Diffusion Time Constants Research Articles

A novel pulse-modulated closed-loop artificial pancreas based on intravenous administration of insulin and glucagon

Current attempts to implement and apply automated control systems for the management of glucose homeostasis in individuals with diabetes are partly successful. In most semi and closed loop control systems to mimic the action of a normal pancreas in diabetes patients, insulin is administered subcutaneously. However, the parameters for insulin diffusion and transport time constants are relatively large and have wide individual variations. Therefore, deviation from a normal meal can result in suboptimal euglycemic control. Stable and reliable closed loop feedback control using continuous glucose monitoring under these conditions is difficult and needs regular interventions from the user. This article describes the translation of the endocrine physiology of a normal pancreas to an electronic equivalent. With this translation, the complex effects of a direct intravenous pulsatile method of insulin and glucagon administration can be simulated in accordance with physiological observations in a healthy subject and has been built with standard electronic components. This device is applicable for any individual and under any condition to automatically maintain the desired optimal euglycemic condition. The insulin and glucagon response control is presented in the same physiological pulsatile manner as is observed in healthy individuals.

Adsorption equilibria and kinetics of CO2 and N2 on silica-alumina gels for reducing gas production by CO2 removal from iron flue gases

In industrial fields, hydrogen-containing flue gases generally contain a certain amount of CO2 and N2. After removing CO2, the raffinate gases can be used in various applications to mitigate carbon emissions. Furthermore, the concentrated CO2-rich extract contributes to efficient CO2 capture. Silica-alumina gel can be a high potential adsorbent because of its high CO2 adsorption capacity and energy-saving desorption. Three different silica-alumina gels with different amounts of alumina (FJ, KD, and WS) were compared to study the impact of the alumina content on the adsorption characteristics. The adsorption behaviors of CO2 and N2 were measured at 293K, 308K, and 323K at pressures up to 1000 kPa. The isotherms correlated with the dual-site Langmuir model, showing that the adsorption capacity for CO2 was much higher than that for N2 across all the adsorbents. The surface areas and adsorption capacities followed the opposite order of alumina content: FJ > KD > WS. However, the difference intervals in the isotherms did not align consistently with the alumina content, but the micropore volume changed by alumina contributed highly to the adsorption capacity. In the adsorption kinetic analysis, the diffusional time constants and kinetic parameters in a non-isothermal adsorption kinetic model showed minor dependence on pressure and temperature. The adsorption rate was higher for N2 than for CO2. It increased with alumina content, following the order WS > KD >> FJ. The results contribute to the establishment of guidelines for the selection of silica-alumina gel and the design of the adsorption process.

Mechanically robust MOF beads for efficient ethylene/ethane separation with fast adsorption kinetics

Shaping of metal–organic frameworks (MOFs) without obvious mass transfer resistance is crucial for scaled-up adsorption separation process. Herein, mechanically robust ultramicroporous MOF (ZU-901, also termed CPL-1-CH3) beads with hierarchical porosity were structured through the calcium alginate (CA) method, which preserved the adsorption separation performance and demonstrated fast gas diffusion rates. The highly crosslinked networks of CA contributed to the excellent mechanical properties of 95 % ZU-901@CA beads with Young’s modulus up to 1.81 MPa and a low abrasion rate of 0.32 %, superior to that of 95 % ZU-901@HPC extrudates (1.13 %). Compared with powder of ZU-901, the C2H4 uptake capacity of ZU-901@CA beads showed only 3 % loss at 298 K. The C2H4 diffusion time constant of 95 % ZU-901@CA beads (7.06 × 10−3 s−1) was comparable to that of the powders (7.61 × 10−3 s−1) and higher than that of ZU-901@HPC extrudates (2.95 × 10−3 s−1). Breakthrough experiments confirmed that ZU-901@CA beads performed well in the dynamic separation of C2H4/C2H6 binary mixture, and could be easily regenerated at room temperature. In addition, after post modification on the exterior of ZU-901 beads, the water contact angle increased from 0° to 119°. Therefore, the mechanically robust ZU-901@CA beads showed great potential in energy-efficient C2H4/C2H6 separation.

Topology Reconfiguration of Anion-Pillared Metal-Organic Framework from Flexibility to Rigidity for Enhanced Acetylene Separation.

