Synthesis of VPI-5 - American Chemical Society

Chapter 21

Synthesis of VPI-5 1

1

2

Mark E. Davis , Consuelo Montes , and Juan M. Garces Downloaded by IOWA STATE UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch021

1

Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Dow Chemical Company, Midland, MI 48640 2

We report here, for the first time, the synthetic procedures used to crystallize the aluminophosphate molecular sieve VPI-5. Two synthesis methods are i l l u s t r a t e d . The step by step procedures are discussed i n d e t a i l and reveal the precise nature of synthesizing VPI-5.

The f i r s t discovery of a z e o l i t e was recorded i n 1756 (1). Since that time numerous n a t u r a l and s y n t h e t i c z e o l i t e s , silica polymorphs, and aluminophosphate-based molecular sieves have been reported. The l a r g e s t r i n g i n these materials c o n s i s t s of 12 tetrahedral (12 T) atoms. This boundary has been i n existence for over 180 years since the f i r s t z e o l i t e to contain 12 T-atom rings, g m e l i n i t e , was discovered i n 1807 (1). Recently, we have synthesized the f i r s t molecular sieve with rings that possess greater than 12 T-atoms (2,3). V i r g i n i a Polytechnic I n s t i t u t e number 5 (VPI-5) i s a family of aluminophosphate based molecular sieves with the same three-dimensional topology. The extra-large pores of VPI-5 contain unidimensional channels circumscribed by r i n g s which have 18 T-atoms and possess f r e e diameters of approximately 12 À (2,3). We report here, for the f i r s t time, the synthesis procedures used to c r y s t a l l i z e the extra-large pore, aluminophosphate, molecular sieve VPI-5. Synthesis techniques f o r c r y s t a l l i z a t i o n of element substituted VPI-5 are forthcoming (Davis, M. E. , et a l . , Zeolites '89, i n press) Experimental Section Pseudoboehmite alumina (Catapal-B) and 85 wt% H3PO4 were used e x c l u s i v e l y as the aluminum and phosphorus s t a r t i n g materials. Aqueous (55 wt%) tetrabutylammonium hydroxide (TBA) and n-dipropylamine (DPA) were purchased from A l f a and A l d r i c h , respectively. 0097-6156/89/0398-0291$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

292

Z E O L I T E

SYNTHESIS

A t y p i c a l synthesis procedure involves the following steps: ( i ) alumina i s s l u r r i e d i n water, ( i i ) phosphoric a c i d i s d i l u t e d i n water, ( i i i ) the phosphoric a c i d s o l u t i o n i s added to the alumina s l u r r y , ( i v ) the aluminophosphate precursor mixture i s aged at ambient c o n d i t i o n s , (v) an organic i s added to the precursor mixture and aged with rapid a g i t a t i o n to form the f i n a l gel, which (vi) i s charged into the autoclave and heated. The gel composition can be written as χ R · y AI2O3 · ζ Ρ 0

Downloaded by IOWA STATE UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch021

