Influence of calcination temperature for LaTi0.2Fe0.8O3 on catalytic pyrolysis of bagasse lignin

Journal of Rare Earths 37 (2019) 837e844

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Influence of calcination temperature for LaTi0.2Fe0.8O3 on catalytic pyrolysis of bagasse lignin* Haiying Wang a, b, Hongjing Han a, b, Yanan Zhang a, b, Jinxin Li a, b, Yanguang Chen a, b, *, Hua Song a, b, Enhao Sun c, Hongzhi Zhao a, b, Mei Zhang a, b, Dandan Yuan a, b a b c

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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 August 2018 Received in revised form 22 October 2018 Accepted 23 October 2018 Available online 8 March 2019

LaTi0.2Fe0.8O3 (LTF) was prepared by the sol-gel method, and the effects of calcination temperature on the structure and properties were investigated. A new method of preparing aryl oxygen-containing compounds from bagasse lignin (BL) by the catalytic pyrolysis over LTF was proposed. The results show that LTF has cubic crystal phase and porous structure and its optimal calcination temperature is 800
C (LTF800). In the test for catalytic pyrolysis of BL, with the addition of LTF-800, the yield of liquid product reaches the maximum; the contents of phenolics, guaiacols, syringols, phenylates and furans increase obviously, while those of benzenes, esters and carboxylic acid decrease. The total content of aryl oxygencompounds (including phenolics, guaiacols, syringols and phenylates) in liquid product is more than 74 wt% with the addition of LTF-800, larger than that obtained by single BL pyrolysis (62 wt%). LTF could avoid oxygen-containing functional groups from being excessively destroyed. It has nice regeneration performance by controlled combustion of char even after 5 cycles. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Keywords: Bagasse lignin Perovskite Catalytic pyrolysis Calcination temperature Aryl oxygen-containing compounds Rare earths

1. Introduction Sugarcane bagasse is the fiber that remains after the sugars have been extracted with approximately 32 wt%e34 wt% cellulose, 19 wt%e24 wt% hemicellulose, 25 wt%e32 wt% lignin, 6 wt%e12 wt% extractives and 2 wt%e6 wt% ash.1,2 Amongst these, cellulose and hemicellulose have been widely used in many aspects, while despite unique characteristics of bagasse lignin (BL), it is mostly used for low-value commercial applications3 due to its complex and stable structure. Lignin is a kind of natural amorphous, threedimensional biopolymer,4 obtained from polymerization of three cinnamyl alcohols: p-hydroxy cinnamyl alcohol, coniferyl alcohol, and sinapyl alcohol, and each of these monomers gives rise to

* Foundation item: Project supported by the National Natural Science Foundation of China (51674089), Heilongjiang Provincial Science Fund for Distinguished Youth Scholar (JC2018002), Postdoctoral Scientific Research Development Fund of Heilongjiang Province (LBH-Q16037) and Postgraduate Innovative Research Projects of Northeast Petroleum University (YJSCX2017-014NEPU). * Corresponding author. College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China. E-mail address: [email protected] (Y.G. Chen).

distinct aromatic nuclei in the lignin structure: p-hydroxyphenyl (H), guaiacyl (G), and syringyl nuclei (S), respectively.5,6 These structural units are connected by ether (beOe4, 4eOe5, aeOe4, etc.) and CeC bonds (5e5, be5, be1, beb, etc.).6 Lignin is regarded as the sole renewable resource for the production of aromatics for the relatively low cost, great abundance and low environmental impact.3 Production methods of high value-added products from lignin are investigated to achieve full use.7 Lignin pyrolysis is the treatment that lignin is heated to certain temperature range to obtain low molecule products. It has attracted extensive attention for its advantages such as low cost, easy separation for the desired products, etc., and it is an important way to realize high-valued and resource utilization of lignin.8 Peng et al.9 found that carbonate additives facilitated the production of methoxy-phenols, and more alkyl-phenols were produced with the hydroxide additives during the pyrolysis process of lignin. The addition of metal chlorides plays an active role for increasing the contents of phenolic compounds from the pyrolysis of lignin.10 Maldhure et al.11 found that the addition of ZnCl2 gave more liquid products under 300e400
C. To reveal the mechanism of lignin pyrolysis, different model compounds12e15 and multiple characterization methods16,17 have been used. Whether lignin or

https://doi.org/10.1016/j.jre.2018.10.020 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

