Acid Sites Of Moderate Strength Research Articles

Wasteless Synthesis and Properties of Highly Dispersed MgAl2O4 Based on Product of Thermal Activation of Gibbsite

The study showed that the interaction of the product of centrifugal thermal activation of gibbsite with an aqueous solution of magnesium nitrate at a cationic ratio Mg:Al = 1:2 leads to the formation of mixed double hydroxides both under hydrothermal treatment at 150 °C and at room temperature. The subsequent thermal treatment at 550 °C of the product of mild interaction leads to ~90% alumina-magnesia spinel and ~10% MgO, while the treatment of the hydrothermal interaction product leads to ~100% spinel with the stoichiometric composition MgAl2O4. The obtained spinel samples possess a high specific surface area (above 100 m2/g) and a hierarchical pore structure formed by the micron-level particles of different sizes (1–2 and 10–20 μm) consisting of ~70 nm crystallites with ~3 nm pores; the samples differ mostly in the total volume and quantitative ratio of the pores. The samples have Lewis acid sites of moderate strength on the surface, the amount of which is much lower to how it is when compared with a sample prepared by precipitation in that they also differ by quantity from each other as well (503 μmol/g for stoichiometric spinel and 304 μmol/g for sample with admixture of MgO). As the calcination temperature is raised to 850 °C, the acidity decreases—only weak Lewis acid sites are observed, the amount of which is also higher for stoichiometric spinel (161 and 39 μmol/g, respectively). The method proposed for the synthesis of alumina-magnesia systems provides a high dispersion and a much lower surface acidity for the oxides; in addition, it minimizes or completely excludes wash water, in distinction to the precipitation method.

Guaiacol hydrodeoxygenation over Ni2P supported on 2D-zeolites

Abstract Different 2D zeolites, namely lamellar ZSM-5 (L-ZSM-5), the corresponding silica pillared ZSM-5 (PI-ZSM-5) and an MCM-22 sample, were investigated as nickel phosphide supports and evaluated as catalysts for guaiacol hydrodeoxygenation (HDO). Characterization confirmed that all loaded materials preserved a high crystallinity degree with formation of the Ni2P phase. The incorporation of Ni2P reduced adsorption capacity, affecting differently the micro- or mesoporous systems, depending on the zeolitic support. Ni2P deposition also modified the acidic properties of the zeolites due to the generation of new acid sites of moderate strength attributed to both Niδ+ species (Lewis acid sites) and the presence of residual P−OH moieties (weak Bronsted acid sites). The catalysts were tested in the HDO of guaiacol at 220 and 260 °C and 40 bar of H2. In all cases, ZSM-5 based catalysts exhibited higher conversions and HDO efficiencies than Ni2P/MCM-22, which denotes that MFI zeolitic structure favors the deoxygenation of phenolic compounds, yielding cyclohexane as major reaction product. This excellent catalytic performance is attributed to the combination of high surface area and optimal acidic properties since a synergetic effect was achieved between the acid sites of the raw zeolitic support and those provided by nickel phosphide having an enhanced accessibility. Thus, the best HDO performance was obtained over Ni2P supported on the pillared ZSM-5, showing a 78 % of guaiacol conversion and 95 % of selectivity towards deoxygenated molecules.

Influence of SBA-15 Zeolitization Conditions on Structural-Sorption and Acidic Properties of Micro-Mesoporous AluminosilicateS SBA-15/ZSM-5

Partly zeolitized micro-mesoporous aluminosilicates SBA-15/ZSM-5 were synthesized by conversion of dry gel of template-containing SBA-15 in the presence of tetrapropylammonium hydroxide. They have a hexagonally ordered mesostructure and developed surface and mesoporosity for which the concentration and strength of the Bronsted and Lewis acid sites are higher than in AlSi-SBA-15. The formation of Bronsted acid sites of moderate strength in the samples with a low degree of zeolitization is due to the presence of ZSM-5 precursors as well as crystallites in the obtained aluminosilicates.

