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Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure-directing agent-free gel
Chinese Journal of Catalysis 35 (2014) 1800–1810
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Article
Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure‐directing agent‐free gel Bumei Zheng a, Yufeng Wan a, Weiya Yang b, Fengxiang Ling b, Hong Xie a, Xiangchen Fang b, Hongchen Guo a,* State Key Laboratory of Fine Chemicals, Department of Catalytical Chemistry and Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China b Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun 113001, Liaoning, China a
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 March 2014 Accepted 21 March 2014 Published 20 November 2014
Keywords: Organic structure‐directing agent‐free synthesis Zeolite Beta Seeding Seed residues Dissolved seed fragment
The organic structure‐directing agent‐free synthesis of zeolite Beta was carried out using several zeolite Beta seeds that differed in SiO2/Al2O3 ratio and crystal size. The synthesis was studied using X‐ray diffraction, X‐ray fluorescence, scanning electron microscopy, transmission electron micros‐ copy, ultraviolet‐Raman spectroscopy, infrared spectroscopy, and N2 physisorption. Synthesis was successful using different zeolite Beta seeds including pure silica seeds. During the induction period, the seeds underwent dissolution. The SiO2/Al2O3 ratio and crystal size, pretreatment (calcination), and seed addition time had a significant influence on seed dissolution behavior, crystallization pro‐ cess, and product. Morphological studies revealed that the seed residues produced by dissolution (except for pure silica) resulted in the formation of “immobilized” surface nuclei, which allowed for the dense growth of fresh small zeolite Beta crystals. The dissolved small seed fragments yielded dispersed nuclei, which formed relatively scattered small zeolite Beta crystals thought to be the main nuclei source of the pure silica seed. It is suggested that the use of an appropriately high SiO2/Al2O3 ratio, small size, and precalcined zeolite Beta seed is helpful for the synthesis of highly crystalline and pure zeolite Beta from the organic structure‐directing agent‐free gel. © 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction High‐silica zeolite Beta (BEA) has three‐dimensional inter‐ sected channels and 12‐membered ring pore openings [1,2]. These properties make it useful as a catalyst in aromatic alkyla‐ tion, isomerization, alkene hydration, aromatic acylation, and nitration syntheses in petrochemical industries [3–9]. Formation of the zeolite Beta structure was achieved origi‐ nally using tetraethylammonium hydroxide (TEAOH) as an organic structure‐directing agent (OSDA) [10]. However, its use has increased the zeolite Beta cost significantly, thereby limit‐ ing its industrial application. For this reason, recent studies
related to zeolite Beta have focused mainly on low‐cost synthe‐ sis using more affordable OSDAs such as tetraethylammonium bromide (TEABr) [11–14]. Xie et al. [15] demonstrated for the first time that zeolite Beta could be synthesized by adding cal‐ cined nanocrystalline zeolite Beta seeds into OSDA‐free alumi‐ nosilicate gel. This synthesis route attracted much attention [16–23]. Iyoki et al. [22,23] proposed that the key factor in successful zeolite synthesis in the absence of OSDA was the common composite building unit contained in both the seeds and zeolite obtained from the gel after heating without seeds. Majano et al. [16] showed that zeolite Beta could be obtained at low crystallization temperature with non‐calcined seeds. They
* Corresponding author. Tel/Fax: +86‐411‐84986120; E‐mail: [email protected] DOI: 10.1016/S1872‐2067(14)60089‐9 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 11, November 2014
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
believe that the active surface of the as‐synthesized seeds pro‐ moted crystallization while calcination of the seeds would be harmful because of seed surface deactivation. Kamimura et al. [17,18] reported that zeolite Beta synthesized with OSDA‐free gel could be used as a seed to generate more zeolite Beta. They concluded that the seed was partially dissolved during the ini‐ tial stage in the hydrothermal environment and that subse‐ quent crystallization took place on the surface of the residual Beta seeds exposed to the liquid. They also observed that zeo‐ lite Beta with SiO2/Al2O3 ratio greater than 52 could not be used as seeds because of their complete dissolution in the ini‐ tial stage in the hydrothermal environment. Xie et al. [19] showed that the residues partially dissolved Beta seeds rather than the dissolved species of Beta seeds helped crystallization. They proposed a core–shell growth mechanism to describe the crystallization of OSDA‐free gel on the surface of partially dis‐ solved Beta seeds. Although it is well known that seeding is useful for the syn‐ thesis of many zeolites, the mechanism by which this occurs has been interpreted in various ways: some believe that seeds behave as crystal nuclei supplying a surface for the crystalliza‐ tion of amorphous aluminosilicate gel; others suggest that the seeds dissolve partially and then the residual seed, or dissolved species, or both form nuclei [23,24]. An investigation into the seeding mechanism therefore holds significance not only for OSDA‐free synthesis of zeolite Beta but also for improving syn‐ thesis techniques of other zeolites. In this work, we carried out a detailed study on the OSDA‐free synthesis of zeolite Beta using different seeds. X‐ray diffraction (XRD), field emission‐scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT‐IR), and ultraviolet (UV)‐ Raman spectroscopy were used to characterize the solid prod‐ ucts obtained before and during the synthesis. Emphasis was given to the structure‐directing effect of different seeds, key aspects of the seed that influence the structure‐directing effect, and the seeding mechanism.
2. Experimental 2.1. Synthesis of zeolite Beta seeds Al‐containing zeolite Beta seeds were synthesized as fol‐ lows. First, silica sol (SiO2, 30 wt%, Qingdao Haiyang Chemical Co.) or fumed silica (Shenyang Chemical Co.) was mixed with OSDA solutions, which included a TEAOH solution (20 wt%, Hejian Qingfeng Shimian Chemical Co.), TEABr solution (99 wt%, Jintan Huadong Chemical Research Institute), ammonia and NaOH, or solution of TEABr and TEAOH. A NaAlO2 solution (Al2O3, 41 wt%, Sinopharm Chemical Reagent Co.) was added to the mixture with vigorous stirring for 40 min. The mixture was transferred to an autoclave and crystallized at 145 °C un‐ der static or stirred conditions for ~3–6 d. Pure silica zeolite Beta seed was synthesized as follows. Fumed silica was mixed with the TEAOH solution (40 wt%, condensed in laboratory). Next, NH4F was added and the mix‐ ture stirred thoroughly. The sticky gel obtained was placed in a
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Teflon‐lined stainless steel autoclave and crystallized at 150 °C for 4 or 5 d. All solid products were filtered, washed, dried overnight at 110 °C, and calcined in air at 540 °C for 6 h to remove the OSDA before they were used as seeds. 2.2. Seed‐assisted synthesis of OSDA‐free zeolite Beta A gel with molar composition Na2O:Al2O3:SiO2:H2O = 11.4:1:40:640 was prepared as follows. Fumed silica was dis‐ solved in the NaOH solution before addition of the NaAlO2 solu‐ tion under vigorous stirring. Calcined zeolite Beta was then added as seed (10 wt% in relation to silica source). The mix‐ tures were agitated to form a homogenous gel. The gel was transferred into a 100‐ml autoclave and crystallized statically at 140 °C. Sampling was conducted at time intervals. The solid product was isolated from the resultant mixture by filtration, washed with deionized water, and dried at 110 °C for charac‐ terization. 2.3. Characterization XRD patterns were collected on a Rigaku D/max‐2004 dif‐ fractometer using Cu Kα radiation (40 kV, 100 mA) and a scan‐ ning rate of 0.02°/min (2θ). The crystallinity of the obtained Beta phase was calculated based on the intensity of the peak at reflection (3 0 2) (2θ = ~22.4°). FE‐SEM micrographs were recorded on a Hitachi S‐4800 microscope. TEM micrographs were obtained on a JEOL JEM‐2100 electron microscope operating at 200 kV with a point resolution of 0.23 nm. TEM samples were dispersed in ethanol and a droplet of this mixture was deposited on a Cu grid. FT‐IR spectra in the hydroxyl region were obtained on a Ni‐ colet iS10 FTIR instrument according to the following proce‐ dure. The sample was pressed into a self‐supporting wafer and placed into a quartz IR cell with CaF2 windows. The FT‐IR spec‐ tra were recorded with a resolution of 4 cm−1 at room temper‐ ature. Prior to measurement, the sample was dehydrated under vacuum at 400 °C for 4 h. UV‐Raman spectra were recorded on a DL‐2 Raman spec‐ trometer. The 244‐nm line of a LEXEL LASER was used as the excitation source. An Acton triple monochromator was used as a spectrometer for Raman scattering. Spectra were collected using a Prinston charge‐coupled device detector. The power of the laser line at the sample was below 3 mW. Elemental analyses were performed using a Bruker SRS 3400 X‐ray fluorescence spectrometer. N2 adsorption‐desorption isotherms were recorded at –196 °C on a Micromeritics ASAP 2000 instrument after activating the sample under vacuum at 350 °C for at least 6 h. The surface area was evaluated using the BET method. The external surface area, micropore surface area, and micropore volume were cal‐ culated using the t‐plot method. The total pore volume was determined from the amount adsorbed at a relative pressure of approximately 0.99, and the mesopore volume was calculated from the difference between the total and micropore volume.