2

5

· g

H 0. 2

Exploratory syntheses were accomplished i n 15 ml Teflon-lined autoclaves which were s t a t i c a l l y heated at autogenous pressure i n forced convection ovens. Larger autoclaves (300 ml, 600 ml, and above) have also been s u c c e s s f u l l y employed. At s p e c i f i e d times, the autoclaves were removed from the oven, quenched i n c o l d water, and the pH of the contents measured. Product VPI-5 was recovered by s l u r r y i n g the autoclave contents i n water, decanting the supernatant l i q u i d , f i l t e r i n g the white s o l i d , and drying the crystals i n ambient a i r . A Stoe 12 X-ray diffractometer was used to c o l l e c t X-ray powder d i f f r a c t i o n data. Figure 1 i l l u s t r a t e s the X-ray powder d i f f r a c t i o n pattern for VPI-5. Scanning electron micrographs were obtained using a Cambridge Instruments Stereoscan 200 scanning electron microscope. Results and Discussion We have synthesized VPI-5 with a v a r i e t y of organic agents such as amines and quaternary ammonium cations. The synthesis procedure depends upon the type of organic agent and we w i l l i l l u s t r a t e an "amine" synthesis with DPA and a "quat." synthesis with TBA. Table I l i s t s reproducible procedures. We have provided an example of a "small" scale synthesis (DPA) and a f a i r l y "large" s c a l e synthesis (TBA) to show that these procedures can be scaled-up. Below we discuss the e s s e n t i a l d e t a i l s of these two procedures. VPI-5 that has been c r y s t a l l i z e d with the use of DPA and TBA w i l l be denoted DPA-VPI-5 and TBA-VPI-5, respectively. Synthesis Using DPA. The synthesis of DPA-VPI-5 i s summarized i n Table I and i s f u l l y described as follows. Upon combining the alumina s l u r r y with the phosphoric a c i d s o l u t i o n , the pH of the precursor mixture r i s e s with aging (see F i g . 2). Thus, the phosphoric a c i d i s slowly reacting with the alumina. The pH s t a b i l i z e s around 1.2-1.3 a f t e r approximately 1.5 hours. This aging process i s important f o r the formation of VPI-5. I f the precursor mixture i s not aged and a l l other steps of the procedure f o l l o w e d , H3 (4,J>) i s u s u a l l y c r y s t a l l i z e d . H3 i s an aluminophosphate hydrate ( A I P O 4 · 1.5 H 0) f i r s t synthesized by d'Yvoire (4) . The s t r u c t u r e of H3 has been solved (5) and contains 4,6, and 8 membered r i n g s . Aging times as long as approximately 10 hours s t i l l y i e l d VPI-5. Upon a d d i t i o n of the DPA to the s t i r r i n g precursor mixture the pH immediately increases to above 3 and then g r a d u a l l y climbs to a f i n a l value of approximately 3.75 (see F i g . 2). Again, the aging of the complete 2

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Synthesis ofVPI-5


DAVIS ET AL.

Figure 1

0

2©

d(Â)

5.38 9.32 10.75 14.26 16.16 18.68 21.76 21.92 22.39 22.56 23.59 24.46 26.12 27.17 28.19 28.96 29.48 30.28 30.88 32.71 34.05 35.86 38.32

16.43 9.49 8.23 6.21 5.48 4.75 4.08 4.05 3.97 3.94 3.77 3.64 3.41 3.28 3.17 3.08 3.03 2.95 2.90 2.74 2.63 2.50 2.35

100 2 14 6 2 6 20 22 14 15 10 4 2 16 5 7 4 8 5 7 2 3 3

X-ray powder d i f f r a c t i o n pattern of VPI-5,



gel composition:

o

DPA · Al 0_ · P.0_ · 40 H_0 2 3 2 5 2

The reaction mixture i s heated at 142°C f o r 20-24 hours

6.

5.

4.

3.

2.

6.9 g of pseudoboehmite are s l u r r i e d i n 20 g of water 10 g of water are added to 11.5 g of phosphoric acid The phosphoric acid solution i s added to the alumina s l u r r y to form a precursor mixture The precursor mixture i s aged f o r 1.5-2 hours at ambient conditions with a g i t a t i o n 5.1 g of DPA are added to the precursor mixture and the resulting g e l aged with agitation f o r 1.5-2 hours at ambient conditions

TBA . A l 0

. Ρ 0

. 50

HO

55 g of pseudoboehmite are s l u r r i e d i n 150 g of water 100 g of water are added to 90 g of phosphoric acid The phosphoric acid solution i s added to the aluminium s l u r r y to form a precursor mixture The precursor mixture i s aged f o r 1.5-3 hours at ambient conditions with no a g i t a t i o n 186 g of 55 wtj TBA are added to the precursor mixture and the r e s u l t i n g g e l vigorously agitated f o r approximately 2 hours at ambient conditions The reaction mixture i s heated at 150 C f o r 24 hours

gel composition:

6.

5.

4.

3.

2.

1.

TBA

Synthesis Procedures f o r VPI-5 With DPA and TBA

1·

DPA

Table I.


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Synthesis ofVPI-5

gel i s important. Since the pH slowly r i s e s during t h i s time period, a chemical reaction i s occurring. I f the gel i s aged for too long of period, e.g., 24 hours, H3 i s c r y s t a l l i z e d rather than VPI-5.

