Preparation of Aromatic Hydrocarbons from Catalytic Pyrolysis of

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Biofuels and Biomass

Preparation of Aromatic Hydrocarbons from Catalytic Pyrolysis of Microalgae/Palm Kernel Shell Using PKS Biochar-based Catalysts Hongchao Wang, Guozhang Chang, Pengyu Qi, Xiao Li, and Qingjie Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03590 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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Graphical abstract

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Preparation of Aromatic Hydrocarbons from Catalytic Pyrolysis of Microalgae/Palm Kernel Shell Using PKS Biochar-based Catalysts

Hongchao Wanga, Guozhang Changa, Pengyu Qia, Xiao Lia, Qingjie Guoa,b*

a

Key Laboratory of Clean Chemical Processing of Shandong Province, College of Chemical

Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China b

State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, College

of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China

*Corresponding author. Address: College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. Tel.: +86 053184022257. E-mail address: [email protected] (Q. Guo).


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Abstract: The objective of this work was to prepare a biochar-based catalyst with higher reactivity and longer life cycle to convert biomass to aromatic hydrocarbons. The palm kernel shell biochar-based supports were initially prepared at various temperatures and with different durations of steam activation, followed by catalytic fast pyrolysis tests. The effects of the variables on the biochar-based support properties in terms of aromatic hydrocarbons were investigated. At last, the biochar-based catalysts were prepared by impregnation method. Results indicated that the biochar-based supports were obtained at the optimum conditions of a temperature of 800 °C and a steam activated time of 50 min, for converting the mixed PKS/microalgae to aromatics with relative content values of 38.94 area% and 49.53 area%, respectively. The characteristics of abundant continuously pore size distributions were observed on the surface of the optimum biochar-based support, resulting in a service longer life than the HZSM-5 catalyst during the co-pyrolysis of PKS/microalgae. The Mg-biochar catalyst exhibited the highest relative content of aromatics during the co-pyrolysis. It is expected that biochar-based catalyst can be widely applied for biomass fast pyrolysis. Keywords: Biochar; Biochar-based catalyst; Catalytic fast pyrolysis; Aromatic hydrocarbons; Reusability

1


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1. Introduction With the rapid development of the global economy, the consumption of fossil fuels continues to increase. Fossil fuel resources are increasing insufficiently to meet energy demands.1 Solar energy-derived biomass is an alternative energy resource that has drawn much attention due to its sustainability and renewability.2 Catalytic fast pyrolysis (CFP) is an improving method for the conversion of biomass directed towards the production of high-value chemicals. In the presence of a catalyst, it is possible for biomass pyrolysis to directly yield aromatic products, such as benzene, toluene, xylene and other aromatic hydrocarbons.3-9 The use of catalysts produces more bio-oil than non-catalytic conditions.10, 11 The catalyst is the key part of the catalytic reaction. Therefore, higher catalytic activity, better selectivity and longer catalysis life are the essential characteristics of a superior catalyst. The traditional method of biomass conversion to aromatic compounds usually includes participation of a molecular sieve catalyst, for example HZSM-5. The metal loaded HZSM-5 catalyst has a high selectivity for single-ring aromatics.12-16 Unfortunately coking of the molecular sieve catalyst is a critical problem. Li et al.17 reported that deactivation of molecular sieve catalysts with coke deposition proceeded through three steps in which carbocation precursors of the soluble coke formed on acidic site of HZSM-5, and then stacked to form chaotic filament-like carbon chains, finally developing into graphite carbon. Biochar is the by-product of biomass pyrolysis. Biochar has been applied to various

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fields due to its versatile physicochemical characteristics.18 The biochar as catalyst facilitates the utilization of all components of biomass. There are many factors that affect the properties of biochar. For example, the pyrolysis temperature, feedstock type, production method, reaction atmosphere, retention time and so on. Menéndez et al.19 concluded that biochar from microwave pyrolysis had a higher surface area than that from conventional pyrolysis. The inorganic elements and BET surface area increased with the increase of the pyrolysis temperature, and higher pyrolysis temperatures enhanced the carbon content of biochar and led to stronger thermal stability.20, 21 Yang et al.22 investigated a KOH-activated carbon whose surface area was 3143 m2·g-1. Alkaline-Earth Metals (AAEMs), such as K and Ca, were constituent of biomass and, after pyrolysis, all metals were found in the biochar. Some studies have reported that AAEMs can enhance the reaction of biomass thermo chemical conversion.23, 24 Some researches have studied the biochar as catalyst for tars pyrolysis and found that the presence of the char could promote the formation of alkyl monoaromatics.25, 26 Yao et al.27 studied biomass gasification with biochar as a catalyst/support and found that AAEMs of cotton char were more important than the pore structure towards gas production. Sun et al.28 found that the chemically activated biochar has obvious catalytic effects on pyrolysis oil of mixed plastics. The selectivity of aromatics was significantly improved. Biochar could promoted the cracking of large ring polyaromatic compounds to produce single ring aromatics.29 Meanwhile, the biochar have low production cost, easy prepared, and control pore structure. All of these properties make


