On the synthesis of the aluminophosphate molecular sieve AIPO4-5

On the synthesis of the aluminophosphate molecular sieve ALP04-5 E. Jahn,* D. Miiller and W. Wieker

Central Institute of Inorganic Chemistry, Academy of Sciences of the GDR, Berlin-Adlershof, GDR and J. Richter-Mendau

Central Institute of Physical Chemistry, Academy of Sciences of the GDR, Berlin-Adlershof, GDR

The possibility that formation and stabilization of tetrahedral AI is necessary for the synthesis of a pure AIPO4-5 phase is examined in relation to the preparation of the reaction mixtures. The AI coordination in the reaction mixtures and products has been characterized by means of solid-state high-resolution 27AI n.m.r. It is concluded that the role of the template (triethylamine) in the formation of AIPO4-5 is to stabilize the newly created tetrahedral AI and to prevent conversion to octahedral AI by shielding from attack by water. Keywords: Molecular sieve AIPO4-5; synthesis; spectroscopy; 27AI n.m.r.

INTRODUCTION

EXPERIMENTAL

Microporous AIPO4-5 is a member of the aluminophosphate molecular sieve family. 1'2 In its synthesis, usually pseudo-boehmite is employed as an alumina source in a pH medium between 3 and 9 (Refs. 3-6). At these pH values, A1 tends to have octahedral coordination. However, in A1PO4-5, aluminium is only tetrahedrally coordinated. This raises questions about the origin and the stability of tetrahedral AI both during the preparation of synthesis mixtures and during their hydrathermal treatment. In this study, an attempt has been made to obtain information concerning these problems. T h e influence of the properties of the synthesis mixtures on the crystallization of AIPO4-5 has been investigated by means of solid-state high resolution 27A1 n.m.r. In preliminary studies, we have found that it is advantageous to use very dilute systems for these investigations. The preparation of these dilute reaction mixtures was realized by using a commercially available, aqueous suspension of aluminium hydroxide instead of pseudo-boehmite as source of alumina.

Phosphoric acid was employed as source of the phosphate and triethylamine (Et~N) as the organic amine component exclusively. The method of preparing reaction mixtures was as follows: The suspension of AI(OH)3 and the phosphoric acid were mixed together in the desired mole ratio and stirred for a period of 3 h at ambient temperature. Triethylamine was added to the so-formed aluminiumphosphate gel, and the pH value of this gel was adjusted to 3.0-3.5 by adding mineral acid dropwise. This final gel was stirred prior to heating. Deviations from this preparation method will be described for each individual case below. The final aluminiumphosphate gel was divided into several portions and sealed in identical autoclaves. These were heated at 175°C under autogenous pressure, without rotating, for given periods of time, and progressively removed from heating in order to obtain materials with different degrees of crystallinity. After cooling, each sample was filtered, washed with cold distilled water, and dried at I 10°C for 15 h. Each synthesis run was regarded as finished when the composition in the reaction product remained unchanged over a period of time. X-ray powder patterns were primarily used for the identification of all the samples. The patterns were compared with a r e f e r e n c e collection o f p o w d e r p a t t e r n s of aluminiumphosphate materials. Most of the synthesis products were examined by light microscopy and some by scanning electron microscopy. The 27A1 n.m.r, spectra were recorded at

* Present address: Central Institute of Physical Chemistry, Academy of Sciences of the GDR, DDR-1199 Berlin-Adlershof, GDR Address reprint requests to Dr. Muller, Central Institute of Inorganic Chemistry, Academy of Sciences of the GDR, DDR1199, Berlin-Adlershof, GDR Received 9 October 1987; accepted 19 September 1988

© 1989 Butterworth Publishers

ZEOLITES, 1989, Vol 9, May 177

Aluminophosphate molecular sieve AIPO~-5: E. Jahn et al.

70.4 and 104 MHz, respectively, in combination with magic-angle spinning (MAS) techniques. Spinning rates of about 3 kHz were used. Up to 1000 scans at 0.1 s i-ecycle time were applied. The chemical shifts were measured relative to external standards of aqueous A1CI:~ solution.

