Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3-Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies

The role of solute polarity on methanol-silica interfacial solvation: a molecular dynamics study

The solvation structure and dynamics of small organic molecules at the methanol-silica interface are important for understanding the reaction dynamics in heterogeneous catalysis as well as the transport mechanisms in liquid chromatography. The role of solute polarity in interfacial solvation at the methanol-silica interface has been investigated via umbrella sampling molecular dynamics (MD) simulations and 1,3-propanediol and n-pentane were selected as representative species of polar and apolar solutes. Free energy calculations reveal that it took a similar free energy cost to transfer both solute molecules from the interface to the bulk, despite the huge difference in their polarities. The 1,3-propendiol molecule can penetrate the adsorbed methanol layer and form hydrogen bonds with the silica surface with its backbone perpendicular to the silica surface, resulting in a significant slowdown of translational and rotational dynamics. Further analysis of solvent density and solute orientations suggest that at the minimum of the adsorption free energy curve, the 1,3-propanediol molecule is in a desolvated state, while n-pentane is in a solvated state. The collective effect of solute concentration has also been studied by unbiased MD simulations, and the free energy barriers of transferring the solute molecule from the interface to bulk, as well as the parallel diffusion coefficients at the interface, show a non-monotonic dependence on solute concentration, which can be related to the crowded environment in the interfacial layers.

The selective oxidation of glycerol over metal-free photocatalysts: insights into the solvent effect on catalytic efficiency and product distribution

Solvent effect in selective aerobic oxidation of glycerol over O-doped g-C₃N₄ was studied combining with control experiments and DFT theory calculation. Notably, a novel oxidative esterification of glycerol to yield esters was discovered in CH₃CN.

The joint effect of surface polarity and concentration on the structure and dynamics of acetonitrile solution: a molecular dynamics simulation study

The interfacial properties of the acetonitrile (ACN)-water-silica interface have great implications in both liquid chromatography and heterogeneous catalysis. We have performed molecular dynamics (MD) simulations of ACN and water binary solutions to give a comprehensive study of the collective effect of silica surface polarity and ACN concentration on interfacial structures and dynamics by tuning both surface charges and ACN concentration. MD simulation results indicate that many properties in the liquid-solid interface region undergo a monotonic change as the silica surface is tuned from polar to apolar due to the weakening of hydrogen bonding, while their dependence on ACN concentration is presumably governed by the preferential adsorption of water at the silica surface over ACN. However, at apolar surfaces, the interfacial structures of both water and ACN behave like the liquid-vapor interface, and this resemblance leads to an enrichment of ACN at the interface as well as accelerated dynamics, which is very different from that in the bulk solution. The organization of ACN molecules at both polar and apolar surfaces can be attributed to the amphiphilic nature of ACN, by which the micro-heterogeneity domain formed can persist both in the bulk and at the liquid-solid interface. Moreover, extending diffusion analysis to the second layer of the interface shows that the interfacial transport pathways at polar surfaces are likely very different from that of apolar surfaces. These simulation results give a full spectrum description of the ACN/water liquid-solid interface at the microscopic level and will be helpful for explaining related spectroscopic experiments and understanding the microscopic mechanisms of separation protocols in current chromatography applications.

Oxidative Coupling of Aldehydes with Alcohol for the Synthesis of Esters Promoted by Polystyrene-Supported N-Heterocyclic Carbene: Unraveling the Solvent Effect on the Catalyst Behavior Using NMR Relaxation

Heterogeneous organocatalysts hold great potential as they offer practical advantages in terms of purification and reusability compared with the homogeneous counterpart. A puzzling aspect is the solvent effect on their catalytic performance. Here we propose a new approach whereby T₁/T₂ NMR relaxation measurements are used to evaluate the strength of solvent-surface interactions in the polystyrene-supported N-heterocyclic carbene-promoted oxidation of aldehydes. The results reveal that solvents with high surface affinity lead to a decrease in catalyst activity.

Solvent Water Controls Photocatalytic Methanol Reforming

Understanding the role of different solvent molecules for practical solid-liquid heterogeneous photocatalytic reactions is critical for determining the pathway of the reaction. In this study, the operando nuclear magnetic resonance (NMR) method, combined with density functional theory (DFT) calculations, was employed to evaluate the control effect of solvent water in the photocatalytic reforming mechanism of methanol with a Pt-TiO₂ catalyst. Results indicate that the presence of water effectively promotes the formation of the HCHO intermediate but inhibits the H₂ evolution originating from the switch of the hydrogen source of the H₂ formation from CH₃OH to H₂O. More interestingly, as detected directly in the ab initio molecular dynamics simulation, a small amount of H₂O can dissociate, and the evolved -OH species at Ti_5c site can greatly reduce the C-H activation barrier of -CH₃O, contributing to the formation of oxidation products (e.g., HOCH₂OH and CH₃OCH₂OH) on the Pt-TiO₂ surface.

