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Investigation of the features of zinc oxide-based catalysts for propylene dehydroaromatization
Applied Catalysis, 44 (1988) Elsevier
Science
Publishers
179
X9-188 B.V.. Amsterdam
-
Printed
in The Netherlands
Investigation of the Features of Zinc Oxide-Based Catalysts for Propylene Dehydroaromatization R. SPINICCI*
Dipartimento [Italy) (Received
and A. TOFANARI
di Energetica,
10 March
C’nicersita’di
Firenze,
1988, revised manuscript
Via di Santa Marta 3, 50139 Florence
received
7 July 1988)
ABSTRACT As a followup to a previous propylene dehydroaromatization
work [R. Spinicci and A. Tofanari, Appl. Catal., 1 (1981) 3971, runs were performed on various types of zinc oxide, including
undoped catalysts and catalysts doped with monovalent (Na+, Ag+ ) and trivalent (Fe”- ) cations, to ascertain whether the doping-induced electron excess concentration can modify catalytic activity. Catalytic activity was tested at the same temperatures and flow conditions as examined in the previous work. Results showed that. with the exception of Ag,O-doped catalysts, no significant alteration is brought about by doping. This can be explained on the basis of similar behavior of the Ag+ and Zn’ . ions (demonstrated in electron spin resonance experiments) and on the basis of the different
types of surface-adsorbed
propylene
[demonstrated in temperature-programmed
desorption (TPD) experiments]; in fact, at the temperature used for the dehydroaromatization tests. only one of the types of propylene still remains adsorbed on the surface, desorbing at about 8OO’C. From an examination
of the TPD
on the adsorptive
of these
properties
peaks,
catalysts
doping appears at higher
not to have a significant
temperatures,
and hence,
effect
it does not
substantially influence dehydroaromatization. This means that dehydroaromatization can be performed at temperatures of up to 800’ C as has already been accomplished in this work. Lastly, it was found that high activity reactivity of the Zn-0 bonds.
and selectivity
towards benzene
formation
resulted
from increased
INTRODUCTION
The oxidative dehydroaromatization of light olefins, especially propylene and isobutene, to yield aromatics, has been an intriguing matter since the first results were reported by Trimm and Doerr in 1970 [ 11. The reaction requires dual-functioning catalysts which exhibit both dehydrogenating activity and weak acid properties so that oxidation can proceed through the formation of a n-ally1 intermediate. Electron t.ransfer between the weak acid centers and the n-ally1 group generate radical-like behavior. Subsequent dimerization and cyclization with oxidative dehydrogenation leads to the formation of aromatics products [ 2 1.
0166.9834/88/$03.50
0 1988 Elsevier
Science
Publishers
B.V.
180
The complexity of this path requires certain catalytic features, such as isomerization, oligomerization and metathesis, which can be partially required by other reactions. Hence, notable insight into the adsorption mechanism and the subsequent reaction of light olefins may be gained through examination of the catalysts’ role in dehydroaromatization. Zinc oxide is very interesting in this respect, since it is active towards both olefin isomerization and dehydroaromatization. Moreover, some recent reports [3,4], confirmed by results obtained in our laboratory, show that, at different reaction conditions, this catalyst is also active in the methane-coupling reaction. In our previous investigation [5], it was found that two forms of zinc oxide are active towards the propylene-tbenzene reaction. The results seemed to indicate that both forms have good activity and high selectivity towards benzene if contact time is not too long and if the oxygen propylene ratio of the reactant mixture exceeds one. With the aim of gaining deeper insight into the properties of this catalyst and into the features of its interaction with propylene, the investigation was expanded to include propylene dehydroaromatization over zinc oxide doped with mono- and trivalent cations. This doping might influence the electron excess concentration on the catalyst surface sites, thereby affecting both activity and selectivity of the two forms of zinc oxide. According to refs. 5 and 6, propylene dehydroaromatization is actually favored by electron-rich species (Zn’, ZnO) that are present on sites with low oxygen content. EXPERIMENTAL
As previously described [ 51, the zinc oxide samples were prepared by a Z-h, 400’ C thermal decomposit.ion of basic zinc carbonate (white product) and of oxalate (reddish pink product, whose colour is probably due to the presence of F-centers). Both have a nearly equal surface area (26.5 m2/g and 25.8 m”/g) and the same structure and morphological makeup [ 51. The greater electron excess on the pink zinc oxide is accompanied by a smaller number of surface acid centers [ 51. Both zinc oxides were doped with mono- and trivalent ions: the catalysts were prepared by impregnating white or pink zinc oxide specimens with an aqueous solution of an appropriate metal salt such as sodium nitrate, silver nitrate, and iron(II1) nitrate. The specimens were then evenly heated until moist and dried at 1OO’C. Afterwards, they were heated in lOO”C-steps to 5OO’C, at which temperature they were calcined for 2 h. The samples were prepared with 0.5% in Na,O, Ag,O, and Fe,O,, the doping level selected on the basis of reports indicating that this level allows the highest electron excess concent.ration [ 71. Determination of the excess concentration was carried out according to methods described [ c5,8]. Dehydroaromatization runs were performed in a l-cm O.D. tubular quartz reactor equipped with a coaxial thermocouple. The catalyst samples (weighing
