Catalytic properties of niobium and gallium oxide systems supported on MCM-41 type materials

Applied Catalysis A: General 325 (2007) 328–335 www.elsevier.com/locate/apcata

Catalytic properties of niobium and gallium oxide systems supported on MCM-41 type materials I. Nowak a, M. Misiewicz a, M. Ziolek a,**, A. Kubacka b,1, V. Corte´s Corbera´n c, B. Sulikowski b,* b

a Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, Poznan´, Poland Institute of Catalysis and Surface Chemistry, Niezapominajek 8, 30-239 Krako´w, Poland c Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Marie Curie 2, 28049 Madrid, Spain

Received 28 July 2006; accepted 7 February 2007 Available online 3 March 2007

Abstract Novel MCM-41 modified catalytic materials were synthesized by impregnation of MCM-41 with niobium and gallium salts. A number of techniques, including nitrogen adsorption, X-ray diffraction, FT-IR and Raman spectroscopies, have been used to characterize a series of gallium and niobium-containing composite materials, aimed especially at the rationalization of the nature of oxides species formed. Generally, the presence of highly dispersed NbOx and GaOx moieties in the mesoporous materials could be deduced from this approach. It has been demonstrated that incorporation of gallium and niobium into the MCM-41 type silica matrix leads to formation of active and selective catalysts for the oxidation of hydrocarbons. # 2007 Elsevier B.V. All rights reserved. Keywords: Acid–base and epoxidation properties; MCM-41; Ga2O3; Gallium–niobium interaction effect; Cyclization of acetonylacetone; Dehydration and dehydrogenation of 2-propanol; Oxidation of cyclohexene

1. Introduction Catalytic processes in the liquid phase are of a great interest for industry in the production of fine chemicals and pharmaceutical products. Environmental restrictions led to the tendency of substitution of the homogeneous catalytic processes by the heterogeneous ones. Hence many classical routes for chemical synthesis have been replaced by catalytic processes. A wide range of these reactions comprises, inter alia, oxidation processes. Among the oxidation processes, the liquid phase oxidation with hydrogen peroxide (a ‘‘clean’’ oxidant) plays a very important role. Recently, mesoporous materials containing niobium were pointed out as promising catalysts for the epoxidation of olefins [1–3]. Mesoporous niobiosilicates are preferred in the oxidation of cyclohexene due to their weak

* Corresponding author. Tel.: +48 12 6395 127; fax: +48 12 4251 923. ** Corresponding author. Tel.: +48 61 8291 243; fax: +48 61 8658 008. E-mail addresses: [email protected] (M. Ziolek), [email protected] (B. Sulikowski). 1 Present address: Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Marie Curie 2, 28049 Madrid, Spain. 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.029

acidity and consequently their low selectivity to the formation of diols from epoxides. On the other hand, gallium-containing metal oxides and zeolites can act as efficient catalysts in several industrial processes [4,5]. However, up to now the use of gallium-containing zeolites in the commercial Cyclar process [6] for the production of aromatics from light alkanes and alkenes is the best example, but it was also shown recently that Ga2O3 supported on MCM-41 is very active for Friedel–Crafts type benzylation and acylation reactions [7,8]. Galliumcontaining metal oxides and zeolites are active in hydrocarbon dehydrogenation and cyclization [9] or oxydehydrogenation [10], methane activation [11,12] and methanol to hydrocarbon conversion [13], and thus it was interesting to use them as a promoter or a support. The special properties (redox properties, photosensitivity, acidity and catalytic behaviour) of compounds containing niobium, such as niobium phosphate, niobia mixed oxides and niobium layered compounds are responsible for the strong motivation to understand and rationalize their properties and use them for various catalytic purposes [14–16]. The objective of this study was to prepare the bifunctional catalysts based on the mixed oxide systems containing the two elements, Ga and Nb. The gallium and niobium species were

