3,4-Difluorobenzonitrile, a vital intermediate for manufacturing the high-efficacy rice herbicide cyhalofop-butyl, has traditionally posed significant synthesis challenges. Existing industrial methods suffered from prolonged reaction times exceeding 20 hours, harsh reaction conditions requiring temperatures above 230°C, modest yields below 80%, substantial energy consumption, serious side reactions like dehalogenation and coking, and significant waste generation. These factors hampered production efficiency and increased costs for this indispensable agrochemical component.

A high-efficiency, environmentally friendly catalytic synthesis process has been successfully developed, overcoming the critical drawbacks of prior methods. This optimized approach utilizes spray-dried potassium fluoride as the fluorinating agent and a dual-functional bis-[N,N-bis(dimethylamino)methylene]chloroiminium salt as the phase-transfer catalyst. The reaction proceeds in anhydrous aprotic solvents, preferably 1,3-dimethyl-2-imidazolidinone (DMI), with an azeotropic water removal agent, ideally cyclohexane.

The ingenuity of the process lies in its stepwise temperature control. The synthesis commences with the dehydration of 3,4-dichlorobenzonitrile precursor under reflux at 90-120°C for 1-2 hours to ensure anhydrous conditions. The fluorination reaction then initiates at a lower temperature range of 130-150°C, facilitating the formation of the intermediate 3-chloro-4-fluorobenzonitrile over 2-3 hours without excessive energy input. Crucially, the second fluorination stage proceeds at a slightly elevated temperature of 180-200°C for only 5-6 hours. This targeted thermal profile dramatically minimizes degradation and coking side reactions prevalent in single-stage high-temperature processes. Remarkably, the total reaction duration is compressed to only 7-9 hours.

Post-reaction workup involves diluting the mixture with toluene, followed by vacuum filtration to remove inorganic salts. The filtrate undergoes fractional vacuum distillation (0.08-0.1 MPa) to isolate high-purity 3,4-difluorobenzonitrile, collected between 95-105°C. A pivotal economic and environmental advantage is the direct recycling of the post-distillation mother liquor, containing the valuable catalyst and residual solvent, back into subsequent reaction batches. This eliminates costly catalyst re-synthesis and reduces solvent waste significantly.

Benchmarked against conventional techniques, this novel synthesis demonstrates exceptional performance. Yields consistently surpass 85% with material purity exceeding 99%. The drastically reduced reaction time lowers energy consumption by over 50%. By mitigating harsh conditions and employing closed-loop solvent/catalyst reuse, the process slashes production costs, minimizes hazardous waste generation, and enhances overall safety. The efficient catalyst system maintains high activity and stability even after multiple recycles.

This breakthrough represents a substantial advancement for sustainable agrochemical manufacturing. By delivering high-purity 3,4-difluorobenzonitrile—the critical building block for cyhalofop-butyl—with unprecedented efficiency and reduced environmental footprint, the process enables more reliable and economical production of this crucial rice herbicide. Its operational simplicity, mild conditions, and excellent yield position it as a highly suitable solution for large-scale industrial implementation.