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Meso-/microporous titanium silicalite with controllable pore diameter for cyclohexene epoxidation Yi Zuo, Ting Zhang, Min Liu, Ying Ji, Chunshan Song, and Xinwen Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03719 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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Industrial & Engineering Chemistry Research

Meso-/microporous titanium silicalite with controllable pore diameter for cyclohexene epoxidation Yi Zuo,a,‡ Ting Zhang,a,‡ Min Liu,a,* Ying Ji,a Chunshan Song,b Xinwen Guoa,* a

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School

of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China. b

EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy

& Mineral Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA. *Corresponding Authors: M. Liu. Tel.: +86 411 84986134. Fax: +86 411 84986134. E-mail: [email protected]. X. Guo. Tel.: +86 411 84986133. Fax: +86 411 84986134. E-mail: [email protected].

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ABSTRACT. A meso-/microporous titanium silicalite with controllable pore diameter was synthesized in an easy and new route by using cetyltrimethylammonium bromide (CTAB) and tetrapropylammonium hydroxide (TPAOH) as mesoporous and microporous template, respectively. This route leads to the formation of mesopores prior to the crystallization of microporous MFI topology. The porosity formation sequence makes the two kinds of channels, which are micropores with MFI topology and mesopores with worm-like morphology, distribute homogeneously. The pore diameter of the mesopores can be adjusted from the maximum center of 2.6 nm to that of 6.9 nm by tuning the molar ratio of CTAB to silicon from 0.125 to 0.20. The meso-/microporous titanium silicalite catalysts were evaluated in the epoxidation of cyclohexene, and shew excellent catalytic activities with respect to the conventional microporous TS-1, due to the enhanced diffusion property in the mesopores and higher titanium content near the external surface of the former.

KEYWORDS. Titanium silicalite, formation mechanism, CTAB, pore diameter controllable, epoxidation of cyclohexene.


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1 INTRODUCTION Titanium silicalite-1 (TS-1), which was first hydrothermally synthesized by Taramasso et al.,1 attracts much attention due to its excellent catalytic activity in the oxidation of alkanes,2 epoxidation of alkenes,3-6 hydroxylation of aromatics,7,8 ammoximation of ketones9-11 and oxidative desulfurization.12 In recent years, the hydroxylation of phenol to produce benzene diols, the epoxidation of propene to produce propene oxide and the ammoximation of methyl ethyl ketone to produce oxime catalyzed by TS-1 were commercialized by some corporations successively.13-15 However, the intrinsic microporous channels of TS-1 (0.54 nm × 0.56 nm) limit its application in large molecular reactions, and also lead to a short lifetime. Therefore, many researchers tried to introduce mesopores or macropores in TS-1. Two main approaches are often used to prepare hierarchical materials, which are the postsynthesis with alkaline solutions16-22 and the one-pot hydrothermal synthesis directly with mesoporous templates.23-25 The new generated mesopores in TS-1 during the post-synthesis with alkaline solutions are usually irregular hollows, the pore diameter of which is hard to be controlled. Furthermore, the connection between hollows and the external surface of material is micropores, which still limits the diffusion of substances. In addition, the preparation period is prolonged. The one-pot synthesis of hierarchical titanium silicalite was first reported by Jacobsen et al. using carbon black as hard-template.26 After that, a series of hierarchical titanium silicalites were synthesized by using different carbon based materials.27,28 However, the complexity of the synthesis procedure limited seriously the industrial applications of these hierarchical materials. An attracted method is to utilize suitable surfactant as soft mesoporous templates for the direct synthesis of hierarchical materials.29 Cetyltrimethylammonium bromide (CTAB) is one of the


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most commonly used surfactants. Nevertheless, the mesopores in the CTAB-directed materials are mostly intercrystals. Furthermore, the micropores and mesopores are often phase-separated from each other. Hierarchical titanium silicalite containing uniform mesopores with microporous walls is rarely reported. Vernimmen et al. considered that the synthesis of “true” meso/microporous titanium silicalite is very difficult.30 To achieve the aim of “true” hierarchical titanium silicalite, some researchers introduced TS-1 seed as a precursor.31,32 In this method, 1demensional hexagonal mesopore system can be obtained, but the preparation of TS-1 seed also prolongs the period. Therefore, we tried to find an easy and rapid method for synthesizing hierarchical materials. Ryoo’s group reported a method for synthesizing ordered multilamellar mesostructure MFI zeolite nanosheets with 2-nm thickness by using a diquaternary ammonium group as soft template.33-35 They considered that this material was the first ordered material with 3D-structured zeolite framework. In this work, we obtained a hierarchical titanium silicalite molecular sieve with homogeneous micropores and mesopores by changing the formation mechanism of porosity. CTAB was used as the mesoporous template, which combined with tetrapropylammonium hydroxide (TPAOH), a template of MFI topology, to form a di-template system and to structure-directing synthesize hierarchical titanium silicalite. Furthermore, the mesoporous diameter, coordination states of titanium ions, and titanium content on the external surface and in the bulk related to the molar ratio of CTAB/Si (n(CTAB/Si)) were studied. The catalytic performance of cyclohexene epoxidation over the hierarchical materials was likewise determined.

