MICROPOROUS MATERIALS ELSEVIER
Microporous
Materials 9 (1997) 259-265
The effect of ageing on the microwave synthesis of zeolite NaA P.M. Slangen *, J.C. Jansen, H. van Bekkum Laboratory
of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan The Netherlands Received 3 September 1996: revision 31 October 1996; accepted 8 November
136, 2428 BL De@ 1996
Abstract In the microwave synthesis of zeolite NaA ageing of the synthesis mixture is a prerequisite for fast crystallization. The rapid heating in microwave synthesis requires better preparation of the synthesis mixture compared to conventional synthesis methods. Sufficiently aged mixtures can yield NaA, with crystal sizes ranging from 0.1 to 0.3 pm, after 1 min in the microwave. It was also found that the synthesis of an unaged mixture can be improved dramatically by adding aged synthesis mixture to it. This suggests that the rearrangement of the synthesis mixture to yield nuclei is the bottleneck in a microwave synthesis. 0 1997 Elsevier Science B.V. Keywords:
Ageing;
Microwave;
NaA; Nucleation;
Synthesis; Zeolites
1. Introduction
Numerous papers have by now appeared on the subject of microwave synthesis of zeolites. The authors invariably report of highly shortened synthesis times compared to conventional heating methods [l-6]. Here we report that this shortening of synthesis time is obtained at a certain price: the extra care needed for the preparation of the synthesis mixture. The shortening of synthesis times in microwave heating is caused by two different mechanisms, i.e., the rapid heat-up of the sample and a better heat transfer which results in rapid and thorough heating of the synthesis mixture. This thorough heating can easily result in tiny hot spots throughout the synthesis mixture. The overall effect is an overall higher temperature and faster * Corresponding author. 0927-6513/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO927-6513(96)00119-S
crystallization. This is consistent with results from organic syntheses, where it has been found that the rate accelerations observed in microwave heating were caused by inhomogeneous heating and superheating [ 7,8]. An intrinsic microwave effect, defined as a non-temperature effect, is not likely. We have studied the effects of rapid heating on the synthesis of zeolite NaA. We chose NaA for this study because of its importance as detergent zeolite, and because it is relatively simple to synthesize. Chu et al. were the first to show that microwave heating could be used for the rapid synthesis of zeolites [ 11. The authors reported the synthesis of NaA in 12 min. However, the products were then (still) contaminated with hydroxysodalite (HS); only after prolonged heating was pure NaA obtained. Jansen et al. showed that pure zeolite NaA could be synthesized in as little as 10 min [ 21. An extensive review on the nucleation and growth of zeolite NaA in conventional heating systems has been published by Zhdanov [9].
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Myatt et al. reported that the mean crystal size depends on the total number of nuclei produced and that the breadth of the size distribution is related to the length of the nucleation period [lo]. For a homogeneous crystal size distribution it would therefore be favorable to have a short nucleation period. A large number of nuclei would result in small crystals, a property which can be advantageous in ion exchange applications, since small crystals have a higher surface area per unit weight and are therefore more reactive. We will report here on the synthesis of zeolite NaA in 1 min, and on the prerequisites for a successful rapid synthesis. The rapid heating of the synthesis mixture allows, in a way, the ‘isolation’ of the species present in the synthesis mixture by condensing them rapidly, and not allowing the mixture to rearrange during heat-up. Microwave heating can thus be used to understand some of the processes involved in zeolite crystallization.
2. Experimental
The synthesis mixture used has been tested previously in microwave heating [2] and was originally taken from Breck [ 1 I]. The molar ratio of the synthesis mixture was 1 SiO,:l A1,03: 1.5 Na,0:96.5 H20. The synthesis mixture was prepared by first dissolving the NaOH (Baker) and the NaAlO, (Riedel-de Ha&n) in all of the H,O (demineralized). When this was a clear solution, the silica (Aerosil 200 from Degussa) was added and stirred for the desired time. The moment the silica was added was taken as the beginning of the ageing. The ageing time thus includes the dissolution of the silica into the aluminate solution. A household type microwave oven, equipped with a thermocouple, was used for the synthesis. The mixtures were loaded into a Teflon autoclave with a thermocouple throughput and a safety pressure plate. This set-up has been described by Jansen et al. [2]. In the microwave synthesis the mixture was rapidly heated to 120°C (-90 s) whereafter it was kept at 100°C for 5 min (except for the I-min synthesis experiment). The rather high temperature of 12O’C was chosen to promote a rapid reaction. This could be the nucleation of
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NaA or the ‘nucleation’ of an amorphous phase or hydroxysodalite (HS). This rapid heating allowed to react the gel more or less as it is, without the changes it undergoes during the slow heat-up in conventional synthesis. For comparison, a conventional synthesis of zeolite NaA was done in a polypropylene bottle in a hot air oven at 100°C. Both in microwave and conventional synthesis, approximately 30 ml of synthesis mixture was used per experiment. After cooling the products were isolated by suction filtration, washed and dried. The products were analyzed by XRD and SEM to determine identity, crystal size distribution and morphology of the products.
