Halogen-Decorated Metal-Organic Frameworks for Efficient and Selective CO₂ Capture, Separation, and Chemical Fixation with Epoxides under Mild Conditions

In the present work, three novel halogen-appended cadmium(II) metal-organic frameworks [Cd2(L1)2(4,4′-Bipy)2]n·4n(DMF) (1), [Cd2(L2)2(4,4′-Bipy)2]n·3n(DMF) (2), and [Cd(L3)(4,4′-Bipy)]n·2n(DMF) (3) [where L1 = 5-{(4-bromobenzyl)amino}isophthalate; L2 = 5-{(4-chlorobenzyl)amino}isophthalate; L3 = 5-{(4-fluorobenzyl)amino}isophthalate; 4,4′-Bipy = 4,4′-bipyridine; and DMF = N,N′-dimethylformamide] have been synthesized under solvothermal conditions and characterized by various analytical techniques. The single-crystal X-ray diffraction analysis demonstrated that all the MOFs feature a similar type of three-dimensional structure having a binuclear [Cd2(COO)4(N)4] secondary building block unit. Moreover, MOFs 1 and 2 contain one-dimensional channels along the b-axis, whereas MOF 3 possesses a 1D channel along the a-axis. In these MOFs, the pores are decorated with multifunctional groups, i.e., halogen and amine. The gas adsorption analysis of these MOFs demonstrate that they display high uptake of CO2 (up to 5.34 mmol/g) over N2 and CH4. The isosteric heat of adsorption (Qst) value for CO2 at zero loadings is in the range of 18-26 kJ mol-1. In order to understand the mechanism behind the better adsorption of CO2 by our MOFs, we have also performed configurational bias Monte Carlo simulation studies, which confirm that the interaction between our MOFs and CO2 is stronger compared to those with N2 and CH4. Various noncovalent interactions, e.g., halogen (X)···O, Cd···O, and O···O, between CO2 and the halogen atom, the Cd(II) metal center, and the carboxylate group from the MOFs are observed, respectively, which may be a reason for the higher carbon dioxide adsorption. Ideal adsorbed solution theory (IAST) calculations of MOF 1 demonstrate that the obtained selectivity values for CO2/CH4 (50:50) and CO2/N2 (15:85) are ca. 28 and 193 at 273 K, respectively. However, upon increasing the temperature to 298 K, the selectivity value (S = 34) decreases significantly for the CO2/N2 mixture. We have also calculated the breakthrough analysis curves for all the MOFs using mixtures of CO2/CH4 (50:50) and CO2/N2 (50:50 and 15:85) at different entering gas velocities and observed larger retention times for CO2 in comparison with other gases, which also signifies the stronger interaction between our MOFs and CO2. Moreover, due to the presence of Lewis acidic metal centers, these MOFs act as heterogeneous catalysts for the CO2 fixation reactions with different epoxides in the presence of tetrabutyl ammonium bromide (TBAB), for conversion into industrially valuable cyclic carbonates. These MOFs exhibit a high conversion (96-99%) of epichlorohydrin (ECH) to the corresponding cyclic carbonate 4-(chloromethyl)-1,3-dioxolan-2-one after 12 h of reaction time at 1 bar of CO2 pressure, at 65 °C. The MOFs can be reused up to four cycles without compromising their structural integrity as well as without losing their activity significantly.</p

Caracterização química e mineralógica de agregados de diferentes classes de tamanho de Latossolos Bruno e Vermelho localizados no estado do Paraná Chemical and mineralogical characterization of the different structure size classes of Red-Yellow and Dusky Red Latosols in Paraná, Brazil

O teor e a forma dos minerais da fração argila são determinantes na definição da morfologia dos agregados do solo. Objetivando estudar a mineralogia da fração argila e as propriedades químicas de diferentes classes de agregados de Latossolos (Latossolo Bruno Ácrico húmico - LBd e Latossolo Vermelho Distroférrico húmico - LVdf) originados de rochas basálticas no Estado do Paraná, coletaram-se amostras indeformadas em diferentes profundidades (horizontes Bw1 e Bw2) em perfis de solos localizados em duas toposseqüências (quatro perfis no LBd e três no LVdf). Após secagem e separação das amostras indeformadas em seis classes de agregados (2-4; 1-2; 0,5-1; 0,25-0,5; 0,105-0,25; < 0,105 mm) determinaram-se os teores de Si solúvel em ácido acético 0,5 mol L-1 e de K, Ca, Mg e Al trocáveis. A fração argila das diferentes classes de agregados também foi estudada por diferentes técnicas: difratometria de raios X, análise térmica e análises químicas. Verificou-se homogeneidade nos teores trocáveis de elementos entre as classes de tamanho de agregados dos horizontes Bw1 e Bw2 dos perfis dos Latossolos Bruno e Vermelho. A intensa e contínua pedogênese dos Latossolos não foi suficiente para homogeneizar a mineralogia da fração argila (característica mais estável que a dos teores trocáveis) dos agregados. A maior variação nos teores de minerais, em função da classe de tamanho dos agregados, foi para o perfil localizado na posição mais alta da toposseqüência do LBd: variação de 35 % nos teores de gibbsita no horizonte Bw1 e 44 % nos teores de caulinita no horizonte Bw2. Considerando, ainda, o efeito da classe de tamanho dos agregados sobre a mineralogia dos horizontes Bw1 e Bw2 do LBd e LVdf, verificaram-se variações em algumas características cristalográficos da goethita e hematita - diâmetro médio do cristal (DMC) e intensidade de substituição isomórfica de Fe por Al - e da caulinita (DMC).<br>The content and shap of clay minerals are important in the definition of soil structure morphology. To evaluate the clay mineralogy and chemical properties of different aggregate size-classes of Latosols (Red-Yellow - LBd and Dusky Red - LVdf) derived from basalt in the state of Paraná, Brazil, soil samples of the Bw1 and Bw2 horizons were collected in four LBd and three LVdf profiles, distributed across two distinct toposequences. Dried and undisturbed soil samples were separated into six size-classes (2-4; 1-2; 0.5-1; 0.25-0.5; 0.105-0.25; < 0.105 mm) and the soluble Si in 0,5 mol L-1 acetic acid and exchangeable K, Ca, Mg and Al contents were determined. The clay fraction extracted from each aggregate size-class was investigated by X-ray diffraction, thermal analysis and chemical analysis. The content of exchangeable elements did not vary among the aggregate size-classes in the Bw1 and Bw2 horizons for Red-Yellow and Dusky Red Latosol profiles. In spite of the high and continuous weathering of these soils the mineralogical characteristics of the aggregate clay fraction were not homogenized. The highest variation in the mineral contents, according to the aggregate size class, was observed for the profile in the highest position of the LBd toposequence; the gibbsite contents varied 35 % in a sample of the Bw1 horizon and the kaolinite contents 44 % in the Bw2 horizon. Additionally, changes in crystallographic characteristics of the goethite and hematite (mean crystal diameter (MCD) and isomorphic substitution level of Fe by Al) and the kaolinite (MCD) were observed among aggregate size classes of the same horizon, for both soils