RESEARCH PAPER

CHINESE JOURNAL OF CATALYSIS Volume 30, Issue 6, June 2009 Online English edition of the Chinese language journal

Cite this article as: Chin J Catal, 2009, 30(6): 525–530.

Received date: 13 December 2008. * Corresponding author. Tel: +86-21-62232058; E-mail: ymliu@chem.ecnu.edu.cn Foundation item: Supported by the National High Technology Research and Development Program of China (863 Program, 2007AA03Z342), the National Basic Research Program of China (973 Program, 2006CB202508), the Shanghai Science and Technology Commission (06SR07101, 07QA14017), and the Shanghai Key Disciplines Project (B409). Copyright © 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved. DOI: 10.1016/S1872-2067(08)60115-1

Synthesis of ZSM-23 Zeolite Using Isopropylamine as Template

LIU Ye, WANG Zhendong, LING Yun, LI Xianbo, LIU Yueming*, WU Peng Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China

Abstract: The synthesis of ZSM-23 in the hydrothermal system of isopropylamine-silica sol-NaAlO2-H2O was investigated. Pure ZSM-23 with high crystallinity was synthesized when the SiO2/Al2O3 molar ratio in the gel was between 60 and 150. The SiO2/Al2O3 molar ratio, template concentration, gel alkalinity, and water content were critical in the crystal phase transition. The template concentration and SiO2/Al2O3 molar ratio played important roles in the morphology control of the products. ZSM-23 zeolites with different morphologies were obtained under different experimental conditions.

Key words: ZSM-23 zeolite; ZSM-5 zeolite; template; isopropylamine

ZSM-23 with the MTT topology structure is a medium pore high silica zeolite first synthesized by Plank et al. [1] at Mobil in 1978. The framework topology of this zeolite is composed of 5-, 6-, and 10-rings without intersecting channels. The 10-ring linear channels of ZSM-23 have a pore diameter of 0.45 nm × 0.52 nm, which is smaller than that of ZSM-5. Owing to its unique pore structure and strong surface acidity, ZSM-23 has high catalytic activity and selectivity in the isomerization of butene and catalytic cracking reactions, especially the catalytic cracking of C4 alkenes to ethene and propylene [2,3]. ZSM-23 can be hydrothermally synthesized using pyrrolidine [1], diisopropanolamine [4], Diquat-7 [5], Diquat-8 [6], or Diquat-12 [7] as templates. However, the high cost of these templates has restricted the large scale production and appli-cation of ZSM-23 zeolite. In addition, pyrrolidine leads to plate-like or jujube core-like large crystals [2], which results in slow mass transport in the zeolite channels and consecutive reactions of the intermediate products. According to previous reports, both ZSM-5 and ZSM-23 [8] can be synthesized using isopropylamine (IPA) as the template. Liu et al. [9] reported that the ZSM-5 zeolite synthesized using IPA as the template was a plate-like and 5–10 μm diameter crystal.

In this work, ZSM-23 zeolite was successfully synthesized under dynamic hydrothermal conditions with gel SiO2/Al2O3 molar ratios between 60 and 150 using IPA as the template. The effects of SiO2/Al2O3 molar ratio, alkalinity, template concen-tration, and water content on the crystallization were investi-gated. In addition, crystal transition and morphology change of the product were studied. A crystal competitive growth phe-nomenon was found in this system. Furthermore, ZSM-23 zeolite with different morphologies was obtained in the con-trollable and reproducible process.

1 Experimental

1.1 Zeolite synthesis

The gel mixtures used for the synthesis were prepared using silica sol (30% SiO2), sodium aluminate (CP), sodium hy-droxide (AR), and deionized water as materials, and isopro-pylamine as template. Sulfuric acid was used to adjust the pH of the synthesis mixture. Crystallization of the ZSM-23 zeolite proceeded under rotating synthesis conditions. The molar compositions of the synthesis gel mixtures were SiO2:Al2O3:

LIU Ye et al. / Chinese Journal of Catalysis, 2009, 30(6): 525–530

IPA:OH :H2O = 1:(1/300–1/60):(0–1.2):(0–0.12):12. A general synthesis procedure was as follows. Sodium hy-

droxide solution and sodium aluminate solution were mixed to form solution A. Then IPA was dropped into solution A fol-lowed by some silica sol under vigorous stirring until a ho-mogeneous gel B was formed. Gel B was continuously stirred for 1 h and then was transferred to a 150 ml Teflon-lined stainless-steel autoclave, which was heated at 443 K in an oven for 72 h. After the crystallization, the autoclave was quenched in cold water and the synthesis product was thoroughly filtered and washed with deionized water, and then dried overnight in air to obtain the “as-synthesized” products, which were con-verted to ZSM-23 zeolite by calcination.

