Cation-exchange resin towards low-cost synthesis of high-performance TS-1 zeolites in the presence of alkali-metal ions

Abstract

Zeolite TS-1 is an important commercial catalyst for green production of oxyfunctionalized chemicals such as alcohols, ketones, epoxides, and oximes. However, the extremely high price greatly restricts its wide applications. In this paper, we developed a new route to synthesize highly active TS-1 zeolites by using low-cost tetrapropylammonium hydroxide (TPAOH) as a starting reagent that contains alkali-metal cations such as Na⁺ and K⁺ and is much cheaper than the high-purity TPAOH currently used in reported routes. The key point is the introduction of a cation-exchange resin into the synthetic system to capture the alkali-metal cations via ion exchange between the alkali-metal cations in the synthetic medium and protons in the resin, as well as adjusting the pH value of the TS-1 synthetic system. The present development shows great commercial potential and opens the possibility of preparing cheap TS-1 catalysts by using commercial TPAOH raw materials.

---

Introduction

Zeolites are well-known nanoporous materials for applications in adsorption, separation and catalysis, especially in petrochemistry and fine-chemical synthesis.^1–6 Titanosilicate TS-1,⁷ a titanium-containing high-silica zeolite, is a key commercial heterogeneous catalyst for green oxidation processes with H₂O₂ as an oxidant to produce oxyfunctionalized chemicals, such as alcohols,^8–11 ketones,^12–14 epoxides,^15–23 oximes^24–26 and so on,^27–31 which is a milestone in zeolite and environmentally-benign catalysis research fields. The eco-friendly processes based on TS-1/H₂O₂ have been successfully applied to industry-scale production of diphenols from phenol, cyclohexanone oxime from cyclohexanone, and propylene oxide from propylene.^32,33 Although the catalytic processes based on TS-1 catalyst are desired and beneficial from environmental viewpoint, the production cost of TS-1 is extremely high, which greatly restricts its wide applications. The high price of TS-1 is mainly attributed to the usage of expensive tetrapropylammonium hydroxide (TPAOH) as organic-structure-directing agents, which can contribute as high as >96% of the total cost of all the raw materials used in TS-1 synthesis.³⁴ In order to reduce TS-1 production cost, scientists made great effects on the low usage of TPAOH^34–36 or replacement of TPAOH with relatively cheap agents,^37–41 such as tetrapropylammonium bromide (TPABr). However, the catalytic performance of TS-1 obtained from these methods was not satisfactory, mainly due to the large size of TS-1 crystals or low content of active Ti species in TS-1 zeolites. Therefore, it is necessary to exploit other low-cost routes to synthesize high-performance TS-1 zeolites.

The extraordinarily high price of TPAOH used in TS-1 synthesis is mainly attributed to its high purity free of alkali-metal cations, e.g. Na⁺, K⁺, to meet the demands of synthesizing TS-1 zeolites. The presence, even in very small amounts, of alkali metal cations in synthetic system of TS-1 zeolites could eliminate the catalytic activity of TS-1 catalyst.^7,42 Compared to other raw materials used to synthesize TS-1, TPAOH contains more alkali metal cations and is very difficult to remove them completely. The complicated and expensive processes, such as AgOH method⁴³ and electrolytic method,^44–46 are often employed so as to effectively remove residual alkali-metal ions in the TPAOH raw materials, which greatly raise the price of TPAOH for TS-1 synthesis. Commercially available TPAOH is obtained by simple reaction of TPABr with KOH in methanol and then recrystallized in water, which is much cheaper than the high-purity TPAOH used in TS-1 synthesis but contains alkali-metal cations. Therefore, the synthesis of TS-1 zeolites by using alkali-metal-containing commercial TPAOH is a possible low-cost route if the adverse effects of alkali-metal ions on TS-1 activity could be solved.

Strategies to solve the adverse influence of alkali metal cations in TS-1 synthesis are still a challenging issue. After studies by many groups around the world in the past decades,^42,47–50 it was found that only a very small amount of alkali-metal ions (molar ratio of alkali-metal ions to Si less than 0.01) was tolerable for synthesizing active TS-1.^42,47–50 Higher amount of alkali-metal ions present in the synthetic system would make the catalytic activity of TS-1 decreased significantly. In the case of commercial TPAOH reagent (Aldrich, containing approximately 7000 ppm K⁺ and 350 ppm Na⁺) used as starting template source with the TPAOH/Si molar ration of 0.45 as reported,⁷ alkali-metal ions with K⁺/Si molar ratio of ∼0.08 and Na⁺/Si molar ratio of ∼0.03 should be present in the TS-1 synthetic system, which are much higher than the limit concentration (alkali-metal ion/Si ∼0.01) for synthesizing active TS-1.

