6 Sol-gel preparation, characterization and catalytic behavior of Mg(O,F)

6.1 Introduction

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Sol-gel preparation of catalytic materials has the ability to change physical characteristics such as surface area, pore size distribution and pore volume, and vary compositional homogeneity at a molecular level. Moreover, during sol-gel step, it is possible to introduce several components into solution to prepare multi-component oxides and bimetallic catalysts. A promoter or an active species can also be introduced to catalysts by sol-gel process. With its versatility and excellent control on a product’s characteristics, sol-gel method has played an important role in catalyst preparation and will continue to do so [1].

In chapter 3, MgO catalysts were found too strong solid base to catalyze Michael addition selectively; in contrast, in chapter 5 calcined Mg-Al hydrotalcites (mixed oxides) with suitable acid-base properties are highly efficient and selective catalysts for Michael additions. F and O2– are similar “basic” anions; however, MgF2 is neutral and catalytically inactive, while MgO is a strong base.Thus, the question was if by sol-gel method OH or O2– groups can be introduced into the MgF2 framework and obtain new compounds and tune the acid-base properties of the resulting materials.

Recently, a novel facile sol-gel synthesis route was reported to prepare high-surface-area X-ray amorphous metal fluorides [248, 2], such as AlF3 and MgF2. The resulting high-surface-area AlF3 (HS-AlF3) has not only a very high surface area but also an extremely strong Lewis acid [248]. More recently, the first aluminum alkoxide fluoride was prepared by a sol-gel fluorination method and the process of fluorination was presented in detail [3]. As in the case of oxides, the sol-gel process was proved an efficient method to prepare fluoride compound with special properties. Meanwhile, “F-anion-doped” metal oxo/hydroxidefluorides prepared by fluorination of metal oxides [4,5,6], aluminum chloride fluoride [7] and aluminum bromide fluoride [8] were proved to have some special structure and catalytic properties in many acid-catalyzed reactions. Acidic Al(III), Cr(III), Fe(III) oxyhydroxy-fluorides were also synthesized by Demourgue et al. [9]. A review about fluorinated metal oxides and metal fluorides as heterogeneous catalysts is also available [10].

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Therefore, in this chapter, an amorphous-like magnesium oxide/hydroxidefluoride, Mg(O,F), with high surface area was prepared by soft sol-gel fluorination-hydrolysis method for the first time and the structure properties are studied by XRD, FTIR, XPS, 19F MAS NMR. Magnesium oxide/hydroxidefluoride catalysts were also tested in Michael additions of CH-acid compounds with methyl vinyl ketone.

6.2 Sol-gel preparation

Sol-gel method is widely used to prepare materials with high surface areas, such as ZrO2 [11], Al2O3 [12] and MgO [177,13]. Generally, at its simplest level, sol-gel process can be achieved through the hydrolysis and condensation of a metal alkoxide.

hydrolysis:

─M─OR + H2O ─M─ OH + ROH

condensation:

─M─ OH + XO─M─ ─M─ O─M─ + XOH (X = H or R)

↓102

However, using Mg(OCH3)2, the sol-gel process may involve following reactions:

hydrolysis:

Mg(OCH3)2 + 2H2O Mg(OH)2 + 2CH3OH

(6.1)

 

Mg(OCH3)2 + H2O Mg(OCH3)(OH) + CH3OH

(6.2)

 

Mg(OCH3)(OH) + H2O Mg(OH)2 + CH3OH

(6.3)

condensation:

─Mg─OH + ─Mg─OH ─Mg─O─Mg─ + H2O

(6.4)

 

─Mg─OCH3 + ─Mg─OH ─Mg─O─Mg─ + CH3OH

(6.5)

The formation of gel from the hydrolysis of alkoxides involves simultaneous hydrolysis and polycondensation reactions. It is found the hydrolysis does not complete according to Eq.(6.1) even an adequate amount of water is added. Instead, a stepwise hydrolysis according to Eq.(6.2) and Eq.(6.3) are suggested [ 14]. Eq.(6.3) is also suggested to be reversible reaction, therefore, the hydrolysis may proceed to completion as shown in Eq.(6.2), but not complete according to Eq.(6.3). Moreover, condensation reactions [Eq.(6.4) and Eq.(6.5)] are simultaneous to hydrolysis, which may be the reason why the hydrolysis step does not complete. Finally, an intimate gel of MgOMg(OH)2 Mg(OCH3)(OH) may be obtained.

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In our case of preparation of magnesium oxide/hydroxidefluoride, before the hydrolysis step, Mg(OCH3)2 is first partial fluorinated with anhydrous HF/Et2O solution via following reaction [Eq.(6.6)]:

fluorination:

Mg(OCH3)2 + xHF Mg(OCH3)2-xFx + xCH3OH

(6.6)

If stoichiometric amount of HF is used, it is supposed to obtain pure MgF2. Since MgF2 is a neutral material, which is of less interest, our aim is not to prepare MgF2. Moreover, in the case of preparation of high-surface-area AlF3, under non-aqueous conditions, it was found only aluminum alkoxide fluoride was obtained even stoichiometric amount of HF was used. Thus, a second gas phase fluorination step was applied to get the pure high-surface-area of AlF3 [248]. However, here, after fluorination, further hydrolysis is used to introduce –OH groups into the framework of the final product via following reaction [Eq.(6.7)].

