Crystal Morphology Prediction of Hexahydro-1,3,5-trinitro-1,3,5-triazine by the Spiral Growth Model

The crystal morphology of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was predicted by the advanced Burton–Cabrera–Frank (BCF) model with consideration of non-centrosymmetric growth units. The present modeling showed that the advanced BCF model provides reliable results and understanding of the growth habit of RDX crystals grown from acetone and γ-butyrolactone, in which the {210} and {111} faces are dominantly developed. In the present work, the kink rate, which is the net flux input and output from kink sites along the edge, is found to be a critical factor for the crystal growth of RDX, where the energy required for detaching the growth unit from a kink site determines the existence probability of a growth unit. The {210} and {111} faces were found to show slow kink rates originated from the kink sites with a low existence probability and consequently became morphologically important faces. Moreover, the effect of solvent on the crystal growth habit of RDX, the difference in morphology of RDX grown from acetone and γ-butyrolactone, was confirmed by utilizing local concentration contained in the kink rate term

Molecular Approach to the Effect of Interfacial Energy on Growth Habit of ε‑HNIW

The present work describes how consideration of the onset of supersaturation for 2D nucleation, σ_2D, is very important for prediction of the growth habit of ε-hexanitrohexaazaisowurtzitane (ε-HNIW) when the edge energy decreases extremely in solution. From ethyl acetate, the spiral growth model without considering σ_2D was shown to accurately predict the polyhedral morphology of ε-HNIW, where the {110}, {101}, {111̅}, {002}, and {101̅} faces are mainly constructed. However, that model was found to be inappropriate for predicting the bipyramidal morphology of ε-HNIW from methanol because the {101} face was anticipated to dominate, but a bipyramid shape is possible only if the {101} face grows faster and finally disappears. The present simulation results show that the edge energy of the {101} face is considerably reduced from methanol by forming a hydrogen bond. This gives rise to a decrease in the σ_2D compared with the growth of ε-HNIW from ethyl acetate, which means that the {101} face has a relatively high probability to grow faster by 2D nucleation. As a consequence, a parameter of σ_2D enables us to exclude the morphologically unimportant faces among flat-faces by determining which face tends to grow faster by 2D nucleation

Prediction of Growth Habit of β‑Cyclotetramethylene-tetranitramine Crystals by the First-Principles Models

Experimentally, β-cyclotetramethylene-tetranitramine (β-HMX) crystals were found to dramatically elongate to the [100] direction when a relatively high supersaturation was imposed. A sudden growth of β-HMX to the [100] direction is closely associated with a mechanistic transition from spiral growth to two-dimensional (2D) nucleation for the (110) face. The onset supersaturation for the growth by 2D nucleation, σ_2D, was found to play a key role in the growth of β-HMX. The present simulation results based on first-principles models such as the spiral growth model and the 2D nucleation model show that the values of σ_2D on the (101) and (101̅) faces are smaller than those on the (020), (110), and (011) faces. This leads to the prediction of rapid growth rates for the (101) and (101̅) faces by 2D nucleation at low supersaturation and the appearance of a typical shape of β-HMX. On the other hand, the needle-like shape of β-HMX begins to prevail when the supersaturation exceeds the σ_2D for the (110) face because its growth mechanism is transformed from the spiral growth mechanism to the 2D nucleation mechanism which accompanies rapid growth of the (110) face. As a result, the present predictions are in remarkable agreement with the experiments. Furthermore, the kinetic Monte Carlo (KMC) simulation also shows that the σ_2D for the (110) face is lower than that for the (011) face because the (011) face provides the surface topology on which growth units are unfavorably incorporated into the lattice sites. It evidently shows that the relative positions of σ_2D bring on the advent of needle-like growth of β-HMX

Crystallization of Glycine by Drowning-Out Combined with Fines Dissolution and Cooling Process with in Situ Control using Focused Beam Reflectance Measurement and Attenuated Total Reflection–Fourier Transform Infrared Spectroscopy

The DFC process (a combined process of drowning-out, fines dissolution, and cooling crystallization) was applied for the crystallization of glycine. The DFC process encourages seed crystals to grow further by the dissolution of fines generated by drowning-out. Therefore, glycine crystals obtained through the DFC process have relatively large size and uniform size distribution with high product yield compared with those obtained by conventional crystallization methods. In the present work, an operating strategy of the DFC process with in situ control was proposed. First, antisolvent is injected until generation of fine particles is assumed to be completed. Second, fines dissolution by heating is conducted until fine particles are mostly dissolved. Here, generation and dissolution of fine particles are monitored by focused beam reflectance measurement and attenuated total reflectance–Fourier transform infrared. The main advantage of the in situ controlled approach demonstrated in the present work is no requirement for any predetermined data and a reduction of the operating time compared with the previously proposed DFC process, in which operating profile is predetermined by the mass balance equations

Recent Progress and Novel Applications in Enzymatic Conversion of Carbon Dioxide

Turning carbon dioxide (CO2) into fuels and chemicals using chemical, photochemical, electrochemical, and enzymatic methods could be used to recycle large quantities of carbon. The enzymatic method, which is inspired by cellular CO2 metabolism, has attracted considerable attention for efficient CO2 conversion due to improved selectivity and yields under mild reaction conditions. In this review, the research progress of green and potent enzymatic conversion of CO2 into useful fuels and chemicals was discussed. Furthermore, applications of the enzymatic conversion of CO2 to assist in CO2 capture and sequestration were highlighted. A summary including the industrial applications, barriers, and some perspectives on the research and development of the enzymatic approach to convert CO2 were introduced

Molecular Modeling on the Role of Local Concentration in the Crystallization of l‑Methionine from Aqueous Solution

In the present work, the importance of local concentration of the growth of l-methionine (l-Met) was explored by the spiral growth model and interfacial structure (IS) analysis. The spiral growth model shows that the decrease in the local concentration of growth units at crystal faces plays a key role in an appearance of thin hexagonal plate-like morphology of l-Met crystals from water. The (001)_2 layer with hydrogen bonds on its under surface was found to require a relatively large amount of energy for detachment of growth units from kink sites. This situation results in lower local concentration and leads to the decrease in net flux of solute molecules at kink sites compared with the (001)_1 layer interacting with its under surface by van der Waals force. Furthermore, the interfacial structure analysis provides important information on the growth of l-Met. Hydrophilic NH₃⁺ and COO^– groups of l-Met on the (001)_1 surface were found to have a tendency to form hydrogen bonding with water, and thus l-Met molecules should overcome the energy barrier required to detach water molecules adsorbed on the crystal surface for continuous growth of the (001) face. From those results, it can be concluded that the limitation of approaching growth units for the formation of the (001)_2 layer onto the (001)_1 surface is responsible for very slow growth of the [001] direction of l-Met crystals