HIGH-TEMPERATURE INFRARED SPECTROSCOPY OF OLEFINS
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High-Temperature Infrared Spectroscopy of Olefins Adsorbed on Faujasites
by P. E. Eberly, Jr. Esso Reeearch Laboratories, Humble Oil and Refining Company, Baton Rouge Refinery, Baton Rouge, Louisiana (Received October 11, 1966)
A high-temperature infrared cell for recording spectra of solids and adsorbed species at temperatures up to 650" and pressures from low6to 760 mm is described. Thus, spectra recorded at 427" show that hydrogen faujasite (HY) produced by calcining NH4Y under vacuum to 427" contains three different hydroxyl groups having absorption frequencies of 3740, 3635, and 3540 cm-I. These groups readily exchange with deuterium gas to form their OD analogs. When hexene-1 is adsorbed on HY at 93", the double-bond character disappears. At 150", polymerization and dehydrogenation processes begin to occur to form a conjugated polyene type of structure as evidenced by a band at 1600 cm-l. Upon heating to 260" , cyclization occurs to form a hydrogen-deficient aromatic ring structure characterized by a band at 1580 cm-I. With the exception of ethylene, which formed no adsorbed species capable of detection, similar results were observed with other low molecular weight olefins, although differences in extent of reaction were found. The condensed-ring structure was not produced on other ion-exchanged forms of zeolite Y with the possible exception of AgY in which partial reduction could have occurred to form some HY. The main reaction on other ion-exchanged forms involved the loss of double-bond character.
I. Introduction In the characterization of solid surfaces and adsorbed species by infrared spectroscopy, almost no spectra have been obtained a t elevated temperatures. Since most catalytic reactions of practical importance occur at high temperatures, the need for spectral studies in situ at these conditions is obvious. First of all, with high-temperature spectroscopy, changes in catalyst structure, particularly with regard to the hydroxyl groups, can be directly observed. With in situ measurements, it is possible to avoid partial rehydration which sometimes occurs when the solid is lowered to room temperature for spectral studies. This effect can be particularly serious in systems which are not completely baked out. A second advantage of high-temperature infrared spectroscopy lies in the possibility of studying adsorbed species at actual reaction conditions, provided that their surface concentration is sufficiently large to permit their detection. This paper describes a new high-temperature infrared cell and associated equipment for recording spectra of solids. Results are presented on the interaction of the various ion-exchanged faujasites with hexene-1 and other olefins at 90-427'.
Faujasite, a crystalline sodium alumino-silicate belonging to the zeolite family, can be readily synthesized and is frequently referred to as zeolite Y. This material has a si1ica:alumina molar ratio of near 5 : 1 and consequently is more stable to heat and steam than its counterpart, zeolite X, which has a silica: alumina ratio of only 2.5: 1. Outside of the difference in composition, however, the anionic frameworks are structurally identical.'J Heats of adsorption3 as well as catalytic activity4depend strongly on the nature of the exchangeable ions. A number of investigators have recorded the spectra of various ion-exchanged faujasites at room temperature after activation at elevated temperature^.^-* With the exception of the (1) L. Broussard and D. P. Shoemaker, J . Am. Chem. Soc., 82, 1041 (1960). (2) D.W.Breck, J. Chem. Educ., 41, 678 (1964). (3) P. E. Eberly, Jr., J. Phys. Chem., 66, 812 (1962). (4) J. A. Rabo, P. E. Pickert, D. N. Stamires, and J. E. Boyle, Actes Congr. Intern. Catalyee, P,Paris, 1960,2055 (1961). (5) L. Bertsch and H. W. Habgood, J. Phya. Chem., 67, 1621 (1963). (6) J. L. Carter, P. J. Lucchesi, and D. J. C. Yates, ibid., 68, 1385 (1964). (7) J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, ibid., 69, 2117 (1965).
