气固相法制备的Ti-ZSM-5的表征及其催化氧化性能——母体性质的影响

CHARACTERIZATION AND CATALYTIC PROPERTIES OF Ti-ZSM-5 PREPARED BY GAS-SOLID METHOD——EFFECT OF THE PRECURSORS

分别以B ZSM 5和Al ZSM 5沸石分子筛为母体,预处理后,在高温下与TiCl4进行气 固相反应合成钛硅沸石Ti ZSM 5。以XRD、FT IR、UV Vis、XRF、ICP AES、SEM等手段对催化剂进行了表征,并考察了母体性质对钛进入骨架及其存在形式的影响以及各样品的丙烯环氧化和苯酚羟基化性能。结果表明,母体本身的物化性能直接决定了所合成的Ti ZSM 5分子筛中钛的含量及其存在形式。以脱硼ZSM 5分子筛为母体,所合成的Ti ZSM 5中骨架钛含量高,非骨架钛含量低,催化性能最好。丙烯环氧化反应中,少量锐钛矿型非骨架钛的存在不会对过氧化氢的有效利用率产生太大影响,但当非骨架钛占据多数且为金红石型TiO2时,会使得过氧化氢的有效利用率急剧降低,同时由于反应活性中心少且被覆盖,催化活性降低;苯酚羟基化反应中,骨架钛含量的差异与催化剂晶粒的大小同时起着重要的作用,强酸中心的存在降低了样品的苯酚羟基化活性。

Ti-ZSM-5 samples were successfully synthesized by using B-ZSM-5 and Al-ZSM-5 zeolites respectively as the precursors. The two step gas-solid synthesis was conducted by firstly pretreating the precusors in HCl solution, then the pretreated precursors were titanated with gaseous TiCl_4. The effect of the precursors on the incorporation of titanium into the framework was investigated. The synthesized titanium containing zeolites were characterized by X-ray diffraction (XRD), Fourier-transformed infrared (FT-IR), X-ray fluorescence (XRF), UV-Vis spectroscopy, inductive coupled plasma-atomic emission spectroscopy (ICP-AES), scanning electron microscopy (SEM), and evaluated with propylene epoxidation as well as phenol hydroxylation. The physicochemical properties, specially the n(SiO_2)/n(Al_2O_3) ratio, siginificantly affected the incorporation of titanium into the zeolite framework. More hetero atoms removed from the precursor, more titanium incorporated into the framework was observed. Using B-ZSM-5 as the precursor, the synthesized Ti-containing samples possessed much more framework titanium and less extra framework titanium. For the reaction of propylene epoxidation, a little amount of anatase did not much affect the utlization of H_2O_2. However, a little amount of framework titanium presented as rutile decreased the utlization and the conversion of H_2O_2 greatly. For the hydroxylation of phenol, the amount of framework titanium together with the crystal size played an important role in the conversion of phenol. Phenol protonation caused by strong acid sites hindered the electrophilic reaction of phenol hydroxylation.