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3. Results and discussion 3745 3735
3.1. Characterization of zeolite Beta seeds
S10 S9 S8
3610 Absorbance
S7 S6 S5 S4 S3 S2 S1 3800
3700
3600
3500
3400
Wavenumber (cm1)
Fig. 1. Hydroxyl vibration FT‐IR spectra of zeolite Beta seeds S1 to S10.
phenomenon confirms that seeds S1 to S8 were Al‐containing zeolites and seeds S9 and S10 were pure silica zeolites. The intensities of the 3745 cm−1 band were stronger for seeds S1 to S6, weaker for seeds S7 and S8, and very weak for seeds S9 and S10. This agrees with the fact that seeds S1 to S6 were small crystals while seeds S7 to S10 were large crystals. The UV‐Raman spectra of the seeds are shown in Fig. 2. The most intense Raman scattering of the zeolite Beta framework vibrations appear mainly from 310 to 470 cm−1. According to the literature [28−31], the peaks at 314 and 343 cm−1 should be 314
343 399 465 425 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1
Intensity
To obtain different zeolite Beta seeds, the synthesis of zeo‐ lite Beta was carried out in Al‐containing and Al‐free gels with different silica sources and different OSDA solutions. The re‐ sultant seeds were investigated and the main crystal data are summarized in Table 1. Seeds S1 to S6 were less than 200 nm in crystal size with relatively larger mesopore volume owing to aggregation of small crystals. However, they were different in terms of SiO2/Al2O3 ratio and relative crystallinity. Both the SiO2/Al2O3 ratio and relative crystallinity increased from seed S1 to S6. Seeds S7 and S8 were synthesized in the same gel but with different crystallization times. Both were approximately 3‐μm spherical crystals and their SiO2/Al2O3 ratios were close to that of seed S5. However, the relative crystallinity of S8 was higher than that of S7 because it was synthesized over longer time. Seeds S9 and S10 were also synthesized in the same gel, with S10 being crystallized over longer time but seeds S9 and S10 were pure silica zeolite Beta approximately 9 μm in crystal size and with truncated octahedral morphology. It is worth noting that the relative crystallinity of the seeds calculated from XRD and micropore volume (Vmicro) data was almost the same for S1 to S6 but different for S7 to S10. Differences in seeds S7 to S10 may be caused by crystal size and lattice im‐ perfections [25,26]. In seeds S7 and S8, the microcrystals were composed of nanocrystals while in seeds S9 and S10, the large crystals were all monocrystals. The crystal size may result in weak XRD peaks in S7 and S8 and strong ones in S9 and S10. FE‐SEM micrographs also show that seed S10 and especially seed S9 had high‐density wormhole defects (not shown), which may result in a decrease of micropore volume for S9 and S10. The OH vibration FT‐IR spectra are shown in Fig. 1. Gener‐ ally, the typical zeolite Beta may exhibit six bands, including hydroxyls on a partially hydrolyzed framework Al (3783 cm−1), terminal SiOH (3745 cm−1), internal SiOH at framework defects (3735 cm−1), hydroxyls associated with extra framework Al (3665 cm−1), bridging hydroxyls (3610 cm−1), and hydrox‐ yl‐bonded hydroxyls (broad band at 3400–3700 cm−1) [25,27]. It can be seen from Fig. 1 that seeds S1 to S8 had bridging hy‐ droxyl peaks at 3610 cm−1, while seeds S9 and S10 did not. This
200
400 600 Raman shift (cm1)
800
Fig. 2. UV‐Raman spectra of zeolite Beta seeds S1 to S10.
Table 1 Synthesis conditions and physicochemical characteristics of zeolite Beta seeds. Sample
Synthetic description
Silica source Template S1 silica sol TEAOH S2 silica sol TEABr S3 silica sol TEAOH S4 silica sol TEABr S5 silica sol TEAOH S6 silica sol TEAOH S7 fumed silica TEAOH+TEABr S8 fumed silica TEAOH+TEABr S9 fumed silica TEAOH S10 fumed silica TEAOH a Calculated on the basis of S6.
State stirred stirred stirred stirred stirred stirred static static static static
Crystal size (nm) <100 <100 <100 <100 150 150 3000 3000 8500 9000
SiO2/Al2O3 molar ratio 11 12 23 22 31 38 30 29 ∞ ∞
Sample description Pore volume (cm3/g) Vmicro Vmeso 0.15 0.21 0.17 0.20 0.20 0.28 0.18 0.35 0.20 0.20 0.23 0.25 0.16 0.08 0.23 0.05 0.16 0.07 0.17 0.07
Crystallinity a (%) XRD Vmicro 66 65 75 74 70 87 84 78 87 87 100 100 49 70 86 100 221 70 224 74
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assigned to five‐membered rings, those at 399 and 425 cm−1 to six‐membered rings, and that at 465 cm−1 to four‐membered rings. The peaks at 314, 399, and 465 cm−1 belonged to the rings in the periodic building unit, while the peaks at 343 and 425 cm−1 belonged to the interlayer connectivity mode for polymorphs. Therefore, the intensity ratio of peaks at 314 and 343 cm−1 indicated the presence of interlayer connecting faults and the larger value indicated fewer stacking faults. It can be seen that seeds S1 to S8 had a larger but comparable amount of stacking faults while seeds S9 and S10 had the least. 3.2. Template‐free synthesis of zeolite Beta with different seeds The seeds played a crucial role in the formation of the zeo‐ lite Beta structure in the OSDA‐free gel because no beta phase could be obtained without seed. Beta zeolites were synthesized by adding different seeds as indicated in Table 2. However, when seeds S1 to S5 were used, the synthesized products (BEA1 to BEA5) showed low zeolite Beta crystallinity. When seeds S6 to S10 were used, the synthesized products (BEA6 to BEA10) showed good zeolite Beta crystallinity. For seeds S1 to S6, which had a similar crystal size (less than 150 nm) but in‐ creasing SiO2/Al2O3 ratio, the crystallinity of the synthesized products increased from 16% to 89%. This phenomenon im‐ plies that the SiO2/Al2O3 ratio is a very important parameter for zeolite Beta seeding. To carry out the successful OSDA‐free synthesis of zeolite Beta, it is better to use zeolite Beta seed with a SiO2/Al2O3 ratio of more than 30. Successful synthesis with pure silica seeds (S9 and S10) in this study implies the possibility of a much higher SiO2/Al2O3 ratio range existing for zeolite Beta seed selection. This opinion is different from pre‐ vious research that has succeeded in the OSDA‐free synthesis of zeolite Beta using zeolite Beta seeds with SiO2/Al2O3 ratios of between 20 and 52 [15−20]. According to this research, seeds with SiO2/Al2O3 ratio greater than 52 could not provide a beta phase because of complete dissolution. We assume that the dissolution behavior of the high‐silica zeolite Beta seed during the OSDA‐free synthesis of zeolite Beta may be influenced by its morphological and constructional features, including crystal size and crystal defects. The crystal size of zeolite Beta seeds affected the crystal size of the synthesized zeolite Beta product. As indicated by Table 2
Fig. 3. Effect of seeds on the morphology of zeolite Beta synthesized in OSDA‐free gels.
and Fig. 3, although the zeolite Beta products obtained with different seeds had a similar appearance (intergrown crystals with truncated octahedral morphology), the increase in crystal size of the zeolite Beta seeds resulted in an obvious increase in crystal size of the zeolite Beta product. It is also interesting to note from Fig. 3 that the residual bodies of the larger crystal seeds remained in the obtained zeolite Beta products (BEA8‐b and BEA10‐b). For the Al‐containing large crystal seed (S8), there was an obvious growth of zeolite Beta product on the seed residue surface, as indicated by the crowded small multi‐ dimensional faces on the big seed residue (BEA8‐b). For the pure silica large crystal seed (S10), however, no growth of zeo‐ lite Beta product on the seed residue could be seen. The residue was round and relatively smooth. Despite these influences, the zeolite Beta products obtained with different seeds all had low but roughly the same
Table 2 Influence of seeds on template‐free synthesis of zeolite Beta. Sample a
Seed
Size (nm)
BEA1 S1 — BEA2 S2 — BEA3 S3 — BEA4 S4 100−150 BEA5 S5 150−200 BEA6 S6 150−200 BEA7 S7 300−700 BEA8 S8 300−700 BEA9 S9 450−950 BEA10 S10 450−950 a Without other crystal phases. b Calculated on the basis of BEA9.
