Catalytic Dehydrofluorination of Hydrofluoroalkanes to Fluorinated Olifein Over Ni/AlF3 Catalysts

The hydrofluoric acid-resisting aluminum compounds (AlF3, AlPO4, AlN) supported with Ni catalyst were prepared by the wetness impregnation and tested for dehydrofluorination of hydrofluoroalkane to synthesize fluoroolefins. It is found that Ni/AlF3 catalyst has the best catalytic performance, CF3CFH2 conversion of 29.3% after the reaction at 430 °C for 30 h, CF2HCH3 conversion of 31.8% after the reaction at 250 °C for 30 h, respectively. Comparatively, dehydrofluorination temperature of CF3CFH2 is higher than CF2HCH3 over the aluminum compounds catalyst, and the activity of catalysts is related with Lewis acidity. For the aluminum compounds catalyst, addition of Ni had promoted the activity and stability of Lewis acidic catalysts, it is attributed to synergistic catalysis of Lewis


Introduction
Fluorine-containing olefins are mainly used as monomer raw materials for fluoropolymer, such as vinyl fluoride for the PVF, vinylidene fluoride for the PVDF, and also as intermediates for the fluorine-containing fine chemicals such as trifluorobromoethylene, etc. Fluoroolefins are a kind of important fine chemicals, the synthesis route has been extensively studied in recent years. In many production methods of fluorine-containing olefins, dehydrofluorination (simplified to De-HF) of hydrofluorocarbons (HFCs) is simple and efficient. At present, there are three De-HF ways: pyrolysis [1][2][3], liquid alkali [4] and heterogeneous catalysis [5]. Pyrolysis usually needs a high temperature, rigorous equipment, many by-products. Liquid alkali way requires the use of strong alkali and produces large amount of byproduct salts, those will bring about serious environmental problems. The catalytic De-HF with low temperature and high selectivity is focus on the synthesis of fluorine-containing olefins.
Most of the transition metal (such as Pd, Cu, Ni, Fe, Ir, Ru, etc.) has the role of activated C-H bond, so the transition metal and its supported catalyst in the catalytic dehydrogenation have a very important application. Pd/AlF3 catalyst has the high reactivity for dehydrofluorination of 1, 1-difluorocyclohexane and 1, 1, 1, 3, 3-Pentafluoropropane [6,7], respectively. However, the noble metal catalyst due to its high price and gradually replaced by a large number of cheap non-noble metal, which also meet the objectives of modern chemical sustainable development [8]. Relatively inexpensive Ni is considered to be one of a ideal metal for replacing noble metal. Alonso et al. [9] pointed out that nano-Ni particles can promote the H transfer reaction (such as alkane dehydrogenation, alcohol oxidation dehydrogenation, amine dehydrogenation). Shimizu et al. [8] studied the reaction mechanism of propanol dehydrogenation on Ni/Al2O3 catalyst, and believed that Ni can activate C-H bond and promote the dehydrogenation reaction with the acid-base site on Al2O3. Osaki et al. [10] reduced NiO-Al2O3 aerogels to obtain Ni/Al2O3 catalyst, TEM found that Ni particles with less than 6 nm were highly dispersed on the surface of Al2O3, which had a very good catalytic performance for CH4 reforming. Aluminum compounds supported with Ni catalysts were investigated in the De-HF of HFCs in this paper.
The study related to De-HF mechanism of HFCs is few up to now. Kamiguchi et al. [11] gives a schematic representation of the typical dehydrohalogenation mechanism of HFCs, as shown in Scheme 1. It is generally believed that De-HF is carried out by the E1 mechanism. The reaction initial step is cleavage of C-F bond by activation. Usually, the fluorine of HFCs with strong electronegativity has partially negativity charged, it can be chemically adsorbed by Lewis acid sites and accelerate the cleavage of the C-F bond. Teinz et al. [12] argued that AlF3 belonged to Lewis acid was an effective De-HF catalyst, and the interaction between Lewis acid sites and fluorine atom initiates the De-HF. Li et al. [13] suggested that the weak Lewis acid sites on the P2O7 4-salt is the reaction center, it proposed that the carboniumion mechanism over the Mg2P2O7 catalyst for De-HF of 1, 1, 1-trifluoroethane(CF3CH3) to vinylidene fluoride. Dehydrofluorination of HFCs should involve the cleavage of C-F and C-H bond. For the above studies, they focus the C-F bond activation, however, the synergistic activation of C-F and C-H bond is not proposed up to now. At present, there is no clear conclusion about De-HF mechanism for HFCs on the catalyst surface. In this paper, Aluminum compound supported Ni catalysts were used to catalytic De-HF of CF3CFH2 and CF2HCH3, respectively. BET, XRD, NH3-TPD and FT-IR characterization were conducted to catalysts, and the relationship between activity and structure of catalyst was revealed. The synergistic catalysis mechanism of dehydrofluorination of HFCs was discussed.
The De-HF mechanism of HFCs.