Flexible metal-organic framework (MOF) adsorbents commonly encounter limitations in removing trace impurities below gate-opening threshold pressures. Topology reconfiguration can fundamentally eliminate intrinsic structural flexibility, yet remains a formidable challenge and is rarely achieved in practical applications. Herein, a solvent-mediated approach is presented to regulate the flexible CuSnF6-dpds-sql (dpds = 4,4''-dipyridyldisulfide) with sql topology into rigid CuSnF6-dpds-cds with cds topology. Notably, the cds topology is unprecedented and first obtained in anion-pillared MOF materials. As a result, rigid CuSnF6-dpds-cds exhibits enhanced C2H2 adsorption capacity of 48.61 cm3 g-1 at 0.01bar compared to flexible CuSnF6-dpds-sql (21.06 cm3 g-1). The topology transformation also facilitates the adsorption kinetics for C2H2, exhibiting a 6.5-fold enhanced diffusion time constant (D/r2) of 1.71 × 10-3 s-1 on CuSnF6-dpds-cds than that of CuSnF6-dpds-sql (2.64 × 10-4 s-1). Multiple computational simulations reveal the structural transformations and guest-host interactions in both adsorbents. Furthermore, dynamic breakthrough experiments demonstrate that high-purity C2H4 (>99.996%) effluent with a productivity of 93.9mmol g-1 can be directly collected from C2H2/C2H4 (1/99, v/v) gas-mixture in a single CuSnF6-dpds-cds column.

Epitaxial Growth of Two-Dimensional MWW Zeolite.

Two-dimensional (2D) zeolite, with a high aspect ratio, has more open skeletons and accessible active sites than its three-dimensional (3D) counterpart. However, traditional methods of obtaining 2D zeolites often cause structural damage and widespread skeleton defects, hindering efficient selectivity in molecular separation. In this study, we present, for the first time, a direct epitaxial synthesis of 2D zeolite (Epi-MWW) guided by hexagonal boron nitride (h-BN) with a coincidence matching of site lattices to MWW zeolite. The as-grown Epi-MWW zeolite possesses a high crystallinity and intact hexagonal 2D morphology, with an average thickness of 10 nm and an aspect ratio of over 50. Thanks to its excellent molecular accessibility, the diffusion time constants of o-xylene (OX) and p-xylene (PX) are as 12 and 133 times higher than those of conventional MCM-22, respectively; the PX/OX selectivity of Epi-MWW is 7.4 times better than MCM-22 as calculated by the ideal adsorbed solution theory.

Cationic tantalum oxide nanoparticle contrast agent for micro computed tomography reveals articular cartilage proteoglycan distribution and collagen architecture alterations

ObjectiveCationic tantalum oxide nanoparticles (Ta2O5-cNPs), as a newly introduced contrast agent for computed tomography (CT) of cartilage, offer quantitative evaluation of proteoglycan content and biomechanical properties. However, knowledge on the depth-wise impact of cartilage constituents on NP diffusion, particularly the influence of the collagen network, is lacking. In this study, we aim to establish the depth-dependent relationship between Ta2O5-cNP diffusion and cartilage constituents (proteoglycan content, collagen content and network architecture). MethodsOsteochondral samples (n = 30) were harvested from healthy equine stifle joints (N = 15) and the diffusion of 2.55 nm diameter cationic Ta2O5-cNPs into the cartilage was followed with µCT imaging for up to 96 hours. The diffusion-related parameters, Ta2O5-cNP maximum partition (Pmax) and diffusion time constant, were compared against biomechanical and depth-wise structural properties. Biomechanics were assessed using a stress-relaxation and sinusoidal loading protocols, whereas proteoglycan content, collagen content and collagen network architecture were determined using digital densitometry, Fourier-transform infrared spectroscopy and polarized light microscopy, respectively. ResultsThe Pmax correlates with the depth-wise distribution of proteoglycans (bulk Spearman’s ρ = 0.87, p < 0.001). More open collagen network architecture at the superficial zone enhances intake of Ta2O5-cNPs, but collagen content overall decreases the intake. The Pmax values correlate with the equilibrium modulus (ρ = 0.80, p < 0.001) of articular cartilage. ConclusionThis study establishes the feasibility of Ta2O5-cNPs for the precise and comprehensive identification of biomechanical and structural changes in articular cartilage via contrast-enhanced µCT.

