TS-1 molecular sieves,which have the MFI topology,have attracted widespread attention because of their shape selectivities and excellent catalytic oxidation performance [1, 2, 3, 4, 5]. TS-1 molecular sieves were first developed by EniChem Company in 1983 [6]. In the reported method,a large amount of tetrapropylammonium hydroxide (TPAOH,TPAOH/SiO2 = 0.4-1.0) was used as a structure-directing agent (SDA),therefore the synthesized TS-1 was expensive [6]. Cheap and environmentally friendly methods for active TS-1 synthesis need to be developed.
Tetrapropylammonium bromide (TPABr) is cheaper and more easily obtained than TPAOH. Müller et al. [7] used TPABr to synthesize TS-1 using ammonia as the alkali source. However,the crystal size of the obtained zeolite was often greater than 10 μm,much larger than that typical of TS-1 synthesized using TPAOH as the SDA. Amines have also been used as alkali sources when using TPABr as the SDA,but micron-scale crystals were obtained [8, 9, 10, 11],resulting in poor catalytic performance as a result of severely limited diffusion. Zeolites with small crystallite sizes show enhanced catalytic activities because of increased mass transport and a reduction in coke formation; such zeolites are therefore important [12, 13, 14].
The crystal size can be decreased by promoting nucleation to provide larger numbers of crystal nuclei. Seed addition to zeolite synthesis systems can promote nucleation,reduce the induction time,and enhance the crystallization rate,and therefore reduce the product crystal size [15]. TS-1 crystals or TS-1 precursors are usually used as the seeds for the synthesis of small-crystal TS-1 molecular sieves. Deng et al. [16] reported a two-step or multistep hydrolysis method for the synthesis of TS-1 with a controllable morphology and particle size. In this method,a high TPAOH/SiO2 ratio led to fast hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in the first step,which helped the formation of sufficient nuclei. When the remaining TEOS was added,the nuclei formed in the first step served as seeds and further catalyzed the nucleation and crystallization of TS-1. The amount of TPAOH used as the SDA is significantly decreased by using this two-step or multistep hydrolysis method. However,this process is tedious because it involves the multistep hydrolysis of TEOS and alcohol removal by evaporation. Furthermore,the crystal size increased from ~400 to ~900 nm when the ratio of TPAOH/SiO2 was decreased from 0.12 to 0.065. Xia et al. [17] synthesized TS-1 molecular sieves using tetraethylammonium chloride (TEACl) and tetrabutylammonium chloride (TBACl) as a mixed SDA,amines or ammonia as alkali sources,and TiCl3 as a Ti source. The addition of TS-1 as the seed to the synthesis mixture reduced the crystal size to ~400-600 nm. In addition to the use of TS-1 seeds (1-20 wt%),a large amount of organic SDA ((TEACl + TBACl)/SiO2 = 0.202) was used. Zhang et al. [18] prepared TS-1 using active TS-1 as seeds,inexpensive hexamethyleneimine as the SDA,and fumed silica as the Si source. The crystal size of the obtained TS-1 was about 10 μm,and the zeolite gave a poor performance in the catalytic epoxidation of n-hexene. Zuo et al. [19] reported the synthesis of small-crystal TS-1 using TS-1 mother liquid as the seed,with colloidal silica and TiCl4 as the Si source and Ti source,respectively. TPABr was used as a template,with aqueous ethylamine as the base. The TS-1 sample had small crystals,of size about 600 nm × 400 nm × 250 nm,and gave a good catalytic performance in the epoxidation of propylene and the hydroxylation of phenol. In addition to TS-1 mother liquid as the seed,a certain amount of TPABr (TPABr/SiO2 = 0.15) was used as an SDA. In a previous study [20],we used colloidal silicalite-1 as the seed for the synthesis of hierarchical nanocrystalline TS-1. The obtained TS-1 had a crystal size of about 300-500 nm,and a large amount of mesopores were formed by particle-particle aggregation; the total molar ratio of TPAOH/SiO2 was reduced to 0.12. Huang et al. [21] synthesized TS-1 at a low TPAOH/SiO2 ratio,0.05; a TS-1 precursor sol of molar composition SiO2:0.033TiO2: 0.25TPAOH:20H2O was used as the seed,and simultaneously provided TPAOH as the SDA. The synthesized TS-1 had a uniform crystal size of ~0.3 μm,very high crystallinity,and a large content of framework Ti,resulting in high activity in cyclohexanone ammoximation. However,a large amount of extra n-butylamine (n-BA/SiO2 = 0.6) was needed to increase the alkalinity and to promote TS-1 nucleation. Furthermore,the hydrothermal syntheses had to be performed using a temperature-programmed process. Wang et al. [22] used pre-prepared TS-1 as seeds and achieved a reduction in the TPA+/SiO2 ratio to 0.03,but the obtained zeolite sizes were in the micron range. Zhang et al. [23] reported a novel solid-state transformation strategy for the low-cost synthesis of nano-sized TS-1,using a TS-1 precursor sol. The TPAOH/SiO2 molar ratio was reduced to 0.0 5. The obtained TS-1 particles were of size 200-400 nm and consisted of 50 nm primary crystallites. They gave an excellent catalytic oxidation performance in the epoxidation of 1-hexene. This method is promising because of the low dose of TPAOH and the use of inorganic silica powder as the silica source. It also enhances efficiency of starting raw material use and avoids filtration and washing steps.
