Ind. Eng. Chem. Res. 2000, 39, 3233-3240
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Dealuminated H-Y Zeolites: Influence of the Number and Type of Acid Sites on the Catalytic Activity for Isopropanol Dehydration Costas S. Triantafillidis and Nicholaos P. Evmiridis* Department of Chemistry, University of Ioannina, Ioannina 45 110, Greece
The effect of the number and type of acid sites of H-Y catalysts dealuminated by different techniques on the dehydration of 2-propanol has been studied in this work. The increase in the number of acid sites generally increases the activity for the formation of both main products, propene and diisopropyl ether. The presence of different types of extraframework phases generated by steaming or treatment by SiCl4 or by (NH4)2SiF6 treatment affects the activity and selectivity in different ways. The yield of propene is always higher than that of ether but shows a minimum at 95 °C in the catalysts with few acid sites and Si-Al amorphous phases formed by dealumination with SiCl4. Highly crystalline catalysts with relatively high acidity (parent or dealuminated by (NH4)2SiF6 or steaming/EDTA) give propene yields higher than ether’s only at temperatures higher than 95 °C. The presence of extraframework Al generated by relatively low-temperature steaming reduces the catalyst activity for both products. A statistical analysis of the turnover frequency at different temperatures showed that all acid sites of the crystalline zeolitic phase participate in the formation of both products, while the catalytic sites of the amorphous phase formed by SiCl4 contribute at relatively low temperatures with propene as the main product. A common compensation effect for the formation of both products exists for all the tested crystalline dealuminated H-Y catalysts. Introduction The catalytic activity of dealuminated H-Y zeolites in acid-catalyzed reactions is dependent both on the framework Si/Al ratio (i.e., the framework aluminum content) and on the presence of extraframework molecular species or amorphous phases, which are usually formed during dealumination and consist of Si, Al, or both. Each dealumination method (i.e., steaming,1,2 or treatment with SiCl42-6 or (NH4)2SiF67-10) has a different effect on the framework and extraframework composition of the resulting zeolitic catalyst. How extraframework Al (EFAl) affects the catalytic activity is still not clear, although three possibilities exist.11 First, the EFAl species could interact with the framework Al (FAl) and increase the strength of the Bronsted acid site, which is related to the FAl atom by stabilizing the negative charge of the skeleton. Second, the EFAl species could itself be a catalytic site, probably acting as a Lewis acid site, and adsorb reactant molecules. Third, the EFAl species could promote the reaction by increasing the rate at which a protonated (on the framework Bronsted acid site) reactant molecule reacts to form products. In addition, it seems reasonable to try to explain the effect of EFAl species by considering the phases that are involved and, further, their interaction with other coexisting phases, for example, silica. To address these possibilities, we investigated a series of dealuminated H-Y zeolites using a number of techniques related to their acid catalytic activity. The samples were prepared by different dealumination methods (steaming, steaming-EDTA, (NH4)2SiF6, and SiCl4) and were characterized for crystallinity and microporosity, for the content of bulk and framework * To whom correspondence should be addressed. Tel.: 0030-651-98404. Fax: 00-30-651-44831. E-mail: ktrianta@ cc.uoi.gr.
