Ind. Eng. Chem. Res. 1988,27, 2023-2030
2023
Selective Hydrogenation of 2-Ethylhexenal. 1. Analysis of Sorption Kinetics for 2-Ethylhexenal and 2-Ethylhexanal on Working Ni/Si02, NiS/Si02, and Pd/Si02 Catalysts Gudmund Smedler Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 Goteborg, Sweden
-
--
The consecutive hydrogenation reactions 2-ethylhexenal - 2-ethylhexanal 2-ethylhexanol or A B C were studied in the gas phase in the presence of Ni/Si02, NiS/Si02, and Pd/SiOP catalysts. Transient effects on activity and selectivity were examined in a packed bed flow reactor. The activity as well as the selectivity of the different catalysts was found to change dramatically during a n induction period that lasted up to 50 h. After this period, the NiS/Si02 and Pd/Si02 catalysts were extremely selective for 2-ethylhexanal formation. Kinetics of aldehyde adsorption and desorption were quantitatively evaluated for the preconditioned catalysts in the presence of hydrogen. For comparison, adsorption measurements were performed in the packed bed reactor in nitrogen flow. Both kinds of sorption measurements indicated clearly that the desorption rates of aldehydes are too low t o fulfill the equilibrium requirement of the Langmuir-Hinshelwood kinetic theory, at the prevailing conditions (2' = 378-423 K, PH2 = 300-4000 Pa, Pc, = 150-400 Pa). The separate hydrogenation of monounsaturates like olefinic and carbonyl compounds is among the most extensively studied catalytc processes, and the vast body of reported studies have, in both cases, been summarized in many review articles (e.g., Webb (1978), Tanaka (1986)). Concerning the hydrogenation of conjugated polyunsaturated systems, the case of butadiene (Oliver et al., 1973; Phillipson et al., 1969) is probably the most widely studied. Some of the knowledge extracted from the publications mentioned above may also be relevant for the hydrogenation of a,@-unsaturatedcarbonyl compounds, a class of reactions which has so far received considerably less interest in the scientific literature (Augustine, 1976). The reaction network for the hydrogenation of a,@-unsaturated aldehydes is schematically pictured by the following scheme: ( 6)
'(
9R-CW2-CH-C-H
R - C H A ~ H ~
R-CH=C-CM (A)
H
(C)
%R-CH2-CH-CH2
k
dH
A dH ( D)
In addition to the main hydrogenation reactions, decomposition to lighter hydrocarbons (LHC) is also frequently observed (Boeseken and van Senden, 1913; Suen and Fan, 1942; Hemidy and Gault, 1965; Young and Sheppard, 1967, 1971; Blyholder and Shihabi, 1977). Implied already by the reaction scheme above, challenging selectivity problems arise for the experimentalist. The present study deals with the hydrogenation of 2-ethylhexenal, a process for which quite a few papers on kinetic studies have been published. It is, however, of considerable industrial importance, e.g., for the production of PVC softeners like dioctylphthalate from 2-ethylhexanol and for the manufacture of octanoic acid, through oxidation of 2-ethylhexanal. In this paper, the term selectivity is used for the ratio between the rate of formation of 2-ethylhexanal (aldehyde) and the corresponding rate of 2-ethylhexanol (alcohol) formation. Previous papers have reported that Pd (Macho and Polievka, 1969; Rylander, 1979; Smedler, 1987) and P2 Ni boride (precipitated by the addition of sodium borohydride to an alcoholic solution of nickel acetate, c.f. Collins et al.
(1983)) catalysts very selectively catalyze the hydrogenation of the C=C double bond of 2-ethylhexenal. In this paper, the interest is focused on thermodynamic and kinetic aspects of aldehyde adsorption and their possible relation to the high selectivity of silica-supported NiS and Pd catalysts. It will also be demonstrated that this remarkable selectivity (in comparison to nickel) is a steady-state property developed after many hours of operation. Since the freshly reduced catalyst surfaces exhibited very different activity and selectivity properties than did the quasi-stationary surfaces, it was concluded that relevant information for the working catalyst could only be obtained if the surface is pretreated by hydrogenation until a metastable carbonaceous adlayer is formed. The idea of measuring adsorption kinetics in the presence of hydrogen on catalyst surfaces covered with strongly chemisorbed carbonaceous residues is certainly not easily realized. Several complications such as hydrogen-induced structural changes of surface carbon and the importance of hydrogenation versus desorption rates must be thoroughly considered in the design of the experiments. Nevertheless, there is no longer any dispute concerning the existence of a carbonaceous overlayer on working hydrogenation catalysts, and it is also well-known that the chemical nature of this adlayer is dependent on the presence of hydrogen as well as temperature (Cormack et al., 1966; Somorjai, 1977). In an earlier paper from our group (Niklasson and Smedler, 19871, it was shown that hydrocarbon adsorption experiments carried out in nitrogen atmosphere on a Ni/Si02 catalyst prereduced at 443 K left significant amounts of irreversibly bound surface residues that could only be removed by hydrogen addition, or other more severe treatments like oxidation. The experimental approach to the problem of obtaining relevant data for the kinetic modeling of sorption processes will be further developed in this paper. Some fundamental questions concerning the principal possibility of distinguishing between reactive and nonreactive adspecies will also briefly be discussed. Experimental Section This section covers the catalyst preparation and pretreatment, the experimental systems used, and a descrip-
0888-588518812627-2023$01.50/0 0 1988 American Chemical Society
2024 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table I. Catalyst Characterization Properties
H, uptake,* Ni/SiOz NiS/Si02g Pd/SiO,
metal content,O wt % 14.0 12.6 0.16
mol H/kg 2.24 X 10-I 1.20 x 10-3 6.45 x 10-3
metal dispersion: % 9.4 5.6 X lo-, 42.9
BET area! m2/g 104.0 98.9 108.5
mean pore particle porosity: % diameter! 8, 51 145.5 56
195
ODetermined by AAS (Perkin-Elmer Model 370). bStandard Hz chemisorption (Chemisorb 2800). c D = mole of H adsorbed/mole of metal. N2 absorption (Acusorb). e Determined by Nz adsorption (micro- and mesopores) and by mercury penetration (macropores). fDetermined by N2 adsorption, 2% of all pores exceeded 1500 8,. gThe sulfur content in the NiS/SiOz catalyst was 5.5% by weight.
