Bio-oil Stabilization by Hydrogenation over Reduced Metal Catalysts

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Bio-oil stabilization by hydrogenation over reduced metal catalysts at low temperatures Huamin Wang, Suh-Jane Lee, Mariefel Valenzuela Olarte, and Alan H. Zacher ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01270 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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ACS Sustainable Chemistry & Engineering

Bio-oil stabilization by hydrogenation over reduced metal catalysts at low temperatures Huamin Wang*, Suh-Jane Lee, Mariefel V. Olarte, and Alan H. Zacher Chemical and Biological Process Development Group, Pacific Northwest National Laboratory (PNNL), 902 Battelle Boulevard, Richland, Washington 99352, United States

*Corresponding Author: Huamin Wang E-mail: [email protected] Tel.: +1-509-371-6705

KEYWORDS:

Biomass;

Fast

pyrolysis;

Bio-oil;

Hydrotreating;

Hydrogenation; Catalyst; Deactivation.

ACS Paragon Plus Environment

Stabilization;


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ABSTRACT: The thermal and chemical instability of biomass fast pyrolysis oil (bio-oil) presents significant problems when they are being converted to hydrocarbon transportation fuels. Development of effective approaches for stabilizing bio-oils is critical to the success of the biomass fast pyrolysis and bio-oil upgrading technology. Catalytic hydrogenation to remove reactive species in bio-oil has been considered as one of the most efficient ways to stabilize bio-oil. This paper provides a fundamental understanding of hydrogenation of actual bio-oils over a Ru/TiO2 catalyst under conditions relevant to practical bio-oil hydrotreating processes. The results indicated hydrogenation of various components of the bio-oil, including sugars, aldehydes, ketones, alkenes, aromatics, and carboxylic acids, over the Ru/TiO2 catalyst and 120 to 160oC. Hydrogenation of these species significantly changed the chemical and physical properties of the bio-oil and overall improved its thermal stability, especially by reducing the carbonyl content, which represented the content of the most reactive species (i.e., sugar, aldehydes, and ketones). The change of content of each component in response to increasing hydrogen additions suggests the following bio-oil hydrogenation reaction sequence: sugar conversion to sugar alcohols, followed by ketone and aldehyde conversion to alcohols, followed by alkene and aromatic hydrogenation, and then followed by carboxylic acid hydrogenation to alcohols. Sulfur poisoning of the reduced Ru metal catalysts was significant during hydrogenation; however, the inorganics at low concentrations had minimal impact at short times on stream, indicating that sulfur poisoning was the primary deactivation mode for the bio-oil hydrogenation catalyst. The knowledge gained during this work will allow rational design of more effective catalysts and processes for stabilizing and upgrading bio-oils.


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Introduction

Lignocellulosic biomass is a renewable, abundantly available, and low-cost resource and is considered to be the most potentially sustainable resource for producing fuels, chemicals, and carbon-based materials 1-3. Lignocellulosic biomass can be converted into different forms of intermediates for fuels and chemicals through numerous processes

1-3

,

and pyrolysis integrated with upgrading is the simplest and most cost-effective approach. The resulting pyrolysis oils (bio-oils) have been determined to be the least expensive renewable liquid fuel

1-7

. However, some properties of bio-oil, such as high oxygen and

water content, poor stability, and corrosiveness, present significant problems during the upgrading process or direct utilization

1-8

. Primarily, bio-oils are thermally and

chemically unstable, mainly because of the presence of reactive species such as carbonyl compounds (aldehydes and ketones). These compounds present a major challenge for catalytic hydrotreating of bio-oil because of the occurrence of severe catalyst deactivation and even reactor plugging by carbonaceous species formed by thermal polymerization of the reactive species (Figure 1)

9-15

. Thermal condensation reactions can occur at a

temperature of 100oC and lower, and these reactions accelerate significantly as the reaction temperature increases

10-12

. Thus, treatment of bio-oil at the high temperatures

required for deoxygenation can be extremely challenging. Therefore, to arrive at a sustainable, reliable, and economical process for upgrading bio-oils to fuels, it is critical that effective approaches for stabilizing bio-oils are developed. Several bio-oil stabilization techniques, including physical and chemical methods, have been studied

15

, and some methods have shown to be very promising for



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producing thermally stable bio-oil for high-temperature catalytic treatment such as hydrodeoxygenation

