Isotopic Exchange Process Research Articles

Oxygen Isotope Exchange between Dust Aggregates and Ambient Nebular Gas

Meteorites and their components exhibit a diverse range of oxygen isotope compositions, and the isotopic exchange timescale between dust grains and ambient gas is a key parameter for understanding the spatiotemporal evolution of the solar nebula. As dust grains existed as macroscopic aggregates in the solar nebula, it is necessary to consider the isotopic exchange timescales for these aggregates. Here, we theoretically estimate the isotope exchange timescales between dust aggregates and ambient vapor. The isotope exchange process between aggregates and ambient vapor is divided into four processes: (i) supply of gas molecules to the aggregate surface, (ii) diffusion of molecules within the aggregate, (iii) isotope exchange on the surface of constituent particles, and (iv) isotope diffusion within the particles. We evaluate these timescales and assess which one becomes the rate-determining step. We reveal that the isotope exchange timescale is approximately the same as that of the constituent particles when the aggregate radius is smaller than the critical value, which is a few centimeters when considering the exchange reaction between amorphous forsterite aggregates and water vapor.

Performance of Structured Packing for the Deuterated Water Distillation Process at Above Atmospheric Pressure

The National Research and Development Institute for Cryogenics and Isotopic Technologies (ICSI) was established in 1970 for research focused the industrial pilot plant. ICSI was created with the purpose of developing the heavy water production technology needed mainly for the operation of the CANDU reactors that equip the Cernavoda Nuclear Power Plant in Romania. This technology has been demonstrated and has been successfully transferred to the heavy water production plant. The accumulated experience in the field of isotopic separation allowed for the development of a second pilot plant, also for the purpose of heavy water, but this time for the purpose of separating deuterium and tritium for the recovery of tritium, mainly from the tritiated heavy water of the CANDU reactors. This experimental pilot is based on a technology that combines two hydrogen isotope separation processes: liquid-phase water-hydrogen catalytic isotopic exchange and hydrogen isotope cryogenic distillation. The success of the separation process consists of the technical solution given to the exchange area, its geometry (distillation-isotopic exchange), and last but not least, the performance of the hydrophilic packing, respectively, of the catalyst. Thus, an extensive research program was initiated and carried out over many years for the development of mixed catalytic packages consisting of a hydrophobic Pt/carbon catalyst (with different concentrations of platinum up to 2%) and hydrophilic packing (stainless steel or phosphorous bronze) manufactured within ICSI and made in different configurations (successive layers, random or structured). This program for the improvement of catalytic packages continues by implementing new solutions regarding the improvement of materials that ensure a large contact surface between the liquid and vapors, respectively, vapor and gas, thus increasing the isotopic separation performance. This paper presents an experimental analysis of two types of hydrophilic stainless steel packings developed for the purpose of separating hydrogen isotopes by water distillation to be used in the manufacture of mixed catalytic packages for the catalytic isotopic exchange process. This paper refers to the experiments performed with deuterated water on the hydrophilic packing in conditions of total reflux. The experiments were performed on a column equipped with two types of hydrophilic packing.

Reaction order of near equilibrium calcite dissolution: Uncertainties and ambiguities of isotopic tracer methods

Dissolution of calcite in water is fundamental to geochemistry. Laboratory methods generally give consistent dissolution rates under conditions far from equilibrium, but close to equilibrium the measurement of dissolution using isotopes is complicated by poorly understood calcite-fluid isotopic exchange processes. This near-equilibrium isotopic exchange occurs in laboratory experiments at rates of order 10-9 to 10-13 mol/m2/s and decreases with experiment duration approximately as 1/time where time is measured from the start of the experiment. Such rates are fast enough to compete with dissolution in days-long experiments and consequently, net dissolution rates may not be measurable with isotopic tracer methods except in experiments carried out for long enough to allow the background exchange to dissipate sufficiently. To better define what should be expected for isotopic signals during dissolution, we develop a quasi-1D model for calcite dissolution that includes the mechanistically unconstrained isotopic exchange as well as solid state diffusion. We use the model, the observations reported in the literature, and the calcite dissolution rates in seawater reported by Naviaux et al., (2019a) to illustrate ambiguities in measurement of near-equilibrium calcite dissolution rates. The results suggest that the reaction order of calcite dissolution close to equilibrium could be 2 at both room temperature and the lower temperatures of the seafloor. The reaction order of near-equilibrium calcite dissolution is important for understanding calcite mineral surface processes, long-term behavior of dissolution on the seafloor, and alkalinity fluxes from modern seafloor sediments. Distinguishing between net dissolution, which affects fluid saturation state, and “exchange,” which doesn’t, is important but typically cannot be done with isotopic tracers alone.

