SiO2 Sol-gel Catalysts Designed for Selective Conversion of Chlorinated Alkanes Into Alkenes

91998 Elsevier ScienceB.V. All rights reserved. Preparation of Catalysts VII i B. Delmonet al., editors.

707

Pd-Ag/SiO) sol-gel catalysts designed for selective conversion of chlorinated alkanes into alkenes B. Heinrichs a'* , P. Delhez a, J.-P. Schoebrechts b, and J.-P. Pirard a aLaboratoire de G6nie Chimique, B6a, Universit6 de Liege, B-4000 Liege, Belgium bLaboratoire Central, Solvay, S.A., Rue de Ransbeek, 310, B-1120 Brussels, Belgium Aerogel-like Pd-Ag/SiO2 catalysts were prepared in a one-step sol-gel process by using Pd and Ag complexes containing an alkoxide moiety with ensuing ordinary drying under vacuum. Although they are trapped inside microporous silica particles which makes them sinter-proof, the resulting bimetallic particles are completely accessible. The formation of Pd-Ag alloy crystallites allows to obtain a very high selectivity in ethylene during hydrodechlorination of 1,2-dichloroethane to the detriment of ethane which is the main product when pure Pd is used. 1. INTRODUCTION The common industrially important preparation methods of dispersed metal catalysts are multistep processes consisting of [ 1, 2]: (a) preparation of the support; (b) distribution of the active component precursor over the support surface (by impregnation, ion exchange, anchoring, ...); (c) drying, calcination and reduction of the precursor compound into the active metallic phase. The application of the sol-gel process to catalysis allows to prepare a solid from a homogeneous solution which includes not only the metal precursor, but also the support precursor [3]. Steps (a) and (b) corresponding to classical methods are then gathered in a single step. Several authors synthesized metallic supported catalysts by the sol-gel route and showed its ability to highly disperse catalytic metals on gels whose texture is finely controlled. Most of the time, the metal of interest is introduced in the initial solution (whose main components are for example aluminum tri-sec-butoxide (ATB) or tetraethoxysilane (TEOS) and water in alcohol) in the form of a salt (e.g. H2PtC16, PdC12, Pd(CH3CO2)2, RuC13, etc.) [36]. Schubert and coworkers used a particularly interesting method to homogeneously disperse nanometer-sized metal particles in a silica gel [7-10]. These authors used alkoxides of the type (RO)3Si-X-A in which a functional organic group A, able of forming a chelate with a cation of a metal like palladium, nickel, silver, copper, etc., is connected to the hydrolysable alkoxide moiety (RO)3Si- via an inert and hydrolytically stable spacer X. The cocondensation of such *E-mail: [email protected]

708 molecules with a network-forming reagent such as Si(OC2H5)4 (TEOS) results in materials in which the catalytic metal is anchored to the SiO2 matrix. Schubert et al. applied this method to the preparation of monometallic and bimetallic catalysts supported on silica. In a previous work, we used this method for the preparation of transition metal aerogelsupported catalysts [11]. In the case of a metallic cation with a coordination number of 4 complexed by [3-(2-aminoethyl)aminopropyl]trimethoxysilane H2NCH2CH2NH(CH2)3Si(OCH3)3 (EDAS), the introduction of the active component precursor in the silica framework proceeds as shown in Figure 1. CH2-CH2 / ',, H5020,, /,0C2H5 H2N",. n4L"NH-(CH2}3-Si(OCH3]3 NH3/H20 \Si + ,M, ~ ~ / / "xX ,., CHaOH H5C20 0C2H5 (H3CO}3Si-(CH213-HN. 'NH2 - C2HsOH CH2-CH2 ,,