181
0.5 and 1.0 g) were pretreated in situ at 550°C for 4 h in a flow of heliumoxygen (4:2). The reaction subsequently occurred at the same temperature with a flow of helium (80% ), oxygen (13.3%), and propylene (6.6% ). The composition of the effluent gases was analyzed as a function of the reaction time on a Carlo Erba 4200 gas chromatograph equipped with two l/8 in.-O.D., 16-ft. columns packed with diisodecylphthalate and ethylhexylsebacate on Chromosorb P AW. Temperature-programmed desorption (TPD ) runs were performed in an apparatus as described in ref. 9, where approximately 0.08 g of catalyst was inserted in a quartz reactor and pretreated according to the procedure previously used in the activity runs. After adsorption at the desired temperature, desorption was achieved by heating the catalyst according to a preset program, using a carrier gas flow (helium) at a rate of 100 ml/min. The presence of the adsorbate released in the carrier gas stream was revealed by means on a hot-wire detector.
RESULTS
AND DISCUSSION
The electron excess on zinc oxide samples doped with Na,O, Fe,O,, and Ag,O were measured; the variations obtained using different dopants and zinc oxide preparations are shown in Table 1. Measurements were carried out on samples pretreated at the same conditions as in the dehydroaromatization runs. In fact, since zinc oxide is a typical n-type semiconductor, thermal treatment can remove some of its lattice oxygen, thereby producing a non-stoichiometric excess of zinc with Zn(1) and Zn(0) on the surface and creating an electron excess concentration, especially on the surface. The different effects that doping with mono- and t.rivalent cations have on the electronic properties of zinc oxide can be readily evaluated from Table 1, which shows the effects of adding trivalent cations (increase in concentration) and those of adding monovalent cations (decrease in concentration with Na,O and increase with Ag,O). According to the literature, many catalytic reactions appear to be directly influTABLE
1
Electron excess concentration Values are expressed in ppm.
on undoped and doped zinc oxides
Undoped
Doped 0.5%
Pink (oxalate precursor) White (carbonate precursor)
8.3 1.1
7.5 0.5
Na,O
0.5% Ag,O
0.5% Fe,O,,
10.2 1.8
9.5 1.5
182
enced by the net electron excess concentration, such as, for example, c&2butene isomerization [ 91, while ot.hers, such as ethylene hydrogenation, are apparently unaffected [lo]. Nevertheless, it should be mentioned that our experiments on dehydroaromatization activity to benzene did not produce results which could be relatable to the electron excess concentration of t.he catalysts. As shown in Fig. 1, samples with 0.5% Ag,O exhibit the highest activity rates (with a certain production of hexadienes ), as well as the highest benzene-carbon dioxide ratios. However, the results of experiments performed using this doping on both types of zinc oxide were not fully reproducible with respect to activity and selectivity; the highest activity and selectivity to benzene could be attributed to the white zinc oxide samples. Xo significant differences in catalytic activity were found in the samples doped with sodium and iron (III) oxides. The activity was slightly higher for pink and white samples doped with 0.5% Fe,O,, but this activity decayed at a quicker rate, especially in the pink oxide. These results demonstrate that the type of dopant does not influence the catalytic activity of zinc oxide towards propylene dehydroaromatization to a decisiv-e extent. Generally speaking, pink zinc oxide gave the best results in terms of activity and perhaps in terms of selectivity towards benzene, although after a few hours of activity, a leveling of these properties through progressive decay was observed in both types of catalysts. It would appear that the electron excess introduced by the dopant is not responsible for the enhanced activity towards propylene dehydroaromatization. Therefore, the role of the Zn-0 couple appears more important and the properties of the silver-doped catalysts may be explained by the similar configurations of the Zn’+ and Ag+ ions, and by the reducibility of the Ag+ ions which increases the oxygen reactivity in the Zn-0 bonds. As far as the sodium-doped catalysts are concerned, the unexpected relatively high activity can be explained on the basis of an increase in the reactivity of oxygen in the Zn-0 bond resulting from the electron-donating capabilities of Nat ions in Na,O [12]. Hence, deeper insight must be gained into the catalysts’ adsorptive properties towards propylene so that the active surface centers involved in the propylene dehydroaromatization mechanism can be distinguished from the other sites involved, for example, in cis-2 isomerization, as well as in other dopingaffected reactions. For this reason, the TPD investigation described in ref. 9, was expanded to include the examination of doped and undoped catalysts up to about 850’ C. Two salient features of the results are shown in Fig. 2. (i) A desorption peak is visible in the highest temperature range, with a maximum at about 820’ C for the undoped zinc oxides and those doped with Ag,O and Fe& In the case of Na,O-doped oxides, the peak shifts to lower temperatures (with a maximum at around 750°C). The effluent analysis re-