Study of Gas Phase m-Cresol Alkylation with Methanol on Solid Acid Catalysts

The gas-phase alkylation of m-cresol with methanol was studied at 523 K on Al-MCM-41 and zeolites ZnY, HBEA, HZSM5 and HMCM22. The acidity was determined by ammonia TPD and FTIR of adsorbed pyridine. On acid sites of moderate strength (Al-MCM-41), initially the O-alkylation rate was higher than the C-alkylation rate. In contrast, formation of dimethylphenols by C-alkylation was highly favored on ZnY and HMCM22 which have both strong acidity although different nature; Lewis (ZnY) and Bronsted and Lewis (HMCM22). High selectivity of 2,5-DMP was observed on HZSM5, probably due to diffusional constraint. All catalysts, except Al-MCM-41, showed deactivation by coke formation.

Porous materials obtained by acid treatment processing followed by pillaring of montmorillonite clays

Montmorillonite clay was treated with hydrochloric acid and subsequently pillared with aluminum polyoxocations. The acid-treated samples were evaluated for the removal of structural elements (Al, Fe and Mg) and for conservation of the lamellar organization. The more severe the treatment, the greater the specific area obtained. Despite the loss of structural organization, all samples were pillared, with a displacement of peak (001) to 2θ lower angles. The pillaring of all acid-treated samples promoted an increase in the specific area of these materials. Pillared samples previously treated at 50°C with HCl concentrations of 2 and 4molL−1 underwent an increase in their specific areas of approximately 45%. Pillared samples treated with 4molL−1 of HCl at 80°C underwent the lowest percent increase in surface area, approximately 10%, compared to the untreated samples. All samples possessed greater uniformity in pore size. Acid treatment prior to pillaring promoted an increase in the number of acid sites of moderate strength compared to pillared natural clay.

α-Pinene isomerisation over heteropoly acid catalysts in the gas-phase

The isomerisation of α-pinene was studied in the gas phase over solid heteropoly acid (HPA) catalysts in a fixed-bed continuous flow reactor at 200 °C and ambient pressure. The catalysts included bulk and SiO 2-supported H 3PW 12O 40 and bulk Cs 2.5H 0.5PW 12O 40 possessing strong Brønsted acid sites, as well as composites prepared by supporting 15 wt% H 3PW 12O 40 on Nb 2O 5, ZrO 2 and TiO 2 possessing both Brønsted and Lewis acid sites of moderate strength. The reaction yielded camphene as the main product in a mixture with monoterpene by-products such as limonene, terpinolenes, terpinenes, β-pinene, p-cymene and others. The HPA catalysts with strong Brønsted acid sites exhibited high initial activities, but suffered from catalyst deactivation, resulting in low camphene yields. Conversely, the HPA catalysts supported on Nb 2O 5, ZrO 2 and TiO 2, although weaker acids, showed more stable performance in α-pinene isomerisation. The HPA catalyst supported on TiO 2 gave a camphene yield of 51% and a total camphene and limonene yield of 58%.

Surface characterization of acidic ceria–zirconia prepared by direct sulfation

Acidic ceria–zirconia (SCZ) solid acid catalysts with a nominal surface density of ca 2 SO 4 2−/nm 2 were prepared by a simple route consisting in soaking high specific surface area Ce x Zr 1− x O 2 (with x = 0.21 and 0.69) mixed oxides solutions in 0.5 M sulphuric acid. Characterizations by TPD-MS, TP-DRIFTS and FT-Raman revealed that most of surface structures generated by sulfation are stable at least up to 700 °C under inert atmosphere and consist mainly as isolated sulfates located on defects or crystal planes and to a lesser extent as polysulfates. Investigations by pyridine adsorption/desorption have stated that: SCZ possess both strong Brønsted (B) and Lewis (L) acid sites, some of them being presumably superacidic; the B/L site ratio was found to be more dependent on the temperature and hydration degree than on the composition of the ceria–zirconia. By contrast, the reactivity of the parent Ce x Zr 1− x O 2 materials towards pyridine is mostly driven by redox properties resulting in the formation of Py-oxide with the participation of Lewis acid sites of moderate strength ( cus Ce x+ and Zr x+ cations). Basicity studies by CO 2 adsorption/desorption reveal that SCZ surfaces are solely acidic whereas the number and strength of Lewis basic sites increases with the Ce content for the parent Ce x Zr 1− x O 2 materials.

