EFFECT OF OXYGENON THE GENERATION OF RADICAL IONS
1005
The Effect of Oxygen on the Generation of Radical Ions by Synthetic Zeolites
by F. R. Dollish and W. Keith Hall Melbn Inatitute, Pittsburgh, PanaylvaniQ
(Received January 17, 1966)
The concentration of radical ions which can be formed on the decationated zeolites was shown to depend on both the amount of available oxygen and the presence of dehydroxylated sites. The oxygen may be either in the gas phase or tenaciously chemisorbed during p r e treatment. I n the former case, below a limiting value, a 1 :1 relationship apparently exists between the number of perylene radicals which are formed and the number of O2 molecules added a t room temperature; in the latter case, only a portion (10-35%) of the oxygen was effective. Even the maximum concentrationof radical ions achieved in the presence of excess oxygen ( ~ l O ~ ~was / g )much smaller than the density of dehydroxylated sites (-1O2I/g). When triphenylmethane was the substrate, carbonium ion concentrations of the same magnitude were effected. There was no dependence, however, on chemisorbed or gaseous oxygen. The 0 2 - ion was formed when the dehydroxylated zeolites were irradiated; hence, the 02 which cannot be removed by evacuation overnight at 550" must be chemisorbed as some other species.
Introduction
crystalline zeolites and to correlate these results with what is known concerning the chemistry of the decaThe formation of radical ions from polynuclear tionation-dehydroxylation process on the NH4+aromatic hydrocarbons, arylamines, and phenylalkenes exchanged zeolites. Also, the formation of triphenylon silica-alumina catalysts has been studied by both carbonium ions from triphenylmethane was briefly inIt has been f o ~ n d ~ ~ * J * ~ optical and epr spectroscopy.-la vestigated and these results are compared with those that treatment of silica-alumina with hydrogen at 500" for radical ions. suppressed radicnl-ion formation. The literature conTurkevich, et al.,ll were first to demonstrate the cerning the effects of the addition of oxygen to the ability of decationated NH4+ Y zeolites to generate catalyst system are in part contradictory. Several radical ions; however, they did not examine the st~dies~.~ reported J' that oxygen decreased the radicalion concentration. However, it has been showna that this effect is due to a physical interaction between (1) J. J. Rooney and R. C. Pink, Proc. Chem. SOC.,70 (1961); Trans. radical ions and oxygen molecules, with resulting Faraday Sac., 58, 1632 (1962). changes in line shape and signal width. Porter and (2) D. M. Brouwer, Chem. Ind. (London), 177 (1961); J . Catalysis, Hall,? by comparing results from epr and optical 1 , 372 (1962). spectroscopy, confirmed that the radical-ion concentra(3) J. K. Pogo, J . Phys. Chem., 65, 1919 (1961). (4) W. K. Hall, J . Catalysis, 1, 53 (1962). tion generated from perylene, anthracene, and thian(5) H. P. Leftin, M. C. Hobson, and J. S. Leigh, J . Phys. Chem., 66, threne (when adsorbed on silica-alumina catalysts) 1214 (1962). was increased on addition of oxygen. Flockhart, (6) A. Terenin, Advan. Catalysis, 1 5 , 227 (1964). Scott, and Pinklo have shown that alumina dehydrated (7) R. P. Porter and W. K. Hall, J . Catalysis, 5 , 366 (1966). a t 900" can form perylene radical ions only in the (8) F. R. Dollish and W. K. Hall, J . Phys. Chem., 69, 4402 (1965). presence of molecular oxygen. (9) H. Imai, Y. Ono, and T. Keii, ibid., 69, 1082 (1965). The purpose of the present investigation was to (10) B. D. Flockhart, J. A. N. Scott, and R. C. Pink, Proc. Chem. Sac., 139 (1964); Trans. Faraday Sac., 6 2 , 730 (1966). study the effects of catalyst pretreatment and of oxy(11) D. N. Stamires and J. Turkevich, J . Am. Chem. Soc., 86, 749 gen addition on the formation of radical ions from per(1964) ; J. Turkevich, F. Nosaki, and D. N. Stamires, Proc. Intern. ylene and triphenylamine adsorbed by synthetic C o w . Catalysis, Srd, Amsterdum, 1964, 1, 586 (1965). Volume 71, Number 4 March 1967
F. R. DOLLISH AND W. KEITHHALL
1006
effects of oxygen or catalyst pretreatment and, consequently, were led to suppose that the ionized electrons were held in deep traps associated with the decationated sites of the zeolite. They reported that only about one radical ion was formed per hundred sites or per 20 molecules adsorbed.
