E. López-Salinas

Thermal stability of 12-tungstophosphoric acid supported on zirconia

A freshly precipitated zirconia was impregnated with an ethanol solution of H3[W12PO40]6H2O (TPA) in order to obtain 0‐25 wt.% TPA/ZrO2. The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO2 exist as distorted intact Keggin species interacting with Zr‐OH or Zr C groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO2 calcined at 773 than that at 473 K. ©2000 Elsevier Science B.V. All rights reserved.

Characterizations of the thermal decomposition of brucite prepared by sol–gel technique for synthesis of nanocrystalline MgO

Abstract Brucite was synthesized by the sol–gel technique using magnesium diethoxide in a homogeneous reaction medium. Its decomposition, including dehydroxylation, crystallization and phase transformation was studied by XRD, TGA–DTG, CTEM, HRTEM, SAED and FTIR techniques. It has been proposed that during the dehydroxylation and decomposition of the magnesium diethoxide gel, some vacancies are generated. The decomposition of brucite with hexagonal structure to form periclase with the cubic structure occurs in the temperature range from 473 to 773 K. In this case, nanocrystalline MgO is formed from the break down of the brucite crystalline structure.

Effects of structural defects and acid-basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol-gel alumina catalyst

The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared FTIR spectroscopy of pyridine adsorption and temperature-programmed desorption of CO and NH . The number of acid and 23 basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions—tetrahedral, pentacoordinated and octahedral, which were identified by the 27 Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 2508C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of g-Al O. 23 Bimolecular reaction to isopropylether was largely governed by the pentacoordinated aluminum ions which were related to the coordinately unsaturated aluminum ions. A mechanism for the formation of isopropylether was proposed: oxygen . q vacancies were suggested to involve the adsorption step of isopropanol, an intermediate species, CH HC , reacted with 32

Effect of sulfation methods on TiO2–SiO2 sol–gel catalyst acidity

A series of x TiO2‐SiO2 (xD1.7, 4.0 and 6.7 wt.% TiO2) binary mixed oxides was prepared by a sol‐gel method in acidic and basic conditions. In order to upgrade the acidic properties of the obtained solids, their sulfation was carried out in three different ways: (1) by in situ (H2SO4 was admixed with the gel), (2) impregnating the dried solids with aqueous solutions of H2SO4, and (3) by impregnating as in (2) but with (NH4)2SO4. FTIR-pyridine adsorption and benzene deuteroexchange showed that the acid site strength depends on the sulfation method. The highest values of total acidity, determined by ammonia thermodesorption, correspond to the in situ sulfated samples. The obtained pore size distribution (4.0‐5.0 nm) was narrow and the specific BET areas (250‐350 m 2 /g) were large. X-Ray diffraction, RDF, XPS spectroscopy and catalytic activity in the 2-butanol dehydration show that one step sulfated in situ TiO2‐SiO2 sol‐gel preparations resulted in catalysts with homogeneous and strong acidity.

Skeletal isomerization of 1-butene on 12-tungstophosphoric acid supported on zirconia

A series of 0‐25 wt% H3[W12PO40] (TPA) impregnated on freshly precipitated Zr(OH)4 were prepared. The solids were characterized by a Hammett indicator method, specific surface area and pore size measurements, pyridine adsorption FTIR and later tested as catalysts in the isomerization of 1-butene. Maximum acid strength of 5‐20 wt% TPA/ZrO2 calcined at 673 K is H0aˇ9.3, but dried samples (393 K) showed near superacid strength, i.e. H0ˇ13.75. The sites are mainly strong Lewis acids, but TPA supported on stabilized ZrO2 (calcined at 773 K) shows both strong Bronsted and Lewis acidity. TPA addition to hydrated Zr(OH)4 stabilizes the surface area of the final calcined material in comparison with that of pure ZrO2; the greater the TPA content the higher the resulting surface area. Skeletal rearrangement of 1-butene to isobutylene proceeds on 5‐25 wt% TPA/ZrO2 but not on pure ZrO2, the greater the TPA content the higher the initial selectivity towards isobutene. In contrast, 20 wt% TPA supported on SiO2 formed no isobutylene. # 1998 Elsevier Science B.V. All rights reserved.

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.

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

A series of Ga-containing hydrotalcite-like materials (GaHTs), [Mg1-xGax(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.

