Synthesis and characterization of zsm-5 type zeolites

Applied Catalysis, 5 (1983) 221-248 Elsevier Scientific Publishing Company,

SYNTHESIS III.

AND CHARACTERIZATION

OF ZSM-5 TYPE ZEOLITES

+a, Niels BLOM b and Eric G. DEROUANE

GABELICA

"Facultes

Universitaires

Rue de Bruxelles, Haldor Topsde

'Present

Laboratories,

Mobil Research

concerning

25 October

a'c

de Catalyse

Belgium Nymgllevej,

and Development

P.O. Box 1025, Princeton,

To whom queries

(Received

de Namer, Laboratoire

61, B-5000 - Namur,

Research

address:

Division, d

- Printed in The Netherlands

CATIONS A CRITICAL EVALUATION OF THE ROLE OF ALKALI AND AMMONIUM

Zelimir

b

227 Amsterdam

55, DK-2800

Corporation,

- Lyngby, Central

Denmark

Research

N.J. 08540, USA.

this paper should be sent,

1982, accepted

2 December

1982)

ABSTRACT Various techni.ques were used to investigate the role of alkali and ammonium cations in governing nucleation and growth processes of(flIjZSM-5 zeolites foniled.within the (Na20, M20)-(Pr4N)20-A1203-Si02-H20 synthesis mixtures (MI = Li, Na, NH4, K, Rb, Cs). Morphology, size, chemical composition and homogeneity of the(MI)ZSM-5 crystallites depend on the competitive interaction between Pr4N oralkali cationic species and aluminosilicate polymeric anions at the early stages of the nucleation process. The latter, in turn, is strongly affected by intrinsic properties of the alkali cations such as their size, their structure-forming or structure-breaking role towards water and their salting-out power. Structure-breaking cations such as K+,Rb+or Cs+favour the formation of larqe (15-25 pm) single crystals or twins. In the presence of structure-forming cations (Li+,Na+) a rapid nucleation yields Si-rich crvstallites homoqeneouslv distributed within the 5-15 nm range. Those are coated with numerous smail (1 nm") Al-richer crystallites formed by a secondary nucleation process from the Si-deficient gel. In all cases, combined PIGE (bulk) and EDX (outer shell) analyses of Si and Al reveal that Al is homogeneously distributed within the individual crystallites and that Si/Al ratio increases with the particle size. As a result, K, Rb and Cs ZSM-5 zeolites appear homogeneous in composition while Li and Na polycrystalline aggregates show an apparent Al-enriched outer rim. In presence of NHq+ ions, large single crystals of ZSM-5 having an Al-deficient core and an Al-rich outer shell, as well as small Si-rich crystallites stemmins from a delayed nucleation process, are formed. This particuiar role of NH4+ is explained in terms of its preferential interaction with aluminate rather than with silicate anions, during the nucleation stage. Our various findings suggest that both solution ion transportation and gel phase transformations (surface nucleation)mechanisms can govern simultaneously the nucleation and growth of ZSM-5 zeolites.

0166-9834/83/0000~000/$03.00

0 1983 Elsevier Scientific

Publishing Company

228 INTRODUCTION It has been recognized

that alkali

ions (salts) influence

precursor

gel for most of the synthetic

component

systems Na20-A1203-Si02-H20,

mation of structural the nucleation

subunits

process

and crystallization and morphology

(structure-directing

of the crystallites

of organic

[ 101.

hydrothermally

In particular

tallization

process

(or surface

directing

effect

of the effect qualitative

and sometimes

inorganic

emphasized

in the formation

mixtures

Few studies in reaction

ion-exchange

and (NH,+, Pr4Nt) systems zeolites crystallize the synthesis

mixture

with increasing

through

of type B).

effects

and

a liquid

High crys-

of the tetrapropyl-

of the type B synthesis. cation

The con-

(base) and the structure-

However, while

the role of the organic

studies [ll,lZ],

the essential

the understanding

role of Nat (or k)

Addition

of these alkali as chlorides

[ 141.

In contrast,

the straightforward alkali

cations

ZSM-5 materials

and reported

with similar as chlorides

system under specifical-

but without synthesis

Nat [15,161.

of (MI) ZSM-5 zeolites

(MI) other than N$, probably

can easily

be obtained

that the corresponding

rates.

from

becau-

(Na or H)

the (Na+, Pr4Nt)

(Na) or (NH4) ZSM-5

When Lit, K' or Cst ions are added to

only, euhedral

ZSM-5 crystallites

size in the order Li < K < Cs [22].

Kt ions are added as hydroxydes,

spheroidal

order Li < Na < K are obtained.

These systems

but,have similar rates of crystallization

in the

the size and

it has been reported

Bibby et al. [21] compared

[17-201.

ions, added

that the latter could not be

in the (Pr4N)20-A1203-Si02-H20

containing

se the various M-containing ZSM-5 through

(synthesis

ratios of the reactants

have reported

mixtures

either

(gels) was found to affect the morphology,

that ZSM-5 can crystallize concentration

of

(alkali) cations still remains fragmentary,

the aggregation of the final crystallites

ly defined

aspects

that ZSM-5 can nucleate

mixtures

of ZSM-5 and reported

in the absence of alkali.

same synthesis

bi-

contradictory.

Erdem and Sand [13,14] as hydroxydes,

various mechanistic

of the organic

by complementary

played by the

of various

or Pr4N+) and inorganic

structure-directing

ions are among the advantages

ascertained

precipitation

of type A), or by a solid hydrogel

process

role of Na+ ions were evidenced.

base was further

obtained

(synthesis

rates and accentuated

al~lmOniUl~i (Pr4Nt)

recently

synthesis

nucleation)

junct clathrating-templating

the for-

in various ways

and the final size

in the presence

we have recognized

grow from Na20-(Pr4N)20-A1203-Si02-H20

transformation

role), the subsequent

(usually tetrapropylammonium

phase ion transportation

In the four-

thereby

effect) [8,9]

of the

[ 11.

We have discussed

(usually Nat) cations. such syntheses

influencing

(salting-out

ZSM-5 zeolite can be synthesized nary mixtures

zeolites [l-4].

Al-rich

Na+ ions were shown to stabilize

(nuclei) [3,5,7]

of the zeolite

ihe formation

crystals

[ 231.

