EXAFS studies of catalyst generation

EXAFS studies of catalyst generation

175 Catalysis Todw,9(1991)175-182 ElsevierSciencePublishersB.V.,Amsterdam-PPrintedinTheNetherlands EXAFS STUDIES OF CATALYST GENERATION J.F.W. MOSSEL...

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175 Catalysis Todw,9(1991)175-182 ElsevierSciencePublishersB.V.,Amsterdam-PPrintedinTheNetherlands

EXAFS STUDIES OF CATALYST GENERATION J.F.W. MOSSELMANS, J.M. COREER, J. EVANS, J.T. GAUNTLETT and J.M. RUWMEY Department of Chemistry, The University, Southampton, SO9 5NIi, England. SUMMARY

Two separate catalytic systems, supported molybdenum olefin metathesis catalysts and chromium pillared clays, have been studied by X-ray absorption spectroscopy @AS). Mo2 (OAc),,-derived catalysts are shown to contain carbon atoms at Q& 1.85 A (possiblyalkylidene groups) and tethering oxygens at $j& 2.05 A. The pillared clays initially consist of chains of &-edge-bridged Cro, octahedra, after calcination some tetrahedral chromium(V1) is present but after hydrogen reduction the pillars are disordered clusters of Cr04units.

-INTRODUCTION EXAFS (extended X-ray absorption fine structure) is now a wellestablished technique for determining the local structure around a central absorbing atom (ref. I), while XANES (X-ray absorption nearedge structure) can prove useful in determining local atomic geometry (ref. 2). In this work two types of system have been studied by XAS: alumina-supportedmolybdenumolefinmetathesis catalysts and chromium pillared clays that catalyse methanol dehydration. The molybdenum systems are active at room temperature (293 K} enabling XAS spectra of the reaction to be taken in situ. However, the methanol reaction occurs at 573 K. At this temperature the vibrational energy is such that XAS spectra are distorted by large Debye-Waller factors significantlyreducing the possibilityof obtaining useful structural data from them. Thus XAS data has been recorded at room temperature on the clays. Olefin metathesis has been catalysed by supported-metalcatalysts for many years. Using N.M.R., homogeneous catalysts have been characterised supporting the Chauvin mechanism (ref. 3) which involves the transformationvia a metallacycle or alkylidene to one another (ref. 4). The supported catalysts have been less wellcharacterised,partly because of the harsher conditions in which they operate. Science ~20-~1/91/$03.50 0 1991Elsevier

Publishers

B.V.

176 Walton et al carried out X.P.S. studies on supported catalysts from dimolybdenum tetra-acetate, but these proved

derived

inconclusive

(ref. 5). Iwasawa et al have studied catalyst's formed

from the interaction of ally1 molybdenum compounds with silica and alumina (ref. 6). However, their XAS studies were of species after high temperature reduction or oxidation, as their techniques did not enable them to study the highly unstable species formed initially. Two studies have examined the catalytic properties of chromiumpillared clays in respect to the dehydrogenation of cyclohexane (ref. 7) and

the

conversion

of methanol

to

olefins

(ref. 8). Little

detailed structural information has been published about pillared clay catalysts. Powder X-ray diffraction can give the inter-layer spacing of the clays; EDX, Ultra-Violet

spectroscopy,

and B.E.T.

surface area techniques have also been used to examine these clays, but

these

provide

scant

information

about

the

structure

of the

pillars. THE MOLYBDENUM CATALYSTS Exoerimental The molybdenum catalysts were prepared using Aluminium Oxide C (Degussa AG) which was calcined in air at 823 K for 18 h, then heated to 473 K for lh compound,

b

either

vacua prior to the introduction of the molybdenum in a solution of diethyl

ether

{Li,MozMea.4Et20,

Mo,C1,[P(n-Bu)S],and Mo$l,(SMe,),) or by grinding the alumina and the molybdenum compound together in a dry nitrogen glove box (Mo,(OAc), and Mo,Cl,py,) . The catalysts were activated by being heated in vacua for 1 h at the activation temperature, then they were cooled prior to the introduction of 40 mbar of propene. XAS

experiments

were

carried

out

in

a

stainless

steel

cell

previously described (ref. 9). The molybdenum samples were placed in the cell in a nitrogen dry box, and the cell was then transferred to the beamline. Full spectra of the catalysts prior to the introduction of propene

and under

propene were

recorded.

In addition,

XAWES

spectra were also recorded during the activation for the Moz(OAc), and [Mo$4e,]4-. The molybdenum K-edge spectra were recorded on Station 9.2 of the S.R.S., at the S.E.R.C. Daresbury Laboratory, employing a doublecrystal Si (220) monochromator, using 50% harmonic rejection, in fluorescence mode. All the XAS spectra were background subtracted using the IBM PC resident program PAXAS (ref. 10); curve fitting was performed using

177

EXCURV90 (ref. 11). Owing to the similaritiesin their backscattering properties,

distinguishing between carbon and oxygen atoms is not

always possible; one of the datasets could be fitted eguaily well with either a carbon or an oxygen shell. Errors in the bond distances

dilute samples; because the systems' disorder affects the amplitude of the EXAFS the error in the coordination are

2% for these

numbers is estimated at Itl.

