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A Novel Strategy for the Synthesis of 1,2-Dichlorotetrafluorocyclobutene from Hexachlorobutadiene and Its Reaction Pathway Xiaomeng Zhou, Pingli Zhang, Jinwei He, and Biao Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01166 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Industrial & Engineering Chemistry Research

A Novel Strategy for the Synthesis of 1,2-Dichlorotetrafluorocyclobutene from Hexachlorobutadiene and Its Reaction Pathway Xiaomeng Zhoua,*, Pingli Zhangb, Jinwei Heb, Biao Zhouc a

b

Economics And Management College, Civil Aviation University of China, Tianjin 300300, China

The College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China c

The University of Tokyo, Tokyo 113-8656, Japan *E-mail: [email protected]

ABSTRACT: In this paper, a novel strategy for the preparation of 1,2-dichlorotetrafluorocyclobutene (DTB) was proposed via a catalytic gas-phase process of fluorination using hexachlorobutadiene (HCBD) and anhydrous HF. In order to search for suitable catalysts and reveal the reaction pathway for this synthetic route, a series of studies were carried out. Firstly, CrOx/ZnO catalysts with different promoters (Ni, Cu, In, Al) were prepared by a precipitate method and the optimum reaction conditions were investigated. The highest activity was achieved on the Cr-Ni-Zn catalyst, whose yield of DTB reached 90% by a multiple cycle reaction. Secondly, the effects of different promoters on the properties of catalysts were studied by BET, SEM, XRD, NH3-TPD, and XPS. It was found that the Cr-Ni-Zn catalyst showed the excellent catalytic performances with more CrOxFy species, higher oxygen concentration and widely distributed acid strength on its surface. Thirdly, combining experimental results with theoretical calculations, a reaction pathway has been proposed. This study offers an economic synthetic route for DTB from HCBD, which is a valuable and promising method for an industrial production.

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Keywords: catalytic fluorination, gas-phase reactions, 1,2-dichlorotetrafluorocyclobutene, hexachlorobutadiene, reaction pathway

1. INTRODUCTION Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) were once widely used in many industries, but most of them had been phased out due to their ozone depletion potential (ODP) and high global warming potential (GWP).1,2 Currently, cyclic fluorocarbons are considered as the most promising substitute on account of their short atmospheric life-time, zero ODP and low GWP.3,4 1,2-Dichlorotetrafluorocyclobutene (DTB), a cyclic fluorocarbon, can be widely applied in pharmaceuticals, agrochemicals, and the syntheses of fluorine-containing materials as a raw material or intermediate.5-8 Thus, special attentions have been paid to the manufacture of DTB in industry.

The synthesis of DTB is far more complicated than that of HFCs due to its cyclic structure and double bond. To date, several synthetic methods have been reported for DTB. For example, 2,3-dichloro-1,1,4,4-tetrafluorobutadiene is heated at 176 ℃ for 89 h to prepare DTB.9 However, the yield of DTB in this reaction has not been reported, and the raw material is difficult to obtain, so the application of this route is quite limited in industry. DTB can also be generated by the co-dimerization of 2-chloropropene and 1,1-dichloro-2,2-difluoroethylene at 180 ℃, but the yield of DTB is only 3.6%.10 Alternatively, 3,3,4,4-tetrafluoro-1,1,2,2-tetrachlorocyclobutane, which is prepared by the dimerization of 1,1-dichloro-2,2-difluoroethylene, can be dechlorinated with zinc in n-butanol to give DTB.11,12 The yield of DTB is 80%-85%, which seems feasible for industry, but the high cost of the

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starting material and the complicated route remain problematic. Therefore, a low-cost and efficient synthetic route to DTB is desirable in industry.

Hexachlorobutadiene (HCBD) has been widely used as solvent in the production of rubber and other polymers. However, because HCBD is highly toxic and a persistent organic pollutant (POP), it was banned for use as a final product by the Stockholm Convention in 2013. Consequently, there is a large overcapacity of HCBD in market. Interestingly, it is theoretically possible to produce DTB from HCBD by a cyclization and Cl/F exchange reaction. Until now, however there are no reports about this method in the literature. Moreover, as can be predicted from the possible reaction pathways and potential product distribution, the reaction system is complicated and may be extremely complex when the isomers of the several possible intermediates are taken into account. Therefore, it is necessary to screen appropriate catalysts for this synthetic route. Chromium-based catalysts are widely used in the gas-phase fluorination reactions.13-18 In general, other elements, such as Zn,19 Ni,20 In,21 Cu,22 and Al23,24 added in small amounts to the Cr-based catalyst, could improve the catalytic properties by promoting the total activity related to the Cl/F exchange and inhabiting the contribution of side reaction. It has been established that the presence of Zn on chromium oxide can perturb HF adsorption, increase the dispersion of chromium and fluorinated active species, and lower apparent activation energies involved in the Cl/F exchange.19,25

