Ordered Mesoporous Alumina with Tunable Morphologies and Pore Sizes for CO₂ Capture and Dye Separation

Abstract

We describe a versatile and scalable strategy toward long-range and periodically ordered mesoporous alumina (Al 2 O 3 ) structures by evaporationinduced self-assembly of a structure-directing ABA triblock copolymer (F127) mixed with aluminum tri-sec-butoxide-derived sol additive. We found that the separate preparation of the alkoxide sol−gel reaction before mixing with the block copolymer enabled access to a relatively unexplored parameter space of copolymerto-additive composition, acid-to-metal molar ratio, and solvent, yielding ordered mesophases of two-dimensional (2D) lamellar, hexagonal cylinder, and 3D cage-like cubic lattices, as well as multiscale hierarchical ordered structures from spinodal decomposition-induced macro-and mesophase separation. Thermal annealing in air at 900 °C yielded well-ordered mesoporous crystalline γ-Al 2 O 3 structures and hierarchically porous γ-Al 2 O 3 with 3D interconnected macroscale and ordered mesoscale pore networks. The ordered Al 2 O 3 structures exhibited tunable pore sizes in three different length scales, <2 nm (micropore), 2−11 nm (mesopore), and 1−5 μm (macropore), as well as high surface areas and pore volumes of up to 305 m 2 /g and 0.33 cm 3 /g, respectively. Moreover, the resultant mesoporous Al 2 O 3 demonstrated enhanced adsorption capacities of carbon dioxide and Congo red dye. Such hierarchically ordered mesoporous Al 2 O 3 are well-suited for green environmental solutions and urban sustainability applications, for example, hightemperature solid adsorbents and catalyst supports for carbon dioxide sequestration, fuel cells, and wastewater separation treatments.

Ultrafast Crystallization of Ordered Mesoporous Metal Oxides and Carbon from Block Copolymer Self‐Assembly and Joule Heating

“…SAXS indicates NMS-1-resin retained the p6mm symmetry with a smaller d-spacing of 13.4 nm due to thermal shrinkage (sample (ii) in Figure 2a), while the absence of peaks in the WAXS pattern indicates an amorphous Al 2 O 3 -resin material (sample (ii) in Figure 2b). [21] We then applied Joule heating on the cured Al 2 O 3 -resin monoliths (SNAP method). SAXS (Figure 2a) and transmission electron microscopy (TEM) (Figure 3a) show the crystalline γ-Al 2 O 3 -carbon monoliths retained their p6mm symmetry after Joule heating at 1155 °C for annealing dwells of 1 to 25 s, accompanied by the decrease of d-spacing values by 18% to ≈11 nm (samples (iii) to (v) in Figure 2a).…”

“…We speculate the BCP-derived carbon acts as a rigid support during Al 2 O 3 crystallization, preserving the mesoscale order and macroscale monolithic shape (Figure S3, Supporting Information). [6,8,27,28] It should be noted that obtaining similar-sized crystallites by conventional thermal annealing would require ≈18 h for crystallization into γ-Al 2 O 3 at 900 °C, [21] that is, more than five orders of magnitude longer compared to the SNAP dwell of <5 s.…”

“…Highly crystalline, ordered mesoporous oxide and carbon materials generated by structure-directing surfactants and block copolymers (BCPs) are desirable for their tunable periodic morphologies and multifunctional properties, [1][2][3][4][5][6][7][8][9][10] and have been broadly deployed in many applications ranging from catalysis, [11,12] energy conversion and storage, [13][14][15][16][17][18][19] adsorption and separation, [20,21] sensing, [22,23] and even superconductors. [24,25] In particular, the combination of high crystallinity and ordered mesoscale morphology expands the application space of widely-used industrial materials such as alumina (Al 2 O 3 ), titania (TiO 2 ), and carbon with high surface area and pore volume, pore accessibility and connectivity, pore size, and other functional properties such as thermal stability and electrical conductivity.…”

“…argon, ammonia) for inorganic crystallization, thereby increasing production time and cost. [4][5][6][7][8][20][21][22][23][24] In particular, untamed nucleation and growth of crystallites that occur spontaneously at higher temperatures often result in the collapse of ordered structures. [3,6,8,21] To circumvent some of the limitations, Lee et al reported a two-step thermal annealing approach referred to as combined assembly by soft and hard chemistries (CASH) method to form highly crystalline and ordered mesoporous TiO 2 and niobia (Nb 2 O 5 ) structures in bulk form.…”

“…[4][5][6][7][8][20][21][22][23][24] In particular, untamed nucleation and growth of crystallites that occur spontaneously at higher temperatures often result in the collapse of ordered structures. [3,6,8,21] To circumvent some of the limitations, Lee et al reported a two-step thermal annealing approach referred to as combined assembly by soft and hard chemistries (CASH) method to form highly crystalline and ordered mesoporous TiO 2 and niobia (Nb 2 O 5 ) structures in bulk form. [6] The first step involved thermal annealing of hybrid monoliths at 700 °C under argon, converting the organic BCP into an in situ carbon scaffold to support mesostructural integrity during oxide crystallization.…”