Characterization of RuCl3-Impregnated NaY Zeolite - ACS

Chapter 26

Characterization of RuCl3-Impregnated NaY Zeolite

Downloaded by CORNELL UNIV on August 6, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch026

Cathy L. Tway1, Salvatore J . Bonafede1, A . Mohamad Ghazi2, Christopher P. Reed3, Robert J . DeAngelis3, and Tom M . Apple4 1Department of Chemistry, 2Department of Geology, and 3Department of Mechanical Engineering, University of Nebraska, Lincoln, N E 68588 4Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180 27Al Magic Angle Spinning (MAS) N M R of RuY zeolite reveals that impregnation of the zeolite with RuCl3 causes dealumination of the lattice. 29Si M A S N M R indicates that the as-prepared RuY is highly disordered. A loss of crystallinity is observed after ion-exchange, however this is apparently not simply due to the low pH of the exchange solution. Zeolites which were immersed in HCl solutions of identical pH as the RuCl3 solution show little loss of crystallinity. NaCl treatment of the RuY produces materials with very high percentages of ruthenium by weight. Little structural damage is observed upon low temperature reduction of RuY, however reductions performed at 723 Κ cause sintering and the formation of large clusters of Ru on the outer edges of the crystallites. Large RuO2 particles are formed under oxidizing conditions. Ruthenium is known to catalyze a number of reactions, including the Fischer-Tropsch synthesis of hydrocarbons (7) and the polymerization of ethylene (2). The higher metal dispersions and the shape selectivity that a zeolite provides has led to the study of ruthenium containing zeolites as catalytic materials (3). A number of factors affect the product distribution in Fischer-Tropsch chemistry when zeolites containing ruthenium are used as the catalyst, including the location of the metal (4) and the method of introducing ruthenium into the zeolite (3). Typically, the amine complexes have been used to ion- exchange ruthenium into Y-type zeolites (5,6). RuCl3:3H20 has largely been avoided as an ion-exchange medium because hydrolysis of the R u 3 cation results in acidic solutions which can lead to a loss of crystallinity in less siliceous zeolites (5,7-8). However there appear to be differences in the behavior of the ruthenium if RuC133H20 is used as the source of ruthenium instead of Ru(NH3)6Cl3 (3,6,9). Shoemaker and Apple (9) studied the chemistry of ruthenium species in RuY zeolites prepared from R u C l : 3 H 2 0 under reducing and oxidizing conditions. In contrast to RuY prepared via ion-exchange with Ru(NH3)6Cl3 (6), ruthenium species in RuCl -impregnated RuY did not sinter under oxidizing conditions. After oxidation, Ru02 remained dispersed within the zeolite supercages (9). Large Ru02 particles were found after oxidation in RuY +

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0097-6156/93/0517-0372$06.00/0 © 1993 American Chemical Society

Davis and Suib; Selectivity in Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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RuCl -Impregnated NaY Zeolite 3

prepared from Ru(NH3)6Cl3 (6). In this paper, RuY zeolite prepared via ionexchange with RuCl3 solutions will be characterized using a variety of techniques. Experimental The starting material for all samples was L Z Y - 5 2 zeolite obtained from Union Carbide which was used without further pretreatment. The Si/Al ratio of the starting NaY zeolite was determined from ^ S i Magic Angle Spinning (MAS) solid state N M R to be 2.7. RuY was prepared by exchanging 20 grams of NaY zeolite with 500 mL of a .04M solution of RuCl3 prepared from RuCl3:3H20 (Strem Chemicals). Ionexchange was performed at room temperature for three hours with constant stirring. The resulting RuY zeolite was rinsed with deionized water and dried in air for 17 hours at 383 K . Zeolite samples were digested using a mixture of HF, HNO3 and HCIO4 in screw-top teflon bombs (Savillex). Ruthenium analyses were performed using a V G PlasmaQuad II Inductively Coupled Plasma Mass Spectrometer. A l l samples were spiked with indium to serve as an internal standard. Atomic absorption was used to determine aluminum content. The results from the chemical analyses are shown in Table 1. X-ray powder diffraction data for all samples were collected via a Phillips xray diffractometer with a copper target tube and a diffracted beam monochromater. Ruthenium and Ru02 particle sizes were estimated by x-ray line shape profile analysis using a single profile technique which provides diffracting particle size, rnicrostrain, and the particle size distribution (10). 27Al and ^ S i M A S N M R were used to monitor changes in the zeolite structure. \ \ N M R was performed at 93.8 M H z with a homebuilt magic-angle spinning probe. ^ S i N M R was performed at 71.3 M H z using the same probe. Diffuse reflectance Fourier Transform IR spectra were collected using an Analect RFX-65 FTIR spectrometer. K B r was used as the background material. RuY samples were diluted with K B r in order to obtain better transmittance. The position of the asymmetric T-O stretching vibration was used to monitor changes in the Si/Al ratio (77). A l l reductions were carried out on a glass vacuum system under static H2 (Linde 99.999%) which was dried by passing through Drierite and molecular sieves prior to exposure to the sample. H2 uptakes were monitored using a capacitance manometer (MKS Instruments Inc.) N2 isotherms at 77 Κ were performed on the same vacuum system using pre-purified grade N2 (Linde) which was dried prior to use. Oxidations were performed under flowing O2 (Linde 99.999%) at 773 Κ and under static O2. The samples were evacuated at 623 Κ to a residual pressure of less than 5 X 10"^ torr prior to reduction or N2 isotherm measurements.


