Toward Quantifying the Chemical Sensitivity of Nuclear Spin Surface Relaxivity in Mesoporous Media.

Low-field nuclear magnetic resonance (NMR) relaxation is a promising non-invasive technique for characterizing solid-liquid interactions within functional porous materials. However, the ability of the solid-liquid interface to enhance adsorbate relaxation rates, known as the surface relaxivity, in the case of different solvents and reagents involved in various chemical processes has yet to be evaluated in a quantitative manner. In this study, we systematically explore the surface relaxation characteristics of 10 liquid adsorbates (cyclohexane, acetone, water, and 7 alcohols, including ethylene glycol) confined within mesoporous silicas with pore sizes between 6 and 50 nm using low-field (12.7 MHz) two-dimensional 1H T1-T2 relaxation measurements. Functional-group-specific relaxation phenomena associated with the alkyl and hydroxyl groups of the confined alcohols are clearly distinguished; we report the dependence of both longitudinal (T1) and transverse (T2) relaxation rates of these 1H-bearing moieties on pore surface-to-volume ratio, facilitating the quantification and assignment of surface relaxivity values to specific functional groups within the same adsorbate molecule for the first time. We further demonstrate that alkyl group transverse surface relaxivities correlate strongly with the alkyl/hydroxyl ratio of the adsorbates assessed, providing evidence for a simple, quantitative relationship between surface relaxivity and interfacial chemistry. Overall, our observations highlight potential pitfalls in the application of NMR relaxation for the evaluation of pore size distributions using hydroxylated probe molecules, and provide motivation for the exploration of nuclear spin relaxation measurements as a route to adsorbate identity within functional porous materials.

High-Pressure Gravimetric Measurements for Binary Gas Adsorption Equilibria and Comparisons with Ideal Adsorbed Solution Theory (IAST).

Measurements of gas mixture adsorption equilibria at high pressures are important for assessing actual adsorbent selectivities but are often out of reach, given the challenging nature of the required experiments. Here, we report a high-pressure gravimetric binary gas adsorption equilibrium measurement system based on simultaneous gas density and mixture adsorption measurements in a single gas cell coupled to a magnetic-suspension balance. Compared to traditional techniques which rely on analytical measurements of gas composition, this approach does not require any sampling. Adsorption measurements of two gas mixtures (0.500 N2 + 0.500 CH4 and 0.400 N2 + 0.600 CO2, mole fraction) on a commercially available molecular sieve (NaY, sodium molecular sieve type Y) were carried out in the temperature range 282 to 325 K with a pressure up to 10 MPa. A prediction method for the gas mixture adsorption equilibria in a closed system using the ideal adsorbed solution theory (IAST) model was used to compare the experimental results. For binary mixtures of components with similar adsorption capacities (here N2 and CH4), the system can measure the adsorption equilibria at pressures higher than 1.0 MPa and the result agrees well with the IAST model prediction. For two gases with very different adsorption capacities, the uncertainty in the adsorption equilibrium measurement is much larger. The dominant uncertainty source is the gas density measurement, whose uncertainty could potentially be cut to half if the current titanium sinker is replaced with a sinker made of single-crystal silicon and with a larger volume.

Aqueous Solid Formation Kinetics in High-Pressure Methane at Trace Water Concentrations.

Natural gas containing trace amounts of water is frequently liquefied at conditions where aqueous solids are thermodynamically stable. However, no data are available to describe the kinetics of aqueous solid formation at these conditions. Here, we present experimental measurements of both solid formation kinetics and solid-fluid equilibrium for trace concentrations of (12 ± 0.7) ppm water in methane using a stirred, high-pressure apparatus and visual microscopy. Along isochoric pathways with cooling rates around 1 K·min-1, micron-scale aqueous solids were observed to form at subcoolings of (0.3-8.6) K, relative to an average equilibrium melting temperature of (253 ± 1.9) K at (8.9 ± 0.08) MPa; these data are consistent with predicted methane hydrate dissociation conditions within the uncertainty of both the experiment and model. The 36 measured formation events were used to construct a cumulative formation probability distribution, which was then fitted with a model from Classical Nucleation Theory, enabling the extraction of kinetic and thermodynamic nucleation parameters. While the resulting nucleation parameter values were comparable to those published for methane hydrate formation in bulk-water systems, the observed growth kinetics were distinctly different with only a small percentage of the water in the system converting into micron-scale solids over the experimental time scale. These results may help explain how cryogenic heat exchangers in liquefied natural gas facilities can operate for long periods without blockages forming despite being at very high subcoolings for aqueous solids.

