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Crystallization of zeolite beta in the presence of chiral amine or rhodium complex
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Microporous and Mesoporous Materials 109 (2008) 567–576 www.elsevier.com/locate/micromeso
Crystallization of zeolite beta in the presence of chiral amine or rhodium complex Yukio Takagi a
a,b,*
, Takayuki Komatsu a, Yasuyoshi Kitabata
b
Department of Chemistry, Tokyo Institute of Technology, 2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b Technical Center of Chemical Catalysts, Manufacturing Department, Numazu Plant, N.E. Chemcat Corporation, 678 Ipponmatsu, Numazu, Shizuoka 410-0314, Japan Received 21 August 2006; received in revised form 14 May 2007; accepted 2 June 2007 Available online 12 June 2007
Abstract Crystallization methods of zeolite beta in an acidic medium at low temperature (pH 4, 373 K) were developed. Zeolite beta was crystallized in the presence of chiral amine or rhodium complex as secondary structure directing agents (SDAs) in the acidic condition whose advantage was high solubility of the chiral amines. The secondary SDAs with C2 symmetry induced the crystallization of zeolite beta which contained a somewhat large fraction of polymorph A. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Zeolite beta; Polymorph; Chiral; Structure directing agents; Rhodium complex
1. Introduction Zeolite beta was first synthesized in 1967 using tetraethylammonium hydroxide as a structure directing agent (SDA) [1]. Zeolite beta has three relational structures, polymorph A (P4122 or P4322), polymorph B (C2/c) and polymorph C (P42/mmc). Only polymorph A is chiral [2] but the other polymorphs, B and C, are achiral. Usual zeolite beta synthesized by the original method has 44% of polymorph A and 56% of polymorph B [3]. The usual beta includes only a small percentage of polymorph C because it contains double four member ring (D4MR) cages as secondary building units which have high tension in their structure. Preferential syntheses of achiral polymorphs, B and C, have already been reported. Zhu et al. [4] synthesized Al-free zirconosilicate enriched with polymorph B *
Corresponding author. Address: Technical Center of Chemical Catalysts, Manufacturing Department, Numazu Plant, N.E. Chemcat Corporation, 678 Ipponmatsu, Numazu, Shizuoka 410-0314, Japan. Fax: +81 55 966 1832. E-mail address: [email protected] (Y. Takagi). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.06.005
using TEAOH as an SDA in a fluoride medium. Cantin et al. reported the synthesis and characterization of all-silica materials containing larger portion of polymorph B [5]. Pure polymorph C of silicate [5] and germanosilicate [6] was also synthesized. SSZ-63 which is described as a random intergrowth of polymorph B and C was prepared and structurally elucidated by Burton et al. [7]. Shen et al. [8] reported that polymorph A was more soluble in mineral acids than polymorph B. Selective crystallization of polymorph A remains as one of the challenges in zeolite synthesis. Dartt et al. reported the synthesis of zeolite beta in the presence of chiral organic molecules but the preparation method had not been disclosed. An X-ray powder pattern of their zeolite beta suggested the zeolite had somewhat larger domains of polymorph A than the usual zeolite beta [9]. In spite of the difficulty concerning selective crystallization of polymorph A, there are a few reports describing catalytic applications of zeolite beta for asymmetric reactions such as formation of 1,2-diphenylethane-1,2-diol by hydration of trans-stilbene oxide [9], or hydrogenation of tiglic acid to 2-methylbutanoic acid [10]. Some requirements of
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SDA for the selective synthesis of the chiral polymorph were suggested by Davis and Lobo [11]. According to them, the SDA molecule must have at least three constraints. First, it must be chiral. Second, its length must be at least 1 nm. Third, it must be stable at the synthesis conditions of high pH and temperatures greater than 383 K. If zeolite beta can be crystallized in acidic media at low temperatures, the third constraint is highly relaxed. Zeolite beta is usually synthesized by hydrothermal crystallization in an alkaline medium because the crystallization requires some solubility of silicate and the source of the other framework elements. Hydroxide anion, which ensures the solubility of these sources in the synthetic medium, has a role of a mineralizer. Camblor et al. [12] synthesized zeolite beta in almost neutral condition using fluoride anion in stead of hydroxide anion as a mineralizer. It is not popular to crystallize zeolites in acidic media probably because the acidity extremely decreases the rate of crystallization. Guth et al. [13] synthesized ZSM-5 in an acidic medium (pH 5) at 443 K. However there are no reports disclosing the synthesis of zeolite beta in acidic conditions to our best knowledge. One of advantages of the acidic crystallization is the higher solubility of amines in an acidic medium, compared to neutral or alkaline synthesis conditions. Most of chiral amines such as natural alkaloids are practically insoluble in water but some of them are soluble in acidic aqueous solution. The acidic condition would enable the crystallization in the presence of various chiral amines without any derivatization such as N-alkylation. The acidic condition is also convenient to use coordination complexes or organometallic species as SDAs because the acidity deactivates substitution of ligands of these compounds [14]. Since the metal complexes or organometallic species have a variety in size and shape, they have been used in the synthesis of zeolitic materials [15–19] as SDAs. UTD-1, which has 14 member rings called ultra large pore, was synthesized using organometallic cation, [Co(Cp*)2]+ (Cp* = pentamethylcyclopentadienyl), as an SDA [20]. Gray et al. [21] crystallized special aluminophosphates using coordination complexes of cobalt, such as [Co(en)3]3+ (en = ethylenediamine) or [Co(tn)3]3+ cations (tn = trimethylenediamine). Rhodium complexes are able to form chiral structure similar to those of the cobalt complexes. The chiral rhodium complexes are more stable for racemization than corresponding cobalt ones. [Rh(en)3]3+ has stronger resistance to racemization in boiling water than [Co(en)3]3+ [22]. Even such stable Rh complexes, however, are decomposed or racemized in the usual conditions of zeolite synthesis. Acidic crystallization at low temperature will have an advantage to prevent the chiral complexes from decomposition or racemization. A rhodium complex, [Rh(phen)3]3+ (phen = 1,10-phenanthroline), led to the formation of SAPO-20 [23], which has SOD structure. The rhodium complex is too large to be incorporated in a sodalite cage. This suggests that the rhodium complex played a role of an SDA working outside
zeolitic domains, while usual SDAs work in the domains. The complex might control the intergrowth rather than nucleation in the crystallization of zeolites. In this study, we developed crystallization methods of zeolite beta in an acidic medium at low temperature (pH 4, 373 K) and reported crystallization of zeolite beta in the presence of the chiral rhodium complex or amine which has various symmetries of point groups (1, C2 and D3). The purpose of this study is to search the constraint on the effective SDA for the selective crystallization of polymorph A. 2. Experimental 2.1. The rapid crystallization method for acidic synthesis of zeolite beta The rapid crystallization method [24,25] was adopted for the acidic synthesis. The mixture of tetraethylorthosilicate (TEOS, Tokyo Chemical Industry) and aqueous solution (35% w/w) of tetraethylammonium hydroxide (TEAOH, SACHEM) was stirred at room temperature until the ethanol formed on hydrolysis of TEOS was completely evaporated. Hydrofluoric acid (47% w/w, Wako Pure Chemical) was added to the clear solution and thick gel was formed. The gel composition was 1.0 SiO2:0.54 TEA:0.54 F:7.5 H2O. Nucleation was carried out in a PTFE vessel at 413 K for 50 h under static condition. After quenching, chiral [Rh(bpy)3](HF2)3 was added to the nucleated gel followed by the adjustment of pH into 4 with HF and the evaporation of water at 313 K. The pH of the dense gel was measured with four kinds of test paper (BCG, BPB, PB and PP) because usual glass electrodes are corroded under acidic conditions in the presence of fluoride. The final gel composition was 1.0 SiO2:0.54 TEA:4.0 F:4.5 H2O:0.30 Rh. Crystallization was performed at 373 K for 2 months statically. The solid product obtained was filtered, washed with deionized water and dried at room temperature. The pH of the mother liquids was in the range of 3.5–4.5. No decrease in the optical rotation of the chiral rhodium complex was observed. 2.2. Recrystallization method 2.2.1. Synthesis of zeolite beta seeds The preparation method of zeolite beta for seed crystals was adapted from the process reported by Wadlinger et al. [1] with modification to reduce the content of sodium. Aluminum sulfate hydrate (15.8 g, Wako Pure Chemical) was dissolved in 130.4 g of TEAOH solution. Then, 149.5 g of silica sol (Snowtex-40, 40 wt.% SiO2, 0.6% Na2O in water, Nissan Chemical) was added to the solution with stirring. The molar composition of the mixture was 1.0 SiO2:0.31 TEAOH:0.026 Al2O3:0.015 Na2O:10.46 H2O. The final mixture was transferred into a PTFE vessel and kept at 413 K for 4 days statically. After the vessel was quenched, the precipitate was filtered, washed with deionized water and dried at 373 K.
