Characterization of synthetic hydrocalumite-type [Ca2Al(OH)6]NO3·mH2O: Effect of the calcination temperature

A lamellar hydrocalumite-type [Ca₂Al(OH)₆]NO₃·mH₂O, (HC), was synthesized and characterized by X-ray diffraction analysis (XRD), surface area, pore size measurements, CO₂-Thermal Programmed Desorption, and later tested as catalysts in the double bond isomerization of 1-butene. The layered structure of HC collapses above 523 K yielding an amorphous material at 573 K which upon calcination at 873–973 K transforms into a mixture of CaO and mayenite Ca₁₂Al₁₄O₃₃. The calcination temperature has a marked effect in the formation of basic sites. Thus for example, HC calcined at 1073 K shows 90% of strong basic sites (CO₂ desorption at 1023 K) while they are absent in HC calcined at 573–673 K. HC calcined at 973 K shows high catalytic activity (74% conversion) in the isomerization of 1-butene without any appreciable deactivation after 4 h on stream.     

Effect of Branched C8 Olefins Addition to Mexican Gasoline on Motor Vehicles Exhaust Emissions

Two groups of gasoline blends were prepared using a base-gasoline, containing 7.6% MTBE and added with either lower olefins (C₅-C₆) or butene's dimer (made up of 70% branched C₈-olefins). In each gasoline group, the olefin concentration was varied 5, 10, and 15 vol%. The dimer was produced in our pilot plant facilities from a mixture of n-butenes. An FTP-75 test procedure, performed on a 98 model year vehicle, was used to assess the effect of olefins type (C₅-C₆ or C₈) and their concentration in a gasoline blend on exhaust emissions; namely: CO, total hydrocarbons, NOx, and toxic compounds, such as aldehydes and ketones. Average CO emissions dropped 20-27%, when using 5-15 vol% C₈ olefins-gasoline, in comparison with those using C₅-C₆ olefins-gasolines. Total hydrocarbons emissions, on the other hand, remained practically unaffected either by the type of olefins or their concentrations (5-15 vol%). NOx emission factors decreased 10-14% when using the C₈ olefins-gasolines, relative to those containing lower olefins. Main toxic pollutants were formaldehyde, propanone, and acetaldehyde. In average, acetaldehyde emissions decreased 23-53% in the dimer-gasoline group, but formaldehyde emissions increased 5-39%, depending on the olefins content and in comparison with the lower olefins-gasolines.     

Long-term evaluation of NiMo/alumina–carbon black composite catalysts in hydroconversion of Mexican 538 °C+ vacuum residue

Alumina with (8–18 wt.%) carbon black composite (AMAC) supports was prepared as bimodal extrudates, containing 11–20% of total pore volume as macropores (i.e. >1000 Å). These supports, in spite of containing carbon black and macropores, showed good side crushing strength (0.67–1.19 kg/mm) after pyrolysis in 6% O₂/N₂. AMAC-catalysts were obtained after impregnating these alumina–carbon black supports with Ni and Mo, to obtain 3.5 wt.% NiO and 15 wt.% MoO₃. These catalysts were evaluated for about 700 h in the hydroconversion of a Mexican vacuum residue (538 °C+) at 415 °C, 200 kg/cm², H₂/HC = 6000 ft³/barrel in a pilot plant equipped with a Robinson–Mahoney reactor. In comparison with a commercial bimodal alumina-based catalyst (ComCat), AMAC catalysts showed much fewer sediments and less Conradson carbon formation. Initial HDS in AMAC containing macropores can be as high as 92%, while that in a ComCat is 86%. On average, yields of naphtha and kerosene were 2.6 and 1.34 times higher with AMAC catalysts than those with ComCat, while diesel yields were similar.     

Synthesis and characterization of nanostructured powders of Bi2O3, BiOCl and Bi

In the last years, synthesis and characterization of bismuth compounds have received a great attention due to their potential applications. In this work, nanostructured powders of Bi₂O₃, BiOCl and Bi were prepared by a simple aqueous precipitation method using elemental bismuth as starting material and varying some experimental conditions. The samples obtained were characterized using X-ray diffraction (XRD) and transmission electronic microscopy (TEM); fluorescence, infrared and UV-vis absorption spectroscopies were also used. The obtained powders present a particle size under 100 nm, irregular morphology and agglomeration. A quantum confinement effect can be observed in the optical properties of the samples prepared in this work making them an interesting option for industrial applications.     

