Article pubs.acs.org/IECR
Cr2O3 Catalysts for Fluorination of 2‑Chloro-3,3,3-trifluoropropene to 2,3,3,3-Tetrafluoropropene Zunyun Xie, Jinglian Fan, Yongxiang Cheng, Lingyun Jin, Gengshen Hu, Jiqing Lu, and Mengfei Luo* Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China ABSTRACT: Cr2O3 catalysts were prepared by a precipitation method and tested for vapor phase fluorination of HCFC-1233xf (2-chloro-3,3,3-trifluoropropene) to HFC-1234yf (2,3,3,3-tetrafluoropropene) to investigate the effect of calcination temperature on the catalytic performance. The catalysts were characterized by XRD, Raman, NH3-TPD, and BET techniques. The results show, with increasing calcination temperature, the crystallite size of the catalyst increased while the surface acid sites decreased. It was found that the catalyst calcined at 500 °C exhibited the highest catalytic activity, with a HCFC-1233xf conversion of 63.3% and selectivies to HFC-1234yf and HFC-245eb (1,1,1,2,3-pentafluoropropane) of 59 and 38%, respectively, at a reaction temperature of 320 °C. Moreover, it was found that the carbon deposit on the surface was responsible for the deactivation of the catalyst during the reaction.
1. INTRODUCTION Nowadays, 1,1,1,2-tetrafluoroethane (HFC-134a) is widely used as the second generation of automobile refrigerant. Although it has an ozone depleting potential (ODP) of 0, it stays in the atmosphere for a very long time and has a high global warming potential (GWP), which will cause dangerous global warming.1,2 The EU rules of mobile air-conditioning directive and F-gas regulation have ruled that the refrigerant with a GWP higher than 150 was forbidden in new model cars since 2011 and will be absolutely forbidden in all cars in 2017. Therefore, the automobile refrigerants with higher GWP will be gradually replaced by the third generation of refrigerants with lower GWP. 2,3,3,3-Tetrafluoropropene (HFC-1234yf) has unique properties such as low toxicity, low flammability, and stable chemical nature. It has excellent environment parameters such as GWP = 4 and ODP = 0 and less life cycle climate performance (LCCP) than that of HFC-134a.3 Moreover, its breakdown products in the atmosphere are the same as those of HFC-134a.4 Thus, HFC-1234yf is considered as a “direct substitution” economy plan for HFC-134a and could be a promising candidate for the third generation of refrigerants. Besides, HFC-1234yf can also be used as a flame retardant, heat transfer medium, propellant, foaming agent, gas medium, sterilizing agent carrier, polymer monomer, grinding polishing agent, a replacement for desiccant, and so on. For the synthesis of HFC-1234yf, much effort has been done by Honeywell and Dupont, and a large number of patents were applied. In those patents, various synthetic routes used 1,1,2,3tetrachloropropene as the raw material for heterogeneous synthesis of HFC-1234yf over Cr-based catalysts.5−9 For example, patent WO2010US61716 proposed a three-step synthesis route for HFC-1234yf over Cr2O3−Al2O3 catalysts. In the first step, 1,1,2,3-tetrachloropropene was catalytically converted to HCFC-1233xf (2-chloro-3,3,3-trifluoropropene); in the second step, HCFC-1233xf is converted to HFC-245eb (1,1,1,2,3-pentafluoropropane) and HCFC-244bb (2-chloro1,1,1,2-tetrafluoropropane); finally, HFC-1234yf was produced © 2013 American Chemical Society
via high temperature reaction from HFC-245eb or HCFC244bb. The key of this route is to synthesize HFC-1234yf from HCFC-1233xf10,11 (steps 2 and 3). If the second and third step reactions can be combined into one, which is CF3CClCH2 + HF → CF3CFCH2 + HCl, the reaction process could be significantly simplified and, more importantly, the energy consumption and pollution could be effectively controlled. Therefore, one-step synthesis from HCFC-1233xf to HFC1234yf is very promising for the industrial production of HFC1234yf. Unfortunately, this reaction is mainly reported in patents and very few papers could be found. In this paper, direct synthesis of HFC-1234yf using HCFC1233xf as the raw material has been conducted through a F−Cl exchange fluorine reaction over Cr2O3 catalysts. The effects of crystalline size of Cr2O3 and the surface acidity on the catalytic performance were studied.