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Benzene to phenol hydroxylation with N2O over Fe-Beta and Fe-ZSM-5: Comparison of activity per Fe-site
Applied Catalysis A: General 319 (2007) 128–136 www.elsevier.com/locate/apcata
Benzene to phenol hydroxylation with N2O over Fe-Beta and Fe-ZSM-5: Comparison of activity per Fe-site Igor Yuranov, Dmitri A. Bulushev, Albert Renken, Lioubov Kiwi-Minsker * Ecole Polytechnique Fe´de´rale de Lausanne (LGRC-EPFL), CH-1015 Lausanne, Switzerland Received 18 October 2006; received in revised form 10 November 2006; accepted 20 November 2006 Available online 3 December 2006
Abstract Fe-Beta catalysts with iron content of 0.045–2.0 wt.% were studied in the benzene to phenol transformation with N2O and compared to similar Fe-ZSM-5 catalysts to understand the influence of zeolite structure on the Fe-sites activity. The Fe-containing zeolites were prepared either by a direct hydrothermal synthesis or by a post-synthesis Fe deposition followed by activation (steaming or high temperature treatment in He). Total amount of Fe(II) active sites able to form atomic oxygen (O)Fe, from N2O, was quantified by the transient response method at 523 K measuring the released N2. The fraction of the (O)Fe active in CO oxidation was determined via the amount of CO2 released. The catalyst activity in the benzene to phenol transformation over the activated isomorphously substituted Fe-Beta and Fe-ZSM-5 zeolites was directly proportional to the amount of the (O)Fe active in CO oxidation. The turnover frequency (TOF) over Fe-Beta and Fe-ZSM-5 catalysts was found to be close indicating a similarity in the structures of Fe active sites. The observed 2-fold difference can be attributed to the influence of the zeolite host lattices. The difference between Fe-Beta and Fe-ZSM-5 was also observed in DRIFT spectra of NO adsorbed on iron sites at room temperature. The bands of NO adsorbed on Fe-Beta and Fe-ZSM-5 were at 1873 and 1878/1891 cm1, respectively. The areas of these bands correlate with the amount of (O)Fe active in CO oxidation. Comparison of the DRIFT spectra of adsorbed NO on the zeolites with the spectra on some iron containing compounds allowed to attribute the adsorption sites to Fe(II) sites and not to Fe(III) sites. # 2006 Published by Elsevier B.V. Keywords: Fe-ZSM-5; Fe-Beta; Zeolites; N2O; Benzene hydroxylation; Phenol; Fe active species
1. Introduction Zeolites are known to confine ultra small metal complexes within micropores resulting in catalytically active sites [1,2]. Iron species stabilized in a ZSM-5 zeolite in the form of socalled ‘‘a-sites’’ is an example of this type of confinement [3]. Fe-sites activated by the ZSM-5 microporous matrix interact with N2O generating surface atomic oxygen (‘‘a-oxygen’’). Simultaneously, the ZSM-5 matrix is able to activate benzene molecules leading to selective one-step benzene to phenol transformation with N2O [4]. Several research groups have investigated the nature of Fe active sites in Fe-ZSM-5 zeolites [3–29]. There is a consensus that Fe active species are stabilized in the form of low nuclearity complexes (–Fe(II)–O–) in the extra-framework positions in zeolite micropores. However, the
* Corresponding author. Tel.: +41 21 693 3182; fax: +41 21 693 3190. E-mail address: [email protected] (L. Kiwi-Minsker). 0926-860X/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apcata.2006.11.023
exact structure of the Fe-sites active in hydroxylation of benzene is still a subject of discussion. It is known that only very small amount of deposited iron usually participate in catalysis (<0.1 wt.%). The major part of Fe is inactive and does not catalyze benzene hydroxylation, making difficult active sites characterization by physico-chemical methods [7,24,29]. Little information is available on the influence of the nature of a host matrix on catalyst properties. Most of research efforts have been aimed at the investigation of Fe-ZSM-5 catalysts in benzene hydroxylation and only a few papers are devoted to other Fecontaining zeolites (Fe-Y, Fe-Mordenite, Fe-Ferrierite, Fe-Beta, Fe-MCM-41 and Fe-MCM-22) [30–35]. Among the zeolites only Fe-Beta was found to be active in the reaction. Zeolite Beta (BEA framework class) as well as ZSM-5 (MFI framework class) has a three-dimensional channel system. BEA has 12-ring apertures of 0.66 nm 0.67 nm and 0.56 nm 0.56 nm along [1 0 0] and [0 0 1] directions, respectively, while ZSM-5 has a 3D 10-ring channels system with apertures of 0.51 nm 0.55 nm and 0.53 nm 0.56 nm along [1 0 0] and [0 0 1] directions,
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respectively [36,37]. Note that zeolite Beta, due to the geometrical parameters, seems to be an interesting candidate for selective hydroxylation of the molecules larger than benzene, such as substituted aromatics. Panov and coworkers claimed that Fe-Beta zeolites are able to form ‘‘a-oxygen’’ [32] being active in benzene hydroxylation [3], but no comparison with Fe-ZSM-5 was done. Centi et al. reported a comparable catalytic performance of activated Fe-Beta catalysts in benzene hydroxylation to phenol with respect to Fe-ZSM-5 catalysts of the same iron content [33]. Pe´rez-Ramı´rez et al., studying evolution of isomorphously substituted Fe-ZSM-5 and Fe-Beta upon activation and catalyst activity in N2O decomposition and N2O reduction with CO, concluded that the nature and distribution of Fe active sites were similar, regardless the zeolite matrix [35]. The authors assumed that the amounts of Fe active sites formed in Fe-ZSM-5 and Fe-Beta catalysts with the same Fe loading upon the same activation treatment were equal. However, this assumption was not proven. As mentioned above, only a part of iron deposited on a zeolite can be converted to the active species upon activation and their amount is often <0.1 wt.%. Clearly, this amount may be dependent on the nature of the host zeolite matrix. Recently, we reported a new method to quantify Fe active sites by the transient response techniques [20]. The method allows to measure total amount of Fe(II)-sites able to form atomic oxygen (O)Fe, from N2O, and a fraction of the (O)Fe active in CO oxidation. The latter fraction was shown to correlate with the Fe-ZSM-5 activity in the benzene to phenol transformation [29]. The same was confirmed recently by the Panov’s group [38]. The aim of the present study is to compare the activity per Fe-site in the benzene to phenol reaction with N2O (TOF values) over Fe-ZSM-5 and Fe-Beta catalysts. The reaction rate was referred to the concentration of Fe active sites quantified by the transient response method [20]. Such approach should bring insight into the relation between the zeolite matrix and the formation of active Fe-sites as well as their intrinsic activity. 2. Experimental 2.1. Catalyst preparation The isomorphously substituted Fe-Beta and Fe-ZSM-5 were prepared by hydrothermal synthesis as described previously [29,39]. The Fe-Beta synthesis mixture was prepared by hydrolysis of tetraethylorthosilicate (TEOS) (98%, Fluka) in an aqueous solution of tetraethylammonium hydroxide (TEAOH) (20%, Fluka). Then a solution of Al(NO3)39H2O (98%, Fluka) and Fe(NO3)39H2O (98%, Fluka) in aqueous TEAOH were added and a mixture was kept under stirring for 3 h. Finally, hydrofluoric acid (HF) (48%, Fluka) was added. Chemical composition of the synthesis mixture can be expressed as follows: TEOS:TEAOH:Al(NO3)3:Fe(NO3)3:H2O:HF = 1:0.55:0.02:0.00054–0.0054:7:0.55. After crystallization at 410 K during 7 days, the autoclave was cooled down to room temperature. The product was filtered, washed with deionized water and calcined in air at 823 K for 12 h.
