In this study, we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support. Disciplines Materials Chemistry | Other Chemistry | Physical ChemistryComments NOTICE: this is the author's version of a work that was accepted for publication in Journal of Catalysis. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Catalysis, [291, (2012) AbstractIn this study we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support.

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The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes. ABSTRACT: The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes.

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We examine the opportunities offered by advancements in solid-state NMR (SSNMR) methods, which increasingly rely on the use of high magnetic fields and fast magic angle spinning (MAS), in the studies of coals and other carbonaceous materials. The sensitivity of one-and two-dimensional experiments tested on several Argonne Premium coal samples is only slightly lower than that of traditional experiments performed at low magnetic fields in large MAS rotors, since higher receptivity per spin and the use of 1 H detection of low-gamma nuclei can make up for most of the signal loss due to the small rotor size. The advantages of modern SSNMR methodology in these studies include improved resolution, simplicity of pulse sequences, and the possibility of using J-coupling during mixing. ■ INTRODUCTIONThe ever increasing need to optimize conversion of heavy fossil fuel resources into useful products in an environmentally benign and cost-effective manner requires detailed understanding of the molecular structure and the reactivities. 1 One of the most powerful analytical methods for studying insoluble carbonaceous materials in bulk is solid-state nuclear magnetic resonance (SSNMR) spectroscopy, which for over three decades has been used as the primary source of information about concentrations of various carbon and hydrogen functionalities. 2−11 Numerous early investigations have suggested that the quantitative (to within a few %) 13 C intensities in coals could be best measured at low magnetic field, B 0 of 4.7 T or less, under slow magic angle spinning (MAS), at rates of 10 kHz or less, using variable-contact time cross-polarization (CP) or direct-polarization (DP) MAS experiments. 4−10 Specifically, it was accepted that the seemingly conflicting requirements of using MAS rates that exceed 13 C chemical shift anisotropies (CSAs), yet do not interfere with the CP process, could be best met under such conditions. Second, the inhomogeneously broadened lines in coals scale linearly with B 0 , which partly negates the resolution and sensitivity advantage of a higher field. Lastly, the high-resolution 1 H NMR studies of coals using combined rotation and multiple-pulse spectroscopy (CRAMPS) were also carried out under low-field/slow-MAS conditions. 8,12,13 The continuous development of stronger magnets, more sensitive probes, operating at higher MAS rates, innovative pulse sequences, and improved computational tools has led to dramatic progress in SSNMR spectroscopy. Advances in ultrafast MAS technology, 14 which allow for sample spinning at 40−80 kHz, opened new opportunities for refining multidimensional SSNMR experiments. 15−20 The impact of fast MAS relies on excellent sensitivity per spin, great flexibility in using the radiofrequency (RF) magnetic fields, efficient CP transfer, increased frequency range of the indirect dimension in rotor-synchronized experiments, and elimination of the spinning sidebands in the presence of large CSAs. In addition, fast MAS by itself or in combination with RF pulse sequences (CRAMPS) can be us...

The thermogenic transformation of kerogen into hydrocarbons accompanies the development of a pore network within the kerogen that serves as gas storage locations both in pore space and the surface area for adsorbed gas with source rocks. Therefore, the successful recovery of gas from these rocks depends on the accessible surface area, surface properties, and interconnectivity of the pore system. These parameters can be difficult to determine because of the nanoscale of the structures within source rocks. This study seeks to investigate the pore structure, surface heterogeneity, and composition of isolated kerogens with progressively increasing thermogenic maturities from source rocks at a middle-east reservoir. Prompt gamma-ray activation analysis (PGAA), nitrogen and methane volumetric gas sorption, and small-angle neutron scattering (SANS) are combined to explore the relationship between the chemical composition, pore structure, surface roughness, surface heterogeneity, and maturity. PGAA results indicate that more mature kerogens have lower hydrogen/carbon ratios. Nitrogen gas adsorption indicates that the pore volume and accessible specific surface area are higher for more mature kerogens. The methane isosteric heat at different methane uptakes in the kerogens is determined by methane isotherms and shows that approximately two types of binding sites are present in less mature kerogens while the binding sites are relatively homogeneous in the most mature kerogen. The hysteresis effects of the structure during the adsorption and desorption processes at different CD 4 gas pressures are studied. An extended generalized Porod's scattering law method (GPSLM) is further developed here to analyze kerogens with fractal surfaces. This extended GPSLM quantifies the surface heterogeneity of the kerogens with a fractal surface and shows that more mature kerogen is chemically more homogeneous, consistent with the results from methane isosteric heat. SANS analysis also suggests a pronounced surface roughness in the more mature kerogens. A microporous region circling around the nanopores, which contributes to high surface roughness and methane storage, is shown to develop with maturity.

A nuclear magnetic resonance (NMR)-based method was developed to quickly and accurately determine porosities and densities of tight reservoir rocks from drill cuttings. The method combines low-field NMR and two mass measurements, one in the air and one in a fluid, to determine rock porosity, grain density, and bulk density. The method provides an inexpensive approach to deliver continuous data for evaluation of a drilled well and is especially useful for unconventional reservoirs for evaluating both vertical and horizontal sections of a well.

Spin relaxation, a defining mechanism of nuclear magnetic resonance (NMR), has been a prime method for determining three-dimensional molecular structures and their dynamics in solution. It also plays key roles for contrast enhancement in magnetic resonance imaging (MRI). In bulk solutions, rapid Brownian molecular diffusion modulates dipolar interactions between a spin pair from different molecules, resulting in very weak intermolecular relaxations. We show that in fluids confined in nanospace or nanopores (nanoconfined fluids) the correlation of dipolar coupling between spin pairs of different molecules is greatly enhanced by the nanopore constraint boundaries on the molecular diffusion, giving rise to an enhanced correlation for the spin pair. As a result, the intermolecular dipolar interaction behaves cooperatively, which leads to a large intermolecular dipolar relaxation rate and opposite in sign to the bulk solution. We found that the classical NMR relaxation theory fails to capture these observations in a nanoconfined fluid environment. Hence, we developed a formal theory and experimentally confirmed that enhanced correlation and cooperated relaxation are ubiquitous in nanoconfined fluids. The newly discovered phenomenon and the developed NMR method reveal new applications in a broad range of synthesized and naturally occurring materials in the field of nanofluidics to study molecular dynamics and structure as well as for MRI image enhancement.