These previous studies suggest that the use of a colloidal TS-1 precursor as seeds is effective for the low-cost synthesis of TS-1,in both hydrothermal and solid-state transformation routes. However,care must be taken when synthesizing the TS-1 precursor to ensure appropriate coordination states of the Ti species; tetrabutyl titanate (TBOT) hydrolysis must be performed at low temperatures (usually 0 °C) and then the alcohols formed by TEOS and TBOT hydrolysis need to be removed at higher temperatures (usually 60-80 °C) [21, 23]. These tedious procedures can be avoided by using a pure-silica silicalite-1 precursor. Colloidal silicalite-1 has been widely used as the seed for the synthesis of ZSM-5,which has the same MFI structure as TS-1 [24, 25, 26, 27, 28, 29, 30].
In this study,we investigated the synthesis of small-crystal TS-1 zeolites using pure-silica silicalite-1 as the seed,ammonia as the alkali source,and TPABr as an auxiliary SDA. The catalytic oxidation performance of TS-1 depends closely on both the crystal size and the coordination states of the Ti species [31]. The effects of using various amounts of silicalite-1 on the physicochemical and catalytic properties of the obtained TS-1 samples were investigated. The epoxidation of 1-hexene and cyclohexanone ammoximation were used as probe reactions for evaluation of the catalytic performance.
The colloidal silicalite-1 seed was prepared according to the literature method,with TEOS (Sinopharm Chemical Reagent Co.,Ltd.) as the Si source and TPAOH (25 wt% aqueous solution,Sinopharm) as the SDA,using SiO2:0.35TPAOH:20H2O: 4EtOH (produced by hydrolysis of TEOS) molar ratios [29]. In a typical synthesis,TEOS (21.2 g) and TPAOH (35.0 g) were mixed under stirring at room temperature. After TEOS hydrolysis,H2O (8.2 g) was added and the mixture was treated hydrothermally in a Teflon-lined stainless-steel autoclave at 80 °C for 72 h. The obtained colloidal silicalite-1 was cooled to room temperature and used in the subsequent synthesis without any treatment. The amounts of SiO2 and TPAOH in the colloidal silicalite-1 seed were 9.35 wt% and 11.07 wt%,respectively.
In the synthesis of small-crystal TS-1,aqueous ammonia (25-28 wt% ammonia,Sinopharm Chemical Reagent Co.,Ltd.) was used to hydrolyze TBOT (≥98.0 wt%,Sinopharm) and TEOS,which were used as the Ti and Si sources,respectively. H2O2 was used to disperse the TBOT and to control the hydrolysis. Aqueous ammonia was dissolved in deionized water,and TBOT,H2O2,and TEOS were successively added dropwise under stirring. Full hydrolysis of TBOT and TEOS gave a mixture of molar composition SiO2:0.5NH3·H2O:0.025TiO2:4EtOH: 0.1ButOH:H2O2:20H2O (EtOH and ButOH were produced by hydrolysis of TEOS and TBOT,respectively). The mixture was heated at 60 °C for 6 h in a water bath; a mixture of pH ~7 was obtained. A small amount of TPABr (99 wt%,Sinopharm),used as an auxiliary organic template,and the colloidal silicalite-1 seed were then added to the mixture. The final molar composition for the syntheses was SiO2:0.025TiO2:xTPABr:yTPAOH (cs-c wt%):20H2O (see Table 1 for x and y; cs-c wt% denotes that TPAOH was introduced from the colloidal silicalite-1 seed and the amount of SiO2 in the colloidal seed represented c wt% of the total SiO2 in the batch). The obtained gel was transferred to a Teflon-lined stainless-steel autoclave and heated at 175 °C for 3 d. When crystallization was complete,the autoclave was quenched and cooled to room temperature. The solid product was separated from the mother liquor by filtration. The product was washed several times with deionized water,dried at 100 °C overnight,and calcined at 550 °C for 6 h to remove the organic template. For comparison,a control experiment was performed using a molar composition of SiO2:0.025TiO2: 0.033TPABr:0.07TPAOH:20H2O,by replacing the colloidal seed with TPAOH,with the other components unchanged.
