One-step benzene oxidation to phenol. Part I: Preparation and characterization of Fe-(Al)MFI type catalysts

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

477

One-step benzene oxidation to phenol. Part I: Preparation and characterization of Fe-(A1)MFI type catalysts. G. Giordano 1, A. Katovic 1, S. Perathoner 2, F. Pino 2, G. Centi 2, J. B.Nagy 3, K. Lazar 4 and P. Fejes 5

1 Dipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, 87030 Rende (CS), Italy 2 Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italy 3 Laboratoire de RMN, FUNDP, 61 rue de Bruxelles, 5000 Namur, Belgium 4 Institute of Isotope and Surface Chemistry, 1525 Budapest, P.O. Box 77, Hungary 5 Applied Chemistry Department, University of Szeged, Rerrich B~la t~r 1, 6720 Szeged, Hungary

The characteristics of iron-containing MFI type zeolites synthesized by different methods (direct synthesis, chemical vapor deposition, solid state reaction and ion exchange), in relation to their application for the one-step synthesis of benzene from phenol, were investigated by MSssbauer and NMR techniques. In the direct synthesis, the amount of iron incorporated in the zeolite framework depends on both its amount in the initial hydrogel and the TPABr template concentration. Part of these framework iron ions migrate to extra-framework position during catalytic tests and/or catalyst hydrothermal pretreatment forming active ( F e , A l ) f r a. . . . . k-O-(Fe,A1)extra-fr . . . . . . k pair sites in which iron is in a distorted octahedral coordination. D u r i n g long-term catalytic tests in benzene hydroxylation the iron ions migrate to more stable positions in the zeolite which is one of the cause of catalyst deactivation together with the formation of carbonaceous species and strongly chemisorbed phenol as detected by 13C-NMR. Introduction of iron by post-synthesis methods leads to a lower dispersion of iron and less stable species during the catalytic reaction. 1. I N T R O D U C T I O N Metal-substituted or metal-containing zeolites are used in many industrial processes, such as the production of caprolactam and direct oxidation of cyclohexanone, the isomerization of linear alkanes, the isomerization of linear C4 and C5 olefins into iso-olefines (Shell processes) and the liquid phase alkylation of benzene (Eni and Exxon-Mobil processes) [1-4]. Recently, the possibility of their

478 use as s u l p h u r r e s i s t a n t catalysts for the conversion of polycyclic aromatic hydrocarbons have been also shown [5-8]. A new field of interesting application is also related to the possibility of introducing well (atomically) dispersed transition metals having redox properties. The unique coordination given to the metal ions by coordination to the zeolite framework acting as coordinating ligand allows to obtain a peculiar reactivity [9]. Most of the studies have been focused on the use of these materials in liquid phase oxidation or NOx reduction in the gas phase, but recently the interest also focused on the selective oxidation of benzene to phenol in gas phase using N20 as the oxidant [9-12] and (Fe,A1)-containing MFI zeolite. In fact, N20 decomposed on the iron sites forming specific oxygen species (called a-oxygen) which can directly hydroxylate benzene to phenol. No other type of iron-containing catalyst has been found to m a t c h such a peculiar behaviour and n o t w i t h s t a n d i n g the intense research activity on these catalysts, the exact nature of the sites responsible for the activity is a matter of question. Different procedures are reported for the partial or total substitution of A1 in the zeolite or for the introduction of other metals. Ionic exchange is one of the most used methods [13], but also solid state reaction and t h e r m a l vapor deposition are becoming widely used preparation methods [14-17]. In alternative to these post synthesis methods leading essentially to extra-framework species (if part of the framework A1 ions are not removed before the addition of the metal), the introduction by direct synthesis [18-21] leads to framework substitution. However, by careful thermal or chemical p r e t r e a t m e n t it is possible to have a controlled partial migration of the T-metal such as Fe from framework to extraframework positions. Limited attention has been generally given in literature to analyze how the m e t h o d of introducing the metal affects the n a t u r e and distribution of the transition metal ion species. In this contribution the characteristics of Fe-(A1)-MFI type zeolites prepared by direct synthesis starting from a hydrogel containing iron complexes (oxalate or phosphate) and tetrapropylammonium bromide (TPABr) are compared with those of samples in which iron has been introduced by post synthesis methods: ion exchange, chemical vapor deposition and solid state reaction. The characteristics of the samples and their change during the catalytic reaction are investigated in relation to the use of these catalysts in the direct oxidation of benzene to phenol (see part II in this Volume) [22]. The location of iron in the zeolite have been studied by NMR and MSssbauer techniques. MSssbauer spectroscopy is an appropriate tool for identifying the oxidation and coordination states of iron. For instance, presence and inter-conversion of various ferrous and ferric states were demonstrated in Fe-MFI during reduction and oxidation treatments [23]. 2. E X P E R I M E N T A L

