Langmuir 1994,10, 1832-1836
1832
Nature of Acidic Sites of the Lewis Type at the Surface of Mica and Related Aluminosilicate Materials Sylvie Berhouet and Herv6 Toulhoat’ Institut FranCais du Pktrole, B P 31 1, 92506 Rueil-Malmaison, France Received November 15, 1993. I n Final Form: March 14, 1994@ This paper describes the results of theoretical calculations of the desorption energies of water, ammonia, and pyridine from model acidic surface sites of aluminosilicate materials of the Bronsted (Type I) and Lewis types (I1 and V). We have borrowed our description and nomenclature of surface sites from Bhattacharyya (Langmuir 1992,8,2284)and comparedour theoretical results to his results from temperature programmed desorption (TPD) experiments of ammonia from mica surfaces. We conclude that only Lewis sites of type V, i.e., involving an electron-deficient coordinatively unsatured tetrahedral silicon atom as the adsorption center, are able to bind ammonia with energies (110-250 kJ/mol) comparable to those observed experimentally (76-122 kJ/mol). Coordinatively unsatured tetrahedral aluminum centers do not bind Lewis bases. Bronsted sites develop binding energies lower than 10 kJ/mol. In this context, pyridine is a stronger Lewis base than both ammonia and water.
Introduction In a recent paper’ K. G. Bhattacharyya reported his results from temperature programmed desorption (TPD) of ammonia from freshly cleaved clean mica surfaces. He observed two peaks around 318 K (low temperature, LT) and 450 K (high temperature, HT), associated with activation energies of desorption of 76 and 122 kJ/mol, respectively. The LT state had a saturation coverage about 1 order of magnitude larger than that of the H T state. Bhattacharyya discussed his results with reference to similar studies conducted with zeolites, which also show evidence of chemisorbed states of ammonia2v3a t relatively low temperatures. He tentatively assigned the L T state to adsorption on either the Bronsted acid site of type I resulting from the exchange of a potassium cation by a proton or the Lewis acid site of type 11,which might appear upon dehydroxylation of the former, and which involves a trivalent neutral aluminum atom (See Figure 1). Moreover, the author suggests the possible occurrence of a further step of dehydration of the surface which from site I1 would lead to sites of type V involving an electrondeficient Sid+(coordinatively unsaturated). The type V site may also be viewed as the dehydration product of a protonated silanol group. The strong chemisorption of ammonia on this type of site could correspond to the H T state. In our opinion, these results and the related discussion shed interesting light on the problem of the nature of electron acceptors (acidic sites) at the surface of oxides and in particular of aluminosilicates. This problem has extremely important industrial implications, the best known of which are in heterogenous catalysis. We believe it is also the key for understanding surface properties of minerals in sedimentary rocks, in particular their wetting behavior in the presence of crude 0ils.4 Crude oils generally contain minute amounts of strong polar bases such as aromatic amine^,^ which are likely to be chemisorbed on electron-deficient surface sites, displacing water molecules and imparting a hardly reversible or irreversible oil-wet Abstract published in Advance ACS Abstracts, May 1, 1994. (1)Bhattacharyya, K.G. Langmuir 1992,8, 2284-2289.
(2) Topsoe, N.,Pedersen, K., Derouane, E. G. J. Catal. 1981, 70, 41. (3)Hidalgo, C. V., Itoh, H., Hattori, T., Niwa, M., Murakami, T. J. Catal. 1984, 85,362. (4) Toulhoat, H., Binet, L. In Physical Chemistry of Colloids and Interfaces in Oil Productions; Toulhoat, H., Lecourtier, J., Eds.; Technip: Paris 1992; p 149.
0743-7463/94/2410-1832$04.5010
Lewk aad (11) 0
4 ‘a, 0’
no
0
\
S ‘ I’ ‘0 0’
2%
‘3
II
V
Figure 1.
