English

• 

  0    Introduction

  The nature of the active species and their possible transformation during the reaction remain unclear. Firstly, CrOxFy compounds originated from Cr(Ⅵ) are generally recognized as the active species while Cr₂O₃ are inactive^[23]； while other researches pointed out that low valent Cr species such as Cr₂O₃ also have high reactivity. For example, our previous work revealed that crystalline Cr₂O₃ obtained at high calcination temperature also showed high F/Cl excha-nge reaction activity^[28]. Therefore, the identification of the contributions of Cr species is desirable to obtain a better understanding of the different roles in the reaction, which will be ideal if a quantitative analysis could be conducted. Secondly, in addition to the loss of volatileCrO_xF_y compounds, the transformation mechanism of CrO_xF_y under working conditions and the resulting compounds need further clarification.

  To illustrate the different roles of Cr species in the F/Cl exchange reaction, delicately designed catalysts are necessary. The currently employed Cr catalysts are usually obtained by precipitating Cr precursors such as Cr(NO₃)₃ with further calcination. The resulting samples often contain a mixture of CrOx oxides with different oxidation states, which leads to complexity in the investigation. In this work, we prepared two catalysts with distinct Cr₂O₃ and CrO₃ compositions which represent low and high valent Cr species. The comparison of the catalytic behaviors of these two catalysts provided useful information on the roles of different Cr species in the F/Cl exchange reaction. Moreover, the catalyst stability was investig-ated and the transformation of Cr species during the reaction was also discussed.

  The catalytic gas phase F/Cl exchange reaction is very important in the synthesis of new generation refrigerants^[1-6], in which Cr-based catalysts are comm-only employed^[7-10]. For chromium oxides, Cr species mainly exist in forms of Cr(Ⅵ) and Cr(Ⅲ), such as CrO₃ and Cr₂O₃^[11]. The Cr-based catalysts for the vapor phase synthesis of hydrofluorocarbons mainly include unsu-pported chromium oxides and those supported on various materials^[12-16]. Moreover, it has been recognized that oxidation states of the Cr species have great influences on the catalytic behaviors in F/Cl exchange reactions. For example, high valent Cr species (e. g. Cr(Ⅵ)) have high reactivity^[17-19], while low valent Cr species (e. g. Cr(Ⅲ)) are believed to be inactive^[20], but they are active for selective hydrogenation and methanol synthesis^[21-22]. Quan et al.^[17] compared the properties of chromium oxides treated in different atmospheres and they found that Cr(Ⅵ) species showed higher catalytic activities for the F/Cl exchange reaction than Cr(Ⅲ). The role of high valent Cr species in the reaction was also investigated in our previous study^[23]. It was found that highly dispersed Cr(Ⅵ) species in the CrO_x-Y₂O₃ catalyst transformed to Cr(Ⅴ) and Cr(Ⅲ) species under high temperature calcination, which resulted in a dramatic decline in the reactivity for the fluorination of 2-chloro-1, 1, 1-trifluoroethane. The importance of such high valent Cr species lies the fact that they could be further transformed to CrO_x-Y₂OFy compounds via the reaction with HF, which are believed to be active species for F/Cl exchange reactions^[24-26]. However, such CrO_xF_y compounds are unstable under working condi-tions. Albonetti et al.^[27] reported that highly dispersed CrO₃ could easily react with HF to form CrO₂F₂ that is gaseous at room temperature, which is the main reason for the loss of Cr species during the reaction and consequently the catalyst deactivation.

  1    Experimental

  1.1    Catalyst preparation

  The Cr₂O₃ support was prepared by a precipitation method. A detailed process was as follows: Cr(NO₃)₃·9H₂O (analytical grade, Sinopharm Chemical Reagent Co., Ltd., China) was added to a 10% ammonia aqueous solution (Lanxi Yongli Chemical Co., LTD., China) under stirring until a precipitated slurry was obtained. The resulting slurry was aged for 2 h at room temperature and then separated by centrifugation from the mother liquid, washed several times with deionized water and dried at 100 ℃ overnight. The solid was calcined at 400 ℃ for 2 h in N₂ atmosphere (denoted as Cr₂O₃^-N) and then reduced at 400 ℃ in H₂ atmo-sphere for 2 h to obtain the final catalyst, which is denoted as Cr₂O₃. The CrO₃/Cr₂O₃ catalyst was prepared by impregnating the Cr₂O₃ support with an aqueous solution of H₂CrO₄ (analytical grade, Sinopharm Chemical Reagent Co., Ltd., China) overnight. Then the suspension was evaporated at 80 ℃ to obtain a solid, which was dried at 150 ℃ for 4 h in N₂ atmo-sphere. The nominal content of CrO₃ in the catalyst was 10% (n/n), corresponding to n_Cr(Ⅵ)/n_Cr(Ⅲ)=1:18.

