- Email: [email protected]
Investigation on the pore structure of binderless zeolite 13× shapes
Microporous and Mesoporous Materials 154 (2012) 119–123
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Investigation on the pore structure of binderless zeolite 13 shapes Kristin Schumann a,⇑, Baldur Unger a, Alfons Brandt a, Franziska Scheffler b a b
Chemiewerk Bad Köstritz GmbH, Heinrichshall 2, 07586 Bad Köstritz, Germany Fakultät für Verfahrens-und Systemtechnik, Institut für Chemie (ICH), Otto-von-Guericke-Universität, Universitätsplatz 2, 39106 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 10 May 2011 Received in revised form 19 July 2011 Accepted 22 July 2011 Available online 28 July 2011 Keywords: Zeolite Adsorption Meso pores Macro pores Mineral binder
a b s t r a c t In the last few years novel binderless molecular sieves as high-performance materials were introduced into technical adsorption processes. Besides of the 100% active adsorption matter such material exhibits an advantageous secondary pore system. In this paper a more detailed investigation of the structure and the generation thereof is presented. To characterize the pore structure of the system SEM, EDX, Hg-porosimetry, as well as adsorption measurements (N2 and CO2) were used. For additional investigations XRD and XRF were applied. As a result of the special manufacturing process – granulation of zeolite NaMSX powder with metakaolin/caustic under high energy input followed by an alkaline treatment – two different types of zeolite morphologies were obtained: On the outer surface of the beads typical octahedral zeolite X crystals are monitored as a result of conventional and epitaxial crystal growth. In contrast the interior of the beads exhibits polycrystalline structures consisting of zeolite X in untypical shape, which is most probably the result of zeolite formation in dense matter. Due to the confined space conventional and epitaxial growth is inhibited. Obviously the original zeolite crystals are intergrown by polycrystalline zeolite NaMSX matter originated from converted metakaolin, and by the way forming macro pores. Said structure formation is the cause for the surprisingly high mechanical stability of the binderless shaped bodies. The existence of the superior secondary pore system and the high mechanical stability of binderless type NaMSX shapes can be explained as result of different pathways of the conversion of metakaolin into zeolite matter due to the existence of areas of different densities after the granulation process. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Due to their unique properties zeolite molecular sieves are one of the most utilized adsorbents in industry [1]. Typical applications for the zeolite types A and X are drying, purification and separation of gases and liquids. Especially in dynamic adsorption processes, where the adsorbent has to be regenerated frequently, a relatively free flow of the adsorption media through the fixed bed of molecular sieve is required. Therefore the zeolite powder has to be shaped into mechanically stable macroscopic particles like beads (spheres) or extrudates. Since pure zeolite powder does not exhibit binding abilities for conventional molecular sieves a suitable (mineral) binder needs to be applied. Such binder usually does not contribute to the adsorption. That means the total adsorption capacity is reduced at about the percentage of the added binder. Furthermore – due to its chemical composition – the mineral binder might catalyse undesired chemical reactions, e.g. coking [2]. Moreover, it needs to be considered, that in the shaping process using a binder a secondary
⇑ Corresponding author. E-mail address: [email protected] (K. Schumann). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.07.015
pore system is generated (consisting of meso and macro pores [3]) within which the components to be adsorbed have to move to and from the adsorption sites of the zeolite crystals [4]. Due to the fact, that the shapes need to exhibit a certain mechanical stability, the shaping is connected with a compactation of the material, which may lead to an unfavourable pore size distribution of said secondary pore system, and, thereby to an inhibition of the diffusion of the components inside of the shape. It was found, that one solution to overcome the aforementioned problem is the application of the concept of binderless zeolite molecular sieves. Although binderless zeolite molecular sieves and their superior properties are known since a while [5–7], especially zeolite type X based binderless molecular sieves are only rarely available on the market. In this paper the manufacturing process and properties of binderless molecular sieves of the zeolite type NaMSX (medium silicon zeolite X, molar SiO2/Al2O3 ratio of 2.2–2.45) [8] in comparison to the related binder containing molecular sieves is discussed. Said zeolite X type with a relative low molar SiO2/Al2O3 is particularly suitable for the removal of polar trace components from air and other gases. It can, therefore, be used as effective adsorbent in the front end purification of a cryogenic air separation unit, mainly for the elimination of CO2 from the feed air prior to liquefaction.
