A natural evolution in process development is the replacement of the homogeneous catalysis with a solid base catalyst, change that simplifies extensively the post­processing process. In particular, it makes it easier to operate in continuous mode, eases the catalyst separation and recycling after reaction, avoids the saponification problem, and yields a cleaner biodiesel product, a purer glycerol that can be more easily marketed.

Various types of heterogeneous catalysts are being considered and studied for biodiesel production, including titanium silicates, ion-exchange resins, and zeolite metal-supported catalysts, among others. Extensive reviews of the current status on the use of solid base catalysts can be found in Liu et al. [63] and Di Serio et al. [33]. Peng et al. [75] prepared, characterized, and studied a solid acid catalyst for its activity in the production of biodiesel from several feedstocks with a high FFA content, showing that heterogeneous catalysis is a worthy option to process those types of raw materials.

Heterogeneous catalysts such as zeolites and metals may also allow for the use of feedstocks with a high FFA content [16, 31, 54]. However, some scientific and technical barriers persist relatively to their application at industrial-scale. For instance, Albuquerque et al. [2] concluded that the catalyst activity strongly depends on the metal composition of the oxides used, and new materials with well-defined structures, high surface area, and adequate basic or acid properties have yet to be developed. Although many of the solid catalysts proposed in literature for biodiesel production have good catalytic performances, they require high temperatures and pressure to work properly. Also, it is necessary to address the questions of deactiva­tion, reusability, and regeneration of the catalysts in practical conditions to assess their real potential for using in commercial applications. As heterogeneous cata­lyzed processes have advantages over homogeneous catalyzed and are easier to operate, more commercial applications will certainly be introduced in the near future with important impact on the biodiesel production.