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Chemistry Africa

, Volume 2, Issue 1, pp 39–45 | Cite as

High-efficiency toluene alkylation with tert-butyl alcohol catalyzed by Ce2O3-modified H-beta zeolites

  • Yuanyuan Wang
  • Xinglong Sun
  • Yixian Han
  • Jiaojing Zhang
  • Xueqin Wang
  • Hua SongEmail author
  • Xi Chen
Original Article
  • 161 Downloads

Abstract

A series of xCe2O3/H-beta catalysts with different Ce2O3 loadings were prepared and the catalytic performances were investigated through toluene butylation as the probe reaction. The xCe2O3/H-beta catalysts showed high para-selectivity due to their improved structural and acidic properties, which were confirmed by XRD, SEM, ICP-OES, BET, Py-IR and TG. Compared with the parent H-beta zeolite, the xCe2O3/H-beta catalysts showed slightly lower toluene conversion and significantly higher para-tert-butyl toluene (PTBT) selectivity. Loading of Ce2O3 firstly narrowed the pore size of H-beta zeolite, which benefited the shape-selectivity, and secondly increased the PTBT selectivity by suppressing the isomerization on acid sites of outer surface of the catalysts. Under comparable conditions, the 5%Ce2O3/H-beta exhibited the maximum PTBT selectivity (82.7%) at 180 °C after 4 h. The optimum reaction conditions over xCe2O3/H-beta zeolites were molar ratio of tert-butyl alcohol to toluene = 3:1, reaction temperature = 180 °C, and reaction time = 4 h.

Keywords

Alkylation Toluene tert-Butyl alcohol Ce2O3 H-beta zeolite 

1 Introduction

Aromatics alkylation is an important reaction in organic chemistry that is widely conducted to synthesize fine chemicals, petrochemicals, dyes, agrochemicals, perfumeries and pharmaceuticals [1, 2, 3, 4]. Friedel–Crafts toluene alkylation with tert–butyl alcohol (TBA) to synthesize para-tert-butyl toluene (PTBT) is a valuable reaction, since its oxidation products p-tert-butylbenzaldehyde and p-tert-butylbenzoic acid [5, 6, 7] are well applied in alkylated resins and in production of pharmaceuticals, fragrances and polymerization regulators. The conventional methods to synthesize PTBT include (1) isobutene alkylation with toluene catalyzed by H2SO4 or Al2O3 + SiO2 + Ni; (2) TBA alkylation with toluene catalyzed by H2SO4, TeCl4, BF3-polyphosphoric acid or HF/FSO3H; (3) 2-Chloro-2-methylpropane alkylation with toluene catalyzed by anhydrous AlCl3, Mo(CO)6 or NaCl + AlCl3. However, these conventional catalysts easily cause environmental pollution and equipment corrosion. For this reason, much attention has been paid to environmental-friendly zeolite catalysts [8, 9, 10, 11].

The toluene alkylation with TBA has been studied using various zeolite catalysts, Ni-Y zeolite was used as cayalyst to synthesize PTBT, but the catalytic activity and para-selectivity are very low [12]. Pai et al. studied vapour-phase toluene alkylation with TBA over large-pore zeolite and found H-beta showed the best catalytic activity compared with HY and HMCM-22 [13]. Mravec et al. found zeolite H-MOR (with Si/Al = 17.5) showed the best catalytic activity (toluene conversion near 60%) and para-selectivity (near 90%) after 8 h at 180 °C [14]. Sebastian et al. investigated the influence of acidity of high-silica HM catalysts on toluene tert-butylation and found high-Si/Al-ratio HM catalyst with strong acidity but lower acid site density performed well in conversion [15]. Selvaraj et al. investigated the influence of Si/Al ratios of Al-MCM-41 on butylation. The toluene conversion and the yield and selectivity of PTBT decreased with increasing the Si/Al ratios of Al-MCM-41 catalysts from 21 to 104 [16]. Zhou et al. found the 4% Ti–Al-M48 catalyst showed the best toluene conversion (44%) and PTBT selectivity (78%) [17]. Though zeolite catalysts possess high catalytic activity for PTBT production, the main drawback is their coke deposition after a certain period, which further limits molecular diffusion and accessibility to acid sites and thereby prevents the catalysis.

