I
A. N. WEBB Texaco Research Center, The Texas Co., Beacon, N. Y.
Hydrofluoric Acid and Acidity of Alumina
,The strange decrease in acidity titratable with potassium hydroxide as hydrofluoric acid is added to alumina led to acidity measurements by ammonia chemisorption. The amount thus adsorbed is independent of hydrofluoric acid but its removal is increasingly difficult as acid is added. This may indicate that strength rather than number of acid sites is increased. ALUMINA ALONE or with added halide has been used as a catalyst for acid-catalyzed reactions such as isomerization, alkylation, and polymerization ( 8 ) . Its inherent acidic properties have been useful when used as a support in the preparation of a variety of catalysts, and hydrofluoric acid has been added, at least in part, to enhance this acidity. Anomalous results obtained in attempting to measure the acidity of such treated alumina led to this work which shows that strength of intrinsic acid sites rather than amount of acidity is increased.
Nature of Acidity
The acidity of pure alumina and also that of silica-alumina cracking catalysts probably arises from incompletely (three rather than four or six) coordinated aluminum atoms (70). The lattice defects associated with these sites may involve a single isolated aluminum atom or an aggregation of aluminum atoms in the normally vacant lattice sites of the spinel structure of y-alumina. Either case would provide a localized electron acceptor. For alumina, the surface density of such sites appears to be constant with area changes produced by calcination, unless a transition temperature is exceeded. Then recrystallization must eliminate defects faster than area is lost. In the dehydrated state, the defect sites are acids only in the Lewis sense (electron pair acceptors) but when hydrated they are Brqhsted (protonic) acids
:O:Al:O: .. ..
:a. .. .
Lewis acid
H.. H+
:o: .. ..
:O:Al:O:
..
BrGnsted acid
These two forms are compatible with infrared spectra of ammonia adsorbed on alumina and silica-alumina that show decreasing amounts of ammonium ions arising from reaction with protonic acid sites and correspondingly more ammonia by reaction with Lewis acid sites as the catalyst is dehydrated ( 5 ) . Trambouze ( 7 7) by thermometric titration of silica-alumina with organic bases in a nonaqueous medium, in addition to alkali base titration in aqueous medium, has shown that protonic acid is converted to Lewis acid by dehydration, but the total acidity remains constant. Trambouze (72) has also found that pure alumina behaves similarly. Using the titration technique with alumina gel he observed development of Lewis acidity upon the removal of water. Some surface hydroxyl groups present, even in the dehydrated material, will show no acidic properties because the aluminum atom is fully coordinated. Measurement of Acidity
Acidity has both quantitative and qualitative factors-how much and how strong. The quantitative factor has received the most attention. Milliken, Mills, and Oblad (6) refer to a number of methods which have been used for solid acids such as silica-alumina catalystsLe., reaction with a carbonate solution to evolve carbon dioxide, titration with aqueous alkali metal hydroxides, inversion of sucrose in aqueous solution, ionexchanging capacity, titration with weak bases in anhydrous media, and chemisorption of ammonia or volatile organic nitrogen bases. Ammonia chemisorption was selected for the measurement of the quantity of acidity. Each molecule of ammonia adsorbed is considered to correspond to one acid site. The results
obtained are compared to values obtained with potassium hydroxide titration. The latter method has long been applied to silica-alumina cracking catalysts (4,9) but has been criticized, justifiably, by Milliken and others (6). Evaluation of the strength of solid acids, the qualitative factor, is more difficult. The vapor pressure of ammonia from the ammonium salts is related to the strength of common protonic acids (70). For Lewis acids, the difficulty of removing chemisorbed ammonia should be a measure of acid strength; that fraction remaining after various pumping procedures, is used in this work as a comparative measure of acid strength. Recently, an indicator and dye-transfer method of measuring strengths of solid acids has been used by Johnson (3). Materials and Procedure The alumina used in this investigation is Alcoa F-10, 8-14 mesh granules. The alumina was dried in air either a t 316" C. for 3 hours or at 135' C. overnight. Hydrofluoric acid (J. T. Baker's C.P. grade) in sufficient aqueous solution to wet the alumina was added and allowed to stand overnight. After drying on a steam bath, the samples were calcined 3 hours at 490" C. Measurement of ammonia chemsorption was carried out on 2-gram samples by outgassing and dehydrating a t 500" C. for 16 hours under vacuum by a mechanical pump a t approximately IO-s-mm. pressure; exposing to a measured charge of ammonia at 10 mm. for 30 minutes with the sample at 175" C. ; pumping off the ammonia with a liquid nitrogen trap for 2 hours with the sample at 175' C . ; and then measuring the ammonia removed and calculating that adsorbed. Selection of time and temperature of chemisorption and pump-off is based on a study by Eischens (7) of adsorption by silica-alumina over a wide range of conditions. For studying differences in the strength of chemisorption, the retention of the chemisorbed ammonia was VOL. 49, NO. 2
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261
with 0.1M potassium hydroxide for 30 minutes, filtering and back-titrating to the methyl red end point with hydrochloric acid, and determining the amount of acidity as equal to the potassium hydroxide consumed.
