Zeolite Synthesis - American Chemical Society

Chapter 25

Disordered Pentasil-Type Borosilicates Synthesis and Characterization

Downloaded by UNIV OF LEEDS on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch025

Giovanni Perego, Giuseppe Bellussi, Angela Carati, Roberto Millini, and Vittorio Fattore Eniricerche S.p.A., 20097 San Donato Milanese, Italy

The synthesis of pentasil-type borosilicates, referred to as BOR-E, from hydrogels containing a binary mixture of Me N /(n-Pr) N ,Et N /(n-But) N or Me N /(n-But) N+ cations is described. Mirror plane-based stacking faults, randomly distributed in the ZSM-5-type (BOR-C) inversion center-based stacking of pentasil layers were identified in the framework structure of BOR-E by X-ray diffraction analysis. The probability of the occurrence of stacking faults, p, was estimated from X-ray diffraction data following a method developed for structure determination of a ZSM-11 type borosilicate (BOR-D). ρ ranges from 0 to 0.2, compared to 0 and 0.25 determined for BOR-C and BOR-D, respectively. The probability is dependent on both the cation pair used in the reaction mixture and the resulting relative abundance of the cations within the pores. +

4

+

4

+

4

+

4

+

4

4

Pentasils constitute a well known family of porotectosilicates. The framework structure of the members of t h i s family, based on f i v e membered rings of tetrahedra, can be described i n terms of two d i f f e r e n t stackings of layer pairs related by an inversion center (i-type) and mirror simmetry (σ-type), respectively ( J, ) .ZSM-5 aluminosilicate, the parent b o r o s i l i c a t e BOR-C ( 2 ), and the pure s i l i c a analog S i l i c a l i t e - 1 ( 3 ), represent the most important members; t h e i r framework structure i s based on i-type stacking ( 4 ). For another member, the ZSM-11 aluminosilicate, a c r y s t a l structure was proposed based on σ-type stacking ( 5^ ). However, a more recent X-ray investigation of B0R-D, the parent b o r o s i l i c a t e structure, c l e a r l y demonstrated that the ZSM-11-type framework i s r e a l l y b u i l t up of a disordered layer sequence of i and σ-type stackings ( 6 ). Evidence of intergrowth of i-type and σ-type was obtained also by high-résolution electron microscopic imaging and electron d i f f r a c t i o n i n pentasil-type aluminosilicates, which were crystallized from hydrogels containing a mixture of tetrapropylammonium and t et rabutyl ammonium hydroxides ( T_ ) or tetrabutylammonium bromide with varying amounts of sodium and potassium hydroxides ( 8 ). 0097-6156/89/0398-0360$06.00/0 o 1989 American Chemical Society Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

25. PEREGO ET AL.

Disordered Pentasil- Type Borosilicates

361

Pentasil-type b o r o s i l i c a t e s with d i f f e r e n t extents of disorder in the framework, referred to as BOR-E (9), were obtained i n our laboratories using binary mixtures of d i f f e r e n t quaternary ammonium cations. The present paper describes the synthesis and structural characterization of BOR-E, based on X-ray d i f f r a c t i o n data. EXPERIMENTAL


