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Conclusion -4method for quantitative estimation of the contributions of dispersion, induction, and orientation energies to cohesion in polar organic liquids has been proposed and illustrated with the methyl n-alkyl ketones. Dispersion always predominates, with induction in a minor, but constant, role. Orientation rapidly loses significance as the hydrocarbon side chain is increased.
Induction energy is more important than is generally appreciated, contributing 5-10% to the total cohesion of the higher, “mostly nonpolar” 2-ketones.
Acknowledgment. We acknowledge many stimulating discussions with Dr. Myer Rosenfeld and thank the Director of the U. S. Army Coating & Chemical Laboratory for permission to publish this work.
Infrared Study of Adsorption of Carbon Dioxide, Hydrogen Chloride, and Other Molecules on “Acid” Sites on Dry Silica-Alumina and y-rilumina’
by J. B. Peri Research and Development Department, American Oil Company, Whiting, Indiana
(Received M a y 3, 1866)
The adsorption of various molecules on specially prepared transparent plates of dry, higharea silica-alumina and y-alumina was studied by infrared and gravimetric techniques. Carbon dioxide was selectively and reversibly adsorbed on a few so-called “acid” sites on both silica-alumina and y-alumina, producing an infrared band near 2375 em-‘ for the former and near 2370 cm-l for the latter. Use of adsorbed CO2 as an indicator allowed titration of these sites, tentatively named “CY sites,” with other adsorbates which displaced carbon dioxide. The CY sites on silica-alumina selectiveIy adsorbed carbon dioxide, acetylene, butene, benzene, and hydrogen chloride. They also strongly adsorbed ammonia and water, but these adsorbates were also held by many other sites. Their reaction with hydrogen chloride produced hydroxyl groups. Their surface concentration fell in the range 3-9 X 1012/cm2. Similar sites on y-alumina predried at 800’ had a surface concentration of about 5 X 1012/cm2. The CY sites are apparently formed by condensation of AI-OH groups during dehydration of the surface. They contain a reactive oxide ion (or ions) in close proximity to an exposed aluminum ion. Their role in catalysis is not yet clear.
Introduction Although the important “acidic oxide” catalysts have been studied intensively for many years, little is known about the surface sites responsible for their activity.2sa Active sites are usually thought to be acidic, but the exact nature of the acidity remains controversial. 3,4 Numerous attempts have been made to measure and characterize the acidity by adsorption of NH,, butylThe Journal of Physical Chemistry
amine, or other bases, often using organic indicators. While such methods show wide variations among sites on a given catalyst and among distributions of sites on (1) Presented in part at 145th National Meeting of the American Chemical Society, New York, N. Y., Sept 1963. (2) L. B. Ryland, M. W. Tamele, and J. N. Wilson, “Catalysis,” Vol. 7, P. H. Emmett, Ed., Reinhold Publishing Corp., New York, N. Y.. 1960.
ADSORPTION OF C02, HCl, ETC.,ON SILICA-ALUMINA AND 7-ALUMINA
different catalysts, the results usually correlate poorly with catalytic properties, particularly when different types of catalysts are compared. Silica-alumina cracking catalysts represent a wide range of compositions but usually exhibit similar catalytic properties. y-Alumina also develops cracking activity after it has been dried above 400",but it is very easily poisoned by H20.5 Although these catalysts must differ considerably in surface structure, there is reason to expect that the active sites have common structural features which can be demonstrated and identified. Only a small fraction of the acid sites on the surface of silica-alumina catalysts are active for crackingJ6and even these may be atypical. Special sites which may be catalytically important appear to exist at surface concentrations) of 1013sites/cm2 or although this has been q ~ e s t i o n e d . ~ Infrared studies of y-alumina,svg and silicaalumina12 have provided some information about the surface structure of these oxides, but many questions remain. Infrared studies of NH3 adsorbed on