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Postsynthesis and Effective Baeyer-Villiger Oxidation Properties of Hierarchical FAU-Type Stannosilicate Zhiguo Zhu, Hao Xu, Jingang Jiang, and Peng Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07947 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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The Journal of Physical Chemistry

Postsynthesis and Effective Baeyer-Villiger Oxidation Properties of Hierarchical FAU-type Stannosilicate

Zhiguo Zhu, Hao Xu, Jingang Jiang, Peng Wu*

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, P. R. China

Corresponding author. Prof. Peng Wu Corresponding Address: Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, P. R. China Tel: +86-21-6223-2292 Fax: +86-21-6223-2292 E-mail address: [email protected] (P. Wu)

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Abstract Sn-Y zeolite, with hierarchical pore systems and extremely low Al content, was successfully prepared via a convenient post-synthetic route which involves proper pre-dealumination and subsequent (NH4)2SnCl6 treatment under mild aqueous condition. The Sn ions were incorporated into the framework of properly dealuminated Y zeolite through reacting with the defect sites generated in the industrial steaming treatment, the first-step acid treatment and the second Sn incorporation process under acidic conditions. The acidic medium achieved by adding HCl in the Sn incorporation process affected not only the amount of incorporated Sn and residual Al but also the coordination state of inserted Sn ions. Compared with hydrothermally synthesized Sn-Beta, postsynthesized Sn-Y zeolite exhibited outstanding catalytic performances in the Baeyer-Villiger oxidation reactions of ketones especially when bulky tert-butyl hydroperoxide was employed as the oxidant due to an open pore structure of 3-dimensional 12-membered ring (12-MR) channels of FAU topology as well as the dealumination-derived intracrystal mesoporosity. Moreover, alkali metal ions modification was shown to be an effective approach for enhancing the selectivity of lactones.

Keywords: USY zeolite; Postsynthesis; Stannosilicate; Baeyer-Villiger oxidation; Ketones. 2


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1. INTRODUCTION The Baeyer-Villiger (B-V) oxidation reaction involving the transformation of ketones into the corresponding esters or lactones, was firstly reported by Baeyer and Villiger in 1899.1,2 Subsequently, the reaction is widely used in organic synthesis, such as, synthesis of steroids and antibiotics in the field of pharmaceuticals,3 pheromones for agrochemistry,4 monomers of polymerization ect.5 The B-V oxidation is traditionally carried out in non-catalytic ways using peracids oxidants including perbenzoic acid,6 m-chloro perbenzoic acid7 and trifluoroperacetic acid8 etc. in more than stoichiometric quantities in complex solutions. They are perhaps the most common and appropriate oxidants for the reaction in the past decades. The reaction using these peracids oxidants not only produces useless by-products inevitably but also demands strong acidity oxidants due to low reactivity.2,9 Moreover, the use of peracids leads to a low selectivity for the oxidation reaction when other function groups like unsaturated bonds are substituted in the reactant.10,11 As a consequence, considering the tremendously growing environmental concern, it is necessary to develop alternative ways in which these organic oxidants could be replaced with more environmentally benign agents such as hydrogen peroxide. Different catalyst systems with hydrogen peroxide as the oxidant have been reported for the B-V reactions, including homogeneous catalysts based on Pt,5 Se,12 Re,13 Mo,14 etc. and heterogeneous catalysts based on heteropolyacids,15 hydrotalcites,16-18 mesoporous materials,19 and zeolites.20 Most of these processes, however, encounter limitations such as low catalytic activity and lactone selectivity, 3



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regeneration issues, safety problem, high cost. For example, Pt-complexes showed extremely limited activity and selectivity in the process.5 Mesoporous stannosilicate of Sn-MCM-41 entailed high hydrophilicity and inferior hydrothermal stability because the pore wall is amorphous.19 A novel method to address these problems has been proposed by Corma et al.21 The carbonyl group can be activated by the tetrahedral Sn ions incorporated into the *BEA-type zeolite framework, and then the Criegee adduct is transformed into corresponding lactone with the aid of H2O2. It also has been demonstrated that thus prepared Sn-Beta catalyst is a robust and efficient heterogeneous catalyst for the B-V oxidation reaction of ketones with aqueous H2O2, giving a high selectivity for lactone products even when the double bond is conjugated with the carbonyl group. Whereas, the long crystallization time and limited tin content in the zeolite framework are the major drawbacks of this traditional synthesis approach for Sn-Beta zeolite. In addition, the use of hazardous F- ions in synthesis system not only results in large-size crystals but also causes environment problem.1 In this respect, significant efforts, including utilizing less toxic ammonium fluoride as the fluoride source,22 employing seeding method to decrease the crystallization time23 as well as dry-gel method in F- free medium,24 have been made to solve these problems in synthesizing of Sn-Beta. On the other hand, postsynthesis methods are also employed to solve these issues by introducing the Sn ions into the defects in pre-treated zeolite framework, which are generated by the removal of framework heteroatoms, such as Al and B.25-27 More recently, Sn-Beta zeolite are found to be capable of catalyze biomass conversion, 4


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during which the size of substrates is generally large.23 Besides, some cyclic ketones with bulky molecular sizes are also used as the reactant in the B-V oxidation.28 As a result, Sn-Beta encounters poor diffusion and steric constraints when processing these bulky substrates. Therefore, developing novel stannosilicates with more open pore channels is still desirable. It has been known that Y zeolite possesses 3-dimensional (3D) 12-membered ring channels and supercarges, which is an effective catalyst in the fluid catalytic cracking (FCC).29 Nevertheless, up to now, heteroatom containing Y zeolite cannot be prepared directly via hydrothermal synthetic method due to the fact that extremely high aluminum content and alkaline metal ions are involved in the synthesis system. Besides, the Al ions of high content also retards the catalytic activities of heteroatom containing zeolite in practical applications. Inspired by the post-synthetic methods employed to prepare metallosilicates,25-27 very recently, we have reported the hierarchical FAU-type stannosilicate prepared by the atom-planting method using SnCl4 vapor at elevated temperatures.30 Thus obtained Sn-Y catalyst demonstrates outstanding catalytic performances in the B-V oxidation reaction of 2-adamantanone with hydrogen peroxide (H2O2) or tert-butyl hydroperoxide (TBHP) as the oxidant. However, a certain amount of Al ions (0.39 mmol g-1 or Si/Al ratio of ca. 43) still exist in the Sn-Y zeolite. It is generally acceptable that the remaining Al ions in the zeolite have a harmful effect on the reaction performance of the tin ions in B-V oxidation.31 To address the issue, in this paper a novel FAU-type stannosilicate zeolite, with extremely low Al content and 5



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hierarchical pore structure, was successfully prepared using an alternative post-synthetic method under mild conditions. In addition, we utilize a series of characterization techniques to explore the textural properties of the prepared tin-containing Y zeolite and the mechanism for Sn insertion in detail. The catalytic performance of Sn-Y was assessed in the B-V oxidation of ketones with alkali metal ions in the reaction system to improve the lactone selectivity. Compared with fluorine-mediated Sn-Beta, Sn-Y zeolite exhibited much higher activity with bulky oxidant of tert-butyl hydroperoxide.