Raney Nickel‐Catalyzed Deuterium Labeling of Nitrogen‐Containing Heterocycles and Pharmaceuticals under Continuous Flow Conditions

Deuterium‐labeled compounds play a pivotal role in physical organic chemistry, life sciences, and materials science. This has resulted in a surge of interest in deuterium‐labeled active pharmaceutical ingredients in recent years. In this study, we present a continuous flow Raney nickel‐catalyzed hydrogen isotope exchange process that boasts compatibility with a wide spectrum of nitrogen‐containing heterocycles and pharmaceutical compounds. The broad applicability of the developed method was demonstrated through successful labeling of various purine bases, imidazoles, pyridines, and active pharmaceutical ingredients, including complex structures like abacavir and remdesivir. Control experiments revealed Raney nickel's crucial role in the exchange process, showcasing the superiority of the continuous flow approach over batch reactions. Furthermore, a scaled‐up experiment demonstrated the robustness of the catalyst.

Directing Hydrogen Isotope Exchange with Aryl Carboxylic Acids

AbstractA highly effective and selective ortho-directed hydrogen isotope exchange process for aryl carboxylic acids has been achieved by using an iridium(I) N-heterocyclic carbene/phosphine complex under mild and neutral conditions. Good levels of deuterium incorporation have been delivered across a wide array of examples, including a number of biologically active drug compounds.

Mechanism of boric acid extraction by trioctylamine and tartaric acid

The separation of boron isotopes poses significant challenges due to their similar chemical properties. Previous studies have demonstrated that boron isotope separation can be achieved by solvent extraction of boric acid (H3BO3) using trioctylamine (TOA) and tartaric acid (DL-TA), resulting in a remarkable separation factor of up to 1.04 through chemical exchange. However, the coordination mechanism among these three compounds remains elusive, impeding our understanding of the isotope exchange process. In this study, the species in the aqueous and organic phases were investigated using extraction experiments (maximum loading method), alongside various characterization techniques (FT-IR, 11B NMR, and high-resolution ESI-MS), followed by DFT computations. The results showed that H3BO3 and DL-TA formed anionic complexes in the aqueous phase with stoichiometric ratios of 1:1 and 1:2, respectively. However, the stoichiometric ratios became more intricate in the organic phase, encompassing 1:1, 1:2, 2:2, 2:3, and 3:3. The average stoichiometric ratio of H3BO3 to DL-TA calculated within the range of 1:1 to 1:1.5. Protonated TOA could form ion pairs with these anionic complexes, while unprotonated TOA might form hydrogen bonds with the carboxyl groups in DL-TA, contributing to the complexity of the stoichiometric ratios between H3BO3 and TOA. The DFT calculations indicated that all complexes containing B(4)–O bonds, including 1:1, 1:2, 2:2, 2:3, and 3:3, effectively facilitated the exchange of isotopes with H3BO3. These findings pave the way to process optimization of boron isotope separation by solvent extraction of boric acid.

High-Energy Quantum Dynamics of the 15N + o-14N14N Rovibrational Activation and Isotope Exchange Processes.

