Acid-base properties of aluminium oxide.pdf

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Applied Catalysis 7 1983) 159-168

Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

159

ACID- BASE PROPERTI ES OF ALUMNI UM OXI DE

Z. VfT, J . VALA and J . MALEK

I nst i tute of Chemcal Process Fundamental s, Czechosl ovak Academy of Sci ences,

165 02 Prague 6 - Suchdol , Czechosl ovaki a.

(Recei ved 22 December 1982, accepted 30 March 1983)

ABSTRACT

The ef fect of the acti vati on temperature on acid- base properti es of al umnacatal ysts was studi ed. The acidi ty and basi ci ty of catal ysts were characteri sed

by usi ng the dynamc sorpti on method and by testi ng the catal yst acti vi ty i n the

depol ymeri sati on of paral dehyde and decomposi ti on of di acetone al cohol . For al l

catal ysts, i t was found that the aci di ty i ncreases si gni f i cantl y w th i ncreasi ng

acti vati on temperature up to 55O C, whereas the basi ci ty goes through a maxi mum

at 140°C. The acidi ty of al umna i ncreases w th i ncreasing surface dehydrati on.

Maxi mum basi ci ty corresponds to a surf ace covered compl etel y w th hydroxyl groups.

I NTRODUCTI ON

Al umna i s a bi funct i onal catal yst whose aci d and base si tes catal yse a number

of react i ons. Whereas the surf ace aci di ty has been att ri buted unambi guousl y to

al umni um cati ons and the surface basi ci ty to oxygen ani ons, the hydroxyl groups

can have aci di c as wel l as basi c character [1, 2]. Surface concentrati ons of al l

these si tes depend on the degree of surface dehydrati on whi ch i s i nf l uenced by

the acti vati on temperature [ l ] . Thus, accordi ng to i deas on the al umna surf ace

the aci di ty of hydroxyl group hydrogens shoul d decrease and that of al umni um

cati ons shoul d i ncrease w th the gradual surf ace dehydrati on. At the same ti me,

the basi ci ty should change as a resul t of i ncreased number of oxygen anions and

decreased number of hydroxyl group oxygens on the catal yst surface.

As has been found experi mental l y, the surf ace aci di ty of alumna i ncreasesmarkedl y w th i ncreasing acti vati on temperature [3-61, i n accordance w th the

formati on of strongl y aci di c A13+ si tes.

Onl y a few data are avai l abl e i n the l i terature concerni ng the ef fect of hyd-

rati on on the surface basi ci ty. Usi ng the ti trati on method, Yamadaya and co-

workers [7] have found that the basi ci ty of al umna i ncreases w th i ncreasi ng

hydrati on and that i t reaches a constant value when the amount of sorbed water

corresponds to three mol ecul ar l ayers. From an R study of adsorpti on of carbon

di oxide on a-al umna i t fol l ows t hat the rel ati vel y hi ghest basi ci ty i s shown

by oxygen ani ons occurri ng i n the nei ghbourhood of a hydroxyl group; the surfacebasi ci ty decreases w th i ncreasi ng dehydroxyl ati on [8]. Mochi da and coworkers [9

have studi ed dehydrochl ori nati on of haloalkanes on al umnosi l i cate, Si 02/ KOH

0166- 9834/ 83/ $03.00 0 1983 Elsevier Science Publishers B.V.



16

catal yst, on alumna acti vated at 500°C and alumna doped w th water, whereas

the product composi ti on obtai ned on acti vated al umna was si ml ar to that obtai ned

on alumnosi l i cate, the product composi ti on resul ti ng from the reacti on on doped

alumna was comparabl e to that found w th the Si 02/KOH catal yst. Resul ts of these

experi ments i ndi cate that the aci di c properti es predomnate on the surface of

dehydrated alumna, basi c propert i es are predomnati ng on the surf ace of hydrated

alumna and the surf ace basi ci ty i s i ncreased by the sorbed water rather than by

the presence of oxygen ani ons.

EXPERIMENTAL

Chemcal s

Di acetone al cohol [I O], 1, 3, 5_t ri ni trobenzene [I l l and di methyl zi nc tetrahydro-

furanate [ I 23 were prepared according to known methods. Except n-butyl amne (pure,

Koch-Li ght, UK), p- xyl ene, paral dehyde (both pure compounds, Sojuzchimexport , USSR)

and 1, 3,5_tri methyl benzene (pure, Al dri ch, Bel gi um, the other compounds used

(Lachema, Czechosl ovaki a) were reagent grade chemcal s. Li qui d compounds were

dri ed over mol ecul ar si eve Nal si t A5 or sodi um w re and di sti l l ed under ni trogen

before use.

