Ind. Eng. Chem. Res. 2006, 45, 4977-4984
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Preparation of Zeolite A Tubes from Amorphous Aluminosilicate Extrudates Ays¸ enur O 2 zcan and Halil Kalıpc¸ ılar* Chemical Engineering Department, Middle East Technical UniVersity, Ankara, 06531, Turkey
A method is presented for the hydrothermal synthesis of binderless zeolite A bars and tubes with high purity from amorphous aluminosilicate extrudates. The solid that was separated from a hydrogel was mixed with an aqueous solution of hydroxyethyl cellulose to obtain a paste, which was extruded into bars and tubes. After calcination, amorphous bars and tubes were converted to zeolite A in the liquid phase that was separated from the hydrogel. The zeolite bars and tubes were composed of cubic zeolite A crystals and had only macropores with an average pore size of ∼3 µm besides the zeolite pores. Zeolite A bars exhibited moderate mechanical strength, with a crush strength of 0.4 MPa. 1. Introduction Zeolites, which are versatile materials because of their microporous crystalline structure, have commonly been used in adsorption and catalytic processes. Zeolite A, which is one of the most important commercial zeolites, is used as a water softener in detergents and as an adsorbent in air separation. Zeolites are typically synthesized in powder form. For zeolites to be used effectively, however, most applications require zeolite structures with a certain macroshape, such as beads or pellets,1,2 because fine powder causes a considerable pressure drop in reactors or in adsorption beds. Recently, monolithic structures have also gained much attention, because they provide a high surface area-to-volume ratio, uniform flow distribution, and low pressure drop.3,4 Therefore, zeolite monoliths can potentially be used in adsorption and catalytic processes. Zeolite beads or pellets are prepared commercially by extruding a mixture of zeolite powder and an inorganic binder, such as clays, which provide mechanical stability to the structure during and after preparation.1 Zeolite A, silicalite, and ZSM-5 monoliths were also prepared with this method, using bentonite as binder, and their performances in air separation were evaluated.5-8 However, binders decrease the purity and often reduce the efficiency of the structure by hindering the accessibility to the zeolite crystals.5,9-13 Therefore, great effort has been devoted to the preparation of zeolite structures in a desired geometry without using binders. The binderless zeolite beads were synthesized in the 1960s via the hydrothermal conversion of preformed gel particles14-19 or clay extrudates to zeolite.1,20-22 However, the former is rather intricate and involves many delicate steps. For example, the preformed gel particles were obtained by treating silica sol with an organic liquid to obtain a dense gel, which was then shaped into tiny beads by subsequent dropping it into a shaping oil.14,15,19 Following the shaping, the beads were converted to zeolite via hydrothermal synthesis in aqueous sodium aluminate solutions. The latter, on the other hand, involved hydrothermal conversion of preformed clay beads, mainly metakaolin, to zeolite beads. Afterward, this technique was applied to prepare self-bonded zeolite A, X, and MFI pellets via the hydrothermal conversion of preshaped clay and other natural sources.23-26 However, the type and purity of the zeolite beads were dependent on the composition and crystallinity of the clay mineral and often required adjustment of the composition. * To whom correspondence should be addressed. Tel.: + 90.312.210 4357. Fax: + 90.312.210 1264. E-mail address:
[email protected].
