Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75, classified as a dense, silvery-white transition metal that remains solid at room temperature and exhibits one of the highest melting points of any element at 3,180°C, surpassed only by tungsten and carbon.[1][2] With an atomic mass of 186.2 u and a density of 21.02 g/cm³, it adopts a hexagonal close-packed crystal structure and displays variable oxidation states from -1 to +7, most commonly +7, forming stable oxides such as rhenium(VII) oxide (Re₂O₇).[2][3] In 2025, rhenium was added to the U.S. list of critical minerals due to its economic and national security importance.[4] Discovered in 1925 by German chemists Walter Noddack, Ida Tacke, and Otto Berg through concentration from gadolinite and molybdenum ores, rhenium was the last stable element to be identified in nature, marking the completion of the list of naturally occurring elements up to uranium.[2][3] Its extreme rarity in Earth's crust, at about 0.4 parts per billion, makes it one of the scarcest refractory metals, primarily occurring as a trace component (100–3,000 ppm) in molybdenite (MoS₂) within porphyry copper deposits.[2] In 2024, world mine production of rhenium was 62 metric tons, primarily as a byproduct of copper and molybdenum refining (mainly from roasting flue dust) in countries like Chile, which accounted for 53% of output, with secondary production from recycling estimated at 25 metric tons.[5] Rhenium's exceptional properties, including high ductility, resistance to wear and corrosion, and ability to withstand extreme temperatures, render it indispensable in high-performance applications, with about 80% used in nickel-based superalloys for turbine blades in jet engines and 15% in platinum-rhenium reforming catalysts for producing high-octane gasoline from petroleum.[5] Despite its scarcity, rhenium poses no significant toxicity risks due to low environmental abundance, though mining-associated activities can impact ecosystems.[2] Emerging uses include radioisotopes like ¹⁸⁶Re and ¹⁸⁸Re for targeted cancer therapies and bone pain management in medicine.[2]History
Discovery
The existence of element 75, positioned below manganese in group 7 of the periodic table, was first predicted by Dmitri Mendeleev in 1869 as "dvi-manganese," based on gaps in his periodic table and anticipated chemical properties similar to those of manganese and technetium (element 43).[6] Mendeleev and subsequent chemists, including Henry Moseley through his work on atomic numbers in 1914, reinforced the expectation of this missing element, estimating it would be rare and exhibit refractory metal characteristics.[7] Although spectral lines possibly belonging to rhenium were observed by Japanese chemist Masataka Ogawa in 1908 from thorianite and misidentified as element 43 (nipponium), the element was confirmed in 1925. In June 1925, German chemists Ida Tacke, Walter Noddack, and Otto Berg reported the spectroscopic detection of element 75 in minerals such as columbite (a niobate-tantalate) and gadolinite (a silicate), after analyzing over 1,000 samples using X-ray emission spectroscopy at the Physikalisch-Technische Reichsanstalt in Berlin.[8] Their method involved bombarding ore samples with electrons to excite atomic emissions, revealing previously unobserved spectral lines consistent with atomic number 75, distinct from known elements.[9] This detection, along with the claimed discovery of element 43 (masurium, later confirmed as technetium in 1937), was part of a systematic search for "missing" elements predicted by Mendeleev, focusing on platinum-group ores and rare earth minerals where trace amounts were hypothesized to occur; the masurium claim was controversial and not verified at the time.[10] Walter Noddack and Otto Berg provided independent confirmation later in 1925 through complementary X-ray spectroscopy and chemical separation techniques, isolating perrhenate salts from the same ores to verify the element's presence and properties.[8] The isolation faced significant challenges due to rhenium's extreme rarity—estimated at only 1 part per billion in the Earth's crust—and its chemical similarity to manganese, which complicated separation from co-occurring elements in manganese-rich ores and required laborious enrichment processes.[3] Early attempts had failed because rhenium is not concentrated in typical manganese deposits, leading the team to target unrelated mineral sources.[11] In 1928, Noddack and Berg achieved the first isolation of metallic rhenium by hydrogen reduction of ammonium perrhenate (NH₄ReO₄) at elevated temperatures, yielding approximately 1 gram of the pure metal from processing 660 kilograms of molybdenite ore.[12] This milestone confirmed the element's metallic nature and refractory properties, marking the end of over 50 years of anticipation since Mendeleev's prediction.[8]Naming and Etymology
