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Synthesis of titanium silicalite-1 with small crystal size by using mother liquor of titanium silicalite-1 as seeds (II): Influence of synthesis conditions on properties of titanium silicalite-1
Microporous and Mesoporous Materials 162 (2012) 105–114
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Synthesis of titanium silicalite-1 with small crystal size by using mother liquor of titanium silicalite-1 as seeds (II): Influence of synthesis conditions on properties of titanium silicalite-1 Yi Zuo, Xiangsheng Wang, Xinwen Guo ⇑ State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 8 June 2012 Accepted 12 June 2012 Available online 19 June 2012 Keywords: Titanium silicalite-1 Synthesis condition Seed Purification method Epoxidation of propylene
a b s t r a c t Titanium silicalite-1 with small crystal size (small-crystal TS-1) was synthesized in a TPABr–ethylamine hydrothermal system by using the mother liquor of nano-sized TS-1 as seeds. The synthesis conditions were systematically studied, including the purification methods of the small-crystal TS-1, Si/Ti molar ratio in TS-1, amount of the added seed and the crystallization period. The as-synthesized TS-1 was characterized by X-ray powder diffraction (XRD), Fourier-transform infrared (FT-IR), Ultraviolet–visible diffuse reflectance (UV–vis), Ultraviolet Raman spectroscopy (UV-Raman), nitrogen sorption, elemental analysis, atomic force microscope (AFM) and scanning electron microscopy (SEM). The size of the TS-1 crystals formed (about 600 nm 400 nm 250 nm) was not significantly affected by the synthesis conditions except for the amount of added seed. The catalytic performance of the synthesized small-crystal TS-1 for the epoxidation of propylene was evaluated. The conversion of hydrogen peroxide, the selectivity of propylene oxide and the utilization of hydrogen peroxide reached 92.2%, 98.0% and 97.0%, respectively, when the mother liquor of small-crystal TS-1 was purified three times by precipitation, the Si/Ti molar ratio was 50, the seed/SiO2 weight ratio was 0.06 and the crystallization time was 48 h. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Propylene oxide (PO), a bulk chemical feedstock for producing polyether polyols and propylene glycols, is one of the most important derivatives of propylene. PO is commercially produced by two methods, the chlorohydrin method and the Halcon method. The former may result in environmental pollution due to the use of chlorine and hydrated lime as raw materials. The latter generates many co-products, which reduce profit seriously [1]. Thus, it is very attractive to develop an environmentally friendly process to make propylene oxide free of by-products. Titanium silicalite-1 (TS-1) with MFI topology was first hydrothermally synthesized by Taramasso et al. [2]. The unique catalytic performance of TS-1/H2O2 in selective oxidation reactions, such as the oxidation of alkanes, epoxidation of alkenes and hydroxylation of aromatics, was studied by many researchers [3–12]. The classical method, which was reported by Taramasso et al. using tetrapropyl ammonium hydroxide (TPAOH) as the template [2], can produce TS-1 with very small crystal size (100–200 nm), but the product is difficult to separate from its mother liquor. Moreover, the synthesis must be conducted in a glove box in order to protect the tetraethyl titanate (TEOT) against water and CO2. In addition, ⇑ Corresponding author. Tel.: +86 41184986133; fax: +86 41184986134. E-mail address: [email protected] (X. Guo). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.06.016
the expensive template TPAOH increases the cost of TS-1, and the high cost is one of the main obstacles for the practical application of TS-1 in industry [13]. Therefore, many researchers focus on developing alternative cheap synthesis routes. A method for the fast synthesis of nano-sized (200 nm) TS-1 under mild conditions was invented by Wang et al. using tetraethylorthosilicate (TEOS) and tetrabutyl titanate (TBOT) as the sources of silicon and titanium, respectively, and TPAOH as the template [14]. In this method, the synthesis is done in air, but TPAOH is still indispensible. After crystallization, a suspension of nano-sized TS-1 in the mother liquor is obtained, which does not settle for a long time. The separation of the mother liquor is difficult due to the small crystal size of TS-1. Furthermore, there is excessive TPAOH (about 0.1 mol/L) in the mother liquor. Müller and Steck used tetrapropyl ammonium bromide (TPABr) as template to synthesize TS-1, but the crystal size was larger than that obtained with TPAOH due to the introduction of Br [15]. Since then, many studies have been performed using TPABr as a template to synthesize TS-1 [16–19], but it is difficult to obtain TS-1 crystals smaller than 1 lm. As a consequence, the activity of the catalysts and the selectivity of the main product PO are very low due to diffusion restriction [20]. Zhang et al. synthesized TS1 with a crystal size of 3 lm 2 lm using TiCl4 and colloidal silica in the TPABr–NH3H2O system [21]. They found that addition of powdered nano-sized TS-1 (200 nm) as crystallization seed could
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accelerate the crystallization, decrease the induction period and achieve a higher crystallinity of the resulting TS-1 [22]. Mao et al. [23] modified micro-sized TS-1 (1 lm), which was synthesized in a TPABr–ethylamine system, with dilute TPAOH solution. Transmission electron microscopy (TEM) images of the sample showed that some irregular mesopores appeared in the TS-1 crystals after the modification, minimizing the diffusion influence. Therefore, the catalytic performance was improved significantly. However, this modification extends the preparation period of TS-1, and the expensive TPAOH needs to be used again. In order to shorten the preparation period, we tried to synthesize TS-1 by using the mother liquor of nano-sized TS-1 as the crystallization seed in a TPABr–ethylamine hydrothermal system. With this method, small-crystal TS-1 with a size of about 600 nm 400 nm 250 nm was obtained [24]. The appearance of the small-crystal TS-1 was significantly different from either the modified micro-sized TS-1 or the nano-sized TS-1, and its catalytic performance for propylene epoxidation and phenol hydroxylation was outstanding. In this follow-up work, the synthesis conditions, including method for removal of the mother liquor, the Si/Ti molar ratio in the TS-1 and crystallization factors, were systematically studied to develop a catalyst that has better performance for the epoxidation of propylene.
Optical Emission Spectrometer. Nitrogen sorption measurements were performed at liquid nitrogen temperature on a Quantachrome AUTOSORB-1 physical sorption apparatus. Surface area and pore volume were calculated according to the BET and BJH method, respectively. The t-plot and HK method was used for the analysis of surface area and pore size distribution of micropore, respectively. The roughness and Ra values were performed on a Veeco Nanoma Vs atomic force microscope (AFM). 2.3. Epoxidation of propylene The epoxidation of propylene was carried out in a 400 mL stainless-steel reactor. In a typical run, 0.4 g of small-crystal TS-1, 24 mL of acetone, 8 mL of methanol and 30 wt.% of hydrogen peroxide were added to the reactor; then propylene was charged to reach 0.4 MPa. The initial concentration of hydrogen peroxide in the reactant solution was 1.1 mol/L. After heating the mixture with stirring at 60 °C for 1 h, the residual H2O2 was checked by iodometric titration. The products were analyzed on a Tianmei 7890F gas chromatograph with a FID and a PEG-20 M capillary column (30 m 0.25 mm 0.4 lm). PO was the main product, and propylene glycol (PG) and its monomethyl ethers (MME) were the byproducts. The conversion of H2O2 (X(H2O2)), selectivity of PO (S(PO)) and utilization of H2O2 (U(H2O2)) were calculated with equations 1, 2 and 3, respectively:
2. Experimental
XðH2 O2 Þ ¼ ðn0 ðH2 O2 Þ nðH2 O2 ÞÞ=n0 ðH2 O2 Þ
ð1Þ
2.1. Preparation of TS-1
SðPOÞ ¼ nðPOÞ=ðnðPOÞ þ nðMMEÞ þ nðPGÞÞ
ð2Þ
Small-crystal TS-1 was prepared hydrothermally [24,25], using colloidal silica (30 wt.%) and titanium tetrachloride as silicon and titanium sources, respectively. TPABr was used as template and aqueous ethylamine (65 wt.%) as base. The molar composition of the gel was n(SiO2):n(TiO2):n(TPABr):n(C2H5NH2):n(H2O) = 1:0.01– 0.05:0.15:1.5:25. A solution containing nano-sized TS-1 was prepared according to Ref. [14] and used as seeds. The amount of solid nano-sized TS-1 in this solution was 0.07 g/mL, and the amount of this TS-1 solution added to the hydrothermal system was 5– 10 vol.% of the mother liquor of small-crystal TS-1. The order of the addition of raw material was as follows: titanium tetrachloride was first added to isopropyl alcohol, then the obtained mixture was added to colloidal silica. Finally, TPABr, ethylamine, the seed solution and water were added to the above mixture. The final solution was transferred to a Teflon lined autoclave and crystallized for 6–60 h at 170 °C. The solid TS-1 was obtained after purification according to different methods described in Sections 3.1 and 3.2. It was then dried at 100 °C and calcined at 540 °C for 6 h to remove the template. Nano-sized TS-1 (denoted as sample 14) using the improved conventional method [26] was also synthesized for comparison.
