Christian Fernandez

Hierarchical ZSM-5 zeolites in shape-selective xylene isomerization: role of mesoporosity and acid site speciation.

The isomerization of o-xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mesoporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the deactivation rate. The HCl-washed hierarchical ZSM-5 displayed an approximately twofold increase in p-xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).

Optimized multiple-quantum magic-angle spinning NMR experiments on half-integer quadrupoles

It has been recently shown that second-order anisotropies can be removed from solid phase NMR spectra of quadrupolar nuclei via the combined use of magic-angle spinning (MAS) and bidimensional multiple-quantum (MQ) spectroscopy. The present study investigates the conditions under which the acquisition of such high-resolution MQMAS NMR spectra are optimized. The excitation and conversion pulse lengths that maximize 0 → ±3(t1) → − 1 (t)2) coherence transfer pathway signals for arbitrary spin numbers were calculated, as were the pulse lengths that provide NMR spectra free from dispersive line shape distortions. For the case of spin-5/2 nuclei, the conditions which optimize experiments involving quintuplequantum coherences were also determined.

Framework Stabilization of Ge-Rich Zeolites via Postsynthesis Alumination

The use of organic structure directing agents in zeolite syntheses has dramatically extended the number of zeolite structure types during the past decades. However, for about 20% of all known zeolite structure types, the necessary postsynthesis elimination of organic templates by high-temperature combustion leads to structure collapse, where the particularly strongly affected are Ge-rich zeolites. Here, we present a treatment approach that leads to zeolite structure stabilization by postsynthetic isomorphous substitution of Al for Ge. An important advantage of this new method is that no preliminary elimination of the organic structure directing agent from zeolite pores is required; thus it can be applied to microporous materials that cannot withstand the high temperature combustion of organic templates. The experimental data unambiguously show that besides framework stabilization the postsynthesis treatment facilitates incorporation of active sites in the zeolite framework. The feasibility of this new approach is corroborated by alumination of a BEC-type material. The presented method is expected to broaden the practical utilization of many microporous materials by improving their thermal stability.

High-resolution 1H homonuclear dipolar recoupling NMR spectra of biological solids at MAS rates up to 67 kHz

Two-dimensional (1)H homonuclear correlation NMR spectra of solids of biological interest have been recorded at high magnetic fields (14.1 and 18.8 T) and MAS rates up to 67 kHz, using RN(n)(nu) symmetry-based homonuclear recoupling and CRAMPS decoupling; this method affords exceptional spectral resolution and is well suited to probe (1)H-(1)H proximities in powdered solids.

Probing Quadrupolar Nuclei by Solid-State NMR Spectroscopy: Recent Advances

Solid-state nuclear magnetic resonance (NMR) of quadrupolar nuclei has recently undergone remarkable development of capabilities for obtaining structural and dynamic information at the molecular level. This review summarizes the key achievements attained during the last couple of decades in solid-state NMR of both integer spin and half-integer spin quadrupolar nuclei. We provide a concise description of the first- and second-order quadrupolar interactions, and their effect on the static and magic angle spinning (MAS) spectra. Methods are explained for efficient excitation of single- and multiple-quantum coherences, and acquisition of spectra under low- and high-resolution conditions. Most of all, we present a coherent, comparative description of the high-resolution methods for half-integer quadrupolar nuclei, including double rotation (DOR), dynamic angle spinning (DAS), multiple-quantum magic angle spinning (MQMAS), and satellite transition magic angle spinning (STMAS). Also highlighted are methods for processing and analysis of the spectra. Finally, we review methods for probing the heteronuclear and homonuclear correlations between the quadrupolar nuclei and their quadrupolar or spin-1/2 neighbors.

Progress in Multiple-Quantum Magic-Angle Spinning NMR Spectroscopy

Recent advances in nuclear magnetic resonance spectroscopy of quadrupolar (I>1/2) nuclei with half-integer spins in solids have been reviewed. The advent of multiple-quantum (MQ) magic-angle spinning (MAS) spectroscopy gave new momentum to the study of quadrupolar nuclei in materials of academic and industrial interest such as minerals, ceramics and glasses, microporous and mesoporous solids and biological materials. It is now possible to record high-resolution solid-state NMR spectra of a range of important nuclei, namely (11)B, (17)O, (23)Na, (27)Al, (71)Ga, (91)Nb. Since its introduction in 1995 MQMAS NMR has evolved considerably and, at present, a range of very useful related techniques are available and have been reviewed, in particular satellite transition (ST) MAS, Inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei, such as cross-polarization MQMAS and MQ heteronuclear correlation spectroscopy (HETCOR) and the recently introduced J-coupling based experiments (such as J-HMQC).