character to the mineral surface. It is, a t present, more and more recognized that the recovery of oil from a reservoir rock is strongly affected by its wettability. Accordingly, as we previously developed a theoretical chemistry approach to this p r ~ b l e m Bhattacharyya’s ,~ paper provided us with the opportunity to compare molecular modeling results to experiments. In the present paper, we study the adsorption potentials of water, ammonia, and pyridine on sites of types I, 11, and V, according to the classification of Bhattacharrya. The sites are modeled as clusters with appropriate geometries and number of electrons. Cluster size effects are also considered to some extent. Computer Experiments All calculations were performed on a Silicon Graphics IRIS 4D-35 TG workstation (6 MIPS). Adsorbates and model sites were built interactively using InsightII.6 The model sites for a muscovite mica surface were clipped out from the model of the elementary cell, with atom coordinates of the asymmetric unit cell corresponding to published crystallographic data.7 For the calculations, the site geometries were assumed to be rigid in order to represent the constraints imposed by the solid state. (5) Schmitter, J. M., Ignatiadis,I., Arpino,P.,Guiochon,G.Anu1. Chem. 1983, 55, 1685. (6) Software distributed by BIOSYM TECHNOLOGIES Inc., 10065 Barnes Canyon Rd., San Diego, CA 92121. (7) Radoslovich, E. W. Acta Crystallogr. 1960, 13, 919.
0 1994 American Chemical Society
Langmuir, Vol. 10, No. 6, 1994 1833
Nature of Acidic Sites of the Lewis Type Table 1. Minima of Adsorption Potentials of Adsorbates on Sites of Type V (& in kJ/mol, &,, in nm) site Va site Vb site Vc adsorbate EdEdEmi. dammonia -99.7 0.225 -33.7 0.275 -62.0 0.230 water -70.7 0.210 -24.1 0.340 -29.5 0.220 pyridine -91.4 0.235 -33.4 0.295 -35.8 0.245 Table 2. Effect of the Dielectric Constant t of the Solvent in the Self-Consistent Reaction Field Approach for Ammonia (Emi. in kJ/mol, &,, in nm) site Va site Vb site Vc c EdEdEd1 4 80
-99.7 -83.6 -79.5
0.225 0.225 0.215
-33.7 -32.5 -33.7
0.275 0.270 0.270
-62.0 -64.0 -66.1
0.230 0.230 0.230
Table 3. Minima of Adsorption Potentials of Adsorbates on Sites of Type V (hn in kJ/mol, d h in nm) Va site site Vb MNDO
MNDO DMol adsorbate EdEdEdEdammonia -99.7 0.225 -244.0 0.210 -33.7 0.275 -111.9 0.250 water -70.7 0.210 -152.3 0.210 -24.1 0.340 -67.5 0.255 pyridine -91.4 0.235 -258.5 0.225 -33.4 0.295 -144.1 0.250 DMol
In order to estimate the adsorption potential of adsorbates on model sites, we used the supramolecule formalism, according to which
where E:% is the adsorption potential energy, as a function of d , the siteladsorbate distance defined as the distance between the nucleus of the nitrogen or oxygen atom of the adsorbate and the nucleus of the coordinatively unsaturated silicon or aluminum atom involved in the process. EtLplsxis the total energy of the complex formed by the adsorbate interacting with the site at distance d. E,ib is the total energy of the site and Emoldethe total energy of the moleculeof adsorbate, when they are separated by an infinite distance. All energies are expressed in kilojoules per mole and distances in nanometers. Distances d were varied by steps of 0.01 nm, and EZLpI0.was computed at each step. The total energieson the right-hand side of eq 1were computed by the self-consistent field procedure as solutions of the Schrs dinger equation in the MNDO approximation.8 We used this implementationin either MOPACg or GEOMOSlOpackages.With the latter, we took advantage of the option to include a reaction field effect. In this approach, the solvent was assimilated with a dielectric surrounding a cavity where the solute is placed in a vacuum. The total energies are self-consistently corrected for the polarization energy resulting from the interaction of the distribution of chargesin the solute and its image in the dielectric. The three sites of type I taken in this study, denoted Ia, Ib, and ICby order of increasing size, and the corresponding sites of types I1 (IIa, IIb, and IIc) and V (Va, Vb, and Vc) are represented in Figure 2. The axis of approach is also indicated by an arrow in Figure 2 for each case. The results reported correspond to this axis of approach being superimposed on the molecule's principal axis of symmetry (Czy,Csr,and Czr,respectively, for water, ammonia, and pyridine), with the heavy atom pointing toward the sites. Approaches along an axis orthogonal to the principal axisof symmetryof the adsorbate and intersecting it at the location of the nucleus of the 0 or N atom resulted in all cases in repulsive potentials at all distances. (8)Dewar, M. J. S., Thiel, W. J. Am. Chem. SOC.1977,99,15,48994907. (9)MOPAC, QCPE Program No. 455. (10)GEOMOS, Rinaldi, D., Hoggan, P. E., Cartier, A., Laboratoire de Chimie Th6orique (UACNRSno. 510),UniversitB de Nancy I, Domaine Universitaire Victor-Grignard, BP 239, 54506 Vandoeuvre-16s-Nancy cedex, France.
