synthesis of sio2 xerogels and pd/sio2 cogelled xerogel catalysts from silylated acetylacetonate...

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Journal of Non-Crystalline Solids 343 (2004) 109–120

Synthesis of SiO2 xerogels and Pd/SiO2 cogelled xerogelcatalysts from silylated acetylacetonate ligand

Stephanie Lambert a, Luigi Sacco b, Fabrice Ferauche a, Benoıt Heinrichs a,*,Alfred Noels b, Jean-Paul Pirard a

a Laboratoire de Genie Chimique, B6a, Universite de Liege, B-4000 Liege, Belgiumb Centre d’Enseignement et de Recherche des Macromolecules (CERM), B6a, Universite de Liege, B-4000 Liege, Belgium

Received 7 October 2003

Abstract

SiO2 xerogels and Pd/SiO2 cogelled xerogel catalysts have been prepared in a mixture of tetrahydrofurane (THF) and ethanol

containing tetraethoxysilane (TEOS), and an aqueous ammonia solution of 0.18mol/l, from synthesized new silylated acetylaceto-

nate ligands, respectively, 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione (MS-acac-H), 2,2,6,6-tetramethyl-4-[3-(trimethoxysi-

lyl)propyl]-3,5-heptanedione (MS-dPvM), and 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedione (MS-dBzM), able to

form a chelate with a metal ion such as Pd2+. All samples form homogeneous colored gels. The resulting catalysts are composed

of palladium crystallites with a diameter of about 3.5nm, located inside primary silica particles exhibiting a monodisperse micro-

porous distribution as well as large palladium particles from 20 to 50nm, situated outside the silica aggregates. The silylated organic

ligand has a strong influence on the textural properties of xerogels and catalysts, both before and after calcination and reduction

steps. Changing the nature of the silylated ligand permits tailoring textural properties such as pore volume, pore size and surface

area. Although small palladium crystallites are located inside the silica particles, their complete accessibility, via the micropore net-

work, has been shown. 1,2-Dichloroethane hydrodechlorination over Pd/SiO2 catalysts mainly produces ethane and the reaction

rate increases linearly with palladium dispersion. Hydrodechlorination over Pd/SiO2 cogelled xerogel catalysts is a structure insen-

sitive reaction compared to the ensemble size concept.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Viable preparative routes to silylated organic mole-

cules initially emerged in the 1960s, largely pioneered

by a group at Degussa [1]. Development of these com-

mercial-scale procedures was originally driven by the

interest in the bifunctional reagents as adhesion promot-

ers for inorganic–organic polymer composites such as

glass fiber-reinforced materials and mineral-filled elas-

0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2004.07.049

* Corresponding author. Tel.: +32 4 366 3505; fax: +32 4 366 3545.

E-mail address: [email protected] (B. Heinrichs).

tomers. Further developments arose from the realization

that attachment of catalytically active metal complexesto silica gels via silylated ligands afforded supported

complexes, whose properties combined the positive attri-

butes of both heterogeneous and homogeneous catalysts.

Immobilization of a sylilated ligand or metal complex

during a sol–gel reaction is typically a straightforward

manner of introducing the metal complex into the sol–

gel mixture, either at the same time as other reagents

or after the other precursors have been allowed to reactto form a sol. One of the most common reasons cited for

the use of sol–gel processing is the intimate molecular-le-

vel mixing of components that can be obtained at the

outset of the reactions [2,3]. In this way, several authors

mailto:[email protected]

110 S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120

developed the cogelation sol–gel process to disperse me-

tal particles in a silica matrix [4–6]: organically substi-

tuted alkoxides of the type (RO)3Si–(CH2)3–A in

which an organic amine group A, able to form a chelate

with a cation of a metal such as palladium, silver, cop-

per, etc., is linked to the hydrolysable silyl group(RO)3Si– via an inert and hydrolytically stable spacer

–(CH2)3–. The co-condensation of such molecules with

a network-forming reagent such as tetraethoxysilane

(TEOS), Si(OC2H5)4, results in materials in which the

metal is anchored to the SiO2 matrix. Heinrichs et al.

[7,8] and Lambert et al. [9–11] used this cogelation

method to synthesize mono- and bimetallic aerogel

and xerogel catalysts. By changing the molar ratiobetween the functionalized amino-alkoxide and TEOS,

textural properties such as pore volume, pore size and

surface area can be tailored. Although metal particles

are located inside the silica particles, their complete

accessibility, via the micropore network, after drying,

high temperature calcination and reduction of the gels,

has been shown [7–11].

The main purpose of the present work is to synthesizenew silylated acetylacetonate ligands, able to form a

chelate with a metal ion such as Pd2+ and able to react

with a silica-forming reagent such as TEOS to form

homogeneous silica gels containing highly dispersed org-

ano-metallic complexes. So, after vacuum drying, these

silica xerogels could be used as catalysts in several or-

ganic reactions such as cyclopropanation, Kharasch

reactions, etc. Furthermore, the samples are also cal-cined and reduced to analyze the dispersion, localiza-

tion, and accessibility of palladium and to compare

their catalytic activity to previously synthesized Pd/

SiO2 cogelled xerogel catalysts used for hydrodechlorin-

ation [9,10].

2. Experimental

2.1. Preparation of silylated acetylacetonate ligands and

palladium complexes

Synthesis of silylated ligands and metal complexes

was performed under an atmosphere of dry argon using

standard Schlenk techniques. Argon was purified by pas-

sage through columns of BASF R3-11 catalysts and 4Amolecular sieves. Tetrahydrofurane (THF) was freshly

distilled under argon from benzophenone and sodium

prior to its use. Dichloromethane was distilled under ar-

gon from calcium hydride. Sodium acetylacetonate was

synthesized from acetylacetone (Fig. 1, scheme 1, struc-

ture 1a) according to Charles [12] and dried at a temper-

ature of 100 �C and under a pressure of 0.07Pa. The

above-described synthesis method is also applied to dip-ivaloylmethane (Fig. 1, scheme 1, structure 1b) and dib-

enzoylmethane (Fig. 1, scheme 1, structure 1c).

Acetylacetone functionalized by an alkoxysilane

group is obtained by reaction of sodium or potassium

acetylacetonate with the adequate halogenocompound

in dry dimethylsulfoxide (DMF) or N,N-dimethylform-

amide (DMSO). Indeed, the kind of halogen plays a cru-

cial role in the C- and O-alkylation competition [13].Using (3-iodopropyl)-trimethoxysilane in THF, 1H

NMR and gas chromatography analyses show that the

O-alkylation is drastically minimized to the benefit of

the C-alkylation. This way is also applied to dipivaloy-

methane and dibenzoylmethane (Fig. 1, scheme 1).

Synthesis of 3-[3-(trimethoxysilyl)propyl]-2,4-pen-

tanedione, MS-acac-H (Fig. 1, scheme 1, structure 2a).