Similarly,

when Lif,Nat

with increasing

show differences

are obtained and

size in the

in nucleation

rates

229 When several

alkali

speciesare

gel, the larger cation appears classical

Al-rich

zeolitic

present

zeolite,

ted the (Na+,K+,Pr4N+)system ported the synthesis systematic

[13,14]

investigated

mixture

of the ZK-type [25].

and the patent of Grose and Flanigen

the competitive

who re-

no other cation other

[27].

It was found that replacement

the nucleation,

final size and morphology

Rb+and Cs+)added

resulting

in more detail the role of alkali cations and growth processes

of the corresponding

ZSM-5 zeolites.

mixtures,

as chlorides

in

of larger crystals.

crystallization

ZSM-5 phases were prepared

(Pr4N)20-A1203-Si02-H20

of Na by Cs in

of germ nuclei,

rates and the deposition

In the present work, we investigated

crystalline

role of alkali and Pr4Nt ionic

species at the early stages of the

could impede the formation

crystallization

in governing

of a

experimen-

the work of Erdem and Sand who investiga-

of aluminosilicate

of ZSM-5 materials

the synthesis

various

confirmed

of ZSM-5 with Na-Li and Na-Ba but no Pr N+ 1261, 14 (M = alkali

in the clathration

smaller

zeolites

excepting

the crystallization

This was recently

been reported.

We have recently

synthesis

[ 241.

work known to us in the (Na+,M', Pr4Nt)system

than Na+)has

species

in an aluminosilicate

to direct preferentially

material

tally in the case of high-silicon For pentasil-type

simultaneously

from identical

with the various

alkali

as well as the

For that purpose, initial Na20-

cations

(Li+,Na+,NHq+,K+,

prior to crystallization.

EXPERIMENTAL Syntheses The various B 1.101.

(M) ZSM-5 samples

A sodium

silicate

cate (Natronwasserglass

Merck,

with 6.9 parts of water. aluminium

sulfate

An acidic

(Fluka, purum).

acid

(LiCl, NaCl, NH4Cl, water.

dium silicate Si02 96.5

vessel

of type

1 part of sodium sili-

appears

by adding

1 part of

to 555 parts of water,

followed

anal. grade) and 9 parts of

and acid solution were added at

1 part of alkali metal

chloride

anal. grade) and 10.6 parts of

has a pH between

2 and 4.

to about 9, by further

The final .mixture has the following

It is stirred

addition molar

of the so-

composition

:

: Na20 : A1203 : H2S04 : Pr4NBr : MC1 : H20 28.8

1

17.3

corresponding

to the following

; Pr4N+/Si02

0.019

of silicate

containing

its pH is adjusted

OH/Si02

= 0.2

was prepared

(98 wt % H2S04, Merck,

imnediately

solution.

to the synthesis

by mixing

purum)

KCl, RbCl, CsCl, all Merck,

The gel which

for 2 hours before

solution Merck,

Equal volumes

the same rate to an agitated

according

was prepared

28.5 wt % Si02. 8.8 wt % Na20 and 62.7 wt % H20)

(A12(S04)3.18H20,

by 13.6 parts of sulfuric Pr4NBr

were prepared

solution

8.9

ingredient

= 0.09

molar

47.1 ratios

1888

:

; Si/Al = 49.3 ; Al/Na = 0.035 ; Al/(Na t M) =

; Si/Na = 6.7 ; Si/(Na t M) = 3.7 ; H20/(Si t Al) = 19.2 ; (Si t Al)/Pr4N+ =

230 11.06

; Na20/(PrqN)20

= 1.61

; M20/(Pr4N)20

2.94, which all fall within of 100 % crystalline

In addition,

parable

to the conditions

termediate

between

ZSM-5 [15,21,281

TABLE

literature

ratios are com-

and Sand for their study of the ki-

of ZSM-5 in presence

reported

=

for the preparation

in the recent patent

high (Na20 + M20)/(Pr4N)20

chosen by Mostowicz

the "dilute" condition

ratios sometimes

range recommended

materials

our relatively

netics of crystallization

higher

the conventional

(Na, Pr4N+)ZSM-5

128-301.

= 1.32 and (Na20 + M20)/(Pr4N)20

of various cations

[231 and are in-

chosen by Von Ballmoos 1311

in the literature

and the

for (Na) ZSM-5 or for (NH4)

(Table 1).

1

Comparison paration

of the various M20/(Pr4N)20

ratios used in the literature

for the pre-

of ZSM-5 zeolites

Nature of MI

M20/(Pr4N)20

Reference molar

ratio

(alkali)

0.028

Na and/or NH4

0.67

Na and/or

K

Na

1.18 2-2.67

Li,Na,K

2.94

Von Ballmoos Erdem 113,141 Argauer

(Ba,Sr)

Li,Na,NH4,K,Rb,Cs

10

Na

Chao

18.75

NH4

Bibby

Rollmann 1281

at room temperature

[ 151

pressure,

were stopped after the gelatineous

heavy crystalline the crystalline

products

zeolite

sidered as a qualitative

and transparent

evaluation

with occasional

in Pyrex stirring.

phases had disappeared,

mother

to form and to separate

vation was stopped usually We have verified

phology

[ 211

for 2 hours, the gel was sealed

tubes and heated at 125OC under autogeneous

change either

I231

This work

Na

The syntheses

[ 331

Mostowicz

6

After agitation

[31]

liquors.

from the mother

of the kinetics

phases

Autocla-

had appeared.

in the case of (Na) ZSM-5 that longer autoclavation

the nature of the zeolite or its chemical

for

liquor may be con-

of crystallization.

two days after the crystalline

yielding

The time required

composition,

times did not size and mor-

(table 2 and 3).

The organic-containing

zeolitic

aaterialswere

filtered,

washed

abundantly

cold water and dried at 12O'C for 14h.

