摘要
合成一系列具有不同质量分数Co、Mn的磷铝分子筛CoAPO-5和MnAPO-5;通过X-衍射、扫描电镜、氮气物理吸附、热重分析以及紫外可见漫反射光谱等技术对分子筛的结构、形貌以及Co、Mn原子在分子筛骨架中的存在状态进行了表征;考察了氧气作氧化剂时分子筛在环己烷低温液相氧化反应中的催化性能。结果表明,所合成的CoAPO-5和MnAPO-5具有典型的A lPO4-5分子筛结构,金属M e/P比小于0.1时,分子筛结晶度较高。金属的种类、价态、质量分数及存在形式决定了模板剂与分子筛骨架作用的强度及方式,而有机模板剂的脱附、燃烧温度与这种作用密切相关;部分模板剂需在高温下脱除,说明分子筛骨架与模板剂之间存在着强相互作用。在CoAPO-5和MnAPO-5分子筛的骨架结构中,存在四配位的Co(II)和Mn(II),经过焙烧可以部分氧化为Co(III)和Mn(III),说明在焙烧样中存在着氧化还原活性中心。对于环己烷选择氧化反应,CoAPO-5和MnAPO-5分子筛都具有适中的催化活性,氧化反应产物分布均随反应时间而变化。虽然MnAPO-5的催化活性比CoAPO-5高,但其深度氧化能力也较强;采用MnAPO-5为催化剂,环己烷氧化反应的环己酮选择性及深度氧化产物的量都较高。同时,骨架Co质量分数对环己烷氧化反应活性具有显著影响,Co/P比为0.05的CoAPO-5分子筛催化活性最高,130℃反应24 h,主要目的产物环己醇和环己酮的选择性可达88.5%。
A series of cobalt- or manganese- incorporated aluminophosphates （ CoAPO-5 and MnAPO-5 ） with different metal contents were hydrothermally synthesized. Various methods such as XRD, SEM, nitrogen adsorption, thermogravimetric analysis and UV-Visible diffusion reflectance spectra were used to characterize the structure and crystallinity of the aluminophosphates as well as the chemical state of Co or Mn in the framework of molecular sieves. Moreover, the selective oxidation of cyclohexane was investigated with CoAPO-5 and MnAPO-5 as catalyst. The metal content incorporated in the molecular sieves has a vital influence on the structure, crystallinity and templates removal of CoAPO-5 and MnAPO-5. High crystallinity was obtained with a Me/P （ Me = Co or Mn） mole ratio lower than 0.10, while higher metal content may result in a great deal of extra framework species. The tetrahedral Co^2＋ or Mn^2＋ in the lattice framework of molecular sieves was evidenced, which could be partially oxidized to Co^3＋ or Mn^3＋ to form redox centers upon calcination. The catalytic tests proved that CoAPO-5 and MnAPO-5 are effective catalysts for the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone at low temperature （ 130 ℃ ） using molecular oxygen as oxidant. MnAPO-5 exhibits higher activity for alkane oxidation than CoAPO-5, but gives more products of deep oxidation. The products distribution was also affected by the reaction duration and the contents of the metal ions. CoAPO-5 with a Co/P mole ratio of 0.05 exhibited the best activity and selectivity ; after the reaction at 130 ℃ lasted for 24 h, the selectivity to cyclohexanol and cyclohexanone reached 88.5% with the cyclohexane conversion of 14.7%.
出处
《燃料化学学报》
EI
CAS
CSCD
北大核心
2007年第1期98-103,共6页
Journal of Fuel Chemistry and Technology
基金
国家自然科学基金(20473110
20603044)
国家重点基础研究发展规划(973计划
2006CB202504
2004CB217802)
