Ab Initio Study of Proton-Transfer Surfaces in Zeolite Models

energies of complexation and that no significant proton-transfer barrier exists between the two. ZO-,NH4+ is the most stable species only using a two ...
0 downloads 0 Views 640KB Size
Download PDF
9034

J. Phys. Chem. 1993,97, 9034-9039

Ab Initio Study of Proton-Transfer Surfaces in Zeolite Models E. Kassab,’ J. Fouquet, M. AUavena, and E. M. Evieth Luboratoire de Dynamique des Interactions MolCculaire (CNRS.UPR 271), UniversitC Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris, France

Received: April 1, 1993

-

The proton-transfer surfaces for the passage from ZOH,NH3 ZO-,NH4+ are computed using a b initio methods for two zeolitemodels, ZOH = HsSiOHAlHs and H3SiOAlH20HSiH3, having one and two complexation sites, respectively. The highest level of calculation indicates that both species have approximately the same energies of complexation and that no significant proton-transfer barrier exists between the two. ZO-,NH4+ is the most stable species only using a two site zeolite model at the MP2 level. The energy difference between the ZOH,NH3 and ZO-,NH4+ species is basis set insensitive but correlation sensitive. With use of a two site model it is predicted that the rotation of the NH4+ species at the zeolite complexation sites is a low activation energy process. In addition, it is concluded that deuterium exchange should be rapid.

A. Introduction

CAGE)

Zeolites are widely used as acid-type catalysts. Many of their catalytic properties can be directly related to Bronsted acidity of the proton attached to the oxonium moiety present in the aluminum containing unit, (-O-)3A1-OHSi(-O-)3, of these structures. Ab initio quantum mechanical modeling of these structures began in the early 1980s’-3 and until recently” has dealt only with the structural aspects of small units. Recently, there has been an effort to theoretically model the structures and energeticsof complexes formed by the binding of a proton acceptor base, :B, to the acidic site.3~5-9 The complexation calculations are evolving mainly in two different directions,one treating larger systems using small basis sets for superclusters (e.g. refs 4 and 9) and periodic H F methods. The size of the systems treated requires the use of small basis sets (STO-3G, 3-21G) which compute self-consistent field (SCF) complexation energies considerably higher than those obtained by using larger basis sets. The other direction is aimed at improving the technical features of the small model calculations in order to determine the variation of complexation energetics with basis set and correlation effects. One major problem is modeling the relative energetics of the formation of neutral and contact ion-pair specieson complexation of a proton acceptor :B with a particular zeolite model, ZOH.

+

- -

Z O H :B Z 0 H : B ZO-,H:B+ (1) Correctly modeling these relative energies is important if one is to have confidence in future theoretical studies of reaction mechanismswhich are presumed to involve ion-pair intermediates. In the case of NH3 it is spectroscopically implied1@13that the observed species is ZO-,NH4+ and that NH4+is possibly attached to several oxygens of the aluminosilicate framework at different sites in the four and six tetrahedral (T) membered rings of faujasite-typezeolites.lO These types of zeoliteshave four different proton sites (oxygens 1 4 , Figure 1). Recent neutron diffraction work14 has structurally confirmed that sites 1 and 3 are frequently protonated, site 2 less so, and site 4 not at all. The site 3 proton is directed into the @-cage. Theoretical rationalization of these site preferences is uncertain.15 Although the NH4+ binding has not been structurally characterized in any zeolite, the interaction of water with a D-Y-zeolite yields a protonated species in which the resulting D30+is attached to three O(3) oxygens of the 6-T ring at the site I’ (Figure 1) in the j3-cage.14 Similar types of two oxygen and three oxygen cross-ring complexations are proposed for NH4+ in the other various 4-T and 6-T rings of these types

