Catalytic hydrothermal liquefaction for bio-oil production over CNTs supported metal catalysts

Chemical Engineering Science 161 (2017) 299–307

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Catalytic hydrothermal liquefaction for bio-oil production over CNTs supported metal catalysts Yu Chen a, Rentao Mu b, Mingde Yang a, Lina Fang a,c, Yulong Wu a,d,⇑, Kejing Wu a, Ya Liu c, Jinlong Gong b,⇑ a

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China c School of Food, Shihezi University, Shihezi 832000, Xinjiang, China d Beijing Engineering Research Center for Biofuels, Beijing 100084, China b

a r t i c l e

i n f o

Article history: Received 28 July 2016 Received in revised form 3 November 2016 Accepted 1 December 2016 Available online 21 December 2016 Keywords: Catalytic HTL Bio-oil Microalgae Metal/CNTs Pathway

a b s t r a c t This paper describes catalytic consequence of hydrothermal liquefaction (HTL) of Dunaliella tertiolecta (D. tertiolecta) over carbon nanotubes (CNTs) supported metals catalysts to produce bio-oil. When Co/CNTs is used as catalysts, the conversion and bio-oil yield increase to 95.78 and 40.25 wt.%, respectively. Chemical analysis results showed that the introduction of catalyst significantly affected the chemical composition of bio-oil with a higher percentage of hydrocarbons and a lower content of fatty acid. The introduction of metal into CNTs had no change in the basic CNT skeleton and the loaded metal nanoparticles encapsulated within the CNT enhances the disorder and defects in CNTs. Based on our results and the literature, the plausible general reaction and catalytic HTL pathways of D. tertiolecta are proposed. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In the past decades, the excessive use of fossil fuels, which cause serious environmental pollution, has motivated researchers to seek renewable, sustainable, and environmental friendly energy (Chen et al., 2015b). One alternative approach is to produce bio-oil from biomass, which has the potential to reduce dependence on fossil fuels (Upadhye et al., 2011). Among the types of biomass, microalgae have a great potential for bio-oils production due to its superior photosynthetic efficiency, high biomass yield, and high CO2 utilization capabilities. Therefore, microalgae have been widely demonstrated as a suitable feedstock for biofuels (Barbosa, 2010; Ifrim et al., 2014). The major components of algae can be roughly classified into carbohydrates, lipids, and proteins, which are important distinguishing features compared with those of terrestrial biomass (Chen et al., 2015a). The conventional transformation methods for microalgae are focused on lipid extraction for biodiesel production (Martin and Grossmann, 2013; Zhang et al., 2014). In contrast to this route, in which only the lipids can be utilized, substitutive

⇑ Corresponding authors at: Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China (Y. Wu). E-mail addresses: [email protected] (Y. Wu), [email protected] (J. Gong). http://dx.doi.org/10.1016/j.ces.2016.12.010 0009-2509/Ó 2016 Elsevier Ltd. All rights reserved.

approaches, such as thermochemical conversion routes, involve the conversion of the entire algal composition, including the proteins and carbohydrates, into bio-oil (J.X. Zhang et al., 2013). Among the thermochemical conversion routes, hydrothermal liquefaction (HTL) is one of the most promising methods for the conversion of wet microalgae (Yeh et al., 2013). It has been demonstrated that HTL performed with sub/supercritical water as the reaction medium is a suitable approach to generate renewable bio-oil from microalgae which can avoid the necessary drying step of biomass prior to use (Chen et al., 2015a; Yu et al., 2014; Zhang et al., 2014). However, the undesired properties of bio-oil obtained from direct HTL of microalgae include high heteroatoms content (e.g. O and N), high acid value, low calorific value (HHV), and poor stability. Therefore, the bio-oil cannot be directly suitable for storage, transport and use as a transport fuel (Chen et al., 2015a; Yu et al., 2014). To this end, catalytic HTL can be considered an effective alternative method for improving bio-oil quality in the preparation of biooil from microalgae. So far, many homogenous catalysts, such as Na2CO3, CH3COOH, and KOH, have been involved in the catalytic HTL of microalgae (Ifrim et al., 2014; Ross et al., 2010). However, additional separation steps with high cost and energy consumption are required for catalyst recovery when using homogeneous catalysts. The substitutive approach involves HTL with heterogeneous catalysts, which focuses on molecular sieve, modified molecular

