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One step benzene oxidation to phenol. Part II: Catalytic behavior of Fe-(Al)MFI zeolites
Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
503
One step benzene oxidation to phenol. Part II: Catalytic behavior of Fe(AI)MFI zeolites S. Perathoner 1, F. Pin01, G. Centi l, G. Giordano 2, A. Katovic 2, J. B.Nagy3, K. Lazar 4, P. Fejes5 1 Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italy 2 Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria, 87030 Rende (CS), 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 B61a t6r 1, 6720 Szeged, Hungary
The catalytic behavior in gas phase hydroxylation of benzene to phenol with nitrous oxide on Fe-(A1)MFI zeolites prepared by different methods is reported. The amount of iron, thermal treatment, presence of aluminium ions and method of iron addition to the zeolite are all important parameters which determine the catalyst life time as well as the selectivity and productivity to phenol. Control of these parameters make it possible to improve the catalyst life-time and productivity to phenol. 1. INTRODUCTION Phenol is a large volume chemical (current worldwide production is about 7 million tons) with an expected market growth of about 6% per year, presently being used for various applications such as production of (i) phenolic resins (used in various areas expected to grow in the future such as the plywood, construction, automotive, and appliance industries) and (ii) of caprolactam and bisphenol A (intermediates in the manufacture of nylon and epoxy resins, respectively). The projected expansion of the market, however, is limited by the coproduction of acetone in the current commercial process via cumene as the intermediate, because the projected market for acetone is stationary or even decreasing. Therefore there is industrial interest in developing new processes of direct synthesis of phenol from benzene. There are two possible benzene hydroxylation processes, one in the liquid phase using H202 and the other one in the gas phase using N20 (the productivity to phenol using 02 as the oxidant is very low). A new recent suggestion is the one-step catalytic oxidation of benzene to phenol in the presence of a H2-O2 feed over palladium membrane catalysts [1 ]. H202 is generated over the Pd catalyst in the presence of a the H2-O2 mixture. The gas phase process using N20 has the advantage over the liquid phase process of using a reactant (N20) which can be recovered from chemical processes (adipic acid
504 production, in particular) or produced from waste by-products (NH4NO3). N20 is a powerful greenhouse gas [2] and its emission will soon be restricted. Therefore, its use as a reactant in phenol synthesis provides the multiple environmental benefit of (i) improved ecocompatibility of phenol production (better atom economy, and reduction of process complexity, waste and risks), (ii) reduction of greenhouse gas emissions and (iii) the reuse of waste. The AlphOx Process from Solutia [3,4] uses N20 produced from adipic acid to hydroxylate benzene to phenol which is then hydrogenated to cyclohexanone and reintroduced into the adipic acid synthesis cycle. The process thus offers the double benefit of avoiding N20 emissions and increasing the productivity to adipic acid. Fe-MFI catalysts are used in the reaction of benzene hydroxylation with N20, but a main limitation is the relatively fast deactivation of the catalysts. The development of the process is still in the pilot plant phase. No other classes of catalysts have been reported in literature having comparable performances to Fe-MFI catalysts. The main problem in developing Fe-MFI type catalysts for benzene hydroxylation with N20 is understanding the factors controlling the catalyst deactivation in order to improve catalyst lifetime. Even so, most part of studies reported in the literature on this reaction and class of catalysts, have been focused on the mechanistic investigation of the nature of the active sites responsible for the reaction of benzene hydroxylation with N20. Since deactivation is quite fast (of the order of hours) and reasonably different in terms of rate from catalyst to catalyst, comparison of the catalytic behavior without considering this factor is not reasonable. In addition, the fact that iron can be present as an impurity in the MFI catalysts has not been always taken into consideration. As a consequence, different and contrasting hypotheses have been reported on the nature of the sites responsible for the reaction of benzene hydroxylation with N20. Suzuki et al. [5], Burch et al., [6] and Gubelmann et al. [7] indicated that Bronsted acid sites are necessary for this reaction to proceed. Sobolev et al. [8] concluded,on the contrary, that Bronsted acidity is not required for this reaction, and attributed the catalyst activity only to iron sites in the extra-framework positions (indicated as or-sites). Panov et al. [9] showed that the number of or-sites and the catalyst reactivity in the benzene to phenol reaction are directly related to the amount of iron present in the catalyst, whereas Zhobolobenko [10] proposed that structural defects in the MFI zeolite framework (defects generated during the heat pretreatment of the catalyst) are the active centres which generate the active u-oxygen species upon interaction with N20. They also pointed out that extra-framework aluminium species form during the calcination of MFI catalysts. Motz et al. [11 ], studying MFI catalysts with a higher content of extra-framework aluminium species created by a steaming treatment, attributed the catalyst activity to the presence of Lewis acidity associated with this extraframework aluminium. Recently Pirutko et al. [12], studying the catalytic activity of catalysts prepared by introducing iron in pentasil-type zeolites with different compositions (B-MFI, A1-MFI, Ga-MFI, Ti-MFI), concluded that A1-MFI and Ga-MFI exhibit high activity even in the presence of 0.01-0.03% by wt. Fe, while B-MFI and Ti-MFI zeolites need 10 to 100 times more Fe. In order to clarify this question of the nature of the active sites as well as to obtain a better understanding of the factors controlling the rate of deactivation, the present study was focused on understanding the relationship between catalyst composition and rate of deactivation, but also taking into account the role of the reaction conditions which may also play a fundamental part in determining the deactivation behavior. Comparison of the data
505 obtained in this study with that on the nature of the iron species in the same catalysts (see part 1 [ 13]) should provide some indications on the role of iron species in the catalytic reaction.
