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1. Introduction
Catalytic processing of heavy feedstock can meet the increased demand of energy up to a great extent. It requires the application of acidic catalysts like zeolites. However, the used synthetic catalysts are difficult to recover and reuse and are mostly spent. The use of natural zeolite as spent catalysts may open new perspectives in the chemical use of heavy feed feedstock by chemical conversion. Natural zeolites are not expensive, widely available and environment friendly. Clinoptilolite is a natural, most abundant medium pore size zeolite, consisting of 2-deminsional pore system containing of oxygen-8- membered and oxygen-10-membered rings.Although clinoptilolite is porous and can be acidified by ion exchange or acid treatment. Samples were characterized by XRD regarding crystallinity and phase composition. 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 ion exchange behaviour of natural zeolite tuff contained ca. 90 ma. % of clinoptilite 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. 0.2 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 experiments show that a part of the cations of clinoptilolite readily exchange with ammonium ions and protons supplied by acid treatment. Substitution of the compensating cations by NH4+ followed by calcination and HCl treatment does not produce structural changes in the original material, but it opens the channels and increases acidity and thermal stability. Calcination at higher temperature and exchange with concentrated acid has a more severe impact on the clinoptilolite structure dealumination as indicated by the appearance of 5- and 6-fold coordinated aluminum. Catalytic activity of sample is related to the surface acidity. The presence of acidic protons of medium to strong strength is confirmed by ammonia-TPD and proton NMR measurements.
4. Conclusions
Acidic natural clinoptilolite catalysts prepared via ammonium exchange followed by calcination and acid treatment shows a positive influence on acidity and porosity. Modification creates hierarchical micro-nano porosity. The specific surface area varies between ca. 45 m2/g and 245 m2/g.
Influence of the of dealumination and porosity on the acid sites of natural zeolite clinoptilolite
(2016)
Heavy feedstock from crude and bio oil is a widely available and renewable resource for production of fuel and starting materials for other organic valuables by cracking or hydrocracking. Catalytic processing of heavy feedstock can meet the increased demand of energy up to a great extent. It requires the application of acidic catalysts like zeolites. However, the used synthetic catalysts are difficult to recover and reuse and are mostly spent.
The use of natural zeolite as spent catalysts may open new perspectives in the chemical use of heavy feed feedstock by chemical conversion. Natural zeolites are not expensive, widely available and environment friendly. Clinoptilolite is the most abundant natural zeolite. Clinoptilolite has a crystalline structure with a defined micropore system of medium size showing unique ion exchange and sorption properties. However, it is catalytically active only in the H-form. Also certain porosity is required for improvement of the accessibility of active sites.
This paper deals with the tuning of acid properties and of the mesoporosity of the clinoptilolite by variation of the Si/Al framework ratio, extra-framework aluminum and modification of the porosity by specific acid and water vapor treatment. The preparation of hierarchical pore structures containing interconnected micro-meso-macropores is an important factor influencing the catalytic performance.
The obtained materials have been characterized by XRD, TEM, FTIR, Raman, TG/DSC. The chemical composition has been determined by ICP-AES. The porosity have been investigated by nitrogen adsorption desorption measurements. The acidity has been measured by Ammonia-TPD. The extent of dealumination, stability of the clinoptilolite against acid treatment and the change in the nature of acid sites and their local structure has been studied by solid state 29Si and 27Al MAS NMR spectroscopy in detail. The catalytic activity has been investigated in the acetalization of benzaldehyde with 1,3-butanediol. The impact of the porosity, change of the Si/Al ratio as well as present Al species on catalytic properties will be discussed.
Clinoptilolite is a natural, medium pore size, zeolite. It contains a 2-deminsional pore system containing of oxygen-8- membered and oxygen-10-membered rings. It is one of the most abundant natural zeolites. Clinoptilolite has a crystalline structure showing unique ion exchange and sorption properties. It is mainly used as ion exchanger and adsorbent for removal of toxic elements and compounds from the environment. The use of natural zeolite as spent catalysts may open new perspectives in the chemical use of heavy feed feedstock by chemical conversion. Natural zeolites are not expensive, widely available and environment friendly. Clinoptilolite is the most abundant natural zeolite. Although clinoptilolite is a potential acid catalyst, reports on its application in catalysis are limited so far.
This contribution deals with the modification of the acidity of the natural zeolite clinoptilolite, the characterization of the 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 followed by calcination and acid treatment. The obtained catalysts 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.
The ion experiments show that a part of the cations of clinoptilolite readily exchange with ammonium ions and protons from HCl treatment. The NMR results give first hints for a relation of the catalytic conversion with the presence 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.
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 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
[1] A.Dziedziecka, B. Sulikowski, : Ruggiero-Mikulajczyk, Catal. Today, 2016, 259, 50-58.
Clinoptilolite is a natural, medium pore size zeolite. It contains a 2-deminsional pore system consisting 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 microporous 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 modification of a natural zeolite clinoptilolite, and the study of the influence of the textural and acidic properties on the catalytic performance in the acid catalyzed acetalization of bezaldehyde with butandiol-1,3 and the more demanding etherification of glycerol with t-butanol or isomerization of n-hexane. The samples were acidified by via ammonium-ion exchange followed by calcination and acid treatment. Samples were characterized by XRD regarding crystallinity and phase composition, FTIR, and N2-adsorption for porosity. The morphology was studied by TEM images. The acidity was studied by ammonia-TPD measurements and 1H solid state MAS NMR. Structure and structural changes caused by applied modifications were studied by 27Al- and 29Si MAS NMR measurements and TRAPDOR experiments as well as thermal analysis.
The studied natural zeolite tuff contained ca. 90 ma.-% of clinoptilolite. The ion exchange behavior was studied with ammonium nitrate solution and for comparison with HCl solution of similar concentration. The cation exchange was followed by AES. The activation temperature was varied between 400-600°C. The catalytic test was performed e.g. using ca. 0.2 g of the catalyst and ca.10g of the aldehyde or alcohol using toluene as solvent and under reflux. Reaction water was removed via a by-pass. The isomerization of n-hexane was performed in a fall stream reactor at elevated temperature at gas phase. Water vapour saturated air was passed over clinoptilolite samples at elevated temperature for different time.
The ion exchange experiments shows that ca 45% 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 results 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.
Modified clinoptilpolite is highly active in acid catalyst acetalization of aldehydes, isomerization of n-hexane and etherification of glycerol with t-butanol. Acidic natural clinoptilolite catalysts prepared via ammonium exchange followed by calcination and acid treatment shows a positive influence on acidity and porosity. Modification creates hierarchical micro-nano porosity. The specific surface area varies between ca. 45 m2/g and 257 m2/g.