Development of a Mechanistic Kinetic Model of the Higher Alcohol

A previously developed kinetic model of higher alcohol synthesis over Cs-doped Zn/Cr/O catalysts is tested against chemical enrichment experiments...
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Development of a Mechanistic Kinetic Model of the Higher Alcohol Synthesis over a Cs-Doped Zn/Cr/O Catalyst. 2. Analysis of Chemical Enrichment Experiments Alessandra Beretta, Luca Lietti, Enrico Tronconi, Pio Forzatti,* and Italo Pasquon Dipartimento di Chimica Industriale e Ingegneria Chimica “G. Natta” del Politecnico, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

A previously developed kinetic model of higher alcohol synthesis over Cs-doped Zn/Cr/O catalysts is tested against chemical enrichment experiments. By simulating the addition of selected reaction intermediates to the CO/H2 feed stream and comparing predicted and measured effects on the product distribution, novel pieces of evidence are gained concerning the mechanistic consistency of the model. The adequacy of the kinetic scheme is confirmed. However, inaccuracies due to the lack of previous information and to the numerical complexity of the model have been identified. In particular, an overestimation of the contribution of ketones in the chain-growth process of aldehydes and primary alcohols has been detected. Such inaccuracies were well disguised by the satisfactory fit of the model to standard kinetic data and by the chemical consistency of the parameter estimates and could be put into light only by the present perturbative analysis of the model. Introduction In part 1 of this work (Beretta et al., 1996) the formulation of a detailed kinetic model of higher alcohol synthesis (HAS) has been reported. The model describes with accuracy the reacting system within a wide experimental field. However, it is known that, especially in the case of the kinetics of complex reacting systems, the goodness of the data fit is not by itself a proof of adequacy of the model (Froment and Hosten, 1981). In the case of HAS, in order to describe the interconnected reaction pathways that each oxygenate can undergo on the catalyst surface, a fairly large number of kinetic parameters has to be introduced. Such parameters allow one to differentiate chemical routes and intermediate reactivities in line with the mechanistic observations but, on the other hand, can generate a degree of numerical uncertainty (e.g., multiplicity of solutions in the search of optimal parameters). While a proper combination of the kinetic parameter estimates can always be found that optimizes locally the model responses, not necessarily the same set of parameter estimates can grant a trustable application of the model outside the experimental field to which the kinetic treatment has been fitted. The purpose of the present work is to check thoroughly the mechanistic consistency of the model developed in part 1. This is important both for a critical evaluation of the model and, ultimately, for addressing a focused revision that could increase the confidence and reliability necessary for process design calculations. The diagnostic method proposed and discussed in the following is the simulation of chemical enrichment experiments. These experiments consist of the addition of selected reaction intermediates to the synthesis gas feed stream which cause changes in the typical HAS product distribution. Such perturbations of the reacting system provide information on the preferential chemical routes followed by the injected oxygenates. Smith and Anderson (1984) verified their mechanistic scheme of HAS over K-doped Cu/ZnO catalysts by injecting C1* Author to whom correspondence is addressed. Fax: (+39)2-7063-8173.

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C4 oxygenates. In particular, the results of these experiments proved that the formation of ethanol, propanol, and isobutanol occurs sequentially. Vedage (1984) and Vedage et al. (1985) demonstrated the dominant role of C1 additions in the synthesis of higher oxygenates over Cu-based catalysts after chemical enrichment experiments with C3 intermediates. Riva et al. (1987) studied the effects of adding various oxygenates to the CO/H2 mixture over K/ZnO/Cr2O3 catalysts. Methanol and formaldehyde were observed to behave like synthesis gas, without any significant change in the product distribution; C2+ oxygenates underwent selective β insertions of C1 units. Kienneman et al. (1991) proposed a mechanism for the C1 f C2 step involving R-aldolization of formaldehyde to explain the results of adding C1 and C2 species to the feed stream over a Cu/ZnO/Al2O3 catalyst. Also, the effects of the addition of C2+ oxygenates suggested the existence of common carboxylate intermediates to the formation of higher alcohols and methyl esters. Finally, the chemical enrichment experiments with 13C methanol over Cu/ZnO catalysts presented by Elliott and Pennella (1988) and Nunan et al. (1988) have to be mentioned. They clarified that the mechanism of ethanol formation does not occur via CO insertion but involves C1 oxygenate species related to methanol. The common feature of all these works is the effective application of the chemical enrichment experiments to investigating the reaction network of HAS. In analogy, the simulation of the addition of oxygenates to the reacting system and the comparison of the experimental and calculated effects on the product distribution is expected to provide a deep insight into the model kinetic scheme. In fact, the correct prediction of the experiments requires both a good description of the reaction network and an adequate estimate of the competition among chemical routes. In the following, cases of injection of linear primary alcohols, branched alcohols, ketones, and secondary alcohols and the comparison with the model simulations are discussed. It is to be noted that, due to the high inlet concentrations of the added oxygenates, the simulation of the chemical enrichment experiments forces © 1996 American Chemical Society