/ "" Si-----

CH2-CH2 / ',

"', rig" \ ,.M,,. /oJS~-(CH2}~-HI~" iNH2 0

0

\

-s~ / 0 /

/

0 /

CH2-CH2

-----Si-I

Figure 1. Introduction of Mn+(EDAS)2 in a SiO2 framework This picture points out the risk of confining the metal inside the silica matrix and making it inaccessible or difficult to reach for a fluid phase. The case of Pd/SiO2 aerogel catalysts was examined in detail [ 12]. Figure 2 represents schematically TEM pictures of those samples after supercritical drying. It appeared that the cogelled catalysts (the term "cogelled" refers to the cocondensation of pd2+(EDAS)z with TEOS) are composed of palladium crystallites with a diameter of about 2 nm located inside quasi-monodisperse silica particles whose diameter is between 10 and 20 nm depending on the considered sample. This metal localization inside SiO2 particles is probably a consequence of the ligand used: the hydrolysable functions in EDAS allow the formation of Si-O-Si bonds all around the complex (Figure 1) and the observation of TEM pictures suggests that a group of such complexes can Figure 2. Schematic representation act as a nucleation agent which leads after all to silica of TEM micrographs of Pd/SiO2 particles with a palladium heart. In order to assess the aerogel catalysts validity of this nucleation by palladium complex hypothesis, the relation between silica particle volume (Vp) and TEOS, EDAS and Pd 2+ concentrations ([TEOS], [EDAS], [pd2+]) was examined. It was shown that Vp is directly proportional to the ratio ([TEOS]+[EDAS])/[Pd 2+] which is in agreement with a nucleation phenomenon [12]. It was then reasonably assumed that gel formation occurs via SiO2 particle nucleation by a set of Pd 2+ complexes, particles growth thanks to hydrolysis and condensation of methoxy groups of EDAS and TEOS, and finally particles aggregation.

709 A very important concern about cogeUed catalysts is the accessibility of the active centers. Because palladium is located inside silica particles, there is a risk that it may not be accessible. By performing drying under supercritical conditions, the goal was to maintain the porosity and thus the accessibility as high as possible. In order to check the accessibility of Pd crystallites, their mean size were measured by TEM as well as by carbon monoxide chemisorption. The two techniques gave the same size which was the proof that all Pd particles are accessible for CO and then for any reactants (provided that the molecules are not too large) in a catalytic system. A detailed analysis of the texture showed that the samples exhibit a continuous macroand mesopore size distribution located in voids between silica particles and a narrow monodisperse micropore distribution centered on a pore size of 0.8 nm located inside SiO2 particles. Pd crystallites are then completely accessible via this micropore network. This particular structure is of great importance to the behavior of cogelled aerogel catalysts in relation to sintering. Because they are larger than the micropores of the silica particles in which they are located, the palladium particles are trapped and are then unable to sinter by migration and coalescence [13]. In consequence, those catalysts are sinter-proof during supercritical drying. It was indeed shown with impregnated catalysts, in which Pd crystallites are dispersed on the outer surface of SiO2 particles, that supercritical drying leads to an extensive sintering when the metal particles are not trapped. Knowing that sintering of metals loaded on a support is one of the main causes for deactivation of industrial catalysts [ 14], cogelled catalysts could be potential candidates for high temperature applications. The cogelation method described above was adapted to the preparation of bimetallic catalysts composed of metals from groups VIII and IB supported on xerogels in order to use them for the selective hydrodechlorination of chlorinated alkanes into alkenes [ 15]. Ito et al. [16] indeed recently demonstrated the ability of bimetallic catalysts from groups VIII and IB to convert chlorinated alkanes into less-chlorinated or unchlorinated alkenes. This new process is economically very attractive since it allows the recycling of alkenes which are raw materials for numerous industrial reactions. The purpose of this paper is to explain the mechanisms of formation of those bimetallic xerogel catalysts and to show that many conclusions relative to the above described metal aerogel-supported catalysts still apply. The interest of those materials for selective hydrodechlorination is also emphasized. 2. EXPERIMENTAL

2.1. Catalysts preparation Six samples containing various amounts of Pd and Ag were prepared. The synthesis parameters are presented in Table 1. For each sample, X or A denote the way used to dry the gel, xerogel dried under vacuum or aerogel dried under supercritical conditions, respectively, followed by the nominal overall atomic percentage iof Ag in the sample. The general synthesis procedure was as follows. For mixture A, to a suspension of Pd acetylacetonate powder (Pd(acac)2) in a quart of the total volume of ethanol, EDAS is added; for mixture B, to a suspension of Ag acetate powder (AgOAc) in another quart of ethanol, 3(aminopropyl)triethoxysilane, HEN(CH2)aSi(OC2Hs)3 (AS) is added. For the monometallic samples X0 and X100, only one mixture was prepared in half the total volume of ethanol.

