Lower Surface Acidity Research Articles

Preparation and Selective Acetylene Hydrogenation Catalytic Properties of Supported Pd Catalyst by the in Situ Precipitation−Reduction Method

Pd/MgAl-layered double hydroxide (LDH) was synthesized in situ on the surface of spherical Al2O3 to obtain a Pd/MgAl-LDH/Al2O3 catalyst, using hexamine as both a precipitant for LDH and a reductant for Pd2+. After calcination and reduction, another Pd/MgO-Al2O3 catalyst was obtained. As a comparison, the Pd/Al2O3 catalyst was prepared by the conventional impregnation method. Low-temperature N2 adsorption−desorption, NH3 temperature-programmed desorption, and scanning electron microscopy results showed that Pd/MgAl-LDH/Al2O3 and Pd/MgO-Al2O3 catalysts possessed larger surface area, lower surface acidity, uniform Pd particle size, and specific Pd shape compared with the Pd/Al2O3 catalyst. The catalytic performances of the catalysts were then studied in the selective hydrogenation of acetylene. In comparison to Pd/Al2O3, Pd/MgAl-LDH-Al2O3 and Pd/MgO-Al2O3 catalysts exhibited not only higher activity due to the uniform size and specific shape of Pd particles which provided more catalytic active sites but also...

Metal modification effects on ethanol conversion to propylene by H-ZSM-5 with Si/Al2 ratio of 150

Metal modification effects on ethanol conversion to propylene were examined by use of H-ZSM-5 with Si/Al2 ratio of 150. In La- and Mg-co-modified H-ZSM-5 (La/Al2 ratio: 1.0, Mg/Al2 ratio: 0.10), 32% propylene selectivity was obtained at 0.1 MPa, 550 °C. This value was higher than those of unmodified and La-modified (La/Al2 ratio: 1.0) H-ZSM-5. The number of acid sites and the acid strength (surface acidity) decreased in the order of unmodified, La-modified, and La- and Mg-co-modified H-ZSM-5. In the initial stage of ethanol introduction, ethylene production by dehydration of ethanol increased in La- and Mg-co-modified H-ZSM-5 with lower surface acidity, whereas the formation of aromatics by oligomerization-cracking increased in unmodified H-ZSM-5 with higher surface acidity. The propylene selectivity was 25% after 36.2 h in the La- and Mg-co-modified H-ZSM-5. The selectivity ratio of propylene to ethylene was 0.47 after 36.2 h; a rapid increase in ethylene formation was also depressed. Therefore, La- and Mg-co-modified H-ZSM-5 with Si/Al2 ratio of 150 was highly effective for the conversion of ethanol to propylene.

Mesoporous aluminophosphate materials: influence of method of preparation and iron loading on textural properties and catalytic activity

In this paper the influence of the method of preparation and the iron loading on the textural properties and surface acidity of Iron aluminophosphates (FeAlP), [Fe x Al0.95P1, x = 0.01 mol−0.1 mol] was studied in order to learn how the preparation conditions and iron loading affect catalyst activity in transesterification reaction between diethyl malonate and benzyl alcohol. The catalysts were characterized by BET surface area, N2 sorption studies, NH3-TPD pXRD, SEM, and. FT-IR. Significant differences in the texture, surface acidity and adsorptive properties of FeAlP catalysts were found showing the influence of the preparation conditions and iron loading on the textural and surface properties of these catalysts. The textural and surface properties are correlated in the transesterification reaction between diethyl malonate and benzyl alcohol. FeAlP with 0.025 mol iron loading prepared by drop wise addition of ammonia, resulted a mesoporous material with uniform pore diameter (5–18 nm), higher surface area and higher surface acidity and hence catalytic activity towards the formation of Dibenzyl malonate. Fast addition of ammonia produced nonuniform porous material with lower surface acidity and low transesterification activity. Urea hydrolysis method yielded nonuniform porous materials with low surface area as well acidity.