dry y-aluminaI3 and ~ i l i c a - a l u m i n a , ' ~of~ ~C~ 0 2 on silica16 and alumina," of butene on alumina18and silicaalumina,lg and of acetylene on alumina20 and silicaalumina19 have also been reported. Because a better understanding of the nature of the active sites on acidic oxides is greatly needed, further infrared study of silica-alumina and dry y-alumina was undertaken using various adsorbed molecules as "probes" to identify and characterize adsorption sites. To facilitate study of molecules covering less than 1% of the surface, the oxides were prepared in the form of transparent aerogel plates. Experimental Section Most of the cells, equipment, and procedures have been described.sp18 Perkin-Elmer Model 12C and 112 spectrometers (equipped with CaF2 and LiF prisms) were supplemented with a Beckman IR-9 (prism-grating) spectrometer. The cell most frequently used (cell Cis) permitted sample weight changes and spectral changes to be measured concurrently. In experiments involving reaction of vapor with silica, the special cell shown in Figure 1 was used. It is entirely of glass and fused quartz. Connections to the cell (numbered in Figure 1) were sealed off as will be described. The preparations of y-aluminag and silica" aerogel plates have been described, as have the sources and purification of SH3, HCl, COz, 1-butene, D,, and D20.8,18,19 Acetylene was prepared from calcium carbide and dried with P,OS. All other chemicals were
3169
commercial materials of high purity and were usually dried with P206. Preparation and Properties of Silica-Alumina Aerogels. Silica-alumina aerogel plates were prepared both by reaction of "dry" silica with A1C13 vapor (followed by hydrolysis and redrying) and by reaction of silica hydrogel with solutions of AIC13 or Al(N03)S to which NH40H had been added (followed by established procedures for converting the hydrogel to a transparent, aerogel).lg Because the surface characteristics as weli as the optical and mechanical properties of aerogel plates may be influenced by the method of preparation, preparative procedures are given in detail. Samples SAV-1 and SAV-2 were made from the same batch of silica aerogel plates, whose original surface area (after calcination in O2 and drying by evacuation for 2 hr at 600") was 785 m2/g as measured by If2 adsorption. Evacuation for 1 hr at 800", rehydration for 8 hr at 100" in 15 torr of H 2 0 vapor, and subsequent evacuation for 1 hr at 600" and 1 hr at 800" reduced this to 750 m2/g. The A1C13was purified by repeated sublimation under vacuum and sealed off in tube A (shown in Figure 1). A silica aerogel plate was placed in a quartz holder in tube B and was then calcined in 200 torr of O2 at 600" for 1 hr and dried by evacuation at either 600" (SAV-1) or 800" (SAV-2) for 1 hr. The cell was sealed off at 1 (Figure l), and the break-seal between tubes A and B (3) K. V. Topchieva, G. RI. Panchenkov, M. A. Kaliko, A. V. Agafonov, L. I. Pigusova, N. M.Kamakin, and Ta. V. Mirsky, Proceedings of the Fifth World Petroleum Congress, Section 111, New York, N. Y., 1959, p 133. (4) A. E. Hirschler and J. 0. Hudson, J. Catalysis,3, 239 (1964). (5) W. K. Hall, F. E. Lutinski, and H. R. Gerberich, ibid., 3, 512 (1964). (6) G. A. Mills, E. R. Boedeker, and A. G. Oblad, J. A m . Chem. SOL, 72, 1554 (1950). (7) H. P. Leftin and W. K. Hall, Aetes Congr. Intern. Catalyse, Paris, 1960, 1, 1353 (1961). (8) J. B. Peri and R. B. Hannan, J. Phys. Chem., 64, 1526 (1960). (9) J. B. Peri, ibid., 69, 211 (1965). (10) R. 5. McDonald, ibid., 62, 1168 (1958). (11) J. B. Peri, ibid., in press. (12) 11.R. Basila, ibid., 66, 2223 (1962). (13) J. B. Per], ibid.,69, 220 (1965). (14) J. E. hlapes and R. P. Eischens, ibid., 58, 1059 (1954). (15) D. E. Nicholson, Xature, 186, 630 (1960). (16) R. P. Eischens and W. 4. Pliskin, Adaan. Catalysis, 9, 662 (1957). (17) N. D. Parkyns, "Third Congress on Catalysis," Vol. 11, NorthHolland Publishing Co., Amsterdam, 1965, p 914. (18) J. B. Peri, Aetes Congr. Intern. Catalyse, de, Paris, 1, 1353 (1961). (19) J. B. Peri, "Third Congress on Catalysis," Vol. 11, NorthHolland Publishing Co., Amsterdam, 1965, p 1100. (20) D. J. C. Yates and P. J. Lucchesi, J. C h m . Phys., 35, 243 (1961).
