Postsynthesis and Selective Oxidation Properties of Nanosized Sn

ARTICLE pubs.acs.org/JPCC

Postsynthesis and Selective Oxidation Properties of Nanosized Sn-Beta Zeolite Pei Li, Guanqi Liu, Haihong Wu,* Yueming Liu, Jin-gang Jiang, and Peng Wu* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, China ABSTRACT: Nanosized Beta zeolites were postsynthetically modified through the solid-gas reaction of highly dealuminated Beta zeolite with SnCl4 vapor at elevated temperatures. The incorporation mechanism of Sn ions and the physicochemical properties of resultant Sn-Beta-PS were characterized by various techniques. Its catalytic performance in Baeyer-Villiger oxidation was compared with the micrometer-sized Sn-Beta-F hydrothermally synthesized by conventional fluoride method. The Sn species were inserted into the framework via the reaction of the SnCl4 molecules with the silanols in the hydroxyl nests that were created by dealumination and thus occupied predominately the tetrahedral coordination sites. The Sn content gained by postsynthesis reached up to 6.2 wt %, corresponding to a Si/Sn ratio of ca. 35. The isolated Sn species exhibited Lewis acidity useful for the Baeyer-Villiger oxidation of ketones. Containing higher Sn contents and more importantly proposing less diffusion limitations to the substrates with a large molecular dimension, nanosized Sn-Beta-PS was superior to Sn-Beta-F in the selective oxidation of 2-adamantanone with hydrogen peroxide.

’ INTRODUCTION Aluminosilicate zeolites are widely used as heterogeneous catalysts in a large number of industrial processes because they have uniform channels of subnano scale, strong Br€onsted acidity, high thermal/hydrothermal stability, and unique molecular shapeselectivity.1-3 In addition to serving as solid acids, the discovery of the metallosilicates that contain isolated transition metal ions in the framework of high-silica zeolites has opened up new applications as selective redox catalysts especially for developing green oxidation processes. As a representative metallosilicate, the titanosilicate with the MFI topology (TS-1) is capable of catalyzing efficiently a variety of selective oxidation reactions with H2O2 as an oxidant under mild liquid-phase conditions. TS-1 has been already applied to the industrial processes of cyclohexanone ammoximation and propylene epoxidation, making an important breakthrough in zeolite catalysis.4,5 The Ti active sites are thereafter incorporated into the framework of zeolites with a large porosity. For example, Ti-Beta of 12-membered ring (MR) and Ti-MWW with 12-MR supercages have been developed to overcome the pore restrictions of 10-MR TS-1.6-10 The successes achieved with titanosilicates have stimulated the researchers to look for other metallosilicates with novel functionality in redox catalysis. Following the fluoride method procedures developed for synthesizing Al-free Ti-Beta,11 Lewis acid-related tin and zirconium ions are incorporated into the tetrahedral sites of the BEA framework.12-15 The Ti-Beta and Zr-Beta zeolites thus synthesized exhibit unique catalytic properties different from those of titanosilicates. They are expected to explore new environmentally benign redox reaction processes. For instance, Sn-Beta is able to catalyze the chemoselective Baeyer-Villiger r 2011 American Chemical Society

oxidation of ketones or aldehydes with H2O2 by activating directly the organic functional groups of carbonyl,13,16-18 whereas Ti-Beta and Zr-Beta are inactive. On the other hand, Sn-Beta and Zr-Beta both are active catalysts for the MeerweinPonndorf-Verley reduction of ketones with alcohols.14,15,19,20 More recently, the importance of Sn-Beta zeolite has received great attentions in biocatalysis because of its superiority in activating carbonyl groups. Sn-Beta catalyzes actively the conversion of trioses, for example, glyceraldehyde and dihydroxyacetone, into alkyl lactates via a Lewis-acid mediated isomerization/esterification reaction sequence in the presence of alcohols.21,22 The large-pore Sn-Beta catalyst is also capable of isomerizing glucose to fructose activity and selectively in aqueous media.23 Since Sn-Beta is promising for exploring green oxidation processes, developing new preparation methods thus becomes an important research subject. The Sn content obtained by direct synthesis is not high enough because of the difficulty in incorporating the Sn ions with a too large ionic radius (0.71 Å) into the silicate matrix (0.41 Å for Si4þ). Up to date, the synthesis of Al-free Sn-Beta still relies on the traditional fluoride method.12 The fluoride system possesses the advantages of giving a high product yield and enhancing the hydrophobicity of zeolites. Meanwhile, the lower basicity in fluoride media usually delays the nucleation and crystal growth. A long time is then required to realize a full crystallization. The zeolite crystals coming from fluoride media usually have a large crystal size, Received: August 14, 2010 Revised: January 21, 2011 Published: February 11, 2011 3663