Vanadium-loaded carbon-based monoliths for on-board NO reduction: Influence of nature and concentration of the oxidation agent on activity

A series of polymeric carbon-coated monoliths oxidized with either concentrated or 2N HNO 3, H 2O 2 or H 2SO 4 and subsequently loaded with 3 wt.% vanadium were prepared. The influence of the different oxidation treatment conditions on the SCR (selective catalytic reduction) catalytic activity, texture and chemical surface properties were studied. Similar pore distribution and pore volumes were observed for the four oxidized samples, indicating that surface modification of carbon supports has been successfully made without disrupting the original textural structures of activated coated monoliths. The surface chemistry created by the oxidation treatments has two effects on the catalytic activity. One of these is that higher surface acidity results in higher NO reduction up to a certain extent. The highest acidities seem to promote a sufficiently strong NH 3 adsorption on the surface such that the lack of efficient desorption decreases the overall NO conversion efficiency. The other effect is that a low surface acidity does not seem to promote vanadium dispersion and fixation, thereby also resulting in decreased NO reduction efficiency.

Surface reactivity of zinc-modified hydroxyapatite

Surface reactivity of hydroxyapatite (Hap), Ca 10(PO 4) 6(OH) 2, and zinc-Hap was evaluated using temperature-programmed desorption (TPD) of ethanol and diffuse reflectance infrared spectroscopy (DRIFTS). Hap was prepared by the precipitation method and the Zn-based catalysts were prepared by the ionic exchange method (ZnHap troc) and by zinc oxide deposition (ZnHap dep). X-ray diffraction (XRD) results have shown the Ca 2+ substitution by Zn 2+ ions for the catalyst prepared by ionic exchange method and a zinc oxide formation as a segregated phase after zinc deposition. TPD-ethanol analyses showed that Hap has catalytic activity to ethanol decomposition and low surface acidity. DRIFTS results showed ethoxide species formation at the surface of ZnHap troc, which undergoes transformation into acetate and carbonate species with increasing temperature. The catalyst prepared through the ionic exchange method (ZnHap troc) showed high selectivity to the ethanol dehydrogenation reaction.

Influence of the acid sites density in the acetone transformation over bifunctional Pt/H[Ga]ZSM5 catalysts

The purpose of this work is to elucidate the effect of acid sites density on the catalytic behaviour of a series of bifunctional catalysts prepared by supporting platinum (about 1.00 wt.%) on MFI-type gallosilicates (H[Ga]/ZSM5 with Si/Ga atomic ratio = 16, 46 and 145). The XPS characterization studies of the bifunctional catalysts allowed to detect the presence of framework as well as extraframework gallium species for the catalyst with highest density of acid sites (Si/Ga = 16), while only framework Ga species could be detected for catalysts with lower acid site density. It was also verified in all the bifunctional catalysts the presence of three types of metal sites: Pt (0), Pt–O ads and PtO species. The results obtained in the synthesis of MIBK from acetone with the different bifunctional solids, indicate that as the density of the acid sites increases, the solids start to behave solely as acid catalysts. This phenomenon is probably related to the presence of extraframework gallium species in the catalyst with higher surface acidity, capable to interact with the platinum species and thus provoking the passivation of the metallic phase, while in the solids with lower surface acidity it seems to exist a better balance between the acidic–metallic functionalities.

