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Evaluation of water adsorption on different kinds of zeolite through the Monte Carlo simulation
1-1.~. Karge ancl J. Weitlcamp (Eels.) Zeolite Science 1994: Recent Progress and Discussions
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
229
EVALUATION OF WATER ADSORFFION ON DIFFERENT KINDS OF ZEOLITE
T H R O U G H THE MONTE CARLO SIMULATION T. Inui and Y. Tanaka Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
SUMMARY Adsorption of water molecules on various zeolites was simulated by the Monte Carlo (MC) method. Water molecules were adsorbed surrounding the counter metal cation in zeolites such as Na-ZSM-5 and Fe-ZSM-5 even at a low H 2 0 pressure, while acid sites of protonated zeolites such as H-ZSM-5 and H-Fesilicate were scarcely covered with H 2 0 molecules. These results are consistent with the high tolerance of metaUosilicate catalysts to water vapor such as H-Cosilicate in the N O conversion reaction.
INTRODUCTION Removal of NO in exhaust gases from diesel engines and other lean-burn combustion facilities has been one of the most important but difficult subjects in catalytic chemistry, because 02 in the exhaust gas strongly interferes with N O conversion on the catalyst. In our previous study, it was found that Cu-incorporated zeolite [1] showed a high performance for NO decomposition in the absence of water vapor. However, under practical conditions, the catalyst must have durability against water vapor. For this purpose, H-Co-silicate [2] was adopted and it exerted a high performance in NO conversion even in the presence of a high concentration (10%) of water vapor. Water adsorption properties of zeolitic catalysts are usually m e a s u r e d by an infrared analysis or a nuclear magnetic resonance method; however, the details of the microscopic states of water molecules adsorbed has remained ambiguous. In this study, the microstructure of water molecules was simulated by computational chemistry methods to understand the water resisting property of metallosilicates. EXPERIMENTAL METHOD
Four kinds of MFI-type silicates and NaA zeolites were treated for calculation. Four silicon atoms of the T12 sites in the unit cell of MFI-type silicate, which are the most probable for the evaluation of lowest potential, were replaced by A1 or Fe atoms. Counterpart Na cations for substituted elements in those MFI-type
230 metaUosilicates were exchanged by H, or Fe. In the case of NaA zeolite, half of the silicon atoms were replaced by AI. The positions of exchanged ions were optimized by energy minimization. The charges of the zeolites and H20 molecules were calculated with the charge equilibration method before simulation of H20 adsorption on zeolites. The Monte Carlo simulation [3] was carried out for the five zeolites, Na-ZSM-5, Fe-ZSM-5, H-ZSM-5, H-Fe-silicate and NaA using CERIUS software which is a computational instrument for material research offered by Molecular Simulation Inc.. One cycle of a fixed pressure (grand canonical) algorithm consisted of creation, removal, translation and rotation of a molecule. The amount of H 2 0 adsorbed was counted after this cycle, and the cycle was repeated 250,000 times, under a condition of 300 K and a pressure range from 0.1k to 20kPa by means of graphics super computer TITAN750V (Stardent Inc.). RESULTS AND DISCUSSION Figure 1 shows the geometric distribution of H20 molecules adsorbed in the unit cell of Na-ZSM-5 under 0.1kPa depicted as A-C coordinate, and areas composed of the dots are proportional to the number of adsorbed H20 molecules. In this case, H20 molecules adsorbed surrounding the Na cations. Each ion was covered by two or three H20 molecules. In the case of Fe-ZSM-5, the result was quite similar although the total amount of adsorbed H20 molecules was smaller than that on Na-ZSM-5. The adsorbed amounts of water on H-ZSM-5 (i.e. H-Al-silicate) and H-Fe-silicate were much smaller than those on Na-ZSM-5 and Fe-ZSM-5, and the acid sites on those protonated metallosilicates were not covered by H 2 0 molecules. Such metaUosilicates, in which transition metal elements are stabilized by incorporation in the framework of the silicate, have high durability against H 2 0 vapor as demonstrated by H-Co-silicate [2]. Figure 2 shows the adsorption isotherms of H20 on NaY, Na-ZSM-5, and H-ZSM5. The isotherms of H20 for NaA and Na-ZSM-5 were of the Freundlich type. On the other hand, the isotherm on H-ZSM-5 obeyed the B.E.T. type. This is rationally understood since the interaction between H20 and protons is weaker than the mutual interaction between water molecules, which would occur in NaZSM-5. At higher water pressure, the adsorbed amount increased due to the condensation of water molecules.
231
] ~176 A
1 =<1
0.25 f
O t~
Q.P
._r
8
(3
0. 5i ~0
0"10f
E E 0"05t~ tO 0 4)~ G~"
0 2 4 6 8 10 12 14 16 18 20 A coordinate (A) Figure 1 Distribution of H20 molecules adsorbed on Na-ZSM-5 at 3 0 0 K under 0.1 kPa. o:the position of AI in the framework .,:the position of counter cation (Na+)
I
,
,
,
:=:
0 2 4 6 8 10 12 14 16 18 20 Pressure (kPa) Figure 2 Adsorption isotherms of H20 molecules at 300K on NaA(o), Na-ZSM-5(=). and H-ZSM-5(=).
REFERENCES [1] T. Inui, S. Kojo, M. Shibata, T. Yoshida and S. Iwamoto, Stud. Surf. 5ci. Catal., 69, 355 (1991). [2] T. Inui, T. Hirabayashi and 5. Iwamoto, Catal. Left., under contribution. [3] T. Inui, T. Tanaka, K. Matsuba, N. Goto, Y. Nakazaki and M. Inoue, Preprints 9th Annual Meeting of Zeolite Association of Japan, 1993, p. 52.
