J. Phys. Chem. 1994, 98, 10032-10035
10032
Cluster Quantum Chemical Study of the Interaction of Dimethyl Methylphosphonate with Magnesium Oxide N. U. Zhanpeisov,* G. M. Zhidomirov,' and I. V. Yudanov Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
K. J. Klabunde Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-3701 Received: November 23, 1993; In Final Form: July 11, 1994@
In the framework of a supermolecular approach using the MIND0/3 method the various channels of dimethyl methylphosphonate (DMMP) adsorption on a magnesium oxide surface are considered. On the basis of calculational results the possible mechanism of destructive adsorption of DMMP on MgO is discussed.
Introduction
'-
Organic derivatives of phosphorus-containing mineral acids (P compounds) are mainly toxic and harmful; some of these are used as chemical poisonous agents.' They are secondary products in the production of phosphorus acid, in remaking of phosphates, in fine organic synthesis, etc. Many metal oxides are known to be a good adsorbents and catalysts in decomposition of P compound^.^-^ Among them alkaline-earth metal oxides, in particular MgO, have been extensively studied by modern experimental methods. Thus, using various representatives of P compounds, namely, dimethyl methylphosphonate (DMMP), triethyl phosphate (TEP), trimethyl phosphate (TMP), and MgO samples having a wide range of surface areas, Klabunde et a l . ' ~ ~showed -~ that (i) these compounds adsorb quite strongly on a MgO surface and (ii) their decomposition on the MgO surface proceeds by a stoichiometric decomposition rather than as a catalytic process. For example, two surface MgO moieties (Le., two surface Mg2+ ions) can decompose about one DMMP molecule, and this finding remains true for MgO samples with different surface areas.7 The volatile products formed as a result of DMMP decomposition are formic acid (major product) and methanol. The release of any phosphorus-containing products did not occur showing that phosphorus remains immobilized on the MgO surface. Moreover, these author^^-^ did not observe any differences in the DMMP decomposition rate when contact time was varied. On the basis of these observations they concluded that not only are surface defect sites responsible for DMMP decomposition, but Mg2+ and 02-ions in the regular oxide surface also take part in this process. After analyzing the reaction products (volatile and nonvolatile), they proposed a possible mechanism of DMMP decomposition on MgO, the key concept of which is initial bidentate coordination of residues of a DMMP molecule on two surface Mg2+ ions. In this paper we attempt a theoretical description of these experimental observations. This paper deals with quantum chemical cluster calculations on DMMP molecule adsorption on MgO by the MIND013 method. On the basis of these calculations, two channels of DMMP adsorption and a possible mechanism of destructive decomposition of DMMP are discussed. @
Abstract published in Advance ACS Abstracts, September 1, 1994.
0022-365419412098-10032$04.50/0
/ /
0
0-'
- 0
/ / I
/ /
0
Figure 1. Two-layer molecular cluster of Mg24024. Numerals correspond to coordination number of centers considered: (.) Mg&+; (0) OLCZ-.
