Stephan Jaenicke

The influence of preparation conditions on the surface area of zirconia

Abstract The conditions for preparation of high surface area zirconia were studied. Samples were prepared by precipitation from aqueous solutions of zirconium chloride with ammonium hydroxide. The order of addition of the reactants was found to affect the surface area. Digestion of the hydrous zirconia is the key to high surface area zirconia without the necessity of adding other oxides or dopants. Both the temperature and the time of digestion are important parameters. Zirconia with surface area in excess of 220 m 2 /g after calcination at 500°C have been obtained. The materials maintained a surface area of >90 m 2 /g even after heat treatment at 900°C for 12 h. In addition, digestion led to the formation of the tetragonal allotrope of zirconia. Samples which had been digested for long times at 100°C are tetragonal and maintain this phase up to 1000°C. The effects of digestion seems to be related to a phase transformation of the hydrous precursor at around 80°C. A mechanism based on defect density is postulated to explain the phase stability.

Thermal and hydrothermal stability of framework-substituted MCM-41 mesoporous materials

Abstract The thermal and hydrothermal stability of framework-substituted MCM-41 with a Si/M molar ratio of 40 (M = Al, Ti) is assessed. Si-MCM-41 with 2.5% substitution of Si by either Al or Ti remains mesoporous when calcined in air at temperatures up to 800°C. The pore structure of Si-MCM-41 and Al-MCM-41 collapses at a calcination temperature of above 800°C, while pore collapse occurs at about 100°C higher for Ti-MCM-41. The addition of La2O3 to Al-MCM-41 improves its thermal stability at lower temperatures, but has no effect at higher temperatures. When MCM-41 materials are treated in pure distilled water at 25–100°C for 4 h, Si-MCM-41 suffers a significant loss of BET surface area, while Al-MCM-41 and Ti-MCM-41 are more stable towards such treatment. All the samples investigated are stable in an acidic solution of pH 2, but disintegrate rapidly in a strongly basic medium (pH 12).

A comparison of post-synthesis alumination and sol-gel synthesis of MCM-41 with high framework aluminum content

Abstract Aluminum-containing mesoporous MCM-41 materials were prepared by post-synthesis modification of a purely siliceous MCM-41 using different Al sources: AlCl3, aluminum isopropoxide and NaAlO2. The structure, thermal stability and acidity of these materials have been investigated and compared with Al-MCM-41 prepared by direct hydrothermal synthesis. Irrespective of the preparation method, the surface area, pore diameter, crystallinity and thermal stability of Al-MCM-41 decrease with increasing Al content. Post-synthesis modified materials possess better thermal stability, and this method allows for the incorporation of more aluminum without disintegration of the mesoporous structure as compared to Al-MCM-41 prepared by direct hydrothermal synthesis. The post-synthesized Al-MCM-41 has a moderate acidity, comparable to that of the direct hydrothermally-synthesized material.

Catalytic carbon monoxide oxidation over strontium, cerium and copper-substituted lanthanum manganates and cobaltates

Abstract The influence of either A or B-site substitution in perovskite-type mixed oxides on the catalytic oxidation of carbon monoxide has been studied. The following systems were investigated: (La,Sr) MnO 3 , La(Mn,Cu)O 3 , (La,Sr)CoO 3 and (La,Ce)CoO 3 . Cobaltates are generally more active than the manganates. Substitution in the A or B-site improved the catalytic activity with oxidation starting from 75 °C. A volcano plot of activity versus composition was obtained for each series with up to a 10-fold increase in catalytic activity for the substituted compounds. Lattice oxygen participates in the reaction even under stoichiometric conditions. The catalysts show a positive rate dependence on the carbon monoxide partial pressure so that under reducing conditions, the reaction is not inhibited. A bistability in the rate of catalytic oxidation at high carbon monoxide concentration was observed over La 1− x Sr x MnO 3 and LaMn 1− x Cu x O 3 (0⩽x⩽0.2). This bistability has been attributed to a carbon monoxide-driven reconstruction of the reduced surface, leading to pairs of Mn 2 ions with a Mn-Mn distance comparable to the spacing in the metal. These pairs provide reactive sites for carbon monoxide oxidation and oxygen chemisorption. Such metal-metal pairs are not found in the perovskite lattice but are a structural feature of the closely related hexagonal 4-layered packing which is the normal crystal structure of SrMnO 3 . The change back to the less active state is due to reoxidation of the surface. It was confirmed that a low mobility of lattice oxygen is a necessary condition for hysteresis in these oxides.