2. EXPERIMENTAL 2.1. Synthesis of Catalysts. Sn-Y zeolites were prepared by post-synthetic process, during which controlled dealumination of parent USY zeolite and tin insertion were performed as described in Scheme 1. A commercially available H-USY (Si/Al = ~6, Shanghai Xinnian Petrochemical Additives Co., Ltd.) was utilized to prepare dealuminated H-USY zeolites via acid treatment. First, the purchased H-USY zeolite was subjected to calcination in air at 873 K for 6 h, which was followed by the reflux in 6 M HNO3 aqueous solution at a solid-to-liquid ratio of 1 g : 50 mL for 1 h and 20 h, respectively. The dealuminated products were named as Y-x where x indicated the acid treatment time. Typically, the Sn incorporation process was performed by dissolving 2 g Y-x powder sample in the aqueous solution of (NH4)2SnCl6 with different concentrations (0 - 8 mM) with a solid-to-liquid ratio of 1 g : 80 mL. It should be noted that a certain amount of HCl (36 wt.%) aqueous solution 6


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was also added into the suspension to control the pH if it was necessary. Then the suspension was stirred in a container at different temperatures (298 - 443 K) for different time (0 - 42 h). After the treatment with (NH4)2SnCl6 aqueous solution, the Sn-containing samples were washed with deionized water repeatedly until the chloride ions cannot detected in the filtrate by AgNO3 solution. After drying in oven at 373 K overnight, they were then calcined at 873 K for 6 h and denoted as Sn-Y in the Table 1. For control experiments, Sn-Beta catalyst was also synthesized in our laboratory strictly according to the literature.21 It is worth noting that ammonium fluoride was used as the the F- source. The obtained Sn-Beta zeolite was washed using deionized water repeatedly, dried in oven at 373 K overnight. It was then calcined at 823 K for 6 h with a temperature gradient of 2 K min-1. 2.2. Characterization Methods. The X-ray diffraction (XRD) patterns were carried out on a Rigaku Ultima IV X-ray diffractometer utilizing Cu-Kα radiation (λ=1.5405 Ǻ) at 35 kV and 25 mA. Scanning electron micrographs (SEM) and transmission electron microscope (TEM) were collected using Hitachi S-4800 microscope

and

JEOL-JEM-2100

microscope,

respectively.

The

nitrogen

adsorption-desorption isotherms were measured on a BELSORP-MAX instrument at 77 K after the samples were activated at 573 K for 5 h under vacuum condition. The specific surface area was calculated based on the Brunauer-Emmett-Teller (BET) method utilizing the data in the relative pressure region of P/P0 = 0.05 - 0.25. The amounts of silicon, aluminum and tin were detected by inductively coupled plasma 7



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(ICP) technique on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. After the samples were dehydrated at 673 K under vacuum, UV-visible diffuse reflectance spectra were performed on a Perkin Elmer Lamda 35 UV/VIS spectrometer utilizing BaSO4 as the reference. The thermogravimetric (TG) analyses were carried out on a METTLER TOLEDO TGA/SDTA851e apparatus from 298 K to 873 K at a heating rate of 10 K min-1 in nitrogen atmosphere. Before FT-IR spectra were performed on a Nicolet Nexus 670 FT-IR spectrometer with a spectral resolution of 4 cm-1 in absorbance mode, the sample was firstly pressed into a self-supported wafer. Then it was placed in a quartz cell which was equipped with a vacuum system and sealed with CaF2 windows. The pyridine and acetonitrile-d3 adsorption was carried out by exposing the pretreated wafer to an excess pyridine vapor and acetonitrile-d3 vapor, respectively, at 298 K for 1 h after the samples were evacuated at 723 K for 2 h to remove any water. The adsorbed pyridine was desorbed successively from 423 K to 473 K for 1 h and then the FT-IR spectra of adsorbed pyridine were performed. The FT-IR spectra of adsorbed acetonitrile-d3 were collected with the desorption temperature increasing from 298 K to 373 K for 2 min. Solid-state MAS NMR spectra were measured on a VARIAN VNMRS-400WB NMR spectrometer. Before the measurement for

119

Sn MAS NMR, the sample was

dehydrated at 723 K for 3 h under vacuum condition and then transferred into a rotor quickly with the protection of N2. 2.3. Catalytic Tests. Baeyer-Villiger oxidation of ketones was performed in a 25 mL flask connected with a water condenser under stirring. In a typical run, 2 mmol 8


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2-adamantanone, 4 mmol H2O2 (30 wt.%) or TBHP (65 wt.% in water or 5.5 M in decane), 10 mL chlorobenzene, 0.5 g benzonitrile (as GC internal standard) and 50 mg catalyst were blended and stirred in the flask at 363 K for 8 h. In the case of cyclohexanone, the flask was charged with 5 mmol cyclohexanone, 1.25 mmol H2O2 (30 wt.%) or TBHP (65 wt.% in water or 5.5 M in decane), 12 mL fluorobenzene, 0.5 g toluene (as GC internal standard) and 100 mg catalyst at 358 K for various reaction time. After these reactions, the solid catalysts were separated from the liquid mixture by centrifugation, and then the reaction mixture was subjected to a gas chromatograph (Shimadzu GC-14B, FID detector) equipped with a 30 m DB-1 capillary column to obtain the corresponding conversion and selectivity. The products after reaction were identified with GC-MS (Agilent-6890GC/5973MS).

3. RESULTS AND DISCUSSION 3.1. Catalysts Preparation. A commercial USY zeolite (Si/Al = ~6) was dealuminated by HNO3 treatment at the beginning and subsequent (NH4)2SnCl6 aqueous solution treatment to obtain Sn-Y zeolite as shown in Scheme 1. The XRD patterns of parent USY, Y-1 dealuminated by acid treatment for 1 h as well as Sn-containing zeolites (Sn-Y-6 and Sn-Y-11) are shown in Figure 1A. All of the samples except Sn-Y-6 exhibited well-defined reflections attributed to FAU-type zeolite, which manifested that the crystalline structure was well-preserved in the process of dealumination and subsequent Sn insertion. After USY zeolite was refluxed in the acid solution for 20 h, the zeolite structure was still well-preserved (not shown). 9



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However, the peak intensities of the Sn-Y-6 sample decreased significantly, implying the structure collapse of FAU zeolite. This also suggested that the high treatment temperature had an adverse effect on zeolite structure. Moreover, the acid treatment lead to an obvious shift of the [111] reflection peak to higher angle (Figure 1B) because of the lattice shrinkage derived from the removal of relatively larger Al ions (in comparison to Si ions). However, the Sn insertion process resulted in a slight shift of the [111] reflection peak lower angle by comparing with the dealuminated USY zeolite, indicating the Sn ions with much larger ionic radius were isomorphously incorporated into the framework of FAU-type zeolite.32 The textural properties of FAU-type zeolites under different preparation conditions were summarized in Table 1. The parent USY with considerable mesopores of 0.14 cm3 g-1 contributed by the steaming treatment possessed grooves and voids in the crystal surface revealed by the SEM images (Figure 2A). After the HNO3 treatment for 1 h, the mesopore volume of 0.16 cm3 g-1 determined by the N2 adsorption-desorption isotherm (Figure S1A) was slightly increased, implying newly formed mesopores deriving from further removal of Al ions.33 Besides, the pore size distribution (Figure S1B) showed that the Y-1 sample possessed relatively board distributed mesopores. At the same time, the bulk molar Si/Al ratio in the zeolite increased from 6 to 42. As for the subsequent stannation process, for Sn-Y-11 sample, the mesoporous volume was not changed while the microporous volume was decreased slightly due to some of the micropores were blocked by the the formation of extra-framework Al species. Many bright spots were noticed in the intracrystal 10


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region in the TEM images of Sn-Y-11 sample (Figure S2), implying the formation of mesopores after steaming and acid treatment. In agreement with the XRD results and porosity changes in the process of HNO3 and stannation treatment, the SEM images of Sn-Y-10 and Sn-Y-11 sample appeared to be unchanged compared with the parent material (Figure 2A, B and Figure S3), while amorphous materials were observed at the higher stannation temperature (Figure 2C), indicating partial degradation of the crystals occurred. Postsynthesized FAU-type stannosilicates with (NH4)2SnCl6 as a Sn source were carried out in acidic aqueous solution. The treatment conditions would have great effect on the physicochemical properties which were then closely related to the catalytic performance. Consequently, the influence of the stannation parent and the stannation conditions like treatment temperature, the concentration of (NH4)2SnCl6 and HCl, were investigated in detail as shown in Table 1 and Figure 3. In our previous work,30 the amount as well as the coordinate state of the active sites were closely affiliated with the type of the parent zeolites. Therefore, three kinds of USY samples obtained by acid treatment over the parent USY zeolite for various time, including Y-0, Y-1 and Y-20, were investigated in the Sn incorporation process under the same condition (Table 1, No. 2, 10, and 13). With respect to the parent Y-0 sample (Table 1, No. 2), the crystalline structures were well-preserved (XRD pattern was not shown here) after stannation and the Sn content was relativity high. However, the obtained sample had the lowest Si/Al ratio, i.e. the highest aluminum content, which was detrimental to the catalytic activity. Although the Si/Al ratio was high for 11



