Herman W. Kouwenhoven

Zeolite Catalyzed Aromatic Acylation and Related Reaches

Abstract The development of catalytic procedures in aromatic acylation is a priority because the current industrial methods apply stoichiometric or excess amounts of metal chlorides or mineral acids as “catalysts”. The paper reviews zeolite catalysis in this field and subsequently focusses on the acylation of phenols. Here, two reaction steps are involved: esterification and the so-called Fries rearrangement; both reactions are catalyzed by H-zeolites. The Fries rearrangement has been studied over various zeolites using phenyl acetate as a standard reactant. For the combined reaction especially the system resorcinol/benzoic acid has been examined with several catalysts. Zeolite H-Beta was found to be the best catalyst. When more bulky reactants are involved the new MCM-41 zeolitic material is a promising catalyst.

Zeolites and Fine Chemicals

Abstract The use of zeolites as selective catalysts and adsorbents in organic syntheses is a field of growing importance. A brief discussion of structure, modification and catalytic properties of zeolites relevant to the topic is given. Some non- and semicatalytic applications of zeolites in fine chemical preparation are mentioned. Examples of use of zeolites as catalysts in organic reactions are chosen from the fields of isomerization, aromatic substitution, oxidation, cyclization and heterocyclic ring formation.

Modified Zeolites for Oxidation Reactions

Synthetic Ti-silicates with the MFI structure-type (Ti-silicalites or TS-1) have been reported to be selective catalysts in (ep) oxidation reactions of aqueous H2O2 with unsaturated compounds. We have applied secondary synthesis to introduce Ti into various zeolites by reaction with TiCl4. It appears that Ti is incorporated into the zeolite framework. The catalytic activity of these modified zeolites was screened using the oxidation of phenol with H2O2 as a test reaction. Their performance depends on crystal structure, crystallite size, Ti content and preparation procedure and is compared with that of the non-modified acidic zeolites The product composition is solvent dependent. Deactivated catalysts may be regenerated by conventional techniques.

Template removal from molecular sieves by low-temperature plasma calcination

The oxidative removal of templates from crystals of MFI- and TON-type zeolites using low-temperature oxygen plasma at 370 K has been investigated. This treatment removes tetrapropylammonium inside 5 µm lengths of channels in the completely siliceous MFI structure (silicalite), and diethylamine to a depth of at least 70 µm in the channels of the completely siliceous TON structure. Substitution of B, Al or Fe into the framework of silicalite significantly obstructs the removal of tetrapropylammonium, but complete decomposition of the template can be accomplished in small samples of [Si, B]-MFI. A comparison of plasma calcination with thermal calcination in air indicates that the release of boron into extra-framework positions upon either treatment is caused by the slow desorption of water at the calcination temperature.

Effect of zeolite structure and morphology on intracrystalline n-hexane diffusion in pentasil zeolites studied by the zero-length column method

n-Hexane diffusion in ferrierite and silicalite-1, two molecular sieves belonging to the pentasil family, has been studied. The zero-length column method has been applied. Large, well shaped crystals were synthesised and used. For silicalite-1, the effect of macroscopic twinning has been verified by determining n-hexane diffusivities in ‘twinned’ and in single silicalite-1 crystals. It was found that ‘twinning’ has no effect on the transport diffusivity of n-hexane in silicalite-1. This is in accordance with structural data on ‘twinned’ MFI frameworks, presented in literature. The n-hexane diffusion in ferrierite is four orders of magnitude smaller than that in silicalite-1. This is proposed to be a consequence of structure faulting.

Chapter 26 – Progress in the Use of Zeolites in Organic Synthesis

This chapter discusses the use of zeolites in separation of organic products, and in shifting equilibria by selective adsorption (or selective membrane passage) of products or side products. Zeolites and mesoporous molecular sieves contribute in several ways to organic synthesis. They play an important role in subtle separation and in isomerization of aromatic regio-isomers, allowing low-waste integrated reaction/separation/isomerization processes. Zeolites can in situ remove selectively small by-products by adsorption or membrane action, thus shifting equilibria to completion. A generally overlooked pitfall is that zeolites induce—by ion exchange—some ionization of hydroxylic organic solvents and reactants. Zeolites and mesoporous molecular sieves can be tuned and modified in many ways, and generally, the protocols are available. Therefore, a given reaction a zeolite catalyst can be upgraded to improved performance. By its acid functions, a zeolite or mesoporous system can act as a bifunctional catalyst and can serve in the so-called “cascade reactions.”

