In　this　work,　two　double-site　acidic　ionic　liquids　were　synthesized　by　a　two-step　method　via　the　reaction　of　1,2-bis(diphenylphosphino)　ethane　(DPhPE)　with　1,3-propanesulfonate　(PS)　to　form　a　DPhPE-PS　zwitterion　followed　by　acidification　with　different　acids.　The　obtained　ionic　liquids　were　characterized　and　further　used　as　catalyst　for　the　esterification　of　various　free　fatty　acids　with　methanol　to　produce　biodiesel.　The　suitable　self-solidifying　double-site　acidic　ionic　liquid　[DPhPE-PS][HSO4]2　is　homogeneous　catalytic　due　to　its　solubility　during　the　esterification,　which　is　precipitated　after　reaction.　The　evaporation　of　the　residual　methanol　and　generated　water　combines　the　advantages　of　homogeneous　and　heterogeneous　catalysts,　including　their　excellent　catalytic　activity　and　convenient　recovery.　[DPhPE-PS][HSO4]2　exhibits　98.65%　of　oleic　acid　conversion　under　operating　appropriate　conditions　optimized　by　response　surface　methodology　(catalyst　dosage　of　6.0　wt%,　molar　ratio　of　methanol　to　oleic　acid　of　10.0:1,　reaction　temperature　of　60.0　°C,　and　reaction　time　of　4.0　h)　and　keeps　outstanding　catalytic　performance　after　9　cycles.　Moreover,　[DPhPE-PS][HSO4]2　also　displays　excellent　catalytic　activity　and　good　versatility　in　the　esterification　of　various　free　fatty　acids　to　biodiesel.　Hence,　the　self-solidifying　double-site　acidic　ionic　liquid　[DPhPE-PS][HSO4]2　can　be　used　as　an　environmentally　friendly　and　efficient　catalyst　for　synthesis　of　biodiesel　via　esterification.　©　2021