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SiO2 cogelled xerogel catalysts from silylated acetylacetonate ligand
Journal of Non-Crystalline Solids 343 (2004) 109–120 www.elsevier.com/locate/jnoncrysol
Synthesis of SiO2 xerogels and Pd/SiO2 cogelled xerogel catalysts from silylated acetylacetonate ligand Ste´phanie Lambert a, Luigi Sacco b, Fabrice Ferauche a, Benoıˆt Heinrichs Alfred Noels b, Jean-Paul Pirard a b
a,*
,
a Laboratoire de Ge´nie Chimique, B6a, Universite´ de Lie`ge, B-4000 Lie`ge, Belgium Centre d’Enseignement et de Recherche des Macromole´cules (CERM), B6a, Universite´ de Lie`ge, B-4000 Lie`ge, 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 0.18 mol/l, from synthesized new silylated acetylacetonate ligands, respectively, 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione (MS-acac-H), 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)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 3.5 nm, located inside primary silica particles exhibiting a monodisperse microporous distribution as well as large palladium particles from 20 to 50 nm, 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 network, 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 insensitive reaction compared to the ensemble size concept. 2004 Elsevier B.V. All rights reserved.
1. Introduction Viable preparative routes to silylated organic molecules initially emerged in the 1960s, largely pioneered by a group at Degussa [1]. Development of these commercial-scale procedures was originally driven by the interest in the bifunctional reagents as adhesion promoters for inorganic–organic polymer composites such as glass fiber-reinforced materials and mineral-filled elas-
*
Corresponding author. Tel.: +32 4 366 3505; fax: +32 4 366 3545. E-mail address: [email protected] (B. Heinrichs).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.07.049
tomers. Further developments arose from the realization that attachment of catalytically active metal complexes to silica gels via silylated ligands afforded supported complexes, whose properties combined the positive attributes 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 react to form a sol. One of the most common reasons cited for the use of sol–gel processing is the intimate molecular-level mixing of components that can be obtained at the outset of the reactions [2,3]. In this way, several authors
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S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
developed the cogelation sol–gel process to disperse metal particles in a silica matrix [4–6]: organically substituted 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, copper, 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 ratio between 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 synthesize new 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 organo-metallic complexes. So, after vacuum drying, these silica xerogels could be used as catalysts in several organic reactions such as cyclopropanation, Kharasch reactions, etc. Furthermore, the samples are also calcined and reduced to analyze the dispersion, localization, and accessibility of palladium and to compare their catalytic activity to previously synthesized Pd/ SiO2 cogelled xerogel catalysts used for hydrodechlorination [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 4 A molecular sieves. Tetrahydrofurane (THF) was freshly distilled under argon from benzophenone and sodium prior to its use. Dichloromethane was distilled under argon from calcium hydride. Sodium acetylacetonate was synthesized from acetylacetone (Fig. 1, scheme 1, structure 1a) according to Charles [12] and dried at a temperature of 100 C and under a pressure of 0.07 Pa. The above-described synthesis method is also applied to dipivaloylmethane (Fig. 1, scheme 1, structure 1b) and dibenzoylmethane (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-dimethylformamide (DMSO). Indeed, the kind of halogen plays a crucial 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 dipivaloymethane and dibenzoylmethane (Fig. 1, scheme 1). Synthesis of 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione, MS-acac-H (Fig. 1, scheme 1, structure 2a). To a suspension of 5 g of sodium acetylacetonate in 30 ml of dry THF, 11.9 g of (3-iodopropyl)-trimethoxysilane 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 under reduced pressure and the product extracted with dry dichloromethane. The slightly viscous colorless liquid obtained by distillation (101–104 C/0.07 Pa) can be indefinitely stored at a temperature of 15 C under argon. Yield: 55%. Synthesis of 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)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 distillation (115–118 C/0.07 Pa) and stored at a temperature of 15 C under argon. Yield: 45%. Synthesis of 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-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 acetylacetone readily gives palladium acetylacetonate in a wide variety of solvents, this method does not work with the functionalized diones. A pre-reaction between the dione and potassium t-butoxyde is necessary, which then yields the palladium complex with 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione, Pd(MS-acac)2, 2,2,6, 6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione, Pd(MS-dPvM)2, and 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedione, Pd(MS-dBzM)2 (Fig. 1, scheme 2). Synthesis of palladium complexes from 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione, Pd(MS-acac)2, from 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione, Pd(MS-dPvM)2 and from 1, 3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedione, [Pd(MS-dBzM)2]. To a fine suspension of 1.1 g of potassium t-butoxyde in 150 ml of THF was added in a first time, either 3.4 g of 3-[3-(trimethoxysilyl)propyl]2,4-pentanedione or 3.4 g of 2,2,6,6-tetramethyl-4[3-(trimethoxysilyl)propyl]-3,5-heptanedione or 3.4 g of 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-prop-
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120 O
Scheme 1
O
O
R
O
R
NaOH R
R
Na+
O
R
+
Na 2CO 3 THF
O
1a, 1b,1c
O R
R
O
Si(OCH3)3
Si(OCH3)3
I
MeOH H2O
O R
R Na+ O
111
O
R
Si(OCH3)3 R
OH R
Si(OCH3)3 2a, 2b, 2c
a b c
R -CH3 -C(CH3)3 -Ph
Scheme 2 O
O
R
R
Pd (OAC) 2 KO tBu
(H3 CO) 3 Si
R O
Pd 2+
THF
O R
(H3 CO)3 Si
2 2a , 2b , 2c
3a , 3b, 3c
Fig. 1. Synthesis schemes of silylated acetylacetonate ligands and palladium complexes.
anedione and in a second time a solution of 1 g of palladium acetate in 10 ml of THF. After 20 min under stirring, the brown suspension was filtrated and the filtrate was evaporated under vacuum. A very viscous brown 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(MSdBzM)2 is 50%. The structures of these silylated complexes 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 1
H 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 acetylacetonate ligand or palladium complex. The names of palladium samples are followed by either the letters TE, 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
d
c
b
a
nm the silylated acetylacetonate palladium complex, is either Pd(MS-acac)2, or Pd(MS-dPvM)2, or Pd(MS-dBzM)2. nligand the silylated acetylacetonate ligand, is either MS-acac-H, or MS-dPvM, or MS-dBzM. ndiluant the diluant solvent, is THF or acetone. Weight loss = 100 · (mth ma)/mth, where mth is the theoretical mass and ma is the actual catalyst mass measured after drying, calcination and reduction steps.
