J. Navarrete

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.

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

Study of the stabilization of zinc phthalocyanine in sol-gel TiO2 for photodynamic therapy applications

Photodynamic therapy (PDT) has emerged as an alternative and promising noninvasive treatment for cancer. It is a two-step procedure that uses a combination of molecular oxygen, visible light, and photosensitizer (PS) agents; phthalocyanine (Pc) was supported over titanium oxide but has not yet been used for cell inactivation. Zinc phthalocyanine (ZnPc) molecules were incorporated into the porous network of titanium dioxide (TiO(2)) using the sol-gel method. It was prepared from stock solutions of ZnPc and TiO(2). ZnPc-TiO(2) was tested with four cancer cell lines. The characterization of supported ZnPc showed that phthalocyanine is linked by the N-pyrrole to the support and is stable up to 250°C, leading to testing for PDT. The preferential localization in target organelles such as mitochondria or lysosomes could determine the cell death mechanism after PDT. The results suggest that nanoparticulated TiO(2) sensitized with ZnPc is an excellent candidate as sensitizer in PDT against cancer and infectious diseases.

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.

Selectivity to cumene in the alkylation of benzene with isopropanol on a MCM-41/γ-Al2O3 catalyst

Abstract Alkylation of benzene with isopropanol to produce cumene was investigated using a MCM-41 aluminosilicate as catalyst. The MCM-41/γ-Al 2 O 3 catalyst, activated by means of a liquid extraction method, mainly contained strong Lewis acid sites. After the activity of the composite catalyst was stabilized, it was observed that, in the isopropylation of benzene, cumene was the major product. The high cumene selectivity could be due to the low acid site concentration in the MCM-41 material, and the rapid diffusion of cumene in the broad channels of this mesoporous framework.

Para-selectivity in the alkylation of toluene with isopropanol on a MCM-41/γ-Al2O3 catalyst

Abstract Alkylation of toluene with isopropanol to produce isopropyltoluenes was investigated using a MCM-41 aluminosilicate as catalyst. The MCM-41/γ-Al2O3 catalyst presented mainly strong Lewis sties so that this material was activated by means of a liquid extraction method. It was observed that in the isopropylation of toluene, the isopropyltoluenes fraction contained more para isomer. This is due to the fact that MCM-41 materials contain relatively a few acid sites. In addition, the primary products of alkylation easily diffuse into the broad channels of this mesoporous framework.

Preparation of Ni/ZrO2–SO42− catalysts by incipient wetness method: effect of nickel on the isomerization of n-butane

Abstract A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 to 9.6 wt.%) were prepared by incipient wetness method. Ni and SO 4 2− promoters were co-impregnated to a parent zirconium hydroxide by a solution of Ni(NO 3 ) 2 ·6H 2 O in H 2 SO 4 . After calcination at 948 K, the solids isomerized n -butane at 338 K. Up to 4.5 wt.% Ni content, nickel increases activity of ZrO 2 –SO 4 2− , afterwards, the catalytic activity decreases. The temperature-programmed desorption of ammonia (TPD-NH 3 ) and IR of pyridine adsorbed results show that enhanced activity cannot be completely explained in function of a higher acid strength. The increase of the isomerizating activity is better explained in terms of a bimolecular mechanism, as proposed by Guisnet et al. [M.R. Guisnet, Acc. Chem. Res. 23 (1990) 392], involving olefins as intermediates. In this mechanism, Ni causes an enhancement in the surface concentration of olefins. In spite of the relatively high Ni concentration, X-ray diffraction results showed no evidence of any NiO phase due to this oxide is well-dispersed on the surface of ZrO 2 –SO 4 2− in form of small particles. The inhibition of isomerizating properties of the catalysts when hydrogen was present in the reactor feed confirmed this bimolecular mechanism. Interestingly, unpromoted ZrO 2 –SO 4 2− exhibited also the usual induction period observed on nickel-promoted sulfated zirconia catalysts. Then, this bimolecular mechanism for the n -butane isomerization could also apply in the unpromoted zirconia sulfate catalyst.

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 evolution in air and argon of nanocrystalline MoS2 synthesized under hydrothermal conditions

Abstract Nanocrystalline molybdenum sulfide was synthesized between 150°C and 225°C under hydrothermal conditions starting from ammonium heptamolybdate and thiourea. Samples were characterized with X-ray powder diffraction, electron microscopy, nitrogen adsorption, thermoanalysis and infrared spectroscopy. The hydroxyls involved in the synthesis and adsorbed on crystals surface hindered crystallization and samples still recrystallized after the final dehydroxylation step above 300°C, just when hydroxyls were isolated from each other. This also promoted sulfur bond breaking that gave rise to partial transformation of the MoS 2 into MoO 2 when the annealing atmosphere was argon, and to the total transformation of the sulfide into MoO 3 when it was air. The initial MoS 2 crystals were bend; many of them were isolated, and others associated in bundles that formed worm-like grains interacting with each other to produce spherical grains aggregated in clusters. This morphology gave rise to samples with a low specific surface area.