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the parent of Y-20 (Table 1, No. 13), the Sn content was low and the crystallinity was decreased to 68 %. As a result, Y-1 dealuminated for 1 h in the first stage was chosen as the optimal starting sample. The stannation was also performed at different temperature (Table 1, No. 6 - 10). The increase of the Sn insertion temperature caused an obvious decrease in surface area and microporous volume. Especially, raising the temperature to 443 K caused serious structural deconstruction (Figure 2C and Table 1, No. 6). To preserve the FAU zeolite framework, 298 K was chosen as the optimal stannation temperature in the following experiments. Besides, the Y-1 zeolite was subjected to (NH4)2SnCl6 aqueous treatment with different concentration at 298 K for 24 h. As shown in Figure 3A, the amount of Sn in the zeolite increased monotonously with increasing (NH4)2SnCl6 aqueous solution concentration, while the amount of Al decrease gradually and then almost keep constant, indicating that dealumination occurred during the stannation of Y-1 zeolite. However, the crystallinity also decreased slowly (Figure S4), which was similar to the incorporation of gallium into FAU-type framework with gallium nitrate as reported in the previous literature.34 Considering the amount of Al and Sn in the zeolite and the crystallinity, the suitable concentration of (NH4)2SnCl6 aqueous solution was fixed at 4 mM in the following experiments. Furthermore, the stannation of Y-1 zeolite was carried out with different HCl aqueous concentration (Figure 3B), the amount of Sn in the zeolite increased significantly when a very small amount of HCl (i.e. 0.1 mM) was added into the suspension while it decreased sharply with increasing the concentration of HCl in the suspension. However, the amount of Al in the Y-1 zeolite decreased 12


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sharply at the beginning and then slowly with the concentration of HCl increasing, revealing that dealumination as well as stannation happened at the same time. Exactly, this was also confirmed by the

27

Al MAS NMR spectra of parent USY, Y-1, and

Sn-Y-11 sample (Figure S5). All the samples exhibited two resonances, a resonance at 60 ppm, assigned to tetrahedrally coordinated aluminum, and a small band at around 0 ppm, related to extra-framework aluminium.35,36 Both the resonance band around 0 and 60 ppm of Sn-Y-11 sample was much weaker than that of Y-1 sample. UV-Visible spectroscopy is widely employed to investigate the coordination states of the transition metal ions in zeolites. An adsorption band at ~208 nm derived from the charge transfer from an O2- to dispersed Sn4+ ions is always observed in the Snzeolites.37 The UV-Vis spectra of Sn-Y zeolite prepared with various HCl concentration were shown in Figure 4. All the UV-Vis spectra were performed on the Sn-Y samples dehydrated at 773 K. The Sn-Y zeolites treated with the HCl concentration of 0.1 and 0.5 M demonstrated a main adsorption band at around 210 nm as well as shoulder bands between 230 nm and 250 nm (Figure 4a and b), which suggested that Sn ions were introduced as tetrahedrally coordinated framework ions as well as extra-framework Sn species.25 The amount of extra-framework Sn species could be reduced by increasing the HCl concentration larger than 1 M with the shoulder band in the range of 230 - 250 nm almost disappearing (Figure 4c, d, and e). At the same time, the 210 nm adsorption band was also slightly decreased, because higher acidic concentration would retard Sn incorporation. It could be concluded that the HCl concentration affected not only the inserted Sn species content but also the 13



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coordination state. In order to obtain the highest content of Sn ions in only tetrahedrally coordination, the optimal HCl concentration would be 1 M. As shown in Figure 3C, concerning the stannation treatment time, both of the inserted Sn species and the residual Al in the zeolite decreased slowly in amount when the stannation time exceeded 24 h. It could be inferred that the long stannation was not good for the Sn incorporation. Figure 3D shows the dependence of the content of incorporated Sn on the extracted Al content. The incorporated Sn content was more than the amount of the removed Al at the beginning and then was less than that with the increase of released Al. Based on the analysis of ICP data, the Si contents in the zeolite remained almost the same. From these experimental results, the isomorphous of Sn for the Al or Si in the zeolite framework seems to be impossible. From the above results, it could be inferred that the optimal stannation was performed utilizing Y-1 as the parent with the (NH4)2SnCl6 and HCl concentration of 4 mM and 1 M at 298 K for 24 h. Then the Sn-Y-11 sample was employed as a representative material in the following characterizations (Table 1, No. 11). 3.2. Characterization of Sn Coordination In the Sn-Y Zeolite. As is kown to all, the amount and the coordinated state of the active sites in metallosilicate catalyst are closely related to their activity and selectivity in the corresponding reaction. It has been demonstrated that the isolated Sn ions in the zeolites framework are responsible for high catalytic activities while those in the extra-framework have a harmful effect on the catalytic properties.27,28 As a result, the coordination state of tin ions in Sn-Y-11 was studied carefully before being used as a catalyst. 14


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The FT-IR study of CD3CN adsorption is one of the widely used methods for characterizing the Sn ions in Sn-containing zeolites.24,38,39 As shown in Figure 5, the FT-IR spectra of CD3CN adsorption on Sn-Y zeolites after different desorption temperature was recorded and four bands at 2316, 2307, 2277, and 2267 cm-1 were noticed. The two bands centered at around 2277 and 2267 cm-1, assigned to CD3CN coordinated to silanol groups and physically adsorbed CD3CN,40 decreased obviously with the desorption temperature increasing. The other two bands at around 2316 and 2307 cm-1, attributed to “open” and “closed” Sn sites in the Sn-containing zeolites,40 kept intact even at 353 K. The appearance of “open” and “closed” Sn sites indicated that partially hydrolysed Sn sites and framework-integrated Sn sites existed in Sn-Y zeolite framework. It had been reported that dispersed SnO2 on pure silica Beta didn’t exhibit the bands of 2316 and 2308 cm-1 after adsorbing acetonitrile-d3.24 These results strongly suggested that the Sn-Y sample obtained from the post-synthetic approach indeed possessed isolated Sn in the framework and the local Sn environment consisted of non-hydrolysed and partially hydrolysed framework Sn sites, which was similar to the Sn-Beta sample synthesized in the fluoride system.38 119

Sn MAS NMR measurement was also adopted to further investigate the

coordination state of tin ions in the Sn-Y-11 sample. With respect to the bulk SnO2, a single narrow band at -605 ppm, assigned to octahedrally coordinated tin species,41-43 was noticed as shown in Figure 6a. The -605 ppm resonance was not observed in the spectrum of Sn-Y-11 sample, indicating that the octahedrally coordinated Sn ions were not formed in this zeolite. A broad signal which was centered at -745 ppm was 15