dx.doi.org/10.1021/jp1076966 | J. Phys. Chem. C 2011, 115, 3663–3670

The Journal of Physical Chemistry C

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Scheme 1. Strategy for Postsynthesis of Sn-Beta through Combination of Dealumination with Solid-Gas Reaction (AtomPlanting Method)

for example, the crystals of Sn-Beta is in micrometer-range (0.52 μm). This reduces possibly the molecular diffusion inside the zeolite channels in particular for the liquid-phase reactions involving bulky molecules. Furthermore, the fluoride system may bring about equipment corrosion and environment pollution from viewpoint of industrialization. Therefore, it is desirable to find eco-friendly ways for preparing Sn-Beta with higher Sn loadings and smaller crystal sizes. Postsynthesis is a useful alternative for implanting the transition metal ions into zeolite framework. There have been many examples dealing with the preparation of metallosilicates through the solid-gas reaction of highly siliceous zeolites with metal chloride vapors. This so-called “atom-planting method” has been proven to be an effective way for incorporating Al, Ga, Sb, As, In, and Ti into the Zeolite Socony Mobil-Five (MFI) framework.24-27 It is also applied successfully to the postsynthesis of various metallosilicates with the Mordenite (MOR) topology by isomorphous substitution with Sb, Ga, and Ti ions.28-30 In this study, we carried out the postsynthesis of Sn-Beta (SnBeta-PS) almost free of Al from highly dealuminated Beta zeolite and SnCl4 vapor. The Sn incorporation mechanism and the physicochemical properties of Sn-Beta-PS have been characterized by various techniques. Its catalytic properties were evaluated in the Baeyer-Villiger oxidation of 2-adamantanone with H2O2 by comparing with that of conventional fluoride-mediated Sn-Beta-F.

’ EXPERIMENTAL SECTION Synthesis of Catalysts. The procedures for the preparation of Sn-Beta were similar to the postsynthesis method reported for Ti-MOR.29 As illustrated in Scheme 1, the preparation strategy consists of two steps, that is, dealumination and postincorporation of Sn species into the defect sites. A commercial aluminosilicate H-Al-Beta zeolite (Sinopec, Si/Al atomic ratio of 11) was treated in 65 wt % HNO3 at 353 K for 8 h at a solid-to-liquid ratio of 1 g/50 mL, resulting in a highly siliceous zeolite, De-Al-Beta (Si/Al > 1500). For a typical run of SnCl4 treatment, 2 g of DeAl-Beta was first pretreated in a quartz tubular reactor at 773 K for 2 h under a stream of dry N2. The reactor was then brought to desirable treatment temperatures. The N2 flow was diverted through an anhydrous SnCl4 liquid in a glass bubbler. The SnCl4 vapor with a partial pressure of 1.33 kPa was carried by N2 flow to contact the zeolite bed for a determined period of time (2 min to 2 h). After the treatment, the sample was purged with pure N2 at the same temperature for 1 h to remove any residual SnCl4 from the zeolite powder. After cooling to room temperature in N2, the

treated zeolite was washed with deionized water under stirring until chloride ions were not detected in the filtrate by AgNO3 solution and then was dried in air at 363 K overnight. The product is denoted as Sn-Beta-PS-T, where T represents the SnCl4 treatment temperature. For control experiment, Sn-Beta-F zeolites were synthesized directly following the procedures in literature.20 The synthesis was carried out in a fluoride system using SnCl4 3 4H2O as a Sn source and with the addition of dealuminated Beta zeolite as seeds. The synthetic gels had the compositions of SiO2/ 0.005-0.01 SnCl4/0.56 TEAOH/0.54 HF/7.5 H2O. The crystallization was carried out in a Teflon-lined stainless steel autoclave under static conditions at 413 K for 10-40 days. To realize a full crystallization, it was necessary to prolong the heating time with increasing Sn content. The solids were recovered by filtration, extensively washed with water, dried at 373 K overnight, and finally calcined at 853 K for 6 h to obtain Sn-Beta-F. Characterization Methods. The X-ray powder diffraction (XRD) patterns were measured on a Rigaku Ultima IV X-ray diffractometer using Cu-KR radiation (λ = 1.5405 Å). Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 microscope. The Sn content was determined by inductively coupled plasma emission spectrometry (ICP) on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. The silicon released during the SnCl4 treatment was collected by passing the gas effluent through a trap containing 200 mL of 1 M NaOH and then its amount was analyzed by ICP. The specific surface area was measured by N2 adsorption at 77 K on a BELSORPMAX instrument after activating the sample at 573 K under vacuum for at least 10 h. The UV-visible diffuse reflectance spectra were recorded on a Shimadzu UV-2550 spectrophotometer using BaSO4 as a reference. The IR spectra were collected on Nicolet Nexus 670 FT-IR spectrometer in absorbance mode at a spectral resolution of 2 cm-1. The sample was pressed into a self-supported wafer with 4.8 mg cm-2 thickness. The wafer was set in a quartz IR cell that was sealed with CaF2 windows and connected to a vacuum system. The sample was evacuated at 723 K for 2 h. The pyridine adsorption was carried out by exposing the pretreated wafer to a pyridine vapor at 298 K for 0.5 h. The adsorbed pyridine was desorbed successively at different temperatures (423-723 K) for 1 h. All the spectra were collected at room temperature. The 29Si MAS NMR spectra were measured on a VARIAN VNMRS 400WB NMR spectrometer at a frequency of 79.43 MHz using one pulse method, a spinning rate of 3 kHz, a recycling delay of 60 s, and (CH3)3Si(CH2)3SO3Na as a chemical shift reference. The 119Sn MAS NMR spectra were recorded at a frequency of 149.5 MHz 3664