Recently, a new route claimed that highly crystalline TS-1 could be synthesized in the presence of alkali-metal ions with a concentration similar to that of commercial TPAOH by carefully controlling the hydrolysis of silicon source and titanium source.^51,52 However, the TS-1 products obtained by the method of controlling the hydrolysis of silicon source and titanium source had large crystal size (several micrometers) and significant amounts of non-active extra-framework titanium species, which led to lower catalytic performance than that synthesized in alkali-metal ion-free system. In addition, the cautious operation for matching the hydrolysis rate of Si and Ti sources would also be very difficult for practical production, because many factors would disturb this matching process such as temperature, pH value, agitation, and time.^7,14,53 Therefore, a facile and direct route aiming at the elimination and/or deactivation of alkali-metal cations in TS-1 synthesis is greatly desired.

In this paper, for the first time, cation-exchange resin was chosen and employed as an additive to overcome the challenge in the synthesis of highly active TS-1 with the use of TPAOH raw material that contains a high-concentration of alkali-metal ions (containing approximately ∼10 000 ppm K⁺ and ∼600 ppm Na⁺). Cation-exchange resin was expected to directly capture alkali-metal cations in the synthetic system, as well as reduce the pH value of the TS-1 synthetic system. The results showed that the presence of cation-exchange resin in TS-1 synthetic system not only effectively eliminated most of alkali metal cations from the final TS-1 product, but also assisted in incorporating more titanium(IV) into the framework positions, thereby significantly enhancing the catalytic activity for the oxidation of 1-hexene using H₂O₂ as oxidant.

Experimental

Chemicals and materials

Titanium tetra-n-butoxide and tetraethylorthosilicate were purchased from Aldrich. Tetrapropylammonium hydroxide (TPAOH) and H₂O₂ (30 wt% in water) were from Sigma-Aldrich. Tetrapropylammonium hydroxide that contains approximately ∼10 000 ppm K⁺ and ∼600 ppm Na⁺ was homemade by mixing TPAOH (Sigma-Aldrich) with KOH and NaOH. Cation-exchange resin (Amberlite IR120 hydrogen form) composed of styrene–divinylbenzene matrix with sulfonic acid active group was purchased from Fluka, which had 620–830 μm in size and 1.8 meq mL⁻¹ capacity. Reagents of 1-hexene and 1,2-epoxyhexane were purchased from Aldrich. 0.25 M Ce(SO₄)₂ in sulfuric acid (0.25 N) was from Fluka. All the chemical agents were used without further purification.

Zeolite synthesis

Firstly, titanium tetra-n-butoxide was added to H₂O₂ aqueous solution to form a stable Ti source of Ti-peroxo complex. Then, an aqueous solution of tetrapropylammonium hydroxide that contains approximately ∼10 000 ppm K⁺ and ∼600 ppm Na⁺ was added into above Ti solution followed by the addition of tetraethylorthosilicate under stirring. After 0.5 h, the resultant solution was heated to 353 K to evaporate alcohol generated during the hydrolysis of the Ti and Si precursors. After completely evaporating alcohol, the clear solution was cooled down, and cation-exchange resin composed of styrene-divinylbenzene matrix with sulfonic acid active group (Amberlite IR120 hydrogen form, Fluka, 620–830 μm in size and 1.8 meq mL⁻¹ capacity) was added. The molar composition of the final mixture was given in Table 1. The final mixture was transferred into an autoclave and treated at 443 K for 2 days under static condition. Decantation method was used to remove cation-exchange resin because cation-exchange resin was easy and fast to precipitate at the bottom of vessel while TS-1 crystal suspended in the solution. The suspended TS-1 crystal was centrifuged, washed with distilled water, and dried at 373 K. The obtained product was treated under calcination at 823 K for 6 h to remove the organic templates. The preparation of TS-1 under conventional synthetic route without cation-exchange resin was also conducted as the control experiment.