Experimental Section The X- and Y-type zeolites were furnished in the sodium form by the Linde Division of Union Carbide Corp. and the Ca2+ and NH4+ forms were prepared by conventional cation-exchange procedures using Baker analytical grade Ca(N0J2 and NH4Ac. Excess salts were removed from the exchanged zeolites by ten centrifugations with distilled water. The degree of ion exchange was calculated from the residual sodium content as determined by ffame photometry. The equipment and high-vacuum procedures have been described earlier.4r7J2 Sealed, greaseless systems were used; no solvent was employed with triphenylmethane, but spectroscopically pure benzene was used to transfer perylene and triphenylamine. It was necessary t o seal the tubes (with a torch) under vacuum because small traces of 02,inadvertently picked up during an experiment, would invalidate the results. Four pretreatment procedures were used. When the zeolite was first evacuated at room temperature for several hours before slowly increasing the temperature to 550" where it was evacuated overnight, it was said t o have had the evacuation pretreatment. The standard pvetreatment consisted of treating the zeolite in flowing oxygen for 17 hr at 550' before evacuating it for 17 hr at this temperature. A catalyst was oxygen cooled if, after the O2 treatment, it was slowly cooled to room temperature in the presence of oxygen before evaouation overnight. When the 0 2 treatment was followed by evacuation for 6 hr, treatment with flowing H2 for 24 hr, and a final evacuation for 6 hr, all at 550", the catalyst was said to be reduced. In order to investigate the effect of "added-back" oxygen, samples of NH4+ Y zeolite which had been given the evacuation pretreatment were connected to a doser attached to a gas buret and known amounts of oxygen were introduced at room temperature. The amount of oxygen removed following the standard pretreatment when the Y zeolite was reduced was determined by circulating the Hz over the catalyst at 547" in an all-glass system. The water evolved was collected in a liquid nitrogen trap and was measured volumetricdly (as water vapor) in a gas adsorption system which was connected to the circulation loop through stopcocks. A weighed amount of reagent (usually about 300 mg The Journal
0.f
Phyeical ChenaOtry
of perylene or triphenylamine) was placed in a reagent reservoir with 10 ml of benzene where it was degassed by the freeze-pump-thaw technique and sealed under vacuum. Transfer of the reagent to the zeolite was effected by rupture of a break-off seal between the catalyst and reagent compartments. The epr measurements were conducted in the presence of benzene. Most samples reached a constant radical-ion concentration after 48-hr contact time. Reproducibility of packing of the zeolite in the probe tube was within 10-20%. Epr measurements were made with a Varian X band spectrometer (Model V-4500) with the microwave bridge in the low-power configuration. A 12-in. magnet was employed; the Varian V-4531 multipurpose cavity was used with a modulation frequency of 100 kc/sec. Radical-ion concentrations, expressed as spins per gram of dry zeolite, were determined at room temperature by comparison of the first moment of the overmodulated derivative signals with those of 0.001 M 1,1-diphenyl-2-picrylhydrazyl in benzene, whose radical-ion concentration had been determined spectrophotometrically.la Power entering the cavity was measured with a Hewlett-Packard 431B power meter using a 20-db coupler. Power levels of the order of 0.04 mw were ordinarily necessary completely to unsaturate the epr signals. Optical spectra from triphenylmethane were taken using the apparatus and procedures previously described.*v7J2 Optical platelets weighing about 40 mg were made by pressing a mechanical mixture of 1030% zeolite with 90-70% silica gel or Cab-O-Si1 at 50 tons/in2. The dilution was necessary in order to give an absorbance less than 2.0. After pretreatment, the entire optical cell assembly was wrapped in aluminum foil to exclude light. Contact of the zeolite with degassed triphenylmethane (40 mg) was effected by rupture of a break-off seal before heating in an oven at 100" for 48 hr. I n some experiments, in order to effect contact between zeolite and substrate at room temperature, triphenylmethane was transferred in benzene. The solvent was then removed by vacuum transfer back into the solution vial cooled to -195". The vial was then removed with a torch. This was not a preferred method because it is doubtful that the solvent (and its trace impurities) could be quantitatively recovered from the zeolite. Absorption spectra were taken on 8 Cary Model 14 (12) H.P. Leftin and W. K. Hall, Actea Congr. Intern. Cafdyee, F, Pa&, 1960, 1, 1353 (1961). (13) J. W.Eastman, G. M. Androes, and M. Cdvin, J . C h .Plryr., 36, 1197 (1962).
EFFECT OF OXYGENON THE GENERATION OF RADICAL IONS
1007
~~
Table I: Effect of Zeolite Pretreatment on the Generation of Radical Ions % exchanged Reagent Coverage, moleeules/g Catalyst pretreatment at 550°
1. Evacuation 2. Standard 3. Cooled in 02,evacuated 25' 4. Reduced
Perylene 45% 7 x 10'0
NHI Y z e o l i t e - - - - - - - - . 70 % 79 % Perylene Triphenylamine 7 x 10m 4 x 1020
-Ca2 Y zeolit-NHI X reolite"38% 79 % 48% Perylene Perylene Perylene 7 x 1020 7 x 1020 7 x 1090
-Na
+
Y zeolite-
..
Perylene 7 x 1020
Radical-ion concentration, spins/g X 10-17
1.3 9.5 36"
6.2 32.3 5Bd
1.0 8.1 2Od
0.3 0.3
1.0
1.8
0.7
0.3
0.4c
0.6 9.1 15c
1.o 2.6c
...
0.3
...
None detectedb None detected
...
...
Pretreatment of the X zeolite was conducted a t 500°, since this material is more unstable than the Y zeolite a t high temperatures. Little (515%) or no change in spin-concentration occurred when exposed to sir for 24 hr. When exposed to air for 24 hr, 1.2 X 1019 spins/g developed.
