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Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates
Applied Catalysis A: General 290 (2005) 54–64 www.elsevier.com/locate/apcata
Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates Teruoki Tago a,*, Yoshihito Okubo b, Shin R. Mukai b, Tsunehiro Tanaka c, Takao Masuda a a
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8, Sapporo 060-8628, Japan b Division of Chemical Engineering, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan c Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan Received 24 January 2005; received in revised form 22 April 2005; accepted 16 May 2005 Available online 27 June 2005
Abstract We propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the coadsorption of NH3 and CO2 on catalysts. First CO2 was adsorbed on the catalyst sample; then NH3 was adsorbed on it. Another adsorption sequence of NH3 and CO2, and CO2 and NH3 single adsorptions were also conducted. The TPD measurements were carried out by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min1 in a helium stream under a total pressure of 1.3 kPa. In solid acid catalysts, there is little difference in the NH3-TPD spectra between single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the number of base sites in the solid acid catalyst is very small. In contrast, in a solid acid–base catalyst of alumina, a remarkable difference in the NH3-TPD spectra was observed between single adsorption and co-adsorption systems. The difference in the TPD spectra between single and co-adsorption systems was ascribed to a strong induction effect appearing on the acid and base sites, which was proved by an in situ IR measurement. The validity of the TPD method was examined by correlating the number of the strong acid sites to catalytic activities of dehydrolysis of ethanol over solid acid and solid acid–base catalysts. In solid acid–base catalysts, the number of strong acid sites was calculated from the activation energy distribution for the desorption of NH3 in a co-adsorption system because of the strong induction effect. A proportional relationship between the intrinsic reaction rate constant, which is based on the concentration of ethanol within the catalyst, and the number of strong acid sites could be obtained, regardless of the catalysts or their types or pore structure. # 2005 Elsevier B.V. All rights reserved. Keywords: Temperature-programmed desorption; Acidic and basic properties; Solid acid and base catalyst; Ethanol dehydrolysis; Ammonia and carbon dioxide
1. Introduction A series of active sites with different strengths are distributed over solid catalysts. These distributions are closely related to catalytic activity and selectivity. In order to * Corresponding author. Tel.: +81 11 706 6550; fax: +81 11 706 6550. E-mail address: [email protected] (T. Tago). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.05.010
design catalysts with high activity and selectivity, the properties of active sites should be accurately measured. There have been several studies on the measurement of the active site properties of solid catalysts. The acidic and basic properties of solid catalysts are usually measured by a method utilizing the temperature-programmed desorption spectra of ammonia (NH3-TPD) [1–14] and carbon dioxide (CO2-TPD) [15–18], respectively. Since only the acidic or
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a CA E Ein f(E)
FC G H k kint q QA Rg t T xA
heating rate (K min1) ethanol concentration in gas phase (mol m3) activation energy for desorption of probe molecules (J mol1) activation energy at inflection point of the F(E, T) function (J mol1) density distribution of the activation energy for desorption of probe molecules (mol2 kg J1) production rate of ethylene (mol s1) number of acid sites (mmol kg1) partial factor (–) reaction rate constant (m3 s1 kg1) intrinsic reaction rate constant (m3 s1 kg1) total number of probe molecules retained on acid or base sites at T (mol kg1) ethanol concentration within catalyst (mol m3) gas constant (8.314 J mol1 K1) time (s) desorption temperature (K) ethanol conversion (–)
Greek letters a constant appearing in Eq. (A.4) (s1) b constant appearing in Eq. (A.4) (mol J1) F function defined by Eq. (A.4)
basic properties of a solid catalyst can be measured by these conventional TPD methods employing a single probe molecule, such as NH3 and CO2, it is difficult to simultaneously measure the acidic and basic properties of solid acid–base catalysts. In a catalytic reaction over solid acid–base catalysts, an induction effect between the acid and base sites will appear, leading to enhancement of the strength of the acidic and basic properties. Accordingly, a simultaneous measurement of the acidic and basic properties of solid catalysts is required to obtain accurate information on catalysts. We have also developed a new TPD method for measuring the NH3-TPD spectra of solid acid catalysts under desorption control conditions, denoted as the dc-TPD method [12]. Since the TPD experiments are conducted in a helium stream under reduced pressure in the method, NH3 pre-adsorbed on the non-acidic and weaker acidic surfaces of the catalysts is desorbed at the beginning of the experiment. Furthermore, NH3 molecules desorbed from the catalyst can be immediately removed from the catalyst particles by a helium stream. Thus, the re-adsorption of NH3 desorbed from the catalyst and the resistance to mass transfer inside the catalyst particles are decreased until they are negligible. Accordingly, although the TPD spectra in a conventional TPD method showed two peaks, the spectra
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measured in the dc-TPD method exhibited only one peak, corresponding to the desorption from strong acid sites. The basic properties of solid catalysts will also be measured using CO2 as a probe molecule in the dc-TPD method. Moreover, this method can be applied to simultaneously measure the desorption rates of NH3 and CO2 that are adsorbed on solid catalysts in advance. The NH3- and CO2TPD spectra thus obtained will allow us to accurately evaluate the acidic and basic properties of solid catalysts by taking into account an induction effect between the acid sites and the base sites. The main objective of this work is to propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the coadsorption of NH3 and CO2 on catalysts. Furthermore, the validity of the TPD method in the NH3 and CO2 coadsorption system will be examined by correlating the properties of active sites with the catalytic activities of the dehydrolysis of ethanol over solid acid catalysts and solid acid–base catalysts.
