Combustion Synthesis of Amorphous Al and Cr Composite as the

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Kinetics, Catalysis, and Reaction Engineering

Combustion synthesis of amorphous Al and Cr composite as the catalyst for dehydrofluorination of 1,1-difluoroethane Wenfeng Han, Bing Liu, Xiliang Li, Luteng Yang, Jinchao Wang, Haodong Tang, and Wucan Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02915 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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Combustion synthesis of amorphous Al and Cr composite as the catalyst for dehydrofluorination of 1,1-difluoroethane Wenfeng Han*a,b,c, Bing Liu a, Xiliang Li a, Luteng Yang

a,b,c

, Jinchao Wang

a,b,c

,

Haodong Tang*a, Wucan Liu b,c a

Institute of Catalysis, Zhejiang University of Technology, Hangzhou 310014, Zhejiang, PR China

b

Zhejiang Research Institute of Chemical Industry, Hangzhou 310023, Zhejiang, PR China c

State Key Laboratory of Fluorinated Greenhouse Gases Replacement and Control Treatment, Hangzhou 310023, Zhejiang, PR China

ABSTRACT Interaction or lattice doping between Cr and Al contributes to the acidity and catalytic activity of Cr/Al catalysts, which is difficult to be achieved by the conventional preparation methods while maintaining nano-structures. We suggest that solution combustion synthesis (SCS) is one of the effective ways for this issue. Nanoscale Cr/Al composite catalysts of sheet structure with thickness around 40-100 nm were fabricated via simple solution combustion method with Cr(NO3)3·9H2O and Al(NO3)3·9H2O as the precursors and glycine as the fuel. Following SCS and fluorination pretreatment, Cr species interact with Al species intimately forming amorphous structure. The interaction provides relatively high amounts of defects and is favorable for the formation of active species, resulting in the enhanced Lewis acidity and catalytic activity. At 400 oC, a reaction rate of 33.9 mmol/h/g was achieved for the dehydrofluorination of HFC-152a over composite catalyst which is 30-40% higher than that of Cr and Al catalysts.

KEYWORDS: Dehydrofluorination; HFC-152a; Solution combustion synthesis; Cr/Al composite; Vinyl fluoride; Microwave; Lewis acidity

1


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1. INTRODUCTION Vinyl fluoride (CH2=CHF, VF) is the monomer to produce polyvinyl fluoride (PVF). PVF is a thermoplastic with various piezo-, pyro-, and ferroelectric properties. In addition, it is inert to various solvents, oils, acids with low permeability to different kinds of gases and liquids.1 Hence, it is almost exclusively considered as films and laminates for wood, paper, plastic, rubber, or metal. It can be laminated to and is used for pipe coverings, duct liners, aircraft interiors, truck and trailer sidings, and as release sheets for bag molding. It is one of the most widely used fluoropolymers for protective and decorative coatings.2 Intensive attention has been attracted to PVF recently. At present, VF is produced via two routes. The first one is the hydrofluorination of acetylene. However, significant amounts of HFC-152a (CH3CHF2, 1,1-difluoroethane ) are usually resulted following the further hydrofluorination of VF. Another route is the direct dehydrofluorination of HFC-152a. HFC-152a is one of the major components for the refrigerants, propellants and feedstock of other fluorocarbons.3 As a greenhouse gas, although its 100-year global warming potential is only 138, which is significantly lower than most of other hydrofluorocarbons (HFCs), the global emission of HFC-152a is increasing rapidly.4 Actually, the application of HFCs, including HFC-152a has been regulated by the Kigali Amendment to the Montreal Protocol.5 We suggest that conversion of these HFCs to corresponding fluorinated monomers via catalytic dehydrofluorination would be of significance. As suggested by our previous studies, HFC-152a can be converted to vinyl fluoride also

a

by catalytic pyrolysis over Cr2O3 catalyst.6-7 In addition, Cr2O3 is

potential

catalyst

1,1,1,2,3-pentafluoropropane 2,3,3,3-tetrafluoropropene

for

the

and

catalytic

dehydrofluorination

1,1,1,3,3-pentafluoropropane

(HFO-1234yf)

and

of to

1,3,3,3-tetrafluoropropene

(HFO-1234ze) respectively.8-9 Due to the highly corrosive environment during dehydrofluorination (formation of HF), the selection of catalysts is majorly limited to AlF3,10 Cr2O3 and MgF2 systems.11 However, these catalysts usually suffer serious deactivation following coke deposition or sintering.12 Consequently, the effects of morphology, surface area, phase structure and promoters have been investigated for 2



