To promote the use of low-quality oils in producing biodiesel, a bifunctional acid-base catalyst Fe/DS-HMS-NH2 is fabricated using the two-step condensation technique. The obtained Fe/DS-HMS-NH2 is of a doubled shell structure in spherical shape with a uniform size of 156 nm. Its hollow core (with a diameter of 86 nm) and two spatial shells with different active sites enables the esterification and transesterification reactions to be accomplished in a one-pot synthesis. The influences of four independent reaction variables on the yield of fatty acid methyl esters Y-F was studied, including catalyst loading m(c), reaction time t, reaction temperature T, and the methanol to degummed palm oil mass ratio r(m/o). The highest yield was obtained at 85.36% (w/w) when m(c) = 6% (w/w), t= 4.5 h, T= 60 degrees C, and r(m/o) = 6:1. The Fe/DS-HMS-NH2 shows a good recyclability with Y-F > 80% (w/w) up to three reaction cycles. (C) 2021 Elsevier Ltd. All rights reserved.