Contraindicaciones: 43 

Usually, the first step towards the synthesis of a zeolitic framework is the preparation of the structure-directing agent (SDA), whose role has already been discussed, see Section 1.1.1. For Ge-UTL synthesis, (6R, 10S)-6,10-dimethyl-5- azoniaspiro[4,5]decane, shown in Figure 3.1, was used as the SDA and synthesised by optimising a literature procedure.1-2

The synthesis of (6R, 10S)-6,10-dimethyl-5-azoniaspiro[4,5]decane in its bromide and hydroxide forms is summarised in Table 3.1 and schematically shown in Figure 3.2.

Figure 3.1 Structure of the (6R, 10S)-6,10-dimethyl-5-azoniaspiro[4,5]decane cation used as the SDA in the Ge-UTL synthesis.

Chemical Quantity / mmol Amount

NaOH (pellets) 284 11.36 g

1,4-dibromobutane 284 61.32 g

(2R,6S)-2,6-dimethylpiperidine 284 32.14 g

Chloroform 4×(100 mL) + 2×(50 mL)

Diethyl ether Precipitating and washing agent

Ambersep 900-OH 7×(45 g)

HCl Titration (1 M)

Distilled water ∼400 mL + washing

Figure 3.2 Schematic representation of the synthetic reaction to prepare the SDA in its bromide form and related experimental apparatus.

The synthetic procedure used can be outlined as follows:

- NaOH pellets were dissolved in 280 mL of deionised water and then 1,4- dibromobutane was added to the solution, which was heated up to reflux temperature (105 °C). At the end of this first stage, the reaction mixture was biphasic with 1,4- dibromobutane forming oily drops in the NaOH solution.

- (2R,6S)-2,6-dimethylpiperidine was added drop-wise over 20-30 min under reflux and fast stirring. The reaction mixture was left for 24 h in refluxing conditions and subsequently left to cool to room temperature, resulting in a single orange phase. - A freshly prepared 50% (w/v) iced NaOH solution was added slowly to the reaction mixture at 0 °C and approximately 20 g of NaOH pellets were added to promote the formation of the oily layer containing the reaction product. After this last addition, 30 min of stirring were needed.

- The organic oily phase was extracted with CHCl3, dried with MgSO4, filtered and concentrated by rotoevaporation.

- The precipitating agent, diethylether, was then added and the resulting solid was filtered, washed with diethylether and recovered as a white powder. Yield ≈ 70%. The identity of the synthesised bromide salt was confirmed by a solution-state 1H NMR spectrum recorded in CDCl3, shown in Figure 3.3, and assigned as follows: δ

1.4 ppm (d, 6H) corresponds to the two methyl groups; δ 1.6-2 ppm (m, 6H) corresponds to the three –CH2 groups on the six-membered ring; δ 2.15 ppm (m, 4H) corresponds to the –CH2 groups of the five-membered ring not directly bonded to the nitrogen atom; δ 3.35 ppm (t, 2H) and δ 4.1 ppm (t, 2H) correspond to the two –CH2 groups of the five-membered ring directly bonded to the heteroatom; δ 4.4 ppm (m, 2H) corresponds to the –CH groups directly bonded to the heteroatom on the six- membered ring.

According to the reported synthesis2 of Ge-UTL, the synthesised bromide salt was then ion-exchanged with Ambersep 900-OH resin. Typically, 45 g of (6R, 10S)-6,10- dimethyl-5-azoniaspiro[4,5]decane bromide were dissolved in 100 mL of deionised water to which 45 g of Ambersep 900-OH resin were added. After 24 h of stirring, the resin was filtered and washed. This same procedure was repeated 7 times in total so that a 0.55 M solution of (6R, 10S)-6,10-dimethyl-5-azoniaspiro[4,5]decane hydroxide was obtained.

