Highly Conjugated Organic Polymers as Heterogeneous Catalysts. I. Introduction and Literature Survey

The use of highly conjugated organic polymers as heterogeneous catalysts is reviewed as part of a series of papers relating to their use in reactions involving oxides of nitrogen. The polymers which have been examined and the types of catalysed reaction so far recorded are described. In general the structural features, including extensive conjugation, which are considered important for catalytic activity and selectivity, are also responsible for their high thermal stability. The presence of hetero-atoms, a high stable concentration of delocalised electrons and semiconductivity, have each been proposed as primary requisites for catalytic activity.     

Polymers as Materials

Abstract

The chemistry and technology of polymers has developed rapidly during the last three decades; it concerns itself with materials consisting essentially of the elements C, H, N, and O, and being therefore conventionally classified as organic polymers. However, in many instances other elements such as B, Si, P, S, F, and CI are present in certain proportions and have a more or less important influence on the ultimate properties of the products. Together with the large family of metallic compounds and ceramic systems, organic polymers represent useful and increasingly important engineering materials in the construction of buildings, vehicles, engines, applicanes, textiles, packaging, printing and writing materials, plastics, rubber goods and household articles of all kinds.     

Phosphates as Catalysts

Although carbon is often considered as the dominant element in terms of covalent bond formation, phosphorus can undoubtedly be considered a s a serious competitor for this position [1]. Further, at least in principle, phosphorus is potentially capable of forming more compounds than carbon. For a variety of reasons, both academic and practical, interest in phosphorus compounds has increased markedly over the last decade. It is not unexpected, then, to find that the use of compounds of phosphorus as catalysts has also accelerated. Unfortunately, a s is often the case in science, the practice outruns the theory with such catalysts, but industrialists and academicians alike have recently displayed an enhanced interest in the mechanisms of catalytic activity on such catalysts.     

Pillared Clays as Catalysts

Clays have long been used as catalysts [1] and they retain a continuing important role since they constitute the matrix of many industrial catalysts based on zeolites. Intercalated clays are a new class of materials, in which an homogeneous distribution of micropores can be obtained with pore openings varying from 4 Å to 18 Å according to the type of pillars. These solids are then attractive because they can adsorb and convert large molecules.     

Heparinized Polymers as Thromboresistant Biomaterials

A major research field of recent development concerns the synthesis and evaluation of thromboresistant polymers for use in heart valves, artificial hearts, ventricular assist devices, and external devices such as heart-lung and kidney machines. Although a wide variety of plastics and elastomers is available, none is completely satisfactory for long-term use in contact with blood.     

Molecular Metal Clusters as Catalysts

Abstract

In earlier analyses [1–8] we have established a correlation between metal clusters and metal surfaces with chemisorbed molecules in the specific contexts of (1) the metal frameworks wherein the metal cluster core structures are fragments of cubic and hexagonal close packed or body centered cubic metal bulk structures; (2) ligand stereochemistry where the geometric features of ligands bound to clusters and to metal surfaces are similar; (3) thermodynamic features where the average bond energies for ligand-metal and metal-metal bonds are comparable, for a specific metal, in the metal cluster and the metal surface regime; and (4) mobility of ligands bonded to metal cluster frameworks and to metal surfaces. Nevertheless, there are sharp distinctions between surfaces and clusters. The average coordination numbers for metal-metal interactions and for metal-ligand interactions are distinctly different for clusters and for surfaces: generally, the former are larger for surfaces and the latter are larger for clusters. Additionally, the surface state is typically differentiated from the cluster state in the degree of coordination saturation—the metal atoms in the surface state are typically less coordinately saturated even for the states in which molecules or molecular fragments are chemisorbed at the surface than those metal atoms at the periphery of a molecular metal cluster. In the crucial chemical issue, metal surfaces are far more reactive than metal clusters. Metal surfaces exhibit a wide range and high level of catalytic activity whereas most metal clusters are catalytically inert, at least under modest reaction conditions, Most reported clusters are relatively stable and nonreactive; they are not the products of sophisticated synthesis procedures designed to generate highly reactive metal clusters. They commonly have been the products of reaction mixtures run at forcing conditions and are thermodynamically controlled, not kinetically controlled, products.     

Zeolites as Catalysts.I

Zeolites are crystalline aluminosilicates of the general formula M_v(AlO₂)_x(SiO₂)_y ZH₂O in which M is either a monovalent or divalent cation. In the former case v is equal to x, whereas if M is divalent v is equal to one-half x. The cation can be exchanged reversibly without destroying the aluminosilicate framework. The water can also be removed without destroying the framework and can be replaced by other molecules whose molecular dimensions are smaller than the pore size of the cavities that are regularly distributed in the framework. The zeolites both occur naturally and can be made synthetically on either laboratory or industrial scale. In recent years they have assumed great technical importance as adsorbents and catalysts. They are also important from a structural point of view, since they represent the next degree of complexity of structure after that of large molecules.     

