Mesoporous SnO 2 – SiO 2 and Sn – silica – carbon nanocomposites by novel non-hydrolytic templated sol – gel synthesis †

A novel non-hydrolytic sol – gel (NHSG) synthesis of mesoporous tin silicate xerogels is presented. The polycondensation between silicon tetraacetate, Si(OAc) 4 , and tetrakis(diethylamido)tin, Sn(NEt 2 ) 4 , resulting in acetamide elimination leads to tin silicate xerogels containing Si – O – Sn linkages. The addition of Pluronic P123 or F127 templates provides homogeneous sti ﬀ gels that are, after template removal by calcination at 500 (cid:1) C in air, converted to stable mesoporous silica xerogels with large surface areas (476 m 2 g (cid:3) 1 ) and dispersed SnO 2 nanoparticles (6 – 7 nm). Heat treatment of the as-prepared tin silicate gels in an inert N 2 atmosphere leads to reduction and transformation to Sn nanoparticles (70 – 150 nm) embedded in a silica – carbon matrix. The composition and morphology of the xerogels, volatile reaction byproducts, and thermal transformations were followed by elemental analyses, IR spectroscopy, thermal analysis TG-DSC, nitrogen adsorption measurements, solid-state NMR spectroscopy, DRUV-vis spectroscopy, electron microscopy, and HT powder XRD. The SnO 2 – SiO 2 xerogels were tested as potential catalysts for aminolysis of styrene oxide with aniline and for the Meerwein – Ponndorf – Verley reduction of 4- tert -butylcyclohexanone. The resulting reaction systems displayed good activity and selectivity.


Introduction
Silicates containing tin have attracted much attention due to their surface acidity and catalytic activity in a variety of reactions (hydroxylation of phenols, 1 Baeyer-Villiger oxidations, 2 Meerwein-Ponndorf-Verley reductions, 3 sugar isomerization, 4 epimerization, 5 oxidative dehydrogenation of cyclohexane, 6 and Cannizzaro-type reactions 7 ). SnO 2 /SBA-15 materials or SnO 2 -SiO 2 nanocomposites were studied as gas sensors, 8 photocatalysts for rhodamine B degradation 9,10 or catalysts for benzylation of aromatics 11 and C-C coupling reactions. 12 The efficiency of these processes increases with the specic surface area of SnO 2 particles. The factors which inuence the surface area, such as primary particle diameter and state of aggregation of the particles, are of great interest. 13 In order to prepare individual primary particles with diameters below 10 nm, and avoid their aggregation, templates based on porous materials are widely used as supports in nanoparticle synthesis. 9,14 Many reports are devoted to porous materials such as Sn-ZSM12, Sn-beta zeolites, Sn-MFI, Sn-MCM, Sn-SBA, and Sn-KIT-5. 2,4,8,12,[15][16][17] Preparation procedures for mesoporous tin silicate materials involve addition of a tin precursor (SnCl 2 , SnCl 4 $5H 2 O) into an appropriate mesoporous silica (MCM, SBA, KIT-5) reaction mixture. Aqueous solutions of TEOS with HCl and Pluronic P123 or F127 are used in the case of SBA or KIT-5 supports. 8,18 The resulting mesoporous tin silicates were used in a variety of catalytic reactions and they displayed high activities. For example, Endud used Sn-MCM-48 molecular sieve (1291-1585 m 2 g À1 ) as a catalyst for oxidation of benzyl alcohol with TBHP. 17 An interesting method for the preparation of nanocrystalline Sn-beta zeolite catalysts for ring-opening hydration of epoxides was published by Tang. 16 This approach is based on dealumination of commercial beta aluminosilicate zeolite with nitric acid and a subsequent reaction with (CH 3 ) 2 SnCl 2 . Recently, Reddy successfully applied mesoporous (582-732 m 2 g À1 ) tin silicates as catalysts for acyl Sonogashira coupling reactions with yields up to 97%. 