710 Mixtures A and B were stirred at ambient temperature during 1 h (formation of Pd and Ag complexes) after which they were mixed together. Tetraethoxysilane (TEOS) was then added. Finally, a solution containing aqueous 0.18N NH3 in the remaining ethanol, was slowly added under vigorous stirring. The vessel was then closed tightly and heated to 70~ for 72 h (gelation and aging). In all gels, the molar ratios EDAS / Pd and AS / Ag = 2 and in the pure silver sample X100, the addition of EDAS was necessary to obtain gelation of the solution. After aging, gels X0 to X100 were dried under vacuum at 150~ for 72 h. The resulting samples are xerogels. Gel A67, which is identical to gel X67, was dried under supercritical conditions at 327~ and 13 MPa and is an aerogel. All gels were subsequently calcined under air at 400~ during 12 h and reduced under 1-I2at 350~ during 3h. Table 1 Synthesis parameters Nominal Ag Ag/(Pd+Ag) Gel time Catalyst Pd(acac)2 Ag(OAc) EDAS AS Pd (wt%) (at%) (min) (mmol) (mmol) (mmol) (mmol) (wt%) 0 1.5 0 0 26 5 36 X0 2.69 0 2.77 1.5 0.75 33 15 541 X33 2.70 1.35 5.40 1.5 1.5 50 15 5 48 X50 2.75 2.70 11.06 1.5 3.0 67 13 5.55 X67 2.76 5.48 5.32 0 1.5 100 30 5.32 X100 0 2.65 11.06 1.5 3.0 67 12 5.55 A67 2.77 5.47 TEOS: 300 mmol; 1-120:1500 mmol; NH3:4.9 mmol; C2HsOH: 3100 mmol

2.2. Catalysts characterization and catalytic experiments The real metals contents were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The composition and size of the metallic particles were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDX). Texture was examined by N2 adsorption at 77 K, Hg porosimetry, and TEM. The hydrodechlorination of 1,2-dichloroethane was conducted in a stainless steel tubular reactor at 300~ and 0.3 Mpa. For each experiment, 0.11 g of catalysts pellets were used. The flow of the reactant mixture consisted of CH2C1-CH2C1 (1 N-l/h), H2 (2 N1/h), and He (37 N1/h). More details about catalysts preparation~ characterization and test can be found in [ 17].

3. RESULTS In all xerogels, metals contents measured by ICP-AES are higher than the nominal values and the gap between nominal and experimental contents decreases when the silver loading increases (Tables 1, 2). However, the weight ratio Ag/(Pd+Ag), which is equal to the atomic ratio since Pd and Ag have almost the same atomic weight, remains identical to the nominal ratio. Metals loadings higher than nominal values could therefore be due to a loss of unreacted

