Uhlíkaté monolity s hierarchickou porozitou ze zkapalněné biomasy

Mgr. Arjeta KRYEZIU

Doctoral thesis

Hierarchically porous carbon monoliths based on liquefied biomass

Abstract:

Cellulose is a renewable resource for the production of advanced carbonaceous materials for various applications. In addition to direct carbonization, attention has recently been paid to the preparation of porous carbons from liquid cellulose-based precursors. Possible pathways of cellulose conversion to liquid state suitable for porous carbons preparation are summarized in the Chapter 1. Hydrothermal liquefaction leading to liquid mixtures of low-molecular weight organics is described in details together with less common decomposition techniques (microwave or ultrasound assisted liquefaction, decomposition in strong gravitation field). Another part is focused on dissolution of cellulose without decomposition, with special attention on dissolution of non-derivatized cellulose. For this purpose, cold alkaline, hot acids, ionic liquids or alcohols are commonly used. A simple and environmentally friendly route for the transformation of cellulose to porous carbon monoliths was developed and described in the Chapter 2. The procedure is based on freezing the cellulose solution in aqueous NaOH (?10 °C), followed by a direct regeneration of the frozen material in an ethanol bath and washing the alkaline with water. After atmospheric drying, the obtained cellulose monoliths composed mainly of cellulose II polymorph showed considerable macroporosity related to an ice-templating effect. During carbonization (500-900 °C, carbon yield > 20 %) the macroporosity (?1-100 ?m) was preserved together with monolithic character. The resulting monolithic carbons also showed significant microporosity, with micropore volume up to 0.25 cm3 g?1, depending on carbonization temperature. The influence of NaOH on the pyrolysis process of cellulose and properties of resulting carbons was investigated in Chapter 3. The mixture of microcrystalline cellulose (MCC) with NaOH and cellulosic gel (CMG) prepared by the same procedure as described in Chapter 2 (but without washing out NaOH) were studied. During carbonization from 500 up to 900 °C with increasing heating rate (from 2 up to 20 K min?1), carbon yield of MCC and MCC/NaOH was decreased to 18 %, whereas CMG and CMG/NaOH carbon yield was around 25 %. Kinetic calculations of MCC pyrolysis by using Starink method show fluctuations of activation energy at the end of reaction, where for CMG the Ea is stable with increasing conversion. Presence of NaOH leads to higher reactivity at low temperature. Non-linear regression calculations showed that during pyrolysis, CMG material requires less activation energy then MCC to start the decomposition reaction (from 227 to 97 kJ mol?1). In the case where alkalis are present, Ea is decreased at low conversion levels. Synthesis of carbon monoliths from liquefied cellulose was developed with the additional step of adding solution of metallic salts (Cu(CH3COO)2?4H2O, Mn(CH3COO)2?4H2O and FeSO4?7H2O) into dissolved cellulose solution (Chapter 4). The procedure is based on mixing dissolved cellulose solution and solution of metallic salts with metal concentration of 2 and 5 mmol, followed by regeneration of the material in ethanol and washing out the remaining NaOH with water. After drying, TG analysis of CMG and metal-based cellulosic gels indicates that materials contain residues of possible metal oxides compounds (Fe2O3/Fe3O4, Mn3O4, and CuO/Cu2O). During carbonization (at 500 and 800 °C) the macroporosity and monolithic shape was preserved. With carbonization at 800 °C, iron (III) oxide was reduced to iron carbide and metallic iron, manganese oxide maintained its oxidized form, whereas copper was reduced at the low temperatures to metallic copper. Electrochemical study of metal-based carbons with Fe, Mn and Cu showed capacitance values of 150, 133 and 83 F g-1. Due to higher graphitization of samples with Fe and the presence of mesopores (45 m2 g-1), electrochemical performance is improved.