the improvement of catalytic performance.11, 13 It is well accepted that dehydrofluorination reactions are catalyzed by Lewis acidic sites.14 Consequently, AlF3, Cr2O3 and MgF2 with high surface areas were prepared to increase the acidic sites and catalytic activities.15-19 More recently, nanoscale metal fluorides were suggested to be promising catalyst candidates as they commonly possess high surface areas and unique acidic properties.12 Although the amount and strength of acidic sites are improved following the preparation of nanoparticles, carbon deposition and sintering of catalysts during catalytic reaction remain as the key challenges for the performance of catalysts. Strong Lewis acidic sites tend to promote the formation of coke.20 In addition, catalysts, especially in the form of nanoparticles sinter significantly at elevated temperatures.21 It was confirmed that MgF2 sinters significantly at temperatures below 300 oC.22 Bimetallic composite catalysts are potential candidates as they possess proper acidic strength and stability, such as Cr-Mg,23 Cr-Al24-25 and MgF2-MF2 composites (M = Ca, Sr, Ba).26 Cr-Al composite catalyst was found to be appropriate catalyst for the fluorination and Cl/F exchange reactions as it meets the requirement for the acidic and mechanical strength with high stability.24-25,

27

Consequently, preparation of Cr-Al composite

catalyst has attracted intensive attention. Generally, Cr-Al catalyst was prepared by co-precipitation in the solution with soluble Cr and Al salts as the precursors.24, 28 To obtain higher surface area, impregnation was suggested. AlF3 was impregnated with CrCl3 aqueous solution followed by drying and fluorination at 200-350 oC. Via impregnation, surface area as high as 50 m2/g was achieved.29 Cr2O3-α-AlF3 catalyst was fabricated with the help of carbon as the hard template. With the surface area of 115 m2/g, the catalyst shows high catalytic activity for the dehydrofluorination of 1,1-difluoroethane.25 However, it is difficult to achieve lattice doping or strong interaction between Cr and Al components although they are the effective ways to modify the acidity of catalysts and stabilize the structure.30 Otherwise, calcination at high temperatures is necessary which also results in significant sintering of catalysts.31 The Lewis acidity of metal (III) fluorides strongly depends on the bulk structure. 3


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Usually, the cations are coordinately saturated by fluorine anions and thus, can not act as Lewis acidic sites. For generating Lewis acidity in metal fluorides, doping with other metals is an effective method. However, results of doping in metal fluorides and their impact on the catalytic activity are limited.27 For instance, Murthy discovered that Cr3+ doping to MgF2 catalyst significantly increased the activity for the dismutation of CCl2F2. The catalytic activity is correlated with the enhanced Lewis acidic strength.23 Similar results were observed over Fe3+ doped MgF2 catalysts.32 In addition, Zn and Sn doping to Cr2O3 or MgF2 also led to the enhanced acidity, activity and improved stability.33-34 In the present work, Cr-Al composite catalysts were prepared via simple solution combustion synthesis (SCS). SCS has been adopted as a one-step method for the preparation of nanomaterials and nanoscale catalysts.35-36 SCS mainly includes the heating of a saturated aqueous solution of the required metal nitrates as the oxidizing agents and a suitable organic fuel as the reducing component to the ignition temperature followed by the spontaneous combustion of the mixed solution.35 During SCS, temperatures as high as 1000 oC can be approached in extremely short time.36 As a result, both nanoparticles and lattice doping or strong interaction between components were achieved.35 Inspired by these studies, this work prepared a series of Cr-Al composite catalysts with varied Cr/Al ratios. Then, the morphology, structure, physical and chemical properties of catalysts were investigated. Finally, the dehydrofluorination of HFC-152a

was

adopted

as

the

model

reaction

for

the

evaluation

of

dehydrofluorination performance over these catalysts.37

2. EXPERIMENTAL 2.1 Preparation of catalyst Firstly, 40.00 g Cr(NO3)3·9H2O, 37.51 g Al(NO3)3·9H2O and 50.00 g glycine (the fuel for SCS) were dissolved in 200 mL distillated water. Then, the solution was heated to 70 oC with vigorous stirring in a water bath. Following the evaporation of water in the solution, thick gel was obtained. 4



Scheme 1 Schematic illustration of Cr/Al composite catalysts preparation by microwave assisted solution combustion synthesis (SCS).

For the SCS, the gel was transferred to a porcelain dish and placed in a kitchen microwave oven (Meidi Company, 800 W, 2.45 GHz, 23 L). Following heating by microwave, flame was observed in 3 min. Significant amounts of white smog erupted in the microwave oven. After combustion, foam like sample was obtained. The solid was pressed into pellets and crushed. The particles with diameters from 0.3 mm to 0.7 mm were collected. Finally, it was pre-fluorinated in a fixed bed reactor with the flow of CHClF2 (30 mL /min) at 300 oC for 2 h. The catalyst is denoted as CA1 (with the Cr/Al molar ratio of 1). For catalysts CA3, CA1.2 and CA0.6, the Cr/Al molar ratio were adjusted to 3, 1.2 and 0.6 respectively during SCS. The preparation process is schematically illustrated in Scheme 1. As comparison, pure Cr catalyst and pure Al based catalysts were prepared and they are marked as Cr and Al respectively following similar SCS procedure.