Structural, redox and acid–base properties of V 2O 5/CeO 2 catalysts

CeO 2 supported V 2O 5 catalysts were prepared by the wetness impregnation technique and their surface structures were characterized by O 2 chemisorption, X-ray diffraction (XRD) and Raman spectroscopy (LRS). The surface acidity and basicity were measured by using microcalorimetry and infrared spectroscopy (FTIR) for the adsorption of NH 3 and CO 2. Temperature programmed reduction (TPR) was employed for the redox properties. In particular, isopropanol probe reactions with and without the presence of O 2 were employed to provide the additional information about the surface acidity and redox properties of the catalysts. Variation of loading of V 2O 5 and calcination temperature brought about the changes of surface structures of dispersed vanadium species, and hence the surface acidic and redox properties. Structural characterizations indicated that V 2O 5 can be well dispersed on the surface of CeO 2. The monolayer dispersion capacity was found to be about 8 V/nm 2, corresponding to about 10% V 2O 5 by weight in a V 2O 5/CeO 2 sample with the surface area of 80 m 2/g. Vanadium species in the catalysts (673 K calcined) with loading lower than 10% were highly dispersed and exhibited strong surface acidity and redox ability, while higher loading resulted in the formation of significant amount of surface crystalline V 2O 5, which showed fairly strong surface acidity and significantly weakened redox ability. Calcination of a 10% V 2O 5/CeO 2 at 873 K resulted in the formation of mainly CeVO 4 on the surface, which showed low surface acidity and redox ability. The probe reaction seemed to suggest that the calcination at higher temperature might cause the decrease of surface acidity more than redox ability. Thus, the 10% V 2O 5/CeO 2 catalyst calcined at 873 K exhibited much higher selectivity to benzaldehyde as compared to other V 2O 5/CeO 2 catalysts studied in this work, although its activity for the conversion of toluene was relatively low.

Dichlorodifluoromethane Decomposition to CO2 with Simultaneous Halogen Fixation by Calcium Oxide Based Materials

The decomposition of CCl2F2 to CO2 and accompanying halogen fixation by a CaO based material was studied. To improve the low reactivity of CaO, a consequence of its low surface acidity, transition metal oxides were added. Impregnation of metal acetylacetonate followed by removal of the ligand under vacuum was found to be an effective method. This method resulted in the formation of carbonaceous species and the reduction of metal oxide to metal, both of which were thought to initiate the decomposition reaction. The reactivity of these materials (MOx(a)/CaO-vac) was found to be in the following order: M = Ni > Cu > V = Fe > Mn > Co > Ca. In particular, nickel supported on CaO was most effective for the decomposition of CCl2F2. During the preparation, nickel oxide was reduced to the metal phase. CCl2F2 was decomposed to CO2 with a small amount of CO, and halogens were fixed as CaFCl, without significant deactivation at 723 K.

Vanadium oxide supported on mesoporous SBA-15 as highly selective catalysts in the oxidative dehydrogenation of propane

The combination of different techniques (DR UV–vis, 51V NMR, UV-Raman, FTIR, and H 2-TPR) in the characterization of vanadia supported on mesoporous SBA-15 catalysts shows that the dispersity and the nature of the vanadium species depend strongly on the V loading. In dehydrated catalysts with V contents lower than 2.8 wt%, vanadium is mainly in a tetrahedral environment. Higher V contents in the catalyst lead to the formation of polymeric V 2O 5-like species. Textural, SEM/TEM, and XRD results indicate that the ordered hexagonal mesoporous structure with large pore diameters of the support is retained upon the vanadium incorporation, and therefore high surface areas were obtained on the final catalysts. Vanadium species anchored to the surface show structural properties similar to those on mesoporous V-MCM and conventional VSiO 2 catalysts, but a higher surface concentration of isolated or low polymeric VO x species could be achieved on the V-SBA samples. The number and nature of the acid sites also change with the vanadium loading. The superior performance of the present mesoporous SBA-15 catalysts in the oxidative dehydrogenation of propane has been attributed to a higher dispersion of V species achieved on the SBA-15 support with large pore diameters as well as the low surface acidity of the catalyst.

Highly Effective Oxidative Dehydrogenation of Propane Over Vanadia Supported on Mesoporous SBA-15 Silica

Vanadia-containing mesoporous SBA-15 catalysts were prepared and characterized for the oxidative dehydrogenation (ODH) of propane. It is demonstrated that the vanadia-supported SBA-15 catalysts exhibit a much higher catalytic activity than those reported in the literature obtained over vanadium-supported mesoporous MCM-41 catalysts in the ODH of propane. The high catalytic performance of the mesoporous SBA-15 catalysts is attributed to the particularly large pore diameters and low surface acidity.

