synthesis of aluminophosphate molecular sieve alpo4-11 nanocrystals

Synthesis of aluminophosphate molecular sieve AlPO4-11nanocrystals

Guangshan Zhu a, Shilun Qiu a,*, Feifei Gao a, Gang Wu a, Runwei Wang a,Binsong Li a, Qianrong Fang a, Yafeng Li a, Bo Gao a, Xianzhu Xu a,

Osamu Terasaki b,*

a Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, Chinab Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

Received 5 May 2000; received in revised form 12 August 2001; accepted 12 August 2001

Abstract

AlPO4-11 nanocrystals, 0:05 0:08 lm in size, were synthesized by optimizing the following chemical parameters:the crystallization temperature and time, the H2O content, the molar ratio of phosphorus to organic template (P2O5/R),

and the HF content. The products were characterized by powder X-ray diraction and scanning electron microscope.

AlPO4-11 nanocrystals could not be obtained by optimizing only the crystallization temperature, time and the water

content, but obtained by optimizing the molar ratio of P2O5/R, the organic template, and the HF content. 2001Elsevier Science B.V. All rights reserved.

Keywords: AlPO4-11; Nanocrystal; Synthesis; Aluminophosphate; Molecular sieve

1. Introduction

Due to the dierent diusion-path lengths, thesize of molecular sieve crystals can aect theirperformance when used as catalysts, adsorbents,ion exchangers and containers for guest molecule.As a consequence, controlling the size of molecularsieve crystals is desirable. Currently, there is anincreasing interest in using nanocrystalline-sized

molecular sieves for their catalytic applicationsand potential as the precursors in the productionof the thin lms [1]. Using traditional hydrother-mal crystallization techniques, only a few zeolitenanocrystalline products have been synthesizedso far, viz. SOD [2], LTA, FAU [3,4], MFI [5],LTL [6] and BEA [7], and there has been littleinformation on the synthesis of aluminophosphatemolecular sieves with nanocrystalline-sized dimen-sions in the literature.

Currently, aluminum phosphate molecularsieves are of growing importance. AlPO4-11 has10-membered ring channels, 6:3 3:9 AA [8] in size,and similar to those of medium pore zeolites. Inthe AlPO4-11 framework, the substitution of sili-con (SAPO-11) [9] and a number of divalent metal

Microporous and Mesoporous Materials 50 (2001) 129135

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*Corresponding authors. Tel.: +81-22-217-6472; fax: +81-

22-217-6475 (O. Terasaki); Tel.: +86-431-516-6328; fax: +86-

431-567-1974 (S. Qiu).

E-mail addresses: [email protected] (S. Qiu), tera-

[email protected] (O. Terasaki).

1387-1811/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S1387-1811 (01 )00431-0

ions (MAPO-11) for phosphorus and aluminum,respectively, produces acidic AlPO4-11 [10]. Thepresence of these acid sites makes substitutedAlPO4-11 a good catalyst for aromatic reactionssuch as the alkylation and disproportionation oftoluene, and the isomerization and dispropor-tionation of styrenes, etc. [1012]. The presentpaper reports the synthesis of nanocrystalline-sizedaluminophosphate molecular sieve (AlPO4-11),and the inuence of various synthetic chemicalparameters on the crystal size and the character-ization of the products.

