CATALYSIS BY RARE EARTH PHOSPHATE lII. CHARACTERISATION

OF SAMARIUM PHOSPHATE AND SAMARIUM P H O S P H A T E - C E S I [ ~

HYDROGENOPHOSPHATE AS KEY CATALYSTS FOR O-ALKYLATION

OF PHENOLS

ANNE-MARIE LE GOVIC a~, PASCALE POMMIER a~, ALAIN AUBRY a~,

LAURENT GILBERT b~ AND MARCELLE JANIN b~

a) Rh6ne-Poulenc Recherches, Centre de Recherches d'Aubervilliers, 52 rue de la

Haie Coq. 93308 Aubervilliers Cedex, France

b)Rh6ne-Poulenc Industrialisation, Centre de Recherche, d'Ing6nierie et de

Technologie, 85 Avenue des Fr~res Perret, BP 62, 69192 Saint-Fons Cedex,

France

SUMMARY

Samarium phosphates, impregnated or not by cesium hydrogenophosphate,

selective catalysts for O-alkylation of phenols, have been characterised by various

techniques. This study has shown that :

- the cesium salt added by wetness impregnation (10 % w/w) has a sintering

effect on its calcination. The examination of structural and textural datas shows that

the cesium does not enter the crystalline network. The cesium salt is uniformly

distributed on the crystalline surface and the special morphology of samarium

phosphate makes the cesium retained in the porosity of the solid.

- Samarium phosphate has an intrinsic acidic activity which can only be observed

on products calcinated at a temperature of 700~ and which therefore possess a

monoclinic structure.

Samarium phosphate calcinated at lower temperatures, with an hexagonal

structure has acido-basic characteristics highly dependant on the synthetic route

use : - a totally basic activity is observed for samarium phosphate being neutralized

with ammonia after precipitation.

62

- products which have not been treated according to the previous step present an

acidic activity

- the addition of cesium by wetness impregnation on the dry catalyst produces a

totally basic behavior.

INTRODUCTION

Even if the use of rare-earth phosphates in heterogeneous catalysis for fine

chemicals has been reported from more that 20 years, those catalysts were little

characterized. Recently, doping those catalysts with cesium salts has greatly

improved the activity, selectivity of the transformation as well as the life time of the

catalysts (refs. 1,2).

Particularly, a synergism between cesium hydrogenophosphate and samarium

phosphate has been observed for the O-alkylation of dihydroxy-benzene (ref. 2).

We described in this paper some characterizations of this solid doped or not, that

may allow to explain catalytic results.

EXPERIMENTAL

Samarium phosphate was prepared by wet synthesis starting from samarium

carbonate (Sm2(CO3)3, originated RP). Precipitation of phosphate by phosphoric

acid is conducted at 80~ by addition of a samarium carbonate suspension in a

vessel containing phosphoric acid. After the end of the addition, the solid could be

treated by ammonia at pH - 9. Cesium hydrogenophosphate is introduced by

wetness impregnation of the dried (110~ solid.

Transmission microscopy is realized using a Philips CM30 apparatus at

300 KV. DRX spectra were realized on a diffractometer Philips 1700 by scanning

between 5 to 70 ~ at l~ Porous distribution is determined by mercury intrusion

after elimination of gas over night at 200~ in an oven (Autopore II 9220 V3.01).

Acidobasic properties characteristics of solids were estimated by studying the

reactivity of 2-methyl 3-butyn-2-ol (MBOH) (ref. 3).

RESULTS AND DISCUSSION

In f luence of the c e s i u m salt on t h e r m a l stabi l i ty

We have compared the specific surface area thermal evolution of samarium

phosphate just dried or samarium phosphate impregnated by cesium hydrogeno-

63

phosphate (10 % w/w) between 400~ and 800~ Results are reported in Figure 1

and Table 1 and show a sintering effect of the cesium salt.

