Precursors in Zeolite Synthesis - ACS Symposium Series (ACS

Chapter 3

Precursors in Zeolite Synthesis

Downloaded by PENNSYLVANIA STATE UNIV on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch003

A Critical Review J. J. Keijsper and M. F. M. Post Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research BV), Badhuisweg 3, 1031 CM Amsterdam, Netherlands

Double n-ring (DnR) s i l i c a t e s have been proposed as possible precursor species i n z e o l i t e synthesis since, for example, the formation of ZSM-5 can be e a s i l y envisaged s t a r t i n g from D5R s i l i c a t e s only. In this contribution we have critically examined this hypothe s i s . The observed composition and dynamics of various s i l i c a t e solutions, which conform to the data for silicalite forming mixtures, are i n l i n e with such a possible precursor role but do not give a d e f i n i t e proof. In a number of instances, however, rates of nucleation towards ZSM-5 fail to show a c o r r e l a t i o n with the concentration of D5R s i l i c a t e s present i n the s t a r t i n g mixtures. Moreover, the apparent random d i s t r i b u t i o n of defect s i t e s i n z e o l i t e ZSM-5, the number of which increases with the S i / A l r a t i o , does not support a precursor role for D5R s i l i c a t e s during c r y s t a l l i z a t i o n . Therefore, we conclude that the D5R s i l i c a t e condensation mechanism i s not generally operative i n the synthesis of MFI structures.

Although the use of organics i n z e o l i t e synthesis has had, and i s s t i l l having, an enormous impact on the formation of S i - r i c h forms of already known structures and on the formation of novel materials, the precise role of the organics i s s t i l l a matter of extensive debate. Often only t h e i r templating or structured i r e c t i n g role i s emphasized. However, the clear absence of a one-to-one correspondence between the geometries of the organic

0097-6156/89Α)398-0028$06.25Α) ο 1989 American Chemical Society

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


3. KEUSPER AND POST


29

used and the structure obtained (1) indicates that other possible roles of the organic also have to be considered. In the l i t e r a t u r e at least three roles have been discussed. F i r s t , the organic may exert an influence on the g e l chemistry by, for example, changing d i s s o l u t i o n rates (1,2). I t has been argued that templating effects may only become operative when the r i g h t gel chemistry i s present. Second, the organic can play a s t a b i l i zing role by being incorporated i n S i - r i c h z e o l i t e frameworks. In this case i t prevents the unfavorable i n t e r a c t i o n between water and the hydrophobic framework, which would otherwise tend to y i e l d dense materials such as α-quartz (3). Third, the organic can influence the (alumino)silicate e q u i l i b r i a i n the synthesis mixture and s t a b i l i z e possible z e o l i t e precursor species (4). In the l i t e r a t u r e there i s general agreement, sometimes a f t e r p r i o r deviating views (5), that the nucleation of a z e o l i t e takes place i n the l i q u i d phase of the synthesis gel and that the growth also involves dissolved nutrients (6-8), which are often thought to consist of approximately 10 Τ atoms (3,7,8). Thus, a d i r e c t l i n k exists between the study of z e o l i t e synthesis and the chemistry of basic (alumino)silicate solutions since these solutions can be considered as model systems f o r the l i q u i d phase present i n a synthesis g e l . I t i s generally accepted that i n these types of solutions an equilibrium exists between (alumino)silicates of varying degrees of condensation. Commonly used techniques to characterize these s i l i c a t e species are chemical trapping by t r i m e t h y l s i l y l a t i o n and ( i n - s i t u ) ^Si-NMR spectroscopy. In this way, numerous d i f f e r e n t s i l i c a t e s have been i d e n t i f i e d , ranging from monomeric (Si^) to hexagonal prismatic (Si-j^) (9). E s p e c i a l l y from the NMR work, general trends have become clear, f o r instance, about the e f f e c t of pH and S1O2 concentration on the average connectivity l e v e l . However, while some authors have speculated on the properties of proposed precursor 5-1 secondary-building-unit (SBU) s i l i c a t e anions (10), which have not yet been p o s i t i v e l y i d e n t i f i e d i n solution, i n general no s p e c i f i c species has been proposed as a z e o l i t e precursor for, say, ZSM-5. Previously, we have speculated on the p o s s i b i l i t y that some p a r t i c u l a r l y highly condensed s i l i c a t e anions, the 'double-n-ring' (DnR) s i l i c a t e s , may be l i k e l y candidates f o r z e o l i t e precursors (2,4,1!)· On the basis of an observed relationship between the extent of depolymerization of D4R s i l i c a t e s i n the synthesis g e l and the structure of the z e o l i t e s obtained from that gel, a pos s i b l e precursor role of the D4R s i l i c a t e has been discussed (11). In another study, the D5R s i l i c a t e i n p a r t i c u l a r has been con sidered as a precursor species f o r the formation of f i v e - r i n g - r i c h s i l i c e o u s z e o l i t e s l i k e ZSM-5 (3,4). This proposal was based on the following considerations: - A s h i f t i n the s i l i c a t e equilibrium towards DnR species upon sub s t i t u t i o n of large organic cations such as tetraalkylammonium (TAA) f o r the a l k a l i . These s i l i c a t e s are s t i l l present i n s o l u t i o n at 90 °C, i . e . , close to z e o l i t e formation temperatures. This e f f e c t may be based on a favorable e l e c t r o s t a t i c i n t e r a c t i o n between the large cations and the condensed DnR s i l i c a t e s . The observed s h i f t p a r a l l e l s the o f t e n - f a c i l i t a t e d formation of S i r i c h z e o l i t e s i n the presence of organics. 2