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hydrogen or "decationiaed" form, faujasites do not contain large concentrations of hydroxyl groups. Carter, Lucchesi, and Yates,6 as well as Angell and Schaffer,* detected their existence on zeolites X and Y. Their concentration, however, appears to be small. By a deuterium-exchange technique, Uytterhoeven, Christner, and Hall' determined that the amount of such groups on KaY is 0.15 X 1020 OH/g for 1-p crystals. This quantity is commensurate with the number of groups needed to terminate the external crystal faces. On the other hand, hydrogen faujasite contains nearly 50 times as many hydroxyl groups, which is sufficient to account for the number of "decationized" sites. Although studies of olefin adsorption on other types of solids have been made,9s10work on zeolites has only recently begun to appear. Ethylene adsorption a t room temperature on faujasite-type catalysts has been studied by Carter, et al.," and Liengme and Ha11.12
11. Experimental Section Materials. The composition of sodium faujasite (Nay) is given in Table I. Other forms were obtained from this material by standard methods of ion exchange. HY was prepared by first exchanging the sodium with ammonium ions. For this purpose, 180 g of NaY was treated with a solution of 333 g of r\TH4N03 dissolved in 3 1. of water. The exchange was conducted for 2 hr a t 70". After allowing settling and decanting the supernatant liquid, a fresh solution of NH4N03 was added and the treatment repeated. After a total of five such treatments, the solid was filtered, thoroughly washed, and oven dried at 150" overnight. Analysis of this sample is given in Table I. Table I : Properties of Na and NHa Faujasites
Composition as expressed by empirical formula, moles NalO (NHI~O
AWa SiOz Relative crystallinity Toluene adsorption capacity a t 93" and 1 mm, mmoles/g"
FiaY
NHiY
1.13 0 1.00 4.72 1.00 2.4
0.08 0.87 1.00 4.67 0.93 2.1
Prior to adsorption, samples were degassed a t 427'.
monium form has a 5% deficiency in cations. This small amount may be due to errors inherent in the analyses or to the presence of some alumina not incorporated in the zeolite structure. By comparing the intensity of the X-ray diffraction lines, the crystallinity of the NH4Y was estimated to be over 90% of that of Nay. Adsorption capacity a t low pressure, which constitutes an alternate method for estimating structure retention, gave essentially the same results. To prepare HY, NH4Y was heated in situ in the infrared cell up to 427" under vacuum. This process removed the physically adsorbed water and liberated NHI gas to produce HY in which the hydrogen atoms exist in the form of hydroxyl groups. Other forms of faujasite were prepared by ion exchanging with the chloride salts of the desired ion. For AgY, the nitrate salt was used. In general a threefold excess of the ion was employed and the treatment was repeated three times. Sample Preparation. For infrared examination, the solids were ground with a mortar and pestle. Then they were compressed under 30,000 psi into 1.25-in. diameter disks. These disks were about 8-16 mils thick and contained 12-22 mg of solid/cm2. One of the dificulties in this procedure was the removal of the disk from the die for insertion into the sample disk holder. This was overcome by putting a slight bevel on the circumference of the plunger used for compression. Thus during fabrication, the pressure at the edge of the disk was much less than that a t the middle and the powder did not bind strongly to the die. Apparatus. Spectra were recorded with a CaryWhite Model 90 infrared spectrophotometer. An important advantage of this instrument is its ability to measure spectra at elevated temperatures without undue interference from the furnace radiation. In this double-beam instrument, the infrared radiation is chopped prior to passing through the sample. Since the detection system is designed to respond only to the chopped radiation, the continuous radiation from the hot sample and furnace is not observed. In general, spectra were obtained in the region of 40001200 cm-' at a spectral slit width of 4 cm-' and a scan speed of 3 cm-'/sec. The high-temperature infrared cell used in this (8) C. L. Angell and P. C. Schaffer, J . Phys. Chem., 69, 3463 (1965). R.P. Eischens and W. A. Pliskin, AdGan. CutaZysG, 10, 2 (1958). (10) D. J. C . Yates and P. J. Lucchesi, J . Phys. Chem., 67, 1197
(9)
Since, for electrical neutrality, the sum of the moles of exchangeable cation oxides should equal the moles of alumina, there appears to be a 13% excess of extraneous soda in Nay. On the other hand, the amT h e Journal of Physical Chemistry
(1963). (11) J.
L. Carter, D. J. C. Yates, P. J. Lucchesi, J. J. Elliott, and V. Kevorkian, ibid., 70, 1126 (1966). (12) B. V. Liengme and W. K. Hall, Trans. Faraday Soc., 62, 3229 (1966).
HIOH-TEMPERATURE INFRARED SPECTROSCOPY OF OLEFINS
T. C.
+Y T;C. I
1719
increased heat conduction. In experiments where temperature control was critical, about 1 mm of helium was introduced to the system prior to hydrocarbon injection to eliminate this effect. To cancel out the absorption of infrared radiation by the gas phase, a dummy cell is placed in the reference beam and connected to the same vacuum and gasdosing system.
