Although the reaction pathway in organic microdroplets has been proved different from the bulk phase, the conversion ratios for the two main products 2a and 3a was far away from quantitative synthesis, leading to a necessity to careful optimization of reaction conditions. Six acid catalysts (sulfuric acid, trifluoromethanesulfonic acid, phosphoric acid, hydrochloric acid, acetic acid and trifluoroacetic acid) were first screened for the acid-catalyzed ring opening in organic microdroplets. As demonstrated in Table (Entries 1-4) and Table , sulfuric acid and CF3SO3H were the two suitable acid catalysts for this conversion, and sulfuric acid was selected considering its slight better performance than the latter. An optimization of catalyst loading amount was carried out in the following. As the loading amount of sulfuric acid was increased from 18 mol% to 184 mol%, the yield of 2a by organic microdroplets was significantly improved up to 90% (Table , Entries 7-9, and Table ). The microdroplet reaction was hardly affected on further increasing sulfuric acid to 36.8 equivalents (Table ). For comparison, no vicinal diamine 2a can be synthesized without the catalysis of sulfuric acid (Table , Entry 1). The optimization of catalyst loading amount indicated that sulfuric acid equivalent was very important for the ring opening reaction of imidazoline derivative 1a. A moderate loading amount of 2.3 equiv. sulfuric acid was selected considering both good yield and low catalyst consumption.