ACIDSITESI N
A
2197
MIXED-OXIDE SYSTEM
The Structure and Properties of Acid Sites in a Mixed-Oxide System. I.
Synthesis and Infrared Characterization
by K. H. Bourne, F. R. Cannings,' and R. C. Pitkethly The British Petroleum Company Limited, B P Research Center, Chertsey Road, Sunbury-on-Thames, Middlesex, England (Received August 19, 1969)
By synthesizing materials containing aluminum atoms in different environments in a silica matrix, an attempt has been made to separate and characterize two discrete acid site structures likely to coexist in commercial silica-aluminas. The two structures are aluminum atoms incorporated (a) onto a silica gel surface and (b) into a silica gel lattice. This paper describes the preparation of the materials and their characterization by infrared spectroscopy. A study has been made of the dehydration and dehydroxylation of the two materials and of concurrent changes in acidity as measured by pyridine adsorption. With the aluminum-on-silica species, the interconversion of Brinsted and Lewis acid sites has been demonstrated: each Brinsted acid site gives rise to a Lewis site on dehydration. The aluminum-in-silica species was shown to contain only Brinsted acid sites after dehydration.
Introduction Because of its use as a catalyst for a wide variety of chemical conversion processes, silica-alumina has attracted a great deal of attention from research workers over many years. Since its preparation involves a cohydrolysis to form the mixed oxides, the constituents of silica-alumina are likely to be present in every possible degree of intermixing, from regions where perfect alternation of silica and alumina occurs in the oxide lattice to regions where bulk silica or alumina are found. Say and Rase2 have demonstrated the existence of alumina aggregates in such a material. It is a logical extension of this argument to point out that not all the catalytically active sites in silica-alumina are likely to be of the same type and that, in the extreme case, there may be as many types of active site as there are possible aluminum environments. I n an attempt to study the relationship between catalytic activity and structures in this complex material several workers have investigated the nature of its surface by studying the bonding of amines to the surface using infrared techniques. 3-5 Parry3a and Basila, et al., 3 b have successfully used pyridine to differentiate between Brgnsted and Lewis acid sites onsilica-alumina. The superiority of this amine over ammonia is not in doubt: the former gives rise to sharp absorption bands in the region 1400-1660 cm-', and those arising from a pyridinium ion (pyridine plus Brfinsted acid site) are readily distinguishable fiom those of coordinated pyridine (pyridine plus Lewis acid site). Adsorbed ammonia gives broad, poorly defined bands. Further, because of its strongly basic character, ammonia combines with even the weakest acid site and behaves nonselectively; silica gel will retain an appreciable quantity of ammonia after evacuation a t elevated temperatures.6
However, because of the complex nature of silicaalumina, the infrared absorption and the catalytic activity studies must have measured the average effects of a large number of different, active, acidic species. I n recent work, we have prepared novel materials, resembling silica-alumina, in which the active sites may reasonably be considered to be of discrete types, and have compared their physical and chemical properties using infrared, chemisorption, and hydrocarbon cracking techniques. These materials are of two kinds. I n one, aluminum atoms are placed onto a silica surface (designated aluminum-on-silica) and in the other they are buried within the silica (aluminum-in-silica) . I n each case we believe that the aluminum atoms have only oxygen and silicon atoms as their near neighbors in the oxide lattice and that, as such, they are discrete, potential catalytic sites. This paper describes the preparation of the materials and their characterization by infrared spectroscopy and other techniques. A further paper will report on the differentiation of the chemical properties of the materials by benzene chemisorption and t-butylbenzene cracking studies.
Experimental Section ( a ) Materials. The water used throughout this
work was deionized (resistivity > 1.5 megohms/cm) and (1) Communications arising from the paper should be addressed to this coauthor. (2) G. R. Say and H. F . Rase, Ind. Eng. Chem., Prod. Res. Develop., 250, Sept (1966). (3) (a) E. P. Parry, J . Catal., 2, 371 (1963); (b) M . R . Basila, T. R. Kantner, and K. H. Rhee, J . Phys. Chem., 68, 3197 (1964). (4) M. R. Basila and T. R. Kantner, ibid., 70, 1681 (1966). (5) M . R. Basila and K. H . Rhee, Abstracts, 145th National Meeting of the American Chemical Society, New York, N . Y., Sept 1963, Paper 2 6 1 . (6) G. Blianakov and R.Polikarova, J . Catal., 5 , 18 (1966).
