J. Phys. Chem. 1083, 87, 1713-1722
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Preparation of Molybdenum Zeolites from Molybdenum Hexacarbonyl. 1. Infrared Studies Suheil Abdo+ and Russell F. Howe' Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 5320 1 (Received: May 19, 1982: In Final Form: December 13, 1982)
Infrared spectroscopy has been used to study the adsorption and decomposition of Mo(CO)~ in the hydrogen and sodium forms of zeolite Y. Two forms of adsorbed MO(CO)~ me found in both H-Y and Na-Y: a weakly (physically)adsorbed complex and a more strongly perturbed species. On being heated in vacuo above room temperature, the adsorbed Mo(CO)~decomposes. In H-Y, three distinct subcarbonyl species are formed reversibly. At temperatures of 200 "C or higher, decarbonylation is irreversible, and oxidation of the molybdenum occurs, as indicated by loss of zeolite OH groups and appearance of oxomolybdenum species. Carbon monoxide chemisorbed on oxidized molybdenum sites gives a characteristicinfrared band at 2170 cm-'. Infrared evidence is presented for interaction of ammonia with intra-zeolitic molybdenum cations. Surface area, X-ray diffraction, and low-frequency infrared measurements indicate that the zeolite crystallinity is retained on heating in vacuo up to 500 "C, but loss of structure occurs on heating in oxygen at high temperature. In Na-Y, a single subcarbonyl species is formed reversibly upon initial decomposition of adsorbed Mo(CO)~.A t higher temperatures metallic molybdenum appears to be formed which does not adsorb appreciable quantities of CO. The resistance of the MoNa-Y to oxidation by O2depends on the temperature of prior activation.
Introduction Transition-metal-exchanged zeolites show a wide range of interesting catalytic properties.' These result in part from the unsaturated coordination of intra-zeolitic transition-metal ions and from the steric and electrostatic influences of the zeolite matrices. The regular crystalline structures of zeolites also allow the preparation and characterization of transition-metal complexes in a surface environment which is more well-defined than that of most conventional heterogeneous catalysts.2 Catalysts containing molybdenum are widely used for hydrodesulf~rization,~ hydr~genation,~ oxidation,, and metathesisa6 The characterization of supported or mixed oxide catalysts containing molybdenum has received considerable attention in the literature, but definitive identification of active sites has not in general been possible. There is thus an incentive to examine the properties of molybdenum-exchanged zeolites, both from the viewpoint of favorably modifying the catalytic properties of molybdenum and with a view to modeling the active sites on conventional molybdenum oxide catalysts. Direct ion exchange of molybdenum from solution into a zeolite is difficult, due to the absence of simple salts of molybdenum which are stable under solution ion-exchange conditions. (A recent patent42claims aqueous ion exchange of MoOz2+into zeolite Y.) A solid-state ion exchange of MoCl, with the hydrogen form of zeolite Y (H-Y) was reported by Dai and Lunsford;' however, the exchange reaction resulted in some loss in crystallinity of the zeolite. A less direct method for loading molybdenum into zeolite Y has been described by Gallezot et al.839 The volatile carbonyl Mo(C0)G was adsorbed into H-Y zeolite and then decomposed by heating in vacuo. During decomposition the initially zerovalent molybdenum became oxidized by the zeolite protons, releasing H2. The overall chemistry of the oxidative decarbonylation in the zeolite resembles that found for many transition-metal carbonyl complexes *Address correspondence t o this author a t the Chemistry Department, University of Auckland, Auckland, New Zealand. 'Union Oil Company of California, Science and Technology Division, Brea, CA 92621.
on hydroxylated supports.l0 The purpose of the present study was to examine in detail the decomposition of Mo(CO), adsorbed in H-Y and in the corresponding sodium form of the zeolite and to characterize as completely as possible the molybdenum zeolites produced. Such characterization is a necessary prelude to studies of the chemisorption and catalytic properties of molybdenum in zeolite Y. This paper describes primarily infrared spectroscopic data which complement the earlier results reported by Gallezot et al.9 A second paper1' will describe electron paramagnetic resonance spectra of paramagnetic species produced in molybdenum zeolites.