12 The major problem of hydrolytic sol-gel processes is the disparity in the hydrolysis reaction rates of different precursors. The non-hydrolytic method [19][20][21] involves completely different mechanism of polycondensation reactions in non-aqueous media and offers decisive advantages, such as control of composition, homogeneity, structure, and texture. 22 However, the non-aqueous synthesis of SnO 2 -SiO 2 materials is not well developed and only two reports were published on this topic. The rst describes a procedure based on the reaction of TEOS with tin(IV) acetate in anhydrous acetic acid. 23 The second report introduces the twin polymerization method for the production of SnO 2 -SiO 2 materials. 24 When compared to conventional hydrolytic methods, non-hydrolytic sol-gel chemistry provides effective routes for the synthesis of homogeneous monolithic gels, nanoparticles 25 and heterogeneous catalysts. 19,20 We have recently reported that non-hydrolytic acetamide elimination 26 can be successfully used for the synthesis of mesoporous titanosilicate, zirconium silicate, and aluminosilicate catalysts. [27][28][29] In the work presented here, this reaction was used for the preparation of tin silicate xerogels. Templated onepot sol-gel method provided homogeneous stiff gels with a high content of Sn. We were also able to prepare SnO 2 nanoparticles embedded in mesoporous silica matrix which prevented them from aggregation. Resulting SnO 2 -SiO 2 xerogels were tested as catalysts for aminolysis of styrene oxide. Heating of dried gels in nitrogen at 500 C caused reduction of Sn(IV) species and Snsilica-carbon nanocomposites were obtained.

General procedures
All manipulations were performed in an atmosphere of dry nitrogen using Schlenk techniques or in an M. Braun dry box with both H 2 O and O 2 levels below 1 ppm.

Synthesis of tin silicates
Tin silicate xerogels were synthesized by templated acetamide elimination according to eqn (1).
SiSnFat ambient pressure Sn(NEt 2 ) 4 (1.229 g; 3.054 mmol) was added dropwise with a syringe to a stirred solution of Si(OAc) 4 (0.789 g; 2.99 mmol) and Pluronic F127 (1.92 g; 0.152 mmol) in toluene (40 cm 3 ). Aer the addition, the color changed to light orange-yellow. The reaction mixture was heated to 80 C and a transparent light yellow-brown stiff gel was formed aer 20 min. The reaction temperature was maintained for 168 h, then the volatile byproducts were separated in vacuo and the solid product (yellow gel) was dried under vacuum for 24 h. Yield 3.074 g, DC% ¼ 59%. TG/DSC: (air, 5 C min À1 ) weight loss at 1000 C 69.40%; (N 2 , 5 C min À1 ) weight loss at 1000 C 69.21%.
SiSnF dried xerogel was calcined in a tube furnace at 400 (SiSnF-400) and 500 C (SiSnF-500) for 3 h in air to remove the template. Carbonization of template and reduction of Sn species was performed by heat treatment under N 2 at 400 (SiSnF-400N) or 500 C (SiSnF-500N) for 3 h.
SiSnPat ambient pressure The synthesis with Pluronic P123 was carried out in analogous manner. Results of analytical measurements are discussed in later sections.
SiSnPAin an autoclave (autogenous pressure) Si(OAc) 4 (0.405 g, 1.53 mmol) was dissolved in CH 2 Cl 2 (18 cm 3 ) and mixed with P123 (0.39 g, 0.067 mmol) solution in toluene (2.0 g of solution, 19.7 wt%). Then Sn(NEt 2 ) 4 (0.459 g, 1.12 mmol) was added and the reaction mixture was sealed in a Teon-lined autoclave. The autoclave was heated in an oven at 110 C. Aer 5 days the reaction was stopped and the autoclave was opened in a glove box. The liquid phase was separated and the resulting gel was placed into a Schlenk ask and dried under vacuum. Yield: 0.591 g. TG/DSC: (air, 5 C min À1 ) weight loss at 1000 C 61.71%; (N 2 , 5 C min À1 ) weight loss at 1000 C 61.45%.

Catalytic reactions
Calcined tin silicate xerogels were degassed before the reaction under vacuum at 115 C for 20 min.
