Maleic anhydride (cis-butenedioic anhydride, C₄H₂O₃, CAS 108-31-6) and succinic acid (butanedioic acid, C₄H₆O₄, CAS 110-15-6) represent two adjacent oxidation states in the four-carbon dicarboxylic acid chemical family. The structural distance between them is the loss of two hydrogen atoms and one oxygen atom from the saturated diacid to the unsaturated cyclic anhydride, but this formal stoichiometric difference produces a fundamental change in electronic character, molecular geometry, and industrial function. Maleic anhydride is a rigid planar five-membered ring containing a cis-configured α,β-unsaturated carbon-carbon double bond and a reactive anhydride group; succinic acid is a freely rotating saturated linear dicarboxylic acid with terminal carboxyl groups separated by a central C–C bond. The molar mass of maleic anhydride is 98.06 g/mol, its melting point is 52.8 °C, and its normal boiling point is 202 °C. Succinic acid has a molar mass of 118.09 g/mol, a melting range of 185–187 °C, and a boiling point of 235 °C at which decomposition becomes significant. The density of maleic anhydride is approximately 1.48 g/cm³, while succinic acid solid density is approximately 1.56 g/cm³. In aqueous processing, maleic anhydride hydrolyzes rapidly to maleic acid, which has first and second acid dissociation constants of 1.83 and 6.07; succinic acid has pK_a1 of 4.16 and pK_a2 of 5.61. These differences in acidity and ring strain govern the use of maleic anhydride as a reactive intermediate and the use of succinic acid as a saturated, thermally stable building block.

The industrial significance of maleic anhydride is concentrated in unsaturated polyester resins, copolymerization, and functionalization of hydrocarbons via grafting or ene reactions, whereas succinic acid is consumed in biodegradable polyesters, solvent esters, food acidulants, pharmaceutical excipients, and polyurethane polyols. A maleic anhydride unit introduces an electron-deficient alkene that can participate in free-radical copolymerization, Diels-Alder addition, and Michael addition; a succinic acid unit does not contain a carbon-carbon double bond and therefore cannot participate in these reactions without prior dehydrogenation. The saturated backbone of succinic acid provides greater thermal and oxidative stability, lower corrosivity, lower volatility, and compatibility with aqueous fermentation and crystallization processes. The anhydride ring of maleic anhydride is highly susceptible to nucleophilic attack by water, alcohols, and amines, whereas succinic acid requires esterification catalysts, higher temperatures, or conversion to succinic anhydride before comparable condensation reactions occur. This mechanistic difference explains why maleic anhydride is handled as a molten or briquetted solid in closed systems with low-moisture nitrogen blanketing, while succinic acid can be stored and conveyed as an ordinary crystalline solid with moisture management comparable to other organic acids.

Structural Consequences of cis-Butenedioic Anhydride Versus Butanedioic Acid

The rigid five-membered anhydride ring of maleic anhydride imposes an eclipsed orientation on the cis-substituted carbonyl groups and places the carbon-carbon double bond in conjugation with both carbonyl groups. This conjugation withdraws electron density from the alkene, lowering its frontier molecular orbital energy and making it a highly efficient dienophile in Diels-Alder cycloadditions. Succinic acid has no alkene, and its central C–C single bond allows free rotation among staggered and gauche conformers; the two carboxylic acid groups can adopt intramolecular hydrogen-bonding geometries only when the central bond rotates to a gauche arrangement. In maleic acid, the cis geometry places the two acid groups close enough for a strong intramolecular hydrogen bond between one carboxylate oxygen and the proton of the adjacent carboxyl group, which is reflected in the unusually low first dissociation constant of 1.83 and the high second dissociation constant of 6.07. Succinic acid, with the carboxylic acid groups separated by a two-carbon saturated bridge, shows pK_a values at 4.16 and 5.61 that are more typical of a simple dicarboxylic acid. The difference in ring strain also appears in hydrolysis and esterification behavior: the anhydride ring of maleic anhydride is opened by water, alcohols, and amines at low temperatures, whereas succinic acid requires dehydration at elevated temperature to form succinic anhydride. Thermal analysis of maleic anhydride shows a sharp melting endotherm at 52.8 °C and a high vapor pressure that allows distillation and sublimation; succinic acid does not distill cleanly and decomposes near 235 °C, requiring crystallization or melt processing rather than vapor-phase purification.

