Blitz Jonathan P, Diebel Richard E, Deakyne Carol A, Christensen Jeannine M, Gun'ko Vladimir M
Department of Chemistry, Eastern Illinois University, Charleston, IL 61920, USA.
J Phys Chem B. 2005 Mar 31;109(12):5667-77. doi: 10.1021/jp040483j.
Reactions of trimethylaluminum, triethylaluminum, and diethylaluminum chloride and ethylaluminum dichloride with silica gel have been studied experimentally by infrared spectroscopy and elemental analysis. The silica gel was subjected to different pretreatments to alter surface functionalities prior to reaction. In all cases the extent of surface modification reaction follows the trend unmodified > 600 degrees C pretreated > hexamethyldisilazane (HMDZ) pretreated > 600 degrees C/HMDZ pretreated. All of the aluminum compounds studied completely react non-hydrogen-bonded silanols, while also reacting with hydrogen-bonded silanols and siloxanes. Primarily monomeric surface species result from the surface modification reaction. Ethylaluminum chlorides preferentially react with silanols through cleavage of the Al-C bond rather than the Al-Cl bond. Singly bonded Si(s)-O-AlCl(2) surface species are readily synthesized by reaction of ethylaluminum dichloride with HMDZ-pretreated silica gel. Bridged bonded (Si(s)-O)(2)-AlCl surface species are readily synthesized by reaction of diethylaluminum chloride with HMDZ-pretreated silica gel. Computational ab initio studies of the cluster Si(4)O(6)(OH)(4) as a model to study the reaction of monomeric and dimeric methylaluminum dichloride with a silica silanol are also described. Comparison of the potential energy surface (PES) of monomer and dimer indicates that the energetics favor monomer reaction, consistent with experimental results. The energy cost in the dimer reaction is primarily from cleavage of a bridged Al-Cl bond upon adsorption. This does not occur when the monomer adsorbs. A comparison of the PES for the two reaction pathways resulting from cleavage of either an Al-Cl or Al-C bond indicates that while the former reaction is slightly kinetically favored (E(a) = 23.1 kJ/mol for Al-Cl bond cleavage versus E(a) = 31.1 kJ/mol for Al-C bond cleavage), the latter is strongly thermodynamically favored with an overall free energy difference between the two reaction pathways of 135 kJ/mol favorable to Al-C bond cleavage. These reactions are thermodynamically controlled.
通过红外光谱和元素分析对三甲基铝、三乙基铝、二乙基氯化铝和二氯化乙基铝与硅胶的反应进行了实验研究。在反应前，对硅胶进行了不同的预处理以改变其表面官能团。在所有情况下，表面改性反应的程度遵循未改性>600℃预处理>六甲基二硅氮烷（HMDZ）预处理>600℃/HMDZ预处理的趋势。所研究的所有铝化合物都能完全与非氢键合的硅醇反应，同时也能与氢键合的硅醇和硅氧烷反应。表面改性反应主要生成单体表面物种。氯化乙基铝优先通过Al-C键的断裂而非Al-Cl键与硅醇反应。通过二氯化乙基铝与HMDZ预处理的硅胶反应可轻松合成单键连接的Si(s)-O-AlCl(2)表面物种。通过二乙基氯化铝与HMDZ预处理的硅胶反应可轻松合成桥键连接的(Si(s)-O)(2)-AlCl表面物种。还描述了对簇Si(4)O(6)(OH)(4)进行的从头算计算研究，以作为研究单体和二聚体二氯甲基铝与硅胶硅醇反应的模型。单体和二聚体势能面（PES）的比较表明，能量学上有利于单体反应，这与实验结果一致。二聚体反应中的能量消耗主要来自吸附时桥连Al-Cl键的断裂。单体吸附时不会发生这种情况。对由Al-Cl或Al-C键断裂产生的两种反应途径的PES比较表明，虽然前一种反应在动力学上略有优势（Al-Cl键断裂的活化能E(a)=23.1kJ/mol，而Al-C键断裂的活化能E(a)=31.1kJ/mol），但后一种反应在热力学上强烈有利，两种反应途径之间的总自由能差为135kJ/mol，有利于Al-C键断裂。这些反应是由热力学控制的。