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biochar a promising catalyst for generating aromatics via biomass catalytic fast pyrolysis. The influence of the zeolite pore structure on the thermo chemical conversion of biomass has been explored. Micropores with sizes of 0.52 to 0.59 nm produce the highest aromatic yield, and the large pore zeolites emerged high coke, low aromatic yield.30, 31 Few reports revealed that the structure of biochar exerts an effect on biomass catalytic fast pyrolysis when biochar was used as the catalyst.27, 32 In this paper, palm kernel shell was chosen to prepare biochar-based catalyst under different conditions for in situ catalytic pyrolysis biomass with the aim of enhancing aromatics production. The porous structure of PKS biochar was controlled by the pyrolysis temperature and steam activation time, which made biochar-based catalyst a developed pore structure. The relationship between the biochar-based catalyst structure and aromatics selectivity was investigated. The synergy of metal active component and structure of biochar-based support was also involved. 2. Experimental 2.1. Materials Table 1. Physicochemical properties of microalgae and PKS. Proximate analysis wad/%

Ultimate analysis wad/%

Composition analysis wad/%

Microalgae M

V

FC

A

C

H

Oa

N

protein

polysaccharide

lipid

Othersb

5.00

79.69

10.28

5.03

49.07

7.59

35.63

6.29

44.00

21.00

30.00

5.00

Palm kernel shell M

V

FC

A

C

H

Oa

N

lignin

hemicellulose

cellulose

Othersb

5.21

70.08

22.70

2.10

50.70

6.00

42.80

0.50

50.70

22.30

20.80

6.20

a, b

Defined by difference 4 ACS Paragon Plus Environment

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Dried palm kernel shell (PKS) particles with sizes of 0.20 to 0.45 mm were used as feedstocks to prepare the biomass-based support, and the PKS and microalgae were selected for the pyrolysis to produce aromatics. The two materials were first ground to obtain a particle size of less than 0.2 mm, and then dried at 80 °C in an oven for 6 h. The proximate and ultimate analyses of the raw materials were listed in Table 1. 2.2. Catalysts and its preparation The commercial HZSM-5 catalyst with a Si-to-Al ratio of 80 was purchased from Nankai University Catalyst Factory. The textural properties of HZSM-5 were listed in Table 2. Table 2. Textural properties of HZSM-5.

HZSM-5

SBET (m2·g-1)

Smicro (m2·g-1)

Sext (m2·g-1)

Vtotal (cm3·g-1)

305.14

110.37

194.77

0.223

Vmicro (cm3·g-1) 0.081

Dporesize (nm) 2.929

The biomass-based supports/catalysts were prepared as follows. First, the biochar supports were prepared using carbonization-activation method. The carbonizations of biochars were carried out in an electrical heating tube furnace (OTF-1200X, Hefei). Approximately 6 g of the PKS particles were placed in a ceramic boat, which was firstly inserted into a quartz tube. The N2 (purity of 99.99%, flow rate of 200 mL/min) carrier was injected into the quartz tube to remove the air inside the tube and keep the biomass under an inert atmosphere. When the pyrolysis temperature reached the preset temperature (600 to 850 °C), the ceramic boat was pushed to reaction zone at once, and the reaction time was 15min. The activations of biochars was prepared at different times (5 to 80 min) of steam 5 ACS Paragon Plus Environment

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activation. Approximately 6 g of the PKS particles were placed in a ceramic boat, which was pushed to the reaction zone when the pyrolysis temperature reached 800°C, stay for 15min, then started the steam generator. The steam activated time set at 5, 20, 35, 50, 80 min, respectively. The obtained PKS biochar-based support was then mixed with 5 mol/L HCl, stirring with a Teflon-lined magnetic stirrer at 60 °C for 2 h. The biochar-based support was then washed with deionized water until reaching pH=7 and thereafter dried at 120 °C for 12 h. The biochars obtained at various temperatures were named 600BC, 650BC, 700BC, 750BC, 800BC, and 850BC. Five activated biochars labeled 5minBC, 20minBC, 35minBC, 50minBC, and 80minBC were obtained from the different steam activation times. Then, metal (Cu, Ni, Zn, or Mg)-doped 800BC catalysts were prepared by wet impregnation method. The metal loading was the percentage of metal salt mass account for the metal-biochar catalyst mass. The metal loading of the M-800BC catalyst was 1, 3, 5, 10 wt.%, respectively. 4g as-prepared 800BC was added into 20ml aqueous solutions of Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, Cu(NO3)2·xH2O or Mg(NO3)2·xH2O and stirred at 30 °C for 4 h. Thereafter, the slurry was dried at 100 °C, followed by calcination at 800 °C in N2 atmosphere for 20 min. M(NO3)x·xH2O were obtained from Aladdin, and all reagents used were analytical grade. 2.3. Catalyst characterization The specific surface area of the catalyst was measured by a Micromeritics