τ

ι


Gel

I

0

0

1 40

1

1

1—ι

1

Preporotion

1 80

I

120

1

Γ

Crystallization

I

160

I

I

200 0

Minutes

I

I

8

16

I

24

32

Hours Time

Figure 2.

pH versus time for the synthesis of DPA-VPI-5.

The c r y s t a l l i z a t i o n of VPI-5 occurs a t 142°C, and the c r y s t a l l i z a t i o n time i s fast. From Figures 2 and 3 i t i s observed that w i t h i n f i v e hours the pH o f the autoclave contents has reached a plateau and the product i s quite c r y s t a l l i n e . In Figure 3 we i l l u s t r a t e the degree of c r y s t a l l i n i t y estimated from X-ray powder d i f f r a c t i o n data as a f u n c t i o n o f time. Since VPI-5 quickly c r y s t a l l i z e s , the values shown at short times are rough estimates only. Notice, however, that the DPA-VPI-5 i s not stable for long periods of time i n the mother liquor, and that the loss of c r y s t a l l i n i t y i s not accompanied by a change i n pH. We specify a c r y s t a l l i z a t i o n time of 20-24 hours i n Table I since we have observed that s l i g h t variations i n the procedure normally lengthen the time f o r the onset of c r y s t a l l i z a t i o n . However, we almost always observe the highest c r y s t a l l i n i t y i n the samples which have been c r y s t a l l i z e d for 20-24 hours. The c r y s t a l l i z a t i o n temperature can be varied ± 5°C with no adverse e f f e c t s . At temperatures above 150°C VPI-5 forms with H3 then quickly decomposes and ultimately A l P O ^ - l l i s c r y s t a l l i z e d . At temperatures around 125°C the s o l i d product i s amorphous at 24 hours but eventually forms H3 after several days. Table I I i l l u s t r a t e s the e f f e c t o f gel composition on the f i n a l product c r y s t a l l i z e d using the procedure l i s t e d i n Table I.


In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 1.00 1.10 0.90 0.75 1.25 1.00 1.00 1.00 1.00 1.00 1.00

1.0 1.0

1.0 0.5 2.0

1 2

•Most l i k e l y dense phase aluminophosphate ••Required longer c r y s t a l l i z a t i o n time

9 10 11

4 5 6 7 8

3

Al Ο 23

DPA

1.00 1.00 1.00

1.00 1.00 1.00 1.00 1.00 0.75 1.25 1.00

U

Ρ 0 25

45 40 40

40 40 40 40 40 40 40 35

2

H0

Variation i n Gel Composition With DPA Synthesis

Preparation

Table I I .


VPI-5 VPI-5 VPI-5 VPI-5 + H3 unknown* unknown* H3 + unknown* VPI-5 with s l i g h t l y lower c r y s t a l l i n i t y VPI-5 + H3*» H3 amorphous

Result

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Synthesis of VPI-5

100 -


° 50-

12

16

20

24

Crystallization Time, hours

Figure 3. Degree of c r y s t a l l i n i t y estimated d i f f r a c t i o n data versus c r y s t a l l i z a t i o n time.

from X-ray powder

Notice that small compositional v a r i a t i o n s (preparations 1-3) are allowable. However, ± 25% v a r i a t i o n s i n g e l composition do not c r y s t a l l i z e pure DPA-VPI-5. I t i s i n t e r e s t i n g to note that the water content i s important and that excess DPA (preparation 11) hinders the c r y s t a l l i z a t i o n of VPI-5. Synthesis With TBA. As with the DPA-VPI-5 c r y s t a l l i z a t i o n , the TBA synthesis involves the use of a precursor aluminophosphate mixture. (This i s summarized also i n Table I.) However, there i s an important d i s t i n c t i o n between the two types of syntheses during step 4. When TBA i s used, the precursor mixture i s not agitated. The quiescent mixture exhibits a pH p r o f i l e i n time nearly that with a g i t a t i o n (see Figures 2 and 4). I f the precursor mixture i s agitated during aging, either TBA-VPI-5 i s formed and accompanied by H3 or only H3 i s c r y s t a l l i z e d . The precursor g e l i s vigorously agitated j u s t p r i o r to the a d d i t i o n o f TBAOH. When the TBAOH i s combined with the aluminophosphate mixture the pH i n s t a n t l y r i s e s to around 5. The f i n a l pH i s dependent upon the degree of mixing during addition of TBAOH. Incomplete mixing produces pH's below 5 and can lead to the formation of impure TBA-VPI-5 (with small amounts of H3 present). The TBA g e l c r y s t a l l i z e s VPI-5 rapidly and reaches a f i n a l pH equivalent to that observed with DPA (see Figures 3 and 4). For reasons s i m i l a r to those o u t l i n e d i n the DPA synthesis, we specify approximately 24 hours of c r y s t a l l i z a t i o n time when using TBA ( i n Table I ) . Notice that the TBA-VPI-5 does not decompose i n the mother liquor. We have observed that the TBA-VPI-5 i s stable i n the mother liquor f o r many days. I t i s i n t e r e s t i n g that the f i n a l pH of the TBA and DPA syntheses are approximately the same yet the TBA-VPI-5 i s stable while the DPA-VPI-5 i s not.