We report full quantum reaction probabilities, computed within the framework of time-independent quantum mechanics using hyperspherical coordinates, for the 15N + 14N14N inelastic and reactive collision processes, restricted to total angular momentum J = 0, for kinetic energies up to 4.5 eV. We take advantage of the nonzero (i = 1) nuclear spin of 14N, leading to the existence of two nuclear spin isomers of 14N14N, namely, ortho- and para-14N14N, to restrict the study to the ortho molecular nitrogen species, with even rotational quantum number j = 0, 2, ... states. Specifically, we start with diatomic reagents ortho-14N14N in the initial rotational state j = 0. A comparison with similar works previously published by other groups using time-dependent wave packet and quasi-classical trajectory methods for the 14N + 14N14N fully symmetric collision is given. We find that reactive processes 15N + 14N14N involving atom exchange do not happen for collision energies less than 2.2 eV. Collisions at energies of around 2.0 eV are most effective for populating reactants' rovibrational states, that is, for inelastic scattering, whereas those at energies close to 5.0 eV yield a newly formed 14N15N isotopologue in a wide variety of excited vibrational levels.

Experimental determinations of carbon and hydrogen isotope fractionations and methane clumped isotope compositions associated with ethane pyrolysis from 550 to 600 °C

Methane clumped isotope compositions signify the relative natural abundances of rare, doubly substituted isotopic species of methane (13CH3D and 12CH2D2) and have emerged as a new isotopic tool to trace the sources, sinks, and lifecycles of methane in the environment. Such measurements can identify equilibration (or re-equilibration) temperatures if found to be in isotopic equilibrium or non-equilibrium processes (e.g., kinetically controlled reactions or mixing) if not in isotopic equilibrium. Naturally occurring thermogenic methane—formed by the thermally activated breakdown of larger organic molecules—has been found to have clumped isotope compositions consistent with equilibrium at reasonable gas formation temperatures in some settings and non-equilibrium processes occurring during either formation, migration, storage, or extraction in others. To explore the potential controls on the isotopic composition of thermogenic methane, we conducted isothermal time-series ethane pyrolysis experiments at 550 and 600 °C to measure methane and ethane 13C/12C and D/H fractionations and methane clumped isotope compositions (resolved 13CH3D and 12CH2D2). We explore the effects of modifying the initial clumped isotope composition of ethane and the addition of water vapor to pyrolysis experiments. We observe that ethane and methane 13C/12C are controlled by kinetic isotope effects and Rayleigh distillation processes. In contrast, ethane and methane D/H and methane clumped isotope compositions appear to be controlled by a combination of these processes and hydrogen isotope exchange. The hydrogen isotope exchange processes lead to isotopic equilibrium as reaction completion is approached for both D/H (ethane/methane) and methane clumped isotope compositions. We develop a chemical model based on a mass balance approach that accounts for inheritance vs. hydrogen-abstraction formation pathways for singly and doubly substituted isotopologues of ethane and methane that is compared to the experimental data. The model allows the determination of carbon and hydrogen kinetic isotope effects associated with ethane cracking and hydrogen abstraction reactions that, where applicable, we compare to prior theoretical constraints. From the comparison of the model to the experimental data, we infer that the kinetically controlled ethane and methane bulk isotope compositions and methane clumped isotope compositions are controlled by kinetic isotope effects (both primary and secondary) associated with both CC bond and CH bond cleavage reactions. Specifically, the methane clumped isotope compositions likely result from a combination of clumped isotope effects associated with ethane breakdown and/or assembly of methane isotopologues (expressed in terms of γ-factor parameters ≠ 1) and combinatorial effects that arise probabilistically. We discuss our experimental results in the context of recent pyrolysis experiments and observations of naturally occurring thermogenic methane. We consider a proposal consistent with observations from nature that the hydrogen isotope exchange reactions that promote equilibration of methane isotopic molecules at or near formation temperature may be facilitated by free radicals generated by pyrolysis reactions. In this framework, isotope exchange effectively ceases when pyrolysis effectively ceases locking in compositions that can be consistent with peak formation temperatures.

