GRAPHENE MEMBRANE FOR GAS SEPARATION

Group 3: Krista Melish, Phillip Keller,

James Kancewick, Micheal Jones

Gas Separation in Industry

Hydrogen separation○ From Nitrogen in ammonia plants○ From hydrocarbons in petrochemical applications

CO2 and water removal from natural gasNitrogen separation from airHydrogen Recovery From Tail Gases Air & natural  gas drying Vapor removal Hydrocarbon Separations Helium recovery from natural gas

Pharmaceuticals

Food processing, packaging, and storing

Membranes for Gas Separation

Less waste produced

Less harm on environment

Lower industrial cost

Lower energy consumption

Limitations of Common Membranes Energy intensive Expensive Lack efficiency and

productivity Break easily The material plugs too

easily and becomes resistant to flow

Properties of a Good Membrane

High flux rate (permeability) High selectivity Ideal pore size High surface area Low manufacturing cost Small thickness Mechanically Stable

Flux Rate of Different Gases

Affected by:

• molecule size

• gas concentration

• pressure difference across the membrane

•the affinity of the gas for the membrane material

Mechanisms for Gas Separation in Membranes

Relationships Among Membranes

Fick’s Law

The Flux rate (J) is inversely proportional to membrane thickness (x)

Selectivity vs. Permeability

of Membranes

Graphene

Single layer of carbon atoms Densely packed Hexagonal pattern Sp2 bonded Crystal lattice One atom thick 2-D structure

Properties of Graphene

Tear-resistant Thermal conductor Very Thin Very stiff, but also flexible Mechanically Strong

Stronger than a diamond Electronically conducting

100 times faster than the silicon in computer chips Ductile

Graphene Becomes a Membrane

Graphene is impermeable to all gases due to the electron density of its Aromatic rings

In order to create a membrane, must create pores synthetically

http://www.physics.upenn.edu/~drndic/group/research.html

Two Methods for Creating Nanopores Bottom-up synthesis

chemical building blocks of functionalized phenyl rings "grow" into a 2-D structure on a silver substrate○ pore diameters of a single atom○ pore-to-pore spacing of less than a

nanometer

TEMPuncture

holes by removing carbon rings by electric beam

The unsaturated carbons are passivated by nitrogen○ Control pore

size

Graphene Membrane Thinnest possible membrane (1 atom thick)

Over 20,000 x thinner than other membranes Ideal pore size for separation

Improvement of 500x compared to other membranes

Large surface area(Up to areas of 1 mm ^2)

Resistant to oxidation (for temperature less than 450 celsius)

Very mechanically stable

POROUS GRAPHENE AS THE ULTIMATE MEMBRANE

FOR GAS SEPARATION

Research Article by the Chemical Sciences, Materials Science, and Technology Divisions of Oak ridge National Laboratory

(De-en Jiang, Valentino R. Cooper, and Sheng Dai)

Article taken from:

Nano Letters 2009

Volume 9

No. 12

Pages 4019-4024

All pictures not cited on slide are from this article and belong to the authors

Article Overview

Inspiration for Research No prior research on graphene as a separation membrane

○ Massive possible efficiency gains in the gas separation field

Goals Use first principles models to mathematically prove the viability

of graphene as the ultimate membrane for gas separation Encourage future research and experimentation

MethodDensity Functional Theory

Simulation Results Further Research and Experimentation Ideas

Research Inspiration Graphene first isolated in 2004

Although there has been a boom of graphene research lately, no efforts have been put into analyzing its usefulness as a gas separation membrane.

Gas separation is very energy intensive currently Huge opportunities to increase efficiency

Application to other fields Proton Exchange Membranes for fuel cells Carbon sequestration from flue gases Gas sensors in instrumentation

Research Goals Show Viability of graphene as a gas separation

membraneMathematical modeling from first principles

Inspire future research and experimentationNew nano-pore designsNew nano-pore construction methodsInnovative applications to new fields

Research Method Density Functional Theory based modeling using

Plane wave base○ 300 and 680 eV kinetic energy cutoffs

Periodic boundary conditions

Initial Static Calculations2 methods usedPerdew, Burke, and Erzenhoff functional form of the

generalized gradient approximation (PBE)Rutgers-Chalmers van der Waals density function for

exchange and correlation (vdW-DF)○ Good at evaluating strength of dispersion interactions

between neutral non polar molecules

Model: Nitrogen Functionalized

Hexagonal cell made of graphene15 H2 or CH4 molecules

placed inside the cell for dispersion calculations

One face of the cell contains the nano-poreNano-pore created by

removing two cells (a), leaving 8 dangling carbons

Functionalized with 4 hydrogens and 4 nitrogens (b)