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deposited on the surface of MCM-41 type materials, which were characterized and tested in the 2-propanol decomposition, acetonylacetone cyclization and epoxidation of cyclohexene. The performance of Ga and/or Nb containing MCM-41 samples was discussed in relation to those of the GaOx/Nb2O5 and NbOx/Ga2O3 oxide catalysts. 2. Experimental 2.1. Materials The Ga–Nb oxide systems were prepared by incipient wetness impregnation of niobium oxide, niobia (Alfa-Aesar), with Ga(NO3)310H2O (Aldrich), roughly corresponding to 0.24, 0.48, and 4.8 wt.% of Ga2O3 and equal to the 0.5, 1, and 10 theoretical monolayer(s), respectively. After impregnation the samples were dried at 373 K for 5 h and calcined at 773 K for 4 h in air. Similarly, Nb-promoted gallia (Ga2O3, Aldrich) materials were prepared using ammonium tris(oxalate) complex of niobium(V) (CBMM Brazil) solution, in order to introduce 0.23, 2.3, and 4.6 wt.% Nb2O5 (corresponding to the 0.1, 0.5, and 1 theoretical monolayer). The Ga- and Nbpromoted series of the samples are denoted as follows: Ga/ Nb2O5 A and Nb/Ga2O3 B, where the numeral A stands for the wt.% Ga2O3 and B represents the wt.% Nb2O5. The same incipient wetness impregnation procedure was applied for the preparation of Ga and Nb modified mesoporous molecular sieves of MCM-41 type. MCM-41 was prepared as described previously [17] using cetyltrimethylammonium chloride (CTACl) as the structure-directing agent. An 8.08 g of sodium silicate solution (Aldrich) was mixed with 50 g of water and 0.3 g of sulfuric acid (POCh, Poland). Then, 80.75 g of water CTACl 25 wt.% solution was added under vigorous stirring. The mixture was hydrothermally crystallized at 373 K for 2 days, then filtered off and washed with deionized water. After drying in air overnight, the product was calcined at 823 K for 12 h to remove the template. A post-synthesis modification was performed with Nb (Nb/MCM-41) or Ga (Ga/MCM-41) sources (Ga(NO3)310H2O or ammonium tris(oxalate) complex of niobium(V)) alone or both, step by step—first gallium followed by niobium impregnation for bimetallic catalysts (Nb/ Ga/MCM-41), in such a way that the final materials contained 4.7 wt.% Ga2O3 and/or Nb2O5 corresponding to Si/Nb = 45 or Si/Ga = 32 and Si/(Nb + Ga) = 18. After impregnation the samples were dried at 373 K for 5 h and calcined at 773 K for 4 h in air. 2.2. Characterization High-angle XRD patterns were collected on a TUR62 diffractometer using a monochromatic Cu Ka source (l = 0.154 nm) operated at 40 keV and 30 mA, running 2u from 48 to 608 with a step of 0.058 for crystalline phases observation. Phases were identified by matching the experimental patterns to the JCPDS powder diffraction file. Low-angle diffraction data were collected between 2u of 18 and 108 with a step of 0.028. The characteristic lattice parameter

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(the repeating distance ‘‘a’’ between two pore centers) was calculated from the following equation: a0 = 2d1 0 031/2. Nitrogen adsorption–desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 porosimeter. The volume of adsorbed N2 was normalized to standard temperature and pressure. Samples were outgassed at 573 K for 3 h before measurement, in order to remove water. The BET surface area was calculated with the cross-sectional area of nitrogen molecule taken as 0.162 nm2 by applying the BET equation for relative pressure between 0.05 and 0.20. The total pore volume (Vt) was obtained from the amount of nitrogen adsorbed at a relative pressure ( p/p0) of 0.99. The pore size distribution was calculated from the adsorption branch of nitrogen adsorption isotherms by using the KJS method, which employs in the classical BJH algorithm an accurate statistical film thickness and modified Kelvin equation, both verified on the basis of adsorption data for a series of high quality MCM-41 materials [18]. The mesopore pore volume (Vp) was calculated from the PSD distribution in the range of mesopores (2 nm < w < 50 nm), while primary mesopore diameter, w, was defined as maximum on the PSD [19]. The pore wall thickness t was then estimated from the average pore diameter (w) and the lattice parameter (a0) using the following equation [20]: t ¼ a0  0:95 w: Infrared spectra were recorded at 2 cm1 resolution and 128 scans in dried 0.5 wt.% KBr pellets on a Bruker Vector 22 spectrometer equipped with a globar lamp source, a KBr beam splitter, and a DTGS/KBr detector. All the FT-IR spectra were obtained under ambient conditions and in the 400–4000 cm1 range. Laser FT-Raman spectra of the studies samples were collected with a Bruker RFS100 spectrometer. A Nd:YAG laser was used as the excitation source (l = 1046 nm), the laser power was set to 100 mW. The sample was placed in a Raman cell, and the scattered light was detected with a diode-pumped germanium solid detector, cooled in liquid nitrogen. The correction of background due to the Rayleigh scattering and the correction for white light were performed by use of the Bruker software. 2.3. Catalytic studies The 2-propanol (IPA) decomposition (dehydration and dehydrogenation) process was studied by using a microcatalytic pulse reactor inserted between the sample inlet and the column of a CHROM-5 chromatograph. A series of pulses of 2propanol (Aldrich, 5 ml) were injected into the catalyst at different reaction temperatures 423–573 K (at 50 K intervals). The catalytic measurements were performed under the following conditions: catalyst weight, 0.05 g; flow rate of helium carrier gas, 40 cm3 min1. Before the experiments, the samples were pretreated by in situ heating under helium (40 cm3 min1) for 2 h at 673 K. The 2-propanol pulses were injected successively till constant values of conversion and yield of products were attained (usually 3–4 pulses). The final