Studies of Heteronuclear Dipolar Interactions Between Spin-1/2 and Quadrupolar Nuclei by Using REDOR During Multiple Quantum Evolution

Abstract A new technique for measurements of dipolar interactions in rotating solids is presented that combines the capabilities of multiple quantum magic angle spinning (MQMAS) with the rotational echo double resonance (REDOR). It employs the dipolar recoupling between spin-1/2 ( I ) and quadrupolar ( S ) nuclei by applying a series of π pulses to the I spins. In contrast to the previously reported MQ-REDOR method, the recoupling sequence is applied during the triple quantum, rather than single quantum evolution. As the dipolar effect is enhanced by the MQ coherence order, this new technique exhibits improved sensitivity toward weak dipolar interactions.

Ultrastructure of cellulose crystallites in flax textile fibres

Seven varieties of flax (Linum usitatissimum) fibres were analyzed in order to gain a deeper insight into the morphological features of the crystalline assembly. Different spectroscopic techniques and a chemical bleaching process were used to provide an accurate description of the lateral arrangement of the polysaccharide chains within the fibre cell wall. The flax fibres were analyzed in their natural state and after an extraction treatment of the non-crystalline components such as hemicelluloses, pectins and phenolics. The chemical bleaching process consisted of a Soxhlet extraction in toluene, a sodium chlorite treatment and an alkaline extraction of the residual hemicelluloses. Solid-state 13C nuclear magnetic resonance (NMR) confirmed the sequential removal of the non-cellulosic components from the flax cell wall. Both wide-angle X-ray diffraction (WAXD) and solid-state 13C NMR provided measures of the crystallite thicknesses and overall crystallinities before and after treatment. The existence of non-cellulosic highly ordered paracrystalline domains was also evidenced by proton spin relaxation time calculation. Whereas the overall crystallinity determined by WAXD decreased after treatment, the cellulose crystallinity calculated with the help of the solid-state 13C NMR slightly increased. This is explained by the difference in chemical selectivity between these two techniques and by the paracrystalline state of both hemicelluloses and pectins. Strong adhesion between cellulose crystallites, hemicelluloses and pectins in the fibres was evidenced by low spin–spin relaxation times and by an increase in crystallite thickness after bleaching. A simple model is proposed that describes the rearrangement of the macromolecules during the bleaching process.

Cumene transformations over mordenite catalysts: a 13C MAS NMR study

Abstract Cumene conversion under an excess of benzene is studied over a series of mordenite (MOR) zeolites (Na-MOR, HNa-MOR, H-MOR and dealuminated H-MOR) by in situ 13 C and 1 H MAS NMR spectroscopy in the temperature range 298–473 K. By tracing the fate of 13 C labeled atoms in the benzene molecule, intermolecular transalkylation, disproportionation and isomerization reactions are highlighted. The reactivity of MOR-based catalysts in these reactions is found to increase in the following order: Na-MOR T =298–393 K, weak acidity), while isomerization to n -propylbenzene needs more severe conditions ( T >473 K, strong acid sites). The reaction mechanisms are discussed.

7Li solid-state NMR of lithium amides: structural information from 7Li quadrupolar coupling constants†

Lithium‐7 quadrupolar coupling constants, χ(7Li), were measured for a number of microcrystalline lithium amides by solid‐state 7Li magic angle spinning (MAS) NMR. A linear correlation between χ(7Li) and the structural N—Li—N angle known from x‐ray diffraction work was found: χ(7Li)=(4.1±0.5)∢N—Li—N ‐(110.8±69.0). On this basis, structural information for new systems becomes available. For the polymer material [H2C(CH2)3NLi]n a ladder structure is proposed. The π‐flip of the five‐membered metallacycle formed by the coordination of tetramethylethylenediamine (TMEDA) with lithium in [(H2C(CH2)3NLi)2·TMEDA]2 was detected by 13C cross‐polarization/MAS DNMR spectroscopy. From the analysis of the temperature‐dependent lineshape of the methyl signals, a barrier of ΔG≠(293)=66 kJ mol‐1 was derived for this rotational process in the solid, which compares favourably with data derived for related compounds.