A Mechanism Study of Framework Si-Al Substitution in Y Zeolite During Aqueous Fluorosilicate Treatment

P.A. Jacobs and R.A. van Santen (Editors), Zeolites: Frrcts, Figures, Fiiture

189

0 1989 Elsevicr Science Publishers B.V.. Amsterdam - Printed in The Netherlands

A MECHANISM STUDY OF FRAMEWORK Si-A1 SUBSTITUTION IN Y ZEOLITE DURING AQUEOUS FLUOROSlLICATE TREATMENT

Yigong He, Caiying Li and Enze Min Research Institute of Petroleum Processing, China Petrochemical Corporation, P.O.Box 914-43, Beijing 100083, (China)

ABSTRACT The reaction mechanism for isomorphous substitution of extraneous silicon for aluminum in Y zeolite framework during aqueous fluorosilicate treatment has been studied. The aluminum in zeolite framework is extracted into the reaction solution in the form of A1F 3- complex ion. The monomolecular silicic 6 acid Si(0H) is an important reaction intermediate which can be inserted into 4 the dealuminated defect sites to form the framework silicon enriched zeolite. Reaction mechanism is proposed. INTRODUCTION The framework silicon enriched zeolites prepared by the reaction of fluorosilicate solution with NH4-Y zeolite were reported by Breck and Skeels(ref. 1 ) . They suggested that the framework aluminum was extracted by complexing reaction of fluoride ion with aluminum and then monomeric silicon species in solution were inserted into the defect sites of the framework. A simple reaction scheme was proposed: O

(NH4)2SiF6

+

M

0

\ /

’ 0

0

\ /O

+

(NHq)2ALF5

+ MF

‘0

However, the details of reaction mechanism were not yet fully understood(ref. 2 ) .

The aim of this work is to identify the form of aluminum existing in the reaction solution after extracted from the zeolite and t o study the chemical state of silicon species which can be inserted into the defect sites after dealumination. Reaction mechanism for isomorphous substitution of

extraneous

silicon atom for framework aluminum atom in Y zeolite during aqueous fluorosilicate treatment is finally proposed. EXPERIMENTAL 1. Sample preparation

Using NH -Y as 4

starting material, framework silicon enriched Y

zeolite

I90 (designated as FSY) was obtained according to the method of Breck and Skeels (ref. 3 ) ; Dealuminated Y zeolite (dasignated as DAY) with a 35% removal of framework aluminum was obtained by slow extraction of framework aluminum with

H4EDTA as described by Kerr(ref. 4). The silicic gel was mixed with water and vibrated for 48 hr. After filtration a monomolecular Si(0Hl4 solution with 12-15 mg Si02/100ml was obtained(5). The DAY zeolite was treated with monomolecular Si(OH)4

and polysilicic acid

solution respectively in a NH Ac-HAc buffer solution at 75 co to determine in

4

which form silicon species can be inserted into the dealuminated defect sites. 2. Sample analysis Unit cell constant and crystallinity(re1ative to the starting NH4-Y zeolite) of zeolite samples were analyzed by x-ray powder diffraction

. The

standar KBr

wafer technique was used to measure the IR spectrum in the region 400-1200 cm-.

1

The method described by Breck and Skeels (ref. 3) was used to measure the IR spectrum in the hydroxyl region. The absolute absorbance value measured

at

3710 cm-l was used to quantify the concentration of the defect sites in framework of zeolite sample

. The

29Si,

27Al MAS NMR spectra of zeolite samples

were obtained using Bruke AM-300 spectrometer. The chemical state of aluminum and silicon in the reaction solution were analyzed respectively by spectroscopy and by silicomolybdic acid method(ref.

27Al NMR

6).

RESULTS AND DISCUSSION Physical properties of the FSY zeolites(samp1es F1-3, in order of decreasing A1 content), DAY zeolite and the parent zeolite NH -Y are listed in Table 1.

