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The Stability of Metallic Cations in Zeolites
by D. J. C. Yates Process Research Division, Esso Research and Engineering Company, Linden, New Jersey (Received December 4, 1964)
It has been found that reducible metals, with a suitable vapor pressure, can be removed from zeolites by hydrogen treatment. Detailed experiments have been performed by weighing samples of Cd, Hg, and Zn zeolites in vacuo and in hydrogen. Removal of the metal has been confirmed by chemical analyses. Some experiments have been performed using zeolites exchanged with metals which have high vapor pressure but which are known to be difficult to reduce. No metal could be removed from these materials. Some X-ray work has been done with metals which are readily reducible but which have too low a vapor pressure to permit the metal to leave the zeolite.
Introduction Despite the large amount of work which has been done on the exchange properties of zeolites,l little information is available on the stability of the exchangeable cations in these lattices. The positions of some of the sodium cations in a synthetic near-faujasite have been located by X-ray spectroscopy.2 Although there are 80 sodium cations per unit cell, 32 of these could not be located. More recent work3 with a silversubstituted synthetic near-faujasite has shown that the positions of all the cations can be determined by X-ray examination. Evidently, X-ray spectra, taken under controlled atmospheres, of such materials could be used to give definite information about the stability of the metallic cations in zeolites, but this does not appear to have been done. The only information that seems to be available on this subject is some brief references in the patent literature to the effect that metals such as mercury,4 cadmium,6 and zinc6 can be driven out of zeolites, but few details were given. This problem has been studied by weighing, in controlled atmospheres, various ion-exchanged forms of a synthetic near-faujasite7 and of the synthetic A-type of zeolite.819 It has been found that metal cations, even a t temperatures where the vapor pressure of the metal is high (e.g., cadmium at 450") are not removed from the zeolites in vacuo. If, however, hydrogen is added to a zeolite containing a reducible metal (e.g., Cd, Hg, Zn), then the metal atoms formed diffuse out of the sieve. In addition to the microbalance studies, The Journal of Physical Chemistry
X-ray spectra have been taken of zeolites containing low vapor pressure metals (Ni and Ag) both before and after hydrogen treatment. Crystals of metallic nickel and silver were found after reduction.
Experimental Materials. The near-faujasite starting material for the Cd, Zn, K, Ni, and Ag samples was pure, powdered Linde 13X sieve (containing no binder), Lot No. 13,961. The nominal chemical composition is NazO.A1203.2.67Si02SnHzO, but in some cases this material'O is slightly sodium deficient, having Na :A1 ratios of about 0.95. This topic is discussed in detail elsewhere. l1 For the mercury-faujasite, the starting material was the 13X sieve in pill form, containing (1) See, for example, the review by R. M. Barrer in the 10th Colston Symposium on the structure and properties of porous materials, London, Butterworth and Go., Ltd., London, 1958. (2) L.Broussard and D. P. Shoemaker, J. Am. Chem. Soc., 82, 1041 (1960). (3) L. Broussard, private communication. (4) D. W. Breck and R. M. Milton, U. S. Patent 3,013,982(1961). (5) D. W.Breck and R. M. Milton, U. S. Patent 3,013,983(1961). (6) D.W.Breck, U. S. Patent 3,013,984(1961). (7) R. M. Barrer, F. W. Bultitude, and J. W. Sutherland, Trans. Faraday SOC.,53, 1111 (1957). (8) D.W.Breck, W. G. Eversole, R. M. Milton, T. B. Reed, and T. L. Thomas, J . Am. Chem. SOC.,78, 5972 (1966). (9) R. M. Barrer and W. M. Meier, Trans. Faraday Soc., 54, 1074 (1958). (10) Information supplied by the Linde Co. (11) J. L. Carter, P. J. Lucchesi, and D. 5. C. Yates, J . Phys. Chem., 68, 1385 (1964).