– – – 0 0 0 – – – White gel Colorless gel Light-orange gel 514 514 514 – – – 253 253 253 2.93 3.12 2.91 – – – MS-acac-H MS-dPvM MS-dBzM
48.5 48.3 48.5
Ethanol Ethanol Ethanol
16 14 0 – 1.0 1.0 1.0 1.0 Black gel Black gel Grey gel Black suspension 343 343 343 343 123 136 123 123 230 237 230 230 1.57 1.68 1.59 1.48 0.266 0.273 0.267 0.255 Pd(MS-acac)2TE Pd(MS-acac)2AE Pd(MS-dPvM)2TE Pd(MS-dBzM)2TE
44.5 45.7 44.5 44.6
THF-ethanol Acetone-ethanol THF-ethanol THF-ethanol
Weight lossd ±2% (wt%) Theoretical metal loading (wt%) Gelation behavior nC2H5 OH (mmol) ndiluantc (mmol) Solvent mixture nH2O (mmol) nTEOS (mmol) nligandb (mmol) nma (mmol) Sample
Table 1 Synthesis operating variables of xerogels and Pd/SiO2 cogelled xerogel catalysts
1.2 1.2 1.0 –
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
Actual metal loading ±0.1% (wt%)
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(Si(OC2H5)4) and aqueous ammonia. So a 0.18 N 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. Four Pd/SiO2 gels were prepared from the corresponding silylated acetylacetonate palladium complex –Pd(MSacac)2 or Pd(MS-dPvM)2 or Pd(MS-dBzM)2–, TEOS and aqueous ammonia. In these syntheses, Pd(MSacac)2 or Pd(MS-dPvM)2 or Pd(MS-dBzM)2 was dissolved in a mixture of 10 ml of ethanol and 10 ml of acetone or THF, and in which some amount of silylated acetylacetonate ligand corresponding to the used palladium complex was present to allow gelification. So the molar ratio silylated ligand/TEOS is equal to 0.05 in palladium gels. After addition of TEOS, a 0.18 N NH3 aqueous solution in the remaining ethanol was added to the mixture under vigorous stirring. The volume of final solutions was 30 ml. The hydrolysis ratio, which is the molar ratio H = [H2O]/([TEOS] + 3/4 [sylilated ligand]), and the dilution ratio, which is the molar ratio R = [ethanol]/([TEOS] + [sylilated ligand]) were kept constant at values of 5 and 10 respectively for all samples. 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 1200 Pa after 90 h. The drying oven was then heated at 150 C for 72 h. 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 (0.02 mmol/s); this temperature was then maintained for 12 h in air (0.1 mmol/s). All dried SiO2 xerogels were heat treated in air at 450 C for 72 h 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 (0.23 mmol/s) and maintained at this temperature for 3 h (same flow). 2.4. Characterization of SiO2 xerogels and Pd/SiO2 cogelled xerogel catalysts Nitrogen adsorption–desorption isotherms were measured at 77 K on a Fisons Sorptomatic 1990 after outgassing for 24 h at ambient temperature. After a 2-h outgassing at ambient temperature, mercury porosimetry measurements were performed with sample monoliths using a manual porosimeter from 0.01 to 0.1 MPa and a Carlo Erba Porosimeter 2000 from 0.1 to 200 MPa.
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
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 72 h at 60 C and 60 nm slices were then cut up with a Reichert-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 (0.003 mmol/s) at 350 C for 3 h. Afterwards, this sample was outgassed under vacuum at 340 C for 16 h. A double adsorption method was used: (i) first adsorption isotherm was measured, which includes both physisorption and chemisorption; (ii) after a 2 h outgassing at 30 C, a second isotherm was measured, which includes physisorption only. Both isotherms were determined in the pressure range of 108–2 · 101 kPa. The difference between first and second isotherms gave the CO chemisorption isotherm. The latter theoretically exhibits a horizontal linear region corresponding to the complete coverage of metallic sites by a monolayer of adsorbate. However the linear region of experimental chemisorption 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 was conducted in a stainless steel tubular reactor (internal diameter: 10 mm) at a pressure of 0.3 MPa. 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 chromatography (ThermoFinnigan with FID) using a Porapak Q5 packed column. Prior to each experiment, the Pd/ SiO2 catalysts were reduced in situ at atmospheric pressure in flowing H2 (0.023 mmol/s) while being heated to 350 C at a rate of 350 C/h and were maintained at this temperature for 3 h. After reduction, the Pd/SiO2 catalysts were cooled in H2 to the desired initial reaction temperature of 300 C. The total flow of the reactant mixture was 0.45 mmol/s and consisted of ClCH2– CH2Cl (0.011 mmol/s), H2 (0.023 mmol/s), and He (0.42 mmol/s). The temperature was successively kept at 300 C (10 h to allow activity stabilization after fast initial deactivation), 350 C (2 h), and 300 C (2 h). The effluent was analyzed every 15 min and eight analyses were made at each temperature (40 for the first level). For each catalytic experiment, the mass of catalyst pellets, sieved between 250 and 500 lm, was equal to 2 · 104 kg. Thanks to the very open structure of Pd/ SiO2 cogelled xerogel catalysts, reaction rate values obtained in the present study were not falsified by diffu-
113
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 2841 cm1 for MS-dPvM (Fig. 1, scheme 1, structure 2b), and 2942 and 2840 cm1 for MS-dBzM (Fig. 1, scheme 1, structure 2c). Phenyl groups in MS-dBzM are responsible for the broad signal at 3056 cm1 (C–H elongation). Characteristic absorptions of carbonyl groups occur at 1713 cm1 for MS-dPvM, and 1698 and 1672 cm1 for MS-dBzM. Characteristic elongations of Si–OCH3 in MS-dPvM and MS-dBzM are present at 1199 and 1087 cm1. The absorptions of these functional groups occur at identical frequencies for palladium complexes, while the carbonyl groups are more sensitive to coordination by the metal and resonate at 1535 cm1 for Pd(MS-acac)2 (Fig. 1, scheme 2, structure 3a), 1575 cm1 for Pd(MS-dPvM)2 (Fig. 1, scheme 2, structure 3b), and 1595 cm1 for Pd(MS-dBzM)2 (Fig. 1, scheme 2, structure 3c). 1 H and 13C NMR shift assignments of MS-acac-H (Fig. 1, scheme 1, structure 2a) are reported in the literature [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 unsubstituted dione and the (trimethoxysilyl)propyl-substituted moiety. Only the two singlets corresponding to the two methyl groups in MS-acac-H are replaced by either one singlet at 0.92 ppm for MS-dPvM (18H; t-butyl groups) or one multiplet at 7–8 ppm for MS-dBzM (10H; phenyl groups). The 1H NMR spectra of palladium complexes (Fig. 1, scheme 2, structures 3a, 3b and 3c) and of the respective free ligands are quite similar, although the peaks are slightly broader and the triplets at about 4 ppm are missing in the three palladium complexes, indicating that deprotonation in a-position has occurred to produce the metal complexes. In the 13C NMR spectra, the absorption due to the methyl groups in MS-acac-H (Fig. 1, scheme 1, structure 2a) is replaced by absorptions at 27.5 and 44.2 ppm (tbutyl groups) for MS-dPvM (Fig. 1, scheme 1, structure 2b), or 129.4, 129.8, 134.4 and 137 ppm (phenyl groups) for MS-dBzM (Fig. 1, scheme 1, structure 2c). Carbonyl groups resonate at 210.4 ppm in MS-dPvM and 197 ppm in MS-dBzM. Palladium complexes give similar 13C spectra, with the difference that the a-carbon absorption (seen at about 51 ppm for the free ligands) is shifted downfield to 110 ppm for Pd(MS-acac)2 (Fig. 1, scheme
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S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
2, structure 3a), to 121 ppm for Pd(MS-dPvM)2 (Fig. 1, scheme 2, structure 3b), and to 145 ppm for Pd(MSdBzM)2 (Fig. 1, scheme 2, structure 3c), while C–O absorption is shifted to 186 ppm for Pd(MS-acac)2, to 160 ppm for Pd(MS-dPvM)2, and to 185.5 ppm for Pd(MS-dBzM)2.
