ARTICLE pubs.acs.org/IECR
Modeling of Toluene Acetylation with Acetic Anhydride on H-USY Zeolite Eileen A. Dejaegere,† Joris W. Thybaut,‡ Guy B. Marin,‡ Gino V. Baron,† and Joeri F. M. Denayer*,† † ‡
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Laboratory for Chemical Technology, Universiteit Gent, Krijgslaan 281 S5, 9000 Gent, Belgium
bS Supporting Information ABSTRACT: The liquid-phase acetylation of toluene with acetic anhydride was carried out in a continuous-flow reactor over H-USY zeolites with different Si/Al ratios at 180 °C, at different contact times and feed compositions. H-USY is an active catalyst for this reaction because the main reaction products at all times on stream are the desired methylacetophenone (MAP) and its reaction byproduct acetic acid. Within the different MAP isomers, the selectivity toward 4-MAP equals 85%. Although the initial acetic anhydride conversion is 100%, the zeolite is subject to deactivation. Small amounts of side products such as methylbenzoic acid and isopropenyltoluene were also identified and their formation explained. The data and insights obtained during these experiments were used to obtain models describing the formation of MAP and the other components present in the reactor effluent. The most plausible model, obtained via model discrimination, was validated at different reaction conditions and takes into account adsorption of the chemical compounds, the catalytic reactions, and deactivation of the catalyst. It also includes hydrolysis of acetic anhydride and the formation of side products originating from MAP. According to this model, catalyst deactivation starts from MAP and acetic anhydride, whereby acetic acid is liberated. Fitting of the model to the experimental data shows that the kinetic constant for the formation of 4-MAP is comparable to that of the deactivation reaction.
’ INTRODUCTION FriedelCrafts acylation of aromatic compounds is a wellknown reaction in the chemical industry. The resulting aromatic ketones are largely used as intermediates in the synthesis of fine chemicals. In these contemporary times, alternative and “greener” catalysts are sought to replace the traditional FriedelCrafts acid catalysts (AlCl3 and H2SO4). Zeolites answer many of the inherent disadvantages suffered by these homogeneous acid catalysts, such as equipment corrosion, environmental pollution, and tedious workup. Zeolites as heterogeneous acid catalysts have already successfully substituted the old-fashioned catalysts in many bulk chemical production processes. Two industrial processes using zeolites (H-Beta or H-USY) were developed for the selective synthesis of acetoanisole and acetoveratrole1 at multiton scale. In both cases, noteworthy improvements were achieved by the replacement of conventional technology (Lewis acid catalysts, acetyl chloride as the acylating agent, and a batch reactor) with innovative technology (zeolite catalysts, acetic anhydride, and a recycle fixed-bed reactor).24 However, generalization of the process still requires improvements regarding deactivation of the zeolite catalyst in general.5 To fully develop the potential of zeolites and control their deactivation, a detailed knowledge of all mechanisms taking place during catalytic transformations over zeolite catalysts is required.6 When a chemical reaction is catalyzed by a zeolite, especially in the liquid phase, several processes occur simultaneously. Next to catalytic conversion, the components are subjected to their adsorption behavior, resulting in results different from those expected when a homogeneously catalyzed process is performed. At present, it is known that the action of solid catalysts results from r 2011 American Chemical Society
their capability to adsorb reacting substances. As such, it is felt that insight in the chemical reaction and deactivation of the catalyst cannot be attained without understanding and taking into account the adsorption processes taking place on the zeolite. Deactivation related to strong adsorption of reactants or products has been witnessed by several research groups.712 The typical approach for developing a new process with solid acids involves reaction studies where catalysts are tested under a wide range of conditions. There are many experimental variables (catalyst properties, temperature, contact time, reactant compositions, etc.) that need to be considered. In the present investigation, we report on the reaction study and modeling of the acylation of toluene with acetic anhydride in continuous-flow