In this study, the CrOx/ZnO catalysts with different promoters (Ni, Cu, In, Al) were prepared by precipitation method, and reaction conditions were investigated through a series of experiments.

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Subsequently, the effects of different promoters on the properties of catalysts were studied by BET, SEM, XRD, NH3-TPD, and XPS. Furthermore, by combination of the experimental results and theoretical calculations at CCSD/cc-pVDZ//B3LYP/6-311++G(d,p) level using Gaussian 03 package, a possible reaction pathway can be identified. This study not only offers a valuable and economic synthetic route to produce DTB in industry, but also convert a kind of overcapacity pollutants (HCBD) to a useful product (DTB).

2. EXPERIMENTAL SECTION

2.1 Chemicals 1,1,2,3,4,4-Hexachloro-1,3-butadiene (HCBD) (98%) was obtained from Letai chemical industry Co. Ltd., Tianjin of China. Anhydrous HF (AHF) (>99.9%) and nitrogen gas (>99.9%) were bought from Beijing North Oxygen Specialty Gases Institute Co., Ltd., Beijing of China. NH3·H2O (30%) was gotten from Beijing Chemical Works Co., Ltd. (china). Analytical grade CrCl3·6H2O (>99%), Zn(NO3)2·6H2O (>99%), Ni(NO3)2·6H2O (>99%), Cu(NO3)2 (>99%), In(NO3)3·5H2O (>99%), Al(NO3)3·9H2O (>99%) were purchased from Xilong Chemical Co. Ltd., Guangxi of China. 2.2 Catalysts preparation and activation The CrOx/ZnO catalysts with different promoters (Ni, Cu, In, Al) were prepared by co-precipitation method. The molar ratio of Cr and Zn was 95:5 in the CrOx/ZnO catalyst and the molar ratio of Cr, Zn and promoter was 95:2:3 in the catalysts. Analytical grade metal salts were firstly dissolved in the distilled water until molarity of the total metal ions equal to 0.1mol/L in solution. Then, 30% NH3·H2O was added to the above prepared solution under continuous stirring 4


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until the pH reached 9.0. Subsequently, the precipitated solid was filtered, washed with deionized water and dried at 110 ℃ for 24 h in a drying oven. Finally, the dried solid was ground, mixed with 2 wt.% graphite, and pelleted into cylindrical wafers (diameter: 2 mm; h: 3 mm). The precursors were formed by calcining the pellets at both 250 ℃ for 10 h and 400 ℃ for 10 h with the N2 flow rate of 150 mL/min.

Before reaction, pre-fluorination was carried out to activate precursors. A 20 g sample of the precursor was packed into the reactor. A mixture of N2 (100 mL/min) and AHF (50 mL/min) was passed through the reactor at 150 ℃ for 10 h. Then, the N2 flow rate dropped to 50 mL/min and the AHF flow rate increased to 100 mL/min at 250 ℃ for 10 h. Subsequently, the N2 flow was stopped and the sample was heated at 250 ℃ for 10 h in AHF at a flow rate of 150 mL/min. Finally, the pre-fluorination catalysts, which were denoted as Cr-Zn, Cr-Ni-Zn, Cr-Cu-Zn, Cr-In-Zn, and Cr-Al-Zn, respectively, were formed.

2.3 Characterization The adsorption-desorption isotherm of nitrogen was measured by a Micro-meritics ASAP 2020 automated gas sorption system at -196 ℃, after the sample was degassed under vacuum at 300 ℃ for 3 h. The specific surface areas of all samples were calculated by the BET method, and the average pore diameters were determined by the Barrett-Joyner-Halenda (BJH) method.

The XRD patterns of the samples were recorded on a Rigaku D/max-2500 powder diffractometer using Cu Kα radiation (40 kV and 100 mA) in the 2θ range from 10 to 80° with a scan rate of 0.3° min-1. 5



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SEM analyses were performed in order to observe microstructure of the samples using a JSM-7500F microscope.

