Yijiao Jiang

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.

Insight into the mechanisms of the ethylbenzene disproportionation: transition state shape selectivity on zeolites.

The direct experimental evidence shows that ethylbenzene disproportionation is a transition state shape selective reaction on zeolites: a bimolecular reaction mechanism via diphenylethane-mediated pathway on large-pore zeolites X and Y (ca. 0.74 nm) and a monomolecular reaction mechanism on medium-pore zeolites ZSM-5 (ca. 0.56 nm) via the ethoxy-mediated intermolecular ethyl group transfer. The lifetime of bulky diphenylethane species was prolonged by a fine-tune of FAU-zeolites, which makes this transition state detectable by 13C MAS NMR spectroscopy. Due to tunable catalytic properties and pore shapes, zeolites are promising catalysts toward emulating the efficiency and selectivity in desired reactions.

Increasing the Brønsted Acidity of Flame‐Derived Silica/Alumina up to Zeolitic Strength

Herein,weshow that SAs prepared by flame-spray pyrolysis (FSP SAs)exhibit a strong Bronsted acidity resembling that of zeolites.Some Bronsted acid sites in the FSP SAs were found to beeven stronger than those of H-ZSM-5, which is regarded asthe most acidic zeolite.Bronsted acidity of SAs is generated by having neighbor-ing aluminum and silanol groups. However, inhomogeneouscomposition of the SAs resulting from existing preparationmethods, such as cogelation, grafting, co-precipitation, andhydrolysis, hinder enhancement of their acidity. Control overpH values and high temperature calcinations are oftenrequired in these methods for the diffusion of aluminumand silicon throughout the phase or network.

Tuning Phase Composition of TiO2 by Sn4+ Doping for Efficient Photocatalytic Hydrogen Generation

The anatase-rutile mixed-phase photocatalysts have attracted extensive research interest because of the superior activity compared to their single phase counterparts. In this study, doping of Sn(4+) ions into the lattice of TiO2 facilitates the phase transformation from anatase to rutile at a lower temperature while maintaining the same crystal sizes compared to the conventional annealling approach. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. Characterization results reveal that the Sn(4+) ions entered into the lattice of TiO2 by substituting some of the Ti(4+) ions and distributed evenly in the matrix of TiO2. The substitution induced the distortion of the lattice structure, which realized the phase transformation from anatase to rutile at a lower temperature and the close-contact phase junctions were consequently formed between anatase and rutile, accounting for the efficient charge separations. The mixed-phase catalysts prepared by doping Sn(4+) ions into the TiO2 exhibit superior activity for photocatalytic hydrogen generation in the presence of Au nanoparticles, relatively to their counterparts prepared by the conventional annealling at higher temperatures. The band allignment between anatase and rutile phases is established based on the valence band X-ray photoelectron spectra and diffuse reflectance spectra to understand the spatial charge separation process at the heterojunction between the two phases. The study provides a new route for the synthesis of mixed-phase TiO2 catalysts for photocatalytic applications and advances the understanding on the enhanced photocatalytic properties of anatase-rutile mixtures.

Regioselective H/D Exchange at the Side‐Chain of Ethylbenzene on Dealuminated Zeolite H‐Y Studied by In Situ MAS NMR–UV/Vis Spectroscopy

A number of heterogeneously catalyzed reactions (e.g. cracking, isomerization, dehydrogenation and alkylation) of hydrocarbons are initiated or promoted on solid acid catalysts by activation of the C H bonds of the reactants. Studies of H/D exchange between the reactants and the Bronsted acid sites of solid catalysts at the early stages of acid-catalyzed reactions provide useful information concerning activation mechanisms and intermediates. These activation mechanisms have been the topic of a number of theoretical and experimental studies. 3] The main routes include pentavalent carbonium ions formed via protonation of alkanes on Bronsted acid sites and trivalent carbenium ions due to hydride abstraction by Lewis acid sites. Sommer et al. concluded that the H/D exchange via the first route requires much higher temperatures compared to the second one, and that it proceeds via direct proton transfer between the solid surface and the alkane. The second route via a carbenium ion may result in regioselective H/D exchange following Markovnikov’s rule. c, e] On sulfated zirconia, only the methyl group of propane exchanges hydrogen atoms at 323 K, but both methyl and methylene groups are involved in H/D exchange at higher temperaACHTUNGTRENNUNGtures. Stepanov and Freude et al. reported regioselective H/D exchange between methyl and methylene groups of propane on the zeolite H-ZSM-5, and showed that the H/D exchange rate of the methyl groups is much higher than that of the methylene groups. Bucko et al. suggested that an entropic effect is responsible for the regioselective H/D exchange of propane and isobutane on zeolite clusters. The probability of adsorption of propane via the methylene group is seventeen times lower than that of adsorption via a methyl group; this entropy contribution leads to a higher free-energy barrier for proton-exchange via the methylene group. Herein, H/D exchange at the side-chain of ethylbenzene adsorbed on dealuminated deH-Y zeolites is studied and preferred regioselective exchange at the methyl group (b-carbon) is found at low temperatures. Based on the recently introduced in situ MAS NMR–UV/Vis spectroscopy, which herein is combined with the injection of short pulses of partially deuterated reactant molecules onto the catalyst at the reaction temperature, a reaction mechanism involving both Lewis and Bronsted acid sites in the H/D exchange reaction is suggested. The combination of complementary spectroscopic techniques helps gaining deeper understanding of catalyzed reactions. As an important advantage, in situ pulsed-flow (PF) H MAS NMR–UV/Vis spectroscopy can probe routes of hydrogen transfer via the characteristic NMR signals of the reactants before and after the exchange. Simultaneously, the formation of cyclohexadienyl and arylcarbenium ions (Scheme 1) is studied via their UV/Vis bands. The application of the pulsed-flow technique allows the study of H/D exchange kinetics at elevated temperatures with a well-defined starting point.