Molecular Metal Clusters as Catalysts

In earlier analyses [1-8] we have established a correlation between metal clusters and metal surfaces with chemisorbed molecules in the specific contexts of (1) the metal frameworks wherein the metal cluster core structures are fragments of cubic and hexagonal close packed or body centered cubic metal bulk structures; (2) ligand stereochemistry where the geometric features of ligands bound to clusters and to metal surfaces are similar; (3) thermodynamic features where the average bond energies for ligand-metal and metal-metal bonds are comparable, for a specific metal, in the metal cluster and the metal surface regime; and (4) mobility of ligands bonded to metal cluster frameworks and to metal surfaces. Nevertheless, there are sharp distinctions between surfaces and clusters. The average coordination numbers for metal-metal interactions and for metal-ligand interactions are distinctly different for clusters and for surfaces: generally, the former are larger for surfaces and the latter are larger for clusters. Additionally, the surface state is typically differentiated from the cluster state in the degree of coordination saturation—the metal atoms in the surface state are typically less coordinately saturated even for the states in which molecules or molecular fragments are chemisorbed at the surface than those metal atoms at the periphery of a molecular metal cluster. In the crucial chemical issue, metal surfaces are far more reactive than metal clusters. Metal surfaces exhibit a wide range and high level of catalytic activity whereas most metal clusters are catalytically inert, at least under modest reaction conditions, Most reported clusters are relatively stable and nonreactive; they are not the products of sophisticated synthesis procedures designed to generate highly reactive metal clusters. They commonly have been the products of reaction mixtures run at forcing conditions and are thermodynamically controlled, not kinetically controlled, products.     

Cyclophosphazene-containing Polymers as Imprint Lithography Resists

We present the evaluation of a cyclophosphazene-containing polymer as a patternable resist for imprint lithography. Hexamethacryloxybutoxycyclotriphosphazene layers containing small amounts of photoinitiator can be applied to silicon wafers substrates by spin coating techniques and cured photochemically to give tough, network polymer thin films. The films were characterized by FT-IR. Thin films approximately 200 nm in thickness were subjected to anisotropic O₂ and CF₄ plasmas and the etch rates were determined. The polymer films etch at a rate of 21 Å/s in CF₄ plasma, and as low as 1.6 Å/s in O₂ plasma, which is comparable or lower than the rates observed with commercially available silicon-containing photoresists. The surface chemical composition was surveyed by X-ray photoelectron spectroscopy, which gave results consistent with the formation of an etch resistant phosphorus-rich layer during reaction with O₂ plasma. The polymer was processed by nano-contact molding imprint lithography and replicated 200 nm period test patterns. This report is the first demonstration of a cyclophosphazene-containing polymer as a resist candidate for high-resolution lithography.     

Nitrogen-Containing Carbon Nanostructures as Oxygen-Reduction Catalysts

Nitrogen-containing carbon nanostructure (CN _x ) catalysts developed by acetonitrile pyrolysis have been studied to better understand their role in the oxygen reduction reaction (ORR) in PEM and direct methanol fuel cell environments. Additional functionalization of the CN _x catalysts with nitric acid has the ability to improve both the activity and selectivity towards ORR.     

丁烯烷基化固体酸催化剂的研究进展

阐述了异丁烷 丁烯烷基化催化剂的发展状况,主要涉及了几种新型固体酸催化剂,包括固体超强酸催化剂、分子筛催化剂和负载型杂多酸催化剂。文章着重对SO2-4 MxOy、MCM-41和负载型杂多酸催化剂的制备,活性和选择性作了探讨。同时指出了这些固体酸催化剂存在的问题,预测了烷基化催化剂研究的发展方向。     

Photopolymerized Thiol-Ene Systems as Shape Memory Polymers

In this study we introduce the use of thiol-ene photopolymers as shape memory polymer systems. The thiol-ene polymer networks are compared to a commonly utilized acrylic shape memory polymer and shown to have significantly improved properties for two different thiol-ene based polymer formulations. Using thermomechanical and mechanical analysis, we demonstrate that thiol-ene based shape memory polymer systems have comparable thermomechanical properties while also exhibiting a number of advantageous properties due to the thiol-ene polymerization mechanism which results in the formation of a homogeneous polymer network with low shrinkage stress and negligible oxygen inhibition. The resulting thiol-ene shape memory polymer systems are tough and flexible as compared to the acrylic counterparts. The polymers evaluated in this study were engineered to have a glass transition temperature between 30 and 40 °C, exhibited free strain recovery of greater than 96% and constrained stress recovery of 100%. The thiol-ene polymers exhibited excellent shape fixity and a rapid and distinct shape memory actuation response.     

钙钛矿型材料作为催化剂的研究进展

钙钛矿型材料由于其优异的结构稳定性和物化性能,日益成为材料科学领域的研究热点。本文对对钙钛矿型材料的制备方法及其作为催化剂进行了简要的综述。     

树脂类催化剂在酯化反应中的应用进展

回顾了近年来离子树脂作为酯化催化剂的发展情况。介绍了聚苯乙烯类树脂和Nafion树脂在各类酯化反应中的应用以及发展,对树脂类催化剂的应用前景以及该类催化剂在绿色化学中的重要作用提出展望。