Arianna Marchioro Florian Degueldre High H2 Adsorption by Coordination Framework Materials Xiang Lin, Junhua Jia, Xuebo Zhao, K. Mark Thomas, Alexender J. Black, Gavin S. Walker, Neil R. Champness, Peter Hubberstey, and Martin Schröder
The challenge of H 2 storage Even if enough hydrogen could be produce to use it as an energy source: Current technology: Store it in under high pressure; Store it under liquid form; HOW TO STORE IT? These solutions require a large consume of energy! Alternative option : storage by physical adsorption
Why physical adsorption? Chemical adsorption: Strong interaction chemical bond formation between the adsorbent and the adsorbate on the surface Irreversible Physical adsorption : Weak interaction resulting from Van der Waals forces Reversible Physical adsorption can be simply controlled by varying the pressure on porous materials
Porous materials Zeolites Aluminosilicates Often used as molecular sieves Carbon nanotubes : Actual capacity: 1% wt at room temperature and 4% at 77K Metal coordination frameworks: Crystalline nanoporous material Metal atoms at vertices of a lattice Organic linker molecules Periodic structure Have an exceptional surface area
Background on adsorption Adsorption isotherms: describe the equilibrium for adsorption at constant T Most frequently used isotherms: Langmuir used for chemisorption and high P physisorption Brunauer, Emmett, Teller (BET) used for physisorption Langmuir isotherm: For monolayer processes Relates the fraction of adsorbed molecules to the gas pressure of a medium above the solid surface Gives the number of max molecules absorbed on a specific surface
Background on adsorption (2) BET isotherm: For multilayer processes Also relates the number of adsorbed molecules on the surface to the pressure Find the specific surface in m 2 /g of the adsorbent Pore size can be obtained by sorption data by applying a Dubinin Astakhov analysis
Problem of the article Investigation of the correlation between pore size and gas adsorption behaviour, varying the pressure Three compounds are tested, changing pore size by varying the organic backbone length
Structure of coordination framework materials (1) Based on biphenyl, terphenyl and quaterphenyl tetracarboxylic acids
Structure of coordination framework materials (2) A solvothermal reaction of these compounds with Cu(NO 3 ) 2. 5/2 H 2 0 in acidic conditions gives the solvated structures
Characterisation (1) X Ray diffraction PLATON/SQUEEZE : estimate the contribution of solvent in X Ray diffraction PLATON/SOLV : estimate the percentage of pore volume in respect to the total volume Results : 1 2 3 63.3% 70.4% 75.5%
Characterisation (2) Powder X Ray diffraction Used to confirm the phase purity ; Stable in dry conditions. Loss of porosity and cristallinity in moisture. Thermal gravimetric analysis (TGA) Heat the crystal to remove solvent and weight to verify it
Characterisation (3) Goal : estimation of ratio Cu/L 1 3 have the same thermal behavior 25 120 C : Blue green crystal becomes deep purpleblue loss of solvent From 120 C : No change in weight loss From 300 C : Degradation of materials
Characterisation (4) Dimensions of the pores for each compound Compound BET Area [m 2 g 1 ] Pore Volume* [cm 3 g 1 ] Pore Volume (N 2 ) [cm 3 g 1 ] Pore Size (N 2 ) [Å] 1 1670 0.683 0.680 6.5 2 2247 1.083 0.886 7.3 3 2932 1.284 1.138 8.3 Pore volume and pore size are determined from the N 2 sorption isotherms at 78K. The variation of the ligand length makes the estimation of the pore size difficult by cristallography*. We can see a form of linearity
H 2 sorption isotherm (1) Results for gravimetric sorption at 1 bar and 78 K 1 2 3 2.59 wt% 2.52 wt% 2.24 wt% The smaller the pore are, the higher the H 2 sorption is. H 2 sorption shows good reversibility and an absence of hysteresis.
H 2 sorption isotherm (2) At low pressure region, the difference between 1 and 2 is greater.
Results at higher P (1) Compound 1: 4.02 wt% at 20 bar max estimated adsorption 4.20 wt% nearly saturated at 20 bar Compound 2: 6.06 wt% at 20 bar max estimated adsorption 6.70 wt% Due to larger pore volume and surface area (longer organic ligand)
Results at higher P (2) Compound 3: Longest organic ligand largest pore volume and surface area 6.07 wt%h 2 at 20 bar : comparable to compound 2 Larger max adsorption : 7.01 wt% H 2 Higher H 2 adsorption would be observed at P>20 bar but measurements have not been reported here
Pore size vs. maximum H 2 uptake Ratios of pore volumes for 1 3: From crystallography : 1:1.59:1.88 From N 2 calculations: 1:1.30:1.67 Max theoretical H 2 uptake: 1:1.60:1.67 From 1 to 2, proportional increase in max H 2 uptake is larger than in pore volume: Due to more optimal pore geometry in 2? From 2 to 3, proportional increase in max H 2 uptake is less than in pore volume: Lower affinity between H 2 molecules and the pores of 3
Pore size vs. density Density of liquid H 2 : 0.0708 g cm -3 at 20 K Calculated densities suggest that H 2 is highly compressed within the pores Contrasting trends: increasing max H 2 uptake with increasing pore size decreasing adsorbed H 2 density with increasing pore size There is an optimum pore size! Better adsorption in smaller pores at low P due to the overlap of the potential energy fields of the pore walls Better adsorption at higher P if larger pore volume
Volumetric storage Excellent gravimetric H 2 storage capacities Also highest volumetric storage capacities yet reported for coordination frameworks The value of 43.6 g L 1 for 2 is close to the 2010 U.S. Department of Energy target of 45 g L 1
Conclusion The sorption isotherms of 1 3 are reversible No evidence for chemisorption of H 2 H 2 adsorption is controlled by the available pore volume: Proportional decrease in adsorbate density with increasing pore size Increasing max H 2 uptake with increasing pore size Comprise between larger pore volume strength of the interaction In prospect: synthesizing new coordination frameworks for enhanced storage by searching for new metal building blocks by determining the optimum pore size by optimizing the framework topology
Remarks Advantages Experimental section : complete and allows to reproduce the experiment Desavantages Results sometimes not so well exposed : Tables are better than sentences Simple (understable) explainations Illustrations Interesting and challenging subject