EXHAUST EMISSIONS FROM GASOLINE- AND LPG-POWERED VEHICLES OPERATING AT THE ALTITUDE OF MEXICO CITY

Unburned hydrocarbons (HCs), carbon monoxide (CO), and oxides of nitrogen (NOx) are the compounds regulated as pollutants by an environmental standard in the Metropolitan Area of Mexico City (MAMC). The main fuel used in vehicular transportation is gasoline, and the use of liquefied petroleum gas (LPG) is now an alternative as low emission technology to decrease the environmental impact of transportation operations. The environmental impact of commercial gasoline consumption in the Valley of Mexico was estimated by on-road and FTP-75 testing of three formulations of gasoline (one leaded [octane 81] and two unleaded [one octane 87 and one octane 93]). A fleet of 30 vehicles was used: 10 were chosen that had pre-1990 technology, while 12 were 1991-1996 vehicles equipped with fuel injection, catalytic converters, and air/ fuel ratio control technology. The remaining eight vehicles were high-performance new model vehicles (1995-1996) equipped with the newest technology available for pollution control. Fifteen vehicles in the fleet were also tested for the effect of changing from leaded to unleaded gasoline. Three different LPG formulations were tested using three vehicles representative of the LPG-powered fleet in the MAMC. Two gasoline-to-LPG conversion certified commercial systems were evaluated following the BAR-90 and the HOT-505 procedures. Emissions corresponding to the high-octane (premium) gasoline showed a 15% higher contribution to HCs with a 6% lower reactivity than the 87 octane gasoline; the HCs in the exhaust for premium gasoline are mainly isoparaffins. When the vehicles were tested on the road at high speeds, an average 3% increase in mileage was obtained when vehicles were switched from leaded to unleaded gasoline, while a 5% increase in mileage was observed when vehicles were switched from 87 octane to premium gasoline. The tests of LPG formulations indicated that a change in composition from 60% vol of propane to 85.5% vol reduces levels of HCs and CO emissions; such is not the case for the NOx emissions. The higher the concentration of propane, the higher the levels of NOx that reached values above the maximum limits set by the environmental standard. A value of 70% vol of propane in the LPG mixture, with variations no greater than 4%, seems to be the best method for reducing pollutant emissions in Mexico City.

Evidence for Lewis and Brønsted Acid Sites on MgO Obtained by Sol-Gel

Traditionally, magnesium oxide has been considered a typical basic catalyst, catalyst carrier and/or adsorbent. In this study MgO was prepared using Mg-ethoxide dissolved in ethanol and hydrolyzed with various aqueous inorganic acids and bases. We have found that it is possible to induce Brønsted and Lewis acid sites depending on the method of preparation and, more specifically, on the type of inorganic acids used in the hydrolysis stage. In the FTIR spectra of adsorbed pyridine on MgO obtained using aqueous HCl (pH = 3), the bands corresponding to Lewis (1603, 1496 and 1444 cm−1) and Brønsted (1550 cm−1) acid sites are observed. The fact that upon vacuum and high temperature treatment, i.e., at 773 K, the above mentioned bands are clearly distinguishable indicated that the acid sites are very strong. On the other hand, the MgO materials obtained using aqueous acetic acid as a hydrolysis catalyst (pH = 5), and that without any catalyst (pH = 7) showed only Lewis acid sites which decreased markedly upon higher temperature treatment. When aqueous NH4OH (pH = 9) was used as the hydrolysis catalyst, the MgO obtained showed only Lewis acid sites. In all cases the number of Lewis acid sites was greater than that of Brønsted sites.

THERMAL BEHAVIOR OF HYDROTALCITE-LIKE [Mg1−xGax(OH)2](CO3)x/2·mH2O AS A FUNCTION OF GALLIUM CONTENT

Abstract The thermal behavior of a series of Ga-substituted hydrotalcites [Mg 1− x Ga x (OH) 2 ] (CO 3 ) x /2 · m H 2 O, where x = 0.072−0.35, was examined by means of thermal gravimetric analysis, (TGA), differential thermal analysis (DTA) and x-ray diffraction (XRD) measurements. The weight loss patterns of Ga-hydrotalcites depend on the Mg/Ga molar ratios. For example, a high [CO 3 ] 2− density in the interlayer region of Ga-rich samples (i.e. Mg/Ga = 1.8) makes them more difficult to dehydrate and slower to decompose than Ga-poor samples (i.e. Mg/Ga = 12.9). Removal of interlamellar water molecules in Ga-rich hydrotalcites consumes less energy than that in Ga-poor ones. The decomposition stage, regardless of Mg/Ga ratios, takes place at a constant temperature of ≈ 410 K, but this process requires much less energy in Ga-rich (Mg/Ga = 1.8; 22 J g −1 ) than in Ga-poor samples (Mg/Ga = 12.9; 202 J g −1 ). XRD analysis indicates that the layered structure is destroyed below 573 K, and that between 623 and 973 K, Ga-hydrotalcites convert into MgO-like materials, probably substituted by Ga 3+ . Calcination temperatures above 1073 K yield MgO-MgGa 2 O 4 mixtures.