Synthesis A l l c r y s t a l l i z a t i o n reactions were performed at 170°C during 7-14 days, i n a stainless s t e e l autoclave equipped with a device for s t i r r i n g . Hydrogels were obtained by dissolving boric acid i n aqueous solution containing tetralkylammonium hydroxides (from Fluka) and RPE grade sodium hydroxide (from Carlo Erba), followed by t e t r a e t h y l o r t h o s i l i c a t e (Dynasil A, from Dynamit Nobel) addition. The molar composition of the precursor hydrogel was: 10SÎO .Β 0 .2.5(R NOH, R.NOH).0.8Na0H.380H 0 CR: Me, Et, n-Pr; R*: n-Pr, n-But). After calcination i n a i r at 550°C the crystals were reacted with ammonium acetate solution and f i n a l l y converted to the Η-form by calcination of the NH^ -exchanged crystals i n a i r at 550°C. More d e t a i l s are reported elsewhere ( 9 ). Chemical analysis was carried out on the as^-synthesized samples, using conventional methods. The content of R^N and R,N was e a s i l y derived by taking into account the weight fractions of nitrogen and carbon obtained from elemental analysis. A l i s t of the c r y s t a l l i z e d samples, together with t h e i r chemical composition, i s reported i n Table I. X-ray analysis X-ray powder patterns were recorded by the step-scanning procedure on a P h i l i p s diffractometer equipped with a pulse-height analyzer using CuKa radiation. The powder pattern of H-BOR-E i s intermediate between the patterns of H-BOR-C and H-BOR-D (see Figure 1). In p a r t i c u l a r , the i n t e n s i t i e s of the reflections with h+k+l=2n+l (according to the unit c e l l c h a r a c t e r i s t i c of H-BOR-C) are lower i n the pattern of H-BOR-E with respect to the pattern of H-BOR-C and are accompanied by some l i n e broadening (see for example 133 at 2Θ « 24.5° i n the patterns of Figure 1). The i n t e n s i t i e s of the remaining r e f l e c t i o n s are p r a c t i c a l l y invariant. Moreover a p a r t i a l coalescence of hkl and khl reflections i s observed i n the pattern of H-BOR-E. These effects occur to varying extent i n the patterns of d i f f e r e n t samples of H-BOR-E. The trend observed i n the pattern of H-BOR-D i s similar to that observed i n the pattern of H-BOR-E, but the extent of the phenomenon i s more pronunced for the l a t t e r . The above mentioned features of the X-ray d i f f r a c t i o n pattern of H-BOR-E are presumably due to the same structural disorder occurring in BOR-D and i n the related ZSM-ll-type frameworks. Consequently, a f i t of the experimentally observed patterns was attempted using the

Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

362

ZEOLITE SYNTHESIS

Table I. Unit c e l l composition for b o r o s i l i c a t e s precursors

Number of atoms or groups/u.c. Sample

X

(b)

R N


(c)

+

R;N

+

B

Si

4.9 4.4 2.9 3.7 3.0

91.1 91.6 93.1 92.3 93.0

0.0 0.25 0.0 0.0 0.0

P

( e )

(d)

I

R: n-Pr 1 2 3 4 5

-But R : η 1 1 0 0 0,,33 >0.99 0..50 0,.67 0..67 >0.99

R: Et 6 7 8

R : η-Pr 1 1 0.,50 0..24 0.67 0..12

4.5 0.9 0.4

2.9 3.1

4.3 3.2 3.1

91.7 92.8 92.9

0.0 0.0 0.0

R: Me 9 10 11 12

R : η-Pr 0.50 0..18 0.66 0..32 0.,75 0..36 0.,80 0,.32

0.7 1.5 1.5 1.4

3.3 3.1 2.7 2.9

4.1 4.3 3.9 5.3

91.9 91.7 92.1 90.7

0.03 0.05 0.09 0.08

R: Et 13 14 15 16

R : η-But 0.50 0,.46 0.66 0..60 0.75 0..62 0.80 0..60

1.7 2. 3 2.4 2. 3

2.0 1.5 1.5 1.5

3.8 3.8 3.7 4.1

92.2 92.2 92.3 91.9

0.07 0.08 0. 10 0.09

R: Me 17 18 19 20 21 22 23 24 25 26

R' : η-But 0.50 0.35 0.50 0..37 0.50 0..41 0.50 0.42 0.50 0..44 0.50 0..48 0.25 0..50 0.50 0..53 0.75 0..58 0.25 0.,60

1.4 1.5 1.6 1.7 1.8 2.0 1.9 2. 1 2.4 2.6

2.6 2.5 2.3 2.3 2.2 2.2 1.9 1.9 1.8 1.8

4.1 4.2 4.5 4.1 4.0 4.3 4.9 4.4 4.1 4.6

91.9 91.8 91.5 91.9 92.0 91.7 91.1 91.6 91.9 91.4

0.14 0.17 0.15 0.13 0.11 0.12 0.10 0.08 0.16 0.20

(a) (b) (c) (d) (e)

1

3.5

4.3

3.5 2.4 3.6

1.2

-

1

-

1

1

Na < 0.1 ions/u.c. . x = nR N /(nR N + mR^N ). Reaction mixture. Crystal. Stacking f a u l t probability, see text. 4

4


25. PEREGOETAL.

363

DisorderedPentasil-TypeBorosilicates


(a)

(b)

(C)

(d)

-—'—'—ι—·—τ—τ—ι—ι—ι—.—t—ι—ι—r—ι—ι—τ—τ—1

15 Figure

1.

