H. A . BENESI
970
Vol. 61
ACJDITY OF CATALYST SURFACES. 11. AMINE TITRATION USING HAMMETT INDICATORS BY H. A. BENESI Shell Development Company, Emeryville, California Received March 96#1067
The surface acidities of clays, cracking catalysts and a variety of acids mounted on silica gel have been more nearly completely characterized than heretofore by titrating suspensions of the above materials in benzene with n-butylamine using ad. sorbed Hammett indicators to determine end- oints. In the case of mounted acids, it has been found that the strenghh and the proportion of acid that can be titratel depend on the surface concentration and type of acid. The results obtained upon titrating representative clay samples suggest that such measurements could be used as a tool in differentiating between clay types, most of the acid centers on a kaolinite surface being much stronger than those on attapulgite and montmorillonite surfaces. I n the case of cracking catalysts it is found that most of the acid centers on silica-alumina and Filtrol SR are fantastically strong; most acid centers on magnesia-silica are weak in comparison.
A complete description of surface acidity requires the determination of (1) an intensive factor-acid strength-and (2) an extensive factor-the number of acid centers. The estimation of acid strength from colors of adsorbed Hammett indicators has already been reported.’J The present study deals with the next logical step, the measurement of the number of acid centers, made by titrating benzene suspensions of powdered catalysts with n-butylamine using adsorbed Hammett indicators to determine end-points. The titration technique differs somewhat from that used by Johnson,a the chief novel feature of the present study being the use of a complete set of Hammett indicators to yield titers as a function of acid strength. Since we have found that most catalyst surfaces contain acid centers that can be grouped into several acid strength ranges, such titer distributions can be used to characterize uniquely or “finger print” surface acidity. Experimental The Hammett indicators used in the resent study decrease in basicity from phenylazonaphtlylamine ( p K , = +4.0) to anthraquinone (pK, = -8.2), and are the same as those listed in the first paper of this series.2 I t was there shown that this class of acid-base indicators can be used to estimate the acid strength of solid surfaces in terms of Hammett and Deyrup’s HOscale,4 which can be considered an extension of the p H scale into media where hydrogen ion activity is no longer identical with its stoichiometric concentration. A discussion of this subject, together with the restrictions that apply to the use of acid-base indicators in characterizing acidic surfaces, is given elsewhere.’J Titration by Successive Approximations.-The n-butylamine titration technique develo ed by Johnsona was modified so that the indicators could e! added to portions of the catalyst suspension after the catalyst sample had been equilibrated with n-butylamine, the end-point bein determined by a series of successive approximations. &e procedure given below has been devised so as to exclude traces of moisture, which affect titration results. (1) Ten to fifteen grams of catalyst sample (collected between 60 and 325 mesh sieves) is calcined at the desired temperature by passing a stream of dry air upward through the vertical catalyst bed at a flow rate sufficient to fluidize the catalyst. The catalyst bed is supported by a medium porosity fritted disc sealed in a 28-mm. Pyrex tube. If the sample is to be calcined a t 550” for example, the Pyrex tube plus sample is taken out of the furnace at the end of the calcination period and allowed to cool while dry air is passed through the catalyst bed. As soon as the sample has cooled to 200”, it is transferred to a screw cap bottle and (1) C . Walling, J . Ana. Chsm. Soc., 72, 1164 (1950). (2) 1%. Benesi, ibid., 7 8 , 5490 (1950). (3) 0. Johnson, T H I JOURNAL, S 69,827 (1955). (4)
L. P. Hamrnett and A. J. Deyrup, J . Am. Chem. Soc., 64,
(1932).