UðH2 O2 Þ ¼ ðnðPOÞ þ nðMMEÞ þ nðPGÞÞ=ðn0 ðH2 O2 Þ XðH2 O2 ÞÞ:
ð3Þ
The n0(H2O2) and n(H2O2) represent the initial and final molar content of H2O2, respectively. The n(PO), n(MME) and n(PG) stand for the number of moles of PO, MME and PG, respectively. 3. Results and discussion 3.1. Separation and purification methods With the procedure described in the experimental section, small-crystal TS-1 was obtained after crystallization at 170 °C for 36 h. Due to the small crystal size, it was hard to separate the solid from the mother liquor by filtration. Three methods were explored to remove the mother liquor. The first one was centrifuging the li-
2.2. Characterization of TS-1 XRD patterns were generated on a Rigaku Corporation D/MAX2400 instrument using Cu Ka radiation. FT-IR spectra were recorded on a Bruker EQUINOX55 spectrometer from 4000 to 400 cm1, and the KBr pellet technique was adopted. UV–vis spectra with wave length from 190 to 500 nm were obtained on a Jasco UV-550 spectrometer, and pure BaSO4 was used as reference. UVRaman spectra were recorded on a UV-Raman spectrometer built at the State Key laboratory of Catalysis (Dalian Institute of Chemical Physics, PR China). The morphology of the crystals was detected on a Hitachi scanning electron microscope (SEM). The elemental analysis of TS-1 was carried out on a Bruker SRS3400 X-ray fluorescence spectrometer (XRF) and a Perkin Elmer OPTIMA 2000DV ICP
Fig. 1. XRD patterns of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3).
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Fig. 2. FT-IR spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3).
Fig. 4. UV-Raman spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3). The excitation wave length is 244 nm.
Fig. 3. UV–vis spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3).
quid/solid mixture, and then adding distilled water to the solid and centrifuging again. This operation was repeated until the pH of the separated liquid was 7. The solid obtained after drying and calcination was noted as sample 2-1 (centrifuging separation). The second method was leaving the mixture static for 12 h so that the solid would precipitate by itself. The liquid was decanted and the same amount of distilled water was added to the solid. The decantation and addition of fresh water were repeated two times before the solid was dried and calcined to generate sample 2-2 (precipitating separation). The third method was directly drying the mixture followed by calcination to get sample 2-3. The XRD patterns of the three samples shown in Fig. 1 indicate that they all have the MFI topology with the characteristic peaks at 2h of 7.8°, 8.8°, 23.0°, 23.9° and 24.4°. The relative crystallinity, which was calculated by dividing the total intensity of the characteristic peaks of the samples by that of the standard TS-1, was the lowest (only 86%) for sample 2-3, and was 95% and 99% for samples 2-2 and 2-1, respectively. Washing can remove amorphous substrates such as silica, which decrease the relative crystallinity. Therefore, the different separation methods influence the relative crystallinity and the amount of amorphous substrates essentially.
Fig. 5. UV-Raman spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3). The excitation wave length is 325 nm.
Table 1 Catalytic performance of catalysts prepared by different purification methods for the epoxidation of propylene. Cat.
X(H2O2)/%
S(PO)/%
U(H2O2)/%
2-1 2-2 2-3 2-4 2-5 2-6 1-4
91.7 91.6 20.4 53.3 70.1 91.7 82.0
73.6 97.9 99.9 99.6 99.6 97.5 94.6
95.8 96.9 90.0 89.8 91.4 96.0 93.8
Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.
The relative crystallinity of sample 2-1 was the highest due to the most complete purification.
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Fig. 6. UV–vis spectra of the seven samples with different molar ratio of Si/Ti: n(Si/ Ti) = 20 (3-1), n(Si/Ti) = 25 (3-2), n(Si/Ti) = 45 (3-3), n(Si/Ti) = 50 (3-4), n(Si/Ti) = 55 (3-5), n(Si/Ti) = 60 (3-6) and n(Si/Ti) = 80 (3-7).
Fig. 8. FT-IR spectra of the six samples with weight ratio of crystallization seed/ SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
Table 2 Catalytic performance of the samples with different molar ratio of Si/Ti for the epoxidation of propylene. Cat.
X(H2O2)/%
S(PO)/%
U(H2O2)/%
3-1 3-2 3-3 3-4 3-5 3-6 3-7
82.8 83.2 89.7 91.6 87.5 87.5 86.3
96.1 97.1 97.0 97.9 95.6 82.6 78.4
77.9 88.7 91.1 96.9 96.3 96.4 96.5
Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.
Fig. 9. UV–vis spectra of the six samples with weight ratio of crystallization seed/ SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
Fig. 7. XRD patterns (a) and crystallization curve (b) of the six samples with weight ratio of crystallization seed/SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
Fig. 2 shows the FT-IR spectra of the samples. The bands at 550 and 800 cm1 were assigned to the characteristic bands of the MFI topology [27]. The band at 960 cm1 is considered as the stretching vibration of [SiO4] units strongly influenced by titanium ions in neighboring coordination sites, which is proof of introducing
titanium into the framework [28]. The intensity of this band is related to the amount of framework Ti [29]. Blocking of zeolite channels and covering of framework Ti by organic compounds may cause the decrease of the intensity of this band [30]. In the present study, the covering of active framework Ti by silica and sodium ions had little influence on the band at 960 cm1 in FT-IR spectra, and the intensities of these bands are almost the same for the three samples. The possible reason is that the content of silica in our small-crystal TS-1 is not significant. UV–vis spectroscopy was adopted for detecting the coordination states of Ti (Fig. 3). The band at 210 nm is due to tetrahedrally coordinated Ti (usually called framework Ti), which is considered as the active center for the epoxidation of propylene [31], while the band at 250–280 nm can be assigned to a charge transfer effect in isolated [TiO4] or [HOTiO3] units, such as octahedrally coordinated Ti (usually called non-framework Ti) [32–34]. The band at 310 nm indicates the existence of anatase TiO2 which is deemed to be the catalyst for decomposition of hydrogen
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peroxide [35]. The amount of anatase TiO2 in both samples 2-2 and 2-3 was a little higher than that in sample 2-1, according to the intensity of the band. Apparently, the purification removed some isolated anatase TiO2 with small particle size. UV-Raman spectroscopy is a sensitive tool for examining the chemical environment of metal cations, especially for titanium cations [36]. Figs. 4 and 5 show the UV-Raman spectra of the samples. The wave lengths of excitation lights were 244 and 325 nm. In Fig. 4, the 380 cm1 band is considered as the fingerprint of the MFI structure, while the bands at 490, 530 and 1125 cm1 are assigned to the framework Ti [37]. The ratio of the signal intensity of 1125 cm1 to that of 380 cm1 (I1125/I380), which is 5.0, 4.8 and 3.3 for samples 2-1, 2-2 and 2-3, respectively, can be used as a measurement for the relative content of framework Ti [38]. However, the disparity among the ratios is believed to be due to the various extent of covering of framework Ti by silica and sodium ions, rather than due to the different content of framework Ti (cf. Fig. 3). Washing can hardly transform non-framework Ti or anatase TiO2 to framework Ti, but can remove amorphous substrates,
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which may cover the framework Ti. It is evident that the ratio obtained for sample 2-3 is the smallest among three samples, indicating the lowest catalytic activity of sample 2-3. In Fig. 5, the bands at 390, 516 and 637 cm1 are assigned to the characteristic bands of anatase TiO2. The band at 637 cm1 for sample 2-1 shows a hypochromatic shift, which may be due to an exchange of Na+ by H+ in the purification process, leading to the increase of the amount of hydroxyl groups in TS-1. Elemental analysis of samples 2-1, 2-2 and 2-3 showed that the molar ratio of Si/Ti (n(Si/Ti)) in sample 2-3 (51.2) was higher than those in samples 2-1 (48.1) and 2-2 (48.4). The higher content of amorphous silica in sample 2-3 may be due to the incomplete washing of the mother liquor from the small-crystal TS-1. This could also be the reason that the relative crystallinity of sample 2-3 is the lowest. The deposition of silica on the catalyst can block the channels and cover the active centers, and thus cause a decrease of the catalytic activity. The presence of a small amount of sodium ions, introduced by the silica sol, can neutralize the acid sites on the surface of TS-1, which is helpful for improving the
Fig. 10. SEM images of the six samples with weight ratio of crystallization seed/SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
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selectivity of the main product PO in the epoxidation of propylene [39,40]. However, if the content of Na+ in the hydrothermal synthesis system is too high, the crystallinity of TS-1 will decrease, and the insertion of titanium into the framework will be hindered. It has been reported that a TS-1 with a large amount of sodium exchange deactivated significantly in the oxidation reaction of alkanes [41]. Therefore, the content of Na+ in the hydrothermal system was controlled strictly in our experiment, but the ions are difficult to be removed completely due to the use of silica sol. The content of Na+ in sample 2-3 (0.81 wt.%) was the highest among the three samples, because it was not purified at all, while in sample 2-1 it was the lowest (0.04 wt.%) due to the complete purification. The crystallinity seems to increase with the decrease of the content of Na+. Nevertheless, we believe that the change of crystallinity is not due to the effect of Na+ but to the amorphous silica on the TS-1 surface. In other words, the low content of Na+ in small-crystal TS-1 hardly influences its crystallinity. The catalytic performance of the samples for the epoxidation of propylene is shown in Table 1. Conversions of H2O2 on samples 2-1 and 2-2 were essentially the same, and were much higher than that on sample 2-3. The low conversion on sample 2-3 is believed to be due to the low crystallinity and the covering of active centers by excessive Na+ (cf. XRD patterns and elemental analysis). A little bit of Na+ in sample 2-2 might lead to a high selectivity of PO (97.9%), while the excessive hydroxyl group in sample 2-1 (UV-Raman spectrum in Fig. 5) could be responsible for the low selectivity of PO. Sample 2-6 was prepared by treating sample 2-1 with dilute Na2CO3 solution, and calcining the sample at 540 °C for 6 h. The content of Na+ in sample 2-6 was 0.10 wt.%, which is similar to that in sample 2-2. The selectivity of PO increased to 97.5% after the treatment, which confirms the discussion above. The utilization of H2O2 for sample 2-3 is the lowest among the three samples because there were more impurities, which accelerates the decomposition of H2O2. Summarily, different purification methods influenced the catalytic performance by changing the amount of impurities, namely, the purity of TS-1. The small-crystal TS-1 with precipitating separation shows the best catalytic performance in propylene epoxidation due to its suitable purity. The low conversion of H2O2 on nano-sized TS-1 (sample 1-4) can be attributed to the low relative crystallinity [24].