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sieves, transition metal oxide, supported metal, etc. (Duan and Savage, 2011; Jena et al., 2012, 2011). Meanwhile, the heterogeneous catalysis can be also applied in a subsequent upgrading step after HTL, which could be a potential way of avoiding the poisoning of catalysts due to the high salt content in algae slurries (Lopez Barreiro et al., 2016). Savage et al. reported the catalytic HTL of microalgae for bio-oil production from microalgae using six heterogeneous catalysts. They claimed that the presence of Pt, Ni, and CoMo catalysts lead to bio-oils with lower O/C ratios than the oil produced in their absence. These findings indicate that catalytic deoxygenation or hydrodeoxygenation proceeded during the hydrothermal liquefaction of the microalga. Thus, there may be opportunities for a single-step catalytic liquefaction process to produce a higher quality bio-oil than non-catalytic liquefaction (Duan and Savage, 2011). Biller et al. investigated three catalysts: CoMo-, Ni-, and Pt-based catalysts (Biller et al., 2011). Their results indicated that the bio-oil yield from the HTL of microalgae increased slightly with the introduction of heterogeneous catalysts and that the increase of HHV was up to 10%. The catalytic HTL of microalgae can produce liquid fuels with high bio-oil quality, which is the objective of researchers in this field. More work is required to identify better heterogeneous catalysts for catalytic HTL processing. The transition metal can be used as the active component to improve the bio-oil quality (Biller et al., 2011; Duan and Savage, 2011). However, targeted catalytic materials that resist deactivation during HTL are needed. Therefore, catalysts with high hydrothermal stability are necessary during HTL process. The use of carbonaceous materials (e.g., activated carbon) as support for metal catalysts have been widely accepted as industrial preference for the production of many fine chemicals because of their large surface area and ease of recycling noble metals (Davari et al., 2014; Xu et al., 2015). Carbon nanotubes (CNTs) are an important type of carbonaceous materials with 1D nanocavities (Davari et al., 2014). Recently, efforts have been exerted to prepare water-soluble CNTs and develop methods for the chemical functionalization of CNTs (Xu et al., 2015; Yang et al., 2014). CNTsupported metals have been proven to act as both recoverable emulsifiers and catalysts (Xiao et al., 2015; Yang et al., 2014). Carbonaceous-material-supported metals have been employed as catalysts in the HTL of microalgae (Ross et al., 2010). However, published works involving the HTL of algae with CNT-supported metal as the catalyst are not available. This paper describes the activity of CNT-supported transition metals (e.g., M = Co, Ni, and Pt) for the catalytic HTL of Dunaliella tertiolecta (D. tertiolecta) to produce bio-oil. The plausible general reaction and catalytic HTL pathways of D. tertiolecta are presented on the basis of our experimental results and previous investigations (Chen et al., 2015a; Biller and Ross, 2011; Ifrim et al., 2014; Yeh et al., 2013; Yu et al., 2014).

Ltd. (Nanjing, China). All reagents were used as-received without further purification. 2.2. Experimental procedure The amount of theoretical transition metal (M = Co, Ni, and Pt) loading were approximately equal to 5% of the mass of CNTs used. CNT-supported metals (M/CNTs) were synthesized by the incipient wetness impregnation method reported in a previous paper (Ding et al., 2015). The synthesis process was detailed as follows: in a typical run, 5 wt.% M/CNTs catalyst was prepared by first dissolving metal nitrates [Co(NO3)3, Ni(NO3)3, and Pt(NO3)4] in deionized water (15 mL) and then slowly dropping this solution onto CNTs (2 g) with continuous stirring in a 100 mL beaker. After allowing the metal to permeate into the support for 12 h at ambient temperature, the catalyst was dried at 105 °C for 3 h. The dried sample was then calcined at a heating rate of 10 °C min
1 to 500 °C and maintained at that temperature for 3 h under N2 atmosphere. Thereafter, the sample was disposed by carburization in a H2 flow of 80 mL min
1 from room temperature to 700 °C at 10 °C min
1. The sample was then reduced at 700 °C for 4 h. The obtained catalysts, which were saved in a dryer prior to use, were labeled as Co/ CNTs, Ni/CNTs, and Pt/CNTs, respectively. The HTL procedure performed in a stainless autoclave was mentioned in our previous report (Chen et al., 2015a). In a typical run, D. tertiolecta (approximately 4.0 g), M/CNTs catalyst (the catalyst dosage is equal to 10% of algae), and 40 mL water were fed into the autoclave, sealed, and heated to the desired temperature (320 °C) at the heating rate of 10 °C min
1, maintained for 30 min. The reaction pressure is approximately 12 MPa during the HTL process. After that, the electric furnace was removed and the autoclave was cooled to room temperature. A blank experiment (designated as ‘‘blank”), which was carried out in absence of any catalysts, was used for comparison. The separation of catalytic HTL products was carried out according with our previous work (Chen et al., 2015a). Gaseous products were vented directly. The liquid phase fraction and the autoclave wall were washed with CH2Cl2 thrice, and the contents were separated by dispersion. CH2Cl2 was removed in a rotary evaporator at 40 °C under reduced pressure, and the remaining liquid fraction was called ‘‘bio-oil.” The water-insoluble fraction remaining on the filter paper was dried at 105 °C for more than 24 h, weighed, and designated as ‘‘SR.” The testing of the gaseous and water-soluble reaction products is beyond the scope of this paper. The bio-oil yield and D. tertiolecta conversion were expressed in %w/w and calculated on the basis of dry organic matter as follows:

