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Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis
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Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis Tao Liu , Xiaoxue Yuan , Gang Zhang , Jing Hu , Jing An , Tong Chen , Gongying Wang PII: DOI: Reference:
S0026-265X(19)31410-9 https://doi.org/10.1016/j.microc.2019.104341 MICROC 104341
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
9 June 2019 10 October 2019 13 October 2019
Please cite this article as: Tao Liu , Xiaoxue Yuan , Gang Zhang , Jing Hu , Jing An , Tong Chen , Gongying Wang , Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis, Microchemical Journal (2019), doi: https://doi.org/10.1016/j.microc.2019.104341
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Highlights A
method
for
analysis
of
six
byproducts
in
DPC
by
SBSE-ATSTD-GC-MS was developed. The main factors influencing SBSE and ATSTD were investigated. The method is simple, rapid, accurate, and sensitive with good selectivity. This
work can
provide
data
transesterification byproducts.
support for
studying
the
Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis Tao Liu a,b,c,†, Xiaoxue Yuan c*,†, Gang Zhang a,b, Jing Hu a, Jing An a,b, Tong Chen a*, Gongying Wang a a Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, 610041, P. R. China b National Engineering Laboratory for VOCs Pollution Control Material & Technology, University of Chinese Academy of Sciences, Beijing, 101408, P. R. China c Sichuan Center for Disease Control and Prevention, Chengdu, 610041, P. R. China * Corresponding authors, E-mail: [email protected] (X. X. Yuan); [email protected] (T. Chen) † These authors contributed equally to this work. Abstract: In this study, a fast and sensitive method based on stir bar sorptive extraction (SBSE) and two-step thermal desorption coupled with online gas chromatography-mass spectrometry was established to simultaneously analyse six trace byproducts [phenyl salicylate (PS), xanthone (XA), methyl o-hydroxybenzoate (MS), 2-methyl diphenyl carbonate (PTC), methyl o-methoxybenzoate (MSME) and phenyl o-methoxybenzoate (PSME)] generated in the process of diphenyl carbonate (DPC) synthesis. Analytes were enriched with multiple sorptive stir bars simultaneously, followed by thermal desorption, cryo-focusing, and instantaneous injection, effectively solving such problems as long adsorption time and small sample volumes compared with conventional stir bars. The main factors influencing SBSE and thermal desorption were studied. Optimized stir bar extraction conditions are as follows: the volume fraction of methanol is 10%, the extraction time is 6 min, and the stirring rate is 1200 rpm at room temperature (25 ℃). Optimized two-step thermal desorption conditions are as follows: in the first desorption step, the desorption temperature is 300 ℃, the desorption time is 10 min, and the valve and transmission line temperature is 200 ℃; in the second desorption step, the cold trap temperature is −10 ℃, which is increased at a rate of 100 ℃/s to 320 ℃ and held for 5 min. A splitless mode and a split mode with a split ratio of 20:1 are used in the first and the second desorption step, respectively. Under the optimal conditions, the six byproducts generated in the synthesis of DPC via transesterification were quantitatively analyzed by external standard method. The results showed that, within the range of 1.0−100 ng/L for all of the six transesterification byproducts, this method revealed good linearity with correlation coefficients (r) ≥0.997. The limits of detection (LODs) were between 0.054 ng/L and 0.253 ng/L, the relative standard deviations (RSD) were from 5.2% to 11.5%, and the recoveries were 81.6%−102.6% (n=3). The method is simple, rapid and has a wide linear range, high accuracy, and sensitivity, as well as good stability. It is suitable for simultaneous, rapid and selective analysis of six trace byproducts in the process of DPC synthesis. It can also provide
vigorous data support and industrialization guidance for the selection of transesterification conditions and research on the formation mechanisms, distribution regularity, and even control of transesterification byproducts. Keywords: Stir bar sorptive extraction; two-stage thermal desorption-gas chromatography-mass spectrometry; byproducts; transesterification; diphenyl carbonate synthesis
1. Introduction DPC is an important chemical intermediate, which is non-toxic and can be employed for synthesizing many important polymers and organic compounds [1,2]. Particularly, it can, in lieu of highly toxic phosgene, react with bisphenol A through melt polycondensation to synthesize polycarbonate (PC) with the excellent performance [3]. Transesterification of dimethyl carbonate (DMC) with phenol is the most promising non-phosgene route for DPC synthesis [4−6], which comprises two steps: the first step is the transesterification of DMC with phenol to produce intermediate methyl phenyl carbonate (MPC), and the second step is further transesterification of MPC with phenol to produce the target product DPC, or the self-disproportionation of MPC to produce DPC and DMC [7]. In pilot scale-up reaction, trace and even ultratrace PS, XA, MS, PTC, MSME, and PSME were found in the purified product. Normally, Lewis acids are used as the catalyst for transesterification. Additionally, the transesterification is realized under high temperature and high pressure [8]. It is found that MPC and DPC are prone to Fries rearrangement and DPC, MS and PS are also methylated under these conditions. It can be inferred from the molecular structures of PS, XA, MS, PTC, MSME and PSME, transesterification catalysts and process conditions that both PS and XA are the products of Fries rearrangement of DPC, while MS, MSME, PSME, and PTC are the products of Fries rearrangement of MPC and methylation of MS, PS, and DPC, respectively. The boiling points of all the above transesterification byproducts are high and approximate to that of DPC, and thus they cannot be easily separated during purification of DPC. They are very prone to remain in transesterification product to affect the purity of DPC and thus seriously affect the molecular weight and performance of PC synthesized in the next step [9−11]. DPC and MPC from transesterification have been qualitatively and quantitatively analyzed [12−14], whereas no study on qualitative and quantitative analysis of trace byproducts except anisole has been reported. More important is that the byproducts at trace level cannot be directly detected by GC and GC-MS. Therefore, the establishment of a rapid, accurate, sensitive and highly selective quantitative method for trace or ultratrace byproducts of transesterification between DMC and phenol is of the great significance of industrial reference for probing the formation mechanisms, distribution regularity and controlling of the transesterification byproducts. Since the concentration of transesterification byproducts is generally at trace or even ultratrace levels, and the sample matrices are complicated, an efficient pretreatment method for enrichment is required for accurate determination of them. Conventional sample pretreatment techniques such as liquid-liquid extraction, Soxhlet extraction, column chromatography, purge-and-trap have exhibited good extraction effects. However, most of them require large sample quantities and sophisticated pretreatment devices and have such disadvantages as great organic solvent consumption, long sample treatment time, possible sample loss, large errors and tedious operating procedures [15]. Solid phase extraction performs well in concentrating and purifying samples but is not sufficiently selective to trace components in processing samples with