The surface acidity of the catalysts was measured by NH3-TPD, which was conducted in a quartz U-shaped reactor and monitored by an on-line chemisorption analyzer (Quantachrome Chem Bet 3000). A 50 mg portion of the catalyst was pre-treated at 400 ℃ for 1 h in He flow (30 mL/min), then cooled to 100 ℃ and finally saturated with 5% NH3/He. The sample was subsequently then purged with He for 30 min to eliminate all physically absorbed ammonia, followed by heating of the sample up to 600 ℃ with a ramp of 10 ℃/min. XPS measurements were acquired on a Kratos Axis Ultra DLD multi-technique X-ray photoelectron spectrometer (UK) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV). All XP spectra were recorded using an aperture slot measuring 300×700 microns. Survey and high resolution spectra were recorded with a pass energy of 160 eV and 40 eV, respectively. Accurate ±0.2 eV binding energies were determined with respect to the position of the adventitious C 1s peak at 284.6 eV. 19

FNMR spectra were recorded on a Bruker AV400 instrument at 400 MHz with CFCl3 as an

internal standard.

GC-MS was carried out on a Shimadzu-QP 2010 Ultra series system equipped with a jet separator for the 2010 GC. The capillary column was DB-5 with 0.25 mm i.d. and 30 m length from J&W Scientific Inc.

GC was carried out under the following operating conditions. The capillary column was DB-5 6


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with a 0.25 mm i.d. and 30 m length from J&W Scientific Inc. The column temperature was set at 35 ℃ for 3 min and then heated up to 200 ℃ at a rate of 10 ℃/min, at which was held for 3 min. The injector and detector temperatures were set at 280 ℃ and 200 ℃, respectively. The split ratio was 80:1 and the sample size was 0.1uL.

The apparatus for the gas-phase fluorination reaction was composed of a pump for transferring HCBD (liquid phase), mass flow controllers to control HF and N2, and an electrically heated tubular Inconel reactor (14 mm in diameter and 300 mm in length) equipped with an inner Inconel tube for the insertion of type-K thermocouples with a 1-mm diameter. The thermocouple enters the reactor through a Monel-type fitting and extends into the catalyst bed to measure the temperature changes in different positions along the reactor.

2.4 The synthesis of DTB from HCBD The amount of 5g catalysts prepared by the co-precipitation were put into the reactor. HCBD and HF were then vaporized at 150 ℃ and passed through the stainless steel reactor in the range of 350~410 ℃ with a 12 s contact time. The molar ratio of HF: HCBD was 7:1, as carefully controlled by the HCBD dosage via a Masterflex metering pump and HF dosage using a mass flow controller. The products were collected after being cooled with an ice bath, washed with a dilute KOH solution, and dried with anhydrous sodium sulphate and molecular sieves (4Å). The obtained products were analyzed by GC, GC-MS and 19FNMR (see Supporting information).

As shown in scheme 1, the reaction of HCBD with AHF in the presence of catalysts led to the production of DTB, C4Cl3F3 (1), C4Cl4F2 (2) and C4Cl5F (3). Substances 2 and 3 may include some 7



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isomers. To increase the yield of DTB, the reaction products were recycled through the reactor until the yield of DTB showed no further increase. The collected products were then distilled for high purity DTB.

2.5 Computational method

In order to investigate the reaction pathway of HBCD and HF, theoretical calculations were performed by Gaussian 03 program packages. The geometries of the reactants, products, and transition states were optimized at the B3LYP/6-311++G(d,p) level.26,27 Harmonic vibrational frequencies have been calculated at the same level to determine the nature of the various stationary points and the zero-point vibrational energies (ZPVEs). The number of imaginary frequencies indicates whether a minimum or a transition state has been located. Connections between reactants, transition structures and products were confirmed by the intrinsic reaction coordinate (IRC) calculations at the B3LYP/6-311++G(d,p) level.28,29 The single-point calculations were performed at the coupled-cluster level of theory with the single and double excitations (CCSD) using a diffuse functions basis set cc-pVDZ in order to improve the accuracy of the energies, especially the relative energies of barrier heights.30

3. RESULTS AND DISCUSSION

3.1 Catalytic performance

In order to study catalytic performance, HCBD and AHF were fed into the reactor with no catalyst appearance, the experimental results showed that no DTB was detected in the reaction products. Then, catalysts were put into the reactor and the optimal reaction conditions were 8


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investigated (see reference31 published by our research group). When the molar ratio of HF:HCBD is 7:1, the contact time is 12 s, and the reaction temperature remains at 390 ℃, the conversion of HCBD and the selectivity to DTB are preferable.

