Solution Combustion Synthesis of Cr2O3 Nanoparticles and the Catalytic Performance for Dehydrofluorination of 1,1,1,3,3-Pentafluoropropane to 1,3,3,3-Tetrafluoropropene

Cr2O3 nanoparticles were prepared by solution combustion synthesis (SCS) with chromium nitrate as the precursor and glycine as the fuel. Commercial Cr2O3 and Cr2O3 prepared by a precipitation method were also included for comparison. The morphology, structure, acidity and particle size of fresh and spent Cr2O3 catalysts were investigated by techniques such as XRD, SEM, TEM, BET and NH3-TPD. In addition, catalytic performance was evaluated for the dehydrofluorination of 1,1,1,3,3-pentafluoropropane (CF3CH2CHF2, HFC-245fa) to 1,3,3,3-tetra-fluoropropene (CF3CH=CHF, HFO-1234ze). The catalytic reaction rate of Cr2O3 prepared by SCS method is as high as 6 mmol/h/g, which is about 1.5 times and 2 times higher than that of precipitated Cr2O3 and commercial Cr2O3, respectively. The selectivity to HFO-1234ze for all the catalysts maintains at about 80%. Compared with commercial and precipitated Cr2O3, Cr2O3-SCS prepared by SCS possesses higher specific surface area and acid amount. Furthermore, significant change in the crystal size of Cr2O3 prepared by SCS after reaction was not detected, indicating high resistance to sintering.


Introduction
1,1,1,3,3-Pentafluoropropane (CF 3 CH 2 CHF 2 , HFC-245fa) is a typical hydrofluorocarbon (HFC). It is mostly used as a physical foaming agent [1][2][3]. Due to its zero ozone depletion potential (ODP), HFC-245fa is being considered a third-generation foaming agent. However, its global warming potential (GWP) is about 1030 times higher than that of CO 2 . Therefore, HFC-245fa is controlled as a potent greenhouse gas by Kyoto Protocol and its various amendments. Recently, HFC-245fa was suggested to be the feedstock for producing 1,3,3,3-tetrafluoropropene (CF 3 CH=CHF, HFO-1234ze). It is an effective way for the sustainable development of HFCs. With ODP of 0 and GWP of only 6, HFO-1234ze is considered as one of the new generation of refrigerants and heat transfer working fluids [4]. In addition, HFO-1234ze is also applied in other fields, for example as a monomer for the synthesis of stable and elastic rubber plastics, a raw material for the preparation of agricultural chemicals, and a fire-proof protective gas for melting magnesium or magnesium alloy [5].
In addition, HFO-1234ze can be converted to 2,3,3,3-tetrafluoropropylene (CF 3 CF=CH 2 , HFO-1234yf) in the presence of a suitable catalyst. HFO-1234yf is also a novel refrigerant [6][7][8]. However, as a major by-product of dehydrofluorination, corrosive HF poses significant challenges for the stability of catalysts. To survive in a highly corrosive HF atmosphere, metal fluorides, such as AlF 3 , metal oxides such as Cr 2 O 3 and activated carbon (AC) were explored as the catalysts. Due to the abundant pores, activated carbon as the catalyst exhibits high activity. However, activated carbon is difficult to recover after deactivation due to the carbon deposition, which limits its commercial application.
It was found that the traditional catalysts adopted in fluorochemical industry, such as Cr 2 O 3 and AlF 3 , present high activity for the conversion of HFC-245fa [9][10][11]. However, due to the strong acidity of these catalysts, coke deposition is a major challenge, leading to the rapid deactivation. In order to improve the performance of these catalysts, metal components such as Rh, Ni, Mg and Pd are adopted as the effective promoters to reduce coke deposition and increase the lifespan of the catalysts [10][11][12][13]. However, the addition of precious metals such as Rh and Pd significantly increases the cost of the catalysts. Introduction of oxygen into the feeding gas is one of the solutions to avoid the coke deposition and deactivation [8,14]. However, oxygen introduction leads to the loss of HFC-245fa and products, as HFC-245fa and products react with oxygen forming CO and CO 2 . In addition, the presence of oxygen also increases the cost of separation.