SiO2/Al2O3 molar ratio 11 11 11 10 10 10 10 10 10 10
Pore volume (cm3/g) Vmicro Vmeso — — — — — — — — 0.18 0.06 0.21 0.08 0.20 0.06 0.20 0.05 0.21 0.04 0.20 0.04
Crystallinity b (%)
Yield (%)
16 19 49 60 64 89 93 94 100 97
— — — — — 32.5 32.3 32.6 — 32.4
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SiO2/Al2O3 ratios. The same SiO2/Al2O3 ratio (approximately 10) as well as similar morphology (intergrown crystals with truncated octahedral morphology) imply that the zeolite Beta products obtained in the OSDA‐free gels were mainly defined by special hydrothermal conditions. Figure 4 shows the crystallization curves of OSDA‐free gels with different zeolite Beta seeds while the inset shows the solid product yield of zeolite Beta at the early stages of crystalliza‐ tion. The decline in zeolite Beta yield at the early stages of crystallization indicates that all seeds were partially dissolved in the induction period. The decline in zeolite Beta yield for BEA9 and BEA10 indicates that the pure silica seeds could be dissolved to a high degree. The different zeolite Beta seeds in‐ fluenced the crystallization process in the OSDA‐free gels con‐ siderably. First, the duration of the induction period was relat‐ ed mainly to the crystal size of the zeolite Beta seeds. The smaller seeds tended to have a shorter induction period while the larger seeds tended to have a longer induction period (Fig. 4(b)). This may be because the smaller seeds generally dis‐ solved faster and so a shorter time was needed for the genera‐ tion of zeolite Beta nuclei. Second, the growth rate of the zeolite Beta crystal seemed to relate to both the SiO2/Al2O3 ratio and crystal size of the zeolite Beta seeds. By comparing the crystal‐ lization curves of BEA1 to BEA6, which were seeded with zeo‐ lite Beta (S1 to S6) of largely similar crystal size but different SiO2/Al2O3 ratio, it could be seen that the crystal growth rate as a whole increased moderately with SiO2/Al2O3 seed ratio (Fig. 4(a)). By comparing the crystallization curves of BEA6 to BEA8, which were seeded with zeolite Beta (S6 to S8) of similar SiO2/Al2O3 ratio but different crystal size, it was found that the crystal growth rate decreased significantly as the seed crystal size increased (Fig. 4(b)). Compared with the SiO2/Al2O3 ratio, the crystal size of the seeds had a significant effect on the zeo‐ lite Beta crystal growth rate. The influence of crystal size of seeds on the crystal growth rate of zeolite Beta may be at‐ tributed to the number of nuclei present. The number of nuclei generated by the different seeds is supposed to decrease with increase in crystal size of zeolite Beta. This assumption is con‐ sistent with SEM observations, which indicated that an increase in crystal size of zeolite Beta seeds resulted in an increase in 100
3.3. Structure‐directing behavior of different zeolite Beta seeds 3.3.1. Seeding in fresh gel 3.3.1.1. Calcined seeds OSDA‐free synthesis of zeolite Beta was carried out with calcined seed S7. After calcination, the OSDA molecules in the seed pore channels were removed. The solid products obtained before and during crystallization were characterized by XRD, FT‐IR, and FE‐SEM. Figure 5 shows that the gel‐containing seed S7 (10 wt%) had a weak diffraction peak of zeolite Beta at 2θ = 22.48°. Dur‐ ing the induction period (0–42 h), this diffraction peak shifted to lower 2θ values, which suggests an increase in Al content in the crystal framework (Al−O bond length is larger than that of Si−O). Because the zeolite Beta yield declined in the induction period (Fig. 4(b)), we tend to attribute this peak shift phenom‐ enon to the dissolution of a Si‐rich area of seeds under basic hydrothermal conditions. After the induction period, the gel began to crystallize. This peak shifted slowly to 2θ = 22.14° and increased in intensity. This indicates that Al‐rich zeolite Beta 100 (b)
BEA1 BEA2 BEA3 BEA4 BEA5 BEA6
60
80
Crystallinity (%)
80
10 Yield (%)
40 20
5 0 0
0 4
8
12 Time (h)
16
60
10
40 20
2
4
Time (h)
0
BEA7 BEA8 BEA9 BEA10 BEA6
Yield (%)
(a)
Crystallinity (%)
crystal size of zeolite Beta product. Third, the different zeolite Beta seeds also had a strong influence on the stability of the zeolite Beta product under hydrothermal synthesis. For seeds S1 and S2 (corresponding to the BEA1 and BEA2 curves), crys‐ tallization ceased at approximately 7 h. Crystal transformation from zeolite Beta to mordenite then took place. Therefore, only low crystallinity zeolite Beta products (lower than 20%) were obtained with these seeds. For seeds S3 to S6 (corresponding to the BEA3 to BEA6 curves), crystallization was extended to 15 to 17 h, and the crystallinity of the zeolite Beta products in‐ creased to approximately 50% to 90%. For seeds S7 to S10 (corresponding to the BEA7 to BEA10 curves), a much longer crystallization time was allowed, and the crystallinity of the zeolite Beta products could be increased to more than 90% before crystal transformation occurred (crystal transformation caused a drop‐off in crystallinity in the curves). Therefore, in the OSDA‐free synthesis of zeolite Beta, the high‐silica zeolite Beta seed favors the stability and crystallinity of the zeolite Beta product.
20
5 0 0
0 24
0
20
40
60
80 100 Time (h)
Fig. 4. Effect of seeds on crystallization kinetics of zeolite Beta in OSDA‐free gels.
120
10 20 Time (h)
140
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3614 3745
77 h 65 h 42 h
Absorbance
Intensity
77 h 65 h 42 h 26 h 18 h 7h 4h 2h 0h
26 h 18 h 7h 4h 2h 0h
S7
5
10
15
20
25
30
35
2/( ) o
3800
3700 3600 Wavenumber (cm1)
3500
3400
Fig. 5. XRD patterns of seed S7 and solid samples obtained with S7‐ containing gel.
Fig. 6. Hydroxyl vibration FT‐IR spectra of solid samples obtained with S7‐containing gel.
was obtained. Figure 6 shows that the initial gel had a strong signal at 3745 cm−1, which is attributed to the terminal SiOH of the gel particles. During the induction period, this band became weak. This change may be an indication of the digestion (or rear‐ rangement) of the amorphous gel under hydrothermal condi‐ tions. When the crystallization time was more than 42 h, the solid product showed a new strong absorption peak at 3614 cm−1, which is assigned to bridging hydroxyls. This indicated that a great deal of Al entered the framework during the crystal growth process, which agrees well with the peak shift in the XRD analysis. Figure 7 shows that in the induction period ((b)–(g)) two important changes occurred to the gel: one was the dissolution of seeds, which seemed to start at the exposed surface and de‐
velop towards the inside of the seed balls and the other was the softening of the graining fresh gel and the covering of the ten‐ der gel on the seed balls. By the end of the induction period (Fig. 7(f)–(g)), sprout‐like crystals could be seen penetrating the gel cover from the surface of the inner seed balls. The well‐crystallized product was a mixture of large and small crystals. The small crystals were intergrown with truncated octahedral morphology. The surface of the large crystals was heavily populated with fresh and small zeolite Beta crystals. The OSDA‐free synthesis of zeolite Beta was also studied with other seeds. The TEM micrographs related to the crystal‐ lization of seed S3‐ and S6‐containing gels are shown in Fig. 8(a,b), and the SEM micrographs related to the crystallization of seed S9‐ and S10‐containing gels are shown in Fig. 8(c,d). SEM characterization was used for the larger crystal samples as
Fig. 7. FE‐SEM micrographs of solid samples obtained with S7‐containing gel.
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the TEM characterization was unsuccessful. Figure 8 shows that seed dissolution occurred in all cases except for the crystallization of seed S3‐containing gel. The results for seed S3‐containing gel may arise because of the low extent of dissolution and small crystal size of seed S3, as the dissolution of seed S3 has been confirmed by the decline of zeolite Beta yield during the induction period (Fig. 4(a)). The situation is similar for seeds S1 to S4. The initial gels under‐ went a digestion (or rearrangement), which made the amor‐ phous gels soft and sticky, and tender gels covered the seeds. However, differences did exist between the seeds. For example, for most of the Al‐containing seeds (such as seed S6), dissolu‐ tion tended to take place inside the seeds, which resulted in the formation of hollow seed residues. In subsequent crystalliza‐ tion, the hollow seed residue surface served as substrate for the growth of fresh and small zeolite Beta crystals. In contrast, for Al‐free large seeds (such as seeds S9 and S10), dissolution oc‐ curred on the outside of the seeds, which resulted in their dis‐ integration and the formation of smaller rounded residues. In
subsequent crystallization, there was no visible growth of the fresh and small zeolite Beta crystals on the surface of the rounded seed residues. Based on these observations, we believe that the dissolution of the seeds in the OSDA‐free synthesis of zeolite Beta is a common phenomenon although the degree of dissolution and behavior may be different because of differences such as SiO2/Al2O3 ratio and crystal size of the seeds. The seed residues produced by dissolution assumed the structure‐directing role in the OSDA‐free synthesis of zeolite Beta. In this case, the role of the seed residues is to provide an “immobilized” surface nucleus. The pure silica seed residues, at least, seem to be an exception. The dissolved seed fragments also have an im‐ portant role in the OSDA‐free synthesis of zeolite Beta, as evi‐ denced by the appearance of abundant small crystals in the crystallization products of the large seed‐containing gels. The dissolved seed fragments can definitely serve as nuclei. For the pure silica seeds (seeds S9 and S10), the dissolved seed frag‐ ments are the main nuclei source. The seeding mechanism is
Fig. 8. TEM (a,b) and FE‐SEM (c,d) micrographs of solid samples obtained with S3‐ (a‐1,2,3), S6‐ (b‐1,2,3), S9‐ (c‐1,2,3), and S10‐ (d‐1,2,3) containing gels.
