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

stant and is not much influenced by the presence of cations. This suggests that the M/u.c. is low for both NH4+ and Na+ cations. Indeed, some 1.5 Na/u.c. were determined by the PIGE technique and by wet chemical analysis. The low-temperature peak was previously explained as being due to TPA+ cations neutralizing the SiO– defect groups in the structure [217, 226, 227, 231]. The behavior of the high-temperature peak is more controversial. It might be due to the decomposition of TPA+ ions neutralizing the negative framework charges introduced by the presence of tetrahedral Fe(III) or other trivalent atoms. This is the case for the [Al]-ZSM-5 samples [226, 229, 231]. However, for silicalite-1 or for boroand gallosilicalite-1, this peak also includes the products of decomposition stemming from the low-temperature peak (tripropylamine, dipropylamine, etc.) [184, 217, 226, 227, 231] (Testa F, Chiappetta R, Crea F, Aiello R, Fonseca A, Bertrand JC, Demortier G, Guth JL, Delmotte L, B.Nagy J, submitted for publication). The K-[Fe]-and Cs-[Fe]- silicalite-1 samples exhibit a different behavior (Fig. 15). At low Fe/u.c. values both the high-temperature peak and the low-temperature peak remain quasiconstant as Fe/u.c. increases. At higher Fe/u.c. values, however, only a single peak at an intermediate temperature is detected (Fig. 15). As there is some overlap between the Fe/u.c. values for the two DSC peaks and the single DSC peak due to decomposition of TPA, this suggests that not only the Fe/u.c. values are important, but the M/u.c. values as well, which greatly influence the decomposition temperature. The influence of the cations has already been shown for the [Al]-ZSM-5 [228, 229] and the borosilicalite-1 [226, 231] samples. In the case of the large K+ and Cs+ cations, due also to their higher amount in the samples, their influence can clearly be detected (Fig. 15). Their amount rises to 3.0/u.c. The specific influence of the cations can also be observed, if the amounts of TPA/u.c., which have decomposed at high (HT) and low (LT) temperatures, are plotted as a function of Fe/u.c. (Fig. 16). For the NH4-[Fe]-silicalite-1 samples, TPA/u.c. of the HT peak is always larger than that corresponding to the LT peak. They both decrease with increasing Fe/u.c. values, the TPA/u.c. values of the single peak are also decreasing with increasing Fe/u.c. as they represent the total amount of TPA/u.c. For the Na- [Fe]-silicalite-1 samples, the TPA/u.c. values of the LT peaks are always higher than those of the HT peaks and remain approximately constant as a function of Fe/u.c. On the other hand, for the K-[Fe]-silicalite-1 samples, at low Fe/u.c. values, the HT peak is larger than the LT peak and their TPA/u.c. values do not depend much on the Fe/u.c. values. Finally, for Cs-[Fe]-silicalite-1, the HT peak decreases, while the LT peak increases with increasing Fe/u.c. Of course, the TPA/u.c. values are decreasing with increasing Fe/u.c. for the single peak. Note that for [Al]-ZSM-5 [228], borosilicalite-1 [228] (Testa F, Chiappetta R, Crea F, Aiello R, Fonseca A, Bertrand JC, Demortier G, Guth JL, Delmotte L, B.Nagy J, submitted for publication) and gallosilica- lite-1 [184], the specific influence of the cations M+ could not be distinguished; the alkali cation only influenced the behavior of the TPA+ ions

Fig. 16 Variation of TPA/u.c. decomposed at high (HT) and low (LT) temperatures as a function of Fe/u.c.

through their relative quantities. At present, no clear-cut explanation can be given for this specific behavior of the various cations. In order to better understand their role in the fluorine-route synthesis of zeolites a study is in progress to identify the various Fe-species, before and after calcination of the samples, together with the identification of the various intermediates produced during the partial decomposition of the TPA+ ions.

