Michael Hunger

Base catalysis on microporous and mesoporous materials: recent progress and perspectives

During the past decade, new classes of porous solid bases such as organic bases grafted onto the walls of ordered mesoporous materials and nitrided zeolites have been studied as catalysts in a wide variety of base-catalyzed reactions. Important examples are the mercaptane oxidation in petroleum refining, well known as sweetening of gasoline and kerosene, carbon–carbon bond forming reactions in fine chemistry such as aldol additions and condensations, and (trans-)esterification reactions interesting for food, pharmaceutical and cosmetic industries. The present work gives a survey on these new solid bases, their applications in heterogeneous catalysis and on new spectroscopic techniques allowing a more detailed investigation of base sites and adsorbates on working catalysts.

Br⊘nsted Acid Sites in Zeolites Characterized by Multinuclear Solid-State NMR Spectroscopy

Abstract Nearly all atoms contributing to the local structure of Br⊘nsted acid sites in zeolites exhibit isotopes accessible for multinuclear solid-state nuclear magnetic resonance (NMR) investigations. Therefore, in the last 15 years, NMR spectroscopy has found numerous applications for the determination of the types of hydroxyl proton in zeolites, of their concentration, accessibility, and mobility, and for the characterization of their acid strength and local structure. It allows the study of the role of hydroxyl groups in the formation of adsorbate complexes and in heterogeneously catalyzed reactions. Meanwhile, NMR spectroscopy belongs to the most powerful techniques for the characterization of Br⊘nsted acid sites in zeolites and related materials. The basis of this success is the invention of new sample preparation techniques, external magnetic fields with high-flux densities, effective line-narrowing methods, and new two-dimensional experiments, making the detection of highly resolved solid-state N...

Nuclear magnetic resonance studies on the acidity of zeolites and related catalysts

A review is given on the possibilities of the various nuclear magnetic resonance techniques which have been applied up till now to study quantitatively the acidity of zeolites and related catalysts. From the intensity of the wide line proton magnetic resonance signal of unloaded catalysts, the total concentration of protons can be determined. A loading of the samples with (deuterated) pyridine reduces drastically the mean residence time of the acidic protons at the oxygen atoms of the surface through formation and thermally activated motion of pyridinium ions. From the proton magnetic relaxation of the pyridine molecules and pyridinium ions a microdynamical model for the pyridine loaded H-Y zeolites could be derived. The rate constant for the decomposition of the pyridinium-ion-surface-comp)ex which can be determined in this way is used as a measure for the strength of acidity of the OH groups involved. Through magic angle spinning of thermally activated samples (contained in sealed glass ampoules to prevent adsorption of water) it became possible for the first time to determine quantitatively the concentration of non-acidic OH groups, of OH groups having different strength of acidity, and of residual ammonium ions. The results are related to the catalytic activity (cumene cracking) of the amorphous silica-aluminas, Hmordenites and zeolites H-Y studied in the present paper. In contrast to Br~nsted acidity, the current application of n.m.r, techniques to study Lewis acidity is more complicated because experimental difficulties, mainly due to fast exchange of adsorbed bases between different kinds of adsorption sites, arise.

Multinuclear solid-state NMR studies of acidic and non-acidic hydroxyl protons in zeolites

Among the variety of experimental techniques used for the characterization of aluminosilicate and aluminophosphate-based molecular sieves, NMR spectroscopy has the advantage of providing detailed information on local bonding and solid-state interactions. In the last 15 years, NMR has been used to study zeolitic frameworks, catalytic active centres, adsorbed probe molecules and their chemical interactions. This paper reviews multinuclear solid-state NMR studies of Brønsted acid sites in zeolites. The main topics are the formation, accessibility and localization of hydroxyl protons, dehydroxylation of zeolites, the interaction of hydroxyl protons with probe molecules, and the geometry and parameters of the local structure of OH groups in these materials.

Reactivity of Surface Alkoxy Species on Acidic Zeolite Catalysts

[Reaction: see text]. A solid understanding of the mechanisms involved in heterogeneously catalyzed reactions is of fundamental interest for modern chemistry. This information can help to refine modern theories of catalysis and, in a very practical way, can help researchers to optimize existing industrial processes and develop new ones. To understand the mechanisms of heterogeneous catalysis, we need to observe and identify reaction intermediates on a working catalyst. Motivated by this goal, we have monitored the catalytic events in heterogeneous systems using in situ magic-angle-spinning (MAS) NMR under flow conditions. In this Account, we describe the reactivity and possible intermediate role of surface alkoxy species in a variety of zeolite-catalyzed reactions. First, we isolate the surface alkoxy species on a working zeolite catalyst and then investigate the chemical reactivity with different probe molecules under reaction conditions. Finally, we investigate reaction mechanisms facilitated by these intermediate surface alkoxy species. We examined the reactivity of surface methoxy species (SMS) in terms of C-O bond and C-H bond activation. SMS on acidic zeolite catalysts act as an effective methylating agent when reacted with different probe molecules (including methanol, water, ammonia, alkyl halides, hydrochlorides, aromatics, carbon monoxide, and acetonitrile) through C-O bond activation. At higher reaction temperatures (ca. 523 K and above), the C-H bond activation of SMS may occur. Under these conditions, intermediates such as surface-stabilized carbenes or ylides are probably formed. This C-H bond activation is directly related to the initiation mechanism of the methanol-to-olefin (MTO) process and invites further investigation. Based on our experimental results, we also discuss the reactivity and the carbenium-ion-like nature of surface alkoxy species and recent theoretical investigations in this area.

