Ind. Eng. Chem. Res. 2010, 49, 12191–12196
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Increasing the Productivity of Colloidal Zeolite Beta by Posthydrolysis Evaporation Shun-Yi Fang,† Anthony S. T. Chiang,*,† and Hsien-Ming Kao‡ Departments of Chemical & Materials Engineering and Chemistry, National Central UniVersity, Chung-Li, Taiwan, Republic of China
Colloidal beta zeolite is typically synthesized by the reaction of a clear precursor sol at temperatures below 100 °C. The process usually suffers from limited batch productivity with a long reaction time, because of the large amount of water used in the precursor sol. Reducing the water content is expected to shorten the reaction time, enhance the productivity, and reduce the particle size, but proper procedures are required to achieve a supersaturated, but pourable, composition. A precursor sol having a record low H2O/SiO2 ratio has been prepared by the addition of a posthydrolysis evaporation step. Compared to a conventional composition, which took 14 days to produce >100-nm particles at 60% yield of ∼55-nm redispersible zeolite beta particles in only 6 days. The formation of beta zeolite and the evolution of its particle size during the 90 °C reaction confirmed that aggregation always occurred before colloidal zeolite particles could be extracted. On the other hand, the “amorphous” residue was also found to be microporous, suggesting that it might be smaller zeolite particles that were undetectable by XRD and did not settle even with the high-speed centrifugation employed. Introduction Beta zeolite, having a three-dimensional interconnected channel system with 12-membered elliptical openings of 0.64 × 0.76 nm and a porosity of about 42%, is among the most versatile zeolites. It has been employed as the catalyst for the isomerization1,2 or hydroisomerization (with metal loading) of C6-C8 n-alkenes3 and as an adsorbent for the separation of hexane isomers.4 In addition, the recent success in synthesizing beta zeolite as discrete colloidal nanoparticles has opened up possibilities for many new applications, including protonconductive fillers in fuel cell membranes,5 transparent moisture sorbents in organic light-emitting device (OLED) encapsulation,6 low-refractive-index additives in photopolymers,7 antireflection coatings on glass,8 and low-k materials for the microelectronics industry.9-11 To realize these applications, the production of colloidal zeolite beta has to be scaled up to at least the kilogram scale. The best way to synthesis discrete zeolite nanoparticles is through the low-temperature reaction of a clear precursor sol. For beta zeolite, the temperature is typically 100 °C.7,10,12-17 The precursor sol can be prepared by mixing various silica sources [tetraethylorthosilicate (TEOS), colloidal silica, fume silica, silica sol] with a structure-directing agent, typically tetraethylammonium hydroxide (TEAOH), and water at a H2O/ Si molar ratio of 12-20. A long reaction time (5-15 days at 100 °C) is needed, and the zeolite yield is usually low. The mechanism for producing colloidal zeolites from a clear sol is rather complicated, and many studies have been performed to understand it, mostly with diluted TEOS/TPAOH/H2O systems. It is known that a population of 3-5-nm primary particles, whose number density can be estimated based on the quasi-equilibrium model,18 is formed when the concentration of silica in an aqueous tetraalkylammonium solution goes above a critical aggregation threshold. What happens after that remains * To whom correspondence should be addressed. E-mail: stchiang@ cc.ncu.edu.tw. Fax: +886-3-4252296. † Department of Chemical & Materials Engineering. ‡ Department of Chemistry.
controversial.19,20 In general, viable nuclei are produced through some form of aggregation processes, and they then grow through means such as particle addition, internal restructuring, or oriented aggregation, eventually leading to colloidal zeolites. For MFI zeolite, secondary particles become the colloidal zeolite, but Hould and Lobo21 found that tertiary aggregation seemed to be a necessary step before the formation of beta zeolite. If the nuclei were formed by aggregation of the primary particles, one would expect faster nucleation when there was a larger population of primary particles. In other words, a higher SiO2 concentration should lead to accelerated nucleation and maybe smaller zeolite particles. This was indeed observed by Kuechl et al.,22 who reported that the synthesis time was shortened with decreasing H2O/SiO2 ratio. Unfortunately, there was a limit to this ratio, beyond which a “pourable gel” could not be prepared. In fact, the lowest H2O/SiO2 ratio ever reported was 7.3, which was used in the original patent of Verduijn.23 Verduijn prepared a “slightly opaque solution” by boiling silicic acid powder in TEAOH solution for 10 min and produced ∼50nm colloidal beta zeolite after heating the mixture at 70 °C for 14 days. We have learned from previous experience with the TEOS/ TPAOH/H2O system24 that a transparent viscous, but pourable, precursor sol with a low H2O/SiO2 ratio can indeed be prepared, if one hydrolyzes TEOS with excess water first and then removes the extra water through vacuum evaporation. We demonstrate herein that this posthydrolysis evaporation step leads to a clear sol with a H2O/SiO2 ratio as low as 5.6, which produces small zeolite in short reaction times at high yields. Compared to the typical recipe with H2O/SiO2 ) 20, the reduction of the water content also doubles the amount of colloidal zeolite produced for the same batch volume. Therefore, the proposed posthydrolysis evaporation step is advantageous for the large-scale production of colloidal beta zeolite. Experimental Section The effect of the water-to-silica ratio was investigated with two compositions having molar ratios of 1 SiO2/0.02 Al2O3/
10.1021/ie100796d 2010 American Chemical Society Published on Web 10/25/2010
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0.36 TEAOH/x H2O, where x ) 20 or 5.6, corresponding to SiO2 concentrations of ∼13.3 and 29.09 wt %, respectively. The precursor sol was prepared from tetraethylorthosilicate (TEOS, Merck), TEAOH (35%, V. P. Chemicals), aluminum isopropoxide (AIP, >98%, Merck), and deionized (DI) water. For the case with x ) 20, 5.16 g of AIP was first dissolved in 47.82 g of 35% TEAOH and stirred for 3 h. In a separate container, 131.52 g of TEOS was mixed with 47.85 g of 35% TEAOH (pH ) 14.36) and 188.52 g of H2O with 3 h of stirring. These two solutions were then combined and stirred for another 24 h at ambient temperature to evaporate the alcohols, and then divided into several polypropylene (PP) bottles for thermal treatment. To achieve a lower water-to-silica ratio, the above procedure was repeated but with an additional evaporation step to remove excess water, as described previously for the synthesis of silicalite-1.24 The evaporation was conducted with a Buchi R114 Rotavapor System in a perfluoroalkoxy (PFA) flask, operated at