* To whom correspondence should be addressed. 0022-365419312097-9034$04.00/0

HEXGONAL PRISMS I

Figure 1. Faujasite-type zeolite structure. Shown are the oxygen sites 1 4 on which proton attachment can occur. Some of the cationic complexation sites, I, 11, and I1 are shown only for the 6-T rings. See

refs 3 and 16. of zeolite structures.1° These structures would be analogous to those structurally known for Na+ ~omplexation.~6J~ On the other hand, it is also possible that the complexation occurs on one, two or three oxygens attached to the same A1 atom of an A104 tetrahedral (T) site.* Ab initio modeling of ammonia and water complexation, initially limited to one oxygen site (e.g. refs 6 and 9), has been extendedto two and three oxygens attached to the same aluminum atom.3-5.7s8We will report later calculations of the complexation energies of NH4+ and Na+ to 3-T cyclic aluminosilicates.18 However, the work presented here is concerned with the technical aspects of determining the relative energies of structures like ZOH:NH3 and ZO-,NH4+ and especiallywhether the energy barrier separating them is sufficient for them to coexist in the same zeolite models. This and associated structural characterizations permit speculating on the dynamical behavior of ammonia attached to acid sites in zeolites.

B. Method of Calculation Two zeolite models were used in the ab initio calculations presented here, H3SiOHAIH3 (I) and H3SiOAIH20HSiH3 (11) (Figure 2). The proton-transfer surfaces for I complexed with both NH3 and PH3 were computed in a series of calculations using fmed values of OH and fully optimizingall other geometrical 0 1993 American Chemical Society

Proton-Transfer Surfaces in Zeolite Models

The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9035

L

1.678

I

A

%

9J-p

I .746,'

I

I

I

I

I

I

,'1.780 I

B

Q

ONE SITE

D

TWO

SITES

Figure 2. SCF/6-3 1G* optimizedstructures of the ZO-,NH,+ (A) contact ion pair (upper) and neutral ZOH-NH3 complex (B) of ammonia interacting with the two tetrahedral, one oxygen site model, H3SiOHAlH3. This is the one site model. Structures C and D are the corresponding complexes for the two site model. SK tables for the major geometrical parameters of this structures.

parameters at the SCF/6-31G* level using both the programs Monstergauss'9 and Gaussian 90.20 Extended calculations were done with these geometries at the MP2 level using this basis and 6-31+G** and Dunning (D95**) bases. This 2-T model, possessing only one oxygen atom, only permits single site binding. Some fully optimized 3-21G calculations were done in order to make some comparisonswith recently published calculationsusing rigid geometries.9 The 3-21G level optimizations showed several hindered rotor minima on the potential surface corresponding to positionsof the H3Si-O and O-AlH3 unitsvis-&vis the O H bond. These various rotomers had nearly the same relative 3-21G energies (f1 kcal/mol), and only one configuration was selected for complexation studies (Figure 2, structures A and B). The same computational protocol was applied to the 3-T zeolite model, H3SiOAlH20HSiH3 (11), with the exception that the structure was optimized restrained in C, symmetry along the proton-transfer reaction coordinate. The size of this system

permitted doing calculations only at the HF/6-31G*, MP2/63 1G*//HF/6-3 1G*, and MP2/6-3 lG*//MP2/6-3 1G* levels. Again, only one rotationallyconstrainedconfiguration was selected for a computational study (Figure 2, structures C and D). Since we are mainly interested in the proton-transfer surfaces no basis set superposition error counterpoise (BSSE-CP) corrections were made in the energies reported below. Such a correction is difficult to apply to a reaction surface in the region of the transition state. We do not include any zero point energy (ZPE) corrections. For complexation energies, these combined corrections are important: probably in the region of 5 kcal/mol or more for the structures studied here. In addition, although the structures reported here all have zero energy gradients with regard to geometrical change, in the case of the 3-T zeolite model (11) these are due to the planar symmetries imposed during the optimizations. In any case, small structure modeling of zeolite systems is still only of semiquantitative interest and does not necessarily justify extended computational protocols.

Kassab et al.

