Reviewer 1 Reviewer 1
What is novel? There are many papers that use silica from corn cob ash to make biodiesel, so please clarify what is different about your study. You should compare results from these similar studies to your results.
Thank you for your comment. The comparison results from the similar studies and the statement of novelty has now been added to the Introduction section (paragraph 3 & 4), as follows: Rice husk is abundantly available as agricultural waste in many rice-producing countries, including Malaysia. Rice husk has been reported to have high silica content (90-97%) (Della et al., 2002), which indicates its potential as silica-based heterogeneous catalyst for biodiesel production. At the same time, 2.5 to 3 Mt of palm fatty acid distillate (PFAD), a lower value by-product of palm oil processing is generated worldwide (Idris et al., 2019). The PFAD is rich in fatty acids and is readily available as biodiesel feedstock. Rice husk ash (RHA) has been studied as catalyst precursor for soybean oil biodiesel (Chen et al., 2013), palm oil biodiesel (Chen et al., 2015; Roschat et al., 2016), and waste cooking oil to biodiesel (Hazmi et al., 2020). Hindirawaty et al. (2020) studied RHA as support material for NiSO4 as a solid acid catalyst in PFAD conversion to methyl ester at different MeOH to oil molar ratio, catalyst amount and reusability. PFAD conversion to biodiesel was also studied using kenaf seed cake-based sulfonated catalyst (Akinfalabi et al., 2019). However, to our best knowledge, no work has been published on RHA as catalyst support for H2SO4 as SiA and and NaOH as solid base catalyst for PFAD conversion to biodiesel. Thus, this work focused on the synthesis of silica-based catalyst support from RHA impregnated with H2SO4 (forming Huskcatacid) and NaOH (forming Huskcatbase), respectively. PFAD was used as fatty acid source to examine the potential application of the prepared catalyst for biodiesel production. The formation of biodiesel from PFAD was performed via esterification using Huskcatacid, followed by transesterification using Huskcatbase. 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.
It took a few reads to understand the first sentence of the abstract. I would delete ”using a facile method” for clarity or move it to the end of the sentence, otherwise it sounds like transesterification was the facile method.
Thank you for your suggestion. The phrase ”using a facile method” has been deleted and the abstract has been revised, as follows: Rice husk ash (RHA) has potential as low-cost catalyst support to produce biodiesel. In this study, RHA was used as precursor for heterogeneous silica-based catalyst, Huskcatacid and Huskcatbase for biodiesel production from palm fatty acid distillate (PFAD). The formation of biodiesel from PFAD was performed via esterification using Huskcatacid, followed by transesterification using Huskcatbase. 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. Results showed that Huskcatacid was efficiently employed in esterification of PFAD to afford 91.6% ester in 1:1 (PFAD:MeOH) and 5 wt% catalyst, followed by transesterification in 1:1 (oil: MeOH) using Huskcatbase (1 wt%) to produce 97.5% biodiesel in 30 min, with high percentage of methyl palmitate (34.43%) and methyl oleate (57.86%). Both Huskcatacid and Huskcatbase have high porosity (2.726 x 10-3 cm3/g and 4.985 x 10-3 cm3/g) and surface area (7.362 m2/g and 14.493 m2/g), respectively, which offer efficient esterification, easy glycerol separation and reusability up to three times to afford biodiesel ranging from 90-97% yield. The biodiesel obtained was applied on Megatech®-Mark III engine and demonstrated a proportional increased of torque (ɽ) with the B100 biodiesel loading. Rice husk demonstrated a good potential as solid support heterogeneous catalysts and feedstock for value-added products, which also assists to overcome agricultural waste management issues. Keywords: rice husk ash; heterogeneous catalysts; esterification; biodiesel; FFA conversion
Delete the sentence saying biodiesel competes with food for vegetable oil because that’s generally inaccurate and spreads misconceptions about biodiesel. The vegetable oil most commonly used is excess soybean oil from growing crops for animal feed. When we grow soybeans for animal feed, we get more oil than we eat.
