Fe-silicalite

The catalytic behavior of Fe-MTW zeolites in the direct ammoxidation of propane was investigated. The obtained catalytic results are compared with behavior of Fe-silicalite catalysts whose activity in propane ammoxidation was recently published. It was found that Fe-MTW catalysts exhibit the similar activity as Fe-silicalites but the selectivity to acrylonitrile was substantially lower. On the other hand, Fe-MTW catalysts produce higher amount of propene and have better acrylonitrile-to-acetonitrile ratio. 

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. 

MFI and MTW zeolites with Fe species introduced during zeolite synthesis were investigated. Fe-silicalite and Fe-MTW catalysts were synthesized accordingly to... 

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... 

Fe-MTW catalysts exhibit activity in the direct ammoxidation of propane after steam pretreatment, but the selectivity to demanded product, acrylonitrile, is substantially lower in comparison with Fe-silicalite catalyst. On the other hand, the Fe-MTW catalysts reach the better AN/AcN ratio, it means that they produce less undesirable byproduct, as is acetonitrile. 

Supported Fe-Mn Fischer-Tropsch Catalysts. A much more limited number of studies have dealt with supported Mn-promoted Fe F-T catalysts. In this respect, it is worthwhile to mention the work of Xu et al These authors added MnO to a Fe/silicalite catalyst and observed an enhanced selectivity towards light olefins. Meanwhile the yields for methane as well as for CO2 formation were almost unaffected by MnO addition. Moreover, the conversion of CO was also insensitive to the addition of the MnO promoter. 

Figure 2.12 Effect of vacuum treatment and NH3 adsorption on the DR UV-Vis spectra of a Fe-silicalite catalyst (Fe =...

A special case is the hydroxylation of benzene with nitrous oxide as oxidant, for which commercialization has been announced [55]. The reaction occurs on Fe-silicalite-1, in the gas phase, at temperatures close to 400 °C, producing molecular nitrogen as by-product. Other zeolites and supported metals and metal oxides are less satisfactory catalysts. Toluene, chlorobenzene, and fluorobenzene are similarly hydroxylated, yielding all three possible isomers. Phenol produces catechol and hydroquinone. 

Mesoporous Fe-MFI zeolites have been successfixlly prepared by treatment of isomorphously substituted Fe-silicalite and ion-exchanged Fe-ZSM-5 in alkaline medium. Iron in framework positions directs the silicon extraction towards mesoporosity development, whereas iron in non-framework positions inhibits silicon dissolution and limits mesoporosity development. 

Details on the hydrothermal synthesis of FeMFI zeolites (with Fe-Al-Si and Fe-Si frameworks), and subsequent post-synthesis treatments have been described elsewhere. The isomorphously substituted zeolites were activated in steam (30vol.% H2O in 30 ml (STP) min of N2 flow) at two different temperatures (873 and 1173 K) during 5 h. Hereafter, the catalysts are denoted followed by the steam temperature, e.g. Fe-silicalite... 

A detailed characterization of the as-synthesized FeMFI zeolites upon calcination and steam treatment has been reported elsewhere. The iron content in FeZSM-5 (Si/Al = 31 and 0.67 wt.%) and Fe-silicalite (Si/Al 00 and 0.68 wt.% Fe) catalysts was very similar. The visual appearance of the steamed Fe-Al-Si and Fe-Si catalysts at different temperatures already suggested a different catalyst constitution with respect to iron. FeZSM-5 (873 K) and Fe-silicalite (1173 K) are light brownish, which evidences a certain accumulation of iron oxide/hydroxide in the zeolite. FeZSM-5 (1173 K) presents a darker brownish color. Fe-silicalite (873 K) was nearly white, suggesting the more isolated nature of the iron species in the catalyst. 

The complete absence of the band in the range of 300-450 nm in Fe-silicalite (873 K) indicates that the majority of Fe species in this catalyst is uniform and well isolated. Previous characterization showed that a fraction of iron in this sample remains in the framework, but this fraction is small compared to the extraframework species. The absence of iron clustering in Fe-silicalite (873 K) has been corroborated by voltammetry and HRTEM, where no iron oxide nanoparticles were observed. Increasing the steam-... 

Figure 2 N2O conversion vs. temperature over Fe-silicalite (873 K) (open symbols) and FeZSM-5 (873 K) (solid symbols) in different feed mixtures. Experimental conditions as described in Methods.

The activation of N2O to produce atomic O species is an essential step in the reactions investigated here and is controlled by the presence of extrafi amework iron species. In an earlier work, we demonstrated that extraframework A1 or Ga species, as well as Bronsted sites in the catalysts play a minor role in N2O activation. Taking this into account, the activity differences between FeZSM-5 (873 K) and Fe-silicalite (873 K) in Fig. 2 are strictly related to the distinct nature of the (extraframework) iron species in the catalysts. The similarity between the iron species and activities of FeZSM-5 (873 K) and Fe-silicalite (1173 K) further supports our previous conclusion. 

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. 

Capturing of the Fe + ions in the zeolite a-cation-exchange positions was first considered theoretically in [81], The Fe + ion grafted to the zeolite framework, or captured by a zeolite lattice defect =Si-0-Fe-0-Si= was proposed in [82] as an active center with low coordination of Fe +. Such structure could be emerged as the result of iron immobilization on vicinal hydroxyl zeolite groups. Also, it was found that addition of trimethylaluminum to Fe-silicalite drastically improves the catalyst activity in the process (20.10). Based on these data a conclusion about the formation of FeAlO active species was made [83],... 

Supercritical CO2 activation of a Naflon membrane prior to zeolite deposition was used to modify its structure. The resultant Nafion-zeolite composite membranes showed a dramatic decrease in methanol permeability (if the colloidal rather than the suspended Fe-silicalite-1 particles were used for the deposition) and a 19-fold higher selectivity compared with either the composite membranes prepared without previous supercritical treatment or the pure commercial Nafion-115 membrane. The method of the in situ synthesis of a zeolite inside the membrane pores was found to be very effective for preparing the composites, giving a sixfold higher selectivity for the composite manbrane compared with the pure Nation membranes (Gribov et al. 2007). 

It was postulated that a surface-generated ferryl intermediate was responsible for CH4 activation, leading to the release of CHs-to the gas phase. Results indicated that 70% CH3OH selectivity at 5.7% conversion was possible with 3 1 CH4/air at 53 bar and 416°C, with a GH8V = 530 h. Fe-sodalite calcined at 300°C Fe-silicalite, hydroxysilicalite, and 8i02-supported iron oxide all showed... 

FAPO-5, FAPO-11, FAPO-8, FeVPI-5, FeS-1 Mesoporous Catalytic activity decreases in the following order FAPO-5 > FAPO-11 > FAPO-8 > Fe-VPI-5 > Fe-silicalite-1 Pinacol rearrangement (252)... 

Kondratenko, E. and P rez-Ramfrez, J. (2007). Micro-Kinetic Analysis of Direct N2O Decomposition over Steam-Activated Fe-Silicalite from Transient Experiments in the TAP Reactor, Catal. Today, 121, pp. 197-203. 

Pdrez-Ramfrez, J., Kondratenko, E. and Debbagh, M. (2005). Transient studies on the mechanism of N2O activation and reaction with CO and C3H8 over Fe-silicalite, J. Catal., 233, pp. 442-452. 

More than twice the phenol yield (5.5%) at a much smaller contact time (3.5 s) in a single pass was obtained with a mixed-matrix membrane of Na-Fe-silicalite-1 dispersed in PVDF, which was obtained by a similar technique, by using DMAc as solvent and water as non-solvent. ... 

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