The name "rhenium" was proposed in 1925 by its discoverers, German chemists Walter Noddack, Ida Tacke, and Otto Berg, shortly after they identified the element through spectroscopic analysis of minerals.[6][13] The term derives from "Rhenus," the Latin name for the Rhine River, a major waterway in central Europe that holds deep cultural and historical significance as a symbol of the researchers' German homeland.[14][15] This naming choice reflected the element's initial detection in minerals sourced from European regions, including columbite and gadolinite from Norway, underscoring a patriotic nod to the Rhine's role in German identity and the scientific contributions emerging from the area.[6][15] The Rhine, originating in the Swiss Alps and flowing through Germany and the Netherlands, has long been celebrated in literature and folklore as a cradle of European civilization, making it a fitting tribute for an element so rare and elusive.[14] Prior to its formal identification, element 75 had been anticipated in the periodic table as "dvi-manganese," a provisional name coined by Dmitri Mendeleev in the late 19th century based on its predicted position two rows below manganese.[16] This systematic nomenclature highlighted Mendeleev's foresight in leaving gaps for undiscovered elements, with dvi-manganese expected to exhibit properties akin to an enhanced version of manganese, such as higher atomic weight and density.[16] Ida Tacke played a pivotal role in the discovery and naming process, contributing to the experimental work and co-authoring the seminal 1925 paper announcing rhenium, yet her contributions were often overshadowed by gender biases prevalent in early 20th-century science.[17][9] Despite facing institutional barriers that limited women's recognition in academia, Tacke's insistence on rigorous verification helped solidify the element's identification, marking a significant yet underacknowledged milestone in her career.[17]Occurrence and Abundance
Natural Sources
Rhenium occurs primarily as a trace element in molybdenum sulfide ores, substituting for molybdenum in the mineral molybdenite (MoS₂), with concentrations typically ranging from 100 to 3,000 ppm.[18][19] It is also associated with other minerals, including gadolinite and columbite, as well as certain platinum-group element ores in sulfide-rich environments.[2] Key natural deposits of rhenium are found in porphyry copper-molybdenum mines, such as those at Chuquicamata in Chile, Bingham Canyon in the United States, and Erdenet in Mongolia, where it is disseminated within the ore bodies.[2][20] Rare independent minerals containing rhenium include rheniite (ReS₂), which forms in volcanic fumaroles under high-temperature conditions, as observed at sites like the Kudriavy volcano on Iturup Island in the Kuril Islands.[21] Due to its low crustal abundance, rhenium is almost exclusively obtained as a byproduct of copper and molybdenum mining from these deposits.[22][23]Cosmic and Earth's Abundance
Rhenium exhibits low abundance in cosmic environments, with an estimated 0.05 atoms per 10^6 silicon atoms in the solar system, based on analyses of carbonaceous chondrites and solar photospheric data that serve as proxies for bulk solar composition. This scarcity reflects its position among the heavier elements, primarily synthesized through the rapid neutron-capture process (r-process) during explosive nucleosynthesis in core-collapse supernovae and neutron star mergers, where extreme neutron fluxes enable the buildup of nuclei beyond iron.[24][25][26] In Earth's crust, rhenium ranks among the rarest stable elements, with an average concentration of approximately 0.3–0.7 parts per billion (ppb) in the upper continental crust, far below common lithophile elements and even scarcer than many precious metals like platinum (∼5 ppb) and gold (∼4 ppb).[27][2][6][28][29] Seawater concentrations are similarly trace, averaging around 7.5 parts per trillion (ppt), where rhenium behaves conservatively as the perrhenate ion (ReO₄⁻) under oxic conditions, showing minimal fractionation during ocean circulation. Rhenium displays no significant biological concentration or known essential role in living organisms, consistent with its rarity and lack of incorporation into biochemical pathways.[27][2][6] As a highly siderophile element, rhenium partitions strongly into metallic phases during planetary differentiation, leading to elevated concentrations in Earth's iron-rich core (estimated at ∼230 ppb) compared to the bulk silicate Earth mantle (∼0.35 ppb). Observations in meteorites, such as carbonaceous chondrites with rhenium levels around 40–50 ppb, and lunar samples reveal systematic depletions relative to solar abundances, attributed to volatile loss of rhenium (likely as oxide species) during high-temperature accretion and magma ocean phases in early planetary bodies. These trends underscore rhenium's moderate volatility alongside its siderophile affinity, influencing its distribution across differentiated reservoirs.[27][30][31]Production