sample names are followed by the word ÔdriedÕ in Table 2) and for the same samples after vacuum drying, calcination, 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 low pressure. This appears in the pressurization curve by a sudden change of slope at Pt on the curve of pressure increase. The curve of pressure decrease can also be divided 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 crushing phenomenon 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 WashburnÕs model [19] and the collapse of larger pores below Pt described by PirardÕs 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 ratio is particularly high [17,21]. The samples Pd(MSdPvM)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-
3.2. Characterization of xerogels and Pd/SiO2 cogelled xerogel catalysts For all samples, except Pd(MS-dBzM)2TE, homogeneous gels are obtained after about 7 h in the oven at the temperature of 80 C. No gel is obtained with the sample Pd(MS-dBzM)2TE because a black suspension appears when the complex Pd(MS-dBzM)2 is dissolved in the mixture of 10 ml of ethanol and 10 ml of THF. The actual palladium loading in cogelled catalysts is generally higher than theoretical loading because the actual catalyst mass after drying, calcination and reduction steps, is lower than theoretical mass. In fact, some TEOS often remains unreacted and is volatilized during vacuum drying. This theoretical mass (mth) is calculated from Eq. (1): mth ¼ nm M m þ ðnTEOS þ nligand þ 2nm ÞM SiO2 ;
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 sylilated acetylacetonate ligand in the gel (mmol); MSiO2 is the molecular weight of SiO2, 60.085 g/mol. In this equation, it is assumed that all TEOS and ligand molecules are converted into SiO2. Results in Table 1 shows that there are only weight losses for samples Pd(MSacac)2TE and Pd(MS-acac)2AE after vacuum drying, calcination and reduction steps. The results of textural properties are presented in Table 2 both for samples after vacuum drying step (the
Table 2 Sample textural properties Sample
Pt (MPa)
Vv (cm3/g)
SBET (m2/g)
Pd(MS-acac)2TE dried Pd(MS-acac)2TE Pd(MS-acac)2AE Pd(MS-dPvM)2TE dried Pd(MS-dPvM)2TE
10 8 16 –b 170
3.6 4.2 3.7 1.6 2.1
66 415 700 491 820
–a 16.9 ± 1.6 14.5 ± 1.3 –a 11.2 ± 1.0
MS-acac-H dried MS-acac-H MS-dPvM dried MS-dPvM MS-dBzM dried MS-dBzM
–b 165 –b –b 24 16
1.6 2.1 1.3 1.9 2.4 3.1
471 890 624 1085 432 825
–a 15.0 ± 1.2 –a 8.7 ± 0.9 –a 37.2 ± 5.8
dSiO2 (nm)
Pt: pressure of change of mechanism during mercury porosimetry; Vv: total cumulative specific pore volume; SBET: specific surface area obtained by BET method; dSiO2: silica particle diameter measured by TEM. a Not measurable. b Not applicable, only collapse.
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120 5
1000 (a) Pd/SiO2 xerogel catalysts
800
(a) Pd/SiO2 xerogel catalysts
VN2 (cm3/g)
VHg (cm3/g)
4
115
3 2
600 400 200
1
0 0 0.01
0 0.1
1
10
100
0.2
0.4
1000
0.6
0.8
1
0.6
0.8
1
p/p0
Pressure (MPa) 1000 5
800 (b) SiO2 xerogels
VN2 (cm3/g)
VHg (cm3/g)
4 3 2
600 400 200
1
0
0 0.01
(b) SiO2 xerogels
0.1
1
10
100
1000
Pressure (MPa) 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.
cury after a mercury porosimetry experiment. The entire pore size distribution is determined by PirardÕs collapse model. In Table 2, Pt evolves very strongly with the nature of the silylated acetylacetonate ligands and palladium complexes. 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 adsorption–desorption isotherm is characterized by: (i) at low relative pressure, a sharp increase of the adsorbed volume 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 macroporous adsorbents. The volume adsorbed between p/ p0 = 0.95 and 1 is large, indicating that the sample contains pores of large dimensions; (iii) all samples exhibit a narrow adsorption–desorption hysteresis loop for p/p0 values close to 1, and this hysteresis is characteristic of capillary condensation in large mesopores. The b-type nitrogen adsorption–desorption isotherms have the fol-
0
0.2
0.4
p/p0 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.
lowing characteristics: a nitrogen adsorption–desorption type 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 surface area, SBET. The a-type nitrogen adsorption–desorption isotherm is observed for samples Pd(MS-acac)2TE, Pd(MSacac)2TE dried, Pd(MS-acac)2AE, MS-dBzM and MS-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-acacH, 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), respectively. 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 specific surface area, SBET strongly increases when the dried samples are calcined and reduced. Furthermore, SBET have smaller values for Pd/SiO2 cogelled xerogel cata-
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Cumulative volume (cm3/g)
10 (a) Pd/SiO2 xerogel catalysts 1
0.1
0.01
0.001 0.1
1
10
100
1000
Pore size (nm) Cumulative volume (cm3/g)
10
(b) SiO 2 xerogels 1
0.1
0.01
0.001 0.1
1
10
100
1000
Pore size (nm) 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), MSdPvM dried (h), MS-dBzM (), MS-dBzM dried () and EDAS (·) [16].
lysts than SiO2 xerogels synthesized from the same silylated 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. These curves were obtained by applying a combination of various 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 0.8 nm, followed by a plateau (Fig. 4). In the range of meso- and macropores, one observes that all samples exhibit a broad distribution. These samples then contain micropores (width < 2 nm), mesopores (2 nm < width < 50 nm) and macropores (width > 50 nm). The total cumulative specific pore volume, Vv that is, the pore volume obtained by addition of pore volume measured by mercury porosimetry (width > 7.5 nm) and cumulative volume of micropores and mesopores of widths between 2 and 7.5 nm measured by nitrogen adsorption–desorption, increases when dried SiO2 xerogels are calcined and when Pd/SiO2 cogelled xerogel catalysts 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 acetylacetonate ligand changes. Furthermore, it has been shown in [17] that the relation between the pressure of transition, Pt and the size of SiO2 particles, dSiO2 is given by the relationship d SiO2 1=P 0:75 in the case of the buckling t of the brittle filaments of mineral oxide under an axial compressive stress. In all samples, when dSiO2 increases, Pt decreases and inversely (Table 2). Table 3 gives palladium particle size determined by TEM and CO chemisorption measurements and catalytic 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 3 nm and large crystallites between 20 and 50 nm (Fig. 5(d)). For both catalysts, the mean diameter of small 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 fifteen crystallites. Concerning localization of palladium crystallites, it appears that cogelled catalysts are composed of silica particles arranged in strings or aggregates, and although TEM gives only a 2D view, it seems that small palladium particles are located inside silica particles, whereas large palladium particles are located at their surface (Fig. 5(d)). The sample Pd(MSacac)2AE exhibits a broad distribution of palladium particle sizes, from 2 to 40 nm. A mean diameter, dchem of palladium particles has also been derived from metal dispersion, D, measured by CO chemisorption for Pd/SiO2 cogelled xerogel catalysts as proposed by Lambert et al. [9–11]. For Pd(MSacac)2TE and Pd(MS-dPvM)2TE containing two families 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 represents a mean diameter of all palladium particles in this sample. Pd(MS-acac)2TE, Pd(MS-acac)2AE and Pd(MSdPvM)2TE mainly produce ethane, C2H6, and two secondary 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 hydrodechlorination reaction [22,23]. In the case of 1,2-dichloroethane hydrodechlorination, the noble metal participates in a catalytic cycle, in which the reactant is dechlorinated
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
117
Fig. 5. TEM micrographs of (a) MS-acac-H (500 000·), of (b) MS-dPvM (500 000·), of (c) MS-dBzM (500 000·) and of (d) Pd(MS-acac)2TE (725 000·).
