Methanation ruthenium zeolite

The production of hydrogen from methane over zeolite supported metal catalysts can be examined as an alternative to steam reforming because the concomitant aromatization reactions can increase the economic potential of the process. For methane aromatization, Mo/ZSM5 catalysts have been intensively studied since their first report in 1993 (/, 2). In 1997 (3), the promotional effect of ruthenium over Mo/ZSM5 catalysts was reported. Other second metals have also been studied to improve catalyst activity and stability and a review on this topic is available 4). 

Carbon monoxide is hydrogenated over ruthenium zeolites in both methanation and Fischer-Tropsch conditions. is exchanged in the zeolite as the amine complex. The zeolites used are Linde A, X, Y, and L, natural chabazitey and synthetic mordenite from Norton. The zeolites as a support for ruthenium were compared with alumina. The influence of the nature of the zeolite, the ruthenium metal dispersion and the reaction conditions upon activity and product distribution were investigated. These zeolites are stable methanation catalysts and under the conditions used show a narrow product distribution. The zeolites are less active than other supports. Sintering of ruthenium metal in the zeolite supercages shows only minor effects on methanation activity, although under our Fischer-Tropsch conditions more C2 and C3 are formed. 

Ruthenium zeolites are active and stable methanation catalysts. Under the Fischer-Tropsch conditions used here they show a narrow product distribution. When the size of the ruthenium particles enclosed in the zeolite cages is increased, there is hardly any eflFect found on the methanation activity. Under F. T. conditions a higher amount of C2 and C3 products are formed. Zeolites are generally less active than other supports. In the class of zeolite supports, the less acidic zeolites act as promoters of the CO hydrogenation under methanation conditions the... 

The reactions are catalyzed by transition metals (cobalt, iron, and ruthenium) on high-surface-area silica, alumina, or zeolite supports. However, the exact chemical identity of the catalysts is unknown, and their characterization presents challenges as these transformations are carried out under very harsh reaction conditions. Typically, the Fischer-Tropsch process is operated in the temperature range of 150°C-300°C and in the pressure range of one to several tens of atmospheres [66], Thus, the entire process is costly and inefficient and even produces waste [67]. Hence, development of more economical and sustainable strategies for the gas-to-liquid conversion of methane is highly desirable. 

Comparison of Initial Methanation Activities for Zeolite and Alumina Supported Ruthenium Catalysts... 

The same workers (234) also studied the methanation behavior of bimetallic clusters of Ru/Ni and Ru/Cu in zeolite Y. Such clusters can be formed by metals, such as ruthenium and copper which are immiscible as bulk metals (235, 236). The turnover numbers versus bimetallic cluster composition are shown in Fig. 22. Dilution of ruthenium with copper clearly causes a marked decrease in specific activity. This decrease in activity is also accompanied by a decrease in methanation selectivity. This was attributed to an inhibiting effect of copper on the ruthenium hydrogenolysis activity. 

In this work, experiments at ambient pressure were carried out under methanation and Fischer-Tropsch conditions. The zeolites as a support for ruthenium were compared with a more conventional one (alumina). The influence of the nature of the zeolite, the dispersion of the ruthenium metal and the reaction conditions upon activity and product distribution were investigated. 

Methanation Activity. AcnviTY and Selectivity. In Figure 1 are compared the methanation activity of 0.5 wt % Ru on NaY zeolite and on alumina, and 1% Ni on NaY zeolite. It is seen that the initial activity of the two ruthenium catalysts is comparable, while the nickel catalyst is... 

The selectivity of 5.6 % Ru on zeolite Y is shown in Figure 2. The hydrocarbons formed are exclusively methane and only minor amounts of CO2 (<6 % ). The latter most probably arises from a parallel water gas shift reaction, since ruthenium metal is known to catalyse this reaction at an appreciable rate (10). 

Influence of the Reduction Temperatube. RuNaY zeolite, containing 5.6 % Ru by weight, is taken as a representative catalyst to illustrate the influence of the reduction temperature on the methanation activity. The dispersion of the ruthenium metal phase measured by desorption of chemisorbed hydrogen and by CO chemisorption is given in Table I. 

Irrespective of the nature of the reaction intermediate, enolic type (11) or surface carbide (12), the dechne of the turnover number for the zeolites with higher Si/Al ratio can be explained as follows. For platinum (13) and palladium (14,15) loaded zeolites, support effects are known to exist. The higher the acidity (and the oxidizing power) of the zeolite, the higher will be the electron-deficient character of the supported metal. It also is well established now (16) that the average acidity of hydrogen zeohtes increases with the Si/Al ratio. This explains why the electron deficient character of ruthenium should increase with the Si/Al ratio of the zeolite, and a stronger interaction with adsorbed CO should be expected. Vannice (19,20) reported that the N value for CH4 formation decreases when the heat of adsorption for CO increases. All this explains why the tmnover number of the methanation reaction over ruthenium decreases when the Si/Al ratio of the zeolite support increases. 

Influence of the Nature of the Zeolite. When the Si/Al ratio of structurally diflFerent zeolites is varied, N for methane formation also changes (Figure 4), At least, this is true for the small clusters obtained after a 300 C reduction. In the latter case N decreases considerably with increasing Si/Al ratio of the zeolite. A and Ch zeolites do not follow this relation. This is attributable to the inabihty of these solids to accept the Ru(NH3)e complex in the inner cages. During the ion exchange procedure this complex is decomposed and probably hydrolyzed ruthenium species are adsorbed. The latter are known to result in much less active catalysts after reduction. 

Table II. Kinetic Data for CO Methanation over Ruthenium on NaY Zeolites and on Alumina...

Influence of the Zeolite on the Product Distribution. When a less acidic support was used for ruthenium, better activity was found under methanation conditions. Using the same argument, under F. T. conditions a higher selectivity for formation of higher hydrocarbons is expected when a less acidic support is used. In this respect, pertinent data are given over RuX and Y zeolites in Table V. The X zeolite is known to be less acidic than the Y zeolite. There is indeed a definite influence of the zeolite matrix in the indicated direction higher products are formed over zeolite X. 

---