Methanol, also known as methyl alcohol or wood alcohol, is the simplest aliphatic alcohol and an organic compound with the chemical formulaCH₃OH and a molecular weight of 32.04 g/mol.[1][2] It appears as a colorless, volatile, flammable liquid that is completely miscible with water and has a faintly sweet, pungent odor similar to ethyl alcohol.[1][3] Methanol is highly toxic upon ingestion, inhalation, or skin absorption, primarily due to its metabolism into formaldehyde and formic acid, which cause metabolic acidosis, optic nerve damage, and potential developmental toxicity.[4][5][1]Globally, methanol is one of the most widely produced and utilized chemicals, with annual production exceeding 100 million metric tons, primarily through the steam reforming of natural gas to produce synthesis gas (syngas), followed by catalytic hydrogenation.[6][2] Approximately 70% of methanol is employed as a feedstock in chemical synthesis, yielding key products such as formaldehyde, acetic acid, methyl tert-butyl ether (MTBE) for gasoline additives, and dimethyl ether for fuels and refrigerants.[7][8] The remainder serves as an industrial solvent, antifreeze agent in windshield washer fluids, and a blend component in fuels for vehicles and power generation, including emerging applications in renewable methanol production from biomass or CO₂ hydrogenation to support sustainable energy transitions.[6][9][10] Despite its versatility, methanol's flammability requires careful handling with dry chemical, CO₂, or alcohol-resistant foam extinguishers, and its toxicity necessitates strict regulatory controls in occupational and environmental settings.[11][12]
Properties
Physical properties
Methanol, with the molecular formula CH₃OH, is the simplest primary alcohol, featuring a methyl group (CH₃-) covalently bonded to a hydroxyl group (-OH).[1] Its molar mass is 32.042 g/mol.[13] At standard conditions, methanol appears as a colorless, volatile liquid possessing a mild alcoholic odor.Key thermodynamic properties include a melting point of −97.6 °C and a boiling point of 64.7 °C at standard atmospheric pressure.[1] The density is 0.7918 g/cm³ at 20 °C, and the vapor pressure is 13.02 kPa at the same temperature.[14] These values indicate methanol's relatively low boiling point and high volatility compared to water, facilitating its use in various applications.Methanol is completely miscible with water, as well as with alcohols, ethers, and many organic solvents such as benzene and acetone, due to its polar nature and ability to form hydrogen bonds.[1] Unlike some alcohol-water mixtures, it does not form an azeotrope, allowing separation by simple distillation.[15]Additional physical metrics include a refractive index of 1.3284 at 20 °C, a flash point of 11–12 °C (closed cup), and an autoignition temperature of 455 °C.[1] At 20 °C, its dynamic viscosity is 0.59 mPa·s, and the surface tension is 22.6 mN/m.
Methanol is a highly polar molecule owing to its hydroxyl (-OH) group, which creates a significant dipole moment of 1.70 D, facilitating strong intermolecular hydrogen bonding that influences its reactivity and interactions in solution.[1] This polarity arises from the electronegative oxygen atom pulling electron density toward itself, resulting in a partial negative charge on oxygen and partial positive charges on the carbon and hydrogens.As a weak acid, methanol has a pKa of 15.5 in aqueous solution, indicating limited dissociation under neutral conditions, but it can be deprotonated by strong bases to form the methoxide ion (CH₃O⁻).[16] Conversely, methanol exhibits very weak basicity, with a proton affinity of 754.3 kJ/mol in the gas phase, making protonation to form CH₃OH₂⁺ unfavorable except in highly acidic environments.[17]Methanol's reactivity stems from its functional groups, allowing it to participate in oxidation reactions where it converts to formaldehyde (HCHO) or further to formic acid (HCOOH) under appropriate oxidizing conditions.[18] It also undergoes esterification with carboxylic acids in the presence of catalysts like sulfuric acid, yielding methyl esters and water. Additionally, halogenation reactions with reagents such as phosphorus pentachloride or hydrogen chloride produce chloromethane (CH₃Cl).[18]The autoionization of methanol follows the equilibrium:$2 \ \ce{CH3OH} \rightleftharpoons \ce{CH3OH2+} + \ce{CH3O-}with an autoprotolysis constant K_{ap} of approximately $10^{-16.7} (or about $2 \times 10^{-17}) at 25°C, reflecting its extremely low ionic conductivity compared to water.[19]Spectroscopically, methanol displays characteristic infrared absorption bands, including a C-O stretching vibration at approximately 1030 cm⁻¹, which is indicative of its alcohol functionality.[20] In ¹H NMR spectroscopy, the methyl protons appear as a singlet around 3.4 ppm, while the hydroxyl proton resonates as a broad singlet near 4-5 ppm, though its exact position varies with concentration and solvent due to hydrogen bonding and exchange.[21]Under normal conditions, methanol remains chemically stable, but at high temperatures (above 500°C), it undergoes thermal decomposition primarily to carbon monoxide (CO) and hydrogen gas (H₂) via the endothermic reaction \ce{CH3OH -> CO + 2H2}.[22] This decomposition is relevant in pyrolysis studies but requires elevated temperatures without catalysts.
