Hierarchical TS-1 synthesized effectively by post-modification with TPAOH and ammonium hydroxide

Abstract

Hierarchical TS-1 with intracrystalline voids could be synthesized by post-modification with tetrapropyl ammonium hydroxide (TPAOH), however the process was supposed to be restrained by the charge balance effect. In order to release the constraint, NH₃·H₂O was introduced in the post-modification. The influences of the TPAOH/NH₃·H₂O modification, the TPAOH concentration and the modification time on the physiochemical properties were studied. The dissolution process and recrystallization process were observed in the combined modification, and both of them were intensified by the NH₃·H₂O introduced, indicating that the constraint on the OH⁻ diffusion was released. Owing to the intensified processes, the TPAOH concentration and post-modification time could be reduced, and hierarchical TS-1 with relative high crystallinity, less defect sites and a larger secondary pore volume (about 0.18 cm³ g⁻¹) was achieved in 4 h. However, the combined-modification exerted little influence on the chemical composition and acidity, and the microporous properties were almost the same with that of TS-1 modified with TPAOH only. The secondary porosity in the combined-modified samples were mainly intracrystalline voids, and grooves on the surface were also found. Although extra-framework titanium species were created by the post-modification, part of the titanium was still in the framework position. The catalytic activity was evaluated by phenol hydroxylation and 1-octene epoxidation, and better catalytic activity was achieved for the improved accessibility of the active sites.

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Introduction

Zeolites with a crystalline microporous framework are important materials in industry, they have been widely used in adsorption, filtration, catalysis and detergents.^1,2 Typically, the size of the channels and cavities of industrial available zeolites is less than 1.2 nm, which would exert diffusion constraint for the substrates with a larger molecular size.³ Ordered mesoporous materials (OMMs), such as MCM-41 (ref. 4 and 5) and SBA-15,^6,7 with tunable pore size were synthesized to release the constraint. However, the acidity, thermal stability and hydrothermal stability of OMMs are obstacles for industrial application.^8,9 On the other hand, a rational approach would be to preserve the microporous structure, and improve the accessibility of the active sites by creating secondary porosity in zeolite particles.^3,10 Then, hierarchical zeolites with micropores, supermicropores, mesopores or macropores were prepared by dissolution-recrystallization,^9,11,12 desilication,^13,14 dealumination,¹⁵ silanization,^16,17 templating^18,19 and combined approaches.²⁰

Among the variety of strategies in the preparation of hierarchical zeolites, the dissolution–recrystallization technology was supposed to be one of the most advantageous and versatile methods.²¹ It has been demonstrated that the catalytic activity, selectivity and stability could be improved by the dissolution–recrystallization process (DRP). Lin et al. reported that hierarchical titanium silicalite-1 with intracrystalline voids was synthesized through the DRP by modification with quaternary ammonium hydroxide;^11,22 and for the excellent catalytic activity and stability, it has been successfully industrialized in cyclohexanone ammoximation and used in the industrial trial of propylene epoxidation. During the modification, the quaternary ammonium cations, such as TPA⁺ (0.84 nm) and TBA⁺ (1.04 nm), were too large to diffuse into the channels (0.55 nm), however they may deposit on the surface for the hydrophobicity of the alkyl chains, which protected the surface from the OH⁻ attack.⁹ Thus, the dissolution process was supposed to be mainly in the inner part of TS-1 particles, while the recrystallization process was on the surface. As the quaternary ammonium cations can't enter the channels by diffusion, the diffusion of the OH⁻ in the channels and the dissolution in the particles were restrained considering the charge balance effect. Theoretically, cations could diffuse into the channels should be introduced to release the constraint on OH⁻ diffusion. TS-1 has been post modified with TPAOH and NaOH, and a positive effect of the introduction of NaOH was found, as larger mesopore volume (0.20–0.27 cm³ g⁻¹) was achieved and the geometric constraint for catechol in phenol hydroxylation was released.²³ However, the Na⁺ in the solution prevented the re-incorporation of the titanium ions into the framework position, and the framework titanium content decreased significantly. In order to intensify the DRP and overcome the negative effect of the alkali cations, ammonium hydroxide was used as the additive agent in the present work. The influences of the combined modification with TPAOH and NH₃·H₂O on the physiochemical properties, catalytic activity and the DRP were studied.

