Ind. Eng. Chem. Res. 1988,27, 2030-2039
2030
Greek Symbols f, = fractional surface coverage of component j 0, = calculated fractional surface coverage, according t o 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 Vapor Phase”. J . Am. Chem. SOC. 1942, 64, 1460. 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 i n 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
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2031 Scheme I 2-ethylhexenal
A
r1
2-ethylhexanal
'2
B
Gas Chromatograph N2 and H2
2-ethylhexanol
C
light hydrocarbons
LHC
mathematical model that (i) describes all observed rate effects well and (ii) accounts for all underlying physical phenomena demands the introduction of a very large number of kinetic parameters (rate constants for different changes on the surface with corresponding energies of activation). The accurate determination of all these parameters is very rarely (if ever) possible, due to practical limitations on the independent experimental variables as well as on the experimental accuracy. Many early investigations have therefore focused either on detailed studies of isolated phenomena or on simplified evaluations of the overall rate of reaction data. The lack of agreement between parameters (e.g., adsorption constants) obtained by those two approaches could hardly be overcome unless the separate parameter determinations are carried out on surfaces and at conditions that resemble the ones prevailing in the rate experiments as much as possible. Deuteriation (Newham and Burwell, 1964; Phillipson et al., 1969; Patterson and Burwell, 1971; Magnusson, 19871, 14C tracer (Thomson and Wishlade, 1962; Al-Ammar and Webb, 1978; Berndt et al., 1983),and infrared (Young and Sheppard, 1971; Blyholder and Shihabi, 1977; Jobson and Smedler, 1988) techniques have proven to be powerful tools for the acquisition of qualitative data at relevant hydrogenation conditions. In the previous paper in this issue on adsorption kinetics (Smedler, 1988), an application of a microbalance technique for measurement of sorption kinetics was reported. Those measurements were performed on surfaces pretreated by long-term hydrogenation and appeared to be well described by simple Langmuir adsorption models. In this second part of the investigation, the sorption kinetics obtained in the microbalance will be used for the modeling of hydrogenation kinetics obtained in an integral packed bed reactor, where no single step is assumed to be rate controlling. The different organic components present in the reaction system, 2-ethylhexenal, 2-ethylhexanal, 2-ethylhexanol, and light hydrocarbons, will, as in part 1, be denoted by A, B, C, and LHC, respectively. Experimental Section A. Catalysts. The catalyst preparation and pretreatment was described in detail in the previous paper in this issue (Smedler, 1988). The experiments and results presented in this paper all refer to the properties exhibited by the catalysts after they had been conditioned to steady-state activity. The common characterization properties for the three catalysts in question are given in Table I of the preceding paper in this issue. B. Packed Bed Reactor. The reactor was the same stainless steel tube (d, = 4 cm, L, = 60 cm, d, = 5 mm) as was described in part 1. Repeated measurements of the residence time distribution, using helium as a tracer, showed that the dispersion in the reactor was low enough to fulfill the requirements for plug flow. Blank tests showed, moreover, that the adsorption capacity and the hydrogenation activity of the glass beads and the reactor wall were negligible at these low temperatures (96%). Since, in addition, the partial pressure difference over the bed with respect to hydrogen was kept below 500 Pa, it could be concluded that the volumetric contraction caused by hydrogen consumption was less than 0.5%. The flow rate could thus be considered constant in the bed. If so, the material balance for an infinitesimal fluidphase volume element could be written as
with T = V,/q (see Nomenclature section). For a packed reactor with plug flow characteristics, a good approximation of the true mixing conditions is achieved by considering each cross section of catalyst pellets as a CSTR (HlavaCek and Votruba, 1977). The entire reactor could thus be considered as Lb/d, = N CSTR elements coupled in a series, with the residence time in each CSTR being T / N .This simplifies eq 1to a system of N coupled initial value problems. For an arbitrary cross section (or CSTR) It, the following expression is obtained: (Yjk-1
- Yjk)N RT NE - --(rads, 7 vF
dyjk
jk
- rdes, j k ) = dt (2)
The assymptotic solution of the system (eq 2) yields the steady-state conditions