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In a typical synthesis of Fe-ZSM-5 [29] tetraethylorthosilicate was added to an aqueous solution of tetrapropylammonium hydroxide (TPAOH), NaAlO2 and Fe(NO3)39H2O. The molar ratios between components were TEOS:TPAOH:NaAlO2:H2O = 0.8:0.1:0.02:33 and Si:Fe = 3200. The mixture was stirred for 3 h at room temperature, and the final transparent gel was transferred to an autoclave and kept in an oven at 450 K for 2 days. The product was filtered, washed with deionized water and calcined in air at 823 K for 12 h. The zeolite was then converted into a H-form by an exchange with a NH4NO3 aqueous solution (0.5 M) and subsequent calcination at 823 K for 3 h. Post-synthesis iron deposition was performed by adsorption a precursor on a zeolite H-form from a Fe(CH3COO)3 aqueous solution (0.01 M) during 24 h followed by washing with water, air-drying and calcination at 823 K for 3 h. The catalysts were activated via steaming (H2O partial pressure of 0.3 bar, He flow rate, 50 ml/min) at 823 K for 4 h or treatment at 1323 K in a He flow (50 ml/min) for 1 h. The characteristics of the catalysts used in this study are listed in Table 1. The content of isomorphously substituted Fe (ppm), the content of Fe loaded by post-synthesis deposition (wt.%) as well as the method of activation are indicated in the catalyst designations. The Si/Al ratio was found to be 45 and 42 for the synthesized Fe-Beta and Fe-ZSM-5 samples, respectively. 2.2. Catalyst characterization The chemical composition of the catalysts was determined by atomic absorption spectroscopy (AAS) via a Shimadzu AA6650 spectrometer. The samples were dissolved in hot aqua regia containing several drops of HF. The specific surface areas (SSA) of the catalysts were measured using N2 adsorption–desorption at 77 K via a Sorptomatic 1990 instrument (Carlo Erba) after catalyst pretreatment in vacuum at 523 K for 2 h. The SSA of the samples was calculated employing the BET method while the Dollimore/Heal method was applied for the calculation of pore volume. X-ray diffraction (XRD) patterns of the catalysts were obtained on a Siemens D500 diffractometer with Cu Ka ˚ ). monochromatic radiation (l = 1.5406 A Determination of the Fe(II) sites in the prepared catalysts was performed by transient response method and temperatureprogrammed desorption through a Micromeritics AutoChem 2910 analyzer as described elsewhere [20,29]. The methods allow determining: (1) The amount of N2 formed over the catalyst (523 K) after a switch from pure He to a 2 vol.% N2O + 2 vol.% N O Ar + 96 vol.% He mixture. The reaction N2 O þ ð ÞFe2 ¼ N2 O N2 þ ðOÞFe proceeds through evolution of a stoichiometric amount of N2 into the gas phase and deposition of one oxygen atom per each active site [40]. The amount of N O the iron sites is denoted as CFe2 in Table 1. (2) The amount of O2 evolved during temperature-programmed desorption (523–873 K) from the sample pre-loaded by
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Table 1 The characteristics of the Fe-Beta and Fe-ZSM-5 catalysts Catalyst
Fe loading (wt.%)
Method of catalyst activation
Concentration of Fe(II) sites (1018 site g1) N O
CFe2 Fe-Betan450 Fe-Betacalc 450 Fe-Betast450 Fe-Betast4500 1.9% Fe-Betacalc 1.9% Fe-Betast c Fe-ZSM-5calc 350 Fe-ZSM-5st350 Fe-ZSM-5calc 5800 Fe-ZSM-5st5800 2.1% Fe-ZSM-5calc 2.1% Fe-ZSM-5st a b c
0.045 0.045 0.045 0.45 1.9 1.9 0.035 0.035 0.58 0.58 2.1 2.1
No 1323 K, He a Steamingb Steaming 1323 K, He Steaming 1323 K, He Steaming 1323 K, He Steaming 1323 K, He Steaming
0.55 2.1 1.6 10.3 16.9 4.4 3.0 0.8 11.1 7.3 24.3 28.9
CTPD Fe 1.2 0.5 3.7 9.0 2.0 0.1 10.3 2.4 10.1 11.6
R (mmol h1 g1), 543 K
CCO Fe 0.4 2.1 1.1 6.1 13.8 3.7 2.1 0.5 9.1 3.3 15.1 18.6
0.010 0.045 0.033 0.097 0.065 0.012 0.072 0.032 0.288 0.126 0.202 0.216
‘‘1323 K, He’’ means activation in flow of He (50 ml/min) at 1323 K for 1 h. ‘‘Steaming’’ means activation in water vapor at 823 K for 4 h. The data on the Fe-ZSM-5 catalysts are taken from [29].
TPD TPD oxygen from N2O: ðOÞTPD Fe þ ðOÞFe ¼ O2 þ 2ð ÞFe . The amount of the iron sites determined by this method is denoted as CTPD Fe in Table 1. (3) The amount of CO2 formed in CO oxidation at 523 K over the sample pre-loaded by oxygen from N2O according to CO the reaction: CO þ ðOÞCO Fe ¼ CO2 þ ð ÞFe . The amount of the iron sites is denoted as CCO Fe in Table 1.
2.3. DRIFTS measurements The catalysts were characterized by ‘‘in situ’’ DRIFTS of adsorbed NO at 303 K. A Perkin-Elmer 2000 FTIR spectrometer with an MCT detector was used. The catalyst (0.015 g) ground in an agate mortar was placed into a SpectraTech 003102 DRIFTS cell with CaF2 windows in a set-up described earlier [41]. This set-up allowed quick switching between pure Ar and 0.5 vol.% NO + 99.5 vol.% Ar mixture flows. The flow of a gas passing through the catalyst was always equal to 20 ml (STP)/min. The DRIFT spectra obtained by averaging of 32 scans with a resolution of 4 cm1 were taken every minute. The nitrosyl region (1740–1940 cm1) of the spectra was found to become identical after a 2 min exposure of the catalyst in NO. After 6 min the flow was switched back to Ar. Before every run, the activated catalysts were pre-treated in Ar at 823 K for 60 min. A single beam spectrum of the catalyst was taken before the introduction of NO at a set temperature as a background. The contribution of the gaseous NO to the spectra was negligible, as shown by the spectra of the mixture containing 0.5 vol.% NO over KBr. 2.4. Catalyst testing Benzene hydroxylation with N2O over the synthesized catalysts was carried out in a vertical stainless-steel fixed-bed reactor (inner diameter, 20 mm) at 510–620 K and atmospheric pressure. The powder catalyst (m = 0.3–1.0 g, dp = 0.2– 0.5 mm) was mixed with inert quartz grains and placed on a
plate of sintered stainless-steel fibres forming a uniform catalyst bed of 3–4 mm height in the middle of the reactor. Before the reaction, the catalyst was pre-treated in He at 773 K for 1 h to remove water and other volatile compounds. The mixture of 1 vol.% of benzene, 5 vol.% of N2O and 94 vol.% of He was used through the catalytic tests. 0.5 vol.% of NO was added to the reaction mixture in order to test the influence of this additive on the catalysts activity. The gases were provided by Carbagas (Lausanne, Switzerland, >99.99%) and used as received. Benzene was fed into the reactor by passing He through a thermostated (T = 293 K) bubble column. The gas flow was controlled by mass flow controllers. The total gas flow of 60 ml(STP)/min was used throughout the study. The reaction temperature was monitored by a thermocouple placed in the catalytic bed. The reaction mixture was analyzed on-line by GC (Perkin-Elmer Autosystem XL). The organic components were separated in a SPB-5 capillary column and detected by FID. The light gases (N2, N2O, CO, CO2) were separated in a Carboxen-1010 capillary column and detected by TCD. The measured activities of the Fe-Beta catalysts were compared to the activities of the Fe-ZSM-5 catalysts reported recently [29]. 3. Results and discussion 3.1. Catalysts Fe-ZSM-5 and Fe-Beta catalysts having different iron loadings (Table 1) were used to compare the catalyst performance in the hydroxylation of benzene to phenol with N2O. The content of isomorphously substituted iron in the zeolites was varied from 350 to 5800 ppm and from 450 to 4500 ppm for Fe-ZSM-5 and Fe-Beta, respectively, by changing the Fe(III) nitrate concentration in the silica gel. The synthesized white samples exhibited X-ray diffraction patterns typical for the MFI and BEA frameworks. Postsynthesis deposition led to an increase in the Fe loading up to
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2 wt.%. These samples had a brown colour indicating the formation of Fe2O3. The catalysts were activated by either steaming or high temperature calcination in He (T = 1323 K). Steam treatment is known to lead to dealumination of zeolite lattice and expulsion of isomorphously substituted Fe(III) [8,10,18,30,42]. The process is accompanied by partial Fe(III) reduction and the formation of extra-framework Fe(II) species. The high temperature treatment in He was shown to increase the concentration of Fe(II) sites involved in the surface atomic oxygen (O)Fe formation from N2O [20]. 3.2. Determination of Fe(II) active sites It was shown by the methods developed previously [20,29] that Fe-Beta were able to generate Fe(II) active sites in the same manner like Fe-ZSM-5 catalysts (Table 1). Concentration of the active sites increases considerably after catalyst activation: the treatment in He at 1323 K is more efficient than steaming. The N O total amount of active sites, CFe2 , was determined by the transient response method in N2O decomposition with the formation of N2 and surface atomic oxygen. Fig. 1a represents the concentration-time profiles obtained for the Fe-Betacalc 450 catalyst. The non-ideal reactor behaviour (as compared to a plug-flow reactor) was characterized by inert tracer argon (Ar). As seen from the response, a delay is observed for N2O and N2 as compared to the Ar response. No O2 was detected in N O the outlet, indicating that only the reaction N2 O þ ð ÞFe2 ¼ N2 O N2 þ ðOÞFe takes place. The concentration of Fe(II) sites N O N O participating in the formation of surface oxygen ðOÞFe2 ; CFe2 , was calculated from the amount of N2 released. For example, about 40 and 80% of the total amount of iron atoms in the activated isomorphously substituted Fe-Betacalc and 450 Fe-ZSM-5calc 350 , respectively, are active in the formation of N O ðOÞFe2 from N2O (Table 1). The TPD method was applied to quantify the amount of iron active sites with the deposited atomic oxygen, which was able to recombine under heating giving gaseous O2. The samples N O pre-treated by N2O at 523 K (deposition of ðOÞFe2 ) were heated in He. The O2 profile obtained during the TPD for the Fe-Betacalc 450 catalyst is shown in Fig. 1b. Oxygen evolution from the catalyst appears as a peak with a maximum at 670 K. This temperature is close to the one observed for the Fe-ZSM-5calc 350 catalyst. The concentration of Fe(II) sites participating in the formation of surface oxygen atoms, which are able to desorb as O2 during TPD, CTPD Fe , was calculated by integrating the O2 peak. Similarly to Fe-ZSM-5, this type of Fe(II) sites was assigned to bi- or oligonuclear species. Finally, the concentration of Fe(II) sites generating surface oxygen (O)Fe able to oxidize substrates was measured in CO oxidation by oxygen pre-loaded on the zeolite surface from N2O (Fig. 1c). As was concluded in [29] for Fe-ZSM-5, the iron sites producing atomic oxygen active in CO oxidation are responsible for the zeolite activity in benzene hydroxylation as well. These sites are assumed to have a low (mono-, bi- or oligo-) nuclearity. The concentration of these sites, CCO Fe , is used further for the comparison of catalysts activities.