  1.2    Catalyst characterizations

  Surface areas of the catalysts were determined by the modified BET method from N₂ adsorption isotherms at liquid nitrogen temperature (-195.7 ℃) on a NOVA 4000e Surface Area & Pore Size Analyzer. Before the measurements, the samples were out-gassed at 300 ℃ for 4 h under vacuum. X-ray diffraction (XRD) patterns of the catalysts were recorded on a PANalytical X′Pert PW3040/60 diffractometer with Cu Kα radiation (λ=0.154 2 nm) operated at 40 kV and 40 mA. The patterns were collected in a 2θ range from 10° to 90°. Scanning electron micrographs (SEM) were recorded using Japan Hitachi S-4800 instrument with accelera-tion voltage 20 kV. Raman spectra were collected by a Renishaw RM1000 confocal microprobe under ambient conditions. The wavelength of the excitation laser was 514.5 nm. The scanning range was 200~ 1 800 cm^-1. The reduction properties of the catalysts were measured by hydrogen temperature programmed reduction (H_2-TPR), which was carried out in a fixed-bed (i. d.=6 mm) reactor containing 20 mg of catalyst. The sample was heated in a flow of N₂ to 300 ℃ at a rate of 10 ℃·min^-1, and kept at 300 ℃ for 30 min. After cooling down to 100 ℃, the sample was heated from 100 to 500 ℃ with a heating rate of 10 ℃·min^-1 under a mixture of 5% H₂+95% N₂ (20 mL·min^-1). The amount of H₂ consumption was measured by a gas chromatograph with a thermal conductivity detector (TCD), which was calibrated by the quantitative reduction of a known amount of CuO powder. X-ray photoelectron spectra of the catalysts were obtained on an ESCALAB 250Xi instrument, with an Al Kα X-ray source (1 486.6 eV), under about 2×10^-10 kPa at room temperature and a pass energy of 20 eV. The binding energy (BE) of C1s core level at 284.6 eV was taken as the internal standard.

  1.3    Catalytic testing

  The fluorination reaction was carried out in the same reactor after the catalyst activation. The feed gas consisted a mixture of 2-chloro-1, 1, 1-trifluoroethane (HCFC-133a)+HF+N₂ (3, 30, 10 mL·min^-1, respectiv-ely), corresponding to a space velocity of 860 h^-1. The products were analyzed by a gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector (FID) and a GS-GASPRO capillary column (60 m×0.32 mm) after the HF and HCl in the reaction effluent was neutralized by passing the effluent through an aqueous KOH solution.

  Before reaction, pre-fluorination was carried out in order to activate the catalyst. The pre-fluorination was performed in a stainless steel tubular reactor (10 mm (i. d.) ×300 mm).3 mL of the catalyst (about 4 g) was loaded in the reactor with a thermal couple placed in the middle of the catalyst bed to monitor the reaction temperature. The catalyst was heated at 300 ℃ for 1 h in a N₂ flow (40 mL·min^-1), followed by a mixture of 80% HF+20% N₂ (total flow rate of 50 m L·min^-1) at 400 ℃ for 2 h. Then the catalyst was cooled down to the desired reaction temperature (320 ℃).

  2    Results and discussion

  2.1    Catalytic behaviors for gas phase fluorination of HCFC-133a

  

  Figure 2. Area specific rates of Cr₂O₃ and CrO₃/Cr₂O₃catalysts with time on stream

  图选项

  下载全尺寸图像

  下载幻灯片

  Fig.1 shows the catalytic behaviors of the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts for the fluorination of HCFC-133a at 320 ℃. The Cr₂O₃ shows good stability during the reaction process, giving a steady state HCFC-133a conversion of 18.5% and 2-chloro-1, 1, l-trifluoroethane (HFC-134a) selectivity of 99.0%. As for the CrO₃/Cr₂O₃ catalyst, it gives high initial conversion (about 30.6%), at the expense of low selectivity to HFC-134a (73.0%). However, the catalyst deactivates rapidly, with the conversion declining to 20.0% after 9 h reaction and the selectivity gradually increasing to 99%.Fig.2 shows the area specific rates of Cr₂O₃ and CrO₃/Cr₂O₃ catalysts with time on stream. We observes that the Cr₂O₃ has steady reaction rate during the process while the area specific reaction rate of the CrO₃/Cr₂O₃ rapidly declines in the first 4 h reaction and it reaches the same level as that of the Cr₂O₃ catalyst. It shows that CrO₃/Cr₂O₃ catalyst has a very significant deactivation, while Cr₂O₃ catalyst is very stable.

