Laboratoire des Sciences Analytiques, University of Lyon LSA, UMR CNRS 5180, Université Lyon 1, 43 bd 11 Novembre, 69622 Villeurbanne, France.
Langmuir. 2011 Dec 6;27(23):14188-200. doi: 10.1021/la2029438. Epub 2011 Nov 9.
The thermodynamics of a chemical reaction confined at a solid surface was investigated through kinetic measurements of a model unimolecular reaction. The thermal cleavage of ester groups grafted at the surface of solid silica was investigated together with complementary physicochemical characterization of the grafted species. The ester molecules were chemically grafted to the silica surface and subsequently cleaved into the carboxylic acids. A grafting process of a reproducible monolayer was designed using the reaction of monofunctional organosilane from its gas phase. The thermal deprotection step of the ester end-group was investigated. The thermal deprotection reaction behaves in quite a specific manner when it is conducted at a surface in a grafted layer. Different organosilane molecules terminated by methyl, isopropyl and tert-butyl ester groups were grafted to silica surface; such functionalized materials were characterized by elemental analysis, IR and NMR spectroscopy, and thermogravimetric analysis, and the thermodynamic parameters of the thermal elimination reaction at the surface were measured. The limiting factor of such thermal ester cleavage reaction is the thermal stability of grafted ester group according to the temperature order: tert-butyl < i-propyl < methyl. Methyl ester groups were not selectively cleaved by temperature. The thermal deprotection of i-propyl ester groups took place at a temperature close to the thermal degradation of the organofunctional tail of the silane. The low thermolysis temperature of the grafted tert-butyl esters allowed their selective cleavage. There is a definite influence of the surface on the reaction. The enthalpy of activation is lower than in the gas phase because of the polarity of the reaction site. The major contribution is entropic; the negative entropy of activation comes from lateral interactions with the neighbor grafted molecules because of the high grafting density. Such reaction is an original strategy to functionalize the silica surface by carboxylic acid groups by means of a simple, reproducible, and efficient process involving in situ thermolysis of ester groups.
通过对模型单分子反应的动力学测量，研究了在固体表面受限的化学反应的热力学。研究了固载在固体二氧化硅表面的酯基的热裂解，同时对固载物种进行了补充的物理化学特性研究。酯分子通过其气相中的单官能有机硅烷反应化学接枝到二氧化硅表面，随后裂解释放出羧酸。设计了一种使用单官能有机硅烷从气相进行重复单层接枝的接枝过程。研究了酯端基的热保护反应。当在接枝层的表面上进行时，酯的热保护反应以相当特殊的方式进行。不同的末端为甲基、异丙基和叔丁基酯的有机硅烷分子被接枝到二氧化硅表面；通过元素分析、IR 和 NMR 光谱以及热重分析对这些功能化材料进行了表征，并测量了表面上的热消除反应的热力学参数。根据温度顺序，限制这种热酯裂解反应的因素是接枝酯基的热稳定性：叔丁基 < 异丙基 < 甲基。温度不能选择性地裂解甲基酯基。异丙基酯基的热保护反应发生在接近有机官能硅烷尾部热降解的温度下。接枝叔丁基酯的低热解温度允许它们的选择性裂解。表面对反应有一定的影响。由于反应位置的极性，活化焓低于气相中的值。主要贡献是熵；由于高接枝密度，与相邻接枝分子的侧向相互作用导致活化熵为负。这种反应是通过简单、可重复和高效的原位酯基热解过程，用羧酸基团官能化二氧化硅表面的原始策略。
