Biodiesel From Conventional Feedstocks - Biotechnology in China: Biofuels and Bioenergy - page 59

3.2 Heterogeneous Catalyst-Mediated Process

3.2.1 Heterogeneous Solid Alkali-Catalyzed Reaction

Some disadvantages associated with homogeneous alkali catalysts, such as costly
separation of the catalyst from the reaction mixture, have inspired the development
of heterogeneous solid catalysts for biodiesel production, and solid alkali catalysts
including mainly alkali metal oxides, alkaline-earth metal oxides, hydrotalcite and
hydrotalcite-like solid alkali have thus been developed [ 18 , 19 ]. In general, alkali
metal oxides and alkaline-earth metal oxides are hygroscopic, which leads to
costly separation. In addition, their preparation requires relatively elevated
temperatures and pressures. Anion exchange resin is another solid alkali catalyst
for biodiesel production. Chinese researchers have reported that different anion
exchange resins pretreated with NaOH solution could be used as solid alkali
catalysts for biodiesel production. Some researchers used hydrotalcite and
hydrotalcite-like solid alkali as the catalysts for biodiesel production. In these
systems, strong and medium basicity can be achieved by the presence of O 2- and
OH - species. However, these types of solid alkali catalysts are easily poisoned by
H 2 O and CO 2 . Researchers have also investigated activated carbon, MgO, CaO,
ZrO 2 and TiO 2 as the solid alkali catalysts for biodiesel production [ 18 - 20 ].
Although heterogeneous catalysts have the advantages of being reusable and
easy to separate from the reaction products, their performance is generally poorer
than that of the commonly used homogeneous catalysts. Though many researchers
have investigated heterogeneous catalyst-mediated biodiesel production, few

practical applications have been reported.

3.2.2 Heterogeneous Solid Acid-Catalyzed Reaction

Many scholars are engaged in the study of solid acid-catalyzed biodiesel
production in China. The solid acid catalysts include zeolites, heteroplyacids, and
ion-exchange resins. [ 21 , 22 ]. In general, these solid acid catalysts adapt well to
feedstocks with high FFA content, and have the advantage of easy separation.
However, solid acid catalysts have the disadvantages of low activity, slow reaction
rate, high temperature, and poor yields. Even though some companies have
claimed that solid acid catalyst-mediated biodiesel production technology is
mature, the separation issue as well as high cost hinders its practical application.
Some scholars are also working on acid ionic liquids for biodiesel production
[ 21 , 22 ]. Ionic liquid, as a type of new environmentally friendly solvent and liquid
acid catalyst, shows the high-density reactivity sites of liquid acid and the
non-volatile property of solid acids. It is reported that ionic liquids can be used as a
solvent when it is present as catalysts. Although using ionic liquid as catalysts for
biodiesel production has been reported in China, the cost, adaptation of materials,
and other issues associated with ionic liquid-mediated biodiesel preparation need
to be further studied.