complicated matrices [16]. Solid-phase microextraction (SPME) is a solvent-free sample extraction technique that integrates collection, concentration, purification and injection [17]. Various forms of SPME, such as fiber solid-phase micro-extraction [18], in-tube solid-phase microextraction [19] and SBSE [20, 21], are currently available. SBSE is an SPME technique, where a glass tube with a magnet enclosed therein is coated with polydimethylsiloxane (PDMS) or polyethylene-ethylene glycol modified silicon (EG-Silicon) for the purpose of extraction. SBSE was proposed by Baltussen et al. [22] in 1999 and commercialized by Gerstel GmbH in 2000. Although the same extraction principle SBSE and SPME share, the volume of the extracted phase of SBSE is 50 to 250 times that of SPME, and the extraction capacity is high, which makes the extraction efficiency higher. SBSE has such advantages as convenient and rapid operation, strong enrichment ability, high sensitivity, good reproducibility, and low sample matrix interference and does not need the use of solvents. Stir bars are reusable and can withstand higher stirring rates as compared with SPME. The purpose of extraction and enrichment is achieved while the bar is stirring and, in general, additional stirrers are not needed so that competitive adsorption by magnetic stirrers is avoided. This technique has been extensively applied in the pretreatment of food, environmental and biological samples for the determination of trace volatile and semi-volatile organic compounds [22−26]. With the thermal desorption-direct injection technique, the sample is heated at the injection port of the gas chromatography or target analytes are desorbed from the matrix using a thermal desorber and entered into the gas chromatography, overcoming the weakness of conventional pretreatment methods. With common single-step thermal desorption techniques, target analytes desorbed are directly entered into the column. As a long time may be needed for complete desorption, chromatographic peaks may broaden seriously, and the sensitivity is severely reduced. To solve this problem, a cold trap system has been introduced into two-stage thermal desorption for enrichment and secondary desorption of target analytes, namely, after desorbed, target analytes enter the cold trap, where the temperature is rapidly increased to “flash” the target analytes and enter them into the column, thereby improving the chromatographic resolution and analytical sensitivity [27]. The automated two-stage thermal desorption (ATSTD) system, comprising a carrier gas control module, dispenses with the GC injection port and has such advantages as simple operation, low time and labor consumption and absence of interfering impurities [28]. In addition, desorbed target analytes are completely entered into the GC system. Thereby, the sample consumption is significantly reduced, which makes it possible to analyze trace analytes in samples. On the basis of this, SBSE and ATSTD were coupled with gas chromatography-mass spectrometry (GC-MS) for simultaneous determination of the six trace or even ultratrace byproducts in the synthesis of DPC via transesterification of DMC with phenol for the first time. The six trace byproducts were quantitatively analyzed by external standard method. Main factors influencing SBSE (extraction time, stirring rate, and the volume fraction of methanol) and main conditions influencing ATSTD (desorption temperature, desorption time and cryo-focusing temperature) were optimized. The results showed that the method is rapid, accurate and sensitive with good reproducibility for the simultaneous determination of the transesterification byproducts.
2. Experimental 2.1 Instruments and reagents
C-MAG HS7 magnetic stirrer (IKA, Germany) and Gerstel Twister stir bars (10 mm×0.5 mm, PDMS 25 μL) (Gerstel GmbH, Germany) were used to extract the six byproducts. Unity Series 2 thermal desorber equipped with a cooled injection system (CIS) (Markes, the UK) was used to desorb byproducts. The determination of byproducts was performed on an HP 6890/5973 gas chromatograph-mass spectrometer equipped with an HP-5 MS column (30 m×0.32 mm×0.25 μm) (Agilent Technologies, USA). LAB-T 100 activation apparatus (Guangzhou Appinno Instruments Co., Ltd, China) and Milli-Q ultrapure water system (Millipore, USA) were also used in the experiment. PS and MS were purchased from Alfa Aesar (USA). XA and DPC were obtained from J&K Scientific Ltd. (China) and Nanjing Duly Biotech Co., Ltd (China), respectively. MSME, PSME, and PTC were purchased from Beijing Innochem Science & Technology Co., Ltd (China). DMC and phenol were obtained from Aladdin (USA). Anisole was purchased from Chengdu Kelong Chemical Reagents Co., Ltd (China). Acetone, methanol, and dichloromethane were all obtained from Fisher Scientific (USA). All of the above reagents are chromatographically pure. MPC (≥99%) was synthesized and purified by reference [29. 30] 2.2. Stir-bar sorptive extraction Prior to use of the stir bars, a cleaning step was firstly performed in 2 mL of methanol-dichloromethane (1:1, v/v) mixture and 2 mL of acetone under ultrasound for 5 min. Then, the stir bars were removed with tweezers, wiped dry with dust-free lens paper and cleaned in an activation apparatus under a high-purity N2 flow of 50 mL/min from 100 ℃ (hold for 20 min) to 300 ℃ (hold for 2 h). 10 mL of the sample solution, accurately measured, was transferred to a 25 mL conical flask, followed by addition of methanol of 10% (volume fraction). Four solid-phase extraction stir bars were adsorbed on a cleaned magnetic stirring rod and placed in the sample solution. The sample solution was then stirred with 1200 rpm at room temperature for 6 min. After extraction, the stir bars were taken out with tweezers, washed with distilled water to remove adhering matters (e.g. suspended matters, soluble salts and other adhering matters), dried with dust-free paper and put them into an empty stainless steel tube, which was then inserted into the heating cell of the thermal desorber for analysis. 2.3. Instrumental analysis Thermal desorption unit (TDU) conditions: A splitless mode was used. TDU was heated from room temperature to 300 ℃ at a rate of 100 ℃/min. The desorption time was 10 min, The desorption flow rate was 50 mL/min. CIS conditions: The analytes were cryofocused at −10 ℃ and then cryo-desorbed at 320 ℃ (heating rate was 100 ℃/s) for 5 min. The split ratio was 20:1. The valve and transmission line temperature was 200 ℃. GC-MS conditions: An HP-5 MS capillary column (30 m×0.32 mm×0.25 μm) was used. The carrier gas was high-purity helium at a flow rate of 0.8 mL/min. The oven was programmed from 140 ℃ (hold for 2 min) at 10 ℃/min to 320 ℃ (hold for 5 min). An EI source was used. The electron energy was 70 eV. The ion source temperature was 230 ℃, the transmission line temperature was 280 ℃, and the quadrupole temperature was 150 ℃. The mass scanning range was 45−550 (m/z). The resolution was 5000. The solvent delay was 2.1 min. A full-scan mode was employed for signal acquisition. 2.4. Preparation of standard curves A quantity of PS, XA, MS, PTC, MSME, and PSME was added into acetone to produce a mixed standard stock solution, respectively. Dilution was performed as needed to produce
corresponding standard solutions of 1, 2, 5, 10, 20, 50, and 100 ng/L, respectively. Mixed standard solutions were extracted by the established method, and then the stir bars were placed in empty stainless steel tubes for thermal desorption and analyzed under the optimal experimental conditions finally. The total ion chromatogram of a mixed standard solution is shown in Fig. 1. It can be seen from Fig. 1 that the six analytes were well separated. Standard curves were plotted according to concentrations and response values of the analytes. Linear correlation coefficients of the six byproducts were shown in Table 1. The results indicate each standard curve was well linear with r ≥0.997.