1.2 Characterization

X-ray powder diffraction (XRD, Bruker D8 Advance dif-fractometer) was used to identify the sample phase and deter-mine the crystallinity. XRD data were collected using Cu K radiation with a scanning range of 5°–35° and a scanning rate of 2°/min. Scanning electron microscopy (SEM, Hitachi 4800) was used to determine the morphology and crystal size of the zeolite samples.

2 Results and discussion

2.1 Influence of the SiO2/Al2O3 molar ratio

As we know, the SiO2/Al2O3 molar ratio plays an important role in crystallization and phase transformation. Fig. 1 shows the XRD patterns of the synthesized products from different gel SiO2/Al2O3 molar ratios. It was found that pure ZSM-23 with high crystallinity was obtained only with SiO2/Al2O3 molar ratios between 60 and 150. The crystallinity of the product began to decline with increasing SiO2/Al2O3 molar ratio from 180. An intense phase peak appeared at 2 = 21.8° for SiO2/Al2O3 molar ratios at and above 210. Its intensity in-creased gradually with a larger SiO2/Al2O3 molar ratio. How-ever, the framework structure was sensitive to the Al content in the relatively low SiO2/Al2O3 molar ratio region. Pure ZSM-35 and ZSM-5 were obtained when the SiO2/Al2O3 molar ratios were 20 and 30, respectively, where only amorphous or quite low crystallinity material appeared. Interestingly, a similarly amorphous product was seen when the SiO2/Al2O3 molar ratio remained between 30 and 60. This was first ZSM-5, then an amorphous phase, and then ZSM-23. All of these results can be attributed to the competitive growth of the three crystal phases, ZSM-35, ZSM-5, and ZSM-23, when the SiO2/Al2O3 molar ratio was below 60.

In the production of pure ZSM-23, the SiO2/Al2O3 molar ratio in the product was different from the nominal ratio in the gel. This was confirmed by the data in Fig. 2. According to the

results from ICP elemental analysis, the SiO2/Al2O molar ratios in the products were 54, 83, and 110, respectively, for gel SiO2/Al2O3 molar ratios of 60, 90, and 120. There were only small differences. This can be attributed to the excellent inte-gration capacity for Si and Al atoms into the zeolite frame-work. This was further confirmed by the high crystallinity of the three samples shown in the XRD patterns. However, this integration capacity decreased considerably when the Si con-

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Fig. 1. XRD patterns of products synthesized using various gel SiO2/Al2O3 molar ratios. (1) 20; (2) 25; (3) 30; (4) 40; (5) 60; (6) 90; (7) 120; (8) 150; (9) 180; (10) 210; (11) 240; (12) 300. Gel composition: IPA/SiO2 = 0.6, OH /SiO2 = 0.08, H2O/SiO2 = 12; 170 oC, 72 h. IPA—isopropylamine.

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200 Ideal result ICP analysis result

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Fig. 2. SiO2/Al2O3 molar ratios in the products synthesized using vari-ous gel SiO2/Al2O3 molar ratios.

LIU Ye et al. / Chinese Journal of Catalysis, 2009, 30(6): 525–530

tent in the gel continued to increase. Fig. 3 shows the SEM images of the ZSM-23 samples for

different SiO2/Al2O3 molar ratios. ZSM-23 synthesized with a SiO2/Al2O3 molar ratio of 60 showed uniform needle-like crystals with a length of 150–200 nm. These aggregated gradually to form platelets particles with increasing SiO2/Al2O3 molar ratios, with a disappearing margin between the crystals. It was clear that the SiO2/Al2O3 molar ratio had a pronounced effect on product morphology.

2.2 Influence of the template concentration

Fig. 4 shows the XRD patterns of the products for different template concentrations (gel SiO2/Al2O3 molar ratio, alkalinity, and water content were kept constant). Amorphous products were only obtained without IPA in the gel. The ZSM-5 phase began to appear with increasing IPA/SiO2 ratio to 0.2. The crystallinity continued to increase until the IPA/SiO2 ratio was 0.3, which was followed by a rapid decline. The ZSM-23 phase began to appear with the increase of the IPA/SiO2 ratio to 0.5, and this dominated the crystalline phase for IPA/SiO2 ratios up to 0.6, where there was no ZSM-5 phase. A higher template concentration yielded an intergrowth of ZSM-23 and a dense phase such as -cristobalite or -quartz.