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observed in hydrated Sn-Y-11 sample, which is related to the isolated Sn species in the framework with adsorbed water.41 After dehydration at 723 K for 3 h, a sharp band at around -450 ppm appeared (Figure 6c), which is attributed to the tetrahedral Sn ions in the zeoites framework.28,43 These results indicated that dehydration decreased the Sn ions coordination number. It could also be inferred that Sn ions were mostly tetrahedrally inserted into the framework of dealuminated USY zeolite. In agreement with the UV-Vis and CD3CN adsorption spectra as demonstrated above, the appearance of the -450 ppm band in the 119Sn MAS NMR spectrum also evidences the existence of isolated Sn ions in the zeolite framework of Sn-Y-11 sample. 3.3. Characterization of Lewis Acidity In the Sn-Y Zeolite. The acid property of the Sn-containing FAU zeolites was explored by the FT-IR spectra of the pyridine adsorption (Figure 7A). The bands at 1439, 1445, 1580, and 1596 cm-1, which was assigned to physically adsorbed and hydrogen-bonded pyridine,44,45 completely disappeared after evacuation at the temperature of 473 K, indicative of loosely interacted pyridine molecules. In addition, the 1451 and 1610 cm-1 bands, attributed to Lewis acid sites,46 became intensive with the desorption temperature increasing to 423 K. It was worth noting that the band at around 1540 cm-1 was almost absent in the spectra of Sn-containing Y zeolite, indication of nearly free of Brønsted acid sites, which was well consistent with the extremely low Al content in the zeolites framework characterized by ICP technique (Table 1) and 27Al NMR MAS (Figure S5). Without exception, the bands at 1490 cm-1 corresponding to Lewis and Brønsted acid sites always appeared together with the bands of 1451 and 1610 cm-1. Moreover, the 16


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three bands became weaker at elevated desorption temperature. Therefore, Sn-containing Y catalysts were dominated by Lewis acidity, supporting the fact that Sn4+ ions were tetrahedrally inserted into the framework of FAU-type zeolite. The pyridine adsorption was also performed on the Sn-Y samples with different Sn contents. As shown in Figure 7B, the weak band at 1451 cm-1 associated with Lewis acid sites, observed for parent USY and Y-1, was assigned to the extra-framework Al species evidenced by the 27Al MAS NMR spectrum (Figure S5). They were generated in the process of the steaming and HNO3 treatment in the first step. Besides, the HNO3 treatment of parent USY reduced the content of the Brønsted and Lewis acidity, suggesting the framework Al in the zeolites as well as extra-framework Al species were removed in the process of the HNO3 treatment. The three FT-IR bands at 1610, 1490 and 1451 cm-1 related to Lewis acidity were more intensive for Sn-Y than that for dealuminated Y-1 samples due to the Sn ions incorporation into the FAU frameworks. In addition, the three bands increased in intensity with the Sn content increasing in the zeolites framework. It can be deduced that the Sn ions are mostly tetrahedrally inserted into the framework of dealuminated Y zeolite, and endow Y zeolite with Lewis acidity, which is supposed to serve as active sites in the catalytic reactions. 3.4. The Mechanism of Sn Incorporation. It had been pointed out that the isomorphous substitution mechanism was not suitable for the Sn incorporation process, evidenced by the relationship between the incorporated Sn amount and extracted Si or Al contents in the process of the (NH4)2SnCl6 treatment. Then, the 17



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mechanism for the Sn incorporation was further explored with FT-IR and

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29

Si MAS

NMR technique. Two vibration bands around 3561 cm-1 and 3630 cm-1, assigned to the bridging hydroxyls in α- and β-cages, respectively,47 were observed in the parent USY zeolite (Si/Al = 6) as shown in Figure 8Aa. With the dealumination proceeding, the two bands became weaker in intensity and vanished completely after HNO3 treatment for 20 h (Figure 8Ab and c). Consistent with this result, the molar Si/Al ratio was increased from 6 to 168 after acid treatment time was prolonged to 20 h as shown in Table 1. Meanwhile, another two vibration bands around 3740 cm-1 and 3470 cm-1, attributing to the external and internal silanol groups in zeolite, respectively, emerged in the process of the HNO3 treatment.47,48 The 3470 cm-1 band intensity, which was attributed to hydrogen bonded defects like silanol nests formed by framework Al removal, reached the maximum when the acid treatment was carried out for 1 h (Figure 8Ac). It indicated that the contents of defect sites derived from dealumination increased in the initial stage of the HNO3 treatment and then decreased after the acid treatment of 1 h. This was likely the reason why the Y-1 sample was chosen as the precursor for Sn ions incorporation. These defect sites could be partially vanished under the harsh acid-treatment conditions, where the structural rearrangement of the framework occurred with the migration of silicon ions into the defect sites. This experiment phenomenon has been recorded in previous study during the dealumination process.49 After the (NH4)2SnCl6 solution treatment for Y-1 samples, the 3470 cm-1 band disappeared completely, suggesting that Sn ions consumed nearly 18


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all of the OH groups, forming framework Sn species.25 To further explore the issue, the

29

Si MAS NMR measurement was also

performed for parent USY, Y-1 dealuminated for 1 h, Y-20 as well as Sn-Y-11 sample. As demonstrated in Figure 8B, the spectra could be deconvoluted into several resonances. Since the resonance band attribute to (SiO)3Si(OH) groups (Q3) is easily confused with that of (SiO)3Si(1Al) groups (3Si1Al), the 1H-29Si cross-polarization NMR spectrum (Figure S6) was recorded to help the identification. The resonance band around -100.9 ppm is attributed to Q3 groups while the band around -102.8 ppm is assigned to 3Si1Al groups. The other three bands at around -105.4, -108.1 and -111.2 ppm are attributed to the Q4 groups, (SiO)4Si.25,50-52 As demonstrated in Table S1, the content of 3Si1Al groups was decreased by 8.3 % while that of Q3 groups was increased by 4.4 % after the HNO3 treatment for 1 h, indicative of the formation of defect sites derived from Al removal.51 It is worth noting that a resonance at ca. -95.2 ppm was observed after the acid treatment for 20 h, which was associated with (SiO)2Si(OH)2 groups (Q2),52 meanwhile the Q3 groups amount was decreased by 1.7 %. This strongly implied that the formation of Q2 groups mainly resulted from the partially destroy of the Q3 groups after the HNO3 treatment time exceeded 1 h. This was consistent with the decrease of the vibration band at around 3470 cm-1 in intensity for Y-20 as shown in Figure 8A. The obvious decrease of Q3 groups and no Q2 groups existence after Sn incorporation over Y-1 further confirmed that most of the Q3 groups were consumed through reacting with the (NH4)2SnCl6 to generate the isolated Sn ions in the framework. 19



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The effect of HCl was also investigated by FT-IR technique as shown in Figure 9. When the Y-1 zeolite was suspended in the (NH4)2SnCl6 solution without adding aqueous HCl (Table 1, No. 5), the 3470 cm-1 band of the obtained sample disappeared completely. What’s more, the bulk molar Si/Al and Si/Sn ratio were 89 and 116, respectively (Table 1), implying that the internal silanols in the zeolite reacted with the (NH4)2SnCl6 and dealumination occurred during the stannation process. When the HCl aqueous solution was added into the suspension solely, the intensity of 3470 cm-1 band decreased slightly compared with Y-1 sample. The reason was already explained above when the acid treatment time was prolonged. Once the (NH4)2SnCl6 and aqueous HCl was suspended in the solution at the same time, the 3470 cm-1 band of the obtained sample also vanished. Simultaneously, the Si/Al ratio increased to 150 and the Si/Sn was 86, indicating the presence of HCl contributed significantly to the Sn ions incorporation into FAU framework and dealumination at the same time. The above results suggested that moderate amount of HCl favored the dealumination and subsequently the internal silanols were generated, meanwhile the Sn ions migrate into partial defect sites. Sn-Y zeolite was prepared by controlled acid treatment of USY and subsequently (NH4)2SnCl6 treatment in acidic aqueous solution. It has been confirmed that the Sn4+ ions were inserted into the framework of the FAU-type zeolite in the following two ways. One was that they reacted with the silanol nests generated in the process of the former steaming and the first-step acid treatment. The other one was that Sn ions might in suit migrate into the defect sites formed by HCl in the second-step 20