2. Experimental procedure 2.1. Catalyst Three kinds of zeolites (MFI-type, Y-type and Beta-type) and silica–alumina were employed as solid acid catalysts. Two kinds of alumina and one zirconia were used as solid acid–base catalysts. These catalysts were supplied by the Catalysis Society of Japan (Table 1). Deactivated alumina was prepared by aging the alumina (JRC-ALO4) in air at 1073 K for 1 h (de-ALO). An MFI-type zeolite (protontype) with a SiO2/Al2O3 ratio of 40 (MFI40) was hydrothermally synthesized by using sodium silicate, aluminum sulfate and tetrapropyl-n-ammonium bromide as a template for 72 h at 473 K [19], followed by an ionexchange technique with NH4NO3 water solution and by calcination in an air stream at 723 K for 1 h. The prepared MFI-type zeolites were confirmed to have the same structure as a pentasil type zeolite by X-ray diffraction analysis (Shimadzu, XRD-610). All the catalysts were pelletized without any binders, crushed, and sieved to yield samples with a particle size of 0.18–0.41 mm. Table 1 Catalysts used in this work Catalysts
Sample
SiO2/Al2O3 (–)
MFI-type zeolite MFI-type zeolite HY-type zeolite H Beta-type zeolite Silica–alumina
– JRC-Z5-70H JRC-Z-HY4.8 JRC-Z-HB25 JRC-ASH1
40 70 4.8 25 4.2
MFI40 MFI70 HY4.8 BEA25 SAH
Activated alumina Deactivated alumina Zirconia
JRC-ALO4 – JRC-ZRO1
– – –
ALO de-ALO ZRO
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2.2. Measurements of acidic and basic properties of catalyst by TPD method Fig. 1 schematically shows the apparatus used in the experiment, which was linked with a quadruple mass spectrometer, Q-Mass (ANELVA, M-QA200TS). The chamber was made of stainless steel, and the adsorption column with an electric furnace was set inside it. The inner diameter of the chamber was 63.5 mm, which was long enough compared to the mean free path of probe molecules to avoid the effect of Knudsen diffusion under reduced pressures. It was verified in preliminary experiments that these conditions were achieved [12,20]. The desorbed NH3 and CO2 molecules in helium could be detected by the spectrometer connected with the chamber by an orifice 50 mm in diameter. A catalyst sample of approximately 0.3 g was loaded on a stainless steel net (200 mesh) in an adsorption column, evacuated for 2 h at 873 K, and then cooled to 373 K. The temperature in the column was controlled using a thermocouple located just beneath the catalyst bed. First, NH3 was adsorbed on the catalyst sample at 373 K for 30 min under a pressure of 3.3 kPa, and NH3 adsorbed on the non-acidic and weaker acidic surfaces was desorbed and flushed out at 373 K for 2 h in a helium stream flowing at 20 cm3 min1 under a total pressure of 1.3 kPa. Second, CO2 was adsorbed on it, followed by evacuation at a total pressure of 1.3 kPa in a helium stream (denoted as NH3–CO2 co-adsorption). Another adsorption sequence of CO2 and NH3 was also conducted (denoted as CO2–NH3 coadsorption). The TPD measurements were conducted by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min1 in a helium stream flowing at
20 cm3 min1 under a total pressure of 1.3 kPa. The desorption rates of CO2 and NH3 were simultaneously measured using a gas mass filter. The mass numbers of NH3, CO2 and He were chosen as 15, 44 and 4, respectively, to detect NH3, CO2 and He. By using the flow rate of He and the relative signals of NH3 and CO2 to He, the desorption rates of NH3 and CO2 were obtained. The activation energy distributions for the desorption of NH3 and CO2 were calculated from the TPD spectra thus obtained. The details are described in Appendix A. 2.3. In situ FT-IR analysis In order to identify the adsorption species on acid and base sites, we also measured the in situ FT-IR spectra by use of an FT-IR apparatus (Perkin-Elmer, IR spectrometer Paragon 1000) with wafers of the solid catalysts without any binders [21,22]. After NH3 molecules were adsorbed on the catalyst at 373 K under reduced pressure, an in situ FT-IR measurement (NH3 single adsorption) was carried out. Secondary CO2 molecules were adsorbed on the catalyst on which NH3 molecules were adsorbed in advance, and an in situ FT-IR measurement (NH3–CO2 co-adsorption) was then performed. Another sequence of adsorption, CO2 to NH3, was also conducted (CO2 single adsorption and CO2–NH3 co-adsorption). 2.4. Reaction of ethanol over solid acid catalysts and solid acid–base catalysts The dehydrolysis of ethanol to produce ethylene over the solid catalysts was conducted as a model reaction. The
Fig. 1. Apparatus for measuring TPD spectra employing single and binary probe molecules.
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reaction was carried out using a fixed bed reactor under differential reaction conditions (the conversion of ethanol was less than 5.0%). The time factor (the ratio of the mass of the catalyst to the mass flow rate of the feed) was 0.021– 0.063 kgcat kg1 feed h, and the reaction temperature was 523 K. An isothermal region of 5 cm in length was obtained at the center of the reactor, where the catalysts weighing 50– 150 mg were loaded. Ethanol, which was fed by a microfeeder, was evaporated in an evaporator and introduced to the catalyst bed with a nitrogen flow. The flow rate of the nitrogen gas was 300 cm3 min1. The reaction was continued for 1 h after reaching a steady state. The reactant and products were analyzed by on-line gas chromatography. Since one molecule of ethanol on an acid site is considered to convert to an ethylene molecule by a dehydrolysis reaction, the dehydrolysis rate of ethanol was assumed to be of the first order with respect to the ethanol concentration. Accordingly, the first-order reaction rate constant, k (m3 s1 kg1), which was used as an index for catalytic activity over acid sites, was calculated from the conversion of ethanol, xA, and the production rate of ethylene, F C (mol s1), by the following equation: DFC xA ¼ kCA ¼ kCA;average ffi kCA0 1 (1) Dw 2
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adsorption isotherm exhibited a similar tendency [24]. Accordingly, the partial factor calculated from the adsorption isotherm of benzene was substituted for the partial factor of ethanol. The experimental procedure and H value were described in detail in our previous paper [25]. The reaction rate expressed by r = kCA (Eq. (1)) is also represented by r = kintQA, where kint is the intrinsic reaction rate constant based on the concentration of ethanol within the catalysts. This kint value is directly correlated to the number of acid sites, and can be expressed by the following equation: CA k (3) kint ¼ k ¼ H QA
3. Results and discussion 3.1. TPD spectra employing single and binary probe molecules
H¼
Fig. 2 shows the typical signals of NH3, CO2 and He in the NH3 and CO2 co-adsorption system for alumina (ALO). The ratios of the signals of NH3 and CO2 to that of He were calibrated in advance by use of gases with known compositions. Accordingly, the ratios of signals and the flow rate of He gave the TPD spectra of NH3 and CO2. Fig. 3(a) and (b) shows the TPD spectra of NH3 and CO2 in a single adsorption system with the dc-TPD method for several kinds of solid acid catalysts and solid acid–base catalysts, respectively. The amounts of NH3 and CO2 molecules desorbed from the catalysts are listed in Table 2. Though the amount of desorbed CO2 was much smaller than the amount of desorbed NH3, it was found that CO2 molecules were desorbed on the solid acid catalysts at a high temperature above 500 K, as shown in Fig. 3(a). This result indicates that the solid acid catalysts possess base sites, of which the number is approximately in the range of 1/50–1/ 100 of the number of acid sites. However, in the solid acid– base catalysts of alumina (ALO) and zirconia (ZRO) as shown in Fig. 3(b), the number of acid sites was smaller than those of the solid acid catalysts. However, the amount of CO2 desorbed from the solid acid–base catalysts was
where QA (mol m3) is the concentration of ethanol within the catalysts and CA (mol m3) the concentration of ethanol in the gas phase. Though the adsorption isotherm of ethanol at a reaction temperature of 523 K was required to calculate the ethanol partial factor H, it was difficult to measure the partial factor, because the dehydrolysis of ethanol proceeded at 523 K over the acid sites of the catalysts. However, the partial factors of benzene and propane calculated from their
Fig. 2. Example of changes in mass numbers and temperature with time.
