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Ind. Eng. Chem. Res. 2008, 47, 9304–9313
A New Coated Catalyst for the Production of Diacetone Alcohol via Catalytic Distillation Greg P. Dechaine† and Flora T. T. Ng* Department of Chemical Engineering, UniVersity of Waterloo, 200 UniVersity AVenue West, Waterloo, Ontario, Canada N2L 3G1
A method has been developed for the production of catalytically active coated ceramic distillation saddles for use in catalytic distillation (CD) columns. A thin layer of magnesium acetate was applied to ceramic Norton saddles using a sol-gel dip-coating method. The magnesium acetate coating was converted to the catalytically active magnesium oxide (MgO) via a temperature-ramped calcination program. The kinetic performance of the MgO-coated saddles for the aldol condensation of acetone was determined in a batch reactor and a batch CD column. Although the coated saddles provided lower yields to DAA than Amberlite IRA-900 ion exchange resins in a batch reactor, their improved mass transfer characteristics provided higher yields than the same ion exchange resins held inside fiberglass bags in a CD column at low flow rates. The coated saddles also showed significant improvements in selectivity to DAA compared to the resin catalysts for the aldol condensation reaction carried out both in the batch reactor and in the CD column. 1. Introduction 1
Catalytic distillation (CD) is a green reactor technology which combines a liquid phase reaction over a solid phase catalyst with simultaneous, countercurrent vapor flow. The potential benefits of carrying out a reaction under distillation conditions include reduced capital cost, higher than equilibrium conversion,increasedselectivity,andimprovedenergyefficiency.1,2 However, CD technology does not present a viable solution to all reaction systems, and each specific application must be evaluated independently to determine the suitability of CD. One of the primary challenges of combining heterogeneous catalysis and distillation is devising methods for loading the catalyst in the column. Special support structures are required to contain the catalyst while maintaining a high void fraction to facilitate countercurrent vapor and liquid flow within the reaction zone. Taylor and Krishna2 did a thorough review of the different methods available for this purpose. Unfortunately, the packing structures currently available tend to separate the vapor and liquid into separate streams, with the liquid flowing through a zone filled with catalyst and the vapor flowing in separate channels surrounding these catalyst zones. Separating the liquid and vapor streams in this fashion decreases the efficiency of the distillation occurring in the reaction zone and reduces the impact of simultaneous separation on selectivity and productivity. The separation efficiency in a catalytic distillation process can be improved by making traditional distillation packings catalytically active. According to Taylor and Krishna,2 coated distillation packings have not been used in practice because of high cost and low catalyst loadings per unit volume. Despite these roadblocks, incentives exist for using such coated distillation packings. According to Oudshoorn et al.,3 coated catalysts have much higher effectiveness factors. As well, the most common catalysts used for CD are ion exchange resins which generally have temperature limits of 60-120 °C. In the case of * To whom correspondence should be addressed. E-mail: fttng@ cape.uwaterloo.ca. † Current address: Department of Chemical and Materials Engineering, University of Alberta, 114 St-89 Ave, Edmonton, Alberta, Canada T6G 2G6.
a coated catalyst, the temperature limit can be much higher (depending on the coating) so that CD can be applied to a greater number of reactions. Finally, the use of coated distillation packings will provide much more efficient distillation within the reaction zone of the CD column, possibly leading to improvements in conversion and selectivity for systems limited by mass transfer. Attempts have been made by various investigators to coat structured packings with a catalytically active layer. Oudshoorn et al.3 coated a structured packing with a zeolite coating, Mehrabani et al.4 coated a structured distillation packing with cation-exchange resin, and Beers et al.5 coated monoliths and metal gauze packings with a BEA zeolitic coating. Unfortunately, none of these investigators reported data regarding the performance of these catalysts in a distillation column. A system that stands to benefit from the application of CD is the production of the solvent diacetone alcohol (DAA) via the aldol condensation of acetone1 (Ac), as shown in eq 1. 2Ac T DAA T MO + H2O 6,7
(1)
In addition to being equilibrium limited, DAA is susceptible to dehydration to form the byproduct mesityl oxide (MO) (eq 1). The DAA and MO products can also be further condensed to higher molecular weight nonvolatile products such as isophorone and mesitylene which lead to deactivation of solid catalysts.8,9 Podrebarac et al.10,11 applied CD to the production of DAA using the hydroxide form of a macroporous anionic exchange resin (Amberlite IRA-900). The resin beads were contained within a fiberglass bag surrounded by wire mesh (a catalyst bale), resulting in severe mass transfer limitations within the reaction zone. These mass transfer limitations were due to both external resistances (the catalyst bale) and intraparticle resistance (the catalyst pores), resulting in reduced reaction rates, rapid deactivation of the catalyst, and poor selectivity to DAA. On the basis of these results, it appears that the production of DAA via CD could benefit greatly from the use of a coated distillation packing instead of packing ion exchange resins in fiberglass bags. Many solid base catalysts for the aldol condensation of acetone have been studied in the literature. Zhang et al.12 studied
10.1021/ie800009u CCC: $40.75 2008 American Chemical Society Published on Web 11/05/2008
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9305
Figure 1. Photographs of the alligator clips used for suspending the saddles for dip-coating.
