Search

Copyright of JavaSPEKTRUM is the property of SIGS DATACOM GmbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The generation of multifunctional isolated active sites in zeolite supports is an attractive method for integrating multistep sequential reactions into a single-pass tandem catalytic reaction. In this study, bifunctional TiSn-Beta zeolite was prepared by a simple and scalable post-synthesis approach, and it was utilized as an efficient heterogeneous catalyst for the tandem conversion of alkenes to 1,2-diols. The isolated Ti and Sn Lewis acid sites within the TiSn-Beta zeolite can efficiently integrate alkene epoxidation and epoxide hydration in tandem in a zeolite microreactor to achieve one-step conversion of alkenes to 1,2-diols with a high selectivity of >90%. Zeolite confinement effects result in high tandem rates of alkene epoxidation and epoxide hydration as well as high selectivity toward the desired product. Further, the novel method demonstrated herein can be employed to other tandem catalytic reactions for sustainable chemical production.

Amorphous silica–aluminas (ASAs) are important solid catalysts and supports for many industrially essential and sustainable processes, such as hydrocarbon transformation and biorefining. However, the wide distribution of acid strength on ASAs often results in undesired side reactions, lowering the product selectivity. Here we developed a strategy for the synthesis of a unique class of ASAs with unvarying strength of Brønsted acid sites (BAS) and Lewis acid sites (LAS) using double-flame-spray pyrolysis. Structural characterization using high-resolution transmission electron microscopy (TEM) and solid-state nuclear magnetic resonance (NMR) spectroscopy showed that the uniform acidity is due to a distinct nanostructure, characterized by a uniform interface of silica–alumina and homogeneously dispersed alumina domains. The BAS population density of as-prepared ASAs is up to 6 times higher than that obtained by classical methods. The BAS/LAS ratio, as well as the population densities of BAS and LAS of these ASAs, could be tuned in a broad range. In cyclohexanol dehydration, the uniform Brønsted acid strength provides a high selectivity to cyclohexene and a nearly linear correlation between acid site densities and cyclohexanol conversion. Moreover, the concerted action of these BAS and LAS leads to an excellent bifunctional Brønsted–Lewis acid catalyst for glucose dehydration, affording a superior 5-hydroxymethylfurfural yield.

Carbon recycling reusing emitted carbon dioxide (CO2) as raw materials is nowadays recognized as a concept ahead of CO2capture and storage for controlling its emission to the air. Here, recently discovered alkali-metal-based instantaneous ex situ mineral carbonation under ambient conditions is studied for saponite clay nanoparticles. The cationic concentration of Cs+is varied for seeking the possibility of reusing Cs-contaminated soil in view of carbon recycling. The ex situ mineral carbonation dominantly occurring at the interior surfaces of 10 Å open spaces formed by overlapped nanosheets increases with increasing the Cs concentration up to 0.580 mmol g–1due to the enlargement of the surface area. The ability of the ex situ mineral carbonation, however, deteriorates with further increasing to 0.585 mmol g–1since the 10 Å open spaces are effectively occupied by Cs compounds as Cs2O reducing their fraction. The present findings of the ex situ mineral carbonation relevant to the cationic concentration imply that the practical soil associated with a Fukushima nuclear accident can stabilize CO2without energy consumption as well as chemical solution.

Hydrogen-bonded SiOH defects and Brønsted acidic bridging OH groups, defined as Brønsted acid sites (BAS), yield overlapping signals in 1H MAS NMR and FTIR spectroscopies. Here, a clear distinction in the distribution of SiOH defect sites between industrial ZSM-5 zeolites with Si/Al ratios of 14 and 20 as well as 60 is reported. At a high Si/Al = 60 ratio, hydrogen-bonded SiOH groups appear as signals in the 1H MAS NMR spectrum of H-ZSM-5 between 3 and 8 pm, thus severely overlapping with signals caused by unperturbed and hydrogen-bonded BAS. FTIR spectra are similarly affected. The hydrogen-bonded SiOH groups are paired, and they are clearly identified by 1H double-quantum single-quantum (DQ-SQ) MAS NMR correlation spectroscopy. A selective analysis was carried out by 1H{27Al} rotational echo adiabatic pulse double resonance (REAPDOR) to yield H–Al distances for the BAS, whereas the silanols do not significantly contribute to this experiment due to short T2relaxation times. Spectral overlap of BAS and defect sites is suggested to be of some relevance in particular for zeolites with high Si/Al ratios.

