Addressing Challenges in Catalysis and Energy: Selective Grafting Functionality Onto Mesoporous Silicas and Organic Hydrides for the Regeneration of Ammonia Borane, a Hydrogen Storage Material

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Date
2011-09-12
Authors
Webb, Jonathan Douglas
Keyword
Hantzsch Ester , Hydride Transfer , Mesoporous Silica , Suzuki-Miyaura , Ammonia Borane , Regeneration , Organic Hydride , Selective Grafting
Abstract
Ordered mesoporous silicas have been shown to have a variety of useful applications ranging from adsorbents for containments to supports for catalysts. While these materials have received a good deal of attention in the literature there is still much opportunity for new technologies. We present research describing a novel approach to incorporate functionality onto the pore surfaces of these materials as well as a highly active catalyst for the Suzuki-Miyaura reaction. Our approach to selectively graft functionality on to the pore walls of the mesoporous silicas SBA-15 and MCM-41 involves treating the materials loaded with a structure directing agent (SDA), with hexamethyldisilazane that passivates the external surface through silylation. Once the SDA is removed the mesopores can be functionalized selectively using standard methods. A test designed to look at the passivation layer is also described. The catalyst developed is designated Pd-SBA-15-SH(g) and it is active for the Suzuki-Miyaura reaction. The activity, recyclability and leaching of Pd-SBA-15-SH(g) was found to be superior to related materials. A mechanistic analysis suggests the catalyst is a reservoir for soluble Pd metal. A key challenge that is holding back wide scale application of ammonia borane (NH3BH3) as a hydrogen storage material for mobile applications is the dearth of regeneration strategies. Presented are our forays into the development of an organic hydride based regeneration strategy. The first phase of the project focused on the reaction between Hantzsch esters and B(C6F5)3. N-substituted Hantzsch esters were found to transfer hydride to boron in >90 % yield. Mechanistic analysis of the reaction suggests either a SET mechanism or a highly asynchronous transition state. A novel hydride transfer equilibrium promoted by B(C6F5)3 was observed and it operated at temperatures below -10 ºC. N,N-ditertbutyl-dihydroimidazole is also an effective hydride donor to B(C6F5)3 as well as other Lewis acids that are more relevant mimics to those invoked in regeneration schemes. When the redistribution of B(SPh)3 is carried out with N,N-ditertbutyl-dihydroimidazole in the presence of NEt3 and CH2Cl2 at 50 ºC, BH2(NEt)3(SPh) is formed. CH2Cl2 functions as a thiol scavenger under the reaction conditions. 1-Octene trapping experiments provided indirect evidence for the formation of diborane, a critical component in the regeneration of NH3BH3.
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