Hydrothermal Catalytic Deoxygenation of Fatty Acid and Bio-oil with In

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Hydrothermal Catalytic Deoxygenation of Fatty Acid and Bio-oil with In-situ H2 Chao Miao, Oscar Marin-Flores, Tao Dong, Difeng Gao, Yong Wang, Manuel Garcia-Perez, and Shulin Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02226 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Sustainable Chemistry & Engineering

Hydrothermal Catalytic Deoxygenation of Fatty Acid and Bio-oil with In-situ H2

Chao Miaoa, Oscar Marin-Floresb, Tao Donga, Difeng Gaoa, Yong Wangb, Manuel Garcia-Péreza, Shulin Chena*

a

Department of Biological Systems Engineering, Washington State University, Pullman, WA,

99164 b

Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington

State University, Pullman, WA, 99164

*Corresponding author. Tel.: +1 509 335 3743; fax: +1 509 335 2722. E-mail address: [email protected] (S. Chen)

ACS Paragon Plus Environment

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ABSTRACT: Fatty acid and its derivatives have recently received considerable interest as a possible precursor for producing renewable hydrocarbon. Compared to the traditional hydrotreating method that consumes a significant amount of H2, this study developed a hydrothermal catalytic deoxygenation approach which produced hydrocarbon from fatty acids and bio-oil with in-situ self-sustaining H2. The presence of H2O played a critical role in the generation of in-situ H2 via enhancing water-gas shift and reforming reactions, thus promoted fatty acid decarbonylation and increased paraffin yields. Both saturated and unsaturated fatty acids (stearic and oleic acid) were employed as model compounds, where 100% conversion was achieved with 63.59% paraffin yield from stearic and 47% n-paraffin yield from oleic acid. By investigating the reaction pathway, fatty acid and glycerol reforming, and water-gas shift reactions were found as the major reactions for the in-situ H2 generation. Decarbonylation was found as the major route for fatty acid and bio-oil deoxygenation. Fatty acid reforming and hydrogenoloysis reactions were found as the major reactions for producing short-chain hydrocarbon (C8-C16). We previously developed a sequential hydrothermal liquefaction method to produce bio-oil from yeast biomass. In this study the produced bio-oil was also tested, where 100 wt% bio-oil conversion and 55.17 wt% of liquid n-paraffin yield were achieved at 320 °C. This study demonstrates that this hydrothermal catalytic process is a promising approach for producing liquid paraffin (C8-C15) from fatty acid and bio-oil under no H2 supply condition. KEY WORDS: Hydrothermal catalytic deoxygenation, Fatty acid, Bio-oil, In-situ H2, Decarbonylation, Hydrogenolysis, Reforming, Paraffin


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■ INTRODUCTION Hydrothermal liquefaction has been widely developed as a favorable process to produce bio-oil from aquatic biomass as it avoids the biomass-drying expenses

1-5

. However, the produced bio-

oil from hydrothermal liquefaction tends to be viscous, tar-like 6, and contains significant amount of oxygen, which leads to the issues of high freezing point and low heating value

7-8

and

constrains its utilization as transportation fuel. Thus, the bio-oil needs to be deoxygenated as a part of downstream processing. Hydrothermal process is a promising approach to perform fatty acid deoxygenation by utilizing water as the reaction medium 9. The increased hydronium ions (H3O+), hydroxide ions (OH−), interphase mass transfer coefficient, and fatty acid solubility in subcritical water condition (200374 °C, 5-22MPa) creates a highly reactive environment for oxidation 9, reforming decarboxylation reactions

7,

12-13

10-11

, and

. Savage, et al. investigated fatty acid hydrothermal

decarboxylation over two heterogeneous catalysts, 5% Pd/C and 5% Pt/C 7, 13. Different length of saturated fatty acid, stearic, palmitic, and lauric acid decarboxylation were investigated, resulting in above 75% alkane yield. This approach exhibited a great performance on saturated fatty acid decarboxylation. However, when processing unsaturated fatty acid (oleic and linoleic acid), only ~10% was obtained from oleic acid and ~5% from linoleic acid

7, 13

. Since decarboxylation of

oleic and linoleic acid required a prior saturation of C=C consuming H2 13, the final hydrocarbon yield is low if no external H2 is supplied. Additionally, most of the reported decarboxylation processes were conducted over noble metals, Pd, Pt, Ru, etc. If the noble catalyst activity could not be maintained in a long duration, process cost will be tremendous. Thus, to ensure the synthesized hydrocarbon from HTL bio-oil is cost competitive, the research direction should



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focus on the following three aspects: (i) enable both saturated and unsaturated fatty acid decarboxylation; (ii) over low cost catalyst; (iii) with no or low amount of external H2 supply. We previously developed a hydrothermal deoxygenation process over Ni-based catalyst under inert condition, achieving >60% paraffin yield from palmitic acid 6. Compared to the noble metals, Ni has attracted increasing interest because of its highest fatty acid hydrogenation activity and low cost