  

  Figure 1. Catalytic performance of Cr₂O₃ and CrO₃/Cr₂O₃ catalysts for the fluorination of HCFC-133a at 320 ℃

  图选项

  下载全尺寸图像

  下载幻灯片

  2.2    Characterizations of the catalysts

  

  Figure 7. Cr2p XPS spectra of Cr₂O₃-N,Cr₂O₃ and spent Cr₂O₃ (a~c),as well as as-prepared CrO₃/Cr₂O₃,pre-fluorinated CrO₃/Cr₂O₃ and spent CrO₃/Cr₂O₃ catalysts (d~f)

  图选项

  下载全尺寸图像

  下载幻灯片

  

  Figure 8. Comparison of catalytic performance of N2 thermal treated Cr₂O₃ and Cr₂O₃ (reduced) catalysts

  图选项

  下载全尺寸图像

  下载幻灯片

  

  Figure 6. Raman spectra of Cr₂O₃ and CrO₃/Cr₂O₃ catalystsbefore and after reaction

  图选项

  下载全尺寸图像

  下载幻灯片

  Table2. Surface contents of different Cr species in various catalysts

  表选项

  导出Excel

  下载全尺寸表图像

  Table2. Surface contents of different Cr species in various catalysts

  Fig.8 compares the catalytic performance of the N₂ thermal treated Cr₂O₃ (Cr₂O₃^-N) and reduced Cr₂O₃ catalyst (which was used in the reaction) and it is found that these two catalysts show almost identical performance. Considering the fact that the Cr₂O₃^-N contains a higher surface CrO₃ content than the reduced one (Table 2), it could be safely concluded that such CrO₃ species in the Cr₂O₃ hardly contribute in the reaction and the low-valent Cr(Ⅲ) species in the Cr₂O₃ catalyst accounts for its reactivity. Besides, the stable performance of the Cr₂O₃ during the reaction provides some promising potentials in practical application. Moreover, it is interesting to quantify the contributions of Cr(Ⅲ) and Cr(Ⅵ) species in the reaction. Considering that only the surface Cr species are involved in the reaction, the quantitative calculation of reactivity of the individual Cr species is based on the following assumptions: (1) Assuming that a catalyst crystallite with a size of D (nm) consists of numerous close-packed Cr₂O₃ cells, then the number of Cr₂O₃ cell in the crystallite is N_bulk=πD³/(6abc), where a, b, and c are lattice parameters of the Cr₂O₃ cell (average values of a, b and c are 0.496, 0.496 and 1.359 nm, respectively, Table 1).(2) The number of surface Cr₂O₃ cell in the crystallite is N_surf=πD2/[(ab+ac+bc)/3].(3) Then, the proportion of the Cr₂O₃ cell on the surface is N_surf/N_bulk=3.75/D. Since the Cr₂O₃ catalyst contains dominantly Cr(Ⅲ) species, the initial reactivity of the Cr(Ⅲ)) could be calculated based on the results in Fig.1, which is 4.16×10^-5 molHCFC-133a·molCr(Ⅲ)-1·s^-1. For the CrO₃/Cr₂O₃ catalyst, it could be seen that the enhanced initial reactivity (reaction rate of the CrO₃/Cr₂O₃^-reaction rate of the Cr₂O₃ in 1st hour) compared to the Cr₂O₃ catalyst is due to the contribution of Cr(Ⅵ), and the content of Cr(Ⅵ) species in the CrO₃/Cr₂O₃ catalyst is 0.4 mmol·gcat-1 (based on the result in Fig.5). Therefore, the initial reactivity (reaction time of 1 h) of the Cr(Ⅵ) species is calculated to be 1.71×10^-4 molHCFC-133a·molCr(Ⅵ)-1·s^-1. Such quantitative comparison clearly shows that the Cr(Ⅵ) species have much higher initial reactivity than the Cr(Ⅲ) species (almost by 4-fold), which is in good agreement with the findings that high valent Cr species play important roles in the reaction^[18], due to the formation of some catalytically active species such asCrO_xF_y which was confirmed by the XPS results (Fig.7e).