RESULTS A N D D I S C U S S I O N Synthesis runs The results of a set of synthesis runs differing in the initial batch compositions are given in Table I. In the~se experiments, the mole ratios of the components were varied within the following limits: 1.25 ~< Et3N/AI,,O:~ ~< 3.5 1 <~ P205/AIzO3 ~< 1.7

Table I shows that under the preparation conditions used the following phases were formed: amorphous AI(OH)~, amorphous A1POa-aq, microporous AIPOI5, aluminiumphosphate hydrate (AIPO4-H3), and A1PO4-tridymite. The structure of AIPO4-tridymite corresponds to SiOz-tridymite. The three-dimensional structure of A1PO4-H3 (Ref. 7) is composed of two kinds of sheets having different coordination of A1. The AI of A1PO4-5 is tetrahedrally coordinated, and there is a strict alternation of AI and P throughout the framework. 1-4 As can be seen from Table 1, a reaction mixture with a P205/AI203 ratio of 1 and a low amine content

(run 1) yields only amorphous AI(OH):~ and amorphous AIPO4.aq. Prolonged stirring of a gel of the same PgOs/Al,~O3 for a time period of 24 h prior to adding the amine (run 2) caused the predominant formation of AIPO4-tridymite as well as A1PO r5 and A1PO4-H3. On the other hand, a large amount of AI(OH)3 remained unreacted when the batch of the same P2Os/AI203 ratio was agitated for 24 h in the presence of amine (run 3). But increasing the P2Os/A1203 ratio to 1.3 improves the conversion of the reactant AI(OH)3 significantly and by applying a high initial Et3N/AI203: ratio, AIPOt-5 occurs as the main product. The time required for a complete crystallization was shortened by increasing the P2Os/A120:s ratio (runs 4 and 5). For example, the mixture of run 5 was transformed after 20 h of hydrothermal treatment into a product that was easy to filter, consisting of a heavy crystalline phase and a transparent mother liquor. A scanning electron photomicrograph of AIPO4-5 crystals of this latter run (see Figure 1) shows crystals as single particles with a well-outlined shape typical of hexagonal A1PO4-5. Further increase of both the P,,Os/AIzO:~ and the Et3N/AI203 ratios in relation to the initial batches yielded reaction products containing relatively pure AIPO4-5 (runs 7 and 8). The preparation of the starting mixtures of these runs was performed by mixing the amine and phosphoric acid prior to adding them to the AI(OH)3. The molar ratios of the components were to give an initial pH value of 3.

27A1 n.m.r, investigations Table 1 Synthesis conditions of AIPO4-crystallization runs Batch composition (expressed in mole ratios) Et3N :AI203:P2Os:H20:HNO3 A.

Time

P2OdAI203 = 1

1. 1.25:1:1:300:0.5

48

2. 1.25:1:1:300:0.5"

36

3. 1.25:1:1:300:0.5 b

48

B.

P20~/Ai203 =

Amorphous AI(OH)3 + amorphous AIPO4.aq AIPO4-tridymite + AIPO4-H3 + AIPO4-5 + amorphous AI(OH)z Amorphous AI(OH)3 ~> AIPO4-5 + AIPO4-H3

1.3

4. 1.5:1:1.3:300:0.5 5. 2:1:1.3:300:0.5 6. 2:1:1.3:300:0.5 b C.