8, 14-18

. A key issue of Ni- based catalysts is the demand of H2 in

deoxygenation reaction, because nickel favors decarbonylation 17, which requires one mole of H2 to decarbonylate one mole of fatty acid. Hence, with no H2 supply nickel based catalysts showed a low activity and fatty acid conversion rate ~20% 8, 17, 19. However, in the hydrothermal media, in-situ H2 was found generated via water-gas shift and reforming reaction 6. The self-generated H2 was subsequently used to promote fatty acid decarbonylation and decarboxylation over Ni based catalyst. Recent publications further demonstrated the existence of organic acid reforming in hydrothermal condition which generated CO2 and H2 as the main products generated

H2

could

not

merely

promote

catalyst

efficiency

(Ni)

on

10-11, 20

fatty

. The acid

decarbonylation/decarboxylation, but also to hydrogenate the unsaturated fatty acid and yield hydrocarbon. Therefore, reforming small amount of fatty acid to generate H2 could be a feasible solution for fatty acid, especially unsaturated fatty acid, and bio-oil decarboxylation under inert condition. We previously developed a sequential hydrothermal liquefaction (SEQHTL) process to produce bio-oil from yeast Cryptococcus curvatus 5. Combining the prior work with this study, an integrated hydrothermal process to produce hydrocarbon biofuels from microorganism will be developed through 1) SEQHTL to produce bio-oil; 2) hydrothermal deoxygenation to hydrocarbon. The produced bio-oil is rich in saturate fatty acid (palmitic and stearic) and


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unsaturated fatty acid (oleic and linoleic) accounting for 32.90 wt% and 41.84 wt%, respectively. Thus, we select stearic acid as the model compound to explore the saturate fatty acid reaction pathway and source of H2 in hydrothermal process. Oleic acid as another model compound will be probed for the reaction pathway of unsaturated fatty acid in hydrothermal process with no H2 supply. Finally, SEQHTL derived bio-oil deoxygenation will be studied.

■ EXPRIMENTAL SECTION Catalyst Preparation. Ni supported on ZrO2 was utilized as catalyst in this study. Incipient wetness impregnation was employed to synthesize Ni/ZrO2. Depending on the studied loading of Ni metal (0%, 5%, 10%, 20%), various amount of Ni(NO3)2·6H2O was dissolved in E-pure water then carefully dropped into the ZrO2 with a consistent stirring. After 4 h ambient drying, the solution was further dewatered in an oven for 12 h at 110 °C, then calcined in a 400 °C furnace for 4 h with dry air. Then pure H2 with 100 mL/min volume rate was employed to activate the catalyst (hydrodeoxygenate/reduce NiO to Ni metal) at 500 °C for 6 h at a heating rate of 5 °C/min. 5% Pd/C was purchased from Sigma. All chemicals for catalyst synthesis and testing were also purchased from Sigma Aldrich. Bio-oil used in the experiment was prepared by following the method developed in our previous work 5. Catalyst characterization. Nitrogen sorption was used to determine the sample surface areas. Nitrogen adsorption-desorption isotherms were conducted on a Micromeritics TriStar II 3020 physisorption analyzer at -196 °C using UHP N2. Before measurement, 0.2 g of catalyst was degassed at 300 °C for 1 h under vacuum. The surface areas of the catalysts were calculated based on the Brunauer–Emmett–Teller (BET) model.



Temperature programmed desorption (TPD) of ammonia or carbon dioxide was performed. The catalysts were activated in helium at 500 °C (ramping rate, 5 °C/min) for 1 h. Carbon dioxide or ammonia was adsorbed at 35 °C or 100 °C at a partial pressure of 1 mbar. Subsequently, Helieum volume rate at 30 ml/min was used to purge out the molecules physisorbed in the samples for 2 h. For the TPD experiments, 5 samples were sequentially heated from 100 to 765 °C (ramping rate, 10 °C/min) to desorb ammonia and from 35 to 450 °C (ramping rate, 10 °C/min) to desorb CO2. The rates of desorbing species were monitored by mass spectrometry (Balzers QME 200). To quantify the acidity, the signal was calibrated using a standard HZSM-5 zeolite (Si/Al=45) with a known acid site concentration. The response of the CO2 signal was calibrated based on the decomposition of NaHCO3. Activity Tests. The same experimental protocol with our previous experiment was followed in this study with the same mini-batch reactor (10 mL) assembled from 3/8-inch stainless steel parts 6