Brønsted acid sites (BAS) in dry aluminosilicate zeolites, also known as bridging OH–H groups, may form hydrogen bonds with other framework oxygen atoms, O, in addition to BAS that are free of such interactions. These are referred to as perturbed and unperturbed BAS, respectively. However, hydrogen bonding may occur only for specific geometric orientations between two framework oxygen atoms, allowing an OH–H···O interaction. To evaluate the possibility for such hydrogen bonding, we introduce an alignment angle κ that describes the angle between the Si–OH–Al bisector and the OH–O direction of a potential hydrogen-bonding donor–acceptor pair. A total of 27 zeolite framework topologies were analyzed, and the relevance of this approach was subsequently demonstrated for H-ZSM-5 with two different Si/Al ratios. Here, it could be unequivocally shown by advanced solid-state NMR methods that the debated 1H NMR signal at ∼6 ppm arises due to the formation of hydrogen-bonded BAS.

Siliceous mesoporous MCM-41 materials were synthesized under ultrasonic treatment from 0.5 to 3 h. The obtained U-[Si]MCM-41 catalysts were applied in the Beckmann rearrangement of cyclohexanone oxime (CHO) to ε-caprolactam (CPL). The ultrasonic treatment can increase the pore wall thickness, while the density of surface SiOH groups increased slightly. Quantitative 1H MAS NMR studies of the U-[Si]MCM-41 catalysts hint at the presence of isolated and vicinal silanols. After dehydration at 393 K, 1H MAS NMR signals of poly-hydrogen-bonded vicinal silanols occurred, which are strongly reduced after dehydration at 473 K. Correspondingly, a much higher conversion of CHO to CPL was observed for U-[Si]MCM-41 dehydrated at 393 K in comparison with those dehydrated at 473 K. This finding indicates poly-hydrogen-bonded SiOH groups can promote the reaction and improve the catalytic performance of siliceous catalytic materials in the Beckmann rearrangement of CHO in an energy economic way at a suitable activation temperature.

The interaction and H/D exchange of free and hydrogen-bonded Bro̷nsted acid sites (BAS) of zeolite ZSM-5 (Si/Al = 14) with n-hexane were investigated by 1H MAS NMR spectroscopy and DFT cluster model calculations. Three model sites were selected from the greater variety of possible BAS locations for model calculations to represent free BAS (Al12-O26(H)-Si12, site I) and strong (Al2-O7(H)-Si6, site II) or weakly hydrogen-bonded (Al5-O12(H)-Si4, site III) BAS. The molecule interacts with the BAS via van der Waals and Coulomb interactions. Strong hydrogen bonds of site II in 5-rings remain intact after n-hexane adsorption, whereas a rupture of weak hydrogen bonds of site III in 6-rings occurred. H/D exchange of the BAS with n-hexane is found for sites I and II at 423 K, but site III is not reactive at this temperature, indicating that the reactivity does not correlate with hydrogen bond strength. This is, at least to some extent, due to the strong adsorption of n-hexane in the groove of the zigzag channel of ZSM-5 for site III. These results contribute to deeper insights into hydrocarbon reactions with zeolite catalysts.

The application of 1H{27Al} offset rotational-echo adiabatic-passage double-resonance (REAPDOR) NMR experiments enables the indirect observation of the first-order quadrupolar lineshapes for 27Al nuclei in acidic zeolites H-beta and H,Na-A by observing the neighboring 1H nuclei under the influence of dipolar coupling between them. The technique allows one to resolve the contribution of different Al environments, provided that these are characterized by different quadrupolar broadenings, reflecting different structural distortions. Therefore, Brønsted acid sites that typically show large quadrupolar coupling constants (CQ) can be distinguished from extra-framework Lewis sites, which often have considerably smaller quadrupolar couplings. The 1H{27Al} offset REAPDOR patterns can be simulated employing a Gaussian probability distribution using a library of reference offset REAPDOR curves (depending on quadrupolar coupling constant and asymmetry parameter). Monomodal and bimodal probability distributions of CQwere used to fit the experimental data. This approach provides direct information on the neighborhood between Brønsted and Lewis acid sites. Finally, a theoretically expected frequency shift of the 1H MAS NMR signals as a function of neighboring 27Al nuclei in different spin states with large quadrupolar coupling is discovered experimentally for the first time for a zeolite Brønsted acid site by using offset REAPDOR.

Homologous series of solid catalysts (silica, SBA-15, dealuminated zeolite DeA–Y, zeolite H,Na–Y) with different pore sizes and loaded with different noble metals (Pt, Rh, Ir, Pd) were applied for the gas phase hydrogenation of propene with para-enriched hydrogen (p-H2) with the aim to produce parahydrogen-induced polarization (PHIP). This hyperpolarization formed by the pairwise incorporation of p-H2into propene was studied via in situ 1H MAS NMR spectroscopy under continuous-flow conditions. By evaluating the characteristic 1H NMR antiphase signals of the hyperpolarized reaction products, the experimental parameters and properties of noble-metal-containing porous catalysts were investigated, which affect the formation and relaxation of PHIP. For the catalysts under study, small pore diameters and interactions of the hyperpolarized product molecules with nuclei inside the pores, such as framework aluminum atoms and extra-framework sodium cations, were found to enhance the relaxation of the hyperpolarization formed by the pairwise incorporation of p-H2into olefinic reactants. Furthermore, a very high hydrogenation activity of the noble-metal-containing catalyst (Pd/H,Na–Y) decreases the formation of PHIP, possibly caused by a too large number of reactive spillover H species without spin correlation, which hinders the pairwise incorporation of p-H2into the olefinic reactants.