THESTABILITY OF METALLIC CATIONSIN ZEOLITES
about 20% of a clay binder. The synthetic zeolite type A was obtained as pills in the 4A form, with about 20% of a clay binder present. The nominal chemical composition of this zeolites is NazO.A1203. 1.92Si02. nHzO. From the above starting materials, the sodium was removed by ion exchange with metal salts using procedures similar to those of Barrer and Bratt.12 Deuterium and carbon monoxide were obtained from the Matheson Co., East Rut)herford, N. J., and were stated to have a purity of a t least 99.5%. Hydrogen was obtained from the Linde Go. of Linden, N. J., and had a purity of 99.98%. All gases were dried before use by passage through a trap cooled to 77°K. For the I(,Cd, Xi, and Ag near-faujasites, X-ray analyses were run on the untreated samples. It was found that the samples were crystalline, with only traces of amorphous material present. To prevent the powders from leaving the bucket of the microbalance during evacuation, the zeolites were pressed at 10,000 p.s.i. before use. They were then cut into the desired size for insertion in the bucket. The pilled mai;erials were used as received. Previous work1' has shown, a t least for the Na near-faujasite, that this pressing procedure had no effect on the Xray spectra. There is no u priori reason to expect the other zeolites to behave any differently from the sodium material in thif respect. Apparatus. The weight loss experiments were conducted using a Cahn microbalance (Cahn Instrument Co., Paramount, Calif.), described e1~ewhere.l~The hang-down tube containing the sample was made of fused silica, while the other hang-down tube was made, of identical dimensions, in Pyrex glass. The buckets used were made of fused quartz and weighed about 80-100 mg. each. They were obtained from the Worden Laboratories, Houston 27, Texas. Nickelchromium alloy wires (nichrome) were used to support the buckets from the microbalance beam. As this balance operates with a null servo system, the buckets remain fixed in space during the course of an experiment. This makes it easy to place wells for the measuring and controlling thermocouples just below the buckets. Side arms were placed a t the bottom of the hang-down tubes so that gases could be passed through the balance when needed. The X-ray equipment used for this work consisted of a Philips diffractometer, using copper K a radiation, with a Geiger counter detector. The zeolite samples were exposed to air while their spectra were measured. Two vacuum systems were used; both consisted of and a backing pump' an 80-1'/sec* Oil diffusion trap cooled with liquid nitrogen. One vacuum system
1677
was used solely for evacuation of the microbalance; dynamic vacua in the low torr range were recorded with an Alpert type hot-filament ionization gauge, The other system consisted of a calibrated volumetric adsorption system which was used to add gases to the microbalance and also for the reduction of the Ni-X and Ag-X. Dynamic vacua in the lo-' torr range were obtained with the second system. Procedure. Prior to use, the molecular sieves were generally dried overnight a t 120" to minimize the amount of water to be handled by the vacuum system. I n all cases, samples weighing between 400 and 475 mg. were loaded into the balance. The sample hangdown tube was then replaced and the balance evacuated. Evacuation then continued while the temperature was increased slowly. Some zeolites are damagedI4 by water at temperatures above 200". The particular temperature used with each zeolite is given in the Results section. After the weight was constant to the desired extent a t this temperature, hydrogen was added. Varying pressures (of the order of a few centimeters) were used, but the rate of removal of the metal did not seem to vary over the range of hydrogen pressures used in this work. The weight of the sample was then recorded with time a t constant temperature. When the weight loss became slow, in some cases the temperature was increased in an effort to drive out more of the metal. The specimens used for X-ray work were prepared as follows. About 1.4 g. of the powder was pressed a t 10,000 p.s.i. This material was then cut to the desired size for insertion in the Pyrex adsorption cells. The cells have been described e1se~here.l~After evacuation between room temperature and 200", hydrogen was passed through the samples at 500 cm.a/ min. The temperature was then increased to 400" for Ni (250" for Ag) and the flow was maintained a t this temperature for 16 hr. The hydrogen flow was then stopped, the samples were evacuated while hot, and then they were cooled down to room temperature in uucuo. The samples were taken out of the cells and put in the X-ray spectrometer. The samples used for the reference spectra were from identical batches except that they were not evacuated or reduced. (12) R. M. Barrer and G. C. Bratt, J. Phys. Chem. Solids, 12, 130 (1959). (13) L. Cahn and H. R. Schultz in "Vacuum Microbalance Techniques," Vol. 2, R. F. Walker, Ed., Plenum Press, New York, N. Y., 1962,p, 7. (14) R. M. Barrer and W. I. Stuart, Proc. Roy. SOC.(London), ~ 2 4 9 , 464 (1959). (15) D.J. C.Yates, W. F. Taylor, and J. H. Sinfelt, J. Am. Chem. sot., 86, 2996 (1964).