298

ZEOLITE SYNTHESIS

τ


Gel

0

40

Preparation

80

120

1

Γ

Crystallization

160

200 0

Minutes

8

Γ6

24,

32

Hours

Time

Figure 4.

pH versus time for the synthesis of TBA-VPI-5.

Morphology. F i g u r e s 5 , 6 , and 7 show scanning e l e c t r o n micrographs which i l l u s t r a t e the morphology o f TBA-VPI-5, DPA-VPI-5, and H3 r e s p e c t i v e l y . The TBA-VPI-5 c r y s t a l s have " n e e d l e - l i k e " morphology and are aggregated into bundles. The c r y s t a l s are approximately 10 microns i n length and are submicron i n diameter. We suspect that the c-axis and thus the 12 Â pore i s oriented i n the d i r e c t i o n of the needle length. On the other hand, the DPA-VPI-5 shows large spherical aggregates (greater than 100 microns). Inspection o f these aggregates reveals p a r t i c l e s which adopt a v a r i e t y of morphologies. Spheres of approximately 5 microns are observed as well as needles (see Figure 6 b ) . From adsorption experiments ( r e f . 3, sample 1 - TBA-VPI-5, sample 2 DPA-VPI-5), i t has been determined that the v o i d volume o f TBA-VPI-5 i s equivalent to DPA-VPI-5. Therefore, the 5 micron spheres i n DPA-VPI-5 are not impurities but must also be DPA-VPI-5 with a d i f f e r e n t or very small c r y s t a l habit. The morphology of H3 i s i l l u s t r a t e d since i t t y p i c a l l y i s the impurity present when VPI-5 i s not c r y s t a l l i z e d properly. H3 i s observed as spherical aggregates of approximately 20 micron diameter. These aggregates are e a s i l y d i s t i n g u i s h e d from TBA-VPI-5 by o p t i c a l microscope. However, since DPA-VPI-5 grows i n large spherical aggregates i t i s more d i f f i c u l t to d i f f e r e n t i a t e from H3 with only a o p t i c a l microscope. However, we presently are able to do so and use the difference i n size (Λ/ 100 μ versus Λ/20 μ) as the distinguishing feature. Adsorption. Table I I I shows the adsorption capacity of AlPO^-5 and VPI-5 f o r various adsorbates. A l l values l i s t e d are obtained at P/P 0.4. The d a t a for AIPO4-5 (except f o r 0


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Synthesis ofVPI-5


21.

F i g u r e 5. Scanning e l e c t r o n m i c r o g r a p h s of TBA-VPI-5. s i z e i s 50 m i c r o n s , (B) b a r s i z e i s 20 m i c r o n s .

(A) b a r


ZEOLITE SYNTHESIS


300

Figure 6. Scanning electron micrographs of DPA-VPI-5. size i s 100 microns, (B) bar size i s 20 microns.

(A) bar


DAVIS ET AL.

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Synthesis of VPI-5


21.

Figure 7. Scanning electron micrographs of H3. 100 microns, (B) bar s i z e i s 20 microns.

(A) bar s i z e i s


In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 0.137 0.021

Neopentane Triisopropylbenzene

6.20

8.50

From ref, 6.