Table 3 Palladium average particle size and catalytic activity for 1,2-dichloroethane hydrodechlorination Sample
Pd(MS-acac)2TE Pd(MS-acac)2AE Pd(MS-dPvM)2TE
CO chemisorption
r (mmol/s kgPd)
TOF (s1)
rTEM2 (nm)
dchem (nm)
D (%)
250 C
300 C
250 C
300 C
7.0 13.5 11.0
6.5 8 9.0
16 27 12
71 85 48
169 0.04 112
0.05 0.11 0.04
0.11
TEM dTEM1 (nm)
rTEM1 (nm)
3.2
0.6 22.5 From 2 to 40 0.6 46.2
3.7
dTEM2 (nm)
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: consumption 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 product, C2H6 [9,10,22,23], which is much less useful from an industrial point of view. However, several authors demonstrated the ability of bimetallic catalysts, composed of alloys 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 catalysts results in an increase in ethylene selectivity, and for samples with 1.5 wt% for palladium and 3.0 wt% for silver or copper, this selectivity reaches 100% in the conditions of the catalytic test.
The consumption rate of 1,2-dichloroethane r is calculated from chromatographic measurements of C2H6, C2H5Cl and C2H4 concentrations in the reactor effluent and from the differential reactor equation that is written as follows: r¼
F A þ F Cl þ F E W
ðF A0 ; F Cl0 and F E0 ¼ 0Þ;
ð2Þ
where r is the consumption rate (mmol/kgPd s), 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 at the 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
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inside the reactor (kgPd). For all samples, r has been calculated 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 catalytic 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 inside the silica matrix. It was shown that the localization of the metal inside the silica matrix was induced by a nucleation process initiated by the silylated amine ligand of the metal, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). For example, when palladium is introduced in xerogels synthesized from EDAS, the 2þ PdðEDASÞ2 complexes are much more reactive than 2þ TEOS. Therefore, hydrolyzed PdðEDASÞ2 complexes can act as a nucleation agent leading to silica particles with a hydrolyzed and condensed PdðEDASÞ2þ core 2 and a shell principally made of hydrolyzed and condensed 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 silylated ligand takes place in the case of TEOS as the main silica precursor. Samples MS-acac-H, MS-dPvM and MS-dBzM present small homogeneous entities visible by TEM (Fig. 5(a)–(c)), which seem to be silica elementary particles. These observations show that the nucleation phenomenon 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 observed when palladium is complexed either by MSacac-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 palladium particle sizes from 2 to 40 nm and no spherical structure of silica particles is observed. These observations could be explained by the fact that the use of acetone to dissolve Pd(MS-acac)2 complex in ethanol does not allow preserving the metal complex architecture during 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 catalysts 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 acetylacetonate 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 also present a buckling-intrusion mechanism (Fig. 2(b)), characteristic features of macroporous solids [19]. These samples, MS-dBzM and EDAS, therefore have high values for the total specific pore volume, Vv equal to 3.1 cm3/g (Table 2) and 2.3 cm3/g [21], respectively. The shape of the adsorption isotherms of samples MSacac-H and MS-dPvM is of type IV at high relative pressure (Fig. 3(b), where MS-acac-H dried presents the same isotherm shape) and only a collapse mechanism is observed for both samples during mercury porosimetry measurements (Fig. 2(b), where MS-acac-H dried presents the same porosimetry curve). All these observations 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 silylated ligand influences xerogel textural properties and by 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 performed 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 compared with 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 4.2 cm3/g, 3.7 cm3/g (Table 2) and 3.9 cm3/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 isotherm shape) and a collapse mechanism is only observed for this sample during mercury porosimetry measurements
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
(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(MSacac)2TE, Pd(MS-acac)2AE (Table 2) and Pd(EDAS)2 [10]. So the nature of silylated ligand also influences textural properties of Pd/SiO2 cogelled xerogel catalysts. The specific surface area obtained by BET method, SBET presents a higher value equal to 1085 m2/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 micropores (pore size < 2 nm) is more important for sample MS-dPvM than for samples MS-acac-H, MS-dBzM and 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 < 2 nm) and small mesopores (2 nm < pore size < 10 nm) 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 37.2 nm (Table 2), microporosity is largely located inside silica particles. So samples MS-dPvM and Pd(MSdPvM)2TE are more microporous than all other samples. This higher microporosity with the use of silylated dipivaloylmethane ligand could be the result of its size of about 1 nm against sizes of about 0.6 nm for the other sylilated acetylacetonate ligands and of about 0.35 nm for EDAS (Molecular simulation with Chem3D Ultra Version 7.0). So greater repulsion between silylated dipivaloylmethane ligand molecules could generate more spaces into silica nuclei, increasing the microporosity inside silica particles. The silica particle sizes are also smaller 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 a small number of ligand molecules in silica nuclei. Before calcination and reduction steps, all the samples already have high specific surface areas, SBET (Table 2). When the samples are calcined in air, the organic groups are removed by thermolysis and/or pyrolysis reactions. This results in an increase in porosity and specific surface area [27,28]. For all samples, after calcination and reduction steps, the total cumulative pore volume, Vv slightly increases, whereas the specific surface area, SBET strongly increases, indicating that a large percentage of micropores and small mesopores are created by the removal of the organic groups during calcination and reduction steps (Table 2). Moreover, the shape of the isotherms of dried samples is not changed during calcination and reduction steps. Only micropores are additionally formed, whereas the meso- and macropores are retained after calcination and reduction steps.
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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 reactions are considerably diminished when the complexes are supported on a silica gel material [26]. Such reductions in activity may arise from changes in the coordination state of the complex or other environmental influences. In many cases, however, the reduced activity may simply result from a large fraction of supported metal complexes being rendered inaccessible to solvent or external reagents. Because palladium is located inside silica particles in Pd/SiO2 cogelled xerogel catalysts, there is a risk of inaccessibility. The values of Vv in Table 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 to TEM measurements, which suggests that palladium particles located inside silica particles are completely accessible. In Table 3, the specific consumption rate of 1,2dichloroethane, 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 0.04 s1 at 250 C and to about 0.11 s1 at 300 C. These values are identical to those obtained for 1,2-dichloroethane hydrodechlorination 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 examined in 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 synthesize new 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. This purpose is attained because the use of 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione (MS-acac-H), or 2,2,6, 6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione (MS-dPvM), or 1,3-diphenyl-2-[3-(trimethoxysilyl) propyl]-1,3-propanedione (MS-dBzM) in an ethanolic solution containing tetraethoxysilane (TEOS) and an ammonia solution of 0.18 mol/l gave very homogeneous colored xerogels, the textural properties of
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which depend on the nature of the silylated acetylacetonate ligand. No acetylacetonate deritative has ever been used as a ligand for the synthesis of metal xerogel catalysts. Nevertheless, 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 0.18 mol/l yielded Pd/SiO2 cogelled xerogel catalysts. These samples contain small metal particles of about 3.5 nm located inside silica particles exhibiting a monodisperse microporous distribution and some large metal particles from 20 to 50 nm situated outside the silica matrix. The silylated acetylacetonate ligand has a large influence 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 properties 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 and reduction steps, whereas the specific surface area, SBET strongly increases, indicating that a large percentage of micropores and small mesopores are created by the removal of the organic groups during calcination and reduction steps. Although palladium particles are located inside the silica particles, their complete accessibility, via the micropore network, is suggested. 1,2-dichloroethane hydrodechlorination 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 insensitive reaction compared to the ensemble size concept.