Powder X-ray diffraction (XRD) patterns of the samples were recorded using a Rigaku X-ray diffractometer,with Ni-filtered Cu Kα radiation (λ = 0.15418 nm) at 30 kV and 30 mA,and scanning in the 2θ range 5°-35° at an angular rate of 5°/min. The relative crystallinities of the products were determined from the peak areas in the 2θ range 22.5°-25° using the sample with the highest crystallinity as the reference. Scanning electron microscopy (SEM) images were obtained using a field-emission scanning electron microscope (Hitachi S-4800),operated at an accelerating voltage of 3 kV. N2 adsorption-desorption measurements were performed at -196 °C using a BELsorp-MAX volumetric adsorption analyzer. The samples were out-gassed at 300 °C for 6 h before the adsorption measurements. The specific surface areas were determined by the Brunauer-Emmett-Teller (BET) method using data in the p/p0 range 0.01-0.2. The t-plot method was used to discriminate between micro- and meso-porosity. Fourier-transform infrared (FT-IR) spectroscopy was performed (Nicolet NEXUS 670 FT-IR spectrometer) in the range 400-4000 cm−1 using KBr disks. The bulk SiO2/TiO2 ratio of the small-crystal TS-1 was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES,Thermo IRIS Intrepid II XSP). Ultraviolet-visible (UV-vis) spectra were obtained using a Shimadzu UV-2700 spectrometer equipped with an integrating sphere,using BaSO4 as a reference.
The epoxidation of n-hexene and ammoximation of cyclohexanone were used as probe reactions to evaluate the catalytic performance of the obtained TS-1 molecular sieves. n-Hexene epoxidation was performed in a 50-mL glass reactor equipped with a reflux condenser and a magnetic stirrer. In a typical reaction,catalyst (50 mg),solvent (10 mL),n-hexene (10 mmol),and 30 wt% H2O2 aqueous solution (10 mmol) were introduced into the reactor and the mixture was heated at 80 °C under constant stirring in a water bath for 2 h. The liquid samples were analyzed using a gas chromatograph (Shimadzu GC-2014) equipped with a 30 m DB-wax capillary column and a flame ionization detector. Cyclohexane was used as the internal standard for the determination of the n-hexene conversion and epoxide selectivity. Cyclohexanone ammoximation was performed at 76 °C at atmospheric pressure in a three-necked flask. The obtained TS-1 (150 mg) was used as the catalyst,30 wt% H2O2 aqueous solution was used as the oxidant,and an equimolar mixture of H2O and tert-butanol was used as the solvent. The molar composition of the reaction batch was solvent:NH3·H2O:H2O2:cyclohexanone = 4.0:1.7:1.1:1.0; the catalyst dose was 15.0 g per mole of cyclohexanone. The products were analyzed using a gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector and a J&W 30 m × 320 μm × 0.25 μm capillary column.
Fig. 1 shows the XRD patterns of the TS-1 samples prepared using different amounts of colloidal silicalite-1 as seeds and TPABr as the auxiliary SDA. For comparison,the XRD pattern of the solid product prepared using TPAOH instead of the colloidal silicalite-1 seed is also included. When a SiO2:0.025TiO2: 0.033TPABr:0.07TPAOH:20H2O molar composition was used,TS-1 samples with similar crystallinities were obtained using colloidal silicalite-1 (20 wt%) or TPAOH (Table 1). However,the XRD peaks of the TS-1 sample prepared directly using TPAOH as the SDA were much sharper and narrower than those of the samples obtained using colloidal silicalite-1 as the seed. This could result from the crystal sizes. Highly crystalline TS-1 was still obtained when the amount of colloidal silicalite-1 seed was reduced. The crystallinity of TS-1 synthesized using 15 wt% of colloidal silicalite-1 seed was similar to that of TS-1 obtained using 20 wt% of seed (Table 1,samples b and c). When the amount of colloidal seed was further reduced to 10 wt%,the crystallinity of the obtained product decreased slightly (Table 1,sample d). This might be caused by the low alkalinity and/or loss of structure-directing ability of the synthesis system when the amount of colloidal silicalite-1 seed was reduced. The use of colloidal silicalite-1 as the seed,ammonia as the alkali source,and TPABr as the auxiliary SDA,enabled highly crystalline TS-1 to be produced using TPA+/SiO2 = 0.07; this is much lower than previously reported values using a TS-1 precursor as the seed [19, 20]. Another advantage of this process is the use of ammonia,which avoids the use of large amounts of organic amines,as reported by Huang et al. [21].