2

c

2

Adsorption at room temperature except f o r 0 N or 0 temperatures.

b

a

0

0.117

0.148

0.156

0.198

0.228

VPI-5

which was performed at e i t h e r l i q u i d

From Davis et a l . , J . Am. Chem. S o c , submitted.

0.145

Cyclohexane

6.00

0.146

AlPO^-5

0.139

b

n-Hexane

°2

Adsorbate

= 0.4

Capacity, cm^/g

Adsorption capacity of molecular sieves at P/P

4.30

3.46

K i n e t i c Diameter, X

Table I I I .



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Synthesis of VPI-5

303

triisopropylbenzene) are from Union Carbide (6). The VPI-5 data were obtained using a McBain-Bakr apparatus and the samples were activated by heating to 350°C under vacuum overnight (Davis et a l . , J . Am. Chem. S o c , submitted). ΑΙΡΟ^-δ adsorbs oxygen and the hydrocarbons l i s t e d except f o r t r i i s o p r o p y l b e n z e n e . The triisopropylbenzene i s too large to penetrate the 12 T-atom r i n g . Notice that the adsorption capacity of AlPO^-5 i s the same, within experimental error, f o r a l l adsorbates l i s t e d . VPI-5 reveals two phenomena not observed f o r AlPO^-5. F i r s t , triisopropylbenzene i s adsorbed. Second, the adsorption capacity monotonically decreases with increasing adsorbate s i z e . Since VPI-5 contains pores that are s l i g h t l y l a r g e r than 12 Â, a l l adsorbates other than triisopropylbenzene have the p o s s i b i l i t y of f i t t i n g more than one molecule across the diameter. In other words, packing of adsorbate molecules may be important i n these extra-large pores. Further evidence to support t h i s ideas i s provided elsewhere (Davis et a l . , J . Am. Chem. S o c , submitted). P e r f l u o r o t r i b u t y l a m i n e (PFTBA) has a k i n e t i c diameter greater than 10 Â (Λ, 10.5 Â ) . After repeated attempts to adsorb PFTBA into VPI-5, we were convinced that our data were influenced by e x t r a c r y s t a l l i n e a d s o r p t i o n . Thus, we performed PFTBA desorption experiments u t i l i z i n g a thermogravimetric analyzer i n which the off-gas was t r a n s f e r r e d i n t o a mass spectrometer. A I P O 4 - 5 was used for comparison since PFTBA cannot adsorb i n a 12 T-atom r i n g . A I P O 4 - 5 and VPI-5 were loaded with PFTBA. Next, A I P O 4 - 5 was heated to 100°C i n flowing helium i n the apparatus described previously. A f t e r several hours, a stable weight was obtained (PFTBA loss observed i n the mass spectrometer). Upon heating AlPO^-5 to_550°C, no further weight loss was observed. Thus, PFTBA can be desorbed from an A I P O 4 surface by flowing helium at 100°C. The same treatment was employed f o r VPI-5. A f t e r reaching a stable weight i n flowing helium at 100°C, the sample was heated to 550°C. Several higher temperature desorption peaks were due to the loss of PFTBA. We interpret this r e s u l t to indicate that PFTBA can adsorb w i t h i n the 18 T-atom rings of VPI-5. Thus, adsorption of molecules with k i n e t i c diameters above 10 Â i s possible with VPI-5. Acknowledgments We thank the National Science Foundation and the Dow Chemical Company f o r support of t h i s work through the P r e s i d e n t i a l Young Investigator Award to M.E.D.

Literature Cited 1. 2. 3. 4. 5.

Breck, D. W., Zeolite Molecular Sieves: Wiley: New York, 1974; p. 188. Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988, 331, 698. Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Zeolites 1988, 8, 362. d'Yvoire, F. Bull. Soc. Chim 1962, 1762. Pluth, J. J.; Smith, J. V. Nature 1985, 318, 165.


304 6.

ZEOLITE SYNTHESIS

Wilson, S. T.; Lok, Β.M.;Messina, C. Α.; Flanigen, Ε. M. Proceedings of the Sixth International Zeolite Conference. Butterworths: Survey, 1985; p. 97.


RECEIVED December 22, 1988


Synthesis of VPI-5 - American Chemical Society

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