Among these investigated catalysts (Fig. 1), the Cr-Ni-Zn catalyst exhibited the best catalytic performance (73% conversion of HCBD and 38% selectivity of DTB at 390 ℃). Although the Cr-In-Zn catalyst also improved the conversion of HCBD (65%) greatly in comparison with the Cr-Zn catalyst, the selectivity of DTB (17%) was unsatisfactory. Moreover, the Cr-Al-Zn and Cr-Cu-Zn catalysts showed almost no improvement on the catalytic activity. The product distribution on different catalysts (Table S1) also showed that the Cr-Ni-Zn catalyst has the best properties. Besides that, substances 1, 2, 3 can be further fluorinated to produce DTB so that products are recycled in the reactor to obtain a higher yield of DTB.

The yield of DTB was increased through multiple recycled reactions (Table S2). In a single pass, the yield of DTB was only 27.7%. By the second recycled reaction, the yield of DTB rose up to 49.6% and after the third cycle, the yield of DTB increased more to 65.7%. DTB was separated by distillation after the third cycle. The high boiling products could react with HF again in the reactor. Through the reactions for multiple cycles, the yield of DTB can increase to beyond 90% over the Cr-Ni-Zn catalyst.

Meanwhile, to prove the feasibility of the Cr-Ni-Zn catalyst in industry, the time-on-stream of the Cr-Ni-Zn catalyst was assessed for 800 h (Fig. S1). The conversion of HCBD just slightly declined at the initial stage of reaction, followed by a steady state after 24 h. After 800 h, the

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conversion of HCBD would decrease to approximately 2/3 of the initial value. Surprisingly, the selectivity to DTB was almost unchanged in the all time-on-stream process, indicating that the Cr-Ni-Zn catalyst can maintain its high performance.

3.2 Catalyst characterizations

3.2.1 Structure analyses Table 1 lists the surface areas and average pore diameters of precursors and pre-fluorinated catalysts with HF. The addition of promoter Ni to the Cr-Zn precursor led to a reduction in the surface areas and an increase in the pore size, which occurred the same with addition of Cu, In, and Al. After pre-fluorination with HF, the surface area of Cr-Zn (97.6 m2/g) and Cr-Ni-Zn (78.2 m2/g) catalysts decreased slightly. However, the surface area of Cr-Cu-Zn (63.2 m2/g), Cr-In-Zn (34.4 m2/g), Cr-Al-Zn (44.4m2/g) catalysts reduced remarkably. Besides that, the Cr-Ni-Zn catalyst had the largest average pore size. Therefore, the Cr-Ni-Zn catalyst, with high surface area and large pore size, should be beneficial to conversion and selectivity.

The XRD patterns of all precursors (Fig. S2) were almost identical, and no diffraction peaks related to promoters could be detected, which indicated that the addition of promoter dose not change the crystalline structure of the precursors. After pre-fluorination with HF (Fig. 2), a crystalline CrF3 phase was detected in samples.32 The Cl/F exchange reactions that occur over chromium oxides require pre-fluorination that involves considerable structural and chemical changes and O/F exchanges on the catalyst surface.33 As usual, the Cr2O3-lattice is difficult to disorder by HF incorporation for the Cr-based catalysts.34 The appearance of CrF3 in samples proved that the 10


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promoters are beneficial to the reaction between Cr2O3 and HF. Complementary characterization by SEM analyses (Fig. S3) also showed that the catalysts after pre-fluorination had better crystallines, which can be explained by the appearance of CrF3. Moreover, the formation of the crystalline CrF3 also can result in the decline of catalyst surface area after pre-fluorination.