x HI 0 0.0
A
0.61
(.a3 0 0.10
A
Results
,206. w
za
L05a:
z
004-
4
'03-
0201-
O'
x,. , 260
'
Figure 1
300 PUMP
400
OFF TEMP
500
O C
Desorption of ammonia
determined. Full coverage was first established by the above procedure. The amounts removed by pumping a t temperatures above 175" C. were then measured. The apparatus consists of a vacuum pump, gas-measuring system with a freeze-out bulb, ammonia storage bulb, and sample tube with furnace. The temperature was measured by an ironconstantan thermocoupie placed in a re-entrant well of the sample tube and read with a Leeds & Northrup portable potentiometer. The ammonia was Matheson Co.'s anhydrous grade. Each charge was frozen and evacuated before use to remove noncondensable gases. The aqueous potassium hydroxide method (4,Q), also used to measure acidity, consists of shaking the sample
Table I.
Effect of Hydrofluoric Acid Addition on Amount of Acidity. Two series of samples of alumina with varying amounts of hydrofluoric acid added, have been examined by potassium hydroxide titration and ammonia adsorption (Table I). No corrections were made for change in weight caused by introduction of fluorine; this correction is small and the accuracy of the measurements does not justify it. Within each group, the surface area, as determined by the physical adsorption of nitrogen, is fairly constant and the acidity as determined by ammonia chemisorption is practically unchanged. There is no such constancy in the potassium hydroxide index values. In both series the acidity measured by this method falls steadily. The index for very small or zero amounts of hydrofluoric acid addition, is higher than the ammonia value and falls to as low as one third as the acid addition increases. Effect of Hydrofluoric Acid on Acid Strength. Although no increase in the amount of acidity as a result of hpdrofluoric acid addition was found by either method, it is generally accepted that the activity of alumina for acid-catalyzed reactions such as cracking and isomerization is benefited by such treatment. A possible explanation is that intrinsic acid sites are increased in strength rather than number. Two experiments were carried out in search of such an effect.
Comparison of Potassium Hydroxide and Ammonia Acidity Measurements HF Surface KOH "3
Sample No.
Added, %
6493 6494 6495 6593
0.54 1.03 4.13 16.52
6343 6656 6657 6658
0 0.65 3.23 6.46
Area,
Sq. M./G. 118 119 114 85 159 154 139 141
Index, Meq./G.
Adsorption, Meq./G.
0.295 0.250 0.140
0.239 0.232 0.251 0.213
0.418 0.351 0.144 0.096
0.258 0.244 0.241 0.266
0.177
In both, the series of samples containing
0 to 6.570 hydrofluoric acid was used. In the first experiment, a coverage of chemisorbed ammonia was established by the standard procedure of exposing to ammonia and pumping off a t 175 C. The temperature was raised in steps of 25" C. and the amount that could be removed with a liquid nitrogen trap in 30 minutes at each temperature was measured. Figure 1 shows the fraction of chemisorbed ammonia retained at each pump-off temperature. The spread in amount retained by the different samples is greatest at around 400" C. These values were checked by a second series of determinations using a 2-hour pump-off a t the single temperature of 400" C. (Table 11). Discussion
Data from the experiments to measure acidity in acid-treated alumina by potassium hydroxide titration and ammonia adsorption are compatible with the known chemical behavior of these materials and with an accepted theory of the nature of acid sites. A particular sample of alumina is considered to have a certain number of acid sites per unit area, arising from incompletely coordinated aluminum atoms. When fully dehydrated, they can accept the electron pair of ammonia. When hydrated, they react to form ammonium so that ammonia adsorption should give a true representation of their number under all conditions. In the potassium hydroxide solution, intrinsic acid sites are all hydrated and should react with the base one to one. However, this reaction may extend further and include alumina not involved in an acid site. How this affects the result depends on whether the alumina reacting with potassium hydroxide goes into solution. If it remains undissolved, the apparent acidity will be increased. This may account for higher acidity obtained by titration in some aluminas without added acid. If alumina goes into solution upon reaction with potassium hydroxide, the number of equivalents of acid consumed in backtitration will be more than the number of potassium equivalents reacting. This would cause the apparent acidity to decrease. The following equations illustrate this. Al(0H)a
Table II.
