1.526
J. R. PERIAND R . B. HANNAN
Vol. 64
anism. Neglected is the further competitive reaction 0 B5Hs-+ products which could account for the observed decrease in the rate of pentaborane production. Acknowledgment.-The authors gratefully acknowledge support in part by n'ational Science Foundation grant KSF-G7383.
+
Predicted rates are compared in Table V with the observed rates at the intensities a t which the pentaborane4 rate of production appeared to have reached again a constant value. The lack of compkte agreement indicates an incomplete mech-__-
SURFACE HYDROXYL GR()UPS O S y-ALUMISA' Research and Development Department, Standard Oil Company, Whiting, Indiana Received A p r i l 29, 1960
Even after drying a t 1000° -,-alumina continues t o evolve traces of water on further heating. This water appears to > bound to the surface of alumina and is known to affect its catalytic properties. To investigate the nature of bound water present in less than monolayer amounts, infrared study was made of transparent sheets of alumina aerogel. Hydroxyl groups, as well as water molecules, are initially held on the alumina surface. Heating a t 400" removes all water molecules, but leaves many of the hydroxyl groups. Alumina dried above 650' gives absorption bands a t 3698,3737 and 3795 cm.-l, assignable to "isolated" hydroxyl groups. These three bands can be eliminated from the spectrum either by further heating or by dwteriuin exchange, but they are not all removed at the same rate. Spectral changes ranging from perturbations to removal or creztion of hydroxyl bands are observed as a result of interactions of the hydroxyl groups with adsorbed molecules other than water. Independent variations observed in the intensities of the three hydroxyl bands indicate that these bands represent chemically distinct groups on at least three different types of surface sites. Such groups may play different roles in catalytic reactions on alumina.
Introduction because the aluminas used have caused high The cata1,ytic propert'ies of y-alumina depend scattering losses. The recent development of transparent plates largely upon the extent to which it has been dried.?--" Even after drying at 1000°, it continues of highly porous y-alumina aerogel'? has greatly to evolve traces of water, which is generally as- reduced the problem of scattering losses. Through sumed to be bound as hydroxyl groups and is increased sensitivity and convenience, they permit widely believed to exist on the surface of alumina infrared study of surface hydroxyl groups a t low crystallites. Hydroxyl groups have been as- coverages. Spectral changes resulting from exsumed to supply protons, either directly5 or in- change of surface hydroxyl groups with deuterium, in certain catalytic reactions on alumina. or from reaction with or perturbation by various Removal of hydroxyl groups has been postulated adsorbed molecules, have also h e n studied to as creating on the surface strained sites t'hat are obtain further information. Experimental catalytically Litt>le direct evidence is available, however, concerning the nature of Sol containing 5 to 7 weight % alumina mas prepared from either The adsorbed water or the hydroxyl groups. high-purity aluminum and acetic acid with a mercuric oxide Floating layers of the sol were convertrd12 t o Direct study of hydroxyl groups on alumina has ratalyst. transparent plates of aerogel. After calcination in air a t been handicapped by the lack of good experimental 600" to remove impurities, plates had specific surface areas techniques. Infrared spect'roscopy has sh0n.n of 300 t o 350 m.2/g., as measured by nitrogen adsorption, much promise in studies of surface groups and and were shoxn by X-ray analysis to be -/-alumina. The surface areas decreased by 10 to 20% after prolonged use adsorbed molecules on solid adsorbents.8 How- involving repeated evacuation a t temperatures up to 950' el-er, it has giT-en little detailed information about' and subsequent rehydration, but X-ray analysis showed n: hydroxyl groups or adsorbed water on a l ~ m i n a , ~ -conversion ~~ to a- or &alumina. After vacuum drying a t 600 (1) Presented >it 136th Meeting, American Chemical Society, Atlantic City, September, 1959. ( 2 ) J. C. F. Holm and R. W. Blue, I n d . Eng. Chem., 43, 501 (1951). 1.3) W.S. Brey, J r . , a n d K. A. Krieger, J . Am. Chem. S O C .7, 1 , 3637
!inlo).