To a suspension of 5g of sodium acetylacetonate in30ml of dry THF, 11.9g of (3-iodopropyl)-trimethoxysi-

lane were added. The resulting mixture was stirred under

reflux for five days and the reaction was monitored by

gas chromatography. The solvent was then removed un-

der reduced pressure and the product extracted with dry

dichloromethane. The slightly viscous colorless liquid

obtained by distillation (101–104 �C/0.07Pa) can be

indefinitely stored at a temperature of �15 �C under ar-gon. Yield: 55%.

Synthesis of 2,2,6,6-tetramethyl-4-[3-(trimethoxysi-

lyl)propyl]-3,5-heptanedione, MS-dPvM (Fig. 1, scheme

1, structure 2b). The same procedure described above

was followed to obtain a viscous colorless liquid by dis-

tillation (115–118 �C/0.07Pa) and stored at a tempera-

ture of �15 �C under argon. Yield: 45%.

Synthesis of 1,3-diphenyl-2-[3-(trimethoxysilyl)pro-

pyl]-1,3-propanedione, MS-dBzM (Fig. 1, scheme 1,

structure 2c). The same procedure described above was

followed to obtain a viscous orange liquid stored at a

temperature of �15 �C under argon. Yield: 80%.

Although palladium acetate in the presence of acetyl-

acetone readily gives palladium acetylacetonate in a

wide variety of solvents, this method does not work with

the functionalized diones. A pre-reaction between thedione and potassium t-butoxyde is necessary, which

then yields the palladium complex with 3-[3-(trimeth-

oxysilyl)propyl]-2,4-pentanedione, Pd(MS-acac)2, 2,2,6,

6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptane-

dione, Pd(MS-dPvM)2, and 1,3-diphenyl-2-[3-(trimeth-

oxysilyl)propyl]-1,3-propanedione, Pd(MS-dBzM)2(Fig. 1, scheme 2).

Synthesis of palladium complexes from 3-[3-(tri-

methoxysilyl)propyl]-2,4-pentanedione, Pd(MS-acac)2,

from 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)pro-

pyl]-3,5-heptanedione, Pd(MS-dPvM)2 and from 1,

3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedi-

one, [Pd(MS-dBzM)2]. To a fine suspension of 1.1g of

potassium t-butoxyde in 150ml of THF was added in a

first time, either 3.4g of 3-[3-(trimethoxysilyl)propyl]-

2,4-pentanedione or 3.4g of 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione or 3.4g

of 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-prop-

R

ONaOH

Na+

R

O

Si(OCH3)3Si(OCH3)3 Si(OCH3)3

I

MeOHH2O

Na+

R

O

Na2CO3THF

R

O

R

O OH

Si(OCH3)3

R

O

+

2a, 2b, 2c

1a, 1b,1c

R

O

(H3CO)3Si

Pd(OAC)2KO tBu

THF

R

R

O

O

(H3CO)3Si

Pd 2+

2

2a , 2b , 2c3a , 3b, 3c

Ra -CH3

b -C(CH3)3

c -Ph

R

R

R

R

R

R

O

O

O

O

O

O

R

Scheme 1

Scheme 2

Fig. 1. Synthesis schemes of silylated acetylacetonate ligands and palladium complexes.

S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120 111

anedione and in a second time a solution of 1g of palla-

dium acetate in 10ml of THF. After 20min under stir-

ring, the brown suspension was filtrated and the

filtrate was evaporated under vacuum. A very viscousbrown solid was obtained and stored during three days

at a temperature of �15 �C under argon. The yield of

complexes Pd(MS-acac)2, Pd(MS-dPvM)2 and Pd(MS-

dBzM)2 is 50%. The structures of these silylated com-

plexes are presented in Fig. 1 (scheme 2, structure 3a

for Pd(MS-acac)2, structure 3b for Pd(MS-dPvM)2 and

structure 3c for Pd(MS-dBzM)2).

2.2. Characterization of silylated acetylacetonate ligands

and palladium complexes

1H NMR and 13C NMR spectra were recorded on a

Bruker 400 spectrometer with TMS as internal standard.

Infrared spectra (thin film on NaCl pellets) were re-

corded on a Perkin–Elmer FTIR 1720X. GC analysis

was carried out using a Varian Star 3400CX.

2.3. Synthesis of SiO2 xerogels and Pd/SiO2 cogelled

xerogel catalysts

Sample designation. Three silica xerogels and four Pd/

SiO2 cogelled xerogel catalysts were prepared and their

synthesis parameters are presented in Table 1. Each

sample is named by the corresponding silylated acetyl-

acetonate ligand or palladium complex. The names of

palladium samples are followed by either the lettersTE, for a solvent mixture of THF and ethanol, or by

the letters AE, for a solvent mixture of acetone and

ethanol.

Gel synthesis. Three SiO2 gels were synthesized from

the corresponding silylated acetylacetonate ligand –

MS-acac-H or MS-dPvM or MS-dBzM–, TEOS

Table

1

SynthesisoperatingvariablesofxerogelsandPd/SiO

2cogelledxerogel

catalysts

Sample

nma

(mmol)

nligandb

(mmol)

nTEOS

(mmol)

nH

2O

(mmol)

Solvent

mixture

ndiluantc

(mmol)

nC

2H

5OH

(mmol)

Gelation

behavior

Theoreticalmetal

loading(w

t%)

Weightloss

d±2%

(wt%

)Actualmetal

loading±0.1%

(wt%

)

Pd(M

S-acac)

2TE

0.266

1.57

44.5

230

THF-ethanol

123

343

Black

gel

1.0

16

1.2

Pd(M

S-acac)

2AE

0.273

1.68

45.7

237

Acetone-ethanol

136

343

Black

gel

1.0

14

1.2

Pd(M

S-dPvM) 2TE

0.267

1.59

44.5

230

THF-ethanol

123

343

Greygel

1.0

01.0

Pd(M

S-dBzM

) 2TE

0.255

1.48

44.6

230

THF-ethanol

123

343

Black

suspension

1.0

––

MS-acac-H

–2.93

48.5

253

Ethanol

–514

Whitegel

–0

–

MS-dPvM

–3.12

48.3

253

Ethanol

–514

Colorlessgel

–0

–

MS-dBzM

–2.91

48.5

253

Ethanol

–514

Light-orangegel

–0

–

anmthesilylatedacetylacetonate

palladium

complex,iseither

Pd(M

S-acac)

2,orPd(M

S-dPvM) 2,orPd(M

S-dBzM

) 2.

bnligandthesilylatedacetylacetonate

ligand,iseither

MS-acac-H,orMS-dPvM,orMS-dBzM

.cndiluantthediluantsolvent,isTHForacetone.

dWeightloss

=100·(m

th�

ma)/m

th,wherem

thisthetheoreticalmass

andm

aistheactualcatalyst

mass

measuredafter

drying,calcinationandreductionsteps.