the performance of alkaline earth metal oxides (including MgO) and showed that these basic solids are active for the aldol condensation reaction. Hydrotalcites and layered double hydroxides in various forms have also been shown to be active for this reaction.13-17 MgO was chosen to be the catalytic material in this study since several methods for applying thin films of magnesium oxide (MgO) to ceramic and/or glass substrates18-22 were reported although none of these studies were of a catalytic nature. Ceramic distillation saddles should have similar surface properties as the glass and ceramic substrates, and therefore these methods were chosen to coat the saddles. This paper reports a new method for the preparation of a coated ceramic saddle catalyst for use as a packing in a CD process. Particular attention is given to the methods necessary for applying the precursor coating to the saddle as well as the successful conversion of this precursor coating to a catalytically active form. The performance of these coated catalytic saddles for the aldol condensation of acetone was determined in both a batch reactor and a CD column. The results obtained in a CD column are significant since this is first report on the use of a coated catalytic distillation packing in an actual CD column. These data provide insight into the performance of such a coated catalyst in a CD column for the aldol condensation of acetone. 2. Experimental Section 2.1. Neutralization of Reagent Acetone. ACS grade acetone contains up to 0.0003 mequiv/g of titratable acid,23 which is a combination of dissolved carbon dioxide (0.000 23 mequiv/g) and acetic acid (0.000 033 mequiv/g).24 Because the catalysts used in this study are basic, this acidity must be removed from the reagent acetone to avoid neutralization of the catalyst sites. The reagent acetone was treated using Amberlite IRA-900 ion exchange resin in the hydroxide form. The dried resin was added to bottles of HPLC acetone (EMD Chemicals Inc. #AX01151) and agitated for 45-60 min. The treated acetone was decanted through a vacuum filter and distilled. The water content of the neutralized and distilled acetone was 0.295 wt % measured using Karl Fischer titration (Mitsubishi model CA-06 coulometric KF titrator), and the DAA content was 10 min were not used in the regression analysis to avoid incorporation of data with potential catalyst deactivation.
The initial rate of reaction for these five catalysts is plotted as a function of the mass of catalyst coating in Figure 3b. The mass of coating was determined after the batch reactor test was completed. As a matter of reference, the mass of catalyst was estimated from the mass of precalcined coating assuming that the initial coating was either pure Mg(OAc)2 or pure Mg(OAc)2 · 4H2O. This data is also included in Figure 3b. With the exception of the value for 50 coats, the measured values correspond very closely to those for Mg(OAc)2 · 4H2O, which indicates that the original coating is reacting with atmospheric moisture leading to the hydrate form. Also worth noting is that the catalyst mass for the catalyst with 50 coats is higher than both estimates (which is not possible) and therefore is questionable. The specific activity of the magnesium oxide coating was obtained by regressing the data (excluding the data point for 50 coats) to obtain a value of 178 mmol/(L min gMgO). The regression line was extrapolated to illustrate that the estimated mass of MgO (based on Mg(OAc)2 · 4H2O) for the catalyst with 50 coats lies very close to the line obtained for the other data. Therefore, any further analysis will use this estimated value rather than the (apparently erroneous) measured value for the mass of MgO on the catalyst with 50 coats. The drawback of a thicker catalyst layer is the potential for increased diffusional resistance as a result of longer pores. The impact of pore diffusion on the rate of reaction is usually quantified using the effectiveness factor, η: η)
rate of reaction with pore diffusion limitations intrinsic rate of reaction
(5)
In general, η for an nth-order irreversible reaction is a function of the dimensionless Thiele modulus, φ:29 φ)
V S
n-1 n + 1 (kmcat)CA 2 De
(6)
where V ) volume of catalyst (m3), S ) external surface area of catalyst (m2), n ) the order of the reaction (2 in this case), k ) the intrinsic rate constant for the reaction in the absence of diffusion (mol1-n/m3(1-n) min gcat), mcat ) mass of catalyst (gcat), CA ) external concentration of reactant A (mol/m3), and De ) the effective diffusivity of the reactant/product in the pores of the catalyst (m2/min). Although the functional relationship between η and φ is complex, particularly for the complex shape of the distillation saddles used in the present work, it is still possible to examine the rate data to determine whether or not pore diffusion is significant. In the case where pore diffusional resistance is negligible, η f 1 and the observed rate of reaction should remain constant:29 (robs)2 k2 η2 ) ) )1 η1 (robs)1 k1