Results and Discussion The computed adsorption potentials for neutral adsorbates interacting in vacuum (e = 1)with sites of types IIa, IIb, and IIc are plotted in Figure 3. These potentials remain positive over the whole range of distances d investigated. We conclude thus that Lewis sites of type 11, namely, involving a neutral aluminum atom with one oxygen vacancy, are unable to be chemisorbed on either ammonia or other bases accounted for. Figure 4 represents the potentials obtained for molecules interacting with sites of types Va, Vb, and Vc in a vacuum. We obtain here minima in the curves with negative ordinates corresponding to theoretical adsorption enthalpies or desorption energies. The coordinates of the minima in the adsorption potentials shown in Figure 4 are given in Table 1. The adsorption energy for ammonia ranges from -100 kJ/mol (on Va) to -33 kJ/mol (on Vb). Likewise, the results corresponding to the interaction of neutral species with Bronsted sites of type I are shown in Figure 5. The potentials are either repulsive or slightly attractive, as for ammonia on Ib. The adsorption energy in that case would be of the order of -10 kJ/mol, much lower than the one reported for the L T state on mica. In order to model the effect of the solvation of ammonia, we made some semiempirical calculations using MNDO in the GEOMOS package, by using the option of taking into account the self-consistent reaction field (SCRF)."J2 Thus, the adsorption potentials were determined for values of the dielectric constant of the outer medium equal to 4 (apolar medium, such as hydrocarbon) and to 80 (highly polar medium, such as water). We give the results of the calculations in Table 2. In the SCRF approach, the adsorption energy of ammonia on the different sites of type V is slightly reduced as E increases up to 80 (simulation of an aqueous environment). However, our conclusions remain unchanged. This solvent effect is of the second order. In order to compare our results with those obtained from an ab-initio method, we performed calculations with DMol, a density functional theory (DFT) program from BIOSY M Technologies I ~ C . These ~ J ~ calculations were performed in a vacuum and limited to sites Va and Vb in view of size limitations. The results are given in Table 3 by comparison with the corresponding values resulting from the semiempirical calculations. The main finding in Table 3 is that MNDO and the DFT calculations indicate similar tendencies. DFT calculations accounting for electron correlation effects and including d orbitals in the basis sets give a better approximation of the variational solution of the Schrodinger equation, and in that case higher binding energies. We believe the discrepancy between MNDO and DMol results stems from the higher level of theory in the latter method. Moreover, DFT results more closely approximate the experimentally observed energies of desorption for H T and LT. The differences between the two methods appear to decrease as the size of the sites increases, but this may be fortuitous. The more reliable ab-initio results indicate that pyridine is the strongest Lewis base, and water the weakest. Our calculations also enable us to estimate the relative stabilities of the different types of surface sites considered (11) Rivail, J. L.,Rinaldi, D., Ruiz-Lopez, M. F. In Theoretical and Computational models for Organic Chemistry; Formosinho, S . J., et al., Eds.; Kluwer: Dordrecht, The Netherlands, 1991; pp 79-92. (12)Rivail, J. L., Rinaldi, D. Chem. Phys. 1976,18,233-242. (13)Delley, B. Chem. Phys. Lett. 1986,110,329; J.Chem. Phys. 1990, 92, 508. (14)Hohenberg, P., Kohn,W. Phys. Rev. E 1964,136 (3),864-871.
1834 Langmuir, Vol. 10, No. 6,1994
Berhouet and Toulhoat
-
I H\ I
I
i
* \O I
r
I.\?
\h
'
'
H
\
o
H
\
o
+
I
1
U
0
I
* '0 ,
I
i
Figure 2.
