Production of aryl oxygen-containing compounds via catalytic pyrolysis of bagasse lignin over La0.8M0.2FeO3 (M=La, Ca, Sr, Ba)

Journal of Analytical and Applied Pyrolysis 142 (2019) 104624

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Production of aryl oxygen-containing compounds via catalytic pyrolysis of bagasse lignin over La0.8M0.2FeO3 (M=La, Ca, Sr, Ba)

T

Haiying Wanga,b, Hongjing Hana,b, Enhao Sunc, Yanan Zhanga,b, Jinxin Lia,b, Yanguang Chena,b, , Hua Songa,b, Hongzhi Zhaoa,b, Yue Kanga,b ⁎

a

College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, China Provincial Key Laboratory of Oil & Gas Chemical Technology, Northeast Petroleum University, Daqing, 163318, China c Daqing Petrochemical Research Center, PetroChina, Daqing, 163000, China b

ARTICLE INFO

ABSTRACT

Keywords: Bagasse lignin Catalytic pyrolysis Perovskite Aryl oxygen-containing compounds Biomass conversion

A new method named the production of aryl oxygen-containing compounds via catalytic pyrolysis of bagasse lignin (BL) over perovskites, La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) was proposed. La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) samples were synthesized by the sol-gel method (SG) and characterized by XRD, SEM, BET and XPS, and their catalytic pyrolysis performances were evaluated by the test of TG and the evaluation of the fixed bed microreactor. Under the action of perovskites, the pyrolysis of BL was promoted, the yields of gaseous and solid products decreased while the yield of liquid products increased by 10–20%; decarboxylation and decarbonylation could be inhibited, and it was conducive to the fracture of aliphatic hydrocarbon side chains on aromatic rings, as a result, the selectivity of CO2/CO decreased while the selectivity of hydrocarbons in gaseous products increased; the total selectivity of aryl oxygen-containing compounds increased from 56 wt.% in the liquid product obtained from pure BL pyrolysis to more than 69 wt.% in that of catalytic pyrolysis of BL. The spent catalyst was regenerated after controlled combustion of solid product — char, and it showed good structural and catalytic stabilities after 5 successful redox cycles.

1. Introduction The sustainable development strategy has led to significant attention on biomass as a promising, renewable alternative for fossil resources [1]. Lignin is one of the main components of lignocellulosic biomass, accounting for 15–30% and 40% of the weight and energy content of lignocellulose, respectively [2]. It is an amorphous, three dimensional, high molecular weight (6 × 105–15 × 106 kg·kmol−1) aromatic polymer of phenylpropanes of 3C attached with 6C atom rings, methoxy groups and non–carbohydratic polyphenolic substances, which are connected by ether linkages and CeC linkages [3]. Lignin has been regarded as a useful renewable non-petroleum resource for the production of high-valued aromatic chemicals due to it has a high degree of aromatization, which is similar to several petroleum components [4]. However, its structure is too complex to use widely compared to cellulose and hemicelluloses [5]. Thus, exploration of chemical approaches for efficient utilization of this renewable feedstock would have significant economic and environmental advantages [6]. Pyrolysis is a thermal degradation method in the absence of oxygen, which decomposes lignin and then converts it into products in the form ⁎

of gaseous, liquid and solid [7]. Pyrolysis is considered as one of the simplest and most cost effective conversion technologies [8], and it has been rapidly developed in recent years due to its advantages of materials adaptability, product diversity, etc. In order to reduce the reaction barrier and improve the yields of target products, catalytic pyrolysis of lignin has been developed [9,10]. Zeolites are investigated widely to catalyze pyrolysis reactions, and they are thought to play two roles: the acid sites catalyze the depolymerization of lignin into desirable and more stable products. The small volume inside the pores could prevent repolymerization and coke formation reactions [10], which effectively promotes the generation of hydrocarbon-rich compounds (typically aromatic hydrocarbons) [11]. However, a considerable amount of oxygen-containing functional groups in lignin are not fully utilized. Solid bases or metal-supported catalysts under a hydrogen atmosphere are also used for the conversion of lignin, but these methods have several drawbacks such as corrosion, difficulty in catalyst recovery, sintering of metals, loss of activity, etc. [12]. In our previous work, it was found that perovskites could promote the conversion of bagasse lignin (BL) into high valued oxygen-containing aromatics. Perovskites could avoid oxygen-containing functional groups from being

Corresponding author at: College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, China. E-mail address: [email protected] (Y. Chen).

https://doi.org/10.1016/j.jaap.2019.05.013 Received 26 October 2018; Received in revised form 22 April 2019; Accepted 21 May 2019 Available online 22 May 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.