They were designated

formula

cation added in the synthesis

(MI) ZSM-5, M* being the alkali

chloride.

with

by the abreviated mixture

as

231 Characterization

of the solid precursors

The nature of the zeolites luated by X-ray diffraction meters were determined

as well as their degree of crystallinity

(XRD) as described

earlier [lo].

by using the A values corresponding

to the reflections

a b and 2 values were further (002) (040) and (501) (3.2). The -the (ill),

by considering the smallest

(151),

crystallites

(303)

and (102) reflections.

was estimated

from the broadening

were determined

by Scanning

Electron

The water and the organic heating

the precursors

min. -' in a Stanton

content

Redcroft

gamma-ray

average

(10m4 Torr) chamber

in the vacuum

at 45' with respect ting voltage analysed System

"bremsstrahlung"

Dispersive

multielemental

by computer.

are discussed

The

of the inforelectrons

under the above-given

con-

in a depth of 1.5 to 3um

stems from a 1 to 2 Pm thickness The EDX can therefore

the whole bulk of smaller, was analysed

wet analysis)

by mixing

well isolated

zeolitic

were chosen

as internal

of other alkali

the following

samples

Each

positions

with

of known composition standards

cations,

analytical

larger than

particles.

surface

of the

be considered

having a diameter

twice on two different

Homogeneous

For the analysis

dards were prepared

of 100 eV at

with Z > 10.

For 25 KeV incident electrons,

the outer rims of crystallites

less than 5% divergence.

terminations.

information

with Pyman et al. [36].

analysing

PIGE or chemical

[35].

Kevex Unispec-

by the incident

analysed

X-rays were

Kevex model 2003 Si (Li)

resolution

depths

targets

inclined

like Rb and Cs,in a depth of 3 to 5 Pm, but in all cases we

that the major

pellet or crystallite

emitted

by a proper retreatment

analytical

in detail elsewhere

in agreement

3-4 ym and analysing

detector

introduced

under an accelera-

spectrometer

of all the elements

were eliminated

for the zeolite

The

X-ray

and

and di-

Microscope,

beam and analysed

and guaranteed

dispersed

analyses)was

Electron

as Na, Al, Si and K are detected

such

and heavier elements must consider

crystallites

individual

semiconductor

by

[ 10,341.

by Energy Dispersive

(nX analytical

analysis

by

pellet of 1 cm diameter

of 50-500 A.

Analyser

The effective

X-rays

Lighterelements

as a method

electron

of 0.008 wm thickness

radiations

and the emitted

sample,

support(for

7000) with a high resolution

5.9 KeV, allowing

ditions

zeolite

of 25 kV with a beam diameter

by an Energy

with a Be window

mations

zeolite

of a JEOL JSM-35

to the incident

elsewhere

were also estimated

analyses)or

rectly glued onto a Cu-Zn metallic

sizes

rate of 1OY

zeolites were determined

(PIGE), as described

The sample-target,a

(EDX) 1351.

0.1 cm thickness(for

and crystal 1101.

[lO,llI.

Analyser

of the uncalcined

Si, Al, Na, K, Rb and Cscontents Analysis

(SEM) analyses

of the (M) ZSM-5 phases were evaluated

Thermal

emission

size of

of the characteristic

in a dry air flow (2.4 l.h-l) at a heating

Si, Al, Li and Na contents proton-induced

Microscopy

refined and checked The average

Morphologies

XRD line at c.a. dhk, = 11.05 [(lOl)reflection].

were eva-

The unit cell para-

(from

for Si and Al de-

four other internal

grade pure reagents

:

stan-

232 a) KF/Na2C03/Rb2SOq/Cs2C03

; b) Si02/A1203/K2S04/Na2B40,

and d) Rb2S04/Cs2SOq/K2S04/Na2B407. ve grindings grindings termined

The preparationswere

of the wet powder mixtures

in solid state. accurately,

; c) Si02/A1203/Na2B407

It appeared

followed

that small amounts

while NH4+ (nitrogen)

homogenized

by subsequent

by successi-

evaporations

and

of Na could not be de-

and Li are too light to be even detected.

RESULTS Syntheses Table 2 reports some synthesis the final phases formed talline material ZSM-5

in the 6 different

to appear are similar

(7 days) and principally

cleation

(crystallization)

the presence

data as well as the structural

Times required

(3 to 3.5 days) although

for (NH4) ZSM-5

(93 days), suggesting

effect on the nucleation

mixture

was reported

of

for the crys-

higher for (Cs)

rates are lower in the two latter cases.

of NH4 + ions in the synthesis

an inhibiting

syntheses.

identification

that the nuNote that

earlier

to have

of ZSM-5 phases [21,31].

TABLE 2 Synthesis

data and XRD characterization

Zeolites

Initial Si/Al at.-ratioa

of various

Autoclavation t.sepb (days)

t.a.t.c

(M) ZSM-5 zeolites

XRD analysis Material

D1ol(A)d

Unit cell oarameters 5

b

2

(days 1

(Li) ZSM-5

50.9

(Na) ZSM-5

47.9

3.5

5

ZSM-5

1000

e

(K)

ZSM-5

49.4

3.5

5

ZSM-5

2000

e

(Rb) ZSM-5

47.5

4

5

ZSM-5

3800

e

(Cs) ZSM-5

4i.o

7

9

ZSM-5

> 8000

e

(NH4)ZSM-5

50.5

93

120

ZSM-5

925

e

(Na) ZSM-5

47.9

3

e

(Na,H)ZSM-5

= reference

3

zeolite

5

ZSM-5

450

4

ZSM-5

-

52

ZSM-5

-

120

ZSM-5

20.10

19.96

13.40

e

[37]

20.07

19.92

13.40

aSlight differences are due to variable amounts of Na silicate added to the final mixture as to adjust its pH at about 9. b Time (days) required for the crystalline zeolite to form and to separate from the mother liquor (qualitative measurement). 'Total autoclavation tine. d. Hverage diameter of the smallest particle calculated XRU line corresponding to the reflection (101). eValues similar 0.05 A.

to those obtained

from the broadening

for (Li) ZSM-5, with a maximum

deviation

of the of

233 X-ray diagrams The X-ray diffraction cate the presence orthorhombic hardly

of crystalline

symmetry.

resolved

show a marked

patterns

of the zeolite

are consistent

[ (Li) ZSM-51.

As expected,

by the presence

indi-

with an

of 3.4 A, exchange

The Dlol values

the lattice parame-

of large cations

with the fact that these parameters

but not their Na content (37,381.

have a bare ionic diameter

Thermal

The patterns

or is a broad singlet

affected

(M) ZSM-5 materials

the XRD line near d = 3.82 R is either

from Li to Cs (Table 2).

ters are not significantly This is consistent

ZSM-5 only.