  Fig.4 shows the SEM images of Cr₂O₃ and CrO₃/Cr₂O₃ catalysts under different conditions (fresh, pre-fluorinated and after reaction). The images of the fresh samples show nanoparticles of Cr₂O₃ and no porous structures are detected (Fig.4a and d). The pre-fluorinated and spent catalysts show no obvious differences to the fresh ones, indicating that the pretreatment and reaction conditions exert no pronounced effect on the catalyst morphologies.

  In order to investigate the properties of the catalysts, various characterizations were conducted. First of all, it is found that the CrO₃/Cr₂O₃ catalyst has slightly higher surface area (62 m²·g^-1) than the Cr₂O₃ (55 m²·g^-1) (Table 1) while the spent catalysts have similar surface areas compared to the fresh counter-parts.Fig.3 shows the XRD patterns of the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts before and after reaction. The diffraction peaks due to crystalline Cr₂O₃ (PDF 38-1479) are clearly observed both in the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts. However, no diffraction peaks related to other Cr species are detected. Compared with the fresh catalysts, the spent ones show almost identical diffraction peaks, which implies that crystalline Cr₂O₃ is difficult to be fluorinated in HF atmosphere. In addition, the Cr₂O₃ catalyst has larger crystallite size than the CrO₃/Cr₂O₃ (Table 1). The corresponding spent catalyst has slightly larger crysta-llite size compared to the fresh one, indicating the growth of catalyst particles during the high tempera-ture reaction. However, the essentially constant lattice parameters suggest that the hexagonal structure of the Cr₂O₃ remains intact during the reaction.

  

  Figure 9. Possible reaction pathways over Cr₂O₃ and CrO₃/Cr₂O₃ catalysts at initial and steady states

  图选项

  下载全尺寸图像

  下载幻灯片

  The second issues concerns the catalyst stability and species transformation during the reaction. The deactivation of the CrO₃/Cr₂O₃ catalyst is attributed to two possible reasons. The first one is the loss of active species (e. g. CrOxFy) during the reaction through evaporation of volatileCrO_xF_y species. It was reported that compounds such as CrOF⁴ have low boiling points which could be easily evaporated at 40~50 ℃^[40]. Indeed, the volatilization of such Cr species was confirmed in our experiment. Some precipitate with a color of pale green was observed in the condensed neutralizing solution, which was identified as Cr(OH)3. The generation of such Cr(OH)3 is most likely due to the reaction between gaseous Cr species with KOH when the exhaust gas passed through the neutraliza-tion solution. The second one is the transformation of CrO_xF_y species to stable but inactive CrF3 compound, which is confirmed by XPS (Fig.7c) results. The inac-tiveness of CrF3 compound is clarified by the fact that the CrO₃/Cr₂O₃ catalyst containing considerable surface CrF3 content (Fig.7f) has the same performance as the Cr₂O₃ catalyst (Fig.1), which is similar to the findings by Chung et al.^[39]. Also, the coverage of such inactive CrF3 on the Cr₂O₃ surface could account for the CrO₃/Cr₂O₃ catalyst deactivation (Fig.2).