(4) 8. IC. Hindin a n d S. W. Weller, Advances zn Catalysis, I X , 70 (1937). ( 5 ) A. G. O h l a c , J. U. Llessenger and H. T. Brown, Ind. Enq. Chem.
39, 1.162 f l 9 ! 7 ) . ( t i ) D. 4.Uowden, J . Chem. S o c . , 242 (1950). ' 7 ) E. B. Cornelius, T. H. Milliken, G. A. Mills a n d -4. G. Oblad, Tms J o r - 7 S . i ~59, . 809 (1955). (S) R . P. Ei-cliens and FIT. A. Piiskin, ddeances zn Catal?/sie,X , 1 (1958). (9) R7,A. Pliskin and R. P. Eischens, T H r s JOURNAL, 69, 1156
(1955). (10) A . C . Yang and C. W. Garland, ibid.. 61, 1504 (1957). (11) R. H. Lindquist a n d D. G. Rea, Presented at 132nd Meeting, Smerican Chemic31 Society, New York. September, 1957.
plates up to 5 mm. thick (-80 mg./cm.2) transmitted 90Yc or more of the incident radiation a t 2.5 p . Reagents used were high-grade commercial materials further purified in the laboratory. Deuterium gas (Stewart Oxygen Co.; purity > 99.570) was passed through a trap containing activated charcoal cooled with liquid nitrogen. Deuterium oxide (Stewart Oxygen Co.) was used without further treatment. All other chemicals were dried and then freed of permanent gases by vacuum distillation. These chemicals and the drying agents used were: reagent grade carbon tetrachloride, PZO'; anmonia (Matheson-anhydrous), freshly ignited CaO; 1-butene (Matheson C.P.), magnesium perchlorate; HCl (Matheson anhydrous), P20j. A Model 112 Perkin-Elmer spectrometer equipped n i t h calcium fluoride optics was used. Spectrometer housings were flushed with dry air passed over Ascarite to remove carbon dioxide. Spectral band width in the hydroxyl-stretching region was about 10 cm.?. (12) J. B. Peri a n d R B. Hannan, Speclrochzm. Acta, 16, 237 (1960).
Oct., 1960
SURFACE HYDROXYL GROUPS ON ALUMINA
The infrared cell, shown in Fig. 1, was constructed of Vycor and quartz and permanently mounted in normal position in the spectrometer. Calcium fluoride windows were sealed with silver chloride t o thin silver flanges, which were similarly sealed to the Vycor cell. Glyptal was used to seal any pinholes found in the window seals, and silver chloride or sealing wax was used t o seal the ground joint. The aerogel plate could be moved between the furnace section and the infrared beam by an external magnet. Reproducible positioning of the plate in the beam was assured by the design of the cell and sliding armature. Either the alumina and the surrounding gas or the gas phase alone could be studied before evacuation. Temperat'ure of the furnace section was controlled within 5'. The vacuum system was of conventional design. A mercury diffusion pump permitted evacuation of the cell t o less than 10-5"mm. of mercury. Provision was made for introduction of known amounts of various gases and vapors and for subsequent recovery of gases for analysis. Pressures were measured mith mercury manometers and McLeod gauges. herogel plates were usually evacuated for 1 t o 2 hours a t a given temperature and subsequently cooled to room temperature before spectra were recorded. Longer periods of evacuation were not considered necessary, because removal of m-ater from alumina occurs slowly after the first half hour and the extent of drying could be estimated from the spectra. Adsorption of various compounds was carried out a t low pressures, and an appreciable fraction of the added material was usually adsorbed. At appropriate intervals, the alumina was calcined in situ a t 600" in oxygen to remove possible organic contaminants. It was then rehydrated by admitting water vapor to the cell to about 1 em. pressure and heating a t 100 to 500" for periods of 15 minutes to 16 hours. Deuterium oxide n'as used t o rehydrate the alumina in certain cases to produce a "deuterated" plate. iilternatively, the alumina was deuterated by exchange with deuterium oxide or deuterium gas.