' When this sample was exposed to dry air for 24 hr, paramagnetism equivalent to 0.9 X lo1' spins/g of zeolite developed. O
spectrophotometer. Absorbance values were converted to carbonium ion concentrations by use of the equation
C=
A N L x 10-3 EW
(1)
where C is the concentration of carbonium ions (ions g-'), A the absorbance, N Avogadro's number, L the geometric area of the catalyst platelet (om2), E the extinction coefficient of the absorbing ion (1. mole-' cm-'), and W the weight of the platelet (g). A value of 4.0 X lo41. mole-' cm-' was used in the calculations for the extinction coefficient of the triphenylmethylcarbonium ion.' The measured absorbance was multiplied by a suitable factor (e.g., 10 for platelets containing 10% zeolite, 5 for platelets containing 20% zeolite) in order to obtain the concentration equivalent to a platelet composed entirely of decationated Y zeolite. X irradiations at room temperature were performed with a Picker unit using an AEG-50-T tube with tungsten target and beryllium window. The sample (2.0 g) of decationated Y zeolite in a sealed Pyrex tube to which an epr quartz probe tube was attached by a graded seal was placed about 5 cm from the target. The voltage and current were set at 45 pkv and 35 ma, respectively. The dose rate was 3 X lo6 r hr-l. After irradiation, the zeolite was shaken down into the quartz epr tube and the epr spectra examined at room temperature.
Results A . E f e c t of Zeolite Pretreatment on Formation of Radical Ions. The results of experiments defining the effect of the pretreatment on radical-ion concentration are listed in Table I. The parent Na+ Y zeolite
did not form detectable amounts of perylene radical ion under the usual experimental conditions, although it had a sodium deficiency equivalent to 6 X 1019 sites/g, indicating that a small concentration of decationated sites may already have been present. The Y zeolite consistently gave greater radical-ion concentrations than the X form for all catalyst pretreatments. The number of radical ions formed for any given pretreatment depended on the nature of the exchanged ion (r\rH4+> Ca2+> Na+) and the extent of ion exchange. The variation of radical-ion concentration with pretreatment of zeolite was consistent with that reported earlier3v4'$8 for silica-alumina. The higher epr spin concentrations following the standard and 02-cooled pretreatments (than when the zeolite was reduced or given the evacuation pretreatment) indicated that oxygen plays a fundamental role in the reaction. B. Efect of Oxygen on the Formation of Radical Ions. A quantitative study was made of the increase in the number of perylene radical ions formed on a decationated Y-type zeolite (evacuated at 550") when a known amount of oxygen was added and also of the decrease caused by the removal of oxygen as water when it was treated with H2 as it became reduced. These results are summarized in Table 11. Both methods yielded values of about one perylene radical ion formed per oxygen molecule. It was found that the oxygen added at 25" to a sample given the evacuation pretreatment could be quantitatively recovered by pumping at 25". However, if after the addition of oxygen (lo'* to 1019molecules/g) a t 25" the zeolite was heated at 660" overnight, no oxygen could be removed, but the increase in radicalion concentration after contact with perylene was only 10-2070 of that produced by the same amount of oxygen added at 25". When a reduced zeolite was treated Vo~urne71, Number .G March 1967
F. R. DOLLISH AND W. KEITHHALL
1008
Table II: The Effect of Oxygen on Radical-Ion Formation (Zeolite: NH4+ Y Zeolite, 45% Exchanged; Coverage: 7
x
10" Perylene Molecules/g)
A. Oxygen Sorption by Zeolite Given Evacuation Pretreatment
No. of 01 molecules/g added at 2 6 O
Radical-ion concn, spins/ g x 10-1:
0
1.3 58 35 25
57 40 19
x x x
1017 1017
1017
Increase in radical-ion concn,a spins/ g x 10-1'
Rstio, increase in spin concn: Oa molecules added
...
0 57
1.0 0.84 1.2
34 23
B. Relationship between HzO Removed and Decrease in Radical-Ion Formation -No.
per gram of zeoliExpt 1 Expt 2
1. Radical-ion concentration after standard pretreatment, spins
9.5
x
1017
9.5
x
1017
0.6
x
1017
1.4
x
1017
8.9
x
1017
8.1
x
1017
g-'
2. Radical-ion concentration after reduction pretreatment, spins g-1 3. Decrease in radical-ion concentration due to reduction, epins g-1 4. Amount of HzO removed during reduction,* cc(NTP) g -I
Equivalent 02 molecules g-' A(8pin concentration) 5. Ratio: Oz (2Hz0) removed
0.079
0.074
10.6 x 1017 10.0 x 1017 0.84 spins/ 0.81 spins/ molecule molecule of 0 2 of Oa
Values given in the second column corrected for the ion concentration (1.3 x lO"/g) before addition of specified amount I n these experiments, 4.0 g of catalyst wm used. of oxygen.
