INTRODUCTION 

One of the prerequisites for the utilization of hydrogen as fuel in car engines or aircraft turbines is a device for the safe and easily handled storage and transportation of hydrogen. Several methods have been proposed for this purpose. Among them are high-pressure tanks for gaseous hydrogen, cryogenic vessels for liquid hydrogen and metal hydride storage systems [1-3]. However, the first two options bear the danger of explosion if improperly handled, and the latter method suffers from high cost and weight. As an alternative, one could envisage hydrogen storage by encapsulation in microporous media. Although reversible occlusion of gases in zeolites is a well known phenomenon (e. g., [4,5]) little attention has previously been paid to their use as media for hydrogen storage (e. g., [6-9]). The working principle is that the guest molecules are forced, under elevated temperatures and pressures, into the cavities of the molecular sieve host. Upon cooling to room temperature or below, hydrogen is trapped inside the cavities. It can be released again by raising the temperature.

Almost all previous work in this area was done on the synthetic zeolites A, X, Y and mordenite and on some zeolite minerals [6,7]. An evaluation of these prior publications seems to indicate that, for a hydrogen loading pressure of 8.0 MPa, zeolite A possesses the highest storage capacity if it is ion exchanged with K⁺ [6]. In an attempt to rationalize the conclusions from this earlier work, a more systematic study on the potential of zeolites as media for hydrogen storage was undertaken using selected zeolites of different structures and different compositions. 

RESEARCH PROGRAM 

In the frame of this research project, selected zeolites differing in the dimension of the pore openings and the intracrystalline cavities and in their chemical composition are scrutinized with respect to their ability for reversible hydrogen storage. The experiments are conducted in a high pressure apparatus designed for pressures up to 10.0 MPa and temperatures up 300 ^oC. The hydrogen storage capacities are typically determined in the following manner: 0.7 to 2.5 g of the zeolite are dried and degassed for 12 hours at 450 ^oC and 1 Pa. Upon cooling to the desired uptake temperature, pure hydrogen at a pressure between 2.5 and 10.0 MPa is admitted. The loaded zeolite is then cooled to 25 ^oC or below, where upon the pressure is released and the storage system is evacuated to a pressure below 1 Pa. At this point, the zeolite is heated again whereby the entrapped hydrogen gas is released. The amount of hydrogen coming from the zeolite is determined quantitatively via continuous measurement of the pressure in the known volume of the apparatus. Based on the results of a first screening, a restricted number of zeolites will be selected and further optimized with respect to their hydrogen storage capacity. Especially the conditions for the encapsulation and the release of hydrogen will be systematically varied. Furthermore, it is planned to explore the long-term stability of the zeolites with respect to the possible number of loading/release cycles, preservation of the storage capacity, and the influence of possible contaminations in the hydrogen gas (e. g., water, carbon monoxide, carbon dioxide etc.).

RESULTS 

In the first set of experiments, the influence of the loading pressure on the amount of stored hydrogen was investigated with A-type zeolites which were ion exchanged with K⁺, Na⁺, Rb⁺ and Cs⁺. They were exposed to hydrogen for 15 minutes at 300 ^oC and pressures ranging from 2.5 to 10.0 MPa. It can be seen from Figure 1 that the amount of encapsulated hydrogen increases linearly with the pressure during loading as had been expected in terms of the ideal gas law. The amount of hydrogen which can be entrapped in zeolite A depends markedly on the nature of the cation. Whereas in CsA, practically no hydrogen is encapsulated, the highest capacity observed under the conditions of this study amounted to 5.7 cm³/g (calculated at
273.15 K and 101.3 kPa) for zeolite KA and a loading pressure of 10.0 MPa. At this stage of 

the investigations it was speculated that the location of encapsulated hydrogen are the small (sodalite) cages in the structure of zeolite A, rather than the larger cavities (alpha-cages) behind the 8-membered ring windows as previously suggested by Fraenkel [6]. In an attempt to solve this ambiguity, the study was extended to a variety of other zeolites containing different types of cages: the zeolites sodalite, A and Y do contain sodalite cages like zeolite A, but they are lacking alpha-cages; conversely, zeolites Rho and ZK-5 do contain alpha-cages, whereas no sodalite cages are present. Furthermore, zeolite Sigma-1 containing pentagondodecahedra was also included in this study. The results are presented in Table 1. Among the zeolites used in this study, sodalite possesses the highest storage capacity for hydrogen. As expected, V_H2,N is generally higher for zeolites having a reasonable portion of small cavities in their structure. From the known chemical compositions and the structures of the zeolites containing beta-cages, the number of cages per mass of zeolite (k/mz) was calculated to estimate the number of hydrogen molecules occluded per sodalite cage (n_H2/k). The result is quite surprising: even with the best materials of this study, only approximately one out of four to five sodalite cages is occupied by a hydrogen molecule. Hence, there remains a considerable potential for enhancing the hydrogen storage capacity in zeolites with an appropriate structure. Since zeolite sodalite possesses the highest density of sodalite cages in its structure, it was decided to focus
on the optimization of this zeolite. As a first means, the aluminum content of sodalite was

systematically varied (i. e., n_Si/n_Al = 1, 4 and 40; note, that the data presented for sodalite in Table 1 were obtained with a zeolite sample prepared in a completely different manner). From the data presented in Figure 2 it can be concluded that the nSi/nAl-ratio of the zeolite significantly influences its capacity for hydrogen storage and the temperature at which the release of the encapsulated hydrogen starts. In addition, the maximum storage capacity is considerably higher than with other zeolites tested so far. Currently, an in-depth study is underway in our laboratory to shed more light on this interesting influence of the aluminum content of sodalite on its hydrogen storage capacity and to further optimize the amount of encapsulated hydrogen.

REFERENCES