711 TEOS during drying. In aerogel A67, nominal and experimental metals contents are equal which could be due to the ability of supercritical drying to lead reactions to completion. Table 2 Characterization of metallic phases . . . . ICP-AES (wt%) Ag/(Pd+Ag) (at%) Particle size (nm) XRD XRD TEM Catalyst Pd Ag ICP-AES Phase 1 Phase 2 Phase 1 Phase 2 Small Large X0 3.3 0 0 0 7.7 2.3 28 X33 2.2 1.1 33 31 100 1.9 >30 2.4 9 X50 2.3 2.2 49 43 100 2.1 >30 3.0 7 X67 1.9 3.7 66 46 100 2.0 18 3.0 10 X100 0 1.7 100 100 10.3 1 to 20 A67 1.5 3.0 67 8 100 2.7 6 2.4 10 XRD spectra of bimetallic xerogels X33, X50 and X67 exhibit a broad peak between the Bragg lines of pure Pd and Ag which demonstrates the presence of a solid solution (SS) Pd-Ag (phase 1). The three samples, but particularly catalyst X67, show also peaks characteristic of unalloyed Ag (phase 2). The composition of the solid solution was calculated from the unit cell parameter corresponding to the SS peak [ 18]. Results in Table 2 indicate that the composition of the bimetallic particles is almost the same as the overall ICP-AES composition (= nominal composition) in xerogel X33, but it deviates increasingly from the overall composition for growing Ag loading. The main reason for this composition gap is probably the presence of a pure silver phase but the shift toward a lower silver percentage may also result from surface enrichment with Ag [ 19]. This enrichment results from the lower surface energy of Ag with respect to Pd [20]. The very wide SS peak are due to the small size of the particles and/or to a distribution of composition [ 19]. With the assumption that all the bimetallic particles have the same Pd-Ag composition, their size can be calculated from the peak broadening thanks to the Scherrer equation [18]. Bimetallic particles in xerogels X33, X50, and X67 are found to be finely dispersed, whereas pure silver particles in these samples are much larger. XRD analysis of aerogel A67 shows an almost total segregation of Pd and Ag with small Pd-containing particles and large pure Ag particles. In figure 3 are schematically represented TEM pictures of bimetallic catalysts. TEM analysis show that all Pd containing samples exhibit metal particles distributed in two families of different size (Table 2). Bimetallic xerogels X33, X50, X67 and aerogel A67 exhibit small and large particles with diameters of 2 to 3 nm and 7 to 10 nm, respectively. It appears that there are more large particles in A67 than in X67 and in X67 than in X33 and X50. In the pure Pd sample X0, small particles are about the same size (2.3 nm) but large particles are much larger, with a mean diameter of 28 nm. The pure Ag catalyst X100 exhibits a broad distribution of metal particle sizes from 1 to 20 nm. Except in X100, it appears that the catalysts are composed of silica particles arranged in strings or aggregates, and although TEM gives only a 2D view, it seems that small metal particles are located inside silica particles, whereas big metal particles are located at their external surface.

712 It is interesting to compare the crystallite size determined by TEM and XRD in Table 2. For the monometallic samples X0 and X100, XRD gives an intermediate value consistent with the limits given by TEM. In the bimetallic xerogels as well as in aerogel A67, the size given by XRD for the solid solution (phase 1 in Table 2) is always close to the size of the small particles detected by TEM. Both methods detect a second family of bigger particles, and the big particles detected by XRD correspond to pure silver. These results lead to the conclusion that the small metal crystallites located inside the silica particles would be Pd-Ag alloy crystallites, whereas the big crystallites located outside the silica would consist of pure Ag. This conclusion was confirmed by an additional STEM-EDX analysis of xerogel X67 and aerogel A67. Due to their sufficient size, the focusing of the electron beam on individual big particles was possible and showed that they are composed of pure silver in both samples. On the contrary, it was not possible to focus the beam on individual small particles but X-rays emitted by groups of small particles in sample X67 were characteristic of Pd and Ag. In sample A67, only X-rays corresponding to Pd were observed which is in agreement with the nearly pure small Pd crystallites observed by XRD.

Figure 3. Schematic representation of TEM micrographs of Pd-Ag/SiO2 sol-gel catalysts

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micropores ] - - p o ~ ~ . . . _ _ / f XO

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0.001

I 0.1

1

10

I

I

I

100

1000

10000

Pore size (nm) Figure 4. Pore size distributions

The texture examination by N2 adsorption as well as Hg porosimetry shows that the catalysts prepared in this work exhibit very broad pore size distributions. The cumulated distributions over the complete pore size range shown in Figure 4 were obtained by applying a combination of various methods to their respective validity domains and by additioning the porous volumes corresponding to those domains. The distributions of micropores were calculated by Brunauer's

