Materials Today Chemistry
Synthesis of titanium silicalite-1 (TS-1) zeolite with high content of Ti by a dry gel conversion method using amorphous TiO2–SiO2 composite with highly dispersed Ti species
Introduction
Zeolites are crystalline microporous materials formed by TO4 (T = Si, Ge, Al, P, Ti etc.) tetrahedral units, and have been widely known for their various properties, including catalytic activity, shape selectivity, solid acidity, and ion exchange capacity [1]. As a catalyst, zeolites offer a wide range of usage to obtain high conversion and selectivity in various reactions [2]. In particular, titanium silicalite-1 (TS-1) is one of MFI-type zeolites with partial substitution of titanium atoms for silicon atoms [3]. It has been reported that titanium atoms in the framework of TS-1 act as active sites for selective catalytic oxidation of organic molecules with H2O2 as the oxidant: hydroxylation of benzene and phenol, epoxidation of olefins, and ammoxidation of ketones [4,5]. It is suggested that H2O2 is activated on TS-1 by forming titanium-peroxo complexes before transferring an oxygen atom to react with the organic molecules [6]. With H2O2 as the oxidant, it offers advantages, such as low cost per oxygen atom and possibility to perform “clean reactions” with the absence of by-products [7]. Thus, further optimization to enhance the catalytic activity of TS-1 is desired for this reaction.
One of the important oxidation reaction using TS-1 is epoxidation. Epoxides are highly valuable intermediates for manufacturing a wide range of important products, including bulk chemicals, polymers, and fine chemicals. Most of the industrial production of epoxides use homogeneous catalysts such as peracids and m-chlorobenzoic acid. However, these produce acid waste that become an environmental concern. The development of TS-1 has advanced as a promising catalyst with high catalyst activity and selectivity that produce water as the single by-product [[8], [9], [10]].
For the oxidation reaction with TS-1, the higher the amount of incorporated titanium including open site Ti as the active site is, the higher the catalytic activity is. Conventionally, as shown in Fig. 1, TS-1 is synthesized using a hydrothermal method with maximum amount of incorporated titanium and open site Ti less than 0.03 of Ti/(Si + Ti). Unfortunately, the effort to incorporate more titanium atoms resulted in the formation of non-framework titania (TiO2)–like amorphous and anatase TiO2 [4,11], which are undesirable species because they do not show activity for the selective catalytic oxidation [6,12,13].
Recently, another method called dry gel conversion (DGC) method has been developed to synthesis the conventional TS-1 [[14], [15], [16]]. By using the DGC method, the crystal and porous size of TS-1 can be successfully modified [14]. However, until now, there is no report of the ability to increase of incorporated Ti amount in TS-1 framework also with this method. We focused on the synthesis of heteroatom-incorporated zeolites by a DGC method [[17], [18], [19]]. For example, Fe [17,18] and Ge [19] with large ion radius could be incorporated in the zeolite framework, suggesting that the DGC method is an effective way to incorporate large heteroatoms into the zeolite framework. In addition, the state of Ti in the starting materials is important to synthesize titanosilicate zeolite [20]. Starting TiO2–SiO2 composite with highly dispersed Ti species would be better to synthesize titanosilicate zeolite because Ti species in titanosilicate zeolite is isolated. We focused on sol-gel method to synthesize TiO2–SiO2 composite with highly dispersed Ti species. Here, we propose a new method to synthesize mesoporous TS-1 with high amount of incorporated titanium in the zeolite framework without secondary templates. It combines two methods as follows: (1) a sol-gel method for the synthesis of TiO2–SiO2 composite with highly dispersed Ti species and (2) a DGC method for the crystallization to TS-1. Furthermore, we confirm that the higher content of titanium atoms on mesoporous TS-1 actually effectively increases its catalytic activity in the epoxidation of 1-hexene.
Section snippets
Synthesis of TS-1-DGC
A typical procedure for the preparation of TS-1 using the sol-gel method and the DGC method is shown in Fig. S1. First, 1.4 g of titanium-tetraisopropoxide (TTIP) and 1.03 g of tetraethyl-orthosilicate (TEOS) was mixed into 30 g of tetrahydrofuran (THF) and stirred for 1 h. Then, H2O was added dropwise under vigorous stirring and stirred for another 4 h. The solution was centrifuged to remove the liquid and dried overnight, followed by calcination at 400 °C for 5 h to obtain a composite of TiO2
Results and discussion
Fig. 2 shows the XRD patterns measured for all samples as well as Si-1. TS-1-DGC is composed of pure MFI structure of zeolite. In comparison with the XRD pattern of Si-1 that shows double peaks at 2θ = 24.5° and 29.5°, only a single diffraction peak was observed for TS-1-HT-1%, TS-1-HT-3%, and TS-1-DGC, as shown in Fig. S2. The absence of any high peak at 2θ = 25.4°, as shown in Fig. S3, also suggests that all samples contain a little to no TiO2 crystal phase. The composition of TS-1 zeolite
Conclusion
Mesoporous TS-1 with high Ti active site (Ti/(Si + Ti) > 5%) has been successfully synthesized by using the combined (sol-gel/DGC) method. The XRD peaks shifted with increasing the content of titanium, indicating that titanium species are incorporated into the MFI framework. The TS-1-DGC shows a unique polycrystalline morphology with smaller crystal size and rough surface. TS-1-DGC shows higher yield on 1-hexene epoxidation, indicating that TS-1-DGC has higher catalytic activity than TS-1-HT.
Conflict of interest
There is no conflict of interest to declare.
Acknowledgment
The authors would like to thank the GHAS laboratory at Osaka University for supporting the XRD, FE-SEM and TEM measurements; JEOL resonance team for supporting the ESR measurements; Prof. Takayuki Hirai, Dr. Yasuhiro Shiraishi, and Mr. Jun Imai for supporting the gas chromatography measurement.
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2023, Catalysis TodayCitation Excerpt :It has been reported that acid treatment of TS-1 removes undesirable Ti species, which are formed on the extra-framework, improving the catalytic performance [16]. On the other hand, recently, it has been reported that the extra-framework Ti species have higher epoxidation activity [17–20]. These reports are due to the fact that peroxo Ti species generated by the reaction between Ti species and H2O2 have been considered to be the active site.
A review on titanosilicate-1 (TS-1) catalysts: Research progress of regulating titanium species
2023, Coordination Chemistry ReviewsCitation Excerpt :What is unsatisfactory is that the limit of Ti/(Ti + Si) molar ratio Ti effectively incorporated to the zeolite framework is about 2.3 %, even when starting from cogels with low Si/Ti ratios. Recently, a new synthesis method using a combination of sol–gel method and steam-assisted dry gel conversion was proposed by Soekiman [83]. Different from the hydrothermal method, dry gel method results in slow crystal growth due to precursor nucleation of the precursor in the high density gel.