Adsorption equilibria and kinetics of CO2, CH4, CO, N2, O2, and H2 on silica-based adsorbents for H2 enhancement processes from steel off-gas for the direct reduced iron process

The direct reduced iron (DRI) process is highly advantageous for reducing carbon emissions in the steel mill industry. A well-designed adsorption technology can provide H2 mixtures from H2-containing off-gases for the DRI process and concentrate CO2 for a carbon capture process simultaneously. The adsorption behaviors of steel off-gas components, CO2, CH4, CO, N2, O2, and H2, were studied at 293, 308, and 323 K and at pressures of up to 1000 kPa on two different silica-based pellets. Their weak adsorption affinity and high CO2 selectivity enables relatively easy desorption without vacuum energy. The CO2 and CH4 isotherms were correlated with the dual-site Langmuir model, whereas the single-site Langmuir model adequately predicted the other isotherms. The order of adsorption capacities of both adsorbents was CO2≫CH4>CO>N2≈O2≫H2, and the heat of adsorption of CO2 was notably higher than those of the others. The silica gel with the higher Al content and lower surface area (SG1) showed slightly higher CO2 isotherms up to 100 kPa, whereas the other (SG2) exhibited a greater adsorption capacity at higher pressures. For the other adsorbates, higher isotherms were observed for SG2 at all pressures. In the adsorption kinetic analysis using a non-isothermal adsorption model, the diffusional time constants and kinetic parameters showed a consistent dependence on pressure and temperature. The adsorption rate followed the order of N2>CH4≥CO≫CO2. The isotherm and uptake curves for CO2 between the silica gel pellets and particles revealed that macropore diffusion affected the overall adsorption kinetics of CO2 owing to adsorption heat resistance.

Degradation study for 18650 NMC batteries at low temperature

This research focuses on the identification and quantification of lithium-ion battery degradation indicators at low temperatures. We studied the cycling aging of 18,650 commercial NMC lithium-ion batteries at 10 °C. For this purpose, we carried out life cycle tests and performed Galvanostatic Intermittent Titration Technique (GITT) tests in the voltage range for the charge and discharge processes for different States of Health (SoH). The diffusion time constant and the ohmic overpotential were determined from GITT for different SoH. As the batteries degrade both parameters increase. A degradation mechanism associated with faradaic effects (increase in ohmic drop overpotential) and a thermodynamic effect associated with changes in the active material (diffusional time constant) is observed. Furthermore, we performed Electrochemical Voltage Spectroscopy studies through Incremental Capacity (IC) curves. IC curves peaks and valleys are associated with battery phase transformations due to aging phenomena. Each peak has a unique peak height, area, and position associated with a degradation mode. This research focuses on the IC curves derived from the discharge capacities at 3A and OCP (Open Circuit Potential) to study the thermodynamic and faradaic effects separately. The peak at 3.6 V and 3.4 V for the 3A and OCP studies, respectively, is the main feature for the detection of the degradation mechanism. The valley in the OCP curves intensifies with the decrease in SoH and is shifted to lower potentials, an effect that is not observed in the 3A curves. So, we could associate this with thermodynamic effects, Loss of Lithium Inventory (LLI) and Loss of Active Material (LAM) of both the positive electrode and the negative electrode.

Equilibrium and Kinetic Insights into the Comprehensive Investigation of CO2, CH4, and N2 Adsorption on Cation-Exchanged X and Y Faujasite Zeolites.

The adsorption equilibria and kinetic performance of CO2, CH4, and N2 on pelletized cation-exchanged faujasite zeolites (with alkali, alkaline earth, and transition metal ions) have been investigated by an innovative volumetric apparatus simultaneously. The standard instrumental analytical techniques, including X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), and atomic absorption spectroscopy (AAS), were utilized to characterize binder-free modified zeolites. EDX and AAS analyses revealed that the ion exchange was successfully achieved. The results indicate that the type of cation present in the zeolite framework and the Si/Al ratio can have a significant impact on the adsorption capacity and kinetic performance. The obtained isotherms were determined by three isotherm models, and the Langmuir-Freundlich (Sips) model was found to show the best agreement with the experimental isotherm data for all gases. The CO2 uptakes of KX, MgX, and CaX reached 4.13, 4.79, and 5.48 mmol/g, respectively. The effective binary and kinetic selectivities of CO2/CH4 and CO2/N2 were also calculated. Among all samples, KX showed the highest CO2/CH4 and CO2/N2 selectivities of 54.46 and 91.62, respectively. Pseudo-first-, pseudo-second-order, and Avrami kinetic models were fitted to the experimental kinetic data to analyze the adsorption kinetics. Finally, the macropore diffusion coefficient (Dp) and microporous diffusional time constant (Dc/rc2) were estimated by correlating the micropore-macropore kinetic model with the experimental fractional uptake curves. Among the ion-exchanged zeolite samples, the K+ form exhibits a suitable performance in terms of kinetic behavior and adsorption capacity.

