Adsorbents : fundamentals and applications

1 Introductory Remarks 1 1.1. Equilibrium Separation and Kinetic Separation / 2 1.2. Commercial Sorbents and Applications / 3 1.3. New Sorbents and Future Applications / 6 References / 7 2 Fundamental Factors for Designing Adsorbent 8 2.1. Potential Energies for Adsorption / 8 2.2. Heat of Adsorption / 10 2.3. Effects of Adsorbate Properties on Adsorption: Polarizability (α), Dipole Moment (μ), and Quadrupole Moment (Q) / 11 2.4. Basic Considerations for Sorbent Design / 12 2.4.1. Polarizability (α), Electronic Charge (q), and van der Waals Radius (r) / 12 2.4.2. Pore Size and Geometry / 13 References / 16 3 Sorbent Selection: Equilibrium Isotherms, Diffusion, Cyclic Processes, and Sorbent Selection Criteria 17 3.1. Equilibrium Isotherms and Diffusion / 18 3.1.1. Langmuir Isotherms for Single and Mixed Gases / 18 3.1.2. Potential Theory Isotherms for Single and Mixed Gases / 20 3.1.3. Ideal Adsorbed Solution Theory for Mixture and Similarities with Langmuir and Potential Theories / 22 v vi CONTENTS 3.1.4. Diffusion in Micropores: Concentration Dependence and Predicting Mixed Diffusivities / 23 3.2. Temperature Swing Adsorption and Pressure Swing Adsorption / 27 3.2.1. Temperature Swing Adsorption / 28 3.2.2. Pressure Swing Adsorption / 30 3.3. Simple Criteria for Sorbent Selection / 40 References / 49 4 Pore Size Distribution 54 4.1. The Kelvin Equation / 54 4.2. Horv´ath–Kawazoe Approach / 55 4.2.1. The Original HK Slit-Shaped Pore Model / 57 4.2.2. Modified HK Model for Slit-Shaped Pores / 60 4.2.3. Modified Model for Cylindrical Pores / 68 4.3. The Integral Equation Approach / 74 References / 76 5 Activated Carbon 79 5.1. Formation and Manufacture of Activated Carbon / 79 5.2. Pore Structure and Standard Tests for Activated Carbon / 82 5.3. General Adsorption Properties / 84 5.4. Surface Chemistry and Its Effects on Adsorption / 86 5.4.1. Effects of Surface Functionalities on Gas Adsorption / 89 5.5. Adsorption from Solution and Effects of Surface Functionalities / 92 5.5.1. Adsorption from Dilute Solution (Particularly Phenols) / 93 5.5.2. Effects of Surface Functionalities on Adsorption / 99 5.6. Activated Carbon Fibers / 104 5.6.1. Adsorption Isotherms / 109 5.7. Carbon Molecular Sieves / 109 5.7.1. Carbon Deposition Step / 114 5.7.2. Kinetic Separation: Isotherms and Diffusivities / 115 5.7.3. Carbon Molecular Sieve Membranes / 117 References / 123 CONTENTS vii 6 Silica Gel, MCM, and Activated Alumina 131 6.1. Silica Gels: Preparation and General Properties / 131 6.2. Surface Chemistry of Silicas: The Silanol Groups / 134 6.3. The Silanol Number (OH/nm−1) / 135 6.4. MCM-41 / 139 6.5. Chemical Modification of Silicas and Molecular Imprinting / 141 6.6. Activated Alumina / 146 6.7. Activated Alumina as Special Sorbents / 150 References / 154 7 Zeolites and Molecular Sieves 157 7.1. Zeolite Types A, X, and Y / 158 7.1.1. Structure and Cation Sites of Type A Zeolite / 158 7.1.2. Structure and Cation Sites of Types X and Y Zeolites / 160 7.1.3. Examples of Molecular Sieving / 161 7.2. Zeolites and Molecular Sieves: Synthesis and Molecular Sieving Properties / 164 7.2.1. Synthesis of Zeolites A, X, and Y / 164 7.2.2. Organic Additives (Templates) in Synthesis of Zeolites and Molecular Sieves / 165 7.3. Unique