2. Experimental

2.1. Synthesis

In the synthesis of AlPO4-11, pseudoboehmiteor aluminum tri-isopropoxide [Al(iPrO)3] andphosphoric acid (H3PO4, 85 wt.%) were used asaluminum and phosphorus sources, respectively.Ammonia (25 wt.%), sodium hydroxide (99 wt.%)and hydrochloric acid (36 wt.%) were used to ad-just the pH value. Di-isopropylamine, di-propyl-amine and di-butylamine were used as the organictemplates and, when necessary, aqueous hydrogenuoride was added. The typical synthesis proce-dure is as follows: (1) aluminum and phosphorussources were dissolved separately in distilled water;(2) a phosphoric acid solution was added dropwiseto the aluminum solution, and the mixture wasallowed to age for 30 min at 90 C; (3) after so-lution (2) was cooled to room temperature, it be-came a viscous aluminophosphate gel, to whichthe organic template was added; (4) ammonia,sodium hydroxide solution or a hydrogen uoridesolution was slowly added, dropwise, to the ho-mogeneous slurry to adjust the nal pH of thereaction mixture; (5) having been stirred for30 min, the nal reaction mixture was sealed inTeon-lined autoclaves and heated at 160200 Cunder autogenous pressure for 12 days. Anysolids, larger than 1 lm, were recovered by lter-ing, washing and drying. With centrifugation at20,000 rpm crystalline products less than 1 lmwere obtained. The solid samples were repeatedlycentrifuged for 1530 min, then dispersed in dis-

tilled water using ultrasonic to remove the re-maining mother liquid, the procedure was repeateduntil the pH of the dispersion was near 7. Thenanocrystalline products were obtained after theywere dried at 80 C for 4 h.

2.2. Characterization

Phases were identied on a Rigaku D/MAXIIIA X-ray powder diractometer (XRD) with arotating target and Ni-ltered CuKa (k 1:5418AA) radiation at room temperature. The crystal sizeand morphology were investigated by a HitachiX-650B scanning electron microscope (SEM).

3. Results

3.1. Synthesis

3.1.1. Eect of the crystallization temperature andtime

The inuence of the crystallization temperatureand time on the average size of AlPO4-11 crys-tal was investigated in the system 1.6i-Pr2NH:1.3P2O5:1.1Al2O3:70H2O. In the synthesis of alu-minophosphate molecular sieves AlPO4-n, themolar ratio P2O5/Al2O3 is usually 1 [13]. Accord-ing to the experimental results of this work, itis easy to obtain products with high crystallinityif the molar ratio is higher than 1. As shown inTable 1 (runs 1a1e), the range of the crystalliza-tion temperature is from 160 to 200 C, and withinthis range, low crystallization temperatures fa-vor the formation of larger AlPO4-11 crystals. Inrun 1b, at a crystallization temperature of 160 C,15 20 lm AlPO4-11 crystals were obtained, butin run 1d, at 200 C, 0:5 2 lm crystals wereobtained. When the crystallization temperaturewas increased to over 200 C (run 1e), a densephase was precipitated. In the same system and at200 C, the results obtained in runs 1f1j indicatethat average sizes of AlPO4-11 crystals did notchange much as the crystallization time was de-creased. When the crystallization time was lessthan 24 h, the reaction mixture remained a slurrywithout crystallizing. It seems, therefore, that the

130 G. Zhu et al. / Microporous and Mesoporous Materials 50 (2001) 129135

crystallization time has little eect on the AlPO4-11 crystal size.

3.1.2. Eect of the molar H2O contentTable 2 shows the inuence of the molar H2O

content on the AlPO4-11 crystal size. As shown inruns 2a2c, when the reaction mixtures with thecomposition 1.6i-Pr2NH:1.3P2O5:1.1Al2O3:xH2Owere crystallized at 200 C for 40 h, the averagecrystal size increased from 0:2 0:7 to 0:3 2 lmwhen the value of x(H2O) was changed from 35 to70. When x was further increased to 175 and 200(runs 2d, 2e), AlPO4-11 crystals were precipitatedwith larger average crystal sizes of 1 4 and2 9 lm, respectively. When x was 175 and 200(runs 2f, 2g), at a crystallization temperature of180 C, AlPO4-11 products were obtained withaverage crystal sizes of 2 5 and 3 10 lm, re-spectively.