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The as-made zeolite was dealuminated in order to be used for seed crystals in the synthesis of fluoride method because of the rate of crystallization would be slow in Alrich systems [26]. One gram of the as-made sample was treated with 100 ml of 12 M HCl at 363 K for 3 days in order to eliminate the aluminum. The solid was filtered and washed with deionized water, and dried at 373 K. XRF analysis showed the Si/Al ratio of the dealuminated zeolite beta to be higher than 1000. 2.2.2. Recrystallization of zeolite beta in an acidic medium at low temperature Lithium hydroxide monohydrate (0.8 g) was dissolved in 35 ml of deionized water. The solution was heated to 353 K. The dealuminated beta (1 g) was added to the hot solution with stirring. After 1 h, the hot slurry was filtered through 0.1 lm membrane. Tetraethylammonium fluoride solution (2.94 g, 50 wt.% in water) and 126 ml of water were added to the filtrate. The resulting solution was neutralized with 47% hydrofluoric acid to pH 8–9 and white gel was formed. The molar composition of the gel was 1.0 SiO2:0.78 TEAOH:1.5 LiOH:2.0 HF:700 H2O. The gel was put into the PTFE vessel and kept at 413 K for 4 days for renucleation. To the renucleated gel, a specific amount of the chiral cation was added. The gel was acidified with HF to pH 4, followed by concentrating at 313 K. Typical molar composition of the precursor was 1.0 SiO2:0.78 TEAOH:4 HF:2.0 LiOH:0.12 chiral cation:4.5 H2O. The final precursor was transferred into the PTFE vessel and kept at 373 K for three weeks. The pH of the reacted slurry was in the range 4–5. The product was put into 100 ml of water, stirred for 30 min, filtered and washed with deionized water and dried. 2.3. Preparation of chiral cations Alkaloids and other chiral amines were purchased from Aldrich or Wako Pure Chemical. Tetraethylammonium fluoride solution was obtained by neutralization of TEAOH with HF. AgHF2 was prepared by dissolving of Ag2O into HF. Chiral rhodium complexes were synthesized as follows: (+)-[Rh(en)3](HF2)3 (+)-[Rh(en)3]Cl3 was synthesized from rhodium chloride hydrate (N.E. Chemcat) and ethylenediamine (Wako Pure Chemical) by literature method [27]. (+)-[Rh(en)3](HF2)3 was obtained by metathesis between the corresponding chloride salt and AgHF2. Optically active [Rh(bpy)3](HF2)3 Racemic complex, [Rh(bpy)3]Cl3, was synthesized by literature method [28] with some modification. Rhodium chloride containing 20 g of rhodium was dissolved in 1000 ml of deionized water under nitrogen atmosphere. 105.4 g of 2,2 0 -bipyridine (Wako Pure Chemical) was dissolved in 750 ml of ethanol. The ethanol solution was added
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to the rhodium solution followed by the addition of 0.2 g of hydrazine monohydrochloride. After 16 h of reflux under nitrogen atmosphere, the mixture was evaporated to dryness. The yellow rhodium complex was extracted with water and recrystallized from water/ethanol/acetone with 90% yield. Optical resolution K3{()-[Co(l-cysu)3]} (cysu = cysteinsulfinato) was used as a resolving agent [29]. (+)-[Rh(bpy)3]Cl3 was obtained by fractional crystallization using K3{()-[Co(l-cysu)3]}. The diastereomer of {(+)-[Rh(bpy)3]}{()-[Co(l-cysu)3]} was less soluble in water than the other enantiomer. The diastereomer was precipitated and separated from the mother liquid followed by anion exchange using Sephadex A-25 (Cl-form, Pharmacia). The ()-isomer was obtained from the filtrate followed by recrystallization. (+)-[Rh(bpy)3](HF2)3 was obtained from the respective chloride by metathesis using AgHF2. [Rh(phen)3](HF2)3 and other rhodium complexes were prepared similarly. 2.4. Characterization All samples of the as-made zeolite were characterized by MAC Science X-ray diffractometer MXP21VACE using Cu Ka radiation operating at 40 kV/300 mA. The scanning range was 2h = 3–88°. Peak separation was carried out using XPRESS-1.1.1 (X-ray Powder Research Software System, developed by MAC Science and the University of Oxford). A JEOL JSM-6300 scanning electron microscope was used to measure the morphology of crystallites under 104 Pa. The siliceous samples had to be coated with carbon sputtered prior to the observation. A Thermo Finnigan Italy automatic elemental analyzer EA-1112 was used to determine the contents of carbon and nitrogen in the zeolitic samples. Thermal analysis of the samples was carried out using Rigaku Thermo plus TG8120. Adsorption tests of bis(a-methylbenzyl)amine were performed as follows. Zeolite beta sample (120 mg) was put into 20 ml of mixture of methyl-tert-butylether/2-propanol (98:2 v/v) containing 1.0 lmol of (+)- or ()-bis(a-methylbenzyl)amine. The slurry was shook for 3 h at room temperature and settled. Absorbance at 260 nm of the supernatant was measured with Hitachi spectrophotometer U-3310. 3. Results and discussion 3.1. Crystallization of zeolite beta in the presence of the chiral rhodium complex First, we tried to develop a crystallization method of zeolite beta using chiral rhodium complexes without any racemization in order to study the effect of the complexes on the selectivity to polymorph A. If the rhodium
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added into the precursor gel in order to shorten the total period of crystallization. This method was applied to the acidic crystallization (pH 4, 373 K). The gel mixtures were treated in the condition of usual hydrothermal synthesis till the nuclei were formed. Fig. 1a shows an XRD pattern of the nucleated gel after filtration and drying. A few intense peaks of zeolite beta were observed but other weaker peaks did not appear. An SEM image of the gel is shown in Fig. 2a. The morphology of the gel appears amorphous. These results mean that the gel had been nucleated and had many small structures which were barely detected by XRD but not detected by SEM. The gel would have a lot of the nuclei. A chiral rhodium complex, (+)-[Rh(bpy)3](HF2)3, was added to the gel followed by adjusting it to pH 4.0 with HF and evaporation of water at 313 K to H2O/SiO2 = 4.5. Zeolite beta was crystallized at 373 K in a period of two months in the presence of the chiral complex. An XRD pattern of the product is shown in Fig. 1b. A specific optical rotation of the rhodium complex was maintained and no racemization was observed after the hydrothermal synthesis. Fig. 2b shows the SEM image of the crystallized zeolite beta. The crystals have a bipyramidal habit with a diameter around 0.3 lm. The
complexes can control the intergrowth along C-axis of zeolite beta, the selective synthesis of polymorph A might be achieved. Though rhodium complexes are more stable to racemization than cobalt ones, they are racemized and decomposed in the usual condition (e.g. pH 8.5, 413 K). The thermostability of some chiral rhodium complexes were tested as shown in Table 1. [Rh(en)3]3+ was the most stable and withstood heating to 393 K at pH of 5. [Rh(bpy)3]3+ and [Rh(phen)3]3+ were racemized in this condition. They were stable only up to 373 K at pH of 4.5 or lower. We tried to crystallize zeolite beta by the usual fluoride methods with modification adding hydrochloric acid to keep the pH of the gel mixture at 4.5 at 373 K. The gel composition was 1.0 SiO2:0.54 TEAOH:0.54 HF:7.5 H2O. Even after two months, the product had been still amorphous judging from its XRD powder patterns. In order to promote the crystallization of zeolite beta in the acidic condition, the rapid crystallization method developed by Inui et al. for ZSM-5 [24] and ZSM-34 [25] was adopted. Zeolite synthesis often requires a long incubation period during which the nuclei of the zeolite are formed. In the rapid crystallization method, the preformed nuclei are
Table 1 Thermostability of chiral rhodium complexes Complexes
(+)-[Rh(en)3]Cl3
()-[Rh(bpy)3]Cl3
()-[Rh(bpy)3]Cl3 (+)-[Rh(phen)3]Cl3
Optical rotation [a]D (degree)
Heating condition
Before heating
After heating
Temperature (K)
pH
Period (day)
+83 +83 +83 +242 +242 +263 245 +628 +628
0 (decomposed) +86 +87 +65 +222 +262 255 0 (decomposed) +638
403 393 368 385 381 373 373 373 373
5.0 5.0 5.0 5.0 4.5 4.5 4.0 5.5 4.0
20 20 20 23 31 13 17 17 17
Intensity (cps)
20000
b
10000
a 0 0
20
40
60
80
2θ (degrees) Fig. 1. XRD patterns for (a) nucleated gel and (b) product by the rapid crystallization method in acidic media.
