Recent Advances in Biowaste-Derived Bifunctional Catalysts in Biodiesel Production: A Mini-Review

Introduction

Ever-increasing global energy demand combined with decreasing supply of fossil fuels and fast- rising CO2 emissions have enhanced the interest in the production of biofuels. Biodiesel offers strong substitutes for fossil fuels since it can be made from sustainable and renewable resources (biomass), are safe, and emit less CO and NOx than conventional fossil fuels [1, 2]. Additionally, biofuels may be utilised in ordinary diesel engines by blending them with petrodiesel with just minor modifications [3]. As a result, it is believed that the catalytic transformation of widely available and renewable biomass as a sustainable energy source is essential for supporting the concepts of a carbon- neutral and waste-free economy [4, 5]. Although catalysis is crucial for the generation of biodiesel [6], the elevated reactivity of homogeneous catalysts is overpowered by some significant issues including wastewater formation, instrument corrosion, and catalyst reusability [7, 8]. As a result, solid catalysts have become viable, environmentally friendly alternatives. They generate less harmful waste, have only moderate corrosion problems, and are simple to reuse as they can be easily separable from the reaction mixture. Consequently, the production of biofuels using solid catalysts is a fast-growing area of present-day chemistry [9, 10]. Therefore, it is highly desirable to develop next-generation solid catalysts from renewable resources for the industrial scale production of biodiesel.

A potential liquid biofuel called biodiesel, may be made from both edible and inedible oils, waste cooking oil (WCO), and animal fats [11, 12, 13]. It has various benefits over fossil fuels, including being renewable, non-hazardous producing less CO2 emissions, yet not emitting unburned hydrocarbons or SO2 during burning due to its high incorporation of 10- 12% oxygen [14, 15, 16]. Furthermore, it doesn’t require much modification to be used directly in modern petrodiesel engines [17, 18].

Long-chain triglycerides (TAGs) and free fatty acids (FFAs) are the two components of biomass, which are transformed into biodiesel by esterification and transesterification (Figure 1) [19]. Transesterification can be effectively catalyzed by solid acid and base catalysts to produce biodiesel and glycerol [10, 20]. However, Despite the high reactivity of the base catalyst, it is not desirable for the esterification reaction as it forms soap on reaction with FFA. On the other hand, an acid catalyst can facilitate both esterification and transesterification [21, 14]. The main drawback of making biodiesel is that it is currently manufactured from edible plant oils to a greater than 95% extent. This increased the cost of food and may result in food shortages in emerging and low- income nations [7, 22]. Because feedstock accounts for up to 75% of manufacturing expenses, using this resource is difficult to justify and escalates production costs [23]. In this context, the exploitation of non-edible oils offers a suitable procedure for the cost-effective production of biodiesel [24]. However, the high FFA concentration of non-edible oils prevents the use of solid base catalysts [25, 26]. This has developed the idea of utilizing a concerted, two-step process involving. FFA esterification with acid-active sites followed by TAG transesterification with base active sites in a bifunctional catalyst highly desirable. It would encourage the use of inedible oil feedstocks by minimizing the complicated product isolation and separation steps and presenting a two-step method [27, 28]. Due to its growing popularity, several review papers on the generation of biodiesel utilising heterogeneous catalysts have been published [29, 30, 31, 32, 33]. However, the application biowaste derived bifunctional catalysts towards biodiesel production is not covered extensively [27, 28]. In this mini-review, we bring up-to- date discussion of the different types of biowaste-derived bifunctional catalysts available for the (trans)esterification of non-edible oils specifically.

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Biowaste-Derived Bifunctional Catalyst

There are many substantial downsides to (trans) esterification of non-edible oils employing heterogeneous inorganic catalysts: Extended reaction time, high reaction temperatures, and high methanol/oil molar ratios are not unusual [34]. Aside from these useful considerations, the production of catalysts from inorganic sources usually resulted in materials with high levels of stability (reusability), but it also typically required unsustainable, costly, and toxic chemicals as well as lengthy, multi-step procedures [35]. In this context, biowaste-derived bifunctional catalyst gaining enormous attention as their preparation involves cheap raw materials, easily available, non-toxic and biodegradable [36, 37]. They offer to address the issues with waste disposal and reduce the cost of (trans) esterifying non-edible oils into biodiesel. In this section, various biowaste- derived bifunctional catalysts used in the synthesis of biodiesel are addressed (Table 1).