conditions over H-USY zeolite at 180 °C. The desired reaction product in this reaction is the para isomer of methylacetophenone (MAP), a colorless liquid with a penetrating floral fruity odor that finds applications in perfumery, flavors, and the fragrance industry.13 Zeolite H-USY combines two important features that make it suitable as a catalyst for the FriedelCrafts reaction using aromatic substrates: a strong acidic character and the ability to accommodate large molecules, such as aromatic ketones, inside its pores. Moreover, its thermal stability and catalytic activity can be improved by dealumination. The H-USY studied in detail has a Si/Al ratio of 30, which was shown to deliver good activity in several alkylation reactions14,15 Received: April 13, 2011 Accepted: September 19, 2011 Revised: July 27, 2011 Published: September 19, 2011 11822
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Industrial & Engineering Chemistry Research and in catalytic cracking and hydrocracking reactions.1618 A limited set of tests was performed using H-USY zeolites with varying Si/Al ratios. Although research has been performed on the acylation of toluene with acetic anhydride catalyzed by zeolites,19,20 it has not been reported so far on H-USY zeolites in continuous-flow reactors. Flow reactors have several advantages from an industrial perspective compared to the more commonly studied batch reactor. Also, deactivation is often described as being the main problem for application of zeolite catalysts, especially for liquidphase reactions. However, it is more difficult to find anything on the kinetics or adsorption effects, which is crucial before a chemical reaction can occur on the zeolite. In this study, a series of reaction conditions are tested to find the optimal reaction conditions and gain insight in the chemical reactions occurring on the zeolite. The approach includes high-throughput plug-flow catalytic experiments and adsorption experiments. Contrary to the typical approaches, the acquired insights will be implemented in a model that confirms or eliminates the possibility of certain reaction mechanisms.
’ METHODOLOGY
ARTICLE
following equations: conversion ð%Þ ¼
product distribution ð%Þ ¼
product yield ð%Þ ¼
! 100
½Pi t 100 ½Pj t
∑j
Pi ðmolÞ 100 FC in feed ðmolÞ
product selectivity ð%Þ ¼
ð1Þ
ð2Þ
ð3Þ
Pi ðmolÞ 100 FC consumed ðmolÞ ð4Þ
Conversion, yield, and selectivity are calculated over the duration of the reaction experiment (or until complete deactivation of the zeolite). The contact time was calculated as follows: contact time ðhÞ ¼
Catalyst and Chemicals. The H-USY (CBV-720, Si/Al 15;
CBV-760, Si/Al 30; CBV-780, Si/Al 40) zeolites were purchased from Zeolyst International in powder form. In this study, these zeolites are represented by, e.g., H-USY-15, for a Si/Al ratio of 15. A very extensive study of the physicochemical properties of these commercial zeolites has been provided in Remy et al.21 Toluene (Aldrich, 99.8%) and acetic anhydride (Riedel-deHaen, 99%) were used, after drying by adding zeolite 3A (CECA), under the physical form of beads. The methyl acetophenone isomers were purchased from Fluka. Catalytic Experiments. The zeolite was activated in situ by heating to 400 °C at a rate of 2 °C/min under a constant nitrogen flow. This maximum temperature was maintained for 8 h. The liquid-phase acylation of toluene with acetic anhydride was carried out in a continuous-flow high-throughput frontal analysis setup, capable of performing eight reactions in parallel. The detailed arrangement and procedure of the experiment has been given elsewhere.10 For a standard experiment, a reactor tube with a length of 50 mm and an internal diameter of 4.5 mm was filled with 0.4 g of H-USY (Si/Al 30). A mixture of acetic anhydride and toluene (molar ratio 1:40) was fed to the catalytic column at a temperature of 180 °C. A backpressure at the outlet of the catalytic column was generated with a 2.5-mm loop of capillary tubing, guaranteeing liquid-phase conditions in the catalytic column at this temperature.10 Toluene was used as the solvent, in excess compared to acetic anhydride, to prevent the pores of the zeolite from being filled only with the more strongly adsorbing molecule acetic anhydride.10 This results in a more favorable intrapore distribution of the acylating agent and the aromatic substrate. For the study on the effects of different reaction conditions, the feed flow rate and the catalyst amount were varied, resulting in different contact times of the reactants with the catalyst. The feed composition was also changed by varying the acetic anhydride/toluene molar ratio from 1:80 to 1:20. The reaction temperature was kept at 180 °C. Reactor effluent samples were collected during 6 h of time on stream (TOS). The conversion of the reagent, product distribution, product yield, and product selectivity are calculated using the
