High-pressure Data Research Articles

Supercritical reactors for the production of advanced bio-fuels: A review

Advanced biofuels, when used as components in fuels, offer advantages over fossil-based hydrocarbons and conventional biofuels to realize highly efficient and clean propulsion systems. Among the several chemical routes to produce biofuels, special attention is given to gas-liquid heterogeneous catalyzed reactions to reduce carbohydrate derived platform building blocks. Supercritical reactors offer opportunities to enhance yield of these type of reactions. In this review, we present studies that take advantage of supercritical technologies to reduce platform biobased molecules. In addition, we discuss related high-pressure phase equilibrium experimental data and modeling tools available, as well as the gaps and needs, for engineering supercritical reactors to produce advanced biofuels. Due to the possibility of tuning solubility with density, opportunities arise in the use of supercritical fluids in combined reaction and separation processes. In this regard, the design of less energy intensive processes is needed to boost the production of advanced biofuels.

Redox Behavior of a Dinuclear Ruthenium(II) Complex Bearing an Uncommon Bridging Ligand: Insights from High-Pressure Electrochemistry.

A dinuclear ruthenium complex bridged by 2,3,5,6-pyrazinetetracarboxylic acid (μ-LH22-) was synthesized and characterized by X-ray crystallography, cyclic voltammetry under ambient and elevated pressures, electron paramagnetic resonance (EPR) and UV/vis-NIR (NIR = near-infrared) spectroelectrochemistry, pulse radiolysis, and computational methods. We probed for the first time in the field of mixed-valency the use of high-pressure electrochemical methods. The investigations were directed toward the influence of the protonation state of the bridging ligand on the electronic communication between the ruthenium ions, since such behavior is interesting in terms of modulating redox chemistry by pH. Starting from the [RuII(μ-LH22-)RuII]0 configuration, which shows an intense metal-to-ligand charge transfer absorption band at 600 nm, cyclic voltammetry revealed a pH-independent, reversible one-electron reduction and a protonation-state-dependent (proton coupled electron transfer, PCET) reversible oxidation. Deeper insight into the electrode reactions was provided by pressure-dependent cyclic voltammetry up to 150 MPa, providing insight into the conformational changes, the protonation state, and the environment of the molecule during the redox processes. Spectroelectrochemical investigations (EPR, UV/vis-NIR) of the respective redox reactions suggest a ligand-centered radical anion [RuII(μ-LH2•3-)RuII]- upon reduction (EPR Δg = 0.042) and an ambiguous, EPR-silent one-electron oxidized state. In both cases, the absence of the otherwise typical broad intervalence charge transfer bands in the NIR region for mixed-valent complexes support the formulation as radical anionic bridged compound. However, on the basis of high-pressure electrochemical data and density functional theory calculations the one-electron oxidized form could be assigned as a charge-delocalized [RuII.5(μ-LH22-)RuII.5]+ valence tautomer rather than [RuIII(μ-LH2•3-)RuIII]+. Deprotonation of the bridging ligand causes a severe shift of the redox potential for the metal-based oxidation toward lower potentials, yielding the charge-localized [RuIII(μ-LH3-)RuII]0 complex. This PCET process is accompanied by large intrinsic volume changes. All findings are supported by computational methods (geometry optimization, spin population analysis). For all redox processes, valence alternatives are discussed.

Sound velocities of skiagite–iron–majorite solid solution to 56 GPa probed by nuclear inelastic scattering

High-pressure experimental data on sound velocities of garnets are used for interpretation of seismological data related to the Earth’s upper mantle and the mantle transition zone. We have carried out a Nuclear Inelastic Scattering study of iron-silicate garnet with skiagite (77 mol%)–iron–majorite composition in a diamond anvil cell up to 56 GPa at room temperature. The determined sound velocities are considerably lower than sound velocities of a number of silicate garnet end-members, such as grossular, pyrope, Mg–majorite, andradite, and almandine. The obtained sound velocities have the following pressure dependencies: V p [km/s] = 7.43(9) + 0.039(4) × P [GPa] and V s [km/s] = 3.56(12) + 0.012(6) × P [GPa]. We estimated sound velocities of pure skiagite and khoharite, and conclude that the presence of the iron–majorite component in skiagite strongly decreases V s . We analysed the influence of Fe3+ on sound velocities of garnet solid solution relevant to the mantle transition zone and consider that it may reduce sound velocities up to 1% relative to compositions with only Fe2+ in the cubic site.

