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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, Takayuki Noda and Miki NiwaOOOSiOHAlOOOAnalysis of the relationship between geometric parameters and Brnsted acid strength based on DFT and ammonia TPDWhat controls Brnsted acid strength of zeolite?a1Good afternoon, ladies and gentlemen. I would like to thank the organizers for giving me this opportunity to talk about our study to find an answer to this very important question, what controls the acid strength of zeolite, through the analysis of relationship between such geometric parameters and Broensted acid strength based on DFT.Ammonia IRMS-TPD (Infrared / mass spectroscopy - temperature-programmed desorption) methodAmmonia adsorption energy (Brnsted acid strength) of each OHDFT (density functional theory) calculationsCompareChem. Lett., 36, 1034 (2007) Theoretical analysis of adsorption heat from ammonia TPDJ. 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 adsorption2We have developed a method to calculate the adsorption heat of ammonia from TPD spectrum, and found that the acid strength was mainly controlled by the crystal structure, but the influence of Si/Al ratio was limited. We then developed a method of ammonia IRMS-TPD in order to precisely measure the Bronsted acid strength of zeolite. In a previous study, we have compared the observed values of ammonia adsorption energy with DFT calculations.

AlSiOHFAU Al(1)-O(1)H-Si(1)MFI Al(7)-O(17)H-Si(4)Clusters cut from various frameworksConstant Si/Al ratio, Different crystallographic positions, various geometriesStructurally optimized part3In this calculation, clusters were cut from various zeolites frameworks with approximately constant silicon to aluminum ratio and various structures.

Chem. Lett., 36, 1034 (2007) Ammonia adsorption energy EadsObs. by TPDCalc. by DFT MFI FER FAU MOR *BEA MWW

p: periodic boundary conditionsThe 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.4This shows an agreement in ammonia adsorption energy Eads between the calculated and observed values. Here 3 points were calculated in periodic boundary conditions with higher accuracy. From the calculation, the acid strength varied, indicating that the acid strength is controlled by the geometry. Then, the acid strength was in agreement with the experimentally observed value, suggesting that the assumed models are correct.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.5Therefore, it is expected that the analysis of the correlation between the geometric parameters and Eads in these models will provide an answer to this important question, how does the zeolite structure control the acid strength? In this study, we did it.Methods:Tool Dmol3 (Material studio 4.0, Accelrys)Cluster method : GGA / BLYP / DNPPeriodic method : GGA / HCTH / DNP6For DFT calculations, Dmol3 was used. In the cluster calculations, GGA level approximation and BLYP functional were used, while in some cases, the periodic boundary conditions and HCTH functional were applied.Crystal phase and pore systemPosition of acid siteMethodMFIAl7-O7-Si8ClusterAl7-O17-Si4ClusterAl9-O18-Si6ClusterAl11-O11-Si12ClusterAl12-O24-Si12ClusterFER10MRAl2-O1-Si2ClusterAl2-O7-Si4ClusterAl4-O6-Si4Clusterferrierite cageAl3-O4-Si1ClusterFAUsupercageAl1-O1-Si1ClusterPeriodicMOR12MRAl1-O3-Si2ClusterAl2-O2-Si4ClusterAl4-O2-Si2ClusterAl1-O6-Si1ClusterAl2-O5-Si2ClusterAl4-O10-Si4ClusterPeriodic8MRAl3-O1-Si1ClusterAl3-O9-Si3ClusterBEAAl1-O4-Si8ClusterPeriodicAl3-O10-Si8ClusterAl8-O10-Si3ClusterMWWAl4-O3-Si1ClusterAl3-O11-Si2ClusterAl5-O9-Si2ClusterAl5-O6-Si4ClusterAl5-O8-Si5ClusterAl6-O2-Si1ClusterSiOAlHOOOSiSiSiOOOSiSiSiOOOOOOOOOOOOOOOOOOSiSiSiSiSiSiSiSiSiSiSiSiSiSiSiSiSiSiStructural optimization at inner (yellow) partEads = EH-Z + ENH3(g) ENH4-ZInfluence of the sizes of the yellow and sky blue parts has been analyzedConvergense of the total energy was observed at this size1 Al in T40-50, therefore Si/Al = 40-50Fixing outer (sky blue) partInvolved in the energy calc.Cluster method8In the cluster method, a cluster had one Al and 40 to 50 T sites. The inner part, shown yellow in this picture, was structurally optimized, and the outer, sky blue part was fixed. For the energy calculation, the sky blue part was also involved. This procedure was done for proton form and ammonium form, and the difference in the total energy is thus shown as the ammonia adsorption energy.aOOOOOOSiOHAlStrategy:Crystal structurePositions of surrounding atomsPositions of surrounding atomsLocal structure of SiOHAl unitLocal structureAcid strengthDependence of Eads on local structure a (AlO distance) and (SiOAl angle)The smaller athe 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?9Strategy for analysis is here. We assumed that the crystal structure determines the positions of surrounding atoms, for example, the positions of these oxygens. Then, the positions of surrounding atoms determine the local structure of SiOHAl unit, and the local structure of SiOHAl unit determines the acid strength. Therefore, we have to analyze the 2 relationships, the relationship between the positions of surrounding oxygens and the local geometric parameters, and the relationship between the local geometric parameters and ammonia adsorption energy. Now I am focusing on the dependence of Eads on the local geometry, namely, a, AlO distance and theta, SiOAl angle. But before the study, classic bond theory or ab-initio calculation shows that the smaller a generates the stronger acid site while the larger theta must generate the stronger acid site. We have known it. So, in this study, the main question is which predominant in a real system, a or theta? and does another structural parameter such as torsion affect or not?OOOSiOHAlOOOaPlots of Eads against a10At first, I will plot Eads against a.