Journal of Analytical and Applied Pyrolysis 142 (2019) 104624

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excessively destroyed, and it has nice regeneration performance by controlled combustion of char after multiple regeneration [13–15]. Perovskites have been widely applied due to their tunable geometric and electronic structures, high structural flexibility, high chemical activity, high chemical stability and low cost [16,17]. The defect chemistry of perovskite materials can be controlled to an extent by doping. Introducing cations with low valence to substitute for A-site metal ions results in the formation of oxygen vacancies and a hypostoichiometric perovskite, which is important as these facilitate oxygen via the vacancy, interstitial or interstitialcy mechanisms [18]. Among the perovskite type catalysts, LaMO3 is the most active one. It has been proved that LaMO3 (M = Co, Cu, Fe, Mn and Ni) is an effective catalyst for pollutant degradation. Considering the low toxicity of Fe, LaFeO3 is chosen as the substrate catalyst for catalyzing lignin pyrolysis [19]. Perovskites whose A-site ions are substituted by Ca2+, Sr2+ and Ba2+ partially have shown satisfactory performance for catalyzing redox reactions [20–22]. Mg2+ ions can only enter the A-site partially to form perovskites with defects because the radius of Mg2+ is small (r = 0.066 nm), while the other Mg2+ ions enter the B-site or exist in the form of highly dispersed MgO [23]. The main target of this paper is to investigate the laws of substituting A-site in LaFeO3-base perovskites by Ca2+, Sr2+ and Ba2+ on catalytic pyrolysis of lignin and their regeneration performances.

ysolid =

ygas = 100%

=

cLMF cBL × 100% cBL

(4)

For BL, the contents of C, H, N and S elements were determined by a 2400II CHNS/O elemental analyzer (Perkin Elmer Co., USA), and the mass fraction of O element was obtained by difference. The morphologies of the samples were observed with JSM-6510LV scanning electron microscope (SEM) (JEOL Ltd., Japan), under the accelerating voltage of 10 kV for image capture. Particle distribution was characterized with MasterSizer laser particle size analyzer (Malvin Co., England). A Tensor 27 fourier transform infrared (FT-IR) spectrometer was used to record spectra with the KBr pellet technique (Bruker Optics Co., Germany). XRay diffraction (XRD) spectra were obtained by a D/Max 2200 X-ray diffractometer (Rigaku Co., Japan) with Cu Kα (λ = 0.1542) radiation operated at 40 kV and 40 mA. The XRD patterns were obtained with a step size of 0.1° and residence time of 4 s, at each step in 10–80 º angle range (2θ). The XRD characterization results were treated by MDI Jade5.0. The specific surface area of the samples was determined by applying the Brunner-Emmet-Teller (BET) method to nitrogen adsorption isotherms recorded at −196 °C, using an ASAP2400 instrument (Micromeritics Co., USA). X-ray photoelectron spectroscopy (XPS) characterization was performed by a PHI 1600 spectrometer (Perkin Elmer Co., USA) with a Mg Kα X-ray radiation operated at 15 kV. Thermogravimetric analyses (TG) were performed with a Diamond TG analyzer (Perkin Elmer Co. USA), from 90 °C to 1000 °C at a heating rate of 10 °C min−1 and under N2 flow rate of 100 mL min−1. The gaseous product was injected to be detected by GC-2014 (Shimadzu Co., Japan) equipped with a TCD detector. The liquid product was diluted 20 times volume with anhydrous ethanol. Then the liquid mixture was characterized via 7890-7000 GC/MS (Agilent Technologies Ltd. Co., USA) after drying by anhydrous magnesium sulfate to detect the composition. The injector temperature was kept at 280 °C; the chromatographic separation was performed with a DB-5 ms UI capillary column; the oven temperature was programmed from 40 °C (3 min) to 300 °C (3 min) with a heating rate of 5 °C min−1; the mass spectra were operated in electron ionization (EI) mode. The mass spectra were obtained from m/z 40 to 550. The selectivity of the compound was determined according to the database of NIST library.

2.2. Catalytic performance and catalyst regeneration The perovskite was well mixed with BL (Guangxi Institute of Botany, Chinese Academy of Sciences) with a mass ratio of 1:3. The mixture was pelletized and then crushed into particles of 150–160 μm. Then, the particles were loaded into an alumina tube whose inner diameter is 10 mm and the length is 0.8 m with a quartz wool plug in the middle section for supporting the samples in a fixed bed micro reactor. The reaction temperature was programmed from room temperature to 600 °C at a rate of 10 °C min−1 and kept for 2 h in nitrogen atmosphere at a flow rate of 100 mL·min−1. The temperature was measured with K typed thermocouple located inside the reaction bed. During reaction, nitrogen carried the pyrolysis products out, the gaseous and liquid products were separated after cooling down, and then collected by gas sampling bag of 20 L and liquid accumulator respectively for subsequent characterizations. The products were collected since heating up. After the reaction, in order to regenerate catalysts, the residual solid which was a mixture of spent catalyst and the solid product obtained from BL pyrolysis, was ground into powder, and then calcined in a muffle furnace in air atmosphere at 800 °C for 6 h. The yield of each phase product was calculated by following formulas.

