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Adsorption complexes of ammonia on germanium- and gallium-modified zeolites
ELSEVIER
THEO CHEM Journal of Molecular Structure (Theochem) 312 (1994) 183-194
Adsorption complexes of ammonia on germanium- and galliummodified zeolites Jumras Limtrakul*, Jarungsak Yoinuan, Duangkamol Tantanak, Michael M. Probst 1 Laboratory for Computational and Applied Chemistry, Chemistry Department, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand
(Received 21 December 1993; accepted 30 December 1993)
Abstract Adsorption complexes of ammonia on germanium- and gallium-modified zeolites were investigated by means of ab initio calculations. Full optimization of the structures was done at the DZ, DZP, and TZ2P levels of theory. Two new models of ammonia adsorption on zeotypes are proposed. In one of these, the covalent structures are stabilized at the bridging OH group by two hydrogen bonds with binding energies of -12.20 and -12.80kcalmol- 1 for the germanium and gallium zeotypes, respectively. The other configuration is a type of the ion-pair structure, where the ammonium cation forms two very strong hydrogen bonds with the anionic zeotype clusters. The conversion energy of acid catalyst/ ammonia to ion-pair complexes for zeolite, germanium and gallium zeotypes, was found to be about 2-10 kcal mol-I.
1. Introduction The investigation of Bronsted acidic surface sites capable of donating protons to adsorbed molecules is the basis of many industrially important reactions in heterogenous catalysis [1,2]. Therefore the structures of these active sites and the details of interaction processes, i.e. adsorption/ desorption, protonation/deprotonation and ionpair formation processes, are of prime importance. Recently it has been found that the substitution of the silicon and aluminium in zeolite by other elements can lead to a remarkable variation in catalytic properties [3,4]. However, most work done to date has focused largely on the interaction *Corresponding author. I Permanent address: Institut fuer Theoretische Chemie, Universitaet Innsbruck, Austria.
of electron-pair donors with the zeolite, although the presence of foreign atoms also significantly disturbs the catalytic structure. Simple models of silica, zeolite and modified zeolite (zeotype) have been studied recently, and the Bronsted acidity order found was in excellent agreement with experimental observations [5]. However, the models used cannot account for NHt bonded in the zeotype. Therefore in the present work we applied for the first time new adsorption models for zeolites (=== SiOA1(OHhOHSi === [6-8]) at the DZ, DZP and TZ2P levels to zeotypes containing germanium (Ge) and gallium (Ga) atoms. A cluster that is smaller but still has a tetrahedral form of AI(OH)4" was also investigated, at both SCF and correlated levels of theory. The results obtained do not only show the relationship between zeotype structure and activity at the molecular level, but also help in
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I. Limtrakul et al./l. Mol. Struct. (Theochem) 312 (1994) 183-194
184
Table I Specification of the basis sets used Atom
TZ2P
DZP
H
(4,1)/[2,1]
{31/1}
np = 0.80
(5,2)/[3,2]
{3ll/ll}
np = 0.46;1.39
N
(8,4,1 )/[4,2,1]
{5Ill/31/1}
np = 1.00
(9,5,2)/[5,3,2]
{5llll/3ll/ll}
nd = 0.58;1.73
0'
(8,4,1 )/[4,2,1]
{5Ill/31/1}
nd = 1.20
(9,5,2)/[5,3,2]
{5llll/311/ll }
nd = 0.69;2.08
Al
(11,7,1)/[6,4,1]
{52lll/4Ill/l}
nd = 0.30
(12,9,2)/[5,7,2]
{5211111/51111/11}
nd = 0.17;0.52
Si
(ll,7,1 )/[6,4,1]
{52111/4Ill/l}
nd = 0.40
(13,9,2)/[6,5,2]
{631111/42111/11}
nd = 0.23;0.69
• The relatively large value (nd) for oxygen in comparison with aluminium and silicon takes into account the fact that oxygen is the most electronegative component in the system studied. The basis sets employed for gallium and germanium were of the ECP type [16].
designing efficient and selective "candidates" in the catalytic field.
2. Method
All calculations were performed at the ab initio level using the program TURBOMOLE [9,10], based on the direct SCF method of Almloef et al. [11]. The
conventional method is to calculate and store the two-electron integrals, which are then retrieved and processed in each SCF iteration. To avoid the sheer number of two-electron integrals required, an alternative approach of abandoning integral storage and recomputing the most time-consuming step (calculation of the two-electron integral) in each iteration has been suggested. In order to increase efficiency, the algorithm includes selective storage of expensive and frequently used integrals,
Table 2 Optimized parameters for the molecular model (AI(OH).j) at the SCF and MP2 levels SCF
MP2
STO-3G
3-21G
DZ
DZP
TZ2P
6-3IG*
6-3IG**
Bond lengths (pm) AI-OJ AI-02 AI-03 AI-04 (AI-O) Ol-HI 02-H2 03-H3 04-H4
176.4 176.4 177.0 176.8 (176.7) 98.7 98.7 98.6 98.6
175.1 175.1 175.8 175.2 (175.3) 96.3 96.3 96.5 96.2
176.8 176.8 177.4 176.5 (176.9) 95.3 95.3 95.4 95.2
176.6 176.6 176.6 176.6 (176.6) 94.3 94.3 94.3 94.3
175.5 175.3 176.1 176.6 (175.9) 93.7 93.7 93.7 93.7
178.7 178.7 179.6 180.6 (179.4) 96.8 96.8 97.0 96.9
178.6 178.6 179.5 180.4 (179.30) 96.2 96.2 96.3 96.2
Bond angles (deg.) 02-AI-OJ 03-AI-04 OJ-Al-04 02-AI-03 AI-OJ-HI AI-02-H2 AI-03-H3 AI-04-H4
105.2 104.3 ll1.7 ll2.0 107.2 107.2 107.9 107.2
ll1.5 105.3 llO.9 108.6 ll7.4 ll7.2 ll5.5 120.9
110.7 107.3 110.3 109.0 130.0 127.9 126.3 129.4
106.3 106.3 ll5.8 ll5.9 ll3.3 113.2 113.4 ll3.5
106.8 103.9 llO.3 llO.1 ll4.8 113.8 112.8 115.7
105.1 101.5 113.1 ll3.1 108.8 108.8 105.5 108.9
105.0 101.8 ll3.0 ll3.2 108.9 108.9 105.4 109.0
Table 3 SCF optimized structural parameters for [H3GeOAI(OH)zOGeH3r, H3GeOHAI(OH)zOGeH3' [H3GeOAI(OH)zOGeH3r [NH4t and H3GeOAI(OH)zOGeH3/NH3
Bond lengths (pm) AI-OI AI-02 AI-03 AI-04 (AI-0) Ge5-01 Ge9-02 Ge5-H6 Ge5-H7,8 Ge9-HI0 Ge9-H11,12 O-H9 N-H17 N-HI8 N-H13,14 01-N 02-N Bond angles (deg.) 02-AI-01 03-AI-04 HI7-N-HI8 H13-N-HI4 Ge9-02-H9 Ge9-02-AI Ge5-0l-AI HI5-03-AI 02-H9-N
[H3GeOAI(OH)zOGeH3]
[H3GeOHAI(OH)zOGeH3]
[H3GeOAI(OH)20GeH3]NH4
H3GeOHAI(OH)20GeH3]NH3
DZ
DZP
TZ2P
DZ
DZP
TZ2P
DZ
TZ2P
DZ
DZP
TZ2P
175.6 175.6 176.0 176.0 (175.8) 171.2 171.2 155.2 155.2 155.2 155.2
176.5 176.5 175.8 175.8 (176.2) 172.5 172.5 153.5 153.3 153.5 153.3
175.6 175.6 174.8 174.8 (175.2) 171.7 171.7 154.3 154.1 154.3 154.1
171.0 188.8 171.7 171.7 (175.8) 173.9 185.5 154.1 153.9 152.9 151.6 96.17
170.9 189.6 172.3 172.3 (176.3) 174.5 185.5 152.2 152.3 151.2 150.2 95.00
170.1 189.3 171.3 171.3 (175.5) 174.2 184.1 152.9 153.1 152.0 151.2 94.17
181.4 180.1 174.5 174.5 (177.6) 178.9 178.2 153.9 152.9 153.9 153.2
172.5 172.5 (176.4) 177.7 177.7 152.6 151.6 152.6 151.6
179.7 178.8 171.6 171.6 (175.4) 176.9 176.7 153.5 152.5 153.5 152.5
107.3 111.2 101.0 249.3 255.5
106.0 106.1 100.5 256.1 256.1
105.7 104.2 100.0 260.6 256.4
174.5 185.2 171.9 171.9 (175.9) 175.4 182.6 154.0 153.8 153.0 152.1 102.40 101.9
172.8 187.0 172.5 172.5 (177.5) 175.0 183.0 152.5 152.3 151.2 150.6 97.76 100.9
172.0 186.5 171.6 171.6 (175.4) 174.8 182.6 154.0 154.0 152.0 151.4 96.70 100.4
100.8 293.2 260.0
100.5 328.0 275.9
99.9 308.5 275.7
DZP 180.4 180A
:-.
t-< ~.