with 0 2 and heated at 550" overnight, the radicalion concentration corresponded to 3540% of the 0 2 added. A study of oxygen adsorptive capacity of the 45% exchanged NH4+Y zeolite after standard pretreatment showed that the dehydroxylated form adsorbed 4.6 X 10'8 Oz molecules/g at 22" in 212 torr of oxygen over a period of 24 hr. When this sample was heated overnight in oxygen a t 545", the adsorption increased to 9.0 X lo1"molecules/g; about 5.5 X 10l8/g of these could not be removed by heating at 550". An attempt was made to relate quantitatively the amount of oxygen initially adsorbed by the zeolite after the evacuation pretreatment and the amount which could be subsequently reduced off as water. However, large amounts of COZ and HzO were formed Ths Journal of Phyedcal Chemistry
during the first contact with oxygen, presumably by oxidation of residual carbonaceous material on the zeolite. Also, reaction with impurities (mainly iron and titanium) may have occurred, causing a poor mass balance. After the third oxidation-reduction cycle, however, the amount of oxygen adsorbed by the zeolite at 550" (9.1 X lo1' 02/g) could be reversibly reduced off as water (8.0 X 10'' 02/g) and was equal to that found for the reduction of a zeolite given the standard pretreatment (Table IIB). Therefore, at present it is unclear whether during the initial uptake of oxygen at 550" chemisorption can occur in more than one form; however, it has been established that only that oxygen which can be reversibly chemisorbed and reduced off as water is effective in radical-ion formation. A sample of the 45% exchanged NH4+ Y zeolite, when evacuated at 550" overnight and then X irradiated under vacuum, produced an epr spectrum showing both the six-line hyperfine pattern characteristic of an electron trapped on an aluminum and the spectrum of the superoxide, 02-, ion.16 The spin concentration of the former was about 1 X 10l6spinsjg and the latter about 4 X 10l6 spins/g. Contact of this sample with a solution of perylene in benzene resulted in the disappearance of the two signals and the formation of a perylene epr signal equivalent to 1.3 X 10l8 spins/g. An unirradiated sample under similar conditions gave a perylene radical-ion concentration of 1.3 X 10'' spins/g or tenfold less than that obtained after X irradiation. X irradiation of the same material in the presence of 2.2 X 1019 O2 molecules/g produced only the 0 2 - signal with a concentration of 1.0 X 10l8 molecules/g, while a sample given the standard pretreatment gave an 02-signal equivalent to 1 X 10'' molecules/g. C. Effect of Temperature of Zeolite Activation on the Formation of Radical Ions. The effect of the temperature of zeolite activation OR the generation of perylene radical ions on NH4+ Y zeolite is illustrated in Figure 1. Under vacuum (curve A), there was little difference in the radical-ion concentration with zeolites given the evacuation pretreatment at temperatures from 100 to 800". On addition of oxygen at room temperature to these samples, however, there was a large increase in perylene radical-ion concentration with samples evacuated above 500" as compared with those treated at lower temperatures. Above 700°,
(14) D.N. Stamiras and J. Turkevich, J. Ant. Chem. (1964). (16) P. H.Kaaai, J . C h a . Phya., 43, 3322 (1906).
SOC.,86, 767
EFFECT OF OXYGENON THE GENERATION OF RADICAL IONS
180
1
160 I *
I
3
140
-
>I
Ei g 120 k4
;
-7
100
-
Zeolite: NH:Y-45%
Exchanged
A. s - 7 ~ 1Perylcns 0 ~ ~ Molecules&rom in Vocuurn B .-Samples o s A in Air For 2 4 Hours C. O-2x10*'QlsCH Moltculer/Gram in Vocuurn
.9
2
80-
f
-
0
20 40
i
0 0
100
200
300
400
500
600
700 800
Temperature of evacuation, O C .
Figure 1. Effect of evacuation temperature on formation of perylene radical ions and triphenylcarbonium ions.