dx.doi.org/10.1021/jp1076966 |J. Phys. Chem. C 2011, 115, 3663–3670

The Journal of Physical Chemistry C when the samples packed in a 7 mm zirconia rotor were spun at 5 kHz. The pulses of 4.5 μs and recycle delays of 50 s were employed. Chemical shift was referred to tetramethyltin using SnO2 as a secondary reference (-604 ppm). For the measurements with dehydrated sample, Sn-Beta-PS was evacuated at 673 K for 5 h and then transferred to the rotor within a glovebox under an atmosphere of nitrogen. Catalytic Reaction and Adsorption. The catalytic tests were performed in a 25 mL flask equipped with a condenser and immersed in a thermostatted bath. The reaction mixture typically contained 2 mmol of 2-adamantanone, 4 mmol of H2O2 (50 wt %), 50 mg of catalyst, and 10 mL of solvent. The mixture was stirred vigorously at 363 K for 10 min to 4 h. In the case of cyclohexanone oxidation, 100 mg of catalyst, 5 mmol of ketone, 2.5 mmol of H2O2 (50%), and 8 mL of MeCN were stirred at 348 K for 3 h. The reaction mixture was subjected to GC analysis (Shimadzu GC-14B) to determine the conversion and product selectivity. The products were identified with GC-MAS (Agilent-6890GC/ 5973MS). The liquid-phase adsorption was carried out using 1,3,5-triisopropylbenzene (TIPB) as a solvent.31 In a typical run, 50 mg of adsorbent was stirred continuously in 2 g of adsorbate solution (0.5 wt % of 2-admantanone in TIPB) at 303 K. A small portion of the liquid was taken out periodically for analyzing the remaining adsorbent.

’ RESULTS AND DISCUSSION Catalysts Preparation. Figure 1 shows the XRD patterns of H-Al-Beta, De-Al-Beta, and Sn-Beta-PS. The samples exhibited well-defined reflections due to the BEA topology. The patterns were comparable in diffraction intensity, indicating no collapse of the crystalline structure occurred during dealumination as well as SnCl4 vapor treatment. The SEM image of Sn-Beta-PS prepared

Figure 1. XRD patterns of H-Al-Beta (a), De-Al-Beta (b) and Sn-BetaPS (6.1 wt %) (c). SnCl4 treatment conditions: temp, 773 K; time, 1.5 h.

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with SnCl4 vapor at 773 K for 1.5 h is shown in Figure 2 together with that of Sn-Beta-F with a Sn/Si ratio of 161 (synthesized at Si/Sn = 150). The crystals of Sn-Beta-PS were composed of nanoparticles the size of which ranged from 60 to 80 nm (Figure 2a). The size of the secondary crystal aggregates in SnBeta-F synthesized in fluoride media was more than 1 μm (Figure 2b). Thus, Sn-Beta-PS was much smaller in crystal size than Sn-Beta-F. The former is then presumed to propose less diffusion limitation to the substrate molecules in particular with a bulky dimension. Table 1 summarizes the results of dealumination of Al-Beta and the dependence of the amount of incorporated Sn on the temperature of SnCl4 treatment. The acid treatment with 6 M HNO3 extracted efficiently the Al species from the framework. The bulk Si/Al ratio increased from 11 for the parent Al-Beta to 1770 for De-Al-Beta, indicating the latter was essentially free of Al. The specific surface increased slightly as a result of the formation of Al-deficient vacancies. In agreement with the above XRD patterns, this is indicative of a well-maintained zeolite structure in De-Al-Beta. When De-Al-Beta was subjected to the SnCl4 treatment, the Sn species were incorporated into the zeolite readily, while the amount of Sn depended on the treatment temperature. When the process time was fixed at 1 h, the suitable treatment temperatures were around 673-773 K, where the Sn amount reached the maximum. At higher or lower temperatures, the Sn incorporation became less efficient. The amount of Si released was negligibly small compared with that of Sn incorporated, even when the SnCl4 treatment was carried out at high treatment temperatures (Table 1). The results strongly suggest that the isomorphous substitution of Sn for the framework Si could be ruled out. The phenomena were very similar to those observed for the alumination or titanination of MOR and MFI zeolites with AlCl3 or TiCl4 vapors reported previously,28,29,32 where the Al or Ti ions were clarified to be inserted into the defect sites in the framework, such as hydroxyl nests composed of four internal silanols. The damage or collapse of the crystalline structure is considered to be negligible as evidenced by the high specific surface areas that the Sn-BetaPS samples possessed (Table 1). To investigate the effect of treatment time on the Sn incorporation, we treated De-Al-Beta with SnCl4 vapor at 773 K for the convenience of experiment operation. As shown in Figure 3, the Sn amount increased rapidly with prolonging the process time and nearly leveled off at 30 min, which is indicative of a saturated Sn incorporation. Meanwhile, the amount of Si released from the zeolite was extremely small, less than 3% of the Sn content even after the treatment for 2 h. The results again denied the replacement of the Sn ions for the Si ions in the framework.

