Heterogeneous Catalysis and Parahydrogen-Induced Polarization

Parahydrogen-induced polarization with heterogeneous catalysts (HET-PHIP) has been a subject of extensive research in the last decade since its first observation in 2007. While NMR signal enhancements obtained with such catalysts are currently below those achieved with transition metal complexes in homogeneous hydrogenations in solution, this relatively new field demonstrates major prospects for a broad range of advanced fundamental and practical applications, from providing catalyst-free hyperpolarized fluids for biomedical magnetic resonance imaging (MRI) to exploring mechanisms of industrially important heterogeneous catalytic processes. This review covers the evolution of the heterogeneous catalysts used for PHIP observation, from metal complexes immobilized on solid supports to bulk metals and single-atom catalysts and discusses the general visions for maximizing the obtained NMR signal enhancements using HET-PHIP. Various practical applications of HET-PHIP, both for catalytic studies and for potential production of hyperpolarized contrast agents for MRI, are described.     

Oxidation of secondary alcohols byN-methylmorpholine-N-oxide (NMO) catalyzed by atrans-dioxo ruthenium(VI) complex or perruthenate complex: A kinetic study

Thetrans-dioxo ruthenium (VI) complex, [P(C₆H₅)₃C₆H₅CH₂]⁺[Ru(O)₂OAcCl₂] or tetrapropylammonium perruthenate catalyzes the oxidation of secondary alcohols to ketones byN-methylmorpholine-N-oxide (NMO). Kinetic studies showed the formation of a complex between catalyst and substrate (alcohol) as the first step in the mechanism.     

4‐(N,N‐Dimethylamino)pyridine‐Embedded Nanoporous Conjugated Polymer as a Highly Active Heterogeneous Organocatalyst

We report herein for the first time the incorporation of a versatile organocatalyst, 4‐(N,N‐dimethylamino)pyridine (DMAP), into the network of a nanoporous conjugated polymer (NCP) by the “bottom‐up” approach. The resulting DMAP‐NCP material possesses highly concentrated and homogeneously distributed DMAP catalytic sites (2.02 mmol g^?1). DMAP‐NCP also exhibits enhanced stability and permanent porosity due to the strong covalent linkage and the rigidity of the “bottom‐up” monomers. As a result, DMAP‐NCP shows excellent catalytic activity in the acylation of alcohols with yields of 92–99 %. The DMAP‐NCP catalyst could be easily recovered from the reaction mixture and reused in at least 14 consecutive cycles without measurable loss of activity. Moreover, the catalytic acylation reaction could be performed under neat and continuous‐flow conditions for at least 536 h of continuous work with the same catalyst activity.     

Efficient Tandem Synthesis of Methyl Esters and Imines by Using Versatile Hydrotalcite‐Supported Gold Nanoparticles

Efficient basic hydrotalcite (HT)‐supported gold nanoparticle (AuNP) catalysts have been developed for the aerobic oxidative tandem synthesis of methyl esters and imines from primary alcohols catalyzed under mild and soluble‐base‐free conditions. The catalytic performance can be fine‐tuned for these cascade reactions by simple adjustment of the Mg/Al atomic ratio of the HT support. The one‐pot synthesis of methyl esters benefits from high basicity (Mg/Al=5), whereas moderate basicity greatly improves imine selectivity (Mg/Al=2). These catalysts outperform previously reported AuNP catalysts by far. Kinetic studies show a cooperative enhancement between AuNP and the surface basic sites, which not only benefits the oxidation of the starting alcohol but also the subsequent steps of the tandem reactions. To the best of our knowledge, this is the first time that straightforward control of the composition of the support has been shown to yield optimum AuNP catalysts for different tandem reactions.     

