Héctor Armendáriz-Herrera

The effect of sulfate ion on the synthesis and stability of mesoporous materials

The formation of hexagonal MCM-41 structures was studied using cetyltrimethyl ammonium bromide as the surfactant template and tetraethyl-orthosilicate as the silica source under hydrothermal conditions in the presence of sulfate ions. For the as-synthesized samples sulfate ions improved the hydrothermal stability of the MCM-41 material. Increasing sulfate concentration in the synthesis gel the amount of water lost by the sample decreases, indicating the formation of a solid with a more hydrophobic character. Since sulfate groups also favors of condensation of silicate units, which increase of wall thickness.

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.

On the influence of particle shape and process conditions in the pressure drop and hydrodynamics in a wall-effect fixed bed

ABSTRACT A low tube-to-particle diameter ratio (dt/de,p) fixed bed, packed with spherical and nonspherical catalyst supports, was used to investigate pressure drop at varying temperature (298–673 K) and inlet pressure (245–294 kPa). The dt/de,p ranged from 3 to 6, namely, a large wall-effect fixed bed, with an average void fraction between 0.38 and 0.61. These conditions pertain to multitubular fixed-bed reactors used for exothermic reactions. The pressure drop was notably influenced by the particle size and morphology as well as temperature. The use of particles with dt/de,p < 5 and relatively large bed void fractions (>0.55) appeared suitable for pressure drop control. The fluid velocity profiles were calculated by applying the Navier–Stokes–Darcy–Forchheimer equation computing the respective permeability parameters with refitted state-of-the-art pressure drop correlations. The fluid flow exhibited different velocity zones across the fixed bed, the highest velocity zone being located near the reactor wall. The axial velocity component was influenced by the catalyst morphology, as well as temperature and inlet pressure.

Metal solution precursors: their role during the synthesis of MoVTeNb mixed oxide catalysts

Synthesized via the slurry method and activated at high temperature (873 K), MoVTeNb multimetallic mixed oxides are applied to catalyze the oxidative dehydrogenation of ethane to ethylene (ODHE). Mixed oxides typically contain M1 and M2 crystalline phases, the relative contribution of these phases and the respective catalytic behaviour being notably influenced by the preparation conditions of the metallic aqueous solution precursor, given the complexity of the chemical interactions of metal species in solution. Thus, detailed in situ UV-vis and Raman studies of the chemical species formed in solution during each step of the synthetic procedure are presented herein. The main role of vanadium is to form decavanadate ions, which interact with Mo species to generate an Anderson-type structure. When niobium oxalate solution is added into the MoVTe solution, a yellow-coloured gel is immediately formed due to a common ion effect. When liquid and gel phases are separated, the M1 crystalline phase is produced solely from the gel phase. Attention is also devoted to the influence and role of each metal cation (Mo, V, Te and Nb) on the formation of the active M1 crystalline phase and the catalytic behaviour in the ODHE. The catalyst constituted mostly of M1 crystalline phase is able to convert 45% of the fed ethane, with a selectivity to ethylene of around 90%.

Controlling the redox properties of nickel in NiO/ZrO2 catalysts synthesized by sol–gel

NiO–ZrO2 samples were prepared by the sol–gel method adjusting the nickel content to 3 and 10 wt% and varying the calcination temperature from 500 to 700 °C. The solids were characterized by X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy, high resolution transmission electron microscopy (HRTEM) and scanning and transmission electron microscopy (STEM). For the sample with 3 wt% Ni, diffraction lines related to cubic zirconia were only detected when calcined from 500 to 650 °C, a NiO crystalline phase as well as the transition phase from cubic to tetragonal zirconia appeared when calcined at 700 °C. UV-vis DRS and HRTEM results indicated the presence of a highly dispersed NiO phase at the nanometric scale throughout the main zirconia crystalline phase. The NiO crystalline phase was already detected for the sample with 10 wt% nickel content calcined at 500 °C. The NiO–ZrO2 interaction was modified by reducing the hydrolysis rate during synthesis leading to a sample with a high dispersion of Ni throughout ZrO2, thus modifying the reducibility of NiO. The NiO–ZrO2 samples were catalytically tested for the oxidative dehydrogenation of ethane as a model reaction. Prior to reaction, the calcined catalysts were pre-treated in situ under reducing and oxidant atmospheres to study their redox properties. As the NiO–ZrO2 interaction modifies the electronic properties of both nickel oxide and zirconia, ethane conversion and ethylene selectivity were strongly influenced not only by the nickel content and calcination temperature but also by the in situ pre-treatment before reaction. This effect was particularly evident in the sample prepared with a modified hydrolysis rate, which changed the redox properties of the NiO species.

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.