Proton spin relaxation study with pulsed NMR on the plasticization of Na+ ion-selective electrode membranes prepared from PVCs with different degrees of polymerization

The proton spin–spin relaxation times (T₂) of ion-selective electrode membranes with differences in the polymerization degree of the incorporated poly(vinyl chloride) (PVC) polymers were investigated.

The effect of surface polarity on the structure and collective dynamics of liquid ethanol

Typical configurations of ethanol during polarity modulation.

Origins of complex solvent effects on chemical reactivity and computational tools to investigate them: a review

Origins of solvent-induced enhancement in catalytic reactivity and product selectivity are discussed with computational methods to study them.

Exploiting enhanced paramagnetic NMR relaxation for monitoring catalyst preparation using T1–T2 NMR correlation maps

A new method to characterise the evolution of surface sites during metal-supported catalyst preparation has been developed, which exploits NMR relaxation times and their sensitivity to paramagnetic ions.

Promoting effect of solvent on Cu/CoO catalyst for selective glycerol oxidation under alkaline conditions

Cu/CoO catalysts were employed for the selective oxidation of glycerol in the aqueous phase under basic conditions.

An integrated total neutron scattering – NMR approach for the study of heterogeneous catalysis

By combining total neutron scattering with nuclear magnetic resonance (NeuNMR) in a single experimental apparatus, new insights into the kinetics and mechanisms of heterogeneous catalytic reactions occurring in situ, within the catalyst pore space, are possible.

Exploring catalyst passivation with NMR relaxation

NMR relaxation has recently emerged as a novel and non-invasive tool for probing the surface dynamics of adsorbate molecules within liquid-saturated mesoporous catalysts. The elucidation of such dynamics is of particular relevance to the study and development of solvated green catalytic processes, such as the production of chemicals and fuels from bio-resources. In this paper we develop and implement a protocol using high field ¹H NMR spin-lattice relaxation as a probe of the reorientational dynamics of liquids imbibed within mesoporous oxide materials. The observed relaxation of liquids within mesoporous materials is highly sensitive to the adsorbed surface layer, giving insight into tumbling behaviour of spin-bearing chemical environments at the pore surface. As a prototypical example of relevance to liquid-phase catalytic systems, we examine the mobility of liquid methanol within a range of common catalyst supports. In particular, through the calculation and comparison of a suitable interaction parameter, we assess and quantify changes to these surface dynamics upon replacing surface hydroxyl groups with hydrophobic alkyl chains. Our results indicate that the molecular tumbling of adsorbed methanol is enhanced upon surface passivation due to the suppression of surface-adsorbate hydrogen bonding interactions, and tends towards that of the unrestricted bulk liquid. A complex analysis in which we account for the influence of changing pore structure and surface chemistry upon passivation is discussed. The results presented highlight the use of NMR spin-lattice relaxation measurements as a non-invasive probe of molecular dynamics at surfaces of interest to liquid-phase heterogeneous catalysis.

Effect of paramagnetic species on T1, T2 and T1/T2 NMR relaxation times of liquids in porous CuSO4/Al2O3

This work explores the effect of paramagnetic ions deposited on solid surfaces on T₁, T₂ and T₁/T₂ relaxation times.

Selective glycerol oxidation over ordered mesoporous copper aluminum oxide catalysts

Ordered mesoporous Cu–Al₂O₃ catalysts were employed for the selective oxidation of glycerol into value-added products. Co-solvents such as ethanol, 1-propanol, and tert-butanol strongly improve the reaction kinetics.

Assessing the effect of reducing agents on the selective catalytic reduction of NOx over Ag/Al2O3 catalysts

The selective catalytic reduction (SCR) of NO_x in the presence of different reducing agents over Ag/Al₂O₃ prepared by wet impregnation was investigated by probing catalyst activity and using NMR relaxation time analysis.