Figure 2. SEM micrographs of Sn-Beta-PS (6.1 wt % Sn) (a) and Sn-Beta-F (1.17 wt % Sn) (b). 3665



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Table 1. Dealumination of Al-Beta and Postsynthesis of Sn-Beta with SnCl4 Vapor Treatment at Various Temperatures sample

SnCl4 treatmenta temp (K)

H-Al-Beta De-Al-Beta

Si/Al ratiob

Sn contentb (mmol g-1)

released Sib (mmol g-1)

surface areac (m2 g-1)

PVd (mL g-1)

11

720

0.202

1770

760

0.206

Sn-Beta-PS-473

473

1780

0.356 (44)

0.018

744

0.201

Sn-Beta-PS-573

573

1790

0.361 (43)

0.020

725

0.199

Sn-Beta-PS-673

673

1792

0.500 (35)

0.022

729

0.199

Sn-Beta-PS-773

773

>1800

0.497 (36)

0.025

714

0.198

Sn-Beta-PS-873

873

>1800

0.339 (48)

0.029

690

0.195

a

Other treatment condition: SnCl4 vapor pressure,1.33 kPa; reaction time, 1 h. b Determined by ICP. The number in the parentheses indicate the Si/Sn ratio. c Specific surface area (Langmuir) given by N2 adsortpion at 77 K. d PV, micropore volume.

Figure 3. Effect of SnCl4 treatment time on the incorporation of Sn and release of Si at 773 K.

Figure 4. UV-vis spectra of Sn-Beta-PS samples with a Sn amount of 2.1(a), 4.95 (b), 5.9 (c), and 6.1 wt % (d). The samples were prepared by SnCl4 treatment at 773 K for 15 min, 30 min, 1 h, and 1.5 h, respectively.

UV-visible spectroscopy is a sensitive and convenient tool, which is widely adopted to detect the coordination states of the transition metal ions in zeolites particularly in the case of Ti, Sn, or Zr-containing metallosilicates. Figure 4 gives the UV-vis spectra of the Sn-Beta-PS samples with different Sn loadings prepared by the SnCl4 vapor treatment at 773 K for a different period of time. The main absorption band at 220 nm is characteristic of tetrahedrally coordinated Sn species in the framework positions. Additionally, a weak shoulder band at 255 nm was also observed for the Sn-Beta-PS samples containing relatively high Sn contents (Figure 4c,d). This implies the presence of a part of extraframework Sn species, which is consistent with the results reported for microporous Sn-MFI.33 The 200 nm band increased in intensity monotonically with increasing amount of Sn, suggesting that the Sn ions were incorporated mostly into the framework, although

Figure 5. 119Sn MAS NMR spectra of SnO2 (a), hydrated Sn-Beta-PS (2.1 wt % Sn) (b), and after dehydration at 673 K (c) .

