Raw Material for Polyether Polyols: Epoxide Compounds

The primary polymerization monomers used in the preparation of polyether polyols include ethylene oxide, propylene oxide, epichlorohydrin, and tetrahydrofuran, which are classified as α-oxides. Propylene oxide and ethylene oxide can be considered internal ether compounds of 1,2-ethylene glycol. However, unlike ordinary ethers, they possess high reactivity due to the high ring strain and strong polarity of their three-membered rings. Under certain process conditions, these compounds can undergo C-O bond cleavage and ring-opening polymerization reactions to form the corresponding polyether polyols. Among these, propylene oxide (PO) and ethylene oxide (EO) are the most important epoxide monomers in polyether polyol production, with a wide range of sources. Additionally, halogenated epoxides like trioxane can be used to produce flame-retardant polyether polyols, which are also of interest.

In terms of molecular structure, like ethylene, the valence bonds in alkenes are similar to those in olefins, making them less stable than general C-C and C-O bonds. Propylene oxide (also known as propylene oxide) is a colorless liquid with an ether-like odor and a boiling point of 34.2°C. It is the main raw material for polyether polyols used in foam plastics, with about two-thirds of global PO production dedicated to this use. It is also used to produce propylene glycol, non-ionic surfactants (such as oilfield demulsifiers, pesticide emulsifiers, and wetting agents), flame retardants, and more.

The reactivity of propylene oxide is not as strong as that of ethylene oxide. Due to the asymmetry of the propylene oxide molecule, it can undergo isomerization reactions after ring opening under acidic catalysis.

Propylene Oxide Production Methods:

1.Chlorohydrin Process:

This method involves the direct reaction of propylene with chlorine in water. The chlorohydrin process is a classic industrial method for synthesizing propylene oxide, with a history of over 50 years. It accounts for about 60% of propylene oxide production. The process includes three main steps: chlorohydrinization, saponification, and purification. In the chlorohydrinization reaction, propylene and chlorine react in water to produce chlorohydrin. The reaction pressure is atmospheric or slightly above, with temperatures controlled between 40-90°C. By-products include hydrochloric acid, dichloropropane, dichloroisopropyl ether, and small amounts of chloropropanone. Producing 1 ton of propylene oxide generates approximately 0.11-0.2 tons of propylene dichlorides. The chlorohydrin solution is saponified with lime to produce propylene oxide, with by-products including calcium chloride, propylene glycol, and propionaldehyde.

The chlorohydrin process for propylene oxide has mature technology, a short production process, and relatively safe production. However, it has disadvantages such as many by-products, severe equipment corrosion, significant environmental pollution, high unit consumption, and poor product quality.

2.Co-oxidation Process:

The essence of the co-oxidation process is using a transition metal catalyst to transfer oxygen from a hydroperoxide to propylene, thus obtaining propylene oxide. Organic hydroperoxides such as isobutane, ethylbenzene, isopropylbenzene, and cyclohexane can react with oxygen to form ethylbenzene hydroperoxide, tert-butyl hydroperoxide, and cyclohexane hydroperoxide, respectively, which can then epoxidize propylene to produce PO.

The co-oxidation process, also known as the Halcon process, involves using organic peroxides and catalyzing free radicals in a liquid-phase reaction to transfer oxygen from the molecule to propylene. This process accounts for 30% of the world's propylene oxide production.

3.Non-Co-Product Co-oxidation Process:

Sumitomo Chemical's new process uses a titanium-based catalyst in a fixed-bed reactor. Propylene is converted into PO through a propylene hydroperoxide intermediate without generating by-products. The process uses cumene hydroperoxide (CHP) as the oxidant. CHP epoxidizes propylene to obtain PO and dimethylbenzyl alcohol, the latter of which dehydrates to form α-methylstyrene, which is then hydrogenated to regenerate cumene, oxidized to CHP, and reused. This epoxidation reaction uses a high-performance salt catalyst to achieve high yields of epoxide. This process has technical and economic advantages, producing no by-products and not requiring additional equipment for co-producing styrene.

4.Peracetic Acid Process:

In this method, acetaldehyde is used as a raw material and oxidized to peracetic acid, which then reacts with propylene to produce propylene oxide and acetic acid. The epoxidation reaction occurs in the presence of an iron catalyst. Due to the presence of a small amount of acetic acid in the peracetic acid solution and the co-production of acetic acid during the process, all equipment except that in contact with propylene and propylene oxide needs to be corrosion-resistant.

In China, the production of propylene oxide mainly uses the chlorohydrin process. Although this method requires lower investment, it generates many by-products and limits product quality and production costs. The co-oxidation process has the advantages of lower raw material costs, the use of ordinary carbon steel equipment, low costs, and minimal environmental pollution. However, it requires high-quality raw materials and balancing large quantities of co-products. To address the environmental issues of the chlorohydrin process and the co-products issue of the co-oxidation process, some new processes have been developed in recent years.

5.Direct Oxidation with Hydrogen Peroxide:

This process uses hydrogen peroxide (HP) as an oxidant to directly oxidize propylene to propylene oxide. Compared with the chlorohydrin process, it produces no by-products or nitrogen-containing waste. Compared with the PO/SM and PO/tert-butanol co-production processes, it does not produce co-products. The key technology of the HP-PO process is the use of a stable, high-activity, and selective titanium silicate heterogeneous catalyst, while also using the directly generated peroxide solution as a raw material to reduce costs.

6.Direct Air Oxidation Process:

The direct air oxidation process for preparing PO involves reacting propylene with oxygen in the presence of hydrogen, any diluent, and a catalyst to produce PO. Although the direct air oxidation process for preparing PO is a developing direction, it is still in the exploratory stage.