Aspects of Physical Adsorption Characterization of Nanoporous Materials. Dr. Matthias Thommes Quantachrome Instruments Boynton Beach, FL, USA

Transcription

1 Aspects of Physical Adsorption Characterization of Nanoporous Materials Dr. Matthias Thommes Quantachrome Instruments Boynton Beach, FL, USA

2 CONTENT (1) General Introduction (2) Choice of Adsorptive(s) for Micro/Mesopore analysis (3) Comments to Surface Area Analysis (4) Pore Size Analysis - Recent Progress and Challenges for MOF Pore Size Analysis (5) Adsorption Properties, Gas Storage (6) Summary, Conclusions

3 Physical Adsorption: Van der Waals Forces The three components that constitute van der Waals Forces Interaction Component Origin of Interactions Equation Keesom Dipole-dipole w( r) = 2 u1u 3( 4πε ε o r) k B T 1 6 r Debye Dipole induced dipole w( r) = 2 u α ( 4πε ε o o r ) r London (Dispersion) Induced Dipole Induced Dipole w( r) = 3 2 αo1α ( 4πε o2 2 o ) I1I2 ( I + I 1 2 ) 1 6 r The London component is the most dominant. n = 6 indicates van der Waals forces are short range Ref.: J. Israelachvili, Intermolecular & Surface Forces (Academic Press, 1992)

4 Adsorption Potential as a Function of Pore Size IUPAC Classification(1985): Micropores: < 2 nm Ultramicropore: <.7nm Supermicropore:>.7 nm Mesopores: 2-5 nm Macropores: > 5 nm Supermicropore Mesopore Ultramicropore

5 Physical Adsorption: IUPAC Classification of Adsorptionisotherms (1985) Micropore Heteronegeous Planar Surf Mesopore Heteronegeous Planar Surf. Mesopore ideal planar Surf. IUPAC(1985) Micropores: < 2 nm Mesopores: 2-5 nm Macropores: > 5 nm ISO (current suggestion) Nanoscale : 1 1 nm

6 Argon (87 K) Adsorption in microporous solids Volume [cm 3 g -1 ] [STP] X Linde 5 A Mordenite ZSM-5 socmof Linear-plot socmof Mordenite Volume [cm 3 g -1 ] [STP] X Linde 5 A Mordenite ZSM-5 socmof Mordenite Semi-logarithmic plot socmof P/P P/P Semi-logaithmic plot of low pressure isotherm allows to reveal the pore structure of microporous materials

7 Choice of Adsorptive for Micropore/Mesopore Analysis

8 Gas Adsorption: Selection of Adsorptives for Surface- and Pore size Analysis Nitrogen: at K (liquid nitrogen temperature, T/T c =.61) pore size analysis of micro-,meso and macropores surface area analysis Argon: at K (T T r = K; T r : bulk triple point temperature; T/T c =.5) at K (liquid argon temperature, T/T c =.57 ) pore size analysis of micro-, meso- and macropores surface area analysis CO 2 : at 195 K (T/T c =.63) at 273 K (T/T c =.89) pore size analysis of micropores of widths < 1.5 nm (particularly for microporous carbons) Krypton : at K (T T r = K) measurement of very low surface areas at K (T T r = K) pore size analysis of thin micro/mesoporous films (M. Thommes et al, 26)

9 Volume [cm 3 ] Choice of Adsorptive for Pore Size Analysis by Gas Adsorption: Nitrogen (77.35 K) or Argon (87.27 K)? N2/77K Ar/87 K Ar and N 2 Adsorption on Faujasite type Zeolite. N 2 /77.4 K Volume [cm 3 g -1 ] CMC2-Carbon (Argon, 87.3K) CMC2-Carbon (Nitrogen, 77.3 K) Ar and N 2 Adsorption on CMC2-active carbon P/P ZEOLITE Ar/87.3 K Zeolites: N 2 - quadrupole interactions important! 2 1 N2/77K Ar/87 K Relative Pressure [P/P ] Many Carbons: N 2 quadrupole interactions not relevant Problem: Below 1 mtorr(p/p = 1-5): Regime of extremely slow kinetics, the selection of proper equilibrium parameters is essential for obtaining accurate adsorption isotherm data

10 Choice of Adsorptive for Pore Size Analysis by Gas Adsorption: Nitrogen (77.35 K) or Argon (87.27 K) in amorphous micro/mesoporous silica? 6 5 Argon 87 K Nitrogen 77 K Volume [cc/g] [STP] P/P M. Thommes, B. Smarsly, M. Groenewolt, P.I. Ravikovitch, A.V. Neimark,Langmuir, 22,756 (26)

