水平梯度表面能材料表面上的滴状凝结换热系数

本文在均质表面的单个球缺形液滴换热模型和液滴通用尺度分布规律的基础上,结合梯度表面能材料表面的液滴分布和凝结换热特性,得到了一维水平梯度表面能材料表面上的滴状凝结换热计算式。在此基础上,研究了壁面过冷度、接触角梯度、工质物性等参数对梯度表面能材料表面滴状凝结换热性能的影响。结果表明:随着过冷度的增加和凝结工质汽化潜热的增大和表面张力的减小和接触角梯度的增大,平均表面凝结换热系数会增大。     

甘氨酸在SiO2表面的吸附及热缩合反应

 考察了甘氨酸水溶液平衡浓度和pH值对其在SiO2表面吸附行为的影响,采用DTG,XRD和FT-IR等手段对甘氨酸在SiO2表面的吸附量及晶相结构等进行了表征. 结果表明,甘氨酸在SiO2上的吸附不符合Langmuir吸附模型,当甘氨酸水溶液平衡浓度小于0.073 mol/L时,主要为特定位置吸附,此时SiO2对甘氨酸的吸附源于其表面≡Si-O-基团与甘氨酸离子的-NH+3 端的相互作用; 当平衡浓度大于0.073 mol/L时,呈多层吸附,形成α和β两种晶型的甘氨酸. 用TPD-MS对甘氨酸的热缩合行为进行了考察,结合FT-IR结果表明,产物为环状二聚体哌嗪二酮(DKP),没有检测到线式二聚体以及表面硅酯类物种形成. SiO2对表面吸附的甘氨酸分子产生了诱导活化,使其热缩合温度与体相α甘氨酸相比降低了50 ℃左右.     

含有不凝气体的蒸汽滴状冷凝传热实验研究

本文对饱和水蒸气和空气的混合物在垂直表面上滴状冷凝传热特性进行了实验研究。考察了不同的压力条件下不凝气对冷凝传热的影响。与膜状冷凝实验结果对比表明,滴状冷凝对含有不凝气的蒸汽冷凝传热有强化作用;在较高的冷凝压力下不凝气体对传热的影响相对较小。     

Condensation of β‐Diester Titanium Enolates with Carbonyl Substrates: A Combined DFT and Experimental Investigation

The condensation of dialkyl β‐diesters with various aldehydes promoted by TiCl₄ has been studied by DFT approaches and experimental methods, including NMR, IR and UV/Vis spectroscopy. Various possible reaction pathways have been investigated and their energy profiles evaluated to find out a plausible mechanism of the reaction. Theoretical results and experimental evidence point to a three‐step mechanism: 1) Ti‐induced formation of the enolate ion; 2) aldol reaction between the enolate ion and the aldehyde, both coordinated to titanium; and 3) intramolecular elimination that leads to a titanyl complex. The presented mechanistic hypothesis allows one to better understand the pivotal role of titanium(IV) in the reaction.     

Bifunctional Heterogeneous Catalysis of Silica–Alumina‐Supported Tertiary Amines with Controlled Acid–Base Interactions for Efficient 1,4‐Addition Reactions

We report the first tunable bifunctional surface of silica–alumina‐supported tertiary amines (SA–NEt₂) active for catalytic 1,4‐addition reactions of nitroalkanes and thiols to electron‐deficient alkenes. The 1,4‐addition reaction of nitroalkanes to electron‐deficient alkenes is one of the most useful carbon–carbon bond‐forming reactions and applicable toward a wide range of organic syntheses. The reaction between nitroethane and methyl vinyl ketone scarcely proceeded with either SA or homogeneous amines, and a mixture of SA and amines showed very low catalytic activity. In addition, undesirable side reactions occurred in the case of a strong base like sodium ethoxide employed as a catalytic reagent. Only the present SA‐supported amine (SA–NEt₂) catalyst enabled selective formation of a double‐alkylated product without promotions of side reactions such as an intramolecular cyclization reaction. The heterogeneous SA–NEt₂ catalyst was easily recovered from the reaction mixture by simple filtration and reusable with retention of its catalytic activity and selectivity. Furthermore, the SA–NEt₂ catalyst system was applicable to the addition reaction of other nitroalkanes and thiols to various electron‐deficient alkenes. The solid‐state magic‐angle spinning (MAS) NMR spectroscopic analyses, including variable‐contact‐time ¹³C cross‐polarization (CP)/MAS NMR spectroscopy, revealed that acid–base interactions between surface acid sites and immobilized amines can be controlled by pretreatment of SA at different temperatures. The catalytic activities for these addition reactions were strongly affected by the surface acid–base interactions.     

Acid‐Catalyzed Cyclization of Terpenes Under Homogeneous and Heterogeneous Conditions as Probed Through Stereoisotopic Studies: A Concerted Process with Competing Preorganized Chair and Boat Transition States

Based on stereoisotopic studies and β‐secondary isotope effects, we propose that the acid‐catalyzed cyclization of geranyl acetate proceeds through a concerted mechanism. Under heterogeneous conditions (zeolite Y confinement), a preorganized chairlike transition state predominates, whereas under homogeneous conditions the boat‐ and chairlike transition states are almost isoenergetic. For the case of farnesyl acetate, we propose that under homogeneous conditions a concerted dicyclization occurs with a preorganized boat–chair transition state competing with the chair–chair transition state. Under zeolite confinement conditions, the chair–chairlike dicyclization transition state is highly favorable. The preference of chairlike transition states within the cavities of zeolite Y is attributed to a transition state shape selectivity effect.     

ScIII‐Doped Zeolites as New Heterogeneous Catalysts: Mukaiyama Aldol Reaction

Sc^III‐doped solids based on zeolite materials have been investigated for the first time as catalysts in organic synthesis. Sc^III–USY zeolite proved to be a novel and very efficient heterogeneous catalyst for the Mukaiyama aldol reaction. This easy‐to‐prepare catalyst exhibited wide scope and compatibility with functional groups and is very simple to use, easy to remove (by simple filtration), and is recyclable (up to three times without loss of activity).     

A Tin–Tungsten Mixed Oxide as an Efficient Heterogeneous Catalyst for C?C Bond‐Forming Reactions

Mix and calcinate : The tin–tungsten mixed oxide (Sn–W oxide) prepared by calcination of the Sn–W hydroxide at 800 °C acts as an effective and reusable solid catalyst for C? C bond‐forming reactions, such as the cyclization of citronellal, the Diels–Alder reaction, and the cyanosilylation of carbonyl compounds with trimethylsilyl cyanide (see scheme). The observed catalysis was truly heterogeneous, and the recovered catalyst could be reused without loss of its high catalytic performance.

     

Well‐Defined Silica‐Supported Calcium Reagents: Control of Schlenk Equilibrium by Grafting

Bound to stay heteroleptic : Grafting of homoleptic benzyl and amide calcium reagents onto silica affords well‐defined supported heteroleptic species, catalytically active in hydrosilylation, hydroamination, and styrene polymerization. The fact that attempts to generate a molecular model were thwarted by the Schlenk equilibrium affording the homoleptic compound (see scheme) emphasizes the importance of immobilization as an approach for the syntheses of heteroleptic calcium complexes.

     

Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

Trifunctional green catalysis : In‐depth characterization shows that oxidant‐free selective oxidation of alcohols by silver nanoparticles on γ‐Al₂O₃, as a new heterogeneous catalyst, proceeds through cooperation of silver, acid, and base sites (see figure).