some decomposition of the lattice may have occurred, causing the ion concentration to drop. Stamires and Turkevich" published a curve similar to that of Figure 1B for 1,l-diphenylethylene adsorbed on decationated Y-type zeolite. Evidently, oxygen was not rigorously excluded in their experiments, which were prepared in a drybox. Also, the maximum number of triphenylamine cation radicals (1.2-1.5 X 1019 spins/g on a 90% decationated Y-type sieve) which they observed could only be duplicated in the present work when excess oxygen was available. Both perylene and triphenylamine adsorbed on decationated Y zeolite (79% exchanged) gave a maximum radicalion concentration of 1.2 X 1019spins/g in the presence of air (Table I, footnote d). However, very little change in ion concentration occurred with the NH4+ Y-type zeolite (45% exchanged) or with the Ca2+ zeolite when samples which had been O2 cooled were exposed to air for 24 hr, indicating that the limiting factor was not lack of suffcient 0 2 . D. E$ect of Impurities in Zeolite. The Y zeolite used in most of the experiments contained 479 ppm of iron and 772 ppm of titanium as the major impurities. In order to check whether these had any effect on the formation of radical ions, a Na+ Y zeolite containing only 1-5 pprn of iron and 1-10 ppm of titanium was tested; it was 81% NH4+ exchanged, given a standard pretreatment, and then contacted with 7 X lozoperylene molecules/g. A maximum perylene radical-ion con-
1009
centration of 1.0 X 10'9 radical ions/g was obtained after exposure to air for 24 hr which was nearly the same as that found for the impure zeolite (1.2 X 10'9 radical ions/g) under similar conditions. This shows that the presence of these impurities has no effect on radical-ion formation. E. E$ect of Oxygen on the Formation of TriphenyEcarbonium Ions. The formation of triphenylcarbonium ions from triphenylmethane has been observed7*'6 to occur on silica-alumina catalysts under conditions similar to those for radical-ion formation. Unlike radical-ion formation, however, reduction of the catalyst had little effect. As can be seen in Table 111, the zeolites behaved much as silica-alumina. Hydrogen treatment was, within experimental error, ineffective and the addition of air did not affect the absorbance at 430 mp in the absence of light. Also, ultraviolet irradiation with a Hanovia mercury lamp in the absence of 0 2 did not alter the carbonium ion concentration. Ion concentration did depend upon the degree of decationation in zeolite (compare Table 111-1 and -6) and also upon the temperature of evacuation. I n the optical experiments of Table 111, it was found that carbonium ions formed, even though the catalyst was maintained mainly in the Brgnsted form, once it had been slightly dehydroxylated in the presence of oxygen (Table 111-7). The ion concentration was repressed, however, from that obtained from the same material more extensively dehydroxylated (Table 111-6) but was still tenfold greater than that from the stoichiometric Brgnsted zeolite (Table 111-8). I n order to make as exact a comparison as possible with the radical-ion data of Figure 1, a series of samples was prepared in the manner used in the epr experiments and these were contacted with triphenylmethane (1 X 1021/g) by vacuum transfer at 100" in the dark for 48 hr. The samples were then all examined together and a visual comparison was made; the results of this comparison are indicated by the dashed curve in Figure 1, which is passed through the quantitative data (points) from the optical experiments. The samples which had been evacuated at 250" were white; those evacuated at 300 and 450" were pale yellow (indicating carbonium ion formation), while those treated between 500 and 550" were deep yellow with the maximum intensity occurring at about 540". Samples evacuated at temperatures between 595 and 640" were again white after contact with substrate. The addition of oxygen to samples evacuated at 300 and 640' did not cause a color change when kept in the dark overnight. Irradiation with ultraviolet light in the pres~~
(16) A. E. Hirschler and J. 0.Hudson, J . Catalysis, 3 , 239 (1964).
Volume 71,Number .G March 1967
1010
F. R. DOLLISH AND W. KEITHHALL
~~~
~~
~
Table III: Formation of Triphenylcarbonium Ions from Triphenylmethane by Decationated Y Zeolite (Nominal Coverage: 2 X 10'' (Ct&)&H/g) Normalized carbonium ion conen Catalyst platelet
x
Treatment
1. 10% NH4+ Y zeolite (79% exchanged) 90% silica gel
+
2. 10% NH4+ Y zeolite (79% exchanged) 90% silica gel
3. 10% NH4+ Y zeolite (79% exchanged) 90% silica gel 4. 10% NH,+ Y zeolite (79% exchanged) 90% silica gel
5. 30% Caa+Y zeolite (79% exchanged) 70% Cab-0-Sil
+
+ +
+
6. 20% NHI+ Y zeolite (45% exchanged) 80% silica gel
7. 20% NH,+ Y zeolite (45% exchanged) SO% silica gel
+
+
8. 100% NH&+Y zeolite (45% exchanged)
10-1sa
(a) Standard, heated 48 hr a t 100" (1) Irradiated 1 hr with ultraviolet light (2) Exposed to air 1hr (b) Reduced, heated 48 hr at 100" (1) Irradiated 1 hr with ultraviolet light ( 2 ) Exposed to air 1 hr (c) Standard, heated 48 hr at 100" (1) Irradiated 1 hr with ultraviolet light (2) Exposed to air 1 hr
7.46 7.0 6.9 5.6 4.6 4.9 4.4 4.3 5.0
(a) Standard pretreatment a t 425", water added-back and evacuated at 300", heated 48 hr a t 100"
4.7
(a) Standard, contacted at room temperature for 72 hr (1) Irradiated 2 hr with ultraviolet light (2)Heated 16 hr a t 100"
1.3 2.6 4.1
(a) Evacuated at 300", contacted a t room temperature
0.54
for 2 hr (1) Heated in dark for 24 hr a t 100"
1.2
(a) Standard, heated 48 hr at 100"
1.8
(a) Standard, heated 48 hr at 100" (1) After standing in 150 mm of 0 2 overnight a t room temperature
3.6 3.2
(a) Standard at 480", water added-back and evacuated at 300",heated 48 hr a t 100" (1) After standing in 150 mm of 0 3 overnight in the dark at room temperature
1.8
(a) Evacuated a t 300" overnight, contacted with (Cd&):CH by heating 48 hr at 100"
0.18
(1) After standing in air overnight in the dark a t room temperature (2) Irradiated 2 hr with ultraviolet light
1.8
0.15 0.30
All carbonium ion concentrations were normalized to platelet containing 100% Y zeolite and are given as ions/g of zeolite. same optical platelet was used for parts la-c. Between experiments, the platelet was burned free of reagent in 0 2 a t 550".
ence of 02, however, led to the formation of a deep yellow color, characteristic of the carbonium ion.