Al, and for the composition of the amorphous phases formed during dealumination. These data were used to interpret ammonia-TPD tests and assess the number, distribution, and origin of acid sites. These data have been extensively described in a previous work.12 In the present work, variously dealuminated H-Y samples, with 6-58 Al atoms/unit cell have been tested for their acidic activity in the dehydration of isopropanol. Many previous studies have focused on the reaction mechanism for dehydration of small alcohol molecules as typical acid-catalyzed reactions on X, Y and on ZSM-5 type acid zeolites.3,6,13-19 However, the data for the effect of the number and type of acid sites in dealuminated H-Y zeolites on the activity and product selectivity for dehydration of isopropanol is limited to Y zeolites with low dealumination. Furthermore, the effect of the different types of amorphous phases and extraframework species, formed by different dealumination techniques, on the activity of the framework Bronsted acid sites is not yet clear. The present work shows that dehydration activity increases with the total number of acid sites; however, product selectivity differs between the samples that consist mainly of the zeolitic crystalline phase and those that also possess an amorphous aluminosilicate phase, formed during treatment with SiCl4. Analysis of the turnover frequency (TOF) values of the samples supported the above conclusions. Experimental Section Catalyst Preparation, Structure Characterization, and Acidity Measurements. The parent zeolite was either NH4NaY (Linde LZ-Y62, Si/Al ) 2.48 and Na2O ) 2.00%) or NaY (Linde LZ-Y52, Si/Al ) 2.66 and Na2O ) 9.45%) and was dealuminated by different procedures.12 The methods used were as follows: (i) hydrothermal treatment, with or without subsequent treatment with EDTA, (ii) treatment with ammonium
10.1021/ie000002s CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000
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Table 1. Physicochemical Characteristics of Catalyst Samplesa aluminum contentb (mg-atoms/g) ((0.05)
Si/Alc
sample
treatment
QFAl
QTAl
QEFAl
Si/AlF
Si/AlT
Si/AlEF
SSAd (m2/g)
crystallinitye (%)
HY(NH4Y) DY-4 DY-8 DY-10 DY-6 DY-7
NH4 exchange (NH4)2SiF6 steamed, 600 °C DY-8, EDTA-treated SiCl4 SiCl4
4.95 2.11 2.44 2.50 0.59 0.42
5.12 2.36 4.56 2.83 2.70 1.10
0.17 0.25 2.12 0.33 2.11 0.68
2.3 5.9 5.4 5.6 16.7 32.2
2.4 6.2 2.8 4.9 5.3 14.5
f 8.3
867 627 808 839 723 697
98 88 94 100 63 83
2.1 3.7
a Data taken from ref 12. b Q FAl, QTAl, QEFAl: mg-atoms of framework Al, of total Al, and of extraframework Al, respectively, per g of dehydrated catalyst calculated from the original values in Table 1, ref 12, which refer to hydrated samples. c Si/AlF, Si/AlT, Si/AlEF: framework Si/Al ratio, total Si/Al ratio, and Si/Al ratio of the extraframework phase, respectively. d Multipoint BET specific surface area. e Relative crystallinity of the samples based on the (533) peak and consideration of the parent zeolites as 100% crystalline. f Samples NH4Y, DY-8, and DY-10 have practically very small or no amount of extraframework Si phase.
hexafluorosilicate (AHFS), and (iii) treatment with gaseous SiCl4. The experimental conditions were varied to produce samples with different degrees of framework dealumination. Conventional chemical analyses of the zeolite samples were done using gravimetric, titrimetric, and photometric methods. The zeolites were characterized for relative crystallinity by comparing the (533) XRD peak height of the dealuminated samples with those of the parent NaY or NH4NaY zeolites, which were considered to be 100% crystalline. The number of framework Al atoms were estimated using XRD, mid-infrared (IR), and highresolution solid-state 29Si MAS NMR spectra. The coordination of Al atoms in the different phases of the samples was determined using high-resolution solidstate 27Al MAS NMR spectra. The specific surface area (SSA) and porosity characteristics were determined by isothermal nitrogen adsorption/desorption at 77 K. The number, strength, and distribution of acid sites were tested using temperature-programmed-desorption (TPD) of ammonia. Adsorption of ammonia was done at 100 °C on the samples, which had been previously carefully dehydrated at 430 °C for 3 h under a flow of He. Prior to desorption, the samples were flushed with He to remove physically adsorbed ammonia. The desorbed ammonia was detected on a thermal conductivity detector (TCD) and then it was determined titrimetrically using standard HCl aqueous solution. A detailed description of the characterization techniques has been reported.12 Catalytic Experiments. The activity for catalytic dehydration of isopropranol (IPA) of the parent zeolite HY (NH4Y) and a number of dealuminated samples covering a broad range of framework Si/Al ratios was tested in a flow system at steady-state conditions. The reaction took place in a bench-scale reactor, similar to that previously described.20,21 The reactor consisted of a silica tube 1 cm in diameter with a sealed-in quartz bed onto which 0.2 g of the zeolite catalyst was placed. The system was heated in a tubular furnace to within (0.5 °C. Analyses of reactants and products were carried out by sampling of 1 cm3 of the gases in a Shimadzu GC-15A gas chromatograph equipped with a thermal conductivity detector and connected to a Chromatopac C-R6A integrator. The column used for analysis was 2 m × 1/8 in. s.s., 10% Carbowax 20 M on Chromosorb W-HP, and 80-100 mesh. Helium was used as the carrier gas in the gas chromatograph. A second line fed He (40 ( 1 mL/min) through a saturator containing isopropanol, which was thermostated at 20 °C, to the reactor. The partial pressure of isopropanol was calculated to be 32.82 mmHg and was kept the
Table 2. Acidic Characteristics of Catalyst Samples (NH3-TPD Technique)
sample HY (NH4Y) DY-4 DY-8 DY-10 DY-6 DY-7
number of acid sitesa density of acid sitesb (mmol of NH3/g) (molecules of NH3 ((0.05) × 1017/m2) total weak medium strong ((0.5) 4.95 2.86 2.75 2.61 1.53 0.38
1.06 0.29 0.42 0.39 0.22 0.04
1.51 0.65 0.83 0.79 0.34 0.07
2.38 1.92 1.50 1.43 0.97 0.27
34.4 27.5 20.6 18.8 12.8 3.2
a Expressed as mmol of NH /g of dehydrated sample calculated 3 from the original values in Table 4, ref 12, which refer to hydrated samples. b Calculated from the number of acid sites/g of dehydrated sample and the SSA of the samples.