120
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
2. Experimental 2.1. Materials As parent zeolite component commercial NaMSX powder from Chemiewerk Bad Köstritz GmbH (Germany) with a crystallinity of 98% and a molar SiO2/Al2O3 ratio of 2.35 was used. The binderless spheres, better described as beads were prepared with metakaolin (Kaorock) from Thiele (USA) supplied by Solvadis (Germany). The chemical composition was determined by XRF as follows: 44.1 wt.% Al2O3, 51.9 wt.% SiO2, 1.0 wt.% H2O and some trace components (Ti, Fe, K, SO24 ). Further raw materials were: sodium silicate (Natronwasserglas) from Wöllner GmbH (Germany) with the following composition 8.3 wt.% Na2O, 27.7 wt.% SiO2, sodium hydroxide solution (50 wt.%) from Overlack GmbH (Germany) and sodium aluminate solution (alumin 10Ò) from Remondis AG (Germany) (19.8 wt.% Na2O, 19.6 wt.% Al2O3).
Fig. 2. Zeolite content of binderless molecular sieves after different conversion times, calculated from adsorption measurments.
2.2. Preparation of the beads The process to make binderless molecular sieve starts with mixing and mechanical shaping of the raw materials. Zeolite NaMSX powder (60 wt.% of the total mix), metakaolin, sodium silicate solution, sodium hydroxide solution and water were treated in an Eirich mixer R02E to form granules. Said freshly prepared (not further manufactured) granules are usually and furthermore in this paper called ‘‘green beads’’. After shaping the beads (e.g. with a bead size in the range of 1.6–2.5 mm) are dried, re-moistured and subsequently put into an alkaline solution for aging and hydrothermal conversion of the non-zeolitic components into zeolite matter (16 h at 84 °C). Finally, the material was washed, dried and thermally activated [8]. For comparison, binder containing material commercial NaMSX based molecular sieve (KÖSTROLITH NaMSX K(1.6–2.5 mm)Ò) Chemiewerk Bad Köstritz GmbH (Germany) (about 18 wt.% of clay binder) was used. Fig. 1 shows both binder containing (gloomy – in reality brownish) and binderless (bright – in reality beige) zeolite molecular sieves. 2.3. Characterisation Crystallinity analysis for zeolite powder and binderless molecular sieves was performed on an X-ray diffractometer (D4, Bruker AXS, Germany).The chemical composition of the starting materials was determined by X-ray fluorescence spectroscopy (S4, Bruker AXS,
Germany) using a rhodium anode tube. Nitrogen and carbon dioxide adsorption isotherms were measured with volumetric sorption equipment (Gemini 2370 CO2; Gemini 2365 N2, Micromeritics, USA). Analysis of meso pores and macro pores was performed by a combination of equipments (PASCAL P 140 and PASCAL P 440, Porotec GmbH, Germany). For SEM measurements a field emission electron microscope (ULTRA 55 plus, Zeiss, Germany) was used. EDX measurements were performed using a Trident System (AMETEC GmbH, Germany).
3. Results and discussion The determination and interpretation of the crystallinity of the binderless NaMSX based molecular sieves is discussed elsewhere [9]. It has been found, that adsorption capacities are a better tool to describe the related processes than the XRD crystallinities. In that respect in Fig. 2 the adsorption capacities for CO2 and N2, measured on the parent zeolite powder, on the final product, and on a few intermediates of the conversion process are presented. Due to the already high amount of zeolite powder present in the unconverted beads (60 wt.% – see above) the kinetics of the zeolite formation does not follow the typical s shape [10] of zeolite syntheses. However, at the end material consisting of about 100% zeolite NaMSX (the adsorption capacities compared to those of pure NaMSX powder) is obtained. In comparison, binder containing material consists of about 18 wt.% of adsorptive inert mineral binder. Consequently, the adsorption capacities of such type of material are by approximately Table 1 Comparison of the static adsorption capacity of zeolite NaMSX powder, binder containing and binderless molecular sieves. Zeolite Binder NaMSX containing powder molecular sieve of the zeolite type NaMSX
Fig. 1. Image of binder containing (gloomy) and binderless (bright) zeolite molecular sieves.
Water adsorption capacity 31.5 25.6 (55% r.h. at 298 K for 24 h)/wt.% CO2 adsorption capacity 35.0 27.1 (at 298 K at 2.4 mbar)/Ncm3/g 120.7 92.7 CO2 adsorption capacity (at 298 K at 333.3 mbar)/Ncm3/g 10.7 8.0 N2 adsorption capacity (at 298 K at 1000 mbar)/Ncm3/g 0.327 0.244 Micro pore volume (N2, 77 K)/Ncm3/g
Binderless molecular sieve of the zeolite type NaMSX 31.2 35.2 119.4 10.5 0.324
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
Fig. 3. SEM image of zeolite NaMSX powder.
121
Fig. 6. SEM image of the outer surface of the shape after 2 h of conversion of metakaolin into zeolite NaMSX.
Fig. 4. SEM image of metakaolin. Fig. 7. SEM image of the outer surface of the shape after 6 h of conversion of metakaolin into zeolite NaMSX.
