Hydroxylation of benzene to phenol with nitrous oxide on Fe-silicalites

Hydroxylation of benzene to phenol with nitrous oxide on Fe-silicalites

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights rese...

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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H y d r o x y l a t i o n o f benzene to phenol with nitrous oxide on Fe-silicalites R. Monaci a, E. Rombi "~,M.G. Cutrufelio a, V. Solinas a and G. Berlier l', G. Spotob "Dipartimento di Scienze Chimiche, Universi~ di Cagliari, via Ospedale 72, 09124 Cagliari, Italy bDipartimento di Chimica IFM, Universit~i di Torino, via P. Giuria 7, 10125 Torino, Italy

The title reaction was studied, at 623 K and atmospheric pressure, over Fe-si|icalites with different Fe203 content. The best results were obtained for the sample with a Fe203 content of 0.82 wt %. FTIR spectroscopy was used to investigate the nature of the extraframework iron species in Fe-silicalites using NO as probe molecule.

1. INTRODUCTION Phenol is an important intermediate for the production of petrochemicals, agrochemicals and plastic products. More than 90 % of the phenol produced world-wide is obtained by the multi-step process of cumene oxidation. An economical single-step process by means of the direct hydroxylation of benzene to phenol has been studied for some time. Conventional partial oxidation methods with molecular oxygen as oxidant have not given satisfying results, as the reaction mainly leads to the destruction of the aromatic ring [1]. Better results have been obtained in the hydroxylation of benzene with an oxygen/hydrogen mixture, using silicasupported precious metals as catalysts [2]. More promising results seem to derive from the use of alternative oxidants as nitrous oxide. In the first studies on the benzene oxidation by N20, silica-supported vanadium, molybdenum and tungsten oxides had been used as catalysts [3]. More recently, zeolite catalysts as Fe,Na-ZMS-5 and Fe-silicalites structurally similar to ZSM-5 zeolites have been used, providing very' interesting results [4,5]. The iron species located in the so called a-sites, which are particularly suitable for the N20 decomposition, seem to play a key-role on the catalytic performance of these materials, as shown by Panov and co-workers [6,7]. According to them, the N20 decomposition occurs on extraframework iron microclusters and leads to surface reactive oxygen species (a-oxygen), which are responsible for the oxidation of benzene to phenol [8]. The aim of this work is to investigate the influence of the iron content on the catalytic activity of Fe-silicalites and give useful additional information on the surface properties of these materials, by studying the adsorption of probe molecules by FT1R spectroscopy.

1680 2. EXPERIMENTAL

2.1. Materials Benzene was a > 99 % pure Aldrich reactant; nitrous oxide was a 99.998 % pure SIAD product. 2.2. Catalysts Four different samples of Fe-silicalite, synthesised as reported elsewhere [9], were used. The iron content of the catalysts, expressed as percent by weight of Fe203, is reported in Table 1. 2.3. Apparatus and procedure The experimental runs were performed at atmospheric pressure in a continuous fixed-bed quartz-glass microreactor, with the following operative conditions: temperature, 623 K; space velocity. (WHSV), 4 h-l; R (N20 moles / benzene moles), 5. Hourly collected products samples were dissolved in acetone and analysed by a Carlo Erba 4160 gas chromatograph fitted with a fused silica capillary column (50 m Petrocol DH 50.2, 0.25 mm I.D., 0.25 lure film thickness; Supelco) and a flame ionisation detector. Products were identified by gas chromatography / mass spectroscopy (GC,qVIS) analysis. Phenol was the main product; only at higher temperatures (> 673 K) small amounts of byproducts, formed by further oxidation of phenol, were detected (mainly benzoquinone and benzofuran). The complete oxidation of benzene to carbon dioxide was the main undesired reaction. The results have been expressed in terms of benzene conversion (moles of benzene reacted / moles of benzene feeded) and of selectivity to phenol (moles of phenol formed / moles of benzene reacted). 2.4. IR measurements The IR experiments were carried out on a Bruker IFS 66 FTIR instrument equipped with a cryogenic MCT detector and running at 2 cm -1 resolution. The Fe-silicalite samples were in form of self-supporting pellets suitable for measurements in transmission mode.

3. RESULTS AND DISCUSSION Activity comparison runs were performed on fresh catalyst samples at the previously mentioned standard conditions. To point out the influence of the iron content of the catalysts on their activity, Figure 1 shows the selectivity towards phenol for three Fe-silicalites at conversion values of 10 mol % (3 h on-stream) and 4 mol % (7 h on-stream), for different weight percentages of Fe203. The Fe-Si-1 sample, containing 0.06 % of Fe203, turned out to be inactive. As it can be seen from the figure, selectivity increases with the Fe203 percentage, reaching the highest value for an Fe203 content of 0.82 % (Fe-Si-3). Higher Fe203 contents mainly favour the complete oxidation at the expense of the phenol fbrmation. Figure 2 shows the change in conversion and selectivity to phenol with time-on-stream for the Fe-Si-3 sample. Conversion decreases during the run, whereas selectivity increases. The same trend was shown by all the samples.