Zeolites in organic cascade reactions

This chapter discusses zeolites in organic cascade reactions. The chapter focuses on the use of zeolites in cascade reactions—that is, combined catalytic reactions without intermediate recovery steps. Numerous multistep cascade syntheses are executed in the cells of living organisms without separation of intermediates. In fine chemicals syntheses, a systematic approach is applied in which intermediate products are isolated and purified for each next conversion step. In most bio–bio examples, two enzymes are applied but four examples already exist where eight enzymes operate in concert. Cascade record holder is a 12-step synthesis of hydrogenobyrinic acid—the corrin moiety of vitamin B12—starting from 5-aminolevulinic acid. The chapter concludes with a discussion of zeolite and ordered mesoporous materials that offer many opportunities to operate in two- and multi-step organic cascade conversions. The modification possibilities and tenability of the microporous and mesoporous systems are impressive allowing design of new bi- and multi-functional catalysts. Bi- and multi-zeolite systems can be envisaged in which catalysis and separation (membranes) operates in concert.

Chapter 13 Preparation Of Zeolitic Catalysts

Publisher Summary This chapter discusses that applications of zeolitic catalysts have been established in many conversion reactions. The nature of the reactions varies considerably and the scale of operations covers a wide range. Highly sophisticated catalyst preparation procedures were developed for the large scale applications in the petrochemical industries. The considerable economic incentive for obtaining the optimal yield structure justifies fine tuning of catalysts for each particular application in hydrocarbon processing. Although the number of existing applications in fine chemical reactions is presently small, this area has a large scope and is a challenging field of applied research in catalyst preparation. The chapter discusses that in the preparation and commercial manufacture of zeolitic catalysts, results of fundamental studies in zeolite chemistry are quickly applied. Especially in large applications such as fluidized catalytic cracking FCC and hydrocracking, even a small improvement in catalyst selectivity often has a considerable economic value. Because of the strong research effort in the area of zeolites, new catalysts having an improved performance are produced at a high rate. This lively interaction between fundamental and applied science makes research into the use of zeolitic catalysts very interesting.

Chapter 15 Preparation of zeolite catalysts

Publisher Summary Zeolite catalysts are applied in hydrocarbon processing and in chemical conversion processes. Properties that make zeolites versatile catalyst components are: Size and shape of the crystallites are variables in catalyst design and are determined by the synthesis conditions. The internal surface area is high and thermally very stable compared to those of the amorphous silicas and aluminas used traditionally in catalyst preparation. The pores are well defined and of molecular dimensions, the pore size is determined by the crystallographic structure. The pore openings ruling the accessibility of the internal surface may be subtly changed by ion-exchange, post impregnation or chemical reaction with specific compounds, (pore size engineering). Charge-compensating cations can be removed by ion exchange. T-atoms are in most structures highly accessible; their nature determines the polarity of the surface and its catalytic activity. The chemical composition of the lattice is dependent on the synthesis conditions. In many cases, the Si/AI ratio may be varied within wide limits and for some structures, also the nature of the T-atoms is a variable.

Low-temperature Plasma Calcination of Zeolite NH4, Na-Y

Abstract 70% Ammonium-exchanged zeolite NH 4 , Na-Y is completely deammoniated by low-temperature plasma calcination, while the crystallinity of the samples is not impaired. Neither the plasma calcined nor shallow bed treated material is “pure H-Y” zeolite, and 27 Al MAS NMR indicates a slight dealumination of the zeolitic framework in each case. Hydronium ion exchange to product H,Na-Y from Na-Y using highly dilute acidic medium also resulted in partial dealumination of the sample. Plasma calcination of 87% ammonium-exchanged NH 4 , Na-Y caused a severe loss of crystallinity, possibly because of mineral acid formation at inaccessible sites. Low-temperature plasma calcination is a mild technique for the oxidative removal of ammonia as long as oxidation products can easily leave the crystallite.