Effect Of Si/Al Ratio On Pore Size Modification Of Zeolite Beta By NaOH Treatment

There was a time when zeolites were considered remarkable materials since they possessed a set of extraordinary properties. The great prosperity of zeolites in the past decades can be supposed a result of the combination of some excellent properties of these materials, such as crystallinity, uniform microporosity, high surface area, good hydrothermal stability and acidic properties, as well as of the existence of a large variety of zeolite structures that could be found naturally or synthesized in the laboratory. However, hierarchical zeolites exhibit improved catalytic properties when compared with conventional ones which only have micropores.Treating zeolite Beta in micron size with NaOH solution is an effective method to create mesoporosity, and the Si/Al ratio is found to be a crucial factor to influence the mesopore formation by extraction of framework silicon using NaOH treatment. Mesopore sizes and volumes of zeolite beta by alkali treatment may to some extend be controlled by adjusting the Si/Al ratios, and an evidently increased mesopore surface area of296-322cm3·g-1was obtained. Though alkali treatment led to reduction in the Bronsted acid sites, the best catalytic performance at70℃for30min was still observed on alkali-treated zeolite beta with the mesopore volume of0.23-0.26cm3·g-1, owing to the effective development of mesoporosity. A framework Si/Al ratio less than30is optimal for a substantial intracrystalline mesoporosity combined with most of the original microporosity after alkaline treatment in0.2M NaOH for30min at65℃.Zeolites beta with molar Si/Al ratios of19-32were successfully obtained by treating parent beta (Si/Al=12.5) with0.3-2.0M citric acid solutions. The citric acid treatment alone had slight effect on the crystallinity and textural properties, but the following alkali treatment evidently developed the mesoporosity of zeolites beta. The catalytic performance in a-pinene isomerization was significantly improved owing to the creation of mesoporous zeolites beta. Though alkali treatment of zeolite beta with increasing Si/Al ratios led to gradually decreased crystallinity and acidity, the conversion of a-pinene as high as90.2%was observed at70℃, which was mainly ascribed to the evidently developed mesoporosity by successive citric acid-NaOH treatment of zeolite beta.