Table 3 Catalytic performance of catalysts prepared with different crystallization times for the epoxidation of propylene. Cat.
X(H2O2)/%
S(PO)/%
U(H2O2)/%
4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9 4-10 4-11
17.3 81.9 90.3 91.6 92.2 91.9 82.1 90.1 91.7 91.6 91.3
99.9 90.4 99.7 97.9 98.0 96.7 89.2 96.0 96.8 97.5 97.0
17.3 70.0 85.7 96.9 97.0 95.0 78.8 91.0 93.6 93.7 93.4
Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.
the content of aluminum in the silica sol needs to be controlled strictly. The content of titanium increased slightly after different times of precipitating separation (from 1.52 to 1.60 wt.%). The lost titanium is considered to be isolated anatase TiO2, and most of titanium exists in the framework, which cannot be removed by washing with distilled water. The ratio n(Si/Ti) decreased with increasing washing time, probably due to the loss of amorphous silica in the washing process. The catalytic performance of samples 2-4 and 2-5 is shown in Table 1. The conversion of H2O2 increased with increasing washing time, due to the decrease of the content of amorphous silica and sodium ion in the samples. The selectivity of PO obtained on sample 2-2 is slightly lower than those on samples 2-4 and 2-5. It is believed that losing more sodium ions leads to a higher exposure of acid sites, which may catalyze the side reactions of PO with solvents or itself. The utilization of H2O2 for samples 2-4 and 2-5 was lower than that for sample 2-2, and is similar to that on sample 2-3, because more impurities left in the catalyst results in more serious decomposition of H2O2. In summary, the changing of the residual amount of sodium ions and amorphous silica, either by different purification methods or by different washing times, leads to a change in the properties of small-crystal TS-1.
3.2. Times of precipitating separation In order to further investigate the relationship among crystallinity, sodium ions and the catalytic performance over the smallcrystal TS-1 in the epoxidation of propylene, the effect of precipitating separation times was examined. Sample 2-4 was prepared in the same way as sample 2-2 except that no fresh water was used to wash the crystal solid after the mother liquor was decanted. Another catalyst, sample 2-5, was also synthesized with a similar procedure. The difference between samples 2-4 and 2-5 was that distilled water was added to the solid, which was obtained after the mother liquor was decanted, and was separated by precipitation again. The relative crystallinity increased from 87% for sample 2-4 to 92% for sample 2-5. This increase also proves that washing can remove some amorphous substance which may decrease the relative crystallinity. The content of sodium ion in samples 2-4 and 2-5 decreased with increasing washing time, but the content of aluminum stayed almost constant. Aluminum, which is also introduced by the silica sol, can easily insert into the framework and form strong bonds with framework oxygen. Therefore, it is hard to remove aluminum by washing. The nearly constant content of aluminum (0.08 wt.%) in the samples indicates that most of the aluminum was inserted into the framework. The insertion of aluminum into the framework may generate acid sites [42,43], thus
Fig. 11. XRD patterns of the five samples with weight ratio of crystallization seed/ SiO2 = 0.03 and with different crystallization time: 12 h (4-7), 24 h (4-8), 36 h (4-9), 48 h (4-10) and 60 h (4-11).
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3.3. Molar ratio of Si/Ti Small-crystal TS-1 samples with different n(Si/Ti) were synthesized by adjusting the amount of titanium tetrachloride added to the hydrothermal system. The washing procedure was the same as that used for sample 2-2. The samples with n(Si/Ti) 20, 25, 45, 50, 55, 60 and 80 were denoted as 3-1, 3-2, 3-3, 3-4, 3-5, 3-6 and 3-7, respectively. The peaks in the XRD pattern with 2h from 25.0° to 30.0° were assigned to the signals from anatase TiO2 [44]. There was no clear difference among these peaks of the seven samples (not shown), may be due to the limited sensitivity of XRD to the detection of a small amount of anatase TiO2. Therefore, UV–vis spectroscopy was expected to show some differences. Tetrahedrally coordinated titanium is considered as the active center in the epoxidation of propylene [31], while anatase TiO2 is the catalyst for the decomposition of H2O2 [35]. When a titanium atom, which has a larger size than a silicon atom, inserts into the framework, the lattice will swell, and the lattice constants will increase from a = 20.101 Å, b = 19.877 Å and c = 13.365 Å of silicalite-
111
1 (pure silicon zeolite with MFI topology) to a = 20.111 Å, b = 19.917 Å and c = 13.385 Å of the TS-1 containing 1.1 wt.% Ti [45]. Only a limited amount of titanium can be inserted into the framework. The maximum was reported to be 2.5 wt.% [45,46]. Thus, if the amount of titanium is higher than 2.5 wt.%, the excess Ti will exist as non-framework Ti or anatase TiO2 [47]. The UV–vis spectra of the samples are shown in Fig. 6. When the n(Si/Ti) value was higher than or equal to 60, as for samples 3-6 (60) and 3-7 (80), most of the titanium atoms were in the tetrahedrally coordinated state (characteristic band at 210 nm). Anatase TiO2 (characteristic band at 310 nm) was found in sample 3-5 with n(Si/ Ti) = 55, while other non-framework Ti (characteristic band at 250280 nm) was observed in sample 3-4 with n(Si/Ti) = 50. The amounts of non-framework Ti increased noticeably with the decrease of n(Si/Ti). However, the intensity of the band at 210 nm was hardly affected by the ratio when n(Si/Ti) was below 55. The catalytic performance of the samples with different n(Si/Ti) values in the epoxidation of propylene is shown in Table 2. The conversion of H2O2 increased with decreasing n(Si/Ti) value due to the increase of the number of active centers, until the ratio
Fig. 12. SEM images of the five samples with weight ratio of crystallization seed/SiO2 = 0.03 and with different crystallization time: 12 h (4-7), 24 h (4-8), 36 h (4-9), 48 h (410) and 60 h (4-11).