Orientation of exsolution lamellae in mantle xenolith pyroxenes and implications for calculating exsolution pressures

Exsolution lamellae in pyroxene occurring in mantle lherzolite xenoliths in Cenozoic basalts from the Mingxi area, Fujian Province, China, have been investigated by electron backscattered diffraction (EBSD) to determine epitaxial relationships between host and lamellae. Clinopyroxene (diopside) hosts developed two sets of lamellae: one of orthopyroxene (pigeonite-enstatite) lamellae and the other of clinopyroxene (augite) lamellae. Orthopyroxene (enstatite) hosts developed a single set of clinopyroxene (augite) lamellae. A zone crossing method has been used to determine Miller indices of lamellae, which appear as linear traces on thin sections tested by EBSD. In clinopyroxene hosts, the index of orthopyroxene lamellae is (100) and that of clinopyroxene lamellae is ~(401) at 22° to the c -axis. In orthopyroxene hosts, the index of clinopyroxene lamellae is (100). Published high-pressure crystallographic data for compositions approximating those of the lamellae and host are used to compare cell parameters of lamellae and hosts at different pressures. Exact phase boundary theory is applied to estimate the exsolution pressure, and the data uncertainty of composition, cell parameters and orientation of the lamellae have been analyzed. Uncertainties of composition and cell parameters give rise to only small uncertainties in the exsolution pressure, but that of the orientation of the lamellae generates large uncertainty. Independent high accuracy measurement of the angle between lamellae and c -axis by TEM or other techniques combined with exact phase boundary theory would give more reliable estimates of exsolution pressure.

High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures.

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase II^{'}, unique to deuterium and distinct from the known phase II. Phase II^{'} of D_{2} is not observed in hydrogen, making it the only phase that does not exist in both isotopes and occupies a significant part of P-T space from ∼25 to 110GPa and below 125K. For H_{2}, the data show that below 30K the transition to phase II happens at as low as 73GPa. The transformation from phase II to III commences at around ∼155 GPa and is completed by 170GPa with the average pressure of ∼160 GPa being slightly higher than previously thought. The updated phase diagrams of H_{2} and D_{2} demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of both elements.

Single-crystal equations of state of magnesiowüstite at high pressures

Solid solutions of (Mg,Fe)O with high iron enrichment may be an important component of ultralow-velocity zones at Earth’s core-mantle boundary. However, to date there have been few high-precision studies on the elastic properties of these materials. In this study we present results on the compression of (Mg_(0.22)Fe_(0.78))O magnesiowustite in both neon and helium pressure media using single-crystal diffraction to ~55 GPa. In addition, our sample was characterized by time-domain synchrotron Mossbauer spectroscopy at ambient pressure using an extended time range that resulted in vastly improved energy resolution. The combination of these high-resolution techniques tightly constrains the presence of a defect-structure component at room pressure due to 4.7 mol% tetrahedrally coordinated ferric iron, resulting in a renormalized composition of (Mg_(0.215)Fe_(0.762)□_(0.023))O. Both high-pressure diffraction data sets are well described by a third-order Birch-Murnaghan equation of state. The best fit-parameters for a crystal with cubic structure in helium are K_(0T) = 148(3) GPa, K′_(0T) = 4.09(12), and V0 = 78.87(6) A^3. Increasing differential stress in the neon-containing sample chamber was correlated with increasing apparent distortion of the initially cubic unit cell, requiring a lower-symmetry hexagonal cell to fit the data above ~20 GPa. For fit equations of state, we determine the pressure-dependent correlation ellipses for the equation of state parameters and compare with previously published single-crystal diffraction data from (Mg,Fe)O crystals in a helium medium. We make two main observations from the data sets using a helium pressure medium: K_(0T) decreases as a function of increasing iron content from periclase to wustite and K′_(0T) is consistent with an approximately constant value of 4.0 that is independent of iron content, at least up to the iron concentration measured here. In combination with previously reported thermal parameters, we compute the density of magnesiowustite with this composition at core-mantle boundary conditions and discuss the implications.