4

TABLE 1 Physical properties of zeolite samples Fram

.*

X-ray

Sample

Si/Al

ao(i)

NH -Y DAQ F-1 F-2 F-3

2.41 4.13 4.39 5.95 7.45

24.681 24.678 24.570 24.496 24.451

*

Cryst.

Framework IR

lSym. cryst. calla-temp. Asym. (X) ('c ) (cm- ) 100 89 99 98 97

970 975 1030 1053 1069

Calculated from the method of Sohn(ref.

1014 1019 1031 1045 1049

Defect structure factor (mol fraction)

788 787 802 809 814

0.018 0.152 0.051 0.062 0.079

7).

The results demonstrate that as the framework Si/A1 ratio increases the unit cell size of FSY zeolites constracts significantly. Similarly, a shift of the bands of most of the skeletal vibrations to higher wavenumbers has observed in IR spectra

. The crystal collapse temperatures of

been

FSY zeolites are

191

Si(2Al) Si( 1Al)

-3s

-80

P W

-100 P P

NH -Y

F- 1

4

Fig. 1.

’

V.”.1.........’...l”1

-1do

‘ r

-80

I

-100 PPm

F-3

29Si MAS NMR spectra of zeolite samples( chemical shift relative

to

TMS ).

-

a

lob P P

NH -Y 4 Fig. 2.

‘ 7 -

d

lob ?Pm

F- 1

-

lor0

’ PPm

d

F-3

27Al MAS NMR pectra of zeolite samples( chemical shift relative

AI(H,o)~’+)

.

to

I92 seen to increase steadily with increasing framework Si/A1 ratio. However, although the Si/A1 ratio of DAY zeolite is much higher than that

of the

parent

zeolite NH -Y, the corresponding physical properties of these two zeolite are

4

known to be the same within the limit of analysis error. Besides, no large amount of the defect structure has been found in the FSY zeolites.

All these

results agree well with those reported by Breck and Skeels(ref. 3). The 29Si MAS NMR spectra and 27Al MAS NMR spectra of the FSY zeolite samples (F-1 and F-3) are given in Figure 1 and Figure 2, respectively. the predominant groups in the Si(lA1) and Si(OA1).

Figure 1 ,

In

29Si MAS NMR spectra of FSY zeolite samples are

characteristic of a framework with high Si/Al ratio.

Figure 2, a single, relatively narrow signal with a chemical shift of

about

27Al MAS N M R spectra of F-1 and F-3, correspoding

62 ppm is seen in the

In

to

tetrahedrally coordinated aluminum. All these indicate that no extraframework aluminum species

is presented. Thus the FSY zeolites obtained by

(NH ) SiF solution with NH -Y zeolite are essentially free 6 4 4 2 aluminum and only with a small amount of defect structure.

o I

reaction of extraframework

Breck and Skeels suggested that the aluminum in the zeolite framework was extracted in the form of A1F 2- complex ion. In order to identify chemical state 5 of aluminum existing in the reaction solution during the reaction, samples of

reaction mother liquor were taken out at different reaction time and were investigated by

27Al NMR spectroscopy. The 27Al NMR spectra of thcse solution sam-

ples and thc standard (NH ) A1F solution are given i n Figure 3 . The 27Al NHR 43 6 spectra of these mother liquor samples and the (NH ) A1F solution all show the 43 6 same characteristic peak with a chemical shift of 0 . 5 ppm corresponding to Al(H 0) 3 + . This sharp peak is ascribed to an octhedral aluminum with high synnn2 6 (H O ) ) n - 3 try. These spectra are i n good agreement with the spectra of (A1F 6-n 2 n (n= 0-6) reported from the literature(ref. 8 ) . This indicates that the aluminum i n the zeolite framework is extracted in the form of AlF 3- complex ion

6

into

the reaction solution.

In order to determine the chemical state of silicon existing in the reaction solution, samples of reaction mother liquor as mentioned above were detected for monomolecular .%(OH) Table 2.

