Propylene oxide is generally a colorless, flammable, volatile liquid with an ether-like odor. It is sparingly soluble in water but miscible with most organic solvents. The molecule of propylene oxide contains a highly chemically reactive epoxy group, enabling it to undergo many chemical reactions. The chemical reactivity of propylene oxide is slightly lower than that of ethylene oxide. It reacts with substances containing active hydrogen (such as water, alcohols, acids, and amines), undergoing ring-opening to form glycols, glycol ethers, alkanolamines, and other compounds.

Figure1: Picture of propylene oxide

Basic Introduction

Propylene oxide (PO) is a major industrial product, with a global annual output of over six million tons. Approximately 70% of propylene oxide is used as polypropylene glycol in the production of polyurethane raw materials, while the remainder is consumed as propylene glycol in unsaturated polyester resins, food additives, and cosmetics. To date, the industrialized production methods for propylene oxide can be broadly divided into two categories. The first is the chlorine-based process dedicated solely to propylene oxide production, and the second is the co-production method, in which propylene oxide is generated alongside other byproducts such as styrene monomer. [1]

Synthetic process

Propylene oxide can be produced via the direct oxidation of propylene using molecular oxygen, which offers significant advantages over the existing chlorohydrin and hydroperoxide processes that generate side products and require complex purification schemes. Recent advances in the liquid-phase and gas-phase catalytic oxidation of propylene using only molecular oxygen as the oxidant and in the absence of reducing agents are summarized. Propylene oxide has been obtained in liquid-phase processes involving soluble or insoluble Mo, W, or V catalysts, which provide moderate conversions and selectivities, although these reactions likely proceed via autoxidation through homogeneous chain reactions. In the gas-phase, propylene oxide is most commonly produced using Ag-, Cu-, or TiO?-based catalysts, though other compositions such as Au-, MoO?-, Bi-based catalysts and photocatalysts have also been suggested. The Ag catalysts differ from those used for ethylene oxide production in that they have high silver contents and numerous additives, including solid-phase alkali metals, alkaline earth metals, and halogens, with NaCl and CaCO? being the most common. Nitrogen oxides, either as gas-phase species or nitrates, have also been found effective in enhancing propylene oxide production, and direct epoxidation by surface nitrates is a possibility. Additionally, titania catalysts supported on silicates have been reported, and these achieve higher propylene oxide selectivities at high conversion than silver catalysts. [2]

Polymerization method

Polypropylene ether glycols are used commercially as intermediates for flexible polyurethane foams and elastomers. In order to simultaneously maximize polyurethane molecular weight and polymer properties, highly bifunctional intermediates are required. Polypropylene ether glycols are prepared by the base-catalyzed polymerization of propylene oxide. A side reaction accompanying this polymerization gives rise to some monofunctionality by producing chains terminated with a hydroxyl group on one end and an allyl or propenyl group on the other. Polymerization method: Propylene oxide was polymerized at atmospheric pressure. Propylene oxide was placed in dropping funnel B; 1,2-propanediol (initiator) and potassium hydroxide (catalyst) were placed in the reaction vessel and dissolved by heating to 100?°C. Nitrogen was continuously fed beneath the liquid surface to provide adequate agitation for this operation. After the potassium hydroxide had dissolved, propylene oxide was added slowly. Temperature control was maintained by varying the immersion depth of the reactor in an oil bath, which was set approximately 20?°C higher than the desired polymerization temperature. Despite the slow feed rate, some of the monomer distilled out of the reactor and was collected in a storage vessel, from which it was periodically returned to the dropping funnel. The vaporization of unreacted monomer provided sufficient agitation of the polymerization mixture. During the later stages of the polymerization, a positive nitrogen pressure in funnel B was sometimes required to prevent vapor lock and force propylene oxide into the reactor.[3]

Sales price

Using propylene as the raw material, the synthesis of propylene oxide is divided into four sections: feed material pretreatment, direct oxidation reaction, initial separation, and final distillation. Propylene oxide is produced alongside byproducts including carbon dioxide (CO?), acetaldehyde, acrylic acid, and acrolein. The CO? is separated through an adsorption-desorption cycle with monoethanolamine (MEA), while the other byproducts are removed by distillation. Ultimately, propylene oxide is obtained with a mass purity of 99.9822%. The purchase cost of propylene is 1100 per tonne, and the selling price of propylene oxide is 2,500 per tonne.

Reference

[1] Ishino M. Development of new propylene oxide process[C]//16th Annual Saudi-Japanese Symposium-Catalysts in Petroleum Refining and Petrochemicals, Proceedings. 2006: 145-155.

[2] Khatib S J, Oyama S T. Direct oxidation of propylene to propylene oxide with molecular oxygen: a review[J]. Catalysis Reviews, 2015, 57: 306-344.

[3] Simons D M, Verbanc J J. The polymerization of propylene oxide[J]. Journal of Polymer Science, 1960, 44: 303-311.