III. Results
X
----
u‘ 1 I
Figure 1. Diagram of high-temperature infrared cell and sample holder: A, O-ring seal; B, chromel-alumel thermocouple; C, handle for sample disk holder; D, supporting ring; E, stainless steel sample disk holder; F, water-cooling jacket; G, Glyptal seal; H, 26/8 x in. KBr disk; I, %in. “VycOr” glass tube; J, l1/,-in. diameter sample disk.
study is shown in Figure 1. It was constructed from 2-in. diameter “Vycor” glass tubing and was connected to the associated vacuum and gasdosing system by an O-ring seal (A). Two KBr disks (H) were attached to the ends of the tubing with “Glyptal.” To keep these seals (G) cool during high-temperature operation, water-cooling jackets (F) were installed at each end. The central portion of the cell was wound with heating wire and carefully insulated so that the sample disks could be heated up to 650”. A front view of the sample holder is shown in the right-hand portion of Figure 1. The sample disk (J) is inserted into a slot in the holder. With handle (C), the holder can be inserted into the cell and supported on a glass lip by ring D. A sheath containing two chromel-alumel thermocouples (B) is then inserted into the holder so that the junctions are near the center of the disk. The output of one thermocouple is sent to a recorder and that of the second to a stepless, proportional controller for accurate temperature control. With this system, spectra of solids can be measured in situ up to 650” and 10-h760 mm. Upon introducing gases to the high-vacuum system, the temperature of the disk was sometimes observed to increase momentarily 10-15” above the set point owing to
Hexene-1 Adsorption on Deuterium Faujasite (DY ) . Upon heating to 427” under vacuum, NH4Y loses both adsorbed water and NH, gas to form HY. The hydrogen exists in the form of characteristic OH groups as shown by the solid-line spectrum in Figure 2 taken a t 427’. Three distinct hydroxyl groups are observed at 3740, 3635, and 3540 cm-I . This spectrum is similar to those recorded previously for HY a t room temp e r a t ~ r e . ~No * ~ evidence is observed for decreases in intensity of the OH bands with temperature. Upon exposure of HY to 50 mm of deuterium gas a t 427”, the three OH groups can be almost quantitatively converted to their OD analogs as observed by a shift in the infrared bands to 2750, 2680, and 2610 cm-l, respectively. The spectrum of DY, recorded a t 427”, is given by the dashed line in Figure 2. DY exhibits strong interactions with adsorbed olefins. After recording the dashed-line spectrum in Figure 2, the excess deuterium gas was evacuated from the system and the sample cooled to 93’ under vacuum. The sample was then exposed to 2-mm pressure of hexene-l for 15 min and the spectrum in the bottom portion of Figure 3 was obtained. First of all, hexenel upon adsorption loses its doublebond character as shown by the absence of an
YRIpYaCf
Ill CY.‘=
Figure 2. Infrared spectra of H(D)Y at 427’. The solid line represents the spectrum of HY. The dashed line represents the spectrum obtained after exposure to 50 mm of Dt at 427’. The original disk contained 19 mg/cma of NH,Y.
Volume 71,Number 6 Mag 1067
P. E. EBERLY, JR.
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so
-
so
-
.',. \
.*
FREQllENCY I N CY.'1
Figure 3. Infrared spectra of DY after exposure to hexene-1 a t 2-mm pressure. The bottom spectrum is that obtained upon exposure of DY to 2-mm pressure of hexene-1 a t 93". After heating the sample to 427' the upper spectrum was obtained. The original disk contained 19 mg/cma of NHaY.
olefinic CH stretch band which normally occurs a t 3090 cm-I in pure gas-phase hexene-1. At the same time, a rapid exchange occurs between the hydrogen atoms in the adsorbed species and deuterium in the OD groups. This is shown by the appearance of CD linkages absorbing a t 2210 and 2170 cm-1 and by the regeneration of the characteristic OH frequencies which blank out even a t short exposure times. Relative to the 2610-cm-l band, the band a t 2680 cm-1 has become weaker, suggesting that the hydrocarbon has preferentially adsorbed near these groups causing a lowering in absorption frequency and/or broadening of the band. Upon removal of the adsorbed material by evacuation and heating to 427", the 2680-cm-I band again becomes the more intense, as shown by the spectrum in the upper portion of Figure 3. Adsorption of Various Olejins on Hydrogen Faujasite ( H Y ) . The interaction of hexene-1 with HY was studied in more detail under conditions in which temperature surges were eliminated by prior injection of a small amount of helium. After injection of 1 mm of helium a t 93", hexene-1 was introduced to the system until the pressure reached 2 mm. The spectrum was recorded after a 1-hr exposure. Additional spectra were recorded after a 1-hr exposure a t successively higher temperatures of 149, 204, and 260". Spectra in the OH and CH stretch region are given in Figure 4. As the temperature is raised, progressively less material is adsorbed on the surface, as evidenced by the decreasing absorbance of the CH stretching bands a t 2960, 2930, and 2880 cm-l. Olefinic CH stretch vibrations, which occur above 3000 cm-l, were not observed a t any temperature investigated. The adsorbed material is seen to interact preferentially with the hydroxyl groups a t 3635 cm-l. Such an interaction normally results in the displacement of the band to The Journal of Phyaical Chemistry
0
3000
8900
2800
1700
FREQUENCY IN CY.-1
Figure 4. Infrared spectra in the OH and CH stretch regions of adsorbed species created by exposure of HY to hexene-1. The solid line represents the spectrum of HY exposed to 2 mm of hexene-1 a t 93'. The remaining spectra were taken as the temperature was raised to successively higher values: - -, 149'; -, 204'; and ., 260'. The original disk contained 16 mg/cm2 of NHaY.