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the model in a kinetic regime (large amount of one selected intermediate of the chain growth) significantly far from the synthesis conditions to which it has been adapted. This suggests to limit the comparison between observed and predicted data to a qualitative rather than quantitative basis. Experimental Section In this study, the same Zn/Cr/O + 15 w/w % Cs2O catalyst was used as in the experiments presented in part 1 (Beretta et al., 1996). The procedures of preparation and activation followed are given elsewhere (Tronconi et al., 1992). Catalytic activity runs were performed in a Cu-lined fixed-bed tubular reactor loaded with 1.5 g of catalyst (40-60 mesh) diluted with 3 times its volume of 0.6 mm glass beads. The temperature of the catalyst was measured by a thermocouple sliding inside a capillary tube immersed in the catalyst bed. The reactor was operated under isothermal conditions at 8.0 MPa, 405 °C with H2/CO ) 1/1, GHSV ) 7900 L(STP) h-1 kgcat-1, and a CO2-free feed stream. Chemical enrichment experiments were performed by injecting several oxygenated molecules into the synthesis gas (feed rate: 5-10 µL/min) with a high-pressure liquid pump (Gilson). All experimental data were obtained under steady-state conditions that were usually maintained for 8-12 h before injecting a different oxygenate. Runs with CO/H2 feeds only were performed at the end of each chemical enrichment experiment; they provided proper reference standards to evaluate the effects of oxygenates addition. The effluents of the reactor were sampled every 30 min using an in-line automated heated sampling valve and analyzed in a Hewlett-Packard 5730A gas chromatograph. A 6 m Porapak Q column was used for separation of C1-C5 products. Coupled with the above on-line GC analyses was the analysis of the products that were collected in a dry-ice-cooled trap. This analysis was carried out with a 25 m long Chrompack CpSil 5 capillary column by using a 5890A Hewlett Packard gas chromatograph. Identification of the various reaction products was performed by comparison of their retention times with those of known standards and by gas chromatograph-mass spectrometer analysis of the liquid fraction. The simulation of the chemical enrichment experiments was performed by applying the kinetic scheme presented in part 1 of this work, assuming (i) a plug flow model for the synthesis reactor, (ii) the optimal values reported in Table 4 of part 1 for the parameter estimates, and (iii) inlet concentration of the injected molecule corresponding to the experimental value. The assumption of thermodynamic equilibrium for the hydrogenation reactions resulted in the partitioning of the total amount of oxygenate injected between carbonylic species and alcohol at the inlet section of the reactor. Results 1. Chemical Enrichment Experiments with Linear Primary Alcohols. Figure 1 shows the product distribution experimentally observed over the Cs/Zn/ Cr/O catalyst under injection of ethanol (inlet mole fraction ) 0.2%). The results are compared with those of a reference run where synthesis gas only was fed. The conversion of the injected alcohol was almost complete, in line with the known high reactivity of the C2 intermediate. It is worth recalling that alcohols do not

Figure 1. Experimental effect of the addition of ethanol on HAS product distribution over the Cs/Zn/Cr/O catalyst. T ) 405 °C, P ) 8.1 MPa, GHSV ) 7900 L(STP)/kg cat/h, H2/CO ) 1/1, CO2 ) 0%, ethanol ) 10 µL/min. C1OH ) methanol; C2OH ) ethanol; C3OH ) 1-propanol; C4OH ) 1-butanol; C5OH ) 1-pentanol; 2mC3OH ) 2-methyl-1-propanol; 2mC4OH ) 2-methyl-1-butanol; 2mC5OH ) 2-methyl-1-pentanol; 3mC4OH ) 3-methyl-1-butanol; 2but ) 2-butanone; 3pent ) 3-pentanone; 3m2but ) 3-methyl-2butanone; 2m3pent ) 2-methyl-3-pentanone.

