Basic Processes in Heterogeneous Reactions

Simple Phase Transition Aragonite to Calcite Complex Phase Transformation Complex Phase Transformation Magma to Rock Volcanic Eruption  

Aragonite is decompressed to calcite stability field Magma is cooled to below the liquidus Gas-bearing magma is decompressed to oversaturation 

Calcite embryos nucleate Crystals nucleate Bubbles nucleate 

Calcite crystals grow at the expense of aragonite Crystals grow Other minerals nucleate and grow Bubbles grow volume of bubbly magma expands rapidly 

No more aragonite No more magma A foam is formed almost no dissolved volatiles in magma 

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The hydrogenation of p-unsaturated alcohols is a classical example of competing reaction processes in heterogeneous catalysis. In principle, either isomerization to an enol followed by tautomerization to the ketone can occur, or direct hydrogenation to the saturated alcohol is possible (1). It is fair to state that interest in this type of chemistry is derived from both basic scientific... 

We are basically interested in heterogeneous, high temperature kinetics. At high temperatures, the chemical reactions occur as fast as the reactants can get together, like ionic solution reactions. Hence, we are concerned mainly with how fast the species can get from one part of the system to another. These transport processes are the bottlenecks... 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

As previously mentioned, all biologically initiated reactions are basically heterogeneous. However, for practical reasons, the processes in the suspended phase can be considered homogeneous. Processes in biofilms proceed by exchange of electron donors and electron acceptors with the surrounding bulk water phase. These processes are, therefore, heterogeneous. 

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