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Figure 1. Weight loss curves for cadmium near-faujasite at 450’: run 1 (0),evacuated for 75 min., hydrogen then added; run 2 (A), evacuated for 65 min., hydrogen then added.
To enable accurate comparisons to be made of the effect of reduction, all of the instrumental conditions of the X-ray diffractometer were kept the same for each metal. The scale factor used for the Ni-X spectra was not, however, the same as that used for the Ag-X spectra. The slits and scanning rate were the same for all four complete spectra. A slower scanning rate was used for the spectra used for the crystallite size determinations.
Results Ion-Exchanged Near-Faujasites.
Results obtained with the cadmium material, after evacuation at 450°, are given in Figure 1. To show the reproducibility of the method, two runs (using two samples from the same batch of material) are shown in the figure, and it will be seen that the variation of weight loss with time is very similar for the two experiments. It should be noted that the points plotted on the figure were obtained from the recorder chart. During all of these experiments, the recorder was continuously in operation with the chart speed varying from 0.5 in./min. to 1 in./hr., depending on the rate of loss of weight. Thus, if desired for any form of kinetic analysis, an extremely large number of weight readings are available; the intervals are much closer than can be plotted on a figure showing the whole experiment. The Journal of Physical Chemistry
Soon after adding the hydrogen, a cadmium film deposited on the walls of the hang-down tube just above the furnace. There is a small error in the amount of cadmium (and zinc) lost from the material, as some of the metal condenses on the hand-down wire itself, typical values being about 0.5 to 1 mg. Knowing the percentage of Cd initially in the sieve from the chemical analysis, it is possible to calculate a final percentage of Cd in the material from the weight loss. The calculated value for run 1 is 17.2% while the chemically obtained figure is 16.8%, which is good agreement (see Table I). These experiments with cadmium were run a t 450°, and the vapor pressure of cadmium at this temperature is 4.3 mm., obtained by interpolation from values given by Dushman and Laff ertyI6and the “Handbook of Chemistry and Physics.”17 Table I1 contains vapor pressure values for a series of metals taken from the same sources. During some workll on the infrared spectra of a series of synthetic near-faujasites, the OH groups were deuterated. The experiments were done by adding the deuterium to the same a t various temperatures, ~
(16) S. Dushman and J. M. Lafferty, “Scientific Foundations of Vacuum Technique,” 2nd Ed., John Wiley and Sons, Inc., New York, N. Y.,1962. (17) “Handbook of Chemistry and Physics,” 44th Ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1963.
