Facile Preparation of Ultrafine Aluminum Hydroxide Particles with or without Mesoporous MCM-41 in Ambient Environments

Summary

An ultrafine aluminum hydroxide nanoparticle suspension was prepared via the controlled titration of [Al(H₂O)]³⁺ with L-arginine to pH 4.6 with and without cage-effect confinement within mesoporous channels of MCM-41.

Abstract

An aqueous suspension of nanogibbsite was synthesized via the titration of aluminum aqua acid [Al(H₂O)₆]³⁺ with L-arginine to pH 4.6. Since the hydrolysis of aqueous aluminum salts is known to produce a wide array of products with a wide range of size distributions, a variety of state-of-the-art instruments (i.e., ²⁷Al/¹H NMR, FTIR, ICP-OES, TEM-EDX, XPS, XRD, and BET) were used to characterize the synthesis products and identification of byproducts. The product, which was comprised of nanoparticles (10-30 nm), was isolated using gel permeation chromatography (GPC) column technique. Fourier transform infrared (FTIR) spectroscopy and powder X-ray diffraction (PXRD) identified the purified material as the gibbsite polymorph of aluminum hydroxide. The addition of inorganic salts (e.g., NaCl) induced electrostatic destabilization of the suspension, thereby agglomerating the nanoparticles to yield Al(OH)₃ precipitate with large particle sizes. By utilizing the novel synthetic method described here, Al(OH)₃ was partially loaded inside the highly ordered mesoporous framework of MCM-41, with average pore dimensions of 2.7 nm, producing an aluminosilicate material with both octahedral and tetrahedral Al (O_h/T_d = 1.4). The total Al content, measured using energy-dispersive X-ray spectrometry (EDX), was 11% w/w with a Si/Al molar ratio of 2.9. A comparison of bulk EDX with surface X-ray photoelectron spectroscopy (XPS) elemental analysis provided insight into the distribution of Al within the aluminosilicate material. Furthermore, a higher ratio of Si/Al was observed on the external surface (3.6) as compared to the bulk (2.9). Approximations of O/Al ratios suggest a higher concentration of Al(O)₃ and Al(O)₄ groups near the core and external surface, respectively. The newly developed synthesis of Al-MCM-41 yields a relatively high Al content while maintaining the integrity of the ordered silica framework and can be used for applications where hydrated or anhydrous Al₂O₃ nanoparticles are advantageous.

Introduction

Materials made of aluminum hydroxide are promising candidates for a variety of industrial applications, including catalysis, pharmaceuticals, water treatment, and cosmetics.^1,2,3,4 At elevated temperatures, aluminum hydroxide absorbs a substantial amount of heat during decomposition to yield alumina (Al₂O₃), making it a useful flame-retarding agent.⁵ The four known polymorphs of aluminum hydroxide (i.e., gibbsite, bayerite, nordstrandite, and doyleite) have been investigated u....

Protocol

1. Al(OH)₃ Nanoparticle Synthesis

1. Dissolve 1.40 g of aluminum chloride hexahydrate in 5.822 g of deionized water.
2. Add 2.778 g of L-arginine to the aqueous aluminum chloride solution while under magnetic stirring. Add the L-arginine slowly, so that the added arginine dissolves and does not form large clumps or chunks; furthermore, a slow addition reduces local concentrations of alkalinity and provides conditions for a more controllable hydrolysis.
3. Once all the arginine dissolves into the solution, heat the solution for 72 h at 50 °C; at this point, the solution may appear as a cloudy suspension.

Results

Nanogibbsite Synthesis

Nanogibbsite was prepared by titrating AlCl₃·6H₂O (14 wt%) with L-arginine to a final Arg/Al molar ratio of 2.75. The synthesis of nanogibbsite particles was monitored via SEC, which is a widely used analysis technique for partially hydrolyzed aluminum chloride solutions, capable of discerning five domains arbitrarily designated as peaks 1, 2, 3, 4, and 5¹

Discussion

The preparation of an aqueous aluminum chloride solution entailed the use of a crystalline hexahydrate salt of aluminum chloride. Although the anhydrous form can also be used, it is not preferred due to its significant hygroscopic properties, which make it difficult to work with and to control the concentration of aluminum. It is noteworthy that aluminum chloride solution should be used within several days of preparation because over time, the [Al(H₂O)₆]³⁺ aqua acid hydrolyzes to yield un.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
aluminum chloride hexahydrate	Alfa Aesar	12297
L-arginine	BioKyowa	N/A
aluminum hydroxide	Sigma Aldrich	239186
Bio-Gel P-4 Gel	Bio-Rad	150-4128
Mesoporous siica (MCM-41 type)	Sigma Aldrich	643645