Spectroscopic methods distinguish the two compounds without ambiguity. The infrared spectrum of maleic anhydride exhibits cyclic anhydride carbonyl stretching bands near 1850 cm⁻¹ and 1775 cm⁻¹; the spectrum of succinic acid contains a broad hydrogen-bonded carboxylic acid O–H absorption between 2500 cm⁻¹ and 3300 cm⁻¹ and a carbonyl band near 1690 cm⁻¹. Nuclear magnetic resonance spectroscopy of maleic anhydride shows olefinic protons at a shift characteristic of an electron-deficient double bond, while succinic acid shows two methylene triplets. These analytical markers are relevant in manufacturing because residual maleic anhydride in succinic acid produced by hydrogenation must be quantified by titration, gas chromatography after methanol esterification, or liquid chromatography with ultraviolet detection. The saturated backbone of succinic acid is not susceptible to the free-radical addition reactions that can form insoluble maleic anhydride homopolymer or copolymer networks during storage or high-temperature processing.

What Limits Selectivity During n-Butane Oxidation to Maleic Anhydride?

The dominant industrial route to maleic anhydride is selective vapor-phase oxidation of n-butane over a vanadium phosphorus oxide catalyst. The stoichiometry C₄H₁₀ + 3.5 O₂ → C₄H₂O₃ + 4 H₂O is accompanied by complete oxidation to carbon monoxide and carbon dioxide, and the selectivity is controlled largely by the oxidation state of the catalyst, the gas-phase residence time, and the temperature profile along the reactor tubes. Commercial processes feed n-butane at 1.5–1.8 mol% in air, below the lower flammability limit, into multitubular fixed-bed reactors containing catalyst-filled tubes cooled by circulating molten salt. The salt bath is maintained between 390 °C and 430 °C, with peak catalyst temperatures potentially higher because the reaction is highly exothermic. Published per-pass molar yields for modern VPO systems are typically in the range of 50–60 mol%, with the remainder of the converted n-butane forming carbon oxides and light acids such as acetic acid and acrylic acid. The central process conflict is that increasing n-butane concentration raises productivity but lowers selectivity at a given salt bath temperature because the bed temperature becomes nonuniform and the catalyst surface becomes over-reduced. Conversely, reducing the feed concentration improves selectivity but increases the volume of gas that must be compressed, heated, and passed through the recovery train, lowering the liquid hourly space velocity that can be achieved economically.

The reactor tube diameter, tube length, and molten salt circulation rate are selected to remove the heat of reaction without allowing a hot spot above the temperature at which the VPO lattice loses selective oxygen species. Typical industrial reactors use tubes with internal diameters in the range of 21–25 mm and catalyst fill lengths of several meters; the molten salt side uses forced circulation and steam generation for heat recovery. Hot spots above approximately 460–480 °C accelerate complete oxidation, shorten catalyst life, and can cause localized over-reduction of the VPO surface. The effluent gas leaving the reactor contains maleic anhydride, water, carbon oxides, acetic acid, acrylic acid, and unconverted n-butane. Maleic anhydride is recovered by partial condensation or absorption in water. Direct water absorption converts maleic anhydride to maleic acid, which must then be dehydrated thermally in the presence of an azeotroping solvent such as xylene to regenerate the anhydride. The dehydration step can also isomerize some maleic acid to fumaric acid, a solid byproduct that must be separated. Alternative recovery systems use an organic solvent with low water content to absorb maleic anhydride directly and reduce the formation of maleic acid. The choice between water absorption and solvent absorption is a process conflict between capital cost, energy for dehydration, and byproduct fumaric acid formation.

Published data for specific commercial VPO catalyst formulations and their exact selectivity responses to n-butane feed composition are limited by licensor confidentiality, but the general operating window of 390–430 °C and 1.5–1.8 mol% n-butane is well established in public industrial literature. The lower flammability limit of n-butane in air is approximately 1.86 vol%, and commercial operation below this limit is necessitated by safety requirements rather than by reaction kinetics. The vapor-phase oxidation route has largely displaced older benzene-based maleic anhydride technology because of lower feedstock cost and lower benzene handling risk, but it requires a more selective catalyst and more demanding temperature control. In the reactor, the VPO catalyst operates in a partially oxidized and partially reduced state, and the molten salt system must maintain the tube wall temperature within a narrow band to prevent runaway oxidation or selectivity loss. This is one of the deep-dive processing zones in the maleic anhydride value chain because the economic performance of the entire downstream maleic anhydride derivative business depends on the ability to sustain high selectivity at high feed conversion over multi-year catalyst cycles.

