Large Surface Area is Key to a Valuable Adsorbent

Why is surface area key to a quality adsorbent?

Before we talk about surface area it’s helpful  to understand how adsorption works.

Adsorbents work by adsorbing liquids or vapors into pores on their surface.  The adsorption process doesn’t truly absorb the vapor or liquid that’s running through it (meaning the the liquid or vapor isn’t turned into a solid with the adsorbent),  rather molecules from the vapor or liquid are adsorbed and thus they get stuck on to the adsorbent.  In short an adsorbent acts like a magnet.

The pores on an adsorbent are where adsorbed molecules are kept.  The pores can have diameters between a couple of nanometers to hundreds of nanometers.  The purpose of the pores is to not only store molecules but sometimes to separate certain molecules by size.  The pore sizes can differ by nanometers or Angstroms (1 Angstrom = 1/10,000,000,000th of a meter) so you can separate liquids and gases at a molecular level.

For example if you wanted to separated methane from water you would use a 3A molecular sieve because the pore size on 3A is 3 Angstrom.  Water molecules have diameters up to 2.9 Angstrom and methane molecules have diameters up to 3.8 Angstrom.   The molecular sieve adsorbs the water and doesn’t adsorb the methanol thus separating the two molecules from one another.

Surface area measures how much exposed area there is on solid objects.  It’s important to distinguish that surface area and volume are not the same.  As long as the width, length, and height of an object remain the same the volume will never change.  Surface area, on the other had, can change if you break the object into smaller pieces.  See the example with the cube below.

Surface Area

Surface Area of a Cube = l*w*6

Volume of a Cube = l*w*h

 

 

Cube Length: 10mm

 Cube Width: 10mm

 Cube Height: 10mm

 

 

Cube Volume = 10*10*10=1,000mm³

 Cube Surface Area = 10*10*6=600mm²

The volume of an object will remain the same, but surface area can expand.  For example if you break the cube above into 5 parts you would find the following.

 

 

 Length: 10mm

  Height: 10mm

  Width: 2mm

Number of Cube Shaped Boxes: 5

 

Cube Surface Area:

 (2*10*10) + ( 4*2*10)*5=1,400mm²

 Cube Volume: (2*5)*10*10=1000mm³

 

By breaking the cube up into smaller sections, the surface area of the cube increases while the volume remains constant.

Surface area in adsorbents can be large.  1 gram of activated carbon for example has a surface that’s usually around 500m²

The pores on most adsorbents go only a few molecules deep so what you need is a lot of pores if you want to adsorb a lot of material.  Since pores are on the surface that is why you need a lot of surface area.  More surface area means more pores which means more liquid/gas is adsorbed.

 

Sources:

Size of methane molecule,  Slide 16 http://www.epa.gov/lmop/documents/pdfs/conf/12th/gladstone.pdf

Size of water molecule http://www.mc3cb.com/pdf_chemistry/What%20is%20the%20diameter%20of%20a%20water%20molecule.pdf

 

 

   

The Far Reaching Effect Zeolite Has On Everyday Human Lives

 

 

Before reading this article here are four questions to consider:

1. Have you ever drank a glass of tap water?
2. Have you ever been to a hospital and seen someone receive medical oxygen?
3. Do you heat up your home with gas?
4. Have you ever washed your clothes with laundry detergent?

The majority of people in Western society would have to answer yes to at least one of these questions, if not all of them.  These are just but a few of the many diverse  medical, practical, and  luxury based purposes and products that zeolites have made possible for humans in everyday life.  This broad spectrum of uses makes zeolite one of the most widely used minerals on Earth, yet most people have never heard of it before.  So the question is, what is zeolite and where does it come from?

Zeolite is a natural occurring group of microporous aluminosilicates that are found here, naturally on Earth. Their widespread use amounted to just under 3,000,000 tons of Zeolite being mined around the world in 2010.

Zeolites are naturally formed under low grade metamorphic conditions.  Low grade metamorphism occurs naturally in the cavities of volcanic rocks, where at temperatures between 200 -320 degrees Celsius, and while under low pressure, zeolites can be formed.  However they have been synthetically formed by humans as well, allowing the creation of a wide variety of different zeolites with many different uses.

Note: Some of the most recently created zeolite was made on-board the Columbia Space Shuttle.The reason for creating zeolite in space is to minimize nucleation effects and eliminate sedimentation.