ZEOLITE SYNTHESIS


30

- The observed presence of D5R s i l i c a t e s i n actual ZSM-5 forming mixtures (12). - Both the formation of ZSM-5 and the occurrence of defect (mis sing T) s i t e s i n the framework can be e a s i l y envisaged s t a r t i n g from D5R s i l i c a t e s only (3) (see Figure 1). In t h i s chapter we w i l l reveal some new findings which are r e levant to the D5R ZSM-5 synthesis model proposed e a r l i e r . The f o l lowing items w i l l be discussed: - The composition and dynamic properties of basic s i l i c a t e solu tions and the implications derived therefrom as to the p o s s i b i l i t y of p o s i t i v e l y i d e n t i f y i n g z e o l i t e precursor species. - The q u a n t i f i c a t i o n of D5R and other s i l i c a t e s during a s i l i c a l i t e synthesis at 95 °C from clear solution. - A comparison of the rate of formation of a ZSM-5 phase i n the presence and absence of d i f f e r e n t organics. - The occurrence and d i s t r i b u t i o n of defect s i t e s i n ZSM-5 samples prepared i n d i f f e r e n t ways and how the A l content a f f e c t s t h i s . Experimental Basic s i l i c a t e solutions were prepared by using s i l i c i c acid (ex Baker, dried at 350 °C) and a solution of the organic bases [25 wt % tetramethylammonium hydroxide (TMAOH), 40 wt % t e t r a ethylammonium hydroxide (TEAOH) , 20 wt % tetrapropylammonium hydro xide (TPAOH); ex Fluka] and, optionally, dimethyl sulfoxide (DMSO). A solution of hexamethonium hydroxide [ (ΜββΝΟ^Η-^ΝΜββ) (0H) ] was prepared from the bromide s a l t (ex Sigma) and Ag20. Quantitative chemical trapping of the s i l i c a t e i n solution was performed by the t r i m e t h y l s i l y l a t i o n method by using an i n t e r n a l standard as described e a r l i e r (4,11). The results thus obtained were reproducible to within 20% r e l a t i v e . Si-NMR spectra were recorded on a Bruker WM-250 ( l i q u i d ) or a Bruker CXP-300 Fourier transform magic-angle-spinning (FT MAS) s o l i d state spectrometer. Resonances are r e l a t i v e to tetramethylsilane (TMS). Dynamics of the s i l i c a t e solutions were studied by selective e x c i t a t i o n techniques by using DANTE-type (13) pulse sequences. Elemental analyses were c a r r i e d out by using X-ray f l u o rescence (XRF; S i , Al) and combustion (C, Η, N) methods. X-Ray d i f f r a c t i o n (XRD) powder spectra were recorded on a P h i l i p s PW 1130 instrument. Samples 1-7 ( s i l i c a l i t e ) were prepared i n a Teflon b o t t l e at 95 °C, under s t a t i c conditions from a homogenized, f i l t e r e d , and clear solution of molar composition: 25 S i 0 (ex s i l i c i c acid), 9 TPAOH (20 wt %, ex Fluka), 2 NaOH, 450 H 0, and (samples 5-7) 50 v o l % DMSO. Synthesis times are indicated i n Table I. Samples 10-17 were prepared i n s t i r r e d , Teflon-lined auto claves at 190 °C s t a r t i n g from a homogenized mixture of molar composition: 40 S i 0 (ex Ketjen s o l 40 AS), 1 A l 0 (ex NaA10 ; ex ICN, dried at 120 °C), 5 Na 0, 1000 H 0, and X, with X and the synthesis times indicated i n Table I I . 2

29

2

2

2

2

2

3

2


2


KEUSPER AND POST

Figure 1.


