Functionalized Silica-Based Catalysts

The wide variety of ILs and reaction conditions makes it difficult to extract a trend, but clearly the presence of both acid and base sites are crucial for the cyclic carbonate synthesis. Outstanding results were reported by Takahashi et al. [78] using phosphoniiun quaternary salts as phospho-niiun halides immobilized on SiO (e.g. CO SiO -CjHg-Pfn-BuljBr)) for the synthesis of PC from PO and CO using a continuous fixed-bed reactor. As far as we know it is best catalytic performance ever reported for the synthesis of PC using a continuous flow reactor with a PO conversion 

Silica surface Silica surface Silica surface X Silica surface 

In another work, Han et al. [77] have demonstrated that other efficient alternative to improve the reactivity of ILs is the incorporation different metal ions into imidazolium-based ILs. All Si-IL-M catalysts (M = Ni, Co +, Mn, Cu ) exhibited activity and selectivity that were 

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The catalytic activity of the NHs-grafted mesoporous silica, FSMN, was examined in some base-catalysed condensations (eqn. 1). The results were listed in Table 1. The FSMN catalyst used here was FSMN-5 that was prepared by the pre-activation at 1073 K followed by NH3-treatment at 973 K. The Aldol condensation of benzaldehyde and acetone did not proceed in this condition (entry 1). The Knoevenagel condensation of benzaldehyde and diethyl malonate (entry 2) did not occurred. On the other hand, the reactions with malononitrile (entry 3) and with ethyl cyanoacetate (entry 4) were catalysed by the FSMN-5. This shows that the NHa-grafted mesoporous silica would function as base catalyst. 

This method allows the sol-gel siliea NR latex eompound to be moulded into the desired shape. TESPT was eo-mixed with TEOS and eoneentrated NR latex. Ammonia which functioned as base catalyst was added into the concentrated NR latex. The silica-TESPT-NR latex compound was then subjected to heat to complete the sol-gel silica conversion process. The dried sol-gel silica-NR mixture was compounded as per normal mixing procedure. A good dispersion of silica particles of the size between 100 and 500 nm was achieved. Using the two-level factorial design, it was concluded that the mechanical properties, i.e. tensile properties and tear strength, were significantly affected by the TEOS loading. It was also found that the amount of ammonia present in the concentrated latex, i.e. 0.7% (w/w) was sufficient to convert TEOS into silica. 

Aminosilicas have been widely studied for use in catalysis, either as a base catalyst or as a support for metal complexes (12). For example, amine functionalized silica can be used to catalyze the Knoevenagel condensation, an important C-C bond forming reaction. Also, the amine sites on the silica can be further functionalized to form supported imines, guanidine, and other species... 

Small amounts of other compounds can be added to Ni-based catalysts to improve the functional characteristics of the final catalyst. Typically, they are calcium aluminate to enhance the mechanical resistance of the catalyst pellets, potassium oxide to improve the resistance to coke formation and silica to form a stable silicate with potassium oxide [34]. Promotion with rare earth oxides such as La2C>3 also results in enhanced resistance to coking. 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

Lasperas, M., Llorett, T., Chaves, L., Rodriguez, I., Cauvel, A. and Brunei, D. Amine functions linked to MCM-41-type silicas as a new class of solid base catalysts for condensation reactions. Stud. Surf. Sci. Catal., 1997, 108, 75-82. 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

There is, nevertheless, some evidence (35, 36), based in NiNaY and NiMo/ alumina/Y model catalyst systems, that the amount of coke formed is reduced with increasing intimacy of mixing of the two functions at the submicron level. This concept is further supported by the reported relatively high performance of NiW/ASA (amorphous silica-alumina) cogel HC catalysts which, it is claimed, exhibit an excellent distribution of the NiW hydrogenation function throughout the catalyst particles (37). 

Stronger solid base catalysts can be prepared by grafting guanidine bases to mesoporous silicas. For example, the functionalization of MCM-41 with 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD), as shown in Fig. 2.40, afforded a material (MCM-TBD) that was an effective catalyst for Michael additions with ethylcya-noacetate or diethylmalonate (Fig. 2.41) [136]. 

Amine functions linked to MCM-41-type silicas as a new class of solid base catalysts for condensation reactions. 

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