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Nb-containing mesoporous molecular sieves — a possible application in the catalytic processes
Microporous and Mesoporous Materials 35–36 (2000) 195–207 www.elsevier.nl/locate/micromeso
Nb-containing mesoporous molecular sieves — a possible application in the catalytic processes Maria Ziolek *, Izabela Sobczak, Izabela Nowak, Piotr Decyk, Anna Lewandowska, Jolanta Kujawa A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland Received 8 April 1999; received in revised form 5 July 1999; accepted for publication 13 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science
Abstract Nb-containing MCM-41 mesoporous molecular sieves were modified with ammonium and copper cations and characterised by ESR, FTIR and H -TPR techniques. The prepared materials were tested in the following catalytic 2 reactions: NO decomposition, reduction of NO with NH , hydrosulphurisation of methanol, and oxidation of 3 thioethers with hydrogen peroxide. The oxidising character of the NbMCM-41 mesoporous molecular sieves was revealed, which induces their application as very active and selective catalysts in the oxidation of thioethers with H O to sulphoxides. HNbMCM-41 samples, which exhibit Lewis acidity after dehydroxylation, are selective catalysts 2 2 in the methanethiol synthesis in the reaction between methanol and hydrogen sulphide. Copper modified NbMCM-41 sieves indicate the higher reducibility of cupric cations than that observed when AlMCM-41 matrix for copper is used. The higher reducibility of cupric ions does not cause a higher activity in the catalytic NO decomposition. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Methanethiol synthesis; Nb-containing catalysts; NO decomposition and reduction; Thioether oxidation with H O 2 2
1. Introduction Nb-containing catalysts can exhibit a large variety of catalytic activities depending on the localisation of the Nb element and its surroundings. In catalysis, niobium compounds can play various functions, as follows: promoter or active phase, support, solid acid catalysts or redox materials [1– 3]. In this paper we will consider niobium catalysts based on mesoporous molecular sieves. They can contain Nb in the extra-framework position or in the skeleton of the sieves [4–8]. Depending on the * Corresponding author. Fax: +48-618-658-008. E-mail address: [email protected] (M. Ziolek)
Nb localisation and the activation conditions, the catalysts can reveal Brønsted acid, Lewis acid or redox properties. The Brønsted acidity is the strongest if niobic acid (Nb O · nH O) is formed 2 5 2 during the preparation and calcination of the material. To achieve high Brønsted acidic properties, the catalyst should be calcined at moderate temperatures (373–573 K ). A niobium oxide cationic species [NbO(5−2n)+ ], which occupies the n extra-framework cation position, plays the role of the Lewis acid site and could exhibit redox properties. Nb localised in the framework of the mesoporous MCM-41 sieves provokes Lewis acidity [8] and oxidising properties [9]. When NbMCM-41 mesoporous sieves are used
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as a support for a transition metal oxide species, one can expect a strong metal support interaction (SMSI ), as in the case of Nb O used as a support 2 5 [10]. Such SMSI affects the catalytic activity of the materials (mainly their redox properties). The redox behaviour of the Nb-containing support should enhance the redox properties of the reducible surface metal oxide species (e.g. Cu, V, Cr, etc.). In this paper, the mesoporous molecular sieves of MCM-41 type containing niobium mainly in the framework and their modified forms are studied in various catalytic reactions. The choice of the catalytic reactions was determined by the nature (known or expected) of the active sites on the prepared catalysts, and is as follows. 1. The reaction requiring the redox properties — the catalytic decomposition of NO [11]. 2. SCR reaction (NH +NO) in which Lewis acid 3 centres are involved [12]. 3. A process occurring on pairs of Lewis acid– base centres — the catalytic synthesis of methanethiol in the reaction between methanol and hydrogen sulphide [13]. 4. The oxidation reactions — as an example, the oxidation of thioethers with hydrogen peroxide was studied [14].
Table 1 Symbols and compositions of the catalysts used in this work Catalyst
Si/Ta ( T=Al, Nb, V )
Cu exchange (%)
CuZSM-5-31-96 CuAlMCM-41-32-132 CuNbMCM-41-32-112 CuNbMCM-41-32-161 NbMCM-41-16 NbMCM-41-32 NbMCM-41-64 HNbMCM-41-16 HNbMCM-41-32 HNbMCM-41-64 VMCM-41-32
31 32 32 32 16 32 64 16 32 64 32
96 132 112 161 – – – – – – –
a Represents precursor Si/Nb atomic ratio.
washing with decationated water and drying at 344 K for 2 h. In the case of copper-containing samples, the ion-exchange procedure carried out at room temperature (RT ) was repeated a few times, depending on the desired Cu exchange level, with Cu(CH COO) (0.2 M ) solution (pH 5.5–6). The 3 2 obtained samples, after filtration, were calcined at 673 K for 4 h. 2.2. H -TPR 2
2. Experimental 2.1. Catalysts Niobium was incorporated into the mesoporous molecular sieves of MCM-41 type during the synthesis carried out due to the description presented in Refs. [7,8]. The catalysts used in this work and their compositions are given in Table 1. Hydrogen forms of mesoporous molecular sieves of MCM-41 type were obtained via cation exchange with NH+ ions at 323 K and deammoni4 ation at 673 K, under vacuum or in helium flow depending on the experiments carried out. The ion exchange was performed using a conventional method, i.e. stirring of the solid in aqua solution of NH Cl (0.1 M, pH 6–6.5) for 8 h. The 4 prepared materials were recovered by filtration,
The temperature-programmed reduction ( TPR) of the samples was carried out using H /Ar 2 (10 vol%) as reductant (flow rate=32 cm3 min−1). 0.04 g of the sample was filled in a quartz tube, treated in a flow of helium at 723 K for 1 h and cooled to RT. It was then heated at a rate of 10 K min−1 to 1300 K under the reductant mixture. Hydrogen consumption was measured by a thermal conductivity detector in PulseChemiSorb 2705 (Micromeritics) apparatus. 2.3. FTIR measurements Infrared spectra were recorded with the Vector 22 (Bruker) FTIR spectrometer. The samples were pressed, under low pressure, into thin wafers of ~10 mg cm−2 and placed in the vacuum cell, where they underwent all activation and adsorption treatments. Spectra were recorded at RT. The
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background spectrum was scanned before determinations. This was automatically subtracted from all recorded spectra. Before the measurements, the samples were activated at 723 K for 3 h under vacuum. The IR spectra of the activated samples were subtracted from those recorded after NO adsorption at RT followed by various treatments. The reported spectra are the results of this subtraction.
2.4. ESR study The ESR measurements were conducted after evacuation of the catalyst at various temperatures (RT to 723 K ) and after NO adsorption at RT and desorption at various temperatures. ESR spectra were recorded at 77 K on a RADIOPAN SE/X 2547 spectrometer. The patterns were obtained at n =8.9 GHz. The g value was calcuESR lated according to the commonly used equation: g=hn/m B. B 2.5. NO decomposition and reduction with NH 3 The decomposition of nitric oxide was carried out in a glass flow-through reactor working at atmospheric pressure. The reaction conditions were as follows: 5 vol.% of NO in He was passed downward at a total flow rate of 10 cm3 min−1 through the reactor which contained 0.5 g of ZSM-5 or 0.2 g of MCM-41 materials (various weights of catalysts were used to obtain the same contact time, t$5.3 s). Before the reaction, catalysts were activated at 923 K in a helium flow (70 cm3 min−1) for 4 h. The analyses of NO and the reaction products were made with gas chromatography using a 5A molecular sieve or 13X+ Porapak Q columns working at 333 K. The catalytic reduction of NO with NH was 3 conducted at 473 K using a gas mixture of O (9 2 vol.%), NO (4000 ppm), NH (4200 ppm), and 3 balance He which was passed through the catalyst bed. The unconverted NO was oxidised and flow through the Salzman solution analysed with the spectrophotometric method.
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2.6. The reaction between methanol and hydrogen sulphide (the hydrosulphurisation process) The reactions between methanol and hydrogen sulphide were carried out in a flow system at 543 and 623 K, using H S:CH OH=2:1 molar ratio. 2 3 Portions of 0.2 g of dehydrated catalysts were used. The catalyst crystallites were tabletted without binder, ground, sieved to a 0.5 to 1.0 mm diameter range and activated for 4 h in situ in a pure and dried helium flow at 673 K. A mixture containing Merck research grade H S (3.0– 2 7.5 vol%), methanol (1.5–3.75 vol%) and helium as carrier gas was passed through the catalyst bed. The flow rate was 2.1–5.3×10−3 m3 h−1 depending on the desired contact time. The reagents and reaction products were analysed on-line using a gas chromatograph model SRI with a flame ionisation detector ( FID) and sulphur FPD detector. The catalytic activity is presented as per cent methanol conversion. 2.7. Oxidation of thioethers with hydrogen peroxide The catalytic reaction between thioethers and hydrogen peroxide was carried out in a glass flask equipped with a magnetic stirrer, thermocouple, reflux condenser and membrane for sampling. 0.04 g of a catalyst was placed in the flask where methanol was added. The oxidation was carried out simply by efficiently stirring at first a mixture of methanol (MeOH ) and the catalyst at 303 K. After stirring for ~1 h, 96% n-dibutyl sulphide (n-Bu S, 2 mmol ) was added, followed by the 2 dropwise addition of 35% hydrogen peroxide (2 mmol ) to achieve the stoichiometry under Eq. (1): Bu S+H O Bu SO+H O. 2 2 2 2 2
(1)
The reaction mixtures were analysed for 30 min each with a Chrom-5 chromatograph equipped with a packed column of Apiezon L (10 wt%) on Chromosorb W operated at 443 K and an FID. The first analysis was done after 10 min from the beginning of the reaction. The regeneration of the catalyst was carried out via calcination at 673 K.
