CONVERSION OF GLYCEROL TO PROPANEDIOL USING HETEROGENEOUS CATALYSTS

Abstract

In the last two decades, considerable attention has been paid for the value addition of excess glycerol (~10 wt.%), a co-product of transesterification process, for the economic feasibility of biodiesel industry. In the recent literature, several routes have been proposed for the glycerol value addition process such as esterification, oxidation, hydrogenolysis, steam reforming etc. Conversion of glycerol to 1,2-propanediol (1,2-PDO) is one of the promising routes among all proposed glycerol conversion processes. 1,2-PDO is an essential commodity chemical used as a monomer for the production of polyester resin, antifreeze agent, paints, food additives, cosmetics, and pharmaceuticals. Globally, ~2.2 million tons of 1,2-PDO is produced per year and its increasing demand rate is 4% per annum. The formation of 1,2-PDO from renewable glycerol is an eco-friendly process compared to the existing commercial process. Hydrogenolysis of glycerol is the splitting of C-C and/or C-O bond of a glycerol molecule with the concurrent addition of hydrogen at elevated temperature and pressure. It has been observed that the transformation of glycerol to 1,2-PDO requires a suitable catalyst with the capability to cleave the C-O bond selectively. Presence of acidic and/or basic sites on the catalyst surface favored the dehydration of glycerol and the active metal sites are necessary for hydrogenation of glycerol dehydrated product to produce 1,2-PDO. In this thesis, Cu, Ni, Zn, Fe, Co monometallic as well as bimetallic (Cu:Ni, Cu:Zn, Co:Zn, Cu:Fe, Co:Fe) catalysts supported on γ-Al2O3, MgO, BaO2, La2O3 and MgO-La2O3 and also Cu-Zn-Mg-Al-O catalysts derived from a layered double hydroxides (LDHs) precursor were developed and their performances were evaluated for selective transformation of glycerol to 1,2- PDO in an autoclave reactor in liquid phase. Various techniques were employed to characterize the developed catalysts such as specific surface area (BET), X-ray Diffraction (XRD), NH3- temperature programmed desorption (TPD), H2-temperature programmed reduction (TPR), CO2- temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), atomic absorption spectroscopy (AAS). Further, to enhance the selectivity/yield of 1,2- PDO, the reaction parameters were optimized experimentally by performing the experiment at the different reaction temperature (170-220 oC), pressure (3-6 MPa), glycerol concentration (10- 40 wt.%) and catalyst loading (2-10 wt.%), respectively. Stability and reusability of Cu-Zn-Mg- Al-O LDHs, Cu:Zn(4:1)/MgO and Cu:Zn(4:1)/MgO-La2O3 catalysts were also performed. ii Hydrogenolysis activity of 20 wt.% Cu/γ-Al2O3, Ni/γ-Al2O3 and Cu-Ni(1:1)/γ-Al2O3 catalysts were evaluated. Hydrogenolysis results demonstrated that the bimetallic Cu-Ni(1:1)/γ- Al2O3 catalyst was the most active and selective to 1,2-PDO in comparison to other monometallic catalysts synthesized. Maximum glycerol conversion of 70.3% with 85.6% selectivity towards 1,2-PDO was obtained in presence of this bimetallic catalyst. The higher catalytic activity of bimetallic Cu-Ni(1:1)/γ- Al2O3 catalyst was due to the presence of a new mixed oxide phase as confirmed by XRD, smaller crystallite size, highest acidity and highest degree of reduction. Bi-functional Cu-Mg-Al-O and Cu-Zn-Mg-Al-O catalysts derived from LDHs precursor were synthesized by urea hydrolysis method and the performances of these catalysts were evaluated. Ball-flower shaped particles were identified in SEM images of all the catalysts and a well-defined layered structure of solid lamella has also been identified. Very high catalytic activity (> 85%) with > 90% selectivity towards 1,2-PDO was achieved in presence of all the LDHs catalysts synthesized. Cu-Zn-Mg-Al-O catalyst was the most active which showed ~98% conversion with very high (~92%) selectivity towards 1,2-PDO at 210 oC, at 4.5 MPa pressure after 12 h of reaction. The synergic interaction between the copper and ZnO on LDHs support, higher reducibility, smaller copper particle size, and well-developed curved platelet structure were solely responsible for the better catalytic activity of Cu-Zn-Mg-Al-O catalyst as compared to Cu-Mg-Al-O catalyst. Further, it was found that the inclusion of small amounts of NaOH as an additive in the reaction mixture significantly improved the selectivity (~94.3%) to 1,2-PDO. However, recycle study showed severe deactivation of the catalyst in the successive reuse. Further, a series of monometallic (Cu, Co, Zn, and Fe) catalysts supported on MgO with 35% metal loading were synthesized and their performance was examined. Cu and Co metals were found to be more active as compared to Zn and Fe. Although the selectivity towards 1,2-PDO was ~100% over Zn and Fe based catalyst. It was also found that 35% Cu/MgO catalyst exhibited 96.6% conversion with 92.6% selectivity towards 1,2-PDO at 210 °C, 4.5 MPa pressure after 12 h of reaction. Presence of acidic