Predicting the Formation of Sulfur-Based Brønsted Acids from the Reactions of SO_x with H₂O and H₂S

Abstract

We have performed an extensive computational investigation of the potential energy surfaces for the reactions of SO x (x = 2 or 3) with H 2 S and H 2 O in the gas phase and in aqueous solution at the CCSD(T)/CBS level of theory plus a self-consistent reaction field approach. Formation of a gas-phase H 2 SO 4 from the hydrolysis of SO 3 at lower temperatures requires the presence of additional water molecules. When additional waters are introduced, the barrier for H 2 SO 4 formation is significantly reduced, and a barrierless transition occurs with only three excess waters as well as in aqueous solution. In a mixture of H 2 O, H 2 S, and SO 3 , formation of H 2 S 2 O 3 has a lower barrier in the gas phase than does the formation of H 2 SO 4 . In aqueous solution, no barrier is predicted, and both are likely to be formed. Introduction of an additional water to the SO 3 + H 2 S reaction results in a decrease in barrier height, nearly identical to the ∼18 kcal/mol catalytic effect of the first additional water in SO 3 hydrolysis, with the barrier for H 2 S 2 O 3 formation disappearing altogether in aqueous solution. Formation of H 2 SO 3 by the way of SO 2 hydrolysis is unlikely. Excess waters reduce the barrier for SO 2 hydrolysis; however, the overall endothermicity is increased as waters are added. The formation of H 2 S 2 O 2 from SO 2 and H 2 S via an isostructural pathway to SO 2 hydrolysis is unlikely, with additional water molecules resulting in a small increase in the overall endothermicity and a catalytic effect smaller than that observed for the SO 3 reactions. The results of this work have implications pertaining to the formation of H 2 SO 4 , H 2 S 2 O 3 , H 2 SO 3 , and H 2 S 2 O 2 in the atmospheres of Earth and Venus. These results also question the existence of H 2 S 2 O 2 as an intermediate in the Claus process.

Preferential SOx Adsorption in Mg-MOF-74 from a Humid Acid Gas Stream

“…Both the breakdown of the MOF upon heating and the molecular ion fragments that are subsequently being released were analyzed. TGA–MS data were collected for each sample immediately after removal from the adsorption chamber after 24 h. Each TGA–MS plot shows the loss of molecular ions corresponding to the adsorbed SO 2 gas and chemically/thermally generated gaseous H 2 S species at temperatures between 100–300 and 100–400 °C, respectively (Figure S4a–d for days 1–4). It is well known that from the molecular formula [M 2 (C 8 H 2 O 6 ) (H 2 O) 2 ]·8H 2 O, the as-made Mg-MOF-74 is ∼30% water by weight.…”

“…Both the breakdown of the MOF upon heating and the molecular ion fragments that are subsequently being released were analyzed. TGA−MS data were collected for each sample immediately after removal from the adsorption chamber after 24 h. Each TGA−MS plot shows the loss of molecular ions corresponding to the adsorbed SO 2 gas 56 and chemically/ thermally generated gaseous H 2 S species 58 at temperatures between 100−300 and 100−400 °C, respectively (Figure S4a To analyze the sulfur−oxygen species bound to the magnesium cation, infrared spectroscopy was used as soon as possible after the sample was removed from the adsorption chamber (within 30 min). The Fourier transform infrared spectroscopy (FTIR) spectrum shows clear changes in the peaks upon adsorption of SO x into the framework pore and to the metal center (Figure 2; each new peak highlighted by yellow bubble).…”

Predicting the Mechanism and Products of CO2 Capture by Amines in the Presence of H2O

“…As the lack of explicit solvent molecules might yield significant errors, we also calculate pK a values using proton transfer reactions between the acid studied and a known pK a reference according to Reactions 11 and 12, following previous work by our group, 55,58,97,98 Takano and Houk, 99 and Pliego and Riveros, 100 and they are also listed in the SI.…”

“…These electronic structure level and basis set combinations were selected on the basis of our prior work for similar hydration reactions, which showed good results. 16,54,55 The aug-cc-pVNZ basis sets will be referred to as aN from this point forward. Vibrational frequencies were also calculated at these same levels of theory to verify the nature of each stationary reaction point (reactant/product or transition state) and to provide the zero-point vibrational energy (ZPE) and thermodynamic contributions to the standard enthalpy (H 298K ) and free energy (G 298K ).…”

“…The COSMO SCRF approach was chosen as it has been previously been used successfully by us for the hydration reactions of other Lewis acids. 16,55 The solvation free energies were then calculated according to eq 8…”