Experimental Study on Phase Transitions of Carbon Dioxide Confined in Nanopores: Evaporation, Melting, Sublimation, and Triple Point.

Capillary phase transitions (evaporation, melting, and sublimation) and the pore triple point of CO2 confined in MCM-41 mesoporous media with a pore diameter of 3.5 nm have been studied by using an isochoric heating procedure in a high-pressure low-temperature differential scanning calorimeter over a pressure range of 0.5-40.5 bar. The procedure is validated by the agreement between the measured conditions of bulk evaporation/sublimation and literature data. The main finding in this work is that the solid-to-fluid phase transitions of CO2 in MCM-41 shift to temperatures higher than those of the corresponding bulk phase transitions. It is also found that the formation of a solid phase of CO2 in MCM-41 does not require the presence of a liquid or solid in the bulk. The capillary-melting and capillary-evaporation curves approach each other as temperature decreases until they meet at the pore triple point. The effect of pressure on capillary melting temperature is significant at pressures close to the pore triple point. Furthermore, the capillary-melting curve approaches the bulk saturated vapor-pressure curve as temperature increases, thus hinting an agreement with the prediction by molecular dynamics simulation in the literature that the curves eventually intersect each other at a high temperature and pressure. Based on the measured capillary phase transitions, the pore triple-point temperature and pressure of nanoconfined CO2 are bracketed and found to be much lower than those of the bulk triple point.

Phase Transition and Criticality of Methane Confined in Nanopores.

For the first time, the phase transition and criticality of methane confined in nanoporous media are measured. The measurement is performed by establishing an experimental setup utilizing a differential scanning calorimeter capable of operating under very low temperatures as well as high pressures to detect the capillary phase transition of methane inside nanopores. By performing experiments along isochoric cooling paths, both the capillary condensation and the bulk condensation of methane are detected. The pore critical point of nanoconfined methane is also determined and then used to derive the parameters of a previously developed self-consistent equation of state based on the generalized van der Waals partition function. Using these parameters, the equation of state can predict the capillary-condensation curves that agree well with the experimental data.

First-order and gradual phase transitions of ethane confined in MCM-41.

The first-order phase transition of ethane confined in MCM-41, i.e., capillary condensation, has been measured using an isochoric cooling procedure by differential scanning calorimetry (DSC) under conditions ranging from 206 K and 1.1 bar up to the pore critical point (PCP). The PCP has also been determined using the three-line method developed earlier based on the vanishing heat of phase transition. As in the bulk phase, no first-order phase transition can occur above the critical point, which also implies that vapor can transform into liquid gradually by following a path around the critical point through the supercritical region. For the first time, the gradual phase transition is demonstrated with ethane in MCM-41, which is achieved through a multistep process with paths proceeding around the PCP without crossing the capillary-condensation curve. The occurrence of the gradual phase transition in nanopores, thus the confined supercriticality, is confirmed while our consistent DSC measurements are also well demonstrated.

Prototype equation of state for phase transition of confined fluids based on the generalized van der Waals partition function.

A simple self-consistent prototype equation of state (EOS) based on the generalized van der Waals (vdW) partition function has been demonstrated to describe the phase transition of simple fluids in nanopores with uniform size. Different from those commonly presented in the literature, the new EOS does not need an auxiliary equation that is conventionally applied to provide the capillary pressure derived from surface tension. The encouraging performance of the EOS calls for further extension to applications with more complex fluids and porous media.

Isochoric measurement of the evaporation point of pure fluids in bulk and nanoporous media using differential scanning calorimetry.

As a continuation of recent series of work, a new approach applying an isochoric heating process using differential scanning calorimetry (DSC) is introduced to measure the evaporation point of pure fluids in both bulk phase and nanoporous media, as opposed to the previous approach of isochoric cooling to measure the condensation point [X. Qiu et al., Phys. Chem. Chem. Phys., 2018, 20, 26241-26248; X. Qiu et al., Phys. Chem. Chem. Phys., 2019, 21, 224-231]. Though these two approaches must arrive at the same phase-transition point for a specified density of bulk pure fluids, it is not necessarily true for confined fluids due to hysteresis in a temperature range sufficiently far below the bulk critical point. The isochoric heating process allows one to accurately measure the phase transition of non-volatile fluids that exist in liquid phase at relatively high temperatures. As the new approach operates without an inert gas, which substantially dissolves in the test sample at high pressures if the standard isobaric measurement ASTM E1782 is used, application to the high-pressure range is enabled with higher accuracy. This method can also be extended to confined systems, where the evaporation points of both bulk and confined fluids are successively measured in a single run of experiment. The results reveal that capillary evaporation, i.e., evaporation of fluids confined in nanoporous media, occurs at a higher temperature (isobarically), or at a lower pressure (isothermally), than that in bulk only after the liquid in bulk space is completely evaporated. The method introduced in this work paves a new way to study the condensation/evaporation hysteresis of confined fluids as well as the evaporation point of confined fluid mixtures.

