Hugo Rojas

Liquid-phase hydrogenation of citral over Ir-supported catalysts

Abstract The hydrogenation of citral over a series of supported iridium catalysts was investigated. The nature of the support (TiO 2 and SiO 2 ), the effect of additives (Fe and Ge), and the influence of the reduction temperature on catalyst performance have been examined. It was found that the presence of ionic species, such as, promoters or species generated upon reduction at high temperature of the reducible TiO 2 support, i.e. TiO x moieties, leads to catalysts active and highly selective to the hydrogenation of the carbonyl bond. This is explained in terms of the presence of positively charged (Fe 3+ , Ge 4+ ) species in intimate contact with Ir, which are responsible for the polarisation of the CO bond.

Selective hydrogenation of furfural on Ir/TiO2 catalysts

Titania-supported Ir catalysts were used in the hydrogenation of furfural. Reactions were carried out in a stirred batch type reactor at 0.62MPa and 363K using a 0.10M solution of furfural in a 1:1 mixture n-heptane -ethanol as solvent. Catalysts containing 2 wt% of Ir were reduced in H2 flow at different temperatures in the range 473-773K. The catalysts were characterized by H2 chemisorption, TEM, TPR, TPD of NH3 and XPS. Conversion of furfural is higher at lower reduction temperatures, but leads to byproducts whereas reduction at higher temperatures shows selectivity to furfuryl alcohol close to 100%.

LIQUID-PHASE HYDROGENATION OF m-DINITROBENZENE OVER PLATINUM CATALYSTS

The m-dinitrobenzene hydrogenation to m-phenylenediamine in liquid phase was studied with platinum supported on SiO 2 , TiO 2 , Al 2 O 3 and Nb 2 O 5 catalysts. Incipient impregnation method was used to prepare the catalysts with 1wt% of Pt. The effect of the support and the reduction temperature (473 K or 773 K) were analyzed. The materials were characterized by X-ray diffraction (XRD), nitrogen physisorption (77 K), hydrogen chemisorption (298 K), transmission electron microscopy (TEM), temperature programmed reduction (H 2 -TPR) and temperature programmed desorption of ammonia (NH 3 -TPD). The results showed that Pt/ TiO 2 (HT) catalyst had a higher yield (60%) toward m- phenylenediamine and conversion level (98.2%) of m-dinitrobenzene. This behavior can be attributed to the reduction of PtO x crystallites to metallic Pt, small particle size and the strong metal-support interaction due to the migration of TiO x species on the Pt crystallites.

THE SOLVENT EFFECT IN THE HYDROGENATION OF CITRAL OVER Ir AND Ir-Fe/TiO2 CATALYSTS

The solvent effect in the citral hydrogenation over iridium catalysts has been studied. The catalysts used in this research were, the monometallic Ir/ TiO2 reduced at low or high temperature (LT y HT) and the bimetallic Ir-Fe/TiO2 (LT y HT) catalysts. As solvent n-heptane, an equimolar mixture n-heptane-1-propanol and 1-propanol were used. The reactions were carried out at a hydrogen pressure of 8.27 bar and at 363 K. In the LT catalyst, the catalytic activity increases upon the addition of Fe3+. This positive effect was obtained keeping constant the selectivity to the unsaturated alcohol, being 100% selective to the products obtained by hydrogenation of the C=O bond. In the HT catalysts, both the addition of Fe and the presence of TiOx moieties generated by the strong metal support interaction (SMSI) effect, lead to an increase of surface acidity. In LT and HT catalysts, an increase in the polarity of the solvent increases the catalytic activity, however the reaction pathway is modified by the presence of acid sites which in polar solvents allow the formation of acetals, mainly in the HT catalysts

SYNTHESIS, CHARACTERIZATION AND CATALYTIC BEHAVIOR OF THE Ir/Nb(2)0(5)/Si0(2) SYSTEM IN THE CITRONELLAL HYDROGENATION

The citronellal hydrogenation was studied on Ir/Nb 2 O 5 -SiO 2 catalysts. A series of catalysts was synthesized with different niobia contents (Ir/N1 1.10%; Ir/N2 2.54%; Ir/N3 3.25%; Ir/N4 5.30%wt%) using the wet impregnation method. These catalysts were reduced in hydrogen atmosphere at low or high temperatures, 473K (LTR) or 773K respectively. Their structural and surface properties were investigated by XRD, TEM, XPS, DTP, FTIR, H 2 chemisorption at 298 K and surface specific area by using the BET method. The X ray diffraction studies revealed only the peaks due to niobia, mainly in those with higher niobia loading. No diffraction lines due to Ir were observed due to its lower content and/or to its high dispersion. The catalytic tests in the hydrogenation of citronellal were carried out in a batch reactor operated at 6.2 bar of H 2 and 343 K. All catalysts showed higher selectivity to isopulegol and to their products due to the cyclization of the citronellal molecule.

