Butyraldehyde and 2-Ethylhexanol Production via Propylene Hydroformylation

In the low-pressure oxo process for butyraldehyde production, chemical-grade propylene is contacted with synthesis gas in a liquid-phase homogeneous catalytic system containing rhodium and triphenylphosphine (TPP). The reaction network proceeds through propylene insertion into a rhodium hydride species, followed by carbon monoxide insertion and hydrogenolysis to yield n-butyraldehyde and isobutyraldehyde. The process operates at temperatures between 85 °C and 100 °C, absolute pressures from 1.5 MPa to 2.5 MPa, and a hydrogen-to-carbon monoxide ratio of 1.0:1 to 1.2:1. The normal-to-iso ratio is dictated by the coordination geometry around the rhodium center and the steric demand of the phosphine ligand. Commercial TPP-modified systems typically achieve n/i ratios between 8:1 and 20:1. The propylene conversion across the primary reactor is generally maintained above 95% by balancing liquid hourly space velocity, catalyst inventory, and gas recycle. Unreacted propylene and synthesis gas are recovered in a high-pressure separator, scrubbed, and recycled to the reactor loop. The homogeneous catalyst is separated from the aldehyde products by distillation under reduced pressure, a step that exposes the TPP ligand to thermal stress and governs the purge rate of high-boiling residues. The exothermic nature of the hydroformylation reaction requires continuous heat removal, typically through an external shell-and-tube heat exchanger or internal cooling coils. A thermosiphon reboiler or forced-circulation cooler is used to generate low-pressure steam, and the reactor temperature is interlocked to prevent excursions above 105 °C because TPP oxidation and ligand degradation accelerate sharply above that threshold.

Chemical-grade propylene from steam cracker or fluid catalytic cracking units must be purified before entering the oxo reactor because rhodium catalysts are extremely sensitive to sulfur, acetylenes, and dienes. Methylacetylene and propadiene (MAPD) are hydrogenated in a fixed-bed palladium-on-alumina guard reactor at 40 °C to 80 °C and pressures of 1.5 MPa to 3.0 MPa. The guard reactor is monitored for temperature rise across the catalyst bed, and the outlet is analyzed by gas chromatography using ASTM D5399. Sulfur compounds such as hydrogen sulfide, carbonyl sulfide, and mercaptans are removed to below 0.1 mg/kg because rhodium forms inactive rhodium-sulfur species. Total sulfur in the purified propylene is measured by ASTM D5453. Water and oxygen concentrations in the synthesis gas are controlled below 0.05 mol% and 0.01 mol%, respectively, to limit phosphine oxidation. Failure of the MAPD hydrogenation catalyst can introduce acetylene compounds into the oxo reactor; these species insert into rhodium acyl intermediates and generate branched or unsaturated oxygenated products that oligomerize into high-boiling residues. Field experience indicates that a sulfur excursion of 0.5 mg/kg for more than 24 h can double the rhodium makeup rate in a continuous oxo loop, although published data for this specific configuration is limited. Carbon monoxide and hydrogen are also purified to prevent iron carbonyl and nickel carbonyl formation because volatile metal carbonyls deposit in the phosphine catalyst solution and contribute to fouling of the distillation section.

What Limits Liquid Hourly Space Velocity in TPP-Modified Propylene Hydroformylation?

Liquid hourly space velocity (LHSV) in commercial liquid-phase propylene hydroformylation is constrained by the interaction between propylene conversion, gas-liquid mass transfer, and catalyst thermal degradation. Process design data for low-pressure oxo reactors indicate that LHSV values are generally maintained between 0.3 h⁻¹ and 1.0 h⁻¹ based on reactor liquid volume. Higher space velocities shorten residence time and reduce propylene conversion below the typical 95% target, increasing the energy demand for recycle compression and raising propylene loss. At lower space velocities below 0.2 h⁻¹, the same conversion can be achieved with lower rhodium concentration, but the thermal exposure of the catalyst solution increases because the product is continuously distilled from the recirculating catalyst stream. This creates an operating conflict: low LHSV improves conversion but accelerates ligand degradation and high-boiler formation, while high LHSV preserves catalyst activity but increases propylene slip. The reactor may be a gas-liquid stirred autoclave or bubble column with internal cooling coils and external steam generation. The heat of reaction is removed through a thermosiphon reboiler or shell-and-tube cooler; if the cooling water supply fails, the reactor temperature can exceed 105 °C within minutes. Above 105 °C, triphenylphosphine oxidation to triphenylphosphine oxide increases, and the normal-to-iso ratio decreases. A typical plant control strategy sets the reactor temperature at 90 °C to 95 °C with a high alarm at 100 °C and interlock shutdown at 105 °C. These thresholds are specific to TPP-modified rhodium systems; unmodified cobalt processes operate at much higher temperature and pressure, and are outside the scope of this discussion.