. In a typical run, 500 mg of fatty acid and 350 mg of catalyst (catalyst/substrate ratio at 70%),

with 4.5 mL of E-pure water, were loaded into the reactor with carefully stirring. With the fixed catalyst/substrate ratio at 70%, various water amount (H2O/stearic acid mass ratio at 0, 0.5, 4, 9, 14) was screened to seek the optimal reaction condition and to discover the reaction mechanism. Before the reaction, the reactor was purged with N2 to ensure the inert condition in the whole reactor. The reactor was then placed inside a small furnace, with a fixed reaction temperature 300 °C, excepted the indicated runs at 320 °C. After the reaction, quickly dip and quench the reactor into a cooling bath to reduce the temperature to 20 °C. After 30 min to condense the volatile organic products, gas phase products were analyzed using GC. After gas phase analysis, reactor cap was opened and added with methylene chloride to extract the organic products. The whole mixture of products, catalyst, and methylene chloride passed through a filter paper to


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remove the catalyst. The organic phase (filtrate) will be rotary evaporated to separate the organic solvent (methylene chloride) and the organic products. After collecting all the reaction products, the reactor will be washed with water and methylene chloride separately to remove the residue products. In the reusability test, the removed catalyst after paper filtration was reloaded into the mini-batch reactor after dried under N2 for 6 h. Then following the previous depiction, runs of catalyst reusability test were conducted at 320 °C, 6 h, with 0.2 g catalyst, 0.5 g bio-oil and 4.5 g H2O. SA and OA were selected as model compounds. Conversion rate (C%), yield of product (C%), and selectivity of product (C%) were calculated as below equations. When SEQHTL bio-oil was used as reactant, conversion (wt%) was calculated as the mass of reacted bio-oil divided by total loaded bio-oil. Products yield (wt%) was calculated as the mass of the product divided by total loaded bio-oil. Experiments were repeated three times for statistical purposes. (%) =

! " #$% (%) =

&'

)$%%* ! " #$% (%) = (+%%) = (+%%) =

( &'

,, ,,

,, &' ,,

(1) (2) (3) (4) (5)

GC-MS, GC-FID Analysis. GC-MS and GC-FID analysis protocol for liquid organic phase products followed our previous developed method with eicosane used as the internal standard for products quantification 6. The vapor phase was analyzed with an Agilent CP490 Micro GC equipped with four columns (5Å molecular sieve, PPQ, Al2O3, and non-polar SiO2) and TCD detectors with the method employed by Li et al. 21.



■ RESULTS AND DISCUSSION Catalyst Characterization. The physical and chemical properties of the screened catalysts in this study were summarized in Supplementary Table 1. The highest BET surface areas is Pd/C (904 m2/g), as the lowest surface areas is 20%Ni/ZrO2 (70 m2/g). The BET surface areas of different Ni loading were approximately in the range between 70-85 m2/g, suggesting Ni metal has a relative good dispersion on ZrO2. Catalyst Screening. Deoxygenation activity of stearic acid (SA) over Ni/ZrO2 and Pd/C was observed at hydrothermal condition (Table 1). An increase in the metal loading supported on ZrO2 from 0 to 20% leads to a raise in the SA conversion from 7.86% to 67.08%, as well as an increase in the paraffin yield from 0.07% to 43.63%. The pure ZrO2 support lead to a higher selectivity to stearone 23.77%, but a lower selectivity to n-C17 alkane 0.84%. Following the increase of metal loading from 0 to 5%, the selectivity of stearone was significantly decreased since the dispersed metal atoms reduced the surface area of ZrO2 support. Compared to Ni/ZrO2, Pd/C exhibited a higher selectivity to n-C17 96.96%. Short chain n-paraffins (C8-C16) were more produced over Ni based catalyst and the selectivity was increased with Ni loading from 38.61 (5% Ni/ZrO2) to 44.57% (20% Ni/ZrO2). This is attributed to the Ni activity on C-C cracking by generating shorter chain paraffin and methane

6, 15, 17

. Low selectivity of n-C18 (lower than 1%)

and high selectivity of n-C17 over all nickel supported catalysts suggested that the major route of SA deoxygenation was via decarboxylation or decarbonylation, but not hydrodeoxygenation. According to the SA conversion and paraffins yield, 20% Ni/ZrO2 was found to be the most effective catalyst in this work and used in the following study.


