Modification of mesoporous silicas by incorporation of heteroelements in the framework

Microporous and Mesoporous Materials 27 (1999) 151–169

Modification of mesoporous silicas by incorporation of heteroelements in the framework A. Tuel * Institut de Recherches sur la Catalyse, CNRS, 2 av. A. Einstein, 69626 Villeurbanne, Cedex, France Received 8 October 1997; received in revised form 9 January 1998; accepted 11 January 1998

Abstract Mesoporous silicas (HMS ) prepared at room temperature using primary alkylamines as surfactant molecules have been modified by incorporation of heteroelements in the silica framework. As for zeolites, the modification of the framework provides materials with interesting properties in catalysis. The extent of incorporation as well as the stability of the products towards calcination depends not only on the nature of the incorporated element, but also strongly on the synthesis conditions. Materials with high substitution levels can be relatively easily obtained in their as-synthesized form, but direct calcination in air often leads to a partial degradation of the mesoporous structure with extraction of some of the metal species. This can be circumvented by removing the organic phase by solvent extraction, which prevents undesirable framework modifications usually observed upon thermal treatment. The determination of the nature, location and coordination of metal species in mesoporous silicas is complicated due to the non-crystalline nature of these materials and by the fact that they can readily change with the degree of hydration of the sample. Moreover, in contrast to zeolites, the discrimination between species located on the internal surface of the mesopores and inside the silica walls is not always trivial by means of conventional spectroscopic techniques. This paper presents a piece of the work that has been achieved over the last years in the field of synthesis and characterization of mesoporous silicas modified by different metals. For each metal, examples of specific catalytic reactions are given and briefly discussed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Catalysis; HMS; Mesoporous silicas; Oxidation; Substitution

1. Introduction In the early 1990s, Mobil researches reported the preparation of a new class of silica and silica– alumina based molecular sieves using supramolecular surfactant templates [1,2]. The so-called M41S materials possess a periodic framework of regular mesopores, whose size depends on the alkyl * Corresponding author. Fax: +33 04 72 44 53 99; E-mail: [email protected]

chain length of the organic template and is generally comprised between ca. 2 and 4 nm. The obtained materials possess interesting physical properties which make them potentially attractive as supports or catalysts. They usually have very high surface areas of 1000 m2/g or more, are thermally stable in dry atmosphere at temperatures up to about 800°C, and have hydrocarbon sorption capacities greater than 0.7 ml/g. Originally, materials were synthesized at relatively high temperatures by a cooperative organiza-

1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 25 0 - 9

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tion process between cationic surfactants (S+), namely alkyltrimethylammonium cations with C 8 to C , and anionic inorganic species (I −) [1,2]. 16 Depending on the synthesis conditions, and principally the surfactant/SiO ratio, different phases 2 could be obtained, like the hexagonal phase MCM-41, the cubic one MCM-48 as well as the lamellar compound MCM-50 [1–6 ]. There have been a considerable number of papers and reviews over the last five years dealing with the synthesis and characterization of these materials, particularly the hexagonal member MCM-41 [7,8]. Since their discovery, new synthesis routes have been developed, which differ from the original one by the nature of the electrostatic interactions between the organic and inorganic phases. In addition to the previously mentioned (S+I −) pathway, the (S−I +) pathway involves condensation of an anionic surfactant with cationic inorganic species [6 ]. Two other routes were also developed in which the surfactant and the inorganic phase have similar charges, but are separated from each other by small cations with opposite charge: (S+X−I +) ( X−=Cl−, Br−) and (S−M+I −) with M+=Na+ or K+ [5,6 ]. At the same time, an elegant route was proposed by Tanev and coworkers to prepare mesoporous silicas at room temperature by a neutral templating route (S°I°) [9,10]. In this case, the organic surfactant is not a quaternary ammonium cation but a primary amine, and the assembly involves hydrogen-bonding interactions between neutral primary amines and neutral inorganic precursors. These materials, denoted HMS (hexagonal mesoporous silica) can be considered as special members of the M41S family, but differ significantly from molecular sieves obtained by the electrostatic assembly pathways. They usually possess thicker framework walls (2–3 nm compared with ca. 1 nm for MCM-41), smaller X-ray scattering domain sizes and a textural mesoporosity. The small X-ray scattering domain sizes, evidenced by the absence of narrow 110 and 200 reflections in the X-ray pattern, reflect a quite short range of hexagonal order compared with MCM-41. One of the most important advantages of HMS compared with MCM-41 is that the organic phase can be totally removed from as-synthesized samples by solvent

extraction, which is not possible in the case of the other pathways where strong electrostatic interactions exist between organic and inorganic phases [10]. The solvent extraction prevents the partial degradation of the mesoporous structure that could occur during calcination in air at relatively high temperatures. Pure silica MCM-41 and HMS possess a neutral framework, which limits their applications to catalysis, molecular sieving, supports, adsorbents, etc. [11]. In order to provide solids with potential catalytic applications, it is possible, like for zeolites, to modify the nature of the framework by incorporation of heteroelements. When trivalent cations like Al3+, B3+, Ga3+, Fe3+ substitute for silicon in the walls of the mesoporous silica, the framework possesses negative charges that can be compensated by protons, and solids can be used in acidic reactions. When other cations like Ti4+, V4+, Sn4+, Zr4+ are introduced, the electroneutrality is maintained and the corresponding mesoporous materials are used rather in specific reactions like oxidations. There are, however, great differences between substituted zeolites and mesoporous silicas. The latter can be regarded as mesoporous organized mixed oxides, which means that their physical and catalytic properties will not differ greatly from those of mixed oxides. In particular, while in most of the zeolitic structures all active sites are within the channels and accessible to guest molecules, a large proportion of these sites in mesoporous silicas can be located inside the walls, and are thus not accessible. This will effectively occur when the metal precursor is introduced together with the silica source during synthesis. However, that can be circumvented by grafting active species on the inner surface of the mesopores of a pure silica material. In this case, all catalytically active sites are accessible and it was reported that these materials are more active than samples prepared by direct synthesis [12]. Another difference with zeolites concerns the nature of the framework itself. In pure silica zeolites like silicalite-1, the number of defect groups like silanols is low as they are essentially located on the outer surface of the crystals. This makes materials with a relatively high hydrophobic character, well adapted for oxidation reactions in aqueous media. In contrast,

A. Tuel / Microporous and Mesoporous Materials 27 (1999) 151–169

mesoporous silicas contain large amounts of silanol groups pointing into the mesopores [2,13,14], capable of hydrogen-bonding with water molecules, which will probably affect their efficiency in catalytic reactions with aqueous H O . 2 2 This paper will focus on the various procedures to modify the framework of mesoporous silicas by direct synthesis. As previous reviews have already been dedicated to a more or less similar subject, this work is centered more on modified HMS materials prepared by a neutral templating route. The focus is placed on the location, coordination, and accessibility to guest molecules of heteroelements in the silica framework in relation to the preparation route. For all materials, examples of catalytic applications are given and discussed.