9, 14, 16

. Physical methods, such as solvents addition

15, 17

or char

removal by filtration 15, 18, were both effective approaches; however, they only worked to enhance the storage and transport stability of bio-oils. Chemical methods focused on the removal of reactive oxygenated functional groups through various heterogeneous catalytic processes under mild conditions, such as esterification with alcohols

15, 19

or

mild hydrogenation/hydrodeoxygenation 9, 14-16. Hydrogenation over a metal catalyst at a moderate temperature could convert active species, such as carbonyl groups, to stable alcohols. This approach has been shown to be the most promising and efficient method for stabilizing bio-oil and, therefore, has attracted the most attention. A two-step process with a low-temperature hydrogenation step

to

stabilize

the

bio-oil

prior

to

the

high-temperature

hydrodeoxygenation/hydrocracking step for oxygen removal has been developed, as shown in Figure 1. This process represents the state of the art for upgrading for bio-oil to fuels

5, 7, 9, 14, 16, 20

. The overall operational lifetime depends on the hydrogenation ability

of the first-step catalyst used. A significant increase in operation time from 5 to over 60 days can be achieved by adding a more active reduced Ru catalyst bed to enhance the hydrogenation ability of the sulfided Ru catalyst at low temperatures, as has been demonstrated at Pacific Northwest National Laboratory (PNNL) 14. However, because of the complicated reactions involved, limited information is available regarding the chemistry that occurs during hydrogenation of actual bio-oils. Past research has focused on the overall two-step processes including hydrogenation followed by hydrodeoxygenation with limited efforts to understand the hydrogenation


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step

5, 9, 14, 21, 22

. Most recently, some research has been focused on low-temperature

hydrogenation of bio-oils; however, the primary focus has been on fractions of bio-oil 16, 23-26

or model compounds

5, 27-36

. Ru, Pd, and Ni catalysts have been studied, but the

primary focus has been on the Ru catalyst because of its superior activity in aqueousphase hydrogenation 37. In this work, we began to develop a fundamental understanding of the chemistry that occurs during hydrogenation of actual bio-oils under conditions relevant to an actual biooil hydrotreating process and of the major components in bio-oil that cause catalyst deactivation. We characterized bio-oil feed stocks, bio-oils hydrogenated to different extents, and catalysts to provide insights into the chemical and physical properties of these resources and to understand the correlation of the properties with the composition of the bio-oil and catalysts. The information will allow the rational design of more effective catalysts and a process for stabilizing and upgrading bio-oil.

Experimental Methods

Materials and catalysts Ru/TiO2 catalyst (3 wt%) was prepared by using impregnation methods in PNNL. The catalysts were pelletized and sized to 30-60 mesh for the hydrotreating test. Two woodderived pyrolysis oil feedstocks, Bio-Oil A and Bio-Oil B, were used. Bio-Oil A was produced by the Technical Research Center of Finland (VTT) from softwood forest residues. Bio-Oil B was produced by the Battelle Memorial Institute from pine sawdust. Bio-oil A with sulfur removal was prepared by treating with a nickel catalyst in hydrogen



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at 80oC and 3.5 MPa in the bench-scale hydrotreater described below. Bio-oil B with inorganic material removal was prepared by flowing through a packed ion-exchange resin bed at 40oC and atmospheric pressure. Properties of these bio-oil samples are described in detail in the results section.

Characterization Bio-oils and hydrogenation products were analyzed for elemental components including carbon, hydrogen, nitrogen (ASTM D5291/D5373), O (ASTM D5373mod), S (ASTM D1552/D4239), and water content (Karl Fischer titration, ASTM D6869). Inorganic material and sulfur contents of the bio-oil samples were measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Viscosity and density measurements were conducted on a Stabinger viscometer (Anton Paar SVM 3000) at 40oC. The carbonyl contents of the bio-oil samples were determined using a modified titration method (the modified Faix method)

38, 39

. A modified ASTM standard

method D664 for determining the acid content of petroleum products was used to determine both carboxylic acid numbers (CAN) and total acid numbers (TAN) of bio-oils 40

. The TAN includes carboxylic acids as well as weaker acidic compounds such as

phenolics. Therefore, the phenolic number (PhAN) was calculated by the difference between the TAN and the CAN. Carbon-13 nuclear magnetic resonance (NMR) spectra were taken in a 500-MHz Varian Unity Plus spectrometer with bio-oil in deuterated dimethyl sulfoxide (d6-DMSO) with relaxation agent, chromium (II) acetyl acetonate. Spectra were processed by using Mestre-Nova software. Catalyst samples were characterized for specific surface area by nitrogen adsorption/desorption isotherms at −196°C using Micromeritics ASAP 2020. The


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Brunauer, Emmett, and Teller equation was used for calculations. Elemental analysis of inorganic and sulfur content of the catalysts was performed by ICP-OES and elemental analysis of carbon of the catalysts was conducted by the standard CHN determination method of ASTM D5291/D5373.