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Fig. 2. SEM images showing morphology and size of (a) nucleated gel and (b) product by the rapid crystallization method in an acidic medium.
conventional synthesis in fluoride medium at higher pH gave crystals with the similar morphology but much larger size. The nano-size of the crystals obtained in the acidic synthesis suggests that a large number of nuclei grew slowly during the long crystallization time. The crystallization process of nano-size zeolite beta developed by Mintova et al. [30] was accomplished within the shorter period (3– 11 days) from basic aluminosilicate solutions at 373 K than the acidic process. It is concluded that zeolite beta is crystallized at pH 4 and 373 K in the presence of chiral (+)[Rh(bpy)3](HF2)3 without any racemization using the gel nucleated by the rapid crystallization method.
source. In our condition, sodium might form Na2SiF6 as a side product consuming silica from the gel mixture and prevent the crystallization of zeolite beta. Lithium hydroxide will be appropriate for the dissolving agent because lithium does not form Li2SiF6 or other fluorosilicates and will consume no Si atoms. Lithium ions will be combined with fluoride ions to form only LiF in our acidic condition. Zeolite beta previously dealuminated in boiling hydrochloric acid was dissolved in aqueous solution of lithium hydroxide. It is assumed that this solution will contain structural building blocks of zeolite beta. It was concentrated by evaporation of water. After adjusting pH with hydrofluoric acid, the mixture was kept at 373 K. Zeolite beta was obtained in a limited pH range of 8–9 but no crystals of zeolite beta were given in other pH region. Some kinds of lithium silicates were dominantly crystallized at higher pH than 9 as shown in Fig. 3 (pH 11). Amorphous materials and lithium fluoride were mainly formed at lower pH than 8. Further trick was required for the acidic crystallization. After dissolving zeolite beta in the lithium hydroxide solution, the thin solution (H2O/SiO2 = 700) was maintained in hydrothermal temperature (413 K) for 4 days in pH 8–9 in order to obtain a large number of the nuclei. The nucleated gel mixtures were amorphous as
3.2. Recrystallization method 3.2.1. Recrystallization of zeolite beta Though zeolite beta could be synthesized in an acidic medium under mild condition as shown above, the crystallization rate was extremely slow and long time (two months) was needed for the crystallization. In order to shorten the period of crystallization, we tried to recrystallize zeolite beta. Ogura et al. [31] reported reprecipitation of ZSM-5 from alkali solution containing dissolved ZSM-5. They adopted sodium hydroxide as an alkali
120000
Intensity (cps)
pH 11 80000
pH 8.5 40000
pH 4 0 0
20
40
60
80
2θ (degrees) Fig. 3. XRD patterns for products by recrystallization at pH of 11, 8.5 and 4. s: Li2SiO5 Æ 2H2O, d: Li2SiO3.
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shown in Fig. 4a. The gel was acidified with hydrofluoric acid to pH 4 and concentrated. Then it was preserved at 373 K for several weeks. After 20 days, zeolite beta was crystallized accompanying lithium fluoride as shown in Fig. 4b. This suggests the nucleated gel had many fine nuclei of zeolite beta. A larger number of nuclei will result in the shorter period for the crystallization. The nuclei seemed to be too fine to be detected by XRD since no peaks of zeolite beta were observed (Fig. 4a). Some papers dealing with the nucleation of zeolite beta were recently reported. Larlus and Valtchev [32] developed the two step nucleation procedure, which allows the synthesis of all-silica zeolite beta under basic condition. The crystallization is difficult under such condition in the usual fluoride process. The appropriate nucleation process can extend the range of the condition to prepare zeolite beta. Corma and Dı´az-Caban˜as [33] synthesized microporous materials which have pore systems close to zeolite beta though they showed amorphous characteristics in XRD, IR and electron diffractions. The material related to zeolite beta was prepared within 6 h from dense gel (H2O/ SiO2 = 7.5) at 448 K under quasi-neutral condition. It
can be considered as a zeolite precursor during the synthesis process. It is concluded that zeolite beta is crystallized in a period of 20 days at pH 4 and 373 K using the nucleated gel prepared from thin solution by the nucleation at pH 8–9 and 413 K. 3.2.2. Recrystallization of zeolite beta in the presence of rhodium chiral complexes As zeolite beta had been obtained in the acidic condition in the period of two or three weeks, we tried the crystallization in the presence of the chiral rhodium complex as a secondary SDA. Table 2 indicates the formed phases in the acidic crystallization using various rhodium complexes, whose structures are presented in Fig. 5. Zeolite beta was crystallized in the presence of some rhodium complexes, ()-[Rh(bpy)3](HF2)3, (±)-[Rh(en) (t-Bu2bpy)2] (HF2)3 and (+)-[Rh(en)(t-Bu2bpy)2] (HF2)3. (+)-[Rh(phen)3](HF2)3 and (±)-[Rh(en)(phen)2](HF2)3 gave only lithium fluoride and an amorphous material, while (+)-[Rh(en)3](HF2)3 formed unknown products.
Intensity (cps)
40000
30000
20000
b 10000
a 0 0
20
40
60
80
2θ (degrees) Fig. 4. XRD patterns for (a) nucleated gel and (b) product by the recrystallization method in an acidic medium. s: LiF.
Table 2 Products obtained by acidic crystallization in the presence of chiral rhodium complexes as secondary SDAs No.
1 2 3 4 5 6 7 8 9 10
Complexes
(+)-[Rh(en)3](HF2)3 ()-[Rh(bpy)3](HF2)3 (±)-[Rh(en)(tBu2bpy)2](HF2)3 (+)-[Rh(en)(tBu2bpy)2](HF2)3 (+)-[Rh(phen)3](HF2)3 (±)-[Rh(en)(phen)2](HF2)3 Blank (original method) Blank (original method) Blank (fluoride method) Blank (fluoride method)
Symmetry
D3 D3 C2 C2 D3 C2 – – – –
XRD intensity (103cps)
A/(A + B) (%)
Phase of products
Peak B
Peak A
Peak ratio
Unknown Beta Amorphous, Amorphous, Amorphous, Amorphous, Beta Beta Beta Beta
–
–
– 75.9 86.4 88.2 – – 76.7 79.5 80.3 82.8
LiF, Beta LiF, Beta LiF LiF
20.6 2.7 15.9 – – 313.6 259.6 249.4 124.1
65.0 16.8 118.3 – – 1030.6 1004.1 1014.6 598.9
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H2 N H2N
N
NH2
3+ N
N
Rh
3+
3+ H2N
NH2
N
N
NH2
Rh
N
Rh
N NH2
N
H2 N
No.2 (-)-[Rh(bpy)3]3+
No.1 (+)-[Rh(en)3]3+
H2 N
N
N
)-[Rh(en)(tBu2bpy)2]3+
No.3
NH2 H2 N 3+
N
N
N
Rh
3+ N
3+ N
NH2
N
N
N
N
Rh
N N
No.4 (+)-[Rh(en)(tBu2bpy)2]3+
N
Rh
N
N
No.5 (+)-[Rh(phen)3]3+
No.6
-[Rh(en)(phen) 2]3+
Fig. 5. Structures of the chiral rhodium complexes used in the recrystallization method.
3.3. Composition of polymorphs 3.3.1. Peak separation Composition of polymorphs was evaluated by XRD profiles of zeolite beta. The shape of the first low angle peak of zeolite beta was affected by the composition. The peak was separated into two peaks. The lower angle peak appeared at 2h = 7.34° (Cu Ka) was assigned to (1 1 0) plane of polymorph B, and the higher one at 2h = 7.74°
to (1 0 1) plane of polymorph A (Fig. 6) [34]. The ratio of the integrated intensity of polymorph A to that of the whole peak, A/(A + B), reflects the proportion of polymorph A. As shown in Table 2, the usual beta synthesized by the original method containing 44% of polymorph A [3] had the A/(A + B) ratio of 76.7–79.5% (Entry Nos. 7 and 8). Zeolite beta obtained by the conventional fluoride method gave the ratio of 80.3–82.8% (Nos. 9 and 10). These results support that the fluoride method gives zeolite
e
Intensity (cps)
20000
b 10000
a
c d
0 4
6
8
10
2θ (degrees) Fig. 6. Peak separation of the first low angle peak of zeolite beta (Entry 7): (a) observed data; (b) higher angle peak; (c) lower angle peak; (d) base line data and (e) synthesized profile.