2. EXPERIMENTAL 2.1 Catalyst Syntheses
Details on the method of preparation of the Fe-(A1)MFI samples have been previously reported [14] or described in part I of this work [13]. Fe was introduced both during hydrothermal synthesis of the zeolite, or by different post-synthesis methods. The characteristics of the samples and the code used for indicating them through out the text are summarized in Table 1. In the code, the subscript after the Fe symbol indicates the % w/w iron content of the zeolite and the subscript after the formula indicates the method used to introduce the iron into the zeolite structure: i.e. stands for "ion exchange", CVD for "Chemical Vapor Deposition" (often indicated as sublimation of FeC13), and ssr for "solid state reaction" while h.t. indicates that iron was directly introduced during hydrothermal synthesis. Post-synthesis introduction of iron was made using, as the parent zeolite, a commercial NH4+-MFI zeolite (from Alsi-Penta) with a SiO2/A1203 ratio of 25. H-ZSM5 was obtained from the same parent zeolite by calcination. The iron content in these samples is around 250 ppm. Usual practice was to calcine the dried catalysts at 550~ in air. Table 1. Characteristics of the Fe-(A1)MFI catab rsts used for the tests of benzene hydroxylation. Sample Code ~
Method o f addition o f Fe
Si/AI
H-ZSM5-C
-
13
H-MFI-S
-
> 1000
Fel.IMFIht-S
hydrothermal synthesis
> 1000
-
Fel.IMFIht-A55
hydrothermal synthesis
55
Fe2.2MFIht-A54
hydrothermal synthesis
54
Fe2.3MFIht-A90
hydrothermal synthesis
Fel.sMFIie-A13
ion exchange
Fe3.6MFIie-A13
Fe0.4MFIcvD-A13
AI/Fe
%Fe
(wt.)
Note
< 0.05
1.1
Commercial samples fi"omAlsi Penta (SN27) Pure Fe-fi,ee Hsilicalite-1 Pure Al-free Fesilicalite-1
1.48
1.1
(Fe-A1)ZSM5 sample
0.75
2.2
(Fe-A1)ZSM5 sample
90
0.91
2.2
(Fe-A1)ZSM5 sample
13
3.71
1.8
ion exchange
13
1.85
3.6
CVD*
13
16.7
0.4
200
....
Parent zeolite is commercial Alsi Penta (SN27)
FeC13 is mixed with the zeolite and heated in air * 9FeC13 sublimation. # C indicates commercial sample, S silicalite and A indicates the Si/A1 ratio.
Fe0.sMFIss~-A62
solid state reaction
62
1.97
0.8
2.2 Catalytic Tests
Before the catalytic tests the Fe-(A1)MFI samples were activated in-situ at a temperature ranging from 600 to 700~ in the presence or absence of steam. The catalytic tests were made in a fixed-bed reactor at a temperature typically of 400~ feeding a mixture containing (i) 3% benzene and 6% N 2 0 in h e l i u m o r (ii) 2 0 % benzene and 3% N 2 0 in helium.
506 Sometimes steam (up to about 3%) was also added to the feed. The total flow rate was 3 L/h and the amount of catalyst 0.Sg (contact time of 0.6 s g/ml). The feed was prepared using an already calibrated mixture of N20 in helium and adding benzene using an infusion pump and a vaporizer chamber. The feed could be sent either to the reactor or to a by-pass for its analysis. The feed coming out of the reactor or from the bypass could be sent to vent or to one of two parallel absorbers containing pure toluene as the solvent (plus calibrated amounts of tetrahydrofuran as the internal standard) cooled at about 15~ in order to condense all organic products. The line to the absorbers was heated at about 200~ in order to prevent condensation of the products. The vent, after condensation of the organic products, was sent to a sampling valve for analysis of the residual gas composition. The reactor outlet stream was sent alternatively to the two parallel absorbers for a given time (typically 3 or 5 rain), in order to monitor the change in the catalytic activity averaged over this time. N20, O2, N2 and total oxidation products (CO and CO2) were analyzed using TCD-Gas chromatography and a 60/80 Carboxen-1000 column, whereas benzene and phenol (as well as other minor aromatic by-products) were determined by FID-Gas chromatography using a ECONO-CAP SE-30 "wide bore" column or a Mass-GC equipped with a capillary Chrompack CP-Sil 5CB-MS Fused Silica column.
3. RESULTS AND DISCUSSION
3.1 Role of catalyst composition Reported in Figure 1 is the catalytic behavior (productivity and selectivity to phenol in benzene hydroxylation with N20) as a function of the time-on-stream of a series of Fe(Al)MFI type catalysts prepared by hydrothermal synthesis. The characteristics and composition of these samples are reported in Table 1. The samples were selected in order to summarize the influence of the composition of the catalyst on the activity and rate of deactivation. A commercial H-ZSM5 sample (H-ZSMS-C) showed effectively good activity, in agreement with literature data. However, it deactivated very quickly and in about 1 h the productivity decreased by a factor of about 50. The selectivity to phenol progressively decreases from an initial value of around 65-70% to less than 10%. It should be noted that similarly to most of the commercial samples, H-ZSMS-C contains traces of iron (see Table 1) due to contamination during the industrial preparation. When a pure Fe-free Silicalite-1 catalyst (H-MFI-S) was tested, no formation of phenol at all could be observed (Figure 1). Silicalite-1 has the same MFI structure as ZSM-5, but is A1flee. The synthesis of Silicalite leads to the presence of structural defects (hydroxyl nests), as confirmed by the presence of silanol groups evidenced by FTIR spectroscopy. Pretreatment of the sample at 700~ in helium before the catalytic tests causes dehydroxylation of these hydroxyl nests forming oxygen vacancies which may activate N20, because they behave as F-centres. In agreement, the catalyst shows an initial activity in N20 decomposition (around 22% N20 conversion aiter 5-10 min of time-on-stream). This suggests that defects in the zeolite can activate N20, but are unable to either form active oxygen species in benzene hydroxylation and/or to activate the organic substrate (benzene) for this reaction.