In particular,

in a doublet

increase

of the as-synthesized

vary with the Al content

Moreover,

easily

such as Cs+.

cesium

ions, which

into the ZSM-5 lattice [20].

analysis

The weight

losses due to H2Oororganiccontentreleaseintheappropriatetempera-

ture intervals

]lOl,as

recorded

from theTGcurves,

enable

the computationofthe

nun-

her of H 0 or Pr Nt ions present in the unit cell of the as-synthesized zeolites 4 (table 3;. While the number of Pr4Nt per unit cell is between 3 and 4 as usually observed

for various

of the counterion

MI

ZSM-5 precursors

[10,11,28]

Li to Cs and it is particularly

low in (NH4) ZSM-5.

per cation MI in a unit cell of zeolite of the affinity

and is independant

, the water content is variable. decreases

of that cation towards water

on the nature

It regularly

decreases

from

The number of water molecules

in parallel

with the decrease

(table 3).

SEM analysis Typical

SEM micrographs

of the six (M) ZSM-5 zeolites

and their size distribution spheroidal

ry small platelet-like tion yielding individuals, consists similar "dilute"

shown in fig. 2.

Li

and

2-5 urn and 8-15 urn sized crystalaggregates

smaller

units.

This morphology

crystallites

still keeping

of large lath-shaped to those observed conditions

1311.

very small thick-set ry nucleation crystallites

hexagonal

number of small crystallites

is predominant

50% in mass of the crystalline of rounded

within

of ZSM-5 [40]. Both morphologies

matter.

(K, Rb) or sharp-edged

sing in the order

K < Rb < Cs.

in the same order

(fig. 2).

observed mixture

when a seconda-

Although

(fig. Z), they represent

Their average

Their size distribution

under

with numerous

still containing

are shown in fig. 3.

(Cs).

larger

single crystals

are sprinkled

a synthesis

nuclea-

(NH4) ZSM-5

NH4' ions crystallizing

The K, Rb and Cs zeolites

individuals

formed

form [28,31].

plates which are typically provoked

consist of made up of ve-

and double terminated

containing

In our case, these crystals

is artificially

zeolites

onto the primarily

of their original

well-developped

in mixtures

Na

respectively,

in fig. 1

could stem from a secondary

which deposit

the outline

are compared

the

only about

consist

of twins

size is increa-

becomes more homogeneous

234

TABLE 3 Thermal

analysis

of various

(M) ZSM-5 zeolites H,O content

Zeolite wt.lossa

mol/u.c.

b

Organic

mol/M Ic

(%)

Hydr.d nb.(MI)

wt.lossa (%)

content mol./u.c.e

ZSM-5

3.2

10.5

13.0

2.2

11.1

3.4

(Na)

ZSM-5

3.6

11.8

14.8

3.2

11.8

3.6

(K)

ZSM-5

2.5

8.2

7.5

2.2

12.1

3.7

7.8

2.2

10.9

3.4

(Li)

2.7

8.6

ZSM-5

1.7

5.6

2.8

1.2

11.5

3.6

(NH4) ZSM-5

0.9

2.9

1.7

-

10.8

3.9

(Rb) ZSM-5 (Cs)

aWeight losses (in %) are referred to the weight of the residual zeolite after calcination of the precursor in dynamic air at 600°C for 2h. b Assuming that the weight loss between 100 and 300°C is due to H 0 release. The molecular weight of a unit cell of calcined zeolite is obtained2from chemical analyses (table 4). 'Molecules d Hydration

of water per alkali cation M'inone numbers of alkali

unit cell of zeolite.

cations at infinite

dilution,

at 20°C [39].

eAssuming that the weight loss between 350 and550°C(air flow) stems from the oxidative decomposition of the organic molecules. The latter are present in the zeolite framework either as Pr4NT counterions (the Al/MT ratio is always higher Calculations were made from the molethan l(table 4 ) and/or probably as Pr NOH. cular weight o 1 Pr4N+ entities. With ?he molecular weight of Pr4NOH, a maximum variation of about 8% to lower values of mol/u.c. are obtained.

Figure

1

235

Figure

1

Scanning

electron

of the various

micrographs

comparing

(M) ZSM-5 zeolites.

the sizes and morphologies

236 I

aso z

(NH,) ZSM-5

40

(K) ZSbl-5

& 10

Figure 2

m

10

40

Crystal size distribution of the various (M) ZSM-5 zeolites as computed from Scanning electron micrographs.

Chemical analyses The zeolites.were analysed for Si, Al, Na and M (P16E and EDX). The results are reported in table 4. The (Si/Al) bulk ratios measured for the Rb and Cs zeolites are close to those of the synthesis mixtures (gels) (fig.4)while they are somewhat lower for the other zeolites (they decrease in the order K > Na > Li). This indicates that the initial Si/Al ratio has little influence on the final zeolite framework composition, the latter being thus strongly dependent on the nature of the alkali counterion. Li and Na zeolites show the highest Al content. For Li and NH4 zeolites, the (Si/Al) "surface" (EDX) and bulk (PIGE) ratios are similar (table 4 and fig. 4).

This could result from the smaller size of their

crystallites,as in such case the bulk is equally probed by the two analytical methods. More generally, data of table 4 ang fig. 4 show that the bigger the average crystallite size (from Na to Cs), the greater is the difference between the (Si/Al) ,,SUrfaCe,, and the (Si/Al)bUlk ratios. In all cases, EDX shows that the average "surface" of a pellet composed of crYstallites of various sizes is enriched in Al with respect to the bulk. This suggests thateither all the crystallites have an inhomogeneous Al distribution or

Secondary

Figure 3

crystallization

of small hexagonal

(NHq) ZSM-5 on the original

that the average

Al distribution

with the crystallite contents

size.

on individual

Al/(Al t Si) variations

is homogeneous

the bigger.

The reverse

this observation

as a function

tendency

within

measured

of different

For the Li, Na, K, Rb and Cs samples,

a crystallite

size are reported

the smaller crystallites

is observed that

but variable

by EDX the actual

sizes, for each zeolite.

of the crystal

only, it can be deduced

of

single crystal of the same.

We have therefore

crystallites

crystallites

contain

Si and Al The

in fig. 5. more Al than

in the case of (NH4) ZSM-5.

From

In the first case, the bulk of the

238

30

40

50 (Si/Al)zeol.

