Catalytic Hydrothermal Liquefaction of Microalgae for Bio-oil

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Catalytic HTL of Microalgae for Bio-oil Production over Silylated SBA-15 with High Hydrothermal Stability Qisong Lin, Kejing Wu, Yu Chen, Yin Tang, Yulong Wu, and Mingde Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03746 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Industrial & Engineering Chemistry Research

Catalytic HTL of Microalgae for Bio-oil Production over Silylated SBA-15 with High Hydrothermal Stability Qisong Lina, Kejing Wua, Yu Chena, b, Yin Tanga, Yulong Wua, c, *, Mingde Yanga a.

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China

b.

School of Chemistry, Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University, Guangzhou 510275, P. R. China

c.

Beijing Engineering Research Center for Biofuels, Beijing 100084, PR China

* Corresponding author Yulong Wu Tel: (+86) 10-89796163; Fax: (+86) 10-69771464 E-mail address: [email protected] Postal address: Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China

ACS Paragon Plus Environment


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Abstract This paper mainly describes the application of silylated SBA-15 with high hydrothermal stability in catalytic hydrothermal liquefaction (HTL) of Dunaliella (D.) tertiolecta to produce bio-oil. The results indicated that selection of silylation reagent species contributes to improve hydrothermal stability of SBA-15 at 613K through substituting hydroxyl group on SBA-15 surface. The grafting of silane on SBA-15 had an impact on pore size, wall thickness, specific surface area, pore volume and other structure characteristics, while not influencing the ordered mesoporous pore structure. When silylated SBA-15 were used as catalysts, D. tertiolecta conversion and bio-oil yield decreased slightly due to adsorption of furan and other compounds on mesoporous structure of SBA-15. Silylated SBA-15 inhibited ammonolysis reaction, Maillard and other reactions, resulting in decreased acid, ester, amide and N-containing heterocyclic content in bio-oil. Meanwhile, content of aldehydes and ketones increase, among which furfural derivatives were the main substances (> 70%). Highly selective THL is realized over silylated SBA-15 with high hydrothermal stability. Keywords: Silylated SBA-15; Bio-oil; Catalytic HTL; Microalgae; Hydrothermal stability


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1. Introduction In recent years, climate change, energy crisis and increasingly environmental pollution have become dilemma in the present sustainable global development 1. At the same time, the contradiction between the non-renewable fossil resources and the rapidly growing energy demand require searching for renewable energy 2. Bio-oil produced from biomass is the only kind of renewable energy that can be converted into liquid fuels, and it is potential alternative to petroleum and fuel in the future 3. The use of microalgae (the third generation of bio-energy) to produce bio-oil is considered promising because it has the advantages such as not taking cultivated land, high photosynthetic efficiency, short growth period, high product oil quality and excellent CO2 utilization capabilities

4-5.

Therefore, the conversion of microalgae to bio-oil has attracted the attention of

researchers around the world. Microalgae mainly composed of protein, lipids and carbohydrates, and the traditional usage of microalgae includes lipid extraction from cultivated microalgae with high-ester content 1. However, the energy consumption of pretreatment drying step is high and the waste of the remaining material is serious 6. Microalgae HTL not only eliminates the drying steps, but also effectively converts all the raw materials, which is with high economic efficiency as well as enhances the bio-oil quality 7-8. However, bio-oil obtained from direct microalgae HTL contains high content of N, O and other undesirable compounds, which conflicts with requirement of high calorific value, high stability and low corrosive for transportation fuel

9-10.

Thus, controllable HTL of microalgae is important to

improve bio-oil quality, among which catalytic HTL is a promising choice 11. It is reported that catalytic HTL of microalgae has been considered to be one of the most efficient ways to produce high-quality bio-oil at high temperature (above 473K) and high pressure 12.Homogeneous

catalysts, such as Na2CO3, KOH, CH3COOH, HCOOH, KtB et al, are studied in

catalytic HTL of microalgae11, 13-14, which have a certain positive effect on conversion and bio-oil yield. However, homogeneous catalysts is difficult to be separated from the product, resulting in high production cost and environmental concerns. Thus, heterogeneous catalysts are better choice to overcome these drawbacks15-17. Savage et al. produced bio-oil by catalytic HTL of microalgae (Nannochloropsis sp.) using six different heterogeneous catalysts, including Pd/C, Pt/C, Ru/C,