same in all catalytic tests. Activity measurements were taken randomly in the range of 70-130 °C in 5 °C intervals after lining out first for 30 min. The catalyst activation was performed in situ at 430 °C under He flow (50 mL/min) for 3 h. Results Structural and Acidic Characteristics of the H-Y Zeolite Catalysts. Six catalysts12 prepared using a variety of dealumination conditions and representing a broad range of acidity were used. Detailed compositional and acid sites data for the six catalysts have been reported;12 however, some critical data are also contained in Tables 1 and 2. Table 1 contains the contents of the framework Al (FAl), of the total Al (TAl), and of the extraframework Al (EFAl) in mg-atoms/g of dehydrated catalyst, the Si/Al ratios of the framework, of the bulk, and of the extraframework phase, the specific surface area (SSA), and the relative crystallinity (XRD) of the samples. The number, density, and distribution of acid sites are shown in Table 2. The most important characteristics of the catalysts that may have an effect on their activity for the dehydration of isopropanol are the following:12 (i) The samples NH4Y, DY-4, DY-8, and DY-10 are highly crystalline with little amorphous phase. NH4Y and DY-10 are practically free of extraframework phase while DY-4 has a silicon-rich phase and DY-8 has a phase consisting mainly of octahedral Al. (ii) The samples DY-6 and DY-7 have a significant amount of amorphous material, which consists of both Si and Al (with Si/Al ) 2-4) in a more or less polymerized form. The formation of this phase is typical of the dealumination method used for the preparation of these samples, that is, dealumination with SiCl4.
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Figure 1. Reaction rates versus temperature for (a) conversion of IPA, (b) propene formation, and (c) diisopropyl ether formation: (O) parent HY (NH4-Y), ([) DY-4, (4) DY-8, (+) DY-10, (0) DY-6, and (9) DY-7.
(iii) The acid sites of the samples with no extraframework phases (NH4Y, DY-10) are the hydroxyls connected to FAl atoms (Bronsted acidity); samples DY-4 and DY-8 may possess additional acid sites, the former due to the extraframework Si species (i.e., of type Si-OH) and the latter due to the EFAl species (i.e., positively charged hydroxy-oxidic species); the samples DY-6 and DY-7 may also possess acid sites in addition to the framework -OH, due to their amorphous Si-Al phases, which may give rise to Bronsted or Lewis type acid sites, especially after the thermal treatment before the catalytic tests. Catalytic Activity Parameters for the Dehydration of Isopropanol at Various Temperatures. Conversion of isopropanol at low temperatures produces both propene and diisopropyl ether. Increasing the temperature increases the selectivity to propene up to the highest temperatures where ether formation ceases. The catalytic activity of the various zeolite catalysts for the conversion of isopropanol (IPA) to propene and diisopropyl ether is presented in Figure 1, in the form of reaction rates versus temperature. The rates are calculated as moles s-1 m-2 of total IPA converted (Rtotal, Figure 1a), as IPA converted to propene (Rprop, Figure 1b), and as IPA converted to ether (Rether, Figure 1c).20,21 Figure 1 shows that Rtotal and Rprop increase with reaction temperature up to the highest temperature
Figure 2. Arrhenius plots for (a) conversion of IPA, (b) propene formation, and (c) diisopropyl ether formation: (O) parent HY (NH4-Y), ([) DY-4, (4) DY-8, (+) DY-10, (0) DY-6, and (9) DY-7.
