Biomass Conversion: Production of Oxygenated Fuel Additives

Abstract

Etherification of glycerol using t-BuOH (tert-butyl alcohol) to produce glycerol ethers such as mono-, di-, and tri-tert-butyl and esterification of glycerol in the presence of acetic acid had been studied to produce mono, di-, and triacetins. These reactions were catalyzed by various acids (solid) to achieve maximum selectivity toward the desired products which are important fuel additives. The products obtained from these processes have commercial importance as they are used in many of industrial applications; therefore, it contributes to insure sustainability of environment and economic life of renewable energy resources. Both etherification and alcohol v/s acid reactions (esterification) of glycerol were studied thoroughly with catalysts (solid acids), and it was noticed that the esterification is a promising way to valorize glycerol with CH3COOH (acetic acid) via catalyst (solid acid). In this process, maximum selectivity toward di- and triacetin was noticed at the time of esterification compared to etherification at the same reaction conditions. Each of the products obtained from esterification have their individual importance. Cost-effective and easily synthesized solid acid catalysts were prepared and characterized using X-ray diffraction, FT-IR, SEM/EDX, NH3-TPD, BET surface, thermogravimetric analysis, and mercury porosimeter. These techniques were useful to determine surface area, morphology, particle size, thermal stability, acidity, pore size distribution, etc. The catalytic activity of the various Bronsted-acidic bentonite clay and silica supported heteropoly acid catalysts was investigated to produce fuel additives (oxygenated) by etherification of glycerol. Among all the catalysts evaluated in this work, Mont- KSF/O and 20% DTP/SiO2 catalysts were the most dynamic catalysts. The etherification process was optimized to maximize production of DTBGE and TTBGE by using Mont-KSF/O. Kinetics was studied by using 20% DTP/SiO2 catalyst. Polyvinylpyrrolidone, bentonite, and MCM-41-based heteropoly acid catalysts were studied to increase the selectivity toward triacetin and glycerol conversion via acidic esterification of glycerol. PVP-DTP catalyst was recorded as the excellent catalyst among the studied catalysts. Parameters of the reaction like temperature, amount of catalyst, substrate ratio, etc. were optimized to increase triacetin selectivity and glycerol conversion. Highest selectivity of around 54% was shown by phosphotungstic acid impregnated polyvinylpyrrolidone catalyst (PVP-DTP) in the direction of diacetin, and around 34% was toward triacetin with 100% conversion of glycerol. Phosphotungstic acid impregnated polyvinylpyrrolidone catalyst (PVP-DTP) showed minimum weight loss (0.040 mg, 1.66%) between 25 and 1000 °C due to better thermal stability of DTP on PVP. This catalyst could be utilized at higher temperature without losing its catalytic activity. Etherification and esterification processes were carried out at 110 °C, substrate ratio of 1:20, 3 wt % catalyst, and 6 h of reaction time. 20% DTP/SiO2 catalyst showed 96% glycerol conversion with excellent selectivity of 45% and 23% toward di- and tri-ether, respectively. Mont.KSF/O showed complete conversion with maximum selectivity toward DTBGE and TTBGE.