Acidity of natural zeolite clinoptilolite and catalytic performance in the acetalization of benzaldehyde with butandiol-1,3
- 1. Introduction
Clinoptilolite is a natural, medium pore size, zeolite. It contains a 2-deminsionalpore system containing of oxygen-8- membered and oxygen-10-membered rings. It is one of the most abundant natural zeolites. It is mainly used as ion exchanger and adsorbent for removal of toxic elements and compounds from the environment. Although clinoptilolite is porous and can be acidified by ion exchange or acid treatment and a potential catalyst, reports on its application in catalysis are limited so far. This contribution deals with acidic modification of a natural zeolite sample, characterization of the nature of acid sites and the catalytic activity in the Brønsted acid catalyzed acetalization of benzaldehyde with butandiol-1,3. The samples were acidified by via ammonium-ion exchange and acid treatment. Samples were characterized by XRD regarding crystallinity and phase composition. The morphology was studied by TEM images The acidity was studied by ammonia-TPD measurements and1. Introduction
Clinoptilolite is a natural, medium pore size, zeolite. It contains a 2-deminsionalpore system containing of oxygen-8- membered and oxygen-10-membered rings. It is one of the most abundant natural zeolites. It is mainly used as ion exchanger and adsorbent for removal of toxic elements and compounds from the environment. Although clinoptilolite is porous and can be acidified by ion exchange or acid treatment and a potential catalyst, reports on its application in catalysis are limited so far. This contribution deals with acidic modification of a natural zeolite sample, characterization of the nature of acid sites and the catalytic activity in the Brønsted acid catalyzed acetalization of benzaldehyde with butandiol-1,3. The samples were acidified by via ammonium-ion exchange and acid treatment. Samples were characterized by XRD regarding crystallinity and phase composition. The morphology was studied by TEM images The acidity was studied by ammonia-TPD measurements and 1H solid state MAS NMR as well as REDOR experiments. Structure and structural changes caused by applied modifications were studied by 27Al- and 29Si MAS NMR measurements. and thermal analysis.
2. Experimental Part
The studied natural zeolite tuff contained ca. 90 ma.-% of clinoptilite. The ion exchange behavior was studied in 0.1 and 0.5 M ammonium nitrate solution and for comparison with HCl solution of similar concentration. The activation temperature was varied between 300-600°C.The catalytic test was performed using ca. 0.1 g of the catalyst and ca.10g of the aldehyde and alcohol using toluene as solvent and under reflux. Reaction water was removed via a by-pass.
3. Results and discussion
The ion experiments show that a part of the cations of clinoptilolite readily exchange with ammonium ions and protons supplied by acid treatment. However, exchange with the acid has a more severe impact on the clinoptilolite structure. More severe treatment times leads to a loss of activity in terms of acetal formation: Sample show all high conversions after 4 h of reaction, but show markedly differences after short reaction times. The presence of acidic protons of medium to strong strength is confirmed by ammonia-TPD and proton NMR measurements. The sample activation causes a partial dealumination as indicated by the appearance of 5- and 6-fold coordinated aluminum. Interestingly, the NMR result give first hints for a relation of the catalytic conversion) with the amount of 5-fold coordinated aluminum. The results will be discussed in terms of a collaborative action of Brønsted acid sites and 5-fold coordinated aluminum.
4. Conclusions
Acid natural clinoptilolite catalysts were prepared via ammonium and acid treatment. The clinoptilpolite is highly active in acid catalyst acetalization of aldehydes, affine chemical reaction. Activity was markedly influenced by the activation. A cooperative effect of Brønsted sites and 5-fold coordinated Al is discussed to explain the superior performance of the most active catalyst.
Acknowledgments (times new roman, 9 pts)
We gratefully acknowledge the excellent assistance of Dr. A. Villinger (XRD) and Dr. M. Frank (TEM images).
References
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