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
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100
Calcined Non-calcined
80 60
20 Yield (%)
different from that reported to date in the literature. As stated above, Kamimura et al. [18] have concluded that the crystalli‐ zation of zeolite Beta from the OSDA‐free gel took place on the surface of the residual Beta seeds, and that the seed residues should be exposed to the liquid. Xie et al. [19] have shown that it was the residue of partially dissolved Beta seeds rather than the dissolved species of Beta seeds that assisted in the crystal‐ lization. 3.3.1.2. Non‐calcined seeds In the non‐calcined seeds, the pore channels of the zeolite Beta seeds were expected to be occupied by OSDA molecules. A previous study has shown that the calcined zeolite Beta seed with SiO2/Al2O3 ratio of 52 was unsuccessful in zeolite Beta synthesis from OSDA‐free gel but that the same zeolite Beta seed without calcination was successful [16]. When we inves‐ tigated the influence of pre‐calcination with seed S6, it was found that both the calcined and non‐calcined zeolite Beta seeds led to successful zeolite Beta synthesis. However, the gel‐containing calcined seed showed a higher degree of seed dissolution, shorter induction period, and higher crystal growth rate (Fig. 9). In contrast with the significant inner dissolution of the calcined seed (Fig. 8(a,b)), the non‐calcined seed did not show a significant dissolution in the induction period (Fig. 10). These results were different from the previous study, which showed that the active surface of as‐synthesized seeds pro‐ moted crystallization while the calcination of seeds would be harmful because of the deactivation of the seed surface [16]. Interestingly, for the gel‐containing calcined seed, the crystalli‐
Crystallinity (%)
40 20
10 0 0
4 8 Time (h)
0 0
4
8
12
16
20
24
28
32
Time (h) Fig. 9. Effect of calcined and non‐calcined seed S6 on crystallization kinetics of zeolite Beta.
zation products (from 7 to 16 h) gave a singlet characteristic peak of zeolite Beta around 2θ = 22.14° (Fig. 11(a)). For the gel‐containing non‐calcined seed, however, the crystallization products (from 20 to 33 h) gave a doublet characteristic peak of zeolite Beta in the same region (Fig. 11(b)). The latter case provides evidence of the formation of the core‐shell structure of zeolite Beta. The core was the zeolite Beta seed S6 with higher SiO2/Al2O3 ratio, while the shell was the new zeolite Beta with lower SiO2/Al2O3 ratio. These results indicate that pretreatment is an important factor for influencing the struc‐
Fig. 10. TEM micrographs of solid samples obtained at early stages of crystallization with non‐calcined seed S6 addition gels (arrows indicate the seeds).
(b)
(a)
33 h 30 h
Intensity
Intensity
16 h
15 h
27 h 23 h 20 h 7h 4h
11 h 7h 4h 2h 1h 0h
5
10
15
20 2/(o)
25
30
2h
0h 35
5
10
15
20 2/(o)
25
30
35
Fig. 11. XRD patterns of solid samples obtained at different crystallization times with calcined (a) and non‐calcined (b) seed S6 addition gels.
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Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
large crystals. Seeding an aged gel also yielded many small crystals. Once again, this provides evidence for the struc‐ ture‐directing effect of dissolved seed fragments.
100 Fresh gel Aged gel
4. Conclusions 60 40
Yield (%)
Crystallinity (%)
80
20 0 0
20
40
60 Time (h)
4 2 0 0
10 20 Time (h) 80
100
Fig. 12. Effect of gel aging on crystallization kinetics of OSDA‐free gel‐ containing seed S7.
ture‐directing performance of zeolite Beta seed. 3.3.2. Seeding in aged gel Compared with seeding in a fresh gel, when the same zeolite Beta seed (S7) was added into the aged gel (prehydrothermal treatment of fresh gel for 7 h at the crystallization tempera‐ ture), more seed was dissolved and the seeding resulted in a shorter induction period and faster crystal growth (Fig. 12). The SEM micrographs (Fig. 13) show that during aging the fresh gel became fused and viscous ((a)–(c)), as was seen in the induction period of the foregoing experiments (Fig. 7). The difference is that when the seed was added into the aged gel, the viscous gel did not envelop the seed. This may be why the seed dissolved more rapidly and in higher quantities. In this case, seed dissolution did not result in the formation of hollow balls as was the case for seeding in a fresh gel (Fig. 7), but ap‐ peared to take place on the surface of the seed. At the end of the crystallization, the surface of the seed residues was heavily populated with small fresh zeolite Beta crystals, which formed
The synthesis of zeolite Beta with OSDA‐free gel was achieved using several zeolite Beta seeds, which differed main‐ ly in SiO2/Al2O3 ratio and crystal size. However, the different zeolite Beta seeds influenced the crystallization process and product significantly. As the seed SiO2/Al2O3 ratio increased, the induction period was shortened and crystal growth rate was enhanced. However, the effect of the seed crystal size was the opposite and much stronger. The seed with high SiO2/Al2O3 ratio was favorable for the synthesis of zeolite Beta with high crystallinity, while the seed with large crystal size resulted in an increase in product crystal size. Nevertheless, different zeo‐ lite Beta seeds all produced zeolite Beta with similar crystal morphology and SiO2/Al2O3 ratio. This indicates that the zeolite Beta products obtained in the OSDA‐free gels were mainly de‐ termined by special hydrothermal conditions. In the induction period, the different zeolite Beta seeds all underwent dissolu‐ tion. The increase of seed SiO2/Al2O3 ratio enhanced the extent of dissolution of the seed, while the increase in seed crystal size reduced the dissolution speed. Compared with the as‐ synthe‐ sized seed, precalcined seeds dissolved faster and in higher quantities. Dissolution of the seed commenced on the bare sur‐ face of the seed. However, when this surface was enveloped by the sticky gel, the dissolution of the seed likely developed from the tiny bare surface into the inside of the seed, thus forming a hollow seed ball. Morphology studies showed that the seed residues produced by dissolution could provide an “immobi‐ lized” surface nucleus, which allowed for the dense growth of fresh small crystals of zeolite Beta although the residues of pure silica seed were an exception. Small seed fragments pro‐ duced by dissolution could provide dispersed nuclei that re‐ sulted in the formation of relatively scattered small zeolite Beta
Fig. 13. FE‐SEM micrographs of solid samples obtained before (a,b,c) and after (d,e,f) seeding (seed S7).
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
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Graphical Abstract Chin. J. Catal., 2014, 35: 1800–1810 doi: 10.1016/S1872‐2067(14)60089‐9 Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure‐directing agent‐free gel Bumei Zheng, Yufeng Wan, Weiya Yang, Fengxiang Ling, Hong Xie, Xiangchen Fang, Hongchen Guo * Dalian University of Technology; Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC
The nature of zeolite Beta seeds affected the organic structure‐directing agent‐free synthesis of zeolite Beta. The seeds dissolved during the induction period and the dissolved fragments were involved in the formation of zeolite Beta.
crystals. This may be the main structure‐directing technique of the pure silica zeolite Beta seed. The use of precalcined zeolite Beta seed with smaller crystal size and appropriately high SiO2/Al2O3 ratio will be useful in the crystallization of zeolite Beta from OSDA‐free gel with high crystallinity and purity. References [1] Newsam J M, Treacy M M J, Koetsier W T, De Gruyter C B. Proc R [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Soc Lond A, 1988, 420: 375 Higgins J B, LaPierre R B, Schlenker J L, Rohrman A C, Wood J D, Kerr G T, Rohrbaugh W J. Zeolites, 1988, 8: 446 Varanasi S R, Kumar P, Puranik V R, Umarji A, Yashonath S. Microporous Mesoporous Mater, 2009, 125: 135 Nur H, Ramli Z, Efendi J, Rahman A N A, Chandren S, Yuan L S. Catal Commun, 2011, 12: 822 Wu Y H, Tian F P, Liu J, Song D, Jia C Y, Chen Y Y. Microporous Mesoporous Mater, 2012, 162: 168 Batalha N, Morisset S, Pinard L, Maupin I, Lemberton J L, Lemos F, Pouilloux Y. Microporous Mesoporous Mater, 2013, 166: 161 Horáček J, Šťávová G, Kelbichová V, Kubička D. Catal Today, 2013, 204: 38 Mihályi R M, Lónyi F, Beyer H K, Szegedi Á, Kollár M, Pál‐Borbély G, Valyon J. J Mol Catal A, 2013, 367: 77 Mu X H, Wang D Z, Wang Y R, Lin M, Cheng S B, Shu X T. Chin J Catal, 2013, 34: 69 Wadlinger R L, Kerr G T, Rosinski E J. US Patent 3 308 069. 1967 Caullet P, Hazm J, Guth J L, Joly J F, Lynch J, Raatz F. Zeolites, 1992, 12: 240 Eapen M J, Reddy K S N, Shiralkar V P. Zeolites, 1994, 14: 295 Mostowicz R, Testa F, Crea F, Aiello R, Fonseca A, Nagy J B. Zeolites, 1997, 18: 308 Qi X L, Liu X Y, Lin B X. Chin J Catal, 2000, 21: 75 Xie B, Song J W, Ren L M, Ji Y Y, Li J X, Xiao F S. Chem Mater, 2008, 20: 4533
[16] Majano G, Delmotte L, Valtchev V, Mintova S. Chem Mater, 2009,
21: 4184 [17] Kamimura Y, Chaikittisilp W, Itabashi K, Shimojima A, Okubo T.