Fig. 5. SEM image of a dried green bead (surface; continuous line: intergrown zeolite crystals, dotted line: metakaolin). Fig. 8. SEM image of the outer surface of a binderless NaMSX shape.
the amount of binder lower than those measured on the pure zeolite powder (Table 1). The binderless molecular sieves formation process can be followed more in detail by means of SEM measurements. In Figs. 3 and 4 the microscopic structure of both main raw materials is shown. The typical octahedrally shaped crystals (single or
intergrown) of zeolite X are monitored. Metakaolin is presented as small plates. In the dried green beads the aforementioned typical shapes of the raw materials are still to be seen – intergrown zeolite crystals and small plates of metakaolin (Fig. 5). For the following investigations the observed spots have to be distinguished between the outer
122
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
Fig. 9. SEM image of the interior of a binderless NaMSX shape. Fig. 11. N2 adsorption isotherms (at 77 K) of zeolite NaMSX powder, binder containing and binderless molecular sieves of the zeolite type NaMSX.
Fig. 10. SEM image of the interior of a binderless NaMSX shape, zeolite matter with morphology of metakaolin.
surface and the interior of the shapes. After two hours of hydrothermal treatment a gel-like matter can be identified on the outer surface of the shapes (Fig. 6). With increasing reaction time zeolite crystals grow directly out of the plate-shaped metakaolin. After six hours large crystals from the original zeolite powder and small crystals from the conversion of metakaolin process can be detected (Fig. 7). We therefore assume an epitaxial as well as a conventional crystal growth of zeolite matter. After the metakaolin conversion is completed only zeolite matter with the morphology of intergrown octahedral crystals is monitored (Fig. 8). The observation of the process in the interior of the beads shows a rather different picture: In the result of the chemical conversion of the metakaolin material with an untypical shape is generated. The monitored plane areas grow until the end of the reaction (Fig. 9). Due to the fact, that the final product consist of 100% of NaMSX zeolite matter (said statement is supported by adsorption investigations – see above and EDX investigations – see below) said untypically shaped material should be NaMSX zeolite matter, too. A possible explanation for the mentioned observation is as follows: Due to the mixing and shaping step in a device with high energy input the material is strongly compacted. This leads to lack of space in the interior of the beads, which would be needed to allow a conventional and epitaxial crystal growth of the zeolite crystals. Thus, only irregularly shaped polycrystalline zeolite X matter can be generated in the result of the metakaolin conversion. Most probably, the described structure formation is the reason for the surprisingly high mechanical stability of the binderless shaped bodies [11]. EDX measure-
ments support the existence of zeolite NaMSX with the unusual morphology in the interior of the shapes. A SEM image of zeolite matter having the morphology of metakaolin is shown in Fig. 10. The presence of the two different morphologies (Figs. 8 and 9) indicates that both building mechanisms described in the literature may coexist in the present system: The solution-mediated transport mechanism on the outer surface and the solid hydrogel transformation mechanism in the interior of the shapes. That means nucleation happened at the interface between the solution media and the formed synthesis gel or directly in the solution. Aluminate and silicate ions from the gel can be dissolved and contribute to the crystal growth (solution-mediated transport mechanism) [12]. Due to confined space in the interior of the shapes the diffusion between the synthesis gel and the solution is inhibited. Probably the solid hydrogel transformation mechanism occurs, and the solution is not involved into the crystallisation process [13]. As shown above, the static adsorption behaviour of the binderless material strictly depends on the amount of zeolite matter in the shape. Thus, a completely converted product exhibits the adsorption capacities of a related pure zeolite powder. However, as mentioned in the introduction, zeolite molecular sieve shapes are mainly needed because of their applicability in dynamic (regenerative) adsorption processes, where a more or less unimpeded flow of fluids through a molecular sieve bed needs to be assured. Thus, our material was tested against conventional, binder-containing material in a
Fig. 12. Hg-porosimetry measurements of binder containing molecular sieve.
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
123
4. Conclusions
Fig. 13. Hg-porosimetry measurements of binderless molecular sieve.