All the 29Si NMR spectra are characteristic of ZSM-5 in its orthorhombic form, the corresponding lines are, however, broader due to the presence of paramagnetic Fe(III)ions in the framework. All the experimental spectra were decomposed by using four NMR lines centered at approximately – 102,

– 107, – 113 and – 117 ppm. (In one half of the samples a fifth line was added at approximately 110 ppm) The first two lines include the contribution of the SiOM (M = TPA and H) defect groups and that of Si(OFe)(OSi)3 groups. The latter two lines stem directly from the MFI framework as Si(OSi)4 groups. The amount of SiOM/u.c. was computed from both the – 102 and – 107 ppm lines. As these include both SiOM and Si(1Fe) groups: I–102,–107 ppm = ISiOM + ISiOM(1Fe). ISi(1Fe) is computed from the Si/Fe ratio: ISi(1Fe) = 100/0.25 Si/Fe. The values computed this way are reported in Table 11. Contrary to what has been reported for [Al]-ZSM-5 [227] and [Ga]-ZSM-5 samples [184], no correlation could be found between the Fe/u.c. and the SiOM/u.c. values. The amount of defect groups is rather high, i.e., much higher than in the Aland Ga-containing MFI structures. Only the Cs-[Fe]-ZSM-5 sample does not show any defect groups with 3.4 Fe/u.c. An extension of this work is necessary to

Table 11 Tetrahedral framework Fe and defect groups per unit cell in [Fe]-ZSM-5 samples obtained from the system 10SiO2 – x Fe(NO3)3 · 9H2 O – 15 MF – 1.25TPABr – 300H2O at 170 ◦C for 7 days

a SiOM defect groups, where M = alkali cations, NH4 , H or TPA

Fig. 17 SEM pictures of some final [Fe]-silicalite-1 samples for Si/Fe ratio of 33 in the presence of various cations

get more details on both the as-made and the calcined [Fe]-ZSM-5 samples. Figure 17 shows SEM pictures of some of the final crystalline samples obtained at an Si/Fe ratio of 33 in the slurry in the presence of various cations. The morphology of the crystals depends highly on both the amount of Fe/u.c.

in the structure and its nature and amount in the gel. The Na-[Fe]-silicalite-1 samples contain essentially prismatic crystals, the length of which can reach 130 µm. The Fe/u.c. is below 1 for most of the samples. Even at high Fe/u.c. values, the morphology remains prismatic.

For the NH4-[Fe]-silicalite-1 samples, the morphology is prismatic at low Fe/u.c. (up to 1.3), but it becomes ovoidal for higher Fe/u.c. values (equal to or higher than 2). Note again that the morphology remains ovoidal for more than 3 Fe/u.c., too. When the framework Fe/u.c. increases further, the morphology becomes spherical. These spheres are, in fact, agglomerates of small crystals. This habit can be observed for the K-[Fe]- and Cs-[Fe]- silicalite-1 samples as well. For these samples, the change in the morphology from prismatic to ovoidal, then to spherical occurs at lower Fe/u.c. values, showing that the nature and probably the amount of M+ cations in the samples also influence the crystal morphology. The aspect (length to width) ratio decreases with increasing Fe-content in the initial gels. A rather complex behavior is detected if the aspect ratios are plotted as a function of 1/r, where r is the radius of the bare exchanged cation M. The 1/r function is thus proportional to the electrostatic energy of gel stabilization by the various alkali counter-cations.