Catal ysts

A Cherox 3300 alumna produced from sodi um al umnate (CHZ CSSP Li tvfnov, Czecho-

sl ovakia), a Pural S al umna produced by hydrol ysi s of alumni um al cohol ate (Condea

Cheme, FRG) and an al umna prepared i n thi s l aboratory were used. 4 sample of

the l att er (Al prep) was obtai ned by preci pi tati ng an al umni um ni trate sol uti on

w th aqueous ammoni a at 20°C and by cal ci nati onofthegel i nai rat 550°C for 4 h.

Al l catal ysts (f racti on 0. 07- 0. 125 mm i n di ameter) were cal ci ned i n oxygen at

550°C for 3. 5 h, cool ed to room temperature, rehydrated i n di sti l l ed water f or

16 h and dri ed at 20°C. Pri or to sorpti on measurements as wel l as catal yti c tests,

al l catal ysts were acti vated by drying i n ni trogen (35 m mn-' ) at the desi red

temperature for 1 h. The surface area of catal ysts was measured by Nel sen-Eggert-sen method [ 13]. The l oss of water duri ng act i vati on was determned gravi metr i -

tal l y. The surf ace concentrati ons of resi dual hydroxyl groups on catal ysts, acti -

vated at temperatures z 14O C, were determned by a pul se method [14] based on

the reacti on of gaseous dimethyl zi nc tetrahydrofuranate w th acti ve hydrogen.

Sorpti on measurements

The surface concentrati ons of aci di c (c, ) and basi c ( c, ) si tes of catal ysts were

determned by the dynamc sorpti on method descri bed earl i er [15, 16]. A sampl e of

catal yst (about 0. 4 g) was acti vated by dryi ng at the desi red temperature i n astainl ess steel col umn (50 mmx4 mm i . d. ) and cool ed i n si tu to room temperature.

A 0. 0025 M sol uti on of the sorbate (pyri di ne i n cycl ohexane for the determ nati on



161

of CA or benzoi c aci d i n methanol for the determ nati on of CD) was passed over

the catal yst unti l surface saturati on was reached, whi ch was i ndi cated by a steady

sorbate concentrati on i n the col umn output. The catal yst sample was washed by

200 m of sol vent to remove the weakl y bound sorbate and the remai ning sorbate was

quanti tati vel y desorbed by a 0. 05 M sol uti on of n-butyl amne i n cycl ohexane

(measurement of CA) or by a 0.05 M sol uti on of aceti c aci d i n methanol (measurement

of ~8). Sorbate concentrati ons i n the col umn output were conti nuousl y measured

by means of a UV detector 254 nm (Development Workshops, Czechosl ovak Academy of

Sci ences, Prague) whi ch was connected to a l i near recorder. The overal l amounts

of the adsorbed and di spl aced sorbate were determ ned from the mass-bal ance of

the sorbate. The overal l amount of the adsorbed sorbate corresponded to the

overal l si tes concentrati on APY

and ABa and the amount of the di spl aced sorbate

to the f racti on of strong si tes cA and ~6.

Catal yti c tests

Depol ymeri sati on of paral dehyde (1, 3, 5- tri methyl -2, 4, 6- tri oxane) to acetaldehyde,

a typi cal l y aci d catal ysed reacti on [ 17], was chosen for testi ng the aci di ty of

catal ysts. Decomposi ti on of di acetone al cohol to acetone, whi ch i s known to be

an excl usi vel y base catal ysed reacti on [18], was used for testi ng the basi ci ty

of catal ysts.

Depolymeri sati on of paral dehyde was carr i ed out under pressure i n the l i qui d

phase by usi ng a stainl ess steel batch mcro- reactor [19]. The reactor containi ng

the acti vated catal yst s 0. 16 g and 7. 1 m of benzene was heated to reacti on

temperature 105°C and 1 m of a benzene sol uti on contai ni ng 0.25 m of paral dehyde

and 0.12 m of p-xyl ene (i nternal standard) was added through a septum Sampl es

of the reacti on mxtures were anal ysed at 93°C by GLC usi ng f l ame-i oni zati on de-

tector and a col umn packed w th 15% Carbowax 20M on Chromaton N (carri er gas

ni trogen). The reacti on l ed to a deacti vati on of catal ysts; formati on of smal l

amount of crotonal dehyde < 3%was observed.