Catalyst preparation
The Ni/AlF3, Ni/AlN and Ni/AlPO4 catalyst were prepared by the wetness impregnation in aqueous solution of Ni(NO3)2· 6H2O. The raw materials included Ni(NO3)2· 6H2O, AlF3, AlN and AlPO4 were from the Aladdin. The aluminum compounds impregnated with nickel nitrate was dried at 100 o C overnight. Finally, it was calcined at 500 o C for 3 h at air. Then prepared catalyst was subject to reduction in H2 flow (40 mL/min) at 500 o C for 2 h. Prior to use, prefluorination was carried out in a stainless steel tubular reactor with a diameter of 3 cm and a length of 40 cm.

Dehydrofluorination of HFCs
Dehydrofluorination of HFCs (CF3CFH2, CF2HCH3) was carried out in a continuous-flow fixed-bed metal reactor (15 mm i.d.) operated in the downflow mode at atmospheric pressure. The reaction temperature was determined with a thermocouple in contact with the 2.0 g catalyst. The flow rates of the gas HFCs and N2 were carefully regulated by mass flow controllers. The molar ratio of N2/HFCs was 10 in a total flow rate of 22.5 ml min -1 and the gas hourly space velocity (GHSV) was 675 h -1 . To remove the product HF, the reaction effluent passed an aqueous KOH solution, then, it was analyzed by a gas chromatograph (HP-5890) using a HP-PLOT/Al2O3 column (I. D. 0.53 mm × 50 m) with a FID detector.