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Cr2O3 catalysts were prepared by a precipitation method.12,13 A detailed process was as follows: ammonia (25−28% aqueous solution, Lanxi Yongli Chemical Co., LTD, China) was added to a chromic nitrate (Cr(NO3)3, 0.5 mol/L, analytical grade, Sinopharm Chemical Reagent Co., Ltd., China) solution under stirring until a precipitated slurry was obtained. The pH value of the suspension was controlled at around 8.0. The resulting slurry was aged for 2 h and then separated from the mother liquid, washed with deionized water, and dried at 120 °C overnight. The solid was calcined at 300, 400, 500, and 600 °C for 4 h to obtain the final catalysts, which are denoted as Cr2O3-3, Cr2O34, Cr2O3-5, and Cr2O3-6, respectively. Received: Revised: Accepted: Published: 3295
November 8, 2012 January 21, 2013 February 10, 2013 February 11, 2013 dx.doi.org/10.1021/ie303067d | Ind. Eng. Chem. Res. 2013, 52, 3295−3299
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2.2. Catalyst Characterizations. X-ray diffraction (XRD) patterns of the catalysts were recorded using a PANalytic X’Pert PW3040 diffractometer with Cu Kα radiation operating at 40 kV and 40 mA. The patterns were collected in a 2θ range from 20 to 80°, with a scanning step of 0.15° s−1. In the Scherrer equation, D = kλ/(β cos θ), where k is the shape factor (k = 0.89), λ is the X-ray wavelength (λ = 0.154056 nm), β is the line broadening at half the maximum intensity in radians, and θ is the Bragg angle. Surface areas of the catalysts were determined by the modified BET method from the N2 adsorption isotherms at liquid nitrogen temperature (−195.7 °C) on an NOVA 40000e Surface Area & Pore Size Analyzer. Before measurement, the samples were outgassed at 120 °C for 4 h under a vacuum. The surface acidity of the catalysts was measured by ammonia temperature-programmed desorption (NH3-TPD), which was conducted on a homemade apparatus. A 50 mg portion of the catalyst was loaded in a quartz tubular reactor (i.d. = 6 mm) and was pretreated in Ar at 300 °C for 1 h. Then, it was cooled down to 50 °C. A flow of NH3 (20 mL/min) was introduced to the sample for 30 min. The gaseous or physisorbed NH3 was removed by purging Ar flow at 100 °C for 90 min. Then, the sample was heated to 700 °C with a ramp of 10 °C min −1 . The desorbed NH 3 was monitored continuously via a TCD detector. Raman spectra were collected by a Renishaw RM1000 confocal microprobe under ambient conditions. The excitation laser wavelength was 514 nm. The power of each laser line was kept at about 3 mW to prevent local heating effects, and the resolution of the spectrometer was 1 cm−1 with the diameter of the analyzed spot being ca. 1 μm. 2.3. Catalytic Testing. The Cr2O3 catalysts were subjected to a pre-fluorination process before reaction in order to activate the catalysts. The pre-fluorination of the oxide samples were carried out in a stainless steel tubular reactor (10 mm (i.d.) × 300 mm), equipped with an electric heater. A 3 mL portion of Cr2O3 was loaded into the reactor and dried at 260 °C for 1 h and subsequently at 350 °C for 1 h in N2 with a flow of 30 mL min−1. Then, the N2 flow was stopped and the mixture of HF (40 mL·min−1) and N2 (10 mL·min−1) was introduced at 260 °C for 2 h and subsequently at 350 °C for 3 h. The molar ratio of HF/CF3CClCH2 was 10, and the gas hourly space velocity (GHSV) was 880 h−1. To remove the HF and HCl, the reaction effluent passed an aqueous KOH solution; then, the products were analyzed online using a gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector (FID) and a GS-GASPRO column (60 m × 0.32 mm) capillary column.
Figure 1. XRD patterns of the Cr2O3 catalysts (a) before and (b) after pre-fluorination.
Figure 1b (36.1°), the average crystalline size of Cr2O3 in the Cr2O3-4, Cr2O3-5, and Cr2O3-6 catalysts is 20, 20.5, and 25.7 nm, respectively. This indicates that the crystalline size of Cr2O3 increases with increasing calcination temperature. 3.2. Physical Properties. Figure 2 shows the N 2 adsorption isotherms (Figure 2a) and pore distributions
Figure 2. N2 adsorption isotherms (a) and pore diameters (b) of the pre-fluorinated Cr2O3 catalysts.