To overcome the disadvantages of zeolites, metallic oxide modification of pore structure and acidity is a traditional method to improve the catalytic properties of zeolites [18]. In this study, Ce2O3 was adopted for H-beta modification in order to improve the selectivity for toluene butylation. The modified catalysts were characterized by different physicochemical techniques, their catalytic activities were evaluated, and the PTBT synthesis conditions were optimized.

2 Experimental

2.1 Preparation of catalyst

Na-beta zeolite (purchased from Zeolyst Int.) was used as a starting material. H-beta powder was prepared by ion exchange of Na-beta with NH4NO3 (Sigma Aldrich, reagent grade) aqueous solution according to the reported procedure [19]. Ce2O3-modified H-beta zeolites support with the mass fraction of Ce2O3 (1, 3, 5, 8 and 16%) were synthesized by wetness impregnation with different concentration of Ce(NO3)3 aqueous solutions for 24 h at room temperature, then dried at 60 °C for one night and then calcined at 450 °C for 4 h. The as prepared catalysts were referred to as x Ce2O3/H-beta (1, 3, 5, 8 and 16%), which x corresponds to the Ce2O3 weight content on catalysts.

2.2 Catalyst characterization

The surface morphological details of catalysts were studied by scanning electron microscopy (SEM, Zeiss-ΣIGMA).

X-ray diffraction (XRD) measurements were performed on a D/max-2200PC-X-ray diffractometer, using a Cu-Kα radiation under the setting conditions of 40 kV, 30 mA, scan range from 10 to 80 at a rate of 4°/min.

The elemental contents of all catalysts was obtained by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a PerkinElmer Optima 2000DV apparatus.

The porosity of the zeolites was determined by nitrogen adsorption at − 196 °C using an automatic micromeritics adsorption equipment of NOVA2000e. The total pore volume was then calculated from the adsorbed volume of nitrogen at P/P0 equal to 0.97. The external surface area and micropore volume were determined by the t-plot method using the Harkins–Jura equation.

IR spectra with pyridine adsorption of samples were recorded using a Bruker FT-IR spectrometer (Tensor 27) together with a high temperature vacuum cell. The sample powder was pressed into a self-supporting wafer, and the spectra were recorded in a wavenumber range of 4000–400 cm−1 with a 4 cm−1 resolution. Before each experiment, material samples were pressed into thin pellets (10–30 mg) with diameter of 16 mm and activated in situ during one night under vacuum (10−5 Pa) at 170 °C. Pyridine was introduced in excess at 150 °C after the activation period. The concentrations of the B sites and L sites were determined from the integrated area bands of the PyH+ (located at around 1540 cm−1) and PyL (around 1450 cm−1) species using the values of the molar extinction coefficients of both bands.

TG (Perkin-Elmer Diamond instrument) analyses of the catalysts were carried out under a N2 flow of 30 mL/min by increasing the temperature from 50 to 700 °C at a heating rate of 10 °C/min.

2.3 Catalytic reactions

The liquid-phase butylation of toluene was carried out with H-beta and Ce2O3/H-beta zeolites. The reaction was performed in a automated high pressure stainless steel reactor. A typical batch consisted of 60 ml of cyclohexane (used as solvent), 94 mmol of toluene, 283 mmol of tert-butyl alcohol, 1 g of catalyst. The liquid reaction was analyzed by GC-14C gas chromatograph equipped with SE-30 capillary column (φ 0.25 mm × 50 m).

3 Results and discussion

3.1 Characterization of catalyst

Figure 1 shows the XRD patterns of H-beta and xCe2O3/H-beta. All the xCe2O3/H-beta zeolites maintained the crystal structure of H-beta after Ce2O3 modification. Compared with H-beta zeolite, the peak intensity at 22.5° in xCe2O3/H-beta decreased gradually with the increase of x, suggesting certain crystallinity was lost after rare earth oxide modification and heat treatment. The peaks of Ce2O3 were not observed at Ce2O3 contents less than 5%, but appeared when the Ce2O3 content was up to 8%, indicating Ce2O3 was highly dispersed at Ce2O3 content below 5%.
Fig. 1