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instrument (Quantachrome Autosorb-1C-VP, USA) using the BET method. The microporous and mesoporous of the biochar-based catalysts were calculated by HK and BJH method, respectively. Scanning electron microscopy (SEM) images were recorded to observe the surface morphology of the biochars on a scanning electron microscope (JSM-6700F, Japan). Meanwhile, the SEM was coupled with an Energy Dispersive System (EDS) to investigate the element distribution of the biochars. The surface chemical functional groups of the biochars were recorded by a Fourier transform infra red spectrometer (Tensor 27, Bruker Instruments, Germany). The test condition: 1mg biochar and 300mg potassium bromide, Resolution 4cm-1, and scan spectrum was 500 ~ 4000cm-1. 2.4. Catalytic fast pyrolysis (CFP) of PKS and microalgae

Figure 1. Experimental setup of CFP. Catalytic fast pyrolysis of PKS and microalgae was executed in the electrical heating tube furnace (Figure. 1). The biochar-based support/catalyst was separated via stainless steel gauze from the mixed PKS/microalgae biomass materials, the mass ratio of catalyst and biomass was 1:1. The support was placed above the gauze and the PKS/microalgae mixture was under the gauze. During the reaction, N2 gas was used as 7 ACS Paragon Plus Environment

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the carrier gas. The gas flow was 150 mL/min. Prior to the reaction, the tube was purged with N2 gas to remove the air inside the quartz tube. The catalytic fast pyrolysis reaction temperature was 600 °C, the reaction time was 15 min and the heating rate was approximately 1200 °C /min, which was the optimum condition for acquiring bio-oils according to the prior tests.33 Bio-oil in each test was cooled and absorbed by isopropyl alcohol and was collected with two Monteggia gas washing bottles at an ice/water atmosphere. 2.5. Characterization of the bio-oils from CFP experiments The obtained bio-oils were analyzed by GC/MS (7890A-5975C, Agilent Technologies, USA) to detect and identify the products of fast pyrolysis catalysis of the biomass and the relative yield of the aromatic hydrocarbons. The type of capillary column was HP-5 (60 m×0.25 nm i.d., film thickness). High purity nitrogen was used as carrier gas. The split mode was used with a split ratio of 50:1. The initial temperature was 40 °C for 2 min, increased to 300 °C at 10 °C ·min-1. The final temperature (300 °C) was held for 25 min. 3. Results and discussion 3.1. Effect of temperature on the preparation of the biochar supports 3.1.1. Yields of the biochar supports The yields of the PKS biochar-based supports obtained at different pyrolysis temperatures were shown in Figure 2a). The yield of biochar decreased from 27.40 wt.% at 600 °C to 21.50 wt.% at 850 °C, because more solid constituents of PKS were


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cracked into the bio-oil and non-condensable gas products as the temperature increased.34 35

a) Yield（%（

30

25

20

15

600

650

700

750

800

850

Temperature (°C)

c) Transmittance (%)

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600BC 650BC 700BC 750BC 800BC 850BC

4000 3500 3000 2500 2000 1500 1000 Wavelength (cm-1)

500

Figure 2. The yield and surface physicochemical properties of PKS biochar supports obtained at different temperatures: a) yields; b) SEM-EDS results; c) FTIR spectra.

3.1.2. Surface topography of the biochar supports After acid pickling, ash on the surface and in the pores of biochar obtained at various temperatures was removed, as shown in Figure 2b). As the preparation temperature rose, the pores on the surface of the biochar gradually increased. At 600 °C, the biochar had an uneven surface and a small amount of large size holes containing particles formed. At a pyrolysis temperature of 700 °C, the biochar had a smooth surface, with more holes distributed over its surface. Further warming to 800 °C, led to a porous structure with honeycomb appearance, distributed denseness and 9 ACS Paragon Plus Environment

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well-developed pore structure. EDS analyses of the biochar at different pyrolysis temperatures were shown in Table 3. The carbon content increased with the increasing temperature (90.36 Atomic%, 91.28 Atomic % and 95.45 Atomic %). After the washing process, while the contents of oxygen, silicon, potassium and calcium decreased, indicating that the three major components (lignin, cellulose, hemicellulose) gradually cracking. More ash removed, less left. Then the elements of biochar-based support were analyzed in order to investigate the effect on catalytic pyrolysis reaction. Compared with other elements, carbon had the highest contents. At 600 °C, the content of oxygen was 9.24 wt.%, and calcium had next highest content, with a content of 3.78 wt.%, which came from Ca2CO3. The content of potassium was 1.44 wt.%. Silicon was from SiO2, and its content was 0.94 wt.%. Other biochars produced under different temperatures had the same trend. The element of Cl mainly came from the HCl solution when acid pickling, and did not clear up entirely, and the residual chlorine content was traces, which played a barely role on catalytic activity. Table 3. Elemental analysis of PKS biochars obtained at different temperatures. 600BC Element