Experimental Section The sodium form of zeolite Y was obtained from Linde Co. (SK40). The ammonium form was prepared by double ion exchange with 1 M NH4N03 at 70 "C, which yielded a material in which 70-8070 of the sodium cations were replaced with NH4+. Mo(CO)~(Alfa Inorganics) was used without further purification. The gases used, 02,CO, and anhydrous NH, (Matheson), were purified by conventional methods. 13C-enrichedCO was supplied by Stohler Isotope chemicals. Zeolite samples for infrared experiments were pressed into self-supporting wafers of thickness 4-10 mg cm-2 in (1) See, for example, C. Naccache and Y. Ben Taarit, Pure Appl. Chem., 52, 2175 (1980). (2) See, for example, J. H.Lunsford, Catal. Reu. Sci. Eng., 12, 137 (1975). (3) G. C. Schuit and B. G. Gates, AIChE J., 19, 417 (1973). (4) E. A. Lombardo, M. Ldacono, and W. K. Hall, J. Catal., 64, 150 (1980). ( 5 ) G . W. Keulks, J . Catal. 19, 232 (1970). (6) J. J. Rooney and A. Stewart, Spec. Period. Rep.: Catal., 1, 277 (1977). (7) P. E. Dai and J. H.Lunsford, J. Catal., 64, 173 (1980). (8) G. Coudurier, M. P. Gallezot, H. Praliaud, M. Primet, and B. Imelik, C.R . Hebd. Seances Acad. Sci., Ser. C , 282, 311 (1976). (9) P. Gallezot, G. Coudurier, M. Primet, and B. Imelik, ACS Symp. Ser. No. 40, 144 (1977). (10) See, for example, D. C. Bailey and S. H.Langer, Chem. Reu., 81, 109 (1981), and references therein. (11) S. Abdo and R. F. Howe, J . Phys. Chem., following article in this
issue.
0 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
a stainless-steel die at pressures of 10000-20 000 psi. Samples were placed in a high-vacuum infrared cell of conventional design equipped with a furnace region into which the zeolite wafer could be raised magnetically. Zeolite H-Y was prepared in situ by outgassing NH4-Y in vacuo at 623 K for 2 h, followed by overnight calcination in O2 at 623 K and further evacuation at the same temperature. Infrared spectra of the zeolite were found to be identical before and after the oxygen treatment, which was therefore omitted in some experiments. The Na-Y zeolite was dehydrated in situ by outgassing in vacuo up to 623 K. Adsorption of MO(CO)~ into the pretreated zeolites was carried out at room temperature. A side arm containing previously degassed Mo(CO), was opened to the infrared cell for periods varying from 5 s to 12 h; the room-temperature vapor pressure of Mo(CO), is approximately 0.08 torr.12 Spectra were recorded during the adsorption of Mo(CO), and following subsequent treatment of the zeolites. The infrared spectrophotometers employed were a Beckman IR-12 and a Nicolet MX1 Fourier transform instrument. The IR-12 was operated in a double-beam transmission mode, with a spectral slit width of 5 cm-' at 2000 cm-'. Spectra recorded on the MX1 were the average of typically 100-200 scans at 2-cm-' resolution (although up to 2500 scans were averaged when it was necessary to observe minor changes in the low-frequency region). Surface area and X-ray powder diffraction measurements were carried out in the laboratory of Professor J. Fripiat.13 BET surface areas were determined with N2 at 77 K. Powder diffraction patterns were measured (in air) with a CGR diffractometer using Cu Ka! radiation. Samples were prepared for these measurements in an analogous fashion to the infrared samples.
Results Infrared Spectra of MoH-Y. Three regions of the infrared spectrum were found to be of interest in monitoring the adsorption and decomposition of Mo(CO), in H-Y prepared from NH4-Y. The carbonyl stretching region (2400-1400 cm-') showed changes due to adsorption and decarbonylation of Mo(CO),, and subsequent readsorption of CO. The interaction of zeolite protons with Mo(CO), could be followed by observing the hydroxyl stretching region (4000-3000 cm-'). Perturbations of the zeolite framework and the formation of oxomolybdenum species could be monitored in the low-frequency metal-oxygen stretching region (1000-400 cm-'). Figure 1shows a series of spectra recorded in the carbonyl stretching region following exposure of H-Y to MO(CO)~ vapor for increasing periods of time. Initially, three bands appeared at 1950,2003,and 2046 cm-'. Scale expansion revealed a fourth band at 2123 cm-' (Figure IC). All of the bands grew in intensity as a function of exposure time, and after 15 min the three major bands were too intense to be measured (Figure If). The relative intensities of the four bands varied with the exposure time (and from one experiment to another). The 2123- and 2003-cm-l bands maintained a constant intensity ratio, as did the 2046- and 1950-cm-' bands. A t least two distinct species are therefore present during adsorption of Mo(CO), (hereafter labeled A and B). The relative contribution of species A (2123 and 2003 cm-') to the spectrum increased with increasing exposure time. Subsequent evacuation at room temperature caused no change in the carbonyl spectrum. When the temperature (12) R. It. Monchamp and F. A. Cotton, J . Chem. Soc., 1438 (1960). (13) A. Carlson, unpublished results.