Table 1. Specific surface area of SmPO4, SmPO4, Cs2HPO4 as a function of calcination temperature

Temperature (~

dried

400

500

700

SmP04 (m2/g)

124

107

97

39

SmPO4, Cs2HPO 4 (m2/g)

91

84

67

10

140

120

100

80

60

40

20

;pecific surface �9 area (m2/g)

, | . , ,

200 300 400 500 600 700

|

800

Temperature (~

- - 0 - - S m P 0 4 I SmP04, Cs2HP04

Fig. 1. Specific surface area of SmPO 4" SmPO4, Cs2HPO 4 as a function of calcination temperature

64

~io "3

. 4

0 . : '

O . l

100

~ T I V E I ~ I O l l v l 0 I ~ 4 E T E R

+ t n i l ' m l t o f l . �9 e x t ~ m l l ~

I I IHii-i t - . . . . . ' . .

- i i _ _ _ . . , . . . .

i ! l l ! / I I I I ILL~__$___! ..... iillili I1 i . ~ i ' ~ " i l ~ , -, . . . . . . l!t~'--,r . . . . . i ~1"t - I - i~; - - ! . i -.' . . . . tbt4.-t-,~-/~if.bt-l-i-i .... ! ,, - _i / ! l i t 1 / i ! I i l i l i ] !i �9 :i l i roLL._! ....... ~ - - I -

iO i O.

O I i U t r [ l ~ . ( l i c r o m t t ~ s )

Porous repartition of SmPO4

• ~ - -

~ ii'i _ L ~ _ L . . ,

O . O l

O . I I

0 . 1

' ,.I_4~H_~L y

i ~i l] i l l I ~.

I [tNitl! i [ i,li!!t, t . il II I l

I !l!i]li I-! -~--~l!Ii-[~i- i iii!i!'il !,

CUHUt.ATIVE IHTRU6IOH v l OIAHETEH

+ t n t J ~ l t O n m i x t . r ~ s t o n

, i l t I i. ili]i i ! t 1. ,11, i I 111,~,~ I : t

,l!I t ,~,~i[! L IiLLL l ~

i ti .til tli i .... !it~i!t!i!l 1!1:! ~tli~l i liilli l!il i ! / ' ' ! t : ~ t i ! i [ ! l i ~ ilil . .~!.!!!!!, ~!i!i!l i .. ,

t0 1 o. ! 0.01

o z . u q ~ n ~ . ( e i c r o u t a r ~ )

Fig. 3. Porous repartition of SmPO4, Cs 2 HPO4

65

For the same thermal treatment (fixed duration and temperature), specific

surface area of SmPO4, Cs2HPO4 are systematically weaker of about 20 m2/g (or

even 30 m2/g) than those of SmPO4. On an other hand, we have checked that the

ammonia treatment has no effect on specific surface area.

Those results have been maked up, in the case of solid calcinated at 500~ by

porosity measuremem and by electronic microscopy analysis.

�9 Poro$imetry �9 samples expand a porous volume of 0,69 cm3/g with a

microporous volume of 0,20 cm3/g. Introduction of cesium lower the porous

volume without any change in the porous repartition (Table 2, Figs. 2 and 3).

Table 2. Porous repartition of SmPO, and SmPO4, Cs2HPO4

Catalyst

SmPO4

SmPO4, Cs2HPO4

Total porous voltt3Ine (cm /g)

0,69

0,56

Porous volume between 30 ~,d 0,1 m (cm/g)

0,45

0,40

Porous volume between 0,1 rn

and 37 A

0,20

0,16

Medimn pore diameter (m)

15

12

Medium pore diameter (A)

65

7o

Electronic microscoov The sintering effect of the cesium salt has been made visible by transmission

electronic microscopy. Comparison of electronic microscopic stereotypes of product

calcinated at 500~ (6 h.) without (Fig. 4) or with Cs2HPO4 (Fig. 5) leads to the

following remarks �9

- SmPO4 is formed of agglomerated polydispersed small stick of size between 10

and 100 nm.