For succinic acid, the industrial manufacturing routes are fundamentally different from the vapor-phase oxidation used for maleic anhydride. The petrochemical route hydrogenates maleic anhydride or maleic acid to succinic anhydride over a supported nickel or noble-metal catalyst, followed by hydrolysis to succinic acid. The hydrogenation step is typically carried out in a fixed-bed or slurry reactor at elevated hydrogen pressure, with the anhydride ring opening and the double bond saturating in a single catalytic sequence. Because maleic anhydride is recovered from the VPO process as the anhydride, hydrogenation can be integrated directly with maleic anhydride production. The succinic anhydride intermediate is then hydrated in aqueous solution and crystallized. The fermentation route, which has gained commercial importance for bio-based succinic acid, uses engineered strains of Actinobacillus succinogenes, Basfia succiniciproducens, or Escherichia coli to convert sugars and carbon dioxide to succinic acid under near-neutral pH conditions. The fermentation broth is clarified, the succinate salt is acidified, and the free acid is purified by crystallization, electrodialysis, or reactive extraction. The pH setpoint is critical because below the first pK_a of succinic acid the broth contains increasing concentrations of free acid, which inhibits microbial productivity; above the second pK_a the broth contains predominantly succinate dianion and requires acidification to recover the free acid. This pH balance is a process conflict analogous to the hot-spot control in n-butane oxidation, and it determines the energy and reagent consumption of the downstream purification train.

Succinic acid from either route is isolated as a white crystalline solid with a melting range of 185–187 °C, and it is typically dried to a moisture content below 1.0 wt% before packaging. The solid has a lower vapor pressure and higher thermal stability than maleic anhydride, allowing it to be stored in hoppers and conveyed with standard rotary valves, screw feeders, and dense-phase pneumatic systems. At relative humidity above approximately 60%, succinic acid can absorb surface moisture and become cohesive, so hopper design must account for mass-flow discharge with polished internal surfaces and a hopper half-angle based on wall friction measurements made according to ASTM D6128-16. The material is not considered combustible dust in the same way as some fine organic acids, but grounding and dust control are still required in bulk handling areas. Published data for specific fermentation yield and productivity rates are plant-specific and are limited by commercial confidentiality, but the general order of downstream operations—clarification, acidification, crystallization, centrifugation, drying—is consistent across public industry descriptions.

When Succinic Acid Replaces Maleic Anhydride in Unsaturated Polyester Synthesis

Unsaturated polyester resins are prepared by condensation of maleic anhydride with saturated dibasic acids such as phthalic anhydride and with diols such as propylene glycol, followed by dissolution in styrene or another vinyl monomer. The maleic anhydride unit provides the unsaturation that later participates in free-radical copolymerization with styrene during cure initiated by peroxides or azo compounds. Because succinic acid is saturated, it cannot supply this double bond. If succinic acid is used as a direct molar replacement for maleic anhydride, the number of copolymerizable double bonds per gram of resin decreases, and the final crosslink density after cure is lower. The resulting cured thermoset may show lower glass transition temperature, lower tensile modulus, lower heat deflection temperature, and greater strain at break, but the magnitude depends on the styrene content, the initiator level, the cure schedule, and the ratio of succinic acid to other saturated dicarboxylic acids. Tensile properties are measured according to ASTM D638-14 or ISO 527-2:2012, flexural properties according to ASTM D790-17 or ISO 178:2019, and heat deflection temperature according to ASTM D648-18 or ISO 75-2:2013. The processing window for the resin is also affected because the replacement lowers the exotherm peak during cure and reduces the probability of thermal cracking in thick castings, but it can also produce an undercured surface if the conversion of styrene is incomplete.

The condensation reaction between succinic acid and propylene glycol proceeds through esterification at temperatures in the range of 180–220 °C, while maleic anhydride can undergo ring-opening esterification at lower temperature and then isomerize from the maleate to the fumarate double bond at the higher esterification temperatures used to drive molecular weight. The isomerization of maleate to fumarate is significant because the fumarate double bond is more reactive toward styrene radical copolymerization; process conditions that maximize isomerization therefore improve resin reactivity. Succinic acid has no double bond and no isomerization pathway, so it functions as a flexible saturated dibasic acid. A formulation containing 10–20 mol% succinic acid on total dibasic acid may reduce brittleness and adjust viscosity without eliminating cure, but at high replacement levels the resin may not attain sufficient network density for structural applications. Published data for this specific configuration are limited and formulation-dependent, and resin producers evaluate the balance through gel-permeation chromatography for molecular weight distribution, dynamic mechanical analysis for glass transition, and ASTM D638-14 tensile tests on cast specimens. The styrene content of commercial unsaturated polyester resins is commonly in the range of 30–40 wt%, and any change in maleic anhydride content must be accompanied by a re-evaluation of the styrene-to-double-bond ratio to avoid residual styrene or low crosslink density.

