OUTLINE: Short review of theory Experimental methods and machinery at the lab Case studies

1. N2 – ADSORPTION theory, experiment and application • OUTLINE: • Short review of theory • Experimental methods and machinery at the lab • Case studies

2. ADSORPTION theory adsorbent adsorbate adsorptive • some properties of adsorption (physisorpion): • exothermic, non-specific, adsorption energy low, • not activated, multilayer, no dissociation/electron transfer

3. ADSORPTION INTERACTIONS theory • Adsorption interactions: • adsorbent and adsorptive are important • Examples:

4. ADSORPTION ISOTHERM theory • amount of N2 adsorbed versus relative pressure at constant temperature • temperature: 77 K: boiling/condensation point of N2 at patm=p° • Amount adsorbed • per unit adsorbent mass: • n mol/g • Vads, liq ml/g = cm3/g • Vads, STP ml/g • … • STP = 0 °C, 1 bar

5. ADSORPTION ISOTHERMS: TYPES theory • IUPAC classification:

6. gas vacuum P Liq. N2 • EXPERIMENTAL • how is an adsorption isotherm recorded? • Total volume of system determined: • - “manifold” volume known • - sample volume determined with He • System is evacuated • Start of adsorption: • - amount of gas dosed in manifold • - p measured  qty known • - gas expanded into sample volume • - equilibrium p measured; compared to calcd p • - difference  amount adsorbed at p • p° measured to account for temp changes in liq. N2 • Lower p/p° limit: determined by • pump and pressure measurement manifold sample

7. MACHINERY AT COK experimental Micromeritics Tristar 2 x Coulter Omnisorp • continuous method: • dosing: constant low flow • quasi-equilibrium • measure pressure • infinite amount of points • long measurements • p/p°min: 10-4 – 10-3 (“left” omnisorp) • p/p°min: 10-5 – 10-4 (“right” omnisorp) • discontinuous method: • amounts of gas dosed • wait until equilibration • measure pressure • limited amount of points • short measurement possible • three samples simultaneously • p/p°min: 10-4 – 10-3

8. SPECIFIC SURFACE AREA theory • 2 frequently used methods: • BET • Comparative methods: t-plot or aS-plot • general remark on concept of determination of the specific surface area : • - surface roughness – probe molecule size • - measured area dependent on size of probe molecule: fractal property

9. SPECIFIC SURFACE AREA SPECIFIC SURFACE AREA: BET theory • Two steps: • 1. Evaluation of the monolayer capacity Vm • 2. N2 cross-sectional area aN2: Vm x aN2 = S • Not very accurate (20 % error) because: • BET model assumptions usually not valid • homogeneous surface, no lateral interactions, infinite number of layers possible, n+1: liquefaction energy • 2. value of cross-sectional area not determined accurately/ dependent on adsorbent surface (usually: 0.162 nm2) • 3. Pore size/shape may change interpretation • Mesoporous silica: overestimation Small cilindrical pores: 10 % underestimation

10. SPECIFIC SURFACE AREA SPECIFIC SURFACE AREA: COMPARATIVE theory • compare adsorption behaviour of unknown with behaviour of • macroporous material with similar surface properties • standard silica reference: hydroxylated silica • you can always make a reference yourself • t-plot, as plot

11. SPECIFIC SURFACE AREA SPECIFIC SURFACE AREA: COMPARATIVE theory sample isotherm comparative plot Vads t or as ~Vads, ref p/p° p/p° reference isotherm • adsorption behaviour (isotherm) of reference is plotted on x-axis as t or as

12. SPECIFIC SURFACE AREA SPECIFIC SURFACE AREA: COMPARATIVE theory • reference adsorption scaled as t or as: t-plot: SBET of reference incorporated!

13. SPECIFIC SURFACE AREA SPECIFIC SURFACE AREA: COMPARATIVE theory • evaluation of the surface area from slopes Vads st or sas ~Sext • fit linear portion with straight line • linear = same adsorption behaviour as ref st or sas ~Smeso+Sext t or as • dependent on SBET reference!

14. Vads Vmeso t or as PORE VOLUMES FROM COMPARATIVE METHOD theory Vtotal Vmicro • independent of SBET reference!

15. MICROPORE SIZE theory • micropore size determination: Horvath-Kawazoe model • estimated p/p° at which micropores of diameter d fill • (slit shape, carbon black): • p/p° < 10-5 necessary to investigate first stages of micropore filling • at the lab: only Right Omnisorp machine (p/p° min ~ 10-5)

16. MICROPORE SIZE theory Artefact! 0.63 nm • Zeolites: • MFI (pore size ~ 5.5 Angstrom); right Omnisorp • FAU (window size ~ 7.4 Angstrom); left Omnisorp • Look at results with criticism!