Calculation Method and Surface Model Cluster quantum chemical calculations have been carried out within the framework of the MIND013 method, with parameterization extended in order to consider the geometry and energetics of various chemisorption structures on a dehydroxylated magnesium oxide surface.1°-12 The (001) plane of a regular magnesium oxide surface was modeled by a two layer molecular cluster of Mg24024, where each layer contains 12 atoms of oxygen and magnesium (Figure 1). The geometrical characteristics of this cluster were fixed at the values corresponding to the experimental bulk structure of a MgO crystal. Namely, Mg-0 bond lengths were set equal to 2.1 A, and 0-Mg-0 bond angles were set at 90" and 180". As is normal, a full optimization of geometry of this cluster leads to other structures, especially for edge and comer atoms.I0 Nevertheless, this has no great effect on the results of the investigation of DMMP molecular interaction with a clean (001) MgO surface (only a small difference in the optimized versus the constrained geometry was observed).1° For all results presented here, the internal coordinates of the adsorbate molecules were fully optimized, whereas only the active site coordinates of the MgO cluster were allowed to relax. Results and Discussion As mentioned above, various pathways of DMMP molecular interaction with a MgO surface were considered. These were both molecular adsorption (one- or two-center form of DMMP adsorption on Lewis acid centers - five-coordinated MgSc2+ cations) and dissociative DMMP adsorption via breaking of a 0-C bond. For dissociative chemisorption a methyl group is formed and is connected to a five-coordinated 052- anion, while the phosphorus residue fragment of the dissociated DMMP
0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 40, 1994 10033
Interaction of DMMP with Magnesium Oxide TABLE 1: Exothermicities for Formation of Various Surface Structures upon Interaction of a DMMP Molecule with a Dehydroxylated Magnesium Oxide Surface (Modeled by a Nonoptimized MUON Cluster, Using a MIND0/3 Calculational Method)
FH3
54.4 -58.6
Dissociative Adsorption Mg5?+, Mg5?+, and 052Mg5c2+,0 5 c 2 - , and 0 5 c 2 -
197 155b
CH3
I
o
P
I
0
'0
Molecular Adsorption Mg5?+ Mg5?+ and Mg5?+
-
CH3
H
6E," W/mol
type of adsorption center
SCHEME 1
'0
I
I
I
-Mg-0-W-* Mg////////////w1//77////////m
CH3
\o
I
-Mg-o-L4g-o-Mg-
a Sign (+) corresponds to stabilization upon chemisorption. This value corresponds to dissociative adsorption of DMh4P residue without oxygen--(CH,O)zPCH3 (see text).
CH3
l
CH3
o\
H
.H\y
H
O/CH3
I
1
0
/
/ p \
-0-Mg-0-Mg-0-Mg-
0
I
I//n/"/////// 114.PL\
~
H
,
SCHEME 2
CH3
H
O
I - Mg I -0 -
-0-Mg -0
+ DMMP
/"//////// 1.J
@)
Figure 2. Molecular (a) and dissociative (b) forms of DMMP adsorption on a regular (001)plane of magnesium oxide. Bond lengths are in angstroms, bond angles in degrees. Under adsorption in both forms it was observed that the adsorption complex was distorted from its optimal position in a noninteracted cluster by a particle of DMMP or by its fragments.
molecule is connected to two surface Mg5c2+cations. Table 1 shows the energy (6E) of different forms of DMMP adsorption on a MgO surface. The energy of dissociative chemisorption is calculated as the difference in energy between the adsorption complex and the sum of the noninteracted initial cluster and the DMMP molecule, Le., 6E = E,(AC) - [E,(DMMP) Et(C)], where AC and C stand for adsorption complex and cluster, respectively. Figure 2 shows the most essential geometric characteristics of the two representatives of both molecular and dissociative forms of adsorption of DMMP. First of all it is clear that one-center molecular adsorption is more favorable by energetics than two-center nondissociative molecular adsorption. In the former case DMMP adsorption occurs via donor-acceptor interaction between a lone electron pair of oxygen in the -P=O group and a vacant orbital of the magnesium cation. This corresponds to transfer of electron density from the DMMP molecule to MgSc2+. For example, the effective charge transfer is equal to 0.191e- for one-center DMMP adsorption on Mg5c2+ and leads to a noticeable polarization of the adsorbed DMMP molecule. But as seen from the data in the Table 1, this channel of molecular adsorption is less favorable than dissociative adsorption of DMMP. Thus, for example, according to our calculations, dissociative adsorption of DMMP on a regular (001) surface taking into account five coordinated Mgsc2+,Mg5c2+,and 0 5 c 2 - acid-base centers occurred considerably more favorably compared with molecular adsorption. These results could be used as an essential argument in favor of the experimental data ~ b s e r v e d . ~ -Namely, ~ dissociative adsorption of DMMP takes place on the entire MgO surface. The latter would be limited only by an ease of access
H
+
for fragments of dissociated DMMP molecules on a dehydroxylated magnesium oxide. On the basis of these calculations, we will now discuss the reaction mechanism of DMMP decomposition on the MgO surface. In ref 7 the following mechanism was proposed. At first, molecular adsorption of DMMP takes place. Increasing the interaction temperature leads to dissociative adsorption, but to form one of the volatile products of the destructive decomposition-a methanol molecule-it assumed the strict presence of a surface hydroxy group (Scheme 1). Once formed, this methanol molecule can dissociatively adsorb on the surface. Subsequently, a OCH3 fragment can be oxidized by an incoming DMMP molecule to form formic acid (a volatile product) and deoxygenated DMMP, (CH30)2PCH3 (Scheme 2). From this process, via dissociative adsorption are formed the main nonvolatile product and surface methoxy groups (Scheme 3), and the cycle is repeated for the next DMMP molecule. These authors also noted' that the presence a small amounts of water in the initial DMMP might be beneficial for the decomposition of DMMP.