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beta more enriched in polymorph A than the original hydroxide method [12]. The products obtained using [Rh(en)(t-Bu2bpy)2]3+ (Entry Nos. 3 and 4) gave the higher A/(A + B) ratios
(86.4% for racemate and 88.2% for (+)-enantiomer) than those using ()-[Rh(bpy)3]3+ (No. 2, 75.9%). The former complex has C2 symmetry, while the latter has D3 one. The A/(A + B) ratios obtained with the C2 complexes were
OCH3 OH
N
HO
N
H3CO
N
N
H3C
OCH3
CH3
H
H O O
N
OH
N
No.11 (-)-cinconidine
No.12 (+)-cinconine
No.13 (+)-berbamine
O
CH3
H3 C H
H3 C
N
NH O
N
CH3
O
O
OH H
O
H
H N
H
H CH3
O N
No.14 (-)-eburnamonine
H
N
N
N
N
N
O
H H
No.16 (-)-lobeline
No.15 (+)-emetine
O
H
O
H
O
N
No.19 (+)-BMBA [(R)-(R*,R*)]-(+)-bis (α -methylbenzyl)amine
No.18 (+)-DHQ2PHAL
HO O N
H
O
N N
N
N
OH
NH2
H
O
No.22 (-)-BDtBSCDA (R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)1,2-cyclohexanediamine
No.21 (+)-BINDA (R)-(+)-1,1'-binaphthyl2,2'-diamine
O
N
N
NH2
No.20 (-)-BiPOP 2,6-bis[(4S)-isopropyl-2-oxazolin-2-yl] pyridine
O
CH3
H N H
OCH3
N
No.17 (-)-strychinine
H
H
H3CO
H
N
H3C
H
H
O
N N
N
NH2
N NH2
CH3
H3C
No.23 (+)-BMPOP
No.24 (-)-BPOP
2,6-bis[(4R,5R)-4-methyl-5-phenyl2-oxazolinyl]pyridine
2,6-bis[(4S)-4-phenyl2-oxazolinyl]pyridine
No.25 (+)-DPEN (1R,2R)-(+)-1,2diphenylethylenediamine
Fig. 7. Structures of the chiral amines used in the recrystallization method.
Y. Takagi et al. / Microporous and Mesoporous Materials 109 (2008) 567–576
also higher than those of the usual beta or the fluoride beta. This suggests that C2 symmetry of the secondary SDA might be suitable for the selective crystallization of polymorph A. 3.3.2. Effect of symmetry of SDA on the selectivity to polymorph A Then we investigated the effect of symmetry of chiral amines on selectivity to polymorph A. Their molecular structures are presented in Fig. 7. Table 3 indicates the formed phases in the acidic crystallization using various amines as secondary SDAs. Amines of Entry Nos. 11–17 have 1 symmetry, while those of Nos. 18–25 have C2 symmetry. Among the 1 symmetry amines, zeolite beta was obtained in the presence of some alkaloids, ()-cinconidine, (+)-cinconine or ()-strychinine but was not obtained in the presence of other alkaloids, (+)-berbamine, ()eburnamonine, (+)-emetine or ()-lobeline. The A/ (A + B) ratios of the products were not so high (75.3– 82.8%). Some chiral amines which have C2 symmetry also led to crystallize zeolite beta (Nos. 18, 19, 21, 22 and 23). Some of them, (+)-(DHQ)2PHAL (No. 18), (+)-BMBA (No. 19) and (+)-BMPOP (No. 23) showed the high A/ (A + B) ratio (92.6%, 90.6% and 92.2%, respectively), though no higher angle peaks (e.g. 2h = 9.68° for (1 0 2) plane) unique to polymorph A [33] were observed in XRD measurements. The unique peak will emerge when the portion of polymorph A is lager than 60% according to the simulation [35]. (+)-BMBA (No. 19) and (+)-BMPOP (No. 23) might fit into the beta pore system while (+)-(DHQ)2PHAL (No. 18) appears too large. The carbon and nitrogen contents of the sample were determined by organic elemental analyzer in order to examine if the chiral amines were included in them. Reference materials prepared without any chiral amines had C/N ratio of 8.4 ± 0.7. This value is comparable to
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8.0 which is the C/N ratio of TEA. As the chiral amines have different C/N ratio (16.0 for No. 19 and 5.7 for No. 23) from TEA, it will be judged if the samples included them. The C/N ratio of the samples (Nos. 19 and 23) was 9.3 and 8.6 respectively. (+)-BMBA (No. 19a) might be included in the sample but (+)-BMPOP (No. 23) might not. This suggests that the former sample might include 10–20 mol% of (+)-BMBA (No. 19) compared to TEA. Thermal analysis was carried out in order to investigate the location of the amine. The ignition point of the amine incorporated in the pore system may be higher than that of outer one. Unfortunately, very little information concerning the location of amine was obtained partly because of the very low content of chiral amine compared with TEA. In order to evaluate chirality of the product, (+)- or ()BMBA was adsorbed on the calcined samples. The reference sample (No. 8) adsorbed 56% of (+)-BMBA and 59% of ()-BMBA from the solution (0.05 mol/l of BMBA in 98% MTBE + 2% 2-propanol) while the sample (No. 19) adsorbed 49% of (+)-BMBA and 33% of ()-BMBA. This suggests that zeolite beta recrystallized with (+)-BMBA as a secondary SDA shows slight enantio-selectivity in the adsorption of BMBA. The reference sample adsorbed more BMBA than the recrystallized one probably because the former had more defects than the latter. Camblor et al. reported that zeolite beta synthesized in hydroxide media had a lot of defects [12]. It should be desirable to determine the fraction of polymorph A by further characterization with advanced methods, such as HRTEM [36] and PDF [37]. It is concluded that zeolite beta crystallized in the presence of chiral amines with C2 symmetry consists of a somewhat lager portion of polymorph A than that using other compounds though the portion is up to 60%. This method may give a possibility for the selective synthesis of polymorph A.
Table 3 Products obtained by acidic crystallization in the presence of various chiral amines as secondary SDAs No.
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 a
Amines
()-cinconidine (+)-cinconine (+)-berbamine ()-eburnamonine (+)-emetine ()-lobeline ()-strychinine (+)-(DHQ)2PHAL (+)-BMBA ()-BiPOP (+)-BINDA ()-BDtBSCDA (+)-BMPOP ()-BPOP (+)-DPEN
Symmetry
1 1 1 1 1 1 1 C2 C2 C2 C2 C2 C2 C2 C2
Phase of products
Amorphous, LiF, Beta Amorphous, LiF, Beta Amorphous, LiF Amorphous, LiF Amorphous, LiF Amorphous, LiF Amorphous, LiF, Beta Beta Amorphous, LiF, Beta Amorphous, LiF Unknown, LiF, Beta Unknown, Beta, Amorphous Amorphous, Beta Amorphous Unknown, Beta
XRD intensity (103cps)
A/(A + B)(%)
Peak B
Peak ratio
3.2 14.8 – – – – 17.1 125.6 14.2 – 24.6 38.1 1.2 – –a
Peak A 9.8 65.1 – – – – 82.3 1562.2 136.1 – 122.5 195.2 14.5 – –a
Peak separation was not completed because of the overlap between two peaks assigned to beta and the unknown product.
75.3 81.5 – – – – 82.8 92.6 90.6 – 83.3 83.7 92.2 – –a
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4. Conclusions Two methods of acidic crystallization of zeolite beta have been developed. Zeolite beta is crystallized at pH 4 and 373 K for two months in the presence of the chiral rhodium complex without any racemization by the rapid crystallization method. Zeolite beta is also crystallized in a period of 20 days at pH 4 and 373 K by the recrystallization method. This method is appropriate to use chiral amines as secondary SDAs without further derivatization like N-alkylation. Zeolite beta crystallized in the presence of the chiral rhodium complex or of amines with C2 symmetry consists of somewhat large portion of polymorph A. This method may have a possibility for the selective synthesis of polymorph A. Acknowledgments
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25]
We thank Ms. Yumiko Aoki and Mr. Hiroyuki Fujisawa for measuring the XRD profiles.