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3.4.1. Weight ratio of seed/SiO2 = 0.06 The small-crystal TS-1 crystallized with different periods was synthesized with a m(seed/SiO2) value of 0.06. The crystallization
period was 6, 12, 24, 36, 48 and 60 h, and the corresponding samples were marked as samples 4-1, 4-2, 4-3, 4-4, 4-5 and 4-6, respectively. Fig. 7 shows the XRD patterns and the crystallization curve of the samples with different crystallization periods. When the crystallization period was 6 h, the diffraction peaks were very weak (Fig. 7a), and the relative crystallinity was less than 17% (Fig. 7b). Apparently, the MFI framework just starts to form at 6 h. After 12 h crystallization, the characteristic peaks of MFI structure became more obvious, corresponding to a crystallinity of 67%. The relative crystallinity increased continuously with increasing crystallization time. After 36 h, the crystallinity remained constant. FT-IR and UV–vis spectra of the samples are shown in Figs. 8 and 9, respectively. In Fig. 8, the IR bands at 550 and 960 cm1 of sample 4-1 are very weak, because the MFI framework has not been generated, and almost no titanium could be inserted into the framework. This coincides with the XRD pattern in Fig. 7a. The intensity of the band at 550 cm1 increased with increasing crystallization time, due to the gradual formation of the framework. The intensity of the band at 960 cm1 hardly changed after more than 12 h of crystallization, indicating that the insertion of Ti into the framework was accomplished in less than 12 h. The intensity of the UV–vis band at 250–280 nm (Fig. 9) was very strong for sample 4-1. When extending the crystallization period, the intensity decreased significantly. It indicates that isolated [TiO4] was generated in the synthesis gel first, and then these species combined with each other to produce uniform TS-1 crystals. When the crystallization period was prolonged for more than 24 h, the spectra of the obtained samples were similar, because the TS-1 framework had been generated, and the coordination states of titanium had become stable.
Fig. 13. Nitrogen sorption curves of samples 2-1, 2-2 and 4-9.
Fig. 14. Pore size distribution of samples 2-1, 2-2 and 4-9.
reached 50. After that, the conversion began to decline with decreasing n(Si/Ti) value. Addition of excessive Ti to the hydrothermal system might generate too much anatase TiO2, which could cover the active centers or block the channels of TS-1. In this case, the diffusion of the substrates will be hindered, and the conversion of H2O2 will be reduced. With increasing n(Si/Ti), the selectivity of PO improved slightly, until n(Si/Ti) reached 50. The non-framework Ti can cause side reactions of PO. The reduction of non-framework Ti content by the increase of the n(Si/Ti) value is beneficial for the PO selectivity. When n(Si/Ti) continues to increase, the amount of Si–OH may increase, resulting in an increase in the number of acid sites. Therefore, the selectivity of PO decreases. The presence of anatase TiO2 can accelerate the decomposition of the H2O2, and thus with the decrease of n(Si/Ti) value (Fig. 6) the utilization of H2O2 decreases gradually. 3.4. Weight ratios of seed/SiO2 Introduction of seeds into the system can accelerate the crystallization and shorten the period of synthesis [21,22,24]. Moreover, there is not only nano-sized TS-1 in the seeds, but also excessive TPAOH (0.1 mol/L), which may influence the synthesis of smallcrystal TS-1. Therefore, we examined the influence of the weight ratio m(seed/SiO2) on the properties of small-crystal TS-1. Meanwhile, the effects of crystallization period are discussed in this section. The other synthesis conditions were: n(Si/Ti) 50, and for three times precipitating separation washing.
Y. Zuo et al. / Microporous and Mesoporous Materials 162 (2012) 105–114 Table 4 The BET surface area and pore volume of catalysts. Cat. BET area Pore volume Micropore BET Micropore volume External BET (m2/g) (cm3/g) area (m2/g) (cm3/g) area (m2/g) 2-1 451 2-2 459 4-9 456
0.31 0.29 0.28
416 426 423
0.18 0.18 0.18
351 358 326
Fig. 10 shows the SEM images of the six samples. The TS-1 crystal was not generated in the first 6 h (sample 4-1). When the crystallization time was prolonged to more than 12 h (from sample 4-2 to sample 4-6), cubic crystals appeared, and the size was hardly changed, being 600 nm 400 nm 250 nm. As shown in Table 3, the conversion of H2O2 increased with increasing crystallization time from 12 to 48 h. The selectivity of PO on sample 4-2 was only 90.4%, probably due to the low crystallization and a little bit rich of octahedral coordinated Ti (cf. Fig. 9). The utilization of H2O2 increased with increase of the crystallinity, because the content of impurities, such as silica, decreased. The catalytic performance of sample 4-6 (crystallized for 60 h) was slightly low. This may be because the surface of the crystals became smoother and the number of active centers on the surface decreased.
3.4.2. Weight ratio of seed/SiO2 = 0.03 Small-crystal TS-1 with the m(seed/SiO2) = 0.03 was synthesized, characterized and evaluated in the epoxidation of propylene. The samples with the crystallization time of 12, 24, 36, 48 and 60 h were denoted as samples 4-7, 4-8, 4-9, 4-10 and 4-11, respectively. The XRD patterns (Fig. 11a) show that all samples have the MFI topology, indicating that the framework of small-crystal TS-1 has already been formed when the crystallization period reaches 12 h. Fig. 11b shows the crystallization curve of the samples. The relative crystallinity increased with increase of crystallization time until 36 h and then remained constant. The relative crystallinity of the samples with m(seed/SiO2) = 0.03 was higher than those of m(seed/SiO2) = 0.06, when the crystallization time was the same. The larger crystal size of the samples with m(seed/SiO2) = 0.03 (about 800 nm 550 nm 250 nm, cf. Figs. 10 and 12), is considered to be the reason. A lower number of seed needs a longer induction period during crystallization; thus, the obtained crystals will tend to become bigger. The surfaces of these samples were smoother than those of the samples with m(seed/SiO2) = 0.06, and the features are more like those of micro-sized TS-1 [24]. The roughness of the samples 4-4 and 4-9, which was evaluated by average Ra values, were 4.11 and 3.08 nm, respectively, according to the AFM characterization. The amount of TPAOH, introduced by seeds, was considered as a factor to influence the roughness of crystals. The more TPAOH was added to the hydrothermal system, the rougher would the crystal surface be, and vice versa. The external surface of the nano-sized TS-1 was rougher than that of the micro-sized TS-1, as TPAOH was used in the synthesis of former, but TPABr in the latter. The nitrogen sorption isotherms and pore size distribution of samples 2-1 (centrifuging purification), 2-2 (precipitating purification) and 4-9 are shown in Figs. 13 and 14, respectively. There is no significantly difference in the three isotherms. A small hysteresis loop can be observed in each isotherm, due to the inter-crystal space. The main peak in the pore size distribution is at about 0.5 nm, which is the pore of the five member ring in the MFI topology. The total surface area of sample 4-9 is similar to that of samples 2-1 and 2-2 (Table 4), but the external surface area is less, due to the larger crystal size. The micropore volumes of the three samples are almost the same, but the total pore volumes of samples 2-
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1 and 2-2 are larger than that of sample 4-9. In other words, the mesopore (the inter-crystal space) volumes of the samples 2-1 and 2-2 are larger than that of sample 4-9. Catalytic performance of the samples with m(seed/SiO2) = 0.03 for epoxidation of propylene was measured, and the results are listed in Table 3. The conversion of H2O2, the selectivity of PO and the utilization of H2O2 increase with the increase of crystallization period until 36 h, and then remained constant. The crystal surface became smoother, which may cause a decrease of the active centers on the external surface. Therefore, the conversion of H2O2 on the samples with m(seed/SiO2) = 0.03 were a little lower than of the samples with m(seed/SiO2) = 0.06. 4. Conclusions Different conditions for synthesis of small-crystal TS-1 were systematically studied in a TPABr–ethylamine hydrothermal system by using the mother liquor of nano-sized TS-1 as seeds. The size of the obtained TS-1 crystal was about 600 nm 400 nm 250 nm, which was not affected significantly by the synthesis conditions, except for the amount of the additional seed, after the formation of crystals. The small-crystal TS-1 showed an excellent catalytic performance in the epoxidation of propylene (X(H2O2) = 92.2%, S(PO) = 98.0%, U(H2O2) = 97.0%), when the mother liquor was separated by three-time precipitation with a n(Si/Ti) value of 50, a m(seed/SiO2) value of 0.06 and a crystallization period of 48 h. A small amount of sodium ions can improve the selectivity of PO in the epoxidation of propylene, but a large amount of sodium ions will cover the active centers on the TS-1, resulting in a decrease of the catalytic activity. Acknowledgement This work was financially supported by the Program for New Century Excellent Talent in University (NCET-04-0268) and the Plan 111 Project of the Ministry of Education of China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2012. 06.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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Synthesis of titanium silicalite-1 with small crystal size by using mother liquor of titanium silicalite-1 as seeds (II): Influence of synthesis conditions on properties of titanium silicalite-1 Yi Zuo, Xiangsheng Wang, Xinwen Guo ⇑ State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 8 June 2012 Accepted 12 June 2012 Available online 19 June 2012 Keywords: Titanium silicalite-1 Synthesis condition Seed Purification method Epoxidation of propylene