Selected

H-BOR-C, sample

ced

portion

25

o f X-ray

30 20 powder

patterns

o f (a)

1, (b) H-BOR-E, sample 13, ( c ) H-BOR-E, sample

21 and (d) H-BOR-D, sharp

20

reflections

sample 2. Arrows

indicate

o f H-BOR-C, which have t h e i r

i n H-BOR-E, w h i l e becoming d i f f u s e

the p o s i t i o n o f intensity

i n H-BOR-D.


redu

364

ZEOLITE SYNTHESIS

model successfully applied to the c r y s t a l structure determination of H-BOR-D ( 6 ).


Calculation of the scattered intensity Calculation of X-ray p r o f i l e s was performed i n steps of 0.04° throughout the 6-50° 2Θ angular region by applying the procedure used in the structure determination of H-BOR-D (6). In t h i s procedure the instrumental broadening was simulated by convoluting the sample p r o f i l e s with two Lorentzian l i n e functions, with a 2:1 intensity r a t i o and a f u l l width at half maximum of 0.1° 2Θ, representing the contribution of Κα. and Ka^ lines, respectively. The calculated patterns were scaled by equating the i n t e n s i t i e s to the experimental values after integrating over the 11-50° 2Θ angular range and subtracting the background. The angular range below 11° was neglected because i t i s very sensitive to the presence of extra-framework species. Calculation of i n t e n s i t i e s was performed by considering tetrahedral s i t e s to only be occupied by S i . The isotropic temperature parameter Β was fixed at 1.0 and 2.0 for S i and 0, respectively. The disordered model as well as the expression derived f o r calculation of scattered i n t e n s i t i e s were described previously ( 6 ). Very b r i e f l y , the model i s based on a pentasil layer sequence i n which the p r e v a i l i n g i-type stacking i s interrupted by σ-type stacking faults. In the c r y s t a l l a t t i c e description adopted for ZSM-5 ( 4 ) the layers l i e i n the b-c plane while being stacked along a. ρ i s the p r o b a b i l i t y of a stacking f a u l t occurrence and p* the probability of a general layer to be at a c r y s t a l boundary. Both the stacking faults and the c r y s t a l termination are regarded as purely random independent events according to Bernoullian s t a t i s t i c s . L, and L are average c r y s t a l size along the b and c directions, respectively. According to the expression used for the calculations ("Equation 15" i n ( 6 )) the scattered intensity i s characterized by a Lorentzian line p r o f i l e . The atomic f r a c t i o n a l coordinates reported for the c r y s t a l structure of ZSM-5 ( 4 ) were used as i n t r i n s i c atomic parameters f o r the pentasil layers as was done f o r the structure determination of H-BOR-D. The calculations were performed f o r four selected samples, characterized by d i f f e r e n t i n t e n s i t i e s f o r the reflections with h+k+l=2n+l, using the t r i a l and error method. F i r s t l y , L^, L^, p* and unit c e l l parameters were adjusted to f i t the d i f f r a c t i o n p r o f i l e of reflections with h+k+l=2n (invariant i n t e n s i t y ) . Subsequently, a value was selected f o r p, i n order to f i t the i n t e n s i t i e s of r e f l e c t i o n s with h+k+l=2n+l. Due to the high s e n s i t i v i t y of the calculated p r o f i l e to any v a r i a t i o n of p, the estimated value of ρ has confidence limits of ± 0.02. The best f i t , i d e n t i f i e d by the lowest value of the intensity disagreement factor, R (see Figures 2 and 3), was obtained using the values l i s t e d i n table II. As for the case of H-BOR-C and H-BOR-D, the resulting values of unit c e l l parameters are s i g n i f i c a n t l y lower than the corresponding parameters c h a r a c t e r i s t i c of the pure s i l i c a analog, S i l i c a l i t e - 1 , which supports framework incorporation of boron. A s l i g h t difference between the values of a and b parameters exists f o r H-BOR-E and