participate directly in HAS but are related through hydrogenation-dehydrogenation reactions to the true reacting species in the chain growth, namely, the corresponding aldehydes and ketones. Thus, the apparent reactivity of ethanol was properly due to the evolution of acetaldehyde, of which the added alcohol represented a sort of reservoir. The injection of ethanol significantly promoted the formation of higher linear alcohols (1-propanol, 1-butanol, 1-pentanol); among them 1-butanol presented the strongest enhancement. This indicated the important contribution of acetaldehyde as a nucleophilic reactant in normal aldol condensations with C1, C2, and C3 species and also that these reactions have second-order kinetics. The production of branched oxygenates was favored, too, especially 2-methyl alcohols. These species are produced not only from the condensations of linear precursors with a C1 intermediate but also from normal aldol condensations wherein propanaldehyde behaves as a nucleophilic reactant. The promotion was moderate in the case of isobutanol (C3 + C1), strong in the case of 2-methylbutanol (C4 + C1 and C3 + C2), and intermediate in the case of 2-methyl-1-pentanol (C5 + C1 and C3 + C3), which reflected the extents of increment of the corresponding reactant concentrations. Concerning the formation of ketones, the addition of ethanol enhanced the productivity of 2-butanone, 3-pen-

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Figure 2. Comparison between experimental and calculated effects of the addition of ethanol over the major classes of HAS products; the bars represent the average percentage increment of outlet mole fraction with respect to reference conditions (no ethanol injected). CH3OH ) methanol; L.A. ) linear alcohols (1-propanol, 1-butanol and 1-pentanol); B.A. ) branched alcohols (2-methylpropanol, 2-methylbutanol, 2-methylpentanol, and 3-methylbutanol); KET. ) ketones (2-butanone, 3-pentanone, 3-methyl-2butanone, and 2-methyl-3-pentanone).

tanone, and 2-methyl-3-pentanone. These species are products of cross-reversal aldol condensations involving acetaldehyde and propionaldehyde as reactants (C2 + C2, C2 + C3, and C3 + C3, respectively). The same ketones can also be formed via ketonization reactions (C2 + C3, C3 + C3, and 2-methyl-C3 + C3, respectively). Figure 2 reports the results of the model simulation in a class-grouped representation. Each bar is the average percentage increment of outlet concentration predicted by the model for a specific class of products after addition of ethanol, the reference being the results of a standard run (e.g., the bar relative to ketones is the average of the percentage increments of 2-butanone, 3-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone concentrations). In the class of linear alcohols the contribution of ethanol was neglected. Figure 2 shows also the experimental observations in the same form for comparison. Correctly, the model predicted an almost complete conversion of ethanol and an overall promotion of HAS. All the chemical routes evidenced by the addition of ethanol are, in fact, included in the kinetic scheme. Nevertheless, an overestimation of the promotion of linear alcohols with respect to branched alcohols was apparent in the model predictions. Further tests indicated that, due to the adsorption terms in the rate expressions (see Table 3 of Part 1), the addition of ethanol was estimated to slow down the formation of terminal products from the intermediate species. This resulted in a preferential promotion of the direct products of ethanol evolution with respect to the branched consecutive products. It is also evident from Figure 2 that the model predicted a much higher enhancement of ketone pro-