1527
7 IHt EM0 WE L .
+
-
8"
Fig. 1.-Infrared
cell.
Results and Discussion TJndried y-alumina aerogel shows strong broad adsorption bands near 3300 and 1650 cm.-I, corresponding to stretching and bending frequencies found in the spectrum of liquid water. These bands disappear after evacuation at 400'. A group of poorly resolved bands remains near 3700 em.-' with a hroad "tail" at' lower frequencies. =is the alumina is dried at, higher temperatures, the "tail" disappears and the remaining bands decrease in number a,nd intensit'y but become more sharply defined. After evacuation at 650 to 70Oo, y-alumina has well-defined absorption maxima at 3698, 3737 and 3795 cm.-'.13 These frequencies are within or above the range expected for the &retching vibrations of hydroxyl groups that are not hydrogenbonded.14 Such groups will be referred to as "isolated.!' Exposure of the alumina to deuterium oxide vapor and re-evacuation at 700" removed these three bands and produced three new ones a t 2733, 2759 and 2803 cm.-'. These frequencies are in the deuterosyl-st'retchiiig range, and each is related to t'he original frequency by t'he factor 0.738. All three of the original frequencies must therefore hare been due to hydroxyl-stretching vibrations. Because isotopic substitution shifted each band by the same factor, the multiplicity of bands cannot be due to combinat'ions of a single hydroxyl frequency with alumina-lattice vibrations.
2700 2800 2900 Frequency, cm. -1, Fig. 3.-Removal of deuteroxyl groups by evacuation.
113) R e a and Lindquist reported a t t h e 136th Meeting of t h e American Chemical Society (Atlantic City, September, 1959) their finding of three hydroxyl bands similar in frequency to those reported here. Their study was made on pressed disks of 7-alumina powder prepared from aluminum isopropoxide and dried a t 600°. (14) L. J. Bellaniy, "?'he Infrared Spectra of Complex Molecules," 2nCl Edition, John TT'ilea- and Eons, Inc., New York, N. Y., 1958.
Adsorption of large molecules generally reduces the peak intensity of the surface-hydroxyl bands and broadens and shifts them to lower frequencies. The effects observed when carbon tetrachloride is adsorbed on alumina predried at 800" are shown
1
3600 3800 4000 Frequency, cm.-l. of adsorbed CC1, on hydroxyl bands.
3400
Fig. 2.-Effect
I 0' 2600
~
J. B. PERI.4SD R. B. HAXKAX
528 0.0
f
d
0.5
z92 -3
0 1.o
2200 2600 3000 :3 400 Frequency, Fig. 4.-Removal of deuteroxyl groups by cvnciintion (replotted spectra). 1800
i di . t
0
2700
~3600
1 1 _ I_
~
2800 3800 Frequency, cm. -1. Fig. 5.--Exchange between Dz and hydroxyl groups at 250'.
I O
'
i
h
3400 3600 3800 4000 Frequency, cm.-l. Fig. 6.- Removal of hydroxyl bands by desorption of carbon tetra-hloride a t indicated temperatures.