Volume 70, iVumber 10 October 1966
J. B. PERI
3170
was opened. Then the AICIa was sublimed back and forth between A and C through the heated (600") cell until spectra showed that all OH groups had been removed from the silica surface. The evolved HCI was collected and sealed off in C for later measurement. All glass external to the furnace was flamed to drive excess AlCla back into A (cooled with liquid N2) which was then sealed off. The cell was again connected to the vacuum system through a break-seal, and hydrolysis was carried out by repeated admission of small doses of H 2 0 vapor at loo", with intervening heating a t 400 to 600" to desorb HC1 H1O. The evolved HCI was collected and dried with P20bbefore subsequent measurement. Data obtained in preparation of the two SAV samples are shown in Table I. Analyses for A120a were ohtained on final samples. The calculated per cent A120s for SAV-1 assumes that each AICla molecule reacted with two OH groups. The higher value "found" indicates that some 1: 1 reaction occurred.
+
Table I: Data from Preparation of SAV Aerogels
w
HCI neovend, om: (STP) From initid
AlCli
0"
Sample
reaction
hydrolysis
SAV-I SAV-2
8.6 4.6
Estd-4.3 5.9
% AhO, Cdcd
(8.9) 6.3
c
on fins1gel
Found
9.5 6.2
The final samples were too small to permit chloride analyses, but analyses of similar preparations indicated that very little chloride was left after hydrolysis. I n preparation of SAV-1, some of the HC1 evolved on hydrolysis was accidentally lost, preventing independent check on chloride retention, but the amount of HCI recovered in preparation of SAV-2 was in agreement with that expected from removal of all chloride. Surface areas were not determined directly for SAV-1 and SAV-2. The areas were assumed to he the same as those determined after similar treatment, except for AICla reaction, for the silica aerogel (SAV-1, 780 m2/g; SAV-2, 750 m2/g). Surface areas obtained for other preparations of silica-alumina made by similar procedures support this assumption. Dry weights for the silica plates used in preparation of SAV samples were SAV-1, 0.110 g, and SAV-2, 0,113 g. The SAA samples were prepared from silica hydrogel plates that were repeatedly washed in distilled H 2 0 and then equilibrated with solutions of AICla (SAA-1 and -2) or AI(NO& (SAA-3) which either had been
I I I I I L
Figure 1. Cell used in studying reaction of AlCl, with surface OH groups: a, Pyrex-to-quartz graded seal; b, thermowell; c, furnace; d, cell body of I-in. square fused-quartz tubing; e, aerogel plate.
(SAA-2 and -3) or were subsequently (SAA-1) titrated with NHlOH solution to a final pH of -3.5. Final solutions were clear. After equilibration for several days, the gel was heated in the solution for 6 hr at 100" in an autoclave. During the following week, it was washed six to eight times with H20 and four or five times with methanol, which was then removed above its critical temperature in an autoclave. The final plates were almost as transparent as typical silica aerogel plates. The dry weights of the plates studied were 0.21 0.01 g. Differences in pH, concentration of solutions, and other variables gave gels having the compositions and surface areas listed in Table 11. A commercial silica-alumina (Nalco HA-l), containing 26% A l 2 0 1 and having a surface area of 495 m2/g, was also studied briefly. It was pressed at 12,OOO psi into a thin (-28 mg/cm2) self-supporting wafer for infrared study with the Beckman IR-9. After drying a t 600" or higher, all silica-alumina catalysts were active for butene isomerization and polymerization, and there is little doubt that the aero-
*
ADSORPTION OF COz, HC1,
ETC., ON
SILICA-ALUMINA AND T-ALUMINA
Table I1
wt SAA-1 SAA-2 SAA-3
%
Surface area,
AlzOa
mz/g
8.4 32.7 13.6
592 618 495
gels were as active for hydrocarbon cracking as typical silica-alumina catalysts. l 9 Procedures. Typical procedures used in pretreatment and infrared study of the catalysts have also been described.*Il8 The catalysts were normally cooled to room temperature before admitting adsorbates or recording spectra. For titrations of surface sites using adsorbed COz as an indicator, 2 or 3 torr of COz was first admitted and, after brief contact, mostly removed by evacuation for 1 to 5 min. With the spectrometer set at 2375 cm-l (COz band) and the chart drive running, a small dose of the vapor used for titration was admitted. When the intensity was not reduced further, a second dose was added, etc. Equilibration normally took a t least 30 min per dose. I n experiments with the SAA aerogels, weight changes were recorded concurrently; they agreed within 10% with values expected from the known dosages. Because quartz cells and thick samples were used in most of the work, spectral study was usually confined to the region above 2000 cm-', but some study was made in cells with CaF2 or XaC1 windows to locate spectral features of possible significance at lower frequencies.