Discussion A . Decatimation of Ammonium Y Zeolites. The Unit cell composition of the Na+ Y zeolite is Na+ss(A102-)a6(Si02)la6. On exchange of part of the Na+ with NH4+ and heating, NHa is evolved, i.e. A
N~+~B,("~+),(A~OZ-)S~(S~~Z)~~B + zNHs Na+se-zH+Z(A102-)~a(SiO~)lae (2)
+
This process has been shown to be complete a t about 300" for Y zeolites containing 80% NH4+.I7 On
The
heating to 550°, the decationated (Brgnsted) sites dehydroxylate to give water and anion and cation vacancy pair sites, i.e.
HO
0
\
/"
A1
0
\/ Si
/V'\
0
0
A ---t
0
Brdnsted site (17) J. B. Uytterhoeven, L. G. Chrktner, and W. K. Hdl, J . Phw. Chm., 69, 2117 (1966).
EFFECTOF OXYGENON
0 HzO
+
0
\
/I
THE
A1
GENERATION OF RADICAL IONS
0
I @Si + \-/O\A1 /\o’\ /\
0 0 0 0 anion vacancy
0
0
/”
Si
(3)
0
cation vacancy
The cation-anion vacancy pairs of the right-hand member of eq 3 are deduced on the basis of stoichiometry. It is not known what (if any) rearrangement occurs beyond this stage. The X-ray patterns of the Y zeolite show slight changes in intensity and position after dehydroxylation but indicate that the gross structure remains intact.ls On the other hand, eq 3 is not easily reversible. Therefore, the representation used for the anon and cation vacancy pair sites should be regarded as a complexion of the system; the indicated formal separation of charge probably does not exist as such because an electronic readjustment would lower the energy of the system. The coulombic energy cost to separate the charge on the sites of eq 3 is about 1.4 ev/pair. Whereas this could be compensated bg re-formation of the tetrahedral structure at the cation vacancy, it is unlikely that this would minimize the energy. In the nomenclature of Rabo,lg Pickert, et a2.,zo and Stamires and Turkevichl1’Sz1the sites formed by elimination of water, as in eq 3, are termed “decationized.” Uytterhoeven, Christner, and Hall” preferred the term “decalionated” for sites formed by elimination of NH3 (eq 2)) which resulted in the appearance of OH bands in the infrared at 3570 and 3660 cm-’ and the term “dehydroxylated” for those formed by the subsequent elimination of HzO. Both terminologies are used in this paper. B. The Role of Oxygen in RadicabIon Formation. The decrease in radical-ion concentration, brought about by treatment of silica-alumina with Hz at elevated temperatures, was first noted by F0g0;~ it was confirmed by Hall14who also found that an amount of HzO could be collected during the Hztreatment which could be equated to the decrease in paramagnetism due to the elimination of one perylene cation radical per oxygen molecule removed. It was pointed out that it was not possible to distinguish whether oxygen was present before reduction, or if the catalyst was oxygen deficient after the treatment, i.e., whether a reactant in the production Oxygen of cation radicals or if the decreased effectiveness of the catalyst resulted from a weakening or elimination of Lewis acid sites. The crystalline zeolites offered the possibility of cetermining the point of stoichiometry by decomposing the NH4+ form under vacuum. The
1011
findings (Table I) that the stoichiometric and reduced zeolites produced nearly equal radical-ion concentrations and that these were much lower than those for the same catalysts after the standard pretreatment indicated that catalysts heated in 0 2 contained excess oxygen, even after evacuation at high temperatures. Furthermore, the nearly stoichiometric relationship between the number of cation radicals produced and the amount of this oxygen (Table 11) showed that the latter was probably a reactant. This was supported by the fact that the same oxygen could be reacted to HzOby heating in Hz. Atkinson, Jones, and Baughan22found that perylene (Pn) in liquid SbCL was stoichiometrically oxidized to the radical ion on the addition of oxygen according to 0 2
+ 4Pn + 6SbCl8 -+4Pn- +
+ 4SbCL- + 2SbOC1
(4)
The data illustrated in Figure 1 show that the Y zeolite in the Brgnsted form did not form radical ions when exposed to oxygen, but that the dehydroxylated zeolite did. Dehydroxylation begins to become significant above 500”.n Recently HirschlerZ3suggested that in the conversion of polynuclear aromatic to cation radicals on solid acid catalysts, the electron removed from the aromatic is trapped by acid protons with the oxygen serving as a catalyst rather than as an electron acceptor. The present results are not in accord with this view since there was a nearly 1 : l correspondence between the number of oxygen molecules added or removed from the system and the change in the number of ions formed and since the ion concentration was inversely related to the number of Brgnsted sites left on the catalyst. Therefore, the picture which emerged, in agreement with Atkinson, Jones, and Baughan,22was that radical ions form on electrophilic sites with (adsorbed) molecular oxygen being the oxidizing agent. The question of the nature of the oxygen chemisorption is intriguing. Since the oxygen sorbed during the standard pretreatment was not removed during overnight evacuation at 550°, it was suggested7 that (18) J. A.Rabo, P. E. Pickert, and J. E. Boyle, U.S. Patent 3,130,006 (1964). (19) J. A. Rabo, P. E. Pickert, D. N. Stamires, and J. E. Boyle, Actes Congr. Intern. Catalyse, P,Paris, 1960, 2 , 2055 (1961). (20) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, PTOC.Intern. Congr. Catalysis, .%d, Amsterdam, i g s q , i , 714 (1965). (21) D.N. Stamires and J. Turkevich, J . Am. Chem. Soc., 86, 749 (1964). (22) J. R. Atkinson, T. P. Jones, and C. E. Baughan, J . Chem. Soc., 5808 (1964). (23) A. E. Hirschler, J. Catdysis, 5, 196 (1966).