Low-silica AlPO-34 materials with similar crystal sizes but different Brønsted acid site densities were prepared and investigated as catalysts in methanol-to-olefin (MTO) conversion. The effect of Brønsted acid site density on catalyst activity and the dominant reaction mechanism during the MTO conversion was investigated viaTGA, GC-MS, solid-state NMR spectroscopy, and in situUV/vis spectroscopy together with the catalytic performance. For the catalysts with lower Brønsted acid site densities, the olefin-based cycle mechanism is the dominant mechanism during the MTO conversion. Long-chain alkenes, e.g., C5–C6alkenes, act as intermediates that are cracked to lower olefins, or are converted to dienes viahydride transfer reactions, and can also diffuse out of the cages of low-silica AlPO-34 catalysts as the products. With decreasing Brønsted acid site density or reaction temperature, the methylation route of the olefin-based cycle was found to be much more favored than the cracking route. Therefore, a higher selectivity to C5–C6alkenes (∼50%) is achieved. Simultaneously, dienes are the predominant deposits occluded in the used catalysts. For catalysts with slightly higher Brønsted acid site densities, the long-chain alkenes are rapidly transformed to aromatics and, subsequently, an aromatic-based cycle mechanism contributes to the MTO conversion. Interestingly, the catalyst with the most suitable Brønsted acid site density can well balance the above-mentioned two reaction cycles accompanied by a low deactivation rate, leading to a long catalyst lifetime of up to 15 h.

Isolated acid sites were selectively generated by postsynthesis modification of a borosilicate zeolite beta. To this end, samples were prepared with pairs of adjacent boron sites balanced by Ca2+ions, whereas isolated boron in the zeolite framework was balanced by NH4+ions. To avoid undesired boron leaching, these ion exchange reactions were carried out in methanol solution rather than in water. Trigonal boron forms near the NH4+ions by decomposing the latter into NH3(and a proton), and selective extraction of the trigonal boron is achieved by water treatment, whereas the tetrahedral boron near Ca2+ions remains in the zeolite framework. The vacancies were refilled with aluminum by treatment with an aqueous Al(NO3)3solution. Two Brønsted acid sites with 1H chemical shifts of 4.0 and 5.0 ppm exist in the dehydrated samples. 1H–27Al REAPDOR solid-state NMR measurements yield quantitative information on the local H–Al distances of isolated H–Al two-spin and H–Al–Al three-spin systems. The nearest H–Al distance is determined at 2.50 Å with an accuracy of 2% (or better) by fitting the oscillatory part of the REAPDOR curves, which was not observed before for zeolite acid sites. The second nearrest Al neighbors show a much larger distance of about 5 Å for the acid protons with a chemical shift of δ = 4 ppm. A second acid site at δ = 5 ppm has an approximately 50% occupation of a second Al neighbor at 3.73 Å, possibly within the same six-ring. This high resolution of dipolar interaction is not observed in a standard zeolite Al-beta prepared by direct synthesis. The method is suitable to identify well-defined local ordering in Al distributions of zeolites.

The intrinsic hydrogenation activities of homologous series of noble-metal-containing zeolites Y were studied by in situ solid-state NMR spectroscopy under semibatch conditions. For the hydrogenation of acrylonitrile, reaction rates in the sequence Pd/H,Na–Y > Rh/H,Na–Y > Pt/H,Na–Y > Ir/H,Na–Y were determined. The dehydrogenation of propane at these zeolites gave a sequence of the turnover frequencies of Ir/H,Na–Y > Rh/H,Na–Y > Pd/H,Na–Y, while Pt/H,Na–Y zeolites showed significantly higher activities. The temperature-programmed desorption of hydrogen (H2-TPD) was utilized for studying the strength of H2/metal interactions. The positions of the high-temperature peaks were arranged according to 2.8Pd/H,Na–Y (723 K) > 2.3Rh/H,Na–Y (713 K) > 4.7Ir/H,Na–Y (663 K). Comparison of these data indicates that strong H2/metal interactions are accompanied by a preferred formation of surface hydrogen atoms, which are the reason for the high hydrogenation activity of Pd/H,Na–Y zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. In the case of the propane dehydrogenation, the strong H2/Pd interactions in Pd/H,Na–Y zeolites hinder the desorption of the reaction product H2, explaining the lower dehydrogenation activity of these zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. For the high catalytic activities of the Pt/H,Na–Y zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt clusters is discussed.