3. Results and discussion
The synthesis time of zeolite NaA in a conventional hot air oven at 100°C is 2 h. Ageing here has a profound effect on crystal size, as can be seen from the SEM pictures in Fig. 1. The NaA crystals from the unaged mixture have crystal sizes ranging from 0.2 to 1.5 urn, which is in the same range as a commercial NaA as it is being used in detergent formulations. The crystals are cubic with knotted edges, indicative of relatively slow crystal growth. The crystals from the overnight aged synthesis mixture are smaller, between 0.1 and 0.4 urn. Here also, the cubes seem to have knotted edges. This suggests that during the ageing time pre-nuclei were formed. Although these nuclei are more abundant in the longer aged synthesis mixture, the necessary synthesis time does not change significantly. This means that the formation of nuclei is not a limiting factor in the conventional synthesis of zeolite NaA. Ageing has, however, a distinct effect on the crystal size. We investigated the effect of ageing on the microwave synthesis of NaA. In Table 1 the products of a 5-min synthesis after different ageing times are given. It can be seen that shortly aged (5 min) synthesis mixtures yield amorphous material. After 1 and 2 h of ageing some hydroxysodalite (HS) is observed. As can be seen in the SEM picture (Fig. 2(a)), only after 3 h of ageing is NaA synthesized with some HS impurities and very little
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(b) Fig. 1. SE
1 pictures
of NaA,
synthesized
in 2 h in a hot air oven
at IOO’C
after
5 min (a), and after
overnight
(b)
ageing.
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Table 1 Products of a 5-min microwave synthesis at 100°C after different times of ageing: product compositions were determined from XRD and SEM Ageing time (min) 5 60 120 180 240 20h
Products (a) 100% amorphous < 10% HS + > 90% amorphous < 10% HS + > 90% amorphous > 80% NaA + < 10% HS + i 10% amorphous >90% NaA+
amorphous material. The NaA crystals have knotted edges and range in size from 0.4 to 2 urn. After 4 h some minor HS impurities are still present in the NaA product from the 5-min microwave synthesis. The crystals are still fairly large, up to 2 urn. Only when the synthesis mixture is aged overnight is pure NaA synthesized (Fig. 2(b)). This pure NaA consists of very small crystals, 0.1-0.4 urn in size, which is similar to the products of the conventional synthesis after overnight ageing. However, for NaA synthesis the entire synthesis mixture does not need to be aged; when 10 wt.% of an overnight aged mixture is added to a freshly prepared synthesis mixture and stirred well, zeolite NaA is obtained with only minor amorphous impurities, as can be seen in the SEM pictures (Fig. 3). The crystals again have knotted edges, and the size ranges from 0.2 to 1.5 urn. When the procedure described above was done with 1 wt.% seeding, the result was incomplete crystallization of NaA. Therefore, in the microwave synthesis the formation of nuclei appears to be the limiting factor. We tentatively define nuclei as the smallest viable system capable of forming an identifiable crystal phase that can induce crystal growth. This nucleus is envisaged to be a few nanometers is size in three directions. For the formation of these nuclei, first the synthesis mixture should be mixed thoroughly to ensure mixing on a molecular scale. When the ingredients of the synthesis mixture are added together, pockets of silica and alumina exist. In time, these pockets will be mixed to form silicaalumina oligomers, with a silicon to aluminum
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ratio of one. This mixing on a molecular scale is essential for the nucleation of NaA. However, the entire mixture does not need to be mixed on a molecular scale, only that part of the mixture which will have to form the nuclei. This can be understood from the observation that adding 10 wt.% of the aged (mixed on a molecular scale) synthesis mixture to the fresh (unmixed) synthesis mixture, results in the formation of NaA. Apart from being essential to the formation of nuclei, the thorough mixing also removes any pockets in which HS can nucleate. A synthesis mixture which is aged for 3 or 4 h is mixed well enough (in most places) to nucleate NaA, but still contains some pockets of inadequate mixing where HS can nucleate (or no nucleation takes place, resulting in amorphous material). Furthermore, the adding of overnight aged mixture to a synthesis mixture that had been aged for 1 h resulted in NaA crystallization with HS impurities. This indicates that, although the nuclei for NaA were supplied, pockets suitable for crystallization of HS still existed. Therefore, in microwave synthesis, the mixing and organization has to be done at room temperature, before the heat-up, whereas in conventional synthesis the mixing can take place during the slow heat-up. In situ monitoring of the changes in a zeolite A synthesis mixture during conventional heating at 65°C by NMR did not give a distinct picture of the changes that take place in the synthesis mixture before crystallization starts (although crystal growth could be followed by NMR) [ 121. The formation of HS as a side product in the synthesis of NaA is not very surprising and can be explained by inadequate mixing. The results after 1 and 2 h mixing indicate that the HS can nucleate before the NaA. However, it does not become a major component of the product. Tassopoulos and Thompson reported that the successive transformation of species into their more stable counterparts occurs only when the synthesis mixture moves into the range in which this species nucleates [ 131. Prolonged heating then results in dissolution and regrowth of crystals of that composition, e.g., when a NaA mixture moves into a HS range, eventually, all NaA will be transformed into HS. However, when the synthesis mixture
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Fig. 2. SEiM pictures of NaA, synthesized in the microwave oven in 5 min at 100°C after 3 h (a) and overnight ILb) ageing.