To inhibit the coke deposition and improve the performance of catalysts, the nanoscale or mesoporous catalysts were prepared by various methods [15][16][17]. At present, research is intensively focused on the preparation of nano-Cr 2 O 3 [18,19]. With SBA-15 or Ca 3 (PO 4 ) 2 as the hard templates, Sun et al. [20] prepared Cr 2 O 3 nanorods or nanoparticles with high specific surface areas. Mouni Roy et al. [21] synthesized Cr 2 O 3 nanocubes with porous structure by solvothermal method. In our previous work [22], we fabricated nano-Cr 2 O 3 catalyst successfully by solution combustion synthesis (SCS) method. The nano-Cr 2 O 3 exhibits improved catalytic dehydrofluorination of 1,1-difluoroethane (HFC-152a, CH 3 CHF 2 ) to the monomer of vinyl fluoride (VF, CH 2 =CHF). To ensure the complete formation of crystallized Cr 2 O 3 , the catalyst was calcined in air at 500 • C. Unfortunately, as reported, the calcination at 400 • C led to the increase in the particle size by more than 50% [23]. In addition, the pre-fluorination by CHClF 2 before reaction also results in the partial coke deposition.
Precipitation is one of the most conventional routes for the preparation of oxide catalyst. However, precipitation usually produces significant amounts of waste solution. In addition, it is difficult to achieve uniform nanoparticles. By contrast, it is well accepted that the solution combustion synthesis is an important method for preparing nano-catalysts [24][25][26]. SCS method is simple, convenient and scalable. For the preparation of catalysts, SCS is mainly carried out by heating corresponding metal nitrates as oxidants and desired amounts of organics as the fuels through combustion. Following SCS, the product is usually crystallized with high surface area. During the combustion, large amounts of gases are produced which flush the solid agglomerates resulting in the fine powder. Herein, we synthesize Cr 2 O 3 catalyst by SCS method with chromium nitrate as the precursor and glycine as the fuel for the dehydrofluorination of HFC-245fa to HFO-1234ze. It is emerging as one of the new generations of refrigerant and heat transfer working fluid. In the present investigation, Cr 2 O 3 was evaluated as the catalyst for the catalytic dehydrofluorination of 1,1,1,3,3-pentafluoropropane (CF 3 CH 2 CHF 2 , HFC-245fa) to HFO-1234ze. It provides a potential way for the preparation value added HFO-1234ze. To avoid the sintering, no calcination of catalyst was adopted. Also, Cr 2 O 3 tends to be partially fluorinated by the reactant, HFC-245fa and the dehydrofluorination product, HF. Consequently, different from the previous study, no pre-fluorination treatment was adopted.

Evaluation of Catalytic Activity
The catalytic activities of Cr 2 O 3 samples prepared by precipitation method (denoted as Cr 2 O 3 -P), solution combustion synthesis (SCS) and commercial Cr 2 O 3 (denoted as Cr 2 O 3 -C) for the pyrolysis of HFC-245fa to HFO-1234ze are shown in Figure 1. Pyrolysis of HFC-245fa was carried out at the pressure of 1 atm and GHSV (gas hourly space velocity, HFC-245fa) of 150 h −1 . During the reaction, HFO-1234ze was detected as the major product. Minor by-products include HFO-1234yf and trace amounts of 3,3,3-trifluorine-1-propyne (CF 3 CF≡CH 2 ). As displayed in Figure 1a, the conversion level of HFC-245fa increases with reaction temperature significantly for all the catalysts. The Cr 2 O 3 -P and Cr 2 O 3 -SCS catalysts commence to catalyze the decomposition of HFC-245fa at temperatures below 175 • C. By contrast, the Cr 2 O 3 -C catalyst starts to promote the reaction at temperatures above 250 • C. Clearly, Cr 2 O 3 -SCS catalyst exhibits highest activity among these catalysts. Furthermore, the activity differs dramatically between Cr 2 O 3 -SCS and the other two catalysts with the increase in reaction temperature. The conversion rate of HFC-245fa over Cr 2 O 3 -SCS catalyst is about 1.5 mmol/h/g and close to 6 mmol/h/g at reaction temperature of 175 • C and 350 • C, respectively. Clearly, the reaction rate of Cr 2 O 3 -SCS catalyst is about 1.5 times and 2 times higher than that of precipitated Cr 2 O 3 and commercial Cr 2 O 3 at 350 • C. The selectivity to HFO-1234ze over all catalysts maintain at about 80% at temperatures between 175 and 350 • C. As presented in Figure 1c,d, all the catalysts show stable catalytic performance within a time on stream (TOS) of 10 h at 300 • C. During the reaction, significant amounts of HF were produced which may react with Cr 2 O 3 changing the composition of the catalyst. Consequently, Cr 2 O 3 was partially converted to CrOxFy by HF. It was suggested that CrOxFy is the active species in dehydrofluorination reactions [27][28][29]. Therefore, Cr 2 O 3 exhibits stable activity in HF atmosphere. The catalytic activities of Cr2O3 samples prepared by precipitation method (denoted as Cr2O3-P), solution combustion