A cage-specific hydrate equilibrium model for robust predictions of industrially-relevant mixtures.

Understanding the thermophysical properties and phase behaviour of gas hydrates is essential for industrial applications ranging from energy transport and storage, CO2 capture and sequestration, to gas production from hydrates found on the seabed. Current tools for predicting hydrate equilibrium boundaries typically use van der Waals-Platteeuw-type models which are over-parameterised containing terms with limited physical basis. Here we present a new model for hydrate equilibrium calculations with 40% fewer parameters than existing tools but with equivalent accuracy, including for multicomponent gas mixtures and/or systems with thermodynamic inhibitors. By eliminating multi-layered shells from the model's conceptual basis and focusing on Kihara potential parameters for guest-water interactions specific to each hydrate cavity type, this new model provides insight into the physical chemistry governing hydrate thermodynamics. The model retains the improved description of the empty lattice developed recently by Hielscher et al. but couples the hydrate model with a Cubic-Plus-Association Equation of State (CPA-EOS) to describe fluid mixtures with many more components including inhibitors such as methanol and mono-ethylene glycol used by industry. An extensive database of over 4000 data points was used to train and evaluate the new model and compare its performance against existing tools. The absolute average deviation in temperature (AADT) achieved with the new model is 0.92 K for multicomponent gas mixtures, compared with 1.00 K for the widely-known model of Ballard and Sloan, and 0.86 K for the CPA-hydrates model implemented in the MultiFlash 7.0 software package. With fewer, more physically justified parameters, this new cage-specific model provides a robust basis for improved hydrate equilibrium predictions particularly for industrially-important, multi-component mixtures containing thermodynamic inhibitors.

Surface-Enhanced Raman Scattering Imaging of Cetylpyridinium Chloride Adsorption to a Solid Surface.

Surface active agents (surfactants) have found a variety of critical technological applications, from helping infant lungs breathe to fugitive dust control at industrial sites. Surfactant molecules adsorb to an interface and facilitate a decrease in the surface free energy (interfacial tension) between two immiscible phases. However, a limited number of methods (e.g., holography and fluorescence microscopy) achieved visualization of surfactant molecule distribution in multiphase systems qualitatively. To probe the efficacy and/or adsorption density of surfactants at such interfaces quantitatively, we demonstrate here a direct observation of surfactant adsorption by surface-enhanced Raman scattering (SERS). This work details the development of a research platform to study surfactant adsorption using Raman imaging. The imaging and analysis were successfully benchmarked against conventional interfacial tension measurements and thermodynamic theory employed to estimate surfactant adsorption at equilibrium. This in situ Raman-based experimental method provides a platform to interrogate structure-function relationships that inform the design process for new surfactant species.

Low-Field NMR Relaxation Analysis of High-Pressure Ethane Adsorption in Mesoporous Silicas.

Understanding the behaviour of short-chain hydrocarbons confined to porous solids informs the targeted extraction of natural resources from geological features, and underpins rational developments in separation, storage and catalytic conversion processes. Herein, we report the application of low-field (12.7 MHz) 1 H nuclear magnetic resonance (NMR) relaxation measurements to characterise ethane dynamics within mesoporous silica materials exhibiting mean pore diameters between 6 and 50 nm. Our measurements provide NMR-based adsorption isotherms within the range 25-50 bar and at ambient temperature, incorporating the ethane condensation point (40.7 bar at our experimental temperature of 23.6 °C). The quantitative nature of the acquired data is validated via a direct comparison of NMR-derived excess adsorption capacities with ex situ gravimetric ethane adsorption measurements, which are demonstrated to agree to within 0.2 mmol g-1 of the observed ethane capacity. NMR T2 relaxation time distributions are further demonstrated as a means to decouple interparticle and mesopore dominated adsorption phenomena, with unexpectedly rapid relaxation rates associated with interparticle ethane gas confirmed via a direct comparison with NMR self-diffusion analysis.

The rational design of Li-doped nitrogen adsorbents for natural gas purification.

