Stabilisation and catalytic properties of high surface area zirconia
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Transcript of Stabilisation and catalytic properties of high surface area zirconia
CatalysisTodrry,10(199~)405-407 Elsevier Science Publishers B.V., Amsterdam
405
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
407
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.