Removal of free fatty acid in waste frying oil by esterification with methanol on zeolite catalysts

The removal of free fatty acid (FFA) in waste frying oil by esterification with methanol was conducted using various zeolite catalysts. The ZSM-5 (MFI), mordenite (MOR), faujasite (FAU), beta (BEA) zeolites, and silicalite were employed with different Si/Al molar ratio in the reaction. The effects of acidic properties and pore structure of the zeolite catalysts were discussed relating to the conversion of the FFA. The MFI zeolite induced an improvement of the removal efficiency of FFA by cracking to the FFA in its pore structure due to its narrow pore mouth. The catalytic activity for FFA removal was lowered with decreasing of acid strength of the zeolites. The strong acid sites of zeolites induced the high conversion of FFA comparatively. The acid strength and pore structure of acidic zeolites affected the catalytic activity in FFA removal.     

A comparative study of the adsorption of water and methanol in zeolite BEA: a molecular simulation study

Grand canonical Monte Carlo simulations were carried out to study the equilibrium adsorption concentration of methanol and water in all-silica BEA zeolite and HBEA zeolites with different Si/Al ratios over a wide range of temperatures and loadings. These zeolites have oval-shaped channels with one side longer than the other. Water sorption into the hydrophobic BEA zeolite had a sharp transition with its sorption going from zero to near full capacity over a very small pressure range. Methanol sorption was much more gradual with respect to pressure. With the addition of hydrophilic sites for the HBEA zeolites by decreasing the Si/Al ratio, adsorption at lower pressures increased significantly for water and methanol. At higher loadings, water and methanol adsorption were found to behave in fundamentally different ways. Water structures in the zeolite channels formed hydrogen-bonded chains while maximising contact with the surfaces on the longer edges of the zeolite channels. Methanol molecules, in contrast, formed very few hydrogen bonds between themselves, with their hydroxyl groups primarily binding with surface of the shorter edge of the zeolite channels and their methyl groups located near the middle of the zeolite channels. The addition of hydrophilic groups in the HBEA zeolites strongly influenced positions of the methanol hydroxyl groups at high loadings, but did not have a significant effect on water structure.     

New zeolite adsorbents for downstream processing of polyphenols from renewable resources

Commercial materials with polyvinylpolypyrrolidone and polymeric amberlites (XAD7HP, XAD16) are commonly used for the adsorptive downstream processing of polyphenols from renewable resources. In this study, beta‐zeolite‐based adsorbent systems were examined, and their properties were compared to organic resins. Batch adsorption experiments were conducted with synthetic solutions of major polyphenols. Adsorption isotherms and desorption characteristics of individual adsorbent were determined based on these results. Maximum adsorption capacities were calculated using the Langmuir model. For example, the zeolites had capacities up to 203.2 mg/g for ferulic acid. To extend these results to a complex system, additional experiments were performed on rapeseed meal and wheat seed extracts as representative renewable resources. HPLC analysis showed that with 7.5% w/v, which is regarded as the optimum amount of zeolites, zeolites A and B could bind 100% of the major polyphenols as well as release polyphenols at high yields. Additionally, regeneration experiments were performed with isopropyl alcohol at 99°C to evaluate how zeolites regenerate under mild conditions. The results showed only a negligible loss of adsorption capacity and no loss of desorption capacity. In summary, it was concluded that beta‐zeolites were promising adsorbents for developing new processes to isolate polyphenols from renewable resources.     

Multiple adsorption of CO on Na-exchanged Y faujasite: a DFT investigation

Carbon monoxide is involved in many chemical and industrial processes, and its removal is of great importance to reduce detrimental environmental and climate impacts. CO is also useful to characterise the metal exchanged in zeolites. Multiple adsorption of CO in zeolite faujasites containing Na cations is investigated through quantum chemical calculations. Density functional theory (DFT) calculations were chosen to investigate the structure of sodium-exchanged cations at site II in Y faujasite and to investigate multiple CO adsorption with Na to predict the structure and the infrared CO stretching signal. DFT analysis using B3LYP, B3LYP-D and M062X showed significant differences in the coordination of Na at site II when three CO are adsorbed. From these investigations, polyadsorption of CO in NaY could lead to threefold-coordinated Na at site II in six-membered rings (6MRs) containing two Al and twofold-coordinated Na at site II in 6MRs containing one Al. These results suggest that introduction of non-bonding interactions is necessary to study polyadsorption of CO in NaY.     