Hydrotreating test A bench-scale hydrotreater was used for the bio-oil hydrogenation tests. It was configured as a single-pass, concurrent, continuous, down-flow reactor (see Figure 2). It is described in detail by Elliott et al. 20. The system can operate at up to 12.4 MPa (1800 psig) with a maximum catalyst temperature of 400oC. The reactor was 63.5-cm-long 316 stainless-steel tubing with a 1.3 cm internal diameter. Catalyst bed configurations and temperature profiles were shown in Figure 2. It was important to place catalysts on the top portion of the reactor (70-140oC, pre-isothermal zone) instead of using inert beads. It was because the thermochemical condensation reactions could occur at temperatures stating from 80oC and catalytic hydrogenation could suppress condensation reactions at the temperature zone with catalyst loaded. Prior to the test, the catalyst was treated in-situ in flowing 10% hydrogen in nitrogen at 1.0 MPa with heating from ambient temperature to 300oC at a rate of 140oC/h and then holding for 2 hours. Each test was conducted over 48 hours on stream, and the liquid products and outlet gas analysis data by a micro gas chromatography (micro-GC) were collected over the entire period with individual products and data sets collected in an operating window of 6 hours. The hydrogen consumption in mol H2 / g feed was calculated based on the bio-oil flow rate and the difference of hydrogen inlet and outlet flowrate. The hydrogen outlet flowrate was calculated by the total outlet flow rate, based



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on the measured outlet flowrate by a volumetric flow meter with deduction of flowrate increase caused by accumulation of liquid in the reactor system, and the hydrogen concentration measured by the mirco-GC. The yield of oil and gas products were determined based on the weight of oil product and the outlet gas flow rate and composition. After each test, the catalyst beds were rinsed with acetone, and spent catalysts were then collected for analysis.

Results and Discussion

Bio-oil hydrogenation on Ru catalyst Bio-oil A, a typical fast pyrolysis bio-oil from softwood forest residues, was used to conduct hydrogenation experiments on the Ru/TiO2 catalyst at 10.3 MPa, liquid hourly space velocity of 0.40 h-1, and hydrogen-to-bio-oil ratio of 2500 L/L. Two separate tests from a fresh catalyst were performed at 120oC and 160oC, and samples and data were collected every 6 hours on stream. Table 1 listed the yields of products, including liquid products, the hydrogenated bio-oil, and gaseous products. The mass balance was always nearly 100%. The major products were hydrogenated bio-oil and gas yields were minor at around 0.005 to 0.017 g/g bio-oil at 160oC and early TOS and lower than 0.005 g/g biooil at late TOS or 120oC. The gas products were primary methane and CO2, probably from decomposition of carboxylic acids in bio-oil, such as formic acids or acetic acid, at a low conversion. The hydrogenated bio-oil products were not as homogenous as the initial bio-oil feed and formed two indistinctly segregated phases, a light phase on top and a dense phase on bottom, upon standing for a period of over 1 hour at room


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temperature. Such phase segregation could be the result of the miscibility change of species after hydrogenation. The hydrogenated bio-oil samples were well shaken until homogeneous macroscopically before sampling for analysis. No solid products were observed macroscopically in the collected products. As will be shown below, some carbonaceous species were detected in the spent catalysts. However, the maximum yield of these carbonaceous species were below 0.0012 g/g bio-oil, which was negligible and not taken into account for the mass balance. Hydrogen addition was the amount (in mmol) of hydrogen consumed per gram of biooil and is used here to represent the activity of the catalysts at different TOS and temperature. As shown in Figure 3 and Table 1, hydrogen addition of the bio-oil decreased linearly with TOS at both 160 and 120oC, indicating deactivation of the catalysts used. As discussed below, the major cause of deactivation was poisoning of the Ru metal by sulfur-containing species and the consequent catalyst fouling by carbonaceous species because of lowered hydrogenation activity of the poisoned catalysts. Catalyst deactivation is slower at 120oC than at 160oC, probably because of slower adsorption of sulfur species and formation of carbonaceous species. The same deactivation progression of bio-oil hydrogenation catalysts was also observed for the other catalysts such as Ru/C and RuSx/C 14. Thirteen hydrogenated bio-oil samples with hydrogen additions increased from 3.8 to 12.4 mmol/g bio-oil were obtained for detailed analysis (see the next section) to understand the hydrogenation chemistry of bio-oil.