Fig. 1. The total ion current chromatogram of six kinds of mixed standard solutions. 1: MS; 2: MSME; 3: PS; 4: XA; 5: PSME; 6: PTC.
2.5. Quality Control New empty stainless steel tubes were cleaned on an activation apparatus under 100 mL/min and at 340 ℃ for 3 h. Used empty stainless steel tubes were cleaned at 340 ℃ for more than 1 h to remove sample residues. Activated empty stainless steel tubes were sealed by capping two ends immediately with polytetrafluoroethylene caps and stored in sealed bags or protective tubes at 4 ℃ in a box with activated carbon or a desiccator. Activated stainless steel tubes were used within two weeks. Prior to use, blank tests were carried out for stir bars and empty stainless steel tubes to ensure the absence of chromatographic peaks. Reagent blank test, sample blank test, and parallel sample test were carried out for each batch of samples. The relative standard deviations for the determination of target compounds in parallel samples should be less than 25% [31]. An intermediate-concentration control point was included in each batch of samples. The relative standard deviation between the measured value of the intermediate-concentration control point and the concentration of the corresponding point on the standard curve was controlled within 20%. In case the requirement was not met, the intermediate-concentration standard solution was re-prepared. If the requirement was still not met, a new standard curve was plotted. All the glassware used in the experiment was cleaned with ultrapure water, soaked in acetone for 1 h and then rinsed with ultrapure water. No detergent was used during the cleaning to avoid possible interferences introduced by the residues of detergent. Afterward, the glassware was dried in an oven at 100 ℃ for 1 h.
3. Results and Discussion 3.1. Selection of SBSE conditions
3.1.1. Effect of extraction time on the extraction efficiency Because the adsorption equilibrium between the solution and the stir bar is affected by the extraction time [32]. Therefore, five different extraction times (2, 4, 6, 8, and 10 min) was investigated. Each extraction time was tested 3 times with a stirring rate of 1200 rpm and a volume fraction of methanol of 5%. The total peak areas of the six byproducts obtained with different extraction times are compared in Fig. 2. As seen in Fig. 2, with the prolongation of the extraction time, the peak area of byproducts increased gradually. When the extraction time was 6 min, an adsorption equilibrium was reached and thus the peak area showed no significant change after 6 min. With the theory of equilibrium distribution and extraction efficiency taken into account, the extraction time was selected as 6 min.
Fig. 2. The effect of extraction time on the extraction efficiency with SBSE.
3.1.2. Effect of stirring rate on the extraction efficiency Stirring samples can increase the extract efficiency and shorten the extraction time. The effect is particularly pronounced for the extraction of substances with high diffusion coefficients and high molecular weights [33]. Therefore, five different stirring rates (400, 600, 800, 1000, and 1200 rpm) were evaluated. The experiment was completed with an extraction time of 6 min and a volume fraction of methanol of 5%. As shown in Fig. 3, with the stirring rate increased from 400 rpm to 1000 rpm, the total peak area of the byproducts increased gradually. Although the total peak area showed no significant change from 1000 rpm to 1200 rpm, 1200 rpm was more suitable than 1000 rpm due to its lower relative standard deviation. Therefore, 1200 rpm was selected in this study.
Fig. 3. The effect of stirring rate on the extraction efficiency with SBSE.
3.1.3. Effect of methanol on the extraction efficiency Using methanol as a dispersant for extraction is an effective route for improving the extraction efficiency of PDMS for weakly polar compounds [34]. Different volume fraction (0%, 5%, 10%, 15%, and 20%) of methanol were added to investigate the extraction efficiency of the six byproducts. As can be known from Fig. 4, the total peak area increased gradually with the increase of the volume fraction of methanol, reaching a maximum when the volume fraction of methanol was 10% and gradually declined as the volume fraction of methanol continued to increase. It is probably because the addition of methanol can increase the distribution coefficient of PDMS between acetone and byproducts, but a high concentration of methanol may cause byproducts to precipitate and increase the adsorption of the byproducts by the conical flask. Therefore, a volume fraction of methanol of 10% was selected in this experiment.
Fig. 4. The effect of volume fraction of MeOH on the extraction efficiency with SBSE.
3.2. Selection of ATSTD conditions 3.2.1. Effect of desorption temperature on the desorption efficiency Considering the boiling points of all the six transesterification byproducts are relatively high and that the coating of the stir bars requires an experimental temperature of ≤300 ℃ [35], the desorption efficiency of the six byproducts at 260, 280, and 300 ℃ was investigated with the desorption time maintained at 10 min, the desorption flow at 70 mL/min and the cryo-focusing temperature at −10 ℃. According to the results calculated on the basis of data shown in Fig. 5, the desorption efficiency of the byproducts was the highest at 300 ℃, which was 1.0 to 2.5 times as much as that at 260 ℃ and 280 ℃. So a desorption temperature of 300 ℃ was selected in the experiment.
Fig. 5. Influence of desorption temperature on the desorption efficiency with ATSTD.