There are 45 different minerals that are classified as zeolites but they only have three different structure types.  These three structures include chain structure, sheet structure, and framework structure.  Chain structure has crystal pores that form prism shaped crystals, sheet structure has crystal pores that are flat, and framework crystal pores have relatively equal sized pore dimensions.

As of November 2011 there are 201 different frameworks (pore classifications) for each of the three different structure types that have been discovered or synthesized by humans.  This combination of having variable structures and having many different pore (framework) sizes and shapes give zeolite the ability to perform many different tasks because of all of the different variations it can be produced in.

How does zeolite work?

Zeolite is microporous.  On its surface are millions of tiny pores that adsorb different materials which is based on the size and shape of the pore and what type of mineral the zeolite is.  Zeolite is also used to make other adsorbents like molecular sieve which is very effective at separating and purify chemicals.  These tiny pores can filter out material that is not needed for a specific application.

Referring to the questions asked at the beginning:

Have you ever drank a glass of tap water?

In the case of tap water, zeolite or molecular sieve collects contaminants in water and removes them so you can drink it.

Have you ever been to a hospital and seen someone receive medical oxygen?

Medical oxygen requires pure 100% oxygen before it can be used.  This pure oxygen is frequently made by removing the other elements like nitrogen and argon from the air that occurs naturally here on Earth.  In this case a 13X molecular sieve is used to remove all other components (that are not oxygen) in our atmosphere so that pure oxygen can be made and administered to patients.

Do you heat up your home with gas?

When natural gas (which is turned into the gas that heats your home) is first harvested from the Earth, it is harvested with a lot of other different elements that could be dangerous for human consumption.  Water also needs to be removed from natural gas streams and again these processes require the use of zeolite based molecular sieve.

Have you ever washed your clothes with laundry detergent?

Laundry detergent uses zeolite as a water softener by removing calcium and magnesium from water.  These elements can interfere with the cleaning benefits that the soaps in the laundry detergent provide.

These are only a few of the many different functions zeolites can provide a person, but there importance in the development of technology and in our everyday lives is undeniable.

 

Sources:

Metamorphism: http://www.tulane.edu/~sanelson/geol111/metamorphic.htm

Zeolite grown in space: http://www.tubitak.gov.tr/tubitak_content_files//spaceworkshop/presentations/Bac.Nurcan.pdf

Zeolite structures: http://www.galleries.com/Zeolite_Group

Zeolite production: http://minerals.usgs.gov/minerals/pubs/commodity/zeolites/mcs-2011-zeoli.pdf

Amount of Zeolite mined: http://minerals.usgs.gov/minerals/pubs/commodity/zeolites/mcs-2011-zeoli.pdf

More Structures: http://www.iza-structure.org/

How are they different?

 

Activated alumina and molecular sieve, they look similar (they both come in a small spherical beaded shape),  they perform the same process (they adsorb material at a molecular level), and they both have regenerative properties (you can re-use the material once the desiccant has reached capacity) yet they are two completely different products.  So the question is, with so many similarities, how are they different?

What they are made of is a good starting point.  Activated alumina is made out of aluminum oxide that is highly porous, while molecular sieve is made out crystalline metal alumino-silicates.  What this means is the pores on molecular sieve can be shaped into specific sizes most commonly seen as 3A, 4A, 5A, and 13X, where as activated alumina’s pores do not have specifically measured sizes.  This means molecular sieve can be used to separate certain molecules of specific sizes from one another, for example removing ammonia from natural gas streams.

From an application standpoint, here is how they differ.  Activated alumina has a real strong water adsorption capacity, it can adsorb a lot more water than molecular sieve, this makes it a very useful material in air compressors or for certain natural gas processing applications. The durability of the material allows it withstand a lot of pressure along with high levels of humidity.

Activated alumina can’t adsorb the large variety of materials or separate certain molecules from one another like molecular sieves can, making it ineffective in a process like ethanol dehydration.  This is because activated alumina would be able to adsorb both ethanol and water molecules and thus no separation would occur.

Molecular sieve may not be able to adsorb as much water but if you needed to reduce water to very low amount, up to 0.1ppm, molecular sieve would be your absorbent of choice because this is something other adsorbents besides molecular sieve have been incapable of doing.

Molecular sieve can also be used to separate specific molecules from one another, due to the customization of their pore sizes.  For example you can separate water from ethanol, and carbon dioxide, ammonia, and larger hydrocarbons from natural gas streams, which is something activated alumina can’t do, or won’t do with same efficiency.