Si/Al and metal loading effects on the templated synthesis of Ni nanowires in CAN and MOR zeolite frameworks

Zeolites with 1D pore channels, such as cancrinite (CAN) and mordenite (MOR), have the potential to be used as templates in the synthesis of subnanometre metal nanowires. Previous studies show a strong correlation between the location of Al atoms in zeolites and the positioning of the metal atoms inside the zeolite framework. Thus, Metropolis Monte Carlo simulations were used here to study the behaviour of Ni atoms within the CAN- and MOR-type zeolites at different Si/Al ratios and Ni loadings. It was found for both zeolite frameworks that a lower Si/Al ratio favoured energetically the positioning of Ni atoms in the 1D pore channels and that higher loadings of Ni promote the formation of 1D Ni structures. These results suggest that it is possible to use zeolites with the CAN and MOR frameworks and low Si/Al ratio as effective templates for the synthesis of metal nanowires.     

Adsorption of Adenine and Thymine on Zeolites: FT-IR and EPR Spectroscopy and X-Ray Diffractometry and SEM Studies

The interactions of adenine and thymine with and adsorption on zeolites were studied using different techniques. There were two main findings. First, as shown by X-ray diffractometry, thymine increased the decomposition of the zeolites (Y, ZSM-5) while adenine prevented it. Second, zeolite Y adsorbed almost the same amount of adenine and thymine, thus both nucleic acid bases could be protected from hydrolysis and UV radiation and could be available for molecular evolution. The X-ray diffractometry and SEM showed that artificial seawater almost dissolved zeolite A. The adsorption of adenine on ZSM-5 zeolite was higher than that of thymine (Student-Newman-Keuls test-SNK p<0.05). Adenine was also more greatly adsorbed on ZSM-5 zeolite, when compared to other zeolites (SNK p<0.05). However the adsorption of thymine on different zeolites was not statistically different (SNK p>0.05). The adsorption of adenine and thymine on zeolites did not depend on pore size or Si/Al ratio and it was not explained only by electrostatic forces; rather van der Waals interactions should also be considered.     

Molecular simulations of physical and chemical adsorption under gas and liquid environments using force field- and quantum mechanics-based methods

Here we review our simulations of adsorption on metal–organic frameworks (MOFs) and platinum (Pt) catalysts, focusing on the modelling methods required to understand these two very different systems. MOFs are porous, crystalline materials with large surface areas, which are promising for a variety of adsorption applications. We review our simulations of gas uptake in PCN-53 (porous coordination network) as well as gas storage in MOFs functionalised with metal alkoxide sites. While fluid–solid interactions in both systems can be modelled quite well using algebraic force fields, the alkoxide sites in the functionalised MOFs require specialised versions, in order to describe the stronger adsorption energies. We discuss grand canonical Monte Carlo (GCMC) simulations of both systems. Pt is a common catalyst, and simulations have proven quite useful for providing molecular level details to understand its functionality. This involves understanding adsorption phenomena, which often requires quantum mechanical calculations. We describe our periodic boundary condition density functional theory (DFT) simulations of Pt-catalysed NO oxidation, focusing on adsorbate geometries and coverage effects. Finally, we describe one of the current ‘grand challenges’ in molecular simulations of adsorption, modelling catalytic activity in aqueous phase, which requires a combination of algebraic force fields, DFT and GCMC.     

Influence of force fields on the selective diffusion of para-xylene over ortho-xylene in 10-ring zeolites

A molecular dynamics study of diffusion of p-xylene and o-xylene has been performed over three different pure silica 10-ring zeolites, MFI, SFG and TUN. The shape selective properties of the frameworks of these three materials have been tested using four different types of force fields commonly used based on united atom, rigid-ion and core-shell approximations. The performance of each force field is analysed in order to find which force fields can give sufficiently accurate estimations that allow to select appropriate zeolites for selective separation of para/ortho xylene. This performance was found to depend on the quality of the structural properties of the zeolite, in particular the size and shape of the 10 rings which act as bottlenecks for the diffusion. The computational results allow us to define some optimum characteristics for the selective diffusion of p-xylene.     

Continuous enzymatic production and adsorption of laminaribiose in packed bed reactors

Bienzymatic production of laminaribiose from sucrose and glucose was combined with adsorption on zeolite BEA to introduce a first capture and purification step. Downstream processing including washing and desorption steps was characterized and optimized on a milliliter scale in batch mode. Results were then transferred to a packed bed system for enzymatic production and adsorption where the influence of adsorbent particle diameter on purity and productivity was evaluated. Finally, a continuous enzymatic production of laminaribiose was conducted over 10 days. The subsequent downstream processing of the loaded zeolites led to purities of over 0.5 g_{Laminaribiose} g_sugar^?1 in the desorbate with a total productivity of 5.6 mg_{Laminaribiose} L_{enzyme bed}^?1 h^?1 without the use of recycles.     