B. Kinetic Model. Surface Model. The systems of eq 2 and 3 could be solved numerically for any arbitrary model for the sorption and reaction kinetics. The adsorptive properties of silica-supported metal catalysts have been extensively studied by many different techniques. Convincing experimental evidence for the existence of many different adsorption sites (TPD studies of hydrogen by Konvalinka et al. (1981)) as well as for fundamentally different states of adsorption for olefins (W-tracer studies by Al-Ammar and Webb (1978),Berndt et al. (1983), and Webb (1978)),and aldehydes (IR studies by Young and Sheppard (1971) and by Blyholder and Shihabi (1977)) has been published through the years. A rigorous kinetic theory for the system studied in this present work seems, in light of the reports above, to yield extremely complicated rate models. At this point, it must be stressed, however, that all kinds of adsorption sites are not relevant for the catalytic action and that all surface intermediates are not reactive. As an example of these discrepancies, the very significant differences between adsorption properties obtained on clean (Somorjai, 19771, prereduced (Niklasson and Smedler, 1987), and conditioned (Smedler, 1988) catalysts may be mentioned. The success of simple Langmuirian one-site models obtained in part 1 of this work justifies the initial assumption that one site (and one state of adsorption) is of major importance. Lumped Model for LHC Formation. The unintentional formation of low boiling point compounds (LHC) was accounted for by assuming their formation to occur in the following parallel sequences
/xH*s
B*s
- xH*a
Pes
LHC(g)
This scheme results from the assumption that partially hydrogenated surface intermediates (P-s) that rapidly decompose into low molecular weight species are formed from adsorbed A, B, and hydrogen. According to literature reports, the lighter products that are formed through aldehyde decomposition are mainly heptene, heptane, carbon monoxide, methanol, methane, and water (Boeseken and van Senden, 1913; Suen and Fan, 1942; Tsuji et al., 1965; Hemidy and Gault, 1965). Further, it was observed (cf. part 1)that these products were not formed unless hydrogen was present. The independently observed material balance for hydrogen indicated an average value of two hydrogen atoms ( x = 2) per unit of LHC formed. This suggests that the formation of methane and methanol (six and four H atoms per carbon monoxide) is not of dominating importance. Instead, the decomposition of 2-ethylhexenal to carbon monoxide and (eventually) heptane (three H atoms) and of 2-ethylhexanal to carbon monoxide and heptane (one H atom) is probably the major route. For all components smaller than C8, a one-component lump (LHC) was applied. They were found to desorb considerably faster than the aldehydes. For this reason, and in absence of reliable quantitative data, the products (C and LHC) were assumed to desorb very fast, Le., always having low fractional coverage. Model for Hydrogen Adsorption. It is a generally accepted view that the dissociation of an H2 molecule requires the participation of two adjacent active centers (whatever their nature may be). The number of equivalent centers required for the adsorption of organic compounds is subject to considerably more debate, even for very simple olefins (Webb, 1978) and aldehydes (Tanaka, 1986). The di-a-bonded adsorbed state would result in the derivation of the dual-site rate model, where a "site" for H2 dissociation would be the same as for aldehyde chemisorption. If, on the other hand, only one-half H2 center is required for aldehyde chemisorption,which is frequently suggested for the case of a-or 7r-allylic adspecies, a completely different rate model will be derived. The nonstationary material balances for hydrogen on the surface are given below: dual-site model (Lee and Butt, 1977; Chaudhari et al., 1986) k,~&&
- k d ~ +~ CViHzri 8 ~ ~= d e ~ , / d t
where 8~~ = [ H * ~ ] ( V F / N ~ ) OA = [A.SI(VF/N,)
OB = [B*~I(VF/N,) s = s1 + s1 (two adjacent centers) 8~ = 1 - 8A - 8~
- 8H2
two-site hydrogen model (cf. Magnusson (1987))
(4)
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2033
+ Cvijri= dOj/dt
kepjOF - kaOj
j = A, B
7
(6)
It should be emphasized that these models both presuppose hydrogen dissociation, even though the dual-site model is mathematicallysimilar to a nondissociative model. Both models are simplified in as much that they do not account for the possible blocking of sites neighboring the occupied ones. Combining the equations for the surface with the previously derived (eq 2) fluid-phase balances yields the complete system of equations, here given for the dual-site model: bjk-1
- Yjk)N
RT N S (kajPYjk6Fk
vF
7
= Yjk(0)
- kdj6jk
k&jk6,k
6jk
t
Read initial guessesof parameters to be astimated
1-1 t o n
+
Read conditions for data point I
dyjk =dt
f
(t I O )
yjo = yoj Yjk
- kdj6jk)
Read known and invariant propertias
(t = 0)
+ C V i j r i k = d6jk/dt
= 6jk(o)
(t = 0)
(7)
This system of coupled initial value problems was solved by a Gear routine NAG routine W2EBF), specially designed for stiff problems. For the steady-state data, the evaluation is much less computer time consuming. Combination of the stationary fluid phase balances with the corresponding surface equations yields the following system of nonlinear algebraic equations:
Next point
/-oN-*
Parameter upd;
El
Figure 2. Algorithm for estimation of kinetic parameters in rate models without any rate-controlling step.