2

2

2

7

2

Table I. Chemical Analysis of RuY Samples Zeolite Sample RUY NaCl Treated RuY AgN03 Treated RuY, 298 Κ AgN03 Treated RuY, 383 Κ

Percent Ru by Weight

Percent A l by Weight

ÏL7 34.6 11.4 11.9

ÎL9~ 9.9 10.8 5.6


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SELECTIVITY IN CATALYSIS

Results and Discussion Crystallinity of RuY. The RuY samples which were prepared from RuCl3 solutions are less crystalline than the starting material. Significant loss of intensity in the X-ray diffraction peaks is observed after the ion-exchange procedure. N2 isotherm data indicate that ion-exchange results in a loss of around 50% of the original surface area and microporosity (Table 2). The ^ S i M A S N M R spectrum shown in Figure 1 illustrates just how severe the structural damage is in these samples. The individual crystallographic silicon species can not be observed and there is a broad component indicating the presence of amorphous material. 2


Table II. N2 Adsorption Data 2

Zeolite

BET Surface Area (m /g)

NaY RuY pH=3.2 Treated NaY pH=1.26 Treated NaY NaCl Treated RuY Calcined RuY NaCl Treated Calcined RuY Calcined RuY, Reduced at 773 Κ

Microporosity (cc/g) .339 .178 .313 .292 .192 .152 .142 .140

710 360 612 601 420 326 300 290

The starting ruthenium exchange solution is somewhat acidic, possessing a pH of 3.2. Due to the instability of Ru(H20)6^ in aqueous solution (7), the pH of the solution quickly drops to a pH of 1.26. In order to test whether the lattice destruction was simply due to the p H of the exchange solutions, the ion-exchange procedure was repeated with solutions containing no ruthenium whose pH were adjusted to 1.26 and 3.2 using HC1. The 29Si M A S N M R results shown in Figure 2 clearly indicate that the structural damage is not simply due to pH. In both of the acid treated samples, the individual crystallographic silicon species can be observed and the shoulder denoting the amorphous material is absent. This conclusion is also supported by the N2 BET surface area and microporosity results listed in Table 2. In all cases, the values for the RuY are much lower than those of the acid treated samples. Our results are in agreement with the work of Lee and Rees (72) who found that HC1 solutions do not dealuminate Y-type zeolites when the hydrogen ion concentration is less than .22 mmole [H+]/gram zeolite. For our experimental conditions, this corresponds to a pH of 2.06. Because the solutions start at a pH of 3.2 and rise upon the addition of NaY zeolite to a p H of 5, we would not expect the acid-derived dealumination to be very significant in the RuY samples. In contrast, Kim et al. (13) found that the amount of aluminum removed from the zeolite lattice during ion-exchange with solutions of metal chlorides was directly related to the p H of the solution and that the presence of the metal cation played no part in the dealumination. Our work more closely mirrors that of Bailar and co workers (14-16) who found that solutions of CrCl3 under reflux conditions could dealuminate a variety of zeolites to a much greater extent than the pH of the CrCl3 solutions would predict. To explain their results, they proposed that the chromium cations could complex with hydrolyzed aluminum ions in the zeolite through the formation of ol bridges" which then diffused out of the zeolite. Therefore, +

M



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RuCl -Impregnated NaYZeolite

j ι ι ι ι ι ι ι r—— i ι ,•. — i

Γ 50

375

3

-50

0

-100

-150

-200

-250 PPM

Figure 1. 29Si M A S N M R spectrum of as-prepared RuY.