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values reported are the mean of values obtained in three or four successive pulses. The reaction products: propene, 2-propanone and diisopropyl ether, were analyzed by GC with a TCD. The column was 2 m long packed with 4 wt.% Carbowax 400 on Gas Chrom R (Altech) and operated at 353 K. Both the reactor and quartz wool were found to have negligible activity under the experimental conditions used. The materials were also tested in acetonylacetone (AcAc) cyclization as a probe reaction. A tubular, down-flow reactor surrounded by an electric heater was used in experiments which were carried out at atmospheric pressure using nitrogen as the carrier gas. The catalyst bed (0.05 g) was first activated for 2 h at 723 K under nitrogen flow (40 cm3 min1). Afterwards, a 0.5 cm3 of acetonylacetone (Fluka, GC grade) was being passed continuously (flow rate 1 cm3 h1) over the catalyst at 623 K. The substrate was delivered with a kdScientific pump system and vapourized before it was passed through the catalyst bed in the presence of a flow of nitrogen carrier gas (40 cm3 min1). Reaction products were collected downstream of the reactor in the cold trap (solid CO2) and analyzed with a CE InstrumentsGC8000Top chromatograph equipped with a 30 m capillary column DB-1 (1.5 mm, 30 m  0.53 mm). The oxidation of cyclohexene was carried out in a glass reactor (two-neck flask) under reflux working in the batch mode. The general procedure was as follows: catalyst (0.04 g) was added to 10 cm3 of acetonitrile. The resulting suspension was magnetically stirred at 318 K for 15 min before adding separately an equimolar mixture of 0.2 cm3 cyclohexene (2 mmol) and 0.17 cm3 hydrogen peroxide (2 mmol). The reaction was kept at 318 K using a water bath and was performed under atmospheric pressure. The first analysis was done after 30 min from the beginning of the reaction. The reaction mixture was injected into a gas chromatograph (CE Instruments-GC8000Top) and analyzed by FID. The products were examined using a capillary column of DB-1. Moreover, the GC–MS (AMD402) analyses were performed in order to identify the reaction products. 3. Results and discussion Important information relevant to the discussion of the location of niobium and gallium in MCM-41 type materials is provided by the few techniques, e.g. low temperature N2 physisorption isotherms, X-ray diffraction, IR and Raman spectroscopies. These techniques are also useful for studying the structural changes occurring after the impregnation. The parent MCM-41 shows (Fig. 1) a typical stepped type IV isotherm with two regions of strong nitrogen uptake. The first step at low relative pressure of about 0.3 corresponds to the capillary condensation of N2 within the primary mesochannels of MCM-41, while the second one seen at high relative pressure (above 0.8) is attributed to the capillary condensation of nitrogen in larger secondary pores (presumably these are interparticle voids) [21]. The KJS-BJH analysis for the physisorption of N2 on the mesoporous materials gives a remarkable narrow pore size distribution with a pore size of ca. from 3.3 to 3.6 nm. The pores