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H.Y. Wang et al. / Journal of Rare Earths 37 (2019) 837e844

model compounds, catalysts play important roles in converting raw materials efficiently and selectively, therefore, it is desirable to explore recyclable catalyst with high stability to produce high value-added chemicals from energy security, economics and environmental standpoints. The perovskite oxides have the general formula ABO3, in which A represents a large electropositive cation such as alkaline earth or rare earth metalions (r  0.090 nm) and B is a small transition metalion.18 Substituting or doping at A or/and B sites would regulate the composition and symmetry of the oxides, create oxygen vacancies and enhance many performances. Perovskite-based materials have been intensively explored due to their fascinating physico-chemical properties, high electrochemical stability, costeffectiveness, and environmental friendliness,19,20 and they are proposed as the promising catalysts in many fields such as partial oxidation of methane to syngas,21 VOC oxidation,22 NOx reduction,23 organics synthesis24 and hydrogen production,25 etc. Some investigations prove that perovskite-typed oxides could release enough oxygen in anoxic condition to catalyze the combustion or conversion of gaseous or solid fuels.26 Deng et al.27 investigated the influence of LaCo1exCuxO3 in the catalytic wet aerobic oxidation (CWAO) of lignin, and the results demonstrated that the generation of anion vacancies increased the amount of adsorbed oxygen surface active site [Co(surf)3þO2] species with increasing Cu content.28 Due to the nontoxic29 and low cost of iron, the perovskites containing iron in the B site are attractive. Perovskites could be synthesized by various methods, such as solid-state reaction,30 solgel,31 hydrothermal crystallization,32 co-precipitation,33 solution combustion,34 microemulsion35 and reverse microemulsion36. Among them, the sol-gel method (SG) is widely used for its good chemical homogeneity and low reaction temperature. Calcination temperature is a critical factor for the formation of perovskite phase. The aim of this work is to study the effect of calcination temperature on both the crystal phase of LTF perovskite by SG and their catalytic activities for the production of aryl oxygencontaining compounds in the pyrolysis process of BL. 2. Experimental 2.1. Catalysts preparation To prepare perovskite samples with single phase by SG, stoichiometric amounts of citric acid (CA), ethylenediaminetetraacetic acid (EDTA), and NH3$H2O were mixed to form a transparent solution. Calculated amounts of La(NO3)3$6H2O and Fe(NO3)3$9H2O were subsequently added to the CA-EDTA-NH3$H2O solution under the constant stirring at 50
C to form solution “A”. The mole ratio of CA:EDTA:total metal ions was 1.6:1.0:1.0, and the pH value was kept at 8.0e8.5. To introduce the titanium precursor, a stabilized titanium solution, consisting of calculated amounts of tetrabutyl titanate, lactic acid, acetic acid and ethanol (the mole ratio was 1:1:1:2), was introduced into solution “A”. 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 calcination at different temperatures for 6 h in air to obtain the perovskite samples. For convenience, LTF calcined at t temperature was abbreviated to LTFt. La2O3, Fe2O3 and TiO2 were prepared by the same method as well.