½FC in effluentt 1 ½FC in feedt
catalyst volume ðm3 Þ volumetric feed flow rate ðm3 =hÞ
ð5Þ
Analytical Techniques. The liquid samples were analyzed by a gas chromatograph [GC; Hewlett-Packard 6890 Series (G1530A)] equipped with a CTC PAL autoinjector and a flame ionization detector. The GC was fitted with a HP-5 capillary column (Agilent; 30 m, i.d. 320 μm, film thickness 0.25 μm). The components in the reaction mixture were identified by a GC coupled to a time-of-flight mass spectrometer (GC-TOF-MS; LECO Pegasus III). The GC unit was equipped with the same column as that used for GC analysis. Most of the components could be identified directly. The isomers of the acylation products were identified by a comparison of their retention times to those of the pure isomeric products. Adsorption Experiments. Adsorption isotherms of acetic anhydride, MAP, and acetic acid on zeolite H-USY were determined at room temperature using the batch adsorption technique.22 Toluene was used as the solvent. The experimental data were fitted with the single-component Langmuir equation:
q ¼ qs
bc 1 þ bc
ð6Þ
Modeling. The model setup to describe the transient response of the reaction system is based on a plug-flow reactor model because the acetylation of toluene with acetic anhydride was performed in catalytic plug-flow reactors.23 The flow through the zeolite bed is described by an axial dispersion model.24
∂ci ∂2 ci ∂ci 1 ε hðqi qi Þ ¼ Dax 2 v ε ∂t ∂z ∂z
ð7Þ
The last term in eq 7 describes the mass transfer between the liquid phase and the adsorbed phase. It is convenient to correlate mass-transfer rates in terms of an effective mass-transfer rate (h), defined according to a linear driving force equation.24 Equation 7 only specifies the variation of the concentration c in the liquid phase. The variation in time of the concentration of component 11823
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i in the adsorbed phase is written as follows: ∂qi ¼ hðqi qi Þ ( ri ∂t
ð8Þ
The reaction rate equations are written in terms of the adsorbed concentration q, e.g., ri ¼ k1 qA qB k2 qi þi :::
ð9Þ
The selected reaction mechanism will impose the way this equation is drawn up for every component. The multicomponent Langmuir isotherm was chosen to describe the adsorbed concentration at thermodynamic equilibrium (q*). In this work, deactivation is expressed by the irreversible adsorption of larger components (coke precursors), which gradually fill in the zeolite pore volume. As a result, the adsorption capacity decreases with increasing degrees of deactivation. ! qi, sat bi ci qcp ð10Þ 1 q i ¼ qcp, sat 1 þ bj cj
∑j
The correction for deactivation influences all reaction rates by decreasing the extent of adsorption, be it by pore blocking or irreversible adsorption. This leads to a set of partial differential equations (PDEs), in which the number of PDEs is determined by the number of components involved in the reaction: each component has equations describing the liquid and adsorbed phases. The coke precursors are only described by their adsorbed phase because it is assumed that these components do not desorb into the liquid phase. These equations were implemented in the Athena Visual Studio Software Package (Stewart and Associates Engineering Software, Inc.) and their parameters fitted to the experimental data. Parameter estimation for the unknown reaction parameters, the kinetic constants, is done by coupling the PDE solving method to the method of least squares. The PDEs were solved by applying a central finite differences discretization scheme with 20 discretization points. The method of least squares minimizes the sum of the squared residuals (SSRs), i.e., difference between the data predicted by the model and the experimental data. The number of responses in the data fitting was 6, namely, the concentration at the reactor exit as a function of the TOS of acetic anhydride, acetic acid, the three MAP isomers, and a lumped fraction of the other products. For each catalytic run, 36 reactor effluent samples obtained at different degrees of deactivation were analyzed, giving a total of 216 individual data points for each catalytic run.