tested, while Rether reaches a maximum and then falls to zero again. The above rate maximum decreases significantly and shifts to higher temperatures with a decreasing number of total acid sites of the zeolite. To calculate the apparent activation energies (Eapp) for the conversion of isopropanol (Etotal) to propene (Eprop) and ether (Eether), the relevant Arrhenius plots were made. The experimentally determined rates in the range of 1%-25% conversion of IPA for isopropanol decomposition (Rtotal) and for propene and ether production, (Rprop) and (Rether), were used for the Arrhenius plots. The straight lines obtained had very good correlation coefficients (r > 0.99), as shown in Figure 2. The Eapp values calculated from the slopes of the straight lines of Figure 2 as well as the intercept values that correspond to the logarithm of the pre-exponential factor in the Arrhenius equation (ln A) and are proportional to the entropy of activation are both given in Table 3 for the catalysts tested. Acid Sites and Catalytic Activity at Various Temperatures. General. Reaction rates for the conversion of IPA are plotted versus the total number of acid sites obtained from ammonia-TPD experiments, expressed as mmol of desorbed ammonia/g of dehydrated catalyst (Figure 3a) and as molecules of desorbed
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Table 3. Kinetic Parameters from the Arrhenius Plots for Conversion of Isopropanol to Propene and Ether apparent activation energy (Eapp) (kJ/mol)
logarithm of the pre-exponential factor (ln A)
sample
conversion of IPA
propene production
ether production
conversion of IPA
propene production
ether production
HY (NH4Y) DY-4 DY-8 DY-10 DY-6 DY-7
121 105 123 107 113 107
126 115 136 121 110 113
117 103 117 98 117 113
27.09 21.51 27.15 22.07 23.24 20.09
27.96 23.90 30.38 25.71 21.73 21.49
25.57 20.41 24.92 19.04 23.68 20.96
Figure 4. (a) (%) Yield of propene and (b) yield ratio (propene/ diisopropyl ether) versus total number of acid sites (ammonia TPD) at (O) 70 °C, (0) 90 °C, (9) 95 °C, and (4) 100 °C. Figure 3. Reaction rates versus total number of acid sites (ammonia TPD) expressed as (a) mmol of NH3/g of catalyst and (b) molecules of NH3/m2 of surface area, for the conversion of IPA at (O) 70 °C, ([) 80 °C, (0) 85 °C, (9) 95 °C, and (4) 100 °C.
ammonia/m2 of surface area (Figure 3b). The plots were made for reaction temperatures from 70 to 100 °C. It can be seen that the conversion of IPA increases smoothly as the total acidity increases. We attribute the scatter in the data to the different types and strengths of the acid sites present in the H-Y catalysts tested. The relationship between the activity (rates) and the number of acid sites is more linear at lower conversion (i.e., low reaction temperatures) or at lower acidities. TOFs. Turnover frequencies for each catalyst at different reaction temperatures have been calculated by dividing the rates of conversion of IPA and of the formation of propene and ether by (a) the total acidity (number of total acid sites), (b) the strong acidity (number of strong acid sites), and (c) the sum of the weak and medium strength acidity (sum of the number of weak and medium strength acid sites). The results are tabulated in Table 1-A (Appendix). Acid Sites and Selectivity at Various Temperatures. The effect of the number of acid sites on the yield
and selectivity for propene, at various temperatures, is shown in Figure 4a,b, respectively. Both graphs show that the catalyst samples can be separated into two groups. The first group (i.e., group A) consists of the DY-6 and DY-7 samples, which have low acidities and significant amorphous material. The second (i.e., group B) consists of the remaining four “crystalline” samples that have relatively high acidity. On one hand, the yield and selectivity to propene for the samples in group B increases only slightly with the increase in acidity and rises monotonically with reaction temperature. The yield of propene is around 20%-30% at 70 °C and increases to ≈70% at 100 °C. On the other hand, the yield of propene for the two SiCl4-treated samples in group A is less affected by temperature and decreases slightly as the number of acid sites increases. Propene as a proportion of the product decreases from ≈70%80% at 70 °C to ≈55% at 95 °C and increases to ≈60%65% at 100 °C. It never drops below 55%. Diisopropyl ether is the other principle product and its selectivity behavior complements that of propene. The dependence of product yield on reaction temperature is also presented in Figure 5. Here, the products’ yields of the catalysts with the two extreme cases of total
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3237
Figure 5. (%) Yield of ([) propene and (4) diisopropyl ether versus reaction temperature of (a) DY-7 and (b) parent NH4Y.