Unconventional seed-assisted strategy for Al-rich hierarchical ZSM-48 zeolite

Inferior diffusion capacity and insufficient acid density hinder the practical application of ZSM-48 zeolite. Finding a simple and practical strategy to simultaneously address these two defects remains a challenge. In response to this dilemma, we developed an unconventional seed-assisted synthesis strategy for Al-rich hierarchical ZSM-48 zeolite. This approach allows for achieving a broader range of silica to alumina ratio and accelerates the entire crystallization process through the selection of unconventional seeds. The synergy between the seed and organic template was demonstrated to play a pivotal role in facilitating nucleation. Direct evidence from 1H-29Si CP MAS NMR, TG, and IR results demonstrates that hexamethonium ions (HM2+) electrostatically adsorb at the defect sites on the seed, thereby promoting nucleation sites formation. Smaller seed crystals undergo more etching during the induction period, resulting in additional defects and enhanced nucleation ability. The obtained catalyst exhibits a diffusion time constant (Deff/L2) nine times that of conventional ZSM-48 zeolite when using p-xylene as a probe molecule. In m-xylene isomerization reaction, Al-rich hierarchical ZSM-48 demonstrates excellent stability along with higher selectivity and yield for p-xylene compared to typical ZSM-5 catalysts. Remarkably, long-term testing of 1000 h yields over 22.5 % of p-xylene, indicating the potential of this catalyst as an alternative for xylene isomerization reaction. This work not only advances the practical application process of ZSM-48 catalyst but also provides valuable insights for optimizing other zeolites.

A Fast Activation Energy Derivation (FAED) approach for Lumped Single Particle model in lithium-ion battery module-level heat generation prediction

The performance of electric vehicles (EVs) heavily relies on the durability and longevity of lithium-ion cells, which are strongly affected by operating temperatures. Various electrochemical models have been developed to investigate the impact of operating temperature on the dynamic heat generation at cell level. Among these, the Lumped Single Particle (LSP) model stands out due to its lower computational cost and higher accuracy under >1C-rates. However, key electrochemical parameters (i.e. the ohmic overpotential, exchange current, and diffusion time constant) in the LSP model are temperature-sensitive, requiring Arrhenius dependencies. Conventional activation energy measuring approaches, relying on testing under discrete fixed temperatures to obtain activation energies, are time-consuming. To address this, this paper proposes a FAED approach that fits the temperature-sensitive parameters and corresponding activation energies to experimental data with a single discharge process, reducing the testing time to a minimum of one hour. This approach was experimentally validated on a Tesla battery module (74P6S), demonstrating a good agreement between simulation and experimental results up to 1.5 C-rates, ranging from 16 ℃ to 46 ℃. Furthermore, this approach is suitable for cells with different lithium chemistries, formats, and properties, making it an effective method for thermal analysis of EV packs.

Influence of sodium and potassium proportion on the adsorption of methanol and water on LTA zeolites at high temperature

Catalytic hydrogenation of CO2 or CO, obtained from biomass gasification, is the most used technology to produce renewable methanol. The main problem is that the reactions are controlled by equilibrium, achieving low conversions to methanol, leading to high purification costs and reagent recirculation rates. At 250 °C and 50 bar, the equilibrium conversion of CO2 is around 25%, and methanol selectivity is about 64%. The process can be improved by a sorption-enhanced reaction process (SERP), removing the reaction products in situ and overcoming the equilibrium. To carry out the process, adsorbents with high affinity to the products at high temperatures, like LTA zeolites, are required. Experimental data on methanol and water adsorption at reaction temperatures is quite scarce in the literature, and it is usually assumed that methanol is not adsorbed onto 4A and 3A zeolites. This information is required to design a SERP process with this adsorbent. In this work, the influence of sodium and potassium proportion on the adsorption of methanol and water on LTA zeolites is studied by measuring Henry's adsorption equilibrium constants and reciprocal diffusion time constants. Sodium LTA zeolites (4A) strongly adsorb water and methanol. Potassium-rich LTA zeolites (3A) have higher water/methanol selectivity than sodium-rich LTA zeolites.

