Patricia Pérez-Romo

Synthesis of silicalite-1 from organo-silicic gels

Well crystallized silicalite-1 has been obtained from three sources of amorphous silica, namely, rice hull ashes, commercial Davisil, and a fume silica from Aldrich. The silicas were first dissolved in glycerol according to a recently described reaction. This reaction transforms rapidly and efficiently large surface area silicates into poly-alkoxide gels. It can be schematized as an etherification of an alcohol function of glycerol by the weakly acid surface silanol groups. The facile hydrolysis of the alkoxide permits the preparation of relatively pure and reactive silica, keeping the mesoporous character of the parent starting material. We insist on the mesoporous character of the solids obtained upon hydrolyzing the organo-silicic gel because we believe the gel plays a role of template in the secondary synthesis of mesoporous structures. The hydrolysis is carried out in presence of a structure directing agent, namely tetra-propylammonium hydroxide, TPAOH. After aging, the residue is dried and calcined. The first advantage of using the organo-silicic gel is probably related to the high degree of depolymerization of silica, witness by the C/Si ratio. The second one, more subtle to define, is to provide an intermediate silica with hydrophilic a hydrophobic regions, interfering differently with the surfactant. After calcination at 500 degrees C, well crystallized silicalite-1 is obtained. The texture of the starting silica influences the textural characteristics of the final silicalite-1.

Effect of chitosan on the performance of NiMoP-supported catalysts for the hydrodesulfurization of dibenzothiophene

Chitosan-added NiMoP catalysts supported on alumina and alumina-titania were studied in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The preparation of catalysts containing Mo (12 wt%), Ni (3 wt%), P (1.6 wt%), and chitosan/nickel = 2 (mol ratio) was accomplished by sequential pore-filling impregnation varying the order of chitosan integration. Materials were characterized by DRIFTS, TPR, TG-DTA, and XPS techniques. The TG-DTA study showed that the nature of the support influences the degradation of chitosan onto the catalytic materials and also influences the HDS of DBT and the product distribution as well. The series of catalysts supported on alumina presented the most remarkable effect of chitosan, in which the OH and NH groups of the organic molecule interact with acid sites of the support weakening the interaction between alumina and deposited metal phases. In all cases, DBT was converted mainly through direct sulfur removal. The catalysts ChP3/A (alumina support impregnated with chitosan in phosphoric acid solution, prior to NiMoP deposition) and ChP4/AT (alumina-titania support impregnated with NiMoP solution, prior to contacting with a solution comprising chitosan and phosphorus) exhibited the best performance in HDS reactions and also showed the highest selectivity in biphenyl formation. Presence of carbonaceous residua on the catalysts surface, as shown by XPS, could enhance the HDS activity over the ChP4/AT sample.

29Si MAS-NMR study of the silica condensation in the synthesis of mesoporous materials

A series of silica mesoporous materials was prepared adding ammonium sulfate into reaction mixture. The 29 Si MAS NMR showed that the Q 4 signal increased when the sulfate content in the synthesis mixture increased thus suggesting that the OH/Si of the material decrease hence confirming that silica had a higher degree of condensation. Accordingly, an increase in the unit cell parameters and in the wall thickness of the synthesized materials was observed.

Physicochemical properties of FSM-16 modified by fluoride and HPA

Mesoporous material FSM-16 has been modified by addition of ammoniumfluoride into the sysnthesis mixture (in-situ modification).After calcination at 863 K the samples were impregnated by phosphotungstic acid (HPA) and characterizedby XRD, FT-IR and 31 P MAS NMR. The long-range order of the impregnated samples were improved. In addition, HPA promotes the formation of both Bronted (B) and Lewis (L) acid sites. The presence of both F and HPA modifiers increases the B acid sites density, and the catalytic activity performance. Agood correlation between the B sites density and conversion ofn-hexane activity was observed.

Adsorption of nitrogen compounds from diesel fuels over alumina-based adsorbent towards ULSD production

ABSTRACT The aim of this work is to estimate some nitrogen (N)-adsorption properties and the N-adsorption capacity (q) required for commercial application in the ultra-low sulfur diesel production. Hydroxyl (OH) groups and interactions among the commercial adsorbent Selexsorb® CDX (CDX) and pyridine and indole were studied by means of Fourier transform infrared spectroscopy. The adsorption of N-compounds from three diesel fuels over CDX was estimated at CDX/fuel ratio: 0.01–0.09 g/g, Contact time: 1–60 m, 303 K, and 0.078 MPa in a batch setup. It was concluded that an appropriate N-adsorbent should have high densities of suface OH groups and Lewis and Brønsted strong-acid sites and a q ≥ 0.70 mmol/g.

Role of Na in the ionic equilibrium ruling the synthesis of mesoporous FSM-16 materials

FSM-16 was prepared from sodium disilicate. In the synthesis mixture, the Na/Si ratio was varied in order to study the effect of Na on the formation and stability of mesoporous materials while the surfactant/Si was maintained constant. Increasing the sodium content in the precursor favors the formation of a pure hexagonal phase. The thermal structural stability is correlated with the OH density.

Synthesis conditions and properties of Ti-MCM-41

Abstract A series of mesoporous Ti-MCM-41 materials was prepared at hydrothermal conditions (HT) as well as room temperature (RT). A constant Si/Ti ratio (26.1) in the initial gel and varying amounts of ammonium sulfate (SO 4 /Si = 0, 0.01, 0.1, 0.3, 0.5) were used. For solids prepared at SO 4 /Si below 0.3 their XRD spectra showed the pattern of a MCM-41 hexagonal array. The structural order decreased as the amount of sulfate increased. In materials prepared at RT higher sulfate concentrations favored the insertion of Ti and the wall thickness while the surface area decreased.