Figure 4

Correlation between (Si/Al) ratio in the gel precursors and the (Si/Al) ratio in the crystalline (M) ZSM-5 zeolites,measured by EOX (0) and by PIGE (I).

TABLE 4 Chemical surface (EDX) and bulk (PIGE) analyses of the various (M) ZSM-5 samples

Zeol ite

Si/Al PIGE a EDX a

(L i) ZSM-5 32.4 (Na) ZSM-5 36.1 (K) ZSM-5 38.5 (Rb) ZSM-5 47.5 (Cs) ZSM-5 45.2 (NH 4)ZSM-5 41.2

31.0 33.5 28.2 35.0 35.5 38.0

A1/(Al+Si )b AliNa Si/Na Al/M Al lu. cdNa/u. c?r~/u. c .f( Na+i\l) I u.c. PIGE EOX (PIGEl(PIGE)a(EOX)c (PIGE)(PIGE) (EOX) 2.99 3.13 14.2 460 3.5g 2.87 0.07 0.81g 0.88 c 2.69 2.90 3.2 116 2.4 2.56 0.80 0.80 2.53 3.42 13.9 535 2.2 2.43 0.17 1.1 1. 27 2.06 2.78 9.1 432 1.8 1. 98 0.22 1.1 1.32 2.16 2.74 10.0 452 1.1 2.08 0.21 2.0 2.21 2.37 2.56 70.0 2920 1.35h 2.27 0.03 1. 68h 1. 71

aAtomic ratio ; average error : ± 5% bX 100 CAverage error ± 15% for Na and ± 5% for K, Rb and Cs [35] dSi + Al = 96 with Si/Al as determined by PIGE eFrom Al/u.c. and AliNa as determined by PIGE fFrom Al/M as determined by EOX gLi determined by PIGE as it could not be detected by EOX hThe NH 3 content was measured by TG/OTG [11].

239

t”

’

>

r-----r

*

a(K) ZSM-5

" 5 a+

0

o(Cs) ZSM-5 0 (Rb)

5

Variation

ZSM-5

srllall crystallites

is richer

ratios

consistent

is probably being

dependent

,rogeneousAl

I,lllland suggests growthrough

mostly

confirms

the same order. oe present

(where Al/Cs

ratios(and

This indicates the

can

per unit

(from the Na silicate)

is added to the synthesis in the case of (Li) and

(and also the total alkali

that for Li, Na, K and Rb, zeolites, framework

negative

on the nature of the alkali

charge, while

essentially

of Pr4N+ cations

we have observed

striking

the M content

= l), Pr4Nt must be present

and that the actual amount

Finally,

ho-

mechanism

from Na to Cs, the Al/M ratios are decreasing

that Pr4N+ ions act both as structure-directing

iS dependent

have a

nucleation

thus the number of Na+ions

alkali cation

This is particularly

is increasing

to compensate

that Al

its concentration

Li, K, Rb and Cs (but not NH4) zeolites

that very little sodium

On the other hand, while

(M + Na) content)

of a surface

particles,

concluded

The fact that (Na) ZSM-5 zeolites

in the zeolite when another

as its chloride.

(NH4) ZSM-5.

the crystallites,

however,

mechanism.

cell, table 4) are low, indicating incorporated

(M) ZSM-5 zeolites.

It is therefore

the existence

that, in that respect,

Except for (Na) ZSM-5, the Al/Na

mixture

within

on their size.

an identical

by EDX) as a function

in Si than that of the smaller

EDX analyses.

homogeneously

distribution

(measured

I

in Al than the outer shell of the bigger ones.

with the "average"

distributed

40

ZSM-5

by SEM) for various

of the latter must also be richer

tiecore

tobe

size (measured

ZSM-5

0 (NHd

-I

of the Al/(Al+Si)

of crystal

ZSM-5

A (Na)

< : 4

Figure

0 (Li)

species

occluded

counterion

that, in contrast

as Pr4NOH.

in (Cs) ZSM-5 This confirms

and as exchange

within

in

Pr4Ntionsnust

cations

the zeolite framework

added in the synthesis.

to Si/Al ratios,

the Al/M ratios

is

240 do not depend

on the size of the crystallites.

and 2 for all the zeolites. corporation

of the alkali

amount of Al which

This observation

Their values confirms

were found between

1

that in all cases the in-

ions in the zeolite framework

is mostly

governed

by the

is present.

DISCUSSION General

behaviourofalkali

Alkali

cations

cations

in presence

(salts) may influence

gels (~01s) in various ways, depending duced in an aqueous medium, dipoles

of water molecules

a first consequence, electrical

double

the nucleation

them into a firm hydration

which

and the hydrophobic

this effect

of the so-formed

arranged

decreasing

series

effect) [8,9].

of the cation

out power

tural subunits

precursors

241 and, in the particular zed to fulfil a template

[10,42].

also linked to the salting-out

effect

der water molecules

regular

species.

Larger

in forming

cations such as k,

less with H20 and will even disrupt bonding

(structure-breaking

from the solvent

will increase

structure-breaking

[ 43,451.

number and/or the effective

can be

series) of

to order the struc-

of various Al-rich

hydrated

Na+ions

structural

zeolites

property

i.e. to their ability

entities

[3,5,

have also been recogni-

This structure-forming

its regular

structure

However,

is to or-

with the aluminosilicate

stabilized

of the surrounding

anions,

water molecules

are expected

H-

such as

is effectively so that they

by building

both the structure-forming

to yield

through

very large cations

Their charge

(K, Rb, Cs, NH4) alkali cations

tent of such a competition

ties are reported

are believed

towards water.

to order the subsequent

of

on its

Rb+, Cs+ I431 and also NH4+ 1441 interact

As a result,

ferent ways for aluminosilicate eventually

The alkali cations

(H20) by the four bulky propyl substituents

the structure

water clathrates

The efficiency

or Hoffmeister

of the cations

effect) 1431.