ductivities than experimentally observed. This seems to suggest that the model overestimates the rate of ketonization reactions. However, also reversal aldol condensations have an important role in the formation of ketones from aldehydes; the model assumes them to be faster than the corresponding normal aldol condensations (see the estimate of ORR4 in Table 4 of Part 1). In the present case, for instance, the model calculated that the condensation of two molecules of acetaldehyde to form 2-butanone was more probable than the condensation to form butanal and 1-butanol, which is in contrast with the experimental indications. Similar results were obtained comparing the experimental and simulated effects of enriching the CO/H2 stream with 1-propanol. The case will be briefly discussed. As for the experiment with ethanol, the addition of the C3 intermediate was observed (Herman and Lietti, 1994) and predicted to affect the productivity of all the classes of oxygenates. The measured conversion of propanol was lower than that of ethanol, confirming the lower reactivity of propionaldehyde compared to acetaldehyde. The same ratio of reactivities was predicted by the model. As in the previous case, all the chemical routes revealed by the perturbation with propanol are included in the kinetic treatment: reversal and normal aldol condensations where propanaldehyde behaves as both a nucleophilic and electrophilic reactant, ketonization reactions, and reversal and normal R-additions. The last reaction, in particular, accounts for the observed and calculated promoting effect of propanol on the formation of 1-butanol. As mentioned in part 1 (Beretta et al., 1996), normal R-addition was never directly evidenced in the course of the past experiences with probe molecules at atmospheric pressure, but it was invoked in the kinetic model to explain the monotonically decreasing distribution of C3+ linear alcohols under synthesis conditions (see Figure 1 in Beretta et al., 1996). Also the experiment with propanol, though, indicated inaccuracies of the model in simulating the relative promotion of primary alcohols and ketones; in particular, the addition of the C3 alcohol affected the growth of 3-pentanone (C3 + C3 ketonization product) and 2-methyl-3-pentanone (C3 + C3 reversal aldol condensation product) to a lesser extent than predicted by the kinetic treatment. In line with the model assumptions, chemical equilibrium between ethanol and acetaldehyde and between propanol and propanaldehyde was approached upon addition of the C2 and C3 alcohols, respectively. Table 1 summarizes the main indications provided by the chemical enrichment experiments on the real and estimated reactivity of the intermediate species. 2. Chemical Enrichment Experiments with Secondary Alcohols and Ketones. The observed effect of adding 2-butanol (inlet mole fraction ) 0.65%) to the CO/H2 feed stream is reported in Figure 3. The conversion of the secondary alcohol was almost complete, as the total mole fraction of 2-butanol and 2-butanone in the outlet stream was lower than 0.01%; this is an indication of the significant reactivity of the ketone. The comparison with the results of a reference run points out that 2-butanone selectively contributed to the formation of higher ketones. The productivity of linear and branched primary alcohols was unaffected. Instead, increments of productivity as high as 1 order of magnitude for 3-pentanone and 3 times for 3-methyl-2butanone were observed. These two ketones are products of normal aldol condensations of 2-butanone with

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2157 Table 1. Reactivity of C2-C3 Intermediates: Experimental Evidence versus Kinetic Model reaction

experimental evidence

kinetic model

aldol condensation

major route C2 and C3 behaves both as nucleophilic and electrophilic reactants C2 reactivity >> C3 reactivity N route prevails on reversal route in cross condensations of C2+ oxygenates significant route approaching TD equilibrium in std conditions, far from it upon addition of reactants TD equilibrium approached

high rate accounted for well reproduced N ≈ reversal (see ORR4) included always approaching TD equilibrium TD equilibrium approached

R-addition ketonization hydrogenation

Figure 4. Comparison between experimental and calculated effect of the addition of 2-butanol. CH3OH ) methanol; L.A. ) linear alcohols (ethanol, 1-propanol, 1-butanol, and 1-pentanol); B.A. ) branched alcohols (2-methylpropanol, 2-methylbutanol, 2-methylpentanol, and 3-methylbutanol); KET. ) ketones (3-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone).

Figure 3. Experimental effect of the addition of 2-butanol on HAS product distribution. Reaction conditions and species nomenclature as in Figure 1. 2-Butanol feed: 5 µL/min.

a C1 electrophilic species; the preferential promotion of the former indicates that the R-methyl group (CH3CO-CH2-CH3) exhibited stronger nucleophilic character than the methylenic group (CH3-CO-CH2-CH3). Also the formation of 2-methyl-3-pentanone, a product of consecutive condensations of a C1 intermediate with either 3-pentanone or 3-methyl-2-butanone, was significantly enhanced. No specific evidence about the condensation of 2-butanone with C2+ intermediates was shown by the experiment. Likewise, no evidence for the electrophilic character of 2-butanone was found. The same data are compared in Figure 4 with the predictions of the kinetic model obtained by simulating the addition of 2-butanol to the feed stream. In contrast with the experimental observation, the calculated effect of injecting the secondary alcohol involved the whole HAS product distribution. A contribution of 2-butanone conversion to the growth of both higher ketones and aldehydes was in fact predicted. This can be primarily attributed to the inclusion of reversible rate expressions for the ketonization reactions in the kinetic model. As