The zeolite samples of Fe-MFI and Fe,A1-MFI were p r e p a r e d in static conditions and under autogeneous pressure at 170 ~ The molar composition of the starting hydrogel was: x Na20 - y T P A B r - z A1203 - SiO2 - q Fe2OJp H A - 20 H20 where x = 0.1-0.32; y = 0.02 - 0.08; z = 0.0 - 0.05; q = 0.0-0.025; the ratio p/q = 3 and HA stays for HzC204 or H3PO4.

479 The synthesis procedure was the following: sodium aluminate (Carlo Erba) is added to a sodium-hydroxide (Carlo Erba) solution and after the homogenization the organic compound (TPABr from Fluka) and the silica source (silica-gel BDH) are added. In a n o t h e r b a k e r a solution of iron complex w i t h oxalic or ortophosphoric acid, starting from iron nitrate (Carlo Erba) and the acid (Carlo Erba) is prepared. This solution was slowly added to the hydrogel and after 1 hour of homogenization was transferred into the autoclaves. For the synthesis of the silica form, the procedure is identical but without the introduction of the aluminum source. Post-synthesis introduction of iron was made with the following methods: i) a direct ionic exchange of NH4+-A1-MFI form with a 0.1 M iron ammonium sulphate solution (stirred for 2 hrs at 60~ and then calcination at 550~ ii) chemical vapour deposition was carried out at 300 ~ by mixing the A1MFI samples with an anhydrous FeC13 in controlled atmosphere (twice for 2 hours). iii) a solid state exchange reaction was performed by mixing FeC13 (5% wt.) with H-MFI and treating the mechanical mixture for 8 hr at 600 ~ The solid products were recovered in a usual m a n n e r and checked by powder X-ray diffraction (XRD). The zeolite samples were f u r t h e r characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), thermal analysis, chemical analysis by atomic absorption, NMR and MSssbauer spectroscopies. MSssbauer spectra were obtained on samples in as received state at 77 K. Spectra are decomposed to Lorentzian lines. Isomer shift data are related to the center of metallic a-iron. The accuracy of positional parameters is ca.+_ 0.03 mm/s. Other experimental details are reported in [23]. 3. R E S U L T S AND D I S C U S S I O N 3.1 T h e r o l e o f t h e p r e p a r a t i o n

The main p a r a m e t e r s t h a t affect the preparation of Fe-MFI type zeolite are summarized in Table 1. Pure MFI type zeolite may be obtained even for a pH close to 11 and large ranges of Si/Fe and Si/A1 ratios. The crystallization time increases (from 1 day to 12 days) with the a m o u n t of iron and a l u m i n i u m present in the initial reaction mixture. The presence of A1 in the s t a r t i n g hydrogel containing Fe leads to a shorter crystallization time (see sample 2 and 10, Table1). Apparently the amount of TPABr in the starting hydrogel does not affect the reaction products. In fact, pure MFI type zeolite was obtained also in absence of organic molecules (see Table 1 samples 1-8, 9 and 10). The samples obtained in the presence of 0.08 moles of TPABr show a very good t h e r m a l stability. As a m a t t e r of fact, no transformation of MFI phase is observed after longer reaction times and even after a thermal t r e a t m e n t at 850 ~ The chemical analyses of as-made MFI samples and those after t r e a t m e n t are reported in Table 2. It may be noted that (i) in TPABr-rich hydrogel the amount of iron incorporated in the zeolitic framework is related to its content in the initial reaction mixture, and (ii) the iron is incorporated preferentially with respect to the A1 (see samples 3, 7 and 8, Table 2).