2.2 Characterization of catalysts The details of characterization are given in Supporting Information. The acidity of catalyst was determined by NH3-TPD technique (ammonia temperature programmed desorption) over a fixed-bed reactor. Purged with He and adsorption of NH3, the temperature was increased from 50 oC to 700 oC with a ramp rate of 10 5


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o

C/min. The desorption of NH3 was recorded by a TCD detector. FT-IR with pyridine

adsorption on Nicolet 6700 was adopted for the identification of Brønsted and Lewis acidic sites. The morphology was investigated by SEM (scanning electron microscope, Hitachi S-4700) with the accelerating voltage of 29 kV and TEM (transmission electron microscopy (JEOL JEM-1200EX) equipped with an X-ray energy spectrometer (EDS). During the characterization of TEM, the sample was crushed into fine powder and dispersed in ethanol by ultrasonic treatment for 30 min. To get clear images, the samples were sprayed with Pt. Images were taken at operating voltage of 160 kV. XRD (X-ray diffraction), EPR (Electron Paramagnetic Resonance), Solid state NMR of 19F and 27Al and SERS (surface-enhanced Raman scattering) of Raman were adopted to explore the structures of catalysts. Surface areas of catalyst were evaluated by Autosorb-1/C gas adsorption analyzer (Quantachrome Instruments) with N2 adsorption at -196 oC. Prior to the analysis, the catalysts were degassed at 200 oC for 5 h. X-ray photoelectron spectroscopy (XPS, ESCALAB210, VG Co., photoelectron spectroscopy with a monochromatized microfocused Mg Ka-ray source) was employed to study the binding energy of Cr and Al in the catalysts. The binding energy was adjusted by the binding energy of adventitious carbon (C 1s) at 284.50 eV. Prior to the measurements, the powder sample, pressed into self-supporting disks, was loaded into a sub-chamber and then evacuated at 25 °C for 4 h. 2.3 Catalytic activity test Catalytic activity tests were carried on a fixed bed reactor (stainless steel, 7.5 mm i.d.). The temperature uniform zone of the reactor was determined to be 50 mm. The reaction temperatures ranged from 200 °C to 400 °C. The blank experiment showed that no noticeable reactions were detected. Prior to the reaction, 2 mL catalyst (0.3-0.7 mm) was first purged with N2 at 300 o

C for 1 h. Then, the temperature of the reactor was decreased to 200 oC and the feed 6



was switched to HFC-152a balanced by N2 (20 mL/min HFC-152a and 20 mL/min N2, controlled by the mass flowrate controllers respectively). The reaction temperature varies from 200 °C to 400 °C and while the pressure was kept at atmospheric pressure. Following reaction, the products were analyzed by an online gas chromatograph (Jiedao GC 1690) equipped with thermal conductivity detector (TCD) and a packed column (2mm×600mm) filled with 5% ODPN. The temperature of column was maintained at 80 oC and 100 oC for the sample port and TCD. Before the GC analysis, HF formed and moisture were captured by a caustic scrubber and NaOH pellets. The solubility of CH3CHF2 and CH2=CHF in water solution is negligible.6

3. RESULTS AND DISCUSSION 3.1 Morphology of pre-fluorinated catalysts The morphology of pre-treated catalysts was investigated by SEM technique and the results are displayed in Figure 1. Following pretreatment with HCFC-22 at 300 oC for 2 h, irregular chromia catalyst was obtained in the absence of Al doping. Clearly, small and flat chromia particles closely accumulate forming dense morphology. This result is consistent with our previous study for the synthesis of Cr2O3 catalyst by SCS.6 With the doping of small amounts of Al during SCS (CA3, Cr to Al molar ratio of 3), composite catalyst with the shape of plate structures was achieved. The thickness of the plates was estimated to be around 40-100 nm. The width ranges from 100 nm to 1000 nm. With the decrease in the Cr to Al molar ratio (from 3 to 0.6), the plates become thicker and the width also augments. In the absence of Cr (Al catalyst), the catalyst is in the form of thick plates with thickness of 120 to 220 nm and width of 500 to 1500 nm. CA1 presents relatively uniform and small plates among all the catalysts.

7


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Cr

CA3

CA1.2

CA1

CA0.6

Al

Figure 1 SEM images of Cr based, Cr-Al composite and Al based catalysts prepared by solution combustion synthesis (SCS). All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the characterization.