Fig. 1. (a) Transient response (523 K) obtained after the switch from He to the 2 vol.% N2O + 2 vol.% Ar + 96 vol.% He mixture over Fe-Betacalc 450 catalyst (pre-treatment in He, 1323 K, 1 h; mcat = 0.51 g); (b) temperature-programmed desorption of oxygen from Fe-Betacalc 450 catalyst after the surface oxygen loading from N2O; (c) transient response (523 K) obtained after the switch from He to the 3 vol.% CO in He over Fe-Betacalc 450 catalyst after the surface oxygen loading from N2O.
In general, CCO Fe was found to be dependent on catalyst preparation, Fe loading and activation methods. At the same time, the isomorphously substituted Fe-Beta and Fe-ZSM-5 synthesized and activated in the same way have close concentrations of the active sites, CCO Fe (Table 1). Thus, we can conclude that the abilities of the ZSM-5 and Beta microporous matrices to generate ðOÞCO active in low Fe temperature CO oxidation are similar. In the samples prepared by post-synthesis Fe deposition, the absolute amount of formed iron sites active in CO oxidation, CCO Fe , is substantially higher as
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compared to isomorphously substituted zeolites. However, the relative concentration of these sites is lower. Post-synthesis loading leads preferentially to the formation of inactive agglomerates of iron(III) oxide within micropores of FeZSM-5 and Fe-Beta zeolites and on the external surface of the zeolite crystals. In the case of Fe-Beta catalysts such a process is more pronounced.
However, up to now there is still no consensus concerning the assignment of the observed bands that can be explained by different conditions of material syntheses (chemical composition, activation, pre-treatment, etc.) and DRIFT spectroscopy measurements (NO partial pressure, NO treatment time, temperature, etc.). Fe-Beta zeolites have been scarcely characterized by adsorbed NO probe IR spectroscopy [53,54].
3.3. DRIFTS measurements
3.3.1. 1806–1834 cm1 region Weak bands in this region are observed for all Fe-ZSM-5 and only for some of Fe-Beta samples (Fig. 2). The couples of the bands 1806–1819 cm1 and 1915 cm1 (not seen clearly in Fig. 2) were earlier assigned to dinitrosyl Fe(II)(NO)2 [47,53,55,56] or trinitrosyl Fe(II)(NO)3 [43,50] species. It is
The catalysts were investigated by DRIFT spectroscopy of adsorbed NO at room temperature. This method of characterization of Fe-sites in zeolites might give some hints concerning their oxidation state, nuclearity and coordination [43–52].
Fig. 2. DRIFT spectra of nitrosyl region recorded for (a) Fe-ZSM-5 and (b) Fe-Beta catalysts in the presence of NO (0.5 vol.%) at 303 K. Spectrum of NO adsorbed on dehydrated FeCl2 is shown for comparison.
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synthesized zeolites. No band in the region of 1855–1895 cm1 was observed for Fe(III) containing compounds Fe2O3 and Fe2(SO4)3 (aq) (99%, Fluka) pre-treated in the DRIFT cell in He at 823 or 473 K for 1 h, respectively. Contrary, a strong band of adsorbed NO was observed for Fe(II) compounds—FeSO4 (aq) and FeCl2 (aq) (99%, Fluka) dehydrated at 373 or 473 K for 1 h. The band of NO adsorbed on FeCl2 is located at the same position (1873 cm1) which was found for NO adsorbed on the activated Fe-Beta zeolites (Fig. 2b). Thus, the bands observed in the 1855–1895 cm1 region spectra of Fe-ZSM-5 and Fe-Beta zeolites should correspond to the NO adsorbed on Fe(II) sites and not on Fe(III) sites. Fig. 3. Dependence of the adsorbed NO peak area (1870–1890 cm1) on the iron sites concentration ðCCO Fe Þ determined for Fe-ZSM-5 (&) and Fe-Beta (4) catalysts via titration of loaded oxygen by CO.
evident that the iron species accepting two or three NO molecules have high coordinative insaturation and are probably isolated [34,43,50]. This adsorbed NO being in equilibrium with the gas phase can be easily removed from the surface in an inert gas flow or vacuum. As mentioned above, the isolated iron species are not seen in the spectra of some Fe-Beta zeolites. At the same time, all Fe-Beta catalysts were found to be active in benzene hydroxylation. These facts can indicate that the presence of isolated Fe(II) species is not crucial for the reaction. 3.3.2. 1855-1895 cm1 region This region represents commonly the most intensive bands, which were assigned by different authors to mononitrosyls either with Fe(II) [16,22,46–49,53,57] or Fe(III) [34,50,54] sites. The authors [52] noticed that the band at 1877 cm1 corresponds to hydrated and dehydrated iron species, but in a close proximity to Al. Earlier a special study was performed with Fe-MFI (without Al) and zeolites containing Al and Ga [49]. In this work the band at 1886 cm1 was assigned to Fe(II) in isolated positions and/or (FeO)x clusters inside the zeolite channels, whereas the 1874 cm1 band—to iron species in FeAlOx nanoparticles of 2 nm size. The nitrosyl region spectra of the synthesized Fe-ZSM-5 catalysts in the presence of gaseous NO demonstrate two strong bands at 1891 and 1878 cm1 (Fig. 2a). The intensities ratio of these bands was constant for all catalysts activated in He at 1323 K but some changes were observed in the case of the steamed samples. This suggests that these bands could be attributed to NO adsorbed on two different iron species. By contrast, only one strong symmetric band of NO vibrations was observed at 1873 cm1 in the case of the Fe-Beta zeolites (Fig. 2b). This difference between the spectra of NO adsorbed on two zeolites suggests an influence of the zeolite matrices on the Fe-sites structure. The adsorbed NO is stable in an Ar flow at room temperature pointing out its strong interaction with iron. It is known that adsorption of NO on Fe(III) is much weaker than on Fe(II) [46,58]. A special study was performed with different iron compounds to elucidate the state of iron in
3.3.3. Correlation of adsorbed NO bands intensities with concentration of active iron sites The activation of Fe-ZSM-5 and Fe-Beta zeolites involves dehydroxylation, formation of extra-framework Fe species and their reduction to Fe(II). N2O interacts with Fe(II) sites forming active oxygen which can interact with CO giving CO2. Fig. 3 shows a dependence of the intensity of the adsorbed NO bands (1870–1895 cm1) on the amount of oxygen CCO Fe which can be measured by a transient response method during CO oxidation. The higher is the concentration of the active iron sites, the higher is the intensity of the IR bands. It is important that the dependences are similar for both Fe-ZSM-5 and Fe-Beta zeolites despite of the difference of the adsorbed NO bands positions. Hence, the bands in the 1870–1895 cm1 region should be assigned for both zeolites to NO adsorbed on Fe(II) active sites. Thus, DRIFTS of NO adsorbed on the Fe-ZSM-5 and FeBeta zeolites indicates some difference in the structures of Fe(II) active sites generated in different host zeolite matrices. At the same time, a similar behaviour of the sites with respect to NO adsorption and active oxygen generation is observed (Fig. 3). Two types of mononitrosyls bonded to (–Fe(II)–O–) species detected in the region 1870–1895 cm1 for the FeZSM-5 spectra indicate a heterogeneity of the Fe(II) sites. Often this region is assigned to adsorption of NO on iron species associated with Al(III) cation [49,52]. 3.4. Catalysts activity in benzene hydroxylation The time dependence of benzene conversion over the Fe-Betacalc 450 catalyst at different temperatures is presented in Fig. 4. The catalyst behaviour is similar to the one reported previously for the isomorphously substituted Fe-ZSM-5calc catalysts with a low iron content [29]. After an activation period, the catalyst performance was observed to be stable at temperatures as high as 540–560 K within 2.5 h. The selectivity of benzene transformation to phenol in this temperature range is high (95–98%). At temperatures >570 K, a pronounced catalyst deactivation was observed due to coke formation [25,33]. In order to avoid the deactivation, the comparison of Fe-ZSM-5 and Fe-Beta catalysts activities was done at 543 K after 2 h on stream. The obtained phenol productivities R are collected in Table 1. The reaction rate R plotted against the amount of Fe(II) active sites active in CO oxidation, CCO Fe , is
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Fig. 4. Conversion of benzene to phenol over Fe-Betacalc 450 catalyst at different temperatures: mcat = 1.0 g; C6H6:N2O = 1:5; total gas flow = 60 ml min1.