synthesis (SCS) and commercial Cr2O3 (denoted as Cr2O3-C) for the pyrolysis of HFC-245fa to HFO-1234ze are shown in Figure 1. Pyrolysis of HFC-245fa was carried out at the pressure of 1 atm and GHSV (gas hourly space velocity, HFC-245fa) of 150 h −1 . During the reaction, HFO-1234ze was detected as the major product. Minor by-products include HFO-1234yf and trace amounts of 3,3,3-trifluorine-1-propyne (CF3CF≡CH2). As displayed in Figure 1a, the conversion level of HFC-245fa increases with reaction temperature significantly for all the catalysts. The Cr2O3-P and Cr2O3-SCS catalysts commence to catalyze the decomposition of HFC-245fa at temperatures below 175 °C. By contrast, the Cr2O3-C catalyst starts to promote the reaction at temperatures above 250 °C. Clearly, Cr2O3-SCS catalyst exhibits highest activity among these catalysts. Furthermore, the activity differs dramatically between Cr2O3-SCS and the other two catalysts with the increase in reaction temperature. The conversion rate of HFC-245fa over Cr2O3-SCS catalyst is about 1.5 mmol/h/g and close to 6 mmol/h/g at reaction temperature of 175 °C and 350 °C, respectively. Clearly, the reaction rate of Cr2O3-SCS catalyst is about 1.5 times and 2 times higher than that of precipitated Cr2O3 and commercial Cr2O3 at 350 °C. The selectivity to HFO-1234ze over all catalysts maintain at about 80% at temperatures between 175 and 350 °C. As presented in Figure 1c,d, all the catalysts show stable catalytic performance within a time on stream (TOS) of 10 h at 300 °C. During the reaction, significant amounts of HF were produced which may react with Cr2O3 changing the composition of the catalyst. Consequently, Cr2O3 was partially converted to CrOxFy by HF. It was suggested that CrOxFy is the active species in dehydrofluorination reactions [27][28][29]. Therefore, Cr2O3 exhibits stable activity in HF atmosphere.

Morphology and Structure of Cr 2 O 3 Catalysts
The morphology and structure of catalysts were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results are displayed in Figure 2. Due to the large amounts of gases released by glycine combustion, the surface of Cr 2 O 3 -SCS catalyst appears to be rather rough and porous. Abundant pores provide higher specific surface area which is a key parameter affecting the catalytic performance. Furthermore, the particle size is less than 100 nm, which is much smaller than the other two catalysts. The small particle size improves the exposure of catalyst surface, providing much more active sites. Thus, developed pores and small particles of Cr 2 O 3 -SCS catalyst result in higher catalytic activity. There are no clear porous channels on the surface of Cr 2 O 3 -P and larger particles are observed. The particle of Cr 2 O 3 -C is irregular, smooth and solid, which may partly contribute to the low catalytic performance over Cr 2 O 3 -C catalyst.

Morphology and Structure of Cr2O3 Catalysts
The morphology and structure of catalysts were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results are displayed in Figure 2. Due to the large amounts of gases released by glycine combustion, the surface of Cr2O3-SCS catalyst appears to be rather rough and porous. Abundant pores provide higher specific surface area which is a key parameter affecting the catalytic performance. Furthermore, the particle size is less than 100 nm, which is much smaller than the other two catalysts. The small particle size improves the exposure of catalyst surface, providing much more active sites. Thus, developed pores and small particles of Cr2O3-SCS catalyst result in higher catalytic activity. There are no clear porous channels on the surface of Cr2O3-P and larger particles are observed. The particle of Cr2O3-C is irregular, smooth and solid, which may partly contribute to the low catalytic performance over Cr2O3-C catalyst.
To further investigate the micro-structure of Cr2O3-SCS catalyst, TEM characterization was carried out, and the images are demonstrated in Figure 2d1,d2 and d3. Clearly, Cr2O3-SCS is composed of ultra-fine particles ( Figure 2d1). As displayed in Figure 2d3, the distances between the lattice fringes are confirmed to be 0.363, 0.245 and 0.167 nm, respectively, which are assigned to the d-spacing values of the (111) and (200) planes in Cr2O3 crystalline. According to the selected area electron diffraction (SAED) patterns in Figure 2d2, a large number of diffraction points are arranged around the center point, indicating that Cr2O3-SCS is close to the single crystal structure. In addition, there are also many disorderly multiple diffraction points, implying that the surface of Cr2O3-SCS is partly covered by carbon produced during combustion. In conclusion, differences in surface morphology and structure of the three catalysts may lead to the differences in specific surface area, which further affects the exposure level of active site and catalytic activity.