3.2.2. Effect of the desorption time on the desorption efficiency An inadequate desorption time will lead to incomplete desorption, resulting in decreased desorption efficiency of extracts. Therefore, the effect of different desorption times (6, 8, 10, and 12 min) on the desorption efficiency was investigated with the desorption temperature maintained at 300 ℃, the desorption flow at 70 mL/min and the cryo-focusing temperature at −10 ℃. According to the results calculated on the basis of data shown in Fig. 6, the desorption efficiency was 74.7% and 87.3% when the desorption time was 6 min and 8 min, respectively. When the desorption time was 10 min, the desorption efficiency reached up to 99.6%. When the desorption time was 12 min, the desorption efficiency was equivalent to that of 10 min. The results suggest that the desorption time of 10 min is sufficient for thermal desorption of the six byproducts, and thus a desorption time of 10 min was selected.
Fig. 6. Influence of desorption time on the desorption efficiency with ATSTD.
3.2.3. Effect of cryo-focusing temperature on the desorption efficiency The cryo-focusing temperature is an important factor influencing the desorption efficiency of each compound. For compounds with different boiling points, different trapping temperatures are required, and the desorption efficiency may also change. So the effect of different cryo-focusing temperatures (−30, −20, −10, 0, and 10 ℃) on the desorption efficiency was investigated. The results showed that MS and MSME with relatively low boiling points were better trapped in the cold trap at low temperatures, while better desorption effects were exhibited for PS, XA, PSM,
and PTC at relatively high temperatures. As a compromise, a cryo-focusing temperature of −10 ℃ was selected. 3.3. Limit of detection, precision, and accuracy of the method and analysis of practical samples Under the optimum conditions, the sensitivity of the method was evaluated through LODs and limits of quantitation (LOQs), which were calculated on the basis of a signal-to-noise ratio of three and ten, respectively. RSD and recoveries were used to evaluate the precision and accuracy of the method, respectively. As shown in Table 1, LODs for the six byproducts ranged from 0.054 ng/L to 0.253 ng/L. LOQs ranged from 0.180 ng/L to 0.843 ng/L. Spiked blank samples containing the six byproducts (1.0, 10, and 100 ng/L) were analyzed, and the RSD ranged between 5.2% and 11.5%. Recovery experiment was carried out on practical samples spiked with 5, 20, and 80 ng/L of analytes were added, respectively. Both practical and spiked samples were analyzed by the same method. Recoveries of the six byproducts were between 81.6% and 102.6%. Moreover, the results showed that the average extraction efficiency and desorption efficiency were 99.8% and 99.7%, respectively. In addition, the practical samples from the transesterification with different reaction temperatures and pressures were quantitatively analyzed by the method. The results were presented in Fig. 7. As seen in Fig. 7 (A) and Fig. 7 (B), six byproducts with different concentrations were detected in all samples. The results also showed that high pressure and high temperature were all beneficial to the formation of the six transesterification byproducts. The total ion chromatogram of the sample is shown in Fig. 7 (C). The result also revealed that phenol, DMC, byproduct anisole, intermediate product MPC, and DPC did not interfere with the determination of the six byproducts. This method is suitable for the simultaneous and selective determination of the six trace byproducts from transesterification between DMC with phenol. Table 1 Correlation coefficient, LOD, LOQ, relative standard deviations, and recoveries. RSD (%) Correlation
LOD
LOQ
coefficient
(ng/L)
(ng/L)
Compound
Recoveries (%)
1.0
10.0
100
5.0
20.0
80
ng/L
ng/L
ng/L
ng/L
ng/L
ng/L
PS
0.999
0.077
0.257
6.8
5.2
7.1
98.7
96.5
89.0
XA
0.998
0.093
0.310
5.7
6.6
8.3
86.2
102.6
100.8
MS
0.999
0.205
0.683
11.2
9.4
8.9
101.7
89.9
96.9
MSME
0.998
0.186
0.620
10.1
11.5
6.9
99.3
100.2
87.4
PSME
0.997
0.253
0.843
9.6
8.8
8.0
84.7
81.6
99.0
PTC
0.998
0.054
0.180
7.0
5.7
6.4
101.2
88.3
93.3
4. Conclusion In this paper, six trace byproducts, that cannot be directly detected by GC-MS, in the synthesis of DPC via transesterification of DMC with phenol were first determined by SBSE-ATSTD-GC-MS. The main factors influencing SBSE and ATSTD were investigated. Under the optimal conditions, the method can detect the six byproducts at ng/L levels in the transesterification. The established method is simple, rapid, accurate, sensitive with high extraction efficiency, desorption efficiency, wide linear range, and good selectivity. It is suitable for the simultaneous and rapid determination of six trace and even ultratrace byproducts in the pilot-scale process of DPC synthesis. This work can also provide a theoretical and data basis for
studying and controlling the transesterification byproducts produced by Fries rearrangement and methylation reaction in the synthesis of DPC.
Fig. 7. The effects of temperature (A), pressure (B) on the byproducts from transesterification between phenol and DMC, and the total ion current chromatogram of the byproducts from transesterification (C). 1: DMC; 2: anisole; 3: phenol; 4: MPC; 5: MS; 6: MSME; 7: DPC; 8: PS; 9: XA; 10: PSME; 11: PTC. Reaction conditions: (A) Phenol: 15 g; n(phenol) = n(DMC); catalyst (dibutyltin oxide): 0.30 g; reaction time: 8 h; reaction pressure: 0.30 MPa. (B) Phenol: 15 g; n(phenol) = n(DMC); catalyst (dibutyltin oxide): 0.30 g; reaction time: 8 h; reaction temperature: 220 ℃.
Declaration of competing interest The authors declare that they have no conflict of interest.
Acknowledgments This work was supported by the Major Demonstration Program of Innovation Academy for Green Manufacture, Chinese Academy of Sciences (IAGM-2019-A10), the Science and Technology Demonstration Program for Transformation of Sichuan Province (No. 18ZHSF0011), the National High-tech R&D Program of China (863 Program, No. 2013AA031703), the National Key R&D Program of China (No. 2016YFB0301900), the Science and Technology Support Program of Sichuan Province (No. 2018JY0615), and the Science and Technology Innovation Program for
Youth Team of Sichuan Province (No. 2013TD0010).
Declaration of competing interest The authors declare that they have no conflict of interest.