-
10034 J. Phys. Chem., Vol. 98, No. 40, 1994
Zhanpeisov et al.
SCHEME 3
- 0 - Mg - Mg-
-0-Mg
+
(CH30)2PCH3
CH3
(C)
Figure 3. Molecular clusters of (a) Mg909, (b) Mg12012, and (c) Mg32032. Numerals correspond to coordination number of various active centers. (.) MgL& (0)O&.
SCHEME 4 CH3
I
+
- 0 - Mg -0-
-b-thg
H20
Mg-
17/"//7/N///// (1)
1
CH3
I
,
0, H
0 '
I Mg 1 -0 -*
81.6
51.0 18.1
-10.0
CH3
p-.
0
TABLE 2: Exothermicities of Various Surface Structures upon Interaction of a DMMP Molecule with a Dehydroxylated Magnesium Oxide Surface (Modeled by Optimized Mg32032 Cluster, Using a MIND013 Calculational Method) type of adsorption center dE," kJlmol Molecular Adsorption Mg3cZ+ 135
+ CH30H
I
-Mg-O-Mg-
According to our hypothesis decomposition of DMMP on the MgO surface proceeds as follows. As in ref 7, an initial adsorption state corresponds to a molecular form of DMMP adsorption. Then, if the temperature is high enough, dissociation of the DMMP molecule takes place. But, in contrast to ref 7, the presence of a surface hydroxy group is not necessary for methanol molecule formation. Instead, methanol could be formed via reaction of a surface chemisorption complex (I) with a water molecule (Scheme 4). The subsequent gain in energy, according to our calculations, is equal to 40 W/mol. Once formed, CH30H may undergo oxidation to from formic acid (Scheme 2 ) which dominates in the mixture of volatile products, while the (CH30)2PCH3 residue could be strongly dissociatively adsorbed on the MgO surface (Scheme 3). Table 1 indicates that dissociative adsorption of (CH30)2PCH3 proceeds with a gain in energy of 155 W/mol when 05c2-, Ok2-, and Mg5c2+ acid-base type of centers are considered. It should be noted that our hypothesis on decomposition of DMMP on the MgO surface is entirely based on thermochemistry of the processes considered. As is seen in this work we could not perform any calculations on possible transition states. Of course, such calculations could give additional insight regarding the most favorable reaction channel. Unfortunately, such calculations are very expensive due to the large cluster sizes needed and will have to await a later time.
Dissociative Adsorption MgcZ+,Mg5?+, and 032Mg4c2+,Mgs?', and 06'a See footnote of Table 1.