Separation of nitrogen (N2) and methane (CH4) is one of the most challenging and energy-intensive processes in the natural gas industry, due to their close physico-chemical properties. The quest for an effective N2-selective adsorbent has long been the focus of research; however, the results have been sparse. In this work, a first-principle study has been used to construct and investigate Li-doped polycyclic aromatic hydrocarbons (PAHs) for N2 rejection in natural gas purification. We doped lithium on a series of linear/nonlinear PAHs consisting of two to six benzene rings. The adsorption affinity of the Li-doped organic molecular systems toward N2 and CH4 was evaluated by calculating the interaction energy using density functional theory. From the gas adsorption selectivities for different Li-doped PAHs, Li-doped phenanthrene and chrysene showed the highest N2 over CH4 equilibrium selectivities, with values of 119.7 and 80.8, respectively. It was found that the Li atom enabled the π bond of the aromatic substrate to interfere with the N2 lowest unoccupied molecular orbital, resulting in strong physisorption of N2. These results indicate the high potential of Li-doped phenanthrene and chrysene for N2 removal from natural gas.

Flexible Adsorbents at High Pressure: Observations and Correlation of ZIF-7 Stepped Sorption Isotherms for Nitrogen, Argon, and Other Gases.

Stepped adsorption isotherms with desorption hysteresis were measured for nitrogen, argon, ethane, carbon dioxide, and methane at pressures up to 17 MPa on zeolitic imidazolate framework-7 (ZIF-7) using a gravimetric sorption analyzer. Such stepped sorption isotherms have not been previously reported for nitrogen or argon on ZIF-7, and required the application of pressures as high as 15 MPa to trigger the ZIF-7 structural phase transition at temperatures around 360 K. The stepped hysteretic sorption isotherms measured for carbon dioxide, methane, and ethane were consistent with previous observations reported in the literature. To correlate these stepped hysteretic sorption isotherms, a semi-empirical model was developed by combining a three-parameter Langmuir equation to describe the Type I aspect of the isotherm, with a model designed to describe the temperature-dependent ZIF-7 structural phase transition. Excellent fits of the combined adsorption and desorption branches were achieved by adjusting nine parameters in the temperature-dependent model, with root-mean-square deviations within 2.5 % of the highest measured adsorption capacity. Each parameter of the new semi-empirical model has a physical basis, allowing them to be estimated or compared independently.

Low-field NMR relaxation-exchange measurements for the study of gas admission in microporous solids.

Understanding the uptake and storage of gases by microporous materials is important for our future energy security. As such, we demonstrate here the application of two-dimensional NMR relaxation experiments for probing the admission and corresponding exchange dynamics of methane within microporous zeolites. Specifically, we report low-field (12.7 MHz) 1H NMR relaxation-exchange correlation measurements of methane within commercial LTA zeolites (3A and 4A) at 25 and 35 bar and ambient temperature. Our results demonstrate the clear identification of bulk-pore and pore-pore exchange processes within zeolite 4A, facilitating the calculation and comparison of effective exchange rate dynamics across varying diffusion length scales and gas pressures. Additional data acquired for zeolite 3A reveals the sensitivity of NMR relaxation phenomena to size-exclusive gas admission phenomena, illustrating the potential of benchtop NMR protocols for material screening applications.

Hydrate nucleation and growth on water droplets acoustically-levitated in high-pressure natural gas.

Hydrate formation was studied using water droplets acoustically levitated in high-pressure natural gas. Despite the absence of solid interfaces, the droplets' area-normalised nucleation rate was about four times faster than in steel autoclave measurements with interfacial areas roughly 200 times larger. Multiple stages of stochastic, template-free hydrate growth were observed.

Accurate High-Pressure Measurements of Carbon Monoxide's Electrical Properties.

Accurate measurements of carbon monoxide's electrical properties were carried out at high pressure for the first time enabling stringent comparisons with theoretical values calculated ab initio. Dielectric permittivity measurements were conducted utilising a microwave re-entrant cavity resonator over the temperature range from (255 to 313) K and at pressures up to 8 MPa with a relative combined expanded uncertainty (k=2) less than or equal to 52 ppm. The new data enable carbon monoxide's molar polarizability to be correlated within 0.5 %, significantly improving upon existing literature data, which have a relative scatter of about 10 %. The measured molecular polarizability and electric dipole moment of carbon monoxide were determined to be 2.176×10-40  C2 m2 J-1 and 0.107 D. Literature values from ab initio calculations for these properties are within 0.28 % and 3.9 %, respectively, of the measured quantities. Moreover, our measurement of the electric dipole moment at finite temperature agrees within 2.2 % with the value derived from accurate spectroscopic measurements for the ground rovibrational state. The second dielectric virial coefficient of carbon monoxide was determined experimentally for the first time to be bϵ =(1.015±0.044) cm3 mol-1 , which compares reasonably with ab initio estimates.

