Molecular Sieves - Science and Technology - Vol. 6 - Characterization II / 06-Isomorphous Substitution in Zeolites

Fig. 5 a A typical 71Ga MAS NMR spectrum of [Ga]-ZSM-5 sample synthesized from 10SiO2 – 18CsF – 0.3Ga(NO3)3 –1.25TPABr–330H2O at 170 ◦C (Ga/u.c. = 1.7). b A typical 29Si NMR spectrum of a [Ga]-ZSM-5 sample synthesized from 10SiO2 – 18CsF – 0.1Ga(NO3 )3 – 1.25TPABr – 330H2 O at 170 ◦ C (Ga/u.c. = 0.9)

with increasing Ga/u.c. (Fig. 6). The SiOH/u.c. values are higher for the K-containing samples than for the Naand Cs-containing ones. The TPA+ ions are incorporated intact into the zeolitic channels. The 13C NMR spectra are characteristic of TPA+ in a well-crystallized ZSM-5 zeolite synthesized either in F-containing media [215, 216] or in alkaline media [217]. The chemical shifts and linewidths are as follows: N–CH2O–: δ = 62.7 ppm (∆H = 298 Hz) with a shoulder at 65 ppm; –CH2O–: δ = 16.4 ppm (∆H = 215 Hz) and CH3–: δ = 11.4 ppm (∆H = 100 Hz) and δ = 10.2 ppm (∆H = 100 Hz). The decomposition of the TPA+ cations occurs either in one or two steps, similarly

Table 9 SiOX/u.c. (X = H, M and TPA), SiOTPA/u.c., SiOH/u.c., sum of positive charges (TPA+M)/u.c. (M = NH4, Na, K and Cs), and sum of negative charges (Ga+F)/u.c. of [Ga]-ZSM-5 samples

a X = H, M and TPA, where M = NH4 , Na, K, Cs

b SiOTPA/u.c. = TPA/u.c. decomposed at low temperature (420 ◦ C) c SiOH/u.c. = SiOX/u.c. – (SiOTPA/u.c. + SiOM/u.c.)

to that of the [Al]-ZSM-5 samples (Fig. 7) [213, 216]. In one sample synthesized in the presence of 18 NH4F and 0.3 Ga(NO3)3, essentially one hightemperature peak at ca. 480 ◦C is observed, characteristic of the decomposition of TPA+ (SiOGa)-ion pairs. In this case, the value of Ga/u.c. (3.2) is close to that of TPA/u.c. (3.0) decomposed at 480 ◦C (Table 8). At lower Ga content, two decomposition peaks are observed at ca. 420 and 480 ◦C. The low-temperature peak can be attributed, as in the case of [Al]-ZSM-5 samples, to TPA+ ions linked to SiO– defect groups or to TPA+ F– (or OH–) ion pairs [213, 216, 217]. This interpretation has nevertheless to be taken with some caution. Indeed, when the TPA/u.c. decomposed at higher temperature and is plotted as a function of Ga/u.c., the experimental points are rather scattered, and the difference between TPA/u.c. and Ga/u.c. becomes greater for low Ga/u.c. values. This suggests, that the high-temperature decomposition peak is related not only to the decomposition of TPA+ ... (SiOGa)– species but also to the decomposition of the products that were formed during the decomposition of the TPA+ F– ion pairs. Several peculiarities have also been observed in the presence of 18 KF and 18 CsF (Fig. 7). An intermediate DTA peak was also observed at 450 ◦C in the presence of 18 KF. With 18 CsF

Fig. 6 Variation of SiOH/u.c. as a function of Ga/u.c. for the various [Ga]-ZSM-5 samples synthesized with NaF, KF and CsF

Fig. 7 DTG and DTA of [Ga]-ZSM-5 samples synthesized from 10SiO2-xMF-y Ga(NO3)3- 1.25TPABr -330 H2 O at 170 ◦C: a x = 9NH4 F, y = 0.3; b x = 18NH4 F, y = 0.3; c x = 18 KF, y = 0.1; d x = 18 CsF, y = 0.1

(and also with 9 CsF) (Fig. 7), a new DTA peak also appeared at 530 ◦C. This peak is characteristic of silicalite-1 and is due to the decomposition of the products occurring in the temperature range of the first DTA peak. The presence of this peak suggests that the crystallites are not homogeneous and contain highly siliceous regions, as it was shown already in [223].