tion of the considerations underlying the choice of experimental procedures. A, Catalyst Preparation. Pd/Si02 Catalyst. (i) The impregnation solution was prepared by dissolution of palladium nitrate in aqua regia (3 parts of concentrated HCl mixed with 1 part of concentrated HN03), followed by dilution with deionized water, yielding a palladium concentration of 42.4 kmol/m3 and pH 0.5. An amount of 100 g of porous spherical (5-mm) S O 2pellets (Girdler T-1571) was impregnated by 600 cm3 of the Pd solution for 2 h at 328 K. (ii) The catalyst was dried at 348 K in an air flow (5.0 cm3/s) for 8 h. (iii) Calcination a t 573 K in an air flow (5.0 cm3/s) for 1 2 h. (iv) Treatment at 473 K for 7 h by a mixed flow of air, nitrogen, and hydrogen: pH2= 1.67 X lo4 Pa, PO, = 3.33 x lo3 Pa, p N 1 = 8.0 x IO4 Pa, and qtot = 6.0 cm3/s. (v) Reduction a t 473 K for 12 h in a mixture of N2 and H2: p H , = 2.5 X lo4 Pa, p N p = 7.5 X lo4 Pa, and qtot = 5.0 cm3/s. Ni/Si02 Catalyst. Two hundred grams of the silica support already mentioned was subjected to the following procedure: (i) impregnation at 300 K for 22 h in an aqueous circulating solution of Ni(N03)2 (2.0 X lo3 mol/m3); (ii) drying a t 348 K in a flow of N2 (6.0 cm3/s) for 24 h; (iii) calcination a t 573 K in an air flow (5.0 cm3/s) for 6.5 h; (iv) reduction at 623 K in a mixed flow of H2 and N2 ( p ~=, 2.5 X 104 Pa, p?, = 7.5 X lo4Pa, qtot = 5.0 cm3/s) for 17 h. Procedures i-iv were carried out 3 times. NiS/Si02 Catalyst. Thirty grams of the Ni/SiO, catalyst was sulfided by the following treatment: (i) reduction of surface oxides at 573 K in hydrogen flow (10.0 cm3/s) for 14 h; (ii) equilibrium sulfidation by a mixed flow of H2 and H2S (pH, = 9.95 X lo4 Pa, p ~ f=i 50 Pa, qtot = 10.5 cm3/s) at 573 K for 16 h; (iii) treatment in nitrogen flow (6.0 cm3/s) at 750 K for 1.5 h. All partial pressures and flow rates are related to inlet and STP conditions, respectively. The common characterization properties (BET area, H2 uptake, computed metal surface area, and total metal content) are summarized in Table I. B. Experimental Equipment. Adsorption and desorption experiments were carried out according to two different methods. The most reliable was a gravimetric method, using a step response technique in a low-pressure (-lo3 Pa) microbalance (Cahn 2000) flow system, c.f. Figure 1. The output signal from the microbalance unit and the pressure inside the microbalance chamber were continuosly monitored. A t selected time intervals, observed values were recorded by an A/D converter card and stored by the microcomputer (see Figure 1). The second method for sorption measurement was based on the time-dependent change of gas-phase composition, as a stepwise change of the inlet composition was performed in a packed bed flow reactor. The reactor outlet was periodically analyzed for the aldehyde concentration by means of a Perkin-Elmer 3920 gas chromatograph supplied with a 2-m OV225 column and an FI detector.
Gasmixing
unit
u
TCF
onloff valve
@
flow c o n t r o l needle Valve
@
streom
e3
vacuum
TCE
temperature
GC
g a s chromotogroph
temperature &SA&%
contiolled
healed
furnoces
S~C:!OP
seiection valve expansion v a l v e controlled
evaporators
Figure 1. Continuous flow microbalance reactor system.
These experiments were carried out in N2 flow. Prior to the adsorption measurements, the same reactor was used for catalyst conditioning to a stationary activity level. Moreover, it was used for hydrogenation experiments, which will be examined in detail in part 2 of this work (Smedler, 1988). In the hydrogenation runs, the outlet was analyzed even for hydrogen by means of a hot wire detector (HWD) in connection with a zeolite 5A column. The sampling procedure was automatic and was controlled by a relay card that was governed by the real-time microcomputer. The reactor dimensions were L, = 600 mm, d, = 40 mm, and d, = 5 mm. The reactor was packed in three sections, a glass bead section (-350 mm) in the lower part of the tube, the catalyst bed, and an upper part with glass beads (-200 mm). A thermocouple in the catalyst section, which was employed for temperature measurement and control, was used to ensure that the bed was isothermal. The flow pattern as well as the delay time was evaluated by means of the step method, using helium as a tracer, and it was shown that the dispersion was low enough for plug flow characteristics (Pea = 50 or thereabouts). C. Catalyst Pretreatment Prior to Adsorption/ Desorption Measurements. As will be shown in a later section, the freshly reduced catalysts appeared to undergo dramatic changes with respect to activity and selectivity during an induction period which, due to the high ratio between catalyst mass and aldehyde feed rate, lasted for up to 50 h of continuous hydrogenation. An experimental series aiming at the establishment of sorption kinetics for the working catalyst must therefore be carried out on a conditioned surface, where the irreversible deposition of carbon has already occurred. Since the amount as well as the chemical nature of different “strongly chemisorbed surface species” varies over wide ranges, there is no obvious or sharp borderline which distinguishes between reversible and irreversible adspecies. It could thus always be argued that every arbitrary choice of experimental standardization or coke base level definition is more or less erroneous. Though such objections are certainly reasonable, it is nevertheless clear that sorption measurements on clean
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2025 Table 11. Transient Changes of Activity and Selectivity NiISiO,' ,
time, h 2 5 10 20 35 50
YA
YB
0.02 0.10 0.10 0.15 0.15
.
