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Secondary building units and composite building units (S/CBUs) are recognized in zeolite frameworks and built up by tetrahedron TO4 units. They were not only identified in the precursor gel for zeolite synthesis but also proposed to be of great significance in interzeolite conversion. Poly-silicic acids of S/CBU structures (S/CBU PAs) are precursors and structural analogues of S/CBUs in zeolites. We performed an exhaustive density functional theory (DFT)-based investigation on thermostability and reaction thermodynamics along potential pathways for the formation and evolution of PAs containing 1–7 T atoms. We showed that most intermolecular condensations (IECs) are exergonic and IECs are among the major mechanisms for the growth and evolution of PAs. The exergonicity of an intramolecular condensation (IAC) depends strongly on the degree of condensation of the reactant PA and it can be even endergonic if the TO4 tetrahedrons are distorted by the T–O–T connection formed. Although S/CBU PAs can only be formed on specific pathways, it is general that the exergonicity for their formation is always superior over that for their non-S/CBU counterparts originated from the same PA. Evolution of S/CBU PAs to non-S/CBU PAs would be less exergonic or even endergonic as compared with the competing formation of S/CBU PA. These data suggest the superior thermostability of S/CBU PAs over non-S/CBU PAs formed by competing IECs and IACs. We expect these findings would help to understand the hydrothermal, solvothermal, and interzeolite conversion processes for the synthesis of zeolites.

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