Table 1 Synthetic conditions and composition of final TS-1 samples

Samples	Gel composition			Left TPA^+^c (%)	pH	Product of TS-1 zeolites
	Si/Ti (mol/mol)	Alkali metals	Resin(SO3H)/Si (mol/mol)			Si/Ti (mol/mol)	Si/K (mol/mol)	Si/Na (mol/mol)
a Molar ratio: TPAOH/SiO2 = 0.45; H2O/SiO2 = 35.b Molar ratio: TPAOH/SiO2 = 0.36; OH^−/SiO2 = 0.47; K^+/SiO2 = 0.10; Na^+/SiO2 = 0.01; H2O/SiO2 = 35.c Left TPA^+ percentage in synthetic media after exchange with resins, which was determined by TG characterization of left solution after removing the exchanged resin. The abbreviation of n.a. means not added.
TS-1-Con^a	40	n.a.	n.a.	—	12.3	51	—	—
TS-1-Alk^b	40	K, Na	n.a.	100	12.4	57	45	194
TS-1-resin-1^b	40	K, Na	0.33	74	11.5	41	105	>1000
TS-1-resin-2^b	40	K, Na	0.37	72	10.3	40	123	>1000
TS-1-resin-3^b	40	K, Na	0.39	67	9.8	39	146	>1000
TS-1-resin-4^b	40	K, Na	0.42	39	6.6	29	214	>1000

---

Zeolite characterization

PXRD measurements were performed on a Bruker Powder D8 Advance diffractometer at 40 kV and 40 mA using CuKα radiation (λ = 1.5418 Angstrom). DRUV/Vis spectra were recorded on a SHIMADZU UV-2450 spectrophotometer at 298 K using BaSO₄ as a reference. IR spectra were recorded as KBr pellets on a SHIMADZU IRPrestige-21 spectrometer. Nitrogen adsorption–desorption isotherms were measured on a TriStar II 3020 sorption analyzer at 77 K. Thermogravimetry (TG) was performed with a TA SDT Q600 apparatus. Elemental analyses (Si, Ti, Na and K) were performed on an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer ICP Optima 2000DV). Field-emission SEM images were obtained on a JEOL JSM-7600F microscope operated at 5 kV. TEM observations were performed on a JEOL JEM-1400 TEM microscope, working at 100 kV. All samples subjected to TEM measurements were dispersed in ethanol ultrasonically and were dropped on copper grids.

Catalytic reactions

Oxidation reactions were performed in a 20 mL glass reactor immersed in a 60 °C oil bath in the presence of H₂O₂ (30 wt% in water). In a typical run, the reactions were carried out with catalyst (25 mg), 1-hexene (5.0 mmol) and H₂O₂ (5.0 mmol) in methanol (5.0 mL) with vigorous stirring for 2 h. After the reaction, the mixture was analyzed by gas chromatography. The amount of unconverted H₂O₂ was determined by titrating with Ce(SO₄)₂ aqueous solution (0.25 M). The products were verified using authentic chemicals commercially available or determined by mass spectrometer.

Results and discussion

Synthetic conditions and composition of final TS-1 samples

The synthetic conditions and the compositions of the TS-1 products are summarized in Table 1. The presence of alkali-metal cations in the synthetic system slightly decreases the titanium content in the final product when comparing sample TS-1-Alk (TS-1 synthesized by using alkali-metal-ion-containing TPAOH) with sample TS-1-Con (TS-1 synthesized via the conventional route by using high-purity TPAOH free of alkali-metal ions). Alkali metal cations could reside in the final product if they are present in the starting synthetic system. Introducing cation-exchange resin into the synthetic media can effectively reduce the pH value and lead to the increase of titanium content, as well as the decrease of the alkali-metal cation amounts in the final product. The decrease of pH value indicates the cation exchange between the resin that contains protons and synthetic medium that contains alkali-metal and TPA cations. The more the resin added, the lesser the alkali-metal cations resided in the final products. This implies that cationic ion-exchange resin can effective exchange with alkali-metal cations in the synthetic media. Meanwhile, the exchange between H⁺ in resin and TPA⁺ also occurs, which can adsorb and remove TPA cations from synthetic solution. The left amounts of TPA cations in synthetic media decrease with the increase of exchange resin added (Table 1). It should be noted that the cationic ion-exchange resin used here is stable with hard spherical morphology (620–830 μm in diameter) during the whole synthetic processes, and can be easily separated from the final synthetic medium.