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hydrolysis:

Mg(OCH3)2-xFx + y H2O Mg(OCH3)2-x-yFx(OH)y + yCH3OH (6.7)

Consequently, the sol-gel process for the preparation of magnesium oxide/hydroxidefluoride with different F contents (see Table 6.1, MOF-0.4MOF-2.0) involved two steps (scheme 6.1): (1) Synthesis of fluoride precursor gel under non-aqueous conditions by partial fluorination of magnesium methoxide with anhydrous HF/Et2O solution (HF is dangerous, which must be handled with care); 2) one-pot hydrolysis of the precursors by adding moderate excess of water to the gel. By this soft sol-gel method, fluoride and hydroxide groups can be simultaneously introduced into the network of the final product. Typically, dried CH3OH (100 mL) was added to magnesium metal (5.0 g, small turnings, 99.98%, Aldrich) in a round-bottom flask with a reflux condenser. After the mixture reacted for about 1─2 h, the Mg(OCH3)2 solution was transferred to a PTFE (polytetrafluoroethylene) bottle. Then a certain amount of HF/Et2O (17.4 mol/L) was added to Mg(OCH3)2 drop by drop at 0 °C in an ice bath and the resulting mixture was stirred for 4─6 h in the tightly closed PTFE bottle. After that, certain amount (a two-fold stoichiometric amount) of distilled water was then added and the reaction mixture was stirred overnight. The finial gel was evaporated under vacuum at around 50 °C to produce dry powder. The dry powder was calcined in a flow-through tube furnace at 350 °C under Ar for 3 h to get the catalyst for Michael addition. The detailed preparation conditions were shown in Table 6.1.

Scheme 6.1 Sol-gel preparation of the Mg(O,F) samples

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Samples MgO-s and MgF2-s (Table 6.1) were also prepared by only hydrolysis step with four-fold stoichiometric amount of H2O (MgO-s) and only fluorination step with 2.5-fold stoichiometric amount of HF (MgF2-s), respectively. Meanwhile, the precursor samples were also prepared by only partial fluorination step (i.e. without hydrolysis step, see Table 6.1, MF-1.0–MF-2.0) as reference materials.

The series of samples (MOF and MF) are denoted in the following way: MOF-1.6 for the magnesium oxyfluoride [Mg(O,F)] fluorinated with a preparation F/Mg ratio of 1.6 and hydrolyzed; MF-1.6 for the magnesium oxyfluoride precursor fluorinated with a F/Mg ratio of 1.6 without hydrolysis; MgO-s and MgF2-s are MgO and MgF2 prepared by sol-gel method, respectively. MgF2-c is a commercially available MgF2 (Aldrich, 99%).

Table 6.1 Preparation conditions, elemental analysis results and texture properties of the samples

Sample

HF/Mg

(mol ratio)

H2O/Mg

(mol ratio)

C (wt%)a

X (wt%) after calcinationb

F/Mge

O/Mge

C

H

F

  

MgO-s

0

4.0

5.19

1.34

1.18

-

-

1.1

MOF-0.4

0.4

3.2

5.86

3.15

2.12

10.19

0.3

1.0

MOF-0.8

0.8

2.4

7.85

3.94

2.26

23.36

0.5

0.9

MOF-1.2

1.2

1.6

5.93

2.86

1.57

28.10

0.8

0.7

MOF-1.6

1.6

0.8

5.01

1.51

1.02

40.60

0.9

0.5

MOF-2.0

2.0

0.8

3.12

0.20

0.23

49.19

-

-

MF-1.0

1.0

0

17.87

11.67

3.02

23.99

-

-

MF-1.6

1.6

0

10.00

2.95f

1.20f

n.d.

-

-

MF-2.0

2.0

0

8.47

n.d.

n.d.

n.d.

-

-

MgF2-s

2.5

0

0.58

0.05

0.04

56.17

-

-

MgF2-cc

-

-

-

0.03c

0c

59.52c (62d)

-

-

a: before calcination
b: after calcination in Ar at 350 °C
c: commercial product without further calcination
d: calculated value
e: determined by XPS
f: after calcination in air at 350 °C

6.3 Characterization of Mg(O,F) samples

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Elemental analysis

The elemental analysis results of samples are summarized in Table 6.1. High carbon contents (38 wt%) on the as-prepared samples could be attributed to remaining OCH3 groups [263,15] or adsorbed organic compounds. The remaining OCH3 groups could not even totally substituted by the second hydrolysis step. The carbon contents of dried precursor gels (MF-1.0, MF-1.6 and MF-2.0, prepared without further hydrolysis step) are much higher than that of corresponding final dried Mg(O,F) samples after hydrolysis (e.g. 10 wt% carbon content for MF-1.6 and only 5 wt% carbon content for MOF-1.6). Compared the carbon contents, it indicates the second hydrolysis completed around 50% during the sol-gel process. Thus, the hydrolysis step seems to be critical to the substitution of alkoxy groups with hydroxy groups, yet the carbon contents indicates that the hydrolysis step does not result in complete replacement of the methoxy groups with hydroxy groups despite the two-fold stoichiometric amount of water used, which is same as the previous results [263]. Hence, in our case, Mg (OCH3)2-x- yFx(OH)y (x and y depended on the HF/Mg and H2O/Mg ratios) was probably formed after the fluorination and hydrolysis of the solution of Mg(OCH3)2 in methanol.