25. PEREGOETAL.

15

365

DisorderedPentasil-Type Borosilicates

20

25

30

35

40

Figure

2. E x p e r i m e n t a l ( — ) and c a l c u l a t e d

tion

profile

insets

show

f o r H-BOR-E,

a plot

sample

o f the i n t e n s i t y

10

45

(··) X-ray d i f f r a c -

(a) and

disagreement

23

( b ) . The

f a c t o r (see

T a b l e I I ) v s . t h e f a u l t p r o b a b i l i t y parameter, p.


ZEOLITE SYNTHESIS


366

IS

10

IS

Figure

3. E x p e r i m e n t a l ( — )

tion

profile

insets

show

f o r H-BOR-E,

a plot

30

55

and c a l c u l a t e d samples

o f the i n t e n s i t y

Table I I ) v s . the f a u l t p r o b a b i l i t y

19

«0

45

(··) X-ray d i f f r a c (a) and

disagreement

26

( b ) . The

f a c t o r (see

parameter, p.


25. PEREGOETAL.

367

DisorderedPentasil-TypeBorosilicates

H-BOR-D, though reduced with respect to H-BOR-C, i n agreement with an excess of i-type stackings occurring i n both structures (see table II). (a) Table II. Values of the structural parameters obtained i n the best f i t of X-ray scattering profile f o r selected samples of H-BOR-E Unit c e l l parameters


Sample

ρ

ρ*

(b)

(c)

10 23 19

0. 05 0.019 0. 10 0. 024 0. 15 0. 026 0. 20 0. 033 ( f ) BOR-D^^ 0. 25 0.037 B0R-C S i l i c a l i t e - .! 2 6

1^

L

a(Â) b(Â)

c(Â)

3

V(Â )

(d) (d) 300 250 250 200 300

300 300 350 300 300

UJ

(a) (b) (c) (d) (e) (f) (g)

c

R (e)

19..822 19..832 19..835 19..835 19..92 19..82 19..874

19.,991 19.,979 19.,973 19.,968 20.,02 20..01 20..117

5285.,4 5280.,2 5284.,8 5287.,4 5324..0 5294..6 5345..5

13.338 13.326 13.340 13.350 13.35 13.35 13.371

0. 11 0. 12 0. 14 0. 10 0. 09

According to "Equation 15" i n ( 6 ). Stacking fault probability. Crystal termination p r o b a b i l i t y along a (stacking d i r e c t i o n ) . Average c r y s t a l size along b (1^) and c ( L ) . Intensity disagreement factor. Ref. ( 6 ). Monoclinic, α = 90.62°, Ref.(2). c

ρ values were found to correlate well with intensity variations i n the experimental patterns. In p a r t i c u l a r , a linear correlation was v e r i f i e d f o r the r a t i o between the intensity of 102 r e f l e c t i o n (29*14°) and the intensity of 301-031 multiplet (2Θ*15°), see Figure 4. The resulting regression of Figure 4 was used for determining the value of ρ for a l l other samples investigated. Results and discussion +

+

+

+

H-B0Ç-E was c r y s t a l l i z e d using Me N /(n-Pr),N , E t N / ( n - B u t ) N and Me N /(n-But),N binary mixtures. Both alkylammonium cations of each pair contained i n the reaction mixture were found i n precursor with only few exceptions. In most cases their molar ratios were found to be s i g n i f i c a n t l y different from the corresponding r a t i o i n the reaction mixture. The results are summarized i n Table I I I . For the Et.Ν /(n-Pr) N cation pair the formation of the ordered structure of BOR-C i s consistent with expectation, the same structure being formed i n the presence of the individual cations. The (n-But) N cation favors the formation of disordered framework structures, evident from the synthesis of BOR-D, when mixed witlji Me N and E t N but not (n-Pr) N . In the l a t t e r case, the (n-Pr) N cation i s greatly favored i n the c r y s t a l l i z a t i o n process and the 4

4

4

+

4

4

4

4

4


4

368

ZEOLITE SYNTHESIS


0.80

Figure

4.