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Ni/SiO2-Al2O3, sulfided Co-Mo/γ-Al2O3 and zeolite. The yield of bio-oil increased from 35 to 57 wt.% when Pd/C was used, and bio-oil produced in the presence of Pd, Pt, Ru, Ni and Co-Mo catalysts possesses lower O/C ratios than using zeolite and blank18. Chen et al. investigated influences of ZrO2/SO42-, HZSM-5, MgO/MCM-41 and CNT-supported transition metals on HTL of D. tertiolecta, and the introduction of catalysts greatly affected the chemical composition and boiling point distribution of the bio-oil as well as conversion pathway19-20. Compared with other heterogeneous catalysts, ordered silicon-based mesoporous zeolites possess highly ordered pore structure, narrow pore size distribution, regulated pore size and large specific surface area, which benefit easy diffusion of macromolecule through the channel

21-22.

In

this article, SBA-15 is investigated because of its relatively high hydrothermal stability23. However, SBA-15 are easily inactivated by hydrogen ions and hydroxide under HTL conditions at 473-633K12. Specific surface area of SBA-15 decreases from 930m2·g-1 to 31 m2·g-1 after hydrothermal treatment at 473K for 12 h

24.

Therefore, hydrothermal stability of SBA-15 under typical HTL conditions

should be further improved for practical HTL. Carbon coating25 and metal heteroatom doping26 can significantly improve hydrothermal stability of SBA-15. However, weak interaction between SBA-15 and carbon (or metal atoms) cannot efficiently cover surface hydroxyl and other hydrophilic functional groups, which is the main reason for low hydrothermal stability12. Therefore, reducing hydroxyl and other hydrophilic functional groups is an efficient way to improve hydrothermal stability of SBA-15. According to literatures, Si-H in silanes can react with Si-OH in SBA-15 to form Si-O-Si bond and release H2, thus, post-grafting silylation can efficiently reduce hydroxyl groups on SBA-15 surface27. Silylation is an important way to modify SBA-15 for certain reactions such as Fischer-Tropsch synthesis28, benzene hydroxylation29, and propylene epoxidation30. As for catalytic HTL of microalgae, post-grafting silylation is a promising way for surface modification of SBA-15. Notably, silylation results in reduced Si-OH bond and increased strong Si-O-Si bond, which is crucial important for improvement of hydrothermal stability. In this article, the surface hydroxyl groups of SBA-15 were modified by grafting various silanes to improve hydrothermal stability for catalytic HTL of D. tertiolecta. Hydrothermal stability of


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differently silylated SBA-15 were tested at 573K, 593K and 613K, under which the microalgae HTL were carried out. N2 physical adsorption analysis, small angle X-ray diffraction (SXRD), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM) were used to characterize the catalysts before and after hydrothermal test. The bio-oil obtained from catalytic HTL of microalgae were analyzed by thermogravimetric analysis (TA), FT-IR, elemental analysis and gas chromatography-mass spectrometry (GC-MS).

2. Experimental 2.1. Chemicals P123 (MW = 5800, EO20PO70EO20), tetraethyl silicate, tris (pentafluorophenyl) borane, diphenylsilane, polymethylhydrogensiloxane, 1,1,3,3-tetramethyldisiloxane, 1,1,3,3-tetramethyl disilazane, dimethylphenylsilane, triphenylsilane are all purchased from Sigma reagent. Dichloromethane, concentrated HCl and anhydrous ethanol are purchased from Beijing Chemical Factory. N-hexane are purchased from Tianjin Guangfu Technology Development Co. Ltd. All above reagents are analytical pure. D. tertiolecta was purchased from Xi'an Wei Te Biotechnology Co., Ltd.

2.2. Preparation of pure SBA-15 8.0 g of the template triblock copolymer P123 was dissolved in 60 mL of deionized water under stirring at 308 K, and then, 240 mL of 2 mol·L-1 HCl was added. After template was completely dissolved, 17 g of tetraethyl silicate was added with stirring at 308 K for 24 h, and subsequently, all transferred to a high pressure autoclave with a tetrafluoroethylene liner keeping at 353 K for 24 h. Obtained products were filtered, washed and dried at room temperature to yield a white solid, which was calcined in a muffle furnace for 6 h to remove the template agent. The calcination procedure was 1 K min-1 from room temperature to 773 K to obtain SBA-15.

