Generation Of Strong Acid Sites Research Articles

Generation of Strong Acid Sites on Yttrium-Doped Tetragonal ZrO2-Supported Tungsten Oxides: Effects of Dopant Amounts on Acidity, Crystalline Phase, Kinds of Tungsten Species, and Their Dispersion

A tungsten oxide catalysts supported on yttrium- or ytterbium-doped crystalline zirconium oxide (WOx/Y(Yb)-ZrO2) with different dopant concentrations but the same tungsten loadings and calcination temperatures were prepared to investigate the mechanism responsible for the strong solid acidity of so-called tungstated zirconia. The surface acid–base properties of WOx/Y(Yb)-ZrO2 and the support were characterized by catalyses of n-butane skeletal isomerization, alkylation of anisole with benzyl alcohol, and 2-butanol decomposition. The structural characterization and quantitative analysis were conducted by XRD, XAFS, and UV–Vis techniques. Crystalline zirconium oxides were effective as the starting material for tungsten–zirconium-based strong solid acid catalysts instead of amorphous Zr(OH)x if the crystalline form was tetragonal. Tetragonal-phase Y(Yb)-ZrO2 substitutional solid solutions with dopant concentrations of 3–5 mol % was a suitable support for stabilization of active small WO3 clusters. Tungsten species aggregated to form inactive large WO3 particles on the monoclinic ZrO2 polymorph with Y-dopant amount less than 2 mol % even though the surface density of tungsten species and calcination temperature were the same as those for the active catalysts. Inactive yttrium–tungsten binary oxides formed when the yttrium doping level exceeded 10 mol %. Intrinsic role of crystalline phase of tetragonal zirconia on generating the strong solid acidity is confirmed to stabilize WO3 small clusters without aggregation.

Performance of Ti-pillared montmorillonite supported Fe catalysts for toluene oxidation: The effect of Fe on catalytic activity

In this study, TiO2-pillared montmorillonite (Ti-PILC) supported Fe catalysts were synthesized and used for the catalytic oxidation of toluene. The prepared catalysts were characterized by a combination of XRD, N2 adsorption-desorption isotherms, XPS, H2-TPR and NH3-TPD. The Fe introduction preserves the mesoporous structure and porosity of Ti-PILC. In the steady-state catalytic test and kinetic analysis, the toluene oxidation over studied catalysts follows the Mars-Van Krevelen mechanism. By the generation of strong acid sites and abundant surface oxygen species, Fe introduction obviously improves the catalytic activity of Ti-PILC for toluene oxidation. But with the increase of Fe content, the activity initially enhances, and then decreases. The effect of Fe species on the promoted toluene oxidation over Ti-PILC was discussed in view of reaction mechanism and above-mentioned variations of physicochemical properties. Furthermore, the Ti-PILC supported Fe catalysts display high stability and superior activity in the presence of water vapor, which present an applied interest.

Protonated titanate nanotubes as a highly active catalyst for the synthesis of renewable diesel and jet fuel range alkanes

Protonated titanate nanotube (PTNT) was first reported as a good catalyst for the hydroxyalkylation/alkylation (HAA) of 2-methylfuran (2-MF) and n-butanal from lignocellulose. Compared with other inorganic solid acids, PTNT has higher catalytic activity and efficiency. Over it, high yield of HAA product (77%) was achieved under mild reaction conditions. PTNT is also effective for the HAA (or alkylation) reactions between 2-MF and other lignocellulosic carbonyl compounds. According to the characterization results, the hydrothermal treatment of TiO2 P25 with NaOH solution and subsequent acidification significantly increases its BET surface area and the amount of acid sites on the surface of catalyst. Moreover, these pretreatments also lead to the generation of strong acid sites and Brönsted acid sites on the surface of catalyst. All of these changes are responsible for the excellent performance of PTNT. As the final aim of this work, the HAA product of 2-MF and n-butanal was converted to diesel and jet fuel range brached alkanes by the hydrodeoxygenation (HDO) over zeolite loaded Ni catalysts. Among them, Ni/H-ZSM-5 demonstrated the best catalytic performance and good stability, which makes it a promising catalyst in future application.

Platinum catalysts supported on silicas: effect of silica characteristics on their catalytic activity in carbon monoxide oxidation

Platinum catalysts supported on silicas with various physicochemical properties were prepared in order to investigate the effect of silica characteristics on their platinum dispersion and catalytic activity in the oxidation of carbon monoxide. Although titania-incorporation into silica and further treatment of the impregnated platinum precursor with hydrogen peroxide were effective for improving the dispersion and stability of platinum catalysts supported on silicas, regardless of the characteristic of the silicas, the platinum catalysts supported on fumed silica with a medium level of surface hydroxyl group concentration exhibited the highest catalytic activity among those supported on mesoporous silica, silica gel, and precipitated silica. The required properties of the highly active platinum catalyst seemed to be a high dispersion of platinum, the formation of a stable titania layer, and the generation of strong acid sites. By contrast, the precipitated silica with a small surface area and high concentration of surface hydroxyl groups was not appropriate for a catalytic support for platinum catalysts.

NO reduction by CH4 in the presence of excess O2 over Mn/sulfated zirconia catalysts

Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH3–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N2 adsorption and the H2–TPR of the catalysts demonstrated that the presence of the SO42− species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH4 by O2 and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO42−. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO2 poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO2.

Strong Acid Sites Created on Small–Sized Anatase

Abstract The acid properties of anatase having different crystalline sizes were estimated by measuring the temperature-programmed desorption (TPD) of ammonia as well as the infrared spectrum of adsorbed pyridine at elevated temperature. The size-controlled anatase was prepared by hydrolysis of the titanium tetraisopropoxide dissolved in propyl alcohol using various amounts of distilled water. The acid properties of the anatase were confirmed to be strongly affected by the crystalline size. Especially, the strength of acid sites was found to remarkably increase with a decrease in the crystalline size. The specific activity and the activation energy for 1-butene isomerization on the anatase also depended on the crystalline size. The generation of strong acid sites on the surface is discussed in terms of the crystalline size.

The acidic properties of TiO2-SiO2 and its catalytic activities for the amination of phenol, the hydration of ethylene and the isomerization of butene

Abstract The generation of strong acid sites was observed on the binary oxide, TiO2-SiO2, which was obtained by a coprecipitation method. The acidity and acid strength of the mixed oxide of a certain composition were estimated to be higher than those of SiO2-Al2O3 containing 20 wt% of Al2O3 by n-butylamine titration and by the adsorption of basic molecules on the surface. The acidity depended both on pretreating temperature and the catalyst composition. The highest acidity per unit weight of catalyst was observed when TiO2-SiO2 (mole ratio = 1) was heated at 500 °C. The binary oxide showed high catalytic activity and selectivity for the amination of phenol with ammonia to produce aniline, but low activity and selectivity for the hydration of ethylene. The isomerizations of butenes were also catalyzed; the products ratio of cis-/trans-2-butene was 1–2. The catalytic action for three reactions is well interpreted in terms of the acidic property. The mechanism of the generation of acid sites by mixing TiO2 with SiO2 is discussed.