The preparation of high surface area zirconia — Influence of precipitating agent and digestion

Abstract High surface area zirconia was produced by digestion of the hydrous oxide at 100°C for various lengths of time. Precipitation of the hydrous zirconia was effected by potassium hydroxide and sodium hydroxide, the pH during precipitation being maintained at 14. The zirconia obtained after calcination of the undigested hydrous precursors at 500°C for 12 h had a surface area of ∼40–50 m 2 /g. With digestion, surface areas as high as 250 m 2 /g could be obtained. NaOH-digested samples were found to have higher surface areas than KOH-digested ones. An optimum digestion time of 12–24 h was found for both precipitants. The zirconia was thermally stable up to 800°C with a surface area of 170 m 2 /g. Longer-digested samples were more thermally stable than shorter-digested ones. The effect of digestion on the surface area was attributed to the aggregation of particles followed by strengthening of inter-particle contacts. Hence, the high surface area of the hydrous oxides was retained in the zirconia formed after calcination. The crystal phase of zirconia depends on digestion and the precipitating agent. Zirconia prepared from the undigested precursors comprised of both monoclinic and tetragonal phases. NaOH-digested samples crystallize only in the tetragonal phase while KOH-digested zirconia was a mixture of monoclinic and tetragonal phase (∼20 : 80). These observations were explained by the incorporation of sodium and potassium ions in the lattice. In all digested samples, the tetragonal phase was observed up to 1000°C; a sudden change to the monoclinic structure occurred above this temperature.

Organic–inorganic hybrid catalysts for acid- and base-catalyzed reactions

Abstract Stable heterogeneous catalysts with adjustable base strength have been prepared by grafting organic amine bases into the the pores of the inorganic mesoporous material, MCM-41, and to a porous styrene–divinylbenzene resin. The activities of these catalysts are compared for the formation of the monoglyceride from lauric acid and glycidol, and the Knoevenagel condensation of heptaldehyde with benzaldehyde to form α-n-amylcinnamaldehyde (jasminaldehyde). Also included in the comparison are catalysts prepared by incorporating K2O, BaO and K2O/La2O3 into MCM-41. The resin-based catalyst suffers from poor thermal and mechanical stability. The organic–inorganic hybrid material containing the strong hindered amine base, TBD (1,5,7-triazabicyclo[4,4,0]dec-5-ene), performs well for the monoglyceride reaction at 110°C, and the catalyst can be re-used for at least 11 cycles with little loss of activity. However, it deactivates if it is used in the coupling reaction with aldehydes. This is obviously caused by loss of the base at the higher reaction temperature of 170°C, and by poisoning of the strong basic sites with benzoic acid which is formed by oxidation or through the Cannizzaro disproportionation of benzaldehyde. The more weakly basic catalysts based on MCM-41 with K2O/La2O3 can be used at a higher reaction temperature to compensate for their lower intrinsic activity, and their activity can be restored by calcination.

Supported zirconium propoxide—a versatile heterogeneous catalyst for the Meerwein–Ponndorf–Verley reduction

Grafting of zirconium 1-propoxide on SBA-15 resulted in highly active catalysts for the MPV reduction. The activity increased with zirconium loading up to a monolayer coverage. In most cases, there were no side products other than the desired alcohol. Electron-donating groups adjacent to the carbonyl group in the substrate facilitate the reaction. The grafted zirconium catalysts did not lose their activity in the presence of moisture or on exposure to ambient atmosphere, making them easy to handle and reuse. No leaching of the grafted zirconium 1-propoxide into the reaction mixture was observed. The addition of pyridine and water to the reaction medium had only a small effect on its activity while benzoic acid led to severe deactivation. The deactivation is attributed to strong adsorption of benzoic acid at the Zr metal centres which could be reversed on removal of the poison. Aluminum 2-propoxide grafted on SBA-15 resulted in a less active catalyst than the zirconium catalysts. The good resistance to hydrolysis of the zirconium catalysts makes them superior to the aluminum 2-propoxide catalysts.