I 0

1

1

1

1

1

I -50

1

1

1

1

1

I -100

1

1

I

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-150

2 9

Figure 2. S i M A S N M R spectra of: A) as-prepared RuY; B) NaY treated at pH=3.2; C) NaY treated at pH=1.26.



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dealumination would proceed until all of the chromium was tied up in these complexes or until the p H of the solution was such that a sufficient amount of hydroxyl groups could not form (14). However, we do not have any evidence to support this type of dealumination process in the RuY system. During the exchange process, greater than 95% of the starting ruthenium is deposited in the zeolite, leaving very little to remain in the solution as a complex with aluminum. A l N M R spectra of the exchange solutions feature a single peak at 0 ppm corresponding to an octahedral aluminum species and no signal that would indicate the presence of an aluminum-ruthenium complex. In addition, A l M A S N M R spectra of the asprepared RuY indicate the presence of extra framework detrital aluminum in an octahedral environment (Figure 3). When chelating agents are used to dealuminate zeolites, typically only tetrahedral aluminum species are observed (8). The actual mechanism for dealumination in the RuY system has not yet been determined. 2 7


2 7

Back Ion-exchange of Ruthenium. Several attempts were made to back ionexchange the ruthenium out of the RuY in accordance of the work of Bailar et al. (1416). Five grams of RuY were immersed in 100 mL of 1 M A g N 0 3 solution, both under room temperature and reflux conditions. No ruthenium was removed from the zeolite during these treatments (Table 1). When the procedure was repeated using 100 mL of a solution of 1 M NaCl at room temperature, significant desilication was observed with no loss of ruthenium. The materials produced from the NaCl treatment contained 36.4 % ruthenium by weight (Table 1). Some extra-framework aluminum is also removed resulting in a material with an Al/Ru ratio of 1.1. Because this Al/Ru ratio is close to one, the ruthenium can not be acting simply as a charge compensating tri-valent cation in the NaCl treated RuY. The inability to back ion-exchange the ruthenium from the zeolite, suggests that even in the starting RuY the ruthenium is not present in a highly dispersed form. Precipitation of ruthenium species has been observed in RuCl3 solutions in the pH range of 3-7 (17). Because the addition of NaY zeolite to our solution caused the p H to rise to 5, it is very likely that polymeric ruthenium was deposited in the zeolite. Bailar et al. (14-16) noted that a similar NaCl treatment of their chromium dealuminated samples resulted in desilication. However chromium cations were exchanged at the same time resulting in only small changes in the chromium content in the treated material. In agreement with their findings, we find that the NaCl treated material is more crystalline than the starting RuY. Figure 4 shows S i M A S N M R spectra of the as-prepared RuY and the NaCl treated material. Following the NaCl treatment a loss in the intensity of the amorphous shoulder as well as some resolution of the silicon resonances for silicon in different chemical environments is observed. The loss of intensity of the amorphous shoulder without loss of Ru upon NaCl treatment suggest that the amorphous shoulder is not due simply to the presence of a Ru atom in the neighborhood of silicon. The N2 isotherms for these materials are nearly identical in shape although the NaCl treated material adsorbs more N2 at lower relative pressures than the starting RuY (Figure 5), which is reflected in a higher microporosity and BET surface area for the NaCl treated material (Table 2). The surface area of the NaCl treated material is actually very similar to that of the starting NaY with a surface area of 660 m /gram zeolite if the weight of the ruthenium is not included. Diffuse reflectance Fourier Transform IR was also used to monitor the effect of the NaCl treatment on the RuY sample. The position of the asymmetric stretching T-O vibration is directly related to the Si/Al ratio of the zeolite, with higher wavenumbers corresponding to higher Si/Al ratios (77). After the NaCl treatment, the position of this band drops from 1038 to 1025 cm"* indicating a loss of silicon 2 9

2



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RuCl Impregnated NaY Zeolite

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f

80

60

40

-20

0

20

PPM

2 7

Figure 3. A l M A S N M R spectrum of as-prepared RuY.