Fraction of Ammonia Retained a t
HF, %
By Single Pump-Off
By Stepwise Pump-Off
Av. Fraction of Sites Affected"
0 0.65 3.23 6.46
0.13 0.17 0.23 0.28
0.09 0.18 0.26 0.31
0.07 0.14 0.18
Difference between the average of two values-zero
fluoric acid.
262
A103---
400" C.
INDUSTRIAL AND ENGINEERING CHEMISTRY
..
and varioua concentrations of hydro-
+ 30H+ 6Hf
-+ A103---
--+.
AI+++
+ 3Hz0
+ 3Hz0
Analyses of potassium hydroxide solutions from aluminas with and without hydrofluoric acid addition show that in both cases approximately 0.3 millimole of aluminum per gram of sample has gone into solution or passed through the filter. No decrease in the measured acidity with increasing acid addition can be ascribed to this cause. However,
PREPARING CATALYSTS IN THE LABORATORY the solution from the sample with 6.46% of hydrofluoric acid extracted 4.2 millimoles of fluorine per gram of sample; thus the solution is 0.1M with respect to fluoride ions which consume hydrogen ions to form undissociated hydrofluoric acid to such an extent that excess hydrochloric acid is required in back-titration to the methyl red end point. This can easily account for the decrease in potassium hydroxide indexes for acid-treated aluminas. The potassium hydroxide method of determining acidity does not seem generally applicable whether or not the concept of Milliken, Mills, and Oblad (6, 7) is accepted-i.e., that coordination shifts and other changes are induced by the base and water. Either titration of the solid acid in a nonaqueous medium with a base such as n-butylamine used by Tamele (70)or adsorption of a basic gas such as quinoline used by Oblad and others (7), appears free of objection. Quinoline gives somewhat lower results, perhaps because of its large molecular size. Ammonia chemisorption is also suitable, since the physically and chemically adsorbed states are easily distinguished; there is no possibility of decomposition at high desorbing temperatures such as those used in strength measurements. The technique of Eischens and others ( 2 ) in obtaining infrared spectra of adsorbed molecules was used to determine the nature of the chemisorbed ammonia. Such spectra with and without acid addition was entirely that of ammonia with no evidence of ammonium ions; thus, the form of chemisorbed ammonia on dried alumina is unchanged by the presence of fluorine. The alumina used was Alon C, a small-particle-size yalumina, 5 to 20 mp, and enough hydrofluoric acid (3.23%) was added to cover the surface with fluorine atoms. The constancy of ammonia adsorption indicates that hydrofluoric acid does not affect the number of intrinsic acid sites. Probably replacement of oxygen with fluorine occurs without altering the subnormal coordination number of the aluminum atoms responsible for the acidity. The strength of the chemisorbing bond formed by the acceptance of unshared electron pairs of ammonia by aluminum atoms is reflected in the difficulty of removing the chemisorbed alumina. The successively higher fractions of such ammonia retained by samples containing more hydrofluoric acid, is an indication of an increase in the electron acceptor (acidic) properties of the aluminum atom. Replacement of an oxygen atom adjacent to the incompletely coordinated aluminum atom with them oreelectronegative fluorine atom would produce this effect.
Table 111.