Mechanism and catalytic performance for direct dimethyl ether synthesis by CO2 hydrogenation over CuZnZr/ferrierite hybrid catalyst.

Direct synthesis of dimethyl ether (DME) by CO2 hydrogenation has been investigated over three hybrid catalysts prepared by different methods: co-precipitation, sol-gel, and solid grinding to produce mixed Cu, ZnO, ZrO2 catalysts that were physically mixed with a commercial ferrierite (FER) zeolite. The catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption of CO2 (CO2-TPD), temperature programmed desorption of NH3 (NH3-TPD), and temperature programmed H2 reduction (H2-TPR). The results demonstrate that smaller CuO and Cu crystallite sizes resulting in better dispersion of the active phases, higher surface area, and lower reduction temperature are all favorable for catalytic activity. The reaction mechanism has been studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methanol appears to be formed via the bidentate-formate (b-HCOO) species undergoing stepwise hydrogenation, while DME formation occurs from methanol dehydration and reaction of two surface methoxy groups.

Experimental Study on the Criticality of a Methane/Ethane Mixture Confined in Nanoporous Media.

For the first time, the critical region of a methane/ethane mixture confined in nanoporous media (SBA-15) is experimentally investigated using differential scanning calorimetry with an isochoric cooling procedure. The results reveal that the supercritical region of the confined fluid mixture exists at a lower pressure than its counterpart in the bulk space. The shift of the critical region is dependent on the pore size, which is similar to that of pure fluids [Tan et al., J. Phys. Chem. C, 2019, 123, 9824-9830]. Specifically, compared to that in bulk, the shift is greater for smaller pore size. The heat of capillary condensation of this mixture is also discussed. The findings in this work would shed some light on the understanding of confined phase behavior, especially criticality, in investigations toward more complex confined mixtures encountered in practical engineering application, for example, oil and gas recovery from unconventional reservoirs.

A generalized van der Waals model for light gas adsorption prediction in IRMOFs.

By making reasonable simplifications to the structures of isoreticular metal-organic frameworks (IRMOFs) and defining important attractive regions of square-well potential, an adsorption model derived from the generalized van der Waals partition function is proposed to describe the isotherms of light gas adsorption in IRMOFs. The simplification of the structures is based on the geometries of the accessible surfaces and the dimensions of the frameworks, and the locations of the attractive regions are defined by examining the distribution of the adsorbate molecules. Grand Canonical Monte Carlo (GCMC) simulations using the simplified structures with square-well potentials and the complete atomic structures with Lennard-Jones and coulombic potentials are performed and compared to verify the reliability of the simplification. The adsorption model proposed in this work can predict adsorption isotherms of IRMOFs accurately by calculating the adsorbed amounts in different attractive regions of the simplified frameworks. It is also demonstrated that the model with the five parameters fitted to the adsorption isotherm at one temperature can accurately predict the isotherms at other temperatures.

Novel isochoric measurement of the onset of vapor-liquid phase transition using differential scanning calorimetry.

A novel method for measuring the onset of vapor-liquid phase transition applying an isochoric procedure in a high-pressure micro differential scanning calorimeter is introduced for the first time. Isochoric dew-point measurement is used to measure vapor pressures of CO2 at different boiling temperatures and dew points of a methane/ethane gas mixture at different pressures or temperatures. The isochoric two-phase bubble-point measurement, similar to the isobaric method, is also demonstrated to measure vapor pressures of methanol at different boiling temperatures. All results are in agreement with the literature data. The isochoric method is found to be superior to the widely used isobaric method. It can be used to measure the onset of vapor-liquid phase transition for a wide range of substances and mixtures, including the ones for which the isobaric method is inapplicable, and it eliminates difficulties usually encountered in the isobaric method. The proposed method along with the findings of this study can pave the way for experimental measurements of phase equilibria in more complex systems.

