FTIR study of interaction of Re2(CO)10 with silica and alumina supports

JournalofMoleculnr Structure,

174(1988)331-336

331

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

FTIR

STUDY

OF INTERACTION OF Re2(CO),9

WITH

SILICA

AND

ALUMINA

SUPPORTS

S. Dobos, A. Beck, and L. Guczi Institute of Isotopes, Hungarian Budapest, P.O.B. 77, Hungary

Academy

of

Sciences,

H-1525

ABSTRACT Re2(CO),9 adsorbed on hydrated alumina and silica supports was found to be bonded to the surface by weak physisorption. On silica no surface reaction was detectable. On alumina support a slow reaction with the surface occured, producing (OH)4Re4(CO) very stable surface molecule of catalytic activity, bond&a' b; multiple hydrogen bonds to the support. INTRODUCTION Catalysts consisting of highly dispersed rhenium

supported

on

high area oxides are known to be active in the hydrogenation of CO (refs. l-2). Kirlin et al. silica

rhenium

supported

prepared

by

the

recently reported an active and stable

thermal

for

catalyst activation

of

alkane a

metathesis,

surface

complex

(OH)4Re4(C0),2(O;lerived from co-ordinatively H4Re4(C0).,2, a cluster, in presence of water (ref. 3). Due to

unsaturated Re the special

effects

of

cluster-support

interaction,

supported

metal catalysts derived from carbonyl clusters are a new class

of

catalysts, often having activities several times as great as their conventional

analogous.

The

characterization of interaction

present

paper

of

Re2(CO)10 silica and alumina by infrared spectroscopy.

deals with

with

the

hydroxylated

EXPERIMENTAL Alumina and silica were obtained CABOT CORPORATION (Cab-0-Sil HS5), vacua at 573 K for 16 hours (by this

from

DEGUSSA

respectively, treatment

(Alon and

the

C)

heated alumina

and in and

silica still remained hydrated: hydroxile monolayer coverage about 60% (refs. 5-6)), Re2(CO).,0.

The

then contacted with supported cluster was

0022-2860/88/$03.50 0 1988Elsevier Science Publishers B.V.

the pentane solution of dried under vacuum.

332 Fig.

Infrared

1.

Re2(CO),O

v,

Calculated

supported wafers cell.

loadings

the

ranging

cluster

from

was dried which

recorded.

spectra,

303

(n-pentane,

to 700 K.

equipped

wafers

were

of CO

vacuum

solutions

and

were

recorded

of

a

pellet

into

vacuum

ir

at temperatures

Re2(CO)10

KBr

on the

and pressed

in a heatable

in

have

solutions also

been

200-scan data accumulation -1 , using a DIGILAB FTS-20C

of 4 cm

NOVA

used as references.

corrected

and 1 and 2% experiments

for 6 hours

Spectra

stretching

baseline

modes

To analyse

of

was fitted

3 computer.

the

the

surface

to the data

Silica spectra reaction

points

of the

spectra.

AND DISCUSSION

Spectra spectra

of solutions

recorded

supported spectra

during

on silica recorded

Re2(CO),0 forming

1% on silica

with DATA GENERAL

the sum of Gaussians

bands

(C) in KBr pellet.

then placed

reported

product,

RESULTS

and

in vacuum

were

out at a resolution

in the region

(B)

dichloromethane

decomposition

under

For all spectra

interferometer

in (A) n-pentane,

in

about

dichloromethane)

was carried

or alumina

were

temperature

(10 mg/cm2), Infrared

of

-1

cm

metal At

alumina.

spectra

should

are quite

room

are

state

crystallites:

stay close

referring

that

in

Fig.

in vacuum

shown

in

of

However,

of

rather the

molecules the two

the molecules

2A-C.

suggest

on the support

of the

1.

must

The

Re2(CO)10

Figs.

(303 K)

the positions

to those

2014 and 1976 cm-').

broadened

are shown

treatment

temperature

be in adsorbed

well defined

(2070,

thermal

and alumina at

essentially

solvents

and KBr pellet

The that than

spectral in

polar

last

bands

be distorted

473K 573K 743K

(D) I

2100

I

1900

(El I

2100

I

1900

(F) I

I

!lOO

u , cm

7 900

-1

Fig. 2. Infrared spectra recorded during thermal Ke3(CO),a supported (A) on silica (metal loading aliimina'lmetal loading 1%) and (C) on alumina (metal Characteristic difference spectra (D,E,F) calculated respectively.

treatment of 1%); (B) on loading 2%). from A, B, C,

334 TABLE 1 Numerical data

of

the

analysis

fitting

by

Gaussians

to

the

experimental spectra of Fig. 3. T(K) Frequencies(cm-') A1

Ea

Eb

Intensities(arb.un.)

Int. ratios

ItA,) I(Ea) I(Eb)

I(Ea+Eb) I(A, 1

FEZ+

I

Ee,(CO),O(l%)/A1203 473 573

2033 2034

1923

1922

1874 la79

52.4 45.4

85 72

279 245

4.2 4.4

0.6 0.6

249 227 206 174

384 356 321 268

4.6 4.8 4.5 4.4

3.8 3.4 3.6 3.9

142 128

2%)/AI203 473 523 573 623

2031 2032 2032 2033

1926 1926 1926 1925

1891 la92 1695 1899

68.5 61.6 58.5 49.3

65.9 67.8 56.9 44.8

I

I

I

I

2lbO

1900

2100

1900

I

2100 v, cm -1

I

1900

Fig. 3. Expanded spectra (B' ,C') of Figs. 2B and 2C, with Gaussian components.

on the surface. By heating the samples, an interesting phenomenon takes as manifested in the difference spectra of Figs.