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Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis Tao Liu , Xiaoxue Yuan , Gang Zhang , Jing Hu , Jing An , Tong Chen , Gongying Wang PII: DOI: Reference:
S0026-265X(19)31410-9 https://doi.org/10.1016/j.microc.2019.104341 MICROC 104341
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
9 June 2019 10 October 2019 13 October 2019
Please cite this article as: Tao Liu , Xiaoxue Yuan , Gang Zhang , Jing Hu , Jing An , Tong Chen , Gongying Wang , Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis, Microchemical Journal (2019), doi: https://doi.org/10.1016/j.microc.2019.104341
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Highlights A
method
for
analysis
of
six
byproducts
in
DPC
by
SBSE-ATSTD-GC-MS was developed. The main factors influencing SBSE and ATSTD were investigated. The method is simple, rapid, accurate, and sensitive with good selectivity. This
work can
provide
data
transesterification byproducts.
support for
studying
the
Stir bar sorptive extraction and automatic two-stage thermal desorption-gas chromatography-mass spectrometry for trace analysis of the byproducts from diphenyl carbonate synthesis Tao Liu a,b,c,†, Xiaoxue Yuan c*,†, Gang Zhang a,b, Jing Hu a, Jing An a,b, Tong Chen a*, Gongying Wang a a Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, 610041, P. R. China b National Engineering Laboratory for VOCs Pollution Control Material & Technology, University of Chinese Academy of Sciences, Beijing, 101408, P. R. China c Sichuan Center for Disease Control and Prevention, Chengdu, 610041, P. R. China * Corresponding authors, E-mail: [email protected] (X. X. Yuan); [email protected] (T. Chen) † These authors contributed equally to this work. Abstract: In this study, a fast and sensitive method based on stir bar sorptive extraction (SBSE) and two-step thermal desorption coupled with online gas chromatography-mass spectrometry was established to simultaneously analyse six trace byproducts [phenyl salicylate (PS), xanthone (XA), methyl o-hydroxybenzoate (MS), 2-methyl diphenyl carbonate (PTC), methyl o-methoxybenzoate (MSME) and phenyl o-methoxybenzoate (PSME)] generated in the process of diphenyl carbonate (DPC) synthesis. Analytes were enriched with multiple sorptive stir bars simultaneously, followed by thermal desorption, cryo-focusing, and instantaneous injection, effectively solving such problems as long adsorption time and small sample volumes compared with conventional stir bars. The main factors influencing SBSE and thermal desorption were studied. Optimized stir bar extraction conditions are as follows: the volume fraction of methanol is 10%, the extraction time is 6 min, and the stirring rate is 1200 rpm at room temperature (25 ℃). Optimized two-step thermal desorption conditions are as follows: in the first desorption step, the desorption temperature is 300 ℃, the desorption time is 10 min, and the valve and transmission line temperature is 200 ℃; in the second desorption step, the cold trap temperature is −10 ℃, which is increased at a rate of 100 ℃/s to 320 ℃ and held for 5 min. A splitless mode and a split mode with a split ratio of 20:1 are used in the first and the second desorption step, respectively. Under the optimal conditions, the six byproducts generated in the synthesis of DPC via transesterification were quantitatively analyzed by external standard method. The results showed that, within the range of 1.0−100 ng/L for all of the six transesterification byproducts, this method revealed good linearity with correlation coefficients (r) ≥0.997. The limits of detection (LODs) were between 0.054 ng/L and 0.253 ng/L, the relative standard deviations (RSD) were from 5.2% to 11.5%, and the recoveries were 81.6%−102.6% (n=3). The method is simple, rapid and has a wide linear range, high accuracy, and sensitivity, as well as good stability. It is suitable for simultaneous, rapid and selective analysis of six trace byproducts in the process of DPC synthesis. It can also provide
vigorous data support and industrialization guidance for the selection of transesterification conditions and research on the formation mechanisms, distribution regularity, and even control of transesterification byproducts. Keywords: Stir bar sorptive extraction; two-stage thermal desorption-gas chromatography-mass spectrometry; byproducts; transesterification; diphenyl carbonate synthesis
1. Introduction DPC is an important chemical intermediate, which is non-toxic and can be employed for synthesizing many important polymers and organic compounds [1,2]. Particularly, it can, in lieu of highly toxic phosgene, react with bisphenol A through melt polycondensation to synthesize polycarbonate (PC) with the excellent performance [3]. Transesterification of dimethyl carbonate (DMC) with phenol is the most promising non-phosgene route for DPC synthesis [4−6], which comprises two steps: the first step is the transesterification of DMC with phenol to produce intermediate methyl phenyl carbonate (MPC), and the second step is further transesterification of MPC with phenol to produce the target product DPC, or the self-disproportionation of MPC to produce DPC and DMC [7]. In pilot scale-up reaction, trace and even ultratrace PS, XA, MS, PTC, MSME, and PSME were found in the purified product. Normally, Lewis acids are used as the catalyst for transesterification. Additionally, the transesterification is realized under high temperature and high pressure [8]. It is found that MPC and DPC are prone to Fries rearrangement and DPC, MS and PS are also methylated under these conditions. It can be inferred from the molecular structures of PS, XA, MS, PTC, MSME and PSME, transesterification catalysts and process conditions that both PS and XA are the products of Fries rearrangement of DPC, while MS, MSME, PSME, and PTC are the products of Fries rearrangement of MPC and methylation of MS, PS, and DPC, respectively. The boiling points of all the above transesterification byproducts are high and approximate to that of DPC, and thus they cannot be easily separated during purification of DPC. They are very prone to remain in transesterification product to affect the purity of DPC and thus seriously affect the molecular weight and performance of PC synthesized in the next step [9−11]. DPC and MPC from transesterification have been qualitatively and quantitatively analyzed [12−14], whereas no study on qualitative and quantitative analysis of trace byproducts except anisole has been reported. More important is that the byproducts at trace level cannot be directly detected by GC and GC-MS. Therefore, the establishment of a rapid, accurate, sensitive and highly selective quantitative method for trace or ultratrace byproducts of transesterification between DMC and phenol is of the great significance of industrial reference for probing the formation mechanisms, distribution regularity and controlling of the transesterification byproducts. Since the concentration of transesterification byproducts is generally at trace or even ultratrace levels, and the sample matrices are complicated, an efficient pretreatment method for enrichment is required for accurate determination of them. Conventional sample pretreatment techniques such as liquid-liquid extraction, Soxhlet extraction, column chromatography, purge-and-trap have exhibited good extraction effects. However, most of them require large sample quantities and sophisticated pretreatment devices and have such disadvantages as great organic solvent consumption, long sample treatment time, possible sample loss, large errors and tedious operating procedures [15]. Solid phase extraction performs well in concentrating and purifying samples but is not sufficiently selective to trace components in processing samples with