Thiophenic compounds adsorption on Na(I)Y and rare earth exchanged Y zeolites: a density functional theory study

We have theoretically investigated the adsorption of thiophene, benzothiophene, dibenzothiophene on Na(I)Y and rare earth exchanged La(III)Y, Ce(III)Y, Pr(III)Y Nd(III)Y zeolites by density functional theory calculations. The calculated results show that except benzothiophene adsorbed on Na(I)Y with a stand configuration, the stable adsorption structures of other thiophenic compounds on zeolites exhibit lying configurations. Adsorption energies of thiophenic compounds on the Na(I)Y are very low, and decrease with the increase of the number of benzene rings in thiophenic compounds. All rare earth exchanged zeolites exhibit strong interaction with thiophene. La(III)Y and Nd(III)Y zeolites are found to show enhanced adsorption energies to benzothiophene and Pr(III)Y zeolites are favorable for dibenzothiophene adsorption. The analysis of the electronic total charge density and electron orbital overlaps show that the thiophenic compounds interact with zeolites by π-electrons of thiophene ring and exchanged metal atom. Mulliken charge populations analysis reveals that adsorption energies are strongly dependent on the charge transfer of thiophenic molecule and exchanged metal atom.     

The effects of structural parameters of zeolite on the adsorption of hydrogen: a molecular simulation study

The adsorption isotherm of hydrogen in zeolites FAU, LTA, KFI, RWY, RHO and TSC has been simulated employing grand canonical Monte Carlo procedure for a temperature range of 77 to 95 K and different pressures. The effects of structural composition, unit cell volume, framework density and specific surface area of zeolite on hydrogen adsorption in zeolites were investigated. The results clearly show that the adsorption of hydrogen in zeolites with the same silica density is a function of oxygen density at low pressures, and it is approximately the same at intermediate pressures. Nevertheless, at high pressures, the adsorption of hydrogen is a function of pore diameter for zeolites with same silica density. The effect of specific surface area on the adsorption isotherm of hydrogen on zeolites with approximately the same specific surface area is significant at low and high pressures. The results clearly indicate that the adsorption of hydrogen in RWY zeolite has maximum value at 77 K and at high pressures. The optimum condition of pressure for hydrogen adsorption isotherm in RWY zeolite is determined to be 600 bar. At a temperature of 77 K and a pressure of 600 bar, the adsorption of hydrogen in RWY zeolite is 6.93 wt %.     

Formation of a non-classical “η-H2-Ag” hydride in zeolitic materials

Quantum chemical calculations including DFT, MP2, MP4 and QCISD predict the formation of non-classical η²-hydride of Ag⁺ coordinated to two oxygen atoms, in the frameworks of various zeolites. These model calculations are compared with existing zeolite structural data, other modelling works, and Inelastic Neutron Scattering (INS) spectroscopy data of dihydrogen adsorbed in a Ag-exchanged zeolite with LTA topology. It is also found that, although a little less stable, the corresponding η²-H₂-Ag⁺ exist as well in the gas phase.     

Carbon doped boron phosphide nanotubes: A computational study

A computational study based on density functional theory (DFT) calculations has been performed to investigate the properties of the electronic structure of carbon doped boron phosphide nanotube (C-doped BPNT). Pristine and the C-doped structures of two representative (6,0) zigzag and (4,4) armchair BPNTs have been investigated. At first, the geometries of the structures have been allowed to relax by optimization. Subsequently, NMR parameters have been calculated in the optimized structures. The results indicated that the influence of C-doping was more significant on the geometries of the zigzag model than the armchair one. The difference of band gap energies between the pristine and C-doped armchair BPNT was larger than the zigzag model. Significant differences of NMR parameters of those nuclei directly contributed to the C-doping atoms have been observed.     

Effect of Si/Al ratio on tetralin adsorption on Y zeolite: a DFT study

Probing the adsorption of tetralin on zeolite is of prime scientific and industrial importance with the aim to upgrade the industrial process of tetralin cracking. In this work, the effect of Si/Al ratio ranging from 12 to 39 on tetralin adsorption property on Y zeolite is studied by DFT calculations. Tetralin adsorption on Y zeolite corresponds to a π-stacking adsorption mechanism between double bonds of aromatic ring and Brønsted acid sites. Therefore, the number of Brønsted acid sites influences the adsorption properties. Lower Si/Al ratio with more Brønsted acid sites interacting with the aromatic ring of tetralin leads to a higher adsorption energy. Furthermore, the charge and frontier molecular orbital analysis are also performed to understand the influence of Si/Al ratio on adsorption performance. Y zeolite with lower Si/Al ratio shows larger charge difference values and lower HOMO–LUMO gap, which directly manifests the stronger adsorption ability of tetralin and indicates bigger possibility of reacting.     