The normal-to-iso ratio in TPP-modified rhodium hydroformylation is controlled more strongly by free phosphine concentration and carbon monoxide partial pressure than by reactor temperature alone. A molar phosphorus-to-rhodium ratio of 100:1 to 200:1 is common, and the free TPP concentration in the catalyst loop defines the coordination sphere at rhodium. Under high carbon monoxide partial pressure, rhodium complexes with multiple carbonyl ligands become less selective toward linear aldehyde. The n/i ratio therefore falls when the hydrogen-to-carbon monoxide ratio is reduced or when the gas feed distribution is uneven across the reactor. In a commercial reactor, the normal-to-iso ratio may be maintained at 10:1 to 12:1 at 90 °C and 1.8 MPa absolute, while certain ligand-stabilized systems with bisphosphite modifications can exceed 20:1. The product split is determined by gas chromatographic analysis on the reactor effluent and the recovered butyraldehyde stream, with ASTM D5399 used for quantification. Because n-butyraldehyde is the desired intermediate for 2-ethylhexanol, the economics of the oxo unit are highly sensitive to changes of 0.1 in normal-to-iso selectivity.

Following the reactor section, the mixed butyraldehyde product is separated from the high-boiling catalyst solution by fractional distillation under reduced pressure. The reactor effluent is flashed to remove unreacted gases, and the liquid enters a stripping column. n-Butyraldehyde boils at approximately 75 °C at atmospheric pressure, while isobutyraldehyde boils near 64 °C. The proximity of these boiling points requires high-efficiency structured packing with 100 to 200 theoretical stages in the butyraldehyde splitter. The column overhead is controlled to minimize water carryover because n-butyraldehyde forms a water azeotrope; excess water in the aldehyde product promotes aldol condensation during storage. Reboiler temperature is typically kept below 140 °C to reduce TPP degradation and high-boiler formation in the bottoms. The bottoms stream containing catalyst is recycled to the reactor, but a small purge is taken to control viscosity and high-boiling aldol condensation byproducts. In plants where isobutyraldehyde is not separately valorized, the isobutyraldehyde stream may be hydrogenated to isobutanol or used as a feedstock for neopentyl glycol. The splitter operates under vacuum, with pressure set at 30 kPa to 80 kPa absolute, depending on plant elevation and cooling water temperature. Published design data for specific column internals are limited, but structured packing with a specific surface area of 250 m²/m³ to 500 m²/m³ is representative of the installed equipment.

When MAPD Breakthrough Shifts Normal-to-Iso Selectivity and Accelerates Fouling in the Oxo Loop

Propylene entering the oxo reactor must be free of methylacetylene and propadiene because these alkynes and dienes are not inert diluents. They compete with propylene for rhodium coordination and can insert into rhodium-acyl intermediates, producing branched or unsaturated oxygenated species that oligomerize into high-boiling residues. In the fixed-bed palladium-on-alumina guard reactor, the propylene stream is hydrogenated at 40 °C to 80 °C and 1.5 MPa to 3.0 MPa, with the exotherm monitored across the catalyst bed. If the guard bed is bypassed or spent catalyst is not changed, MAPD breakthrough above 5 mg/kg is observed in the reactor feed; the resulting high-boiler formation increases the purge rate from the catalyst loop and reduces n/i selectivity. The catalyst solution viscosity may rise, and the downstream distillation column reboiler can foul. In one typical control strategy, the MAPD hydrogenation reactor outlet is sampled by gas chromatography every 4 h using ASTM D5399 or an equivalent refinery gas method. The total sulfur in the same stream is controlled below 0.1 mg/kg because rhodium-thiolate and rhodium-sulfide species are inactive. Carbonyl sulfide is particularly problematic because it passes through some sulfur guard beds and decomposes slowly, releasing hydrogen sulfide inside the oxo reactor. Field experience indicates that a sulfur excursion of 0.5 mg/kg for more than 24 h can double the rhodium makeup rate, though published data for this specific configuration is limited.

