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

for “solid-state reaction”, while H.T. indicates that iron was directly introduced during hydrothermal synthesis. Post-synthesis introduction of iron was made, using as parent zeolite a commercial NH4+-MFI zeolite (from AlsiPenta) with a SiO2/Al2O3 ratio of 25. H-ZSM-5 was obtained from the same parent zeolite by calcination. The iron content in these samples is around 250 ppm. Usual practice was to calcine the dried catalysts at 550 ◦C in air.

Prior to the catalytic tests, the [Fe,Al]-MFI samples were activated in-situ at temperatures ranging from 600 to 700 ◦C in the presence or absence of steam. The catalytic tests were made in a fixed-bed reactor at a temperature typically of 400 ◦C feeding a mixture containing (i) 3 mol % benzene and 6 mol % N2O in helium or (ii) 20 mol % benzene and 3 mol % N2O in helium. Sometimes steam (up to about 3 mol %) was also added to the feed. The total flow rate was 3 L h–1 and the amount of catalyst 0.5 g (contact time of 0.6 s g ml–1). The feed was prepared using a calibrated mixture of N2O in helium and adding benzene using an infusion pump and a vaporizer chamber. The feed could be sent either to the reactor or to a bypass for its analysis. The feed coming out of the reactor or from the bypass could be sent to vent or to one of two parallel absorbers containing pure toluene as the solvent (plus calibrated amounts of tetrahydrofuran as the internal standard) cooled at about – 15 ◦C in order to condense all organic products. The line to the absorbers was heated at about 200 ◦C in order to prevent condensation of the products. The vent, after condensation of the organic products, was sent to a sampling valve for analysis of the residual gas composition. The reactor outlet stream was sent alternatively to the two parallel absorbers for a given time (typically 3 or 5 min), in order to monitor the change in the catalytic activity averaged over this time. N2O, O2, N2 and total oxidation products (CO and CO2) were analyzed using TCD-gas chromatography and a 60/80 Carboxen-1000 column, whereas benzene and phenol (as well as other minor aromatic by-products) were determined by FID-gas chromatography using a ECONO-CAP SE-30 “wide bore” column or a Mass-GC equipped with a capillary Chrompack CP-Sil 5CB-MS fused silica column.

3.4.4

Role of the Catalyst Composition

Reported in Fig. 26 is the catalytic behavior (productivity and selectivity to phenol in benzene hydroxylation with N2O) as a function of time on stream over a series of [Fe,Al]-MFI-type catalysts prepared by hydrothermal synthesis. The characteristics and composition of these samples are reported in Table 15. The samples were selected in order to summarize the influence of catalyst composition on the activity and rate of deactivation. A commercial H-ZSM-5 sample (H-ZSM-5-C) showed good activity in agreement with literature data. However, it deactivated very quickly, and in about 1 h the productivity decreased by a factor of about 50. The selectivity to phenol pro-

activate N2O, because they behave as F-centers. In agreement, the catalyst shows an initial activity in N2O decomposition (around 22% N2O conversion after 5–10 min of time on stream). This suggests that defects in the zeolite can activate N2O, but are unable to either form active oxygen species in benzene hydroxylation and/or to activate the organic substrate (benzene) for this reaction.

When an Al-free [Fe]-silicalite (Fe1.1MFIH.T.-S) is used, the productivity to phenol is low but quite stable. No change in the productivity was observed

up to about 20 h of time on stream. The selectivity to phenol is initially low and progressively increases up to final constant values of about 50%. The conversion of N2O is low (about 5%) and nearly constant with increasing time on stream. Therefore, in the absence of Al the activity of the catalyst towards phenol synthesis is low, but remarkably stable, suggesting that Al sites may play a role in the synthesis of phenol, but also in the side-reactions leading to catalyst deactivation. When both Fe and Al are present in the zeolite