Surface Density of Acid Sites and Added Fluorine
HF, % Surface area, sq. m./g. “a adsorption, meq./g. -Area per acid site, A2. Area per F atom, AB. F atoms per acid site
0 159 0.258 102
... 0
The actual strength of chemisorption or equivalent acidity has not been measured, but if comparison can be made to the decomposition temperature of ammonium salts of protonic acids, the adsorption sites of alumina are indeed strong acids. This is not in agreement with the results obtained with indicators by Johnson (3). Pure alumina did not give the acid color with basic indicators with a pK as high as 5. The reason for this is not clear, but the concept of acid strength based on dissociation in aqueous solution may not generally apply to solids which are acidic only in the Lewis sense. The observed differences between the fraction of ammonia retained at 400’ C. by samples of different fluorine content can be interpreted as fractions of acid sites affected by hydrofluoric acid addition. These are minimum values, since it is assumed that no ammonia is desorbed from the acid sites affected by fluorine. The consequences of this interpretation are reasonable in view of the relationship between the surface density of acid sites and of added fluorine (Table
111). In all cases, there is approximately one acid site per 100 sq. A. Assuming that all added fluorine is uniformly distributed on the surface, there is in the sample with 0.65% hydrofluoric acid slightly more than one fluorine atom per 100 sq. A. This fluorine can be expected to influence only about one tenth of the area; thus, it is reasonable that only 7y0of the acid sites are strengthened so as to retain ammonia. In the sample containing 6.4670 acid, however, sufficient fluorine is present to cover the surface (at 7 sq. A. per atom) yet only about 20% of the sites are affected. Additional amounts of hydrofluoric acid are not nearly as effective in increasing the acid strength as the first 0.5%. The reason is probably that larger amounts of acid react with bulk alumina to form fairly large crystallites of basic aluminum fluoride. This is indicated by detection of this compound by x-ray diffraction. Summary
T h e fact that the amount of ammonia chemisorbed by alumina is independent of hydrofluoric acid addition is interpreted to mean that the number of acidic
0.65
3.23
6.46
154 0.244
139 0.241 96 14 6.68
141 0.266 96 7 12.1
105 79 1031
sites is not affected. The increasing difficulty of removing such ammonia with acid addition indicates that hydrofluoric acid increases the acid strength of alumina. The first 0.570 of addition is most effective and appears to remain on the surface and influence acid sites directly in proportion to surface area covered. An amount of acid sufficient to influence the entire surface does not appear to do so. Instead a large fraction forms crystallites of basic aluminum fluoride by reacting with bulk alumina. Potassium hydroxide indexes are not a satisfactory measure of acidity for aluminas, especially those treated with hydrofluoric acid. Acknowledgment
The author is indebted to L. C. Roess, R. P. Eischens, and M. D. Riordan for many helpful discussions in defining and solving this problem. T h e work of L. W. Cook in preparing the samples, of P. H. Lewis and L. L. Gent in the x-ray diffraction analysis, of J. M. Crone and N. H. Sherman on potassium hydroxide indexes, and the assistance of 0.B. Purdy in chemisorption measurements are gratefully acknowledged. literature Cited
(1) Eischens, R. P., unpublished work. (2) Eischens, R. P., Francis, S. A., Pliskin, W. A., J. Phys. Chem. 60, 194 (1956). (3) Johnson, O., Zbid., 59, 827 (1955). (4) M. W. Kellogg Co., Petroleum Research Div., “Potassium Hydroxide Method for Estimation of Catalytic Activity,” Dec. 30, 1944. (5) Mapes, J. E., Eischens, R. P., J . Phys. Cham. 58, 1059 (1954). (6) Milliken, T. H., Jr., Mills, G. A., Oblad, A. G., Discussions Faraday SOC.8, 279 (1950). (7) Oblad, A. G., Milliken, T. H., Jr., Mills, G. A., “Advances in Catalysis,” vol. 111, p. 199, Academic Press, New York, 1951. (8) Russel, A. S., “Alumina Properties,” Tech. Paper 10, p. 24, Aluminum Co. of America, Pittsburgh, 1953. (9) Scheumann, W. M., Rescorla, A. R., Oil Gas J . 46, 231 (1947). (10) Tamele, M. W., Discussions Faraday Sot. 8, 270 (1950). (11) Trambouze, Y., Compt. rend. 234,1770 (1952). (12) Ibbid., 236, 1262 (1953). RECEIVED for review May 11, 1956 ACCEPTED October 9, 1956 VOL. 49, NO. 2
FEBRUARY 1957
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