  XPS experiments were conducted to further investigate the surface structures of the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts (Fig.7) and the detailed results are summarized in Table 2. For the N₂ thermal treated Cr₂O₃ catalyst (Cr₂O₃^-N, Fig.7a), the Cr2p3/2 peak was deconvoluted into three peaks at binding energies of 578.1, 576.6 and 575.5 eV, assigned to CrO₃, amor-phous Cr₂O₃ and crystalline Cr₂O₃^[21], respectively. The Cr₂O₃ catalyst (reduced sample as used in reaction) and the spent one (Fig.7b and c) show essentially the same features as the fresh one, suggesting the intact surface structure of this catalyst during the reaction which could explain the stability of the Cr₂O₃. The presence of CrO₃ in the Cr₂O₃ at all stages (N₂ thermal treated, reduced and spent) is in good agreement with the Raman results (Fig.6). Moreover, the XPS analyses provide more reliable information on the Cr(Ⅵ) species compared to the Raman results (Fig.6). However, the concentrations of CrO₃ species differ in these three catalysts (Table 2). For example, the surface concentration of the Cr(Ⅵ) species in the N₂ thermal treated Cr₂O₃ (Cr₂O₃^-N, 22.8%) is higher than those of the reduced Cr₂O₃ (15.8%), which implies the reduction of surface CrO₃ species. Also, compared to the reduced Cr₂O₃, the spent Cr₂O₃ catalyst contains a lower content of surface Cr(Ⅵ) but a higher content of crystalline Cr₂O₃, which indicates that some CrO₃ species transform to crystalline Cr₂O₃ during the high temperature reaction. Nevertheless, the finding of CrO₃ implies this species is quite stable in the catalyst, possibly due to the strong interaction between CrO₃ and Cr₂O₃ in this catalyst. For the fresh CrO₃/Cr₂O₃ catalyst (Fig.7d), it shows three deconvoluted peaks at binding energies of 578.1, 576.8 and 575.9 eV, assigned to CrO₃, amorphous Cr₂O₃ and crystalline Cr₂O₃, respectively^[23]. The spectrum of the fresh CrO₃/Cr₂O₃ contains the same features as the Cr₂O₃ catalyst (Fig.7b), except that the surface concentration of CrO₃ in the CrO₃/Cr₂O₃ is 42.4%. This much higher value compared to that in the Cr₂O₃ (15.8%) suggests that the CrO₃/Cr₂O₃ contains more CrO₃ species than the Cr₂O₃, which again agrees with the Raman results (Fig.6). For the pre-fluorinated CrO₃/Cr₂O₃ catalyst (Fig.7e), the Cr2p3/2 core level could be deconvoluted into three peaks at 579.1, 576.8 and 575.9 eV. The assignment of the peak at 579.1 eV is a little compli-cated and it might be due to a combination of CrO_xF_y species at 578.8 eV^{[28, 35]} and CrF₃ species at 579.4 eV^[18]. The formation of suchCrO_xF_y species may be due to the fluorination of CrOx during the pretreatment with HF to form someCrO_xF_y species such as CrO₂F₂, CrOF⁴, CrOF³, CrOF₂ and CrOF^[36-37] and these species are regarded as the active species for F/Cl exchange reaction^[38-39]. Moreover, the formation of CrF3 is prob-ably due to the decomposition of suchCrO_xF_y species as it was reported that CrOF³ could decompose to stable CrF3 at 350 ℃^[40]. After reaction, the spent CrO₃/Cr₂O₃ (Fig.7f) contains CrF3 species with a BE of 579.4 eV, which could be due to the decompositionCrO_xF_y species during the reaction^[40]. In addition, since the surface CrF3 content in the spent catalyst is less than 28.5% and the contents of crystalline and amor-phous Cr₂O₃ in this sample increase compared to the pre-fluorinated one, it implies that some CrO₃ species could transform to Cr₂O₃ during the reaction, which is similar to the finding in the Cr₂O₃ catalyst (Fig.7c).

  Fig.5 shows the H_2-TPR profiles of the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts. The Cr₂O₃ catalyst shows a very weak reduction peak in range of 350~600 ℃, due to the reduction of trace high-valent Cr species in the catalyst^[29]. The presence of Cr(Ⅵ) species in the Cr₂O₃ catalyst are probably due to the formation of CrO₃ via the oxidation of Cr₂O₃ compound during the calcina-tion process at 400 ℃ although it was then reduced. Similar findings of Cr(Ⅵ) were also reported in Y2O₃^-Cr₂O₃ catalyst systems^[30]. For the CrO₃/Cr₂O₃ catalyst, one intense but broad reduction peak is observed at 300~600 ℃, which could be attributed to the reduction of high valent Cr species (e. g. Cr⁵⁺, Cr⁶⁺)^[29]. Also, the calculated H₂ consumptions on the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts are 0.02 and 0.61 mmol·g^-1, respectively. According to the H₂ consumption values and assuming that all the high-valent Cr species are Cr(Ⅵ), the contents of Cr(Ⅵ) species in these catalysts could be calculated, assuming the reduction during 300~600 ℃ is due to 2CrO₃+3H₂ → Cr₂O₃+3H₂O. The Cr(Ⅵ) contents in the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts are 0.013 and 0.40 mmol·gcat-1, respectively. Note that for the CrO₃/Cr₂O₃ catalyst, the nominal Cr(Ⅵ) content is 0.68 mmol·gcat-1. Therefore, the lowered actual Cr(Ⅵ) content (0.40 mmol·gcat-1) compared to the nominal value implies that part of the CrO₃ could transform to Cr₂O₃ during the thermal treatment (150 ℃ in N₂). These values suggest that in the Cr₂O₃ the Cr species dominantly exist in the form of Cr₂O₃ (Cr3+), while the Cr species exist in forms of mixed high valent Cr6+ and Cr₂O₃ in the CrO₃/Cr₂O₃.Fig.6 shows the Raman spectra of the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts before and after reaction. For all the catalysts, six Raman bands at 302, 342, 545, 610, 676 and 1 350 cm^-1 are observed, which are attributed to Cr₂O₃ species^[31]. Compared to the Cr₂O₃, the intensities of these bands are significantly lower in the CrO₃/Cr₂O₃ catalyst. In addition, a rather weak band at 1 000 cm^-1 assigned to Cr(Ⅵ) is also detected in all the catalysts^[32-34]. This presence of Cr(Ⅵ)) species in these fresh catalysts is in consistent with the H_2-TPR results (Fig.5). Moreover, calculation of I1 000/I545 ratio is used to quantitatively compare the concentrations of Cr(Ⅵ) in the catalysts (Fig.6), where I1 000 and I545 refer to the intensity of band at 1 000 and 545 cm^-1, respectively. For the fresh catalysts, the I1 000/I545 ratio is 0.05 for the Cr₂O₃ catalyst, which is much lower than that for the CrO₃/Cr₂O₃ catalyst (0.28). This semi-quantitative comparison suggests that the Cr(Ⅵ) content is much higher in the CrO₃/Cr₂O₃ catalyst than that in the Cr₂O₃ catalyst, which is in good agreement with the H_2-TPR results (Fig.5). For the spent CrO₃/Cr₂O₃ catalyst, the ratio is 0.12. The lowered ratio compared with that for the fresh CrO₃/Cr₂O₃ catalyst (0.28) suggests either the loss of Cr(Ⅵ) species during the reaction process or the transformation of such species to low valent Cr species. Besides, the color of the CrO₃/Cr₂O₃ catalyst changed from black to green after 9 h reaction, which is the characteristic color of Cr(Ⅲ) species. This observation again confirms the transformation of Cr species during the reaction process.