(Si(OC2H5)4) and aqueous ammonia. So a 0.18N NH3

aqueous solution in half of the total ethanol volume

was added, under stirring, to a mixture containing

TEOS, MS-acac-H or MS-dPvM or MS-dBzM, and

the rest of ethanol. The molar ratio silylated ligand/

TEOS must be equal to 0.06 to allow gelification. FourPd/SiO2 gels were prepared from the corresponding sil-

ylated acetylacetonate palladium complex –Pd(MS-

acac)2 or Pd(MS-dPvM)2 or Pd(MS-dBzM)2–, TEOS

and aqueous ammonia. In these syntheses, Pd(MS-

acac)2 or Pd(MS-dPvM)2 or Pd(MS-dBzM)2 was

dissolved in a mixture of 10ml of ethanol and 10ml of

acetone or THF, and in which some amount of silylated

acetylacetonate ligand corresponding to the used palla-dium complex was present to allow gelification. So the

molar ratio silylated ligand/TEOS is equal to 0.05 in pal-

ladium gels. After addition of TEOS, a 0.18N NH3

aqueous solution in the remaining ethanol was added

to the mixture under vigorous stirring. The volume of fi-

nal solutions was 30ml. The hydrolysis ratio, which is

the molar ratio H = [H2O]/([TEOS] + 3/4 [sylilated lig-

and]), and the dilution ratio, which is the molar ratioR = [ethanol]/([TEOS] + [sylilated ligand]) were kept

constant at values of 5 and 10 respectively for all sam-

ples. The vessel was then tightly closed and heated up

to 80 �C for 10 days (gelling and aging [14]).

Drying. The wet gels were dried under vacuum

according to the following procedure: the flasks were

opened and put into a drying oven at 80 �C, and the

pressure was slowly decreased (to prevent gel bursting)to reach the minimum value of 1200Pa after 90h. The

drying oven was then heated at 150 �C for 72h. The

resulting samples are xerogels [14].

Calcination. The calcination conditions for Pd/SiO2

catalysts were as follows: the sample was heated up to

400 �C at a rate of 120 �C/h under flowing air

(0.02mmol/s); this temperature was then maintained

for 12h in air (0.1mmol/s). All dried SiO2 xerogels wereheat treated in air at 450 �C for 72h to remove organic

residues.

Reduction. The Pd/SiO2 catalysts were heated up to

350 �C at a rate of 350 �C/h under flowing H2

(0.23mmol/s) and maintained at this temperature for

3h (same flow).

2.4. Characterization of SiO2 xerogels and Pd/SiO2

cogelled xerogel catalysts

Nitrogen adsorption–desorption isotherms were mea-

sured at 77K on a Fisons Sorptomatic 1990 after out-

gassing for 24h at ambient temperature. After a 2-h

outgassing at ambient temperature, mercury porosi-

metry measurements were performed with sample mon-

oliths using a manual porosimeter from 0.01 to 0.1MPaand a Carlo Erba Porosimeter 2000 from 0.1 to

200MPa.


SiO2 and metal particles sizes were examined by

transmission electron microscopy (TEM) with a Philips

CM100 microscope. All samples were impregnated with

an epoxy resin (EPON 812) to which an amine was

added to serve as a hardener. Hardening goes on for

72h at 60 �C and 60nm slices were then cut up with aReichert-Jung Ultracut E microtome. Finally, these

slices were put on a copper grid.

Metal dispersion in Pd/SiO2 cogelled xerogel catalysts

was determined from CO chemisorption at 30 �C on a

Fisons Sorptomatic 1990 device. Before measurements,

the calcined sample was reduced in situ in flowing H2

(0.003mmol/s) at 350 �C for 3h. Afterwards, this sample

was outgassed under vacuum at 340 �C for 16h. A dou-ble adsorption method was used: (i) first adsorption iso-

therm was measured, which includes both physisorption

and chemisorption; (ii) after a 2h outgassing at 30 �C, asecond isotherm was measured, which includes physi-

sorption only. Both isotherms were determined in the

pressure range of 10�8–2 · 10�1kPa. The difference be-

tween first and second isotherms gave the CO chemi-

sorption isotherm. The latter theoretically exhibits ahorizontal linear region corresponding to the complete

coverage of metallic sites by a monolayer of adsorbate.

However the linear region of experimental chemisorp-

tion isotherms often exhibits a slope and the monolayer

uptake was obtained by back extrapolation of the linear

region to zero pressure [15].

The Pd/SiO2 cogelled xerogel catalysts were tested for

1,2-dichloroethane hydrodechlorination, which wasconducted in a stainless steel tubular reactor (internal

diameter: 10mm) at a pressure of 0.3MPa. The reactor

was placed in a convection oven. A constant flow of

each reactant was maintained by a Gilson piston pump

for ClCH2–CH2Cl and Brooks mass flow controllers for

H2 and He. The effluent was analyzed by gas chroma-

tography (ThermoFinnigan with FID) using a Porapak

Q5 packed column. Prior to each experiment, the Pd/SiO2 catalysts were reduced in situ at atmospheric pres-

sure in flowing H2 (0.023mmol/s) while being heated to

350 �C at a rate of 350 �C/h and were maintained at this

temperature for 3h. After reduction, the Pd/SiO2 cata-

lysts were cooled in H2 to the desired initial reaction

temperature of 300 �C. The total flow of the reactant

mixture was 0.45mmol/s and consisted of ClCH2–

CH2Cl (0.011mmol/s), H2 (0.023mmol/s), and He(0.42mmol/s). The temperature was successively kept

at 300 �C (10h to allow activity stabilization after fast

initial deactivation), 350 �C (2h), and 300 �C (2h). The

effluent was analyzed every 15min and eight analyses

were made at each temperature (40 for the first level).

For each catalytic experiment, the mass of catalyst pel-

lets, sieved between 250 and 500lm, was equal to

2 · 10�4kg. Thanks to the very open structure of Pd/SiO2 cogelled xerogel catalysts, reaction rate values ob-

tained in the present study were not falsified by diffu-

sional limitations [16]. Thus, these values reflect the

intrinsic chemical kinetics.