Zeolite characterization

As shown in Fig. 1, powder X-ray diffraction (PXRD) patterns indicate that all the samples had the MFI structure with high purity. The diffraction intensity of sample TS-1-Alk was lower than that of sample TS-1-Con, indicating lower crystallinity of sample TS-1-Alk synthesized in the presence of alkali-metal cations. After adding a small amount of cation-exchange resin in the synthetic system, as shown in Fig. 1c, the diffraction intensity significantly increased as compared with TS-1-Alk. Further increase of the added amounts of cation-exchange resin caused a slight decrease in the diffraction intensity (Fig. 1d–f).

Fig. 1 PXRD patterns of final TS-1 products (a) TS-1-Con, (b) TS-1-Alk, (c) TS-1-resin-1, (d) TS-1-resin-2, (e) TS-1-resin-3, and (f) TS-1-resin-4.

Scanning electron microscope (SEM) images indicate that TS-1-Con possesses cubic morphology and uniform crystal size of 100–200 nm. The presence of alkali metal ions did not affect the morphology and crystal size of TS-1 (TS-1-Alk) except for some impure small particles around the TS-1 crystals (Fig. 2a and b). These impure particles may be amorphous, which is the reason of low intensity in PXRD pattern (Fig. 1b). After introducing cationic ion-exchange resin, the crystal size slightly increased to 300–400 nm but the surface of crystal was not smooth (Fig. 2c–f). Fig. 2f (TS-1-resin-4) indicates that more resin added would result in the formation of some impurity.

Fig. 2 SEM images of final TS-1 products (a) TS-1-Con, (b) TS-1-Alk, (c) TS-1-resin-1, (d) TS-1-resin-2, (e) TS-1-resin-3, and (f) TS-1-resin-4.

The size and the morphology of final TS-1 products are further measured by transmission electron microscopy (TEM), as shown in Fig. 3. The crystal size determined from TEM images is consistent with the results of SEM measurement. In addition, Fig. 3c showed a dark black image of the crystals, indicating well-crystallized particles. TEM images in Fig. 3d and e reveal that every particle seems to exhibit co-growth of several small crystals, which is probably the reason of slightly low PXRD intensity (Fig. 1d and e). Fig. 3f also showed dark black image of crystals with small particles. The relatively low diffraction intensity in PXRD was attributed to these small particles, which probably was some non-crystalline silica or titania due to the low pH and low TPA cations of the synthetic system.

Fig. 3 TEM images of final TS-1 products (a) TS-1-Con, (b) TS-1-Alk, (c) TS-1-resin-1, (d) TS-1-resin-2, (e) TS-1-resin-3, and (f) TS-1-resin-4. The scale bars are 200 nm in all images.

The surface area and micropore volume of final TS-1 products are summarized in Table 2. Sample TS-1-Alk synthesized by using alkali-metal-ion-containing TPAOH has much lower surface area and microporous volume than TS-1-Con synthesized via the conventional route, indicating alkali metal ions adversely affect the crystallinity of final TS-1 product. This is consistent with the low-intensity results of PXRD. The low surface area and microporous volume may be attributed to some impure small particles around the TS-1 crystals as observed in SEM and TEM images (Fig. 2b and 3b), giving solid evidence of amorphous structure of impure small particles.

Table 2 Structural parameters of final TS-1 samples

Samples	Si/Ti (mol/mol)	SBET^a (m^2 g^−1)	Vmicro^b (cm^3 g^−1)
a Brunauer–Emmett–Teller (BET) surface area estimated by nitrogen adsorption/desorption measurements.b Micropore volume.
TS-1-Con	51	414	0.15
TS-1-Alk	57	306	0.08
TS-1-resin-1	41	398	0.14
TS-1-resin-2	40	392	0.14
TS-1-resin-3	39	379	0.14
TS-1-resin-4	29	374	0.14

---

After introducing resins into the synthetic system, all the TS-1 samples synthesized exhibited higher surface area and microporous volume than TS-1-Alk. In addition, TS-1 samples synthesized in the presence of resin exhibited lower surface area and microporous volume than TS-1-Con synthesized via the conventional route. This is probably due to the low concentration of TPA cations and low pH value after ion-exchange between the cations in synthetic media and protons in resin as shown in Table 1. TPA cations that are used as organic structure-directing agents were partly removed by resin, which would result in the low micro-porosity. The low pH value would lead to low crystallinity of final TS-1 products, which also affected the surface area and microporous volume.