Abdo and
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C M-' Flgure 1. Infrared spectra of Mo(CO), adsorbed in H-Y: (a) base-line spectrum of H-Y; (b) exposed to MO(CO)~ vapor at 0.08 torr for 20 s; (c) 1OX expansion of b; (d) 45 s; (e) 90 s; (f) 15 min.
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CM-l Flgure 2. Infrared spectra of MoH-Y following activation in vacuo for 30 min at (a) 50, (b) 75, (c) 100, (d) 125, (e)150, (f) 175, and (9)200 OC.
was raised however, the spectrum began to change in a complex fashion. Figure 2 shows a series of spectra recorded after activation in vacuo for 30 min at 50 "C and
IR Studies of Molybdenum Zeolites
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TABLE I: Carbonyl Stretching Frequencies of Species Observed during Adsorption and Decomposition of Mo(CO), frequencies: cm" assignment Mo(CO), in H-Y (This Work) A B
2123, 2003 2045,1950 1931, 1854, 1753 1909, 1833, 1668 2066, 2013, 1980
C D
E
Mo( CO), (physically adsorbed) Mo(CO), (chemisorbed) anionic dinuclear species ( ? ) Mo(CO),O,
Mo(CO), in Na-Y (This Work) 2120,2000 Mo( CO), (physically absorbed) 2040,1970 Mo(CO), (chemisorbed) 1951, 1917, 1790, Mo,(CO),,~- (?) 1762 Mo(CO), in H-Y (Ref 9 ) Mo(CO),
2125, 2050,1995, 1955 2045, 1965, 1905, 1835,1670 2070, 2020,1985 a
not assigned 1
Mo(CO),O,
+ 2 em-'.
at successive 25 "C intervals. The various carbonyl bands which appeared and decayed together are labeled in the figure, and their frequencies listed in Table I. The five distinct groups of bands thus correspond to a minimum of five different carbonyl species. Identification of these species is discussed below. The relative concentrations of the different species varied with the extent of Mo(CO)~ uptake (time of exposure to Mo(CO)~vapor). For example, the lower frequency bands (species D) were more intense relative to those of B and E in samples exposed to Mo(CO)~ for 1 2 h than in samples exposed to Mo(CO)~for short periods only. Activation at 200 "C almost completely removed all carbonyl bands (Figure 2g). Decarbonylation was accompanied by sample color changes from white through red-brown to black. Admission of CO to samples activated at temperature up to 200 "C reversed in part the decarbonylation process. For example, the spectrum in Figure 3a was obtained by adding 50 torr of CO to a sample activated at 100 "C and is similar (but with reduced intensity) to that obtained on initial adsorption of Mo(CO)~(Figure 1). At the same time, the original white color was restored. The spectrum could be cycled back and forth between Figures 3a and 2c by successive addition of CO and outgassing at 100 "C. The color changes were also completely reversible. Admission of 13C0to a sample activated at 100 "C gave the spectrum shown in Figure 3b. This is identical with the spectrum obtained with l2C0 (Figure 3a) except that all bands are shifted 40-50 cm-' to lower frequency, indicating that complete exchange with 13C0has occurred. In contrast, no exchange took place if I3CO was admitted following exposure to l2COand evacuation of the gas phase. Figure 3c shows a spectrum obtained by adding CO to a sample activated at 200 "C. The same bands appeared as in Figure 3a, but 1 order of magnitude less intense. Samples activated above 200 "C contained no residual carbonyl bands, but addition of CO caused the appearance of very weak bands above 2000 cm-'. In order to see these bands more clearly, we performed experiments with zeolite wafers which were 1order of magnitude thicker (about 50 mg cmV2)than those used to study the decomposition of Mo(CO)@Figure 4 shows transmittance spectra (obtained with the Beckman spectrophotometer) of CO adsorbed on MoH-Y activated at 400 "C for 30 min (spectrum a) and for 12 h (spectrum b). The increased sample thickness can be seen by comparing the intensity of the zeolite band at
2300
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C M-' Figure 3. Infrared spectra of MoH-Y: (a) activated at -100 "C in vacuo, then exposed to 80 torr of CO at 25 "C;(b) activated at 100 "C in vacuo, then exposed to 45 torr of 13C0 at 25 "C;(c) activated at 200 "C in vacuo, then exposed to 45 torr of CO at 25 "C.