480 Table 1. Influence of the m a i n p a r a m e t e r s on the products of the system: x N a 2 0 - y T P A B r - z A1203 - SiO2 - q Fe2Ogp H A - 20 H20 Sample Si/A1 gel Si/Fe gel TPABr pH Time (d) Product 1 oo 100 0.08 11.5 1 MFI 2 ~ 10 0.08 11.2 12 MFI 3 100 100 0.08 11.5 1 MFI 4 100 50 0.08 11.5 2 MFI 5 50 oo 0.08 11.6 1 MFI 6 50 100 0.08 11.4 2 MFI 7 50 50 0.08 11.5 2 MFI 8 25 25 0.08 11.3 4 MFI 9 10 20 0.02 11.2 7 MFI 10 10 10 0.0 10.8 9 MFI Samples 4, 5, 6 and 7 corresponding, respectively to the samples Fe2.3MFIht-A90, Feo.sMFIssr-A62, Fel.lMFIht-A55, Fe2.2MFIht-A54 of Table 1 p a r t II of this work [22].

Instead, the a l u m i n u m a t o m s are p r e f e r e n t i a l l y i n c o r p o r a t e d into the MFI s t r u c t u r e w h e n the TPABr content decreases or in its absence (sample 9 and 10, Table 2). Consequently, a small a m o u n t of iron is detected in the final products.

Table 2. Chemical analyses of MFI samples as-made and after t r e a t m e n t . Sample Si/A1 gel Si/Fe gel TPABr Si/A1 zeol

Si/Fe zeol

1 oo 100 0.08 106.5 2 oo 10 0.08 13.8 3 100 100 0.08 93.0 73.8 4 100 50 0.08 91.5 35.9 4 calc. 100 50 0.08 90.1 39.1 5 50 oo 0.08 57.2 5 ssr 50 oo 0.08 62.1 121.7 6 50 100 0.08 56.8 80.3 6 calc. 50 100 0.08 55.1 81.7 7 50 50 0.08 51.6 37.3 7 calc. 50 50 0.08 54.1 40.4 8 25 25 0.08 42.3 29.6 9 10 20 0.02 25.0 307 10 10 10 0.0 18.1 269 ll*ie 13 23.8 12*cvd 13 222 calc.= calcination at 600 ~ ssr= solid state reaction; ie= ion exchange; cvd= chemical v a p o u r deposition. * commercial s a m p l e s from Alsi P e n t a (SN27). Sample 11 e 12 corresponding respectively to Fe3.6MFIet-A13 and Feo.4MFIcvd-A13 of Table 1 p a r t II of this work [22].

481 An explanation for this behaviour can be attributed to the different preference showed by the solution cations for the iron- and aluminium- containing anions. Different authors have demonstrated [24] that Na § interacts better with [Si-OA1], whereas TPA § prefers [Si-O-Fe] groups. The calcination process does not affect the amount of iron incorporated in the zeolite structure. In fact, a small amount of iron is lost in the samples after the calcination (see samples 4, 4 calc., 6, 6 calc., 7 and 7 calc., Table 2). The incorporation of iron by post-synthesis methods results in large introductions of iron only with the ion exchange method, while solid state reaction and chemical vapor deposition exhibit a small iron incorporation (see sample 5ssr, l l i e and 12cvd, Table2). As reported in the part II of this work [22] the method of iron incorporation strongly affects the catalytic behaviour of the samples, especially their stability. 3.2 T h e n a t u r e of t h e i r o n s p e c i e s

The MSssbauer spectra at 77 K of selected samples and related parameters after deconvolution are reported in Figure 1 and Table 3, respectively. The spectrum of the as-synthesized sample exhibits a broad Fe 3§ singlet, with a comparatively low isomer shift (IS) value. The singlet (lack of quadrupole splitting, QS) indicates a symmetric environment, whereas the low isomer shift (IS77 K < 0.4 ram/s) is indicative of tetrahedral coordination. Thus, this spectrum is a typical one characteristic for the isomorphously substituted ferric ions in the as synthesized sample, prior to the removal of template molecules used in the synthesis (see e.g. in [25]). The activation of the sample results in a partial removal of iron from the framework. Combined (Fe, A])framework-O-(Fe, A1)extra_framework pairs may be formed as reflected in the appearance of quadrupole splitting [23]. The symmetry is extended to a distorted octahedral one, as shown by the increase of the IS and (QS values as well).