700BC Weight

Atomic%

Element

%

800BC Weight

Atomic%

Element

%

Weight

Atomic%

%

C

84.41

90.36

C

82.80

91.28

C

92.16

95.45

O

9.24

7.43

O

5.16

4.27

O

4.25

3.30

Si

0.94

0.43

Si

0.81

0.38

Si

0.12

0.06

Al

0.11

0.05

Cl

8.09

3.02

Cl

2.74

0.95

K

1.44

0.48

K

0.86

0.29

K

0.38

0.12

Ca

3.78

1.21

Ca

2.29

0.76

Ca

0.35

0.11


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The FTIR spectra of the PKS biochars obtained at different pyrolysis temperatures were shown in Figure 2c). The absorbance peaks located between 3540 cm-1 and 3300 cm-1 were related to the stretching vibration of –OH, which mainly came from hydroxyl groups. The band between 1700 and 1300 cm-1 represented the stretching vibrations of polycondensed aromatics, conjugated olefins.17 The peak at 1680 cm-1 was representative of C=O from the carboxyl and carbonyl groups.35 The 1635 cm-1 peak was the C=C from the polycyclic aromatics. The C-C stretching and C=C stretching vibrations at 1570 cm-1 mainly came from aromatic rings and olefins.36, 37 The absorption peak at 1433 cm-1 indicated the presence of C-C skeleton vibrations from aromatic rings. The intensity of the 1680 cm-1 and 1570 cm-1 peak decreased, while the 1433 cm-1 and 1635 cm-1 band increased, indicating that the biochar cracked with the increased of the pyrolysis temperature, and olefins transformed into aromatics via hydrogen transfer or aromatization. The peaks between 1057 and 1103 cm-1 were assigned to phenol C-O stretching vibrations.38 The bands from 820 cm-1 to 760 cm-1 were attributed to the stretching vibration of C-H from aromatics.38 The band at 480 cm-1 was caused by the symmetric stretching vibration of Si-O-Si, which indicated the presence of SiO2.39 Table 4. Textural properties of PKS biochars obtained at different temperatures. 600BC

650BC

700BC

750BC

800BC

850BC

Surface area (m2/g)

36.70

365.52

300.92

269.87

248.27

87.52

Average pore radius (nm)

4.72

1.97

2.08

2.13

2.28

3.06

Total pore volume (cm3/g)

0.09

0.18

0.16

0.14

0.14

0.067

Most probable pore size (nm)

1325.13

3.69

3.29

3.71

3.69

14.83


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3.1.3. Pore structures of the biochar supports During biomass catalytic fast pyrolysis, the pore structure of the catalyst was a vital condition for the selectivity of aromatic hydrocarbon. So, the pore structure of the biochar-based support was analyzed in this section. The surface area, average pore radius, total pore volume, and most probable pore size were the most important factors to evaluate the pore structure of the PKS biochars. The evolution of the pore structure of biochar was caused by the change of pyrolysis temperature. BET dates of the PKS biochars obtained at different temperatures were listed in Table 4. The average radius (1-4 nm) mainly consisted of micropores and mesopores. At a pyrolysis temperature of 600 °C, the 600BC had the largest average pore radius (4.72 nm) and lowest surface area (36.70 m2·g-1). This result implied that the pyrolysis reaction was incomplete because of the accumulation of coke deposit on the biochar orifice or in the channel interior, which blocked the pores. Furthermore, the surface of the PKS biochar was rough. The SEM images and BET data suggested that only a few large holes were on the surface. For pyrolysis between 650 to 850 °C, the average pore radius gradually increased, while the surface area and pore volume decreased.40 The pores grew in size, and the SEM images showed that the surface of the biochar become smooth. At a pyrolysis temperature of 850 °C, the surface area dropped rapidly because the high temperature caused the carbon skeleton structure of the biochar to collapse.41, 42 Most probable pore size (nm) refers to the pore size with the greatest probability.43, 44


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All of the biochars had the most probable pore size approximately 3.6 nm, except for the biochars formed at 600 °C and 850 °C (which had average pore sizes of 1325.13 nm and 14.83 nm, respectively). The pore size distributes mainly in 3-4 nm, with in a range of 650-800 °C. The pore diameter of the biochar was studied and results were illustrated in Figure 3. It is clear that the pore of biochar showed one peak at about 3-4 nm. And there is a peak in the range of 10-20 nm, its intensity increased by increasing temperature from 650 to 850 °C. It indicated that increasing pyrolysis temperature enhancing mesoporosity. 0.0035

0.0020

0.0020

-1

0.0015 0.0010 0.0005

0.0015 0.0010 0.0005

0.0000

750BC

0.0015  V(d) (cc ·g -1 )

0.0025  V(d) (cc·g )

 V(d) (cc·g -1 )

700BC

0.0030

0.0025

0.0000 4

6

50 8 Pore Diameter (nm)

100

150

6

8

10

50

100

0.0005

150

4

0.00020

800BC

0.0012

0.0010

0.0000 4

Pore Diameter (nm)

0.0014

6 8 50 Pore Diameter (nm)

100

150

850BC

0.0010

0.00015  V(d) (cc ·g -1 )

0.0008 0.0006 0.0004 0.0002 0.0000 -0.0002

0.0020

0.0035 650BC

0.0030

 V(d) (cc ·g -1 )

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

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0.00010 0.00005 0.00000

4


100

150

4


100

150

Figure 3. Pore diameter distribution of the biochars obtained at different temperature.