(Fe1.1MFIH.T.-A55 and Fe2.2MFIH.T.-A54) the productivity to phenol increases by a factor of about 10 or 20, and the selectivity to phenol also markedly

increases reaching values higher than 90% for Fe1.1MFIH.T.-A55. The productivity to phenol is approximately proportional to the amount of iron (compare

phenol productivities for Fe1.1MFIH.T.-A55 and Fe2.2MFIH.T.-A54; the latter has twice the iron content and initially twice the productivity than that of

the former, while the Si/Al content is the same). The selectivity to phenol, on the contrary, is higher in the case of the sample having lower iron content. The trend with time on stream is quite similar for the two catalysts. The productivity to phenol decreases, although much less dramatically than in the case of H-ZSM-5-C, while the selectivity to phenol increases. The conversion of N2O is initially around 40% and then decreases to about 10–15%

for Fe1.1MFIH.T.-A55. When this result is compared with that of the sample having a comparable iron content, but no aluminum ions (Fe1.1MFIH.T.-S), it is evident that the presence of Al ions (or probably better sites related

to Al, such as Al-OH Brønsted acid) together with iron ions is a condition necessary for efficient activation of N2O. However, sites for the decomposition of N2O are also present which progressively become inactivated by the carbonaceous-type species formed on the catalyst, causing its deactivation. In

fact, the rate of N2O conversion in Fe1.1MFIH.T.-A55 decreased by a factor of about 6–8 during the 2.5 h of the experiments (Fig. 26), while the productiv-

ity to phenol decreased by a factor of about 2. The side decomposition of N2O produces O2 (which is also detected at the reactor outlet), and probably O2 determines the total combustion of benzene, phenol or reaction intermediates and thus the formation of CO2 (CO is usually observed only in traces). This explains why in parallel to the decrease in N2O conversion, an increase in the selectivity to phenol is observed. With increasing iron content (Fe2.2MFIH.T.- A54) the initial conversion of N2O is about 65% and then decreases to about 20–25% after 2.5 h of time on stream. This indicates that the increase in iron

content increases the number of active sites for phenol synthesis (the productivity is almost doubled), but probably also increases the formation of a second type of iron species. These species are probably small iron-oxide particles within the zeolite cavities formed in the process of partial migration of iron from framework to non-framework positions during the initial catalyst pretreatment. The increased amount of iron favors the aggregation of these iron species and thus, a higher amount of aggregated iron oxide is thought to be responsible for the decomposition of N2O to N2 + O2 instead

of N2 + α-O. As a consequence, the productivity of phenol in Fe2.2MFIH.T.- A54 is higher than in Fe1.1MFIH.T.-A55, but the selectivity to phenol is lower. Decreasing the amount of Al, while maintaining the amount of iron (com-

pare Fe2.3MFIH.T.-A90 with Fe2.2MFIH.T.-A54) constant leads to an increase in both the productivity and selectivity, although the general trend remains

again similar. This indicates that probably an Al/Fe ratio close to 1 is the optimal compromise to maximize both productivity and selectivity to phenol, suggesting that the active sites for phenol synthesis comprise both Al and Fe.

3.4.5

Role of Methodology in Iron Introduction in [Fe]-MFI Catalysts

In order to analyze in a concise way the effect of different methods of addition of iron into [Al]-MFI samples on the catalytic performances (activity and deactivation), catalytic data have been summarized in Table 16. The following parameters are reported: (i) the maximum observed phenol productivity, which is generally obtained at the beginning of the reaction or after about 20 min of time on stream (see Fig. 26), (ii) phenol productivity after 2 h, indicating catalyst stability, and (iii) the selectivity in correspondence with the maximum phenol productivity and after 2 h time on stream. Table 16 compares the behavior of some selected samples in which Fe was introduced post-synthetically by (i) an ion-exchange method (Fe1.8MFII.E.-A13 and Fe3.6MFII.E.-A13) using an aqueous solution of iron-ammonium-sulfate, (ii) CVD (contacting the dehydrated zeolite at 300 ◦C with a flow of FeCl3 in N2) and (iii) solid-state reaction (FeCl3 and the zeolite are mixed homogeneously and heated to 400 ◦C). For better comparison, data for two samples prepared by the hydrothermal method and containing both Fe and Al

(Fe1.1MFIH.T.-A55 and Fe2.2MFIH.T.-A54) as well as data for the parent zeolite used for ion exchange and CVD (H-ZSM-5-C) are also reported.