(Figure 2b) of the pre-fluorinated Cr2O3 catalysts. All isotherms are type IV according to the IUPAC classification, and the pore sizes are centered at 20 nm. Other parameters such as surface area, pre-volume, and pore size of the catalysts are summerized in Table 1. The surface areas are 96, 110, 56, and 28 m2 g−1 for Table 1. Physical Properties of the Pre-Fluorinated Cr2O3 Catalysts physical properties
3. RESULTS AND DISCUSSION 3.1. Phase Analysis of Cr2O3 Catalysts. Figure 1 shows the XRD patterns of the Cr2O3 catalysts before (Figure 1a) and after pre-fluorination (Figure 1b). It can be seen that the diffraction peaks of each individual catalyst are almost identical before and after pre-fluorination, indicating that the prefluorination hardly changes the bulk structure of the catalyst. Diffraction peaks due to Cr species are not observed for the catalyst calcined at 300 °C, suggesting that Cr species are amorphous in this sample (Cr2O3-3). With increasing calcination temperature, Cr2O3 translates from amorphous structure into crystalline phase, and the intensity of the diffraction peaks gradually becomes stronger. On the basis of the Scherrer equation and the most intensive diffraction peak in
catalyst
SBET (m g )
V (cm3 g−1)
D (nm)
Cr2O3-3 Cr2O3-4 Cr2O3-5 Cr2O3-6
96 110 56 28
0.27 0.38 0.23 0.19
21.8 18.5 21.0 26.3
2
−1
the Cr2O3-3, Cr2O3-4, Cr2O3-5, and Cr2O3-6 catalysts, respectively, indicating that the surface area of catalyst calcined at 400 °C is the largest and it remarkably declines with further increasing calcination temperature. Figure 3 shows the EDX spectra of the catalysts. It is found that the molar fraction of F in the catalyst decreases with increasing calcination temperature, implying the fact that the Cr2O3 catalyst calcined at high temperature (e.g., 600 °C) is difficult to be fluorinated compared to those calcined at low temperature (e.g., 300 °C). Moreover, comparison of the fresh 3296
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Figure 3. EDX spectra of pre-fluorinated catalysts (a, b, c, d) and the spent Cr2O3-5 catalysts (e).
and spent Cr2O3-5 catalyst (Figure 3c and e) reveals that the F content in the catalyst remains constant after reaction, indicating there is no F loss during the reaction. Moreover, carbon species are also detected in the spent Cr2O3-5 catalysts. 3.3. Surface Acidity of the Cr2O3 Catalysts. Figure 4 shows the NH3-TPD profiles of the pre-fluorinated Cr2O3
Table 2. Conversion of HCFC-1233xf and Selectivities to HFC-1234yf, HFC-1234ze, and HFC-245eb over the Cr2O35 Catalysts Pretreated with Different HF/N2 Ratios (total flow = 50 mL min−1)a selectivity (%) HF/N2
conversion (%)
HFC-1234yf
HFC-1234ze
HFC-245eb
2/1 4/1 6/1 8/1
62.9 63.3 62.5 63.5
58.5 58.2 58.3 58.2
2.3 2.1 2.4 2.3
36.4 36.7 36.2 36.2
a Reaction temperature, 320 °C; catalyst loading, 3g; GHSV, 880 h−1; molar ratio, HF/CF3CClCH2 = 10/1.
that have reacted on the unit surface area (obtained at 1 h of reaction time) in unit time. It can be seen from Table 3 that the specific rate of reaction increases with increasing calcination temperature. However, the calcination temperature has no obvious effect on the selectivities to HFC-1234yf, HFC-1234ze, and HFC-245eb. For all of the catalysts, the selectivities to HFC-1234yf, HFC-1234ze, and HFC-245eb vary from 58 to 60%, 0.5 to 2.1%, and 37 to 40%, respectively. However, since there are few published works on this reaction, a comparison of the catalytic results could not be successfully made. Figure 5a shows the stability of the catalysts for the fluorination of HCFC-1233xf at 320 °C. It can be seen that the conversion of HCFC-1233xf decreases with time on stream for all the catalysts, indicating the obvious deactivation of the catalysts. Among the catalysts, the Cr2O3-3 sample suffered the most severe deactivation compared to other catalysts. However, the selectivities to HFC-1234yf, HFC-1234ze, and HFC-245eb are quite stable (Figure 5b). It was reported that the catalytic behaviors of Cr-based materials for F/Cl exchange reactions are closely related to the catalyst properties such as F content and surface acidity. The presence of F in the catalyst plays an important role due to the formation of the CrOxFy active phase in the F/Cl exchange reactions.14−16 Also, the surface acidity of the catalyst is essential because the F/Cl exchange reaction is an acidcatalyzed reaction, but too many surface acid sites are detrimental because it may cause carbon formation on the catalyst surface which could block the active sites and consequently the reactivity.17 For example, Lee et al. reported that the surface acidity would weaken the C−Cl bond and thus be beneficial for the F/Cl exchange. However, the strong acid site would adsorb haloalkane species which are the coke precursors, and the major cause of deactivation is the blocking of the active sites by the coke.17
Figure 4. NH3-TPD profiles of the pre-fluorinated Cr2O3 catalysts.
