oil studied. The higher concentration of multi-ring compounds and the greater saturation of these compounds during hydrotreating explains why the real CFP oils consume somewhat more hydrogen than the models do (Table 2 and Figure 3). Note the tendency in the models and the real CFP oil to partially hydrogenate the polyaromatics to single aromatic rings and leave the single-ring aromatics unreacted. This is analogous to only one ring becoming saturated in two-ring compounds in the model oils (Figure and Figure ). There were no 3-ring aromatics detected in the hydrotreated model oil. This suggests a low rate of ring growth and consequently low coke formation even though there may be mechanisms of coke formation that do not involve much 3- or 4-ring aromatics and so would not be detected by this GC method that does not detect heavier polyaromatic hydrocarbons. This suggestion is consistent with the low rate of coking reported in Table 3. There was also no evidence of ring growth in the real CFP oil but some ring compounds were formed from the real raw oil during hydrotreating. The ring growth in the raw oil can be explained by the presence of more reactive compounds, such as aldehydes (~40% of the REAL Raw IN nonring group in Figure 7, none detected in the others) and levoglucosan. Overall, the GC/MS comparison shows that similar proportions of hydrocarbons and oxygenates were detected in both the model and real oils. Similar types of reactions occurred during hydrotreating in both sets of oils despite the real oils having more multi-ring compounds. The reactions included conversion of methoxyphenols into phenolics and saturated ring compounds (cyclohexanols and cyclohexanones, and eventually cyclohexanes) and saturation of C-C double bonds and aromatic rings in multi-ring compounds with typically one aromatic remaining. The similarities confirm that the much simpler model oils can be used successfully for studying reactions during hydrotreating of CFP oils. 17 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
CONCLUSIONS A noble metal catalyst Pd/C was successfully applied to hydrotreating of model and real catalytic fast pyrolysis (CFP) and non-catalytic biomass fast pyrolysis (raw) oils. High carbon efficiencies (>95%) were obtained for the catalytic (CFP) oils, and they consumed less hydrogen per mass of final product than the non-catalytic (raw) pyrolysis oils did. GC/MS analysis confirms that there is hydrogenation at lower temperatures but hydrodeoxygenation of the most recalcitrant compounds, phenols and furans only at 360°C. The aromatic rings in phenols became hydrogenated prior to deoxygenation leading to the formation of cyclohexanones and cyclohexanols as intermediates. Alkenes and one ring of naphthalene derivatives are readily hydrogenated but single aromatic hydrocarbon rings resist hydrogenation under the conditions studied. There was no formation of three-ring aromatics in the model oils—this suggests low coke formation from the compounds present in the models. The model CFP oils gave similar performance to the real CFP oils with respect to chemical transformations, carbon yield, and deoxygenation. The higher presence of multi-ring aromatics in the real CFP oils causes the real oils to consume slightly more hydrogen than the models do, since the multi-ring compounds hydrogenated readily to the point of containing a single aromatic ring. These experiments did not produce the desired oxygen level of
