A DFT study of the effect of NNN Al atom on strength of Brönsted acid sites of HY zeolite

A density functional theory study has been employed to investigate the effect of next-nearest-neighbours aluminium (NNN Al) atom on the acid strength of Brönsted acid site in Y zeolite. Additionally, the distribution of NNN Al atom was also studied. The obtained results show that thermodynamically, the favourable sites for location of NNN Al atom are the diagonal sites of the four-membered ring and meta-position of hexagon I. Furthermore, the increase of NNN Al atoms causes the nonlinear reduction of the strength of Brönsted acid sites. When the number of NNN Al atoms is greater than two, the increasing extent of acid strength is not obvious with the reduction in the number of NNN Al atoms. But the acid strength will increase linearly with the further reduce of the number of NNN Al atoms. Compared with deprotonation energy (E_D), ammonia adsorption energy (E_ads) could give a more reasonable measuring result for the acid strength.     

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.     

Entrapping Molecules in Zeolites Nanocavities: A Thermodynamic and Ab-Initio Study

Adsorption enthalpies of Ar, N₂, CO, H₂O, CH₃CN and NH₃ on H-BEA and H-MFI zeolites and on Silicalite, have been measured calorimetrically at 303K in order to assess the energetic features of dispersive forces interactions (confinement effects), H-bonding interactions with surface silanols and specific interactions with Lewis and Brønsted acidic sites. The adsorption of the molecular probes with model clusters mimicking surface silanols, Lewis and Bronsted sites has been simulated at ab-initio level. The combined use of the two different approaches allowed to discriminate among the different processes contributing to the measured (-Δ_adsH). Whereas CO and N₂ single out contributions from Lewis and Br{\o}nsted acidic sites, Ar is only sensitive to confinement effects. For H₂O, CH₃CN and NH₃ the adsorption on Brønsted sites is competitive with the adsorption on Lewis sites. The energy of interaction of H₂O with all considered zeolites is surprisingly higher than expected on the basis of -Δ_adsH vs PA correlation.     

Use of comparative triazolinium triflate fragmentation rates as a tool to assay relative competency of Brønsted bases in N→N proton transfer

Brønsted acid-induced fragmentation of a triazoline is used as a tool to identify Brønsted base additives capable of playing the role of a proton shuttle. Relative to water, dimethyl formamide accelerates proton transfer substantially under these conditions. A series of alcohols and ethers were also used to demonstrate that the Brønsted basicity of additive functionality, not their Brønsted acidity, is responsible for their ability to accelerate proton transfer from triazoline N3 to N1. This knowledge was then used to develop a convenient two step protocol for the synthesis of oxazolidine diones from benzyl azide and an unsaturated imide that employs a substoichiometric additive for triazolinium fragmentation.     

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.     

General base-catalyzed ester hydrolysis as a model of the “charge-relay” system

The validity of the “charge-relay” system in serine esterases was examined by use of the general base-catalyzed hydrolysis of ethyl chloroacetate (I) as a model system. The general base catalytic rate for 2-benzimidazoleacetic acid (II) exhibited an eightfold positive deviation from the Brønsted plot including benzimidazole, imidazole, N-methylimidazole (V), and acetate ion, though in nucleophilic catalysis of the hydrolysis of p-nitrophenyl acetate, the point for II conformed to the Brønsted relationship together with imidazole and benzimidazole derivatives. The positive deviation of II from the Brønsted plot for the general base-catalysis was attributed to the cooperativity of the carboxyl group of II, the imidazolyl group of II, and the hydroxyl group of water. The present result provides support for the “charge-relay” system. Furthermore, the (essentially) total loss of the enzymatic activity due to N-3 methylation of histidine-57 in α-chymotrypsin is discussed in comparison to the general base-catalysis of V in the hydrolysis of I, which is also favorable for the “charge-relay” system.     

Cation redistribution upon dehydration of Na58Y faujasite zeolite: a joint neutron diffraction and molecular simulation study

Using neutron scattering and Monte Carlo simulation, we investigate the distribution of cations in Na₅₈Y faujasite upon (de)hydration. We introduce a new method for the assignment of cations to specific sites in molecular simulations from their local environment. This allows us to bypass the need of the coordinates of crystallographic sites, which vary as water adsorption induces changes in the zeolite framework structure. Although the agreement between experiments and simulation is excellent at high temperature, some differences are observed below 150°C. We show that these differences are due to the presence of water and that temperature itself as well as adsorption-induced deformation of the framework play a less important role. We demonstrate the migration of sodium to sites III upon water adsorption, not observed for other Si:Al ratios.     

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.     