Liquid phase microcalorimetry applied to evaluate surface polarity of MCM-41 type aluminosilicates

Three calorimetric methods have been used to evaluate the surface polarity of three MCM-41 materials characterised by different Si:Al ratios (referred to as SiAl xC14, where x is the moles of Si:Al ratio, and 14 the chain length of the surfactant template). In the liquid phase, the heats of adsorption of 1-butanol from n-heptane and the heats of immersion in water, n-heptane and formamide were compared with the corresponding heat values obtained for two reference solids, viz. silica XOB015 and alumina. Surface (internal) energy of all MCM-41 samples appeared to be smaller than that of any reference solid, the differences being more marked in the case of alumina. In the gas phase, the combination of ammonia gas adsorption and mirocalorimetry measurements was used to evaluate the number and strength of acidic surface sites. With the exception of SiAl8C14, which contains 46% of strongly acidic sites, SiC14 and SiAl32C14 have low surface acidity. The broad distribution of adsorption energy values indicates the energetic heterogeneity of the acidic sites. Incorporating Al into the silicate matrix caused the surface polarity of the materials to increase.

Number and Strength of Surface Acidic Sites on Porous Aluminosilicates of the MCM-41 Type Inferred from a Combined Microcalorimetric and Adsorption Study

A combined microcalorimetry and adsorption study has been used to characterize the surface acidity of two series of MCM-41 aluminosilicates (referred to as SiAlxCn, where x is the mole Si:Al ratio and n the chain length of the surfactant template). 29Si magic angle spinning NMR spectra of a selected sample (SiAl32C16) indicates the presence of siloxane groups, Si(OSi)4, and three types of silanol groups, that is, single (SiO)3−Si−OH, hydrogen-bonded (SiO)3−SiOH···HO−Si−(SiO)3, and geminal (SiO)2−Si(OH)2. It is also possible to detect the contributions from Si(3Si,1Al) and Si(2Si,2Al) sites. Generally, the width of NMR spectra is characteristic of amorphous materials and suggests a large variety of Si−O−Si and Si−O−Al bond angles and lengths. The 27Al MAS NMR study made on the hydrogen-exchanged SiAl32C14 sample (H+−SiAl32C14) and the one rich in aluminum (SiAl8C14) clearly points to the presence of extraframework six-coordinate Al in the calcined materials. The volumetric and calorimetric measurements of gas ammonia adsorption at 353 K were used to determine the number and strength of surface acidic sites. With the exception of H+−SiAl32C14 and SiAl8C14, all samples have low surface acidity. The density of acidic sites ranges between 0.04 and 0.09 μmol m-2, increasing a little with decreasing pore size. These sites are relatively homogeneous and weak. For H+−SiAl32C14, surface acidity is enhanced as a result of the formation of strongly acidic Brönsted-type sites and extraframework aluminum. The density of acidic sites increases to 0.11 μmol m-2 and the fraction of strongly acidic sites is about 60%. In the case of the SiAl8C14 sample, the density of acidic sites reaches a value of 0.3 μmol m-2. These sites are very heterogeneous: the corresponding differential enthalpy of ammonia adsorption decreases from −160 to −80 kJ mol-1. Following the pyridine-TPD study on this sample, Lewis acid sites producing surface pyridine complexes constitute the strongest acidic sites.

Introduction of acidic groups at the surface of activated carbon by microwave-induced oxygen plasma at low pressure

Functional groups at the surface of a commercial granulated activated carbon were modified at low temperature in an oxygen plasma. Changes in the chemistry of the surface were monitored by thermal desorption, titration of acidic groups, and XPS. A comparison of untreated and plasma-treated carbons, both before and after acid-washing and crushing, showed that reactions in oxygen plasma yield a low surface acidity with minimal modification of the initial sample porosity. Carboxylic groups, which are largely responsible for the surface acidity, appear to be unstable in the presence of the most reactive oxygen species of the plasma.