(7)
On the other hand, when there is strong pore diffusion resistance, η f 1/φ and the ratio of observed reaction rates is inversely proportional to the ratio of the Thiele moduli:29 φ1 η2 ) η1 φ2
(8)
The volume of MgO on the surface of the saddle is given by mcat/FMgO, where mcat is the mass of MgO coating on the saddles. For the thin coatings present on the saddles, the external surface area, S, will remain relatively constant. Finally, since CA and
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9309
De are constant, then, for the case of strong pore diffusional resistance, eqs 6 and 8 combine to give
( )()
φ1 mcat,1 η2 ) ) η1 φ2 mcat,2
3/2
k1 k2
no. of coats
MgO mass (g)
k1,0 × 1000 (L/(mol min gcat))
η/η20 ) k/k20
φ/φ20 (eq 9)
10 20 30 40 50
0.0190 0.0179 0.0318 0.0667 0.1907a
0.273 0.844 0.803 1.206 1.066
0.32 1.00 0.95 1.43 1.26
1.61 1.00 0.43 0.12 0.03
1/2
(9)
Therefore, if there is significant pore diffusion resistance, the ratio of effectiveness factors should be inversely proportional to the mass of catalyst coating on the saddles. The aldol condensation of acetone (eq 1) is second order in acetone concentration (CAc) while the reverse reaction is first order in DAA concentration (CDAA), yielding the following rate expression:6 rDAA )
Table 2. Effect of Coating Mass on Catalyst Effectiveness Factor
1 dCDAA ) k1CAc2 - k2CDAA mcat dt
(10)
where k1 ) the rate constant for the forward reaction (L/(mol min gcat)), k2 ) rate constant for the reverse reaction (min-1 gcat-1), and mcat ) mass of catalyst (gcat). At the start of a batch run (t ) 0), the concentration of DAA is zero, and therefore the second term in eq 10 disappears. Therefore, using the initial rate of reaction allows eq 6 to be used even though the aldol condensation reaction is reversible. The initial rate and the mass of catalyst can be used together with the initial acetone concentration (12.91 mol/L at 54 °C) to calculate the initial rate constant (k1,0) for the forward reaction. The values of k1,0, as well values of η/η20 and φ20/φ (i.e., the values for 20 coats were used as the baseline) are summarized in Table 2. Both η and φ were referenced to the values for the catalyst with 20 coats since the initial reaction rate for the catalyst with 10 coats seems artificially low. If there were significant diffusional resistances within the pores of the MgO coating, then eq 9 would apply and the ratio of the effectiveness factors would decline rapidly according to the values calculated in the last column in Table 2. However, the ratio of the effectiveness factors remains relatively constant with a value around one, indicating that eq 7 applies and that pore diffusion has very little effect on the rate of reaction with the coated catalyst. This analysis was performed assuming an irreversible reaction (eq 6) when it was stated earlier that the aldol condensation reaction is reversible (eq 1). However, in the case of a reversible reaction, diffusional resistances within the pores would result in lower effectiveness factors than the irreversible case, and therefore the effects would be even more pronounced. The analysis to this point does not give any indication about the durability of the coating. Although the catalyst produced with 50 coats had the highest yield, its integrity was questionable. A large portion of the coating layer flaked off upon addition to the reactor. A similar effect was seen for the catalyst produced with 40 coats, although to a lesser degree. This effect was likely due to aging of the coating solution as the coating process proceeded. Toward the end of the coating process, the coating solution became much thicker and more viscous due to evaporation and general consumption of the coating solution. This caused more coating to be deposited per dip, which in turn led to a large excess of coating; particularly for the last batch (i.e., 50 coats). The catalysts produced with 30 coats or less did not exhibit this flaking behavior and remained intact even after 7 h of use in the batch reactor. Therefore, the optimum number of coats in the batch reactor was 30 coats at a withdrawal speed of 1.25 mm/s. This recipe was used for the remainder of this work to produce the coated saddles.