× 100%

(3)

2.3. Characterization

To prepare La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) with single phase by the sol-gel method (SG), stoichiometric amounts of citric acid (CA), ethylene diamine tetraacetic acid (EDTA), and NH3·H2O were mixed to form a transparent solution. Calculated amounts of metal nitrates were subsequently added to the CA-EDTA-NH3·H2O solution under the constant stirring at 50 °C. The mole ratio of CA: EDTA: total metal ions was 1.6:1.0:1.0, and the pH value was kept at 8.0-8.5. The mixture was then heated to 80 °C and stirred for 3 h to form a gel. The gel was then dried at 120 °C for 12 h followed by being calcined at 800 °C for 6 h to obtain the La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) perovskite samples.

mliq

ysolid

Where δ, cLMF, cBL presented the selectivity change percentage, the selectivity of certain component obtained by the catalytic pyrolysis of BL under the action of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) and that obtained by the pyrolysis of pure BL, respectively.

2.1. Catalysts preparation

mBL

yliq

(2)

Where ygas, yliq and ysolid presented the yield of gaseous, liquid and solid product, mliq, mBL, mLMF and mtotal were the mass of liquid product, BL, La0.8M0.2FeO3 (M = La, Ca, Sr, Ba), and the final mass of residual solid, respectively. The selectivity change of a certain component after adding La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) was calculated by following formula.

2. Experimental methods

yliq =

m total mLMF × 100% mBL

3. Results and discussion 3.1. Lignin characterization The molecular structure of BL is quite complex, hence various characterization techniques were applied to analyze its morphological and chemical composition. As shown in Fig. 1(a), BL granule presented irregular ellipsoidal with the size of approximately 15–20 μm. This result was consistent with that obtained by laser particle size analysis as

(1) 2

Journal of Analytical and Applied Pyrolysis 142 (2019) 104624

H. Wang, et al.

Fig. 1. (a) SEM images and (b) particle size distribution of BL. Table 1 Elemental composition of BL. Component

C

H

Oa

N

S

Contents (wt.%)

61.55

6.32

30.15

0.79

1.19

a

By difference.

shown in Fig. 1(b). The main constituent elements of BL are listed in Table 1, including C, O and H, which accounted for approximately 98% weight of BL. Moreover, the oxygen content was above 30%, indicating that there were considerable oxygen-containing functional groups in BL [24]. A small amount of N and S may be from protein on cell walls [25]. FT-IR of BL is shown (Fig. 2) and the functional groups corresponding to vibration of different frequencies are also marked [26]. The absorption at 2933 cm−1 was originated from the C–H stretch in methyl, methylene and methyne groups. The absorption at 1383 cm−1 corresponded to aliphatic C–H stretching and CH3O- stretching. The absorption peak at 1690 cm−1 ascribed to C]O stretch in unconjugated ketone, carbonyl and ester groups. The absorption at 1632 cm-1 was originated from C]O stretch in conjugated carbonyl. OeH stretching was reflected by the band of 3406 cm−1. The absorptions at 1598 cm−1, 1508 cm−1 and 1331 cm−1 corresponded to aromatic skeleton vibrations. The bands in the region of 1266 cm−1 and 1124 cm−1 showed the existence of G and S rings, respectively. Combining the characterization results of elemental analysis and FT-IR, it can be concluded that there were a variety of oxygen-containing functional groups in BL molecular. Río et al. [27] already found BL had a syringyl-rich structure and the H:G:S molar composition was 2:38:60.

Fig. 3. XRD patterns of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba).

3.2. Characterization results of perovskites

XRD (Fig. 3). The diffraction patterns for La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) corresponded to cubic crystal phase and the results were identical to that reported by Sastre [20]. No stray peaks were observed, suggesting that majority of metal ions had been incorporated into the perovskite skeleton [28]. The order of crystallinity from large to small was as following: La0.8Ba0.2FeO3 > La0.8Sr0.2FeO3 > La0.8Ca0.2FeO3 > LaFeO3, it was concluded that the crystallinity could be improved by doping alkaline-earth metal ions. The cation radius from large to small is sorted: rBa > rSr > rLa > rCa. However, the cell volume (Vcell) of LaFeO3 was close to that of La0.8Ba0.2FeO3, which may be the result of lattice distortion [24,29]. The length of unit cell (a), and Vcell were calculated and summarized in Table 2. The morphology of perovskite samples was characterized with SEM (Fig. 4). As can be seen, La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) had porous flake structure, which was beneficial to the full contact with reactants.