Bio-oil yield ¼

x1  100% x0

D: tertiolecta conversion ¼ 1
2. Experimental 2.1. Materials Microalgae D. tertiolecta was purchased from Xi’an Victory Biochemical CO., Ltd. (Xi’an, China). The samples were dried at 105 °C for 12 h, ground, and sieved with an 80 mesh standard sieve. The proximate and ultimate analysis results of D. tertiolecta and its chemical constituents are reported in our previous work (Chen et al., 2015a). The metal nitrates of Co(NO3)3, Ni(NO3)3, and Pt(NO3)4 were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Multi-walled Carbon nanotubes (CNTs, specific surface area P110 m2 g
1) were purchased from XFNANO Materials Tech Co.,

ð1Þ

x2  100% x0

ð2Þ

where xo (g), x1 (g), and x2 (g) were defined as the mass of D. tertiolecta, bio-oil, and SR, respectively. The mass of D. tertiolecta used for Eqs. (1) and (2) is calculated on a dry weight basis and ash-free. 2.3. Analysis of bio-oil Thermogravimetric analysis (TGA) experiments were conducted by a SDT Q600 thermogravimetric analyzer (TA Instruments, America). Alumina crucible was used to hold 10 mg of sample uniformly at the bottom, and the crucible was placed at the same position of the beam platform of the thermal analyzer. Subsequently, the sample was heated from room temperature to 800 °C at a heating rate of 10 °C min
1 under a N2 flow of 100 mL min
1. Every experiment

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▲

was analyzed thrice at the same conditions, and the average result was taken as the final result. The C, H, N, and O content of the sample were measured using a CE440 elemental analyzer. The measurements were repeated in duplicate and a mean value is reported. The HHV of the sample was calculated according to the formula of Dulong (Huang et al., 2011) based on the sample’s elemental composition:
1

HHV ðMJ kg Þ ¼ 0:3383C þ 1:422  ðH
O=8Þ

䖩 䖩

Pt/CNTs

䖩

▲

▲

■ 䖩

■

Ni/CNTs

■

ð3Þ

GC–MS analysis was conducted on a Trace DSQ GC–MS system with an AB-5MS capillary column (30 m  0.25 mm id, 0.25 lm film thickness), and helium was used as carrier gas with a flow rate of 1 mL min
1. The column temperature was programmed from 70 to 300 °C at a rate of 10 °C min
1 after an initial two-minute isothermal period and was kept at the final temperature for 10 min. The inlet temperature was set to 300 °C, and the split ratio was 1:50. The mass spectrometer was set to an ionizing voltage of 70 eV with a mass range from 35 to 650 amu. 2.4. Catalyst characterization XRD patterns were measured on a D8 ADVANCE diffractometer by using Cu Ka radiation (k = 0.1541 nm, 36 kV, 2 mA, scanning step = 2°/min
1). The diffraction patterns were recorded by scanning at an angle ranging from 5 to 80°. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were performed on an IRIS Intrepid II XSP atomic emission spectrophotometer (Experimental samples were incinerated, and then digested using concentrated HNO3 following EPA method 3051A prior to ICP analysis.). Raman spectra were recorded with a JY/Horiba LabRam HR 800 Raman microspectrometer (Horiba Jobin Yvon, France). A diode laser of 531.95 nm (frequency doubled Nd: YAG) was focused through the 10 objective of a microscope (Tokyo, Japan). Raman signal was collected in the spectral interval of 400–4000 cm
1. XPS measurements were conducted by using a PHI Quantera microprobe (ULVAC-PHI Inc., Japan) equipped with an aluminum anode as the monochromatized X-ray source (1486.7 eV run at 10 kV and 15 mA in fixed analyzer transmission mode). The peak fitting procedure was performed with XPS Peak 4.1. The C–C peak was set to 284.8 eV. 3. Results and discussions 3.1. Catalysts characterization CNTs can be used as catalyst support materials wherein metal particles with catalytic activity may decorate along the external walls or be filled in the interior (Zhang et al., 2008). The XRD patterns of the CNTs and M/CNTs catalysts are shown in Fig. 1. The results indicate that the diffraction peaks at 26° and 43° are attributed to the typical graphite structure of CNTs (Li et al., 2005; Nakhaei Pour et al., 2014), which were unchanged after the introduction of metal. As can be seen from Fig. 1, with the addition of Co, two new peaks appeared at 44° and 52°, thus showing that the Co with the respective form of cubic cobalt structure dispersed on the CNT skeleton (Davari et al., 2014). For Ni/CNTs, the characteristic diffraction peaks appeared at 44.5°, 51.8°, and 76.3°, thus indicating that the added Ni existed in the form of metal Ni (Saidi et al., 2014). Pt/CNTs have major diffraction peaks at 2h = 40°, 47°, and 68°, which can be attributed to the metal Pt fcc structure (Li et al., 2005; Zhou et al., 2009). The XRD results suggest that with the introduction of metals, the respective form of the metal state entering the pores of the CNT framework does not change the basic skeleton of CNTs. It is worth noting that if the oxidation states of