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Impact of Water on Hydrothermal Catalytic Deoxygenation of SA over Ni/ZrO2. Figure 1 exhibited the impact of water on deoxygenation of SA over Ni/ZrO2 catalyst under inert condition with each run carbon balance over 95%. It exhibits that 100% conversion of SA and 64% yield of total liquid paraffins were obtained with water/SA ratio at 4:1. SA conversion was significantly increased from 37% at 0:1 water/SA ratio to 100% at 4:1. And liquid paraffins yield were also increased from 2.8% at 0:1 water/SA ratio to 64% at 4:1. Further increase in water/SA ratio (>4:1) shows a slight decrease in both conversion and paraffin yield. This could be attributed to the excessive dilution of SA that reduced the reaction rate, or excess amount of water impeding SA decarbonylation reaction (water is the product of decarbonylation). In the gaseous phase, H2, CO2 and CH4, are the major gaseous products (Figure 2). H2 yield was gradually increased from 0.89% at no presence of H2O to 13% at 9:1 H2O/SA ratio. The formation of H2 was reported via water-gas shift and reforming of fatty acid

24

, thereby the

presence of H2O improved the H2 yield. CH4 and CO2 were also increased from 0.64% and 0.51% at no presence of H2O to 19% and 8% at 4:1 H2O/SA ratio. Little CO was detected after the reaction, which suggests any CO formed was consumed in water-gas shift and/or methanation reaction

25

. Also, trace amount of short chain fatty acid C9-C17, stearone, and stearyl stearate

were also observed as side products of the reaction (Supplementary Figure 1-2). Short chain liquid paraffins (C8-C16) were observed in SA deoxygenation, which also gradually increased with water/SA ratio. With no presence of water, short chain paraffins (C8-C16) only accounts for 17% of total liquid paraffins which then significantly increased to 64% at 4:1 water/SA ratio (Figure 1). The result suggests that cracking reaction was enhanced by the presence of water. Although Ni is a well-known catalyst for breaking C-C of hydrocarbon 11, 15, a run of pure heptadecane under the same condition did not show any cracking products. Thus, the



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short chain paraffins was not formed by long chain paraffins cracking, but via the cracking of long chain fatty acid to short chain fatty acid, followed by decarbonylation of the formed short chain fatty acids to paraffin. The observed short chain C9-C17 fatty acids (1.7 % yield at 0.4:1 water/SA ratio, Supplementary Figure 1) after SA deoxygenation further demonstrates the existence of the cracking pathway of long chain fatty acid to short chain fatty acid. Two pathways are plausible to scission C-C bond of long chain fatty acid to short chain fatty acid: (i) hydrothermal reforming of fatty acid

26

; (ii) hydrogenolysis

24

. The generation of

significant amount of H2 demonstrated the existence of hydrothermal reforming reaction. Also, Saruul Idesh reported a hydrothermal reforming reaction of fatty acid over Ni/C catalyst at 350 °C 26. Another C-C scission reaction is hydrogenolysis. The predominant amount of CH4 in gaseous phase and the successive decreased yield from n-hexadecane to n-octane demonstrated that cleavage of C-C bond also occurred through a successive hydrogenolytic cracking (Table 2) 24

. The overall reaction pathway of stearic acid was depicted as Figure 3. Effect of Reaction Residence Time on Hydrothermal Catalytic Deoxygenation of Stearic

Acid over 20% Ni/ZrO2. Figure 4-6 presents the results of stearic acid hydrothermal catalytic deoxygenation under different reaction time, with each run carbon balance over 95%. The conversion of SA was gradually increased from 38% to 100% with increasing reaction time from 2 h to 12 h, accompanied with a yield increase of liquid paraffins from 17% to 51% (Figure 4). After 6 h reaction, despite the continuous increase on fatty acid conversion, paraffin yield exhibited only a slight increase due to the increased formation of methane. In liquid phase, n-C17, short-chain paraffin (C8-C16), and short-chain fatty acids (C9-C17) are the major products (Figure 5). Yield of n-C17 was increased with the reaction time from 6.62% to 14.61% from 2 h to 6 h, and then changed slightly after 6 h because of the reduced SA


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concentration along the continue of reaction. Yield of short-chain fatty acids was initially increased to 2.27% at 2 h, with a formation rate of 0.069 mmol g-1 h-1, then reached a maximum yield 3.2% at 4 h followed by the gradually decrease to 0% at 12h. Short-chain paraffin increased with the reaction time from 10.72% to 36.4% from 2 h to 12 h. The yields of different lengths of short-chain paraffin (C8-C16) and short-chain fatty acid (C9-C17) were in a gradually decreased trend with the decrease of carbon length (Table 2). This indicated SA was first cracked by cleaving the end C-C bond to form heptadecanoic acid. Heptadecanoic acid could be further: (1) cracked to form hexadecanoic acid, or (2) decarbonylated/decarboxylated to form n-hexadecane. Since the formed n-hexadecane was relatively stable which did not crack to form shorter chain nparaffin, the formed hexadecanoic acid will repeat the pathway to form shorter chain fatty acid and n-paraffin. The detectable minimal carbon number molecule is n-octane and nonanoic acid. -. /01 22/ + 2/5 2 → -7 /00 22/ + 25 + 3/5 -. /01 22/ + /5 → -. /07 + 2 + /5 2

(6) (7)

-. /01 22/ + /5 → -7 /00 22/ + /9

(8)