Screening the Optimal Loading of a Pt-Catalyzed Zeolite for Hydrogen Isotope Exchange

The separation of tritiated sources from the exhaust stream of a nuclear fusion system remains a key area of study. While current hydrogen isotope separation technologies are effective at separating gaseous elemental hydrogen, they require additional costly and time-intensive electrolysis steps to be applied toward tritiated water. Previous work has proposed a capture and exchange method, which this work has applied to screen for an optimal weight loading of platinum onto a zeolite molecular sieve. Several samples of various weight loadings were cycled using a series of isotope exchange processes, and it was determined that a weight loading between 0.65 to 0.80 wt% Pt is optimal to separate heavier isotopes of hydrogen from a water waste stream.

Temperature dependence of hydrogen isotope effect in LaNi5-H(D) system

To study inverse effect of hydrogen isotopes in LaNi5 for hydrogen isotope separation at ambient temperature, temperature dependence of hydrogen isotope effect in LaNi5－H(D) system was investigated in the temperature range -70 °C to 3 °C. Isotherms of hydrogen isotopes were tested in an automated Sieverts apparatus with a thermostat, and pressure differences were studied between H2 and D2. Isotope exchange process was carried out and H2－HD－D2 equilibrium was judged by online chromatography, separation factors were calculated to investigate the thermodynamic isotope effect. In the H2, D2 isotherms, average time interval (0.5h) between each equilibrium point was relatively short due to high precision temperature control. From the plateau pressures at a constant hydrogen (or deuterium) concentration in LaNi5 (2D or 2H atoms/mole of LaNi5) the partial molar enthalpies, ⊿H, and entropies, ⊿S of formation were calculated. ⊿H and ⊿S kept good accordance with the values from references, which indicated good performance of the Sieverts apparatus. The isotope exchange equilibrium was reached within 1h. The results showed that ratio between H2 pressure and D2 pressure, r(H－D) = P(D2)/P(H2), dropped as temperature decreased and the inverse effect was found (r(H－D)<1.0) below -5 °C according to the absorption branch of isotherms, whereas the effect was estimated to appear below 60 °C from the desorption branch. The separation factor of the H2－HD－D2 mixture, decreased as temperature increased and the inverse effect happened below -20 °C. The inverse effect was affected by the isotherm hysteresis and HD existence.

Alkali-metal bases in catalytic hydrogen isotope exchange processes.

The preparation of compounds labelled with deuterium or tritium has become an essential tool in a range of research fields. Hydrogen isotope exchange (HIE) offers direct access to said compounds, introducing these isotopes in a late stage. Even though the field has rapidly advanced with the use of transition metal catalysis, alkali-metal bases, used as catalysts or under stoichiometric conditions, have also emerged as a viable alternative. In this minireview we describe the latest advances in the use of alkali-metal bases in HIE processes, showcasing their synthetic potential as well as current challenges in the field. It is divided in different sections based on the isotope source used, emphasizing their benefits, disadvantages and limitations. The influence on the choice of alkali-metal in these processes as well as their possible mechanistic pathways are also discussed.

PLATINUM ON REDUCED GRAPHENE OXIDE NANOSHEETS OBTAINED UNDER MICROWAVE IRRADIATION, USING REDUCING AGENTS

Hydrophobic catalysts used in catalytic detritiation process of heavy water consist of platinum impregnated on charcoal (commercial material) and PTFE. For this type of material, Pt is found as agglomerates leading to an increase of hydrophilicity in the final catalyst and also, to a low number of functional groups for isotopic exchange. Therefore, in order to avoid these drawbacks, another type of platinum-based material was considered, with a higher platinum dispersion which would lead to an increase number of functional groups for the isotopic exchange process. There are different methods to synthesize Pt on graphene composites, but using microwaves involves advantages like fewer stages and shorter time to produce. The obtaining method uses reduction of H2PtCl6 in the presence of ethylene glycol and other reducing agents (NaBH4 and KOH) and graphene oxide suspension. Samples were then microstructurally investigated using SEM, BET, X-ray diffraction, RAMAN and TGA.

Improved Lumped-Parameter and Numerical Modeling of Unsaturated Water Flow and Stable Water Isotopes.

