Fe Zeolites

Second Concept in Catalyst Design. One-Pot Synthesis of Fe Zeolite Catalysts... 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

So, in the latter case the apparent activation energy is increased by the heat of adsorption of CO, amounting to about 40-60 kJ/mol as calculated from the IR experiments. Hence, for both the Co and the Cu samples E is slightly larger than 2 (table 2) while for iron ai is considerably lower. All these values are compatible with values reported in the literature for Fe-zeolites [6,7,10,11] or dilute solid solutions of Co in MgO [31]. The kinetic and IR results with NO indicate that, like CO, it can remove the oxygen from the... 

Catalytic oxidative dehydrogenation of propane by N20 (ODHP) over Fe-zeolite catalysts represents a potential process for simultaneous functionalization of propane and utilization of N20 waste as an environmentally harmful gas. The assumed structure of highly active Fe-species is presented by iron ions balanced by negative framework charge, mostly populated at low Fe loadings. These isolated Fe sites are able to stabilize the atomic oxygen and prevent its recombination to a molecular form, and facilitate its transfer to a paraffin molecule [1], A major drawback of iron zeolites in ODHP with N20 is their deactivation by accumulated coke, leading to a rapid decrease of the propylene yield. 

Fe-zeolites were prepared using the NH4 form of BEA Si/Al = 13.5. Parent BEA zeolite (average particle size of 300 nm or 1pm) was dried at 150 °C for 4 h and then mixed with a solution of FeCl3 in acetyl acetone. After 12 h of mixing, excess of the solution was removed, the solid was dried at room temperature and heated under vacuum at 350 °C for 4 h. A sample was washed with distilled water and dried in an air at room temperature. Then, the remaining organic species in the Fe-zeolites was removed by calcination at 450 °C in air for 10 h. The produced catalysts contain 0.6 wt% of Fe. This preparation procedure predominantly provides iron introduction into cationic sites [3], Two types of catalysts were prepared, Fe-BEA with a particle size of 1 pm (Fe/m-BEA) and Fe-BEA with particle size of 300 nm (Fe/n-BEA). 

Ammoxidation of propane over Fe-zeolites effect of reaction variables, catalyst composition and catalyst structure... 

The increasing volume of chemical production, insufficient capacity and high price of olefins stimulate the rising trend in the innovation of current processes. High attention has been devoted to the direct ammoxidation of propane to acrylonitrile. A number of mixed oxide catalysts were investigated in propane ammoxidation [1]. However, up to now no catalytic system achieved reaction parameters suitable for commercial application. Nowadays the attention in the field of activation and conversion of paraffins is turned to catalytic systems where atomically dispersed metal ions are responsible for the activity of the catalysts. Ones of appropriate candidates are Fe-zeolites. Very recently, an activity of Fe-silicalite in the ammoxidation of propane was reported [2, 3]. This catalytic system exhibited relatively low yield (maximally 10% for propane to acrylonitrile). Despite the low performance, Fe-silicalites are one of the few zeolitic systems, which reveal some catalytic activity in propane ammoxidation, and therefore, we believe that it has a potential to be improved. Up to this day, investigation of Fe-silicalite and Fe-MFI catalysts in the propane ammoxidation were only reported in the literature. In this study, we compare the catalytic activity of Fe-silicalite and Fe-MTW zeolites in direct ammoxidation of propane to acrylonitrile. 

Figure 1. DR UV-vis spectra of hydrated Fe-zeolite catalysts pretreated by calcination in flow of dry oxygen at 540 °C (black curves) and steam-treated in the flow of water vapor at 540 °C (gray curves). A) Fe-sil-12900, B) Fe-MTW-11500, C) Fe-MTW-14700, Fe-MTW-18900.

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

Table 1 Catalytic activity of Fe-zeolites in propane ammoxidation at 540 °C...

Table I. Selectivity in oxidations using Fe /zeolites and hydrogen peroxide (22)...

Since the decomposition is regulated by the redox cycle Fe"/Fem represented by Eqs. 1,2, the kinetics can be treated by the very classical Mars and van Krevelen model (23, 40). Moreover, it was experimentally demonstrated that inhibition by excess 02 is of low extend for Co- and Fe-zeolite, the rate law can thus be expressed as ... 

A detailed kinetic study of N20 decomposition over Co-, Cu-, and Fe-zeolite has been reported by Kapteijn et al. (40) with analysis of the boosting effect of CO and NO by several rate laws. Regarding the kinetics of N20 + NH3 + 02 reaction to N2 over Fe-BEA (41), the dependency of the rate with respect to N20, NH3 and 02 pressures are presented in Figure 16.4. A rate law was proposed to account for the volcano-shaped dependency of the rate vs. NH3 pressure, and based on the strong adsorption of NH3 on Ferr sites. 

Highly loaded Fe/zeolite catalysts show good activity and a reasonable stability under realistic conditions, but the light-off temperature is too high and a significant amount of CO is formed... 