catalysts.12,14 It can be seen that the catalysts calcined at both 300 and 400 °C have large desorption peaks while the desorption peaks are much weaker for the catalysts calcined at 500 and 600 °C. The relative amount of surface acid site of the catalyst can be calculated on the basis of the desorption peak area. The surface acid densities of the Cr2O3-3, Cr2O3-4, Cr2O35, and Cr2O3-6 are 29, 18, 8, and 3 (μmol/m2), respectively. This implies that the amount of surface acid sites dramatically decreases with increasing calcination temperature, which is probably due to the decline in specific areas of the catalysts. 3.4. Reaction Performance of Cr2O3 Catalysts. Table 2 shows the conversion of HCFC-1233xf and selectivity to HFC1234yf, HFC-1234ze, and HFC-245eb over the Cr2O3-5 catalysts pretreated with different HF/N2 ratios. It can be seen that the conversion of HCFC-1233xf and selectivities to HFC-1234yf, HFC-1234ze, and HFC-245eb are almost constant. Thus, the HF/N2 ratio (2−8) in the pretreatment has little impact on the catalytic performance. Table 3 shows the specific rate, conversion of HCFC-1233xf, and selectivities to HFC-1234yf, HFC-1234ze (1,3,3,3-tetrafluoropropene), and HFC-245eb over the Cr2O3 catalysts. Note that the products were analyzed after the mixture passed through the KOH solution, and thus, the above results are based on the observed volatiles only. The specific rate of HCFC-1233xf is defined as the amount of reactant molecules 3297
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Table 3. Specific Rate, Conversion of HCFC-1233xf, and Selectivity to HFC-1234yf, HFC-1234ze, and HFC-245eb over Cr2O3 Catalystsa selectivity (%)
a
catalyst
conversion (%)
specific rate (10−6 mol min−1 m−2)
HFC-1234yf
HFC-1234ze
HFC-245eb
Cr2O3-3 Cr2O3-4 Cr2O3-5 Cr2O3-6
56.7 62.7 63.3 56.2
0.31 0.33 0.59 1.07
58.4 59.0 58.2 60.7
0.2 0.5 2.1 1.8
39.8 38.5 36.7 37.2
Reaction temperature, 320 °C; catalyst loading, 3g; GHSV, 880 h−1; molar ratio, HF/CF3CClCH2 = 10/1.
Figure 5. Stability of the catalyst for the fluorination of HCFC-1233xf at 320 °C (a) and the selectivities to HFC-1234yf, HFC-1234ze, and HFC-245eb for the Cr2O3-5 catalyst (b). Figure 6. Raman spectra of fresh and spent Cr2O3-5 catalyst.