Occurrence
In the interstellar medium
Methanol was first detected in the interstellar medium in 1970 through radio telescope observations toward Sagittarius B2, marking it as one of the earliest complex organic molecules identified in space.[23] This discovery highlighted its abundance in molecular clouds, where it serves as a key tracer of dense, star-forming regions.[24]In these environments, methanol forms primarily through two mechanisms: gas-phase reactions, such as the combination of hydrogen atoms with methoxy radicals (H + CH₃O → CH₃OH), and surface catalysis on interstellar dust grains involving the successive hydrogenation of carbon monoxide (CO) adsorbed onto icy mantles.[24] The grain-surface pathway dominates in cold, dense clouds, where atomic hydrogen accretes and reacts at low temperatures, leading to methanol-rich ices.[25] Abundance levels vary, with gas-phase abundances up to 10⁻⁶ relative to H₂ in hot corinos, while solid methanol constitutes a few to tens of percent of water ice; in comets like Hale-Bopp, it constitutes up to 2% of the volatile content relative to water.[24][26]Astronomers map methanol distributions using its rotational spectral lines, particularly the transitions at 36.2 GHz (J=4₁-3₀ E) and 48.3 GHz, which emit from excited states in warm gas and enable high-resolution imaging of molecular cloud structures.[27] In astrobiology, methanol acts as a precursor to more complex organic molecules, facilitating the synthesis of prebiotic compounds through further reactions in icy environments. Its deuterated isotopologues, such as CH₂DOH, exhibit high fractionation ratios that signify formation on dust grains at temperatures around 10 K, providing insights into the cold chemistry preceding star and planet formation.[28]Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) up to 2025 have revealed gaseous methanol in protoplanetary disks, such as around HD 100546, demonstrating both thermal and non-thermal desorption processes that release it from ices into the disk atmosphere.[29] These findings underscore methanol's role in delivering organics to nascent planetary systems.[29]
On Earth
Methanol is naturally produced on Earth through various biological processes, primarily in plants via the demethylation of pectin in cell walls catalyzed by pectin methylesterases (PMEs), which releases methanol as a byproduct during cell expansion and growth.[30] Emissions rates can reach up to 38 μg g (dry mass)⁻¹ h⁻¹, with increased release upon mechanical damage or stress.[31] Decaying vegetation also contributes significantly, as microbial breakdown of plant material, including pectin, liberates methanol during decomposition in soils and litter layers.[32]Microbial production further adds to Earth's methanol pool, particularly in anaerobic environments where bacteria reduce CO₂ or degrade organic matter to form methanol as an intermediate. For instance, certain anaerobic bacteria performing nitrate-dependent anaerobic methane oxidation can produce methanol.[33] In wetlands, methanol concentrations in soils can reach 0.48–2.6 mM due to such microbial activity amid decaying biomass, while in oceans, phytoplankton generate methanol through metabolic pathways, sustaining concentrations around 0.1–0.4 μM in seawater.[34][35] These microbial sources are balanced by rapid degradation, with ocean bacteria oxidizing methanol to CO₂ at rates of 2–8 nmol L⁻¹ day⁻¹, facilitating its cycling in aquatic and terrestrial ecosystems.[35][36]Geological sources provide trace amounts of methanol, emitted through volcanic activity and geothermal vents, where high-temperature reactions in fumaroles and hydrothermal systems form it from CO₂ reduction or organic precursors deep in the Earth's crust.[37]In the atmosphere, methanol maintains global average concentrations of 0.5–5 ppb, reflecting a balance between natural emissions and sinks like oxidation; levels spike to several ppb during biomass burning events, where incomplete combustion of vegetation releases large pulses.[38] In oceans and soils, methanol persists at low levels—approximately 0.1–0.4 μM dissolved in seawater and variable in wetland soils—cycling primarily through microbial degradation that converts it back to CO₂, preventing accumulation.[35]The environmental flux of methanol from natural sources is substantial, with annual global emissions estimated at around 100 million tons, predominantly from terrestrial plants (about 101 Tg yr⁻¹) and oceanic biogenic processes (gross ~24 Tg yr⁻¹), based on 2021 modeling.[38][39]
History
Early discovery and characterization
In 1661, Robert Boyle first isolated pure methanol through the destructive distillation of boxwood, referring to it as the "spirit of box" or "adiaphorus spiritus lignorum" after rectifying crude wood vinegar over milk of lime.[40] This marked the initial laboratory-scale production of the compound, obtained as a volatile, flammable liquid distinct from other distillates like acetic acid. Boyle's work laid the groundwork for recognizing methanol as a unique substance derived from organic matter, though its chemical nature remained unclear for over a century.[40]During the early 19th century, advancements in analytical chemistry enabled more precise characterization. In 1834, Jean-Baptiste Dumas and Eugène-Melchior Péligot conducted key experiments on "wood spirit," confirming its elemental composition as carbon, hydrogen, and oxygen through combustion analysis and derivative preparations. Their studies demonstrated that methanol differed from ethanol in reactivity and properties, such as forming distinct ethers and halides, establishing it as a separate alcohol rather than a variant of spirit of wine. Dumas and Péligot introduced the term "méthyle" for the radical CH₃, derived from the Greek "methy" (wine) and "hyle" (wood), reflecting its origins; this led to the naming of "methyl alcohol," later shortened to methanol in 1892 by international nomenclature standards.Purification techniques evolved in the mid-19th century with the refinement of fractional distillation, allowing separation of methanol from water, acetone, and other wood distillation byproducts through repeated vaporization and condensation cycles.[40] By the late 1800s, methanol was widely recognized as the simplest aliphatic alcohol, with its structure confirmed as CH₃OH. Early medical applications, such as in solvents and antiseptics, revealed its toxicity, including risks of blindness and acidosis from ingestion, as documented in pharmacological studies like those by J.F. McFarlan in 1855.