In addition, these plates look like the broken Raschig rings. It is worth noting that numerous amounts of gases were produced during SCS in a very short time (less than 3 min in the present study).6, 35 We suggest that the gel (derived from the condensation of Cr(NO3)3·9H2O, Al(NO3)3·9H2O and glycine before SCS) was flushed by these gases. Large amounts of bubbles may form initially and burst finally by the increasing amounts of gases. As a result, solid catalyst broke into Raschig ring-like plates. The catalysts prepared by SCS and pre-treated with HCFC-22 were further analyzed by TEM and the results are demonstrated in Figure 2. Consistent with SEM results, pure chromia catalyst shows dense and irregular particles. By contrast, pure alumina catalyst exhibits thick and flat plates. According to the TEM images, Cr/Al composite catalysts presents sheet-like structure. Clearly, CA1 (with Cr to Al molar ratio of 1 during SCS) has thinner and smaller sheets.

8



Cr

CA3

CA0.6

Al

CA1

Figure 2 TEM images of Cr based, Cr-Al composite and Al based catalysts prepared by solution combustion synthesis (SCS). All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the characterization.

For pure chromia catalyst, clear fringe spaces of 0.161 and 0.271 nm, corresponding to the (116) and (104) planes of Cr2O3 were observed. This is further confirmed by the diffraction ring of selected area electron diffraction (SAED) indicating the polycrystalline structure of Cr2O3. However, no such clear fringes were observed for other catalysts. Based on the morphology and structure change in Cr/Al composite catalysts, it implies that the lattice doping or strong interaction took place between Cr and Al species during SCS. In addition to the TEM, elemental analysis was conducted for CA1 and the results are illustrated in Figure 3. CA1 is composed of Cr, Al, F, O, and small amounts of C. No other impurities were detected. Clearly, following the pretreatment of fluorination by CHClF2 at 300 oC for 2 h, some of oxides were fluorinated. Furthermore, elemental mapping analysis indicates that Cr and Al elements distribute in the catalyst uniformly.

9


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Figure 3 Elemental mapping analysis of CA1 catalyst. The catalyst was pre-fluorinated with HCFC-22 (CHClF2) at 300

o

C for 2 h prior to the

characterization.

3.2 Structure of pre-fluorinated catalysts The phase structures of catalysts were studied by XRD and the results are shown in Figure 4. XRD confirms that pure Cr catalyst is in the form of Cr2O3 with hexagonal structure (R-3c, PDF #38-1479) and no other phase structures or impurities were observed. However, following the doping of Al during SCS, amorphous structures were obtained. As indicated in Figure 4, even with relatively small amounts of Al doping (CA3 with Cr to Al molar ratio of 3), all the Cr2O3 diffraction peaks disappear and no clear diffraction peak is observed from the spectrum of CA3. With further increase in Al doping (CA1 and CA0.6), the spectra suggest the amorphous structure of Cr/Al composite catalysts. 10



110 012 104

116 300 113 024 214

Cr

Intensity

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

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CA3 CA1 CA0.6 Al 10

20

30

40

50

60

70

80

2 Theta（ º）

Figure 4 XRD patterns of Cr based, Cr-Al composite and Al based catalysts prepared by solution combustion synthesis (SCS). All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the characterization.

It is unexpected that pure Al based catalyst prepared by SCS presents amorphous structure. As suggested by Zhuravleva38 that the formation of α-Al2O3 and γ-Al2O3 requires high temperature annealing or highly excessive amounts of fuel during SCS. Under conditions similar with the present study, the combustion temperature of Al(NO3)3·9H2O was determined to be only 600-800 oC.38 However, formation of crystal structure of corundum at relatively low annealing temperatures of 600–800 °C needs the time of at least 1–2 h. Clearly, combustion of 3 min in SCS is far too short for the formation of α-Al2O3 and γ-Al2O3. Consequently, amorphous Al2O3 structure was resulted. In addition, as demonstrated in Figure S1, no significant sintering was detected over all these catalysts after reaction at 400 oC for 10 h as almost identical XRD patterns were obtained compared with Figure 4. Clearly, these catalysts are relatively stable under the reaction conditions adopted in the present study.

11


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(a)

Al CA1

33.5 ppm -15.4 ppm

61.4 ppm

5.6 ppm

-200 -150 -100 -50

0

50

100 150 200

δ Al (ppm) (b)

CA1

-1

12 cm

Al

Cr

100

200

300 400 500 600 -1 Raman shift (cm )

700

800

(c) Cr CA1 Al

g=2.00

300

Figure 5 (a)

27

400 500 600 Magnetic field (mT)

700

Al MAS NMR spectra of Al catalyst and CA1 catalyst after

pretreatment with HCFC-22 at 300 oC for 2 h. (b) SERS spectra of Cr, CA1 and Al catalysts enhanced by Ag sol. (c) EPR spectra of Cr, CA1 and Al catalysts. All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the characterization.