a
This is the catalyst mass estimated from the original mass of coating, assuming that the coating was Mg(OAc)2 · 4H2O.
3.3. Catalytic Performance of Coated Saddles. Once the procedure for producing the coated saddles (i.e., 30 coats) was finalized, the catalytic performance of the saddles was studied further. The performance of the coated saddles in a batch reactor (92 saddles, 30 coats) is shown in Figure 4. The number of saddles loaded to the batch reactor was doubled to 92 saddles (vs 46 saddles in the previous section) in order to reduce the time required for completing the experiment. Also, these experiments were performed using neutralized acetone (see section 2.1) to reduce the effects of deactivation. The data from one of the batch reactor experiments of Podrebarac et al.6,30 for the strongly basic macroporous anion exchange resin Amberlite IRA-900 is included in Figure 4 for comparison. The resin catalyst used by Podrebarac et al.6,30 exhibits a higher conversion of acetone, although the majority of the increased conversion arises from the production of byproduct MO rather than DAA. Since the dehydration of DAA to MO is acid catalyzed, the use of un-neutralized acetone (i.e., acetone containing trace acetic acid) could neutralize some of the basic sites on the resin, and the trace acetic acid could potentially increase the conversion of DAA to MO. In any case, the resin catalyst does reach the equilibrium conversion to DAA much faster than the coated saddles, implying a higher activity. In an effort to compare the activities of these two catalysts, the initial rates of the reaction are summarized in Table 3. The turnover frequency (TOF) for each catalyst is also included in Table 3. The number of basic sites present on the coated saddles was estimated assuming a basic site density of 0.15 mequiv/g as measured by Matsuda et al.31 for MgO derived from Mg(OAc)2. This method of calculating TOF assumes that all sites present on the catalyst are of equal activity for the aldol condensation reaction. The TOF for the coated saddles are 2 orders of magnitude higher than those for the resin catalyst, suggesting stronger basic sites on the MgO coating than on the ion exchange resins. The higher initial reaction rate of the resin catalysts is therefore likely due to a much larger number of basic sites present in the reactor. Since the ion exchange resins are not stable beyond 60 °C, it is not possible to use temperatureprogrammed desorption of CO2 to compare the distribution and strength of the basic sites of the coated saddles and the resins. Therefore, further discussions on the effect of basic site strength and density on the relative rates of these two different types of catalysts are not warranted. In terms of the selectivity of the catalyst toward DAA, Figure 4b indicates that the coated saddles are significantly more selective to DAA than the resin catalysts. The most likely reason for this difference in selectivity is the difference in the porosity of the two catalysts, although the acidity in the un-neutralized acetone used by Podrebarac et al.6 may result in increased MO production. According to Podrebarac et al.,6 diffusional limitations cause the concentration of DAA within the pores of the resin catalyst to remain at the equilibrium value even if the bulk concentration is not. This leads to increased production of MO since the active sites within the pore are exposed to a higher concentration of DAA. The coated saddles, on the other hand,
9310 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 4. Comparison of the acetone conversion and DAA selectivity of the coated saddles (92 coated saddles, 30 coats, neutralized acetone, Ar atmosphere) and the resin catalyst of Podrebarac et al.6,30 (Amberlite IRA 900 macroporous resin, 2 mL resin, 0.49 mmol H+/mL Resin, un-neutralized acetone, air atmosphere) at 54 °C. Table 3. Comparison of the Initial Rate of DAA Production and the Turnover Frequency (TOF) for the Coated Saddles and the Resin Catalyst Used by Podrebarac et al.6,30 (Batch Reactor, 100 mL Acetone at 54 °C) initial rate catalyst
(mmol/ L · min)
(mmol/ (L min gcat))
no. of sitesa (mequiv)
TOFb (min-1)
current work (92 saddles, 30 coats) 23b (0.0640 g MgO) 23c (0.0539 g MgO) 24a (0.0940 g MgO) 24b (0.0563 g MgO)
17.5 21.6 28.5 16.0
273 401 303 284
0.0096 0.0081 0.0141 0.0084
182 268 202 189
resin catalyst6,30 (Amberlite IRA 900, 2 mL resin, 0.49 mmol/mL) expt 5 expt 6 expt 7
61.7 60.8 60.0
0.98 0.98 0.98
6.29 6.21 6.12
a
The number of base sites for the coated saddles was estimated assuming a basic site density of 0.15 mequiv/g as measured by Matsuda et al.31 for Mg(OAc)2 derived MgO. b TOF ) turnover frequency based on the initial rate.