acidity (the SiCl4-treated sample DY-7 and the parent NH4Y) are plotted as a function of temperature. The yield of propene shows a minimum at 95 °C with DY-7, but it is always higher than the yield of ether (Figure 5a). However, with the highest acidity sample parent NH4Y, the propene yield increases monotonically with temperature from 70 °C up to the highest temperature tested and only above 95 °C does it become higher than the yield of ether (Figure 5b). However, in both cases, it is clear that above 95 °C the production of propene is strongly favored over that of ether. These results indicate that propene becomes the thermodynamically favored product at higher temperatures.22 Acid Sites and Activation Energy. In Figure 6, Eapp ((5 kJ/mol) for the conversion of IPA and the production of propene and ether are plotted versus the total number of acid sites. Figure 6 shows that the moderately dealuminated catalysts DY-4 and DY-10 (≈50% framework dealumination) have the most active sites (lower activation energy) for the production of ether, suggesting that the framework Bronsted acid sites are the principle active sites for ether formation. The activation energy for ether formation in the catalysts DY-6, DY-7, and DY-8 is relatively high (115 kJ/mol), and the effect of the type of EFAl species present in these catalysts on the activation energy is the same (Figure 6c). In the case of propene production, DY-6 and DY-7 have the lowest activation energies while DY-8 has the highest (Figure 6b). This suggests, on one hand, that the samples with low framework Bronsted acidity and Si-Al amorphous phases formed by the SiCl4 method have active sites for propene formation that have a somewhat lower activation energy than the framework Bronsted acid sites of the moderately dealuminated DY-4 and DY-10 catalysts. On the other hand, DY-8 contains EFAl molecular species formed by the relatively low-temperature steam-
Figure 6. Apparent activation energy versus total number of acid sites (ammonia TPD): (a) conversion of IPA, (b) propene formation, and (c) diisopropyl ether formation.
ing procedure and has a higher activation energy for sites in the crystalline phase (compare with DY-4 and DY-10). Discussion Mechanistic Considerations. When a series of dealuminated H-Y zeolitic catalysts are tested for isopropanol dehydration, the scatter of the TOF for each catalyst around the mean TOF for all catalysts can be used as a measure of the dissimilarity of the acid sites, with regard to their type and/or their acidity strength. In addition, statistical significance tests can quantify the probability that the catalysts may be separated in groups based on the similarity of their TOFs. The selectivity, S, of the formation of the specific products can be described by the following equation (1), which refers to the selectivity for propene over ether upon isopropanol dehydration:
S(product propene) ) yield of propene/ yield of ether ) Rpropene/Rether ) constant × f(T) (1)
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Table 4. Mean TOF (Rate/Number of Acid Sites) Values for the Set of the Six Catalysts at Different Reaction Temperatures for the Conversion of Isopropanol (IPA) and the Formation of Propene and Ether IPA conversion
propene formation
T (°C)
meana R/TAS
meanb R/SAS
meanc R/WMAS
70 80 85 95 100
0.081 0.226 0.398 1.028 1.500
0.136 0.387 0.688 1.752 2.534
0.214 0.579 1.007 2.660 3.930
a
mean R/TAS
mean R/SAS
diisopropyl ether formation
mean R/WMAS
mean R/TAS
mean R/SAS
mean R/WMAS
0.097 0.229 0.412 1.361 2.611
0.047 0.142 0.243 0.506 0.501
0.082 0.248 0.426 0.866 0.843
0.117 0.350 0.595 1.296 0.081
Group of the Six Catalysts 0.034 0.054 0.084 0.138 0.155 0.261 0.521 0.885 0.999 1.692
R/TAS ) rate/total acid sites. b R/SAS ) rate/strong acid sites. c R/WMAS ) rate/(weak + medium) acid sites.