the diffuse reflectance spectra are not definitely reliable for quantification. 119 Sn MAS NMR spectra were further measured to provide a strong proof for the incorporation of tetrahedral Sn species by SnCl4 treatment. In contrast to a sharp resonance at -604 ppm observed on pure SnO2, hydrated Sn-Beta-PS showed a very broad signal that was centered at -720 ppm (Figure 5a,b). The resonances in the range of -690 to -740 ppm are generally attributed to the SnIV species with the adsorption of water.16 The dehydration made the resonance shift to -450 ppm (Figure 5c), which is characteristic of the Sn species with a lower coordination state, probably tetrahedrally coordinated in the framework. Fully in agreement with what has been reported on directly synthesized Sn-Beta,16 these results ruled out the presence of octahedral SnO2 in Sn-Beta-PS and verified that the incorporated Sn species were mainly located in the framework. Sn Incorporation Mechanism. The aforementioned results suggested the formation of the tetrahedral Sn species took place via the routes other than the isomorphous substitution for thermodynamically stable framework Si ions. The Sn ions were likely incorporated through the reaction of SnCl4 molecules with the internal silanols. IR and NMR spectroscopies were thus applied to study this issue. The IR spectra in hydroxyl stretching vibration region of H-Al-Beta, De-Al-Beta, and Sn-Beta-PS are shown in Figure 6. The spectra were measured in absorbance mode from the self-supported wafers after a complete dehydration under vacuum, which eliminated the contamination of physically or chemically adsorbed H2O.34 The parent H-Al-Beta sample exhibited two bands at 3745 and 3610 cm-1 (Figure 6a), which are assigned to terminal silanol groups and structural Si(OH)Al groups, respectively.28 Although without any dealumination, it also showed a much broader band at 3500 cm-1, attributed to the delocalized hydrogen-bonded internal silanols. 3666



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Scheme 2. Illustration of the Reaction between SnCl4 and SiOH Groups in the Hydroxyl Nest

Figure 6. IR spectra in the hydroxyl stretching vibration region of H-AlBeta (a), De-Al-Beta (b), and Sn-Beta-PS (6.1 wt % Sn) (c).

Figure 8. IR spectra of De-Al-Beta (Si/Sn = ¥) (A) and Sn-Beta-PS (6.1 wt % Sn) (B) after pyridine adsorption at 298 K for 30 min and desorption at 423 (a), 523 (b), 623 (c), and 723 K (d) for 1 h, respectively.

Figure 7. 29Si MAS NMR spectra of H-Al-Beta (a), De-Al-Beta (b), and Sn-Beta-PS samples with a Sn content of 3.1 (c), 4.95 (d), and 6.1 wt %. The Sn-Beta-PS samples were prepared by SnCl4 treatment at 773 K for 15 min, 30 min, and 1.5 h, respectively.

These silanols are mainly generated from the stacking faults in Beta polymorphs. A deep dealumination made the 3610 cm-1 band vanish completely (Figure 6b). This is in accordance with the ICP investigation shown in Table 1 to verify that De-Al-Beta was highly siliceous. Meanwhile, the dealumination enhanced obviously the bands at 3745 and 3500 cm-1. This can be taken as a clear evidence for the formation of defect sites on the external surface and inside the framework as well. Those hydroxyl groups in the framework are believed to be clustered as hydroxyl nests as reported in previous dealumination/realumination study.28 After the SnCl4 vapor treatment at 773 K for 1.5 h, the 3745 cm-1 band decreased slightly in intensity (Figure 6c), indicating that the SnCl4 molecules interacted with the terminal silanols to certain extent. This reaction probably generated a part of extraframework Sn species in Sn-Beta-PS as shown in UV-vis spectra (Figure 4). More importantly, the 3500 cm-1 band decreased greatly in intensity after the SnCl4 vapor treatment. This is indicative of the reaction of SnCl4 molecules with the internal silanols, implying that the incorporation of Sn atoms into the framework sites took place in the same manner as alumination or titanination of highly siliceous MOR zeolites.28,29 For quantitative comparison, the 29Si MAS NMR spectra were measured without cross-polarization technique for H-Al-Beta, DeAl-Beta, and Sn-Beta-PS samples (Figure 7). The strong signals around -112 ppm attributed to the framework Si(OSi)4 (Q4) units were observed for all samples. A weak signal at -103 to -105 ppm was also observable for H-Al-Beta (Figure 7a), which is contributed by the overlapped signals of the (OSi)3Si(OH) (Q3) and Si(1Al) groups.35,36 The signal was intensified by dealumination and turned