After calcination, the carbon contents decrease more or less due to the remove of residual organics. However, the remaining OCH3 groups could not even totally removed by calcination at 350 °C in Ar (Table 6.1), which agrees with the results in literature [ 16 ]. When the nominal ratio of HF/Mg was increased, the final carbon contents after calcination decreased to some extent; the experimental fluorine contents increased linearly with the amount of HF added (MOF series). It is interesting to note that the carbon content of MgF 2 -s prepared by this method was only 0.58% and 0.05%, respectively, before and after calcination. The low carbon contents found for MgF 2 -s suggests that the addition of excess HF does lead to the complete removal of –OCH 3 groups. On the other hand, the fluorine content of this sample (56.17 wt% ) indicates incomplete fluorination (MgF 2 : 62 wt%). Oxygen as either O 2– or OH must be present in the network. Compared the nominal F contents, it was found that the F contents were less than the corresponding theoretical values, even for MgF 2 -s, because HF could not completely react with Mg(OCH3)2 under the conditions. The measured and calculated F content of commercial MgF 2 are 59.52% and 62%, respectively. The difference comes from the measurement.

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Textural properties

The Mg(O,F) samples after calcination at 350 °C in Ar have very high surface areas (270390 m2/g, Table 6.2) with high pore volumes (0.370.56 cm3/g, Table 6.2) except that MOF-2.0 has a relative lower surface area of 104 m2/g with pore volume of 0.38 cm3/g.However, the surface area of MgF2-s is only 27 m2/g, which is similar to that of MgF2 prepared by normal methods [17]. Commercially available MgF2 has only surface area of 0.4 m2/g. The surface areas of the fluorinated MOF samples (MOF-0.4–1.6) may be roughly related to the carbon (–OCH3) contents (high C, high S BET), whereas MgO-s has a low carbon content (1.5 wt%) but an incredibly large surface area (373 m2/g). This suggests a different relationship between the carbon content and surface area in systems containing only magnesium and oxygen.

Table 6.2Texture properties of the samples

Sample

SBET (m2/g)

VBJH-d (cm3/g)b

davg (Å)c

MgO-s

373

0.58

62

MOF-0.4

327

0.37

46

MOF-0.8

387

0.45

48

MOF-1.2

363

0.56

62

MOF-1.6

278

0.46

66

MOF-2.0

104

0.38

146

MgF2-s

27

0.16

230

MgF2-ca

0.4

0.005

427

a: commercial product without further calcination
b: BJH desorption cumulative pore volume of pores between 17.0 and 3000.0 Å diameter
c: Average pore diameter by BET

↓108

The adsorption/desorption isotherms of all the samples are of type IV, which are obtained for mesoporous adsorbents. The isotherms and hysteresis loops are slightly different depending on the F contents (Fig. 6.1). Combination of the isotherm sharp with the analysis of the shape and the width of the hysteresis loop can give the main information on the texture characteristics of a solid [ 18]. Small mesopores in MgO-s and MOF-0.4–1.6 are indicated by a closing of the hysteresis loop at P/Po values of ca. 0.4. Larger mesopores are found in the samples with higher fluorine contents, MOF-2.0 and MgF2-s, whose loops close at around 0.6 and 0.7, respectively. All the samples exhibit a limited amount of macroporosity (i.e., slight increase in adsorption between 0.95 and 1.00), which can be attributed to N2 adsorption between particles. The observed loops in Fig.6.1 indicate that the types of pore formed depend on the fluorine content from small cone-shaped pores (MgO-s) to ink-bottle pores (MOF-0.4 and MOF-0.8) to cylinder-shaped pores and parallel plates (MOF-2.0 and MgF2-s, respectively) [267].MOF-1.2 and MOF-1.6 exhibit two desorption steps but it is unclear if this is due to two different types of pores or to pores with two different cross sections [19].

Fig. 6.1 N2 adsorption/desorption isotherms of calcined Mg(O,F) samples

The pore size distributions of calcined Mg(O,F)samples are shown in Fig.6.2. The pore size distributions of MOF-0.4–MOF-1.6 have maxima at 35 Å, except that MOF-1.6 has an additional maximum at about 55 Å. However, MOF-2.0, MgO-s and MgF2-s exhibit broader pore size distributions with maxima at higher values about 90, 60 and 135 Å, respectively.