Pr4Nt are also structure-ordering shielded

species

case of ZSM-5,

function

the

1411

alkali cations

to nucleating

char-

:

Li > Na > K > NH4 > Rb > Cs On the other hand, small hydrated

en-

sols will

to become hydrated,thus

(called lyotropic

salting

The

the aluminosilicate

aluminosilicate

and hence on its size (ionic radius).

in the following

approximately

sphere with, as

of the solution.

alkali cations with, as a result,

gel (salting-out

depends on the tendency

charge density

stabilize

(Si-rich)

When intro-

In basic medium, these negatively

colloid.

ged species will interact with the hydrated precipitation

properties.

will interact with the

of the (super) saturation

layer and the solvation

into a macromolecular

gels

process of aluminosilicato

on their intrinsic

bare alkali cations

and orient

an increase

tities will be weakened floculate

charged

of aluminosilicate

stable

(Li, Na) and

to compete

in dif-

stable sols and then gels and

aluminosilicate

framework.

Obviously

the ex-

depends on the size of the bare ion, on its hydration radius of the hydrated

in table 5.

cation.

Some of these proper-

241 TABLE 5 Some intrinsic

properties

of alkali and related

ionic radii (A)

hydrated

radii

cations

(A)

Mol.hydr.

Hydration numbers b

Shannon'

Marcusb

Nightingale

(46)

(47)

(39)

(48)

(39)

Li+

0.60

0.74

2.00

3.82

34.6

2.2

Nat

0.95

1.02

2.28

3.58

52.1

3.2

K+

1.33

1.38

2.17

3.31

44.2

2.2

NH4+

1.48

1.59

3.31

Rb+

1.48

1.49

2.24

3.30

49.4

2.2

cs+

1.69

1.70

2.07

3.29

38.7

1.2

Pr4Nt

4.94

-

4.94

A13+

0.50

0.53

Ions

Pauling

-

(39)

(2.0)

4.75

aFor six-coordination bAt infinite %!olar

dilution

hydrated

at 298 K

ionic volumes

Effect of alkali cations Upon mixing anions,in entities

on nucleation

the initial

equilibrium

cate negatively

tetrahedral

species,are

to the building

anions will condense charged

complexes.

species

consists

units

displacement

cations will influence

such a nucleation

Firstly,

because

they will favour

then their further

the intimate

lead to zeolite the various Thirdly,

of their variable

mixture

alkali

cations

the alkali

cations

cess by controlling gel.

the formation

Alkali

ways. around

them,

of macromole-

and precipitations complexes

as a gel.

with Pr4Nt which

can

or impeded by the size and the charge of competitively

incorporated framework.

of specific

with the same anions.

with Pr4Nt as charge In particular,

(Li, Na) will act as templates

the formation

ei-

the negati-

that of a water cla-

in three different

agglomeration

are finally

process,

between

to order water molecules

of aluminosilicate

sating units for Al in the crystalline structure-forming

ressembling

of the solution,

which also interact

cations

anions to form aluminosili-

bonded water molecules.

process

ability

nuclei, will be favoured

charged

At pH above

The latter order around them the

of hydrogen

the supersaturation

cular sol species, Secondly,

framework.

in an interaction

in a framework

thrate, by progressive

silicate

the only negatively

of the zeolite

with aluminate

and the Pr4Nt ions.

aluminosilicate

rates the monomeric

The first step in the nucleation

ther in acidic or in basic medium, vely charged

in acidic medium,

with polysilicate

that participate

9, (poly)silicate

and crystallization

ingredients

in the nucleation

aluminosilicate

compen-

the (water)

entities

pro-

within

the

242 Effect of a structure-breaking

cation

(cesium) on the formation

of ZSM-5

In acidic medium, hydrated Cs+ions, the smallest positive entities (table 5) will compete silicate-Al

more for the anions and the other possible and silicate-Pr4Nt,

number of nuclei

(because silicate-Pr4Nt

Si-rich

silicate-Al(H20):

(because

little Na (because silicate-Na

slower

is to desorganize silicate

and yield

their clathrated

occurs.

large crystals

interactions

are impeded),

are impeded)

are empeded)

Consequently,

anions and yield Al-rich

"C$Pr4Nt-aluminosilicate" smaller Al-richer

the nuclei will grow with a slow rate process

observations

sol or gel.

can then appear

separately

on (Cs) ZSM-5 are entirely

study of the Al-Al

anions

interactions

in the presence

slowly

(average

(SEM data), which are Si-rich

amounts

of aluminiumand(charge

TG data

(table 3) support less hydration

Pr4Nt are present (tripropylamine

tallites

CsAanion

interaction)

crystals

consists

role by preventing

containing

The present few but large

and contain

equal

(table4).

: (Cs) ZSM-5 as synthesized, for Cs+) and about 3.6

in the form of Pr4NOH n.m.r.

(fig. 2) shows that most of the crys-

individual

ofAl-deficient ZSM-5 salting-out

through

process,yielding

The morphology

of the (Cs) ZSM-5

units, often twinned

(fig. 1).

This

slowgrowing]28,31].

effect of the &ions

also plays a

The small amount of nuclei formed

a mechanism

similar to the liquid pha-

larger well-defined

and discussed

rates

(due to the preferred

crystals formedupon

a rapid gel precipitation.

of type A, described

implies slower crystallization of less nuclei,

less aluminiun.

can then grow, at least partially, se ion transportation

by high

less with alu-

by solid state 13C high resolution

the presence

of well-outlined

is typical

PIGE analysis)

probably

of large particles

in turn suppose

that the less efficient

synthesis

with the

of 25 urn or more and that less small size crystallites

The presence

which

such as Cs+.

(which has little affinity

The size distribution

129,311,

believe

consistent

(table 2), yielding

the above observations

water

could not be detected

have a diameter

morphology

and finally yield

conpensating)CstiansandlittleNa+ions

per unit cell, as usually,

1491).

are present.

amount)

A few Al-richer

in the gel precursor

of large cations

crystals

spectroscopy

Al spe-

(but in smaller

aluminosilicate

data show that (Cs) ZSM-5 crystallizes

contains

(higher pH).

the pH to about 9-10, the negative

" Al n.m.r. 1271, we have shown that Pr4Nt ions interact

minosilicate

a

(leading to

process.

In a preliminary resolution

with, as a result,

ZSM-5 crystallites.

Our experimental above described

nuclei

very

The

with silicate

where ordering

cies, which appear, will condense with the still remaining silicate

and containing

Cs+cations

at the end of the crystallization

On the other hand, upon increasing

a small

essentially

will be formed.

water structure

on the Pr4Nt centers,

silicate-Na,

Consequently,

of the structure-breaking

"precipitation"

the ZSM-5 structure)

interactions

interactions

other effect of the interaction species

interactions,

Will be less preferred.

previously

single crystals(see

1101).