Experimental

Synthesis of TS-1

TS-1 was synthesized following the procedure reported by Thangaraj.²⁴ Tetraethyl orthosilicate (TEOS) was the silica source, tetrabutyl titanate (TBOT) was the titanium source, and aqueous TPAOH (25 wt%) was both the structure directing agent and alkali source. TEOS was firstly hydrolyzed with TPAOH for about 3 hours, then TBOT dissolved in anhydrous isopropyl alcohol was dropped into the hydrolyzed solution. Finally, water was added and the mixture was stirred at 80 °C for 3 hours to remove alcohol. The chemical composition of the final gel was SiO₂ : TiO₂ : TPAOH : H₂O = 1 : 0.025 : 0.5 : 40, and it was transferred to a Teflon-lined stainless-steel autoclave. The crystallization was carried out under autogenous pressure at 175 °C for 4 days. The mixture obtained was filtered, washed with distilled water, dried at 110 °C for 6 h, and calcined at 550 °C for 5 h.

Post-synthesis

The post-synthesis was carried out as follows, 5 g of the previous prepared TS-1 was mixed with 20 mL mixed solution of TPAOH (0–0.15 mol L⁻¹) and ammonium hydroxide (0 or 0.1 mol L⁻¹). The mixture obtained was transferred to a Teflon-lined stainless-steel autoclave, and the post-synthesis was carried out under autogenous pressure at 150 °C for some time (1–24 h). Finally, the mixture was filtered, washed with distilled water, dried at 110 °C for 6 h, and calcined at 550 °C for 5 h. The samples obtained were denoted as TS-1-xP-yN-t, in which x and y respectively represented the molar concentration of TPAOH and NH₃·H₂O, and t was the post-modification duration.

Characterization

X-ray diffraction (XRD) patterns were collected on a Philips Panalytical X'pert diffractometer with nickel-filtered Cu Kα radiation (40 kV, 250 mA). The 2θ scanning ranged from 5° to 35°, and the scanning rate was 0.4° min⁻¹. X-Ray Fluorescence (XRF) experiments were conducted on a Rigaku 3721E spectrometer with W radiation (40 kV). The X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo-Fischer-VG ESCALAB250 with Al Kα radiation, and the framework titanium content and extra-framework titanium content were integral results of the Ti 2p_3/2 peaks at 460.3 eV and 458.7 eV.

Scanning electron microscopy (SEM) images were taken from a Hitachi 4800 microscope (20 kV). Transition electron microscopy (TEM) images were recorded on a JEOL JEM-2100 microscope (200 kV). The samples were dispersed in anhydrous alcohol using the ultrasonic technique, the suspension obtained was added dropwise to a micro grid membrane and dried in air.

N₂ adsorption–desorption isotherms were collected at 77 K on a Micromeritics ASAP 2405 apparatus. The samples were previously dried under vacuum (0.1 Pa) at 300 °C for 6 h. The surface properties were derived from the isotherms using BET and t-plot methods.

Infrared (IR) spectra were recorded on a Thermo Nicolet 750 infrared spectrometer. The samples were evacuated at 500 °C for 2 h under vacuum conditions, then they were grinded with KBr, and the mixture was pressed into a wafer. The scanning ranged from 400 cm⁻¹ to 4000 cm⁻¹. The UV-visible (UV-Vis) spectra were recorded on a JASCO UV-visible 550 spectrometer. The samples were pressed into a self-supported wafer, and the spectra was recorded from 200 nm to 500 nm.

Thermal gravimetric analysis (TGA) was carried out under air atmosphere on a TA SDT Q600 simultaneous thermal analyzer. The results were recorded from 500 °C to 800 °C with a heating rate of 10 °C min⁻¹.

²⁹Si MAS NMR experiments were performed on a Bruker AVANCE III 500WB spectrometer at a resonance frequency of 99.3 MHz using a 7 mm double-resonance MAS probe. The magic-angle spinning speed was 5 kHz in all experiments, and a typical π/6 pulse length of 1.8 μs was adopted for ²⁹Si resonance. The chemical shift of ²⁹Si was referenced to tetramethylsilane.