  

  Figure 3. XRD patterns of the Cr₂O₃ and CrO₃/Cr₂O₃ catalystsbefore and after reaction

  图选项

  下载全尺寸图像

  下载幻灯片

  Table1. Specific surface areas,crystallite sizes and lattice parameters of various catalysts

  表选项

  导出Excel

  下载全尺寸表图像

  Table1. Specific surface areas,crystallite sizes and lattice parameters of various catalysts
  

  Figure 4. SEM images of fresh Cr₂O₃,pre-fluorinated Cr₂O₃ and spent Cr₂O₃ (a~c),as well as fresh CrO₃/Cr₂O₃,pre-fluorinated CrO₃/Cr₂O₃ and spent CrO₃/Cr₂O₃ catalysts (d~f)

  图选项

  下载全尺寸图像

  下载幻灯片

  

  Figure 5. H₂-TPR profiles of Cr₂O₃ and CrO₃/Cr₂O₃ catalysts

  图选项

  下载全尺寸图像

  下载幻灯片

  Therefore, the possible reaction models on the Cr₂O₃ and CrO₃/Cr₂O₃ catalysts are illustrated in Fig.9. For the Cr₂O₃ catalyst (bottom panel), it is stable during the reaction, and HCFC-133a could react with HF on the surface to form HFC-134a. For the CrO₃/Cr₂O₃ catalyst (top panel), in addition to the reaction over the Cr₂O₃ support, theCrO_xF_y compounds generated during the pre-fluorination are very active for the reaction. However, these compounds could either volatilize or transform to stable but inactive CrF3 during the reaction.

  Based on the above results, two important issues could be clarified. The first issue concerns the contributions of different Cr species in the reaction. The high initial activity of the CrO₃/Cr₂O₃ catalyst (30.6%) indicates the important role of the high valent Cr(Ⅵ) species, which is in line with the comprehension that such species are crucial for the formation of catalytically active species such as CrO_xF_y^[41] . However, the finding that the low valent Cr(Ⅲ) compounds such as Cr₂O₃ is also active for the reaction suggests the role of low valent Cr species, which has been rarely recognized in literature^[28]. Since the Cr₂O₃ also contains a surface CrO₃ content of 15.8% (Table 2), an important question may arise that these CrO₃ species could also contribute in the reaction. To clarify this, an additional experiment was conducted.

  3    Conclusions

  This work demonstrated that different species of chrome oxide catalyst have distinctively different behaviors in the vapor phase fluorination of HCFC-133a to synthesize HFC-134a. The Cr₂O₃ support containing dominantly Cr(Ⅲ)) species (amorphous or crystalline Cr₂O₃) is very stable during the reaction, although its reactivity is not high. On the contrary, the CrO₃/Cr₂O₃ catalyst containing high valent Cr species (Cr(Ⅵ)) could react with HF to form some catalytic active species such asCrO_xF_y. These species have high initial reactivity than the Cr₂O₃, but they volatilize during the reaction and could transform to stable but inactive CrF3 species, which accounts for the catalyst deactivation.