= Yoj
Yjo
By making use of the equation of continuity, these equations could be sequentially solved from CSTR to CSTR, given the inlet composition (NAG routine C05PBF). Solving these equations (eq 8) for the different experimental conditions yields the (calculated) outlet composition. A nonlinear regression parameter estimation routine was used to obtain the model parameters that minimized the sums of squared differences between observed and calculated outlet partial pressures. The algorithm is seen in Figure 2. These parameters were, thereafter, used for simulation of the dynamic behavior of the reaction systems under investigation (according to eq 7). Results and Discussion Derivation of Kinetic Models. As was reported in part 1 of this work, the Pd/Si02 and the NiS/SiOz catalysts were extremely selective with respect to the formation of 2-ethylhexanal (B), while Ni/Si02 was active even for the hydrogenation of B to 2-ethylhexanol (C). There is thus no possibility to study that second hydrogenation step for the cases of Pd/SiOz and NiS/SiOz. The reaction networks to be investigated were therefore formulated like A(g)
B(g)
‘aAj[ ‘dA
‘aBjFdB
A*s
- B*s
‘1
LHC(g)
‘2
C(g)
with r2 omitted for the cases of NiS/SiOz and Pd/SiOz. For the sake of simplicity, the hydrogen interaction with the surface and the adsorbed aldehydes is omitted in the schemes above. Postulating that the surface reactions are irreversible elementary steps and that the products (C and LHC) are rapidly desorbing after their formation allows the entire system (cf. eq 7 and 8) to be described by the following rate parameters: kaH,kaA,k,, km2,k a , km, k l , k2, k3, kq, and Ns/ VF. Since ah elementary steps should have an energy of activation E 1 0, the description of the simplified kinetics on a uniform (single-site) surface demands the introduction of 21 parameters. This may explain why the assumption of a rate-controlling step is so frequent in kinetic modeling procedures. In this special case, however, separate information concerning the sorption kinetics and the density of adsorption sites is available. In addition, the temperature dependence of some rate constants may be so low that their corresponding energy of activation is practically zero in the experimental temperature range. Complementary information was obtained from the transient responses of step changes of the inlet composition and from extra runs where the exchange rate between H2and D2was measured simultaneously with the rate of hydrogenation. Table I shows some representative results from the H2/D2 exchange rate measurements. Comparison of the various calculated turnover frequencies (Table I) makes it clearly questionable whether the hydrogen uptake is an adequate measure of the density of active sites. The determination of the set of parameters necessary to give a good description of the steady-state rate data was performed by a stagewise regression procedure. A t each stage, the activation energy of a rate constant was introduced or dropped, facilitating a judgement of the statistical
2034 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table I. Comparison of Rates and Turnover Frequencies for 2-Ethylhexenal Hydrogenation ( r and Hydrogen Dissociation (raD),Expressed as the Rate of HD Formation ~HD,
rl3
catalvst Ni/SiOz
Fmol/(s ke) 2037
pmol/(s k-d 22.1
NiS/SiOp Ni/Si02
242 3559
3.7 250
Pd/SiOz
799
36.7
~ u n s-.' ~ . . I .
9.1 x
u1.a
s-1
9.9 x IO-'
uun.b s-1 . . I .
2.6 x
conditions 51 Pa, PB = 121 Pa, P Hi ~PD, + P H ~
u, -b s-1
2.8
X
lo4
T = 424 K, PA
= 24400 P a 2.0 x 10-1 3.1 x 10-3 8.5 x 10-3 1.3 x 10-4 1.6 X lo-' 1.1 x 4.5 X lo-' 3.1 X T = 432 K, PA = 2235 Pa, PB = 406 Pa, p~~ + p~~ + p~~ = 25 600 Pa 1.24 x 10-1 5.7 x 10-3 2.0 x 10-1 9.1 x 10-3
*Turnover frequency is based on the hydrogen uptake (cf. Table I of the preceding paper in this issue). Turnover frequency is based on mol/kg (Ni/Si02),2.82 X lo-' mol/kg (NiS/Si02), and 4.02 X low3mol/kg (Pd/SiO,). the regressed site densities: 7.89 X
Table 11. Regression Parameters of Hydrogenation Kinetics" Ni NiS Pd N,/Vp 127.5 f 28 45.5 f 7 6.5 f 2.6 6 X lo-' f 2 X 6 X lo+ f 2 X koaH2 2 X 10'' f 7. x IO" -_ 4.1 X 10'' f 1 X IO" 8.5 x 10-5 rl: 4 x 10" 4.1 X 10'' f 2 x 10" 2.7 x 10-5 f 4.1 X f6X 3.8 x 10-5 f I x IO+ 7 x 10-7 9.5 x 10-3 f 2 x 10-3 2.4 X f 4.8 X lo-' f 6 X 1 x 10-2 1.5 x 10-3 h 9.1 x 104 1 x 10-5 7.5 x 10-4i 3 x 10-5 3 x 10-5 2.4 x 10-3 i 3 x 10-5 3.4 x 10-3 i 2 x 104 1.3 x 10-3 f 7 x 10-6 73.4 f 8.3 9.6 f 2.1 32.4 f 16.8 49.2 h 1.0 60.9 f 2.8 47.6 f 0.8 56.8 f 0.7 50.2 rl: 2.8 77.4 f 3.8 1.7 x 10-3 f 2 x 10-4 5.1 x 10-3 rl: 5 x 104 1.6 X lo-' f 4 x 10-3 31.7 f 9.3 4.5 x 10-4 f 9 x 10-5 36.6 f 6.0 1.1 x 104 h 2 x 10-5 4.5 x 10-4i 1.5 x 10-5 2.6 x 10-3 1.2 x 10-3 81.9 i 23.7 62.3 f 11.5 31.6 f 12.6 1.1 x 104f 2 x 10-5 4.5 x 104 1.5 x 10-5 31.6 f 12.6 81.9 f 23.7 0.989 0.990 0.995
*
*
"The superscript (0)corresponds to the value of respective rate constant at the reference temperature (400 K). R = (SST,~- SSh,)/ SSTo,(percentage explained by the model). N./ V, in mol/m3 gas, kw in Pa'ls-l, k, in s-l, E, in kJ/mol, k, in s-l, E, in kJ/mol.