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Shimojima A, Okubo T. J Phys Chem C, 2011, 115: 744 [19] Xie B, Zhang H Y, Yang C G, Liu S Y, Ren L M, Zhang L, Meng X J,
Yilmaz B, Müller U, Xiao F S. Chem Commun, 2011, 47: 3945 [20] Zhang H Y, Xie B, Meng X J, Müller U, Yilmaz B, Feyen M, Maurer S,
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Gies H, Tatsumi T, Bao X H, Zhang W P, Vos D D, Xiao F S. Microporous Mesoporous Mater, 2013, 180: 123 Iyoki K, Itabashi K, Okubo T. Chem Asian J, 2013, 8: 1419 Itabashi K, Kamimura Y, Iyoki K, Shimojima A, Okubo T. J Am Chem Soc, 2012, 134: 11542 Iyoki K, Itabashi K, Okubo T. Microporous Mesoporous Mater, 2014, 189: 22 Cundy C S, Cox P A. Microporous Mesoporous Mater, 2005, 82: 1 Camblor M A, Corma A, Valencia S. Microporous Mesoporous Mater, 1998, 25: 59 Xu R R, Pang W Q. Chemistry: Zeolites and Porous Materials. Beijing: Science Press, 2004. 130 Kiricsi I, Flego C, Pazzuconi G, Parker W O Jr, Millini R, Perego C, Bellussi G. J Phys Chem, 1994, 98: 4627 Blasco T, Camblor M A, Corma A, Esteve P, Guil J M, Martínez A, Perdigón‐Melón J A, Valencia S. J Phys Chem B, 1998, 102: 75 Tosheva L, Mihailova B, Valtchev V, Sterte J. Microporous Mesoporous Mater, 2001, 48: 31 Mihailova B, Valtchev V, Mintova S, Faust A C, Petkov N, Bein T. Phys Chem Chem Phys, 2005, 7: 2756 Majano G, Mintova S, Ovsitser O, Mihailova B, Bein T. Microporous Mesoporous Mater, 2005, 80: 227
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Article
Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure‐directing agent‐free gel Bumei Zheng a, Yufeng Wan a, Weiya Yang b, Fengxiang Ling b, Hong Xie a, Xiangchen Fang b, Hongchen Guo a,* State Key Laboratory of Fine Chemicals, Department of Catalytical Chemistry and Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China b Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun 113001, Liaoning, China a
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 March 2014 Accepted 21 March 2014 Published 20 November 2014
Keywords: Organic structure‐directing agent‐free synthesis Zeolite Beta Seeding Seed residues Dissolved seed fragment
The organic structure‐directing agent‐free synthesis of zeolite Beta was carried out using several zeolite Beta seeds that differed in SiO2/Al2O3 ratio and crystal size. The synthesis was studied using X‐ray diffraction, X‐ray fluorescence, scanning electron microscopy, transmission electron micros‐ copy, ultraviolet‐Raman spectroscopy, infrared spectroscopy, and N2 physisorption. Synthesis was successful using different zeolite Beta seeds including pure silica seeds. During the induction period, the seeds underwent dissolution. The SiO2/Al2O3 ratio and crystal size, pretreatment (calcination), and seed addition time had a significant influence on seed dissolution behavior, crystallization pro‐ cess, and product. Morphological studies revealed that the seed residues produced by dissolution (except for pure silica) resulted in the formation of “immobilized” surface nuclei, which allowed for the dense growth of fresh small zeolite Beta crystals. The dissolved small seed fragments yielded dispersed nuclei, which formed relatively scattered small zeolite Beta crystals thought to be the main nuclei source of the pure silica seed. It is suggested that the use of an appropriately high SiO2/Al2O3 ratio, small size, and precalcined zeolite Beta seed is helpful for the synthesis of highly crystalline and pure zeolite Beta from the organic structure‐directing agent‐free gel. © 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction High‐silica zeolite Beta (BEA) has three‐dimensional inter‐ sected channels and 12‐membered ring pore openings [1,2]. These properties make it useful as a catalyst in aromatic alkyla‐ tion, isomerization, alkene hydration, aromatic acylation, and nitration syntheses in petrochemical industries [3–9]. Formation of the zeolite Beta structure was achieved origi‐ nally using tetraethylammonium hydroxide (TEAOH) as an organic structure‐directing agent (OSDA) [10]. However, its use has increased the zeolite Beta cost significantly, thereby limit‐ ing its industrial application. For this reason, recent studies
related to zeolite Beta have focused mainly on low‐cost synthe‐ sis using more affordable OSDAs such as tetraethylammonium bromide (TEABr) [11–14]. Xie et al. [15] demonstrated for the first time that zeolite Beta could be synthesized by adding cal‐ cined nanocrystalline zeolite Beta seeds into OSDA‐free alumi‐ nosilicate gel. This synthesis route attracted much attention [16–23]. Iyoki et al. [22,23] proposed that the key factor in successful zeolite synthesis in the absence of OSDA was the common composite building unit contained in both the seeds and zeolite obtained from the gel after heating without seeds. Majano et al. [16] showed that zeolite Beta could be obtained at low crystallization temperature with non‐calcined seeds. They
* Corresponding author. Tel/Fax: +86‐411‐84986120; E‐mail: [email protected] DOI: 10.1016/S1872‐2067(14)60089‐9 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 11, November 2014
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
believe that the active surface of the as‐synthesized seeds pro‐ moted crystallization while calcination of the seeds would be harmful because of seed surface deactivation. Kamimura et al. [17,18] reported that zeolite Beta synthesized with OSDA‐free gel could be used as a seed to generate more zeolite Beta. They concluded that the seed was partially dissolved during the ini‐ tial stage in the hydrothermal environment and that subse‐ quent crystallization took place on the surface of the residual Beta seeds exposed to the liquid. They also observed that zeo‐ lite Beta with SiO2/Al2O3 ratio greater than 52 could not be used as seeds because of their complete dissolution in the ini‐ tial stage in the hydrothermal environment. Xie et al. [19] showed that the residues partially dissolved Beta seeds rather than the dissolved species of Beta seeds helped crystallization. They proposed a core–shell growth mechanism to describe the crystallization of OSDA‐free gel on the surface of partially dis‐ solved Beta seeds. Although it is well known that seeding is useful for the syn‐ thesis of many zeolites, the mechanism by which this occurs has been interpreted in various ways: some believe that seeds behave as crystal nuclei supplying a surface for the crystalliza‐ tion of amorphous aluminosilicate gel; others suggest that the seeds dissolve partially and then the residual seed, or dissolved species, or both form nuclei [23,24]. An investigation into the seeding mechanism therefore holds significance not only for OSDA‐free synthesis of zeolite Beta but also for improving syn‐ thesis techniques of other zeolites. In this work, we carried out a detailed study on the OSDA‐free synthesis of zeolite Beta using different seeds. X‐ray diffraction (XRD), field emission‐scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT‐IR), and ultraviolet (UV)‐ Raman spectroscopy were used to characterize the solid prod‐ ucts obtained before and during the synthesis. Emphasis was given to the structure‐directing effect of different seeds, key aspects of the seed that influence the structure‐directing effect, and the seeding mechanism.
2. Experimental 2.1. Synthesis of zeolite Beta seeds Al‐containing zeolite Beta seeds were synthesized as fol‐ lows. First, silica sol (SiO2, 30 wt%, Qingdao Haiyang Chemical Co.) or fumed silica (Shenyang Chemical Co.) was mixed with OSDA solutions, which included a TEAOH solution (20 wt%, Hejian Qingfeng Shimian Chemical Co.), TEABr solution (99 wt%, Jintan Huadong Chemical Research Institute), ammonia and NaOH, or solution of TEABr and TEAOH. A NaAlO2 solution (Al2O3, 41 wt%, Sinopharm Chemical Reagent Co.) was added to the mixture with vigorous stirring for 40 min. The mixture was transferred to an autoclave and crystallized at 145 °C un‐ der static or stirred conditions for ~3–6 d. Pure silica zeolite Beta seed was synthesized as follows. Fumed silica was mixed with the TEAOH solution (40 wt%, condensed in laboratory). Next, NH4F was added and the mix‐ ture stirred thoroughly. The sticky gel obtained was placed in a
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Teflon‐lined stainless steel autoclave and crystallized at 150 °C for 4 or 5 d. All solid products were filtered, washed, dried overnight at 110 °C, and calcined in air at 540 °C for 6 h to remove the OSDA before they were used as seeds. 2.2. Seed‐assisted synthesis of OSDA‐free zeolite Beta A gel with molar composition Na2O:Al2O3:SiO2:H2O = 11.4:1:40:640 was prepared as follows. Fumed silica was dis‐ solved in the NaOH solution before addition of the NaAlO2 solu‐ tion under vigorous stirring. Calcined zeolite Beta was then added as seed (10 wt% in relation to silica source). The mix‐ tures were agitated to form a homogenous gel. The gel was transferred into a 100‐ml autoclave and crystallized statically at 140 °C. Sampling was conducted at time intervals. The solid product was isolated from the resultant mixture by filtration, washed with deionized water, and dried at 110 °C for charac‐ terization. 2.3. Characterization XRD patterns were collected on a Rigaku D/max‐2004 dif‐ fractometer using Cu Kα radiation (40 kV, 100 mA) and a scan‐ ning rate of 0.02°/min (2θ). The crystallinity of the obtained Beta phase was calculated based on the intensity of the peak at reflection (3 0 2) (2θ = ~22.4°). FE‐SEM micrographs were recorded on a Hitachi S‐4800 microscope. TEM micrographs were obtained on a JEOL JEM‐2100 electron microscope operating at 200 kV with a point resolution of 0.23 nm. TEM samples were dispersed in ethanol and a droplet of this mixture was deposited on a Cu grid. FT‐IR spectra in the hydroxyl region were obtained on a Ni‐ colet iS10 FTIR instrument according to the following proce‐ dure. The sample was pressed into a self‐supporting wafer and placed into a quartz IR cell with CaF2 windows. The FT‐IR spec‐ tra were recorded with a resolution of 4 cm−1 at room temper‐ ature. Prior to measurement, the sample was dehydrated under vacuum at 400 °C for 4 h. UV‐Raman spectra were recorded on a DL‐2 Raman spec‐ trometer. The 244‐nm line of a LEXEL LASER was used as the excitation source. An Acton triple monochromator was used as a spectrometer for Raman scattering. Spectra were collected using a Prinston charge‐coupled device detector. The power of the laser line at the sample was below 3 mW. Elemental analyses were performed using a Bruker SRS 3400 X‐ray fluorescence spectrometer. N2 adsorption‐desorption isotherms were recorded at –196 °C on a Micromeritics ASAP 2000 instrument after activating the sample under vacuum at 350 °C for at least 6 h. The surface area was evaluated using the BET method. The external surface area, micropore surface area, and micropore volume were cal‐ culated using the t‐plot method. The total pore volume was determined from the amount adsorbed at a relative pressure of approximately 0.99, and the mesopore volume was calculated from the difference between the total and micropore volume.