2D-F.

all, as for instance the difference spectrum of

373-323

side

quite clearly, the lower frequency

of

place,

First K

of

shows

the

spectrum is a -1 resultant of a decreasing broad band centered at 2014 cm and of centered

an increasing narrower one

at

the

same

frequency.

similar behaviour holds for the band centered at about 1976 It is obvious

there

that

is

an

interconversion

taking

between differently perturbed molecules of Re2(CO).,0: the decreasing bands belong to more perturbed molecules, the basis of the band crystallites,

or

shapes

rather

crystallites

(=

crystallites

decompose

-

be

high

as

fragments

may

of

when

as

dispersion

unit

cells).

These

and

shapes

-

to

those

measured

in

"Physisorption" should supposingly mean hydrogen

on of

incipient at

393

physisorbed molecules with spectral bands very similar frequencies

-

incipient

fragments

producing

heating,

place broader

which

regarded

A

cm-'.

in

K

both

solutions.

bonding

between

carbonyls and surface hydroxo groups: the appearance of the band -1 , which is inactive in infrared

of A., symmetry at about 2130 cm

at the unperturbed molecule, indicates a lowering of the molecular symmetry. At T>423 K, on silica, no carbonyls, consequently no

Re2(CO).,0

or its carbonylic reaction product has been detected. This fact is rather surprising. As learned from thermogravimetric

experiments,

the weight loss midpoint (= temperature at which 50%

of

observed

weight loss of step had occured) for Re2(CO),0 is at 483

K (ref. 4), while supported Re2(CO),0 in our case has left the surface at

a temperature 100 K lower. That means the

surface

molecules

are

linked to the surface by weaker forces than in crystal. On alumina

(Figs.

2B

and

2E)

Re2(CO),0

practically

similar patterns, the only characteristic difference

is

shows

that

at

higher metal loading (2%, Figs. 2C and 2F)

Re2(CO),0 leaves the surface at somewhat higher temperature. However, on alumina above 423 K, after Re2(CO),0 has left

the

surface,

visible: a singlet (A,) at 2032

and

a

new bands become doublet (Ea+Eb) between

1926-1874 cm -1 , as shown on expanded spectra in Figs. 3A

and

3B.

In Table 1 we collected the frequencies, intensities and intensity ratios for the component Al , Ea and

Eb .

Both

the

singlet

doublet are stable up to very high temperatures T=623 K). basis of frequencies,

unusual

thermal

stability

and

On

and the

intensity

ratio

I(Ea+Eb)/A,

A., + E modes thermally

= 4.5

of the complex

activated

or reactive mode might

hydroxo

refer

hydrogen

bonding

between

those

silica

ratio

is estimated

that

(OH)4Re4(C0),2,

opens,

suggests between

supposingly

hydrogen

type

of

group.

is

quite

in comparison

this hypothesis.

E

mode

Re(C0)3

(0.60),

and surface

bondings

the

It is

molecules

to more

bonded

can occur

through

hydroxiles.

Adducts

component

Eb

loading

(I(Eb/Ea)

=

estimated

simply

to

from the spectra

that

that on alumina, the C-O bonds

lower:

soluted

of

the

higher when

at

THF,

The

Fig. onto same

suggesting

silica,

results

adduct

with

value

of 4.8

the complex

carbonyls

in

brought 2.8.

in

on

in the Re(C0)3

the

surface

is

adsorbing

(ref. 7). The much

because

are 3.4-

concentration

(OH)4Re4(C0).,2 directly

in n-octane,

when

with

spectral

metal

for the molecules

analogous

[Re(C0)30H.THF]4 our case

the

= 0.6).

I(E/A,)

its solution

value

the angle

water

of

I(Ea/Eb)

to Ea are of greater

(I(Eb/Ea)

3, in the case of

from

structure

produce

assigned

loading

The intensity 1 of ref.

which

in the case of higher

while

the

to the surface

carbonyls

surface

the

by hydrogen bonding between the hydroxo groups 2and surface 0 anions etc. It is very interesting,

that the molecules

lower metal

of

ratio

to two

Link

and

to

of

of the degenerate

loading

not support

bands

product

The splitting

symmetry

intensity

the

formed

of the complex

-3.9),

does

to the surface.

may be also

preferred

the

that Ea and Eb belong

differently

the

Re2(CO),8

on alumina.

in the case of 1% metal

that of 2% (3.4-3.9) probable

we assign

to the splitting

of C3v local

the fact that

different

between

groups

formally

value),

(OH)4Re4(C0)12,

reaction

due to the lowering However,

(averaged

group

enter

forms

found

a THF in

in situ,

considerably into

multiple

hydroxiles.

REFERENCES 1 2

3

Ogino, J. Chem. sot., Chem. M. Komiyama, T. Okamoto and Y. 618-619. Commun ., (1984) T. Tsunoda, H. Ogasawara, M. Komiyama, S. Ozawa and Y. Ogino, Chem. Lett., (1981) 819. Gates, J. Chem. Chem. P.S. Kirlin and B.C. sot., Commun ., (1984)

4 5 6 7

277-279.

L.M. Fillman and S.C. Tang, Thermochim. Acta, 75 (1984) 71-84. Guczi, Chim. S. Dobos, I. Boszormenyi, J. Mink and L. Inorg. Acta, 120 (1986) 135-143. Inorg. Chim. S. Dobos, I. Boszormenyi, J. Mink and L. Guczi, Acta, 120 (1986) 145-152. Gzbelein, Chem. M. Herberhold, G1 S&s, J. Ellermann and H. 2931-2941. Ber., 111 (1978)