Adsorption Properties: Anionic Oxygens and Isolated Cations / 173 7.4. Interactions of Adsorbate with Cations: Effects of Cation Site, Charge, and Ionic Radius / 175 7.4.1. Cation Sites / 175 7.4.2. Effects of Cation Sites on Adsorption / 180 7.4.3. Effects of Cation Charge and Ionic Radius / 183 References / 187 8 π-Complexation Sorbents and Applications 191 8.1. Preparation of Three Types of Sorbents / 192 8.1.1. Supported Monolayer Salts / 193 8.1.2. Ion-Exchanged Zeolites / 197 8.1.3. Ion-Exchanged Resins / 201 8.2. Molecular Orbital Theory Calculations / 202 8.2.1. Molecular Orbital Theory—Electronic Structure Methods / 202 8.2.2. Semi-Empirical Methods / 203 viii CONTENTS 8.2.3. Density Functional Theory Methods / 203 8.2.4. Ab Initio Methods / 205 8.2.5. Basis Set / 204 8.2.6. Effective Core Potentials / 205 8.2.7. Model Chemistry and Molecular Systems / 206 8.2.8. Natural Bond Orbital / 207 8.2.9. Adsorption Bond Energy Calculation / 208 8.3. Nature of π-Complexation Bonding / 208 8.3.1. Understanding π-Complexation Bond through Molecular Orbital Theory / 209 8.3.2. π-Complexation Bonds with Different Cations / 212 8.3.3. Effects of Different Anions and Substrates / 213 8.4. Bulk Separations by π-Complexation / 216 8.4.1. Deactivation of π-Complexation Sorbents / 216 8.4.2. CO Separation by π-Complexation / 216 8.4.3. Olefin/Paraffin Separations / 219 8.4.4. Aromatics/Aliphatics Separation / 220 8.4.5. Possible Sorbents for Simulated Moving-Bed Applications / 222 8.5. Purification by π-Complexation / 223 8.5.1. Removal of Dienes from Olefins / 224 8.5.2. Removal of Aromatics from Aliphatics / 226 References / 227 9 Carbon Nanotubes, Pillared Clays, and Polymeric Resins 231 9.1. Carbon Nanotubes / 231 9.1.1. Catalytic Decomposition / 233 9.1.2. Arc Discharge and Laser Vaporization / 241 9.1.3. Adsorption Properties of Carbon Nanotubes / 243 9.2. Pillared Clays / 253 9.2.1. Syntheses of PILCs / 253 9.2.2. Micropore Size Distribution / 256 9.2.3. Cation Exchange Capacity / 258 9.2.4. Adsorption Properties / 260 9.2.5. PILC and Acid-Treated Clay as Supports / 262 9.3. Polymeric Resins / 264 9.3.1. Pore Structure, Surface Properties, and Applications / 266 CONTENTS ix 9.3.2. Comparisons of Resins and Activated Carbon / 269 9.3.3. Mechanism of Sorption and Gas-Phase Applications / 271 References / 273 10 Sorbents for Applications 280 10.1. Air Separation / 280 10.1.1. 5A and 13X Zeolites / 282 10.1.2. Li-LSX Zeolite / 283 10.1.3. Type X Zeolite with Alkaline Earth Ions / 288 10.1.4. LSX Zeolite Containing Ag (AgLiLSX) / 289 10.1.5. Oxygen-Selective Sorbents / 296 10.2. Hydrogen Purification / 303 10.3. Hydrogen Storage / 305 10.3.1. Metal Hydrides / 306 10.3.2. Carbon Nanotubes / 308 10.4. Methane Storage / 321 10.5. Olefin/Paraffin Separations / 326 10.5.1. Sorbents / 326 10.5.2. PSA Separations / 328 10.5.3. Other Sorbents / 334 10.6. Nitrogen/Methane Separation / 334 10.6.1. Clinoptilolites / 336 10.6.2. ETS-4 / 341 10.6.3. PSA Simulation: Comparison of Sorbents / 344 10.7. Desulfurization of Transportation Fuels / 344 10.7.1. Fuel and Sulfur Compositions / 347 10.7.2. Sorbents Studied or Used / 349 10.7.3. π-Complexation Sorbents / 350 10.8. Removal of Aromatics from Fuels / 361 10.9. NOx Removal / 363 References / 371 Author Index 383 Subject Index 403