Heterogeneous Catalysis

A catalyst is anything that speeds up the rate of a reaction without being consumed in the process. It refers to processes that are able to selectively choose specific reaction pathways by slowing down unfavorable reactions and speeding up favorable ones. Catalysis is thus the use of catalysts to tune reactions towards desired outcomes.

In general, catalysts have the ability of providing alternative reaction pathways via different transition states with lower activation energies. This translates into faster kinetics as well as energy savings:

Energy Barrier http://en.wikipedia.org/wiki/Catalysis

Catalysts are important in biology as enzymes, in industry for chemical synthesis, in the energy and fuels industries for synthesis and refinement, in food processing and in many other sectors. Common examples of catalysis include the automotive catalytic converter, the fuel cell, hydrogenation of fats, the production of ammonia from natural gas, and enzymes involved with metabolism in our bodies.

The North American Catalysis Society has a great page on the description of catalysts in laymans terms. They also correctly ascertain that catalysts "translate to global energy savings, less pollution, fewer side products, lower cost reactor materials, and ultimately products which reduce global warming." In fact the global market for catalysts is on the order of US$12 billion. " Estimates are that catalysis contributes to greater than 35% of global GDP."

A heterogeneous catalyst is a catalyst that remains in another phase from that with which it is reacting on. Heterogeneous catalysis is perhaps the most important industrial catalysis because it allows for easy separation of the products from the catalyst. The basic mechanism for this type of catalyst is that a reactant molecule first binds to the surface of the catalyst and then proceeds to react with some reaction mechanism. Zeolites are perhaps the most important example of heterogeneous catalysts because of their role in petroleum refining.

 

Zeolites

Zeolites are "tridimensional crystalline aluminosilicates with pores/cavities of molecular dimensions" (Corma A.  Chem. Rev. 1995, 95. 559-614.). Below you can see the regular pore like structure of a common zeolite, ZSM-V.

ZSM-5 Structure http://www.3dchem.com/molecules.asp?ID=86

The figure above shows the alternating silicon to oxygen bond framework that creates these ordered structures. Pure silicon/oxygen zeolites are known as molecular sieves because of their ability to trap water in their pores. These have no acidic behavior. Only upon the introduction of a heteroatom with a lower valency than Si, such as Al, B, Ga, or Fe, and upon protonation, does the zeolite exhibit acidity. In this state the zeolite is known as a solid acid and the acid sites are Bronsted in nature (versus Lewis acidity).

protonated zeolite

Zeolites come in many shapes and sizes. There are over 200 known structure types in existence today (http://iza-online.org). It has been calculated that over 2 million zeolite structures are possible given different crystal arrangements, however, it is likely that most of these are unstable at moderate temperatures and not viable for practical use (Deem et al. "Computational Discovery of New Zeolite-Like Materials" J. Phys. Chem. C DOI: 10.1021) . This means there are many morphologies, that when combined with different heteroatoms, metals and reactants, yield an almost infinite smorgasbord of possibilities.

Known zeolite structures have been classified and organized by the International Zeolite Association. They are typically given 3 letter acronyms to describe their framework type. These include MOR for mordenite, MFI for mobil five (also known as ZSM-V for zeolite Socony Mobil -five after the company that invented it), FAU for faujasite. The origins of these names can be found here.

This paper deals with zeolites H-SAPO-34, H-ZSM-5, and H-SAPO-18, where the H simply means that the zeolite has Bronsted acid sites.

Zeolites are effective catalysts because of their regular pore structure and size. Zeolites have dimensions on the molecular scale which means they can selectively allow certain sized chemicals in or out of the pores. This has implications on the catalytic activity and is commonly referred to as shape selectivity. This can refer to selectivity of the products, reactants and/or transition states. Below we show the typical pore size of ZSM-V (MFI) on the order of 5.5 Angstroms (a carbon-carbon bond is on the order of 1.54 Angstroms):

MFI dimensions

One can infer that a small molecule such as methanol (CH3OH) can easily fit inside the pore, while a larger molecule such as p-xylene might have more trouble. The video below illustrates the size of methanol in a similar sized cavity of ferrierite:

This paper involves the use of acidic zeolites for the catalysis of methanol to olefins.