It has been found that several different states of Ti species exist in TS-1 zeolite, including tetra-coordinated, hexa-coordinated and oligomeric Ti. However, only isolated tetra-coordinated Ti species, namely framework Ti species, are proven to provide the catalytic activity.⁷ The presence of highly coordinated extra-framework Ti and anatase-like TiO₂ particles could adversely affect the catalytic activity and result in the decomposition of hydrogen peroxide. The coordination states of Ti species in TS-1 can be detected by diffuse reflectance ultraviolet-visible (DRUV/Vis) spectroscopy. The absorbance peak at approximately 210 nm is attributed to Ti⁴⁺O²⁻ → Ti³⁺O⁻ ligand-to-metal charge transfer, which is assigned to the framework Ti species.

As shown in Fig. 4, TS-1-Con shows a main peak at 210 nm and a shoulder band at 260 nm, indicating the presence of framework Ti species and a small amount of extra-framework Ti species. A weak band at 310–330 nm suggests the presence of a very small amount of TiO₂ tiny particles in TS-1-Con product. TS-1-Alk sample shows only a main band at 210 nm, indicating that most of Ti species is at framework positions. The low intensity of the band at 210 nm points out lower framework Ti content in this sample. All the TS-1 samples synthesized in the presence of resin exhibit a strong band at 210 nm, indicating that most of the Ti species in TS-1 are framework Ti species. In addition, sample of TS-1-resin-4 exhibits another intense band at 315 nm, indicating the presence of a significant amount of anatase-like TiO₂ in this sample. It is worth noting that TS-1-resin-3 shows only framework Ti species with Si to Ti molar ratio of 39, which is the maximum possible Ti content in the TS-1 framework using current techniques. This observation indicates that the presence of cation-exchange resin in synthetic system is beneficial to the incorporation of titanium cations into the framework positions of zeolite.

Fig. 4 Diffuse reflectance UV/Vis (DRUV/Vis) spectra of final TS-1 products (a) TS-1-Con, (b) TS-1-Alk, (c) TS-1-resin-1, (d) TS-1-resin-2, (e) TS-1-resin-3, and (f) TS-1-resin-4.

The framework Ti in TS-1 could also be probed by IR spectroscopy. The band at 960 cm⁻¹, attributed to a stretching vibration mode of [SiO₄] perturbed by adjacent framework Ti species, has been widely accepted as a proof of isomorphous substitution of Ti in the TS-1 lattice.⁷ Importantly, the band intensity at 960 cm⁻¹ proportionally increases if the amount of the framework Ti in TS-1 increases. However, this feature peak would decrease after exchange with alkali metal ions.^42,47–50 To avoid the influence of adsorbed water on the frequency and intensity of this band, all IR spectra were measured after drying samples at 400 °C for 1 h.

As shown in Fig. 5A, all the samples show the band at 960 cm⁻¹, indicating the presence of framework Ti species, which is consistent with the results of DRUS/VIS measurement. The band intensity at 960 cm⁻¹ decreased for sample TS-1-Alk synthesized in the presence of alkali-metal cations as compared with TS-1-Con synthesized without alkali-metal cations (Fig. 5B). The band intensity at 960 cm⁻¹ was greatly enhanced for the samples obtained from resin-containing system and gradually increased upon increasing the added amounts of resin.

Fig. 5 IR spectra of final TS-1 products (a) TS-1-Con, (b) TS-1-Alk, (c) TS-1-resin-1, (d) TS-1-resin-2, (e) TS-1-resin-3, and (f) TS-1-resin-4.