and/or basic sites, bi-functional nature, high metallic surface area (4.4 m2.g-1), lower copper crystallite size (~28 nm) were the main reasons behind the high catalytic activity and 1,2-PDO selectivity. Further, Zn and Fe were incorporated with Cu and Co to increase the overall selectivity and/or the yield of 1,2-PDO. A series of MgO supported bimetallic catalysts were also prepared and their performances were verified. This study aims to optimize the reaction parameters to maximize glycerol conversion and 1,2-PDO selectivity/yield, catalyst stability, and reusability. Among all the catalyst examined, Cu:Zn(4:1)/MgO catalyst exhibited a maximum of 98.4% conversion with 93.4% selectivity iii towards 1,2-PDO at 210 oC and 4.5 MPa pressure. The addition of zinc into the Cu/MgO catalyst increased the degree of reduction of Cu:Zn(4:1)/MgO catalyst and also lowered its reduction temperature significantly. Zinc enhanced basicity and also the reducibility of catalyst by hydrogen spillover effect. NH3 –TPD and CO2 –TPD results revealed that copper introduced the acidic sites, whereas, Zn introduced the additional basicity in the catalyst. Very high catalytic activity and selectivity over Cu:Zn(4:1)/MgO catalyst was due to the presence of an appropriate combination of acidic (2.13 mmol NH3 gcat-1) and/or basic (1.81 mmol CO2 gcat-1) sites concentration on the catalyst, high hydrogen consumption (6.7 mmol gcat-1), very high degree of reduction (91.7%), and the presence of small average copper particle size (37.1 nm) in the reduced catalyst. Catalyst stability and reusability experiments were performed up to 3rd cycle. Glycerol conversion was found to reduce by ~ 14% after 3rd successive reuse. However, the selectivity towards 1,2-PDO was almost remained same (~94%). The effects of various basic supports (La2O3, CaO, BaO2, and MgO-La2O3) on the performance of Cu-Zn bimetallic catalysts were investigated. The best catalytic activity was obtained in presence of Cu:Zn(4:1)/MgO-La2O3 catalyst, which showed 100% conversion with 93% selectivity towards 1,2-PDO at 210 °C and at 4.5 MPa pressure. After the addition of La2O3 to Cu:Zn(4:1)/MgO catalyst, average crystallite size of the catalyst was decreased from 33.2 to 27.8 nm and the degree of reduction of the catalyst was increased significantly from ~92% to 97%. Higher activity of Cu:Zn(4:1)/MgO-La2O3 catalyst was associated with the presence of smallest average crystallite size (27.8 nm), higher acidic strength (2.12 mmol.gcat-1)/basic strength (1.87 mmol.gcat-1) and higher degree of reduction (97%). To understand the intrinsic kinetic behaviour of glycerol hydrogenolysis reaction, kinetic studies were performed in presence of Cu:Ni(1:1)/γ-Al2O3, Cu:Zn(4:1)/MgO and Cu:Zn(4:1)/MgO-La2O3 catalyst, respectively. Over Cu:Ni(1:1)/γ-Al2O3 catalyst, the kinetic experiments were conducted at the different reaction temperature (180-220 ºC) and pressure (3- 6 MPa), respectively. A series reaction scheme of glycerol conversion to 1,2-PDO followed by the hydrogenolysis of 1,2-PDO to propanol was considered to develop the kinetic model. To develop the kinetic model, 1,2-PDO and propanol (1-PO + 2-PO) were considered as the main reaction products. A more realistic heterogeneous kinetic model based on combined Langmuir– Hinshelwood-Hougen-Watson (LHHW) and an Eley-Rideal (ER) approach was developed. The calculated activation energy was found to be 70.5 kJ.mol-1 for the conversion of glycerol to 1,2- PDO and it was 79.5 kJ.mol-1 for the production of PO from 1,2-PDO, respectively. The parity plot of the experimental and model simulated concentration of reactant and products were fitted iv very well. Further, in presence of Cu:Zn(4:1)/MgO-La2O3 and Cu:Zn(4:1)/MgO catalyst, a simple reaction scheme from glycerol to 1,2-PDO was considered and a kinetic model based on Langmuir–Hinshelwood-Hougen-Watson (LHHW) approach was also developed. Finally, the overall economic feasibility of the liquid phase hydrogenolysis of glycerol to 1,2-PDO was carried out in presence of Cu:Zn(4:1)/MgO catalyst which showed best activity among all the catalysts. Per day 60 kg production of 1,2-PDO was assumed to be the basis of calculation. Production cost included total fixed cost and operating cost. Annual income tax rate (φ) was considered as 30% of profit. Operating cost included material cost, energy cost, reaction cost, and catalyst separation cost and product purification cost. Production cost per kilograms of 1,2-PDO was estimated to be Rs. 1502.4/-, whereas the market price of 1,2 PDO is Rs. 4437.5/- per kg (Alfa aesar = 99.5% 1,2 PDO, Item no 030948). The selling price of one kilogram 1,2- PDO was considered to be Rs. 2000/-. Based on that, the return on investment after taxes (ROI) was found to be 82.82% and the payback period was calculated as 1.12 years. Therefore, the economic analysis suggested that the production of 1,2-PDO from renewable glycerol is extremely profitable and it is very promising for commercial application.