507 1,6 ,-'r "7 ..c -~
1,4
+ --O-~ --v--
1,2
F: E 1,0 ...... or-. 9 0,8 o
H-ZSM5-C Fez2MFI~-A54 H-MFI-S Fel.lMFI~-A55
+ Fel.IMFIht-S - - 0 - - Fe 2 aMFI~A90
0,6
._> 0,4 "o
~o
13. 0,2 0,0 0,0
0,5
1,0
1,5
2,0
2,5
Time, h
9
I
I
100
I
I
I
80 -.
~
0
60
o eeQ.
~
40
2o
0 0,o
.= ~v,
,
h
i 0,5
,
L~,
,
i
1,0
L
L
,
L
J
1,5
L
,
,
,
i
2,0
,
,
,
,v
i
J_
2,5
Time, h
Figure 1. Behavior of the catalysts as a function of the time-on-stream in the productivity and selectivity to phenol by benzene hydroxylation with N20. Catalysts pretreated at 700~ in helium for two hours. Reaction conditions: T=400~ 3% benzene, 6% N20, balance He. When an Al-free Fe-Silicalite (Fel.IMFIht-S) is used the productivity to phenol is low, but quite stable. No change in the productivity was observed up to about 20 hours of time-onstream. The selectivity to phenol is initially low and progressively increases up to final constant values of about 50%. The conversion of N20 is low (about 5%) and nearly constant with increasing time-on-stream. Therefore, in the absence of A1 the activity of the catalyst towards phenol synthesis is low, but remarkably stable, suggesting that A1 sites may play a role in the synthesis of phenol, but also in the side reactions leading to catalyst deactivation. When both Fe and A1 are present in the zeolite (Fel.lMFIht-A55 and Fe2.2MFIht-A54) the productivity to phenol increases by a factor of about 10 or 20, respectively and the selectivity
508 to phenol also markedly increases reaching values higher than 90% for Fel.lMFIht-A55. The productivity to phenol is approximately proportional to the amount of iron (compare phenol productivities for Fe~.tMFIht-A55 and Fe2.EMFIht-A54; the latter has twice the iron content and double the productivity of the former, while the Si/A1 content is the same). The selectivity to phenol, on the contrary, is higher in the case of the sample having the lower iron content. The trend with time-on-stream is quite similar in the two catalysts. The productivity to phenol decreases, although much less dramatically than in the case of HZSMS-C, while selectivity to phenol increases. The conversion of N20 is initially around 40% and then decreases to about 10-15% for Fe1.lMFIht-A55. When this result is compared with that of the sample having a comparable iron content, but no aluminium ions (Fel.lM~Iht-S), it is evident that the presence of A1 ions (or probably better sites related to A1, such as AI-OH Bronsted acid) together with iron ions is a condition necessary for efficient activation of N20. However, sites for the decomposition of N20 are also present which progressively become inactivated by the carbonaceous-type species formed on the catalyst, causing its deactivation. In fact, the rate of N20 conversion in Fel.lMFIht-A55 decreased by a factor of about 6-8 during the 2.5 hours of the experiments (Figure 1), while the productivity to phenol decreased by a factor of about 2. The side decomposition of N20 produces 02 (which is also detected in the reactor outlet) and probably 02 determines the total combustion of benzene, phenol or reaction intermediates and thus the formation of CO2 (CO is usually observed only in traces). This explains why in parallel to the decrease in N20 conversion, an increase in the selectivity to phenol is observed. With increasing iron content (Fe2.2MFIht-A54) the initial conversion of N20 is about 65% and then decreases to about 20-25% after 2.5h of time-on-stream. This indicates that the increase in iron determines an increase in the number of active sites for phenol synthesis (the productivity is about twice), but probably also increases the formation of a higher amount of a second type of iron species. Reasonably these species are small iron-oxide particles within the zeolite cavities which form in the process of partial migration of iron from framework to non-framework positions during the initial catalyst pretreatment. The increased amount of iron favours the process of aggregation of these iron species and thus a higher amount of aggregated iron oxide which is responsible for the decomposition of N20 to N2 + 02 instead of N2 + c~-O. As a consequence, the productivity of phenol in Fe2.2MFIht-A54 is higher than in Fel.lMFIht-A55, but the selectivity to phenol lower. Decreasing the amount of A1, while maintaining constant the amount of iron (compare Fe~.3MFIht-A90 with Fe2.2MFIht-A54) leads to an increase in both the productivity and selectivity, although the general trend remains again similar. This indicates that probably an A1/Fe ratio close to 1 is the optimal compromise to maximize both productivity and selectivity to phenol, suggesting that the active sites for phenol synthesis comprise both A1 and Fe. 3.2 Role of methodology in iron introduction in Fe-MFI catalysts In order to analyze in a concise way the effect of different methods of addition of iron in Fe-(A1)MFI samples on the catalytic performances (activity and deactivation), catalytic data have been summarized in Table 2. The following parameters are reported: (i) the maximum observed phenol productivity, which is generally obtained at the beginning of reaction or aiter about 20 minutes of time-on-stream (see Figure 1), (ii) the phenol productivity alter 2 hours, which gives an indication of the catalyst stability, and (iii) the selectivity in correspondence with the maximum phenol productivity and after 2 hours of time on stream.
