382
J. Phys. Chem. 1980, 84,382-388
Selectivity in the Oxidative Dehydrogenation of Butene on Zinc-Iron Oxide Catalyst H. H. Kung," B. Kundalkar, M. C. Kung, and W. H. Cheng Department of Chemical Engineering, The Materials Research Center, and the Ipatieff Laboratory, Northwestern Universiw, Evanston, Illinois 60201 (Received August 9, 1979) Publication costs assisted by the Department of Energy
The oxidative dehydrogenation of butene to butadiene was studied on a zinc ferrite catalyst. Room temperature adsorption of butene on the catalyst followed by thermal desorption resulted in the production of butene isomers, butadiene, and carbon dioxide. Evidence was presented which suggests that the selective oxidation and the combustion sites are independent, and this made possible the estimation of the densities of these two oxidation sites from the amounts of oxidation products formed. Information on the reactivities of the sities was provided by temperature programmed desorption profiles of butene. Comparison of these results with those on iron oxide suggests that zinc ferrite is a more selective oxidation catalyst because it has a higher density of selective oxidation sites and a lower density of combustion sites, and because its combustion sites are less active than those on iron oxide.
Introduction It has always been of interest to understand the difference in selectivity between two catalysts in a heterogeneous catalytic reaction. This is particularly so if only some of the possible products are of value. For example, in the oxidative dehydrogenation of butene, the desired product is butadiene. However, a waste product of carbon dioxide is also produced. When the selectivity of a reaction, such as the example above, has a strong impact on the economic feasibility of a process, there is a strong incentive to modify a good catalyst chemically in the hope of further improving selectivity. In the oxidative dehydrogenation of butene, it has been found that simple iron oxide, a-Fe203, a t low temperatures catalyzes a rather selective production of butadiene.' This was later substantiated by Rennard and Kehl who further showed that addition of zinc oxide to iron oxide to form ZnFeaOl further increases the selectivity under identical conditions.2 For example, the selectivities for butadiene on Fez03 were 83% at 325 "C and 43% at 375 "C, while on ZnFe204they were 89% at 325 "C and 88% at 375 "C. This enhancement in selectivity has now been established not only for the adIt is then dition of zinc oxide but also for M g W and of interest to understand how the addition of a second oxide enhances the observed selectivity, and, in the case of zinc oxide, what causes the large change in the temperature dependence of selectivity. In general, in a parallel reaction in which a reactant can form two products over a catalyst, the observed selectivity is either determined by the ratio of rate constants of the two pathways if the two products are formed on the same site or is determined by the density of active sites for each pathway and the activity of these sites if the two products are formed on different sites. In an earlier paper, we found that on iron oxide, butadiene and carbon dioxide were formed on two separate sites, and we reported a method that we developed which measures the site densities for the production of butadiene (selective oxidation) and carbon dioxide (combustion).6 Here we report the result on zinc-iron oxide. Again we found that selective oxidation and combustion take place on two separate sites. Comparison of the results here with those on iron oxide allows us to conclude that the difference in selectivity behavior mentioned above for the two materials is primarily due to a combination of the difference in site densities and a difference in the reactivity of the combustion sites. 0022-3654/80/2084-0382$01 .OO/O
The method we developed to measure the site densities is measurement of the amount of product produced under the condition in which there is only one turnover per site. To achieve this, the contact time between the reactant and the catalyst must be short, to prevent multiple reactions, yet long enough that each site is equilibrated with the reactant, and the products of the reaction must be quantitatively determined. In practice, a finite contact time is required for equilibration. Correction for multiple reactions is made by extrapolating the data to zero contact time. Multiple reactions are minimized by allowing the reactant and the catalyst to be in contact a t low temperature. This is followed by thermal desorption for quantitative determination of the product. If indeed the amounts of products we measured in the above manner correspond to the quantities of the sites, these amounts should not vary with the pressure of the reactants over the catalyst, with the rate of purging of the catalyst with helium after equilibration, or with the rate of thermal desorption. This then consists of a list of control experiments. Furthermore, the concept that different products are produced from different active sites implicitly assumes that there is no interconversion of the properties of sites. The validity of this assumption has to be verified.
