Binary caesium–lanthanum oxide supported on MCM-41: A new stable heterogeneous basic catalyst
作者:K. R. Kloetstra、M. van Laren、H. van Bekkum
DOI:10.1039/a606766b
日期:——
Heterogeneous mesoporous stable basic catalysts have been prepared
by wet or solid-state impregnation of MCM-41 with caesium acetate and
lanthanum nitrate followed by thermal decomposition.
133
Cs
MAS NMR data of CsLa/MCM-41 show an increase in the CsâO bond
length in the CsLa mixed oxides compared with that in Cs oxides
supported on MCM-41. A small difference in chemical shift between the
hydrated and dehydrated materials is observed, indicating a weak
interaction of water with Cs
+
cations. CO
2
temperature-programmed desorption (TPD) suggests that these materials
possess small oxide clusters of mild basicity. The thermal stability
of the materials, which is independent of the framework aluminium
content, is reflected by an unaffected CO
2
desorption and
BET surface area on successive thermal cycles. Alkali
metalâlanthanum binary oxides of the Na, K and Rb-type appear to
be thermally less stable, in the sense of a significantly smaller
CO
2
desorption indicative of particle clustering.
23
Na MAS NMR shows several resonances for the
NaLaO
x
materials. The presence of the single
Rb-oxide component in the RbLaO
x
materials is
confirmed by
87
Rb MAS NMR. The mild base strength of the
CsLaO
x
/MCM-41 is demonstrated by its ability to
remove a proton from enolates having a
pK
a
⩽10.2. The presence of lanthanum and the
mesoporous framework govern the activity and product selectivity in
the liquid-phase Michael addition of ethyl cyanoacetate to ethyl
acrylate. The Knoevenagel addition of enolates to benzaldehyde in
aqueous media is also catalysed by the CsLa-oxide/MCM-41 system. The
catalyst is re-usable after regeneration at high temperature without
loss of activity.
通过醋酸铯和硝酸镧湿法或固态浸渍 MCM-41,然后进行热分解,制备出了异质介孔稳定碱性催化剂。
CsLa/MCM-41 的 133 Cs MAS NMR 数据显示,与 MCM-41 上支撑的铯氧化物相比,CsLa 混合氧化物中的 CsâO 键长度有所增加。水合材料和脱水材料之间的化学位移差异很小,这表明水与 Cs + 阳离子之间的相互作用很弱。CO 2 温度编程解吸(TPD)表明,这些材料具有温和碱性的小氧化物簇。这些材料的热稳定性与框架中的铝含量无关,在连续的热循环中,其 CO 2 解吸和 BET 表面积均不受影响。Na、K 和 Rb 型碱金属镧二元氧化物的热稳定性似乎较差,因为它们的 CO 2 解吸附量明显较小,这表明存在颗粒聚集现象。87 Rb MAS NMR 证实了 RbLaO x 材料中存在单一的铷氧化物成分。CsLaO x /MCM-41 能够从 pK a â½10.2 的烯醇中去除一个质子,这证明了它的弱碱强度。在氰乙酸乙酯与丙烯酸乙酯的液相迈克尔加成反应中,镧和介孔框架的存在决定了活性和产物选择性。CsLa-oxide/MCM-41 系统还能在水介质中催化烯醇类与苯甲醛的 Knoevenagel 加成反应。催化剂在高温下再生后可重复使用,且不会失去活性。