It looks: Weak dependence of Eads on a MFI FER FAU MOR *BEA MWW11However, it looks the relationship is weak.A: Bidentates in open spacesH-O distance < 2.7 Coordination environment of NH4

12But we noticed that the coordination environment of NH4 in the NH4 form resulted by the adsorption of ammonia is different among clusters. In some cases, NH4 forms a bidentate in which each one hydrogen is close to one oxygen, and in some cases, one hydrogen is located between two oxygens. This is an example. These species, with a little bit different positions of NH4, is however classified into the same category A, bidentates in relatively open spaces.B: Bidentates in small spaces

C: TridentatesHH* SiOAlOOHHNHHHH=* SiOAlOOHHNHHSiO

13In this species, one hydrogen is surrounded by three oxygens with small distances, meaning it is located in a very small cavity. These are termed B, bidentates in small cavities. This is an example. The ammonium ion is somewhat buried. These are tridentates.

A: Bidentate in open spaceB: Bidentate in small spaceEads may contain steric effects in such cases.Attractive: confinement effect Sauer et al., J. Am. Chem. Soc., 120, 1556 (1998)Repulsive: steric hindrance14Again the pictures are shown. NH4 is in an open space. In this case, it is in a small hollow. The adsorption energy must be affected by specific interactions due to such structures. In some cases, confinement effects may induce the attractive interaction as pointed by researchers, and in other cases, repulsive interaction is speculated due to the steric hindrance, and so that we have to consider the difference of these species.

MFI FER FAU MOR *BEA MWW15I am changing the symbols of this picture.

MFIFERFAUMOR*BEAMWW

A (bidentates in open spaces) showed a trend where the smaller a (AlO distance), the higher EadsExceptions with high EadsConfinement effectExceptions with low EadsSteric hindrance A H=H H=H* H=H=* HHB HH* HH=* C HH=H=** HHH 16The filled symbol shows the bidentates in open spaces. For these species, we can find a relationship where the smaller the AlO distance a, the higher the ammonia adsorption energy Eads. A few points show higher Eads than this relationship. These should be due to confinement effect. These few points should be due to steric hindrance.

MFIFERFAUMOR*BEAMWW

A H=H H=H* H=H=* HHB HH* HH=* C HH=H=** HHH The smaller a (AlO distance), the more positive eOHMulliken charge of OH17I would like to discuss more deeply why the smaller a brings the higher acid strength. These are the plots of mulliken charge of OH against the a. The smaller a gives the more positive charge of OH. Its interesting that this relationship contains also the different corodination environments of NH4.SiOAlHSiOAlHLarger aFar Al from SiOHSiOH is kept neutralSmaller aLewis center (Al) withdraws electron of OMore positive charge of OH More acidic H18It suggests that the larger a means the far aluminum from SiOH. SiOH should be kept neutral. In contrast, the smaller a makes that aluminum lewis center withdraws electron of O, giving the more positive charge of OH and more acidic proton.OOOSiOHAlOOOPlots of Eads against 19Then, I will plot Eads against theta, SiOAl angle.

