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.