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3. Results 3.1. Characterisation of the catalysts NbMCM-41 mesoporous sieves dehydroxylated during the activation at 673–723 K exhibit pairs of Lewis acid–base centres which are as follows:
(2) The presence of both active sites was proven by the FTIR measurements after NO adsorption [9].
species (denoted MMNbMO− in the text below) could play the role of Lewis base or oxidising centre. It was evidenced from NO adsorption experiments that this species exhibits a strongly oxidising character. Nitric oxide adsorbed on NbMCM-41 materials gives rise to the formation of nitrate and nitrite species observed in the IR spectra below 1600 cm−1 [9]. Simultaneous adsorption of NO and oxygen at room temperature on these catalysts leads to the production of nitrate species (the IR bands at 1417 and 1380 cm−1) which are strongly held on the surface even after evacuation at 673 K. If MMNbMO− plays the role of an oxidising centre, it ought to exhibit a radical character. Actually, in the ESR spectra of all niobium-containing mesoporous molecular sieves, paramagnetic centres were observed. Their character depends on the evacuation temperature. Fig. 1 demonstrates the ESR spectra recorded for NbMCM-41-32 evacuated at 573 and 723 K. The observed signals are not due to the paramagnetic Nb4+ centre, because for such a centre one can expect 10 ESR lines with g$1.89 [15]. Moreover, the oxidation state of Nb atoms in the framework of mesoporous MCM-41 was reported as Nb5+ due to the absence of ESR signal (Nb5+ exhibit diamagnetic properties) [16 ]. The evacuation at 573 K gives rise to a signal ( g=2.031 and 2.005) like that described in the literature for Nb O 2 5 doped TiO [17], which was assigned to the oxygen 2
Fig. 1. ESR spectra (recorded at 77 K ) of NbMCM-41-32 sample evacuated at 573 (a) and 723 K (b).
species formed by photo-irradiation of NbNO species which changed into NbMO− species. In the experiment described by Fig. 1, the following evacuation of the sample at 723 K reveals the arising of a sharp signal with g=1.997. Such a signal is characteristic for a hole centre generated by the oxygen present in the semiconductors [18] and was also described for niobium oxide evacuated at 773 K and interpreted as a hole mainly localised on an oxygen atom and near a niobium atom [19,20]. One cannot exclude the origin of this sharp band from carbon radicals formed from residual traces of a template [21]. The presence of Lewis acid sites (LAS) and Lewis basic sites (LBS ) or oxidising centres on the surface of the dehydroxylated NbMCM-41 mesoporous sieves [Eq. (2)] ought to influence the properties of copper cations incorporated into the extra-framework positions via cation exchange in the solution. One can compare the physico-chemical properties of CuNbMCM-41 with those of CuAlMCM-41 materials. The reducibility of copper cations in CuNbMCM-41 is much easier than that on CuAlMCM-41, as evidenced by the ESR study — Fig. 2. In both samples evacuated at RT, Cu2+ cations are coordinated with six H O molecules forming 2 an octahedral structure indicated by g =2.37 and d
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Fig. 2. ESR spectra (recorded at 77 K ) of CuAlMCM-41-32-132 (A) and CuNbMCM-41-32-112 (B) evacuated at various temperatures (indicated in the figure) and after NO adsorption.
A =138 G. When CuAlMCM-41-32-132 is evacud ated at temperatures above 373 K, this structure transforms to a tetrahedral coordinated one, described by g =2.31 and A =164 G [Fig. 2(A)]. d d Such a transformation is not so easily detectable on the CuNbMCM-41-32-112 sample, because the evacuation at 373 K and higher temperatures changes both parameters only slightly, i.e. towards g =2.34 and A =142 G [Fig. 2(B)]. The square d d planar structure found in the case of the evacuation of CuZSM-5 zeolites at higher temperatures [22] and indicated by g =2.27 and A =169 G was not d d observed on both Cu-containing mesoporous materials. Fig. 2(A) and (B) reveals the difference in Cu2+ reducibility depending on the matrix. The ESR signal from the paramagnetic copper almost completely disappears after evacuation of CuNbMCM-41-32-112 at 723 K, whereas it is preserved (with a lower intensity) after treatment of CuAlMCM-41-32-132 under the same conditions. The NO adsorption at RT on the CuNbMCM41-32-112 catalyst evacuated at 723 K causes a partial oxidation of the reduced copper species evidenced by an appearance of the ESR signal (the hyperfine structure) due to Cu2+ cations besides the signal from the Cu+NO complex. The adsorption of NO on the CuAlMCM-41-32-132 material gives rise to the formation of the Cu+NO complex,
but does not change the intensity of the ESR signal from Cu2+. The H -TPR results confirm the above men2 tioned behaviour of the easier reducibility of copper cations on CuNbMCM-41-32-112 than on the CuAlMCM-41-32-132 sample. Fig. 3 exhibits the results obtained on both kinds of material. There is no TPR peak from the reduction of
Fig. 3. H -TPR profiles 2 CuNbMCM-41-32-112.
of
CuAlMCM-41-32-132
and
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Cu2+Cu+ on the CuNbMCM-41-32-112 material because all Cu2+ cations have been already autoreduced during the activation. The assignment of the TPR peaks for this sample could be as follows: the low temperature (LT ) peak at 638 K is due to Cu+Cu0 reduction; at ~757 K a small shoulder is ascribed to extra-framework niobium species reduced with hydrogen; and a high temperature (HT ) peak at 1063 K originates from the reduction of framework niobium species [9]. The copper–metal (Cu0) species was detected in the reduced sample by the XRD study — the XRD pattern exhibits a peak at 2H#43.6° due to Cu0. The latter statement seems to be interesting because it indicates that the framework niobium can be reduced, and therefore one could consider its possible participation in the reactions in which the redox mechanism is involved. However, the reduction temperature of framework niobium is very high compared with that e.g. for copper cations. The H -TPR profile of the 2 CuAlMCM-41-32-132 catalyst shown in Fig. 3 exhibits only one broad peak which covers all copper reduction steps. One should point out that the temperature at which this peak grows is higher than the maximum temperature of the LT peak registered on the Nb-containing molecular sieve, indicating a lower reducibility of the copper species localised on the Al-containing matrix than that registered on the Nb-containing material.
Table 2 Activity of the mesoporous catalysts in the decomposition of NO Catalyst
TOFa×10−3 (s−1)
Temperature (K)
CuZSM-5-31-96 CuAlMCM-41-32-132 CuNbMCM-41-32-112 CuNbMCM-41-32-161 CuNbMCM-41-32-161
2.40 1.60 1.21 0.82 1.55
723 823 823 823 923
a TOF=number of NO molecules converted per Cu ion per second for temperatures at which a maximum conversion was reached (excluding the last sample for which two values at various temperatures are shown).
copper exchanged AlMCM-41 and NbMCM-41 mesoporous materials and for comparison on the CuZSM-5 zeolite. The lower activity of CuNbMCM-41 than that of CuAlMCM-41 is well illustrated by the FTIR study reported in Fig. 4. The formation of Cu+NO complex on the CuAlMCM-41-32-132 material (spectrum a) is clearly visible. This complex is evidenced by the IR band at 1813 cm−1. It is absent on CuNbMCM-41-32-112 under the same adsorption conditions (spectrum b). The latter
3.2. The catalytic decomposition of NO Most of the literature data stress the redox mechanism of the catalytic decomposition of NO (e.g. Refs. [11,23,24]) in which the copper cationic species plays the most important role. In our previous paper [25] it was shown that CuAlMCM-41 mesoporous molecular sieves are less active in the NO decomposition than CuZSM-5 zeolites. However, their activity was not too low and, taking into account the fact that mesoporous sieves do not exhibit Cu2+ in the square planar structure, commonly recognised as the active form in the NO decomposition, the activity of this catalyst shows that the other copper species are also active in this reaction. Table 2. displays turnover frequency ( TOF ) reached on the
Fig. 4. FTIR spectra of NO adsorbed at RT on: CuAlMCM-41-32-132, 0.5 mbar of NO (a); CuNbMCM41-32-112, 0.5 mbar of NO (b); the same catalyst, 1.5 mbar of NO (c); and NbMCM-41-32, 0.5 mbar of NO (d ).