I
NiS/SiO,* . -
Yc
YLHC
YA
YB
0.20 0.30 0.50 0.50 0.55
1 1.78 0.60 0.40 0.35 0.30
0.10 0.20 0.25 0.30 0.40 0.40
0.70 0.65 0.50 0.30 0.35 0.45
Pd/SiOQC , -
Yc
YLHC 0.20 0.10 0.10 0.10 0.10 0.10
0.05 0.15 0.30 0.15 0.05
YA
YB
Yc
YLHC~
0.10 0.10 0.10 0.10 0.10
0.05 0.10 0.40 0.65 0.75 0.75
0.45 0.35 0.20 0.10
0.50 0.45 0.30 0.15 0.15 0.15
Process conditions: poA= 290 Pa, poH2 = lo00 Pa, T = 422 K, q = 1.02 cm3/s (STP), and wmt = 10.0 g. *Process conditions: p " =~ 250 and wMt = 20.0 g. cProces~conditions: p o A = 190 Pa, p o B = 195 Pa, p o ~=218700 Pa, Pa, poHz= 980 Pa, T = 424 K, Q = 6.51 cms/s (STP), T = 421 K, Q = 4.89 cms/s (STP), and wat = 27.7 g. dyj = fraction of component j in the reactor outlet.
and freshly reduced surfaces are a worse imitation of the real working process. Another complication is that the adsorption occurring in the presence of hydrogen is accompanied by hydrogenation. It is, however, possible to minimize the ratio between hydrogenation rate and sorption rate experimentally, most conveniently by carrying out the experiments at relatively low hydrogen pressures (see Results and Discussion section). Other more traditional considerations like the mixing conditions, possible influence of mass-transfer limitations, and flow restrictions for the microbalance system were discussed in a previous paper (Niklasson and Smedler, 1987). Even for this present study, it was proven by standard methods that the use of step response analysis was justified and that diffusional limitations were neglible. The results of repeated RTD experiments, performed according to the step method using helium as a tracer, are shown in Figure 2, together with the fastest response observed for the adsorption experiments. It is obvious from this figure that macromixing is extremely fast in comparison to adsorption. Before starting a series of sorption experiments in the microbalance apparatus, the catalyst to be studied was conditioned to steady-state activity in the packed bed reactor. The conditions for these hydrogenation experiments were 0 Pa < PA < 300 Pa, 0 Pa < p B < 400 Pa, 500 Pa < p H z < 3800 Pa, 0 Pa < pc < 250 Pa. The reactor temperature was varied at the levels 378,393,408, and 423 K. The total flow rate was varied between 1and 7 cm3/s (STP)and the amount of catalyst was 10.0,20.0, and 27.7 g for the Ni/SiOz, NiS/SiOz, and Pd/Si02 catalysts, respectively. A detailed evaluation of the kinetics will be presented in part 2 of this paper (Smedler, 1988). After this long-term hydrogenation treatment, ca. 20 pellets were picked a t random and mounted on a string connected to the sample beam of the microbalance. As counterweight, the corresponding mass of unimpregnated silica was employed. The catalyst was reduced by purified hydrogen in situ (pH2 = 2000 Pa, T = 573 K) and reconditioned by hydrogenation ( p A = 20 Pa, p H z = 40 Pa, T = 423 K) until the recorded change of weight attained a stationary level. Thereafter, the aldehyde flow was shut off, and the loss of weight due to desorption and hydrogenation was observed to asymptotically approach a level above the original base line, indicating reformation of irreversible deposits. The adsorption-desorption sequence just described was then repeated until the amount adsorbed a t equilibrium was found to desorb reversibly, normally after two to three cycles. The zero level at this point is defined as the base level of irreversible adsorption. An isothermal series was, after this treatment, performed in the following sequence: (i) 20 min of hydrogen treatment a t 423 K and p H 2 = 40 Pa; (ii) adsorption and desorption at 423 K, p~~= 40 Pa, and PA = 20 Pa; (iii) cooling to the experimental temperature, T; (iv) adsorption and
0.d'
I
0
50 +
100
150
Tracer response
D Fastest adsorption
*
200
\ 250
L
300 Time/s
response
Figure 2. Macromixing characteristics of the continuous flow microbalance system. The helium sensor is placed in the outlet of the microbalance chamber. The tracer and the fastest adsorption response are both normalized.
desorption at T, p H 2 = 40 Pa, and PA = 20 Pa; (v) repeat of iv; (vi) adsorption and desorption at T, p H , = 40 Pa, and p B = 20 Pa; (vii) adsorption and desorption a t T, p H ? = 20 Pa, and PA = 10 Pa; (viii) adsorption and desorption at T, pH2= 20 Pa, and p B = 10 Pa; (ix) rapid cooling to room temperature. The desorption experiments were carried out at the same hydrogen partial pressures as above and with the aldehyde pressure equal to zero. The total pressure was about lo3 Pa (excess Nz as diluent), and the temperature was varied at the levels 378,393,408, and 423 K. Points iv-viii were used for the kinetic evaluation. About 1500 data points were collected for each aldehyde on each catalyst. I t should be emphasized that the numerical values of partial pressures given are approximate set values, while the real values were determined by GC analysis (FID for aldehydes, HWD for hydrogen) and measurement of the total pressure (Pirani, Balzer PKG 020). The temperature control was better than fl K. Employing the procedure above turned out to yield reproducible measurements, which was verified through repeat experiments. A typical series of adsorption-desorption experiments is shown in Figure 3. Results and Discussion Induction Phenomena in the Packed Bed Reactor. The transient behavior during the induction period for the three catalysts was recorded through periodic sampling and GC analysis of the outlet stream from the plug flow reactor. From these experiments, the most striking observations were as follows: (1)The activity and selectivity properties were gradually changing during a very long time (about 50 h). The outlet compositions a t different times are given in Table 11. (2) The main initial products formed on all catalysts were light hydrocarbons, probably produced through
2026 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
(2) The hydrogen uptake, as determined on the clean, outbaked, and prereduced catalyst, is a very questionable measure of the number of sites that are active in the hydrogenation reactor. Both conclusions combined indicate clearly that the carbonaceous overlayer is interfering with the metal in a most decisive manner. Regardless of the hidden nature of this interaction, these results confirm the conceptions concerning the catalytic importance of surface carbon, discussed by numerous authors, e.g., Somorjai (1977). An obvious and directly observable effect is, however, that the overlayer provides desorption sites for the aldehydes, which are practically irreversibly adsorbed on the freshly reduced catalysts.