↓109

Fig. 6.2 Pore size distributions of calcined Mg(O,F) samples

XRD study

Before calcination, all the dried samples are X-ray amorphous-like materials (Fig. 6.3A). However, broad peaks around 2θof 11 and 61° and the the decaying peak at 2θof 33.5° (Fig. 6.3A: a,b,c,d) indicate the presence of Mg(OH)x(OCH3)y or Mg(OH)x(OCH3)yFz (i.e. the hydrolysis of OCH3 groups is not totally completed) [263]. The XRD pattern is similar to that of Mg(OCH3)1.3(OH)0.7 (PDF No.22-1788). This is also the reason why the dried products have high carbon contents (38 wt%, Table 6.1). Broad characteristic peaks around 2θof 40 and 53° with very low intensity are observed in the patterns of MOF-1.6 (Fig. 6.3A:e) and MOF-2.0 (Fig. 6.3A:f), the samples with high F contents. Reference MgF2-s prepared by this sol-gel method has all the characteristic peaks from MgF2 (PDF No. 41-1443) and these peaks are all broad.

↓110

After calcination at 350 °C in Ar, MgO-s (Fig. 6.3B:a) and MgF2-s (Fig. 6.3B:g) show the patterns of MgO (PDF No. 45-946) and MgF2 (PDF No. 72-1150), respectively. However, the Mg(O,F) samples (MOF-0.4 to MOF-2.0) are still amorphous-like materials. Characteristic peaks from Mg(OH)x(OCH3)yFz disappear more or less in the patterns of MOF-0.4, MOF-0.8 and MOF-1.2 (Fig. 2B: b,c,d). Broad peaks for MgF2 in the patterns of sample MOF-1.6 and MOF-2.0 (Fig. 6.3A: e,f) become a little sharper (Fig. 6.3B: e,f), still with low intensities. In contrast, good crystalline MgF2 is formed (Fig. 6.3B: g) after calcining the reference material MgF2-s under the same conditions. These results indicate that crystallization process promoted by calcination is not obvious for Mg(O,F)samples. Some special species formed during hydrolysis preserved the distortion during the calcination. Further calcination at 550 °C led to the formation of separated crystalline phases MgO and MgF2 (Fig. 6.3C: c,d,e,f). Further calcination at 550 °C led to separate, crystalline phases of MgO and MgF2. The main phase was MgF2 for the samples with

Fig. 6.3 XRD patterns of Mg(O,F) samples (b-f) and reference materials MgO-s (a) and MgF2-s (g) before calcination (A) and after calcination at 350 °C (B) and 550 °C (C) in Ar

F/Mg ratios higher than 1.0. Peaks for both MgF2 and MgO indicated a mixture of these phases in MOF-0.8, whereas the pattern of MOF-0.4 (Fig. 6.3C: b)with 10.2 wt% F exhibited a pure MgO showing two broad MgO peaks and no MgF2 peaks, which can be explained by that small amount of MgF2 is covered by MgO and can not be detected by XRD.

↓111

Thermal analysis

Fig. 6.4 TG- DTA profiles of uncalcined MOF-1.6

The presence of –OH and –OCH3 in the framework was confirmed by simultaneous thermal analysis (performed using a TA-MS device). TG-DTA profiles of the uncalcined samples, MOF-1.6 (5.0 wt% C) is shown in Fig. 6.4. The TG curve (under N2) of MOF-1.6 shows an initial weight loss of about 8.7% below 320 °C mainly due to the desorption of water [m/z = 17 (OH+), 18 (H2O+)]. The corresponding differential thermal analysis (DTA) peak and endothermic effect are around 148 °C. However, the condensation of some unstable –OH groups cannot be excluded completely in this case. The weight loss (10.2%) from 320 to 600 °C corresponds to the loss of –OH and also some –OCH3 species. The corresponding DTA curve with a strong endothermic peak with Ton 347 °C and Tp 429 °C has to be attributed to the condensation reactions of –OHgroups to produce H2O or condensation reactions of –OHand–OCH3 groups to produce CH3OH, which is confirmed by the ionic current signal for m/z = 18 (H2O+) and m/z = 32 (CH3OH+), respectively. The thermal behavior of the sample after calcination at 350 °C in Ar is similar (not shown), which indicates the presence of –OH and –OCH3 even after calcination at 350 °C in Ar. The weight losses of two steps are slightly lower, 3.4 and 7.7%, respectively.

