Probing the Evolution of Adsorption on Nanoporous Solids by In Situ Solid-State NMR Spectroscopy

Low-field NMR investigations on dynamics of crude oil confined into nanoporous silica rods and white powder

In the present study, to mimic the natural confinement of crude oils, model experiments are conducted with crude oils having different physical properties and maltenes of parent crude oils without asphaltenes confined into engineered nanoporous silica rods with pore diameters of 2.5 and 10.0 nm and white powdered nanoporous silica with pore diameters of 2.5 and 4.0 nm. This will help with suggesting potential treatments for enhancing crude oil recovery. Low-field nuclear magnetic resonance (LF-NMR) relaxometry has been applied to achieve this goal. The nanoporous proxies resemble real-life nanoporous rocks of reservoirs. The dynamics of confined crude oils with different ^oAPI gravity deviate from bulk dynamics, and deviation changes depending on the ^oAPI gravity. This suggests that treatments must be decided appropriately before crude oil production. Similar treatments could be applied for light and medium-heavy crude oils. Mathematical analysis of NMR relaxation curves of confined crude oils with different fractions of SARA (saturates, aromatics, resins, asphaltenes) indicates that the conventional SARA approach needs a better definition for the confined state of matter. The NMR relaxation behavior of confined maltenes shows that resin molecules might act like saturates in natural confinement with various scale pores from nano to micro and even macro, or aromatics might show resin-like behaviors. Confinement of brine and a light crude oil into white powdered nanoporous silica proxies demonstrates that brine could be utilized along with some additives such as nanoparticles for oil recovery. Therefore, these issues must be evaluated in deciding the proper treatments for crude oil production.

Activation and catalytic transformation of methane under mild conditions

In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH₄) under mild conditions for both chemical synthesis with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH₄ to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH₄ to value-added intermediate compounds including but not limited to CH₃OH, HCHO, HCOOH, and CH₃COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH₄ under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH₄ to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH₄ or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH₄ to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH₄ under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.

Recent Advances in Solid-State Nuclear Magnetic Resonance Spectroscopy

The sensitivity of nuclear magnetic resonance (NMR) spectroscopy to the local atomic-scale environment offers great potential for the characterization of a diverse range of solid materials. Despite offering more information than its solution-state counterpart, solid-state NMR has not yet achieved a similar level of recognition, owing to the anisotropic interactions that broaden the spectral lines and hinder the extraction of structural information. Here, we describe the methods available to improve the resolution of solid-state NMR spectra and the continuing research in this area. We also highlight areas of exciting new and future development, including recent interest in combining experiment with theoretical calculations, the rise of a range of polarization transfer techniques that provide significant sensitivity enhancements, and the progress of in situ measurements. We demonstrate the detailed information available when studying dynamic and disordered solids and discuss the future applications of solid-state NMR spectroscopy across the chemical sciences.

Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with ¹³C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observed ¹³C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the ¹³C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of ¹³C nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

The initial adsorption process of benzene in double-walled carbon nanotubes studied by in situ solid-state NMR

Adsorption kinetics of benzene in double-walled carbon nanotubes at the very earliest stages were directly monitored for the first time by using in situ solid-state ¹³C NMR.

In situ solid-state (1)H NMR studies of hydration of the solid acid catalyst ZSM-5 in its ammonium form

Hydration of the ammonium form of the solid acid catalyst ZSM-5 is investigated by applying a technique that has been developed recently for carrying out in situ solid-state NMR studies of adsorption processes. From (1)H MAS NMR spectra recorded as a function of time and temperature during the hydration process, insights are established on the nature of the interaction between the adsorbed water molecules and the ammonium cations in the ZSM-5 material. The change in isotropic chemical shift for the ammonium cations is consistent with the formation of N-Hcdots, three dots, centeredO hydrogen bonding with the water molecules. Studies of the adsorption of deuterated water, and dehydration of the hydrated material, are also reported.

Heterogeneous catalysis: enigmas, illusions, challenges, realities, and emergent strategies of design

Predominantly this article deals with the question of how to design new solid catalysts for a variety of industrial and laboratory-orientated purposes. A generally applicable strategy, illustrated by numerous examples, is made possible based on the use of nanoporous materials on to the (high-area) inner surfaces of which well-defined (experimentally and computationally) active centers are placed in a spatially separated fashion. Such single-site catalysts, which have much in common with metal-centered homogenous catalysts and enzymes, enable a wide range of new catalysts to be designed for a variety of selective oxidations, hydrogenations, hydrations and hydrodewaxing, and other reactions that the "greening" of industrial processes demand. Examples are given of new shape-selective, regio-selective, and enantioselective catalysts, many of which operate under mild, environmentally benign conditions. Also considered are some of the reasons why detailed studies of adsorption and stoichiometric reactions at single-crystal surfaces have, disappointingly, not hitherto paved the way to the design and production of many new heterogeneous catalysts. Recent work of a theoretical and high-throughout nature, allied to some experimental studies of well-chosen model systems, holds promise for the identification of new catalysts for simple, but industrially important reactions.