Density and viscosity of propylene glycol at high temperatures and high pressures

The density and viscosity of propylene glycol have been simultaneously measured over the temperature range from (298–452) K and at pressures up to 245 MPa using a combined method of hydrostatic weighing and falling-body techniques, respectively. The combined expanded uncertainty of the density, pressure, temperature, and viscosity measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be (0.15–0.30) %, 0.05%, 0.02 K, and (1.5–2.0) %, respectively. The measured high-temperature and high-pressure PVT data for propylene glycol were used to develop theoretically based Tait-type equation of state (EOS). The measured viscosities were used to develop theoretically based viscosity correlation models (Arrhenius-Andrade and VFT type models) with the pressure dependent parameters.

Glass Transition Temperature and Density Scaling in Cumene at Very High Pressure.

We present a new method that allows direct measurements of the glass transition temperature T_{g} at pressures up to 4.55GPa in the glass-forming liquid cumene (isopropylbenzene). This new method uses a diamond anvil cell and can measure T_{g} at pressures of 10GPa or greater. Measuring T_{g} at the glass→liquid transition involves monitoring the disappearance of pressure gradients initially present in the glass, but also takes advantage of the large increase in the volume expansion coefficient α_{p} at T_{g} as the supercooled or superpressed liquid is entered. Accurate T_{g}(P) values in cumene allow us to show that density scaling holds along this isochronous line up to pressures much higher than any previous study, corresponding to a density increase of 29%. Our results for cumene over this huge compression range yield ρ^{γ}/T=C, where C is a constant and where γ=4.77±0.02 for this nonassociated glass-forming system. Finally, high-pressure cumene viscosity data from the literature taken at much lower pressures and at several different temperatures, corresponding to a large dynamic range of nearly 13 orders of magnitude, are shown to superimpose on a plot of η vs ρ^{γ}/T for the same value of γ.

High-temperature, high-pressure viscosities and densities of toluene

Toluene viscosities and densities, measured with a windowed, variable-volume, rolling-ball viscometer/densimeter (RBVD), are reported at temperatures from 296–535K and pressures to 300MPa. The combined expanded uncertainty in the reported viscosities and densities are 4.0% and 0.8% of the measured values respectively, each with a coverage factor, k=2. Densities obtained with the RBVD exhibit an average absolute percent deviation (ΔAAD) of 0.20% against densities calculated with the Lemmon and Span Equation of State. Viscosities obtained with the RBVD exhibit ΔAAD values of less than 4.0% for data from 18 of 20 available literature sources for high pressure data. The reported viscosities are generally in good agreement with calculated viscosities from the reference equation of Avgeri et al. The reported toluene viscosities extend the available database from 423–535K and 40–300MPa. Toluene viscosities are also reasonably correlated with the empirical Tait equation that allows for reliable interpolation of the data. Free Volume Theory (FVT) matches the reported viscosities to within 3.4% ΔAAD using FVT parameters fit to a viscosity data set limited to 373K and using densities calculated with the Perturbed–Chain Statistical Association Fluid Theory (PC-SAFT). An improved fit of the viscosity data is obtained if temperature-dependent, FVT parameters are used. The challenge remains to predict the temperature dependence in the FVT parameters from first principles.

High-pressure phase diagram, structural transitions, and persistent nonmetallicity of BaBiO3 : Theory and experiment

${\mathrm{BaBiO}}_{3}$ is a mixed-valence perovskite, which escapes the metallic state through a Bi valence (and Bi-O bond) disproportionation or CDW distortion, resulting in a semiconductor with a gap of 0.8 eV at zero pressure. The evolution of structural and electronic properties at high pressure is, however, largely unknown. Pressure, one might have hoped, could reduce the disproportionation, making the two Bi ions equivalent and bringing the system closer to metallicity or even to superconductivity, such as is attained at ambient pressure upon metal doping. We address the high-pressure phase diagram of pristine ${\mathrm{BaBiO}}_{3}$ by ab initio DFT calculations based on GGA and hybrid functionals in combination with crystal structure prediction methods based on evolutionary algorithms, molecular dynamics, and metadynamics. The calculated phase diagram from 0 to 50 GPa indicates that pristine ${\mathrm{BaBiO}}_{3}$ resists metallization under pressure, undergoing instead at room temperature structural phase transitions from monoclinic I2/m to nearly tetragonal P-1 at 7 GPa, possibly to monoclinic C2/m at 27 GPa, and to triclinic P1 at 43 GPa. Remarkably, all these phases sustain and in fact increase the inequivalence of two Bi neighboring sites and of their Bi-O bonds and, in all cases except semimetallic C2/m, the associated insulating character. We then present high-pressure resistivity data, which generally corroborate these results, and show that the insulating character persists at least up to 80 GPa, suggesting that the C2/m phase is probably an artifact of the small computational cell.