1H MAS NMR studies on the acidity of zeolites

Proton magic-angle-spinning nuclear magnetic resonance (‘H MAS NMR) spectra contain quantitative information about Bransted acidity and structure defects in z&&es. The strength of acidity of bridging OH groups increases with the Si/Al ratio from 1.4 to 7 but remains constant above Si/Al ~10. Two signals of acidic hydroxyl protons observed in zeolites HY are correlated with the so-called high- and low-frequency band in infrared spectroscopy.

Magic-angle spinning nuclear magnetic resonance studies of water molecules adsorbed on Brønsted- and Lewis-acid sites in zeolites and amorphous silica–aluminas

On weakly rehydrated dealuminated zeolites and amorphous silica–aluminas the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (1H MAS NMR) measurements. A 1H NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the 1H NMR line of Bronsted-acid sites (bridging OH groups) to lower field for hydrated samples can be interpreted quantitatively by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.

Multinuclear MAS n.m.r, and i.r. spectroscopic study of silicon incorporation into SAPO-5, SAPO-31, and SAPO-34 molecular sieves

27Al, 31P, 29Si, and 1H MAS n.m.r. as well as diffuse reflectance i.r. spectroscopy were used to investigate the isomorphous substitution of silicon into the framework of aluminophosphate molecular sieves with different pore sizes. 29Si MAS n.m.r. was found to be the only direct method to study this type of silicon incorporation. In all SAPOs investigated, the incorporation of silicon generates two different kinds of Bronsted acidic sites irrespective of the number of nonequivalent T sites in the framework. The results of both the i.r. spectroscopy and the 1H MAS n.m.r. spectroscopy in the presence of sorbed molecules corroborate the commonly accepted assignment of these two different species to undisturbed hydroxyls and hydroxyls interacting with further framework oxygen, respectively.

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

Further progress in the field of heterogeneous catalysis depends on our knowledge of the nature and behavior of surface sites on solid catalysts and of the mechanisms of chemical reactions catalyzed by these materials. In the past decades, solid-state NMR spectroscopy has been developed to an important tool for routine characterization of solid catalysts. The present work gives a review on experimental approaches and applications of solid-state NMR spectroscopy for investigating Brønsted and Lewis sites on solid acids. Studies focusing on the generation of surface sites via post-synthesis modification routes of microporous and mesoporous materials support the development of new and the improvement of existing catalyst systems. High-temperature and flow techniques of in situ solid-state NMR spectroscopy allow a deeper insight into the mechanisms of heterogeneously catalyzed reactions and open the way for studying the activity of acidic surface sites. They help to clarify the activation of reactants on Brønsted and Lewis acid sites and improve our understanding of mechanisms affecting the selectivity of acid-catalyzed reactions.

Thermal stability and dehydroxylation of Brønsted acid sites in silicoaluminophosphates H-SAPO-11, H-SAPO-18, H-SAPO-31, and H-SAPO-34 investigated by multi-nuclear solid-state NMR spectroscopy

The thermal stability and dehydroxylation of the microporous silicoaluminophosphates H-SAPO-11, H-SAPO-18, H-SAPO-31, and H-SAPO-34 were investigated after thermal treatment at temperatures of 773–1173 K. While no change of the crystallinity upon thermal treatment at 1173 K was found by X-ray diffraction, 1H MAS NMR spectroscopy indicated a dehydroxylation of 40–50% of the initially existing bridging OH groups. As shown by 27Al and 29Si MAS NMR spectroscopy, this dehydroxylation is not accompanied by a dealumination but rather by a removal of silicon (desilication) in the local structures of the former bridging OH groups. The dehydroxylation and desilication of the silicoaluminophosphate framework do not lead to the formation of defect OH groups as evidenced by 1H MAS NMR spectroscopy. To explain the high stability of the silicoaluminophosphates after dehydroxylation and desilication, a healing process is proposed which is based on a migration of phosphorus atoms to framework vacancies and their transformation to P(OAl)4 species.