3. Results

3.1. Characterization of silylated acetylacetonate ligands

and palladium complexes

The IR spectrum of MS-acac-H (Fig. 1, scheme 1,

structure 2a) is given in the literature [13]. Broad signals

are observed due to the C–H elongation at 2969 and

2841cm�1 for MS-dPvM (Fig. 1, scheme 1, structure

2b), and 2942 and 2840cm�1 for MS-dBzM (Fig. 1,scheme 1, structure 2c). Phenyl groups in MS-dBzM

are responsible for the broad signal at 3056cm�1 (C–H

elongation). Characteristic absorptions of carbonyl

groups occur at 1713cm�1 for MS-dPvM, and 1698

and 1672cm�1 for MS-dBzM. Characteristic elonga-

tions of Si–OCH3 in MS-dPvM and MS-dBzM are pre-

sent at 1199 and 1087cm�1. The absorptions of these

functional groups occur at identical frequencies for pal-ladium complexes, while the carbonyl groups are more

sensitive to coordination by the metal and resonate at

1535cm�1 for Pd(MS-acac)2 (Fig. 1, scheme 2, structure

3a), 1575cm�1 for Pd(MS-dPvM)2 (Fig. 1, scheme 2,

structure 3b), and 1595cm�1 for Pd(MS-dBzM)2 (Fig.

1, scheme 2, structure 3c).1H and 13C NMR shift assignments of MS-acac-H

(Fig. 1, scheme 1, structure 2a) are reported in the liter-ature [13]. The 1H NMR spectra of MS-dPvM (Fig. 1,

scheme 1, structure 2b) and MS-dBzM (Fig. 1, scheme

1, structure 2c) show no major difference for the unsub-

stituted dione and the (trimethoxysilyl)propyl-substi-

tuted moiety. Only the two singlets corresponding to

the two methyl groups in MS-acac-H are replaced by

either one singlet at 0.92ppm for MS-dPvM (18H; t-bu-

tyl groups) or one multiplet at 7–8ppm for MS-dBzM(10H; phenyl groups). The 1H NMR spectra of palla-

dium complexes (Fig. 1, scheme 2, structures 3a, 3b

and 3c) and of the respective free ligands are quite sim-

ilar, although the peaks are slightly broader and the tri-

plets at about 4ppm are missing in the three palladium

complexes, indicating that deprotonation in a-positionhas occurred to produce the metal complexes.

In the 13C NMR spectra, the absorption due to themethyl groups in MS-acac-H (Fig. 1, scheme 1, structure

2a) is replaced by absorptions at 27.5 and 44.2ppm (t-

butyl groups) for MS-dPvM (Fig. 1, scheme 1, structure

2b), or 129.4, 129.8, 134.4 and 137ppm (phenyl groups)

for MS-dBzM (Fig. 1, scheme 1, structure 2c). Carbonyl

groups resonate at 210.4ppm in MS-dPvM and 197ppm

in MS-dBzM. Palladium complexes give similar 13C

spectra, with the difference that the a-carbon absorption(seen at about 51ppm for the free ligands) is shifted

downfield to 110ppm for Pd(MS-acac)2 (Fig. 1, scheme


2, structure 3a), to 121ppm for Pd(MS-dPvM)2 (Fig. 1,

scheme 2, structure 3b), and to 145ppm for Pd(MS-

dBzM)2 (Fig. 1, scheme 2, structure 3c), while C–O

absorption is shifted to 186ppm for Pd(MS-acac)2, to

160ppm for Pd(MS-dPvM)2, and to 185.5ppm for

Pd(MS-dBzM)2.

3.2. Characterization of xerogels and Pd/SiO2 cogelled

xerogel catalysts

For all samples, except Pd(MS-dBzM)2TE, homoge-

neous gels are obtained after about 7h in the oven at

the temperature of 80 �C. No gel is obtained with the

sample Pd(MS-dBzM)2TE because a black suspensionappears when the complex Pd(MS-dBzM)2 is dissolved

in the mixture of 10ml of ethanol and 10ml of THF.

The actual palladium loading in cogelled catalysts is

generally higher than theoretical loading because the ac-

tual catalyst mass after drying, calcination and reduc-

tion steps, is lower than theoretical mass. In fact, some

TEOS often remains unreacted and is volatilized during

vacuum drying. This theoretical mass (mth) is calculatedfrom Eq. (1):

mth ¼ nmMm þ ðnTEOS þ nligand þ 2nmÞMSiO2; ð1Þ

where nm is the amount of palladium complex in the gel

(mmol); Mm is the palladium atomic weight; nTEOS and

nligand are respectively the amounts of TEOS and syli-

lated acetylacetonate ligand in the gel (mmol); MSiO2is

the molecular weight of SiO2, 60.085g/mol. In this equa-

tion, it is assumed that all TEOS and ligand moleculesare converted into SiO2. Results in Table 1 shows that

there are only weight losses for samples Pd(MS-

acac)2TE and Pd(MS-acac)2AE after vacuum drying,

calcination and reduction steps.

The results of textural properties are presented in Ta-

ble 2 both for samples after vacuum drying step (the

Table 2

Sample textural properties

Sample Pt (MPa) Vv (

Pd(MS-acac)2TE dried 10 3.6

Pd(MS-acac)2TE 8 4.2

Pd(MS-acac)2AE 16 3.7

Pd(MS-dPvM)2TE dried –b 1.6

Pd(MS-dPvM)2TE 170 2.1

MS-acac-H dried –b 1.6

MS-acac-H 165 2.1

MS-dPvM dried –b 1.3

MS-dPvM –b 1.9

MS-dBzM dried 24 2.4

MS-dBzM 16 3.1

Pt: pressure of change of mechanism during mercury porosimetry; Vv: total c

BET method; dSiO2: silica particle diameter measured by TEM.

a Not measurable.b Not applicable, only collapse.

sample names are followed by the word �dried� in Table

2) and for the same samples after vacuum drying, calci-

nation, and reduction steps.

Submitted to an increasing mercury pressure, the

samples Pd(MS-acac)2TE, Pd(MS-acac)2TE dried,

Pd(MS-acac)2AE, Pd(MS-dPvM)2TE, MS-acac-H,MS-dBzM and MS-dBzM dried exhibit two successive

behaviors [17,18]. The samples Pd(MS-acac)2TE and

MS-dBzM are shown as examples in Fig. 2(a) and (b),

respectively. At low pressure, the sample collapses under

the isostatic pressure, and above a pressure of transition

(Pt), which is characteristic of the material composition

and microstructure, mercury can enter the network of

small pores not destroyed during compression at lowpressure. This appears in the pressurization curve by a

sudden change of slope at Pt on the curve of pressure in-

crease. The curve of pressure decrease can also be di-

vided into two distinct parts separated by an abrupt

change of slope. At high pressure, there is large volume

variation reversibility with a hysteresis, whereas at low

pressure, there is only a very limited volume variation

indicating the mainly irreversible nature of the crushingphenomenon that occurred during pressurization. Two

models can then be used in order to calculate the pore

size distribution from mercury porosimetry: the mercury

intrusion in small pores above Pt described by Wash-

burn�s model [19] and the collapse of larger pores below

Pt described by Pirard�s model [17,18].