New Experimental Data and Modeling of Glymes: Toward the Development of a Predictive Model for Polyethers

The design and optimization of industrial processes relies on the availability of robust and accurate models and equations of state (EoSs). Considering the further advancement of the use of soft-SAFT (one version of statistical associating fluid theory) EoS, toward its implementation in industrial processes, a methodology to determine the molecular model and transferable molecular parameters of glymes is discussed herein. In addition to the commonly used vapor pressure and saturated liquid densities, the description of the temperature and pressure effect is improved by including one additional density–pressure and one isothermal compressibility isotherm (both at 323 K) for the molecular parameter optimization. For the guiding of the selection and optimization of the soft-SAFT EoS molecular model and parameters, new high-pressure density data (pρT) and derived properties, such as isothermal compressibility and isobaric thermal expansion, of eight glymes (glycol ethers) have been determined in wide ranges o...

LaB Work Function and Structural Stability under High Pressure**Supported by the National Natural Science Foundation of China under Grant Nos 11274030 and 11474281.

The work functions of the (110) and (100) surfaces of LaB are determined from ambient pressure to 39.1 GPa. The work function of the (110) surface slowly decreases but that of the (100) surface remains at a relatively constant value. To determine the reason for this difference, the electron density distribution (EDD) is determined from high-pressure single-crystal x-ray diffraction data by the maximum entropy method. The EDD results show that the chemical bond properties in LaB play a key role. The structural stability of LaB under high pressure is also investigated by single-crystal x-ray diffraction. In this study, no structural or electronic phase transition is observed from ambient pressure to 39.1 GPa.

Pressure and Temperature Study on the Structural Stability of GdNbO4:Eu3+

Lanthanide niobate is an excellent nonlinear optical and high dielectric constant material. However, development and application of these novel properties depend on an understanding of its structural stability. In this work, Raman and luminescence spectra of GdNbO4 with different Eu3+ ions doping concentrations under high pressure and low temperature were obtained. The evolution of Raman spectra indicated that an irreversible structural phase transition of GdNbO4 from monoclinic phase to M′-fergusonite phase occurred at 6.2 GPa, and the phase transition was completed at about 13.1 GPa. An obvious discontinuity on Eu3+ ions luminescence intensity ratio between the 5D0 → 7F2 and the 5D0 → 7F1 transition lines was observed with increasing pressure. The results indicated that the site symmetry of Eu3+ ions in GdNbO4:Eu3+ was changed, which was in agreement with high-pressure Raman spectra data. Meanwhile, Raman and luminescence spectra showed that the crystal structure remained stable in the temperature range...

Relativistic coupled-cluster and density-functional studies of argon at high pressure

The equation of state $P(V,T)$ for solid argon is determined by the calculation of accurate static and vibrational terms in the free energy. The static component comes from a quantum theoretical many-body expansion summing over all energetic contributions from two-, three-, and four-body fragments treated with relativistic coupled cluster theory, while the lattice vibrations are described at an anharmonic level including two- and three-body forces as well as temperature effects. The dynamic part is calculated within the Debye and Einstein approximation, as well as by a more accurate phonon treatment where the vibrational motions in the lattice are coupled. Our results are in good agreement with room-temperature high-pressure experimental data up to $\ensuremath{\sim}20$ GPa. In the 20--100 GPa pressure range, however, we see considerable deviations between experiment and theory, perhaps indicating missing four-body contributions (beyond the quadruple dipole terms included here), missing five and higher-body effects, and the need to go beyond the coupled cluster singles-doubles with perturbative triples treatment in such higher-body forces. This contrasts with the results for solid neon, where excellent agreement has been achieved taking only two- and three-body forces into account [P. Schwerdtfeger and A. Hermann, Phys. Rev. B 80, 064106 (2009)]. We demonstrate that the phase transition from fcc to hcp cannot account for the large discrepancies observed. Density functional calculations give very mixed results in the high-pressure region, but some functionals such as optB88-vdW (proposed by Lundqvist and co-workers) describe the many-body forces in argon reasonably well over the range of pressures investigated. Theoretical investigations of the heavier rare gas solids reaching experimental accuracy in the high-pressure regime therefore remain a significant challenge.