Results Comparison of Spectra of Silica-Alumina with Those of Silica and y-Alumina. Spectra were recorded for all the silica-alumina samples after progressive drying under vacuum at temperatures between 400 and 800". Although there were differences between gels, particularly in the wtention of H-bonded OH groups after drying, there was no band that consistently distinguished silica-alumina from silica. The resemblance was, in general, closer than that reported by Basila.12 Every feature of the OH spectra of silica-alumina aerogel plates dried a t 600' or higher had previously been noted in spectra of silica aerogel." A band near 3650 cm-l may or may not appear in spectra of silicaalumina, depending on the composition and extent of surface hydration, but a similar band or bands can appear in spectra of some pure silica gels. Frequency shifts of the 3750-cm-' band on heating appeared identical with those on silica" for all samples dried above 600'. As with silica, a "hot band" was observed a t
3171
3580 cm-' for silica-alumina heated above 700". No obvious or consistent difference was found in the rate of exchange of residual OH groups with D, or DzO. A weak band near 1394 cm-' was observed under some conditions on Nalco HA silica-alumina, but it disappeared on repeated strong drying and rehydration, and no support was found for assignment12of this band to vibration of a surface A1-0 linkage. A band at 4520 cm-I in the spectrum of silica aerogel assigned to combination of stretching and deformation frequencies for OH groups" prompted investigation of bands in this region on silica-alumina and on yalumina. On silica-alumina (SAA-2 and SAV-l), the band at 4520 cm-' appeared no different from that observed on silica. Because bands in this region are relatively weaker on dry y-alumina, a thick sample of alumina aerogel (-300 mg/cm2) was examined after drying at 750": it showed bands at -4585, 4350,and 4107 cm-' but none a t 4520 cm-I. Adsorption of NH,. Adsorption of NH, on silica-
A
0 3
l I / - " l
-w 0 5
z V
i
1.0 1.5 ~~
3300
3400
3500
3600
W A V E N U M B E R [ CM-']
Figure 2. Spectra of NHa adsorbed on SAA-2. A. Adsorbed a t ~ 3 . 5 'to surface coverages (molecules/100 A*) &s follows: a, 0 (predried at 800"); b, 0.3; c, 0.6; d, 1.6; e, 2.5. B. After further adsorption subsequent evacuation for 0.5 hr a t temperatures as follows: a, ~ 3 5 " ;b, 200"; c, 400"; d, 800" (12C spectrometer, LiF prism).