where CA (mol m3) is the average concentration of ethanol in the gas phase and w (kg) the mass of the catalyst. However, since the ethanol was adsorbed on the surface of the catalysts, it was considered that the concentration of ethanol within the catalysts was higher than that of ethanol in the gas phase. Moreover, the pore structure and surface area of the catalysts used in this study were dependent on the types of catalysts, leading to different adsorption properties of ethanol to the catalysts. Accordingly, in order to investigate the relationship between the acidity and activity of the catalysts, we required the reaction rate constants based on the concentration within the catalysts. For this reason, the partial factor, H, was introduced, which represented the ratio of the concentration of reactant in catalyst particles to that in the gas phase, and was evaluated from the adsorption isotherms [23]. concentration of adsobed molecules within catalyst concentration in gas phase QA ¼ CA (2)
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Fig. 3. TPD spectra of NH3 and CO2 for several kinds of solid acid catalysts (a) and solid acid–base catalysts (b) in single adsorption system with dcTPD method.
approximately half of the amount of NH3 desorbed from these catalysts, which was much larger than that of the solid acid catalysts. These results indicate that the base sites exist on the solid acid–base catalysts in the same order as the acid sites. Figs. 4 and 5 show the NH3-TPD spectra in NH3 single adsorption and the NH3 and CO2 co-adsorption system with the dc-TPD method for solid acid and solid acid–base catalysts, respectively. A series of NH3-TPD spectra obtained for these catalysts by raising the beginning temperature by 25 K is also shown in these figures. The difference between adjoining spectra was considered to represent the desorption spectra from acid sites with a uniform acid strength, which was used to calculate the positive constants of a and b described in Appendix A. In the solid acid catalysts shown in Fig. 4, there is little difference in the NH3-TPD spectra between the single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the Table 2 Amounts of NH3 and CO2 desorbed from solid acid and solid acid–base catalysts in a single adsorption system NH3 (mmol kg1)
CO2 (mmol kg1)
ALO de-ALO ZRO
113 106 17
60 59 1.5
MFI40 BEA25 HY4.8
562 446 945
8.5 7.5 Trace
Fig. 4. NH3-TPD spectra for MFI-type zeolite (MFI40, (a)), Beta-type zeolite (BEA25, (b)) and silica–alumina (SAH, (c)) in single and coadsorption systems with the dc-TPD method.
number of base sites in the solid acid catalyst is very small, as listed in Table 2. Accordingly, the acidic properties of solid acid catalysts can be characterized by the dc-TPD method employing a single probe molecule of NH3. In contrast, in the solid acid–base catalyst of alumina (ALO) shown in Fig. 5, a remarkable difference in the NH3TPD spectra was observed between the single adsorption and co-adsorption systems. The TPD spectra in the coadsorption system were shifted to a high temperature region as compared with the spectra in the single adsorption system. Moreover, it was found that these TPD spectra depended on the adsorption sequences of the probe molecules of NH3 and CO2 on alumina, even though the probe molecules were adsorbed on the same catalyst. In the NH3–CO2 co-adsorption system shown in Fig. 5(a), the
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result, since the strong induction effect appears on the acid site of the solid acid–base catalyst in the CO2–NH3 coadsorption system, the CO2–NH3 co-adsorption system is an appropriate method for characterizing the acidic properties because it takes into account the induction effect. 3.2. In situ IR measurement
Fig. 5. NH3-TPD spectra for alumina (ALO) in single adsorption and coadsorption (NH3–CO2 (a) and CO2–NH3 (b)) systems with the dc-TPD method.
amount of NH3 desorbed from alumina was almost the same as that in a single adsorption system. In contrast, in the CO2– NH3 co-adsorption system shown in Fig. 5(b), the amount of NH3 desorbed from the catalyst was remarkably increased up to 3.7 times compared with the single adsorption system. The difference in TPD spectra between the single and coadsorption systems was ascribed to a strong induction effect appearing on the acid sites during the desorption of NH3. Since the CO2 molecules were adsorbed on the base sites, the strength of the acid sites was enhanced to shift the TPD spectra to a high desorption-temperature region. Moreover, the difference in the amount of the NH3 between NH3–CO2 and CO2–NH3 co-adsorption systems was also ascribed to the induction effect during the adsorption of NH3. In the NH3–CO2 co-adsorption system, after NH3 was adsorbed on the acid sites, CO2 was adsorbed on the base sites. Accordingly, the amount of NH3 adsorbing on alumina in the co-adsorption system was almost the same as that in the single adsorption system. In contrast, in the CO2–NH3 coadsorption system, after CO2 was adsorbed on the base sites, NH3 was adsorbed on the acid sites of alumina. The strength of the acid sites was enhanced by the induction effect of the adsorbed CO2 on the base sites, leading to a remarkable increase in the amount of NH3 adsorbing on alumina. Accordingly, there was much more NH3 desorbed from alumina in the CO2–NH3 co-adsorption system than in the NH3–CO2 co-adsorption and single adsorption systems. As a
As discussed above, the differences in NH3-TPD spectra between the single and co-adsorption systems were observed using the solid acid–base catalyst; this result was ascribed to the induction effect between the acid and base sites. However, it was considered that the coordinate adsorption of NH3 on CO2 molecules might cause an increase in the number of adsorbed NH3 on the solid acid–base catalysts. In order to clarify whether NH3 and CO2 molecules were adsorbed on the acid and base sites of the catalysts, respectively, or not, we measured in situ IR spectra using MFI-type zeolite (MFI40) and alumina (ALO) at 373 K under reduced pressure in the NH3 and CO2 single adsorption and co-adsorption systems. Fig. 6(a) shows the in situ IR spectra of NH3 and CO2 adsorbed on MFI-type zeolite (MFI40) in the single and coadsorption systems. The IR spectrum of CO2 adsorbed on MFI40 in the single adsorption system showed almost the same peaks as the fresh MFI40. This resulted from the presence of very few base sites on the solid acid catalysts shown in Fig. 3. Furthermore, the IR spectrum of NH3 in the single adsorption system was in good agreement with the spectra in the NH3–CO2 and CO2–NH3 co-adsorption systems. This resulted from the presence of only a negligibly small induction effect in the solid acid catalysts; this result supports the above-mentioned experimental results concerning TPD for solid acid catalysts shown in Fig. 4. Fig. 6(b) shows the in situ IR spectra of NH3 and CO2 adsorbed on alumina (ALO). In the single adsorption system, peaks at wave numbers around 1650, 1450–1480 and 1240 cm1 corresponded to the adsorbed CO2 on the base sites. The peaks around 1620 and 1270 cm1 corresponded to the adsorbed NH3 on the Lewis sites [26]. The peaks around 1700, 1470 and 1400 cm1 were assigned to the N–H bending modes corresponding to the adsorbed NH3 on the Brønsted sites [27]. Though the peak around 1650 cm1, which corresponded to the adsorbed CO2, was separated into two peaks around 1625 and 1575 cm1 in the NH3–CO2 coadsorption system, the other peaks in the NH3–CO2 and CO2– NH3 co-adsorption systems were in good agreement with the peaks in the NH3 and CO2 single adsorption methods. Furthermore, no appearance of new peaks due to the coadsorption of NH3 and CO2 was observed, indicating that the probe molecules of NH3 and CO2 were adsorbed on the acid and base sites, respectively, without any adsorption on the adsorption species of other probe molecules. Since the peak around 1620 cm1, which is corresponding to the asymmetric deformation vibration of the adsorbed NH3, has a strong intensity, it is possible to compare the
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Fig. 6. FT-IR spectra of NH3 and CO2 adsorbed on MFI-type zeolite (MFI40, (a)) and alumina (ALO, (b) and (c)) in single and co-adsorption systems.