Decomposi ti on of di acetone al cohol i n the l i qui d phase was perf ormed i n a

magneti cal l y sti rred 50 m gl ass reactor. The reactor contai ni ng 34 m of benzene

and about 0.23 g of acti vated al umna was heated to reacti on temperature 70°C

and a mxture of 4 m of di acetone al cohol and 3 m of 1, 3, 5_tri methyl benzene

(i nternal standard) was added through a septum Sampl es of the reacti on mxture

were anal ysed at 76°C by GLC usi ng catharometer and a col umn packed w th 3%

Si l i cone OV-17 on Gas Chrom Q (carr i er gas hydrogen). The reacti on proceeded w th

hi gh conversi ons and w thout deacti vati on of catal ysts. Mesi tyl oxi de < 4% was

formed as by-product.

In both reacti ons the catal ysts were pl aced i n the reactors w thout contact w th

ai r moi sture. Conversi ons of reactants were determned from thei r decrease i n con-

centrati ons w th respect to i nternal standards. The course of reacti ons was



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163

characterised by initial reaction rates at conversions up to 20 . Thermal decom-

position of reactants did not occur in reactions under study.

RESULTS N DISCUSSION

The effect of the a ct iva ti on temperature Tact) on the proper ties of catalysts

is documented by the data in Table 1. The amount of water released from the ca ta

lysts increases with increasing Tact and this leads to a continuous increase in

the surface areas. Only the Cherox 3300 e xhibit s a maximum at 140°C due to the

structure changes at higher temperatures. The concentrations of residual hyd

roxyl groups cOH decrease with increasing Tact The values of cOH are near to

those reviewed for y-alumina by Hendriksen et al. [20].

From Table 1, i t also follows for all catalysts that the increase of Tact en

hances the overall adsorption of pyridine Apy approximately within one order of

magnitude, whereas the amount of displaced pyridine cA

is increased by more than

two orders of magnitude. The proportion of strong sites t herefore i nc re as es , in

accordance with the literature data [3-5]. The concentrations of basic sites ex

hibit a completely different tendency. Whereas the overall adsorption of benzoic

15rp

mol i1

1

5

o 4 6

act

FIGURE

1 The effect of the ac ti vat ion temperature Tacs) on the catalyt ic act ivityof alumina in the decomposition of diacetone alcohol rDAA

circle points) and

in the depolymerisation of paraldehyde r ~ A square points . [ ] Cherox 3300, Pural S, l prep



164

acid ABa does not change too much the amount of displaced benzoic acid cB

slowly

decreases except for the Cherox 3300). It is noteworthy that also unactivated

catalysts show high concentration of basic sites. Similar result has been obtain

ed by titration with bromthymol blue pKa = 7.2) by Yamadaya and coworkers [7J.

A presence of strongly basic sites is qualitatively evidenced by adsorption of

1 3 5 trinitrobenzene pKa

= 14.4) from a benzene solution. The intensity of

alumina colouration decreases for all catalysts in the order of Tact: 4 > 3 ~550»20°C. Strongly basic sites are therefore present in a significant amount on

the catalysts activated at 140-550°C.

The different effect of Tact on the rate of acid catalysed depolymerisation

of paraldehyde r ~ A and base catalysed decomposition of diacetone alcohol rgAA

is presented in Figure 1. Whereas increases markedly for all alumina samples

with increasing Tact maximum value of rgAA is exhibited by alumina samples acti-

vated at 140°C. In the depolymerisation of paraldehyde on al l catalysts the values

of can be c or re la te d with concentrations cA Figure 2). The concentrations of

these active sites which are expressed by the number of pyridine molecules dis-

placed per 1 m2, correspond for Tact

= 500°C to 6-8 x 1017m

2, a value which is

near to concentration of sites found by Knozinger [21J for the isomeris ation of

mo n

a

FIGURE 2 The dependence of catalytic activity of alumina in the depolymerisation

of paraldehyde r ~ A on the concentration of acid sites cA).

Cherox 3300,

Pural S, Al prep The numbers denote the activation temperatures of catalysts.



165

2

IO3

mol Ii’ ti2

1

0

140

140

5300300

9550 550

082 014 086cB pmol mB2

FIGURE 3 The dependence of catal yti c acti vi ty of alumna i n the decomposi ti on

of di acetone al cohol (r: AA) on the concentrati on of basi c si tes (c, ). ural S,

@ *' prep.The numbers denote the acti vati on temperatures of catal ysts.