THESTABILITY OF METALLIC CATIONS IN ZEOLITES
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Table I: Metal Content of Zeolites before and after Hydrogen Treatment euac"al1.n
at 1 4 0 T
Zeolite type
13X 13X 13X
44
Metal
Metal content, in wt. %, by --chemical methodsBefore After reduction reduction
Cd Hg Zn Zn
23.8 12.6 14.0 14.9
16.8 0.3 6.4 12.6
Metal content, i n w t . %, calcd. from microbalance data after reduotion
17.2 0.2 5.1 12.1
390
0
380
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370
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Table I1 : Temperatures ("C.) for Given Values of Vapor Pressures 7
Metal
0.1
1
Hg
El 321 405 266
126.2 394 487 341
Cd Zn K
Vapor press., mm. 10 40
184.0 484 593 443
228.8 553 673 524
100
261.7 611 736 586
after initial evacuation of the sample a t 450". The first deuteration on Cd-X was done with 15.3 cm. of gas at 150" for 15 min. No unusual effects were observed. After deuterating for 10 min. a t 300" with a pressure of 16.1 cm., a cadmium mirror was formed after evacuating the Dz. Due to the low vapor pressure of Cd at 300" (0.04 mm.), no Cd could escape while the Dz was in the cell. An opaque mirror was formed for about 1 cm. just where the cell emerged from the furnace. After deuterating for 10 min. at 450°, the mirror became much larger-a portion of the tube about 3 em. long became opaque. Despite the vapor pressure of Na being 1.35 mm. at 450", no Xa mirror was formed on evacuating the gas after deuterating the Na-X sieve at 450". In addition to the data given in ref. 11, some experiments were done with K-X. I n no case was a potassium mirror observed, even after deuteration at 450°, where the vapor pressure is 11.5 mm. Data obtained with mercury, a t temperatures from 140 to 300", are shown in Figure 2. I n contrast with the cadmium, the mercury was driven out much faster, the loss rate being small 80 min. after adding hydrogen to the Hg-X, while the rate was quite fast after 100 min. with the Cd-X. The mercury vapor pressure is 1.75 mm. at 140", which is less than that of cadmium at 450". At 155 min., the hydrogen was evacuated, which increased the rate of weight loss for a short time. At 270 rnin., hydrogen was added again, and no change
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Figure 2. Weight loss curve for mercury near-faujasite at a series of temperatures.
in the rate of loss was seen. The temperature was then increased, 200" being reached at 290 min. More mercury was driven out, but the rate of loss soon decreased to a small value. Finally, the temperature was increased to 300" at 400 min. A much greater percentage of the initial metal present was removed with the Hg-X, virtually all of the mercury being removed (see Table I). The vapor pressure of zinc is much lower than that of cadmium and mercury, so the experiment was begun by adding hydrogen at 500", after initial evacuation a t 600". No zinc mirror was seen during the evacuation. The data are shown in Figure 3, the rate of weight loss at 500" in hydrogen being shown as well as values at 600". Despite the high vapor pressure at 600" (11.5 mm.), the rate of loss of zinc is much slower than that of cadmium, the Zn-X losing weight a t a fairly constant rate for 8 hr. Calculated and measured zinc contents a t the end of the experiment are shown in Table I and are in good agreement. To illustrate the type of recordings obtained in this work, Figure 4 was obtained by direct tracing from the recorder chart at about hour 18 of the experiment Volume 69,Number 6 Mag 1986
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Figure 3. Weight loss curve for zinc near-faujasite in hydrogen; kept at 500" for the first 18 hr. and at 600" for the remaining time.
shown in Figure 3. The balance ampliiier was set so that full-scale deflection on the recorder was obtained for a l-mg. change in weight. The figure gives an idea of the typical "noise" obtained a t this sensitivity. To indicate how the mass is obtained from the recorder, the balance amplifier mass dial was set a t 380.500 mg. while that portion of the run shown in Figure 4 was being obtained. At the 30-mh. period (Figure 4) the total mass of the sample was 381.445 mg., as the pen was a t 94.5 divisions on the recorder. I n addition to the pen connected to the microbalance, the recorder has another pen connected to a thermocouple. Until the 30-min. point, the temperature was 500" (as accurately measured with a second thermocouple attached to a potentiometer) and was then increased, reaching 600" at about 38 min. It will be seen that the zinc evolution commenced shortly after the temperature rose above 500". Some experiments have also been done with K-X material. This was chosen as an example a€ a metal with a high vapor pressure (125 mm. a t 600") which is very difticult to reduce with hydrogen. The sample weighed 351.820 mg. after evacuation a t 600", and, on adding 2.2 cm. of hydrogen a t the same temperature, no weight change could be detected and no metallic mirror was seen. The Journal of Physical Chemistry
In addition to the microbalance experiments with metals of high vapor pressure, some 'work has been done with low vapor pressure metals, using X-ray spectroscopy. Portions of the spectrum of Ni-X (scanned at l"/min.) are shown in Figure 5. It will be seen that there are no prominent peaks on the fresh zeolite over the region of 28 from 38 to 48". However, a prominent 111 peak of crystalline nickel is seen on the reduced sample. Similar experiments were performed with Ag-X, and portions of the spectra are shown in Figure 6. In this case, there are two very prominent peaks in the fresh zeolite that are much reduced in intensity after reduction. An extremely intense 111 Ag peak can be seen after reduction, in addition to a strong 200 peak. To obtain information on the average crystallite sizes of the metals driven out of the zeolites, slow Xray scans (0.125O/min.) were taken of the 111 Ni peak and the 200 Ag peak. Bulk Ni and Ag were used to obtain values for the instrumental broadening of the peaks. Following the procedure given by Klug and Alexander,18the sizes were found to be 240 A. for the Ni and 170 A. for the Ag. ~
~~
(18) H. P. Klug and L. E. Alexander, "X-Ray Diffraction Procedures,'' John Wiley and Sons, Ino., New York, N.Y.,1954,p. 491.