Table 5. Relative Standard Deviation (RSD) Values of the TOF (Rate/Number of Acid Sites) Values at Different Reaction Temperatures IPA conversion
propene formation
T (°C)
RSDa R/TAS (%)
RSDb R/SAS (%)
RSDc R/WMAS (%)
70 80 85 95 100
25.9 18.6 24.6 17.2 14.3
27.9 27.1 34.7 24.5 18.0
38.8 23.1 21.2 24.5 27.3
70 80 85 95 100
28.9 10.8 16.0 15.9 17.4
27.7 13.2 21.7 17.4 15.9
a
RSD R/TAS (%)
RSD R/SAS (%)
diisopropyl ether formation
RSD R/WMAS (%)
RSD R/TAS (%)
RSD R/SAS (%)
RSD R/WMAS (%)
84.5 52.8 37.6 27.3 28.5
55.3 42.9 45.7 23.9 16.6
59.8 49.6 52.9 31.4 17.3
51.3 38.8 38.5 24.3 30.0
Group of the Four Crystalline Catalysts (NH4Y, DY-4, DY-8, DY-10) 39.6 26.0 17.5 46.3 35.0 27.4 25.7 25.6 43.7 14.5 23.1 19.4 26.4 30.0 22.2 26.8 14.0 15.2 29.4 21.4 30.3 17.4 14.2 32.1 19.7
35.0 17.4 26.0 23.1 20.5
40.5 24.7 25.3 27.2 27.3
Group of the Six Catalysts 62.0 46.3 32.1 23.2 18.7 20.7 13.6 20.0 17.2 21.0
R/TAS ) rate/total acid sites. b R/SAS ) rate/strong acid sites. c R/WMAS ) rate/(weak + medium) acid sites.
The parameter of selectivity when measured for different catalysts at constant temperature, provided that the catalytic sites that contribute to the formation of all products in the overall reaction are of the same nature and type in all the catalysts, must be constant. This is a conclusive parameter and in combination with the scatter of TOF values may give evidence for the types of sites involved in the catalyst. Additionally, the activation energy for catalysts with the same type of catalytic sites should be the same. If the sites are distributed on different matrix material and have different acid strengths,23 they should follow the compensation rule. All the catalysts tested in the present work are more than ≈65% crystalline (Table 1); therefore, the contribution of the acid sites in the crystalline zeolitic phase to the rates will be the major one; the contribution of acid sites in the amorphous phase or any other type of sites will scatter the TOF, depending on the ability of these sites to modify the activity of the acid sites in the crystalline phase. Additionally, the data indicate that the active sites for the formation of ether do not form ether at higher temperatures. These sites at higher temperatures may produce propene. Effect of the Type of Acid Sites on Product Selectivity. Figure 4 separates the samples into lowacidity group A and high-acidity group B. The selectivity to propene for the samples in group B remains constant. The selectivity to propene for the samples in group A differs within the group and from the samples of group B, especially at temperatures e90 °C. This suggests that more than one type of catalytic acid sites in different proportions are involved in the samples of group A. Furthermore, the temperature dependence of the selectivity in group A is different from that in group B,
suggesting that the different types of catalytic sites among the two groups of catalysts have different activation energies for propene formation. On one hand, if the two SiCl4-treated catalysts of group A have an aluminosilicate amorphous phase (mainly Lewis acidity) and a crystalline zeolitic phase (Bronsted acidity), while the four samples of group B have nearly totally crystalline zeolitic phase, the high selectivity for propene formation at relatively low temperatures can be attributed to acid sites in the amorphous phase formed by SiCl4. On the other hand, the almost constant selectivity of the catalysts of group B at temperatures within the useful range indicates the existence of similar acid sites that are associated with acid sites in the crystalline zeolitic framework. Statistic Analysis of TOF ValuessEffect of the Amorphous Phase on Activity. The mean TOF values at each temperature for the entire set of the six catalysts as calculated on the basis of the total acid sites, the strong acid sites, and the sum of the weak and medium strength acid sites, for the conversion of isopropanol and the formation of propene and ether, are shown in Table 4. The scatter of the TOF values about the mean value, expressed as percentage of relative standard deviation (% RSD), is estimated on the basis of the six catalyst data for each temperature and the results are given in Table 5. The % RSD values for the conversion of isopropanol are relatively smaller for the TOFs based on the total sites compared to the TOFs based on either the strong or weak plus medium strength acid sites. The same procedure has been followed for the rate data of propene formation, and the lowest scatter is obtained when the TOFs are based on total acid sites at relatively high temperatures, that is, g85 °C, and when they are based
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Figure 7. Compensation effect for the formation of ([) propene (line 1) and (4) ether (line 2).