2.3. Preparation of silylated SBA-15 Typical silylation is carried out as follow27. 0.3 g of the prepared SBA-15 was dried in vacuum at 383 K for 12 h, cooled to room temperature, and then, added with 15 mL of dichloromethane and



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1.5 mmol of silylation reagent. After about 2 min, 0.015 mmol of tris (pentafluorophenyl) borane was added and ultrasonic treated for about 5 min. The product was filtered in vacuum, washed with dichloromethane and n-hexane, and dried at 383 K in vacuum. The silylated mesoporous catalysts are named corresponding to different organic reagents and modification mechanism are shown in Fig.1.

2.4. Hydrothermal stability testing The hydrothermal stability testing of SBA-15 and silylated SBA-15 were carried out in a stainless autoclave of 50 mL volume, which was heated by an external electric furnace and the temperature was measured by thermocouple with a temperature correction of ± 1 K. During the testing, the prepared mesoporous catalysts (about 0.5 g) was added to the autoclave containing 30 mL of deionized water at different temperature with PID control for 0.5 h. Then, the reaction vessel was rapidly cooled to room temperature, and the mesoporous materials after the hydrothermal testing were filtrated and dried for analysis.

2.5. Characterization of catalysts The SXRD analysis was measured on a D/max-2500/PC type X-ray diffractometer from Japan. The Cu Kα ray (λ = 0.154 nm, 40 kV, 30 mA) was used with a step of 0.01 from 0.6 ° to 5 °. FT-IR was performed on a Bruker Fourier transform infrared spectrometer (Horiba, Germany). The scanning time of the sample was 2 min with background scanning time of 2 min. The wave scanning range was 400-4000 cm-1. All of the TEM analysis were performed on an HT7700 type electron microscope, and the samples were ultrasonically dispersed in anhydrous ethanol and the suspension was then dropped onto a copper mesh microgrid with a porous carbon film. N2 physical adsorption analysis was carried out in a nitrogen gas at 77 K by means of an ASAP 2050 type adsorbent. All samples were pretreated for 4 h in a vacuum atmosphere of 573 K before N2 physical adsorption analysis. The C and H contents were measured on the Vario EL III type element analyzer (German Elemental Analysis System), and the O content was calculated by the difference methods.


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2.6. Catalytic HTL of D. tertiolecta Similarly with previous work10,

19-20, 31-33

, accurately weighed 3 g of D. tertiolecta powder

(elemental composition analysis are shown in Table S1), 0.15 g catalyst (5 wt.% of catalyst) were mixed well on the weighed paper, then added to reactor with 30 mL of deionized water. Reactor was sealed and placed in an electric furnace, and then, was heated to reaction temperature, keeping for 30 min. After the reaction was completed, the reactor was removed and rapidly cooled to room temperature. The product was washed with dichloromethane for several times, filter to separate liquid phase from residue, the residue was dried in 378 K for 24 h to get solid residue. The filtrate was transferred to a separatory funnel, the lower organic phase was distilled under reduced pressure, and the solvent dichloromethane was distilled off. The bio-oil was weighed and then dissolved by quantitative dichloromethane and stored in a refrigerator to be preserved and analyzed.

2.7. Characterization of bio-oil and solid residue FT-IR, elemental analysis were similar with catalyst characterization. GC-MS analysis was performed on an Agilent 7890-5975 Gas Chromatography-Mass Spectrometer. The chromatographic column is HP-5MS (30 m × 0.25 mm × 0.25 μm) with high purity helium of 1 mL∙min -1 as carrier gas. The injection volume is 1 μL with split ratio of 50: 1 and gasification temperature of 573 K. The ion source temperature is 523 K with electron energy of 70 eV, and the scan rate is 1000 amu∙s-1 with a range of 35-650 amu. GC-MS interface temperature is 523 K. The temperature rise procedure in GC is the initial temperature of 333 K, with 5 k∙min-1 rate to 453 K, constant temperature for 1 min, then rose to 553 K in 10 K∙min-1, constant temperature for 4min. In this paper, the main chemical components of bio-oil were analyzed by GC-MS. TG, DTG curves were performed on a synchronous thermal analyzer (TA Instruments, STD Q600) with sample loading of about 10 mg. The carrier gas is high purity nitrogen at a flow rate of 150 mL∙min-1, and the temperature rises from room temperature to 1073 K at 10℃∙min-1.