2. Experimental 2.1. Synthesis of modified HMS molecular sieves Modified HMS molecular sieves, denoted Me-HMS (where Me is the incorporated metal ), were prepared following a recipe similar to that reported by Tanev and Pinnavaia [10] for pure silica and titanium–silica materials. In a typical preparation, hexadecylamine ( HDA) was added to a solution containing water and ethanol ( EtOH ) and the mixture was stirred until homogeneous. Then tetraethyl orthosilicate ( TEOS ) was added under vigorous stirring. Depending on its nature, the metal precursor could be dissolved either in the water/ethanol/HDA solution before addition of TEOS or in TEOS itself. The final mixture, with the following composition: SiO –xMeO –0.3HDA–7EtOH–35H O 2 2 2 with 0≤x≤0.1 was stirred at room temperature for about 12 h to obtain the products. Solids were then recovered by centrifugation, washed with distilled water, and air-dried at room temperature. Organic molecules occluded in the mesopores were removed either by direct calcination in air at 550°C for 10 h or by solvent extraction. For solvent extraction, 1 g of dried Me-HMS was dispersed in

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20 ml of ethanol containing a small amount of NH Cl and the mixture was refluxed under vigor4 ous stirring for 1 h. NH Cl was added to ethanol 4 to exchange protonated amines in the case of trivalent metal substituted HMS [15]. The solid was then filtered, washed with cold ethanol and the extraction procedure was repeated once. After the complete extraction of the organics, samples were dried at 100°C in an oven. In some cases, the solvent extraction was followed by calcination in air at 500°C for about 5 h. 2.2. Post-synthesis treatments The procedure is adapted from a recipe reported by Fraile et al. [16 ] to prepare amorphous titanium–silica catalysts. 1 g of calcined pure silica HMS was first evacuated under vacuum at 350°C for 3 h. It was then dispersed in 20 ml of dry toluene containing titanium isopropoxide Ti(OiPr) or 4 vanadyl isopropoxide VO(OiPr) and the mixture 3 was refluxed for 3 h under vigorous stirring and inert atmosphere (argon). The solid was then recovered by filtration, washed with cold toluene and dried at 100°C under vacuum. It was then calcined in air at 500°C for 5 h. 2.3. Characterization of the samples X-ray diffraction patterns were recorded between 1 and 30° (2h) on a Philips PW 1710 diffractometer (Cu Ka radiation) using SiO as an 2 external standard. UV–vis spectra were obtained in the reflection mode using a Perkin Elmer Lambda 9 spectrometer. EPR spectra were obtained at 77 K in the X-band mode with a Varian E9 spectrometer. Solid state NMR experiments were performed on a Bruker DSX 400 spectrometer equipped with a 4 mm double bearing probe head. Samples were typically spun at 12 kHz in zirconia rotors. 51V NMR spectra were obtained with the same equipment under static conditions. 27Al, 11B and 51V NMR chemical shifts were measured with respect to Al(H O)3+ , Et OBF 2 6 2 3 and VOCl , respectively. The metal content in 3 Me-HMS was determined by ICP after solubilization of the samples in HF:HCl solutions. N 2 adsorption/desorption isotherms were collected on

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a Catasorb apparatus. Samples were evacuated at 250°C overnight prior to analysis. The pore size and distribution were calculated by the BJH method assuming cylindrical pores. The acidity of substituted HMS was studied by pyridine adsorption. Wafers (typically 15 mg) were placed in a cell equipped with KBr windows, calcined at 500°C for 6 h under O and evacuated 2 under vacuum (10−5 Torr) for 1 h prior to admission of pyridine. A known amount of pyridine was introduced onto the wafer at room temperature. After 5 min, the sample was evacuated first for 15 min at room temperature and then at increasing temperatures for 1 h. IR spectra were collected on a Bruker IFS 48 FTIR spectrometer. For each spectrum, 200 scans were accumulated. 2.4. Catalytic experiments Ti-, V- and Zr-containing HMS materials have been used as catalysts in the oxidation of various substrates with aqueous H O or anhydrous tert2 2 butyl hydroperoxide ( TBHP, 70 wt% in chlorobenzene). All reactions were carried out in roundbottomed flasks equipped with a magnetic stirrer at temperatures between 0°C and 80°C. Products were analyzed by gas chromatography and the oxidant concentration in the reaction mixture was measured by titration with KI.

3. Results and discussion 3.1. Incorporation of trivalent cations As already mentioned, incorporation of trivalent cations in a silica framework creates charge defects that can be converted to acid sites, whose number and strength depend on the amount and nature of the incorporated metal. Due to its potential applications in various petroleum refining processes, aluminum-containing mesoporous silica has received considerable attention over the last years in both the patent and open literature. In particular, two recent reviews by Sayari give an overview of catalytic applications of Al-MCM-41 and an exhaustive list of industrial patents on the synthesis and catalytic applications of these materials [8,17].

The incorporation of aluminum in mesoporous silicas as well as the physical properties and stability of the obtained solids strongly depend on the synthesis conditions [18–20]. As-synthesized materials with Si/Al ratios in the range 100–2 can be obtained, and 27Al NMR shows that all aluminum species are exclusively in a tetrahedral environment [21–23]. However, calcination at relatively high temperatures generally produces a loss in surface area and pore volume, together with a partial extraction of Al species from the framework, principally for materials with high Al loading (Si/Al<20). In fact, dealumination probably arises from hydrolysis of framework aluminum by steam generated during the combustion of the surfactant. Corma et al. [19] have shown that the extent of dealumination could be decreased by calcinating the materials first under N followed 2 by air. Luan et al. [24] also observed that the sodium form of Al-MCM-41 could be calcined in air at 550°C without significant formation of extraframework aluminum, which was not the case for the hydrogen form. Concerning the decrease in specific surface area and pore volume observed after calcination of Al-rich samples, it was stipulated that dense phases could be formed in which aluminum is also tetrahedrally coordinated, and thus cannot be distinguished from framework Al by solid state NMR [22]. We have prepared a series of aluminum-containing HMS in our group using primary amines as surfactant and aluminum nitrate as metal precursor [15] ( Table 1). For Si/Al ratios in the range 100–10, all the metal was incorporated in the silica framework and tetrahedrally coordinated in as-synthesized samples as evidenced by 27Al NMR ( Fig. 1). As for MCM-41 materials, direct calcination of the products in air at 550°C causes a partial collapse of the structure and extraction of Al species from the framework. In contrast, when primary amines are removed by an ethanol/NH Cl extraction, samples retain their 4 mesoporosity and no extraction is observed. Dense phases are probably not formed as, for all samples, ˚ ) and the the pore diameter (approximately 36 A pore volume are the same as those of pure silica material ( Table 1). Moreover, extracted samples can be further calcined in air at temperatures up

A. Tuel / Microporous and Mesoporous Materials 27 (1999) 151–169 Table 1 Chemical composition and characteristics of calcined and solvent-extracted trivalent metal-modified HMS (from ref. [15]) Sample

Al-HMS Al-HMS Al-HMS Al-HMS Ga-HMS Ga-HMS Ga-HMS Ga-HMS Fe-HMS Fe-HMS Fe-HMS Fe-HMS B-HMS B-HMS B-HMS B-HMS

Si/Me Gel

Solid

100 50 20 15 100 50 20 15 100 50 20 15 100 50 15 3

88 38 21 8 86 48 14 13 85 55 18 15 150 95 27 17

S (m2/g)b

˚ )a,b W (A p

1215 (1166) 1188 (1215) 983 (1079) 885 (1252) 1156 (1288) 929 (1179) 1011 (1233) 779 (1023) 1123 (1201) 700 (935) 753 (1032) 698 (1078) 1002 (1056) 1153 (1201) 1237 (1154) 887 (935)

30 30 32 22 32 28 25 22 34 22 25 22 33 35 32 29

(36) (35) (37) (35) (35) (36) (37) (35) (35) (37) (35) (33) (36) (36) (37) (38)

a W is the mean pore diameter obtained from N adsorption p 2 isotherms. b Values in parentheses correspond to solvent-extracted samples.

