Understanding the Nature of Water Bound to Solid Acid Surfaces. Ab Initio Simulation on HSAPO-34

Abstract

Ab initio simulations of water adsorbed in the microporous silicoaluminum phosphate HSAPO-34 are reported. A neutron diffraction study by L. Smith et al., Science 1996, 271, 799, showed that both H2O and H3O+ are present when this solid acid is loaded with water. The ab initio simulations support this; however, new details are revealed which are not accessible by diffraction experiments averaging over all sites. The acid−base reaction requires at least three water molecules per two nearby acidic sites in the micropores of the solid and leads to a protonated water cluster, H3O+(H2O)2.

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…For water, which has a lower proton affinity than methanol (697 kJ/mol vs 761 kJ/mol [77] ), higher loadings are required to obtain similar levels of proton transfer. With sufficient amounts of water, hydronium ions can be stable species, which is in correspondence with the findings of Smith et al en Termath et al [54,55] Figure 4 showed a high probability for framework deprotonation for H-SAPO-34 loaded with 5 methanol or 8 water molecules per acid site. Although these loadings might seem relatively high, the adsorbed guest molecules are still very mobile under the simulated conditions (Supporting Information section S3).…”

“…For water, which has a lower proton affinity than methanol (697 kJ/mol vs 761 kJ/mol [77] ), higher loadings are required to obtain similar levels of proton transfer. With sufficient amounts of water, hydronium ions can be stable species, which is in correspondence with the findings of Smith et al en Termath et al [54,55] (Supporting Information section S3). We selected these two most extreme situations for further analysis of proton motion as during these simulations the best sampling of proton transfer reactions was achieved.…”

“…[50][51][52][53][54][55][56][57][58][59][60][61] Water and methanol are two crucial molecules during the MTO process. In some cases water is added to the methanol feed in the form of process condensate or steam to tune the product selectivity.…”

“…Whether methanol and water forms ion pairs upon adsorption in H-SAPO-34 is a point of discussion in literature and seems to depend on the applied conditions. [50,54,55,58] Furthermore, extensive work was performed by Krishna and co-workers on the effect of hydrogen bonding on adsorption of water-alcohol mixtures in zeolites and the consequences for the diffusivities. [76] To obtain deeper insights into the mobility of protons in the zeolitic system under study, a methanol or water loadings, the probability for framework deprotonation significantly increases.…”

“…Whether methanol and water forms ion pairs upon adsorption in H-SAPO-34 is a point of discussion in literature and seems to depend on the applied conditions. [50,54,55,58] Furthermore, extensive work was performed by Krishna and co-workers on the effect of hydrogen bonding on adsorption of water-alcohol mixtures in zeolites and the consequences for the diffusivities.[76]To obtain deeper insights into the mobility of protons in the zeolitic system under study, a profound analysis of the performed MD simulations with varying loadings of methanol or water in H-SAPO-34 was performed. The positions of the protons and all hydrogen atoms in the system were traced during the 50 ps NPT simulations of H-SAPO-34, loaded with different amounts of methanol (1,3,4 or 5 molecules per acid site) or water (1, 4, 5, 6, 7 or 8 11 molecules per acid site), to detect proton transfer.…”