509 Table 2 compares the behavior of some selected samples in which Fe was introduced postsynthesis by (i) an ion exchange method (Fel.sMFIie-A13 and Fe3.6MFIie-A13) using an aqueous solution of iron-ammonium-sulphate, (ii) CVD (contacting the anhydrated zeolite at 300~ with a flow of FeC13 in N2) and (iii) solid state reaction (FeC13 and the zeolite are mixed homogeneously and heated to 400~ For better comparison, data for two samples prepared by the hydrothermal method and containing both Fe and A1 (Fe1.1MFIht-A55 and Fe2.2MFIht-A54) as well as data for the parent zeolite used for ion-exchange and CVD (HZSM5-C) are also reported. Table 2. Phenol productivity and selectivity for Fe-(A1)MFI samples in which iron was introduced using different methods. Before benzene oxidation the catalysts were pretreated at 700~ in helium for two hours. Reaction conditions as in Figure 1. Selectivity3 Phenol productivity Sample Max1 2 h2 Max3 2h3 Fel.1MFIht-A54
0.38
0.18
81
92
Fe2.2MFIht-A55
0.81
0.62
56
69
H-ZSM5-C
0.44
0.02
61
11
Fel.sMFIie-A13
0.21
0.01
28
12
Fe3.6MFIie-A13
1.03
0#
78
17#
Fe0.aMFIcvo-A13
0.29
0.135
38
12
0.04 0# 7 1 Fe0.sMFIssr-A62 1 Maximumphenol productivity (mmolh"1g-l) at 400~ 2 Phenol productivity(mmol h~ g-l) at 400~ after 2 h of reaction 3 Phenol selectivity (%) at 400~ expressed on the total product basis in corrispondence to the maximum productivity and alter 2h of reaction. # The catalyst is completelydeactivatedjust after 1 h of reaction. The selectivitycorresponds therefore to a reaction time of 50 minutes $ The productivityto phenol does not further decrease in tests up to 20h of time-on-stream. The post-synthesis introduction of iron in the Fe-(A1)MFI catalysts generally leads to lower selectivities and productivities to phenol, although in some cases (e.g. Fe3.6MFIie-A13) good initial behavior is obtained. With respect to the behavior of the parent zeolite (HZSM5-C), the productivity to phenol increases more than twice and an increase in the selectivity to phenol could also be noted. However, similarly to H-ZSM5-C, after 1 h the productivity to phenol becomes negligible. When the iron content is lower, much poorer performances were observed, being higher the rate of side decomposition of N20. Therefore, the addition of iron by ion exchange could lead to catalysts having an initial activity better than the parent zeolite, but the effect of fast deactivation discussed before did not change. The addition of iron by CVD leads to not very selective catalysts due to a high rate of side N20 decomposition, but stable activity was noted after an initial decrease in phenol productivity. According to previous characterization, the CVD (sublimation) method leads to the formation of isolated or binuclear iron species [15], probably having characteristics similar to those of the active species responsible for the generation of the selective ~-O species during the interaction with N20. However, especially when the feed or the zeolite is
510 not fully anhydrated, nanoparticles of iron oxides also form [16], responsible for the decomposition of N20 and lowering of the selectivity. In agreement, the sample prepared by solid state reaction and in which the formation of the latter species is enhanced leads to very few selective and active catalysts in phenol formation. 4. CONCLUSIONS The Si/A1 and Si/Fe ratio in Fe-MFI, as well as also the method of addition of iron to the catalyst, have a marked influence on the catalyst activity and rate of deactivation during onestep oxidation of benzene to phenol. If deactivation is not taken into consideration, the comparison of different catalysts and the analysis of the structure-activity relationship may not be correct. Other factors not reported here due to space limitations determine the catalytic activity and stability, namely the temperature and atmosphere of pre-activation of the catalyst, the zeolite structure and the feed composition (N20/benzene ratio and their concentrations, and presence of H20 in the feed). These factors will be discussed in a subsequent manuscript. 5. REFERENCES
1. S. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Shoji, T. Namba, F. Mizukami, SCIENCE, 295 (2002): 105. 2. G. Centi, S. Perathoner, F. Vazzana, CHEMTECH, 29(12) (1999) 48. 3. A.S. Kharitonov, G.I. Panov, K.G. Ione, V.N. Romannikov, G.A. Sheveleva, L.A. Vostrikova, V.I. Sobolev, US Patent 5 110995 (1992). 4. P.P. Nott6, Topics in Catal., 13 (2000) 387. 5. E.Suzuki, K. Nakashiro, Y. Ono, Chem. Lett. (1988) 953. 6. R. Burch, C. Howitt, Appl. Catal. 86 (1992) 135. 7. M. Gubelmann, P.-J. Tirel, US Patent 5 001 280 and E.P. 0 341 113, both assigned to Rhone-Poulenc. (1988) 8. V.I. Sobolev, K.A. Dubkov, E.A. Paukshits, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov, G.I. Panov, Appl. Catal. A 141 (1996) 185. 9. G.I. Panov, V.I. Sobolev, A.S. Kharitonov, J. Mol. Catal. 61 (1990) 85. 10. Zhobolobenko, Mendeleev Commun., 28 (1993). 11. J.L. Motz, H. Heiniche, W. HSlderich, Studies in Surf. Sci. Catal., 105 (1997) 1053. 12. L.V. Pirutko, V.S. Chemyavsky, A.K. Uriate, G.I. Panov, Appl. Catal. A, 227 (2002) 143 13. G. Giordano, A. Katovic, S. Perathoner, F. Pino, G. Centi, J. B.Nagy, K. Lazar, P. Fejes, Stud. Surf. Sci. Catal., part I, this issue. 14. G. Centi, S. Perathoner, G. Romeo, Stud. Surf Sci. Catal., 135 (2001) 181. 15. B. Wichterlova, J. Dedecek, Z. Sobalik, in Catalysis by Unique Metal Ion Structures in Solid Matrices, G. Centi, B. Wichterlova, A. Bell Eds., NATO Science Series II Vol. 13, Kluwer Acad. Pub.: Dordrecht (The Netherlands), Ch. 3, p. 31. 16. G. Centi, F. Vazzana, Catal. Today, 53 (1999) 683.