A bifunctional solid catalyst made from corncobs (CC) was described for the (trans) esterification of neem seed oil (NSO) into biodiesel [38]. There were three steps in the catalyst’s preparation. To synthesize sulfonated corncob (CC- SO4), the corncobs were first dried in the sun, milled into a powder and partially carbonized at 280°C, then treated with sulfuric acid. The second process involved carbonising dried poultry droppings (PD) at 280°C, treating them with 0.1 M KOH, and then calcining them at 900 °C for 6 h (KOH@ PD). CC-SO4 and KOH@PD were then magnetically stirred together to form the catalyst. The catalyst that resulted had an excellent biodiesel yield of 92.89%. The reusability test revealed that the catalyst can be used for two consecutive cycles with a biodiesel yield of 89.26%. Similarly, KOH- modified sulfonated corncob (KOH@CC-SO4) was utilized in the (trans) esterification of Waste Cooking Oil (WCO) containing high FFA content and showed a high biodiesel yield of 97.8% under the optimized reaction conditions. A two-step process was involved in the preparation of the catalyst. At first, the cleaned and dried corn cobs were milled followed by treated with conc. sulfuric acid to form sulfonic acid functionalized activated carbon (CC-SO4). Secondly, the CC-SO4 was modified with KOH via wet impregnation. The catalyst prepared with a corncob/H2SO4 (weight/volume) ratio of 5:1 at 700°C for 4 h displayed the highest surface area (1311 m2/g) with an acid density of 1.12 mmol/g. Although this catalyst could be employed for up to five consecutive reaction cycles, maintaining a 92% FFA conversion, the authors noted the necessity to address the leaching of KOH sites [39]. Additionally, a solid catalyst derived from rice husk was utilized for the preparation of biodiesel. The carbonized rice husk was impregnated with different amounts of KOH and Fe2O3 to introduce basic and magnetic characteristics, respectively. The maximum base density (4.43 mmol/g) was found in the catalyst loaded with 20% KOH and 5% Fe, while the surface area (57.89 m2/g) and acid density (24.59 mmol/g) were only modest. Since production is known to be dependent on the basic sites of the catalyst, it demonstrated the highest activity of the synthesized catalysts (98.6% biodiesel output). The biodiesel output was decreased to 40% after 6 cycles due to catalyst instability, which may be due to catalyst poisoning [40]. In another study, different amounts of K2O and NiO-modified carbonized rice husk (RH) based magnetic bifunctional catalyst was prepared for the production of biodiesel. The aggregation of potassium oxides and nickel on the carbon framework is responsible for the uneven shape shown in the FESEM micrographs (Figure 2) of the synthesized catalysts. On increasing nickel levels, it was noticed that the carbon framework’s pore size shrank, most likely as a result of pore blockage by crystallization. Among synthesized catalysts, RH-Ni (5%)-K(20%) had the greatest biodiesel production (98.2%, depreciated to 71.0% after 5 cycles) [41].

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A solid bifunctional catalyst derived from the bio-waste palm kernel shell (PKS) was prepared for the synthesis of biodiesel from WCO. The preparation of the catalyst involved a two-step process:

• Calcination of PKS at high temperature to generate activated carbon (AC).
• Impregnation of different amounts of K2CO3 and CuO to introduce surface basicity and acidity, respectively.

The catalyst loaded with 30% K2CO3 and 5% CuO resulted in a very high surface area (438.08 m2/g) with high surface acidity (27.02 mmol/g) and basicity (8.86 mmol/g), respectively, thus giving good biodiesel yield of 95%. Reusability experiments revealed a considerable depreciation in the product yield to 80% after 5 consecutive cycles, suggesting leaching of the catalyst acidic and basic sites [42]. In another study, a hydrothermally carbonized catalyst was derived from waste PKS to produce activated carbon (PKSAC). After being treated with NaOH, the surface area drastically increased from 3.57 to 3368.60 m2/g, and different concentrations of K2CO3 and Cu(NO3)2 were then added to introduce surface acidity and basicity. The combination of PKSAC-K(20%)-Cu(5%) led to a maximum FAME production of 95.4%, which may have been a result of the comparatively high base and acid densities of 5.73 and 1.48 mmol/g, respectively. However, this performance was not sustained throughout the course of further testing [43]. A solid bifunctional catalyst with an Angel wing shell (AWS) basis was recently developed through a two-step procedure.