Volume 74, Number 10 M a y 14,1970
2198
E(. H. BOURNE, F. R. CANNINGS, AND
10
R. C. PITKETHLY
DIFFUSION PUMP A N D
Figure 1. Infrared cell and disk holder.
all chemicals were of Analar grade. Pyridine was degassed and dried over 5A molecular sieve. Sorbosil U30 silica gel of 72-100 mesh particle size, supplied by Messrs. Joseph Crosfield, Warrington, Lancs, was extracted with 0.1 N hydrochloric acid to remove traces of sodium and iron. It was then washed with water in a Soxhlet apparatus for 8 hr and dried at 110" for 24 hr. All aluminum-on-silica preparations were derived from this parent material by contacting it with aqueous solutions of aluminum sulfate hexadecahydrate. The amount of aluminum taken up by the gel may be increased by increasing the concentration of the sulfate solution, by repeating the treatment with fresh solution, or by percolating the solution through a bed of the gel. The incorporation of ions onto a gel surface by such techniques has been described by Maatman and coworkers7 as ion exchange. Following such a percolation procedure with 0.1 144 aluminum sulfate solution, the gel was exhaustively extracted with water to remove surplus salt and dried overnight at 110". Its aluminum content was 1.30/0by weight. Two samples of aluminum-in-silica were made by stirring for 16 hr, a mixture of tetraethylorthosilicate (Monsanto Chemicals, London) with twice its volume of hydrochloric acid, at pH 2. Sufficient aluminum sulfate was dissolved in this mixture to give an aluminum content of 0.5 or 1 wt yo on the silica. Ammonia solution ( 2 N ) was then stirred rapidly into the mix until it gelled, at pH 6. The hydrogel was separated, reslurried with 10 volumes of water, filtered The Journal of Phyaical Chemistry
off, dried for 2 hr at 110", and at 360" for 4 hr. The calcined gel was transferred to a glass column where dilute nitric acid (1 N ) was run over it at a rate of 2 vol/vol hr, until the eluate was free of aluminum, as measured spectrophotometrically at 386 mp using 8-hydroxyquinoline as an indicator. The acid washing procedure lasted 7 days, after which the treated gel was exhaustively extracted with water and dried overnight at 110". The aluminum contents of these silicas were 0.045 and 0.15 wt To. The number of strongly protonic centers on both preparations was estimated by exchange with saturated sodium bicarbonate solution, followed by water washing. The final sodium content was regarded as a direct measure of the number of acid centers. (b) Sample Preparation. Self-supporting 13-mm ' diameter disks of silica, aluminum-on-silica (1.3 wt % aluminum), and aluminum-in-silica (0.045 wt % aluminum) were formed by pressing the oxide powder at 9000 lig/cm2 in a conventional KBr die. Suitably sized powders were produced by prolonged grinding in a large agate pestle and mortar. Unless stated otherwise, approximately 11 mg of sample was used for each disk. (c) Infrared Cell. The cell is shown in Figure 1. The body was machined from a 9 X 5 X 5 cm3stainless steel block, recesses being cut to accommodate 4 O-rings (7) D. L. Dugger, J. H. Stanton, B. N. Irby, B. L. bfcconnell, W. W. Cummings, and R. W. Maatman, J . Phys. Chem., 68, 757 (1964).
ACIDSITESIN
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MIXED-OXIDE SYSTEM
and 25 mm diameter by 3 mm thick circular rock salt plates. These windows were held in place with bolted rings. Flanges A and I3 were sealed with aluminum gaskets. The horizontal side arm C, constructed from a I-in. Kovar-to-Pyrex seal, was graded through glass seals to a quartz finger. This could be surrounded by a thermostatically controlled 500-W furnace rated to 800". The disk was mounted vertically in a stainless-steel holder, E, which clipped onto the silica framework, F. The latter could be moved through grooves in the cell body, back and forth along the side arm, C, by the action of a Pyrex-encapsulated iron core, G, and external magnet. After calcining in the quartz section of the tube, the disk was withdrawn and reproducibly positioned in the analytical beam of the infrared spectrometer. The cell was part of a vacuum manifold connected to conventional, commerically available equipment, capable of maintaining a limiting dynamic Torr. Pyridine and water pressure of ca. 2 X vapor were introduced through greaseless diaphragm stopcocks attached to the manifold. Details of the various dosing operations are given at relevant places in the text. ( d ) Spectra. Spectra were recorded on a GrubbParsons GS 2, double-beam infrared grating spectrometer, range 2-15 pm (5000 to 660 cm-l). The manufacturer's specification of a limiting resolution of r t l cm-1 is more realistically assessed at rt3 cm-' under the conditions of this work. Attenuation of the reference beam, with an adjustable comb, was frequently necessary to compensate for the energy lost in light scattering by the disk. All spectra were run ostensibly at room temperature but this assumption neglects the heating effect of the infrared beam. Absorbances were calculated from band heights.