significance of the parameter in question (for details see Smedler (1987)). The aldehyde sorption parameters were fixed at the previously determined values (part 1). Table I1 summarizes the results of the parameter determination for the three catalysts, along with approximate 95% confidence limits and the percentage explained by the model (R). The three resulting rate models were examined by residual analysis. The residuals of the dependent variables (pj,obs- &), were plotted against contact time, temperature, OHz,IjA,and jjB,yielding no systematic trends. The models could thus be accepted from statistical viewpoints. Trials with the alternate hydrogen adsorption model, where one hydrogen molecule is assumed to dissociate on two aldehyde adsorption sites, resulted in significantly worse fits of the experimental data. The residual sum of squares was at best a factor of 2 higher than for the adopted dual-site model. Consequences of the Rate Models. The resulting rate models could be used to evaluate how the fractional coverages vary at different temperatures and gas-phase compositions. Such calculations could also be used to illustrate the magnitude of the errors that would result if the very frequent assumption of adsorption equilibrium between the gas phase and the catalyst surface is applied.
I / O
).
I
150&
0
I
0
/'
I
100
-
I 1
/ I
/ o I
0
I
The models were derived from nonstationary sorption kinetic measurements, performed in the microbalance system, and from steady-state rate measurements. For this reason, some additional experimental testing concerning their ability to predict dynamic effects is necessary. Three different experimentally observed effects were investigated (i) the transient outlet response of a step change in the reactor inlet; (ii) the reactor shut-down procedure described in the Experimental Section; (iii) the change of infrared spectrum described in detail by Jobson and Smedler (1988). Point iii calls for a brief comment. That work was an infrared investigation of the adsorbate structures formed when A or B was adsorbed in either N2 or H2atmosphere, using a pulse injection technique. Differential spectra (bands from backgrouund carbonaceous overlayer were subtracted) were recorded at different times after a pulse of A (or B) had been injected in the feedstream. The very same Ni/Si02 and NiS/Si02 catalysts as those employed in this present work were investigated, after they had been conditioned in a manner similar to the procedure given in part 1of this present work. It was observed by IR spectroscopy that A, when it was adsorbed in hydrogen flow (pHz= 1 atm), was rapidly converted to B on the Ni/Si02 catalyst. This effect was considerably less pronounced on the NiS/SiOz catalyst, due to its lower activity. By use of the site densities, rate constants, and energies of activation obtained in the preceding regression analysis, it is possible to simulate the time-dependent behavior of the different systems, at the conditions prevailing in the nonstationary experiments (i-iii). This is done by solving
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2035 PjI Pa
PI / Po
2501\
1
2501
'I
I , 200 -
snl
-----___
1 ;
I
//
,'
50'
I
\
/
0 0
-PA/Pa
- -PB/Pa
_ _ _ _p~/pa
Time/s
01
0.40 -
1
O.OOL
0.006
- --PB/Pa
0.008
0.010
- -PC/Pa
0.012 Z/m
PLiPa
Figure 4. Simulated shut-down procedure of the reactor. (I) Hydrogenation to steady state at 379 K (0-10000 s). (11) Bypass of evaporator, N2, and Hzin the reactor inlet (10000-11200 s). (111) Temperature rise from 379 to 422 K,0.035 K/s (11200-12400 s). (IV) Hydrogen is turned off for 2000 s (12400-14400 s). (V) Hydrogen is tumed on again, yielding a substantial increase of products in the outlet (14400-24400 s). NiS/Si02 catalyst. 0.3 0
0.002 0.
-PA I Pa
0.70
- -- - -
0.60
-
0.50
-
0.30
- - _ _ -----__
-
0
-FJH2
0.002
0.OOL
---en
0.006
0.008
--ea
0.010
0.012 2 /m
Figure 6. Steady-state profiles through the Ni/SiOz bed at q = 1.5 cm3/s (STP), poH2= 1800 Pa, p o =~ 300 Pa, and T = 378 K. (a, upper) Partial pressure profiles. (b, lower) Fractional coverage profiles. - - es
-
,- - - /
/
- A'
Timek
The qualitative agreement between observed and simulated transient responses for the different experiments (i-iii) provides satisfactory support for the derived hydrogenation models. In fact, it could be concluded that the models all fulfill the following criteria: adsorptiondesorption rates for the aldehydes and steady-state hydrogenation kinetics are accurately predicted; dynamic hydrogenation experiments are qualitatively predicted with low discrepancies between observed and simulated responses; no observed effect is contradicted by the proposed models. The points above enforce the conclusion that the models could be adopted if the underlying assumptions are valid (which is considerably harder to ascertain). Some interesting consequences arise from the models presented. The calculated plug flow profiles of steady-state partial pressures and fractional coverages, as evaluated for the Ni/Si02 catalyst at the lowest temperature (378 K), are envisaged in parts a and b of Figure 6, respectively. For that catalyst, the combination of a high reaction rate constant (kl)and slow desorption of the product B makes the fractional coverage of B close to the inlet very high, despite the fact that the partial pressure of B is close to zero. Similar patterns were obtained for the NiS/Si02 and Pd/Si02 catalysts, even though their lower rate constants (i.e., lower kl values) made the effect less striking. An interesting test is to simulate the kinetic behavior of the catalysts if the sorption rates were infinitely fast, as is very often assumed. Keeping the adsorption equilibrium and reaction rate constants at the values obtained and raising the adsorption and desorption rate constants by 2 orders of magnitude make such a simulation possible.
