In-situ-IR-, -NMR-, -EPR- und -UV/Vis-Spektroskopie: Wege zu neuen Erkenntnissen in der heterogenen Katalyse

gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-β Zeolite as Studied by Solid-State NMR Spectroscopy

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem-diol-type species in the activation of acetone on Sn-β zeolite. ¹³ C, ¹¹⁹ Sn, and ¹³ C-¹¹⁹ Sn double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)₃ Sn-OH) on Sn-β is responsible for the formation of the surface species. ¹³ C MAS NMR experiments together with density functional theory calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.

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.

In Situ and Operando Spectroscopy for Assessing Mechanisms of Gas Sensing

The mechanistic description of gas sensing on inorganic, organic, and polymeric materials is of great scientific and technological interest. The understanding of surface and bulk reactions responsible for gas-sensing effects will lead to increased selectivity and sensitivity in the chemical determination of gases and thus to the development of better sensors. In recent years, spectroscopic tools have been developed to follow the physicochemical processes taking place in an active sensing element in real time and under operating conditions. Thus, the monitoring of the processes in "living" gas sensors is no longer an unsolvable problem. This Review gives an overview of in situ and operando spectroscopic techniques for the study of gas-sensing mechanisms on solid-state sensors.

Mono- and Dinuclear Oxovanadium(V)calixarene Complexes and Their Activity as Oxidation Catalysts

The background of the investigation is constituted by reactive moieties and intermediates playing relevant roles on the surfaces of vanadiumoxide-based catalysts during the oxygenation/dehydrogenation of organic substrates. With the aim of modeling such species, a series of mono- and dinuclear charged and uncharged vanadium oxo complexes containing p-tert-butylated calix[4]arene and calix[8]arene ligands (denoted H4B and H8B' ', respectively, in the protonated forms) has been synthesized and characterized: PPh4[O=VB] ((PPh41), O=VB(OAc) (2), PPh4[O2V2HB' '] (3), and [mu-O(O=V(OMe))2B(Me2)] (4), where superscripts OAc and Me2 indicate that one or two protons of H4B are substituted by these residues, respectively. These compounds were analyzed both in solution and by means of single-crystal X-ray crystallography; it turned out that the crystal structures are retained on dissolution (2 changed only from the paco to the cone structure). In the case of 4, it could be shown that the bulk product consists of a mixture of two isomers (4t and 4c) differing in the relative positions of the vanadium-bound methoxy groups. Subsequently, all compounds were tested as catalysts for the oxidation of alcohols with O2. It turned out that the two dinuclear complexes efficiently catalyze the oxidation of 1-phenyl-1-propargyl alcohol and fluorenol; in addition, they even show some activity with respect to the oxidation of dihydroanthracene. This may hint to a higher activity of dinuclear sites on the surfaces of heterogeneous catalysts as well.

Formation and decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y investigated by in situ stopped-flow MAS NMR spectroscopy

Methylation of aniline by methanol on zeolite H-Y has been investigated by in situ (13)C MAS NMR spectroscopy under flow conditions. The in situ (13)C continuous-flow (CF) MAS NMR experiments were performed at reaction temperatures between 473 and 523 K, molar methanol-to-aniline ratios of 1:1 to 4:1, and modified residence times of (13)CH(3)OH between 20 and 100 (g x h)/mol. The methylation reaction was shown to start at 473 K. N,N,N-Trimethylanilinium cations causing a (13)C NMR signal at 58 ppm constitute the major product on the catalyst surface. Small amounts of protonated N-methylaniline ([PhNH(2)CH(3)](+)) and N,N-dimethylaniline ([PhNH(CH(3))(2)](+)) were also observed at ca. 39 and 48 ppm, respectively. After increase of the temperature to 523 K, the contents of N,N-dimethylanilinium cations and ring-alkylated reaction products strongly increased, accompanied by a decrease of the amount of N,N,N-trimethylanilinium cations. With application of the in situ stopped-flow (SF) MAS NMR technique, the decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y to N,N-dimethylanilinium and N-methylanilinium cations was investigated to gain a deeper insight into the reaction mechanism. The results obtained allow the proposal of a mechanism consisting of three steps: (i) the conversion of methanol to surface methoxy groups and dimethyl ether (DME); (ii) the alkylation of aniline with methanol, methoxy groups, or DME leading to an equilibrium mixture of N,N,N-trimethylanilinium, N,N-dimethylanilinium, and N-methylanilinium cations attached to the zeolite surface; (iii) the deprotonation of N,N-dimethylanilinium and N-methylanilinium cations causing the formation of N,N-dimethylaniline (NNDMA) and N-methylaniline (NMA) in the gas phase, respectively. The chemical equilibrium between the anilinium cations carrying different numbers of methyl groups is suggested to play a key role for the products distribution in the gas phase.