11 Choice of Adsorptive for Pore Size Analysis by Gas Adsorption: Nitrogen (77.35 K) or Argon (87.27 K)? Ar/87 K N 2 /77 K Me8 socmof Volume [cm 3 g -1 ] N 2 /77.4 K Ar/87.3 K 7 color scheme: carbon, oxygen, nitrogen, indium P/P J. Moellmer, E.B. Celer, R. Luebke, A.J. Cairns, R. Staudt, M. Eddaoudi, M. Thommes, Microporous and Mesoporous Materials 129 (21) 345

12 Volume [cm 3 g -1 ] [STP] High Resolution Argon Adsorption at 87 K : Micro-Pore Size Analysis with 1 Angstroem resolution 13 X Linde 5 A Mordenite ZSM-5 Mordenite ZSM-5 Argon adsorption at 87 K P/P M. Thommes, in Introduction to Zeolite Science, 3 rd Revision, Chapter 15, Studies in Surface Science and Catalysis 168, Elsevier 27 pp Mordenite ZSM-5

13 Aspects of Surface Area Analysis

14 a Comments to the Surface Area Determination of Microporous Materials by Application of the BET equation BET equation in a strict sense not applicable to microporous materials apparent surface area n p/ p C = p p [ 1 ( pp / ) ] nmc nmc 1/[W(P /P) -1] socmof_me8 Ar/87 K BET: m 2 /g Correlation:.997 Pseudo-C constant: - 5 Application of BET equation in classical range, i.e. rel pressure range.5 to.3 : Negative Intercept, i.e. neg. C- constant Less than optimal correlation coefficient P/P

15 . BET (Brunauer, Emmett, and Teller) Surface Area Analysis of Microporous Materials Question: How to find the linear range of the BET plot for microporous materials in a way that it reduces any subjectivity in the assessment of the monolayer capacity Rouquerol et al. suggested: (see for instance: Studies in Surface Science and Catalysis, pp ) to apply the following criteria: The quantity of C must be positive (i.e. any negative intercept on the ordinate of the BET plot is an indication that one is outside the valid range of the BET equation) The application of the BET equation should be limited to the pressure range where the term n (P P) or alternatively n(1 P/P ) continuously increases with P/P.

16 Procedure to find the linear range for application of the BET equation for microporous materials socmof_ar/87 K.7.6 socmof_me8 Ar/87 K n(1-(p/p ) [mol/g] /[W(P/P) -1] P/P Maximum at P/P =.4, Applying BET equation for data points <.4! P/P BET-area: m 2 /g Correlation: C constant:

17 Application of BET Equation for Surface Area Analysis of Materials: Choice of proper adsorptive! Problem: Effective cross-sectional area for nitrogen on surfaces with polar site is not well defined: Quadrupole moment of the nitrogen molecule can lead to specific interactions with polar surface sites (e.g.,hydroxyl groups) causing an orientating effect on the adsorbed nitrogen molecule Significant effect on effective cross-sectional aea, i.e. true N 2 crosssectional area can be up to 2% smaller compared to customary value of 16.2 A 2 for polar surface! L. Jelinek et. Al. : Langmuir 1 (1994) pp A Galarneau et al.,. Micropor. Mesopor. Mat. 27 (1999) pp J. Rouquerol, et al. In: Characterization of Porous Solids, Gregg, S.J., Sing, K.S.W., Stoeckli, H.F. (Eds), Soc. Chem. Ind., London, 1979 pp Alternative/Solution: Argon adsorption (preferably at 87 K), i.e. Argon gives more reliable pore surface area because effective crosssectional area is not so dependent on the surface chemistry. In case of our socmof : Argon (87K): 1148 m 2 /g Nitrogen(77 K): 135 m 2 /g

18 Pore Size/Porosity Analysis

19 Pore Size Analysis by Gas Adsorption Macroscopic, thermodynamic methods Micropores (< 2 mn): e.g., Dubinin-Radushkevitch or more advanced methods such as Horvath-Kawazoe (HK) and Saito-Foley (SF), t-method, alpha-s method Meso/Macropores (2-1 nm): e.g., Kelvin equation based methods such as BJH (Barrett,Joyner, Halenda) Modern, microscopic methods, based on statistical mechanics describe configuration of adsorbed molecules on a molecular level : e.g., Density Functional Theory (DFT), Molecular Simulation these methods are applicable for pore size analysis of both the micro- and mesopore size range An accurate pore size analysis over the complete pore size range can be performed by a single method.