183
60
120
180
240
1180
1260
t
(min)
b)
1
v-
I
60
120
180
240
300
360
t
(min)
C)
60
Fig. 1. Plots
120
180
of propylene conversion
240
300
360
t
versus time at 550°C
(min)
for pink and white zinc oxides doped
with (a) 0.5% Ag,O. (b) 0.5% Na,O, and (c) 0.5% Fe20,. Key: l , conversion to CO, and 0, to C,H, on white samples: n , conversion to CO, and 0, to C,H, on reddish-pink samples.
184
100
300
500
700
T OC
900
Fig. 2. TDP profile of propylene, adsorbed at room temperature on zinc oxide. Undoped doped (---). Flow-rate of helium carrier gas: 100 ml/min and heating rate: B”C/min.
(-)
and
veals the presence of oxidative dehydrogenation and dehydroaromatization products, together with total oxidation products. (ii) The previously identified peak for undoped oxides with a maximum at ca. 440°C is found at the same temperatures for Ag,O-and FezO,-doped oxides, whereas it shifts to lower temperatures (360°C) for Na,O-doped catalysts. The effluent analysis reveals the sole presence of propylene without oxidation products. These results suggest that the investigated temperature range can be divided into two regions and that, while certain reactions taking place at temperatures below 410-450°C can be affected by the type of doping, the dehydroaromatization reaction, which occurs above 440-480’ C, cannot be affected by the type of doping. This is because propylene, retained on the catalyst surface up to 8OO’C, is not affected by doping. On the other hand, the presence of Na,O as a dopant should not lead one to expect special effects, even though the adsorptive properties towards propylene appear slightly modified. These hypotheses have received confirmation from the TPD experiments performed on doped and undoped catalysts with preadsorbed ethylene, under the same conditions as those used for propylene testing. The two peaks in the desorptogram (Fig. 3) are attributable (i) to desorption of slightly chemisorbed or even physisorbed ethylene (maximum at 90 ’ C ) , which can be eliminated by performing adsorption above lOO-12O”C, and (ii) to desorption of ethylene (maximum at 430’ C). No other peak can be found up to the highest temperatures investigated. With the foregoing assumptions, neither doped nor undoped zinc oxides should exhibit any acbivity towards ethylene trimerization and aromatization at temperatures ranging from 500 to 800°C: this has been confirmed in recent experiments [ 131.
185
395
100
3co
435
500
9bo
700
-
T OC
Fig. 3. TPD profile of ethylene adsorbed at room temperature on zinc oxide. Undoped (-) and doped with 0.5% Na,O (---). Flow-rate of helium carrier gas: 100 ml/min and heating rate: 8” C/ min.
All of the above results can be explained by supposing that dehydroaromatization activity is mainly related to the electron excess associated with the Zn+ ions or Zn” atoms, and thus can be correlated to their configuration. Logically, a marked increase in activity can be expected with Ag,O-doped catalysts because of the similarit,es between Zn2+ and Ag+ and between Zn+ and Ag. Accordingly, the reduced zinc species was investigated through electron spin resonance (ESR) spectroscopy, by using both zinc oxides pretreated as in the prior dehydroaromatization runs so that the results could be correlated to those obtained at reaction conditions. In keeping with the literature [ 141, our results confirmed the presence of Zn+ (Fig. 4). Since propylene is retained on the catalyst surface up to 800-850°C dehydroaromatization runs were carried out at about 800” C (Fig. 5). Both doped and undoped catalysts showed high activity (26.5%) and high selectivity (68.6%) to benzene, despite the formation of several intermediate products, especially cyclohexadienes and cyclohexenes which are not shown in Fig. 5. This selectivity value is of notable interest, since it is considerably higher than the result observed on doped and undoped zinc oxide samples at 550’ C. However, it should be noted that such high activity and selectivity values result from a reactant mixture with a propylene:oxygen ratio of 2:l: as this ratio is exactly inverse to the one used in the dehydroaromatization runs at 550 aC, we must necessarily suppose that the lack of oxygen in the reactant mixture must be compensated for by a higher reactivity of surface oxygen species. It would thus appear that the higher the temperature, the greater the importance of the oxygen reactivity and the Zn-0 couple in the reaction mecha-
186
50 G
1 /I ;
g=1.958 A
1
cI-__c/,
L
\\,'I
g=2.01
/Jy+_?-
---I
i I) I g=2.0005
-
\ji'
Fig. 4. ESR spectrum of zinc oxide pretreated in air at 50°C (T='77K, microwave frequency z9.29 GHz: microwave power=5 dB; field modulation intensity=4 Gpp; gain =2.5* 105; scan range 100 G).