¹H–¹³C CP/MAS NMR experiments were performed on a Bruker AVANCE III 600WB spectrometer at a resonance frequency of 150.9 MHz using a 4 mm double-resonance MAS probe at a sample spinning rate of 7.5 kHz. ¹H–¹³C CP/MAS NMR spectra were recorded by using a recycle delay of 5 s and a contact time of 2 ms. The chemical shift of ¹³C was determined using a solid external reference, hexamethylbenzene.

The acidity were measured by the Fourier transform infrared (FTIR) spectra using pyridine as the probe molecules and temperature-programmed desorption (TPD) with NH₃ as the probe molecules. The Py-FTIR spectra were obtained on a FTS3O00 FTIR spectrometer by 64 scans with a resolution of 4 cm⁻¹. Self-supporting thin wafers were pressed and placed in a quartz IR cell with CaF₂ windows. Prior to the measurements, the samples were dehydrated under vacuum (10⁻³ Pa) at 350 °C for 1 h and then cooled down to 50 °C for pyridine adsorption. Before pyridine adsorption, the IR spectra of the samples were recorded at 200 °C. The samples were purged under vacuum (10⁻³ Pa) to higher temperatures at a heating rate of 10 °C min⁻¹ after adsorbing pyridine for 10 s. Then, the IR spectra of pyridine were recorded at 200 °C. The spectra given in this work were difference spectra. The quantitative values of the acid sites were calculated by the integral area of the characteristic peaks at 1450 cm⁻¹ and 1540 cm⁻¹.

The NH₃-TPD experiments were performed on a Micromeritics Autochem II 2920 analyzer equipped with a TCD detector. The samples were firstly purged with pure He for 1 h (25 mL min⁻¹) at 600 °C, then they were cooled down to 100 °C for 10 min. At the same temperature, the NH₃ adsorption was carried out with a mixture gas of 10 wt% NH₃ and 90 wt% He for 30 min, and the physisorped NH₃ were purged with pure He for 60 min. Finally, the samples were heated to 600 °C at a rate of 10 °C min⁻¹. The quantitative values of the acid sites were calculated by the integral area of the characteristic peaks.

Catalytic activity

The catalytic activity was evaluated by phenol hydroxylation and 1-octene epoxidation, both of which were carried out in a round-bottomed flask equipped with a condenser and a magnetic stirrer. In phenol hydroxylation, 0.2 mol phenol was dissolved in 17.2 mL acetone, then 0.9 g catalyst was added. Under stirring, the mixture was heated to 80 °C, and the aqueous H₂O₂ (30 wt%) was gradually added through a metering pump. The molar ratio of H₂O₂ and phenol was 0.3, and the reaction was carried out for 30 min. In 1-octene epoxidation, 0.24 g catalyst was mixed with 4.0 g 1-octene and 6.8 g tert-butyl hydroperoxide (TBHP, 5.5 mol L⁻¹ in decane), the molar ratio of 1-octene and TBHP was 1. The reaction was carried out at 100 °C for 4 h. The reaction products of phenol hydroxylation and 1-octene epoxidation were cooled in ice-water, then they were centrifuged and analyzed by an Agilent 6890N equipped with a FFAP capillary column. The oxidant efficiency was determined by iodometric titration.

Results and discussion

Influences on physicochemical properties

Fig. 1 shows the XRD patterns of TS-1 and the modified samples. As TPAOH was the structure directing agent of zeolite with MFI topology, the five characteristic peaks (2θ = 7.9°, 8.8°, 23.1°, 23.9° and 24.4°) remained unchanged after the modification. Denoting the crystallinity of parent TS-1 as 100%, the relative crystallinity (R.C.) of the modified samples were calculated by comparing the integral area of the characteristic peaks between 22–26° with that of parent TS-1. The R.C. increased by 3.9% when TPAOH was used only, while it decreased significantly by 37.4% after ammonium hydroxide treatment. In the presence of both TPAOH and NH₃·H₂O, the R.C. decreased by about 5–15%. In basic media, the silicon species of the zeolite topology could be preferentially dissolved.^25,26 However if the structure directing agent of the zeolite was in the solution, the topology could be protected due to the recrystallization of the dissolved species.^11,27 The R.C. increased when the recrystallization was stronger, vice versa. The decrease of the R.C. during the TPAOH/NH₃·H₂O modification indicated that the erosion was more severe than in the TPAOH modification, however the R.C. was still relative high.