The post-synthetic introduction of iron in the Fe-[Al]-MFI catalysts generally leads to lower selectivities and productivities to phenol, although in some cases (e.g., Fe3.6MFII.E.-A13) good initial behavior is obtained. With respect to the behavior of the parent zeolite (H-ZSM-5-C), the productivity to phenol increases more than twice, and an increase in the selectivity to phenol could also be noted. However, similarly to H-ZSM-5-C, after 1 h the productivity to phenol becomes negligible. When the iron content is lower,

Table 16 Phenol productivity and selectivity for [Fe,Al]-MFI samples in which iron was introduced using different methods. Before benzene oxidation the catalysts were pretreated at 700 ◦C in helium for two hours. Reaction conditions as described in the text

much poorer performances were observed, because of the higher rate of side decomposition of N2O. Therefore, the addition of iron by ion exchange could lead to catalysts with an initial activity better than that of the parent zeolite, but the effect of fast deactivation discussed before did not change. The addition of iron by CVD leads to not very selective catalysts due to a high rate of N2O decomposition, but stable activity was noted after an initial decrease in phenol productivity. According to the previous characterization, the CVD (sublimation) method leads to the formation of isolated or binuclear iron species [249], probably with characteristics similar to those of the active species responsible for the generation of the selective α-O species during the interaction with N2O. However, especially when the feed or the zeolite is not fully dehydrated, nanoparticles of iron oxides also form [250] being responsible for the decomposition of N2O and lowering selectivity. In agreement, the sample prepared by solid-state reaction—in which the formation of the latter species is enhanced—leads to very selective and active catalysts in phenol formation.

3.5 [Fe]-BEA

Using the reagents shown in the experimental part the synthesis of [Fe]-BEA proposed in [158] could not be reproduced. Indeed, starting from a gel com-

430 J. B.Nagy et al.

position of 40SiO2 – 1.02Fe(NO3)3 · 9H2O – 19.04TEAOH – 4NaOH – 676H2O using 24 h ageing time only an amorphous phase was obtained at 120 ◦C after 12 days. Similarly unsuccessful was the attempt when the ageing was done at 273 ◦C for 3 h. The reaction temperature of 150 ◦C did not lead to any [Fe]-BEA zeolite. Finally, even if a solution of 25% TEAOH in methanol was used, no [Fe]-BEA zeolite could be obtained following the method proposed in [159]. In order to find a well-reproducible method using our reagents, we have systematically varied the amount of iron source, the amount of TEAOH, the ageing time of the gel, the reaction time and the temperature of the reaction [194]. The general composition of the gel was the following: 40SiO2 – xFe(NO3)3 · 9H2O – yTEAOH – 4NaOH – 676H2O with x = 0.38, 0.49, 0.51, y = 19.04, ageing time = 2, 18, or 24 h, reaction time = 7, 9, 10, 20, 21, 22, 26, 28, or 30 days. Only one synthesis led to [Fe]-Beta, with x = 0.49, y = 19.04, 24 h ageing and 20 days of synthesis time at 120 ◦C. Even this synthesis was not reproducible. Note that using fumed silica instead of TEOS as silica source, some unidentified layered compounds were obtained. For x = 0.45, 0.49 and 0.60 and y = 10.88, 13.6 and 16.3, ageing time = 2 or 24 h, reaction time = 16, 18, 20, 21, 22, 25, 29 or 40 days, T = 120 ◦C, [Fe]-BEA cocrystallized in most cases with an unknown phase having a diffraction peak at 2θ = 5.6◦. Note that a similar peak was obtained during the synthesis of Beta zeolite with low Al-content [251]. This peak disappears during calcination at 450 ◦C. As the reaction temperature of 120 ◦C was not adequate to obtain pure [Fe]-BEA in a reproducible way, the reaction temperature was raised to 150 ◦C. At this temperature the amounts of Fe(NO3)3 · 9H2O and TEAOH were also varied in order to optimize the synthesis conditions. The data are reported in Table 17.

