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

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Presentation on theme: "Dependence of Acid Strength of H-form Zeolite on Structural Compression through Shortening of Al-O Distance ( Tottori Univ. ) Naonobu Katada, Katsuki Suzuki,"— Presentation transcript:

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 O O O Si O H Al O O O Analysis of the relationship between geometric parameters and Brønsted acid strength based on DFT and ammonia TPD What controls Brønsted acid strength of zeolite? θ a

2 Ammonia IRMS-TPD (Infrared / mass spectroscopy - temperature-programmed desorption) method Ammonia adsorption energy (Brønsted acid strength) of each OH DFT (density functional theory) calculations Compare Chem. Lett., 36, 1034 (2007) 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) Heat of ammonia adsorption

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

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

5 It is expected that: Analysis of the correlation between the geometric parameters and E ads 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 ・・・ Dmol 3 (Material studio 4.0, Accelrys) Cluster method : GGA / BLYP / DNP Periodic method : GGA / HCTH / DNP

7 Crystal phase and pore systemPosition of acid siteMethod MFI Al7-O7-Si8Cluster Al7-O17-Si4Cluster Al9-O18-Si6Cluster Al11-O11-Si12Cluster Al12-O24-Si12Cluster FER 10MR Al2-O1-Si2Cluster Al2-O7-Si4Cluster Al4-O6-Si4Cluster ferrierite cageAl3-O4-Si1Cluster FAUsupercageAl1-O1-Si1 Cluster Periodic MOR 12MR Al1-O3-Si2Cluster Al2-O2-Si4Cluster Al4-O2-Si2Cluster Al1-O6-Si1Cluster Al2-O5-Si2Cluster Al4-O10-Si4 Cluster Periodic 8MR Al3-O1-Si1Cluster Al3-O9-Si3Cluster BEA Al1-O4-Si8 Cluster Periodic Al3-O10-Si8Cluster Al8-O10-Si3Cluster MWW Al4-O3-Si1Cluster Al3-O11-Si2Cluster Al5-O9-Si2Cluster Al5-O6-Si4Cluster Al5-O8-Si5Cluster Al6-O2-Si1Cluster

8 Si O Al H O O O Si O O O O O O O O O O O OO O O O O O O O O Structural optimization at inner (yellow) part E ads = E H-Z + E NH3(g) – E NH4-Z Influence of the sizes of the yellow and sky blue parts has been analyzed Convergense of the total energy was observed at this size 1 Al in T 40-50, therefore Si/Al = Fixing outer (sky blue) part Involved in the energy calc. Cluster method

9 a O O O O O O Si O H Al θStrategy: Crystal structure → Positions of surrounding atoms Positions of surrounding atoms → Local structure of SiOHAl unit Local structure → Acid strength Dependence of E ads 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?

10 O O O Si O H Al O O O a Plots of E ads against a

11 It looks: Weak dependence of E ads on a ■ MFI ▲ FER ● FAU ▼ MOR ◎ *BEA ◆ MWW

12 A: Bidentates in open spaces H-O distance < 2.7 Å Coordination environment of NH 4

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

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

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

16 MFIFERFAUMOR*BEAMWW A (bidentates in open spaces) showed a trend where the smaller a (AlO distance), the higher E ads Exceptions with high E ads Confinement effect Exceptions with low E ads Steric hindrance A ● H=H− ▲ H=H−* ▲ H=H−* ■ H=H=* ■ H=H=* ▼ H−H− ▼ H−H− B △ H≡H−* ▽ H≡H=* ▽ H≡H=* C □ H≡H=H=** ◇ H−H−H− ◇ H−H−H−

17 MFIFERFAUMOR*BEAMWW A ● H=H− ▲ H=H−* ▲ H=H−* ■ H=H=* ■ H=H=* ▼ H−H− ▼ H−H− B △ H≡H−* ▽ H≡H=* ▽ H≡H=* C □ H≡H=H=** ◇ H−H−H− ◇ H−H−H− The smaller a (AlO distance), the more positive e OH Mulliken charge of OH

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

19 O O O Si O H Al O O O  Plots of E ads against 

20 MFIFERFAUMOR*BEAMWW A ● H=H− ▲ H=H−* ▲ H=H−* ■ H=H=* ■ H=H=* ▼ H−H− ▼ H−H− B △ H≡H−* ▽ H≡H=* ▽ H≡H=* C □ H≡H=H=** ◇ H−H−H− ◇ H−H−H− a (AlO distance) is the most important

21 O O O Si O H Al O O O a triangle A triangle B b  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 E ads H Crystal structure → Positions of surrounding atoms Positions of surrounding atoms → Local structure of SiOHAl unit Local structure → Acid strength

22 a // b a //  Search for suitable X, Y and Z in a = X b + Y  + Z

23 MFIFERFAUMOR*BEAMWW A ● H=H− ▲ H=H−* ▲ H=H−* ■ H=H=* ■ H=H=* ▼ H−H− ▼ H−H− a depends on the sum of b and  a = 0.29 b 

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

25 MFIFERFAUMOR*BEAMWW A ● H=H− ▲ H=H−* ▲ H=H−* ■ H=H=* ■ H=H=* ▼ H−H− ▼ H−H− E ads (kJ mol -1 ) = b (Å)  (degree) The smaller b and  (the stronger compression) gives the higher acid strength E ads

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

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

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

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

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


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