’ RESULTS Reaction Pattern and Products. Figure 1 presents the consumption of acetic anhydride and the concentration profiles of the main components present in the reactor effluent as a function of the TOS over the H-USY (Si/Al 30) zeolite for the acetylation of toluene. Toluene is not included in the graph because its concentration remains almost constant during the whole run. Toluene is used in excess in the reaction feed compared to acetic anhydride to prevent the pores of the zeolite from being filled only with acetic anhydride because the latter component is adsorbed more strongly (see below). The acetic anhydride consumption is complete for the first 15 min of TOS, but the catalytic activity changes with the TOS. In the first 100 min, the conversion of
Figure 1. Acetic anhydride consumption and concentration profiles in the reactor effluent for the acetylation of toluene with acetic anhydride at 180 °C, at 0.033 h contact time over H-USY-30 (molar feed ratio 1:40 AcA/Tol). Inset: magnification of the region 030 min of TOS.
acetic anhydride drops from 100% to 10%. The rapid decrease in activity is attributed to deactivation of the zeolite catalyst. After 100 min, the catalytic activity does not decline much further. A few possible causes for this deactivation have been described in the literature: (i) strong adsorption of MAP and subsequent reactions result in adsorption of coke-like products during the reaction,19,25,26 and (ii) inhibition of MAP production by products of acetic anhydride transformation (condensation products and acetic acid).2528 This deactivation phenomenon will be discussed in more detail below. MAP and the simultaneously produced acetic acid are the main reaction products, regardless of the TOS or acetic anhydride conversion degree. During the first 15 min of TOS, when the conversion level equals 100%, more acetic acid is produced than acetic anhydride consumed (Figure 1). Afterward, when the conversion starts to decrease, the acetic anhydride consumption is equal to acetic acid production for these reaction conditions: for every molecule of acetic anhydride converted, one acetic acid molecule is formed. MAP is formed in lower amounts, but the acetic acid and MAP concentration profiles show the same deactivation as the acetic anhydride conversion. At low TOS ( H-USY-30 > H-USY40. The lower the Si/Al ratio (and thus the higher amount of acid sites on the zeolite), the higher the conversion. As a function of the TOS, this decrease is caused by a faster catalyst deactivation as the Si/Al increases from 15 to 40 (not shown). Looking at the turnover number, it has often been stated21 that dealumination initially increases the catalytic activity of H-USY zeolites, but for the most dealuminated samples, the activity decreases again. This trend can also be seen in Figure 6, where it is clear that the acid sites of zeolite H-USY-30 are the most active for this reaction. In terms of product distribution, there is no significant difference for the three zeolites. As the Si/Al ratio increases, the acetic acid fraction increases very slightly from 79 to 82% and the MAP fraction decreases faintly from 19 to 17%. The para/ortho isomer ratio shows a small increase. The higher conversions of the more alumina-rich zeolite, however, lead to a higher production of
“others”. The extra amount of acid sites seems to make consecutive reactions more probable. Adsorption Properties. Adsorption is closely interlinked with the chemical reaction rates. Therefore, the adsorption/reaction model needs parameters describing the adsorption behavior of the main components. Independent adsorption experiments yield the necessary adsorption parameters applied in the model, such as the Henry and Langmuir constants. However, adsorption experiments cannot be performed at the reaction temperature because in these conditions obviously reaction will also occur. For that reason, adsorption experiments were performed at room temperature, to obtain reasonable values for the adsorption parameters. Figure 7 shows the experimental adsorption isotherms of acetic anhydride, acetic acid, and MAP in toluene data together with the fitted isotherm curves. All of these components are adsorbed selectively from the apolar toluene solvent. On the basis of the steepness of the adsorption isotherms (Figure 7), it is clear that acetic anhydride is adsorbed very selectively and MAP is the least selective from toluene. This can be explained by the presence of two acyl groups on acetic anhydride instead of just one on acetic acid and MAP,10 thereby increasing the driving force for adsorption of acetic anhydride. The saturation capacity is also largest for acetic anhydride. These adsorption isotherms indicate that, during chemical reaction on the zeolite, acetic anhydride will adsorb to a larger extent than the reaction products. Model Discrimination. In a first step, modeling was performed on the