Recent Progress in Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
Shaping of mesoporous molecular sieves Martin Hartmann a *, Sebastian Kunz a , G. Chandrasekara>b and V. Murugesan b
"Advanced Materials Science, University of Augsburg, 86135 Augsburg, Germany Department of Chemistry, Anna University, Chennai 600025, India
1. Intoduction Mesoporous materials such as SB A-15 are synthesized as amorphous powders (particle size 0.1 to 10 urn) and invariably need to be shaped into bodies such as granules, spheres and extrudates prior to their use in fixed-bed adsorbers or reactors. Particle shaping is a complex procedure involving several steps viz. compounding / mixing, shaping, drying and calcination. In the present contribution, SBA-15 powders are shaped into extrudates and the influence of additives such as binder, macropore builder and cross-linking agent on the mechanical strength of the extrudates is tested. Mechanical strength is one of the key parameters for the reliable industrial application of a solid catalyst. Failure of catalyst strength in a fixed-bed reactor causes maldistribution of fluid flow and a large pressure drop through the bed, which results in a low efficiency and (in serious cases) plant failure. Solids catalysts formulations containing zeolites and mesoporous materials are porous and full of defects, dislocations and discontinuations in their bulk phase. The flaws are in the same range of size and nature as the micro-cracks defined by fracture mechanics , which states that expanding of microcracks under tensile stress concentrated around the edges of the flaws is the primary reason for fracture . Variations in size shape and orientation of flaws results in a large scattering range of strength data of catalysts. Therefore, the mechanical strength data have to be treated based on statistical analysis. 2. Experimental Section The synthesis SBA-15 was performed employing optimized procedures as described elsewhere . All mesoporous materials were characterized by XRD (Siemens D5005), nitrogen adsorption (Quantachrome Autosorb 1 sorption analyzer), mercury porosimetry (Micormeritics) and thermogravimetric analysis
(Setaram) before and after shaping into extrudates. The mesoporous material SBA-15 was mixed with bentonite or kaolin as a binder and methyl cellulose in order to controle the macropore structure of the extrudate after calcination. TEOS was added as a cross-linking agent. A typical paste composition is as follows: 7.5 g SBA-15 : 2.0 - 6.0 g bentonite : 1.2 - 6.0 g methyl cellulose : 2.0 - 12.0 g TEOS : 35 - 40 g H 2 O. The components were mixed in ThermoHaake Polylab Rheomix instrument, extruded into cylinders with a diameter of 3 mm, dried at 100°C and subsequently calcined for 24 h at 550°C in air.The vertical crushing strength of the cylinders was measured by Mecmesin strength tester. Prior to the test, the extrudates were cut into discs with a thickness of 2 mm. 3. Results and Discussion
Intensity / ((arb. arb. u nits ) units)
Figure 1 exemplary shows the X-ray diffraction patterns of the SBA-15 extrudates before and after calcination in comparison to the parent powder. The SBA-15 extrudates exhibit at least three well defined reflections, which are somewhat broader than those of the powder sample. The nitrogen adsorption isotherms (not shown) of the powder sample and the extrudates both exhibit type IV isotherms characteristic of well-ordered mesoporous materials. The influences of the different paste components such as bentonite and methyl cellulose on the mechanical strength were evaluated and the results are depicted in Figure 2. The vertical crushing of a disc can be used as a diagnostic test for the mechanical strength of a shaped particle . The method is based on the relationship between the tensile stress and the loading based on elastic theory.
Angle 22 θ/°
Figure 1: XRD patterns of SBA-15 powder, SBA-15 extrudates (mbentonite/mSBA-i5 = 0.5) and calcined extrudates (top to bottom).
The tensile stress cr/i/for a plane-faced disc-like specimen with limited height can be calculated using equation (1) OM = 2-P I (n-d-l), where P denotes the crushing force, d and 1 the diameter and the length of the extrudate, respectively. The tensile stress is increasing with rising amount of binder bentonite (Figure 2a) and the macropore builder methyl cellulose (Figure 2b). However,
simultaneously the specific pore volume and the specific surface area are decreasing due to the increasing amount of (nonporous) binder. Therefore, for industrial use, a compromise between mechanical strength and specific surface area has to be found depending on the targeted application.
4 6 8 10 Amount of bentonite / %
Amount of methyl cellulose / %
Figure 2: Effect of the amount of bentonite (left) and methyl cellulose (right) addition on the crushing strength of the SB A-15 extrudates.
It has been shown that the failure distribution of the crushing strength measurements does not fit a Gaussian-distribution, but rather a so-called Weibull distribution . The two-parameter Weibull equation (2)
was used for the correlation of our data. In Eq. (2), F(crM) is the probability of failure, uM is the maximum tensile stress within the specimen, fi0 a size parameter and m the Weibull modulus. Combining Eqs. (1) and (2), we obtain (3) with P = (3o (2/n-d-[)m. Rearranging and taking the natural logarithm of both sides of equation (3) results in
In order to prove that the stress follows a Weibull distribution, 110 extrudates of one sample (mbentonjte/mSBA-i5 = 0.5) were measured. In Figure 3, the exponential fit as well as linear fit of the Weibull distribution are shown. The Weibull para-meters m = 4.2 and b = 0.15 (from linear regression analysis) confirm a sufficient mechanical strength but indicate a rather broad strength distribution. The failure probability is already ca. 10 % at a crushing stress of 0.9 MPa and increases to 50% at a = 1.44 MPa. In contrast, conventional analysis gave a crushing stress of 1.7 MPa with standard deviation of 0.2 MPa (20 extrudates).
crushing stress a I MPa
Figure 3: Weibull distribution curve (left) and Weibull plot (right) of the VCS data.
4. Conclusion The influence of bentonite and methyl cellulose content on the mechanical stability of SBA-15 extrudates is studied. The statistical treatment reveals that the VCS data of SBA-15 extrudates scatter in rather large ranges, which is an intrinsic property inherited from the brittleness of the solid porous material and the fracture nature of the strength failure. 5. Acknowledgement Financial support of this work by Deutsche (Ha2527/4-2) is gratefully acknowledged.
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