Preparation by microwave irradiation of nanometre-sized AlPO4-5 molecular sieve Hongbin Du, Min Fang,†Wenguo Xu, Xianping Meng and Wenqin Pang Department of Chemistry, Jilin University, Changchun 130023, P. R. China The influence of the synthesis conditions on the crystallization and crystal size of AlPO4-5 molecular sieve is investigated in a (TEA)2O–Al2O3–P2O5–H2O system. The initial mixture composition and the crystallization method affect the crystallization and the crystal size of the product.Microwave heating of the synthesis mixture results in the formation of AlPO4-5 with nanometresized particles. Aluminophosphate molecular sieves are important materials dropwise to the above solution, followed by addition of the remaining water. The mixture was stirred for 6 h, and statically that are commonly used as catalysts, catalyst supports and adsorbents. Among them is the well known AlPO4-5, which aged for 12 h under ambient conditions to form a nearly transparent homogeneous gel.For the conventional hydrother- was first discovered in 1982 by Flanigen and co-workers.1,2 A large number of papers on the synthesis of AlPO4-5 have now mal synthesis of AlPO4-5, the gel was transferred into a 20 ml PTFE-lined stainless steel autoclave and heated at 333 K in been published,1–5 because of both its zeolite properties and its potential applications as advanced materials.6–9 an oven for a specified time, typically 7 h.The product was recovered by centrifugation (at 15000 rpm for 5–15 min), In the utilization of zeolites as catalysts, catalyst supports and adsorbents, the crystal size affects the performance washed repeatedly with distilled water (centrifuged and redispersed in water) and dried at ambient temperature.For the (activity, selectivity, rates of adsorption) simply by altering the diffusion path-length through the crystallites.10,11 Previous preparation under agitation, the autoclave was rotated at ca. 45 rpm in the oven. In the case of the microwave heating studies have shown that the smallest crystals are most effective as catalysts as long as the catalytic reaction proceeds in the preparation of AlPO4-5, the gel was charged into a 20 ml PTFE autoclave. The crystallization was carried out in a internal void.12,13 Currently, there is increasing interest in ultrafine particles of molecular sieves based on their potential modified domestic microwave oven operating at 2450 MHz.The reaction mixture was heated quickly at a heating rate of catalytic applications and their possible use as precursors for thin-film formation.14 However, only a few kinds of zeolites ca. 2K s-1 from room temperature to the crystallization temperature of 323–333 K and then held at the final tempera- with nanometre-sized particles have been synthesized, i.e.sodalite, A, Y, ZSM-5 and L.15–18 To our knowledge, there is very ture for 7–25 min. The products were identified by means of XRD on a Rigaku little information on the preparation of aluminophosphate molecular sieves with ultrafine particles in the literature. D/MAX-IIIA diffractometer with Cu-Ka radiation.Scanning electron images (SEM) and transmission electron images Recently, microwave heating has been applied successfully to the preparation of zeolites such as A,19 Y20 and ZSM-5,20 (TEM) were taken on Hitachi X-650 and JEM-100CXII microscopes, respectively. as well as the recently reported large AlPO4-5 crystals.21 Compared to the conventional hydrothermal crystallization, microwave heating of zeolite synthesis mixtures can drastically Results and Discussion reduce the crystallization time, often accompanied by the Effect of the P2O5 content formation of small crystals.19,20 This prompted us to explore its use as a method for the synthesis of nanometre-sized crystals The effect of the P2O5 content on the crystallization and of AlPO4-5.crystal size of AlPO4-5 is summarized in Table 1.It can be The present paper focuses on the preparation of AlPO4-5 seen that the P2O5 content in the gel plays an important role with nanometre-sized particles. The influence of the synthesis in the crystallization of AlPO4-5 molecular sieve: an excess of conditions on the crystallization and crystal size of AlPO4-5 P2O5 usually results in the formation of an unknown phase, is discussed in a (TEA)2O–Al2O3–P2O5–H2O system.while with insufficient P2O5 in the gel aluminium hydroxide contaminates the AlPO4-5 crystals. Suitable P2O5/Al2O3 molar ratios for the formation of pure AlPO4-5 range from 1.0 to 1.2. Experimental It can also be seen that the P2O5/Al2O3 ratio influences AlPO4-5 molecular sieve was synthesized using orthophos- crystal size.As shown in Fig. 1, a high P2O5/Al2O3 molar ratio phoric acid (H3PO4, 85%), aluminium hydroxide [Al(OH)3, favours the formation of AlPO4-5 composed of large aggre- 