Binderless zeolite beads were also prepared by converting the inorganic binding material to zeolite in beads that contained an aluminosilicate or silicate binder and a type of zeolite, which may act as seed for nucleation. Thus, binderless beads of the same type of zeolite27,28 (or different types of zeolite29,30) were obtained. Recently, many studies have been conducted to prepare binderless zeolite macrostructures with a hierarchical pore structure.31-37 A hierarchical pore structure is desired because it is expected to decrease the diffusional resistance and increase the accessibility to the zeolite pores in catalytic applications.38 The aforementioned studies have essentially focused on the preparation of zeolite beads or pellets with a size of several millimeters. A significant effort has also been devoted to develop methods to prepare larger zeolitesespecially ZSM-5 and silicalitesmacrostructures without a binder. Crea and coworkers39,40 prepared self-supported mordenite and ZSM-5 pellets from dense hydrogels via hydrothermal synthesis. Zeolite structures such as ZSM-5 disks and TS-1 monoliths were synthesized by converting a dry aluminosilicate gel to zeolite in the vapor of water or a mixture of organics.41-44 This method reduced the consumption of organic template molecules and usually yielded high crystallinity. Scheffler and co-workers45,46 prepared monolithic foams of silicalite via the in situ synthesis of silicalite crystals in the pores of a sponge gel, which was then removed by calcination. The foam exhibited a bimodal pore structure that had only micropores and macropores. Zhang et al.47 made monolithic foams by mixing 50-nm silicalite crystals with a starch gel or by adsorbing nanosized crystals in a starch foam. This method was different from the others, because presynthesized zeolite crystals were used to comprise the monolith. A similar approach was followed by Wang et al.48 to prepare silicalite tubes. Recently, Shmizu and Hameda49 proposed bulk materials dissolution technique to synthesize MFI tubes from porous glass tubes, in which a porous quartz tube and SiO2 fabric was converted to MFI in an aqueous solution of tetrapropylammonium hydroxide and hydrogen fluoride. Although the fabric was very fragile after conversion to ZSM5, stable ZSM-5 tubes were obtained with this technique. In the current study, a method was proposed to prepare zeolite A bars and tubes by hydrothermal synthesis of amorphous aluminosilicate extrudates. A hydrogel was filtrated to separate the solid and liquid phases. The solid phase was dried and shaped using hydroxyethyl cellulose (HEC) as an organic binder, which was then removed by calcination before hydrothermal treatment. The liquid phase of the hydrogel was used to convert the extrudates to zeolite A hydrothermally.50,51 The zeolite A
10.1021/ie060011m CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006
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Table 1. Effect of Relative Amounts of Amorphous Powder and Hydroxyethyl Cellulose (HEC) Solutions on the Texture of Paste and on the Properties of Extrudates A/H ratio of the paste
texture of the paste
force applied for extrusion
observations on the state of the extrudates
0.54 0.67 0.82 1
watery normal dry too dry
2 kgf (easy to extrude) 35 kgf (easy to extrude) 52 kgf for bars, 115 kgf for tubes (difficult to extrude) too hard to extrude (balance was broken)
deformed after extrusion have cracks and flaws good shape without cracks and flaws
tubes can be used in adsorption processes such as air separation, and the method can be extended to obtain binderless monoliths of zeolite A and other zeolites, such as zeolite X, and may provide some insights on the crystallization of zeolites. The zeolite A bars and tubes were characterized via X-ray diffraction (XRD) for purity, scanning electron microscopy (SEM) for morphology, and nitrogen adsorption and mercury porosimetry for microstructure. The flexural crush strength of the bars was also measured. 2. Experimental Methods 2.1. Materials. The aluminosilicate hydrogel was prepared using water glass (extra pure, Merck, 0.287Na2O:SiO2: 8.036H2O), sodium hydroxide (pure, NaOH), aluminum hydroxide (pure, Al(OH)3, Merck), and distilled water. The organic binder was 2-hydroxyethyl cellulose (HEC), with an average molecular weight of 720 000 g/mol (Aldrich). Commercial zeolite A powder (Merck, Lot No. 5251610) was used as a reference. 