Analysis of hydrogenated bio-oil with different hydrogen additions



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Detailed analyses of the Bio-Oil A and its hydrogenated products with different hydrogen additions obtained from the two hydrogenation tests using Bio-Oil A feed were performed by elemental analysis for carbon, hydrogen, nitrogen, oxygen, and sulfur contents; Karl Fischer titration for water contents; density and viscosity measurements; carbonyl titrations; acid titrations; and

13

C NMR to understand their properties and

composition. The correlation of properties and compositions of the hydrogenated bio-oils with varied hydrogen additions could help to develop a deep understanding of the reaction mechanisms occurring during actual bio-oil hydrogenation under the relevant reaction conditions. As shown in Figure 4 and Table S1, the hydrogen-to-carbon ratios (dry basis) and oxygen contents (dry basis) were calculated based on elemental analysis for carbon, hydrogen, and oxygen and Karl Fischer titration for water contents. Hydrogen-to-carbon ratios increased linearly as the hydrogen addition increased. The oxygen and water contents, however, varied differently at different reaction temperatures. At 120oC, no changes in oxygen and water contents were observed at different hydrogen additions, and formation of CO2 as a gas product was minimal (100oC (Figure 6 D). It is also possible that condensation and hydrogenation reactions could occur in parallel as discussed above; however, the latter should dominate. Carbonyl content analyzed by carbonyl titration were in good agreement with the above observed conversion of sugar species and aldehyde and ketones. The potentiometric titration (i.e., the modified Faix method) was used to determine the content of carbonylcontaining functional groups, including that in aldehydes, ketones, and simple sugar species 39. As shown in Figure 7 A and Table S1, the carbonyl content in hydrogenated bio-oil initially decreased linearly as the hydrogen addition increased, and then leveled off as hydrogen additions continued (>9 mmol/g). This outcome is consistent with

13

C

NMR results discussed above, indicating the rapid conversion of carbonyl-containing molecules such as aldehydes, ketones, and sugar species. The linear dependence of the carbonyl content with hydrogen additions as well as extrapolation of the curve (carbonyl content with hydrogen addition) to zero matching the number of feed strongly suggests that hydrogenation of these carbonyl-containing molecules is the dominant reaction. If the thermal-chemical reaction, which should be independent hydrogen additions, is significant, one would expect that extrapolation of the curve (carbonyl content with hydrogen additions) to zero should be lower than that of feed.


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The signal of aromatic and olefinic carbon in the range of 100 to 163 ppm of the

13

C

NMR spectrum of hydrogenated bio-oils decreased with increasing hydrogen additions and almost disappeared when hydrogen additions became very high (>11.1 mmol/g biooil), indicating the conversion of aromatics during bio-oil hydrogenation. The aromatics in bio-oil are primarily from lignin and consist of monomers, dimmers, and oligomers of aromatics with varying numbers of oxygen-containing substituents (e.g., phenol, cresol, guaiacol, syringol, benzoic acid, etc.) 51. The content of phenolic compounds (PhAN), a portion of aromatic compounds in bio-oil, was determined using an acid titration method (see Figure 7 and Table S1). The modified potentiometric titration method provides improved detection of the phenolic content of bio-oils, and the results can be used to differentiate carboxylic acids from other acidic components such as phenolic compounds 40

. It is notable that this method might only determine a portion of the phenolic

compounds in bio-oil. A linear decrease in the PhAN with increasing hydrogen additions was observed, which is consistent with our NMR results. Reaction of a phenolic compound to a product that could not be detected by acid titration could be hydrogenation to saturate the ring, hydrogenolysis to remove the hydroxyl ion, or an etherification reaction that produces an ether. However, the latter two reactions should be very slow at the condition used in our study. As reported by Zhao et al