In the present work, the generation and properties of Brønsted acidic bridging OH groups (Si(OH)Al) in Rh-, Ir-, Pd-, and Pt-containing zeolites Y were quantitatively investigated by multinuclear solid-state NMR spectroscopy. The number of Si(OH)Al groups formed per noble metal atom upon their reduction was found to be 0.3 to 1.3 instead of the theoretically expected values of 2 (Pd2+, Pt2+) or 3 (Rh3+, Ir3+). For zeolites Y with low noble metal loadings, a significantly lower acid strength of Si(OH)Al groups in comparison with those of H,Na–Y zeolites occurred. Interestingly, noble metal-containing zeolites Y with lower acid strength have a significant content of not completely reduced noble metal atoms with partially cationic character. The spatial distribution of Si(OH)Al groups in noble metal-containing zeolites Y was found to differ significantly from that in H,Na–Y zeolites. This effect is explained by the migration of a significant content of noble metal atoms into the sodalite cages of zeolites Y already upon calcination of their noble metal complexes. Therefore, a much higher content of Si(OH)Al groups generated by the reduction of noble metal species in zeolites Y are located in sodalite cages compared to those in H,Na–Y zeolites.

Ce(IV)-containing zeolite Beta was successfully prepared by a reproducible and scalable two-step post-synthesis strategy. This consists of creating vacant T sites that are associated with silanol groups by the dealumination of the H-Beta zeolite and a subsequent dry impregnation of the resultant Si-Beta zeolite with cerium (IV) isopropoxide. XRD, FT-IR, UV-Vis and 1H MAS NMR confirmed that Ce(IV) was successfully incorporated into the Beta zeolite framework by the post-synthesis procedures and that they exist as isolated atoms with tetrahedral coordination. The mechanism of Ce incorporation into the framework of Beta zeolite was confirmed by DRIFT spectroscopy. The ring-opening hydration of epoxides to their corresponding 1,2-diols was selected as a model reaction for an evaluation of the catalytic performance of the as-synthesized Ce-Beta zeolite.

The formation and evolution of initial reaction intermediates as well as the reaction mechanism during the early period of the methanol conversion on the silicoaluminophosphate SAPO-41 with one-dimensional and 10-numbered ring pore system was elucidated. According to in situ UV–vis spectroscopy, the formation and nature of intermediates formed on the catalysts in the methanol conversion process were monitored. The intermediates remaining on the catalysts after quenching the methanol conversion were determined by ex situ UV–vis, 1H MAS NMR, and 13C MAS NMR spectroscopy, and the reactivity of these species was investigated by adsorption of ammonia and subsequent solid-state NMR spectroscopy. The above-mentioned spectroscopic studies gave a detailed mechanistic insight into the induction period of the methanol conversion on SAPO-41. Monoenylic carbenium ions, being the dominating species during the initial period of the methanol conversion, were rapidly formed and gradually transferred to dienylic carbenium ions, benzene-based carbenium ions, and trienylic carbenium ions, in addition to three-ring compounds and dienes with different chain lengths. On the basis of these spectroscopic observations and the catalytic results, the olefin-based reaction cycle is disclosed to be the dominating reaction mechanism in the initial period of the methanol conversion on SAPO-41.

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.

In the present work, the mechanism of the methanol-to-hydrocarbon (MTH) conversion over the silicoaluminophosphate SAPO-41 with one-dimensional 10-ring pores has been investigated. For this purpose, catalytic experiments with co-feeding of reactants, in situFTIR and UV/Vis spectroscopy, and 1H MAS NMR spectroscopy of used catalysts upon loading of ammonia were performed. Using these methods, only a low content of aromatics could be observed on the working SAPO-41 catalyst. These findings and the characteristic changes in the ethene selectivity during the co-feeding experiments indicate that alkylaromatics can be excluded as active hydrocarbon pool compounds on SAPO-41 applied as MTH catalysts. On the other hand, dienes and enylic carbenium ions were detected using FTIR and UV/Vis spectroscopy and the formation of amines by reaction of alkenes with ammonia was detected via1H MAS NMR spectroscopy. These results are experimental evidence that large olefins partially existing in their carbenium state dominate the catalytically active hydrocarbon pool on the working SAPO-41. Due to this dominant alkene-based reaction mechanism and the limited formation of aromatics, SAPO-41 is a suitable catalyst for the C3+C4olefin as well as gasoline production.

A rheological mechanism of long-term self-assembly induced by H2O molecules is highlighted for layered saponite nanoparticles based on the results of thermogravimetry and differential thermal analysis, time-resolved length change measurement with high-resolution dilatometry, solid-state nuclear magnetic resonance, positronium annihilation spectroscopy, and molecular dynamics simulation. Prior to self-assembly, saponite nanoparticles exhibit two kinds of local molecular structures, where one and two nanosheets are inserted into interlayer spaces forming open spaces with their sizes of ∼0.3 and ∼0.9 nm, respectively. H2O molecules adsorbed at Na+cations in the interlayer spaces immediately trigger off the onset of the rheological motion of nanosheets in parallel to the layer direction. One of two nanosheets inserted into the interlayer spaces is thus gradually released away, with the local molecular structure with smaller open spaces getting to dominant for the self-assembled saponite nanoparticles.