Dried AWS was calcined at 900 °C for 2 h to convert it into calcined AWS (CAWS) followed by treated with sulfuric acid to introduce acidic sites (CAWS-SO4). Then the catalyst was employed for the (trans) esterification of palm fatty acid distillate (PFAD) to produce biodiesel. To investigate its impact on the acidity and basicity of the catalyst, the concentration of sulfuric acid was changed from 3 to 11 M. Initially, an increase in sulfuric acid concentration led to an increase in the acidity of the catalyst, with CAWS-SO4 (5) and CAWS-SO4 (7) (for example, generated using 5 M or 7 M H2SO4 being discovered to be 2.14 and 4.73 mmol/g, respectively. However, when the quantity of sulfuric acid further increased, the catalyst surface was damaged, which resulted in a reduction in acidity. In contrary, low sulfuric acid concentration CAWS-SO4 (3) led to the introduction of highest base density of 1.33 mmol/g. A high biodiesel yield of 97.4% was accomplished utilizing CAWS-SO4 (7) catalyst, predictable with catalyst activity for esterification being principally subject to the presence of acidic sites. Catalyst reusability, however, once more, proven to be limited [44].

aReaction temperature (°C), reaction time (h), catalyst loading (wt.%), methanol/oil molar ratio, Table 1: Various biowaste-derived solid bifunctional catalysts for the production of biodiesel.

Summary and Concluding Remarks

The synthesis of biodiesel from edible and, more critically moving ahead, non-edible oils has shown significant promise for solid, readily reusable catalysts. Bifunctionalized analogues compared to the monofunctionalized catalyst enhance the selectivity for the intended product(s) and make it easier to catalyze the transformation of more complicated substrates and mixtures in one-pot processes. They also make the product-catalyst separation more effective. These elements work together to produce a variety of economic, safety, and sustainability advantages. Here, current developments in the use of solid bifunctional catalysts with acid-base and Lewis-Brnsted functionalities to synthesise the increasingly attractive fuel/additive biodiesel from sustainable biomass, notably from inedible forms thereof, are reviewed. The important conclusions may be summed up as follows:

• The generation of biodiesel from non-edible oil is an extremely urgent goal since it would resolve the food vs. fuel conflict that affects most of the existing state-of-the- art.
• Furthermore, these catalysts would use readily accessible and inexpensive feedstocks, which would have a negative structural impact on production costs.

• To assist the conversion of TAGs and FFAs, it is extremely desirable to produce a solid catalyst that is affordable, stable, metal-free, sustainable, renewable, and effective.
• Bifunctional catalysts are thought to hold potential for biodiesel generation from non- edible oils, where high FFA concentration is frequently a problem. In contrast, the employment of single-functionality solid catalysts necessitates time-consuming, sometimes expensive two-step methods.
• The improvement of the solid catalyst’s surface hydrophobicity promotes FAME anti- hydrolysis activity and prevents water molecules from reagents or byproducts of FFA esterification from interacting with the catalyst’s active sites, boosting catalytic stability and reusability.
• Due to the simplicity of quick, quantitative separation of the catalyst from the reaction mixture, adding magnetic behaviour to solid bifunctional catalysts improves durability and reusability. While this is already a characteristic of research on, for instance, metal oxide catalysts, it has not yet been established in the field of biowaste-based bifunctional catalysts, where it may be a great approach to assure catalyst recycling.
• The development of bio-waste-derived bifunctional solid catalysts for the generation of biodiesel is a field that is expanding because it uses components that are easily accessible, renewable, non-toxic, and biodegradable. It is tempting to be able to maximise process cost- effectiveness, and concerns with trash disposal are eliminated by reusing bio-waste. However, the area of catalyst stability needs to develop before problems like magnetic cyclability can be looked into. The current state-of-the-art falls behind that of inorganic catalysts in that, as a result of active site degradation, product yields quickly decline with catalyst reuse. catalyst reuse. This now lags behind inorganic catalysts in certain respects; a common aspect of the state-of-the-art is that product yields rapidly decline with catalyst reuse owing to active site degradation.