the samples in a fixed bed micro-reactor. The reaction temperature was measured with K typed thermocouple located inside the reaction bed and kept at 600
C for 2 h. Nitrogen was used as the carrier gas. After the reaction, the residual solid was calcined in a Muffle furnace in the presence of air under 800
C for 6 h, and then the used catalyst was regenerated. 2.3. Characterization The decomposition process of perovskite precursor was carried out in a simultaneous TG-DSC (NETZSCH STA 409 PC/PG). To mitigate the difference between heat and mass transfers, the sample mass was kept at 10 mg. The samples were heated from 50 up to 1000
C at a constant heating rate of 10
C/min, using an ultra-dry nitrogen atmosphere and a flow rate of 100 mL/min. X-ray diffraction (XRD) spectra were obtained using a Rigaku D/Max 2200 X-ray diffractometer (Rigaku Co., Japan) with Cu Ka (l ¼ 0.1542) radiation operating at 40 kV and 40 mA. An approach with a step size of 0.1
and residence time of 4 s at each step in 10ºe80
angle range (2q) was used to generate the XRD patterns. A JEOL JSM6510LV scanning electron microscope (SEM) (JEOL Ltd., Japan) was used to observe the morphologies of the samples. An accelerating voltage of 10 kV was used for image capture. 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 a Micromeritics ASAP2400 instrument (Micromeritics Co., USA). Thermogravimetric analysis (TG) and derivative thermogravimetric analysis (DTG) of the pyrolysis process were performed on a Perkin Elmer Diamond TG/DTA analyzer (Perkin Elmer Co. USA), from 90 to 1000
C at a heating rate of 10
C/min and under N2 flow of 100 mL/min. The gas released during the experiment process was detected by GC-2014 (Shimadzu Co., Japan) equipped with a TCD detector. The composition of liquid products was detected by an Agilent 7890-7000 GC/ MS (Agilent Technologies Ltd. Co., USA). The injector temperature was kept at 280
C; the chromatographic separation was performed with a DB-5ms UI capillary column; the oven temperature was programmed from 40
C (3 min) to 300
C (3 min) with a 5
C/min heating rate; the mass spectra were operated in electron ionization (EI) mode. The mass spectra were obtained from m/z 40 to 550. The yield of the compounds can be determined according to the database of NIST library. X-ray photoelectron spectroscopy (XPS) characterization was performed on a PHI 1600 spectrometer (Perkin Elmer Ltd. Co., USA) with a Mg Ka X-ray radiation operated at 15 kV. 2.4. Yield of three phase products The yield of each phase product was calculated by the following formulas.

yliq ¼

mliq  100% mBL

ysolid ¼

mtotal  mcat  100% mBL

(1)

(2)

2.2. Catalytic properties and catalyst regeneration

ygas ¼ 100%  yliq  ysolid

The LTF perovskite was mixed with BL (Guangxi Institute of Botany, Chinese Academy of Sciences) uniformly and the mass ratio was 1:3. The mixture was pelletized and crushed into particles with the particle size of 150e160 mm and then loaded into an alumina tube with a quartz wool plug in the middle section for supporting

where ygas, yliq and ysolid represent the yield of gaseous, liquid and solid phase; mliq, mBL, mcat and mtotal are the mass of liquid product, BL, catalyst and the final mass of residual solid, respectively. The mass fraction change of a certain component after adding LTF was calculated by the following formula.

(3)

H.Y. Wang et al. / Journal of Rare Earths 37 (2019) 837e844

d¼

cLTF  cBL  100% cBL

839

(4)

where d, cLTF and cBL represent the mass fraction change, the mass fraction of certain component obtained by the catalytic pyrolysis of BL under the action of LTF and that obtained by the pyrolysis of single BL, respectively. 3. Results and discussion 3.1. Characterization results of perovskites A typical thermal profile of the decomposition of LTF precursor obtained by simultaneous TG-DSC is shown in Fig. 1. Five regions of mass loss can be identified in the TG plot during thermal breakdown. Between 50 and 170
C, the weightlessness can be attributed to the removal of moisture and ammonia physically adsorbed with the corresponding endothermic peak in the DSC curve, and the mass loss is approximately 3.4 wt%. A marked mass loss (approximately 40.3 wt%) from 170 to 240
C can be identified due to the cleavage of bonds and the removal of water and ammonia in xerogel. Then there is a gentle and prolonged mass loss of around 26.3 wt% between 240 and 440
C, followed by a significant mass loss of about 23.6 wt% at 440e540
C, which may be attributed to the decomposition of components in xerogel. There is almost no mass loss in the TG spectrum above 540
C, which may be because the decomposition of materials has completed roughly. According to the DSC plot, a marked exothermic characteristic can be observed at 240e540
C. In this stage, the thermal decomposition of free CA and NOe 3 needs to absorb heat, while the auto-combustion of xerogel can release heat, and the heat released is more than the heat absorbed. While there is a marked endothermic characteristics between 440 and 690
C, indicating that there is phase transformation in this stage.37,38 Therefore, the calcination temperature is crucial to the formation of perovskite phase. The XRD patterns are shown in Fig. 2. From Fig. 2(a), there is a distinct difference between LTF and Fe2O3/La2O3/TiO2, which are all calcinated at 800
C. The peaks of LTF-800 at 2q angles of 22.5
, 32.1
, 39.7
, 46.1
, 52.0
, 57.3
, 67.4
and 76.6
correspond to cubic LaFeO3 perovskite-type structure (JCPDS card 75-0541) quite well without impurity peaks. No reflection of lanthanum, iron and titanium oxides is observed by XRD, suggesting that majority of lanthanum, iron and titanium ions have been incorporated into the perovskite structure.39 As can be observed from Fig. 2(b), the