a b s t r a c t Titanium silicalite-1 with small crystal size (small-crystal TS-1) was synthesized in a TPABr–ethylamine hydrothermal system by using the mother liquor of nano-sized TS-1 as seeds. The synthesis conditions were systematically studied, including the purification methods of the small-crystal TS-1, Si/Ti molar ratio in TS-1, amount of the added seed and the crystallization period. The as-synthesized TS-1 was characterized by X-ray powder diffraction (XRD), Fourier-transform infrared (FT-IR), Ultraviolet–visible diffuse reflectance (UV–vis), Ultraviolet Raman spectroscopy (UV-Raman), nitrogen sorption, elemental analysis, atomic force microscope (AFM) and scanning electron microscopy (SEM). The size of the TS-1 crystals formed (about 600 nm 400 nm 250 nm) was not significantly affected by the synthesis conditions except for the amount of added seed. The catalytic performance of the synthesized small-crystal TS-1 for the epoxidation of propylene was evaluated. The conversion of hydrogen peroxide, the selectivity of propylene oxide and the utilization of hydrogen peroxide reached 92.2%, 98.0% and 97.0%, respectively, when the mother liquor of small-crystal TS-1 was purified three times by precipitation, the Si/Ti molar ratio was 50, the seed/SiO2 weight ratio was 0.06 and the crystallization time was 48 h. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Propylene oxide (PO), a bulk chemical feedstock for producing polyether polyols and propylene glycols, is one of the most important derivatives of propylene. PO is commercially produced by two methods, the chlorohydrin method and the Halcon method. The former may result in environmental pollution due to the use of chlorine and hydrated lime as raw materials. The latter generates many co-products, which reduce profit seriously [1]. Thus, it is very attractive to develop an environmentally friendly process to make propylene oxide free of by-products. Titanium silicalite-1 (TS-1) with MFI topology was first hydrothermally synthesized by Taramasso et al. [2]. The unique catalytic performance of TS-1/H2O2 in selective oxidation reactions, such as the oxidation of alkanes, epoxidation of alkenes and hydroxylation of aromatics, was studied by many researchers [3–12]. The classical method, which was reported by Taramasso et al. using tetrapropyl ammonium hydroxide (TPAOH) as the template [2], can produce TS-1 with very small crystal size (100–200 nm), but the product is difficult to separate from its mother liquor. Moreover, the synthesis must be conducted in a glove box in order to protect the tetraethyl titanate (TEOT) against water and CO2. In addition, ⇑ Corresponding author. Tel.: +86 41184986133; fax: +86 41184986134. E-mail address: [email protected] (X. Guo). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.06.016
the expensive template TPAOH increases the cost of TS-1, and the high cost is one of the main obstacles for the practical application of TS-1 in industry [13]. Therefore, many researchers focus on developing alternative cheap synthesis routes. A method for the fast synthesis of nano-sized (200 nm) TS-1 under mild conditions was invented by Wang et al. using tetraethylorthosilicate (TEOS) and tetrabutyl titanate (TBOT) as the sources of silicon and titanium, respectively, and TPAOH as the template [14]. In this method, the synthesis is done in air, but TPAOH is still indispensible. After crystallization, a suspension of nano-sized TS-1 in the mother liquor is obtained, which does not settle for a long time. The separation of the mother liquor is difficult due to the small crystal size of TS-1. Furthermore, there is excessive TPAOH (about 0.1 mol/L) in the mother liquor. Müller and Steck used tetrapropyl ammonium bromide (TPABr) as template to synthesize TS-1, but the crystal size was larger than that obtained with TPAOH due to the introduction of Br [15]. Since then, many studies have been performed using TPABr as a template to synthesize TS-1 [16–19], but it is difficult to obtain TS-1 crystals smaller than 1 lm. As a consequence, the activity of the catalysts and the selectivity of the main product PO are very low due to diffusion restriction [20]. Zhang et al. synthesized TS1 with a crystal size of 3 lm 2 lm using TiCl4 and colloidal silica in the TPABr–NH3H2O system [21]. They found that addition of powdered nano-sized TS-1 (200 nm) as crystallization seed could
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accelerate the crystallization, decrease the induction period and achieve a higher crystallinity of the resulting TS-1 [22]. Mao et al. [23] modified micro-sized TS-1 (1 lm), which was synthesized in a TPABr–ethylamine system, with dilute TPAOH solution. Transmission electron microscopy (TEM) images of the sample showed that some irregular mesopores appeared in the TS-1 crystals after the modification, minimizing the diffusion influence. Therefore, the catalytic performance was improved significantly. However, this modification extends the preparation period of TS-1, and the expensive TPAOH needs to be used again. In order to shorten the preparation period, we tried to synthesize TS-1 by using the mother liquor of nano-sized TS-1 as the crystallization seed in a TPABr–ethylamine hydrothermal system. With this method, small-crystal TS-1 with a size of about 600 nm 400 nm 250 nm was obtained [24]. The appearance of the small-crystal TS-1 was significantly different from either the modified micro-sized TS-1 or the nano-sized TS-1, and its catalytic performance for propylene epoxidation and phenol hydroxylation was outstanding. In this follow-up work, the synthesis conditions, including method for removal of the mother liquor, the Si/Ti molar ratio in the TS-1 and crystallization factors, were systematically studied to develop a catalyst that has better performance for the epoxidation of propylene.
Optical Emission Spectrometer. Nitrogen sorption measurements were performed at liquid nitrogen temperature on a Quantachrome AUTOSORB-1 physical sorption apparatus. Surface area and pore volume were calculated according to the BET and BJH method, respectively. The t-plot and HK method was used for the analysis of surface area and pore size distribution of micropore, respectively. The roughness and Ra values were performed on a Veeco Nanoma Vs atomic force microscope (AFM). 2.3. Epoxidation of propylene The epoxidation of propylene was carried out in a 400 mL stainless-steel reactor. In a typical run, 0.4 g of small-crystal TS-1, 24 mL of acetone, 8 mL of methanol and 30 wt.% of hydrogen peroxide were added to the reactor; then propylene was charged to reach 0.4 MPa. The initial concentration of hydrogen peroxide in the reactant solution was 1.1 mol/L. After heating the mixture with stirring at 60 °C for 1 h, the residual H2O2 was checked by iodometric titration. The products were analyzed on a Tianmei 7890F gas chromatograph with a FID and a PEG-20 M capillary column (30 m 0.25 mm 0.4 lm). PO was the main product, and propylene glycol (PG) and its monomethyl ethers (MME) were the byproducts. The conversion of H2O2 (X(H2O2)), selectivity of PO (S(PO)) and utilization of H2O2 (U(H2O2)) were calculated with equations 1, 2 and 3, respectively:
2. Experimental
XðH2 O2 Þ ¼ ðn0 ðH2 O2 Þ nðH2 O2 ÞÞ=n0 ðH2 O2 Þ
ð1Þ
2.1. Preparation of TS-1
SðPOÞ ¼ nðPOÞ=ðnðPOÞ þ nðMMEÞ þ nðPGÞÞ
ð2Þ
Small-crystal TS-1 was prepared hydrothermally [24,25], using colloidal silica (30 wt.%) and titanium tetrachloride as silicon and titanium sources, respectively. TPABr was used as template and aqueous ethylamine (65 wt.%) as base. The molar composition of the gel was n(SiO2):n(TiO2):n(TPABr):n(C2H5NH2):n(H2O) = 1:0.01– 0.05:0.15:1.5:25. A solution containing nano-sized TS-1 was prepared according to Ref. [14] and used as seeds. The amount of solid nano-sized TS-1 in this solution was 0.07 g/mL, and the amount of this TS-1 solution added to the hydrothermal system was 5– 10 vol.% of the mother liquor of small-crystal TS-1. The order of the addition of raw material was as follows: titanium tetrachloride was first added to isopropyl alcohol, then the obtained mixture was added to colloidal silica. Finally, TPABr, ethylamine, the seed solution and water were added to the above mixture. The final solution was transferred to a Teflon lined autoclave and crystallized for 6–60 h at 170 °C. The solid TS-1 was obtained after purification according to different methods described in Sections 3.1 and 3.2. It was then dried at 100 °C and calcined at 540 °C for 6 h to remove the template. Nano-sized TS-1 (denoted as sample 14) using the improved conventional method [26] was also synthesized for comparison.