Characterization
Surface areas of the catalysts were determined by the modified BET method from the N2 sorption isotherms at 77 K on an automatic NOVA 2200 eapparatus. X-ray diffraction (XRD) patterns were collected on a Foucs D8 powder diffractometer operating at 40 kV and 30 mA using Cu-Kα radiation, in the 2θ range from 10 to 70 o with a scan rate of 0.2 o min −1 .
Pyridine (Py) absorbed infrared spectrum (Py-IR) was also conducted at 200 and 400 o C, under a high vacuum. The IR spectrum a self-supported wafer of the sample was also recorded in the spectral range 1800-1400 cm -1 with 64 scans and at a resolution of 4 cm -1 .
Ammonia temperature-programmed desorption (NH3-TPD) was conducted on an automatic Altamira-100 Characterization System. 100 mg of catalysts was loaded into a quartz tublar reactor and was heated from room temperature to 500 o C and kept for 30 min in a flow of N2 (30 ml min -1 ). Then it was cooled down to 50 o C. A flow of NH3 (20 ml min -1 ) was then introduced for 10 min. The gaseous or physical sorption NH3 was removed by purging N2 flow (30 ml min -1 ) at 80 o C for 1 h. Then the sample was heated to 600 o C with a ramp of 10 o C min -1 . The desorbed NH3 was monitored continuously via a TCD detector. The total amount of NH3 desorbed was exactly determined by the adsorption with an excess of dilute HCl and following the titration with NaOH solution. For the titration, the mixture indicator was adopted, containing 0.1% brom-cresol green ethanol solution and 0.2% methyl red ethanol solution.
High resolution transmission electron microscopy (HRTEM) of the sample was obtained on a JEOL JEM 2100F equipment with a field emissive gun, operating at 200 kV and with a point resolution of 0.24 nm. Since the HF with strong corrosion produced during dehydrofluorination (De-HF) of hydrofluorocarbons (HFCs), choice of catalyst is harsh. Generally, fluorides, phosphates and nitrides have the hydrofluoric acidresistance. It is selected the AlF3, AlN and AlPO4 catalysts for De-HF of HFCs and investigated their catalytic performance. 1, 1-difluoroethane (CF2HCH3) and 1, 1, 1, 2-tetrafluoroethane (CF3CFH2) are considered as typical HFCs molecular for De-HF reaction. Figure 1a) is the activity results of the AlF3, AlN and AlPO4 catalyst in the reaction temperature range of 400~500 o C. It is found that the activity of AlF3 is higher than that of the AlN and AlPO4. The AlN and AlPO4 have little catalytic activity when the reaction temperature is less than 425 o C. Fig.1b) is the relationship between selectivity to trifluoroethylene and reaction temperature. At 480 o C, selectivity of three catalysts is not much different, all above 98%. From Figure 1b), the selectivity of three catalysts to trifluoroethylene is gradually decreased as the reaction temperature increases, due to presence of byproduct CFH=CFH, CF2HCF3 and CF2HCFH2 derived from the cracking or isomerization of CF3CFH2 under the high temperature [14,15]. The temperature is also an important factor affecting the performance of the catalyst. Among the three catalysts, AlF3 catalyst has a good catalytic performance for De-HF of CF3CFH2. Then, the AlF3, AlN and AlPO4 catalysts used to dehydrofluorination of CF2HCH3 to vinyl fluoride. It can be seen from the Figure 2 that AlF3 has the highest activity for De-HF of CF2HCH3, compared with the AlPO4 and AlN catalysts. When the temperature is low (200~250 o C), the selectivity of the three catalysts to vinyl fluoride is not very different (close to 100%). When the reaction temperature is more than 280 °C, selectivity (S) of three catalysts is: SAlN > SAlF3 > SAlPO4. Similarly, the temperature has a significant effect on De-HF of CF2HCH3, and the AlF3 catalyst has a high activity. Supported Ni has a high activity for alkyl dehydrogenation, ammonia selective oxidation, alkylated steam reforming [16], etc., which has attracted much attention in the field of heterogeneous catalysis. The Ni/AlF3, Ni/AlN and Ni/AlPO4 catalysts used to De-HF of CF3CFH2 and CF2HCH3, respectively, and the reactivity results are shown in Table 1. As seen from Table 1 that the activity of the AlF3, AlN and AlPO4 catalysts has decreased with the reaction time for De-HF of HFCs (CF3CFH2 and CF2HCH3). It is found that activity of three catalysts is improved after 2%Ni supported on the AlF3, AlN and AlPO4 catalysts. Although the activity of catalyst supported Ni also slows down, the decrease rate is obviously lower. So, the addition of Ni can improve activity for De-HF of HFCs. Dehydrofluorination temperature of CF3CFH2 is significantly higher than that of CF2HCH3, which is due to CF3CFH2 molecules with more strong electronwithdrawing F and higher dissociation of the C-F bond.

Characterization
The surface area, pore diameter, pore volume of catalysts listed in Table 2. Surface area of the AlF3, AlPO4 and AlN is 52 m 2 g -1 , 29 m 2 g -1 , 85 m 2 g -1 , respectively. After supported Ni, their surface areas have little decrease, with no difference of pore diameter and vlolume. AlN catalytst has the low activity, it may due to the small surface area (29 m 2 g -1 ).   In this paper, Py-IR technique was used to detect the surface acid types of the AlF3, AlN and AlPO4 catalysts. Figure 4 shows the Py-IR results of the AlF3, AlN and AlPO4 catalysts. The vibrational peaks are attributed to Lewis acids at 1450 cm -1 , 1608 cm -1 and 1652 cm -1 (ν19b and ν8a, respectively), and 1540 cm -1 (ν19b) attributed to Brönsted acid [17,18]. It can be seen from Figure 4 that the vibration peaks (ν19b) of the AlF3, AlN and AlPO4 catalysts appear at 1450 cm -1 , indicating that the surface of these catalysts is predominantly Lewis acid. After Ni was loaded, the surface acid types of the Ni/AlF3, Ni/AlN and Ni/AlPO4 catalysts did not change significantly. In Figure 5.1) is the NH3-TPD curve for the AlF3, AlN and AlPO4 catalysts. Compared with three catalysts, the AlN3 has the largest NH3 desorption peak area, followed by the AlPO4, that of AlN is the smallest, so the acidity (A) order of the three catalyst is: AAlF3 > AAlPO4 > AAlN, the acid amounts are listed in Table 2. Figure 5.2) is the NH3-TPD results for the Ni/AlF3, Ni/AlPO4 and Ni/AlN catalysts. By comparing the catalyst doped and undoped Ni from Figure 5, it was found that the distribution of acid sites (NH3 desorption peak position) and the peak area were not obviously different. Their acidity results are shown in Table 2. The acid amount of the catalyst before and after the loading of Ni did not change significantly. Thus, the loading Ni did not significantly alter the Lewis acid properties of catalysts.