The structure of the catalysts was further investigated by MAS-NMR spectroscopy (Figure 5a), Raman spectroscopy (Figure 5b) and EPR spectroscopy 12



(Figure 5c). The local environment of Al3+ was elucidated by

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27

Al MAS-NMR. For

CA1 catalyst pretreated with HCFC-22 exhibits the strong signal at chemical shift of about -15.4 ppm which is assigned to AlF6 octahedra.39-40 By contrast, no such peak is identified over Al catalyst. Instead, three peaks at the chemical shifts of 5.6 ppm, 33.5 ppm and 61.4 ppm were observed, respectively. They are attributed to 6-fold (AlVI), 5-fold (AlV), and 4-fold coordination (AlIV) of aluminum, respectively.41 Although 19F MAS NMR spectrum (Figure S2) was recorded for pretreated Al catalyst and no obvious difference is noted between Al catalyst and CA1 catalyst, it indicates that only trace amounts of alumina were fluorinated during pretreatment over Al catalyst. Clearly, doping of Cr facilitates the fluorination of alumina forming AlF6 octahedra. In addition, as suggested by Yang etc., 5-fold (AlV) coordination of aluminum is responsible for the doping of metal.42 As indicated in Figure 5a, derived from SCS, 5-fold (AlV) coordination is the major state of aluminum in Al2O3 framework. Hence, following SCS, doping of Cr to Al2O3 frame work is highly favorable. With SERS（surface-enhanced Raman scattering）technique, Al, CA1 and Cr catalysts were characterized respectively (Figure 5b). For the Cr catalyst, the spectrum agrees well with that of Cr2O3.43 In addition, spectrum of Al catalyst is consistent with Al2O3 reported in literature.44 Following the doping of Al to Cr catalyst, the signal of Al keeps almost unchanged. However, for Cr signal, only two peaks were detected over CA1 catalyst. Probably, the interaction between Cr and Al results in the disappearance of the other three peaks. This hypothesis is reinforced by the Raman shift of the major peak at ~624 cm-1. Compared with Cr catalyst, this peak in CA1 shifts toward high wave number by 12 cm-1. As suggested by Hardcastle, this shift is attributed to the formation of Cr-Al composite.43 As the result of interaction between Cr and Al species, the formation of defects over CA1 catalyst is enhanced significantly. Electron paramagnetic resonance (EPR) allows the study of paramagnetic centers which may occur as more or less isolated defects in primarily diamagnetic crystals.45 All the spectra of Al, CA1 and Cr show a peak at g=2.00 (Figure 5c). This peak is assigned to the vacancy defect of different metal sites. Al catalyst prepared by SCS exhibits very weak EPR signal, indicating 13


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minor vacancy defects existing in the Al catalyst. Clearly, CA1 catalyst demonstrates improved defects due to the formation of Cr3+-Al3+ pairs.45 This result is consistent with that of Raman experiments, reinforcing the interaction between Cr and Al species. Following the interaction and improved amounts of defects, formation of increased amounts of Lewis acidic sites is expected. Based on the above discussion, we suggest that Cr and Al form composite similar with solid solution as Cr2O3 diffraction peaks disappear completely over CA composite catalyst. The radius of Al3+ is around 0.053 nm, which is close to that of Cr3+ (with the radius of about 0.062 nm). The radius difference is less than 15%, and hence it is feasible for the formation of lattice substitution under SCS conditions.

3.3 Surface properties of solution combustion synthesized catalysts It is well accepted that surface area is one of the key issues for the performance of catalysts. Therefore, the surface area was measured by N2 adsorption and calculated according to the BET equation. The N2 adsorption–desorption isotherms are presented in Figure S3. All the catalysts show typical type III isotherms and H3 hysteresis indicating the slit-shape pores by the aggregation of particles.46 The surface areas determined by BET equation are listed in Table S1. Pure Al and Cr catalysts possess rather low surface area (15 m2/g and 58 m2/g respectively). The doping of Al to Cr significantly enhances the surface area. For CA3 with Cr/Al molar ratio of 3, surface area of as high as 164 m2/g is obtained which is even higher than that of catalyst prepared by carbon hard template method (115 m2/g).15, 25 By co-precipitation, similar surface area was achieved over Cr/Al composite catalyst (1:1).47

14



(a)

1610 cm-1

1450 cm-1

Absorbance

1540 cm-1 CA1 Cr Al

1700 1650 1600 1550 1500 1450 1400 Wavenumber, cm-1

(b) TCD signal (a.u.)