' 2, 3-di methyl - I -butene. Al though the aci d si tes concentrati ons obtai ned for i n-

di vidual Tact do not di f fer very much, the catal ysts di spl ay consi derabl e di f fer-

ences i n the acti vi ty. That may be caused by vari ous trace i mpuri ti es such as

cati ons of other metal s ori gi nati ng f rom the preparati on of catal ysts. The val ues

Of +A*correl ate l i nearl y w th the concentrati ons c5 onl y for Pural S and Al

prepcatal ysts acti vated at 140-550°C (Fi gure 3).

Taki ng i nto consi derati on that the dependence of r i AA on c5 does not pass through

the zero val ue, we can not excl ude the possi bi l i ty that the measurement of basi ci ty

by the sorpti on method i nvol ves a certain proporti on of catal yti cal l y i nacti ve

si tes.

Figures 4 and 5 show the dependences of t -E* and ri AA on the concentrati ons of

resi dual hydroxyl groups co, , of the catal ysts, demonstrati ng cl earl y the ef fect

of hydrati on on acid-base properti es of al umna. As i t can be seen, the i ncreasi ng

dehydrati on of al umna surface l eads to a rapi d i ncrease i n the rate of the acid

catal ysed depolymeri sati on of paral dehyde ( Figure 4). In agreement w th sorpti on

measurements, thi s evi denced that aci di ty of al umna i s connected w th the de-

hydrated part of the surface and that chemsorbed water i n thi s range of surf ace

coverage behaves as a catal yti c poi son of aci d si tes. I n thi s range of surf ace



166

mo I t-i’n-i2

4-

2-

0_

0 4 8 12- 18

cOH number m2 10

FIGURE 4 The dependence of catal yti c acti vi ty of al umna i n the depolymeri sati on

of paral dehyde ( ' ; A) on the concentrati on of resi dual hydroxyl groups (CO, ).

Cl Cherox 3300 m Put-al S, m Al prep. The numbers denote the acti vati on tem

peratures of catal ysts.

coverage, the hydroxyl groups act as basi c si tes for decomposi ti on of di acetone

alcohol at the same ti me. The rate of thi s reacti on i ncreases w th surface con-

centrati on of hydroxyl groups (Fi gure 5). The hi ghest acti vi ti es are possessed

by catal ysts acti vated at 14O C, thei r surface coverage by hydroxyl groups i s

about of a monolayer capaci ty. Thi s capaci ty i s assumed for y-al umna to be about

12. 5 x 10180H mW2 [ZO]. The hi gher content of sorbed water excessi ng thi s capaci ty

decreases the concentrati on of strong si tes and therefore the catal yti c acti vi ty

ow ng to the occupati on of hydroxyl groups. Thi s i s the reason of negl i gi bl e

retroaldol i sati on acti vi ty of unacti vated Cherox 3300 and Pural S catal ysts whi ch

contain over 20% water, i n contrast to unacti vated Alprep

catal yst w th smal l er

water content (Tabl e 1). Thi s l owwatercontentdoesnot suf f i ce to occupy al l

basi c OH groups and therefore thi s catal yst posses rel ati vel y hi gh acti vi ty

(Figure 1). However, the concentrati ons of basi c si tes cB obtai ned by the sorpti on

method do not exceed 10% of the number of present hydroxyl groups. I t i s possi bl e,

that the real number of strong basi c hydroxyl groups i s represented by a smal l

f racti on, whi ch i s proporti onal to the overal l concentrati on of hydroxyl groups.



167

2

Ololll

o 4 8 2

- 2 8COH

number m

FIGURE 5 The dependence of catalyt ic act ivi ty of alumina in the decomposition

of diacetone alcohol rg

AA

on the concentration of residual hydroxyl groups

cOH) Cherox 3300, Pural S, t Al prep The numbers denote the activation

temperatures of catalysts

It follows from the above results that acid-base properties of alumina depend

markedly on the degree of surface hydration and they can be influenced by acti-

vation temperature in a broad range. For all studied catalysts independently

of their different origins i t was found that acidity of alumina increases with

increasing surface dehydration whereas maximum basicity is reached when alumina

surface is completely hydroxylated.

ACKNOWLEDGEMENTS

The authors express their thanks to Dr. L. Beranek for providing samples of

commercial catalysts and to Dr. D Tomanova for determination of surface areas

of catalysts

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