2.8. Calculation formula The D. tertiolecta conversion is calculated by the following formula:



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Conversion = (1−W1 / W0) × 100%

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(1)

where, W1 is the residue mass (g), and W0 is the microalgae mass (g). The oil yield is calculated by the following formula: Oil yield = W2 / W0 × 100%

(2)

where, W2 is the bio-oil mass(g). The calorific value is calculated based on mass percentage of each element, according to the Du Long formula 31: HHV (MJ ∙ kg-1) = 0.3383C + 1.422 (H − O / 8)

(3)

3. Results and discussion 3.1. Characteristics of SBA-15 and silylated SBA-15 As shown in Table 1, the modified SBA-15 grafted with the silylating agent pore size was reduced while the wall thickness was increased with respect to the unmodified SBA-15, where SBA-15-1, SBA-15-2 , SBA-15-3 and SBA-15-4 (abbreviation correspondence are shown in Fig. 1) had a large reduction in pore size as well as wall thickness which indicated a certain correlation between them, while pore size and wall thickness change of SBA-15-5 and SBA-15-6 were relatively small. As for specific surface area, SBA-15-6 slightly increased while the other modified SBA-15 decreased, especially that SBA-15-2, SBA-15-3 and SBA-15-4 decreased most significantly by 19%, 28% and 20%, respectively. In addition, the mesopore volume of SBA-15-1, SBA-15-2 and SBA-15-3 significantly reduced, while SBA-15-4, SBA-15-5, SBA-15- 6 remained essentially unchanged. It can be seen that the SBA-15 pore size reduced and the wall thickness increased by grafting the silylated group, and the specific surface area tend to decrease, which was because the grafted silylated group on the SBA-15 surface, the corresponding structural parameters in the modified SBA-15 are not consistent due to the nature and structure of the silylated agent itself. SXRD results of silylation-modified SBA-15 and unmodified SBA-15 before and after the hydrothermal testing at different temperatures are shown in Fig.2. It can be found that the modified SBA-15 prepared by grafting silylated agent has the strongest diffraction peak (100 crystal face) at 2θ ≈ 0.9 ° in the SXRD pattern, two lower intensity diffraction peaks (110 crystal face) and (200


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crystal face) appeared at 1.5 °and 1.8 °. The results showed that SBA-15 can maintain the original two-dimensional (2D) hexagonal P6mm symmetry structure after grafting the silylated group, and the grafting process did not destroy the ordered mesoporous structure of SBA-15. After the hydrothermal testing at 573 K, the SXRD patterns of mesoporous materials obtained by grafting diphenylsilane,

polymethylhydrogensiloxane

and

1,1,3,3-tetramethyldisiloxane

(SBA-15-1,

SBA-15-2 and SBA-15-3, respectively) are fully consistent with pure SBA-15, while the peak intensity of the (100) diffraction peak of 1,1,3,3-tetramethyldisilazane, dimethylphenylsilane and triphenylsilane modified SBA-15 (SBA-15-4, SBA-15-5 and SBA-15-6, respectively) decreased comparing to SBA-15, however, each modified SBA-15 all had three diffraction peaks, indicating that the hydrothermal conditions of 573 K for 0.5 h had no significant effect on the modified SBA-15 and pure SBA-15. When the hydrothermal testing temperature increased to 593 K, the SXRD diffraction peak of the unmodified SBA-15 almost disappeared completely, indicating that ordered mesoporous structure of the unmodified SBA-15 had been obviously destroyed after hydrothermally treated at 593 K for 0.5 h, while the SBA-15 modified by silylated reagent maintained (100) diffraction peaks, indicating that the ordered mesoporous structure remained with slight damage. When the temperature was further increased to 613 K, the SXRD diffraction peak of the unmodified SBA-15 molecular sieve completely disappeared, which indicated that the mesoporous structure of unmodified SBA-15 had been completely destroyed at 613 K for 0.5 h. SBA-15 modified by silylated reagent showed significant differences. SBA-15-1 and SBA-15-2 ordered pore structure had been destroyed, but still retain part of the mesoporous structure, so when compared with SBA-15, their hydrothermal stability improved. All three diffraction peaks of SBA-15-3 and SBA-15-6 maintained intact, indicating that even after hydrothermal treatment at 633K for 0.5h, SBA-15-3 and SBA-15 6 can still maintain the ordered mesoporous structure. SBA-15-4 and SBA-15-5 SXRD diffraction peak completely disappeared, indicating that its ordered mesoporous structure was completely destroyed. It was found that SBA-15-6 had the best hydrothermal stability while SBA-15-1 and SBA-15-5 were the worst, which may be related to the type and amount of the substituents of diphenylsilane, dimethylphenylsilane and triphenylsilane. The number of benzene ring substituents may be the main