3.2.1. Crystal phase and morphology The crystal structures of the perovskites were characterized with

3.2.2. BET characterization The specific surface area (S), pore volume (Vpore) and average pore

Fig. 2. (a) FT-IR spectra and (b) partial enlargement of FT-IR spectra of BL. 3

Journal of Analytical and Applied Pyrolysis 142 (2019) 104624

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were assigned to 2p3/2 and 2p1/2 states of Fe3+ [33,37]. For ideal stoichiometric LaFeO3, the valence of Fe is +3, however, oxygen atoms escape from LaFeO3 and oxygen vacancies are formed in calcination, and the electrons released by oxygen vacancies are captured by Fe3+ and Fe3+ is reduced to Fe2+. Indeed, the coexistence of Fe2+ and oxygen vacancies is common in ferrite perovskites [37]. The peak area ratio of Fe2+ to Fe3+ was sorted: La0.8Sr0.2FeO3 > La0.8Ca0.2FeO3 > La0.8Ba0.2FeO3 > LaFeO3. As shown in Fig. 5(c), the O1 s XPS signal can be divided into two peaks, corresponding to two kinds of oxygen [38]. The peak at low binding energy corresponds to the lattice oxygen species Olat (O2− and O−), which reflects the redox behavior of the metal [39]. The peak at high binding energy corresponds to the adsorption oxygen species Oads (O2− and O22−), whose content reflects the concentration of oxygen vacancies in the compound [40]. The contents of oxygen species are listed in Table 3. The ratio of Oads/Olat was as following: La0.8Sr0.2FeO3 > La0.8Ca0.2FeO3 > La0.8Ba0.2FeO3 > LaFeO3, which was consist with the order of the peak area ratio of Fe2+ to Fe3+. More adsorption oxygen indicates the presence of more oxygen vacancies and higher content of quasi-free electrons in the perovskite oxides [41]. By trapping electrons, adsorption oxygen leads to the formation of O2−, which is the active center for the oxidation [42], meanwhile it can facilitate the deep oxidation of organics [33]. The existence of oxygen vacancies is also beneficial to the movement of lattice oxygen.

Table 2 Parameters of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba).

Crystal type Crystallinity a/Å Vcell/ Å3 S/(m2·g−1) Vpore/(cm3·g−1) dpore/nm

LaFeO3

La0.8Ca0.2FeO3

La0.8Sr0.2FeO3

La0.8Ba0.2FeO3

Cubic 93.79% 3.923 60.40 5.24 0.0109 7.90

Cubic 95.72% 3.894 59.07 7.78 0.0238 11.33

Cubic 99.54% 3.905 59.56 12.29 0.0450 13.79

Cubic 99.83% 3.924 60.41 10.40 0.0392 14.47

size (dpore) of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) were determined with BET (Table 2). The value of S increased after lanthanum ions were substituted by alkaline-earth metal ions partially as reported by Bradha [30]. This may be because after the high-valence metal ions were partially substituted by low-valence ions, the interatomic interaction in crystal cells was weakened, as a result, the cell volume expanded and the density of the particles decreased [31]. The specific reasons still need further study. The values of Vpore and dpore also increased after the partial substitution. In our previous study, S of the perovskite prepared by SG was about 10 times larger than that of the sample prepared by the solid-state method [32]. 3.2.3. XPS characterization XPS analyses were performed to present the chemical state of La 3d, Fe 2p and O 1s for the prepared samples (Fig. 5). For La 3d spectra of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) shown in Fig. 5(a), there were two doublets peaks, namely La 3d3/2 and La 3d5/2. The results were similar with the form of the standard trivalent La 3d spectrum, indicating that the valence of La was +3 [33,34]. The binding energies of La in La0.8M0.2FeO3 (M = Ca, Sr, Ba) were lower than those in LaFeO3 due to the change of coordination environment after doping alkaline earth metal ions, suggesting higher covalence of La-O bond and higher structural stability [35]. The XPS spectra of Fe 2p exhibited two peaks at 710.0 eV and 723.7 eV, which corresponded to the peaks of Fe 2p3/2 and Fe 2p1/2 in La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) as shown in Fig. 5(b), respectively. The peak at 718.3 eV was owing to shake-up satellites [36]. Peaks can be further deconvoluted into four sub-peaks centered at 709.5 eV, 722.7 eV and 710.9 eV, 724.4 eV, the former two sub-peaks corresponded to 2p3/2 and 2p1/2 states of Fe2+, and the later two sub-peaks

