Cogelled Xerogel Catalysts Research Articles

Ethylene polymerization and hydrodechlorination of 1,2-dichloroethane mediated by nickel(II) covalently anchored to silica xerogels

Ni/SiO2 cogelled xerogel catalysts have been prepared in ethanol containing nickel acetylacetonate, tetraethoxysilane (TEOS), an aqueous ammonia solution of 0.54 mol L−1 and either a commercial sylilated ligand, 3-(2-aminoethyl)aminopropyltrimethoxysilane (EDAPMS), or a home-made new silylated pyrazolypyridine ligands, respectively 2-[4-[3-(trimethoxysilyl)propyl]-3,5-dimethyl-1H-pyrazol-1-yl]pyridine (MS-PzPy) and 2-[4-[3-(trimethoxysilyl)propyl]-3,5-dimethyl-1H-pyrazol-1-yl]-6-methylpyridine (MS-PzPyMe), able to form a chelate with a metal ion such as Ni2+. All samples form homogeneous and very highly dispersed Ni/SiO2 cogelled xerogel catalysts. The resulting catalysts are composed of nickel nanoparticles with a diameter of about 2.8 nm, located inside primary silica particles exhibiting a monodisperse microporous distribution. The silylated organic ligand has a strong influence on the textural properties of cogelled xerogel 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. Homogenous nickel complexes synthesized from pyrazolylpyridine derivatives are inactive for ethylene polymerization. In opposite, heterogenous nickel-based catalysts onto silica xerogel synthesized from pyrazolylpyridine derivatives bearing a tethered trialkoxysilyl group allow increasing ethylene polymerization activity. Although nickel nanoparticles are located inside the silica crystallites, their complete accessibility, via the micropore network, has been shown. For 1,2-dichloroethane hydrodechlorination over Ni/SiO2 catalysts, the conversion of 1,2-dichloroethane is high at the temperature of 350 °C and mainly ethane is produced.

Development by the sol–gel process of highly dispersed Ni–Cu/SiO2 xerogel catalysts for selective 1,2-dichloroethane hydrodechlorination into ethylene

Ni–Cu/SiO2 xerogel catalysts have been synthesized by cogelation of industrial tetraethoxysilane (Dynasil) and chelates of Ni and Cu with industrial 3-(2-aminoethylamino)propyltrimethoxysilane (Dynasylan DAMO) in industrial ethanol denatured with diethyl phthalate. Despite the use of industrial grade reagents, highly dispersed bimetallic Ni–Cu/SiO2 xerogel catalysts were obtained. These samples are composed of completely accessible Ni–Cu alloy crystallites with sizes of 1.6–3.4nm located inside silica particles exhibiting a monodisperse microporous distribution. It appears that the bimetallic complex acts as a nucleation agent in the formation of silica particles. The combination of results obtained from the calculation of the metal ratio in catalysts, H2 chemisorption and transmission electron microscopy allowed calculating the surface composition of the nickel–copper particles in Ni–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a very pronounced surface enrichment with copper. While 1,2-dichloroethane hydrodechlorination over pure nickel mainly produces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity. The specific consumption rate of 1,2-dichloroethane decreases when copper loading increases. The turnover frequency, that is, the number of catalytic cycle per active site (nickel atom and its surrounding copper atoms) and per second, seems to be independent of surface composition of alloy particles.

Structural feature and catalytic performance of Cu―SiO2―TiO2 cogelled xerogel catalysts for oxidative carbonylation of methanol to dimethyl carbonate

Abstract Cu―SiO 2 ―TiO 2 cogelled xerogel catalysts were prepared, characterized and tested for oxidative carbonylation of methanol to dimethyl carbonate (DMC). The catalyst with 12.5 mol% of Cu loading shows better catalytic performance and reduced corrosion as compared with the CuCl catalytic system, which is attributed to highly dispersed Cu + active centers as well as lower chloride content. Further copper loading resulted in loss of copper during the preparation of the catalyst and decreased catalytic activity in DMC synthesis.

On the structure-sensitivity of 2-butanol dehydrogenation over Cu/SiO2 cogelled xerogel catalysts

Cu/SiO2, cogelled xerogel catalysts were synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Cu with 3-(2-aminoethylamino)-propyltrimethoxysilane (EDAS). The resulting catalysts are composed of metallic crystallites with a diameter of about 3 nm located inside microporous silica particles and larger metallic particles with a diameter of 8-30 nm located outside silica network. Cu/SiO2, catalysts were tested for 2-butanol dehydrogenation. This reaction over Cu/SiO2 cogelled xerogel catalysts is a structure-sensitive reaction: large metallic particles located outside silica particles are responsible for the dehydrogenation reaction, while small metallic particles located inside silica particles do not participate to the reaction. (C) 2007 Elsevier B.V. All rights reserved.

