The B3LYP optimized structures of adsorption complexes formed upon adsorption of PY and 44BPY on BAS are illustrated in Figure 2
and Figure 3
, respectively, and listed in Table 1
(see also Supplementary Materials
). In the case of the interaction of PY with BAS located either in the open region or in the closed region, our calculation results show that the B3LYP geometry optimization of this system leads to the formation of monodentate ion pair complex PYH+
by transferring the acidic proton to the ligand whatever the Si/Al ratio. The PYH+
pyridinium remains close to the AlO4
tetrahedron, forming a strong hydrogen bond O…H-N with the oxygen atom. In the open region, the hydrogen bound is almost linear (O...H-N = 172°), while, in the closed region, due to the steric constraints, the hydrogen bond elongates a little and moves away from linearity, (O...H-N = 153°). As can be seen in Table 1
, the Si/Al ratio has no effect on the structures of the pyridine adsorption complexes. Indeed, in the case of the Si/Al = 15 ratio, the position of the second Al atom is very far from the cation PYH+
. For the optimized structures of the 44BPY adsorption complexes, the situation is a little different, since the adsorbed molecule occupies the two straight channel regions. In the case of monodentate ion pair complex 44BPYH+
, when pyridinium ring is in one region, the pyridyl ring is in another region of ionic or covalent character depending on the Si/Al ratio of 15 or 31, respectively. Therefore, the effect of the Si/Al ratio could not be negligible on some intermolecular geometrical parameters. In the open region, the monodentate adsorption complexes are formed by a strong hydrogen bond O…H-N, a little longer and much less linear than in the case of PY complexes. On the other hand, the hydrogen bond is more bent in the case of the cluster of Si/Al = 31 (O...H-N = 138°) where the pyridyl ring is in front of SiO4
tetrahedron than in the case of Si/Al = 15 where the pyridyl ring is in front of acidic proton of AlO4
tetrahedron (O...H-N = 148°). In the closed region, the proton transfer between BAS and 44BPY leading to the formation of the ion pair complex does not occur spontaneously. It passes through the formation of the slightly stable hydrogen bound complex, and a very low barrier that can be crossed by the vibrational zero point energy [15
]. In this case, the 44BPYH+
cation within the 10R channel moves away from AlO4
tetrahedron. The optimized O…H-N distance is 2.88 Å. Therefore, it is not complexed to the zeolite framework by a strong hydrogen bond, but is rather considered as a solvated ion in the zeolite cavity. Indeed, the molecule adjusts its position and orientation in order to reduce the steric constraints of the confinement effects of the zeolite framework and thus tends to become parallel to the 10R channel axis (denoted (z) in Figure 1
). It should be noted that for Si/Al = 15, the bidentate 44BPYH22+
ion pair complex could be formed by the transfer of the second proton (with some barrier) from either the open region or the closed region to the 44BPYH+
cation within closed or open monodentate ion pair complex, respectively [29
]. In this bidentate ion pair complex, the position of each pyridinium cation is almost similar to that of the corresponding monodentate ion pair complex [29
]. As a consequence, the 44BPYH22+
dication must bend around the C-C interring axis.
The corrected adsorption energies (Eads
) of PY and 44BPY ligands calculated at the B3LYP and M06-2X-D3 levels are listed in Table 2
. For all adsorption complexes considered here, Eads
values calculated at the B3LYP level are much lower compared to the M062X-D3 values. At the B3LYP level, except for the adsorption of PY in the open region where there is free space available, PY and 44BPY adsorbed on the BAS of the 32T cluster model are not or little bounded. This is not surprising, owing to the lack of dispersive vdW interactions in the B3LYP functional and the importance of the steric effect due to the lack of free space, especially in the closed region. However, B3LYP calculations allow us to evaluate the steric effect. For example, the destabilization due to steric constraints of the adsorbed PY in the closed region can be approximately estimated as the B3LYP interaction energy difference between the closed and open regions (19.5 kcal/mol). At the M06-2X-D3 level, due to the dispersive vdW interactions that are accounted for at this level, the stability of the two ion pair complexes formed in the open and closed regions increases considerably, especially in the closed region where the dispersion energy Edisp
is calculated to be more than twice higher (~25.0 kcal/mol) than that of the open region (~11.9 kcal/mol). The dispersive vdW interactions represent ~34% and ~88% of the total corrected adsorption energies of ~−35.2 and ~−28.4 kcal/mol for PY adsorbed in the open and closed regions, respectively. The adsorption energy difference between both complexes decreases from 19.5 kcal/mol calculated at the B3LYP level to 6.8 kcal/mol estimated by M06-2X-D3 calculations. It should be emphasized that whatever the level of calculation, the Si/Al ratio has no influence neither on the total corrected adsorption energy nor on the dispersion energy of pyridine because the Al sites are spaced far enough apart to have properties of isolated sites.
In the case of the 44BPY adsorption in the straight channel of H-ZSM-5 zeolite, since it is a large bidentate ligand, there is a large molecule/zeolite surface contact area. Obviously, the confinement effect is much more important for 44BPY than for PY. The 44BPY bidentate ligand could thus interact with two Brønsted acid sites associated with two Al atoms quite apart from each other and located in the open and closed regions. Consequently, the Si/Al ratio could have effects on the energetic properties of 44BPY adsorption. For Si/Al = 31, when 44BPY interacts with the BAS, only a monodentate ion pair complex is formed either in the open region, or in the closed region depending on the position of Al site. The stability of 44BPY adsorption complexes are completely due to the dispersive vdW interactions executed by confinement effect of the zeolite framework. As in the case of PY adsorption, the corrected adsorption energy calculated at the M062X-D3 level of 36.8 kcal/mol for 44BPY adsorbed in the open region is larger by 11.3 kcal/mol than that calculated for 44BPY adsorbed in the closed region. This relative stability of 44BPY adsorption complexes is mainly due to the steric effect, which is more important in the case of the closed region.
For Si/Al = 15, the 32T cluster has two BAS located in two different regions. Thus, three types of adsorption complexes are involved in the proton transfer from BAS of zeolite to 44BPY ligand. In addition to the two monodentate ion pair complexes, a bidentate ion pair complex is formed by the transfer of the second proton to one or the other monodentate ion pair complex. The stability of these complexes is also entirely governed by the confinement effect. The adsorption energies of these complexes increase in the following order: (44BPYH+
. The mechanism of the double proton transfer reaction leading to the formation of the bidentate ion pair complex was largely discussed in our previous papers [27
]. In comparison with Si/Al = 31, the corrected adsorption energies are larger, especially for closed monodentate ion pair complex and the relative stability of these complexes is smaller. It does not exceed 5.2 kcal/mol. The high stability of the adsorption complexes for Si/Al = 15 with respect to that for Si/Al = 31 is due to the fact that, in the case of Si/Al = 15, the second pyridyl ring of the monodentate ion pair complexes is not far from the second Brønsted acid site to interact with it.