To elucidate the evolution of catalyst crystallization before and after the reaction, all catalysts were characterized by X-ray diffraction (XRD) and the results are demonstrated in Figure 3. As indicated in Figure 3, the XRD patterns of Cr2O3 derived from different preparation methods agree well with that of standard Cr2O3 pattern (PDF #38-1479, the space group: R-3c (167)), indicating that all samples possess the phase of Cr2O3 crystalline. In our previous study, the Cr2O3-SCS was calcined in air at 500 °C to obtain the complete crystal structure [22]. Clearly, calcination results in the partial sintering. Figure 3 conforms that that pure Cr2O3 can be derived without calcination. As a result, the particle size of Cr2O3-SCS is smaller than 100 nm, and while it is up to about 200 nm in our previous study.  Figure 2d 2 , a large number of diffraction points are arranged around the center point, indicating that Cr 2 O 3 -SCS is close to the single crystal structure. In addition, there are also many disorderly multiple diffraction points, implying that the surface of Cr 2 O 3 -SCS is partly covered by carbon produced during combustion.
In conclusion, differences in surface morphology and structure of the three catalysts may lead to the differences in specific surface area, which further affects the exposure level of active site and catalytic activity.
To elucidate the evolution of catalyst crystallization before and after the reaction, all catalysts were characterized by X-ray diffraction (XRD) and the results are demonstrated in Figure 3. As indicated in Figure 3, the XRD patterns of Cr 2 O 3 derived from different preparation methods agree well with that of standard Cr 2 O 3 pattern (PDF #38-1479, the space group: R-3c (167)), indicating that all samples possess the phase of Cr 2 O 3 crystalline. In our previous study, the Cr 2 O 3 -SCS was calcined in air at 500 • C to obtain the complete crystal structure [22]. Clearly, calcination results in the partial sintering. Figure 3 conforms that that pure Cr 2 O 3 can be derived without calcination. As a result, the particle size of Cr 2 O 3 -SCS is smaller than 100 nm, and while it is up to about 200 nm in our previous study. as obtained samples are highly crystallized. However, the intensities of the crystal diffraction peaks based on (012), (104) (110), (113), (024), (116), (214) and (300) planes differ significantly among the three catalysts. Compared with Cr2O3-P and Cr2O3-SCS, although with identical diffraction peaks, the intensity of Cr2O3-C is much stronger, indicating Cr2O3-C catalyst possesses the highest degree of crystallinity among three catalysts. To further investigate the growth of Cr2O3 crystalline during reaction, we calculated the crystal sizes of fresh and spent Cr2O3 based on diffraction peaks of Cr2O3 phase. All the data of crystal sizes are calculated according to Scherrer formula [30] and the results are listed in Table 1.  Cr2O3-C-fresh >100 Cr2O3-C-spent >100 The crystal size of Cr2O3-SCS catalyst increases slightly after reaction based on all the crystal facets. It indicates that the no significant sintering is observed for Cr2O3-SCS catalyst after time on stream of 10 h. By contrast, the crystal size of fresh Cr2O3-P catalyst is relatively larger than that of Cr2O3-SCS. In addition, it sinters significantly following reaction. Cr2O3-C has a crystal size of more than 100 nm and a larger particle size (It is out of the calculation range of Scherrer equation when the exact crystal size is larger than 100 nm).
In summary, the Cr2O3-SCS possesses higher sintering resistance, and while Cr2O3-P catalyst sinters facilely under reaction conditions. In addition, we suggest that the carbon produced in the process of combustion for Cr2O3-SCS catalyst prohibits the particle from growing. Figure 4 demonstrates the N2 adsorption-desorption isotherms for all the catalysts. The isotherms of Cr2O3-P and Cr2O3-SCS exhibit type IV characteristic (according to the IUPAC classification) with a well-defined capillary condensation step and H3 hysteresis loops which are usually observed with the aggregates of particles giving rise to slit-shape pores [31]. The pores are  Table 1. The crystal size of Cr 2 O 3 -SCS catalyst increases slightly after reaction based on all the crystal facets. It indicates that the no significant sintering is observed for Cr 2 O 3 -SCS catalyst after time on stream of 10 h. By contrast, the crystal size of fresh Cr 2 O 3 -P catalyst is relatively larger than that of Cr 2 O 3 -SCS. In addition, it sinters significantly following reaction. Cr 2 O 3 -C has a crystal size of more than 100 nm and a larger particle size (It is out of the calculation range of Scherrer equation when the exact crystal size is larger than 100 nm).