713 method, which is based on the multilayer adsorption assumption, applied to the t-plots derived from nitrogen adsorption isotherms [21]. The distributions of mesopores smaller than 7.5 nm were calculated by the Broekhoff-de Boer method based on N2 capillary condensation [21 ]. Mesopores larger than 7.5 nm and macropores were examined by Hg porosimetry using the collapse model at low Hg pressure and the intrusion model at high pressure [22, 23]. Palcontaining xerogels (X0, X33, X50 and X67) exhibit similar distributions an example of which is given in Figure 4 with sample X0. In the micropore domain, those four catalysts exhibit a very narrow pore size distribution centered on a mean value of about 0.8 nm which corresponds to the steep volume increase followed by a plateau. Contrary to Pd-containing xerogels, the pure Ag sample X100 exhibits a broad micropore size distribution from 0.7 to 2 nm, and aerogel A67 does not contain any micropores. In the range of meso- and macropores, one observes that all samples exhibit a broad distribution and that for xerogels this distribution shifts toward the large pores from X0 to X100. It is particularly interesting to note that the pore volumes VHg of those xerogels, contained between 2 and 6 cm3/g, are in the same order of magnitude as the pore volume of aerogel A67 (9.2 cm3/g) or other aerogels described in literature [24]. For this reason, those materials are called "low-density xerogels". The texture was also examined by TEM. Micrographs of samples X0, X33, X50, and X67 show that when Ag loading is increased, the xerogel structure evolves from an arrangement of silica particles in strings to an arrangement in aggregates which become more compact. This is schematically represented in Figure 3. Aerogel A67 exhibits dense aggregates comparable in size to X67, and xerogel X100 contains much bigger aggregates where silica particles do not appear clearly as it is the case in Pd containing samples. Pure silver xerogel X100 and aerogel A67 were completely inactive for the hydrodechlorination of 1,2c2 dichloroethane. On the ~ 75 ".~ contrary, xerogels X0, X33, X50, and X67 convert ~ 5o CH2C1-CH2C1 with an increasing selectivity in g ethylene. Figure 5 shows .~ 25 conversions and selectivities e =o measured at 300~ and at 0 stationary state obtained after 0 20 40 60 80 16 h. The pure palladium Ag/(Pd+Ag) (overallat%) sample X0 mainly produces ethane with a selectivity of Figure 5. Hydrodechlorination conversion and selectivities about 85 %. Ethyl chloride (o: 1,2-dichloroethane conversion; o: ethylene selectivity; CH3-CH2C1, which is not observed with the other o: ethane selectivity; A: ethyl chloride selectivity) catalysts, is the secondary product. The introduction of silver into the catalyst leads to a drastic change in selectivity towards C2H4, and when the Ag loading is high enough, this selectivity reaches 100 %.

714 4. DISCUSSION As mentioned above, the examination of Pd containing xerogels shows a structure with increasingly compact aggregates of SiO2 clusters or particles as the Ag content is increased (Figure 3). In agreement with the work of Brinker and Scherer [25], we assume that gel formation occurs in two successive steps: 1- formation of silica particles by hydrolysis and condensation of TEOS and 2- gelation by aggregation of those clusters. Let us consider the second step first. As explained in [25], if the colliding clusters always stick together (sticking probability = 1), the rate of aggregation is determined by transport kinetics, and the process is known as diffusion-limited cluster-cluster aggregation (DLCCA). In this case, attachment tends to occur at the cluster periphery which leads to an open structure. In many cases, the sticking probability is much lower than unity; thus many collisions will occur before two clusters link together. This process, called reaction-limited cluster-cluster aggregation (RLCCA), allows more opportunity for the clusters to interpenetrate and leads to more compact aggregates. The RLCCA mechanism is expected to dominate when a repulsive electrostatic barrier, which decreases the sticking probability, is present between the colliding particles. In his study of the stability of aqueous silica sols, Iler indicates that pH 2 is approximately the point of zero charge [26]. Above this value, the surface of the silica particles becomes negatively charged by deprotonation, and their mutual repulsion increases with pH. This repulsion effect is also observed for silica gel synthesized from TEOS in mixed alcoholwater systems [25]. In precursor solutions of Pd-containing gels, the AgOAc and AS concentrations and, therefore, the basicity are increased from X0 to X67 (Table 1). Silica particles in those solutions are then characterized by a growing repulsive negative electrostatic barrier, and their aggregation is limited by an increasingly slow reaction. As explained above, this progressively slower RLCCA mechanism would have to lead to more compact aggregates which is in agreement with TEM observations schematized in Figure 3. If we consider the aggregation step only, gel time is supposed to increase with the concentrations of AgOAc and AS. On the contrary, gel times reported in Table 1 indicate that gel formation is faster with increasing Ag content. This initially paradoxical result can be explained by considering the first step of gel formation, that is the formation of silica particles by hydrolysis and condensation of TEOS. These two reactions are accelerated by an increasing basicity [25], and the gel time decrease leads to the conclusion that the formation of silica particles is probably the rate determining step in the overall process of gel formation. It was shown previously that the complex pd2§ seems to act as a nucleation agent in the formation of silica particles (see introductory part and [12]). This nucleation effect is observed again in the pure Pd sample X0: TEM shows small Pd crystallites located inside SiO2 particles. Nevertheless, in this xerogel, big Pd crystallites are also observed at the surface of SiO~ particles. As explained before, this might result from migration and coalescence during thermal treatments of smaller Pd particles which are not trapped inside silica particles [ 12, 13]. On the contrary, in the pure Ag sample, a nucleation effect by Ag is not observed. Whereas Pd-containing samples exhibit strings or aggregates of SiO2 particles enclosing metal crystallites, this structure does not appear in sample X100. Ag particles are distributed over a broad range of sizes. Small crystallites seem to be located inside a porous SiO2 matrix but not inside individual silica particles having a monodisperse micropore size distribution (Figure 4). Unlike the Pd complex, the Ag complex is, therefore, supposed not to act as a nucleation

