Downstream processing of biochemical products strongly depends on the interaction between biomolecules and material surfaces that can be influenced greatly by the pH level. In this study, the influence of the pH value on the adsorption of the amino acids glycine, alanine, and lysine in MFI-type zeolite was investigated using density functional theory (DFT) and microcalorimetric experiments. Different pH values were modeled by varying the amino acids protonation states. The investigated protonation states exhibit two different adsorption motifs in the pores, with the neutral α-C-amino group motifs being significantly less stable than the protonated ones. In the case of neutral glycine and alanine, the adsorption energy is insufficient to overcome the adsorption of the four water molecules usually present in the pores. This result explains the pH-dependent adsorption behavior that has also been observed experimentally and provides an avenue for designing efficient adsorption processes.

Zeolites can adsorb small organic molecules such as alcohols from a fermentation broth. Also in the zeolite-catalyzed conversion of alcohols to biofuels, biochemicals, or gasoline, adsorption is the first step. Several studies have investigated the adsorption of alcohols in different zeolites experimentally, but computational investigations in this field have mostly been restricted to zeolite MFI. In this study, the adsorption of C1-C4 alcohols in BEA and MOR was investigated using density functional theory (DFT). Calculated adsorption geometries and the corresponding energies of the designed cluster models were comparable to periodic calculations, and the adsorption energies were in the same range as the corresponding computational and experimental values reported in the literature for zeolite MFI. Thus, BEA and MOR may be good adsorption materials for alcohols in the field of downstream processing and catalysis. Aside from the DFT calculations, adsorption isotherms were determined experimentally in this study from aqueous solutions. For BEA, the adsorption of significant amounts of alcohol from aqueous solution was observed experimentally. In contrast, MOR was loaded with only a very small amount of alcohol. Although differences were found between the affinities obtained from gas-phase DFT calculations and those observed experimentally in aqueous solution, the computational data presented here represent molecular level information on the geometries and energies of C1-C4 alcohols adsorbed in zeolites BEA and MOR. This knowledge should prove very useful in the design of zeolite materials intended for use in adsorption and catalytic processes, as it allows adsorption behavior to be predicted via judiciously designed computational models.

Experimentally, zeolites were shown to adsorb amino acids from the aqueous phase. However, there has not been a satisfactory theoretical explanation on the effect of water on the underlying adsorption mechanisms yet. In this study, the effect of water on the pH-dependent adsorption behavior of glycine and alanine in zeolite BEA was investigated using density functional theory (DFT). Using a microsolvation model, the coadsorption and formation of an intermolecular H-bond system between water molecules, amino acid, and zeolite was shown. In addition, different pH values were modeled by varying the amino acids protonation states with the protonated states being significantly more stable in the BEA pores than the net neutral ones. This behavior was experimentally approved by isothermal titration calorimetry (ITC) and adsorption isotherm measurements. These results provide new molecular level information on the adsorption of amino acids in zeolites from the aqueous phase and could be used to support the experimental side of adsorption process design in industrial biotechnology by qualitatively predicting binding behavior by means of DFT calculations and simplified model systems.