↓112

FTIR

Fig. 6.5 FTIR spectra of calcined MOF-0.4–MOF-1.6 (b–e), MgO-s (a), MgF2-s (f), and commerical Mg(OH)2 (g) and MgF2-c (h)

To further investigate the framework of the Mg(O,F) samples, FTIR of calcined samples was performed to clarify whether OH or (OCH3) or O2– or all are present in the network. FTIR was also carried out on MgO-s, MgF2-s, commercially available Mg(OH)2 and MgF2 (MgF2-c) reference samples (Fig. 6.5g and h). The FTIR spectra of KBr pellets of calcined Mg(O,F) samples are shown in Fig. 6.5 and the assignment based on [265] is shown in Table 6.3. The very strong, broad band at 1460 cm–1 can be attributed to not only δasC-H, but also bicarbonates formed by adsorbed carbonate on the basic surface (see chapter 5). This explains the large intensity differences between the νC-H and δasC-H bands. The 1640-cm–1H–OH) band and a broad band between 3675 and 3290 cm–1O–H) could correspond to adsorbed water on the sample surface and is found in all of the spectra (Fig. 6.5a–h).In [20], absorption at 1620 cm–1 was attributed to bridging hydroxy or oxy groups in intermediate alkoxy/hydroxy compounds. A very weak band at 872 cm–1C O) could indicate a negligible amount of methanol in the sample.

↓113

Table 6.3 Assignment of the FTIR absorption bands

Wave number (cm-1)

Mode assigned

Group or compound

3700

νO─H

Hydroxyl group

3630 (3750─3000)

νO─H

Adsorbed water

2750─2970

νC─H

Alkoxy group

1630─1650

δH─OH

Adsorbed water

1400─1500

νC─O

Carbonate ion

1050─1130

νC─O

Alkoxy group

500─600

νMg─O

Mg–OR

400─500

νMg─F

Mg–F

The spectrum of MgO-s (Fig. 6.5a) shows a band at 3700 cm–1 for isolated OH groups. Very weak vibrations observed between 3000 and about 2750 cm–1C H) and 1087 cm 1C H or νC O) suggests the presence of very few methoxy groups in this sample. The spectra of the MOF samples and MgO-s vary in their band intensities except for the band(s) at 1087 cm–1 (and 1001 cm–1 in Fig. 6.5e) and the region above 3000 cm–1 and below 600 cm–1. The following changes are observed in the MOF spectra with increasing fluorine contents: the OH-band at 3700 cm–1 decreases in intensity and shifts to lower wave number; a second band at 3609 cm–1 appears (Fig. 6.5d) and increases in intensity (Fig. 6.5e); the 1080-cm–1 band becomes stronger (Fig. 6.5b–d) and then decreases in intensity (Fig. 6.5e), while a weaker band appears at 1000 cm–1 (Fig. 6.5e); bands below 600 cm–1 are shifted. It seems the isolated OH groups become bridged when higher amounts of fluorine are present in the network. The spectrum of Mg(OH)2 (Fig. 6.5g) has a very strong band at 3700 cm–1 for the isolated OH group characteristic. Carbonates are also found on the surface of Mg(OH)2 (1460 cm–1 band). Whereas adsorbed water is observed on the surface of the hydroxide, very little water is found on the surface of MgF2 (MgF2-s and MgF2-s, Fig. 6.5f and h). The only strong bands for these two samples are observed below 600 cm–1 and can be attributed to νMg F [21].The main band is at 460 cm–1 with a shoulder at 550 cm–1 and a sharp but weak band at 410 cm–1. The crystal structure of MgF2 (ICSD No. 56506, PDF No. 41-1443) contains two Mg–F bonds (2.012 and 1.988 Å).

In the range of 400–600 cm 1, the bands of calcined Mg(O,F) samples (Fig. 6.5b–e) aredifferent with that of MgF2 (Fig. 6.5f and 6.5e) and MgO-s (Fig. 6.5a). The band at 550 cm 1 (Mg–OR stretching [22]) are observed in the spectra (Fig. 6.5b and 6.3c) of calcined MOF-0.4 and MOF-0.8, which contain low F content (< 25 wt% F). However this band does not appear in the spectrum of calcined MgO-s. Meanwhile, the band of Mg–F stretching mode for MgF2 (MgF2-s and MgF2-c) around 460 cm 1 (Fig. 6.5f and 6.3h) is not observed in all the spectra of calcined Mg(O,F) samples (Fig. 6.5b–e). These results can be explained by the coexistence of Mg–O(R) and Mg–F in the different coordinated Mg spheres of calcined Mg(O,F) samples.

↓114

19F MAS NMR

Fig. 6.6 19F MAS NMR spectra of MOF samples after calcination at 350 °C: (a) MOF-0.4, (b) MOF-0.8, (c) MOF1.2, (d) MOF-1.6, (e) MOF-2.0, (f) MgF2-s, and (g) MgF2-c

From the structure of the crystal solvate of magnesium methoxide with methanol, Mg atom is octahedrally coordinated [23]. During the fluorination-hydrolysis process, F and OH could substitute for –OCH3 groups to form MgOxF6-x octahedrally coordinated units. The coordinative state depends on the relative F and O contents.

