Synthesis of epichlorohydrin from 1,3-dichloropropanol using solid base

Abstract

In this work, the synthesis of epichlorohydrin (ECH) from 1,3-dichloropropanol (DCP) by using solid sodium hydroxide (NaOH) is carefully investigated. Inert organic solvent, 1-octanol, is introduced to ensure reaction intensity under control. The reaction performances with respect to apparent kinetics and selectivity are determined to explore optimized reaction conditions and confirm potentials for enhancing productivity in one batch. The dissolution and liquid phase reaction mechanism and instant reaction assumption are proposed and verified through process analysis. A process design towards free additional water is schematically figured out to manipulate solid NaOH, by-product, and unreacted starting materials to realize a nearly closed circuit. This process allows high selectivity over 97% and complete DCP conversion at 323.2 K within a reaction time less than 20 min. Other advantages include near-zero wastewater emission, economically possible NaOH regeneration from NaCl, and robust operating condition window.

Introduction

Epichlorohydrin (ECH) is an organochlorine compound and an epoxide with high reactivity. The ring-open reactions of ECH may occur under attacks from nucleophiles, electrophiles or radicals due to the different electron densities of its three carbon atoms and oxygen atom [1]. ECH and its ramifications are widely used in polymer science, either as crosslink reagents in polymer synthesis or as raw materials of epoxy adhesives, coatings and composite materials [2], [3], [4]. The conventional ECH production process is originated from allyl chloride [5], [6], [7]. Recently, with the fast development of biodiesel production, the main byproduct — glycerol — is becoming an important starting material [8], [9]. Although different starting materials correspond to various process routes, there exists a common step — the cyclization of dichloropropanol (DCP) through reacting with sodium hydroxide (NaOH) aqueous solution or calcium hydroxide (Ca(OH)₂) slurry at 90 °C or higher temperature. The DCP has two isomers, 1,3-DCP and 2,3-DCP. The 1,3-DCP is almost the unique intermediate in the synthesis of ECH with glycerol as starting material. The cyclization of DCP abides an internal nucleophilic substitution mechanism, i.e., the internal nucleophile (O⁻) attacks the carbon with leaving group to form a CO bond and break the CCl bond [10]. The reactivity of DCP is strongly affected by the position of substituent groups and their electron withdrawing characters. Although there exists different reports on the reaction orders of the cyclization of DCP [11], [12], researchers have common recognition that the synthesis from 1,3-DCP to ECH, much faster than that from 2,3-DCP to ECH, is easy to reach high conversion. But, the critical challenges still need to be met that excessive introduction of water leads to heavy burdens with respect to ECH separation and wastewater treatment [13], and high risk of ECH hydrolysis resulting in unsatisfied selectivity (lower than 95%) [14], [15].

Solid base is an important branch of reagents and catalysts, particularly known as heterogeneous base catalysts. Reactions catalyzed by heterogeneous catalysts are highly preferred in industry for the convenience to separate the catalysts and products. The syntheses of some fine chemicals, such as transesterification of oil to give biodiesel, isomerization of unsaturated hydrocarbons and addition reactions, have been widely reported to use strong solid base catalysts like alumina coated with alkali and alkaline-earth metal oxides, or weak solid base catalysts like zeolites and hydrotalcites [16], [17], [18], [19]. In contrast, using solid base as reactant is rarely reported, of which main disadvantage is high cost without considerations of wastewater treatment and base regeneration. It should be noted that the production of NaOH by sodium chloride (NaCl) electrolysis is a well-known commercial process, in which considerable amount of energy is consumed by concentrating NaCl aqueous solution. Using solid NaOH instead of NaOH aqueous solution may greatly improve the economic feasibility of NaOH regeneration and near-zero emission of wastewater in ECH production. A non-aqueous environment is also worth expecting for enhanced selectivity.

In this work, the synthesis of ECH from 1,3-dichloropropanol (DCP) by using solid NaOH was carefully investigated. Inert organic solvent, 1-octanol, was introduced to ensure reaction intensity under control. The reaction performances with respect to apparent kinetics and selectivity were determined to explore optimized reaction conditions and confirm potentials for enhancing productivity in one batch. The reaction mechanism was discussed to understand how the reaction occurs and how to establish a rational reaction system. Furthermore, a process design towards free additional water was proposed to manipulate solid NaOH, by-product, and unreacted starting materials to realize a nearly closed circuit. The usage of solid NaOH is meaningful for water saving, wastewater decrement, and selectivity enhancement in ECH production.