A gel with a molar composition of 1 SiO2: 0.5 GeO2: 0.5 SDA, as summarised in Table 3.2, was prepared by dissolving germanium dioxide in 150 mL of a 0.55 M (6R, 10S)-6,10-dimethyl-5-azoniaspiro[4,5]decane hydroxide solution. Silicon dioxide was subsequently added in small portions into the solution and the resulting mixture was stirred at room temperature for 30 min.

1.5 2.0 2.5 3.0 3.5 4.0 4.5 1_{H (ppm)} 1.0 5.0

Figure 3.3 1H solution-state NMR spectrum recorded at 7.05 T in CDCl3 of the as-

SiO2 (Cab-o-sil) GeO2 (99.999 % Acros) SDA Quantity / mmol 178.1 88.6 81.8 Molar ratio 1 0.5 0.5 Amount 10.7 g 9.3 g 150 mL of 0.55 M sol.

The gel was transferred into a 300 mL Teflon-lined Parr® autoclave, as schematically shown in Figure 3.4, and heated at 175 °C for 7 days in static conditions. The solid was recovered by filtration, washed with copious amounts of distilled water, air dried overnight (15.5 g) and characterised by PXRD, as shown in Figure 3.5.

Figure 3.4 Schematic representation of Ge-UTL synthetic reaction. The autoclave sketch has been adapted from Parr® 4744 and 4749 general purpose acid digestion bomb instruction manual.

In order to remove the SDA, calcination has to be performed on the as-synthesised zeolite. Therefore, the Ge-UTL zeolite was heated up to 575 °C at a rate of 1 °C/min, held at this constant temperature for 6 h and cooled to 25 °C at a rate of 2 °C/min under an atmosphere of air. The 29Si MAS NMR spectrum of the calcined zeolite in Figure 3.6 clearly shows the presence of an ordered material (with multiple Q4 sites, as described in Chapter 4), while its PXRD pattern proves its phase purity, as shown in Figure 3.7.3 The Si/Ge ratio of the material was determined to be 4.4 ± 0.3 by EDX.

Figure 3.6 29_{Si (9.4 T, 10 kHz) MAS NMR spectrum of calcined Ge-UTL. For this} spectrum 432 transients were averaged with a recycle interval of 120 s.

Figure 3.5 PXRD pattern of as-made Ge-UTL.

Si (ppm)

5 10 15 20

2 ( )

The 29Si-enriched Ge-UTL was synthesised within the group following a similar procedure, but starting from a different silicon source, tetraethoxysilane (TEOS), Si(OEt)4, a part of which, as summarised in Table 3.3, was 99% 29Si-enriched

(CortecNect).

Si(OEt)4 (TEOS)

∼18% 29Si-enriched GeO2 SDA Quantity / mmol 1.6 (99%

29_{Si-enriched) +}

10.1 (nat. ab.) 5.8 5.8

Molar ratio 1 0.5 0.5

Amount 0.333 g + 2.094 g 0.609 g 10 mL of 0.58 M sol.

A gel with a molar composition of 1 SiO2: 0.5 GeO2: 0.5 SDA, as summarised in

Table 3.3, was prepared by dissolving germanium dioxide in 10 mL of a 0.58 M (6R, 10S)-6,10-dimethyl-5-azoniaspiro[4,5]decane hydroxide solution. TEOS, with an overall 29Si-enrichment level of approximately 18%, was subsequently added and the resulting reaction mixture was stirred for 2 h to remove EtOH. This gel was transferred into a 23 mL Teflon-lined Parr® autoclave and heated at 175 °C for 14 days. The solid was recovered by filtration, washed with copious amounts of distilled

Figure 3.7 PXRD patterns of calcined Ge-UTL (black) and a pattern simulated for the UTL framework from IZA database (red).3

5 10 15 20 25 30 2 ( )

35 40

water and air dried overnight. The PXRD of the as-made zeolite is shown in Figure 3.8 and can be compared to Figure 3.5. In order to remove the SDA, calcination of the as-synthesised zeolite was performed, as previously described. The corresponding NMR spectrum of the 29Si-enriched starting Ge-UTL sample is shown in Chapter 4.