Catalytic test

The catalytic oxidation of 1-hexene in the presence of H₂O₂ was chosen to evaluate the catalytic performance of the TS-1 catalysts prepared in the presence of alkali metal ions (Fig. 6 and Table 3). For TS-1 catalysts synthesized using high-purity TPAOH free of alkali-metal cations (TS-1-Con), a conversion rate of 12.7% was obtained. However, TS-1-Alk synthesized from alkali-metal cation-containing TPAOH showed almost no catalytic activity for 1-hexene epoxidation although it contains a certain amount of framework Ti species as observed by ICP, DRUV/VIS and IR measurements, indicating that alkali-metal cations do greatly affect the catalytic performance of TS-1 zeolites. In addition, the low crystallinity and low microporous volume of TS-1-Alk are also responsible for its low catalytic activity. Moreover, the catalytic activity of TS-1-Alk (conversion rate < 4%) cannot effectively restore by using as-reported acid washing post-treatment method,⁴² mainly due to the low content of framework titanium(IV). Significantly, after introducing a small amount of cation-exchange resin into the synthetic system, the obtained TS-1-resin-1 sample with catalytic activity of 6.5% conversion was detected. With further increases in the added amounts of resin, TS-1-resin-2 sample with catalytic performance surpassing TS-1-Con was achieved. More importantly, the catalytic performance of TS-1 sample further increased to the conversion of 22.5% when more resin was present in the synthetic system, as seen from TS-1-resin-4. It is exciting to successfully achieve the one-pot synthesis of highly active TS-1 in the presence of high-concentration alkali-metal cations. The developed methodology paves the way to the feasible synthesis of high-performance TS-1 by using commercial TPAOH that are much cheaper but contains alkali-metal cations. It should be noted that further increase of the added amounts of resin would result in the formation of only non-crystalline materials without any catalytic activities. This implies crystallinity and porosity of final catalysts will affect the catalytic activity. Only crystalline catalysts with certain amounts of micropores have the possibility to possess catalytic activity for 1-hexene oxidation.

Fig. 6 Catalytic activity for 1-hexene oxidation with H₂O₂ by using (a) TS-1-Con, (b) TS-1-Alk, (c) TS-1-resin-1, (d) TS-1-resin-2, (e) TS-1-resin-3, and (f) TS-1-resin-4.

Table 3 Catalytic oxidation of 1-hexene with H₂O₂^a

Entry	Catalyst	Si/Ti (mol/mol)	Conversion (%)	Selectivity (%)			TON (mol/mol-Ti)	SH2O2^d (%)
				Epoxide	Diol^b	Others^c
a Reaction condition: catalyst (25 mg), methanol (5 mL), 1-hexene (5 mmol), H2O2 (5 mmol), temperature, 60 °C, 2 h.b Containing the diol derivative from ring opening reaction.c The products of homolytic oxidation and further oxidation.d Selectivity in the use of H2O2 towards the oxidation of 1-hexene.e The abbreviation of n.d. means not determined.
1	TS-1-Con	51	12.7	97.2	0	2.8	80	69
2	TS-1-Alk	57	0.2	73.4	26.6	0.1	2	n.d.^e
3	TS-1-resin-1	41	6.5	92.9	4.5	2.7	33	33
4	TS-1-resin-2	40	14.3	96.1	1.5	2.4	71	71
5	TS-1-resin-3	39	18.1	95.0	3.0	2.0	87	87
6	TS-1-resin-4	29	22.5	96.4	1.2	2.4	82	82

---

Conclusions

We have developed a low-cost route to synthesize TS-1 zeolites with the use of cheap raw materials that contain a high-concentration of alkali-metal ions. The key point is that cation-exchange resin was used to capture alkali-metal cations in synthetic system so as to eliminate their adverse effects on TS-1 synthesis, as well as reduce the pH value of TS-1 synthetic system. SEM and TEM characterizations indicate that the crystals are 300–400 nm in diameter. DRUV/VIS and IR spectra demonstrated that high content of framework Ti species are present in the TS-1 crystal when introducing cation-exchange resin. Specifically, sample TS-1-resin-3 achieved 2.0 wt% framework Ti content, which is the maximum possible Ti content in the TS-1 framework using current techniques. The catalytic tests indicate that active TS-1 zeolites cannot be obtained without cation-exchange resins if alkali-metal ion-containing raw materials are used in the synthesis. The introduction of cation-exchange resin can effectively eliminate the adverse effects of alkali-metal cations on the catalytic activity of final TS-1 products, significantly enhancing the catalytic activity for epoxidation of 1-hexene with H₂O₂ as oxidant. The working mechanism may be that cation-exchange resin can capture alkali-metal cations in synthetic system as well as reduce the pH value of TS-1 synthetic system. Since alkali-metal cations compete with titanium cations in the interaction with zeolite framework, the elimination of alkali-metal cations by cation-exchange resin is beneficial to the incorporation of titanium cations into the framework positions. The decrease of the pH value would adjust the crystallization processes for matching the hydrolysis and condensation rate of Si and Ti sources of TS-1 zeolites as well as weakening the interaction between the alkali-metal cations and zeolite framework,^42,47–50 which is beneficial to forming more framework titanium species in final TS-1 zeolites. The present development shows a great commercial potential and opens an alternative low-cost route of preparing titanium-containing zeolites by using cheap raw materials containing alkali-metal cations.

