Physical Adsorption of Gases onto High- Area Substrates An - - PowerPoint PPT Presentation

Physical Adsorption

• f Gases onto High-

Area Substrates

An edutainment feature presentation by Jo Melville

Chemistry 125: Physical Chemistry Laboratory October 24th, 2014

• Atmospheric CO2 up 25% in past 50 years
• Global temperatures up 0.8℃ in past 50

years

• Gas storage could make greener fuels

economically/energetically viable

• Gas capture could scrub existing

greenhouse gases from the atmosphere

Why Gas Sorption?

Hansen, J., et al. (2006) "Global temperature change". Proc. Natl. Acad. Sci. 103: 14288-14293 Chu, S. Science 2009, 325, 1599. Granite, E. J.; Pennline, H. W. Ind. Eng. Chem. Res. 2002, 41, 5470.

Why Gas Sorption?

• Two key elements:

○ Gas Storage ■ Stable, high-density ways to store gases ■ Often for gaseous fuel sources (CH4, H2, etc.) ■ Requires controllable adsorption/desorption ○ Gas Capture ■ Rapid, high-density ways to sequester gases from air, or scrub waste products ■ Often for greenhouse gases (CO2, etc.) ■ Primarily requires strong adsorption only

Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal–Organic Framework mmen-Mg2(dobpdc) Thomas M. McDonald, Woo Ram Lee, Jarad A. Mason, Brian

• M. Wiers, Chang Seop Hong, and Jeffrey R. Long. Journal of the American Chemical Society 2012 134 (16), 7056-7065

Experimental Goals

• Test uptake of N2 gas on silica as proof of concept:

○ Construct adsorption isotherm ■ Presence of “steps” allow for rapid modulation of gas sorption through small changes in pressure ○ Measure density of gas storage: ■ Order-of-magnitude estimate gives a baseline for energy densities of adsorbed gases ○ Measure enthalpy of adsorption: ■ Provides reference value for CO2 sequestration, determines viability of gas storage for energy use

Experimental Design

• Silica sample placed under

vacuum

• Calibration with inert He gas

through use of gas bulbs

• N2 gas introduced, pressure drop

measured, process repeated until pressure drop is zero

• Plot relative pressure p/p° vs.

cumulative volume adsorbed

Experimental Theory

• Langmuir models single

monolayer formation as molecules adsorb to a surface

• BET models multiple layers

as molecules adsorb onto already- adsorbed molecules in discrete monolayers ○ Breaks down for too few or too many layers (.05<(p/p°)<0.3)

BET isotherm model. θ: number of monolayers x: relative pressure (p/p°) c: temperature-dependant constant

http://www.cchem.berkeley.edu/molsim/teaching/fall2011/CCS/Group7/images/structure/models.jpg A Model for Multilayer Adsorption of Small Molecules in Microporous Materials. Janina Milewska-Duda,*,†, Jan T. Duda,*,‡, Grzegorz Jodłowski,† and, and Mirosław Kwiatkowski. Langmuir 2000 16 (18), 7294-7303

Data: Silica Gel Isotherm

• Type II Isotherm

○ 0<x<0.1: monolayer formation ○ 0.1<x<0.6: multiple layer formation ○ 0.6<x<0.8: N2 (l) condensation

• Multiple isotherms

account for different methods of calibrating the bulbs with mercury

Data: Linearization of Silica Gel Isotherm

• Plotting x vs.

x/(v(1-x)) linearizes the isotherm

• Most linear

0.05<x<0.3

Data: Linearization of Silica Gel Isotherm

• Focus on x=

[0.05, 0.3] to maximize fit

• Temperature

dependence and monolayer volume can be derived from slope/intercept

• f fit

Results: Surface Area, Heat of Adsorption

• Lit: Langmuir surface area = 592 m2/g

○ Langmuir model does not account for formation of multiple layers

• Values are orders of magnitude higher than competing chemisorption methods (σ=0.3350 m2/g for H2 gas on a

Ru metal sponge) ○ Order-of-magnitude energy density of this adsorbed gas (~0.56 MJ/L) is roughly an order of magnitude higher than gaseous H2, though an order of magnitude lower than compressed liquid H2

• qads unfortunately an order of magnitude below comparable adsorption energies for zeolites (~55 kJ/mol)

Monolayer volume (νm) (cm3) Surface area (σ) (m2/g) Heat of adsorption (qads) (J/mol) 3-Term 64(2) 270(10) 9200(600) 2-Term 42(1) 180(0) 9200(600)

http://www.azom.com/article.aspx?ArticleID=1114 College of the Desert, “Module 1, Hydrogen Properties”, Revision 0, December 2001 Hydrogen Properties. Retrieved 2014-06-08. Peereboom, Lars. Adsorption of Bio-renewable Substrates on Supported Metal Catalyst in Water. pp. 30 Adsorption of CO2 on Zeolites at Moderate Temperatures. Ranjani V. Siriwardane,*, Ming-Shing Shen, and, Edward P. Fisher, and James Losch. Energy & Fuels 2005 19 (3), 1153-1159

A Proposed Method for Gas Sorption

• Repeating array of organic “linkers”
• Periodic metal binding sites
• Crystal framework creates micropores for

gas storage

• “Folding” of structure could create multiple

isotherm “steps” for easy sorption/desorption

• Metal
• Entangled
• Ligand
• Very
• Ideal for
• Localizing
• Lots of
• Energy
• r, for short:
• Jmetal
• Organicframework

A Proposed Method for Gas Sorption

http://1.bp.blogspot.com/_vfImvyorvjQ/S66l4_2V8AI/AAAAAAAADJ4/EUM0153hLaI/s1600/bath+mof+figforpress1.jpg http://www.sigmaaldrich.com/content/dam/sigma-aldrich/materials-science/learning-center/mof/mof-header.jpg

Conclusions

• Validation of BET theory as an improvement on Langmuir

theory

• Order-of-magnitude estimates suggest commercially-viable

energy densities for hydrogen storage are plausible

• Greater surface area/porosities are necessary for carbon

capture/sequestration

• Next steps:

○ adsorption of different gases (H2, CH4) ○ MELVILLE/JO synthesis to maximize substrate surface area

Questions?

• Thanks to:

○ The Melville Group ○ Jo Melville ○ Jo Melville ○ Jonathan “Jo” Melville ○ Jonathan F. Melville ○ Melville, J. ○

• J. Melville

○ Jonathan “Herman” Melville ○ Jonathan “Ishmael” Melville