Commercial inquiry volumes for methyl tert-butyl ether repeatedly conflate three identifiers: the definitive CAS registry number 1634-04-4, the EU Community identifying number 216-653-1, and the abbreviation MTBE. The compound is a branched aliphatic ether with molecular formula C₅H₁₂O and molar mass 88.15 g/mol. It is produced by liquid-phase etherification of methanol with isobutylene; its major outlets are gasoline oxygenate blending, catalytic cracking to high-purity isobutylene, and selected low-boiling process and laboratory solvent applications. Synonyms appearing in procurement documents include 2-methoxy-2-methylpropane, tert-butyl methyl ether, TBME, and methyl tertiary-butyl ether.

What Is MTBE? CAS 1634-04-4, EC 216-653-1

The substance name methyl tert-butyl ether describes a methyl ether of tert-butanol; the structure is (CH₃)₃COCH₃. The CAS registry number 1634-04-4 resolves the identity of the substance irrespective of the naming convention applied. The EC number 216-653-1 is the corresponding EINECS/REACH identifier used in EU regulatory submissions. Fuel-grade MTBE is typically supplied at 99% w/w minimum purity; chemical-grade material for downstream cracking may carry tighter methanol and water specifications. The oxygen mass fraction of MTBE is 18.15% w/w, a value that anchors gasoline oxygenate calculations under EN 228 and ASTM D4815. The branched tertiary structure contributes to lower boiling point and higher octane blending response than the linear methyl n-butyl ether, which is a separate substance with different regulatory and physical properties.

Fixed-Bed Resin Deactivation Is Linked to Feed Dienes and Methanol Ratio

The synthetic route reacts isobutylene with methanol over a macroreticular sulfonic acid ion-exchange resin. Production units typically operate with a methanol-to-isobutylene molar ratio of 1.05:1 to 1.2:1 and reactor inlet temperatures from 40 °C to 80 °C. The etherification reaction is exothermic; adiabatic fixed beds are often arranged with interstage cooling or quench methanol, because the temperature rise can exceed 25 K at high isobutylene conversion. Outlet temperatures above 100 °C are avoided because resin sulfonic acid groups deactivate and dimethyl ether byproduct increases. Feed quality is critical: C₄ streams from steam crackers must be selectively hydrogenated to reduce diene content below about 0.2% m/m. Higher diene content accelerates gum formation on the resin, raises bed pressure drop, and shortens the production cycle. The single-pass isobutylene conversion in a two-stage design can exceed 90% at methanol excess; the second stage is operated at lower temperature to shift etherification equilibrium. Final fuel-grade material above 99% w/w is recovered after methanol removal by water wash or extractive distillation. Chemical-grade material for high-purity isobutylene cracking commonly specifies methanol below 50 mg/kg because methanol acts as a catalyst poison in the downstream cracking unit.

In refinery gasoline oxygenate blending, MTBE is added at 11% v/v to 15% v/v in some premium-grade oxygenated fuels. At 15% v/v, the oxygen contribution is approximately 2.6% m/m to 2.7% m/m in finished gasoline, depending on gasoline density; this approaches the EN 228 oxygen ceiling of 2.7% m/m. Refinery laboratories use ASTM D4815 gas chromatography to speciate MTBE, ETBE, TAME, DIPE, and C₁–C₄ alcohols in finished fuel. Inline near-infrared analyzers on the blend header provide closed-loop oxygenate control; a deviation above 0.2% absolute from the certified tank concentration triggers automatic divert-to-rundown in many terminal automation systems. The vapour pressure interaction is non-linear: adding MTBE without reducing butane can raise the finished fuel Reid vapour pressure by 5–10 kPa, affecting seasonal volatility compliance. Reformulators therefore adjust butane and light straight-run gasoline simultaneously with MTBE dosage. Octane response is also significant: research octane number values for MTBE are typically cited in the range 112–118, with motor octane number values of 97–101, but the final blending octane depends on fuel matrix and co-solvent composition.