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factors that contribute to the different hydrothermal stability of SBA-15-1, SBA-15-5 and SBA-15-6. 1,1,3,3-tetramethyldisiloxane and 1,1,3,3-tetramethyldisilazane had similar structure, but the Si-O and Si-N made the hydrothermal stability of SBA-15-3 and SBA-15-4 significantly different, the reason may be that Si-O-Si is an angular and symmetrical structure, and Si-N-Si is connected to a hydrogen on N, which forms a asymmetric triangular pyramid structure, eventually resulting in Si-O-Si bond more stable than Si-N-Si bond. Polymethylhydrogensiloxane contains a large amount of substituted hydrogen that can be multi-site grafted on SBA-15 surface by dehydrogenation reaction with hydroxyl, thus improving the hydrothermal stability of SBA-15-2, however, due to the polymer chain is too long, it is easy to collid with water molecules in the high temperature water phase and break polymer chain off, causing lower hydrothermal stability of SBA-15-2 than SBA-15-3 and SBA-15-6. The silylation grafting process is in fact a process in which a silylated agent is introduced into the mesoporous material by the catalytic reaction with the hydroxyl groups on the surface of the mesoporous material. FT-IR can effectively analyze whether the alkylation reagent is successfully grafted. The results of FT-IR analysis of SBA-15 and silylated SBA-15 are shown in Fig.3. It can be seen from Fig.3 that all the samples showed strong broad peaks near the 1063 cm-1 due to the absorption of the antisymmetric stretching vibration of Si-O-Si, and strong in the vicinity of 1063 cm-1 before and after silylation modification, the sharp absorption peak indicated that the siloxane of SBA-15 is mainly composed of silicon tetrahedral structure and the organic modification did not destroy the skeleton structure of SBA-15 34. The absorption peak at 3200-3600 cm-1 is specified as the stretching vibration and bending vibration of the O-H bond on the surface of the mesoporous material

35.

For SBA-15, the vibration absorption peak of O-H was obvious, indicating that the

surface of SBA-15 contains a certain amount of hydroxyl, which was due to the hydroxyl-terminated structure while for silylated mesoporous material the vibration absorption peak of O-H significantly weakened or even completely disappeared. It can be considered that the silylation reagent successfully grafted to the surface of SBA-15, so that the reduction of surface hydroxyl increased the hydrothermal stability of the mesoporous catalyst after the silylation. In addition, after the introduction of the silylated group, an absorption peak at 2800-3000 cm-1 appeared, which was


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caused by the absorption of the C-H stretching vibration, thereby strongly demonstrating that the silylation group had been introduced onto the SBA-15. At the same time, after the introduction of the silylated agent, there appeared some absorption peaks in the range of 500-1000 cm-1, which corresponded to the functional groups of the introduced silylated agents, thereby further illustrating the alkyl group was successfully introduced onto the SBA-15. The absorption peak near 1632 cm-1 was the bending vibration absorption of the adsorbed water on the inner and outer surfaces of SBA-15

36

, which is mainly related to the hydrophilicity of the surface hydroxyl groups. After the

silylation modification, the peaks decreased or diappeared, which further showed that the organic modified group successfully grafted on the SBA-15 surface resulting in the hydrophilic of SBA-15. In order to further determine the morphological structure of silylated SBA-15, the resulting silylated SBA-15 were subjected to TEM analysis. It can be seen from Fig.4 that, like unmodified SBA-15, the silylated SBA-15 exhibited a highly ordered hexagonal crystal structure consisting of 2D channels along the (100) and (110) direction, which was exactly the same as the SXRD analysis. In addition, the SBA-15 ordered hexagonal structure remained intact after silylation, indicating that the lattice of SBA-15 was not destroyed by the introduced silylation reagent. The morphologies of SBA-15-3 and SBA-15-6 were selected to compare with SBA-15 after hydrothermal testing as shown in Fig.5. After hydrothermal treatment for 0.5 h at 613K, the hexagonal structure of the 2D channels of SBA-15 could not be observed, indicating that the ordered skeleton structure had collapsed and the ordered mesoporous structure had been to complete destruction. While the silylated SBA-15 still maintained a good structure, indicating that the grafted silylation agent was beneficial to improve the hydrothermal stability of the mesoporous materials and prevent the collapse of structure. In view of all, diphenylsilanes, polymethylhydrogensiloxanes, 1,1,3,3-tetramethyldisiloxane, 1,1,3,3-tetramethylbissilane, dimethylphenylsilane and triphenylsilane were successfully grafted onto SBA-15 using tris (pentafluorophenyl) borane as catalyst. SBA-15-3 and SBA-15-6, modified by triphenylsilane and 1,1,3,3-tetramethyldisiloxane, respectively, had the most favorable hydrothermal stability and can maintain a highly ordered 2D hexagonal structure.