on strong acid sites at lower temperatures. Finally, the TOFs of ether formation showed less scatter about the mean value when they were based on the weak plus medium strength acid sites for temperatures e95 °C. The scatter, however, becomes smaller as the temperature increases in nearly all cases. The scatter of the TOF values for the four crystalline catalysts only (group B), for the conversion of IPA and the formation of propene and ether (% RSD values in Table 5), is lower when the TOFs are based on the total acid sites or strong acid sites (for all temperatures tested), suggesting that the total acidity or strong acidity is a better measure for the number of catalytic sites than the sum of the weak and medium strength acid sites. The scatter of TOF values (based on the six catalysts) in Table 5 indicates that isopropanol conversion best correlates with the total acidity and formation of propene with the total acidity or the strong acidity while ether formation correlates best with the sum of weak and medium strength acid sites. This change in the scatter between the crystalline zeolite catalysts (group B) and the entire set of six catalysts indicates that the amorphous phase of samples DY-6 and DY-7 (group A) affects the catalytic activity of the acid sites in the crystalline zeolitic framework. However, we believe that not just the strong acid sites but also the total acid sites catalyze propene formation because the same sites that catalyze ether formation, catalyze propene formation at higher temperatures. This argument implies that in highly crystalline catalysts the active sites are the total acid sites measured by TPD of NH3. A significance test between the TOFs of the group of the crystalline catalysts and the TOFs of the group of the SiCl4-treated catalysts revealed that the effect of the amorphous Si-Al phase is significant at relatively low reaction temperatures, that is, e85 °C, where the TOFs for propene production are increased by the amorphous phase active sites, while the TOFs for ether production are decreased. Activation Energy and Compensation Effect. The difference in the Eapp for propene formation (Figure 6b) between the SiCl4-treated catalysts (DY-6 and DY7) and the steamed catalyst (DY-8) indicates that the extraframework phase formed by different dealumination methods has a different effect on the activity of the catalysts. A compensation effect has been observed in the set of catalysts used in this work, upon plotting of the Eapp versus ln A values.23 The mean, parallel straight lines in Figure 7 are for propene formation (line
1) and for ether formation (line 2) by the four crystalline catalysts. The correlation coefficients (>0.99) suggest the existence of a common compensation effect for both reaction routes for these four crystalline catalysts. The compensation effect arises from surface heterogeneity of the catalysts,23 which suggests possible configurational or entropy of activation differences between crystalline and amorphous phase sites. Our acidity results (Table 2) and the data in the literature23,24 show that the parent NH4Y zeolite sample with a high total acidity has a lower proportion of strong acidity than the moderately dealuminated Y samples, that is, DY-4 and DY-10. Consequently, the apparent activation energy values of DY-4 and DY-10 are lower than the values of NH4Y, as is discussed above (Figure 6), resulting in the observed compensation effect. The low-temperature steamed sample DY-8, which contains EFAl species with little amorphous phase, has selectivity profiles similar to the purely crystalline samples, and the plots in Figure 7 show that it fits well within the crystalline group of catalysts. The experimental points in Figure 7 that correspond to DY-6 and DY-7 samples (which possess amorphous material in addition to the crystalline phase) show a small deviation from the mean lines of the crystalline samples, and this can be attributed to the different types of acid sites that are involved in the amorphous phase of these samples.