Fig. 1. N2 physisorption isotherms of MCM-41 composites with different heteroatom (Nb, Ga) loadings.

size distribution is only slightly affected by the modification. The sharp pore size distribution, with a ca. 0.6–0.8 nm full width at half-height—FWH (Table 1), shows that the mesopores are indeed very uniform. However, the FWH decreases after introduction of Nb and/or Ga species. The BET surface area measured for the calcined support was 940 m2 g1, which decreases after the modification with one metal down to 840 m2 g1 and further to 800 m2 g1 after introducing the second metal. Moreover, the main conclusions which may be drawn from the results in Table 1 are that when Ga and Nb are introduced, the average channel pore size appear to change, however only a small fraction (10% for mono- and 20% for bimetallic samples, respectively) of the channel space, i.e. mesopore volume, declines, while the wall thickness increases. Niobium and gallium species coat the walls as the wall thickness was inferred, and thus metallic species are wetting the silica inner surface. The low-angle XRD patterns of calcined MCM-41, Nb/ MCM-41, Ga/MCM-41, and Nb/Ga/MCM-41 are shown in Fig. 2 and all display four reflection peaks at ca. 2.4, 4, 4.6, and 6.2 that can be indexed to the (1 0 0), (1 1 0), (2 0 0), and (2 1 0) diffraction lines characteristic of the hexagonal structure. From the d(1 0 0)-spacing one can calculate the lattice constant 4.3 nm, which also denotes the distance between the centers of adjacent channels. The data indicate that the parent calcined MCM-41 silica is of high quality, and that the ordered hexagonal mesostructure remains intact after adding gallium and/or niobium. Moreover, it is also interesting to note that the unit cell parameter decreased in the following way: SiMCM41 > Nb/SiMCM-41 ffi Ga/SiMCM-41 > Nb/Ga/MCM-41, that confirms the partial obstruction of the mesochannels by the incorporated species. The intensity of the three less intense

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Table 1 Structural/textural data of MCM-41 before and after introducing metal species Material

a0

BET surface area (m2 g1)

Vt (cm3 g1)

Vp (cm3 g1)

w (nm)

t (nm)

FWH (nm)

MCM-41 Nb/MCM-41 Ga/MCM-41 Nb/Ga/MCM-41

4.32 4.25 4.25 4.18

940 830 840 800

0.84 0.70 0.72 0.65

0.78 0.65 0.66 0.59

3.6 3.3 3.3 3.3

0.89 0.99 0.98 1.04

0.77 0.68 0.64 0.58

a0, lattice parameter; Vt, total pore volume; Vp, mesopore volume; w, pore width; t, wall thickness; FWH, full width at half-height of the pore volume peak in PSD.

reflections gradually decreases after addition of niobium or gallium. Thus one might conclude that there is only partial filling of the channels after single impregnation. However, the peak intensities for the bimetallic sample were significantly lowered as compared with those corresponding to the purely siliceous MCM-41, thus suggesting that the long-range regularity of the hexagonal arrays of mesopores of MCM-41 was affected to higher extend after the introduction of two metal species. This was also clearly seen in the drop in pore volume and surface area (Table 1). Direct characterization of the extra lattice niobium and gallium oxide phases generated by calcination proved to be very difficult probably on account of their small size and fine deposition in the silica matrix. Both XRD in the wide-angle range and IR spectroscopy did not allow detecting any oxide particles in the samples, indicating that the latter must be amorphous and/or smaller in size than few nanometers. X-ray diffraction patterns of niobium oxide, Nb2O5, and gallium oxide, Ga2O3, modified with Ga and Nb species, respectively, indicated a presence of the monoclinic b-Ga2O3 form [22] and a pseudohexagonal Nb2O5 phase [23,24]. In Fig. 3, X-ray diffraction patterns of niobia and gallia modified

with Nb or Ga and the corresponding pristine phases are presented. XRD patterns showed that gallium or niobium incorporation did not induce any change in crystallinity of the samples in comparison with pure materials. Moreover, the diffractograms did not indicate the presence of characteristic signals due to crystalline Nb or Ga oxides phases along with very intense lines due to the matrix. It can be deduced that the active phases introduced were indeed homogeneous. Moreover, one can suggest that the dispersion of niobium and gallium oxides over pristine oxides is rather high and/or that the phases formed are amorphous. The reflections of the modified materials were very sharp and no broadness of reflections due to the matrices was observed, which suggest the lack of changes in the size of crystallites. Moreover, the FT-IR spectra of impregnated oxides showed the bands of a lower intensity

Fig. 2. XRD patterns of MCM-41 before and after introducing metallic species.