XRD patterns of H-beta and xCe2O3/H-beta. (1) H-beta, (2) 3%Ce2O3/H-beta,(3) 5% Ce2O3/H-beta,(3) 8% Ce2O3/H-beta,(5) 16% Ce2O3/H-beta

The SEM images in Fig. 2 showed the morphology and size of 5%Ce2O3/H-beta were nearly unchanged compared with the parent H-beta zeolite. The average particle sizes of 5%Ce2O3/H-beta were around 200–300 nm.
Fig. 2

SEM micrographs of H-beta and 5%Ce2O3/H-beta

The actual load of Ce2O3 in each xCe2O3/H-beta catalyst determined from ICP was close to its theoretical load. The Si/Al ratio of xCe2O3/H-beta increased with the Ce2O3 loading increased from 1% to 16%, which could be due to dealumination from zeolite framework (Table 1). The textural properties of parent H-beta and xCe2O3/H-beta were shown in Table 1. The parent H-beta zeolite exhibited the highest specific surface area (492 m2/g), pore volume (0.48 cm3/g) and pore size (3.90 nm). The Ce2O3 loading increasing from 1 to 16% led to a decrease of specific surface area from 481.2 m2/g to 372.8 m2/g, total pore volume from 0.47 to 0.34 cm3/g and pore size from 3.86 to 3.42 nm. The specific surface area and total porous volume of the 16%Ce2O3/H-beta decreased severely due to the blocking of pores by excessive Ce2O3.
Table 1

The structural parameters of parent H-beta zeolite and xCe2O3/H-betazeolites

Zeolites

Content of Ce2O3 (%)a

Si/Al ratioa

BET surface area (m2/g)b

Pore volume (cm3/g)c

Pore size (nm)

H-beta

25.0

492.5

0.48

3.90

1%Ce2O3/H-beta

1.1

25.2

481.2

0.47

3.86

3%Ce2O3/H-beta

2.9

25.5

476.3

0.47

3.83

5%Ce2O3/H-beta

5.2

26.1

465.2

0.45

3.78

8%Ce2O3/H-beta

7.8

27.0

443.6

0.41

3.64

16%Ce2O3/H-beta

15.7

28.2

372.8

0.34

3.42

aDeduced from ICP analysis

bSpecific surface area calculated by the BET method

cTotal pore volume determined at P/P0 = 0.99

The acidity of zeolites is very important in catalytic activity and product selectivity for any catalyzed reaction. The acid properties of samples were characterized by pyridine adsorption at 150 °C followed by IR analysis (Fig. 3 and Table 2). The B acidity and L acidity of H-beta both changed significantly after the Ce2O3 loading (Table 2). As expected, the acidity (both B acid and L acid) linearly decreased with the Ce2O3 loading from 1 to 16%, but the B/L ratio first increased and then dropped, indicating L acidity decreased more drastically than B acidity. The reason might be that the special electronic structure or namely the empty orbits of the rare metal element can withdraw electrons from O–H bonds and thereby increase the B acidity [20]. In addition, the total acid amount of the 16%Ce2O3/H-beta was sharply reduced, because the high Ce2O3 loading blocked the pores of H-beta.
Table 2

Acidic properties of catalysts determined by Py-IR at 150 °C

Catalyst

B acid (μmol/g)a

L acid (μmol/g)b

Total acid (μmol/g)

B/L Ratio

H-beta

84.2

47.2

131.4

1.78

3%Ce2O3/H-beta

81.4

44.4

125.8

1.83

5%Ce2O3/H-beta

78.9

41.3

120.2

1.91

8%Ce2O3/H-beta

75.4

37.4

112.8

2.01

16%Ce2O3/H-beta

63.2

32.5

95.7

1.94

aDeduced from the intensity of the band located at around 1540 cm−1

bDeduced from the intensity of the band located at around 1450 cm−1

Fig. 3

Py-IR curves of parent H-beta and x Ce2O3/H-beta. (1) H-beta, (2) 3% Ce2O3/H-beta, (3) 5% Ce2O3/H-beta,(3) 8% Ce2O3/H-beta,(5) 16% Ce2O3/H-beta