Fig. 2. XRD patterns for Fe2O3, La2O3, TiO2 and LTF which are all calcinated at 800
C and the standard card of LaFeO3 (a), and LTF calcinated under different temperatures (b).

perovskites are strongly influenced by calcination temperature. There is no perovskite phase formed when the precursor is calcinated below 450
C. With increasing calcination temperature, the intensity of characteristic peaks increases gradually. The crystallinity was calculated by MDI Jade 5.0 and is summarized in Table 1. It increases with temperature increasing and it almost no longer increases above 800
C. For the sol-gel method, chemical reagents are mixed homogeneously in molecule or atom scale during the solation and gelation processes, a three-dimensional reticular structure is formed by hydrolysis and polycondensation. Then the gel shrinks, cracks and collapses after drying to obtain the xerogel precursor. During the calcinating process, the increase of calcination temperature is beneficial to the continuous crystallization on the surface of crystal particles, the crystal particle size increases continuously which is in accordance with XRD characterization results.37,40 Table 1 Parameters of LTF determined by XRD and BET.

Fig. 1. TG-DSC of LTF precursor.

T (oC)

Crystallinity

S (m2/g)

V (cm3/g)

d (nm)

450 600 800 900 1000

e 84.68 96.32 96.70 96.85

13.74 24.28 12.95 9.96 7.23

0.0384 0.0790 0.0543 0.0464 0.0355

11.2 13.0 16.3 18.6 19.6

840

H.Y. Wang et al. / Journal of Rare Earths 37 (2019) 837e844

The SEM images of LTF samples calcinated under different temperatures are shown in Fig. 3. The samples calcinated at 450 and 600
C are composed of particles with small size, which may be the result of immature development of crystal particles. The suitable temperature leads to large particle size with rough and porous surfaces which is beneficial to maintaining high specific surface area of LTF. While excessive temperature results in particle aggregation. The BET characterizations, including the specific surface area (S), pore volume (V) and pore diameter (d) are summarized in Table 1. It reveals that calcination temperature has an important effect on specific surface area and pore structure. Sorting S and V of LTF calcined under different temperatures from large to small is LTF600 > LTF-800 > LTF-900 > LTF-1000. On the contrary, d reverses. It is the results of the significant merging and the pore structure collapses or disappears during annealing at high temperature. Fig. 4. Distribution of gaseous, liquid and solid products from BL pyrolysis catalyzed by LTF calcined under different temperatures.

3.2. Catalytic performances of LTF 3.2.1. Products yield and composition It can be found from Fig. 4 that with increasing the calcination temperature from 450 to 1000
C, the yield of gaseous products decreases initially then increases. The yield rules of liquid and solid products reverse, and yliq reaches the maximum value, 22.17% in the presence of LTF-800, which has increased by 34.69% compared to that in the absence of perovskites, and the yields of gaseous and solid products are 42.20 wt% and 35.63 wt%, which have decreased by 1.61% and 12.35%, respectively. The main components of gaseous products are CO2, CO, CH4, CnHm (n ¼ 2e4, m ¼ 2nþ2 or m ¼ 2n) as shown in Table 2. The release of CO2 and CO below 400
C is mainly a result of the cracking and reforming of thermolabile carboxyl, carbonyl and ether groups in phenylpropane side chains. In addition, the formation of CO at high temperature range (450e600
C) is probably caused by the breaking of diaryl ether groups and secondary pyrolysis of volatiles. Demethylation of the methoxyl groups and the profound rupture of aromatic rings can also contribute to the release of CO2. CnHm is mainly generated from the fragmentation of alkyl side chains on aromatic rings.41 With the addition of LTF-800, the contents of CO2 and CO decrease by 8.01% and 30.70%, respectively. While the content of CH4 increases by 30.43%, that of CnHm (n ¼ 2e4, m ¼ 2nþ2 or m ¼ 2n) is almost unchanged, which indicates that LTF-800 could inhibit decarboxylation, decarbonylation

Table 2 The components and contents of gaseous products (wt%).