complicated matrices [16]. Solid-phase microextraction (SPME) is a solvent-free sample extraction technique that integrates collection, concentration, purification and injection [17]. Various forms of SPME, such as fiber solid-phase micro-extraction [18], in-tube solid-phase microextraction [19] and SBSE [20, 21], are currently available. SBSE is an SPME technique, where a glass tube with a magnet enclosed therein is coated with polydimethylsiloxane (PDMS) or polyethylene-ethylene glycol modified silicon (EG-Silicon) for the purpose of extraction. SBSE was proposed by Baltussen et al. [22] in 1999 and commercialized by Gerstel GmbH in 2000. Although the same extraction principle SBSE and SPME share, the volume of the extracted phase of SBSE is 50 to 250 times that of SPME, and the extraction capacity is high, which makes the extraction efficiency higher. SBSE has such advantages as convenient and rapid operation, strong enrichment ability, high sensitivity, good reproducibility, and low sample matrix interference and does not need the use of solvents. Stir bars are reusable and can withstand higher stirring rates as compared with SPME. The purpose of extraction and enrichment is achieved while the bar is stirring and, in general, additional stirrers are not needed so that competitive adsorption by magnetic stirrers is avoided. This technique has been extensively applied in the pretreatment of food, environmental and biological samples for the determination of trace volatile and semi-volatile organic compounds [22−26]. With the thermal desorption-direct injection technique, the sample is heated at the injection port of the gas chromatography or target analytes are desorbed from the matrix using a thermal desorber and entered into the gas chromatography, overcoming the weakness of conventional pretreatment methods. With common single-step thermal desorption techniques, target analytes desorbed are directly entered into the column. As a long time may be needed for complete desorption, chromatographic peaks may broaden seriously, and the sensitivity is severely reduced. To solve this problem, a cold trap system has been introduced into two-stage thermal desorption for enrichment and secondary desorption of target analytes, namely, after desorbed, target analytes enter the cold trap, where the temperature is rapidly increased to “flash” the target analytes and enter them into the column, thereby improving the chromatographic resolution and analytical sensitivity [27]. The automated two-stage thermal desorption (ATSTD) system, comprising a carrier gas control module, dispenses with the GC injection port and has such advantages as simple operation, low time and labor consumption and absence of interfering impurities [28]. In addition, desorbed target analytes are completely entered into the GC system. Thereby, the sample consumption is significantly reduced, which makes it possible to analyze trace analytes in samples. On the basis of this, SBSE and ATSTD were coupled with gas chromatography-mass spectrometry (GC-MS) for simultaneous determination of the six trace or even ultratrace byproducts in the synthesis of DPC via transesterification of DMC with phenol for the first time. The six trace byproducts were quantitatively analyzed by external standard method. Main factors influencing SBSE (extraction time, stirring rate, and the volume fraction of methanol) and main conditions influencing ATSTD (desorption temperature, desorption time and cryo-focusing temperature) were optimized. The results showed that the method is rapid, accurate and sensitive with good reproducibility for the simultaneous determination of the transesterification byproducts.
2. Experimental 2.1 Instruments and reagents
C-MAG HS7 magnetic stirrer (IKA, Germany) and Gerstel Twister stir bars (10 mm×0.5 mm, PDMS 25 μL) (Gerstel GmbH, Germany) were used to extract the six byproducts. Unity Series 2 thermal desorber equipped with a cooled injection system (CIS) (Markes, the UK) was used to desorb byproducts. The determination of byproducts was performed on an HP 6890/5973 gas chromatograph-mass spectrometer equipped with an HP-5 MS column (30 m×0.32 mm×0.25 μm) (Agilent Technologies, USA). LAB-T 100 activation apparatus (Guangzhou Appinno Instruments Co., Ltd, China) and Milli-Q ultrapure water system (Millipore, USA) were also used in the experiment. PS and MS were purchased from Alfa Aesar (USA). XA and DPC were obtained from J&K Scientific Ltd. (China) and Nanjing Duly Biotech Co., Ltd (China), respectively. MSME, PSME, and PTC were purchased from Beijing Innochem Science & Technology Co., Ltd (China). DMC and phenol were obtained from Aladdin (USA). Anisole was purchased from Chengdu Kelong Chemical Reagents Co., Ltd (China). Acetone, methanol, and dichloromethane were all obtained from Fisher Scientific (USA). All of the above reagents are chromatographically pure. MPC (≥99%) was synthesized and purified by reference [29. 30] 2.2. Stir-bar sorptive extraction Prior to use of the stir bars, a cleaning step was firstly performed in 2 mL of methanol-dichloromethane (1:1, v/v) mixture and 2 mL of acetone under ultrasound for 5 min. Then, the stir bars were removed with tweezers, wiped dry with dust-free lens paper and cleaned in an activation apparatus under a high-purity N2 flow of 50 mL/min from 100 ℃ (hold for 20 min) to 300 ℃ (hold for 2 h). 10 mL of the sample solution, accurately measured, was transferred to a 25 mL conical flask, followed by addition of methanol of 10% (volume fraction). Four solid-phase extraction stir bars were adsorbed on a cleaned magnetic stirring rod and placed in the sample solution. The sample solution was then stirred with 1200 rpm at room temperature for 6 min. After extraction, the stir bars were taken out with tweezers, washed with distilled water to remove adhering matters (e.g. suspended matters, soluble salts and other adhering matters), dried with dust-free paper and put them into an empty stainless steel tube, which was then inserted into the heating cell of the thermal desorber for analysis. 2.3. Instrumental analysis Thermal desorption unit (TDU) conditions: A splitless mode was used. TDU was heated from room temperature to 300 ℃ at a rate of 100 ℃/min. The desorption time was 10 min, The desorption flow rate was 50 mL/min. CIS conditions: The analytes were cryofocused at −10 ℃ and then cryo-desorbed at 320 ℃ (heating rate was 100 ℃/s) for 5 min. The split ratio was 20:1. The valve and transmission line temperature was 200 ℃. GC-MS conditions: An HP-5 MS capillary column (30 m×0.32 mm×0.25 μm) was used. The carrier gas was high-purity helium at a flow rate of 0.8 mL/min. The oven was programmed from 140 ℃ (hold for 2 min) at 10 ℃/min to 320 ℃ (hold for 5 min). An EI source was used. The electron energy was 70 eV. The ion source temperature was 230 ℃, the transmission line temperature was 280 ℃, and the quadrupole temperature was 150 ℃. The mass scanning range was 45−550 (m/z). The resolution was 5000. The solvent delay was 2.1 min. A full-scan mode was employed for signal acquisition. 2.4. Preparation of standard curves A quantity of PS, XA, MS, PTC, MSME, and PSME was added into acetone to produce a mixed standard stock solution, respectively. Dilution was performed as needed to produce
corresponding standard solutions of 1, 2, 5, 10, 20, 50, and 100 ng/L, respectively. Mixed standard solutions were extracted by the established method, and then the stir bars were placed in empty stainless steel tubes for thermal desorption and analyzed under the optimal experimental conditions finally. The total ion chromatogram of a mixed standard solution is shown in Fig. 1. It can be seen from Fig. 1 that the six analytes were well separated. Standard curves were plotted according to concentrations and response values of the analytes. Linear correlation coefficients of the six byproducts were shown in Table 1. The results indicate each standard curve was well linear with r ≥0.997.
