Zirconium Rhymes With Titanium

Zirconium has a long, illustrious history in jewelery. The element's name derives from an ancient word for the gemstone, zircon.  The element is more commonly found in cubic zirconia, which--thanks to the Soviet method for making it using the so-called skull process--is cheap and plentiful.

Zirconium rhymes with titanium. Often, such transition-metal family members mimic each other. Zirconium, like titanium, is a valuable catalyst for making plastics. I know--yawn. But catalysis is an intellectually interesting aspect of chemistry--one which has "real-life" analogy--much like status quo and change.

Catalysts are classified as "heterogeneous" or "homogeneous" depending on whether they mix freely with hoi polloi substrates. "Heterogeneous" means that the catalyst stays in a different phase than whatever it's working on--e.g., a catalytic converter working on gas phase exhaust. Homogeneous catalysts swim in a liquid phase like everything else around it--in a single phase.

First generation Ziegler-Natta catalysts were heterogeneous. Catalysis happened at the edges or face of a chunk or pellet.  Obviously, a lot of unused catalyst lies buried inside- and is wasted. Homogeneous catalysts are known for their "atom efficiency"-- a concept that becomes more important for rarer platinum group metals.

How Titanium Gets All Touchy-Feely with Carbon

A titanium chloride catalyst holds one end of the growing polymer chain. The same titanium atom simultaneously binds another incoming ethylene and stabilizes the contortions leading to the insertion of the next link into the growing chain. Titanium does this by polarizing ethylene's electrons while stabilizing a migration:



Original is here


Polarization, followed by attack, followed by depolarization...polarization, followed by attack, followed by depolarization...polarization, followed by attack, followed by depolarization... link

Karl Ziegler: "Consequences and Development of an Invention"

Karl Ziegler, German chemist (1898-1973)

Karl Ziegler, then director of the Max-Planck-Institute-for Coal Research, describing what he and co-workers discovered ten years prior to winning the 1963 Nobel Prize in Chemistry:

The catalyst is prepared simply by simultaneously pouring, with exclusion of air, two liquid materials into about two liters of a gasoline-like hydrocarbon, after which ethylene is introduced, while stirring. The gas is absorbed quickly; within an hour one can easily introduce 300-400 liters of ethylene into the two liters of liquid. At the same time, a solid substance precipitates, and can scarcely be stirred anymore. If the brown catalyst* is then destroyed, by the addition of some alcohol and by the introduction of air, the precipitate becomes snow-white and can be filtered off. In its final state it will accumulate in amounts of 300-500 g, as a dry, white powder.

~Karl Ziegler "Consequences and development of an invention"

______________
*The two co-catalysts were titanium and aluminum chlorides

Polyethylene had been known earlier. A British company, ICI, held patents for what they called "polythene" (hmm, maybe related to the Beatles' "plasticene"?), but ICI's polyethylene was different animal than Ziegler's polyethylene. The difference is at the atomic level. Though both plastics were polymers of ethylene, the older, inferior product was highly branched:

Ziegler's new process for making polyethylene essentially made perfectly linear chains of polymer with very little branching. The bulk properties of the two were markedly different. The density differences are akin to what one expects from trying to pack together a bunch of branches versus bunches of straight sticks.

Ziegler and his Institute became independently wealthy as the plastic age began in earnest.

Titan Macht Kohlenstoff

Titanium is the carbon of transition metals. The Germans realized this early on. Look again at this "carbo-centric" Periodic Table of the Elements from 1926:

Follow the color code (carbon is black) down and to the left. The association runs right through titanium and zirconium, down to hafnium and thorium. To the right, the rhymes-with-carbon pathway runs down through germanium, tin, and lead; I'll get to the latter later after having some fun with the first row transition metals.

Titanium has probably touched more carbon atoms than any other metallic element. And there are two words for this Benjamin--just two words I want to say to you. Are you listening Benjamin?

Ziegler-Natta!