Ί

Variation

reflections in

the

X-ray

o f t h e r a t i o between t h e i n t e n s i t y o f t h e

occurring powder

at 2 $ pattern

f a u l t p r o b a b i l i t y parameter,

1

ζ 14° and 2 Î : 1 5 ° ,

respectively,

o f H-BOR-E, as a f u n c t i o n

o f the

p.


25. PEREGO ET AL.

369

Disordered Pentasil-Type Borosilicates

ordered BOR-C phase i s formed. Probably ^BOR-E may be obtained by working with a large excess of (n-But)^N cations i n the reaction mixture. A similar behaviour was observed when the synthesis of disordered pentasil-type aluminosilicates from the (n-Pr)^n /(n-But).N system was attempted (7). The most interesting results concern the behaviour of Me^N . This cation favors the formation of BOR-A (Nu-l-type framework), when used alone i n the reaction mixture (2). In the above binary mixtures, i t behaves d i f f e r e n t l y , leading to the formation of pentasil-type structure, even when i t s concentration greatly exceeds that of the other alkylammonium cation i n the reaction mixture. Moreover, t h i s cation seems to be more e f f i c i e n t than (n-But)^N in producing the formation of ^BOR-E. A s a matter of fact, BOR-E i s e a s i l y obtained from the Me.N /(n-Pr).N system, but not from the (n-But) N /(n-Pr) N system. * +

+


4

4

Table I I I . Summary of the results Cation pair RN , RJ^N

x ^

Borosilicate phase

p ^

4

R:

1

Et

R : n-Pr

0 0 * 0.2 1

BOR-C BOR-C BOR-C

0 0 0

n-Pr

n-But

0 0.7 * 1

BOR-D BOR-C

0.25 0

Me

n-Pr

0 0.2 f 0.4 1

BOR-C BOR-E. . B0R-A

0 0.03 - 0.09

0.25 0.07 - 0.10 0

U;

Et

n-But

0 0.4 * 0.6 1

BOR-D BOR-E BOR-C

Me

n-But

0 0.3 τ 0.6 1

BOR-D BOR-E . B0R-A

0.25 0.08 - 0.20

r

U;

(a) Cations abundance i n the precursor, x=nR N /(nR N + mR N ). (b) Frequency of stacking f a u l t s , see text, (c) Nu-1 type, Ref.( 2 ). 4

4

4

The stacking fault probability ,p, i n H-BOR-E depends on both the cation pair used i n the reaction mixture and the resulting cation abundance, defined by the molar f r a c t i o n x=nR N /(riCJtf +mR N ) within the pore structure of the precursor. In the Me N /(n-But) N system, ρ decreases l i n e a r l y from 0.25 (H-BOR-D phase, x=0) to ca. 0.1 f o r x=0.5. For higher values of x, the trend i s inverted 4

4

4


+

4


370

ZEOLITE SYNTHESIS

abruptly, ρ going up to 0.2 f o r x=0.6 (see Figure 5a). ^ linear dependence of ρ on χ i s al^so observed for the Me,Ν /(n-Pr),Ν system (Figure 5b). For the Et,N /(n-But).Ν system, values of 0.08 - 0.10 are observed for x«0.6 (see Table Γ and III) near those expected for a linear v a r i a t i o n between the values c h a r a c t e r i s t i c of H-BOR-D (x=0, p=0.25) and H-BOR-C (x=l, p=0), respectively. The occurrence of a σ-type stacking fault i n the framework of pentasils may be regarded as a 'twin* involving two domains of ZSM-5 type. Such a 'twin' probably occurs frequently i n pentasil-type c r y s t a l s , though i t i s not e a s i l y detected by X-ray analysis when present at low abundance (p

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