The MPV reduction 31 of 4-tert-butylcyclohexanone was carried out in a 50 cm 3 round-bottom Schlenk ask equipped with a reux condenser connected to a dry N 2 source. The reaction mixture containing the calcined tin silicate catalyst (25 mg), 4-tert-butylcyclohexanone (500 mg, 3.54 mmol), and dry 2propanol (15 cm 3 , 0.20 mol) was stirred under reux for 1 h.
The products were analyzed by 1 H NMR spectroscopy and the GC-MS method using 0.100 cm 3 of nonane as an internal standard.
Characterization IR spectra were recorded on a Bruker Tensor 27 FTIR spectrometer using KBr pellets or on a Bruker Alpha-Platinum ATR system. GC-MS measurements were performed on a mass spectrometer TSQ Quantum XLS coupled with a gas chromatograph Trace GC Ultra by Thermo Scientic. The gas chromatograph was equipped with a TS-SQC column (length 15 m, diameter 0.25 mm, lm thickness 0.25 mm) using a temperature program: 50 C (0 min), 5 C min À1 to 80 C, 15 C min À1 ramp to 120 C, 35 C min À1 ramp to 200 C, held at this temperature for 0.5 min. Injection split mode, injector, the interface, and detector temperatures were all set to 200 C. The column pressure was 31.5 kPa and the ionization energy was 70 eV. For liquid phase 1 H and 13 C NMR measurements, a Bruker Avance III DRX 300 MHz spectrometer was used. The spectra were referenced to the residual proton or carbon resonances of benzene-d 6 (7.15 and 128.0 ppm) and CDCl 3 (7.20 and 77.0 ppm). 29 Si solid state NMR spectra were measured on a Bruker Avance III HD 700 MHz spectrometer with a MAS DVT 700S4 BL4 N-P/H probe. Chemical shis were referenced externally to 29 Si d [(Me 3 SiO) 8 Si 8 O 20 ]: 11.72 ppm; 13 C d [adamantane] 38.68 ppm. High temperature powder XRD diffractograms were recorded on an X'Pert PRO diffractometer equipped with a Co Ka X-ray tube and a HTK 16 high-temperature chamber (Anton Paar, Graz, Austria) with a Pt holder. Samples were measured from 500 to 1250 C in 50 C increments. The sample was held during scanning at a constant temperature for 12 min. Powder diffraction patterns were collected with the PANalytical X'Pert PRO diffractometer equipped with conventional X-ray tube (Cu K a radiation, 40 kV, 30 mA) and a linear position sensitive detector PIXcel with an anti-scatter shield. Qualitative analysis was performed with HighScorePlus soware package (PANalytical, The Netherlands, version 4.1.0), and Match3. For quantitative analysis of samples, we used Diffrac-Plus Topas (Bruker AXS, Germany, version 4.2). This program permits to estimate the weight fractions of crystalline phases by means of Rietveld renement procedure. 33 Nitrogen adsorption/ desorption experiments were performed at 77 K on a Quantachrome Autosorb-1MP porosimeter. Surface areas (SA) and total pore volumes (V tot at p/p 0 ¼ 0.98) were determined by volumetric techniques. 34,35 Prior to the measurements, the samples were degassed at 100 C for at least 24 h until the outgas rate was less than 0.4 Pa min À1 . The adsorptiondesorption isotherms were measured for each sample at least three times. The specic surface area was determined by the multipoint BET method using at least ve data points with relative pressures between 0.05 and 0.30. 34,35 The valid pressure ranges were established by the Rouquerol transform and equivalent BET surface areas were calculated. Pore size distributions were calculated by the NLDFT method from the adsorption branch of the isotherms with the use of a nitrogenon-silica kernel and by the QSDFT method for carbonized samples. Thermal analysis (TG/DSC) was performed on a Netzsch STA 449C Jupiter apparatus in a stream of air (70 cm 3 min À1 ) with a temperature gradient of 5 C min À1 to 1000 C, in a Pt/Rh crucibles. Tin and silicon contents were determined on an ICP optical emission spectrometer iCAP 6500 Duo (Thermo, UK) equipped with a solid-state generator with a frequency of 27.12 MHz and a maximum power input 1350 W. For Si analysis, wavelengths of 212.4 and 251.6 nm were used. Sn was measured at 189.9 nm. Elemental analysis (C, H, and N) was performed on a Flash 2000 CHNS Elemental Analyzer (Thermo Scientic) at Palacky University, Olomouc. TEM images were obtained from a Philips CM 12 TEM/STEM system with EDAX Phoenix EDS, and JEOL JEM2010 and 3010 microscopes equipped with EDX detectors and CCD with resolution 1024 Â 1024 pixels. Studies of surface acidity were performed on air-calcined xerogels (cca 50 mg) which were dried under vacuum (1 h, 115 C) before adsorption. Then, the samples were exposed for 30 min to pyridine vapors under its autogenous pressure at 25 C. Aer adsorption, samples were dried under vacuum at r.t. for 2 hours. Adsorbed pyridine was characterized by the IR technique.