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catalyst exhibits a small band at ~1813 cm−1 when a higher amount of NO is applied (spectrum c). On both catalysts, besides the above mentioned IR band, Cu2+(NO) (NO−) or nitrite/nitrate 2 species are detected at ~1612 and 1606 cm−1. The first species is formed with the participation of Cu2+O− active sites generated during the autoreduction of the catalyst [26 ]. The comparison of the FTIR spectra of NO adsorbed on the NbMCM-41 material (spectrum d) and that obtained on its modified form (CuNbMCM41-32-112, spectra b, c) indicates various origins of the bands at ~1630 and 1606 cm−1. The latter can be due to the bridging nitrate species described in the literature when NO was adsorbed on metal oxides [27,28]. It is most probably formed on two various active sites, i.e. Cu2+O− and MMNbMO−, and therefore is absent on the NbMCM-41-32 sample. The latter molecular sieve exhibits the band at ~1630 cm−1 which could be due to the nitrite species formed on niobium centres. 3.3. The SCR process The selective catalytic reduction (SCR) of NO with ammonia requires the presence of the Lewis acid centres involved in the chemisorption of NH , and some authors also stressed that the 3 redox centres taking part in the NO adsorption are also required [12]. Therefore, one could expect the activity of NbMCM-41 mesoporous sieves in this reaction because they exhibit either mentioned centres. The catalytic test showed the lower activity of both types of material in comparison with that noted for the commercial catalyst. The NO conversion at 473 K was as follows: 32% on NbMCM-41-16; 43% on NbMCM-41-32; and ~95% on the commercial catalyst. These results correlate well with the number of Lewis acidic centres, which is higher for the sample with Si/Nb=32 than that on the catalyst exhibiting Si/Nb=16 [8].
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iol (MeSH ) in the reaction between methanol (MeOH ) and hydrogen sulphide on faujasite type zeolites with alkali metal cations [13]: MeOH+H SMeSH+H O. (3) 2 2 Depending on the catalyst, dimethyl sulphide (Me S) can also be formed as follows: 2 2MeOH+H SMe S+2H O. (4) 2 2 2 NbMCM-41 molecular sieves which exhibit the above mentioned active centres were supposed to be selective catalysts in the methanethiol synthesis. Actually, our earlier study indicated the high selectivity of Nb-containing mesoporous molecular sieves in the reaction of methanol with H S 2 towards MeSH [7,29]. In this paper we wish to consider the possible reaction pathways. Therefore, a study in the region of very low methanol conversion was carried out, i.e. at 540 K (the methanol conversion was below 5%). Fig. 5 shows the changes in the selectivity to organic sulphur compounds depending on the contact time in the reaction performed on HNbMCM-41-16 at 540 K. The selectivity to MeSH increases with the growth of the contact time, whereas that to dimethyl sulphide is going through a maximum. It is important to point out the formation of products other than the organic sulphur one. They are: propane, propene, dimethyl ether and acetone. Especially interesting seems to be the formation of acetone, which can be produced in the oxohydra-
3.4. Hydrosulphurisation of methanol The pairs of LAS and LBS were stressed as the active sites in the selective formation of methaneth-
Fig. 5. The effect of the contact time on the selectivity to methanethiol and dimethyl sulphide in the reaction of methanol with hydrogen sulphide carried out at 540 K on HNbMCM-41-16.
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occurs according to the following scheme:
Fig. 6. The MeSH selectivity and yield variations versus the MeOH conversion in the reaction of methanol with hydrogen sulphide carried out at 623 K on HNbMCM-41-64 (a), HNbMCM-41-32 (b), HNbMCM-41-16 (c), and HAlMCM-41-16 (d ).
tion of propene. Water, whose presence is required in this reaction, is produced in the hydrosulphurisation process, and the active oxygen has to be on the surface of the catalyst. The formation of acetone was also detected when methanol dehydration was carried out under the same conditions as the hydrosulphurisation process. In this reaction, dimethyl ether is the main product (~55% selectivity), but ca. 30% selectivity to acetone was detected. In the catalytic experiments carried out at 623 K on various niobium-containing MCM-41 materials, both the activity and the selectivity to MeSH change with increasing niobium content (i.e. from Si/Nb=64 to Si/Nb=16). The MeOH conversion increases but the selectivity to MeSH decreases in this order. The results for HNbMCM-41 with various Si/Nb ratios and HAlMCM-41-16 are summarised in Fig. 6 showing the MeSH selectivity and the yield variations versus the MeOH conversion. It clearly evidences that the increase of niobium content in the hydrogen form of the catalyst improves the MeSH yield.
H2O2 H2O2 Bu S Bu SO Bu SO . (5) 2 2 2 2 sulphoxide sulphone The high selectivity to sulphoxide can be reached when the initial activity of the catalyst is high, because the formation of sulphone from sulphoxide is a slow reaction. The comparison of the activity of NbMCM-41 materials with various Si/Nb ratios with that of the VMCM-41-32 sample is shown in Fig. 7. The plot for the reaction carried out without the catalyst is also given. For both types of catalyst there is a characteristic absence of the inductivity period at the beginning of the reaction, usually observed when the other catalysts (e.g. Ti-containing materials) are used [30,31]. There is not a big difference in the Bu S conversion on all Nb-containing materials. 2 Only the catalyst with the lowest Nb content, i.e. NbMCM-41-64, exhibits a short inductivity period when the Bu S conversion increases from 78 to 2 88% in the first hour of the reaction. It was indicated in the literature [32] that vanadiumcontaining catalysts are more active in the oxidation of thioethers than Ti-containing materials, commonly recognised as the active catalysts in the reactions with hydrogen peroxide participation. Fig. 7 shows that the VMCM-41-32 sample does not exhibit an inductivity period like NbMCM-41-16 and -32 materials, but its activity is lower than that for the niobium catalysts. In our previous work [33], the possibility of the
3.5. Thioethers oxidation with hydrogen peroxide As an example of this type of oxidation process, the reaction between dibutyl sulphide (Bu S ) and 2 H O has been studied in this work. The reaction 2 2
Fig. 7. The conversion of dibutyl sulphide in the reaction with H O on various mesoporous molecular sieves. 2 2
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Fig. 8. The effect of the catalyst regeneration on the dibutyl sulphide conversion on Nb- and V-containing catalysts as well as without catalysts.
regeneration of Nb-containing catalysts was pointed out. Fig. 8 shows the influence of the regeneration of both V- and Nb-containing catalysts on the Bu S conversion. The hydrogen form 2 of the NbMCM-41 sieve shown in this figure exhibits a similar activity to the pure NbMCM-41 material. HNbMCM-41-32 does not change significantly its activity after regeneration, even if it was carried out twice. On the contrary, the VMCM-41-32 catalyst is not stable and loses its activity after the first regeneration process. The removal of vanadium from the solid was observed during the reaction and was evidenced by the yellow colour of the solution after the reaction. This colour indicates the presence of V complex with V5+ in octahedral coordination [34]. The removal of vanadium from the catalyst is a reason for the drastic decrease in Bu S conversion after 2 the regeneration procedure.
