On January 1, 2013, Ron Stites was awarded his fifth patent. This particular patent describes an alternative method for improving catalyst performance by adding trace amounts of desired metals (primarily alkali) to the catalyst. We will describe the process in general.
A catalyst is a chemical (often a solid) that helps speed up a reaction without actually being consumed in the reaction. A typical example would be the very important Haber reaction which is the synthesis of ammonia from nitrogen and hydrogen. These two gases can be mixed, heated and even put under very high pressure with virtually no reaction. Nevertheless, if these hot, pressurized gases are passed over a finely iron powder that has a small amounts of potassium, calcium, silicon or aluminum they quickly react to make ammonia in good yields. Without this catalyst being commercialized around 1913 much of the world would be starving, but that is a different story.
The “magic” of a catalyst is to change the rate of a reaction. Reactions that go very slowly (like the ammonia reaction above) can be sped up by hundreds and even thousands of times. This can make the difference between a commercially useful reaction and one that is just a laboratory curiosity. These catalysts are often solids and can be metals, basic oxides, silicates, aluminates and even something as mundane as charcoal. The actual active ingredient of the catalyst might be present as an almost pure material or be present in only trace amounts on an otherwise inert support. Catalysts come in all kinds of sizes and shapes but they share some common features.
One of these is the ability to have intimate contact between the active ingredient of the catalyst and the reactants of the reaction. Hence, the active ingredient almost always has a very high surface area to weight ratio. This usually means very small particles that are widely and uniformly dispersed. One of the great challenges for the catalyst maker is to have the active ingredient a small particle without all the particles packing together so tightly that in interior particles are useless. This is often accomplished by mixing the active ingredient with a porous material that can be pressed or molded into a shape. Those shapes can be complex or very simple depending on the porous material. They might be donuts, stars, wafers or just pellets. The porous material holds the small particles apart while still allowing the reactant gases to reach them. Alternatively, the active ingredient can be placed as a very thin layer on the surface of an essentially impervious material. These are usually round balls and the catalysts are called egg shell type catalysts referring to the thin layer of active ingredient. There are other schemes, but these are two of the most common.
A second feature of catalysts is the presence of trace but very important secondary and even tertiary active ingredients. Early catalysts were often pure materials. It was quickly learned, however, that small amounts of a second or a third material could make a huge difference in performance. These minor materials cannot only change the speed of the reaction, but can even change the type of products being made. Because they can “promote” desired effects, these minor components are often called “promoters.”
Uniformly and consistently dispersing these minor materials into the primary active ingredient can be a challenge – especially if these materials are quite dissimilar. Sometimes these minor materials can simply be mixed with the major ingredient, but frequently this simply will not work. It is quite difficult to get dissimilar solids to mix uniformly and stay together as the blend is made into a physical shape. Often they segregate and the desired effect can be lost. Hence, many schemes were developed to add these promoters as a separate controllable step. Some of these are:
- Vaporize the promoter with heat and condense it on the catalyst
- Make the promoter into a volatile carbonyl gas and react it onto the catalyst
- Dissolve the promoter in water and then dry it onto the catalyst
All of these techniques have advantages and disadvantages. Nevertheless, they do not work well for adding alkali metals (Na, K, Cs, etc.) to some types of catalysts, especially those that are water sensitive. It is these types of catalysts that are often used in alternative fuels. Many are water sensitive and many require alkali metal promoters. Hence, this invention is especially useful in the alternative fuels industry.
It was noted that alkali metals react directly with most alcohols (but especially methanol) to make alcohol soluble alkoxide salts. Hence, alkali metals can be dissolved in alcohol rather than water. This solution can be used to uniformly dope water sensitive catalysts with the desired alkali metal. The excess alcohol is simple evaporated leaving behind the alkoxide salt. These salts are readily converted to the free alkali ion and appropriate counter ion (often oxide or carbonate) under the operating conditions of the catalyst.
This method was used to promote two important alcohol catalysts with potassium. These were Cu/ZnO/Al2O3 (a methanol catalyst) and MoS2 (a low activity WGS catalyst). Both tests indicated that the promotion was successful as evidenced by the shift in products toward heavier alcohols (the desired effect).
The procedure was also used with several other transition metals. It was learned that cobalt acetate, rhenium oxide and rhodium oxide all had significant alcohol solubility. Hence, it was shown that existing catalysts could be promoted with trace amounts of these transition metals. And finally, other metal oxides, hydroxides and acetates have significant alcohol solubility and may be amenable to this technique.
This method was developed by Ron Stites while Director of Research for Range Fuels. The patent application was picked up and pursued by Albemarle Corporation, an international catalyst company headquartered in Baton Rouge, LA.