to be centered at -103 ppm as the signal due to the Si(1Al) groups disappeared (Figure 7b). The removal of Al ions left the vacancies in the framework. De-Al-Beta is then deduced to contain a high concentration of internal SiOH groups forming the hydroxyl nests. The SnCl4 vapor treatment decreased gradually the intensity of the Q3 signal. The more the Sn ions were incorporated, the less intensive the Q3 signal became in the spectra of Sn-Beta-PS (Figure 7c-e). After the SnCl4 vapor treatment at 773 K for 1.5 h, a maximized Sn loading of 6.1 wt % was achieved, and the intensity of the 103 ppm signal decreased the most. On the other hand, the Q4 signals around -112 ppm were almost intact after dealumination and subsequent Sn incorporation. The fact that the concentration of the Si(OSi)3(OH) became lower after atom-planting treatment strongly supports that the Sn ions were incorporated into the defect sites by the reaction between the SnCl4 molecules and the SiOH groups in the hydroxyl nest as described in Scheme 2. Figure 3 depicted that the treatment for 1 h was sufficient to saturate the bulk amount of Sn in Sn-Beta-PS, that is, the SnCl4 molecules could react with the internal silanols available to a deep extent within 1 h. This can explain why the Q3 signal at -103 ppm disappeared almost for the Sn-Beta-PS sample prepared by the SnCl4 vapor treatment for 1.5 h (Figure 7e) Acidic Properties of Sn-Beta Zeolites. The catalytic properties of Sn-zeolites are closely related to their Lewis acidity. Pyridine was then employed as a probe molecule to provide detailed information about the amount and strength of Lewis acid sites in Sn-Beta-PS. The IR spectra of adsorbed pyridine in the range of pyridine ring-stretching modes were measured after desorption at various temperatures. As shown in Figure 8A, De-Al-Beta showed the vibrations of stretching modes of hydrogen-bonded (hb) and physically (ph) adsorbed pyridine at 1599 (hb, mode 8a), 1581 (hb, ph, mode 8b), 1483 (pb, mode 19a), 1446 (hb, mode 19b), and 1440 cm-1 (ph, mode 19b).37-39 The bands at 1483 and 1440 cm-1 associated with physically adsorbed pyridine diminished completely after the desorption at 523 K. The bands at 1599 and 1446 cm-1 associated with hydrogen-bonded 3667



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Table 2. Compariosn of Baeyer-Villiger Oxidation of Cyclohexanone with Hydrogen Peroxide between Sn-Beta-F and Sn-Beta-PSa no.

catalyst

Si/Sn

ketone conv (%)

lactone selb (%) 70.0

1

Sn-Beta-F

150

20.2

2

Sn-Beta-PS1

135

25.7

62.5

3

Sn-Beta-PS2

57

34.1

57.8

a

Reaction conditions: cat., 100 mg; cyclohexanone, 5 mmol; H2O2 (50%), 2.5 mmol; MeCN, 8 mL; temp, 348 K; time, 3 h. b The main byproduct was 6-hydroxycaproic acid. Figure 9. IR spectra of De-Al-Beta (a) and Sn-Beta-PS with a Sn content of 0.93 wt % (b), 3.1 wt % (c), 4.95 wt % (d), and 6.1 wt % (e). The adsorbed pyridine on the sample was evacuated at 523 K for 1 h.

pyridine were more resistant against evacuation but decreased in intensity with a rising desorption temperature. On the other hand, the bands at 1490 cm-1 characteristic of both Br€onsted and Lewis acid sites and at 1450 cm-1 corresponding to Lewis acid sites were absent in the spectra of De-Al-Beta regardless of desorption temperature, suggesting that De-Al-Beta was almost free of Lewis acid sites. In the spectra of Sn-Beta-PS with a relatively high Sn content, we observed two distinct bands at 1611 and 1490 cm-1 and a shoulder one at 1450 cm-1 in addition to similar bands shown by De-Al-Beta (Figure 8B). These new bands absent for De-Al-Beta were more evacuation temperature resistant and remained even after desorption at 723 K (Figure 8Bd). The bands at 1611, 1490, and 1450 cm-1 apparently correlated to the different vibration modes of the pyridine-rings adsorbed on the Sn species incorporated into the matrix of Beta zeolites.39-41 The C5H5N 3 3 3 Sn(IV) adduct with pyridine molecule (ligand) coordinated to the Sn(IV) ion (center) may contribute to these bands. These three bands can be taken as the evidence for the presence of Lewis acid sites in Sn-Beta-PS. At the same time, the spectra did not show an obvious band at 1550 cm-1 due to the vibration of pyridinium ions, which is commonly used as an evidence for the presence of Br€onsted acid sites. Therefore, SnBeta-PS was featured by Lewis acidity rather than Br€ onsted acidity. The pyridine adsorption was further carried out on the Sn-Beta-PS samples with various Sn contents. Figure 9 compares the spectra of the samples after removing weakly adsorbed pyridine by evacuation at 523 K. The bands at 1611, 1490, and 1450 cm-1, that were absent for De-Al-Beta but Sn species-dependent, increased in intensity with increasing Sn content. Thus, with the introduction of tetrahedrally coordinated Sn in Sn-Beta-PS the amount of Lewis acid sites increased gradually. Nevertheless, the 1599 and 1446 cm-1 bands due to hydrogen-bonded pyridine were still visible after desorption at 523 K. It is deduced that the Sn-BetaPS samples contained the strong Lewis acid sites and also the moderate acidity contributed by the hydroxyl groups. Catalytic Properties of Sn-Beta-PS. The catalytic properties of Sn-Beta were first investigated in the Baeyer-Villiger oxidation of cyclohexanone. Both Ti-Beta-F and Ti-Beta-PS showed comparable activity at similar Si/Sn ratios (Table 2, nos. 1 and 2). The ketone conversion increased reasonably with increasing Sn content in the case of Ti-Beta-PS ((Table 2, no. 3). The product was mainly ε-caprolactone with a selectivity of 58-70%. Meanwhile, the hydrolysis of ε-caprolactone with water, probably catalyzed by the weak acid sites on Sn-Bteta, also took place to certain extant, which gave 6-hydroxycaproic acid as a byproduct. With the purpose to avoid the influence of byproduct formation on the evaluation of the catalytic properties of Sn-Beta and also to