amount of adsorbed NH3 in the single adsorption system with the amount in the co-adsorption system by using the area. Fig. 6(c) shows the peaks around 1620 cm1. The measured peaks around 1500–1700 cm1 are obtained as the sum of the series of elemental peaks of CO2 and NH3. To clarify these elemental peaks, we carried out the deconvolution of the peaks as shown in the figure. Compared with the area of the adsorbed NH3 in the single adsorption system, changes in the areas corresponding to the amount of adsorbed NH3 in the NH3–CO2 and CO2–NH3 co-adsorption systems were 0.9 times and 3.8 times, respectively. These values were in good agreement with the increases in the amount of NH3 desorbed from strong acid sites, which were measured by dc-TPD experiments.
3.3. Activation energy distributions in single and co-adsorption systems Fig. 7 shows the activation energy distributions for the desorption of NH3 and CO2 from solid catalysts calculated from the TPD spectra shown in Figs. 4 and 5 using Eqs. (A.7)–(A.10) with the values of a and b listed in Tables 3 and 4. In solid acid catalysts, the TPD spectra in the NH3 single adsorption system were employed to obtain the activation energy distributions (Fig. 7(a)), because little difference in the TPD spectra between single and coadsorption was observed. In contrast, in the solid acid–base catalyst of alumina, the TPD spectra in single and coadsorption systems were employed (Fig. 7(b) and (c)). In
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from that of the peak positions in the TPD spectra, which exhibited almost the same peak positions of MFI40, HY4.8 and BEA25. The same tendency was reported in our previous paper [12]. Accordingly, the activation energy distributions can give more accurate information concerning the acidic properties of solid catalysts as compared with the TPD spectra. In alumina as a solid acid–base catalyst, differences in the distribution curves between single and co-adsorption systems were clearly observed. The curves in the co-adsorption systems were shifted to a higher energy region, indicating that the numbers of acid sites and base sites with an activation energy above 100 kJ mol1 increased. This resulted from a strong induction effect appearing between the acid and base sites, as discussed in Section 3.1. In particular, in the CO2– NH3 co-adsorption system, the number of strong acid sites with an activation energy above 100 kJ mol1 greatly increased to a value, which was comparable to that of the solid acid catalysts shown in Fig. 7(a). As shown in these figures, the acid sites distributed on each catalyst have strength-distributions in the range of 50– 250 kJ mol1. Katada et al. have reported that the strong acid sites of solid acid catalysts possess an adsorption enthalpy of NH3 approximately above 100 kJ mol1 [8]. Moreover, Masuda et al. have reported a relationship between the activation energy for NH3 desorption and the adsorption enthalpy of NH3, in which the enthalpy above 100 kJ mol1 corresponds to the activation energy approximately above 100 kJ mol1 [13]. Accordingly, it can be concluded that the strong acid sites distributed on the catalysts possess activation energy values above 100 kJ mol1. Moreover, the number of acid sites with an activation energy above 100 kJ mol1 was denoted as the number of strong acid sites, Gm,100 kJ. 3.4. Dehydrolysis of ethanol over solid acid and solid acid–base catalysts Fig. 7. Activation energy distributions for the desorption of NH3 from solid acid catalysts (a) in a single adsorption system, and the distributions for desorption of NH3 and CO2 from alumina (ALO) as a solid acid–base catalyst in NH3–CO2 (b) and CO2–NH3 (c) co-adsorption systems with the dc-TPD method.
these figures, a large value of activation energy, E, corresponded to stronger acid and base sites. In the solid acid catalysts shown in Fig. 7(a), the acid strength followed the order from the peak position of f(E); SAH < HY4.8 < BEA25 < MFI40. The order was different Table 3 Parameters used in Eq. (A.2) for solid acid catalysts in an NH3 single adsorption system
MFI40 HY4.8 BEA25 SAH
a (s1)
b (mol kJ1)
4.02 1.47 3.03 0.288
0.157 0.151 0.157 0.174
In order to clarify the relationship between the acid properties and catalytic activities, we carried out the catalytic dehydrolysis of ethanol over solid acid and solid acid–base catalysts. Fig. 8 shows the plots of the reaction rate constant, k, based on the concentration of ethanol in the gas phase against the number of acid sites, Gm,373 K. This Gm,373 K represents the amount of NH3 desorbed above 373 K in the TPD experiment in the NH3 single adsorption system. The reaction rates of zeolites were 20–60 times higher than silica–alumina (SAH) and alumina (ALO). However, it seemed difficult to easily relate the activities to the number of acid sites. As described in Section 2, the concentration of ethanol within the catalysts is different from that in the gas phase, which depends on the type of catalysts. Accordingly, the intrinsic reaction rate constant, kint = k/H, based on the concentration of ethanol within catalysts was employed in place of the reaction rate constant, k, in order to examine the
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Table 4 Parameters used in Eq. (A.2) for alumina in NH3 and CO2 single and co-adsorption systems Single adsorption
ALO a (s1) b (mol kJ1)
NH3–CO2 co-adsorption
CO2–NH3 co-adsorption
NH3
CO2
NH3
CO2
NH3
CO2
1.37 10 3 9.10 102
21.0 0.150
0.634 0.177
0.640 0.214
0.590 0.181
1.46 10 2 0.118
relationship between the catalytic activity and the acidity. Moreover, the number of strong acid sites should be expressed as that of acid sites with activation energy for desorption above 100 kJ mol1, Gm,100 kJ. This value can be calculated from the activation energy distribution. Fig. 9 shows the correlation of the intrinsic reaction rate constant to the Gm,100 kJ. The dimensionless rate constant in the vertical axis was expressed as the ratio of the intrinsic reaction rate constant, kint, to that of silica–alumina, kint,SAH; the number of acid sites in the horizontal axis, Gm,100 kJ, was obtained from the area of the activation energy distribution above 100 kJ mol1. In the solid acid catalyst, the activation energy distributions calculated from the TPD spectra in the NH3 single adsorption system were used to obtain the number of strong acid sites, Gm,100 kJ. It was found that the activity was linearly increased as the number of strong acid sites increased, and that the activity of various solid acid catalysts lies on a straight line, except for Y-type zeolite (HY4.8). In the case of Y-type zeolite, the density of the acid sites was higher than the densities of other catalysts, and so the reaction mechanism would be different: such as bi-molecular reaction. In the solid acid–base catalysts, the open triangle symbols represent correlation by employing the Gm,100 kJ value calculated from the TPD spectra in an NH3 single adsorption system. However, the symbols did not lie on the line representing the relationship between the activity and the
Fig. 8. The reaction rate constant, k, for dehydrolysis of ethanol over solid catalysts as a function of the number of acid sites, Gm,373 K, of which the value was calculated from the amount of NH3 desorbed above 373 K in a NH3 single adsorption system.