Ag/SiO2, Cu/SiO2 and Pd/SiO2 cogelled xerogel catalysts for benzene combustion: Relationships between operating synthesis variables and catalytic activity

The aim of this work is to explore the applicability of the sol-gel method for the preparation of Ag/SiO2, Cu/SiO2 and Pd/SiO2 catalysts and to see whether such a method can yield silver, copper and palladium species stabilized by the carrier in the case of benzene oxidation. So Ag/SiO2, Cu/SiO2 and Pd/SiO2 xerogel catalysts were synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Ag, Cu and Pd with 3-(2-aminoethylamino)-propyltrimethoxysilane (EDAS). The resulting catalysts are composed of completely accessible metallic crystallites with a diameter of about 3 nm located inside silica particles. (c) 2006 Elsevier B.V. All rights reserved.

A TEM study on the localization of metal particles in cogelled xerogel catalysts

The localization of metal and alloy particles in monometallic Pd/SiO 2 and bimetallic Pd–Ag/SiO 2 sol–gel catalysts prepared by cogelation was examined by rotating transmission electron microscopy. The analysis of the resulting micrograph series provides a direct proof that the 2-to 3-nm active metal nanoparticles in those catalysts are located inside microporous silica particles or aggregates. Such localization has long been suspected but with, up to now, no direct evidence.

Pd–Ag/SiO 2 and Pd–Cu/SiO 2 cogelled xerogel catalysts for selective hydrodechlorination of 1,2-dichloroethane into ethylene

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. 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).

Synthesis of SiO 2 xerogels and Pd/SiO 2 cogelled xerogel catalysts from silylated acetylacetonate ligand

SiO 2 xerogels and Pd/SiO 2 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 Pd 2+. 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/SiO 2 catalysts mainly produces ethane and the reaction rate increases linearly with palladium dispersion. Hydrodechlorination over Pd/SiO 2 cogelled xerogel catalysts is a structure insensitive reaction compared to the ensemble size concept.

Study of textural properties and nucleation phenomenon in Pd/SiO2, Ag/SiO2 and Cu/SiO2 cogelled xerogel catalysts

Pd/SiO2, Ag/SiO2 and Cu/SiO2 xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Pd, Ag and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). The resulting materials are composed of completely accessible metallic crystallites with a diameter of about 3 nm, located inside silica particles exhibiting a monodisperse microporous distribution centered on a pore width of about 0.8 nm. Voids between silica particles and between aggregates of silica particles constitute the material meso- and macroporous texture. The EDAS/TEOS ratio is the main operating variable to tailor the material morphology. When the EDAS/TEOS ratio decreases, metallic particle size decreases, whereas silica particle size increases. Thus the meso- and macropores between silica particles are larger. So the total pore volume can reach 5 cm3/g when the EDAS/TEOS ratio decreases. The remarkable morphology of these materials can be explained by the rule in which the EDAS–metal complex acts as a nucleation agent in the formation of silica particles. A nucleation mechanism by EDAS–metal complex based on mass equations and the nuclei formation kinetics allows fitting the entire results.

Determination of surface composition of alloy nanoparticles and relationships with catalytic activity in 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–copper nanoparticles in Pd–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a very pronounced surface enrichment with copper. Surface compositions obtained with this method, which combines three different experimental techniques, are in agreement with the literature data previously obtained for surface segregation in Pd–Cu/SiO2 catalysts by other techniques as low energy ion scattering and X-ray photoelectron spectroscopy. While 1,2-dichloroethane hydrodechlorination over pure palladium mainly produces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity, to reach 100% in ethylene selectivity for the sample containing 1.4wt.% of palladium and 3.0wt.% of copper.