20 Density Functional Theory (DFT): Characteristic density profile of a Lennard-Jones fluid in micro-and mesopores 8 6 Micropore Mesopore ρ z* From: S. Lowell, J. Shields, M. Thomas, M. Thommes, Characterization of porous solids and Powders: Surface Area, Pore Size and Density, Kluwer Academic Publ, 24, J.P.R.B. Walton and N. Quirke, Mol. Sim (1989)

21 Calculation of the DFT Pore Size Distribution

22 Calculation of DFT Pore Size Distribution Curves from Experimental Adsorption/Desorption Isotherms ( ) MAX = NPP (, W) f( W)dW NPP W W MIN

23 Recommendations for Application of DFT/MC methods for Pore Size Analysis Make sure that you have a proper match of experimental (adsorptive/adsorbent system) and chosen DFT kernel Compare the DFT theoretical isotherm with the experimental sorption isotherm

24 3 24 H-Mordenite 13X NLDFT_Zeolite Fit_(spherical pore model) NLDFT-Zeolite Fit (cylindrical pore model) Volume [cc/g] P/P X-Zeolite structure Mordenite structure Volume [cc/g] Zeolite X- type DFT-Fitting : cylindrical pore model DFT-Fitting : spherical pore model P/P 13X zeolite: Pressure where pore filling occurs is controlled by the adsorption potential exerted by the (spherical) cage, and not the 7.4 A wide aperture) The cylindrical model does not fit the Zeolite X isotherm

25 Argon adsorption at 87 K on a 5:5 mixture of ZSM-5 +MCM-41: Combined micro/mesopore analysis by NLDFT Adsorption, [mmol/g] MCM-41 ZSM dv/dd [cm 3 /g] ZSM-5 MCM-41 histogram integral Vcum [cm 3 /g] P/Po D, [Å] M. Thommes, In: Introduction to Zeolite Science 3rd Revised Revision (eds. Cejka et al,) Chapter 15, Studies in Surface Science and Catalysis 168, Elsevier 27 pp

26 Theoretical predictions of the pore size dependence of the relative pressure of the equilibrium pore condensation/evaporation transition N 2 /77 K in cylindrical silica pores. Neimark AV, Ravikovitch P.I., Grün M., Schüth F., Unger K.K, (1998) J. Coll. Interface Sci. 27,159

27 N 2 sorption (77 K) in MCM-41 and Pore Size Analysis by Modified Kelvin eq. (BJH method) and Nonlocal- Density Functional Theory (NLDFT) Volume [1-6 m 3 /g] N 2 (77 K): ads N 2 (77 K): des DFT-Fitting Dv(d) [cc/å/g] BJH-Pore size distribution DFT-Pore size distribution BJH NLDFT RELATIVE PRESSURE p/p Pore Diameter [Å] Classical methods (i.e. BJH, based on Kelvin equation) underestimate the pore diameter up to ca. 25 %! NLDFT allows to calculate an accurate pore size distribution

28 Volume [cc/g] Nitrogen sorption in hierarchically structured Silica (KLE) N 2 sorption isotherm RUN 1 (Ads) RUN1 (Des) RUN 2( Des) RUN 2 (Ads) dv(cc/å/g) Pore size distribution KLE-Silica NLDFT-PSD(spherical pore model) 1.3 nm 13.9 nm Relative Pressure P/P NLDFT analysis (spherical mesopores, cylindrical micropores) Mesopore Size: N 2 -sorption: 13.9 nm TEM: Ca. 13 nm SAXS: 13.8 nm Excellent agreement between SAXS and new NLDFT approach! KLE M. Thommes, B. Smarsly, M. Groenewolt, P.I. Ravikovitch, A.V. Neimark,Langmuir, 22,756 (26)

29 Argon Sorption at 87 K in Mesoporous ZSM-5 and NLDFT Pore Size Analysis ZSM-5 Data from: D. Serrano, J. Aguado, G. Morales, J. Rodriguez, A. Peral, M. Thommes, J.D. Epping, B.F. Chmelka, Chemistry of Materials Vol (29)

30 Pore Blocking/Percolation and Cavitation Phenomena Wc = critical neck diameter; 5 nm for N 2 (77 K) Theoretical and molecular simulation work on Cavitation: L.D. Sarkisov, P.A. Monson, Langmuir 17, 76 (21); P.I Ravikovitch, A.V. Neimark, Langmuir 18, 983(22); A. Vishnyakov, A.V. Neimark, Langmuir. 23,19, 324; Experiments: M. Thommes, B. Smarsly, M. Groenewolt, P. Ravikovitch A.V. Neimark, Langmuir, 22,756 (26); O. Sel, A.Brandt, D. Wallacher, M. Thommes B. Smarsly, Langmuir 23, (27) 4724