- In the case of SmPO4, the periphery of those sticks is well defined (frame

bones). - In the case of SmPO4, Cs2HPO4, the periphery of the sticks is badly defined and

they are linked by amorphous zone of molten aspect enriched in cesium, as shown

on cartography analysis (Fig. 6). The STEM-EDS cartography analysis does not

show if the cesium is uniformly widespread on each cristallite or if it creates a solid

solution in the cristalline structure of the phosphate. It is the reason why a DRX

structural analysis was realized.

66

Samarium phosphate precipitates as an hexagonal phase and shows a phase

transition between 600 and 700~ to form a more closed monoclinic phase. Those

results are in good agreement with schneider's published datas (ref. 4) reviewing

crystalline structure studies about rare earth orthophosphates.

Rare earth orthophosphates can be subdivided into several families according to

their crystalline structures and the polymorphic modification as a function of the

temperature. The first family regroups light rare earth (so called ceric phosphates)

including the following elements : La, Ce, Pr, Nd, Sm, Eu. Those phosphates are

dimorphiques. Indeed, they precipitate at low temperature under hexagonal phase

and evoluate at higher temperature to the thermodynamically stable phase, the

monoclinic one, isomorphic to monasite CePO4. The phase transition temperature is

accompanied by an exothermic phenomena linked to the cation ionic radius and is

higher as the cation radii is lower (ref. 5).

�9 DRX

DRX studies of SmPO4 and SmPO4, Cs2PO4 calcinated at various temperatures

(between 200 and 800~ show that cesium has no visible effect on crystalline

structure of products :

- the crystalline phase transition (from hexagonal to monoclinic) occurs between

600 and 700~ independently on the presence of cesium (Table 3)

- the mesh parameters are similar for SmPO4 and SmPO4, CszHPO 4 (Table 4)

DRX shows no formation of cesium pyrophosphate which is usually obtained as

early as 300 ~

The complete analysis of the crystalline structure by DRX and EXAFS of

impregnated structure shows that the cesium does not enter the crystalline network

in SmPO4. The comparison of those results and the electronic microscopy analysis leads to

the conclusion that the cesium is uniformly distributed on crystallite surface and that

the excess of cesium is retained in the porosity of the solid, probably as amorphous

cesium phosphate.

67

Fig 5. Electronic microscopy of SmPO4, CsHPO 4

68

Fig. 6-2. STEM - EDS cartography of Sm PO4, Cs2HPO 4 | localisation of P

69

Fig. 6-3. STEM - EDS cartography of Sm PO 4, | localisation of Cs

Cs2HPO 4

ACIDO-BASIC PROPERTIES

Characterization of SmPO4

We have examined the influence of surface chemistry of SmPO4 on its acido-

basic properties. Characterization by reactivity of MBOH was realized for product

treated at pH = 9 with an ammoniacal solution or not. The MBOH test permits to

determine without any doubt the acido-basic characteristic of surface site.

We have reported in Table 5, methylbutynol conversion at the 12th pulse and

the acidic (A), basic activities (B) and activity due to acid base pairs (B) obtained

for each samarium phosphate. The evolution of acido properties - conversion of

MBOH and selectivity in the various products formed as a function of preparation

methods and calcination temperatures are reported Figures 7 and 8.

�9 The acidic selectivity is the sum of selectivity in 3-methyl 3-buten-l-yne

(MBYNE) and in prenal which are formed on acidic sites.

�9 The basic selectivity is the selectivity in acetone or acetylene which are formed on basic sites.

70

�9 The acid base pairs selectivity is the sum of selectivity in 3-hydroxy 3-methyl

butan-2-one, in 3-methyl 3-buten 2-one and methylisopropylketone which are

formed supposedly to be on acid-base pairs.