503
One step benzene oxidation to phenol. Part II: Catalytic behavior of Fe(AI)MFI zeolites S. Perathoner 1, F. Pin01, G. Centi l, G. Giordano 2, A. Katovic 2, J. B.Nagy3, K. Lazar 4, P. Fejes5 1 Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italy 2 Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria, 87030 Rende (CS), 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 B61a t6r 1, 6720 Szeged, Hungary
The catalytic behavior in gas phase hydroxylation of benzene to phenol with nitrous oxide on Fe-(A1)MFI zeolites prepared by different methods is reported. The amount of iron, thermal treatment, presence of aluminium ions and method of iron addition to the zeolite are all important parameters which determine the catalyst life time as well as the selectivity and productivity to phenol. Control of these parameters make it possible to improve the catalyst life-time and productivity to phenol. 1. INTRODUCTION Phenol is a large volume chemical (current worldwide production is about 7 million tons) with an expected market growth of about 6% per year, presently being used for various applications such as production of (i) phenolic resins (used in various areas expected to grow in the future such as the plywood, construction, automotive, and appliance industries) and (ii) of caprolactam and bisphenol A (intermediates in the manufacture of nylon and epoxy resins, respectively). The projected expansion of the market, however, is limited by the coproduction of acetone in the current commercial process via cumene as the intermediate, because the projected market for acetone is stationary or even decreasing. Therefore there is industrial interest in developing new processes of direct synthesis of phenol from benzene. There are two possible benzene hydroxylation processes, one in the liquid phase using H202 and the other one in the gas phase using N20 (the productivity to phenol using 02 as the oxidant is very low). A new recent suggestion is the one-step catalytic oxidation of benzene to phenol in the presence of a H2-O2 feed over palladium membrane catalysts [1 ]. H202 is generated over the Pd catalyst in the presence of a the H2-O2 mixture. The gas phase process using N20 has the advantage over the liquid phase process of using a reactant (N20) which can be recovered from chemical processes (adipic acid
504 production, in particular) or produced from waste by-products (NH4NO3). N20 is a powerful greenhouse gas [2] and its emission will soon be restricted. Therefore, its use as a reactant in phenol synthesis provides the multiple environmental benefit of (i) improved ecocompatibility of phenol production (better atom economy, and reduction of process complexity, waste and risks), (ii) reduction of greenhouse gas emissions and (iii) the reuse of waste. The AlphOx Process from Solutia [3,4] uses N20 produced from adipic acid to hydroxylate benzene to phenol which is then hydrogenated to cyclohexanone and reintroduced into the adipic acid synthesis cycle. The process thus offers the double benefit of avoiding N20 emissions and increasing the productivity to adipic acid. Fe-MFI catalysts are used in the reaction of benzene hydroxylation with N20, but a main limitation is the relatively fast deactivation of the catalysts. The development of the process is still in the pilot plant phase. No other classes of catalysts have been reported in literature having comparable performances to Fe-MFI catalysts. The main problem in developing Fe-MFI type catalysts for benzene hydroxylation with N20 is understanding the factors controlling the catalyst deactivation in order to improve catalyst lifetime. Even so, most part of studies reported in the literature on this reaction and class of catalysts, have been focused on the mechanistic investigation of the nature of the active sites responsible for the reaction of benzene hydroxylation with N20. Since deactivation is quite fast (of the order of hours) and reasonably different in terms of rate from catalyst to catalyst, comparison of the catalytic behavior without considering this factor is not reasonable. In addition, the fact that iron can be present as an impurity in the MFI catalysts has not been always taken into consideration. As a consequence, different and contrasting hypotheses have been reported on the nature of the sites responsible for the reaction of benzene hydroxylation with N20. Suzuki et al. [5], Burch et al., [6] and Gubelmann et al. [7] indicated that Bronsted acid sites are necessary for this reaction to proceed. Sobolev et al. [8] concluded,on the contrary, that Bronsted acidity is not required for this reaction, and attributed the catalyst activity only to iron sites in the extra-framework positions (indicated as or-sites). Panov et al. [9] showed that the number of or-sites and the catalyst reactivity in the benzene to phenol reaction are directly related to the amount of iron present in the catalyst, whereas Zhobolobenko [10] proposed that structural defects in the MFI zeolite framework (defects generated during the heat pretreatment of the catalyst) are the active centres which generate the active u-oxygen species upon interaction with N20. They also pointed out that extra-framework aluminium species form during the calcination of MFI catalysts. Motz et al. [11 ], studying MFI catalysts with a higher content of extra-framework aluminium species created by a steaming treatment, attributed the catalyst activity to the presence of Lewis acidity associated with this extraframework aluminium. Recently Pirutko et al. [12], studying the catalytic activity of catalysts prepared by introducing iron in pentasil-type zeolites with different compositions (B-MFI, A1-MFI, Ga-MFI, Ti-MFI), concluded that A1-MFI and Ga-MFI exhibit high activity even in the presence of 0.01-0.03% by wt. Fe, while B-MFI and Ti-MFI zeolites need 10 to 100 times more Fe. In order to clarify this question of the nature of the active sites as well as to obtain a better understanding of the factors controlling the rate of deactivation, the present study was focused on understanding the relationship between catalyst composition and rate of deactivation, but also taking into account the role of the reaction conditions which may also play a fundamental part in determining the deactivation behavior. Comparison of the data
505 obtained in this study with that on the nature of the iron species in the same catalysts (see part 1 [ 13]) should provide some indications on the role of iron species in the catalytic reaction.