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3.2. Catalytic HTL of D. tertiolecta with silylated SBA-15 3.2.1.

The influence of introduction of silylated SBA-15 on conversion ratio and bio-oil yield

The production of bio-oil from HTL of D. tertiolecta catalyzed by silylation modified mesoporous catalysts were studied. The reaction temperature was 573 K and the reaction time was 0.5h referring to our previous work 10, 19-20, 31-33. The relationship of bio-oil yield and feedstock conversion between blank and using catalyst were shown in Fig.6. The bio-oil yield of using silylated SBA-15 to catalyze HTL of D. tertiolecta were somewhat lower than that of the blank experiment, which may be related to the limitation of the pore structure. It had been reported that due to the limitation of catalyst pores, adsorption of strongly irreversible furan derivatives in the pores had a detrimental effect on the selectivity of furan, and thus the formation of more polymers in the pores of the catalyst further decreased the apparent conversion rate 37. The calculation of conversion in this paper was based on the quality of the solid residue, so the conversion of the furan derivative in the catalyst pore led to the increase of the residue mass, therefore, the conversion ratio of the blank experiment was slightly higher than that of the catalytic HTL process. In order to further explain the reduced conversion, the solid residue obtained by blank experiment and catalytic HTL were analyzed by TA, as shown in Fig.7. The results of DTG showed that the maximum weight loss rate of the solid residue obtained in the blank experiment is about 693 K, while the HTL is about 593 K. In addition, the maximum weight loss rate of the SBA-15-1 and residue of the blank mixture was also at 693 K. The results should be attributed to the adsorption in the catalyst, since the weight loss temperature of the polymer is lower than that of the solid residue after HTL, independent of the thermal decomposition of the catalyst. The results also showed that the composition of solid residue had changed significantly using silylated SBA-15 to catalyze HTL relative to the non-catalytic HTL.

3.2.2.

Characteristics of bio-oil

In this paper, the bio-oil obtained from catalytic and non-catalytic HTL of microalgae were analyzed, and the corresponding calorific value was also calculated, the experimental results are shown in Table 2. The content of C and H in bio-oil was much higher than that in D. tertiolecta, and


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the content of O was greatly reduced. There were differences in the contents of C and O in the bio-oil obtained by silylated SBA-15, which may be related to the presence of different silylated reagents. Since the changes in C and O content among bio-oil were not significant, so the addition of the catalyst was not significant for the increase in calorific value. However, the addition of the catalyst significantly reduced the N content in the bio-oil, in other words, the catalyst used facilitates the transfer of the N-containing coumpounds to the aqueous or gas phase, thereby reducing the N content in bio-oil, which was of great significance for the conversion of microalgae biomass into fuel. It can be seen from Fig.8 that the absorption peak in the range of 3100-3600 cm-1 in the bio-oil weakened obviously relative to the FT-IR pattern of D. tertiolecta reported before 38, indicating that the protein and carbohydrate in the microalgae had been effectively converted to produce other substances, resulting in reduction of N-containing and hydroxyl-containing compounds. In addition, the FT-IR results of all bio-oil had a certain similarity at many absorption peak locations, indicating that the organic matter in the bio-oil were very similar in the composition, for example, the wavenumber at 2850-3050 cm-1 represented methyl and methylene groups; a characteristic absorption peak of -OH or -NH in the range of 3200-3800 cm-1; carbonyl (including ester, ketone, aldehyde, carboxylic acid, etc.) in the range of 1650-1720 cm-1 . However, there was still a significant difference in the bio-oil obtained from the catalytic HTL bio-oil and the blank, compared with the blank experiment, the absorption peak near 1700 cm-1 obviously moved toward the low wave number, which indicated that the bio-oil contained more aldehydes or ketones, the strong and broad absorption peaks near 3200 cm-1 moved to higher wavenumber, which indicated that the content of carboxylic acid in the bio-oil was reduced and the content of the alcohols was more, the absorption peak in the range of 1000-1100 cm-1 showed that the content of the material containing C-O bond increased, 1522 cm-1 is the characteristic peak when the C=C bond wass conjugated with the carbonyl group. The GC-MS analysis of Fig.9 can help us better understand the chemical composition of bio-oils. GC-MS analysis was used to identify major chemical components of catalytic and non-catalytic bio-oils. In most cases, the identification of GC-MS was based on the NIST 98. It is worth noting