Acknowledgments S.L. is grateful to the Belgian Fonds pour la Formation a` la Recherche dans lÕIndustrie et dans lÕAgriculture, F.R.I.A., for a PhD grant. The authors also thank the Belgian Fonds National de la Recherche Scientifique, the Fonds de Bay, the Fonds de la Recherche Fondamentale et Collective, the Ministe`re de la Re´gion Wallonne and the Ministe`re de la Communaute´ Franc¸aise (Action de Recherche Concerte´e 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 ligand Ste´phanie Lambert a, Luigi Sacco b, Fabrice Ferauche a, Benoıˆt Heinrichs Alfred Noels b, Jean-Paul Pirard a b
a,*
,
a Laboratoire de Ge´nie Chimique, B6a, Universite´ de Lie`ge, B-4000 Lie`ge, Belgium Centre d’Enseignement et de Recherche des Macromole´cules (CERM), B6a, Universite´ de Lie`ge, B-4000 Lie`ge, 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 0.18 mol/l, from synthesized new silylated acetylacetonate ligands, respectively, 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione (MS-acac-H), 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)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 3.5 nm, located inside primary silica particles exhibiting a monodisperse microporous distribution as well as large palladium particles from 20 to 50 nm, 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 network, 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 insensitive reaction compared to the ensemble size concept. 2004 Elsevier B.V. All rights reserved.
1. Introduction Viable preparative routes to silylated organic molecules initially emerged in the 1960s, largely pioneered by a group at Degussa [1]. Development of these commercial-scale procedures was originally driven by the interest in the bifunctional reagents as adhesion promoters for inorganic–organic polymer composites such as glass fiber-reinforced materials and mineral-filled elas-
*
Corresponding author. Tel.: +32 4 366 3505; fax: +32 4 366 3545. E-mail address: [email protected] (B. Heinrichs).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.07.049
tomers. Further developments arose from the realization that attachment of catalytically active metal complexes to silica gels via silylated ligands afforded supported complexes, whose properties combined the positive attributes 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 react to form a sol. One of the most common reasons cited for the use of sol–gel processing is the intimate molecular-level mixing of components that can be obtained at the outset of the reactions [2,3]. In this way, several authors
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S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
developed the cogelation sol–gel process to disperse metal particles in a silica matrix [4–6]: organically substituted 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, copper, 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 ratio between 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 synthesize new 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 organo-metallic complexes. So, after vacuum drying, these silica xerogels could be used as catalysts in several organic reactions such as cyclopropanation, Kharasch reactions, etc. Furthermore, the samples are also calcined and reduced to analyze the dispersion, localization, and accessibility of palladium and to compare their catalytic activity to previously synthesized Pd/ SiO2 cogelled xerogel catalysts used for hydrodechlorination [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 4 A molecular sieves. Tetrahydrofurane (THF) was freshly distilled under argon from benzophenone and sodium prior to its use. Dichloromethane was distilled under argon from calcium hydride. Sodium acetylacetonate was synthesized from acetylacetone (Fig. 1, scheme 1, structure 1a) according to Charles [12] and dried at a temperature of 100 C and under a pressure of 0.07 Pa. The above-described synthesis method is also applied to dipivaloylmethane (Fig. 1, scheme 1, structure 1b) and dibenzoylmethane (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-dimethylformamide (DMSO). Indeed, the kind of halogen plays a crucial 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 dipivaloymethane and dibenzoylmethane (Fig. 1, scheme 1). Synthesis of 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione, MS-acac-H (Fig. 1, scheme 1, structure 2a). To a suspension of 5 g of sodium acetylacetonate in 30 ml of dry THF, 11.9 g of (3-iodopropyl)-trimethoxysilane 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 under reduced pressure and the product extracted with dry dichloromethane. The slightly viscous colorless liquid obtained by distillation (101–104 C/0.07 Pa) can be indefinitely stored at a temperature of 15 C under argon. Yield: 55%. Synthesis of 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)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 distillation (115–118 C/0.07 Pa) and stored at a temperature of 15 C under argon. Yield: 45%. Synthesis of 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-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 acetylacetone readily gives palladium acetylacetonate in a wide variety of solvents, this method does not work with the functionalized diones. A pre-reaction between the dione and potassium t-butoxyde is necessary, which then yields the palladium complex with 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione, Pd(MS-acac)2, 2,2,6, 6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione, Pd(MS-dPvM)2, and 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedione, Pd(MS-dBzM)2 (Fig. 1, scheme 2). Synthesis of palladium complexes from 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione, Pd(MS-acac)2, from 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione, Pd(MS-dPvM)2 and from 1, 3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedione, [Pd(MS-dBzM)2]. To a fine suspension of 1.1 g of potassium t-butoxyde in 150 ml of THF was added in a first time, either 3.4 g of 3-[3-(trimethoxysilyl)propyl]2,4-pentanedione or 3.4 g of 2,2,6,6-tetramethyl-4[3-(trimethoxysilyl)propyl]-3,5-heptanedione or 3.4 g of 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-prop-
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120 O
Scheme 1
O
O
R
O
R
NaOH R
R
Na+
O
R
+
Na 2CO 3 THF
O
1a, 1b,1c
O R
R
O
Si(OCH3)3
Si(OCH3)3
I
MeOH H2O
O R
R Na+ O
111
O
R
Si(OCH3)3 R
OH R
Si(OCH3)3 2a, 2b, 2c
a b c
R -CH3 -C(CH3)3 -Ph
Scheme 2 O
O
R
R
Pd (OAC) 2 KO tBu
(H3 CO) 3 Si
R O
Pd 2+
THF
O R
(H3 CO)3 Si
2 2a , 2b , 2c
3a , 3b, 3c
Fig. 1. Synthesis schemes of silylated acetylacetonate ligands and palladium complexes.
anedione and in a second time a solution of 1 g of palladium acetate in 10 ml of THF. After 20 min under stirring, the brown suspension was filtrated and the filtrate was evaporated under vacuum. A very viscous brown 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(MSdBzM)2 is 50%. The structures of these silylated complexes 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 1
H 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 acetylacetonate ligand or palladium complex. The names of palladium samples are followed by either the letters TE, 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
d
c
b
a
nm the silylated acetylacetonate palladium complex, is either Pd(MS-acac)2, or Pd(MS-dPvM)2, or Pd(MS-dBzM)2. nligand the silylated acetylacetonate ligand, is either MS-acac-H, or MS-dPvM, or MS-dBzM. ndiluant the diluant solvent, is THF or acetone. Weight loss = 100 · (mth ma)/mth, where mth is the theoretical mass and ma is the actual catalyst mass measured after drying, calcination and reduction steps.