4. Discussion 4.1. Characterisation of the catalysts In the earlier paper [8], LAS were identified on the surface of the NbMCM-41 mesoporous molecular sieves and detected by IR spectroscopy after lutidine adsorption on the samples. The amount of LAS is higher when the dehydroxylation of hydrogen forms, i.e. HNbMCM-41 materials, is
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carried out. LAS are assigned to the cationic Nb species. Their formation is accompanied by the generation of the other species, denoted MMNbMO−, in which oxygen bonded to the lattice niobium exhibits a negative charge. One could expect that such a species plays the role of a Lewis basic site. However, at the moment, there is no strong evidence for a basic character of this species, whereas the ESR results presented in this paper (Fig. 1) indicate the radical character of the NbO− species. It is well known that O− radicals belong to the strongest oxidants [35]. The activated oxygen forms — like the neutral singlet O and the ionic 2 O− and O− species — are strongly electrophilic 2 reactants which attack the molecule in the region of its highest electron density. The oxidising properties of the MMNbMO− species in Nb-containing mesoporous molecular sieves were documented by NO adsorption as well as NO+O adsorption, 2 leading to the formation of nitrite and nitrate species registered in the FTIR spectra. These species are formed on the surfaces exhibiting an oxidising character. The oxidising properties of Nb- and HNbMCM-41 materials were also evidenced by their high activity in the reaction between Bu S 2 and H O , which will be discussed in one of the 2 2 following paragraphs. The comparison of the physico-chemical properties of NbMCM-41 and AlMCM-41 mesoporous molecular sieves indicates the fundamental difference of both kinds of material. Therefore, one can expect that their application as matrices for e.g. transition metal cations should reveal various properties of the catalysts. Indeed, the study of Cu-containing materials shows that copper incorporated into the extra-framework position of AlMCM-41 samples exhibits lower reducibility (during the activation and in H -TPR experiments) 2 than that registered for CuNbMCM-41 samples ( Fig. 3). That, of course, should influence their activity in various catalytic processes. 4.2. The catalytic decomposition and reduction of NO Similarly as in other Cu-containing molecular sieves, copper in CuNbMCM-41 is reduced during
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the activation under vacuum or in He (N ) flow 2 according to the scheme: 2Cu2+(OH )−Cu2+O−+Cu++H O. (6) 2 When Nb-containing matrix is applied, copper is more easily reduced than if Cu is introduced into the AlMCM-41 material, as evidenced by the results reported in Section 3.1. A negative charge located on MMNbMO− in the skeleton of the mesoporous material makes the copper reduction easier. The higher reducibility of copper in CuNbMCM-41 than in CuAlMCM-41 does not correlate in a simple way with the activity in NO decomposition. One could expect higher NO conversion on the CuNbMCM-41 sample thanks to the easier copper reduction. It is known that the reducibility of copper is an important parameter in the NO decomposition [36 ]. However, as the results presented in this paper show, if the reduction of Cu2+ is too easy as in CuNbMCM-41, a partial reduction towards Cu0 takes place. Therefore, the formation of Cu+NO complex — an active intermediate in the NO decomposition — is clearly visible on CuAl- but not on CuNbMCM-41 ( Fig. 4). The application of a higher copper exchange level (161%) in the NbMCM-41-32 material leads to an increase of the maximum conversion of NO, which is like that on CuAlMCM-41-32-132 but reached at a higher temperature, i.e. 923 K ( Table 2). The results presented reveal that not only Cu+NO complex takes part in the decomposition of NO, but also Cu2+OMNO and bridging nitrate species. NbMCM-41 materials, which after the activation (dehydration) exhibit a negative charge on the oxygen in the MMNbMO− species, interact more strongly with all copper cationic species than the AlMCM-41 matrix. Therefore, the copper species located in the NbMCM-41 matrix do not adsorb NO too strongly, which affects their activity in the NO decomposition. Vanadia-based catalysts are commonly used in NO SCR by NH [12] and there is a general x 3 agreement on the fact that the active sites are/contain vanadium ions. Many authors have considered the mechanism of this reaction, and the main question is whether the SCR reaction can occur on a single site or needs two adjacent sites,
as proposed by Wachs and coworkers [37,38]. Many data suggest that the reaction occurs on the Lewis acid sites, and most authors agree that NO does not necessarily adsorb before reacting. In the case of the other transition metal cations, such as Fe-, Cu-, Cr- and Mn-based catalysts, there is a better agreement among authors according to the reaction mechanism [12]. They think that the reaction occurs on the transition metal cationic centres (Lewis sites), where both ammonia and NO could adsorb and the adsorption of both is supposed to take place before the reaction. In the opinion of Busca et al. [12], there is no doubt that the reaction needs two functions of the catalyst, namely ‘redox’ and ‘acidic’. The results of NO SCR carried out on Nb-containing MCM-41 materials showed their lower activity to compare with the commercial vanadia-based catalysts. However, the relationship between the NO conversion and the number of Lewis acid sites in NbMCM-41 samples was well registered. The higher the LAS, as in NbMCM-41-32, the higher the activity found in NO SCR. 4.3. Hydrosulphurisation of methanol In the reaction between H S and methanol, the 2 main organic sulphur products are methanethiol and dimethyl sulphide. The routes of their formation may be different. It has been suggested that they are obtained via an independent path on zeolites [13,39–41] according to Eqs. (3) and (4). However, the view that dimethyl sulphide is a secondary product arising from methanethiol predominates [42]. This is based on the results obtained on metal oxides used as catalysts, and is also proposed for such zeolites as exhibit mainly LAS–LBS pairs of centres [13]. The formation of methoxy species which next react with H S to 2 form MeSH is stressed as the first reaction step. It is known that methoxy species are formed from methanol much more easily when the Brønsted acid sites of the catalysts participate in this process. In our earlier work [43] the chemisorption of MeOH was studied on Nb-containing MCM-41 catalysts using FTIR measurements. It has been pointed out that the dissociation of the methanol
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molecules occurs on all Nb-containing MCM-41 materials. The results presented in this paper suggest that dimethyl sulphide is either formed as the first product leading to the formation of thiol according to the following route: H2S 2MeOH Me SMeSH+HC (7) 2 or is formed in the independent path. Such a suggestion is based on the observed selectivity changes with increase of the contact time of reagents ( Fig. 5). The selectivity to MeSH increases, whereas that to Me S first increases and next 2 decreases with growth of the contact time. Moreover, with increasing methanol conversion the selectivity to MeSH decreases, suggesting that thiol is a final product in the reaction route. The study of the hydrosulphurisation process at lower temperatures gave an opportunity to find interesting oxohydration properties. Such properties were suggested based on the formation of acetone from propene (formed in the dehydration of methanol ) and oxygen (from the catalyst lattice) in the presence of water (formed in the dehydration of methanol ). This process points out the oxidising properties of NbMCM-41 materials. However, it needs a wider study to propose the reaction routes or mechanism. 4.4. Thioethers oxidation with hydrogen peroxide The catalytic oxidations can be divided into three basic categories [14]: (1) free-radical autooxidation; (2) oxidation of coordinated substrates followed by reoxidation of the reduced metal ion; and (3) catalytic oxygen transfer. The catalytic oxygen transfer is the reaction of an oxygen donor, for example H O or RO H, with an organic 2 2 2 substrate in the presence of metal catalysts. The active oxidant in such processes may be an oxometal ( XMMNO) or peroxometal (MMO R) 2 species formed in the reaction between a catalyst (MMX ) and an oxygen donor (RO H ). It was 2 stated that early transition elements ( Ti, Zr, Mo, W ) react via peroxometal intermediates. Very often it is difficult to distinguish between a heterolytic, Mars–van Krevelen type mechanism and competing homolytic pathways via radical intermediates (free radical auto-oxidation pathways).
205
The results presented in this paper do not allow the establishment of the pathway for the reaction between thioethers and hydrogen peroxide. It could be the peroxometal pathway, because any changes in the oxidation state of niobium (in NbMCM-41 mesoporous molecular sieves) were noticed. In such a case, ions increase the oxidising power of the peroxo group by acting as a Lewis acid. However, one cannot exclude the reaction route via radical intermediates, because the radical species have been found on the surface of the NbMCM-41 material (Fig. 1). The answer to the question about the reaction pathway in the thioether oxidation on Nb-containing sieves requires further study. For an industrially useful redox molecular sieve catalyst it is important that the metal is not leached from the internal surface of the molecular sieve during the reaction. Actually, a rapid leaching of metal ions is a general problem associated with the use of heterogeneous metal catalysts in liquidphase oxidations. This problem seems to be resolved by the application of NbMCM-41 mesoporous molecular sieves as catalysts in the oxidation with H O . Vanadium-containing catalysts, 2 2 which were recognised as the more active catalysts in the thioethers oxidation than Ti-containing samples, are not stable in the reaction with hydrogen peroxide. Neuman and Khenkin [44] have found leaching of vanadium from V-MCM-41 mesoporous molecular sieves during the oxidation of alkanes with H O . The same was 2 2 observed in our experiments when Bu S was oxi2 dised with hydrogen peroxide on VMCM-41-32. Niobium-containing MCM-41 mesoporous sieves are not only more active in the oxidation of Bu S with H O than the VMCM-41 catalyst 2 2 2 ( Fig. 7), but also more stable. As Fig. 8 indicates, the HNbMCM-41 catalyst can be regenerated a few times without changing its activity. This behaviour suggests that maybe niobium is not leached from the solid during the reaction with H O . It 2 2 will be checked precisely in our further study.
5. Conclusions $
The oxidising character of NbMCM-41 is probably due to the presence of Nb-oxygen radical species, which reveal electrophilic properties.
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NbMCM-41 mesoporous molecular sieves are attractive catalysts for the selective oxidation of thioethers to sulphoxides with H O thanks to 2 2 their high activity (higher than that of the VMCM-41 catalysts) and high stability (no leaching of Nb from the solid was detected during the reaction). The thioether oxidation pathway on Nb-containing MCM-41 catalysts can be via a peroxometal intermediate and/or a radical autooxidation route. The oxidising properties of Nb-containing MCM-41 catalysts in the presence of water were registered in the reactions where methanol was a reactant. Propene formed in the first step of the methanol decomposition was oxidised to acetone. NbMCM-41 mesoporous molecular sieves applied as a matrix for copper vary from AlMCM-41 and lead to easier reducibility of copper. However, this feature does not increase the activity of the catalyst in the NO decomposition. The Lewis acidity of Nb-containing mesoporous molecular sieves is responsible for the activity in the SCR with NH and the hydrosulphurisa3 tion of methanol. HNbMCM-41 catalysts exhibit a high selectivity of methanethiol formation in the reaction between methanol and hydrogen sulphide. It seems that methanethiol is produced in an independent reaction pathway or is the final product in the longer route of the reaction.