䖩

䖩䕺

Co/CNTs

䕺

䖩

䖩

CNTs 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

2θ /degree Fig. 1. XRD patterns of various metals supported on CNTs. Legend: (w) CNTs, (r) Co, (j) Ni, and (N) Pt.

metals were well dispersed in the catalyst, it is difficult to obtain the correlative information from the XRD spectra. To this end, XPS can give more information on the valence state of the metal. Raman features arising from the photophysical and resonant scattering process in nanocarbon materials, e.g., graphite, graphene, CNTs and CNFs, are well known (Reinhold-López et al., 2015). Therefore, Raman analysis of the CNTs and M/CNT catalysts are shown in Fig. 2 and the related Raman data are listed in Table 1. Both the G band (4–16 cm
1) and D0 band (6–14 cm
1) of M/CNTs have blue1 shifts (Fig. 2 and Table 1) compared to that of intrinsic CNTs. However, only little blue shifts of 2 and 4 cm
1 is observed in the D band of Pt/CNTs and Ni/CNTs, whereas no blue shifts are observed for the D band of Co/CNTs. The Raman shifts of the G and D0 band position, as well as the change in ID/IG ratios consistent with the introduction of metal into CNTs, asymmetrically affects the CNT structure. Furthermore, the addition of metal into CNTs increases the ID/IG ratios, thus indicating the enhancement of disorder and defects in CNTs. The amounts of theoretical transition metal loading were equal to 5% of the mass of CNTs used. The actual test information on the metal loading amounts is listed in Table 2. ICP-AES values all exhibit much higher than theoretical values while the XPS values are very lower than the theoretical values. The reasons for this phenomenon may include as follows: Firstly, due to the procedures of carburization and calcination during the catalysts preparation processes, some volatiles species were lost resulting in the decrease of CNTs amounts. Therefore, ICP-AES values are much higher than the theoretical values. Secondly, the surface contents of metals as measured by XPS is much lower than the theoretical values, This result confirms that the fair amount of added metal nanoparticles are encapsulated within inner wall of CNTs (Wang et al., 2014; Xu et al., 2015). The XPS analysis of M/CNTs is shown in Fig. 3. Fig. 3(A) represents the photoelectron spectra of Co/CNTs, the two photoemission maxima at binding energies of 781.1 and 796.4 eV are designated as Co 2p3/2 and Co 2p1/2, respectively (Davari et al., 2014). The peaks appearing at approximately 786.1 eV for 2p3/2 may be attributed to the C–Co structure. Furthermore, the position of the main peak in the Co 2p photoemission spectrum is shifted to a lower

1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.

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D G

D` Pt-CNTs

Ni-CNTs

Co-CNTs

CNTs 400

800

1200

1600

2000

2400

2800

3200

3600

4000

-1

Raman shift (cm ) Fig. 2. Raman spectrums of various metals supported on CNTs.

Table 1 Raman data of various metals supported on CNTs.

3.2. Effect of catalysts on bio-oil yields and conversions

Raman shift (cm
1)

Samples

CNTs Co-CNTs Ni-CNTs Pt-CNTs

ID/IG

D

G

D0

1340 1340 1344 1342

1569 1577 1585 1573

2674 2687 2688 2683

1.18 1.40 1.55 1.25

Table 2 Metal loading amounts of various metals supported on CNTs. Samples

Metal loading amounts (wt.%)

Co-CNTs Ni-CNTs Pt-CNTs

ICP-AES results

XPS results

8.85 10.56 5.93

0.01 1.39 0.07

binding energy, indicating the interaction between CNTs and Co metal, which is similar to the Raman results. As can be seen from Fig. 3(B), the Ni 2p peaks at 855.5 eV stand for metallic Ni combined with CNTs (higher than the standard Ni0 binding energy of 853.0 eV), and the peaks at 873.1 and 861.5 eV are considered NiO and Ni(OH)2 (M.J. Zhang et al., 2013). Therefore, Ni may be in the form of a mixture of Ni, NiO, and Ni(OH)2 in

A

Co 2P

Ni/CNTs. In this paper, due to the well dispersed of the oxidation states of Ni in the catalyst, we indicated that the Ni may in the metallic Ni form based on XRD results, however, we here obtain more information of the valence state of the metal in the catalyst through XPS analysis. It come to conclusion that Ni may be in the form of a mixture of Ni, NiO, and Ni(OH)2 in Ni/CNTs according to combinational results of XRD and XPS. The results consist with the published papers (Saidi et al., 2014; M.J. Zhang et al., 2013). For Pt/CNTs, the metal photoelectron spectrum is shown in Fig. 3(C), the Pt 4f7/2 and Pt 4f5/2 lines appear at 70.2 eV (Pt0) and 73.8 eV (Pt2+), respectively. The comparison of the binding energies indicates that Pt is present in the form of a mixture of zerovalent metal and Pt2+ because of the peaks at 73.8 eV (Park et al., 2002). However, the oxidation states of Ni and Pt cannot be obtained from the XRD pattern because the small peak is disregarded. In summary, although the introduction of metals (Co, Ni, and Pt) into CNTs does not change the basic skeleton of CNTs, the added metal nanoparticles are encapsulated within CNTs, thus resulting in the enhancement of the disorder and defects in CNTs. Except for the Co presented in the zerovalent metallic state, the Ni and Pt are in the form of a mixture of metallic and oxides state.