-----__, Timeis
- - e,
Figure 5. Simulated fractional coverages after injection of a pulse of A to the IR cell flow reactor. T = 403 K,PH = 0.85 cm3/s, and W,, = 50 mg. (a, upper) Ni/SiOz catalyst. lower) NiS/Si02 catalyst.
(b,
the system of differential equations (eq 7) describing the dynamic changes. Figures 3 and 4 show examples of the simulated outlet responses for experiments of type i and ii, respectively. In Figure 5, parts a (Ni) and b (NiS),the fractional coverages of A and B,as simulated for the conditions in the differential IR cell reactor, are given. The qualitatively observed effects are clearly predicted by the models.
2036 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
ej
PA 1 Pa
1
0.751,
100
50
0
I\ k \
0
\I
0.005 -PA/Pa
I 0,010
0.015
0.020
0.025
0.030
0.035 Zlm
Figure 7. Steady-state partial pressure profiles in the Ni/Si02, NiS/SiOz, and Pd/SiOz beds, calculated for the actually determined parameter values (curve I) and for the (hypothetic) case of fast adsorption and desorption (curve 11). Conditions: q = 1.5 cm3/s (STP), p " =~1800 ~ Pa, p o =~300 Pa, and T = 378 K.
Figure 7 shows the partial pressure profiles that would result if the sorption rates were very fast, together with the profiles for the actual cases. These figures make it very clear that the slow sorption rates significantly influence the overall kinetics, most dramatically on the Ni/Si02 catalyst. For that catalyst, the increase of the sorption rate constants by a factor of 100 is not sufficient to make the overall rate of reaction 1 completely controlled by the surface reaction. This was discovered by comparing the results when the sorption rate constants were increased by different factors (e.g., 10, 50, 100, and 200) from the measured values. It should be mentioned that the difference in temperature dependence between desorption and surface reaction rate constants (cf. Table 11)makes the relative importance of sorption kinetics less pronounced at higher temperatures. Although rate predictions for experimental conditions outside the investigated domain are quite unreliable, it may be noted that the rate models for the catalysts predict that the assumption of adsorption equilibrium (surface reaction control) becomes more reasonable at increasing temperature. Hydrogen equilibrates more rapidly with the surface. This became clear from the experimentally observed response curves, where the hydrogen outlet partial pressure was found to approach a stationary level after about 700, 200, and 400 s for the Ni, NiS, and Pd catalysts, respectively. After these time intervals, the aldehydes were still far from their steady-state levels, which was reached after typically 5000 s (depending on temperature). Independent mass spectrometric measurements (cf. Table I) confirm that the hydrogen interaction with the surface is very rapid compared to the rate of hydrogenation. These experimental observations are also predicted by the rate models. Parts a, b, and c of Figure 8 show the fractional coverages according to the regression model and the corresponding equilibrium fractional coverages for Ni, NiS, and Pd, respectively. It was concluded that hydrogen (closely) equilibrates on the part of the surface that is not occupied by A or B, while the aldehyde fractional coverages differ significantly from equilibrium. Effect of Hydrogen on the Selectivity (Ni/SiO,). It should be mentioned that our previously reported kinetic analysis of the nickel-catalyzed hydrogenation of 2ethylhexenal (Niklasson and Smedler, 1987) was performed in quite a different manner than that presented in the preceding sections of the present paper.
1.00
,,$-- - - --- -
--
-1
-
. -1
i-
0.4 0
--
I
i ------------
35
-OH2 -eA.cg
- - _ -BH2 eq
_ _ BA
0,
---Be
Zlm eq
Figure 8. Comparison between predicted fractional coverage profiles and the corresponding profiles that would result if (local) equilibrium were established between the surface and the gas phase. The calculations were performed for the conditions q = 1.5 cm3/s (STP), poH = 1800 Pa, poA = 300 Pa, and T = 378 K. (a,upper) Ni/SiOz. middle) NiS/SiOz. (c, lower) Pd/SiOz.
(k,
In that earlier study, the experimental conditions were similar to the present ones, even though the reactant partial pressures were somewhat higher. As frequently reported for consecutive hydrogenation reactions, it was observed that the rate of the second hydrogenation step (B C) more rapidly increased with increasing hydrogen pressure than did the first step (A
-
-
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2037
0
500
1000
1500
2000
2500
3000
%‘ Pa Figure 9. Hydrogen effect on the steady-state selectivity ( r l / r 2 )of the Ni/Si02 catalyst. The rates are calculated by means of the parameters obtained by regression analysis of the experimentaldata (cf. Table 11). T = 423 K,pA= pB = 150 Pa.