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THESTABILITY OF METALLIC CATIONS IN ZEOLITES
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Figure 4. Diagram obtained by direct tracing from the recorder chart a t about hour 18 of the zinc near-faujasite experiment (Figure 3). I
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Figure 6. X-Ray diffraction patterns of Ag-X zeolite: ( a ) before reduction; (b) after reduction with hydrogen a t 250’ for 16 hr.
+ + 38
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Degrees (281
Figure 5. X-Ray diffraction patterns of Ni-X zeolite: (a) before reduction; (b) after reduction with hydrogen at 400’ for 16 hr.
48
Ion-Exchanged “A” T y p e Zeolite. Only one example of this zeolite was investigated, the zinc-exchanged form. As this was in the form of pellets, containing a clay binder, it is diacult to compare rates of removal of the zinc from the Zn-A with the Zn-X. The purpose of the experiment was to see if a metal could be removed from the A type of zeolite, which has much smaller cavities than does the near-faujasite. It will be noticed in Figure 7 that there are two breaks a.t the regions representing 29 and 49.5 hr. These occurred when the balance amplifier was changed from a setting giving full scale recorder deflection for a l-mg. weight change to a 10-mg. full-scale setting and back again to 1 mg. The cause of this is unknown, but it is probably due to a slight error in the initial setting up of the balance. However, if the curve obtained over the first 29-hr. period is extrapolated, it fits over the Volume 69,Number 6 May 1966
D. J. C. YATES
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Figure 7. Weight loss curve for zinc "A" type of zeolite at 600"; evacuated for 70 min., hydrogen then added.
last portion of the graph, indicating that reproducible results can be obtained over long periods on a given range. No other discontinuities of this type were observed on all the other runs when the recorder range was changed in the same way. The results shown in Figure 7, obtained a t 600', show that, despite the smaller pores, the Zn can indeed be driven out of the A zeolite. The rate seems slow, but the effect of the clay binder present in the Zn-A i s unknown. Good agreement was obtained between the calculated and measured zinc content a t the end of the experiment (Table I).