Figure 6 TEM images of Ni/AlF3 catalyst
As can be seen from Table 1, the addition of Ni improved the activity of catalysts. As the Ni/AlF3 catalyst has good activity for De-HF of HFCs, Ni supported on the catalyst surface are characterized by TEM, as shown in Figure 6. The lattice spacing of AlF3 was calculated to be 0.35214 nm, which was attributed to the 012 crystal plane of AlF3 according to the standard card of XRD. Distribution of Ni Particle Size As shown in the histogram in Figure 6, the size of Ni particles on the surface is mainly distributed at about 3 nm, which perform the excellent activity for De-HF of HFCs in this region of Ni particles.

Effect of Lewis acid sites and Ni
The Lewis acid sites play an important role to dehydrofluorination (De-HF) of hydrofluorocarbons (HFCs). De-HF of HFCs requires more stringent conditions of reaction, such as high temperatures. The dissociation energy of C-F is 522.0 ± 8.4 kJ mol -1 , which is the higher than 414 kJ mol -1 of C-H, so the cleavage of C-F bond is considered to be a key step of catalytic De-HF of HFCs. It is well known that Lewis acid are the reactivity centers of the activated C-X (X=F, Cl, Br, I) bonds [12]. AlF3 catalyst has a high activity for De-HF of HFCs due to the fact that AlF3 has large number of Lewis acid sites (0.326 mmol g -1 ). For the AlF3, AlN and AlPO4 catalyst, it was also found that Lewis acid sites were proportional to activity of De-HF, the same results reported in our previous works [19].
According to the result of  CF2HCH3). Besides, the stability of AlF3 catalyst obviously improved by addition of Ni as seen from the Table 1. The activity of AlF3 catalyst decline with time on stream, the long-chain fluorocarbons and/or coke coving the Lewis acid centres might result in deactivation of catalysts. The long-chain fluorocarbons and coke derived from the coupling and cracking of carbonpositive ion intermediates [13], carbon-positive ion intermediate generated by cleavage of C-F bond over strong Lewis acid sites which considering as the first step [12]. The Ni/AlF3 catalysts possessed good activity and stability, it may be due to the fact that Ni restrain the formation of carbon-positive ion intermediate through the synergistic catalysis.
Scheme 3 shows the De-HF process of HFCs to fluoroolefins over the bi-functional Ni/AlF3 catalyst. Carmichael Group [20,21] proposed two De-HF mechanism of CF3CH3 and CF2HCH3, which is the homolytic and heterolytic cleavage in the C-F and C-H bond fracture. For the AlF3 catalyst, the catalytic mechanism due to the carbon-positive ion (heterolytic cleavage), the activation of C-F bond over Lewis acid sites is the initial step. However, for the Ni/AlF3 catalyst, the synergistic catalysis took place on the surface of catalyst (homolytic cleavage), the synergetic activation of C-F and C-H bond by Lewis acid sites and Ni showed in schematic diagram (Scheme 3). So, in the E2 elimination mechanism of HFCs, Lewis acid sites and Ni activated C-F and C-H bond, respectively. This mechanism could avoid the carbon-positive ion intermediates as much as possible. That may be main reason that Ni/AlF3 catalyst has a good catalytic performance.

Conclusion
Catalytic performance of AlF3, AlN and AlPO4 catalyst were investigated for dehydrofluorination of CF3CFH2 and CF2HCH3, respectively. By the IR and NH3-TPD characterization, the activity of AlF3, AlN and AlPO4 catalyst were related to their Lewis acid sites. After additive Ni in the three Al catalysts, catalytic performance of catalysts were improve obviously, and of that Ni/AlF3 catalyst is the best, the Ni particle is about 3 nm. The dehydrofluorination of HFCs to fluoroolefin over the Ni/AlF3 catalyst were subjected to a synergistic catalysis of Lewis acid sites and Ni.