B). Since such kinetic behavior could never be explained by a single-site model, as long as adsorption equilibrium is established, we were led to the conclusion that the two reactions occur on different sites. Such a model was proven to predict the observed hydrogen effect on the selectivity with respect to B. In the present study, the experimental technique for measurement of sorption kinetics has been improved, due to the partial failure of the technique used in the previous work. The single-site kinetic model, including nonequilibrium adsorption behavior, evidently predicts transient as well as stationary hydrogenation data. Analyzing the stationary data for Ni/SiOz by means of power law rate equations yields apparent hydrogen orders of reaction of 0.51 and 0.79 for reaction steps 1 and 2, respectively. The corresponding apparent energies of activation were found to be 12.8 and 6.6 kJ/mol. In our earlier report (Niklasson and Smedler, 1987), very similar results were obtained (Elgpp = 13.6 kJ/mol, Ez = 10.1 kJ/mol, $Iz1 = 0.16, and 8~~ = 0.46). This means%at there is no reason to believe that different mechanisms are operating in the two different investigations. The parameters determined by regression analysis (eq 8) of the rate data for the Ni/SiOz catalyst could be used to simulate the steady-state selectivity ( r l / r z ) . Figure 9 shows the results of such calculations, performed at PA = pB = 150 P a and at T = 423 K. Obviously, the hydrogen effect is predicted. It could therefore be concluded that a marked hydrogen influence on the selectivity could well result from nonequilibrium adsorption, and there is in that case no need to complicate the picture of the surface by the introduction of multisite models. In addition, the model presented here is in good qualitative agreement with the experimental observations made by Niklasson and Smedler (1987). If the adsorption and desorption rate constants were increased by a factor of 1000, while all other parameters are kept constant, the rates of reactions 1 and 2 will not be influenced by the sorption rates. In that case (adsorption equilibrium), the selectivity ( r l / r z )at 423 K and PA = pB = 150 Pa will be 25.3 and independent of the hydrogen pressure. Concluding Discussion In part 1 of this work, two independent experimental techniques for estimation of sorption kinetics were compared, namely, differential microbalance measurements in the presence of hydrogen and absolute measurements of the change of gas-phase composition after a step change
in the inlet of a flow reactor. In the latter case, no hydrogen was present. Both experiments implied that the sorption rate constants were too small to justify the frequent assumption of adsorption equilibrium during hydrogenation. The results from the microbalance experiments were, because of their closer resemblance to hydrogenation conditions (i.e., qualitative Hz presence), used for the quantitative evaluation of the sorption kinetics. In this second part, the hydrogenation kinetics have been modeled on the basis of the previously obtained sorption kinetics, without postulating any special elementary step as rate controlling. The resulting models have been proven to give a good description of transient as well as stationary data, a fact that strongly supports their validity. For the given set of experimental observations, it is seemingly the case that the influence of the slow sorption rates must be accounted for, especially in the case of Ni/SiOa. Higher aldehyde pressures and higher temperatures would diminish the relative influence of adsorption and desorption rates, respectively, but the most important conclusion is that the influence of the sorption rates of larger organic compounds is, in principal, not to be neglected unless such an approximation is experimentally verified. A much too frequently used description of catalytic processes is the sequential subdivision: (1) adsorption of reactants, (2) surface reaction, and (3) desorption of products. Such a description could often lead to the misunderstanding that these steps are truly sequential (as for entirely physical steps like mass transfer across phase boundaries) and that one of them must be rate controllling. In fact, reactant desorption and product adsorption rates should also be included and, as has been demonstrated here, different steps contribute to the overall rate in such a complicated way that the identification of a single rate-determining step is not always possible. The reasoning above may be illustrated by the results on the Ni/SiOz catalyst at 378 K, p o H 2 = 1800 Pa, and poA= 300 Pa. In this case, the ratio between the rate of adsorption of A and the overall rate of consumption of A is as low as 1.2. This does, however, not justify the conclusion that the overall rate is, almost entirely, controlled by the adsorption rate of A. The reason for the adsorption rate of A being so low is, instead, the result of a high rate of formation of B that is slowly desorbing. The combined effect of these kinetic properties leads to a high steadystate fractional coverage of B, which in turn reduces the probability for adsorption of A. The steady-state rates of adsorption and desorption for A and B are illustrated by parts a and b of Figure 10, at 378 and 423 K, respectively. Clearly enough, it is not easy to identify a rate-determining step, and moreover, a change of reactor temperature or gas-phase composition would significantly alter the relative magnitude of the rates of different steps. From a critical viewpoint, it could certainly be argued that the choice of a single-site model of the surface and the assumption that the surface reactions are elementary steps are positively not in full agreement with the underlying physical reality. The surface reactions should, if possible, be subdivided into their respective schemes of possible elementary events, e.g., hydrogen abstraction and addition, conjugation, carbonium ion formation, etc. Among such ”truly” elementary steps, reversible as well as irreversible and parallel as well as sequential changes may exist. It should thus be emphasized that the assumption of “irreversible elementary surface reactions” that was applied in the forgoing examination of the kinetics
2038 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
enough to make the overall rate of reaction nearly independent of the hydrogen sorption rates. (3) In the case where the influence of aldehyde sorption rates is most pronounced (Ni, 378 K), an increase of all adsorption and desorption rate constants by a factor of 100 would, if the surface reaction rate constants remain unchanged, yield an increase in the overall rate of reaction 1by a factor of 7 and a substantial (cf. Figure 7) increase in the potential yield of B. The relatively slow sorption rates are thus undesirable from activity as well as selectivity viewpoints.