Gas Hydrate Formation Probability Distributions: The Effect of Shear and Comparisons with Nucleation Theory.

Gas hydrate formation is a stochastic phenomenon of considerable significance for any risk-based approach to flow assurance in the oil and gas industry. In principle, well-established results from nucleation theory offer the prospect of predictive models for hydrate formation probability in industrial production systems. In practice, however, heuristics are relied on when estimating formation risk for a given flowline subcooling or when quantifying kinetic hydrate inhibitor (KHI) performance. Here, we present statistically significant measurements of formation probability distributions for natural gas hydrate systems under shear, which are quantitatively compared with theoretical predictions. Distributions with over 100 points were generated using low-mass, Peltier-cooled pressure cells, cycled in temperature between 40 and -5 °C at up to 2 K·min-1 and analyzed with robust algorithms that automatically identify hydrate formation and initial growth rates from dynamic pressure data. The application of shear had a significant influence on the measured distributions: at 700 rpm mass-transfer limitations were minimal, as demonstrated by the kinetic growth rates observed. The formation probability distributions measured at this shear rate had mean subcoolings consistent with theoretical predictions and steel-hydrate-water contact angles of 14-26°. However, the experimental distributions were substantially wider than predicted, suggesting that phenomena acting on macroscopic length scales are responsible for much of the observed stochastic formation. Performance tests of a KHI provided new insights into how such chemicals can reduce the risk of hydrate blockage in flowlines. Our data demonstrate that the KHI not only reduces the probability of formation (by both shifting and sharpening the distribution) but also reduces hydrate growth rates by a factor of 2.

Hydrate Shell Growth Measured Using NMR.

Benchtop nuclear magnetic resonance (NMR) pulsed field gradient (PFG) and relaxation measurements were used to monitor the clathrate hydrate shell growth occurring in water droplets dispersed in a continuous cyclopentane phase. These techniques allowed the growth of hydrate inside the opaque exterior shell to be monitored and, hence, information about the evolution of the shell's morphology to be deduced. NMR relaxation measurements were primarily used to monitor the hydrate shell growth kinetics, while PFG NMR diffusion experiments were used to determine the nominal droplet size distribution (DSD) of the unconverted water inside the shell core. A comparison of mean droplet sizes obtained directly via PFG NMR and independently deduced from relaxation measurements showed that the assumption of the shell model-a perfect spherical core of unconverted water-for these hydrate droplet systems is correct, but only after approximately 24 h of shell growth. Initially, hydrate growth is faster and heat-transfer-limited, leading to porous shells with surface areas larger than that of spheres with equivalent volumes. Subsequently, the hydrate growth rate becomes mass-transfer-limited, and the shells become thicker, spherical, and less porous.

Application of Raman Spectroscopy for Sorption Analysis of Functionalized Porous Materials.

Functionalized porous materials could play a key role in improving the efficiency of gas separation processes as required by applications such as carbon capture and storage (CCS) and across the hydrogen value chain. Due to the large number of different functionalizations, new experimental approaches are needed to determine if an adsorbent is suitable for a specific separation task. Here, it is shown for the first time that Raman spectroscopy is an efficient tool to characterize the adsorption capacity and selectivity of translucent functionalized porous materials at high pressures, whereby translucence is the precondition to study mass transport inside of a material. As a proof of function, the performance of three silica ionogels to separate an equimolar (hydrogen + carbon dioxide) gas mixture is determined by both accurate gravimetric sorption measurements and Raman spectroscopy, with the observed consistency establishing the latter as a novel measurement technique for the determination of adsorption capacity. These results encourage the use of the spectroscopic approach as a rapid screening method for translucent porous materials, particularly since only very small amounts of sample are required.

Low-Field Functional Group Resolved Nuclear Spin Relaxation in Mesoporous Silica.

Solid-fluid interactions underpin the efficacy of functional porous materials across a diverse array of chemical reaction and separation processes. However, detailed characterization of interfacial phenomena within such systems is hampered by their optically opaque nature. Motivated by the need to bridge this capability gap, we report low-magnetic-field two-dimensional (2D) 1H nuclear spin relaxation measurements as a noninvasive probe of adsorbate identity and interfacial dynamics, exploring the relaxation characteristics exhibited by liquid hydrocarbon adsorbates confined to a model mesoporous silica. For the first time, we demonstrate the capacity of this approach in distinguishing functional group-specific relaxation phenomena across a diverse range of alcohols and carboxylic acids employed as solvents, reagents, and liquid hydrogen carriers, with distinct relaxation responses assigned to the alkyl and hydroxyl moieties of each confined liquid. Uniquely, this relaxation behavior is shown to correlate with adsorbate acidity, with the observed relationship rationalized on the basis of surface-adsorbate proton-exchange dynamics. Our results demonstrate that nuclear spin relaxation provides a molecular-level perspective on sorbent/sorbate interactions, motivating the exploration of such measurements as a unique probe of adsorbate identity within optically opaque porous media.

