Rhenium

Rhenium is a chemical element; it has symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion, rhenium is one of the rarest elements in the Earth's crust. It has one of the highest melting and boiling points of any element. It resembles manganese and technetium chemically and is mainly obtained as a by-product of the extraction and refinement of molybdenum and copper ores. It shows in its compounds a wide variety of oxidation states ranging from −3 to +7.
Rhenium was originally discovered in 1908 by Masataka Ogawa, but he mistakenly assigned it as element 43 rather than element 75 and named it nipponium. It was rediscovered in 1925 by Walter Noddack, Ida Tacke and Otto Berg, who gave it its present name. It was named after the river Rhine in Europe, from which the earliest samples had been obtained and worked commercially.
Nickel-based superalloys of rhenium are used in combustion chambers, turbine blades, and exhaust nozzles of jet engines. These alloys contain up to 6% rhenium, making jet engine construction the largest single use for the element. The second-most important use is as a catalyst: it is an excellent catalyst for hydrogenation and isomerization, and is used for example in catalytic reforming of naphtha for use in gasoline. Because of the low availability relative to demand, rhenium is expensive, with price reaching an all-time high in 2008–09 of US$10,600 per kilogram. As of 2018, its price had dropped to US$2,844 per kilogram due to increased recycling and a drop in demand for rhenium catalysts.

History

In 1908, Japanese chemist Masataka Ogawa announced that he had discovered the 43rd element and named it nipponium after Japan. In fact, he had found element 75 instead of element 43: both elements are in the same group of the periodic table. Ogawa's work was often incorrectly cited, because some of his key results were published only in Japanese; it is likely that his insistence on searching for element 43 prevented him from considering that he might have found element 75 instead. Just before Ogawa's death in 1930, Kenjiro Kimura analysed Ogawa's sample by X-ray spectroscopy at the Imperial University of Tokyo, and said to a friend that "it was beautiful rhenium indeed". He did not reveal this publicly, because under the Japanese university culture before World War II it was frowned upon to point out the mistakes of one's seniors, but the evidence became known to some Japanese news media regardless. As time passed with no repetitions of the experiments or new work on nipponium, Ogawa's claim faded away. The symbol Np was later used for the element neptunium, and the name "nihonium", also named after Japan, along with symbol Nh, was later used for element 113. Element 113 was also discovered by a team of Japanese scientists and was named in respectful homage to Ogawa's work. Today, Ogawa's claim is widely accepted as having been the discovery of element 75 in hindsight.
Rhenium received its current name when it was rediscovered by Walter Noddack, Ida Noddack, and Otto Berg in Germany. In 1925 they reported that they had detected the element in platinum ore and in the mineral columbite. They also found rhenium in gadolinite and molybdenite. In 1928 they were able to extract 1 g of the element by processing 660 kg of molybdenite. It was estimated in 1968 that 75% of the rhenium metal in the United States was used for research and the development of refractory metal alloys. It took several years from that point before the superalloys became widely used.
The original mischaracterization by Ogawa in 1908 and final work in 1925 makes rhenium perhaps the last stable element to be understood. Hafnium was discovered in 1923 and all other new elements discovered since then do not have stable isotopes.

Characteristics

Rhenium is a silvery-white metal with one of the highest melting points of all elements, exceeded by only tungsten. It also has one of the highest boiling points of all elements, and the highest among stable elements. It is also one of the densest, exceeded only by platinum, iridium and osmium. Rhenium has a hexagonal close-packed crystal structure.
Its usual commercial form is a powder, but this element can be consolidated by pressing and sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact solid having a density above 90% of the density of the metal. When annealed this metal is very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K; tungsten-rhenium alloys are also superconductive around 4–8 K, depending on the alloy. Rhenium metal superconducts at.
In bulk form and at room temperature and atmospheric pressure, the element resists alkalis, sulfuric acid, hydrochloric acid, nitric acid, and aqua regia. It will however, react with nitric acid upon heating.

Isotopes

Rhenium has one stable isotope, rhenium-185, which nevertheless occurs in minority abundance, a situation found only in two other elements. Naturally occurring rhenium is only 37.4% ¹⁸⁵Re, and 62.6% ¹⁸⁷Re, which is unstable but has a very long half-life. A kilogram of natural rhenium emits 1.07 MBq of radiation due to the presence of this isotope. This lifetime can be greatly affected by the charge state of the rhenium atom. The beta decay of ¹⁸⁷Re is used for rhenium–osmium dating of ores. The available energy for this beta decay is the second lowest known among all radionuclides, only behind the decay from ¹¹⁵In to excited ¹¹⁵Sn*. The isotope rhenium-186m is notable as being one of the longest lived metastable isotopes with a half-life of around 200,000 years. There are 33 other unstable isotopes that have been recognized, ranging from ¹⁶⁰Re to ¹⁹⁴Re, the longest-lived of which is ¹⁸³Re with a half-life of 70 days.