Model: Nitrogen Functionalized

Resulting pore electron density isosurface (red) leaves a rectangular pore3.0 Angstroms by 3.5

AngstromsNitrogen slightly attractiveHydrogen slightly repulsive0.05 eV Barrier to H2 versus

0.33-0.41 eV for CH4

Nitrogen yellow, Hydrogen blue

Model: Nitrogen Functionalized Graph shows the

interaction energy between H2 and the nitrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.

Red line and squares are calculated using vdW-DF method.

Black line and dots are calculated using PBE method.

Relatively flat curve shows little repulsion as molecule approaches the pore.

Model: Nitrogen Functionalized

Graph shows the interaction energy between CH4 and the nitrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.

Red line and squares are calculated using vdW-DF method.

Black line and dots are calculated using PBE method.

Curvature shows the repulsion of the molecule as it approaches the pore

Model: Nitrogen Functionalized

Results Selectivity for H2 / CH4 with the

nitrogen functionalized pore is

108 (Arrhenius)

Selectivity is high compared to

traditional polymer membranes

and silica membranes with

selectivities ranging from 10-103

Graphene is also much more

resilient than other membrane

materials that are more

susceptible to Hydrogen

damage

Difficulties Such functionality will be

hard to specify during manufacture ○ i.e. The placement of the

Nitrogens and Hydrogens will be random around the edge of the pore

Much easier to functionalize the poor using only Hydrogen

Next calculations are for a Hydrogen only functionalized pore

Model: Hydrogen Functionalized

(a) Face of Hexagonal cell with nano-pore functionalized with only Hydrogen (blue)

Created by removing 2 neighboring rings from the graphene sheet like before.

(b) Pore-electron density isosurface showing effective pore size

Dimensions are now 2.5 Angstroms by 3.5 Angstroms

Now it will be harder for both species to pass through

Model: Hydrogen Functionalized

Graph shows the interaction energy between H2 and the hydrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.

Red line and squares are calculated using vdW-DF method.

Black line and dots are calculated using PBE method.

Relatively flat curve shows little repulsion as molecule approaches the pore just like before.

Model: Hydrogen Functionalized Graph shows the

interaction energy between CH4 and the hydrogen functionalized pore as a function of adsorption height and orientation of the molecule within the pore.

Red line and squares are calculated using vdW-DF method.

Black line and dots are calculated using PBE method.

Curvature shows the repulsion of the molecule as it approaches the pore with values significantly higher than before.

Model: Hydrogen Functionalized

ResultsSelectivity for Hydrogen over methane raised to 1023

New barriers were 0.22 eV for H2 and 1.6 eV for CH4 which translates to a pass through frequency of 109 atoms of H2 per second at room temperature.

Conducted further research to judge the effect of inevitable errors in future manufacture such as removing three neighboring rings versus just 2 resulting in a width of 3.8 Angstroms.○ Found that this small error resulted in the pore becoming

useless (neither species impeded)○ Demands absolute precision in manufacture

Conclusions Although these are just mathematical models, they show the

viability of graphene as a new generation super membrane material.

This research applies universally to the separation of gaseous molecules based on size.

If findings can be reproduced in real life, this will seriously advance many industries including “green technologies” like fuel cells and carbon capture projects.

Next efforts should be focused into two main areas: 1. Further modeling to test new pores for more systems of gases 2. Experimentation to physically construct the pores being modeled.

Further Research As mentioned previously, further modeling and manufacture

processes need to be investigated.

Interesting Systems to model would be exhaust gases of common combustion engines, air separation, ethylene/ethane, and any other difficult distillation systems

Future manufacturing techniques using electron beams to punch holes into graphene need experiments focused on reducing the diameter of the beam to widths capable of targeting groups of 2-3 carbon atoms.

New functionalizing groups for liquids Desalination of sea water Wastewater treatment: community and industrial Biological screening

○ This would require functional group modeling that accounts for both mechanical and electrical interactions. Require many different equations for modeling, which will increase the time and computing

power needed.

Literature Cited

Article taken from:Nano Letters 2009

Volume 9

No. 12

Pages 4019-4024

All pictures not cited on slide are from this article and belong to the authors.

References

Graphene Source Sciencedirect Graphene Website