Reactions over Microporous Solid Acids

Hydrocarbon cracking Y-zeolites ZSM-5 and Beta zeolite additives Si02-AI2O3 

350-500 °C 15-30 bar Important intermediates for solvents, insecticides, herbicides, pharmaceuticals and detergents 

300 0°C20bar Ethylbenzene, ethyltoluene precursors for styrenes used for polymers, isopropyl benzene precursor for phenols 

400-500 °C ca. 1 bar More valuable lower molecular weight fractions 

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Stopped flow experiment was performed, in which the reactant flow was stopped and replaced with an inert gas flow as the sample is cooled to room temperature, leaving only NNN-TMA ions in the zeolite. Subsequent heating above 200 °C (in a sealed system) resulted in conversion of the NNN-TMA to the other species, showing the NNN-TMA is indeed reactive and part of the reaction mechanism. Analysis of the transient effects of sharp changes in reactant composition by in situ NMR in this way is likely to be very powerful in understanding reaction mechanisms over microporous solid acids. 

Scheme 8.11 Steps in the reaction of methanol over microporous solid acids.

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

Membrane-like heteropoly acid-blended polymer film catalysts were prepared using a common solvent (or mixed solvents) and they were tested as fixed-bed catalysts for the ethanol conversion reaction in a continuous flow reactor. It was found that heteropoly acid catalyst was finely and uniformly distributed through the polymer matrix. All the film catalysts showed the higher selectivity to acetaldehyde than the bulk solid catalyst. Conversion and selectivity over the film catalysts were also affected by the nature of solvent and polymer. Microporosity of the film catalyst was controlled by the phase separation method. The microporous film catalyst could be regarded as a highly dispersed heteropoly acid catalyst supported on polymer matrix. The film catalysts were characterized by IR, TPD, SEM, EDX, DSC, and ESCA. 

The announcement in 1972 of the discovery of a new acidic solid with reproducible micropore structure generated a new wave of interest in zeolitic materials (ref. 1). The demonstration of the catalytic efficiency of the ZSM family of zeolites in the conversion of methanol to gasoline-range hydrocarbons was particularly timely in view of the worldwide concern over the continued availability of oil supplies. There followed a burst of activity relating to the structure of the ZSM catalysts, their shape selective properties, their acidic characteristics, their catalytic activities in a variety of reactions and the mechanism of the methanol conversion process (refs. 2-4). Efforts in this area have continued. 

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