9036 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993

TABLE I: Basis Set Variation of the Relative Energies, E, of the Interaction of NH3 with the ZOH Model HSiOHAlHj Darameters 6-31G* r(Si0) r(Al0) LSiOAl r(OW r(HN) LOHN r(NW LHNH SCF6-31G' E, (kcallmol) MP2//6-3 lG* E, (kcallmol) SCF 6-31+G1*//6-31G* E, (kcallmol) MP2/6-3 l+G**//6-3 1G* E, (kcal/mol) SCF D95**//6-31G* E, (kcall mol) MP2/D95**//6-31G1 E, (kcallmol) a

ZOH + NH3

ZOH..NHI~

ZO.H.NH3

ZO-..NH,+

ZO- + NHI+

1.IO0 2.037 131.2 0.952

1.683 1.984 130.8 0.983 1.780 173.2 1.003 (1.004) 107.7 106.6 -665.98676 (-15.5) -666.48 118 (-20.0) -666.0173 1 (-13.3) -666.58181 (-17.0) -666.04 109 (-13.1) -666.60163 (-1 8.1)

1.643 1.890 130.0 1.316 1.171 171.6 1.005 (1.006) 110.0 (107.4) -665.97358 (-1.3) -666.47729 (-17.6) -666.00400

1.633 1.867 131.7 1.474 1.091 169.3 1.006 (1.007) 110.8 (107.3) -665.91395 (-7.5) -666.47588 (-16.7) -666.00373 (-4.8) -666.57491 (-13.0) -666.02515 (-4.3) -666,59315 (-12.7)

1.575 1.789 178.4

m

1.003 107.2 -665.96200 (0.0) -666.44928 (0.0) -665.99608 (0.0) -666.55414 (0.0) -666.01932 (0.0) -666.57285 (0.0)

(-5.0)

-666.57726 (-14.5) -666.02645 (-4.5)

-666.59615 (-14.6)

m

1.013 109.47 -665.8041 1 (+99.1) -666.29501 (+96.8) -665.83659 (+100.1) -666.39417 (+100.4) -665.85417 (+103.6) -666.40741 (+103.8)

Single site binding, see Figure 3. Geometrical parameters given in angstroms and degrees. Ab initio energies are in atomic units.

TABLE II: Basis Set Variation of the Relative Energies, E, of the Interaction of PH3 with the ZOH Model H3SiOHAlH3 parameters ZOH + PH3 ZOH..PH,' ZO.H.PH3 ZO-..PH4+ ZO- + PH4+ 6-31G'

r(Si0) r(Al0) LSiOAl r(OH)

4-w LOHP

1.705 2.024 136.8 0.951 m

r(PH)

1.403

LHPH

95.4

SCF 6-31G*

E, (kcal/mol) MP2//6-3 lG*

-952.22560 (0.0) -952.64108

E, (kcal/mol)

(0.0)

SCF 6-31+G**//6-31G* E, (kcal/mol) MP2/6-31+G**//6-3 1G*

-952.25031

1.696 2.020 134.0 0.960 2.599 170.6 1.399 (1.398) 97.0 97.3 -952.23231 (-4.2) -952.65180 (-6.7) -952.25661

(0.0)

(-4.0)

-952.74261

-952.15312 (-6.6) -952.26113 (-4.4) -952.77105 (-6.4)

E, (kcallmol)

(0.0)

SCF D95**/6-31GS

-952.26064

E, (kcal/mol)

(0.0)

MP2/D95**//6-31GS

-952.16093

E, (kcal/mol)

(0.0)

1.634 1.853 134.1 1.483 1.491 163.6 1.391 (1.380) 103.1 (106.2) -952.19055 (+22.1) -952.62754 (+12.3) -952.21505 (+22.2) -952.12514 (+10.6) -952.22238 (+24.0) -952.73780 (+ 14.5)

1.617 1.I95 139.5 1.926 1.389 146.1 1.398 (1.374) 102.5 (1 11.9) -952.19189 (+2 1.2) -952.62241 (+15.4) -952.21567 (+21.8) -952.12108 (+ 13.5) -952.22208 (+24.2) -952.73214 (+18.1)

1.575 1.189 178.4 m

1.380 109.47 -952.03493 (+119.7) -952.45589 (+120.0) -952.06213 (+118.1) -952.55478 (+117.9) -952.06272 (+124.2) -952.55926 (+ 126.5)

