Effect of confinement by porous materials on chemical reaction kinetics

Organic transformations in the confined space of porous organic cage CC2; catalysis or inhibition

Porous organic cages have shape persistent cavities which provide a suitable platform for encapsulation of guest molecules with size suitably fitting to the cavity. The interactions of the guest molecule with the porous organic cage significantly alter the properties of the guest molecule. Herein, we report the effect of encapsulation on the kinetics of various organic transformations including 2 + 4 cycloaddition, 1,5-sigmatropic, 6π-electrocyclization, ring expansion, cheletropic, dyotropic, trimerization and tautomerization reactions. Non-bonding interactions are generated between the CC2 cage and encapsulated species. However, the number and nature/strength of interactions are different for reactant and TS with the CC2 cage and this difference detects the reaction to be accelerated or slowed down. A significant drop in the barrier of reactions is observed for reactions involving strong interactions of the transition state within the cage. However, for some reactions such as the Claisen rearrangement, reactants are stabilized more than the transition state and therefore an increase in activation barrier is observed. Furthermore, non-covalent analyses of all transition states (inside the cage) confirm the interaction between the CC2 cage and substrate. The current study will promote further exploration of the potential of other porous structures for similar applications.

Porous organic cages have shape persistent cavities which provide a suitable platform for encapsulation of guest molecules with size suitably fitting to the cavity.

Water-Gas Shift Reaction to Capture Carbon Dioxide and Separate Hydrogen on Single-Walled Carbon Nanotubes

In view of the increasingly severe global warming and ocean acidification caused by CO₂ emissions, we report a new procedure, named "reactive separation", to capture CO₂. We used advanced Monte Carlo and molecular dynamics methods to simulate the water-gas shift reaction in single-walled carbon nanotubes. We found that (11,11) carbon nanotubes with a diameter of 0.75 nm have the best ability to capture CO₂ generated in the water-gas shift reaction. When the feed water-gas ratio is 1:1, the pressure is 3 MPa, and the temperature is 473 K, the storage capacity of CO₂ reaches 2.18 mmol/g, the molar fraction of CO₂ and H₂ inside the carbon nanotube is 0.87 and 0.09, respectively, the conversion of CO in the pore is as high as 97.6%, and the CO₂/H₂ separation factor is 10.3. Therefore, utilizing the reaction and separation coupling effect of carbon nanotubes to adsorb and store the product CO₂ formed in the water-gas shift reaction, while separating the generated clean energy gas H₂, is a promising strategy for developing novel CO₂ capture technologies.

BN-Doped Graphene and Single-Walled Carbon Nanotubes for the Catalysis of SN2 Reactions: Insights from Density Functional Theory Modeling

The inner space of carbon nanotubes has already been proven to provide a type of confinement, which can dramatically alter the energetics of chemical reactions when compared to the gas phase. Moreover, BN doping can be used to fine-tune electronic properties, which might influence the enthalpy and activation energy of chemical reactions that take place inside their inner space. The energy profile of the prototype Menshutkin S_N2 reaction between ammonia and chloromethane has been analyzed in a variety of carbon-based materials at the DFT (density functional theory) level. Pristine zigzag (9,0) and (12,0) single-walled carbon nanotubes and graphene sheets have been doped with boron and nitrogen at different stoichiometries, namely, BN, BNC, BNC₂, and BNC₄, that resulted in remarkable variations of their catalytic effects. Graphene has revealed to be the support material, which depends less on doping in terms of enthalpy and energy barrier of the reaction. However, when graphene is rolled up into tubular forms, the influence of doping becomes increasingly stronger as the nanotube radius decreases. In the case of BNC₄ doping of (9,0) nanotubes, the activation energy drops 10 kcal/mol when compared to the pristine case, and the reaction became even exothermic by more than 15 kcal/mol.

The effects of confinement in pores built of folded graphene sheets on the equilibrium of nitrogen monoxide dimerisation reaction

In the current work we have used reactive Monte Carlo simulations to systematically study the effects of graphene folding on equilibria of NO dimerisation occurring at isolated surfaces and in porous networks built of corrugated graphene sheets. It has been demonstrated that the folding of isolated graphene sheets significantly improves the yield of reactions occurring on their surface. Then, it has also been shown that in slit-like pores formed by the folded graphene sheets the reaction yield depends on the corrugation and arrangement of the pore walls. It has been found that the reaction yield increases when the walls' corrugation is high because of the appearance of narrow regions and/or wedge-like regions in the pores. The condensation of reacting fluid in such places, where the bulges at both walls are close one to another, leads to much higher reaction yield than on the surface of isolated sheets. Thus, we recommended the highly corrugated graphene to control the chemical reactions.