↓115

High-solution solid-state 19F MAS NMR was used to study the fluorine coordination species in calcined Mg(O,F) samples. The spectra of the samples with different fluoride contents and the commercial MgF2 (MgF2-c, without further treatment) are shown in Fig. 6.6. For MgF2-c (Fig. 6.6f), only signal around –198 ppm is observed which corresponds to the bridging fluorine in octahedrally coordinated MgF6 sphere [251,24]. Crystalline, tetragonal MgF2 (ICSD 56506, PDF No. 41-1443) has only one crystallographically unique fluorine atom, which coordinates three Mg atoms (Mg–F lengths: 2.0119, 2.0119, and 1.9883 Ǻ and Mg–F–Mg bond angles: 129.9, 129.9, and 100.2°). Calcined MgF2-s has similar spectrum as commercial MgF2 with an additional small signal at –184 ppm, which suggests a second type of fluorine coordination species in this sample. After calcination of the same sample at 550 °C, a very weak peak in the XRD pattern (Fig.6.3C: f) indicated a separate and minor phase of MgO. Thus, it seems there are minor mixed lattice regions (MgO x Fy) in the network, in which the fluorine coordination (Mg–F bond lengths and Mg–F–Mg angles) is different than that found in MgF2. With the decreasing fluorine concentrations of the Mg(O,F) samples, the 19F MAS NMR signals (Fig. 6.6d–a) become broader. MOF-2.0, MOF-1.6 and MOF-1.2 have the same maximum at around –198 ppm as MgF2-c. Meanwhile MOF-1.6 has two shoulder signals at around –192 and –184 ppm, and MOF-1.2 has another maximum at –192 ppm. MOF-0.8 and MOF-0.4 have much broader signals. Maximums at around –192 and –177 ppm with shoulders are observed, respectively. The dominant fluorine species in these samples has a different coordination compared to that in MgF2. Based on the spectra of all the samples, six resonance positions from high to low field may be distinguished at about the chemical shifts of about –158, –168, –177, –184, –192, and –198 ppm, which are supposed to be MgO5F, MgO4F2, MgO3F3, MgO2F4, MgOF5, MgF6 (oxygen may come from oxo or hydroxyl group, fluorine may come from bridge or terminal coordinated fluorine specie) coordination species in the first octahedrally coordinated Mg sphere. The number of fluorine atoms coordinated to Mg increases with fluorine chemical shift decreasing from high to low chemical shift field. A similar trend between the number of fluoride atoms coordinated to aluminum and the fluorine chemical shift was also observed in hydroxyl/oxyfluorides aluminum before (see Fig.6.7) [ 25]. In this case, the broad signals may be ascribed to the coexistence of some different fluorine coordination species. Meanwhile, the high disorder structure of Mg(O,F) samples indicated by XRD may be another reason for the broad signals. The former interpretation is preferred because two maxima are observed in the spectrum of MOF-1.2 (Fig. 6.6c), which cannot simply be explained by the degree of the disorder of the sample.

Fig. 6.7 Typical 19F chemical shifts of octahedral aluminum environments with oxygen and fluorine in the first coordination sphere, AlO6- x F x , versus x [274]

In 19F MAS NMR spectrum of MOF-1.6 calcined at 550 °C (Fig. 6.8) the signal around –198 ppm with some weak signals (–150 to –190 ppm) appeared instead of the broad signal with a maximum at –198 ppm of the sample calcined at 350 °C (Fig. 6.6d). This indicates high temperature calcination can destroy the framework of Mg(O,F) and result in crystalline MgF2. This result also agrees with the presence of crystalline MgF2 in XRD patterns of the samples calcined at 550 °C (Fig. 6.3C). However, the weak signals demonstrate negligible amount of certain or some unknown fluorine species were formed during high temperature calcination, which can not be detected by XRD.

↓116

Fig. 6.8 19F MAS NMR spectra of MOF-1.6 after calcination at 550 °C

XPS

The XPS (Mg 1s, F 1s, O 1s) results of selected calcined Mg(O,F) samples and commercially available MgF2 are shown in Fig. 6.9. The surface atomic ratios, F/Mg and O/Mg, for the MOF samples were determined by integration of the XPS peaks in Fig. 6.9 and are given in Table 1. The XPS spectra of commercially available MgF2 (MgF2-c) was measured and discussed in [251].

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From the surface atomic ratio F/Mg and O/Mg, with the nominal F/Mg ratio increase, the surface F/Mg increases and the O/Mg decreases. The O/Mg of MgO-s is 1.1, which indicates that the surface species is MgO. This is consistent with the XRD result. For calcined MOF-0.4, MOF-0.8 and MOF-1.2, the values of (2O/Mg+F/Mg) are more than 2.0, which indicates that besides O2– species on the surface there is OH or (OCH3) species. This result agrees with the FTIR result. However, on the surface of calcined MOF-1.6, the oxygen species is more likely O2– since the value of (2O/Mg+F/Mg) is almost equal to 2.0, which means this sample seems to be a true magnesium fluoride exclusively with O2– species.

