Organic catalysis over zeolites: A perspective on reaction paths within micropores

Catalysis has been an important field for polyurethane chemistry and many improvements have been accomplished in recent years. Some of the greatest challenges however still remain. There is not yet a satisfactory catalytic way for the direct oxidation of propene to propene oxide in spite of ruthless activities by all major players over decades. New methods of catalysts screening and improved tools for catalyst characterization may help to solve this problem in the near future. Catalyst development in the area of polyether synthesis has recently opened new routes to completely new products. New raw materials may become attractive and new polyether polyol structures will be accessible with new catalysts. The range of new polymers will open up new opportunities. In the field of isocyanates the optimization of all manufacturing steps (nitration, hydrogenation, phosgenation) is an ongoing process since commercial production started an incremental improvements are accomplished and implemented constantly. Major improvements for existing processes, including all their catalytic reaction steps, are not to be expected in the near future. Significant improvements seem only possible by alternative ways of isocyanate manufacturing. In this area, catalytic reactions and the modern tools of catalysts research will play a key-role. However, as for all new developments, they will have only a chance on realization on a commercial scale, if they are economically competitive to well established processes.

Trends in industrial catalysis in the polyurethane industry

Publisher Summary The rapid development of zeolite catalysis in the synthesis of various types of organic compounds is reflected in the considerable body including many patents and patent applications. It discusses that the size of the molecules to be converted or synthesized seems limited by the pore dimensions of zeolites. However, a growing number of examples related to outer surface catalysis, or to selective scavenging of side products or impurities or to slow release of reagents explains that zeolites can be of much value also in the synthesis or conversion of large molecules or materials. Recently discovered super-large pore molecular sieves enlarge the spectrum of molecules having access to zeolites considerably. Another limiting factor is the relatively rapid decay of zeolite catalysts due to coke formation. Many examples exist- as shown in the foregoing of stabilizing zeolites by adding components or by adapting the reaction mixture, example by adding water or carbondioxide. Also, process conditions-fluidized bed with continuous regeneration is applied to overcome this trouble. The chapter also reviews that that a further exponential growth of the use of zeolites and related materials takes place. Zeolite catalysis and technology will be future cornerstones of a clean environmentally friendly organic chemicals industry.

Chapter 16 “Zeolites in Organic Syntheses”

A H-Ga-ZSM5 zeolite catalyst was applied for gas phase hydroxylation of benzene to phenol with nitrous oxide as oxidant. In a wide range of temperatures and partial pressures phenol was the main product. Up to reactor temperatures of 400°C benzoquinone was the main by-product. Above 400°C total oxidation of benzene started. Increasing partial pressures of benzene and particularly of nitrous oxide led to an increase of the reaction rate. Selectivity to the main product phenol was strongly affected by the benzene feed concentration. Rising benzene partial pressure led to increasing selectivity to phenol.

Hydroxylation of benzene with nitrous oxide on H-Ga-ZSM5 zeolite

The conversion of methane to methanol over zeolitic cr-oxygen sites has been demonstrated using Fe-ZSM-5. To discriminate between mono- and poly-nuclear active sites, we prepared the [Fe]-ZEO with iron in the ZEOlite lattice via direct synthesis and Fe, -ZEO, by dispersion of x wt.% iron on the ZEOlite. Shape-selective formation of nano-clusters of iron oxides with various sizes is realized inside the pore-sizes varying from 10.0 to 8.0 and 6.3 to 4.3 Angstrom of the CFI, MOR, MFI, and CHA zeolites. The Fe-K edge X-ray absorption data were obtained for the Fe-CIT-5, Fe-ZSM-5, Fe-MOR and Fe-CHA zeolites containing iron clusters. In Mossbauer spectroscopy the absence and presence of a hyperfine magnetic field (HMF) for [Fe]-CIT-5 and Fe-CIT-5 are seen. The quantum mechanics calculations analyze the different environments of iron, e,g, the tetrahedral lattice occluded species, the di-nuclear sites attached to the zeolite, the nano-phase hematite sites. The molecular mechanics calculations involve a new molecular mechanics force field, the universal force field (UFF). alpha -Oxygen can be formed on di-nuclear iron rites in zeolites by N2O decomposition at elevated temperatures and is dependent on the zeolite structure utilized. Fe-chabazite (CHA), Fe-mordenite (MOR) and Fe-CIT-5 (CFI) were found to be less active than Fe-ZSM-5. A range of preparative and activation conditions were studied preceding methane conversion. Proper activation is essential to maximize catalyst actvity, e.g. pretreatment under vacuum at 800-900 degreesC, activation with N2O at 250 degreesC and reaction with methane at 20 degreesC, Extraction of methanol from the catalyst is performed with H2O Structure-activity effects are discussed. (C) 2001 Elsevier Science B.V, All rights reserved.

Methane partial oxidation in iron zeolites: theory versus experiment

Silica-supported benzenesu1fonic acids were found to be highly active catalysts for the vapor-phase nitration of benzene with nitrogen dioxide to yield nitrobenzene at 443 K. The decay of the activities was not observed during the course of the reaction. The quantitative conversion of both benzene and nitrogen dioxide was accomplished. A direct synthesis of phenol was found to be possible through a vapor-phase reaction of benzene with dinitrogen monoxide over H-ZSM-5 zeolite at 603 K. The phenol yield on benzene basis was 6%.

K. Tohmori

Papers

Functionalizatiow of Benzene by its Reaction with Nitrogen Oxides Over Solid-Acid Catalysts