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Preparation on nanostructured TiO2 supported platinum catalysts by photochemical deposition

Publication . Machado, Bruno; Gomes, Helder; Faria, Joaquim

Nowadays, thanks to a growing environmental awareness, selectivity is seen as the major driving force in catalyst research and development. In order to design the best possible catalyst there are several parameters that can be fine tuned depending on the type of reaction. Type of metal, support or thermal surface treatments are variables known to influence selectivity, as in the hydrogenation of organic substrates containing unsaturated functional groups. Selectivity towards the normally unfavoured hydrogenation of the carbonyl group instead of the C=C olefinic bond can be improved in presence of metal particles with sizes above 3 nm1. To a certain extent the metal deposition step allows particle size control, and innovative techniques, some more simple other more complex, are currently being developed. In this field, photochemical deposition of noble metals in different supports is gaining importance due to its simplicity and advantages. The main advantage is the ability of spreading very effectively the metal throughout the support, leading to high metal dispersions with particle size in the desired range. Additionally, if the support is a semiconductor, like TiO2, the deposited metal ions are also automatically reduced mainly through the action of conduction band electrons. Titanium dioxide supported platinum catalysts (1 and 3 wt.%) were prepared by liquid phase photodeposition of H2PtCl6⋅6H2O precursor. Thermal treatments under N2 and H2 were performed at 773K (1Pt/TiO2-773 and 3Pt/TiO2-773) providing materials with variable particle sizes at the nanometer scale. The prepared catalysts were tested on the liquid phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol (100mL stainless steel autoclave at 363K and 10bar). The preferred unsaturated alcohol is obtained by the reduction of the carbonyl group as opposed to hydrogenation of C=C olefinic bond to yield the saturated aldehyde. Allylic alcohols are valuable intermediates in the production of fragrances, flavoring additives, pharmaceuticals and agrochemicals. Under laboratory conditions thermally treated 3Pt/TiO2-773 catalyst revealed a selectivity towards cinnamyl alcohol of 55% at 87% conversion, against 28% selectivity at 27% conversion under similar conditions for the untreated catalyst. These results were explained by a combined effect of SMSI and platinum particle size increase. The catalysts were also tested in oxo-steroid selective hydrogenation and extremely promising results were obtained.

2007Conference object

Open access

Carbon xerogel supported noble metal catalysts for fine chemical applications

Publication . Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, Philippe; Figueiredo, José; Faria, Joaquim

Carbon xerogel, a mesoporous material, was produced by polycondensation of resorcinol and formaldehyde. A concentrated nitric acid solution was subsequently used to introduce high amounts of oxygenated groups on the surface. The carbon xerogel served as a support for Pt, Ir and Ru monometallic catalysts by using organometallic precursors. The catalysts were tested in the liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. The introduction of surface groups was important to increase metal dispersion but proved to limit selectivity towards the unsaturated alcohol. After a thermal treatment at 973 K the catalysts showed excellent thermal stability and a narrow metal size distribution. Regarding the catalytic results measured at 50% conversion, Pt catalysts exhibited the highest selectivity to cinnamyl alcohol (73 %) followed by Ir with 65 % and finally Ru with only 32 %.

2008Conference object

Open access

TiO2-carbon xerogel composites as supports in the preparation of Pt catalysts for selective hydrogenation

Publication . Gomes, Helder; Machado, Bruno; Silva, Adrián; Dražić, Goran; Faria, Joaquim

In the last decades, considerable efforts have been devoted to the development of heterogeneous catalysts able to perform efficient selective hydrogenation of the carbonyl function in α,β-unsaturated aldehydes, to yield the corresponding unsaturated alcohols, which are important intermediates in organic synthesis of fine chemicals for several industries, such as flavor, fragrance and pharmaceutical. Unfortunately, high selectivities towards these alcohols are difficult to achieve, since thermodynamics favors the hydrogenation of the C=C over the C=O bond and due to kinetic reasons, as the reactivity of the olefinic bond is higher than that of the carbonyl.

2009Conference object

Open access

Carbon xerogel supported noble metal catalysts for fine chemical applications

Publication . Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, Philippe; Figueiredo, José; Faria, Joaquim

Carbon xerogels are mesoporous materials obtained upon pyrolysis of the dried gels resulting from polycondensation of resorcinol and formaldehyde. Treatment with nitric acid under severe conditions introduces high amounts of oxygen containing functional groups onto the surface of thematerial, leading however to the collapse of its porous structure. The resulting material is then used to support 1 wt.% Pt, Ir and Ru monometallic catalysts by wet impregnation using organometallic precursors. The catalysts are characterized by N2 adsorption–desorption isotherms at 77 K, temperature programmed desorption coupled with mass spectrometry, scanning and transmission electron microscopy, and H2 chemisorption. The liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol is used in order to assess the catalytic performance of the prepared materials. Pt and Ru catalysts are initially very selective towards the hydrogenation of the olefinic double bond, while Ir is mostly selective towards the carbonyl group. After a thermal post-reduction treatment at 973 K, selectivity towards cinnamyl alcohol is significantly improved regardless of the metal nature. The Pt catalyst exhibits the best behavior, a complete shift in C=C to C=O hydrogenation being detected. The improvement in selectivity is rationalized in terms of both an increase in metal particle size and a modification in the surface chemistry of the catalyst after the post-reduction treatment.

2010Journal article

Restricted access

Carbon aerogel supported platinum catalysts for selective hydrogenation of cinnamaldehyde

Publication . Machado, Bruno; Morales-Torres, Sergio; Gomes, Helder; Pérez-Cadenas, A.F.; Maldonado-Hódar, F.J.; Carrasca-Marín, F.; Figueiredo, José; Faria, Joaquim

This paper describes the preparation and characterization of 1% wt. Pt catalysts supported in carbon aerogels for the application in the liquid-phase selective hydrogenation of cinnamaldehyde. Carbon aerogel supports with different textures were activated with hydrogen peroxide and ammonium peroxydisulfate leading to large amounts of surface groups but keeping unchanged their textural properties. After introducing Pt, the surface chemistry and morphology of the catalysts was characterized by analytical techniques like SEM, TPD, N2 adsorption isotherms, mercury porosimetry and H2 chemisorption. Catalysts prepared with activated aerogels exhibited good selectivity towards the desired product, cinnamyl alcohol. A high temperature post-treatment of the catalysts failed to enhance their selectivity, mainly leading to hydrocinnamaldehyde production.

2008Conference object

Open access