TG analysis of spent H-beta and spent Ce2O3/H-beta catalysts are performed and results listed in Fig. 4. The first mass loss all took place between 100 and 150 °C and corresponded to the removal of H2O from the zeolites [21]. The curves after 150 °C, which might relate to the decomposition of coke deposition stuck inside the zeolite, the mass change in the curve of spent H-beta from 150 to 800 °C was much greater than the spent Ce2O3/H-beta, which indicated that Ce2O3 modification of H-beta zeolite could prohibit carbon deposition to some extent.
Fig. 4

TG curves of spent H-beta and spent Ce2O3/H-beta catalysts

3.2 Catalytic mechanism

The alkylation reaction was investigated over parent H-beta and xCe2O3/H-beta zeolites. The mechanism is shown in Scheme 1: (1) dehydration of TBA to isobutylene; (2) catalytic protonation of isobutylene to tert-butyl carbocation; (3) electrophilic substation between tert-butyl carbocation and toluene to form PTBT and MTBT; (4) isomerization of PTBT to MTBT; (5) further alkylation of PTBT/MTBT with tert-butyl carbocation to 3,5-DTBT; (6) dealkylation of PTBT to toluene and isobutylene; (7) further reaction of excessive isobutylene with each molecule to form oligomers; (8) reaction of oligomers with excessive toluene to form longer-alkyl-chain alkyltoluenes.
Scheme 1

Reaction mechanism for the alkylation of touene with tert-butanol

3.3 Catalytic performance

The toluene conversion and PTBT selectivity in the toluene alkylation with TBA were shown in Table 3. In all cases, the main products were PTBT (para-isomer) and MTBT (meta-isomer), while OTBT (ortho-isomer) was not detected. Trace 3,5-di-tert-butyltoluene (3,5-DTBT) from further alkylation of PTBT or MTBT was also detected.
Table 3

Products selectivity and catalyst activity

Catalyst

Toluene conversion (%)

Yield of products (%)

PTBT selectivity (%)

PTBT

MTBT

Other

H-beta

58.4

39.3

15.7

3.4

67.3

1%Ce2O3/H-beta

58.3

47.3

10.1

0.9

81.1

3%Ce2O3/H-beta

56.3

45.8

9.8

0.7

81.3

5%Ce2O3/H-beta

56.1

46.4

9.2

0.5

82.7

8%Ce2O3/H-beta

53.2

44.1

8.6

0.5

82.9

16%Ce2O3/H-beta

45.4

37.7

7.3

0.4

83.0

The xCe2O3/H-beta exhibited slightly lower catalytic activity but much higher para-selectivity compared with the parent H-beta. The highest catalytic activity from H-beta (58.4% at 190 °C after 4 h) was reasonable owing to its highest B acidity [16]. The toluene conversions of all the xCe2O3/H-beta catalysts were lower than that of H-beta, and decreased gradually along with the increasing Ce2O3 loading. This result agreed well with the trend of B acidity in the xCe2O3/H-beta catalysts (Table 2). The toluene conversion was 56.1% over the 5%Ce2O3/H-beta, but drastically decreased over the 16%Ce2O3/H-beta. XRD and BET analysis showed excessive Ce2O3 covered the surface acid sites and blocked the pores and channels, which led to the poor catalytic activity.

All the xCe2O3/H-beta catalysts exhibited higher PTBT selectivity than H-beta, which could be explained from two aspects. (1) Since molecule diffusion in pores is directly influenced by the pore size and H-beta is a zeolite with larger pores than the kinetic diameters of PTBT and MTBT [14], the shape-selectivity has less effect on the product selectivity to PTBT. Nevertheless, Ce2O3 loading could narrow the pore size of the H-beta zeolite, which benefits the shape-selectivity. (2) PTBT (kinetics favored) easily isomerizes to MTBT (thermodynamics favored) on the external surface, without showing any steric hindrance, but the deactivation of acid sties caused by the Ce2O3 loading increases the PTBT selectivity by suppressing the further isomerization of PTBT on these acid sites.