Isomerization of 1-butene catalyzed by surfactant-modified, Al2O3-pillared clays

AbstractRecent studies indicate that a template method for creating Al-pillared clays, in which surfactant micelles foster the creation of a homogeneous mesoporous network within the pillar, effectively enhance catalyst performance and adsorbent properties. No studies, however, have described the relative effects of the surfactant concentration and Al content on the textural and acidic properties and on the catalytic activity of the Al-pillared clays. The purpose of the present study was to fill this gap, using the isomerization of 1-butene as the test process for catalytic activity. Modified pillared clays (MPC) were prepared from a synthetic clay, TS-1, using different amounts of a non-ionic surfactant (Igepal CO-720) and a fixed concentration of a solution containing the Al polycation [Al13O4-(OH)24]7+. MPC with a fixed amount of surfactant and different amounts of Al were also prepared. The catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), temperature-programmed desorption of ammonia, 27Al magic-angle spinning nuclear magnetic resonance (27Al MAS NMR), and N2 adsorption/desorption isotherms. Isomerization of 1-butene at 250°C was used to test the catalytic activity. Analyses by XRD and XRF showed that the synthesized solids were amorphous and that the amount of pillaring by Al increased with the amount of Al complex used. Interestingly, the surface area and pore volume were directly proportional to the amount of surfactant employed and decreased with increasing amounts of Al pillaring. All solids showed activity for 1-butene isomerization, with a maximum conversion of ∼75%. Only cis- and trans-2-butene were observed. The absence of isobutene suggested that acid sites of moderate strength were formed, in agreement with the results obtained from the desorption of ammonia.

Xylene isomerisation catalyst

Different 2D zeolites, namely lamellar ZSM-5 (L-ZSM-5), the corresponding silica pillared ZSM-5 (PI-ZSM-5) and an MCM-22 sample, were investigated as nickel phosphide supports and evaluated as catalysts for guaiacol hydrodeoxygenation (HDO). Characterization confirmed that all loaded materials preserved a high crystallinity degree with formation of the Ni2P phase. The incorporation of Ni2P reduced adsorption capacity, affecting differently the micro- or mesoporous systems, depending on the zeolitic support. Ni2P deposition also modified the acidic properties of the zeolites due to the generation of new acid sites of moderate strength attributed to both Niδ+ species (Lewis acid sites) and the presence of residual P−OH moieties (weak Brønsted acid sites). The catalysts were tested in the HDO of guaiacol at 220 and 260 °C and 40 bar of H2. In all cases, ZSM-5 based catalysts exhibited higher conversions and HDO efficiencies than Ni2P/MCM-22, which denotes that MFI zeolitic structure favors the deoxygenation of phenolic compounds, yielding cyclohexane as major reaction product. This excellent catalytic performance is attributed to the combination of high surface area and optimal acidic properties since a synergetic effect was achieved between the acid sites of the raw zeolitic support and those provided by nickel phosphide having an enhanced accessibility. Thus, the best HDO performance was obtained over Ni2P supported on the pillared ZSM-5, showing a 78 % of guaiacol conversion and 95 % of selectivity towards deoxygenated molecules.

Transition-metal-free microporous and mesoporous catalysts for the epoxidation of cyclooctene with hydrogen peroxide

Transition-metal-free microporous and mesoporous materials were studied as heterogeneous epoxidation catalysts. MCM-41-type materials and zeolites with various porous structures and aluminium or gallium content but devoid of any transition metal were prepared and tested in the epoxidation of cyclooctene with aqueous hydrogen peroxide. The most promising catalytic results were obtained with Al-MCM-41, Ga-MCM-41, and ultra-stable zeolite Y (USY) with a low Si/Al ratio. All of these catalysts have acid sites of moderate strength. With Ga-MCM-41, 95% conversion of cyclooctene with 96% selectivity toward the epoxide was reached after 24 h of reaction. The catalyst could be recycled with no loss of catalytic properties.