n-Butyraldehyde is converted to 2-ethylhexenal by base-catalyzed aldol condensation in a continuous stirred-tank or loop reactor. Fresh n-butyraldehyde is mixed with dilute aqueous sodium hydroxide at 1 wt% to 3 wt% NaOH, creating a two-phase system in which n-butyraldehyde is the organic phase and the aqueous caustic phase provides hydroxide ion for enolate formation. The condensation reaction is exothermic and occurs at 80 °C to 130 °C; the reaction rate accelerates with temperature, so a control band of ±3 °C around setpoint is typically specified. If the temperature exceeds 140 °C, the aldehyde can undergo further condensation and dehydration to high-boiling oligomers, and the aqueous phase can promote Cannizzaro side reactions that consume n-butyraldehyde. The reactor effluent is separated into an organic 2-ethylhexenal-rich phase and an aqueous caustic phase. The aqueous phase is partially recycled, but a purge is taken to remove sodium salts of organic acids and to control the accumulation of dissolved aldol intermediates. The organic phase is washed with water to reduce sodium content below 10 mg/kg before it enters the hydrogenation section. On a commercial scale, the aldol reactor is often a vertical cylindrical vessel with external circulation through a heat exchanger; the heat exchanger is designed with high recirculation flow to prevent localized boiling and fouling. Published data for the exact fouling rate in this specific configuration is limited, but operators schedule caustic side cleaning at intervals determined by pressure drop across the circulation loop.

After the exothermic aldol step, the hydrogenation of 2-ethylhexenal to 2-ethylhexanol is performed in a fixed-bed reactor over copper-zinc oxide or nickel-based catalysts. The reaction is carried out in the liquid phase or trickle-bed mode at 140 °C to 180 °C and hydrogen partial pressures of 3.0 MPa to 10.0 MPa. In the primary hydrogenator, the aldehyde group and the carbon-carbon double bond are both saturated; the reaction is strongly exothermic, and hot-spot formation is the dominant operational risk. Reactor temperature is controlled by cold hydrogen quench injections along the catalyst bed or by external recycle of hydrogenated product to dilute the feed. The hydrogen-to-2-ethylhexenal molar ratio at the reactor inlet is typically maintained between 1.05:1 and 1.3:1. If hydrogen is insufficient, the intermediate 2-ethylhexanal accumulates, and this aldehyde can undergo further condensation into high-boiling dimers. If hydrogen is excessive, the product can be over-hydrogenated or hydrodeoxygenated to light hydrocarbons; published data for this specific catalyst configuration is limited. The crude hydrogenation product is then distilled in a series of vacuum columns. Light ends are removed first, then 2-ethylhexanol is taken overhead or as a side draw at a typical final column pressure of 5 kPa to 20 kPa absolute. Product quality is monitored by ASTM D5399 for assay, ASTM D1209 for color, ASTM E203 for water, and ASTM D1613 for acidity. Typical commercial 2-ethylhexanol specifications require an assay above 99.5 wt%, a color below 10 Pt-Co, water below 0.05 wt%, and acidity below 0.01 wt% as acetic acid.