* Single site binding. Geometrical parameters given in angstroms and degrees. Ab initio energies are in atomic units. C. Discussion and Results 1. The Two Tetrahedral, One Site Model, HfiiOHAlH,. The critical points on the potential energy surface for the proton transfer between H3SiOHAlH3 and NH3 and PH3 are given in Tables I and 11. The two bases NH3 and PH3 have gas-phase proton affinities of 204 and 189 kcal/mol, respectively.2' The use of these two examples will demonstrate how a relatively small change in proton affinities affects the proton-transfer reaction coordinate. Although the calculations have been performed at 15 different OH distances, Tables I and I1 only report four situations; the energies and structures of (i) separated species, ZOH + :B, (ii) the proton-transfer transition state, ZO-H-:B, (iii) the contact ion pair, ZO-,H:B+, and (iv) the separated ion pair, ZO- + H:B+. The latter energy is of only formal interest since it cannot be related to an experimental measureable. Complexation with NHj. Ab initio calculations of the complexation of NH3 with HpSiOHAlH3 have been published in several previous studies.69 As seen from Table I, thecomplexation energy of ZOH + NH3 giving ZOH:NH3 varies with basis set

and correlation. The three DZP and larger basis sets employed yielded SCF complexation energies between 13 and 16 kcal/mol, and these were uniformly raised by about 4 kcal/mol at the MP2 level to give values between 17 and 20 kcal/mol. These energies would be lowered by at least several kcal/mol if both BSSE-CP and ZPE corrections were applied.7.8 The effect of correlation on the relative energy difference between the ZOH:NH3 and ZO-,NH4+ forms was much larger than with the complexation energies. First, as seen from Figure 3, and by direct calculation, the ZO-,NH4+ structure has a fragile SCF/6-3 lG* minimum on the hypersurface. This minimum does not exist on the MP2/ /SCF/6-3 1G* surface. The SCF energy differences between the ZOH:NH3 and ZO-,NH4+ 6-3 lG* optimized structures are in the 8-9 kcal/mol region but reduced significantly at the MP2 level. The MP2 surface indicates that the ZOH:NH3 and ZO-,NH4+ structures cannot coexist. This observation is in keeping with previous theoretical work showing that there is a near zero intrinsic barrier for proton transfer between oxygen and nitrogen acid-base systems.22

The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9037

Proton-Transfer Surfaces in Zeolite Models

-9

22' -11

18'

MP2 -13

14.

IO' -15

-

6 .

0

2'

E 5

-

E e Y

5 - 1 7

Y

.-cx

.-c

g-ls

-?'

W c

w c -6.

-2 I

- 10'

-23

-25 -22

-26

4 -27

{ 0 . 0

1.0

1.2

1 . 4

i

I .E

I .I

F i e 3 . SCF/6-31Gt and MP2//SCF/6-31GS abinitioproton-transfer surfaces for the one oxygen site model, HsSiOHAIH3, interacting with ammonia and phosphine. Of particular technical note is the marginal proton-transfer barrier that occurs at the SCF level is removed at the MP2 level.

TABLE 111: Effect of Electric Field on the 6316' SCF Energies of the Single Site ZOH,NH3 and ZO-,N& Complexes' electric . .... ...

field E, (au)

energy (au) CCX ZOH,NHP

0 -0.005 -0.010

-665.98676 -665.99756 -666.00979

0 4.005 -0.010

-665.97395 -665.99262 -666.01276

-3.432 -4.528 -4.528

PZ

Pr

5.916 5.851 6.579

6.840 7.398 7.986

9.302 9.866 10.607

10.217 10.892 11.571

ZO-,NH,+ -4.226 4.612 -4.624

I

1 1 . 0

1 . 1

1.2

1.3

1 . 4

1.5

1.6

1.7

1.

e

R(0-H) in Angstroms

R(0-H) in Angstroms

' S e e Figure 3 orientation of the x-, y-, and z-axis. The z-axis is definedalongtheO-Hbond. Thereis nodipolemomentin they-direction. The electric field is given in atomic units, 1 au field unit = 51.4 V/A.