3.2.2 Surface Acidity NH3-TPD was used to investigate the acid strength of the catalysts’ surface. As can be observed from Fig. 3, both Cr-Zn and Cr-Al-Zn had one peak at 350 ℃, which indicates medium-strength acid.35 Cr-Cu-Zn had two peaks at 225 ℃ and 350 ℃ , which indicate weak acid and medium-strength acid, respectively. Cr-In-Zn had a small peak at 210 ℃, which indicate weak acid. A broad desorption profile in the range of 150-450 ℃ was found on the Cr-Ni-Zn surface with a maximum peak at around 250 ℃, which suggest a wide distribution of weakly acidic sites on the Cr-Ni-Zn catalyst. It can be obviously found that medium-strength acid predominated on the surface of Cr-Zn, Cr-Al-Zn and Cr-Cu-Zn, while Cr-Ni-Zn and Cr-In-Zn had more weak acid.

The syntheses of DTB from HCBD and HF was involved in Cl/F exchange and cyclization reactions, which required that catalysts possess different acid strength. It is obvious that the Cr-Ni-Zn catalyst consisted of a wide distribution of weakly acidic sites, which seem advantageous for fluorination of HCBD.

3.2.3 Surface composition To obtain further insight into the properties of catalysts, the compositions of catalyst surfaces were measured by XPS (Table 2). It can be observed that the oxygen concentration decreases with 11



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increasing fluorine concentration, since Cr-O bonds are replaced by Cr-F bonds when precursors are treated with HF at high temperatures.23,34 There are three main types of surface fluoride present in the catalytic reaction: HF (weakly adsorbed), labile F, and CrIII-F. In these three surface fluorides, the labile F, which is formed initially by breaking the H-F bond of weakly adsorbed HF, plays a key role in the catalytic activity. Both the weakly adsorbed HF and labile F are deactivated by the conversion to the CrIII-F bond, which is an irreversible process unfortunately. The presence of excess oxygen prevents the labile HF from converting irreversible CrIII-F, which is beneficial to fluorination reaction.32,34,36 From Table 2, it can be seen that Cr-Ni-Zn is of more advantages due to its highest oxygen concentration.

The chemical states of chromium in catalysts were detected by XPS (see Fig. 4). For Cr-Zn, Cr-Ni-Zn, Cr-Cu-Zn and Cr-Al-Zn samples, the Cr2p profiles are similar and can be resolved into three peaks at 576.8, 578.4 and 581.7 eV, which are assigned to Cr2O3, CrOxFy and CrF3/CrF3·3H2O, respectively.32 Many studies have demonstrated that CrOxFy plays an important role in providing active sites for the Cl/F exchange reactions.37-39 As observed in Table S3, Cr-Ni-Zn has the largest proportion of CrOxFy (80.7%). These results further confirm that the Cr-Ni-Zn catalyst was suitable for preparation DTB.

3.3 The reaction pathway Fig. 5 depicts a plausible reaction pathway for the synthesis of DTB using HCBD and HF over the Cr-Ni-Zn catalyst. The catalytic gas-phase fluorination process of HCBD is obviously involved in Cl/F exchange reaction and cyclization reaction. Considering HCBD is of stability and no

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self-cyclization reaction even at 500 ℃,40,41 it seems reasonable that a Cl/F exchange rather than a cyclization reaction to HCBD takes place. Furthermore, our experimental results proved that no new products were detected when HCBD was exposed to the Cr-Ni-Zn catalyst at 390 ℃ without HF. In contrast, when hexachlorocyclobutene was heated to 195 ℃, HCBD from the ring opening reaction was found in the products.42 This proved that HCBD was more stable than hexachlorocyclobutene. In order to realize the cyclization of HCBD more clearly, the energy barrier and reaction enthalpy were calculated at the B3LYP/6-311++G(d, p) level by Gaussian 03 package. The calculation results showed that the barrier height for the HCBD cyclization was 34.68 kcal/mol, and that the barrier height for the hexachlorocyclobutene ring-opening reaction was 32.14 kcal/mol. All these results validated that the cyclization of HCBD is difficult to take place during the DTB synthesis process.

NMR analyses indicated that the Cl/F exchange primarily occurred at the terminal CCl2 groups rather than the inner CCl group due to the steric influence of the Cl atoms. As a result of the large covalent and van der Waals radius of the Cl atom, HCBD exists in a gauche configuration. Consequently, the positively charged carbon atoms in the middle of the molecule are entirely surrounded by electron-dense chlorine atoms, which prevents the Cl/F exchange reaction from taking place.43,44

After the generation of fluorine-containing dienes by the Cl/F exchange, these dienes will undergo cyclization reactions. Although an intra-molecular cyclization has been known in the cases of hydrocarbon dienes and polyenes, only a few researchers have investigated halocarbon cyclizations. An issued patent implied that conjugated di-olefins, containing at least one fluorine on the unsaturated carbon atoms, can cyclize at temperatures above 400 ℃.9 The literature also 13



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confirmed that some perfluorinated dienes were heated to give cyclic compounds owing to an intramolecular cyclization of diene with an excess of Lewis acid catalyst.45 However, none of them mentioned details about fluorochlorocarbon cyclization.