[26]
References
[27]
[1] R.L. Wadlinger, G.T. Kerr, E.J. Rosinski, US Patent No. 3,308,069, 1967. [2] M.M.J. Treacy, J.M. Newsam, Nature 332 (1988) 249. [3] J.M. Newsam, M.M.J. Treacy, W.T. Koetsier, C.B. de Gruyter, Proc. R. Soc. Lond. A 420 (1988) 375. [4] Y. Zhu, G. Chuah, S. Jaenicke, J. Catal. 227 (2004) 1. [5] A. Cantin, A. Corma, M.J. Dı´az-Caban˜as, J.L. Jorda, M. Moliner, F. Rey, Angew. Chem. Int. Ed. 45 (2006) 8013. [6] A. Corma, M.T. Navarro, F. Rey, J. Rius, S. Valencia, Angew. Chem. Int. Ed. 40 (2001) 2277. [7] A.W. Burton, S. Elomari, I. Chan, A. Pradhan, C. Kibby, J. Phys. Chem. B 109 (2005) 20266. [8] J.P. Shen, J. Ma, J. Guo, D.Z. Jinag, E.Z. Min, Chin. Chem. Lett. 5 (1994) 1075. [9] C.B. Dartt, M.E. Davis, Catal. Today 19 (1994) 151. [10] Q.H. Xia, S.C. Shen, J. Song, S. Kawi, K. Hidajat, J. Catal. 219 (2003) 74. [11] M.E. Davis, R.F. Lobo, Chem. Mater. 4 (1994) 756. [12] M.A. Camblor, A. Corma, S. Valencia, Chem. Commun. (1996) 2365. [13] J.L. Guth, H. Kessler, R. Wey, in: Y. Murakami, A. Iijima, J.W. Ward (Eds.), New Developments in Zeolite Science and Technology,
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Microporous and Mesoporous Materials 109 (2008) 567–576 www.elsevier.com/locate/micromeso
Crystallization of zeolite beta in the presence of chiral amine or rhodium complex Yukio Takagi a
a,b,*
, Takayuki Komatsu a, Yasuyoshi Kitabata
b
Department of Chemistry, Tokyo Institute of Technology, 2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b Technical Center of Chemical Catalysts, Manufacturing Department, Numazu Plant, N.E. Chemcat Corporation, 678 Ipponmatsu, Numazu, Shizuoka 410-0314, Japan Received 21 August 2006; received in revised form 14 May 2007; accepted 2 June 2007 Available online 12 June 2007
Abstract Crystallization methods of zeolite beta in an acidic medium at low temperature (pH 4, 373 K) were developed. Zeolite beta was crystallized in the presence of chiral amine or rhodium complex as secondary structure directing agents (SDAs) in the acidic condition whose advantage was high solubility of the chiral amines. The secondary SDAs with C2 symmetry induced the crystallization of zeolite beta which contained a somewhat large fraction of polymorph A. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Zeolite beta; Polymorph; Chiral; Structure directing agents; Rhodium complex
1. Introduction Zeolite beta was first synthesized in 1967 using tetraethylammonium hydroxide as a structure directing agent (SDA) [1]. Zeolite beta has three relational structures, polymorph A (P4122 or P4322), polymorph B (C2/c) and polymorph C (P42/mmc). Only polymorph A is chiral [2] but the other polymorphs, B and C, are achiral. Usual zeolite beta synthesized by the original method has 44% of polymorph A and 56% of polymorph B [3]. The usual beta includes only a small percentage of polymorph C because it contains double four member ring (D4MR) cages as secondary building units which have high tension in their structure. Preferential syntheses of achiral polymorphs, B and C, have already been reported. Zhu et al. [4] synthesized Al-free zirconosilicate enriched with polymorph B *
Corresponding author. Address: Technical Center of Chemical Catalysts, Manufacturing Department, Numazu Plant, N.E. Chemcat Corporation, 678 Ipponmatsu, Numazu, Shizuoka 410-0314, Japan. Fax: +81 55 966 1832. E-mail address: [email protected] (Y. Takagi). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.06.005
using TEAOH as an SDA in a fluoride medium. Cantin et al. reported the synthesis and characterization of all-silica materials containing larger portion of polymorph B [5]. Pure polymorph C of silicate [5] and germanosilicate [6] was also synthesized. SSZ-63 which is described as a random intergrowth of polymorph B and C was prepared and structurally elucidated by Burton et al. [7]. Shen et al. [8] reported that polymorph A was more soluble in mineral acids than polymorph B. Selective crystallization of polymorph A remains as one of the challenges in zeolite synthesis. Dartt et al. reported the synthesis of zeolite beta in the presence of chiral organic molecules but the preparation method had not been disclosed. An X-ray powder pattern of their zeolite beta suggested the zeolite had somewhat larger domains of polymorph A than the usual zeolite beta [9]. In spite of the difficulty concerning selective crystallization of polymorph A, there are a few reports describing catalytic applications of zeolite beta for asymmetric reactions such as formation of 1,2-diphenylethane-1,2-diol by hydration of trans-stilbene oxide [9], or hydrogenation of tiglic acid to 2-methylbutanoic acid [10]. Some requirements of
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SDA for the selective synthesis of the chiral polymorph were suggested by Davis and Lobo [11]. According to them, the SDA molecule must have at least three constraints. First, it must be chiral. Second, its length must be at least 1 nm. Third, it must be stable at the synthesis conditions of high pH and temperatures greater than 383 K. If zeolite beta can be crystallized in acidic media at low temperatures, the third constraint is highly relaxed. Zeolite beta is usually synthesized by hydrothermal crystallization in an alkaline medium because the crystallization requires some solubility of silicate and the source of the other framework elements. Hydroxide anion, which ensures the solubility of these sources in the synthetic medium, has a role of a mineralizer. Camblor et al. [12] synthesized zeolite beta in almost neutral condition using fluoride anion in stead of hydroxide anion as a mineralizer. It is not popular to crystallize zeolites in acidic media probably because the acidity extremely decreases the rate of crystallization. Guth et al. [13] synthesized ZSM-5 in an acidic medium (pH 5) at 443 K. However there are no reports disclosing the synthesis of zeolite beta in acidic conditions to our best knowledge. One of advantages of the acidic crystallization is the higher solubility of amines in an acidic medium, compared to neutral or alkaline synthesis conditions. Most of chiral amines such as natural alkaloids are practically insoluble in water but some of them are soluble in acidic aqueous solution. The acidic condition would enable the crystallization in the presence of various chiral amines without any derivatization such as N-alkylation. The acidic condition is also convenient to use coordination complexes or organometallic species as SDAs because the acidity deactivates substitution of ligands of these compounds [14]. Since the metal complexes or organometallic species have a variety in size and shape, they have been used in the synthesis of zeolitic materials [15–19] as SDAs. UTD-1, which has 14 member rings called ultra large pore, was synthesized using organometallic cation, [Co(Cp*)2]+ (Cp* = pentamethylcyclopentadienyl), as an SDA [20]. Gray et al. [21] crystallized special aluminophosphates using coordination complexes of cobalt, such as [Co(en)3]3+ (en = ethylenediamine) or [Co(tn)3]3+ cations (tn = trimethylenediamine). Rhodium complexes are able to form chiral structure similar to those of the cobalt complexes. The chiral rhodium complexes are more stable for racemization than corresponding cobalt ones. [Rh(en)3]3+ has stronger resistance to racemization in boiling water than [Co(en)3]3+ [22]. Even such stable Rh complexes, however, are decomposed or racemized in the usual conditions of zeolite synthesis. Acidic crystallization at low temperature will have an advantage to prevent the chiral complexes from decomposition or racemization. A rhodium complex, [Rh(phen)3]3+ (phen = 1,10-phenanthroline), led to the formation of SAPO-20 [23], which has SOD structure. The rhodium complex is too large to be incorporated in a sodalite cage. This suggests that the rhodium complex played a role of an SDA working outside
zeolitic domains, while usual SDAs work in the domains. The complex might control the intergrowth rather than nucleation in the crystallization of zeolites. In this study, we developed crystallization methods of zeolite beta in an acidic medium at low temperature (pH 4, 373 K) and reported crystallization of zeolite beta in the presence of the chiral rhodium complex or amine which has various symmetries of point groups (1, C2 and D3). The purpose of this study is to search the constraint on the effective SDA for the selective crystallization of polymorph A. 2. Experimental 2.1. The rapid crystallization method for acidic synthesis of zeolite beta The rapid crystallization method [24,25] was adopted for the acidic synthesis. The mixture of tetraethylorthosilicate (TEOS, Tokyo Chemical Industry) and aqueous solution (35% w/w) of tetraethylammonium hydroxide (TEAOH, SACHEM) was stirred at room temperature until the ethanol formed on hydrolysis of TEOS was completely evaporated. Hydrofluoric acid (47% w/w, Wako Pure Chemical) was added to the clear solution and thick gel was formed. The gel composition was 1.0 SiO2:0.54 TEA:0.54 F:7.5 H2O. Nucleation was carried out in a PTFE vessel at 413 K for 50 h under static condition. After quenching, chiral [Rh(bpy)3](HF2)3 was added to the nucleated gel followed by the adjustment of pH into 4 with HF and the evaporation of water at 313 K. The pH of the dense gel was measured with four kinds of test paper (BCG, BPB, PB and PP) because usual glass electrodes are corroded under acidic conditions in the presence of fluoride. The final gel composition was 1.0 SiO2:0.54 TEA:4.0 F:4.5 H2O:0.30 Rh. Crystallization was performed at 373 K for 2 months statically. The solid product obtained was filtered, washed with deionized water and dried at room temperature. The pH of the mother liquids was in the range of 3.5–4.5. No decrease in the optical rotation of the chiral rhodium complex was observed. 2.2. Recrystallization method 2.2.1. Synthesis of zeolite beta seeds The preparation method of zeolite beta for seed crystals was adapted from the process reported by Wadlinger et al. [1] with modification to reduce the content of sodium. Aluminum sulfate hydrate (15.8 g, Wako Pure Chemical) was dissolved in 130.4 g of TEAOH solution. Then, 149.5 g of silica sol (Snowtex-40, 40 wt.% SiO2, 0.6% Na2O in water, Nissan Chemical) was added to the solution with stirring. The molar composition of the mixture was 1.0 SiO2:0.31 TEAOH:0.026 Al2O3:0.015 Na2O:10.46 H2O. The final mixture was transferred into a PTFE vessel and kept at 413 K for 4 days statically. After the vessel was quenched, the precipitate was filtered, washed with deionized water and dried at 373 K.
