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Fast Synthesis of Gibbsite Nanoplates and Process Optimization Using Box-Behnken Experimental Design † ‡ § † † Xin Zhang,*, Xianwen Zhang, Trent R

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Fast Synthesis of Gibbsite Nanoplates and Process Optimization Using Box-Behnken Experimental Design † ‡ § † † Xin Zhang,*, Xianwen Zhang, Trent R Article Cite This: Cryst. Growth Des. 2017, 17, 6801−6808 pubs.acs.org/crystal Fast Synthesis of Gibbsite Nanoplates and Process Optimization using Box-Behnken Experimental Design † ‡ § † † Xin Zhang,*, Xianwen Zhang, Trent R. Graham, Carolyn I. Pearce, B. Layla Mehdi, ∥ † † † ⊥ † Alpha T. N’Diaye, Sebastien Kerisit, Nigel D. Browning, Sue B. Clark, , and Kevin M. Rosso*, † Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡ School of Automobile and Transportation Engineering, Hefei University of Technology, Hefei, Anhui 230009, China § ⊥ The Voiland School of Chemical and Biological Engineering and Department of Chemistry, Washington State University, Pullman, Washington 99164, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States *S Supporting Information ABSTRACT: Developing the ability to synthesize composi- tionally and morphologically well-defined gibbsite particles with high yield is an ongoing need that has not yet achieved the required level of rational design. Here we report optimization of a clean inorganic synthesis route based on statistical experimental design examining the influence of Al(OH)3 gel precursor concentration, pH, and aging time at temperature. At 80 °C, the optimum synthesis conditions of gel concentration at 0.5 M, pH at 9.2, and time at 72 h maximized the reaction yield up to ∼88%. The resulting gibbsite product is composed of highly uniform euhedral hexagonal nanoplates within a basal plane diameter range of 200−400 nm. The independent roles of key system variables in the growth mechanism are considered. On the basis of these optimized experimental conditions, the synthesis procedure, which is both cost-effective and environmentally friendly, has the potential for mass production scale-up of high quality gibbsite material for various fundamental research and industrial applications. ■ INTRODUCTION systems such as nuclear waste, gaining precise control of α particle crystallinity, size, morphology, and structural perfection Gibbsite, -Al(OH)3, is an important natural and industrial 3,11−21 material that is used in a wide variety of applications including of crystallite facets is critical. Thus, there is an ongoing flame retardants, adsorbents, polishing agents, fillers, coating need for developing the ability to synthesize gibbsite materials − materials, and as a rheology enhancer.1 5 Gibbsite has also that are nanoscale with uniform physical properties. been used in the biomedical industry, for instance, as a vaccine Gibbsite particles tend to adopt a platelet morphology. Gibbsite has a hexagonal crystal structure, comprised of quasi- adjuvant and a substrate for direct treatment of stomach 3+ diseases.6,7 Furthermore, gibbsite is one of the most important two-dimensional layers of edge-sharing octahedral Al (OH)6 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. precursors for the preparation of various alumina products, sheets each having two-thirds of the octahedral cavities α occupied, and sheets are weakly bound together along the c- Downloaded via PACIFIC NORTHWEST NATL LABORATORY on December 27, 2018 at 17:02:35 (UTC). such as high surface area -Al2O3, which have a broad range of applications in the ceramics and metallurgical industries and as axis by hydrogen bonding (Figure S1). This structure thus adsorbents or supports for catalysts.3,8 Finally, gibbsite is a tends to feature a dominant (001) basal surface bound by relatively stable (110) and (010) edge facets and less stable major constituent in high-level nuclear waste stored in large 22 quantities at the Hanford Site, Washington, USA, and at the (100) facets. Savannah River Site, South Carolina, USA, with future Synthesis is achievable through various routes. Typically it is prepared based on the industrial process of manipulating Bayer processing plans dependent on developing a predictive 14,16,19 understanding of the growth and dissolution behavior of this liquors. Such gibbsite, usually several to hundreds of 9 micrometers in size, shows very rough surfaces at the material in particular. 