This will be discussed in more detail in connection with the study of the crystallization rate (see below). The specific effects of the alkali cations are also clearly visible, if the variation of lengths (L), widths (W) and aspect ratios (L/W) are plotted as a function of log R, where R is the crystallization rate in % h–1 (Fig. 18). Both L and W are decreasing very rapidly and almost linearly as log R increases (Fig. 18a and b). NH4-[Fe]- and Na-[Fe]-silicalite-1 samples show different variations, although the slopes of the lines are similar. On the other hand, K-[Fe]-and Cs-[Fe]-silicalite-1 samples show a large similarity. Interestingly, the aspect ratios, L/W, are increasing with increasing log R values. The pairs (NH4, Fe)-(Na, Fe) and (K, Fe)-(Cs, Fe) silicalite-1 show similar behavior (Fig. 18c). All these results show convincingly that one of the most characteristic features in the physical appearance of zeolitic crystals, namely the aspect ratio, does not only depend on the rate of crystallization, but also on the specific interactions of the exchange cations with the various crystal faces. The crystallization curves representing the percentage of crystallinity as a function of time are shown in Fig. 19. The peak at 2θ = 23.3◦ in the X-ray diffractogram was used as a measure of crystallinity, taking an ultrasonically treated final crystalline sample as standard for the 100% crystalline sample. When log(1/tind) is plotted as a function of log R (tind being the induction time, i.e., the time necessary for the appearance of 4% crystallinity), a general tendency can be observed: the rate of induction increases with the increasing rate of crystallization. The points are rather scattered, suggesting that the possible influence of the various factors (pH, Fe(NO3)3-content and MF-content of the gels) is not exactly the same on the rates of induction and crystallization. In the following, general trends will only be indicated for the

Fig. 18 Variation of the length L (a), the width W (b), and the L/W ratio (c) of the crystals as a function of log R (R is the crystallization rate in % h–1)

variation of the induction and crystallization rates. Due to the roughly parallel

variation of log(1/tind) and log R, only the variation of log R will be analyzed in more detail. From the observation that the pH values of the mother liquor

of the initial slurry (pHi) are always higher than the final pH (pHf) it follows that OH– ions are consumed during the dissolution of the solid phase and the final condensation of the building units (BUs) to form the crystalline [Fe]-silicalite-1. log R increases with increasing pH, although the experimental points are scattered, here too. A slope of 0.4 can be proposed for the general

Fig. 19 Crystallization curves of [Fe]-silicalite-1 samples obtained from the mixture 10SiO2 – 0.2Fe(NO3 )3 · 9H2 O – 15MF – 1.25TPABr – 300H2 O at 170 ◦C

trend showing the positive influence of the OH– ions on the crystallization rate. A similar slope was determined (0.3) for the synthesis of silicalite-1 in fluoride-containing media (1 mol NaF in a given slurry) within the pH range of 2.5–6.5 [232] and for the synthesis of gallosilicalite-1 within the pH range of 7.5–10 [184]. However, increasing the NaF content (to 10 mol, under otherwise identical conditions) a steeper slope was obtained for silicalite-1 within the pH range of 6–7.5 [233] and for borosilicalite-1 within the pH range of 7–9.5 (Testa F, Chiappetta R, Crea F, Aiello R, Fonseca A, Bertrand JC, Demortier G, Guth JL, Delmotte L, B.Nagy J, submitted for publication). The positive role played by the OH– ions emphasizes the fact that even in fluoridecontaining media the OH– ions play an important mineralizing role. log R slightly decreases with increasing Fe(NO3)3-content in the gel. Sometimes, a minimum appears as in the case of NH4-[Fe]-silicalite-1. A similar decrease in the crystallization rates with trivalent substituting elements (like Al, B, Ga) has previously been observed showing the difficulty of the replacement of silicon by foreign substituents in tetrahedral framework positions [184, 227, 234] (Testa F, Chiappetta R, Crea F, Aiello R, Fonseca A, Bertrand JC, Demortier G, Guth JL, Delmotte L, B.Nagy J, submitted for publication).