Formation of ZSM-5, including possible 'missing Τ s i t e s ' defects (·), s t a r t i n g from D5R s i l i c a t e s only.



D4R

D5R

2

R

a

Ρ

XRD

2

Si, wt % 2

x

c

c

0.3(14) 0.9(11) 0.8(11) 0.8(11) 0.8(10) 2

0.7(37) 5.9(48) 5.8(44) 4.2(46) 4.7(47)

98 72 70 69 61 2

18 20 23 30

100 100 100 100

0.9(15) 0.9(12) 1.3(13) 1.8(15)

1.5(26) 1.7(22) 2.1(21) 3.9(33)

1.7(29) 1.8(25) 2.1(21) 2.5(22)

1.9(30) 3.5(41) 4.5(45) 3.8(31)

94 89 62 50 30 85 95

. 3 28 38

2

4.4 4.4 4.4 4.3

96 96 96 96

. 96 96 96

_

2.1 3.4 3.5

-

46 35 25

25 27 30 27

z

5 6 7

-

1 2 3 4

Sample Number

a: Absolute percentages are given with, i n brackets, the r e l a t i v e amounts, i . e . , as i f no polymeric s i l i c a t e s and s i l i c a l i t e p r e c i p i t a t i o n were present. M+D - mono- and dimeric s i l i c a t e s ; D4R: double-four-ring s i l i c a t e ; D5R: d o u b l e - f i v e - r i n g s i l i c a t e ; R - other small s i l i c a t e s ; Ρ - polymeric s i l i c a t e s as d e r i v e d from q u a n t i t a t i v e chemical t r a p p i n g procedure. For Ρ the percentage o f S i recovered i n the products has a l s o been evaluated. b: XRD: % c r y s t a l l i n i t y as compared to 100% c r y s t a l l i n e TPA s i l i c a l i t e . S i , wt %: Percentage S i recovered (based on elemental a n a l y s i s ) . U n i t c e l l : Composition o f u n i t c e l l (based on 96 S i s i t e s ) as d e r i v e d from elemental analysis. c: Longer synthesis times do not a f f e c t e i t h e r the s i l i c a t e composition o f the s o l u t i o n or the product p r o p e r t i e s .

0 24 72 400

0.2(9) 0.9(11) 0.9(12) 0.7(10) 0.9(11)

Composition: 25 S10 /9 TPAOH/2 NaOH/450 Η 0 + 50 v o l % DMSO

0.8(40) 2.3(30) 2.5(33) 2.3(33) 2.6(32)

Molar S o l u t i o n

0 24 48 96 168

y

Unit C e l l (Si0 ) (TPA0H) (H 0)

Properties o f Product**

Molar S o l u t i o n Composition: 25 S10 /9 TPA0H/2 NaOH/450 H 0; Non-Stirred

M+D

Time, Composition o f S o l u t i o n , mol % h

Table I . S i l i c a t e Composition o f a S i l i c a l i t e - F o r m i n g S o l u t i o n at 95 °C and Product Properties Obtained


In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 22

22 50

16

10 Hexane-1,6dlol

3 NaAl0 4 NaOH

360 MeOH

360 MeOH -I- 3 wt % Na,H-ZSM-5

a: b: c: d: e:

40

5 TEAOH

e

17

16

15

14

13

12

29

26

90, I

90, Ζ

8

20

95, Μ

100, Ζ

30

100, Ζ

22

23

Si/Al

27

b

100, Ζ

90, Ζ

75, Ζ

10

11

Cryst., %

Sample c

3

2

90

82

90

85

95

80

80

88

7.5

-

-

5.5

3.8

3.5

-

-

Si Organic/ Recovered, U n i t C e l l vt %

2

d

Time when c r y s t a l l i n i t y reaches i t s highest value ( c f . Figure 6 ) . % C r y s t a l l i n i t y , determined by XRD: Ζ - ZSM-5, M - mordenite; I - ISI-1. Based on elemental analyses. Number o f organic molecules per 96 Τ s i t e s based on elemental analyses, At 170 °C;

Precursors in Zeolite Synthesis - ACS Symposium Series (ACS

Recommend Documents