Zeolites as supports for an enzymatic alcoholysis reaction

A recombinant cutinase from Fusarium solani pisi was immobilized by adsorption on several zeolites and its activity towards the alcoholysis reaction of butyl acetate with hexanol, in organic media (isooctane), was measured as a function of the water content and water activity. The effects of the zeolite framework composition (including cation nature) and acidity were studied. The results were compared with other commonly used supports: polyamide Accurel-PA6, silica and alumina. Both the nature of the cation and the silica:alumina (Si:Al) ratio of the framework revealed to be important parameters. The most promising results were obtained for supports with little acidity and with lower Si:Al ratio. This last observation is in accordance with the results obtained with silica and alumina.     

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.     

Identical Ring Cleavage Products during Anaerobic Degradation of Naphthalene, 2-Methylnaphthalene, and Tetralin Indicate a New Metabolic Pathway

Anaerobic degradation of naphthalene, 2-methylnaphthalene, and tetralin (1,2,3,4-tetrahydronaphthalene) was investigated with a sulfate-reducing enrichment culture obtained from a contaminated aquifer. Degradation studies with tetralin revealed 5,6,7,8-tetrahydro-2-naphthoic acid as a major metabolite indicating activation by addition of a C₁ unit to tetralin, comparable to the formation of 2-naphthoic acid in anaerobic naphthalene degradation. The activation reaction was specific for the aromatic ring of tetralin; 1,2,3,4-tetrahydro-2-naphthoic acid was not detected. The reduced 2-naphthoic acid derivatives tetrahydro-, octahydro-, and decahydro-2-naphthoic acid were identified consistently in supernatants of cultures grown with either naphthalene, 2-methylnaphthalene, or tetralin. In addition, two common ring cleavage products were identified. Gas chromatography-mass spectrometry (GC-MS) and high-resolution GC-MS analyses revealed a compound with a cyclohexane ring and two carboxylic acid side chains as one of the first ring cleavage products. The elemental composition was C₁₁H₁₆O₄ (C₁₁H₁₆O₄-diacid), indicating that all carbon atoms of the precursor 2-naphthoic acid structure were preserved in this ring cleavage product. According to the mass spectrum, the side chains could be either an acetic acid and a propenic acid, or a carboxy group and a butenic acid side chain. A further ring cleavage product was identified as 2-carboxycyclohexylacetic acid and was assumed to be formed by β-oxidation of one of the side chains of the C₁₁H₁₆O₄-diacid. Stable isotope-labeling growth experiments with either ¹³C-labeled naphthalene, per-deuterated naphthalene-d₈, or a ¹³C-bicarbonate-buffered medium showed that the ring cleavage products derived from the introduced carbon source naphthalene. The series of identified metabolites suggests that anaerobic degradation of naphthalenes proceeds via reduction of the aromatic ring system of 2-naphthoic acid to initiate ring cleavage in analogy to the benzoyl-coenzyme A pathway for monoaromatic hydrocarbons. Our findings provide strong indications that further degradation goes through saturated compounds with a cyclohexane ring structure and not through monoaromatic compounds. A metabolic pathway for anaerobic degradation of bicyclic aromatic hydrocarbons with 2-naphthoic acid as the central intermediate is proposed.     

Identical ring cleavage products during anaerobic degradation of naphthalene, 2-methylnaphthalene, and tetralin indicate a new metabolic pathway

Anaerobic degradation of naphthalene, 2-methylnaphthalene, and tetralin (1,2,3,4-tetrahydronaphthalene) was investigated with a sulfate-reducing enrichment culture obtained from a contaminated aquifer. Degradation studies with tetralin revealed 5,6,7,8-tetrahydro-2-naphthoic acid as a major metabolite indicating activation by addition of a C(1) unit to tetralin, comparable to the formation of 2-naphthoic acid in anaerobic naphthalene degradation. The activation reaction was specific for the aromatic ring of tetralin; 1,2,3,4-tetrahydro-2-naphthoic acid was not detected. The reduced 2-naphthoic acid derivatives tetrahydro-, octahydro-, and decahydro-2-naphthoic acid were identified consistently in supernatants of cultures grown with either naphthalene, 2-methylnaphthalene, or tetralin. In addition, two common ring cleavage products were identified. Gas chromatography-mass spectrometry (GC-MS) and high-resolution GC-MS analyses revealed a compound with a cyclohexane ring and two carboxylic acid side chains as one of the first ring cleavage products. The elemental composition was C(11)H(16)O(4) (C(11)H(16)O(4)-diacid), indicating that all carbon atoms of the precursor 2-naphthoic acid structure were preserved in this ring cleavage product. According to the mass spectrum, the side chains could be either an acetic acid and a propenic acid, or a carboxy group and a butenic acid side chain. A further ring cleavage product was identified as 2-carboxycyclohexylacetic acid and was assumed to be formed by beta-oxidation of one of the side chains of the C(11)H(16)O(4)-diacid. Stable isotope-labeling growth experiments with either (13)C-labeled naphthalene, per-deuterated naphthalene-d(8), or a (13)C-bicarbonate-buffered medium showed that the ring cleavage products derived from the introduced carbon source naphthalene. The series of identified metabolites suggests that anaerobic degradation of naphthalenes proceeds via reduction of the aromatic ring system of 2-naphthoic acid to initiate ring cleavage in analogy to the benzoyl-coenzyme A pathway for monoaromatic hydrocarbons. Our findings provide strong indications that further degradation goes through saturated compounds with a cyclohexane ring structure and not through monoaromatic compounds. A metabolic pathway for anaerobic degradation of bicyclic aromatic hydrocarbons with 2-naphthoic acid as the central intermediate is proposed.     