Fig. 1 XRD patterns of TS-1 and the modified samples.

The N₂ adsorption–desorption isotherms collected at 77 K are illustrated in Fig. 2(a). The isotherm of TS-1-N-24 was attributed to type I, while the isotherms of parent TS-1 and TS-1 modified with TPAOH or TPAOH/NH₃·H₂O were type IV with H4 hysteresis loop. As the hysteresis loops of TS-1-0.15P-24 and the combined modified samples suddenly closed at P/P₀ = 0.45, the adsorption isotherm was preferred to calculate the pore size distribution,²⁸ and the results were shown in Fig. 2(b). For parent TS-1, mesopores with diameter of 2–4 nm and 8–16 nm were observed. The NH₃·H₂O modification decreased the mesopore number for the severe etching. Most of the mesopores created by the TPAOH modification and the TPAOH/NH₃·H₂O modification were about 16–32 nm, and there were more mesopores in the range of 4–16 nm in the combined modified samples. The surface properties derived from the isotherms are illustrated in Table 1. Owing to the severe erosion, all the surface properties of TS-1-N-24 decreased significantly by more than 33%. When TS-1 was modified with 0.15 M TPAOH for 24 h, the specific surface (S_BET), microporous surface (S_MI) and microporous volume (V_MI) decreased respectively by 2.2%, 4.4% and 6.8%, while the matrix area (S_MA) and secondary pore volume (V_SP) increased by 13.0% and 88.9%. In the presence of 0.15 M TPAOH and 0.1 M NH₃·H₂O, the S_BET, S_MI and V_MI decreased separately by 1.5%, 5.9% and 6.8% after 24 h, while the S_MA and V_SP increased more significantly by 34.8% and 104.4%. The decrease of the microporous properties (S_MI and V_MI) and the increase of the V_SP indicated that the secondary porosity was created at the expense of part of the micropores. Comparatively, the introduction of NH₃·H₂O would help synthesizing hierarchical TS-1 with larger V_SP and S_MA, however it exerted little influences on the microporous properties.

Fig. 2 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution of TS-1 and the modified samples.

Table 1 The surface properties of TS-1 and the modified samples^a

Sample	SBET (m^2 g^−1)	SMA (m^2 g^−1)	SMI (m^2 g^−1)	VMI (cm^3 g^−1)	VSP (cm^3 g^−1)
a SBET: the specific surface area, calculated by BET method. SMA: external surface area, calculated by t-method. SMI: microporous surface area, calculated by t-method. VMI: microporous volume, calculated by t-method. VSP: secondary pore volume, subtraction of total pore volume and microporous volume.
TS-1	454	46	409	0.190	0.090
TS-1-N-24	285	13	272	0.124	0.055
TS-1-0.15P-N-2	453	61	399	0.180	0.153
TS-1-0.15P-24	444	52	391	0.177	0.170
TS-1-0.15P-N-24	447	62	385	0.177	0.184
TS-1-0.03P-N-24	424	48	376	0.170	0.166

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The SEM images of the samples are shown in Fig. 3. The parent TS-1 particles were aggregates of small crystals with smooth surface. The surface was severely eroded by ammonium hydroxide, and it remained relative smooth due to the protection effect of TPA⁺ cations in TPAOH solution.⁹ When TPAOH and NH₃·H₂O were simultaneously used, grooves on the surface could be observed, which illustrated that the etching in the combined modification was intensified by the NH₃·H₂O introduced.

Fig. 3 SEM images of TS-1 and the modified samples (1) TS-1 (2) TS-1-0.15P-24 (3) TS-1-N-24 (4) TS-1-0.15P-N-24 (5) TS-1-0.03P-N-24 (6) TS-1-0.15P-N-2.