Results and Discussion (a) Sodium Treatment of Aluminum-Silicas.
When various aluminum-on and aluminum-in-silicas were contacted with sodium bicarbonate solution, carbon dioxide was evolved and sodium incorporated onto the gel. Not all this sodium could be leached from the gel by exhaustive water extraction; neither silica nor alumina shows this ability to retain sodium ions. The data in Table I show that each aluminum atom retained one sodium ion (or "half" a divalent ion) ; this, in turn, suggests that each behaved as a discrete protonically acidic species, whether or not the aluminum was buried within the silica structure (the analytical errors a t the lowest A1 contents are relatively high). Washing with 0.1 N hydrochloric acid readily removed all the aluminum from the aluminum-on-silica. This behavior immediately distinguishes it from aluminum-in-silica and indicates that all the aluminum is a t the silica surface in the former material. (b) Water on Silica Gel and Aluminum-on-Silica.
2199 Table I : Analysis of Variously Treated Aluminium Silicas
Hoat material
Al-on-silica
Al-indica
AI,
% wt
0.16 0.70 1.36 1.20 0.28 0.15
Naa guest, % wt
Other guest
0.14 0.63 1.24
... ...
...
ions,*
% wt
...
...
1.30 Ni 0.15 Mg
0.20
...
Treatment with sodium bicarbonate. respective metal sulfates.
Guest ion/Al atom ratio
1.o 1.1 1.1 0.5 0.6 1.6
Treatment with the
Few differences appeared in the infrared spectra of disks of the parent silica and of aluminum-on-silica on dehydration and dehydroxylation at temperatures up to 600" in a vacuum, although the latter retained water rather more strongly. The 1630-cm-' band, assignable to water deformation, was small and stayed sensibly constant on pumping the silica at temperatures in the range 25-600", but with aluminum-on-silica a decrease in absorbance was observed down to a value close to that found for the silica. The band was never completely removed; it seems likely that the residue can be attributed to an Si0 overtones rather than to water trapped by capillary collapse on disk formationQ,since the two samples approached a similar final state following different dehydration patterns. In the hydroxyl stretching region, dehydration gave rise to the anticipated sharpening of the broad 3-pm band with the emergence, at treatments above 500°, of a peak at 3744 cm-' assignable to isolated silanol groups. Q-ll Bands at 3795, 3737, and 3698 cm-I, assigned by Peril1 to A1-OH stretch, did not appear at any degree of dehydration of the aluminum-on-silica up to treatment temperatures of 600". Failure to discern such bands was probably due to the low concentration of aluminum in the sample and to the fact that any such OH groups would necessarily interact with other hydroxyl groups in their vicinity. They would then merely contribute to the broad 3-pm band. Table I1 shows results of a study of the relative rehydration properties of silica and aluminum-onsilica samples. The absorbance of the 1630-cm-' band was utilized to estimate molecular water contents. After a preliminary 1 hr evacuation at 25", spectra were obtained from 6.5-mg disks of silica and aluminum-on-silica. The disks were then calcined at the temperature indicated, cooled, dosed with water vapor, and again pumped for 1hr at 25". (8) H. A. Benesi and A . C. Jones, J . Phus. Chem., 63, 179 (1959). (9) F. H. Hambleton, J. A. Hockey, and J. A . G . Taylor, Nature, 208, 138 (1965). (10) R. S. McDonald, J . Phys. Chern., 62, 1168 (1956). (11) J. B. Peri, ibia'., 70, 2937 (1966). Volume 74, Number 10 M a y 1JV1970
K. H. BOURNE, F. R. CANNINGS, AND R. C. PITKETHLY
2200 Table 11: Absorbance of 1630-cm-' Band Treatment of disk
Table 111: Pyridine Assignments in the Range 1660-1400 cm-1 Al-onsilica
Silica
Evacuation 1 hr at 25" Evacuation 1 hr at 350" Water added at 25" Evacuation 1 hr at 25" Evacuation 1 hr at 500" Water added at 25" Evacuation 1 hr at 25" Evacuation 1 hr at 600" Water added at 25" Evacuation 1 hr at 25"
0.049 0.044 0.255" 0.047 0.045 0. 162a 0.050 0.050 0.299" 0.049
0.074 0.071 0 , 516a 0.086 0.058 0.417" 0,094 0.057 0.271" 0.091
a Variations caused by differences in the amount of water added.
