synthesis of alpo4-5 powder by microwave heating influence of starting gel ph and reaction time

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Microporous and Mesoporous Materials 29 ( 1999) 329–337

Synthesis of AlPO4-5 powder by microwave heating:

Influence of starting gel pH and reaction time

Tetsuya Kodaira a,b,*, Kohji Miyazawa a,1, Takuji Ikeda c, Yoshimichi Kiyozumi b

a National Institute for Ad vanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba, Ibaraki 305-8562, Japan

b National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

c Institute of Applied Physics, Uni versity of Tsukuba, Ten-noudai 1-1, Tsukuba, Ibaraki 305-8573, Japan

Received 10 August 1998; accepted 21 December 1998

Abstract

Extremely pure AlPO4-5 powder was synthesized using a microwave heating method. The yield as well as the size

distribution of the AlPO4

-5 crystals depended on the pH value of the starting gel. On reducing the pH value from 7.0

to 4.0, the average size of the product crystal was found to increase, the size distribution became wider, and the yield

increased. The pH value of the supernatant liquid after the synthesis was always ca. 7.5, independent of the pH values

of the starting gels. During the first step of crystallization, disk-like crystals are formed, and they grow in the c-axis

direction resulting in the formation of a hexagonal rod-like shape. The channels of the crystals are completely open,

and, therefore, the crystals obtained are suitable for use as a host to stabilize guest materials. © 1999 Elsevier ScienceB.V. All rights reserved.

Keywords: AlPO4

-5; Microwave heating; pH; Reaction time; Synthesis

1. Introduction number of crystal nuclei are simultaneously gener-ated during the rapid increase in the gel temper-ature. Therefore, it is believed to be a promisingA synthetic method using a microwave (MW)technique for preparing zeolite crystals with aheating technique [1] has recently been applied tonarrow size distribution.produce zeolites in a short time compared with a

Recently, it has become possible to induce newconventional (CV ) heating method. The synthesesproperties into materials by incorporating themof many kinds of zeolites, materials related tointo the nano-space of zeolites; for example, thezeolites, and mesoporous materials using the MWnon-linear optical property of p-nitroaniline inheating method have already been reported [2– channel-type zeolites [11,12], or the ferromagne-10]. In this method, it is believed that a largetism of arrayed potassium clusters in K-type LTA[13]. For the accurate measurements of some of

* Corresponding author.the physical properties of the guest materials stabi-1Present address: Toyota Central Research and Developmentlized in the zeolites, e.g. heat capacity and magneticLaboratories Co. Ltd., 41-1, aza Yokomichi, oaza Nagakute,

Nagakute cho, Aichi gun, Aichi, 480-1131, Japan. susceptibility using a single crystal, a single crystal

1387-1811/ 99/ $ – see front matter © 1999 Elsevier Science B.V. All rights reserved.

PII: S 1 3 8 7 - 1 8 1 1 ( 9 9 ) 0 0 0 0 2 - 5



330 T. Kodaira et al. / Microporous and Mesoporous Materials 29 (1999) 329–337

of millimeter size is required. It is, however, diffi- paper in comparison with that obtained by the

CV heating.cult to obtain such large crystals by the CV heating

method. On the other hand, although the size of

each crystal is small, the use of powdered zeolites,

with uniform size and without impurities, may be 2. Experimentalmore practical for physical measurements.

AlPO4-5 and SAPO-5, both with an AFI struc- The synthesis of AFI-type zeolites (AlPO

4-5 and

SAPO-5) have been reported by many groups [ 15– ture, are possible candidates as containers in which

one-dimensional guest materials may be isolated 22] using diff erent Al sources and organic tem-

plates. In this study, alumina sol (10 wt.%,and stabilized in their channels [12,14]. AlPO4-5

and SAPO-5 crystals are known to be easily synthe- Kawaken Fine Chemical ) and ortho-phosphoric

acid [H3PO

4] (85 wt.%, Wako) were employed assized in sizes larger than 100 mm in the c-axis

direction. The anisotropy of the structure can be the Al and P sources, respectively. Triethylamine

[(C2H

5)3N: Et

3N](99+ wt.%, Wako) was used asobserved in the optical spectra using one single

crystal of the size mentioned above. It is, however, an organic template. The mole fractions of these

starting materials, dissolved in water, were aswell known, and also shown later in this paper,

that large crystals of AlPO4-5 and SAPO-5 follows:

obtained by the CV heating methods are often1 Al

2O

3: 1.03 P

2O

5: 3.56 Et

3N: 250 H

2O

accompanied by a wide size distribution, aggrega-

tions, and byproducts. A few years ago, Girnus These materials are mixed following the pro-

cedure of Demuth et al. in three steps [17]. (1)et al. systematically investigated the synthetic con-

ditions of AlPO4-5 using the MW heating method Solution A was prepared by dropwise adding the

alumina sol into two-thirds of the total water[2]. They succeeded in synthesizing large crystals

within a short synthetic time using hydrofluoric content. (2 ) Solution B was prepared by dropwise

addition of Et3N to ice-cold diluted H

3PO

4 aq.acid (HF). For obtaining high quality products,

they also used a secondary synthetic procedure, containing one-third the total water content. ( 3)

Solution B was added dropwise to solution Awhich is the re-irradiation with the MW of the

supernatant liquid after the removal of the large under vigorous stirring using a mechanical mixer.In the third step, the viscosity of the mixtureAlPO

4 crystals with the byproducts.