Industrial development
The industrial production of methanol began in the 1830s through the destructive distillation, or pyrolysis, of wood, a process that yielded approximately 1-2% methanol by weight from hardwood feedstock.[41][42] This method, often referred to as "wood alcohol" production, supplied small-scale needs for solvents and chemicals until the early 20th century, when global output was limited to around 10,000 tons per year due to the labor-intensive nature and low efficiency of wood-based processes.[43]The transition to synthetic production marked a pivotal shift, with initial experiments on syngas (a mixture of carbon monoxide and hydrogen) dating back to the 1890s, but commercial viability arrived in 1923 with BASF's high-pressure process. This breakthrough utilized a ZnO-Cr₂O₃ catalyst at pressures of 300–400 atm and temperatures of 350–400 °C, enabling efficient conversion of syngas derived primarily from coal into methanol and scaling production dramatically.[44] Post-World War II advancements further revolutionized the industry; in 1966, Imperial Chemical Industries (ICI) introduced a low-pressure process employing a Cu/ZnO/Al₂O₃ catalyst, operating at 50–100 atm and 220–260 °C, which reduced energy costs and improved yields, facilitating widespread adoption.[45]Feedstock evolution paralleled these technological leaps, with coal dominating pre-1970s production due to its availability for syngas generation, though it accounted for a significant portion of global output until natural gassteam reforming emerged as the preferred method. By 2000, natural gas reforming supplied about 80% of global methanol, driven by lower costs and cleaner processing compared to coal gasification.[41]Coal remains prominent in specific regions, particularly China, where it underpins roughly 40% of worldwide production as of 2025, reflecting the country's resource base and policy support for coal-to-chemicals pathways.[41][46]Global capacity has expanded exponentially, from modest levels in the 1920s to approximately 170 million tons per year by 2024, underscoring methanol's role as a foundational chemical intermediate.[47] Projections indicate a 25% capacity increase by 2030, fueled by demand in emerging applications and regional expansions, particularly in the Middle East.[48]Key milestones have shaped the industry's trajectory, including the 1980s boom in methyl tert-butyl ether (MTBE) production, which consumed vast methanol volumes as a gasoline oxygenate to meet clean air standards.[49] In the 2010s, China's adoption of methanol-to-olefins (MTO) technology, exemplified by the 2010 startup of the world's first commercial DMTO unit in Baotou, integrated methanol into petrochemical chains and boosted domestic capacity.[50] The 2020s have seen a surge in green methanol initiatives, with over 170 low-carbon projects announced since 2021 to address decarbonization goals through renewable feedstocks like biogas and captured CO₂.[51] In 2025, notable advancements include the validation of the world's largest green methanol project in China and the launch of wind-to-hydrogen methanol facilities, such as the 548 million USD Tiaobinshan project, marking accelerated commercialization of sustainable production.[52][53]
Production
Conventional production from synthesis gas
The conventional production of methanol primarily relies on the catalytic conversion of synthesis gas (syngas), a mixture of carbon monoxide (CO) and hydrogen (H₂), derived from fossil feedstocks such as natural gas or coal. Syngas is generated through steam reforming of natural gas, where methane reacts with steam to produce CO and H₂ according to the endothermic reaction CH₄ + H₂O → CO + 3H₂, typically at temperatures of 800–1000 °C and pressures of 2–3 MPa over nickel-based catalysts. Alternatively, partial oxidation of coal or heavy hydrocarbons can be employed, involving the reaction of the feedstock with oxygen and steam at high temperatures (1200–1500 °C) to yield syngas with a lower H₂/CO ratio, necessitating adjustment via the water-gas shift reaction (CO + H₂O ⇌ CO₂ + H₂). These methods account for the majority of global methanol output, with natural gas-based routes favored in regions with abundant low-cost gas supplies due to higher efficiency and lower emissions compared to coal gasification.[9]The core methanol synthesis step involves the exothermic hydrogenation of CO (and CO₂) over a catalyst: CO + 2H₂ ⇌ CH₃OH (ΔH = -90.7 kJ/mol), conducted under equilibrium-limited conditions at 5–10 MPa and 230–260 °C to favor conversion while managing heat release. The process requires a stoichiometric H₂/CO ratio of approximately 2:1, achieved by blending syngas streams or incorporating CO₂ (via CO₂ + 3H₂ ⇌ CH₃OH + H₂O). Modern plants employ fixed-bed tubular reactors packed with copper-zinc oxide (Cu/ZnO/Al₂O₃) catalysts, which offer selectivity exceeding 99% toward methanol and operate for 3–5 years before replacement; slurry-phase reactors are used in some large-scale facilities for better heat dissipation. Catalyst poisoning by sulfur compounds, present in raw syngas, is prevented through upstream desulfurization to below 0.1 ppm using zinc oxide absorbers.[9][54]In a typical process flow, purified syngas is compressed to reaction pressure, preheated, and fed into the synthesis reactor, where per-pass conversion is limited to 20–30% to avoid equilibrium constraints and hotspots; unreacted gases are recycled after cooling and separation. The reactoreffluent, containing 5–15% methanol vapor along with water, light hydrocarbons, and inerts, undergoes multistage cooling and condensation to recover crude methanol (70–90% purity), followed by distillation in two or three columns: a pre-distillation column removes lights and dissolved gases, a water column extracts water (boiling point 100 °C), and a final refining column yields high-purity methanol (>99.85 wt%) by separating higher-boiling impurities like ethanol. This integrated loop achieves overall carbon efficiency of 90–95%, with tail gas purged to prevent inert buildup.[9][55]The process is energy-intensive, requiring approximately 30–40 GJ per metricton of methanol, including feedstock consumption (about 28–30 GJ/ton for natural gas) and utilities for compression, reforming, and distillation; modern plants optimize via heat integration to approach 31 GJ/ton. Yields in world-scale facilities reach 5,000–7,000 tons per day, with global production capacity projected at around 116 million tons per year in 2025, of which approximately 60% derives from natural gas and 40% from coal. Economics vary by feedstock and location, with natural gas-based production costing 200–300 USD per ton in 2024, driven by gas prices (typically 3–6 USD/MMBtu) and capital costs of 400–600 million USD for a 5,000 tpd plant; coal-based routes are cheaper in coal-rich areas but incur higher environmental compliance expenses. China is the largest producer, accounting for about 50% of global capacity, primarily via coal gasification, while the Middle East contributes about 15–20% through efficient natural gas reforming.[56][57][58][59][60][61]