Acknowledgements

This research project is supported by the program for scientific research innovation team in colleges and universities of Shandong province, the National Research Foundation (NRF), Prime Minister's Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) Programme-Singapore Peking University Research Centre for a Sustainable Low-Carbon Future, the NTU-A*Star Silicon Technologies Centre of Excellence under the program grant No. 1123510003, as well as Japan Society for the Promotion of Science (JSPS) fellowship and research funds.

Notes and references

1. K. Varoon, X. Y. Zhang, B. Elyassi, D. D. Brewer, M. Gettel, S. Kumar, J. A. Lee, S. Maheshwari, A. Mittal, C. Y. Sung, M. Cococcioni, L. F. Francis, A. V. McCormick, K. A. Mkhoyan and M. Tsapatsis, Science, 2011, 334, 72–75.
2. K. Na, C. Jo, J. Kim, K. Cho, J. Jung, Y. Seo, R. J. Messinger, B. F. Chmelka and R. Ryoo, Science, 2011, 333, 328–332.
3. M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki and R. Ryoo, Nature, 2009, 461, 246–249.
4. J. L. Sun, C. Bonneau, A. Cantin, A. Corma, M. J. Diaz-Cabanas, M. Moliner, D. L. Zhang, M. R. Li and X. D. Zou, Nature, 2009, 458, 1154–1157.
5. H. Lee, S. I. Zones and M. E. Davis, Nature, 2003, 425, 385–388.
6. S. Mitchell, N. L. Michels, K. Kunze and J. Perez-Ramirez, Nat. Chem., 2012, 4, 825–831.
7. M. Taramasso, G. Perego and B. Notari, US Pat. 4,410,501, 1983.
8. D. R. C. Huybrechts, L. Debruycker and P. A. Jacobs, Nature, 1990, 345, 240–242.
9. T. Tatsumi, M. Nakamura, S. Negishi and H. Tominaga, J. Chem. Soc., Chem. Commun., 1990, 476–477.
10. C. B. Khouw, H. X. Li, C. B. Dartt and M. E. Davis, ACS Symp. Ser., 1993, 523, 273–280.
11. J. G. Wang, Y. L. Zhao, T. Yokoi, J. N. Kondo and T. Tatsumi, ChemCatChem, 2014, 6, 2719–2726.
12. C. F. Shi, B. Zhu, M. Lin, J. Long and R. W. Wang, Catal. Today, 2011, 175, 398–403.
13. L. Wang, J. Sun, X. J. Meng, W. P. Zhang, J. Zhang, S. X. Pan, Z. Shen and F. S. Xiao, Chem. Commun., 2014, 50, 2012–2014.
14. W. B. Fan, R. G. Duan, T. Yokoi, P. Wu, Y. Kubota and T. Tatsumi, J. Am. Chem. Soc., 2008, 130, 10150–10164.
15. L. N. Wang, Y. Q. Wang, G. Q. Wu, W. P. Feng, T. Zhang, R. M. Yang, X. Jin, H. N. Shi and S. H. Wang, Asian J. Chem., 2014, 26, 943–950.
16. J. G. Wang, L. Xu, K. Zhang, H. G. Peng, H. H. Wu, J. G. Jiang, Y. M. Liu and P. Wu, J. Catal., 2012, 288, 16–23.
17. K. Na, C. Jo, J. Kun, W. S. Ahn and R. Ryoo, ACS Catal., 2011, 1, 901–907.
18. K. F. Lin, L. Li, B. F. Sels, P. A. Jacobs and P. P. Pescarmona, Catal. Today, 2011, 173, 89–94.
19. M. G. Clerici, G. Bellussi and U. Romano, J. Catal., 1991, 129, 159–167.
20. S. B. Kumar, S. P. Mirajkar, G. C. G. Pais, P. Kumar and R. Kumar, J. Catal., 1995, 156, 163–166.
21. M. Sasidharan, P. Wu and T. Tatsumi, J. Catal., 2002, 205, 332–338.
22. J. Vernimmen, M. Guidotti, J. Silvestre-Albero, E. O. Jardim, M. Mertens, O. I. Lebedev, G. Van Tendeloo, R. Psaro, F. Rodriguez-Reinoso, V. Meynen and P. Cool, Langmuir, 2011, 27, 3618–3625.