Extraction Processes
Rhenium is primarily recovered as a byproduct during the processing of molybdenite concentrates from copper-molybdenum ores, where it volatilizes as rhenium heptoxide (Re₂O₇) during roasting and collects in the flue dusts.[2] The extraction begins with roasting the molybdenite in the presence of soda ash (sodium carbonate), which converts molybdenum to water-soluble sodium molybdate while oxidizing rhenium to the soluble perrhenate ion (ReO₄⁻); subsequent leaching of the calcine or flue dust with water or dilute alkali yields a solution rich in perrhenate.[32] This leaching step typically achieves high dissolution rates for rhenium, often exceeding 90% from the dust, forming perrhenic acid (HReO₄) or its salts in the pregnant liquor.[33] The perrhenate-containing leach solution also includes molybdenum, necessitating separation techniques to isolate rhenium. Ion exchange using anion-exchange resins preferentially adsorbs ReO₄⁻ over molybdate (MoO₄²⁻) due to differences in ionic charge and complexation, allowing selective elution of rhenium with solutions like sodium hydroxide or perchloric acid.[34] Alternatively, solvent extraction employs extractants such as tributyl phosphate (TBP) in kerosene, which selectively complexes perrhenate in acidic media (pH 1-3), achieving separation efficiencies of over 95% for rhenium while leaving molybdenum in the raffinate; stripping with ammonia then precipitates ammonium perrhenate (NH₄ReO₄).[35] These methods ensure high-purity rhenium recovery from the mixed liquor. The separated ammonium perrhenate is then reduced to metallic rhenium powder via hydrogen gas in a tubular furnace. The reduction occurs in stages: initial decomposition at around 200-300°C releases ammonia and water, followed by progressive reduction of intermediate oxides (Re₂O₇ to ReO₃, ReO₂, and finally Re) at 300-500°C, yielding fine powder with particle sizes typically 1-20 µm.[36] The overall reaction can be represented as: $2\mathrm{NH_4ReO_4} + 7\mathrm{H_2} \rightarrow 2\mathrm{Re} + 8\mathrm{H_2O} + 2\mathrm{NH_3} This equation derives from the stepwise thermal decomposition of NH₄ReO₄ to Re₂O₇, NH₃, and H₂O, followed by hydrogen reduction of Re₂O₇ to Re (Re₂O₇ + 7H₂ → 2Re + 7H₂O), with stoichiometry balanced for two moles of perrhenate.[37] Overall recovery efficiencies from copper-molybdenum ores range from 40-60%, limited by initial volatilization losses and separation selectivity.[38] For high-purity rhenium (>99.99%), alternative processes bypass powder formation. Electrolysis of perrhenic acid (HReO₄) in sulfuric acid electrolytes deposits rhenium metal on cathodes like platinum or copper at potentials of -0.5 to -1.0 V vs. SHE, producing compact deposits suitable for electronics.[39] Chemical vapor deposition (CVD) involves the hydrogen reduction of rhenium hexafluoride (ReF₆) or carbonyl precursors at 500-800°C, yielding thin films or freestanding structures with exceptional purity for aerospace components.[40]Global Supply and Economic Aspects
Global rhenium production was approximately 63 tonnes in 2023 and an estimated 62 tonnes in 2024, according to revised data, driven by recovery from molybdenum processing amid rising demand for high-performance alloys.[5] Projections indicate a potential rise to around 70-85 tonnes annually by 2030 when including expansions in secondary recovery and new processing capacities in key regions.[41] Chile remains the leading producer, accounting for about 47% of global output through byproduct recovery from copper and molybdenum mines, followed by the United States and Poland at roughly 15% each.[5] China, while a major consumer representing over 40% of global demand, contributes domestic production of around 8% and relies heavily on imports.[41][5] The rhenium supply chain is highly concentrated, with over 90% derived as a byproduct of molybdenite roasting during molybdenum and copper production, exposing it to fluctuations in those markets and geopolitical risks in mining regions.[42] This dependency has led to supply vulnerabilities, including periodic shortages when primary metal prices deter recovery from low-grade ores. Rhenium prices have exhibited significant volatility, surging from an average of $1,030 per kg in 2020 to $1,070 per kg for metal pellets in 2023, before escalating sharply to $2,300–2,400 per kg for ammonium perrhenate in mid-2025 due to heightened demand from aerospace superalloys and speculative trading.