MgO insertion endowed strong basicity in mesoporous alumina framework and improved CO2 sorption capacity

https://doi.org/10.1016/j.jcou.2020.101294Get rights and content

Highlights

• Highly mesoporous alumina 2D nanoflakes with tunable textural properties were developed.

• A superior textural properties corresponding to >300 m2/g was obtained without the aid of templating agents.

• The incorporation of MgO in to the alumina framework enhance the basic properties.

• The MgO-Al2O3 composites show excellent cyclic stability for elevated temperature CO2 capture.

Abstract

Fine tuning the electronic properties of amorphous mesoporous alumina has been extensively studied due to its wide range of applications. Luckily, fundamental changes via secondary metal atom insertion has been broadly recognized and pave the way towards development of novel materials. In this study, structure-property correlation of mesoporous alumina framework has been evaluated for elevated temperature application via MgO insertion. The pristine Al2O3 possess an excellent specific surface area corresponding to 319 m2/g with average pore diameter of 14.8 nm. However, surface area and pore diameter were linearly decreased as a function of hydrothermal temperature. Remarkably, MgO insertion improved its specific surface area to 357 m2/g with narrow pore size distribution at 7.1 nm. The improved surface area is due to the coexistence of MgO with Al2O3 in its amorphous framework. Meanwhile, the improved CO2 sorption capacity attributed to the induced strong basic sites via MgO insertion and better kinetics is due to the appropriate pore structure of the derived nano-composite. Further, the heterogeneity of the alumina framework with 6-coordinated and 4-coordinated environment established improved basicity as suggested by solid state 27Al-NMR and CO2-TPD results. The developed composite shows excellent thermal stability for temperature swing CO2 desorption with 95% retainment in its sorption capacity.

Introduction

Mesoporous alumina-based materials with tunable textural properties including large specific surface area and narrow pore size distribution have gained greater attention owing to their potential applications as a catalyst support and adsorbent for environmental applications.[1] Due to their remarkable physicochemical properties and promising applications including catalysis, adsorption, energy storage and gas sensing these have attracted extensive studies.[[2], [3], [4], [5], [6], [7], [8]] Considering their wide applicability, numerous strategies have been proposed which mainly focus on the improvement of textural properties. Among the various approaches, hard/soft templating methods have been extensively studied to induce the desired mesoporous structure.[[9], [10], [11], [12]] Though, these approaches have promising outcome including unique crystalline framework and ordered mesopore structure of metal oxides, but still are proved to be inefficient. The main reason for this is, mesopre structure of the metal oxides depend on the structural framework of hard template. Also, removal of the templates requires additional step which often results in the collapse of the mesostructure.

Despite their advantageous textural properties, pristine Al2O3 has limited utilization due to its inherently unreducible nature. Contrarily, alumina has been chosen as a prominent candidate for the development of the various binary composites with diverse physicochemical properties. This offers an opportunity to alter the inherently inert unreducible oxide in to functional oxide with varied basicity and acidity.[13,14] In amorphous alumina framework, the secondary metal atom coexist with inherent defects potentially influenced by local structure and chemical environment. This approach of bonding secondary metal cations in to Al2O3-framework is normally expected to improve their thermal stability and significantly tune the acid/base strength of the resulting composites.[8,15] In order for this to happen, alumina must possess better specific surface area and amorphous nature so that the heteroatom could be efficiently fused. Also, as alumina has different polymorphs such as γ-Al2O3, δ-Al2O3 and α-Al2O3, the desired phase could be obtained via a controlled calcination at different temperatures.[16] Most often, γ-Al2O3 is chosen as candidate of interest to incorporate the secondary metal cations as this phase allows the better distribution of secondary metal cations in its framework while retaining significant surface area and porosity. Nevertheless, this is valid by the amorphous nature of γ-Al2O3.

For elevated temperature CO2 capture, modifying MgO by incorporating alumina, silica or zirconium as stabilizers are worthy to explore. High theoretical sorption capacity (1.09 g of CO2 per gram of MgO) and wide temperature range makes MgO particularly attractive.[[17], [18], [19], [20], [21]] Among the various basic oxides, MgO possess the lowest desorption temperature (<500 °C) which makes it an ideal candidate. However, due to low sorption capacity and sorption decay under temperature swing desorption limited its large scale applications.[19,[22], [23], [24]] The decay in cyclic stability is due to temperature-assisted desorption. Due to formation of stable MgCO3 is highly favored at 200-400 °C, temperature assisted desorption at high temperature (∼500 °C) is utilized, which often results in deterioration in subsequent cycles. Surprisingly, addition of Al2O3 in MgO proved to be advantageous as Al2O3 helps to retain the cyclic stability at high temperature. This is due to the composite structure of MgO-Al2O3 which restricts the high reactivity of MgO and thereby controls its agglomeration at high temperature. Several approaches have been reported to improve the cyclic stability of MgO at high temperature.[[25], [26], [27]] However, addition of Al2O3 still remained as an effective technique.[[28], [29], [30], [31], [32], [33]]

Considering these advantageous features of mesoporous alumina which acts as host for heteroatom and MgO which acts as active site for elevated temperature CO2 capture, a facile synthesis approach is presented via hydrolysis and condensation of butoxide groups. The basicity and basic sites was evaluated by using solid state 27Al-NMR and CO2-TPD. The cyclic stability was evaluated under temperature-swing desorption. Present approach displays a simplified method for the development of nanomaterials for environmental applications. As well, the synthesis method could be easily scalable and opens up new pathways to derive various composites for energy and environmental applications.

Section snippets

Synthesis of 2D intertwined nanosheet Al2O3 via hydrothermal method

2D intertwined nanosheets Al2O3 samples were synthesized via a template-free hydrothermal method via hydrolysis and condensation of metal alkoxide at different temperature. In brief, 25 mM aluminium tert-butoxide (Al[OC(CH3)3]3), technical grade, Sigma Aldrich) was dissolved in 50 ml isopropanol at room temperature. Then, the temperature was raised to 80 °C and the solution was heated under vigorous stirring for 30 min. Subsequently, required amount of DI water was added dropwise under vigorous

Results and Discussion

The facile formation of Al2O3 nanostructure is shown in the Fig. 1(a). The single step hydrothermal precipitation follows hydrolysis and condensation which after calcination yield γ-Al2O3. In order to elucidate this assumption, the surface morphology of Al2O3@120 °C was examined by using the FE-SEM characterization and the corresponding micrographs are shown in Fig. 1(b&c). The sample exhibits a disordered morphology with agglomerates of several nanometers. However, a closure look reveals the

Conclusions

In summary, we have presented a facile, green and template-free synthesis of mesoporous Al2O3 for effective insertion MgO to generate strong basicity. Remarkably, the strong basicity has been endowed eliminating the loss of active surface area. The approach has shown scalable mesoporous structure with excellent surface area corresponding to 315 m2g/ with narrow pore size distribution. Moreover, the insertion of MgO in to mesoporous framework of alumina as heteroatom has shown enhanced surface

CRediT authorship contribution statement

Vishwanath Hiremath: Conceptualization, Methodology, Investigation, Writing - original draft. Bezawit Tatek Shiferraw: Formal analysis, Writing - review & editing. Jeong Gil Seo: Conceptualization, Visualization, Resources, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A2C1090304. This work was also supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2020R1C1C1010085). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2020R1A5A1019131).

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