In summary, the Cr 2 O 3 -SCS possesses higher sintering resistance, and while Cr 2 O 3 -P catalyst sinters facilely under reaction conditions. In addition, we suggest that the carbon produced in the process of combustion for Cr 2 O 3 -SCS catalyst prohibits the particle from growing. Figure 4 demonstrates the N 2 adsorption-desorption isotherms for all the catalysts. The isotherms of Cr 2 O 3 -P and Cr 2 O 3 -SCS exhibit type IV characteristic (according to the IUPAC classification) with a well-defined capillary condensation step and H3 hysteresis loops which are usually observed with the aggregates of particles giving rise to slit-shape pores [31]. The pores are majorly generated between particles gaps. As expected, the Cr 2 O 3 -C catalyst shows a very low nitrogen adsorption, indicating the limited porosity. It is consistent with the SEM results that the surface of Cr 2 O 3 -C particles is very smooth. Furthermore, the Cr 2 O 3 -P and Cr 2 O 3 -SCS catalysts exhibit two capillary condensation steps. The capillary condensation step at relative pressure (P/P 0 ) of 0.1-0.8 results from the adsorption of nitrogen in micropores, indicating that there are very few micropores in the pore walls between the adjacent nanorods. Another step at higher pressures (above 0.8) is derived from the adsorption of nitrogen in mesopores.
the specific surface area of catalyst Cr2O3-C is as low as 0.6 m 2 /g, and while that of Cr2O3-SCS catalyst is as high as 58.2 m 2 /g. The specific surface area of Cr2O3-P is almost the average of them. Clearly, Cr2O3-SCS catalyst has developed pores compared with the other catalysts. It is consistent with SEM and TEM results that the Cr2O3-SCS catalyst with coarse surface possess a larger specific surface area, and a smaller specific surface area with smooth and non-porous surface over Cr2O3-C. As mentioned above, higher specific surface area usually provides more active sites. Thus, specific surface area and pore structure is one of the reasons for the difference in reaction rates over the three catalysts (Cr2O3-SCS > Cr2O3-P > Cr2O3-C).
It is worth noting that the surface area of Cr2O3-SCS is about 32 m 2 /g following calcination at 500 °C [22]. As displayed in Figure 3, the temperature during SCS is sufficient for the formation of Cr2O3 crystalline. Clearly, without calcination, sintering is avoided leading to improved surface area in this study.   Figure 5 presents the results of X-ray photoelectron spectroscopy (XPS) experiments for Cr2O3-SCS and Cr2O3-C. Very similar peaks are observed for both catalysts. According to the deconvolution of Cr 2p3/2 peaks, there are three Cr species both for Cr2O3-SCS and Cr2O3-C. The peak with binding energy of 576.1 eV indicates the existence of Cr(OH)3 [19,32] and the peak at binding energy of 577.3 eV is suggested to be the typical peak of Cr2O3 [33,34]. The peak at 578.7 eV is attributed to the CrO3 species [19,35]. Clearly, Cr(OH)3, Cr2O3, and CrO3 co-exist on the surface of Cr2O3-SCS and Cr2O3-C. The emergence of CrO3 is most probably attributed to oxidation of Cr2O3 at high temperatures. To confirm the effect of pore structure on catalytic performance, the textural parameters such as specific surface area, pore volume and pore size distribution are summarized in Table 2. As expected, the specific surface area of catalyst Cr 2 O 3 -C is as low as 0.6 m 2 /g, and while that of Cr 2 O 3 -SCS catalyst is as high as 58.2 m 2 /g. The specific surface area of Cr 2 O 3 -P is almost the average of them. Clearly, Cr 2 O 3 -SCS catalyst has developed pores compared with the other catalysts. It is consistent with SEM and TEM results that the Cr 2 O 3 -SCS catalyst with coarse surface possess a larger specific surface area, and a smaller specific surface area with smooth and non-porous surface over Cr 2 O 3 -C. As mentioned above, higher specific surface area usually provides more active sites. Thus, specific surface area and pore structure is one of the reasons for the difference in reaction rates over the three catalysts (Cr 2 O 3 -SCS > Cr 2 O 3 -P > Cr 2 O 3 -C). It is worth noting that the surface area of Cr 2 O 3 -SCS is about 32 m 2 /g following calcination at 500 • C [22]. As displayed in Figure 3, the temperature during SCS is sufficient for the formation of Cr 2 O 3 crystalline. Clearly, without calcination, sintering is avoided leading to improved surface area in this study.  [19,32] and the peak at binding energy of 577.3 eV is suggested to be the typical peak of Cr 2 O 3 [33,34]. The peak at 578.7 eV is attributed to the CrO 3 species [19,35]. Clearly, Cr(OH) 3 Table 3 lists their respective surface compositions. According to the XPS results, the dominant phase on the surface of the sample is confirmed to be Cr2O3. The Cr2O3-SCS catalyst has more CrO3 phase. It may be resulted from the combustion of glycine which produces high temperature instantaneously facilitating the oxidation of Cr2O3. Another phase, Cr(OH)3 transforms into Cr2O3 readily at temperature above 300 °C. Hence, Cr(OH)3 plays a role in the formation of Cr2O3 although there is noticeable difference between Cr2O3-SCS and Cr2O3-C. However, the significant difference in CrO3 contents on the surface catalyst plays a major role in the catalytic performance as high-valent Cr species such as Cr (VI) are vital for the reaction because they could be transformed to the active species such as CrOxFy. As demonstrated in Table 4, the spent Cr2O3-SCS contains significant amounts of fluorine element, indicating that there are species such as CrOxFy in the surface.  Table 4. Surface composition of fresh and spent Cr2O3-SCS determined by X-ray energy spectrometer (EDS). It is generally accepted that the active site of dehydrofluorination reaction is the surface acidic site of the catalyst [36]. Therefore, the activity of catalyst increases with surface acid content. Unfortunately, the acidic site is also the coke deposition center which is majorly responsible for the stability of catalyst [37][38][39]. The surface acidity of catalysts was characterized by temperatureprogrammed desorption of ammonia (NH3-TPD) as illustrated in Figure 6. The NH3-TPD is usually used for the investigation of the acid strength and acid amount on the surface of catalysts. A broad desorption profile in the range of 100-700 °C in Cr2O3-SCS is observed with three peaks at around 60 °C, 430 °C and 620 °C respectively. Five peaks appear in Cr2O3-P at around 195 °C, 255 °C, 340 °C, 450 °C, 620 °C. Desorption of NH3 on Cr2O3-C were minor.  Table 3 lists their respective surface compositions. According to the XPS results, the dominant phase on the surface of the sample is confirmed to be Cr 2 O 3 . The Cr 2 O 3 -SCS catalyst has more CrO 3 phase. It may be resulted from the combustion of glycine which produces high temperature instantaneously facilitating the oxidation of Cr 2 O 3 . Another phase, Cr(OH) 3 transforms into Cr 2 O 3 readily at temperature above 300 • C. Hence, Cr(OH) 3 plays a role in the formation of Cr 2 O 3 although there is noticeable difference between Cr 2 O 3 -SCS and Cr 2 O 3 -C. However, the significant difference in CrO 3 contents on the surface catalyst plays a major role in the catalytic performance as high-valent Cr species such as Cr (VI) are vital for the reaction because they could be transformed to the active species such as CrOxFy. As demonstrated in Table 4, the spent Cr 2 O 3 -SCS contains significant amounts of fluorine element, indicating that there are species such as CrOxFy in the surface. It is generally accepted that the active site of dehydrofluorination reaction is the surface acidic site of the catalyst [36]. Therefore, the activity of catalyst increases with surface acid content. Unfortunately, the acidic site is also the coke deposition center which is majorly responsible for the stability of catalyst [37][38][39]. The surface acidity of catalysts was characterized by temperature-programmed desorption of ammonia (NH 3 -TPD) as illustrated in Figure 6. The NH 3 -TPD is usually used for the investigation of the acid strength and acid amount on the surface of catalysts. A broad desorption profile in the range of 100-700 • C in Cr 2 O 3 -SCS is observed with three peaks at around 60 • C, 430 • C and 620 • C respectively. Five peaks appear in Cr 2 O 3 -P at around 195 • C, 255 • C, 340 • C, 450 • C, 620 • C. Desorption of NH 3 on Cr 2 O 3 -C were minor.