MFIFERFAUMOR*BEAMWW

A H=H H=H* H=H=* HHB HH* HH=* C HH=H=** HHH a (AlO distance) is the most important20No relation was found, Also for other local geometric parameters, we found no relation. So I can say that a is the most important.OOOSiOHAlOOOatriangle Atriangle BbPositions of 2 triangles are controlled mainly by the crystal topologyb: distance between the centers of 2 triangles: planar angle between the 2 trianglesInfluences of b and on a and EadsHCrystal structurePositions of surrounding atomsPositions of surrounding atomsLocal structure of SiOHAl unitLocal structureAcid strength21Now I told the relationship between the local structure and acid strength. Then, lets move to the relationship between the positions of surrounding atoms and the local structure. In order to express numerically the positions of these atoms, 2 triangles are assumed, and Influences of b, distance between the centers of 2 triangles and omega, planar angle, on a and Eads are analyzed. a // ba // Search for suitable X, Y and Z in a = Xb + Y + Z

22These are the plots of a against b and the plots of a against omega. Weak relationships are seen, but it seems complex. We assumed a simple equation in which a is sum of b and omega, and we searched numerically for suitable coefficients.

MFIFERFAUMOR*BEAMWW

A H=H H=H* H=H=* HHa depends on the sum of b and a = 0.29b + 0.0037 + 0.5623From the very simple minimum square method, we find this equation, in which a depends on the sum of b and omega.Compression of SiOHAl unit from both sides makes b and smallerand finally a smaller.Simple geometryOOOOSiOAlOOabH

24This means that the compression of SiOHAl unit from both sides makes b and omega smaller, and finally it makes a smaller. This is very simple geometry.MFIFERFAUMOR*BEAMWW

A H=H H=H* H=H=* HH

Eads (kJ mol-1) = 394 - 58b () - 0.79 (degree)The smaller b and (the stronger compression) gives the higher acid strength Eads25Finally, we found this relationship. The smaller b and omega, namely the stronger compression from both sides, give the higher acid strength Eads. The equation is this.The stronger compression brings the shorter AlO distance and the higher acid strengthWe propose how the zeolite structure controls the acid strengthAl

SiOH26We propose how the zeolite structure controls the acid strength. The stronger compression brings the shorter AlO distance and the higher acid strength.OOOSiOAlOOOHLarge poreAcid site on a wall with a low curvatureLarge Weak acidityFurther speculation: Relationship between the acid strength and pore size27I am showing further speculations. Our conclusion suggests a relationship between the acid strength and pore size. In large pore, the acid site is located on a wall with a low curvature. Omega should be large and the acidity should be weak.Small poreAcid site on a wall with a high curvatureSmall Strong acidityOOOOSiOAlOOH28In contrast, in small pore, with acid site on a wall with a high curvature, omega should be small, and the acid strength should be strong. And, in many cases, b should be small because it is compressed through bonds on this side.Predicted general trend, the smaller pore, the stronger acidityIn agreement with the empirical trends: MFI > BEA > FAU in MOR: 8MR > 12MRNiwa et al., J. Phys. Chem., B, 109, 18749 (2005) in FAU: sodalite cage > supercageSuzuki et al., J. Phys. Chem., C, 111, 894 (2007) No reports of strong acidity on so-called large pore zeolites, mesoporous silicas, nor mesoporous zeolitesNecessary to consider for design of new zeolites29Of course, exceptions should be possible, but we know an inverse relationship between pore size and acid strength. The following experimental observations are in agreement with this speculation. The acid strength is in the order of MFI, BEA, FAU, inverse order of micropore size. In MOR and FAU, the acid strength is stronger in smaller pores among pores or cavities with different sizes. And we have not seen reports of strong acidity on so-called large pore zeolites, mesoporous silicas and mesoporous zeolites. This is my speculation, but I think, for the design of the zeolite catalyst with strong acidity and high accessibility, this should be considered.AlThe stronger compression gives the higher acid strength

SiOH30But OK, lets go back to the conclusion of this study. The stronger compression brings the stronger acidity. Thank you for your kind attention. Sma4Win ver. 1.1 SMA file 1[FIG3.SMP] Sma4Win ver. 1.5 SMP fileConverted from Ngraph ver. 5.3 file

8[TPD.TXT]TPD.TXT0 0 0 1 0 2 15 2 16 40 255 300 0 0 0 00 4 500 2550 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 6 19 7 40 16711935 300 0 0 0 00 2 500 167119350 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 2 27 2 40 16711808 300 0 0 0 00 8 500 167118080 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 1 30 1 40 16711680 300 0 0 0 00 10 500 167116800 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 7 31 10 40 16711680 300 0 0 0 00 10 500 167116800 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 3 41 4 40 16711680 300 0 0 0 00 10 500 167116800 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 1 45 1 40 16711680 300 0 0 0 00 10 500 167116800 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[TPD.TXT]TPD.TXT0 0 0 1 0 2 0 60 -1 40 16711680 300 0 0 0 00 10 500 167116800 0 1 0 0 0 16777215 50 0xy0 0 0 0 0 0 0 0 01 40 0 300 1