Results

36 20 20

AIPO,-H3 >> AIPOctridymite AIPO4-5 > AIPO4-H3 AIPO4-5 > AIPO4-H3

15 15

AIPO4-5 ~> AIPO4-H3 AIPO4-5

24

AIPO4-5 ~> AIPO4-H3

P2Os/AI203 = 1.7

7. 2.5:1:1.7:300 8. 3.5:1:1.7:300 O. m

9. 2:1:1.3:300:3

a Agitation of the aluminiumphosphate gel prior to the addition of the Et3N for 24 h at ambient temperature bAgitation of the aluminiumphosphate gel in presence of the amine for 24 h at ambient temperature

178

ZEOLITES, 1989, Vol 9,

May

In order to investigate the influence of the preparation conditions on the formation of the various AIPO4 phases in more detail, the solid phases of the reaction mixtures were investigated by means of solid-state high-resolution 27A1 n.m.r, measurements. The samples examined were separated from the liquid phase by centrifugation both prior and after the addition of the triethylamine. T h e solid gel phases were dried at 50°C for 50 h and were shown to remain X-ray amorphous under these preparation conditions. The A1 in the aluminiumhydroxide suspension utilized as the alumina source was shown to possess A1 exclusively in octahedral coordination surrounded by six oxygen atoms of water molecules or hydroxide ions.

Reaction mixtures with molar ratios P2OJAI203 of 1 The 27A1 n.m.K spectra of the solid phase of the gel with a P2OJAI203 ratio of 1, recorded prior to (a) and after (b) the addition of the amine, respectively, are shown in Figure 2. Both spectra are characterized by an intense signal with a chemical shift 5(A1) at about 0 ppm. This reveals that the AI is still largely octahedrally coordinated as in the starting material. On the basis of the appearance of a signal at 8(AI) = 40 ppm in the spectra a and b of Figure 2 t h a t can be assigned to AI tetrahedrally coordinated by phos-

Aluminophosphate molecular sieve ALP04-5: E. Jahn et al.

Figure 1 Scanning electron photomicrograph of AIPO4-5 crystals from the system 2 Et3N:I AI203:1,3 P2Os:300 H20:0.5 HNO3

phate tetrahedra, 8 it is concluded that the depolymerization of the aluminiumhydroxide under the influence of the phosphoric acid provokes the conversion of octahedral A1 into tetrahedral coordination. The expected instability of the tetrahedral AI at low pH values in these systems is confirmed by spectrum c of Figure 2, which is taken from the solid phase of a gel after a stirring period of 48 h in the absence of any amine• Besides the signals at nearly 0 and 40 ppm, this spectrum involves two signals at - 1 0 and - 1 5 ppm, which are typical for octahedral A1 coordinated simultaneously by phosphate tetrahedra and water molecules. 8 This result can be explained by assuming that phosphate tetrahedra and/or water molecules enter into the coordination sphere of A1 and convert it from tetrahedral to octahedral. The continuous generation of octahedral A1 leads to the formation of a completely amorphous aluminiumphosphate gel that is characterized by the predominant existence of octahedral AI along with a little tetrahedral A1. The 27A1 n.m.r, spectra b and c of the Figure 2 reflect the structural properties of these aluminiumphosphate gels employed as synthesis mixtures of run 1 of Table 1 (sample corresponding to spectrum b) and of run 2 of Table 1, respectively (sample corresponding to spectrum c, but after the addition of the amine).

Reaction mixtures with molar ratios P 20 5/A120 3 of 1.3 Figure 3 shows the 27A1 n.m.r, spectra of the solid

phases of aluminiumphosphate gels with P2Os/AI,_,O:~ ratio of 1.3 that have been recorded prior to (a) and after (b) the introduction of the amine into the gel. The appearance of strong signals with chemical shifts of - 1 5 and +40 ppm, respectively, in spectrum a indicates an accelerated depolymerization of the starting aluminiumhydroxide and a rapid formation of an aluminium-phosphate gel in synthesis batches with P2Os/A120~ ratios > 1. This might be expected from the work of Callis et al. '~ who reported that a g g r e g a t e p o l y m e r s can f o r m from A1 and orthophosphate in acid medium, and these exhibit a degree of polymerization that is extremely sensitive to the environment. The polymeric character is very evident from the high viscosities and colloidal properties. Callis gave hypothetical structures of these polymers, which include octahedral AI and AI/P < 2. • 10 Kmep reported that high-viscous aluminiumphosphate gels with AI/P ratios ~< 2 already tend to form those coordination polymers that represent the building units of the aluminiumphosphates crystallizing in these systems• In the systems of the present study, the only detected aluminiumphosphate phase occurring along with A1PO4-5 is A1PO4-H3 (see Table I). Therefore, we assume that the coordination polymers usually present in similar aluminiumphosphate gels in our initial batches are also building units of AIPO4-H3. The 27A1 n.m.r, spectrum of a pure AIPO4-H3 sample is given in the Figure 3c. The appearance of two signals in the spectrum agrees with the occurrence of A1 in two distinct kinds of coordination of the sheet structure of AIPO4-H3 (Ref. 7). The signal