-
-
a
+
lower frequencies. However, no discrete, additional bands were found to occur, the spectrum merely exhibiting a small depression in transmission throughout the whole OH stretch region. In addition to these effects, reactions leading to cyclization and formation of aromatic ring structures were also observed. Spectra illustrating these phenomena are shown in Figure 5 and were taken a t the same conditions as those in Figure 4. In this region of the spectrum, bands due to C=C stretching modes, aromatic ring vibrations, and CH bending modes occur. At 93", where the concentration of adsorbed material is highest, as indicated by the intense CH band absorption a t 1460 and 1380 cm-', no olefinic vibration a t 1630 cm-' owing to the C=C stretching mode is observed. The broad, low-intensity band in this region is due to some vibration of the solid, since it is present even before exposure to the olefin. At 149", however, a new band appears at 1600 cm-I which is indicative of the formation of olefinic groups in a conjugated polyene type of s t r ~ c t u r e . ' ~This ~ ~ ~structure must be highly unsaturated, since no olefinic CH stretch vibrations are observed. At the same time, the intensity (13) R. N. Jones and C. Sandorfy, "Chemical Applications of Speotroscopy," Interscience Publishers, Inc., New York, N. Y., 1956, Chapter 4. (14) E.R. Blout, M. Fields, and R. Karplus, J. Am. Chem. Soc., 7 0 , 194 (1948).
HIGH-TEMPERATURE INFRARED SPECTROSCOPY OF OLEFINS
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Table 11: Absorbance" of Bands in 1600-1580-cm-1 Region on HY at 2-mm Pressure Temp, OC
Hexene-1
93 149 204 260
0 0.93 (1600)* 3.48 (1590) 4.13 (1580)
' Per gram of solid.
Pentene-1
Butene-1
Isobutylene
Propylene
0 0.71 (1600) 0.67 (1595) 4.18 (1580)
0 0.92 (1600) 1.04 (1590) 2.45 (1585)
0 3.91 (1590) 3.48 (1590) 4.27 (1585)
0 2.00 (1600) 5.38 (1590) 9.40 (1580)
' Numbers in parentheses represent wavenumbers (cm-l).
"t
I
60
0
1100
1
WOO
I 1500
I 1400
1300
1700
FRllQVENCY
IN
I 1600
I IS00
I
1400
I
1300
CY.
Figure 5. Infrared spectra in the 170&1200-~m-~region of adsorbed species created by exposure of HY to hexene-1. The lines have the same significance as those in Figure 4.
of the saturated CH bending frequencies decreases. Upon successive increases of temperature to 204 and 260", the new band becomes more intense and shifts progressively to a lower frequency near 1580 cm-'. This latter band is Characteristic of an aromatic ring vibration associated with the carbon skeleton and is not due to CH linkage~.'~J6 Its intensity, in fact, varies inversely with those associated with carbonhydrogen vibrations. This aromatic species is tightly adsorbed, since the 1580-cm-' band cannot be lowered in intensity by further evacuation and heating up to 427". The discharged disk was brownish in color. Similar results are observed with other low-molecular weight olefins, although differences in extent of reaction exist. Data are listed in Table I1 and were obtained in the same manner as described for hexene-1. With ethylene under the same conditions, no bands due to adsorbed species were observed. At 150", all the other olefins produce the new band a t 1600 cm-' due to a conjugated polyene structure. At higher temperatures, the band in all cases shifts to 1585-1580 cm-' characteristic of the aromatic ring structure.
