A systematic investigation of synthesis conditions on the silanol defects of silicalite-1

Understanding of the main factors affecting the silanol defects (Si–OH) of zeolite is vital for its catalysis and adsorption properties. Taking silicalite-1 (MFI) as an example, a systematic investigation of synthesis temperature, TPAOH addition, ethanol addition, and K addition on the silanol defects of silicalite-1 was conducted using pure reagents excluding the effect of metal impurities. Silicalite-1 synthesized form pure tetraethyl orthosilicate (TEOS) and tetrapropylammonium hydroxide (TPAOH) contains a relatively high concentration of silanol nests with a Q 3 /Q 4 ratio of 10.0% in the 29 Si MAS NMR spectra. The introduction of trace alkali metal ions above 0.02 mol% effectively reduces the defect concentration of silicalite-1. Moreover, the effect of K reaches a plateau as K/Si ratio beyond 0.2 mol% does not lead to further reduction of structural defects. Lowering the synthesis temperature to 125 °C results in smaller crystal sizes, irregular morphologies, and a slight decrease in silanol nests. Decrease of the TPAOH amount increases the crystal size from 139 nm to 224 nm, but has little effect on the silanol defect concentration. Ethanol from controlled addition reduces the silanol defects somewhat. • Trace alkali metal ions (>0.02 mol%) reduce defects in silicalite-1, reaching a plateau when K/Si exceeds 0.2 mol%. • Synthesis at 125 °C yields smaller, irregular crystals with slightly fewer silanol nests. • Reducing the TPAOH amount increases crystal size from 139 to 224 nm with little effect on silanol defects. • Ethanol reduces the silanol defects somewhat.