Fig. 1. The total ion current chromatogram of six kinds of mixed standard solutions. 1: MS; 2: MSME; 3: PS; 4: XA; 5: PSME; 6: PTC.
2.5. Quality Control New empty stainless steel tubes were cleaned on an activation apparatus under 100 mL/min and at 340 ℃ for 3 h. Used empty stainless steel tubes were cleaned at 340 ℃ for more than 1 h to remove sample residues. Activated empty stainless steel tubes were sealed by capping two ends immediately with polytetrafluoroethylene caps and stored in sealed bags or protective tubes at 4 ℃ in a box with activated carbon or a desiccator. Activated stainless steel tubes were used within two weeks. Prior to use, blank tests were carried out for stir bars and empty stainless steel tubes to ensure the absence of chromatographic peaks. Reagent blank test, sample blank test, and parallel sample test were carried out for each batch of samples. The relative standard deviations for the determination of target compounds in parallel samples should be less than 25% [31]. An intermediate-concentration control point was included in each batch of samples. The relative standard deviation between the measured value of the intermediate-concentration control point and the concentration of the corresponding point on the standard curve was controlled within 20%. In case the requirement was not met, the intermediate-concentration standard solution was re-prepared. If the requirement was still not met, a new standard curve was plotted. All the glassware used in the experiment was cleaned with ultrapure water, soaked in acetone for 1 h and then rinsed with ultrapure water. No detergent was used during the cleaning to avoid possible interferences introduced by the residues of detergent. Afterward, the glassware was dried in an oven at 100 ℃ for 1 h.
3. Results and Discussion 3.1. Selection of SBSE conditions
3.1.1. Effect of extraction time on the extraction efficiency Because the adsorption equilibrium between the solution and the stir bar is affected by the extraction time [32]. Therefore, five different extraction times (2, 4, 6, 8, and 10 min) was investigated. Each extraction time was tested 3 times with a stirring rate of 1200 rpm and a volume fraction of methanol of 5%. The total peak areas of the six byproducts obtained with different extraction times are compared in Fig. 2. As seen in Fig. 2, with the prolongation of the extraction time, the peak area of byproducts increased gradually. When the extraction time was 6 min, an adsorption equilibrium was reached and thus the peak area showed no significant change after 6 min. With the theory of equilibrium distribution and extraction efficiency taken into account, the extraction time was selected as 6 min.
Fig. 2. The effect of extraction time on the extraction efficiency with SBSE.
3.1.2. Effect of stirring rate on the extraction efficiency Stirring samples can increase the extract efficiency and shorten the extraction time. The effect is particularly pronounced for the extraction of substances with high diffusion coefficients and high molecular weights [33]. Therefore, five different stirring rates (400, 600, 800, 1000, and 1200 rpm) were evaluated. The experiment was completed with an extraction time of 6 min and a volume fraction of methanol of 5%. As shown in Fig. 3, with the stirring rate increased from 400 rpm to 1000 rpm, the total peak area of the byproducts increased gradually. Although the total peak area showed no significant change from 1000 rpm to 1200 rpm, 1200 rpm was more suitable than 1000 rpm due to its lower relative standard deviation. Therefore, 1200 rpm was selected in this study.
Fig. 3. The effect of stirring rate on the extraction efficiency with SBSE.
3.1.3. Effect of methanol on the extraction efficiency Using methanol as a dispersant for extraction is an effective route for improving the extraction efficiency of PDMS for weakly polar compounds [34]. Different volume fraction (0%, 5%, 10%, 15%, and 20%) of methanol were added to investigate the extraction efficiency of the six byproducts. As can be known from Fig. 4, the total peak area increased gradually with the increase of the volume fraction of methanol, reaching a maximum when the volume fraction of methanol was 10% and gradually declined as the volume fraction of methanol continued to increase. It is probably because the addition of methanol can increase the distribution coefficient of PDMS between acetone and byproducts, but a high concentration of methanol may cause byproducts to precipitate and increase the adsorption of the byproducts by the conical flask. Therefore, a volume fraction of methanol of 10% was selected in this experiment.
Fig. 4. The effect of volume fraction of MeOH on the extraction efficiency with SBSE.
3.2. Selection of ATSTD conditions 3.2.1. Effect of desorption temperature on the desorption efficiency Considering the boiling points of all the six transesterification byproducts are relatively high and that the coating of the stir bars requires an experimental temperature of ≤300 ℃ [35], the desorption efficiency of the six byproducts at 260, 280, and 300 ℃ was investigated with the desorption time maintained at 10 min, the desorption flow at 70 mL/min and the cryo-focusing temperature at −10 ℃. According to the results calculated on the basis of data shown in Fig. 5, the desorption efficiency of the byproducts was the highest at 300 ℃, which was 1.0 to 2.5 times as much as that at 260 ℃ and 280 ℃. So a desorption temperature of 300 ℃ was selected in the experiment.
Fig. 5. Influence of desorption temperature on the desorption efficiency with ATSTD.