23. Y. Kuwahara, K. Nishizawa, T. Nakajima, T. Kamegawa, K. Mori and H. Yamashita, J. Am. Chem. Soc., 2011, 133, 12462–12465.
24. L. Xu, H. G. Peng, K. Zhang, H. H. Wu, L. Chen, Y. M. Liu and P. Wu, ACS Catal., 2013, 3, 103–110.
25. J. LeBars, J. Dakka and R. A. Sheldon, Appl. Catal., A, 1996, 136, 69–80.
26. T. Tatsumi and N. Jappar, J. Catal., 1996, 161, 570–576.
27. T. H. Yu, Q. Zhang, S. Xia, G. Y. Li and C. W. Hu, Catal. Sci. Technol., 2014, 4, 639–647.
28. M. Moliner and A. Corma, Microporous Mesoporous Mater., 2014, 189, 31–40.
29. J. LeBars, J. Dakka and R. A. Sheldon, Appl. Catal., A, 1996, 136, 69–80.
30. A. Bhaumik, P. Mukherjee and R. Kumar, J. Catal., 1998, 178, 101–107.
31. L. H. Chen, X. Y. Li, G. Tian, Y. Li, J. C. Rooke, G. S. Zhu, S. L. Qiu, X. Y. Yang and B. L. Su, Angew. Chem., Int. Ed., 2011, 50, 11156–11161.
32. B. Notari, Adv. Catal., 1996, 41, 253–334.
33. P. Bassler, H.-G. Gobbel and M. Weidenbach, Chem. Eng. Trans., 2010, 21, 571–576.
34. T. Iwasaki, M. Isaka, H. Nakamura, M. Yasuda and S. Watano, Microporous Mesoporous Mater., 2012, 150, 1–6.
35. Y. L. Ding, Q. P. Ke, T. T. Liu, W. C. Wang, M. Y. He, K. Q. Yang, H. L. Jin, S. Wang and T. D. Tang, Ind. Eng. Chem. Res., 2014, 53, 13903–13909.
36. D. G. Huang, X. Zhang, T. W. Liu, C. Huang, C. H. Chen, C. W. Luo, E. Ruckenstein and Z. S. Chao, Ind. Eng. Chem. Res., 2013, 52, 3762–3772.
37. Q. H. Xia and Z. Gao, Mater. Chem. Phys., 1997, 47, 225–230.
38. G. Li, X. W. Guo, X. S. Wang, Q. Zhao, X. H. Bao, X. W. Han and L. W. Lin, Appl. Catal., A, 1999, 185, 11–18.
39. J. L. Grieneisen, H. Kessler, E. Fache and A. M. Le Govic, Microporous Mesoporous Mater., 2000, 37, 379–386.
40. Q. Zhao, X. H. Bao, X. W. Han, X. M. Liu, D. L. Tan, L. W. Lin, X. W. Guo, G. Li and X. S. Wang, Mater. Chem. Phys., 2000, 66, 41–50.
41. X. S. Wang, X. W. Guo and G. Li, Catal. Today, 2002, 74, 65–75.
42. C. B. Khouw and M. E. Davis, J. Catal., 1995, 151, 77–86.
43. B. Sun, W. Wu and E. Wang, Pet. Process. Petrochem., 2002, 33, 22–27.
44. C. H. Huang and T. W. Xu, Environ. Sci. Technol., 2006, 40, 5233–5243.
45. H. Z. Feng, C. H. Huang and T. W. Xu, Ind. Eng. Chem. Res., 2008, 47, 7552–7557.
46. J. N. Shen, J. Yu, J. Huang and B. Van der Bruggen, Chem. Eng. J., 2013, 220, 311–319.
47. G. Bellussi and V. Fattore, Stud. Surf. Sci. Catal., 1991, 69, 79–92.
48. B. Notari, Stud. Surf. Sci. Catal., 1991, 60, 343–352.
49. T. Tatsumi, K. A. Koyano and Y. Shimizu, Appl. Catal., A, 2000, 200, 125–134.
50. P. X. Yao, Y. Q. Wang, T. Zhang, S. H. Wang and X. Y. Wu, Front. Chem. Sci. Eng., 2014, 8, 149–155.
51. A. Vasile and A. M. Busuioc-Tomoiaga, Mater. Res. Bull., 2012, 47, 35–41.
52. H. C. Guo, L. Zhang, H. L. Li, Y. H. Jia and C. Y. Liu, China Pat. 201110295596.9, 2011.
53. A. Thangaraj, M. J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites, 1992, 12, 943–950.

---