Fig. 1. 27Al NMR spectra of Al-HMS (Si/Al=21). As-synthesized (a), calcined in air (b), solvent-extracted (c) and calcined after solvent extraction (d ) (from ref. [15]).

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to ca. 500°C without removal of framework Al. Indeed, in that case, water, ethanol and ammonia remaining in the mesopores after extraction are expelled from the solid at low temperature, thus preventing the formation of steam at higher temperatures. Adsorption of pyridine on the samples shows that Al-HMS contains both Lewis and Bro¨nsted acid sites. Bro¨nsted sites are weak and their strength is approximately the same as that of amorphous Al O –SiO gels. This was not the 2 3 2 conclusion of Mokaya and Jones [25,26 ], who found that directly calcined Al-HMS with Si/Al= 5 possesses acid sites very similar in strength to those of HY zeolite. In particular, the proportion of Bro¨nsted sites was higher than on a sample of Al-MCM-41 prepared with a cationic surfactant and having the same Al content. These data were supported by catalytic results in the cracking/ dehydrogenation of cumene. Both Al-HMS and HY showed high conversions at temperatures in the range 300–500°C compared with Al-MCM-41. Boron can also be successfully incorporated in the framework of pure silica MCM-41 or HMS [15,27–31]. For B-MCM-41 or B-HMS, it was found that boron was tetrahedrally coordinated in as-synthesized samples for a wide range of boron contents (Si/B ratios from 100 to about 5). Using 11B and 29Si NMR, Sayari et al. [27] proposed that boron species are attached to the silica walls of MCM-41 through interactions with surface hydroxyl groups, which means that no boron substitutes for silicon inside the walls. Calcination of as-synthesized B-MCM-41 in air converts part of the four-coordinated B in trigonal BO species. Interestingly, in contrast to what is 3 usually observed in zeolites, four-coordinated sites remain even after a prolonged treatment in air at 550°C. This suggests that different environments of boron are probably present in as-synthesized products, that cannot be distinguished by their 11B NMR chemical shift alone. In the case of B-HMS, elimination of the organic phase by solvent extraction does not remove B from the framework, and all boron species retain their tetrahedral coordination, probably because of the presence of water or alcohol in their coordination sphere [15] ( Fig. 2). All the above mentioned

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Fig. 2. 11B NMR spectra of B-HMS (Si/B=17). As-synthesized (a), solvent-extracted (b), freshly calcined in air (c) and exposed to ambient atmosphere for 2 days after calcination (d) (from ref. [15]).

papers [15,27,31] report that a new species, characterized by a 11B NMR line at ca. 19.5 ppm/Et OBF , appears when calcined materi2 3 als are exposed to moisture or ambient atmosphere. Sayari et al. [27] attributed this new species to extraframework boron, probably a hydrated form of boric acid. Due to the fact that the corresponding NMR line is narrow and at high field, Liu et al. [31] proposed that it may arise from four-coordinated boron in a non-framework borate phase, formed by reaction of water with three-coordinated framework B species. The real nature of this phase is still unknown, but using 1H–11B CP/MAS NMR, Liu et al. [31] showed that this newly created four-coordinated boron is not protonated. Surprisingly, new species can be reconverted into framework four-coordinated boron upon further hydration in moist air for 2 days. We also observed a similar behavior on calcined B-HMS materials contacted with air for various periods of time [15] ( Fig. 2). The reconversion of extraframework species into four-coordinated framework species has never been observed

in zeolitic materials, but is possible in mesoporous silica due to the non-crystalline nature of the framework. In a similar way, Hamdan et al. [23] have shown that re-incorporation of extraframework aluminum in a mesoporous silica could be achieved by heating a pure silica MCM-41 in the presence of an aqueous solution of NaAlO at 2 temperatures below 120°C. 27Al NMR of the treated samples showed only four-coordinated framework species without any signal in the region corresponding to six-coordinated extraframework species. B-MCM-41 and B-HMS materials possess a very low acidity, essentially a Lewis acidity. This is probably the reason why no catalytic data have yet been reported with these materials. As for aluminum, gallium can be incorporated into the pure silica materials, and samples with Si/Ga ratios from 100 to about 10 can be synthesized [15,32,33]. 71Ga NMR spectra of as-prepared samples show a broad line between ca. 140 and 160 ppm, characteristic of four-coordinated tetrahedral Ga species. Cheng et al. [33] have shown that the intensity of the NMR line was proportional to the Ga content in as-made products, thus supporting the incorporation. However, after calcination, the signal at 140 ppm decreased, particularly for Ga-rich samples with Si/Ga<20, indicating partial extraction of the four-coordinated framework species. Nevertheless, it seems that Ga is more stable than Al in the framework of mesoporous silicas [32]. When galloaluminosilicate products were prepared with Si:Ga:Al=120:1:3, about 7% of Al species were expelled from the structure upon calcination, whilst no Ga was extracted for a similar sample with Si:Ga:Al=120:3:1. We have observed that removing the organics from as-synthesized Ga-HMS by solvent extraction prevented leaching of Ga species from the framework, and that template-free samples with Si/Ga ratios of about 10 could be obtained with all Ga in tetrahedral coordination [15] ( Table 1). This shows once more the advantage of the neutral pathway compared with the conventional ionic routes to prepare trivalent metal-containing mesoporous silicas with relatively high metal contents. The literature concerning Fe-containing meso-

A. Tuel / Microporous and Mesoporous Materials 27 (1999) 151–169

porous silicas is scarce. Yuan et al. [34] have reported on the synthesis of Fe-containing MCM-41, and concluded the incorporation of Fe species in the framework on the basis of essentially EPR spectroscopy. He et al. [35] prepared Fe-MCM-41 with SiO /Fe O ratios from 124 to 2 2 3 32 and claimed the incorporation of FeIII species in as-made solids from EPR and Mo¨ssbauer spectroscopies. However, calcination of the materials led to drastic modifications of EPR spectra, in particular the disappearance of the signal at g= 4.3, suggesting that Fe species were removed from the framework, which was also supported by Mo¨ssbauer spectroscopy. When Fe-HMS materials are prepared with hexadecylamine (Si/Fe from 100 to 15) ( Table 1), the EPR spectra of as-made solids are the same as that of Fe-containing silicalite-1 (Fig. 3) [15]. In particular, for a sample with Si/Fe=18, we observe two main signals at g=2 and g=4.3, together with a shoulder at intermediate g values ( g#2.4). The interpretation of such signals is not trivial and numerous attributions have been proposed in the literature [36,37]. It is usually assumed that the signal at g=2 corresponds to highly symmetric four-coordinated Fe3+ species, whilst that at g=4.3 is due to a

Fig. 3. EPR spectra of Fe-HMS (Si/Fe=85) recorded at 77 K. As-synthesized (a), solvent-extracted (b) and calcined in air (c). For comparison, the spectrum of a sample of Fe-silicalite-1 (Si/Fe=92) is reported (d) (from ref. [15]).