in Fig. 2 . ' 5 The spectra were obtained with the indicated pressures of carbon tetrachloride in the cell. These effects demonstrate that the hydroxyl bands correspond to groups capable
Val. 64
of directly interacting with adsorbed large moleoules. The hydroxyl groups must therefore be on the surface, rather than within the alumina lattice. Changes in deuteroxyl bands were produced by evacuating "deuterated" alumina at progressively higher temperatures. Almost all hydroxyl groups on the alumina had previously been converted to deuteroxyl groups by exchange with deuterium oxide. The observed results, shown in Fig. 3, are analogous to those for the hydroxyl bands. The deuteroxyl spectra are shown rather than the hydroxyl spectra because of the better resolution of the fluorite prism in this region. The central band decreased in intensity more rapidly than the other two, but all three clearly remained after drying a t 850". A better comparison of relative intensitirs is shown in Fig. 4, where the spectra are replotted on a linear absorbance scale over a wider frequency ranqe. The "tail" a t lower frequencies is evident for the alumina dried at 450". Hydroxyl groups can also be exchanged with deuterium gas. Between 250 and 500' exchange occurs at a convenient rate, depending on the degree of hydration of the alumina and the pressure of the deuterium gas. During siich exchange, as shown in Fig. 5, the central band in both regions generally changed in intensity more slowly than the other bands. The lowest frequency band frequently changed faster than either of the others. At 250" complete equilihrium between deuteroxyl and hydroxyl bands was not reached even after 18 hours. Above 500", however, equilibration occurred rapidly between all hydroxyl and deuteroxyl hands. Further evidence of independent behavior of the three deuteroxyl bands was obtained during experiments involving butene and deuterated alumina predried at 800". At 200°, only the band at 2733 cm.-' showed exchange of hydrogen with butene, the other two bands remaining unchanged. After this band had been completely con\-erted to the corresponding hydroxyl band a t 3698 cm.-', the cell was evacuated and the alumina heated to 400". At this temperature, the bands at 2759 and 3698 em.-' appeared t o exchange or inter-convert to yield bands a t 2733, 2750, 3696 and 3737 c*rn.-' in isotopic equilibrium. The band a t 2803 em.-! remained essentially unchanged. Independent behavior of the individual hydroxyl or deuteroxyl bands was also observed in other experiments. An example of such behavior during removal of adsorbed carbon tetrachloride from alumina predried at 800" is shown in Fig. 6. The alumina originally held adsorbed carbon tetrachloride in equilibrium n-ith the i-apor at 32 mm. pressure. Evacuation at room temperature removed most of the adsorbed carbon tetrachloride and largely restored the three isolated hydroxyl hands. Evacuation at progressii ply higher temperatures, howei-cr, eliminated these hands in order from highest to lowest frequency, apparent1,v as a result of reaction of hydroxyl groups with (15) The spectra shown in the figures are, unless otherwise noted, smoothed tracings of the origina! spectra. Tht? noise level in t h e original spectra was typically 1 t o 2%.
SURFACE HYDROXYL GROUPS ON Y-ALUMINA
Oct., 1960
residual carbon tetrachloride. After evacuation at 750", all hydroxyl bands have disappeared from the alumina spectrum, t.he weak bands remaining being caused principally by residual water vapor in the spectrometer. A black deposit-apparently carbon-was left on the alumina. Changes observed in hydroxyl and N-H stretching bands on alumina during adsorption and desorption of ammonia are shown in Fig. 7. When the surface held a large amount of adsorbed ammonia, the hydroxyl bands a t highest and lowest frequency were greatly reduced in intensity and apparently shifted to lower frequencies. The central band, however, although somewhat shifted in frequency was reduced in intensity much less than the other two. When the alumina was subsequently heated in vacuo at 400" to desorb most of the ammonia, the lowest-frequency hydroxyl band increased beyond its original intensity. Concurrent changes in the N-H stretching spectra showed that the "ammonia" retained a t 400" differed in character from the ammonia originally adsorbed a t room temperature; hence transfer of a proton from the ammonia to an oxide ion may have occurred on heating "3( 0 - 4 XH2OH-). 