CA1

CA3 CA0.6

Cr Al 100

200 300 400 o Temperature( C)

500

600

(C) 100

Conversion of CFC-12（ %）

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

80 60

CA1 CA3 CA0.6 Al Cr

40 20 0

100

150 200 250 300 350 400 o Reaction temperature（ C）

450

Figure 6 (a) FT-IR spectra of adsorbed pyridine, (b) NH3-TPD profiles with a heating rate of 10 °C/min in He atmosphere, and (c) conversion of CFC-12 over Cr based, Al based and Cr/Al composite catalysts for the Cl/F exchange reaction of CFC-12 as a function of reaction temperature (atmospheric pressure, 1200 h-1 with the feed gas composition of 25% CFC-12 and 75% N2). All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the characterization. 15


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Dehydrofluorination of HFC-152a to vinyl fluoride is one of the most typical reactions exclusively catalyzed by Lewis acidic sites.48-49 Hence, the acidity of catalysts was analyzed by FT-IR and NH3-TPD techniques. Figure 6 shows the FT-IR spectra (Figure 6a), NH3-TPD profiles (Figure 6b) and Cl/F exchange reaction (Figure 6c) for Cr based, Cr-Al composite and Al based catalysts prepared by solution combustion synthesis (SCS) followed by the pre-fluorination with HCFC-22 (CHClF2) at 300 oC for 2. FT-IR spectra were measured following the adsorption of pyridine and vacuuming at 200 oC. As displayed in FT-IR spectra, strong adsorption peaks at 1610 cm-1 and 1450 cm-1 were detected, which are representative peaks of Lewis acidic sites. By contrast, adsorption peak at 1540 cm-1 responsible for the Brønsted acidic sites are rather weak for all the catalysts.50 Clearly, following solution combustion synthesis (SCS), Lewis sites are responsible for the acidity of all the catalysts. The number of acidic sites was further evaluated by NH3-TPD with a heating rate of 10 °C/min in He atmosphere. For all the catalysts, two ammonia desorption peaks were observed with desorption temperatures at around 150-180 oC and 430-440 oC respectively. However, the peaks over Cr and Al catalysts are rather weak indicating small amounts of acidic sites on the surface of Cr and Al catalysts. Clearly, two major ammonia desorption peaks correspond to the weak acidic sites and strong acidic sites respectively.51-54 Similar with the trend of surface area, both the amounts of weak and strong acidic sites are improved dramatically following the formation of Cr-Al composite catalysts. Dismutation of CFC-12 (CCl2F2) is one of the typical model reactions for the investigation of Lewis acidity.37 The catalytic performances of Cr based, Al based and Cr/Al composite catalysts for the Cl/F exchange reaction were evaluated at temperatures between 100 °C to 400 °C, atmospheric pressure and gas hourly space velocity (GHSV) of 1200 h-1. The conversion levels of CFC-12 over the catalysts are presented in Figure 6c. According to the experiments, Al based catalyst is almost inactive for the dismutation of CFC-12 (conversion is lower than 20% even at 450 oC). Compared with Al catalyst, pure Cr catalyst shows relatively higher activity. For Cr catalyst, it commences to 16



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exhibit activity at reaction temperature around 250 oC. The conversion of CFC-12 increases with reaction temperature dramatically. At 420 oC, the conversion of 74% is achieved. Consistent with previous study, Cr/Al is an efficient catalyst for Cl/F exchange reaction.55 As shown in Figure 6c, Cr/Al composite catalysts commence to catalyze the Cl/F exchange reaction at temperatures lower than 100 oC. Similar with Cr catalyst, conversion of CFC-12 increases with reaction temperature significantly. For CA1, conversion of higher than 80% is achieved at 300 oC.

(b)

(a)

74.2 eV

577.2 eV 576.9 eV 575.8 eV Al 75.2 eV

Cr 74.1 eV

577.0 eV 578.6 eV 579.0 eV

582

CA1 77.0 eV

CA1 577.0 eV CA3

75.0 eV CA3

84

580 578 576 574 Binding energy, eV

80 76 72 68 Binding energy, eV

64

Figure 7 XPS spectra Cr based, Cr-Al composite and Al based catalysts prepared by solution combustion synthesis (SCS). (a) Cr 2p3/2 and (b) Al 2p. All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the characterization.