presented in Fig. 5. As shown, R is directly proportional to CCO Fe obtained for the isomorphously substituted catalysts, suggesting the iron sites active in low temperature CO oxidation as responsible also for the catalysts activity in benzene hydroxylation. The observed slopes of the straight lines associated to Fe-ZSM-5 and Fe-Beta catalysts are slightly different. The reaction rate of benzene hydroxylation per active iron site (TOF) for Fe-ZSM-5 zeolites is 2-fold higher the TOF for FeBeta zeolites. Such a small difference indicates a similarity in the structures of Fe active sites stabilized in Beta and ZSM-5 zeolites and a weak influence of the zeolite host lattices. An interesting observation is that the catalysts prepared by the post-synthesis treatment (2.1% Fe-ZSM-5 and 1.9% FeBeta) do not follow the dependences described above (Fig. 5). Although they are able to generate a large amount of iron sites 19 1 active in CO oxidation (CCO Fe 1–2 10 site g ), the rate of benzene hydroxylation measured for these catalysts is relatively low (Table 1). This is probably due to Fe2O3 nanoparticles occurring in large quantities within micropores of post-synthesis treated zeolite [22,29], which limit access of ‘‘large’’ molecules of benzene (phenol) to the active sites hindering the catalyst performance. At the same time, the
Fig. 5. The rate of benzene to phenol oxidation as a function of iron sites concentration ðCCO Fe Þ determined via titration of loaded oxygen by CO: mcat = 1.0 g; C6H6:N2O = 1:5; total gas flow = 60 ml min1; T = 543 K.
activity of the 2.1% Fe-ZSM-5 catalysts was always higher as compared to the 1.9% Fe-Beta catalysts. An effect of the presence of NO in the reaction feed on the benzene hydroxylation was tested over the synthesized catalysts. As discussed above, NO adsorbs strongly on iron stabilized in zeolite micropores. At the same time, it was shown that adsorbed NOx species do not block Fe(II) sites for active oxygen (O)Fe loading and can accelerate N2O decomposition over Fe-ZSM-5 catalysts [11,27,59]. It was also demonstrated that the presence of NO inhibited low temperature reduction of N2O with CO over the Fe-ZSM-5 [60]. The dual role of NO, acting as a promoter in N2O decomposition and as an inhibitor in N2O reduction with CO, was explained by the presence of different (isolated and oligonuclear) iron species involved in the reaction mechanisms. In the latter case, NO adsorbs on specific Fe-sites responsible for CO activation, blocking them. On the base of the EPR spectra of benzene adsorbed on Fe-ZSM-5 zeolite, the authors [61] claimed that the (O)Fe does not participate in the formation of acceptor sites responsible for the ionization of benzene molecules. As seen in Fig. 6, 0.5 vol.% of NO inhibits the benzene hydroxylation over both Fe-ZSM-5 and Fe-Beta catalysts. The calc activity of the Fe-ZSM-5calc 350 falls to zero, while the Fe-Beta450 deactivates only partially. At the temperature studied direct decomposition of N2O to N2 and O2 does not take place even in the presence of adsorbed NO [62]. Scavenging of active oxygen species formed from N2O by NO with gaseous NO2 formation seems to be also insignificant. Therefore, under the reaction conditions NO molecules compete with benzene molecules for specific adsorption sites, thus influencing the reaction rate. These specific sites differ from the Fe(II) active sites responsible for the active oxygen (O)Fe formation, and their properties seem to depend on the host zeolite framework. Both MFI and BEA zeolite frameworks are able to generate similar (–Fe(II)–O–)n (probably (–Fe(II)–O–)n–Al–) sites of a low nuclearity responsible for the formation of surface atomic oxygen (O)Fe. At the same time, additional sites responsible for the activation of benzene molecules seem to exist in the vicinity of the (–Fe(II)–O–)n species. The nature of these sites is still unclear and requires further investigation. As mentioned above,
Fig. 6. Influence of NO on benzene to phenol oxidation over the catalysts at 543 K: mcat = 1.0 g; C6H6:N2O = 1:5; total gas flow = 60 ml min1.
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ZSM-5 and Beta zeolites are comprised of three-dimensional pore systems, consisting of intersecting sets of tubular channels with maximum free diameter of 0.58 and 0.70 nm, respectively. Since the maximum kinetic diameter of benzene molecules (0.58 nm) [63] is very close to channel diameters, the molecules are strongly distorted when penetrating through the zeolite frameworks. They are subjected to diverse energy fields: dispersion, repulsion, polarization, adsorption interaction, acid–base interaction, etc. Thus, the zeolite framework itself can be considered as a site responsible for the benzene activation. At the same time, strong acidic sites originating from the zeolite framework are also candidates for the activation of hydrocarbons [24,64]. Specific extra-framework Fe-containing species (isolated or fragments of oligonuclear ones) not participating in (O)Fe generation might be considered as well. Obviously, the mutual arrangement of the sites responsible for the formation of the surface atomic oxygen and the sites responsible for the benzene activation seem to determine the reaction rate. The geometry of the ensemble of active sites stabilized in Fe-ZSM-5 is likely to suit better to the reaction intermediates and results in the higher rate of benzeneto-phenol transformation as compared to Fe-Beta catalysts. 4. Conclusions (1) Both Fe-ZSM-5 and Fe-Beta zeolites generate Fe(II) active sites able to decompose N2O producing surface atomic oxygen (O)Fe and molecular nitrogen. A fraction of this oxygen is active in CO oxidation and the benzene hydroxylation by N2O to phenol. (2) The concentration of iron sites responsible for this activity can be measured by the transient response method during CO oxidation by atomic oxygen (O)Fe pre-loaded from N2O. (3) The reaction rate in the benzene hydroxylation with N2O per active iron site (TOF) was found 2-fold higher over FeZSM-5 catalysts as compared to Fe-Beta. These catalysts have probably Fe(II) active sites of similar structure. The observed difference in the activity indicates a weak influence of the zeolite host lattices as confirmed by the DRIFT spectra of NO adsorbed on iron at room temperature. (4) Active species responsible for the activity of Fe-Beta and Fe-ZSM-5 catalysts in benzene hydroxylation are likely to contain a (–Fe(II)–O)n–Al– fragment of a low nuclearity in the extra-framework positions in the zeolites micropores. The fragment is responsible for the surface atomic oxygen (O)Fe formation. At the same time, an additional site responsible for activation of benzene molecule seems to exist in the vicinity. (5) The geometry of the ensemble of active sites stabilized in Fe-ZSM-5 zeolites is likely to suit better to the reaction intermediates of benzene-to-phenol transformation. Acknowledgements The authors thank the Swiss National Science Foundation for the financial support. Participation of Martin Wettstein in experiments is highly appreciated.
