Silicalite-1 crystals with modified morphology: HRTEM imaging and synthesis of b-oriented films

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Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

SILICALITE-1 CRYSTALS WITH MODIFIED MORPHOLOGY: HRTEM IMAGING AND SYNTHESIS OF B-ORIENTED FILMS Diaz, I.*, Bonilla, G., Lai, Z., Terasaki, 0 . \ Vlachos, D.G.^ and Tsapatsis, M. Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003-3110, USA. Tel: +1-413-577-0136. *E-mail: diaz(a>ecs.umass.edu. ^Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden. ^Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA.

ABSTRACT Silicalite-1 crystals with a modified shape have been synthesized using organic poly cations (dimer and trimer TPA) as zeolite shape modifiers to enhance the relative growth rate along the b-axis (straight channels). The crystal surface microstructure and subsequent identification of the crystallographic faces has been analyzed using High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED). The modified morphology enabled the fabrication of b-oriented silicalite-1 membranes that show the best performance reported up to now in xylene separation. A method based on growth of an oriented seed layer to a well-packed film is also reported here. Keywords: Crystal morphology, silicalite-1 membranes, xylene separation, HRTEM, b-oriented

INTRODUCTION Zeolites are crystalline materials with periodic arrangements of cages and channels of nanometer dimensions. Their tailored structure, stability, and activity have led to a broad variety of applications in industry as molecular sieves, catalysts, adsorbents, and ion exchangers with high capacities and selectivities. Thus, zeolites are important for the production of fuels, petrochemicals, in the efficient use of raw materials, in energy efficiency, and in pollution abatement. Zeolites grown as films can be used as membranes that offer novel opportunities for use as membrane reactors, chemical sensors, and in electronic and thermoelectric applications [1]. In order to further exploit their use as membranes, it is crucial to develop methods to manipulate their preferred orientation. Our research attempts to understand the mechanisms that govern the crystal growth process of silicalite-1 (structure type MFl) and alter aspects of these mechanisms at the molecular level to produce the desired membrane microstructure. Experimental strategies have been developed to bias the crystal and membrane growth towards the desired morphology. In addition to a silica source and water, a pure silica molecular sieve, silicalite-1 (structure type MFI), is synthesized in the presence of a structure-directing agent (SDA), namely an organic amine. These organic SDAs have a pronounced influence on the crystal growth step and, therefore, on the crystal growth rate, size, and morphology. In addition, our previous modelling and simulation studies [2] show that for membranes prepared using the secondary growth technique, the morphology of crystals grown in solution and the resulting membrane microstructure at the same experimental conditions are strongly related. In the secondary growth technique [3], the seed layer can be randomly oriented and the preferred orientation develops as a result of the competitive growth of the crystals. As growth proceeds, crystals are overgrown by adjacent crystals and the number of them extending on the surface decreases progressively. The crystals that survive during growth eventually dominate the overall microstructure and dictate the preferred orientation of the final film. However, from our previous experiments [4], the growth along the b-axis is always the slowest. The newly observed b-elongated crystal shape coupled with the theoretical studies is the basis for a new methodology of the rational design of membranes with a desired orientation and microstructure by tailoring the individual crystal morphology. In this paper, we demonstrate that changing the SDA leads to a fundamentally different silicalite-1 crystal morphology. A High Resolution Transmission Electron Microscopy (HRTEM) study provides clear evidence for crystal shape modification, and allows imaging of surface microstructure. The fabrication of b-oriented silicalite-1 membranes that show the best performance reported up to now in xylene separation, is also reported.