Discussion It is striking that no cadmium or zinc is removed from zeolites on evacuation a t high temperatures but that these cations are readily driven out by the addition of hydrogen a t the same temperature. Evidently the ions are strongly held in the lattice, but, when the ions gain an electron from the hydrogen, the atoms are held only by weak van der Waals forces in the lattice. Despite the very strong electrical field in the cavities of these materials, it is evidently not high enough to retain the atoms in their original positions in the lattice. This observation suggested a search for reducible metals with vapor pressures greater than, say, 1 mm. over the temperature region where these zeolites are stable (below about 700'). Examination of low pressure vapor pressure datal6 shows that very few metals meet these criteria. The only ones, in addition to The Journal of Physical Chemistry
those used in this work, seem to be the group VI elements selenium (vapor pressure = 1 mm. a t 347'), polonium (vapor pressure = 1 mm. a t 472'), and tellurium (vapor pressure = l mm. at 520'). As far as is known, no faujasites have been prepared with these cations. However, they offer serious difficulties in investigation. Polonium is radioactive, while both selenium and tellurium melt a t temperatures where their vapor pressure is small (m.p. of Se, 217'; m.p. of Te, 450'). However, if selenium and tellurium zeolites can be prepared, it is possible that the metals could be removed, assuming these metals can be reduced by hydrogen. A further point is that hydrogen seems to be the best agent for neutralizing the metallic cations. An experiment was performed with Cd-X, where the sample was evacuated a t 450'. At the end of this period, the sample weighed 353.610 mg. Carbon monoxide was then added to the sample, but no change in weight could be detected (to within *0.05 mg.) after 10 min. The carbon monoxide was then removed from the balance, and hydrogen added. Immediate evolution of cadmium was observed, the sample losing 8.0 mg. in 10 min. There seem to be only a few metals which can be readily driven out of zeolites, namely, Cd, Hg, and Zn with certainty, and possibly Se, Po, and Te. Nevertheless, the general concept that metallic ions in zeolites can be converted into neutral atoms can be important for all reducible metals, whatever their vapor pressure. This is shown by the X-ray work with nickel and
THESTABILITY OF METALLIC CATIONS IN ZEOLITES
silverlgzeolites. In both cases, reduction with hydrogen neutralized these cations. The metal atoms thus formed are only held loosely to the lattice at the temperature of reduction, as shown by the formation of crystalline metallic nickel and silver. The average crystallite size (240 A. for Ni and 170 8. for Ag) is so large, compared to the cavities of the zeolites2 that it seems very likely that the metal has migrated to the external surface of the zeolite crystals. The crystals have then grown from the aggregation of these metal atoms. It is, however, not known whether the diffusing entities in the zeolitic cavities are metal atoms or extremelyo small crystallites of diameter less than about 13 A. I t is, of course, also possible that a considerable number of the metal atoms may remain as individual atoms, either in the zeolite cavities or on the external surface of the zeolite. The reduction of the intensity of the pair of zeolite lines in Figure 6 a t values of 20 between 40 and 42', after reduction, indicates that some considerable change has occurred in the Ag-X zeolite structure. This may be simply due to the decrease in number of Ag ions in their normal positions in the lattice, or it may be due to some type of breakdown of the zeolite structure. However, no detailed investigations of this effect have been made. It is of interest that the mobility of the Ni and Ag atoms is so pronounced at temperatures considerably lower than their melting points. The zeolites were reduced a t 400 and 250', and the respective melting points of the metals are 1452 and 961'. This indicates that the attractive forces between a free nickel or silver atom and the zeolite cavities are less than the Ni-Ni and Ag-Ag attractive forces in the bulk metal. The vapor pressure of Ni and Ag is extremely low a t the
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reduction temperatures used. It is 10" mm. for Ni at 1142' and mm. for Ag at 757'. These are the lowest temperatures for which vapor pressure data are available. Other observations have shown the mobility of silver in Ag-X zeolite after reduction. During work on the deuterium exchange of this zeolite, using infrared spectroscopy, it was noticed that the sample had turned from white to a dull yellow after evacuation at 450'. The infrared transmission, after cooling down to room temperature, was then normal for this type of nearfaujasite. The sample was deuterated (with Dz) at 300' for 10 min. and was then seen to be black and to have essentially zero infrared transmission. As metallic crystallites of the size found in the X-ray experiments absorb (and scatter) infrared radiation strongly, this indicates in an even more striking fashion the high mobility of silver atoms (in hydrogen) on zeolite surfaces a t 300'. This work has shown that the ions of reducible metals can readily be neutralized, with hydrogen or deuterium, on zeolites of the near-faujasite type7 and of the "A" type.*vg It is probable that similar effects will occur with other zeolites. If this is correct, consideration has to be given to the stability of all these materials in reducing atmospheres a t elevated temperatures.
Acknowledgments. The author wishes to thank E. M. Kelley for help with the microbalance measurements. Thanks are also due to Dr. L. Broussard for helpful discussions and for communicating his unpublished X-ray results on silver zeolites. (19) Similar effects of silver migration have been found with another form of zeolite by L. Broussard (private communication).
Volume 60,Number 6 May 1066