nQds(mmol/kg1
5r .*+ *** ”*+
L.
r‘
.++
I
31
i
.**
Timels
Figure 3. Experimental microbalance responses during adsorption-desorption experiments (2-ethylhexanal on NiS/Si02). (1)PHz = 16.3 Pa, PB = 6.1 Pa, T = 378 K; (2) P H =~ 17.2 Pa, P B = 6.4 Pa, T = 393 K; (3) P H =~ 17.5 Pa, pB = 6.3 Pa, T = 407 K; (4) PH*= 18.9 Pa, PB = 6.6 Pa, T = 423 K. Table 111. Selected Initial Rate Data for Activity Comparison between the Catalysts
model I1
rl,
T,K
P H ~ ,Pa
423 378 423 378
1800 1800 900 900
300 300 300 300
wol/(s kg cat) Ni/SiOz 47.0 16.6 33.6 16.4
423 378 423 378
1800 1800 900 900
300 300 300 300
NiS/Si02 6.6 3.1 4.2 2.5
423 378 423 378
1800 1800 900 900
300 300 300 300
Pd/SiOz 4.8 1.5 3.3 0.8
PA,
Pa
rl, rmol/(s kg metal)
Sorption Kinetics in the Presence of Hydrogen The microbalance measurements of adsorption and desorption kinetics were analyzed by three different Langmuirian change-of-weight rate models: model I d8j/dt = kipj(l - 8;) - k ~ 8 , ( j = A or B) (1)
ul? s-l
d8j/dt = kGpj (1 - 8j) - kdj8 - k18jp~, model I11
336 119 240 117 52.4 24.6 33.3 19.8 2982 932 2050 524
2.1 x 7.4 x 1.5 X 7.3 x
104 10-5 lo4 10-6
5.5 x 10-3 2.6 X 3.5 x 10-3 1.4 X 7.4 x 2.3 x 5.1 x 1.3 X
10-4 10-4 10-4 lo4
a The turnover frequency for reaction 1 is based on the assumption that the density of active sites equals the hydrogen uptake.
cracking and reverse oxo reactions (Boeseken and van Senden, 1913; Suen and Fan, 1942; Young and Sheppard, 1967, 1971). (3) The Pd and the NiS catalysts did readily catalyze the hydrogenation of B, contrary to their later stationary performance. The hydrogenation data taken after the catalyst activity had stabilized were used to obtain empirical correlations for the rates of reaction in terms of temperature and reactant partial pressures. The sole purpose of that correlation was to provide a rough tool for prediction of the rates at the lower pressures in the microbalance experiments. Representative steady-state rate data for the three catalysts are given in Table 111. The low relevance of the hydrogen uptake as a universal measure of the number of active sites is indeed apparent in the results obtained on the NiS catalyst, for which the high turnover frequency is a direct consequence of the low hydrogen uptake (cf. Table I). In summary, two important conclusions could be drawn from the hydrogenation experiments: (1)The explanation of the extreme selectivity exhibited by the Pd and the NiS catalysts is not to be found by examination of the catalyst sites in their juvenile, freshly reduced state.
The first model presupposes that all hydrogen effects are unimportant for the measured rates of sorption (but certainly not for the site properties). The three terms of models I1 and I11 account for the rate of adsorption, desorption, and hydrogenation, respectively. While model I11 is derived from the assumption that hydrogen is rapidly equilibrating on the surface (equal rates of hydrogen adsorption and desorption), model I1 is based on the additional assumption that the hydrogen fractional coverage is very low within the pressure and temperature range studied. According to a recent study on hydrogen sorption kinetics by Niklasson and Andersson (19871, it seems very reasonable to assume a very low fractional coverage with respect to hydrogen on silica-supported metal catalysts. When p H D 5 40 Pa, the coverage would be less than 0.1% on the Ni/Si02 catalyst used in this work. The parameters to be estimated by nonlinear regression analysis, when the three models are fitted to the six experimental series (one for A and one for B on each one of the three catalysts), are as follows: for model I, koaj,E,+ ko,, E,, and w,~; for model 11, as in model I plus kol and El; for model 111, as in model I1 plus KoH and Al-€H. The superscript (0)refers to the value of the rate constant at the reference temperature (400 K) and w,, is the total adsorption capacity of the respective catalyst (in millimoles/ kilogram). Model I relies on the postulate that the rate of hydrogenation is much slower than the rate of desorption. A comparison of the results (sum of squares and possible trends in the residuals) obtained by model I and those obtained by either model I1 or I11 could reveal information concerning the importance of hydrogenation. For model 111, it was found for all three catalysts that the value of KoH was so low that the term ( K H ~ H , ) ”was ~ neglible in the denominator. This made an accurate de-
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2027 Table IV. Regression Parameters of Sorption Kinetics" system koa, P a - l d E,, kJ/mol A on Ni/Si02 4.1 x 10-5 f1.6 X 10" B on Ni/SiOz 2.7 x 10-5 f6.8 X lo-' A on NiS/SiOz 4.1 x 10-5 fl.1 x 10" B on NiS/SiOz 4.1 x 10-5 f5.4 x 10-7 A on Pd/SiOz 8.5 X f4 x 10" €3 on Pd/Si02 3.8 x 10-5 f l x 10"
ko,+ s-l 1.54 x 10-3 f2.6 x 10-5 1.25 x 10-3 f7.0 x 10-5 9.1 x 10-4 *i.3 x 10-5 2.38 x 10-3 f3.4 x 10-5 7.5 x 10-4 *3 x 10-5 3.4 x 10-3 f2.1 x 104
Ed, kJ/mol
'The activation energy of adsorption was in no case different from zero. The superscript a t 400 K. R is the fraction of the total variation that is explained by the model.