Comparison of the Peroxidase‐Like Activity of Unmodified,Amino‐Modified,and Citrate‐Capped Gold Nanoparticles

The origin of the peroxidase‐like activity of gold nanoparticles and the impact of surface modification are studied. Furthermore, some influencing factors, such as fabrication process, redox property of the modifier, and charge property of the substrate, are investigated. Compared to amino‐modified or citrate‐capped gold nanoparticles, unmodified gold nanoparticles show significantly higher catalytic activity toward peroxidase substrates, that is, the superficial gold atoms are a contributing factor to the observed peroxidase‐like activity. The different catalytic activities of amino‐modified and citrate‐capped gold nanoparticles toward 3,3′,5,5′‐tetramethylbenzidine (TMB) and 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) diammonium salt (ABTS) show that the charge characteristics of the nanoparticles and the substrate also play an important role in the catalytic reactions.     

A metal-free C–C/C–O bond formation for the synthesis of 2-amino-5-oxo-4-aryl-4H,5H-pyrano[3,2-c]chromene-3-carboxamide catalyzed by polystyrene-supported p-toluenesulfonic acid (PS-PTSA)

An efficient and eco-friendly procedure for the synthesis of 2-amino-5-oxo-4-aryl-4H,5H-pyrano[3,2-c]chromene-3-carboxamide has been developed through a one-pot three-component condensation of 4-hydroxycoumarin with aldehydes and cyanoacetamide, in the presence of catalytic amount of polystyrene-supported p-toluenesulfonic acid (PS-PTSA) as a highly active and reusable heterogeneous acid catalyst in EtOH at 80?°C conditions. This new procedure offers several advantages such as shorter reaction times, excellent yields, a wide range of functional group tolerance, easy experimental work-up procedure, operationally simple under metal-free reaction conditions and C-C/C-O bond formation. The catalyst can be recovered and reused for at least four runs without any significant impact on the product yields.     

Spin-polarized current and tunnel magnetoresistance in heterogeneous single-barrier magnetic tunnel junctions

Current in heterogeneous tunnel junctions is studied in the framework of the parabolic conduction-band model. The developed model of the electron tunneling takes explicitly into account the difference of effective masses between ferromagnetic and insulating layers and between conduction subbands. Calculations for Fe/MgO/Fe-like structures have shown the essential impact of effective mass differences in regions (constituents) of the structure on the tunnel magnetoresistance of the junction.     

纳米LiH的热稳定性与催化加氢反应活性

考察了纳米LiH在不同温度热处理后比表面积的变化及其与Cp2TiCl2组成催化剂的加氢反应活性之间的关系,300度热处理后LiH比表面积已呈较大幅度的下降,但催化剂的加氢反应活性反而更高,说明比表面积是克杨纳米LiH高活性的一个主要原因,但不是唯一 的因素。     

气固相法合成的Ti-ZSM-5催化剂上苯乙烯氧化反应的宏观动力学

 采用TiCl4气固相同晶取代法制得的Ti－ZSM－5作催化剂,对H2O2氧化苯乙烯反应的宏观动力学进行了研究,考察了催化剂、苯乙烯和H2O2用量及反应温度对苯乙烯氧化反应速率的影响．结果表明,催化剂Ti－ZSM－5和底物苯乙烯对苯乙烯氧化反应速率的贡献均为一级,而H2O2为1／2级;苯乙烯氧化反应的表观活化能Ea＝48．14kJ／mol．当以丙酮为溶剂,在n（PhCH∶CH2）／n（H2O2）＝7．91,催化剂用量为20g／L,反应温度为343K的条件下,反应360min时,苯乙醛选择性和H2O2利用率分别可达91．9％和88．6％．     

极谱催化波法测定苯甲醛

在磷酸盐缓冲溶液中 ,苯甲醛与联氨反应生成的产物苯甲醛腙在K2 S2 O8存在时于 - 1.2 2V(vs.SCE)产生一灵敏的极谱催化波。基于此 ,拟定了测定痕量苯甲醛的新方法。在 8.0× 10 -2 mol/LKH2 PO4 NaOH(pH 8.0± 0 .2 ) 2 .5× 10 -2 mol/L联氨 - 3.5× 10 -2 mol/LK2 S2 O8底液中 ,极谱催化波峰电流与苯甲醛浓度在 1.0× 10 -9～ 6 .2× 10 -8mol/L范围内呈线性关系 ,比无K2 S2 O8存在时灵敏度提高约 6 0倍。本方法适用于生活污水中苯甲醛含量的测定