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rhombic distortion of the tetrahedral coordination of Fe3+. The presence of intermediate signals, particularly around g=2.4, is generally thought to arise from small extraframework Fe–O clusters or tiny ferric oxide particles. Calcination of the sample did not significantly modify the EPR spectrum. In particular, no intense signal due to extraframework oxide was observed when the spectrum was recorded at room temperature. That was also confirmed by Mo¨ssbauer spectroscopy; no signal characteristic of small iron oxide clusters was observed at liquid He temperature [38]. More details about the characterization of Fe-modified HMS materials will be given in a forthcoming paper. However, the analogy with Fe-silicalite-1, and the absence of tiny extraframework oxide particles in calcined samples, strongly suggests that part of Fe is tetrahedrally coordinated and substitutes for Si in the silica framework. 3.2. Incorporation of titanium Due to the remarkable properties of the titanium-containing silicalite-1 ( TS-1) in oxidation reactions at low temperature with dilute H O 2 2 solutions [39–41], the synthesis of Ti-modified large pore zeolites and mesoporous materials has received considerable attention over the past years [42–45]. Indeed, TS-1 is a medium pore zeolite ˚ ), which limits its catalytic (pore diameter ca. 5.6 A applications to small organic substrates. Ti-MCM-41 was first reported in 1993 in the patent literature [46 ]. During the following years, several groups have reported the synthesis and characterization of Ti-containing MCM-41 [47– 50], MCM-48 [51] and HMS [9,52,53]. More recently, Bagshaw et al. [54] also prepared Ti-MSU-1, a titanium-containing mesoporous silica prepared in the presence of non-ionic polyethylene oxide template. Both Ti-MCM-41 and Ti-HMS can be obtained within a quite broad range of compositions (Si/Ti ratios from 100 to 10). For Ti-HMS materials, all titanium originally introduced into the synthesis mixture is incorporated in the final solids, and the composition of the mesoporous materials reflects quite well those of the precursor gels ( Table 2). Addition of titanium to the synthesis mixture results in an increase

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Table 2 Characteristics of calcined Ti-HMS materials Si/Ti Gel

Product

100 50 30 20

85 52 33 19.5

S (m2/g)

W ˚p )a (A

V (cm3/g)b

a 0˚ (A )c

WT ˚ )d (A

1286 1178 1103 1023

34 34 35 35

0.67 0.70 0.69 0.65

51.6 53.9 55.0 58.5

17.6 19.9 20.0 23.5

a W is the mean pore diameter obtained from N adsorption p 2 isotherms. b V is the mesopore volume measured at p/p =0.5 in the N 0 2 isotherm. c a is the unit cell parameter calculated from the XRD distance 0 by a =2d /앀3. 0 100 d Wall thickness=a −W . 0 p

of the unit cell size and framework wall thickness of the mesoporous materials, thus supporting the incorporation [55]. Indeed, an increase in unit cell parameters was observed upon incorporation of titanium into the framework of various zeolites like silicalite-1 or zeolite beta as well. The coordination and nature of Ti species in Ti-modified mesoporous silicas have been studied using a variety of spectroscopic techniques. UV–vis spectroscopy is one of the most commonly used techniques to access the local Ti environment in titaniumcontaining silicates [56–58]. Most of the data reported in the literature related to Ti-modified mesoporous silicas concern materials with relatively low Ti loading, typically <2.5 wt%. For such solids, the UV–vis spectrum is usually composed of an absorption band at ca. 220 nm together with a shoulder in the range 250–280 nm. The spectrum is different from that of TS-1, for which a sharp signal is observed at 210 nm without any signal beyond 270 nm [57]. Nevertheless, the absence of a distinct peak at 330 nm in calcined samples indicates that no anatase-like phases are formed upon calcination of the products [56 ]. It is usually assumed that the signal around 210 nm in the UV–vis spectrum characterizes four-coordinated Ti species that substitute for Si in the silica framework [57]. Absorptions at higher wave numbers probably arise from Ti sites with a higher coordination (five- and six-coordinated), either isolated in the framework or in very small titania

nanodomains [57,58]. The intensity of the UV–vis signal at 270 nm increases relative to that at 210 nm with the Ti content in Ti-HMS materials ( Fig. 4). For a sample with Si/Ti=19.5, only one broad line is observed with a maximum of absorption at ca. 250 nm [52]. Calcination of as-made materials results in a decrease in intensity of the line at 270 nm, which suggests that part of the five- and six-coordinated Ti species is hydrated, and probably generated by hydration of tetrahedral four-coordinated species. However, in contrast to what is observed for TS-1, a signal at 270 nm persists even after evacuation of the calcined sample in vacuum at 450°C (Fig. 4). This may indicate that some of the five- and six-coordinated species are not hydrated and could be in the form of small TiO clusters in the silica walls of 2 the mesoporous material. Similar conclusions were obtained by X-ray absorption spectroscopy at the Ti K edge [55]. Although Ti-MCM-41 and Ti-HMS samples do not clearly show the multiple pre-edge peaks characteristic of octahedral titanium, the pre-edge peak characteristic of tetrahedral Ti sites shifted slightly and has a lower intensity compared with TS-1, in which titanium is supposed to be tetrahedrally coordinated. XANES spectra similar to those of Ti-containing mesoporous silicas were reported by Liu and Davis [59] for Ti–Si mixed oxides. For a sample with Si/Ti=8, only one peak was observed at ca.

Fig. 4. UV–vis spectra of calcined Ti-HMS samples with Si/Ti= 85 (a), 45 (b), 45 after dehydration at 400°C (c) and 19.5 (d).