3.3. Catalytic performances 3.3.1. TG analyses TG results for the pyrolysis processes are provided in Fig. 6. With increasing temperature, three weight loss regions in TG curves were observed. The whole thermal decomposition process could be distinguished: (1) The first weight loss region was principally resulted from water evaporation. The end temperatures of the first stage were basically same for the curves, indicating that the addition of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) took few effects at this stage. (2) The second weight loss was obvious due to BL decomposition. BL was depolymerized and transferred into the glass transition, then ester bonds and CeC bonds were cracked, small-molecule gases and aryl oxygencontaining compounds were generated. At this stage, the end pyrolysis temperature was sorted: BL > BL + LaFeO3 > BL + La0.8Ba0.2FeO3 > BL + La0.8Ca0.2FeO3 > BL + La0.8Sr0.2FeO3, while the mass of residual solid decreased in turn. That was to say, BL pyrolysis was promoted

Fig. 4. SEM images of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba). (a) LaFeO3 (b) La0.8Ca0.2FeO3 (c) La0.8Sr0.2FeO3 (d) La0.8Ba0.2FeO3. 4

Journal of Analytical and Applied Pyrolysis 142 (2019) 104624

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Fig. 5. XPS spectra of (a) La 3d, (b) Fe 2p and (c) O 1s. Table 3 The content of oxygen species determined by O 1s XPS. Oxygen specie

LaFeO3

La0.8Ca0.2FeO3

La0.8Sr0.2FeO3

La0.8Ba0.2FeO3

Olat/% Oads/%

52.47 47.53

59.81 40.19

64.92 35.08

54.38 45.62

Fig. 7. Yields of gaseous, liquid and solid products obtained from pyrolysis processes. (a) BL (b) BL + LaFeO3 (c) BL + La0.8Ca0.2FeO3 (d) BL + La0.8Sr0.2FeO3 (e) BL + La0.8Ba0.2FeO3. Table 5 The compositions of gaseous products.

Fig. 6. TG diagrams for pyrolysis processes. Table 4 n, E and A in the temperature range of 200–357 °C. Catalyst

n

E/(KJ mol−1)

A/min−1

R2

None LaFeO3 La0.8Ca0.2FeO3 La0.8Sr0.2FeO3 La0.8Ba0.2FeO3

2 2 2 2 2

98.11 81.43 68.60 55.26 71.78

1.58 × 107 1.37 × 105 3.99 × 104 2.96 × 104 8.87 × 104

0.990 0.996 0.996 0.991 0.992

Sample

CO2/(wt.%)

CO/(wt.%)

CH4/(wt.%)

CnHm/(wt.%)

BL BL + LaFeO3 BL + La0.8Ca0.2FeO3 BL + La0.8Sr0.2FeO3 BL + La0.8Ba0.2FeO3

53.93 50.80 47.81 43.59 47.15

4.69 3.47 2.98 3.34 3.08

19.16 22.96 23.93 28.10 25.39

22.22 22.77 25.28 24.97 24.38

amorphous carbon [43,44]. These results were identical to those reported by Wang et al [45]. The Coats-Redfern method [46] was applied to analyze the kinetic characteristics of BL pyrolysis catalyzed by the perovskites. The reaction order (n), activity energy (E) and pre-exponential factor (A) in the temperature range of 200–357 °C are summarized (Table 4). Within this temperature range, the pyrolysis processes were conformed to the second-order kinetics law, the values of R2 were all above 0.99, and the order of E and A from large to small is: BL >

with the addition of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba). (3) The weight loss in last stage was small, suggesting that the primary pyrolysis of BL was basically completed, manifested as the gradually leveled-off TG curves. At this stage, aromatic molecules were condensed into 5

Journal of Analytical and Applied Pyrolysis 142 (2019) 104624

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Table 6 The compositions of liquid products. No.

1 2 3 4 5 6 7 8 9

Compound

phenol furfural o-xylene p-xylene ethylbenzene 3-methyl-phenol 4-methyl-phenol p-cresol 3-ethoxypropionic acid