Acknowledgements This work was partially supported by the Polish Committee for Scientific Research ( KBN ) under Grant 3 T09A 099 12.
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Nb-containing mesoporous molecular sieves — a possible application in the catalytic processes Maria Ziolek *, Izabela Sobczak, Izabela Nowak, Piotr Decyk, Anna Lewandowska, Jolanta Kujawa A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland Received 8 April 1999; received in revised form 5 July 1999; accepted for publication 13 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science
Abstract Nb-containing MCM-41 mesoporous molecular sieves were modified with ammonium and copper cations and characterised by ESR, FTIR and H -TPR techniques. The prepared materials were tested in the following catalytic 2 reactions: NO decomposition, reduction of NO with NH , hydrosulphurisation of methanol, and oxidation of 3 thioethers with hydrogen peroxide. The oxidising character of the NbMCM-41 mesoporous molecular sieves was revealed, which induces their application as very active and selective catalysts in the oxidation of thioethers with H O to sulphoxides. HNbMCM-41 samples, which exhibit Lewis acidity after dehydroxylation, are selective catalysts 2 2 in the methanethiol synthesis in the reaction between methanol and hydrogen sulphide. Copper modified NbMCM-41 sieves indicate the higher reducibility of cupric cations than that observed when AlMCM-41 matrix for copper is used. The higher reducibility of cupric ions does not cause a higher activity in the catalytic NO decomposition. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Methanethiol synthesis; Nb-containing catalysts; NO decomposition and reduction; Thioether oxidation with H O 2 2
1. Introduction Nb-containing catalysts can exhibit a large variety of catalytic activities depending on the localisation of the Nb element and its surroundings. In catalysis, niobium compounds can play various functions, as follows: promoter or active phase, support, solid acid catalysts or redox materials [1– 3]. In this paper we will consider niobium catalysts based on mesoporous molecular sieves. They can contain Nb in the extra-framework position or in the skeleton of the sieves [4–8]. Depending on the * Corresponding author. Fax: +48-618-658-008. E-mail address: [email protected] (M. Ziolek)
Nb localisation and the activation conditions, the catalysts can reveal Brønsted acid, Lewis acid or redox properties. The Brønsted acidity is the strongest if niobic acid (Nb O · nH O) is formed 2 5 2 during the preparation and calcination of the material. To achieve high Brønsted acidic properties, the catalyst should be calcined at moderate temperatures (373–573 K ). A niobium oxide cationic species [NbO(5−2n)+ ], which occupies the n extra-framework cation position, plays the role of the Lewis acid site and could exhibit redox properties. Nb localised in the framework of the mesoporous MCM-41 sieves provokes Lewis acidity [8] and oxidising properties [9]. When NbMCM-41 mesoporous sieves are used
1387-1811/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 22 0 - 6
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as a support for a transition metal oxide species, one can expect a strong metal support interaction (SMSI ), as in the case of Nb O used as a support 2 5 [10]. Such SMSI affects the catalytic activity of the materials (mainly their redox properties). The redox behaviour of the Nb-containing support should enhance the redox properties of the reducible surface metal oxide species (e.g. Cu, V, Cr, etc.). In this paper, the mesoporous molecular sieves of MCM-41 type containing niobium mainly in the framework and their modified forms are studied in various catalytic reactions. The choice of the catalytic reactions was determined by the nature (known or expected) of the active sites on the prepared catalysts, and is as follows. 1. The reaction requiring the redox properties — the catalytic decomposition of NO [11]. 2. SCR reaction (NH +NO) in which Lewis acid 3 centres are involved [12]. 3. A process occurring on pairs of Lewis acid– base centres — the catalytic synthesis of methanethiol in the reaction between methanol and hydrogen sulphide [13]. 4. The oxidation reactions — as an example, the oxidation of thioethers with hydrogen peroxide was studied [14].
Table 1 Symbols and compositions of the catalysts used in this work Catalyst
Si/Ta ( T=Al, Nb, V )
Cu exchange (%)
CuZSM-5-31-96 CuAlMCM-41-32-132 CuNbMCM-41-32-112 CuNbMCM-41-32-161 NbMCM-41-16 NbMCM-41-32 NbMCM-41-64 HNbMCM-41-16 HNbMCM-41-32 HNbMCM-41-64 VMCM-41-32
31 32 32 32 16 32 64 16 32 64 32
96 132 112 161 – – – – – – –
a Represents precursor Si/Nb atomic ratio.
washing with decationated water and drying at 344 K for 2 h. In the case of copper-containing samples, the ion-exchange procedure carried out at room temperature (RT ) was repeated a few times, depending on the desired Cu exchange level, with Cu(CH COO) (0.2 M ) solution (pH 5.5–6). The 3 2 obtained samples, after filtration, were calcined at 673 K for 4 h. 2.2. H -TPR 2
2. Experimental 2.1. Catalysts Niobium was incorporated into the mesoporous molecular sieves of MCM-41 type during the synthesis carried out due to the description presented in Refs. [7,8]. The catalysts used in this work and their compositions are given in Table 1. Hydrogen forms of mesoporous molecular sieves of MCM-41 type were obtained via cation exchange with NH+ ions at 323 K and deammoni4 ation at 673 K, under vacuum or in helium flow depending on the experiments carried out. The ion exchange was performed using a conventional method, i.e. stirring of the solid in aqua solution of NH Cl (0.1 M, pH 6–6.5) for 8 h. The 4 prepared materials were recovered by filtration,
The temperature-programmed reduction ( TPR) of the samples was carried out using H /Ar 2 (10 vol%) as reductant (flow rate=32 cm3 min−1). 0.04 g of the sample was filled in a quartz tube, treated in a flow of helium at 723 K for 1 h and cooled to RT. It was then heated at a rate of 10 K min−1 to 1300 K under the reductant mixture. Hydrogen consumption was measured by a thermal conductivity detector in PulseChemiSorb 2705 (Micromeritics) apparatus. 2.3. FTIR measurements Infrared spectra were recorded with the Vector 22 (Bruker) FTIR spectrometer. The samples were pressed, under low pressure, into thin wafers of ~10 mg cm−2 and placed in the vacuum cell, where they underwent all activation and adsorption treatments. Spectra were recorded at RT. The
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background spectrum was scanned before determinations. This was automatically subtracted from all recorded spectra. Before the measurements, the samples were activated at 723 K for 3 h under vacuum. The IR spectra of the activated samples were subtracted from those recorded after NO adsorption at RT followed by various treatments. The reported spectra are the results of this subtraction.
2.4. ESR study The ESR measurements were conducted after evacuation of the catalyst at various temperatures (RT to 723 K ) and after NO adsorption at RT and desorption at various temperatures. ESR spectra were recorded at 77 K on a RADIOPAN SE/X 2547 spectrometer. The patterns were obtained at n =8.9 GHz. The g value was calcuESR lated according to the commonly used equation: g=hn/m B. B 2.5. NO decomposition and reduction with NH 3 The decomposition of nitric oxide was carried out in a glass flow-through reactor working at atmospheric pressure. The reaction conditions were as follows: 5 vol.% of NO in He was passed downward at a total flow rate of 10 cm3 min−1 through the reactor which contained 0.5 g of ZSM-5 or 0.2 g of MCM-41 materials (various weights of catalysts were used to obtain the same contact time, t$5.3 s). Before the reaction, catalysts were activated at 923 K in a helium flow (70 cm3 min−1) for 4 h. The analyses of NO and the reaction products were made with gas chromatography using a 5A molecular sieve or 13X+ Porapak Q columns working at 333 K. The catalytic reduction of NO with NH was 3 conducted at 473 K using a gas mixture of O (9 2 vol.%), NO (4000 ppm), NH (4200 ppm), and 3 balance He which was passed through the catalyst bed. The unconverted NO was oxidised and flow through the Salzman solution analysed with the spectrophotometric method.
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2.6. The reaction between methanol and hydrogen sulphide (the hydrosulphurisation process) The reactions between methanol and hydrogen sulphide were carried out in a flow system at 543 and 623 K, using H S:CH OH=2:1 molar ratio. 2 3 Portions of 0.2 g of dehydrated catalysts were used. The catalyst crystallites were tabletted without binder, ground, sieved to a 0.5 to 1.0 mm diameter range and activated for 4 h in situ in a pure and dried helium flow at 673 K. A mixture containing Merck research grade H S (3.0– 2 7.5 vol%), methanol (1.5–3.75 vol%) and helium as carrier gas was passed through the catalyst bed. The flow rate was 2.1–5.3×10−3 m3 h−1 depending on the desired contact time. The reagents and reaction products were analysed on-line using a gas chromatograph model SRI with a flame ionisation detector ( FID) and sulphur FPD detector. The catalytic activity is presented as per cent methanol conversion. 2.7. Oxidation of thioethers with hydrogen peroxide The catalytic reaction between thioethers and hydrogen peroxide was carried out in a glass flask equipped with a magnetic stirrer, thermocouple, reflux condenser and membrane for sampling. 0.04 g of a catalyst was placed in the flask where methanol was added. The oxidation was carried out simply by efficiently stirring at first a mixture of methanol (MeOH ) and the catalyst at 303 K. After stirring for ~1 h, 96% n-dibutyl sulphide (n-Bu S, 2 mmol ) was added, followed by the 2 dropwise addition of 35% hydrogen peroxide (2 mmol ) to achieve the stoichiometry under Eq. (1): Bu S+H O Bu SO+H O. 2 2 2 2 2
(1)
The reaction mixtures were analysed for 30 min each with a Chrom-5 chromatograph equipped with a packed column of Apiezon L (10 wt%) on Chromosorb W operated at 443 K and an FID. The first analysis was done after 10 min from the beginning of the reaction. The regeneration of the catalyst was carried out via calcination at 673 K.