32

, phenol could

only be hydrogenated to cyclohenxanol at 200oC on a Pd/C catalyst, and direct breaking of a C-O bond in phenol was extremely slow at 200oC. Therefore, hydrogenation of aromatics in bio-oil occurred during bio-oil hydrogenation at the conditions tested here (Figure 6 E). However, hydrogenation of oligomers of the aromatics might be slow. Aromatic compounds also contribute to the instability of bio-oil. For instance, phenolic



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compounds could act as catalysts for acid-catalyzed condensation reactions and also directly react with other species, such as aldehydes, to form a polymer, such as phenolaldehyde resins, due to its aromatic ring that was subject to reaction via electrophilic substitution with the aid of acid 12. Aromatics with an active substituent, such as vanillin with a carbonyl group, had a higher tendency for condensation through its carbonyl groups

12

. Saturation of aromatic ring as well as the carbonyl groups could significant

mitigate the reactivity of these molecules and enhance the thermal stability of bio-oil. Carbonyl carbon in carboxylic acids and esters, which showed peaks in the range between 163 to 180 ppm in the

13

C NMR spectrum, changed slightly at low hydrogen

additions ( 7 mmol/g bio-oil), more than 90% of detected carbon by 13C NMR were alkyl carbons and methoxy/hydroxy carbons in the hydrogenated products, compared to 54% of those species in the feed stock, indicating a significant change in the composition of the bio-oil and, therefore, its thermal stability. The acidity of bio-oil, mainly because of volatile acids such as acetic and formic acid, and phenolic compounds, make pyrolysis bio-oils corrosive, especially at elevated temperatures 8. The modified TAN method was used to measuring the acidity of fast pyrolysis bio-oils and their hydrotreating products 40. As shown in Figure 7 B and Table S1, the TAN for Bio-Oil A feed was 131.8 mg KOH/g, a value higher than the value determined by the standard TAN method (~80 mg KOH/g)

52

. This result is consistent

with the fact of that the modified method used in our study provided more sensitive detection for the phenolic compounds in bio-oils over the standard method 40. A decrease in the TAN of hydrogenated bio-oils was observed, and the number decreased as the hydrogen addition increased. The TAN number decreased more than 50% while hydrogen addition was greater than 10 mmol/g. These results agree with outcome of the hydrogenation of carboxylic acids and phenolics discussed above. The decrease of acidity could significantly reduce the corrosiveness of bio-oils and improve their processing. Physical properties of bio-oils such as density and viscosity are important parameters, especially when directly used as a fuel 8. Lower density and viscosity provide better efficiency in terms of combustion and emissions. It was found that the density and



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viscosity normally depended on the water and alcohol contents and the water insoluble components 8. Aging or thermal treatment of bio-oil could lead to increased viscosity and density. We found that, after catalytic hydrogenation, the density and viscosity of hydrogenated bio-oils were always lower than the values of the Bio-Oil A feed and decreased as the hydrogen addition increased, as shown in Figure 8 and Table S1. The density of the hydrogenated bio-oil decreased from 1.18 to 1.14 at the lowest hydrogen addition and to 1.08 at the highest hydrogen addition. An approximately 40 to 50% decrease of viscosity was observed after hydrogenation of bio-oil. Clearly, the decrease in density and viscosity was the result of hydrogenation of sugars and other oxygenates to form alcohols. For some samples, the increase in water content also played a role. By assuming that the decrease of the contents of the major components measured by titration methods solely due to the hydrogenation reaction and the hydrogenation reaction of these components occurred via the reaction network showed in Figure 6, the hydrogen addition to the individual components were calculated. The selectivity of hydrogen addition to these components was then determined as the percentage of calculated hydrogen addition of the individual components to measured overall hydrogen addition to bio-oil. As shown in Figure 9, the calculated hydrogen addition represented 65~75% of the total hydrogen addition measured during the reaction, indicating that the titration method captured most of the desired components in the bio-oil. The components that were not included (labeled as “Other” in Figure 9) probably were non-phenolic aromatics, olefins, carbonyl in esters, and some high-molecular-weight phenolic and sugar species that were not titrated by the methods used. The hydrogen-addition selectivity clearly indicated that hydrogenation of carbonyls in sugar, aldehyde, and


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ketones consumed ~50% of the overall hydrogen addition, which is in agreement with the high concentration of these species in bio-oils

53

. Phenolic compounds were the second

group of components that consumed hydrogen additions, and carboxylic acids consumed the least hydrogen. The decrease of hydrogen selectivity to carbonyl groups and increase to phenolics and carboxylic acids with increasing hydrogen additions occurs because of the almost complete conversion of carbonyl groups at the high hydrogen additions, which agrees with the disappearance of peaks for sugar and carbonyl carbon on the