The SEM images in Fig. 2. show the morphologies of the obtained TS-1 samples. TS-1 synthesized using a molar composition of SiO2:0.025TiO2:0.033TPABr:0.07TPAOH:20H2O had a bulk morphology when TPAOH was used directly (Fig. 2(a)); this is in agreement with its sharper and narrower XRD peaks. The crystal size clearly decreased when colloidal silicalite-1 seed was used. TS-1 consisting of spherical particles of size about 100 nm was obtained when 20 wt% of colloidal seed was used (Fig. 2(b)). When the amount of colloidal seed was reduced to 15 wt%,the obtained TS-1 showed intergrowth,and the primary crystal size increased to about 250 nm × 150 nm × 50 nm (Fig. 2(c)). On further decreasing the amount of colloidal seed to 10 wt%,the primary particle size remained the same,but the morphology was tabular (Fig. 2(d)). Amorphous phases were not observed in the SEM images,although the sample prepared using 10 wt% of colloidal silicalite-1 seed showed the lowest XRD crystallinity. The small-crystal TS-1 prepared by Zuo et al. [19] using TS-1 mother liquid as the seed had a crystal size of about 150 nm,and the particle sizes of the TS-1 samples obtained by Huang at al. [21] using a TS-1 precursor sol as the seed were about 300 nm. The TS-1 samples prepared in this study had much smaller particle sizes,especially when the total amount of TPA+ was low and the use of an extra organic amine as the alkali source was avoided.
N2 sorption measurements were performed to determine the textural properties of the prepared TS-1 samples; the isotherms are shown in Fig. 3. TS-1 synthesized using a molar composition of SiO2:0.025TiO2:0.033TPABr:0.07TPAOH:20H2O showed a typical type I isotherm when TPAOH was used directly,indicating microporous properties. The isotherms of the samples prepared using different amounts of colloidal silicalite-1 as the seed showed increased N2 uptake in the higher relative pressure range; this can be ascribed to condensation of N2 in the interparticle voids formed by the small crystallites. Correspondingly,the external surface areas (Sext) and secondary pore volumes (Vsec) of the small-crystal TS-1 were larger than those of the bulk material obtained when TPAOH was used directly (Table 1). Furthermore,the larger the amount of colloidal seed used,the higher the pore volume of the synthesized zeolite was. The sample synthesized using 20 wt% of colloidal seed had Sext and Vsec values of 86 m2/g and 0.15 cm3/g,respectively,much higher than those of the sample synthesized directly using TPAOH as the SDA. All the samples had BET surface areas (SBET) of larger than 400 m2/g. The sample prepared using 10 wt% of colloidal seed adsorbed the lowest amount of N2,corresponding to the smallest SBET,400 m2/g,micropore surface area (Smic),340 m2/g,and micropore volume (Vmic),0.15 cm3/g. The other samples had Smic values higher than 360 m2/g and Vmic values larger than 0.16 cm3/g. These results are in accordance with the XRD results.
FT-IR spectra of the synthesized TS-1 samples are shown in Fig. 4. The absorption peaks at 1100,800,and 450 cm−1 are assigned to internal vibrations of (Si,Ti)O4; these are also present in materials such as amorphous SiO2,quartz,and christobalite. The absorption peaks at 1230 and 550 cm−1 are ascribed to the double-ring tetrahedral vibrations and asymmetric stretching of SiO4 and TiO4 tetrahedra in the zeolite framework,respectively. The intensity ratio of the peaks at 550 and 450 cm−1 (I550/I450) is generally used to evaluate the crystallinity of a zeolite with the MFI topology,and is called the IR crystallinity [32, 33]. The I550/I450 ratios for the samples synthesized using 10,15,and 20 wt% of colloidal silicalite-1 seed were 0.61,0.72,and 0.73,respectively; that for the bulk sample prepared directly using TPAOH as the SDA was 0.73. It was reported that I550/I450 for pure-silica silicalite-1 with a particle size of 2-4 μm was 0.8,and the value for directly synthesized NH4-ZSM-5 was in the range 0.69-0.72 [29, 34, 35]. This demonstrates that the small-crystal TS-1 synthesized in this study using colloidal silicalite-1 seed had high crystallinity. The I550/I450 ratio for the sample prepared using 10 wt% of colloidal silicalite-1 seed was lower than those of the others,showing that the sample had lower crystallinity; this is consistent with the XRD results. All the prepared samples showed a clear absorption band at ~960 cm−1,which is usually assigned to the stretching vibration of [SiO4] units strongly influenced by Ti ions in neighboring coordination sites. This absorption,which is not observed for pure silicate molecular sieves,is a strong indicator of Ti incorporation into the TS-1 framework [36]. The intensity ratio for the bands at ~960 and 800 cm−1,I960/I800,is proportional to the content of framework Ti and is usually used to evaluate the level of Ti incorporated into the framework [37]. The I960/I800 ratio of the bulk TS-1 sample prepared directly using TPAOH as the SDA was much lower than those of the small-crystal TS-1 samples prepared using colloidal silicalite-1 as the seed,suggesting that use of a colloidal seed facilitated Ti incorporation into the TS-1 framework.