Acid sites are associated with framework A1 or other trivalent atoms. The number of the acid sites is proportional to the concentration of framework A1 or other trivalent atom. The strength of the acid sites in most zeolites is inversely proportional to the concentration of framework A1 up to about a silica/alumina ratio of 10. The nature of the heteroatom also affects acid strength. A1 zeolites are much more acidic than Ga- or Fe-zeolites. B-zeolites have very weak acidity. ALPO4-S have no exchangeable cations and therefore no acidity. 

In an attempt to synthetize novel zeolite materials with different catalytic or thermal properties, zeolitic structures related to ZSM-5 zeolite have been obtained. The purpose was either to get better catalytic performances in shape selectivity and resistance to aging or to overcome Mobil patents. In such a way Al free ZSM-5 (designated silicalite) (67), B-ZSM-5 (Al replaced by B in boralite ( 60) or borosilicates ( )) Fe-ZSM-5 (Al replaced by Fe) ( ) zeolites have been prepared. Also, Cr, V, Ge, Zr, Ti,. .. were reported to have been used to replace Al or Si in zeolites. Some interesting improvements in catalytic properties have then been claimed. The key question is whether or not the substituted element is the active site or if residual Al is doing all the work. ... 

Besides the use as a model system, copper exchanged zeolites themselves are of technical interest, e.g. as hydrocracking catalyst components [1]. The copper ion redox behaviour is important for cocatalytic effects during catalyst activation (reduction of Fe-zeolites ) and catalyst regeneration. 

N20-decomposition was employed as a model reaction to test the derived Fe-zeolite catalysts. Activity tests were carried out in a parallel-flow reactor system, which typically consumes 50 mg of catalyst particles (125-250 pm). The catalysts were tested in pure N20/He conditions (4.5 mbar N2O) at a total pressure of 3 bar-a. The space time, W/F°(N20), was 900 kgxsxmoT (W is the catalyst mass and F°(N20) the molar flow of N2O at the reactor inlet). The products were analyzed by gas chromatography (Chrompack CP 9001) and continuously analyzed with a chemiluminescence NOx analyzer (Ecophysics CLD 700 EL). The catalysts were pretreated in He at 673 K for 1 h, and cooled down in the same gas to the starting reaction temperature. Typically, one hour ensures steady state operation for this reaction at the conditions described above. 

The performance of the catalysts was tested in N2O decomposition. This reaction is well catalyzed by Fe-zeolites, and therefore the appropriate model reaction for this type of materials. The performance was compared with two Fe-zeolite catalysts prepared through conventional ion-exchange of NH4-form zeolites. As indicated in Figure 3 (right) the performance of the one-pot catalyst was even superior to the conventionally prepared. The reason for this is ascribed to the minimisation of the FeOx formation, which takes place in the classical preparation (TPR profiles not shown). 

Targeted preparation of Fe-zeolites with iron prevailing in extraframework cationic positions... 

Two main EPR signals were observed for Fe-zeolite samples with Fe/Al < 0.3 after oxidation and followed by evacuation at 450°C (see Fig. 2). The signal at... 

The unification of the various interpretations with respect to the active sites is extremely complicated due to the intrinsic heterogeneous nature of iron species in the catalyst. Particularly challenging in practice is suppressing clustering of iron species into large inactive iron oxide particles. A further complicating aspect for a rational unification is the application of Fe-zeolites in a wide range of catalytic reactions with a different mechanism. 

The preparation method of iron-zeolites has been recomized as critical in order to obtain reproducible catalysts with a desired performance." A distribution of iron species is normally obtained upon activation of catalysts by available methods. Suppressing clustering of iron species into iron oxide is convenient, since these species are proven inactive at low temperatures in the various reactions catalyzed by Fe-zeolites. " Steam activation of isomorphously substituted FeMFI zeolites enables a certain control of the degree of iron clustering, and thus on the relative amount of certain species in the final catalyst, as compared toother methods. A rather unique achievement has been attained here with Fe-silicalite (873 K), in view of the remarkable uniform nature of extraframework species in isolated positions. A minor association of iron species is present... 

The results presented evidence possibilities of tailoring uniform iron sites in FeMFI zeolites, under specific synthesis and activation conditions. Preparation of steam-activated Fe-silicalite containing mainly isolated iron species in extraffamework positions is essential to derive stmcture-activity relationships in various N2O conversion reactions over iron zeolite catalysts. The activity of the cluster-free Fe-silicalite was significantly higher in N2O reduction with CaHg and CO. However, some level of association of iron species leads to higher activities in direct N2O decomposition. Due to the intrinsic reaction mechanism, this result demonstrates the sensitivity of reactions for the form of the iron species in Fe-zeolites, rather than the existence of a unique active site. 

Sample Si/Al gel Si/Fe gel Si/Al zeolite Si/Fe zeolite Morphology Cryst. size... 

P-06 - Effect of the reductant nature on the catalytic removal of N2O on a Fe-zeolite-beta catalyst... 

---