Phthalic anhydride (PA, C₈H₄O₃) is an important aromatic intermediate produced primarily through the gas-phase catalytic oxidation of o-xylene (CAS 95-47-6) using air or oxygen-enriched air.
o-Xylene (ortho-xylene) Properties
| Physical Properties | |
| Molecular weight | 106.17 |
| Boiling point | 144.41°C |
| Vapor pressure | 6 Torr at 20°C |
| Freezing point | -25.18°C |
| Refractive index | 1.5054 at 20°C |
| Density | 0.8802 g/mL (7.346 lb/gal) at 20°C |
| 0.8760 g/mL (7.311 lb/gal) at 25°C | |
| Dielectric constant | 2.57 at 20°C |
| Dipole moment | 0.45 D at 25°C |
| Solvent group | 7 |
| Polarity index (P') | 2.5 |
| Eluotropic value on alumina | 0.26 |
| Viscosity | 0.81 cP at 20°C |
| Surface tension | 30.03 dyn/cm at 20°C |
| Solubility in water | 0.018% at 25°C |
| Regulatory and Safety Data | |
| DOT Hazard Class | 3, Flammable Liquid |
| Packing Group | II |
| UN Identification Number | UN1307 |
How Is Phthalic Anhydride Produced from o-Xylene?
Phthalic anhydride (C8H4O3) is manufactured through the catalytic vapor-phase oxidation of o-xylene (CAS 95-47-6) (C8H10) with air, carried out in a fixed-bed, multi-tubular reactor. The process uses a catalyst composed of vanadium pentoxide (V2O5) and titanium dioxide (TiO2), operating at a temperature of 340–450°C. The core chemical reaction is:
C8H10+3O2→C8H4O3+3H2O
1. Reaction System and Reactor Type
The industrial production of phthalic anhydride is typically carried out in fixed-bed multitubular reactors featuring a shell-and-tube configuration, wherein the tubes are packed with catalyst particles:
The tubes are filled with the catalyst, while a heat-transfer medium circulates through the shell side to remove excess reaction heat.
The adoption of this specific structure is crucial for the following reasons:
- The oxidation reaction generates a substantial amount of heat.
- Temperature must be strictly controlled to prevent over-oxidation (i.e., the formation of COx).
- Localized overheating points (hot spots) can shorten the catalyst's service life and compromise its selectivity.
2. Catalyst System
The reaction is catalyzed by a heterogeneous catalyst, the typical components of which include:
- Vanadium pentoxide (V₂O₅) - the active oxidizing component
- Titanium dioxide (TiO₂) - the structural support (framework)
- Often modified with promoters (e.g., compounds of potassium or sulfur)
V₂O₅ provides the redox activity necessary for selective oxidation; TiO₂ enhances the catalyst's dispersion and mechanical stability; and the promoters improve selectivity toward phthalic anhydride while inhibiting deep oxidation reactions.
3. Process Production Conditions
- Temperature: 340–450°C
- Pressure: Slightly above atmospheric pressure
- Feedstock: A mixture of vaporized o-xylene and air
Oxygen-to-Hydrocarbon Ratio: Must be strictly controlled to prevent combustion reactions
Key Control Parameters:
- Feed concentration (o-xylene content in the air stream)
- Space velocity (the rate at which the gas flows through the catalyst bed)
- Temperature uniformity across the various tubes within the reactor
4. Reaction Mechanism (Simplified Industrial Perspective)
The oxidation process is not accomplished through a single, direct conversion step, but rather proceeds gradually through a series of intermediate steps:
ortho-xylene undergoes partial oxidation to form intermediate products; these subsequently form phthalic acid-related species on the catalyst surface, which then undergo a dehydration reaction to ultimately yield phthalic anhydride.
5. Product Recovery and Separation
After the reaction, the product stream contains phthalic anhydride vapor, water vapor, unreacted oxygen and nitrogen, and trace by-products. The hot gas is then rapidly cooled in condensers to recover phthalic anhydride, which solidifies as flakes or is collected in molten form depending on the system design. Further purification steps such as melting and filtration are applied to remove impurities, and the final product is stored and transported either as solid flakes or in molten form depending on downstream application requirements.
6. Industrial Applications of Phthalic Anhydride
Phthalic anhydride produced from o-xylene serves as a crucial intermediate in the following fields:
-
Plasticizers (PVC Industry)
Used for the synthesis of phthalate plasticizers
Applied in flexible PVC products (e.g., cables, films, flooring materials)
-
Unsaturated Polyester Resins (UPR)
Used in the manufacture of Fiber-Reinforced Plastics (FRP)
Application areas: Marine vessels, construction, wind energy equipment components
-
Alkyd Resins (Coatings Industry)
Used in paints, varnishes, and various industrial coatings
Imparts excellent adhesion and film-forming properties to coatings
Conclusion
o-Xylene (CAS 95-47-6) is a key aromatic intermediate primarily utilized in the production of phthalic anhydride; the latter, in turn, supports major industrial sectors such as polyesters, plasticizers, resins, and coatings. Its significance lies in its role as a fundamental structural building block within large-scale petrochemical conversion processes.
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