It is clearly seen from Table 17 that under certain conditions pure [Fe]-BEA can be obtained in a reproducible manner at 150 ◦C. The conditions are 0.45 or 0.60 Fe(NO3)3 · 9H2O, 13.6 or 16.3 TEAOH, 24-h ageing time and 4–8 days reaction time. The narrow crystallization fields are reported in Fig. 27, where the crystallization field of [Fe]-BEA is surrounded at higher Fe content by a phase where [Fe]-BEA coexists with an unknown phase U. Note that at low

Table 17 Synthesis conditions for the [Fe]-BEA obtained from gels of composition 40 SiO2

– xFe(NO3)3 · 9H2O – yTEAOH – 4NaOH – 676H2O at 150 ◦C a

Fig. 27 Crystallization fields of [Fe]-BEA (β = BEA) zeolite from gels of composition 40SiO2 – xFe(NO3)3 · 9H2O – yTEAOH – 4NaOH – 676H2O at 150 ◦C, aging time 24 h

ageing time of the gels (2 h), only an amorphous phase was obtained in all cases. The white color of all the final crystalline [Fe]-BEA zeolite samples suggests that Fe(III) occupies tetrahedral framework sites in the structure. In the zone of crystallization of [Fe]-BEA—i.e., 0.60 Fe(NO3)3 · 9H2O and 16.3 TEAOH—[Fe,Al]-BEA and [Al]-BEA were also synthesized maintaining the sum of moles of iron and aluminum source equal to 0.6. The data are reported in Table 18. It is seen that only the crystalline phases [Fe,Al]-BEA or [Al]-BEA were obtained in all synthetic runs already at 3 or 4 days of crystallization time. These results reinforce the existence of the zone where pure [Fe]-BEA zeolite samples could be obtained.

The [Fe]-BEA zeolite was also obtained in the presence of oxalic acid (see experimental part). In Table 19 the results on the synthesis of BEA zeolitetype are reported. First, when the reaction temperature increases from 140 to 170 ◦C, the crystallization time is shortened, but the thermal stability of the product is decreased. As a matter of fact, from the reaction system B2 the BEA-type zeolite was obtained after 7 days at 140 ◦C and after 4 days at 170 ◦C, but if the reaction time was prolonged to 8 days at 170 ◦C, the products were BEA and quartz. On the contrary, at 140 ◦C only the BEA phase was

Table 18 Synthesis conditions for [Fe,Al]- and [Al]-BEA obtained from gels of composition 40SiO2 – xFe(NO3)3 · 9H2 O – zAl(OH)3 – 16.3TEAOH – 4NaOH – 676H2 O at 150 ◦ C a

a Aging time: 24 h

Moles of reactants per mol of SiO2

formed even after 18 days. The BEA-type zeolite is also obtained in the presence of a mixture of TEAOH and TEABr, but the preliminary experiments carried out with this mixture showed that for a complete crystallization it is necessary to double the crystallization time.

As in the case of the MOR-type zeolite (see below), it was not possible to obtain the Al-free (or [Fe]-BEA) starting from the studied hydrogel system. Consequently, the experiments for the iron introduction were carried out at the lowest Al content in the hydrogel (equal to 0.02). First, it must be noted that the pH values play an important role in the formation of BEA-type zeolite. As in the case of the MOR-type zeolite, the pH value of the starting hydrogel must be higher than 13. The crystallization field of Fe, [Al]-BEA is more limited than that of the corresponding form of the MOR zeolite. In fact, the iron-containing [Al]-BEA zeolite crystallizes from the hydrogels having a Si/Fe ratio ranging from 100 to 50. Higher amounts of iron in the initial reaction mixture (Si/Fe = 33.3) did not lead to crystallization of BEA, in fact, after long reaction times the only detected crystalline phase was the mordenite. The crystallization time is a function of the iron content in the starting hydrogel. As a matter of fact, it increases with the iron content, e.g., the reaction time was 7 days in absence of iron, (sample B2) and 18 days with (Si/Fe = 50, sample B4).

3.6 [Fe]-MOR

At the beginning it is useful to underline that an unsuccessful attempt to obtain a pure iron form of BEAand MOR-type zeolites was undertaken, but evidently the absence of aluminum in the initial hydrogel does not allow the formation of crystalline phases even after long reaction times.