termination of KoH and AHHimpossible and justified the assumption underlying model 11; the fractional hydrogen coverage is always low in the region of experimental observations (cf. Niklasson and Andersson (1987)). The judgement of the significance of hydrogenation influence on the measured rates of sorption could therefore only be performed by comparison of models I and 11. When comparing different models concerning goodness of fit, an F test (cf. Draper and Smith (1966))could be used for hypothesis testing. The observed F value will be the mean s u m of squares of model I divided by the mean sum of squares of model 11. It was concluded that the hypothesis "model I is not less probable than model 11" could in no case be discarded. The determination of the kinetic parameters of model I was performed by a jackknife procedure that has been described in detail elsewhere (Smedler, 1987). Table IV shows the parameter values, along with approximate 95% confidence limits and the percentage of the total variation that are explained by the model. Plotting the-residuals (0, - 8. ) against temperature, pH2, p,, and 0, did, in generat:thow a satisfactory randomness of the residuals. The only exception was the adsorption of A on the Pd/ SiOz catalyst, where the residual showed a minor correlation with pH2. With that small reservation, it was concluded that the single-site Langmuirian adsorption model (I) could be accepted from a statistical point of view. Before the final acceptance of the models, the parameters should be tested according to their physical likelihood. As is evident from Table IV, the adsorption was found to be nonactivated in all cases; i.e., the estimated energy of activation for adsorption was not significantly different from zero. This result is not surprising and is in agreement with earlier results (Niklasson and Smedler, 1987). Consequently, the enthalpies of adsorption would equal the negatives of the corresponding activation energies of desorption. The entropies of adsorption could thus easily be calculated for the standard state in the gas phase a t 1atm; i.e., if the equilibrium constants are expressed as
(e),
The values of A H and A S are given in Table V. It could be verified that they all fulfill the inequality for thermodynamic constraints suggested by Boudart et al. (1967): 10 < -AS < (12.2 - 0.0014AH) (cal/(mol K)) (5)
A comparison between the obtained adsorption parameters suggests a partial explanation for the high selectivity of the Pd catalyst: The adsorption rate constant ratio
49.2 fl.O 50.2 f2.8 47.6 *0.8
56.8 f0.7 60.9 f2.8 77.4 f3.8 (0)refers
W-,
mmol/kg 30.9 50.33 30.9 f0.33 21.8 f0.21 21.8 f0.21 6.5 f1.6 6.5 f1.6
R 0.978 0.958 0.975 0.972 0.960 0.972
to the value of respective rate constant
Table V. Thermodynamics of Adsorption
m ~ , ASA, m ~ , ASB, KA/KB catalyst kJ/mol J/(mol K) kJ/mol J/(mol K) (400 K) Ni/SiOz -49.2 -56.7 -50.2 -61.7 1.2 NiS/SiOz -47.6 -50.1 -56.8 -80.6 2.6 Pd/SiOz -60.9 -86.9 -77.4 -134.1 10.1 ( I z ~ / I z is ~ )2.2 and the equilibrium constant ratio (KAIKB) is 10.1 at the reference temperature. The weak adsorption of saturated carbonyl compounds on palladium catalysts has been suggested previously (Sungbom and Tanaka, 1982). These authors arrived at this conclusion indirectly by examination of liquid-phase hydrogenation kinetics. The conclusion expressed in this present paper, resulting from adsorption measurements, is believed to be the first direct experimental evidence for this frequently suggested effect. An interesting result is that the sorption kinetics on the two nickel-based catalysts seem to be surprisingly similar, in view of the very large selectivity difference. Even though a comparison of KA/KB (1.1on Ni and 2.67 on NiS) indicates a higher selectivity for the sulfided catalyst, the difference seems too small to provide a satisfactory explanation for the observed selectivity of the NiS catalyst. Since the NiS/Si02 catalyst has been proven to dissociate hydrogen at a rate that is very much higher than the rate of 2-ethylhexenal hydrogenation (Smedler, 1987), the source of the extreme selectivity of this catalyst is most likely not found by examination of adsorption kinetics, neither for hydrogen nor for the aldehydes. Instead, the surface reaction is probably subject to kinetic hindrance, caused either by the structure of adsorbed 2-ethylhexanal or by some kind of geometric site requirement that is not fulfilled on the sulfided surface. These possibilities will be investigated in a forthcoming paper by means of infrared studies of adsorbed structures on the Ni and the NiS catalysts (Jobson and Smedler, 1987). A general experimental implication of the results discussed above is that measurements of the overall sorption kinetics reflect the changes on all reversible adsorption sites. Since some of these sites, or some of these adsorption states, may be insignificant for the hydrogenation reactions, it is not certain that the measured amount of adsorbed species is equal to (or even proportional to) the fraction of all adsorbed species that are adsorbed in a reactive state. It is, principally, not possible to distinguish between reactive and nonreactive adspecies by measuring the rate of change of surface coverage. The measured adsorption capacity and the measured rate of adsorption define, instead, upper limits for these properties on the reactive sites. In spite of this problem, the fact that the proposed models explain the observations well and that the param-
2028 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 200
Table VI. Adsorbed Amounts in Nitrogen Atmosphere (Packed Bed Flow Reactor)"
150
T, K
p ' ~ Pa ,
378.8 394.4 407.8 424.2 380 393.7 407.8 423.2
239 269 276 228 178 166 165 157
379.5 393.9 407.4 423.4 379.2 393.5 407.3 423.6 379.4 393.9 398.6 408 424
254 255 249 245 0 0 0 115 109 107 104 94
0 287 296 283 270 125 113 109 106 94
379.2 394.7 407.5 424.4 379.5 400.9 411.9 423.9 380.0 394.4 398.4 407.5 422.4
263 253 224 225 0 0 0 0 137 144 139 138 140
0 0 0 0 430 467 450 459 166 166 158 153 157
100
50
0
5000
15000
25000
~.daA,
P'B, Pa
0 0
Figure 4. Simultaneous coadsorption of 2-ethylhexenal (0) and 2-ethylhexanal (+) at 378 and 423 K in N2 flow on the NiS/SiOz catalyst. The flow rate was 1.07 cm3/s (STP) and 2.08 cm3/s (STP) at 378 and 423 K, respectively.