Pore structure modification of ZIF-8 by ligand exchange for separation mono- and di-branched isomers of hexane via thermodynamic and kinetic mechanism

Isomeric C6 alkanes are high-quality raw materials for producing high octane gasoline. Among them, the octane number of di-branched C6 alkanes is significantly higher than that of mono-branched C6 alkanes. Therefore, efficient and economical separation of mono- and di-branched C6 alkanes has received widespread attention. Herein, ZIF-8 with mixed ligands (named as ZIF-8-IM) was successfully synthesized by exchanging some 2-methylimidazole (MIM) ligands in the framework of ZIF-8 with imidazole (IM), and the microstructure of ZIF-8-IM were analyzed from N2 adsorption. Adsorption thermodynamic and kinetic properties of n-hexane (nHEX), 3-methylpentane (3MP) and 2,3-dimethylbutane (23DMB) on ZIF-8-IM were determined by IGA-100 intelligent weight analyzer. Column breakthrough experiments were carried out to confirm the separation performance of of 3MP/23DMB binary mixture and nHEX/3MP/23DMB ternary mixture on ZIF-8-IM and ZIF-8. Results show that the microporous structure of ZIF-8 is changed by ligand exchange, which means the most probable pore diameters at 1.08 nm and 1.34 nm of ZIF-8 are converted to 1.06 nm and 1.19 nm of ZIF-8-IM. The adsorption equilibrium and kinetic characteristics of mono-branched 3MP are significantly enhanced due to the microstructural changes in ZIF-8-IM. The equilibrium adsorption capacity of 3MP on ZIF-8-IM is 1.6 times that of 23DMB, and the diffusional time constant is 4–13 times that of 23DMB. By relying on the synergistic effect of thermodynamic and kinetic characteristics, efficient adsorption and separation of 3MP/23DMB mixtures can be achieved with a separation factor of 2.24 on ZIF-8-IM. In addition, ZIF-8-IM can also achieve clear separation of nHEX/3MP/23DMB ternary mixtures, obtaining individual nHEX, 3MP, and 23DMB products. The results of this study show excellent application prospects in the production of the raw materials for high octane gasoline and molecular oil refining processes.

Online Aging Diagnostics Using Optimally Designed Experiments

Degradation of lithium-ion batteries is the result of many complex phenomena occurring simultaneously at varying time and length scales. The underlying electrochemical and mechanical phenomena have received much attention from researchers [1]. Physics-based models of these effects support the mechanistic understanding of degradation modes and can thereby help reduce their severity. Few studies target changing electrochemical parameters such as diffusion coefficients or reaction rate constants that have a direct impact on model accuracy and manifest themselves in observable aging. Lyu et al. [2] used a simplified electrochemical model and monitored battery degradation by following changes in diffusion time constants, electrode balancing, reaction rate coefficients, and ohmic resistance. However, several of the parameters they attempted to track could only be identified with low accuracy as they used the same data-set to identify all parameters. In this study, we investigate parameters of a full order Newman-type model [3] over the course of a batteries lifetime under real-world load-cycles. To ensure parameter identifiability, optimally designed experiments are used for parameter estimation.In a previous study [4] the feasibility of optimal experiment design for parametrization of electrochemical battery models was demonstrated. We now extend this work and re-evaluate key parameters over the course of an aging study on commercial, nickel-rich 18650 lithium-ion batteries. We highlight how quantifying changes in physical battery parameters can extend standard performance metrics for a batteries state-of-health by, e.g., including degradation in rate-capability. Additionally, the importance of battery usage conditions such as C-rate or state-of-charge window on model parameter trajectories is investigated and their relationship with conventional performance metrics such as the bulk cell resistance or rate-capability determined.Quantifying how specific mechanisms contribute to apparent capacity or power fade is a major step towards battery lifetime optimization. This could enable designs more tailored for specific applications and significantly extend batteries useful lifetime. Furthermore, updating parameters is essential for electrochemical control strategies relying on accurate model predictions of battery states as illustrated in Figure 1. This re-calibration would make a battery management system aging-sensitive and enable more efficient utilization and a physics-informed state-of-health.Figure 1: The central plot shows how parameters change during aging. If this change is not considered, model performance deteriorates between beginning-of-life (BOL) and end-of-life (EOL) (in blue, right-hand side). This is normally handled by using conservative battery management systems and over-sizing systems. The proposed strategy (orange) achieves higher model accuracy during the entire useful life and the parameter estimates can be used to formulate an extended state-of-health.