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Benzene to phenol hydroxylation with N2O over Fe-Beta and Fe-ZSM-5: Comparison of activity per Fe-site Igor Yuranov, Dmitri A. Bulushev, Albert Renken, Lioubov Kiwi-Minsker * Ecole Polytechnique Fe´de´rale de Lausanne (LGRC-EPFL), CH-1015 Lausanne, Switzerland Received 18 October 2006; received in revised form 10 November 2006; accepted 20 November 2006 Available online 3 December 2006
Abstract Fe-Beta catalysts with iron content of 0.045–2.0 wt.% were studied in the benzene to phenol transformation with N2O and compared to similar Fe-ZSM-5 catalysts to understand the influence of zeolite structure on the Fe-sites activity. The Fe-containing zeolites were prepared either by a direct hydrothermal synthesis or by a post-synthesis Fe deposition followed by activation (steaming or high temperature treatment in He). Total amount of Fe(II) active sites able to form atomic oxygen (O)Fe, from N2O, was quantified by the transient response method at 523 K measuring the released N2. The fraction of the (O)Fe active in CO oxidation was determined via the amount of CO2 released. The catalyst activity in the benzene to phenol transformation over the activated isomorphously substituted Fe-Beta and Fe-ZSM-5 zeolites was directly proportional to the amount of the (O)Fe active in CO oxidation. The turnover frequency (TOF) over Fe-Beta and Fe-ZSM-5 catalysts was found to be close indicating a similarity in the structures of Fe active sites. The observed 2-fold difference can be attributed to the influence of the zeolite host lattices. The difference between Fe-Beta and Fe-ZSM-5 was also observed in DRIFT spectra of NO adsorbed on iron sites at room temperature. The bands of NO adsorbed on Fe-Beta and Fe-ZSM-5 were at 1873 and 1878/1891 cm1, respectively. The areas of these bands correlate with the amount of (O)Fe active in CO oxidation. Comparison of the DRIFT spectra of adsorbed NO on the zeolites with the spectra on some iron containing compounds allowed to attribute the adsorption sites to Fe(II) sites and not to Fe(III) sites. # 2006 Published by Elsevier B.V. Keywords: Fe-ZSM-5; Fe-Beta; Zeolites; N2O; Benzene hydroxylation; Phenol; Fe active species
1. Introduction Zeolites are known to confine ultra small metal complexes within micropores resulting in catalytically active sites [1,2]. Iron species stabilized in a ZSM-5 zeolite in the form of socalled ‘‘a-sites’’ is an example of this type of confinement [3]. Fe-sites activated by the ZSM-5 microporous matrix interact with N2O generating surface atomic oxygen (‘‘a-oxygen’’). Simultaneously, the ZSM-5 matrix is able to activate benzene molecules leading to selective one-step benzene to phenol transformation with N2O [4]. Several research groups have investigated the nature of Fe active sites in Fe-ZSM-5 zeolites [3–29]. There is a consensus that Fe active species are stabilized in the form of low nuclearity complexes (–Fe(II)–O–) in the extra-framework positions in zeolite micropores. However, the
* Corresponding author. Tel.: +41 21 693 3182; fax: +41 21 693 3190. E-mail address: [email protected] (L. Kiwi-Minsker). 0926-860X/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apcata.2006.11.023
exact structure of the Fe-sites active in hydroxylation of benzene is still a subject of discussion. It is known that only very small amount of deposited iron usually participate in catalysis (<0.1 wt.%). The major part of Fe is inactive and does not catalyze benzene hydroxylation, making difficult active sites characterization by physico-chemical methods [7,24,29]. Little information is available on the influence of the nature of a host matrix on catalyst properties. Most of research efforts have been aimed at the investigation of Fe-ZSM-5 catalysts in benzene hydroxylation and only a few papers are devoted to other Fecontaining zeolites (Fe-Y, Fe-Mordenite, Fe-Ferrierite, Fe-Beta, Fe-MCM-41 and Fe-MCM-22) [30–35]. Among the zeolites only Fe-Beta was found to be active in the reaction. Zeolite Beta (BEA framework class) as well as ZSM-5 (MFI framework class) has a three-dimensional channel system. BEA has 12-ring apertures of 0.66 nm 0.67 nm and 0.56 nm 0.56 nm along [1 0 0] and [0 0 1] directions, respectively, while ZSM-5 has a 3D 10-ring channels system with apertures of 0.51 nm 0.55 nm and 0.53 nm 0.56 nm along [1 0 0] and [0 0 1] directions,
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respectively [36,37]. Note that zeolite Beta, due to the geometrical parameters, seems to be an interesting candidate for selective hydroxylation of the molecules larger than benzene, such as substituted aromatics. Panov and coworkers claimed that Fe-Beta zeolites are able to form ‘‘a-oxygen’’ [32] being active in benzene hydroxylation [3], but no comparison with Fe-ZSM-5 was done. Centi et al. reported a comparable catalytic performance of activated Fe-Beta catalysts in benzene hydroxylation to phenol with respect to Fe-ZSM-5 catalysts of the same iron content [33]. Pe´rez-Ramı´rez et al., studying evolution of isomorphously substituted Fe-ZSM-5 and Fe-Beta upon activation and catalyst activity in N2O decomposition and N2O reduction with CO, concluded that the nature and distribution of Fe active sites were similar, regardless the zeolite matrix [35]. The authors assumed that the amounts of Fe active sites formed in Fe-ZSM-5 and Fe-Beta catalysts with the same Fe loading upon the same activation treatment were equal. However, this assumption was not proven. As mentioned above, only a part of iron deposited on a zeolite can be converted to the active species upon activation and their amount is often <0.1 wt.%. Clearly, this amount may be dependent on the nature of the host zeolite matrix. Recently, we reported a new method to quantify Fe active sites by the transient response techniques [20]. The method allows to measure total amount of Fe(II)-sites able to form atomic oxygen (O)Fe, from N2O, and a fraction of the (O)Fe active in CO oxidation. The latter fraction was shown to correlate with the Fe-ZSM-5 activity in the benzene to phenol transformation [29]. The same was confirmed recently by the Panov’s group [38]. The aim of the present study is to compare the activity per Fe-site in the benzene to phenol reaction with N2O (TOF values) over Fe-ZSM-5 and Fe-Beta catalysts. The reaction rate was referred to the concentration of Fe active sites quantified by the transient response method [20]. Such approach should bring insight into the relation between the zeolite matrix and the formation of active Fe-sites as well as their intrinsic activity. 2. Experimental 2.1. Catalyst preparation The isomorphously substituted Fe-Beta and Fe-ZSM-5 were prepared by hydrothermal synthesis as described previously [29,39]. The Fe-Beta synthesis mixture was prepared by hydrolysis of tetraethylorthosilicate (TEOS) (98%, Fluka) in an aqueous solution of tetraethylammonium hydroxide (TEAOH) (20%, Fluka). Then a solution of Al(NO3)39H2O (98%, Fluka) and Fe(NO3)39H2O (98%, Fluka) in aqueous TEAOH were added and a mixture was kept under stirring for 3 h. Finally, hydrofluoric acid (HF) (48%, Fluka) was added. Chemical composition of the synthesis mixture can be expressed as follows: TEOS:TEAOH:Al(NO3)3:Fe(NO3)3:H2O:HF = 1:0.55:0.02:0.00054–0.0054:7:0.55. After crystallization at 410 K during 7 days, the autoclave was cooled down to room temperature. The product was filtered, washed with deionized water and calcined in air at 823 K for 12 h.