+
Volume 70, Number 10 October 1966
3172
alumina was unselective and complicated as illustrated by the spectra of NH3 on SAA-2 shown in Figure 2. Bands differed somewhat for different silica-alumina samples. On dry silica-alumina, NH3 was held principally as NH3 on Lewis acid sites (bands in range 33003400 and a t 1630 cm-l) as also found by others,14v16but even very dry samples showed some NH4+ (band near 1470 cm-l). In some cases there was also evidence for Si-NHz (weak bands near 3450 and 1560 em-l). When silica-alumina holding NH3 adsorbed at room temperature was heated under vacuum a t progressively higher temperatures, the appearance of the NH stretching bands changed markedly. Many more sites hold NH3 than are catalytically active, and various types of adsorption sites exist. Because chemisorption of NH3 on y-alumina forms some NHZ and OH groups13 it would be interesting to know whether similar adsorption of NH3 occurs on active sites on silica-alumina. Attempts 1 o answer this question were unsuccessful, owing to slow similar chemisorption on dry silicall and to rapid exchange of H between NH3 and preexisting OD groups on deuterated silica-alumina which prevented readily establishing possible formation of a few new OH groups. Adsorption of COz on Silica-Alumina and Silica. A few surface sites on silica-aluminas predried at 600” or higher invariably held COz fairly strongly. Such adsorption produced a strong band near 2375 cm-’, part of which typically persisted after 5 min evacuation at room temperature. Figure 3 shows the characteristics of this band as well as smaller, associated bands or shoulders a t 2406 and 2355 cm-’. As COz was progressively removed by evacuation, the main band generally shifted slightly to a higher frequency. The most strongly held COz gave a band a t 2377 cm-’. The extent of C02 adsorption varied with different silica-aluminas and depended not only on temperature and pressure but also on the predrying. Drying a t 800” rather than 600” markedly increased retention. At higher pressures, the COz adsorbed on silica-alumina also iiicluded a more weakly held component, as evidenced by a second major band (-2345 em-l). For example, spectra illustrating the adsorption of COz on SAA-1 and SAA-3 as a function of pressure are shown in Figure 4. Some contribution from gaseous COz is also present at the higher pressures. Xegligible C 0 2was usually adsorbed on silica aerogel below 2 torr. At higher pressures, it produced a band near 2345 cm-’ (as also on Cabosi116),but it was usually held weakly and was rapidly removed by evacuation. However, after 1 ewek of contact with C02 (263 torr), highly dried silica showed trace retention (band a t 2345 The Journal of P h y s h l Chemistry
J. B. PERI
0.0
I
I
I
I
I
0 1
yl
U
z a
g
0.3
: a
0,s
2374
I .c I.: 2320
1360
1400
40
W A V E N UM B E R [ CM-’l
Figure 3. Spectra of COZadsorbed on SAA-2: a, predried a t 800°, exposed to CO, a t 3 torr followed by 30-see evacuation; b, after further evacuation (IR-9 spectrometer, double-beam operation).
cm-l) even after extended evacuation. This apparently indicates slow penetration of COz into “closed pores.” Weights recorded concurrently with the spectra and at the pressures shown in Figure 4, together with similar data for COzadsorption on silica aerogel, are represented by Figure 5. For SAA-1, the open points represent data obtained after drying a t 600” a t intervals throughout prolonged use. One point, specially designated, was obtained after “strong” adsorption of C02had been blocked by prior chemisorption of HC1 as will be discussed below. The dashed curves, which represent “strongly adsorbed” COZ on silica-alumina, were obtained by subtracting the adsorption on silica from the adsorption on silica-alumina. Below 5 torr most of the difference in GOz adsorption between silica-alumina and silica must have been due to the COz responsible for the 2375-cm-l band, because there was no indication of other adsorbed forms of COZ. On this basis, we can conclude that adsorption producing the 2375-cm-l band must have amounted to a t least 2 X 10l2molecules/cm2 for SAA-1 dried a t 600”, and to 7 x 1012molecules/cmZ for SAA-3 dried at 800”. A maximum number of sites for such adsorption can be taken as the maximum difference shown by the
3173
ADSORPTION OF COz, HCI, ETC., ON SILICA-ALUMINA AND Y-ALUMINA
lA
10
WAVENUMBER (CM-’)
Figure 4. Spectra of COZadsorbed on silica-alumina. A. SAA-1: a, after drying a t 600”; b, after COZaddition, P = 30 torr; torr; g, 1.0 X 10+ torr; h, 3.5 X torr. B. SAA-3: a, after drying c, 19.0 torr; d, 9.8 torr; e, 2.5 torr; f, 4.4 X torr; e, 5.0 X torr (12C spectrometer, a t 800’; b, after COz addition, P = 4.2 torr; c, 0.2 torr; d, 1.0 X LiF prism). I
I
I
I
P (TORR1
Figure 5. Adsorption of COZon silica aerogel and on silica-aluminas (-35’): a, on silica aerogel; b, on SAA-1; c, on SAA-3; d, difference between adsorption on silica and on SAA-1 (= b - a); e, difference between adsorption on silica and on SAA-3 ( = c - a ) ; f, adsorption on SAA-1 after chemisorption of HCI.