Catalysts
Clearly, compared with Cr2O3-SCS and Cr2O3-P, Cr2O3-C contains very low level of acidity. As mentioned previously, acid sites are the active centers of dehydrofluorination reaction. Thus, it is reasonable that the reaction rate is very low over Cr2O3-C. Furthermore, the peak areas of NH3 desorption (represent the number of acidic sites) for the three catalysts were estimated. It is confirmed that the acid amount of Cr2O3-SCS is two times higher than that of Cr2O3-P, and about 8 times higher that of Cr2O3-C. It explains the results that Cr2O3-SCS has the highest catalytic activity, followed by Cr2O3-P. Therefore, acid content and acid species are the main reasons for the difference in catalytic activity of catalysts. As discussed previously, different from our previous study [22], no calcination was adopted during the catalyst preparation. Clearly, as demonstrated in Figure 1 and Figure 3, high activity and well crystallized Cr2O3 are achieved without calcination. As demonstrated in Figure 7a, following calcination at 500 °C for 2 h, compared with the sample without calcination (Figure 2b), significant sintering is observed. In addition, the XRD patterns (Figure 7b) reinforce the conclusion. Following calcination, although exact the same diffraction peaks were detected, the peak intensities increase dramatically, indicating the growth of Cr2O3 crystalline.  Clearly, compared with Cr 2 O 3 -SCS and Cr 2 O 3 -P, Cr 2 O 3 -C contains very low level of acidity. As mentioned previously, acid sites are the active centers of dehydrofluorination reaction. Thus, it is reasonable that the reaction rate is very low over Cr 2 O 3 -C. Furthermore, the peak areas of NH 3 desorption (represent the number of acidic sites) for the three catalysts were estimated. It is confirmed that the acid amount of Cr 2 O 3 -SCS is two times higher than that of Cr 2 O 3 -P, and about 8 times higher that of Cr 2 O 3 -C. It explains the results that Cr 2 O 3 -SCS has the highest catalytic activity, followed by Cr 2 O 3 -P. Therefore, acid content and acid species are the main reasons for the difference in catalytic activity of catalysts.

Catalysts Preparation
As discussed previously, different from our previous study [22], no calcination was adopted during the catalyst preparation. Clearly, as demonstrated in Figures 1 and 3, high activity and well crystallized Cr 2 O 3 are achieved without calcination. As demonstrated in Figure 7a, following calcination at 500 • C for 2 h, compared with the sample without calcination (Figure 2b), significant sintering is observed. In addition, the XRD patterns (Figure 7b) reinforce the conclusion. Following calcination, although exact the same diffraction peaks were detected, the peak intensities increase dramatically, indicating the growth of Cr 2 O 3 crystalline. Clearly, compared with Cr2O3-SCS and Cr2O3-P, Cr2O3-C contains very low level of acidity. As mentioned previously, acid sites are the active centers of dehydrofluorination reaction. Thus, it is reasonable that the reaction rate is very low over Cr2O3-C. Furthermore, the peak areas of NH3 desorption (represent the number of acidic sites) for the three catalysts were estimated. It is confirmed that the acid amount of Cr2O3-SCS is two times higher than that of Cr2O3-P, and about 8 times higher that of Cr2O3-C. It explains the results that Cr2O3-SCS has the highest catalytic activity, followed by Cr2O3-P. Therefore, acid content and acid species are the main reasons for the difference in catalytic activity of catalysts. As discussed previously, different from our previous study [22], no calcination was adopted during the catalyst preparation. Clearly, as demonstrated in Figure 1 and Figure 3, high activity and well crystallized Cr2O3 are achieved without calcination. As demonstrated in Figure 7a, following calcination at 500 °C for 2 h, compared with the sample without calcination (Figure 2b), significant sintering is observed. In addition, the XRD patterns (Figure 7b) reinforce the conclusion. Following calcination, although exact the same diffraction peaks were detected, the peak intensities increase dramatically, indicating the growth of Cr2O3 crystalline.

Solution Combustion Synthesis Method
Similar to our previous work [22], Cr(NO 3 ) 3 ·9H 2 O (>99.5%, Aladdin Company, Shanghai, China) was used as the Cr 2 O 3 precursor. In a typical experiment, 20 g Cr(NO 3 ) 3 ·9H 2 O was first dissolved in 75 mL distilled water. Under vigorous stirring, 12.5 g glycine (>99%, Aladdin Company) as the fuel was added and mixed well. Then, the mixed solution was condensed at 70 • C in a furnace. As a result, the gel-like paste was obtained. Following condensing, the paste was loaded to a microwave oven (800 W, 2.45 GHz, 23 L, Midea Company, Foshan, Guangdong, China. Assisted by microwave, large amounts of smoke were released and combustion flame was observed in about less than 3 min. Following combustion, the sample was cooled to room temperature and green foam-like Cr 2 O 3 powder was obtained. The sample is denoted as Cr 2 O 3 -SCS.