Direct synthesis of methyl isobutyl ketone in gas-phase reaction over palladium-loaded hydroxyapatite

Several compositions of palladium-loaded calcium hydroxyapatite Pd( x)/CaHAp were prepared and characterized by XRD, IR, and UV–visible spectroscopy and temperature-programmed reduction (TPR). The acid–base properties of the samples were studied with the use of butan-2-ol conversion and IR spectroscopy coupled with lutidine adsorption/desorption. Calcium hydroxyapatite bears Lewis acid sites of moderate strength and a limited number of Brønsted acid sites, which produce butenes by dehydration. The selectivity of the dehydrogenation reaction (formation of methyl ethyl ketone) is very limited in the absence of oxygen but indicates the existence on the surface of phosphate of basic sites that are as essential as the acid sites in the direct synthesis of methyl isobutyl ketone (MIBK) from acetone and H 2. Dispersion of Pd on the hydroxyapatite introduces redox properties and the bifunctionality needed in the hydrogenation of mesityl oxide (MO). Temperature-programmed reduction (TPR) of Pd( x)/CaHAp catalysts shows a sharp peak of H 2 consumption between 268 and 276 K, depending on palladium loading, followed by a negative peak at about 338 K, which is associated with the decomposition of β-PdH. Hydrogen chemisorption measurements carried out at 353 K allowed the determination of the size of palladium particles, which varies between 8 and 28 nm. The Pd( x)/CaHAp catalysts showed good performance in the low-pressure one-step synthesis of MIBK. They are stable under the experimental conditions, and they exhibit a MIBK and MIBC selectivity that reaches 92% for a conversion of 22% and an optimal Pd loading of 2 wt%. Other parameters of the reaction (such as temperature of reaction, residence time, H 2/acetone ratio) were studied. They all play an important role in acetone conversion and product distribution. However, the reaction temperature has the most significant influence, since a maximum of the global conversion was observed at 423 K.

Skeletal isomerization of n-butenes to isobutene over acid-treated natural clinoptilolite zeolites

The proton form of the natural clinoptilolite zeolite (HNZ) was modified by treatment with various acids. Effects of modification on catalytic performance were investigated for the skeletal isomerization of n-butenes to isobutene. Among HNZs treated with various acids, only those modified with boric acid (B-HNZ) showed an improved catalytic performance of skeletal isomerization in terms of activity and stability. Furthermore, B-HNZ exhibited higher isobutene selectivity compared to unmodified HNZ at the same conversion of n-butenes. The performance was comparable to that of a widely known ferrierite catalyst at the same reaction conditions. The main effect of boric acid treatment on HNZ was the selective increase in the number of acid sites of moderate strength required for efficient catalysts in skeletal isomerization without collapse of its framework. These acid sites of moderate strength were considered to be selective and stable reaction sites for skeletal isomerization of n-butenes. Coke deposition poisoned non-selective strong acid sites and imposed a spatial restriction on zeolite channels, leading to suppression of undesired dimerization reactions. Factors affecting selectivity to isobutene are discussed.

Effect of nickel on the surface acidity of γ-alumina and alumina-supported nickel—molybdenum hydrotreating catalysts

The influence of nickel on the acid strength distribution of a γ-Al 2O 3 support and a series of alumina-supported hydrotreating catalysts such as Ni&z.sbnd;Mo/γ-Al 2O 3and Ni&z.sbnd;P/γ-Al 2O 3 in their oxidic form in the concentration range close to commercial catalysts were examined by temperature-programmed desorption of ammonia. A substantial increase in the concentration of strong acid sites was observed when nickel was added to γ-Al 2O 3. On the other hand, addition of molybdenum to γ-Al 2O 3 resulted in the neutralization of the strong acid sites with a significant increase in the number of acid sites of medium strength. The influence of phosphorus on the acidity was similar to that of molybdenum. Addition of nickel to MoO 3/γ-Al 2O 3 and PO 4/γ-Al 2O 3 systems was found to increase the number of strong acid sites considerably, with a corresponding decrease in concentration of the acid sites of moderate strength. A mechanism based on the formation of a nickel aluminate spinel on the surface of alumina and its effect in increasing the net positive charge or the coordinatively unsaturated sites of the surface is proposed in order to rationalize the influence of nickel on increasing the acid strength.