– – – 0 0 0 – – – White gel Colorless gel Light-orange gel 514 514 514 – – – 253 253 253 2.93 3.12 2.91 – – – MS-acac-H MS-dPvM MS-dBzM
48.5 48.3 48.5
Ethanol Ethanol Ethanol
16 14 0 – 1.0 1.0 1.0 1.0 Black gel Black gel Grey gel Black suspension 343 343 343 343 123 136 123 123 230 237 230 230 1.57 1.68 1.59 1.48 0.266 0.273 0.267 0.255 Pd(MS-acac)2TE Pd(MS-acac)2AE Pd(MS-dPvM)2TE Pd(MS-dBzM)2TE
44.5 45.7 44.5 44.6
THF-ethanol Acetone-ethanol THF-ethanol THF-ethanol
Weight lossd ±2% (wt%) Theoretical metal loading (wt%) Gelation behavior nC2H5 OH (mmol) ndiluantc (mmol) Solvent mixture nH2O (mmol) nTEOS (mmol) nligandb (mmol) nma (mmol) Sample
Table 1 Synthesis operating variables of xerogels and Pd/SiO2 cogelled xerogel catalysts
1.2 1.2 1.0 –
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
Actual metal loading ±0.1% (wt%)
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(Si(OC2H5)4) and aqueous ammonia. So a 0.18 N 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. Four Pd/SiO2 gels were prepared from the corresponding silylated acetylacetonate palladium complex –Pd(MSacac)2 or Pd(MS-dPvM)2 or Pd(MS-dBzM)2–, TEOS and aqueous ammonia. In these syntheses, Pd(MSacac)2 or Pd(MS-dPvM)2 or Pd(MS-dBzM)2 was dissolved in a mixture of 10 ml of ethanol and 10 ml of acetone or THF, and in which some amount of silylated acetylacetonate ligand corresponding to the used palladium complex was present to allow gelification. So the molar ratio silylated ligand/TEOS is equal to 0.05 in palladium gels. After addition of TEOS, a 0.18 N NH3 aqueous solution in the remaining ethanol was added to the mixture under vigorous stirring. The volume of final solutions was 30 ml. The hydrolysis ratio, which is the molar ratio H = [H2O]/([TEOS] + 3/4 [sylilated ligand]), and the dilution ratio, which is the molar ratio R = [ethanol]/([TEOS] + [sylilated ligand]) were kept constant at values of 5 and 10 respectively for all samples. 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 1200 Pa after 90 h. The drying oven was then heated at 150 C for 72 h. 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 (0.02 mmol/s); this temperature was then maintained for 12 h in air (0.1 mmol/s). All dried SiO2 xerogels were heat treated in air at 450 C for 72 h 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 (0.23 mmol/s) and maintained at this temperature for 3 h (same flow). 2.4. Characterization of SiO2 xerogels and Pd/SiO2 cogelled xerogel catalysts Nitrogen adsorption–desorption isotherms were measured at 77 K on a Fisons Sorptomatic 1990 after outgassing for 24 h at ambient temperature. After a 2-h outgassing at ambient temperature, mercury porosimetry measurements were performed with sample monoliths using a manual porosimeter from 0.01 to 0.1 MPa and a Carlo Erba Porosimeter 2000 from 0.1 to 200 MPa.
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
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 72 h at 60 C and 60 nm slices were then cut up with a Reichert-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 (0.003 mmol/s) at 350 C for 3 h. Afterwards, this sample was outgassed under vacuum at 340 C for 16 h. A double adsorption method was used: (i) first adsorption isotherm was measured, which includes both physisorption and chemisorption; (ii) after a 2 h outgassing at 30 C, a second isotherm was measured, which includes physisorption only. Both isotherms were determined in the pressure range of 108–2 · 101 kPa. The difference between first and second isotherms gave the CO chemisorption isotherm. The latter theoretically exhibits a horizontal linear region corresponding to the complete coverage of metallic sites by a monolayer of adsorbate. However the linear region of experimental chemisorption 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 was conducted in a stainless steel tubular reactor (internal diameter: 10 mm) at a pressure of 0.3 MPa. 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 chromatography (ThermoFinnigan with FID) using a Porapak Q5 packed column. Prior to each experiment, the Pd/ SiO2 catalysts were reduced in situ at atmospheric pressure in flowing H2 (0.023 mmol/s) while being heated to 350 C at a rate of 350 C/h and were maintained at this temperature for 3 h. After reduction, the Pd/SiO2 catalysts were cooled in H2 to the desired initial reaction temperature of 300 C. The total flow of the reactant mixture was 0.45 mmol/s and consisted of ClCH2– CH2Cl (0.011 mmol/s), H2 (0.023 mmol/s), and He (0.42 mmol/s). The temperature was successively kept at 300 C (10 h to allow activity stabilization after fast initial deactivation), 350 C (2 h), and 300 C (2 h). The effluent was analyzed every 15 min and eight analyses were made at each temperature (40 for the first level). For each catalytic experiment, the mass of catalyst pellets, sieved between 250 and 500 lm, was equal to 2 · 104 kg. Thanks to the very open structure of Pd/ SiO2 cogelled xerogel catalysts, reaction rate values obtained in the present study were not falsified by diffu-
113
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 2841 cm1 for MS-dPvM (Fig. 1, scheme 1, structure 2b), and 2942 and 2840 cm1 for MS-dBzM (Fig. 1, scheme 1, structure 2c). Phenyl groups in MS-dBzM are responsible for the broad signal at 3056 cm1 (C–H elongation). Characteristic absorptions of carbonyl groups occur at 1713 cm1 for MS-dPvM, and 1698 and 1672 cm1 for MS-dBzM. Characteristic elongations of Si–OCH3 in MS-dPvM and MS-dBzM are present at 1199 and 1087 cm1. The absorptions of these functional groups occur at identical frequencies for palladium complexes, while the carbonyl groups are more sensitive to coordination by the metal and resonate at 1535 cm1 for Pd(MS-acac)2 (Fig. 1, scheme 2, structure 3a), 1575 cm1 for Pd(MS-dPvM)2 (Fig. 1, scheme 2, structure 3b), and 1595 cm1 for Pd(MS-dBzM)2 (Fig. 1, scheme 2, structure 3c). 1 H and 13C NMR shift assignments of MS-acac-H (Fig. 1, scheme 1, structure 2a) are reported in the literature [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 unsubstituted dione and the (trimethoxysilyl)propyl-substituted moiety. Only the two singlets corresponding to the two methyl groups in MS-acac-H are replaced by either one singlet at 0.92 ppm for MS-dPvM (18H; t-butyl groups) or one multiplet at 7–8 ppm for MS-dBzM (10H; phenyl groups). The 1H NMR spectra of palladium complexes (Fig. 1, scheme 2, structures 3a, 3b and 3c) and of the respective free ligands are quite similar, although the peaks are slightly broader and the triplets at about 4 ppm are missing in the three palladium complexes, indicating that deprotonation in a-position has occurred to produce the metal complexes. In the 13C NMR spectra, the absorption due to the methyl groups in MS-acac-H (Fig. 1, scheme 1, structure 2a) is replaced by absorptions at 27.5 and 44.2 ppm (tbutyl groups) for MS-dPvM (Fig. 1, scheme 1, structure 2b), or 129.4, 129.8, 134.4 and 137 ppm (phenyl groups) for MS-dBzM (Fig. 1, scheme 1, structure 2c). Carbonyl groups resonate at 210.4 ppm in MS-dPvM and 197 ppm in MS-dBzM. Palladium complexes give similar 13C spectra, with the difference that the a-carbon absorption (seen at about 51 ppm for the free ligands) is shifted downfield to 110 ppm for Pd(MS-acac)2 (Fig. 1, scheme
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S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
2, structure 3a), to 121 ppm for Pd(MS-dPvM)2 (Fig. 1, scheme 2, structure 3b), and to 145 ppm for Pd(MSdBzM)2 (Fig. 1, scheme 2, structure 3c), while C–O absorption is shifted to 186 ppm for Pd(MS-acac)2, to 160 ppm for Pd(MS-dPvM)2, and to 185.5 ppm for Pd(MS-dBzM)2.