2. EXPERIMENTAL 2.1 Catalyst Syntheses
Details on the method of preparation of the Fe-(A1)MFI samples have been previously reported [14] or described in part I of this work [13]. Fe was introduced both during hydrothermal synthesis of the zeolite, or by different post-synthesis methods. The characteristics of the samples and the code used for indicating them through out the text are summarized in Table 1. In the code, the subscript after the Fe symbol indicates the % w/w iron content of the zeolite and the subscript after the formula indicates the method used to introduce the iron into the zeolite structure: i.e. stands for "ion exchange", CVD for "Chemical Vapor Deposition" (often indicated as sublimation of FeC13), and ssr for "solid state reaction" while h.t. indicates that iron was directly introduced during hydrothermal synthesis. Post-synthesis introduction of iron was made using, as the parent zeolite, a commercial NH4+-MFI zeolite (from Alsi-Penta) with a SiO2/A1203 ratio of 25. H-ZSM5 was obtained from the same parent zeolite by calcination. The iron content in these samples is around 250 ppm. Usual practice was to calcine the dried catalysts at 550~ in air. Table 1. Characteristics of the Fe-(A1)MFI catab rsts used for the tests of benzene hydroxylation. Sample Code ~
Method o f addition o f Fe
Si/AI
H-ZSM5-C
-
13
H-MFI-S
-
> 1000
Fel.IMFIht-S
hydrothermal synthesis
> 1000
-
Fel.IMFIht-A55
hydrothermal synthesis
55
Fe2.2MFIht-A54
hydrothermal synthesis
54
Fe2.3MFIht-A90
hydrothermal synthesis
Fel.sMFIie-A13
ion exchange
Fe3.6MFIie-A13
Fe0.4MFIcvD-A13
AI/Fe
%Fe
(wt.)
Note
< 0.05
1.1
Commercial samples fi"omAlsi Penta (SN27) Pure Fe-fi,ee Hsilicalite-1 Pure Al-free Fesilicalite-1
1.48
1.1
(Fe-A1)ZSM5 sample
0.75
2.2
(Fe-A1)ZSM5 sample
90
0.91
2.2
(Fe-A1)ZSM5 sample
13
3.71
1.8
ion exchange
13
1.85
3.6
CVD*
13
16.7
0.4
200
....
Parent zeolite is commercial Alsi Penta (SN27)
FeC13 is mixed with the zeolite and heated in air * 9FeC13 sublimation. # C indicates commercial sample, S silicalite and A indicates the Si/A1 ratio.
Fe0.sMFIss~-A62
solid state reaction
62
1.97
0.8
2.2 Catalytic Tests
Before the catalytic tests the Fe-(A1)MFI samples were activated in-situ at a temperature ranging from 600 to 700~ in the presence or absence of steam. The catalytic tests were made in a fixed-bed reactor at a temperature typically of 400~ feeding a mixture containing (i) 3% benzene and 6% N 2 0 in h e l i u m o r (ii) 2 0 % benzene and 3% N 2 0 in helium.
506 Sometimes steam (up to about 3%) was also added to the feed. The total flow rate was 3 L/h and the amount of catalyst 0.Sg (contact time of 0.6 s g/ml). The feed was prepared using an already calibrated mixture of N20 in helium and adding benzene using an infusion pump and a vaporizer chamber. The feed could be sent either to the reactor or to a by-pass for its analysis. The feed coming out of the reactor or from the bypass could be sent to vent or to one of two parallel absorbers containing pure toluene as the solvent (plus calibrated amounts of tetrahydrofuran as the internal standard) cooled at about 15~ in order to condense all organic products. The line to the absorbers was heated at about 200~ in order to prevent condensation of the products. The vent, after condensation of the organic products, was sent to a sampling valve for analysis of the residual gas composition. The reactor outlet stream was sent alternatively to the two parallel absorbers for a given time (typically 3 or 5 rain), in order to monitor the change in the catalytic activity averaged over this time. N20, O2, N2 and total oxidation products (CO and CO2) were analyzed using TCD-Gas chromatography and a 60/80 Carboxen-1000 column, whereas benzene and phenol (as well as other minor aromatic by-products) were determined by FID-Gas chromatography using a ECONO-CAP SE-30 "wide bore" column or a Mass-GC equipped with a capillary Chrompack CP-Sil 5CB-MS Fused Silica column.