CO2 breakthrough test, which is often used as test for the applicability of such molecular sieves in the front-end purification of cryogenic air separation plant (e.g. for purifying a gas stream contaminated by CO2, hydrocarbons and/or nitrogen oxides [14]). In this particular case, the time until the breakthrough of CO2 in a concentration of 5 ppm was measured (see e.g. [14]). First results show, that by the use of binderless material the breakthrough time can be extended by 55% as compared to the conventional material based on the same zeolite. This fact can not only be explained by the zeolite matter in the shape, but must have something to do with the secondary pore system, needed for the movement of the components within the shape. This item will be outlined below: In Fig. 11 the nitrogen adsorption isotherms measured at 77 K on pure NaMSX powder, on the conventional binder-containing NaMSX, and on the related binderless NaMSX molecular sieve are depicted. Both the pure zeolite powder and the binderless shape exhibit nearly identical isotherms, whereas the one measured on the binder-containing material differs in both the position in the pattern and the shape from the other two. The position of the isotherm is clearly determined by the amount of adsorption active zeolite matter in the shape – which should be lower in the case of the binder containing sample. The difference in the shape, however, indicates that there may exists some transport limitations in the binder-containing material caused by the existence of an essential amount of smaller pores (meso pores). This assumption is supported by mercury intrusion measurements (Figs. 12 and 13). As it clearly can be shown the conventional binder-containing molecular sieves have much more meso pores than binderless ones. The ratio of meso pores to overall transport pores decrease from 0.30 in binder containing material to 0.05 in binderless molecular sieves. Thus, besides of the high mechanical stability of the related shapes, using the described procedure a material with a very open secondary pore system (>90% of macro pores) is obtained. This is a very important factor as especially in technical dynamic adsorption processes macro pores facilitate faster kinetics. Conventional molecular sieves contain in addition to macro pores a relatively high amount of meso pores which inhibit a fast movement of the molecules that should be adsorbed/desorbed (Knudsen diffusion [15]).
Mechanically stable binderless zeolite molecular sieves of NaMSX type with superior adsorption properties can made in a stable large scale proved process [8]. The related material exhibits two different morphologies of the zeolite matter – typical octahedral crystals on the outer surface of the shapes and polycrystalline morphology in the interior of the shapes, which can be assigned to different mechanisms of formation. As expected, such binderless molecular sieves show an enhanced static adsorption capacity as compared to binder containing molecular sieves of the same zeolite type. Due to the existence of a unique secondary pore system in advantageous combination with the 100% zeolite content the new material shows remarkably better dynamic adsorption properties in comparison to the conventional material. For the mechanical stability of mineral clay binder containing molecular sieves it is needed to treat the clay binder at about 800 K [16]. Such high temperatures may, especially in the case of NaMSX type zeolites already lead to a partial degradation of the zeolite structure. As a further advantage of the new material it should be noted, that binderless molecular sieves, however, do not need to be treated at such high temperatures, as their mechanical stability is – as described above – based on the intergrowth between the original zeolite crystals. Thus, only the dehydration temperatures (about 650 K) are required in order to transfer a binderless molecular sieve in its activated state. Acknowledgment The authors gratefully acknowledge Fraunhofer IKTS Hermsdorf for SEM, EDX and Hg porosimetry measurements, and the Thüringer Aufbaubank for financial contribution. References [1] R.T. Yang, Adsorbents Fundamentals and Applications, 1st ed, John Wiliey & Sons, Inc., Hoboken, 2003 (pp.1–6). [2] D.M. Ruthven, Principles of Adsorption & Adsorption Processes, first ed., John Wiley & Sons, New York, 1984 (pp. 19–28). [3] M.E. Davis, Nature 417 (2002) 813–821. [4] D. Bathen, M. Breitbach, Adsorptionstechnik, first ed., Springer, Berlin, 2001. [5] R. L. Taggart, G. L. Ribaud, US Patent 3119 659 1964. [6] R. J. Nozemack, C. W. Chi, J. J. Schwonke, US Patent 4381 255 1983. [7] L. Puppe, G. Ulisch, DE Patent 3401 485 1985. [8] A. Brandt, J. Schmeißer, B. Unger, H. Tschritter, U. Henkel, B. Gojdar, D. Gruhle, G. Winterstein DE Patent 10 2008 046 155 2009. [9] 22nd German Zeolite Conference, Munich, March 3-5, 2010, Kristin Schumann, Alfons Brandt, Baldur Unger, Jens Schmeißer, Franziska Scheffler, Formation of binderless 13 zeolite shapes, Book of Abstracts, Munich, 2010, 207–208. [10] K. Byrappa, M. Yoshimura, Handbook of Hydrothermal Technology–A Technology for Crystal Growth and Materials Processing, first ed., Noyes Publications/William Andrew Publishing LLC, Norwich, 2001. pp. 354–364. [11] 23rd German Zeolite Conference, Erlangen, March 2-4, 2011, Kristin Schumann, Alfons Brandt, Baldur Unger, Franziska Scheffler, Binderless Zeolite 13 Shapes with Different Morphologies, Book of Abstracts, Erlangen, 2011, pp. 47-48. [12] R. Xu, W. Pang, J. Yu, Q. Huo, J. Chen, Chemistry of Zeolites and Related Porous Material–Synthesis and Structure, first ed., John Wiley & Sons Asia Pte Ltd, Singapore, 2007. pp. 289–294. [13] D.W. Breck, J. Chem. Educ. 41 (1964) 678–689. [14] C. Lutz, P.-G. Schmitt, US Patent 2008/0156190 2008. [15] K. Malek, M.-O. Coppens, J. Chem. Phys. 119 (2003) 2801–2811. [16] F. Bergaya, B.K.G. Theng, G. Lagaly, Handbook of Clay Science, first ed., Elsevier, Amsterdam, 2006.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Investigation on the pore structure of binderless zeolite 13 shapes Kristin Schumann a,⇑, Baldur Unger a, Alfons Brandt a, Franziska Scheffler b a b