2 EXPERIMENTAL SECTION 2.1 Synthesis of meso-/microporous titanium silicalites


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The meso-/microporous titanium silicalite was synthesized according to the reference,36 except for adding CTAB as the mesoporous structure-directing agent. Tetraethyl orthosilicate (TEOS, Sinopharm Chemical Reagent Co., Ltd) and tetrabutyl titanate (TBOT, Sinopharm Chemical Reagent Co., Ltd) were used as silicon and titanium sources without any further purification. A typical synthesis procedure was mixing TEOS (17.7 mL), CTAB (0-5.83 g, Sinopharm Chemical Reagent Co., Ltd), TPAOH (25 wt%, 7.15 g, Shanghai Cainorise Chemicals Co., Ltd) and water (25.9 mL) in a flask firstly, and hydrolysing at 313 K for 5 h. The addition of CTAB to the hydrolysis of TEOS is considered as a key point of this method. Secondly, a solution of TBOT (0.68 mL), isopropyl alcohol (6.5 mL), TPAOH (25 wt%, 5.84 g) and water (13.0 mL) was added in this flask. The molar composition of the gel was n(SiO2) : n(TiO2) : n(TPAOH) : n(CTAB) : n(H2O) = 1 : 0.025 : 0.2 : (0-0.20) : 27. Thirdly, the gel was transferred to a Teflonlined autoclave and crystallized under static condition at 443 K for 72 h. The obtained suspension was separated by centrifugation, and the solid was washed with ethanol, dried at 353 K over night and calcined at 813 K for 6 h. The obtained meso-/microporous titanium silicalite was given a signature of MTS-n, where n standed for the n(CTAB/Si). A conventional microporous TS-1 synthesized according to the reference was introduced for comparison.37

2.2 Characterization of meso-/microporous titanium silicalites X-ray powder diffraction (XRD) patterns were recorded on a Rigaku Corporation SmartLab 9 X-ray diffractometer equipment using Cu Kα radiation. Fourier transform infrared (FTIR) spectra were collected on a Bruker EQUINOX55 spectrometer from ν=4000 to 400 cm-1, and the KBr pellet technique was used. Ultraviolet visible diffused reflectance (UV/vis) spectra were obtained on a Jasco UV-550 spectrometer from λ=190 to 500 nm, and pure BaSO4 was used as a


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reference. Nitrogen physisorption measurements were performed at liquid argon temperature on a Quantachrome Autosorb iQ2 physical sorption apparatus. Total surface area and pore volume were calculated according to the BET model and t-plot method, respectively. The appearances of the crystals and porosities were determined on a Tecnai G220 S-Twin transmission electron microscope (TEM) instrument. The elemental compositions of the samples on the surface and in the bulk were supplied by X-ray photoelectron spectroscopy (XPS, Thermo VG ESCALAB250 instrument using Al Kα radiation and operating at a constant power of 260 W) and inductively coupled plasma-optical emission spectroscopy (ICP-OES, Perkin Elmer OPTIMA 2000DV instrument), respectively. A home-made physical adsorption apparatus was employed for detecting the cyclohexene adsorption curves of the meso-/microporous titanium silicalites. The weight of adsorbed cyclohexene was measured at 298 K with respect to different adsorption time.

2.3 Epoxidation of cyclohexene The epoxidation of cyclohexene was carried out in a 400 mL glass batch reactor equipped with a reflux condenser. The evaluation process was as follows: Feeding the catalyst (0.2 g), acetonitrile (18 mL) and aqueous H2O2 (0.7 mL, 50 wt%) into the reactor. Cyclohexene (1.6 mL) was then added to the reactor, and heated the reactor to 333 K. The reaction was completed after heating for 6 h under magnetic stirring. The residual H2O2 was measured by iodometric titration. The products were analyzed by a Tianmei 7890F gas chromatogragh equipped with a flame ionization detector (FID) and a PEG-20M capillary column (30 m × 0.25 mm × 0.5 µm). The main product is cyclohexene oxide (CHO), and the by-products are 2-cyclohexen-1-ol, 2cyclohexen-1-one and 1,2-cyclohexanediol. The conversion of H2O2 (X(H2O2)), conversion of


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cyclohexene (X(CH)), selectivity of CHO (S(CHO)) and utilization of H2O2 (U(H2O2)) were calculated according to the equations (1), (2), (3) and (4), respectively. X(H2O2) = (n0(H2O2) - n(H2O2)) / n0(H2O2)

(1)

X(CH) = (n0(CH) - n(CH)) / n0(CH)

(2)

S(CHO) = n(CHO) / (n(CHO) + n(Others))

(3)

U(H2O2) = (n(CHO) + n(Others)) / (n0(H2O2) · X(H2O2))

(4)

The n0(H2O2) and n(H2O2) represent the initial and final molar numbers of H2O2, respectively. The n0(CH) and n(CH) stand for the initial and final molar numbers of cyclohexene, respectively. The n(CHO) is the molar number of CHO, while n(Others) is the total molar number of the byproducts.

3 RESULTS AND DISCUSSION 3.1. Characterization of hierarchical titanium silicalite. A series of characterizations were performed to study the properties of these materials. The XRD patterns of the meso-/microporous titanium silicalites in small-angle region (

microporous titanium silicalite with ... - ACS Publications

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