Minimum ignition energies and laminar burning velocities of ammonia, HFO-1234yf, HFC-32 and their mixtures with carbon dioxide, HFC-125 and HFC-134a.

Given the safety issues associated with flammability characteristics of alternative environmentally-friendly refrigerants, it is vital to establish measurement systems to accurately analyse the flammability of these mildly flammable refrigerants. In this study, we used a customised Hartmann bomb analogue to measure the minimum ignition energy (MIE) and laminar burning velocity (BV) for refrigerant/air mixtures of pure ammonia (R717), R32, R1234yf and mixtures of R32 and R1234yf with non-flammable refrigerants of R134a, R125 and carbon dioxide (R744). The MIEs of R717, R32, and R1234yf were measured at an ambient temperature of 24 °C to be (18.0 ± 1.4), (8.0 ± 1.5) and (510 ± 130) mJ at equivalence ratios of 0.9, 1.27 and 1.33, respectively. Adding the non-flammable refrigerants R134a, R125 and R744 along with R32 at volumetric concentrations of 5% each to R1234yf reduced the latter compound's flammability and increased its MIE by one order of magnitude. The laminar burning velocities of pure R717 and R32 were measured at an equivalence ratio of 1.1 using the flat flame method and found to be 8.4 and 7.4 cm/s, respectively. Adding 5% R1234yf to R32 decreased the laminar burning velocity by 11%, while a further 5% addition of R1234yf resulted in a decrease of over 30% in the laminar burning velocity.

Temperature dependence of adsorption hysteresis in flexible metal organic frameworks.

"Breathing" and "gating" are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli. These effects are often associated with eminent steps and hysteresis in sorption isotherms. Despite significant mechanistic studies, the accurate description of stepped isotherms and hysteresis remains a barrier to the promised applications of flexible MOFs in molecular sieving, storage and sensing. Here, we investigate the temperature dependence of structural transformations in three flexible MOFs and present a new isotherm model to consistently analyse the transition pressures and step widths. The transition pressure reduces exponentially with decreasing temperature as does the degree of hysteresis (c.f. capillary condensation). The MOF structural transition enthalpies range from +6 to +31 kJ·mol-1 revealing that the adsorption-triggered transition is entropically driven. Pressure swing adsorption process simulations based on flexible MOFs that utilise the model reveal how isotherm hysteresis can affect separation performance.

Influence of Mineral Composition of Chars Derived by Hydrothermal Carbonization on Sorption Behavior of CO2, CH4, and O2.

The doping of SiO2 and Fe2O3 into hydrochars that were produced by the hydrothermal carbonization of cellulose was studied with respect to its impact on the resulting surface characteristics and sorption behavior of CO2, CH4, and O2. During pyrolysis, the structural order of the Fe-doped char changed, as the fraction of highly ordered domains increased, which was not observed for the undoped and Si-doped chars. The Si doping had no apparent influence on the oxidation temperature of the hydrochar in contrast to the Fe-doped char where the oxidation temperature was reduced because of the catalytic effect of Fe. Both dopants reduced the micro-, meso- and macroporous surface areas of the chars, although the Fe-doped chars had larger meso- and macroporosity than the Si-doped char. However, the increased degree in the structural order of the carbon matrix of the Fe-doped char reduced its microporosity relative to the Si-doped char. The adsorption of CO2 and CH4 on the chars at temperatures between 273.15 and 423.15 K and at pressures up to 115 kPa was slightly inhibited by the Si doping but strongly suppressed by the Fe doping. For O2, however, the Si doping promoted the observed adsorption capacity, while Fe doping also showed an inhibiting effect.

An optimal trapdoor zeolite for exclusive admission of CO2 at industrial carbon capture operating temperatures.

High purity molecular trapdoor chabazite with an optimal Si/Al ratio (1:9) was prepared from fly ash. Gas adsorption isotherms and binary breakthrough experiments show dramatically large selectivities for CO2 over N2 and CH4, which are the highest among physisorbents at operating temperatures suitable for postcombustion carbon capture and natural gas separations.