here, on the basis of the total energy of the isolated sites. Comparisons can only be made for isomeric and isoelectronic sites, i.e., I, 11, and V + OH- (as I1 = V OH-). Actually, the difference between I1 and V + OH-represents the binding energy of OH- to V. The total energies of
+
sites of types I1 and V + OH- minus that of sites of type I are reported in Table 4. As expected, the formation of sites of type V requires the largest amount of energy. This requirement apparently decreases as the size of the model system increases. This
Langmuir, Vol. 10, No. 6, 1994 1835
Nature of Acidic Sites of the Lewis Type Site Ila
0.0
0.1
Site V a in
in E = t
0.2
0.3
t =I
0.5
0.4
Site II b in E = l
,,
I
5004
1 I
-40A
Site V c in E =f 0.0
0.1
0.2 Site II c in
0.3
0.4
0.5
'"1
E =1
0.1
Ib+
0.3
0.4
I
I
Oi5 d (nm)
L ,
-
-
400
:
. I
-20-
2 U y1
: -40*
-60-
d (nm) " I
00
1
I
1
1
01
02
03
04
-1
05
Figure 3. MNDO adsorption potentials for ammonia (dotted line),water (full l i e ) , and pyridine (broken line) on sites of type 11. is understandable as the result of the delocalization of coordinative unsaturation and the positive charge on a larger number of heavy atoms. We can therefore extrapolate that charged surface vacancies such as sites of Type V are more likely to occur (as metastable states) a t the surface of extended crystallites. Our findings show that, although more energetic than sites of type 11, and thus less frequent, sites of type V only would be able to behave as Lewis acids. In summary, our results indicate that the L T state of ammonia adsorbed on mica surfaces and on related
.
.. ....
e
.
Figure 4. JNDO adsorption potentials for ammoc., (dotted line),water (full line),and pyridine (broken line) on sites of type
v.
Table 4. MNDO Total Energy Differences of Isolated Model Sites I1 and V + OH- with Respect to That of Site I ( A h in kJ/mol) size site I1 site V + OHa 7.0 1353.4 b 86.2 1375.9 C 1239.4 79.8
aluminosilicate surfaces is not likely to correspond to the Bronsted acidlbase interaction postulated by Bhattacharyya. Moreover, this author points out that the saturation coverage measured for this L T state a t 103 molecules/cmz is only one-tenth of the coverage expected for site I, taking a 1:l equivalence with surface potassium ions.
Berhouet and Toulhoat
1836 Langmuir, Vol. 10, No. 6, 1994 Site I a in
E =I
d (nml
Site I b in
1
35 25-
E =i
a
1
:i
relatively strong binding of ammonia onto a mica surface. He suggests that the H T state could be interpreted by this type of interaction. We would rather maintain that both L T and HT states reflect electron donation interactions of the ammonia's nitrogen atom lone pair with type V sites. The differences in desorption energy, or rather the existence of a biomodal spectrum of desorption energies, would simply reflect the variety of environments of type V sites available on a cleaved mica surface. In our calculations, the Vc site is more representative for a Si+ surface atom embedded in a 001 plane of a mica sheet. It is a defect less likely to occur a t this location, known from atomic force microscopy (AFM) studies to be molecularly smooth over very large ranges. The Va sites are by contrast better models for sites located at the edges of 001 sheets (edges of crystallites, steps and kinks in the basal faces). Steric constraints reduce the computed MNDO adsorption energies on Vc sites to -30 and -62 kJ/mol for water and ammonia, respectively. According to our interpretation, the T P D spectra reported in ref 1would therefore mean that the LT state corresponds to basal V sites (best modeled by Vc) and the H T state to the more energetic edge V sites (best modeled by Va). The high ratio basal area/edge area expected for cleaved mica would account for the ratio of the LT/HT site coverage, corrected by the ratios of V site average areal densities on both types of crystal faces. This hypothesis might be tested by undertaking ammoniaTPD experiments on micas or related phyllosilicate samples covering a range of known ratio basal area/edge area.
Conclusions
,A Figure 5. MNDO adsorption potentials for ammonia (dotted line), water (fullline),and pyridine (broken line) on sites of type I.
However, our calculations very nicely support Bhattacharyya's hypothesis of the type V site to explain the
We describe the results of theoretical calculations of the adsorption potentials of water, ammonia, and pyridine on model acid sites likely to exist at the surface of mica and related aluminosilicates. These results are compared to experimental data of ammonia TPD on muscovite mica reported by Bhattacharyya. We conclude that the fairly strong desorption energies corresponding to the observed low temperature (LT) and high temperature (HT) adsorbed states are consistent only with acidibase interactions of the Lewis type. The Lewis acid site on the mica is identified with an electron-deficient trivalent silicon atom (site of type V postulated by Bhattacharyya). We propose that the distribution of desorption energies on such sites corresponds to different locations involving more-or-less steric hindrances to the adsorption of Lewis bases. Sites from the basal planes would thus be identified with the LT state, and sites from edge planes and steps or kinks would be identified with the HT state.