715 agent. In the wet gel, this molecule would be spread rather randomly through the SiO2 network. The bimetallic xerogels X33, X50 and X67 exhibit a structure similar to the pure Pd xerogel X0. TEM shows small Pd-Ag alloy crystallites located inside SiO2 particles with a monodisperse micropore size distribution (Figure 3 and 4). Several larger pure Ag particles are also observed at the surface of the SiO2 particles. A careful examination of aerogel A67 is helpful in order to understand the mechanism of Pd-Ag alloy formation. It has been shown that this sample contains small, nearly pure Pd crystallites (8 at% of Ag) located inside SiO2 particles, as well as numerous large pure Ag crystaUites (Table 2). Its texture is very different from that ofxerogel samples, since no micropores are present (Figure 4). Finally, A67 does not exhibit any catalytic activity which leads to the conclusion that Pd is not accessible for the gas phase. Aerogel A67 is then probably composed of small, nearly pure palladium crystallites confined inside non porous silica particles and big pure silver crystallites (inactive for hydrodechlorination) spread over their surface. All these results are in favor of the following mechanism for the Pd-Ag alloy formation in bimetallic xerogels: in wet gels, Pd complex groups would be located inside porous SiO2 particles because of the nucleation effect, while the Ag complex would be spread randomly through the silica network. During subsequent thermal treatments, silver would migrate through micropores inside SiO2 particles and combine with trapped palladium to form small alloy particles. In the course of their migration, silver atoms and/or particles could meet and form bigger particles. Some of them, observed experimentally, would become too large to diffuse through the micropores and to combine with palladium. In aerogel A67, the same mechanism would probably occur, but, for an unknown reason, micropores inside SiO2 particles would close before the majority of silver reaches palladium. This leads to the observed nearly complete segregation of the two metals. The catalytic resuks shows that the formation of alloy particles is a determining factor for the obtaining of a high selectivity in C2H4during hydrodechlorination and the role of this alloy in the reaction mechanism is at present under investigation. 5. CONCLUSIONS When dried under vacuum, the Pd-Ag/SiO2 xerogels exhibit a texture similar to that of Pd/SiO2 aerogels dried under supercritical conditions with pore volumes in the range of 2-6 cm3/g and pores ranging from micro- to macropores of several hundred nanometers. As in aerogels and due to the nucleation by Pd2+(EDAS)2 effect, completely accessible metal crystallites are trapped inside microporous silica particles which makes them sinter-proof. Formation of Pd-Ag alloy particles by migration of Ag toward Pd allows to obtain a very high selectivity in ethylene during hydrodechlorination of 1,2-dichloroethane. The possibility to obtain an aerogel structure by avoiding the difficult step of supercritical drying is of great importance for the development of such materials in the future. REFERENCES 1. K. Foger, in "Catalysis: Science and Technology", J.R. Anderson and M. Boudart (eds.), Vol. 6, p. 227, Springer-Verlag, Berlin, 1984.

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