We note now that these above results are in disagreement with thoseobtainedat theSCF/3-21Glevel using fixed, crystalrelated geometries.9 These latter calculations show 3-21G complexation energies in the 30-40 kcal/mol and ZO-,NH4+ being more stable than ZOH:NH3 regardless of cluster size. We fully optimized the H3SiOHAlH3,NH3system at the SCF/3-21G level and found that the complexation energy of ZOH:NH3 is actually in the order of 26 kcal/mol but that the ZO-,NHd+ structure was not a minimum on the surface. Using fixed N-H distances for the proton attached to the oxygen to ZO- and fully optimizing all other parameters gave an SCF/3-21G ZO-,NH4+ energy 9 kcal/ mol less stable than the ZOH:NH3 form. Similar results are found on other zeolite models even at the STO-3, 3-21G, and 6-31G* levels18 and in previous work at the 6-31G*//STO-3G level.' These results, in combination with the larger basis set calculations presented here, indicate that the ZOH:NH3/ZO-,NH4+ energy difference is fairly basis set

Figure4. SCF/6-31GS andMP2//SCF/6-31G* abinitioproton-transfer

surfaces for the two oxygen site model, H3SiOAlH20HSiH3,interacting with ammonia. Of particular technical note is the marginal protontransfer barrier that occurs at the SCF level is removed at the MP2 level and that the order of stabilities of the contact ion pair and neutral species are inversed.

insensitive. One also anticipates that this energy differenceshould be BSSE-CP insensitive. The large basis set small model ammoniation energies calculated here (below 20 kcal/mol) are still very far from experimental values ranging in the 30-40 kcal/ mol region.'.* In any case, the previously found 3-21G complexation energies of 30-40 kcal/mol are only in fortuitous agreement with the experimental values. In a technical sense the large basis set completely optimized calculations are of a better quality than small basis set rigid geometry values. However, the rigid geometry calculations are interesting in their own right in showing that an inversion of ZOH:NH3,ZO-,NH4+ order of stability can occur using other reasonable geometrical configurations and approach modeling the experimental values. The fact that the energy difference between the neutral and zwitterionic complexes can approach a zero value indicates that a change in the electrostatic environment in the region of the acidic site could affect the relative stabilities of these species. One expects both the ZOH and ZOH:NH3 forms to have some polar character but that the ZO-,NH4+ structure should be much more polar. In fact, the variation of the SCF dipole moment along the proton-transfer reaction coordinate is significant but not large. The change in going from the ZOH:NH3, 6.8 D, to the ZO-,NH4+ structure, 10.2 D, is only 3-4 units. These two forms only differ in SCF energy by about 8 kcal/mol, and that order could be inverted if the electric field gradient is sufficient. Table I11 shows that the order is inverted if a field of about 0.5 V A-1 is applied along the z-axis (Figure 2), which is approximately in the direction of the 0-H-N linkages. The electric field value is lower or of the same order as those estimated in complexation regions of zeolite materials (e.g. ref 4). Therefore, the electrostatic environment created by external ionic components not treated in the small cluster model could reasonably affect whether the species

9038 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993

Kassab et al.

TABLE I V Basis Set Variation of the Relative Energies, E, of the Interaction of NH3 with the ZOH Model HfiiOAlH2OHSiHHJ. parameters ZOH + NH3 ZOH..NH3* ZO.H.NH3 ZO-..NH,+c

ZO- + NH4+

6-31G*

r(Si0) r(OA1) r(0Si) LSiOAl LOA10 LAlOSi r(OH) r(HN) r(NH) r(HO) fOHN LHNN r(NH) LHNH

SCF 6-3 lG* E, (kcal/mol) MP2//6-31GS E, (kcal/mol) MP2//MP2/6-3 lG* E, (kcal/mol)

1.700 2.003 1.734 1.615 135.5 99.7 154.9 0.954 m

1.003 107.2 -103 1.02060 (0.0) -1031.761 56

(0.0)