Fig. 3. XRD patterns of different oxide systems.

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Fig. 4. FT Raman spectra of niobia systems (A): (a) Nb2O5; (b) 0.24% Ga/Nb2O5; (c) 0.48% Ga/Nb2O5; (d) 4.8% Ga/Nb2O5; gallia systems (B): (a) Ga2O3; (b) 0.23% Nb/Ga2O3; (c) 2.3% Nb/Ga2O3; (d) 4.6% Nb/Ga2O3.

than those of the pure oxide. It is in line with the statement concerning the suggestions derived from XRD patterns on the coverage of crystallites with amorphous oxides phases. In the characterization of metal-containing solid materials, Raman spectroscopy has been extensively used for the determination of isolated/clubbed metal sites, and it is generally accepted that the results obtained from this measurement are conclusive [25]. In Fig. 4, the Raman spectra of the oxide systems are reported. The spectra of modified samples are dominated by the typical features of parent materials, i.e. niobia or gallia. The spectrum of niobium oxide system (Fig. 4A(a)) is certainly dominated by the band at 690 cm1, typical of all niobium oxides [26,27], due to vibrations of Nb–O–Nb bridges with nearly octahedral niobium oxide species. The weak band around 820 cm1 can be assigned to the symmetric stretching mode of surface Nb O, while Raman bands between 200 and 300 cm1 to the bending modes of Nb–O–Nb linkages [26,28]. The spectra of the oxide systems with small gallium loadings (1 monolayer) do not show any modifications of the relative intensities of the bands characteristic of niobia. This indicates that, in accordance with XRD results, the support was not modified by deposition of GaOx. However, for loading of 4.8% a decrease of the signals intensity was observed. In Fig. 4B, the Raman spectra of all the NbOx/Ga2O3 samples are depicted. The Raman spectrum of pure b-Ga2O3 used as a support for the Nb-modified catalysts is also shown in Fig. 4B(a) for reference purposes. As seen, pure Ga2O3 exhibits five main peaks at around 200, 346, 417, 654 and 766 cm1 [29] and moreover, the spectrum baseline was almost plain. The peak position and relative intensity are consistent with those reported in literature [29,30]. The bands in the range 300– 600 cm1 correspond to the bending vibrations, while the bands above 600 cm1 are due to the Ga–O4 tetrahedral stretching

modes [29,31]. In Raman spectra of NbOx/Ga2O3 systems (Fig. 4B(b)–(d)) all the bands characteristic for the gallium oxide (Fig. 4B(a)) are seen. The intensity of the peaks decreases with the number of NbOx monolayers. There are no detectable Raman bands characteristic for the surface NbOx species supported on Ga2O3, probably because of the relatively weak surface NbOx Raman bands in comparison to the much stronger Raman signals of the Ga2O3 support. The only visible features which can be assigned to the presence of NbOx species in the Raman spectra of NbOx/Ga2O3 systems are: (i) the asymmetry of the peak at 767 cm1 emerging with the increase of NbOx monolayer number, and (ii) the negligible rise of the baseline (630 cm1) suggesting the possible presence of a very weak band. In conclusion, the monolayer surface coverage of the NbOx/Ga2O3 catalysts could not be determined by Raman spectroscopy [4]. The spectra of all the MCM-41 supported samples (not shown) were similar to that of the pristine MCM-41 support. The Raman bands observed at 480 and 620 cm1 arise from the threefold and fourfold siloxane rings, whereas at 810 and 970 cm1 are assigned to the siloxane bridges and the silanol groups, which may arise from the inorganic backbone of the amorphous MCM-41 silica surface [32]. Especially, the first two bands are relatively intense. In the Raman spectra of the calcined Nb/, Ga/ and Nb/Ga/MCM-41 samples, there were no bands corresponding to the crystalline niobia and gallia, indicating that the metal oxide species are very well dispersed inside the framework of MCM-41. This was further confirmed by the absence of peaks corresponding to appropriate oxide phases in the wide-angle X-ray diffractograms. The same was also found for other MCM-41 modified with transition metal compounds [33]. For example, the absence of a sharp Raman band at 680 cm1 (symmetric stretching modes of niobia