The determination of the surface energy of conducting polymers by inverse gas chromatography at infinite dilution

Inverse gas chromatography (IGC) has been used at infinite dilution to determine the dispersive and acid–base contributions to the surface energy of inherently conducting polymer (ICP) powders and nanocomposites, with an emphasis on polypyrrole bulk powder. New and previously published results are discussed and show that polypyrrole and polyaniline are high-surface energy materials by contrast to conventional polymers and undoped poly(3-octyl thiophene), thus reflecting a behaviour comparable to that of metals or graphite. In the case of polypyrrole, it was found that the dispersive and acid–base properties are correlated, and that the high energetic sites are predominantly acidic. Ageing and low doping levels have a significant impact on polypyrrole surface thermodynamics as they resulted in low surface energy and acidity of this ICP.

An FT-IR and flow reactor study of the selective catalytic oxy-dehydrogenation of C3 alcohols on Mn 3O 4

The C3 alcohols 1-propanol, 2-propanol and prop-2-en-1-ol (allyl alcohol) are catalytically oxidized in He/oxygen to the corresponding carbonyl compounds propanal, acetone and acrolein with high selectivities and yields over Mn 3O 4. The reaction occurs in the temperature range 400–550 K and yields far over 85% are obtained in the synthesis of acrolein and acetone. The reaction rate is faster for the secondary alcohol 2-propanol, than for the primary ones. The oxidation of the unsaturated allyl alcohol is faster than for the saturated one, 1-propanol. Selectivity is limited by overoxidation of the carbonyl compounds. IR studies show that the high yields obtained in carbonyl compounds production over this catalyst are mainly due to their very weak adsorption. The main factor limiting the selectivity to acetone and, mainly, propanal is their tendency to give enolate anions that further convert into acetaldehyde in both cases. Overoxidation of aldehydes to the corresponding car☐ylate species is a less efficient mechanism limiting selectivity. The selectivity/activity behavior is definitely different than that observed for stoichiometric alcohols oxidations with Mn(III) compounds and also than that observed on other catalysts. This difference is associated to the low surface acidity of the Mn 3O 4 catalyst.

Sorption‐Desorption of Atrazine and Simazine by Model Soil Colloidal Components

AbstractSorption‐desorption of atrazine and simazine on montmorillonite, ferrihydrite, and humic acid was determined using the batch equilibration procedure. Sorption of atrazine and simazine was higher on humic acid than on montmorillonite. Ferrihydrite does not sorb triazine herbicides. Enhanced sorption of atrazine and simazine on montmorillonite was measured after increasing the surface acidity of the clay. Results indicate sorption of s‐triazine herbicides on montmorillonite as protonated species must be preceded by sorption as molecular species on hydrophobic microsites of the clay unless the pH of the bulk solution is close to the pKa of the herbicide. In this latter case, cation exchange would be also operative. Sorption‐desorption of atrazine and simazine was more hysteretic for humic acid than for montmorillonite, indicating that these herbicides desorb more difficultly from organic matter than from montmorillonite because of the contribution of hydrophobic interactions with humic acid. It is suggested that, besides organic matter, smectites of low charge density and high surface acidity should increase the retention of s‐triazine herbicides in soils.