2.2. Preparation of Aluminosilicate Gel: Shaping and Synthesis. To prepare 100 g of hydrogel, water glass (12.5 g) was diluted with 35 g distilled water to obtain a sodium silicate solution. Aluminum hydroxide (5.2 g), sodium hydroxide (5.3 g), and distilled water (42 g) were mixed at room temperature until all sodium hydroxide was dissolved. This mixture was then heated with stirring until the aluminum hydroxide was dissolved. When cooled, the water that had evaporated during the heating was compensated by adding distilled water. The hydrogel that was obtained by adding sodium aluminate solution onto sodium silicate solution had a molar composition of 2.5Na2O:Al2O3: 1.7SiO2:150H2O. The hydrogel was filtrated using Whatman No. 41 filter paper to separate the solid phase from the liquid phase. The solid phase was washed with distilled water until the pH of the filtrate decreased to ∼7.5, and then it dried at 80 °C for 24 h. The solid was equilibrated with water in a desiccator with 86% relative humidity for at least 24 h and then was ground to a fine powder to obtain the aluminosilicate powder that was used for shaping. The liquid phase was kept in plastic cups with tightly closed lids. It was transparent and stable for almost three weeks. The aluminosilicate powder was mixed with 4 wt % HEC solution in a porcelain mortar, and the mixture was kneaded to obtain a paste. The paste was extruded into the form of bars and tubes, using a homemade ram extruder, which was placed on a bathroom scale. The ram extruder and the scale were then placed on the lower leg of a hydraulic press. The piston of the ram extruder was pressed by the upper leg of the hydraulic press. The reading of the bathroom scale increased until the paste started to emerge from the die, and then it remained constant. This reading was recorded as the force needed for extrusion. The extrudates were smashed in the mortar and extruded again to avoid air bubbles and to increase the homogeneity of the paste. This was repeated four times. Bar-shaped extrudates were 20-60 mm in length, with a diameter of 5.85 mm, and tube-
shaped extrudates were 55-60 mm in length with an inside diameter of 3.95 mm and an outside diameter of 9.85 mm. The extrudates were calcined in air at 600, 800, or 1000 °C for 2 h in a muffle furnace. The heating rate was 2 °C/min. The calcined extrudates were then treated at 80 °C for 72 h in autoclaves with polytetrafluoroethylene (PTFE) inserts in the liquid phase that had been obtained by filtering the gel. The ratio of the weight of liquid to the weight of solid extrudate was 14:1 or 28:1 in each autoclave. After hydrothermal treatment, the bars and tubes were washed with distilled water. 2.3. Characterization. The extrudates were crushed for XRD analysis, which was performed with a Philips model PW 1729 X-ray diffractometer with nickel-filtered Cu KR radiation at a scan rate of 0.05°/min. The voltage and current were 30 kV and 24 mA, respectively. The XRD crystallinities were determined via the intensity summation method, using the 12 strongest peaks and commercial zeolite A powder as a reference. SEM photographs were taken with a JEOL model JSM6400 SEM system that was operating at an accelerating voltage of 20 kV. The samples were broken into small pieces, and one of the pieces was analyzed via SEM without any further treatment. The samples for SEM analysis were coated with gold before analysis. The mercury porosimetry measurements were conducted using a Poremaster 60 porosimeter (Quantochrome Corporation) in the high-pressure region, where the maximum pressure was 55 000 psi. The nitrogen adsorption measurements were performed using Quantochrome Corporation Autosorb-1C/MS equipment at 77 K. For nitrogen adsorption measurements, the tubes and bars, which were synthesized in sodium form, were turned into calcium form by ion exchange in a 1 M CaCl2 solution. Before the mercury porosimetry and nitrogen adsorption measurements, all samples were dried at 673 °C for 3 h, and outgassing was performed additionally at 300 °C for 1.5 h before nitrogen adsorption. The chemical compositions of liquid and solid phases were determined with inductively coupled plasma-mass spectroscopy (ICP-MS) (Perkin-Elmer model DRC II). The flexural crush strength of the bars was measured using a Lloyd model NR30K strength test machine with the threepoint loading technique. A bar is placed on two supports and force is applied on the bar as the third point (midpoint). The length of the bending span was 24 mm. 3. Results and Discussion 3.1. Texture of the Paste. Bars and tubes were obtained from a paste that had been prepared by mixing HEC solution and aluminosilicate powder that had been separated from the hydrogel. Table 1 summarizes the effect of the A/H ratio on the texture of the paste (where A is the weight of the amorphous aluminosilicate and H is the weight of 4 wt % HEC solution in the paste) and the properties of the extrudates. As the A/H ratio decreased, the force needed for extrusion decreased; however, the extrudates were weak and deformed easily. The force needed for extrusion indicates the consistency (or viscosity) of the paste. Apparently, as the viscosity of paste increases, the force for flow of paste through the extruder die increases.
Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4979 Table 2. Percent Crystallinity of Zeolite A Macrostructures Prepared with Different A/H Ratios, Structures, Calcination Temperatures, and Liquid/Solid Ratios
Figure 1. X-ray diffraction (XRD) patterns of (a) a dried bar at 25 °C for 24 h, (b) a calcined bar at 600 °C for 2 h, (c) a zeolite A bar hydrothermally converted at 80 °C for 72 h, (d) a zeolite 4A tube, and (e) zeolite 4A powder obtained from the initial hydrogel. The lines show the XRD pattern reported by the International Centre for Diffraction Data (ICDD) (Powder Diffraction File Card No. 39-0222).
Figure 2. Photographs of zeolite 4A bars and tubes with different lengths and diameters. All were prepared from pastes with an A/H ratio of 0.82, calcined at 600 °C, and converted to zeolite 4A at 80 °C, after 72 h in an autoclave, with a liquid/solid ratio of 14:1.
Self-standing extrudates with the smoothest surface were obtained from pastes with an A/H ratio of 0.82. The force applied for extrusion of this paste into bars and tubes was ∼52 and ∼115 kgf, respectively, as shown in Table 1. The lateral surface area that was in contact with the paste was higher in the tube extruder (8.8 cm2) than in the bar extruder (1.9 cm2). As the paste was exposed to more friction in the tube extruder, the force needed for extrusion increased. 3.2. Crystallization of Binderless Zeolite A Bars and Tubes. Figure 1e shows the XRD pattern of the powder obtained from the initial hydrogel by hydrothermal treatment at 80 °C for 24 h and the pattern of zeolite A reported by the International Centre for Diffraction Data (ICDD) (PDF File Card No. 390222). The product of the hydrogel contains only zeolite A. The extrudates were amorphous, with a hump that was characteristic to the amorphous aluminosilicates centered at a Bragg angle of 28° after drying at room temperature, as shown in Figure 1a. No phase change occurred after calcination at 600 °C, and at 800 °C, all extrudates were amorphous but with a smaller hump (see Figure 1b). After hydrothermal treatment at 80 °C for 72 h with a liquid/solid ratio (by weight) of 14:1 in the autoclave, zeolite A was the only crystalline phase in the bars and tubes (see Figures 1c and 1d, respectively). Figure 2 shows photographs of the zeolite A macrostructures. Because the organic binder was removed from the structure by
A/H ratio of the paste
shape of extrudate
calcination temp (°C)
liquid/solid ratio (by weight) in autoclave
crystallinity (%)
0.54 0.67 0.82 0.82 0.82 0.82 0.82
bar bar bar bar bar tube tube
600 600 600 800 1000 600 600
14 14 14 14 14 14 28
87 90 90 62 nepheline 90 96
calcination before hydrothermal treatment, zeolite A bars and tubes had no binding material in their structure. No flaws or cracks were observed on the surface of most tubes and bars other than a slight bending, which can be addressed by improving the design and operation of the extruder. Nevertheless, extrusion is a useful technique for shaping ceramics, because of its simplicity and flexibility, so that the method can be furthered to prepare more-complicated zeolitic macrostructures, such as honeycomb monoliths that have potential for use in adsorption processes.5 The XRD crystallinities of zeolite A structures are shown in Table 2. No difference is discerned in the crystallinities of the bars that were prepared with three different A/H ratios: all were highly crystalline zeolite A. Similarly, the crystallinities of the zeolite A bars and tubes prepared from extrudates calcined at 600 °C were similar, regardless of the liquid/solid ratio in the autoclave. However, the calcination temperature strongly influenced the crystallinity. Zeolite A was the only crystalline phase in the bars that were synthesized from extrudates calcined at 800 °C, as shown in Figure 3, with a crystallinity of only 62%, despite being subjected to the same period of crystallization as the bars that were synthesized from extrudates calcined at 600 °C, indicating that a significant amount of amorphous aluminosilicate remained in the bars after hydrothermal treatment. Calcination at 1000 °C resulted in conversion of amorphous extrudate to mainly nepheline (NaAlSiO4), while preserving its shape. Zeolites in powder form are typically crystallized from hydrogels that are composed of solid and liquid phases, and the composition of hydrogel determines the type of zeolite to be formed.1,41 In the present study, the liquid phase in the hydrogel was separated from the solid phase by filtration. The solid phase was washed with distilled water and dried to obtain the solid powder, which was then used for shaping. Table 3 shows the chemical compositions of the liquid solution and solid
Figure 3. XRD patterns of zeolite A bar synthesized at 80 °C for 72 h after calcining the bar (a) at 600 °C, (b) at 800 °C. Panel c shows the pattern of the bar calcined at 1000 °C, and panel d shows the XRD pattern of commercial zeolite A powder.