SOx removal by calcined MgAlFe hydrotalcite-like materials: effect of the chemical composition and the cerium incorporation method

Sulfur oxides are one of the most hazardous atmospheric pollutants since they contribute directly to acid rain formation. Consequently, stringent environmental regulations limit atmospheric SOx emissions, motivating research on efficient ways to reduce them. To supply an alternative to reduce these emissions in fluid catalytic cracking units, this study discloses efficient SOx reducing materials based on calcined MgAlFe hydrotalcite-like compounds (HT's). Thus, HT materials were synthesized by several methods including cerium addition. The adsorption of SO2 was carried out by contacting the calcined solid with a mixture of SO2 (1%) in air at 650 degrees C. It was demonstrated that the isomorphic incorporation of iron increased its reduction capability which was reflected in higher reduction rates and metal sulfate reduction grade at 550 degrees C. Moreover, when cerium was present in the iron-containing materials the saturation rate was improved, because cerium oxide promotes the oxidation of SO2 to SO3. The way cerium is incorporated influences the SO2 adsorption capacity.     

Remote sensing study of emissions from motor vehicles in the Metropolitan Area of Mexico City

Remote sensing was employed for the first time to measure nitric oxide (NO) levels of on-road light-duty motor vehicles of the Metropolitan Area of Mexico City (MAMC). The sensor placed at 12 different sites also measured the concentration of CO2, CO, and total hydrocarbons (THC) in the exhaust emissions. A database was compiled containing 122 800 readings, of which 84 650 (69%) records were valid emissions measurements. CO, HC, and NO valid readings were 68.9, 63.4, and 62.9%, respectively, of the total attempted measurements. Furthermore, 42 822 vehicles were number-plate-matched to model year with the information provided by the Inspection/Maintenance Program. The mean emissions of total valid readings for CO, HC, and NO were determined to be 1.31 vol %, 440 ppm (propane), and 914 ppm, respectively. In 1991 and 1994, remote sensing measurements of CO and HC tailpipe emissions were performed in the MAMC in five different locations (30 000 valid readings). Large drops in both pollutants were observed for the intervening years, but sufficient vehicle information was not available at that time to fully explain the observed trends. Compared with those reports, our results point out to a steady decrease in CO and HC exhaust emissions with vehicle model year. The fleet emissions measured exhibit a gamma-distribution, with 10% of the most polluting fleet studied being responsible for 45%, 25%, and 29% of the CO, HC, and NO emissions, respectively. NO emissions in taxis are the highest among the vintage of vehicles, a matter of concern since according to the distance traveled per year, they represent 22% of the total activity in the MAMC.     

Oxidative dehydrogenation of n-butane: a comparative study of thermal and catalytic reaction using Fe–Zn mixed oxides

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier. This revised version was published online in July 2006 with corrections to the Cover Date.     

New Gallium-substituted hydrotalcites: [Mg1-x Ga x (OH)2](CO3) x/2 ·mH2O

A series of Ga-containing hydrotalcite-like materials (GaHTs), [Mg_1-x Ga _x (OH)₂](CO₃) _x/2 ·mH₂O wherex = 0.072 0.35 (Mg/Ga = 12.9 1.8), was synthesized by a coprecipitation method. The resulting solids were characterized by means of X-ray diffraction, thermal gravimetric and infrared analyses. All GaHTs showed diffraction patterns typical of Mg-Al hydrotalcites. Small amounts of brucite, Mg(OH)₂, were detected only in the GaHT with Mg/Ga = 12.9. Attempts to obtain Ga-richer hydrotalcites, Mg/Ga < 1.8, resulted in solids with an invariably constant Mg/Ga = 1.8 ratio, which appears to be the maximum Ga content limit. Judging from the TGA pattern of a GaHT (Mg/Ga = 7.7), the layered structure is stable up to ca. 573 K, and at 873 K the resulting solid shows a MgO-like diffraction pattern, suggesting that Ga³⁺ may be replacing some Mg²⁺ ions in MgO. Once their layered structure collapses (i.e., at 873 K), GaHT can be easily converted back into the original layered material by treating in a carbonate-containing aqueous solution, i.e., they show the memory effect typical of Mg-Al hydrotalcites.     

Sulfated SnO2-SiO2 superacid catalysts by Sol-Gel method

A series of sulfated SnO₂-SiO₂ binary oxides were prepared by the Sol-Gel method with different SnO₂ molar content. Tin chloride and tetraethylorthosilicate were used as precursors. The resulting solids were characterized by Fourier Transform Infrared spectroscopy (FTIR), ammonia thermodesorption, surface area measurements and also titrated using Hammett indicators. To correlate the acidic properties with catalytic activity, solids were tested in the 2-propanol decomposition reaction. SnO₂ addition to SiO₂ produce a twofold synergetic effect: an increase in the surface area in most of the resulting mixed oxides and high catalytic activities for the dehydration reaction. The titrated materials, using Hammett indicators, are of superacid nature (Ho –13.7).