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The as-made zeolite was dealuminated in order to be used for seed crystals in the synthesis of fluoride method because of the rate of crystallization would be slow in Alrich systems [26]. One gram of the as-made sample was treated with 100 ml of 12 M HCl at 363 K for 3 days in order to eliminate the aluminum. The solid was filtered and washed with deionized water, and dried at 373 K. XRF analysis showed the Si/Al ratio of the dealuminated zeolite beta to be higher than 1000. 2.2.2. Recrystallization of zeolite beta in an acidic medium at low temperature Lithium hydroxide monohydrate (0.8 g) was dissolved in 35 ml of deionized water. The solution was heated to 353 K. The dealuminated beta (1 g) was added to the hot solution with stirring. After 1 h, the hot slurry was filtered through 0.1 lm membrane. Tetraethylammonium fluoride solution (2.94 g, 50 wt.% in water) and 126 ml of water were added to the filtrate. The resulting solution was neutralized with 47% hydrofluoric acid to pH 8–9 and white gel was formed. The molar composition of the gel was 1.0 SiO2:0.78 TEAOH:1.5 LiOH:2.0 HF:700 H2O. The gel was put into the PTFE vessel and kept at 413 K for 4 days for renucleation. To the renucleated gel, a specific amount of the chiral cation was added. The gel was acidified with HF to pH 4, followed by concentrating at 313 K. Typical molar composition of the precursor was 1.0 SiO2:0.78 TEAOH:4 HF:2.0 LiOH:0.12 chiral cation:4.5 H2O. The final precursor was transferred into the PTFE vessel and kept at 373 K for three weeks. The pH of the reacted slurry was in the range 4–5. The product was put into 100 ml of water, stirred for 30 min, filtered and washed with deionized water and dried. 2.3. Preparation of chiral cations Alkaloids and other chiral amines were purchased from Aldrich or Wako Pure Chemical. Tetraethylammonium fluoride solution was obtained by neutralization of TEAOH with HF. AgHF2 was prepared by dissolving of Ag2O into HF. Chiral rhodium complexes were synthesized as follows: (+)-[Rh(en)3](HF2)3 (+)-[Rh(en)3]Cl3 was synthesized from rhodium chloride hydrate (N.E. Chemcat) and ethylenediamine (Wako Pure Chemical) by literature method [27]. (+)-[Rh(en)3](HF2)3 was obtained by metathesis between the corresponding chloride salt and AgHF2. Optically active [Rh(bpy)3](HF2)3 Racemic complex, [Rh(bpy)3]Cl3, was synthesized by literature method [28] with some modification. Rhodium chloride containing 20 g of rhodium was dissolved in 1000 ml of deionized water under nitrogen atmosphere. 105.4 g of 2,2 0 -bipyridine (Wako Pure Chemical) was dissolved in 750 ml of ethanol. The ethanol solution was added
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to the rhodium solution followed by the addition of 0.2 g of hydrazine monohydrochloride. After 16 h of reflux under nitrogen atmosphere, the mixture was evaporated to dryness. The yellow rhodium complex was extracted with water and recrystallized from water/ethanol/acetone with 90% yield. Optical resolution K3{()-[Co(l-cysu)3]} (cysu = cysteinsulfinato) was used as a resolving agent [29]. (+)-[Rh(bpy)3]Cl3 was obtained by fractional crystallization using K3{()-[Co(l-cysu)3]}. The diastereomer of {(+)-[Rh(bpy)3]}{()-[Co(l-cysu)3]} was less soluble in water than the other enantiomer. The diastereomer was precipitated and separated from the mother liquid followed by anion exchange using Sephadex A-25 (Cl-form, Pharmacia). The ()-isomer was obtained from the filtrate followed by recrystallization. (+)-[Rh(bpy)3](HF2)3 was obtained from the respective chloride by metathesis using AgHF2. [Rh(phen)3](HF2)3 and other rhodium complexes were prepared similarly. 2.4. Characterization All samples of the as-made zeolite were characterized by MAC Science X-ray diffractometer MXP21VACE using Cu Ka radiation operating at 40 kV/300 mA. The scanning range was 2h = 3–88°. Peak separation was carried out using XPRESS-1.1.1 (X-ray Powder Research Software System, developed by MAC Science and the University of Oxford). A JEOL JSM-6300 scanning electron microscope was used to measure the morphology of crystallites under 104 Pa. The siliceous samples had to be coated with carbon sputtered prior to the observation. A Thermo Finnigan Italy automatic elemental analyzer EA-1112 was used to determine the contents of carbon and nitrogen in the zeolitic samples. Thermal analysis of the samples was carried out using Rigaku Thermo plus TG8120. Adsorption tests of bis(a-methylbenzyl)amine were performed as follows. Zeolite beta sample (120 mg) was put into 20 ml of mixture of methyl-tert-butylether/2-propanol (98:2 v/v) containing 1.0 lmol of (+)- or ()-bis(a-methylbenzyl)amine. The slurry was shook for 3 h at room temperature and settled. Absorbance at 260 nm of the supernatant was measured with Hitachi spectrophotometer U-3310. 3. Results and discussion 3.1. Crystallization of zeolite beta in the presence of the chiral rhodium complex First, we tried to develop a crystallization method of zeolite beta using chiral rhodium complexes without any racemization in order to study the effect of the complexes on the selectivity to polymorph A. If the rhodium
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added into the precursor gel in order to shorten the total period of crystallization. This method was applied to the acidic crystallization (pH 4, 373 K). The gel mixtures were treated in the condition of usual hydrothermal synthesis till the nuclei were formed. Fig. 1a shows an XRD pattern of the nucleated gel after filtration and drying. A few intense peaks of zeolite beta were observed but other weaker peaks did not appear. An SEM image of the gel is shown in Fig. 2a. The morphology of the gel appears amorphous. These results mean that the gel had been nucleated and had many small structures which were barely detected by XRD but not detected by SEM. The gel would have a lot of the nuclei. A chiral rhodium complex, (+)-[Rh(bpy)3](HF2)3, was added to the gel followed by adjusting it to pH 4.0 with HF and evaporation of water at 313 K to H2O/SiO2 = 4.5. Zeolite beta was crystallized at 373 K in a period of two months in the presence of the chiral complex. An XRD pattern of the product is shown in Fig. 1b. A specific optical rotation of the rhodium complex was maintained and no racemization was observed after the hydrothermal synthesis. Fig. 2b shows the SEM image of the crystallized zeolite beta. The crystals have a bipyramidal habit with a diameter around 0.3 lm. The
complexes can control the intergrowth along C-axis of zeolite beta, the selective synthesis of polymorph A might be achieved. Though rhodium complexes are more stable to racemization than cobalt ones, they are racemized and decomposed in the usual condition (e.g. pH 8.5, 413 K). The thermostability of some chiral rhodium complexes were tested as shown in Table 1. [Rh(en)3]3+ was the most stable and withstood heating to 393 K at pH of 5. [Rh(bpy)3]3+ and [Rh(phen)3]3+ were racemized in this condition. They were stable only up to 373 K at pH of 4.5 or lower. We tried to crystallize zeolite beta by the usual fluoride methods with modification adding hydrochloric acid to keep the pH of the gel mixture at 4.5 at 373 K. The gel composition was 1.0 SiO2:0.54 TEAOH:0.54 HF:7.5 H2O. Even after two months, the product had been still amorphous judging from its XRD powder patterns. In order to promote the crystallization of zeolite beta in the acidic condition, the rapid crystallization method developed by Inui et al. for ZSM-5 [24] and ZSM-34 [25] was adopted. Zeolite synthesis often requires a long incubation period during which the nuclei of the zeolite are formed. In the rapid crystallization method, the preformed nuclei are
Table 1 Thermostability of chiral rhodium complexes Complexes
(+)-[Rh(en)3]Cl3
()-[Rh(bpy)3]Cl3
()-[Rh(bpy)3]Cl3 (+)-[Rh(phen)3]Cl3
Optical rotation [a]D (degree)
Heating condition
Before heating
After heating
Temperature (K)
pH
Period (day)
+83 +83 +83 +242 +242 +263 245 +628 +628
0 (decomposed) +86 +87 +65 +222 +262 255 0 (decomposed) +638
403 393 368 385 381 373 373 373 373
5.0 5.0 5.0 5.0 4.5 4.5 4.0 5.5 4.0
20 20 20 23 31 13 17 17 17
Intensity (cps)
20000
b
10000
a 0 0
20
40
60
80
2θ (degrees) Fig. 1. XRD patterns for (a) nucleated gel and (b) product by the rapid crystallization method in acidic media.