Note that the specific rate increases while the density of surface acid sites decreases (Figure 3) in the catalyst; this indicates that high surface acid density is unfavorable for the fluorination of HCFC-1233xf. On the basis of the results, reaction pathways of this reaction could be proposed as follows:
for the Cr2O3-5 catalyst is due to the carbon formed on the catalyst surface, which is consistent with the detection of carbon on the spent Cr2O3-5 catalyst by the EDX technique (Figure 3e). To further investigate the surface carbon species, the spent Cr2O3-5 catalyst was analyzed by the thermo-gravimetric (TG) method, as shown in Figure 7. The slight weight loss at around
CF3CClCH 2(HCFC‐1233xf) + HF → CF3CFClCH3(HCFC‐244bb) + HCl
(1)
CF3CFClCH3 + HF → CF3CF2CH3(HFC‐245eb) + HCl (2)
CF3CF2CH3 ↔ CF3CFCH 2(HFC‐1234yf) + HF
(3)
CF3CFClCH3 → CF3CFCH 2(HFC‐1234yf) + HCl (2′)
The main products detected in the reaction are HFC-245eb and HFC-1234yf. Thus, the reaction may proceed as follows: HCFC-1233xf reacts with HF through an addition reaction to a saturated compound HCFC-244bb (eq 1).18 HCHC-244bb can react with HF through F/Cl exchange reaction to form HFC245eb (eq 2), which could be decomposed to HFC-1234yf, but it is a reversible reaction (eq 3). Also, HCFC-244bb could be directly decomposed to HFC-1234yf (eq 2′). Figure 6 shows the Raman spectra of fresh and spent Cr2O35 catalyst. The Raman band at 552 cm−1 assigned to chromium(III) species was observed for both of them.19 This suggests that the active species in the Cr2O3-5 catalyst has no change before and after the reaction, and the deactivation should not be ascribed to the phase transformation. However, new Raman bands at 1600 and 1350 cm−1 assigned to carbon deposit are observed on the only used catalyst.20 This indicates that there is the formation of carbon during the reaction. Therefore, it could be concluded that the general deactivation
Figure 7. TG-DTA curves of the spent Cr2O3-5 catalyst.
100−200 °C was due to the evaporation of moisture absorbed on the surface. At the temperature range 300−450 °C, there is other weight loss (1.44%) due to the oxidation of carbon species.21 Since the F content in the catalyst before and after reaction remains unchanged (Figure 3c and e), it is not likely that the catalyst deactivation is due to the loss of F in the catalyst during the reaction. Thus, the deposition of carbon on the surface could be the main cause of catalyst deactivation. 3298
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(14) Zhu, Y.; Fiedler, K.; Rüdiger, St.; Kemnitz, E. Aliovalentsubstituted chromium-based catalysts for the hydrofluorination of tetrachloroethylene. J. Catal. 2003, 219, 8−16. (15) Adamczyk, B.; Boese, O.; Weiher, N.; Schroeder, S. L. M.; Kemnitz, E. Fluorine modified chromium oxide and its impact on heterogeneous catalyzed fluornation reactions. J. Fluorine Chem. 2000, 101, 239−246. (16) Kemnitz, E.; Hess, A.; Rother, G.; Troyanov, S. Characterization of the Structure and Catalytic Behavior ofAlF3‑x(OH)x with Aluminum Successively Replaced by Chromium and Magnesium. J. Catal. 1996, 159, 332−339. (17) Lee, H.; Jeong, H. D.; Chung, Y. S.; Lee, H. G.; Chung, M. J.; Kim, S.; Kim, H. S. Fluorination of CF3CH2Cl over Cr-Mg Fluoride Catalyst The Effect of Temperature on the Catalyst Deactivation. J. Catal. 1997, 169, 307−316. (18) Wendlinger, L.; Jarrest, S. E.; Pigamo, A.; Francheville; Pomimique, D. B.; Charly. Catalytic gas phase fluorination of 243db to 1234yf. US8207384132, 2012. (19) Arkatova, L. A. The deposition of coke during carbon dioxide reforming of methane over intermetallides. Catal. Today 2010, 157, 170−176. (20) Barshilia, H. C.; Rajam, K. S. Growth and characterization of chromium oxide coatings prepared by pulsed-direct current reactive unbalanced magnetron sputtering. Appl. Surf. Sci. 2008, 255, 2925− 2931. (21) Li, C.; Stair, P. C. An advance in Raman studies of catalysts: ultraviolet resonance Raman spectroscopy. Stud. Surf. Sci. Catal. 1996, 101, 881.
Also, the inhibition of carbon formation during the reaction seems to be the key to improving the performance of the catalysts.
4. CONCLUSION With increasing calcination temperature, the crystalline size of Cr2O3 catalysts increases while the amount of surface acid sites decreases, which leads to the increase of the specific rate of fluorination of HCFC-1233xf. However, the catalyst calcined at 500 °C shows the highest activity, with the HCFC-1233xf conversion of 63.3% and selectivities to HFC-1234yf and HFC245eb of 59 and 38%, respectively. The carbon deposit on the surface of the catalysts may be the main reason for the deactivation of the catalyst during the reaction.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-0579-82283910. Fax: +86-579-82282595. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 20873125) and the Program for Changjiang Scholars and Innovative Research Team in Chinese Universities (IRT0980).
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REFERENCES
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