13

C NMR

spectrum and the leveling off of carbonyl content to below 0.5 mmol/g at hydrogen additions >9 mmol/g bio-oil. All the results presented above indicate the occurrence of hydrogenation of various components in bio-oil, including sugars, aldehydes, ketones, alkenes, aromatics, and carboxylic acids, over the Ru/TiO2 catalyst and 120 to 160oC. Hydrogenation of these species significantly changed the chemical and physical properties of the bio-oil and improved the thermal stability, especially by reducing the carbonyl content, which contains most reactive species (sugar, aldehydes, and ketones). Changing the content of each component by increasing hydrogen additions, which represents increasing reaction temperature or number of active sites on the catalyst, also suggests the following reaction sequence during bio-oil hydrogenation at 120-160oC on a Ru catalyst: sugar conversion to sugar alcohols, then ketones and aldehyde conversion to alcohols, then alkane and aromatic hydrogenation, and then carboxylic acid hydrogenation to alcohols. Other reactions, such as condensation of sugar decomposition products and aldehydes and ketones as well as decomposition of carboxylic acids, could also occur, however, at a much slower rate than hydrogenation reactions. This sequence is in agreement with



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Page 20 of 43

reports of the reaction temperature required for hydrogenating model compounds representing the above components of bio-oil

27, 29-34

. The dependence of bio-oil

properties and composition on hydrogen additions could help determine the appropriate amount of hydrogen to be used to maximize overall performance and economics associated with consumption of hydrogen, which is expensive. Our ongoing research is focusing on the evaluation of the thermal stability of the hydrogenated bio-oil in the practical process, such as hydrotreating at a high temperature of around 400oC, upgrading in a fluid catalytic cracking unit, or using directly as fuels. As mentioned above, we has already demonstrated a significant increase in operation time of high-temperature hydrodeoxygenation/hydrocracking step by a deeper hydrogenation of bio-oil

14

. The

transformation of reactive species in bio-oil into alcohols could potentially make the biooil suitable for being upgraded by catalytic cracking. Some relevant research reported the co-cracking of alcohols and unsaturated compounds in bio-oil could generate liquid hydrocarbons 54, 55.

Effect of contaminants in bio-oil on the hydrogenation performance As shown in Figure 3, decreasing the hydrogen addition and increasing TOS indicates deactivation of the bio-oil hydrogenation catalyst for hydrogenating bio-oil A. Deactivation of bio-oil hydrotreating catalysts was observed widely and is considered to be a major challenge for upgrading bio-oils 9, 13. A previous study showed that the major deactivation mode of a sulfided Ru catalyst for bio-oil hydrogenation in a two-stage process for bio-oil upgrading was fouling of catalysts by carbonaceous species formed by the condensation reaction of active species in the bio-oil and possible poisoning by


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inorganic elements from the bio-oil

13

. A detailed analysis of spent Ru/TiO2 catalysts

after the hydrogenation test of bio-oil A at 160oC in this research was conducted by elemental analysis and surface area and pore volume measurements, as shown in Table 3. A slight decrease of surface area, from 53.5 to 45.3 m2/g, and pore volume, from 0.26 to 0.20 mL/g, was observed, indicating a blocking of pores by carbonaceous species, consistent with the presence of around 6% C on the spent catalyst. It also explained the slight decrease in Ru content from 2.8 to 2.6 wt%. However, the loss of surface area and the estimated carbonaceous species content observed here were much less than the reported numbers on the sulfided Ru catalyst

13

, which showed an almost complete loss

of surface area (from ~300 to ~2 m2/g) and around 30 wt% of the carbonaceous species in the spent catalyst. It indicated much improved hydrogenation performance of the catalyst used in the current research as the reduced form, which could well compete with the thermal-chemical reaction and significantly mitigate catalyst fouling by carbonaceous species. Such a slight decrease of surface area and pore volume did not agree with the significant decrease of catalyst activity (a hydrogen addition from 12.4 to 5.4 mmol/g bio-oil). The carbonaceous species could be removed by hydrogen treatment at 400oC and, as shown in Table 3, surface area, pore volume, and Ru content were fully recovered after the hydrogen treatment, and the carbon content decreased to

Bio-oil Stabilization by Hydrogenation over Reduced Metal Catalysts

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