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that the listed percentages only refer to the relative content of the compound in the bio-oil that can be gasified under the detection conditions (573 K) and can pass through the GC column. Compared with the blank experiment, the bio-oil obtained by catalytic HTL contained a lot of aldehydes (mainly furfural and its derivatives), and the content is up to 70% (Fig.10). 5-hydroxymethyl furfural (5-HMF) became the component with largest amount in the bio-oil while the content of 5-HMF and its derivatives detected in the blank experiment was very small. 5-HMF and its derivatives were derived from the conversion of carbohydrate, thus the results implied that silylated SBA-15 favored the conversion of carbohydrate to 5-HMF and its derivatives. This results was consistent with the TA analysis of solid residue. At the same time, alcohols were detected in the bio-oil obtained by catalytic HTL while the blank experiment was not detected. The alcohols mainly derived from the hydrogenation reaction of the acid so it can be seen that the silylated SBA-15 has the effect of hydrogenation during HTL. In addition, amino and N-containing heterocyclic substances all dereased when using catalysts, this was because the introduction of catalysts reduced ammonolysis reaction and Maillard reaction. The reduction of acid and esters should be attributed to the inhibition of the hydrolysis of the lipid by the catalyst or the promotion of transfer of the acid and the ester to the aqueous phase.

4. Conclusion Silylation reagents can be effectively grafted onto SBA-15 as selective HTL catalyst. During the grafting process, most of the surface hydroxyl groups on SBA-15 are replaced by silylated groups. Wall thickness, specific surface area, pore volume and other mesoporous pore structure are changed after grafting, while grafting does not destroy the order of the original mesoporous channels. Silylated SBA-15 with high hydrothermal stability is able to tolerate 613K hydrothermal environment. The selection of silylation reagents plays an important role in improving the hydrothermal stability of SBA-15, and 1,1,3,3-tetramethyldisiloxane and triphenylsilane were outstanding. During the HTL, silylated SBA-15 inhibited the ammonolysis reaction, Maillard and other reactions, which can effectively reduce the acid, esters, amides and N-containing heterocyclic compounds content in the bio-oil, at the same time, catalytic HTL can effectively catalyze the


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conversion of carbohydrates into 5-HMF and its derivatives with a content up to 70%.

Acknowledgements This study was supported financially by National Natural Science Foundation of China (No. 21376140, No. 21376136 and No. 21576155).

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Table caption Table 1 Physical properties of SBA-15 and other modified SBA-15

Table 2 Elemental analysis of bio-oil obtained from HTL process


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Figure caption Fig.1. Silylation mechanism of SBA-15 with different silicanes, and related silicanes are marked as SBA-15-X, where X range from 1 to 6.

Fig.2. SXRD results of SBA-15 and silylated SBA-15 after hydrothermal test at 613, 593, and 573 K for 0.5 h. Coordinate axis range of intensity and 2θ is the same for different figures.

Fig.3. FT-IR profiles of silylated SBA-15, ( black) SBA-15, ( red) SBA-15-1, ( blue) SBA-15-2, ( dark cyan) SBA-15-3, ( magenta) SBA-15-4, ( dark yellow) SBA-15-5, ( Navy) SBA-15-6. Fig.4. TEM images of as-synthetized SBA-15 and silylated SBA-15

Fig.5. TEM images of SBA-15, SBA-15-3 and SBA-15-6 after hydrothermal testing at 633K for 0.5h

Fig.6. Conversion and bio-oil yield of different microalgae catalytic HTL processes

Fig.7. DTG results of solid residues from catalytic HTL of microalgae using silylated SBA-15, where None refers to blank HTL without catalyst

Fig.8. FT-IR results of bio-oil obtained from microalgae catalytic HTL

Fig.9. Normalized content of compound categories from different catalytic HTL, ( hydrocarbon, (

red) alcohol, (

dark yellow) amide, (

blue) aldehyde ketone, (

dark cyan) acid, (

magenta) ester, (

Navy) N-heterocyclic.