number of strong acid sites. This resulted from the induction effect that appeared between the acid sites and the base sites of the solid acid–base catalyst. In this reaction, it is considered that H2O molecules produced by the dehydrolysis of ethanol are adsorbed on the base sites as well as ethanol, which leads to the enhancement of the strength of the acid sites. Accordingly, the activation energy distributions calculated from the TPD spectra in the CO2–NH3 co-adsorption system were applied to obtain the Gm,100 kJ value in the solid acid– base catalyst, as shown by the closed circle symbols in the figure. The closed circle symbols were in good agreement with the correlation line obtained for the solid acid catalysts. Thus, the intrinsic reaction rate constants represented by k/H can be correlated with the number of strong acid sites, regardless of the catalyst type or their pore structure. Moreover, the proportional relationship between the intrinsic reaction rate constant, k/H, and the number of strong acid sites, Gm,100 kJ, was expressed by the following equation: k ¼ kint / Gm;100 kJ H
(4)
As a result, the proposed TPD method in the co-adsorption system can provide the accurate information concerning the number of strong acid sites where the catalytic reaction proceeds, by taking into account the induction effect appearing between the acid sites and the base sites.
Fig. 9. The intrinsic reaction rate constant, kint, for dehydrolysis of ethanol over solid acid catalysts as a function of the number of acid sites, Gm,100 kJ, of which the value was calculated from the area of the activation energy distribution above 100 kJ mol1.
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4. Conclusion A new TPD method for the simultaneous characterization of the acidic and basic properties of solid catalysts was proposed and the following conclusions were obtained: (1) Since there are few base sites in the solid acid catalyst, the induction effect between the acid and base sites is negligible. Thus, the acidic properties of solid acid catalysts can be characterized by the dc-TPD method employing single probe molecules of NH3. (2) The TPD spectra in single adsorption systems reveal that the amounts of acid sites and of base sites existing on the solid acid–base catalysts are in the same order. Moreover, the TPD spectra and the activation energy distribution exhibited a remarkable difference in acid site and base site distributions, such as the sharpness, peak position and area, between single and coadsorption systems, due to the strong induction effect between the acid and base sites, which was proven by the in situ IR measurements. (3) The validity of the new TPD method in the NH3 and CO2 co-adsorption system was examined by correlating the number of acid sites to the catalytic activities. By modifying the reaction rate constants using a partial factor, H, which represents the ratio of the concentration of reactant in the catalyst particles to that in the gas phase, one could obtain the intrinsic reaction rate constant based on the concentration of ethanol within catalysts. Furthermore, the number of strong acid sites was defined as the number of acid sites with activation energy for desorption over 100 kJ mol1. As a result, a proportional relationship between the activities and the number of strong acid sites calculated from the TPD spectra in the co-adsorption system could be obtained, regardless of the catalyst type or their pore structure. Appendix A. Calculation procedure for activation energy distributions of molecules desorbed from catalysts A series of acid sites with different strengths were distributed over solid catalysts. NH3 molecules pre-adsorbed on these acid sites were desorbed at different desorption rates. Accordingly, the observed TPD spectra are the sum of the desorption spectra from these acid sites. The desorption rates of ammonia from acid sites with an activation energy E can be expressed by dðDqÞ E ¼ k0 ðEÞ exp ðDqÞ (A.1) dt Rg T where Rg (J molS1 KS1) is the gas constant, and Dq t he amount of ammonia retained on acid sites having an activation energy between E and E + DE at the desorption temperature, T. The following relation, called the compensation
63
effect [12,28,29], frequently holds between k0(E) and E: k0 ðEÞ ¼ a exp½bE
(A.2)
where a and b are positive constants. The temperature, T, increases linearly with time, t, at a heating rate a: T ¼ T0 þ at
(A.3)
Substituting Eqs. (A.2) and (A.3) into (A.1), and integrating the resulting differential equation yields Z Dq a ebE T E ¼ exp exp dT Dq0 Rg T a T0 a ebE T ex ¼ FðE; TÞ ffi exp (A.4) a x where x = E/(RgT). The total amounts of NH3 initially adsorbed (q0 (mol kgS1)) and retained at temperature, T (q (mol kgS1)), on the acid sites of the catalyst were expressed by the following equations: Z 1 f ðEÞ dE (A.5) q0 ¼ 0
q¼
Z
1
FðE; TÞ f ðEÞ dE
(A.6)
0
The f(E) (mol2 JS1 kgS1) is a density distribution function of the activation energy for desorption, E. Eqs. (A.4) and (A.6) were converted to a relationship between the distribution function, f(E), and the overall desorption rate of NH3, (Sdq/dT), as follows: ð1 þ wÞ2 dq f ðEÞ ¼ dT Rg ufðu þ U=uÞðe=2 þ wÞ þ 2U ug (A.7) where u, w, E and U were related to T by the following equations: E¼
Ein ð2w þ eÞ 2ðw þ 1Þ
u¼
Ein ; Rg T
w ¼ u bEin ;
(A.8)
U¼
2ðu þ 2Þ wþ1
(A.9)
Here, Ein is the activation energy at the point of inflection of the F(E, T) curve at T, and was related to both T and the heating rate a by the following equation: aEin Ein ¼ exp (A.10) Rg T 2 a exp½bEin Rg T A series of TPD spectra from acid sites with uniform acid strength was required to determine the constants a and b. We reported an experimental method to produce such spectra by utilizing a series of TPD spectra measured by raising the beginning temperature, T0, in desorption experiment by a small amount of DT. The calculation procedure for the values of a and b was described in our previous paper [12]. When the
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value of the parameters of a and b are known, all the terms except (Sdq/dT) on the right-hand side of Eq. (A.7) can be calculated in advance for a selected value of T by use of Eqs. (A.8)–(A.10). Since the term (Sdq/dT) is measured as the TPD spectrum, the activation energy distribution for desorption, f(E), can be calculated for various temperatures along the TPD spectrum. The total amounts of CO2 adsorbed on the base sites of the catalyst and the activation energy distribution for CO2 desorption were also obtained by the same procedure, using the TPD spectra of CO2.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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[12] T. Masuda, Y. Fujikata, H. Ikeda, S. Matsushita, K. Hashimoto, Appl. Catal. A 162 (1997) 29. [13] T. Masuda, Y. Fujikata, S.R. Mukai, K. Hashimoto, Appl. Catal. A 165 (1997) 57. [14] T. Masuda, Y. Fujikata, S.R. Mukai, K. Hashimoto, Appl. Catal. A 172 (1998) 73. [15] S.V. Bordawekar, E.J. Doskocil, R.J. Davis, Langmuir 14 (1998) 1734. [16] J.L. Di-Cosimo, C.R. Apesteguia, M.J.L. Gines, E. Iglesia, J. Catal. 190 (2000) 261. [17] D.E. Jiang, B.Y. Zhao, Y.C. Xie, G.C. Pan, G.P. Ran, E.Z. Min, Appl. Catal. A 219 (2001) 69. [18] S.K. Bej, C.A. Bennett, L.T. Thompson, Appl. Catal. A 250 (2003) 197. [19] T. Masuda, T. Asanuma, M. Shouji, S.R. Mukai, M. Kawase, K. Hashimoto, Chem. Eng. Sci. 58 (2003) 649. [20] T. Masuda, Y. Fujikata, H. Ikeda, K. Hashimoto, Micropor. Mesopor. Mater. 38 (2000) 323. [21] T. Tanaka, S. Matsuo, T. Maeda, H. Yoshida, T. Funabiki, S. Yoshida, Appl. Surf. Sci. 121 (1997) 296. [22] T. Masuda, N. Fukumoto, M. Kitamura, S.R. Mukai, K. Hashimoto, T. Tanaka, T. Funabiki, Micropor. Mesopor. Mater. 48 (2001) 239. [23] T. Masuda, Catal. Surv. Asia 7 (2003) 133. [24] T. Masuda, Y. Okubo, S.R. Mukai, M. Kawase, K. Hashimoto, A. Shichi, A. Satsuma, T. Hattori, Y. Kiyozumi, Chem. Eng. Sci. 56 (2001) 889. [25] T. Masuda, K. Fukada, Y. Fujikata, H. Ikeda, K. Hashimoto, Chem. Eng. Sci. 51 (1996) 1879. [26] D.P. Sobczyk, J.J.G. Hesen, J. Van Grondelle, D. Schuring, A.M. De Jong, R.A. Van Santen, Catal. Lett. 94 (2004) 37. [27] J. Medema, J.J.G.M. Van Bokhoven, A.E.T. Kuiper, J. Catal. 25 (1972) 238. [28] K. Hashimoto, K. Miura, T. Watanabe, AIChE J. 28 (1982) 737. [29] K. Miura, Energy & Fuels 9 (1995) 302.
Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates Teruoki Tago a,*, Yoshihito Okubo b, Shin R. Mukai b, Tsunehiro Tanaka c, Takao Masuda a a
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8, Sapporo 060-8628, Japan b Division of Chemical Engineering, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan c Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan Received 24 January 2005; received in revised form 22 April 2005; accepted 16 May 2005 Available online 27 June 2005
Abstract We propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the coadsorption of NH3 and CO2 on catalysts. First CO2 was adsorbed on the catalyst sample; then NH3 was adsorbed on it. Another adsorption sequence of NH3 and CO2, and CO2 and NH3 single adsorptions were also conducted. The TPD measurements were carried out by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min1 in a helium stream under a total pressure of 1.3 kPa. In solid acid catalysts, there is little difference in the NH3-TPD spectra between single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the number of base sites in the solid acid catalyst is very small. In contrast, in a solid acid–base catalyst of alumina, a remarkable difference in the NH3-TPD spectra was observed between single adsorption and co-adsorption systems. The difference in the TPD spectra between single and co-adsorption systems was ascribed to a strong induction effect appearing on the acid and base sites, which was proved by an in situ IR measurement. The validity of the TPD method was examined by correlating the number of the strong acid sites to catalytic activities of dehydrolysis of ethanol over solid acid and solid acid–base catalysts. In solid acid–base catalysts, the number of strong acid sites was calculated from the activation energy distribution for the desorption of NH3 in a co-adsorption system because of the strong induction effect. A proportional relationship between the intrinsic reaction rate constant, which is based on the concentration of ethanol within the catalyst, and the number of strong acid sites could be obtained, regardless of the catalysts or their types or pore structure. # 2005 Elsevier B.V. All rights reserved. Keywords: Temperature-programmed desorption; Acidic and basic properties; Solid acid and base catalyst; Ethanol dehydrolysis; Ammonia and carbon dioxide
1. Introduction A series of active sites with different strengths are distributed over solid catalysts. These distributions are closely related to catalytic activity and selectivity. In order to * Corresponding author. Tel.: +81 11 706 6550; fax: +81 11 706 6550. E-mail address: [email protected] (T. Tago). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.05.010
design catalysts with high activity and selectivity, the properties of active sites should be accurately measured. There have been several studies on the measurement of the active site properties of solid catalysts. The acidic and basic properties of solid catalysts are usually measured by a method utilizing the temperature-programmed desorption spectra of ammonia (NH3-TPD) [1–14] and carbon dioxide (CO2-TPD) [15–18], respectively. Since only the acidic or
T. Tago et al. / Applied Catalysis A: General 290 (2005) 54–64
a CA E Ein f(E)
FC G H k kint q QA Rg t T xA
heating rate (K min1) ethanol concentration in gas phase (mol m3) activation energy for desorption of probe molecules (J mol1) activation energy at inflection point of the F(E, T) function (J mol1) density distribution of the activation energy for desorption of probe molecules (mol2 kg J1) production rate of ethylene (mol s1) number of acid sites (mmol kg1) partial factor (–) reaction rate constant (m3 s1 kg1) intrinsic reaction rate constant (m3 s1 kg1) total number of probe molecules retained on acid or base sites at T (mol kg1) ethanol concentration within catalyst (mol m3) gas constant (8.314 J mol1 K1) time (s) desorption temperature (K) ethanol conversion (–)
Greek letters a constant appearing in Eq. (A.4) (s1) b constant appearing in Eq. (A.4) (mol J1) F function defined by Eq. (A.4)
basic properties of a solid catalyst can be measured by these conventional TPD methods employing a single probe molecule, such as NH3 and CO2, it is difficult to simultaneously measure the acidic and basic properties of solid acid–base catalysts. In a catalytic reaction over solid acid–base catalysts, an induction effect between the acid and base sites will appear, leading to enhancement of the strength of the acidic and basic properties. Accordingly, a simultaneous measurement of the acidic and basic properties of solid catalysts is required to obtain accurate information on catalysts. We have also developed a new TPD method for measuring the NH3-TPD spectra of solid acid catalysts under desorption control conditions, denoted as the dc-TPD method [12]. Since the TPD experiments are conducted in a helium stream under reduced pressure in the method, NH3 pre-adsorbed on the non-acidic and weaker acidic surfaces of the catalysts is desorbed at the beginning of the experiment. Furthermore, NH3 molecules desorbed from the catalyst can be immediately removed from the catalyst particles by a helium stream. Thus, the re-adsorption of NH3 desorbed from the catalyst and the resistance to mass transfer inside the catalyst particles are decreased until they are negligible. Accordingly, although the TPD spectra in a conventional TPD method showed two peaks, the spectra
55
measured in the dc-TPD method exhibited only one peak, corresponding to the desorption from strong acid sites. The basic properties of solid catalysts will also be measured using CO2 as a probe molecule in the dc-TPD method. Moreover, this method can be applied to simultaneously measure the desorption rates of NH3 and CO2 that are adsorbed on solid catalysts in advance. The NH3- and CO2TPD spectra thus obtained will allow us to accurately evaluate the acidic and basic properties of solid catalysts by taking into account an induction effect between the acid sites and the base sites. The main objective of this work is to propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the coadsorption of NH3 and CO2 on catalysts. Furthermore, the validity of the TPD method in the NH3 and CO2 coadsorption system will be examined by correlating the properties of active sites with the catalytic activities of the dehydrolysis of ethanol over solid acid catalysts and solid acid–base catalysts.