In manufacturing equipment, the replacement of maleic anhydride with succinic acid changes the reactor profile because succinic acid is a solid with a melting point of 185–187 °C that must be melted or charged as a powder, whereas maleic anhydride can be charged as a melt at approximately 60–70 °C. Esterification reactors equipped with partial condensers, water removal columns, and vacuum systems can accommodate both acids, but the feed system must handle the higher melting solid for succinic acid. Agitation torque and heat transfer are also affected because the initial reaction mixture with solid succinic acid is a suspension rather than a homogeneous melt. The endpoint is controlled by acid number, typically measured according to ASTM D4662-20, and by viscosity of the final resin solution measured at 25 °C with a Brookfield or cone-and-plate viscometer. The acid number specification ensures that the polycondensation has reached the target molecular weight before the resin is cooled and blended with styrene. Because succinic acid is a weaker acid than maleic acid, its esterification kinetics are slower unless a catalyst such as an organotin compound or tetrabutyl titanate is used, and the water removal rate must be monitored to avoid foaming caused by rapid dehydration.

Ring-Opening Chemistry in Paper Sizing and Lubricant Dispersancy

Maleic anhydride is widely used as a reactive intermediate in the preparation of alkenyl succinic anhydride paper-sizing agents and polyisobutenyl succinic anhydride lubricant dispersants. In both applications the anhydride ring is opened by a nucleophile—water or a polyamine—after the maleic anhydride has been grafted onto a hydrophobic hydrocarbon chain. For lubricant dispersants, polyisobutylene is first reacted with maleic anhydride under thermal or chlorine-assisted conditions to form polyisobutenyl succinic anhydride, which is then condensed with polyamines to form succinimide dispersants. The imide and amide products contain polar sites that adsorb onto soot and sludge particles while the hydrocarbon tail maintains solubility in the base oil. The total base number of the finished dispersant is determined by potentiometric titration according to ASTM D2896-21, and the kinematic viscosity of the dispersant or its concentrate is measured according to ASTM D445-21. The process conflict in dispersant manufacture is that residual maleic anhydride or acid must be removed before amine condensation to avoid gelation and inconsistent nitrogen content; unreacted maleic anhydride can also corrode downstream equipment. Reactors are typically agitated pressure vessels with overhead systems for removing water of reaction and excess maleic anhydride under reduced pressure. The grafting reaction is carried out at temperatures above 150 °C and under nitrogen, with the endpoint followed by acid number titration according to ASTM D664-18 or ASTM D974-14.

In paper sizing, maleic anhydride is used to modify rosin, producing maleated rosin esters or fortified rosin sizes that are dispersed in water and precipitated onto pulp with alum at acidic or near-neutral wet-end pH conditions. The ring-opening reaction converts the anhydride to a dicarboxylic acid or ester, which provides the carboxyl groups needed for emulsification and retention on cellulose fibers. The amount of maleic anhydride used in rosin fortification is often in the range of 3.0–10.0 wt% based on rosin solids, and the reaction is followed by acid number and softening point. The hydrophilic-lipophilic balance of the resulting size is sensitive to the degree of maleation, and excess maleic anhydride can produce water-sensitive paper or low stock sizing efficiency. Paper sizing performance is evaluated by the Cobb test according to ISO 535:2023 and by the Hercules size test. The maleic anhydride anhydride ring is more reactive toward rosin acid conjugated double bonds than succinic acid would be, which is why succinic acid is not a direct replacement in fortified rosin sizes. The saturated diacid can be used as a raw material for succinic anhydride, but only after dehydration at high temperature in a separate step.