sample names are followed by the word ÔdriedÕ in Table 2) and for the same samples after vacuum drying, calcination, 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 low pressure. This appears in the pressurization curve by a sudden change of slope at Pt on the curve of pressure increase. The curve of pressure decrease can also be divided 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 crushing phenomenon 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 WashburnÕs model [19] and the collapse of larger pores below Pt described by PirardÕs 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 ratio is particularly high [17,21]. The samples Pd(MSdPvM)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-
3.2. Characterization of xerogels and Pd/SiO2 cogelled xerogel catalysts For all samples, except Pd(MS-dBzM)2TE, homogeneous gels are obtained after about 7 h in the oven at the temperature of 80 C. No gel is obtained with the sample Pd(MS-dBzM)2TE because a black suspension appears when the complex Pd(MS-dBzM)2 is dissolved in the mixture of 10 ml of ethanol and 10 ml of THF. The actual palladium loading in cogelled catalysts is generally higher than theoretical loading because the actual catalyst mass after drying, calcination and reduction steps, is lower than theoretical mass. In fact, some TEOS often remains unreacted and is volatilized during vacuum drying. This theoretical mass (mth) is calculated from Eq. (1): mth ¼ nm M m þ ðnTEOS þ nligand þ 2nm ÞM SiO2 ;
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 sylilated acetylacetonate ligand in the gel (mmol); MSiO2 is the molecular weight of SiO2, 60.085 g/mol. In this equation, it is assumed that all TEOS and ligand molecules are converted into SiO2. Results in Table 1 shows that there are only weight losses for samples Pd(MSacac)2TE and Pd(MS-acac)2AE after vacuum drying, calcination and reduction steps. The results of textural properties are presented in Table 2 both for samples after vacuum drying step (the
Table 2 Sample textural properties Sample
Pt (MPa)
Vv (cm3/g)
SBET (m2/g)
Pd(MS-acac)2TE dried Pd(MS-acac)2TE Pd(MS-acac)2AE Pd(MS-dPvM)2TE dried Pd(MS-dPvM)2TE
10 8 16 –b 170
3.6 4.2 3.7 1.6 2.1
66 415 700 491 820
–a 16.9 ± 1.6 14.5 ± 1.3 –a 11.2 ± 1.0
MS-acac-H dried MS-acac-H MS-dPvM dried MS-dPvM MS-dBzM dried MS-dBzM
–b 165 –b –b 24 16
1.6 2.1 1.3 1.9 2.4 3.1
471 890 624 1085 432 825
–a 15.0 ± 1.2 –a 8.7 ± 0.9 –a 37.2 ± 5.8
dSiO2 (nm)
Pt: pressure of change of mechanism during mercury porosimetry; Vv: total cumulative specific pore volume; SBET: specific surface area obtained by BET method; dSiO2: silica particle diameter measured by TEM. a Not measurable. b Not applicable, only collapse.
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120 5
1000 (a) Pd/SiO2 xerogel catalysts
800
(a) Pd/SiO2 xerogel catalysts
VN2 (cm3/g)
VHg (cm3/g)
4
115
3 2
600 400 200
1
0 0 0.01
0 0.1
1
10
100
0.2
0.4
1000
0.6
0.8
1
0.6
0.8
1
p/p0
Pressure (MPa) 1000 5
800 (b) SiO2 xerogels
VN2 (cm3/g)
VHg (cm3/g)
4 3 2
600 400 200
1
0
0 0.01
(b) SiO2 xerogels
0.1
1
10
100
1000
Pressure (MPa) 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.
cury after a mercury porosimetry experiment. The entire pore size distribution is determined by PirardÕs collapse model. In Table 2, Pt evolves very strongly with the nature of the silylated acetylacetonate ligands and palladium complexes. 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 adsorption–desorption isotherm is characterized by: (i) at low relative pressure, a sharp increase of the adsorbed volume 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 macroporous adsorbents. The volume adsorbed between p/ p0 = 0.95 and 1 is large, indicating that the sample contains pores of large dimensions; (iii) all samples exhibit a narrow adsorption–desorption hysteresis loop for p/p0 values close to 1, and this hysteresis is characteristic of capillary condensation in large mesopores. The b-type nitrogen adsorption–desorption isotherms have the fol-
0
0.2
0.4
p/p0 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.
lowing characteristics: a nitrogen adsorption–desorption type 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 surface area, SBET. The a-type nitrogen adsorption–desorption isotherm is observed for samples Pd(MS-acac)2TE, Pd(MSacac)2TE dried, Pd(MS-acac)2AE, MS-dBzM and MS-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-acacH, 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), respectively. 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 specific surface area, SBET strongly increases when the dried samples are calcined and reduced. Furthermore, SBET have smaller values for Pd/SiO2 cogelled xerogel cata-
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Cumulative volume (cm3/g)
10 (a) Pd/SiO2 xerogel catalysts 1
0.1
0.01
0.001 0.1
1
10
100
1000
Pore size (nm) Cumulative volume (cm3/g)
10
(b) SiO 2 xerogels 1
0.1
0.01
0.001 0.1
1
10
100
1000
Pore size (nm) 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), MSdPvM dried (h), MS-dBzM (), MS-dBzM dried () and EDAS (·) [16].
lysts than SiO2 xerogels synthesized from the same silylated 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. These curves were obtained by applying a combination of various 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 0.8 nm, followed by a plateau (Fig. 4). In the range of meso- and macropores, one observes that all samples exhibit a broad distribution. These samples then contain micropores (width < 2 nm), mesopores (2 nm < width < 50 nm) and macropores (width > 50 nm). The total cumulative specific pore volume, Vv that is, the pore volume obtained by addition of pore volume measured by mercury porosimetry (width > 7.5 nm) and cumulative volume of micropores and mesopores of widths between 2 and 7.5 nm measured by nitrogen adsorption–desorption, increases when dried SiO2 xerogels are calcined and when Pd/SiO2 cogelled xerogel catalysts 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 acetylacetonate ligand changes. Furthermore, it has been shown in [17] that the relation between the pressure of transition, Pt and the size of SiO2 particles, dSiO2 is given by the relationship d SiO2 1=P 0:75 in the case of the buckling t of the brittle filaments of mineral oxide under an axial compressive stress. In all samples, when dSiO2 increases, Pt decreases and inversely (Table 2). Table 3 gives palladium particle size determined by TEM and CO chemisorption measurements and catalytic 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 3 nm and large crystallites between 20 and 50 nm (Fig. 5(d)). For both catalysts, the mean diameter of small 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 fifteen crystallites. Concerning localization of palladium crystallites, it appears that cogelled catalysts are composed of silica particles arranged in strings or aggregates, and although TEM gives only a 2D view, it seems that small palladium particles are located inside silica particles, whereas large palladium particles are located at their surface (Fig. 5(d)). The sample Pd(MSacac)2AE exhibits a broad distribution of palladium particle sizes, from 2 to 40 nm. A mean diameter, dchem of palladium particles has also been derived from metal dispersion, D, measured by CO chemisorption for Pd/SiO2 cogelled xerogel catalysts as proposed by Lambert et al. [9–11]. For Pd(MSacac)2TE and Pd(MS-dPvM)2TE containing two families 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 represents a mean diameter of all palladium particles in this sample. Pd(MS-acac)2TE, Pd(MS-acac)2AE and Pd(MSdPvM)2TE mainly produce ethane, C2H6, and two secondary 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 hydrodechlorination reaction [22,23]. In the case of 1,2-dichloroethane hydrodechlorination, the noble metal participates in a catalytic cycle, in which the reactant is dechlorinated
S. Lambert et al. / Journal of Non-Crystalline Solids 343 (2004) 109–120
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Fig. 5. TEM micrographs of (a) MS-acac-H (500 000·), of (b) MS-dPvM (500 000·), of (c) MS-dBzM (500 000·) and of (d) Pd(MS-acac)2TE (725 000·).