LO
Acknowledgment The financial support provided by the Swedish National Board of Technical development is gratefully acknowledged. Nomenclature Indexes I = observation index, steady-state kinetic runs i = Reaction index ( i = 1-4) j = component index 0' = A, B, C, LHC, H2) k = CSTR index ( k = 1 to N ) Symbols
L,;
0 - ,
0
\ I
OW2 r Cdk
--
,
1
0004
_____
,
I
0006
radss
,
I
0008
,
--
1
0010
r des,
0012 Zlm
-- 'desB
Figure 10. Steady-staterate profiles for the rates of adsorption and desorption of A and B on the Ni/Si02 catalyst, [rads] = [).des] (in pmol/(s kg catalyst)). (a, upper) T = 378 K, p = 1.5 cm3/s (STP), poHz = 1800 Pa, p o A= 300 Pa. (b, lower) T = 423 K, q = 1.5 cm3/s (STP),p ' = ~ 1800 ~ Pa, p ' ~= 300 Pa.
is identical with the assumption that for each surface reaction a rate-controlling substep exists. The validity of this assumption could clearly be questioned for the Ni/ SiOz and the NiS/SiOz catalysts, due to the fact that the estimated energies of activation for reaction 1 were not significantly different from zero on these catalysts. For that reason, it does not seem wise to speculate too much concerning mechanistic aspects of the surface reactions. Such interesting phenomena could hardly be penetrated unless the rates of the different substeps could be measured. If this could be done during actual hydrogenation conditions, the method of kinetic modeling that has been used in this work could be extended further to more sophisticated views on the surface reaction. The problems associated with increased mathematical complexity and statistical correlation between different parameters could, however, be limiting factors for such models. In summary, the kinetic analysis implies the following conclusions: (1)The overall rate of reaction 1 is severely influenced by the rates of aldehyde adsorption and desorption on the Ni/Si02 catalyst. This effect is also observed on the NiS/SiOz and the Pd/Si02 catalysts, but in these latter cases, the influence is less important. The rate of reaction 2 that is measurable only on the Ni/Si02 catalyst is only slightly influenced by the sorption rates. (2) For all three catalysts, the hydrogen adsorption equilibrium constant is much lower than for the aldehydes. The adsorption and desorption rates of hydrogen are large
d, = catalyst particle diameter, m d, = tube reactor diameter, m Ei = energy of activation, surface reaction i, J/mol Eiapp= apparent energy of activation,power law model, J/mol Eaj= adsorption energy of activation for component j , J/mol Edj= desorption energy of activation for component j , J/mol ki = rate constant for surface reaction i , s-l koi = value of ki at reference temperature (400 K), s-' kaj = adsorption rate constant of component j , Pa-' s-l koaj= value of k , at reference temperature (400 K), Pa-' s-l kdj = desorption rate constant of component j , s-l kodj = value of k , at reference temperature, s-l Lb = length of catalyst bed, m L, = length of tube reactor, m n = number of independent kinetic runs N = number of CSTRs in respective reactor model N,= amount of surface sites, mol NE/VF = surface site concentration, mol/m3 of gas P = total pressure, Pa p j = partial pressure of component j , Pa p o j = inlet partial pressure of component j , Pa pj,obs = observed outlet partial pressure of component j , Pa r j j = calculated outlet partial pressure of component j , Pa q = volumetric flow rate, m3/s R = equation of state constant, J/(mol K) rada,j k = rate of adsorption of component j , CSTR k , s-l rdes,jrk = rate of desorption of component j , CSTR k , s-l ri = rate of surface reaction i, s-l rik = rate of surface reaction i, CSTR k , s-l
T = absolute temperature, K t = time, s VF = total gas volume in catalyst bed, m3 y; = mole fraction of component j y,k = mole fraction of j in CSTR k
Greek Symbols 6H = apparent hydrogen order of reaction in reaction i, power yaw model
OF = fraction of unoccupied sites 6 ~ k= fraction of unoccupied sites, CSTR k 6; = fractional coverage of component j
Ind. Eng. Chem. Res. 1988,27, 2039-2043
e,,
= fractional coverage of component j i n CSTR k = stoichiometric coefficient of component j , reaction i vl = turnover frequency of reaction 1, s-l vHD = turnover frequency of the H2/D2 exchange reaction, T = residence time of an inert component in the catalyst bed, S
Registry No. Ni, 7440-02-0; NiS, 11113-75-0;Pd, 7440-05-3; 2-ethylhexenal, 645-62-5.