Comparative studies on the catalytic activity and structure of a Cu-MOF and its precursor for alcoholysis of cyclohexene oxide

The wide use of metal–organic frameworks (MOFs) in heterogeneous catalysis has been limited due to the fact that the coordination sphere of the metal ions in most known MOF structures is completely blocked by the organic linkers. In this work, a comparative structural study of the precursor Cu(BF4)2·nH2O (active metal salt catalyst) and the derived Cu-MOF was carried out by using solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques combined with the study of the interaction of the materials with different reactant molecules by thermogravimetry coupled with mass spectrometry (TG-MS). We clarify how an ideal MOF catalyst can keep the same high reactivity as its metal precursor, and how the existing efficient metal salt/complex catalysts can be linked to analogously reactive MOF catalysts. By choosing proper precursors and organic linkers, MOFs can keep an unsaturated and flexible coordination sphere, which endows them with unique guest-induced reactivity and reactant shape selectivity. The labile coordination facilitates MOFs to perform as well as metal complexes in alcoholysis of cyclohexene oxide but affording good recyclability.

Confined Au-Pd Ensembles in Mesoporous TiO2 Spheres for the Photocatalytic Oxidation of Acetaldehyde

Titanium dioxide (TiO2) is the most widely used photocatalyst for environmental remediation and solar-fuel production, owing to its low cost, nontoxicity, and abundance. In TiO2based photocatalysis, the photogenerated electrons and holes separate and migrate to the surface to be involved in the surface redox reactions. However, a high recombination rate of the photogenerated charge carriers leads to low photocatalytic efficiency. Various noble metals have been introduced to trap photogenerated electrons across the interface and, thus, improve the charge separation within the TiO2 semiconductor. [2]

One‐Step Room‐Temperature Synthesis of [Al]MCM‐41 Materials for the Catalytic Conversion of Phenylglyoxal to Ethylmandelate

Mesoporous [Al]MCM‐41 materials with nSi/nAl ratios of 15 to 50 suitable for the direct catalytic conversion of phenylglyoxal to ethylmandelate have been successfully synthesized at room temperature within 1 h. The surface areas and pore sizes of the obtained [Al]MCM‐41 materials are in the ranges of 1005–1246 m2 g−1 and 3.44–3.99 nm, respectively, for the different nSi/nAl ratios. For all [Al]MCM‐41 catalysts, most of the Al species were tetrahedrally coordinated with Si in the next coordination sphere of atoms. 1H and 13C magic‐angle spinning NMR spectroscopic investigations indicated that the acid strength of the SiOH groups on these [Al]MCM‐41 catalysts and the density of these surface sites are enhanced with increasing Al content in the synthesis gels. These surface sites with enhanced acid strength were found to be catalytically active sites for phenylglyoxal conversion. The [Al]MCM‐41 material with nSi/nAl=15 showed the highest phenylglyoxal conversion (93.4 %) and selectivity to ethylmandelate (96.9 %), whereas the [Al]MCM‐41 material with nSi/nAl=50 reached the highest turnover frequency (TOF=99.3 h−1). This is a much better catalytic performance than that of a dealuminated zeolite Y (TOF=1.7 h−1) used as a reference catalyst, which is explained by lower reactant transport limitations in mesoporous materials than that in the microporous zeolite.

Brønsted acid sites based on penta-coordinated aluminum species

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (AlIV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-AlIV and -AlV show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of 1H and 27Al sites demonstrates that BAS-AlV co-exists with BAS-AlIV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.

Yolk-Shell-Structured Aluminum Phenylphosphonate Microspheres with Anionic Core and Cationic Shell

Spherical materials with yolk‐shell structure have great potential for a wide range of applications. The main advantage of the yolk‐shell geometry is the possibility of introducing different chemical or physical properties within a single particle. Here, a one‐step hydrothermal synthesis route for fabricating amphoteric yolk‐shell structured aluminum phenylphosphonate microspheres using urea as the precipitant is proposed. The resulting microspheres display 3D sphere‐in‐sphere architecture with anionic core and cationic shell. The controllable synthesis of aluminum phosphates with various morphologies is also demonstrated. The anionic core and cationic shell of the aluminum phenylphosphonate microspheres provide docking sites for selective adsorption of both cationic methylene blue and anionic binuclear cobalt phthalocyanine ammonium sulphonate. These new adsorbents can be used for simultaneous capture of both cations and anions from a solution, which make them very attractive for various applications such as environmental remediation of contaminated water.