Critical behavior of Mg1–xFexO at the pressure-induced iron spin-state crossover

We present a high-pressure study of $\mathrm{M}{\mathrm{g}}_{1--x}\mathrm{F}{\mathrm{e}}_{x}\mathrm{O}$ (ferropericlase, Fp) single crystals $0.04(1)\ensuremath{\le}x\ensuremath{\le}0.33(1)$ with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ of the ``FeO high spin lattice contribution'' in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter ${a}_{C}$ (defined as the minimum ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of $\mathrm{F}{\mathrm{e}}^{2+}$.

Studies of the mechanical and extreme hydrothermal properties of periodic mesoporous silica and aluminosilica materials

In order to assess the suitability of mesoporous materials for applications in energy harvesting/storage processes occurring under extreme conditions, their mechanical, thermal and hydrothermal properties need to be fully investigated. In this study, the bulk mechanical and extreme hydrothermal properties of periodic mesoporous SBA-15 type silica and SBA-15 type aluminosilica (Al-SBA-15) were investigated using in situ small angle x-ray scattering (SAXS). In situ SAXS measurements were made on dry mesoporous SBA-15 silica and Al-SBA-15 aluminosilica samples as a function of pressure (at room temperature) to ∼ 12 GPa and on the same mesoporous materials under extreme hydrothermal conditions (to 255 °C and ∼114 MPa) using the diamond anvil cell (DAC). The analyses of the high-pressure SAXS data indicate that the mesoporous Al-SBA-15 aluminosilica has substantially greater bulk mechanical stability (isothermal bulk modulus κ = 34.7(4.5) GPa) than the mesoporous SBA-15 silica (κ = 12.0(3.0) GPa). Our molecular dynamics (MD) simulations are able to accurately model the bulk mechanical stability properties of mesoporous SBA-15 silica but underestimate the same properties of Al-SBA-15 aluminosilica. Analysis of the in situ SAXS data measured under extreme hydrothermal conditions indicates swelling of the pore walls due to water incorporation that is more significant in mesoporous Al-SBA-15 aluminosilica (∼2x) than in SBA-15 silica. In addition, the Al-SBA-15 aluminosilica clearly exhibits superior hydrothermal stability compared to SBA-15 silica under the extreme experimental temperature and pressure conditions.

High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestanding superconducting ($T_{c}$ = 4.3\,K) boron doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30\,GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients and the grain boundary components undergo an irreversible phase change at 14\,GPa. We show that the blue shift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_{0}$=510$\pm$28\,GPa. The comparative analysis of high-pressure data clarified that the sp$^{2}$ carbons in the grain boundaries transform into hexagonal diamond.

Experimental and topological determination of the pressure-temperature phase diagram of morniflumate, a pharmaceutical ingredient with anti-inflammatory properties

The pressure-temperature phase diagram of morniflumate (niflumic acid β-morpholinoethyl ester) has been obtained by high-pressure thermal analysis. In addition, calorimetric melting data (TI→L=(348.1±0.4) K and ΔHI→L=(89±2) J·g−1) and the specific volumes of the solid and the liquid state have been obtained under normal pressure. Comparison of the measured high-pressure melting data with the equilibrium curve obtained through the Clapeyron equation indicates that the initial slopes are the same (dP/dT=(2.96±0.06) MPa·K−1) at the melting point under normal pressure. The fact that the Clapeyron equation can be used to construct topological phase diagrams may be of interest for the food and pharmaceutical industries.