The TEM images of the samples are shown in Fig. 4. Accumulated pores were observed in parent TS-1, and the particles were severely eroded by the NH₃·H₂O treatment. When TS-1 was post-modified with aqueous TPAOH or mixed solution of TPAOH and NH₃·H₂O, the erosion was mainly in the inner part of the crystals, and intracrystalline voids were created. Moreover, crystals with much smaller size and no intracrystalline voids were found in the combined modified samples, which should be the products of the recrystallization process.

Fig. 4 TEM images of TS-1 and the modified samples (1) TS-1 (2) TS-1-0.15P-24 (3) TS-1-N-24 (4) TS-1-0.15P-N-24 (5) TS-1-0.03P-N-24 (6) TS-1-0.15P-N-2.

The ¹H–¹³C CP/MAS NMR spectra of the uncalcined TS-1-0.15P-N-24 and TS-1-0.15P-24 are shown in Fig. 5. In the ¹H–¹³C CP/MAS NMR spectra, the characteristic signals of the TPAOH entrapped in the channels were at 10.7, 11.7, 16.9, and 63.9 ppm. The splitting at 10.7 and 11.7 ppm is supposed to be a result of the differences in the van der Waals interactions between the protons of the methyl group and the silicon atoms in the framework position of the two different channel structure.^29,30 The spectra in Fig. 5 gave direct information that TPAOH could be incorporated into the channels through the recrystallization process in the presence of both TPAOH and NH₃·H₂O.

Fig. 5 ¹H–¹³C CP/MAS NMR spectra of TS-1-0.15P-24 and TS-1-0.15P-N-24.

The thermal gravimetry technology was used to calculate the amount of the TPAOH entrapped in the channels. The mass loss between 200 °C and 550 °C of TS-1-0.15P-24 was 4.38%, and it increased significantly by about 40% to 6.28% when NH₃·H₂O was introduced. Combined with the grooves on the surface, the larger V_SP and the smaller crystals without intracrystalline voids in the combined-modified samples, it can be inferred that the introduction of NH₃·H₂O intensified both the dissolution process and the recrystallization process.

The ²⁹Si NMR spectra of parent TS-1 and the TS-1 modified are shown in Fig. 6. In the ²⁹Si NMR spectra, the signals at about −103 ppm and −113 ppm were respectively attributed to Q3 species (Si(OSi)₃(OH)) and Q4 species (Si(OSi)₄), while the (Si(OSi)₃(OTi)) species were supposed to exhibit a broad peak at about −116 ppm.^31–33 After peak fitting, the ratio of the integral area (Q4 : Q3) could be used to illustrate the defect sites in the TS-1 crystals. The Q4 : Q3 of TS-1 was 17.6; and it increased significantly to 27.7 after the TPAOH modification, indicating that there were less defect sites in TS-1-0.15P-24. Although the NH₃·H₂O introduced in the combined modification intensified the DRP, the Q4 : Q3 in TS-1-0.15P-N-24 increased to 25.4, which illustrated that the TPAOH/NH₃·H₂O modification could also decrease the defect sites.

Fig. 6 The ²⁹Si NMR spectra of parent TS-1 and the modified samples.

The chemical composition of the samples are shown in Table 2. The molar ratio of SiO₂ and TiO₂ in parent TS-1 was 40.5. As a result of the selective dissolution of the silicon species in basic condition, the SiO₂ : TiO₂ decreased to 37.9 after NH₃·H₂O modification. However, the SiO₂ : TiO₂ remained almost the same due to the recrystallization process in the presence of TPAOH, and the introduction of NH₃·H₂O exerted little influence on the chemical composition.

