Aluminas modifier effect

The effects of post-synthesis alumination on purely siliceous MCM-41 material with A1(NC>3)3 on acidity have been studied by FTIR, NH3-TPD, and IPA decomposition reaction. The FTIR results of pyridine absorption show that both Lewis and Bronsted acid sites are increased by the post-modification. The amount of NH3 adsorbed on the alumina-modified MCM-41 samples increases with the loading of Al onto the surface of MCM-41. Due to the improved acidity, the alumina-modified MCM-41 materials show considerably higher catalytic activity for dehydration of isopropanol than purely siliceous MCM-41. In addition, XRD and N2 adsorption results show that all MCM-41 samples maintained their uniform hexagonal mesoporous structure well after they have been subjected to post-synthesis alumination with the loading of Al species on Si-MCM-41 varied from 0.1 wt. % up to 10 wt. % (calculated based on AI2O3). 

Compensation effects have also been reported for the dehydration of alcohols on alumina (280) and on alumina modified with 10% potassium chloride (281). Values of A referred to unit area of active surface were not included in these reports, but for data in the latter article (281) we calculate e = 0.0836 and at = 0.0075. 

The same qualitative conclusions can be drawn from the analysis of curves in Fig.2b. At free pH (pH 6.5 before adsorption), alumina sample represents a moderately-charged hydrophilic substrate because the pH value is close to the pzc. The maximum quantity of adsorption corresponds to the area of 0.52 nm per one adsorbed molecule (cf. 0.35 nm at the water-air interface). For pH 3 surfactant ions achieve a close-packed arrangement in the adsorbed bilayer and the density of bilayer adsorption at the plateau (0.11 nm /molecule) is even less than the air-water interfacial density. At the same time, the cmc is markedly diminished by a decrease in the pH. Both effects can be attributed to the appearance of a non-ionized surfactant species in a solution. The neutral form of the surfactant is less soluble in water and thus exhibits a greater affinity for a hydrophobic surface of alumina modified with grafted aliphatic chains. The decresed repulsion between uncharged heads causes a closer packing of the adsorbate in a mixed surface structure. 

Table 1 presents the properties of aluminas modified by various additives. It is seen that lanthanum is the most effective additive considering stabilisation of phase composition and... 

Alumina supported on silica undergoes unforeseen modifying effect during catalytic titration with organic acid, base and electron acceptor. The observed phenomenon seems to be interesting from the practical point of view, however it would be an essential limitation for the catalytic titration method using the mentioned poisons. 

Alumina has found wide application in GSC. It is a highly polar material with a typical surface area of 250 m g . It interacts strongly with polar molecules such as water, and also has some catalytic activity, for example, it may convert acetone to diacetone alcohol or dehydration may occur. Much of the early work on alumina was carried out by Scott who subsequently concentrated his research on surface modified materials [37,38]. He measured polarity in terms of the retention of ethylene relative to non-polar ethane and propane. Activation by heating at temperatures up to 500°C increased the polarity by loss of water and adsorption of water reduced polarity until a minimum was reached when the amount required for a monolayer had been adsorbed. Further water continued to reduce the activity (by reducing the surface area) but increased the polarity [39-41]. Scott later extended his work on alumina to substances modified with sodium hydroxide and obtained results similar to those obtained for the water-modified alumina without the need to presaturate the carrier gas. Retention of benzene relative to heptane increased markedly on alumina modified with sodium salts in the order OH > Cl > Br > I . A recent comparative study of the modifier effects alkali... 

When supported on y-alumina (modified with 30% w/w K as KOH) this [Ru3Ni] catalyst had activity in ammonia synthesisj It was again more effective than the corresponding [OssNi] catalyst (prepared from the analogous [Os3Ni(/<3-H)Cp(CO)9]) but the monometallic ruthenium catalyst derived from [Ru3(CO)i2] was the most active. ... 

The reflection of y-alumina and PdO ciystals can be distinguished on the samples calcined at 500 °C. When the calcination temperature is increased fi om 500 "C to 1000 "C, alumina undergoes changes of crystal phase as reported in previous studies [17], This modification of the crystal structure is usually accompanied by a decrease of the surface area. In the case of Ba- and La-modified material, this modification of the crystal structure is noticeably diminished. On the samples calcined at 1000 C (right part of the figure), the increase in crystalline character affects the palladium more than the alumina support. The presence of La and Ba within the alumina structure effectively avoids these transformations. In contrast, the samples supported on pure alumina suffer from the formation of various phases, mostly 0-AI2O3. However, it is possible to observe traces of a-ALOs phases. 