Ne

243 The EDX analysis confirms

of Si/Al as a function

that big crystals

Al (fig. 5).

contain

In addition,

all the crystallites,

Al (and Cs) must be homogeneously

as suggested

and the apparent

Al-richer

can be explained

if we consider

crystallites portant

between

the silicate-Cs charge

the complex

(H2D)i

andthus

will favour

that cannot be probed

is expected

(Cs) ZSM-5, crystals distributed tent with

Moreover,

because

the water ordering

our experimental

effect

observations

within

Pr4Nt (more nuclei)

higher average ratio higher

Al content

lite crystals

nal nuclei still exist

in growing

that the nucleation

ZSM-5 but, in the present

by the presence

The most striking

difference

morphology

of the latter

and mosaic

structure

Al upon growth 128,311. starts

initially

(fig. 2). nuclei)

incorporation by the

and an Al/Na

are also present

in the zeo-

crystallites in the origi-

Very large Si-rich

in the size histograms

crystals

of fig. 2).

to the one proposed

of an Al-richer

of the structure-forming

between

This is consis-

of Al present

is similar

to

size and are

crystallite

and hydrated

for (Cs) to

Na ions.

(Cs) ZSM-5 and (Na) ZSM-5 is the particular

: medium sized units (3 to 15 pm) with less developped

faces and covered with small growth

by the amount

case, the probability

form is increased

In contrast

of the individual

crystallites).

mechanism

gel

These conside-

(PIGE), higher Na content

(fig. 5) but are rare (not computed

This suggests

1101.

This is also substantiated

(table 4). The EDX Si/Al analysis

(or incorporated

will oc-

the amorphous

for (Na)ZSM-5.

that Pr4Nt cations

shows that their final size is governed

species

the hydrous gel

of the N&ions)

and Al (Al-richer

process.

of the crystallites

than 1, thus confirming

delo-

less for

silicate

have a smaller average

the 5-15 urn range

at the early stages of the nucleation

than

of their structure-forming

in the alumino

(type B) mechanism

of (Na)ZSM-5 are numerous,

more homogeneously an easier

to be smaller

and even hydratedAlcationicspeciesmay

with the solid hydrogel

rations can explain

of ZSM-5

The latter will then compete

formed because of the strong salting-out

consistent

by EDX.

of the larger size and therefore

rapidly and a larger number of nuclei are formed within

phase,

small

The rather im-

nuclei. As a result,thenucleationof

theforcationofzeolite

(rapidly cur

because

sodium ions.

stage of the process.

interactatthis

of the Al-rich

(sodium) on the formation

anions with the Pr4N+ cations

properties,Na+ions

analysis

Si/Al ratio from EDX and from PIGE (fig. 4)

silicate-Na(H20):

interaction

of the hydrated

throughout

The latter observation

outer shell of the bigger ones.

cation

contain more

EDX crystallite

that EDX probes the entirety

pH, the interaction

crystallites

distributed

EDX analysis).

core of the large crystals,

Effect of a structure-forming

calized

(average

the average

must be due to the Si-rich

At acidic

by both the individual

surface

but only the Si-rich

difference

of the size of individual

little Al and that small crystals

(%O.lrm) crystallites was also reported

We may suppose

from the silicate

(fig. 1).

Such a spherulitic

for (Na) ZSM-5 that incorporate

that the growth of the crystal

species

available

in solution.

more

nuclei

Once the gel is

244 enriched

with Al, it will yield,

crystallites

through a second nucleation,

which either deposit

separately

served for (Cs) ZSM-5) oragglomerateand/or tes.

This is not in contradiction

small percentage explains

as isolated units (such as those obrecover

the primary

Si-rich

with our average EDX analyses

of the Si-richer

the small difference

small platelet-like

crystalli-

because

only a

cores are not probed by this technique.

between Al/S1 found on the "surface"

This

(EDX) and in

the bulk (PIGE) (table 4 and fig. 4).

Effect of Li, K and Rb cations (Li) ZSM-5 crystals, cation,

grow rapidly

as (Na) ZSM-5

also formed

(fig. 1).

Some other properties

ratio, the very slight difference 4), which

is consistent

even more a restricted with respect embedded

ble 4).

to the M'contents

cations

The properties

of K and Rb zeolites

hydrated

than for Na.

is the largest

logically

In particular,

crystals

fall between

(ta-

ionic

The latter,

ion among the alkali

those reported

: Li > Na > K > Rb > Cs.

H20 content

grown from a mixture

The size and morphology

and principally

containing

however,

for

Most

morphology)

(water) structure-breaking

both the (K) and the (Rb) materials (fig. 2) where,

show bidisper-

the number of large (essen-

predominates. of (K) ZSM-5 are comparable

and Sand for the similar material

corresponds

is found

(M) ZSM-5 zeolites

value of the molar

for Cs)

trend in the order

(size, composition,

sion in the size distribution

tion, the regular

complexes,

as well as the Al/Li ratio,

and the Al/M ratios in other by the smaller

(table

(fig. E),characterize

very little sodium

the Li content,

Si/Al

(table 5).

materials

tially single)

Surprisingly,

to one series of data [39],

of these properties

alkali cations.

average

the surface and bulk Al-content

to the one reported

Na and for Cs, with a regular

characterize

characteristics

such as the smallest

of Li(H20)z with aluminosilicate

while

This could be explained

volume for Li (comparable

hydrated

between

interaction

the crystallites,

at least according

of a (water) structure-forming

with the very small crystal sizes

to that of Pr4N+ species.

within

are similar

in presence

(table 2) and present the same morphological

increase

to the similar

synthesized

in the average increase

to those reported

in the presence

by Erdem

of KOH 1141.

In addi-

sizes from Li to Cs that we observe,

observed

for Li, Na and K [221 or Li, K and

Cs 1231 ZSM-5 materials.

The particular

behaviour

In aqueous solution, molecules

of NH4+ ions the interaction

is weak so that the "hydrated

of NH4 + ions with the surrounding structure"

of NH4+, which

of "hydrated radii" of 3.31 8, [48] (table 1) is questionable. to consider NH4 + ions as simply replacing structure

[ 441.