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(NH4)2SnCl6 treatment (Scheme 1). It could be inferred that the stannation process consisted of two incorporation ways mentioned above at the beginning and then tin ions were mainly reacted with the defect sites which was generated in the HCl medium, evidenced by Figure 3D. Both of the acid treatment time in the first step and the amount of HCl in the second step affected the quality and quantity of inserted Sn ions. Moreover, the addition of HCl in the second step favored the Al removal. When the stannation was performed utilizing the Y-1 sample that the internal silanol nests were maximum as precursor with the aqueous HCl concentration of 1 M, the obtained catalyst should have the best catalytic performance. 3.5. Catalytic Properties of Sn-Y In the Baeyer-Villiger Oxidation. The B-V oxidation of ketones into the corresponding esters or lactones is particularly attractive for practical applications due to its simplified processing conditions and minimized reaction substrate as well as limited waste production.28 So the catalytic performance of Sn-Y materials is evaluated in the Baeyer-Villiger oxidation reaction of 2-adamantanone and cyclohexanone utilizing H2O2 or TBHP as the oxidant. The Sn-Y zeolites obtained from the dealuminated Y-1 zeolite using various HCl concentration were evaluated in the B-V oxidation reaction of 2-adamantanone as shown in Figure 10. A “volcanic type” profile of the 2-adamantanone conversion was observed. The lactone selectivity was generally as high as 100 % for these catalysts because 2-adamantanone possessed a rigid structure as well as a large molecular size.28 The Sn-containing Y catalyst prepared with HCl concentration of 1 M exhibited the highest activity in the reaction due to the high-quality in terms of 21



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coordination state and sufficient content in zeolite framework. This was well consistent with the conjecture that discussed in the Figure 4. As a result, the Sn-Y-11 sample was utilized to catalyze the B-V oxidation of cyclohexanone with aqueous H2O2 in the following experiments. The B-V oxidation of cyclohexanone gives ε-caprolactone as the main product, which is a significant chemical intermediate.28 The oxidation reaction was first carried out at 358 K using the H2O2 as the oxidant in fluorobenzene medium. Unfortunately, the selectivity of ε-caprolactone was as low as 87.1 % with the conversion of 12.2 %. In order to improve the selectivity of the ε-caprolactone, alkali ions (in this case KCl) were added directly to the reaction system. Firstly, the effect of the concentration of added alkali in the form of KCl was studied as illustrated in Figure 11. Surprisingly, the selectivity increased gradually and the conversion remained almost constant in the early stage with the concentration of KCl increasing. For concentration smaller than the optimum, the amount of alkali ions was not enough to improve the selectivity. N. Rai et al have inferred that the alkali ions could make modification on the active sites in the form of exchanging onto an adjacent silanol group of the “open” Sn sites.53,54 Therefore this may be one reason of the phenomenon because the “open” and “closed” Sn active sites existed in the Sn-Y zeolites as evidenced by CD3CN adsorption-desorption. The improvement in selectivity could also arise from neutralization of acidity from defects and Brønsted acid sites in the framework, thereby deterring the ring-opening reaction and the formation of byproducts. This phenomenon was well consistent with the previous repots.55 Further increasing the 22


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concentration of KCl, the selectivity increased to ~96 % while too much KCl in the reaction system caused the decline of the conversion due to poisoning some of the active sites, which was reported in the literature.55,56 Thus, the results indicated that adding alkali metal ions into the reaction system was an effective and efficient approach for enhancing the reaction activity. And an unprecedented high selectivity of 95.4 % and the conversion of 12.2 % were obtained with the optimum KCl concentration of 0.069 mM. Then different types of alkali metal ions were also investigated with the concentration of 0.069 mM (Table 2). Alkali metal ions containing sodium, potassium, rubidium, and cesium were directly added in the oxidation of cyclohexanone with H2O2 at 358 K for 30 min. Remarkably, the increase in selectivity of ε-caprolactone was observed and the conversion didn’t appreciably alter when the alkali metal ions were present. Among all of the alkali chloride salts, cesium chloride possessed the best catalytic properties with the conversion and selectivity of 12.6 % and 96.5 %, respectively (Table 2, No.5). It was well known that the alkalinity of cesium ions was the strongest among all the alkali metal ions.57 The ability in terms of neutralization of acidity from defects and Brønsted acid sites in the zeolites framework was superior. In case of K2CO3, it possessed excellent alkalinity deriving from potassium and carbonate ions, which gave rise to the unprecedented activity with the conversion and selectivity of 12.3 % and 97.8 %, respectively (Table 2, No. 6). To illustrate the advantage of the Sn-containing FAU-type stannosilicate, Sn-Y was compared with fluorine-mediated Sn-Beta in the B-V oxidation reaction of 23



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cyclohexanone with fluorobenzene as the solvent (Table 3). The lactone selectivity of Sn-Beta (99.1 %) was slightly higher than that of Sn-Y (97.8 %). The Sn-Y zeolite showed a higher yield of ε-caprolactone compared with Sn-Beta within 30 min. This may be predominantly associated with the open pore structure derived from 3D 12-MR channels, dealumination-derived intracrystal mesoporosity and high Sn content in the Sn-Y catalyst. What’s more, the conversion and efficiency of H2O2 were comparable for Sn-Y and Sn-Beta catalysts. Because of the different Sn contents in the catalysts, site-time-yield (STY) was thus used to compare their specific catalytic performance. Unfortunately, the Sn-Y was inferior to Sn-Beta in the oxidation of cyclohexanone into corresponding lactone, achieving 64 in 30 min with the addition of K2CO3. As reported in the literature, the STY values decreased gradually with the active sites increasing.58 The Sn-Beta possessed low Sn contents because of the limitation of the synthesis method. So it was concluded that the catalytic activities of Sn-Beta in terms of STY were superior to that of Sn-Y. TBHP, much larger than H2O2, was also employed as the oxidant in the B-V oxidation of ketones (Table 4). TBHP aqueous solution as well as TBHP in decane was studied as the oxidant. Both 2-adamantanone and cyclohexanone were studied in the B-V oxidation with TBHP. First, the control experiment was performed to study any homogeneous catalytic contributed by the solvent or the added alkali ions. Undoubtedly, no appreciable corresponding lactones were obtained in the absence of Sn-containing catalyst. Besides, after these reactions, the Sn content in the catalysts determined by ICP technique was almost unchanged, indicative of no Sn leaching 24


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during the reactions. In all the cases, the activity of the both catalysts in the TBHP system was inferior to that in the H2O2 system. It was reasonable that the reactant or the intermediate products generated during the reaction met severe diffusion limitation when employing TBHP with large molecule size as the oxidant.30,59 When TBHP/water and TBHP/decane were served as oxidant, the lactone selectivity (nearly 100%) was comparable for these two Sn-containing catalysts. The catalytic performance employing 2-adamantanone as the reactant was superior to that utilizing cyclohexanone as the reaction substrate, which was in line with previous reports.60 When the TBHP/decane solution was employed as oxidant, the reaction was proceeded in anhydrous condition. Sn-Y was capable to convert much more 2-adamantanone and cyclohexanone when compared with Sn-Beta under the same reaction condition, mainly because hierarchical Sn-Y catalyst alleviated the diffusion limitation of the bulky oxidant as well as intermediates generated in the reaction. Once the TBHP in water (65 wt.%) was utilized in the reaction, the catalytic activity of the two Sn-containing zeolites decreased obviously. Some water molecules would enter the pore systems to hinder the substrate molecules approach the active sites. This phenomenon was more obvious for those hydrophilic materials. As shown in Figure S7, the Sn-Y (4.8 wt.%) absorbed more water compared with Sn-Beta (3.3 wt.%). Accordingly, it was presumed that Sn-Beta synthesized in the fluorine systems was more hydrophobic than Sn-Y, from which it was anticipated to be more active in the oxidation of ketone with TBHP in water. However, the opposite results were observed, indicating that the diffusion problem may dominate the reaction instead of 25



