The development of new catalytic processes requires the synthesis of catalysts with highly specific active sites on an appropriate support. To avoid the mass transfer limitations that affect the catalyst activity and selectivity, the support must have a suitable porous texture. Sol-gel chemistry is an efficient tool to control the morphology and reactivity of solids. The sol-gel process consists in the synthesis of a three-dimensional network by chemical reaction in liquid phase. After vacuum drying and calcination, mineral materials with preserved porosity and tailored morphology can be obtained.
An original synthesis method has been developped in the case of monometallic catalysts Pd/SiO2, Ag/SiO2 and Cu/SiO2 and bimetallic catalysts Pd-Ag/SiO2 and Pd-Cu/SiO2. This method allowed synthesizing in a single step both the active sites (metallic particles) and the porous support (silica) of the catalyst. The use of 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to complex Pd, Ag and Cu in an ethanolic solution containing TEOS and aqueous ammonia allows obtaining cogelled catalysts with a hierarchic texture. Complexes Mn+(EDAS)x induce a nucleation mechanism because of their higher reactivity compared to the network-forming reagent with ethoxy groups (TEOS). Through this nucleation mechanism by complexes Mn+(EDAS)x, metal particle size, silica particle and aggregate sizes, density, pore size distribution and specific surface area can be tailored by appropriately choosing the molar ratio EDAS/TEOS. Texture characterization and metal dispersion assessment lead to the conclusion that the metal wears the form of crystallites having a size of 2-3 nm and located within microporous silica particles. These metallic catalysts are found to be very active and selective for chlorinated alkanes hydrodechlorination into less or not chlorinated alkanes (Pd/SiO2) or alkenes (Pd-Ag/SiO2, Pd-Cu/SiO2) and for volatile organic compounds oxidation into H2O and CO2 (Ag/SiO2, Cu/SiO2). These catalysts are sinterproof due to the fact that the metallic crystallites cannot migrate because they are trapped inside the microporous SiO2 particles.
In the case of Pd-Ag/SiO2 and Pd-Cu/SiO2 cogelled xerogel catalysts, the combination of results from carbon monoxide chemisorption, X-ray diffraction, and transmission electron microscopy allowed calculating the surface composition of the palladium-silver or palladium-copper particles. Values obtained indicate a very pronounced surface enrichment with silver or copper. While 1,2-dichloroethane hydrodechlorination over pure palladium mainly produces ethane, increasing silver or copper content in bimetallic catalysts results in an increase in ethylene selectivity. Used alone, these metals deactivate rapidly due to their covering by chlorine atoms. Thanks to its activation power of hydrogen by dissociative chemisorption, palladium present in thePd-Ag or Pd-Cu alloy supplies hydrogen atoms for the regeneration of the chlorinated silver or copper surfaces into metallic silver and copper. The specific consumption rate of 1,2-dichloroethane decreases when silver or copper loading increases. The turnover frequency, that is, the number of catalytic cycle per active site (palladium atom and its surrounding silver or copper atoms) and per second, seems to be independent of surface composition of alloy particles and 1,2-dichloroethane hydrodechlorination is insensitive to the atom's nature (silver or copper).
Before this work, no silylated acetylacetonate deritative has ever been used as a ligand for the synthesis of metal xerogel catalysts and the use of silylated acetylacetonate palladium complexes yields Pd/SiO2 cogelled xerogel catalysts with the same structural characteristics as Pd/SiO2 cogelled xerogel catalysts synthesized from EDAS. These new catalysts, in which catalytically active metal complexes are attached to silica gels via silylated ligands, allows combining the positive attributes of both heterogeneous and homogeneous catalysts.