The ascending thermal treatments had no effect on the 1630-cm-l band of silica and consequently the amount of molecular water retained by this sample at all stages must be regarded as zero. With aluminum-on-silica the variations in band intensity suggest that dehydration produces sites which are subsequently able to chemisorb mater strongly. These sites are probably Lewis acid centers formed from Brghsted sites according to the following scheme.
H@ HO
OH
/ / \
\e
0
A1
dehydration +
0 H
HO
0
0
I
€10
0
0
0
This subject is further discussed in a later section of the paper. The data also indicate that some molecular water is very strongly adsorbed by aluminum-onsilica, since evacuation a t 350" does not completely remove it from the surface. ( c ) Spectra of Adsorbed Pyridine. Parry3& and Basila, et aZ.,3bhave defined the solid acid sites on commercial silica-alumina surfaces in terms of the infrared spectra of adsorbed pyridine in the region 1660-1400 cm-l. By calibration against synthesized standards, the spectra obtained in the current work have been used to classify the types of site effecting adsorption as Lewis (LPy) Brprnsted (BPy) or surface hydroxyl (HPy). Table I11 lists the assignments of the different bands; the 19 b modes have been used to distinguish between different types of adsorbed pyridine. 12,13 T h e Journal of Physical Chemistry
PY ,a
HPy,b
Type
cm-1
cn-1
BPy: cm-1
Modee 8a 8b 19a 19b
1582
1614 1593 1490 1438
1639 1613 1489 1539
... 1483 1440
LPY,d
cm-1
1617 ..,
1495 1451
a A dilute solution in chloroform. Taken from Sidorov.12 From synthesized pyridinium hydrochloride. d From synthesized pyridine-aluminum chloride. e Designations taken from Turkevitch.13
c
The assignments are in accord with those in the work of Gill, et al., and of Cook14on LPy and BPy complexes. It can be seen that the pyridinium ion alone produced a band in the vicinity of 1540 cm-l and the appearance of this band in the spectrum of a disk is taken as indication of the presence of BrZnsted acidity. Coordinately bonded or Lewis pyridine generated a unique band a t 1451 cm-' where the pyridinium ion does not absoib. Pyridine itself gives a band a t 1440 cm-l. ( d ) Pyridine Adsorbed on Silica Gel. Figure 2b shows the spectrum of pyridine adsorbed onto the parent silica, pretreated by evacuation a t 400" for 1 hr. Addition of water did not significantly change the spectrum. Evacuation for 10 min at room temperature removed all the bands ascribable to physisorbed pyridine and those remaining, a t 1599 and 1447 cm-I (spectrum c), are assignable to hydrogen bonded pyridine. These were lost on pumping for 24 hr at 25". All bands attributable to adsorbed pyridine were rapidly removed by pumping a dosed sample a t 100". The lack of a band at 1546 cm-' implies the absence of BPy and the 1447-cm-' band is at too low a frequency to be attributed to LPy. The pyridine is only weakly retained and adsorption occurs on or adjacent to hydroxyl groups. The effect was observed on hydroxyl stretch intensity at 3700-3740 cm-l. The preferential location for attachment seems to be wealtly H-bonded OH groups. Isolated hydroxyl groups were not affected at low pyridine concentrations. These results are in general agreement with those of Parry.2 ( e ) Acid Sites on Aluminum-on-Silica. Evacuation for 1 hr a t 400" of a disk of aluminum-on-silica (1.3 wt % aluminum), that had previously been contacted with pyridine vapor at room temperature, produced the spectrum shown in Figure 3a. The bands at 1624,1496,1462 (shoulder), and 1456 cm-l are typical of very firmly held LPy, and demonstrate the strength of the adsorption sites. The splitting in the (12) A. N. Sidorov, Opt. Spektrosk., 8, 806 (1960). (13) C. H. Kline a n d J. Turkevitch, J . Chem. Phys., 12, 300 (1944). (14) N. S.Gill, R. H. N u t t a l l , D. E. Scaife, a n d D. W. A. Sharp, J . Inorg. N u c l . Chem., 18, 79 (1961); D. Cook, Can. J . Chem., 39,
2009 (1961).