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hydrophobicity. To illustrate the issue, the adsorption and diffusion of cyclohexanone inside the channels of the two samples were carried out with liquid-phase adsorption technique utilizing 1,3,5-triisopropylbenzene (1,3,5-TIPB) as the solvent. As shown in Figure 12, the adsorption capacity of Sn-Y as well as the adsorption rate was higher than that of Sn-Beta under anhydrous conditions. Once very few water was appended into the adsorption system, the adsorption capacity and rate of the two Sn-containing materials decreased significantly. In the liquid-phase adsorption of cyclohexanone involving H2O, water would adsorb preferably on the Sn active sites, especially for those in a hydrophilic micro-environment.60,61 The adsorption capacity of Sn-Y decreased by 23.2 % when adding H2O into the adsorption system, and it was more than that of Sn-Beta (19.5 %) synthesized in the fluorine medium. This indicated that Sn-Beta had a more hydrophobic framework, which has been evidenced by TG technique. Despite of the decrease of adsorption capacity in hydrous condition, Sn-Y still showed higher adsorption capacity, implying that Sn-Beta was inferior to Sn-Y in the perspective of diffusion property. As a result, even though Sn-Y synthesized by post-treatment method was less hydrophobic than Sn-Beta, Sn-Y zeolite still exhibited higher activity when processing reactions involving bulky molecules, because of the released diffusion limitation mainly contributed by the dealumination-derived intracrystal mesoporosity.

4. CONCLUSIONS A hierarchical FAU-type stannosilicate with extremely low Al content can be 26


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prepared via post-synthetic method of treating dealuminated USY with (NH4)2SnCl6 solution under mild acidic condition at 298 K. The optimal HNO3 treatment time is 1 h under refluxing condition, and then the Sn ions are inserted into zeolite by reacting with the defect sites in two ways. One is that they reacted with the silanol nests formed in dealumination process achieved by steaming as well as acid treatment, and the other one is that Sn ions may in suit migrate into the part of the defects sites generated by aqueous HCl. The optimal Sn-Y zeolite which possesses a Si/Sn ratio of ca. 86 in tetrahedrally coordinated configuration and a Si/Al ratio of ca. 150 is obtained. Subsequently, thus prepared Sn-Y catalyst is modified by adding alkali metal ions directly into the reaction medium in order to improve the selectivity of the ε-caprolactone. The Sn-Y zeolite demonstrates an excellent catalytic performance in the B-V oxidation of ketones compared with Sn-Beta synthesized in the fluorine medium due to an open pore structure derived from 3D 12-MR channel systems as well as dealumination-derived mesopores.

ACKNOWLEDGMENTS We gratefully acknowledge the financial supports from the NSFC of China (21533002, 21373089, 21403069), China Ministry of Science and Technology under contract of 2016YFA0202804.

APPENDIX A. SUPPLEMENTARY MATERIAL Supplementary data associated with this article can be found in the online 27



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107-120. (48) Verboekend, D.; Keller, T. C.; Mitchell, S.; Pérez-Ramírez, J. Hierarchical FAU- and LTA-Type Zeolites by Post-Synthetic Design: A New Generation of Highly Efficient Base Catalysts. Adv. Funct. Mater. 2013, 23, 1923-1934. (49) Wu, P.; Komatsu, T.; Yashima, T. IR and MAS NMR Studies on the Incorporation of Aluminum Atoms into Defect Sites of Dealuminated Mordenites. J. Phys. Chem. 1995, 99, 10923-10931. (50) Lutz, W.; Enke, D.; Einicke, W.; Taschner, D.; Kurzhals, R. Mesopores in USY Zeolites II. Z. Anorg. Allg. Chem. 2012, 638, 2189-2192. (51) Xu, H.; Zhang, Y.; Wu, H.; Liu, Y.; Li, X.; Jiang, J.; He, M.; Wu, P. Postsynthesis of Mesoporous MOR-type Titanosilicate and Its Unique Catalytic Properties in Liquid-phase Oxidations. J. Catal. 2011, 281, 263-272. (52) Van Aelst, J.; Haouas, M.; Gobechiya, E.; Houthoofd, K.; Philippaerts, A.; Sree, S. P.; Kirschhock, C.; Jacobs, P.; Martens, J. A.; Sels, B. F.; Taulell, F. Hierarchization of USY Zeolite by NH4OH. A Postsynthetic Process Investigated by NMR and XRD. J. Phys. Chem. C 2014, 118, 22573-22582. (53) Rai, N.; Caratzoulas, S.; Vlachos, D. G. Role of Silanol Group in Sn-Beta Zeolite for Glucose Isomerization and Epimerization Reactions. ACS Catal. 2013, 3, 2294-2298. (54) Bermejo-Deval, R.; Orazov, M.; Gounder, R.; Hwang, S.; Davis, M. E. Active Sites in Sn-Beta for Glucose Isomerization to Fructose and Epimerization to Mannose. ACS Catal. 2014, 4, 2288-2297. 34


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(55) Tolborg, S.; Sádaba, I.; Osmundsen, C. M.; Fristrup, P.; Holm, M. S.; Taarning, E. Tin-containing Silicates: Alkali Salts Improve Methyl Lactate Yield From Sugars. ChemSusChem 2015, 8, 613-617. (56) Goa, Y.; Wu, P.; Tatsumi, T. Catalytic Performance of [Ti,Al]-Beta in the Alkene Epoxidation Controlled by the Postsynthetic Ion Exchange. J. Phys. Chem. B 2004, 108, 8401-8411. (57) Kuwahara, Y.; Nishizawa, K.; Nakajima, T.; Kamegawa, T.; Mori, K.; Yamashita, H. Enhanced Catalytic Activity on Titanosilicate Molecular Sieves Controlled by Cation-π Interactions. J. Am. Chem. Soc. 2011, 133, 12462-12465. (58) Dijkmans, J.; Demol, J.; Houthoofd, K.; Huang, S.; Pontikes, Y.; Sels, B. Post-synthesis Snβ: An Exploration of Synthesis Parameters and Catalysis. J. Catal. 2015, 330, 545-557. (59) Dutta, B.; Jana, S.; Bhunia, S.; Honda, H.; Koner, S. Heterogeneous Baeyer–Villiger Oxidation of Cyclic Ketones Using tert-BuOOH as Oxidant. Appl. Catal. A: Gen. 2010, 382, 90-98. (60) Luo, H. Y.; Bui, L.; Gunther, W. R.; Min, E.; Román-Leshkov, Y. Synthesis and Catalytic Activity of Sn-MFI Nanosheets for the Baeyer–Villiger Oxidation of Cyclic Ketones. ACS Catal. 2012, 2, 2695-2699. (61) Harris, J. W.; Cordon, M. J.; Iorio, J. D.; Vega-Vila, J. C.; Ribeiro, F. H.; Gounder, R. Titration and Quantification of Open and Closed Lewis Acid Sites in Sn-Beta Zeolites That Catalyze Glucose Isomerization. J. Catal. 2016, 335, 141-154.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE CAPTIONS Figure 1. Wide angle XRD patterns (A) and enlarged [111] reflections (B) of USY (a), Y-1 (b), Sn-Y-11 (c), Sn-Y-6 (d). Figure 2. Scanning electron micrographs of USY (A), Sn-Y-11 (B), and Sn-Y-6 (C). Figure 3. Effects of (NH4)2SnCl6 concentration (A), HCl concentration (B), treatment time (C) on the amount of Al or Sn in the zeolite and the relationship profile between the amount of released Al and the amount of incorporated Sn (D). Stannation conditions: (A) HCl, 0.1 M; time, 24 h; temp., RT; (B) (NH4)2SnCl6, 4 mM; temp., RT; time, 24 h; (C) (NH4)2SnCl6, 4 mM; HCl, 1 M; temp., RT; (D) (NH4)2SnCl6, 4 mM; HCl, 1 M; temp., RT. The liquid to solid was 80 mL : 1 g for all cases. Figure 4. UV-vis spectra of different Sn-Y samples obtained by Sn-incorporation in the presence of 0.1 M (a), 0.5 M (b), 1 M (c), 2 M (d), and 4 M (e) HCl solution. Other stannation conditions: (NH4)2SnCl6, 4 mM; liquid to solid ratio, 80 mL :1 g; temp., RT; time, 24 h. Figure 5. FT-IR spectra of Sn-Y-11 after adsorption of CD3CN at 298 K for 1 h and desorption at 313 K (a), 333 K (b), 353 K (c), and 373 K (d) for 2 min, respectively. Figure 6.