eter values are physically reasonable does imply that the real adsorption process is accurately described by a uniform Langmuirian surface model. The models should, however, be tested for consistency with hydrogenation kinetics, as will be done in the second paper of this work (Smedler, 1988).
Sorption Kinetics in N2at Higher Pressure Another class of methods for the investigation of adsorption properties (mainly adsorption equilibrium constants) is the analysis of the effluent response to a pulse or a step in the inlet of a catalyst bed. Since these methods require that the experiments be carried out in an inert atmosphere, they will reflect the sorption on the catalyst surface when it is short of hydrogen. This will positively lead to irreversible adsorption and a resulting catalyst surface state that differs from the conditions during hydrogenation (Niklasson and Smedler, 1987). Moreover, there is no straightforward way to separate adsorption on the active sites from that on the support. The wide use of such methods and the possibility of obtaining information about competition effects when the two aldehydes adsorb simultaneously does, however, motivate a comparison between such a method and the microbalance experiments. After the catalysts had been conditioned to a stationary activity level, three different series were performed with A, B, or a mixture of A and B fed to the inlet of the packed bed reactor. The aldehyde inlet pressures (Poj) ranged from 100 to 450 Pa, and the catalyst temperature was, again, varied a t the levels 378,393,408, and 423 K. The amount adsorbed of each component could easily be evaluated from the relation
A significant competition effect, the preferential adsorption of A in comparison to B, is clearly seen in Figure 4. From this figure it can also be noted that the outlet partial pressure of B does in fact go through a maximum that is higher than poB This is due to the chromatographic effect of any plug flow reactor, which is basically caused by the different adsorption behavior of different components. In addition, the rather slow desorption is clearly shown by Figure 4. As is seen in Table VI, the adsorbed amount of B is significantly lower when it is adsorbed simultaneously with A. When the two aldehydes were adsorbing separately, the responses were strikingly similar for A and B. It seems
n.ds,B*
mmol/kg
fA
fB
10.5 5.8 6.2 4.9
0.86 0.88 0.82 0.87 0.65 0.54 0.59 0.64
0.68 0.73 0.57 0.59
18.8 14.8 11.5 5.4 21.9 16.3 10.8 7.1
0 0 161 152 160 176
35000 Ti mels
mmol/kg Pd/SiO, , -
Ni/SiOz
0
0
37.2 32.7 22.5 20.2
0 0
28.5 17.8 17.3 14.7 11.8
0.77 0.75 0.94 0.88 42.0 34.1 27.1 17.9 15.9 8.3 7.9 6.9 5.1
0.40 0.51 0.50 0.53 0.44
0.56 0.87 0.77 0.99 0.48 0.70 0.69 0.66 0.68
NiS/Si02 29.6 22.0 14.4 11.3
25.4 19.1 16.3 13.3 11.5
0.89 0.91 0.99 1.0 47.2 28.9 20.7 24.5 17.0 10.9 9.7 7.3 6.9
0.54 0.61 0.65 0.66 0.71
0.54 0.70 0.71 0.61 0.57 0.67 0.66 0.72 0.62
f A and f B are the fractions of the amounts of adsorbed A and B that were found to desorb within 4 h, without Hzadmission.
therefore reasonable to assume that they are competing for the same kind of active sites. Table IV does moreover show that the adsorption was, in many cases, to a significant extent irreversible. Among the more advanced evaluation methods for experiments of this kind, the Kubin-Kucera method is often applied (Lidefelt, 1983; Magnusson, 1983). As an alternative, the complete time-dependent material balance equations for the solid and the fluid phases could be solved without restrictions. For this purpose, the packed reactor was modeled as a finite series of CSTRs, one for each cross section of catalyst pellets (HlavaCek and Votruba, 1977). This yields the following system of nonlinear differential equations for an abitrary CSTR k and component j : gas phase
pjk(t = 0 ) is known pjo = poj
(I' = A or B)
catalyst phase O;k(t = 0) is known
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2029 surface reaction is thus, not necessarily, the rate-controlling step.
0
c;." 0
'
. . . boo0
,
1
8000
'
12'000
16000
'
20000 Time i s
Figure 5. Observed (symbols) and simulated (lines) responses for adsorption in N l , 2-ethylhexenal (circles), and 2-ethylhexanal (crosses). Open and closed symbols refer to coadsorption at 423 and 378 K, respectively.