Submitted to an increasing mercury pressure, the

samples Pd(MS-dPvM)2TE dried, MS-acac-H dried,MS-dPvM and MS-dPvM dried only collapses under

mercury pressure, as observed on aerogels [18,20] and

xerogels for which the silylated amine ligand/TEOS ra-

tio is particularly high [17,21]. The samples Pd(MS-

dPvM)2TE dried and MS-acac-H dried are shown as

examples in Fig. 2(a) and (b), respectively. Furthermore,

these samples do not contain any trace of trapped mer-

cm3/g) SBET (m2/g) dSiO2(nm)

66 –a

415 16.9 ± 1.6

700 14.5 ± 1.3

491 –a

820 11.2 ± 1.0

471 –a

890 15.0 ± 1.2

624 –a

1085 8.7 ± 0.9

432 –a

825 37.2 ± 5.8

umulative specific pore volume; SBET: specific surface area obtained by

0

1

2

3

4

5

0.01 0.1 1 10 100 1000

Pressure (MPa)

VH

g(c

m3 /g

)

(a) Pd/SiO2 xerogel catalysts

0

1

2

3

4

5

0.01 0.1 1 10 100 1000

Pressure (MPa)

VH

g(c

m3 /g

)

(b) SiO2 xerogels

Fig. 2. Mercury porosimetry curves (volume variation as a function of

mercury pressure) of (a) samples Pd(MS-acac)2TE (m and n) and

Pd(MS-dPvM)2TE dried (j and h), of (b) samples MS-acac-H dried

(m and n) and MS-dBzM (� and �). The dark points represent the

curve of pressure increase and the empty points represent the curve of

pressure decrease.

0

200

400

600

800

1000

0

200

400

600

800

1000

0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

p/p0

VN

2(c

m3 /g

)


p/p0

VN

2(c

m3 /g

) (b) SiO2 xerogels

Fig. 3. Nitrogen adsorption–desorption isotherms of (a) samples

Pd(MS-acac)2TE (m and n) and Pd(MS-dPvM)2TE dried (j and

h), of (b) samples MS-acac-H dried (m and n) and MS-dBzM (� and

�).The dark points represent the curve of pressure increase and the

empty points represent the curve of pressure decrease.


cury after a mercury porosimetry experiment. The entire

pore size distribution is determined by Pirard�s collapsemodel.

In Table 2, Pt evolves very strongly with the nature ofthe silylated acetylacetonate ligands and palladium com-

plexes. Furthermore, for each sample, Pt value is always

higher before calcination and reduction steps (dried

samples) than after.

The nitrogen adsorption analysis reveals the presence

of two types of isotherms. The a-type nitrogen adsorp-

tion–desorption isotherm is characterized by: (i) at low

relative pressure, a sharp increase of the adsorbed vol-ume is followed by a plateau which corresponds to type

I isotherm according to BDDT classification [19], which

is characteristic of microporous adsorbents; (ii) at high

pressure, the adsorbed volume increases quickly, like

in type II isotherm, which is characteristic of macropor-

ous adsorbents. The volume adsorbed between p/

p0 = 0.95 and 1 is large, indicating that the sample con-

tains pores of large dimensions; (iii) all samples exhibit anarrow adsorption–desorption hysteresis loop for p/p0values close to 1, and this hysteresis is characteristic of

capillary condensation in large mesopores. The b-typenitrogen adsorption–desorption isotherms have the fol-

lowing characteristics: a nitrogen adsorption–desorptiontype IV isotherm according to the BDDT classification

[19] with a broad hysteresis. This observation could be

explained by the absence of macropores and very large

mesopores and, therefore an increase of the specific sur-

face area, SBET.

The a-type nitrogen adsorption–desorption isotherm

is observed for samples Pd(MS-acac)2TE, Pd(MS-

acac)2TE dried, Pd(MS-acac)2AE, MS-dBzM andMS-dBzM dried. The samples Pd(MS-acac)2TE and

MS-dBzM are shown as examples in Fig. 2(a) and (b),

respectively. The b-type is characterized for samples

Pd(MS-dPvM)2TE, Pd(MS-dPvM)2TE dried, MS-acac-

H, MS-acac-H dried, MS-dPvM and MS-dPvM dried.

The samples Pd(MS-dPvM)2TE dried and MS-acac-H

dried are shown as examples in Fig. 2(a) and (b), respec-

tively. When each dried sample is calcined and reduced,the porosity in samples increases because at low relative

pressure, the plateau corresponding to micropores

strongly increases and the hysteresis loop shifts towards

smaller p/p0 values, which is characteristic of capillary

condensation in smaller mesopores. In Table 2, the spe-

cific surface area, SBET strongly increases when the dried

samples are calcined and reduced. Furthermore, SBET

have smaller values for Pd/SiO2 cogelled xerogel cata-

0.001

0.01

0.1

1

10

0.1 1 10 100 1000Pore size (nm)

0.1 1 10 100 1000

Pore size (nm)

Cum

ulat

ive

volu

me

(cm

3 /g)


(b) SiO2 xerogels

0.001

0.01

0.1

1

10

Cum

ulat

ive

volu

me

(cm

3 /g)

Fig. 4. Pore size distributions of (a) samples Pd(MS-acac)2TE (m),

Pd(MS-acac)2TE dried (n), Pd(MS-acac)2AE (d), Pd(MS-dPvM)2TE

(j), Pd(MS-dPvM)2TE dried (h) and Pd(EDAS)2 (·) [18], of (b)

samples MS-acac-H (m), MS-acac-H dried (n), MS-dPvM (j), MS-

dPvM dried (h), MS-dBzM (�), MS-dBzM dried (�) and EDAS (·)[16].


lysts than SiO2 xerogels synthesized from the same silyl-

ated ligand.

Fig. 4 shows the evolution of the cumulative volume

distributions over the entire pore size range for SiO2

xerogels and Pd/SiO2 cogelled xerogel catalysts. Thesecurves were obtained by applying a combination of var-

ious methods to their respective validity domains and by

adding the porous volume distributions corresponding

to each domain [8,17]. All samples are characterized

by a steep volume increase around 0.8nm, followed by

a plateau (Fig. 4). In the range of meso- and macro-

pores, one observes that all samples exhibit a broad dis-

tribution. These samples then contain micropores(width < 2nm), mesopores (2nm < width < 50nm) and

macropores (width > 50nm).

The total cumulative specific pore volume, Vv that is,

the pore volume obtained by addition of pore volume

measured by mercury porosimetry (width > 7.5nm)

and cumulative volume of micropores and mesopores

of widths between 2 and 7.5nm measured by nitrogen

adsorption–desorption, increases when dried SiO2 xero-gels are calcined and when Pd/SiO2 cogelled xerogel cat-

alysts are calcined and reduced (Table 2).