ZEOLITES, 1989, Vol 9, May 179

Aluminophosphate molecular sieve ALP04-5: E. Jahn et al.

60

absence of an amine, respectively, leads to the conclusion that tetrahedrally coordinated A! originating from the depolymerization of AI(OH):~ is stable in an amine-containing gel, even at low pH values. Combining this result with the results of the hydrothermal treatment of the corresponding synthesis mixtures (runs 5 and 6 of Table 1), it becomes evident that the successful crystallization of AIPO4-5 requires conditions that prevent the conversion of tetrahedral AI into octahedral A1. It can be assumed that under the synthesis conditions used, the building units first formed can act as seeds in the crystal growth process of A1POr5. But this needs further examination. The ability of the amine to favor the nucleation and crystallization of AIPO4-5 in competition with the formation of A1POr H3 depends on its concentration in the synthesis batches. This conclusion is corroborated by the results of a synthesis run utilizing a batch composition with a low amine supply (run 4 of Table, l). This batch yields AIPO4-H3 preferentially; It seems possible that the amine content of this system is not sufficient to impede the conversion of tetrahedral AI into octahedral AI by water molecules entering into the

~2~5.0 42.0 I

I

39.5

c

A

40.2

B0

41.2

Q

Figure 2 2>AI MAS n.m.r, spectra (70.4 MHz) of aluminiumphosphate gels with a P20s/AI203 ratio of 1. (a) Prior to the addition of Et3N, (b) after the addition of Et3N, (c) after agitation for 48 h in the absence of Et3N

at/5(Al) = 41.2 corresponds to tetrahedrally coordinated A1 linked via oxygen bridges to four phosphate tetrahedra, and the signal with a chemical shift 8(AI) = - 1 2 ppm is due to octahedrally coordinated A1 with four phosphate and two water molecules in its coordination sphere, simultaneously. The broad signal at - 1 2 ppm may be caused by the presence of different next neighbors in the octahedra of A1. The comparison of spectra b and c of the FiDtre 3 makes the existence of building units of A1PO4-H3 in the aluminiumphosphate gel quite likely because of the intense peak at 8(AI) = 42.9 ppm and the shoulder peak at/5(AI) = - 1 4 ppm in spectrum b. This falls also in line with the reaction products obtained after the hydrothermal treatment of the corresponding synthesis mixture (run 5 of Table I). Moreover, these results provide evidence for the conclusion that octahedral AI, once created during the preparation of the synthesis mixture, is stable and can induce the nucleation of unwanted phases prior to or as well as the nucleation of AIPO4-5. Agitation of the aluminiumphosphate gel in the presence of the amine for a period of 24 h did not enhance the signal at - 1 4 ppm, in contrast to agitation without any amine. This different behavior of aluminiumphosphate gels in the presence and the

180

ZEOLITES, 1989, Vol 9, May

-12.0

42.g 6.4 -14.0

B0 Figure 3 (a,b) 27AI MAS n.m.r, spectra (104 MHz) of aluminiumphosphate gels with a P2Os/AIz03 ratio of 1.3 (a) prior to the addition of Et3N, (b) after the addition of Et3N. (c) 27AI MAS n.rn.r, spectrum of AIPO4-H3

Aluminophosphate molecular sieve AIPO~-5: E. Jahn et al. -

41.6

is supported by the appearance of the 27A1 n.m.r. spectrum obtained from the solid phase of the synthesis mixture of run 7 (Figure 4). On the basis of this spectrum, the tetrahedral coordination of AI by phosphate tetrahedra is shown to be the most favored coordination of AI in this system.