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polyhedra) and additional Raman bands between 200 and 300 cm1 (bending modes of Nb–O–Nb linkages) demonstrates that crystalline Nb2O5 nanoparticles [24,26,27] are not present in any of the Nb/MCM-41 and Nb/Ga/MCM-41 samples. The lack of signals at 1070 and 920 cm1 attributed to the perturbed silica surfaces may be caused by the interference of neighbouring niobium atoms or by the presence of Si–O–Nb or Si–O–Ga moieties [28]. The surface properties of the prepared catalysts were tested in 2-propanol (IPA) decomposition, acetonylacetone cyclization (the gas phase reactions) and cyclohexene oxidation with hydrogen peroxide in acetonitrile as the reaction medium (the liquid phase reaction). In all the reactions studied the activity of the catalysts based on a mesoporous support was several times higher than that obtained for the systems in which niobia or gallia were used as the matrices. In general, the dehydration of 2-propanol to propene requires the presence of acid sites and the dehydrogenation of 2-propanol to acetone involves the presence of base or redox sites [34,35]. The dehydration of 2-propanol to propene was the main reaction in the case of all Nb-containing samples (i.e. supported on either gallium oxide or the MCM-41 matrix) revealing predominantly the acidic character of the samples (Table 2). Gallium oxide shows a high selectivity to acetone (propanone), indicating that the dehydrogenation of IPA, a base (redox)-catalyzed reaction, was a major one. However, when Ga2O3 is loaded with niobium species, the basic (i.e. redox) activity is totally suppressed. The materials (niobia and MCM41 samples) impregnated with the gallium salt exhibited low basicity (diisopropylether and/or 2-propanone selectivity was below 10%) in contrast to the Nb-containing materials free from Ga species, which demonstrated acidic character only. The increase of Ga content from 0.48 to 4.8% led to the higher selectivity towards propanone. The base-type activity of the latter sample was also proved by the results on the acetonylacetone cyclization process (Table 3). The intramolecular condensation of acetonylacetone (1,4diketone) is known to undergo both by acid- and base-catalyzed intramolecular cyclization, leading to a quite distinct products:

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Table 3 Cyclization of acetonylacetone at 623 K after 30 min Material

Nb2O5 Ga/Nb2O5 4.8% Ga2O3 Nb/Ga2O3 4.6% MCM-41 Ga/MCM-41 Nb/MCM-41 Nb/Ga/MCM-41

Conversion (%)

6.3 6.2 4.2 5.1 2.8 77.9 97.1 81.5

Selectivity (%) DMF

MCP

100 81 100 100 100 100 100 100

0 19 0 0 0 0 0 0

2,5-dimethyl-furan (DMF) in the first case and 3-methyl-2cyclopentenone (MCP) in the latter one [36]. It is, therefore, a very good test reaction confirming the acid–base character of the oxide systems. As expected, for niobium doped gallia the only product formed was 2,5-dimethylfuran. On the contrary, 3methyl-2-cyclopentenone is observed when a high amount of gallium (4.8%) has been introduced into the oxide systems. For the MCM-41 based catalysts formation of 2,5-dimethyl-furan (DMF) was observed only. The activity of Nb/MCM-41 and Nb/Ga/MCM-41 was higher in this reaction than that of Ga/ MCM-41. It is clear that niobium species dispersed in the MCM-41 materials enhance the catalytic activity in this reaction. The results of cyclohexene oxidation on mesoporous materials are briefly summarized in Fig. 5 and Table 4. Pure silica MCM-41 sample showed almost no activity in the

Table 2 Decomposition of 2-propanol at 573 K at the second pulse Material

Nb2O5 Ga/Nb2O5 0.24% Ga/Nb2O5 0.48% Ga/Nb2O5 4.8% Ga2O3 Nb/Ga2O3 0.46% Nb/Ga2O3 2.3% Nb/Ga2O3 4.6% MCM-41 Ga/MCM-41 Nb/MCM-41 Nb/Ga/MCM-41