Table 5. Acidobasicity of samarium phosphate determined by reaction of MBOH

Catalyst

S m P O 4

S m P O 4

treated by an

ammoniacal solution

SmPO4

Cs2HPO4 (10 % p/p) ,,

SmPO4,

Cs2HPO4 (10 % p/p)

(treated by an

ammoniacal solution)

Calcination

temperature

Conversion of

MBOH (%)

dried 11

500~ 14

700~ 30

dried 27

500~ 24

700~

dried

500~

0,7

0,5

32

39

dried

500~

A B AB

94 1 5

97 1 2

98 1 1

2 98 -

7 93 -

91 8 1 _ _ _

100

100

If we compare products calcinated at 500~ SmPO4 without treatment has a

totally acidic behaviour while the sample treated at pH = 9 as a totally basic

behaviour. The basic behaviour observed for SmPO4 treated at pH = 9 indicates

the presence of residual anions coming from the ammoniacal neutralization step.

When calcinated at 700~ both products present comparable behaviour with an

higher acidity for the phosphate not treated. At 700~ we find the intrinsic acidic

activity of samarium phosphate.

SmPO4, Cs2HPO4 was also characterized by the MBOH test. Results reported in

Table 5 show that the presence of the cesium salts exalt the surface basicity. The

addition of the cesium salts induces a totally basic like behaviour of this catalyst.

The observed difference in activity should be interpreted with some caution due to

the high basic activity of cesium oxide, the presence of which, even in small

quantities, can not be excluded.

Conversion of 40-, MBOH (%)

3O A , . , w

20-

1 0 0 ~ [

i

0

100 200 300 400 500 600 700

--43- SmPO4 without treatment

SmPO4 treated at pH = 9

Calcination temperature of SmPO4 (~

A 4 0 -

30 J

20~,

L

10~ ~ ,

0 '

100

v

200 300 400 500 600 700

- -u- SmPO4 without treatment

SmPO4 treated at pH = 9

Calcination temperature of SmPO4 (~

A �9 activity = mmol MBOH transformed per surface unit and per hour

Fig. 7. Acido-basic properties of samarium phosphate

Selectivity (%)

100

80

60 I,

40 ~ 2o ~

100 200 300 400

S m P 0 4

--{3--

---o- % MBYNE (Acidity) ~

% Prenal (Acidity)

500 600 700

Calcination temperature of SmP04 (~

72

Selectivity (%)

100

v

80 -i L

60

40 ~

2 0 -

O ~

_20100

SmPO4 treated at pH = 9

200 300 400 500 600 700

- - o - % Acetylene (basicity) i

% Acetone (basicity)

- - I - % MBYNE (acidity) :

r % Prenal (acidity)

Calcination temperature of SmPO 4 (~

Fig. 8. Acido-basic properties of samarium phosphate �9 selectivity on each catalyst

CONCLUSION

This study leads to the following conclusions.

Cesium salt added by wetness impregnation (10 % w/w) has a sintering effect

on the calcination of samarium phosphate. The examination of structural and

textural data shows that the cesium does not enter the crystalline network. The

cesium salt is uniformly distributed on crystallites surface and the special

morphology of samarium phosphate makes the cesium retained in the porosity of the

solid.

Samarium phosphate has an intrinsic acidic activity which can only be observed

on products calcinated at 700~ and therefore with a monoclinic structure.

Samarium phosphates calcinated at a lower temperature, with an hexagonal

structure has acido-basic characteristics highly dependant on the synthetic route

used

- a totally basic activity is observed for samarium phosphate being neutralized

with ammoniac after preparation

- products which have not been treated according to the previous step present an

acidic activity

- the addition of cesium by wetness impregnation on the wet product gives it a

totally basic activity.

73

References 1. P.J. Tirel, C. Doussain, L. Gilbert, M. Gubelmann, H. Pernot, J.M. Popa, Studies in

surface science and catalysis, 78,693, (1983) 2. L. Gilbert, M. Janin, A.M. Le Govic, P. Pommier, A. Aubry, Preceeding paper in this

issue 3. H. Lauron-Pernod, F. Luck, J.M. Popa, Applied Catalysis, 78,213, (1991) 4. L. Niinist6, M. Leskelii in "Handbook on the Physics and Chemistry of rare earth"

F.A. Gschneider, J.R. Eyring, L. Eyring Eds., Vol. 9, Chapter 59, p. 91. 5. R. Kijkowna, Nieorg. Zwiazki Fosforowe, 7,239, (1976)

74