Jaime S. Valente

Thermokinetic study of the rehydration process of a calcined MgAl-layered double hydroxide.

The rehydration process of a calcined MgAl-layered double hydroxide (LDH) with a Mg/Al molar ratio of 3 was systematically analyzed at different temperatures and relative humidity. Qualitative and quantitative experiments were done. In the first set of samples, the temperature or the relative humidity was varied, fixing the second variable. Both adsorption and absorption phenomena were present; absorption process was associated to the LDH regeneration. Of course, in all cases the LDH regeneration was confirmed by other techniques such as TGA, solid state NMR, and SAXS. In the second set of experiments, a kinetic analysis was performed, the results allowed to obtain different activation enthalpies for the LDH regeneration as a function of the relative humidity. The activation enthalpies varied from 137.6 to 83.3 kJ/mol as a function of the relative humidity (50 and 80%, respectively). All these experiments showed that LDH regeneration is highly dependent on the temperature and relative humidity.

Low Concentration Fe-Doped Alumina Catalysts Using Sol-Gel and Impregnation Methods: The Synthesis, Characterization and Catalytic Performance during the Combustion of Trichloroethylene

The role of iron in two modes of integration into alumina catalysts was studied at 0.39 wt% Fe and tested in trichloroethylene combustion. One modified alumina was synthesized using the sol-gel method with Fe added in situ during hydrolysis; another modification was performed using calcined alumina, prepared using the sol-gel method and impregnated with Fe. Several characterization techniques were used to study the level of Fe modification in the γ-Al2O3 phase formed and to correlate the catalytic properties during trichloroethylene (TCE) combustion. The introduction of Fe in situ during the sol-gel process influenced the crystallite size, and three iron species were generated, namely, magnetite, maghemite and hematite. The impregnated Fe-alumina formed hematite and maghemite, which were highly dispersed on the γ-Al2O3 surface. The X-ray photoelectron spectra (XPS), FT-IR and Mössbauer spectroscopy analyses revealed how Fe interacted with the γ-Al2O3 lattice in both catalysts. The impregnated Fe-catalyst showed the best catalytic performance compared to the catalyst that was Fe-doped in situ by the sol-gel method; both had better catalytic activity than pure alumina. This difference in activity was correlated with the accessibility of the reactants to the hematite iron species on the surface. The chlorine poisoning for all three catalysts was less than 1.8%.

CO2 Capture at Low Temperatures (30–80 °C) and in the Presence of Water Vapor over a Thermally Activated Mg–Al Layered Double Hydroxide

The carbonation process of a calcined Mg-Al layered double hydroxide (LDH) was systematically analyzed at low temperatures, varying the relative humidity. Qualitative and quantitative experiments were performed. In a first set of experiments, the relative humidity was varied while maintaining a constant temperature. Characterization of the rehydrated products by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and solid-state NMR revealed that the samples did not recover the LDH structure; instead hydrated MgCO(3) was produced. The results were compared with similar experiments performed on magnesium oxide for comparison purposes. Then, in the second set of experiments, a kinetic analysis was performed. The results showed that the highest CO(2) capture was obtained at 50 °C and 70% of relative humidity, with a CO(2) absorption capacity of 2.13 mmol/g.

Thermochemical and Cyclability Analyses of the CO2 Absorption Process on a Ca/Al Layered Double Hydroxide

Hydrocalumite ([Ca2Al(OH)6]2CO3·mH2O) was synthesized by precipitation and thermally activated at 300 and 550°C. Its CO2 chemisorption capacity was evaluated and compared with that of calcium oxide (CaO). Initial thermal analyses showed that CaAl-550 sample has better properties as CO2 sorbent than CaO, evaluated under similar conditions. It was determined that both materials (CaAl-550 and CaO) have similar kinetic behavior, and the presence of Ca12Al14O33 on the CaAl-550 sample did not reduce or interfere with the CO2 capture. Moreover, when the CO2 absorption-desorption cyclability was analyzed, the CaAl-550 sample apparently possessed better CO2 capture efficiency and thermal stability than CaO. In fact, different characterization analyses (nuclear magnetic resonance and scanning electron microscopy ) suggest that CO2 capture efficiency and thermal stability observed on the CaAl-550 sample can be attributed to the aluminum presence, as Ca12Al14O33.

Novel SOx removal catalysts for the FCC process: Manufacture method, characterization, and pilot-scale testing

A novel method for preparing SOx removing (ReSOx) catalysts for the fluid catalytic cracking (FCC) process, based on multimetallic layered double hydroxides (LDHs), is presented. The synthesis procedure is industrially feasible and environmentally friendly. Ceria is incorporated in varying amounts as oxidation promoter, to obtain catalysts that are able to work efficiently in either partial or full combustion regenerator modes. The manufacturing process presented herein enables obtaining, by spray drying, microsphere particles with mechanical properties that are adequate for fluidization, without requiring addition of binding agents. The physicochemical properties of the catalysts are examined by several characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). A uniform dispersion of the metal components is observed throughout the particles. Furthermore, the microspheroidal catalysts are tested for the reduction of SOx emissions in a pilot-scale FCC plant, circulating together with a conventional cracking catalyst, during the conversion of a sulphur-containing industrial gas oil obtained from a Mexican refinery. ReSOx catalyst addition to the pilot FCC unit, in only 0.2 wt% of the total catalyst inventory, results in a maximum %SO2reduction of 74–80%. Additionally, the effect of the ReSOx catalyst on the main cracking reactions is studied; it is shown that the disruption of the cracking catalysts activity and selectivity is minimal, at a concentration of 1.5 wt% of the total catalyst inventory. Thus, the use of these SOx removal catalysts appears as a viable, low investment, flexible and effective option for in situreduction of SOx flue gas emissions.

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.

Electrochemical characterization of carbon paste electrodes modified with MgZnGa and ZnGaAl hydrotalcite-like compounds

Two hydrotalcite-like compounds (HTs) were synthesized by coprecipitation. The electrochemical behavior of MgZnGa ([Mg0.58 Zn0.17Ga0.25 (OH)2] (CO3)2−0.125 1.5 H2O) and ZnGaAl ([Zn0.75Ga0.19 Al0.06 (OH)2] (CO3)2−0.125·1.5 H2O) Hydrotalcite-like compounds (HTs) was studied in NaOH at different concentrations. Voltammetric and chronoamperometric studies were performed to identify oxidation and reduction processes and the effect of the cations in total reactivity. Electrocatalytic effect of HTs on hydroxide electrochemical oxidation shows better performance of MgZnGa; this is apparently due to the presence of Mg and a greater amount of Ga3+ in the lattice of this HT. As far as reduction is concerned, Zn(II) reduction process is observed within the lattices of both HTs and is influenced by both the amount of OH− in the solution and the potential which the previous oxidation has been performed.

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.