[GRAPH]4000 7000 14000 14000100 1 1 040 0 300 16777215

[AXIS-0]0 5 1 0 0 10000 -1 -1 0 1 0 0 1 5 5 1 12 0 1 1 1 1 0 100 0 300 100-24 0 0 0 400 0 0 0 0 0 0 2 18Times New Roman1 0 300 40 0 300 11 0 150 40 0 300 1

[AXIS-1]100 160 10 0 0 10000 -1 -1 0 1 0 0 1 5 5 1 12 0 1 1 1 1 -100 -400 0 1200 100-24 0 0 0 400 0 0 0 0 0 0 2 18Times New Roman1 0 300 40 0 300 11 0 150 40 0 300 1

[AXIS-2]0 5 1 0 0 10000 -1 -1 -1 1 0 0 1 5 5 1 12 0 1 1 1 0 0 -100 0 900 100-24 0 0 0 400 0 0 0 0 0 0 2 18Times New Roman1 0 300 40 0 300 11 0 150 40 0 300 1

[AXIS-3]100 160 10 0 0 10000 -1 -1 -1 1 0 0 1 5 5 1 12 0 1 1 1 0 100 0 0 0 100-24 0 0 0 400 0 0 0 0 0 0 2 18Times New Roman1 0 300 40 0 300 11 0 150 40 0 300 1

[TITLE]63500 16100 0 1[Al] - [Na] / mol kg^-1@-24 0 0 0 400 0 0 0 0 0 0 2 18Times New Roman-24 0 0 0 400 0 0 0 128 0 0 2 18

-2300 12700 0 1Adsorption heat of ammonia / kJ mol^-1-24 0 -900 900 400 0 0 0 0 0 0 2 18Times New Roman-24 0 -900 900 400 0 0 0 128 0 0 2 18

7200 5100 255 1Al-MOR-24 0 0 0 400 0 0 0 0 3 2 1 18Times New Roman-24 0 0 0 400 0 0 0 128 0 0 2 18

3600 7500 16711935 1Al-MFI-24 0 0 0 400 0 0 0 0 3 2 1 18Times New Roman-24 0 0 0 400 0 0 0 128 0 0 2 18

4000 9200 8388736 1Al-BEA-24 0 0 0 400 0 0 0 0 3 2 1 18Times New Roman-24 0 0 0 400 0 0 0 128 0 0 2 18

10200 11200 16711680 1Al-FAU-24 0 0 0 400 0 0 0 0 3 2 1 18Times New Roman-24 0 0 0 400 0 0 0 128 0 0 2 18

[ARROW]0[DOT]0[OTHERS] 20 0 0 0 0 0 0 0 0-24 0 0 0 400 1 0 0 2 3 2 1 18Symbol

[MASKS] 000

[DATA][TPD.TXT] 1997-10-6 20:52:50[M]-[Na]A0dH+sigma-sigmaMOR1.9921.573141+9-92.3861.494139+9-90.2990.269145+9-90.0000.0000.4260.448141+5-50.7670.777143+6-61.3801.195144+6-61.6361.590144+7-71.1011.095145+4-40.7160.797145+6-60.3250.249144+7-70.6510.568144+7-70.9890.996143+4-41.3011.285144+5-51.0761.046146+7-70.3870.408144+1-1MFI0.000.0080.4330.647128+6-61.0541.095129+5-51.1101.294127+3-30.7490.737134+5-50.3840.448133+0-00.2940.339134+1-1BEA1.230.83122+9-91.190.55122+11-11FAU4.840.86118+10-100.000.020.430.13117+3-31.560.71110+4-480C1.520.71109+3-31.721.10108+2-280C2.021.02109+4-42.5080C3.241.34112+9-93.531.72115+11-1180C4.29(0.1780C0.740.38112+3-380C0.890.92106+5-50.990.73108+2-280C2.4380C3.161.30113+8-80.79(0.20USYGa-MFI0.770.717126.5+9-90.570.508129+6-60.460.511129.5+4-40.330.493123.5+2-20.240.302131.0+1-10.230.277129.5+5-50.190.311128.0+1-10.160.208130.0+4-40.120.175131.0+2-20.130.202129.0+2-20.210.264128.0+4-4[End of Data]

1.0541.095129