99%], tetraethylammonium hydroxide (TEAOH, 25%) and gates. At P2O5/Al2O3 ratios from 1.0 to 1.1, uniform plate-like distilled water as reactants. The chemical composition of the crystallites are obtained.initial gel was 1.0Al2O35xP2O55y(TEA)2O5zH2O, where x, y and z are changed systematically with x=1.1, y=0.7 and z= Effect of the template content and pH value in the gel 50 as the basis to study the influence of the gel composition The influence of the template content on the synthesis of on the crystallization and crystal sizes. AlPO4-5 is shown in Fig. 2. It seems that tetraethylammonium A typical synthesis procedure is described as follows. An hydroxide as a template favours the formation of small crystal- appropriate amount of aluminium hydroxide was added to the lites of AlPO4-5, in contrast to triethylamine as the template, hot orthophosphoric acid which was diluted by ca. 1/3 of the which usually favours the formation of large AlPO4-5 crystals.5 total water.After stirring for ca. 1 h, TEAOH was added In the (TEA)2O/Al2O3 ratio range from 0.6 to 1.0, pure AlPO4-5 with small crystallites is obtained. Scanning electron † Current address: Department of Chemistry, University of New Brunswick, Fredericton, NB, Canada E3B 6E2. images show that the (TEA)2O content also affects the crystal J. Mater. Chem., 1997, 7(3), 551–555 551Fig. 2 SEM images of AlPO4-5 synthesized with a (TEA)2O/Al2O3 Fig. 1 SEM images of AlPO4-5 synthesized with a P2O5/Al2O3 ratio of (a) 1.2, (b) 1.1 and (c) 1.0 ratio of (a) 0.6, (b) 0.8 and (c) 1.0 Table 1 Influence of the P2O5 /Al2O3 ratio on the crystallization gel composition no. (TEA)2O Al2O3 P2O5 H2 O product crystal habit F1600 0.7 1.0 1.4 50 unidentified — F1601 0.7 1.0 1.2 50 AlPO4-5 irregular, sphere, 10–30 mm F1602 0.7 1.0 1.1 50 AlPO4-5 uniform, 0.3 mm F1603 0.7 1.0 1.0 50 AlPO4-5 uniform, 0.8 mm F1604 0.7 1.0 0.8 50 AlPO4-5+Al(OH)3 — morphology (Fig. 2). When the (TEA)2O/Al2O3 ratio decreases similar to those caused by the (TEA)2O content, i.e. a high from 0.6 to 1.0, the average crystal size of AlPO4-5 decreases (TEA)2O content results in an increase in the pH value in the and the crystal-size distribution becomes narrower.These facts gel, thus accelerating the nucleation rate and leading to the indicate that at the highest (TEA)2O content more nuclei that formation of small crystallites with a narrow crystal-size are responsible for nucleation and subsequent crystallization distribution. are formed, and the nucleation rate to crystal growth rate ratio increases.Similar observations have been reported by Finger et al.5 Effect of water content Since an increase in the (TEA)2O content enhances the Variation of the water content results in changes to both the alkalinity, the effect of pH is investigated in a separate experi- nucleation rate and the crystal growth rate, as indicated in ment by adding hydrochloric acid (pH 5.4 and 5.8).The Fig. 4. Dilution of the reaction gel decreases the nucleation standard gel has a pH value of 6.4 [(TEA)2O/Al2O3=0.7]. As rate. The products obtained from diluted gels contain large can be seen from Fig. 3, the growth of AlPO4-5 is rather spherical agglomerates and tiny needle-shaped crystallites. On sensitive to changes in the pH value. At the lowest pH in the the other hand, concentration of the gel enhances the gel, large aggregates form.As the gel pH is increased from 5.4 nucleation rate, resulting in the formation of uniform small to 5.8 to 6.4 smaller crystals result (see Fig. 3). This suggests AlPO4-5 crystals. However, a further decrease in the water increased nucleation, at the expense of growth, as the pH is increased within this limited region. These phenomena are content leads to the formation of crystallites with various sizes 552 J.Mater. Chem., 1997, 7(3), 551–555Fig. 4 SEM images of AlPO4-5 synthesized with an H2O/Al2O3 ratio Fig. 3 SEM images of AlPO4-5 synthesized with a pH value in the gel of (a) 40, (b) 50 and (c) 72 of (a) 5.4, (b) 5.8 and (c) 6.4 [Fig. 4(a)], probably due to the inhomogeneity of the condensed gel.Effect of crystallization conditions The relationship between the crystallization temperature and the crystal size of the products was investigated by fixing the crystallization time. Good crystalline products are obtained at 413–453 K, and the crystal size is not distinctly dependent on the temperature. The crystallization method has an influence on the crystal Fig. 5 TEM image of AlPO4-5 synthesized under stirring size of AlPO4-5. Results from Fig. 5 show that stirring of the gel during the crystallization period is an important factor in determining the crystal size of AlPO4-5. Stirring leads to the broader reaction mixture composition range. Moreover, the AlPO4-5 crystals thus obtained usually have smaller