3.1.3. Eect of the molar ratio of P2O5/RThe molar P2O5/R ratio aects the pH value of

the nal reaction mixture (Table 3). The rangeof pH suitable for the crystallization was from5 to 9, and for values higher than 9 or lower than5,no AlPO4-11 crystals were obtained. At a pH6.5 (run 3a), the size of the produced crystals was0:3 1 lm. When the pH of the nal reactionmixture was increased to 8 by adding ammonia orsodium hydroxide solution (run 3a0), or reducedto 5.5 by adding hydrochloric acid (run 3a00),the size of the produced crystals was 0:3 1 and0:5 1:2 lm, respectively. When the pH of thenal reaction mixture was increased to 7.5 and 8.0by reducing the P2O5/R molar ratio (runs 3c, 3b),the precipitated crystals were 1 5 and 1015 lm in size, respectively. When the pH of thenal reaction mixture was reduced to 5.5 by in-creasing the P2O5/R molar ratio (runs 3d3f), thesize of the product crystals was reduced to0:05 0:08 lm. When di-butylamine (runs 3g, 3h)or di-propylamine (runs 3i, 3j) was used instead ofdi-isopropylamine as the organic template, underthe same synthesis conditions as those of 3f and 3c,the size of the produced crystals was 0:5 1:2 and0:05 0:08, and 0:3 0:8 and 0:5 1:2 lm, re-spectively.

Table 1

The eects of the crystallization temperature and time on the

average size of AlPO4-11 crystals

No. Tempera-

ture (C)Time

(h)

Average crys-

tal size (lm)Phase

1a 140 40 Gel

1b 160 40 15 20 AlPO4-111c 180 40 0:8 5 AlPO4-111d 200 40 0:5 2 AlPO4-111e 240 27 Dense

phase

1f 200 48 0:5 3 AlPO4-111g 200 40 0:5 2 AlPO4-111h 200 35 0:3 2 AlPO4-111i 200 30 0:3 2 AlPO4-111j 200 27 0:3 1 AlPO4-11

Table 2

The eect of the H2O content on the average size of AlPO4-11

crystals with the gel composition 1.6i-Pr2NH:1.3P2O5:1.1Al2O3:xH2O

No. x(H2O) Tempera-ture (C)

Time (h) Average crys-

tal size (lm)

2a 35 200 40 0:2 0:72b 60 200 40 0:4 12c 70 200 40 0:3 22d 175 200 40 1 42e 200 200 40 3 92f 175 180 40 2 52g 200 180 40 3 10

Table 3

The eects of molar P2O5/R ratio and the templates on the

average size of AlPO4-11 crystals with the gel composition

xP2O5:yR:1.1Al2O3:70H2O at 200 C

No. x(P2O5) y(R) pH Average crystalsize (lm)

3a 1.3 1.6a 6.5 0:3 13a0 1.3 1.6a 8.0 0:3 13a00 1.3 1.6a 5.5 0:5 1:23b 1.14 2.2a 8.0 10 153c 1.3 2.0a 7.5 1 53d 1.3 1.2a 6.0 0:8 43e 1.5 1.6a 6.0 0:1 0:53f 1.73 1.6a 5.5 0:05 0:083g 1.73 1.6b 5.5 0:5 1:23h 1.3 2.0b 7.5 0:05 0:083i 1.73 1.6c 5.5 0:3 0:83j 1.3 2.0c 7.5 0:5 1:2aR i-Pr2NH.bR Bu2NH.c Pr2NH.

G. Zhu et al. / Microporous and Mesoporous Materials 50 (2001) 129135 131

3.1.4. Eect of the HF contentIn the reaction mixture with the composi-

tion 1.6i-Pr2NH:1.3P2O5:1.1Al2O3:70H2O:xHF (seeTable 4), the average crystal size was increasedfrom 0:3 0:8 to 2 10 lm at 200 C when themolar ratio of HF x increased from 0.1 to 0.5(runs 4a4c). However, when x was further in-creased to 0.8 and 1.2 (runs 4d, 4e), the size wasreduced to 0:1 0:5 lm. When the amount of HFwas increased to 3, only an aluminophosphatedense phase was precipitated. Under the samesynthesis conditions as run 4d and at the crystal-lization temperature of 160 C (4d0), nanocrystals,0:05 0:08 lm in size, were precipitated.

