1. INTRODUCTION
The demand for sustainable alternative to petroleum diesel fuel is
increasing due to the depletion of fossil resources (Huang et al.,
2012). Biodiesel produced from vegetable oil was traditionally used as
an alternative biodiesel feedstock. However, converting vegetable oil to
biodiesel has caused great concerns due to food crisis issues.
Consequently, non-edible biodiesel feedstock from
agricultural waste are in high
demand for biodiesel production (Chongkhong et al., 2007; Atadashi et
al., 2010). For example, palm fatty acid distillate (PFAD), an
agricultural waste massively generated from palm oil industry is a
potential biodiesel feedstock attributed to its high free fatty acids
(FFAs) content (72.7 – 92.6%) with triglycerides residues (Zahan and
Kano 2018).
Catalyst plays a vital role in the
transesterification reaction of free fatty acids (FFA) to form
biodiesel. Sulfuric acid (H2SO4) and
hydrochloric acid (HCl) are common acid catalysts used to accelerate the
transesterification process for a higher yield of biodiesel (Chongkhong
et al., 2007; Canakci and Van
Gerpen, 1999; Fadhil et al., 2012; Zuo et al., 2013). Major issues of
acid catalysts are the corrosive effect on the reactor and high amount
of alcohol required, which incur high production cost (Leung et al.,
(2010). Homogeneous base-catalyst such as sodium hydroxide (NaOH) and
potassium hydroxide (KOH) are employed as an alternative to acid
catalyst in transesterification process to exhibit faster and
non-corrosive reaction (Ma and Hanna (1999). Employing homogeneous base
catalyst, however, suffers disadvantage due to saponification effect
which reduce the yield of biodiesel
(Leung et al., 2010; Romero et
al., 2011).
Heterogeneous base-catalyst such as calcium oxide and silica has been
reported to produce a higher yield of biodiesel compared to homogeneous
base catalyst (Leung et al., 2010). Nevertheless, the catalyst has a
difficulty to diffuse into the product mixture to form oil, alcohol and
catalyst (Mbaraka and Shanks, 2006). This limitation has led to the
introduction of catalyst support to overcome the weaknesses of
heterogeneous catalyst (Mbaraka and Shanks, 2006; Zabeti et al., 2010).
Catalyst support is a solid carrier impregnated with
an active species (Mabena et al.,
(2011). NaOH/Al2O3,NaOH/SiO2 and ZS/Si are examples of catalyst supports
used in the production of biodiesel (Taufiq-Yap et al., 2011; Jacobson
et al., 2008; Arzamendi et al., 2007). The incorporation of active
species into catalyst support can increase the reaction with
triacylglycerol (Zabeti et al., 2010).
We have reported on using Imperatacid and Imperatabase as
silica-based heterogeneous catalysts from Imperata cylindrica(I. cylindrica ) for a facile transesterification reaction of palm
oil mill sludge (POMS) (Ngaini et al., 2016). The high porosity
of silica in I.cylindrica offers convenient esterification and
transesterification in higher percentage yield. A very low amount of
silica content (1-8%) in I. cylindrica has led our study
to search for agricultural waste with higher silica content for
preparation of catalyst support. Rice husk has been reported to have
high silica content (90-97%) (Della et al. , 2002) and is
abundantly available as agricultural waste in South East Asia,
especially Malaysia.
Herein we report on the production of biodiesel from PFAD employing
silica-based catalyst support from rice husk impregnated with
H2SO4 and
NaOH as solid acid catalyst (SiA)
and solid base catalyst (SiB), respectively. The formation of biodiesel
from PFAD was performed via esterification using SiA followed by
transesterification using SiB. The biodiesel produced was evaluated
based on several reaction parameters such as oil to methanol ratio,
amount of catalyst, reaction times and temperature and the optimization
of parameters was carried out.