Texture and morphology of SiO2 xerogels and Pd/

SiO2 cogelled xerogel catalysts have also been examined

by TEM and the sizes of SiO2 elementary particles, dSiO2

have been evaluated. Sizes given in Table 2 represent the

arithmetic mean on 50 particles. Samples MS-acac-H,

MS-dPvM and MS-dBzM are shown in Fig. 5(a)–(c),

and it is observed that the silica particle size, dSiO2

strongly varies when the nature of silylated acetylaceto-nate ligand changes. Furthermore, it has been shown in

[17] that the relation between the pressure of transition,

Pt and the size of SiO2 particles, dSiO2is given by the

relationship dSiO2� 1=P 0:75

t in the case of the buckling

of the brittle filaments of mineral oxide under an axial

compressive stress. In all samples, when dSiO2increases,

Pt decreases and inversely (Table 2).

Table 3 gives palladium particle size determined byTEM and CO chemisorption measurements and cata-

lytic activity for 1,2-dichloroethane hydrodechlorination

over Pd/SiO2 cogelled xerogel catalysts.

TEM analysis indicates that Pd(MS-acac)2TE and

Pd(MS-dPvM)2TE exhibit metal particles distributed

in two families of different sizes: small crystallites of

about 3nm and large crystallites between 20 and 50nm

(Fig. 5(d)). For both catalysts, the mean diameter ofsmall crystallites, dTEM1 is the arithmetic mean of 50

diameters of small palladium particles measured on

TEM micrographs. The number of larger crystallites is

much smaller than for small crystallites, and their mean

diameter, dTEM2 is then estimated from an average of fif-

teen crystallites. Concerning localization of palladium

crystallites, it appears that cogelled catalysts are com-

posed of silica particles arranged in strings or aggre-gates, and although TEM gives only a 2D view, it

seems that small palladium particles are located inside

silica particles, whereas large palladium particles are lo-

cated at their surface (Fig. 5(d)). The sample Pd(MS-

acac)2AE exhibits a broad distribution of palladium par-

ticle sizes, from 2 to 40nm.

A mean diameter, dchem of palladium particles has

also been derived from metal dispersion, D, measuredby CO chemisorption for Pd/SiO2 cogelled xerogel cata-

lysts as proposed by Lambert et al. [9–11]. For Pd(MS-

acac)2TE and Pd(MS-dPvM)2TE containing two fami-

lies of particle sizes, values of dchem are between values

of dTEM1 and dTEM2 For Pd(MS-acac)2AE exhibiting a

broad distribution of palladium particle sizes, dchem rep-

resents a mean diameter of all palladium particles in this

sample.Pd(MS-acac)2TE, Pd(MS-acac)2AE and Pd(MS-

dPvM)2TE mainly produce ethane, C2H6, and two sec-

ondary products are observed: ethyl chloride, C2H5Cl,

and ethylene, C2H4 as in the case of Pd/SiO2 cogelled

xerogel catalysts synthesized with sylilated amine ligands

[9,10]. Indeed, noble metals catalysts (Group VIII), and

particularly palladium, are very active for the hydrode-

chlorination reaction [22,23]. In the case of 1,2-dichloro-ethane hydrodechlorination, the noble metal participates

in a catalytic cycle, in which the reactant is dechlorinated

Fig. 5. TEM micrographs of (a) MS-acac-H (500000·), of (b) MS-dPvM (500000·), of (c) MS-dBzM (500000·) and of (d) Pd(MS-acac)2TE

(725000·).

Table 3

Palladium average particle size and catalytic activity for 1,2-dichloroethane hydrodechlorination

Sample TEM CO chemisorption r (mmol/skgPd) TOF (s�1)

dTEM1 (nm) rTEM1 (nm) dTEM2 (nm) rTEM2 (nm) dchem (nm) D (%) 250�C 300�C 250�C 300�C

Pd(MS-acac)2TE 3.2 0.6 22.5 7.0 6.5 16 71 169 0.05 0.11

Pd(MS-acac)2AE From 2 to 40 13.5 8 27 85 0.04 0.11

Pd(MS-dPvM)2TE 3.7 0.6 46.2 11.0 9.0 12 48 112 0.04 0.10

dTEM1, dTEM2: mean diameter of small and large palladium particles respectively measured by TEM; rTEM1, rTEM2: standard deviations associated

with dTEM1 and dTEM2, respectively; D: metal dispersion measured by chemisorption; dchem: mean diameter derived from dispersion D; r: con-

sumption rate of 1,2-dichloroethane; TOF: turnover frequency.


by chlorination of the metal surface, which is then itself

dechlorinated by reduction with hydrogen. Because of

the high reactivity of hydrogen on noble metals, the

dechlorinated organics, C2H4 in the present case, is

immediately converted into the fully hydrogenated prod-

uct, C2H6 [9,10,22,23], which is much less useful from an

industrial point of view. However, several authors dem-

onstrated the ability of bimetallic catalysts, composed ofalloys such as Pd–Ag [8,11], Pt–Cu [24,25], Pd–Cu [11],

to convert chlorinated alkanes selectively into less or

not chlorinated alkenes. So in a previous work over

Pd–Ag/SiO2 and Pd–Ag/SiO2 cogelled xerogel catalysts

[11], increasing silver or copper content in bimetallic cat-

alysts results in an increase in ethylene selectivity, and for

samples with 1.5wt% for palladium and 3.0wt% for sil-

ver or copper, this selectivity reaches 100% in the condi-tions of the catalytic test.

The consumption rate of 1,2-dichloroethane r is cal-

culated from chromatographic measurements of C2H6,

C2H5Cl and C2H4 concentrations in the reactor effluent

and from the differential reactor equation that is written

as follows:

r ¼ F A þ F Cl þ F E

WðF A0; F Cl0 and F E0 ¼ 0Þ; ð2Þ

where r is the consumption rate (mmol/kgPds), FA is the

molar flowrate of ethane at the reactor outlet (mmol/s),

FA0 is the molar flowrate of ethane at the reactor inlet

(mmol/s), FCl is the molar flowrate of ethyl chloride atthe reactor outlet (mmol/s), FCl0 is the molar flowrate

of ethyl chloride at the reactor inlet (mmol/s), FE is

the molar flowrate of ethylene at the reactor outlet

(mmol/s), FE0 is the molar flowrate of ethylene at the

reactor inlet (mmol/s) and W is the palladium mass


inside the reactor (kgPd). For all samples, r has been cal-

culated from Eq. (2) at the temperatures of 250 and

300 �C and these results are presented in Table 3. It is

observed that r increases with palladium dispersion.