Figure 6 exhibited the gaseous phase products yield at different reaction residence time. In gaseous phase, CO2, H2 and CH4 are the major products. CO was not detected by GC due to the water-gas shift reaction and/or methanation

25

. Molar yield of H2 decreased from 18% to 11%

with the prolonged reaction time from 2 h to 9 h. This implied that H2 was initially produced by SA reforming, but then gradually consumed by hydrogenolysis and deoxygenation, which lead to the yield increase of CH4 and CO2. According to the oxygen balance before and after reaction, occurrence of SA reforming could be further demonstrated. Before the reaction total oxygen is 0.0035 mol (calculated based on oxygen mole in 0.5g SA), while after reaction CO2 has contained 0.0058 mol oxygen (calculated based on 12 h reaction), which is ~60% more of



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oxygen than in reactant. This demonstrated CO2 and H2 were generated via SA reforming and water-gas shift reactions. Supplementary Figure 3 showed the conversion of SA and different products yields at various reaction temperatures and residence time. Temperature exhibited a positive effect on the conversion of SA. SA conversion was achieved at 100% at 320 °C in 2 h, while only 37.9% SA conversion was obtained at 300 °C in 2 h. The overall reaction rate of SA was calculated at 300 °C as 0.95 mmol g-1 h-1 which significantly increased to 3.56 mmol g-1 h-1 at 320 °C, thus temperature increased the reaction rate. However, the higher liquid yield was obtained at low temperature (300 °C) with long residence time (12h), which was attributed to the enhanced hydrogenolysis and reforming reactions occurred at high temperature. Compared to n-C17 formation rate increase from 300 °C to 320 °C (0.18±0.04 mmol·g-1·h-1 to 0.53±0.04 mmol·g-1·h1

), short-chain compounds formation rate exhibited a significantly promotion from 0.41±0.08

mmol·g-1·h-1 at 300 °C to 1.83±0.1 mmol·g-1·h-1 at 320 °C. The increased formation rate demonstrated that cracking reaction (reforming and hydrogenolysis) was favored by high temperature 27. Hydrothermal

Catalytic

Deoxygenation

of

Oleic

Acid.

Hydrothermal

catalytic

deoxygenation of OA over 20%Ni/ZrO2 was operated in 300 °C, 2-6 h, with no external H2 with each run carbon balance over 95%. Complete conversion of OA was achieved with stearic acid (SA) and liquid paraffin (C10-C18) as the major products. The increased reaction time promoted hydrocarbon yield and reduced SA yield. Around 78% SA was produced with 17% paraffin yield in a 2 h run, while 28% SA was produced with 42% paraffin in a 6 h run (Figure 7). Compared to SA deoxygenation, paraffin yield produced from OA was relatively lower before 4 h. After 6 h,


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OA and SA reached similar paraffin yield. The prolonged reaction time also increased the fraction of short-chain paraffin (C8-C16/total paraffin) from 23% at 2 h to 56% at 6 h (Figure 7b). OA hydrothermal deoxygenation occurred through two steps: hydrogenation of OA to SA and decarboxylation of SA to paraffin 13. Although no external H2 was input, the in-situ generated H2 was supplied for OA hydrogenation. A complete hydrogenation of OA was completed after 2 h with no OA observed. As reaction proceeds, the formed SA suffered further deoxygenation to produce paraffin. Thus, compared to SA deoxygenation at the same condition, OA exhibited a lower paraffin yield at the beginning period (2 h and 4 h in Figure 7), then reached a similar paraffin yield after 6 h. Another worthy noted variance is the fraction of short-chain paraffin (C8C16/total liquid paraffin) obtained from OA and SA (Figure 7b). SA exhibited a similar fraction around 60-65% during 2-9 h, while only 23% short-chain paraffin fraction was obtained from OA in 2h which then gradually increased to 56% at 6 h. This implied that OA cracking reaction rate was lower than SA cracking reaction rate and OA hydrogenation reaction rate, otherwise short-chain paraffin ratio should be higher at the early stage of OA reaction. Therefore, the overall OA hydrothermal deoxygenation pathway over Ni/ZrO2 initiated with the reforming of OA to generate in-situ H2 which saturate OA to SA, then followed by SA reaction pathway to form various length of n-paraffin. Hydrothermal Catalytic Deoxygenation of Bio-oil. Table 3 showed the fatty acid profile in bio-oil produced via SEQHTL at 240 °C 5 with 85.16% of fatty acid content. Deoxygenation of SEQHTL bio-oil was investigated with various reaction temperatures and residence time. Conversion was increased from 55.39% at 300 °C to 100% at 320 °C and 340 °C (Table 4). Paraffin yield was increased from 31.33% at 300 °C to 55.17% at 320°C and then reduced to 49.40% at 340 °C. This is attributed to the enhanced cracking rate of bio-oil to gaseous products