Hydrogenation of m-Dinitrobenzene Over Pt Supported Catalysts on TiO2–Al2O3 Binary Oxides

Catalytic hydrogenation of m-dinitrobenzene over Pt/TiO2–Al2O3 catalysts was investigated. The effect of the enrichment of TiO2 was analyzed. The results showed that the increase in TiO2 content produced an enhancement in the activity of the catalyst. In these consecutive reactions, both rate constants (k1 and k2) were determined and it whatever the catalyst, k1 is higher than k2. The limiting step of the reaction was the transformation of m-nitroaniline to m-phenylenediamine. This behavior was explained by the modification in the acceptor-donor electron character of the group in the meta position.Graphical Abstract

Preyssler Heteropolyacids in the Self‐Etherification of 5‐Hydroxymethylfurfural to 5,5′‐[Oxybis(methylene)]bis‐2‐furfural Under Mild Reaction Conditions

The synthesis of 5,5′‐[oxybis(methylene)]bis‐2‐furfural (OBMF) from 5‐hydroxymethylfurfural (5‐HMF) was studied using bulk and alumina‐supported Preyssler heteropolyacids (HPAs). The formation of OBMF was related to the amount of Brønsted acid sites, and the lowest yield of OBMF was obtained with supported HPAs. However, the Lewis acidity of the HPA supported on Al2O3 favored the formation of 2,5‐dimethylfurane. The effects of solvent, catalyst loading, temperature, and reaction time on the selectivity to OBMF from 5‐HMF were studied to optimize OBMF production using bulk Preyssler HPAs; a yield of 84 % to OBMF was obtained at 5 h and 343 K. These results demonstrate that bulk Preyssler HPA is a good candidate for OBMF synthesis under mild reaction conditions.

KINETIC STUDY OF THE HYDROGENATION OF CITRAL ON Ir PROMOTED Au/TiO2 CATALYST

ABSTRACT A kinetic study of citral hydrogenation over an Au-Ir/TiO 2 catalyst was performed with the aim to understand the effect of iridium on gold in this catalytic system. Au-Ir/TiO 2 catalyst was prepared by co-deposition precipitation in an atomic ratio of 3/1. The effect of citral concentration, hydrogen pressure and temperature effect were also studied. The product distribution obtained is related with the proportion of Me δ+ /Me 0 sites. The deactivation of the catalyst occurs in the whole studied temperature range, 363 to 403 K, being more drastic as temperature increases due to the irreversible adsorbed CO blocks principally Ir 0 sites. From initial reaction rates treatment an apparent global order close to 1 was determined. A Langmuir-Hinshelwood-type kinetic model involving the surface reaction as the rate limiting step between adsorbed citral and hydrogen on active sites with different nature shows good agreement with experimental initial reaction rates. Keywords: Citral, Au- Ir catalyst, kinetic study.

New application of decaniobate salt as basic solid in the synthesis of 4 H -pyrans by microwave assisted multicomponent reactions

This study focuses on the search of new applications of polyoxometalates known as decaniobate ions [Nb10O28]6−. The decaniobates can be used as basic solid catalysts in multicomponent reactions to obtain good yields to 4H-pyrans (> 95%) under microwave radiation and solvent-free conditions, using several aldehydes with different electron-withdrawing or electron donor substituents. The synthesis of (TMA)6 [Nb10O28]6H2O was performed following a simple protocol (HPNb), which was modified to decrease the number of hexaniobate species that are formed as impurities (HPNb-HF).

Synthesis of 1,4-dihydropyrimidines with immobilized urease: effect of method immobilization on magnetic supports

Abstract The effect of the urease immobilization method was studied on magnetic supports for the Biginelli/Hantzsch reaction. For this purpose, Fe3O4/SiO2 was modified with 3-aminopropyl-triethoxysilane and then activated with glutaraldehyde. A ratio of 500 mg of enzyme per gram of support at 4°C and 18 h were sufficient for the physical adsorption, while 24 h were required for covalent bonding. The Biginelli and Hantzsch reactions were used to evaluate urease application in multicomponent reactions (MCRs). The synthesis of 1,4-dihydropyrimidines was successfully performed using immobilized urease favoring the Hantzsch product. The magnetic properties of the supports allow easy separation, and the urease immobilized by both methods improved the enzymatic activity compared to that of free urease.