Table 2 The XRF and XPS results of TS-1 and the modified samples

Sample	SiO2 : TiO2 (XRF)	XPS/%
		Framework Ti	Extra-framework Ti
TS-1	40.5	100	0
TS-1-N-24	37.9	56.7	43.3
TS-1-0.15P-N-2	40.7	69.1	30.9
TS-1-0.15P-24	40.5	80.9	19.1
TS-1-0.15P-N-24	40.8	68.3	31.7
TS-1-0.03P-N-24	40.4	70.6	29.4

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The FT-IR spectra of parent TS-1 and hierarchical TS-1 are shown in Fig. 7. The signal at 550 cm⁻¹ is attributed to the five-rings of zeolite with MFI topology,³⁴ and the band at 960 cm⁻¹ is supposed to be the stretching vibration of the Si–O bond in Si–O–Ti bridges.³⁵ Although the crystals were eroded by post-treatment, the MFI topology remained unchanged and titanium was still in the framework position.

Fig. 7 FT-IR spectra of TS-1 and the modified samples.

The UV-Vis spectra of the samples are shown in Fig. 8. The characteristic bands of anatase and four-coordinated isolated titanium ions are respectively at 330 nm and 210 nm, while the broad band in the region of 260–290 nm is generated by five- or six-coordinated extra-framework titanium species.^36–38 The titanium species in parent TS-1 were mainly four-coordinated. After post-modification with TPAOH or NH₃·H₂O, although the band intensity at 210 nm was strong as usual, the intensity of the bands at 260–290 nm and 330 nm increased. Similar phenomena was also observed in the combined-modified samples. The changes of the bands intensity gave direct information that part of the titanium in the modified samples was still four-coordinated framework titanium species, however extra-framework titanium was introduced.

Fig. 8 UV-Vis spectrum of TS-1 and the modified samples.

The XPS results in Table 2 further illustrated the oxidation state of titanium on the surface. In the Ti 2p_3/2 spectra, the characteristic peaks at 460.3 eV and 458.8 eV are respectively attributed to framework and extra-framework Ti⁴⁺,^39,40 and the molar ratio of which could be obtained from the integral areas after peak fitting. On the surface of parent TS-1, the framework titanium content was almost 100%. On TS-1-N-24, the framework titanium content decreased significantly to 56.7% for the severe etching. Although extra-framework titanium were also created in the presence of TPAOH, 80.9% of the titanium on TS-1-0.15P-24 was still four-coordinated, and about 70% of the titanium on the combined modified samples was in the framework position.

The acidity of TS-1 and the modified samples were measured by NH₃-TPD and Py-FTIR. In the NH₃-TPD curves of the samples, a desorption peak at about 160 °C could be observed. Although the attribution of the peaks among the low-temperature region (<400 °C) is controversial, they were usually ascribed to weak acid sites.^41,42 In the Py-FTIR spectra, the band at about 1540 cm⁻¹ was attributed to pyridine adsorbed on Brönsted acid sites, while the band at about 1450 cm⁻¹ was associated with pyridine on Lewis acid sites.^42,43 In the Py-FTIR spectra of TS-1 and the modified samples, a weak band at about 1450 cm⁻¹ was observed, indicating that there were Lewis acid sites in the samples, however the Brönsted acid sites were absent. The quantitative values of the acid sites from the NH₃-TPD curves and Py-FTIR spectra are shown in Table 3. The amount of the NH₃ and pyridine adsorbed on TS-1 was respectively about 0.71 mL g⁻¹ and 20.3 μmol g⁻¹. Although secondary porosity and extra-framework titanium species were introduced by etching part of the microporous framework, the post-modification exerted little influences on the acidity of TS-1. The NH₃-TPD value of the modified samples was 0.69–0.77 mL g⁻¹, while the Py-FTIR value was 17.6–20.7 μmol g⁻¹.