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3. Results 3.1. Characterisation of the catalysts NbMCM-41 mesoporous sieves dehydroxylated during the activation at 673–723 K exhibit pairs of Lewis acid–base centres which are as follows:
(2) The presence of both active sites was proven by the FTIR measurements after NO adsorption [9].
species (denoted MMNbMO− in the text below) could play the role of Lewis base or oxidising centre. It was evidenced from NO adsorption experiments that this species exhibits a strongly oxidising character. Nitric oxide adsorbed on NbMCM-41 materials gives rise to the formation of nitrate and nitrite species observed in the IR spectra below 1600 cm−1 [9]. Simultaneous adsorption of NO and oxygen at room temperature on these catalysts leads to the production of nitrate species (the IR bands at 1417 and 1380 cm−1) which are strongly held on the surface even after evacuation at 673 K. If MMNbMO− plays the role of an oxidising centre, it ought to exhibit a radical character. Actually, in the ESR spectra of all niobium-containing mesoporous molecular sieves, paramagnetic centres were observed. Their character depends on the evacuation temperature. Fig. 1 demonstrates the ESR spectra recorded for NbMCM-41-32 evacuated at 573 and 723 K. The observed signals are not due to the paramagnetic Nb4+ centre, because for such a centre one can expect 10 ESR lines with g$1.89 [15]. Moreover, the oxidation state of Nb atoms in the framework of mesoporous MCM-41 was reported as Nb5+ due to the absence of ESR signal (Nb5+ exhibit diamagnetic properties) [16 ]. The evacuation at 573 K gives rise to a signal ( g=2.031 and 2.005) like that described in the literature for Nb O 2 5 doped TiO [17], which was assigned to the oxygen 2
Fig. 1. ESR spectra (recorded at 77 K ) of NbMCM-41-32 sample evacuated at 573 (a) and 723 K (b).
species formed by photo-irradiation of NbNO species which changed into NbMO− species. In the experiment described by Fig. 1, the following evacuation of the sample at 723 K reveals the arising of a sharp signal with g=1.997. Such a signal is characteristic for a hole centre generated by the oxygen present in the semiconductors [18] and was also described for niobium oxide evacuated at 773 K and interpreted as a hole mainly localised on an oxygen atom and near a niobium atom [19,20]. One cannot exclude the origin of this sharp band from carbon radicals formed from residual traces of a template [21]. The presence of Lewis acid sites (LAS) and Lewis basic sites (LBS ) or oxidising centres on the surface of the dehydroxylated NbMCM-41 mesoporous sieves [Eq. (2)] ought to influence the properties of copper cations incorporated into the extra-framework positions via cation exchange in the solution. One can compare the physico-chemical properties of CuNbMCM-41 with those of CuAlMCM-41 materials. The reducibility of copper cations in CuNbMCM-41 is much easier than that on CuAlMCM-41, as evidenced by the ESR study — Fig. 2. In both samples evacuated at RT, Cu2+ cations are coordinated with six H O molecules forming 2 an octahedral structure indicated by g =2.37 and d
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Fig. 2. ESR spectra (recorded at 77 K ) of CuAlMCM-41-32-132 (A) and CuNbMCM-41-32-112 (B) evacuated at various temperatures (indicated in the figure) and after NO adsorption.
A =138 G. When CuAlMCM-41-32-132 is evacud ated at temperatures above 373 K, this structure transforms to a tetrahedral coordinated one, described by g =2.31 and A =164 G [Fig. 2(A)]. d d Such a transformation is not so easily detectable on the CuNbMCM-41-32-112 sample, because the evacuation at 373 K and higher temperatures changes both parameters only slightly, i.e. towards g =2.34 and A =142 G [Fig. 2(B)]. The square d d planar structure found in the case of the evacuation of CuZSM-5 zeolites at higher temperatures [22] and indicated by g =2.27 and A =169 G was not d d observed on both Cu-containing mesoporous materials. Fig. 2(A) and (B) reveals the difference in Cu2+ reducibility depending on the matrix. The ESR signal from the paramagnetic copper almost completely disappears after evacuation of CuNbMCM-41-32-112 at 723 K, whereas it is preserved (with a lower intensity) after treatment of CuAlMCM-41-32-132 under the same conditions. The NO adsorption at RT on the CuNbMCM41-32-112 catalyst evacuated at 723 K causes a partial oxidation of the reduced copper species evidenced by an appearance of the ESR signal (the hyperfine structure) due to Cu2+ cations besides the signal from the Cu+NO complex. The adsorption of NO on the CuAlMCM-41-32-132 material gives rise to the formation of the Cu+NO complex,
but does not change the intensity of the ESR signal from Cu2+. The H -TPR results confirm the above men2 tioned behaviour of the easier reducibility of copper cations on CuNbMCM-41-32-112 than on the CuAlMCM-41-32-132 sample. Fig. 3 exhibits the results obtained on both kinds of material. There is no TPR peak from the reduction of
Fig. 3. H -TPR profiles 2 CuNbMCM-41-32-112.
of
CuAlMCM-41-32-132
and
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Cu2+Cu+ on the CuNbMCM-41-32-112 material because all Cu2+ cations have been already autoreduced during the activation. The assignment of the TPR peaks for this sample could be as follows: the low temperature (LT ) peak at 638 K is due to Cu+Cu0 reduction; at ~757 K a small shoulder is ascribed to extra-framework niobium species reduced with hydrogen; and a high temperature (HT ) peak at 1063 K originates from the reduction of framework niobium species [9]. The copper–metal (Cu0) species was detected in the reduced sample by the XRD study — the XRD pattern exhibits a peak at 2H#43.6° due to Cu0. The latter statement seems to be interesting because it indicates that the framework niobium can be reduced, and therefore one could consider its possible participation in the reactions in which the redox mechanism is involved. However, the reduction temperature of framework niobium is very high compared with that e.g. for copper cations. The H -TPR profile of the 2 CuAlMCM-41-32-132 catalyst shown in Fig. 3 exhibits only one broad peak which covers all copper reduction steps. One should point out that the temperature at which this peak grows is higher than the maximum temperature of the LT peak registered on the Nb-containing molecular sieve, indicating a lower reducibility of the copper species localised on the Al-containing matrix than that registered on the Nb-containing material.
Table 2 Activity of the mesoporous catalysts in the decomposition of NO Catalyst
TOFa×10−3 (s−1)
Temperature (K)
CuZSM-5-31-96 CuAlMCM-41-32-132 CuNbMCM-41-32-112 CuNbMCM-41-32-161 CuNbMCM-41-32-161
2.40 1.60 1.21 0.82 1.55
723 823 823 823 923
a TOF=number of NO molecules converted per Cu ion per second for temperatures at which a maximum conversion was reached (excluding the last sample for which two values at various temperatures are shown).
copper exchanged AlMCM-41 and NbMCM-41 mesoporous materials and for comparison on the CuZSM-5 zeolite. The lower activity of CuNbMCM-41 than that of CuAlMCM-41 is well illustrated by the FTIR study reported in Fig. 4. The formation of Cu+NO complex on the CuAlMCM-41-32-132 material (spectrum a) is clearly visible. This complex is evidenced by the IR band at 1813 cm−1. It is absent on CuNbMCM-41-32-112 under the same adsorption conditions (spectrum b). The latter
3.2. The catalytic decomposition of NO Most of the literature data stress the redox mechanism of the catalytic decomposition of NO (e.g. Refs. [11,23,24]) in which the copper cationic species plays the most important role. In our previous paper [25] it was shown that CuAlMCM-41 mesoporous molecular sieves are less active in the NO decomposition than CuZSM-5 zeolites. However, their activity was not too low and, taking into account the fact that mesoporous sieves do not exhibit Cu2+ in the square planar structure, commonly recognised as the active form in the NO decomposition, the activity of this catalyst shows that the other copper species are also active in this reaction. Table 2. displays turnover frequency ( TOF ) reached on the
Fig. 4. FTIR spectra of NO adsorbed at RT on: CuAlMCM-41-32-132, 0.5 mbar of NO (a); CuNbMCM41-32-112, 0.5 mbar of NO (b); the same catalyst, 1.5 mbar of NO (c); and NbMCM-41-32, 0.5 mbar of NO (d ).