do not experience such a phenomenon since as previously discussed (section 3.2), there are essentially no diffusional limitations within the pores of the coated saddles. Concentrations of DAA within the pores will not likely increase to the same extent as in the resin catalyst resulting in improved selectivity toward DAA with the coated saddles. 3.4. Batch CD Column Experiments. The performance of the coated saddles was measured in a batch CD column at three different reboiler settings: 55, 70, and 85 V. For each experiment, a fresh batch of coated saddles was produced. As anticipated, the temperature in the reboiler rises steadily throughout the experiment as a result of increasing concentrations of DAA and MO. The temperatures within the reaction zone did not vary with time by more than 0.2 °C for any of the runs, indicating a pseudo-steady state within the column despite the composition changes occurring in the reboiler. The results of the first experiment at 85 V are shown in Figure 5. The mass of both DAA and MO in the reboiler increases linearly with time over the 12 h of the experiment. Both data sets were regressed (with a forced intercept of 0), and the results of the regressions are included in Figure 5. The slopes obtained from these regressions give the productivity (in g/h) of each of the two products. This same analysis was repeated for all batch CD experiments, including those of Podrebarac et al.,10,30 and
Figure 5. Mass of DAA and MO in the reboiler as a function of time for CD run 1 (reboiler set at 85 V, 230 saddles with 30 coats, 500 mL of acetone charged to the reboiler).
the results and operating conditions for each run are summarized in Table 4. The productivity (in g/h) remained constant over the first 12 h of each experiment, and therefore these can be taken as pseudo-steady-state values. These pseudo-steady-state values were converted to conversion (X), yield (Y), and selectivity (S)27 per unit time (see eqs 2-4), and the results are shown in Figure 6. 3.4.1. DAA Productivity. According to the data in Table 4 and Figure 6, the coated saddles have higher absolute production rates of DAA at lower flow rates within the column. It is only once the flow in the column exceeds 13 g/min that the yield of DAA from the resin catalysts exceeds that produced from the coated saddles. This is in stark contrast to the results observed with the batch reactor, where the resin catalysts significantly out produced the coated saddles. In an attempt to understand the observed productivity of these two catalysts in the CD column, the TOF obtained in the CD column was divided by the mean TOF obtained in the batch reactor studies (see section 3.3), and these results are included in Table 4. Neither catalyst achieved the same TOF as was observed in the batch reactor. This suggests external mass transfer is a limiting factor in the CD column at the flow rates used in the CD runs. This is not surprising since external mass transfer resistances were eliminated in the batch reactor experiments by providing adequate mixing. The effect of the external mass transfer in the CD column is much more pronounced in
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9311 Table 4. Summary of CD Operating Data and Productivities for the Coated Saddles and Ion Exchange Resin6,30 Catalysts productivity (g/h) reboiler setting (V)
reflux flow (g/min)
catalyst volume (mL)
reaction zone volume (mL)
DAA
MO
no. of sitesa (mequiv)
coated catalyst (230 saddles, 30 coats)
55
5.59
43.7
43.7
6.09
0.18
0.0211
resin catalyst6,30
70 85 85 55 65 75 85
9.01 13.3 13.3 5.49 8.76 12.27 16.36
43.7 43.7 43.7 20.5 21.8 21.4 23.3
43.7 43.7 43.7 55.7 55.7 55.7 55.7
12.3 12.0 11.4 2.22 5.05 13.2 35.9
0.23 0.15 0.24 2.94 4.01 5.81 7.54
0.0211 0.0211 0.0211 10.0 10.7 10.5 11.4
catalyst
TOF (min-1)
TOFCD/ TOFbatchb (%)
41.4
19.7
83.7 81.7 77.6 0.0317 0.0678 0.181 0.451
39.9 38.9 36.9 0.5 1.1 2.9 7.3
a The number of sites for the coated saddles was estimated using the mean mass of catalyst/saddle from Table 3 as well as the assumed 0.15 mequiv/ g basic site density.31 b The mean value of the TOF values for the batch reactor was used to make this calculation.