To probe the cyclization of fluorinated dienes in the DTB synthesis, the geometrical parameters of HCBD, possible intermediates and products were optimized at the B3LYP/6-311++G(d, p) level with the Gaussian 03 package (Fig. S4). Linear structures and their isomers, cyclic structures and their isomers were denoted as L and L′, R and R′, respectively. The barrier height and enthalpy change of HCBD and possible cyclization intermediates were calculated, as shown in Fig. S5. Cyclic products were generated by the con-rotatory reaction of linear structures in accordance with the Woodward-Hoffmann rules and frontier orbital theory.46 It is obvious in Fig. S5 that the number and position of the Cl atoms have an important impact on the cyclization barrier height, which are 2.53, -0.81, -3.96, -4.20, -5.71, -7.74 kcal/mol for HCBD, C4Cl5F, trans-C4Cl4F2, cis-C4Cl4F2, C4Cl3F3, C4Cl2F4 , respectively. It proved that the ring structures became preferably stable with increasing number of fluorine atoms. Therefore, the cyclization of HCBD and C4Cl5F was more difficult than that of C4Cl4F2 and C4Cl3F3. Moreover, the stability of the ring structure was related to its steric effects. Compared the bond strain in DTB with that of HCBD, the presence of fluorine strengthened the 4C-2C bond, while chlorine had the opposite effect due to its significant steric hindrance (PBE0/6-311++G(d, p)).47 As a result, the highly fluorinated olefins ring structure was favorable, but the linear structure was preferable for the highly chlorinated olefins, and this was consistent with the 4C-2C bond distance. Therefore, DTB is more stable than other ring structures at high temperatures.

Moreover, the cyclization of fluorinated dienes is affected by the steric effect from the Cl atoms 14


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and by the high electronegativity of fluorenes as well. Therefore, C4Cl2F4 is more difficult to get cyclized when compared with C4Cl3F3 and C4Cl4F2. According to the above discussions, HCBD first undergoes a Cl/F exchange reaction to yield C4Cl4F2 and C4Cl3F3. Then, cyclization reactions are going to take place for the formation of c-C4Cl4F2 and c-C4Cl3F3. A further Cl/F exchange reaction of c-C4Cl4F2 and c-C4Cl3F3 will lead to DTB successfully. Although it is showed here that the properties of the catalyst play an important role in the DTB production, the detailed mechanism requires further studies.

4. CONCLUSIONS In order to successfully produce DTB from HCBD via a catalytic gas-phase fluorination method, a series of the CrOx/ZnO catalysts with different promoters (Ni, Cu, In, Al) had been studied. The experimental results showed that the Cr-Ni-Zn catalyst exhibited the best catalytic performance，and the yield of DTB could reach beyond 90% by multiple-cycle reactions. To prove the feasibility of the Cr-Ni-Zn catalyst in industry, the time-on-stream of the Cr-Ni-Zn catalyst was assessed for 800 h. The selectivity to DTB was almost unchanged and the conversion of HCBD was decrease slightly, which proved that the Cr-Ni-Zn catalyst has wide application potentials in industry. Furthermore, in order to recognize structural properties, the prepared catalysts were characterized by means of BET, SEM, XRD, NH3-TPD, and XPS. It revealed that the Cr-Ni-Zn catalyst, when compared to the Cr-Al-Zn, Cr-Cu-Zn, and Cr-In-Zn catalysts, was of the highest surface area, largest pore size, more CrOxFy species, the highest oxygen concentration and widely distributed acid strength, all of which were beneficial to the catalytic activity. Moreover, it was found that the reaction pathway included

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three reactions based upon experimental results and theoretical calculations. The first was the Cl/F exchange of HCBD to form C4Cl4F2 and C4Cl3F3. The second was the cyclization of C4Cl4F2 and C4Cl3F3 to produce c-C4Cl4F2 and c-C4Cl3F3. The third was the further Cl/F exchange of c-C4Cl4F2 and c-C4Cl3F3 to get DTB. This study was the key to the synthetic technology of DTB, and was the important impact of large-scale industrial application.