[5][41] As of November 2025, spot prices have reached $4,300–4,800 per kg amid ongoing supply tightness and jet engine sector growth.[43] Recycling has become increasingly vital, supplying 20–30% of global rhenium through recovery from spent petroleum catalysts and superalloy scrap, with worldwide secondary production estimated at 25 tonnes in 2024 and expected to grow to support total effective supply of up to 85 tonnes annually.[5][44] In November 2025, the United States designated rhenium as a critical mineral in its final list, underscoring efforts to enhance domestic recycling and reduce import reliance, which stood at 65% of apparent consumption.[4][45]Physical and Atomic Properties
Physical Characteristics
Rhenium appears as a silvery-white, lustrous transition metal and adopts a close-packed hexagonal crystal structure.[46][47] With a density of 21.02 g/cm³ at 20°C, it ranks as the fourth-densest element, following osmium, iridium, and platinum.[48][49] Rhenium exhibits one of the highest melting points of any element at 3186°C, exceeded only by tungsten and carbon, and has a boiling point of 5596°C.[50] In terms of mechanical properties, rhenium is highly ductile, with an elongation at break of up to 24% and a tensile strength of about 1.13 GPa at room temperature.[48] It also displays excellent creep resistance at elevated temperatures, maintaining structural integrity under prolonged high-stress conditions, as evidenced by its 100-hour rupture stress of 10 MPa at 2200°C.[48] Rhenium's thermal conductivity measures 48 W/(m·K), while its electrical conductivity is 5.6 × 10⁶ S/m.[50] The coefficient of linear thermal expansion is 6.2 × 10⁻⁶ K⁻¹.[51]Atomic and Electronic Structure
Rhenium (Re) is a transition metal with atomic number 75, positioned in group 7 and period 6 of the periodic table.[1] Its ground-state electron configuration is [\ce{Xe}] 4f^{14} 5d^5 6s^2, featuring five unpaired electrons in the 5d subshell that influence its chemical and physical behaviors.[52] The empirical atomic radius of rhenium is 137 pm, reflecting its compact atomic size typical of late transition metals.[53] Ionic radii depend on oxidation state and coordination environment; for instance, the six-coordinate \ce{Re^{7+}} ion has a Shannon ionic radius of 53 pm, highlighting the contraction in higher oxidation states due to increased effective nuclear charge.[54] Rhenium possesses an electronegativity of 1.9 on the Pauling scale, indicating moderate electron-attracting ability consistent with its group position.[52] The first three successive ionization energies are 760 kJ/mol, 1260 kJ/mol, and 2510 kJ/mol, respectively, with the increasing values reflecting the progressive removal of electrons from increasingly stable inner shells.[53] In the solid metallic state, rhenium's electronic band structure is dominated by contributions from its 5d orbitals, which overlap extensively to form delocalized bands that strengthen metallic bonding and contribute to its high melting point of 3186 °C.[55] This d-orbital involvement results in robust cohesion among atoms, distinguishing rhenium from lighter transition metals. Rhenium exhibits paramagnetic behavior arising from its unpaired 5d electrons, with a mass magnetic susceptibility of $4.56 \times 10^{-9} m³/kg at room temperature.[50]Isotopes
Rhenium has two stable isotopes: rhenium-185 (^{185}\mathrm{Re}), with an atomic mass of 184.952 u and a natural abundance of 37.4%, and rhenium-187 (^{187}\mathrm{Re}), with an atomic mass of 186.955 u and an abundance of 62.6%. These isotopes contribute to the element's standard atomic weight of 186.207(1) u. In addition to the stable isotopes, 34 radioactive isotopes of rhenium have been characterized, ranging in mass number from 160 to 194, with the most stable being rhenium-186 (^{186}\mathrm{Re}), which has a half-life of 3.7213(6) days and decays primarily by beta emission. Another notable radioactive isotope is rhenium-188 (^{188}\mathrm{Re}), with a half-life of 16.98(3) hours, also decaying via beta emission followed by gamma emission. These isotopes, particularly ^{186}\mathrm{Re} and ^{188}\mathrm{Re}, are used in brachytherapy for treating certain cancers due to their beta-particle emissions. The isotope ^{187}\mathrm{Re} exhibits natural radioactivity, undergoing beta decay to osmium-187 (^{187}\mathrm{Os}) with an extremely long half-life of 4.35(12) × 10^{10} years. This decay process forms the basis for the rhenium-osmium (Re-Os) geochronology method, which dates ancient geological events by measuring the ratio of ^{187}\mathrm{Re} to ^{187}\mathrm{Os} in meteorites and ore deposits. Radioactive isotopes of rhenium for medical and research applications are typically produced via neutron activation in nuclear reactors or by proton bombardment in cyclotrons; for instance, ^{186}\mathrm{Re} is generated from stable ^{185}\mathrm{Re} through thermal neutron capture, while ^{188}\mathrm{Re} is often obtained from the decay of tungsten-188 produced in a reactor. Due to the natural isotopic composition, samples of rhenium exhibit a slight variation in density of about 0.1%, arising from differences in isotopic masses, though no significant isotopic fractionation occurs in natural processes.Chemical Properties