a close approach to equilibrium is assumed, the excess 2-butanone was estimated to decompose to acetaldehyde and propionaldehyde; consequently, the increment of concentration of these two intermediates resulted in the promotion of linear and branched primary alcohols, similarly to the cases of ethanol and 1-propanol injection. Furthermore, according to the model ketones can originate aldehydes also via reversal aldol condensations. For instance, the kinetic model predicted a promotion of the formation of 1-pentanol via reversal aldol condensation of 2-butanone with formaldehyde: no evidence of this reaction was given by the experimental results. With respect to the predicted evolution of 2-butanone toward higher ketones, deviations of the model predictions from the experimental results were detected, too. These are due to some simplifying assumptions of the model, namely, that the methyl and the methylenic groups have comparable nucleophilic character and that the ketone may combine equally well with C1 or C2+ carbonyl species. Experiments with addition of acetone and 3-methyl2-butanone to the syngas stream were carried out and simulated, too. Again, while a selective contribution to

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Table 2. Reactivity of Ketones: Experimental Evidence versus Model Assumptions reaction aldol condensation

R-addition ketonization hydrogenation

experimental evidence

kinetic model

normal mode (nucleophilic character) occurs with C1 species occurs with C1-C4 species reactivity -CH3 ) -CH2reactivity -CH3 >> reactivity -CH2normal mode (electrophilic character) no evidence included reversal mode: absent or negligible included absent or negligible not included approaching TD equilibrium in std conditions, always approaching TD equilibrium far from it upon addition of ketones TD equilibrium approached TD equilibrium approached

with C2 and C3 aldehydes). Even if the model includes a role of isobutanal in normal R-additions as experimentally observed, a decrease in the formation of higher branched oxygenates was predicted. This was caused, as already mentioned in the case of addition of ethanol, by an overestimated saturation of the surface due to the simulated injection of the oxygenate: this resulted in a reduced evolution of the C2 and C3 intermediates to higher species. The main comments upon the reactivity of 2-methylbranched species are summarized in Table 3. Critical Analysis

Figure 5. Experimental and calculated effects of the addition of isobutanol on HAS product distribution. CH3OH ) methanol; L.A. ) linear alcohols (ethanol, 1-propanol, 1-butanol, and 1-pentanol); B.A. ) branched alcohols (2-methylbutanol, 2-methylpentanol, and 3-methylbutanol); KET. ) ketones (2-butanone, 3-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone). Reaction conditions as in Figure 1. Isobutanol feed: 5 µL/min.

ketones chain growth was experimentally observed, a more diffuse promotion of the whole HAS process was instead predicted. In Table 2 the major observations about the reactivity of ketones are summarized and compared with the assumptions of the kinetic model. 3. Chemical Enrichment Experiment with 2Methyl-1-propanol. An experiment with addition of isobutanol to the feed stream was performed (inlet mole fraction ) 0.65%) in order to verify the role of isobutanal and 2-methyl species in HAS. The results are reported in Figure 5 in grouped form. A negligible conversion of the oxygenate fed was observed. No significant influence on any class of HAS products was in fact evident. Only a slight promotion of 3-methyl-1-butanol production was measured with respect to reference conditions. This proves that the carbonyl group of isobutanal was subject to the nucleophilic attack of a C1 intermediate, via normal R-addition. The model simulation, also reported in Figure 5, predicted a promoting effect of isobutanal on the formation of ketones such as 3-methyl-2-butanone and 2-methyl-3-pentanone (the ketonization products of isobutanal

The simulation of the chemical enrichment experiments provided novel elements for a critical overview of the mechanistic model for HAS developed in part 1 (Beretta et al., 1996). Each fundamental aspect of the treatment (reaction scheme, species reactivities, rate expressions) has been severely tested by the perturbative approach herein adopted. Reaction Scheme. The reaction network included in the model has been substantially confirmed. All the reactions experimentally demonstrated by the addition of oxygenates are accounted for by the kinetic treatment. This was crucial in particular for simulating the perturbations of HAS caused by the enrichment of acetaldehyde (the very first intermediate of the chain-growth process); the effects were correctly predicted to spread and propagate over the whole reacting system. However, simplifications of the kinetic scheme appear to be possible. For instance, reverse ketonization reactions and ORR aldol condensations involving ketones as reactants were found to be negligible. These reactions were previously included in the model for lack of direct evidence. The present study identified them as the source of an unrealistic dependence of primary alcohols growth from ketones evolution. Relative Reactivities of the Intermediates. While it is confirmed that both aldehydes and ketones manifest nucleophilic character, the chemical enrichment experiments seemed to indicate that formaldehyde has the strongest electrophilic character among all the oxygenates. The model accounts for the specific role of the CN + C1 aldol condensations in the formation of primary alcohols through dedicated parameters (see ORR1, ORR2, ORR3 in Table 2 of Beretta et al., 1996). However, this specificity is neglected in the case where the nucleophilic reactant is a ketone; comparable probability is, in fact, assigned to the condensations of ketones with either a C1 or a C2+ species. A sensitivity analysis indicated that this inaccuracy has only minor effects on the prediction of the product distribution. Thus, the introduction of further parameters diversifying the reactivities of the electrophilic species does not seem necessary. It appears more important, on the