14,16 In most cases, the chemical behavior of gibbsite in these microscale. Because Bayer liquors entail very high systems depends strongly on its physical properties, such as concentrations of sodium aluminate and typically contain particle size and shape. For example, in applications as an impurities, these undesirable components comprise a large adsorbent, rheology enhancer, vaccine adjuvant, or as a α precursor for the preparation of high surface area -Al2O3,it Received: October 2, 2017 is desirable for the gibbsite to be nanoscale.3,8,10,11 Developing a Revised: November 3, 2017 fundamental understanding of gibbsite reactivity in complex Published: November 13, 2017 © 2017 American Chemical Society 6801 DOI: 10.1021/acs.cgd.7b01400 Cryst. Growth Des. 2017, 17, 6801−6808 Crystal Growth & Design Article fraction of the adsorbed residual.14,19 Recently, alternative nm and 100−200 nm, respectively. This short review of synthesis protocols that more specifically emphasize size and previous work highlights the sensitivity of the gel-based shape control have been developed. For example, using more approach to aging time, temperature, and pH. It also dilute sodium aluminate solution as the aluminum precursor, exemplifies the present lack of systematic understanding of and adding a surfactant such as cetyltrimethylammonium the relative importance of these three master variables. Finally, bromide (CTAB) as a particle capping agent, prismatic “bar- with respect to production scale-up of nanoparticles with shaped” gibbsite that is disproportionately extended along the precisely defined sizes and shapes, all the above-mentioned c-axis and features prominent edge facets can be prepared methods have drawbacks related to the apparent inverse hydrothermally.23 However, the size of these bar-shaped correlation between synthesis time and particle size. particles (longest dimension along c-axis) was up to several A systematic study of the effects of major synthesis variables micrometers, with a specific surface area too low for most is required to achieve high yields while retaining size and shape applications. control. In this study, we take advantage of statistical The sensitive control of gibbsite physical properties available experimental design via the Box-Behnken strategy28,29 to gain via synthesis with organic additives has led to exploration of insight into the most important synthesis variables with respect these effects with promising albeit empirical results with to size, shape, and yield. The result of this effort is a simple and unpredictable yields. For example, particles approaching the optimized hydrothermal method to fast-synthesize euhedral nanoscale can be prepared by a simple thermal treatment of hexagonal gibbsite nanoplates with diameters from 200 to 400 aluminum alkoxides mixed with hydrochloric acid nm and thicknesses from 8 to 20 nm. The synthesis approach 11,20,21,24 (HCl). Generally, the procedure entails two steps: consists of reacting a Al(OH)3 gel precursor, prepared by · ° (1) obtain a clear sol via mixing an aqueous solution of adding NaOH (aq.) to Al(NO3)3 9H2O (aq.), at 80 C for 72 aluminum alkoxides with HCl followed by stirring for 7−10 h. The effect of aging temperature and time, pH and precursor days at room temperature; (2) heat the sol solution to 100 °C concentration on the size and morphology of gibbsite and for around 3 days to precipitate gibbsite with a yield of 41− product yield was studied in detail. Furthermore, the product 66%. Typical products are nanoplates approximately 200 nm in yield achievable via this synthesis route was maximized to 88% diameter. However, this procedure is slow, consuming more through statistical experimental design. Confirmatory evidence than 10 days. A modified method shortens the synthesis period for the formation of high-quality gibbsite nanoplates was to 4 days by using a single aluminum alkoxide as the precursor obtained using powder X-ray diffraction (XRD), scanning and introducing an intermediate preheating step to accelerate electron microscopy (SEM), scanning/transmission electron the precursor’s hydrolysis/peptization.3 But, unfortunately, microscopy (S/TEM), synchrotron X-ray absorption spectros- although again nanoplates approximately 200 nm in size result, copy (XAS), and atomic force microscopy (AFM). the yield decreases to less than 30%. Incidentally, platelets of larger size (up to 1 μm) could be prepared by repeating the ■ EXPERIMENTAL METHODS seed growth procedure (step two above). However, in all these Synthesis of Gibbsite Nanoplates. Samples were grown using a cases with organic additives, similar to the case for CTAB, there · ≥ hydrothermal method. Al(NO3)3 9H2O( 98%, Sigma-Aldrich) was is a purity concern for the platelet surfaces due to organic dissolved into deionized water under stirring to form a homogeneous precursor residue. solution with a concentration of 0.25 M at room temperature, followed For clean inorganic routes, the most flexible approach entails by addition of 1 M NaOH (≥98%, Sigma-Aldrich) aqueous solution to use of amorphous aluminum hydroxide [Al(OH)3] gel as the adjust the pH to around 5.0. After continuous stirring for 1 h, the precursor and control of the properties of the gibbsite product solution was centrifuged to collect gel-like precipitates. The gel was through aging time, temperature, and solution
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