Table 3 Palladium average particle size and catalytic activity for 1,2-dichloroethane hydrodechlorination Sample
Pd(MS-acac)2TE Pd(MS-acac)2AE Pd(MS-dPvM)2TE
CO chemisorption
r (mmol/s kgPd)
TOF (s1)
rTEM2 (nm)
dchem (nm)
D (%)
250 C
300 C
250 C
300 C
7.0 13.5 11.0
6.5 8 9.0
16 27 12
71 85 48
169 0.04 112
0.05 0.11 0.04
0.11
TEM dTEM1 (nm)
rTEM1 (nm)
3.2
0.6 22.5 From 2 to 40 0.6 46.2
3.7
dTEM2 (nm)
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: consumption 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 product, C2H6 [9,10,22,23], which is much less useful from an industrial point of view. However, several authors demonstrated the ability of bimetallic catalysts, composed of alloys 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 catalysts results in an increase in ethylene selectivity, and for samples with 1.5 wt% for palladium and 3.0 wt% for silver or copper, this selectivity reaches 100% in the conditions of the catalytic test.
The consumption rate of 1,2-dichloroethane r is calculated from chromatographic measurements of C2H6, C2H5Cl and C2H4 concentrations in the reactor effluent and from the differential reactor equation that is written as follows: r¼
F A þ F Cl þ F E W
ðF A0 ; F Cl0 and F E0 ¼ 0Þ;
ð2Þ
where r is the consumption rate (mmol/kgPd s), 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 at the 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
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inside the reactor (kgPd). For all samples, r has been calculated 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 catalytic 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 inside the silica matrix. It was shown that the localization of the metal inside the silica matrix was induced by a nucleation process initiated by the silylated amine ligand of the metal, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). For example, when palladium is introduced in xerogels synthesized from EDAS, the 2þ PdðEDASÞ2 complexes are much more reactive than 2þ TEOS. Therefore, hydrolyzed PdðEDASÞ2 complexes can act as a nucleation agent leading to silica particles with a hydrolyzed and condensed PdðEDASÞ2þ core 2 and a shell principally made of hydrolyzed and condensed 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 silylated ligand takes place in the case of TEOS as the main silica precursor. Samples MS-acac-H, MS-dPvM and MS-dBzM present small homogeneous entities visible by TEM (Fig. 5(a)–(c)), which seem to be silica elementary particles. These observations show that the nucleation phenomenon 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 observed when palladium is complexed either by MSacac-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 palladium particle sizes from 2 to 40 nm and no spherical structure of silica particles is observed. These observations could be explained by the fact that the use of acetone to dissolve Pd(MS-acac)2 complex in ethanol does not allow preserving the metal complex architecture during 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 catalysts 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 acetylacetonate 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 also present a buckling-intrusion mechanism (Fig. 2(b)), characteristic features of macroporous solids [19]. These samples, MS-dBzM and EDAS, therefore have high values for the total specific pore volume, Vv equal to 3.1 cm3/g (Table 2) and 2.3 cm3/g [21], respectively. The shape of the adsorption isotherms of samples MSacac-H and MS-dPvM is of type IV at high relative pressure (Fig. 3(b), where MS-acac-H dried presents the same isotherm shape) and only a collapse mechanism is observed for both samples during mercury porosimetry measurements (Fig. 2(b), where MS-acac-H dried presents the same porosimetry curve). All these observations 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 silylated ligand influences xerogel textural properties and by 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 performed 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 compared with 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 4.2 cm3/g, 3.7 cm3/g (Table 2) and 3.9 cm3/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 isotherm shape) and a collapse mechanism is only observed for this sample during mercury porosimetry measurements
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(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(MSacac)2TE, Pd(MS-acac)2AE (Table 2) and Pd(EDAS)2 [10]. So the nature of silylated ligand also influences textural properties of Pd/SiO2 cogelled xerogel catalysts. The specific surface area obtained by BET method, SBET presents a higher value equal to 1085 m2/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 micropores (pore size < 2 nm) is more important for sample MS-dPvM than for samples MS-acac-H, MS-dBzM and 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 < 2 nm) and small mesopores (2 nm < pore size < 10 nm) 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 37.2 nm (Table 2), microporosity is largely located inside silica particles. So samples MS-dPvM and Pd(MSdPvM)2TE are more microporous than all other samples. This higher microporosity with the use of silylated dipivaloylmethane ligand could be the result of its size of about 1 nm against sizes of about 0.6 nm for the other sylilated acetylacetonate ligands and of about 0.35 nm for EDAS (Molecular simulation with Chem3D Ultra Version 7.0). So greater repulsion between silylated dipivaloylmethane ligand molecules could generate more spaces into silica nuclei, increasing the microporosity inside silica particles. The silica particle sizes are also smaller 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 a small number of ligand molecules in silica nuclei. Before calcination and reduction steps, all the samples already have high specific surface areas, SBET (Table 2). When the samples are calcined in air, the organic groups are removed by thermolysis and/or pyrolysis reactions. This results in an increase in porosity and specific surface area [27,28]. For all samples, after calcination and reduction steps, the total cumulative pore volume, Vv slightly increases, whereas the specific surface area, SBET strongly increases, indicating that a large percentage of micropores and small mesopores are created by the removal of the organic groups during calcination and reduction steps (Table 2). Moreover, the shape of the isotherms of dried samples is not changed during calcination and reduction steps. Only micropores are additionally formed, whereas the meso- and macropores are retained after calcination and reduction steps.
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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 reactions are considerably diminished when the complexes are supported on a silica gel material [26]. Such reductions in activity may arise from changes in the coordination state of the complex or other environmental influences. In many cases, however, the reduced activity may simply result from a large fraction of supported metal complexes being rendered inaccessible to solvent or external reagents. Because palladium is located inside silica particles in Pd/SiO2 cogelled xerogel catalysts, there is a risk of inaccessibility. The values of Vv in Table 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 to TEM measurements, which suggests that palladium particles located inside silica particles are completely accessible. In Table 3, the specific consumption rate of 1,2dichloroethane, 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 0.04 s1 at 250 C and to about 0.11 s1 at 300 C. These values are identical to those obtained for 1,2-dichloroethane hydrodechlorination 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 examined in 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 synthesize new 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. This purpose is attained because the use of 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione (MS-acac-H), or 2,2,6, 6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione (MS-dPvM), or 1,3-diphenyl-2-[3-(trimethoxysilyl) propyl]-1,3-propanedione (MS-dBzM) in an ethanolic solution containing tetraethoxysilane (TEOS) and an ammonia solution of 0.18 mol/l gave very homogeneous colored xerogels, the textural properties of
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which depend on the nature of the silylated acetylacetonate ligand. No acetylacetonate deritative has ever been used as a ligand for the synthesis of metal xerogel catalysts. Nevertheless, 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 0.18 mol/l yielded Pd/SiO2 cogelled xerogel catalysts. These samples contain small metal particles of about 3.5 nm located inside silica particles exhibiting a monodisperse microporous distribution and some large metal particles from 20 to 50 nm situated outside the silica matrix. The silylated acetylacetonate ligand has a large influence 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 properties 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 and reduction steps, whereas the specific surface area, SBET strongly increases, indicating that a large percentage of micropores and small mesopores are created by the removal of the organic groups during calcination and reduction steps. Although palladium particles are located inside the silica particles, their complete accessibility, via the micropore network, is suggested. 1,2-dichloroethane hydrodechlorination 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 insensitive reaction compared to the ensemble size concept.
Acknowledgments S.L. is grateful to the Belgian Fonds pour la Formation a` la Recherche dans lÕIndustrie et dans lÕAgriculture, F.R.I.A., for a PhD grant. The authors also thank the Belgian Fonds National de la Recherche Scientifique, the Fonds de Bay, the Fonds de la Recherche Fondamentale et Collective, the Ministe`re de la Re´gion Wallonne and the Ministe`re de la Communaute´ Franc¸aise (Action de Recherche Concerte´e n 00-05-265) for their financial support.
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