1161 EXPERIMENTAL SECTION Synthesis and characterization of modified MFI crystals In order to study the growth of silicalite-1 crystals, we start with a colloidal suspension of calcined seeds that were initially formed in the presence of monomer TPA [5]. The silicalite-1 particles used for the seeding are -100 nm spherical-like seeds that are characterized using SEM. These seeds are then deposited inside a clear growth solution containing water, the template, and a silica source (Tetraethylorthosilicate, Aldrich). The seeded growth of initially spherical particles is subsequently monitored over time subject to different growth conditions (i.e., temperature, silica/organic ratio, silica/water ratio, pH of the solution, etc.,) and templates. Synthesis temperatures typically range from 90 °C - 200 °C. The crystal shape and individual growth rates of the crystal faces are determined using scanning electron microscopy (SEM). An average of -20 crystals are used to calculate the aspect ratio of the growing crystals. XRD experiments give information on the degree of crystallinity of the collected samples. The secondary growth of silicalite-1 seeds was analyzed for a synthesis composition of 40 Si02: 9 SDA: 9500 H2O: 160 Ethanol at 175 °C for 24 hours, where the SDAs used were either Tetrapropylammonium hydroxide (TPA), Bis-l,6-(tripropylammonium) hexamethylene dihydroxide (dimer-TPA) or Bis-N,N-(tripropylammoniumhexamethylene) di-N, N-propylammonium trihydroxide (trimer-TPA), shown in Figure 1. XRD patterns were collected on a Philips X-Pert system using CuKa radiation. X-ray powder diffraction was performed in a 9/20 geometry. SEM was performed on a JEOL 100 CX microscope operating in SEM mode at 15 kV. The samples were coated with gold before SEM observation. For the transmission electron microscopy experiments, the samples were crushed on an agate mortar, dispersed in acetone and dropped on a holey carbon microgrid. HRTEM images were taken with a JEOL ARM 1250 microscope operating at 1250kV(Cs=1.654mm).

B is-1,6- (tripropylammomum)he xamethylene dihydroxide (Diitier TPA)

3 OH B is-N, N- (tripf opylammoniumhe xam ethylene) di- N, Npropylammonium trihydroxide (Trbtier TPA) Figure 1. Chemical formula of the Structure-Directing Agents (SDA). Membrane preparation and xylene isomer separation For the growth of membranes, we used a seeded growth procedure [6, 7]. A homemade alumina support (pore size 200 nm) [8] was coated with a top layer of mesoporous silica (pore size 2 nm) by using the sol-gel technique developed by Brinker and co-workers [9]. The seeds with particle dimensions of 500 nm by 200 nm by 100 nm were synthesized by hydrothermal growth at 130°C for 12 hours in a mixture with a molar composition of 5Si02: ITPAOH: 500H2O: 20EtOH. They were subsequently washed by repeated centrifugation and decanting, and then they were calcined at a temperature of 525°C for 10 hours before seed

1162 layer deposition. The method of Ha et al. [10] has been followed in order to obtain a uniform and monolayer coverage of the support. For the xylene isomer separation measurements, the feed partial pressure is 0.45 kPa for para-xylene and 0.35 kPa for ortho-xylene. The permeation conditions, setup, and analysis procedure are described in [8] and references therein. RESULTS AND DISCUSSION The MFI type structure is described as a combination of two interconnected channel system with orthorhombic (Pnma) symmetry. The framework forms sinusoidal channels along the direction of the a-axis, interconnected with lOMR straight channels running along the b direction (Figure 2A). A tortuous path is present along the c-direction. Siliceous ZSM-5, silicalite-1, is typically synthesized in the presence of tetrapropylammonium hydroxide (TPA), which acts as a structure-directing agent (SDA) through incorporation at the channel intersections (Figure 2A). The charge distribution, size, and geometric shape of the SDA are believed to induce its structure directing properties [11, 12].

Figure 2. (A) shows a schematic representation of MFI crystal structure. (B) Scanning electron microscopy micrographs of silicalite-1 crystals prepared from TPA, (C) dimer-TPA and (D) trimer-TPA as structure directing agent. The characteristic crystal shape of silicalite-1, using TPA as the SDA, is that of a coffin shape (Figure 2B). Sinusoidal channels run along the a-axis, while straight channels are along the b-axis, the shortest dimension of the crystal. Tortuous channels are present along the c-direction, the longest dimension of the crystal. Systematic studies varying synthesis temperature, silica/TPA, silica/H20 and pH of the solution have been carried out in order to modify the axis length ratio, however, the crystal dimensions are consistently longer along the c-axis and smaller along the b-axis (Lc > La > Lb). The crystallization rate for the dimer and trimer is slower than that of the monomer [13]. In addition, the crystal morphology using the dimer and trimer is extremely different from the observed shape with the monomer. For both dimer and trimer, the hOl and hOO crystal faces present in the characteristic coffin shape