NOTES

The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (No. 51176078).

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI:

The data of GC-MS and

19

FNMR of products, the yields of products with different catalysts

(Table S1), the yields of DTB through three-times reaction with the Cr-Ni-Zn catalyst (Table S2), amount of Cr species of different components on catalysts (Table S3), stability of Cr-Ni-Zn for conversion of HCBD and DTB selectivity (Fig. S1), XRD patterns of different precursors (Fig. S2), electron micrographs of the Cr-Ni-Zn catalyst (Fig. S3), optimized structures and geometrical parameters of various species (Fig. S4), the barrier heights and enthalpy changes for the cyclization of fluorinated dienes (Fig. S5).

17



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C.;

Fadli,

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Sawerysyn,

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Experimental

Study

on

the

Thermal

Oxidation

of 20


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Page 22 of 30

Scheme 1. Reaction products for the reaction of HCBD with HF.

Table 1 Physical properties of precursors and catalysts pre-treated with HF.

Samples

Catalysts pre-treated

Precursors

with HF Surface area

Pore size

Surface area

Pore

(m2/g)

(Å)

(m2/g)

size (Å)

Cr-Zn

134.4

55.9

97.6

53.5

Cr-Ni-Zn

128.4

66.2

78.2

83.3

Cr-Cu-Zn

147.9

51.3

63.2

71.7

Cr-In-Zn

159.9

26.7

34.4

36.0

Cr-Al-Zn

178.3

24.4

44.4

29.7

22


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Table 2 Surface composition analysis of catalysts by XPS.

Samples

Fresh catalysts % (molar content) Cr

F

O

Zn

promoter

Cr-Zn

9.7

39.0

50.9

4.5

0

Cr-Ni-Zn

11.5

20.8

65.0

1.5

1.2

Cr-Cu-Zn

16.2

26.4

55.3

1.6

1.1

Cr-In-Zn

11.4

25.2

60.7

1.7

1.0

Cr-Al-Zn

18.9

28.2

49.8

1.7

1.4

23



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Figure Captions Fig. 1 The effects of promoters on the catalytic activity in different temperatures (HF:HCBD = 1:7, contact time = 12 s). Fig. 2 XRD patterns of the catalysts pre-treated with HF. Fig. 3 NH3-TPD profiles of catalysts pre-treated with HF. Fig. 4 XP spectra of the Cr2p level for catalysts pre-treated with HF. Fig. 5 Overview of a plausible reaction pathway. (Note: black arrows: Cl/Fl exchange reactions; pink arrows: cyclizations; dashed arrows: no reaction; green: cis-trans isomers; numbers: barrier height (kcal/mol).)

24


Page 25 of 30

Figure 1

80

40

70

35

60

30

50 40 30

Cr-Zn Cr-Ni-Zn Cr-Cu-Zn Cr-In-Zn Cr-Al-Zn

20 10

Selectivity of DTB/%

Conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 20 15 10 5

0 340 350 360 370 380 390 400 410 420

0 340 350 360 370 380 390 400 410 420

Temp ( )

Temp ( )

25



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Page 26 of 30

Figure 2

26


Page 27 of 30

Figure 3

Cr-Ni-Zn

Cr-In-Zn

Rate of NH3 desorption/a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr-Cu-Zn

Cr-Al-Zn

Cr-Zn

150

200

250

300

350

400

450

500

Temperature/

27



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Page 28 of 30

Figure 4

Cr-Zn

Cr-Ni-Zn

Cr-Cu-Zn

Cr-In-Zn

Cr-Al-Zn

570

575

580

585

590

595

Binding energy (eV)

28


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Figure 5

Cl

Cl

Cl Cl

Cl Cl

CClF=CClCCl=CCl2

CClF=CClCCl=CClF

Cl

Cl

Cl Cl Cl

Cl F Cl

Cl

Cl F Cl

F

Cl 34.68

32.14 Cl Cl

Cl Cl Cl

Cl

F/Cl

Cl

Cl

Cl

F

Cl

Cl

F

F Cl

F Cl Cl

Cl Cl

F

F Cl

Cl CClF=CClCCl=CF2

+HF

Cl

F F

CCl2 FCHClCCl=CCl2

F

Cl

Cl

Cl

F

F

F Cl

F F

F

F

F

29



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For Table of Contents Only

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