Abstract

keywords: aluminophosphate zeolite, hydrophilicity, force field

Aluminophosphate zeolite (AlPO4)[1] is an artificial material in which tetrahedral AlO4 and PO4 are linked alternately in three dimensions to be a regular pore structure with no defect. This structure makes the AlPO4 electronically neutral with neither the counter cations nor Broenstead acid sites like standard natural zeolites. Consequently, AlPO4 is expected to have adsorption, catalysis, and separation properties different from standard zeolites. Based on the cluster model from quantum mechanics (QM) calculations[2], we have used molecular dynamics (MD) simulations to study the hydrophilicity of AlPO4.

Despite the lack of a hydrophilic site, AlPO4 has a strong hydrophilicity as confirmed by isothermal adsorption experiments[3]. One might assume that this hydrophilicity is merely a consequence of the difference in electronegativity between Al(1.5) and P(2.1). However, recent experimental data [4] shows that some aluminum sites (denoted as a site) can adsorb two water molecules whereas other sites (denoted as b site) adsorb none. This implies that structural character must play a role in hydrophilicity of AlPO4. In the AlPO4 frame, the a site is located at the intersection of two rings with 4 oxygen atoms, and the b site is located at the intersection of a ring with 4 oxygen atoms and a ring with 6 oxygen atoms.

To investigate the hydrophilicity of AlPO4, we made cluster models from the hydrate [4] and dehydrate[5] forms of VPI-5 [6] (an aluminophosphate containing rings with 18 oxygen atoms). We carried out ab initio QM calculations (Hartree-Fock level with basis set superposition error correction) on the reaction of a water molecule with a cluster model of VPI-5. We found the following results which were confirmed quantitatively using the energy decomposition method [7]:

(1) No water molecule is adsorbed at the a Al sites of the dehydrate form and b Al sites of the both hydrate and dehydrate form.The repulsion results from electron exchange interactions between the oxygen atom of the water molecule and the oxygen ligands of the Al atom.This repulsive interaction exceeds the attractive interaction between the Al site and the water molecule;

(2) The geometric deformation around the a Al site moves the oxygen atom ligands away from the direction of approach for the water molecule. This deformation reverses the relative magnitude between above two components, making adsorption favorable.Thus the detailed balance between these two kinds of interaction causes the hydrophilicity differences in the Al site.

Since the hydrophilicity of AlPO4 depends on the local geometric deformation, the site-specific hydrophilicity must be explained by differences in the stiffness of the local deformations at the two sites, which is determined by the location within the pore structure.

In order to test these ideas, we need a reliable force field (FF) capable of representing the stiffness of the framework structure while allowing the local deformation required for reaction with the water as determined from the QM. Consequently, we adopted the form of the new MS-Q FF proposed by Cagin[8]; Demiralp, Cagin, and Goddard [9] have already applied this FF for describing silica, glasses, minerals, and zeolites. The MS-Q FF adopts the charge calculated using the charge equilibration method (QEq) by Rappe and Goddard [10].This allows the charges to readjust with geometry. The only other terms in the FF are two-body Morse terms.

Thus the interaction between atom i and j is given by (1a,1b)

U_ij(R_ij)=D₀{kai_ij² - 2 k_ij ] + q_i q_j/R_ij (1a)

kai_ij=exp[(-gamma_ij/2)(R_ij/R₀-1)] (1b)

where D0 is the bond strength in kcal/mol, R0 is the bond distance in A, gamma is a scale factor, and q_i is the partial charge of i-th atom. The results [11-13] for the dehydrate VPI-5 [5] are summarized in Table I. We find net partial charges +1.4 for Al, +0.56 for P, and -0.5 for O. The FF parameters are summarized in Table II. We compare them with results from van Beest et al. [14] and de Vos Burchart et al.[15]. The FF of van Beest et al. uses large fixed partial charges on the P(+3.4) and O(-1.2).While the FF of de Vos Burchart et al. uses smaller fixed charges of Al(0.56), P(0.25), and O(-0.18).

Table III shows the MS-Q FF well reproduces the experimental structure of VPI-5.

Acknowledgement

References

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Atom  	  MS-Q a    BKS b   BBGV c

Al1 	 1.4209     1.4     0.56
Al2 	 1.3957     1.4     0.56
P1  	 0.5834     3.4     0.25
P2  	 0.5531     3.4     0.25
O1  	-0.5262    -1.2    -0.18
O2  	-0.5005    -1.2    -0.18
O3  	-0.4759    -1.2    -0.18
O4  	-0.4820    -1.2    -0.18
O5  	-0.4691    -1.2    -0.18
O6  	-0.5192    -1.2    -0.18
O7  	-0.4915    -1.2    -0.18

	     Ro(A)    Do(kcal/mol)       gamma 

O-O         3.7910     0.53633          10.4112
Al-Al       3.4103     0.29556          11.7139
P-P         3.4103     0.29556          11.7139
Al-O        1.6848    27.315             8.4642
P-O         1.4468    51.381             8.3633
Al-P        4.2419     0.29556          11.7139

               Cell             Bond 
             Parameters      Distances		   Angles
 	     a      c       Al-O   P-O   P-O1-Al   P-O2-Al     P-O3-Al

Expta      18.9    8.1     1.68   1.54   172.9	   126.7       174.9
MS-Q FFb   19.1    8.6     1.67   1.58   168.8     137.0       177.8
BKS-FFc    18.9    8.4     1.75   1.51   161.3     137.0       173.9

a) Reference 5.
b) Present work.
c) Reference 13.