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Fig. 2. SEM images showing morphology and size of (a) nucleated gel and (b) product by the rapid crystallization method in an acidic medium.
conventional synthesis in fluoride medium at higher pH gave crystals with the similar morphology but much larger size. The nano-size of the crystals obtained in the acidic synthesis suggests that a large number of nuclei grew slowly during the long crystallization time. The crystallization process of nano-size zeolite beta developed by Mintova et al. [30] was accomplished within the shorter period (3– 11 days) from basic aluminosilicate solutions at 373 K than the acidic process. It is concluded that zeolite beta is crystallized at pH 4 and 373 K in the presence of chiral (+)[Rh(bpy)3](HF2)3 without any racemization using the gel nucleated by the rapid crystallization method.
source. In our condition, sodium might form Na2SiF6 as a side product consuming silica from the gel mixture and prevent the crystallization of zeolite beta. Lithium hydroxide will be appropriate for the dissolving agent because lithium does not form Li2SiF6 or other fluorosilicates and will consume no Si atoms. Lithium ions will be combined with fluoride ions to form only LiF in our acidic condition. Zeolite beta previously dealuminated in boiling hydrochloric acid was dissolved in aqueous solution of lithium hydroxide. It is assumed that this solution will contain structural building blocks of zeolite beta. It was concentrated by evaporation of water. After adjusting pH with hydrofluoric acid, the mixture was kept at 373 K. Zeolite beta was obtained in a limited pH range of 8–9 but no crystals of zeolite beta were given in other pH region. Some kinds of lithium silicates were dominantly crystallized at higher pH than 9 as shown in Fig. 3 (pH 11). Amorphous materials and lithium fluoride were mainly formed at lower pH than 8. Further trick was required for the acidic crystallization. After dissolving zeolite beta in the lithium hydroxide solution, the thin solution (H2O/SiO2 = 700) was maintained in hydrothermal temperature (413 K) for 4 days in pH 8–9 in order to obtain a large number of the nuclei. The nucleated gel mixtures were amorphous as
3.2. Recrystallization method 3.2.1. Recrystallization of zeolite beta Though zeolite beta could be synthesized in an acidic medium under mild condition as shown above, the crystallization rate was extremely slow and long time (two months) was needed for the crystallization. In order to shorten the period of crystallization, we tried to recrystallize zeolite beta. Ogura et al. [31] reported reprecipitation of ZSM-5 from alkali solution containing dissolved ZSM-5. They adopted sodium hydroxide as an alkali
120000
Intensity (cps)
pH 11 80000
pH 8.5 40000
pH 4 0 0
20
40
60
80
2θ (degrees) Fig. 3. XRD patterns for products by recrystallization at pH of 11, 8.5 and 4. s: Li2SiO5 Æ 2H2O, d: Li2SiO3.
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shown in Fig. 4a. The gel was acidified with hydrofluoric acid to pH 4 and concentrated. Then it was preserved at 373 K for several weeks. After 20 days, zeolite beta was crystallized accompanying lithium fluoride as shown in Fig. 4b. This suggests the nucleated gel had many fine nuclei of zeolite beta. A larger number of nuclei will result in the shorter period for the crystallization. The nuclei seemed to be too fine to be detected by XRD since no peaks of zeolite beta were observed (Fig. 4a). Some papers dealing with the nucleation of zeolite beta were recently reported. Larlus and Valtchev [32] developed the two step nucleation procedure, which allows the synthesis of all-silica zeolite beta under basic condition. The crystallization is difficult under such condition in the usual fluoride process. The appropriate nucleation process can extend the range of the condition to prepare zeolite beta. Corma and Dı´az-Caban˜as [33] synthesized microporous materials which have pore systems close to zeolite beta though they showed amorphous characteristics in XRD, IR and electron diffractions. The material related to zeolite beta was prepared within 6 h from dense gel (H2O/ SiO2 = 7.5) at 448 K under quasi-neutral condition. It
can be considered as a zeolite precursor during the synthesis process. It is concluded that zeolite beta is crystallized in a period of 20 days at pH 4 and 373 K using the nucleated gel prepared from thin solution by the nucleation at pH 8–9 and 413 K. 3.2.2. Recrystallization of zeolite beta in the presence of rhodium chiral complexes As zeolite beta had been obtained in the acidic condition in the period of two or three weeks, we tried the crystallization in the presence of the chiral rhodium complex as a secondary SDA. Table 2 indicates the formed phases in the acidic crystallization using various rhodium complexes, whose structures are presented in Fig. 5. Zeolite beta was crystallized in the presence of some rhodium complexes, ()-[Rh(bpy)3](HF2)3, (±)-[Rh(en) (t-Bu2bpy)2] (HF2)3 and (+)-[Rh(en)(t-Bu2bpy)2] (HF2)3. (+)-[Rh(phen)3](HF2)3 and (±)-[Rh(en)(phen)2](HF2)3 gave only lithium fluoride and an amorphous material, while (+)-[Rh(en)3](HF2)3 formed unknown products.
Intensity (cps)
40000
30000
20000
b 10000
a 0 0
20
40
60
80
2θ (degrees) Fig. 4. XRD patterns for (a) nucleated gel and (b) product by the recrystallization method in an acidic medium. s: LiF.
Table 2 Products obtained by acidic crystallization in the presence of chiral rhodium complexes as secondary SDAs No.
1 2 3 4 5 6 7 8 9 10
Complexes
(+)-[Rh(en)3](HF2)3 ()-[Rh(bpy)3](HF2)3 (±)-[Rh(en)(tBu2bpy)2](HF2)3 (+)-[Rh(en)(tBu2bpy)2](HF2)3 (+)-[Rh(phen)3](HF2)3 (±)-[Rh(en)(phen)2](HF2)3 Blank (original method) Blank (original method) Blank (fluoride method) Blank (fluoride method)
Symmetry
D3 D3 C2 C2 D3 C2 – – – –
XRD intensity (103cps)
A/(A + B) (%)
Phase of products
Peak B
Peak A
Peak ratio
Unknown Beta Amorphous, Amorphous, Amorphous, Amorphous, Beta Beta Beta Beta
–
–
– 75.9 86.4 88.2 – – 76.7 79.5 80.3 82.8
LiF, Beta LiF, Beta LiF LiF
20.6 2.7 15.9 – – 313.6 259.6 249.4 124.1
65.0 16.8 118.3 – – 1030.6 1004.1 1014.6 598.9
Y. Takagi et al. / Microporous and Mesoporous Materials 109 (2008) 567–576
573
H2 N H2N
N
NH2
3+ N
N
Rh
3+
3+ H2N
NH2
N
N
NH2
Rh
N
Rh
N NH2
N
H2 N
No.2 (-)-[Rh(bpy)3]3+
No.1 (+)-[Rh(en)3]3+
H2 N
N
N
)-[Rh(en)(tBu2bpy)2]3+
No.3
NH2 H2 N 3+
N
N
N
Rh
3+ N
3+ N
NH2
N
N
N
N
Rh
N N
No.4 (+)-[Rh(en)(tBu2bpy)2]3+
N
Rh
N
N
No.5 (+)-[Rh(phen)3]3+
No.6
-[Rh(en)(phen) 2]3+
Fig. 5. Structures of the chiral rhodium complexes used in the recrystallization method.