As mentioned above, the addition of catalyst resulted in the enhancement of bio-oil yield and bio-oil quality (Duan and Savage, 2011). Generally, the undesired properties of bio-oil are caused by high heteroatoms (O and N) content. In particular, high O content results in low HHV, poor storage stability, and corrosion. Thus, removing heteroatoms is a necessary step to improve the quality of bio-oil. Different metal catalysts exhibited different catalytic performance aimed at different microalgal biochemical constituents during the HTL process (Biller and Ros, 2011). To this end, several CNT-supported metals were employed in the current study for the production of bio-oil via the HTL of D. tertiolecta. Metal catalysts have been tested for upgrading of bio-oils due to appropriate accessibility surfaces and corresponding high activity (Saidi et al., 2014). The bio-oil yields and conversions are plotted against the types of catalyst, as shown in Fig. 4. As can be seen in this figure, both bio-oil yield and conversion increase with the introduction of catalysts compared with that of the blank value, i.e., the employed M/CNTs catalysts are conducive to the improvement of microalgal conversion and bio-oil yield. When Co/CNTs was used as the catalyst, the conversion and bio-oil yield of 95.78 and 40.25 wt.%, respectively, is slightly higher than the blank value and other catalysts. All catalytic HTL experiments were conducted in the absence of H2. The obtained results may be related to the main biochemical components in D. tertiolecta. For the blank value (non-catalytic

B

Ni 2 p

C

Pt 4f

XPS simulated data Baseline 2+ Pt 0 Pt XPS simulated data Baseline Metallic Ni Ni (OH)2

XPS simulated data Baseline Co 2P3/2 Co-C Co 2P1/2

775

780

785

790

795

800

Binding energy (eV)

NiO

805

810

845 850 855 860 865 870 875 880 885 890

Binding energy (eV)

66

68

70

72

74

76

78

80

Binding energy (eV)

Fig. 3. Metal photoelectron spectra of various metals supported on CNTs. (A) Co/CNTs; (B) Ni/CNTs; and (C) Pt/CNTs.

82

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120

Conversion Bio-oil yield 100

Percent (%)

80

60

40

20

0 Blank

Co/CNTs

Ni/CNTs

Pt/CNTs

Catalyst Fig. 4. Conversion and bio-oil yield via catalytic HTL on various M/CNTs catalysts.

800 700

Co/CNTS Ni/CNTS Pt/CNTS Blank

500

o

Boiling point ( C)

600

400 300

Diesel fraction 200 100 0

0

10

20

30

40

50

60

70

80

90

Mass (%) Fig. 5. Distribution of boiling point of the obtained bio-oil over various catalysts.

HTL), the conversion of three components in D. tertiolecta (proteins, carbohydrates, and lipids) decreases in the sequence of lipids > proteins > carbohydrates (Biller and Ross, 2011). The introduction of catalyst, which may influence the liquefaction process to some extent, results in the enhancement of conversion and bio-oil yield. The conversion (95.78%) is slightly higher when Co/ CNTs are used as the catalysts than when Ni/CNTs (92.36%) and Pt/CNTs (93.25%) are employed as the catalysts. However, the bio-oil yields showed no obvious difference over the three catalysts. The use of Ni/CNTs and Pt/CNTs promotes the high recovery of lipids, and the Co/CNT catalyst is related to the decrease of heteroatoms, thus resulting in the enhancement of the quality of the obtained bio-oil. In addition, H2 was not added in catalytic HTL; thus, the removal of heteroatoms may not be attributed to the hydrodeoxygenation reaction. Catalytic HTL does not need high H2 pressure, thus increasing safety in the production process. Therefore, Co/CNT catalyst, which can decrease the content of heteroatoms in the bio-oil and improve bio-oil quality, may be considered a promising catalyst for the catalytic HTL of microalgae. 3.3. Analysis of the bio-oil from catalytic HTL The bio-oil obtained from the HTL of microalgae is a complex mixture. The TGA applied in simulated distillation is considered a miniature ‘‘distillation” (Chen et al., 2015a). Fig. 5 displays the boil-