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4970 eV, but UV–vis spectroscopy unambiguously revealed the presence of five and six-coordinated Ti species. In that case, the intensity of the preedge peak increased upon dehydration of the sample, thus confirming that some of the five- and six-coordinated species result from the hydration of four-coordinated framework sites. XPS data on Ti-HMS were also consistent with a majority of four-coordinated Ti species in the silica framework [60]. The Ti 2p binding energy was found to be 3/2 approximately 459.9 eV, i.e. more than 1 eV below that of octahedral Ti in TiO [61]. Adsorption of 2 pyridine on a series of Ti-HMS materials shows that samples with low Ti loading (Si/Ti>100) possess essentially Lewis sites, as observed on pure silica HMS (Fig. 5). As far as the Ti content is increased, Bro¨nsted sites appear, whose relative concentration increases with the amount of titanium in the materials. A similar trend was reported by Klein et al. [62] for microporous Ti–Si mixed oxides. However, comparison of IR spectra in Fig. 5 with those of mixed oxides containing the same Ti loading shows that the proportion of Bro¨nsted sites is higher in Ti-HMS, suggesting a lower dispersion of Ti species in the latter. The absence of Bro¨nsted sites in samples with low Ti loading suggests that most of the Ti species are isolated, tetrahedrally coordinated and substituted

Fig. 5. Infrared spectra of pyridine adsorbed on Ti-HMS samples with Si/Ti=85 (a), 45 (b) and 19.5 (c). Spectra were obtained after evacuation of pyridine at 100°C for 30 min. The arrow indicates the adsorption characteristics of pyridinium ions associated with Bro¨nsted sites.

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for Si in the framework. At high Ti loadings, the presence of titania nanodomains with five- and six-coordinated Ti could explain the formation of Bro¨nsted sites. Ti-MCM-41 and Ti-HMS have been used in a variety of oxidation reactions with either H O or 2 2 anhydrous TBHP as oxidizing agent [47,48,50,53]. For bulky olefins like a-terpineol or norbornene, the materials showed better activities than Ti-beta, due to less severe diffusional limitations [48]. Ti-HMS are also excellent catalysts for the hydroxylation of 2,6-di-tert-butyl phenol into the corresponding ketone, as reported by several groups [9,17,53,55,63]. Most of these applications have been reviewed recently by Sayari [17]. We have also shown that Ti-HMS materials could catalyze the oxidation of substituted aniline into the corresponding azoxy or nitroso compounds, depending on the temperature of the reaction [64–66 ]. The activity of Ti-HMS was very high compared with that of TS-1, and comparable with that of the large pore zeolite Ti-beta. Table 3 shows the results of the oxidation of cyclohexene over a series of Ti-containing mesoporous silicas with both H O 2 2 and TBHP as oxidizing agent. With anhydrous TBHP, we observe the selective formation of cyclohexene oxide. When aqueous H O is used as the 2 2 oxidant, two major products, namely cyclohexene oxide and cyclohexenol, are formed. This is characteristic of a radical pathway involving OH · radicals, formed by decomposition of H O on 2 2 titanium sites. We have previously reported that the epoxidation of cyclohexene with H O over 2 2 Ti/SiO catalysts proceeds via two different routes: 2 a non-radical route leading to cyclohexene oxide and a radical one leading to cyclohexene oxide and cyclohexenol [67]. The relative contribution of these two mechanisms depends on reaction conditions, principally the presence of water in the mixture. In the absence of water, the first one predominates, which explains the high selectivity in epoxide obtained with TBHP. As far as water is present, the first pathway is inhibited and cyclohexenol is formed together with the epoxide by a bimolecular reaction involving cyclohexenyl hydroperoxide and cyclohexene itself. Interestingly, the amount of cyclohexenol is low when samples have been subjected to ethanol

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Table 3 Oxidation of cyclohexene over a series of Ti-modified mesoporous silicas Sample

Ti-HMS Ti-HMS Ti-HMS Ti-HMS Ti-HMS Ti-MCM-41c Ti-HMSd

Si/Ti

184 85 52 52 19.5 48 53

Ox.

HO 2 2 HO 2 2 HO 2 2 TBHP HO 2 2 HO 2 2 HO 2 2

C

ox

85 97 95 92 97 72 98e

(%)a

Select. (%)b Epox.

Alc.

Diol.

50 (86) 52 (82) 48 94 41 57 45

35 (4) 30 (6) 30 2 33 29 35

15 (10) 18 (14) 22 4 26 14 10

Reaction conditions: 0.5 g catalyst, 0.1 mol cyclohexene, oxidant/cyclohexene=0.2, T=70°C, solvent: acetonitrile. Data are obtained after 3 h reaction. a Oxidant conversion. b Selectivities in cyclohexene oxide ( Epox.), cyclohexenol (Alc.) and cyclohexanediol (Diol.). Values in parentheses correspond to extracted samples. c Ti-MCM-41 was synthesized according to ref. [55]. d Ti-HMS was obtained by reaction of titanium isopropoxide with pure silica HMS following ref. [16 ]. e Data were obtained after 1 h 30 min.

extraction without further calcination. For a sample with Si/Ti=184, the percentage in alcohol decreases from 35% for a calcined material to 4% for an extracted catalyst. This indicates that the solvent extraction increases the hydrophobic character of Ti-HMS, probably by partial esterification of internal silanol groups with ethanol. We can also notice that the amount of cyclohexanediol increases with the metal content, which can be directly correlated with the presence of Bro¨nsted acid sites in Ti-rich samples. As already observed by other groups, Ti-HMS materials are much more active than the corresponding Ti-MCM-41 samples ( Table 3). Zhang et al. [55] and Tanev et al. [9] attributed this difference in activity to the presence of a textural mesoporosity in HMS materials, which facilitates the accessibility of substrate molecules to the active sites. In fact, examination of N 2 adsorption/desorption isotherms did not reveal great differences between our samples, and Ti-HMS materials do not significantly differ from Ti-MCM-41 from a textural mesoporosity point of view. This suggests that mesoporosity may not be the only factor that can explain the difference in activity. Since spectroscopic techniques do not permit a clear discrimination between the two materials, one of these factors could be the location

of Ti sites in the silica walls. We have observed that a sample prepared by grafting Ti onto the inner surface of the mesopores of a pure silica HMS is more active than a sample in which Ti is incorporated during synthesis ( Table 3). Oldroyd et al. [12] also observed that a Ti-grafted MCM-41 material showed a high activity compared with a conventional Ti-MCM-41 in the epoxidation of cyclohexene with 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH ). For such samples, all Ti sites are accessible to substrate molecules since no active sites are located inside the walls of the mesoporous silica. Therefore, if we assume that, in contrast to Ti-MCM-41, Ti species in Ti-HMS are not randomly distributed in the silica framework but rather located on the surface of the mesopores, the difference in activity can be explained even in the absence of additional textural mesoporosity. Ti-MCM-41 and Ti-HMS are indeed synthesized under very different conditions. For Ti-HMS, the formation of the mesoporous structure necessitates the rapid hydrolysis and condensation of the precursors Ti(OiPr) and TEOS. 4 At basic pH values, hydrolysis of the titanium alkoxide is fast compared with that of TEOS, and the relative concentration in titanate ions immediately after mixing of the TEOS/Ti(OiPr) and 4 HDA solutions is probably high. Under such

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conditions, it is conceivable that the first inorganic layer around surfactant micelles contains a large proportion of Ti species compared with the internal regions of the walls (Fig. 6). A higher Ti concentration at the surface of the mesopores could also explain the great population of Bro¨nsted sites observed by pyridine adsorption.