The competitive growth of the two zeolite structures, ZSM-5 and ZSM-23, existed during the whole crystallization period. IPA is a small amine molecule and yields ZSM-23 framework [8] with a smaller channel dimension in contrast to ZSM-5. In general, a basic environment is necessary for zeolite crystalli-zation. In the course of dissolving polysilicate (or polyalumi-nosilicate), OH is consumed, and the pH decreases. As a result, the crystallization is slowed dramatically and even stopped. According to Rollmann et al. [8], the small amine plays a pe-

culiar role, namely, a “pH-stabilizing” role, in addition to its well-known “structure-directing” role. A low IPA/SiO2 ratio in the gel had a stronger negative effect on ZSM-23 growth than on ZSM-5 growth because ZSM-5 crystallization took rela-tively little time and was less sensitive to the pH decline. In contrast to this, ZSM-23 growth was seriously restricted be-cause of its lengthy crystallization period. In the presence of

Fig. 3. SEM images of ZSM-23 zeolites synthesized using various gel SiO2/Al2O3 molar ratios. (a) 60; (b) 90; (c) 120; (d) 200.

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Fig. 4. XRD patterns of products synthesized under different template contents. IPA/SiO2 molar ratio: (1) 0; (2) 0.2; (3) 0.3; (4) 0.4; (5) 0.5; (6) 0.6; (7) 0.8; (8) 1.0; (9) 1.2. Gel composition: SiO2/Al2O3 = 60, OH /SiO2

= 0.08, H2O/SiO2 = 12; 170 oC, 72 h.

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adequate amine, due to its alkalinity, the effect of OH con-sumption was retarded, and the pH decline was much less. Hence, a relatively stable pH could be maintained during the crystallization period, which gave the conditions for the nu-cleation and growth of ZSM-23 crystals. A higher IPA/SiO2 molar ratio was preferred to produce the ZSM-23 phase.

As shown in Fig. 5, the morphology of ZSM-23 changed with the IPA concentration. For a synthesis gel with the same Na/SiO2 molar ratio, needle-like ZSM-23 crystals with lengths of 150–200 nm were formed when the IPA/SiO2 molar ratio was 0.6. The lengths of the ZSM-23 crystals increased to 0.6–1

m with increasing IPA/SiO2 molar ratios from 0.6 to 1.8–1.2. With the increase of the IPA/SiO2 molar ratio to 2.0, the mor-phology of the ZSM-23 crystals changed to be more regular and uniform. The length increased to 2 m, which showed a more (001)-oriented growth of the ZSM-23 crystals, parallel to the linear channels. However, when the IPA concentration was further increased, instead of this continuous growth, the nee-dle-like particles tended to aggregate and the length of a single particle decreased. This implied that the (010)-oriented growth had occurred prior to the (001)-oriented growth.

2.3 Influence of the alkalinity

Fig. 6 shows the XRD patterns of the products for different alkalinity (gel SiO2/Al2O3 molar ratio, template content, and water content were kept constant). An amorphous product was only obtained when the OH /SiO2 molar ratio was 0.03. The ZSM-23 phase began to appear with increasing OH /SiO2

molar ratio from 0.04. The crystallinity continued to increase until the OH /SiO2 molar ratio was 0.06, where ZSM-23 was formed with the best crystallinity. When the OH /SiO2 molar

ratio reached 0.08, the ZSM-5 phase began to appear and there was the coexistence of the two structures, ZSM-5 and ZSM-23. When the OH /SiO2 molar ratio reached up to 0.1, there was only ZSM-5. A higher OH /SiO2 molar ratio yielded hard and amorphous products, which undoubtedly had a lower crystal-linity for ZSM-5.

It is well known that most aluminosilicate zeolites are formed in a basic gel environment. Flanigen [10] indicated that

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Fig. 6. XRD patterns of products synthesized under various alkalinities. OH /SiO2 molar ratio: (1) 0.03; (2) 0.04; (3) 0.06; (4) 0.08; (5) 0.1; (6) 0.12. Gel composition: SiO2/Al2O3 = 60, IPA/SiO2 = 0.4 , H2O/SiO2 = 12; 170 oC, 72 h.

Fig. 5. Influence of template concentration on the morphology of the ZSM-23 zeolites. IPA/SiO2 molar ratio: (a) 0.8; (b) 1.0; (c) 1.2; (d) 2.0.

