Effect of structural and thermodynamic factors on the sorption of hydrogen by metal–organic framework compounds

Abstract

UDC 546.03,544.723 S. V. Kolotilov and V. V. PavlishchukThe characteristics of the sorption of hydrogen by metal-organic framework compounds (MOF) were examined, and the structural and thermodynamic factors that favor the sorption of H 2 by such substances were determined. The effect of the structure of the MOF and the size and geometry of the pores on the sorption characteristics was analyzed.The marked worldwide shortage of such sources of natural hydrocarbon fuel as natural gas and petroleum, the need to diversify energy resources in order to reduce modern economic risks, and a series of ecological problems (both real and imaginary) in recent years have stimulated attention to hydrogen fuel as an alternative energy source capable in the coming decades of substantially reducing if not completely displacing hydrocarbon raw material as the main type of fuel in stationary power plants and transportation media. The transition to the new energy resource will bring global consequences in world industry and economics, and this has forced politicians and economists in Europe and the USA to talk about the appearance of a new type of economics -"the hydrogen economy" [1][2][3][4][5]. Considering the strategic importance of this problem the European Commission in 2002 created a high-level group on the technologies of the use of hydrogen and fuel cells. In 2003 this group presented its analysis and proposals, which laid the foundation for the creation of a general European strategy in this field [1,2].Of course, the replacement of one type of energy source by another involves the need to solve a series of scientific and technological problems, the most important of which are the efficient production, purification, storage, and utilization of hydrogen. One of the most important tasks is to create convenient and safe materials and devices based on them for the stockpiling and storage of hydrogen, particularly hydrogen for use in transportation facilities. A leading role in media for the storage of hydrogen at the moment is played by media for the accumulation of hydrogen with various types of adsorbents, which must combine reversibly and repeatedly with as large an amount of hydrogen as possible [6][7][8]. Unfortunately, most types of traditional adsorbents have only slight affinity for hydrogen. The department of energy of the USA has defined the conditions that adsorbents of hydrogen, essential for the economics of the change to hydrogen fuel in the automobile industry, must satisfy. In particular, it was established that for the adsorbents it is necessary to achieve 6 wt.% hydrogen capacity by 2010 and 9 wt.% by 2015 [9][10][11][12].Various types of adsorbents, such as carbon materials (including nanotubes and fullerenes), zeolites, hydrides, and polymeric materials (including electrically conducting materials), were used as the basis for the creation of prospective materials for the stockpiling and storage of hydrogen. However, it was shown by calculations that the upper theoretical limit for the adsorption of hydrogen b...

Structures and sorption properties of the coordination polymers built up of 3d metal carboxylate polynuclear complexes

“…The assessment of hydrogen density in pores of these compounds at the max imum filling gives values 0.016 and 0.020 g cm -3 for 10 and 11, respectively, and equals 23 and 28% of the hydro gen density at 20 K. These values of H 2 density in pores of the coordination polymers are close to those found earlier. 18 In conclusion, in the present work we found the con ditions for the formation of coordination polymers Fe 2 MO(AcO) 6 (bpy) 1.5 ] n (M = Co, Ni) in the reaction of trinuclear acetate complexes Fe 2 MO(AcO) 6 (H 2 O) 3 with 4,4´ bipyridine. It was shown that the prepared compounds [Fe 2 MO(AcO) 6 (bpy) 1.5 ] n are able to adsorb nitrogen and hydrogen.…”

Role of the chemical structure of metal–organic framework compounds in the adsorption of hydrogen

“…In the crystal structure of this MOF there are S-shaped channels in which there are coordinated (to the Ni 2+ ions) and uncoordinated water molecules. It was shown by derivatography that the action of heat on NaNi 3 (L13) 2 (OH)(H 2 O) 5 to 350°C leads to the removal of all the water molecules; during dehydration the green color of the complex changes to yellow-brown [indicating changes in the coordination sphere of the nickel(II)], and the surface area increases sharply -the value of S, determined by the sorption of nitrogen, increases from zero (for the undehydrated sample) to 743 m 2 /g and then decreases to 640 m 2 /g for the samples dehydrated at 300 and 350°C respectively [91]. From the order of the signs it is possible to form an opinion about the presence of the coordination-unsaturated nickel ions in the dehydrated form of the complex -in NaNi 3 (L13) 2 (OH)(H 2 O) 5 there are four coordinated molecules of H 2 O for the three nickel ions, and it is unlikely that all the positions in the coordination spheres of the nickel ions will be occupied by donor atoms after dehydration; moreover, rehydration takes place very quickly and is accompanied by a change in the color of the complex.…”

“…From the order of the signs it is possible to form an opinion about the presence of the coordination-unsaturated nickel ions in the dehydrated form of the complex -in NaNi 3 (L13) 2 (OH)(H 2 O) 5 there are four coordinated molecules of H 2 O for the three nickel ions, and it is unlikely that all the positions in the coordination spheres of the nickel ions will be occupied by donor atoms after dehydration; moreover, rehydration takes place very quickly and is accompanied by a change in the color of the complex. Dehydration leads to the formation of the compound NaNi 3 (L13) 2 (OH) (according to calculation from the loss in weight), the structure of which is evidently similar to the structure of NaNi 3 (L13) 2 (OH)(H 2 O) 5 , although it has not been possible to fully interpret the structure of the dehydrated phase [83]. [83] reached a conclusion about bonding of the hydrogen by the coordination-unsaturated nickel ions.…”

“…Metal-organic framework compounds (MOF) or porous coordination polymers are regarded as one of the most promising sorbents for the storage of hydrogen [1][2][3][4][5][6][7][8][9]. The problem of achieving an acceptable sorption capacity for MOFs with respect to hydrogen (9 wt.% and 81 g/L before 2015 [10]) can only be solved on the condition of an understanding of the factors influencing the ability of such compounds to sorb H 2 .…”

“…In a first approximation it is possible to distinguish two groups of factors influencing the sorption capacity of MOFs, and they can be called arbitrarily macroscopic (characteristics nor directly dependent on the chemical structure of the MOF -surface area, volume of pores) and microscopic (characteristics of the structural elements interacting directly with the H 2 molecule). Analysis of the array of accumulated data indicates that if only macroscopic physicochemical factors are taken into account the sorption capacity of the MOF for hydrogen is determined by the surface area and the volume of the pores in the compound [1, 4,5,9,[11][12][13]. Since such an approach does not take account of the characteristics of the chemical structure of these compounds the question remains open as to how much the structure of the ligands affects the ability of MOFs to sorb hydrogen.…”