Adsorbents are amazing materials. Some types can perform the job of purification by removing contaminants from fluids (gases or liquids). Other types of adsorbents can perform the job of bulk separation of one kind of molecule from another. The properties of each adsorbent are optimized for the specific job they are intended to perform. Many books have been written on this subject. The following is a quick overview.

What are the Key Properties of Adsorbents?
The key properties of adsorbents that give them the power to do specific jobs are:
• Internal Surface Area – provides adsorbing power/capacity & speed of action of adsorbents
• Pore Size – used to separate molecules that fit into or do not fit into the pores of the adsorbent
• Surface Chemistry – used to enhance the removal of specific types of molecules & atoms
• Particle Size – is selected to balance the conflicting goals of low pressure drop vs. the speed of action of
  an adsorbent.

What Materials are used to make Adsorbents?
Adsorbents can be made from many different raw materials, but they fall into several classes of materials. The raw material an adsorbent is made from determines many of its key properties.





 Surface Area, m2/g 

 Coconut shells,
 Coal, wood

 Aluminum TriHydrate 

 Zeolite Type:
 Alumina (Al2O3)
 and Silica (SiO2)

 Carbon Molecular 
 Sieves: Coal

 Raw Materials 

 ≥ 1,000

 200 - 250

 600 - 700

 MicroPore Size, Å 

 Range: 15 - 30

 Range: 30 - 200

 DISCRETE SIZES:   3Å, 4Å, 5Å, 10Å

 Hydrophobic / Hydrophyllic 





 Can be either



 Unmodified version used
 mostly to remove 

 Hydrocarbons, VOC’s 

 Water; Hydrocarbon
 Dew-Point Control 

 Water, H2S,
 Mercaptans, CO2

Other well-known types of adsorbents that do not fall into the above three classes:
Iron-Sponge: wood chips impregnated with iron-oxide to remove H2S from gas.
Improved iron-oxide media: porous inorganic substrate (~ 20 m2/g) coated with iron-oxide particles
Granulated Metal Oxide: typically, CuO ground to a small particle-size & bound together into round beads to remove H2S &/or elemental Hg. Internal surface area of 15-40 m2/g.

How does Surface Area determine the Power/Capacity of an Adsorbent?
The easiest way to understand this principle is to picture in your mind what happens on a molecular level. Surface Area is like warehouse space: the more warehouse space you have, the more things you can store. So, the more internal surface area you have, the more places you have to remove and store molecules or atoms removed from the fluid being processed.

How does Pore Size determine the Speed-of-Action of an Adsorbent?
When a fluid passes through a vessel filled with adsorbent, there is a zone in the vessel within which the contaminant is removed from the fluid & is “stored” on the adsorbent. If an adsorbent is “slow acting,” then the size of this removal zone is large. If the adsorbent is “fast acting,” the removal zone is smaller. We call the removal zone the mass transfer zone (MTZ) because it is the zone in the vessel that is required for the total mass (or quantity) of contaminant in the fluid to transition from the fluid-phase to the adsorbed-phase.

Smaller pores contribute to faster removal action, thus a smaller MTZ. For example: zeolite molecular sieves vs. activated alumina for removing water from a gas. The zeolite has smaller pores than the alumina, which contributes to the faster action of the zeolite for removing water.

What Other Adsorbent Properties contribute to Faster Action (smaller MTZ)?
• Higher Surface Area
Surface area is related to pore size: smaller pores mean more total internal surface area. You can visualize this concept better if you do a simple thought experiment. Imagine a cube, 1 cm length per side. What is its total surface area? What is the total surface area if you divide the cube into smaller cubes, each with 0.1 cm sides? What if you divide the cube into cubes with 0.01 cm sides? Or even 0.001 cm sides? More storage area (more internal surface area) means faster contaminant removal.

• Greater Heat-of-Adsorption
When molecules adsorb, they change to a lower energy state, just as they do when they condense from a gas to a liquid. The greater the heat of adsorption of an atom or molecule, the lower energy state it is in after it adsorbs on the internal surface of the adsorbent. Water molecules have a higher heat-of-adsorption (lower adsorbed energy state) on zeolite molecular sieve than they do on activated alumina.

• Smaller Adsorbent Particle Size
The smaller the diameter of the pellet, extrudate, bead, or granule, the less distance the contaminant molecule has to diffuse(travel) to reach the adsorbent’s internal surface. Another way to look at this: Smaller diameter particles have more external surface area per unit of mass. The more external surface area there is, the more places the contaminant atom/molecule has to enter the adsorbent’s pore structure.

How can we do bulk removal with an adsorbent?
Examples of this are the use of 5Å zeolite molecular sieves to separate straight-chain (normal-paraffin) hydrocarbon molecules from cyclic, aromatic, and iso-paraffins. Because straight-chain hydrocarbons will fit into 5Å diameter pores, they will adsorb. Cyclics, iso’s, & aromatics will not fit into 5Å pores, thus they will not adsorb and therefore pass through the adsorber vessel into the effluent stream. The adsorbed normal-paraffins can be recovered by desorbing them with a lighter gas at a higher temperature. Another example of bulk separation is done with either zeolite or carbon molecular sieves due to the RATE of adsorption of molecules of almost the same molecular or atomic diameter, such as oxygen and nitrogen. A rapid adsorption cycle is used so that the process never comes to equilibrium. This principle is used in Rapid-PSA Units for separating either oxygen or nitrogen gas from air.

What is “Physical” Adsorption?
Some adsorbents work by using the principle of “physical adsorption,” which is a pure state of attraction of the surface of the adsorbent for the target contaminant molecule. In reality, most adsorbents work by using a combination of physical and chemical forces to attract the target contaminant molecule. Within a given class of molecule, the physical attractive force of the surface area is greater as the m.w. of the molecule increases. For example, ethane is more strongly adsorbed on activated carbon than is methane, and butane is more strongly adsorbed than ethane. In general, though, with physical adsorption, the adsorption force is not very strong, and the adsorbent molecules can be thermally-removed (de-sorbed) by passing a hot gas through the adsorbent.

What is “Chemical” Adsorption or “Chemi-sorption?”
Chemical adsorption uses a much stronger attractive force of the surface of the adsorbent for the target contaminant molecule. In fact, the contaminant molecule becomes chemically bound to the surface of the adsorbent, and cannot easily be removed.

What are “impregnated” adsorbents and why are they used?
Most adsorbents work by using the principle of “physical adsorption,” but some target molecules or atoms are only very weakly attracted to the adsorbent surface by physical forces alone. For example, elemental mercury atoms (Hg) will adsorb on activated carbon, but only weakly and not to very high capacity, and can be easily desorbed. However, by taking advantage of the very high internal surface area of the activated carbon, and depositing (impregnating) elemental sulfur on the internal surface of the carbon, we change it into a chemi-sorbent. Thus, sulfur-impregnated activated carbon strongly attracts elemental-Hg, ionic-Hg & mercury compounds. The adsorbed Hg is chemically bound to the carbon, and cannot be easily desorbed. If we want to use metal oxide (CuO) for removing mercury, we impregnate the carbon internal surface with CuO, which then can be sulfided in-situ by a process fluid containing H2S or by pre-sulfiding in-situ with H2S injection.

Thus, whatever chemi-sorption chemistry you prefer, you “get more of it” with an impregnated activated carbon, because the carbon has the highest surface area of all the adsorbents and will out-perform adsorbents with less surface area.

Not all impregnated adsorbents are the same, because it is the dispersion of the impregnant available for adsorption that creates the adsorbing power of the product.