If the bed is modeled by N CSTRs, this problem is solved by the simultaneous numerical integration of 2 X 2 X N coupled initial value problems (two components (A and B), two equations (gas and solid phases), and N CSTRs). Due to the fact that the CPU-time consumption for one set of sorption parameters is quite high (machine and parameter dependent) and that the experiments were carried out a t somewhat unrealistic (hydrogen-free) conditions, a complete regression analysis of the transient responses does not seem worthwhile. Instead, a number of trial simulations was performed for the experimental results from the NiS series. The parameters obtained from the microbalance experiments did not match the adsorption data from the packed bed reactor well. A set of favorable parameter values were instead found to be the following: kod = 8 X lo4 Pa-' s-', kodA = 2.5 X s-', k 0 d = 8 x lo4 Pa-' S-', kodB = 5 x S-l, Edq = E d B = 50 kJ/mol, and N,/VF = 70 mol/m3 of gas. Simulating the effluent responses by means of these parameters shows reasonably good agreement with the observed responses, as is seen in Figure 5. The fits a t the two intermediate temperatures were also satisfactory. It could thus be noted that these experiments are best described if the adsorption rate constants are lowered by a factor of 5, the desorption rate constants are increased by a factor of 2.5, and the site density is increased by a factor of about 2, in comparison to the optimal parameter values obtained from the regression analysis of the microbalance data. Adsorption on the support is one possible explanation for the increased site density and desorption rate constant. The heats of adsorption do, however, remain almost unchanged. Regardless of the experimental drawbacks of this second method and the exact reasons for the differences with respect to parameter values, it has hereby been demonstrated by a completely independent method that the orders of magnitude of the adsorption and desorption rate constants are Pa-' s-l and s-', respectively. Combining this information with the more reliable parameter values from the microbalance experiments provides strong support for the most important conclusion of this paper; adsorption and desorption of these relatively large molecules (2-ethylhexenal and 2-ethylhexanal) are, contrary to what is frequently assumed in kinetic model fitting procedures, not instantaneous but rather slow processes on working supported metal catalysts. It should be stressed, however, that the adsorption rate will grow considerably at industrial aldehyde pressures (- lo5 Pa), but the desorption rate will still be bound by the desorps-') as an upper limit. The tion rate constant
Concluding Remarks It has been verified by two independent experimental techniques that the adsorption properties of a conditioned catalyst surface, with substantial amounts of carbonaceous deposits, is very different from the case of a clean surface (cf. Webb (1978)). It has also been demonstrated that the activity and selectivity properties of the freshly reduced surfaces differ strongly from those seen when the catalyst properties have stabilized, due to saturation of irreversible carbon deposition. The obvious experimental implication is that activity and selectivity criteria, based on arguments concerning kinetics and thermodynamics of elementary steps, demand that these steps be studied a t conditions as close to the working conditions as possible. Quantitative evaluation of sorption kinetics in the presence of hydrogen permitted the proposal of physically and statistically acceptable adsorption models for all three catalysts under investigation. The model obtained for the Pd/Si02 catalyst provides a partial explanation for the very high selectivity exhibited by this catalyst. Further study is needed for an explanation of the selectivity of the NiS/Si02 catalyst. The sorption kinetics that were obtained in this present study suggest that a nonreactive adsorbate structure rather than a low fractional coverage of B will unravel the reason for the remarkable selectivity of NiS/Si02. Nomenclature d, = diameter of catalyst pellet, m d, = diameter of tube reactor, m El = energy of activation for surface reaction 1,kJ/mol EG = adsorption energy of activation for componentj , kJ/mol E* = desorption energy of activation for componentj , kJ/mol AH = enthalpy change of adsorption, kJ/mol j = component index, j = N2, H2, A, B, C, or LHC k = CSTR index, 12 = 1 to N Kj = adsorption equilibrium constant for component j , Pa-' ka, = adsorption rate constant for component j , Pa-' s-' k0u = adsorption rate constant at the reference temperature (400K) k , = desorption rate constant for component j , s-' koa= desprotion rate constant at the reference temperature kl = rate constant for surface reaction 1, Pa-' 9-l or s-' kol = rate constant at the reference temperature L, = length of tube reactor, m N = total number of CSTRs N s / VF = concentration of adsorption sites, mol sites/m3 of gas n&i = amount adsorbed of componentj , mmol/kg of catalyst Pea = Peclet number for axial dispersion p , = partial pressure of component j , Pa poi = inlet partial pressure of component j p,k = partial pressure of component j in CSTR k qj = volumetric flow rate of component j , m3/s qtot = total volumetric flow rate R = equation of state constant, J/(mol K) rl = rate of reaction 1, mol/(s kg of catalyst) or mol/(s kg of metal) A S = entropy change of adsorption, J/(mol K) T = absolute temperature, K t = time, s td = delay time in the packed reactor W,, = adsorption site density, mmol/kg of catalyst Wcat = charged catalyst mass, kg yi = mole fraction of component j
Ind. Eng. Chem. Res. 1988,27, 2030-2039
2030
Greek Symbols f, = fractional surface coverage of component j 0, = calculated fractional surface coverage, according to a regression model o ] k = fractional surface coverage of component j in CSTR k &k = fraction of unoccupied sites in CSTR k v, = frequency factor of adsorption vd = frequency factor of desorption 7 = mean residence time in the catalyst bed Registry No. Ni, 7440-02-0; NiS, 16812-54-7;P d , 7440-05-3; 2-ethylhexenal, 645-62-5; 2-ethylhexanol, 123-05-7.