Pt-alumina modified with DHVin proved to be less effective than that modified with Cnd for the hydrogenation of EtPy ee reached only 30% (Tungler, Sheldon... 

The repeated use of Pt-alumina modified by dihydrocinchonidine was studied by Balazsik and Bartok for the enantioselective hydrogenation of ethyl pyruvate under mild experimental conditions in toluene and AcOH. In toluene, depending on the reaction conditions, an increase of ee by 10-20% was observed on the reused catalysts. The same effect, however, was not found in AcOH. The phenomenon was attributed to an intrinsic feature of the Pt-alumina-cinchona system, in which the restructuring of the Pt surface may play an important role. 

Wachowski et al. [13] studied Co-Mo sulphides supported on alumina-modified with La203 for the WGS reaction. Results reveal that only higher concentrations of La203 increase the WGS activity. After that they studied the effect of support (alumina, titania, zirconia) on the WGS activity of Mo and Mo-Ni catalysts. Mo-impregnated Ti02 catalyst shows much better activity compared to alumina and zirconia supported catalysts. The addition of Ni to... 

Varghese OK, Gong D, Paulose M, Ong KG, Grimes CA, Dickey EC (2002) Highly ordered nanoporous alumina films effect of pore size and uniformity on sensing performance. J Mater Res 17(5) 1162-1171 Vijayamohanan K, Mulla IS, Pradhan SD (1996) Humidity-sensing behaviour of surface-modified zirconia. Sens Actuators A 57 217-221... 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

In this work, various Ru-BINAP catalysts immobilized on the phosphotungstic acid(PTA) modified alumina were prepared and the effects of the reaction variables (temperature, H2 pressure, solvent and content of triethylamine) on the catalytic performance of the prepared catalysts were investigated in the asymmetric hydrogenation of dimethyl itaconate (DMIT). 

Supported Co, Ni, Ru, Rh, Pd and Pt as well as Raney Ni and Co catalysts were used for the hydrogenation of dodecanenitrile to amines in stirred SS autoclaves both in cyclohexane and without a solvent. The reaction temperature and the hydrogen pressure were varied between 90-140 °C and 10-80 bar, respectively. Over Ni catalysts NH3 and/or a base modifier suppressed the formation of secondary amine. High selectivity (93-98 %) to primary amine was obtained on Raney nickel, Ni/Al203 and Ru/A1203 catalysts at complete nitrile conversion. With respect to the effect of metal supported on alumina the selectivity of dodecylamine decreased in the order Co Ni Ru>Rh>Pd>Pt. The difference between Group VIII metals in selectivity can be explained by the electronic properties of d-band of metals. High selectivity to primary amine was achieved on base modified Raney Ni even in the absence of NH3. 

Since the initial work of Onto et al. (1) a considerable amount of work has been performed to improve our understanding of the enantioselective hydrogenation of activated ketones over cinchona-modified Pt/Al203 (2, 3). Moderate to low dispersed Pt on alumina catalysts have been described as the catalysts of choice and pre-reducing them in hydrogen at 300-400°C typically improves their performance (3, 4). Recent studies have questioned the need for moderate to low dispersed Pt, since colloidal catalysts with Pt crystal sizes of <2 nm have also been found to be effective (3). A key role is ascribed to the effects of the catalyst support structure and the presence of reducible residues on the catalytic surface. Support structures that avoid mass transfer limitations and the removal of reducible residues obviously improve the catalyst performance. This work shows that creating a catalyst on an open porous support without a large concentration of reducible residues on the Pt surface not only leads to enhanced activity and ee, but also reduces the need for the pretreatment step. One factor... 

The true role of incorporation of anions in the formation of anodic alumina is being intensively discussed. Baker and Pearson183 have considered the anion effect in modifying the structure of anodic oxides to be due to the coordinative ability of anions to replace alumina tetrahedra in the body of the oxides. Dorsey184,185 has postulated that in porous oxides, anions stabilize the network of alumina tetrahedra and octahedra. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Du 1986). This reflects the importance of smaU pores in order to apply effectively capillary condensation as a separation mechanism. Uhlhom (1990) demonstrated the effect of multilayer diffusion of propylene through a modified y-alumina membrane at 0°C. The separation factor for the N2/CjHg mixture was 27, where propylene is the preferentially permeating component, while the permeability increased to 7 times the Knudsen diffusion permeability. Although this mechanism appears to be very effective because of a high separation factor and a high permeability, it is limited by the obvious need for a condensable component. This in turn restricts the applicability range, due to limits set by temperature and pressure, needed for formation of multilayers or capillary condensation. 

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