Catalyst Deactivation Signatures in a Rhodium-Based Oxo Loop

Deactivation of a TPP-modified rhodium hydroformylation catalyst is characterized by three concurrently observed signatures: an increase in the purge rate of high-boiling residues, a gradual loss of normal-to-iso selectivity, and a measurable increase in triphenylphosphine oxide concentration. The catalyst solution is sampled from the reactor loop and analyzed by ³¹P nuclear magnetic resonance spectroscopy to quantify free TPP, rhodium-bound TPP, and triphenylphosphine oxide. In commercial practice, the free TPP concentration is kept above 5 wt% in the recirculating catalyst solution; when it falls below this threshold, the n/i ratio declines. High-boiling byproducts formed by aldol condensation of butyraldehydes accumulate in the distillation bottoms and raise the viscosity of the catalyst stream. The viscosity increase reduces heat transfer coefficients in the reboiler and can restrict flow through the reactor circulation pump. A purge stream from the catalyst loop is typically maintained between 1% and 3% of the total recirculating flow. The purge stream is treated to recover rhodium values by extraction or thermal decomposition. If the purge is too small, high-boiler concentration increases and eventually precipitates in the reboiler; if the purge is too large, rhodium makeup costs increase. The operational boundary is therefore narrow. The triphenylphosphine oxide concentration is usually controlled below 10 wt% of total phosphorus because higher concentrations reduce the free ligand activity and increase the viscosity of the catalyst solution. Published data for the exact deactivation rate in continuous plants is limited, but field data indicate that a well-controlled TPP system can operate for 18 to 36 months before a major rhodium recovery campaign.

The Phase Behavior of Aqueous Sodium Hydroxide in n-Butyraldehyde Condensation Determines Yield and Fouling Boundaries

The aldol condensation system operates as a liquid-liquid dispersion, and the phase behavior determines both reaction selectivity and fouling. n-Butyraldehyde has limited water solubility, and the aqueous sodium hydroxide phase is the catalyst carrier. At 90 °C to 120 °C, the organic phase contains less than 5 wt% water, but the aqueous phase can contain 5 wt% to 20 wt% n-butyraldehyde and 2-ethylhexenal as dissolved or emulsified species. The interfacial area between the two phases is generated by a static mixer or a high-shear circulation pump. If the aqueous phase becomes saturated with organic sodium salts, the interfacial tension decreases and the phases can emulsify. This emulsion stabilizes the dispersion but makes downstream separation difficult and increases sodium carryover into the hydrogenation catalyst. Sodium carryover above 10 mg/kg in the organic feed to the hydrogenator can reduce catalyst activity and cause surface fouling. The caustic concentration must be maintained between 1 wt% and 3 wt%; below 1 wt%, the reaction rate is too low, and above 3 wt%, the Cannizzaro side reaction is enhanced. A process analyzer measuring pH in the aqueous recycle loop is often used to maintain the caustic concentration. The purge rate from the aqueous loop is set by the total organic acid sodium salt concentration, typically controlled below 10 wt% to avoid precipitation. Published data for the equilibrium compositions at industrial temperature and pressure is limited, but these operating ranges are consistent with process chemistry handbooks and field observations from continuous aldol-hydrogenation trains.

Both butyraldehyde and 2-ethylhexanol require oxygen-free storage. n-Butyraldehyde undergoes autoxidation to n-butyric acid and peroxy compounds when exposed to air; peroxide formation can be suppressed by a nitrogen blanket and by maintaining the storage temperature below 30 °C. Isobutyraldehyde is even more sensitive to oxidation and may be stored with a free-radical inhibitor. 2-Ethylhexanol is less prone to autoxidation but can form esters when stored with residual acid or aldehyde impurities. The final product is typically transferred to carbon steel or stainless steel tanks, but carbon steel is acceptable only if water content is below 0.05 wt% and the tank is nitrogen blanketed. Nitrogen blanketing pressure is usually 0.5 kPa to 2.0 kPa gauge. If the product is used for plasticizer production, trace aldehyde can react with phthalic anhydride and cause color; therefore the 2-ethylhexanal content is often limited to below 0.1 wt%. The storage tanks are equipped with conservation vents and flame arrestors; API 2000 or EN 14015 may be referenced for vent sizing. Product stability is verified by ASTM D1209 color and ASTM D1613 acidity after 30-day storage at ambient temperature. Published data for specific long-term storage stability of 2-ethylhexanol in unlined carbon steel is limited, but field experience indicates that iron contamination can rise if the tank is not passivated.