Volunte 71,Number 4 March 1967
F. R. DOLLISH AND W. KEITHHALL
1012
it is held as 02-on anion vacancies, the electrons being furnished by donor sites (presumably cation vacancies) in the solid. This was consistent with current thought24 on oxygen chemisorption on oxides. However, the results of the X irradiation experiments showed that if 02were present before the irradiation, it should have been detected by epr. Since no paramagnetism was found under these circumstances, the chemisorbed oxygen must have been held in some other way. Moreover, this oxygen must be able to react with perylene and triphenylamine to form ion pairs, e.g., P n . +02-. No epr spectrum of 02-was found together with that of the cation radical, but this is usual with donoracceptor systems. 25 C. Radical-Ion Formation on Calcium Y Zeolites. Very little radical-forming ability was found for the calcium-exchanged Y zeolite containing 36% Ca2+, even in the presence of oxygen (Table I). However, a t 79% Calf the results were comparable with those of the NH4+ Y zeolites. These results are consistent with the data of Rabo, et al.,19*20 who found that the catalytic activity of Ca2+Y zeolite was very small up to an exchange level of 45%, but increased rapidly above 65% exchange. At low-exchange levels, the Ca2+ will first occupy the SI site positions.20 These are in the hexagonal prisms which bridge adjacent cuboctahedra and will accommodate Ca2+ equivalent to about 50% of the base-exchange capacity. The other type sites are located in or near the planes of the sixmembered oxygen rings facing the large cavities. Originally, on the average, there was about one Na+ for each such ring. Thus, complete exchange with Ca2+ would leave about half the rings empty. Under these circumstances, a large separation of charge occurs,2o creating a positive site and a site bearing a partial negative charge (cation vacancy). Conceivably, in the presence of small amounts of water, the calcium Y zeolite can exist as rz6
0
HO
\
\/
/I’
A1
Si
/\o’\
0
0
0
+
0
Ca(0H) + 0 0 \-/O\ / A1 \OF\ 0 0 0
,
(5)
On heating, CaO may be precipitated out leaving the zeolite decationized. has shown that this process occurs with the divalent montmorillonites and saponites. The site on the left of eq 5 is identical with that formed on the NH4+ Y zeolite as shown in eq 3. While one cannot be categorical a t present, it is at least possible that at high levels of exchange, these sites on the Ca2+Y zeolite can undergo the same The Journal of Physical Chemistry
chemistry as described herein for the decationated zeolites. D . Magnitude of Radical-Ion Concentration. A salient feature of the results was the magnitude of radical-ion concentrations as related to the site concentrations thought to be present. The 79% NH4+ Y zeolite contained about 1 X loz1anion vacancy sites per gram when completely dehydroxylated; this, in agreement with others,”J1 is over two orders of magnitude higher than the highest radical-ion concentrations observed. Evidently, there is some limitation to the extent of reaction. Although it is not possible a t present to be certain what this factor is, several suggestions based on the zeolite structure may be advanced. One possible reason is that the substrate molecules may be excluded from the interconnecting channels of the zeolite crystals. These channels have a free diameter in the large cavities of about 13 A, but the channel openings are only 8-9 A in diameter (Naf form). The molecular size of triphenylmethane and triphenylamine is about 10 A while perylene is 6.5 X 9.1 A. Rabo, Pickert, and Boylel* have shown that the maximum number of molecules which can be sorbed by the Y zeolite depends greatly on the level of decationation as well as the critical size of the sorbate molecule. An Na+ zeolite sorbed 6.2 X 1OZo/g molecules of tri-n-butylamine (critical dimension = 9.1 A) at room temperature. When, however, the ammonium ion exchange exceeded about 20y0,there was an apparent decrease in pore size after decationating. A 65y0NH4+-35% Naf Y-type sieve then sorbed only 1.4 X 1OZo/g,a decrease of about 80%. By contrast, perfluorotri-n-butylamine (critical dimension = 11.5 A) was sorbed by both the Na+ and K ” 4 + forms to about the same extent, Le., about 1.8 X 1019/g. No change in sorption capacity (13 X 1Oz0/g)between sodium and decationated Y zeolite was found for molecules of critical dimension 7.5 A. The dimensions of the perylene and triphenylmethane molecules are sufficiently large to restrict the adsorption and, perhaps, sterically prevent ion formation of those molecules which do manage to pass the shrunken openings of the decationated zeolites and enter the large cavities. Whereas we have no information concerning the distribution throughout the solid of the substrates used in the pres(24) H. B. Charman. R. M. Dell, and SOC.,59, 453, 470 (1963).
