Microporous zeolites with tunable framework compositions and variable crystalline structures have a wide range of applications such as catalysis,adsorption,ion exchange and separation [1, 2, 3]. Layered zeolites are a class of unique microporous materials composed of the crystalline layers of unit cell level,which are usually interlinked each other via hydrogen bondings in assynthesized precursors [4, 5]. This kind of weak and variable interlayer linkage endows layered zeolites with structural flexibility and diversity in contrast to conventional 3-dimentional (3D) zeolites [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. In view of this feature,many modifications have been applied to lamellar precursors to obtain new structures with larger pore sizes or higher external surface area,such as pillaring [18],full or partial delamination [19, 20, 21],intercalation [22] and interlayer silylation [23]. Acid treatment is a common technique for modifying lamellar precursors [4, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37]. Taking one of the most widely studied materials,MCM-22(P) as an example, interlayer expanded material IEZ-MWW was post-synthesized under harsh acid treatment condition [23],while a material structurally analogous to MCM-56 was formed under mild conditions,e.g. acid treatment at room temperature. This material is classified as a sub-zeolite of MWW topology [4, 24, 25], possessing more exposed 12-MR side cups and higher external surface areas. Other sub-zeolites have also been produced by acid treatment,such as sub-NSI zeolite and ECNU-4 [33, 36]. PLS-3 is a kind of layered silicate with a crystal size of nanometer scale. It is converted to 3D FER topology via topotactic conversion upon direct calcination [38]. In this study,with the purpose to prepare oxidation catalysts suitable for processing bulky molecules,we conducted the post-synthesis of titanosilicate Ti-ECNU-8 with a sub-zeolite structure of FER by combining HCl-EtOH treatmentinduced structural reorganization and solid-gas titanation. Consisting of disordered stacking of FER sheets with a lower longrange crystallinity but possessing a high external surface area than corresponding 3D FER counterpart,Ti-ECNU-8 exhibited an excellent catalytic performance for the epoxidation of cycloalkenes with H2O2. 2. Experimental
The layered precursor PLS-3 silicate was synthesized according to previously reported methods [38],using protonated kanemite as silica source and tetraethylammonim hydroxide (TEAOH) as the structure-directing agent (SDA). The molar composition of the starting mixture was 1.0SiO2:0.2TEA+:0.04NaOH:6.5H2O. The synthetic gel was charged into a stainless autoclave equipped with a Teflon liner and heated at 443 K for 24 h under static conditions. ECNU-8 was prepared by post-synthesis of PLS-3. In a typical run,1.0 g of PLS-3 was treated in a Teflon-lined autoclave with 30 mL of 1.0 mol/L HCl-EtOH solution at 443 K for 40 min. Then the product underwent filtration,washing,drying and calcination,the final material named as ECNU-8.
Calcined ECNU-8 and PLS-3 were placed into a quartz tube reactor (i.d. 3 cm) and pretreated at 773 K for 2 h in a dry N2 flow. Then the reactor was brought to a treatment temperature of 673 K, meanwhile the N2 flow went through an anhydrous TiCl4 solution to carry it into the reactor for 1 h. After cooling to room temperature in N2 flow,the resultant was washed with deionized water and dried in air at 353 K overnight,named as Ti-ECNU-8 or Ti-PLS-3,respectively.
All the catalytic tests were performed in a 50 mL flask equipped with a condenser under stirring. Temperature was controlled with an oil bath. The detailed reaction conditions are shown in the footnotes of Table S2 in Supporting information. 3. Results and discussion
The XRD pattern of PLS-3 precursor exhibited a characteristic diffraction at 2θ = 7.6° that was due to the [2 0 0] plane of layered structure (Fig. 1a). After direct calcination,the [2 0 0] diffraction shifted to a higher region at 2θ = 9.4° as a result of interlayer condensation (Fig. 1b),forming the 3D FER structure,which was consistent to literature [38]. However,the sub-zeolite ECNU-8 prepared by HCl-EtOH treatment showed a different XRD pattern (Fig. 1c). The layer-related [2 0 0] diffraction moved to a higher angle as a result of interlayer condensation. Meanwhile,the [h k l] diffractions with h ≠ 0,such as [2 0 0],[2 2 0],[4 0 0] and [6 0 0] planes,became broaden and less intensive. However,those assigned to the indexes with h = 0,such as [0 2 0],[0 1 1], [0 3 1],[0 0 2] and [0 4 0] planes,were relatively well retained. This phenomenon indicated the HCl-EtOH treatment for a short time of 40 min caused a reorientation of the interlayer hydrogen bond moieties,which made the layer-stacking disordered in ECNU-8. This result was very similar to the difference observed between MCM-22 and MCM-56 [24, 25],indicating the formation of disordered sub-zeolite which may possess more exposed external surface. The reason for this outcome may be an inhomogeneous removal of the organic SDA species occluded between the layers. After the calcination of ECNU-8,the [2 0 0] diffraction further shifted to a higher angle,having the same position as 3D FER (Fig. 1d). After TiCl4 treatment of calcined ECNU-8,the resultant showed the same characteristic peaks with a little loss of peak intensity (Fig. 1e),verifying the preservation of the structure during this procedure.
