CatalysisTodrry,10(199~)405-407 Elsevier Science Publishers B.V., Amsterdam

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STABILISATIOW AND CATALYTIC PROPERTIES OF HIGH SURFACE ARFA ZIRCONIA

Ruth FmINla, Peter GOULDINGL, Jean HAVILAND2, Richard W. JOyNERl*, Ian

McALPINE 2*

, Peter MOLES2*, Colin NORMAN3,and Trevor NOWELL',

1 Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, PO Box 147, Liverpool, L69 3BK. UK

2Magnesium Elektron Ltd., PO Box 6, Swinton, Manchester, M27 2LS, UK

3 Alcan Chemicals, Chalfont Park, Gerrards Cross, Buckinghamshire, SL9 OQB, UK

ABSTRACT Doping zirconia with silica or lanthana results stabilisation of the surface area, with values of 70 -

in2 +gnificant 85 m g obtained

after calcination at 973K. The decomposition of propan-l-01 has been used as a test reaction, and indicates that the undoped zirconia surface shows both weak acidic and basic properties. The reactivity of the surface is only weakly changed by adding lanthana, while silica strongly promotes acidic behaviour.

INTRODUCTION

There is considerable current interest in the use of partially reducible

oxides, such as ceria and titania, as catalyst supports. Birconia also has

potentially wide usefulness, since it can show both acidic and basic

character. It is of interest for example as a component in automotive

exhaust catalysts, (l), and in the Fischer - Tropsch hydrocarbon synthesis,

(2). The wider applicability of this important material will be further

enhanced if stable, high surface areas can be achieved, and sintering'whicb

is associated with the tetragonal to monoclinic phase transition reduced.

In this paper we report the successful use of silica and lanthana dopants

to stabilise the surface area of zirconia. The extent to which the

presence of dopants modifies the catalytic properties of the zirconia

surface has been studied, using the conversion of propan-l-01 as a test

reaction. There are two possible decomposition routes: dehydration

yielding propene is taken to indicate acidic sites , whereas dehydrogenation

to acetone, (propanone), shows tbat basic sites are present.

z Present Address, Factory Inspectorate, Stanley Rd., Bootle, Werseyside. Authors to whom correspondence may be addressed.

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EXPERINENTAL

siiica was impregnated onto high purity zirconia by a number of methods,

including from enhydrous solution. Zirconia / lanthana catalysts were

prepared by coprecipitation, since, contrary to a previous report, (3), we

found that impregnation of lanthanum was not effective in stabilising the

zirconia surface area. All catalysts were dried at 360 - 370 K and

calcined at 973 X for 2 h. Surface areas were measured using nitrogen and

single or multi-point BET methods. Changes occurring during calcination

were probed by thermogravimetric methods and the calcined catalysts were

studied by X-ray diffraction.

Catalytic activity was measured in a flow microreactor, with a charge of

ca 2 g. A nitrogen carrier stream was used and analar propanol-2-d

injected by an HPLC pump into a vaporiser / preheater before the catalyst

bed, at a liquid hourly space velocity of 1 h-l. Reaction products were

analysed by gas chromatography, (Varisn, Vista 4600), using a flame

ionisation detector. No reaction was observed in the absence of a

catalyst.

RESULTS AND DISCUSSION

Surface areas for all the catalysts of present interest are given in

Table 1, which also lists phase compositions determined by X-ray

diffraction. Catalytic results are s ummarised in Table 2, with the

temperature required to convert 20% of the alcobol taken as s measure of

activity. This table &so shows the selectivity of the catalysts end

quotes some specific activities. Catalytic activity was measured over an

8h period and none of the catalysts deactivated in use.

Both silica ana lanthana additives are effective in increasing the

surface area of zirconia, in each case by a factor of about three. The

reasons for the stsbilisation of surface area will be considered elsewhere,

but the results for the lanthana doped catalyst sbow that retention of the

tetragonal phase of zirconia is clearly an important factor.

The catalytic measurements indicate that zirconia itseLf shows both

weakly acidic and basic properties. Fdidition of lenthana. itself a basic

oxide, slightly suppresses acidic behaviour, with the result that the

specific activity of the catalyst is decreased, while the selectivity to

acetone increases slightly. The influence of added silica is much more

surprising. The activity increases markedly, even after the increase in

surface area is discounted, the specific activity increases by almost an

order or magnitude. Added silica pram&es the acidic properties of the

catalyst, since selectivity swings wholly towards dehydration and the

form&ion of propene. Possible reasons for this dramatic change in

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behaviour as a result of the addition of silica will be considered

elsewhere.

TABLE 1.

Surface Areas of Zirconia Catalysts

Dopant wt/ % Surface Area/ Phase Composition / % 2 -1

my Tetragonal Monoclinic

Undoped - 29 2 ND*

98 Silica 0.37 55 ND Silica 1.12 75 ND ND Silica 1.87 85 NTI ND Silica 3.5 85 12 28 Lanthana 4 58 100 0 Lanthana 8 74 100 0

* ND = Not Determined

Table 2

Performance of Zirconia Catalysts

Dopant/ Loading

T/ K for 20% Relative SELECTIVITY/ % Conversion of Specific Dehydration Dehydrogenation Propan-2-01 Activity

Undoped Si/O.37% Si/1.12% Si/1.87% Si/3.5%

Z$% La/0% La203

518 1 90 10 563 1.2 96 4 543 2.1 100 0 513 8.3 100 0 523 9.0 100 0 593 < 0.03 90 10 513 0.4 92 8 553 0.a 02 18 628 Not Determined 66 34

REFERENCES

1) H.K. Stepien, W.B. Williamson and H.S. Gandhi, Amer. Sot. Auto. Eng., Paper 800,843, 1980.

2) see e.g. EP 0 110 449 and 0 127 220, assigned to Shell Research. 3) P. Turlier, J.A. Dalton, G.A. Martin and P. Vergnon, ~ppl. Catal., 29,

(1987) 305.