s. s. Teale,
Trans. Faraday
(25) L. S. Singer and J. Kommandeur, J . Chem. Phys., 34, 133 (1961). (26) D. N. Stamires and J. Turkevich, J . Am. Chem. Soc., 85, 2557 (1963). (27) J. D. Rus,sell, Trans. Faraday SOC.,61, 2284 (1965)
EFFECT OF OXYGENON THE GENERATION OF RADICAL IONS
ent work, Stamires and TurkevichZ1 reported that a maximum of 3.4 X 1020/gmolecules of triphenylamine could be sorbed by a decationized zeolite, indicating that the sorption is restricted, as with tri-n-butylamine. This was still more than 20 times the number of radical ions formed. Uytterhoeven, Christner, and Hall" have calculated the number of hydroxyl groups required to terminate the external crystal faces of the zeolite used in this work, uix., 5 X 1019/g. Since about 40% of these would be expected to be associated with AlO2- tetrahedra, there would be 79% X 40% X 5 X 1019 = 1.6 X lOI9/g Lewis acid sites on the external faces, a value near the maximum ion concentration achieved (1.2 X 1019/g). It should be pointed out that the sites developed on the crystal faces should correspond to the conventional Lewis acid sites of silica-alumina catalysts; anion vacmcies may not occur. There are several reasons for believing that the ion formation was not limited to the crystal faces. Firstly, it mas shown that chemisorbed oxygen will effect the reaction. Presumably, O2 is uniformly distributed throughout the solid after overnight evacuation at 550", yet a t least 30y0 of this was available for reaction a t room temperature. If reaction occurred only at the crystal faces, this tightly bound oxygen would have to migrate with great rapidity throughout the solid. Secondly, the radical and carbonium ion concentrations (per gram) corresponded fairly well with those reported7 for silica-alumina catalysts (-10l8/g). Here, the surface area was mainly in pores larger than 30 A, so that the steric restriction should have been much less severe Another possibility is that the reaction is thermodynamically restricted, perhaps by an increase in the potential energy of the solid as it takes on electrons. Porter and Hall7 noted that radical-ion concentration on silica-alumin a did not correlate well with surface area but appeared to depend on mass as well. Also, in the formation of perylene-iodine charge-transfer complexesz5 the concentration of neutral perylene molecules is about 30 times greater than the radicalion concentration.21 If ion formation is restricted in this way, then reaction may take place throughout the solid. The results of' the oxygen adsorption study (part B of Results) suggested that radical-ion formation was limited by the maximum amount of oxygen which could be chemisorbed at 550". This amount (obtained after heating at 545") was 9 X 10l8 O2 molecules/g of
1013
zeolite which is of the same order of magnitude as the maximum number of radical ions formed on this same zeolite. Higher values were not found when O2 was present in large excess. Oxygen was ineffective unless the zeolite had been decationized. As found for alurnina,l0 ion formation was repressed unless the solid had been dehydroxylated at high temperature. There is a distinct possiblity that the sites responsible have not yet been characterized. E. The Formation of Triphenylcarbonium Ions. The significant difference between carbonium ion and radical-ion formation was in the response to oxygen. The electron acceptor for radical ions probably was 02, as in s o l ~ t i o n , ~not ~ J ~catalyst hydroxyl groups as suggested by Hir~chler.?~ Oxygen mas not essential for the reaction of triphenylmethane in the absence of light. The photochemical formation of triphenylmethyl hydroperoxide would understandably increase the carbonium ion concentIration when the system is irradiated in the presence of 02. Treatment of the zeolite with Hz removed the oxygen (as HzOj which effected reaction of perylene and triphenylamine. It is therefore clear that oxygen does not participate in carbonium ion formation as it does in radical-ion formation. It is not necessary to oxidize triphenylmethane to triphenylcarbinol (or hydroperoxide) preliminary to carbonium ion formation as claimed.16 By the same token, however, the true mechanism is still uncertain. Direct hydride ion abstraction by the cation-anion vacancy pair sites7 is a possibility, but as with radical-ion formation the maximum concentration of ions developed is many times less than the density of sites available. Again, it is possible that the reaction occurs on defects developed during dehydroxylation, which have not yet been characterized. The loss in ability to form carbonium ions when the zeolite is completely dehydroxylated (Figure 1) is not understood and is under further investigation.
Acknowledgment. This work was sponsored by the Gulf Research and Development Co. as part of the research program of the Multiple Fellowship on Petroleum. Special thanks are due to Dr. P. E. Pickert and to the Linde Co., Division of Union Carbide Corp., for the special sample of ultrapure zeolite. (28) W. I. Aalbersberg, Thesis, Free University of Amsterdam, The Netherlands, 1960. (29) W. I. Aalbersberg, G. J. Hoijtink, E. L. Mackor, and W. P. Weijland, J. Chem. Soc., 3049, 3058 (1959).
Volume 71, Number 4
March 1967