Hot climate inhibits volcanism on Venus: Constraints from rock deformation experiments and argon isotope geochemistry

The disparate evolution of sibling planets Earth and Venus has left them markedly different. Venus’ hot (460°C) surface is dry and has a hypsometry with a very low standard deviation, whereas Earth’s average temperature is 4°C and the surface is wet and has a pronounced bimodal hypsometry. Counterintuitively, despite the hot Venusian climate, the rate of intraplate volcano formation is an order of magnitude lower than that of Earth. Here we compile and analyse rock deformation and atmospheric argon isotope data to offer an explanation for the relative contrast in volcanic flux between Earth and Venus. By collating high-temperature, high-pressure rock deformation data for basalt, we provide a failure mechanism map to assess the depth of the brittle–ductile transition (BDT). These data suggest that the Venusian BDT likely exists between 2 and 12km depth (for a range of thermal gradients), in stark contrast to the BDT for Earth, which we find to be at a depth of ∼25–27km using the same method. The implications for planetary evolution are twofold. First, downflexing and sagging will result in the sinking of high-relief structures, due to the low flexural rigidity of the predominantly ductile Venusian crust, offering an explanation for the curious coronae features on the Venusian surface. Second, magma delivery to the surface—the most efficient mechanism for which is flow along fractures (dykes; i.e., brittle deformation)—will be inhibited on Venus. Instead, we infer that magmas must stall and pond in the ductile Venusian crust. If true, a greater proportion of magmatism on Venus should result in intrusion rather than extrusion, relative to Earth. This predicted lower volcanic flux on Venus, relative to Earth, is supported by atmospheric argon isotope data: we argue here that the anomalously unradiogenic present-day atmospheric 40Ar/36Ar ratio for Venus (compared with Earth) must reflect major differences in 40Ar degassing, primarily driven by volcanism. Indeed, these argon data suggest that the volcanic flux on Venus has been three times lower than that on Earth over its 4.56billion year history. We conclude that Venus’ hot climate inhibits volcanism.

Methane Adsorption Rate and Diffusion Characteristics in Marine Shale Samples from Yangtze Platform, South China

Knowledge of the gas adsorption rate and diffusion characteristics in shale are very important to evaluate the gas transport properties. However, research on methane adsorption rate characteristics and diffusion behavior in shale is not well established. In this study, high-pressure methane adsorption isotherms and methane adsorption rate data from four marine shale samples were obtained by recording the pressure changes against time at 1-s intervals for 12 pressure steps. Seven pressure steps were selected for modelling, and three pressure steps of low (~0.4 MPa), medium (~4.0 MPa), and high (~7.0 MPa) were selected for display. According to the results of study, the methane adsorption under low pressure attained equilibrium much more quickly than that under medium and high pressure, and the adsorption rate behavior varied between different pressure steps. By fitting the diffusion models to the methane adsorption rate data, the unipore diffusion model based upon unimodal pore size distribution failed to describe the methane adsorption rate, while the bidisperse diffusion model could reasonably describe most of the experimental adsorption rate data, with the exception of sample YY2-1 at high pressure steps. This phenomenon may be related to the restricted assumption on pore size distribution and linear adsorption isotherm. The diffusion parameters α and β/α obtained from the bidisperse model indicated that both macro- and micropore diffusion controlled the methane adsorption rate in shale samples, as well as the relative importance and influence of micropore diffusion and adsorption to adsorption rate and total adsorption increased with increasing pressure. This made the inflection points, or two-stage process, at higher pressure steps not as evident as at low pressure steps, and the adsorption rate curves became less steep with increasing pressure. This conclusion was also supported by the decreasing difference values with increasing pressures between macro- and micropore diffusivities obtained using the bidisperse model, which is roughly from 10−3 to 100, and 10−3 to 10−1, respectively. Additionally, an evident negative correlation between macropore diffusivities and pressure lower than 3–4 MPa was observed, while the micropore diffusivities only showed a gentle decreasing trend with pressure. A mirror image relationship between the variation in the value of macropore diffusivity and adsorption isotherms was observed, indicating the negative correlation between surface coverage and gas diffusivity. The negative correlation of methane diffusivity with pressure and surface coverage may be related to the increasing degree of pore blockage and the decreasing concentration gradient of methane adsorption. Finally, due to the significant deviation between the unipore model and experimental adsorption rate data, a new estimation method based upon the bidisperse model is proposed here.