In the template-assisted reactions between Si(OAc) 4 and Sn(NEt 2 ) 4 , the yield of the product as well as the mass of starting precursors were precisely weighed to allow gravimetric estimation of the degree of condensation, DC ¼ 100(n total À n residual )/ n total , where n total is the molar amount of organic groups in the starting materials and n residual is molar amount of residual organic groups in the xerogel is based on the difference of theoretical and experimental yield. As the condensation reactions were never quantitative, the degree of condensation represents the relative difference between the maximum theoretical loss of Et 2 NC(O)CH 3 (eqn (1)) in comparison to what is experimentally observed. This difference also denes the number of acetoxy groups on silicon and diethylamide groups on tin that are le in the matrix.

Results and discussion
Tin silicate xerogels were synthesized by novel non-hydrolytic acetamide elimination according to eqn (1). We successfully applied this approach for the sol-gel synthesis of titanosilicates, zirconium silicates, and aluminosilicates. This polycondensation results in the formation of gels containing Si-O-M (M ¼ Ti, Zr, Al) networks. [26][27][28][29] These xerogels are unfortunately microporous and aer calcination nonporous. A signicant improvement is achieved with the addition of block-copolymer templates (Pluronic P123, F127) as structure directing agents. This modication allows obtaining stiff transparent gels. The templates are then removed by calcination in air at 500 C and the nal products exhibit mesoporous character with high surface areas. In this work we apply template-assisted acetamide elimination for the preparation of mesoporous tin silicates. The stoichiometric equation represents a complete polycondensation leading to the Si-O-Sn network and quantitative elimination of N,N-diethylacetamide. However, under experimental conditions, the condensation is incomplete and unreacted acetate and amide functional groups were observed in the xerogels. The reaction parameters are summarized in Table 1. For the comparison and modication of textural properties and surface areas, the reaction was performed also in a Teon-lined autoclave in dry CH 2 Cl 2 solvent. The precursors were mixed together in the autoclave inside a glove box. Then, the autoclave was placed into an oven and heated at 110 C.
All reactions proceed with the formation of transparent yellowish gels which are dried under vacuum to dark orange rubbery products. GC-MS analysis of volatile byproducts conrmed N,N-diethylacetamide as the only reaction byproduct of heterocondensation (Fig. 1S †). Templates were removed by calcinations in air at 400 and 500 C. Heat treatment in the atmosphere of N 2 at 400 and 500 C was employed as an effective method for carbonization and reduction of Sn species to metallic tin nanoparticles.
IR spectra of dried gels presented in Fig. 1 show vibrational bands corresponding to Si-O-Sn linkages 18,36,37 at 940-956 cm À1 , Si-O-Si at 793-799 and 1014-1030 cm À1 , and Sn-O-Sn 38 at 667 cm À1 . The residual acetoxy groups are represented by asymmetric and symmetric COO stretches at 1564 and 1450 cm À1 , respectively. The difference between symmetric and asymmetric carboxylate vibrational bands is 110-120 cm À1 and according to Deacon-Phillips rules, this is indicative of their bidentate bridging mode on metal centers. 39 Diethylamido groups display an absorption band at 1370 cm À1 . An intense band at 1100 cm À1 is attributed to C-O-C vibrations of templates.