Isotherms and kinetics of water sorption onto MOFs for adsorption cooling applications

• Three adsorbents: MOF 801, aluminum fumarate, and MIL 100 (Fe), were synthesized. • Water sorption isotherms onto the MOFs were thermogravimetrically measured. • The diffusion coefficients were determined using the TALDF sorption kinetics model. • COP th and SCE were computed for the studied pairs. • MIL 100 (Fe)/water had the fastest kinetics, highest net uptake with a high COP of 0.8. MOFs (metal organic frameworks), a new type of porous coordination polymers (PCPs), has emerged as a novel class of adsorbent material that has piqued the interest of academics working on a variety of sorption-related applications all over the world. Adsorption heat pump research is no exception, and it has accelerated since the MOF adsorbents were introduced. In this study, water adsorption isotherms onto three distinct synthesized MOFs, MOF 801, aluminum fumarate, and MIL 100 (Fe), were experimentally obtained at three different temperatures (30, 50, and 70 °C) with varying evaporating pressures using the thermogravimetric technique. The S-shaped adsorption isotherms were correlated with the well-known Sun-Chakraborty model. P- T -W diagrams were generated using the adsorption isotherms data to determine the net adsorption uptake within a certain adsorption heat pump working condition. Water sorption kinetics were also analyzed using the time-adapted linear driving force model. Adsorption enthalpy, specific cooling effect, and coefficient of performance of the studied pairs were calculated and compared with the results of the PS-I/water pair. Results suggested that, for a water-based adsorption cooling system, the studied MOF adsorbents significantly outperform the traditional polymer-based adsorbent PS-I. Among the MOF/water pairs, MIL 100 (Fe)/water pair showed the fastest sorption kinetics with a diffusion time constant of 5.4 × 10 −4 s −1 . This pair also exhibited the highest specific cooling effect of 569.42 kJ kg −1 and a coefficient of performance of 0.80, which was rather impressive. These results will be valuable in determining suitable adsorbent-adsorbate pairs for next-generation adsorption cooling systems.

Adsorption equilibria and kinetics of ethane and ethylene on zeolite 13X pellets

To establish a design basis of the cyclic adsorption process for ethylene purification, the adsorption equilibria and kinetics of ethane and ethylene on zeolite 13X pellets were measured by a volumetric method at 303–343 K under pressure up to 600 kPa. The Sips model showed better prediction of ethane in the full pressure range, but the dual-site Langmuir (DSL) model was more accurate for ethylene in the pressure range of <250 kPa. The strong cation-π interaction between ethylene and Na+ in zeolite 13X led to higher adsorption capacity and affinity than those of ethane. It resulted in a greater isosteric heat of adsorption (Qst) of ethylene at a low loading amount, while Qst variance in ethane was almost linearly increased with a dominant lateral interaction. At 303 K, the adsorption amount and affinity of ethylene at <5 kPa were slightly greater than those of propane but lower than those of propylene. However, the adsorption isotherms of ethane/ethylene became higher than those of propane/propylene above a certain pressure. The experimental uptake curves of ethane and ethylene were well predicted by a non-isothermal sorption model, considering the adsorption thermal effects. The difference in the apparent reciprocal diffusional time constant (Dapp/R2) between ethane and ethylene was mainly attributed to the thermal effects by the heat of adsorption. The comparison of Dapp/R2 values between zeolite 13X pellet and powder indicated that macropore diffusional resistance also contributed to the adsorption kinetics.