Simple and accurate isochoric differential scanning calorimetry measurements: phase transitions for pure fluids and mixtures in nanopores.

Various types of nanopores are encountered in many different engineering and science applications. Due to incomplete understanding of the phase behavior of fluids in nanosize confined space, the improvement of such applications has been largely based on experience and empirical approaches. Therefore, experimental studies on the phase behavior of confined fluids that are simple but accurate are still urgently needed. We recently developed a new isochoric procedure using a Differential Scanning Calorimeter (DSC) to measure the onset of vapor-liquid phase transitions, which has been successfully used in experiments measuring the vapor pressures of pure substances and the dew points of a bulk mixture in the absence of nanopores [Qiu et al., Phys. Chem. Chem. Phys., 2018, 20, 26241-26248]. It is the purpose of this work to extend the new method to confined fluids. To demonstrate the superior ability of the new method, we measure the capillary condensation of CO2 and the dew points of a binary methane/ethane gas mixture confined in SBA-15 with different pore diameters.

Synthesis of methanol from CO2 hydrogenation promoted by dissociative adsorption of hydrogen on a Ga3Ni5(221) surface.

Catalytic carbon dioxide (CO2) hydrogenation to liquid fuels including methanol (CH3OH) has attracted great attention in recent years. In this work, density functional theory (DFT) calculations have been employed to study the reaction mechanisms of CO2 hydrogenation to CH3OH on Ga3Ni5(221) surfaces. The results show that all intermediates except for the O atom prefer to adsorb on Ni sites, and dissociative adsorption of hydrogen (H2) on the Ga3Ni5(221) surface is almost barrierless and highly exothermic, favoring CO2 hydrogenation. Moreover, the presence of Ga indeed enhances the dissociative adsorption of H2, and this is verified by the projected density of states (PDOS) analysis. Importantly, three possible reaction pathways based on formate (HCOO) and hydrocarboxyl (COOH) formations and reverse water gas shift (rWGS) with carbon monoxide (CO) hydrogenation have been discussed. It is found that CO2 reduction to CH3OH in these pathways prefers to occur entirely via the Langmuir-Hinshelwood (L-H) mechanism. COOH generation is the most favorable pathway because the HCOO and rWGS with CO hydrogenation pathways have high energy barriers and the resulting HCOOH intermediate in the HCOO pathway is unstable. In the COOH reaction pathway, CO2 is firstly hydrogenated to trans-COOH, followed by the formation of COH via three isomers of COHOH, its hydrogenation to trans-HCOH, and then the production of CH3OH via a CH2OH intermediate.

Accurate Monte Carlo simulations on FCC and HCP Lennard-Jones solids at very low temperatures and high reduced densities up to 1.30.

Canonical Monte Carlo simulations on face-centered cubic (FCC) and hexagonal closed packed (HCP) Lennard-Jones (LJ) solids are conducted at very low temperatures (0.10 ≤ T(∗) ≤ 1.20) and high densities (0.96 ≤ ρ(∗) ≤ 1.30). A simple and robust method is introduced to determine whether or not the cutoff distance used in the simulation is large enough to provide accurate thermodynamic properties, which enables us to distinguish the properties of FCC from that of HCP LJ solids with confidence, despite their close similarities. Free-energy expressions derived from the simulation results are also proposed, not only to describe the properties of those individual structures but also the FCC-liquid, FCC-vapor, and FCC-HCP solid phase equilibria.

Monte Carlo simulation and equation of state for flexible charged hard-sphere chain fluids: polyampholyte and polyelectrolyte solutions.

The thermodynamic modeling of flexible charged hard-sphere chains representing polyampholyte or polyelectrolyte molecules in solution is considered. The excess Helmholtz energy and osmotic coefficients of solutions containing short polyampholyte and the osmotic coefficients of solutions containing short polyelectrolytes are determined by performing canonical and isobaric-isothermal Monte Carlo simulations. A new equation of state based on the thermodynamic perturbation theory is also proposed for flexible charged hard-sphere chains. For the modeling of such chains, the use of solely the structure information of monomer fluid for calculating the chain contribution is found to be insufficient and more detailed structure information must therefore be considered. Two approaches, i.e., the dimer and dimer-monomer approaches, are explored to obtain the contribution of the chain formation to the Helmholtz energy. By comparing with the simulation results, the equation of state with either the dimer or dimer-monomer approach accurately predicts the excess Helmholtz energy and osmotic coefficients of polyampholyte and polyelectrolyte solutions except at very low density. It also well captures the effect of temperature on the thermodynamic properties of these solutions.