TG/DSC analyses of dried products performed under air atmosphere show the highest mass loss between 200 and 400 C (54-60%). This decrease is caused by oxidation and decomposition of the residual organic groups and templates. An interesting behavior is observed by the DSC method where samples show exothermic peaks between 350 and 400 C. These effects are attributed to oxidation of copolymer template. Exothermic peak between 400 and 530 C can be connected with the crystallization of SnO 2 in silicate matrix ( Fig. 2 and 2S †). This crystallization effect displayed by the sample SiSnPA (Fig. 3S †) prepared in an autoclave at 110 C is weaker than in the case of samples SiSnP and SiSnF (Fig. 2S † and 2) synthesized at 80 C. This may imply better stabilization of Sn in the Sn-O-Si matrix in xerogels prepared by polycondensation at higher temperature. A higher degree of condensation and crosslinking agrees with a lower organic content and thus a higher residual mass le aer heating to 1000 C during the TG/DSC experiments (Table 1). DSC measurements performed in nitrogen atmosphere revealed endothermic peaks between 390 and 410 C. These effects could correspond to reduction of Sn-O-Si species and subsequent formation of Sn metallic nanoparticles. 40 In the case of SiSnP sample synthesized with P123, the maximum of endothermic peak is located at 399 C (Fig. 4S †). The DSC curve of SiSnF sample prepared with F127 template shows an exothermic effect followed by an endothermic peak with the maxima at 406 C (Fig. 2). The SiSnPA DSC curve (Fig. 5S †) of TG/DSC analysis performed under N 2 illustrates weak exothermic and endothermic effects.  The presence of SnO 2 and metallic Sn particles mentioned above was established by the powder XRD diffraction analysis. The tin silicate xerogels calcined in air at 400 and 500 C display diffractions of tetragonal cassiterite SnO 2 (PDF 41-1445) ( Fig. 3 and 6S †). Considering the width of SnO 2 diffraction lines, the size of crystallites is very small and remains almost constant on heating up to 1200 C (Fig. 7S, Table 1S †). The Rietveld renement of the high temperature powder XRD patterns shows an average size of crystallites from 7 to 12 nm ( Fig. 7S and 8S †). The size of about 7 nm is retained to 950 C. A signicant increase of the SnO 2 nanoparticle size is observed between 1000 and 1200 C when the size is increased from 8 to 12 nm (Fig. 7S †). In the case of the autoclave-synthesized sample, SiSnPA-500, SnO 2 nanoparticles with the size of 5.9 nm were observed ( Fig. 9S and 10S †). Aer calcination at 1200 C the SnO 2 nanocrystal size increased to 8.3 nm (Fig. 11S †). From this behavior we can conclude that the SnO 2 nanocrystals are stabilized to high temperatures as they are enclosed in the pores of silicate matrix and their growth is largely prevented. The powder XRD patterns of the samples heated in N 2 at 500 C show diffractions assigned to metallic Sn (PDF 04-0673), SnO 2 (PDF 41-1445), and also SnO (PDF 06-0395). The diffractions of metallic tin prove that partial reduction by carbon takes place during heating under N 2 (Fig. 3  and 6S †). The size of Sn nanoparticles in the nanocomposites SiSnF-500N and SiSnPA-500N computed from Rietveld renement is about 146 and 107 nm, respectively ( Fig. 12S and 13S †). Powder XRD diffractogram of the SiSnF-400N sample (heated in N 2 at 400 C) (Fig. 14S †) does not display the lines of metallic Sn and only weak diffractions of SnO 2 were observed. This result is in an agreement with the DSC measurement where the endothermic effect of Sn reduction is observed at 406 C. The SnO 2 phase in samples heated under N 2 can be formed by a phase separation from tin silicate during heat treatment.