ACIDSITESIN
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220 1
MIXED-OXIDE SYSTEM
lowest energy band indicates the presence of two distinct forms of Lewis acid center.ls On addition of water vapor (spectrum b), the 1456cm-1 peak shrank and revealed a very small band at 1446 cmV1,arising from HPy. The 19a mode at 1496 cm-' was converted to absorption at 1490 cm-', and a broad band appeared at 1547 cm-'. All these observations are consistent with the generation of BPy at the expense of LPy. The variation in the intensity of the 19a mode is shown in Figure 3 and the shift from 1496 cm-l to 1490 cm-' is easily detected. This change in the frequency has not been reported previously; it was very apparent in this work.
57
I667
1400
WAVE
NUM6ER
1400
WAVE NUMBER cm-I
Figure 3. The infrared spectra of pyridine on aluminum-onsilica: a, disk dosed with pyridine at 25" then evacuated for 1 hr at 400'; b, water vapor admitted at 25'; c, more water vapor plus Eiome pyridine admitted at 25'; d, disk evacuated for 10 min at 25'; e, disk evacuated 1 hr at 25".
Lm''
Figure 2. The infrared spectra of pyridine on silica (transmission vs. cm-l): a, silica evacuated for 1 hr at 400'; b, pyridine added; c, evacuated for 10 min at 25".
When more water together with pyridine was added (spectrum c), the absorbances of the bands at 1490 and 1547 cm-l changed minimally from 0.248 to 0.248, and 0.056 to 0.054, respectively. Those of the band at 1446 cm-l increased from 0.010 to 0.193. It is difficult, therefore, to believe that the band at 1490 cm-' contains more than a very small contribution from HPy. This view is supported by the spectra of pyridine on silica, displaying bands only a t 1599 and 1447 cm-I. Evacuation for the times given produced a reduction, then elimination, of HPy after pumping for 1 hr. The increase in Lewis acidity during this process can be followed in Figure 3. Finally, the spectrum reverts to (a) after 1 hr evacuation at 400". We conclude that conversion of these Lewis acid sites to Brgnsted is reversible and depends on the combination of water with the surface. This statement is consistent
with the simple representation of an acid-center of aluminum-on-silica, given on a preceding page. Pyridine must slightly modifiy the stability of the sites and probably facilitates the removal of HzO. The generation of BPy occurs whether pyridine or water is added first to a dehydrated aluminum-onsilica. This implies that the water molecule can react with a center that is obstructed by pyridine. Pyridine is the stronger base and generally remains after a calcination which detaches chemisorbed water molecules from the surface, but some sites can lose pyridine before water, as will be explained later. Figure 4 shows results from a more extensive examination of the interrelationship of Lewis and Brgnsted acidities. A disk of the aluminum-on-silica, was successively evacuated for 1 hr at Tx" (referred to as treatment X), dosed with pyridine, dosed with water vapor, and then evacuated again for 1 hr a t 25" (the (15) F. R. Cannings, J. Phys. Chem., 72, 4691 (1968). Volume 74, Number 10
Mag l 4 ! 1970
IC. H. BOURNE,F. R. CANNINGS, AND R. C. PITKETHLY
2202
Table IV : Total Acidity of One Disk of Aluminum-on-Silica, Based on the LPy Scale of Measurement (Arbitrary Units) after Each Cycle Y Treatment Evacuation temp, OC (ie., treatment X)------------, 25
BPy ( h a ? ) BPy converted to LPy scale LPY Total acidity on LPy scale
200
0.075 0.194 0.154 0.348
0.072 0.189
0.151 0.340
4.06
6.06
I
I
I
I
I
1
I40
POD
300
400
500
TEMPCLATUCI
OF
recArt.mr
X,
600
1
7OC
OL
Figure 4. Relative numbers of coexisting acid sites and total acidity of 1.3 wt yo aluminum-on-silica after cycle Y, against temperature of evacuation in treatment X.