BL BLþLTF-800

CO2

CO

CH4

CnHm

53.93 49.61

4.69 3.25

19.16 24.99

22.22 22.15

and avoid oxygen-containing functional groups from being excessively destroyed, and it is conducive to the fracture of aliphatic hydrocarbon side chains on aromatic rings. The components and contents of compounds in liquid pyrolysis products catalyzed by different catalyzers are summarized (Fig. 5 and Table 3). The main aryl oxygen-containing compounds in liquid products are phenolics, guaiacols, syringols and phenylates, the rest are benzenes, esters, furans and carboxylic acid. After adding LTF-800, the contents of phenolics, guaiacols, syringols, phenylates and furans increase by 9.34%, 18.36%, 25.38%, 23.81% and 62.43%, respectively, while the contents of benzenes, esters and carboxylic acid decrease by 43.06%, 44.83% and 16.22%, respectively. Hence, we can surmise that the addition of LTF-800 is conducive to the selective fracture of chemical bonds in BL, and more oxygen-containing functional groups go into the liquid product, which is consistent with the change of gaseous product. BL has a syringyl-rich structure, and it is primarily b-O-4 alkyl-aryl

Fig. 3. SEM images of LTF calcinated under different temperatures. (a) 450
C; (b) 600
C; (c) 800
C; (d) 900
C; (e)1000
C.

H.Y. Wang et al. / Journal of Rare Earths 37 (2019) 837e844

841

groups in BL, so the pyrolysis reaction is often accompanied by complex reactions such as depolymerization, condensation and esterification. The total content of aryl oxygen-compounds in liquid product is more than 74 wt% with LTF-800 as catalyst, while it is about 62 wt% in liquid product obtained by single BL pyrolysis. The solid product is char, its coverage on the surface of LTF-800 causes the deactivation of the catalyst.

Fig. 5. GC diagrams of liquid products.

ether substructures.42 The crack of ether bonds in BL can generate syringol-type and guaiacol-type compounds, etc., particularly under relatively low temperature.14,43 Then the cleavage of methoxyl groups on syringol-type and guaiacol-type compounds can form phenol-type compounds and aromatic hydrocarbons, particularly under elevated temperature.44 There are many kinds of functional

3.2.2. TG-DTG analyse Fig. 6 shows the TG-DTG patterns for the pyrolysis of single BL and the pyrolysis of BL under the action of LTF-800. With increasing temperature, there are three mass loss regions observed for single BL pyrolysis in TG curve. According to the mass loss calculated in each region, the whole thermal decomposition process can be distinguished as below. (1) The first mass loss region in the temperature range of 90e200
C is caused by water evaporation, and the mass loss is approximately 2.5 wt%. (2) The second mass loss at 200e477
C is the result of BL decomposition and the mass loss is approximately 66 wt%. BL is depolymerized and translated into glass transition, then ester bonds and CeC bonds crack, and smallmolecule gases (including CO2, CO, CH4 and CnHm) and aryl oxygencontaining compounds are generated. The mass loss rate reaches the maximum at 439
C as observed from DTG curve. (3) The mass loss in the last stage which is above 477
C is very little, it suggests that the primary pyrolysis of BL has basically completed, and the

Table 3 The components and contents of compounds in liquid products analyzed by GC/MS. No.