experimental observations obtained in the following reaction conditions: a reaction temperature of 180 °C, a feed concentration of 0.2 M acetic anhydride in toluene, and a contact time of 0.033 h (cf. Figure 1). On the basis of the chemistry proposed before (Schemes 14) and the experimental observations, a series of reaction schemes (Table 1) were tested to see whether they adequately described the concentration profiles of the acetylation of toluene with acetic anhydride in plug-flow conditions. These reaction schemes are incorporated into the adsorptionreaction model and fitted with the experimental data. The main difference between these reaction schemes is the formation of coke precursors, which are responsible for catalyst deactivation caused by pore blockage. The formation of the acylated products is identical for all reaction models because their formation is believed to be the 11828
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Scheme 4. Reaction Model for the Acetylation of Toluene with Acetic Anhydride at 180 °C over H-USY Zeolite
Table 1. Reaction Schemes for the Acetylation Reaction of Acetic Anhydride on Toluenea
Table 2. SSRs for All Components and for the MAP Isomers Separately and F Values for the Tested Reaction Model Numbers 16 (cf. Table 1)
General Reaction Mechanism Used for All Models acetylation reaction side reaction k1
sf 2-MAP þ AA Tol þ AcA
k2
sf 3-MAP þ AA k3
sf 4-MAP þ AA
SSR
k4
AcA sf 2AA
model
all components
MAP
F value
1
0.3081
0.1046
2.76 104
2
0.7121
0.2674
1.09 104
3
0.1762
0.0642
4.78 104
4
0.6547
0.2626
1.05 104
5
0.9033
0.3938
1.04 104
6
0.9588
0.3014
7.22 103
k5
4-MAP þ 4-MAP sf others
Specific Side Reactions Used for Different Models model number side reaction k6
1
AcA þ AcA sf coke precursors þ 2AA
2
others þ AcA sf coke precursors þ AA
3
4-MAP þ AcA sf coke precursors þ AA
4
4-MAP þ 4-MAP sf coke precursors
k6
k6
Table 3. Kinetic Constants for the Acetylation of Toluene with Acetic Anhydride on H-USY-30 at 180 °C (for the Reactions as Illustrated in Scheme 4) 95% confidence interval
k6
ki
k6
5
others þ 4-MAP sf coke precursors
6
k6
others þ others sf coke precursors
a
Tol = toluene, AcA = acetic anhydride, MAP = methylacetophenone, and AA = acetic acid.
straightforward acetylation of acetic anhydride on toluene (Scheme 1). This reaction is described by three kinetic constants for production of the three different MAP isomers. The kinetic constants for the formation of these isomers can therefore be seen as a measure for the selectivity toward the separate MAP isomers. Hydrolysis of acetic anhydride is also always included. The formation of “others” is thought to be understood (Schemes 2 and 3). Because the formation of “others” includes quite a few reaction steps, lumping of several components based on chemical reaction schemes was done. In all models, the tolylacylium ion and isopropenyltoluene are considered part of the MAP lump. This is justified by Scheme 2, which shows that for every MAP either one tolylacylium ion or one isopropenyltoluene is formed. In addition, 4-MAP is by far the dominant MAP isomer present in the reactor effluent, so all secondary reactions are more likely to involve this isomer. For this reason, only the combination of 4-MAP with the 4-MAP isomer is withheld for the production of other products. These “others”, which mainly include methylbenzoic acid, dimethylbenzophenones, and dimethylanthracenediones, will remain grouped during the modeling. The main focus of attention will be on isopropenyltoluene (part of the MAP fraction), which is formed
3 1 1
2
ki/k3
2
4.1 10
( 1.3 10
k2 [(mol/m3)1 s1]
1.9 102
( 7.6 103
0.04
k3 [(mol/m3)1 s1]
4.6 101
( 1.2 101
1.00
k4 (s1)
6.5 103
( 1.3 103
0.01
k5 [(mol/m3)1 s1]
3.1 101
( 8.4 102
0.67
k6 [(mol/m3)1 s1]
4.8 101
( 1.4 101
1.04
k1 [(mol/m )
s ]
0.09
Table 4. Selected Reaction Conditions for Validation of the Best Reaction Model condition
feed concn of AcA in toluene (M)
contact time (h)
A
0.28
0.017
B C
0.20 0.17
0.033 0.004
D
0.15
0.008
together with these larger components but is only detected in very small amounts. This reactive compound will be consumed in other reactions. Also, the acylium ion is not directly present in the reaction schemes but is lumped together with acetic anhydride. This species undergoes instant further reaction and does not exist as such. The main difference between the models is the manner of deactivation. Deactivation is described as the coke precursors that are irreversibly adsorbed in the zeolite pores, causing pore blockage. The first series of models includes deactivation originating directly from acetic anhydride or rather its ketene. This was 11829
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Figure 8. Parity plot (a) and concentration profiles for acetic anhydride (b), acetic acid (c), and MAP (d) for the different reaction conditions listed in Table 4. The lines represent the simulated concentration profiles.