complete treatment is known as cycle Y). The temperature] TxoIof treatment X was increased from 25 to 200" and then in increments of 100" until 700" was reached, while the procedure for the rest of cycle Y remained unchanged. Spectra were run at the stages shown in Table IV. It was possible, from the work on water dosing alone, to establish the numerical relationship between the absorption coefficients of the 1547 (BPy) and 1456 (LPy) cm-l bands. The assumption was made that water converted all LPy remaining on the sample after evacuat,ion a t 400" to BPy. This is justified] to a first approximation, by the appearance of spectra a and b of Figure 3. The relevant absorbances] calculated from peak heights, are 0.049 and 0.128, and the conversion factor Airbe ~-
- 2.61
A1647
is obta,ined. T h e Journal of Physical Chemistry
300
400
500
600
700
0.047 0.123 0.217 0.340
0.048 0.124 0.233 0.357
0.046 0.120 0.228 0.348
0.030 0.078 0.236 0.314
0.030 0.078
0.141 0.219
On this basis Table IV was compiled, showing the total acidity (here absorbance and acidity are equated), on the LPy scale, of aluminum-on-silica and the results are plotted in Figure4. This exercise illustrates the conservation of recoverable acid site numbers. They remain sensibly constant after calcinations up to 500". At 600" there is an irreversible loss of 10% of the total acidity, and the Brpinsted acid sites suffer depletion. The more severe calcination temperature of 700" probably altered the oxide lattice. In all these measurements] HPy existed only in trace quantities. I n one additional experiment, after evacuation a t 600°, dosing and subsequent room temperature evacuation] the disk stood for 70 hr in a pyridine-water vapor atmosphere. The absorbances of the acid site bands then became (after the standard 25" evacuation) BPy 0.062; LPy 0.146. Total acidity remained a t 0.31, but there was a considerable conversion of LPy to BPy, when compared with the corresponding values in Table IV. This evidence suggests that a slow process of rehydroxylation occurs on some of the aluminum atoms and those involved may well be the sites which previously have been observed to strongly chemisorb molecular water. The relative proportions of the two acid site types depend on the immediate history of the sample. Referring to those regions of Figure 4 where plateaux occur, after 100" evacuation, 5 Brpinsted to 4 Lewis sites exist. The ratio changes to 1 Brpinsted to 2 Lewis after 400" evacu at'ion. The amount of pyridine retained by the disk after treatment X a t temperatures up to 700", is plotted in Figure 5. BPy disappeared above 30OoI whereas some LPy remained at 700". There is some empirical evidence to suggest that the elimination of the 1547-cm-l vibration is not solely due to the loss of water from Brpinsted acid sites. On readmission of dry pyridine after the evacuation, a small quantity of BPy is found, when the temperature of evacuation does not exceed 500". This did not occur during 600" determinations] so the likelihood of some water accompanying the pyridine dose is disproved. The results from a separate series of experiments in which the sequence of operations for cycle Y was followed, except that all water dosing was omitted, are shown in Figure 6. The constancy of acid site numbers is maintained to 500".
ACIDSITESIN
A
2203
MIXED-OXIDE SYSTEM
Table V : Absorbances of Pyridine on a Partially Sodium Exchanged Aluminum-on-Silica'
---
-Wave numbe1624 om-1
1598 om-1
1546 om-1
LPY
HPY
BPY
...
0.048
0.045
0.047
0.074 0.092
...
...
0.075
0.020
0.063
0,088
0.012
Treatment
Disk evacuated 1 hr at 25' ; dosed with pyridine and water and evacuated for 1 hr at 25" Evacuated 1 hr at 300" Dosed with pyridine and water and evacuated for 1 hr at 25' Evacuated 1 hr at 600' Dosed with pyridine and water and evacuated for 1 hr at 25'
1456 cm-1
1445 om-'
LPY
HPY
0.135
0.070
0.086
0.034 0.058
0.082 0.115
0.135
0.053
0.070
0.189
1493 om-1
... +
Nil
4
Using 0.71 wt % Na; using 1.3 wt % ' Al.
TEMPERATURE OF TREATMENT X ,
t
Figure 6. Relative numbers of coexisting acid sites on 1.3 wt 7' aluminum-on-silica after cycle Y, but omitting all water dosing, against temperature of evacuation for treatment X. TEMPERATURE OF TREATMNT X , * C
Figure 5. Relative numbers of coexisting acid sites detected on 1.3 wt yo aluminum-on-silica after treatment X, against temperature of treatment X.