SAPO-34 catalysts obtained after MTO conversion times of 0–30 min were investigated by PFG NMR spectroscopy for studying the self-diffusivities of ethane and ethene. Furthermore, the catalysts were investigated by in situ UV/vis, 13C MAS NMR, and 1H MAS NMR spectroscopy, giving insights into the organic deposits and acid sites in different periods of the catalyst lifetime. The ratio of the self-diffusivities of ethene and ethane, Dethene/Dethane, used as a measure of the diffusion selectivity, was found to increase to the benefit of the olefin compared to the alkane for higher adsorbate loadings and increasing MTO conversion times. This change of the diffusion selectivity is one of the reasons for the variation of the product selectivity during the catalyst lifetime. Variable-temperature PFG NMR studies gave a significantly larger apparent activation energy for the self-diffusivity of ethane (Ea= 7.6 kJ/mol) compared with ethene (Ea= 4.2 kJ/mol).

Feel the burn: Silica/alumina was prepared in milliseconds by flame‐spray pyrolysis (see picture) with a homogeneous composition and at super‐high calcination temperatures. This solid acid has tunable acidity ranging from weak or moderate to strong Brønsted acidity. This unique property makes this material versatile for desired industrial applications.

1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, CQ, of the framework aluminum atoms in dehydrated MIL-53 from CQ= 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, CQ, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the CQvalues (CQ= 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2centers, the third change of the CQvalue to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the CQvalue of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility.

No Abstract

No Abstract

Zeolites Al,Na−X and Al,Na−Y with defined numbers of extraframework aluminum cations were prepared by exchange in an aqueous solution of aluminum nitrate. A maximum concentration of Brønsted acidic bridging OH groups in supercages (SiOHsupAl) was reached upon dehydration of zeolites Al,Na−X and Al,Na−Y at 423 K. Further raising of the dehydration temperature led to a dehydroxylation of zeolites due to the recombination of aluminum hydroxyl groups with hydroxyl protons of bridging OH groups. High-field 27Al multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy was utilized to study zeolites Al,Na−X/61 and Al,Na−Y/63 dehydrated at 423 K. Second-order quadrupolar effect parameters of 10.1−11.0 MHz for tetrahedrally coordinated framework aluminum atoms, compensated in their negative charge by hydroxyl protons (AlIV/H) and aluminum cations (AlIV/Alx), 3.6−4.4 MHz for tetrahedrally coordinated framework aluminum atoms compensated by sodium cations (AlIV/Na), and 5.6−7.6 MHz for pentacoordinated extraframework aluminum cations (Alxcat.) were obtained. Comparison of the number of AlOH groups with the number of pentacoordinated extraframework aluminum cations determined by one-dimensional high-field 27Al MAS NMR spectroscopy gave a ratio near 1:1. This finding and the five-fold coordination of the cationic extraframework aluminum species hint to the presence of HO−Al−O−Al−OH compounds, but also a minor number of Al(OH)2and AlOspecies could exist. The enhanced acid strength of bridging OH groups in zeolites Al,Na−X and Al,Na−Y in comparison with zeolites H,Na−X and H,Na−Y, as found by adsorption of acetonitrile, may be due to a polarizing effect of cationic extraframework aluminum species in the vicinity of Brønsted acid sites.

By using 13C MAS NMR spectroscopy (MAS=magic angle spinning), the conversion of selectively 13C‐labeled n‐butane on zeolite H‐ZSM‐5 at 430–470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13C‐label in the n‐butane molecule, and 2) oligomerization–cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl‐substituted cyclopentenyl cations and condensed aromatic compounds. In situ 13C MAS NMR and complementary ex situ GC–MS data provided evidence for a monomolecular mechanism of the 13C‐label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to 13C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (Ea=75 kJ mol−1for the scrambling and 71 kJ mol−1for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n‐butane as being the rate‐determining stage of the n‐butane conversion on zeolite H‐ZSM‐5.

The first syntheses of highly ordered bifunctional periodic mesoporous organosilicas (PMOs) containing different amounts of aromatic thiophene and benzene bridging groups are reported. Employing the triblock copolymer Pluronic P123 as well as the oligomeric Brij 76 surfactant under acidic conditions as supramolecular structure-directing agents, the syntheses of two series of bifunctional aromatic PMO materials with pore sizes in the range of 4.8–5.4 and 3.3 nm, respectively, have been realized. Independent of the molar ratios of the organosilanes in the initial reaction mixtures, highly ordered PMO materials with 2D hexagonal mesostructures have been obtained in all cases. After a one-off calibration based on 29Si MAS NMR measurements, the quantification of the organic functional groups has been carried out for the first time, in case of PMO materials, using Raman spectroscopic methods.