3.2. Effect of the activation time on the preparation of activated-biochar supports 3.2.1. Yields of the activated-biochar supports The yields of PKS activated-biochar supports obtained at different steam activation times were shown in Figure 4a). From the diagram, it was clearly observed that the yields decreased as the residence time of activation increased. Compared with the yields 13 ACS Paragon Plus Environment

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of biochars obtained at various pyrolysis temperatures, the activated-biochar supports had a higher weight loss, even though 5minBC had a yield of only 18.2 wt.%. These values revealed that more fixed carbon was converted into molecular components, such as CO, CO2, and CH4, in a steam activation atmosphere. 30

a Yield （%（

20

10

0 0

20

40

60

80

Time min

c) 5minBC

Transmittance (%)

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

20minBC 35minBC 50minBC 80minBC

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavelength(cm )

Figure 4. The yield and surface physicochemical properties of PKS activated-biochar supports obtained at various time of steam activation: a) Yields; b) SEM-EDS results; c) FTIR spectra.

3.2.2. Surface topography of the activated-biochar catalysts The typical surface morphologies of the PKS activated-biochar supports obtained at different steam activation times were shown in Figure 4b). The SEM images of activated-biochars showed obvious porous structures, which were found to be advanced in porosity in comparison to biochars obtained through pyrolysis. The 5minBC sample had small pores on its surface and an irregular porous structure. Extension of the


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activation time caused an obvious change in surface pores and the samples had a denser rough surface. The morphologies of 80minBC were remarkably changed, demonstrating a smooth surface with a compact porous structure. Interestingly, 80minBC presented a honeycomb briquette shape with larger holes. The results illustrated the components were further consumed and promoted the formation of porous structure of biochar under the steam activation. Table 5. Elemental analysis of PKS activated-biochar supports. 5minBC

35minBC

80minBC

Elemen

Weight

Atomic

Elemen

Weight

Atomic

Elemen

Weight

Atomic

t

%

%

t

%

%

t

%

%

C

91.46

95.36

C

89.19

94.33

C

87.00

92.46

O

5.22

4.09

O

5.63

4.47

O

6.91

5.51

Si

0.12

0.05

Si

1.27

0.57

Si

3.16

1.44

Cl

0.92

0.33

Cl

1.13

0.41

Cl

1.12

0.40

Ca

0.09

0.03

Ca

--

--

Ca

0.11

0.04

K

--a

--

K

0.16

0.05

K

0.16

0.05

a Not

detected

Considering the EDS data presented in Table 5, the activated-biochar support was mainly carbon. For comparison, the elemental contents of biochars obtained under different steam activation conditions varied from those obtained at different pyrolysis temperatures. The content of carbon was found to decrease (91.46 wt.% to 87.00 wt.%) with the increase of the activation time, while the content of oxygen increased (5.22wt.% to 6.91 wt.%) as the steam activation time increased. An upward trend was found in regard to the content of silicon (0.12 wt.% to 3.16 wt.%). In addition, when the steam activation time was over 35min, the content of silicon was higher than in biochar formed under pyrolysis conditions. As the reaction of activation progressing, more ash 15 ACS Paragon Plus Environment

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content was produced. The contents of calcium and potassium were trace amounts that have barely changed. Figure 4c). showed the FTIR spectra of activated-biochar supports obtained at various steam activation time. In comparison to the pyrolysis conditions, the amount of functional groups in the biochars obtained in a steam activation atmosphere was fewer. Among them, some groups (C=O, C-H, Si-O) were similar to those of biochars obtained at various temperatures. As observed from the presented spectra, some new functional groups were found. Aliphatic C-H stretching vibrations were indicated by the absorption peak at 2950 cm-1.36 The number of chemical functional groups of the activated-biochar support decreased, manifested the components of PKS gradually fell off under steam activation. 3.2.3. Pore structures of the activated-biochar catalysts 0.008

5minBC

0.004 0.003 0.002

0.04 -1

0.005

 V(d) (cc·g )

 V (d) (cc ·g -1 )

 V(d) (cc ·g -1 )

0.015

0.006

0.03 0.02 0.01

0.001 2

6

4

9

40

100

200

0.00

1

Pore Diameter (nm)


40

100

200

0.010

0.005

0.000

2

4


100

200

0.030

0.024 50minBC

0.025

0.016

0.020

 V(d) (cc ·g -1 )

0.020

0.012 0.008 0.004 0.000

35minBC

20minBC

0.05

0.007

0.000

0.020

0.06

0.009

 V(d) (cc ·g -1 )

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

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80minBC

0.015 0.010 0.005

2

4

10 Pore Diameter (nm)

40

100

200

0.000

2

4

10 Pore Diameter (nm)

40

100

200

Figure 5. Pore size distribution of PKS activated-biochar supports.