nism. The greater reactivity of the Zn-0 couple at higher temperatures is confirmed by the oxygen TPD runs performed on a series of doped and undoped samples (Fig. 6). As the relative desorptograms show an oxygen peak at relatively high temperatures (maximum at 630 a C), it can be presumed that at higher temperatures, the catalyst surfaces are characterized by an enhanced degree of reduction. As reported [ 141, the result of high-temperature heating is an increase in the number of surface Zn+ ions (confirmed in our experiments by the increased intensity in the ESR signal shown in Fig. 4) and a decrease in the number of surface 0, ions. Hence, in accordance with the hypothesis suggested in ref. 13, the number of n-ally1 intermediates formed during propylene adsorption is lowered and, during formation of this species, propylene can be adsorbed as a partially oxidized intermediate. This reasoning, which supports the role of the Zn-0 couple in zinc oxide reactivity, was confirmed by our experiments on catalytic activity at ca. 800’ C. In summation, the oxidative reaction of propylene can proceed in such a way as to produce mostly benzene in the presence of oxygen-deficient reactant mixtures: in such a case, the experimentally observed rapid activity decay appears logical (Fig. 6), since, after a brief time lapse, the lattice oxygen cannot be restored and the Zn-0 couples rapidly decrease. In conclusion, all of the above factors contribute to enhancing the importance of the zinc species and the Zn-0 couple features in the dehydroaroma-
187
60
120
180
240
300
360
t
360
t
(min)
b)
1
A
60
120
180
240
300
(min)
Fig. 5. Plots of propylene conversion versus time at. 800°C for pink and white zinc oxides doped with (a) 0.5% Ag,O and (b) 0.5% Na,O. Key: 0, conversion to CO, and 0, to C,Hs on white samples; n , conversion to CO, and 0, to C,H6 on reddish-pink samples.
tization reaction, where doping is apparently less important than, for example, in isomerization reactions. Moreover, at high temperatures, the behaviour of t,he zinc oxide towards dehydroaromatization explains the absence of benzene in the methane-coupling reaction products. In an examination of zinc oxide activity towards oxi-
!OO
300
500
700
900 T 'C
Fig. 6. TDP profile of oxygen adsorbed at room tempoerature on zinc oxide. Undoped (-) and doped with 0.5% Na,O (---), Flow-rate of helium carrier gas: 100 ml/min and heating rate: 8’C/ mion.
dative met,hane conversion into ethylene at temperatures of ca. 8OO”C, no benzene was produced according to a schematic pathway: C,H,+C,H,+C,H,; conversely, on calcium oxide, ethylene produces TPD desorptograms with peaks centered at 340’ C and 840 j C. Therefore, a certain amount of ethylene can still be present at the temperatures used in the methane conversion, thereby producing benzene via ethylene trimerization according to a patern previously reported in ref. 13. ACKNOWLEDGEMENT
We are grateful to R. van der Kwast for his contribution
to this work.
REFERENCES 1
D.L. Trimm
2 3
E.A. Memdor, React. Kinet. Catal. Lett., 11 (1979) 11. T. Ito, J.X. Wang, C.H. Linand J.H. Linsfard, J. Am. Chem. Sot., 107 (1985)
4 5
G.E. Keller and M.M. Basin, 73 (1973) 9. R. Spinicci and A. Tofanari, Appl. Catal., 1 (1981)
387.
6
T. Uematsu and H. Hashimoto,
48.
5 8
I. Aso, S. Abe, N. Yamazoe and T. Seyiama, J. Catal., 59 (1979) V.J. Norman, Analyst (London), 82 (1957) 474.
9
R. Spinicci
and L.A. Doerr, Chem. Comm.,
and A. Tofanari,
1303 (1970).
