Hydrothermal synthesis of K2CO3-promoted hydrotalcite based on hydroxide-form precursors for high temperature CO2 sorbent

다국어 초록 (Multilingual Abstract)

In many materials for CO2 sorption, hydrotalcite is attracting substantial attention as high temperature (200-500 °C) CO2 sorbent due to its fast sorption/ desorption kinetics and easy regenerability. However, the CO2 sorption capacity of conventional hydrotalcite is relatively low for large-scale commercial use. To enhance the CO2 sorption capacity, hydrotalcite is impregnated with K2CO3 and/or Na2CO3. Although K2CO3/Na2CO3 promoted hydrotalcite has high CO2 sorption capacity, the synthesis method takes long time and is inconvenient because hydrotalcite synthesis step and alkali metal impregnation step are separated. Conventionally, hydrotalcite is synthesized from water soluble nitrate form precursors by a co-precipitation method. Although hydrotalcite structure is easily developed by a co-precipitation method, product crystallinity is low. Moreover, product should be repeatedly washed because large amount of basic solution such as NaOH or KOH are added during the synthesis step to control the pH of solution.
In this study, K2CO3-promoted hydrotalcite was newly synthesized by a simple and eco-friendly method without a solvent-consuming washing step. Analysis based on X-ray diffraction indicated that all samples had structures of well-defined hydrotalcite crystalline and un-reacted Mg(OH)2 precursor. Moreover, K2CO3 was successfully co-precipitated in sample during the synthesis step. Morphology of hydrotalcite was confirmed by the scanning electron microscopy and showed the hexagonal plate-like structure. The analysis based on N2 adsorption data showed the decrease in surface area with increase in the amount of co-precipitated K2CO3. Thermogravimetric analysis was used to measure CO2 sorption capacity and the results revealed that CO2 sorption capacity increased by co-precipitation of K2CO3 and K2CO3 co-precipitated amount had an optimum value. The CO2 sorption isotherm was measured at 240 °C temperature and 0.05-10 bar pressure range and fitted to Langmuir isotherm model. Moreover, the regeneration ability of sorbents was assessed by sorption/desorption cycles.

목차 (Table of Contents)

• CONTENTS
• ABSTRACT ⅰ
• 국 문 요 약 ⅲ
• CONTENTS v
• LIST OF FIGURES viii
• LIST OF TABLES xii
• 1. INTRODUCTION 1
• 1.1. Global warming phenomena 1
• 1.2. Carbon capture and storage (CCS) technology 5
• 1.3. CO2 sorption technology 8
• 2. THEORETICAL BACKGROUND 9
• 2.1. Necessity of high temperature CO2 sorbent 9
• 2.1.1. Direct CO2 capture from plant off-gas 9
• 2.1.2. Sorption enhanced reaction 9
• 2.2. What makes a good CO2 sorbent? 12
• 2.3. Various high temperature CO2 sorbents 13
• 2.3.1. Calcium oxide 13
• 2.3.2. Lithium/sodium zirconate 13
• 2.3.3. Hydrotalcite 14
• 2.3.3.1. What is hydrotalcite? 14
• 2.3.3.2. Research trend 17
• 2.3.3.3. Synthesis method 17
• 3. EXPERIMENTS 20
• 3.1. New synthesis method for hydrotalcite based on hydroxide- form precursors 20
• 3.2. Characteristics 23
• 3.3.1. Composition 23
• 3.3.2. Morphology 23
• 3.3.3. Structure 23
• 3.3.4. N2 adsorption 23
• 3.3.5. CO2 sorption 24
• 4. RESULTS AND DISCUSSION 27
• 4.1. Synthesis method for hydrotalcite based on hydroxide-form precursor 27
• 4.1.1. Reaction temperature effect 27
• 4.1.2. Reaction time effect 31
• 4.1.3. Mg/Al ratio effect 34
• 4.2. Mg/Al ratio effect on CO2 sorption 37
• 4.2.1. Morphology 37
• 4.2.2. N2 adsorption 37
• 4.2.3. Structure 38
• 4.2.4. CO2 sorption 38
• 4.3. K2CO3/Al ratio & synthesis temperature effect 45
• 4.3.1. Composition 45
• 4.3.2. Morphology 46
• 4.3.3. N2 adsorption 46
• 4.3.4. Structure 47
• 4.3.5. CO2 sorption 48
• 4.4. Reconstruction effect 62
• 4.4.1. Morphology 62
• 4.4.2. N2 adsorption 62
• 4.4.3. Structure 63
• 4.4.4. CO2 sorption 63
• 5. CONCLUSIONS 69
• REFERENCES 72