Fig.10. Normalized content of 5-HMF in bio-oil produced by using different catalysts


black)


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Table 1 Physical properties of SBA-15 and other modified SBA-15 Samples

da100(nm)

a0b(nm)

Dc(nm)

Wd(nm)

SBET(m2/g)

Vmice(cm3/g)

Vmesf(cm3/g)

SBA-15

8.9

10.3

5.5

4.7

669

0.060

0.633

SBA-15-1

8.8

10.2

4.7

5.5

606

0.053

0.483

SBA-15-2

8.6

9.9

4.3

5.6

543

0.028

0.514

SBA-15-3

9.1

10.5

4.8

5.7

481

0.009

0.480

SBA-15-4

9.0

10.4

4.9

5.5

536

0.050

0.629

SBA-15-5

9.1

10.5

5.3

5.2

631

0.029

0.633

SBA-15-6

9.1

10.5

5.4

5.1

682

0.063

0.632

a

Calculated by SXRD results

b

Cell parameters a0 = d1 0 0 × 2/

c

Pore diameter from BET by BJH Adsorption method.

d

Wall thickness W= a0 - D

e

Microporous volume calculated by t-Plot method

f

Mesoporous volume calculated by BJH Adsorption method


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Table 2 Elemental analysis of bio-oil obtained from HTL process Blank

SBA-15-1

SBA-15-2

SBA-15-3

SBA-15-4

SBA-15-5

SBA-15-6

C/wt.%

64.89

65.50

66.8

66.80

67.50

65.60

63.80

H/wt.%

7.9

7.9

8.1

8.1

8.0

7.9

7.6

O/wt.%a

26.9

26.47

24.96

24.94

24.39

26.38

28.38

N/wt.%

0.31

0.13

0.14

0.16

0.11

0.12

0.22

H/C

1.46

1.46

1.46

1.47

1.43

1.45

1.42

O/C

0.31

0.30

0.28

0.28

0.27

0.30

0.33

N/C

0.06

0.02

0.02

0.03

0.02

0.02

0.04

CH1.46O0.31

CH1.46O0.30

CH1.46O0.28

CH1.46O0.28

CH1.46O0.27

CH1.46O0.3

CH1.46O0.33

N0.06

N0.02

N0.02

N0.03

N0.02

N0.02

N0.04

28.51

28.80

29.79

29.80

29.99

28.85

27.45

Formula

HHV/ MJ·kg-1 a

Calculated by difference



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Fig.1. Silylation mechanism of SBA-15 with different silicanes, and related silicanes are marked as SBA-15-X, where X range from 1 to 6.


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Fig.2. SXRD results of SBA-15 and silylated SBA-15 after hydrothermal test at 613, 593, and 573 K for 0.5 h. Coordinate axis range of intensity and 2θ is the same for different figures.



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

Fig.3. FT-IR profiles of silylated SBA-15, ( black) SBA-15, ( red) SBA-15-1, ( blue) SBA-15-2, ( dark cyan) SBA-15-3, ( magenta) SBA-15-4, ( dark yellow) SBA-15-5, ( Navy) SBA-15-6.


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Fig.4. TEM images of as-synthetized SBA-15 and silylated SBA-15



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Fig.5. TEM images of SBA-15, SBA-15-3 and SBA-15-6 after hydrothermal test at 633K for 0.5h


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Fig.6. Conversion and bio-oil yield of different microalgae catalytic HTL processes



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Fig.7. DTG results of solid residues from catalytic HTL of microalgae using silylated SBA-15, where None refers to blank HTL without catalyst


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Fig.8. FT-IR results of bio-oil obtained from microalgae catalytic HTL



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Fig.9. Normalized content of compound categories from different catalytic HTL, ( hydrocarbon, (

red) alcohol, (

dark yellow) amide, (

blue) aldehyde ketone, (

dark cyan) acid, (

Navy) N-heterocyclic.


black)

magenta) ester, (

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Fig.10. Normalized content of 5-HMF in bio-oil produced by using different catalysts



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Table of Content graphic


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Catalytic Hydrothermal Liquefaction of Microalgae for Bio-oil

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