J. Catal. 47 (1977)
J. Therm.
Anal., 23 (1982)
5062.
375.
45.
10 11
T. Uematsu, K. Mamura, K. Irai and H. Hashimoto, J. Catal., 45 (1976) J. Aigesporse and S.J. Teichner, J. Catal., 2 (1963) 359.
12 13 14
W.D. Mross, Catal. Rev.-Sci. Eng., 25 (1983) 591. R. Spinicci and A. Tofanari, unpublished results. E.G. Ismailov, V.F. Anufrien, N.G. Maksiummov and V.D. Sokolovskii,
15
Lett., 3 (19i5) 301. G.W. Kaulks and M. Yu, React. Kinet. Catal. Lett., 35 (1987)
360.
68.
React. Kinet. Catal.
Science
Publishers
179
X9-188 B.V.. Amsterdam
-
Printed
in The Netherlands
Investigation of the Features of Zinc Oxide-Based Catalysts for Propylene Dehydroaromatization R. SPINICCI*
Dipartimento [Italy) (Received
and A. TOFANARI
di Energetica,
10 March
C’nicersita’di
Firenze,
1988, revised manuscript
Via di Santa Marta 3, 50139 Florence
received
7 July 1988)
ABSTRACT As a followup to a previous propylene dehydroaromatization
work [R. Spinicci and A. Tofanari, Appl. Catal., 1 (1981) 3971, runs were performed on various types of zinc oxide, including
undoped catalysts and catalysts doped with monovalent (Na+, Ag+ ) and trivalent (Fe”- ) cations, to ascertain whether the doping-induced electron excess concentration can modify catalytic activity. Catalytic activity was tested at the same temperatures and flow conditions as examined in the previous work. Results showed that. with the exception of Ag,O-doped catalysts, no significant alteration is brought about by doping. This can be explained on the basis of similar behavior of the Ag+ and Zn’ . ions (demonstrated in electron spin resonance experiments) and on the basis of the different
types of surface-adsorbed
propylene
[demonstrated in temperature-programmed
desorption (TPD) experiments]; in fact, at the temperature used for the dehydroaromatization tests. only one of the types of propylene still remains adsorbed on the surface, desorbing at about 8OO’C. From an examination
of the TPD
on the adsorptive
of these
properties
peaks,
catalysts
doping appears at higher
not to have a significant
temperatures,
and hence,
effect
it does not
substantially influence dehydroaromatization. This means that dehydroaromatization can be performed at temperatures of up to 800’ C as has already been accomplished in this work. Lastly, it was found that high activity reactivity of the Zn-0 bonds.
and selectivity
towards benzene
formation
resulted
from increased
INTRODUCTION
The oxidative dehydroaromatization of light olefins, especially propylene and isobutene, to yield aromatics, has been an intriguing matter since the first results were reported by Trimm and Doerr in 1970 [ 11. The reaction requires dual-functioning catalysts which exhibit both dehydrogenating activity and weak acid properties so that oxidation can proceed through the formation of a n-ally1 intermediate. Electron t.ransfer between the weak acid centers and the n-ally1 group generate radical-like behavior. Subsequent dimerization and cyclization with oxidative dehydrogenation leads to the formation of aromatics products [ 2 1.
0166.9834/88/$03.50
0 1988 Elsevier
Science
Publishers
B.V.