Figure 6. Yield and selectivity of DAA for the coated saddles and ion exchange resins6,30 in a batch CD column as a function of reflux flow.
the case of the resin catalysts as indicated by the much lower yield of DAA and the lower percentage of the maximum TOF obtained in the CD column. This effect is magnified at lower flow rates in the CD column where the mass transfer rates are reduced. The primary goal of synthesizing the coated saddles was to improve external mass transfer within the reaction zone of a CD column. These results indicate that indeed the coated catalytic saddles improved the external mass transfer in the reaction zone, leading to improved reaction rates. It should also be noted that at higher flow rates the performance of the coated saddles reaches a plateau. For flow rates above 9.0 g/min, neither the yield of DAA nor the TOF increases. This suggests that the external mass transfer has reached its maximum and that the reaction rates are kinetically limited rather than mass transfer limited. However, despite being kinetically controlled, the TOF obtained within the CD column is still much lower than that observed in the batch reactor. Possible explanations for this phenomenon include liquid distribution and the wall effect in the CD column in addition to the differences in the hydrodynamics between the batch reactor and the CD column. According to Porter and Templeman,32 for pilot and laboratory scale columns, a significant portion of the total flow in the column will be along the wall region rather than on the packing. In this study, the ratio of column diameter (1 in.) to saddle size (1/4 in.) is 4:1, which according to Porter and Templeman32 would result in >70% of the total flow along the wall region (i.e., not flowing over the catalyst). If a larger
proportion of the total flow were to contact the catalyst, then one would expect the productivity to increase, resulting in an increase in the TOF. This could be one of the reasons for the lower TOF observed for the coated saddles in the CD column than those in the batch reactor despite being kinetically controlled. It should also be noted that the fiberglass bags used by Podrebarac et al. are the first generation of physical structures used for loading catalyst particles such as ion exchange resins into a CD column. Newer packings such as structured catalytic distillation packings overcome some of the external mass transfer limitations presented by the fiberglass bags. Therefore, such next generation packing structures would likely result in increased production of DAA at lower flow rates, although to what extent is unknown. 3.4.2. DAA Selectivity. The selectivity toward DAA for both the resin catalysts and the coated saddles is also shown in Figure 6. Both catalysts show decreased selectivity to DAA in the CD column relative to a batch reactor. In the case of the coated saddles, the selectivity in the batch reactor exceeded 99.5% for conversions below the equilibrium conversion to DAA. However, in the CD column the selectivity ranged from 96.6% to 98.5%. In the case of the resin catalysts the selectivity in the CD column ranged from 38.9% to 80.1%, compared to 91.5%-94.4% in the batch reactor at equilibrium conversion to DAA. Both catalysts showed increases in selectivity with higher flow rates in the column, indicating that the selectivity is sensitive to changes in mass transfer. Even for the coated saddles, the selectivity increases as the flow rate in the column increases albeit to a lesser degree. The improved mass transfer characteristics of the coated saddles resulted in selectivities much closer to those obtained in the batch reactor. Therefore, the enhanced mass transfer characteristics of the coated saddles compared to the resin catalyst held in fiberglass bags do lead to significant improvements in selectivity to DAA in the CD experiments. 3.4.3. Deactivation. As indicated previously, deactivation is a major concern for any basic catalyst used for the aldol condensation of acetone. Results of the batch CD experiments for the coated saddles, however, showed very little sign of deactivation over the first 12 h of operation as indicated by the linearity of both the DAA and MO production data. In order to assess the stability of the coated saddles, two of the four batch CD experiments were carried out for 24 h (one run at 85 V and the run at 55 V). The results of these extended runs are shown in Figure 7. The regression lines obtained using the first 12 h of data were extended to determine whether the productivity remained constant after 24 h. In both cases, the productivity of MO did not remain constant over 24 h of operation, indicating some deactivation. As for
9312 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 7. DAA and MO yield with coated saddles after 24 h in a batch CD column.