Chemiewerk Bad Köstritz GmbH, Heinrichshall 2, 07586 Bad Köstritz, Germany Fakultät für Verfahrens-und Systemtechnik, Institut für Chemie (ICH), Otto-von-Guericke-Universität, Universitätsplatz 2, 39106 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 10 May 2011 Received in revised form 19 July 2011 Accepted 22 July 2011 Available online 28 July 2011 Keywords: Zeolite Adsorption Meso pores Macro pores Mineral binder
a b s t r a c t In the last few years novel binderless molecular sieves as high-performance materials were introduced into technical adsorption processes. Besides of the 100% active adsorption matter such material exhibits an advantageous secondary pore system. In this paper a more detailed investigation of the structure and the generation thereof is presented. To characterize the pore structure of the system SEM, EDX, Hg-porosimetry, as well as adsorption measurements (N2 and CO2) were used. For additional investigations XRD and XRF were applied. As a result of the special manufacturing process – granulation of zeolite NaMSX powder with metakaolin/caustic under high energy input followed by an alkaline treatment – two different types of zeolite morphologies were obtained: On the outer surface of the beads typical octahedral zeolite X crystals are monitored as a result of conventional and epitaxial crystal growth. In contrast the interior of the beads exhibits polycrystalline structures consisting of zeolite X in untypical shape, which is most probably the result of zeolite formation in dense matter. Due to the confined space conventional and epitaxial growth is inhibited. Obviously the original zeolite crystals are intergrown by polycrystalline zeolite NaMSX matter originated from converted metakaolin, and by the way forming macro pores. Said structure formation is the cause for the surprisingly high mechanical stability of the binderless shaped bodies. The existence of the superior secondary pore system and the high mechanical stability of binderless type NaMSX shapes can be explained as result of different pathways of the conversion of metakaolin into zeolite matter due to the existence of areas of different densities after the granulation process. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Due to their unique properties zeolite molecular sieves are one of the most utilized adsorbents in industry [1]. Typical applications for the zeolite types A and X are drying, purification and separation of gases and liquids. Especially in dynamic adsorption processes, where the adsorbent has to be regenerated frequently, a relatively free flow of the adsorption media through the fixed bed of molecular sieve is required. Therefore the zeolite powder has to be shaped into mechanically stable macroscopic particles like beads (spheres) or extrudates. Since pure zeolite powder does not exhibit binding abilities for conventional molecular sieves a suitable (mineral) binder needs to be applied. Such binder usually does not contribute to the adsorption. That means the total adsorption capacity is reduced at about the percentage of the added binder. Furthermore – due to its chemical composition – the mineral binder might catalyse undesired chemical reactions, e.g. coking [2]. Moreover, it needs to be considered, that in the shaping process using a binder a secondary
⇑ Corresponding author. E-mail address: [email protected] (K. Schumann). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.07.015
pore system is generated (consisting of meso and macro pores [3]) within which the components to be adsorbed have to move to and from the adsorption sites of the zeolite crystals [4]. Due to the fact, that the shapes need to exhibit a certain mechanical stability, the shaping is connected with a compactation of the material, which may lead to an unfavourable pore size distribution of said secondary pore system, and, thereby to an inhibition of the diffusion of the components inside of the shape. It was found, that one solution to overcome the aforementioned problem is the application of the concept of binderless zeolite molecular sieves. Although binderless zeolite molecular sieves and their superior properties are known since a while [5–7], especially zeolite type X based binderless molecular sieves are only rarely available on the market. In this paper the manufacturing process and properties of binderless molecular sieves of the zeolite type NaMSX (medium silicon zeolite X, molar SiO2/Al2O3 ratio of 2.2–2.45) [8] in comparison to the related binder containing molecular sieves is discussed. Said zeolite X type with a relative low molar SiO2/Al2O3 is particularly suitable for the removal of polar trace components from air and other gases. It can, therefore, be used as effective adsorbent in the front end purification of a cryogenic air separation unit, mainly for the elimination of CO2 from the feed air prior to liquefaction.
120
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
2. Experimental 2.1. Materials As parent zeolite component commercial NaMSX powder from Chemiewerk Bad Köstritz GmbH (Germany) with a crystallinity of 98% and a molar SiO2/Al2O3 ratio of 2.35 was used. The binderless spheres, better described as beads were prepared with metakaolin (Kaorock) from Thiele (USA) supplied by Solvadis (Germany). The chemical composition was determined by XRF as follows: 44.1 wt.% Al2O3, 51.9 wt.% SiO2, 1.0 wt.% H2O and some trace components (Ti, Fe, K, SO24 ). Further raw materials were: sodium silicate (Natronwasserglas) from Wöllner GmbH (Germany) with the following composition 8.3 wt.% Na2O, 27.7 wt.% SiO2, sodium hydroxide solution (50 wt.%) from Overlack GmbH (Germany) and sodium aluminate solution (alumin 10Ò) from Remondis AG (Germany) (19.8 wt.% Na2O, 19.6 wt.% Al2O3).