Chemistry in confined space through the eyes of surface science-2D porous materials

There are a rapidly growing number of studies showing exciting new opportunities in the way confinement effects on surfaces affect the properties of materials and their chemistry. These effects have been observed recently under two-dimensional (2D) van der Waals materials such as a graphene and boron nitride and for the case of supported 2D-porous oxides, including silicates, aluminosilicates and zeolite nanosheets. This review summarizes the current state of the art in this area of research and how confinement effects in 2D systems relate to those found in 3D porous and layered materials. The focus of this review is put in 2D-materials with inherent porosity, such as 2D-porous oxides. An outlook is also given for the future of this exciting emerging area.

Carbon Nanohorns as Reaction Nanochambers - a Systematic Monte Carlo Study

Carbon nanohorns (CNHs, one of the newest carbon allotropes) have been subjected to intensive experimental and theoretical studies due to their potential applications. One of such applications can be their use as reaction nanochambers. However, experimental studies on the reaction equilibria under confinement are extremely challenging since accurate measurements of the concentrations of reacting species in pores are a very hard task. So, the main ways to examine such phenomena are theoretical methods (e.g. the reactive Monte Carlo, RxMC). We have presented the first systematic RxMC study on the influence of the CNH’s geometric parameters (the apex angle, the diameter, and the length) on reaction equilibria, taking the nitrogen monoxide dimerisation as an example. All the investigated parameters significantly affect the reaction yield at low and moderate coverages. Short and narrow CNHs have been found to be preferred. However, the key factor influencing the reaction equilibria is the presence of a conical part. Energetics of interactions between the reacting molecules in this fragment of a nanohorn maximises the effects of confinement. In consequence, CNHs have the advantage over their nanotube counterparts of the same diameter. The obtained results have confirmed that CNHs can be considered as potential reaction nanochambers.

Confinement Effects on Carbon Dioxide Methanation: A Novel Mechanism for Abiotic Methane Formation

An important scientific debate focuses on the possibility of abiotic synthesis of hydrocarbons during oceanic crust-seawater interactions. While on-site measurements near hydrothermal vents support this possibility, laboratory studies have provided data that are in some cases contradictory. At conditions relevant for sub-surface environments it has been shown that classic thermodynamics favour the production of CO2 from CH4, while abiotic methane synthesis would require the opposite. However, confinement effects are known to alter reaction equilibria. This report shows that indeed thermodynamic equilibrium can be shifted towards methane production, suggesting that thermal hydrocarbon synthesis near hydrothermal vents and deeper in the magma-hydrothermal system is possible. We report reactive ensemble Monte Carlo simulations for the CO2 methanation reaction. We compare the predicted equilibrium composition in the bulk gaseous phase to that expected in the presence of confinement. In the bulk phase we obtain excellent agreement with classic thermodynamic expectations. When the reactants can exchange between bulk and a confined phase our results show strong dependency of the reaction equilibrium conversions, [Formula: see text], on nanopore size, nanopore chemistry, and nanopore morphology. Some physical conditions that could shift significantly the equilibrium composition of the reactive system with respect to bulk observations are discussed.

Monte Carlo study of chemical reaction equilibria in pores of activated carbons

Systematic Monte Carlo studies concerning relationships between the porous structure of activated carbons and the equilibria of reactions under confinement are presented.

The effect of nano confinement on the C-h activation and its corresponding structure-activity relationship

The C–H activation of methane, ethane, and t-butane on inner and outer surfaces of nitrogen-doped carbon nanotube (NCNTs) are investigated using density functional theory. It includes NCNTs with different diameters, different N and O concentrations, and different types (armchair and zigzag). A universal structure-reactivity relationship is proposed to characterize the C–H activation occurring both on the inner and outer surfaces of the nano channel. The C–O bond distance, spin density and charge carried by active oxygen are found to be highly related to the C–H activation barriers. Based on these theoretical results, some useful strategies are suggested to guide the rational design of more effective catalysts by nano channel confinement.

Wonderful nanoconfinement effect on redox reaction equilibrium

Redox reactions have been found to be favoured with nanoscale confinement of solid matrices. Softening of reaction conditions as well as enhancement of reaction kinetics in confined nanospaces has been demonstrated.