UðH2 O2 Þ ¼ ðnðPOÞ þ nðMMEÞ þ nðPGÞÞ=ðn0 ðH2 O2 Þ XðH2 O2 ÞÞ:
ð3Þ
The n0(H2O2) and n(H2O2) represent the initial and final molar content of H2O2, respectively. The n(PO), n(MME) and n(PG) stand for the number of moles of PO, MME and PG, respectively. 3. Results and discussion 3.1. Separation and purification methods With the procedure described in the experimental section, small-crystal TS-1 was obtained after crystallization at 170 °C for 36 h. Due to the small crystal size, it was hard to separate the solid from the mother liquor by filtration. Three methods were explored to remove the mother liquor. The first one was centrifuging the li-
2.2. Characterization of TS-1 XRD patterns were generated on a Rigaku Corporation D/MAX2400 instrument using Cu Ka radiation. FT-IR spectra were recorded on a Bruker EQUINOX55 spectrometer from 4000 to 400 cm1, and the KBr pellet technique was adopted. UV–vis spectra with wave length from 190 to 500 nm were obtained on a Jasco UV-550 spectrometer, and pure BaSO4 was used as reference. UVRaman spectra were recorded on a UV-Raman spectrometer built at the State Key laboratory of Catalysis (Dalian Institute of Chemical Physics, PR China). The morphology of the crystals was detected on a Hitachi scanning electron microscope (SEM). The elemental analysis of TS-1 was carried out on a Bruker SRS3400 X-ray fluorescence spectrometer (XRF) and a Perkin Elmer OPTIMA 2000DV ICP
Fig. 1. XRD patterns of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3).
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Fig. 2. FT-IR spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3).
Fig. 4. UV-Raman spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3). The excitation wave length is 244 nm.
Fig. 3. UV–vis spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3).
quid/solid mixture, and then adding distilled water to the solid and centrifuging again. This operation was repeated until the pH of the separated liquid was 7. The solid obtained after drying and calcination was noted as sample 2-1 (centrifuging separation). The second method was leaving the mixture static for 12 h so that the solid would precipitate by itself. The liquid was decanted and the same amount of distilled water was added to the solid. The decantation and addition of fresh water were repeated two times before the solid was dried and calcined to generate sample 2-2 (precipitating separation). The third method was directly drying the mixture followed by calcination to get sample 2-3. The XRD patterns of the three samples shown in Fig. 1 indicate that they all have the MFI topology with the characteristic peaks at 2h of 7.8°, 8.8°, 23.0°, 23.9° and 24.4°. The relative crystallinity, which was calculated by dividing the total intensity of the characteristic peaks of the samples by that of the standard TS-1, was the lowest (only 86%) for sample 2-3, and was 95% and 99% for samples 2-2 and 2-1, respectively. Washing can remove amorphous substrates such as silica, which decrease the relative crystallinity. Therefore, the different separation methods influence the relative crystallinity and the amount of amorphous substrates essentially.
Fig. 5. UV-Raman spectra of centrifuging separated TS-1 (2-1), precipitating separated TS-1 (2-2) and directly dried TS-1 (2-3). The excitation wave length is 325 nm.
Table 1 Catalytic performance of catalysts prepared by different purification methods for the epoxidation of propylene. Cat.
X(H2O2)/%
S(PO)/%
U(H2O2)/%
2-1 2-2 2-3 2-4 2-5 2-6 1-4
91.7 91.6 20.4 53.3 70.1 91.7 82.0
73.6 97.9 99.9 99.6 99.6 97.5 94.6
95.8 96.9 90.0 89.8 91.4 96.0 93.8
Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.
The relative crystallinity of sample 2-1 was the highest due to the most complete purification.
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Fig. 6. UV–vis spectra of the seven samples with different molar ratio of Si/Ti: n(Si/ Ti) = 20 (3-1), n(Si/Ti) = 25 (3-2), n(Si/Ti) = 45 (3-3), n(Si/Ti) = 50 (3-4), n(Si/Ti) = 55 (3-5), n(Si/Ti) = 60 (3-6) and n(Si/Ti) = 80 (3-7).
Fig. 8. FT-IR spectra of the six samples with weight ratio of crystallization seed/ SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
Table 2 Catalytic performance of the samples with different molar ratio of Si/Ti for the epoxidation of propylene. Cat.
X(H2O2)/%
S(PO)/%
U(H2O2)/%
3-1 3-2 3-3 3-4 3-5 3-6 3-7
82.8 83.2 89.7 91.6 87.5 87.5 86.3
96.1 97.1 97.0 97.9 95.6 82.6 78.4
77.9 88.7 91.1 96.9 96.3 96.4 96.5
Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.
Fig. 9. UV–vis spectra of the six samples with weight ratio of crystallization seed/ SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
Fig. 7. XRD patterns (a) and crystallization curve (b) of the six samples with weight ratio of crystallization seed/SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
Fig. 2 shows the FT-IR spectra of the samples. The bands at 550 and 800 cm1 were assigned to the characteristic bands of the MFI topology [27]. The band at 960 cm1 is considered as the stretching vibration of [SiO4] units strongly influenced by titanium ions in neighboring coordination sites, which is proof of introducing
titanium into the framework [28]. The intensity of this band is related to the amount of framework Ti [29]. Blocking of zeolite channels and covering of framework Ti by organic compounds may cause the decrease of the intensity of this band [30]. In the present study, the covering of active framework Ti by silica and sodium ions had little influence on the band at 960 cm1 in FT-IR spectra, and the intensities of these bands are almost the same for the three samples. The possible reason is that the content of silica in our small-crystal TS-1 is not significant. UV–vis spectroscopy was adopted for detecting the coordination states of Ti (Fig. 3). The band at 210 nm is due to tetrahedrally coordinated Ti (usually called framework Ti), which is considered as the active center for the epoxidation of propylene [31], while the band at 250–280 nm can be assigned to a charge transfer effect in isolated [TiO4] or [HOTiO3] units, such as octahedrally coordinated Ti (usually called non-framework Ti) [32–34]. The band at 310 nm indicates the existence of anatase TiO2 which is deemed to be the catalyst for decomposition of hydrogen
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peroxide [35]. The amount of anatase TiO2 in both samples 2-2 and 2-3 was a little higher than that in sample 2-1, according to the intensity of the band. Apparently, the purification removed some isolated anatase TiO2 with small particle size. UV-Raman spectroscopy is a sensitive tool for examining the chemical environment of metal cations, especially for titanium cations [36]. Figs. 4 and 5 show the UV-Raman spectra of the samples. The wave lengths of excitation lights were 244 and 325 nm. In Fig. 4, the 380 cm1 band is considered as the fingerprint of the MFI structure, while the bands at 490, 530 and 1125 cm1 are assigned to the framework Ti [37]. The ratio of the signal intensity of 1125 cm1 to that of 380 cm1 (I1125/I380), which is 5.0, 4.8 and 3.3 for samples 2-1, 2-2 and 2-3, respectively, can be used as a measurement for the relative content of framework Ti [38]. However, the disparity among the ratios is believed to be due to the various extent of covering of framework Ti by silica and sodium ions, rather than due to the different content of framework Ti (cf. Fig. 3). Washing can hardly transform non-framework Ti or anatase TiO2 to framework Ti, but can remove amorphous substrates,
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which may cover the framework Ti. It is evident that the ratio obtained for sample 2-3 is the smallest among three samples, indicating the lowest catalytic activity of sample 2-3. In Fig. 5, the bands at 390, 516 and 637 cm1 are assigned to the characteristic bands of anatase TiO2. The band at 637 cm1 for sample 2-1 shows a hypochromatic shift, which may be due to an exchange of Na+ by H+ in the purification process, leading to the increase of the amount of hydroxyl groups in TS-1. Elemental analysis of samples 2-1, 2-2 and 2-3 showed that the molar ratio of Si/Ti (n(Si/Ti)) in sample 2-3 (51.2) was higher than those in samples 2-1 (48.1) and 2-2 (48.4). The higher content of amorphous silica in sample 2-3 may be due to the incomplete washing of the mother liquor from the small-crystal TS-1. This could also be the reason that the relative crystallinity of sample 2-3 is the lowest. The deposition of silica on the catalyst can block the channels and cover the active centers, and thus cause a decrease of the catalytic activity. The presence of a small amount of sodium ions, introduced by the silica sol, can neutralize the acid sites on the surface of TS-1, which is helpful for improving the
Fig. 10. SEM images of the six samples with weight ratio of crystallization seed/SiO2 = 0.06 and with different crystallization time: 6 h (4-1), 12 h (4-2), 24 h (4-3), 36 h (4-4), 48 h (4-5) and 60 h (4-6).