I 0

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1

' I -50

1

1

1

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1

I -100

1

1

1

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I -150

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-200

PPM 2 9

Figure 4. S i M A S N M R spectra of: A) as-prepared RuY; B) NaCl treated RuY. Davis and Suib; Selectivity in Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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during the NaCl treatment. However, the position of this band in the NaCl treated material is still higher than that of the starting NaY (Table 3). Table ΠΙ. Diffuse Reflectance FT-IR Results for RuY Samples 1

Asymmetric T-O Stretching Vibration (cm" )

Zeolite

1017 1038 1025 1042 1071


NaY RuY NaCl Treated RuY Calcined RuY Calcined RuY, Reduced at 773 Κ

The NaCl treatment also hinders the reduction of the ruthenium in the zeolite. In the starting RuY 2.6 moles Η are used per mole ruthenium during reduction at 298 K . In the NaCl treated material, this ratio drops to 0.4 moles H/mole Ru. However both of these ratios are much lower than the ratio of six predicted from the following reaction: Ru3+ + 3H2 + 3ZO- —> Ru°:3H ds + 3ZOH a

where ZO" represents the anion sites in the zeolite (9). The low H/Ru ratio in the starting RuY supports the presence of polymeric ruthenium. The presence of a large amount of CI" ions in the NaCl solution may allow the formation of ruthenium chloro complexes which would hinder the reduction of ruthenium in the NaCl treated material. Such chloro complexes are known to exist in solutions of RuCl3 (77). In RU/AI2O3 and Ru/Si02 catalysts prepared from RuCl3 solutions contamination with CI" ions even after reduction at elevated temperatures has been observed (18-19). The amount of residual CI" ions in the reduced ruthenium catalyst is also related to the type of support, where CI" ions are more prevalent in RU/AI2O3 than in Ru/Si02 (79). We are currently investigating the amount of residual CI" ions in the RuY system. Effects of Reduction. Reduction of both the RuY and the NaCl treated RuY at 773 Κ results in the formation of large ruthenium particles which have migrated out of the zeolite supercages. This finding is in agreement with prior work with RuY prepared from R u C l 3 : 3 H 2 0 (9). The x-ray diffraction data feature a strong ruthenium pattern for both samples. Using line width analysis of the x-ray peaks we estimate the size of the ruthenium particles to be 12.0 nm in the RuY and 7.4 nm in the NaCl treated material. The smaller particles in the NaCl treated RuY indicates that this material is harder to reduce even at high temperature, supporting the presence of CI" contarnination in this sample. The lattice destruction that occurs during the formation of these large particles was followed using N2 isotherms (Figure 6). Although the adsorption capacity drops with high temperature reduction, the shape of the isotherm does not change. Evidently, the migration of ruthenium during the reduction process does not result in the formation of mesopores. Effects of Calcination. Calcination of RuY or NaCl treated RuY under flowing O2 at 773 Κ results in the formation of large Ru02 particles which are observable


TWAY ET AL.

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RuClflmpregtiated NaY Zeolite


160

RuY NaCl Treated RuY - -A

— • —

60

0.2

0.4

0.6

0.8

Relative Pressure (P/Po) Figure 5. N2 isotherms at 77 K for as-prepared RuY and NaCl treated RuY.




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RuCl^Impregnated NaY Zeolite

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using x-ray diffraction. The formation of large Ru02 particles was observed in RuY zeolite prepared from the Ru(NH3)6Cl3, although the particles were not large enough to be observed with x-ray diffraction analysis (6). However Shoemaker and Apple (9) found no evidence of the migration of ruthenium under oxidizing conditions and the formation of large particles of Ru02 in their work with RuY prepared from RuCl3. A l l of their samples were evacuated prior to oxidation under static conditions (9). When our samples are evacuated at 773 Κ and then exposed to oxygen at 773 Κ we observe the formation of large Ru02 particles, so the reason for the discrepancy is unclear. Because Ru02 is not soluble in acidic solutions, only the ruthenium that is not in the zeolite as Ru02 will be dissolved during the acid digestion used in the ICP/MS analysis. The solutions above the undissolved Ru02 particles were analyzed and the results indicate that only 3% of the ruthenium is not incorporated into Ru02 particles in the calcined RuY samples. Calcination of RuY does provide some stabilization of the silicon, probably due to migration of silicon into the defect sites produced during ion-exchange. Figure 7 shows 2 S i M A S N M R spectra of the starting R u Y and the calcined material. Calcination results in a decrease in intensity of the amorphous shoulder. In addition, NaCl treatment of the calcined material does not result in an increase in the surface area, but instead a decrease. However, the surface area of the calcined material is lower than that of the starting RuY due to the formation of Ru02 particles (Table 2). The Ru02 particles can not be reduced at room temperature, but reduce readily at 773 K . The ruthenium particles produced after this reduction procedure are estimated to be 16 nm in diameter from x-ray diffraction line width analysis. The reduction results in further loss of crystallinity, reflected by a drop in surface area and microporosity (Table 2). In addition, the position of the asymmetric T-O stretching vibration is at 1071 cm~l, indicating a very silicon-rich material. 9