Nanoconfining affinity materials for pH-mediated protein capture–release

Based on the nanoconfinement effect, two new affinity materials that have a pH-responsive capture–release ability for proteins were developed.

Combining reactive and configurational-bias Monte Carlo: confinement influence on the propene metathesis reaction system in various zeolites

In order to efficiently calculate chemical equilibria of large molecules in a confined environment the reactive Monte Carlo technique is combined with the configurational-bias Monte Carlo approach. To prove that detailed balance is fulfilled the acceptance rule for this combination of particular Monte Carlo techniques is derived in detail. Notably, by using this derivation all other acceptance rules of any Monte Carlo trial moves usually carried out in combination with the configurational-bias Monte Carlo approach can be deduced from it. As an application of the combination of reactive and configurational-bias Monte Carlo the influence of different zeolitic confinements (MFI, TON, LTL, and FER) on the reaction equilibrium and the selectivity of the propene metathesis reaction system was investigated. Compared to the bulk phase the conversion is increased significantly. The authors study this reaction system in the temperature range between 300 and 600 K, and the pressure range from 1 to 7 bars. In contrast to the bulk phase, pressure and temperature have a strong influence on the composition of the reaction mixture in confinement. At low pressures and temperatures both conversion and selectivity are highest. Furthermore, the equilibrium composition is strongly dependent on the type of zeolite. This demonstrates the important role of the host structure in catalytic systems.

Effect of solvation and confinement on the trans-gauche isomerization reaction in n-butane

The effect of solvation and confinement on the conformational equilibria and kinetics of n-butane is examined using molecular dynamics simulations of the bulk and confined fluids and compared to appropriately chosen reference states. Clear evidence for a solvent shift of the preferred conformation in bulk n-butane is found. At a temperature of 292 K and a density of 6.05 nm-3 a small solvent shift in favor of gauche is observed (similar to previously reported values), and the shift increases substantially with an increase in density to 8.28 nm-3. The rate of torsional interconversion from the trans to the gauche state, calculated using the relaxation function method, was found to increase with increasing temperature and density. The rate constants kTG and kGT have an Arrhenius temperature dependence yielding activation energies significantly lower than the trans-gauche and gauche-trans barrier heights in the torsional potential for a free molecule, depending on the density. In the confined phase, we considered the same densities as simulated in the bulk phase, and for four different values of the physical pore width (approximately 1.5-4.0 nm). At the high density, we find that the position of the trans-gauche equilibrium is displaced towards excess trans compared with the bulk phase, reflecting the confinement and interactions of the molecules with the pore wall. The isomerization rate is found to decrease with decreasing pore width. Again, we find that the kinetics obeys an Arrhenius rate law and the activation energy for the trans-gauche and gauche-trans interconversions is slightly smaller than that of the bulk fluid at the same density.

Mesoscale simulation of polymer reaction equilibrium: Combining dissipative particle dynamics with reaction ensemble Monte Carlo. I. Polydispersed polymer systems

We present a mesoscale simulation technique, called the reaction ensemble dissipative particle dynamics (RxDPD) method, for studying reaction equilibrium of polymer systems. The RxDPD method combines elements of dissipative particle dynamics (DPD) and reaction ensemble Monte Carlo (RxMC), allowing for the determination of both static and dynamical properties of a polymer system. The RxDPD method is demonstrated by considering several simple polydispersed homopolymer systems. RxDPD can be used to predict the polydispersity due to various effects, including solvents, additives, temperature, pressure, shear, and confinement. Extensions of the method to other polymer systems are straightforward, including grafted, cross-linked polymers, and block copolymers. To simulate polydispersity, the system contains full polymer chains and a single fractional polymer chain, i.e., a polymer chain with a single fractional DPD particle. The fractional particle is coupled to the system via a coupling parameter that varies between zero (no interaction between the fractional particle and the other particles in the system) and one (full interaction between the fractional particle and the other particles in the system). The time evolution of the system is governed by the DPD equations of motion, accompanied by changes in the coupling parameter. The coupling-parameter changes are either accepted with a probability derived from the grand canonical partition function or governed by an equation of motion derived from the extended Lagrangian. The coupling-parameter changes mimic forward and reverse reaction steps, as in RxMC simulations.

