Synthesis of fly ash-based zeolites for use as catalysts in the transesterification of waste-derived maggot oil for biodiesel production

Abstract

From nearly a century, biodiesel production has demonstrated its potential as an alternative to petroleum diesel mainly due to environmental and energy security. The principal biodiesel production route from different oil feedstocks is dependent on catalysts, of which solid catalysts have attracted attention for greener production. Fly ash solid waste derived from coal power station is identified as a suitable and cheaper catalyst feedstock. In spite of escalating market, biodiesel industry has been faced with the challenge of overall production cost, owing to conventional vegetable oil feedstocks high cost of most solid catalysts derived from conventional feedstocks and synthesis routes. This study therefore introduces and explores the use of maggot oil as a novel waste-derived feedstock for biodiesel production, aims to develop cheaper solid catalysts by various routes using coal fly ash, in bid to make biodiesel production more economical. The characterisation of physicochemical properties was employed on the as-received oil feedstock. These include free fatty acid (FFA) analysis via a gas chromatogram (GC); titrimetric determination of acid value (AV) and saponification value (SV); test for Iodine value (IV) by Wij's method; density, viscosity and refractive index using appropriate equipment. Further oil feedstock characteristic was carried out via screening transesterification tests using conventional homogeneous catalysts. Maggot oil revealed to contain triglyceride characteristic components and was found to be highly saturated (SFAs) with lauric acid in major proportion (40 %). The oil exhibited a high acid value ranging between 7-10 mg KOH/g oil, a low-density value (0.883 g/cm3) and viscosity (43.16 mm2/s) suitable for biodiesel feedstock characteristics. An iodine value of 44.27 g I2/g of the oil was reported as reflective of oil good reactive characteristic. Based on these, maggot oil demonstrated potential by feedstock converting to fatty acid methyl esters (FAMEs) with high yield (65.50-70.02 %) and quality-compliant biodiesel over homogeneous catalysts. Catalyst characterisation employed on the prepared and modified catalyst samples included X-Ray diffraction (XRD), Fourier transform infrared (FT-IR), Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), and the Brunauer–Emmett–Teller (BET) coupled with the Barret-Joyner-Halenda (BJH) measurement. In exception to BET, the above characterisations were also conducted on the as-received Arnot coal fly ash. Characterisation and analytics employed for biodiesel products were as similar as those used for the feedstock oil. Coal fly ash showed inherent material characteristic in terms of mineralogical composition, structural configuration, and the material showed compositionally characteristic of class F type fly ash. Zeolite samples were hydrothermally prepared from coal fly ash via the direct method and fusion-assisted synthesis route. The effect of pre-synthesis chemical and physical parameters, followed by the actual hydrothermal synthesis time and temperature, were investigated for the direct method synthesis route. Fusion-assisted synthesis route, based on optimum conditions from the direct method, was achieved by investigation of hydrothermal synthesis time. Zeolite was successfully produced from coal fly ash by both the direct and the fusion-assisted hydrothermal method. The direct method synthesis revealed the formation of HS-zeolite from as low as 11-20 % to 100 % phase crystallinity, induced majorly by increase in hydrothermal synthesis time. Established optimum presynthesis conditions obtained were water-to-coal fly ratio mixture of 5:1, ageing at 60 ᵒC for 1.5 hours, NaOH-to-coal fly ratio of 1.2:1, and hydrothermal synthesis time of 72 hours at 140 ᵒC; including an extended synthesis time of 144 hour at 100 ᵒC. The resultant sample (HS-72) demonstrated a considerably purer phase HS zeolite by 85 %, and this was extended to 91 % with the decreased temperature at extended synthesis time (HS-144(100ᵒC)).. The fusion-assisted synthesis route comparably resulted to samples of low phase crystallinity range (< 30 %), with 52 % optimum crystallinity and 80 % phase purity HS zeolite obtained at 144-hour extended synthesis time at 100ᵒC. The derived sample at the optimum condition (HSF-144) exhibited similar hexagonal cubic crystal morphology as those obtained via the direct method, and showed a comparably higher surface area and pore volume characteristic (44.98 m2/g and 0.148 cm3/g). The direct method synthesis resulted as a more energy efficiency approach, thus, was deemed as an economic and more feasible route for upscale synthesis of overall high quality HS zeolite. Selected prepared catalysts were modified by ion exchange method using potassium hydroxide (KOH) and by bifunctional method, to boost their characteristic properties and activity in biodiesel production in this work. Sample modification by both methods, demonstrated no effect on the phase identity of HS, neither considerably impacted on its crystal morphology. Both modification methods, caused a decrease in sample crystallinity to below 30 %, decrease in crystal sizes, and accounted for minor induction of basic properties. The catalytic activity of the synthesised HS zeolite samples on the transesterification of maggot oil, was investigated on a batch production unit at fixed pre-optimised conditions (1.5 hour, 60 ᵒC, 15:1 MeOH/oil ratio). Activity tests using the prepared samples were also carried out on sunflower oil for comparison. The above was followed by process optimisation of maggot oil transesterification using the design of central composite (CCD) for research methodology studies (RSM), where the influence of methanol-to-oil ratio, agitation rate and reaction time on the yield of biodiesel, was investigated. HS zeolite obtained via the fusion hydrothermal method led to high activity with regard to a higher biodiesel yield (84.10 %), a higher FAME content (64.95 %) and an overall more compliant biodiesel quality, compared to catalyst samples obtained via the direct method. The high activity majorly owed to larger surface area, higher mesoporosity and a possibly better acidity shown by the fusion-assisted HS zeolite catalyst. The catalyst showed a greater activity in the transesterification of sunflower oil (biodiesel yield, 89.70 %; FAME content, 90.93 %), and gave the RSM optimum conditions of MeOH/oil ratio of 6:1, agitation rate of 400 rpm and reaction time of 1.5 hour. Under these conditions, maggot oil-derived biodiesel yield and FAME content were as highest as 85.47 % and 84.76 % respectively, and the obtained biodiesel complied with the standard specifications. Kinetic study for the transesterification of maggot oil was conducted to predict the yield of biodiesel with reaction time, using the pseudo first-order mechanism. The mechanism held true for reaction time below 1.5 hour and revealed that a higher biodiesel yield could have been achieved from a shorter reaction time of 0.25 hour. A liner kinetic model resulted and was associated with a considerable reaction rate constant of 1.15×10-3 min-1 and gradual increase in the rate of reaction (5.37×10-6 - 6.13×10-6 mol/cm3.min) with an increase in reaction time. This study has guaranteed feasibility for upscale production of HS as a novel heterogeneous catalyst and an economic approach of biodiesel production using both coal fly ash and maggot oil as industrial waste-derived feedstocks. The study recommends improving major qualities associated with enhanced catalyst activity of produced zeolites and further RSM optimisation of biodiesel production to enhance the FAME yield of biodiesel.