Characterizing unsaturated water flow in the subsurface is a requirement for understanding effects of droughts on agricultural production or impacts of climate change on groundwater recharge. By employing an improved lumped-parameter model (LPM) approach that mimics variable flow we have interpreted stable water isotope data (δ18 O and δ2 H), taken over 3 years at a lysimeter site located in Germany. Lysimeter soil cores were characterized by sandy gravel (Ly1) and clayey sandy silt (Ly2), and both lysimeters were vegetated with maize. Results were compared with numerical simulation of unsaturated flow and stable water isotope transport using HYDRUS-1D. In addition, both approaches were extended by the consideration of preferential flow paths. Application of the extended LPM, and thus varying flow and transport parameters, substantially improved the description of stable water isotope observations in lysimeter seepage water. In general, findings obtained from the extended LPM were in good agreement to numerical modeling results. However, observations were more difficult to describe mathematically for Ly2, where the periodicity of seasonal stable water isotope fluctuation in seepage water was not fully met by numerical modeling. Furthermore, an extra isotopic upshift improved simulations for Ly2, probably controlled by stable water isotope exchange processes between mobile soil water and quasi-immobile water within stagnant zones. Finally, although LPM requires less input data compared with numerical models, both approaches achieve comparable decision-support integrity. The extended LPM approach can thus be a powerful tool for soil and groundwater management approaches.

Superhydrophobic Pt@SBA-15 catalyst for hydrogen water isotope exchange reactions

In this work, Santa Barbara amorphous-15 (SBA-15) mesoporous silica was modified to fabricate hydrophobic microscopic reaction environments that enhanced the catalytic activity of the liquid phase catalytic exchange (LPCE) reaction. The hydrophobicity or hydrophilicity of the environments synthesized using modified SBA-15 was verified by conducting N2 and water vapor adsorption-desorption experiments. A porous composite catalyst, Pt@SBA-15-x/PVDF, was synthesized via a two-solvent method. The water contact angle of Pt-modified SBA-15 was 152°, proving its superhydrophobicity and granting it excellent catalytic performance for LPCE techniques in hydrogen-water isotope exchange processes. The catalytic performance of the hydrophobic catalyst did not decrease significantly after 30 days of usage.

Synthesis of Deuterated Heterocycles with Me2NCD(OMe)2 and Evaluation of the Products for Metabolism by Aldehyde Oxidase.

The metabolic oxidation of drug-like small molecules by aldehyde oxidase (AO) has commonly been mitigated through the incorporation of deuterium at the oxidation site. We report that dimethylformamide dimethyl acetal and related compounds undergo rapid CH to CD isotopic exchange upon exposure to methanol-d and similar deuterated alcohols. This isotopic exchange process can be used to synthesize Me2NCD(OMe)2 and has significant implications for the use of Me2NCD(OMe)2 in the synthesis of specifically deuterium-labeled compounds. The application of Me2NCD(OMe)2 to the synthesis of various heterocycles that have been associated with AO metabolism is described, and we report the impact of deuteration on the rate of in vitro AO-mediated metabolism.

Molecular dynamics simulations of hydrogen isotope exchange in tungsten vacancies

Solute hydrogen can cause many damaging processes in the lattices of metals, such as deformation of the material, which can take place in large scales through blistering and embrittlement. Especially in nuclear fusion applications, the trapped hydrogen isotope Tritium in the reactor wall materials can pose a radiological safety hazard. Techniques for hydrogen removal from metals usually require high temperatures. However, an efficient low temperature method to remove hydrogen is the so-called isotope exchange mechanism, where one isotope is being removed from the material by replacing it by another isotope introduced in the material. The atomic scale exchange mechanism of isotope exchange has not yet been determined. In this study we use molecular dynamics simulations to provide an atomic-scale explanation to the processes related to hydrogen isotope exchange in bulk materials. The results show that the lattice mono-vacancies and small vacancy clusters, usually produced in irradiation experiments, exhibit isotope exchange at low temperatures. The isotope exchange process should also be seen in other hydrogen trapping defects with similar trapping properties as vacancies.