For the first time, the dry-gel conversion method was applied for the preparation of MCM-41 materials with partially crystalline framework. The X-ray diffraction pattern of the material obtained after a dry-gel conversion time of 36 h consists of reflections of both long-range ordered MCM-41 material and zeolite ZSM-5. The nitrogen adsorption/desorption isotherm confirms the presence of micropores and uniform mesopores with a narrow pore size distribution. By 29Si MAS NMR spectroscopy it was evidenced that a significant portion of the amorphous MCM-41 framework was transformed into a partially crystalline framework. TEM and SEM images indicate the occurrence of mesopores with disturbed arrangements, but no formation of separate zeolite ZSM-5 crystals occurred. By 1H MAS NMR spectroscopy of the above-mentioned material, the formation of hydroxyl groups with a chemical shift of ca.4 ppm and a concentration of ca.0.1 mmol/g was observed. The acid strength of these hydroxyl groups, as studied by adsorption of the 13C MAS NMR probe molecule 13C-2-acetone, was found to agree with that of bridging OH groups in zeolite H-ZSM-5. After conversion times of 48 h and 72 h, an extensive to total transformation of the dry gel into zeolite ZSM-5 occurred.

27Al spin-echo, high-speed MAS νrot 30 kHz, and MQMAS NMR spectroscopy in magnetic fields of B0 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated γ-Al2O3and non-hydrated aluminum-exchanged zeolite Y Al,Na-Y and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations δiso 70 ± 10 ppm, CQCC 15.0 ± 1.0 MHz. The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters δiso 65 ± 5 ppm, CQCC 8.0 ± 0.5 MHz. The residual two signals were attributed to aluminum cations δiso 35 ± 5 ppm, CQCC 7.5 ± 0.5 MHz and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters δiso 10 ± 5 ppm, CQCC 5.0 ± 0.5 MHz. By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y81.5 in the non-hydrated state, the aluminum distribution in this material was determined.

Aniline alkylation with methanol on zeolite H–Y has been studied using in situ13C MAS NMR spectroscopy under batch conditions. To clarify the main reaction pathways, the conversion of methanol as well as the interaction of aniline with surface methoxy groups were investigated under similar conditions. Methanol-13C and methyl iodide-13C were used as labeled reactants. Co -adsorption of aniline and methanol-13C on zeolite H–Y led to strongly adsorbed aniline molecules, assigned to aniline H-bonded to zeolite Brønsted acid sites, and three types of methanol species of different mobility: mobile methanol molecules with a liquid-like characteristics and two types of rigid methanol species with solid-like characteristics attributed to a methanol adsorption complex with aniline and surface methoxy groups, respectively. Among all the methanol species observed, only surface methoxy groups were shown to be responsible for aniline alkylation which takes place at temperatures from 373 to 523 K. The formation of surface methoxy groups was found to be a limiting step of the overall reaction. The primary alkylation product is N-methylaniline. Toluidines and N-methyltoluidines are formed at temperatures from 523 to 623 K after complete conversion of methanol to N-methylaniline. Therefore, isomerization or disproportionation of N-methylaniline was proposed to account for their formation.

The development of new solid catalysts for use in industrial chemistry has hitherto been based to a large extent upon the empirical testing of a wide range of different materials. In only a few exceptional cases has success been achieved in understanding the overall, usually very complex mechanism of the chemical reaction through the elucidation of individual intermediate aspects of a heterogeneously catalyzed reaction. With the modern approach of combinatorial catalysis it is now possible to prepare and test much more rapidly a wide range of different materials within a short time and thus find suitable catalysts or optimize their chemical composition. Our understanding of the mechanisms of reactions catalyzed by these materials must be developed, however, by spectroscopic investigations on working catalysts under conditions that are as close as possible to practice (temperature, partial pressures of the reactants, space velocity). This demands the development and the application of new techniques of in situ spectroscopy. This review will show how this objective is being achieved. By the term in situ (Lat.: in the original position) is meant the investigation of the chemical reactions which are taking place as well as the changes in the working catalysts directly in the spectrometer.