Changes in the chemical environment of an atom may be followed by changes in the photoelectron energies. From the binding energy (BE) of Mg 1s, it seems that when more F is bonded to Mg from MgO-s to MOF-1.6, the Mg 1s BE shift to high value from 1302.7 to 1304.0 eV. This means the introduction of F does change the chemical environment of Mg. The BEs of F 1s varied from sample to sample as shown in Fig.6.9, while there is not a direct correlation between the BE and the F content. It is surprising to note that in the F 1s spectrum of calcined MOF-1.2, there is an additional peak at around 687.6 eV besides the peak at around 684.6 eV, which may be ascribed to the significantly different F chemical environment. From the 19F MAS NMR spectra, obviously, calcined MOF-1.2 is the only sample that has two separated peaks. It is the same case for calcined MOF-1.2 in the O 1s spectra. Calcined MOF-1.2 has two peaks at around 530.8 and 533.5 eV, respectively. Again, there is no direct correlation between the BE and the O content. The BE varies from 530.8 to 533.5 eV.

Fig. 6.9 XPS results of Mg(O,F) samples after calcination at 350 °C in Ar

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TEM

TEM was used to study the morphology of calcined Mg(O,F) sample. The TEM of calcined MOF-1.6 at 350 °C in Ar is shown in Fig. 6.10. Irregular shapes of particles (around 10 nm) are observed, which has low contrast and are probably agglomerates of much smaller particles. The selected-area electron diffraction patterns of the particles are not very diffused, which indicates the particles are somehow crystalline. Because the particles are very small, in XRD pattern of this sample, only broad peaks are observed. According to the lattice distances measured from the diffraction rings, the predominant phase may be MgF2 and some isolated particles may be MgO. Probably, MgO particles also coalesce with MgF2 particles since very broad diffraction rings are also observed. Because of the very close lattice distances (d values) of MgF2 and MgO, O atom partially incorporation in the lattice of MgF2 may not change the structure and only slightly changes lattice distances. Thus, lattice distances measured from the diffraction rings can not be distinguished with that of MgF2 and can not indicate the existence of the Mg(O,F) compound in this case. In contrast, the coexistence of O and F in the frameworks of Mg(O,F) samples has been proved by FTIR, 19F MAS NMR and XPS. Energy-dispersive X-ray spectroscopy (EDX) confirmed qualitatively that calcined MOF-1.6 consists of Mg, F and O.

Fig. 6.10 TEM of calcined MOF-1.6

6.4 Catalytic behavior of Mg(O,F)

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Catalytic behaviour of Mg( O,F) in Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone

Fig. 6.11 Michael addition of 2-methyl-cyclohexane-1,3-dione to methyl vinyl ketone with the MOF samples calcined at 350 °C. Yield (A) and selectivity (B) of Michael adduct versus reaction time. Reaction temperature: room temperature (RT)

The catalytic activity of the MOF samples was first screened for the Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone at room temperature; these results are shown in Fig. 6.11. Although supported fluoride ion (e.g. KF/Al2O3) as a solid base shows very good activities in many reactions [93], including Michaeal additions, the pure, crystalline MgF2 samples, MgF2-c and MgF2-s, were totally inactive. The MOF samples, MOF-0.4–MOF-2.0, MgO-s and commercially available Mg(OH)2 exhibited diverse catalytic activities. MgO-s (Fig. 6.11) shows a very unselective catalytic activity similar to that found for MgO synthesized in aqueous medium in chapter 3: high initial yield (ca. 70%, 1 h) with a decrease to below 20% over 24 h caused by a consecutive intramolecular aldol cyclization (see chapter 3). The product yield achieved with commercial Mg(OH)2 went through a maximum of over 70% yield after 4 h but did not decrease as drastically as in the case of MgO (final yield: 45%, not 20%). In chapter 5, the activity and selectivity of the catalyst were attributed to the type of base site and presence of Lewis acid sites. MgO had a low amount of Brønsted base sites (OH), but more and stronger Lewis base sites (O2–), whereas Mg(OH)2 only has Brønsted base sites (strong absorption at 3700 cm–1 for isolate OH groups in IR spectrum). This could indicate that not only Brønsted base sites but also Lewis base sites can catalyzed the reaction, however, the presence of acid sites may be important for high reaction selectivity.

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The different F contents of the MOF-0.4–MOF-2.0 samples also have a direct effect on catalytic activity. The sample with the lowest fluorine contents (MOF-0.4, 10.2 wt% F) exhibits the highest initial yield of about 90% after 2 h, after which the yield decreases parallel to that of MgO-s giving a final yield of ca. 25%. In the case of MOF-1.2, maximum yield was reached at a longer reaction time (4 h) and was lower (65%). The decrease in yield for this sample thereafter resembles that of Mg(OH)2 and not MgO (final yield: 45%). The yields of the samples with higher fluorine contents (MOF-1.6, 40 wt% F and MOF-2.0, 49.2 wt% F) no longer went through a maximum of yield; a consecutive reaction did not occur. Both of these samples demonstrated the most selective activities and achieved the highest final yields: 65% for MOF-2.0 and 90% for MOF-1.6. Thus, a preliminary conclusion is that the combination of Brønsted base sites, Lewis base sites, and Lewis acid sites is optimal in these two samples. Reaction with MOF-2.0 is however significantly slower than with MOF-1.6.Fig. 6.11Bshows that the reaction selectivity achieved with MOF-0.4 and MOF-1.2 are in between that of MgO-s (MgO) and Mg(OH)2. Based on the results of calcined hydrotalcite (chapter 5), it is assumed that optimum acid–base properties, compared to MgO and Mg(OH)2, could make MOF-1.6 and MOF-2.0 more selective catalyst in the reaction. On the other hand, the results indicate that the acid-base properties on Mg(O,F) can be adjusted by adjusting the F. From the catalysis results, it is shown that the acid-base property of the material may be modified by varying the relative ratio of the anions.