(f) Sodium Exchanged Aluminum-on-Silica. Examination of an aluminum-on-silica disk, partly exchanged with sodium, Na = 0.71 ult %, A1 = 1.3 wt % (fully exchanged would be 1.1 wt % sodium), produced spectra summarized in Table V. LPy and BPy contents are quite high, reflecting the lack of sodium neutralization of some of the acid sites. The ratio LPy/BPy after 300" evacuation, pyridine and water dosing and evacuation at room temperature, is similar for this material and the parent aluminum-on-silica, again indicating that the two forms of acid are located a t the same center. The self-consistency of the overall method is demonstrated by a simple calculation. After cycle Y at 300°, BPy absorbance is 0.047 (Table IV) for the parent aluminum-on-silica, Taking into account the weights of material used for the disk (11 mg for the aluminum-
on-silica parent, 13 mg for the sodium exchanged sample) and the neutralization by sodium, on a one sodium atom for one aluminum atom basis, the absorbance of the partly exchanged disk can be predicted thus A1646
= 0.047
X
13 1.1 - 0.71 -X 11 1.1
=
0.021
This value is in excellent agreement with that found in practice (Table V). Apart from supporting the technique, the validity of the calculation shows either that all the Brdnsted sites detected on the aluminumon-silica are of equal strength (on the assumption that the stronger acid sites would exchange preferentially with sodium bicarbonate) or that all Brdnsted sites are strong enough to retain pyridine to the completion of cycle Y. The latter is the more likely explanation, confirming that pyridine is a very satisfactory selective base for solid acid site investigation. Another feature of the spectra of partly sodium exchanged aluminum-on-silica is the increase of the 1445cm-l band with increased temperature of treatment X Volume 74,Number 10 M a y 14, 1970
K. H. BOURNE, F. R. CANNINGS, AND R. C. PITKETHLY
2204 (columns 3 and 7 of Table V). This absorption probably arises from pyridine associated with the sodium ion, rather than pure HPy. Sodium obviously modifies the behavior of the acid site, and healing modifies the effect of the sodium. This change may be the initial step in the formation of new, weaker Brgnsted acid sites, a subject referred to by Flockhart and Pink.16 The change is accompanied by a decrease in BPy concentration. Acid Sites on Aluminum-in-Silica, Whereas with aluminum-on-silica brief washing with a dilute mineral acid removes all aluminum from the gel, aluminum-insilica contains a residue of aluminum that cannot be leached out by acid. The aluminum is believed to be buried within the silica lattice, bonded via oxygen atoms to four silica atoms and to carry a proton thus
Si
Si
\ H e 0
0
/
\/ Ale / \
/
Si
0
Type I Brgnsted
0
\
Si
Evacuation and heating should not so readily remove protons from this material, because of lack of neighboring hydroxyl groups. The center should behave as a permanent Brgnsted acid. A 25 mg disk gave spectra, after dosing and pumping at 25", showing the presence of HPy and BPy, with the former predominating. Evacuation at 200" eliminated all the HPy, and some BPy was lost without a concomitant increase in LPy. Evacuation at 270" removed all pyridine. A similar treatment of a disk of 0.16% wt aluminumon-silica (i.e., of comparable aluminum content) produced a spectrum, after evacuation at 270°, that contained the bands of both LPy and BPy. Therefore, it is unlikely that with the former material, the generation of Lewis acid structures from Brginsted has escaped notice, particularly when one considers the relative extinction coefficients of the two bands. The bonding of pyridine to the site appears weaker than in the case of aluminum-on-silica and this must be due t o the different environment of the aluminum. The results are consistent with the structure as originally envisaged, one that precludes the facile formation of Lewis acidity and provides a source of heat stable protons. Pyridine dosing of disks of aluminum-in-silica that had previously been calcined a t 400 and 500" initially produced only HPy. On standing, BPy slowly developed; the formation of BPy was accelerated by the addition of water vapor. KOLPy was detected. The Journal of Physical Chemistry
The likelihood of free unhydrated protons in the solid oxide is regarded by Hall and as doubtful. They demonstrated that on deammination of NH, zeolite, the liberated protons attached themselves to, and opened, A1-0-Si bridges and formed hydroxyl groups. It could be that with aluminum-in-silica a buried proton is sufficiently stabilized by the proximity of the oxygen lattice to remain intact. I n the hydrated state it becomes available at the surface of the oxide, to undergo ion exchange or form BPy with adsorbed pyridine. Evidence of hydrogen ion migration in zeolites has been recorded18 and the same mechanism would explain these observations. The migration is probably enhanced by a basic adsorbate. The Structures of the Acid Sites. Any theory we propose to explain the observed interchange of acid types has to include the formation of two distinct Lewis acid centers, corresponding to the major absorption of LPy a t 1455 cm-' and the shoulder found at 1462 cm-l. The frequency of the latter indicates that it is the stronger electrophilic site. We believe that the geometry of the predominant site produced by aluminum exchange with a fully hydroxylated silica, can be represented by a two-bond attachment to the silica lattice. The aluminum assumes tetrahedral coordination and carries two hydroxyl groups and a formal negative charge that is balanced by an exchangeable proton. Additional evidence for this aluminum environment has been obtained from the reaction of carbon tetrachloride with a fully sodium exchanged 1.3 wt % aluminum-on-silica. Substitution of the hydroxyls located on the aluminum occurs, and the chlorine to aluminum ratio found is 2 to 1. (The technique has been described. 19) The reactions of this type of site that occur on calcination can be represented by the following scheme.
