孔道结构化合物(H_3NCH_2CH_2NH_3)_3[(VO)_4(PO_4)_2(HPO_4)_4]的钒氧化态与热稳定性关系研究

热处理脱除有机模板易导致VPO体系孔道结构化合物结构破坏,从而阻碍此类材料的实际应用。以孔道结构化合物(H3NCH2CH2NH3)3(VO)4 (PO4 )2 (HPO4 )4 ](简称V2P3 en)为研究对象,利用热分析(DSC TG)、动态原位高温X 射线衍射(XRD)及多功能X 射线光电子能谱(XPS)等方法,研究了该化合物在不同气氛热处理过程中模板脱除及结构演化规律,重点探讨了钒氧化态的变化对结构稳定性的影响。结果表明,加热过程中随着有机模板的分解脱除,形成较强的还原环境,骨架中的部分钒被还原(V4+→V3+),使原有的配位环境(VⅣO5]三角双锥、VⅣO6]畸变八面体)与钒的氧化态不符而导致结构重组。因此,钒氧化态的变化是影响热稳定性的重要因素之一。

Influences of vanadium oxidation states on the thermal stability of vanadium phosphates with open framework

Open framework compounds of VPO system synthesized with organic templates must be calcined to remove the organic molecules from the channels. This can provides the porosity of the corresponding solids that leads to industrial applications in the fields of catalysis, molecular sieves, ionic exchange and gas separation. However, it is always difficult to extract templates without the collapse of the structures. In order to investigate the factors influencing the thermal stability of these compounds, a vanadium phosphate (H_3NCH_2CH_2NH_3)_3\(VO)_4(PO_4)_2(HPO_4)_4\] (V_2P_3-en for short) with open-framework is characterized by DSC-TG, in situ XRD and XPS. The thermal degradation of V_2P_3-en in the course of heating treatment under Ar and air atmospheres respectively, including the removal of organic templates, phase transition and change of vanadium oxidation state, has been studied systematically. It is shown that the structure of the compound collapses with the decomposition of organic template (en), and the oxidation states of vanadium atoms become variable (i.e. some vanadium atoms are reduced from V~(4+) to V~(3+)). In terms of crystal chemistry, the structure has to be rebuilt because the original coordination environments of V~(4+) (distorted octahedron and trigonal bipyramid) cannot match those for V~(3+) (regular octahedron). Therefore, the rational control of the pretreatment conditions must be taken into account in order to hinder vanadium atoms from being reduced.