.,...
;>;-
!2.. ~
~
~
,...~ ~ ?
!:
"' ~
'c"
";:,'" :li 'v..
......
'"...... "' '0
~
'-
107.4 108.9
173.3 173.3 131.2
107.6 106.8
136.9 136.9 116.3
106.3 106.3
100.6 114.7
133.0 133.0 116.5
121.8 124.4 171.3 146.8
97.7 115.3
120.3 128.6 154.2 127.5
95.7 115.0
119.0 129.4 145.7 125.5
99.9 112.2 94.6 110.8
100.3 109.3 97.3 110.3
99.8 109.1 98.3 110.3
132.6 130.0 125.3
126.4 126.4 125.3
126.0 126.5 123.7
102.5 115.3 111.3 120.9 126.1 149.2 148.2 163.5
98.1 115.2 107.4 118.5 131.6 141.9 126.9 175.9
99.8 114.7 108.0 116.8 127.7 135.7 125.5 166.0
......
~
I ......
'0 .....
00
v.
00
0\
Table 4 SCF optimized structural parameters for [H3SiOGa(OHhOSiH3r, H 3SiOHGa(OHhOSiH 3, [H3SiOGa(OHhOSiH3nNHtJ and H 3SiOGa(OHhOSiH 3/NH 3 [H3SiOGa(OHhOGaH3J DZ Bond lengths (pm) Ga-OI Ga-02 Ga-03 Ga-04 (Ga-O) Si5-OJ Si9-02 Si5-H6 Si5-H7,8 Si9-H10 Si9-HII,12 O-H9 N-HI7 N-HI8 N-H13,14 01-N 02-N Bond angles (deg.) 02-Ga-OI 03-Ga-04 HI7-N-Hl8 HI3-N-HI4 Si9-02-H9 Si9-02-Ga Si5-01-Ga HI5-03-Ga 02-H9-N
DZP
TZ2P
[H3SiOHGa(OHhOSiH3]
[H 3SiOGa(OHhOSiH 3]NH4
DZ
DZ
DZP
TZ2P
DZP
TZ2P
[H3SiOHGa(OHhOSiH3]NH3 DZ
DZP
TZ2P ~
t-.
180.4 180.4 180.1 180.1 (180.3) 163.2 163.2 149.3 149.3 149.3 149.3 -
182.1 182.1 179.4 179.4 (180.8) 158.6 158.6 149.5 149.5 149.5 149.5
182.1 182.1 178.9 178.9 (180.5) 157.3 157.3 149.1 149.1 149.1 149.1
-
175.2 194.9 176.5 176.5 (180.8) 165.7 175.2 148.3 148.2 147.5 146.4 96.28 -
-
-
-
-
177.4 203.9 175.5 175.5 (183.1) 161.8 170.3 148.3 148.4 147.4 146.7 95.17 -
-
177.2 206.4 174.6 174.6 (183.2) 160.4 168.1 147.9 148.0 147.1 146.4 94.36 -
-
184.2 184.3 177.3 177.3 (180.8) 168.7 168.7 148.5 147.7 148.5 147.7
186.9 187.2 176.8 176.8 (181.9) 163.5 163.7 149.0 147.9 149.0 147.9
186.8 186.9 176.1 176.1 (181.5) 161.6 161.7 148.7 147.6 148.7 147.6
108.0 108.1 101.1 253.6 253.5
104.5 104.8 100.6 262.5 261.0
103.5 104.0 100.1 264.0 262.6
-
178.5 190.1 177.1 177.1 (182.0) 166.8 172.8 148.4 148.2 147.8 146.8 103.90 101.7 100.9 297.5 257.5
177.9 195.2 176.9 176.9 (183.3) 160.9 168.9 148.9 148.5 147.7 146.9 99.00 100.8 100.6 340.6 271.6
177.6 197.9 176.1 176.1 (181.9) 160.3 166.9 148.3 148.0 147.2 146.6 97.93 100.1 99.9 315.3 271.9
§. ::; ~
1't"
~ ~ ~
~ ~
:-
~ .,
'"
~ .......
;;!
"c:::" "~
..., .....
'-
tv
....... ..... '0
~
106.2 109.6 174.7 174.7 122.6 -
104.2 109.9
-
157.1 157.1 114.4
103.4 109.4
148.6 148.6 114.4
98.7 116.5
91.4 115.4
-
-
123.4 127.1 170.8 134.3 -
119.8 134.0 155.1 119.6 -
90.6 115.0 119.3 133.1 148.2 118.6
-
101.2 117.3 95.8 110.8 133.1 133.1 134.9 -
98.6 112.1 99.1 110.3
98.1 112.1 99.9 110.2
131.2 131.2 120.3
131.3 131.4 118.9
103.3 117.0 111.0 122.6 126.9 150.1 134.1 166.6
100.2 124.3 107.0 118.1 129.5 133.1 136.3 175.9
96.9 118.6
-
107.8 117.6 129.8 141.2 120.1 173.3
'-
.....
00 ...,
.....I
~
Table 5 SCF optimized structural parameters for [H 3SiOAI(OHhOSiH 3
r, H 3SiOHAI(OHhOSiH 3, [H 3SiOAI(OHhOSiH 3nNH ]+ and H3SiOAI(OHhOSiH3/NH3 4
[H 3SiOAI(OHhOAlH 3]
[H 3SiOHAI(OHhOSiH3]
DZ
DZ
DZP
TZ2P
DZP
TZ2P
[H 3SiOAI(OHhOSiH3]NH4
[H3SiOHAI(OH)20SiH3INH3
DZ
DZ
DZP
TZ2P
DZP
TZ2P ~
Bond lengths (pm) AI-OJ AI-02 AI-OJ AI-04 (AI-O) Si5-01 Si9-02 Si5-H6 Si5-H7,8 Si9-HIO Si9-HII,12 O-H9 N-HI7 N-HI8 N-HI3,14 Ol-N 02-N Bond angles (deg.) 02-AI-OJ 03-AI-04 HI7-N-HI8 HI3-N-HI4 Si9-02-H9 Si9-02-AI Si5-02-AI HIS-OJ-AI 02-H9-N
t-< §. 175.5 175.5 175.6 175.6 (175.6) 162.9 162.9 149.4 149.4 149.4 149.4 -
177.1 177.1 174.4 174.4 (175.8) 158.2 158.2 149.5 149.5 149.5 149.5
106.6 109.1
104.9 118.2 -
-
175.6 175.6 173.7 173.7 (174.6) 156.7 156.7 149.1 149.1 149.1 149.1
-
-
171.1 190.1 170.9 170.9 (175.8) 165.2 174.9 148.5 148.2 147.4 146.5 96.31 -
172.2 197.0 170.2 170.2 (177.4) 161.4 170.5 148.5 148.3 147.4 146.6 95.06
97.8 116.0
94.4 114.1 -
-
171.0 195.9 169.6 169.6 (176.5) 160.1 168.7 148.0 148.0 147.1 147.3 94.35
-
-
-
-
-
179.9 179.8 171.6 171.6 (175.7) 168.2 168.2 148.6 147.8 148.6 147.8
181.7 181.7 171.3 171.3 (176.5) 163.4 163.4 149.1 147.9 149.1 147.9
179.9 180.4 170.8 170.8 (175.5) 161.4 161.6 148.8 147.6 148.8 147.6
108.1 108.1 101.0 253.4 253.4
104.5 104.5 100.6 261.8 261.7
103.2 104.1 100.2 265.3 262.2
101.7 116.7 94.9 110.9
99.7 111.2 98.7 116.3
135.6 135.6 150.6 -
132.5 132.5 128.3
99.7 110.8 99.6 110.2 133.2 133.8 125.6
-
-
173.2 186.1 171.6 171.6 (175.6) 165.6 172.6 148.8 148.2 147.8 146.8 103.07 101.3
171.3 192.2 171.0 171.0 (176.6) 160.7 169.0 149.1 148.4 147.8 146.9 98.70 100.7
172.1 191.9 170.0 170.0 (176.0) 159.7 167.4 148.5 148.0 147.3 146.5 97.50 100.1
100.9 313.5 260.1
100.7 342.8 273.3
99.9 332.1 275.3
103.4 115.9
100.6 115.1 106.7 117.1 129.3 163.1 127.8 175.5
98.8 112.7
-
-
:, ~
E.~
~ ~
~ ,.... ~ .... I:
~
---~
'c"
";:,~
'-.