Scheme 3. Baeyer-Villiger Oxidation of 2-Adamantanone with Hydrogen Peroxide

Table 3. Baeyer-Villiger Oxidation of 2-Admantanone with Hydrogen Peroxide Catalyzed by Sn-Beta-PS (6.1 wt % Sn) in Different Solventsa no.

solvent

1

b

conv (%)

sel (%)

MTBE

18.4

88

2

MeCN

15.6

80

3

C6H5CN

19.8

72

4

C6H5CH2OH

25.6

75

5 6

1,4-dioxane C6H5Cl

5.7 85.7

60 >99

7

C6H5NO2

83.4

>99

8c

C6H5Cl

80.5

>99

a

Reaction conditions: Sn-Beta-PS, 50 mg; 2-adamantanone, 2 mmol; H2O2 (50%), 4 mmol; solvent, 10 mL; temp, 343 K; time, 3 h. b MTBE, 323 K. c The results of fifth use. The used catalyst was recovered by washing with chlorobenzene.

investigate the possibility of Sn-Beta for processing bulky ketones, we have further carried out extremely selective oxidation of 2-admantanone with H2O2 as illustrated as Scheme 3. The multicyclic ketone thus chosen has a large molecular dimension and then requires an open reaction space to be converted catalytically. This reaction is considered to be suitable for evaluating the ability of zeolite catalysts for catalyzing bulky reactions. The effect of organic solvent on the 2-adamantone oxidation was first examined by using Sn-Beta-PS (6.1 wt %) as a catalyst. The reactions were carried out at 343 K for 3 h except for methyl tert-butyl ether (MTBE) at 323 K. As shown in Table 3, the reaction took place slowly in the solvents such as MTBE, acetonitrile, benzonitrile, benzyl alcohol, and 1,4-dioxane. Since the main reaction of ketone oxidation to corresponding lactone product retarded greatly in these solvents, the lactone selectivity was lower than 60% (Table 3, nos. 1-5). The most suitable solvents proved to be chlorobenzene and nitrobenzene, in which the ketone conversion was higher than 83%, whereas the lactone selectivity was nearly 100% (Table 3, nos. 6 and 7). Meanwhile, the utilization efficiency of H2O2 was in the range of 65-70%. These two solvents are miscible with aqueous H2O2, making the 3668



Figure 10. The time course of Baeyer-Villiger oxidation of 2-admantanone over Sn-Beta-PS (6.1 wt % Sn). Reaction conditions: solvent, chlorobenzene; others, see Table 2.

reaction occurr in a triphasic system (two liquid phases and one solid phase). The hydrophilic zeolite catalyst would prefer to stay in the aqueous phase, whereas the organic product stayed mainly in the organic solvent. This then may avoid constitutive reactions of lactone such as condensation, hydrolysis, and deep oxidation. There is still a large room to make clear these significant solvent effects observed with Sn-Beta. In the following experiments, we chose chlorobenzene as a suitable solvent to carry out the oxidation. With respect to the reusability and stability of SnBeta-PS, the used catalyst was washed with chlorobenzene first and then subjected to the reaction again under the same conditions. The admantanone was still high as 80.5% after the fifth use, indicating Sn-Beta-PS was a robust and reusable catalyst (Table 3, no. 8). As shown in Figure 10, the ketone conversion increased with the reaction time and the lactone selectivity to the product kept over 99% in the reaction time range from 10 min to 4 h. Generally the reaction took place rapidly in the initial stage of 0-1 h and then slowed down in the next time. The conversion reached nearly 99% after the reaction for 4 h. Figure 11 shows the effect of the amount of Sn on the conversion and compares the catalytic properties between SnBeta-PS and Sn-Beta-F. The number shown on the bars represents the Sn content of each catalyst. Taking into account the fact that two series of Sn-Beta catalysts differed in Sn content, the reaction results are arranged in an order of increasing Sn content. After the reaction for 4 h, even the Sn-Beta-PS catalyst with a much lower Sn-content (0.65 wt %) was capable of giving a conversion of 91.0%. With the Sn content of Sn-Beta-PS increased, the ketone conversion increased further and reached nearly 99% at 6.1 wt % of Sn loading. In the case of Sn-Beta-F, the limitation of direct synthesis made the Sn content changeable only within an extremely narrow region for fully crystallized samples. Although with a low Sn loading, Sn-Beta-F was highly active for the reaction and was capable of giving an admantanone conversion over 80%. Nevertheless, the Sn-Beta-PS zeolite was slightly more active than Sn-Beta-F if making a reasonable comparison at a similar Sn content. For example, the conversion on Sn-Beta-F (0.74 wt % Sn) was 10% lower than Sn-Beta-PS (0.65 wt % Sn). The Sn-Beta-F zeolite of micrometer scale would be more hydrophobic than nanosized Sn-Beta-PS, and then it is expected to be more effective in the ketone oxidation involving aqueous oxidant of H2O2. The opposite results actually observed here suggest the diffusion issue may dominate the reaction over the hydrophobic nature of the catalyst. The adsorption and