Table 3 The acidity of TS-1 and the modified samples

Sample	NH3-TPD/(mL g^−1)	Py-FTIR/(μmol g^−1)
		Lewis	Brönsted
TS-1	0.71	20.3	0
TS-1-N-24	0.77	17.6	0
TS-1-0.15P-N-2	0.70	19.6	0
TS-1-0.15P-24	0.74	20.7	0
TS-1-0.15P-N-24	0.74	18.4	0
TS-1-0.03P-N-24	0.69	19.5	0

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Influences of the TPAOH concentration

When the TPAOH concentration was 0.03–0.15 mol L⁻¹, hierarchical TS-1 was synthesized by TPAOH/NH₃·H₂O modification. The surface properties derived from the N₂ adsorption–desorption isotherms illustrated that whether NH₃·H₂O was introduced or not, the S_MI and V_SI decreased, while the V_SP increased. The V_SP obtained is shown in Fig. 9. The V_SP increased with increasing TPAOH concentration, and larger V_SP was achieved in the combined modification, indicating that the TPAOH concentration could be reduced by adding NH₃·H₂O into the DPR. The SEM and TEM images in Fig. 3 and 4 showed that the secondary porosity of the combined modified samples (TS-1-0.03P-N-24 and TS-1-0.15P-N-24) were grooves on the surface and intracrystalline voids.

Fig. 9 The influences of TPAOH concentration on secondary pore volume.

The ²⁹Si NMR spectra illustrated that the defect sites decreased even when TPAOH was 0.03 mol L⁻¹. The XRF, FT-IR, UV-Vis and XPS analyses indicated that the TPAOH concentration in the combined modification exerted little influence on the chemical composition and titanium coordination. For TS-1-0.03P-24, the SiO₂ : TiO₂ was 40.4, and 70.6% of the titanium on the surface was four-coordinated.

Influences of the post-modification time

When TS-1 was post-modified with TPAOH/NH₃·H₂O for 2 h, grooves on the surface and intracrystalline voids were introduced in the TS-1 crystals, and the V_SP was more than 0.135 cm³ g⁻¹. The influences of post modification time on the V_SP are illustrated in Fig. 10. The V_SP increased with increasing post-modification time when TPAOH was used only, and larger V_SP could be achieved under the same condition in the TPAOH/NH₃·H₂O modification. Moreover, during the combined modification, the V_SP increased significantly in 4 h, and it remained at about 0.18 cm³ g⁻¹ after 4 h, indicating that the dissolution process and recrystallization process reached equilibrium more quickly when NH₃·H₂O was introduced.

Fig. 10 The influences of post-modification time on secondary pore volume.

During the combined modification, the chemical composition and titanium coordination remained almost the same. For TS-1-0.15P-N-2, the SiO₂ : TiO₂ was 40.7, and the framework titanium on the surface was 69.1%. Moreover, the defect sites decreased with increasing time, as the Q4 : Q3 was 17.6 for parent TS-1, which increased substantially to 23.1 and 25.6 after 2 h and 24 h.

Catalytic activity

The catalytic activity of the samples were firstly evaluated by phenol hydroxylation, and the results are illustrated in Table 4. When phenol hydroxylation was catalyzed by TS-1, catechol (CAT) and hydroquinone (HDQ) were the main products, and a small part of the hydroquinone could be deeper oxidized to p-benzoquinone (BZQ).⁴⁴ Moreover, diffusion constraint for phenol and geometric constraint for CAT were observed, however CAT could also be synthesized inside the channels.^45,46 For parent TS-1, the phenol conversion, H₂O₂ efficiency, BZQ selectivity and molar ratio (n_ortho : n_para) of ortho-product (CAT) and para-product (BZQ and HDQ) was respectively 22.8%, 76.1%, 0.34% and 1.09. As the TS-1 crystals were severely eroded by the NH₃·H₂O modification, the extra-framework titanium species created may help decomposing hydrogen peroxide and starting a homolytic hydroxylation pathway, resulting in a decrease of the activity and dihydroxybenzenes (DHB) selectivity.^47,48 For TS-1-N-24, the catalytic activity, H₂O₂ efficiency and n_ortho : n_para decreased to 17.5%, 64.1% and 0.81, while the BZQ selectivity increased significantly to 9.90%. After TPAOH modification, intracrystalline voids were created in the crystals, which would release the diffusion constraint for phenol, leading to an increase of the phenol conversion and H₂O₂ efficiency. The positive effect of the intracrystalline voids should be more stronger than the negative effect of the extra-framework titanium species in TS-1-0.15P-24, the catalytic activity and H₂O₂ efficiency increased respectively to 25.50% and 85.4%, however the BZQ selectivity and n_ortho : n_para remained almost the same. When grooves on the surface, intracrystalline voids and larger V_SP were simultaneously introduced, the constraints for phenol and catechol were both improved, which would lead to an increase of the phenol conversion, H₂O₂ efficiency and catechol selectivity. Owing to the much more stronger effect of the positive effect of the secondary porosity in TS-1-0.15P-N-24, the catalytic activity, H₂O₂ efficiency and n_ortho : n_para increased respectively to 28.4%, 96.6% and 1.29.