Fig. 2. Zeolite content of binderless molecular sieves after different conversion times, calculated from adsorption measurments.
2.2. Preparation of the beads The process to make binderless molecular sieve starts with mixing and mechanical shaping of the raw materials. Zeolite NaMSX powder (60 wt.% of the total mix), metakaolin, sodium silicate solution, sodium hydroxide solution and water were treated in an Eirich mixer R02E to form granules. Said freshly prepared (not further manufactured) granules are usually and furthermore in this paper called ‘‘green beads’’. After shaping the beads (e.g. with a bead size in the range of 1.6–2.5 mm) are dried, re-moistured and subsequently put into an alkaline solution for aging and hydrothermal conversion of the non-zeolitic components into zeolite matter (16 h at 84 °C). Finally, the material was washed, dried and thermally activated [8]. For comparison, binder containing material commercial NaMSX based molecular sieve (KÖSTROLITH NaMSX K(1.6–2.5 mm)Ò) Chemiewerk Bad Köstritz GmbH (Germany) (about 18 wt.% of clay binder) was used. Fig. 1 shows both binder containing (gloomy – in reality brownish) and binderless (bright – in reality beige) zeolite molecular sieves. 2.3. Characterisation Crystallinity analysis for zeolite powder and binderless molecular sieves was performed on an X-ray diffractometer (D4, Bruker AXS, Germany).The chemical composition of the starting materials was determined by X-ray fluorescence spectroscopy (S4, Bruker AXS,
Germany) using a rhodium anode tube. Nitrogen and carbon dioxide adsorption isotherms were measured with volumetric sorption equipment (Gemini 2370 CO2; Gemini 2365 N2, Micromeritics, USA). Analysis of meso pores and macro pores was performed by a combination of equipments (PASCAL P 140 and PASCAL P 440, Porotec GmbH, Germany). For SEM measurements a field emission electron microscope (ULTRA 55 plus, Zeiss, Germany) was used. EDX measurements were performed using a Trident System (AMETEC GmbH, Germany).
3. Results and discussion The determination and interpretation of the crystallinity of the binderless NaMSX based molecular sieves is discussed elsewhere [9]. It has been found, that adsorption capacities are a better tool to describe the related processes than the XRD crystallinities. In that respect in Fig. 2 the adsorption capacities for CO2 and N2, measured on the parent zeolite powder, on the final product, and on a few intermediates of the conversion process are presented. Due to the already high amount of zeolite powder present in the unconverted beads (60 wt.% – see above) the kinetics of the zeolite formation does not follow the typical s shape [10] of zeolite syntheses. However, at the end material consisting of about 100% zeolite NaMSX (the adsorption capacities compared to those of pure NaMSX powder) is obtained. In comparison, binder containing material consists of about 18 wt.% of adsorptive inert mineral binder. Consequently, the adsorption capacities of such type of material are by approximately Table 1 Comparison of the static adsorption capacity of zeolite NaMSX powder, binder containing and binderless molecular sieves. Zeolite Binder NaMSX containing powder molecular sieve of the zeolite type NaMSX
Fig. 1. Image of binder containing (gloomy) and binderless (bright) zeolite molecular sieves.
Water adsorption capacity 31.5 25.6 (55% r.h. at 298 K for 24 h)/wt.% CO2 adsorption capacity 35.0 27.1 (at 298 K at 2.4 mbar)/Ncm3/g 120.7 92.7 CO2 adsorption capacity (at 298 K at 333.3 mbar)/Ncm3/g 10.7 8.0 N2 adsorption capacity (at 298 K at 1000 mbar)/Ncm3/g 0.327 0.244 Micro pore volume (N2, 77 K)/Ncm3/g
Binderless molecular sieve of the zeolite type NaMSX 31.2 35.2 119.4 10.5 0.324
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
Fig. 3. SEM image of zeolite NaMSX powder.
121
Fig. 6. SEM image of the outer surface of the shape after 2 h of conversion of metakaolin into zeolite NaMSX.
Fig. 4. SEM image of metakaolin. Fig. 7. SEM image of the outer surface of the shape after 6 h of conversion of metakaolin into zeolite NaMSX.