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Table 3. Chemical Compositions of Solid Phase (Separated from the Hydrogel, Washed with Distilled Water, and Dried To Prepare the Extrudates) and the Liquid Phase Composition sample
Al
Na
1 2
8193 ( 87 ppm 7867 ( 75 ppm
Liquid Phase 32910 ( 507 ppm 31720 ( 447 ppm
1 2
12.8% ( 0.2% 11.1% ( 0.1%
Solid Phase 10.4% ( 0.2% 8.9% ( 0.1%
Si 282 ( 4 ppm 274 ( 1 ppm 18.2% ( 0.1% 17.8% ( 0.2%
powder; the chemical composition for the liquid corresponds to 4.72Na2O:Al2O3:0.066SiO2, and that for the solid corresponds to 0.95Na2O:Al2O3:2.65 SiO2 (in dry basis). During the hydrothermal conversion of amorphous extrudates to zeolite A, the compositions of liquid solution and solid extrudate were similar, regardless of the relative amounts of liquid and solid in an autoclave, because both were obtained from the hydrogels with the same batch composition. In addition, the solid phase was not distributed uniformly in the liquid phase, as opposed to powder synthesis. All of these observations suggest that the conversion of amorphous extrudates to zeolite A is likely to be dependent on the local composition in and around the solid extrudate, hence, the compositions of solid and liquid phases.51,52 3.3. Crystal Morphology in the Zeolite A Bars and Tubes. Figure 4 shows fractured cross-sectional SEM images of the bars that were prepared from the paste with an A/H ratio of 0.82. Each sample was obtained after a step as the bars were going through drying at room temperature, calcination at 600 °C, and synthesis with a liquid/solid ratio of 14:1. The dried
bar has a very porous, spongelike appearance, which also indicates the amorphous structure of the bar. The calcined bar, on the other hand, has a denser appearance, so that the pores cannot be detected visually in the image. Based on mercury porosimetry, a calcined bar has a narrow pore size distribution, with an average pore size of ∼0.1 µm, and a macroporosity of 46%, with an intrusion volume of 0.698 cm3/g. Calcination leads to denser structures as well to ∼10 % shrinkage in the length and diameter of the dried bars. In contrast to calcination, the bar dimensions remained almost the same after crystallization. Zeolite A crystals with their characteristic cubic shape forming a very porous structure can be seen in Figure 4c and 4d. There are no other crystalline phase and amorphous particles, indicating the high purity of bars, which is in agreement with the XRD results. The size of the crystals in the bar varies over a range of 1.6-4.5 µm, with an average of 3.3 µm, which was obtained by measuring the length of the crystal edges of ∼180 crystals from cross-sectional SEM images. A similar crystal morphology was observed in zeolite A bars that have been prepared at different times, which shows the reproducibility of the synthesis method. Figure 5 shows the surface view of outer and inner surfaces of a tube, as well as the cross-sectional view of the tube that was made with a liquid/solid ratio of 14:1. The core of the tube is very porous and consists of twinned and highly interlocked zeolite A crystals with rounded edges. The crystal morphology at both surfaces is very similar to the morphology in the core. However, some gel-like particles, which were probably deposited onto the surface from the liquid phase during the hydrothermal synthesis, can be seen at the outer surface, as opposed to the inner surface and the core of tubes. The crystals in the
Figure 4. Scanning electron microscopy (SEM) images of the bars (a) dried at 25 °C for 24 h, (b) calcined at 600 °C for 2 h, and (c, d) hydrothermally converted to zeolite A at 80 °C for 72 h in an autoclave with liquid/solid ratio of 14:1 at two different magnifications (500× for panel c, 2000× for panel d). The A/H ratio of the paste was 0.82.