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3. Results and discussion 3745 3735
3.1. Characterization of zeolite Beta seeds
S10 S9 S8
3610 Absorbance
S7 S6 S5 S4 S3 S2 S1 3800
3700
3600
3500
3400
Wavenumber (cm1)
Fig. 1. Hydroxyl vibration FT‐IR spectra of zeolite Beta seeds S1 to S10.
phenomenon confirms that seeds S1 to S8 were Al‐containing zeolites and seeds S9 and S10 were pure silica zeolites. The intensities of the 3745 cm−1 band were stronger for seeds S1 to S6, weaker for seeds S7 and S8, and very weak for seeds S9 and S10. This agrees with the fact that seeds S1 to S6 were small crystals while seeds S7 to S10 were large crystals. The UV‐Raman spectra of the seeds are shown in Fig. 2. The most intense Raman scattering of the zeolite Beta framework vibrations appear mainly from 310 to 470 cm−1. According to the literature [28−31], the peaks at 314 and 343 cm−1 should be 314
343 399 465 425 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1
Intensity
To obtain different zeolite Beta seeds, the synthesis of zeo‐ lite Beta was carried out in Al‐containing and Al‐free gels with different silica sources and different OSDA solutions. The re‐ sultant seeds were investigated and the main crystal data are summarized in Table 1. Seeds S1 to S6 were less than 200 nm in crystal size with relatively larger mesopore volume owing to aggregation of small crystals. However, they were different in terms of SiO2/Al2O3 ratio and relative crystallinity. Both the SiO2/Al2O3 ratio and relative crystallinity increased from seed S1 to S6. Seeds S7 and S8 were synthesized in the same gel but with different crystallization times. Both were approximately 3‐μm spherical crystals and their SiO2/Al2O3 ratios were close to that of seed S5. However, the relative crystallinity of S8 was higher than that of S7 because it was synthesized over longer time. Seeds S9 and S10 were also synthesized in the same gel, with S10 being crystallized over longer time but seeds S9 and S10 were pure silica zeolite Beta approximately 9 μm in crystal size and with truncated octahedral morphology. It is worth noting that the relative crystallinity of the seeds calculated from XRD and micropore volume (Vmicro) data was almost the same for S1 to S6 but different for S7 to S10. Differences in seeds S7 to S10 may be caused by crystal size and lattice im‐ perfections [25,26]. In seeds S7 and S8, the microcrystals were composed of nanocrystals while in seeds S9 and S10, the large crystals were all monocrystals. The crystal size may result in weak XRD peaks in S7 and S8 and strong ones in S9 and S10. FE‐SEM micrographs also show that seed S10 and especially seed S9 had high‐density wormhole defects (not shown), which may result in a decrease of micropore volume for S9 and S10. The OH vibration FT‐IR spectra are shown in Fig. 1. Gener‐ ally, the typical zeolite Beta may exhibit six bands, including hydroxyls on a partially hydrolyzed framework Al (3783 cm−1), terminal SiOH (3745 cm−1), internal SiOH at framework defects (3735 cm−1), hydroxyls associated with extra framework Al (3665 cm−1), bridging hydroxyls (3610 cm−1), and hydrox‐ yl‐bonded hydroxyls (broad band at 3400–3700 cm−1) [25,27]. It can be seen from Fig. 1 that seeds S1 to S8 had bridging hy‐ droxyl peaks at 3610 cm−1, while seeds S9 and S10 did not. This
200
400 600 Raman shift (cm1)
800
Fig. 2. UV‐Raman spectra of zeolite Beta seeds S1 to S10.
Table 1 Synthesis conditions and physicochemical characteristics of zeolite Beta seeds. Sample
Synthetic description
Silica source Template S1 silica sol TEAOH S2 silica sol TEABr S3 silica sol TEAOH S4 silica sol TEABr S5 silica sol TEAOH S6 silica sol TEAOH S7 fumed silica TEAOH+TEABr S8 fumed silica TEAOH+TEABr S9 fumed silica TEAOH S10 fumed silica TEAOH a Calculated on the basis of S6.
State stirred stirred stirred stirred stirred stirred static static static static
Crystal size (nm) <100 <100 <100 <100 150 150 3000 3000 8500 9000
SiO2/Al2O3 molar ratio 11 12 23 22 31 38 30 29 ∞ ∞
Sample description Pore volume (cm3/g) Vmicro Vmeso 0.15 0.21 0.17 0.20 0.20 0.28 0.18 0.35 0.20 0.20 0.23 0.25 0.16 0.08 0.23 0.05 0.16 0.07 0.17 0.07
Crystallinity a (%) XRD Vmicro 66 65 75 74 70 87 84 78 87 87 100 100 49 70 86 100 221 70 224 74
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
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assigned to five‐membered rings, those at 399 and 425 cm−1 to six‐membered rings, and that at 465 cm−1 to four‐membered rings. The peaks at 314, 399, and 465 cm−1 belonged to the rings in the periodic building unit, while the peaks at 343 and 425 cm−1 belonged to the interlayer connectivity mode for polymorphs. Therefore, the intensity ratio of peaks at 314 and 343 cm−1 indicated the presence of interlayer connecting faults and the larger value indicated fewer stacking faults. It can be seen that seeds S1 to S8 had a larger but comparable amount of stacking faults while seeds S9 and S10 had the least. 3.2. Template‐free synthesis of zeolite Beta with different seeds The seeds played a crucial role in the formation of the zeo‐ lite Beta structure in the OSDA‐free gel because no beta phase could be obtained without seed. Beta zeolites were synthesized by adding different seeds as indicated in Table 2. However, when seeds S1 to S5 were used, the synthesized products (BEA1 to BEA5) showed low zeolite Beta crystallinity. When seeds S6 to S10 were used, the synthesized products (BEA6 to BEA10) showed good zeolite Beta crystallinity. For seeds S1 to S6, which had a similar crystal size (less than 150 nm) but in‐ creasing SiO2/Al2O3 ratio, the crystallinity of the synthesized products increased from 16% to 89%. This phenomenon im‐ plies that the SiO2/Al2O3 ratio is a very important parameter for zeolite Beta seeding. To carry out the successful OSDA‐free synthesis of zeolite Beta, it is better to use zeolite Beta seed with a SiO2/Al2O3 ratio of more than 30. Successful synthesis with pure silica seeds (S9 and S10) in this study implies the possibility of a much higher SiO2/Al2O3 ratio range existing for zeolite Beta seed selection. This opinion is different from pre‐ vious research that has succeeded in the OSDA‐free synthesis of zeolite Beta using zeolite Beta seeds with SiO2/Al2O3 ratios of between 20 and 52 [15−20]. According to this research, seeds with SiO2/Al2O3 ratio greater than 52 could not provide a beta phase because of complete dissolution. We assume that the dissolution behavior of the high‐silica zeolite Beta seed during the OSDA‐free synthesis of zeolite Beta may be influenced by its morphological and constructional features, including crystal size and crystal defects. The crystal size of zeolite Beta seeds affected the crystal size of the synthesized zeolite Beta product. As indicated by Table 2
Fig. 3. Effect of seeds on the morphology of zeolite Beta synthesized in OSDA‐free gels.
and Fig. 3, although the zeolite Beta products obtained with different seeds had a similar appearance (intergrown crystals with truncated octahedral morphology), the increase in crystal size of the zeolite Beta seeds resulted in an obvious increase in crystal size of the zeolite Beta product. It is also interesting to note from Fig. 3 that the residual bodies of the larger crystal seeds remained in the obtained zeolite Beta products (BEA8‐b and BEA10‐b). For the Al‐containing large crystal seed (S8), there was an obvious growth of zeolite Beta product on the seed residue surface, as indicated by the crowded small multi‐ dimensional faces on the big seed residue (BEA8‐b). For the pure silica large crystal seed (S10), however, no growth of zeo‐ lite Beta product on the seed residue could be seen. The residue was round and relatively smooth. Despite these influences, the zeolite Beta products obtained with different seeds all had low but roughly the same
Table 2 Influence of seeds on template‐free synthesis of zeolite Beta. Sample a
Seed
Size (nm)
BEA1 S1 — BEA2 S2 — BEA3 S3 — BEA4 S4 100−150 BEA5 S5 150−200 BEA6 S6 150−200 BEA7 S7 300−700 BEA8 S8 300−700 BEA9 S9 450−950 BEA10 S10 450−950 a Without other crystal phases. b Calculated on the basis of BEA9.