sizes.formation of smaller crystallites than those obtained under static conditions, probably because more nucleation centres From Table 2, one can see that the P2O5/Al2O3 ratio in the reaction mixture is crucial in determining the products and are created by agitation of the gel. Moreover, it is of interest to note that microwave heating the crystal size. AlPO4-5 is formed in the P2O5/Al2O3 ratio range from 1.1 to 1.8, outside this range either Al(OH)3 coexists of the aluminophosphate gel produces AlPO4-5 crystallites with much smaller crystals in comparison with those obtained with AlPO4-5 or an unknown phase instead of AlPO4-5 is crystallized.At P2O5/Al2O3=1.1, nanocrystals of AlPO4-5 can by the conventional heating (Fig. 6). Similar results have been observed in the syntheses of zeolites A, Y and ZSM-5,19,20 be obtained when (TEA)2O/Al2O3 is fixed at 0.7 [Fig. 6(a)]. With the increase in the P2O5/Al2O3 ratio, the yields of the which are attributed to simultaneous and abundant nucleation under microwave radiation. products decrease and large crystallites are easily obtained. It seems that a relatively low P2O5/Al2O3 ratio favours the Under microwave heating conditions, the influence of the synthesis conditions on the crystal size was investigated.The formation of AlPO4-5 with small crystals. Similar results have been reported by Girnus et al.21 synthesis conditions and the crystallization products are summarized in Tables 2 and 3. Compared with the conventional The (TEA)2O/Al2O3 ratio also plays an important role in the crystallization, as shown in Table 3.Alow (TEA)2O content hydrothermal synthesis (P/Al=1.0–1.2 in the gel in this work), preparation of AlPO4-5 by microwave heating is possible in a usually results in the formation of AlPO4–C, and with an J. Mater. Chem., 1997, 7(3), 551–555 553Fig. 7 XRD patterns of AlPO4-5 samples synthesized by using (a) microwave heating (sample W126) and (b) conventional hydrothermal methods (sample F1602) microscopy shows that the specimen consists of fine particles with sizes as small as ca. 50 nm [Fig. 6(a)]. Fig. 6 TEM images of AlPO4-5 synthesized by microwave heating: (a) sample W126, (b) sample W131 and (c) sample W133 Conclusions Aluminophosphate AlPO4-5 crystals with small sizes were excess of (TEA)2O in the gel amorphous phases are obtained.Pure AlPO4-5 is crystallized in the (TEA)2O/Al2O3 ratio range synthesized using TEAOH as a template. Various synthesis parameters such as P2O5/Al2O3 , (TEA)2O/Al2O3, H2O/Al2O3 from 0.7 to 1.1. The synthesized AlPO4-5 samples usually consist of nanometre-sized crystals or loose agglomerates, as ratios and the crystallization method influence the crystallization and the crystal size of AlPO4-5.The synthesis of shown in Fig. 6. The XRD pattern of sample W126 (see also Tables 2 and 3) AlPO4-5 with uniform small crystals requires appropriate reaction mixture compositions. By microwave heating, is shown in Fig. 7(a). The peak positions are similar to those for AlPO4-5 prepared by conventional hydrothermal synthesis AlPO4-5 is synthesized successfully, and the product usually consists of smaller particles than those synthesized by the [Fig. 7(b)], but the intensities are different. The X-ray diffraction line-broadening of this sample is most probably due to conventional hydrothermal method. At (TEA)2O/Al2O3= 0.7–1.1 and P2O5/Al2O3=1.1, AlPO4-5 with nanometre-sized the size effect of the small particles. Similar observation has been reported in the case of zeolite L.18 Transmission electron is crystallized. Table 2 Influence of the P2O5/Al2O3 ratio on the crystallization of AlPO4-5 under microwave heating gel composition no.(TEA)2O Al2O3 P2O5 H2O P2O5/Al2O3 product av. size/nm W142 0.7 1.4 1.1 50 0.8 AlPO4-5+Al(OH)3 — W126 0.7 1.0 1.1 50 1.1 AlPO4-5 50 W138 0.7 0.8 1.1 50 1.4 AlPO4-5 200 W137 0.7 0.6 1.1 50 1.8 AlPO4-5 300 W136 0.7 0.5 1.1 50 2.2 unidentified — Table 3 Influence of the (TEA)2O/Al2O3 ratio on the crystallization of AlPO4-5 under microwave heating gel composition no.(TEA)2O Al2O3 P2O5 H2 O (TEA)2O/Al2O3 product av. size/nm W125 0.5 1.0 1.1 50 0.5 AlPO4-C — W126 0.7 1.0 1.1 50 0.7 AlPO4-5 50 W131 0.9 1.0 1.1 50 0.9 AlPO4-5 <50 W133 1.1 1.0 1.1 50 1.1 AlPO4-5 60 W142 1.3 1.0 1.1 50 1.3 ama — aAm=amorphous. 554 J. Mater. Chem., 1997, 7(3), 551–55511 K. Rajagopalan, A. W. Peters and G. C. Edwards, Appl. Catal., We wish to thank Prof. Jiesheng Chen for helpful discussions. 1986, 23, 69. We also acknowledge support of this work by the National 12 V. P. Shiralkar, P. M. Joshi, M. 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D. Barthormeuf, E. G. Derouane and W. Ho�lderich, NATO Plenum Press, New York, 1989, vol. 221, p. 53. Paper 6/06132J; Received 5th September, 1996 J. Mater. Chem., 1997, 7(3), 551–555 555