3.3. Incorporation of vanadium As for titanium, incorporation of vanadium into the framework of mesoporous silicas may provide the latter with interesting properties in oxidation reactions with organic hydroperoxides or H O . 2 2 Indeed, vanadium-containing zeolites catalyze reactions like the hydroxylation of aromatics [68– 70], the oxidation of primary amines [71], and the selective side-chain oxidation of alkylaromatics [72,73], the latter not being possible over Ti-containing materials. The synthesis and characterization of V-MCM-41 [60,74,75], V-MCM-48 [76,77], and V-HMS [78–81] have been reported in the recent literature. Luan et al. [75] synthesized V-MCM-41 at 95°C using vanadyl sulfate as metal precursor. Gels with Si/V ratios from 10 to 80 were prepared, but chemical analysis of as-made products showed that vanadium was not totally incorporated. In a similar way, Reddy et al. [74] prepared one sample of V-MCM-41 with dodecyltrimethylammonium cations in which 80% of the vanadium originally introduced in the gel was incorporated. The solid was obtained after heating the precursor gel at 100°C for 6 days. V-HMS can be synthesized at room temperature using primary amines as surfactant molecules with Si/V ratios between ca. 400 and 35 ( Table 4) [78,79,81]. For samples with Si/V>50, materials possess high surface areas and the pore diameter increases from ˚ for products prepared with dodecylamine to 25 A Table 4 Characteristics of various calcined V-HMS (from ref. [79]) na

10 12 12 12 16 16 16

Fig. 6. Organization of silicate and titanate species around primary amines to form Ti-HMS by the neutral (S°I°) pathway.

Si/V Gel

Prod.

100 100 50 33 100 50 33

120 108 62 36 109 57 30

S (m2/g)

˚ )b W (A p

788 998 1015 794 1124 1015 753

25 28 28 28.5 34 34.5 35

a Number of carbon atoms in the alkylamine C H NH used n 2n+1 2 as surfactant molecule. b W is the mean pore diameter obtained from N adsorption p 2 isotherms.

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˚ for solids synthesized with hexadecylamine. 35 A For lower Si/V ratios, we observed a significant decrease in surface area upon calcination in air, suggesting that materials with high V contents are not thermally stable [79]. The coordination and oxidation state of vanadium species in as-made and calcined V-modified mesoporous silicas have been studied using UV–vis, EPR and NMR spectroscopy. UV–vis spectra of as-synthesized V-MCM-41 prepared with vanadyl sulfate show two absorptions at about 275 and 340 nm that were assigned to charge transfer transitions between oxygen atoms and a central V5+ ion, as already observed for V5+ centers in zeolitic structures [75]. Since the products were synthesized with a V4+ precursor, the authors concluded that most of the V4+ species were oxidized to V5+ during synthesis. However, the presence of an EPR signal showed that some of the vanadium species were not oxidized and remained in the form of vanadyl VO2+ species. Two different V4+ sites were observed, as for V-silicalite-1 [82], with g values and hyperfine coupling constants typical of VO2+ ions in a square pyramidal coordination. Exposure of as-made products to air for long periods resulted in a drastic decrease in the intensity of the EPR signal, suggesting that VO2+ species could easily be oxidized to V5+ at room temperature. This is probably one of the reasons why Sayari and coworker [78] did not observe EPR signals on their as-synthesized V-MCM-41 sample, which was synthesized at higher temperature and for a longer period. Nevertheless, these authors attributed the lack of the EPR signal in as-made solids to the presence of V4+ species in highly symmetrical lattice positions, thus preventing their observation even at 77 K. Since V-HMS are prepared at room temperature, oxidation of the vanadium precursor does not occur and all V species in freshly prepared materials are in the 4+ oxidation state. When samples are prepared with vanadyl acetylacetonate, the UV–vis spectra of as-made samples are composed of a single narrow absorption band at ca. 315 nm, very similar to that observed for an aqueous solution of the vanadium salt [79] (Fig. 7). The EPR spectra show a well-defined

Fig. 7. UV–vis spectra of as-synthesized ( left) and calcined (right) V-HMS (Si/V=109). Freshly calcined (a) and exposed to ambient atmosphere for 5 min (b), 10 min (c) and 30 min (d ) (from ref. [79]).

signal typical of isolated VO2+ in a square pyramidal coordination (Fig. 8). Attempts to remove the occluded template from the mesopores by solvent extraction resulted in an almost complete removal of vanadium from the solids. This indicated that vanadium is probably occluded in the mesopores of as-made materials, but not yet anchored to the silica, with no significant amounts of V species inside the walls. However, the observation of an

Fig. 8. EPR spectra of V-HMS (Si/V=109) recorded at 77 K. As-synthesized sample (a), calcined (b) and contacted with toluene at 25°C for 1 h after calcination (c) (from ref. [79]).

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EPR spectrum at room temperature suggested that the mobility of VO2+ species inside the channels was very low. For both V-MCM-41 and V-HMS materials, calcination of as-made products in air results in the oxidation of VO2+ species to V5+ as evidenced by a decrease in the intensity of the EPR signal and drastic modifications in UV–vis spectra [75,79,81]. Luan et al. [75] observed that the EPR signal did not completely disappear upon calcination, and concluded that some of the vanadyl species are at inaccessible sites. Following these authors, accessible sites would be on the internal surface of the mesopores whilst inaccessible sites would be inside the silica walls, thus supporting the observation of two EPR signals in as-made solids. In contrast, calcination of V-HMS materials resulted in a complete disappearance of the EPR signal (Fig. 8). This was consistent with V4+ species entrapped inside the mesopores with templating molecules in as-made materials and attached to the surface of the pores after calcination. Therefore, calcined V-HMS materials are probably similar to V-MCM-48 materials reported by Morey et al. [76 ] and obtained by treating a pure silica MCM-48 with vanadyl isopropoxide. Immediately after calcination, dried V-HMS are white and the corresponding UV–vis spectra show a single absorption band around 245 nm, that can be unambiguously assigned to V5+ species in a tetrahedral environment (Fig. 7). However, immediately after exposure of the samples to ambient atmosphere, the color changes from white to bright yellow and an additional peak emerges at about 380 nm in the UV–vis spectra. The intensity of the latter signal increases with time whilst that at 245 nm decreases. This is consistent with a change in coordination of V5+ species by direct coordination of water molecules to the vanadium centers. The situation is completely reversible and the initial spectrum is restored upon outgasing the sample at 200°C for 3 h. The reversibility indicates that no V–O–V linkages were formed upon calcination and further hydration of V-HMS samples. According to the literature, UV–vis bands at 380 nm have been assigned to VV species with a short VNO bond associated with three longer V–O bonds in interaction with water molecules