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In a typical synthesis of Fe-ZSM-5 [29] tetraethylorthosilicate was added to an aqueous solution of tetrapropylammonium hydroxide (TPAOH), NaAlO2 and Fe(NO3)39H2O. The molar ratios between components were TEOS:TPAOH:NaAlO2:H2O = 0.8:0.1:0.02:33 and Si:Fe = 3200. The mixture was stirred for 3 h at room temperature, and the final transparent gel was transferred to an autoclave and kept in an oven at 450 K for 2 days. The product was filtered, washed with deionized water and calcined in air at 823 K for 12 h. The zeolite was then converted into a H-form by an exchange with a NH4NO3 aqueous solution (0.5 M) and subsequent calcination at 823 K for 3 h. Post-synthesis iron deposition was performed by adsorption a precursor on a zeolite H-form from a Fe(CH3COO)3 aqueous solution (0.01 M) during 24 h followed by washing with water, air-drying and calcination at 823 K for 3 h. The catalysts were activated via steaming (H2O partial pressure of 0.3 bar, He flow rate, 50 ml/min) at 823 K for 4 h or treatment at 1323 K in a He flow (50 ml/min) for 1 h. The characteristics of the catalysts used in this study are listed in Table 1. The content of isomorphously substituted Fe (ppm), the content of Fe loaded by post-synthesis deposition (wt.%) as well as the method of activation are indicated in the catalyst designations. The Si/Al ratio was found to be 45 and 42 for the synthesized Fe-Beta and Fe-ZSM-5 samples, respectively. 2.2. Catalyst characterization The chemical composition of the catalysts was determined by atomic absorption spectroscopy (AAS) via a Shimadzu AA6650 spectrometer. The samples were dissolved in hot aqua regia containing several drops of HF. The specific surface areas (SSA) of the catalysts were measured using N2 adsorption–desorption at 77 K via a Sorptomatic 1990 instrument (Carlo Erba) after catalyst pretreatment in vacuum at 523 K for 2 h. The SSA of the samples was calculated employing the BET method while the Dollimore/Heal method was applied for the calculation of pore volume. X-ray diffraction (XRD) patterns of the catalysts were obtained on a Siemens D500 diffractometer with Cu Ka ˚ ). monochromatic radiation (l = 1.5406 A Determination of the Fe(II) sites in the prepared catalysts was performed by transient response method and temperatureprogrammed desorption through a Micromeritics AutoChem 2910 analyzer as described elsewhere [20,29]. The methods allow determining: (1) The amount of N2 formed over the catalyst (523 K) after a switch from pure He to a 2 vol.% N2O + 2 vol.% N O Ar + 96 vol.% He mixture. The reaction N2 O þ ð ÞFe2 ¼ N2 O N2 þ ðOÞFe proceeds through evolution of a stoichiometric amount of N2 into the gas phase and deposition of one oxygen atom per each active site [40]. The amount of N O the iron sites is denoted as CFe2 in Table 1. (2) The amount of O2 evolved during temperature-programmed desorption (523–873 K) from the sample pre-loaded by
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Table 1 The characteristics of the Fe-Beta and Fe-ZSM-5 catalysts Catalyst
Fe loading (wt.%)
Method of catalyst activation
Concentration of Fe(II) sites (1018 site g1) N O
CFe2 Fe-Betan450 Fe-Betacalc 450 Fe-Betast450 Fe-Betast4500 1.9% Fe-Betacalc 1.9% Fe-Betast c Fe-ZSM-5calc 350 Fe-ZSM-5st350 Fe-ZSM-5calc 5800 Fe-ZSM-5st5800 2.1% Fe-ZSM-5calc 2.1% Fe-ZSM-5st a b c
0.045 0.045 0.045 0.45 1.9 1.9 0.035 0.035 0.58 0.58 2.1 2.1
No 1323 K, He a Steamingb Steaming 1323 K, He Steaming 1323 K, He Steaming 1323 K, He Steaming 1323 K, He Steaming
0.55 2.1 1.6 10.3 16.9 4.4 3.0 0.8 11.1 7.3 24.3 28.9
CTPD Fe 1.2 0.5 3.7 9.0 2.0 0.1 10.3 2.4 10.1 11.6
R (mmol h1 g1), 543 K
CCO Fe 0.4 2.1 1.1 6.1 13.8 3.7 2.1 0.5 9.1 3.3 15.1 18.6
0.010 0.045 0.033 0.097 0.065 0.012 0.072 0.032 0.288 0.126 0.202 0.216
‘‘1323 K, He’’ means activation in flow of He (50 ml/min) at 1323 K for 1 h. ‘‘Steaming’’ means activation in water vapor at 823 K for 4 h. The data on the Fe-ZSM-5 catalysts are taken from [29].
TPD TPD oxygen from N2O: ðOÞTPD Fe þ ðOÞFe ¼ O2 þ 2ð ÞFe . The amount of the iron sites determined by this method is denoted as CTPD Fe in Table 1. (3) The amount of CO2 formed in CO oxidation at 523 K over the sample pre-loaded by oxygen from N2O according to CO the reaction: CO þ ðOÞCO Fe ¼ CO2 þ ð ÞFe . The amount of the iron sites is denoted as CCO Fe in Table 1.
2.3. DRIFTS measurements The catalysts were characterized by ‘‘in situ’’ DRIFTS of adsorbed NO at 303 K. A Perkin-Elmer 2000 FTIR spectrometer with an MCT detector was used. The catalyst (0.015 g) ground in an agate mortar was placed into a SpectraTech 003102 DRIFTS cell with CaF2 windows in a set-up described earlier [41]. This set-up allowed quick switching between pure Ar and 0.5 vol.% NO + 99.5 vol.% Ar mixture flows. The flow of a gas passing through the catalyst was always equal to 20 ml (STP)/min. The DRIFT spectra obtained by averaging of 32 scans with a resolution of 4 cm1 were taken every minute. The nitrosyl region (1740–1940 cm1) of the spectra was found to become identical after a 2 min exposure of the catalyst in NO. After 6 min the flow was switched back to Ar. Before every run, the activated catalysts were pre-treated in Ar at 823 K for 60 min. A single beam spectrum of the catalyst was taken before the introduction of NO at a set temperature as a background. The contribution of the gaseous NO to the spectra was negligible, as shown by the spectra of the mixture containing 0.5 vol.% NO over KBr. 2.4. Catalyst testing Benzene hydroxylation with N2O over the synthesized catalysts was carried out in a vertical stainless-steel fixed-bed reactor (inner diameter, 20 mm) at 510–620 K and atmospheric pressure. The powder catalyst (m = 0.3–1.0 g, dp = 0.2– 0.5 mm) was mixed with inert quartz grains and placed on a
plate of sintered stainless-steel fibres forming a uniform catalyst bed of 3–4 mm height in the middle of the reactor. Before the reaction, the catalyst was pre-treated in He at 773 K for 1 h to remove water and other volatile compounds. The mixture of 1 vol.% of benzene, 5 vol.% of N2O and 94 vol.% of He was used through the catalytic tests. 0.5 vol.% of NO was added to the reaction mixture in order to test the influence of this additive on the catalysts activity. The gases were provided by Carbagas (Lausanne, Switzerland, >99.99%) and used as received. Benzene was fed into the reactor by passing He through a thermostated (T = 293 K) bubble column. The gas flow was controlled by mass flow controllers. The total gas flow of 60 ml(STP)/min was used throughout the study. The reaction temperature was monitored by a thermocouple placed in the catalytic bed. The reaction mixture was analyzed on-line by GC (Perkin-Elmer Autosystem XL). The organic components were separated in a SPB-5 capillary column and detected by FID. The light gases (N2, N2O, CO, CO2) were separated in a Carboxen-1010 capillary column and detected by TCD. The measured activities of the Fe-Beta catalysts were compared to the activities of the Fe-ZSM-5 catalysts reported recently [29]. 3. Results and discussion 3.1. Catalysts Fe-ZSM-5 and Fe-Beta catalysts having different iron loadings (Table 1) were used to compare the catalyst performance in the hydroxylation of benzene to phenol with N2O. The content of isomorphously substituted iron in the zeolites was varied from 350 to 5800 ppm and from 450 to 4500 ppm for Fe-ZSM-5 and Fe-Beta, respectively, by changing the Fe(III) nitrate concentration in the silica gel. The synthesized white samples exhibited X-ray diffraction patterns typical for the MFI and BEA frameworks. Postsynthesis deposition led to an increase in the Fe loading up to
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2 wt.%. These samples had a brown colour indicating the formation of Fe2O3. The catalysts were activated by either steaming or high temperature calcination in He (T = 1323 K). Steam treatment is known to lead to dealumination of zeolite lattice and expulsion of isomorphously substituted Fe(III) [8,10,18,30,42]. The process is accompanied by partial Fe(III) reduction and the formation of extra-framework Fe(II) species. The high temperature treatment in He was shown to increase the concentration of Fe(II) sites involved in the surface atomic oxygen (O)Fe formation from N2O [20]. 3.2. Determination of Fe(II) active sites It was shown by the methods developed previously [20,29] that Fe-Beta were able to generate Fe(II) active sites in the same manner like Fe-ZSM-5 catalysts (Table 1). Concentration of the active sites increases considerably after catalyst activation: the treatment in He at 1323 K is more efficient than steaming. The N O total amount of active sites, CFe2 , was determined by the transient response method in N2O decomposition with the formation of N2 and surface atomic oxygen. Fig. 1a represents the concentration-time profiles obtained for the Fe-Betacalc 450 catalyst. The non-ideal reactor behaviour (as compared to a plug-flow reactor) was characterized by inert tracer argon (Ar). As seen from the response, a delay is observed for N2O and N2 as compared to the Ar response. No O2 was detected in N O the outlet, indicating that only the reaction N2 O þ ð ÞFe2 ¼ N2 O N2 þ ðOÞFe takes place. The concentration of Fe(II) sites N O N O participating in the formation of surface oxygen ðOÞFe2 ; CFe2 , was calculated from the amount of N2 released. For example, about 40 and 80% of the total amount of iron atoms in the activated isomorphously substituted Fe-Betacalc and 450 Fe-ZSM-5calc 350 , respectively, are active in the formation of N O ðOÞFe2 from N2O (Table 1). The TPD method was applied to quantify the amount of iron active sites with the deposited atomic oxygen, which was able to recombine under heating giving gaseous O2. The samples N O pre-treated by N2O at 523 K (deposition of ðOÞFe2 ) were heated in He. The O2 profile obtained during the TPD for the Fe-Betacalc 450 catalyst is shown in Fig. 1b. Oxygen evolution from the catalyst appears as a peak with a maximum at 670 K. This temperature is close to the one observed for the Fe-ZSM-5calc 350 catalyst. The concentration of Fe(II) sites participating in the formation of surface oxygen atoms, which are able to desorb as O2 during TPD, CTPD Fe , was calculated by integrating the O2 peak. Similarly to Fe-ZSM-5, this type of Fe(II) sites was assigned to bi- or oligonuclear species. Finally, the concentration of Fe(II) sites generating surface oxygen (O)Fe able to oxidize substrates was measured in CO oxidation by oxygen pre-loaded on the zeolite surface from N2O (Fig. 1c). As was concluded in [29] for Fe-ZSM-5, the iron sites producing atomic oxygen active in CO oxidation are responsible for the zeolite activity in benzene hydroxylation as well. These sites are assumed to have a low (mono-, bi- or oligo-) nuclearity. The concentration of these sites, CCO Fe , is used further for the comparison of catalysts activities.