31 Argon (87.3 K) and Nitrogen (77.4 K) Adsorption into Pillared Clay AL P Nitrogen 77 K(Al-P5) Argon 87 K(Al-P5).15 Nitrogen 77 K (Desorption) Argon 87 K (Desorption) Volume [cm 3 g -1 ] STP Dv(d) [cm 3 /Å/ g -1 ] Ar 83 K N2 77K Pseudo-Pore Size Analysis from Desorption Branch Disagreement in PSD s Cavitation P/P Pore Diameter [Å] Disagreement between Ar-and N2 pseudo desorption PSD s due to Cavitation transition

32 H3 Hysteresis in disordered Catalyst Adsorption Desorption 1.8 Adsorption Desorption Volume STP [cc/g] N2/77K sorption on disordered alumina catalyst Dv(log d) [cc/g] BJH-PSD Artifact Relative Pressure P/P Pore Diameter [Å] The typical step-down of the desorption branch at ca..4 for nitrogen adsorption is indicative of cavitation, i.e. which occurs because large mesopores are only accessible through narrow mesopores or micropores.

33 Application of methods based on DFT and molecular simulation for pore size analysis in industry and academia DFT is meanwhile widely used for micro/mesopore analysis Comprehensive library of DFT methods for various adsorptive/adsorbent pairs is available Since 27: NLDFT methods for pore size analysis are featured/recommended in standards of the International Standard Organization (ISO, i.e. ISO ) Facilitates the application/use of DFT methods for pore size analysis in industry However: NLDFT does not take into account surface heterogeneity /roughness Development of QSDFT (Quenched solid density functional theory) by Alex Neimark and co-workers!

34 QSDFT vs. NLDFT: Nitrogen adsorption (77.4 K) on activated carbon fiber NLDFT Fit QSDFT Fit Experimental data, N 2 (77 K)/ACF QSDFT NLDFT Volume [cm 3 g -1 ] Dv(d) [cm 3 /Å/g] P/P Pore Diameter [Å] Activated carbon fiber ACF-15 A.V Neimark, P.I Ravikovitch, Y. Lin, M. Thommes, Carbon 47 (29) 1617 QSDFT provides a much more realistic approach for the pore size analysis of heterogeneous activated carbon!

35 Aspects of MOF Pore Size/Porosity Analysis

36 Choice of Adsorptive for Pore Size Analysis by Gas Adsorption: Nitrogen (77.35 K) or Argon (87.27 K)? Volume [cm 3 ] N2/77K Ar/87 K Faujasite type Zeolite. N 2 /77.4 K Volume [cm 3 g -1 ] Ar/87 K N 2 /77 K Me8 socmof N 2 /77.4 K Ar/87.3 K 7 Ar/87.3 K P/P ZEOLITE P/P Problem: Below 1 mtorr(p/p = 1-5 ): Regime of extremely slow kinetics, the selection of proper equilibrium parameters is essential for obtaining accurate adsorption isotherm data

37 color scheme: carbon, oxygen, nitrogen, indium Key to superior hydrogen storage materials: Narrow pores < 1nm and High surface area and High localized charge density M. Eddaoudi and co-workers, Angew. Chem. INt. Ed., 46, 3278 (27)

38 Pore Size Analysis of soc-mof based on Argon (87K) Adsorption and NLDFT Volume cm 3 g -1 [STP] Experiment NLDFT-FIT (cylindrical NLDFT/zeolite pore model) NLDFT-Fit (cylindrical NLDFT/carbon model) Dv(d) [cm 3 /Å/g] Ar(87K)/Zeolite NLDFT, Cylindrical Pore Ar(87K)/Carbon NLDFT, Cylindrical Pore 6.2 Å (NLDFTzeolite model)!! 12 Å (NLDFT carbon model) P/P Pore Diameter [Å] Pore size obtained from cylindrical NLDFT pore model (6.2 Å nm) assuming an oxidic(zeolitic) surface agrees well with the accessible/internal pore diameter (D in = D - σ ss, where D is the distance between surface atoms, and σ ss is diameter of the adsorptive

39 Pore Size Analysis of Chromium 2,6- Naphthalenedicarboxylate with MIL-11 Topology V a / cm 3 (STP) g MIL-11 MIL-11_NDC N 2 /77 K Sorption NLDFT 2,,2,4,6,8 1, p/p Andreas Sonnauer, Frank Hoffmann,Michael Fröba Lorenz Kienle, Viola Duppel, Christian Serre, Gérard Férey, Matthias Thommes, and Norbert Stock*, Angewandte Chemie, (21) BJH