The variation of the crystallization rate as a function of the amount of MF salts in the slurry shows an interesting behavior. log R decreases with increasing MF content for NH4-[Fe]-silicalite-1 samples. As the F– ions are mineralizing agents in these syntheses, the decrease might be attributed to a particular behavior of the NH4+ ions. Indeed, for the other two cations, Na+ and K+, log R increases with increasing MF content. For the KF:0.1Fe(NO3)3 system even a minimum is found for 15 KF in the gel. The specific influence of the various alkali cations on the crystallization rate can also be recognized, if log R is plotted as a function of 1/r for 15 moles MF in the slurry. A similar variation is found for the system 24MF:0.3Fe(NO3)3. Cs+ is the most effective cation for accelerating crystallization, followed by K+ and Na+. NH4+ is the least effective cation. As mentioned above, the function 1/r is proportional to the electrostatic stabilization energy of a negatively charged surface

by the positive cations. As no variation in parallel is found, this means that the electrostatic stabilization energy is not the main factor influencing the crystallization rate for various exchange cations. Note that as a rough increase is observed for log R as a function of pHi, the pHi values show a similar variation as in the function of 1/r (not shown). This shows once again that the cations act more through their chemical characteristics than by solely physical (electrostatic) interaction in the fluoride-containing media. It is worth mentioning that the pHi values are the lowest in the presence of NH4F. As both NH4+ and F– ions stem from a weak base and a weak acid, respectively, in dilute aqueous solutions the approximate pH is given by pH = (pKNH4 + pKHF)/2 = 5.6. This value is equal to 5.6 because pKNH4 = 9.25 and pKHF = 2.0. The observed values are between 6.5 and 7.0. If, instead of Fe(NO3)3, Ga(NO3)3 is used in similar synthesis slurries, the initial pHi is markedly higher, reaching 6.7–8.5. Thus, specific characteristics of the substituting ions (like their propensity to hydrolysis of the respective F–-containing ions, or their specific adsorption/reaction on the silica component) and not least the resulting ionic strength in the fairly concentrated slurries, all influence greatly, and in a fairly complex way, the initial pHi of the slurry and with it the induction and crystallization rates as well. These results emphasize the need for further investigation of the “solution chemistry” of the initial crystallization slurry. Whereas in the fluoride route, using slightly acidic or neutral media, the complex stability of various soluble Fe(III) fluoro-complexes determine the (near-)equilibrium concentrations of various iron species in the slurry, their chemical identification is currently hampered, because the temperature of synthesis and the resulting ionic strength are too high. Nevertheless, one point seems to be important and worth noting. Irrespective of the chemical identity of the Fe(III)species (subsequently referred to as BU), it is certain that this BU is a mononuclear iron species, because the products are absolutely free of extra-framework (EFW) iron oxide/hydroxide which, when present even in trace amounts, imparts rusty coloration of various grades (from buff to a bright orange-red rusty color). The mononuclearity of the BU is strongly supported by Mössbauer spectroscopy studies carried out on zeolite samples synthesized within the frame of this research work (vide infra). This behavior is in contrast with synthesis routes carried out in alkaline media. In these methods, gelation of iron silicate is always preceded by the preparation of a strongly acidic Fe(III) salt solution (using sulphuric or rather perchloric acid). After cooling, the iron silicate zeolite-precursor gel is formed by the addition of cool waterglass solution, rich in monomeric silica species. However, earlier magnetic [235], spectroscopic [236] and po-

tentiometric [237] measurements showed that a number of complex ions (like [Fe(OH)(H2O)5]2+, [(H2O]5Fe-O-Fe(H2O)5]4+ , [Fe2(OH)2(H2O)8]4+ where

the two Fe(III)-ions of octahedral, Oh, coordination share common edge, etc.) are formed when the pH is changed. Whilst at lower pH (< 0.5) the whole iron appears to be monomeric, above pH = 2 at least 90% of the Fe(III)ions