HO He
OH
\e/
HO
A-Hz0
A1
/ \
0
B
\
+ Hz0
0
0
A1
/ \
0
Type I1 Brgnsted 1546 cm-' Type IV Lewis 1455 cm-I fslow
HO
0
\ J\ AI / \
0
H
0
Type IVa Lewis 1455 cm-I (16)
B. D. Flockhart and R.
C. Pink, J. Catal., 4, 90 (1965).
ACIDSITESIN
A
MIXED-OXIDESYSTEM
2205
The route followed on rehydration can be either B or C. The latter corresponds to the slow recovery of Brgnsted acidity (BPy), observed when the sample was contacted with a pyridine-water mixture for 70 hr. The appropriate 19b vibration of adsorbed pyridine is also shown. The existence of Type IVa sites explains the presence of molecular water detected after rehydration of calcined specimens. It should be added that coordination of water, leading to slow regeneration of Rrgnsted sites, may also occur on the Type VI site (see below). The Type IV site is a Lewis acid producing a pyridine band at 1455 cm-l. I n addition we consider that a second type of aluminum site, in much lower concentration, is formed during the aluminum sulfate exchange process. This only appears at favorable surface environments and the site closely resembles those found on zeolite,17 involving three A1-O-Si linkages. This site can behave either as a Lewis or Brgnsted acid, depending on the degree of ionization of a nearby silanol group. 0 He 0
0
0
€1 0
0
\ / \ / \ / Si Si A1 e e / \ / \ / \ 0
0
0
0
/
AI
/ \
0 0
0
Type I11 Brgnsted 1546 cm-' Type V Lewis 1455 cm-' The presence of Type I11 sites would explain the Brgnsted acidity found even after evacuation at 500". A slight decrease of the isolated hydroxyl band intensity detected after pyridine dosing and evacuation is confirmatory evidence. A condensation reaction between two Type V species, as suggested by Stamires and TurkevichZ0to account for radical ion forming electron traps, could generate a stronger Lewis acid
0 2
OH
\ / Si / \
0
0
0
/ / \ AI
0
+
0
HZO
+
0
0
0
0 0
\ / Si / \
0
\e/ AI
/ \
0
0
+ 0
0 \
@
Si
/ \
0
AI 0
/
/ \
0
0
Type VI Lewis 1462 cm-1 I n a previous study of mordeniteJ15Type VI sites were believed to be responsible for the 1462-cm-l band of adsorbed pyridine. Their structure leads one to expect them to be stronger acids than Type IV sites and the shift in frequency of the adsorbed pyridine is consistent with this. The probable ratio of Type I1 to Type I11 in the fully hydrated state is not known but is likely to be greater than 1O:l. The apparent loss in acid site numbers at 600" may be partially due to the formation of Type VI.
Summary The data have established that aluminum-in-silica behaves differently from aluminum-on-silica on dehydration, and an explanation, in terms of structural changes, has been offered. Reaction of aqueous aluminum sulfate with the surface of silica gel produces two types of Brgnsted acid site in which aluminum is bonded to the surface through two or four oxygen bridges (Types I1 and 111, respectively). Both can dehydrate to Lewis acid sites (Types IV and VI). A third type of Brgnsted site exists (Type I) with aluminum completely enclosed in the silica lattice by four oxygen bridges, as in aluminumin-silica, and is resistant t o dehydration. The Lewis acid sites of Type VI are present only in small numbers and are similar to those found in much higher concentration in mordenite. Acknowledgment. Permission to publish this paper has been given by The British Petroleum Company Limited. (17) J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, J. Phys. Chem., 69, 2117 (1965). (18) J. W. Ward, ibid., 73, 2086 (1969). (19) British Patent Application No. 35490/68. (20) D. N. Stamires and J. Turkevich, J. Amer. Chem. SOC.,8 6 , 749, 757 (1964).
Volume 74, Number 10 M a y l 4 * 1970