Literature Cited Al-Ammar, A. S.; Webb, G. “Hydrogenation of Acetylene over Supported Metal Catalyst. Part I”. J. Chem. SOC.,Faraday Trans 1 1978, 74, 195. Berndt, G. F.; Thomson, S. J.; Webb, G . “Hydrogenation of Acetylene over Supported Metal Catalyst. Part IV”. J. Chem. Soc., Faraday Trans 1 1983, 79, 195. Blyholder, G.; Shihabi, D. “Infrared Spectral Observation of the Interaction of Acetone with Silica-Supported Ni and Con. J. Catal. 1977, 46, 91. Boeseken, J.; van Senden, G. H. “Zerstijrungdes Heptylalkohols bei 220’ in Ggw. von fein verteiltem Nickel”. Red. Trav. Chim. Pays-Bas 1913, 32, 23. Chaudhari, R. V.; Jaganathan, R.; Kohle, D. S.; Emig. G.; Hoffmann, H. ”Kinetic Modelling of a Complex Consecutive Reaction in a Slurry Reactor: Hydrogenation of Phenyl Acetylene”. Chem. Eng. Sci. 1986, 41, 2696. Froment, G. F.; Hosten, L. “Catalytic Kinetics: Modelling”. In Catalysis; Springer Verlag: Berlin, 1981; Vol. 2, p 98. Grant, J.; Moyes, R. B.; Oliver, R. G.; Wells, P. B. “The Hydrogenation of Alkadienes. Part VII”. J. Catal. 1976, 42, 213. 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: Engelwood Cliffs, NJ, 1977; Chapter 6. Jobson, E.; Smedler, G. “Infrared Investigation of 2-Ethyl-Hexenal and 2-Ethylhexanal Adsorbed on Working Ni/Si02 and NiS/Si02 Catalysts”. Submitted for publication in J. Catal. 1988. Konvalinka, J. A.; van Oeffelt, P. H.; Scholten, J. J. F. “Temperature Programmed Desorption of Hydrogen from Nickel Catalysts”. Appl. Catal. 1981,1, 141.
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Lee, H. C.; Butt, J. B. “Kinetics of the Desulfurizationof Thiophene: Reactions of Thiophene and Butene”. J. Catal. 1977, 49, 320. Magnusson, J. “H2/D2Exchange as a Model Reaction for Studying Hydrogen Adsorption on a-A1203-SupportedCopper and Nickel Catalysts”. Ind. Eng. Chem. Res. 1987,26, 874. Newham, J.; Burwell, R. L. “Reactions between Secondary Alcohols, Ketones, and Hydrogen on Metallic Catalysts”. J. Am. Chem. SOC. 1964,86, 1179. 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. Patterson, W. R.; Burwell, R. L. “Isotopic Exchange Reactions Involving Alcohols, Ketones and Deuterium on Silica, on Palladi1971, 93, 833. um/Silica, and on Alumina”. J. Am. Chem. SOC. Phillipson, J. J.; Wells, P. B.; Wilson, G. R. “The Hydrogenation of A 1969, 1351. Alkadienes. Part 111”. J. Chem. SOC. Smedler, G. “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”. Znd. Eng. Chem. Res. 1988, preceding paper in this issue. 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. Somorjai, G. A. “Active Sites in Heterogeneous Catalysis”. Adu. Catal. 1977, 26, 2-68. Suen, T.-J.; Fan, S. “Catalytic Hydrogenation of Heptaldehyde in 1942, 64, 1460. Vapor Phase”. J. Am. Chem. SOC. Tanaka, K. “Studie? in Surface Science and Catalysis”. In Catalytic Hydrogenation, Cerveny, L., Ed.; Elsevier: Amsterdam, 1986; Vol. 27. Thomson, S. J.; Wishlade, J. L. “Radiochemical Studies of Chemi1962,58, sorption and Catalysis. Part IV”. Trans. Faraday SOC. 1170. Tsuji, J.; Ohno, K.; Kajimoto, T. “Organic Synthesis by Means of Noble Metal Compounds. Part XX”. Tetrahedron Lett. 1965, 4565. 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. Part V”. J. Catal. 1971, 20, 340. Received f o r review November 17, 1987 Accepted June 13, 1988
Hydrocracking of n -Heptane with a NiO-Mo03/HY Ultrastable Zeolite as Catalyst. The Network of the Reaction M. Isabel Vgzquez, A g u s t b Escardino,* and Antonio Aucejo Departamento de Ingenierla Qujmica, Uniuersitat de Valdncia, Doctor Moliner, 50, 46100 Burjassot, Valencia, Spain
The hydrocracking of n-heptane has been studied in a continuous, tubular, plug flow reactor using a 4 wt % Ni0-8 wt % MoO,/HYUS zeolite as catalyst, to try to obtain a network of reactions to account for the formation of the various products observed. In view of the products obtained and depending on whether they are primary or secondary, a series of simultaneous parallel reaction schemes have been proposed to explain the network of the reaction. The kinetic parameters of these reactions have been obtained from the initial selectivities of the products. The values of the apparent activation energies obtained for the isomerization, hydrogenolysis, cracking, and disproportionation reactions were 99.1, 169.6, 221.4, and 195.9 kJ/mol, respectively. Hydrocracking is a c o m b i n a t i o n of cracking and hyd r o g e n a t i o n which is carried out at relatively h i g h pressures and temperatures lower than catalytic cracking. At these operating conditions, the isomerization reactions are more favored than those of cracking. In any case, the extent of these reactions can be varied by a d e q u a t e l y se-
* T o whom correspondence should be addressed. 0888-5885/88/2627-2039$01.50/0
lecting t h e o p e r a t i n g conditions and varying the hydrog e n a t i o n f c r a c k i n g r a t i o of the catalyst. In the hydrocracking of hydrocarbons, a large number of simultaneous, parallel, and consecutive reactions take place (Langlois and Sullivan, 1970). In order to s t u d y these reactions, t h e initial selectivities method could be applied. This procedure has been satisfactorily employed to s t u d y n-heptane cracking on CrHNaY and HY zeolite catalysts
0 1988 American Chemical Society