Precipitation Method
As a comparison, Cr 2 O 3 was also prepared by conventional precipitation method. 0.1 mol Cr(NO 3 ) 3 ·9H 2 O was first dissolved in 200 mL distilled water and an excessive amount of aqueous ammonia solution was added stepwise. After vigorous stirring for 2 h, the solution was filtered by Buchner funnel with a vacuum pump. Then the obtained solid was dried at 80 • C for 12 h. The sample is denoted as Cr 2 O 3 -P.

Commercial Cr 2 O 3
Analytically pure Cr 2 O 3 received from Aladdin Company was adopted as a reference catalyst. The samples were ground and pressed into pellets at 20 MPa followed by crushing and sieving Cr 2 O 3 particles between 0.4 and 0.8 mm. The sample is denoted as Cr 2 O 3 -C.

Catalytic Activity
Catalytic activity evaluation for dehydrofluorination reaction of 1,1,1,3,3-pentafluoropropane (HFC-245fa) to HFO-1234ze was carried out with a fixed-tube reactor (stainless steel with i.d. of 7.5 mm, Golden Eagle Technology, Tianjin, China. Cr 2 O 3 catalyst (2 mL) was loaded into the isothermal zone of the reactor. A thermal couple in the middle of the catalyst bed functioned as the detector of the reaction temperature. Prior to reaction, the reactor was first purged with pure nitrogen to remove water vapor and air at reaction temperatures. The gas-phase HFC-245fa with GHSV (gas hourly space velocity) of 150 h −1 , balanced by four times of nitrogen, passed through the reactor. Prior to the experiments, all the catalysts were ground and pressed into pellets at 20 MPa followed by crushing and sieving. Cr 2 O 3 particles between 0.3 and 0.7 mm were collected and were loaded to the reactor. The gaseous effluent from the reactor passed through a scrubber containing about 1 M KOH solution (850 mL) to remove HF, followed by the composition analysis with a GC-9790 gas chromatograph (Fuli Instruments, Taizhou, Zhejiang, China) equipped with a thermal conductivity detector (TCD).

Catalyst Characterization
SEM images were used for the investigation of morphology were obtained on a FESEM system (Hitachi S-4700, Hitachi, Tokyo, Japan) with the accelerating voltage of 15 kV. It is equipped with an EDS. XRD patterns of catalysts were obtained by an X'Pert Pro analytical instrument (PANalytical B.V., Almelo, Netherlands). XPS was conducted at 3 mA and 15 kV on ESCALAB MkII (Waltham, MA, USA). To avoid the surface charging effect, binding energies were referenced to C1s binding energy of carbon, taken to be 284.6 eV. The XPS spectra were analyzed by the XPS peak software (XPS PEAk Fit 4.1, Systat Software, Inc., San Jose, CA, USA). TEM images included to further investigate the microstructure of catalysts were obtained with a 2100F Transmission Electron Microscope (JEOL, Akishima, Tokyo, Japan) at an acceleration voltage of 200 kV. The BET surface area and total pore volume of catalysts were measured by N 2 adsorption-desorption at −196 • C with Quantachrome Autosorb Automated Gas Sorption System (Quantachrome, Boynton beach, FL, USA). The catalyst samples were degassed at 200 • C for 6 h under vacuum before measurements. NH 3 -TPD was carried out in a self-made instrument and a thermal conductivity detector (TCD) was used for detecting the NH 3 signal.

Conclusions
The Cr 2 O 3 catalyst was successfully prepared by solution combustion synthesis method with Cr(NO 3 ) 3 ·9H 2 O as the Cr precursor and glycine as the fuel. In addition, the catalysts was evaluated for the dehydrofluorination of HFC-245fa producing HFO-1234ze. Cr 2 O 3 prepared by SCS exhibits very high reaction rate of HFC-245fa with 6 mmol/h/g, which is about 1.5 times and 2 times higher than that of precipitated Cr 2 O 3 and commercial Cr 2 O 3 , respectively. It remains stable after 10 h time on stream. Compared with commercial and precipitated Cr 2 O 3 , Cr 2 O 3 prepared by SCS possesses higher specific surface area and acid amount. Furthermore, no significant sintering of Cr 2 O 3 prepared by SCS under reaction was detected, indicating high resistance to sintering.