303

ing point distribution of the bio-oil from non-catalytic and catalytic HTL of microalgae. Although the boiling point distribution graph does not provide exact information for each chemical in the bio-oil, the HTL of the microalgae is likely to decompose the microalgal biomass into small molecules (Na et al., 2012). The bio-oil obtained from the catalytic HTL of microalgae over Co/CNTs exhibits different boiling point distributions of bio-oil. Compared with the non-catalytic and Ni/CNTs or Pt/CNTs catalytic HTL process, the bio-oil obtained from the Co/CNTs catalytic HTL represents smaller molecular weight compounds. The mass portions of bio-oil below 350 °C of boiling point over Ni/CNTs, blank and Co/ CNTs are 54%, 59% and 65%, respectively. The difference in boiling point distribution may be related to the difference of components in the bio-oil caused by different reaction pathways. GC–MS analysis was used to identify the main components in the bio-oil. The identification of GC–MS was based on a comparison with the spectra of the NIST 98 spectrum library. The detailed components and percentages of components in the bio-oil are detailed in Table 3. The current percent area values only illustrate the relative content of each compound in the bio-oil that can be vaporized and passed through the GC column. As can be seen from Table 3, the main compounds of the bio-oil included n-hexadecanoic acid, hexadecanamide, and 2,6,10,14-tet ramethyl-2-hexadecene during the HTL of microalgae with or without catalysts. Previous works used GC–MS to investigate the composition of bio-oil from HTL of microalgae (Chen et al., 2015a; 2012; Zou et al., 2010a,b). The results showed that fatty acids and amides were the main components of the obtained bio-oil. However, a significant difference existed in the main components of the four types of bio-oil. For instance, when Ni/CNTs and Pt/CNTs were used as catalysts, the content of n-hexadecanoic acid is over 50% (53.64% and 53.46%, respectively), whereas the use of Co/CNTs catalyst results in a low n-hexadecanoic acid content of only 26.36%. Furthermore, cyclopentanone derivatives (2-methyl-cyclopentanone, 2-methyl2-cyclopenten-1-one, 2,3-dimethyl-2-cyclopenten-1-one and 2,3,4-trimethyl-2-cyclopenten-1-one) and hydrocarbons (ethyl-benzene, decane, 1,3,5-ttrimethyl-benzene, undecane, 1,2,4,5-tetramethyl-benzene, dodecane, 2,6-dimethyl-undecane, 7-methyl-tridecane, and tridecane), which is close to 40%, are the main products in bio-oil obtained from Co/CNTs. On the contrary, the content of cyclopentanone derivatives and hydrocarbons is approximately 15% and some of them were not involved in the bio-oil over Ni/CNTs and Pt/CNTs catalysts. The component distribution of the bio-oil obtained from Pt/CNTs catalysis is more similar to Co/CNTs catalysis than that of Ni/CNTs catalysis. To conduct an in-depth study on the compound classes in the obtained bio-oil, the major components in the bio-oil were categorized into groups on the basis of functionalities and shown in Fig. 6. In this figure, there are a high content in fatty acids, ketones and N-containing compounds, and low hydrocarbons content in the bio-oil obtained from blank one. The bio-oils produced from Ni/CNTs and Pt/CNTs as the catalysts, as well as the blank value, contain a high content of fatty acids compared with Co/CNTs. For the Ni/CNTs and Pt/CNTs used as catalysts, the main compounds in the bio-oil have similar tendencies: fatty acids > hydrocarbons > N-containing compounds P ketones. When Co/CNTs were used as catalyst, the constituent compounds include hydrocarbons (51.63%), fatty acids (26.36%), ketones (13.98%), and N-containing compounds (8.01%), i.e., a bio-oil with high percentage of hydrocarbons and low fatty acid content was obtained when Co/CNTs were used as the catalysts. In theory, the bio-oil obtained from Co/CNTs catalysts is closer to the requirements of transport fuel than that of others. Furthermore, the cyclopentanone derivatives can be transformed from carbohydrates; this result has been proven in

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Table 3 GC–MS results of the bio-oil obtained from catalytic HTL over various catalysts. No.

Retention time (min)

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

3.30 3.55 4.11 5.62 6.22 6.54 7.66 7.83 9.26 9.42 9.82 10.34 10.70 11.72 12.23 12.94 14.25 14.71 17.05 17.78 17.97 18.09 18.89 23.49 23.84 25.42 25.68 28.06 28.28 32.13 37.64 46.58

2-Methyl-cyclopentanone Ethyl-benzene 2-Methyl-2-cyclopenten-1-one Decane 1,3,5-Trimethyl-benzene 2,3-Dimethyl-2-cyclopenten-1-one 2,3,4-Trimethyl-2-cyclopenten-1-one Undecane 1,2,4,5-Tetramethyl-benzene 3-Isopropylidene-5-methyl-hex-4-en-2-one 2-Methyl-5-hydroxypyridine Dodecane 2,6-Dimethyl-undecane 5-Methyl-2-cyclohexen-1-one 7-Methyl-tridecane Tridecane 1-Butyl-2-pyrrolidinone 1,2,3,4-Tetrahydro-1,1,6-trimethyl-naphthalene 2,6,10-Trimethyl-dodecane 1-Pentadecene 1,2-Dimethyl-1H-indole 2-Methyl-1-tetradecene 5,6,7,7a-Tetrahydro-4,4,7a-trimethyl-2(4H)-Benzofuranone Nonadecane DL-Alanyl-l-leucine 3,7,11,15-Tetramethyl-2-hexadecene 2,6,10,14-Tetramethyl-2-hexadecene 7-Methyl-9-olefinic undecanoate n-Hexadecanoic acid Hexadecanamide 3-Hydroxybutyl-quinoline Vitamin E