dashed curves (-5 X 1012/cm2for SAA-1 and -16 X 1012/cm2 for SAA-3). The existence of an upper limit for the concentration of these sites was also established
by blocking them with adsorbates held more strongly than COz, as will be shown. For SAA-1 the “net” adsorption approximates a Langmuir isotherm for a surface containing about 5 X 1OI2 sites/cm2. The plot of Plweight adsorbed us. P does not give a straight line, however, being instead concave downward, particularly at lower pressures. Similar curvature was obtained by plotting Plabsorbance us. P using intensities of the 2375-em-’ band a t pressures between 2 torr torr. The adsorption sites thus apand 3.5 X parently vary in strength as also indicated by the shift in frequency of the 2375-em-’ band as surface coverage decreased. The heat of adsorption of CO, at low coverage on these sites wsts estimated as roughly 15 kcal/mole from spectra and pressure measurements at 40, 80, and 120” (using the Clausius-Clapeyron equation). The sites on which COz giving a band in the 2360-2380-cm-’ region is held will be called cr sites. Adsorption of HCZ. On all of the silica-aluminas, some HC1 mists chemisorbed to form new OH groups. This was evidenced either by a marked increase in the H-bonded “tail” below the isolated OH band at 3750 em-’ or by a distinct band near 3610 em-’, as shown in Figure 6. The intensity of the 3750-cm-l band was not reduced. When DC1 was added, a new OD band was produced near 2660 cm-’. Chemisorption was rapid a t room temperature but Volume 70, Number 10
October 1966
J. B. PERI
3174
n n
-'-I
I
I
2300
2340
2380
0
W A V EN UM B E R J-CM-')
Figure 6. Hydroxyl bands on SAA-3: a, after drying a t 800'; b, after subsequent chemisorption of HCl (12C spectrometer, LiF prism).
Figure 7. Spectra showing effect on COZadsorption of prior chemisorption of HCl: a, SAA-1 dried at 600"; b, after addition of COZ ("3 torr); c, after evacuation, adsorption of 0.4 cm3 (STP) of HCI, and readdition of COZ ( ~ torr); 3 d, after evacuation, further adsorption of HCl, and readdition of COZ (12 torr) (12C spectrometer, LiF prism).
not extensive even with large additions of HCI. The extent varied with silica-alumina sample, with predrying temperature, and with contact time but after short contact was usually less than 2 X 1013molecules/ cm2. Chemisorption of HC1 completely blocked subsequent adsorption of COz on a sites. Following the experiments on COz adsorption, SAA-1 was redried under vacuum at 600" (1 hr) and exposed to about 3 torr of COz a t room temperature. No major change was noted in the number of a sites. After a 15-min evacuation to remove adsorbed COz, a small dose (0.48 em3 (STP)) of HC1 was admitted to the cell, and it was adsorbed almost completely. After evacuation for 5 min, 83% of the HCl remained on the aerogel. When COz was then added as before, the spectrum, shown in Figure 7, indicated only a small fraction of the original CO, adsorption. The adsorbed COzwas again removed by evacuation and a large dose (5.35 em3 (STP)) of HC1 was admitted. This addition increased retention of HCl after 5 min evacuation by 33%. When COz was then admitted a t 12 torr, none was adsorbed on the a sites. Maximum chemisorption of HCI was 1.1 X 1013 molecules/cm2. The number of a sites was evidently somewhat lower than this; from the extent of site-blocking by the first addition of HC1, the maximum was estimated as 9 X 1012/cm2. After reevacuation a t
600°, the plate was exposed to 5 torr of HC1 for 15 min. Permanent retention of HC1 in this instance was nearly 1.6 X l O I 3 molecules/cm2. Thus after the rapid and fairly selective blocking of the a sites, HC1 continued to chemisorb, but more slowly. For comparison, SAA-3 (predried a t 800") was later slowly titrated with just enough HCl to displace the COz held after a 5-min evacuation. The apparent number of a sites was 2.6 X 10*3/cm2. Blocking of QI Sites by NH3 or HzO. Compared with HCI, two to four times as many molecules of NH3were required to block the a sites for subsequent adsorption of COz. This was partly caused by slow rearrangement of adsorbed NH3 on the surface. Equilibration overnight at room temperature or heating to 200" and slow cooling somewhat decreased the amount of NH3 required. Equilibration for 0.5 hr at room temperature was, however, normally allowed. Figure 8 illustrates data obtained on SAA-1. As shown in curve d, the complete adsorption of 1.0 cm3 (STP) of KH3failed to block COz adsorption completely after 0.5-hr equilibration. After 16 hr, however, COZ was no longer adsorbed. The NHa added corresponded to 2.0 X 1013 molecules/cm2. Over three times this amount was retained after the sample had been exposed to 50 torr of NHB followed by evacuation for 0.5 hr a t room temperature.