Maleic anhydride also reacts with polyolefins in the melt to form maleic anhydride-grafted polypropylene or polyethylene coupling agents. These grafted polymers are produced on co-rotating twin-screw extruders with L/D ratios of 40:1 or higher, using peroxide initiators and maleic anhydride feeding at a controlled rate. Grafting levels of 0.5–2.0 wt% are typical, and the residual free maleic anhydride is removed by vacuum devolatilization in the extruder to meet odor and food-contact requirements. The grafted anhydride groups hydrolyze to carboxylic acid groups on the polymer backbone and promote adhesion to glass fibers, minerals, or metal surfaces in composites and multilayer films. The degree of grafting is measured by acid-base titration after dissolution, infrared spectroscopy, or nitrogen analysis after imide derivatization. Succinic acid cannot be grafted directly onto polyolefins by this mechanism because it does not possess the activated alkene or anhydride required for radical grafting; it would need to be converted to succinic anhydride first, and even then the saturated anhydride lacks the activated double bond of maleic anhydride and is less efficient as a radical grafting agent.

Succinic acid is a monomer for polybutylene succinate, a biodegradable aliphatic polyester produced by esterification of succinic acid with 1,4-butanediol, typically catalyzed by tetrabutyl titanate or another organometallic catalyst. The first stage is carried out at 180–200 °C under nitrogen to form oligomers and remove water; the second stage is a polycondensation at 220–240 °C under vacuum to achieve high molecular weight. The melt flow rate of polybutylene succinate is specified according to ISO 1133-1:2022 at 190 °C under a 2.16 kg load, and tensile properties are measured according to ISO 527-2:2012 on injection-molded specimens. Compostability is evaluated according to ISO 14855-1:2012 or EN 13432:2000 under controlled composting conditions at 58 °C. The processing window for polybutylene succinate is narrow relative to polyolefins because the polyester is susceptible to thermal degradation above approximately 250 °C and requires pre-drying to a moisture content below 0.05 wt% before melt processing. Moisture above this level hydrolyzes the ester bonds during extrusion and lowers molecular weight, causing a measurable decrease in melt viscosity and a loss of mechanical properties in injection-molded parts. Succinic acid used for polymer production must therefore meet strict moisture and acid value specifications, and the monomer is often dried in a vacuum dryer or fluidized-bed dryer before entering the esterification reactor.

Beyond polybutylene succinate, succinic acid is converted to ester solvents such as dimethyl succinate and diethyl succinate, which are used in coatings, cleaning formulations, and pharmaceutical processing. These esters are produced by acid-catalyzed esterification with methanol or ethanol, with water removal by distillation or azeotropic separation. The purity of the succinic acid feedstock influences the color and acid number of the ester product, and unreacted succinic acid is removed by neutralization or distillation. Succinic acid itself is listed in the United States as a direct food substance affirmed as generally recognized as safe under 21 CFR 184.1091, and it is used as a pH regulator and flavoring acid in foods. In pharmaceutical manufacturing, succinic acid is used as a buffer and as an excipient in solid dosage forms; its purity is controlled by pharmacopeial monographs that specify assay, residue on ignition, and heavy-metal limits. The saturated structure of succinic acid makes it stable to air oxidation, whereas maleic anhydride and maleic acid can undergo free-radical reactions and require storage under oxygen-limited conditions. Succinic acid also serves as a precursor to succinimides, N-methylpyrrolidone, and other solvents through amidation and reduction chemistry, but these routes require high-temperature dehydration and hydrogenation equipment that is more capital-intensive than the direct esterification used for succinate esters.

Succinic acid-derived polyols are produced by esterification of succinic acid with diols such as diethylene glycol, 1,4-butanediol, or 1,6-hexanediol to form polyester polyols for polyurethane systems. These polyols are characterized by hydroxyl number according to ASTM D4274-21 and acid number according to ASTM D4662-20. The absence of carbon-carbon double bonds in succinic acid confers hydrolytic and oxidative stability to the resulting polyol, but it also reduces the aromatic content and glass transition temperature relative to phthalic anhydride-based polyols. In polyurethane applications, the saturated polyester polyol may require adjustment of the isocyanate index and catalyst package to achieve the desired hardness and cure profile. The processing temperature for succinic acid polyester polyols is typically above the melting point of succinic acid to maintain a homogeneous melt phase, and the reaction is conducted under nitrogen with a partial condenser to return glycols while removing water. The final polyol is filtered to remove unreacted succinic acid or oligomer gels before storage. The molecular weight distribution, acid number, and hydroxyl number are the primary release criteria, and the polyol viscosity is measured using a rotational viscometer at a specified temperature.