3. RESULTS AND DISCUSSION
3.1 Role of catalyst composition Reported in Figure 1 is the catalytic behavior (productivity and selectivity to phenol in benzene hydroxylation with N20) as a function of the time-on-stream of a series of Fe(Al)MFI type catalysts prepared by hydrothermal synthesis. The characteristics and composition of these samples are reported in Table 1. The samples were selected in order to summarize the influence of the composition of the catalyst on the activity and rate of deactivation. A commercial H-ZSM5 sample (H-ZSMS-C) showed effectively good activity, in agreement with literature data. However, it deactivated very quickly and in about 1 h the productivity decreased by a factor of about 50. The selectivity to phenol progressively decreases from an initial value of around 65-70% to less than 10%. It should be noted that similarly to most of the commercial samples, H-ZSMS-C contains traces of iron (see Table 1) due to contamination during the industrial preparation. When a pure Fe-free Silicalite-1 catalyst (H-MFI-S) was tested, no formation of phenol at all could be observed (Figure 1). Silicalite-1 has the same MFI structure as ZSM-5, but is A1flee. The synthesis of Silicalite leads to the presence of structural defects (hydroxyl nests), as confirmed by the presence of silanol groups evidenced by FTIR spectroscopy. Pretreatment of the sample at 700~ in helium before the catalytic tests causes dehydroxylation of these hydroxyl nests forming oxygen vacancies which may activate N20, because they behave as F-centres. In agreement, the catalyst shows an initial activity in N20 decomposition (around 22% N20 conversion aiter 5-10 min of time-on-stream). This suggests that defects in the zeolite can activate N20, but are unable to either form active oxygen species in benzene hydroxylation and/or to activate the organic substrate (benzene) for this reaction.
507 1,6 ,-'r "7 ..c -~
1,4
+ --O-~ --v--
1,2
F: E 1,0 ...... or-. 9 0,8 o
H-ZSM5-C Fez2MFI~-A54 H-MFI-S Fel.lMFI~-A55
+ Fel.IMFIht-S - - 0 - - Fe 2 aMFI~A90
0,6
._> 0,4 "o
~o
13. 0,2 0,0 0,0
0,5
1,0
1,5
2,0
2,5
Time, h
9
I
I
100
I
I
I
80 -.
~
0
60
o eeQ.
~
40
2o
0 0,o
.= ~v,
,
h
i 0,5
,
L~,
,
i
1,0
L
L
,
L
J
1,5
L
,
,
,
i
2,0
,
,
,
,v
i
J_
2,5
Time, h
Figure 1. Behavior of the catalysts as a function of the time-on-stream in the productivity and selectivity to phenol by benzene hydroxylation with N20. Catalysts pretreated at 700~ in helium for two hours. Reaction conditions: T=400~ 3% benzene, 6% N20, balance He. When an Al-free Fe-Silicalite (Fel.IMFIht-S) is used the productivity to phenol is low, but quite stable. No change in the productivity was observed up to about 20 hours of time-onstream. The selectivity to phenol is initially low and progressively increases up to final constant values of about 50%. The conversion of N20 is low (about 5%) and nearly constant with increasing time-on-stream. Therefore, in the absence of A1 the activity of the catalyst towards phenol synthesis is low, but remarkably stable, suggesting that A1 sites may play a role in the synthesis of phenol, but also in the side reactions leading to catalyst deactivation. When both Fe and A1 are present in the zeolite (Fel.lMFIht-A55 and Fe2.2MFIht-A54) the productivity to phenol increases by a factor of about 10 or 20, respectively and the selectivity
508 to phenol also markedly increases reaching values higher than 90% for Fel.lMFIht-A55. The productivity to phenol is approximately proportional to the amount of iron (compare phenol productivities for Fe~.tMFIht-A55 and Fe2.EMFIht-A54; the latter has twice the iron content and double the productivity of the former, while the Si/A1 content is the same). The selectivity to phenol, on the contrary, is higher in the case of the sample having the lower iron content. The trend with time-on-stream is quite similar in the two catalysts. The productivity to phenol decreases, although much less dramatically than in the case of HZSMS-C, while selectivity to phenol increases. The conversion of N20 is initially around 40% and then decreases to about 10-15% for Fe1.lMFIht-A55. When this result is compared with that of the sample having a comparable iron content, but no aluminium ions (Fel.lM~Iht-S), it is evident that the presence of A1 ions (or probably better sites related to A1, such as AI-OH Bronsted acid) together with iron ions is a condition necessary for efficient activation of N20. However, sites for the decomposition of N20 are also present which progressively become inactivated by the carbonaceous-type species formed on the catalyst, causing its deactivation. In fact, the rate of N20 conversion in Fel.lMFIht-A55 decreased by a factor of about 6-8 during the 2.5 hours of the experiments (Figure 1), while the productivity to phenol decreased by a factor of about 2. The side decomposition of N20 produces 02 (which is also detected in the reactor outlet) and probably 02 determines the total combustion of benzene, phenol or reaction intermediates and thus the formation of CO2 (CO is usually observed only in traces). This explains why in parallel to the decrease in N20 conversion, an increase in the selectivity to phenol is observed. With increasing iron content (Fe2.2MFIht-A54) the initial conversion of N20 is about 65% and then decreases to about 20-25% after 2.5h of time-on-stream. This indicates that the increase in iron determines an increase in the number of active sites for phenol synthesis (the productivity is about twice), but probably also increases the formation of a higher amount of a second type of iron species. Reasonably these species are small iron-oxide particles within the zeolite cavities which form in the process of partial migration of iron from framework to non-framework positions during the initial catalyst pretreatment. The increased amount of iron favours the process of aggregation of these iron species and thus a higher amount of aggregated iron oxide which is responsible for the decomposition of N20 to N2 + 02 instead of N2 + c~-O. As a consequence, the productivity of phenol in Fe2.2MFIht-A54 is higher than in Fel.lMFIht-A55, but the selectivity to phenol lower. Decreasing the amount of A1, while maintaining constant the amount of iron (compare Fe~.3MFIht-A90 with Fe2.2MFIht-A54) leads to an increase in both the productivity and selectivity, although the general trend remains again similar. This indicates that probably an A1/Fe ratio close to 1 is the optimal compromise to maximize both productivity and selectivity to phenol, suggesting that the active sites for phenol synthesis comprise both A1 and Fe. 3.2 Role of methodology in iron introduction in Fe-MFI catalysts In order to analyze in a concise way the effect of different methods of addition of iron in Fe-(A1)MFI samples on the catalytic performances (activity and deactivation), catalytic data have been summarized in Table 2. The following parameters are reported: (i) the maximum observed phenol productivity, which is generally obtained at the beginning of reaction or aiter about 20 minutes of time-on-stream (see Figure 1), (ii) the phenol productivity alter 2 hours, which gives an indication of the catalyst stability, and (iii) the selectivity in correspondence with the maximum phenol productivity and after 2 hours of time on stream.
