M.L. Guzmán-Castillo

Active sulfated alumina catalysts obtained by hydrothermal treatment

AbstractThe structural and surface properties of sulfated γ -Al 2 O 3 prepared by hydrothermally aging precipitates obtained from aluminum sulfateand ammonia have been studied. The Lewis sites are formed on coordinately unsaturated fourfold alumina. No pentacoordinated aluminumhas been found, probably as a consequence of the aging process. Residual sulfate is distributed among several species characterized by theirdesorption temperatures. Bronsted sites of high strength are probably located on sulfate species desorbing between 900 and 1000 ◦ C. Theymay have the following schematic structure (O=) 2 S(–OH)(–O–Al). The decomposition of 4-methylpentan-2-ol shows clear differencesaccording to the types of acidity present on the surface. The sulfated aluminas have favorable textural properties: pore size ∼=50 nm andspecific surface area ∼=200 m 2 / g stable up to more than 700 ◦ C.  2003 Elsevier Inc. All rights reserved. Keywords: Sulfated alumina; Hydrothermal preparation; Acid sites; Catalytic properties

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.

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.

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.

On the effect of a high reactive sulfur species on sulfur reduction in gasoline

Varying solutions are available for reducing the amount of sulfur contained in gasoline from fluid catalytic cracking units (FCCUs). However, the use of selective catalysts or additives can be the easiest to implement and potentially represent the most cost effective solution. In order to improve overall performance a better understanding of the effects of the feed properties on catalyst activity is needed. In this work we study the effect of a high reactive sulfur compound on the sulfur distribution in cracked products. Experiments were carried out in a fixed fluidized bed reactor using a REUSY equilibrium catalyst (Ecat), a commercial additive for gasoline sulfur reduction and an industrial feed spiked with hexane-2-thiol. The acidity of the materials was studied using FTIR spectra of chemisorbed pyridine. With spiked feeds both Ecat and the additive exhibited lower cracking activity indicating that the hexane-2-thiol had a strong poisoning effect on the materials acid sites. The effect of the type of acidity of the materials used is discussed and a selective adsorption of hexane-2-thiol on Lewis acid sites is proposed to explain the observed results. Similar trends were generated with a high sulfur feed derived from a Mayan crude oil thus leading to the conclusion that the presence of nonthiophenic sulfur compounds in the feed might have a strong negative effect on sulfur reduction. The potential use of an alumina material, possessing Bronsted and Lewis acidity, for sulfur reduction is presented and discussed.