3.4 Optimization of reaction conditions

3.4.1 Effect of catalyst loading

The effect of catalyst (Ce2O3/H-beta) loading on toluene butylation reaction was studied (Fig. 5). As the catalyst loading increased, the conversion of toluene first increased and then decreased. Since more acidic sites would directly increase the reaction rate. However, excess acidic sites on the surface of zeolites would accelerate the subsidiary reaction (isomerization and oligomerization), which resulted in the decreasing of toluene conversion and PTBT selectivity.
Fig. 5

Effect of catalyst loading on TBA conversion and product selectivity

3.4.2 Effect of TBA/toluene molar ratio

PTBT is the desired product of the toluene butylation, and a high PTBT selectivity may be more favorable. So for toluene butylation, the TBA/toluene molar ratio is at least > 1. Since the alkylation of toluene is reversible reaction, excess TBA can suppress the reverse reaction, promote the selectivity to PTBT. The effect of TBA/toluene molar ratio on butylation reaction was studied with the TBA/toluene molar ratio range from 1:1 to 5:1. As shown in Fig. 6, Along with effect of TBA/toluene molar ratio from 1:1 to 3:1, the toluene conversion increased, while the selectivity to PTBT decreased. With a further increase in the TBA/toluene molar ratio from 3:1 to 5:1, the selectivity to PTBT decreased. Therefore, TBA/toluene molar ratio of 3:1 was chosen in the subsequent experiments.
Fig. 6

Effect of TBA/toluene molar ratio on TBA conversion and product selectivity

3.4.3 Effect of reaction temperature

The effect of toluene conversion and PTBT selectivity on reaction temperature was studied. The alkylation reaction using 5.0%Ce2O3/H-beta catalyst were conducted at 140, 160, 180, 200 and 220 °C, respectively. As shown in Fig. 7, the toluene conversion increased as temperature raised from 140 to 180 °C after 4 h. However, the toluene conversion decreased from 180 to 220 °C. Since alkylation of toluene with TBA is a reversible and exothermic reaction, the reaction would be limited by thermodynamics when the reaction temperature was high, and the reverse reaction enhanced, which led to an increase in dealkylation rate of PTBT. The selectivity of PTBT decreased with the temperature increasing, it was because the isomerization of PTBT to MTBT, which was more stable at the higher temperature.
Fig. 7

Effect of reaction temperature on toluene conversion and PTBT selectivity

4 Conclusions

Several x Ce2O3/H-beta catalysts with different Ce2O3 loadings were prepared. The Ce2O3 loading led to a loss of crystallinity and slightly decreased the pore size and specific surface area. The B acidity and total acidity decreased with the increase of Ce2O3 loading.

The xCe2O3/H-beta exhibited slightly lower catalytic activity but much higher para-selectivity compared with H-beta. The toluene conversion was 56.1% with PTBT selectivity to 82.7% over the 5%Ce2O3/H-beta at 180 °C after 4 h. The PTBT selectivity was improved significantly from 67.3% to 82.7%, which could be explained from two aspects. (1) Since molecule diffusion in pores is directly influenced by the pore size and H-beta is large-pore zeolite, the shape-selectivity has less effect on PTBT selectivity. However, the Ce2O3 loading narrows the pore size of H-beta zeolite and thereby benefits the shape-selectivity. (2) The isomerization of PTBT (kinetics favored) to MTBT (thermodynamics favored) mainly occurs on the external surface, without showing any steric hindrance, but the deactivation of acid sties caused by Ce2O3 contributes to para-selectivity by suppressing the isomerization of PTBT.

Notes

Acknowledgements

This work was supported by the Natural Science Foundation of Heilongjiang Province of China (QC2017005), the Youth Fund of Northeast Petroleum University (2018QNL-26). We are also grateful for the measurement assistants from Analysis & Testing Center of Northeast Petroleum University.

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Copyright information

© The Tunisian Chemical Society and Springer Nature Switzerland AG 2018

Authors and Affiliations

  • Yuanyuan Wang
    • 1
  • Xinglong Sun
    • 2
  • Yixian Han
    • 1
  • Jiaojing Zhang
    • 1
  • Xueqin Wang
    • 1
  • Hua Song
    • 1
    Email author
  • Xi Chen
    • 1
  1. 1.Chemistry and Chemical EngineeringSchool of Northeastern Petroleum UniversityDaqingChina
  2. 2.Daqing Petrochemical Engineering Co., Ltd.DaqingChina

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