Hexene-1 Adsorption on Other Ion-Exchanged Faujasites. The degree of interaction of hexene-1 with various faujasites depends on the nature of the cation. The reactions to produce aromatic ring structures appear to be peculiar to HY and AgY. With the latter material, however, some reduction of the silver occurred during exposure to the hydrocarbon. This could have resulted in the formation of HY, accounting for their similarity in behavior. With the other ion-exchanged faujasites, the main effect was the loss of double-bond character in the adsorbed material. An estimate of this loss is obtained by dividing the absorbance a t 1630 cm-I due to C=C stretching by that a t 1460 cm-I due to CH band vibrations. Values are listed in Table 111. In all cases, the C=C vibration occurs near 1630 cm-I which represents a shift of 12 cm-' from that in liquid hexene-1. For the alkali metal forms, the intensity ratio is about the same as observed for liquid-phase hexene-1. The ratio is markedly lower for the divalent forms, and with AgY and HY, no double frequency is observed. ~
~
Table 111: Hexene-1 Adsorption at 93' and 2 mm. Amount of Double-Bond Character in Adsorbed Phase %
Solid
exchange
AiaadAuso
NaY LiY KY CaY MgY CdY AgY HY
100 64 95 75 67 73 100 92
1.53 1.45 1.34 0.84 0.35 0.20 0 0
IV. Discussion High-temperature infrared spectroscopy shows that the OH groups on hydrogen faujasite exhibit nearly (15) C. N. R. Rao, "Chemical Applications of Infrared Spectroa-
copy," Academic Press Inc., New York, N. Y., 1963, Chapter 2.
Volunze 71, Number 6 May 18137
1722
the same infrared spectrum a t 427" as that a t 93". In this range, we see no evidence for proton delocalization. This effect has been observed on micas16 and also on decationized faujasite." Since the predominant OH bands blank out at both conditions, it is possible that some loss of intensity may have occurred which would have been beyond our detection. This loss, however, could not have been large. The OH groups absorb at 3740,3635, and 3540 ern-'. The groups a t 3740 cm-' are almost universally observed on silica-containing materials and are not believed to be necessarily characteristic of HY. Since zeolites are rarely completely free of amorphous material, it is possible that they could be associated with such an impurity rather than the cryscalline faujasite. The OH groups a t 3635 and 3540 cm-l appear to be peculiar to HY. Some authors have attributed the lower frequency band to interaction between two adjacent hydroxyl groups.* Liengme and Ha11,12however, suggested that the two bands may result from OH groups in different, crystallographic locations. The data in the present report favor the latter explanation. The groups a t 3635 cm-l are believed to be located inside the adsorption cages near six-membered oxygen rings. The other groups a t 3540 cm-l are in inaccessible bridge positions located between two sodalite units. The assignment of the lower frequency to the bridge position is consistent with the possibility of the proton being able to interact with twice as many oxygen anions as its counterpart in the cage position. Also, it explains the preferential interaction of olefins with the 3635-cm-' band. The fact that both groups in their deuterated form can be transformed into their hydrogen
The Journal of Physical Chemistry
P. E. EBERLY, JR.
analogs upon exposure to hexene-1 is probably due to the formation of hydrogen during the adsorption process. HY reacts strongly with adsorbed olefins causing them to polymerize, dehydrogenate, and cyclize to form aromatic ring structures. These reactions occur on the surface at 150-260", which is an unusually low temperature since normal aromatization processes operate a t 50Cb600". The polymerization reaction is the first to occur near 150", as evidenced by the appearance of a band at 1600 cm-l, indicative of a polyene structure. Propylene and isobutylene, being the most easily polymerized, show the greatest intensity of this band. At higher temperatures, the band shifts to 1585-1580 cm-l, characteristic of the aromatic ring structure. This sequence of reactions is one of the mechanisms of carbon formation. A band in this region has been reported for cokes isolated from silica-alumina catalysts.18 The interaction cf olefins with other ion-exchanged faujasites is less intense. The spectra of hexene-1 on alkali metal forms are not too dissimilar from that of pure liquid hexene-1. With divalent zeolites, there is a loss in double-bond character, as indicated by a decrease in intensity of the 1630-cm-l band. This may result from an interaction of the T electrons with the surface to form carbon to surface bonds or to the occurrence of some polymerization. (16) J. J. Fripiat, P. Rouxhet, and J. Jacobs, Am. Mineralogbt, 50, 1937 (1965). (17) Private communication, (18) P. E. Eberly, Jr., C. N. Kimberlin, Jr., W. H. Miller, and H. V. Drushel, Ind. Eng. Chem. Process Design Develop., 5, 193 (1966).