10 11 12 13 14

dihydrobenzofuran 1-methoxy-4-methyl-benzene guaiacol 4-methyl-guaiacol 3-methoxy-1,2 -dihydroxyhenzene 15 ethyl 3-ethoxyacrylate 16 3-ethoxypropionic acid ethyl ester 17 4-vinyl-guaiacol 18 1,2,3-trimethoxy-benzene 19 4-hydroxy-3-methyl-guaiacol 20 syringol 21 3-propane acrylate 22 4-methyl-syringol 23 1-(2,3,4-trihydroxyphenyl) -ethanone 24 1-(4-hydroxy-3-methoxyphenyl) −2-propanone 25 4-vinyl-syringol 26 1,2,3-trimethoxy-5-methyl -benzene 27 4-hydroxy-3,5-dimethoxy -benzaldehyde 28 syringaldehyde 29 3,4,5-trimethoxyphenol 30 2,6-dimethoxy-4 -(2-propenyl)-phenol 31 4-acetyl-syringol 32 1-(4-hydroxy-3,5 -dimethoxyphenyl)-1-propanone 33 4-hydroxy-3,5-dimethoxy -benzoic acid hydrazide Total selectivity

Total selectivity of aryl oxygen-containing compounds Not detected Total

RT /min

Type

5.4 5.9 6.6 6.9 7.4 8.0 8.3 8.6 9.2 9.7 10.3 11.1 12.0 12.4

phenolic furan benzene benzene benzene phenolic phenolic phenolic carboxylic acid furan phenylate guaiacol guaiacol phenolic

13.1 14.0 14.8 15.9 17.2 18.9 19.6 20.8 21.2

Selectivity of liquid product/(wt.%) a

b

c

d

e

5.58 3.05 10.87 9.30 2.77 0.00 3.75 4.98 3.73

13.43 3.57 10.10 2.26 2.19 2.01 6.60 0.00 1.09

7.82 4.02 7.60 1.66 1.25 0.00 3.93 1.84 1.71

10.76 2.88 3.01 2.30 1.42 0.00 7.31 2.23 2.47

10.51 3.23 3.38 2.66 1.19 2.36 5.15 2.99 1.53

0.00 0.00 4.87 3.14 0.00

0.00 4.51 5.42 3.17 2.55

1.66 5.51 3.06 3.06 2.51

2.45 3.53 2.88 5.15 0.00

1.91 3.86 2.76 5.62 0.00

ester ester guaiacol syringol guaiacol syringol ester syringol phenolic

4.19 0.00 5.08 2.25 3.14 6.75 0.00 2.32 2.43

0.00 0.00 8.53 3.09 0.00 9.15 3.01 0.00 0.00

0.00 0.00 11.95 3.69 0.00 10.80 3.49 0.00 0.00

2.97 2.18 13.04 2.24 0.00 12.07 1.73 0.00 0.00

4.34 0.00 9.97 2.55 0.00 11.39 2.11 0.00 0.00

22.1

guaiacol

0.00

1.27

1.89

2.72

2.84

22.5 23.0

syringol phenylate

1.98 3.80

2.77 0.00

4.26 0.00

4.42 0.00

3.51 0.00

23.6

syringol

0.00

2.80

3.95

1.86

1.39

24.1 25.2 26.6

syringol phenylate syringol

0.00 0.00 0.00

1.44 2.05 1.17

3.08 2.90 0.00

1.59 1.74 0.00

3.63 2.38 0.00

27.3 29.4

syringol syringol

2.51 1.68

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

31.5

syringol

2.20

0.00

0.00

0.00

0.00

16.74 16.23 19.69 3.80 22.94 4.19 3.05 3.73 56.46 9.63 100

24.59 18.39 20.42 6.56 14.55 3.01 3.57 1.09 69.96 7.82 100

16.10 19.96 25.78 8.41 10.51 3.49 5.68 1.71 70.25 8.36 100

20.30 23.79 22.18 5.27 6.73 6.88 5.33 2.47 71.54 7.05 100

21.01 21.19 22.47 6.24 7.23 6.45 5.14 1.53 70.91 8.74 100

phenolics guaiacols syringols phenylates benzenes esters furans carboxylic acid

(a) BL (b) BL + LaFeO3 (c) BL + La0.8Ca0.2FeO3 (d) BL + La0.8Sr0.2FeO3 (e) BL + La0.8Ba0.2FeO3.

BL + LaFeO3 > BL + La0.8Ba0.2FeO3 > BL + La0.8Ca0.2FeO3 > BL + La0.8Sr0.2FeO3, the results were consistent with the results of TG. It indicated that the catalytic activity increased after doping alkaline earth metal ions into LaFeO3.

perovskites, the yields of liquid products increased by 24.1–56.3%, and the yields of gaseous and solid products decreased by 4.2–7.7% and 4.7–15.1%, respectively, and the maximum yield of liquid product, 25.73 wt.% was obtained under the action of La0.8Sr0.2FeO3.