Although Al itself is inactive in the present work, the doping of Al to Cr catalyst improves the catalytic activity strikingly. However, the activity does not increase with the doping of Al monotonically. According to Figure S4, the order of activity follows CA1 > CA1.2 > CA3 > CA0.6 > Cr > Al. Apparently, the content of Al is in the 17


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sequence of Al > CA0.6 > CA1 > CA1.2 > CA3 > Cr. Hence, the high acidity of CA1 can not be explained by the content of Cr or Al. We suggest that Cr and Al composite with interaction similar with solid solution facilitates the formation of defects and acidic sites. Catalysts were further characterized by XPS for the surface chemistry on theses catalysts. For Cr/Al composite catalysts, as exhibited in Figure 7a, Cr 2p3/2 is deconvoluted into two peaks indicating that there are two Cr states in composite catalysts. The peak with binding energy around 579 eV is assigned to Cr (VI) species.56 The peak with binding energy of 577 eV is attributed to Cr2O3.57 For pure Cr catalyst, additional peak with binding energy of 575.8 eV is detected which is believed to be Cr(OH)3.56 Similar with our previous study, small amounts of fluorinated Cr species are observed following pretreatment of CHClF2 at 300 oC for 2 h (F content of 6.2 at% as listed in Table S3).6-7 In addition to the difference in peak deconvolution, the binding energy of Cr (VI) species shifts significantly following the doping of Al during SCS. By contrast, no noticeable shift is observed for Cr2O3 peak (at binding energy of 577.0 eV). It implies that Cr (VI) species may strongly interact with doping Al species or form lattice doping and while Cr2O3 almost has no direct interaction with Al doping. In addition, following the doping of Al, the F concentration is almost doubled (Table S3). As only AlF3 is observed in NMR (Figure 5a), Al species in composite catalysts tend to be fluorinated. It is consistent with the results by XRF measurement. Clearly, F contents in the bulk of composite catalysts are much higher than that of other catalysts (Table S4). XPS spectra of Al 2p are shown in Figure 7b. For pure Al catalyst, only one peak is detected with binding energy of 74.2 eV which is the typical binding energy of Al2O3.58-59 Following the preparation of Cr/Al composite catalysts, a new peak at the binding energy of 75-77 eV (75.2 eV for CA1 and 77.0 eV for CA3) is deconvoluted. These new species are suggested to be AlOxFy or AlClxFy (ACF) and AlF3, respectively. As discussed previously, Cr (VI) species strongly interact with Al doping. Clearly, this interaction facilitates the fluorination of Al2O3 forming AlOxFy or 18



AlClxFy (ACF) structures. The binding energy shifts of F 1s and O 1s in Figure S5 reinforce the strong interaction or lattice doping of Cr (VI) species with Al species over composite catalysts.

3.4 Catalytic activity for the dehydrofluorination of HFC-152a to vinyl fluoride The catalytic activities for the dehydrofluorination of HFC-152a to vinyl fluoride over Cr, CA1 and Al catalysts were evaluated at temperatures between 200 oC and 450 oC, atmospheric pressure and gas hourly space velocity (GHSV) of 1200 h-1. The reaction rate as a function of reaction temperature is illustrated in Figure 8a. As the selectivity to target product, VF is close to 100% over all the catalysts, the selectivity is not shown in the figure.

Reaction rate( mmol/h/g)

(a)

35

Cr CA3 CA1 Al

30 25 20 15 10 5 0

200

400 450 Reaction temperature( ºC)

250

300

350

(b) 32

Reaction rate （ mmol/h/g）

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

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CA1 28 CA3 24 20

Cr

16

Al

12 8 0.00

0.05 0.10 0.15 0.20 0.25 Weak acidic site amount, mmol/g 19


0.30

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(c)100 Conversion of HFC-152a (%)

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


90 80 70 60 50 0

10 20 30 40 50 Time on stream (h)

60

Figure 8 (a) Reaction rate of HFC-152a dehydrofluorination over Cr, Al and Cr/Al composite catalysts as a function of reaction temperature (atmospheric pressure, 1200 h-1 with the feed gas composition of 50% HFC-152a and 50% N2). (b) Weak acidic site dependence of reaction rate at 350 oC. All the catalysts were pre-fluorinated with HCFC-22 (CHClF2) at 300 oC for 2 h prior to the catalytic activity rest. (c) Conversion of HFC-152a over CA1 composite catalysts as a function of time on stream (atmospheric pressure, 400 oC and 1200 h-1 with the feed gas composition of 50% HFC-152a and 50% N2).