3.1.5. CharacterizationFig. 1 shows the XRD patterns of the as-syn-

thesized AlPO4-11 samples from runs 1b (a) and4d0 (b). Both patterns show a pure AlPO4-11 phasethough some peak broadening was observed forpattern (b).

Fig. 2 shows the SEM micrographs of the as-synthesized AlPO4-11 samples with average crystalsizes of 15 20 (a), 10 15 (b), 3 10 (c), 2 5(d), 0:3 1 (e) and 0:05 0:08 lm (f), obtainedfrom runs 1b, 3b, 2g, 2f, 3a and 4d0, respectively,which indicates a reduction in size from 20 to 0.08lm in the length with all the crystals having asingle uniform morphology and narrow particlesize distribution.

4. Discussion

An ecient procedure for the synthesis of na-nometer-sized crystals of AlPO4-11 was found by

studying, systematically, the factors aecting thecrystallization process, including synthesis tem-perature and time, the molar ratios of H2O, P2O5/R, HF and the nature of the templates. Before thecrystallization, the aging of the initial reactionmixture for 30 min at 90 C is necessary, otherwisea broad size distribution and other phases appearbecause organic templates like di-propylamine canbe used to synthesize other aluminophosphatemolecular sieves [13,14]. Aging the reaction mix-ture to become homogeneous, which is benecialto narrow the particle size distribution [6,15,16].In order to obtain small and uniform AlPO4-11crystals, the water content is minimized (run 2a),which causes the higher supersaturation and pro-motes the nucleation rate and competition for theavailable chemical nutrients. The reaction tem-perature is maximized (run 1d) as this promotesthe rapid formation of nuclei and consumptionof chemical nutrients. However, it should be em-phasized that nanometer-sized AlPO4-11 crystalscannot be obtained only by optimizing the watercontent and the reaction temperature. Optimizing

Table 4

The inuence of the presence of HF on the average size of

AlPO4-11 crystals with the gel composition of 1.6i-Pr2NH:1.3P2O5:1.1Al2O3:70H2O:xHF

No. x(HF) Tempera-ture (C)

Time (h) Average crys-

tal size (lm)

4a 0.1 200 24 0:3 0:84b 0.3 200 24 0:5 1:24c 0.5 200 24 2 104d 0.8 200 24 0:8 24d0 0.8 160 24 0:05 0:84e 1.2 200 24 0:1 0:54f 3.0 200 24 Dense phase

Fig. 1. XRD patterns of as-synthesized AlPO4-11 in (a) run 1b

and (b) run 4d0.


the molar ratio of P2O5/R, selecting a proper or-ganic template, and adding a proper amount ofHF also play crucial roles in the formation ofnanometer-sized AlPO4-11 crystals.

The proper pH range for synthesizing pureAlPO4-11 is 59, which can be adjusted by addingammonia or sodium hydroxide solution, or hy-drochloric acid, however, the average size of theproduct crystals changes slightly (run 3a3a00).Changing the molar ratio of P2O5/R in the nalreaction mixture can also make the pH valuevaried. For example, when the molar ratio ofP2O5/R was increased from 0.52 to 1.08 (runs 3b3f), the pH changed from 8 to 5.5. At the sametime, the sizes of the crystals were decreased from10 15 to 0:05 0:08 lm. Accordingly, the molarratio of P2O5/R (R: di-isopropylamine) in the re-action mixture is one of the key roles which aectthe average size of the nal products.