4

by silicomolybdic acid method. The results are given

in

It was found that the monomolecular Si(OH)4 existed through the reaction of (NH ) SiF with NH -Y during the preparation of samples F-1 and F-3. This 4 2 6 4 indicates that the silicon in the (NHq)2SiF6 is changed into Si(OH)4 during

.

the reaction It would seem reasonable to suggest that the monomolecular Si(OH)4

is

the

reaction intermediate which can be inserted into the dealuminated defect sitcs of DAY zeolite. In order to verify this suggestion, experiments were designed

193 i-.

2

OD

a a

50

- 50

0 PPM

i-.

0

a.

0

50

a.

,

Fig. 3.

PPM

b.

C

0

CL

50

- 50

0

50

P a

= a

I

- 50

PPM

'9

/

I

I

I

(

I

0

I

I

r

,

- 50

I

I

,

,

r

,

,

50

,

0

PPM

PPM

d.

e.

r

,

,

,

~

I

l

-50

,

1

50

,

,

,

(

,

0

,

,

,

,

PPM

f

27Al NMR spectra of solution samples( chemical shift relative

Al(t1~0)~~' ). a. (NH4)3A1F6 standard sample, b.c.d.e. tion mother liquor samples at 0.5,1.0,

to

are respectively reac-

2.0hr and after reaction during

preparation of F-1 zeolite sample, f. is reaction mother liquor sample reaction during the preparation of F-3 zeolite sample.

l

-50

the after

I94 TABLE 2 Identification of existence of monomolecular Si(OH)4

in the reaction solution

Existence of Si(0H) 4 at various reaction time

Reaction system

0.5hr

l.Ohr

2.0hr

3.5hr

4.0hr

+ +

+ +

+ +

+

+

t

+

NH -Y - (NH AC-HAC) 4 4 system

-

-

-

-

-

(NH ) SiF6 -(NH Ac42 4 HAc) system

-

-

-

-

-

-

NH4-Y - (NH4)2SiF 6 (NH Ac-HAc) system

*

4

F-1 sample F-3 sample

*

11+11

Si(0H)

and "-" express respectively the presence and absence of monomolecular in the reaction solution.

4

to support it. A DAY zeolite sample with known amount of defect sites was treated with monomolecular Si(OH)4 of Si(OH)4

in NH Ac-HAc buffer solution. The concentration

4

in the solution is very low( 12-15 mg Si02/100ml ), therefore, the

solution were renewed several times during the experiments. The results are given in Table 3 along with the result of DAY sample treated with a polysilicic acid solution. TABLE 3 Reaction of DAY zeolite with monomolecular Si(0H) Exist. of Si(0H) Reaction system DAY DAY DAY DAY DAY acid DAY -

4

at various time

0.5hr

l.Ohr

3.0hr

I

I +

+

Si(OH)4-2f + Si(OH)4-6# + Si(0H) -9# + polysi4icic

-

(NH4Ac-HAc) -

+ + -

I

6.0hr

I

4 polyslficfc acid Framework IR ao(li)

t

-

-

-

24.680 24.679

+

Sym. ( cm-l)

24.678 24.661 24.632 24.590

-

ASP.

1019 1021 1023 1026

787 791 794 796

1019 1019

787 786

#: No. of times of renewal of solution. It is shown in Table 3 that when DAY zeolite was reacted with fresh monomosolution again and again, the concentration of Si(0H) 4 4 in the reaction solution decreased gradually and disappeared finally. On the other lecular Si(0H)

hand, unit cell sizes of the product zeolites contracted and a s p e t r i c

and

symmetric stretch wavenumbers shifted to higher values. At the same time, by IR

I95 TABLE 4 The content of defect sites in the framework of zeolite samples x

(Alx Siyaz)02

Sample

y

x

z

Defect strcture factor (mol fraction)

Amount of Si inserted

Percentageof Si inserted

DAY

0.170

0.680 0.152

0.152

0.000

0.0

A2**

0.169

0.703

0.129

0.129

0.023

17.4 42.4

A4

0.171

0.736

0.096

0.096

0.033

A6

0.110

0.754

0.078

0.078

0.018

56.1

As

0.170

0.780

0.052

0.052

0.026

15.0

A9

0.168 0.799

0.033

0.033

0.019

90.2

NH4-Y

0.284

0.010

0.018

0.000

0.0

*

**

Defect A

0.697

structure in the unit cell.