3.3.2. Yields of products The yields of gaseous, liquid and solid products obtained from pyrolysis processes are shown (Fig. 7). There are different aryl oxygencontaining compounds in liquid products, which could be applied as raw materials or important intermediates in fine chemical industries, such as pesticide, medicine, printing and dyeing, spices, etc. [47,48]. Therefore, the yield of liquid product is an important indicator to measure the catalytic performance of perovskites. It was found that compared with the pyrolysis of pure BL, after adding different

3.3.3. Products composition The compositions of gaseous products were CO2, CO, CH4, CnHm (n = 2–4, m = 2n+2 or m = 2n) as shown in Table 5. The release of CO2 and CO was mainly derived from the cracking of thermolabile carboxyl, carbonyl and ether groups in the phenylpropane side chains. CH4 and CnHm (n = 2–4, m = 2n+2 or m = 2n) were mainly generated from the fragmentation of alkyl side chains on aromatic rings [49]. With the addition of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba), the selectivity of CO2 and CO decreased by 5.8–19.2% and 26.0–36.5% respectively, 6

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Fig. 8. (a) XRD patterns of fresh, spent and regenerated La0.8Sr0.2FeO3 (5 cycles); SEM images of (b) spent and (c) regenerated La0.8Sr0.2FeO3 (5 cycles).

Fig. 9. (a) Fe 2p and (b) O 1s XPS of fresh and regenerated La0.8Sr0.2FeO3 (5 cycles).

while the selectivity of CH4 and CnHm (n = 2–4, m = 2n+2 or m = 2n) increased by 19.8–46.7% and 2.5–13.8 % respectively. The results indicated that chemical bonds in BL tended to crack selectively during pyrolysis catalyzed by perovskites. The compositions of liquid products are summarized in Table 6. The major aryl oxygen-containing compounds were phenolics, guaiacols, syringols and phenylates, the rest was benzenes, furans, esters and carboxylic acid. The compositions of liquid products were obvious different due to the crack of various bonds in the relative complex structure of BL during pyrolysis accompanied by complex reactions such as depolymerization, condensation and esterification, etc. [50]. With the addition of La0.8M0.2FeO3 (M = La, Ca, Sr, Ba), the total selectivity of aryl oxygencontaining compounds increased, while the selectivity of benzenes dramatically decreased. The cleavage of typical internal structure unit linkages under relatively low temperature produced guaiacol-type (G) and syringol-type (S) compounds. The cleavage of methoxyl groups on the aromatic rings through demethoxylation, demethylation and dehydroxylation reactions produced the notable increase of phenol-type (H) compounds and aromatic hydrocarbons, particularly under elevated temperature [51]. After adding perovskites, the selectivity of phenylates/

esters in liquid products increased while that of carboxylic acid slightly decreased, which was attributed to the reaction of carboxylic acid with the compounds containing hydroxyl groups, namely esterification. The selectivity of furans increased slightly with the addition of perovskites. Furfural was the pyrolytic product of pentosane, which was not completely separated out when extracting BL. The C–H bonds in allyl groups which connected to the methylene-quinone structures in BL were relatively weak during pyrolysis. After H· was dissociated from C–H bonds, allyl radicals were formed and then cyclized to form dihydrobenzofuran [52]. The total selectivities of aryl oxygen-compounds in liquid products were more than 69 wt.% with La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) as catalysts, while it was about 56 wt.% in the liquid product obtained by pure BL pyrolysis. The dissociation energies of certain bonds in BL are very large, thus these bonds are hard to be broken, while they are prone to cyclize or repolymerize to form the solid product, char [53]. Char covers on the surface of the catalysts and makes them deactivate. The pyrolysis of BL experiences extremely complex intertwined network of elementary and consecutive reactions, its detailed mechanism and kinetics have not yet been fully known, thus further study would be required [45]. 7

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after partial substitution of La3+ by alkaline earth metal ions. The positive charge defects could be balanced either by the formation of oxygen vacancies or by a higher oxidation state ion at the B-site. However, in practice, the charge compensation mechanism in LaFeO3based perovskites is mainly associated with the generation of oxygen vacancies [37,54], at the same time, there are a small amount of Fe2+ ions in the perovskites [55]. Oxygen vacancies constitute oxygen ions transport channels locally in the crystal, so that oxygen ions could be transported easily [56,57]. The cubic perovskite has relatively wide internal channels and the maximum number of equivalent positions of oxygen, which are conducive to the migration of oxygen ions. When the concentration of oxygen vacancies in perovskite is too high, the oxygen vacancies become ordered, as a result, the migration energy of oxygen ions increases. Therefore, the doping amount must be controlled within a certain range [58]. In this experiment, La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) samples were all cubic, however there were more oxygen vacancies in La0.8M0.2FeO3 (M = Ca, Sr, Ba) due to doping alkli-earth metal ions, which could adsorb more oxygen in the air, hence they showed better catalytic performance than LaFeO3. Perovskite oxides could release oxygen, thus making gaseous or solid fuel convert even under anaerobic environment, and the oxygen could be supplemented during redox processes [24]. With increasing temperature, Oads was desorbed firstly for the pyrolysis reaction (Scheme 1, line I). Then, a small portion of surface lattice oxygen escaped to participate in the reaction (Scheme 1, line II). With the further increasing of temperature, a very small amount of internal lattice oxygen was transferred to the surface of the catalyst along the transport channels, and then reacted with BL (Scheme 1, line III) [42]. However, XRD characterization results showed that the crystal phase of the perovskite was not changed after reaction, indicating that the amount of lattice oxygen involved in the reaction was too small to cause structural variation. The catalytic activities of spent perovskites could be recovered by recombining with atmospheric oxygen. The short pore length facilitates the rapid diffusion of both bulk oxygen and lattice oxygen [59].