High surface area zirconia by digestion of zirconium propoxide at different pH

The microstructure of hydrous zirconia (ZrO2) prepared by the hydrolysis of zirconium propoxide followed by digestion at pH 1, 3 and 9 and the ZrO2 derived from this precursor have been studied. The undigested hydrous oxides formed plate-like aggregates with low porosity. After digestion at pH 9, the hydrous oxides became highly porous with surface areas up to 550 m2/g. Calcination at 500°C for 12 h results in ZrO2 with surface areas of more than 380 m2/g. ZrO2 formed after digestion in acidic medium had lower surface areas of less than 100 m2/g and were mixed in tetragonal and monoclinic phases. The water/propoxide ratio during gel formation influences the surface area of the resulting ZrO2. The thermal stability of the material is remarkable: ZrO2 resulting from long digestion retained up to 100 m2/g after successive heating to 900°C. The observed microstructural properties are attributed to the nature of the alkoxide precursor formed under different water/alkoxide ratios and to the effects of digestion under varying pH conditions.

Zirconium–Beta zeolite as a robust catalyst for the transformation of levulinic acid to γ-valerolactone via Meerwein–Ponndorf–Verley reduction

Zr–Beta zeolite is a robust and active catalyst for the Meerwein–Ponndorf–Verley reduction of levulinic acid to γ-valerolactone, a versatile intermediate for bio-fuels and chemicals. In a batch reactor, γ-valerolactone was formed with a selectivity of >96%. In a continuous flow reactor, >99% yield of γ-valerolactone was obtained with a steady space-time-yield of 0.46 molGVLgZr−1 h−1 within 87 h, on a par with that of noble metal based catalysts. The high activity of this catalyst was attributed to the presence of Lewis acidic sites with moderate strength. Due to the relatively few basic sites, it is not poisoned by the acidic reactant. Its robustness in liquid and gas phase reactants coupled with good thermal stability makes Zr–Beta a green regenerable catalyst that can be used directly on levulinic acid without the need for derivatization.

Propylene epoxidation with hydrogen peroxide catalyzed by molecular sieves containing framework titanium

Abstract The catalytic epoxidation of propylene with aqueous hydrogen peroxide over four Ti-containing silicates, namely titanium silicalite (TS-1), a TiO2–SiO2 xerogel, Ti-MCM-41, and a TiCl4-modified HZSM-5 zeolite has been investigated. It was found that only the crystalline molecular sieve TS-1 and the TiCl4 modified HZSM-5 zeolite were active, whereas the TiO2–SiO2 xerogel and the mesoporous Ti-MCM-41 were almost inactive. The structure of these Ti-containing silica materials plays an important role in determining their catalytic performances. Shape-selectivity and the hydrophobic nature of the material are the two most important factors. In the presence of TS-1, propylene oxide was the predominant product, whereas propylene diol and its mono-methyl ethers were formed with a TiCl4 modified HZSM-5. It is suggested that the acidity of catalyst is a crucial factor in determining product selectivity. Propylene diol and the mono-methyl ethers are produced by further reactions of propylene oxide with H2O and methanol. The oxirane ring opening reactions are catalyzed on acid sites. The H2O2 turnover frequency based on the titanium content increased while the selectivity to propylene oxide decreased with an increase of the Si/Ti ratio in TS-1 catalyst. The influence of CH3OH/H2O ratio in the solvent as well as the reaction temperature on the title reaction and the regeneration of deactivated catalyst have also been investigated.