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

Podsumowanie

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.

Streszczenie

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.

Wprowadzenie

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 using computational and experimental techniques to improve our understanding of the formation and structures thereof⁶. The preparation of nanoscale particles is of particular interest due to their potential to exhibit quantum effects and properties differing from those of their bulk counterparts. Nanogibbsite particles with dimensions on the order of 100 nm are easily prepared under various conditions^{7,8,9,10,11,12,13,14}.

Overcoming inherent challenges associated with reducing the particle sizes further is difficult; therefore, only a few cases exist where nanogibbsite particles have dimensions on the order of 50 nm.^14,15,16,17 To the best of our knowledge, there have been no reports of nanogibbsite particles smaller than 50 nm. In part, this is attributed to the fact that nanoparticles tend to agglomerate due to electrostatic instability and the high probability for the formation of hydrogen bonds between the colloidal particles, especially in polar protic solvents. Our objective was to synthesize small Al(OH)₃ nanoparticles by using exclusively safe ingredients and precursors. In the current work, aqueous particle aggregation was inhibited by incorporating an amino acid (i.e., L-arginine) as a buffer and stabilizer. Moreover, it is reported that the guanidinium-containing arginine prevented aluminum hydroxide particle growth and aggregation to yield an aqueous colloidal suspension with average particle sizes of 10-30 nm. It is proposed here that the amphoteric and zwitterionic properties of arginine mitigated the surface charge of aluminum hydroxide nanoparticles during the mild hydrolysis to disfavor particle growth beyond 30 nm. Although arginine was not capable of reducing the particle size below 10 nm, such particles were achieved by taking advantage of the "cage" confinement effect within the mesopores of MCM-41. Characterization of the Al-MCM-41 composite material revealed ultrafine aluminum hydroxide nanoparticles within the mesoporous silica, which has an average pore size of 2.7 nm.

Protokół

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.

2. Precipitating Al(OH)₃ with NaCl

1. Prepare a GPC column that is 49 in long and 1.125 in diameter. Pack the gel in successive steps of adding gel and allowing water to flow through the column to ensure proper packing, with minimal space between the gel beads. Pack the gel to about 80% of the column; the amount of gel packed varies every time and only affects the retention time of the separated species.
2. Introduce 10 mL of as-synthesized Al(OH)₃ nanoparticle suspension (prepared in step 1.3) into the column using an HPLC pump with a 10 mL injector loop. Custom-make the injector loop using tubing with an external diameter of approximately 0.125 in and a length that is calibrated to deliver 10 mL of injected sample.
3. Collect the column elution in intervals correlating with the dRI peak location. Connect the GPC output to the input of a differential refractive index (dRI) detector.
   NOTE: As separated species come out of the GPC, they appear on the dRI detector as a peak and are then collected in 125 mL bottles. The GPC column produces two well-resolved peaks, which are both collected and analyzed with size-exclusion chromatography (SEC) and elemental analysis (EA) to discern arginine from aluminum species. The total volume collected will depend on size of the GPC column, the total amount of packing material used, and the flow rate of the deionized water used to elute the column.
   1. Collect the majority of the peak 1 fraction over 100 min at a 0.2 mL/min flow rate.
   2. Collect the eluent in 30 min intervals once a peak emerges on the RI detector of the GPC column.
      NOTE: Changing the interval range will change the concentration and purity of the resulting purified peak 1 material. It is better to collect small intervals of the peak at first to determine which portion contains the highest concentration and purity of peak 1 species for a specific column.
4. Prepare 1 wt% of NaCl.
5. Add the prepared NaCl solution dropwise to 10 mL of purified Al(OH)₃ nanoparticles; the material prepared using NaCl precipitation is not used for further experiments.

3. Preparation of Al-MCM-41

1. Activate approximately 1.0 g of MCM-41 at 120 °C under vacuum for 3 h in a vacuum oven.
2. Prepare 50.0 g of aluminum chloride solution by combining 9.6926 g of AlCl₃·6H₂O with 40.3074 g of deionized water.
3. Add 0.7 g of activated MCM-41 to 50.0 g of aluminum chloride solution (prepared in step 3.2).
4. Allow adequate mixing time (1 h) to ensure homogeneity of the AlCl₃ diffused throughout the MCM-41 channels.
5. Add L-arginine to the heterogeneous mixture to an Arg/Al molar ratio of 2.75 under magnetic stirring. Similarly to step 1.2, add the arginine slowly enough so as to allow the instantaneously formed flocculates to redissolve and reduce the clumping of the arginine before continuing the addition.
6. Once homogeneous, heat the mixture at 50 °C for 72 h.
7. Filter the obtained heterogeneous solution using a Buchner funnel, under vacuum and equipped with qualitative 90 mm filter paper circles (or any other appropriate filter papers).
8. Wash the filtered white powder with excess deionized water to ensure the removal of unreacted aluminum chloride, arginine, or water-soluble byproducts from the produced Al-MCM-41 material.

Wyniki

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 5^1...

Dyskusje

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...

Materiały

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