4. Discussion

An important advantage of the cogelation method is

the possibility of homogeneously distributing the cata-

lytic metal through the whole material, that is, inside

the silica particles. In previous studies [7–11,21], TEM

micrographs indeed seemed to exhibit metal particles in-

side the silica matrix. It was shown that the localizationof the metal inside the silica matrix was induced by a

nucleation process initiated by the silylated amine ligand

of the metal, 3-(2-aminoethylamino)propyltrimethoxysi-

lane (EDAS). For example, when palladium is intro-

duced in xerogels synthesized from EDAS, the

PdðEDASÞ2þ2 complexes are much more reactive than

TEOS. Therefore, hydrolyzed PdðEDASÞ2þ2 complexes

can act as a nucleation agent leading to silica particleswith a hydrolyzed and condensed PdðEDASÞ2þ2 core

and a shell principally made of hydrolyzed and con-

densed TEOS [7,9,10,21]. Furthermore, in the case of

SiO2 xerogels [21], when the silylated ligand contains

methoxy groups, the nucleation phenomenon by the sil-

ylated ligand takes place in the case of TEOS as the

main silica precursor.

Samples MS-acac-H, MS-dPvM and MS-dBzM pre-sent small homogeneous entities visible by TEM (Fig.

5(a)–(c)), which seem to be silica elementary particles.

These observations show that the nucleation phenome-

non by the silylated acetylacetonate ligand takes place

as in the case of SiO2 xerogels synthesized from EDAS

[21]. On the one hand, TEM micrographs obtained for

Pd(MS-acac)2TE and Pd(MS-dPvM)2TE show small

palladium crystallites inside microporous silica particles(Fig. 5(d)). So the nucleation phenomenon is also ob-

served when palladium is complexed either by MS-

acac-H or by MS-dPvM in a THF-ethanol mixture.

Pd(MS-acac)2 and Pd(MS-dPvM)2 complexes in a

THF-ethanol mixture are more reactive than TEOS

and allow a nucleation process initiated by the silylated

acetylacetonate ligand. On the other hand, the sample

Pd(MS-acac)2AE exhibits a broad distribution of palla-dium particle sizes from 2 to 40nm and no spherical

structure of silica particles is observed. These observa-

tions could be explained by the fact that the use of ace-

tone to dissolve Pd(MS-acac)2 complex in ethanol does

not allow preserving the metal complex architecture dur-

ing sol–gel immobilization and prevents the nucleation

phenomenon by the silylated acetylacetonate ligand to

take place [26].Thanks to the large difference in steric volume of the

three silylated acetylacetonate ligands, the textural prop-

erties of SiO2 xerogels and Pd/SiO2 cogelled xerogel cat-

alysts show a large influence of the nature of silylated

acetylacetonate ligand and metal complex on the final

texture of xerogels and catalysts, both before as well

as after calcination and reduction steps.

In the case of SiO2 xerogels, the silylated acetylaceto-nate ligand/TEOS molar ratio is equal to about 0.06 and

the samples MS-acac-H, MS-dPvM and MS-dBzM are

compared with a SiO2 xerogel synthesized from EDAS

and TEOS and in which the EDAS/TEOS molar ratio

is also equal to 0.06 [21]. The shape of the adsorption

isotherms of the samples MS-dBzM (Fig. 3(b)) and

EDAS [21] is of type II at high relative pressure. During

mercury porosimetry measurements, both samples alsopresent a buckling-intrusion mechanism (Fig. 2(b)),

characteristic features of macroporous solids [19]. These

samples, MS-dBzM and EDAS, therefore have high val-

ues for the total specific pore volume, Vv equal to

3.1cm3/g (Table 2) and 2.3cm3/g [21], respectively. The

shape of the adsorption isotherms of samples MS-

acac-H and MS-dPvM is of type IV at high relative pres-

sure (Fig. 3(b), where MS-acac-H dried presents thesame isotherm shape) and only a collapse mechanism

is observed for both samples during mercury porosime-

try measurements (Fig. 2(b), where MS-acac-H dried

presents the same porosimetry curve). All these observa-

tions are characteristic features of mesoporous solids

[19], which therefore present smaller values of Vv than

MS-dBzM and EDAS (Table 2). So the nature of the sil-

ylated ligand influences xerogel textural properties andby changing the nature of the silylated ligand, e.g.,

EDAS, MS-acac-H, MS-dPvM or MS-dBzM, textural

properties such as pore volume, pore size and surface

area can be tailored. The same development can be per-

formed for Pd/SiO2 cogelled xerogel catalysts in which

the silylated acetylacetonate ligand/TEOS molar ratio

is equal to about 0.05. The samples Pd(MS-acac)2TE,

Pd(MS-acac)2AE and Pd(MS-dPvM)2TE are comparedwith a Pd/SiO2 catalyst synthesized from EDAS and

TEOS in which palladium loading is equal to 1.5%

and the EDAS/TEOS molar ratio is equal to 0.05 [10].

The shape of the adsorption isotherms of the samples

Pd(MS-acac)2TE (Fig. 3(a)), Pd(MS-acac)2AE and

Pd(EDAS)2 is of type II at high relative pressure, and

during mercury porosimetry measurements, all these

samples present a buckling-intrusion mechanism (Fig.2(a)), characteristic features of macroporous solids.

These samples, Pd(MS-acac)2TE, Pd(MS-acac)2AE

and Pd(EDAS)2, therefore have high values for the total

specific pore volume, Vv equal to 4.2cm3/g, 3.7cm3/g

(Table 2) and 3.9cm3/g (10), respectively. The shape of

the adsorption isotherm of sample Pd(MS-dPvM)2TE

is of type IV at high relative pressure (Fig. 3(a), where

Pd(MS-dPvM)2TE dried presents the same isothermshape) and a collapse mechanism is only observed for

this sample during mercury porosimetry measurements


(Fig. 2(a), where Pd(MS-dPvM)2TE dried presents the

same porosimetry curve). All these observations are

characteristic features of a mesoporous solid, which

therefore presents smaller value of Vv than Pd(MS-

acac)2TE, Pd(MS-acac)2AE (Table 2) and Pd(EDAS)2[10]. So the nature of silylated ligand also influences tex-tural properties of Pd/SiO2 cogelled xerogel catalysts.

The specific surface area obtained by BET method,

SBET presents a higher value equal to 1085m2/g for the

sample MS-dPvM than those of samples MS-acac-H,

MS-dBzM (Table 2) and EDAS [21]. Furthermore, in

Fig. 4(b), the cumulative volume associated to micro-

pores (pore size < 2nm) is more important for sample

MS-dPvM than for samples MS-acac-H, MS-dBzMand EDAS. For Pd/SiO2 cogelled xerogel catalysts, SBET

value of Pd(MS-dPvM)2TE (Table 2) and the cumulative

volume associated to micropores (Fig. 4(a)) are much

higher than those of Pd(MS-acac)2TE, Pd(MS-acac)2AE

and Pd(EDAS)2 [10]. As it is well-known that micropores

(pore size < 2nm) and small mesopores (2nm < pore

size < 10nm) contribute essentially in SBET values [19]

and the silica particle sizes of SiO2 xerogels and Pd/SiO2 cogelled xerogel catalysts are equal from 8.7 to

37.2nm (Table 2), microporosity is largely located inside

silica particles. So samples MS-dPvM and Pd(MS-

dPvM)2TE are more microporous than all other samples.

