Results and Discussion - Chapter VI: Investigation of MIL-53 (Al, Ga) for acid Brønsted-type ca

6. Chapter VI: Investigation of MIL-53 (Al, Ga) for acid Brønsted-type catalysis

6.3. Results and Discussion

6.3.1. Kinetic Results

Figure 67 shows the results of the kinetic study carried out over different catalysts. After 30 minutes, MIL-53 (Ga) leads to total conversion with 40% selectivity toward 4-tert-Bu-toluene. In order to ensure that the experiment takes place in heterogeneous phase, an experiment at room temperature is carried out. This experiment shows total conversion after one day. This result confirms not only the high activity of MIL-53 (Ga), but also that the catalyst is not soluble during the reaction.

0 1 2 3 4

0 20 40 60 80 100

Conversion [%]

Time [Hrs]

Figure 67: Conversion of toluene alkylation with tert-BuCl at 100°C: MIL-53 (Ga) (red), H-BEA (green) and MOF-69c (blue).

Moreover, the chemical analysis carried out after filtration does not reveal the presence of Ga cation inside the solution. This test confirms that MIL-53 (Ga) is not destroyed during the reaction. In view of the high activity of this material, other reactions were attempted using various alkyl agents: chlorobenzene, phenol, n-BuCl, 1-hexene and tert-BuOH. None of these reactions proceeded. One reason could be the combination of the setup and the organic framework of the MOF. Indeed, since these molecules are more difficult to alkylate, the temperature and/or pressure generally have to be increased. Unfortunately, one of the weaknesses of MOFs is

performed at higher temperature simply destroy the catalyst. An alternative could be to increase the pressure, but our setup does not permit this. Under these conditions, we cannot draw conclusions regarding the activity of MIL-53 (Ga) toward these compounds.

6.3.2. IR Study

3720 3660 3600 3540

2200 2100

Absorbance(a.u.)

Wavenumber cm-1

a )

2200 2100

3720 3660 3600 3540

Absorbance(a.u.)

Wavenumber cm-1

b )

Figure 68: FTIR spectra of CO adsorbed at 100 K: spectra collected at RT under vacuum (red);

spectra collected at 100 K in the presence of 40 mbar of CO (black); and spectra collected at decreasing coverage and increasing temperatures (blue). Part a) represents MIL-53(Al); part b), MIL-

53(Ga).

Theoretical calculations are in very good agreement with experimental data for Q(OH) bands: 3709 (th) and 3704 cm^-1 (exp) for MIL-53(Al); and 3663 (th) and 3669 cm^-1 (exp) for MIL-53(Ga). The cooling of the samples induces small blue hydroxyl shifts of 'Q= +10 and +15 cm^-1 for Al and Ga samples, respectively. This phenomenon is probably due to a response of the flexible structure of the framework to a change in temperature. Upon CO adsorption¹³⁴, rather small red shifts appear in the –OH vibration regions, together with very small blue and red shifts in the CŁO region.

In particular, for MIL-53(Al), 'Q(OH) = - 30/-50 cm^-1 while for MIL-53(Ga), 'Q=

-50/-100 cm^-1 are observed. Surprisingly, a parallel perturbation in the CO stretching

around the CO liquid-like frequency (ca. 2140 cm^-1): namely doublets at 2145-2137 cm^-1 and 2147-2137 cm^-1, respectively. These bands are reversible only upon prolonged outgassing, which indicates that they are associated with some stable adduct.

Theoretical calculations indicate four main adsorption modes for CO:

1 - The traditional "C adduct" -OH···CO 2 - A reverse "O adduct" -OH···OC

3 - A bridging adduct -OH···OC···OH- at the centre of the cavity

4 - A benzene adduct, B···CO is near the benzene ring (B) of the linker. Several orientations have been studied (see Supporting Information). Contrary to the results obtained by Camarota et al.¹³⁵ on phenylene periodic mesoporous organo-silica with crystalline walls, it is not possible to stabilize any M-OH···CO···B or M-OH···OC···B adducts due to excessive distances between M-OH and B.

Adsorption energy and frequency calculations show that these four kinds of adducts exhibit quite similar stabilities. The C adduct, however, remains the most stable, and leads to the highest (OH), but to very small expected blue shifts in the CO region (2145 cm^-1), in agreement with experiments. The three other modes induce very small calculated (OH) and slightly negative CO shifts (from 2142 to 2131 cm^-1), which may explain the lower frequency component of the experimental doublet. As a synopsis, the experiments in combination with theory show that the CO region is very poorly discriminating with respect to the relative acidities of these materials, but the OH region allows us to conclude that the Brønsted acidity follows the order: MIL-53(Al) < MIL-53(Ga). Note that only the MIL-53(Al) sample shows a small presence of Lewis sites (band at 2230 cm^-1), likely due to the external surface of the material. The higher Brønsted acidity shown by MIL-53(Ga) is further confirmed upon CD3CN adsorption (Figure 3)¹³⁶. In both samples, however, both OH and CN shifts are very modest, lower than those observed for silicalite¹³⁶. Also, for both samples, a very small fraction of Lewis sites is revealed (weak bands at around 2320 cm^–1)¹³⁷. Calculations predict -OH···NC-CH3 adducts, with a higher red / blue shift in the OH / CN region for MIL- 53(Ga) (for example, ǻȞCN = +17 and +23 cm^-1 for MIL-53(Al) and MIL-53(Ga), respectively).

2320 2240

3600 3400 3200 3600 3400 3200

2320 2240

Wavenumber cm^-1

a) b)

Erreur ! Source du renvoi introuvable.: FTIR spectra of adsorbed CD₃CN: spectra collected at RT under vacuum after activation (red); spectra collected in presence of vapour pressure of CD3CN

(black). Part a) represents MIL-53(Al); part b), MIL-53(Ga).

a, b – Steamed HY c- Mordenite d- HEMT [Si/Al=3.8]

e-HY [Si/Al=5.4]

f- H-MFI g-[Ga] H-MFI h- HY [Si/Al=2.9]

i- HY [Si/Al=2.7]

j- HNaX

k- phosphated SiO₂ l- Steamed HNaY m-Silica & MIL-53 [Ga]

Ȟ(CO) [cm^-1] ǻȞ(OH) [cm-1]