3.3

Influence of Alkali Cations on the Incorporation of Al, B and Ga Into the MFI Framework

The T atoms are essentially incorporated in the zeolite framework as it is clearly shown by 11B, 27Al and 71Ga NMR measurements [181, 184, 219]. Indeed, the position of the maxima are at – 4.0 ppm vs. Et2O·BF3 for 11B NMR, + 52 ppm vs. Al(H2O)5+6 for 27Al NMR spectra and + 152 ppm vs. Ga(H2O)3+6 for 71Ga NMR spectra. Table 10 shows the composition of the final [T]-MFI phases synthesized from 10SiO2 – 9MF – yT – 1.25TPABr – 330H2O at 170 ◦C, with M = NH4, Na, K or Cs; T = 0.1 H3BO3, 0.16 Al(OH)3 and 0.1 Ga(NO3)3. At such a low initial T atom content of the gels, the introduction of the TIII atoms is quite efficient. Indeed, the Si/T ratio of the initial gels is equal to 100 for B and Ga, and 62.5 for Al, while the Si/T ratios in the final samples vary between 119 and 73.

The fluorine contents are similar in all samples and vary between 1.1 and 4.8 F/u.c. The M/u.c. values are rather low showing that the tetrapropylammonium (TPA+) ions neutralize the framework negative charges associated with the presence of TIII atoms in the framework. The TPA/u.c. values are equal to 3.8, which is close to the maximum theoretical value of 4. In this case all the channel intersections are occupied by TPA+ ions. The amount of T/u.c. in the structure increases with increasing amount of T atoms in the initial gels. Up to 8 B/u.c. can be introduced into the framework starting from gel compositions of 10SiO2 – 6H3BO3 – 9KF – 1.25TPABr – 330H2O, up to 6.5

Table 10 Comparison of the final zeolitic [T]-ZSM-5 phases (T = B, Al, Ga) synthesized from gels 10 SiO2 – y T – 9 MF – 1.25 TPABr – 330 H2O at 170 ◦ C (X = H, M and/or TPA, and M = NH4, Na, K, Cs)

Al/u.c. starting from 10SiO2 – 15KF – 6Al(OH)3 – 1.25TPABr – 330H2O, up to 3.2 Ga/u.c. starting from 10SiO2 – 18NH4F – 0.3Ga(NO3)3 – 1.25TPABr – 330H2O. As it can be seen from these results, K+ ions are the preferred cations for the introduction of boron and aluminum, while NH4+ ions are more efficient in the case of Ga. (This sequence is not followed at low TIII/u.c. and these values are generally close to 1.) From the comparison of

the initial and final Si/T ratios (Si/Bgel = 1.7 and Si/B of the zeolite is equal to 1.1; Si/Algel = 10 and Si/Alzeolite = 14; Si/Gagel = 33 and Si/Gazeolite = 29) the following conclusion can be drawn. Ga is introduced with the high-

est yield, followed by Al and the introduction of B is the less effective one for these maximal conditions. The amount of the SiOX defect groups (X = H, M and/or TPA) was determined from 29Si NMR measurements. It is much higher for [Al]- and [Ga]-ZSM-5 samples, while only a small amount of SiOX is found in [B]-ZSM-5 zeolites. Moreover, the NH4-[T]-ZSM-5 zeolites show generally the smallest value of SiOX/u.c. The relative SiOX/u.c. values do not seem to be linked to the initial and final pH values being in the range of 8–9 for most cases. It is also interesting to compare both the relative amount and the temperature of decomposition of the two different TPA+ species, whether the counter-cations balanced the negative charges of the TOSi– (high-temperature DTA peak) or the SiO– defect groups (lowtemperature DTA peak). While the relative amount depends on the T/u.c. values, the decomposition temperature is greatly influenced by the M/u.c. values of the counter-cations. When the temperatures of the extrema of the two peaks and of the single peak are plotted as a function of the Al/u.c., interesting variations can be noted (Fig. 8). At low Al content two peaks were observed merging into a single peak at high Al content (around 7–8 Al/u.c. curves a and b). A similar behavior was observed previously for samples synthesized in the presence of NH4F. This relationship only holds, however, for samples prepared in the presence of NH4F and for those prepared with a low amount of aluminum and hence containing only a low amount of M/u.c. If the samples contain more than 1.5 M/u.c. (there is only one exception), the two DTA peaks are merging much faster as a function of Al/u.c. (curve c in Fig. 8). Indeed, if the data for NaF, KF and CsF are plotted as a function of M/u.c., a clear-cut variation can be observed (Fig. 9). At low M/u.c. values two peaks are well detected (the only exception is that obtained with CsF, Al/u.c. = 2.5 and Cs/u.c. = 0.5). These two peaks merge into one for M/u.c. ≥ 1.5. This means that the alkali cations neutralize more preferentially the F– ions and/or the defect SiO– groups, while the remaining TPA+ ions interact better with the negative charges (SiOAl)– of the framework. Hence, with a high M/u.c. content no TPA+ ions are directly interacting with either F– or SiO– defect groups. The decomposition of TPA+ cations in Ga-containing samples occurs either in one or two steps, similarly to that of the [Al]-ZSM-5 samples [213, 216] (Fig. 10).