[EMmim][NTf2 ]-a Novel Ionic Liquid (IL) in Catalytic CO2 Capture and ILs' Applications.

Ionic liquids (ILs) have been used for carbon dioxide (CO2 ) capture, however, which have never been used as catalysts to accelerate CO2 capture. The record is broken by a uniquely designed IL, [EMmim][NTf2 ]. The IL can universally catalyze both CO2 sorption and desorption of all the chemisorption-based technologies. As demonstrated in monoethanolamine (MEA) based CO2 capture, even with the addition of only 2000 ppm IL catalyst, the rate of CO2 desorption-the key to reducing the overall CO2 capture energy consumption or breaking the bottleneck of the state-of-the-art technologies and Paris Agreement implementation-can be increased by 791% at 85 °C, which makes use of low-temperature waste heat and avoids secondary pollution during CO2 capture feasible. Furthermore, the catalytic CO2 capture mechanism is experimentally and theoretically revealed.

Kinetics, thermodynamics, and physical characterization of corn stover (Zea mays) for solar biomass pyrolysis potential analysis.

Solar pyrolysis of agricultural waste has huge potential for sustainable production of fuel and chemical feedstock. In this paper, the kinetics, thermodynamics, and physical characterization of corn stover (CS) collected from Wyoming, USA was conducted with respect to solar pyrolysis. The kinetics and thermodynamics of the CS pyrolysis was analyzed in detail using the methods described by KAS (Kissinger-Akahira-Sunose) and FWO (Flynn-Wall-Ozawa), from which the activation energy, Gibbs energy, Arrhenius pre-exponential factor, enthalpy, and entropy were derived.14 other kinetics models based on reaction order, diffusion, nucleation, geometric contraction, power models were also examined, and models based on diffusion was found to be best suited. The CS was used for solar pyrolysis of biomass and the products were analyzed by mass spectroscopy, ICP-MS, GPC, micro-GC, and Elemental analyzer. The results show that CS is suitable for solar pyrolysis to produce chemicals and other fuels.

Catalyst-TiO(OH)2 could drastically reduce the energy consumption of CO2 capture.

Implementing Paris Climate Accord is inhibited by the high energy consumption of the state-of-the-art CO2 capture technologies due to the notoriously slow kinetics in CO2 desorption step of CO2 capture. To address the challenge, here we report that nanostructured TiO(OH)2 as a catalyst is capable of drastically increasing the rates of CO2 desorption from spent monoethanolamine (MEA) by over 4500%. This discovery makes CO2 capture successful at much lower temperatures, which not only dramatically reduces energy consumption but also amine losses and prevents emission of carcinogenic amine-decomposition byproducts. The catalytic effect of TiO(OH)2 is observed with Raman characterization. The stabilities of the catalyst and MEA are confirmed with 50 cyclic CO2 sorption and sorption. A possible mechanism is proposed for the TiO(OH)2-catalyzed CO2 capture. TiO(OH)2 could be a key to the future success of Paris Climat e Accord.

Statistical associating fluid theory coupled with restrictive primitive model extended to bivalent ions. SAFT2: 2. Brine/seawater properties predicted.

Statistical associating fluid theory coupled with restricted primitive model (SAFT2) represents the properties of aqueous multiple-salt solutions, such as brine/seawater. The osmotic coefficients, densities, and vapor pressures are predicted without any additional parameters using the salt hydrated diameters obtained for single-salt solutions. For a given ion composition of brine, the predicted vapor pressure, osmotic coefficient, activity of water, and density are found to agree with the experimental data.

Statistical associating fluid theory coupled with restrictive primitive model extended to bivalent ions. SAFT2: 1. Single salt + water solutions.

Statistical associating fluid theory coupled with the restricted primitive model is extended to multivalent ions by relaxing the range of the square-well width parameter, which leads to a new dispersion term approximation and calls for a new set of salt and ion parameters. This new approximation, referred to as SAFT2, requires a single set of parameters derived from the salt (mean ionic) activity coefficients and liquid densities of single-salt solutions for five cations (Li(+), Na(+), K(+), Ca(2+), Mg(2+)), six anions (Cl(-), Br(-), I(-), NO(3)(-), SO(4)(-2), HCO(3)(-)), and 24 salts. These parameters, in turn, are shown to predict the osmotic coefficients for single salt + water solutions.