Separation of factors responsible for lysozyme catalysis

The four major factors contributing to lysozyme facilitation of acetal hydrolysis are general acid catalysis, distortion, electrostatic stabilization of the transition state and entropy effects. The importance of general acid catalysis is assessed from the Brønsted relationship using an a coefficient of 0·5 based on model compounds. The degree of electrostatic stabilization is assessed from the electric field interactions of Asp 52 and Glu 35. Interaction of lysozyme with transition state analogs is used to calculate the contribution of distortion to catalysis. The remaining acceleration factor is assigned to entropy effects. The value of the entropy effect suggests that the transition state has some freedom of motion on the enzyme.     

Brønsted acid-catalyzed Nazarov cyclization of pyrrole derivatives accelerated by microwave irradiation

The Brønsted acid-catalyzed Nazarov cyclization of pyrrole derivatives was developed. Microwave irradiation accelerated the Nazarov cyclization significantly at 40 °C to give cyclopenta[b]pyrrole derivatives in excellent yields with high trans selectivity.     

Squalene hopene cyclases: highly promiscuous and evolvable catalysts for stereoselective CC and CX bond formation

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Highlights► Squalene hopene cyclases as mimics of chiral Brønsted acid catalysts. ► Terpene cyclases are inherently promiscuous and evolvable catalysts. ► The plasticity of squalene hopene cyclases can be used to produce heteropolycycles.     

On the reactivity of 23-methoxycarbonyl furospirostanes

Brønsted and Lewis acid-catalysed reactions of the 23-methoxycarbonyl furospirostanic side chain are described. While bromination, deuteration and BF₃·Et₂O/AcOH treatment involve regioselective F-ring opening with exclusive participation of Δ²²-furostenic intermediates, BF₃·Et₂O/Ac₂O treatment leads to irreversible E- or F-ring cleavage.     

乙醇催化脱水制乙烯反应条件及飞温现象

以HZSM-5分子筛为乙醇脱水催化剂,考察了催化剂的硅铝摩尔比和反应工艺条件对乙醇转化率及生成乙烯选择性的影响,分析了反应初期的飞温现象,用X射线衍射（XRD）、环境扫描电镜（SEM）和热分析（TG-DTA）对其催化剂进行了表征。结果表明：硅铝摩尔比为50∶1的HZSM-5催化乙醇脱水制乙烯反应在温度300℃、液体质量空速7h-1时,乙醇转化率大于99.8%,乙烯选择性达99.1%。HZSM-5催化乙醇脱水反应初期的飞温过程生成的聚合物堵塞和覆盖部分催化剂孔道,导致催化剂活性下降和乙烯选择性显著降低,适当降低进料温度可有效控制飞温现象发生。     

Protophilicity index and protofelicity equalization principle: new measures of Brønsted-Lowry-Lewis acid–base interactions

The simultaneous contributions of proton and electron transfer to the Brønsted-Lowry and Lewis acid–base properties of a set of p-substituted phenols are reported in this work. As a result of the analysis, a novel protophilicity index considered as the second-order energy change of a Brønsted-Lowry base as it is saturated with protons, a combined Brønsted-Lowry-Lewis acidity index (with a corresponding basicity index), and a protofelicity equalization principle (a parallel of the electronegativity equalization principle) are presented.     

Preparation of biodiesel from waste oils catalyzed by a Brønsted acidic ionic liquid

Preparation of biodiesel from waste oils catalyzed by a novel Brønsted acidic ionic liquid with an alkane sulfonic acid group was investigated. The acidity and the activity of the ionic liquid were very low at lower temperature when the ionic liquid was crystalloid; they recovered at higher temperature when the crystallized ionic liquid was liquefied. When methanol:oils:catalyst molar ratios were 12:1:0.06, the yield of fatty acid methyl esters can reach 93.5% after the reaction of acidic oil with methanol had taken place at 170 °C for 4 h. In addition, the ionic liquid had a good reusability and can be easily separated from the biodiesel by simple decantation.     

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.