Regulating mesopore structures of support toward enhanced selective hydrogenation of dimethyl oxalate to methyl glycolate on Ag catalysts

Precisely regulating mesopore structures toward enhanced diffusion of reactants and targeted product is of great significance for achieving simultaneously improved activity and selectivity, which is exemplified by this work employing mesoporous silica with distinct structured mesopores as support for Ag catalysts to selectively hydrogenate dimethyl oxalate (DMO) into methyl glycolate (MG). Mesoporous silica nanosphere (MSNS) with center-radial pores shows shorter pore length than SBA-15 with parallel mesopores, which is beneficial for the diffusion process and thus gives rise to enhanced hydrogenation activity. Inspired by these insights, MSNSs sized in 50, 90, 120 and 160 nm are further synthesized to regulate pore length and systematically explore their effects on the reaction. The diffusion process is enhanced with decreasing the size of MSNS from 160 to 50 nm, as elucidated by the effective diffusion time constant (De/R2) determined by zero-length column (ZLC) method. Accordingly, DMO can facilely diffuse into the mesopores with shorter length to access the active sites while the formed MG product can easily diffuse out to avoid the over-hydrogenation process, which is corroborated by the comparison for performances of these Ag/MSNSs catalysts. Notably, a moderately short pore length, balancing well the diffusion and hydrogenation process, is more preferable for achieving both higher activity and better selectivity to MG, and MSNS sized in 90 nm supported Ag catalyst thus exhibits the greatest performances, with MG selectivity up to 96.6 % at 99.7 % of DMO conversion.

Assessing potential correlation between T2 relaxation and diffusion of lactate in the mouse brain.

PurposeWhile diffusion and T2 relaxation are intertwined, little or no correlation exists between diffusion and T2 relaxation of intracellular metabolites in the rodent brain, as measured by diffusion‐weighted MRS at different TEs. However, situation might be different for lactate, since it is present in both extracellular and intracellular spaces, which exhibit different diffusion properties and may also exhibit different T2. Such a TE dependence would be crucial to account for when interpreting or modeling lactate diffusion. Here we propose to take advantage of a new diffusion sequence, where J‐modulation of lactate is canceled even at long TE, thus retaining excellent signal, to assess potential T2 dependence on diffusion of lactate in the mouse brain.MethodsUsing a frequency‐selective diffusion‐weighted spin‐echo sequence that removes J‐modulation at 1.3 ppm, thus preserving lactate signal even at long TE, we investigate the effect of TE between 50.9 and 110.9 ms (while keeping diffusion time constant) on apparent diffusivity and kurtosis in the mouse brain.ResultsRegardless of the metabolites, no difference appears for the diffusion‐weighted signal attenuation with increasing TE. For lactate, apparent diffusivity and kurtosis remain unchanged as TE increases.ConclusionNo significant TE dependence of diffusivity and kurtosis is measured for lactate in the 50–110 ms TE range, confirming that potential T2 effects can be ignored when interpreting or modeling lactate diffusion.

Investigation of water kinetics in zeolite Linde‐Type‐A crystals by a concentration‐swing frequency response

AbstractA new concentration‐swing frequency response system was developed to evaluate water kinetics in non‐binder containing zeolite Linde Type A (LTA) crystals with frequencies up to 1 Hz. For commercial micron‐sized 3A crystals, the mass transfer rate is too fast to be determined accurately due to reduced sensitivity near the system limits. Larger LTA crystals (~15 μm) were synthesized in house and ion exchanged to three potassium concentrations (0, 48, and 77 wt%) to aid in the accurate determination of mass transfer mechanism and rates. Micropore diffusion with parallel sites, not a surface barrier, described all the data well, consistent with the bimodal distribution of crystal sizes from scanning electron microscope images. The transport diffusivity for water in zeolite 3A crystals near saturated loadings at 25°C is about 1 × 10−12 m2/s, 10 times slower than 4A crystals around 10−11 m2/s, both supporting fast micropore diffusion time constants &gt;1 s−1 for micron‐sized crystals.

A novel adsorption differential volumetric apparatus to measure mass transfer in nanoporous materials

A novel adsorption differential volumetric apparatus was developed for the determination of diffusional time constants in nanoporous materials and applied to diffusion of nitrogen and argon in commercial pellets of 4A zeolite. The system is designed for high rates of data acquisition allowing to determine mass transfer time constants of seconds over the pressure range from vacuum to 130 kPa. Diffusion of N2 and Ar on a single pellet and fragments obtained from the pellet are studied between –10 °C and 35 °C and 0.8 to 55 kPa. These systems are chosen as representing weak adsorption to demonstrate the sensitivity of the apparatus that gives a good signal-to-noise ratio even with a single pellet in the entire pressure range. The systems studied confirm micropore diffusion control and an isothermal diffusion model was shown to reproduce accurately the observed kinetics using reduced pressure plots. As the crystal size in the pellet is not known accurately, the resulting activation energies and the ratio of diffusional time constants of N2 and Ar were used to validate the results against known literature values.