Experimental Section Materials. Zinc ferrite was prepared by the freeze-dry techniques6 A dilute solution of zinc nitrate (Spex, Hi Pure Chemical) and iron nitrate (ROC/RIC, ultra pure) of the correct metal stoichiometry was first neutralized with ammonium hydroxide to the point just before precipitation. The solution was then sprayed into liquid nitrogen, and the resulting solid was dried by vacuum sublimation at about -50 "C. The dried powder was decomposed by heating in vacuo at 350 O C , and later converted to oxide by heating in air at 525 O C for 7 h. Debye-Sherrer X-ray diffraction of the oxide showed only spinel zinc ferrite. The BET area as determined by nitrogen adsorption was 7.8 m2/g. Apparatus and Procedure. The apparatus was the same as the one used in an earlier s t ~ d y .Briefly, ~ it consisted of a conventional flow system with a glass U-tube reactor of 30 mL. The inlet and outlet of the reactor were connected to a switching valve which served to either direct the flow of inert carrier (helium) through the reactor or 0 1980 American Chemical Society
Oxidatiwe Dehydrogenation of Butene
bypass it. Pin injection valve upstream from the reactor was used to introduce the reactant cis-2-butene or other materials for experiment or for calibration of gas chromatograph. A Chromosorb trap downstream from the reactor, when cooled to liquid nitrogen temperature, was used to collect the hydrocarbons and COz purged out of the reactor. This trapped material was then flashed into the carrier gas for gas chromatographic analysis with a column packed with 20% dimethylsulfolane coated on 80-100 mesh Chromosorb w(aw). Normally, about 60 mg of zinc ferrite was used which formed a layer of less than 0.2 cm thick in the reactor. The small amount was used to minimize the complication due to readsorption of the desorbed material. However, to obtain temperatwe-programmed desorption profiles, about 270 mg was used to enhance signal-to-noise ratio. The standard pretreatment of the catalyst consisted of heating it in a strearn of oxygen (Linde, extra dry, no further purification) of' 40 mL/min a t 400 "C for 0.5 h. Then the catalyst was cooled in oxygen to 300 "C for 10 min when the flow was switched from oxygen to helium (Linde, high purity, purified with a molecular sieve trap in liquid nitrogen,). After 0.5 h at 300 "C, the catalyst was cooled to room temperature in He and then used for further experiments. The He flow was always maintained a t 40 mL/min. In most experiments, a pulse of known amount of cis2-butene was introduced into the helium carrier via the injection valve. After the entire pulse was carried into the reactor, the reactor was isolated from the rest of the system, and the pulse of butene was trapped for equilibration with the catalyst for a set period (referred to as trapping time). During equilibration and subsequent purging, the catalyst was maintained at 22 f 1.5 "C. Then the He carrier was flowed through the reactor again for a set period (referred to as purging time) to remove the unadsorbed or weakly adsorbed molecules. These purged out molecules were Collected in the Chromosorb trap for quantitative analysis. Thermal desorption then followed. The desorbed molecules were also collected in the Chromosorb trap for quantitative analysis. Unless specified, the heating rate was 15 "C/min and the catalyst was heated to 400 "C. If temperature-programmed desorption profiles were desired, the molecules still adsorbed on the catalyst after purging were desorbed by heating the catalyst a t 15 "C/ min from room temperature to 400 "C. The desorbed molecules were detected directly downstream by a thermal conductivity cell. A lag time of about 1.5 min between the catalyst bed tmd the detector has been corrected for in the profiles reported later. The amount of air that could have passed over the catalyst was 0.8 X IO" molecules/h with He purging and 1.5 X 1016molecules/h in the trapping mode.6 There was negligible reaction of butene in a reactor without catalyst. In experiments in which the adsorbate was butadiene, the procedure was identical with that which used butene as adsorbate except that butadiene was introduced for equilibration. In the pulse reaction experiments, the catalyst was first heated in oxygen and helium in a standard manner. After the heating in helium, the temperature of the catalyst was adjusted to the reaction temperature. A pulse of cis-2butene of 2.65 X l0ls molecules was then passed over the catalyst. The products formed were collected in the Chromosorb trap downstream from the reactor for analysis. The data ireported here have all been obtained from experiments repeated at least twice and often more on the
The Journal of Physical Chemistry, Voi. 84, No. 4, 1980 383
(x 1 0 ' ~ ) TOTAL C,H6
o
2.5
L L ' ' ' . '
0 10 20
40
60
80
Purging time,
100
120
140
160
18(
min
Flgure 1. Product distribution in cis-2-butene adsorption and desorption as a function of purging time: trapping time 60 min; about 0.06 g of ZnFe20,; (0)butadiene purged out; (X) CO, thermally desorbed; (e) butadiene thermally desorbed; (0)total butadiene produced equals the sum of that purged out and that thermally desorbed.
same sample of catalyst. Except for a few experiments, most have also been repeated with different samples from the same preparation. The relatively large scattering of some of the data as can be seen in the figures and the tables was due to the small quantities of these products.