Fig. 1. (a) Transient response (523 K) obtained after the switch from He to the 2 vol.% N2O + 2 vol.% Ar + 96 vol.% He mixture over Fe-Betacalc 450 catalyst (pre-treatment in He, 1323 K, 1 h; mcat = 0.51 g); (b) temperature-programmed desorption of oxygen from Fe-Betacalc 450 catalyst after the surface oxygen loading from N2O; (c) transient response (523 K) obtained after the switch from He to the 3 vol.% CO in He over Fe-Betacalc 450 catalyst after the surface oxygen loading from N2O.
In general, CCO Fe was found to be dependent on catalyst preparation, Fe loading and activation methods. At the same time, the isomorphously substituted Fe-Beta and Fe-ZSM-5 synthesized and activated in the same way have close concentrations of the active sites, CCO Fe (Table 1). Thus, we can conclude that the abilities of the ZSM-5 and Beta microporous matrices to generate ðOÞCO active in low Fe temperature CO oxidation are similar. In the samples prepared by post-synthesis Fe deposition, the absolute amount of formed iron sites active in CO oxidation, CCO Fe , is substantially higher as
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compared to isomorphously substituted zeolites. However, the relative concentration of these sites is lower. Post-synthesis loading leads preferentially to the formation of inactive agglomerates of iron(III) oxide within micropores of FeZSM-5 and Fe-Beta zeolites and on the external surface of the zeolite crystals. In the case of Fe-Beta catalysts such a process is more pronounced.
However, up to now there is still no consensus concerning the assignment of the observed bands that can be explained by different conditions of material syntheses (chemical composition, activation, pre-treatment, etc.) and DRIFT spectroscopy measurements (NO partial pressure, NO treatment time, temperature, etc.). Fe-Beta zeolites have been scarcely characterized by adsorbed NO probe IR spectroscopy [53,54].
3.3. DRIFTS measurements
3.3.1. 1806–1834 cm1 region Weak bands in this region are observed for all Fe-ZSM-5 and only for some of Fe-Beta samples (Fig. 2). The couples of the bands 1806–1819 cm1 and 1915 cm1 (not seen clearly in Fig. 2) were earlier assigned to dinitrosyl Fe(II)(NO)2 [47,53,55,56] or trinitrosyl Fe(II)(NO)3 [43,50] species. It is
The catalysts were investigated by DRIFT spectroscopy of adsorbed NO at room temperature. This method of characterization of Fe-sites in zeolites might give some hints concerning their oxidation state, nuclearity and coordination [43–52].
Fig. 2. DRIFT spectra of nitrosyl region recorded for (a) Fe-ZSM-5 and (b) Fe-Beta catalysts in the presence of NO (0.5 vol.%) at 303 K. Spectrum of NO adsorbed on dehydrated FeCl2 is shown for comparison.
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synthesized zeolites. No band in the region of 1855–1895 cm1 was observed for Fe(III) containing compounds Fe2O3 and Fe2(SO4)3 (aq) (99%, Fluka) pre-treated in the DRIFT cell in He at 823 or 473 K for 1 h, respectively. Contrary, a strong band of adsorbed NO was observed for Fe(II) compounds—FeSO4 (aq) and FeCl2 (aq) (99%, Fluka) dehydrated at 373 or 473 K for 1 h. The band of NO adsorbed on FeCl2 is located at the same position (1873 cm1) which was found for NO adsorbed on the activated Fe-Beta zeolites (Fig. 2b). Thus, the bands observed in the 1855–1895 cm1 region spectra of Fe-ZSM-5 and Fe-Beta zeolites should correspond to the NO adsorbed on Fe(II) sites and not on Fe(III) sites. Fig. 3. Dependence of the adsorbed NO peak area (1870–1890 cm1) on the iron sites concentration ðCCO Fe Þ determined for Fe-ZSM-5 (&) and Fe-Beta (4) catalysts via titration of loaded oxygen by CO.
evident that the iron species accepting two or three NO molecules have high coordinative insaturation and are probably isolated [34,43,50]. This adsorbed NO being in equilibrium with the gas phase can be easily removed from the surface in an inert gas flow or vacuum. As mentioned above, the isolated iron species are not seen in the spectra of some Fe-Beta zeolites. At the same time, all Fe-Beta catalysts were found to be active in benzene hydroxylation. These facts can indicate that the presence of isolated Fe(II) species is not crucial for the reaction. 3.3.2. 1855-1895 cm1 region This region represents commonly the most intensive bands, which were assigned by different authors to mononitrosyls either with Fe(II) [16,22,46–49,53,57] or Fe(III) [34,50,54] sites. The authors [52] noticed that the band at 1877 cm1 corresponds to hydrated and dehydrated iron species, but in a close proximity to Al. Earlier a special study was performed with Fe-MFI (without Al) and zeolites containing Al and Ga [49]. In this work the band at 1886 cm1 was assigned to Fe(II) in isolated positions and/or (FeO)x clusters inside the zeolite channels, whereas the 1874 cm1 band—to iron species in FeAlOx nanoparticles of 2 nm size. The nitrosyl region spectra of the synthesized Fe-ZSM-5 catalysts in the presence of gaseous NO demonstrate two strong bands at 1891 and 1878 cm1 (Fig. 2a). The intensities ratio of these bands was constant for all catalysts activated in He at 1323 K but some changes were observed in the case of the steamed samples. This suggests that these bands could be attributed to NO adsorbed on two different iron species. By contrast, only one strong symmetric band of NO vibrations was observed at 1873 cm1 in the case of the Fe-Beta zeolites (Fig. 2b). This difference between the spectra of NO adsorbed on two zeolites suggests an influence of the zeolite matrices on the Fe-sites structure. The adsorbed NO is stable in an Ar flow at room temperature pointing out its strong interaction with iron. It is known that adsorption of NO on Fe(III) is much weaker than on Fe(II) [46,58]. A special study was performed with different iron compounds to elucidate the state of iron in
3.3.3. Correlation of adsorbed NO bands intensities with concentration of active iron sites The activation of Fe-ZSM-5 and Fe-Beta zeolites involves dehydroxylation, formation of extra-framework Fe species and their reduction to Fe(II). N2O interacts with Fe(II) sites forming active oxygen which can interact with CO giving CO2. Fig. 3 shows a dependence of the intensity of the adsorbed NO bands (1870–1895 cm1) on the amount of oxygen CCO Fe which can be measured by a transient response method during CO oxidation. The higher is the concentration of the active iron sites, the higher is the intensity of the IR bands. It is important that the dependences are similar for both Fe-ZSM-5 and Fe-Beta zeolites despite of the difference of the adsorbed NO bands positions. Hence, the bands in the 1870–1895 cm1 region should be assigned for both zeolites to NO adsorbed on Fe(II) active sites. Thus, DRIFTS of NO adsorbed on the Fe-ZSM-5 and FeBeta zeolites indicates some difference in the structures of Fe(II) active sites generated in different host zeolite matrices. At the same time, a similar behaviour of the sites with respect to NO adsorption and active oxygen generation is observed (Fig. 3). Two types of mononitrosyls bonded to (–Fe(II)–O–) species detected in the region 1870–1895 cm1 for the FeZSM-5 spectra indicate a heterogeneity of the Fe(II) sites. Often this region is assigned to adsorption of NO on iron species associated with Al(III) cation [49,52]. 3.4. Catalysts activity in benzene hydroxylation The time dependence of benzene conversion over the Fe-Betacalc 450 catalyst at different temperatures is presented in Fig. 4. The catalyst behaviour is similar to the one reported previously for the isomorphously substituted Fe-ZSM-5calc catalysts with a low iron content [29]. After an activation period, the catalyst performance was observed to be stable at temperatures as high as 540–560 K within 2.5 h. The selectivity of benzene transformation to phenol in this temperature range is high (95–98%). At temperatures >570 K, a pronounced catalyst deactivation was observed due to coke formation [25,33]. In order to avoid the deactivation, the comparison of Fe-ZSM-5 and Fe-Beta catalysts activities was done at 543 K after 2 h on stream. The obtained phenol productivities R are collected in Table 1. The reaction rate R plotted against the amount of Fe(II) active sites active in CO oxidation, CCO Fe , is
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Fig. 4. Conversion of benzene to phenol over Fe-Betacalc 450 catalyst at different temperatures: mcat = 1.0 g; C6H6:N2O = 1:5; total gas flow = 60 ml min1.