Thank you for your comment. The sentence implying biodiesel competes with food has been deleted in the revised manuscript. The whole Introduction section has now been rewritten to focus on the catalyst for biodiesel production (previously focused on biodiesel production). Revised Introduction section is as follows: Catalyst plays a vital role in the transesterification reaction of free fatty acids (FFA) to form biodiesel. Thus, the synthesis of ideal catalyst for biodiesel production has gained attention of researchers in recent decade. 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) exhibited faster and non-corrosive reaction during production of biodiesel (Ma and Hanna (1999). However, homogeneous base-catalyst suffers disadvantage due to saponification effect which reduce the yield of biodiesel (Leung et al., 2010; Romero et al., 2011). Heterogeneous catalysts like tungstate (Sunita et al., 2008), ferric alginates (Peng-Lim et al., 2012), silica nanoparticles (Yang et al., 2017) and titanium dioxide (Carlucci et al., 2019) were then explored as alternatives available to homogenous biodiesel catalysts. However, the drawbacks of these catalyst are costly materials, complicated synthesis routes, and low catalytic activity (Akinfalabi et al., 2019). 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), but 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 for heterogenous base-catalyst increase the reaction with triacylglycerol (Zabeti et al., 2010). Catalyst support is a solid carrier impregnated with an active species (Mabena et al., (2011). For instance, NaOH/Al2O3, NaOH/SiO2 and ZS/Si has been used as catalyst supports in the production of biodiesel (Taufiq-Yap et al., 2011; Jacobson et al., 2008; Arzamendi et al., 2007). The use of biomass-based catalyst from agricultural waste in biodiesel production reduce the overall cost of biodiesel production. For instance, silica based catalyst from corn germs and corn cobs gave optimum production of corn oil biodiesel with catalyst amount of only 19.8% (Milan et al 2018). Silica-based heterogeneous catalysts derived from Imperata cylindrica offers convenient esterification (>90 wt% oil) and transesterification to afford 80% biodiesel (Ngaini et al., 2016). However, Imperata cylindrica has low silica content (1-8%), which requires a lot of feedstock for catalyst preparation. Rice husk is abundantly available as agricultural waste in many rice-producing countries, including Malaysia. Rice husk has been reported to have high silica content (90-97%) (Della et al., 2002), which indicates its potential as silica-based heterogeneous catalyst for biodiesel production. At the same time, 2.5 to 3 Mt of palm fatty acid distillate (PFAD), a lower value by-product of palm oil processing is generated worldwide (Idris et al., 2019). The PFAD is rich in fatty acids and is readily available as biodiesel feedstock. Rice husk ash (RHA) has been studied as catalyst precursor for soybean oil biodiesel (Chen et al., 2013), palm oil biodiesel (Chen et al., 2015; Roschat et al., 2016), and waste cooking oil to biodiesel (Hazmi et al., 2020). Hindirawaty et al. (2020) studied RHA as support material for NiSO4 as a solid acid catalyst in PFAD conversion to methyl ester at different MeOH to oil molar ratio, catalyst amount and reusability. PFAD conversion to biodiesel was also studied using kenaf seed cake-based sulfonated catalyst (Akinfalabi et al., 2019). However, to our best knowledge, no work has been published on RHA as catalyst support for H2SO4 as SiA and and NaOH as solid base catalyst for PFAD conversion to biodiesel. Thus, this work focused on the synthesis of silica-based catalyst support from RHA impregnated with H2SO4 (forming Huskcatacid) and NaOH (forming Huskcatbase), respectively. PFAD was used as fatty acid source to examine the potential application of the prepared catalyst for biodiesel production. The formation of biodiesel from PFAD was performed via esterification using Huskcatacid, followed by transesterification using Huskcatbase. 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.
Make sure to include all necessary details to repeat your experiments. For example, how was sulfonation executed? What temperature and volume was the activated ash soaked in NaOH? Was there mixing during esterification and transesterification?
The details of experimental procedures have been added for clarity, as follows: Preparation of activated rice husk ash Rice husk (1000 g) were cleaned and dried in the oven for 3 days. The dried rice husk was calcined in a closed furnace at 800°C for 16 h, cooled to room temperature to obtain rice husk ash RHA (230.0 g). RHA (50.0 g) was soaked in NaOH solution (50 mL, 20%) and stirred for 2 h at room temperature. The mixture was filtered, rinsed with distilled water and dried for 24 h. Concentrated sulphuric acid (99% H2SO4) was added dropwise, stirred and washed with distilled water to pH 7 (Wahi and Senghie, 2011). The sample was dried in oven at 80 °C for 24 h to produce activated rice husk ash (ARHA) (47.0 g). Preparation of Huskcatacid ARHA (20.0 g) was soaked in concentrated H2SO4 (20 mL, 95%) and stirred for 30 min, decanted heated in a furnace for 12 h at 100°C (Kastner et al., 2012). The sample was cooled and rinsed with distilled water to pH 2. The sample was dried in the oven at 80 °C for 24 h, cooled to room temperature to produce Huskcatacid (18.9 g) and stored in desiccator prior to use. Preparation of Huskcatbase ARHA (20.0 g) was stirred in NaOH solution (20 mL, 5M) for 24 h at room temperature. Excess NaOH solution was decanted and the solid was heated at 700°C for 10 min (Refaat, 2011). The sample was cooled to room temperature to yield Huskcatbase (19.2 g) and kept in desiccator prior to use.