Table 3. MSssbauer parameters extracted from 77 K spectra QS Sample Component IS 7 as-made 7 after 3 h of reaction 7 after 3 cycles of reaction/regeneration IS= isomer shift, related FWHM= full line width contribution, %

Fe3+~tr Fea§ Fe 2+ Fe3+oct

0.33 0.45 1.10 0.42

1.08 3.04 0.92

FWHM

RI

2.10 0.68 0.34 0.87

100 96 4 100

to (z-iron, mnds; QS= quadrupole splitting, mm/s; at half maximum, mm/s; RI= relative spectral

482 During long term catalytic tests in benzene hydroxylation and r e p e a t e d activation-reaction cycles, a change in the iron species is noted which can be i n t e r p r e t e d as a migration of iron from the initial positions in the zeolite framework deriving from framework to extra-framework migration to more stable locations. This change is indicated by a decrease of line width (FWHM). Presence of Fe 2§ component detected in a minor amount (RI - 4 %) in the sample after exposure to the reaction mixture attests that a reversible Fe2*+~Fe 3+ cycle is involved in the reaction. It may be noted, that the samples were exposed to air after being removed from the catalytic reactor, and therefore a p a r t i a l reoxidation of Fe 2+ probably occurred. More complete information is expected from in situ MSssbauer measurements, but the detection of Fe 2* suggests t h a t iron ions are reduced during the catalytic reaction.

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Figure 1. 77 K MSssbauer spectra of as-synthesized sample (bottom); after 3 cycles of activation/reaction (middle); and exposed to 3 h of reaction (top).

483 A further aspect worth to be noted is the absence of magnetically split components in the spectra. This indicates that there is no superparamagnetic relaxation at 77 K, i.e. neither presence of extended oxidic (antiferromagnetic) Fe-O-Fe clusters, nor carbidic (ferromagnetic) Fe-C-Fe chains are detected. It may be noted that the threshold size of particles sufficient to exhibit magnetic component is a few nanometers. Thus, high dispersion of iron ions, most probably close to ionic one, is additionally proven. The 29Si-NMR spectra of the as made MFI samples and of the spent Fe-MFI catalysts show interesting behaviours. The as-synthesized MFI samples are white showing clearly that there are no extra-framework ions species in the samples. In these cases, 1H-29Si cross polarized spectra could be taken, where the intensities of the -103 ppm and-106 ppm lines are smaller in the cross polarized spectra. Oppositely, in the colored samples, where extra-framework ion species are present no cross polarized spectra could be measured. It was also the case of the spent samples. In the spent catalysts, the intensity of the -100 ppm line is quite small showing that the defect groups due to the silanol groups have been eliminated during the reaction. The spent catalysts certainly contain extra-framework ion species. After three hours of reaction, the catalyst becomes black and shows a 13CNMR spectrum centered at ca 140 ppm which is quite broad (A -- 4000 Hz) and could be due to carbonaceous species or coke. The 13C-NMR spectrum of the catalyst after three cycles of reaction-regeneration shows a broad line (A = 1000 Hz) at ca 126 ppm which can be attributed to strongly adsorbed phenol. 4. CONCLUSIONS The preparation procedure utilizing iron complexes is a good method for introducing iron into MFI type zeolite in a large crystallization field. The amount of the iron incorporated into the zeolitic framework depends on its content in the initial hydrogel for the TPABr-rich systems. In absence or in poor TPABr systems A1 is preferentially incorporated and only a small amount of Fe is detected in the crystals. This suggests that with a control of the amount of TPABr in the hydrogel, or with the addition of other organic molecules [24], it is possible to modulate the incorporation of iron into the MFI framework. The NMR and MSssbauer data indicate that from direct synthesis the iron is incorporated in tetrahedral position into the zeolitic framework. After the catalytic reaction extra-framework iron is detected. The deactivation of catalysts is due to the formation of carbonaceous species or coke. The addition of iron by post-synthesis methods is also possible, but the catalytic behaviour is strongly affected by the procedure of iron incorporation. REFERENCES

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