presented in Fig. 5. As shown, R is directly proportional to CCO Fe obtained for the isomorphously substituted catalysts, suggesting the iron sites active in low temperature CO oxidation as responsible also for the catalysts activity in benzene hydroxylation. The observed slopes of the straight lines associated to Fe-ZSM-5 and Fe-Beta catalysts are slightly different. The reaction rate of benzene hydroxylation per active iron site (TOF) for Fe-ZSM-5 zeolites is 2-fold higher the TOF for FeBeta zeolites. Such a small difference indicates a similarity in the structures of Fe active sites stabilized in Beta and ZSM-5 zeolites and a weak influence of the zeolite host lattices. An interesting observation is that the catalysts prepared by the post-synthesis treatment (2.1% Fe-ZSM-5 and 1.9% FeBeta) do not follow the dependences described above (Fig. 5). Although they are able to generate a large amount of iron sites 19 1 active in CO oxidation (CCO Fe 1–2 10 site g ), the rate of benzene hydroxylation measured for these catalysts is relatively low (Table 1). This is probably due to Fe2O3 nanoparticles occurring in large quantities within micropores of post-synthesis treated zeolite [22,29], which limit access of ‘‘large’’ molecules of benzene (phenol) to the active sites hindering the catalyst performance. At the same time, the
Fig. 5. The rate of benzene to phenol oxidation as a function of iron sites concentration ðCCO Fe Þ determined via titration of loaded oxygen by CO: mcat = 1.0 g; C6H6:N2O = 1:5; total gas flow = 60 ml min1; T = 543 K.
activity of the 2.1% Fe-ZSM-5 catalysts was always higher as compared to the 1.9% Fe-Beta catalysts. An effect of the presence of NO in the reaction feed on the benzene hydroxylation was tested over the synthesized catalysts. As discussed above, NO adsorbs strongly on iron stabilized in zeolite micropores. At the same time, it was shown that adsorbed NOx species do not block Fe(II) sites for active oxygen (O)Fe loading and can accelerate N2O decomposition over Fe-ZSM-5 catalysts [11,27,59]. It was also demonstrated that the presence of NO inhibited low temperature reduction of N2O with CO over the Fe-ZSM-5 [60]. The dual role of NO, acting as a promoter in N2O decomposition and as an inhibitor in N2O reduction with CO, was explained by the presence of different (isolated and oligonuclear) iron species involved in the reaction mechanisms. In the latter case, NO adsorbs on specific Fe-sites responsible for CO activation, blocking them. On the base of the EPR spectra of benzene adsorbed on Fe-ZSM-5 zeolite, the authors [61] claimed that the (O)Fe does not participate in the formation of acceptor sites responsible for the ionization of benzene molecules. As seen in Fig. 6, 0.5 vol.% of NO inhibits the benzene hydroxylation over both Fe-ZSM-5 and Fe-Beta catalysts. The calc activity of the Fe-ZSM-5calc 350 falls to zero, while the Fe-Beta450 deactivates only partially. At the temperature studied direct decomposition of N2O to N2 and O2 does not take place even in the presence of adsorbed NO [62]. Scavenging of active oxygen species formed from N2O by NO with gaseous NO2 formation seems to be also insignificant. Therefore, under the reaction conditions NO molecules compete with benzene molecules for specific adsorption sites, thus influencing the reaction rate. These specific sites differ from the Fe(II) active sites responsible for the active oxygen (O)Fe formation, and their properties seem to depend on the host zeolite framework. Both MFI and BEA zeolite frameworks are able to generate similar (–Fe(II)–O–)n (probably (–Fe(II)–O–)n–Al–) sites of a low nuclearity responsible for the formation of surface atomic oxygen (O)Fe. At the same time, additional sites responsible for the activation of benzene molecules seem to exist in the vicinity of the (–Fe(II)–O–)n species. The nature of these sites is still unclear and requires further investigation. As mentioned above,
Fig. 6. Influence of NO on benzene to phenol oxidation over the catalysts at 543 K: mcat = 1.0 g; C6H6:N2O = 1:5; total gas flow = 60 ml min1.
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ZSM-5 and Beta zeolites are comprised of three-dimensional pore systems, consisting of intersecting sets of tubular channels with maximum free diameter of 0.58 and 0.70 nm, respectively. Since the maximum kinetic diameter of benzene molecules (0.58 nm) [63] is very close to channel diameters, the molecules are strongly distorted when penetrating through the zeolite frameworks. They are subjected to diverse energy fields: dispersion, repulsion, polarization, adsorption interaction, acid–base interaction, etc. Thus, the zeolite framework itself can be considered as a site responsible for the benzene activation. At the same time, strong acidic sites originating from the zeolite framework are also candidates for the activation of hydrocarbons [24,64]. Specific extra-framework Fe-containing species (isolated or fragments of oligonuclear ones) not participating in (O)Fe generation might be considered as well. Obviously, the mutual arrangement of the sites responsible for the formation of the surface atomic oxygen and the sites responsible for the benzene activation seem to determine the reaction rate. The geometry of the ensemble of active sites stabilized in Fe-ZSM-5 is likely to suit better to the reaction intermediates and results in the higher rate of benzeneto-phenol transformation as compared to Fe-Beta catalysts. 4. Conclusions (1) Both Fe-ZSM-5 and Fe-Beta zeolites generate Fe(II) active sites able to decompose N2O producing surface atomic oxygen (O)Fe and molecular nitrogen. A fraction of this oxygen is active in CO oxidation and the benzene hydroxylation by N2O to phenol. (2) The concentration of iron sites responsible for this activity can be measured by the transient response method during CO oxidation by atomic oxygen (O)Fe pre-loaded from N2O. (3) The reaction rate in the benzene hydroxylation with N2O per active iron site (TOF) was found 2-fold higher over FeZSM-5 catalysts as compared to Fe-Beta. These catalysts have probably Fe(II) active sites of similar structure. The observed difference in the activity indicates a weak influence of the zeolite host lattices as confirmed by the DRIFT spectra of NO adsorbed on iron at room temperature. (4) Active species responsible for the activity of Fe-Beta and Fe-ZSM-5 catalysts in benzene hydroxylation are likely to contain a (–Fe(II)–O)n–Al– fragment of a low nuclearity in the extra-framework positions in the zeolites micropores. The fragment is responsible for the surface atomic oxygen (O)Fe formation. At the same time, an additional site responsible for activation of benzene molecule seems to exist in the vicinity. (5) The geometry of the ensemble of active sites stabilized in Fe-ZSM-5 zeolites is likely to suit better to the reaction intermediates of benzene-to-phenol transformation. Acknowledgements The authors thank the Swiss National Science Foundation for the financial support. Participation of Martin Wettstein in experiments is highly appreciated.
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