Conversion (%)

0.4 2.0 2.6 3.3 1.6 1.2 1.5 2.8 0.1 10.3 2.2 19.6

Selectivity (%) Propene

Diisopropylether

2-Propanone

100 92 96 72 44 100 100 100 100 94 100 95

0 0 0 5 0 0 0 0 0 6 0 3

0 8 4 23 56 0 0 0 0 0 0 2

Fig. 5. Comparison of Ga and/or Nb modified MCM-41 catalysts for epoxidation of cyclohexene.

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Table 4 Oxidation of cyclohexene with hydrogen peroxide in acetonitrile at 318 K Material

Nb2O5 Ga/Nb2O5 0.24% Ga/Nb2O5 0.48% Ga/Nb2O5 4.8% Ga2O3 Nb/Ga2O3 0.46% Nb/Ga2O3 2.3% Nb/Ga2O3 4.6% MCM-41 Ga/MCM-41 Nb/MCM-41 Nb/Ga/MCM-41 a

Cyclohexene conversion (%) a

1.0 7.8 12.1 8.5 3.4 6.7 10.3 15.5 15.0 15.6 37.7 33.7

Selectivity (%) a

Epoxide selectivity at 3% cyclohexene conversion

Epoxide

Diol

Others

5 27 25 23 9 38 41 54 11 3 33 36

95 73 75 77 80 62 59 46 77 97 62 58

0 0 0 0 11 0 0 0 12 0 5 6

1 29 27 23 6 32 31 29 15 2 96 100

After 40 h.

cyclohexene epoxidation. It is very well evidenced that the insertion of Nb species to the Ga-containing MCM-41 system causes a slight increase in the initial activity coupled with a significant increase of the selectivity to epoxide (Fig. 5). However, Nb-free gallium-containing oxide and MCM-41 reveal diol formation as the dominating reaction pathway. The significant epoxide formation was observed after introducing of niobium species (Table 4), while gallium ones caused the diol formation. It is interesting to note that both oxides reveal high diol formation yield. The highest selectivity to epoxide, for the same cyclohexene conversion (3%), is observed on the bimetallic mesoporous system (Nb/Ga/MCM-41) indicating the promoting role of niobium–gallium interaction. Moreover, it seems that the proper isolation of active species is required for the high activity in this process and this can be achieved using a high area support such as a mesoporous molecular sieve. Finally, we note that the niobium impregnated samples based on the mesoporous MCM-41 matrix exhibit the much higher cyclohexene conversion than those prepared on the metal oxide supports, which is clearly due to the high surface areas of the former materials. 4. Conclusions Combination of the characterization results obtained by different techniques (N2 isotherms, XRD, IR and Raman spectroscopies) for niobia and gallia supported on mesoporous MCM-41 did not allow the identification of metal oxide species on the MCM-41 surface. However, the difference in the catalytic activity proves the modification of the matrix surface. The catalysts based on oxide supports, Nb2O5 and Ga2O3, exhibit a several times lower catalytic activity than the Nb- and/ or Ga-impregnated mesoporous MCM-41 materials. All the niobium-containing oxide systems show evidence of acidic properties, as demonstrated by observing the 2-propanol dehydration route. The deposition of Ga species on the matrix induces base/redox properties. The efficiency of cyclohexene epoxidation on the NbMCM-41 catalyst is considerably increased after insertion of Ga species, thus indicating the

promoting role of gallium in the activity of niobium species located on the MCM-41 surface. Acknowledgements MZ research team appreciates the extensive long-term and fruitful collaboration with J.C. Volta in the field of oxidative catalysis and thanks the Polish State Committee for Scientific Research (grant no. 3T09A 10026; 2004–2007) for the financial support. AK, BS and VCC are grateful to the EU for the Marie Curie Action grant TOK-CATA (no. MTKD-CT-2004-509832). Ammonium tris(oxalate) complex of niobium was kindly supplied by CBMM (Companhia Brasileira de Metalurgia e Minerac¸a˜o, Brazil). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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