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Ammonia IRMS-TPD (Infrared / mass spectroscopy - temperature-programmed desorption) method

Dependence of Acid Strength of H-form Zeolite on Structural Compression through Shortening of Al-O Distance （ Tottori Univ. ） Naonobu Katada , Katsuki Suzuki, Takayuki Noda and Miki Niwa.

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Ammonia IRMS-TPD (Infrared / mass spectroscopy - temperature-programmed desorption) method

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  1. Dependence of Acid Strength of H-form Zeolite on Structural Compression through Shortening of Al-O Distance （Tottori Univ.） Naonobu Katada, Katsuki Suzuki, Takayuki Noda and Miki Niwa Analysis of the relationship between geometric parameters and Brønsted acid strength based on DFT and ammonia TPD What controls Brønsted acid strength of zeolite? H O O O Al Si a θ O O O O

  2. Theoretical analysis of adsorption heat from ammonia TPD J. Phys. Chem., B,101, 5969 (1997) Acid strength is mainly controlled by the crystal structure. Influence of Si/Al ratio is small. Catal. Surveys Asia,8, 161 (2004) Ammonia IRMS-TPD (Infrared / mass spectroscopy - temperature-programmed desorption) method Ammonia adsorption energy (Brønsted acid strength) of each OH Compare Heat of ammonia adsorption Chem. Lett.,36, 1034 (2007) DFT (density functional theory) calculations

  3. Clusters cut from various frameworks Constant Si/Al ratio, Different crystallographic positions, various geometries FAU Al(1)-O(1)H-Si(1) MFI Al(7)-O(17)H-Si(4) Al Si O H Structurally optimized part

  4. Ammonia adsorption energy Eads ■ MFI ▲ FER ● FAU ▼ MOR ◎ *BEA ◆ MWW p: periodic boundary conditions Calc. by DFT The acid strength (Eads) varied with geometry. → The acid strength is controlled by the geometry. Eads was in agreement with the experimentally observed value． → The assumed models are reliable. Obs. by TPD Chem. Lett.,36, 1034 (2007)

  5. It is expected that: Analysis of the correlation between the geometric parameters and Eads in these models will provide an answer to an important question: How does the zeolite structure control the acid strength? The correlation was analyzed in this study.

  6. Methods: Tool ・・・ Dmol3 (Material studio 4.0, Accelrys) Cluster method : GGA / BLYP / DNP Periodic method : GGA / HCTH / DNP

  7. Structural optimization at inner (yellow) part Fixing outer (sky blue) part Involved in the energy calc. Si Si Si Si O O O O Si Si Si Si Si O O Si O O O O H Si O O Si Si Si O O O O O O O Si O O Si Si Al Si Si Si Si O O O O Si Si Si Si Cluster method 1 Al in T40-50, therefore Si/Al = 40-50 Influence of the sizes of the yellow and sky blue parts has been analyzed Convergense of the total energy was observed at this size Eads = EH-Z + ENH3(g) – ENH4-Z

  8. Strategy: Crystal structure　→　Positions of surrounding atoms Positions of surrounding atoms　→　Local structure of SiOHAl unit Local structure　→　Acid strength H O O O Al Si O O O O a θ Dependence of Eads on local structure a (AlO distance) and  (SiOAl angle) The smaller a，the larger  must generate the stronger acid site. I. N. Senchenya et al., J. Phys. Chem.,90, 4857 (1986) H. Kawakami et al, J. Chem. Soc., Faraday Trans.,2,80, 205 (1984) Which is predominant in real zeolites, a or ? Does other structural parameter (torsion) affect or not?

  9. H O O O Al Si a O O O O Plots of Eads against a

  10. ■ MFI ▲ FER ● FAU ▼ MOR ◎ *BEA ◆ MWW It looks: Weak dependence of Eads on a

  11. Coordination environment of NH4 H-O distance < 2.7 Å A: Bidentates in open spaces

  12. H H H H O N N H H O O H H Si O O Si Si O O Al Al H≡H−* H≡H=* B: Bidentates in small spaces C: Tridentates

  13. A: Bidentate in open space B: Bidentate in small space Eads may contain steric effects in such cases. Attractive: confinement effect Sauer et al., J. Am. Chem. Soc.,120, 1556 (1998) Repulsive: steric hindrance

  14. ■ MFI ▲ FER ● FAU ▼ MOR ◎ *BEA ◆ MWW

  15. MFI FER FAU MOR *BEA MWW Exceptions with high Eads Confinement effect A (bidentates in open spaces) showed a trend where the smaller a (AlO distance), the higher Eads A ● H=H− ▲ H=H−* ■ H=H=* ▼ H−H− B △ H≡H−* ▽ H≡H=* C □ H≡H=H=** ◇ H−H−H− Exceptions with low Eads Steric hindrance

  16. MFI FER FAU MOR *BEA MWW The smaller a (AlO distance), the more positive eOH Mulliken charge of OH A ● H=H− ▲ H=H−* ■ H=H=* ▼ H−H− B △ H≡H−* ▽ H≡H=* C □ H≡H=H=** ◇ H−H−H−

  17. H Larger a Far Al from SiOH SiOH is kept neutral O Si Al H Smaller a Lewis center (Al) withdraws electron of O More positive charge of OH More acidic H O Si Al

  18. H O O O Al Si θ O O O O Plots of Eads against 

  19. a (AlO distance) is the most important MFI FER FAU MOR *BEA MWW A ● H=H− ▲ H=H−* ■ H=H=* ▼ H−H− B △ H≡H−* ▽ H≡H=* C □ H≡H=H=** ◇ H−H−H−

  20. Crystal structure　→　Positions of surrounding atoms Positions of surrounding atoms　→　Local structure of SiOHAl unit Local structure　→　Acid strength H H O O O Al Si a triangle B triangle A O O b O O  Positions of 2 triangles are controlled mainly by the crystal topology b: distance between the centers of 2 triangles : planar angle between the 2 triangles Influences of b and  on a and Eads

  21. a // b a //  Search for suitable X, Y and Z in a = Xb + Y + Z

  22. a = 0.29b + 0.0037 + 0.56 MFI FER FAU MOR *BEA MWW a depends on the sum of b and  A ● H=H− ▲ H=H−* ■ H=H=* ▼ H−H−

  23. H O O O Al Si a O b O O  O Compression of SiOHAl unit from both sides makes b and  smaller，and finally a smaller. Simple geometry

  24. The smaller b and (the stronger compression) gives the higher acid strength Eads MFI FER FAU MOR *BEA MWW A ● H=H− ▲ H=H−* ■ H=H=* ▼ H−H− Eads (kJ mol-1) = 394 - 58b (Å) - 0.79 (degree)

  25. O Si Al We propose how the zeolite structure controls the acid strength． The stronger compression brings the shorter AlO distance and the higher acid strength． H

  26. Further speculation: Relationship between the acid strength and pore size H O O O Al Si O O O O Large pore Acid site on a wall with a low curvature Large  Weak acidity

  27. H O O O Al Si O O O O Small pore Acid site on a wall with a high curvature Small  Strong acidity

  28. Predicted general trend, the smaller pore, the stronger acidity In agreement with the empirical trends: MFI > BEA > FAU in MOR: 8MR > 12MR Niwa et al., J. Phys. Chem., B,109, 18749 (2005) in FAU: sodalite cage > supercage Suzuki et al., J. Phys. Chem., C,111, 894 (2007) No reports of strong acidity on so-called large pore zeolites, mesoporous silicas, nor mesoporous zeolites Necessary to consider for design of new zeolites

  29. O Si Al The stronger compression gives the higher acid strength． H

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