In this study, we report the synthesis of high increases resulting in the formation of a gel. We

stirred this gel long enough to make it homogen-quality AlPO4-5 powder with small crystal sizes, a

narrow size distribution, and a high yield without eous. The pH value of this gel was ca. 7.6. The

pH value of the starting gel was adjusted byusing HF and a secondary synthesis procedure.

The pH value of the starting gel and the total MW titration with sulfuric acid [H2SO

4](50 wt.%,

Wako). The pH values of the gels prepared areheating time are selected as the parameters of the

synthesis. The quality of the powder obtained was shown in Table 1. The total amount of water was

increased by the titration, e.g. ca. 10% for the gelevaluated based on scanning electron microscopy

(SEM) images and X-ray powder diff raction with pH=4.0. This increase of water in the gel,

however, did not influence the quality of the final( XRD) patterns. It was found that the pH valueof the starting gel plays an important role in product. In the other gels, the increase in the

amount of water by the titration was less than thiscontrolling the average crystal size, the size distri-

bution, and the yield. Furthermore, it was clearly value. These starting gels were kept at room tem-

perature for 1 h before the MW heating.observed in the optical microscopy images of

Se-loaded AlPO4-5 crystals that the channels of The gel was heated using a microwave oven

(Questron: Q-wave 3000). The frequency and max-the crystals obtained are open throughout the

crystal with very few defects or impurities. The imum power of the microwave radiation were

2.45 GHz and 1 kW, respectively. The gel of ca.characteristics of the crystal growth of AlPO4-5

obtained by the MW heating is discussed in this 50 ml was set in a Teflon-lined polyimide autoclave




(a) (b)

(c) (d)

Fig. 1. SEM images of AlPO4

-5 crystals in samples #1-1– #1-4 obtained from the gels with diff erent pH values: (a) #1-1 (pH=7.0);(b ) #1-2 (pH=6.0); (c) #1-3 (pH=5.0); (d) #1-4 (pH=4.0).

and the XRD patterns confirm the fact that the decrease of the pH value. The pH value of the

supernatant liquid after the MW irradiation wasobtained products are the AlPO4-5 single phase.

For comparison, a sample of AlPO4-5 labeled found to be independent of the pH value in the

starting gel, and was 7.5±0.1.#2 was synthesized using the CV heating method.

The starting gel with the same composition of

sample #1-2 in a metal autoclave was heated in an 3.2. E ff ect of MW irradiation time

electric oven. The gel was kept at 190°C for 4 days.

An example of the SEM image of this AlPO4-5 is The duration of MW heating was varied from10 to 40 min for samples #3-1– #3-4 as shown inshown in Fig. 3. Although large hexagonal rod-

like single crystals are present, there is a wide size Table 1. The pH values of the starting gels in these

samples were 6.0. The SEM images of thedistribution of AlPO4-5 crystals. Aggregates of the

small AlPO4-5 crystals and amorphous-like depos- AlPO

4-5 crystals for samples #3-1– #3-4 are shown

in Fig. 4(a)–(d), respectively. The average sizes of its on the surface of the AlPO4-5 crystals are also

observed. the crystals are shown in Table 1. In sample #3-1,

hexagonal disk-like shaped crystals have alreadyThe yields of AlPO4-5 powder in weight per

volume of the starting gel, in units of g l−1, are formed. With the increase in the MW heating time,

the crystals grow only in the c-axis direction andshown in Table 1. The yield increases with the



333T. Kodaira et al. / Microporous and Mesoporous Materials 29 (1999) 329–337

Fig. 2. XRD patterns of the AlPO4

-5 powders: (a) #1-2; (b ) #1-4.

heating. The pH value of the supernatant fluid

was 7.5 for sample #3-3.

3.3. Crystallinity of the AlPO4-5 obtained

Se atoms were loaded into the AlPO4-5 channels

of sample #1-2. The transparent crystals of

AlPO4-5 changed to orange because of the loading

of Se atoms under saturated conditions, ca. 5

atoms per unit cell. Fig. 5 is a monochromatic

photograph of a polarized optical transmission

micrograph image of Se-loaded AlPO4-5 crystals.