The influence of the steam activation time on the specific surface area and porous


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Energy & Fuels

structure of the activated-biochar support was given in Table 6. Unlike PKS biochars generated under pyrolysis conditions, the surface area of activated-biochar supports increased from 488.60 to 1175.86 m2·g-1 with activated time increased from 5 min to 80 min. The increased surface area could be due to more carbon reacting with H2O. Similar to the surface area, the average pore radius of activated-biochar support increased (2.452nm - 4.719nm) with the increasing activation time. In addition, the micropore volume rose during the first stage (5-35 min) and then decreased (35-80 min). The results completely agreed with the idea that the increasing activation time influenced the porous structure due to the loss of carbon and led to the transformation of micropores into mesopores or even macropores. Table 6. Textural properties of PKS activated-biochar supports.

Surface area

5minBC

20minBC

35minBC

50minBC

80minBC

488.604

812.822

1103.275

1152.556

1175.864

0.185

0.251

0.289

0.248

0.193

2.452

2.890

3.397

3.963

4.719

0.489

0.381

0.493

0.488

0.490

679.864

963.065

1379.958

1392.117

1312.070

1.179

0.679

1.483

1.483

1.269

(m2·g-1) Micropore volume (cm3·g-1) Average pore radius (nm) Average hole width (nm) Micropore surface area (m2·g-1) Most probable pore size (nm)

The pore size distribution of activated-biochar catalysts were shown in Figure 5. With the increase of the reaction time, the amount of mesopores and macropores continued to increase. However, micropores increased at first and then decreased, with the maximum value occurring at 35min. Although the amount of mesopore was highest 17 ACS Paragon Plus Environment

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in 80minBC, the amount of macropore simultaneously increased, implying that the structure of the activated-biochar catalyst was partially destroyed. Compared with the biochars of different pyrolysis temperature, the pore size of activated-biochar support shown two or more sharp peak at about 0-2 nm, indicating that the steam activation contributed to the form of micropore. With the reaction time increased, two broad peaks at about 5-10 nm in mesopore region, and the peak area increased. The results illustrated that steam activation appear to affect pore diameter distribution obviously, which could increase the mesoporosity of activated-biochar support. The steam activation deteriorated biomass macromolecular structure and made the activated-biochar porous structure abundant. Meanwhile, the activated-biochar catalysts had both micropore and mesopore properties, the wider pore size distribution range made the catalyst avoid deactivated easily. 3.3. Preparation of aromatics from catalytic fast co-pyrolysis experiments In this section, the interactions between the yield of aromatic hydrocarbons and porous structure of biochar-based catalysts were studied by carrying out experiments using catalytic fast pyrolysis microalgae/palm kernel shell with a quality ratio of 1. The use of microalgae as third generation biofuel feedstock has been proposed as a solution to the problem of lignocellulosic biomass supply shortage. The meaning of co-pyrolysis of microalgae and PKS was improving the aromatics yield. Then the biochar-based catalyst was designed to further improving the aromatics yield. In previous study, the mixed pyrolysis of microalgae/palm kernel shell demonstrated a synergistic effect in the


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Energy & Fuels

preparation of aromatics.33 The amount of aromatics of microalgae/palm kernel shell mass ratio of 1:1 co-pyrolysis was higher than sole pyrolysis of microalgae and palm kernel shell. The synergistic effect for aromatic hydrocarbons was the substitution reaction of free radicals with phenols in the primary bio-oil of co-pyrolysis, as a result of the yield of aromatics increased significantly. The experiment was conducted at 600 °C using PKS biochar-based support as the catalyst for 15 min. The biomass and catalyst were arranged in layers, with the biomass on the bottom and the catalysts on top. In addition, the biomass and catalyst were separated by a stainless steel wire. The non-condensable gases, bio-oil and biochar were obtained by catalytic fast pyrolysis of biomass. The increase of aromatics yield of biomass catalytic fast pyrolysis due to the presence of biochar-based catalyst was shown in Figure 6, which indicated that the activated-biochar catalysts have a higher selectivity of aromatics than biochars. According to the BET data, the apertures of biochars under different pyrolysis temperatures were mainly micropores. And the activated-biochar catalysts were consisted of micropore and mesopore. Simultaneously, the surface area of activated-biochar catalyst was more 3.2 times than biochar. Surface area reflected the advanced pore structure. As shown in Figure 6, when the biochars obtained at different pyrolysis temperatures were used as catalysts, the yield of aromatic hydrocarbons first increased then decreased, with the highest aromatics yield (38.94 area%) obtained using 800BC as the catalyst. The surface areas of biochars obtained at different temperatures decreased