When di-butylamine was used instead of di-isopropylamine as the organic template, under thehigh P2O5/R molar ratio (run 3g) and the sameconditions as those of run 3f, which results in theformation of nanocrystalline products, crystalswith 0:5 1:2 lm in size were precipitated. With alow P2O5/R molar ratio (run 3h) and the sameconditions as those of run 3c, which are the con-ditions forming the products 1 5 lm in size,nanocrystalline products, 0:05 0:08 lm in size,were precipitated. When di-propylamine was usedas the organic template, the average crystal size didnot change with high or low P2O5/R molar ratios(runs 3i, 3j). This suggests that the nature of theorganic templates aects the average crystal sizein dierent ways, even under the same synthesisconditions. In order to obtain nanocrystals, theP2O5/R molar ratio must be optimized for onekind of organic template.

Fig. 2. SEM photographs of as-synthesized AlPO4-11 with average crystal sizes of 15 20 (a), 10 15 (b), 3 10 (c), 2 5 (d), 0:3 1(e) and 0:05 0:08 lm (f), obtained in runs 1b, 3b, 2g, 2f, 3a and 4d0, respectively.


The synthesis of molecular sieves in the pres-ence of HF, rst reported by Guth et al. [14], wasdeveloped by Qiu et al. [17] for the preparationof single crystals. It is believed that the reactionspecies form a uoride complex, during the initialstage of crystallization, which is then slowly hy-drolyzed to form other less uorinated specieswhich slowly and continuously supply the chemi-cal nutrients for the growth of single crystals.For synthesizing AlPO4-11 (runs 4a4c), when themolar ratio of HF in the reaction mixture wasbelow 0.5, the uoride ions formed the uoridecomplexes AlF36 and PF

6 with aluminate and

phosphate species present before crystallization.Those complexes hydrolyzed to form less uori-nated aluminate and phosphate species, supplyingchemical nutrients to promote the crystal growth.Thus the nal AlPO4-11 crystal size increased withincreasing HF content. However, when a largeramount of HF added (run 4d), the uoride com-plexes were dicult to hydrolyze, which inhibitedfurther crystal growth and adding an excess of HF,gave aluminophosphate dense phase (run 4f). Bychanging only the molar ratio of HF in the reac-tion mixture, the products with the maximumand minimum average sizes of 2 10 and 0:10:5 lm were precipitated (runs 4c, 4e), respec-tively, from the reaction mixture composition 1.6i-Pr2NH:1.3P2O5:1.1Al2O3:70H2O:xHF at 200 C.When another factor, crystallization tempera-ture, was changed to 160 C (run 4d0), nanocrys-tals as smaller as 0:05 0:08 lm were obtained inthe gel.

5. Conclusions

(1) Supersaturation in liquid reaction systems isaected by some synthetic chemical parameters,such as crystallization temperature and time, H2Ocontent, the molar ratio of P2O5/R and the natureof organic template. Since the size of microporouscrystals appears to be controlled by the supersat-uration of the liquid reaction system, crystals withdierent average size can be synthesized by opti-mizing synthetic parameters. AlPO4-11 crystalswith dierent average sizes from 0.05 to 20 lm

have been synthesized by changing the syntheticparameters. It also notes that both the P2O5/Rratio and the nature of the templates play key rolesin the synthesis of AlPO4-11 nanocrystalline par-ticles.

(2) The presence of HF aects the average sizeof AlPO4-11 nanocrystals. The crystals with theaverage size from 0.5 to 10 lm can be obtained byadjusting the HF content in the reaction mixture.By optimizing other synthetic parameters, such ascrystallization temperature and the HF content,nanocrystals, 0:05 0:08 lm in size, have beenobtained.

Acknowledgements

This work was supported by the NationalNatural Science Foundation of China, the StateBasic Research Project (G2000077507), andCREST, Japan Science and Technology Corpo-ration (JST). We thank Dr. Frank Lincoln, Uni-versity of Western Australia for critical readingof the manuscript and for his helpful suggestions.

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synthesis of aluminophosphate molecular sieve alpo4-11 nanocrystals

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