The bulk SiO2/TiO2 molar ratios of the obtained TS-1 samples were ~43,slightly higher than the batch SiO2/TiO2 ratio (Table 1). This could be caused by low incorporation of Ti species. Zhao et al. [38] reported that only 90% of the Ti species in the synthesis gel were found in the solid product. The Ti coordination states in the TS-1 samples were examined using diffuse-reflectance UV-vis spectrometry,which is sensitive to the electronic states of Ti species [32, 39]; the spectra are shown in Fig. 5. A strong absorption band centered at 210 nm was observed for all the obtained samples,indicating that most Ti atoms were tetrahedrally coordinated. A weak absorption at about 270 nm was also observed for the samples prepared using colloidal silicalite-1 as the seed. This absorption arises from charge transfer in isolated [HOTiO3] units,i.e.,tetrahedrally coordinated Ti species with less attachment to the silica support through Ti-O-Si linkages [40]. An absorption band at 320 nm,which is ascribed to non-framework Ti species [31],was observed in the spectra,particularly for the bulk sample prepared using TPAOH as the SDA and the sample synthesized using 20 wt% of colloidal seed. If too much colloidal silicate-1 is used,an increased amount of alcohol is introduced. This might be unfavorable to Ti incorporation. This means that use of an appropriate amount of the colloidal seed could facilitate Ti incorporation into the zeolitic framework.
The catalytic performance of the TS-1 samples was investigated in the epoxidation of n-hexene and ammoximation of cyclohexanone. 1-Hexene conversions in selective oxidation by H2O2 catalyzed by the synthesized TS-1 samples were in the range 10%-14%,irrespective of the crystal size (Table 2). This can be ascribed to the small 1-hexene molecules having easy access to the active Ti sites in the catalysts. The epoxidation of n-hexene is not limited by intracrystalline diffusion and is therefore not influenced by the crystal size. When cyclohexanone ammoximation was used as the probe reaction,the conversion of cyclohexanone clearly differed depending on the crystal size. Only 16% cyclohexanone conversion was achieved when the catalyst was bulk TS-1 obtained directly using TPAOH as the SDA. When the small-crystal TS-1 samples prepared using various amounts of colloidal silicalite-1 as the seed were used as the catalyst,the cyclohexanone conversion increased to higher than 90% (Table 2). Cyclohexanone is relatively large and the reaction is diffusion controlled. The use of small-crystal TS-1 facilitated the diffusion of bulky molecules to and from the active Ti centers. Furthermore,the small crystals have larger numbers of active tetrahedral Ti centers,which might also lead to higher conversion than that achieved using the bulk material. However,it should be pointed out that the 1-hexene conversions in selective oxidation by H2O2 catalyzed by the small-crystal TS-1 samples synthesized here were slightly lower than that reported previously [20]. The zeolite catalytic activity is closely related not only to the type of reaction,but also to the zeolite crystallinity,the coordination state,and location of the active centers [16, 20, 31, 41]. A comprehensive investigation of the catalytic activities of these TS-1 samples is still needed.
Small-crystal TS-1 was synthesized using pure-silica colloidal silicalite-1 as the seed,a small amount of TPABr as an auxiliary SDA,and ammonia as the alkali source. The use of a colloidal seed reduced the total TPA+/SiO2 ratio needed to 0.07. The small-crystal TS-1 samples synthesized in this study had high crystallinities,and the crystal sizes were in the range 100-300 nm. An appropriate amount of the colloidal seed facilitated Ti incorporation into the zeolitic framework. The catalytic performance of the small-crystal TS-1 samples in cyclohexanone ammoximation was better than that of bulk TS-1 prepared directly using TPAOH as the SDA; this was the result of improved diffusion and larger numbers of active tetrahedral Ti centers.
2015, Vol. 36 Issue (11): 1928-1935
PDF (860 KB)
2015, Vol. 36 Issue (11): 1928-1935
PDF (860 KB)