In Table 20 the results of the synthesis of the [Al]-MOR and [Al,Fe]-MOR zeolites are shown [195]. As expected, the amount of Al in the starting hydrogel is the crucial parameter during the crystallization process. Indeed, the zeolite MOR was obtained from the mixtures with a Si/Al ratio lower than 33.3. The alkali content influences the nature of the products. Starting from low Al content, the dense phase (quartz) co-crystallizes with a zeolitic phase

Moles of reactants per mol of SiO2

changing into zeolite BEA when the amount of sodium hydroxide in the starting hydrogel decreased, and a layered phase appeared when the lowest alkali content was tested. This confirms that a large amount of sodium hydroxide is necessary for the synthesis of MOR-type zeolite [252]. In any case, for the formation of [Al]- or [Fe,Al]-MOR zeolite the pH value of the starting hydrogel must be higher than 13. In the presence of iron only, this reaction system leads to the formation of quartz and in the presence of aluminum (Al2O3) ranging from 0.005 to 0.015 no crystalline phases were detected even after long reaction times. Since with this hydrogel system it was impossible to obtain mordenite in the absence of aluminum, the iron incorporation was tested in the system containing the lowest amount of Al2O3 (equal to 0.02). The maximum amount of iron that could be introduced into the zeolite was equal to 0.015 Fe2O3 (Si/Fe = 26.6). The crystallization time became longer when the iron content in the starting hydrogel increased. In the system without iron (sample M4), MOR crystallized in 2.5 days, on the contrary for a higher Si/Fe ratio (equal to 26.6, sample M9) 11 days were required. Only for the highest iron loading it was necessary to increase the amount of sodium hydroxide in order to maintain the pH of the hydrogel higher than 13, the value which was previously found to be essential to obtain pure MOR-type zeolite.

3.7

[Fe]-TON, [Fe]-MTW

[Fe]-MTW and [Fe]-TON zeolites were also synthesized [189]. For [Fe]-MTW the initial hydrogel composition was: xNa2O – 0.2MTEABr – SiO2 – qFe2O3 – pC2H2O4 – 20H2O where x = 0.1–0.16; q = 0.005–0.02 and the ratio p/q = 3. MTEABr stands for methyltriethylammonium bromide. [Fe]-TON was ob-

tained from gels: xNa2O – 15CH3OH – SiO2 – qFe2O3 – pC2H2O4.

The effects of Si/Fe ratio, pH values and crystallization time on the nature of the products are reported in Table 21. The results show that it is possible to obtain Fe-zeolites in a wide range of OH–/SiO2 and Si/Fe ratios using the

iron-oxalate complex. The various types of zeolite show different behavior regarding the crystallization process.

Due to the observation that the nature of iron complex does not affect the nature of the crystalline products and the reaction time, for the syntheses of the [Fe]-MTW and [Fe]-TON zeolites only oxalic acid is used. In the case of the [Fe]-MTW zeolite it can be observed that it crystallizes in a narrower range of Si/Fe ratios, compared to the [Fe]-MFI zeolite. The lower limit of the Si/Fe ratio is about 30, as a matter of fact for a lower ratio (25) no crystalline products are found after a very long reaction time (32 days). Another observation is that in this kind of synthesis the pH value plays an important role. The [Fe]-MTW crystallizes in this system in a very narrow range of the pH value. For pH value greater than 12 the MTW-type zeolite co-crystallizes with other crystalline phases such as magadiite. For a pH value smaller than 11.5 (sample C4 in Table 21) no crystalline product is found after a long reaction time (27 days). The [Fe]-TON-type zeolite crystallizes in a range of the Si/Fe ratio larger than the range of [Fe]-MTW zeolite but more restricted than for the [Fe]-MFI zeolite. In fact, it is possible to obtain [Fe]-TON for a ratio of Si/Fe = 25, for lower ratio the formation of the [Fe]-TON zeolite is not completed even after a long synthesis time (20 days). Similar to [Fe]-MTW zeolite crystallization, the pH values play an important role in the formation of [Fe]- TON. As a matter of fact, higher pH values—higher than 12—lead to the co-crystallization of TON with a dense phase such as cristobalite. Lower pH values, smaller than 11, do not lead to the formation of any crystalline phases even after long reaction times (22 days). All the samples of iron zeolites, MFI-, MTWand TON-type, show white color even after overnight calcination at 600 ◦C. This is an indication that no extra-framework iron is present in the

Table 21 Influence of Si/Fe ratio, sodium hydroxide content and pH value of the initial reaction mixture on the nature of the final products