For all the catalysts, the reaction rate increases with reaction temperature monotonically. According to the evaluation, Cr/Al composite catalysts exhibit higher reaction rates than that of Cr and Al catalysts. CA1 composite catalyst shows the highest reaction rate at elevated temperatures, and while Al presents relatively low performance. To elucidate the difference of catalytic activity over these catalysts, the correlation between reaction rate and surface area is attempted and the results are displayed in Figure S6. Apparently, surface area plays a major role in the dehydrofluorination of HFC-152a to vinyl fluoride as higher reaction rate was obtained with higher surface area. However, highest reaction rate is achieved over CA1 catalyst at 350 oC although its surface area is much lower than that of CA3. It implies that other factors may also play a role in the dehydrofluorination of HFC-152a. 20



As argued previously, dehydrofluorination of HFC-152a to vinyl fluoride reaction is exclusively catalyzed by Lewis acid site. Hence, the reaction rate is correlated with the amount of acidic site derived from NH3-TPD spectra in Figure 6 and Table S2. As displayed in Fig S7, no obvious relationship was observed between total amounts of acidic site. In addition, similar case was found between the amount of strong acidic site and reaction rate. We suggest that dehydrofluorination of HFC-152a is able to be catalyzed by relatively weak acidic sites. Although strong acidic sites are more feasible to promote dehydrofluorination, they deactivate significantly under reaction conditions.60 This hypothesis is reinforced by the correlation of reaction rate with weak acidic sites (Figure 8b). Clearly, the reaction rate agrees well with the amount of weak acidic sites. At reaction temperature of 400 oC, no clear deactivation was observed following time on stream of about 60 h (Figure 8c). By contrast, as we reported previously, conversion of HFC-152a over commercial Cr2O3 catalyst decreased from 85% to 65% in time on stream of 60 h.6-7 Therefore, CA1 show both high stability and activity. As confirmed by XPS in Fig 7, small amounts of Cr (VI) were detected over CA1 catalyst. In addition, as demonstrated in Figure 5, Cr (VI) species interact with Al species intimately. It is possible to form solid solution like composite catalyst in with the Cr/Al ratio of 1. Clearly, the interaction is favorable for the formation of active phases with Lewis acidity, resulting in the enhanced acidity and catalytic activity. Consequently, the composite catalysts dramatically facilitate the dehydrofluorination of HFC-152a. We suggest that the interaction of Cr and Al species is favorable for the formation of active sites. As mentioned previously, prior to the activity test, the catalysts were pre-fluorinated and characterized with HCFC-22 (CHClF2) at 300 oC for 2 h. As displayed in Figure 8c, following fluorination, catalyst exhibits stable activity for the dehydrofluorination of HFC-152a. No induction period and deactivation of catalyst were observed. Clearly, fluorination in the present study is sufficient to achieve active state for the catalyst. Otherwise, an induction period will be observed. As no deactivation was observed in time on stream of 60 h, characterization of spent catalyst 21


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is not included in the present work. As reported in our previous study,6-7 no significant change in structure and composition were detected after dehydrofluorination of HFC-152a. However, small amounts of carbon deposition were found by TG. Clearly, although it is possible to characterize the spent catalyst, the signals of XRD, XPS and NH3-TPD will be rather weak due to the deposition of coke. Hence, the catalytic activity is correlated with the characteristics of fluorinated catalysts rather than spent catalysts. 4. CONCLUSIONS Cr based, Cr-Al composite and Al based catalysts were synthesized by solution combustion method with Cr(NO3)3·9H2O and Al(NO3)3·9H2O as the precursors and glycine as the fuel. Following SCS, irregular chromia catalyst was obtained in the absence of Al doping. With the doping of small amounts of Al during SCS (CA3, Cr to Al molar ratio of 3), composite catalyst with the shape of plates was achieved. The thickness of the plates is estimated to be around 40-100 nm. The width ranges from 100 nm to 1000 nm. Following the doping of Al to Cr catalysts during SCS, amorphous structures were obtained. Following SCS and pretreatment of fluorination, Cr (VI) species interact with Al species intimately and while Cr2O3 almost has no direct interaction with Al doping. It is possible to form solid solution like composite catalyst. The interaction is favorable for the formation of active phases with Lewis acidity, resulting in the enhanced acidity and catalytic activity. The composite catalysts dramatically facilitate the dehydrofluorination of HFC-152a.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of catalyst characterization, tables of surface area, porosity, acidity and elemental composition of catalysts, XRD spectra of spent catalysts,

19

F MAS NMR

spectra, N2 adsorption-desorption isotherms, conversion of CFC-12 for the Cl/F 22



exchange reaction of CFC-12 as a function of Cr/Al in the catalysts, XPS spectra, Surface area dependence of reaction rate, total acidic site and strong acidic site dependence of reaction rate. AUTHOR INFORMATION Corresponding authors *Tel: +86-15158074035. E-mail: [email protected] (W.F. Han) [email protected] (H.D. Tang) ORCID

Wenfeng Han: 0000-0003-2252-9311 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The financial supports from the National Natural Science Foundation of China (Grant No. 21776257), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY12B03007), the Qianjiang Talent Project B in Zhejiang Province (2013R10056), and Special Programs for Research Institutes in Zhejiang (2015F50031) are acknowledged.

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Combustion Synthesis of Amorphous Al and Cr Composite as the

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