Effect of confinement by porous carbons on the unimolecular decomposition of formaldehyde

As part of an effort to understand the effect of confinement by porous carbons on chemical reactions, we have carried out density functional theory calculations on the unimolecular decomposition of formaldehyde within graphitic carbons. Our results show that the interactions with the carbon walls result in a lowering of the reaction barrier. For larger pores, there is also a shift of the equilibrium towards the formation of carbon monoxide and hydrogen at low temperatures. This trend is reversed for small pore sizes.

Chemical reaction equilibrium in nanoporous materials: NO dimerization reaction in carbon slit nanopores

We present a molecular-level simulation study of the effects of confinement on chemical reaction equilibrium in nanoporous materials. We use the reaction ensemble Monte Carlo (RxMC) method to investigate the effects of temperature, nanopore size, bulk pressure, and capillary condensation on the nitric oxide dimerization reaction in a model carbon slit nanopore in equilibrium with a bulk reservoir. In addition to the RxMC simulations, we also utilize the molecular-dynamics method to determine self-diffusion coefficients for confined nonreactive mixtures of nitric oxide monomers and dimers at compositions obtained from the RxMC simulations. We analyze the effects of the temperature, nanopore width, bulk pressure, and capillary condensation on the reaction equilibrium with respect to the reaction conversion, fluid structure, and self-diffusion coefficients. We show that the influence of the temperature, nanopore size, and capillary condensation on the confined reaction equilibrium is quite dramatic while the effect of the bulk pressure on the reaction equilibrium in the carbon slit nanopore is only moderate. This work is an extension of previous work by Turner et al. [J. Chem. Phys. 114, 1851 (2001)] on the confined reactive nitric oxide system.

Proton Transfer in Nanoconfined Polar Solvents. II. Adiabatic Proton Transfer Dynamics

The reaction dynamics for a model phenol-amine proton transfer system in a confined methyl chloride solvent have been simulated by mixed quantum-classical molecular dynamics. In this approach, the proton vibration is treated quantum mechanically (and adiabatically), while the rest of the system is described classically. Nonequilibrium trajectories are used to determine the proton transfer reaction rate constant. The reaction complex and methyl chloride solvent are confined in a smooth, hydrophobic spherical cavity, and radii of 10, 12, and 15 A have been considered. The effects of the cavity radius and the heavy atom (hydrogen bond) distance on the reaction dynamics are considered, and the mechanism of the proton transfer is examined in detail by analysis of the trajectories.

Reactive Monte Carlo and grand-canonical Monte Carlo simulations of the propene metathesis reaction system

The influence of silicalite-1 pores on the reaction equilibria and the selectivity of the propene metathesis reaction system in the temperature range between 300 and 600 K and the pressure range from 0.5 to 7 bars has been investigated with molecular simulations. The reactive Monte Carlo (RxMC) technique was applied for bulk-phase simulations in the isobaric-isothermal ensemble and for two phase systems in the Gibbs ensemble. Additionally, Monte Carlo simulations in the grand-canonical ensemble (GCMC) have been carried out with and without using the RxMC technique. The various simulation procedures were combined with the configurational-bias Monte Carlo approach. It was found that the GCMC simulations are superior to the Gibbs ensemble simulations for reactions where the bulk-phase equilibrium can be calculated in advance and does not have to be simulated simultaneously with the molecules inside the pore. The confined environment can increase the conversion significantly. A large change in selectivity between the bulk phase and the pore phase is observed. Pressure and temperature have strong influences on both conversion and selectivity. At low pressure and temperature both conversion and selectivity have the highest values. The effect of confinement decreases as the temperature increases.

Proton Transfer in Nanoconfined Polar Solvents. 1. Free Energies and Solute Position

The reaction free energy curves for a model phenol-amine proton-transfer system in a confined CH3Cl solvent have been calculated by Monte Carlo simulations. The free energy curves, as a function of a collective solvent coordinate, have been obtained for several fixed reaction complex radial positions (based on the center-of-mass). A smooth, hydrophobic spherical cavity was used to confine the solvent, and radii of 10 and 15 A have been considered. Quantum effects associated with the transferring proton have been included by adding the proton zero-point energy to the classical free energy. The results indicate the reaction complex position can be an important component of the reaction coordinate for proton-transfer reactions in nanoconfined solvents.