Compound

RT (min)

Type

Content of liquid product (wt%) BL

BLþLTF-800

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Phenol Furfural o-xylene p-xylene Ethylbenzene 4-methyl-phenol p-cresol 3-ethoxypropionic acid Dihydrobenzofuran 1-methoxy-4-methyl-benzene Guaiacol 4-methyl-guaiacol Ethyl 3-ethoxyacrylate 4-vinyl-guaiacol 1,2,3-trimethoxy-benzene 4-hydroxy-3–methyl-guaiacol Syringol 3-propane acrylate 4-methyl-syringol 1-(2,3,4-trihydroxyphenyl)-ethanone 1-(4-hydroxy-3-methoxyphenyl) -2-propanone 4-vinyl-syringol 1,2,3-trimethoxy-5-methyl-benzene Syringaldehyde 3,4,5-trimethoxyphenol 4-acetyl-syringol 1-(4-hydroxy-3,5-dimethoxyphenyl) -1-propanone 4-hydroxy-3,5-dimethoxy -benzoic acid hydrazide

5.4 5.9 6.6 6.9 7.4 8.3 8.6 9.2 9.7 10.3 11.1 12.0 13.1 14.8 15.9 17.2 18.9 19.6 20.8 21.2 22.1

Phenolic Furan Benzene Benzene Benzene Phenolic Phenolic Carboxylic acid Furan Phenylate Guaiacol Guaiacol Ester Guaiacol Syringol Guaiacol Syringol Ester Syringol Phenolic Guaiacol

6.18 3.38 12.02 10.28 3.06 4.15 5.51 4.13 0 0 5.39 3.48 4.64 5.62 2.49 3.48 7.47 0 2.57 2.69 0

7.94 3.59 7.83 2.36 4.25 2.33 7.86 3.46 1.90 2.32 7.19 3.95 0 4.88 5.28 2.54 14.01 2.56 3.36 2.13 2.71

22.5 23.0 24.1 25.2 27.3 29.4

Syringol Phenylate Syringol Phenylate Syringol Syringol

2.19 4.2 0 0 2.78 1.86

0 0 2.46 2.88 0 2.21

31.5

Syringol

2.43

0

18.53 17.97 21.79 4.2 25.36 4.64 3.38 4.13

20.26 21.27 27.32 5.2 14.44 2.56 5.49 3.46

22 23 24 25 26 27 28 Total content/wt%

Phenolics Guaiacols Syringols Phenylates Benzenes Esters Furans Carboxylic acid

842

H.Y. Wang et al. / Journal of Rare Earths 37 (2019) 837e844

mass loss is approximately 6.5 wt%. In this stage, aromatic molecules are condensed into amorphous carbon.38,45 There are also three mass loss regions for the pyrolysis of BL under the action of LTF-800: 90e200
C, 200e428
C, and above 428
C, and the mass loss for each stage is 4 wt%, 70.5 wt% and 3.3 wt%, respectively. For the pyrolysis of BL under the action of LTF-800, the ending temperature of the second stage is 49
C lower than that of single BL pyrolysis, the mass loss rate reaches the maximum at 371
C, and the amount of residual solid is less. It indicates that LTF-800 could promote the pyrolysis of BL.46 3.3. Regeneration performance

Fig. 6. TG-DTG patterns.

Fig. 7. XRD patterns for used and regenerated LTF samples (5 cycles).

3.3.1. Characterization results of regenerated perovskites XRD patterns for used and regenerated (5 successive redox cycles) LTF samples are shown in Fig. 7. Through comparing Fig. 7 with Fig. 2(b), it can be concluded that the crystal phases of used and regenerated LTF samples are kept except LTF-450. The used and regenerated LTF-450 have cubic perovskite phase due to the phase transformation at high temperature during reaction and regeneration processes. The intensities of used samples are lower than those of regenerated ones because of the solid productdchar which could impede mass and heat transfer coats on the surface of LTF samples. After the char is removed by controlled combustion, the catalytic performance is restored. There are irregular lumps of char deposited on the surface of LTF-800 after the catalytic pyrolysis of BL, as can be observed in Fig. 8(a). The coverage of the initial large interstices among perovskite grains leads to the loss of catalytic activity. After being regenerated by the combustion of char under oxygen atmosphere, the porous morphology is recovered as shown in Fig. 8(b). Changes of BET results after catalytic pyrolysis and regeneration reactions are also investigated. S, V and d of the used LTF-800 are 1.37 m2/g, 0.0023 cm3/g and 6.8 nm, the values of regenerated LTF800 are 12.62 m2/g, 0.0531 cm3/g and 16.2 nm, respectively. S, V and d of the used sample are much smaller than those of the fresh and the regenerated LTF-800 because the char deposites on its surface. After regeneration, the char reacts with oxygen in air and is removed, and the surface and pore structure of LTF-800 are recovered. The metal ions in the A-site are reported to be chemically inert,47 herein the XPS spectra of O 1s and Fe 2p in fresh and regenerated (5 successive redox cycles) LTF-800 samples are analyzed as showed in Fig. 9. The O 1s spectra consist of two peaks, which correspond to two forms of oxygen, i.e., lattice oxygen Olat and adsorption oxygen Oads.44 The peak at the binding energy of 527.5e530.0 eV corresponds to the lattice oxygen species (O2, O), which reflects the redox behavior of the metal, and the peak at

Fig. 8. SEM images of used (a) and regenerated (b) LTF-800 (5 cycles).