.... ......
'"
...... ---'0 :'f
'-.
-
176.8 176.8 132.7
104.5 108.6
168.5 168.5 118.2 -
166.6 166.6 117.7
-
109.5 127.9 171.8 150.1 -
94.3 113.5
-
119.6 131.8 164.3 128.3 -
118.8 131.9 157.2 125.8
-
-
111.0 120.5 126.9 158.9 148.6 171.0
-
......
....00I ......
~
107.6 116.8 129.0 152.5 125.4 176.5
00 -.I
l. Limtrakul et al.ll. Mol. Struct. (Theochem) 312 (1994) 183-194
188
improved integral bonds for prescreening, and minimization of difference density matrices, as described in detail in Ref. 9. The three basis sets used were of DZ, DZP and TZ2P quality, constructed from Huzinaga's primitive sets [12] augmented with polarization functions (Table 1). Geometry optimization was terminated when the gradient norm with respect to the internal coordinates was less than 10- 3 E h ail'. The energy change at that time was below 5 x 10-6 E h . The details of the interaction processes were evaluated as follows. The adsorption energy of NH 3 on the modified zeolite (M-ZH) is the energy of the reaction
(1) whilst for an anion M-Z- and NHt it is the energy of the reaction M-ZH
----+
M-Z-
NH 3 + H+
----+
NHt
+ H+
(2a) (2b)
The energy for reaction (2a) is called the deprotonation energy (or gas phase acidity) and the protonation energy is represented by Eq. (2b). For an ion pair ([M-Z-][NHtD the complexation energy is the energy of the reaction: M-Z-
+ NHt
----+
[M-Z-][NHtJ
(3)
Finally, the conversion energy of a covalent structure to an ion pair structure is the sum of the energies of the three reactions above. All calculations were carried out on a HewlettPackard 700 series workstation at the Laboratory for Computational and Applied Chemistry, Kasetsart University. 3. Results and discussion 3.1. Computed equilibrium structures of zeotypes in comparison with zeolite, and related work
The structures computed for the smaller model compound (Al(OH)4) at each of the six levels (STO-3G, DZ, DZP, TZ2P SCF, 6-31G*/MP2 and 6-31G**/MP2) are listed in Table 2. It can be
seen from the table that the results for Al(OH)4 and the structural parameters computed at the DZP and TZ2P levels are substantially similar and close to the more accurate MP2 structure. The computed OH bond lengths at the DZP level are too short by 1.9 pm. The average (AI-O) bond length is too short by 2.7 pm. The 02-AI-Ol angle is too wide by 1.30. On going from the DZ to DZP basis set the AI-O-H bond angles decrease significantly. Further enlargment of basis set from DZP to TZ2P has only a minor effect on these angles. These results imply that for the correct description of these compounds a basis set of at least doublezeta plus polarization (DZP) is required. 3.2. Acid germanium and gallium zeotypes and their deprotonated forms
The results of the full geometry optimization of the acid germanium and gallium zeotypes and their deprotonated forms obtained at the DZ, DZP and TZ2P SCF levels are given in Tables 3 and 4. The results show the expected pattern. There is close agreement between the DZP and TZ2P results: bond lengths differ by almost 1.4 pm for the germanium zeotype and by 2.5 pm for the gallium zeotype. Substituting gallium for aluminium in the zeolite framework appears to have little effect on the Si5-01 and Si9-02 bonds. The difference between the (Ga-O) and (AI-O) bond lengths for the protonated gallium zeotype and acid zeolite framework is 6.7pm (see Table 5). A more interesting change is seen in some of the angles. The largest discrepancy (9°) is seen in the predicted Si5-01-Al and Si5-01-Ga angles of protonated forms. The calculated bridging hydroxyl angles are different by more than 1pm. In the germanium zeotype (see Table 3) the influence of the germanium atoms on Al(OH)4 is most pronounced on the AI-02 distance, which is 6.6 pm shorter than that in the acid zeolite (see Table 5). Another large discrepancy can be seen in the observed Ge-O-Al (157.2°) and Si-O-Al (145.7°) angles. The changes in the electron distribution and the equilibrium OH bond lengths resulting from the substitution of aluminium for gallium and
J. Limtrakul et alp. Mol. Struct. (Theochem) 312 (1994) 183-194
189
(b)
(a)
HI4 H13
H14
99. 9CCt~~0.1
H13
qplOO.87
~HI7
~~17
178.30
H6
257.84
H7
H7
-
81 82 83 84
H16
H15 81 82 83 84
129.0 98.8 152.5 116.8
127.7 99.8 135.7 127.7
(c) H18
H19~ 100.
HI7
H15 81 129.8 82 96.9 83 141. 2 84117.6
Fig. I. Schematic representation of the molecular models and the atom labelling used for the systems' Bronsted site (H 3YOHX(OHhOYH 3) and negatively charged framework ([H 3YOX(OHhOYH3n. (a) Covalent structure, H 3SiOAI(OHhOSiH 3/NH 3. (b) H 3GeOAI(OHhOGeH 3/NH 3. (c) H 3SiOGa(OHhOSiH3/NH 3 of the surface complexes of ammonia on acidic catalysis.
190
I. Limtrakul et al.jl. Mol. Struct. (Theochem) 312 (1994) 183-194
silicon for germanium can be used to determine the relative acidity of zeotypes. The results suggest that the germanium zeotype has a lower acidity than the gallium zeotype. The results are similar for the deprotonated species. Only some of the angles are too large with the DZ basis set: Si-O-Al (176.8° versus 166.6°), Ge-O-Al (173.3° versus 136.9°) and Si-O-Ga (174.7° versus 148.6°). This is due to the known poor description of such angles at this level [13]. Comparison between the structure of the acid germanium zeotype and its deprotonated framework show some major differences in the geometries. Removal of a proton from the acid zeotype results in contraction of the bonds in the framework. The Ge9-02 bond is decreased by 12.4 pm and by over 2pm for Ge5-01, while the AI-02 bond is decreased by 13.7pm and AI-Ol is increased by 5.5 pm. The average (AI-O) bond distances of the four AI-O bonds of the deprotonated and protonated models are almost the same (176.2 and 175.5 pm, respectively). For the gallium zeotype (see Table 4), the deprotonated structure has a Si9-02 bond that is shortened by about 11 pm and a Ga-02 bond that is shortened by about 24 pm. The average (Ga-O) bond length is found to be almost 183.2 and 180.5 pm for the protonated and deprotonated forms, respectively. This strengthening characteristic of these bonds in the gallium zeotype is somewhat similar to zeolitic molecular models. Using the systematic deviations of the experimental and theoretical data for zeolite, together with the results listed in Tables 3 and 4, we can predict for the first time the local geometries of the gallium and germanium zeotypes.
