Exploratory study of mesopore templating with carbon during zeolite synthesis

Here we present an exploratory study on the generation of mesopores in zeolite crystals by templating with carbon during synthesis. With nitrogen physisorption, scanning electron microscopy and (three-dimensional) transmission electron microscopy we have studied the influence of the carbon source and zeolite synthesis conditions on the mesopores generated after the carbon has been burned off. In particular, silicalite-1 crystals have been synthesized in the presence of a carbon matrix. After burning off the carbon, zeolite samples with mesopore volumes between 0.05 and 0.71 ml/g were obtained. With dissolved SiO2 at a crystallization temperature of 150 degreesC large crystals (15 mum length) were obtained, whereas the use of tetraethylorthosilicate at a crystallization temperature of 170 degreesC yielded crystals with a size between 0.5 and 6 mum. Low heating rates (0.5 degreesC/min) to a final crystallization temperature of 170 degreesC led to rather uniform small zeolite crystals (0.6-1 mum), whereas the crystals were less uniform in size when the autoclave was rapidly heated to the final temperature, i.e. also much larger crystals were found. Both carbon nanofibers and carbon black aggregates were capable to act as a template for cylindrical mesopores that start at the external surface of the zeolite crystals. However, the tortuosity of the mesopores templated by the carbon black aggregates was much higher than the cylindrical mesopores templated by the carbon nanofibers. With increasing zeolite crystal size the chance that carbon aggregates are completely surrounded by the zeolite increases, leading to inkbottle type mesopores. The results show for the first time carbon nanofiber skeins to be highly promising secondary templates to obtain cylindrical mesopores with a low tortuosity in zeolite crystals. (C) 2003 Elsevier Inc. All rights reserved.