1 00

Hydrocarbons Ketones Fatty acids Nitrogen-containing compounds

90 80

Relative content (%)

Bio-oil

70

(%)

60 50 40

Blank

Co/CNTs

Ni/CNTs

Pt/CNTs

2.59 a a a a a a a a 2.66 3.59 a a 2.16 a a a 1.66 a 3.69 a a 9.60 a 5.75 3.12 6.11 8.73 41.34 3.47 2.16 3.37

3.13 5.20 a 5.19 2.83 6.71 2.51 1.37 4.99 a a 1.31 1.65 a 1.86 2.26 a a a a a a a a a a 4.39 a 26.36 8.01 a a

a a 2.02 4.41 a 2.25 a a a a a a a 1.73 a a a a a 3.83 a a 1.82 2.52 a 2.6 5.54 a 53.64 5.23 a a

a 3.02 a 4.72 1.82 4.94 2.33 a 4.19 a a a a 1.72 a a 2.22 a 1.02 1.11 1.96 1.15 a 1.68 a 2.6 3.37 a 53.46 8.68 a a

i.e., Co/CNT catalysts may be suitable for the conversion of carbohydrates to hydrocarbons. Therefore, the reaction mechanism of Co/CNT catalyst may be different from Ni/CNT and Pt/CNT catalysts during the catalytic HTL of microalgae. Furthermore, nitrogencontaining compounds, which are important components in the bio-oil obtained from non-catalytic HTL of microalgae, show low bio-oil content from catalytic HTL, particularly in Co/CNTs catalysis. The amino acids can undergo a deamination reaction to produce other compounds, and this process can remove the nitrogen from microalgae effectively.

30

3.4. Predicted reaction pathway for the catalytic HTL of D. tertiolecta

20

As reported in previous works (Biller and Ross, 2011; Ifrim et al., 2014; Yeh et al., 2013), hydrolysis and repolymerization reactions are the two key reactions in the HTL processing of microalgae. At the early stage of HTL, the substrates are all first hydrolyzed into small intermediate molecules. With the increase of reaction temperature, other reactions, such as repolymerization, decomposition, rearrangement, ammonolysis, dehydration, and condensation, may occur by the interaction of intermediates. The general reaction network of HTL of D. tertiolecta represent in Fig. 7 was proposed on the basis of the experimental results and previous studies (Chen et al., 2015a; Biller and Ross, 2011; Ifrim et al., 2014; Yeh et al., 2013; Yu et al., 2014). In general, protein is hydrolyzed to produce amino acids and short chain fatty acids. Lipid is hydrolyzed to produce glycerol and long chain fatty acids. Carbohydrate is hydrolyzed to produce monosaccharides and oligosaccharides. With the increase of reaction temperature, amino compounds are produced due to the decarboxylation of amino acids, and carbon dioxide is released during this process. In this way the oxygen is removed from the microalgae. Furthermore, alkenes (1-pentadecene) are produced

10 0 Blank

Co/CNTs

Ni/CNTS

Pt/CNTs

Catalyst Fig. 6. Major compound classes of the bio-oil obtained from catalytic HTL over various catalysts.

our previous work (Chen et al., 2015a). Components such as 2-methyl-cyclopentanone, 2,3-dimethyl-2-cyclopenten-1-one, and 2,3,4-trimethyl-2-cyclopente-1-one can be observed in the bio-oil over the Co/CNTs catalyst. Compared with other conditions, the yield of these components are low and are not even observed in certain conditions. Furthermore, the hydrocarbon compounds (e.g., undecane, dodecane, 2,6-dimethyl-undecane, 7-methyl-tridecane, and tridecane), which can also be derived from carbohydrates via a series of reactions such as condensation, dehydration, deoxygenation, and denitrogenation, is enhanced when using Co/CNT,

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305

Fig. 7. General reaction network of HTL of D. tertiolecta.

from the decarboxylation of fatty acids obtained from hydrolysis of the lipids, amino acids (DL-alanyl-l-leucine) are derived from the hydrolysis of proteins, and cyclic oxygenates (2-methylcyclopentanone and 5-methyl-2-cyclohexen-1-one) are produced from carbohydrates. Furthermore, the hydroxyl groups in long chain fatty acids are replaced by ammonia to produce amide (hexadecanamide). N-containing compounds, including pyrrolidinone, pyrrolidinedione, indole, pyridine, quinoline, and their derivatives (2-methyl-5-hydroxypyridine, 1,2-dimethyl-1H-indole and 3-hydroxybutyl-quinoline), are produced from the Maillard reaction between amino acids and monosaccharides. Long chain fatty acids can react with alcohol to form ester (7-methyl-9olefinic undecanoate). Furthermore, vitamins, which are also components of microalage, undergo dehydration and rearrangement to produce 3,7,11,15-tetramethyl-2-hexadecene, 2,6,10,14-t etramethyl-2-hexadecene, and aromatic compounds (1,2,3,4-tetra hydro-1,1,6-trimethyl-naphthalene and 5,6,7,7a-tetrahydro-4,4,7a -trimethyl-2(4H)-benzofuranone). As mentioned before, the employed catalysts affected the conversion, bio-oil yield, and chemical compositions during the HTL processing of D. tertiolecta. The effects of the employed catalysts on the variation of chemical compositions will be related to the catalytic HTL pathway. According to the aforementioned results and previous studies, the probable catalytic HTL reaction pathway is represented in Fig. 8. The dashed line in Fig. 8 represented the

