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磷铝分子筛的合成初期,小分子间发生缩合反应形成二聚体、三聚体、四聚体等聚合物,这一阶段的物质形成时间很短,在实验中难以测得。本文采用量子化学方法对沸石合成初期低聚物的稳定结构进行探索和研究。使用密度泛函(DFT)理论,B3LYP泛函,6-31G(d,p)基组,从56种初始结构中我们得到了4种二聚体,7种三聚体,6种四聚体的稳定结构。根据分子相对自由能,在生成的稳定结构中,链状的水合物与环状化合物更为稳定。这些稳定结构在连续介质水溶剂化模型(PCM)下计算得到的相对自由能表明:在水环境下(Al-P-Al)三元环以及四元环状化合物更为稳定。其中环状四聚体与实验中观察到沸石骨架中广泛存在的四元环结构相吻合。计算得到的结构数据、振动频率和NMR化学位移与AIPO沸石相似。说明这种环状结构可能是形成沸石的关键结构单元,在合成初期的溶液中大量存在。最后我们探讨了在中性条件下缩合反应的机理。
In the early stage of the synthesis of phosphorous-aluminum molecular sieves, condensation reaction occurs between small molecules to form polymers such as dimer, trimer and tetramer. The formation time of this material is very short and difficult to measure in experiments. In this paper, the quantum chemical method of Zeolite synthesis of oligomers in the early stable structure of exploration and research. We have obtained four kinds of dimers, seven kinds of trimers, six kinds of tetramers from 56 kinds of initial structures using density functional theory (DFT), B3LYP functional, 6-31G (d, p) Stable structure. According to the relative free energy of molecules, chain-like hydrates and cyclic compounds are more stable in the resulting stable structure. The calculated relative free energies of these stable structures under successive aqueous solvation (PCM) show that the (Al-P-Al) ternary and quaternary cyclic compounds are more stable under aqueous conditions. Among them, the cyclic tetramer is in agreement with the widely observed four-membered ring structure in the zeolite framework observed experimentally. The calculated structure data, vibrational frequencies and NMR chemical shifts are similar to AIPO zeolites. This ring structure may be the formation of the key zeolite structural units, early in the synthesis of a large number of solutions. Finally, we explored the mechanism of condensation under neutral conditions.