the productivity of DAA, the run at 55 V remained constant while the run at 85 V showed a slight decline after 24 h. Therefore, it appears slight deactivation is occurring in the CD column based on the very limited data. Podrebarac et al.10 also observed a similar decline in MO productivity with time, and they attribute this to the formation of higher molecular weight products due to further condensation reactions involving MO on the catalyst surface.9 The fact that MO is a precursor to the deactivation could explain why the effects of deactivation are more prevalent in the formation of MO than DAA. However, more experiments at much longer reaction time are required before any conclusions could be made regarding the apparent deactivation. Another major concern when using the coated saddles is the potential for the MgO coating to erode and leach from the saddle substrate leading to catalyst in the reboiler. As mentioned previously, the aldol condensation of acetone is an equilibriumlimited reaction. At 54 °C, the equilibrium concentration of DAA is 4.3 wt %.6 During the batch CD experiments, the temperature in the reboiler ranged between 55.2 and 63.5 °C. At these temperatures, the equilibrium concentration of DAA should be less than 4.3 wt %. However, the concentration of DAA in the reboiler exceeded this equilibrium value after only 2 h for experiments at 85 V and after 3 h at 55 V. At this point, any possible MgO catalyst in the reboiler will catalyze the reverse reaction (i.e., the decomposition of DAA to form acetone). As the concentration of DAA in the reboiler increases, the rate of this reverse reaction would increase. If there were significant amounts of catalyst in the reboiler due to leaching from the coated saddles, the decomposition of DAA would result in a decrease in DAA productivity rather than the observed increase in DAA concentration. The fact that the productivity of DAA continued for 24 h indicates that very little (if any) of the coating layer was removed from the saddles over the course of the experiment. As well, no solids were observed in the reboiler when the experiment was terminated, further supporting the claim that the coating did not leach off during the course of the experiment. Whether or not the coating could remain intact under our CD experimental conditions for periods significantly longer than 24 h is unknown.
sol-gel dip coating method followed by calcination to produce a catalytically active MgO layer. The initial rate of production of DAA increases proportionally with the mass of coating, and hence there is incentive to deposit as much coating mass as possible. However, the durability of the coating degenerates with an increase in the mass of coating, resulting in a tradeoff between yield and durability. The durability of the coating is also significantly affected by the calcination method used and requires slow temperature ramping during both heating and cooling to ensure adequate structural integrity of the catalytic coating. The initial rate of DAA production of the coated catalyst saddles in a batch reactor was significantly lower than that for the ion-exchange resin catalyst used by Podrebarac et al.6 but with a much higher selectivity to DAA. The coated saddles are much more active than the ion exchange resins based on calculated TOF. The higher rates exhibited by the resins likely resulted from the much higher concentrations of basic sites on the resins. Despite having much lower concentrations of basic sites, the coated saddles performed very well in a CD column. The coated saddles do not experience the severe mass transfer limitations that were encountered by the resin catalyst held in fiberglass bags. At lower flow rates in the CD column, the coated catalysts produce more DAA than the resin catalysts held in the fiberglass bags. At higher column flow rates, however, the resin catalysts held within fiberglass bags benefit from improved mass transfer, resulting in a higher production of DAA than the coated saddles. The coated saddles however provided a higher selectivity to DAA than the resin catalysts in the CD experiments due to the reduced external mass transfer resistance and the intraparticle diffusional limitations of the resins. Therefore, coated catalytic saddles would be particularly advantageous for reaction systems where high selectivity is required. Slight deactivation of the coated saddles apparently occurred in the CD experiments when operated for extended periods (i.e., 24 h) while the productivity of both DAA and MO over a 12 h period remained constant. Erosion of the coated catalyst did not occur over the 24 h of CD experiments. Acknowledgment
4. Conclusions A method has been developed for the production of catalytically active coated ceramic distillation saddles for use in CD. Mg(OAc)2 was deposited onto ceramic Norton saddles using a
The authors acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) in the form of research grants and a scholarship for Greg Dechaine.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9313
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ReceiVed for reView January 2, 2008 ReVised manuscript receiVed September 14, 2008 Accepted September 23, 2008 IE800009U