The changes in the structural parameters on ammonia adsorption to germanium- and galliummodified zeolites are minute but very impressive. The results are in accordance with Gutmann's rules [14,15]. In the gallium-modified zeolite (Fig. l(c» the 02-H9 bond is lengthened, the Ga-02 and Si9-02 bonds are shortened, and the Ga-04 and Si9-H bonds are lengthened. The same trends are observed for the germanium zeotype. The corresponding changes are virtually identical with the DZP and TZ2P basis sets. The optimized geometrical parameters for the zeotypejammonia complexes (Figs. (lb) and (1c» show that the two oxygen atoms of the zeotype framework act on one site as an acid (02 atom) and on the other site (01 atom) as a base. The intermolecular 02··· Nl distance of the gallium zeotypejNH 3 complex increases on basisset expansion: 257.5 pm (DZ), 271.6 pm (DZP), and 271.9 pm (TZ2P). Small basis sets often lead to too-small intermolecular distances [13]. This is also reflected in the calculated interaction energies (Table 6), which decrease on basis-set expansion. The intermolecular 02-H9··· N angles are 166.6° (DZ), 175.9° (DZP), and 173.3° (TZ2P). The same trends are observed for the germanium zeotypej NH 3 complex. The results of the intermolecular interaction energies of the gallium zeotype, obtained with the different basis sets are -18.90 (DZ), -15.40 (DZP), and -12.80kcalmol- 1 (TZ2P). Note that the DZ result is almost 48% too large compared with the TZ2P result. The stability of complexes at the TZ2P level is found to decrease in the order: zeolitejNH 3 ~ gallium zeotype-NH 3 (-12.80 kcal mol-I) > germanium zeotypejNH 3 (-12.20 kcal mol-I).
3.3. Ammonia adsorption on gallium and germanium zeotypes
3.4. The interaction of ammonium ion with gallium and germanium zeotypes
The fully optimized geometrical structures of the ammonia complexes, constrained to Cs symmetry, for the gallium and germanium zeotypes are reported for the first time. The optimized parameters obtained with the basis sets considered are summarized in Tables 3 and 4 and shown in Figs. l(b) and l(c).
The optimized structural parameters of the [H 3SiOGa(OHhOSiH 3r[NH 4t and [H 3GeOAl(OHhOGeH3nNH4]+ complexes are here reported at the DZ, DZP and TZ2P levels for the first time. For the gallium zeotypejNH 3 complex (Fig. 2(c» the Ga-Ol and Ga-02 bonds are nearly equal in length, being about 5pm longer
1. Limtrakul et al./l. Mol. Struct. (Theochem) 312 (1994) 183-194
191
Table 6 Computed total and intermolecular binding energies System
Basis set
Total energy, -Eh (a.u.)
H 3SiOAl(OHhOSiH 3
DZ DZP TZ2P
1123.932471 1124.157606 1124.401552
H 3GeOAI(OHhOGeH 3
DZ DZP TZ2P
553.447984 553.567183 553.766538
H 3SiOGa(OHhOSiH3
DZ DZP TZ2P
883.963992 884.162022 884.382896
H 3SiOHAl(OHhOSiH3
DZ DZP TZ2P
1124.448846 1124.661050 1124.901783
H 3GeOHAI(OHhOGeH 3
DZ DZP TZ2P
553.981726 554.106127 554.298188
H 3SiOHGa(OHhOSiH 3
DZ DZP TZ2P
884.479656 884.666642 884.885870
H 3SiOA1(OHhOSiH3/NH 4
DZ DZP TZ2P
1180.625210 1180.866645 1181.129527
124.64 107.94 104.75
H3GeOAI(OH)20GeH3/NH4
DZ DZP TZ2P
610.146067 610.304022 610.516240
128.00 125.39 118.38
H 3SiOGa(OHhOSiH3/NH4
DZ DZP TZ2P
940.658122 940.873617 941.113058
125.51 109.55 106.12
H 3SiOHAl(OHhOSiH3/NH 3
DZ DZP TZ2P
1180.626373 1180.871930 1181.136910
18.26 15.32 12.80
H 3GeOHAI(OHhOGeH 3/NH 3
DZ DZP TZ2P
610.158203 610.313688 610.532349
17.61 13.24 12.20
H 3SiOHGa(OHhOSiH 3/NH 3
DZ DZP TZ2P
940.658198 940.877652 941.120994
18.90 15.40 12.80
than the corresponding bonds in the anionic structure. The same trends are observed for the Si-O bonds. The Ga· .. N intermolecular bond distance in the ion-pair structure is 342.0 pm, which is about 50 pm shorter than the corresponding bond in the covalent structure. The N-H bonds are significantly lengthened (3 pm longer).
Binding energy, -!:!.E (kcalmol- 1)
In the germanium-modified zeolite complexes with ammonium ion (Fig. 2(b)) the Ge-O bonds of complexes are about 5 pm longer than the corresponding bond in the deprotonated structure. The (AI-O) of the germanium zeotype is 1.3 pm longer than that in its zeolite analogue (Fig. 2(a)). The AI· .. N distance of germanium zeotype is
192
J. Limtrakul et aLIJ. Mol. Struct. (Theochem) 312 (1994) 183-194
(a) (b)
-
03
04 03
61 62 63
133.2 99.7 133.8 99.6
e4
04
61 62 63 64
126.0 99.8 126.5 98.3
(c)
-
03
04 61 62 63 64
131.3 98.1 131. 4 99.9
Fig. 2. Schematic representation of the molecular models and the atom labelling used for the systems' Bronsted site (H) YOHX(OHhOYH) and negatively charged framework ([H) YOX(OHhOYH)n. (a) ion pair structure, [H)SiOAl(OHhOSiH)r fNH4]+. (b) [H)GeOAl(OHhOGeH 3r (NHt]+· (c) [H)SiOGa(OHhOSiH3r (NHt]+ of the surface complexes of ammonia on acidic catalysis.
8 pm shorter than that in the zeolite corresponding to a higher binding energy. This result again indicates that this zeotype is a stronger base than the zeolite.
The reaction energies of ammonia adsorption on the zeolite and the two zeotypes are illustrated in Schemes 1-3, where the numbers in parentheses are the energies calculated at the TZ2P level.
J. Limtrakul et al./!. Mol. Struct. (Theochem) 312 (1994)
ZH+NH 3
95.94
96.68
(96.58)
(98.30)
----+
Z- +NHt
15.32 (12.80)
T
3.32
15.40
(-104.75)
(12.80)
1
T
----+
1
- 109.55
(-106.12)
2.53
(4.63) [ZH] [NH 3]
Ga-Z- +NHt
Ga-ZH+NH 3
- 107.94
193
183~194
(4.98) [Z-] [NHtl
Scheme J. Conversion energy of zeolite/ammonia (kcalmol- 1).
In summary, the conversion energy of acid catalyst/ammonia to the ion pair complexes for the zeolite and the gallium and germanium zeotypes is found to be about 2-10 kca1 mol- 1
[Ga-ZH][NH 3]
----+
[Ga-Z-][NHt]
Scheme 3. Conversion energy of gallium zeotype/ammonia (kcalmol- 1).
Acknowledgements
Financial support by the KURDI Research Fund is gratefully acknowledged. Our sincere thanks are due to Professor R. Ah1richs (Karlsruhe) for his continued support of this work. M.M.P. thanks the Austrian Research Council (FWF) for support (Project No. P901O/MOB).
References
118.21 (116.29) Ge-ZH+NH 3 13.24 (12.20)
T
----+
Ge-Z- +NHt - 125.39
1
(-118.38)
6.06 (10.11) [Ge-ZH] [NH 3]
----+
[Ge-Z-] [NHt]
Scheme 2. Conversion energy of germanium zeotype/ammonia (kcalmol- 1).
[1] J. Klinoski, Chern. Rev., 91 (1991) 1459. [2] J.M. Thomas, Sci. Am., 266 (1992) 82. [3] C.T.W. Chu and C.D. Chang, J. Phys. Chern., 89 (1985) 1569. [4] B.M. Lok, C.A Messian, R.L. Patton, R.T. Gajek, T.R. Cannon and E.M. Flanigen, J. Am. Chern. Soc., 106 (1984) 6092. [5] J. Limtrakul and S. Polman-Hannongbua, J. Mol. Struct. (Theochem), 280 (1993) 139. [6] J. Limtrakul, J. Mol. Struct., 288 (1993) 105. [7] G.J. Kramer, R.A VanSanten, C.A Emeis and AK. Nowak, Nature, 363 (1993) 529. [8] J. Sauer, H. Horn, M. Haeser and R. Ahlrichs, Chern. Phys. Lett., 173 (1989) 26. [9] M. Haeser and R. Ahlrichs, J. Comput. Chern., 10 (1989) 104. [10] R. Ahlrichs, M. Baer, M. Haeser, H. Horn and C. Koelmel, Chern. Phys. Lett., 162 (1989) 165. [11] J. Almloef, K. Faegri, Jr., and K. Korsen, J. Comput. Chern., 3 (1982) 385.