SiO2/Al2O3 molar ratio 11 11 11 10 10 10 10 10 10 10
Pore volume (cm3/g) Vmicro Vmeso — — — — — — — — 0.18 0.06 0.21 0.08 0.20 0.06 0.20 0.05 0.21 0.04 0.20 0.04
Crystallinity b (%)
Yield (%)
16 19 49 60 64 89 93 94 100 97
— — — — — 32.5 32.3 32.6 — 32.4
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Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
SiO2/Al2O3 ratios. The same SiO2/Al2O3 ratio (approximately 10) as well as similar morphology (intergrown crystals with truncated octahedral morphology) imply that the zeolite Beta products obtained in the OSDA‐free gels were mainly defined by special hydrothermal conditions. Figure 4 shows the crystallization curves of OSDA‐free gels with different zeolite Beta seeds while the inset shows the solid product yield of zeolite Beta at the early stages of crystalliza‐ tion. The decline in zeolite Beta yield at the early stages of crystallization indicates that all seeds were partially dissolved in the induction period. The decline in zeolite Beta yield for BEA9 and BEA10 indicates that the pure silica seeds could be dissolved to a high degree. The different zeolite Beta seeds in‐ fluenced the crystallization process in the OSDA‐free gels con‐ siderably. First, the duration of the induction period was relat‐ ed mainly to the crystal size of the zeolite Beta seeds. The smaller seeds tended to have a shorter induction period while the larger seeds tended to have a longer induction period (Fig. 4(b)). This may be because the smaller seeds generally dis‐ solved faster and so a shorter time was needed for the genera‐ tion of zeolite Beta nuclei. Second, the growth rate of the zeolite Beta crystal seemed to relate to both the SiO2/Al2O3 ratio and crystal size of the zeolite Beta seeds. By comparing the crystal‐ lization curves of BEA1 to BEA6, which were seeded with zeo‐ lite Beta (S1 to S6) of largely similar crystal size but different SiO2/Al2O3 ratio, it could be seen that the crystal growth rate as a whole increased moderately with SiO2/Al2O3 seed ratio (Fig. 4(a)). By comparing the crystallization curves of BEA6 to BEA8, which were seeded with zeolite Beta (S6 to S8) of similar SiO2/Al2O3 ratio but different crystal size, it was found that the crystal growth rate decreased significantly as the seed crystal size increased (Fig. 4(b)). Compared with the SiO2/Al2O3 ratio, the crystal size of the seeds had a significant effect on the zeo‐ lite Beta crystal growth rate. The influence of crystal size of seeds on the crystal growth rate of zeolite Beta may be at‐ tributed to the number of nuclei present. The number of nuclei generated by the different seeds is supposed to decrease with increase in crystal size of zeolite Beta. This assumption is con‐ sistent with SEM observations, which indicated that an increase in crystal size of zeolite Beta seeds resulted in an increase in 100
3.3. Structure‐directing behavior of different zeolite Beta seeds 3.3.1. Seeding in fresh gel 3.3.1.1. Calcined seeds OSDA‐free synthesis of zeolite Beta was carried out with calcined seed S7. After calcination, the OSDA molecules in the seed pore channels were removed. The solid products obtained before and during crystallization were characterized by XRD, FT‐IR, and FE‐SEM. Figure 5 shows that the gel‐containing seed S7 (10 wt%) had a weak diffraction peak of zeolite Beta at 2θ = 22.48°. Dur‐ ing the induction period (0–42 h), this diffraction peak shifted to lower 2θ values, which suggests an increase in Al content in the crystal framework (Al−O bond length is larger than that of Si−O). Because the zeolite Beta yield declined in the induction period (Fig. 4(b)), we tend to attribute this peak shift phenom‐ enon to the dissolution of a Si‐rich area of seeds under basic hydrothermal conditions. After the induction period, the gel began to crystallize. This peak shifted slowly to 2θ = 22.14° and increased in intensity. This indicates that Al‐rich zeolite Beta 100 (b)
BEA1 BEA2 BEA3 BEA4 BEA5 BEA6
60
80
Crystallinity (%)
80
10 Yield (%)
40 20
5 0 0
0 4
8
12 Time (h)
16
60
10
40 20
2
4
Time (h)
0
BEA7 BEA8 BEA9 BEA10 BEA6
Yield (%)
(a)
Crystallinity (%)
crystal size of zeolite Beta product. Third, the different zeolite Beta seeds also had a strong influence on the stability of the zeolite Beta product under hydrothermal synthesis. For seeds S1 and S2 (corresponding to the BEA1 and BEA2 curves), crys‐ tallization ceased at approximately 7 h. Crystal transformation from zeolite Beta to mordenite then took place. Therefore, only low crystallinity zeolite Beta products (lower than 20%) were obtained with these seeds. For seeds S3 to S6 (corresponding to the BEA3 to BEA6 curves), crystallization was extended to 15 to 17 h, and the crystallinity of the zeolite Beta products in‐ creased to approximately 50% to 90%. For seeds S7 to S10 (corresponding to the BEA7 to BEA10 curves), a much longer crystallization time was allowed, and the crystallinity of the zeolite Beta products could be increased to more than 90% before crystal transformation occurred (crystal transformation caused a drop‐off in crystallinity in the curves). Therefore, in the OSDA‐free synthesis of zeolite Beta, the high‐silica zeolite Beta seed favors the stability and crystallinity of the zeolite Beta product.
20
5 0 0
0 24
0
20
40
60
80 100 Time (h)
Fig. 4. Effect of seeds on crystallization kinetics of zeolite Beta in OSDA‐free gels.
120
10 20 Time (h)
140
160
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3614 3745
77 h 65 h 42 h
Absorbance
Intensity
77 h 65 h 42 h 26 h 18 h 7h 4h 2h 0h
26 h 18 h 7h 4h 2h 0h
S7
5
10
15
20
25
30
35
2/( ) o
3800
3700 3600 Wavenumber (cm1)
3500
3400
Fig. 5. XRD patterns of seed S7 and solid samples obtained with S7‐ containing gel.
Fig. 6. Hydroxyl vibration FT‐IR spectra of solid samples obtained with S7‐containing gel.
was obtained. Figure 6 shows that the initial gel had a strong signal at 3745 cm−1, which is attributed to the terminal SiOH of the gel particles. During the induction period, this band became weak. This change may be an indication of the digestion (or rear‐ rangement) of the amorphous gel under hydrothermal condi‐ tions. When the crystallization time was more than 42 h, the solid product showed a new strong absorption peak at 3614 cm−1, which is assigned to bridging hydroxyls. This indicated that a great deal of Al entered the framework during the crystal growth process, which agrees well with the peak shift in the XRD analysis. Figure 7 shows that in the induction period ((b)–(g)) two important changes occurred to the gel: one was the dissolution of seeds, which seemed to start at the exposed surface and de‐
velop towards the inside of the seed balls and the other was the softening of the graining fresh gel and the covering of the ten‐ der gel on the seed balls. By the end of the induction period (Fig. 7(f)–(g)), sprout‐like crystals could be seen penetrating the gel cover from the surface of the inner seed balls. The well‐crystallized product was a mixture of large and small crystals. The small crystals were intergrown with truncated octahedral morphology. The surface of the large crystals was heavily populated with fresh and small zeolite Beta crystals. The OSDA‐free synthesis of zeolite Beta was also studied with other seeds. The TEM micrographs related to the crystal‐ lization of seed S3‐ and S6‐containing gels are shown in Fig. 8(a,b), and the SEM micrographs related to the crystallization of seed S9‐ and S10‐containing gels are shown in Fig. 8(c,d). SEM characterization was used for the larger crystal samples as
Fig. 7. FE‐SEM micrographs of solid samples obtained with S7‐containing gel.