Biosynthetic production
Methylotrophic bacteria, particularly aerobic methanotrophs such as Methylococcus capsulatus and Methylosinus trichosporium, naturally oxidize methane to methanol as the first step in their carbon assimilation pathway. This process is catalyzed by methane monooxygenases (MMOs), which insert one oxygen atom from O₂ into methane to form methanol, with the other reduced to water using reducing equivalents from NADH. The key reaction is:\mathrm{CH_4 + O_2 + NADH + H^+ \rightarrow CH_3OH + H_2O + NAD^+}In these organisms, methanol is typically a transient intermediate quickly oxidized further to formaldehyde by methanol dehydrogenase (MDH), but accumulation can be achieved under oxygen-limited conditions or by inhibiting MDH activity. For instance, the thermoacidophilic methanotrophMethylacidiphilum fumariolicum SolV has demonstrated continuous methanol production from methane, reaching titers of up to 4.1 mM (approximately 0.13 g/L) in chemostat cultures under oxygen limitation and without lanthanides that activate MDH.[62]Engineered biosynthetic routes have focused on enhancing methanol production by modifying methanotrophs or introducing MMO pathways into non-native hosts to accumulate methanol from methane or biogas, rather than further metabolizing it. For example, genetic engineering of Methylosinus trichosporium OB3b has achieved methanol titers of 1.1 g/L from 50% methane in batch cultures by optimizing MMO expression and MDH inhibition. Efforts to produce methanol from CO₂ or syngas via pathways like the Wood-Ljungdahl pathway in acetogens (e.g., Clostridium ljungdahlii) have primarily targeted acetate or ethanol, with limited direct methanol output due to thermodynamic challenges in reducing CO₂ to methanol anaerobically; however, hybrid approaches combining syngas fermentation with methanol extraction show promise for low-yield C1 conversion. Molybdenum-containing enzymes, such as formate dehydrogenases in some acetogens, play auxiliary roles in syngas utilization but are not central to methanol synthesis.[63]Biosynthetic processes typically involve aerobic or microaerobic fermentation in bioreactors, where methane or biogas serves as the feedstock, and methanol is harvested via gas stripping or pervaporation to prevent over-oxidation. Yields remain low, ranging from 0.1 to 0.5 g methanol per g substrate consumed, due to competing oxidation pathways, poor gas-to-liquid mass transfer, and slow microbial growth rates (0.3–0.4 h⁻¹). Advances include adaptive laboratory evolution for improved methane tolerance and multi-stage reactors separating oxidation from growth phases, with lab-scale demonstrations reaching 1 g/L titers. As of 2025, biosynthetic methanol constitutes less than 1% of global production (dominated by chemical synthesis at ~100 million metric tons annually), primarily in research settings; commercial pilots remain limited to ethanol-focused efforts like those by LanzaTech on steel mill off-gases.[62][64]
Green and sustainable production
Green and sustainable production of methanol emphasizes renewable feedstocks and low-carbon processes to align with global climate objectives, such as reducing greenhouse gas emissions in hard-to-abate sectors like chemicals and shipping. These methods leverage renewable electricity, biomass, and captured carbon dioxide to produce methanol with near-zero net emissions, contrasting with conventional fossil-based routes that rely on natural gas or coalsyngas. Key pathways include e-methanol synthesis via electrochemical reduction and biomassgasification, both of which have seen rapid project announcements amid policy support for decarbonization. As of August 2025, the project pipeline includes 134 e-methanol initiatives with 23.4 million tons capacity by 2030 and 104 biomethanol projects.[41][65][66]E-methanol, also known as electro-methanol, is produced through the hydrogenation of captured CO₂ using green hydrogen generated from water electrolysis powered by renewable electricity. The core reaction is the catalytic conversion of CO₂ and H₂ into methanol:\ce{CO2 + 3H2 -> CH3OH + H2O}This process requires high-purity CO₂ from industrial sources or direct air capture, combined with H₂ from electrolyzers, typically operating at 200–300°C and 50–100 bar pressure using copper-based catalysts. E-methanol production addresses CO₂ utilization while storing intermittent renewable energy, making it suitable for scaling with abundant solar or wind resources.[67][68]Biomass-derived methanol involves gasification of lignocellulosic feedstocks, such as forestry residues or agricultural waste, to produce syngas (CO + H₂), followed by conventional methanol synthesis. The gasification step occurs at high temperatures (800–1,000°C) in oxygen-steam environments to yield a hydrogen-rich gas, which is then cleaned and converted to methanol via the same catalysts as fossil routes. Typical yields reach approximately 0.4–0.5 tons of methanol per ton of dry biomass, depending on feedstock composition and process efficiency, with overall energy conversion efficiencies of 50–60%. This pathway utilizes sustainable biomass supplies without competing with food production, supporting circular bioeconomy goals.