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at high temperature. Short chain n-paraffin yield was increased with temperature from 0.98% at 300 °C to 2.73% at 320°C which is also due to the enhanced cracking rate. Reaction time played a significant role in bio-oil conversion and paraffin yield. At 320°C, as reaction time prolonged from 1 h to 6 h bio-oil conversion and paraffin yield increased from 44.79% to 100% and 20.55% to 55.17%, respectively. In the gaseous phase, 7.66% gas was generated where H2, CH4, and CO2 are the major products. H2 was produced from bio-oil and increased with temperature (300 to 320 °C) and time (1 to 6 h). Three major reactions contributed to the formation of H2, reforming of fatty acid, reforming of glycerol, and water-gas shift. Due to bio-oil contained 27% acylglycerides (TAG, DAG, and MAG) (Table 3), acylglycerides were first hydrolyzed to glycerol and fatty acid. The released glycerol was subsequently reformed to generate H2 under this condition

24

. The released fatty

acid after hydrolysis also suffered hydrothermal reforming reaction to generate H2. Therefore, compared to model compounds (SA and OA), hydrothermal deoxygenation of bio-oil produced higher yield of H2 under the same condition. Compared to other bio-oil deoxygenation work 28-29

13,

, conversion of unsaturated fatty acid was achieved with a relatively high paraffin yield (~42%

from OA) in this study. Since bio-oil contained high amount of unsaturated fatty acid (~52%), deoxygenation of unsaturated fatty acid is a key factor affecting the overall bio-oil conversion rate. Figure 8 showed the effect of catalyst reusability on the bio-oil conversion and paraffin yield. The catalyst was reused for two times. Compared to the fresh catalyst, both the reused catalyst (reused one and two times) showed a complete conversion of bio-oil and a comparable yield of n-paraffin. This demonstrated 20% Ni/ZrO2 has the potential to be reused for deoxygenating biooil to produce n-paraffin. Our previous study also demonstrated the reusability of Ni/ZrO2


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through EDX and XRD study 6. Since this study employed a small batch reactor for the test, the more detailed and careful reusability test, e.g. 1000 h run on a continuous reactor, should be further conducted but beyond the scope of this work.



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■ CONCLUSION This study demonstrates that this developed method (hydrothermal catalytic deoxygenation) is an effective approach to produce n-paraffin from fatty acid with no external input of H2. 20% Ni/ZrO2 was selected as the most effective catalyst in the screened catalysts due to its high activity on fatty acid deoxygenation and the suppression of side reactions. A complete conversion of stearic acid with 63.59% liquid paraffin yield was achieved at 300 °C, 6 h, SA/water mass ratio at 1:4, with no external H2 supply. Three major reactions predominated the fatty acid hydrothermal deoxygenation over Ni/ZrO2: (a) fatty acid/glycerol hydrothermal reforming to generate H2; (b) fatty acid decarbonylation to form n-paraffin; (c) fatty acid reforming and hydrogenolysis to form short-chain fatty acids followed by decarbonylation to yield short-chain paraffin. The presence of water enhanced the formation of in-situ H2 via watergas shift and reforming reactions, thus promoted fatty acid decarbonylation and increased paraffin yields. Meanwhile, fatty acid cracking was also enhanced by water and yielded various lengths of n-paraffin from C8-C18 and fatty acid from C9-C18, as well as significant amount of methane. This approach also exhibited a high activity on unsaturated fatty acid (OA), where 100% conversion and 47% paraffin yield was achieved at 300 °C, 9h with no external supply of H2. SEQHTL derived bio-oil deoxygenation was also tested using this approach, resulting in 100% bio-oil conversion with 55.17 wt% of liquid paraffin at 320 °C, 6h. Compared to the traditional deoxygenation process (hydrodeoxygenation and decarboxylation), this work developed a promising approach to deoxygenate both saturated and unsaturated fatty acid, and bio-oil with no external input of H2.


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■ ACKNOWLEDGEMENTS This work was supported in part by the Department of Energy (DOE) and the Washington State University Agricultural Research Center. The authors would like to thank Jonathan Lomber and Moumita Chakraborty for their assistance in developing fatty acids, hydrocarbons, and other organic and gaseous products through the GC-MS analysis procedure. The authors also would like to thank Zhehao Wei, Yan Li, and Stephen Davidson for their help and support on catalyst characterization.