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Fig. 3. SEM picture of NaA, synthesized overnight aged mixture was added.
Fig. 4. SEM
picture
of NaA,
in the microwave
synthesized
oven in 5 min at 100-C
in I min at 120-C
in the microwave
from
unaged
oven from
synthesis
overnight
mixture
to which
aged synthesis
IO wt.%
mixture.
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stays in the NaA region, the only HS that can nucleate is in the inhomogeneous pockets. Under conventional synthesis methods the slow heat-up allows the formation of sufficient nuclei for a successful synthesis, and crystal growth becomes the limiting factor. The crystal growth rate can be accelerated by raising the temperature [ 141. However, when the temperature is raised too fast, nuclei have not formed and crystal growth cannot take place. When sufficient nuclei have formed, NaA can be synthesized in 1 min at 12O’C. A SEM picture of this NaA is depicted in Fig. 4, and shows that the crystals are quite small, between 0.1 and 0.3 pm. In addition, fast heat-up can be regarded as a tool for studying the synthesis mixture as it is, not as it has become during heat-up. Furthermore, when it is possible to understand what species have to be present in the synthesis mixture, this may be of use in improving the synthesis of zeolites. The rapid heating of the synthesis mixture more or less allows the ‘isolation’ of the species present in the synthesis mixture by condensing them rapidly, and not allowing the mixture to rearrange during heat-up. However, this also means that for a successful microwave synthesis, much more care needs to be taken with regard to the preparation of the synthesis mixture and that mixtures that readily crystallize in a conventional oven do not necessarily crystallize in a microwave oven.
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Acknowledgment
Akzo Nobel Chemicals The Netherlands, is gratefully financial support for microwave synthesis. Dr. H.T. Rijnten and thanked for useful discussions.
bv, Amsterdam, acknowledged for studies in zeolite Dr. J. Nieman are
References [I] [2]
[3] [4]
[6] [7]
We have demonstrated the effect of ageing on the microwave synthesis of NaA. It was found that ageing is a prerequisite for the successful rapid synthesis of NaA, and this is in contrast to the conventional synthesis. During the ageing, mixing on a molecular scale is envisaged to allow the formation of nuclei necessary for the crystallization of NaA. In conventional synthesis, the mixing and formation of nuclei can take place during the heat-up, so that here no ageing is necessary for a successful synthesis. When the synthesis mixture has been aged sufficiently, the synthesis of NaA can be done in 1 min by microwave heating, leading to small crystals with a relatively homogeneous crystal size distribution.
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Apart from accelerating a zeolite synthesis, microwave heating can be used as a tool to understand some of the processes that take place in zeolite synthesis. It allows the almost instantaneous heat-up of the synthesis mixture, thus letting the reaction take place in a synthesis mixture that has hardly changed during the heat-up. This allows, in a way, the isolation of species present in the reaction mixture at room temperature.
[5]
4. Conclusion
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[8] [9]
[lo] [ 1I ] [ 121 [ 131 [14]
P. Chu, F.G. Dwyer and J.C. Vartuli. US Patent, 4.778.666 (1988). J.C. Jansen, A. Arafat, A.K. Barakat and H. van Bekkum, in M.L. Occelli and H.E. Robson (Eds.), Molecular Sieves, Van Nostrand Reinhold, New York, 1992, pp. 507-521. A. Arafat, J.C. Jansen. A.R. Ebaid and H. van Bekkum, Zeolites, 13 ( 1993) 162. I. Girnus. K. Jancke, R. Vetter, J. Richter-Mendau and J. Caro, Zeolites, 15 (1995) 33. S.L. Cresswell, J.R. Parsonage, P.G. Riby and M.J.K. Thomas, J. Chem. Sot. Dalton Trans., ( 1995) 2315. C. Wu and T. Bein. J. Chem. Sot. Chem. Commun., (1996) 925. A.G. Whittaker and D.M.P. Mingos. J. Microwave Power Electromagnetic Energy. 29 (1994) 195. K.D. Raner. C.R. Strauss, F. Vyskoc and L. Mokbel. J. Org. Chem., 58 (1993) 950. S.P. Zhdanov, in E.M. Flanigen and L.B. Sand (Eds.), Molecular Sieve Zeolites, Adv. Chem. Ser. 101, ACS, Washington, DC, 1971. pp. 20-43. G.J. Myatt, P.M. Budd. C. Price, F. Hollway and S.W. Carr. Zeolites, 14 (1994) 190. D.W. Breck. Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974, p. 270. J. Shi, M.W. Anderson and SW. Carr. Chem. Mater., 8 (1996) 369. M. Tassopoulos and R.W. Thompson, R.W., Zeolites, 7 (1987) 243. W. Meise and F.E. Schwochow, in W.M. Meier and J.B. Uytterhoeven (Eds.), Molecular Sieves, ACS, Washington DC, 1973, p. 169.