The solid state NMR spectroscopy was used to study structural sites of calcined xerogels. The 29 Si CPMAS NMR spectrum (Fig. 4) of calcined sample SiSnF-500 displays resonances with chemical shis of À110, À102, and À92 ppm. These signals can be attributed to silicon environments represented by Si(OSi) 4 , Si(OSi) 3 (OSn), and Si(OSi) 2 (OSn) 2 species (and possibly also Q 3 and Q 2 sites). 41,42 These resonances represent additional hint pointing to the presence of Si-O-Sn bonds in calcined xerogels.   The coordination environment of tin centers in the air calcined xerogel SiSnP-500 was examined by DRUV-vis spectroscopy (Fig. 5). Absorption maxima observed at 222 nm are attributed to tetrahedrally coordinated tin atoms. 17,36,37,41 The shoulder at about 266 nm is indicative of hexacoordinated polymeric Sn species. This spectrum indicates that a part of the tin atoms in the calcined xerogels are tetrahedrally coordinated. These species are not separated as SnO 2 aer calcination, but form a solid solution of SnO 2 in SiO 2 .
Air calcined tin silicate xerogels exhibit surface areas from 164 to 476 m 2 g À1 and total pore volume from 0.28 to 0.58 cm 3 g À1 (Table 2). Adsorption/desorption isotherms belong to type IV with H2 hysteresis at higher p/p 0 pressures, characteristic of mesoporous solids. The average pore diameters reach up to 9.8 nm. The mesopores can be in this case occupied by SnO 2 nanoparticles and this probably accounts for the relatively low pore volumes. On the other hand, their embedding in the pores prevents growth of SnO 2 crystals during heat treatment. The pore diameters are in close agreement with the sizes of SnO 2 nanoparticles computed by Rietveld renement 33 (Table 1S †). The highest surface area of 476 m 2 g À1 with pore volume of 0.58 cm 3 g À1 and pore diameter of 6.6 nm (Fig. 15S, † Table 2) was reached with the calcined product of the reaction performed in an autoclave (SiSnPA-500). This could be a result of a lower Sn content, more extensive crosslinking due to higher reaction temperature, and resulting higher homogeneity. Moreover, the different polarity of CH 2 Cl 2 , as compared to toluene, can affect the arrangement of the template in the reaction mixture. Aer heating under N 2 (500 C) the sample SiSnPA-500N exhibits specic surface area of 123 m 2 g À1 . In this case mesoporosity is retained the average pore diameter is surprisingly higher (9.8 nm) than for the sample calcined in air (SiSnPA-500) ( Fig. 16S and 17S †), and also higher than the pore diameters of other N 2 heated samples. This difference could be caused by a lower content of Sn, higher condensation degree and resulting stability of this sample during the heating in N 2 , and/or by the formation of a lower amount of SnO 2 as compared to the sample calcined in air. The tin silicate xerogel SiSnF-500N heated under nitrogen at 500 C exhibits a lower apparent surface area (121 m 2 g À1 , 0.14 cm 3 g À1 ) than the sample calcined in air (Fig. 6). This decrease is caused by reduction of Sn species and separation of Sn nanoparticles. The pore diameter according to QSDFT    is 3.4 nm and it shows that mesoporous character is retained (Fig. 7). In this case, the hysteresis type is H4 which is characteristic for slit-shaped pores and/or plate-like particles. 43 The same xerogel SiSnF heated in N 2 atmosphere at 400 C reached a surface area of 223 m 2 g À1 (Fig. 6, Table 2). At 400 C, the xerogel framework does not collapse and Sn species are not reduced to metallic tin nanoparticles. This observation is in a good agreement with the powder XRD pattern where diffractions of metallic tin is not observed at this temperature and the DSC endothermic peak for the metallic Sn formation appears above 406 C (Fig. 2). TEM images illustrate the morphology and structure of air calcined and N 2 heated xerogels (at 500 C). Air calcined xerogels display SnO 2 nanoparticles embedded in silica (Fig. 8 le,  Fig. 18S †). Composites formed during heating in N 2 display relatively uniform Sn metallic particles in silica/carbon matrix (Fig. 8 right). The size of these particles is approximately 70-150 nm.