40 Effect of Temperature and Pore Size on Adsorption

41 H 2 Adsorption and Isosteric Heat for MOFs 24 1 Volume [cm 3 g -1 ] STP K H 2 ads/des 87 K H 2 ads/des 97 K H 2 ads/des 17 KH 2 ads/des 77 K 87 K 97 K 17 K Isosteric Heat of Adsorption [kj/mol] H 2 Ads. [77, 87 K] H 2 Ads. [77, 87, 97 K] H 2 Ads [77, 87, 97, 17 K] Isosteric Heat of Adsorption Pressure [Torr] Volume adsorbed [cm 3 g -1 ] Isotherms obtained at (at least) three different temperatures should be used for isosteric heat analysis

42 Effect of Thermodynamic State of Adsorptive on Adsorptionisotherm: Methane and Hydrogen Adsorption at 17 K in socmof Hydrogen 17 K Methane 17 K T < T c,b 3 Hydrogen 17 K Methane 17 K Volume [cm 3 g -1 ] STP CH 4 : T/T c =.56 H 2 : T/T c = 3.23 T > T c,b Pressure [Torr] Volume [cm 3 g -1 ] STP CH 4 H Pressure [Torr] Significant differences in adsorbed amount of H2 an CH 4 at 17 K are mainly due to differences in thermodynamic state!

43 Schematic phase diagram of pore fluid (e.g. in silica mesopore) and bulk fluid pore W1 > pore W 2 M. Thommes, In Nanoporous Materials Science and Engineering, (edited by Max Lu and X.S Zhao, eds.), Imperial College Press, Chapter 11, , (24)

44 Effect of Temperature on Excess Adsorption Planar Surface Mesopore Micropore

45 CO 2 (273 K) Adsorption on socmof over a wide pressure range from P/P =1-5 to Surface Excess n A [mmol g -1 ] σ n A = n + n A Total adsorbed amount ρ g σ n = ρ 1 ρ V g liq P Pressure [MPa] (1) n A = n σ + ρ g V a; ; n A = total adsorbed amount; : nσ = surface excess V a = volume of adsorbate V p (pore Volume, for sufficiently small pores) n A = n σ + ρ g V p (2) If V a varies according to V a = n a /ρ a, and if ρ A ρ liq n A = n σ /(1- ρ g / ρ liq ), :

46 Adsorption of CO 2 and CH 4 on socmof over a wide temperature and pressure range CO 2 n σ [mmol g -1 ] CO K n σ CO 2 (273K) Ads soc-mof n σ CO 2 (298K) Ads soc-mof n σ CO 2 (323K) Ads soc-mof n A CO 2 (273K) Ads soc-mof n A CO 2 (298K) Ads soc-mof n A CO 2 (323K) Ads soc-mof CH Pressure [MPa] n [mmol g -1 ] n σ CH 4 (273K) Ads soc-mof n σ CH 4 (298K) Ads soc-mof n σ CH 4 (323K) Ads soc-mof n A CH 4 (273K) Ads soc-mof n A CH 4 (298K) Ads soc-mof n A CH 4 (323K) Ads soc-mof 273 K 298 K 323 K -ΔH iso [kj mol -1 ] CO 2 CH 4 Isosteric heat CH 4 5 CH 4 (273K, 298K, 323K) Ads soc-mof CO 2 (273K, 298K, 323K) Ads soc-mof Pressure [MPa] n A [mmol g -1 ]

47 Summary and Conclusions We reviewed some important aspects of physical adsorption characterization: Argon 87 K adsorption in addition to nitrogen77k recommended for a comprehensive and accurate micro-mesopore analysis Microscopic methods (e.g, DFT, molecular simulation) allow one to obtain a much more accurate and comprehensive pore size analysis compared to the classical macroscopic, thermodynamic methods We addressed some open questions concerning the surface and pore size characterization of MOF s by using physical adsorption; however, much more work is still needed in order to arrive at a comprehensive surface area, porosity and pore size characterization. The shape of sorption isotherms is affected by the pore structure and surface characteristics of the adsorbent, but also by the states of pore and bulk fluid phases this is particularly important for the interpretation of adsorption isotherms within the context of gas storage and separation applications

48 Acknowledgement Special Thanks to Reiner Staudt, Jens Moellmer as well as Mohamed Eddaoudi and Amy Cairns R. Staudt & J. Moellmer wish to thank the DFG for support (DFG-Projekt SPP 1362 MOF STA428/17-1) M. Eddaoudi gratefully acknowledge the financial support: National Science Foundation (DMR ) Department of Energy DOE-BES (DEFG2-7ER467).