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Table 1. Iron content of Fe-silicalite catalysts. Catalyst

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1682 As reported by other authors [ 10], the benzene oxidation by N20 is accompanied by coke fbrmation and subsequent catalyst deactivation. Removing coke from the catalysts, by burning it out in air flow, resulted in the recovery of their catalytic activity. The decrease in the conversion value during the run and the complete deactivation of the catalyst within 10 hours on-stream can therefore be ascribed to the production of phenol and polyphenols, which are coke precursors. A possible interpretation of the selectivity increase with time-on-stream could be the presence, on the surface, of catalytic sites which can activate N20 in different ways and then produce phenol or favour the total oxidation. The nature of the extraframework iron species was studied by FTIR spectroscopy, using NO as probe molecule, on the most selective catalyst (Fe-Si-3). The IR spectrum of the sample (previously outgassed at 773 K), in equilibrium with 10 torr of NO at room temperature, is shown in Figure 3 (dotted line). In the same figure, the effect of the gradual decrease in the equilibrium pressure of NO is also reported (full and dashed lines). Examining the figure, it can be said that: i) at the maximum NO coverage, the spectrum is dominated by a strong absorption at 1808 cm ~ superimposed on a second band centred at 1839 cm -~. Weaker bands appear at 1914 and 1765 cm-1; ii) reducing the NO pressure, the bands at 1914 and 1808 cm- are progressively erosed and in the end completely disappear. This phenomenon is 1.5

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1683 accompanied by the simultaneous (and parallel) intensification of the signals at 1839 and 1765 cm ~ (which finally dominate the spectrum: dashed line in Figure 3). According to literature data [11,12], the bands at 1914, 1839, 1808 and 1765 cm -~ can be assigned to nitrosylic complexes formed on extraframework (or partially extraframework) ferrous species (FeA sites). As previously seen (Figure 2), during the first hours on-stream the complete oxidation of benzene prevails; this reaction leads to the formation of three moles of H20 for every mole of benzene reacted. Therefore, to test the influence of water on the surface of Fe-silicalites, FTIR measurements on the Fe-Si-3 sample containing preadsorbed H20 were carried out. Figure 4 shows the spectra collected with the maximum NO coverage on a sample simply outgassed at 773 K (dashed line) and on a sample previously contacted with 5 torr of H20 at room temperature, evacuated under high vacuum for 10 minutes at the IR beam temperature and finally contacted with NO (full line). It can be noted that by pretreating with water: i) the bands at 1914, 1808 and 1765 cm -~ become stronger; ii) the increase in their intensity is accompanied by a decrease in the abso~tion at 1839 cm -1. The erosion of the band at 1839 cm-, due to the water adsorption, can also be seen at low coverage [ 13]. These findings lead to the conclusion that the absorption at 1839 Cln-~ originates from the presence of at least two different components, one of which is selectively depleted by the absorption of water. To explain this thct it can be thought that on the Fe-silicalite surface, together with the FeA sites, another family of ferrous sites (FEB) is present. On the samples without water, these centres form, by NO adsorption, mononitrosylic complexes, which absorb at 1839 cm ~. After H20 adsorption, the sites become able to further interact with NO, forming polynitrosylic complexes, which are spectroscopically indistinguishable from those formed on the FeA sites. Thus, water translbrms the Fen sites into a new species with NO absorption properties similar to those of the FeA centres. As regards the nature of these t~rrous centres, their different tendency to adsorb NO (at least a trinitrosyl can be tbnned by FeA and only a mononitrosyl by Few), is a clear clue to different coordinative states, higher for FeB than for FeA. Spectroscopic results suggest that the FeA sites are isolated, low coordinated iron centres (probably mononuclear) ~afted to the framework, while FeB species belong to (FeO)n extraframework (or partially extraframework) clusters which can be broken up by H20, producing more reactive Fea-type species. It is therefore probable that the catalytic surface initially consists of a certain number of Fea and FeR sites, the former being more selective towards phenol and the latter being responsible for the reaction of complete oxidation. During the first hours on-stream, the complete oxidation prevails, as shown by the high quantity of CO2 initially produced. That also implies a remarkable production of Ha0, whose effect on the sites already present on the surface is to transform the FeR centres into FeA-type, as shown by the IR spectra. The increase in the number of FeA-type sites can be correlated with the higher selectivity towards phenol (Figure 2). Thus, as the Fe~ sites turn into FeA-type, the production of CO2 decreases in favour of phenol. The variation in selectivity, due to the presence of the different ferrous sites, FeA and FeB, can be explained assuming that they adsorb N20 producing oxygen species of different nature. In our opinion, the prevailing formation of phenol could be ascribed to a nucleophilic species like 02-, responsible tbr the partial oxidation; on the other hand, species with an

1684 electrophilic character, like O2- and O, would attack the aromatic ring in the region of the highest electronic density, leading to the rupture of the benzene ring and the prevailing formation of CO2.

4. CONCLUSIONS Within the tested series of Fe-silicalite catalysts, the best results were obtained with the sample containing an Fe203 percentage around 1%. All the catalysts deactivated during the run and the selectivity towards phenol seemed to be correlated with extraframework ferrous sites, identified by FTIR spectroscopy, which would be able to provide nucleophilic oxygen species responsible for the partial oxidation.

AKNOWLEDGEMENTS The financial support ofMURST (Cofin 98) is gratefully acknowledged.

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