194
I. Limtrakul et al.II. Mol. Struct. (Theochem) 312 (1994) 183-194
[12] S. Huzinaga, Approximate Atomic Function, Division of Theoretical Chemistry, University of Alberta, 1971. [13] J. Limtraku1 and R. Ah1richs, Chern. Phys. Lett., 160 (1989) 478. [14] V. Gutmann, The Donor-Acceptor Approach to
Molecular Interaction, Plenum Press, New York, 1978. [15] V. Gutmann, G. Resch and W. Linert, Coord. Chern. Rev., 43 (1982) 133. [16] G. Igel-Mann, H. Stoll and H. Preuss, Mol. Phys., 65 (1988) 1321.
THEO CHEM Journal of Molecular Structure (Theochem) 312 (1994) 183-194
Adsorption complexes of ammonia on germanium- and galliummodified zeolites Jumras Limtrakul*, Jarungsak Yoinuan, Duangkamol Tantanak, Michael M. Probst 1 Laboratory for Computational and Applied Chemistry, Chemistry Department, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand
(Received 21 December 1993; accepted 30 December 1993)
Abstract Adsorption complexes of ammonia on germanium- and gallium-modified zeolites were investigated by means of ab initio calculations. Full optimization of the structures was done at the DZ, DZP, and TZ2P levels of theory. Two new models of ammonia adsorption on zeotypes are proposed. In one of these, the covalent structures are stabilized at the bridging OH group by two hydrogen bonds with binding energies of -12.20 and -12.80kcalmol- 1 for the germanium and gallium zeotypes, respectively. The other configuration is a type of the ion-pair structure, where the ammonium cation forms two very strong hydrogen bonds with the anionic zeotype clusters. The conversion energy of acid catalyst/ ammonia to ion-pair complexes for zeolite, germanium and gallium zeotypes, was found to be about 2-10 kcal mol-I.
1. Introduction The investigation of Bronsted acidic surface sites capable of donating protons to adsorbed molecules is the basis of many industrially important reactions in heterogenous catalysis [1,2]. Therefore the structures of these active sites and the details of interaction processes, i.e. adsorption/ desorption, protonation/deprotonation and ionpair formation processes, are of prime importance. Recently it has been found that the substitution of the silicon and aluminium in zeolite by other elements can lead to a remarkable variation in catalytic properties [3,4]. However, most work done to date has focused largely on the interaction *Corresponding author. I Permanent address: Institut fuer Theoretische Chemie, Universitaet Innsbruck, Austria.
of electron-pair donors with the zeolite, although the presence of foreign atoms also significantly disturbs the catalytic structure. Simple models of silica, zeolite and modified zeolite (zeotype) have been studied recently, and the Bronsted acidity order found was in excellent agreement with experimental observations [5]. However, the models used cannot account for NHt bonded in the zeotype. Therefore in the present work we applied for the first time new adsorption models for zeolites (=== SiOA1(OHhOHSi === [6-8]) at the DZ, DZP and TZ2P levels to zeotypes containing germanium (Ge) and gallium (Ga) atoms. A cluster that is smaller but still has a tetrahedral form of AI(OH)4" was also investigated, at both SCF and correlated levels of theory. The results obtained do not only show the relationship between zeotype structure and activity at the molecular level, but also help in
0166-1280/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(94)03714- V
I. Limtrakul et al./l. Mol. Struct. (Theochem) 312 (1994) 183-194
184
Table I Specification of the basis sets used Atom
TZ2P
DZP
H
(4,1)/[2,1]
{31/1}
np = 0.80
(5,2)/[3,2]
{3ll/ll}
np = 0.46;1.39
N
(8,4,1 )/[4,2,1]
{5Ill/31/1}
np = 1.00
(9,5,2)/[5,3,2]
{5llll/3ll/ll}
nd = 0.58;1.73
0'
(8,4,1 )/[4,2,1]
{5Ill/31/1}
nd = 1.20
(9,5,2)/[5,3,2]
{5llll/311/ll }
nd = 0.69;2.08
Al
(11,7,1)/[6,4,1]
{52lll/4Ill/l}
nd = 0.30
(12,9,2)/[5,7,2]
{5211111/51111/11}
nd = 0.17;0.52
Si
(ll,7,1 )/[6,4,1]
{52111/4Ill/l}
nd = 0.40
(13,9,2)/[6,5,2]
{631111/42111/11}
nd = 0.23;0.69
• The relatively large value (nd) for oxygen in comparison with aluminium and silicon takes into account the fact that oxygen is the most electronegative component in the system studied. The basis sets employed for gallium and germanium were of the ECP type [16].
designing efficient and selective "candidates" in the catalytic field.
2. Method
All calculations were performed at the ab initio level using the program TURBOMOLE [9,10], based on the direct SCF method of Almloef et al. [11]. The
conventional method is to calculate and store the two-electron integrals, which are then retrieved and processed in each SCF iteration. To avoid the sheer number of two-electron integrals required, an alternative approach of abandoning integral storage and recomputing the most time-consuming step (calculation of the two-electron integral) in each iteration has been suggested. In order to increase efficiency, the algorithm includes selective storage of expensive and frequently used integrals,
Table 2 Optimized parameters for the molecular model (AI(OH).j) at the SCF and MP2 levels SCF
MP2
STO-3G
3-21G
DZ
DZP
TZ2P
6-3IG*
6-3IG**
Bond lengths (pm) AI-OJ AI-02 AI-03 AI-04 (AI-O) Ol-HI 02-H2 03-H3 04-H4
176.4 176.4 177.0 176.8 (176.7) 98.7 98.7 98.6 98.6
175.1 175.1 175.8 175.2 (175.3) 96.3 96.3 96.5 96.2
176.8 176.8 177.4 176.5 (176.9) 95.3 95.3 95.4 95.2
176.6 176.6 176.6 176.6 (176.6) 94.3 94.3 94.3 94.3
175.5 175.3 176.1 176.6 (175.9) 93.7 93.7 93.7 93.7
178.7 178.7 179.6 180.6 (179.4) 96.8 96.8 97.0 96.9
178.6 178.6 179.5 180.4 (179.30) 96.2 96.2 96.3 96.2
Bond angles (deg.) 02-AI-OJ 03-AI-04 OJ-Al-04 02-AI-03 AI-OJ-HI AI-02-H2 AI-03-H3 AI-04-H4
105.2 104.3 ll1.7 ll2.0 107.2 107.2 107.9 107.2
ll1.5 105.3 llO.9 108.6 ll7.4 ll7.2 ll5.5 120.9
110.7 107.3 110.3 109.0 130.0 127.9 126.3 129.4
106.3 106.3 ll5.8 ll5.9 ll3.3 113.2 113.4 ll3.5
106.8 103.9 llO.3 llO.1 ll4.8 113.8 112.8 115.7
105.1 101.5 113.1 ll3.1 108.8 108.8 105.5 108.9
105.0 101.8 ll3.0 ll3.2 108.9 108.9 105.4 109.0
Table 3 SCF optimized structural parameters for [H3GeOAI(OH)zOGeH3r, H3GeOHAI(OH)zOGeH3' [H3GeOAI(OH)zOGeH3r [NH4t and H3GeOAI(OH)zOGeH3/NH3
Bond lengths (pm) AI-OI AI-02 AI-03 AI-04 (AI-0) Ge5-01 Ge9-02 Ge5-H6 Ge5-H7,8 Ge9-HI0 Ge9-H11,12 O-H9 N-H17 N-HI8 N-H13,14 01-N 02-N Bond angles (deg.) 02-AI-01 03-AI-04 HI7-N-HI8 H13-N-HI4 Ge9-02-H9 Ge9-02-AI Ge5-0l-AI HI5-03-AI 02-H9-N
[H3GeOAI(OH)zOGeH3]
[H3GeOHAI(OH)zOGeH3]
[H3GeOAI(OH)20GeH3]NH4
H3GeOHAI(OH)20GeH3]NH3
DZ
DZP
TZ2P
DZ
DZP
TZ2P
DZ
TZ2P
DZ
DZP
TZ2P
175.6 175.6 176.0 176.0 (175.8) 171.2 171.2 155.2 155.2 155.2 155.2
176.5 176.5 175.8 175.8 (176.2) 172.5 172.5 153.5 153.3 153.5 153.3
175.6 175.6 174.8 174.8 (175.2) 171.7 171.7 154.3 154.1 154.3 154.1
171.0 188.8 171.7 171.7 (175.8) 173.9 185.5 154.1 153.9 152.9 151.6 96.17
170.9 189.6 172.3 172.3 (176.3) 174.5 185.5 152.2 152.3 151.2 150.2 95.00
170.1 189.3 171.3 171.3 (175.5) 174.2 184.1 152.9 153.1 152.0 151.2 94.17
181.4 180.1 174.5 174.5 (177.6) 178.9 178.2 153.9 152.9 153.9 153.2
172.5 172.5 (176.4) 177.7 177.7 152.6 151.6 152.6 151.6
179.7 178.8 171.6 171.6 (175.4) 176.9 176.7 153.5 152.5 153.5 152.5
107.3 111.2 101.0 249.3 255.5
106.0 106.1 100.5 256.1 256.1
105.7 104.2 100.0 260.6 256.4
174.5 185.2 171.9 171.9 (175.9) 175.4 182.6 154.0 153.8 153.0 152.1 102.40 101.9
172.8 187.0 172.5 172.5 (177.5) 175.0 183.0 152.5 152.3 151.2 150.6 97.76 100.9
172.0 186.5 171.6 171.6 (175.4) 174.8 182.6 154.0 154.0 152.0 151.4 96.70 100.4
100.8 293.2 260.0
100.5 328.0 275.9
99.9 308.5 275.7
DZP 180.4 180A
:-.