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the TEM characterization was unsuccessful. Figure 8 shows that seed dissolution occurred in all cases except for the crystallization of seed S3‐containing gel. The results for seed S3‐containing gel may arise because of the low extent of dissolution and small crystal size of seed S3, as the dissolution of seed S3 has been confirmed by the decline of zeolite Beta yield during the induction period (Fig. 4(a)). The situation is similar for seeds S1 to S4. The initial gels under‐ went a digestion (or rearrangement), which made the amor‐ phous gels soft and sticky, and tender gels covered the seeds. However, differences did exist between the seeds. For example, for most of the Al‐containing seeds (such as seed S6), dissolu‐ tion tended to take place inside the seeds, which resulted in the formation of hollow seed residues. In subsequent crystalliza‐ tion, the hollow seed residue surface served as substrate for the growth of fresh and small zeolite Beta crystals. In contrast, for Al‐free large seeds (such as seeds S9 and S10), dissolution oc‐ curred on the outside of the seeds, which resulted in their dis‐ integration and the formation of smaller rounded residues. In
subsequent crystallization, there was no visible growth of the fresh and small zeolite Beta crystals on the surface of the rounded seed residues. Based on these observations, we believe that the dissolution of the seeds in the OSDA‐free synthesis of zeolite Beta is a common phenomenon although the degree of dissolution and behavior may be different because of differences such as SiO2/Al2O3 ratio and crystal size of the seeds. The seed residues produced by dissolution assumed the structure‐directing role in the OSDA‐free synthesis of zeolite Beta. In this case, the role of the seed residues is to provide an “immobilized” surface nucleus. The pure silica seed residues, at least, seem to be an exception. The dissolved seed fragments also have an im‐ portant role in the OSDA‐free synthesis of zeolite Beta, as evi‐ denced by the appearance of abundant small crystals in the crystallization products of the large seed‐containing gels. The dissolved seed fragments can definitely serve as nuclei. For the pure silica seeds (seeds S9 and S10), the dissolved seed frag‐ ments are the main nuclei source. The seeding mechanism is
Fig. 8. TEM (a,b) and FE‐SEM (c,d) micrographs of solid samples obtained with S3‐ (a‐1,2,3), S6‐ (b‐1,2,3), S9‐ (c‐1,2,3), and S10‐ (d‐1,2,3) containing gels.
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
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100
Calcined Non-calcined
80 60
20 Yield (%)
different from that reported to date in the literature. As stated above, Kamimura et al. [18] have concluded that the crystalli‐ zation of zeolite Beta from the OSDA‐free gel took place on the surface of the residual Beta seeds, and that the seed residues should be exposed to the liquid. Xie et al. [19] have shown that it was the residue of partially dissolved Beta seeds rather than the dissolved species of Beta seeds that assisted in the crystal‐ lization. 3.3.1.2. Non‐calcined seeds In the non‐calcined seeds, the pore channels of the zeolite Beta seeds were expected to be occupied by OSDA molecules. A previous study has shown that the calcined zeolite Beta seed with SiO2/Al2O3 ratio of 52 was unsuccessful in zeolite Beta synthesis from OSDA‐free gel but that the same zeolite Beta seed without calcination was successful [16]. When we inves‐ tigated the influence of pre‐calcination with seed S6, it was found that both the calcined and non‐calcined zeolite Beta seeds led to successful zeolite Beta synthesis. However, the gel‐containing calcined seed showed a higher degree of seed dissolution, shorter induction period, and higher crystal growth rate (Fig. 9). In contrast with the significant inner dissolution of the calcined seed (Fig. 8(a,b)), the non‐calcined seed did not show a significant dissolution in the induction period (Fig. 10). These results were different from the previous study, which showed that the active surface of as‐synthesized seeds pro‐ moted crystallization while the calcination of seeds would be harmful because of the deactivation of the seed surface [16]. Interestingly, for the gel‐containing calcined seed, the crystalli‐
Crystallinity (%)
40 20
10 0 0
4 8 Time (h)
0 0
4
8
12
16
20
24
28
32
Time (h) Fig. 9. Effect of calcined and non‐calcined seed S6 on crystallization kinetics of zeolite Beta.
zation products (from 7 to 16 h) gave a singlet characteristic peak of zeolite Beta around 2θ = 22.14° (Fig. 11(a)). For the gel‐containing non‐calcined seed, however, the crystallization products (from 20 to 33 h) gave a doublet characteristic peak of zeolite Beta in the same region (Fig. 11(b)). The latter case provides evidence of the formation of the core‐shell structure of zeolite Beta. The core was the zeolite Beta seed S6 with higher SiO2/Al2O3 ratio, while the shell was the new zeolite Beta with lower SiO2/Al2O3 ratio. These results indicate that pretreatment is an important factor for influencing the struc‐
Fig. 10. TEM micrographs of solid samples obtained at early stages of crystallization with non‐calcined seed S6 addition gels (arrows indicate the seeds).
(b)
(a)
33 h 30 h
Intensity
Intensity
16 h
15 h
27 h 23 h 20 h 7h 4h
11 h 7h 4h 2h 1h 0h
5
10
15
20 2/(o)
25
30
2h
0h 35
5
10
15
20 2/(o)
25
30
35
Fig. 11. XRD patterns of solid samples obtained at different crystallization times with calcined (a) and non‐calcined (b) seed S6 addition gels.
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large crystals. Seeding an aged gel also yielded many small crystals. Once again, this provides evidence for the struc‐ ture‐directing effect of dissolved seed fragments.
100 Fresh gel Aged gel
4. Conclusions 60 40
Yield (%)
Crystallinity (%)
80
20 0 0
20
40
60 Time (h)
4 2 0 0
10 20 Time (h) 80
100
Fig. 12. Effect of gel aging on crystallization kinetics of OSDA‐free gel‐ containing seed S7.
ture‐directing performance of zeolite Beta seed. 3.3.2. Seeding in aged gel Compared with seeding in a fresh gel, when the same zeolite Beta seed (S7) was added into the aged gel (prehydrothermal treatment of fresh gel for 7 h at the crystallization tempera‐ ture), more seed was dissolved and the seeding resulted in a shorter induction period and faster crystal growth (Fig. 12). The SEM micrographs (Fig. 13) show that during aging the fresh gel became fused and viscous ((a)–(c)), as was seen in the induction period of the foregoing experiments (Fig. 7). The difference is that when the seed was added into the aged gel, the viscous gel did not envelop the seed. This may be why the seed dissolved more rapidly and in higher quantities. In this case, seed dissolution did not result in the formation of hollow balls as was the case for seeding in a fresh gel (Fig. 7), but ap‐ peared to take place on the surface of the seed. At the end of the crystallization, the surface of the seed residues was heavily populated with small fresh zeolite Beta crystals, which formed
The synthesis of zeolite Beta with OSDA‐free gel was achieved using several zeolite Beta seeds, which differed main‐ ly in SiO2/Al2O3 ratio and crystal size. However, the different zeolite Beta seeds influenced the crystallization process and product significantly. As the seed SiO2/Al2O3 ratio increased, the induction period was shortened and crystal growth rate was enhanced. However, the effect of the seed crystal size was the opposite and much stronger. The seed with high SiO2/Al2O3 ratio was favorable for the synthesis of zeolite Beta with high crystallinity, while the seed with large crystal size resulted in an increase in product crystal size. Nevertheless, different zeo‐ lite Beta seeds all produced zeolite Beta with similar crystal morphology and SiO2/Al2O3 ratio. This indicates that the zeolite Beta products obtained in the OSDA‐free gels were mainly de‐ termined by special hydrothermal conditions. In the induction period, the different zeolite Beta seeds all underwent dissolu‐ tion. The increase of seed SiO2/Al2O3 ratio enhanced the extent of dissolution of the seed, while the increase in seed crystal size reduced the dissolution speed. Compared with the as‐ synthe‐ sized seed, precalcined seeds dissolved faster and in higher quantities. Dissolution of the seed commenced on the bare sur‐ face of the seed. However, when this surface was enveloped by the sticky gel, the dissolution of the seed likely developed from the tiny bare surface into the inside of the seed, thus forming a hollow seed ball. Morphology studies showed that the seed residues produced by dissolution could provide an “immobi‐ lized” surface nucleus, which allowed for the dense growth of fresh small crystals of zeolite Beta although the residues of pure silica seed were an exception. Small seed fragments pro‐ duced by dissolution could provide dispersed nuclei that re‐ sulted in the formation of relatively scattered small zeolite Beta
Fig. 13. FE‐SEM micrographs of solid samples obtained before (a,b,c) and after (d,e,f) seeding (seed S7).
Bumei Zheng et al. / Chinese Journal of Catalysis 35 (2014) 1800–1810
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Graphical Abstract Chin. J. Catal., 2014, 35: 1800–1810 doi: 10.1016/S1872‐2067(14)60089‐9 Mechanism of seeding in hydrothermal synthesis of zeolite Beta with organic structure‐directing agent‐free gel Bumei Zheng, Yufeng Wan, Weiya Yang, Fengxiang Ling, Hong Xie, Xiangchen Fang, Hongchen Guo * Dalian University of Technology; Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC
The nature of zeolite Beta seeds affected the organic structure‐directing agent‐free synthesis of zeolite Beta. The seeds dissolved during the induction period and the dissolved fragments were involved in the formation of zeolite Beta.
crystals. This may be the main structure‐directing technique of the pure silica zeolite Beta seed. The use of precalcined zeolite Beta seed with smaller crystal size and appropriately high SiO2/Al2O3 ratio will be useful in the crystallization of zeolite Beta from OSDA‐free gel with high crystallinity and purity. References [1] Newsam J M, Treacy M M J, Koetsier W T, De Gruyter C B. Proc R [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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