'The independent variations in intensity of the hydroxyl and deuteroxyl bands are difficult to explain except on the basis that these three bands correspond to three chemically distinct types of hydroxyl groups on the alumina surface. The same data also show that groups of a single type cannot be readily converted to other types, and that the various types do not rapidly interchange hydrogen a t temperatures up to at least 230". The number of hydroxyl groups on the alumina was obtained in several experiments by measuring the number of hydrogen atoms that could be exchanged with deuterium gas. Measured amounts of deuterium were exchanged with the alumina a t GOO" until the hydroxyl and deuteroxyl bands showed similar configurations and no longer changed with time. h'lass-spectrometer analyses were made on the resulting mistures of H2, HD and D,. Upon th(: assumption that each hydroxyl group occupies 8 A.2 on R completely covered surface, typical results ranged from 40% coverage after drying a t 400" to about El,after drying at 800". Although coverage depends primarily on drying temperature, it is also affected by the duration and rate of evncuation, as well as the number and distribution of hydroxyl groups initially present. From spwtra obtained during exchange with deuterium, the average infrared absorptivity of isolated hydroxyl groups was calculated to be about 8 X lo4 cm.3/mole. Beer's law can probably be used in comparing relative amounts of hydroxyl a t coverages below l5%, where no hydrogen bonding appears to exist. Coverages after drying above 900" were estimated from absorbance to be 1% or less. The three hydroxyl bands were nevertheless still evident on such very dry alumina. All three isolated hydroxyl bands could be restored in the spectrum of highly dried alumina by rehydrating the alumina with water vapor and subsequently evacuating above GOO". To investigate rehydration, enough water vapor to yield a final
+
1520
0 _-_____---3200
3400 3600 3800 4000 Frequency, cm. -l. Fig. 7.--Adsorption of NHJ on alumina predried at 800"
+
- J
2600 Fig. 8.-Detailed
2700 2800 2900 Frequency, cm. -1. changes in 0-D stretching bands on evacuation.
coverage of 40% mas adsorbed at room temperature on alumina previously dried a t 800". Absorption bands produced were similar to those of the stretching and bending vibrations of liquid water. The hydroxyl band at 3795 cm.-l was replaced by a band near 3500 ern.-', but most of the isolated hydroxyl groups apparently were not perturbed by the adsorbed water. After standing at room temperature for one week, the alumina showed no significant spectral changes. Subsequent heating for half an hour in the closed system a t 300" greatly reduced the "liquid" water bands, increased the isolated hydroxyl bands, and restored the band at 3795 cm.-1. These results show that most of the added water did not react to form hydroxyl groups a t room temperature but remained as molecular mater. If some hydroxyl groups were produced, they became isolated groups only after heating. Water molecules did react with alumina at 300" to form hydroxyl groups and many of these n-ere isolated. The greatest increase was observed a t 3698 cm.-I.
1530
sfl. 11.('IONSTANCE
~ E F F L E R-4ND FREDERICK D. ROSSINI
The spectral changes observed on drying alumina can be accounted for logically. After calcination a t 600" and exposure to moist air, the alumina surface at room temperature holds both molecular water and surface hydroxyl groups. As it is heated during drying, water molecules not desorbed and removed from the system react to form hydroxyl groups. At higher temperatures, the rcverse reaction causes surface hydrinyl groups to form water molecule-.. n-hich are desorbed and removed. Ultimately. three types of hydroxyl groups remain, which represent isolated groups 011 different types of surface sites. Figure 8 shons detailed changes in the spectrum of a fairly thick plate of "deuterated" aluminri as it is dried above 500". The spectra recorded after drying at 300 and 600" have more than three absorption maxima. Several bands disappear betwelen 500 and 700". Some of these bands are probably caused by hydrogen-bonding between closely spaced hydroxyl groups. The broad tail at frequencies helow those of the isolated hydroxyl bands is also believed to be caused by vibrations of hydrogc?n-bondedhydroxyl groups. (16) 31 -5toii and D E. Willinins, J Cham i'hijs, 31, 32q ilq59)
Vol. 64
Conclusion The high frequencies of the bands remaining after drying at high temperatures suggest-but do not prove-that the attachment of hydroxyl groups to the surface is largely ionic in character. The observed frequencies lie above those normally found for metallic hydroxides which are not hydrogen honded.16probably as a result of the location of the hydroxyl groups on the surface. The largely ionic character of alumina" appears to support this interpretation. The groups correspoiiding to the band at 3698 rm.-l are apparently the most "acidic" of the three types of hydroxyl groups, as shown by the greater ease with which they exchange hydrogen. This behavior appears consistent with t heir lower vibrational frequency. The different types of hydroxyl groups may well play different roles in catalytic reactions on alumina. Variations in the relative abundance of these types may be more important than the total amount of chemically bound water. Further work is in progress to elucidate the nature of t h e v groups and the sites to which they are bound. (17) R. Bersolin, zbid., 29, 326 (19%).
