Page 130 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Application of heterogeneous acid catalyst derived from biomass for biodiesel process 101

4.7 Reusability of catalyst

Reusable nature is considered to be one of the major advantages in solid biomass
acid catalyst. When allowed to continuously catalyze esterification and transesterifi-
cation reactions, its activity can decrease. Change in this catalytic activity is due to
the fact that sulfonated catalyst has a closed structure at dry state because of strong
hydrogen bonds in its structure. This has more flexibility when it swells in any
swelling agent such as water, which again affects the diffusion of reactants as well
as products and increases the swelling efficiency in polymers. The use of catalyst
having smaller and more hydrophilic molecules in larger and more hydrophobic
species would be less efficient swelling agents. This resultant catalyst can be used
for multiple cycles. Consequently, these cycles reveal the combined influence of
catalyst swelling along with deactivation due to SO 3 H leaching. When swelling
effect is seen to exceed the leaching of SO 3 H group, there will be a considerable
improvement in the activity of the catalyst. Biomass-derived solid catalyst and their
cycles of reusability is listed in Table 4.3.
Regeneration is the process of reactivating the spent catalyst active site by sulfo-
nation process and recovering its original activity for its use in both esterification
and transesterification. A study showed that the total acid density of the catalyst
decreased from 6.85 to 3.42 mmol/g post reaction, while on regeneration, it showed
a considerable increase upto 6.12 mmol/g which indicates that most of the acid sites
have been recovered by this step (Horgnies et al., 2013). Hence, loss in catalyst
activity is probably due to some active acid sites being covered by adsorbed inter-
mediates or product species. Reactivation of these acid sites can be carried out with
the help of simple regeneration techniques.

4.8 Current challenges and future prospects

The current review aims to explain the importance of biomass-derived solid acid
catalyst to produce biodiesel which is an emerging field of research in the current
scenario. Wider uses have been reviewed extensively in the case of both esterifica-
tion and transesterification reactions. The main challenges include acid sites stabil-
ity and control of surface properties to favor the feedstock diffusion toward the acid
site and to eliminate the polar compound such as glycerol from the catalyst struc-
ture. When compared to the effect of solid alkaline catalysts on transesterification
reaction, the acid catalysts derived from biomass show a slower rate of reaction.
Hence, more quantity of catalyst, high temperature, and longer time for the reaction
are required for transesterification process. For enhancing biodiesel production the
above mentioned problems need to be addressed through intensive researches with
respect to carbon-based acid catalyst. The other drawbacks, causes, and possible
remedies are listed in Table 4.4. Therefore, it is a crucial step to identify efficient
acid catalysts to rectify these problems in future. Also, diversifying the feedstocks