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Figure 5. SEM images of a zeolite 4A tube prepared from a paste with an A/H ratio of 0.82. The tubular extrudate was calcined at 600 °C and crystallized in an autoclave with a liquid/solid ratio of 14:1 at 80 °C.
core, the size of which is in the range of 2.8-7.5 µm, with an average of 4.8 µm, are slightly smaller than the crystals at the surfaces. A relatively wide particle size distribution that was observed in both tubes and bars may suggest that they nucleate at different times or that the rate of crystal growth changes from one point to another in the extrudate. For both nucleation and crystal growth, the liquid phase must be absorbed and diffuse into the solid extrudate. Zeolite crystals nucleate at the favored sites where the liquid phase can reach53-55 and grow by consuming the amorphous aluminosilicate that surrounds the nuclei.56 The liquid phase diffuses to the core from both inner and outer surfaces of a tube; however, it diffuses to the core only from a single surface during the synthesis of bars. The slight variations between the crystallization environments inside the tubular and bar extrudates may result in some morphological differences not only between a tube and a bar but also along the cross section of the extrudates.32,44,57,58 Despite the macroporous structure of zeolite A bars and tubes, they were self-standing, because of twinning of the crystals. In addition, assuming that solution-mediated crystal growth occurred,56 the dissolution rate of the amorphous aluminosilicate is likely to be much slower than the rate of crystal growth, so that the extrudates preserved their shapes. Calcination of the extrudates probably increased the stability of the aluminosilicate against a highly alkaline medium and reduced the dissolution rate of extrudates. 3.4. Mechanical Stability of Zeolite A Bars. The crush strength of bar-shaped extrudates increased with the A/H ratio of the paste, and the strongest bars were made at an A/H ratio of 0.82 (Figure 6). The texture of the paste, which is dependent on the A/H ratio, had a significant effect on the mechanical stability of dried bars. As the A/H ratio of the paste was
Figure 6. Average crush strength of bars prepared from paste with different A/H ratios.
increased, drier pastes and hence stronger bars were obtained after drying. The dried bars were also stronger than the calcined and synthesized bars, because of the presence of hydroxyethyl cellulose before calcination. Following the calcination at 600 °C, the bars were very porous and free of binder. Because no sintering occurred at this temperature, the crush strength of the bars decreased with calcination. However, calcination at 800 °C resulted in a drastic increase in the crush strength, probably because of the lower macroporosity of the bars.50 The mechanical strength of the bars increased after hydrothermal treatment, as a result of extensive intergrowth among the crystals. Zeolite A bars prepared from bars calcined at 600 °C had an average crush strength of 0.42 MPa, and those calcined at 800 °C had an average crush strength of 2.47 MPa. Nevertheless, calcination at 800 °C yielded zeolite A bars with lower purity. Lower macroporosity, obtained by calcination at a higher temperature, introduces additional diffusion resistance to the liquid phase during hydrothermal crystallization and increases the alkaline stability of the bar, which are likely to
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Figure 7. SEM image of zeolite A bar prepared using bentonite as a binder. Zeolite A and bentonite contents of the bar are 85% and 15%, respectively.
Figure 8. Nitrogen adsorption-desorption isotherms for NaA and CaA bars and tubes at 77 K.
result in zeolite A bars with lower crystallinity. It can be deduced that the crush strength of zeolite A bars is dependent on the texture of the paste, the macroporosity of the calcined bar, and the degree of intergrowth among the crystals. For comparison purposes, zeolite A bars were extruded from pastes prepared using commercial zeolite A powder (85%) and bentonite (15%), which is often used as a binder in commercial zeolite beads. The SEM image of this bar (Figure 7) shows that bentonite particles are smeared on the cubic zeolite A crystals, which is very similar to the view of zeolite A monoliths prepared with bentonite by Li et al.5 and commercial zeolite A beads reported by Grande et al.59 The bars that contain bentonite had a crush strength of 0.45 MPa, which is slightly higher than the
Figure 9. Pore size distribution of zeolite NaA bar and tube, determined by mercury porosimetry.
zeolite A bars that had been prepared from aluminosilicate extrudates with an A/H ratio of 0.82 and calcined at 600 °C. 3.5. Microstructure of Zeolite A Bars and Tubes. The Zeolite NaA bar did not adsorb nitrogen: the volume adsorbed is