This higher microporosity with the use of silylated dipi-

valoylmethane ligand could be the result of its size of

about 1nm against sizes of about 0.6nm for the other syl-

ilated acetylacetonate ligands and of about 0.35nm forEDAS (Molecular simulation with Chem3D Ultra Ver-

sion 7.0). So greater repulsion between silylated dipiva-

loylmethane ligand molecules could generate more

spaces into silica nuclei, increasing the microporosity in-

side silica particles. The silica particle sizes are also smal-

ler for samples MS-dPvM and Pd(MS-dPvM)2 (Table 2).

This feature could also be the result of repulsion between

silylated dipivaloylmethane ligand molecules, involving asmall number of ligand molecules in silica nuclei.

Before calcination and reduction steps, all the sam-

ples already have high specific surface areas, SBET (Ta-

ble 2). When the samples are calcined in air, the

organic groups are removed by thermolysis and/or pyro-

lysis reactions. This results in an increase in porosity and

specific surface area [27,28]. For all samples, after calci-

nation and reduction steps, the total cumulative porevolume, Vv slightly increases, whereas the specific sur-

face area, SBET strongly increases, indicating that a

large percentage of micropores and small mesopores

are created by the removal of the organic groups dur-

ing calcination and reduction steps (Table 2). Moreover,

the shape of the isotherms of dried samples is not chan-

ged during calcination and reduction steps. Only micro-

pores are additionally formed, whereas the meso- andmacropores are retained after calcination and reduction

steps.

Accessibility to active sites is of crucial importance

for an efficient catalytic activity. Site accessibility will

naturally depend on the material structure, which is

dependent on the composition of the material and on

processing conditions. It is frequently found that the

specific activities of metal complexes in catalytic reac-tions are considerably diminished when the complexes

are supported on a silica gel material [26]. Such reduc-

tions in activity may arise from changes in the coordina-

tion state of the complex or other environmental

influences. In many cases, however, the reduced activity

may simply result from a large fraction of supported me-

tal complexes being rendered inaccessible to solvent or

external reagents. Because palladium is located insidesilica particles in Pd/SiO2 cogelled xerogel catalysts,

there is a risk of inaccessibility. The values of Vv in Ta-

ble 2 show that porosity remains high after drying under

vacuum, although this does not prove accessibility to

palladium. Nevertheless, it is observed in Table 3 that

dTEM1 6 dchem 6 dTEM2 for Pd(MS-acac)2TE and Pd

(MS-dPvM)2TE. So palladium particle sizes obtained

by CO chemisorption measurements are related toTEM measurements, which suggests that palladium par-

ticles located inside silica particles are completely

accessible.

In Table 3, the specific consumption rate of 1,2-

dichloroethane, r, increases with palladium dispersion

and the turnover frequency (TOF), that is, the number

of molecules consumed per surface metal atom and

per second, is equal to about 0.04s�1 at 250 �C and toabout 0.11s�1 at 300 �C. These values are identical to

those obtained for 1,2-dichloroethane hydrodechlorina-

tion over Pd/SiO2 cogelled xerogel catalysts synthesized

with EDAS as silylated ligand [9,10]. Although the

structure sensitivity of C–Cl hydrogenolysis with the

ensemble size concept has been pointed out by several

authors [22,29,30], it seems that the 1,2-dichloroethane

hydrodechlorination over Pd/SiO2 catalysts examinedin the present study, is a structure insensitive reaction,

as already previously observed [9,10].

5. Conclusions

The main purpose of the present work was to synthe-

size new silylated acetylacetonate ligands, able to form achelate with a metal ion such as Pd2+ and able to react

with a silica-forming reagent such as TEOS. This

purpose is attained because the use of 3-[3-(trimethoxy-

silyl)propyl]-2,4-pentanedione (MS-acac-H), or 2,2,6,

6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptan-

edione (MS-dPvM), or 1,3-diphenyl-2-[3-(trimethoxysi-

lyl) propyl]-1,3-propanedione (MS-dBzM) in an

ethanolic solution containing tetraethoxysilane (TEOS)and an ammonia solution of 0.18mol/l gave very homo-

geneous colored xerogels, the textural properties of


which depend on the nature of the silylated acetylaceto-

nate ligand.

No acetylacetonate deritative has ever been used as a

ligand for the synthesis of metal xerogel catalysts. Nev-

ertheless, in this study, the use of silylated palladium

complexes of the same silylated acetylacetonate ligands,in a mixture of tetrahydrofurane (THF) and ethanol

containing tetraethoxysilane (TEOS) and an ammonia

solution of 0.18mol/l yielded Pd/SiO2 cogelled xerogel

catalysts. These samples contain small metal particles

of about 3.5nm located inside silica particles exhibiting

a monodisperse microporous distribution and some

large metal particles from 20 to 50nm situated outside

the silica matrix.The silylated acetylacetonate ligand has a large influ-

ence on the textural properties of xerogels and catalysts,

both before and after calcination and reduction steps.

By changing the nature of the silylated ligand, e.g.,

MS-acac-H, MS-dPvM or MS-dBzM, textural proper-

ties such as pore volume, pore size and surface area

can be tailored. For all the samples, the total cumulative

pore volume, Vv slightly increases after calcination andreduction steps, whereas the specific surface area, SBET

strongly increases, indicating that a large percentage of

micropores and small mesopores are created by the re-

moval of the organic groups during calcination and

reduction steps.

Although palladium particles are located inside the

silica particles, their complete accessibility, via the micro-

pore network, is suggested. 1,2-dichloroethane hydrode-chlorination over Pd/SiO2 catalysts mainly produces

ethane, and the hydrodechlorination activity increases

with palladium dispersion. Hydrodechlorination over

Pd/SiO2 xerogel cogelled catalysts is a structure insensi-

tive reaction compared to the ensemble size concept.

Acknowledgments

S.L. is grateful to the Belgian Fonds pour la Forma-

tion a la Recherche dans l�Industrie et dans l�Agricul-

ture, F.R.I.A., for a PhD grant. The authors also

thank the Belgian Fonds National de la Recherche Sci-

entifique, the Fonds de Bay, the Fonds de la Recherche

Fondamentale et Collective, the Ministere de la Region

Wallonne and the Ministere de la Communaute Franc-aise (Action de Recherche Concertee n� 00-05-265) for

their financial support.

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synthesis of sio2 xerogels and pd/sio2 cogelled xerogel catalysts from silylated acetylacetonate...

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