1163 are no longer observed in the SEM images. Instead, we observe fully developed and well-defined elongated oval shaped crystals (Figures 2C and ID, respectively). An La/Lb ratio less than 1 is observed for a variety of synthesis conditions using both the dimer and trimer of TPA. These results are consistent with molecular mechanics simulations [14] reported in the literature. Moreover, our results with the trimer show that under appropriate synthesis conditions the Lc/Lb ratio is approaching 1.0 (Figure 2D). This offers the possibility of synthesizing, for the first time, silicalite-1 crystals with the b-axis as the longest dimension of the crystal. NMR experiments confirm that both the dimer and trimer are occluded intact within the zeolite framework [15]. High resolution transmission electron microscopy (HRTEM) in conjunction with selected area electron diffraction (SAED) allows us to index the crystal faces that are present. As it can be seen in Figure 3, [100] and [010] zone axes of silicalite-1 are readily identified by electron diffraction. In the top figure, the new elongated b-axis is observed when the crystal lies perpendicular to the a-axis, yielding a diffraction pattern indexed as the [100] zone axis. This orientation corresponds to the projection of sinusoidal channels running along the a-axis, as shown in the scheme, and in the experimental HRTEM image (Figure 3). On the other hand, the diffraction pattern obtained when the crystal shows the narrow and flat oval shaped face is recognized as [010], equivalent to the step-free OkO face usually observed in the coffin shaped morphology. This projection is the one that lets us observe the lOMR pore opening of the straight channels running along the b-axis (see Figure 3), which corroborates the desired change in the crystal dimensions and pore alignment. In the experimental HRTEM image, the 5 and 6MR can be easily distinguished thanks to the high resolution of the images. flM

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Figure 3. Low magnification transmission electron microscopy images, selected area electron diffraction patterns, framework scheme and HRTEM experimental images of silicalite-1 dimer-TPA crystals, with oval shape, along [100] and [010] orientations. HRTEM, in addition to providing clear evidence for crystal shape modification, allows imaging of surface microstructure. Figure 4 shows TPA grown silicalite-1 crystals that have the equilibrium shape. The top curved surface is a combination of (101) and (001) crystal faces in such a way that only three unit cells along the a-axis are observed in the top curved edge (Figure 4A). Figure 44B also shows a few (001) + (101) steps that allows for a certain degree of curvature. Figure 4C demonstrates that the most abundant crystal face expressed in the curvature of the coffin shaped crystals is (101), which does not have channels perpendicular to the surface. A step size of c. a. 1.2 nm in height is in good agreement with the terraces observed by Agger et al. [17] and could correspond to the thickness of the pentasil chains measured along [010] zone axis. Only in the proximity of the meeting of the curved and sharp step free (100) face, the (001) planes disappear (Figure 4B), leading again to a more stepped surface, but formed in this case by (100) + (101) steps (Figure 4D).

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Figure 4. HRTEM images and electron diffraction pattern of silicalite-1 crystals, synthesized with TPA, showing coffin shape, along the [010] orientation In contrast, for dimer and trimer TPA grown crystals, mainly (100) planes are present in the curved surface as can be observed in Figure 5C. The curvature in this case is formed only by (100) and (101) planes, with an increasing amount of steps in the proximities of the sharp-pointed edge (Figures 5 A and 5B). The use of a high voltage transmission electron microscope (1250kV), combined with minimal exposure of the specimen to the electron beam, allows for high resolution images of the surface microstructure. However, since the crystals are too thick for quantitative surface analysis, we are conducting further HRTEM

1165 studies of silicalite-1 oval-shaped nanocrystals. We will attempt to relate these findings to a recently reported physicochemical study of silicalite-1 crystal growth [16].

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lOnm Figure 5. HRTEM images of silicalite-1 dimer-TPA crystals with oval shape, along the [010] orientation. Figure 6 shows the membrane morphologies synthesized by the modified seeded growth procedure starting from b-oriented seed monolayers and using trimer-TPA as structure directing agent. The membrane is a polycrystalline dense layer with a thickness of about 1 |im. From the cross section, Figure 6A, the membrane is well intergrown throughout the entire thickness, with no overlap of grains during the secondary growth, which should help to reduce diffusion resistance. Most grains have the oval shape, which is consistent with the crystal morphology grown from the bulk solution. Thus, the flat shape of each grain should be the (010) face, as verified by HRTEM. Therefore, from the top view SEM image in Figure 6B, the membrane is b-oriented.

