Direct transesterification of wet microalgae to biodiesel using phosphonium carboxylate ionic liquid catalysts

In this study, four types of tetrabutylphosphonium carboxylate ionic liquids (ILs) were synthesized and used for a one-pot transesterification of wet Chlorella vulgaris (C. vulgaris) microalgae into fatty acid methyl esters (FAME) in the presence of methanol, as well as refined oils (sunflower, canola, and corn oil). The resulting process removed the need for complete drying and lipid extraction steps typically needed for biodiesel production from microalgae. The leading candidate ionic liquid catalyst, tetrabutylphosphonium formate ([P4444][For]), was further optimized using response surface methodology to minimize material consumption, increase water compatibility, reduce processing time while maximizing yield from wet microalgae biomass. The optimized conditions resulted in a yield of 98.0 ± 2.48% using biomass containing 40 wt% water. Finally, the reusability of the ionic liquid was confirmed and [P4444][For] could be recycled with less than 2% loss between cycles. Due to the simple and low cost of synthesizing this IL, [P4444][For] is a promising catalyst for a one-pot transesterification process for biodiesel production from wet microalgae biomass. [Display omitted] •Four phosphonium carboxylate ionic liquids were tested for transesterification activity.•[P4444][formate] was identified as the most promising IL catalyst.•Response surface methodology was used to optimize reaction conditions and time.•[P4444][formate] is simple to synthesize and easy to recycle.