Differences and similarities in binding of pyruvate and L-lactate in the active site of M4 and H4 isoforms of human lactate dehydrogenase

We present QM/MM calculations that show differences in geometries of active sites of M(4) and H(4) isoforms of human LDH ligated with oxamate, pyruvate or L-lactate. As the consequence of these differences, binding isotope effects of the methyl hydrogen atoms of pyruvate and l-lactate may be used to experimentally distinguish these isoforms. Based on the FEP calculations we argue that L-lactate is a better candidate for the experimental studies. Our calculations of energies of interactions of ligands with the active site residues provide explanation for the observed experimentally sensitivity to inhibition of the M(4) isoenzyme isoform and pinpoint the differences to interactions of the ligand with the histidine residue. We conclude that pyruvate interacts much stronger in the active site of H(4) than M(4) isoform and that the latter interactions are weaker than with water molecules in the aqueous solution.     

A density functional theory study of hydrogen adsorption on Be-, Mg-, and Ca-exchanged LTL zeolite clusters

Hydrogen molecule adsorption on frameworks consisting of alkaline earth metal atoms (Be, Mg, or Ca) in LTL zeolite was investigated via density functional theory. A 24T zeolite cluster model was used in this study. HOMO and LUMO energy, chemical potential, chemical hardness, electronegativity, adsorption energy, and adsorption enthalpy values were calculated. The Mg-LTL and Ca-LTL clusters were found to have much lower chemical potentials and adsorption energies than those of the Be-LTL cluster. Additionally, the calculations indicated that the Mg-LTL and Ca-LTL clusters are softer (considering their lower chemical hardness values) and more chemically reactive than the Be-LTL cluster. The calculated hydrogen adsorption enthalpies were ?14.7 and ?9.4 kJ/mol for the Mg-LTL and Ca-LTL clusters, respectively, which are significantly larger than the enthalpy of liquefaction for the hydrogen molecule. These results imply that the Mg-LTL and Ca-LTL zeolite structures are promising cryoadsorbents for hydrogen storage.

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Graphical abstract Hydrogen adsorption was theoretically investigated on Be-, Ca- and Mg-LTL clusters. Ca- and Mg-LTL zeolites are potential cryoadsorbent materials for hydrogen storage.

     

Accurate aqueous proton dissociation constants calculations for selected angiotensin-converting enzyme inhibitors

Studies that allow computing values of aqueous proton dissociation constants (pKa), gas phase proton affinities, and the free energy of solvation have been performed for six members of angiotensin-I-converting enzyme (ACE) inhibitor family (captopril, enalaprilat, imidaprilat, ramiprilat, perindoprilat, and spiraprilat). Density functional theory (DFT) calculations using PBE1PBE functional on optimized molecular geometries have been carried out to investigate the thermodynamics of gas-phase protonation. The conductor-like polarizable continuum model (CPCM) solvation method at various levels of theory was applied to calculate the free energy of solvation for the ACE inhibitors and their respective anions. The CPCM solvation calculations were performed on both gas-phase and solvent-phase optimized structures. The combination of gas-phase and solvation energies according to the thermodynamic cycle enabled us to compute accurate pKa values for the all studied molecules.     

Theoretical Ab initio study of the effects of methylation on the nature of hydrogen bonding in A:T base pair

We report the results of a theoretical ab initio study of methylation in Watson-Crick A:T base pairs. Equilibrium geometries were obtained without symmetry restrictions by the gradient procedure at DFT level of theory with the standard 6-31G(d) basis set. Each local minima was verified by energy second derivative calculations. Single-point calculations for the DFT geometries have been performed at the MP2/6-31G(d,p), MP2/6-31++G(d,p), and MP2/6-311++G(2d,2p) levels of theory. The geometrical parameters, relative stabilities and counterpoise corrected interaction energies are reported. In addition, using a variation-perturbation energy decomposition scheme, we have found the important contributions to the total interaction energy.     

Carbon dioxide capture using covalent organic frameworks (COFs) type material—a theoretical investigation

The present work deals with a density functional theory (DFT) study of porous organic framework materials containing – groups for CO₂ capture. In this study, first principle calculations were performed for CO₂ adsorption using N-containing covalent organic framework (COFs) models. Ab initio and DFT-based methods were used to characterize the N-containing porous model system based on their interaction energies upon complexing with CO₂ and nitrogen gas. Binding energies (BEs) of CO₂ and N₂ molecules with the polymer framework were calculated with DFT methods. Hybrid B3LYP and second order MP2 methods combined with of Pople 6-31G(d,p) and correlation consistent basis sets cc-pVDZ, cc-pVTZ and aug-ccVDZ were used to calculate BEs. The effect of linker groups in the designed covalent organic framework model system on the CO₂ and N₂ interactions was studied using quantum calculations.