The presence of Lewis and Brønsted acid sites on the surface of air-calcined xerogel (SiSnP-500) was examined by pyridine adsorption followed by IR spectroscopy. Vibrational bands in the IR spectra (Fig. 19S †) were assigned to Lewis (1454 cm À1 ), Lewis + Brønsted (1489 cm À1 ) and Brønsted (1540 cm À1 ) acidic sites. 44 These data indicate that calcined tin-silicate materials possess mixed Lewis and Brønsted surface acidity.

Catalytic studies
The catalytic performance of air-calcined tin silicate xerogels was evaluated in a model reaction of aminolysis of styrene oxide with aniline. 32 For the catalytic tests, 25 mg of a dried (vacuum, 115 C, 1 h) calcined xerogel was used. Reaction mixtures were analyzed by 1 H NMR spectroscopy during the tests. The 1 H NMR spectra of the reaction mixture (in C 6 D 6 ) display resonances attributed to two products: 2-phenyl-2-(phenylamino)ethanol (I) and 1-phenyl-2-(phenylamino)ethanol (II) (Fig. 20S and 21S †). These products were also conrmed by the GC-MS technique. Yields of products were measured aer 5, 60, and 120 min. The tin silicate catalysts exhibit a good activity (up to 63%) and high selectivity for the product I (Table 3). To the best of our knowledge, there are no reports related to SnO 2 -SiO 2 aminolysis catalysts for comparison with our results. However, the Sn-beta zeolite was used to catalyze aminolysis. 45 The details and catalytic activities are given in Table 4. These results show that we achieved comparable conversion and higher selectivity. However, Tang et al. 45 used much lower tin content in their catalyst and TON number is therefore increased. The second type of testing reaction was Meerwein-Ponndorf-Verley reduction 3 of 4-tert-butylcyclohexanone in isopropanol (Fig. 22S †). The resulting products were analyzed by the GC-MS technique.   In this case we observed 1-isopropoxy-4-tert-butylcyclohexane 46 (yield 7%) instead of the 4-tert-butylcyclohexanol as the expected product (Fig. 23S †). This could be caused by the presence of Brønsted active sites on the catalyst.

Conclusions
In this work we presented a novel non-hydrolytic templated acetamide elimination as an efficient one-pot sol-gel method for the preparation of tin silicate xerogels with a high content of tin (up to 52.4 wt%). The polycondensation reaction provides homogeneous transparent stiff gels containing Si-O-Sn linkages and releases N,N-diethylacetamide as the condensation byproduct. Addition of Pluronics P123 and F127 as templates and gelation agents to the reaction mixture was crucial for achieving mesoporous character of xerogels and for preventing pore collapse upon calcination. We also demonstrated the use of autoclave in these reactions and this modication brings the advantage of lower organic residual content and provides higher condensation degree. By varying the heat treatment conditions, the synthesized gels were processed to two types of materials. On one hand, calcination in air atmosphere leads to mesoporous SnO 2 -SiO 2 xerogels with surface areas up to 476 m 2 g À1 and containing stabilized SnO 2 nanoparticles with the size of $6 nm. Their diameter remains constant even up to 950 C and only increases to $12 nm at 1200 C. Air calcined xerogels were tested as catalysts for aminolysis of styrene oxide and featured 63% conversion aer 2 h. Catalysis of MPV reduction of 4-tert-butylcyclohexanone resulted in the 1-isopropoxy-4-tertbutylcyclohexane (7%) as a catalytic product instead of expected alcohol.
On the other hand, heating in N 2 atmosphere resulted in the Sn-silica-carbon nanocomposites. These materials display mesoporous character with a small amount of micropores. The size of Sn particles is in the range of 70-150 nm. Potential application of these nanocomposites could be found in electrode materials. 47 From these data we can conclude that non-hydrolytic templated acetamide elimination can be used as an effective one pot synthetic method for the preparation of SnO 2 -SiO 2 and Snsilica-carbon nanocomposites.