2721
stored in a desiccator until the sample is titrated. In most cases, titrations are begun the day following the calcination. (2) The calcined sample is transferred to a dry box containing a series (usually six to eight) of weighed one-ounce screw-cap bottles plus caps. Roughly one gram of sample is transferred to each of the one-ounce bottles, a 2-ml. scoop being a handy aid in measuring out the one-gram portions. The capped one-ounce bottles are then removed from the dry box, reweighed to obtain the sample weights to the nearest milligram and stored in a desiccator over Driente until used. (3) Ten ml. of dry benzene is added to each of three weighed samples. Enough 0.1 IC€ n-butylamine in benzene is added from a 10-ml. buret to each of the three samples so as to bracket the expected titer by the appropriate number of millimoles of n-butylamine per gram of sample. (Silica-alumina catalyst having a surface area of 500 m.2/g. has an anticipated titer of 0.4 mmole/g. The amounts added should thus be 0.30, 0.40 and 0.50 mmole of butylamine/g.) The tightly capped samples are then equilibrated in a rotator at least four hours (or overnight) a t room temperature. (4) Two-ml. portions of the equilibrated catalyst SUBnensions are added to test-tubes and tested with each of the aammett indicators. After arranging the test-tubes in order of increasing n-butylamine content, it is easy to determine a t which stage enough n-butylamine has been added to neutralize catalyst acidity to the particular indicator used. (5) Steps 3 and 4 are repeated as often as is deemed necessary using smaller stepwise increases in n-butylamine content between the limits established in the previous trial. Since, in the general case, the titer is not the same for all indicators, the preparat,ion of sets of catalyst samples containing different ranges of n-butylamine contents is often necessary. In the present study, the titers of catalyst samples have been bracketed to 0.025 mmole/g. when the titer is ca. 0.5 mmole/g., and to 0.01 mmole/g. when the titer is ca. 0.1 mmole/g. As an example, if the titer has been bracketed between 0.35 and 0.375 mmole/g., it is reported as 0.36 mmole/g., the uncertainty being *0.01 mmole/g. The above procedure for determining end-points hrts several advantages over that in which n-butylamine solution is added directly to a catalyst sample containing an adsorbed indicator. The new procedure enables one to titrate a catalyst using as many as ten indicators to determine end-points with only a little more effort than that needed using only one indicator. Then, too, the n-butylamine can be equilibrated with the catalyst sample without introducing traces of moisture in the course of the titration. Finally, equilibrium is more quickly attained by adding indicator after n-butylamine rather than by adding 12butylamine until the strongly adsorbed acid form of the indicator has been displaced. Titrations have been carried out in both benzene and isooctane with identical results. It is also found that raising the equilibrcttion temperature from 25 to 50’ has no effect on the titration results. Preparation of Solutions.-All of the benzene used in the above procedure is purified as follows. Two liters of benzene (Baker Chemical Company) is agitated overnight with 500 ml. of concentrated sulfuric acid. The benzene layer
July, 1957
AMINE TITRATION USING HAMMETT INDICATORS
is then passed through a column (30 cm. long, 4 cm. diameter) of Davison Grade 912 silica gel to remove dissolved acid and sulfonated impurities. The resulting benzene is tested with phenylazonaphthylamine to make sure that all acidic impurities have been removed. It is then stored over l/(''-g mesh activated alumina (Alcoa Grade F-1). One tenth per cent. indicator solutions in benzene are used to test catalyst suspensions as described in the earlier work.2 The n-butylamine (Eastman Kodak white label) is purified by distillation through a 15-plate still a t a reflux ratio of 15:l.. The 0.1 M n-butylamine solution is prepared by weighing 1.0 ml. of purified n-butylamine in a 100-ml. volumetric flask and making up to volume using purified benzene. Mounted Acids.-Davison Grade 70 silica gel (60 to 200 mesh) was chosen as the base for mounted acids because it was relatively pure, available in large quantities and had a high average pore diameter (86 k ) . The surface of Grade 70 silica gel was found to be slightly acidic, presumably because of the presence of aluminum (0.07%). Its titer, 0.01 mmole/g. in the -3.0 to -8.2 Ho range, was subtracted from each of the acidities reported in Tables I and 11. Acids were mounted on the above silica gel by an impregnation procedure described elsewhere.2 All mounted acids were freshly dried a t 120' for 16 hours before they were titrated. Clays.-The atta ulgite used (Attaclay, Attapulgus Clay Company, Philadeghia) had the com osition 67% SiOn, 12.5%, Al?Oa, Il.O%, MgO 4.9% Fen& and 2.5% CaO. The kaolinite (Velvex) was described in the previous paper.2 The montmorillonite used was a bentonite from Clay Spur, Wyoming,' and had a base-exchange capacity of 0.98 meq./ g. of clay (dried a t 120"). This material had been selected because of its freedom from other clay minerals. Characterization by differential thermal analysis and X-ray diffraction had established that this was truly a pure montmorillonite. Clays were dried a t 120' for 16 hours before they were titrated. Cracking Catalysts .-The two silica-alumina ( Aerocat fluid) catalysts used were obtained from American Cyanamid Company and had the compositions 11% AltOa, 89% Si02 and 23% A1203,.77% SiOz. They were denoted as MS-A-1 and MS-A-3 silica-alumina, respectively. Filtrol SR catalyst was obtained from Filtrol Corporation and had the composition 37% A1203, 61% SiOz and 2% Fe,203. The silica-magnesia catalyst used6 was prepared in this Laboratory, contained 31% MgO, 69% SiOz, and had an average pore diameter of 38 A. Cracking catalysts were calcined three hours at 550' the day before they were titrated.