• 1. [1]

     Cheng Y X, Fan J L, Xie Z Y, et al. J. Fluorine Chem., 2013, 156:66-72 doi: 10.1016/j. jfluchem.2013.08.016

  2. [2]

     Cochon C, Corre T, Celerier S, et al. Appl. Catal. A, 2012,413(3):149-156

  3. [3]

     Mao W, Wang B, Ma Y B, et al. Catal. Commun., 2014,49(2):73-77

  4. [4]

     Yoshimura T, Homoto Y, Yamada Y, et al. US Patent, 6011185. 1996-08-29.

  5. [5]

     Brunet S, Boussand B, Martin D. J. Catal., 1997,171(3):287-292

  6. [6]

     于洪波,王月娟,彭小波,等.无机化学学报, 2012,28(5):905-909 http://www. wjhxxb. cn/wjhxxbcn/ch/reader/view_abstract. aspx?flag=1&file_no=20120507&journal_id=wjhxxbcnYU Hong-Bo, WANG Yue-Juan, PENG Xiao-Bo, et al. Chinese J. Inorg. Chem., 2012,28(5):905-909 http://www. wjhxxb. cn/wjhxxbcn/ch/reader/view_abstract. aspx?flag=1&file_no=20120507&journal_id=wjhxxbcn

  7. [7]

     Ünveren E, Kemnitz E, Lippitz A, et al. J. Phys. Chem. B, 2005,109(9):903-1913

  8. [8]

     York S C, Cox D F. J. Phys. Chem. B, 2003,107(13):5182-5189

  9. [9]

     钟永辉,周琪,刘家琴,等.无机化学学报, 2013,29(10):2133-2139 http://www. wjhxxb. cn/wjhxxbcn/ch/reader/view_abstract. aspx?flag=1&file_no=20131017&journal_id=wjhxxbcnZHONG Yong-Hui, ZHOU Qi, LIU Jia-Qin , et al. Chinese J. Inorg. Chem., 2013,29(10):2133-2139 http://www. wjhxxb. cn/wjhxxbcn/ch/reader/view_abstract. aspx?flag=1&file_no=20131017&journal_id=wjhxxbcn

  10. [10]

      Alonso C, Morato A, Medina F, et al. Appl. Catal. B, 2003, 40(3):259-269

  11. [11]

      Weckhuysen B M, Wachs I E, Schoonheydt R A. Chem. Rev., 1996,96:3327-3349 doi: 10.1021/cr940044o

  12. [12]

      Rao J M, Sivaprasad A, Rao P S, et al. J. Catal., 1999,184(4):105-111

  13. [13]

      Lee H, Kim H S, Kim H, et al. J. Mol. Catal. A:Chem., 1998,136(3):85-89

  14. [14]

      Cho D H, Kim Y G, Chung M J, et al. Appl. Catal. B, 1998, 18(3):251-261

  15. [15]

      Teinz K, Manuel S R, Chen B B, et al. Appl. Catal. B, 2015, 165(5):200-208

  16. [16]

      Sekiya A, Quan H D, Tamura M, et al. J. Fluorine Chem., 2001,112:145-148 doi: 10.1016/S0022-1139(01)00483-3

  17. [17]

      Quan H D, Tamura M, Matsukawa Y, et al. J. Mol. Catal. A: Chem., 2004,219(2):79-85

  18. [18]

      Kohne A, Kemnitz E. J. Fluorine Chem., 1995,75:103-110 doi: 10.1016/0022-1139(95)03320-D

  19. [19]

      Brunet S, Requieme B, Colnay E, et al. Appl. Catal. B, 1995,5(4):305-317 doi: 10.1016/0926-3373(94)00048-4

  20. [20]

      He J, Lu J Q, Xie G Q. Indian J. Chem. Technol., 2009,48A: 489-497

  21. [21]

      Huang J J, Fu Y P, Wang J Y, et al. Mater. Res. Bull., 2014, 51(2):63-68

  22. [22]

      Mierczynski P, Maniecki T P, Maniukiewicz W, et al. React. Kinet. Catal. Lett., 2011,104(2):139-148

  23. [23]

      He J, Xie G Q, Lu J Q, et al. J. Catal., 2008,253:1-10 doi: 10.1016/j. jcat.2007.11.005