about 1850 cm-l with that in Figure la. After prolonged activation at 400 "C a single band due to chemisorbed CO appeared at 2170 cm-' (Figure 4b; the lower frequency shoulders were removed on evacuation of the gas phase). This band could be removed by outgassing at 200 "C. The 2170-cm-l band also appeared along with two lower frequency bands (Figure 4a) when CO was adsorbed in samples activated less severely. Exposure of MoH-Y to oxygen at any point during the decomposition of adsorbed Mo(CO)~caused a complete and irreversible removal of all remaining carbonyl ligands. Changes in the OH stretching region following adsorption and decomposition of Mo(CO)~ in H-Y are shown in Figure 5. The H-Y as prepared in situ by decomposition of NH,-Y has a characteristic OH spectrum consisting of bands at 3644 and 3546 cm-l.14 Exposuie to Mo(CO), vapor caused complete removal of the 3644-cm-' band and the appearance of a new broad band centered at about 3480 cm-' which overlapped completely the 3546-cm-l band (Figure 5b). Evacuation at 100 "C partially restored the 3644-cm-' band and almost completely removed the broad 3480-cm-' band, although the original 3546-cm-l band was not restored to its original intensity (Figure 5c). On outgassing at successively higher temperatures (30 min at each temperature) the overall intensity of both OH bands gradually decreased, and after 90 min at 500 "C (Figure 5g) the zeolite was almost completely dehydroxylated. Subsequent reduction in hydrogen at 450 "C partially restored both bands, to about 10% of their original intensity. The infrared spectra of zeolites show intense bands below 1300 cm-' due to vibrations of the aluminosilicate framework. The most intense of these, at about 1150 and 1050 cm-', can be observed only by diluting the zeolite in a KBr pellet.15J6 The less intense lower frequency bands can however be observed with a pure zeolite wafer (sufficiently thin). Figure 6a shows a low-frequency spectrum (14)J. W. Ward. Adu. Chem. Ser.. No.101.380 (1971). (15) E. M. Flanigen, H. Khatami, k d H. A: Szymanski, Adu. Chem.
Ser., No. 101,201 (1971). (16)E.M.Flanigen in "Zeolite Chemistry and Catalysis", J. f i b o , Ed., American Chemical Society, New York, 1976,ACS Monogr., No. 171,p 80.
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Abdo and Howe r
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4000
38
30
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1
34
32
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CM-' Figure 5. Infrared spectra in v ( 0 H ) region: (a) H-Y; (b) exposed to Mo(CO), vapor at 0.08 torr for 15 min; activated in vacuo for 30 min at (c) 100, (d) 200, (e) 300, and (f) 400 OC and (9) at 500 OC for 90 min.
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C M-' Figure 1. Infrared spectra of CO adsorbed in MoH-Y following activation at 400 OC for (a) 30 min and (b) 12 h.
of H-Y prepared by in situ decomposition of NH4-Y. The bands above 1000 cm-' are cut off by the absorbance limit of the spectrophotometer, but three major lower frequency bands are resolved. Adsorption of Mo(CO)~at room temperature caused the appearance of three new bands (845, 666, and 595 cm-') indicated by arrows in Figure 6b, and possibly some broadening of the zeolite framework bands a t 805 and 755 cm-'. Activation in vacuo a t 100 " C gave the spectrum shown in Figure 6c. The new bands appearing on adsorption of Mo(CO)~were removed, but an intense band appeared a t 930 cm-'. Exposure to CO at room temperature reversed these changes; the 930-cm-' band was removed, and the 595-cm-l band restored to about 20% of its original intensity (the 845- and 666-cm-I bands were then to weak to be detected).
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