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
229
EVALUATION OF WATER ADSORFFION ON DIFFERENT KINDS OF ZEOLITE
T H R O U G H THE MONTE CARLO SIMULATION T. Inui and Y. Tanaka Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
SUMMARY Adsorption of water molecules on various zeolites was simulated by the Monte Carlo (MC) method. Water molecules were adsorbed surrounding the counter metal cation in zeolites such as Na-ZSM-5 and Fe-ZSM-5 even at a low H 2 0 pressure, while acid sites of protonated zeolites such as H-ZSM-5 and H-Fesilicate were scarcely covered with H 2 0 molecules. These results are consistent with the high tolerance of metaUosilicate catalysts to water vapor such as H-Cosilicate in the N O conversion reaction.
INTRODUCTION Removal of NO in exhaust gases from diesel engines and other lean-burn combustion facilities has been one of the most important but difficult subjects in catalytic chemistry, because 02 in the exhaust gas strongly interferes with N O conversion on the catalyst. In our previous study, it was found that Cu-incorporated zeolite [1] showed a high performance for NO decomposition in the absence of water vapor. However, under practical conditions, the catalyst must have durability against water vapor. For this purpose, H-Co-silicate [2] was adopted and it exerted a high performance in NO conversion even in the presence of a high concentration (10%) of water vapor. Water adsorption properties of zeolitic catalysts are usually m e a s u r e d by an infrared analysis or a nuclear magnetic resonance method; however, the details of the microscopic states of water molecules adsorbed has remained ambiguous. In this study, the microstructure of water molecules was simulated by computational chemistry methods to understand the water resisting property of metallosilicates. EXPERIMENTAL METHOD
Four kinds of MFI-type silicates and NaA zeolites were treated for calculation. Four silicon atoms of the T12 sites in the unit cell of MFI-type silicate, which are the most probable for the evaluation of lowest potential, were replaced by A1 or Fe atoms. Counterpart Na cations for substituted elements in those MFI-type
230 metaUosilicates were exchanged by H, or Fe. In the case of NaA zeolite, half of the silicon atoms were replaced by AI. The positions of exchanged ions were optimized by energy minimization. The charges of the zeolites and H20 molecules were calculated with the charge equilibration method before simulation of H20 adsorption on zeolites. The Monte Carlo simulation [3] was carried out for the five zeolites, Na-ZSM-5, Fe-ZSM-5, H-ZSM-5, H-Fe-silicate and NaA using CERIUS software which is a computational instrument for material research offered by Molecular Simulation Inc.. One cycle of a fixed pressure (grand canonical) algorithm consisted of creation, removal, translation and rotation of a molecule. The amount of H 2 0 adsorbed was counted after this cycle, and the cycle was repeated 250,000 times, under a condition of 300 K and a pressure range from 0.1k to 20kPa by means of graphics super computer TITAN750V (Stardent Inc.). RESULTS AND DISCUSSION Figure 1 shows the geometric distribution of H20 molecules adsorbed in the unit cell of Na-ZSM-5 under 0.1kPa depicted as A-C coordinate, and areas composed of the dots are proportional to the number of adsorbed H20 molecules. In this case, H20 molecules adsorbed surrounding the Na cations. Each ion was covered by two or three H20 molecules. In the case of Fe-ZSM-5, the result was quite similar although the total amount of adsorbed H20 molecules was smaller than that on Na-ZSM-5. The adsorbed amounts of water on H-ZSM-5 (i.e. H-Al-silicate) and H-Fe-silicate were much smaller than those on Na-ZSM-5 and Fe-ZSM-5, and the acid sites on those protonated metallosilicates were not covered by H 2 0 molecules. Such metaUosilicates, in which transition metal elements are stabilized by incorporation in the framework of the silicate, have high durability against H 2 0 vapor as demonstrated by H-Co-silicate [2]. Figure 2 shows the adsorption isotherms of H20 on NaY, Na-ZSM-5, and H-ZSM5. The isotherms of H20 for NaA and Na-ZSM-5 were of the Freundlich type. On the other hand, the isotherm on H-ZSM-5 obeyed the B.E.T. type. This is rationally understood since the interaction between H20 and protons is weaker than the mutual interaction between water molecules, which would occur in NaZSM-5. At higher water pressure, the adsorbed amount increased due to the condensation of water molecules.
231
] ~176 A
1 =<1
0.25 f
O t~
Q.P
._r
8
(3
0. 5i ~0
0"10f
E E 0"05t~ tO 0 4)~ G~"
0 2 4 6 8 10 12 14 16 18 20 A coordinate (A) Figure 1 Distribution of H20 molecules adsorbed on Na-ZSM-5 at 3 0 0 K under 0.1 kPa. o:the position of AI in the framework .,:the position of counter cation (Na+)
I
,
,
,
:=:
0 2 4 6 8 10 12 14 16 18 20 Pressure (kPa) Figure 2 Adsorption isotherms of H20 molecules at 300K on NaA(o), Na-ZSM-5(=). and H-ZSM-5(=).
REFERENCES [1] T. Inui, S. Kojo, M. Shibata, T. Yoshida and S. Iwamoto, Stud. Surf. 5ci. Catal., 69, 355 (1991). [2] T. Inui, T. Hirabayashi and 5. Iwamoto, Catal. Left., under contribution. [3] T. Inui, T. Tanaka, K. Matsuba, N. Goto, Y. Nakazaki and M. Inoue, Preprints 9th Annual Meeting of Zeolite Association of Japan, 1993, p. 52.