2. Experimental procedure 2.1. Catalyst Three kinds of zeolites (MFI-type, Y-type and Beta-type) and silica–alumina were employed as solid acid catalysts. Two kinds of alumina and one zirconia were used as solid acid–base catalysts. These catalysts were supplied by the Catalysis Society of Japan (Table 1). Deactivated alumina was prepared by aging the alumina (JRC-ALO4) in air at 1073 K for 1 h (de-ALO). An MFI-type zeolite (protontype) with a SiO2/Al2O3 ratio of 40 (MFI40) was hydrothermally synthesized by using sodium silicate, aluminum sulfate and tetrapropyl-n-ammonium bromide as a template for 72 h at 473 K [19], followed by an ionexchange technique with NH4NO3 water solution and by calcination in an air stream at 723 K for 1 h. The prepared MFI-type zeolites were confirmed to have the same structure as a pentasil type zeolite by X-ray diffraction analysis (Shimadzu, XRD-610). All the catalysts were pelletized without any binders, crushed, and sieved to yield samples with a particle size of 0.18–0.41 mm. Table 1 Catalysts used in this work Catalysts
Sample
SiO2/Al2O3 (–)
MFI-type zeolite MFI-type zeolite HY-type zeolite H Beta-type zeolite Silica–alumina
– JRC-Z5-70H JRC-Z-HY4.8 JRC-Z-HB25 JRC-ASH1
40 70 4.8 25 4.2
MFI40 MFI70 HY4.8 BEA25 SAH
Activated alumina Deactivated alumina Zirconia
JRC-ALO4 – JRC-ZRO1
– – –
ALO de-ALO ZRO
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T. Tago et al. / Applied Catalysis A: General 290 (2005) 54–64
2.2. Measurements of acidic and basic properties of catalyst by TPD method Fig. 1 schematically shows the apparatus used in the experiment, which was linked with a quadruple mass spectrometer, Q-Mass (ANELVA, M-QA200TS). The chamber was made of stainless steel, and the adsorption column with an electric furnace was set inside it. The inner diameter of the chamber was 63.5 mm, which was long enough compared to the mean free path of probe molecules to avoid the effect of Knudsen diffusion under reduced pressures. It was verified in preliminary experiments that these conditions were achieved [12,20]. The desorbed NH3 and CO2 molecules in helium could be detected by the spectrometer connected with the chamber by an orifice 50 mm in diameter. A catalyst sample of approximately 0.3 g was loaded on a stainless steel net (200 mesh) in an adsorption column, evacuated for 2 h at 873 K, and then cooled to 373 K. The temperature in the column was controlled using a thermocouple located just beneath the catalyst bed. First, NH3 was adsorbed on the catalyst sample at 373 K for 30 min under a pressure of 3.3 kPa, and NH3 adsorbed on the non-acidic and weaker acidic surfaces was desorbed and flushed out at 373 K for 2 h in a helium stream flowing at 20 cm3 min1 under a total pressure of 1.3 kPa. Second, CO2 was adsorbed on it, followed by evacuation at a total pressure of 1.3 kPa in a helium stream (denoted as NH3–CO2 co-adsorption). Another adsorption sequence of CO2 and NH3 was also conducted (denoted as CO2–NH3 coadsorption). The TPD measurements were conducted by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min1 in a helium stream flowing at
20 cm3 min1 under a total pressure of 1.3 kPa. The desorption rates of CO2 and NH3 were simultaneously measured using a gas mass filter. The mass numbers of NH3, CO2 and He were chosen as 15, 44 and 4, respectively, to detect NH3, CO2 and He. By using the flow rate of He and the relative signals of NH3 and CO2 to He, the desorption rates of NH3 and CO2 were obtained. The activation energy distributions for the desorption of NH3 and CO2 were calculated from the TPD spectra thus obtained. The details are described in Appendix A. 2.3. In situ FT-IR analysis In order to identify the adsorption species on acid and base sites, we also measured the in situ FT-IR spectra by use of an FT-IR apparatus (Perkin-Elmer, IR spectrometer Paragon 1000) with wafers of the solid catalysts without any binders [21,22]. After NH3 molecules were adsorbed on the catalyst at 373 K under reduced pressure, an in situ FT-IR measurement (NH3 single adsorption) was carried out. Secondary CO2 molecules were adsorbed on the catalyst on which NH3 molecules were adsorbed in advance, and an in situ FT-IR measurement (NH3–CO2 co-adsorption) was then performed. Another sequence of adsorption, CO2 to NH3, was also conducted (CO2 single adsorption and CO2–NH3 co-adsorption). 2.4. Reaction of ethanol over solid acid catalysts and solid acid–base catalysts The dehydrolysis of ethanol to produce ethylene over the solid catalysts was conducted as a model reaction. The
Fig. 1. Apparatus for measuring TPD spectra employing single and binary probe molecules.