Table 4 The catalytic activity in phenol hydroxylation^a

Sample	Phenol conv./%	Selectivity/%		H2O2 efficiency/%	northo : npara
		BZQ	DHB
a BZQ: p-benzoquinone, DHB: dihydroxybenzenes, including catechol and hydroquinone.
TS-1	22.8	0.34	99.66	76.1	1.09
TS-1-N-24	17.5	9.90	90.10	64.1	0.81
TS-1-0.15P-N-2	27.3	0.13	99.87	91.4	1.22
TS-1-0.15P-24	25.5	0.27	99.73	85.9	1.08
TS-1-0.15P-N-24	28.4	0.57	99.43	96.6	1.29
TS-1-0.03P-N-24	28.4	0.22	99.78	94.6	1.18

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As the DRP was intensified by NH₃·H₂O, hierarchical TS-1 could be synthesized with less TPAOH and shorter post-modification time, the catalytic activity of which increased all the same for the secondary porosity and larger V_SP. For TS-1-0.03P-N-24 and TS-1-0.15P-N-2, the catalytic activity in phenol hydroxylation increased to 28.4% and 27.3%, while the H₂O₂ efficiency and n_ortho : n_para was about 92% and 1.2.

The catalytic activity were further evaluated by 1-octene epoxidation using TBHP as the oxidant. The results in Table 5 illustrated that the 1,2-epoxyoctane selectivity of all samples was about 99.0%, while the TBHP efficiency was about 87.0%. The 1-octene conversion of parent TS-1 was 9.6%, which increased slightly to 10.0% after NH₃·H₂O modification; and 10.9% of the 1-octene could be converted when TS-1-0.15P-24 was used. More significantly, the 1-octene conversion of the combined-modified samples increased to more than 12.1%. Thus, although the extra-framework titanium species were created, the accessibility of the active centers was improved for the secondary porosity introduced, and better catalytic activity could be achieved, especially when NH₃·H₂O was introduced in the DRP.

Table 5 The catalytic activity in 1-octene epoxidation^a

Sample	1-Octene conv.%	1,2-Epoxyoctane selectivity/%	TBHP efficiency/%
a TBHP: tert-butyl hydroperoxide.
TS-1	9.6	99.0	86.7
TS-1-N-24	10.0	98.8	86.3
TS-1-0.15P-N-2	12.7	99.2	86.9
TS-1-0.15P-24	10.9	99.0	88.1
TS-1-0.15P-N-24	13.4	99.0	88.6
TS-1-0.03P-N-24	12.1	98.9	88.4

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Conclusion

Hierarchical TS-1 with relatively high crystallinity, grooves on the surface and intracrystalline voids was synthesized by post-modification with TPAOH and NH₃·H₂O. The dissolution and recrystallization processes were observed in the TPAOH/NH₃·H₂O modification, both of which were intensified by the NH₃·H₂O introduced, indicating that the processes could be restrained by the charge balance effect when TPAOH was used only. The TPAOH/NH₃·H₂O modification was more efficient than TPAOH treatment in the hierarchical TS-1 synthesis, the TPAOH concentration could be reduced, and hierarchical TS-1 with less defect sites and larger secondary pore volume could be achieved in 4 h. The combined modification exerted little influences on the chemical composition, microporous porosity and acidity; and although extra-framework titanium species were created, part of the titanium was still in the framework position. Due to the secondary porosity and larger secondary pore volume, the diffusion constraint was released, the accessibility of the active centers was improved, and the catalytic activity in phenol hydroxylation and 1-octene epoxidation increased.

Acknowledgements

The authors gratefully thanks the financial support from SINOPEC.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06657g

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