Fig. 5. SEM image of a dried green bead (surface; continuous line: intergrown zeolite crystals, dotted line: metakaolin). Fig. 8. SEM image of the outer surface of a binderless NaMSX shape.
the amount of binder lower than those measured on the pure zeolite powder (Table 1). The binderless molecular sieves formation process can be followed more in detail by means of SEM measurements. In Figs. 3 and 4 the microscopic structure of both main raw materials is shown. The typical octahedrally shaped crystals (single or
intergrown) of zeolite X are monitored. Metakaolin is presented as small plates. In the dried green beads the aforementioned typical shapes of the raw materials are still to be seen – intergrown zeolite crystals and small plates of metakaolin (Fig. 5). For the following investigations the observed spots have to be distinguished between the outer
122
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
Fig. 9. SEM image of the interior of a binderless NaMSX shape. Fig. 11. N2 adsorption isotherms (at 77 K) of zeolite NaMSX powder, binder containing and binderless molecular sieves of the zeolite type NaMSX.
Fig. 10. SEM image of the interior of a binderless NaMSX shape, zeolite matter with morphology of metakaolin.
surface and the interior of the shapes. After two hours of hydrothermal treatment a gel-like matter can be identified on the outer surface of the shapes (Fig. 6). With increasing reaction time zeolite crystals grow directly out of the plate-shaped metakaolin. After six hours large crystals from the original zeolite powder and small crystals from the conversion of metakaolin process can be detected (Fig. 7). We therefore assume an epitaxial as well as a conventional crystal growth of zeolite matter. After the metakaolin conversion is completed only zeolite matter with the morphology of intergrown octahedral crystals is monitored (Fig. 8). The observation of the process in the interior of the beads shows a rather different picture: In the result of the chemical conversion of the metakaolin material with an untypical shape is generated. The monitored plane areas grow until the end of the reaction (Fig. 9). Due to the fact, that the final product consist of 100% of NaMSX zeolite matter (said statement is supported by adsorption investigations – see above and EDX investigations – see below) said untypically shaped material should be NaMSX zeolite matter, too. A possible explanation for the mentioned observation is as follows: Due to the mixing and shaping step in a device with high energy input the material is strongly compacted. This leads to lack of space in the interior of the beads, which would be needed to allow a conventional and epitaxial crystal growth of the zeolite crystals. Thus, only irregularly shaped polycrystalline zeolite X matter can be generated in the result of the metakaolin conversion. Most probably, the described structure formation is the reason for the surprisingly high mechanical stability of the binderless shaped bodies [11]. EDX measure-
ments support the existence of zeolite NaMSX with the unusual morphology in the interior of the shapes. A SEM image of zeolite matter having the morphology of metakaolin is shown in Fig. 10. The presence of the two different morphologies (Figs. 8 and 9) indicates that both building mechanisms described in the literature may coexist in the present system: The solution-mediated transport mechanism on the outer surface and the solid hydrogel transformation mechanism in the interior of the shapes. That means nucleation happened at the interface between the solution media and the formed synthesis gel or directly in the solution. Aluminate and silicate ions from the gel can be dissolved and contribute to the crystal growth (solution-mediated transport mechanism) [12]. Due to confined space in the interior of the shapes the diffusion between the synthesis gel and the solution is inhibited. Probably the solid hydrogel transformation mechanism occurs, and the solution is not involved into the crystallisation process [13]. As shown above, the static adsorption behaviour of the binderless material strictly depends on the amount of zeolite matter in the shape. Thus, a completely converted product exhibits the adsorption capacities of a related pure zeolite powder. However, as mentioned in the introduction, zeolite molecular sieve shapes are mainly needed because of their applicability in dynamic (regenerative) adsorption processes, where a more or less unimpeded flow of fluids through a molecular sieve bed needs to be assured. Thus, our material was tested against conventional, binder-containing material in a
Fig. 12. Hg-porosimetry measurements of binder containing molecular sieve.
K. Schumann et al. / Microporous and Mesoporous Materials 154 (2012) 119–123
123
4. Conclusions
Fig. 13. Hg-porosimetry measurements of binderless molecular sieve.
CO2 breakthrough test, which is often used as test for the applicability of such molecular sieves in the front-end purification of cryogenic air separation plant (e.g. for purifying a gas stream contaminated by CO2, hydrocarbons and/or nitrogen oxides [14]). In this particular case, the time until the breakthrough of CO2 in a concentration of 5 ppm was measured (see e.g. [14]). First results show, that by the use of binderless material the breakthrough time can be extended by 55% as compared to the conventional material based on the same zeolite. This fact can not only be explained by the zeolite matter in the shape, but must have something to do with the secondary pore system, needed for the movement of the components within the shape. This item will be outlined below: In Fig. 11 the nitrogen adsorption isotherms measured at 77 K on pure NaMSX powder, on the conventional binder-containing NaMSX, and on the related binderless NaMSX molecular sieve are depicted. Both the pure zeolite powder and the binderless shape exhibit nearly identical isotherms, whereas the one measured on the binder-containing material differs in both the position in the pattern and the shape from the other two. The position of the isotherm is clearly determined by the amount of adsorption active zeolite matter in the shape – which should be lower in the case of the binder containing sample. The difference in the shape, however, indicates that there may exists some transport limitations in the binder-containing material caused by the existence of an essential amount of smaller pores (meso pores). This assumption is supported by mercury intrusion measurements (Figs. 12 and 13). As it clearly can be shown the conventional binder-containing molecular sieves have much more meso pores than binderless ones. The ratio of meso pores to overall transport pores decrease from 0.30 in binder containing material to 0.05 in binderless molecular sieves. Thus, besides of the high mechanical stability of the related shapes, using the described procedure a material with a very open secondary pore system (>90% of macro pores) is obtained. This is a very important factor as especially in technical dynamic adsorption processes macro pores facilitate faster kinetics. Conventional molecular sieves contain in addition to macro pores a relatively high amount of meso pores which inhibit a fast movement of the molecules that should be adsorbed/desorbed (Knudsen diffusion [15]).