119

Sn MAS NMR spectra of SnO2 oxide (a), hydrated Sn-Y-11 (b),

dehydrated Sn-Y-11 (c). Spinning side bands are highlighted by asterisks. Figure 7. (A) FT-IR spectra of Sn-Y-11 (Si/Sn=86) after pyridine adsorption at 298 K for 1 h and desorption at 323 K (a), 373 K (b), 423 K (c), and 473 K (d) for 1 h, respectively. (B) FT-IR spectra of Sn-free parent USY (a), Sn-free Y-1 (b) and Sn-Y with a Sn content of 0.91 wt.% (c), 1.98 wt.% (d), 2.31 wt.% (e) after pyridine adsorption at 298 K for 30 min and desorption at 373 K for 1 h. Figure 8. FT-IR spectra in the region of hydroxyl stretching vibration (A) and 29Si 37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MAS NMR spectra (B) of USY (a), Y-1 (b), Y-20 (c), Sn-Y-11 (d). Figure 9. FT-IR spectra in the region of hydroxyl stretching vibration of Sn-Y-5 (a), Sn-Y-4 (b), Sn-Y-11 (c). Figure 10. The conversion of 2-adamantanone in function of the HCl concentration in process of (NH4)2SnCl6 treatment. Reaction conditions: cat, 50 mg; 2-adamantanone, 2 mmol; H2O2 (30 wt.%), 4 mmol; chlorobenzene, 10 mL; temp., 363 K; time, 10 min. Figure 11. Conversion and selectivity in the B-V oxidation of cyclohexanone versus the concentrations of KCl in fluorobenzene. Reaction conditions: cat, 100 mg; cyclohexanone, 5 mmol; H2O2 (66.9 wt.%), 1.25 mmol; fluorobenzene, 12 mL; temp, 358 K; time, 0.5 h. Figure 12. Liquid-phase adsorption of cyclohexanone over Sn-containing catalysts. Sn-Y-11 under anhydrous conditions (a), Sn-Beta under anhydrous conditions (b), Sn-Y-11 under hydrous conditions (c), Sn-Beta under hydrous conditions (d). Adsorption conditions: cat., 50 mg; 1.0 wt.% cyclohexanone in 1,3,5-TIPB, 2 g; temp., 298 K; water if added, 0.2 g.

38


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Two-step Strategy for Post-synthesis of Sn-Y.

Mesopore

Mesopore

Mesopore

6 M HNO3 Mesopore

Mesopore

reflux Mesopore

Mesopore

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 56

Table 1. Textural Properties of Various FAU Zeolites.a

No.

Stannation condition

Parent

Crys.c

Si/Al

C[(NH4)2SnCl6]

C(HCl)

Temp.

(mM)

(M)

(K)

-

-

-

100

6

Sn-Y-2

4

0.1

298

84

Sn-Y-3

-

-

-

Sn-Y-4

0

1.0

Sn-Y-5

4

Sn-Y-6

sampleb

d

Si/Sn

d

SBETe

Pore volume (cm3 g-1)

(m2 g-1)

Vmicrof

Vmesog

Vtotale

∞

831

0.31

0.14

0.45

18

136

820

0.30

0.14

0.44

95

42

∞

805

0.28

0.16

0.44

298

92

99

∞

800

0.27

0.16

0.43

0

298

89

89

116

810

0.27

0.16

0.43

4

0.1

443

12

253

135

363

0.08

0.21

0.29

4

0.1

343

79

145

42

595

0.20

0.18

0.38

Sn-Y-8

4

0.1

323

80

141

46

612

0.21

0.17

0.38

Sn-Y-9

4

0.1

303

82

140

43

636

0.23

0.16

0.39

Sn-Y-10

4

0.1

298

89

127

43

794

0.28

0.16

0.44

Sn-Y-11

4

1.0

298

86

150

86

763

0.27

0.16

0.43

-

-

-

90

168

∞

729

0.27

0.18

0.45

4

0.1

298

68

186

190

515

0.21

0.19

0.40

Sn-Y-1

(%)

Y-0(6)

Sn-Y-7

Y-1(42)

Sn-Y-12 Y-20(168) Sn-Y-13 a

Stannation conditions: liquid to solid ratio, 80 mL : 1 g; time, 24 h.

b

The parent USY zeolites with different dealumination degrees were used for stannation. The numbers in the parentheses indicate the Si/Al ratio.

c

The relative crystallinity was calculated from the summed intensity of the XRD diffractions at 2 theta of 26.1, 10.2, 11.9, 15.9, 20.6, and 23.9º by assuming that of parent USY to be 100%.

d

Determined by ICP.

e

Determined by N2 adsorption at 77 K.

f

Calculated by t-plot method.

g

Vmeso = Vtotal – Vmicro.

40


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Baeyer-Villiger Oxidation of Cyclohexanone with H2O2 Over Sn-Y Treated with Various Alkali Metal Ions.a

a

No.

Treatment

1

Ketone conv.

Lactone sel.

Lactone yield

H2O2 (%)

(%)

(%)

(%)

Conv.

Eff.b

-

12.2

87.1

10.6

53.1

91.8

2

NaCl

11.2

89.6

8.8

51.2

87.7

3

KCl

12.0

95.4

11.5

52.0

92.3

4

RbCl

12.4

96.3

11.9

52.3

93.2

5

CsCl

12.6

96.5

11.4

52.8

92.8

6

K2CO3

12.3

97.8

12.1

53.2

92.5

Reaction conditions: cat, 100 mg; cyclohexanone, 5 mmol; H2O2 (66.9 wt.%), 1.25

mmol; fluorobenzene, 12 mL; concentration of alkali metal ions, 0.069 mM; temp., 358 K; time, 0.5 h. b

Utilization efficiency of H2O2 = amount of product/amount of H2O2 consumed ×

100%.

41



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Page 42 of 56

Table 3. Catalytic Activity of Sn-containing Catalysts for the Baeyer-Villiger Oxidation of Cyclohexanone with H2O2a.

a

H2O2 (%)

STYd

Ketone conv.

Lactone sel.

Lactone yield

(%)

(%)

(%)

Conv.

Eff.c

(h-1)

86

12.3

97.8

12.0

53.2

92.5

64

116

11.2

99.1

11.1

51.3

92.1

78

Catalyst

Si/Snb

Sn-Y Sn-Beta

Reaction conditions: cat, 100 mg; cyclohexanone, 5 mmol; H2O2 (66.9 wt.%), 1.25

mmol; fluorobenzene, 12 mL; K2CO3, 0.069 mM; temp., 358 K; time, 0.5 h. b

Molar ratio determined by ICP.

c

Utilization efficiency of H2O2 = amount of product/amount of H2O2 consumed ×

100%. d

Site-time-yield (STY), moles of cyclohexanone converted per mol of Sn active sites per hour.

42


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Oxidation of Cyclic Ketone with TBHP Over Sn-containing Catalysts.

2-Adamantanonea (%) Catalyst

Si/Sn

TBHP in waterc conv.

sel.