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2159 Table 3. Reactivity of 2-Methyl-Branched Species: Experimental Evidence versus Model Assumptions reaction

experimental evidence

aldol condensation R-addition ketonization hydrogenation

negligible contribution weak contribution absent or negligible TD equilibrium approached

contrary, to introduce a distinction between the nucleophilic character of the methyl group and that of the methylenic group of a 2-ketone. While neglected by the kinetic model, this was proven to affect significantly the simulation of the chain-growth process of ketones and secondary alcohols. Finally, it has been established that 2-methyl-branched species have terminal behavior. This is in line with the model assumptions (see the estimate of kiso in Table 4 of Beretta et al., 1996). However, a contribution of isobutanal in the formation of branched terminal ketones (marginal products of HAS) via ketonization reaction is included in the kinetic scheme; the chemical enrichment experiment with isobutanol showed that this reaction does not occur. Rate Expressions. The results of this study converge in indicating that the kinetic model overestimates the role of ketonization reactions. Contrary to the case of standard runs, the chemical enrichment experiments have shown remarkable deviations of these reactions from thermodynamic equilibrium. As a matter of fact, it was impossible to prove the existence of the reverse ketonizations. Thus, the equilibrium constraint imposed by the model appears questionable. The regression procedure forced the model to satisfy such a constraint through the assumption of a high reaction rate. The experiments showed on the contrary that ketonizations are slow reactions in comparison with the prevailing aldol condensation routes. This can be corrected by replacing the reversible rate expressions included in the model with irreversible kinetics of formation of ketones from aldehydes. It is worth emphasizing that only the present experiments, perturbing the relative amounts of aldehydes and ketones, could evidence the real rate of ketonization reactions and the model inaccuracies. Concerning aldol condensations and R-additions, the changes in the product distribution after addition of oxygenates have, in general, confirmed that these reactions have second-order kinetics, as assumed in the model. A weaker dependence than predicted from the surface saturation is, however, suggested for the reaction rates. Finally, the chemical enrichment experiments have indicated that the reactions of hydrogenation-dehydrogenation between carbonyl species and alcohols are governed by thermodynamic equilibrium. This supports the adequacy of the model assumptions and, in particular, the structure of the set of mass balance equations that takes advantage of the existence of the equilibria to reduce the number of model variables. Conclusions In the first part of this work, the results of an extensive mechanistic investigation on the high-temperature synthesis of higher alcohols have been combined in the formulation of a detailed kinetic model. By fitting the model to isothermal data, optimal values have been estimated for the set of kinetic parameters; the estimates were found to be in line with the general knowledge of the chain-growth process. Also, a wide distribution of products including primary and second-

kinetic model a very low reactivity of 2-m-C3 as electrophilic reactant is included as in aldol condensations always approaching TD equilibrium TD equilibrium approached