[65][69]Carbon capture integration enhances sustainability by pairing direct air capture (DAC) or point-source CO₂ with green H₂ for methanol synthesis, enabling negative emissions when using biogenic CO₂. A notable example is the first commercial-scale e-methanol facility in Denmark, operational since May 2025, which uses CO₂ from biogas and waste incineration alongside renewable H₂ to produce 42,000 tons annually, demonstrating viable integration of DAC-like capture with electrolysis. In Iceland, Carbon Recycling International's pioneering plant, expanded in recent years, utilizes geothermal-powered electrolysis and DAC-derived CO₂, though at smaller initial scales of around 4,000–5,000 tons per year, highlighting early commercialization in regions with cheap renewables. These projects underscore the feasibility of CO₂-to-methanol as a carbon sink technology.[70][71][72]Globally, green methanol accounted for about 0.2% of total production in 2023, with capacity at roughly 0.5 million tons amid overall methanol output of 110 million tons. However, as of July 2025, 230 projects have been announced worldwide, targeting a combined capacity of 41 million tons per year by 2030, driven by demand in shipping and chemicals. The market is projected to grow at a compound annual growth rate (CAGR) of 34% through 2034, fueled by investments in electrolysis and capture infrastructure.[73][74][75]Key developments include India's first CO₂-to-green methanol plant, announced in November 2024 by Ohmium International in collaboration with Spirare Energy and JNCASR, which integrates PEM electrolyzers with CO₂ capture from a thermal power plant for initial production targeting industrial applications. In the United States, the overall methanol market, including green segments, is valued at USD 5.81 billion in 2025, with capacity growth supported by incentives like the Inflation Reduction Act for low-carbon fuels. The European Union has advanced mandates through the FuelEU Maritime regulation, effective from 2025, requiring progressive greenhouse gas intensity reductions for shipping fuels—up to 80% by 2050—positioning green methanol as a compliant drop-in alternative to marine diesel.[76][77][78]Economically, green methanol production costs range from 800–1,500 USD per ton in 2025, primarily due to hydrogen expenses, but are expected to decline to 500–800 USD per ton with scaling and cheaper renewables by 2030. Lifecycle emissions for green methanol are below 10 g CO₂ equivalent per MJ, compared to over 50 g CO₂/MJ for fossil-based methanol, enabling up to 90% reductions when accounting for full supply chains including capture and transport. These metrics position green methanol as cost-competitive in carbon-priced markets, particularly for high-value uses like sustainable aviation and maritime fuels.[41][79][80]
Applications
Chemical synthesis
Methanol serves as a fundamental building block in the chemical industry, with approximately 75% of global production in 2024 directed toward the synthesis of derivatives such as formaldehyde, acetic acid, and olefins.[58] This utilization underscores its versatility as a C1 feedstock, enabling the production of high-volume commodities essential for materials like resins, plastics, and fuels additives. Global methanol demand reached about 91 million metric tons (MMT) in 2023, with chemical applications dominating due to established catalytic processes that convert methanol efficiently under controlled conditions.[57]Formaldehyde is the largest derivative, accounting for roughly 26% of methanol consumption, or approximately 24 MMT annually based on recent demand figures.[57] The primary industrial process involves the partial oxidation of methanol with oxygen:\mathrm{CH_3OH + \frac{1}{2}O_2 \rightarrow HCHO + H_2O}This reaction occurs over silver catalysts at 600–700 °C or iron-molybdate catalysts at 400–500 °C, achieving high selectivity (>90%) in fixed-bed or fluidized-bed reactors.[81] The silver process, historically dominant, operates in excess air to minimize side reactions like complete combustion, while the iron-molybdate route uses steam-diluted feeds for better heat management. Formaldehyde production reached approximately 26 MMT globally in 2024, with methanol comprising over 90% of the feedstock in modern plants.[82]Acetic acid production consumes about 9% of methanol, equivalent to roughly 8 MMT yearly. The Monsanto process, introduced in the 1970s, employs rhodium-iodide catalysis for the carbonylation of methanol:\mathrm{CH_3OH + CO \rightarrow CH_3COOH}Conducted at 150–200 °C and 30–40 atm in a liquid-phase reactor, this method yields >99% selectivity, with iodide promoters enhancing catalyst activity.[83] Modern variants, like the iridium-based Cativa process, further improve efficiency and reduce precious metal usage, supporting acetic acid output of over 17 MMT worldwide as of 2024.[84]Methyl tert-butyl ether (MTBE) synthesis utilizes around 11% of methanol, primarily through acid-catalyzed etherification:\mathrm{CH_3OH + (CH_3)_2C=CH_2 \rightarrow (CH_3)_3COCH_3}Ion-exchange resins or zeolites serve as catalysts in reactive distillation columns at 100–150 °C, converting isobutene from refinery streams with >98% yield.[85] While MTBE production was phased out in the United States during the 2000s due to groundwater contamination concerns, it remains significant in China, where annual capacity exceeds 22 MMT to meet gasoline oxygenate demand as of 2024.[86]The methanol-to-olefins (MTO) process, capturing 17% of methanol use, transforms methanol into ethylene and propylene (C₂–C₄ olefins) via dehydration and aromatization over SAPO-34 zeolite catalysts at 400–500 °C in fluidized-bed reactors. This non-petrochemical route achieves 80–85% olefin selectivity, with the "hydrocarbon pool" mechanism sustaining the reaction.[50]China dominates MTO capacity, operating over 20 MMT annually as of 2024, driven by downstream polyethylene and polypropylene production.[57]Other notable syntheses include dimethyl ether (DME), which accounts for 6% of methanol consumption through dehydration:$2 \mathrm{CH_3OH \rightarrow CH_3OCH_3 + H_2O}This equilibrium-limited reaction uses alumina or zeolite catalysts at 250–400 °C, yielding >99% conversion in fixed-bed setups. Methylamines (monomethylamine, dimethylamine, trimethylamine) are produced by reacting methanol with ammonia over silica-alumina catalysts at 300–450 °C, with selectivities tuned by feed ratios to favor desired isomers for applications in surfactants and pharmaceuticals. These processes collectively highlight methanol's pivotal role in chemical manufacturing, with ongoing innovations focusing on catalyst stability and sustainability.[87][88]