H.Y. Wang et al. / Journal of Rare Earths 37 (2019) 837e844

843

530.0e531.5 eV corresponds to the adsorption oxygen species 2 (O 2 , O2 ), whose content reflects the concentration of oxygen vacancy in the compound. By trapping electrons, adsorption oxygen leads to the formation of O 2 , which is the active center for the oxidation.48 Besides, the oxygen vacancies of perovskite would further influence the redox processes for the interaction between LTF and BL.49. The contents of oxygen species are listed in Table 4. It is supposed that the perovskite could provide oxygen especially Oads to react with BL,50 meanwhile its crystal phase are not affected. After regeneration, the contents of oxygen species and the catalytic activity are recovered.51 The XPS spectra of Fe 2p exhibit that the doublet peaks located at about 710 and 724 eV could be assigned to 2p3/2 and 2p1/2 states of Fe,52 and the peak at 718.7 eV corresponds to the satellite peak. It represents that the valence of iron does not change after regeneration. 3.3.2. The stability evaluation of LTF-800 Five successive redox cycles were carried out to investigate the catalytic stability of LTF-800. It can be seen from Fig. 10 that with the cycle number increasing, the yields of gaseous, liquid and solid products almost unchange. The components and contents of the gaseous and liquid products obtained after multiple redox cycles are basically the same as those of products obtained by fresh LTF-800. It suggests that LTF-800 has nice catalytic stability for the conversion of BL to aryl oxygen-containing compounds. 4. Conclusions

Fig. 9. XPS of O 1s (a) and Fe 2p (b).

Table 4 The contents of oxygen species (wt%). Oxygen specie

Olat

Oads

Fresh Regenerated

43.35 42.82

56.65 57.18

The optimal calcination temperature of LTF is 800
C, LTF-800 is porous with cubic crystal phase and its S, V and d are 12.95 m2/g, 0.0543 cm3/g and 16.3 nm, respectively. When LTF-800 is used to catalyze the pyrolysis of BL, yliq reaches the maximum 22.17%, ygas and ysolid are 42.20 wt% and 35.63 wt%, respectively. The yield of liquid product increases by 34.69% and those of gaseous and solid products decrease by 1.61% and 12.35% compared with the pyrolytic products of single BL, respectively. The main components in gaseous products are CO2, CO, CH4 and CnHm (n ¼ 2e4, m ¼ 2nþ2 or m ¼ 2n); the main aryl oxygen-containing compounds in liquid products are phenolics, guaiacols, syringols and phenylates, the rest are benzenes, furans, esters and carboxylic acid. By adding LTF-800, the contents of CO2 and CO decrease by 8.01% and 30.70%, respectively, while the content of CH4 increases by 30.43%, and that of CnHm (n ¼ 2e4, m ¼ 2nþ2 or m ¼ 2n) is almost unchanged; the contents of phenolics, guaiacols, syringols, phenylates and furans increase by 9.34%, 18.36%, 25.38%, 23.81% and 62.43%, respectively, while the contents of benzenes, esters and carboxylic acid decrease by 43.06%, 44.83% and 16.22%, respectively. It indicates that LTF-800 could inhibit decarboxylation, decarbonylation and avoid oxygencontaining functional groups from being excessively destroyed, and it is conducive to the fracture of aliphatic hydrocarbon side chains on aromatic rings and dehydrogenation of the methoxyl groups. The used catalyst can be regenerated after controlled combustion of solid product-char. LTF-800 has nice regeneration performance for the catalytic conversion of BL after 5 cycles. References

Fig. 10. The catalytic stability of LTF-800 after 5 redox cycles.

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