Roger Gläser

Characterization of the acid sites in MCM-41-type materials by spectroscopic and catalytic techniques

Temperature-programmed desorption of pyridine, FTIR and multinuclear NMR spectroscopy and catalytic test reaction have been applied to investigate acid sites in mesoporous materials with silicon/aluminum ratios of 266 and 20 and in an amorphous aluminosilicate with n Si /n Al =5.4. The results obtained by these methods indicate the presence of weak Bronsted acid sites in the aluminosilicate samples. Based on the spectroscopic data, a model of Bronsted acid sites in the aluminum-containing MCM-41 is proposed which consists of silanol groups with 3‐4 framework aluminum atoms in their local structure. In the dehydrated ammonium form of the mesoporous aluminosilicate, these framework aluminum atoms have a tetrahedral oxygen coordination. Deammoniation leads to a strong broadening of the 27Al echo NMR signal due to framework aluminum atoms which indicates a variation of their local geometry and/or oxygen coordination. 27Al echo NMR studies of calcined and subsequently reammoniated samples indicate that this variation in the bonding state of framework aluminum atoms is a reversible process.

The catalytic opportunities of near-critical water: a benign medium for conventionally acid and base catalyzed condensations for organic synthesis

Near-critical water (NCW; 250–350 °C, 40–90 bar) provides an environmentally benign alternative medium for conducting organic synthesis. The Claisen–Schmidt condensation of benzaldehyde with 2-butanone was investigated and is used to demonstrate the ability to conduct conventionally acid or base catalyzed reactions homogeneously using NCW without the addition of a catalyst. Kinetic investigations of the Claisen–Schmidt condensation yielded activation energies of 24.7 and 22.6 kJ mol−1 for the formation of products consistent with those formed under classical acidic and basic conditions, respectively. Investigations of other conventionally base catalyzed condensations were performed in uncatalyzed NCW for completeness, including the self-condensation reaction of butyraldehyde, the benzaldehyde–acetone cross-aldol condensation, the intramolecular Claisen condensations of ethyl-4-acetylbutyrate and ethyl levulinate, as well as the intramolecular Dieckmann condensation of diethyl adipate.

Influence of the aluminum content on the adsorptive properties of MCM-41

Abstract Experimentally determined adsorption isotherms of nitrogen, water and various aromatics and alcohols show that the sorption capacity of MCM-41 is only slightly affected by the aluminum content in the structure. Capillary condensation in the intracrystalline volume occurs independent of the nature of the adsorbed species. The experimental equilibrium data can be described well by a simplified model which distinguishes between the sorption in the inter- and the intraparticle voids. Furthermore, it is shown that the affinity between the sorbed species and MCM-41 increases with the molar polarizability a.

MCM-41-type molecular sieves as carriers for metal-phthalocyanine complexes

MCM-41 is used for the first time as inorganic mesoporous host for the immobilization of copper- and iron-phthalocyanine complexes. The results of an indepth characterization by physico-chemical methods suggest that most of the macrocycles are indeed located inside the mesopores and not at the external surface of the MCM-41 particles. Whereas immobilized CuPc is inactive for the oxidation of n-hexane at 20°C, using tertiary-butylhydroperoxide as oxidant, FePc in MCM-41 is considerably more active than the free FePc complex. However, fast and irreversible deactivation of the catalysts occurs which is tentatively attributed to an oxidative degradation of the free and the immobilized FePc complex.

Acid/base-catalyzed ester hydrolysis in near-critical water

Hydrolyses of substituted benzoic acid esters in near-critical water (250–300 °C) show autocatalytic kinetic behavior and surprisingly give the same rate constant regardless of substituent, suggesting that an acid-catalyzed mechanism predominates under our reaction conditions.

Selective Oxidation of 1- and 2-Propanol with Molecular Oxygen by Noble Metal Catalysis in “Supercritical” Carbon Dioxide

The selective oxidation of 1- and 2-propanol by molecular oxygen over supported platinum catalysts was investigated in “supercritical” carbon dioxide as an environmentally benign and safe reaction medium. The reaction occurs exclusively to acetone or propionic aldehyde and propionic acid in a single-phase region at 100–190 bar and at a mild temperature (40 °C). Compared to conversions in aqueous solution, catalyst stability is significantly enhanced in “supercritical” carbon dioxide and depends on the oxygen concentration in the reaction medium. Thus, at least a fourfold higher substrate/catalyst ratio than with water as a solvent can be used. Platinum catalysts with nanoporous silica (MCM-41, silicalite-1) as a support are also active for the oxidation of 2-propanol in “supercritical” carbon dioxide.

Probing the hydrophobic properties of MCM-41-type materials by the hydrophobicity index

The surface properties of MCM-41-type materials with different n Si /n Al -ratios and at temperatures from 30 °C to 120 °C were characterized by the Hydrophobicity Index (HI). As expected, HI increases with decreasing aluminum content of the molecular sieve. It is demonstrated that MCM-41-type materials are truly hydrophobic and possess high adsorption capacities for organic compounds. A model is developed which describes the binary adsorption equilibrium in terms of a superposition of sorption and capillary condensation.

Acylation of activated aromatics without added acid catalyst

Phenol and resorcinol can be acetylated to the corresponding esters and ketones in aqueous and neat acetic acid at high temperature (250–300 °C) to substantial equilibrium conversion without any added acid catalyst.

Metal-TUD-1 Catalyzed Aerobic Oxidation of Cyclohexane: A Comparative Study

The relative performance of various metal-incorporated mesoporous silicas of the TUD-1 structure type (M-TUD-1, where M = Ti, Cr, Co, Fe, Cu, Mn) in the selective aerobic oxidation of cyclohexane with tert-butylhydroperoxide and cyclohexylhydroperoxide (CHHP) to the respective hydroperoxides and subsequent decomposition to the alcohol and ketone is reported. In particular, relationships regarding metal type and loading, silica structure type and peroxide initiator are elucidated, to show that it is possible to tune catalysts for either CHHP formation or CHHP decomposition.

The Application of Zeolites in Catalysis

In this chapter, the most important applications of zeolite catalysts in the refining and petrochemical industries are highlighted. In an introductory section, the key features of zeolites and the methods for tailoring their properties related to their application as catalysts are briefly discussed. A short overview of the preparation of synthetic zeolites is also included. One major section deals with zeolite applications in petroleum refining. Processes such as catalytic cracking, isomerisation of light gasoline and light alkenes, isobutane alkylation and conversion of methanol to gasoline or olefins are addressed. The versatility of zeolite catalysts is reflected in the section on applications in petrochemistry. Here, the manufacture of alkylaromatics as well as oxidation, ammoxidation and amination reactions are treated.