1.685 1.957 1.748 1.615 130.0 98.2 144.4 0.988 1.746 1.004 2.471 173.6 94.8 1.004 (1.003) 107.9 107.7 -103 1 .O4521 (-15.4) -103 1.79510 (-21.0) -103 1.80019 (-24.2)‘

1.655 1.880 1.782 1.621 128.8 98.9 135.9 1.225 1.24 1.012 2.01 1 168.5 99.5 1.012 (1.004) 109.8 (109.5) -103 1.03708 (-10.3) -103 1.79777 (-22.7)

1.629 1.826 1.826 1.629 130.5 100.7 130.4 1.678 1.041 1.041 1.678 151.1 100.0 1.041 (1.005) 111.5 (111.4) -103 1.04229 (-13.6) -1031.80147 (-25.0) -103 1.80458 (-26.7)‘

1.579 1.774 1.774 1.579 179.3 107.8 179.3 m

1.013 109.47 -1030.86865 (+95.4) -103 1.61356 (+92.9)

C- Geometrical parameters given in angstroms and degrees. Ab initio energies are in atomic units. Single site binding. c Two site binding, see Figure 4. The Er values are computed from the MP2//6-31GZ energies of the monomers. The geometries of the MP2 optimized structures are not given in this table. That of the ZOH-NH, structure showed partial proton transfer with the 0-H and N-H bonds in the hydrogen bond at 1.08 and 1.48 A, as compared to the SCF values of 0.99 and 1.75 A.

is Z0H:NHs or ZO-,NH,+ and whether these species are bound to a single site or to multiple sites. Complexation with PHJ. As seen from both Figure 3 and Table 11, the ZOH,PHs is a less interesting system than ZOH,NH3. In this system there is a large energy difference between the two forms and computationally the ZO-,PH4+ is not bound to the zeolite model vis-A-vis the energy of the separated species, ZOH PH3. However, one feature of the calculation is there is the same larger relative stabilization of the ZO-,PH4+ species at the MP2 level as found in the case of ZO-,NH,+. It should bementioned that thephosphonumspecies,P K + , proton attached structure is not a global minimum in this structural region. The hydrogen atoms of the PH4+ are really not protonic and water and ammonia prefer to complex directly to the phosphorous atom.23 2. The Three Tetrahedral, Two Site Model, HfiiOAlHzOHSMJ. The two oxygen site model, H3SiOAlH20HSiH3, permits examining the difference in energy between three configurations, the one site complexations, ZOH:NH3 and ZO-,NH4+, and the two site complexationof NH4+ bound to two oxygens of the ZO- anion (Figure 2, structures C and D). The proton-transfer surface connecting ZOH:NH3 with the two site complex, ZO-,NH4+, is shown in Figure 4 and the structural and energetic details of the geometriesof the SCF minima are shown in Table I. As seen from Figure 4 the SCF surface exhibits a significant barrier between the two species. However, the ZOH: NH3 structure is only 2 kcal/mol more stable than the two site ZO-,NH4+ complex. However, the MP2//SCF surface shows an inversion of this order and the lack of a minimum for the ZOH:NH3 structure. When one uses the extended protocol of optimizing at the MP2 level (Table IV), the ZOH:NH3 minimum reappears. The MP2 optimized barrier between the two species was not recomputed, but it cannot be greater than 1-2 kcal/mol. These computational features are of purely technical interest since the overall conclusion is that, at this basis set and correlation level, both structures are of comparable stability to within several kilocalories per mole. Of particular notice is that the MP2//SCF/6-31GZ or MP2/ /MP2/6-31G* binding energy in the two site model is on the