3.3. Composition of polymorphs 3.3.1. Peak separation Composition of polymorphs was evaluated by XRD profiles of zeolite beta. The shape of the first low angle peak of zeolite beta was affected by the composition. The peak was separated into two peaks. The lower angle peak appeared at 2h = 7.34° (Cu Ka) was assigned to (1 1 0) plane of polymorph B, and the higher one at 2h = 7.74°
to (1 0 1) plane of polymorph A (Fig. 6) [34]. The ratio of the integrated intensity of polymorph A to that of the whole peak, A/(A + B), reflects the proportion of polymorph A. As shown in Table 2, the usual beta synthesized by the original method containing 44% of polymorph A [3] had the A/(A + B) ratio of 76.7–79.5% (Entry Nos. 7 and 8). Zeolite beta obtained by the conventional fluoride method gave the ratio of 80.3–82.8% (Nos. 9 and 10). These results support that the fluoride method gives zeolite
e
Intensity (cps)
20000
b 10000
a
c d
0 4
6
8
10
2θ (degrees) Fig. 6. Peak separation of the first low angle peak of zeolite beta (Entry 7): (a) observed data; (b) higher angle peak; (c) lower angle peak; (d) base line data and (e) synthesized profile.
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beta more enriched in polymorph A than the original hydroxide method [12]. The products obtained using [Rh(en)(t-Bu2bpy)2]3+ (Entry Nos. 3 and 4) gave the higher A/(A + B) ratios
(86.4% for racemate and 88.2% for (+)-enantiomer) than those using ()-[Rh(bpy)3]3+ (No. 2, 75.9%). The former complex has C2 symmetry, while the latter has D3 one. The A/(A + B) ratios obtained with the C2 complexes were
OCH3 OH
N
HO
N
H3CO
N
N
H3C
OCH3
CH3
H
H O O
N
OH
N
No.11 (-)-cinconidine
No.12 (+)-cinconine
No.13 (+)-berbamine
O
CH3
H3 C H
H3 C
N
NH O
N
CH3
O
O
OH H
O
H
H N
H
H CH3
O N
No.14 (-)-eburnamonine
H
N
N
N
N
N
O
H H
No.16 (-)-lobeline
No.15 (+)-emetine
O
H
O
H
O
N
No.19 (+)-BMBA [(R)-(R*,R*)]-(+)-bis (α -methylbenzyl)amine
No.18 (+)-DHQ2PHAL
HO O N
H
O
N N
N
N
OH
NH2
H
O
No.22 (-)-BDtBSCDA (R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)1,2-cyclohexanediamine
No.21 (+)-BINDA (R)-(+)-1,1'-binaphthyl2,2'-diamine
O
N
N
NH2
No.20 (-)-BiPOP 2,6-bis[(4S)-isopropyl-2-oxazolin-2-yl] pyridine
O
CH3
H N H
OCH3
N
No.17 (-)-strychinine
H
H
H3CO
H
N
H3C
H
H
O
N N
N
NH2
N NH2
CH3
H3C
No.23 (+)-BMPOP
No.24 (-)-BPOP
2,6-bis[(4R,5R)-4-methyl-5-phenyl2-oxazolinyl]pyridine
2,6-bis[(4S)-4-phenyl2-oxazolinyl]pyridine
No.25 (+)-DPEN (1R,2R)-(+)-1,2diphenylethylenediamine
Fig. 7. Structures of the chiral amines used in the recrystallization method.
Y. Takagi et al. / Microporous and Mesoporous Materials 109 (2008) 567–576
also higher than those of the usual beta or the fluoride beta. This suggests that C2 symmetry of the secondary SDA might be suitable for the selective crystallization of polymorph A. 3.3.2. Effect of symmetry of SDA on the selectivity to polymorph A Then we investigated the effect of symmetry of chiral amines on selectivity to polymorph A. Their molecular structures are presented in Fig. 7. Table 3 indicates the formed phases in the acidic crystallization using various amines as secondary SDAs. Amines of Entry Nos. 11–17 have 1 symmetry, while those of Nos. 18–25 have C2 symmetry. Among the 1 symmetry amines, zeolite beta was obtained in the presence of some alkaloids, ()-cinconidine, (+)-cinconine or ()-strychinine but was not obtained in the presence of other alkaloids, (+)-berbamine, ()eburnamonine, (+)-emetine or ()-lobeline. The A/ (A + B) ratios of the products were not so high (75.3– 82.8%). Some chiral amines which have C2 symmetry also led to crystallize zeolite beta (Nos. 18, 19, 21, 22 and 23). Some of them, (+)-(DHQ)2PHAL (No. 18), (+)-BMBA (No. 19) and (+)-BMPOP (No. 23) showed the high A/ (A + B) ratio (92.6%, 90.6% and 92.2%, respectively), though no higher angle peaks (e.g. 2h = 9.68° for (1 0 2) plane) unique to polymorph A [33] were observed in XRD measurements. The unique peak will emerge when the portion of polymorph A is lager than 60% according to the simulation [35]. (+)-BMBA (No. 19) and (+)-BMPOP (No. 23) might fit into the beta pore system while (+)-(DHQ)2PHAL (No. 18) appears too large. The carbon and nitrogen contents of the sample were determined by organic elemental analyzer in order to examine if the chiral amines were included in them. Reference materials prepared without any chiral amines had C/N ratio of 8.4 ± 0.7. This value is comparable to
575
8.0 which is the C/N ratio of TEA. As the chiral amines have different C/N ratio (16.0 for No. 19 and 5.7 for No. 23) from TEA, it will be judged if the samples included them. The C/N ratio of the samples (Nos. 19 and 23) was 9.3 and 8.6 respectively. (+)-BMBA (No. 19a) might be included in the sample but (+)-BMPOP (No. 23) might not. This suggests that the former sample might include 10–20 mol% of (+)-BMBA (No. 19) compared to TEA. Thermal analysis was carried out in order to investigate the location of the amine. The ignition point of the amine incorporated in the pore system may be higher than that of outer one. Unfortunately, very little information concerning the location of amine was obtained partly because of the very low content of chiral amine compared with TEA. In order to evaluate chirality of the product, (+)- or ()BMBA was adsorbed on the calcined samples. The reference sample (No. 8) adsorbed 56% of (+)-BMBA and 59% of ()-BMBA from the solution (0.05 mol/l of BMBA in 98% MTBE + 2% 2-propanol) while the sample (No. 19) adsorbed 49% of (+)-BMBA and 33% of ()-BMBA. This suggests that zeolite beta recrystallized with (+)-BMBA as a secondary SDA shows slight enantio-selectivity in the adsorption of BMBA. The reference sample adsorbed more BMBA than the recrystallized one probably because the former had more defects than the latter. Camblor et al. reported that zeolite beta synthesized in hydroxide media had a lot of defects [12]. It should be desirable to determine the fraction of polymorph A by further characterization with advanced methods, such as HRTEM [36] and PDF [37]. It is concluded that zeolite beta crystallized in the presence of chiral amines with C2 symmetry consists of a somewhat lager portion of polymorph A than that using other compounds though the portion is up to 60%. This method may give a possibility for the selective synthesis of polymorph A.
Table 3 Products obtained by acidic crystallization in the presence of various chiral amines as secondary SDAs No.
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 a
Amines
()-cinconidine (+)-cinconine (+)-berbamine ()-eburnamonine (+)-emetine ()-lobeline ()-strychinine (+)-(DHQ)2PHAL (+)-BMBA ()-BiPOP (+)-BINDA ()-BDtBSCDA (+)-BMPOP ()-BPOP (+)-DPEN
Symmetry
1 1 1 1 1 1 1 C2 C2 C2 C2 C2 C2 C2 C2
Phase of products
Amorphous, LiF, Beta Amorphous, LiF, Beta Amorphous, LiF Amorphous, LiF Amorphous, LiF Amorphous, LiF Amorphous, LiF, Beta Beta Amorphous, LiF, Beta Amorphous, LiF Unknown, LiF, Beta Unknown, Beta, Amorphous Amorphous, Beta Amorphous Unknown, Beta
XRD intensity (103cps)
A/(A + B)(%)
Peak B
Peak ratio
3.2 14.8 – – – – 17.1 125.6 14.2 – 24.6 38.1 1.2 – –a
Peak A 9.8 65.1 – – – – 82.3 1562.2 136.1 – 122.5 195.2 14.5 – –a
Peak separation was not completed because of the overlap between two peaks assigned to beta and the unknown product.
75.3 81.5 – – – – 82.8 92.6 90.6 – 83.3 83.7 92.2 – –a
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4. Conclusions Two methods of acidic crystallization of zeolite beta have been developed. Zeolite beta is crystallized at pH 4 and 373 K for two months in the presence of the chiral rhodium complex without any racemization by the rapid crystallization method. Zeolite beta is also crystallized in a period of 20 days at pH 4 and 373 K by the recrystallization method. This method is appropriate to use chiral amines as secondary SDAs without further derivatization like N-alkylation. Zeolite beta crystallized in the presence of the chiral rhodium complex or of amines with C2 symmetry consists of somewhat large portion of polymorph A. This method may have a possibility for the selective synthesis of polymorph A. Acknowledgments
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25]
We thank Ms. Yumiko Aoki and Mr. Hiroyuki Fujisawa for measuring the XRD profiles.
[26]
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