t-< ~.
.,...
;>;-
!2.. ~
~
~
,...~ ~ ?
!:
"' ~
'c"
";:,'" :li 'v..
......
'"...... "' '0
~
'-
107.4 108.9
173.3 173.3 131.2
107.6 106.8
136.9 136.9 116.3
106.3 106.3
100.6 114.7
133.0 133.0 116.5
121.8 124.4 171.3 146.8
97.7 115.3
120.3 128.6 154.2 127.5
95.7 115.0
119.0 129.4 145.7 125.5
99.9 112.2 94.6 110.8
100.3 109.3 97.3 110.3
99.8 109.1 98.3 110.3
132.6 130.0 125.3
126.4 126.4 125.3
126.0 126.5 123.7
102.5 115.3 111.3 120.9 126.1 149.2 148.2 163.5
98.1 115.2 107.4 118.5 131.6 141.9 126.9 175.9
99.8 114.7 108.0 116.8 127.7 135.7 125.5 166.0
......
~
I ......
'0 .....
00
v.
00
0\
Table 4 SCF optimized structural parameters for [H3SiOGa(OHhOSiH3r, H 3SiOHGa(OHhOSiH 3, [H3SiOGa(OHhOSiH3nNHtJ and H 3SiOGa(OHhOSiH 3/NH 3 [H3SiOGa(OHhOGaH3J DZ Bond lengths (pm) Ga-OI Ga-02 Ga-03 Ga-04 (Ga-O) Si5-OJ Si9-02 Si5-H6 Si5-H7,8 Si9-H10 Si9-HII,12 O-H9 N-HI7 N-HI8 N-H13,14 01-N 02-N Bond angles (deg.) 02-Ga-OI 03-Ga-04 HI7-N-Hl8 HI3-N-HI4 Si9-02-H9 Si9-02-Ga Si5-01-Ga HI5-03-Ga 02-H9-N
DZP
TZ2P
[H3SiOHGa(OHhOSiH3]
[H 3SiOGa(OHhOSiH 3]NH4
DZ
DZ
DZP
TZ2P
DZP
TZ2P
[H3SiOHGa(OHhOSiH3]NH3 DZ
DZP
TZ2P ~
t-.
180.4 180.4 180.1 180.1 (180.3) 163.2 163.2 149.3 149.3 149.3 149.3 -
182.1 182.1 179.4 179.4 (180.8) 158.6 158.6 149.5 149.5 149.5 149.5
182.1 182.1 178.9 178.9 (180.5) 157.3 157.3 149.1 149.1 149.1 149.1
-
175.2 194.9 176.5 176.5 (180.8) 165.7 175.2 148.3 148.2 147.5 146.4 96.28 -
-
-
-
-
177.4 203.9 175.5 175.5 (183.1) 161.8 170.3 148.3 148.4 147.4 146.7 95.17 -
-
177.2 206.4 174.6 174.6 (183.2) 160.4 168.1 147.9 148.0 147.1 146.4 94.36 -
-
184.2 184.3 177.3 177.3 (180.8) 168.7 168.7 148.5 147.7 148.5 147.7
186.9 187.2 176.8 176.8 (181.9) 163.5 163.7 149.0 147.9 149.0 147.9
186.8 186.9 176.1 176.1 (181.5) 161.6 161.7 148.7 147.6 148.7 147.6
108.0 108.1 101.1 253.6 253.5
104.5 104.8 100.6 262.5 261.0
103.5 104.0 100.1 264.0 262.6
-
178.5 190.1 177.1 177.1 (182.0) 166.8 172.8 148.4 148.2 147.8 146.8 103.90 101.7 100.9 297.5 257.5
177.9 195.2 176.9 176.9 (183.3) 160.9 168.9 148.9 148.5 147.7 146.9 99.00 100.8 100.6 340.6 271.6
177.6 197.9 176.1 176.1 (181.9) 160.3 166.9 148.3 148.0 147.2 146.6 97.93 100.1 99.9 315.3 271.9
§. ::; ~
1't"
~ ~ ~
~ ~
:-
~ .,
'"
~ .......
;;!
"c:::" "~
..., .....
'-
tv
....... ..... '0
~
106.2 109.6 174.7 174.7 122.6 -
104.2 109.9
-
157.1 157.1 114.4
103.4 109.4
148.6 148.6 114.4
98.7 116.5
91.4 115.4
-
-
123.4 127.1 170.8 134.3 -
119.8 134.0 155.1 119.6 -
90.6 115.0 119.3 133.1 148.2 118.6
-
101.2 117.3 95.8 110.8 133.1 133.1 134.9 -
98.6 112.1 99.1 110.3
98.1 112.1 99.9 110.2
131.2 131.2 120.3
131.3 131.4 118.9
103.3 117.0 111.0 122.6 126.9 150.1 134.1 166.6
100.2 124.3 107.0 118.1 129.5 133.1 136.3 175.9
96.9 118.6
-
107.8 117.6 129.8 141.2 120.1 173.3
'-
.....
00 ...,
.....I
~
Table 5 SCF optimized structural parameters for [H 3SiOAI(OHhOSiH 3
r, H 3SiOHAI(OHhOSiH 3, [H 3SiOAI(OHhOSiH 3nNH ]+ and H3SiOAI(OHhOSiH3/NH3 4
[H 3SiOAI(OHhOAlH 3]
[H 3SiOHAI(OHhOSiH3]
DZ
DZ
DZP
TZ2P
DZP
TZ2P
[H 3SiOAI(OHhOSiH3]NH4
[H3SiOHAI(OH)20SiH3INH3
DZ
DZ
DZP
TZ2P
DZP
TZ2P ~
Bond lengths (pm) AI-OJ AI-02 AI-OJ AI-04 (AI-O) Si5-01 Si9-02 Si5-H6 Si5-H7,8 Si9-HIO Si9-HII,12 O-H9 N-HI7 N-HI8 N-HI3,14 Ol-N 02-N Bond angles (deg.) 02-AI-OJ 03-AI-04 HI7-N-HI8 HI3-N-HI4 Si9-02-H9 Si9-02-AI Si5-02-AI HIS-OJ-AI 02-H9-N
t-< §. 175.5 175.5 175.6 175.6 (175.6) 162.9 162.9 149.4 149.4 149.4 149.4 -
177.1 177.1 174.4 174.4 (175.8) 158.2 158.2 149.5 149.5 149.5 149.5
106.6 109.1
104.9 118.2 -
-
175.6 175.6 173.7 173.7 (174.6) 156.7 156.7 149.1 149.1 149.1 149.1
-
-
171.1 190.1 170.9 170.9 (175.8) 165.2 174.9 148.5 148.2 147.4 146.5 96.31 -
172.2 197.0 170.2 170.2 (177.4) 161.4 170.5 148.5 148.3 147.4 146.6 95.06
97.8 116.0
94.4 114.1 -
-
171.0 195.9 169.6 169.6 (176.5) 160.1 168.7 148.0 148.0 147.1 147.3 94.35
-
-
-
-
-
179.9 179.8 171.6 171.6 (175.7) 168.2 168.2 148.6 147.8 148.6 147.8
181.7 181.7 171.3 171.3 (176.5) 163.4 163.4 149.1 147.9 149.1 147.9
179.9 180.4 170.8 170.8 (175.5) 161.4 161.6 148.8 147.6 148.8 147.6
108.1 108.1 101.0 253.4 253.4
104.5 104.5 100.6 261.8 261.7
103.2 104.1 100.2 265.3 262.2
101.7 116.7 94.9 110.9
99.7 111.2 98.7 116.3
135.6 135.6 150.6 -
132.5 132.5 128.3
99.7 110.8 99.6 110.2 133.2 133.8 125.6
-
-
173.2 186.1 171.6 171.6 (175.6) 165.6 172.6 148.8 148.2 147.8 146.8 103.07 101.3
171.3 192.2 171.0 171.0 (176.6) 160.7 169.0 149.1 148.4 147.8 146.9 98.70 100.7
172.1 191.9 170.0 170.0 (176.0) 159.7 167.4 148.5 148.0 147.3 146.5 97.50 100.1
100.9 313.5 260.1
100.7 342.8 273.3
99.9 332.1 275.3
103.4 115.9
100.6 115.1 106.7 117.1 129.3 163.1 127.8 175.5
98.8 112.7
-
-
:, ~
E.~
~ ~
~ ,.... ~ .... I:
~
---~
'c"
";:,~
'-.