Mechanically stable binderless zeolite molecular sieves of NaMSX type with superior adsorption properties can made in a stable large scale proved process [8]. The related material exhibits two different morphologies of the zeolite matter – typical octahedral crystals on the outer surface of the shapes and polycrystalline morphology in the interior of the shapes, which can be assigned to different mechanisms of formation. As expected, such binderless molecular sieves show an enhanced static adsorption capacity as compared to binder containing molecular sieves of the same zeolite type. Due to the existence of a unique secondary pore system in advantageous combination with the 100% zeolite content the new material shows remarkably better dynamic adsorption properties in comparison to the conventional material. For the mechanical stability of mineral clay binder containing molecular sieves it is needed to treat the clay binder at about 800 K [16]. Such high temperatures may, especially in the case of NaMSX type zeolites already lead to a partial degradation of the zeolite structure. As a further advantage of the new material it should be noted, that binderless molecular sieves, however, do not need to be treated at such high temperatures, as their mechanical stability is – as described above – based on the intergrowth between the original zeolite crystals. Thus, only the dehydration temperatures (about 650 K) are required in order to transfer a binderless molecular sieve in its activated state. Acknowledgment The authors gratefully acknowledge Fraunhofer IKTS Hermsdorf for SEM, EDX and Hg porosimetry measurements, and the Thüringer Aufbaubank for financial contribution. References [1] R.T. Yang, Adsorbents Fundamentals and Applications, 1st ed, John Wiliey & Sons, Inc., Hoboken, 2003 (pp.1–6). [2] D.M. Ruthven, Principles of Adsorption & Adsorption Processes, first ed., John Wiley & Sons, New York, 1984 (pp. 19–28). [3] M.E. Davis, Nature 417 (2002) 813–821. [4] D. Bathen, M. Breitbach, Adsorptionstechnik, first ed., Springer, Berlin, 2001. [5] R. L. Taggart, G. L. Ribaud, US Patent 3119 659 1964. [6] R. J. Nozemack, C. W. Chi, J. J. Schwonke, US Patent 4381 255 1983. [7] L. Puppe, G. Ulisch, DE Patent 3401 485 1985. [8] A. Brandt, J. Schmeißer, B. Unger, H. Tschritter, U. Henkel, B. Gojdar, D. Gruhle, G. Winterstein DE Patent 10 2008 046 155 2009. [9] 22nd German Zeolite Conference, Munich, March 3-5, 2010, Kristin Schumann, Alfons Brandt, Baldur Unger, Jens Schmeißer, Franziska Scheffler, Formation of binderless 13 zeolite shapes, Book of Abstracts, Munich, 2010, 207–208. [10] K. Byrappa, M. Yoshimura, Handbook of Hydrothermal Technology–A Technology for Crystal Growth and Materials Processing, first ed., Noyes Publications/William Andrew Publishing LLC, Norwich, 2001. pp. 354–364. [11] 23rd German Zeolite Conference, Erlangen, March 2-4, 2011, Kristin Schumann, Alfons Brandt, Baldur Unger, Franziska Scheffler, Binderless Zeolite 13 Shapes with Different Morphologies, Book of Abstracts, Erlangen, 2011, pp. 47-48. [12] R. Xu, W. Pang, J. Yu, Q. Huo, J. Chen, Chemistry of Zeolites and Related Porous Material–Synthesis and Structure, first ed., John Wiley & Sons Asia Pte Ltd, Singapore, 2007. pp. 289–294. [13] D.W. Breck, J. Chem. Educ. 41 (1964) 678–689. [14] C. Lutz, P.-G. Schmitt, US Patent 2008/0156190 2008. [15] K. Malek, M.-O. Coppens, J. Chem. Phys. 119 (2003) 2801–2811. [16] F. Bergaya, B.K.G. Theng, G. Lagaly, Handbook of Clay Science, first ed., Elsevier, Amsterdam, 2006.