  24. [24]

      Zhu Y, Fiedler K, Rüdiger St, et al. J. Catal., 2003,219:8-16 doi: 10.1016/S0021-9517(03)00183-0

  25. [25]

      Adamczyk B, Boese O, Weiher N, et al. J. Fluorine Chem., 2000,101:239-246 doi: 10.1016/S0022-1139(99)00165-7

  26. [26]

      Kemnitz E, Hess A, Rother G, et al. J. Catal., 1996,159(3): 332-339

  27. [27]

      Cuzzato P, Alberani P, Zappoli S, et al. Appl. Catal. A, 2007,326(5):48-54

  28. [28]

      Xie Z Y, Fan J L, Cheng Y X, et al. Ind. Eng. Chem. Res., 2013,52(13):3295-3299

  29. [29]

      Xia Y S, Dai H, Jiang H Y, et al. Environ. Sci. Technol., 2009,43(11):8355-8360

  30. [30]

      Wang F, Fan J L, Zhao Y, et al. J. Fluorine Chem., 2014,166(2):78-83

  31. [31]

      Yim S D, Nam I S. J. Catal., 2004,221(7):601-611

  32. [32]

      Groppo E, Damin A, Bonino F, et al. Chem. Mater., 2005,17(9):2019-2027

  33. [33]

      Weckhuysen B M, Wachs I E. J. Phys. Chem., 1996,100: 14437-14442 doi: 10.1021/jp960543o

  34. [34]

      Kim D S, Tatibouet J M, Wachs E. J. Catal., 1992,136(3): 209-221

  35. [35]

      Loustaunau A, Fayolle-Romelaer R, Celerier S, et al. Catal. Lett., 2010,138(3):215-223

  36. [36]

      Xing L Q, Lu J Q, Bi Q Y, et al. J. Raman Spectrosc., 2011, 42:1095-1099 doi: 10.1002/jrs.2811

  37. [37]

      Hope E G, Jones P J, Levason W, et al. J. Chem. Soc., Dalton Trans., 1985:529-533

  38. [38]

      Kemnitz E, Hansen G, Kohne A. J. Mol. Catal., 1992,77(2): 93-200

  39. [39]

      Chung Y S, Lee H, Jeong H D, et al. J. Catal., 1998,175(3): 220-225

  40. [40]

      Hope E G, Jones P J, Tajik M, et al. J. Chem. Soc., Dalton Trans., 1984:2445-2447

  41. [41]

      Jia W Z, Jin L Y, Wang Y J, et al. J. Ind. Eng. Chem., 2011, 17(6):615-620

• 

  Figure 1  Catalytic performance of Cr₂O₃ and CrO₃/Cr₂O₃ catalysts for the fluorination of HCFC-133a at 320 ℃

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Area specific rates of Cr₂O₃ and CrO₃/Cr₂O₃catalysts with time on stream

  下载: 全尺寸图片 幻灯片
  

  Figure 3  XRD patterns of the Cr₂O₃ and CrO₃/Cr₂O₃ catalystsbefore and after reaction

  下载: 全尺寸图片 幻灯片
  

  Figure 4  SEM images of fresh Cr₂O₃,pre-fluorinated Cr₂O₃ and spent Cr₂O₃ (a~c),as well as fresh CrO₃/Cr₂O₃,pre-fluorinated CrO₃/Cr₂O₃ and spent CrO₃/Cr₂O₃ catalysts (d~f)

  下载: 全尺寸图片 幻灯片
  

  Figure 5  H₂-TPR profiles of Cr₂O₃ and CrO₃/Cr₂O₃ catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Raman spectra of Cr₂O₃ and CrO₃/Cr₂O₃ catalystsbefore and after reaction

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Cr2p XPS spectra of Cr₂O₃-N,Cr₂O₃ and spent Cr₂O₃ (a~c),as well as as-prepared CrO₃/Cr₂O₃,pre-fluorinated CrO₃/Cr₂O₃ and spent CrO₃/Cr₂O₃ catalysts (d~f)

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Comparison of catalytic performance of N2 thermal treated Cr₂O₃ and Cr₂O₃ (reduced) catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Possible reaction pathways over Cr₂O₃ and CrO₃/Cr₂O₃ catalysts at initial and steady states

  下载: 全尺寸图片 幻灯片

  Table 1.  Specific surface areas,crystallite sizes and lattice parameters of various catalysts

  下载: 导出CSV

  Table 2.  Surface contents of different Cr species in various catalysts

  下载: 导出CSV
•