Cyclohexanoneb (%)

TBHP in decaned

TBHP in waterc

TBHP in decaned

conv.

sel.

conv.

sel.

conv.

sel.

Sn-Y

86

12.8

99.1

24.3

99.3

1.6

96.7

4.8

98.9

Sn-Beta

116

2.8

99.4

9.6

99.6

0.9

98.6

3.1

98.8

a

Reaction conditions: cat, 50 mg; 2-adamantanone, 2 mmol; TBHP, 4 mmol;

chlorobenzene, 10 mL; temp., 363 K; time, 8 h. b

Reaction conditions: cat, 100 mg; cyclohexanone, 5 mmol; TBHP, 1.25 mmol;

fluorobenzene, 12 mL; temp., 358 K; time, 3 h. c

TBHP, 65 wt.% in H2O; K2CO3 (0.069 mM) added.

d

TBHP, 5.5 M in decane.

43



[111]

[111]

A Intensity (a.u.)

d

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 56

c b

c

b a

a 5

10

15

20

25

30

B

35

5.5

6.0

6.5

7.0

2 Theta (°)

2 Theta (°)

Figure 1. Wide angle XRD patterns (A) and enlarged [111] reflections (B) of USY (a), Y-1 (b), Sn-Y-11 (c), Sn-Y-6 (d).

44


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A

C

B 500nm

500nm

2µm

2µm

500nm 500nm

2µm

Figure 2. Scanning electron micrographs of USY (A), Sn-Y-11 (B), and Sn-Y-6 (C).

45


-1

-1

Al Sn

A

0.6

Amount of Al or Sn (mmol g)

0.8

0.4 0.2 0.0 0 2 4 6 8 Concentration of (NH4)2SnCl6 (mM)

0.3 0.2 0.1 0.0 0

20

40 Time (h)

60

Al Sn

B 0.3 0.2 0.1 0.0

0

1 2 3 4 Concentration of HCl (M)

0.32

Al Sn

C

0.4

Page 46 of 56

D

-1

0.4

Incorporated Sn (mmol g)

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



80

0.24 0.16 0.08 0.069

0.138 0.207 0.276 -1 Released Al (mmol g)

Figure 3. Effects of (NH4)2SnCl6 concentration (A), HCl concentration (B), treatment time (C) on the amount of Al or Sn in the zeolite and the relationship profile between the amount of released Al and the amount of incorporated Sn (D). Stannation conditions: (A) HCl, 0.1 M; time, 24 h; temp., RT; (B) (NH4)2SnCl6, 4 mM; temp., RT; time, 24 h; (C) (NH4)2SnCl6, 4 mM; HCl, 1 M; temp., RT; (D) (NH4)2SnCl6, 4 mM; HCl, 1 M; temp., RT. The liquid to solid was 80 mL : 1 g for all cases.

46


Page 47 of 56

210 a

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

255

c d e

200 250 300 350 400 450 500 Wavelength (nm) Figure 4. UV-vis spectra of different Sn-Y samples obtained by Sn-incorporation in the presence of 0.1 M (a), 0.5 M (b), 1 M (c), 2 M (d), and 4 M (e) HCl solution. Other stannation conditions: (NH4)2SnCl6, 4 mM; liquid to solid ratio, 80 mL :1 g; temp., RT; time, 24 h.

47



2277

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2267

2316 2307 a b c d

2320 2300 2280 2260 2240 2220 Wavenumber (cm-1)

Figure 5. FT-IR spectra of Sn-Y-11 after adsorption of CD3CN at 298 K for 1 h and desorption at 313 K (a), 333 K (b), 353 K (c), and 373 K (d) for 2 min, respectively.

48


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Page 49 of 56

-450

-745 -605

∗ ∗

c

b ∗ ∗

-200

-400

a

∗ ∗

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-600

-800

-1000

Chemical shift (ppm) Figure 6.

119

Sn MAS NMR spectra of SnO2 oxide (a), hydrated Sn-Y-11 (b),

dehydrated Sn-Y-11 (c). Spinning side bands are highlighted by asterisks.

49



A

B

1445 1451

1596 1610 1580

1439

1490

a

1544

1451

1610

1490

e

Absorbance (a.u.)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 56

b c

d c b a

d 1600

1550

1500

1450

1400

Wavenumber (cm-1)

1600

1550

1500

1450

1400

Wavenumber (cm-1)

Figure 7. (A) FT-IR spectra of Sn-Y-11 (Si/Sn=86) after pyridine adsorption at 298 K for 1 h and desorption at 323 K (a), 373 K (b), 423 K (c), and 473 K (d) for 1 h, respectively. (B) FT-IR spectra of Sn-free parent USY (a), Sn-free Y-1 (b) and Sn-Y with a Sn content of 0.91 wt.% (c), 1.98 wt.% (d), 2.31 wt.% (e) after pyridine adsorption at 298 K for 30 min and desorption at 373 K for 1 h.

50


Page 51 of 56

5

A

Q

3470

4

d

d 3

4

B 3740

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

Q

Si (1Al) 3 Q

c

c 2

b

3630 3561

1

b

a 0 4000

a 3800

3600

3400

3200

3000

Wavenumber (cm-1)

-90

-95

-100

-105

-110

-115

-120

Chemical shift (ppm)

Figure 8. FT-IR spectra in the region of hydroxyl stretching vibration (A) and 29

Si MAS NMR spectra (B) of USY (a), Y-1 (b), Y-20 (c), Sn-Y-11 (d).

51



5 3740

4

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3470

c

3

b

2

a 1 0 4000

3800

3600

3400 3200 Wavenumber (cm-1)

3000

Figure 9. FT-IR spectra in the region of hydroxyl stretching vibration of Sn-Y-5 (a), Sn-Y-4 (b), Sn-Y-11 (c).

52


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Page 53 of 56

100

Conversion of ketone (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0

0

1 2 3 4 Concentration of HCl (mM)

Figure 10. The conversion of 2-adamantanone in function of the HCl concentration in process of (NH4)2SnCl6 treatment. Reaction conditions: cat, 50 mg; 2-adamantanone, 2 mmol; H2O2 (30 wt.%), 4 mmol; chlorobenzene, 10 mL; temp., 363 K; time, 10 min.

53



100 15 80 10

60

5

0

0.0

40

Selectivity of lactone (%)

20

Conversion of ketone (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 56

20

0.3

0.6 1.2 1.5

Concentration of KCl (mM) Figure 11. Conversion and selectivity in the B-V oxidation of cyclohexanone versus the concentrations of KCl in fluorobenzene. Reaction conditions: cat, 100 mg; cyclohexanone, 5 mmol; H2O2 (66.9 wt.%), 1.25 mmol; fluorobenzene, 12 mL; temp, 358 K; time, 0.5 h.

54


Page 55 of 56

1.6 a b c

-1

Adsorbed ketone (mmol g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2

d 0.8

0.4

0.0 0

20

40

60

80

100 120

Adsorption time (min) Figure 12. Liquid-phase adsorption of cyclohexanone over Sn-containing catalysts. Sn-Y-11 under anhydrous conditions (a), Sn-Beta under anhydrous conditions (b), Sn-Y-11 under hydrous conditions (c), Sn-Beta under hydrous conditions (d). Adsorption conditions: cat., 50 mg; 1.0 wt.% cyclohexanone in 1,3,5-TIPB, 2 g; temp., 298 K; water if added, 0.2 g.

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Toc graphic FAU-type stannosilicate (Sn-Y) with hierarchical pore structure and extremely low Al content, postsynthesized by dealumination-induced mesopore formation and subsequent isomorphous substitution with (NH4)2SnCl6, shows unique catalytic properties in the Baeyer-Villiger oxidation of ketones using hydrogen peroxide or tert-butyl hydroperoxide as the oxidant.

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Postsynthesis and Effective BaeyerâVilliger ... - ACS Publications

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Postsynthesis and Effective BaeyerâVilliger ... - ACS Publications