ary alcohols, aldehydes and ketones, methane, and higher hydrocarbons was successefully reproduced as a function of feed composition, contact time, and pressure. In the second part, the mechanistic consistency of the model was probed in order to evaluate more critically the reliability of its predictions. An original approach was adopted, consisting of the simulation of chemical enrichment experiments and the comparison of the model calculations with experimental measurements. A substantial adequacy of the kinetic scheme included in the treatment was found. However, the present perturbation analysis revealed the fictitious nature of a limited number of model assumptions, as discussed in the previous section. While such inaccuracies could be thoroughly conciliated with the standard experimental data to which the model had been fitted, they have been clearly identified by testing the model against independent data which were specifically informative on the relative significance of the chemical routes and reactivities involved in HAS. This approach appears fully in line with the methodology of “attacking the problem from various angles” which Frohlich and Cryder (1930) first proposed and applied as the only efficient strategy for the rationalization of the HAS reaction mechanism. The indications presently obtained are highly valuable for future development of the kinetic model. Due to the improved consistency with the HAS chemistry, it is expected that this will provide a simulation tool applicable with some confidence also to extrapolation purposes, as may be required by process design applications. Acknowledgment The authors express their appreciation to Dr. Domenico Sanfilippo, of SNAMPROGETTI SpA, for useful discussions and critical suggestions in the field of higher alcohol synthesis. L.L. expresses his gratitude to Professor Kamil Klier and to Dr. Richard Herman, Lehigh University, Bethlehem, PA, for allowing him to perform some of the experiments in their laboratory during his stay at Lehigh. Literature Cited Beretta, A.; Tronconi, E.; Forzatti, P.; Pasquon, I.; Micheli, E.; Tagliabue, L.; Antonelli, G. B. Development of a Mechanistic Kinetic Model of the Higher Alcohol Synthesis over a Cs-doped Zn/Cr/O Catalyst. 1. Model Derivation and Data Fitting. Ind. Eng. Chem. Res. 1996, 35, 2144-2153. Elliott, D. J.; Pennella, F. Mechanism of Ethanol Formation from Synthesis Gas over CuO/ZnO/Al2O3. J. Catal. 1988, 114, 9099. Frohlich, K.; Cryder, D. S. Catalysts for the Formation of Alcohols from Carbon Monoxide and Hydrogen. VI. Investigation of the Mechanism of Formation of Alcohols Higher than Methanol. Ind. Eng. Chem. 1930, 22, 1051-1057. Froment, G. F.; Hosten, L. H. Catalytic Kinetics: Modelling. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 2, Chapter 3. Herman, R. G.; Lietti, L. Controlling Factors in the Synthesis of Higher Alcohols over Alkali-Promoted Low-Temperature Cu/ Zn/(Cr or Al) and High-Temperature Zn/Cr Catalysts. In

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Proceedings of the 11th International Pittsburgh Coal Conference; Chiang, S.-H., Ed.; University of Pittsburgh, Center for Energy Research, Pittsburgh, 1994; Vol. 1, p 68-73. Kiennemann, A.; Idriss, H., Kieffer, R.; Chaumette, P.; Durand, D. Study of the Mechanism of Higher Alcohol Synthesis on CuZnO-Al2O3 Catalysts by Catalytic Tests, Probe Molecules, and Temperature Programmed Desorption Studies. Ind. Eng. Chem. Res. 1991, 30, 1130-1138. Nunan, J. G.; Bogdan, C. E.; Klier, K.; Smith, K. J.; Young, C.W.; Herman, R. G. Methanol and C2 Oxygenate Synthesis over Cesium Doped Cu/ZnO and Cu/ZnO/Al2O3 Catalysts: A Study of Selectivity and 13C Incorporation Patterns. J. Catal. 1988, 113, 410-433. Riva, A.; Trifiro`, F.; Vaccari, A.; Busca, G.; Mintchev, L.; Sanfilippo, D.; Mazzanti, W. The Role of Cr and K in Catalysts for Highpressure and High-temperature Methanol and Higher-alcohols Synthesis. J. Chem. Soc., Faraday Trans. 1 1987, 83, 22132225. Smith, K. J.; Anderson, R. B. A Chain Growth Scheme for the Higher Alcohols Synthesis. J. Catal. 1984, 85, 428-436.

Tronconi, E.; Lietti, L.; Groppi, G.; Forzatti, P.; Pasquon, I. Mechanistic Kinetic Treatment of the Chain Growth Process in Higher Alcohol Synthesis over a Cs-Promoted Zn-Cr-O Catalyst. J. Catal. 1992, 135, 99-114. Vedage, G. The Mechanism of Methanol Synthesis and Catalyzed Pathways to Low Alcohols. Ph.D. Thesis, Lehigh University, Bethlehem, PA, 1984. Vedage, G.; Himelfarb, P. B.; Simmons, G. W.; Klier, K. AlkaliPromoted Copper-Zinc Oxide Catalysts for Low Alcohol Synthesis. ACS Symp. Ser. 1985, 279, 295-312.

Received for review October 11, 1995 Accepted April 2, 1996X IE950618V

X Abstract published in Advance ACS Abstracts, May 15, 1996.