Fuel applications
Methanol serves as a gasoline additive to enhance octane rating, with blends limited to up to 3% (M3) in the United States and European Union, where it provides a research octane number (RON) boost of approximately 133, improving engine efficiency without significant vehicle modifications.[89] In China, higher blends such as M15 (15% methanol) to M85 (85% methanol) are more prevalent, particularly in provinces like Shanxi and Shaanxi, where over 70,000 taxis have been converted to run on M85 or M100, driven by methanol's lower cost relative to gasoline.[90] These blends reduce carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) emissions compared to pure gasoline, though aldehyde emissions may increase slightly, manageable with catalytic converters; methanol has also aided the phase-out of methyl tert-butyl ether (MTBE) as an oxygenate in reformulated gasoline due to MTBE's groundwater contamination issues.[89]As a direct fuel, methanol is gaining traction in marine shipping to meet International Maritime Organization (IMO) goals for net-zero greenhouse gas emissions by 2050, with a regulatory framework adoption targeted for 2025 to enforce compliance starting in 2028.[91] E-methanol, produced from renewable sources like green hydrogen and captured CO₂, enables zero-carbon operations in dual-fuel vessels; for instance, A.P. Moller-Maersk ordered 18 large dual-fuel methanol container ships in 2024, with over 10 delivered by mid-2025, each capable of carrying over 16,000 TEU and reducing lifecycle GHG emissions by at least 65% compared to fossil fuels.[92]In fuel cell applications, direct methanol fuel cells (DMFCs) oxidize methanol directly at the anode via the reaction:\mathrm{CH_3OH + H_2O \rightarrow CO_2 + 6H^+ + 6e^-}operating typically at 60–130 °C to balance kinetics and methanol crossover.[93] These cells achieve power densities of 100–200 mW/cm², as demonstrated in systems with advanced catalysts yielding up to 181 mW/cm² at 80 °C under low methanol concentrations, making DMFCs suitable for portable and automotive power sources despite challenges like anodepoisoning.[94]The methanol-to-gasoline (MTG) process converts methanol to hydrocarbons over ZSM-5 zeolite catalysts at 350–400 °C, following the simplified reaction:\mathrm{CH_3OH \rightarrow \text{hydrocarbons} + \mathrm{H_2O}with high selectivity for gasoline-range products.[95] A commercial demonstration occurred at the New Zealand Synthetic Fuels plant from 1985 to 1997, producing 14,500 barrels per day of gasoline from natural gas-derived methanol, validating the technology's viability for stranded gas resources.[95]Methanol acts as an energy carrier for hydrogen storage through steam reforming:\mathrm{CH_3OH + \mathrm{H_2O \rightleftharpoons CO_2 + 3H_2}(or partial reforming to CO + 2H₂), offering a volumetric energy density of approximately 18.8 MJ/L, over twice that of liquid hydrogen at 8.2 MJ/L, facilitating safer and more efficient transport and on-site generation.[96]As of 2023, energy applications (fuels and biodiesel) account for about 14% of global methanol consumption, with projections indicating growth to around 20% by 2030 driven by adoption of green methanol in transportation sectors.[57] Demand for green methanol in shipping and aviation is projected to rise at a compound annual growth rate (CAGR) of 32% through 2032, driven by decarbonization mandates and scalable production from renewables.[97]
Other uses
Methanol functions as a versatile solvent in numerous industrial applications, particularly for dissolving resins, dyes, and other substances in the formulation of paints, adhesives, and inks. Approximately 10-15% of global methanol production is dedicated to such solvent uses and other miscellaneous applications (including biodiesel and denaturants) as of 2023, leveraging its ability to mix readily with water and many organic compounds.[57] In biodiesel manufacturing, methanol serves as the primary reactant in the transesterification process, reacting with triglycerides from vegetable oils or animal fats to yield fatty acid methyl esters and glycerol.[1][2][41]As a denaturant, methanol is incorporated into ethanol at concentrations ranging from 0.5% to 5% by volume, rendering the mixture undrinkable and exempt from beverage alcohol taxes for industrial purposes such as cleaning agents and fuel additives. This practice is standardized in formulas like specially denatured alcohol (SDA) 3-A, which includes 5% methanol to ensure compliance with regulatory requirements.[98][99][100]In pharmaceutical production, methanol acts as a solvent for synthesizing vitamins such as B12 and niacin, as well as hormones, antibiotics like streptomycin, and other active ingredients. At low concentrations, it exhibits antiseptic properties suitable for certain formulations. High-purity grades of methanol are also utilized for cleaning electronic components in semiconductor manufacturing, where its low residue and effective solvency prevent contamination.[1][1]Methanol finds niche applications as an antifreeze additive in windshield washer fluids, typically comprising 30–50% of the mixture to provide freeze protection down to -20°C or lower, and in de-icing solutions for aircraft and roadways. Emerging roles include its use as a physical absorptionsolvent in carbon capture technologies, notably the Rectisol process, which employs chilled methanol to selectively remove CO2 and other acid gases from syngas streams in gasification plants.[101][102]
Quality and regulatory aspects
Specifications
Methanol specifications vary by intended application, with industry standards ensuring consistent quality, purity, and performance across chemical, fuel, solvent, and reagent uses. The International Methanol Producers and Consumers Association (IMPCA) establishes reference specifications for AA grade methanol, the most common for fuel and chemical synthesis, requiring a minimum purity of 99.85% w/w on a dry basis.[103] This grade limits water content to a maximum of 0.10% w/w to prevent phase separation and corrosion issues, and sulfur to ≤0.5 mg/kg to reduce emissions and catalyst poisoning.[103][104] Key impurities such as ethanol are restricted to ≤50 mg/kg and acetone to ≤30 mg/kg in AA formulations, while color is limited to ≤5 Pt-Co units to ensure visual clarity and absence of oxidative degradation products.[103]For solvent applications in electronics and pharmaceuticals, high-purity grades demand ≥99.9% w/w purity, with low metal content (<1 mg/kg total for elements like iron, copper, and lead) to avoid contamination in sensitive manufacturing. This prioritizes minimal non-volatile residues and absorbance properties for clean processing. Fuel grade methanol, aligned with ASTM D1152 for baseline purity (99.85% w/w), limits water to ≤0.10% w/w to maintain combustion stability.[105][106] Green methanol variants require additional sustainability certifications, such as ISCC EU or RSB, verifying low-carbon feedstocks and emissions reductions through third-party audits.[107][108]