Conclusion The data in the present study show that the activity of isopropanol dehydration increases monotonically with the number of total acid sites measured by ammonia for a wide range of framework Al content in dealuminated H-Y zeolites. However, amorphous phases that exist in some catalysts affect the activity for both propene and ether formation. Different behaviors in activity and product yield selectivity is found for the nearly crystalline catalysts, which have been dealuminated by AHFS and steaming/EDTA, and the catalysts that possess an amorphous phase formed by the SiCl4 dealumination. The yield of propene in the latter group of catalysts (SiCl4) shows a minimum at 95 °C, but it is always higher than that of ether, while in the former group the yield of propene is higher than that of ether only at temperatures higher than 95 °C. The EFAl molecular species formed by the relatively low temperature steaming inhibits the reactivity of the acid sites of the crystalline zeolitic structure for IPA conversion and increases the apparent activation energy for the formation of both propene and ether. On one hand, the Si-Al amorphous phase formed by the SiCl4 method imposes low apparent activation energy for the formation of propene. On the other hand, this amorphous phase possesses sites for ether formation with higher activation energy than the ones in the moderately dealuminated type-Y crystalline zeolitic phase. A common compensation effect exists for all the dealuminated H-Y catalysts for both the formation of propene and the formation of ether. Appendix See Table 1-A.
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Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000
Table 1-A. TOF for All the Catalysts at Different Reaction Temperatures TOF (molecules of IPA × 10-3/(site s)) based on total acid sites
TOF (molecules of IPA × 10-3/(site s)) based on strong acid sites
reaction temperature (°C)
TOF (molecules of IPA × 10-3/(site s)) based on weak + medium acid sites
reaction temperature (°C)
sample
70
80
85
95
100
70
NH4Y DY-4 DY-8 DY-10 DY-6 DY-7
0.067 0.092 0.060 0.112 0.061 0.093
0.237 0.266 0.217 0.276 0.176 0.182
0.444 0.430 0.375 0.549 0.290 0.297
0.940 1.101 1.005 1.337 0.819 0.963
1.177 1.601 1.514 1.820 1.386 1.503
0.139 0.137 0.109 0.205 0.095 0.130
NH4Y DY-4 DY-8 DY-10 DY-6 DY-7
0.021 0.030 0.016 0.023 0.040 0.072
0.079 0.090 0.050 0.059 0.102 0.122
0.172 0.157 0.107 0.141 0.161 0.191
0.507 0.583 0.451 0.621 0.442 0.523
NH4Y DY-4 DY-8 DY-10 DY-6 DY-7
0.046 0.062 0.044 0.089 0.021 0.021
0.158 0.175 0.167 0.216 0.074 0.061
0.273 0.273 0.269 0.408 0.128 0.105
0.433 0.518 0.552 0.716 0.377 0.437
80
85
reaction temperature (°C)
95
100
70
80
85
95
100
1.953 1.641 1.836 2.448 1.284 1.348
2.444 2.386 2.767 3.332 2.173 2.104
0.129 0.280 0.132 0.247 0.168 0.326
0.457 0.808 0.479 0.608 0.485 0.638
0.857 1.308 0.829 1.210 0.800 1.040
1.813 3.347 2.219 2.948 2.261 3.370
2.269 4.867 3.344 4.013 3.826 5.260
0.818 1.145 1.009 1.238 0.811 0.974
Formation of Propene 0.045 0.164 0.357 1.053 0.045 0.134 0.235 0.869 0.029 0.092 0.195 0.824 0.042 0.109 0.259 1.137 0.063 0.159 0.253 0.693 0.101 0.170 0.268 0.732
1.698 1.706 1.845 2.266 1.272 1.364
0.041 0.091 0.035 0.050 0.111 0.252
0.153 0.273 0.111 0.131 0.280 0.426
0.332 0.479 0.235 0.311 0.446 0.670
0.978 1.773 0.996 1.369 1.220 1.830
1.577 3.480 2.229 2.729 2.239 3.410
0.359 0.456 0.509 0.582 0.572 0.529
0.095 0.092 0.080 0.164 0.032 0.030
Formation of Ether 0.328 0.566 0.261 0.407 0.304 0.491 0.396 0.746 0.116 0.201 0.085 0.147
0.746 0.680 0.931 1.066 0.896 0.740
0.088 0.188 0.097 0.197 0.057 0.075
0.304 0.533 0.368 0.477 0.205 0.212
0.526 0.831 0.594 0.899 0.354 0.368
0.835 1.573 1.219 1.579 1.041 1.530
0.692 1.387 1.125 1.284 1.578 1.850
Conversion of IPA 0.492 0.923 0.396 0.641 0.397 0.686 0.505 1.005 0.275 0.454 0.255 0.416
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0.899 0.771 1.009 1.311 0.591 0.612
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Received for review January 9, 2000 Revised manuscript received April 26, 2000 Accepted April 29, 2000 IE000002S