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the cations of the alkali metals play two roles in zeolite syn-thesis. First, by providing OH , they accelerate the dissolving of silica. Second, the cations guide SiO4 or AlO4 tetrahedra to replace H2O molecules, arranging these orderly around them-selves. Then the SiO4 or AlO4 tetrahedra condense onto a cav-ity, which is commonly considered to be the fundamental structure unit for the zeolite framework. There is some corre-sponding but non-specific relationship between the cations and cavities mentioned above, and the former makes it possible to direct different zeolites under different gel environments. In the gel we investigated, a relatively low concentration of sodium cations was preferable for the formation of the fundamental structure unit of ZSM-23, but a high concentration was good for the growth of the ZSM-23 framework, so an intergrowth of a double crystal phase was inevitable. A higher alkalinity re-sulted in a better crystallinity of ZSM-5 and smaller proportion of ZSM-23 in the intergrowth phase. In other words, a highly

alkaline gel was preferred to direct the ZSM-5 framework. Thus, in order to produce a pure ZSM-23 phase, a strict alka-linity control was essential.

To further investigate the influence of template concentra-tion and alkalinity on crystal phase transformation, a group of crossover experiments were conducted. As shown in Table 1, a more alkaline medium gave ZSM-23 with an intergrowth of ZSM-5, whereas too high template concentration produced a dense phase. Therefore, to produce ZSM-23 zeolite with a single phase, high crystallinity, and no dense phase, the alka-linity and template concentration must be restricted.

2.4 Influence of water concentration

Fig. 7 shows the XRD patterns of the products formed with different gel water concentrations (gel SiO2/Al2O3 molar ratio, template content, and alkalinity were kept constant). It was found that only the ZSM-5 phase was obtained when the H2O/SiO2 molar ratio was 10. The ZSM-23 phase began to appear while the ZSM-5 phase disappeared when the H2O/SiO2

molar ratio was increased to 12. Above this, the crystal phase of the products remained ZSM-23. These results showed that water concentration also played an important role in crystal phase transformation. A water-poor environment, which means that the alkaline gel was condensed and had strong alkalinity, would certainly lead to ZSM-5 crystallization. Meanwhile, an environment with adequate water and relatively weak alkalin-ity would favor the formation of ZSM-23.

3 Conclusions

The influences of SiO2/Al2O3 molar ratio, template concen-tration, alkalinity, and water concentration on the synthesis of ZSM-23 zeolite were investigated with the hydrothermal sys-tem of isopropylamine-silica sol-NaAlO2-H2O. It was found that the competitive growth of three crystal phases, ZSM-35, ZSM-5, and ZSM-23, was due to the effect of the SiO2/Al2O3 molar ratio. Pure, high crystallinity ZSM-23 can be synthesized

Table 1 Influence of template concentration and alkalinity on product crystallinity

Product crystallinity at various OH /SiO2 molar ratios IPA/SiO2 molar ratio 0 0.02 0.04 0.06 0.08 0.10 0.12

0 amorphous amorphous amorphous amorphous amorphous amorphous amorphous 0.2 amorphous amorphous amorphous ZSM-5(T) ZSM-5(L) ZSM-5 ZSM-5 0.3 amorphous amorphous ZSM-5(T) ZSM-5 ZSM-5 ZSM-5 ZSM-5 0.4 amorphous ZSM-5(T) ZSM-5(T) ZSM-5(T) ZSM-5(L) ZSM-5 ZSM-5 0.6 amorphous amorphous ZSM-23(T) ZSM-23 ZSM-5(L) ZSM-5 ZSM-5/-23 0.8 amorphous amorphous ZSM-23 ZSM-23 ZSM-23(D) ZSM-5(L) ZSM-5/-23(D) 1.0 amorphous amorphous ZSM-23 ZSM-23 ZSM-23(D) ZSM-5/-23(D) ZSM-5/-23(D) 1.2 amorphous ZSM-23(L) ZSM-23(D) ZSM-23(D) ZSM-23(D) ZSM-23(D) ZSM-5/-23(D)

T: trace; L: low crystallinity; D: dense phase; ZSM-5/-23: intergrowth of ZSM-5 and ZSM-23. Gel composition: SiO2/Al2O3 = 60, H2O/SiO2 = 12; 170 oC, 72 h.

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Fig. 7. XRD patterns of products synthesized under various water con-centrations. H2O/SiO2 molar ratio: (1) 10; (2) 12; (3) 15; (4) 20. Gel composition: SiO2/Al2O3 = 60, IPA/ SiO2 = 1.0 , OH /SiO2 = 0.08; 170 oC,72 h.

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with SiO2/Al2O3 molar ratios between 60 and 150. The SiO2/Al2O3 molar ratio and template concentration had a direct influence on the morphology of the product. A needle-like ZSM-23 zeolite with a length of 150–200 nm was synthesized successfully when the SiO2/Al2O3 molar ratio was between 60 and 150 and the IPA/SiO2 ratio was 0.6.

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