Porous boron nitride supports obtained from molecular precursors.: Influence of the precursor formulation and of the thermal treatment on the properties of the BN ceramic

Boron nitride (BN) porous samples have been prepared in order to be used as noble metal catalyst support from various molecular precursors, using classical thermal methods to expand and preceramise the precursors. Three types of precursors have been tested: ammonia borane, polyborazylene and several aminoborazines and derived polymers. Using trimethylamino borazine (MAB) and MAB polymers, the surface of the foams was shown to be decreasing when the polymerisation advancement of the precursor increased meanwhile the crystallisation of the samples decreased. All the samples issued from MAB presented the same SEM morphology: large pores with a glassy skin surface covering a disorganised inner part, hidden under the thin homogeneous BN skin, which appeared through the sample breaks. This was related to the melting property of MAB polymers which imbedded the internal part of the sample and avoided its expansion. Attempts were made, using the bulkier amino group as anilino- and benzyl-aminoborazine to enhance the porosity of the samples using a precursor with a lower ceramic yield. The change in the specific area was not important. However, a surface of 30 m2 g−1 was currently obtained. The ability of the precursor to lead to high specific area seemed to be correlated to the expansion possibilities of the precursor in the solid state prior to fusion. Foams exhibiting a surface of more than 50 m2 g−1 have been obtained when polyborazylene based precursor was pyrolysed.