Compounds

Rhenium compounds are known for all the oxidation states between −3 and +7 except −2. The oxidation states +7, +4, and +3 are the most common. Rhenium is most available commercially as salts of perrhenate, including sodium and ammonium perrhenates. These are white, water-soluble compounds. Tetrathioperrhenate anion ⁻ is possible.

Halides and oxyhalides

The most common rhenium chlorides are ReCl₆, ReCl₅, ReCl₄, and ReCl₃. The structures of these compounds often feature extensive Re-Re bonding, which is characteristic of this metal in oxidation states lower than VII. Salts of ²⁻ feature a quadruple metal-metal bond. Although the highest rhenium chloride features Re, fluorine gives the d⁰ Re derivative rhenium heptafluoride. Bromides and iodides of rhenium are also well known, including rhenium pentabromide and rhenium tetraiodide.
Like tungsten and molybdenum, with which it shares chemical similarities, rhenium forms a variety of oxyhalides. The oxychlorides are most common, and include ReOCl₄, ReOCl₃.

Oxides and sulfides

The most common oxide is the volatile yellow Re₂O₇. The red rhenium trioxide ReO₃ adopts a perovskite-like structure. Other oxides include Re₂O₅, ReO₂, and Re₂O₃. The sulfides are ReS₂ and Re₂S₇. Perrhenate salts can be converted to tetrathioperrhenate by the action of ammonium hydrosulfide.

Other compounds

is a hard compound having a hardness similar to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride.

Organorhenium compounds

is the most common entry to organorhenium chemistry. Its reduction with sodium amalgam gives Na with rhenium in the formal oxidation state −1. Dirhenium decacarbonyl can be oxidised with bromine to bromopentacarbonylrhenium:
Reduction of this pentacarbonyl with zinc and acetic acid gives pentacarbonylhydridorhenium:
Methylrhenium trioxide, CH₃ReO₃ is a volatile, colourless solid that has been used as a catalyst in some laboratory experiments. It can be prepared by many routes, a typical method is the reaction of Re₂O₇ and tetramethyltin:
Analogous alkyl and aryl derivatives are known. MTO catalyses for the oxidations with hydrogen peroxide. Terminal alkynes yield the corresponding acid or ester, internal alkynes yield diketones, and alkenes give epoxides. MTO also catalyses the conversion of aldehydes and diazoalkanes into an alkene.

Nonahydridorhenate

A distinctive derivative of rhenium is nonahydridorhenate, originally thought to be the rhenide anion, Re⁻, but actually containing the anion in which the oxidation state of rhenium is +7.

Occurrence

Rhenium is one of the rarest elements in Earth's crust with an average concentration of 1 ppb; other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust. Rhenium is probably not found free in nature, but occurs in amounts up to 0.2% in the mineral molybdenite, the major commercial source, although single molybdenite samples with up to 1.88% have been found. Chile has the world's largest rhenium reserves, part of the copper ore deposits, and was the leading producer as of 2005. It was only recently that the first rhenium mineral was found and described, a rhenium sulfide mineral condensing from a fumarole on Kudriavy volcano, Iturup island, in the Kuril Islands. Kudriavy discharges up to 20–60 kg rhenium per year mostly in the form of rhenium disulfide. Named rheniite, this rare mineral commands high prices among collectors.

Production

Approximately 80% of rhenium is extracted from porphyry molybdenum deposits. Some ores contain 0.001% to 0.2% rhenium. Roasting the ore volatilizes rhenium oxides. Rhenium oxide and perrhenic acid readily dissolve in water; they are leached from flue dusts and gasses and extracted by precipitating with potassium or ammonium chloride as the perrhenate salts, and purified by recrystallization. Total world production is between 40 and 50 tons/year; the main producers are in Chile, the United States, Peru, and Poland. Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons per year. Prices for the metal rose rapidly in early 2008, from $1000–$2000 per kg in 2003–2006 to over $10,000 in February 2008. The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures:
There are technologies for the associated extraction of rhenium from productive solutions of underground leaching of uranium ores.