put forward as a possible reaction in several papers.25,26,28 Model 1 represents the polymerization of ketene according to Bonati et al.28 The next two models represent the cause of deactivation as the addition of ketene onto benzophenones and anthracenediones (model 2) and onto isopropenyltoluene (included into MAP) (model 3). In the second series, deactivation is proposed by consecutive reactions starting from MAP, leading to large molecules that block the pores.19,25,26 To judge the fitting of the modeling results, several graphs and values are used. Parity plots, experimental and fitted concentration profiles as a function of the TOS, and the MAP isomer product distribution versus acetic anhydride conversion allow for a visual comparison. A statistical significance factor (F value) and the SSRs for all products and for the MAP isomers separately allow for a more mathematical distinction between the models (Figure 1 in Supporting Information). On the basis of these two types of selection criteria, the best model can be chosen. As can be seen in Table 2, the SSRs are the smallest for model 3. Of all models, model 3 also has the highest statistical significance. A closer look at the parity plots and concentration profiles (Figure 1 in Supporting Information) yields more insight into the performance of the tested reaction models. Models 1 and 3 allow the best fitting of the experimental data, as can be observed from the parity plots. Models 2 and 46 show very bad correlations between the data and the fitting. The main shortcoming of model 1 is that it predicts total deactivation: the acetic anhydride concentration reaches feed level concentrations, and the acetic acid and MAP concentrations decrease to zero. This clearly does not match the experimental data. For models 2 and
46, too much of the MAP or “others” fraction is necessary to achieve decent deactivation, which can only continue if enough MAP (and acetic acid) is produced. The coke precursors are then a “sink” for the MAP molecules, while acetic acid is produced in too high amounts. Model 3 describes the 100% acetic anhydride conversion at the start of the experiment properly. It shows the correct deactivation profiles, and the predicted concentration levels for acetic acid and MAP match the experimental data. The most discriminating performance criterion is the product distribution of the MAP isomers (Figure 1 in Supporting Information). Model 3 (Scheme 4) is able to describe the MAP product distribution properly and, therefore, is identified as the most plausible model in this series of tested reaction schemes. The kinetic constants obtained for model 3 are given in Table 3, together with the 95% confidence intervals. From this table, it can be observed that the kinetic constant for the formation of 4-MAP is comparable to that of the deactivation reaction. Model Validation and Discussion. The model selected in the previous section performed best for one specific set of reaction conditions. To validate the model, reactor elution profiles were simulated at different contact times and feed concentrations at 180 °C and compared with the experimental observations. The selected data for this validation were those listed in Table 4. The predicted results for the acetic anhydride, acetic acid, and MAP concentration profiles are plotted against the experimental observation for these reaction conditions in Figure 8a. For these different contact times and feed concentrations, the reaction model adequately describes the time evolution of catalyst deactivation and the concentration profiles for the main species involved in the reaction. 11830
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Industrial & Engineering Chemistry Research The most plausible model describes coke precursor formation starting from the MAP lump and acetic anhydride, whereby acetic acid is liberated. As explained above, the MAP lump contains the MAP isomers, the tolylacylium ion, and isopropenyltoluene. Isopropenyltoluene is produced in equimolar amounts to the tolylacylium ion, and the latter is known to lead to the production of “others”, e.g., dimethylbenzophenones and -anthracenediones. Isopropenyltoluene is, however, only found in extremely small amounts (