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Figure 6. (A) Cross section and (B) Top view Scanning Electron Microscopy images of the membrane made by secondary growth of the seed layers at 175 °C. The separation performance of the b-oriented MFI membrane was tested by xylene isomers and the result is shown in Figure 7. This separation is industrially important and has been widely investigated in the past. However, poor separation performances were achieved on those membranes although some of them showed very good selectivity for other gas mixtures such as butane isomers. As an example, Figure 7 compares the xylene separation results on the c- and hOh-oriented MFI membranes as reported in our previous work [1]. Obviously, there is a significant improvement of both the separation factor as well as permeation flux for the b-oriented MFI membranes.

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CONCLUSIONS Use of the Dimer and Trimer of TPA as structure directing agents drastically affects the MFI crystal morphology. High resolution transmission electron microscopy combined with selected area electron diffraction, in addition to providing clear evidence for crystal shape modification, allows imaging of surface microstructure. We found that for dimer and trimer TPA grown crystals, surface termination with (001) planes, that are present in TPA grown crystals, is rare. We found that the curved surface is a combination of (100) and (101) crystal faces. This morphology enabled the synthesis of the first functional b-oriented MFI membranes that exhibit the highest flux and selectivity for xylene isomer separation.

1167 REFERENCES 1. Lai, Z.P., Bonilla, G., Diaz, I., Nery, J.G., Sujaoti, K., Amat, M.A., Kokkoli, E., Terasaki, O., Thompson, R.W., Tsapatsis, M., and Vlachos, D.G., Science, 300 (2003) 456. 2. Bonilla, G., Tsapatsis, M., and Vlachos, D.G., Microporous Mesoporous Mater., 42 (2001) 191. 3. Lovallo, M.C. and Tsapatsis, M., Nanocrystalline Zeolites: Synthesis, Characterization, and Application with Emphasis on Zeolite L Nanoclusters, in Advanced Techniques in Catalyst Synthesis, W.R. Moser, Editor. 1996, Academic Press: San Diego, p. 307. 4. Gouzinis, A. and Tsapatsis, M., Chem. Mater., 10(1998) 2497. 5. Schoeman, B.J., Sterte, J., and Otterstedt, J.E., J. Chem. Soc. Chem. Commun., 12 (1993) 994. 6. Hedlund, J., Sterte, J., Anthonis, M., Bons, A.J., Carstensen, B., Corcoran, N., Cox, D., Deckman, H., De Gijnst, W., de Moor, P.P., Lai, F., McHenry, J., Mortier, W., and Reinoso, J., Microporous Mesoporous Mater., 52 (2002) 179. 7. Lovallo, M.C. and Tsapatsis, M., AIChE J., 42 (1996) 3020. 8. Xomeritakis, G., Lai, Z.P., and Tsapatsis, M., Ind. Eng. Chem. Res., 40 (2001) 544. 9. Lu, Y., Ganguli, R., Drewien, C.A., Andersen, M.T., Brinker, C.J., Gong, W.L., Guo, Y.X., Soyez, H., Dunn, B., Huang, M.H., and Zink, J.I., Nature, 389 (1997) 364. 10. Ha, K., Lee, Y.J., Jung, D.Y., Lee, J.H., and Yoon, K.B., Adv. Mater., 12 (2000) 1614. 11. Lok, B.M., Cannan, T.R., and Messina, C.A., Zeolites, 3 (1983) 282. 12. Burkett, S.L. and Davis, M.E., J. Phys. Chem., 98 (1994) 4647. 13. Beck, L.W. and Davis, M.E., Microporous Mesoporous Mater., 22 (1998) 107. 14. de Vos Burchart, E., Jansen, J.C., van de Graaf, B., and van Bekkum, H., Zeolites, 13 (1993) 216. 15. Bonilla, G., Diaz, I., Tsapatsis, M., and Vlachos, D.G., in preparation (2003). 16. Nikolakis, V., Tsapatsis, M., and Vlachos, D.G., Langmuir, 19 (2003) 4619. 17. Agger, J.R., Hanif, N., Cundy, C.S., Wade, A.P., Dennison, S., Rawlinson, P.A., and Anderson, M.W., J. Am. Chem. Soc, 125 (2002) 830.