Scheme 1. Scheme for the catalytic pyrolysis of BL over perovskites.

3.4. Regeneration performances Based on above results, it suggested that La0.8Sr0.2FeO3 showed satisfactory catalytic activity for the pyrolysis of BL, therefore, its regeneration performance was investigated. XRD patterns of fresh, spent (5 cycles) and regenerated (5 cycles) La0.8Sr0.2FeO3 are shown in Fig. 8(a). The diffraction peaks were closely inspected, which revealed that the crystal phases of the three samples were same. The diffraction peak intensity of spent La0.8Sr0.2FeO3 was lower than that of two other samples because irregular lumps of char deposited on the surface of the catalyst, which could be observed in Fig. 8(b). The char could impede mass and heat transfer. After being regenerated by the control combustion of char under air atmosphere, the intensity of regenerated La0.8Sr0.2FeO3 was recovered to be the same as the fresh one, displayed as the porous morphology as shown in Fig. 8(c), and the catalytic performance was also recovered. The S, Vpore and dpore of regenerated La0.8Sr0.2FeO3 (5 cycles) determined by BET were 12.02 m2 g−1, 0.0437 cm3 g−1 and 13.36 nm, respectively, and the values were nearly unchanged compared with those of the fresh one (the values were 12.29 m2 g−1, 0.0450 cm3 g−1, 13.79 nm, respectively). The results showed that La0.8Sr0.2FeO3 did not sinter obviously after 5 successful redox cycles, indicating its nice thermal stability. The XPS spectra of Fe 2p and O 1s for fresh and regenerated (5 cycles) La0.8Sr0.2FeO3 samples are provided (Fig. 9). The regenerated sample had larger peak area ratio of Fe2+ to Fe3+ than that of the fresh one, and the content of Oads in regenerated La0.8Sr0.2FeO3 (67.14%) was a little higher than that in the fresh one (64.92%). It was resulted from that during the catalytic processes, quite a small amount of surface lattice oxygen in perovskite escaped and then participated in the reaction, forming oxygen vacancies in the lattice. Then, more O2 in the air adsorbed on the surface of perovskite [40]. Fortunately, it didn’t affect the structural and catalytic stability of La0.8Sr0.2FeO3.

4. Conclusions La0.8M0.2FeO3 (M = La, Ca, Sr, Ba) samples were prepared by SG to be used as catalysts for catalyzing BL pyrolysis. The perovskites had cubic phase and porous flake structure. The perovskites promoted the pyrolysis of BL and reduced activation energies, and it showed better effects after doping alkaline earth metal ions especially Sr2+, benefitting from more oxygen vacancies were formed and more oxygen was adsorbed. The pyrolysis processes were conformed to the second-order kinetics law. During pyrolysis, the main gaseous products were CO2, CO, CH4 and CnHm (n = 2–4, m = 2n+2 or m = 2n), the main liquid products were aryl oxygen-containing compounds, mainly including phenolics, guaiacols, syringols and phenylates. The addition of perovskites facilitated the cleavage of chemical bonds in BL selectively, as a result, more liquid products were formed, the contents of hydrocarbons in gaseous products and aryl oxygen-containing compounds in liquid products increased, but the formations of CO2, CO and benzenes were inhibited. The perovskite showed nice structural and catalytic stability after 5 cycles of redox. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51674089), Heilongjiang Provincial Science Fund for Distinguished Youth Scholar (No. JC2018002), Postdoctoral Scientific Research Development Fund of Heilongjiang Province (No. LBH-Q16037), Postgraduate Innovative Research Project of Northeast Petroleum University (No. YJSCX2017-014NEPU).

3.5. The mechanism of catalytic BL pyrolysis The electron-neutrality of LaFeO3-based perovskites is modified 8

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