509 Table 2 compares the behavior of some selected samples in which Fe was introduced postsynthesis by (i) an ion exchange method (Fel.sMFIie-A13 and Fe3.6MFIie-A13) using an aqueous solution of iron-ammonium-sulphate, (ii) CVD (contacting the anhydrated zeolite at 300~ with a flow of FeC13 in N2) and (iii) solid state reaction (FeC13 and the zeolite are mixed homogeneously and heated to 400~ For better comparison, data for two samples prepared by the hydrothermal method and containing both Fe and A1 (Fe1.1MFIht-A55 and Fe2.2MFIht-A54) as well as data for the parent zeolite used for ion-exchange and CVD (HZSM5-C) are also reported. Table 2. Phenol productivity and selectivity for Fe-(A1)MFI samples in which iron was introduced using different methods. Before benzene oxidation the catalysts were pretreated at 700~ in helium for two hours. Reaction conditions as in Figure 1. Selectivity3 Phenol productivity Sample Max1 2 h2 Max3 2h3 Fel.1MFIht-A54
0.38
0.18
81
92
Fe2.2MFIht-A55
0.81
0.62
56
69
H-ZSM5-C
0.44
0.02
61
11
Fel.sMFIie-A13
0.21
0.01
28
12
Fe3.6MFIie-A13
1.03
0#
78
17#
Fe0.aMFIcvo-A13
0.29
0.135
38
12
0.04 0# 7 1 Fe0.sMFIssr-A62 1 Maximumphenol productivity (mmolh"1g-l) at 400~ 2 Phenol productivity(mmol h~ g-l) at 400~ after 2 h of reaction 3 Phenol selectivity (%) at 400~ expressed on the total product basis in corrispondence to the maximum productivity and alter 2h of reaction. # The catalyst is completelydeactivatedjust after 1 h of reaction. The selectivitycorresponds therefore to a reaction time of 50 minutes $ The productivityto phenol does not further decrease in tests up to 20h of time-on-stream. The post-synthesis introduction of iron in the Fe-(A1)MFI catalysts generally leads to lower selectivities and productivities to phenol, although in some cases (e.g. Fe3.6MFIie-A13) good initial behavior is obtained. With respect to the behavior of the parent zeolite (HZSM5-C), the productivity to phenol increases more than twice and an increase in the selectivity to phenol could also be noted. However, similarly to H-ZSM5-C, after 1 h the productivity to phenol becomes negligible. When the iron content is lower, much poorer performances were observed, being higher the rate of side decomposition of N20. Therefore, the addition of iron by ion exchange could lead to catalysts having an initial activity better than the parent zeolite, but the effect of fast deactivation discussed before did not change. The addition of iron by CVD leads to not very selective catalysts due to a high rate of side N20 decomposition, but stable activity was noted after an initial decrease in phenol productivity. According to previous characterization, the CVD (sublimation) method leads to the formation of isolated or binuclear iron species [15], probably having characteristics similar to those of the active species responsible for the generation of the selective ~-O species during the interaction with N20. However, especially when the feed or the zeolite is
510 not fully anhydrated, nanoparticles of iron oxides also form [16], responsible for the decomposition of N20 and lowering of the selectivity. In agreement, the sample prepared by solid state reaction and in which the formation of the latter species is enhanced leads to very few selective and active catalysts in phenol formation. 4. CONCLUSIONS The Si/A1 and Si/Fe ratio in Fe-MFI, as well as also the method of addition of iron to the catalyst, have a marked influence on the catalyst activity and rate of deactivation during onestep oxidation of benzene to phenol. If deactivation is not taken into consideration, the comparison of different catalysts and the analysis of the structure-activity relationship may not be correct. Other factors not reported here due to space limitations determine the catalytic activity and stability, namely the temperature and atmosphere of pre-activation of the catalyst, the zeolite structure and the feed composition (N20/benzene ratio and their concentrations, and presence of H20 in the feed). These factors will be discussed in a subsequent manuscript. 5. REFERENCES
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