Reagent grade methanol adheres to ACS standards for analytical work, with purity ≥99.8% and testing often involving water compliant with ISO 3696 Grade 3 to ensure accuracy in dilutions and extractions.[110] Regional regulations, such as EU REACH, impose impurity controls indirectly through substance registration and restriction dossiers, mandating reporting of contaminants like heavy metals and ensuring methanol meets Annex XVII limits for safe handling in mixtures.[111]As of 2025, marine fuel standards under ISO 6583:2024 specify three grades: Marine Methanol Grade A (MMA, highest purity with additional lubricity requirements), Grade B (MMB, aligned with IMPCA), and Grade C (MMC, wider tolerances), tightening sulfur limits to ≤0.5 mg/kg for all grades to align with global emission controls, including the Mediterranean ECA's 0.1% sulfur cap.[109][112] Traceability requirements for green claims have also strengthened, with certifications like ISCC and RSB mandating blockchain or audit trails for feedstock origins and lifecycle emissions.[107][108] These evolutions support methanol's role in sustainable applications without compromising end-product integrity.
Analysis methods
Gas chromatography (GC), particularly with flame ionization detection (FID), serves as the primary analytical technique for detecting and quantifying impurities in methanol, such as higher alcohols and trace hydrocarbons, at parts-per-million (ppm) levels.[113] This method separates volatile components based on their interaction with a stationary phase, allowing precise identification and measurement of contaminants that could affect methanol's use in chemical synthesis or fuels. For instance, standards like ASTM D7920 outline GC procedures to assess impurities in fuel-grade methanol, ensuring compliance with quality thresholds by resolving peaks for compounds like ethanol or acetone with detection limits below 10 ppm.[114] While GC-FID excels for organic impurities, thermal conductivity detection (TCD) variants can also quantify water content in the low ppm range, though it is less sensitive for alcohols compared to FID.[115]Karl Fischer titration remains the gold standard for determining water content in methanol, offering high accuracy across a wide range from 0.01% to 100%, with coulometric variants achieving detection limits as low as 10 ppm for trace moisture.[116] The method involves dissolving the sample in an anhydrous methanol-based medium and titrating with iodine-containing reagent, where the reaction stoichiometrically consumes water to form methyl iodate and other products, enabling endpoint detection via electrometric or colorimetric means.[117] This technique is essential for quality control in anhydrous methanol production, as even minor water contamination can catalyze side reactions in downstream applications. Coulometric Karl Fischer is preferred for ultra-low water levels in high-purity grades, providing results with relative standard deviations under 2%.[118]Spectroscopic methods complement chromatographic techniques for targeted impurity analysis and structural verification in methanol. Ultraviolet-visible (UV-Vis) spectroscopy detects aldehydes, such as formaldehyde, by measuring absorbance at approximately 340 nm, where carbonyl groups exhibit characteristic n-π* transitions, allowing quantification of trace levels (down to 1 ppm) without sample preparation.[119] Nuclear magnetic resonance (NMR) spectroscopy, particularly ¹H-NMR, confirms methanol's structural integrity by identifying the distinct singlet peaks for the methyl (δ ≈ 3.4 ppm) and hydroxyl (δ ≈ 4.8 ppm, variable due to hydrogen bonding) protons, while also detecting impurities like ethanol through additional multiplet signals.[120] These non-destructive methods are widely used in research and quality assurance, with NMR providing quantitative purity assessments via integration ratios when internal standards are employed.[121]Additional techniques address specific impurities beyond volatiles. High-performance liquid chromatography (HPLC) with UV detection quantifies acetates, such as methyl acetate, in methanol by separating polar compounds on reversed-phase columns, achieving resolutions for concentrations from 0.1% to trace levels.[122]Inductively coupled plasma mass spectrometry (ICP-MS) determines metal contaminants, like sodium or iron, at sub-ppm levels by ionizing the sample in an argonplasma and analyzing mass-to-charge ratios, crucial for preventing catalytic poisoning in industrial processes.[123] Simpler physical measurements, including density and viscosity, serve as rapid bulk checks for overall composition, correlating deviations from standard values (e.g., density 0.791 g/mL at 20°C) to potential contamination.[124]For real-time process control in production facilities, on-line Raman spectroscopy enables continuous monitoring of methanol purity by analyzing vibrational spectra, where characteristic C-O stretch bands at around 1030 cm⁻¹ distinguish methanol from water or ethanol impurities without sampling disruptions.[125] This non-invasive approach supports immediate adjustments in synthesis gas conversion, maintaining product consistency at scales from laboratory to industrial.[126]Regulatory standards guide these analyses, with EPA Method 8260 specifying GC-MS protocols for volatile organics in environmental samples containing methanol, ensuring detection of associated impurities like benzene at low ppb levels.[127] Modern laboratory norms achieve accuracies of ±0.01% for major components like methanol itself, reflecting advancements in instrumentation and calibration traceable to NIST standards.