Where are the error bars on the % conversion and torque graphs? How many independent replicates were completed? The measurement was conducted manually, therefor the error bars on the % conversion and torque graphs was not obtained no replication was performed.
The formatting for your equations is inconsistent. Equation 4 is formatted fine, but equations 1 and 2 should be formatted like equation 4 (2 lines for a fraction). Also equation 4 should be numbered equation 3 because there is no equation 3.
Equations 1 and 2 has been formatted as required. Equation 4 has been renumbered as Equation 3 in the revised manuscript, as follows: \[Acid\ value,\frac{\text{mgKOH}}{g}=\ \frac{N_{\text{KOH}}\text{\ x\ }\text{Mw}_{\text{KOH}}\text{\ x}\text{\ V}_{t}}{W_{s}}x100\%\ \ (Eq.\ 1)\] \(FFA,\%=\frac{\text{AV}_{s}}{2}x100\%\ \ (Eq.\ 2)\) \(Conversion,\ \%=\frac{Weight\ of\ biodiesel\ produced,\ g}{Weight\ of\ feedstock,\ g}x100\%\ \ (Eq.\ 3)\)
You say there’s only a small amount of smoke emission, but did you quantify that? You should have data to support that claim. Just saying the smoke is normally more black is not adequate.
Thank you very much for the comment. The data was obtained during application of biodiesel via observation study. This could be important for future work which focusing on biodiesel rather than the solid support catalyst. We have therefor, revised the sentences in the following sections, and add statement on the required quantification of smoke data.

Combustion capability test

The combustion capability test of PFAD biodiesel (B100) was demonstrated on the alternative diesel engine (Megatech- Mark III) (Fig. 8). Ethanol was initially introduced into the combustion chamber to achieve 400-500°C before B100 was introduced. It was observed that, a bright orange flame with less smoke was released upon combustion of B100 in the chamber in 10-15 mins. In contrast, CD has been reported to emit black smoke due to incomplete combustion (Akihama, et al., 2002). A slight, odourless smoke was produced during combustion of B100 indicated a sufficient and complete combustion of the engine. However, the smoke observed through visual and relative smell observation, which require further quantification for verification. CONCLUSION Huskcatacid and Huskcatbase have been successfully developed from rice husk ash and employed as heterogenous solid support for the production of biodiesel from PFAD. The catalysts were conveniently prepared and employed in the esterification and transesterification reaction to afford high yield of biodiesel (>97% yield) with reusability up to three times. The biodiesel performance was demonstrated on alternative diesel engine and visual observation showed only small amount of smoke emission compared to the commercial diesel. This study indicated that rice husk demonstrated a good potential as solid support heterogeneous catalysts and feedstock for value-added products, which also assists to overcome agricultural waste management issues.
Reviewer 2 Reviewer 2
This manuscript describes the synthesis and applicatoin of rice husk derived ’catalysts’ for the synthesis of biodiesel from Palm Fatty Acid Distillate. The authors describe the synthesis of H2SO4 and NaOH impregnated rice husk derived silica and their application in transesterification/esterification of Palm Fatty Acid Distillate. While the use of such materials may aid production of biodiesel the materials cannot be described as a heterogeneous catalyst. These are merely supported reagents and as a consequence of their leaching there is little improvement in the overall process as a requirement for quenching and removal of alkali impurities are still required. For this reason I cannot recommend publication. If however the authors discuss their work in the context of a supported reagent and benefits over homogeneous catalysts then the manuscript could be reconsidered.
Thank you for the suggestion. The title, abstract and Introduction section have been revised to discussed H2SO4 and NaOH impregnated rice husk ash as catalyst support. New Title: Rice Husk Ash as Heterogeneous Silica-Based Catalyst Support for Palm Fatty Acid Distillate Conversion to Biodiesel The revised abstract and Introduction section is per our response to Reviewer 1.
Specific comments [2]1.There is no elemental analysis on the final materials to quantify acid and base site loading. Thank you. Quantitative element composition of the of Huskcatacid and Huskcatbase was characterized using X-Ray Diffraction (XRD) and Energy Dispersive X-ray (EDX) analyses (as depited in Table 1). Other reported, similar studies used the same approach in characterization of biodiesel catalysts (Fu, et al., 2011; Fu, et al., 2012; Akinfalabi et al., 2019; Hyndryawati et al., 2020; Idris et al., 2020).