3500
3600
3700
3800
WAVENUMBER (CM-')
The Journal of Physical Chemistry
ADSORPTION OF COz, HCl,
0 1
ETC., ON
SILICA-ALUMINA AND 7-ALUMINA
-
w V
1 0 -
I S (0-
2300
1
I
I
I
2340
$
8
"
2380
"
'
-
2420
3175
would indicate 9.3 X 10l2 sites/cm2, but the spectra showed dimer and possibly higher polymers, indicating 1 4 . 6 X 10l2CY sites/cmz. Like butene, acetylene was adsorbed very selectively by the CY sites. Apparently, it retained its triple bond, as evidenced by acetylenic C-H stretching bands near 3240 and 3275 cm-l.19 No band was observed near 2000 cm-l ( C z C stretch). However, silica-alumina adsorbs strongly in this region, so a weak C=C stretching band would have been difficult to detect. Benzene, carbon disulfide, and ethyl bromide were also adsorbed with high selectivity on the CY sites. Fewer than 1013molecules/cm2of any of them completely displaced COz on SAA-1 predried at 600". Pyridine, like NH, and HtO, was held strongly at room temperature not only on the CY sites but on many others as well. Compared with Cot, NzO as such was strongly held on about the same number of sites (and probably on the same sites), but direct displacement of NzO with COz or vice versa was not attempted.
WAVENUMBER (CM-')
Figure 8. Spectra showing effect on CO2 adsorption of prior adsorption of NHs: a, SAA-1 dried at 600'; b, after addition of COz (-3 torr); c, after evacuation, adsorption of 0.4 cm3 (STP) of NHa, and readdition of COn ("3 torr); d, after evacuation, adsomtion of 1.0 om3 (STP) . . of NHs. and readdition of COZ i"3 torr); e, after standing for 1 hr following d ; f, after evacuation for 5 min (12C spectrometer, LiF prism).