H20 molecules

water

leads to values

Other people prefer

in the unchanged

solvent

245

If we consider the very small size of the bare ion (1.48 A) or if we take the "hydrated"

value (1.54 A) reported

teraction

between

the aluminosilicate

in the synthesis

mixture

ions, with respect medium).

(table l), then the in-

anions and the positively

independantly

structure-breaking

of Pr4Nt-aluminosilicate

1391

will be the most important

to the far biggerPr4Nt,

In addition,

preaominant

in the literature

charged

species

in the case of small NH4+

Na(H20):

(in acidic

and/or Al(H20):

of their size, NH4+ ions will also play a

role towards water.

associations

As a result,

to occur will be reduced,

will be formed and they will grow slowly, yielding

the probability very few nuclei

very large crystallites

at the

end of the process. This is entirely consistentwith (NH4) ZSM-5. are formed hibiting

our observations

A small number of unusually Bibby [21]

indeed.

function

large

of

(up to 25 x 45 pm) single crystals

and van Ballmoos

of NH40H to explain

in the case of synthesis

[311 have also invoked

an in-

the large size of the (NH4) ZSM-5 crystals

they have prepared. However

we also observe,after

but small hexagonal onto the primary previously

platelets

which grow profusly,

large crystals

(fig. 2).

The particular

121,311.

that they have appeared process.

a rather long period,

The average

analyses ofindividual cy

configuration

EDX and PIGE analyses

We propose negative

the Al-rich

species.

silicate

However

yield

large units.

gelation occur.

(table 4).

aluminate (50)

in Al.

NH4+ ions strongly

:

- NH4 ' interactions

Al-richer

silicate

into the growing crystallites

will predominate

favours

The resulting

transported

solution

surface,

still contains

to

of large single

from the solution, and incorporated

the large (NH4) ZSM-5

to adsorb NH4+ ions which

Al-rich

nuclei will

is expected

the formation

At the end of the process,

for their negative

over

effect of NH4' ions,

are slowly exhausted

so formed will have tendency

a great affinity

core.

interact with

with a slow rate and finally

of the very poor salting-out

species will be progressively

crystallite.

of both small

so that a small number of Si-rich

species

tenden-

As the combined.

of a pellet composed

They will grow accordingly

As the silicate

the EDX

very little Al, while the outer shell

will be impeded and a liquid phase ion transportation

1101.

nucleation

although

However

must still have a Si-richer

interpretation

Because

a secondary

suggests

that the core of the single crystals

surface

We have shown that such a mechanism

crystals

stopped.

still suggest

- NH4 ' interactions

beformedinpreference.

through

was found enriched

big crystals

the following

surface

to contain

less Al than the average

and big units,

or sprinkled

was not mentioned

reveal for the first time an unexpected

appear

of the big and rare single crystals average

separately

for Si/Al are comparable,

(average)

crystallites

of numerous

of the small crystallites

after the large ZSM-5 units,

: the small crystallites

must contain

either

Such a phenomenon

EDX and PIGE analyses

still in favour of an Al-enriched

the formation

and the growth

Pr4Nt and Al-depleted

still have process

is

alumino-

246 silicate

anions while the residual

This favoursthesecondary rapidly

numerous

the solution

concentration

formation

small Si-rich

is exhausted

of NH4+ ions is now very low.

of a large amount of new nuclei, yielding

crystallites.

Their growth

from its ingredients,

size of the new crystallites

is stopped

which explains

as soon as

why the average

remains small.

CONCLUSIONS Our work demonstrates ring the synthesis

the essential

of ZSM-5 zeolites.

and their structure-forming size of the resulting

(Si, Al,Pr4N%nd major

under identical techniques

catalytic

properties

conditions,

size, morphology,

larities,

methods,

differences

individual

processes,

usually

synthesis

mixture,

homogeneously

distributed

alkali

within

cation

with crystal

size.

is therefore

a function

randomly

homogeneity

Our findings transformation processes

distributed

that Pr4Nt

The larger on their sur-

that both Pr4Nt and Pr4NOH

framework. in yielding

large single crystals

onter rim

that stem from a secondary

together

nucleation.

of ZSM-5

with very small Si-

(NH4) ZSM-5 contains

directly yields

the corres-

form.

suggest

that solution

importance

depends

ion transportation(A)

(B) mechanisms

can occur simultaneously.

(essentially

The

polycrystalli-

in size, are rapidly obtained.

core and an Al-rich

(surface nucleation)

their relative

of the particles.

and close to 1, indicating

very little sodium so that its thermal decomposition protonated

They

Al is

of the Si/Al ratio in such

alkali cation such as Litor Na+,smaller

in the zeolite

in the

but the S-i/Al ratio

less Al than the small ones which crystallize

NH4 having an Al-deficient

in the

as counterions.

+ ions behave more particularily

ponding

and very little Nat.

crystallites

The Al/M ratio is higher than 1, indicating are present

use

on simi-

composition

added as chloride

of the size distribution

for all the zeolites

With a structure-forming

units still countain

use of various

the combined

indications

chemical

(Rb+,Cs+)is

the individual

are only acting to a limited extent

rich crystallites

emphasized

obtained

(or twins) of (M) ZSM-5 are formed.

The average

increases materials

ne aggregates,

can play a

by the complementary

the new alkali Mt as counterion

Al/M ratio is constant

towardsthe

and composition

of each zeolite.

large single crystals

contain

linkedtothe

(M) ZSM-5 materials,

local variationsofthe

du-

properties

on these zeolites.

PIGE and EDX, which give important

or slight

crystallites

essentially

entities

conducted

of the six different

were characterized

When a structure-breaking

face.

dispersion

(XRD, TG, SEM), we have more particularly

of two analytical

cations

role towards water,

The control of these properties

cation content).

While the physical

alkali

their salting-out

ions , can be used to direct the synthesis

of desired

role in numerous

In particular,

or structure-breaking

hydrated

formationofcrystallites

role played by various

All other synthesis strongly

structure-forming/breaking

and solid gel phase

are not separated. variables

on the nature of the alkali

role).

Both

being fixed, cations

247 ACKNOWLEDGMENTS The authors continuous

wish to thank Or. J. Rostrup-Nielsen

interest

(Haldor Topsde A/S) for his

in this work and for his valuable

suggestions.

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