Property and Stability Boundaries in Bulk Storage

Bulk terminal operations require vapour balancing and nitrogen blanketing because the closed-cup flash point of -33 °C and vapour pressure of 26.8 kPa at 20 °C place MTBE in the high-volatility fuel category. Storage tanks are commonly specified to API 650 with internal floating roofs or fixed roofs equipped with nitrogen padding. The Reid vapour pressure at 37.8 °C is approximately 55–62 kPa by ASTM D323, which is used in blending calculations. Water bottoms collected from tank water draws can contain MTBE at concentrations up to the saturation value shown in the table; therefore, drain water should be routed to an oil-water separator before discharge. Long-term contact with air can generate trace organic peroxides, so maintenance procedures should include peroxide screening before hot work or vessel entry. The liquid is not corrosive to carbon steel, but seal and gasket selection must be verified against supplier immersion data because elastomer resistance varies significantly; PTFE and 316L stainless steel are generally acceptable for wetted service. Loading systems should be bonded and grounded, and high-level shutoffs should be linked to the terminal tank management system.

Names Buyers Get Wrong and Their CAS Consequences

In tender documents, methyl tert-butyl ether is frequently misdescribed as “methyl butyl ether,” a term that can be interpreted as methyl n-butyl ether, CAS 628-28-4, which has different physical properties and is not the same gasoline oxygenate. The shortened term “tert-butyl ether” omits the methyl group and can resolve to di-tert-butyl ether, CAS 6163-66-2. The abbreviation “MTB” is not a recognized unique identifier. Buyers also conflate MTBE with ETBE, CAS 637-92-3, and TAME, CAS 994-05-8, partly because all three compounds appear in ASTM D4815 oxygenate calibration sets. A specification that lists only “methyl ether oxygenate” is insufficient: ETBE has an oxygen mass fraction of 15.66% w/w, not 18.15% w/w, and a boiling point of 72.8 °C, which changes volatility, driveability, and tank blending calculations. The term “methyl tertiary-butyl ester” is chemically inaccurate and can create customs or safety-data-sheet discrepancies because an ester is not an ether. Unambiguous purchase orders should use CAS 1634-04-4 and EC 216-653-1 as primary identifiers; synonyms may be added but should not replace the registry number. Buyers should also specify intended grade limits for methanol, water, C₄ residue, and gum content rather than relying on a vague term such as “MTBE 90,” which has no universal grade definition.

High-purity isobutylene producers use MTBE as a chemical intermediate by cracking the ether over solid acid catalysts at reactor temperatures typically between 150 °C and 300 °C. The cracking step produces isobutylene and methanol; downstream product recovery includes quenching, methanol removal, and distillation to reject diisobutylene and other C₈ oligomers. Methanol in the MTBE feed below 50 mg/kg is preferred because residual methanol competes for acid sites and reduces cracking efficiency. This application has stricter purity requirements than fuel blending; purchasers should not assume that conventional fuel-grade MTBE is suitable without a low-methanol certificate and a water specification. Published data for specific catalyst configurations is limited; vendor pilot evaluations are normally required to set conversion and selectity targets.

Laboratory solvent applications demand additional quality controls. MTBE used as an aprotic solvent in organometallic or extraction procedures is often dried over activated molecular sieves to water levels below 10 mg/kg. Reclaimed solvent should be monitored by Karl Fischer titration, ASTM E203, because water and methanol impurities terminate Grignard initiators and alter extraction selectivity. Peroxide-stabilized or oxygenated solvent grades are not universal; the buyer should define inhibitor content, packaging atmosphere, and lot-to-lot water variability in the specification.

When ETBE or TAME Is Not Equivalent in Oxygenate Blending

Fuel ethers are not direct substitutes for MTBE solely because they appear in the same ASTM D4815 calibration. ETBE has a boiling point of 72.8 °C and oxygen mass fraction 15.66% w/w; TAME has a boiling point near 86.3 °C and similar oxygen mass fraction. MTBE, with a boiling point of 55.2 °C, contributes more oxygen per unit mass and exerts a larger front-end volatility effect. Switching from MTBE to ETBE at equal volume reduces oxygen delivery and alters the distillation curve; the oxygen-compliant blend may require a 2–5% relative volume adjustment. Reformulators must also recertify against EN 228 or ASTM D4814 because driveability index, vapour lock index, and cold-start emission profiles are affected by the different boiling behaviour. Tank blend reports should use the correct compound-specific response factors; using MTBE factors for ETBE or TAME produces laboratory bias exceeding 0.1% to 0.3% in oxygenate speciation depending on concentration.

Transport classification for methyl tert-butyl ether is UN 2398, Class 3, Packing Group II. EU safety data sheets are tied to EC 216-653-1; the harmonized entry carries Flam. Liq. 2, H225.

Import documentation should state CAS 1634-04-4 and EC 216-653-1 together; one identifier without the other is a recurring cause of clearance delay when the cargo is described only as “MTBE oxygenate.”