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selectivity of the main product PO in the epoxidation of propylene [39,40]. However, if the content of Na+ in the hydrothermal synthesis system is too high, the crystallinity of TS-1 will decrease, and the insertion of titanium into the framework will be hindered. It has been reported that a TS-1 with a large amount of sodium exchange deactivated significantly in the oxidation reaction of alkanes [41]. Therefore, the content of Na+ in the hydrothermal system was controlled strictly in our experiment, but the ions are difficult to be removed completely due to the use of silica sol. The content of Na+ in sample 2-3 (0.81 wt.%) was the highest among the three samples, because it was not purified at all, while in sample 2-1 it was the lowest (0.04 wt.%) due to the complete purification. The crystallinity seems to increase with the decrease of the content of Na+. Nevertheless, we believe that the change of crystallinity is not due to the effect of Na+ but to the amorphous silica on the TS-1 surface. In other words, the low content of Na+ in small-crystal TS-1 hardly influences its crystallinity. The catalytic performance of the samples for the epoxidation of propylene is shown in Table 1. Conversions of H2O2 on samples 2-1 and 2-2 were essentially the same, and were much higher than that on sample 2-3. The low conversion on sample 2-3 is believed to be due to the low crystallinity and the covering of active centers by excessive Na+ (cf. XRD patterns and elemental analysis). A little bit of Na+ in sample 2-2 might lead to a high selectivity of PO (97.9%), while the excessive hydroxyl group in sample 2-1 (UV-Raman spectrum in Fig. 5) could be responsible for the low selectivity of PO. Sample 2-6 was prepared by treating sample 2-1 with dilute Na2CO3 solution, and calcining the sample at 540 °C for 6 h. The content of Na+ in sample 2-6 was 0.10 wt.%, which is similar to that in sample 2-2. The selectivity of PO increased to 97.5% after the treatment, which confirms the discussion above. The utilization of H2O2 for sample 2-3 is the lowest among the three samples because there were more impurities, which accelerates the decomposition of H2O2. Summarily, different purification methods influenced the catalytic performance by changing the amount of impurities, namely, the purity of TS-1. The small-crystal TS-1 with precipitating separation shows the best catalytic performance in propylene epoxidation due to its suitable purity. The low conversion of H2O2 on nano-sized TS-1 (sample 1-4) can be attributed to the low relative crystallinity [24].
Table 3 Catalytic performance of catalysts prepared with different crystallization times for the epoxidation of propylene. Cat.
X(H2O2)/%
S(PO)/%
U(H2O2)/%
4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9 4-10 4-11
17.3 81.9 90.3 91.6 92.2 91.9 82.1 90.1 91.7 91.6 91.3
99.9 90.4 99.7 97.9 98.0 96.7 89.2 96.0 96.8 97.5 97.0
17.3 70.0 85.7 96.9 97.0 95.0 78.8 91.0 93.6 93.7 93.4
Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.
the content of aluminum in the silica sol needs to be controlled strictly. The content of titanium increased slightly after different times of precipitating separation (from 1.52 to 1.60 wt.%). The lost titanium is considered to be isolated anatase TiO2, and most of titanium exists in the framework, which cannot be removed by washing with distilled water. The ratio n(Si/Ti) decreased with increasing washing time, probably due to the loss of amorphous silica in the washing process. The catalytic performance of samples 2-4 and 2-5 is shown in Table 1. The conversion of H2O2 increased with increasing washing time, due to the decrease of the content of amorphous silica and sodium ion in the samples. The selectivity of PO obtained on sample 2-2 is slightly lower than those on samples 2-4 and 2-5. It is believed that losing more sodium ions leads to a higher exposure of acid sites, which may catalyze the side reactions of PO with solvents or itself. The utilization of H2O2 for samples 2-4 and 2-5 was lower than that for sample 2-2, and is similar to that on sample 2-3, because more impurities left in the catalyst results in more serious decomposition of H2O2. In summary, the changing of the residual amount of sodium ions and amorphous silica, either by different purification methods or by different washing times, leads to a change in the properties of small-crystal TS-1.
3.2. Times of precipitating separation In order to further investigate the relationship among crystallinity, sodium ions and the catalytic performance over the smallcrystal TS-1 in the epoxidation of propylene, the effect of precipitating separation times was examined. Sample 2-4 was prepared in the same way as sample 2-2 except that no fresh water was used to wash the crystal solid after the mother liquor was decanted. Another catalyst, sample 2-5, was also synthesized with a similar procedure. The difference between samples 2-4 and 2-5 was that distilled water was added to the solid, which was obtained after the mother liquor was decanted, and was separated by precipitation again. The relative crystallinity increased from 87% for sample 2-4 to 92% for sample 2-5. This increase also proves that washing can remove some amorphous substance which may decrease the relative crystallinity. The content of sodium ion in samples 2-4 and 2-5 decreased with increasing washing time, but the content of aluminum stayed almost constant. Aluminum, which is also introduced by the silica sol, can easily insert into the framework and form strong bonds with framework oxygen. Therefore, it is hard to remove aluminum by washing. The nearly constant content of aluminum (0.08 wt.%) in the samples indicates that most of the aluminum was inserted into the framework. The insertion of aluminum into the framework may generate acid sites [42,43], thus
Fig. 11. XRD patterns of the five samples with weight ratio of crystallization seed/ SiO2 = 0.03 and with different crystallization time: 12 h (4-7), 24 h (4-8), 36 h (4-9), 48 h (4-10) and 60 h (4-11).
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3.3. Molar ratio of Si/Ti Small-crystal TS-1 samples with different n(Si/Ti) were synthesized by adjusting the amount of titanium tetrachloride added to the hydrothermal system. The washing procedure was the same as that used for sample 2-2. The samples with n(Si/Ti) 20, 25, 45, 50, 55, 60 and 80 were denoted as 3-1, 3-2, 3-3, 3-4, 3-5, 3-6 and 3-7, respectively. The peaks in the XRD pattern with 2h from 25.0° to 30.0° were assigned to the signals from anatase TiO2 [44]. There was no clear difference among these peaks of the seven samples (not shown), may be due to the limited sensitivity of XRD to the detection of a small amount of anatase TiO2. Therefore, UV–vis spectroscopy was expected to show some differences. Tetrahedrally coordinated titanium is considered as the active center in the epoxidation of propylene [31], while anatase TiO2 is the catalyst for the decomposition of H2O2 [35]. When a titanium atom, which has a larger size than a silicon atom, inserts into the framework, the lattice will swell, and the lattice constants will increase from a = 20.101 Å, b = 19.877 Å and c = 13.365 Å of silicalite-
111
1 (pure silicon zeolite with MFI topology) to a = 20.111 Å, b = 19.917 Å and c = 13.385 Å of the TS-1 containing 1.1 wt.% Ti [45]. Only a limited amount of titanium can be inserted into the framework. The maximum was reported to be 2.5 wt.% [45,46]. Thus, if the amount of titanium is higher than 2.5 wt.%, the excess Ti will exist as non-framework Ti or anatase TiO2 [47]. The UV–vis spectra of the samples are shown in Fig. 6. When the n(Si/Ti) value was higher than or equal to 60, as for samples 3-6 (60) and 3-7 (80), most of the titanium atoms were in the tetrahedrally coordinated state (characteristic band at 210 nm). Anatase TiO2 (characteristic band at 310 nm) was found in sample 3-5 with n(Si/ Ti) = 55, while other non-framework Ti (characteristic band at 250280 nm) was observed in sample 3-4 with n(Si/Ti) = 50. The amounts of non-framework Ti increased noticeably with the decrease of n(Si/Ti). However, the intensity of the band at 210 nm was hardly affected by the ratio when n(Si/Ti) was below 55. The catalytic performance of the samples with different n(Si/Ti) values in the epoxidation of propylene is shown in Table 2. The conversion of H2O2 increased with decreasing n(Si/Ti) value due to the increase of the number of active centers, until the ratio
Fig. 12. SEM images of the five samples with weight ratio of crystallization seed/SiO2 = 0.03 and with different crystallization time: 12 h (4-7), 24 h (4-8), 36 h (4-9), 48 h (410) and 60 h (4-11).