Reaction ensemble molecular dynamics: direct simulation of the dynamic equilibrium properties of chemically reacting mixtures

A molecular simulation method to study the dynamics of chemically reacting mixtures is presented. The method uses a combination of stochastic and dynamic simulation steps, allowing for the simulation of both thermodynamic and transport properties. The method couples a molecular dynamics simulation cell (termed dynamic cell) to a reaction mixture simulation cell (termed control cell) that is formulated upon the reaction ensemble Monte Carlo (RxMC) method, hence the term reaction ensemble molecular dynamics. Thermodynamic and transport properties are calculated in the dynamic cell by using a constant-temperature molecular dynamics simulation method. RxMC forward and reverse reaction steps are performed in the control cell only, while molecular dynamics steps are performed in both the dynamic cell and the control cell. The control cell, which acts as a sink and source reservoir, is maintained at reaction equilibrium conditions via the RxMC algorithm. The reaction ensemble molecular dynamics method is analogous to the grand canonical ensemble molecular dynamics technique, while using some elements of the osmotic molecular dynamics method, and so simulates conditions that directly relate to real, open systems. The accuracy and stability of the method is assessed by considering the ammonia synthesis reaction N2 +3 H2 <-->2N H3 . It is shown to be a viable method for predicting the effects of nonideal environments on the dynamic properties (particularly diffusion) as well as reaction equilibria for chemically reacting mixtures.

Dual control cell reaction ensemble molecular dynamics: A method for simulations of reactions and adsorption in porous materials

We present a simulation tool to study fluid mixtures that are simultaneously chemically reacting and adsorbing in a porous material. The method is a combination of the reaction ensemble Monte Carlo method and the dual control volume grand canonical molecular dynamics technique. The method, termed the dual control cell reaction ensemble molecular dynamics method, allows for the calculation of both equilibrium and nonequilibrium transport properties in porous materials such as diffusion coefficients, permeability, and mass flux. Control cells, which are in direct physical contact with the porous solid, are used to maintain the desired reaction and flow conditions for the system. The simulation setup closely mimics an actual experimental system in which the thermodynamic and flow parameters are precisely controlled. We present an application of the method to the dry reforming of methane reaction within a nanoscale reactor model in the presence of a semipermeable membrane that was modeled as a porous material similar to silicalite. We studied the effects of the membrane structure and porosity on the reaction species permeability by considering three different membrane models. We also studied the effects of an imposed pressure gradient across the membrane on the mass flux of the reaction species. Conversion of syngas (H2/CO) increased significantly in all the nanoscale membrane reactor models considered. A brief discussion of further potential applications is also presented.

Simulations of time-dependent fluorescence in nano-confined solvents

The time-dependent fluorescence of a model diatomic molecule with a charge-transfer electronic transition in confined solvents has been simulated. The effect of confining the solvent is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size (radii of 10-20 angstroms). In previous work [J. Chem. Phys. 118, 6618 (2002)] it was found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. Here we examine the effect of cavity size on the time-dependent fluorescence, a common experimental probe of solvent dynamics. The present results confirm a prediction that motion of the solute in the cavity after excitation can be important in the time-dependent fluorescence. The effects of solvent density are also considered. The results are discussed in the context of interpreting time-dependent fluorescence measurements of confined solvent systems.

Computational elucidation of the transition state shape selectivity phenomenon

The most commonly cited example of a transition state shape selective reaction, m-xylene disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136 kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated pathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the course of reaction. Barrier height shifts on the order of 10-20 kJ/mol are achievable. Observed selectivities do not agree with the transition state characteristics calculated here and, hence, are most likely due to product shape selectivity. Further examination of the pathways highlights the importance of mechanistic steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state shape selectivity.

Molecular simulation of shocked materials using the reactive Monte Carlo method

We demonstrate the applicability of the reactive Monte Carlo (RxMC) simulation method [J. K. Johnson, A. Z. Panagiotopoulos, and K. E. Gubbins, Mol. Phys. 81, 717 (1994); W. R. Smith and B. Tríska, J. Chem. Phys. 100, 3019 (1994)] for calculating the shock Hugoniot properties of a material. The method does not require interaction potentials that simulate bond breaking or bond formation; it requires only the intermolecular potentials and the ideal-gas partition functions for the reactive species that are present. By performing Monte Carlo sampling of forward and reverse reaction steps, the RxMC method provides information on the chemical equilibria states of the shocked material, including the density of the reactive mixture and the mole fractions of the reactive species. We illustrate the methodology for two simple systems (shocked liquid NO and shocked liquid N2), where we find excellent agreement with experimental measurements. The results show that the RxMC methodology provides an important simulation tool capable of testing models used in current detonation theory predictions. Further applications and extensions of the reactive Monte Carlo method are discussed.