+

order of -25 kcal/mol. This is a net gain of about 8 kcal/mol over the one site ZO-,NH4+ MP2//SCF/6-31GZ value shown in Table I. However, part of this energy gain is due to the different structural nature of the H3SiOHAlH3 and H3SiOAlHzOHSiHp units. The single site calculation for the two site ZO-,NH4+ complex is not technically possible for two reasons. First, the complex does not represent a minimum on the hypersurface and, secondly, the supposedly noncomplexating oxygen is still near the N-H unit. One structure was obtained by optimizing all the geometrical parameters of C, (H3SiOAlH2OSiH3)-,NH4+ while imposing linearity on the O-H-N bond. This structure has zero gradients except for the bending coordinate connecting this structure with the two site ZO-,NH4+ structure. This structure (Figure 5, structure A) still has a short NH-other oxygen bond distance (2.02 A) and is not authentically a single site structure. This structure is conceptually a minimum on the proton-transfer coordinatebut not with respect to yielding the two site ZO-,NH4+ complex. The energy of this configuration is only 2.6 and 3.5 kcal/mol above that of the two site ZO-,NH4+ structure at the SCF and MP2 levels, respectively. Its MP2 complexation energy is 21.5 kcal/mol which is 5 kcal/mol more stable than the corresponding authentic single site complex (Table I). Another structure was also computed (Figure 5, structure B) which represents a transition state for the rotation about one O-HNH3+ axis. This is a bifurcated transition state structure of a type found in other ion-molecule rotational isomerization mechanisms.23J4 This structure has an energy of 2.5 and 3.9 kcal/mol at the SCF and MP2 levels, respectively, less stable than the two site structure. These values represent an estimate of the activation energy for rotation of the NH4+ at a zeolite site. These values are higher than found in experimental studies (ca. 1 kcal/mol13) but demonstrate that the zeolite model used does predict a loose structure in which a number of geometrical configurations are accessible at room temperature. Finally, recent work’**also showed less than 1 kcal/mol separating the two and three site ZO-,NH4+ complexes for ZO- = H2Al(OH)2- and HAI(OH)3-. The relative energies of the C and D structures in Figure 2 and A and B structures in Figure 5 also show that the deuterium scramblingin the reaction ZOD,NH3 ZOH,NDH2

-

The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9039

Proton-Transfer Surfaces in Zeolite Models

of NH3 from one oxy-aluminum center to another since NH3 would be required to migrate over several intervening low binding Si-O-Si oxygen sites. D. Conclusions The calculations presented here indicate only a marginal energeticadvantage for ammonia existing in an NH4+ form rather than the NH3 form using the zeolite models employed here. The advantage found is not particularly sensitive to the basis set employed. The combined effect of both correlation and using a two site model favors the formation of NH4+. We also conclude that there is no significant barrier for proton transfer between the neutral ZOH,NH3 and ZO-,NH4+ forms and that they will not coexist at the same ZOH site. However, the dynamic behavior of the system with regard to deuterium exchange may give the appearance that they do.

A BIFURACTED STRUCTURE Energies: SCF/B-SlG* -1051.05824, MP2 -1051.79522

References and Notes

*& RELATIVE ENERGY SCF +2.5

, MP2 +5.9

kcal/mol

h

Si

(1) Hass, E. C.; Mezey, P. G.; Plath, P. J. J. Mol. Struct. 1981,76,389; 1982, 87, 261. (2) Sauer, J. Chem. Rev. 1989,89,199-225. This review gives a coverage of the literature prior to 1989. See Table 25, p 242. (3) Sauer, J. In ModelingofStructure andReactivity inZeolites; Catlow, C. R. A., Ed.; Academic Press: London, 1992; pp 183-216. (4) Ahlrichs, R.; Bar, M.; Haser, M.; Kolmel, C.; Sauer, J. Chem. Phys. Lett. 1989, 164, 199. (5) Sauer, J.; Horn, H.; Haser, M.; Ahlrichs, R. Chem. Phys. Lett. 1990, 26, 173. (6) (a) Kassab, E.; Seiti, K.; Allavena, M. J. Phys. Chem. 1991, 95, 9425. (b) Allavena, M. Seiti, K.; Kassab, E.; Ferenczy, Gy.; Angyan, J. G. Chem. Phys. Lett. 1990, 168,461. (7) Teunissen, E.; van Duijneveldt, F. M.; van Santen, R. A. J. Phys. Chem. 1992,96, 266. (8) Teunissen, E.; van Santen, R. A,; Jansen, A. P. J.; van Duijneveldt, F. M.J. Phys. Chem. 1993, 97, 1993. (9) Brand, H. V.; Curtiss, L. A.; Iton, L. E. J. Phys. Chem. 1992, 96, 7725. (10) Ozin,G.A.;Baker,M.D.;Godber,J.;Gil,C. J. J.Phys. Chem. 1989, 93. 2899. (11) Vega, A. J.; Luz, Z. J. Phys. Chem 1987, 91, 365. (12) Earl, W. L.; Fritz, P. 0.;Lunsford, J. H. J. Phys. Chem. 1987, 91, 2091. (13) Udovic, T. J.; Cavanagh, R. R.; Rush, J. J.; Wax, M. J.; Stucky, G. D.; Jones, G. A.; Corbin, D. R. J. Phys. Chem. 1987, 91, 5968. (14) (a) Czjek, M.; Jobic, H.; Fitch, A. N.; Vogt, T. J.Phys. Chem. 1992, 96,1535. (b) Jobic, H.; Czjek, M.; van Santen, R. A. J . Phys. Chem. 1992, 96, 1540. (15) Schroder, K.-P.; Sauer, J.; Leslie, M.; Catlow, C. R.; Thomas, J. M. Chem. Phys. Lett. 1992, 188, 320. (16) Smith, J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.;