.... ......
'"
...... ---'0 :'f
'-.
-
176.8 176.8 132.7
104.5 108.6
168.5 168.5 118.2 -
166.6 166.6 117.7
-
109.5 127.9 171.8 150.1 -
94.3 113.5
-
119.6 131.8 164.3 128.3 -
118.8 131.9 157.2 125.8
-
-
111.0 120.5 126.9 158.9 148.6 171.0
-
......
....00I ......
~
107.6 116.8 129.0 152.5 125.4 176.5
00 -.I
l. Limtrakul et al.ll. Mol. Struct. (Theochem) 312 (1994) 183-194
188
improved integral bonds for prescreening, and minimization of difference density matrices, as described in detail in Ref. 9. The three basis sets used were of DZ, DZP and TZ2P quality, constructed from Huzinaga's primitive sets [12] augmented with polarization functions (Table 1). Geometry optimization was terminated when the gradient norm with respect to the internal coordinates was less than 10- 3 E h ail'. The energy change at that time was below 5 x 10-6 E h . The details of the interaction processes were evaluated as follows. The adsorption energy of NH 3 on the modified zeolite (M-ZH) is the energy of the reaction
(1) whilst for an anion M-Z- and NHt it is the energy of the reaction M-ZH
----+
M-Z-
NH 3 + H+
----+
NHt
+ H+
(2a) (2b)
The energy for reaction (2a) is called the deprotonation energy (or gas phase acidity) and the protonation energy is represented by Eq. (2b). For an ion pair ([M-Z-][NHtD the complexation energy is the energy of the reaction: M-Z-
+ NHt
----+
[M-Z-][NHtJ
(3)
Finally, the conversion energy of a covalent structure to an ion pair structure is the sum of the energies of the three reactions above. All calculations were carried out on a HewlettPackard 700 series workstation at the Laboratory for Computational and Applied Chemistry, Kasetsart University. 3. Results and discussion 3.1. Computed equilibrium structures of zeotypes in comparison with zeolite, and related work
The structures computed for the smaller model compound (Al(OH)4) at each of the six levels (STO-3G, DZ, DZP, TZ2P SCF, 6-31G*/MP2 and 6-31G**/MP2) are listed in Table 2. It can be
seen from the table that the results for Al(OH)4 and the structural parameters computed at the DZP and TZ2P levels are substantially similar and close to the more accurate MP2 structure. The computed OH bond lengths at the DZP level are too short by 1.9 pm. The average (AI-O) bond length is too short by 2.7 pm. The 02-AI-Ol angle is too wide by 1.30. On going from the DZ to DZP basis set the AI-O-H bond angles decrease significantly. Further enlargment of basis set from DZP to TZ2P has only a minor effect on these angles. These results imply that for the correct description of these compounds a basis set of at least doublezeta plus polarization (DZP) is required. 3.2. Acid germanium and gallium zeotypes and their deprotonated forms
The results of the full geometry optimization of the acid germanium and gallium zeotypes and their deprotonated forms obtained at the DZ, DZP and TZ2P SCF levels are given in Tables 3 and 4. The results show the expected pattern. There is close agreement between the DZP and TZ2P results: bond lengths differ by almost 1.4 pm for the germanium zeotype and by 2.5 pm for the gallium zeotype. Substituting gallium for aluminium in the zeolite framework appears to have little effect on the Si5-01 and Si9-02 bonds. The difference between the (Ga-O) and (AI-O) bond lengths for the protonated gallium zeotype and acid zeolite framework is 6.7pm (see Table 5). A more interesting change is seen in some of the angles. The largest discrepancy (9°) is seen in the predicted Si5-01-Al and Si5-01-Ga angles of protonated forms. The calculated bridging hydroxyl angles are different by more than 1pm. In the germanium zeotype (see Table 3) the influence of the germanium atoms on Al(OH)4 is most pronounced on the AI-02 distance, which is 6.6 pm shorter than that in the acid zeolite (see Table 5). Another large discrepancy can be seen in the observed Ge-O-Al (157.2°) and Si-O-Al (145.7°) angles. The changes in the electron distribution and the equilibrium OH bond lengths resulting from the substitution of aluminium for gallium and
J. Limtrakul et alp. Mol. Struct. (Theochem) 312 (1994) 183-194
189
(b)
(a)
HI4 H13
H14
99. 9CCt~~0.1
H13
qplOO.87
~HI7
~~17
178.30
H6
257.84
H7
H7
-
81 82 83 84
H16
H15 81 82 83 84
129.0 98.8 152.5 116.8
127.7 99.8 135.7 127.7
(c) H18
H19~ 100.
HI7
H15 81 129.8 82 96.9 83 141. 2 84117.6
Fig. I. Schematic representation of the molecular models and the atom labelling used for the systems' Bronsted site (H 3YOHX(OHhOYH 3) and negatively charged framework ([H 3YOX(OHhOYH3n. (a) Covalent structure, H 3SiOAI(OHhOSiH 3/NH 3. (b) H 3GeOAI(OHhOGeH 3/NH 3. (c) H 3SiOGa(OHhOSiH3/NH 3 of the surface complexes of ammonia on acidic catalysis.
190
I. Limtrakul et al.jl. Mol. Struct. (Theochem) 312 (1994) 183-194
silicon for germanium can be used to determine the relative acidity of zeotypes. The results suggest that the germanium zeotype has a lower acidity than the gallium zeotype. The results are similar for the deprotonated species. Only some of the angles are too large with the DZ basis set: Si-O-Al (176.8° versus 166.6°), Ge-O-Al (173.3° versus 136.9°) and Si-O-Ga (174.7° versus 148.6°). This is due to the known poor description of such angles at this level [13]. Comparison between the structure of the acid germanium zeotype and its deprotonated framework show some major differences in the geometries. Removal of a proton from the acid zeotype results in contraction of the bonds in the framework. The Ge9-02 bond is decreased by 12.4 pm and by over 2pm for Ge5-01, while the AI-02 bond is decreased by 13.7pm and AI-Ol is increased by 5.5 pm. The average (AI-O) bond distances of the four AI-O bonds of the deprotonated and protonated models are almost the same (176.2 and 175.5 pm, respectively). For the gallium zeotype (see Table 4), the deprotonated structure has a Si9-02 bond that is shortened by about 11 pm and a Ga-02 bond that is shortened by about 24 pm. The average (Ga-O) bond length is found to be almost 183.2 and 180.5 pm for the protonated and deprotonated forms, respectively. This strengthening characteristic of these bonds in the gallium zeotype is somewhat similar to zeolitic molecular models. Using the systematic deviations of the experimental and theoretical data for zeolite, together with the results listed in Tables 3 and 4, we can predict for the first time the local geometries of the gallium and germanium zeotypes.
