Technical Abstract: Biodiesel, typically fatty acid methyl esters (FAME), has received much attention because it is a renewable biofuel that contributes little to global warming compared to petroleum-based diesel fuel. The most common method used for biodiesel production is based on the alkali-catalyzed transesterification of first-use refined oils and fats with an alcohol (e.g., methanol). These technologies, however, require significant modification when applied to second use materials such as greases because of their higher free fatty acid (FFA) content. Recently, we reported a series of insoluble porous polymer grafted diphenylammonium salts that efficiently esterified the FFA in greases to FAME. In this work, the diphenylammonium salts were supported onto two robust mesoporous silicas. The resulting catalysts had high esterification activity with > 99% of the FFA in greases converted to FAME, and the FFA content in the treated greases was reduced to < 1 wt%. The mesoporous silica-supported catalysts displayed minimal transesterification activity.