HEATS OF COMBUSTIOS ,4ND FORI\IATIOi\; OF THE HIGHER SORMAL ALKJ'I, C'\7CI,0PESTANES, CYCLOHEXANES, BENZENES ASD 1-SLKEYES I N THE LIQUID STATE AT %501 BY SISTERM. COSSTASCELOEFFLER, R.S.M., AND FREDERICK D. ROSSIXI? Chemicol and Petroleum Resmrch Laboratory, Carneyie Institute of Txhnology, Pittsburgh 13, Pennsylvania Rrceiiud 4 p 1 z l $9, 1960
Measurements were made of the heats of combustion, relative to that of n-hexadecane, of n-decylcyclopentane, n-decylcyclohexane, I-hexadecene and n-decylbenzene, in the liquid state a t 25'. The data confirm the work of Fraser and Prosen that, within the limits of present-day measurements, the increment per CH, group, in the heat of combustion or formation, ij, constant for the higher members of these normal alkyl series of hydrocarbons in the liquid state a t 25". Recommended values are given for the heats of combustion and formation, for the liquid state a t 2 5 O , for the members of theee normal alkyl series of hydrocarbons having more than three carbon atoms in t h e normal alkyl chain. Values for the corresponding heats of vaporization x t 25' are also given.
the same value as for the series of normal parafI. Introduction It has been shown by Prosen and R o ~ s i n i , ~fins. from measurements on 8 liquid normal paraffins in the rsnge C5to CM,t)hat the increment per CH, group in the h.eat of formation of the higher normal paraffin hydrocarbons in the liquid state at 25" is a constanr, within the limits of present-day measurements. Lat'er, Fraser and Prosen" reported data on one higher member of each of four other normal alkyl series of hydrocarbons and concluded that the increment per CH, group in the heat of formation of the higher members of t'hese other normal alkyl series of hydrocarbons in t8he liquid stat'e at 26" is also a constant of subst,aiitially (1) This investigation was supported in part b y a grant from the Sational Science Foundation. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry a t the Carnegie Institute of Technology. ( 2 ) University o t Notre Dame, Notre Dame, Indiana. (3) E. J. Prosen .and F. D. Rossini, J . Research N a f 7 . Bur. Standards, 3 4 , 263 (1945). (4) F. AI. Fraser and E. J. Prosen, ibid., 56, 329 (1955).
In the assembly of the extensive set of precise and internally consistent tables of thermodynamic properties of hydrocarbons and related compounds, prepared by the American Petroleum Institute Research Project 44, the values for the normal paraffins assume a great importance in providing a base-line framework from which values for many compounds of other classes are calculated without experimental mea~urement.~Where possible, it is desirable to use the increments per CH2 group derived from extensive measurements on the paraffin hydrocarbons in calculating values of appropriate thermodynamic properties for the members of other series of normal alkyl hydrocarbons. ;iccordingly, because of the great importance of ( 6 ) E'. D. Roszini. IC. R. Pitzer, R . L. Arnett, R. AI. Braiin and G. C. Pimentel, "Selected values of physical and thermodynamic properties of hydrocarbons and related compounds," A P I Research Project 44. Carnegie Press, Pittsburgh, Pennsylvania, 1953.