Results and Discussions The n-butylamine titers plotted as a function of acid strength in Figs. 1-3 are cumulative; L e . , they are measures of the number of acid centers (in mmoles/g.) having an Ho equal to or lower than the pKa value of the indicator used. (It should be remembered that as Ho decreases and approaches more negative values, the acid strength of the surface increases.) The actual amount of surface acid in a given Ho range is given by the difference between n-butylamine titers using the two indicators bracketing that Ho range. I n Tables I-V the amount of surface acid in each acid strength range is reported together with the total surface acidity, which is, of course, also equal to the n-butylamine titer a t the highest Ha value. Surface areas measured using the Brunauer-Emmett-Teller method' are reported for all solids studied so that acid concentrations can be converted from a unit weight (mmole/g.) to a unit surface (pmoles,/m.2) basis. ( 5 ) Kindly furnished b y Dr. D. R. Lewis, Shell Development Company, Houstm, Texas. (6) Kindly furnished b y Dr. R. E. Van Dyke, Shell Development Company, Emeryville, California. (7) 8. Brunauer, P. H. E m m e t t and E. Teller, J4Am. Chem, Xoc. 60, BO9 (1938).
. M
971
2.01
1
-
0 -2 -4 -b INCREASING ACID STRENGTH (Ho UNITS).
$2
+4
-8
Fig. 1.-Butylamine titers us. acid strength for mounted acids dried at 120'. Acid concentration is 1.0 mmole/g. of silica gel. Vertical lines denote titer uncertainties. \
w
ATTAPIJLGITE
I +2
0.00 $4
-2
0
-4
INCREASING ACID STRENGTH (Ho UNITS).
-6
Fig. 2.-Butylamine titers us. acid strength for clays dried a t 120'. Vertical lines denote titer uncertainties.
t.. .
\\SILICA-MAGNESIA
\
\ \
0.01 t4
SILICA-AL$JMINA (MS:A- I )
,*
-f.
\
I
t2
I
1'-
I
0 -2 -4 INCREASING AClD STRENGTH (Ho UNITS).
I
-b
-0
Fig. 3.-Butylamine titers us. acid strength for cracking catalysts calcined at 550". Vertical lines denote titer uncertainties.
Mounted Acids.-Surface acidities of some of the more common non-volatile acids mounted on silica gel were measured to see if the results would seem reasonable in the light of the wellknown behavior of the acids in dilute aqueous solution. In Fig. 1 are plotted n-butylamine titers of H3B03, H3P04 and H2SO4mounted on silica gel as a function of acid strength. The curves shown in Fig. 1 illustrate the striking difference between
H. A. BENESI
972
the three acids mounted on silica gel, the strength increasing in the order expected, H3B03 < HrP04 < HzS04. The results listed in Table I allow more detailed examination of the differences between the three mounted acids studied. It can be seen that the amount of titratable acid does not fall into a single acid strength range but is spread over several acid strength ranges. The results show that both hydrogen ions per H2S04molecule can be titrated, hut that only one hydrogen ion per H3P04 and 0.2 hydrogen ion per H3B03 molecule is titratable. The above findings agree with those made by Johnson13who concluded that an acid with a dissociation constant of in aqueous solution is the weakest mounted acid that can be titrated using butter yellow (pKa = +3.3) as the indicator. TABLE I ACIDITYOF MOUNTED ACIDS' DRIEDAT 120' Butylamine titer (mmoles/g.) in the Ho range Acid HiBOa HAPO~ H,SO4
Surface area, m.*/g.
, 4-4.0 to -3.0
357 330 307
0.15
.IO .SO
-3.0 to -5.6
-5.6 to -8.2
0.02 .80 .30
0.00 .07
.BO
a 1 .O mmole of acid/g. of silica gel. A0.05 mmole/g.