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with the increase in temperature because the average pore radius increased and the range of pore radius distribution of biochars exhibited higher selectivity for aromatics. However, the biochars obtained at different pyrolysis temperatures had relatively lower surface areas. The porous structure of activated-biochar support was developed than biochars. Overall, an appropriate porous radius benefited the shape-selectivity for aromatic hydrocarbons. Additionally, the amount of pores is an important factor that influences the catalyst on aromatics yield. When the activated-biochar supports were used to catalytic pyrolysis of biomass, the yield of aromatic hydrocarbons was 2.37 times higher than that of the non-catalyst process. Moreover, the yield of aromatic compounds increased as the biochar activation time increased. The maximal aromatics yield (49.53 area%) occurred with 50minBC as the catalyst. Additionally, the increase of the activation time led to an increase of the surface area of the activated-biochar catalyst. The average pore radius was distributed between 2.50 and 4.00 nm. Meanwhile, the existence of micropores and mesopores demonstrated that the optimum pore radius was from 3.96 to 4.72 nm for the selectivity of aromatic hydrocarbons. As a comparison to a standard catalyst, HZSM-5 was used to catalyze fast pyrolysis of biomass to produce aromatic hydrocarbons. The yield of the aromatic compounds was given in Figure 6. Additionally, the result showed that the yield was 38.96 area %. This result was same as another study on catalytic pyrolysis of biomass utilizing HZSM-5.45 The HZSM-5 catalyst had a Si/Al ratio of 80, pore size from 0.51-0.56 nm, and the catalytic pyrolysis of microalgae under HZSM-5 obtained


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the maximum aromatics yield at the Si/Al ratio of 80.46 50 Relative content (Area%)

40 30 20 10

5m 20 inB m C 35 inBC m 50 inBC m 80 inB mi C nB C HZ SM -5 No -ca t al ys t

0 60 0B 65 C 0B 70 C 0B 75 C 0B 80 C 0B 85 C 0B 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

Energy & Fuels

Figure 6. The relative content of Aromatic hydrocarbons for the bio-oil.

3.4. Reusability of biochar-based catalyst To investigate the reusability of biochar-based catalysts, 50minBC, which had the optimal aromatics yield, was chosen. Repeated catalytic pyrolysis experiments were performed at 600 °C with a catalyst-to-biomass ratio of 1. In addition, contrasting experiments were conducted under the same conditions using the HZSM-5 molecular sieve as catalyst. Ten repetitions of catalytic pyrolysis experiments were carried out to obtain results regarding the aromatics yield. The results of group 1, group 5 and group 10 were recorded. As shown in Figure 7, similar trends were observed for both the HZSM-5 and 50minBC catalytic pyrolysis experiments. In group 1, the catalysts maintained a high activity and high aromatics production selectivity. As was observed, the aromatics yield decreased gradually with the increase of recycling times. Meanwhile, 50minBC appeared to have greater activity than HZSM-5. The aromatics yield of group 5 reached 26.378 area % of activated-biochar catalyst, and the


Energy & Fuels

deactivation phenomenon of two kind catalysts were apparent in group 10. The activated-biochar catalyst had the advantage of higher resistance for catalyst deactivation because its structure both micropores and mesopores, wherein the micropores structure are similar in to the HZSM-5 molecular sieve. With the increase of reaction times, HZSM-5 was blocked and deactivated because the macromolecule oxygenates could not passed through the pores or accumulate in the pore opening. Under the action of 50minBC catalyst, monocyclic aromatic hydrocarbons could passed through appropriate micropores, while some macromolecular oxygenates could passed through larger mesopores and macropores. In this process, secondary reactions occurred and macromolecular compounds be cracked into smaller molecules during the catalytic pyrolysis process. Hence, the activated-biochar catalyst did not suffer from serious coke deposition. As a consequence, activated-carbon catalyst had a longer service life than HZSM-5. 50 Relative content (Area%)

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

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HZSM-5 50minBC

40 30 20 10 0

2

4

6

8

10

Figure 7. Aromatics relative content of ten times recycle experiments.

3.5. The influence of metal loading on biochar-based catalyst


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Figure 8. The distribution of Metal actives sites: a) 3wt.% Cu -800BC, b) 3wt.% Zn -800BC, c) 3wt.% Mg -800BC, d) 3wt.% Ni-800BC, e) 10wt.% Ni-800BC.