17. MESOPORE SIZE DISTRIBUTION theory • BJH model: mesopore filling is due to • Pressure dependant film formation on mesopore walls • Classical t-curves: • Condensation pressure – pore width • Kelvin equation • Use desorption isotherm with hemispherical meniscus (rm) • rp = rk + t (Halsey: usually not preferable)

18. Vads 0.4 p/p° MESOPORE SIZE DISTRIBUTION theory • BJH model: problems with ordered mesoporous materials • Adsorbed layer thickness underestimated (wall curvature) • Use of desorption branch: • Instability of condensate at p/p° ~ 0.4 (4 – 5.5 nm) during desorption: • tensile strength effect • Desorption branch more prone to material imperfections (network effects)

19. MESOPORE SIZE DISTRIBUTION theory • improved BJH model: KJS (Kruk – Jaroniec – Sayari) • widely used to determine mesopore size OMM • Corrected t-curve based on standard adsorption isotherm • 2. Correction for increased film thickness due to mesopore curvature • 3. Use of ADsorption isotherm (Kelvin for hemispherical meniscus) • Application between 2 – 10 nm; above 6 nm accuracy decreases • Application for cilindical pores (MCM-41) or cilinder-like pores (MCM-48)

20. MESOPORE SIZE: OTHER METHODS theory • methods for hexagonal mesoporous materials (e.g. MCM-41, SBA-15) • the “4V/S” method • from mesopore volume and XRD • Void fraction via • geometry • density • d100 = first reflection in XRD pattern • = 2.2 g/cm3 for amorphous silica

21. 1 µm CASE STUDY I: nano Silicalite-1 • synthesis: • TPA + silica  nano Silicalite-1 particles  porous nano S1 particles • structure calcination HT Micropore diameter: ~ 5.5 Angstrom Particle size ~ 100 nm

22. CASE STUDY I: nano Silicalite-1 • adsorption isotherms (Right Coulter) • fresh: ~ type III : TPA in pores/on surface: apolar surface • calcined: type I: microporous • p/p° > 0.9: interparticle capillary condensation

23. CASE STUDY I: nano Silicalite-1 • adsorption isotherms: logarithmic plot • fresh: very low uptake at low p/p°: apolar surface • calcined: micropores give high uptake below p/p°= 10-3

24. DETERMINATION OF THE SURFACE AREA nano Silicalite-1 1. BET method fresh calcined washed fitted p/p° range: 0.05 – 0.3 • good linearity for the washed sample, difficult for calcined • calcined sample: negative intercept

25. DETERMINATION OF THE SURFACE AREA nano Silicalite-1 • BET method • analysis results (fitted range: 0.05 – 0.3): • validity check of BET model: 50 < C < 200 • washed sample: too low C value •  low adsorbent-adsorbate interaction (TPA on surface) • gives only indication of surface area • calcined: negative C: BET model not valid • Overlap micropore filling – monolayer formation BET not applicable on organic surfaces and microporous solids

26. DETERMINATION OF THE SURFACE AREA nano Silicalite-1 2. Comparative method t-plot; SBET, REF = 25 m2/g fresh • intercept < 0 • no linear regions: adsorption behaviour ≠ reference (organic surface) • t-plot not applicable: bad reference

27. DETERMINATION OF THE SURFACE AREA nano Silicalite-1 2. Comparative method t-plot; SBET, REF = 25 m2/g calcined • t<0.2: highly nonlinear: micropore filling • good linearity in region t= 0.4 – 1 : reference behaviour • intercept: 0.12 mL/g=micropore volume • slope: 140 m2/g

28. DETERMINATION OF THE SURFACE AREA nano Silicalite-1 • Conclusions: • fresh sample: BET better method (40 m2/g) • calcined sample: comparative method preferable (140 m2/g) • although this method relies indirecly on the BET method, • via the BET determination of the reference surface area) • comparative method gives also micropore volume

29. 10 nm 100 nm CASE STUDY II: Mesopore size of Zeotile-6 • Ordered mesoporous material prepared from MFI nanoslabs • 2D hexagonal P6m XRD TEM 100 100 intensity 110 110 200 200 210 210 300 300 220 220 310 310 0 0 2 2 4 4 6 6 8 8 10 10 2 theta / 2 theta / ° °

30. ISOTHERM Zeotile-6 calcined • type IV with small hysteresis loop

31. COMPARATIVE – as Zeotile-6 *BET area = 1100 m2/g: probably overestimated

32. MESOPORE SIZE Zeotile-6 • the “4V/S” method • From SBET: From Sas: • Highly dependent on S! • from mesopore volume and XRD • ramorphous silica (2.2 g/ml): rSilicalite-1 (1.8 g/ml): • Only slightly dependent on r

33. MESOPORE SIZE DISTRIBUTION Zeotile-6 • Classical: 3.2 nm • Modified: 3.9 nm

34. MESOPORE SIZE: SUMMARY Zeotile-6 • Best methods for hexagonal mesoporous materials: • Vmeso and XRD: size • KJS: size distribution

35. CONCLUSIONS • surface area : better with comparative method than BET • IF good reference • pore volumes: easily determinable with comparative methods • micropore size: limited possibilities in the lab • mesopore size: know that • - traditional BJH underestimates pore size of • mesoporous silicas, especially in the range of 2 – 6 nm • - alternative/improved methods • if detailed analysis needed, verify software settings

36. SPECIFIC SURFACE AREA SPECIFIC SURFACE AREA: COMPARATIVE theory sample isotherm comparative plot Vads t or as ~Vads, ref p/p° p/p° reference isotherm • adsorption behaviour (isotherm) of reference is plotted on x-axis as t or as

37. SURFACE AREA – BET Zeotile-6 Nicely linear