Conclusion Ion-exchange of NaY zeolite with RuC13 solutions produces dealuminated materials. However the dealumination is not simply due to the low pH of die exchange solution. NaCl treatment of these materials allows the preparation of zeolitic materials containing very high ruthenium content with good microporosity. These novel materials may prove to be useful for a variety of catalytic reactions. Unfortunately, the ruthenium sinters and migrates out of the zeolite under reducing and oxidizing conditions, limiting the applicability of these materials. Acknowledgments This work was supported by the National Science Foundation under grant CHE8718850. This material is based upon work supported under a National Science Foundation Graduate Fellowship. We thank Prof. Bernard C. Gerstein for discussions pertaining to CI" ion contamination of supported ruthenium catalysts. Literature Cited 1. 2. 3.

Vannice, M. A . In Solid State Chemistry of Energy Conversion and Storage; Goodenough, J. B . ; Whittingham, M. S., Eds.; Advances in Chemistry Series 163; American Chemical Society: Washington, D. C., 1977, 15-32. Yashima, T.; Ushida, Y.; Ebisawa, M.; Hara, N . J. Catal. 1975, 36, 320. Chen, Y. W.; Wang, H . T.; Goodwin, J. G. Jr. J. Catal. 1983, 83, 415.



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9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Nijs, H.; Jacobs, P.; Uytterhoeven, J. J. Chem. Soc., Chem. Commun. 1979, 181. Lunsford, J. H. In Metal Microstructures in Zeolites; Jacobs, P. Α.; Jaeger, P.; Jiru, P.; Schulz-Ekloff, G., Eds.; Studies in Surface Science and Catalysis 12; Elsevier Scientific Publishing Company: Amsterdam, The Netherlands, 1982, 1-13. Verdonck, J. J.; Jacobs, P. Α.; Genet, M.; Poncelet, G. J. Chem. Soc., Faraday Trans.I,1980, 76, 403. Wehner, P.; Hindman, J. C. J. Am. Chem. Soc. 1950, 72, 3911. Scherzer, J. In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T. E., Jr.; Dalla Betta, R. Α.; Derouane, E. G.; Baker, R. T. K., Eds.; American Chemical Society Symposium Series, 248; American Chemical Society: Washington, D. C.,1984; 157-200. Shoemaker, R.; Apple, T. J. Phys. Chem. 1987, 91, 4024. Reed, C. P.; DeAngelis, R. J.; Zhang, Y. X.; Liou, S. H. In Advances in X -ray Analysis 34; Barrett, C. S.; Gilfrich, J. V.; Noyan, I. C.; Huang, T. C.; Perdecki, P. K., Eds.; Plenum Press: New York, 1991; 557-565. Flanigen, Ε. M.; Szymanski, Η. Α.; Khatami, H. In Molecular Sieve Zeolites; Advances in Chemistry Series 101; American Chemical Society, Washington, D. C., 1971, Vol. 1; 201-230. Lee, E. F. T.; Rees, L. V. C. J. Chem. Soc., Faraday Trans. I 1987, 83, 1531. Kim, J. T.; Kim, M. C.; Okamoto, Y.; Imanaka, T. J. Catal. 1989, 115, 319. Garwood, W. E.; Chen, Ν. Y.; Bailar, J. C., Jr. Inorg. Chem. 1976, 15, 1044. Garwood, W. E.; Lucki, S. J.; Chen, Ν. Y.; Bailar, J. C., Jr. Inorg. Chem. 1978, 17, 610. Garwood, W. E.; Chu, P.; Chen, Ν. Y.; Bailar, J. C., Jr. Inorg. Chem. 1988, 27, 4331. Khan, M. M. T.; Ramachandraiah, G.; Rao, A. R. Inorg. Chem. 1986, 25, 665. Narita, T.; Miura, H.; Sugiyama, K.; Matsuda, T.; Gonzalez, R. D.J.Catal. 1987, 103, 492. Bossi, Α.; Garbassi, F.; Petrini, G.; Zanderighi, L. J. Chem. Soc., Faraday Trans.I,1982, 78, 1029.

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Characterization of RuCl3-Impregnated NaY Zeolite - ACS

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