Heterogeneously catalyzed disproportionation

Keywords Disilanes / Lewis Base / Lewis Acid / Heterogeneously Catalyzed Disproportionation / Polysilane... 

The heterogeneously catalyzed disproportionation of 1,1,2,2-tetrachlorodimethyldisilane leads via oligo(chloromethylsilane)s to highly branched poly(chloromethylsilane)s. The disilane derived oligomer formation can be controlled by the nature of the Lewis base catalyst. 

Summary Recently we investigated the effectiveness of various Lewis base catalysts in the heterogeneously catalyzed disproportionation of 1,1,2,2-tetrachlorodimethyl-disilane. The preparation and properties of these catalysts are discussed in view of their significant catalytic parameters. 

Table 1. Results of the heterogeneously catalyzed disproportionation of MeChSi-SiChMe and properties of the catalyst surfaces.

Table 2. Results of the heterogeneously catalyzed disproportionation of MeCl2Si-SiCl2Me and properties of the catalyst surface Si-NMR (EtO)3Si(CH2)m-imidazole m = 8 = -57.7 ppm m = 2 6 = -49.1 ppm (CDCI3/TMS).

Hofmann and coworkers (327-330) have reported a series of studies on the deactivation kinetics for the heterogeneously catalyzed disproportionation of ethyl benzene to benzene and diethyl benzene under SCF conditions. Kinetic studies have been conducted in both a loop reactor using a protonated Y-faujasite (Z-14) catalyst (327) and in a continuous concentration-controlled recycle reactor using an HY-zeolite (HYZ) (329,330) and USY-zeolite, H-ZSM-5, and H-mordenite (328) under supercritical conditions T > 373 C, P > A5 bar). Coke extraction by SCFs was found to be strongly dependent on the type of catalyst used, and the Lewis acid centers were determined to play an important role in the coke formation and activity of the catalysts. A simple kinetic model for the catalyst deactivation was proposed (329) for SCF conditions and high ethyl benzene concentration. Based on the relatively high estimated deactivation energy of about 147 kJ/mol and first-order deactivation, the authors concluded that the catalyst deactivates much slower under SCF conditions than under atmospheric pressure. 

Instead ef the name metathesis, the term disproportionation is frequently applied to the reaction, and sometimes the term dismutation. For historical reasons the name disproportionation is most commonly used for the heterogeneously catalyzed reaction, while the homogeneously catalyzed reaction is usually designated as metathesis. The name disproportionation is correct in the case of the conversion of acyclic alkenes according to Eq. (1) however, this name is inadequate in most other situations, such as the reaction between two different alkenes, and reactions involving cycloalkenes. Similar objections apply to the name dismutation. The name metathesis is not subject to these limitations and, therefore, is preferred. 

As expected, the aptitude for disproportionation of the aromatic compound depends upon the nature of the alkyl group, and the order of reactivity is isopropyl > ethyl > methyl. Due to their higher nucleophilicity, polyalkylbenzenes react faster than monoalkylbenzenes. This effect is pronounced in the case of methylbenzenes. Toluene itself shows little reactivity over Nafion-H at 193°C. Diethylbenzenes react much faster than dimethylbenzenes. The rate of conversion of diethylbenzenes over Nafion-H at 193°C is 5 10-5 mol min 1 g-1 of catalyst.269 This is a low rate when compared with that using AICI3 HC1 in the liquid phase at room temperature (10-4 mol min-1 g-1 of catalyst).272 However, one should bear in mind that Nafion-H is a truly insoluble heterogeneous catalyst, whereas in the case of AICI3—HC1 a soluble complex is formed with the hydrocarbon and therefore the rates are not directly comparable. The equilibrium composition of the acid-catalyzed disproportionation of diethylbenzenes depends upon the nature of the catalyst. 

Base-Catalyzed Disproportionation of Tetrachlorodimethyldisilane — Investigations of the Heterogeneous Catalysts... 

Results are summarized in Fig. 5. Over SOaCrOz and SA, the reaction took place smoothly at 373 K, though the disproportionation of E2 proceeded higher temperature. Modified clay minerals also catalyzed the reaction at an appreciable rate. Alumina and HY showed low activity and other catalysts were almost inactive for the reaction. This clearly shows that strong solid acids can catalyze the alkylation reaction. The heterogeneous alkylation of E2 is an acid catalyzed reaction. 

The common point that makes the Schrock and Basset catalysts active in alkene metathesis is the alkoxy or aryloxy ligand in Schrock s catalysts and the silyloxy ligand in Basset s catalysts. Also note that Basset s catalysts are in fact Schrock s complexes or catalysts that are grafted on silica in order to link the metal by a Si-O-M bond and make catalysts heterogeneous. Schrock s high oxidation state compounds also give o-bond metathesis upon a-H elimination, but they do not catalyze alkane disproportionation. 

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