Figure 1. XANES of fMo&4eaJb‘ -derived catalyst during activation at 573 K. (Spectra are taken at seven minute intervals).

TABLE 1 Catalytic results for the molybdenum catalysts. Precursor Mo~(OAC)~

activation temperature /R 573

turnover no. / min" 0.3

343 0.2 448 0.1 573 0.03 473 (Turnover number is number of propene mo&%es molybdenum atom per minute.)

Li~Mo~e~.rlE~O MozCl,CP(n-W,lS Mo,Cl,py, Mo$l,We2),

converted per

pesults and Discussion The catalytic results for the systems (Table 1) show that when activated at the optimum temperature the Mo~(OAC)~ and [Mo$4e,]“ systems display the same order of activity as the allyl-molybdenum derived

system8

precursors inconclusive

used

studied produced

by

Iwasawa

less

#z

active

EXAFS results, which

a.& (ref. 12). The catalysts

and

other

also

suggested more disorder

some

in the

1'78 chloride-derived catalysts. The XANES of the Moz(OAc),-derived catalyst

shows a definite change

during the activation as a pre-edge peak grows (ref. 9). The XANES of the [Mo$eJ4--derived

catalysts does not show much change during

the activation process (Figure 1); as it is an active catalyst when activated at room temperature, this is perhaps not so surprising. The systems were studied by XAS on two separate samples to try to ensure a degree of experimental reproducibility in the experiments. The pre-propene data for the Mo,(OAc),-derived catalysts proved to be insoluble for both experiments. An R factor of 40% was not achieved either;

this

suggests

environment.

Both

spectra

for

a

large

however

disorder have

a

in

the

component

molybdenum at

high

k

indicating, perhaps, molybdenum neighbours between 2.7 and 3.0 A. TABLE

2

EXAFS

results

for the

Moz(OAc),-derived catalysts

after

exposure to 40 mbar propene Atom Shell Radius/A Coordination Debye-Waller Number Factor1 A2 Type J_, 0.52% MO' C 1.88 2.0 0.013 0 2.06 2.0 0.003 C 2.28 3.2 0.012 II. 0.60% &fQ C 1.81 0.8 0.004 0 2.04 1.6 0.008 C 2.26 1.9 0.007 C 2.64 2.5 0.017' C 2.98 2.3 0.008 MO 3.01 a) Fourier window l.3-3.0°iy R 24.8 %; b) FouiieDr12window0.6-3.7 A,

The in situ post-propene data show no signs of molybdenum atoms within

bonding

distances.

Both experiments

show carbon

distances suitable for a molybdenum-alkylidene

atoms at

link and tethering

oxygen links of ca. 2.05 fi. There is one published structure of a molybdenum-alkylidene, Mo(CH-t-Bu) (NAr) (OSO,CF,),(dme) (Ar = 2,6diisopropylphenyl, dme = 1,2_dimethoxyethane) (ref. 13), and the geometries of a number of tungsten alkylidenes have been determined by x-ray crystallography

(ref. 14). The former has a molybdenum-

carbon distance of 1.90 A and two types of oxygen ligand at distances of

g&

2.10

(anionic)

and

2.31

A

(neutral),

while

the

molybdenum-nitrogen distance is 1.72 A. The more dilute sample EXAFS could be fitted satisfactorily by a three-shell model (C, 0 and C); the other sample, however, afforded evidence for six shells. The statistical

significance

of these shells

179 has been validated by two methods. Initially, as a coarse test, the integral of the EXAFS due to each shell and thus its contribution to the total EXAFS is calculated using the 'Fitstatt option in EXCuRV90. Finally and more rigorously the statistics test of Joyner et 15) was

applied

as

each

shell

was

added.

One

of

(ref.

these

shells

indicates the presence of a molybdenum atom at around 3.0 A. This is too long for a metal-metal bond but could be the result of an oxygenbridged species. The inner three shells fit as either three oxygen shells or the chemically more plausible

C, 0, C arrangement. The

shell at 2.58 A is difficult to assign, it is too long for a direct bond but implies an angle of around 100' at the carbene carbon; it may be part of a metallacyclobutane ring. The carbons at 2.98 A give an angle of 126' at the carbene carbon which is more reasonable. Hence there are indications of alkylidene groups in the major species present in the active catalyst. The actual structure of the catalyst may thus be precursor-dependent, although the pre-edge peak in the XANES seems to indicate significant distortion from a centrosymmetric species. THE CHROMIUM CLAYS Exverimental The

clays

were

pillared

Cr(N0,),/Na,(C03), heated

at

using 368 K

an for

aqueous 36 h

solution

prior

to

of

O.lM

use.