Results Results of the experiments are presented in three sections: those from adsorption and desorption of cis-2butene, those from adsorption and desorption of butadiene, and those from pulse reaction studies. cis-2-Butene. The standard condition for cis-2-butene adsorption was to introduce 5.0 X 10l8 molecules for equilibration and to purge the reactor with helium at 40 mL/min after equilibration. Desorption was accomplished by heating a t 15 OC/min. The effect of deviation from these conditions on the products formed will be presented later. During equilibration, isomerization of cis-2-butene to trans-2-butene and l-butene occurred readily. Oxidative dehydrogenation to butadiene also occurred and some butadiene desorbed into the gas phase. These products can be purged out at room temperature for analysis. S3ome butene isomers, most of the butadiene, and carbon dioxide (or their precursors) adsorb strongly on the oxide and can only be thermally desorbed. As will be substantiated later by carbon balance, the three isomers of butene, butadiene, and carbon dioxide accounted for over 95% of the products in the reaction. For a given trapping time of 60 min, the quantities of some products varied with different purging times. It was found that the amount of l-butene purged out at room temperature was constant at about 2.3 X IO1' molecules/m2 of catalyst, independent of purging time. The amount of trans-%butene increased slowly from about 9 x lo1' to 12 X molecules/m2 when the purging time increased from 10 to 180 min. The amount of butadiene purged out also increased with increasing purging time as shown in Figure 1. The amounts of products thermally desorbed as a function of purging time are shown in Figures 1 and 2. Except for COz, the amounts decreased with increasing purging time as expected. Similar to the explanation given earlier,5 the increase in C02 was probably due to the ox-
The Journal of Physical Chemistty, Vol. 84, No. 4, 1980
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Kung et al.
TABLE I: Carbon Balance in cis-2-Butene Adsorption and Desorption o n Z n F e 2 0 4 a
DESORBED 0
trans- 2- CH ,8
0
cis-2-C,H8
amount ( X introduced
i-C,He
2.12
i
2.0-
2.65 I 1
3.18 i: 3.71 i 4.24 ri
0
IO 20
60
40
100
120
Purging time,
min
80
140
160
thermally desorbedb
1.29 i 0 0 2 1.18 f 0.02 0.05 2.04 i 0.04 1 . 6 8 i 0.04 1.69 i 0.04 0.06 2.27 z 0.04 2.08 i 0.04 0.07 2 . 7 0 ? 0.05 0.08 3.33 I- 0.06 0.04
0.89 i 0.03 0.85 t 0.03 0.85 i 0.03 0.92i 0.03 0.95 i 0.03 0.90 i 0.03 0.91 I 0.03 0.89 i 0.03 0.89 ? 0.03
total detectedC 2.18 2.03 2.89 2.60 2.64 3.17 2.99 3.59 4.22
i f f
i i _t
i i k
0.05 0.05 0.07 0.07 0.07 0.07 0.07 0.08 0.09
a Exgerimental conditions: 0.1472 g of ZnFe,O,, 60min trapping, 60-min purging, standard flow rate and desorption rate. These values are number of C, molecules or equivalent. ' Sum of those purged o u t and those thermally desorbed.
180
Figure 2. Product distribution in cis-2-butene adsorption and desorption as a function of purging time: trapping time 60 min; about 0.06 g of ZnFe,O,; (X) 1-butene thermally desorbed; (0)cis-2-butene thermally desorbed; (0)trans-2-butene thermally desorbed. I
purged outb
l o L 8molecules)
1
TABLE 11: Effect of Pulse Size of cis-2-Butene on the Production of CQ, and C,H, on ZnFe,O,a _____
pulse size (X 1 O l 8 molecules) 5.03 10.1
products ( X 10" molecules/rn') _ l l l
total butadieneb
CO,
2. 8c 2.7 2.9 3.0
l.gc 2.3 1.7 1.7
a Experimental conditions: about 0.060 g of ZnFe,O,, 90-min trapping, 15-min purging, standard flow rate and desorption rate. Sum of butadiene purged o u t and thermally desorbed. From Figure 3.