163

[83]. It is interesting to note that the position of the newly formed species is different from that observed by Morey et al. [76 ] for hydrated V-MCM-48. Indeed, these authors observed that hydration of the samples resulted in the appearance of UV–vis peaks at 415 and 440 nm, attributed to square pyramidal and octahedral VV species, respectively. However, our results are in very good agreement with those obtained by other groups on V-MCM-41 and V-HMS [75,81]. As already reported for V-containing zeolites and aluminophosphates, VV species in calcined V-HMS can be readily reduced at low temperature by organic molecules like polymethylbenzene [72,84]. Indeed, 90% of the EPR intensity was restored upon contacting a calcined dry V-HMS sample with toluene at 25°C (Fig. 8). This indicated that VV species are easily accessible to organic molecules and that they can readily change their oxidation state at low temperature, which makes V-HMS interesting candidates for redox catalytic reactions. The change in coordination of V centers in calcined V-HMS has also been monitored by 51V NMR. Previous studies on supported V O cata2 5 lysts and V-containing zeolites have shown that, at high fields, the line shape is dominated by the chemical shift anisotropy, which is characteristic of the coordination of the V centers [76,85]. Due to the large chemical shift anisotropy of V5+, static measurements are often preferable to MAS experiments as spinning the sample at the magic angle results in the observation of numerous side bands, not always very easy to interpret. Typically four-coordinated V species in a distorted tetrahedral environment show a rhombic anisotropic spectrum with a maximum at d between −500 ) and −800 ppm, whilst distorted octahedral species are characterized by a nearly axially symmetric spectrum with d in the range −300 to ) −500 ppm/VOCl [86 ]. As-made V-HMS materi3 als do not show any NMR signal, consistent with the absence of V5+ species. This was not the case for V-MCM-41 synthesized at high temperature, for which part of the V4+ precursor species was oxidized during synthesis. Indeed, Luan et al. [75] observed a 51V NMR line at ca. −560 ppm for as-synthesized V-MCM-41, characteristic of tetrahedrally coordinated VV species. Freshly calcined

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dry V-HMS materials show a broad signal with a maximum at ca. −550 ppm, consistent with the presence of tetrahedral VV species and in agreement with UV–vis data ( Fig. 9). The NMR line shape and position are very similar to those reported by Morey et al. [76 ] for V-MCM-48 solids. Upon hydration, the spectrum is broadened and a new signal is observed with a maximum intensity around −320 ppm relative to VOCl , 3 which characterizes octahedrally coordinated VV species. Nutation experiments have shown that this new signal was not due to polycondensed oxide species, which was also supported by its disappearance upon outgasing the samples at 200°C [79]. Similar changes in 51V NMR spectra upon hydration of calcined samples have also been observed by Luan et al. [75] for V-MCM-41 and by Reddy et al. [81] for V-HMS. Recent papers have reported that V-modified mesoporous silicas catalyze the oxidation of various organic substrates in the liquid phase with both H O and TBHP as oxidizing agent. In 2 2 particular, Reddy et al. [81] reported high conversions in the oxidation of 2,6-di-tert-butyl phenol with excellent quinone selectivities using TBHP as oxidant. Other substrates like phenol, naphthalene,

Fig. 9. 51V NMR spectra of calcined V-HMS (Si/V=109). Freshly calcined (a) and after exposure to ambient atmosphere for 1 h (b) (from ref. [79]).

cyclododecanol were also selectively oxidized into the corresponding ketones with H O or TBHP. 2 2 However, as underlined by the authors, catalysts were not stable in the presence of dilute H O and 2 2 most V species were leached from the solids after the first reaction run. The stability was increased in the presence of TBHP but seemed to depend on a lot of factors like the solvent or substrate nature, as well as on the nature of the vanadium precursor used to prepare the catalysts. This was also the conclusion of Neumann et al. [87], who performed the oxidation of linear and cyclic alkanes over V-MCM-41 with TBHP. Nevertheless, these authors claimed that V-MCM-41 could activate molecular oxygen, and took advantage of this property to form in situ peroxo radicals from isobutyraldehyde at room temperature. These radicals further reacted with secondary carbons, thus leading to the very selective formation of the corresponding ketones. According to this work, no leaching was observed and the catalyst could easily be regenerated by calcination in air. 3.4. Incorporation of zirconium We have previously reported that it was possible to prepare zirconium-containing mesoporous silicas using a neutral assembly pathway [63,88]. Zr-HMS materials are obtained following the recipe used for Ti-HMS but using zirconium isopropoxide Zr(OiPr) instead of Ti(OiPr) . 4 4 Products with high surface areas, typically >900 m2/g, can be prepared with Si/Zr ratios in the range 1000–25 ( Table 5). They possess all the characteristics of previously reported HMS materials, in particular regular mesopores with diameters ˚ and a pore volume of ca. 0.7 cm3/g. of ca. 36 A As for titanium, all zirconium introduced into the precursor gel is incorporated in the final solids. Extraction of the organic phase with boiling ethanol did not modify the chemical composition of the products, indicating that, in contrast to vanadium, Zr species were strongly bonded to the silica framework in as-made materials. The dispersion of Zr species in silica was studied by UV–vis spectroscopy. As-made and extracted samples showed a sharp absorption peak at ca. 210 nm without any signal typical of bulky ZrO ( Fig. 10). 2

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Table 5 Characteristics of calcined Zr-HMS samples (from ref. [63]) Si/Zr Gel

Prod.

1000 500 100 50 25

1085 525 125 47 26

S (m2/g)

˚ )a W (A p

V (cm3/g)b

1021 1035 1049 990 890

34 34 33 36 36

0.71 0.70 0.72 0.65 0.62

a W is the mean pore diameter obtained from N adsorption p 2 isotherms. b V is the mesopore volume measured at p/p =0.5 in the N 0 2 isotherm.

However, as for titanium, the position of the UV band did not completely exclude the presence of tiny oxide particles within the mesopores. The absence of reliable data concerning UV–vis spectra of Zr-containing molecular sieves and silicates did not allow us to conclude the real nature and coordination of Zr species in Zr-HMS. Adsorption of pyridine on the samples revealed the presence of exclusively Lewis acid sites, thus contrasting with Ti-modified materials where Bro¨nsted acid sites were observed for relatively high Ti loading ( Fig. 11). Moreover, temperature programmed desorption of ammonia showed that these Lewis sites were relatively strong, similar to those observed in Al-HMS [63].

Fig. 10. UV–vis spectra of as-synthesized Zr-HMS (Si/Zr=47) (a) and bulky ZrO (b). 2

Fig. 11. Infrared spectra of pyridine adsorbed on Ti-HMS (Si/Ti=19.5) (a) and Zr-HMS (Si/Zr=26) (b). Same experimental conditions as in Fig. 5.