The changes in the structural parameters on ammonia adsorption to germanium- and galliummodified zeolites are minute but very impressive. The results are in accordance with Gutmann's rules [14,15]. In the gallium-modified zeolite (Fig. l(c» the 02-H9 bond is lengthened, the Ga-02 and Si9-02 bonds are shortened, and the Ga-04 and Si9-H bonds are lengthened. The same trends are observed for the germanium zeotype. The corresponding changes are virtually identical with the DZP and TZ2P basis sets. The optimized geometrical parameters for the zeotypejammonia complexes (Figs. (lb) and (1c» show that the two oxygen atoms of the zeotype framework act on one site as an acid (02 atom) and on the other site (01 atom) as a base. The intermolecular 02··· Nl distance of the gallium zeotypejNH 3 complex increases on basisset expansion: 257.5 pm (DZ), 271.6 pm (DZP), and 271.9 pm (TZ2P). Small basis sets often lead to too-small intermolecular distances [13]. This is also reflected in the calculated interaction energies (Table 6), which decrease on basis-set expansion. The intermolecular 02-H9··· N angles are 166.6° (DZ), 175.9° (DZP), and 173.3° (TZ2P). The same trends are observed for the germanium zeotypej NH 3 complex. The results of the intermolecular interaction energies of the gallium zeotype, obtained with the different basis sets are -18.90 (DZ), -15.40 (DZP), and -12.80kcalmol- 1 (TZ2P). Note that the DZ result is almost 48% too large compared with the TZ2P result. The stability of complexes at the TZ2P level is found to decrease in the order: zeolitejNH 3 ~ gallium zeotype-NH 3 (-12.80 kcal mol-I) > germanium zeotypejNH 3 (-12.20 kcal mol-I).
3.3. Ammonia adsorption on gallium and germanium zeotypes
3.4. The interaction of ammonium ion with gallium and germanium zeotypes
The fully optimized geometrical structures of the ammonia complexes, constrained to Cs symmetry, for the gallium and germanium zeotypes are reported for the first time. The optimized parameters obtained with the basis sets considered are summarized in Tables 3 and 4 and shown in Figs. l(b) and l(c).
The optimized structural parameters of the [H 3SiOGa(OHhOSiH 3r[NH 4t and [H 3GeOAl(OHhOGeH3nNH4]+ complexes are here reported at the DZ, DZP and TZ2P levels for the first time. For the gallium zeotypejNH 3 complex (Fig. 2(c» the Ga-Ol and Ga-02 bonds are nearly equal in length, being about 5pm longer
1. Limtrakul et al./l. Mol. Struct. (Theochem) 312 (1994) 183-194
191
Table 6 Computed total and intermolecular binding energies System
Basis set
Total energy, -Eh (a.u.)
H 3SiOAl(OHhOSiH 3
DZ DZP TZ2P
1123.932471 1124.157606 1124.401552
H 3GeOAI(OHhOGeH 3
DZ DZP TZ2P
553.447984 553.567183 553.766538
H 3SiOGa(OHhOSiH3
DZ DZP TZ2P
883.963992 884.162022 884.382896
H 3SiOHAl(OHhOSiH3
DZ DZP TZ2P
1124.448846 1124.661050 1124.901783
H 3GeOHAI(OHhOGeH 3
DZ DZP TZ2P
553.981726 554.106127 554.298188
H 3SiOHGa(OHhOSiH 3
DZ DZP TZ2P
884.479656 884.666642 884.885870
H 3SiOA1(OHhOSiH3/NH 4
DZ DZP TZ2P
1180.625210 1180.866645 1181.129527
124.64 107.94 104.75
H3GeOAI(OH)20GeH3/NH4
DZ DZP TZ2P
610.146067 610.304022 610.516240
128.00 125.39 118.38
H 3SiOGa(OHhOSiH3/NH4
DZ DZP TZ2P
940.658122 940.873617 941.113058
125.51 109.55 106.12
H 3SiOHAl(OHhOSiH3/NH 3
DZ DZP TZ2P
1180.626373 1180.871930 1181.136910
18.26 15.32 12.80
H 3GeOHAI(OHhOGeH 3/NH 3
DZ DZP TZ2P
610.158203 610.313688 610.532349
17.61 13.24 12.20
H 3SiOHGa(OHhOSiH 3/NH 3
DZ DZP TZ2P
940.658198 940.877652 941.120994
18.90 15.40 12.80
than the corresponding bonds in the anionic structure. The same trends are observed for the Si-O bonds. The Ga· .. N intermolecular bond distance in the ion-pair structure is 342.0 pm, which is about 50 pm shorter than the corresponding bond in the covalent structure. The N-H bonds are significantly lengthened (3 pm longer).
Binding energy, -!:!.E (kcalmol- 1)
In the germanium-modified zeolite complexes with ammonium ion (Fig. 2(b)) the Ge-O bonds of complexes are about 5 pm longer than the corresponding bond in the deprotonated structure. The (AI-O) of the germanium zeotype is 1.3 pm longer than that in its zeolite analogue (Fig. 2(a)). The AI· .. N distance of germanium zeotype is
192
J. Limtrakul et aLIJ. Mol. Struct. (Theochem) 312 (1994) 183-194
(a) (b)
-
03
04 03
61 62 63
133.2 99.7 133.8 99.6
e4
04
61 62 63 64
126.0 99.8 126.5 98.3
(c)
-
03
04 61 62 63 64
131.3 98.1 131. 4 99.9
Fig. 2. Schematic representation of the molecular models and the atom labelling used for the systems' Bronsted site (H) YOHX(OHhOYH) and negatively charged framework ([H) YOX(OHhOYH)n. (a) ion pair structure, [H)SiOAl(OHhOSiH)r fNH4]+. (b) [H)GeOAl(OHhOGeH 3r (NHt]+· (c) [H)SiOGa(OHhOSiH3r (NHt]+ of the surface complexes of ammonia on acidic catalysis.
8 pm shorter than that in the zeolite corresponding to a higher binding energy. This result again indicates that this zeotype is a stronger base than the zeolite.
The reaction energies of ammonia adsorption on the zeolite and the two zeotypes are illustrated in Schemes 1-3, where the numbers in parentheses are the energies calculated at the TZ2P level.
J. Limtrakul et al./!. Mol. Struct. (Theochem) 312 (1994)
ZH+NH 3
95.94
96.68
(96.58)
(98.30)
----+
Z- +NHt
15.32 (12.80)
T
3.32
15.40
(-104.75)
(12.80)
1
T
----+
1
- 109.55
(-106.12)
2.53
(4.63) [ZH] [NH 3]
Ga-Z- +NHt
Ga-ZH+NH 3
- 107.94
193
183~194
(4.98) [Z-] [NHtl
Scheme J. Conversion energy of zeolite/ammonia (kcalmol- 1).
In summary, the conversion energy of acid catalyst/ammonia to the ion pair complexes for the zeolite and the gallium and germanium zeotypes is found to be about 2-10 kca1 mol- 1
[Ga-ZH][NH 3]
----+
[Ga-Z-][NHt]
Scheme 3. Conversion energy of gallium zeotype/ammonia (kcalmol- 1).
Acknowledgements
Financial support by the KURDI Research Fund is gratefully acknowledged. Our sincere thanks are due to Professor R. Ah1richs (Karlsruhe) for his continued support of this work. M.M.P. thanks the Austrian Research Council (FWF) for support (Project No. P901O/MOB).
References
118.21 (116.29) Ge-ZH+NH 3 13.24 (12.20)
T
----+
Ge-Z- +NHt - 125.39
1
(-118.38)
6.06 (10.11) [Ge-ZH] [NH 3]
----+
[Ge-Z-] [NHt]
Scheme 2. Conversion energy of germanium zeotype/ammonia (kcalmol- 1).
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[12] S. Huzinaga, Approximate Atomic Function, Division of Theoretical Chemistry, University of Alberta, 1971. [13] J. Limtraku1 and R. Ah1richs, Chern. Phys. Lett., 160 (1989) 478. [14] V. Gutmann, The Donor-Acceptor Approach to
Molecular Interaction, Plenum Press, New York, 1978. [15] V. Gutmann, G. Resch and W. Linert, Coord. Chern. Rev., 43 (1982) 133. [16] G. Igel-Mann, H. Stoll and H. Preuss, Mol. Phys., 65 (1988) 1321.