The arrow in the figure indicates the direction of

Fig. 3. SEM image of the AlPO4-5 crystals obtained by the the electric field of the polarized light. We canconventional heating method. clearly see that the crystals whose c-axes are paral-

lel to the electric field of the transmission light are

uniformly darkened. As reported in Ref. [14],become close to hexagonal rod-like shape with no

size change in the a – b plane. It was also observed anisotropic coloration originates from the forma-

tion of an Se chain in a channel. Fig. 5 indicatesthat no disk-like or smaller crystal was formed in

samples #3-2– #3-4. In the SEM images of samples that Se atoms were homogeneously loaded into

the channel with no blocking by contamination,#3-3 and #3-4, we can hardly see the diff erence in

size and shape of the crystals. This means that the etc. This result shows that the synthesized

AlPO4-5 crystals using the MW heating methodcrystal growth stops after ca. 30 min of MW




(a) (b)

(c) (d)

Fig. 4. SEM images of the AlPO4

-5 powders obtained using diff erent MW heating times: (a) #3-1 (10 min); (b ) #3-2 (20 min); (c)#3-3 (30 min); (d) #3-4 ( 40 min).

have completely opened channels, namely from

one hexagonal face of a crystal to the opposite

face. They are suitable as the container of guest

materials and allow the detailed analysis of their

physical properties.

4. Discussion

4.1. Comparison with the previous study using the

MW heating method

In a previous study [2], the secondary synthesis

method was proposed for obtaining AlPO4-5 crys-

tals with a homogeneous size without byproducts.

We found, however, that this technique is notFig. 5. Transmission polarized optical microscope image of

necessarily used for synthesizing AlPO4-5 crystalsSe-loaded AlPO

4-5. The direction of the electric field of the

incident light is indicated by the arrow. with high quality. The diff erence between the pre-




the pH of the solution increases with the growth MW. The dielectric loss occurs all over the solu-of the crystals. However, it is quite difficult to tion. In contrast, in the case of the CV heatingexplain why the crystal growth stops at pH=ca. method, the solution is warmed in the autoclave7.5. Aluminum has a tendency to have an octahe- by the conduction of heat. Therefore, using MW

dral coordination between pH=3 and 9. Jahn irradiation, the gel is quickly and uniformly heatedet al. pointed out that phosphoric acid and amine compared with the CV heating method. For themolecules play an important role during the stabili- MW heating, the temperature over all the gelzation of the tetrahedrally coordinated Al species quickly reaches the condition that starts the reac-[26 ]. This, however, gives us no information about tion. Thus, the process we imagine is as follows.the termination of the reaction at pH#7.5. The Many crystal nuclei (precursors) are simulta-amine molecules in the gel are supposed to form neously formed all over the gel. Once the nucleiclusters with AlO

4 and PO

4, and shield the tetrahe- are generated with a high density, the residual Al

dral coordination of Al from attack by water [26]. and P sources are used only for the growth of The form of the cluster may change as a result of these nuclei that form crystals until the pH of thethe variation in the pH value. The clusters at pH=

gel is 7.5. This explains why the crystals have a7.5 may not be able to shield the tetrahedra from

tendency to have a homogeneous size comparedthe attack by water. with the CV heating method.

The duration of synthesis of the AlPO4-5 crystals

4.4. Morphology of the crystals during theneeded to terminate the crystal growth was ca.

synthesis30 min in the present work. The synthesis duration

is shorter than that using the CV heating method.As described in Section 3.2, the crystals growIn a previous study using the MW heating method,almost homogeneously in size for between 10 andAlPO

4-5 crystals were obtained in only a few30 min of MW heating. This means that the crystal

minutes [2 ]. This diff erence may originate fromnuclei are mostly formed within the MW heatingof 10 min. This is the first observation of the the diff erence in the Al source and the acid. Thus,simultaneous generation of many nuclei during the HF is believed to accelerate the speed of the crystalMW heating method. growth, and the use of aluminum isopropylate

The aspect (length-per-width) ratio of the may lead to a more uniformly dispersed Al sourceAlPO

4-5 crystal varied during the synthetic reac- than that of the alumina sol. The particle size of

tion. The crystals grow mainly in the a–b plane the initial alumina sol is 20–200 A. The Al sourceduring the first 10 min in which disk-like crystals may be gradually supplied for the growth of theare formed. After 10 min, the crystals do not grow crystals in the present case. Girnus et al. proposedin the a–b plane any more but in the c-axis a model to explain the significantly short synthesisdirection. This kind of behavior has been reported time in MW heating from a microscopic view pointin several kinds of zeolites using the CV heating [2]. Hydrogen bonds between the water moleculesmethod [27] . During the synthesis of AlPO

4-5 are destroyed by the MW irradiation forming

using the CV heating method, the length in the active water molecules which accelerate the speeda–b plane is constant with the variation in the gel of the crystal growth.density, and a large aspect ratio (needle-like) can

be obtained for the diluted starting gel [15]. In thepresent study, the concentration of the solution

5. Conclusionsdecreases with the growth of the crystal, and,consequently, the crystals grow only in the c-axis

Powdered AlPO4-5 crystals of high quality can

direction after a certain reaction time.be synthesized using a MW heating technique. In

the MW heating method, the procedure for the4.5. Comparison with the CV heating method

preparation of the gel aff ects the quality of the

final AlPO4-5 product. It is concluded that theGenerally, in the MW heating method, the solu-

tion is directly warmed by the dielectric loss of the crystal growth depends on the pH of the starting

synthesis of alpo4-5 powder by microwave heating influence of starting gel ph and reaction time

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