Then

sufficient clay to give 12 mm01 Cr/ meq clay, of particle size less than 2pm3 and predispersed

in water

(O.O3g/ml), was added and the

mixture stirred for 1.5 h. The clay was separated in a centrifuge and air-dried at 323 K. After drying two further treatments were applied to the clay. It was calcined in air or vacuum at 523 K for 2 h. After calcination they were hydrogenated in a stream of hydrogen for 2 h at 523 K. The do01 -spacings, as determined by powder X-ray diffraction, of the pillared clays are ca. 17 A for bentonite and beidellite samples and suggest the layers are being separated by one Cr04 unit.(A value for laponite could not be obtained, indicating delamination.) Catalysis experiments were performed at 473 K in a sealed vessel for 2 h

Using

0.1

ml of methanol and 0.1 g of clay. The reaction

products typically consisted ca. 10 0 methane, 45 % Ct-C4 and 45 b G,C, including branched, linear and cyclic alkanes and dimethoxymethane and dimethoxyethane. Station

7.1

monochromator

The chromium K-edge spectra were recorded on the S.R.S. using a double crystal Si(ll1) detuned to give 50% harmonic rejection. The clay

of

180 samples were diluted with boron nitride to 10% Cr and held between 'Sellotape'

in

aluminium

sample

holders.

Data

was

collected

in

transmission mode except for the solution, for which a thallium-doped sodium

iodide

scintillation

counter

was

used

to

record

the

fluorescence spectrum. k/A-’

12F

Fig. 2. k3-weighted EXAFS (top) and Fourier transform (bottom) for Cr-pillared beidellite calcined in vacua (experimental,- -theoretical). Besults and Discussion The chromium environment of the pillared clays has been examined by XAS in four different states, after pillaring, after calcination in air

or &I

vacua

and after calcination

in

vacua

followed

by

reduction with hydrogen. The results of the EXAFS analyses are listed in Table 3 for the most active sample based on beidellite. Moreover spectra

of

the

reduced

catalyst

after

use

showed

the

chromium

environment to be unaffected by the methanol dehydration. The reduced pillared beidellite is about 6 times more efficient than untreated beidellite

in the

catalysing

the methanol

dehydration

reaction.

Initially the chromium in the clay is in an octahedral environment

181

with a mean chromium-oxygen distance of 1.97 A with two close chromium atoms at 2.97 A (Figure 2). This is an environment essentially unchanged from that of the pillaring solution. This is typical of edge-linked octahedral units. The ratio of the two r(Cr...Cr) distances indicates that the polymeric chromium species are &-linked octahedral sites. After calcination in air a small proportion of the chromium is tetrahedrallycoordinated with shorter chromium-oxygen distances of 1.64 A. This part of the chromium has been oxidised from Cr(II1) to Cr(V1). The proportion is dependent on the type of clay with the order laponite > bentonite > beidellite. The reduced clay is all octahedral with a mean chromium-oxygenbond length of 1.97 AL,though now there are about three close Cr atoms at 2.95 b. TABLE 3. EEAFS results for the chromium-pillaredclays. Atom Interatomic Sample Coordination Debye-Waller Tvne Distance (Ait Number Factor (A2) 0 Pillaring* 1.97 6 0.011 2 0.016 Cr 2.97 Solution2 0.024 Cr 3.89 0.5 0 1.65 0.010 Beidelliteb 4.5 0.012 0 1.98 after air 2 0.010 Cr 2.95 calcination 2 0.017 Cr 3.95 6 0.016 0 1.95 Beidellite= 2 0.016 Cr 2.93 after vacuum 1 0.025 Cr 3.90 calcinatfon 6 0 1.96 0.012 Beidellited 3 0.013 Cr 2.95 after reduction 3.89 0.023 a) R 28 O; b) R"i5 %; c) R 14 %; d)'R 15 %. The d,, spacing does not change during these treatments. Assuming the octahedral units are initially arranged in zigzag chains, the EXAFS-derived coordination number of two for the closest chromium shell implies that the pillars are chains containing at least 8-10 chromium atoms [using the formula (2n-2112, where n = mean chain length]. The chromium coordination number of three for the hydrogenated species suggests a more two-dimensional pillar containing a larger number of chromium atoms. However the number of atoms in the second chromium shell is too small for any model with three chromiums in the first shell. This suggests there has been a large disordering in the structure of the pillars caused by the calcination/reduction, as in EEAFS significant disorder leads to shells not being seen or having severely reduced amplitude to that expected.

182 The study shows that the environment of most of the chromium is essentially unchanged after the oxidation, though some tetrahedral chromium

is formed. After

reduction

the pillars

appear

to have

changed from being chains into clusters. Hence the improved catalytic performance may be due to a greater number of sites being available after these treatments. How these processes affect the availability of BrCinsted acid/base sites are probably critical in determining the relative catalytic activity of the clay. ACKNOWLEDGEMENTS We wish to thank the S.E.R.C. for financial support, studentships (J.M.C., J.F.W.M, J.M.R.) and access to the facilities of the S.R.S.. Thanks also go to the B.P. Research Centre for support to J.M.C.. We are

grateful

to

the

laboratory's

staff

for

assistance

with

our

experiments. We acknowledge the help of Dr. N.A. Young and R.J. Perry in recording XAS spectra. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15.

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