Abstract:

Cellulose is a renewable resource for the production of advanced carbonaceous materials for various applications. In addition to direct carbonization, attention has recently been paid to the preparation of porous carbons from liquid cellulose-based precursors. Possible pathways of cellulose conversion to liquid state suitable for porous carbons preparation are summarized in the Chapter 1. Hydrothermal liquefaction leading to liquid mixtures of low-molecular weight organics is described in details together with less common decomposition techniques (microwave or ultrasound assisted liquefaction, decomposition in strong gravitation field). Another part is focused on dissolution of cellulose without decomposition, with special attention on dissolution of non-derivatized cellulose. For this purpose, cold alkaline, hot acids, ionic liquids or alcohols are commonly used. A simple and environmentally friendly route for the transformation of cellulose to porous carbon monoliths was developed and described in the Chapter 2. The procedure is based on freezing the cellulose solution in aqueous NaOH (?10 °C), followed by a direct regeneration of the frozen material in an ethanol bath and washing the alkaline with water. After atmospheric drying, the obtained cellulose monoliths composed mainly of cellulose II polymorph showed considerable macroporosity related to an ice-templating effect. During carbonization (500-900 °C, carbon yield > 20 %) the macroporosity (?1-100 ?m) was preserved together with monolithic character. The resulting monolithic carbons also showed significant microporosity, with micropore volume up to 0.25 cm3 g?1, depending on carbonization temperature. The influence of NaOH on the pyrolysis process of cellulose and properties of resulting carbons was investigated in Chapter 3. The mixture of microcrystalline cellulose (MCC) with NaOH and cellulosic gel (CMG) prepared by the same procedure as described in Chapter 2 (but without washing out NaOH) were studied. During carbonization from 500 up to 900 °C with increasing heating rate (from 2 up to 20 K min?1), carbon yield of MCC and MCC/NaOH was decreased to 18 %, whereas CMG and CMG/NaOH carbon yield was around 25 %. Kinetic calculations of MCC pyrolysis by using Starink method show fluctuations of activation energy at the end of reaction, where for CMG the Ea is stable with increasing conversion. Presence of NaOH leads to higher reactivity at low temperature. Non-linear regression calculations showed that during pyrolysis, CMG material requires less activation energy then MCC to start the decomposition reaction (from 227 to 97 kJ mol?1). In the case where alkalis are present, Ea is decreased at low conversion levels. Synthesis of carbon monoliths from liquefied cellulose was developed with the additional step of adding solution of metallic salts (Cu(CH3COO)2?4H2O, Mn(CH3COO)2?4H2O and FeSO4?7H2O) into dissolved cellulose solution (Chapter 4). The procedure is based on mixing dissolved cellulose solution and solution of metallic salts with metal concentration of 2 and 5 mmol, followed by regeneration of the material in ethanol and washing out the remaining NaOH with water. After drying, TG analysis of CMG and metal-based cellulosic gels indicates that materials contain residues of possible metal oxides compounds (Fe2O3/Fe3O4, Mn3O4, and CuO/Cu2O). During carbonization (at 500 and 800 °C) the macroporosity and monolithic shape was preserved. With carbonization at 800 °C, iron (III) oxide was reduced to iron carbide and metallic iron, manganese oxide maintained its oxidized form, whereas copper was reduced at the low temperatures to metallic copper. Electrochemical study of metal-based carbons with Fe, Mn and Cu showed capacitance values of 150, 133 and 83 F g-1. Due to higher graphitization of samples with Fe and the presence of mesopores (45 m2 g-1), electrochemical performance is improved.

Keywords

cellulose, liquefaction, dissolution, regeneration, porosity, monoliths, carbon, kinetic, activation energy, metals

Language used: English

Date on which the thesis was submitted / produced: 4. 7. 2022

Accessible from:: 31. 12. 2999

Thesis defence

• Date of defence: 26. 8. 2022
• Supervisor: doc. RNDr. Václav Slovák, Ph.D., doc. RNDr. Václav Slovák, Ph.D.

University of Ostrava

Faculty of Science

Doctoral programme / field:
Analytical Chemistry / Analytical chemistry of Heterogenous Systems