■ SUPPORTING MATERIALS Supporting materials is to exhibit some supporting results to the main content Supplementary Table 1 illustrates the properties of catalysts used in this study Supplementary Figure 1 shows the influence of stearic/water mass ratio on the C9-C17 fatty acid yield Supplementary Figure 2 shows the influence of stearic/water mass ratio on the selectivity to stearone and stearyl stearate Supplementary Figure 3 shows the temporal variations of stearic acid conversion and products yield at different temperatures


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1 2 3 ■ FIGURES AND TABLES 4 5 6 7 Table 1. Stearic acid conversion over different catalysts at hydrothermal condition 8 9 Conversion paraffin yield Selectivity (C%) 10catalyst (%) (%) C8-C16 n-C17 n-C18 Aa Ba Ca 11 12ZrO2 7.86±1 0.07±1 0.00±0 0.84±0.1 0.00±0 0.00±0 23.77±2 0.00±0 13 5%Ni/ZrO2 27.65±3 18.55±2 38.61±4 27.55±3 0.42±0.1 6.32±1 0.20±0.1 1.54±0.2 14 10%Ni/ZrO 59.30±4 28.17±3 38.08±4 18.87±2 0.33±0.1 4.12±0.5 0.17±0.1 0.29±0.1 2 15 43.63±4 44.57±4 19.91±2 0.45±0.1 1.14±0.1 0.52±0.1 0.00±0 1620%Ni/ZrO2 67.08±3 175%Pd/C 35.21±4 34.54±4 0.5±0.1 96.96±1 0.13±0 0.00±0 0.00±0 6.82±1 18 a a a a A= C6-C17 fatty acid, B= alkene, C= stearone, D= stearyl stearate 19 20 Experimental conditions: SA (0.5 g), H2O (4.5 ml), 300 °C, 6h, catalyst loading (0.2 g) 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 ACS Paragon Plus Environment 60

Da 0.00±0 0.79±0.1 0.78±0.1 0.00±0 0.00±0


Table 2. Various length of n-paraffin and fatty acids distribution in organic products

Compound proportion a proportion b C8 0.06% 0.18% C9 0.60% 0.76% C10 2.56% 1.94% C11 6.61% 3.98% C12 9.88% 5.26% C13 11.82% 6.20% C14 11.19% 5.85% C15 11.65% 5.98% C16 10.46% 5.65% C17 31.03% 18.98% C18 0.71% 0.36% C19 0.16% 0.13% >C19 0.35% 0.18% C7FA 0.00% 0.00% C8FA 0.00% 0.00% C9FA 0.00% 0.00% C10FA 0.00% 0.07% C11FA 0.00% 0.15% C12FA 0.00% 0.16% C13FA 0.00% 0.21% C14FA 0.00% 0.23% C15FA 0.00% 0.27% C16FA 0.00% 0.55% C17FA 0.00% 0.37% C18FA 0.00% 40.01% a n-paraffin and FA proportion in organic products at the condition of 4:1 H2O/SA ratio b n-paraffin and FA proportion in organic products at the condition of 9:1 H2O/SA ratio Experimental conditions: SA (0.5 g), 300 °C, 6h, catalyst loading (0.2 g)


Page 24 of 36

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Table 3. GC analysis of bio-oil Fatty acids Yeast biomass wt% wt% a TAG 93.92% FFA 5.33% DAG 0.54% MAG 0.22% Fatty acids Yeast biomass wt% wt% a 14:0 0.37 16:0 18.58 16:1 0.25 18:0 17.67 18:1 47.84 18:2 13.25 18:3 1.07 20:0 0.82 Saturated 37.44 MUFA 48.09 PUFA 14.32 Total 42.85b a: calculation based on the total lipid weight

Bio-oil wt % a 9.85% 72.72% 12.28% 5.15% Bio-oil wt % b 0.34 16.44 0.183 15.36 41.65 10.24 0.087 0.76 32.90 41.84 10.43 85.16

b:

calculation based on the biomass dry weight


Page 26 of 36

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Table 4. Effect of reaction temperature and time on bio-oil hydrothermal catalytic deoxygenation 320 °C 1h

320 °C 2h

320 °C 4h

320 °C 6h

300 °C 6h

340 °C 6h

Reaction condition

(wt%)

(wt%)

(wt%)

(wt%)

(wt%)

(wt%)

Conversion

44.79%±4%

81.23%±2%

97.85%±2%

100.00%±0% 55.39%±5%

100.00%±0%

Liquid paraffin yield

20.55%±2%

50.20%±5%

54.48%±5%

55.17%±5%

49.40%±5%

Short-chain paraffin

0.69%±0.1% 1.82%±0.2% 2.05%±0.5% 2.73%±0.5%

0.98%±0.3% 2.73%±0.5%

Total gas yield

7.66%±1%

8.78%±1%

H2 yield

0.27%±0.1% 0.60%±0.1% 0.73%±0.2% 1.87%±0.2%

0.34%±0.1% 0.56%±0.1%

CH4 yield

0.74%±0.1% 1.28%±0.1% 3.29%±0.4% 4.15%±0.4%

0.91%±0.1% 10.65%±1%

CO2 yield

6.65%±1%

7.52%±1%

12.91%±1%

11.03%±1%

22.12%±3%

18.10%±2%

32.54%±4%

26.52%±3%

31.33%±3%

Experimental conditions: Bio-oil (0.5 g), H2O (4.5 g), catalyst loading (0.2 g)