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Figure 11. A comparison of 2-adamantanone oxidation between SnBeta-PS and Sn-Beta-F. Reaction conditions: solvent, chlorobenzene; time, 4 h; others, see Table 2.

Figure 12. A comparison of adsorption properties between Sn-Beta-PS (6.1 wt % Sn) and Sn-Beta-F (1.17 wt % Sn). Adsorption conditions: adsorbent, 50 mg; adsorbate (0.5 wt % 2-adamantanone in 1,3,5-TIPB), 2 g; temp, 303 K.

diffusion of 2-admantanone inside the channels of these two kinds of Sn-Beta samples were then measured using liquid-phase technique with 1,3,5-TIBP as a solvent. As depicted in Figure 12, Sn-Beta-PS showed a faster adsorption rate as well as a slightly higher adsorption capacity than Sn-Beta-F. The 2-adamantanone molecules adsorbed readily on Sn-Beta-PS and reached an equilibrium within 20 min at 303 K, but they took 50 min to reach the equilibrium on Sn-Beta-F under the same conditions. Even though Sn-Beta-PS was more hydrophilic after two posttreatment steps in comparison to Sn-Beta-F, its smaller crystal size and less diffusion limitation to the bulky reactant molecules would advantageous to the oxidation of 2-admantanone. On the other hand, we have investigated the dependence of the initial reaction rate (R0) of 2-adamantanone oxidation and the Lewis acidity on the Sn content in Sn-Beta-PS. The Lewis acidity was estimated from the intensity of the 1490 cm-1 band since it appeared singly in the IR spectra and the samples were free of Br€onsted acidity. As shown in Figure 13, De-Al-Beta showed a very low reaction rate because it did not Lewis acid site but only hydroxyl groups of a moderate acidity. The amount of Lewis acid sites was nearly proportional to the Sn content in SnBeta-PS. Simultaneously the initial reaction rate increased with an increasing Sn content. It is clear that the catalytic property of Sn-Beta-PS in the oxidation of 2-adamantanone was closely relative to the Lewis acid sites but independent of the hydroxyl 3669



Figure 13. Dependence of the initial reaction rate of 2-adamantanone oxidation and the Lewis acidity on the Sn content in Sn-Beta-PS. Reaction conditions: Sn-Beta-PS, 50 mg; 2-adamantanone, 2 mmol; H2O2 (50%), 4 mmol; chlorobenzene 10 mL; temp, 363 K.

groups with a moderate acidity. This is in accordance with the mechanism of Baeyer-Villiger oxidation proposed on Sn-Beta-F in which the carbonyl groups of the ketones are activated through the coordination to the Lewis acid sites of the Sn active species.13,18

’ CONCLUSIONS Nanocrystalline Sn-Beta-PS zeolites can be prepared to have a high Sn content from highly dealuminated Beta zeolites and SnCl4 vapor by atom-planting method. The Sn incorporation is realized by the SnCl4 treatment at an optimal temperature of 773 K. The Sn ions are mainly incorporated into the defect sites through the reaction between SnCl4 with the SiOH groups in the hydroxyl nests. The nanosized Sn-Beta-PS catalysts possess a high Sn content and propose less diffusion limitations to the bulky molecules and thus are more active than microsized SnBeta-F in the Baeyer-Villiger oxidation of 2-adamantanone with H2O2. This research implies that more promising catalytic systems are expectable if direct hydrothermal synthesis methods are developed for preparing nanosized Sn-Beta at high Sn loadings. ’ AUTHOR INFORMATION Corresponding Author

*Tel/Fax: þ86-21-6223-2292. E-mail: [email protected] (P.W.); [email protected] (H.W.).

’ ACKNOWLEDGMENT We gratefully acknowledge the NSFC of China (20890124, 20925310, 20873043), the Science and Technology Commission of Shanghai Municipality (09XD1401500, 08JC1408700), 973 Program (2006CB202508), 863 Program (2007AA03Z342, 2008AA030801), and the Shanghai Leading Academic Discipline Project (B409). ’ REFERENCES

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Postsynthesis and Selective Oxidation Properties of Nanosized Sn

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