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catalyst exhibits a small band at ~1813 cm−1 when a higher amount of NO is applied (spectrum c). On both catalysts, besides the above mentioned IR band, Cu2+(NO) (NO−) or nitrite/nitrate 2 species are detected at ~1612 and 1606 cm−1. The first species is formed with the participation of Cu2+O− active sites generated during the autoreduction of the catalyst [26 ]. The comparison of the FTIR spectra of NO adsorbed on the NbMCM-41 material (spectrum d) and that obtained on its modified form (CuNbMCM41-32-112, spectra b, c) indicates various origins of the bands at ~1630 and 1606 cm−1. The latter can be due to the bridging nitrate species described in the literature when NO was adsorbed on metal oxides [27,28]. It is most probably formed on two various active sites, i.e. Cu2+O− and MMNbMO−, and therefore is absent on the NbMCM-41-32 sample. The latter molecular sieve exhibits the band at ~1630 cm−1 which could be due to the nitrite species formed on niobium centres. 3.3. The SCR process The selective catalytic reduction (SCR) of NO with ammonia requires the presence of the Lewis acid centres involved in the chemisorption of NH , and some authors also stressed that the 3 redox centres taking part in the NO adsorption are also required [12]. Therefore, one could expect the activity of NbMCM-41 mesoporous sieves in this reaction because they exhibit either mentioned centres. The catalytic test showed the lower activity of both types of material in comparison with that noted for the commercial catalyst. The NO conversion at 473 K was as follows: 32% on NbMCM-41-16; 43% on NbMCM-41-32; and ~95% on the commercial catalyst. These results correlate well with the number of Lewis acidic centres, which is higher for the sample with Si/Nb=32 than that on the catalyst exhibiting Si/Nb=16 [8].
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iol (MeSH ) in the reaction between methanol (MeOH ) and hydrogen sulphide on faujasite type zeolites with alkali metal cations [13]: MeOH+H SMeSH+H O. (3) 2 2 Depending on the catalyst, dimethyl sulphide (Me S) can also be formed as follows: 2 2MeOH+H SMe S+2H O. (4) 2 2 2 NbMCM-41 molecular sieves which exhibit the above mentioned active centres were supposed to be selective catalysts in the methanethiol synthesis. Actually, our earlier study indicated the high selectivity of Nb-containing mesoporous molecular sieves in the reaction of methanol with H S 2 towards MeSH [7,29]. In this paper we wish to consider the possible reaction pathways. Therefore, a study in the region of very low methanol conversion was carried out, i.e. at 540 K (the methanol conversion was below 5%). Fig. 5 shows the changes in the selectivity to organic sulphur compounds depending on the contact time in the reaction performed on HNbMCM-41-16 at 540 K. The selectivity to MeSH increases with the growth of the contact time, whereas that to dimethyl sulphide is going through a maximum. It is important to point out the formation of products other than the organic sulphur one. They are: propane, propene, dimethyl ether and acetone. Especially interesting seems to be the formation of acetone, which can be produced in the oxohydra-
3.4. Hydrosulphurisation of methanol The pairs of LAS and LBS were stressed as the active sites in the selective formation of methaneth-
Fig. 5. The effect of the contact time on the selectivity to methanethiol and dimethyl sulphide in the reaction of methanol with hydrogen sulphide carried out at 540 K on HNbMCM-41-16.
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occurs according to the following scheme:
Fig. 6. The MeSH selectivity and yield variations versus the MeOH conversion in the reaction of methanol with hydrogen sulphide carried out at 623 K on HNbMCM-41-64 (a), HNbMCM-41-32 (b), HNbMCM-41-16 (c), and HAlMCM-41-16 (d ).
tion of propene. Water, whose presence is required in this reaction, is produced in the hydrosulphurisation process, and the active oxygen has to be on the surface of the catalyst. The formation of acetone was also detected when methanol dehydration was carried out under the same conditions as the hydrosulphurisation process. In this reaction, dimethyl ether is the main product (~55% selectivity), but ca. 30% selectivity to acetone was detected. In the catalytic experiments carried out at 623 K on various niobium-containing MCM-41 materials, both the activity and the selectivity to MeSH change with increasing niobium content (i.e. from Si/Nb=64 to Si/Nb=16). The MeOH conversion increases but the selectivity to MeSH decreases in this order. The results for HNbMCM-41 with various Si/Nb ratios and HAlMCM-41-16 are summarised in Fig. 6 showing the MeSH selectivity and the yield variations versus the MeOH conversion. It clearly evidences that the increase of niobium content in the hydrogen form of the catalyst improves the MeSH yield.
H2O2 H2O2 Bu S Bu SO Bu SO . (5) 2 2 2 2 sulphoxide sulphone The high selectivity to sulphoxide can be reached when the initial activity of the catalyst is high, because the formation of sulphone from sulphoxide is a slow reaction. The comparison of the activity of NbMCM-41 materials with various Si/Nb ratios with that of the VMCM-41-32 sample is shown in Fig. 7. The plot for the reaction carried out without the catalyst is also given. For both types of catalyst there is a characteristic absence of the inductivity period at the beginning of the reaction, usually observed when the other catalysts (e.g. Ti-containing materials) are used [30,31]. There is not a big difference in the Bu S conversion on all Nb-containing materials. 2 Only the catalyst with the lowest Nb content, i.e. NbMCM-41-64, exhibits a short inductivity period when the Bu S conversion increases from 78 to 2 88% in the first hour of the reaction. It was indicated in the literature [32] that vanadiumcontaining catalysts are more active in the oxidation of thioethers than Ti-containing materials, commonly recognised as the active catalysts in the reactions with hydrogen peroxide participation. Fig. 7 shows that the VMCM-41-32 sample does not exhibit an inductivity period like NbMCM-41-16 and -32 materials, but its activity is lower than that for the niobium catalysts. In our previous work [33], the possibility of the
3.5. Thioethers oxidation with hydrogen peroxide As an example of this type of oxidation process, the reaction between dibutyl sulphide (Bu S ) and 2 H O has been studied in this work. The reaction 2 2
Fig. 7. The conversion of dibutyl sulphide in the reaction with H O on various mesoporous molecular sieves. 2 2
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Fig. 8. The effect of the catalyst regeneration on the dibutyl sulphide conversion on Nb- and V-containing catalysts as well as without catalysts.
regeneration of Nb-containing catalysts was pointed out. Fig. 8 shows the influence of the regeneration of both V- and Nb-containing catalysts on the Bu S conversion. The hydrogen form 2 of the NbMCM-41 sieve shown in this figure exhibits a similar activity to the pure NbMCM-41 material. HNbMCM-41-32 does not change significantly its activity after regeneration, even if it was carried out twice. On the contrary, the VMCM-41-32 catalyst is not stable and loses its activity after the first regeneration process. The removal of vanadium from the solid was observed during the reaction and was evidenced by the yellow colour of the solution after the reaction. This colour indicates the presence of V complex with V5+ in octahedral coordination [34]. The removal of vanadium from the catalyst is a reason for the drastic decrease in Bu S conversion after 2 the regeneration procedure.