To study the effect of biochar-based catalyst loaded with metal active components on the yield and selectivity of aromatic hydrocarbons, four kinds of biochar-based catalyst loaded with Cu, Ni, Zn and Mg were prepared and used to catalytic fast pyrolysis of biomass, respectively. The 1, 3, 5 and 10 wt% (metal nitrate mass accounts for catalyst mass) metal-biochar catalysts were prepared by conventional wet impregnation method. 800BC was used as support. The distribution of metal actives sites in M-800BC were shown in Table 8. The metal active component was loaded on the biochar-based support perfectly. Surface area of M-800BC catalysts was slightly reduced, average pore radius was increased. It indicated that metal particles distributed on the surface and in the pores, blocked smaller pores, and thus the average pore size 23 ACS Paragon Plus Environment

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was increased. The experimental conditions for catalytic fast pyrolysis of biomass were the same as the above-mentioned. Production yields and aromatic selectivity for the biomass catalytic pyrolysis was shown in Figure 9. The introduction of metal active components had great influence on the selectivity of aromatic hydrocarbons in bio-oil obtained by biomass catalytic pyrolysis. Four metal- 800BC catalysts enhanced the selectivity of aromatics. Bio-oil contained a large amount of styrene under the action of Cu, and the yield of aromatic hydrocarbons decreased (40.23-33.82 area %) with the increase of metal loading. Under the action of Ni, Zn and Mg, the content of benzene and toluene increased obviously. The introduction of Ni in the Ni-800BC catalyst, the yield of aromatics increased significantly. With the increase of Ni loading, the yield of aromatics also showed a downward trend (46.61-30.01 area %). The influence of Zn loading on the aromatics yield was opposite to that of Cu and Ni, and when the Zn loading was 10 wt%, the maximum yield of aromatic hydrocarbons obtained. The addition of Mg has a significant improved in the yield of aromatics, especially benzene, toluene. The aromatics yield reached a maximum of 49.10 area % when the Mg loading was 3 wt.%. The introduction of the metal active site had a synergistic effect with pore structure, and reduced the content of macromolecular oxygenates in bio-oil, the selectivity to aromatics was significantly improved. Various in the type of metals and metal loading had a different impact on the production in bio-oil. The combination of the metal component treated by impregnation with the biochar support blocked the micropores and reduced the porous structure. On the other hand, metal component


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provided more active sites for the biomass catalytic pyrolysis. The chance of pyrolysis vapor occurred secondary reaction increased and improved the reaction rate. 55 50

Relative content (area%)

45 40 35

Benzene Toluene Ethyl benzene P-xylene O-xylene Styrene Ethyltoluene Hemimellitol

30 25 20 15 10 5 0 Cu Cu 1% Cu 3% Cu -5% -1 0% Ni -1 Ni % Ni 3% Ni -5% -1 0% Zn Zn -1% Zn 3% Zn 5% -1 0% M gM 1% gM 3% g M -5% g10 % 80 0B 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

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Figure 9. Yields and aromatic selectivity for the biomass catalytic pyrolysis using M-800BC catalyst.

4. Conclusion Biochar-based catalysts played an important role during catalytic pyrolysis because of their special porous structure. The biochars had structures that mainly contained micropores and mesopore. The highest aromatics yield (38.94 area%) occurred at 800BC as catalyst. On the other hand, the activated-biochar supports had wider range of pore size distribution, making it a developed porous structure, which favored the produce of aromatic hydrocarbons. 50minBC had the best selectivity for aromatics production (49.53 area%). In comparison with HZSM-5, activated-biochar catalysts had higher resistance for deactivation. Hence, biochar-based support/catalyst had a lower risk of coke deposition. The introduction of metal active components played a synergy with pore structure, further improved the aromatic hydrocarbons.


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Conflict of interest The authors declare no competing financial interest. Acknowledgements This work is supported by the Key Project of the Natural Science Foundation of Shandong Province (ZR2015QZ02), the Key Research & Development Program of Shandong Province (2018GGX104013), and the introduction of scientific and technological innovation team of Ningxia Hui Autonomous Region (2016). References (1) Perego, C.; Bosetti, A., Biomass to fuels: The role of zeolite and mesoporous materials. Microporous Mesoporous Mat. 2011, 144 (1-3), 28-39. (2) Lu, Q.; Li, W. Z.; Zhu, X. F., Overview of fuel properties of biomass fast pyrolysis oils. Energy Convers. Manage. 2009, 50 (5), 1376-1383. (3) Chang, G.; Miao, P.; Yan, X.; Wang, G.; Guo, Q., Phenol preparation from catalytic pyrolysis of palm kernel shell at low temperatures. Bioresour. Technol. 2018, 253, 214-219. (4) Bridgwater, T., Challenges and Opportunities in Fast Pyrolysis of Biomass: Part II Upgrading options and promising applications in energy, biofuels and chemicals. Johns. Matthey Technol. Rev. 2018, 62 (2), 150-160. (5) Chen, H.; Cheng, H.; Zhou, F.; Chen, K. Q.; Qiao, K.; Lu, X. Y.; Ouyang, P. K.; Fu, J., Catalytic fast pyrolysis of rice straw to aromatic compounds over hierarchical HZSM-5 produced by alkali treatment and metal-modification. J. Anal.Appl. Pyrolysis


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Preparation of Aromatic Hydrocarbons from Catalytic Pyrolysis of

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