180
The complexity of this path requires certain catalytic features, such as isomerization, oligomerization and metathesis, which can be partially required by other reactions. Hence, notable insight into the adsorption mechanism and the subsequent reaction of light olefins may be gained through examination of the catalysts’ role in dehydroaromatization. Zinc oxide is very interesting in this respect, since it is active towards both olefin isomerization and dehydroaromatization. Moreover, some recent reports [3,4], confirmed by results obtained in our laboratory, show that, at different reaction conditions, this catalyst is also active in the methane-coupling reaction. In our previous investigation [5], it was found that two forms of zinc oxide are active towards the propylene-tbenzene reaction. The results seemed to indicate that both forms have good activity and high selectivity towards benzene if contact time is not too long and if the oxygen propylene ratio of the reactant mixture exceeds one. With the aim of gaining deeper insight into the properties of this catalyst and into the features of its interaction with propylene, the investigation was expanded to include propylene dehydroaromatization over zinc oxide doped with mono- and trivalent cations. This doping might influence the electron excess concentration on the catalyst surface sites, thereby affecting both activity and selectivity of the two forms of zinc oxide. According to refs. 5 and 6, propylene dehydroaromatization is actually favored by electron-rich species (Zn’, ZnO) that are present on sites with low oxygen content. EXPERIMENTAL
As previously described [ 51, the zinc oxide samples were prepared by a Z-h, 400’ C thermal decomposit.ion of basic zinc carbonate (white product) and of oxalate (reddish pink product, whose colour is probably due to the presence of F-centers). Both have a nearly equal surface area (26.5 m2/g and 25.8 m”/g) and the same structure and morphological makeup [ 51. The greater electron excess on the pink zinc oxide is accompanied by a smaller number of surface acid centers [ 51. Both zinc oxides were doped with mono- and trivalent ions: the catalysts were prepared by impregnating white or pink zinc oxide specimens with an aqueous solution of an appropriate metal salt such as sodium nitrate, silver nitrate, and iron(II1) nitrate. The specimens were then evenly heated until moist and dried at 1OO’C. Afterwards, they were heated in lOO”C-steps to 5OO’C, at which temperature they were calcined for 2 h. The samples were prepared with 0.5% in Na,O, Ag,O, and Fe,O,, the doping level selected on the basis of reports indicating that this level allows the highest electron excess concent.ration [ 71. Determination of the excess concentration was carried out according to methods described [ c5,8]. Dehydroaromatization runs were performed in a l-cm O.D. tubular quartz reactor equipped with a coaxial thermocouple. The catalyst samples (weighing
181
0.5 and 1.0 g) were pretreated in situ at 550°C for 4 h in a flow of heliumoxygen (4:2). The reaction subsequently occurred at the same temperature with a flow of helium (80% ), oxygen (13.3%), and propylene (6.6% ). The composition of the effluent gases was analyzed as a function of the reaction time on a Carlo Erba 4200 gas chromatograph equipped with two l/8 in.-O.D., 16-ft. columns packed with diisodecylphthalate and ethylhexylsebacate on Chromosorb P AW. Temperature-programmed desorption (TPD ) runs were performed in an apparatus as described in ref. 9, where approximately 0.08 g of catalyst was inserted in a quartz reactor and pretreated according to the procedure previously used in the activity runs. After adsorption at the desired temperature, desorption was achieved by heating the catalyst according to a preset program, using a carrier gas flow (helium) at a rate of 100 ml/min. The presence of the adsorbate released in the carrier gas stream was revealed by means on a hot-wire detector.
RESULTS
AND DISCUSSION
The electron excess on zinc oxide samples doped with Na,O, Fe,O,, and Ag,O were measured; the variations obtained using different dopants and zinc oxide preparations are shown in Table 1. Measurements were carried out on samples pretreated at the same conditions as in the dehydroaromatization runs. In fact, since zinc oxide is a typical n-type semiconductor, thermal treatment can remove some of its lattice oxygen, thereby producing a non-stoichiometric excess of zinc with Zn(1) and Zn(0) on the surface and creating an electron excess concentration, especially on the surface. The different effects that doping with mono- and t.rivalent cations have on the electronic properties of zinc oxide can be readily evaluated from Table 1, which shows the effects of adding trivalent cations (increase in concentration) and those of adding monovalent cations (decrease in concentration with Na,O and increase with Ag,O). According to the literature, many catalytic reactions appear to be directly influTABLE
1
Electron excess concentration Values are expressed in ppm.