C,H,
$1 01
1 0
DESORBED
--
"
0
I
,
50
io0
150
I80
TRAPPING TIME (min.) Figure 3. Product distribution in cis-2-butene adsorption and desorption as a function of trapping time: purging time 15 min; about 0.06 g of ZnFe,O,; ( 0 )butadiene purged out; (X) CO, thermally desorbed; ( 0 ) butadiene thermally desorbed; (0)total butadiene produced equals the sum of that purged out and that thermally desorbed.
ygen in the carrier stream that passed over the catalyst during purging. It should be noted that by using about 60 mg of catalyst, an oxygen content of less than 1 ppm is enough to account for the increase in COz. This explanation is substantiated by the experiment in which the amount of COzdesorbed after 60-min trapping and 20-min purging was found to increase from 1.8 X 1017to 2.6 X molecules/m2 if three pulses of oxygen were deliberately passed over the catalyst during purging. The product butadiene, however, did not seem to be affected by the oxygen pulses. The products formed were also investigated as a function of trapping time. To avoid possible complication due to oxygen leak with long purging, the data were obtained with a 15-min purging. Figure 3 shows the data for the purged-out butadiene and thermally desorbed COz and butadiene. It is of interest to note that both the amount of desorbed C02 and the total amount of butadiene pro-
duced, which is the sum of that purged out and desorbed, were independent of trapping time. The amount of isomers that were purged out increased linearly with increasing trapping time at a rate of 2.6 X 1OI5 mdecules/(m2 min of trapping) for 1-butene, and 17.6 X loi5 molecules/(m2 min) for trans-2-butene. Since trapping time is equivalent to reaction time, the Increase is expected. On the other hand, the amounts of isomers that were thermally desorbed were constant independent of trapping time. The amounts desorbed were 0.26 X 1017,1.8 X lo1', and 2.3 X 1017molecules/m2 of 1-butene, cis-2-butene, arid trans-2-butene, respectively. Experiments whose conditions were different from the standard ones were performed to check the carbon balance. The results are shown in Table I. To improve accuracy, a larger quantity of catalyst and a smaller quantity of cis-2-butene than standard were used. The total carbon detected which is the sum of those purged out and those thermally desorbed appeared to agree with the total carbon introduced. This suggests that most of the products in our experiments were detected and that there was little carbonaceous residue left on the catalyst after each experiment. The latter was substantiated by the results of the experiments in which we attempted to burn off any carbonaceous residues on the catalyst after an adsorption and desorption cycle by passing oxygen pulses over the catalyst at 400 "C.The carbon dioxide thus produced was less than 3% of the total carbon adsorbed on the catalyst before desorption. The total amount of butadiene and carbon dioxide produced were also investigated as a function of the pulse size of cis-2-butene introduced for equilibration, the flow rate of carrier over the catalyst during purging, and the heating rate in thermal desorption. Table II shows the results when the pulse size of butene was twice the
Oxidative Dehydrogenation of Butene
The Journal of Physical Chemistry, Vol. 84, No. 4, 7980
385
TABLE 111: Effect of Flow Rate during Purging o n the Production of CO, and C,H, o n ZnFe,O,' (X
products 10" molecules/m*)
He flow rate, mL/min
trapping time, min
total butadieneb
40 40 40 60 60 40 40 97 97
90 90 90 90 90 60 60 60 60
2.7 2.6 2.7 2.8 2.3 3.2 3.0 2.8 3.0
CO, 2.6 2.3 1.7 1.6 1.9 2.0
1.8 1.7 1.6
//
a Experimental conditions: about 0.06 g of ZnFe,O,, 15-min purging, other conditions standard. Sum of butadiene purged o u t and thermally desorbed.
standard size. Since the total amount of butadiene and of carbon dioxide produced did not vary with trapping time, the pulse size effect was investigated at one trapping time. Within experimental error the quantities of these products were unchanged. On the other hand, the quantities of l-butene and trans-2-butene isomers that were purged out increased by about 50% on doubling the pulse size. The amount of isomers thermally desorbed increased by about 10%. Results on the effect of carrier flow rate during purging are shown in Table 111. Within experimental error, the production of butadiene and C02 was unaffected by the flow rate in the range investigated. Varying the flow rate did not have any effect on the isomer production either. The effect of heating rate was investigated in experiments by using standard conditions and 60-min purging, 60-min trapping. The quantities of butadiene and C02 produced did not appear to vary systematically for heating rates of 2.4,15, and 30 "C/min. There was no systematic change in the quantities of isomers produced either. When the adsorbed cis-2-butene was desorbed in temperature-programmed desorption, the resulting desorption profile (Figure 4b) can be well separated into two regions. Product analysis showed that the low temperature region consisted of only C4 hydrocarbons, with the isomers of butene desorbing a t lower temperature (