Table 111: Results of Titration of Silica-Aluminas with CdH, HC1, and NH3 PreSample
SAV-1
Results with other silica-aluminas were similar. On SAV-1 adsorption of a total of 1.15 cm3 (STP) of NH3 blocked 85% of the CY sites after 0.5-hr equilibrsG tion, and 95% after 16 hr. After the same sample had been reevacuated a t 600" to restore the original CY sites, the addition of only 0.53 cm3 (STP) of HCl was adequate to block all of them. I n blocking CY sites, HzO resembled NHa. Compared with HC1, two to five times as many molecules of HzO were needed to block COz adsorption completely. Blocking of CY Sites 012 Silica-Alumina by Butene, Acetylene, and Other Molecules. 1-Butene was adsorbed on a sites wit>hvery high selectivity, and it remained olefinic at first (band at 3020 cm-' Is). Compared with HC1, fewer butene molecules were needed to block COz adsorption completely. Normal titration procedure was followed except with SAA-1. After SAA-1 had been predried a t 600°, tested for COz adsorption, and evacuated to remove most COZ,a larger than usual dose 1.3 cm3 (STP) of butene was added. After 10 min, the cell was briefly evacuated. The 0.7 wt yo butene retained on the surface completely blocked COZ adsorption on CY sites. Adsorption of butene as monomer
SAV-2 SAA-1 SAA-2 SAA-3
drying temp, 'C
600 800 800 600 800 800
No./cmz (X10-13) ---to block a sitesC4H8 HC1 NHa
0.53 0.81 0.73 0.4 0.31 0.79
No./oml ( X 10-18) max retention of NHa
No/A1 max retention of NHs
0.87
1.7
C=O Possibly, the 2370-cm-l and 1870-cm-I bands are related through an equilibrium such as 0
ll
//
-
The Journal of Physical Chemistry
0
C
C
0
//
02-
Al+ Al+
---.f
+--
/\
0
0
A1
A1
I
I
The concentration of CY sites on silica-alumina is known within fairly narrow limits. It can hardly be greater than that indicated by HC1 chemisorption and hardly less than that indicated gravimetrically at pressures below 2 torr. Steric factors4 cannot explain the low concentrations of a sites found in this study, because C02 and the other molecules used as titrants are all fairly small. The NH3 titrations show, moreover, that the CY sites represent only a fraction of a larger group of sites which are virtually equivalent for adsorption of NH3 and presumably also of n-butylamine. Thus, a large steric factor is not required to explain differences in the number of sites measured by Hirschler4 and by Leftin and Hall.’ Characterization of the strength and number of acid sites by titration with n-butylamine using organic indicators may be very misleading. If the acid color of the indicator is produced (22) A. Terenin and V. Filimonov, L‘Hydrogen Bonding,” D, Hadsi, Ed., Pergamon Press, Inc., New York, N. Y., 1959,p 545. (23) E.B. Cornelius, T. H. Milliken, G. A. Mills, and A. G. Oblad, J . Phys. Chem., 59, 809 (1955). (24) J. Overend, private discussion. (25)B. M. Gatehouse, S. E. Livingstone, and R. S. Nyholm, J . Chem. sot., 3137 (1958).
ADSORPTION OF COS, HCl,
ETC., ON
SILICA-ALUMINA AND T-ALUMINA
on only a few sites belonging to a large class wherein all sites hold butylamine strongly, titration will falsely indicate that all sites in the class are as strong as those holding the indicator. The difference between the acid sites on the SAA-2 and SAA-3 shown in Table IV apparently does not fairly reflect differences between these catalysts. Probably the presence of a few very strong acid sites on SAA-3 has in this instance caused classification as pK. < -8.0 of sites which would otherwise have been titrated as pK, = -5.6 sites. Description as pK, = - 5.6, moreover, may properly apply to only a small fraction of the sites normally so characterized. The QI sites show important adsorptive properties, but although these sites are either active or similar in nature to sites which are active, they are not essential for catalytic activity. Blocking all QI sites by chemisorption of HC1 increases activity for polymerization of butene.lg Possibly, slow desorption of products normally limits the catalytic contribution of the CY sites, while similar sites which hold olefins less strongly are responsible for most of the catalyzed reaction. Possibly, a cooperative effect is needed between an CY site and an adjacent Brpinsted acid site, but no direct evidence presently supports such speculation.
3179
The a sites, which preferentially and strongly adsorb many unsaturated or polar molecules, are evidently ionic and therefore should promote the ionic dissociation of adsorbed hydrocarbons. Yet butene and acetylene seem to be held on such sites on silica-alumina with relatively little change in their character; there is no spectral evidence for hydride or proton abstraction (or for proton addition) to form organic ions. Ions of such hydrocarbons are probably formed only transiently on active sites and readily escape detection because of their very low steady-state concentration. The ability to identify and study separate types of surface sites and to establish the type of site on which adsorption of a given molecule occurs is essential for ultimate solution of the complex problems presented by oxide catalysts. Some progress has been made in this direction, but much more remains to be done in relating individual types of sites, or combinations of these, to the actual catalytic behavior of a surface.
Acknowledgments. Special acknowledgment is made of the contribution of R. J. Bertolacini, who measured the acidities given in Table IV. The assistance of J. Kekich in most of the experimental work is also gratefully acknowledged.
Volume YO, Number 10 October 1866