form dimeric species. It must not be forgotten that, in order to attain gelation, the pH has to be raised at least to 4.2–4.4. In previous publications [41, 225], using XP and Mössbauer spectroscopies, we showed that these binuclear iron species cannot be separated into monomeric ones; they are present in the iron silicate gel too, and because only one of them can be built into the zeolitic framework, a relatively high percentage (using sulphuric acid at the onset, about 30%) of iron is present in EFW positions from the very beginning. In other words, these samples, depending on the total iron content, are more or less rust-colored. It is surprising, indeed, that we find hints in the literature [238] that in strongly alkaline media, at or above 180 ◦C, these colored gels (in the presence of TMA cations) crystallize into white sodalite (SOD) specimens. This seems to be possible only if the binuclear iron species get separated from each other (probably via forming ferrate anions) during the arduous conditions of the (sodalite) synthesis. We believe that this mechanism cannot be operative in the synthesis of ZSM-5 (MFI) zeolites. From the point of view of catalytic activity both the fluoride-and alkaline routes might result in excellent catalysts; nonetheless, for scientific (e.g., spectroscopic) studies the fluoride-route should be preferred, because it leads to products with framework (FW) iron in unique tetrahedral, Th coordination.

Table 12 The synthesized MFI products through the variation of the starting hydrogel composition, in the type-A and type-B systems

EG: ethylene glycol; A: without EG; B: with EG

3.4.2

Alkaline Route

In Table 12 the products obtained for both systems A and B in the presence of oxalic acid or phosphoric acid are reported.

3.4.2.1

[Fe]-MFI Synthesized by the Alkaline Route

[Fe]-MFI products were obtained in alkaline media using either oxalic or phosphoric acid as complexing agents [35]. In addition, the initial gels were prepared either without ethylene glycol (system A) or in the presence of ethylene glycol (system B) (Table 12).

The general compositions of the gels were: xNa2O – yTPABr – zAl2O3 –

SiO2 – qFe2O3 – pHA – 20H2O where x = 0.1 – 0.32; y = 0.02 – 0.08; z = 0 – 0.05; q = 0.005 – 0.025; the ratio p/q = 3 and HA stands for H2C2O4 or

H3PO4. The second system was prepared from the following initial reaction mixture: 0.16Na2O – xTPABr – yEG – zAl2O3 – SiO2 – qFe2O3 – pHA – 10H2O where x = 0.08; y = 0 – 6.0; z = 0 – 0.05; q = 0.005 – 0.1; the ratio p/q = 3 and HA stands for H2C2O4 or H3PO4 and EG for ethylene glycol.

First, it can be observed that, if only TPABr is present in the initial gel (system A), [Fe]-MFI zeolite can be obtained in a large Si/Fe range and reaction time of 1–4 days (samples 1A–7A in Table 12). The obtained [Fe]- MFI products are thermally stable. Indeed, if the reaction time is increased, the transformation of the MFI structure into more stable phases is not observed. Even after a thermal treatment up to 850 ◦C, the MFI is the sole phase detected. The crystallization time increases with increasing iron and/or aluminum content of the starting hydrogel (compare samples 1A and 2A, and samples 4A and 6A, respectively).

In system B, the formation of [Al]-MFI is very easy in the presence of ethylene glycol (EG) only (sample 21B). This result confirms the data reported in [239]. In the presence of EG, only Al-rich hydrogels were studied, because no pure MFI zeolite could be synthesized in a silica-rich system [213]. EG is also able to direct the formation of the MFI structure in the presence of both Al and Fe (samples 22B and 23B). If TPABr is also present in the initial gel, the reaction time decreases (see samples 11B and 18B, and 12B and 19B). In addition, the stability of the final MFI products also increases with increasing TPABr content of the gel. Indeed, a pure [Fe]-MFI sample is obtained with 0.08 TPABr (sample 8B) even after 6 days of crystallization, while sample 14B synthesized in the presence of 0.02 TPABr shows the co-crystallization of MFI with quartz and cristobalite after 4 days of crystallization. The increasing TPABr content in the initial hydrogel results in decreasing reaction time (see samples 14B and 8B). On the other hand, the increase in the initial Al or Fe content in the hydrogels leads to the increase in reaction time: compare sam-