Literature Cited Augustine, R. L. “The Stereochemistry of Hydrogenation of a,@Unsaturated Ketones”. Adv. Catal. 1976, 25, 1. Blyholder, G.; Shihabi, D. “Infrared Spectral Observation of the Interaction of Acetone with Silica-SuportedNi and Con. J . Catal. 1977, 46,91. Boeseken, J.; van Senden, G. H. “Zerstorung des Heptylalkols bei 220° in Ggw. von fein verteiltem Nickel”. Rec. Trau. Chim. PU~S-BU 1913, S 32, 23. Boudart, M.; Mears, D. E.; Vannice, M. A. “Kinetics of Heterogeneous Catalytic Reactions”. Znd. Chim. Belg. 1967, 32, 281. Collins, D. J.; Grimes, D. E.; Davis, B. H. “Kinetics of the Catalytic Hydrogenation of 2-Ethyl-2-Hexenal”. Can. J. Chem. Eng. 1983, 61, 36. Cormack, D.; Thomson, S. J.; Webb, G. “Radiochemical Studies of Chemisorption and Catalysis, Part VI”. J . Catal. 1966, 5, 224. Draper, N. R.; Smith, H. In Applied Regression Analysis; Wiley; New York, 1966. Hemidy, J. F.; Gault, F. G. “RBactions de Contact du butanal sur Chim. Fr. 1965, 1710. Film de Palladium”. Bull. SOC. HlavaCek, V.; Votruba, J. In Chemical Reactor Theory, a Reuiew; Lapidus, L., Amundson, N. R., Eds.; Prentice-Hall: Englewood Cliffs, NJ, 1977; Chapter 6. Jobson, E.; Smedler, G. “Infrared Investigation of 2-Ethylhexenal and 2-Ethylhexanal Adsorbed on Working Ni/SiOz and NiS/SiOz Catalysts”. Submitted for publication in J . Catal. 1987. Lidefelt, J.-0.“Adsorption Equilibrium Constants of Methyl Oleate and Methyl Linoleate in Vapor Phase on Supported Copper and 1983, 60(3), 1. Nickel Catalysts”. J . Am. Oil Chem. SOC. Macho, V.; Polievka, M. “Selektivna Hydrogenlcia 2-Ethyl-2-Hexenalu na Palidiu v Parnef Fkze”. Chem. Prum. 1969, 19(5),215.
Magnusson, J. “Rate Factors in Vapor Phase Hydrogenation of Methyl Esters of Fatty Acids”. Doctoral Thesis, Chalmers University of Technology, Goteburg, Sweden, 1983. Niklasson, C.; Andersson, B. “The Adsorption and Reaction of H2 and Dz on a Ni/SiOz Catalyst”. Submitted for publication in J . Catal. 1987. Niklasson, C.; Smedler, G. “Kinetics of Adsorption and Reaction for the Consecutive Hydrogenation of 2-Ethylhexenal on a Ni/Si02 Catalyst”. Ind. Eng. Chem. Res. 1987, 26, 403. Oliver, R. G.; Wells, P. B.; Grant, J. In Proceedings of the 5th International Congress on Catalysis; Hightower, J. D., Ed.; NorthHolland: Amsterdam, 1973; Vol. 1, p 659. Phillipson, J. J.; Wells, P. B.; Wilson, G. R. “The Hydrogenation of Alkadienes. Part 111”. J. Chem. Soc. A 1969, 1179. Rylander, P. N. Catalytic Hydrogenation in Organic Synthesis; Academic: New York, 1979; p 74. Smedler, G. “Kinetic Analysis of the Liquid Phase Hydrogenation of 2-Ethylhexenal in the Presence of Supported Ni, Pd and NiS Catalysts”. Can. J . Chem. Eng. 1987, in press. Smedler, G. “Selective Hydrogenation of 2-Ethylhexenal. 2. Analysis of Transient and Stationary Hydrogenation Kinetics on Working Ni/SiOz, NiS/Si02, and Pd/SiOp Catalysts“. Ind. Eng. Chem. Res. 1988, following paper in this issue. Somorjai, G. A. ‘Active Sites in Heterogeneous Catalysis”. Adu. Catal. 1977, 26, 2-68. Suen, T.-J.; Fan, S. “Catalytic Hydrogenation of Haptaldehyde in 1942, 64, 1460. Vapor Phase”. J . Am. Chem. SOC. Sungbom, C.; Tanaka, K. “Concentration Dependence of Ketone Hydrogenation Catalyzed by Ru, Pd and Pt. Evidence for Weak Ketone Adsorption of Pd Surface”. Bull. Chem. Soc. Jpn. 1982, 55, 2275. Tanaka, K. “Studies in Surface Science and Catalysis”. In Catalytic Hydrogenation;Cerveny, L., Ed.; Elsevier: Amsterdam, 1986;Vol. 27. Webb, G. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1978; Vol. 20. Young, R. P.; Sheppard, N. “Infrared Spectroscopic Studies of Adsorption and Catalysis: Acetone and Acetaldehyde on Silica and Silica-Supported Nickel”. J . Catal. 1967, 7, 223. Young, R. P.; Sheppard, N. “Infrared Spectroscopic Studies of Adsorption and Catalysis V. Acetaldehyde on Silica-Supported Nickel”. J . Catal. 1971, 20, 340. Received for reuiew November 17, 1987 Accepted June 13, 1988
Selective Hydrogenation of 2-Ethylhexenal. 2. Analysis of Transient and Stationary Hydrogenation Kinetics on Working Ni/Si02, NiS/Si02, and Pd/Si02 Catalysts Gudmund Smedler Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 Gateborg, Sweden
The kinetics of the reaction network in Scheme I have been examined in the gas phase at atmospheric pressure, in a packed bed flow reactor, using inlet composition, contact time, and temperature (378-423 K) as independent variables. The kinetics were studied in the presence of three different catalysts: Ni/Si02, NiS/Si02, and Pd/Si02. Previously reported data on adsorption kinetics were employed for the development of dynamic rate models, where no single step was postulated to be rate limiting. These models explained the measured rate data well, and it could be concluded that the slow rate of aldehyde desorption had an important influence ( t h o u g h not rate controlling) on the overall hydrogenation rate. This effect was observed on all three catalysts and was most pronounced at low temperature. In addition, it was found that the aldehydes were much more strongly adsorbed on the active sites than was hydrogen ( K A> 10KH,). The modeling of the kinetics of different heterogeneous hydrogenation reactions has been discussed b y numerous authors i n the fields of applied catalysis and reaction engineering. A very challenging problem is the synthesis of the growing knowledge concerning surface phenomena and structures of surface i n t e r m e d i a t e s (e.g. Webb (1978),
Grant et al. (1976)) w i t h the established experience in applied statistics and regression analysis (e.g., Froment and Hosten (1981)). The complexity of hydrocarbon processes in the presence of supported metal catalysts includes the interaction between a large number of different phenomena. A complete
0888-5885/88/2627-2030$01.50/0 0 1988 American Chemical Society