Safety
Toxicity
Methanol is metabolized primarily in the liver through oxidation by alcohol dehydrogenase to formaldehyde, which is rapidly converted by aldehyde dehydrogenase to formic acid; this metabolite inhibits mitochondrial cytochrome c oxidase, disrupting cellular respiration and leading to tissue hypoxia, particularly in the optic nerve and brain.[4] The biological half-life of methanol in human blood is approximately 2–3 hours under normal conditions, though this can be prolonged by competitive inhibition or metabolic saturation.[4]Acute exposure to methanol produces severe toxic effects via inhalation, ingestion, or dermal absorption, with inhalation LC50 values reported at 41,000 ppm (approximately 54 g/m³) in mice over 6 hours, indicating high potency in mammalian models.[101] Symptoms typically emerge after a latent period of 12–24 hours and include nausea, vomiting, abdominal pain, and central nervous system depression; visual disturbances such as blurred vision, photophobia, and snowfield vision arise from formic acid-induced optic nerve damage, with blindness possible at ingested doses of 3–10 g in humans.[128]Chronic exposure to methanol can result in cumulative damage to the liver and kidneys, manifesting as elevated liver enzymes and renal tubular dysfunction with prolonged low-level contact.[11] Occupational exposure limits are established to mitigate these risks, with the OSHA permissible exposure limit (PEL) set at 200 ppm as an 8-hour time-weighted average, the ACGIH threshold limit value (TLV) at 200 ppm (8-hour TWA) and 250 ppm (STEL) as of 2025, and the NIOSH immediately dangerous to life or health (IDLH) concentration at 6,000 ppm.[129][130]In humans, ingestion of as little as 30 mL of pure methanol can be fatal due to severe metabolic acidosis from formic acid accumulation, which is exacerbated in individuals with folate deficiency as hepatic tetrahydrofolate is required for formatedetoxification to carbon dioxide.[4] Vulnerable populations, including children due to lower body mass and immature metabolic pathways, and alcoholics from induced alcohol dehydrogenase activity leading to faster toxic metabolite formation, face heightened risks of severe outcomes from even sublethal exposures.[4] Methanol is not classified as a carcinogen by the International Agency for Research on Cancer (IARC Group 3), indicating inadequate evidence of carcinogenicity in humans.[131]Biomonitoring of exposure often involves measurement of urinary formate levels, which correlate with internal dose and aid in assessing occupational health risks.[132] Its volatility facilitates inhalation as a primary route in industrial settings.[4]Management of methanol poisoning focuses on inhibiting metabolism, correcting acidosis, and removing the toxin. Fomepizole is the preferred antidote to block alcohol dehydrogenase, with ethanol as an alternative; folinic acid or folic acid enhances formate elimination; sodium bicarbonate addresses acidosis; and hemodialysis is indicated for severe cases or methanol levels >50 mg/dL.[4][133]
Fire and explosion hazards
Methanol is classified as a Class IB flammable liquid under NFPA 30 standards, characterized by a flash point below 22.8°C (73°F) and a boiling point at or above 37.8°C (100°F), indicating high ignitability at ambient temperatures.[1] Its vapors form explosive mixtures with air, with a lower explosive limit (LEL) of 6% by volume and an upper explosive limit (UEL) of 36% by volume, allowing ignition over a wide concentration range.[134] A distinctive hazard is that methanol flames are nearly invisible in daylight, complicating detection and increasing the risk of unnoticed fire spread.[11]Explosion risks are particularly acute from vapor cloud formation in confined or poorly ventilated spaces, where methanol's low minimum ignition energy of 0.14 mJ enables ignition from minor sparks, such as those from static electricity or mechanical equipment.[135] The autoignition temperature is 464°C (867°F), meaning sustained heat sources can trigger combustion without an open flame.[134] These properties necessitate careful control of ignition sources during transfer, storage, or processing to prevent vapor accumulation leading to deflagration or detonation.Safe handling requires storage in well-ventilated areas to disperse vapors, using grounded metal containers or approved non-sparking equipment to minimize static buildup, as methanol's conductivity can generate electrostatic charges during flow.[136] It is incompatible with strong oxidizers, such as peroxides or chromic acid, which can cause violent reactions or accelerated decomposition.[12]Compliance with NFPA 30 guidelines for Class IB liquids dictates limitations on container sizes, separation distances from ignition sources, and fire-resistant construction for storage facilities.[137]In the event of a fire, appropriate extinguishing agents include dry chemical, carbon dioxide (CO₂), or alcohol-resistant foam, as water alone may spread burning liquid without fully suppressing vapors.[11] For spills, methanol evaporates quickly due to its high vapor pressure, but residual liquid poses slip hazards and persistent ignition risks from lingering vapors; containment with absorbent materials and ventilation is essential to avoid vapor cloud formation.[138]Regulatory classifications designate methanol as UN 1230, a Class 3 flammable liquid under U.S. Department of Transportation (DOT) rules, requiring specific packaging, labeling, and placarding for transport.[1] For green methanol variants used in sustainable applications, 2025 updates in maritime guidelines maintain similar flammability profiles but introduce enhanced protocols for fuel systems, including improved leak detection and crew training to address equivalent volatility and fire risks.[139]Historical incidents in chemical plants during the 1970s underscored static electricity as a key ignition source during methanol handling and transfer, often resulting in vapor ignitions and fires that prompted stricter grounding and bonding requirements.[140] Modern mitigation relies on continuous monitoring with flame detectors, gas sensors, and automated shutdown systems, substantially reducing the incidence of such events in contemporary facilities.[141]