Table Of Contents

Everyt-Jan topic

Full Professor, van ‘t Hoff Institute for Molecular Sciences

Prof. dr. Evert Jan Meijer

Computation Chemistry Group.

Adsorption and Diffusion in Nanoporous materials

Associate Professor, van ‘t Hoff Institute for Molecular Sciences

Dr. David Dubbeldam

Computation Chemistry Group since 2009. Lead-author of the iRASPA and RASPA molecular simulations codes.

Ordered crystalline porous materials offer the potential for selective adsorption by exploiting differences in molecular configurations. Zeolites are readily available, very stable, and cheap. A zeolite should have the right combination of high adsorption selectivity, combined with an adequate capacity for use in traditionally used fixed-bed devices. Recently, new classes of nanoporous materials have been designed that have good stability, high void volumes, and well-defined tailorable cavities of uniform size. Example are metal–organic frameworks (MOFs), covalent organic frameworks (COFs), and zeolitic imidazolate frameworks (ZIFs). These novel materials possess almost unlimited structural variety because of the many combinations of building blocks that can be imagined. MOFs are synthesized under solvo- or hydrothermal conditions in the presence of a base. During synthesis the building blocks self-assemble into crystalline materials that, after evacuation, can find applications in adsorption separations, air purification, gas storage, chemical sensing, and catalysis.

Ordered crystalline porous materials offer the potential for selective adsorption by exploiting differences in molecular configurations. Zeolites are readily available, very stable, and cheap. A zeolite should have the right combination of high adsorption selectivity, combined with an adequate capacity for use in traditionally used fixed-bed devices. Recently, new classes of nanoporous materials have been designed that have good stability, high void volumes, and well-defined tailorable cavities of uniform size. Example are metal–organic frameworks (MOFs), covalent organic frameworks (COFs), and zeolitic imidazolate frameworks (ZIFs). These novel materials possess almost unlimited structural variety because of the many combinations of building blocks that can be imagined. MOFs are synthesized under solvo- or hydrothermal conditions in the presence of a base. During synthesis the building blocks self-assemble into crystalline materials that, after evacuation, can find applications in adsorption separations, air purification, gas storage, chemical sensing, and catalysis.

Regenerative adsorber systems can operate continuously by using:

  • multiple fixed beds of an adsorbent; 
  • fluidized bed contactors with separate adsorption and desorption vessels; or
  • rotary bed adsorbents that cycle continuously between adsorption and desorption operations.

Pressure swing adsorption (PSA) is an industrial separation technique at atmospheric temperature and high pressure. In practice a batch setup with various vessels in parallel is used; some are in adsorption mode, while others are in regeneration mode. In pressure swing the regeneration is done at low pressure and temperature, while thermal swing adsorption uses high temperature and pressure. PSA is less sensitive to diffusional effects and largely dominated by adsorption. This is in contrast to membrane technology, where the difference in mobility of species leads to separation. The difficulty of screening lies in determining what “most suitable” means. The economics of PSA is crucially dependent on adsorption selectivity. High adsorption selectivity leads to smaller equipment volumes and lower capital and energy requirements. However, high selectivity means a strong affinity of the most adsorbing species and therefore higher regeneration costs. Besides selectivity, it is also the capacity of the material that determines the efficiency of a separation device. In pressure swing adsorbers, high adsorption capacities are desirable because they result in lower frequencies in the PSA regeneration cycles. Other crucially important factors are cost of the material and stability (especially temperature and water stability); once the MOF is evacuated it is likely to become air and moisture sensitive. A reliable screening methodology should take all these considerations into account.

Pressure swing adsorption (PSA) is an industrial separation technique at atmospheric temperature and high pressure. In practice a batch setup with various vessels in parallel is used; some are in adsorption mode, while others are in regeneration mode. In pressure swing the regeneration is done at low pressure and temperature, while thermal swing adsorption uses high temperature and pressure. PSA is less sensitive to diffusional effects and largely dominated by adsorption. This is in contrast to membrane technology, where the difference in mobility of species leads to separation. The difficulty of screening lies in determining what “most suitable” means. The economics of PSA is crucially dependent on adsorption selectivity. High adsorption selectivity leads to smaller equipment volumes and lower capital and energy requirements. However, high selectivity means a strong affinity of the most adsorbing species and therefore higher regeneration costs. Besides selectivity, it is also the capacity of the material that determines the efficiency of a separation device. In pressure swing adsorbers, high adsorption capacities are desirable because they result in lower frequencies in the PSA regeneration cycles. Other crucially important factors are cost of the material and stability (especially temperature and water stability); once the MOF is evacuated it is likely to become air and moisture sensitive. A reliable screening methodology should take all these considerations into account.