Activated Carbons: In Vitro Affinity for Aflatoxin B1 and Relation of Adsorption Ability to Physicochemical Parameters

Affinity in vitro tests were conducted of the efficacy of 17 activated carbons (ACs) in binding aflatoxin B1 from solution. Relationships between adsorption ability and physicochemical parameters of the ACs (surface area, iodine number, methylene blue index, and surface acidity) were tested. Using 5 ml of a 4 μg/ml aqueous solution of aflatoxin B1 and 2 mg of an AC, adsorption abilities ranged from 44.47% to 99.82%. Four ACs showed very high adsorption abilities, binding more than 99% of the available aflatoxin B1. In comparative testing five ACs showed a greater ability to bind aflatoxin B1 than hydrated sodium calcium aluminosilicate (HSCAS). Three ACs also showed high adsorption abilities (ca. 99%) at increasing aflatoxin B1 concentrations (50 and 250 μg/ml) whereas HSCAS adsorption ability greatly declined. With the exception of three ACs, aflatoxin B1 adsorption was significantly correlated with all the physicochemical parameters, confirming a close relationship between molecule trapping and the surface physicochemical adsorption process. The methylene blue index was more reliable than iodine number and surface area in predicting AC adsorptive ability. The results suggested that ACs with a high methylene blue index and low surface acidity have a very high in vitro affinity for aflatoxin B1; however, their efficacy in protecting against aflatoxicosis should be verified further by in vivo tests.

Studies on ferric oxide hydroxides

The effects of sulfate ions on the formation and physico-chemical properties of ferric oxide hydroxides were studied using various techniques. Samples prepared from 0.1 M Fe(NO3)3 solution without sulfate ions contain mixtures of goethite and hematite, whereas basic sulfates of varying composition were formed from solutions containing sulfate: iron (III) ratio greater than and equal to one. Goethite samples of very narrow particle size range, spherical shape, microporous nature, high surface area, high true density and low packing density and surface acidity, which are very necessary for good catalysts and ferrite making, were obtained at low sulfate: iron (III) ratios (0.25∶ 0.5).

Effect of concentration of surface acidic functional groups on electric double-layer properties of activated carbon fibers

The relationship between electric double-layer properties in an organic electrolytic solution and a concentration of surface acidic functional groups (surface acidity) of a phenolic resin-based activated carbon fiber (ACF) was characterized in a system of ACF/electrolytic solution/ACF The lower surface acidity of ACF resulted in the lower apparent leakage current of the electric double layer. The capacitance and the dc resistance of the layer were almost independent of the surface acidity. Further heat treatment of ACF in nitrogen atmosphere decreased the surface acidity of ACF. Types of C-O bonds in surface acidic functional groups were identified as carboxy and ester groups for ACF as-prepared and as phenolic hydroxy and quinone groups for ACF heat treated. The ACF obtained from the phenolic resin-based fibers showed an extremely low surface acidity in comparison with other kinds of ACF. The polarizable electrodes of the phenolic resin-based ACF resulted in electric double-layer capacitors with high capacitance and low leakage current.

Modified alumina supports for catalytic oxidation of benzene to maleic anhydride over vanadia

Aluminas, intermediate in the transformation of γ to α-phase, have been tested as supports for vanadia in the oxidation of benzene to maleic anhydride. Treatment of γ-alumina in air or steam results in an initial rapid loss of half the original surface area corresponding to closing off of small pores with no development of bulk α-phase. However, 27Al NMR measurements, and determinations of surface Lewis acidity using the dehydration of methanol to dimethyl ether as a test reaction, each show that conversion to the α arrangement is well advanced at the local and surface levels. Further reduction in area parallels formation of crystalline α-phase. Aluminas with surface areas of 20 to 40 m 2 g −1 contain 30–60% α-phase and have very low surface acidity. Catalyst comprising 9% V 2O 5 on such supports are able to convert benzene to maleic anhydride with selectivities of up to 83%, in contrast to zero selectivity for comparison catalysts using γ-alumina supports. The activity and selectivity patterns with the partially transformed aluminas as supports can be rationalised in terms of an optimal average number of V 2O 5 layers of three to four. With less than this, some support is exposed and selectivity and activity per unit area of catalyst is degraded. Thicker layers lead to lower than optimal activity per unit weight of V 2O 5 and some loss of the favourable effect of an α-alumina surface on the selectivity of overlying V 2O 5 layers.