39.95%±4%

28.74%±3%

100

80

70

Conversion or paraffin yield (%)

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 28 of 36

80 60 60

50

40

40

30 Conversion Paraffin yield C8-C16 paraffins/total paraffins

20

20

0

10 0

2

4

6

8

10

12

14

16

Water/SA mass ratio (w/w)

Figure 1. Influence of SA/water mass ratio on the conversion of SA and the yield of paraffins Experimental conditions: SA (0.5 g), 300 °C, 6h, catalyst loading (0.2 g)


Fraction of C8-C16 paraffins in total paraffins (%)


Page 29 of 36

25

16 CH4 CO2

14

H2 20

12 10

15

8 10

6 4

5 2 0

0 0

2

4

6

8

10

12

14

Water/SA mass ratio (w/w)

Figure 2. Influence of SA/water mass ratio on the gaseous products yield Experimental conditions: SA (0.5 g), 300 °C, 6h, catalyst loading (0.2 g)


16

H2 yield (mol %)

CH4 or CO2 yield (C%)

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



(a) Decarboxylation -CO2

+H2 Decarbonylation

C17H36

-H2O, -CO

C17H35COOH

Reforming -CO2

H2

Decarbonylation Hydrogenolysis -CH4

+H2

C9-C17 fatty acid

+2H2, Hydrogenation -H2O

-H2O, -CO

C8-C17 Paraffins

+H2, Hydrogenation

C18H37OH

-H2O

C18H38

(b) Decarbonylation

Hydrogenolysis

C15H31COOH +H2

+H2

Reforming

+H2O

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

H2

CO

+H2O

+H2

Water-gas shift

CH4

Methanation

CO2 +H2 Methanation

Figure 3. Reaction pathway (a) liquid phase; (b) gaseous phase


Page 30 of 36

Page 31 of 36

100

Conversion of SA or Yield of paraffin (%)

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


Conversion Paraffins yield 80

60

40

20

0 0

2

4

6

8

10

12

Reaction Time (h) Figure 4. Conversion of SA and yield of paraffins under different reaction time Experimental conditions: SA (0.5 g), H2O (0.2 g), 300 °C, catalyst loading (0.2 g)


14


50

Yield of n-C17 or short chain compounds (%)

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 32 of 36

n-C17 (%) Short-chain paraffin (%) Short-chain fatty acid (%) 40

30

20

10

0 0

2

4

6

8

10

12

14

Reaction Time (h) Figure 5. Short-chain fatty acid, short-chain paraffin, and n-C17 yields under different reaction time Experimental conditions: SA (0.5 g), H2O (0.2 g), 300 °C, catalyst loading (0.2 g)


Page 33 of 36

3.0e-1

CH4 (C%), CO2 (C%), H2 (mol %) Yield

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


CH4 (C%) H2 (mol %) CO2 (C%)

2.5e-1

2.0e-1

1.5e-1

1.0e-1

5.0e-2

0.0 0

2

4

6

8

10

12

Reaction Time (h) Figure 6. Gaseous yields yield under different reaction time Experimental conditions: SA (0.5 g), H2O (0.2 g), 300 °C, catalyst loading (0.2 g)


14


80

50 SA OA

Short-chain paraffin fraction (%)

70

40

Paraffin yield (%)

30

20

60

50

40

30 SA OA

20

10

10 0

2

4

6

8

0

10

2

4

6

8

10

Time (h)

Time (h)

70

60

50

SA yield (%)

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 34 of 36

40

30

20 SA OA

10

0 0

2

4

6

8

10

Time (h)

Figure 7. Comparison of (a) paraffin yield, (b) short-chain paraffin fraction (C8-C16/total liquid paraffin), and (c) SA yield, produced from OA and SA hydrothermal deoxygenation at different reaction time Experimental conditions: SA/OA (0.5 g), H2O (0.2 g), 300 °C, catalyst loading (0.2 g)


Page 35 of 36

100

Conversion of bio-oil or paraffin yield (%)

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


80

Conversion Paraffin yield

60

40

20

0 0

1

2

Catalyst reused time Figure 8. Effect of catalyst reusability on bio-oil conversion and paraffin yield Experimental conditions: Bio-oil (0.5 g), H2O (4.5 g), 320 °C, 6h, catalyst loading (0.2 g)



For Table of Contents Use Only

This work developed a catalytic thermal process, which is able to produce hydrocarbon (nparaffin) from fatty acid (especially unsaturated fatty acid) and bio-oil (derived from yeast by hydrothermal liquefaction) by using in-situ H2 with no external H2 input.


Page 36 of 36

Hydrothermal Catalytic Deoxygenation of Fatty Acid and Bio-oil with In

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