7.9

CONCLUSIONS

B0 Figure 4 27AI MAS n.m.r. (104 MHz) spectrum of the solid gel phase of the reaction mixture of the composition 2.5 Et~N:I AI203:1.7 PzOs:300 H20

coordination sphere of the Al atoms. In this way, the generation of building units of AIPO4-H3 prevails over the creation of other species leading to the predominant formation of AIPOa-H3.

Reaction mixtures with molar ratios P 20 5/A120 3 of 1.7 In order to obtain information on the mode of action of the triethylamine in the formation of AIPO4-5, the following experiments were carried out, involving different techniques to prepare the reaction mixtures (runs 8 and 9 of Table 1). To formulate the reaction mixture of run 9, a certain amount of amine was separately neutralized with a mineral acid other than phosphoric acid and then added to the reactant AI(OH)~. After stirring this suspension with the phosphoric acid for a given period, the pH of the system was adjusted to 3 by addition of further amine and then exposed to hydrothermal heating. In the case of run 8 (Table I), the amine and the phosphoric acid were mixed together and then combined with the required amount of AI(OH)3. The mole ratios of the components were chosen such that a pH value of 3 in the final system was attained. Only by employing the latter described preparation procedure, no AIPO4-H3 could be detected in the reaction products. This suggests that species bearing both amine and phosphate fulfill an important function in the crystallization of A1PO4-5. The latter

The results of the present study support the conclusion that the formation of a pure A1PO4-5 phase requires preparative conditions that promote the formation of tetrahedral A1 species, surrounded by O atoms of four phosphate tetrahedra, and their stabilization against the conversion into A1 octahedra prior to their incorporation into the crystals. These requirements appear to be fulfilled by employing simultaneously high P2Os/AI20~ and h i g h Et~N/ AI20~ ratios in the synthesis mixtures by adding premixed amine and phosphate to the alumina source. High P2OJAI20 ~ ratios possibly favor tetrahedral A1 species, and high Et~N/AI2Os ratios may ensure their stabilization, leading to formation of A1PO4-5. A possible explanation of the stabilization is as follows: The amine molecules are bound via the phosphate tetrahedra to the initial species built up of AIO4 and PO4 tetrahedra. These amine molecules could shield such species from the attack of water and therefore stabilize the tetrahedral coordination of AI.

REFERENCES 1 Bennett, J.M., Cohen, J.P., Flanigen, E.M., Pluth, J.J. and Smith, J.V., ACS Symp. Ser. 218, Am. Chem. Soc., 1983, p. 109 2 MiJller, D., Jahn, E., Fahlke, B., Ladwig, G. and Haubenreisser, U. Zeolites 1985, 5, 53 3 Wilson, S.T., Lok, B.M. and Flanigen, E.M., US Pat. 4 310 440 (1982) 4 Wilson, S.T., Lok, B.M., Messina, C.A., Cannan, T.R. and Flanigen, E.M.J. Am. Chem. Soc. 1982, 104, 1146 5 Wilson, S.T., Lok, B.M., Messina, C.A. and Flanigen, B.M., in Proceedings of the Sixth International Zeolite Conference, Butterworths, Guildford, UK, 1985, p. 97 6 Wilson, S.T., Lok, B.M., Messina, C.A. and Flanigen, E.M., ACS Symp. Ser. 218, Am, Chem. Soc., 1983, p. 3 7 Pluth, J.J. and Smith, J.V. Nature 1985, 318, 165 8 MLiller, D., Grunze, I., Hallas, E. and Ladwig, G. Z. Anorg. AIIg. Chem. 1983, 500, 80 9 Callis, C.F., van Wazer, J.R. and Aryan, P.G. Chem. Rev. 1954, 54, 777 10 Kniep, R. Angew. Chem., 1986, 98, 520

ZEOLITES, 1989, Vol 9, May

181