The Kinetic Model of Diffusion and Reactions in Powder Catalysts during Temperature Programmed Oxygen Isotopic Exchange Process

The mathematical model of diffusion in powder oxide catalysts during the process of temperature programmed oxygen isotopic exchange is proposed. The diffusion is considered together with the homogeneous and heterogeneous oxygen isotopic exchange processes. The matrix forms of exchange rate equations of simple and complex heteroexchange, and homoexchange reactions which obtain symmetrical forms are analyzed. The quantitative values of model parameters are found from the fitting of experimental data taken from literature of temperature programmed oxygen isotopic exchange process in catalysts ZrO2 and CeO2. The fittings show a good matching of model results with experimental data. The shapes of kinetic curves registered during temperature programmed oxygen isotopic exchange process are analyzed and the influence of various process parameters such as activation energies of simple and complex heteroexchange, oxygen surface concentration of catalyst, ratio of catalysts surface and volume of reactor, diffusion activation energy is considered. The depth profiles of diffusing oxygen species in oxide catalysts powder are calculated.

Low‐Temperature Protonic Ceramic Fuel Cells through Interfacial Engineering of Nanocrystalline BaCe0.7Zr0.1Y0.1Yb0.1O3−δ Electrolytes

Nanocrystalline BaCe0.7Zr0.1Y0.1Yb0.1O3−δ (BCZYYb) is designed by a novel strategy with improved proton transport properties at low temperatures (&lt;300 °C). In situ Raman spectroscopy and electrical conductivity relaxation (ECR) are used to quantitatively evaluate the surface exchange coefficients during the hydrogen isotope exchange process. Similar surface exchange coefficients are measured via in situ Raman spectroscopy and ECR measurements, representing new tools to better understand proton transport behaviors at the materials’ interface. The surface exchange coefficient in nanocrystalline BCZYYb is nearly four times higher than that in conventional dense BCZYYb at 300 °C, indicating higher surface mobility of protonic species in the designed BCZYYb membrane. The improved performance originates from the combined interfacial and bulk effects for proton transport at low temperatures. In addition, low‐temperature protonic ceramic fuel cells (PCFCs) are built based on a nanocrystalline BCZYYb electrolyte with improved single‐cell performance at 300 °C, which indicates enhanced proton transport properties in contemporary energy conversion and storage materials can be achieved through interfacial engineering.

Residence time distribution study in a pilot-scale liquid phase catalytic exchange (LPCE) column packed with a mixture of hydrophobic and hydrophilic catalysts

Residence time distribution (RTD) measurements were carried out in a packed bed column designed for exchange of hydrogen isotopes. The main objective of the study was to characterize the liquid phase mixing under various processes and operating conditions. The packed bed was composed of a mixture of two different types of catalytic packing materials, i.e., a hydrophobic material and a hydrophilic material. Technitium-99m (99mTc) as sodium pertechnetate was used as a radiotracer for RTD measurements. From the measured RTD curves, mean residence times (MRTs), liquid holdup and degree of mixing of liquid phase were evaluated. An axial dispersion model exchange with stagnant zones was used to simulate the measured RTD curves. The results of model simulation showed that volume fraction of hydrophobic to hydrophilic packing and gas/liquid superficial velocities affect the liquid holdup, bed pressure drop and liquid phase dispersion/mixing characteristics. The results of the present study will help to screen packing, optimize the volume of the packing fractions, design and construct the catalyst and optimize the operating conditions for scale up of the isotope exchange process.

Comparison of Properties of the Hydrophobic Catalyst RCTU-3SM in Reactions of Isotope Exchange Between Hydrogen and Water Vapors and Oxidation of Trace Hydrogen in Gas Flows

A comparative study of the efficiency of the hydrophobic catalyst RCTU-3SM in chemical isotope exchange reactions of hydrogen with water and the oxidation of trace amounts of hydrogen in relation to the detritiation tasks of technological streams was carried out. It is shown that, depending on the equilibrium conditions of the isotope exchange process, there is an optimum temperature at which the reaction rate has a maximum. It was found that the rate of oxidation reaction of trace hydrogen depends on the content of oxygen in the purified stream. With oxygen concentration reducing, the oxidation rate initially increases, and when the ratio of oxygen and hydrogen concentrations is less than 100, the rate remains constant within the experimental error.