B、Al、La同晶取代EU-1分子筛酸性的密度泛函理论计算

采用量子力学的密度泛函理论(DFT)方法,研究了B、Al、La同晶取代EU-1分子筛骨架的Si后可能的存在位置,确定了与平衡电荷质子结合的骨架O位置,考察了杂原子B、La进入骨架后对EU-1分子筛Brnsted酸(B酸)强度的影响。计算所采用的26T簇模型均在DFT的BLYP方法和DNP基组下完成。对合成的分子筛进行了NH3-TPD表征。能量分析表明,Al和B取代EU-1分子筛骨架最可能的位置为T(1)、T(2)、T(3)、T(6)、T(7)和T(8)位,La取代最可能的位置为T(1)、T(2)和T(8)位。B酸落位点分别为Al(1)-O(12)H-Si(2)和Al(1)-O(13)H-Si(6)位,B(8)-O(29)H-Si(10)位,La(1)-O(12)H-Si(2)、La(1)-O(13)H-Si(6)、La(2)-O(12)H-Si(1)位。与硅铝EU-1分子筛相比,B和La进入骨架会导致EU-1分子筛的B酸强度有所减弱,Al-EU-1、La-EU-1、B-EU-1三者中,Al-EU-1的B酸强度最强,B-EU-1的最弱。B和La进入EU-1分子筛骨架后,酸性减弱,弱酸量增加,理论计算与实验结果一致。     

Reactivity of Brönsted acid ionic liquids as dual solvent and catalyst for Fischer esterifications

Several water-stable ionic liquids with different acidity and affinity were synthesized and applied as both solvents and acid catalysts for Fischer esterification of ethanol reacting with four aliphatic carboxylic acids (acetic acid, n-hexanoic acid, lauric acid, and stearic acid). Among the studied ionic liquids, [(n-bu-SO₃H) MIM][HSO₄] (3-butyl-1-(butyl-4-sulfonyl) imidazolium sulfate) and [(n-bu-SO₃H) MIM][p-TSO] (3-butyl-1-(butyl-4-sulfonyl) imidazolium toluenesulfonate) show higher reactivity for the production of ethyl esters. The catalytic activities of these ionic liquids are strongly dependent on the acidity of their anions and cations, as well as their hydrophilicity and affinity with the reactants. Water refluxing through the condenser may be another important reason for obtaining high conversion of esterification, indicating a water-sequester process is still needed in order to obtain a higher yield of ester in the ionic liquid catalyzed esterification system. Kinetics studies show the conversions of the acids increase with reaction temperature and time, and reach equilibrium within about two hours. The apparent activation energies are 39.1±2.0, 49.7±2.5, 51.4±2.5 and 59.3±3.0 kJ·mol⁻¹ for the formation of ethyl acetate, ethyl n-hexanoate, ethyl laurate and ethyl stearate, respectively.     

Acid centers in sulfated, phosphated and/or aluminated zirconias

On sulfated ZrO₂, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm^?1 suggests that besides structural Lewis sites on the surface of ZrO₂, strong Lewis sites are made from chemisorbed SO₃. Upon adsorption of water, SO₃ is converted, partially, into a surface sulfated species which may act as strong Brønsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO₄. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P–OH groups working as weak Brønsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.     

Br⊘nsted Acid Sites in Zeolites Characterized by Multinuclear Solid-State NMR Spectroscopy

Nearly all atoms contributing to the local structure of Br?nsted acid sites in zeolites exhibit isotopes accessible for multinuclear solid-state nuclear magnetic resonance (NMR) investigations. Therefore, in the last 15 years, NMR spectroscopy has found numerous applications for the determination of the types of hydroxyl proton in zeolites, of their concentration, accessibility, and mobility, and for the characterization of their acid strength and local structure. It allows the study of the role of hydroxyl groups in the formation of adsorbate complexes and in heterogeneously catalyzed reactions. Meanwhile, NMR spectroscopy belongs to the most powerful techniques for the characterization of Br?nsted acid sites in zeolites and related materials. The basis of this success is the invention of new sample preparation techniques, external magnetic fields with high-flux densities, effective line-narrowing methods, and new two-dimensional experiments, making the detection of highly resolved solid-state NMR spectra and the separation of spectral parameters possible. This article gives a review of these techniques and a summary of the most important applications of multinuclear solid-state NMR spectroscopy for the characterization of Br?nsted acid sites in dehydrated zeolites.     

Theoretical study of the enhanced Brønsted acidity of Zn2+‐exchanged zeolites

The effect of Zn²⁺ exchange on the Brønsted acidity of a protonic zeolite has been studied by the ab initio DFT (density functional theory) approach using the BLYP generalized gradient approximation. Three different zeolite cluster models have been compared: two 6“T” models (two 4“T” rings with an oxygen atom bridge) with Si/Al=1 and Si/Al=2 and a 4“T” model (ring form) with Si/Al=1. The Brønsted acidity has been probed by computation of the acetonitrile adsorption and the cluster deprotonation energy. The presence of Zn²⁺ does not affect the cluster Brønsted acidity but it creates a very strong Lewis site (Zn²⁺) in all models studied. On the other hand, the presence of ZnOH⁺ enhanced the Brønsted site acidity in the case of the 6T model with Si/Al=1. This enhancement is due to a change in cluster geometry and position of OH group in ZnOH⁺ upon acetonitrile adsorption.     

Acetonylacetone conversion on AlPO4–cesium oxide (5–30 wt%) catalysts

The effect of hydrogen spillover over Pd/SiO₂ + USY on pyridine chemisorbed on acid sites of USY zeolite was investigated. According to FTIR measurement, pyridine adsorbed on Lewis acid sites migrated to Brønsted acid sites under hydrogen-flowing atmosphere. The rate of migration was drastically promoted by physically mixed Pd/SiO₂ catalysts. This suggests that spilt-over hydrogen enhance the migration of pyridine adsorbed on Lewis acid sites.