4. Discussion 4.1. Characterisation of the catalysts In the earlier paper [8], LAS were identified on the surface of the NbMCM-41 mesoporous molecular sieves and detected by IR spectroscopy after lutidine adsorption on the samples. The amount of LAS is higher when the dehydroxylation of hydrogen forms, i.e. HNbMCM-41 materials, is
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carried out. LAS are assigned to the cationic Nb species. Their formation is accompanied by the generation of the other species, denoted MMNbMO−, in which oxygen bonded to the lattice niobium exhibits a negative charge. One could expect that such a species plays the role of a Lewis basic site. However, at the moment, there is no strong evidence for a basic character of this species, whereas the ESR results presented in this paper (Fig. 1) indicate the radical character of the NbO− species. It is well known that O− radicals belong to the strongest oxidants [35]. The activated oxygen forms — like the neutral singlet O and the ionic 2 O− and O− species — are strongly electrophilic 2 reactants which attack the molecule in the region of its highest electron density. The oxidising properties of the MMNbMO− species in Nb-containing mesoporous molecular sieves were documented by NO adsorption as well as NO+O adsorption, 2 leading to the formation of nitrite and nitrate species registered in the FTIR spectra. These species are formed on the surfaces exhibiting an oxidising character. The oxidising properties of Nb- and HNbMCM-41 materials were also evidenced by their high activity in the reaction between Bu S 2 and H O , which will be discussed in one of the 2 2 following paragraphs. The comparison of the physico-chemical properties of NbMCM-41 and AlMCM-41 mesoporous molecular sieves indicates the fundamental difference of both kinds of material. Therefore, one can expect that their application as matrices for e.g. transition metal cations should reveal various properties of the catalysts. Indeed, the study of Cu-containing materials shows that copper incorporated into the extra-framework position of AlMCM-41 samples exhibits lower reducibility (during the activation and in H -TPR experiments) 2 than that registered for CuNbMCM-41 samples ( Fig. 3). That, of course, should influence their activity in various catalytic processes. 4.2. The catalytic decomposition and reduction of NO Similarly as in other Cu-containing molecular sieves, copper in CuNbMCM-41 is reduced during
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the activation under vacuum or in He (N ) flow 2 according to the scheme: 2Cu2+(OH )−Cu2+O−+Cu++H O. (6) 2 When Nb-containing matrix is applied, copper is more easily reduced than if Cu is introduced into the AlMCM-41 material, as evidenced by the results reported in Section 3.1. A negative charge located on MMNbMO− in the skeleton of the mesoporous material makes the copper reduction easier. The higher reducibility of copper in CuNbMCM-41 than in CuAlMCM-41 does not correlate in a simple way with the activity in NO decomposition. One could expect higher NO conversion on the CuNbMCM-41 sample thanks to the easier copper reduction. It is known that the reducibility of copper is an important parameter in the NO decomposition [36 ]. However, as the results presented in this paper show, if the reduction of Cu2+ is too easy as in CuNbMCM-41, a partial reduction towards Cu0 takes place. Therefore, the formation of Cu+NO complex — an active intermediate in the NO decomposition — is clearly visible on CuAl- but not on CuNbMCM-41 ( Fig. 4). The application of a higher copper exchange level (161%) in the NbMCM-41-32 material leads to an increase of the maximum conversion of NO, which is like that on CuAlMCM-41-32-132 but reached at a higher temperature, i.e. 923 K ( Table 2). The results presented reveal that not only Cu+NO complex takes part in the decomposition of NO, but also Cu2+OMNO and bridging nitrate species. NbMCM-41 materials, which after the activation (dehydration) exhibit a negative charge on the oxygen in the MMNbMO− species, interact more strongly with all copper cationic species than the AlMCM-41 matrix. Therefore, the copper species located in the NbMCM-41 matrix do not adsorb NO too strongly, which affects their activity in the NO decomposition. Vanadia-based catalysts are commonly used in NO SCR by NH [12] and there is a general x 3 agreement on the fact that the active sites are/contain vanadium ions. Many authors have considered the mechanism of this reaction, and the main question is whether the SCR reaction can occur on a single site or needs two adjacent sites,
as proposed by Wachs and coworkers [37,38]. Many data suggest that the reaction occurs on the Lewis acid sites, and most authors agree that NO does not necessarily adsorb before reacting. In the case of the other transition metal cations, such as Fe-, Cu-, Cr- and Mn-based catalysts, there is a better agreement among authors according to the reaction mechanism [12]. They think that the reaction occurs on the transition metal cationic centres (Lewis sites), where both ammonia and NO could adsorb and the adsorption of both is supposed to take place before the reaction. In the opinion of Busca et al. [12], there is no doubt that the reaction needs two functions of the catalyst, namely ‘redox’ and ‘acidic’. The results of NO SCR carried out on Nb-containing MCM-41 materials showed their lower activity to compare with the commercial vanadia-based catalysts. However, the relationship between the NO conversion and the number of Lewis acid sites in NbMCM-41 samples was well registered. The higher the LAS, as in NbMCM-41-32, the higher the activity found in NO SCR. 4.3. Hydrosulphurisation of methanol In the reaction between H S and methanol, the 2 main organic sulphur products are methanethiol and dimethyl sulphide. The routes of their formation may be different. It has been suggested that they are obtained via an independent path on zeolites [13,39–41] according to Eqs. (3) and (4). However, the view that dimethyl sulphide is a secondary product arising from methanethiol predominates [42]. This is based on the results obtained on metal oxides used as catalysts, and is also proposed for such zeolites as exhibit mainly LAS–LBS pairs of centres [13]. The formation of methoxy species which next react with H S to 2 form MeSH is stressed as the first reaction step. It is known that methoxy species are formed from methanol much more easily when the Brønsted acid sites of the catalysts participate in this process. In our earlier work [43] the chemisorption of MeOH was studied on Nb-containing MCM-41 catalysts using FTIR measurements. It has been pointed out that the dissociation of the methanol
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molecules occurs on all Nb-containing MCM-41 materials. The results presented in this paper suggest that dimethyl sulphide is either formed as the first product leading to the formation of thiol according to the following route: H2S 2MeOH Me SMeSH+HC (7) 2 or is formed in the independent path. Such a suggestion is based on the observed selectivity changes with increase of the contact time of reagents ( Fig. 5). The selectivity to MeSH increases, whereas that to Me S first increases and next 2 decreases with growth of the contact time. Moreover, with increasing methanol conversion the selectivity to MeSH decreases, suggesting that thiol is a final product in the reaction route. The study of the hydrosulphurisation process at lower temperatures gave an opportunity to find interesting oxohydration properties. Such properties were suggested based on the formation of acetone from propene (formed in the dehydration of methanol ) and oxygen (from the catalyst lattice) in the presence of water (formed in the dehydration of methanol ). This process points out the oxidising properties of NbMCM-41 materials. However, it needs a wider study to propose the reaction routes or mechanism. 4.4. Thioethers oxidation with hydrogen peroxide The catalytic oxidations can be divided into three basic categories [14]: (1) free-radical autooxidation; (2) oxidation of coordinated substrates followed by reoxidation of the reduced metal ion; and (3) catalytic oxygen transfer. The catalytic oxygen transfer is the reaction of an oxygen donor, for example H O or RO H, with an organic 2 2 2 substrate in the presence of metal catalysts. The active oxidant in such processes may be an oxometal ( XMMNO) or peroxometal (MMO R) 2 species formed in the reaction between a catalyst (MMX ) and an oxygen donor (RO H ). It was 2 stated that early transition elements ( Ti, Zr, Mo, W ) react via peroxometal intermediates. Very often it is difficult to distinguish between a heterolytic, Mars–van Krevelen type mechanism and competing homolytic pathways via radical intermediates (free radical auto-oxidation pathways).
205
The results presented in this paper do not allow the establishment of the pathway for the reaction between thioethers and hydrogen peroxide. It could be the peroxometal pathway, because any changes in the oxidation state of niobium (in NbMCM-41 mesoporous molecular sieves) were noticed. In such a case, ions increase the oxidising power of the peroxo group by acting as a Lewis acid. However, one cannot exclude the reaction route via radical intermediates, because the radical species have been found on the surface of the NbMCM-41 material (Fig. 1). The answer to the question about the reaction pathway in the thioether oxidation on Nb-containing sieves requires further study. For an industrially useful redox molecular sieve catalyst it is important that the metal is not leached from the internal surface of the molecular sieve during the reaction. Actually, a rapid leaching of metal ions is a general problem associated with the use of heterogeneous metal catalysts in liquidphase oxidations. This problem seems to be resolved by the application of NbMCM-41 mesoporous molecular sieves as catalysts in the oxidation with H O . Vanadium-containing catalysts, 2 2 which were recognised as the more active catalysts in the thioethers oxidation than Ti-containing samples, are not stable in the reaction with hydrogen peroxide. Neuman and Khenkin [44] have found leaching of vanadium from V-MCM-41 mesoporous molecular sieves during the oxidation of alkanes with H O . The same was 2 2 observed in our experiments when Bu S was oxi2 dised with hydrogen peroxide on VMCM-41-32. Niobium-containing MCM-41 mesoporous sieves are not only more active in the oxidation of Bu S with H O than the VMCM-41 catalyst 2 2 2 ( Fig. 7), but also more stable. As Fig. 8 indicates, the HNbMCM-41 catalyst can be regenerated a few times without changing its activity. This behaviour suggests that maybe niobium is not leached from the solid during the reaction with H O . It 2 2 will be checked precisely in our further study.
5. Conclusions $
The oxidising character of NbMCM-41 is probably due to the presence of Nb-oxygen radical species, which reveal electrophilic properties.
206 $
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NbMCM-41 mesoporous molecular sieves are attractive catalysts for the selective oxidation of thioethers to sulphoxides with H O thanks to 2 2 their high activity (higher than that of the VMCM-41 catalysts) and high stability (no leaching of Nb from the solid was detected during the reaction). The thioether oxidation pathway on Nb-containing MCM-41 catalysts can be via a peroxometal intermediate and/or a radical autooxidation route. The oxidising properties of Nb-containing MCM-41 catalysts in the presence of water were registered in the reactions where methanol was a reactant. Propene formed in the first step of the methanol decomposition was oxidised to acetone. NbMCM-41 mesoporous molecular sieves applied as a matrix for copper vary from AlMCM-41 and lead to easier reducibility of copper. However, this feature does not increase the activity of the catalyst in the NO decomposition. The Lewis acidity of Nb-containing mesoporous molecular sieves is responsible for the activity in the SCR with NH and the hydrosulphurisa3 tion of methanol. HNbMCM-41 catalysts exhibit a high selectivity of methanethiol formation in the reaction between methanol and hydrogen sulphide. It seems that methanethiol is produced in an independent reaction pathway or is the final product in the longer route of the reaction.
Acknowledgements This work was partially supported by the Polish Committee for Scientific Research ( KBN ) under Grant 3 T09A 099 12.
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