Component mobility by a minute quantity of the appropriate solvent as a principal motif in the acceleration of solid-supported reactions

The effects solvents have on fluoride-promoted heterogeneous hydrolysis and alcoholysis of various organo-phosphorus (OP) compounds on the surface of KF/Al2O3 are described. Solid-state magic angle spinning NMR analyses and SEM microscopy have shown that not only is the identity of the solvent important in these reactions but also its quantity. That is, minimal solvent amounts are favored and much more effective in such solid-supported reactions (and maybe generally) than those featuring solvent-free or excess solvent (>50 wt %) conditions. The addition of a minute quantity of the correct solvent (3-10 wt %, molar equivalent scale) avoids reagents leaching from the matrix, permits mobility (mass transport) of the reaction components and ensures their very high local concentration in close proximity to the solid-support large porous surface area. Accordingly, significant acceleration of reactions rates by orders of magnitude is obtained. Fascinatingly, even challenging phosphoesters with poor leaving groups, which were found to be very stable in the presence of solvent-free KF/Al2O3 or wetted with excess water, were efficiently hydrolyzed with a minute amount of this solvent.

Probing hydrogen-bonding in binary liquid mixtures with terahertz time-domain spectroscopy: a comparison of Debye and absorption analysis

Terahertz time-domain spectroscopy is used to explore hydrogen bonding structure and dynamics in binary liquid mixtures, spanning a range of protic–protic, protic–aprotic and aprotic–aprotic systems.

Interpretation of NMR relaxation as a tool for characterising the adsorption strength of liquids inside porous materials

Nuclear magnetic resonance (NMR) relaxation times are shown to provide a unique probe of adsorbate–adsorbent interactions in liquid-saturated porous materials. A short theoretical analysis is presented, which shows that the ratio of the longitudinal to transverse relaxation times (T₁/T₂) is related to an adsorbate–adsorbent interaction energy, and we introduce a quantitative metric e_surf (based on the relaxation time ratio) characterising the strength of this surface interaction. We then consider the interaction of water with a range of oxide surfaces (TiO₂ anatase, TiO₂ rutile, γ-Al₂O₃, SiO₂, θ-Al₂O₃ and ZrO₂) and show that e_surf correlates with the strongest adsorption sites present, as determined by temperature programmed desorption (TPD). Thus we demonstrate that NMR relaxation measurements have a direct physical interpretation in terms of the characterisation of activation energy of desorption from the surface. Further, for a series of chemically similar solid materials, in this case a range of oxide materials, for which at least two calibration values are obtainable by TPD, the e_surf parameter yields a direct estimate of the maximum activation energy of desorption from the surface. The results suggest that T₁/T₂ measurements may become a useful addition to the methods available to characterise liquid-phase adsorption in porous materials. The particular motivation for this work is to characterise adsorbate–surface interactions in liquid-phase catalysis.

Evaluation of the plasticization of ion-selective electrode membranes by pulsed NMR analyses

The potentiometric polymeric membranes for ion-selective electrodes were evaluated by analyses of the proton spin-spin relaxation times T2 with pulsed NMR. The T2 measurements were performed using the Hahn-Echo, Solid-Echo and CPMG pulse sequences. The T2 values and fractions F of each component were obtained by analyses of the FID signals measured with the Hahn-Echo pulse sequence. The softer potentiometric polymeric membrane possessed the main fraction F(L), providing a relatively longer T(2L) value. A linear relationship existed between the weight ratio of the membrane solvent and ln T(2L) (or ln total T2×F). This analysis method could quantify the degree of hardness or softness of the potentiometric polymeric membranes with the differences in the membrane solvent weight. The normalized derivative spectra were acquired from the transverse magnetization M(t) data measured by using the Solid-Echo and CPMG pulse sequences. In the normalized derivative spectra of the potentiometric polymeric membranes, most PVC peaks in the short time region shifted to a larger area of long time regions by plasticization, and the softer potentiometric polymeric membrane incorporating more membrane solvent exhibited a relaxation peak in the relatively longer time region. Thus, the normalized derivative spectra were effective in elucidating the compatibility of the PVC with the membrane solvent.

Assessing the surface modifications following the mechanochemical preparation of a Ag/Al2O3 selective catalytic reduction catalyst

Surface modifications of Ag catalysts prepared using mechanochemistry and wet-impregnation.

Deactivation studies of a carbon supported AuPt nanoparticulate catalyst in the liquid-phase aerobic oxidation of 1,2-propanediol

The aerobic oxidation of 1,2-propanediol in alkaline aqueous solvent over bimetallic AuPt/C catalysts has been studied and catalyst reusability has been assessed.