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3.4.1. Weight ratio of seed/SiO2 = 0.06 The small-crystal TS-1 crystallized with different periods was synthesized with a m(seed/SiO2) value of 0.06. The crystallization
period was 6, 12, 24, 36, 48 and 60 h, and the corresponding samples were marked as samples 4-1, 4-2, 4-3, 4-4, 4-5 and 4-6, respectively. Fig. 7 shows the XRD patterns and the crystallization curve of the samples with different crystallization periods. When the crystallization period was 6 h, the diffraction peaks were very weak (Fig. 7a), and the relative crystallinity was less than 17% (Fig. 7b). Apparently, the MFI framework just starts to form at 6 h. After 12 h crystallization, the characteristic peaks of MFI structure became more obvious, corresponding to a crystallinity of 67%. The relative crystallinity increased continuously with increasing crystallization time. After 36 h, the crystallinity remained constant. FT-IR and UV–vis spectra of the samples are shown in Figs. 8 and 9, respectively. In Fig. 8, the IR bands at 550 and 960 cm1 of sample 4-1 are very weak, because the MFI framework has not been generated, and almost no titanium could be inserted into the framework. This coincides with the XRD pattern in Fig. 7a. The intensity of the band at 550 cm1 increased with increasing crystallization time, due to the gradual formation of the framework. The intensity of the band at 960 cm1 hardly changed after more than 12 h of crystallization, indicating that the insertion of Ti into the framework was accomplished in less than 12 h. The intensity of the UV–vis band at 250–280 nm (Fig. 9) was very strong for sample 4-1. When extending the crystallization period, the intensity decreased significantly. It indicates that isolated [TiO4] was generated in the synthesis gel first, and then these species combined with each other to produce uniform TS-1 crystals. When the crystallization period was prolonged for more than 24 h, the spectra of the obtained samples were similar, because the TS-1 framework had been generated, and the coordination states of titanium had become stable.
Fig. 13. Nitrogen sorption curves of samples 2-1, 2-2 and 4-9.
Fig. 14. Pore size distribution of samples 2-1, 2-2 and 4-9.
reached 50. After that, the conversion began to decline with decreasing n(Si/Ti) value. Addition of excessive Ti to the hydrothermal system might generate too much anatase TiO2, which could cover the active centers or block the channels of TS-1. In this case, the diffusion of the substrates will be hindered, and the conversion of H2O2 will be reduced. With increasing n(Si/Ti), the selectivity of PO improved slightly, until n(Si/Ti) reached 50. The non-framework Ti can cause side reactions of PO. The reduction of non-framework Ti content by the increase of the n(Si/Ti) value is beneficial for the PO selectivity. When n(Si/Ti) continues to increase, the amount of Si–OH may increase, resulting in an increase in the number of acid sites. Therefore, the selectivity of PO decreases. The presence of anatase TiO2 can accelerate the decomposition of the H2O2, and thus with the decrease of n(Si/Ti) value (Fig. 6) the utilization of H2O2 decreases gradually. 3.4. Weight ratios of seed/SiO2 Introduction of seeds into the system can accelerate the crystallization and shorten the period of synthesis [21,22,24]. Moreover, there is not only nano-sized TS-1 in the seeds, but also excessive TPAOH (0.1 mol/L), which may influence the synthesis of smallcrystal TS-1. Therefore, we examined the influence of the weight ratio m(seed/SiO2) on the properties of small-crystal TS-1. Meanwhile, the effects of crystallization period are discussed in this section. The other synthesis conditions were: n(Si/Ti) 50, and for three times precipitating separation washing.
Y. Zuo et al. / Microporous and Mesoporous Materials 162 (2012) 105–114 Table 4 The BET surface area and pore volume of catalysts. Cat. BET area Pore volume Micropore BET Micropore volume External BET (m2/g) (cm3/g) area (m2/g) (cm3/g) area (m2/g) 2-1 451 2-2 459 4-9 456
0.31 0.29 0.28
416 426 423
0.18 0.18 0.18
351 358 326
Fig. 10 shows the SEM images of the six samples. The TS-1 crystal was not generated in the first 6 h (sample 4-1). When the crystallization time was prolonged to more than 12 h (from sample 4-2 to sample 4-6), cubic crystals appeared, and the size was hardly changed, being 600 nm 400 nm 250 nm. As shown in Table 3, the conversion of H2O2 increased with increasing crystallization time from 12 to 48 h. The selectivity of PO on sample 4-2 was only 90.4%, probably due to the low crystallization and a little bit rich of octahedral coordinated Ti (cf. Fig. 9). The utilization of H2O2 increased with increase of the crystallinity, because the content of impurities, such as silica, decreased. The catalytic performance of sample 4-6 (crystallized for 60 h) was slightly low. This may be because the surface of the crystals became smoother and the number of active centers on the surface decreased.
3.4.2. Weight ratio of seed/SiO2 = 0.03 Small-crystal TS-1 with the m(seed/SiO2) = 0.03 was synthesized, characterized and evaluated in the epoxidation of propylene. The samples with the crystallization time of 12, 24, 36, 48 and 60 h were denoted as samples 4-7, 4-8, 4-9, 4-10 and 4-11, respectively. The XRD patterns (Fig. 11a) show that all samples have the MFI topology, indicating that the framework of small-crystal TS-1 has already been formed when the crystallization period reaches 12 h. Fig. 11b shows the crystallization curve of the samples. The relative crystallinity increased with increase of crystallization time until 36 h and then remained constant. The relative crystallinity of the samples with m(seed/SiO2) = 0.03 was higher than those of m(seed/SiO2) = 0.06, when the crystallization time was the same. The larger crystal size of the samples with m(seed/SiO2) = 0.03 (about 800 nm 550 nm 250 nm, cf. Figs. 10 and 12), is considered to be the reason. A lower number of seed needs a longer induction period during crystallization; thus, the obtained crystals will tend to become bigger. The surfaces of these samples were smoother than those of the samples with m(seed/SiO2) = 0.06, and the features are more like those of micro-sized TS-1 [24]. The roughness of the samples 4-4 and 4-9, which was evaluated by average Ra values, were 4.11 and 3.08 nm, respectively, according to the AFM characterization. The amount of TPAOH, introduced by seeds, was considered as a factor to influence the roughness of crystals. The more TPAOH was added to the hydrothermal system, the rougher would the crystal surface be, and vice versa. The external surface of the nano-sized TS-1 was rougher than that of the micro-sized TS-1, as TPAOH was used in the synthesis of former, but TPABr in the latter. The nitrogen sorption isotherms and pore size distribution of samples 2-1 (centrifuging purification), 2-2 (precipitating purification) and 4-9 are shown in Figs. 13 and 14, respectively. There is no significantly difference in the three isotherms. A small hysteresis loop can be observed in each isotherm, due to the inter-crystal space. The main peak in the pore size distribution is at about 0.5 nm, which is the pore of the five member ring in the MFI topology. The total surface area of sample 4-9 is similar to that of samples 2-1 and 2-2 (Table 4), but the external surface area is less, due to the larger crystal size. The micropore volumes of the three samples are almost the same, but the total pore volumes of samples 2-
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1 and 2-2 are larger than that of sample 4-9. In other words, the mesopore (the inter-crystal space) volumes of the samples 2-1 and 2-2 are larger than that of sample 4-9. Catalytic performance of the samples with m(seed/SiO2) = 0.03 for epoxidation of propylene was measured, and the results are listed in Table 3. The conversion of H2O2, the selectivity of PO and the utilization of H2O2 increase with the increase of crystallization period until 36 h, and then remained constant. The crystal surface became smoother, which may cause a decrease of the active centers on the external surface. Therefore, the conversion of H2O2 on the samples with m(seed/SiO2) = 0.03 were a little lower than of the samples with m(seed/SiO2) = 0.06. 4. Conclusions Different conditions for synthesis of small-crystal TS-1 were systematically studied in a TPABr–ethylamine hydrothermal system by using the mother liquor of nano-sized TS-1 as seeds. The size of the obtained TS-1 crystal was about 600 nm 400 nm 250 nm, which was not affected significantly by the synthesis conditions, except for the amount of the additional seed, after the formation of crystals. The small-crystal TS-1 showed an excellent catalytic performance in the epoxidation of propylene (X(H2O2) = 92.2%, S(PO) = 98.0%, U(H2O2) = 97.0%), when the mother liquor was separated by three-time precipitation with a n(Si/Ti) value of 50, a m(seed/SiO2) value of 0.06 and a crystallization period of 48 h. A small amount of sodium ions can improve the selectivity of PO in the epoxidation of propylene, but a large amount of sodium ions will cover the active centers on the TS-1, resulting in a decrease of the catalytic activity. Acknowledgement This work was financially supported by the Program for New Century Excellent Talent in University (NCET-04-0268) and the Plan 111 Project of the Ministry of Education of China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2012. 06.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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