Zr-HMS showed interesting catalytic properties in a variety of oxidation reactions with both H O and TBHP as oxidizing agent. In the oxida2 2 tion of substituted amines, activities and selectivities were similar to those obtained over Ti-HMS [63]. Turnover numbers increased with Zr content, indicating that Zr species were effectively the active sites. Bulky substrates like 2,6-di-tert-butyl phenol were also selectively oxidized to the corresponding quinone with selectivities higher than 85%, and we never observed any leaching of Zr from the solids even after several catalytic runs. For epoxidation reactions, Zr-HMS behaved differently from Ti-HMS. Activities and turnover numbers were comparable for samples with the same metal content, but selectivities in epoxide were systematically lower on Zr-HMS. As an example, Table 6 compares the results of the epoxidation of norbornene with H O over Ti- and 2 2 Zr-HMS. The selectivity in epoxide, which is close to 70% for Ti-HMS, decreases to about 20% for Zr-HMS. The same trend was observed during the oxidation of cyclohexene with H O ( Table 7). 2 2 The selectivity in cyclohexene oxide decreased from 75% to 55% from Ti-HMS to Zr-HMS, whilst that in cyclohexanediol increased from 22% to 45%. These differences in selectivities were attributed to the higher strength of the acid sites in Zr-HMS as evidenced by TPD of ammonia, thus capable of catalyzing the ring opening of the epoxide. As for Ti-HMS materials, extracted samples

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Table 6 Epoxidation of norbornene over calcined Ti- and Zr-HMS (from ref. [63]) Catalyst

Si/Me

Substrate conv. (%)

H O conv. (%) 2 2

Epoxide sel. (%)

Alcohol sel. (%)

Zr-HMS Ti-HMS

47 52

7.5 6.5

74 82

20 70

80 30

Reaction conditions: 0.5 g catalyst, 0.1 mol norbornene, H O /norbornene=0.2, T=70°C, solvent: acetonitrile. Data were obtained 2 2 after 3 h reaction.

showed high epoxide selectivities compared with calcined samples, essentially due to the lower formation of cyclohexenol by a radical mechanism ( Table 7). 3.5. Incorporation of other elements A few recent publications report on the synthesis and basic characterization of mesoporous silicas modified by metals like tin [89,90], chromium [77,80,91], manganese [92] or molybdenum [80]. Sn-MCM-41 with Si/Sn ratios between 178 and 83 were successfully prepared by Das et al. [89]. The authors observed that XRD d spacings, pore 100 diameters and sorption capacities increased with Sn content of the materials. Incorporation of tin in the silica framework was also supported by an increase in intensity and a shift with respect to pure silica materials of the IR band at 970 cm−1. These materials showed interesting catalytic properties in the oxidation of aromatics with TBHP Table 7 Oxidation of cyclohexene over various Zr-HMS samples Si/Zr

1085 525 125 47 26

C 2 (%)a H2O

79 87 90 92 97

Select. (%)b Epox.

Alc.

Diol.

50 (98) 52 (81) 48 (72) 32 30

35 (2) 30 (9) 27 (6) 23 22

15 (0) 18 (10) 25 (22) 45 48

Reaction conditions: 0.5 g catalyst, 0.1 mol cyclohexene, H O /cyclohexene=0.2, T=70°C, solvent: acetonitrile. Data 2 2 are obtained after 3 h reaction. a H O conversion. 2 2 b Selectivities in cyclohexene oxide ( Epox.), cyclohexenol (Alc.) and cyclohexanediol (Diol.). Values in parentheses correspond to extracted samples.

and the hydroxylation of phenol and 1-naphthol with dilute hydrogen peroxide. Sn-MCM-41 were much more active than Sn-impregnated pure silica MCM-41, and the difference was attributed to the low dispersion of the metal species in the latter. Abdel-Fattah and Pinnavaia [90] prepared Sn-HMS at room temperature with dodecylamine as surfactant molecule. The material was characterized by means of XRD and N adsorption/ 2 desorption without any indication on the nature and coordination of Sn species. The solid was used to form poly -lactic acid with relatively high molecular mass and low polydispersity. The same reaction performed with Sn-doped SiO or 2 bulky SnO yielded polymers with lower molecular 2 mass and higher polydispersities. Ulagapan and Rao [91] synthesized Cr-MCM-41 materials with different chromium sources. The solids possessed relatively low surface areas (550 m2/g) and no indication on the Cr content in the materials was reported. However, the authors observed an increase in the XRD d reflection with Cr content in the gel, which 100 was considered as a proof for chromium incorporation in the silica framework. EPR and UV–vis spectroscopy clearly showed the presence of CrIII species in as-prepared materials that were oxidized to CrVI upon calcination. Cr-MCM-41 was significantly active in the hydroxylation of phenol, 1-naphthol and in the oxidation of aniline with aqueous H O . However, the possibility of leaching 2 2 of Cr species in the presence of H O , which is 2 2 currently observed for most Cr-containing zeolites and aluminophosphates, was not mentioned. Cr-MCM-41 and Cr-HMS with 0.61 and 1.4 wt% Cr, respectively, also showed an activity in the hydroxylation of benzene with H O [80]. 2 2 Cr-MCM-48 was also found to be active in the

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oxidation of methyl methacrylate and styrene with H O , but the possibility of homogeneous catalysis 2 2 could not be refuted as Cr species were rather grafted onto the surface of the mesopores and partially leached in the presence of dilute acetic acid [77]. Mn-modified MCM-41 as well as the cubic analog MCM-48 have been synthesized by Zhao and Goldfarb [92] in both basic and acidic media with very low Mn/Si ratios (0.0004–0.09). On the basis of EPR measurements performed on as-synthesized and calcined samples, they concluded that Mn species were reduced from MnIII to MnII upon calcination and that they were not located in the silica walls but rather inside the mesopores.

4. Concluding remarks The recent discovery of mesoporous silica-based molecular sieves has opened new opportunities in various fields of applications including catalysis. Their framework composition can easily be modified by the incorporation of a number of trivalent or tetravalent elements, thus providing solids with potential applications in acid or redox reactions. As far as trivalent metals are concerned, modified mesoporous silicas can be synthesized within a quite broad range of composition. Products with high metal contents can be obtained but they usually suffer from a lack of stability upon thermal treatments. This can be circumvented by using primary amines as surfactant molecules, which can be removed from as-made products by solvent extraction. Ti-, V- and Zr-containing mesoporous molecular sieves have also been successfully synthesized using both ionic and neutral assembly pathways. Metal species are usually highly dispersed in the silica framework, with a coordination slightly different from that observed in zeolites. Indeed, in contrast to zeolites, incorporated species can be located either on the internal surface of the mesopores or within the silica walls. When species are located on the surface of the mesopores, they are coordinated to water molecules, but can readily change their coordination upon outgasing the samples at low temperature. Even though trivalent

167

metal-containing mesoporous silicas like AlMCM-41 or Al-HMS possess a moderate acidity, they have shown interesting activities in reactions that do not require the presence of strong acid sites. Ti-, V- and Zr-modified systems have also been used in the oxidation of bulky olefins with both H O and organic hydroperoxides, for which 2 2 zeolite-based systems are inactive due to steric restrictions. Whilst Ti- and Zr-containing catalysts are relatively stable under reaction conditions, it was reported that V species could easily be leached from the silica framework, which limits their applications as true heterogeneous catalysts. It is usually observed that these catalysts show a higher activity than the corresponding mixed oxides. This can be attributed to their high surface area and the improved accessibility of the active sites. Moreover, their unique behavior in some of the reactions is also a consequence of their hydrophobic/hydrophilic character that was found to be intermediate between those of zeolites and mixed oxides.

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