.

B IMPOSED LINEAR 0-H-N STRUCTURE Energies: SCF/651G* -1051.05815, MP2 -1031.79597 RELATIVE ENERGY SCF +2.6, MP2 +5.5 kcal/mol

Figure 5. An optimized planar structure (structure A) in which linearity is imposed on the 0-H-N bond in order to simulate a single site structure and a transition state for rotation of NH4 around a 9,axis (structure

B). should be rapid by a combined rotation and rocking motion between the two oxygen sites. The SCF dipole moment change along the proton-transfer reaction coordinate in this two site model is smaller than that found in the one site model, ranging from about 7 D for structure ZOH:NH3 to 9 D for the two site complex ZO-,NH4+. In this case, the zwitterionic structure for the two site complex is more compact than the one site complex. Even so, given the uncertainties expressed above as to the electrostatic effect of the local environment, the calculations presented in this section indicate that one and two site ZO-,NH4+ structures have nearly the same energies and are near the energy of the ZOH:NH3 structure. Using a three site model, one can easily envisage the rapid migration-rotation motion of NHs/N€&+ species in moving from one-to-two and two-to-three site complexation around the A104 tetrahedon. However, in low aluminum zeolites, the deep well in which the complex exists would prevent any rapid migration

ACS MonographSeries 171;AmericanChemicalSociety:Washington, D.C., 1976; pp 3-79. (17) Catlow, C. R. In ref 3. (18) Evleth, E. M.; Kassab, E.; Allavena, M. Manuscript in preparation. Binding of Na+ and NH,+ to a cyclic -O-AIH&-SiH&-SiHr showed three site binding in the case of Na+ and two site in the case of N&+. In the latter case, only the two oxygen atoms adjacent to the A1 atom were complexed. In the latter case, theNH, SCF binding energies to the protonated zeolite model were-24.0, -25.0, and -15.3 kcal/mol at the STO-3G, 3-21G, and 6-31G* levels, respectively, and the NH,+ structure was less stable than the neutral complex by 9.3, 9.8, and 7.5 kcal/mol, respectively. (19) Poirier, R. Monstergauss; Department of Chemistry, Memorial University of Newfoundland St. Johns, Newfoundland, 1988. (20) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.;Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. Gaussian 90, Revision I; Gaussian, Inc.: Pittsburgh PA, 1990. (21) Lias, S.G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Re$ Dara 1984. 13. 695. (22) Cao, H. Z.; Allavena, M.; Tapia, 0.; Evleth, E. M. J. Phys. Chem. 1985, 89, 1581. (23) Evleth, E. M.;Hamou-Tahra, Z . D.;Kassab. E. J. Phys. Chem. 1991, 95, 1213. (24) Kassab, E.; Evleth, E. M.; Hamou-Tahra, Z. D. J . Am. Chem. Soc. 1990, 112, 103. ~