Fig. 12 Variation of the induction rate (1/tind) and of the crystallization rate (R) as a function of 1/r (r: radius of the bare cation) for the [B]-ZSM-5, [Al]-ZSM-5 and [Ga]-ZSM-5 zeolites synthesized in fluoride-containing media

Na+ ions. Note that the sequence found for K+ ions follows the sequence of Sanderson electronegativities, i.e., Ga > B > Al [224]. On the other hand, the solubility of MF fluorides gives the sequence NaF < KF < NH4F < CsF. Only the crystallization rate of [Al]-ZSM-5 follows this sequence. For the majority of the other cases (both induction and crystallization rates), KF influences the reaction rates the most. The effect of both the amount of T and the amount of MF was systematically explored on both the induction and crystallization rates. It can be concluded that the M+ ions not only exert their coulombic stabilization effect (which is inversely proportional to r), but they also interact specifically with the various anionic species at the solid-liquid interface, where both nucleation and crystallization take place. Alkali cations deeply influence the incorporation of trivalent (T) atoms in metallosilicalite crystals. In particular K+ ions are preferential cations for the introduction of boron and aluminum, while NH4+ ions are more efficient for Ga. The physico-chemical properties of metallosilicalites are also markedly influenced by alkali cations.

3.4 [Fe]-MFI

3.4.1

Fluoride Route

The gels with the composition 10SiO2–xFe(NO3)3 · 9H2O–yMF–1.25TPABr– 300H2O, where x = 0.1, 0.2 and 0.3, y = 3, 6, 9, 12, 15, 18, 21 and 24 and M = NH4, Na, K and Cs all led to crystalline MFI structures at 170 ◦C except for x = 0.1, y = 3 and M = Cs, where the gels remained amorphous. All the prepared gels were white before heating, because the iron species were not precipitated as hydroxides, but they formed soluble complexes with fluoride ions. Note that this is not the case for gels prepared in alkaline media, which were generally pale yellow or even “rusty” [225]. The as-synthesized products were all white powders, with a light greenish hue; only in some cases

Fig. 14 DSC curves of Cs-[Fe]-silicalite-1 samples prepared with various amounts of CsF from the gel 10SiO2 – xFe(NO3)3 · 9H2O – yCsF – 1.25TPABr – 300H2 O at 170 ◦C with y = 6, 12, 18 and 24

Fig. 15 Variation of the temperatures of the peaks in the DSC curves as a function of Fe/u.c.

Demortier G, Guth JL, Delmotte L, B.Nagy J, submitted for publication) or [Ga]-silicalite-1 [184]. The variation of the temperature of both the highand low-temperature DSC peaks as a function of Fe/u.c. leads to interesting observations (Fig. 15). For the NH4-[Fe]-silicalite-1 samples both the high-and low-temperature peaks decrease slightly with increasing Fe/u.c. On the other hand, for the (Na, Fe)-silicalite-1 samples both the low-temperature and the hightemperature peaks remain quasi-constant with increasing Fe/u.c. For NH4+ and Na+ cations the interaction of TPA+ with the framework is con-