(n= 2,4,"'9)

the product zeolites of the nth reaction of DAY zeolite

with monomolecular Si(OH)&.

I

0

2

4

6

8

10

N Fig. 4. The defect structure factor(z) of the product zeolites versus times(N) for the reaction of DAY zeolite with monomolecular Si(0H)

4

solution.

I96 spectra in hydroxyl region(ref. 3), it was found in Table 4 and Figure 4 that the concentration of defect sites in the framework of product zeolites decreased gradually and disappeared finally with increasing reaction times. results indicate that monomolecular Si(0H)

Those

4 is the species which can be inserted

into the defect sites left by dealuminum. However, when the DAY zeolite was reacted with polysilicic acid, no monomolecular Si(0H)

4 was discovered

during

the reaction and physico-chemical properties of the product zeolite were changed obviously. This shows that polysilicic acid can not be inserted

not into

the defect sites in the framework. All the above results show that.monomo1ecular Si(0H)

4

is an important inter-

mediate and a precursor €or extraneous silicon atom to replace aluminum atom in zeolite framework during the reaction of (NH ) SiF solution with NH -Y 6 4 4 2 zeolite. From the above experiment results, the following reaction mechanism

for

isomorphous substitution of extraneous silicon for aluminum in zeolite framework

is proposed:

(NH4)2SiF6 SiF6

2-

2NH:

=

H20 =

t

SiF5(0H)’-

t

SiF6’-

t

SiF (OH)25

H20 =

+ F- + + F- + H+

SiF4

SiF4(OH)22-

+ H20

=

SiF (OH) 2- + 3 3

SiF3(OH)32-

+ H20

=

Si(OH)4

t

3F-

Si(OH)4

+

6F-

SiF6

2-

6F- +

+ 4H20

=

o,NH;,o

+

A1

’ 0

‘OH

/ +

/OH

t

H+

t

H+

+ 4H+

/

+ A ~ F ~ ~NH;-

(3)

OH OH,
( zeolite )

Si(0HI4

OH

4Ht =

0 ‘

‘OH

F-

OH

=

OH

\

0,

0 ’

P

Si,

+ 4H20

(4)

0

(FSY zeolite)

CONCLUSIONS The (NH ) Sip6 undergoes astepise hydrolysis to produce six free F-ion and 4 2 a monomolecular Si(0H) during the reaction The aluminum in zeolite framework

4

.

is extracted into the reaction solution in the form of

ALP 3- complex

6

ion by

I97 the reaction of six free F- ion with a framework aluminum atom, then monomolecular Si(0H)

which forms a tetrahedral configuration is 4 defect sites left by dealumination.

inserted into

the

ACKNOWLEDGMENTS The authors express their sincere thanks

to Dr. Wanzen

Lu and Professor

Zi Gao for helpful discussions and comnents. We are also gratefulto Jun Hou for runing the n.m.r.

spectra.

REFERENCES 1

2 3

4 5

6 7 8

D.W. Breck and G.W. Skeels, A.C.S. Symposium Series 218, American Chemical Society, Washington, D.C., (1983). G.W. Skeels and D.W. Breck, Proceeding of the Sixth International Conference on Zeolite, Reno, 1984, pp. 87-96. D.W. Breck and G.W. Skeels, USP. 4,503,023 , 1985. G.T. Kerr, J. Phys. Chem., 72 (1968) 2594-2596. Zeren He, Preparation Handbook of Inorganic Compounds, Ed. Fuel Chemistry Publishing Sercice, China, 1972, pp. 424. J.D. Strickland, J. Am. Chem. SOC., 74 (1952) 862. J.R. Sohn, S.J. DeCanio and J.H. Lunsford, Zeolites, 6 (1986) 225-227. R.K. Harris and B.E. Mann, NMR and the Periodic Table, Academic Press Inc.(London) Ltd. London, 1978, pp. 279.