Die Entwicklung neuer Festkörperkatalysatoren für den Einsatz in der industriellen Chemie beruht bisher noch zu einem sehr großen Teil auf dem empirischen Erproben einer Vielzahl unterschiedlicher Materialien. Nur in wenigen Ausnahmefällen ist es gelungen, durch die Aufklärung der einzelnen Teilaspekte einer heterogen katalysierten Reaktion den gesamten, meist sehr komplexen Mechanismus der chemischen Umsetzung zu verstehen. Mit modernen Konzepten der kombinatorischen Katalyseforschung ist es möglich geworden, innerhalb kurzer Zeiträume eine große Zahl unterschiedlicher Materialien zu präparieren und zu testen und damit schneller als vorher geeignete Katalysatoren zu finden oder ihre chemische Zusammensetzung zu optimieren. Unser Verständnis der Mechanismen der von diesen Materialien katalysierten Reaktionen muss jedoch durch spektroskopische Untersuchungen an arbeitenden Katalysatoren unter möglichst praxisnahen Bedingungen (Temperatur, Partialdruck der Reaktanten, Katalysatorbelastung) entwickelt werden. Dies erfordert die Entwicklung und den Einsatz neuer Techniken der In-situ-Spektroskopie. Der vorliegende Aufsatz soll zeigen, auf welchen Wegen dieses Ziel erreicht wird. Der Begriff in situ (lat.: an Ort und Stelle) beinhaltet im eigentlichen Sinne die Untersuchung der direkt im Spektrometer ablaufenden chemischen Reaktionen und die mit ihnen verbundenen Veränderungen des Katalysators.

Applying in situ 13C MAS NMR spectroscopy under continuous flow (CF) conditions and on-line gas chromatography, the conversion of methanol on silicoaluminophosphates SAPO-34 and SAPO-18 was studied. At reaction temperatures of 573 to 673 K, a strong increase of the yield of propylene and ethylene was observed by on-line gas chromatography. Simultaneously with the occurrence of in situ 13CC CF MAS NMR signals at 126–135 ppm indicates the formation of a mixture of aliphatic and/or cyclic olefins and alkylated aromatics with carbon numbers of C6–C12. All these support the “hydrocarbon pool” mechanism for the formation of light olefins on the SAPO-34 and SAPO-18.

Lanthanum-exchanged zeolites X, Y, and EMT, containing either sodium or potassium as residual cations, were used as catalysts in two acid-catalyzed hydrocarbon reactions, viz. the disproportionation of ethylbenzene and the isomerization of n-octane (for the latter reaction, Pd/γ-Al2O3was admixed to the acid zeolites to make the catalysts bifunctional). All pairs of LaM-type zeolites (where Mstands for Na or K) showed remarkable activity differences, the LaK form being always less active than the LaNa form. IR and 1H NMR spectroscopy revealed that, for a given zeolite type and degree of lanthanum exchange, the concentration of bridging hydroxyl groups in the large cages was at least 20% lower than that for the LaK form. 139La NMR spectroscopy indicated that the concentration of lanthanum ions in the large cages of the as-exchanged LaK forms was always lower than that in the corresponding LaNa forms. Surprisingly, already in the as-exchanged LaK forms, some La3+migration into the small cages appears to occur, and this was corroborated by framework strains indicated by 29Si and 27Al NMR. It is proposed that potassium cations in the large cages cause an enhanced migration of lanthanum cations into the small cages and a preferential formation of Brønsted acid sites in these small cages during the thermal dehydration of lanthanum cations (Hirschler–Plank mechanism). A similar effect was found in HNaY and HKY zeolites: The presence of the bulkier potassium cations causes a significant diminution of the accessible Brønsted acid sites in the large cages and of the catalytic activity. Overall, one must conclude from these results that in zeolites possessing both large and small cages, the nature of the residual alkali cations can exert a pronounced influence on the local distribution of the Brønsted acid sites, regardless of how these sites were generated. Copyri ght 2001 Academic Press.

A new technique is introduced allowing simultaneous in situ MAS NMR investigations of hydrocarbon conversions on solids under flow conditions and on-line gas chromatography. For adsorption of methanol on zeolite HBeta, equal amounts of adsorbed molecules were determined by both analytical methods. Studying the synthesis of methyl tertiary-butyl ether (MTBE) on zeolite HBeta using an MAS NMR rotor reactor, a constant yield of MTBE of Ymtbe= 27% was obtained up to a weight hourly space velocity of 1.4 h-1. The variation of the reaction temperature led to a simultaneous change of the 13C MAS NMR signals of isobutoxy species and of the yield of MTBE determined by on-line gas chromatography which indicates that isobutoxy species act as chemically active compounds. In this first application, the new in situ technique has demonstrated its advantage for a simultaneous investigation of compounds with a long residence time on the catalyst surface and of compounds rapidly leaving the catalyst surface.

Silica gel, dealuminated zeolite USY and cesium-exchanged zeolite Y were impregnated with alkali metal hydroxides or acetates followed by calcination. Surface methoxy groups formed upon loading these samples with methyl iodide were applied as ¹³C NMR spectroscopic probes to investigate the formation of well dispersed basic guest compounds. The variation of the isotropic ¹³C NMR shifts of methoxy groups bound to different guest compounds on silica gel (49.6–48.7 ppm) shows the same tendency as the electronegativities of alkali metal atoms introduced by impregnation. ¹³C CP/MAS NMR studies of surface methoxy groups bound to guest compounds on silica gel and zeolites Y revealed an influence of the support on the ¹³C chemical shift tensor. The application of this NMR spectroscopic method on cesium-exchanged zeolite Y impregnated with cesium hydroxide allows one to distinguish at least two types of oxidic guest compounds. Copyright © 1999 John Wiley & Sons, Ltd.

In situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography have been applied to study the onset of the conversion of methanol on zeolite HZSM‐5 at temperatures between 373 and 573 K. In the steady states of methanol conversion at T⩾ 523 K, by on‐line gas chromatography mainly the formation of ethene and propene was observed. Simultaneously recorded in situ 13C MAS NMR spectra show signals at 12–25 ppm and at ca. 125–131 ppm indicating the presence of adsorbed C4–C6olefins. The observation of these adsorbates on a working catalyst supports the “hydrocarbon pool” mechanism previously proposed for the methanol‐to‐hydrocarbon conversion on acidic zeolites. Methanol conversion at 473 and 573 K and subsequent purging of the catalyst with dry nitrogen at 293 K led to a 13C MAS NMR signal at 59 ppm due to methoxy groups. No hints to the presence of ethoxy, propoxy or butoxy groups and the formation of alkyl oxonium ions were found by in situ 13C MAS NMR spectroscopy under flow conditions.

The synthesis of the high-silica zeolite NU-87 and its physicochemical characterization are reported. The catalytic properties of zeolite HNU-87 were investigated in the conversion of 1- and 2-methylnaphthalene. Both isomerization to the corresponding methylnaphthalene and transalkylation to naphthalene and dimethylnaphthalenes are observed. The influence of reaction temperature and the modified residence time on the product distributions is reported and discussed in terms of shape selectivity effects in the peculiar pore system of NU-87. For comparison, typical results obained with zeolite HMCM-22, which possesses a similar pore architecture, are also included. The results of methylnaphthalene conversion over HNU-87 are compared to those obtained in the alkylation of naphthalene with methanol on the same zeolite catalyst.

IR and1H MAS NMR confirm that extra-framework aluminium present in dealuminated (ultrastable) zeolite Y is reintroduced into the framework by treatment with strongly basic solutions at elevated temperatures. The realuminated sample contains twice as many Brønsted acid sites than the ultrastable precursor and, with an accuracy of 20%, the same number of acid sites as the parent as-prepared zeolite. However, not as many hydroxyl groups associated with framework Al in the product are acccesible to pyridine as in the parent sample.

Eine Zunahme der Basizitätdes zeolithischen Wirts, die durch die oxidische Gastkomponente bedingt ist, bewirkte dessen Imprägnierung mit Caesiumhydroxid (Ausschnitt aus der Struktur siehe rechts; ○ Kationen, • Gastkomponente). Die Anwendung solcher Materialien als basische Festkörperkatalysatoren könnte für die chemische Industrie interessant sein.

The conversion of propan-2-ol on zeolites HY and LaNaY has been investigated by gas chromatography (GC) andin situ1H and13C MAS NMR spectroscopy under continuous-flow conditions using a new MAS NMR microreactor with cylindrical catalyst bed. At reaction temperatures ofT=373 K andT=393 K a propan-2-ol conversion of 50 and 100%, respectively, and the formation of propene, diisopropyl ether, and small amounts of acetone was determined by GC. Applyingin situ1H and13C MAS NMR spectroscopy, the initial step of the reaction was found to be the physisorption of propan-2-ol on Brønsted acid sites. A formation of isopropoxy species could be excluded by13C MAS NMR spectroscopy.1H MAS NMR spectroscopy indicated that the Brønsted acid sites of the zeolites LaNaY and HY were hydrated by water molecules in the first part of the induction period. These water molecules were formed in result of the propan-2-ol dehydration. The strong low-field shift of the1H MAS NMR signals of the hydrated Brønsted acid sites is due to a partial protonation of adsorbed water molecules. AtT=393 K, a significant13C MAS NMR signal of strongly bonded acetone molecules appeared at 220 ppm in the spectra of zeolites LaNaY and HY. As demonstrated by propan-2-ol conversion on a partially dealuminated zeolite HY, this by-reaction is promoted by extra-framework aluminium species. The formation of coke precursors which caused13C MAS NMR signals at 10–50 ppm is explained by an oligomerization of propene.In situ13C MAS NMR experiments carried out under a continuous flow of propene showed that the above-mentioned coke precursors are also formed on partially rehydrated zeolite HY.

AbstractNearly all atoms contributing to the local structure of Brønsted 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 Brønsted 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 NMR spectra and the separation of spectral parameters possible. This article gives a review of these techniques and a summary of the most important applications of multinuclear solid-state NMR spectroscopy for the characterization of Brønsted acid sites in dehydrated zeolites.