on undoped and doped zinc oxides
Undoped
Doped 0.5%
Pink (oxalate precursor) White (carbonate precursor)
8.3 1.1
7.5 0.5
Na,O
0.5% Ag,O
0.5% Fe,O,,
10.2 1.8
9.5 1.5
182
enced by the net electron excess concentration, such as, for example, c&2butene isomerization [ 91, while ot.hers, such as ethylene hydrogenation, are apparently unaffected [lo]. Nevertheless, it should be mentioned that our experiments on dehydroaromatization activity to benzene did not produce results which could be relatable to the electron excess concentration of t.he catalysts. As shown in Fig. 1, samples with 0.5% Ag,O exhibit the highest activity rates (with a certain production of hexadienes ), as well as the highest benzene-carbon dioxide ratios. However, the results of experiments performed using this doping on both types of zinc oxide were not fully reproducible with respect to activity and selectivity; the highest activity and selectivity to benzene could be attributed to the white zinc oxide samples. Xo significant differences in catalytic activity were found in the samples doped with sodium and iron (III) oxides. The activity was slightly higher for pink and white samples doped with 0.5% Fe,O,, but this activity decayed at a quicker rate, especially in the pink oxide. These results demonstrate that the type of dopant does not influence the catalytic activity of zinc oxide towards propylene dehydroaromatization to a decisiv-e extent. Generally speaking, pink zinc oxide gave the best results in terms of activity and perhaps in terms of selectivity towards benzene, although after a few hours of activity, a leveling of these properties through progressive decay was observed in both types of catalysts. It would appear that the electron excess introduced by the dopant is not responsible for the enhanced activity towards propylene dehydroaromatization. Therefore, the role of the Zn-0 couple appears more important and the properties of the silver-doped catalysts may be explained by the similar configurations of the Zn’+ and Ag+ ions, and by the reducibility of the Ag+ ions which increases the oxygen reactivity in the Zn-0 bonds. As far as the sodium-doped catalysts are concerned, the unexpected relatively high activity can be explained on the basis of an increase in the reactivity of oxygen in the Zn-0 bond resulting from the electron-donating capabilities of Nat ions in Na,O [12]. Hence, deeper insight must be gained into the catalysts’ adsorptive properties towards propylene so that the active surface centers involved in the propylene dehydroaromatization mechanism can be distinguished from the other sites involved, for example, in cis-2 isomerization, as well as in other dopingaffected reactions. For this reason, the TPD investigation described in ref. 9, was expanded to include the examination of doped and undoped catalysts up to about 850’ C. Two salient features of the results are shown in Fig. 2. (i) A desorption peak is visible in the highest temperature range, with a maximum at about 820’ C for the undoped zinc oxides and those doped with Ag,O and Fe& In the case of Na,O-doped oxides, the peak shifts to lower temperatures (with a maximum at around 750°C). The effluent analysis re-
183
60
120
180
240
1180
1260
t
(min)
b)
1
v-
I
60
120
180
240
300
360
t
(min)
C)
60
Fig. 1. Plots
120
180
of propylene conversion
240
300
360
t
versus time at 550°C
(min)
for pink and white zinc oxides doped
with (a) 0.5% Ag,O. (b) 0.5% Na,O, and (c) 0.5% Fe20,. Key: l , conversion to CO, and 0, to C,H, on white samples: n , conversion to CO, and 0, to C,H, on reddish-pink samples.
184
100
300
500
700
T OC
900
Fig. 2. TDP profile of propylene, adsorbed at room temperature on zinc oxide. Undoped doped (---). Flow-rate of helium carrier gas: 100 ml/min and heating rate: B”C/min.
(-)
and
veals the presence of oxidative dehydrogenation and dehydroaromatization products, together with total oxidation products. (ii) The previously identified peak for undoped oxides with a maximum at ca. 440°C is found at the same temperatures for Ag,O-and FezO,-doped oxides, whereas it shifts to lower temperatures (360°C) for Na,O-doped catalysts. The effluent analysis reveals the sole presence of propylene without oxidation products. These results suggest that the investigated temperature range can be divided into two regions and that, while certain reactions taking place at temperatures below 410-450°C can be affected by the type of doping, the dehydroaromatization reaction, which occurs above 440-480’ C, cannot be affected by the type of doping. This is because propylene, retained on the catalyst surface up to 8OO’C, is not affected by doping. On the other hand, the presence of Na,O as a dopant should not lead one to expect special effects, even though the adsorptive properties towards propylene appear slightly modified. These hypotheses have received confirmation from the TPD experiments performed on doped and undoped catalysts with preadsorbed ethylene, under the same conditions as those used for propylene testing. The two peaks in the desorptogram (Fig. 3) are attributable (i) to desorption of slightly chemisorbed or even physisorbed ethylene (maximum at 90 ’ C ) , which can be eliminated by performing adsorption above lOO-12O”C, and (ii) to desorption of ethylene (maximum at 430’ C). No other peak can be found up to the highest temperatures investigated. With the foregoing assumptions, neither doped nor undoped zinc oxides should exhibit any acbivity towards ethylene trimerization and aromatization at temperatures ranging from 500 to 800°C: this has been confirmed in recent experiments [ 131.
185
395
100
3co
435
500
9bo
700
-
T OC
Fig. 3. TPD profile of ethylene adsorbed at room temperature on zinc oxide. Undoped (-) and doped with 0.5% Na,O (---). Flow-rate of helium carrier gas: 100 ml/min and heating rate: 8” C/ min.