compounds that are considered intermediate products. Compared with non-catalytic HTL processing (Fig. 7), Fig. 8 shows two important reactions during the catalytic HTL processing of microalgae, including deoxygenation and denitrogenation. Based on the experimental results in Table 3 and Fig. 6, bio-oil with high percentage of hydrocarbon components, low content of fatty acid, and low content of amide was obtained when Co/CNTs were used as the catalyst. The hydrocarbon components include benzene derivatives (e.g., 1,3,5-trimethyl-benzene, ethyl-benzene, and 1,2,4,5-tetramethyl-benzene) and short chain hydrocarbon (e.g., decane, undecane, dodecane, 2,6-dimethyl-undecane, 7-methyl-tridecane, and tridecane), which are produced from the N-/O-containing components via deoxygenation and denitrogenation and furfural derivatives via condensation and dehydration, respectively. These products are all derived from hydrolysis products of carbohydrates. The content of carbohydrates is approximately 52.31% reported in our previous work (Chen et al., 2015a), and the Co/CNT catalysts enhance the effective conversion of carbohydrates. For Ni/CNTs and Pt/CNTs, although the conversion and bio-oil yield are higher than that of the blank value, the content of fatty acid and N-containing components are all higher than that of the Co/CNT value with a low percentage of hydrocarbon compounds. This result can be attributed to the low percentage of chain hydrocarbons, which are produced from the furfural derivatives via con-

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Fig. 8. Proposed pathways scheme for HTL of D. tertiolecta over M/CNTs.

densation and dehydration, i.e., Ni/CNTs and Pt/CNTs catalysts are not conducive to the transformation of carbohydrates. Furthermore, when Ni/CNTs and Pt/CNTs are used as catalysts, the bio-oil contains a low percentage of ketones and Ncontaining compounds and high content of fatty acids. This result indicates that Ni/CNTs and Pt/CNTs hinder the ammonolysis reaction between fatty acids and ammonia. Therefore, the type of catalyst greatly affects the chemical composition and boiling point distribution of the HTL bio-oil and the catalytic HTL pathway. As can be seen from Table 3, with the introduction of catalysts, the proportions of various components changed significantly, and the proportions of some components suitable for use as fuel also increased greatly. Thus, the catalytic performance (involving conversion and bio-oil yield) integrated with the chemical compositions of bio-oils indicated that the selected catalyst was beneficial to the conversion process, although minimal difference was observed in the elemental composition of the different biooil samples. However, microalgae have a complex mixture of chemical constituents. Both the non-catalytic and catalytic HTL of microalgae, the cross-linked reactions among the constituents are complex, thus indicating that a certain chemical composition in the reaction mixture may be formed via different reaction pathways. For example, the hydrocarbons in bio-oil may be produced from the decarboxylation of long chain fatty acids and/or the condensation of ketones. Therefore, the proposed pathways in this study are merely possible pathways. This result can also reasonably explain the generation of specific components in bio-oil. The detailed reaction kinetic study of different possible pathways will be recommended for future study.

4. Conclusions On the basis of the results presented above, some conclusions can be drawn as follows: (1) M/CNTs (M = Co, Ni, and Pt) are conducive to the enhancement of the conversion and bio-oil yield compared with a blank value. When Co/CNTs are used as catalysts, the conversion and bio-oil yield increase to 95.78 and 40.25 wt.%, respectively. (2) The introduction of catalyst significantly affected the chemical composition of HTL bio-oil, and the type of catalyst greatly affects the chemical composition, boiling point distribution of the bio-oil and the catalytic HTL pathway. Furthermore, bio-oil with a higher percentage of hydrocarbons, a lower content of fatty acid and a lower N-containing compounds was obtained over Co/CNTs. (3) Based on our results and previous works, the plausible general reaction and catalytic HTL pathways of D. tertiolecta are proposed. The M/CNT catalysts enhance the conversion of carbohydrates. Meanwhile, the selected catalysts hinder the ammonolysis reaction between fatty acid and ammonia resulting in the decrease of nitrogen content in the bio-oils.

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

Y. Chen et al. / Chemical Engineering Science 161 (2017) 299–307

No. 21176142), Program for New Century Excellent Talents in University (No. NCET-12-0308), and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13026).

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