The schematic illustration for this procedure is given in Fig. 2. Starting from PLS-3 lamellar precursor,direct calcination burned off the organic SDA species and leaded to dehydration between the layers,forming the 3D FER structure with 10 × 8-MR. Differently,a controlled HCl-EtOH treatment removed a part of SDA species, which would disturb the regular stacking of the FER sheets along the [1 0 0] direction and generated a sub-zeolite structure ECNU-8 with a higher external surface area than normal 3D FER structure. A further titanation with TiCl4 vapor introduced the Ti species into the framework of ECNU-8,giving rise to titanosilicate Ti-ECNU-8 with well-retained sub-zeolite structure.
The textural properties of calcined PLS-3 and ECNU-8 were investigated by N2 sorption. As given in Table S1 in Supporting information,the material prepared by acid treatment had lower microporosity and total surface area. The total surface area of ECNU-8 was 359 m2 g-1,which was 23.1% smaller than that of PLS- 3 (467 m2 g-1) (Table S1,Nos. 1 and 2),this is presumably because of the disordered stacking of the FER sheets. The external surface area calculated by t-plot method was 167 m2 g-1 and 130 m2 g-1 for ECNU-8 and PLS-3,respectively (Table S1),indicating that the disordered stacking of FER sheets led to a more exposed external surface. Fig. 3 compares the N2 adsorption isotherms of Ti-ECNU-8 and PLS-3. The former showed a lower adsorption capacity in the low P/P0 region (0-0.8) than PLS-3,but a similar amount at higher P/P0,indicating a more exposed external surface of Ti-ECNU-8. The external surface area Ti-ECNU-8 was 160 m2 g-1 (Table S1,Nos. 4). It was very comparable to that of ECNU-8,suggesting the TiCl4 treatment did not bring about an obvious structural change,which was in agreement with XRD characterization. With an enlarged external surface area in comparison to directly calcined PLS-3,Ti- ECNU-8 was expected to be a potential catalyst for processing bulk substrates.
UV-vis spectroscopy is a sensitive method to detect the coordination states of the transition metal ions in zeolites particularly in the case of Ti,Sn or Zr-containing metallosilicates [39]. The UV-vis spectrum of Ti-ECNU-8 gave a main absorption band at 215 nm (Fig. 4A),which was characteristic peak of tetrahedrally coordinated Ti species in the framework positions. Meanwhile,a weak band at 260 nm was also observed,implying the presence of a little amount of hexa-coordinated extraframework Ti species. IR spectra in the structural vibration region are widely used to characterize titanium zeolites,as they exhibited a characteristic IR band approximately at 960 cm-1 [40]. The FT-IR spectrum of Ti-ECNU-8 showed more enhanced symmetric band at 960 cm-1 than calcined PLS-3 (Fig. 4B),verifying again the formation of framework Ti species. The Si/Ti ratio of Ti-ECNU-8 was 135,which was much lower than that of Ti-PLS-3 (Table S1, Nos. 3 and 4). Since the TiCl4 molecules with a relatively large molecular dimension can hardly enter into the channels of 10-MR window,the more Ti introduced into Ti-ECNU-8 than PLS-3 could ascribe to a more exposed surface in former material. This could be also taken as an evidence of the presence of more accessible reaction spaces in Ti-ECNU-8. The SEM image indicated that Ti- ECNU-8 was composed of rod-like shaped crystals of 100-150 nm (Fig. 4C),which was similar to PLS-3 reported previously [38]. Table S2 provides the catalytic performances for the epoxidation of 1-hexene over various titanosilicates. TS-1 with the 10-MR MFI structure showed the highest conversion than the other materials,demonstrating it was an efficient catalyst for linear alkenes. In terms of specific activity per Ti site,Ti-ECNU-8 showed a higher turnover number (TON). Ti-PLS-3 gave an extremely lower conversion than Ti-ECNU-8 owing to its low Ti content and less accessible surface. To further explore the catalytic property of Ti-ECNU-8,the epoxidation of a series of cycloalkenes was also conducted (Fig. 4D). For catalyzing small molecule cyclopentene, TS-1 gave the highest TON. However,with increasing molecular dimension of cycloalkenes,TS-1 lost its superiority due to the steric restrictions of its 10-MR channels to bulky molecules. Meanwhile, Ti-ECNU-8 with a high external surface area exhibited the advantages by showing the highest TON for catalyzing cyclohexene and cycloheptene. Nevertheless,the catalytic activity decreasing behavior of Ti-ECNU-8 with increasing molecular size of substrates was different from Ti-Beta with a 3D 12-MR pore system. Although showing a higher absolute TON than Ti-Beta,Ti-ECNU-8 decreased in activity more sharply with increasing molecular size of substrates. As a result,Ti-Beta and Ti-ECNU-8 gave very comparable TONs for cyclooctene. Thus,Ti-ECNU-8 would possess a smaller reaction space than 12-MR Ti-Beta. 4. Conclusion
A sub-zeolite of FER topology named as Ti-ECNU-8 has developed by combining a controlled acid treatment of PLS-3 precursor and solid-gas titanation with TiCl4 vapor. Possessing a structure composed of disordered stacking of FER sheets but with a more exposed external surface,Ti-ECNU-8 serves as an excellent catalyst for catalyzing the epoxidation of bulk alkenes with hydrogen peroxide.
AcknowledgmentsWe gratefully acknowledge the National Natural Science Foundation of China (Nos. 21373089,U1162102),PhD Programs Foundation of Ministry of Education (No. 2012007613000),the National Key Technology R&D Program (No. 2012BAE05B02),and the Shanghai Leading Academic Discipline Project (No. B409)
Appendix A. Supplementary dataSupplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.09.003.
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