3.2.2. Effect of the desorption time on the desorption efficiency An inadequate desorption time will lead to incomplete desorption, resulting in decreased desorption efficiency of extracts. Therefore, the effect of different desorption times (6, 8, 10, and 12 min) on the desorption efficiency was investigated with the desorption temperature maintained at 300 ℃, the desorption flow at 70 mL/min and the cryo-focusing temperature at −10 ℃. According to the results calculated on the basis of data shown in Fig. 6, the desorption efficiency was 74.7% and 87.3% when the desorption time was 6 min and 8 min, respectively. When the desorption time was 10 min, the desorption efficiency reached up to 99.6%. When the desorption time was 12 min, the desorption efficiency was equivalent to that of 10 min. The results suggest that the desorption time of 10 min is sufficient for thermal desorption of the six byproducts, and thus a desorption time of 10 min was selected.
Fig. 6. Influence of desorption time on the desorption efficiency with ATSTD.
3.2.3. Effect of cryo-focusing temperature on the desorption efficiency The cryo-focusing temperature is an important factor influencing the desorption efficiency of each compound. For compounds with different boiling points, different trapping temperatures are required, and the desorption efficiency may also change. So the effect of different cryo-focusing temperatures (−30, −20, −10, 0, and 10 ℃) on the desorption efficiency was investigated. The results showed that MS and MSME with relatively low boiling points were better trapped in the cold trap at low temperatures, while better desorption effects were exhibited for PS, XA, PSM,
and PTC at relatively high temperatures. As a compromise, a cryo-focusing temperature of −10 ℃ was selected. 3.3. Limit of detection, precision, and accuracy of the method and analysis of practical samples Under the optimum conditions, the sensitivity of the method was evaluated through LODs and limits of quantitation (LOQs), which were calculated on the basis of a signal-to-noise ratio of three and ten, respectively. RSD and recoveries were used to evaluate the precision and accuracy of the method, respectively. As shown in Table 1, LODs for the six byproducts ranged from 0.054 ng/L to 0.253 ng/L. LOQs ranged from 0.180 ng/L to 0.843 ng/L. Spiked blank samples containing the six byproducts (1.0, 10, and 100 ng/L) were analyzed, and the RSD ranged between 5.2% and 11.5%. Recovery experiment was carried out on practical samples spiked with 5, 20, and 80 ng/L of analytes were added, respectively. Both practical and spiked samples were analyzed by the same method. Recoveries of the six byproducts were between 81.6% and 102.6%. Moreover, the results showed that the average extraction efficiency and desorption efficiency were 99.8% and 99.7%, respectively. In addition, the practical samples from the transesterification with different reaction temperatures and pressures were quantitatively analyzed by the method. The results were presented in Fig. 7. As seen in Fig. 7 (A) and Fig. 7 (B), six byproducts with different concentrations were detected in all samples. The results also showed that high pressure and high temperature were all beneficial to the formation of the six transesterification byproducts. The total ion chromatogram of the sample is shown in Fig. 7 (C). The result also revealed that phenol, DMC, byproduct anisole, intermediate product MPC, and DPC did not interfere with the determination of the six byproducts. This method is suitable for the simultaneous and selective determination of the six trace byproducts from transesterification between DMC with phenol. Table 1 Correlation coefficient, LOD, LOQ, relative standard deviations, and recoveries. RSD (%) Correlation
LOD
LOQ
coefficient
(ng/L)
(ng/L)
Compound
Recoveries (%)
1.0
10.0
100
5.0
20.0
80
ng/L
ng/L
ng/L
ng/L
ng/L
ng/L
PS
0.999
0.077
0.257
6.8
5.2
7.1
98.7
96.5
89.0
XA
0.998
0.093
0.310
5.7
6.6
8.3
86.2
102.6
100.8
MS
0.999
0.205
0.683
11.2
9.4
8.9
101.7
89.9
96.9
MSME
0.998
0.186
0.620
10.1
11.5
6.9
99.3
100.2
87.4
PSME
0.997
0.253
0.843
9.6
8.8
8.0
84.7
81.6
99.0
PTC
0.998
0.054
0.180
7.0
5.7
6.4
101.2
88.3
93.3
4. Conclusion In this paper, six trace byproducts, that cannot be directly detected by GC-MS, in the synthesis of DPC via transesterification of DMC with phenol were first determined by SBSE-ATSTD-GC-MS. The main factors influencing SBSE and ATSTD were investigated. Under the optimal conditions, the method can detect the six byproducts at ng/L levels in the transesterification. The established method is simple, rapid, accurate, sensitive with high extraction efficiency, desorption efficiency, wide linear range, and good selectivity. It is suitable for the simultaneous and rapid determination of six trace and even ultratrace byproducts in the pilot-scale process of DPC synthesis. This work can also provide a theoretical and data basis for
studying and controlling the transesterification byproducts produced by Fries rearrangement and methylation reaction in the synthesis of DPC.
Fig. 7. The effects of temperature (A), pressure (B) on the byproducts from transesterification between phenol and DMC, and the total ion current chromatogram of the byproducts from transesterification (C). 1: DMC; 2: anisole; 3: phenol; 4: MPC; 5: MS; 6: MSME; 7: DPC; 8: PS; 9: XA; 10: PSME; 11: PTC. Reaction conditions: (A) Phenol: 15 g; n(phenol) = n(DMC); catalyst (dibutyltin oxide): 0.30 g; reaction time: 8 h; reaction pressure: 0.30 MPa. (B) Phenol: 15 g; n(phenol) = n(DMC); catalyst (dibutyltin oxide): 0.30 g; reaction time: 8 h; reaction temperature: 220 ℃.
Declaration of competing interest The authors declare that they have no conflict of interest.
Acknowledgments This work was supported by the Major Demonstration Program of Innovation Academy for Green Manufacture, Chinese Academy of Sciences (IAGM-2019-A10), the Science and Technology Demonstration Program for Transformation of Sichuan Province (No. 18ZHSF0011), the National High-tech R&D Program of China (863 Program, No. 2013AA031703), the National Key R&D Program of China (No. 2016YFB0301900), the Science and Technology Support Program of Sichuan Province (No. 2018JY0615), and the Science and Technology Innovation Program for
Youth Team of Sichuan Province (No. 2013TD0010).
Declaration of competing interest The authors declare that they have no conflict of interest.
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