Reactivity and Oxidation States
Rhenium metal exhibits notable chemical inertness at room temperature, resisting dissolution in most acids such as hydrochloric acid and hydrofluoric acid, as well as alkalis and aqua regia under ambient conditions. However, it dissolves readily in hot concentrated nitric acid, forming perrhenic acid (HReO₄), and can also react with concentrated sulfuric acid upon heating.[56][57] Rhenium displays a broad range of oxidation states spanning from -3 to +7, excluding -2, with compounds documented across this spectrum. The most stable states are +4, exemplified by rhenium(IV) oxide (ReO₂), and +7 under oxidizing environments, as seen in the perrhenate anion (ReO₄⁻). Lower states like -3 occur in specialized organometallic or cluster compounds, while higher states predominate in oxo species due to rhenium's tendency to form strong metal-oxygen bonds.[58][59] In terms of reactivity trends, rhenium forms volatile oxides, notably the yellow Re₂O₇, which sublimes at relatively low temperatures and contributes to mass loss during high-temperature oxidation. The metal and its oxides can be reduced to elemental rhenium using hydrogen gas at temperatures exceeding 500°C, a process commonly employed in purification. A representative oxidation reaction is 2Re + 3O₂ → 2ReO₃, where kinetics shift from chemically rate-limited below 500°C to diffusion-controlled above this threshold in oxygen atmospheres. Rhenium demonstrates strong affinity for sulfur, preferentially forming sulfides in natural and synthetic systems, and reacts vigorously with halogens upon heating to yield halides like ReF₆ and ReCl₅. The standard reduction potential for ReO₄⁻ / Re is +0.37 V, indicating moderate thermodynamic favorability for reduction under acidic conditions.[60][61][62][2][56][63]Thermodynamic Data
The standard enthalpy of formation (ΔH_f°) for elemental rhenium in its solid state, Re(s), is defined as 0 kJ/mol by convention for elements in their standard state at 298.15 K and 1 bar pressure. For the perrhenate ion, ReO₄⁻(aq), the value is -1120 kJ/mol, reflecting the high stability of this species in aqueous solution derived from perrhenic acid. The standard Gibbs free energy of formation (ΔG_f°) for Re(s) is also 0 kJ/mol at 298.15 K, consistent with its elemental standard state. The standard entropy (S°) for Re(s) is 36.9 J/(mol·K) at 298.15 K, determined from low-temperature heat capacity measurements extrapolated to room temperature. The molar heat capacity at constant pressure (C_p) for Re(s) is 25.5 J/(mol·K) at 298.15 K, indicating moderate thermal response typical of transition metals. In Ellingham diagrams, which plot the standard free energy change for oxide formation versus temperature, rhenium oxides occupy a position indicating relatively low stability compared to other refractory metals, as the formation lines slope positively due to the entropy increase from gaseous oxygen. This is exemplified by Re₂O₇, which sublimes at approximately 250°C under standard conditions, leading to volatility that limits oxide persistence at elevated temperatures.[64] The Re–O bond dissociation energy in perrhenate compounds is approximately 500 kJ/mol, underscoring the robustness of these bonds in high-oxidation-state rhenium species and contributing to the thermodynamic favorability of Re(VII) formation. Phase diagram data for the rhenium–tungsten system reveal a continuous solid solution with no distinct eutectic, but a liquidus minimum near 50 at% Re at around 3180°C, which is relevant for high-temperature alloy processing and establishes the scale for melting behavior in Re–W binaries.[65]| Property | Value | Conditions | Source |
|---|---|---|---|
| ΔH_f° (Re(s)) | 0 kJ/mol | 298.15 K, 1 bar | NIST Standard Reference Data |
| ΔH_f° (ReO₄⁻(aq)) | -1120 kJ/mol | 298.15 K, 1 bar | NBS Technical Note 270 |
| ΔG_f° (Re(s)) | 0 kJ/mol | 298.15 K, 1 bar | NIST Standard Reference Data |
| S° (Re(s)) | 36.9 J/(mol·K) | 298.15 K | CRC Handbook of Chemistry and Physics (2023)[53] |
| C_p (Re(s)) | 25.5 J/(mol·K) | 298.15 K | KnowledgeDoor Periodic Table[24] |
| Re–O BDE (perrhenates) | ~500 kJ/mol | Gas phase approximation | J. Phys. Chem. 1996 |
| W–Re liquidus minimum | ~3180°C | ~50 at% Re | J. Nucl. Mater. 1999[65] |