All of the above results can be explained by supposing that dehydroaromatization activity is mainly related to the electron excess associated with the Zn+ ions or Zn” atoms, and thus can be correlated to their configuration. Logically, a marked increase in activity can be expected with Ag,O-doped catalysts because of the similarit,es between Zn2+ and Ag+ and between Zn+ and Ag. Accordingly, the reduced zinc species was investigated through electron spin resonance (ESR) spectroscopy, by using both zinc oxides pretreated as in the prior dehydroaromatization runs so that the results could be correlated to those obtained at reaction conditions. In keeping with the literature [ 141, our results confirmed the presence of Zn+ (Fig. 4). Since propylene is retained on the catalyst surface up to 800-850°C dehydroaromatization runs were carried out at about 800” C (Fig. 5). Both doped and undoped catalysts showed high activity (26.5%) and high selectivity (68.6%) to benzene, despite the formation of several intermediate products, especially cyclohexadienes and cyclohexenes which are not shown in Fig. 5. This selectivity value is of notable interest, since it is considerably higher than the result observed on doped and undoped zinc oxide samples at 550’ C. However, it should be noted that such high activity and selectivity values result from a reactant mixture with a propylene:oxygen ratio of 2:l: as this ratio is exactly inverse to the one used in the dehydroaromatization runs at 550 aC, we must necessarily suppose that the lack of oxygen in the reactant mixture must be compensated for by a higher reactivity of surface oxygen species. It would thus appear that the higher the temperature, the greater the importance of the oxygen reactivity and the Zn-0 couple in the reaction mecha-
186
50 G
1 /I ;
g=1.958 A
1
cI-__c/,
L
\\,'I
g=2.01
/Jy+_?-
---I
i I) I g=2.0005
-
\ji'
Fig. 4. ESR spectrum of zinc oxide pretreated in air at 50°C (T='77K, microwave frequency z9.29 GHz: microwave power=5 dB; field modulation intensity=4 Gpp; gain =2.5* 105; scan range 100 G).
nism. The greater reactivity of the Zn-0 couple at higher temperatures is confirmed by the oxygen TPD runs performed on a series of doped and undoped samples (Fig. 6). As the relative desorptograms show an oxygen peak at relatively high temperatures (maximum at 630 a C), it can be presumed that at higher temperatures, the catalyst surfaces are characterized by an enhanced degree of reduction. As reported [ 141, the result of high-temperature heating is an increase in the number of surface Zn+ ions (confirmed in our experiments by the increased intensity in the ESR signal shown in Fig. 4) and a decrease in the number of surface 0, ions. Hence, in accordance with the hypothesis suggested in ref. 13, the number of n-ally1 intermediates formed during propylene adsorption is lowered and, during formation of this species, propylene can be adsorbed as a partially oxidized intermediate. This reasoning, which supports the role of the Zn-0 couple in zinc oxide reactivity, was confirmed by our experiments on catalytic activity at ca. 800’ C. In summation, the oxidative reaction of propylene can proceed in such a way as to produce mostly benzene in the presence of oxygen-deficient reactant mixtures: in such a case, the experimentally observed rapid activity decay appears logical (Fig. 6), since, after a brief time lapse, the lattice oxygen cannot be restored and the Zn-0 couples rapidly decrease. In conclusion, all of the above factors contribute to enhancing the importance of the zinc species and the Zn-0 couple features in the dehydroaroma-
187
60
120
180
240
300
360
t
360
t
(min)
b)
1
A
60
120
180
240
300
(min)
Fig. 5. Plots of propylene conversion versus time at. 800°C for pink and white zinc oxides doped with (a) 0.5% Ag,O and (b) 0.5% Na,O. Key: 0, conversion to CO, and 0, to C,Hs on white samples; n , conversion to CO, and 0, to C,H6 on reddish-pink samples.
tization reaction, where doping is apparently less important than, for example, in isomerization reactions. Moreover, at high temperatures, the behaviour of t,he zinc oxide towards dehydroaromatization explains the absence of benzene in the methane-coupling reaction products. In an examination of zinc oxide activity towards oxi-
!OO
300
500
700
900 T 'C
Fig. 6. TDP profile of oxygen adsorbed at room tempoerature on zinc oxide. Undoped (-) and doped with 0.5% Na,O (---), Flow-rate of helium carrier gas: 100 ml/min and heating rate: 8’C/ mion.
dative met,hane conversion into ethylene at temperatures of ca. 8OO”C, no benzene was produced according to a schematic pathway: C,H,+C,H,+C,H,; conversely, on calcium oxide, ethylene produces TPD desorptograms with peaks centered at 340’ C and 840 j C. Therefore, a certain amount of ethylene can still be present at the temperatures used in the methane conversion, thereby producing benzene via ethylene trimerization according to a patern previously reported in ref. 13. ACKNOWLEDGEMENT
We are grateful to R. van der Kwast for his contribution
to this work.
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