Catalytic behaviour of MOF-1.6 in Michael addition of 2-methylcyclohexane-1 ,3 -dione to methyl vinyl ketone at different temperatures

The Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone was performed using MOF-1.6 as catalyst at 50 and 75 °C. The results were shown in Fig. 6.12.

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With the reaction temperature increase, the yield of Michael adduct went through a maximum of over 90% yield after 2 h at reaction temperature of 50 °C and over 85% after 1 h at 75 °C. The yield of Michael adduct decreased to 76% and 35%; and the yield of bridged ketol increased to 20% and 57% after 8 h at 50 °C and 75 °C, respectively because of the consecutive reaction described in chapter 3. These results indicted that at higher reaction temperature, the behavior of MOF-1.6 became similar to that of MgO at room temperature.

Fig. 6.12 Michael addition of 2-methyl-cyclohexane-1,3-dione to methyl vinyl ketone with the MOF-1.6 at RT, 50 and 75 °C. Yield of Michael adduct (A) and bridged ketol (B) versus reaction time

Michael addition of 2-acetylcyclopentanone, 2-acetylcyclohexanone and 2-methoxycarbonyl cyclopentanone to methyl vinyl ketone over MOF-1.6

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Based on its high activity and selectivity, MOF-1.6 was chosen to test in three Michael additions (see Scheme 6.2) with liquid CH-acids of different strengths (pK a values): 2-acetylcyclopentanone (7.8), 2-acetylcyclohexanone (10.1), and 2-methoxycarbonyl cyclopentanone (10.3) at room temperature. The result was shown in Fig. 6.13. As in the reaction with 2-methyl-cyclohexane-1,3-dione to methyl vinyl ketone, high product yields (75–90% after 24 h) with 100% selectivities were obtained successfully with MOF-1.6 that were directly dependent on the pK a of the CH-acid. The addition of reactant with lower pK a to methyl vinyl ketone proceeds faster. Interesting enough, MOF-1.6 had a similar catalytic behavior as the most active calcined hydrotalcite catalyst CHT0.6 (see chapter 5). It means by fluorination-hydrolysis sol-gel process, it was successful to design a new efficient catalyst system for Michael additions.

Scheme 6.2 Michael additions of 2-methylcyclohexane-1,3-dione (1), 2-acetylcyclopentanone (2a), 2-acetylcyclohexanone (2b) and 2-methoxycarbonyl cyclopentanone (3) to methyl vinyl ketone (4). Michael adduct: 2-methyl-2-(3-oxo-butyl)-cyclohexane-1,3-dione (5), 2-acetyl-2-(3-oxo-butyl)-cyclopentanone (6a), 2-acetyl-2-(3-oxo-butyl)-cyclohexanone (6b) and 2-oxo-1-(3-oxo-butyl)-cyclopentanecarboxylic acid methyl ester (7)

Fig. 6.13 Michael addition of 2-acetylcyclopentanone (pK a = 7.8), 2-acetylcyclohexanone (pK a = 10.1) and 2-methoxycarbonyl cyclopentanone (pK a = 10.3) to methyl vinyl ketone with calcined MOF-1.6. Yield of Michael adduct versus reaction time. Reaction temperature: room temperature (RT).

6.5 Conclusions

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Amorphous-like material Mg(O,F) with high surface area can be prepared by two-step soft fluorination-hydrolysis sol-gel process.

Different fluorine species in Mg(O,F) are detected by 19F MAS NMR and XPS. A periclase MgO or tetragonal MgF2 structure is formed during low-temperature calcination at 350 °C depending on the fluorine contents of sample. As the fluorine content decreases in the sample, the structural distortion of the fluorine coordination becomes greater. Low fluorine contents in the MgO network results in diverse fluorine coordinations. Two distinct fluorine coordinations are present in the mixed lattice of MgO and MgF2. As the fluorine content increases, the FMg3 species becomes the dominant species. High-temperature calcination of Mg(O,F) leads to the formation of separated crystalline MgO and MgF2 phases.

Successfully controlled introduction of F into the MgO network makes it possible to tune the acid-base property of the resulting material.Mg(O,F) with suitable F content can act as an efficient and selective catalyst for Michael additions. In the Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone. The catalyst with a F/Mg mol ratio of 1.6 achieved the best catalytic results producing the Michael addition product in a yield of 90% with a selectivity of 100% at room temperature within 24 h. This catalyst was successfully applied in other Michael additions of 2-acetylcyclopentanone, 2-acetylcyclohexanone, and 2-methoxycarbonylcyclopentanone as further CH-acids to methyl vinyl ketone to obtain high final yields and selectivities.


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