269 141
Appendix Chemical activity of MgO in catalytic reactions is often correlated with surface defects such as low coordinated (LC) magnesium and oxygen ions (MgLC2+ and OLC~-)of various surface irregularities-faces, edges, comers, etc. In the process under consideration these centers should also be the most active. For an illustration of this statement the corresponding calculations have been performed within the framework of MIND0/3 method. A magnesium oxide surface was modeled by the clusters of Mg909, Mg12012, and Mg32032 (Figure 3), containing all the types of low coordinated magnesium and oxygen ions. As in refs 10-12, a full optimization of geometry of both initial clusters and the DMMP molecule and various chemisorption complexes was carried out. Mg909 and Mg12012 clusters have been used to search all possible forms of DMMP adsorption on magnesium oxide, and a relatively large Mg32032 cluster has been used to define more precisely the most preferable forms of adsorption. Table 2 shows corresponding energies of DMMP adsorption on magnesium oxide calculated using the big32032 cluster. As expected, as the coordination number of the active surface cations and anions increases, the corresponding adsorption energy decreases. The most energetically favorable molecular adsorption occurs as a one-center event on a threecoordinated Mg3c2+site. It should also be noted that two-center molecular adsorption is less favorable compared with one-center adsorption. Taking into account the lower concentration of three coordinated centers than four and five coordinated, we can conclude that one center molecular adsorption will be favorable for Mg4c2f ions on edges and steps of a magnesium oxide
J. Phys. Chem., Vol. 98, No. 40, 1994 10035
Interaction of DMMP with Magnesium Oxide surface. But, as seen from the data in Table 2, this channel of molecular adsorption turns out to be less profitable than dissociative adsorption. Thus, for example, a dissociative DMMP adsorption on Mgc2+, MgSc2+, and 0c2-acid-base type of centers proceeds with essential gain in energy compared with one-center molecular adsorption on Mgk2+. Moreover, if a 032- basic center is complexed to the dissociated CH3 fragment of a DMMP molecule, then this gain in energy will be still higher (see Table 2). Acknowledgment. K.J.K. gratefully acknowledges support for destructive adsorption chemistry investigations from the Army Research Office. References and Notes (1) Ekerdt, J.; Klabunde, K. J.; Shapley, J. R.; White, J. M.; Yates, J. T. J. Phys. Chem. 1988, 92, 6182. (2) Baier, R. W.; Weller, S. W. Ind. Eng. Chem. Process Des. Dev. 1967, 6, 380. (3) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. SOC. 1985,107, 774.
(4) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. Soc. 1985,107, 91. ( 5 ) Kuiper, A. E. T.; van Bokhoven, J. J. G. M.; Medema, J. J. Card. 1976, 43, 154.
(6) Atteya, M.; Klabunde, K. J. Chem. Mater. 1991, 3, 182. (7) (a) Li, Y.-X.; Klabunde, K. J. Langmuir 1991, 7, 138. (b) In Figure 2 it should be noted that four oxygen moieties are necessary, three from DMMP and one from the surface. Thus, it is difficult to ascertain if OCH3 is lost and a surface oxygen serves as a bridge, or if CH3 is lost and a surface oxygen binds to the CH3. Perhaps this is actually irrelevent since oxygen scrambling is probably rapid (see: Li, Y.-X.; Klabunde, K. J. Chem. Mater. 1992, 4, 611). (8) Li, Y.-X.; Schlup, J. R.; Klabunde, K. J. Lungmuir 1991, 7, 1394. (9) Li, Y.-X.; Koper, 0.;Atteya, M.; Klabunde, K. J. Chem. Mater. 1992, 4, 323. (10) (a) Zhanpeisov, N. U.; Pelmenshikov, A. G.; Zhidomirov, G. M. Kiner. Karal. 1990,31,570 (translated by Plenum). (b) Actually experimental work demonstrates that the surface of MgO undergoes little surface relaxation compared with the bulk. Welton-Look, M. R.; Bemdt, W. J. Phys. C. 1982, IS, 5691. Urano, T.; Kanaji, T.; Kaburagi, M. Surf: Sci. 1983, 134, 109. Maksym, P. A. Surf: Sci. 1985, 149, 157. (11) Zhidomirov, G. M.; Zhanpeisov, N. U. C a r d Today 1992,13,5 17. (12) Zhanpeisov, N. U.; Zhidomirov, G. M. Mendeleev Commun. 1992, 111.