Improvement of metal dispersion in Pd/SiO2 cogelled xerogel catalysts for 1,2-dichloroethane hydrodechlorination

The study of the influence of synthesis operating variables (nature and concentration of complexing silane, palladium percentage, temperatures of gelling, aging and vacuum drying of xerogels, molar ratio between the complexing silane and palladium, molar concentration of ammonia solution, and use of tetramethylammonium hydroxide as base instead of NH3) allows improving metal dispersion in Pd/SiO2 cogelled xerogel catalysts. The use of 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) or 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) to complex palladium in an ethanolic solution containing tetraethoxysilane (TEOS) and an ammonia solution of 0.54mol/l allows obtaining a Pd/SiO2 xerogel catalyst with a mean metal particle diameter of 2.4nm located inside silica particles. Indeed, complexes Pd(EDA(E)S)xn+ induce a nucleation mechanism because of their higher reactivity compared to the network-reagent (TEOS). Although metal particles 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 specific hydrodechlorination rate per gram of palladium increases proportionally with palladium dispersion. Hydrodechlorination over Pd/SiO2 cogelled xerogel catalysts is a structure insensitive reaction with regard to the ensemble size concept.

Synthesis of Pd/SiO 2, Ag/SiO 2, and Cu/SiO 2 cogelled xerogel catalysts: study of metal dispersion and catalytic activity

Pd/SiO 2, Ag/SiO 2, and Cu/SiO 2 xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Pd, Ag, and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). It appears that the metal complex acts as a nucleation agent in the formation of silica particles. The resulting catalysts are then composed of completely accessible metallic crystallites with a diameter of about 3 nm located inside silica particles exhibiting a monodisperse microporous distribution. The metal dispersion has been determined from CO and O 2 chemisorption, TEM, and X-ray diffraction. Although metallic particles are located inside silica particles, their complete accessibility, via the micropore network, has been shown. 1,2-Dichloroethane hydrodechlorination over Pd/SiO 2 catalysts mainly produces ethane and the specific hydrodechlorination rate per gram of Pd decreases when metal loading increases. Hydrodechlorination over Pd/SiO 2 catalysts is a structure-insensitive reaction with regard to the ensemble size concept. Benzene oxidation over Ag/SiO 2 and Cu/SiO 2 catalysts produces H 2O and CO 2 only and specific oxidation rate per gram of metal decreases when silver and copper loadings increase. Furthermore, it is concluded that benzene oxidation is a structure-insensitive reaction.

Mass transfer in low‐density xerogel catalysts

AbstractTexture and morphology of a Pd‐Ag/SiO2 hydrodechlorination low‐density xerogel catalyst prepared by a cogelation sol‐gel process were characterized in detail to examine mass transfer in such catalysts. The catalyst consists of active Pd‐Ag nanocrystallites trapped inside elementary 20 nm microporous silica particles arranged in larger aggregates, which constitute the macroscopic pellet. To reach active sites, reactants must first diffuse through large pores located between aggregates of SiO2 particles and then through smaller pores located between those elementary particles inside the aggregates. Finally, they diffuse through micropores located inside silica particles. Diffusion in such a “funnel” structure cannot be described assimilating the pellet to a pseudo‐continuum. Diffusion should be examined carefully at three levels of decreasing size: the macroscopic pellet, the aggregate of silica particles, and the elementary silica particle. This approach shows that cogelled xerogel catalysts have remarkable mass‐transfer properties.

Valence state of chromium in active centers of a potassia-chromia-alumina dehydrogenation catalyst

Ni–Cu/SiO2 xerogel catalysts have been synthesized by cogelation of industrial tetraethoxysilane (Dynasil) and chelates of Ni and Cu with industrial 3-(2-aminoethylamino)propyltrimethoxysilane (Dynasylan DAMO) in industrial ethanol denatured with diethyl phthalate. Despite the use of industrial grade reagents, highly dispersed bimetallic Ni–Cu/SiO2 xerogel catalysts were obtained. These samples are composed of completely accessible Ni–Cu alloy crystallites with sizes of 1.6–3.4 nm located inside silica particles exhibiting a monodisperse microporous distribution. It appears that the bimetallic complex acts as a nucleation agent in the formation of silica particles. The combination of results obtained from the calculation of the metal ratio in catalysts, H2 chemisorption and transmission electron microscopy allowed calculating the surface composition of the nickel–copper particles in Ni–Cu/SiO2 cogelled xerogel catalysts. Values obtained indicate a very pronounced surface enrichment with copper. While 1,2-dichloroethane hydrodechlorination over pure nickel mainly produces ethane, increasing copper content in bimetallic catalysts results in an increase in ethylene selectivity. The specific consumption rate of 1,2-dichloroethane decreases when copper loading increases. The turnover frequency, that is, the number of catalytic cycle per active site (nickel atom and its surrounding copper atoms) and per second, seems to be independent of surface composition of alloy particles.