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reaction was carried out using a fixed bed reactor under differential reaction conditions (the conversion of ethanol was less than 5.0%). The time factor (the ratio of the mass of the catalyst to the mass flow rate of the feed) was 0.021– 0.063 kgcat kg1 feed h, and the reaction temperature was 523 K. An isothermal region of 5 cm in length was obtained at the center of the reactor, where the catalysts weighing 50– 150 mg were loaded. Ethanol, which was fed by a microfeeder, was evaporated in an evaporator and introduced to the catalyst bed with a nitrogen flow. The flow rate of the nitrogen gas was 300 cm3 min1. The reaction was continued for 1 h after reaching a steady state. The reactant and products were analyzed by on-line gas chromatography. Since one molecule of ethanol on an acid site is considered to convert to an ethylene molecule by a dehydrolysis reaction, the dehydrolysis rate of ethanol was assumed to be of the first order with respect to the ethanol concentration. Accordingly, the first-order reaction rate constant, k (m3 s1 kg1), which was used as an index for catalytic activity over acid sites, was calculated from the conversion of ethanol, xA, and the production rate of ethylene, F C (mol s1), by the following equation: DFC xA ¼ kCA ¼ kCA;average ffi kCA0 1 (1) Dw 2
57
adsorption isotherm exhibited a similar tendency [24]. Accordingly, the partial factor calculated from the adsorption isotherm of benzene was substituted for the partial factor of ethanol. The experimental procedure and H value were described in detail in our previous paper [25]. The reaction rate expressed by r = kCA (Eq. (1)) is also represented by r = kintQA, where kint is the intrinsic reaction rate constant based on the concentration of ethanol within the catalysts. This kint value is directly correlated to the number of acid sites, and can be expressed by the following equation: CA k (3) kint ¼ k ¼ H QA
3. Results and discussion 3.1. TPD spectra employing single and binary probe molecules
H¼
Fig. 2 shows the typical signals of NH3, CO2 and He in the NH3 and CO2 co-adsorption system for alumina (ALO). The ratios of the signals of NH3 and CO2 to that of He were calibrated in advance by use of gases with known compositions. Accordingly, the ratios of signals and the flow rate of He gave the TPD spectra of NH3 and CO2. Fig. 3(a) and (b) shows the TPD spectra of NH3 and CO2 in a single adsorption system with the dc-TPD method for several kinds of solid acid catalysts and solid acid–base catalysts, respectively. The amounts of NH3 and CO2 molecules desorbed from the catalysts are listed in Table 2. Though the amount of desorbed CO2 was much smaller than the amount of desorbed NH3, it was found that CO2 molecules were desorbed on the solid acid catalysts at a high temperature above 500 K, as shown in Fig. 3(a). This result indicates that the solid acid catalysts possess base sites, of which the number is approximately in the range of 1/50–1/ 100 of the number of acid sites. However, in the solid acid– base catalysts of alumina (ALO) and zirconia (ZRO) as shown in Fig. 3(b), the number of acid sites was smaller than those of the solid acid catalysts. However, the amount of CO2 desorbed from the solid acid–base catalysts was
where QA (mol m3) is the concentration of ethanol within the catalysts and CA (mol m3) the concentration of ethanol in the gas phase. Though the adsorption isotherm of ethanol at a reaction temperature of 523 K was required to calculate the ethanol partial factor H, it was difficult to measure the partial factor, because the dehydrolysis of ethanol proceeded at 523 K over the acid sites of the catalysts. However, the partial factors of benzene and propane calculated from their
Fig. 2. Example of changes in mass numbers and temperature with time.
where CA (mol m3) is the average concentration of ethanol in the gas phase and w (kg) the mass of the catalyst. However, since the ethanol was adsorbed on the surface of the catalysts, it was considered that the concentration of ethanol within the catalysts was higher than that of ethanol in the gas phase. Moreover, the pore structure and surface area of the catalysts used in this study were dependent on the types of catalysts, leading to different adsorption properties of ethanol to the catalysts. Accordingly, in order to investigate the relationship between the acidity and activity of the catalysts, we required the reaction rate constants based on the concentration within the catalysts. For this reason, the partial factor, H, was introduced, which represented the ratio of the concentration of reactant in catalyst particles to that in the gas phase, and was evaluated from the adsorption isotherms [23]. concentration of adsobed molecules within catalyst concentration in gas phase QA ¼ CA (2)
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Fig. 3. TPD spectra of NH3 and CO2 for several kinds of solid acid catalysts (a) and solid acid–base catalysts (b) in single adsorption system with dcTPD method.
approximately half of the amount of NH3 desorbed from these catalysts, which was much larger than that of the solid acid catalysts. These results indicate that the base sites exist on the solid acid–base catalysts in the same order as the acid sites. Figs. 4 and 5 show the NH3-TPD spectra in NH3 single adsorption and the NH3 and CO2 co-adsorption system with the dc-TPD method for solid acid and solid acid–base catalysts, respectively. A series of NH3-TPD spectra obtained for these catalysts by raising the beginning temperature by 25 K is also shown in these figures. The difference between adjoining spectra was considered to represent the desorption spectra from acid sites with a uniform acid strength, which was used to calculate the positive constants of a and b described in Appendix A. In the solid acid catalysts shown in Fig. 4, there is little difference in the NH3-TPD spectra between the single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the Table 2 Amounts of NH3 and CO2 desorbed from solid acid and solid acid–base catalysts in a single adsorption system NH3 (mmol kg1)
CO2 (mmol kg1)
ALO de-ALO ZRO
113 106 17
60 59 1.5
MFI40 BEA25 HY4.8
562 446 945
8.5 7.5 Trace
Fig. 4. NH3-TPD spectra for MFI-type zeolite (MFI40, (a)), Beta-type zeolite (BEA25, (b)) and silica–alumina (SAH, (c)) in single and coadsorption systems with the dc-TPD method.
number of base sites in the solid acid catalyst is very small, as listed in Table 2. Accordingly, the acidic properties of solid acid catalysts can be characterized by the dc-TPD method employing a single probe molecule of NH3. In contrast, in the solid acid–base catalyst of alumina (ALO) shown in Fig. 5, a remarkable difference in the NH3TPD spectra was observed between the single adsorption and co-adsorption systems. The TPD spectra in the coadsorption system were shifted to a high temperature region as compared with the spectra in the single adsorption system. Moreover, it was found that these TPD spectra depended on the adsorption sequences of the probe molecules of NH3 and CO2 on alumina, even though the probe molecules were adsorbed on the same catalyst. In the NH3–CO2 co-adsorption system shown in Fig. 5(a), the
T. Tago et al. / Applied Catalysis A: General 290 (2005) 54–64
59
result, since the strong induction effect appears on the acid site of the solid acid–base catalyst in the CO2–NH3 coadsorption system, the CO2–NH3 co-adsorption system is an appropriate method for characterizing the acidic properties because it takes into account the induction effect. 3.2. In situ IR measurement
Fig. 5. NH3-TPD spectra for alumina (ALO) in single adsorption and coadsorption (NH3–CO2 (a) and CO2–NH3 (b)) systems with the dc-TPD method.
amount of NH3 desorbed from alumina was almost the same as that in a single adsorption system. In contrast, in the CO2– NH3 co-adsorption system shown in Fig. 5(b), the amount of NH3 desorbed from the catalyst was remarkably increased up to 3.7 times compared with the single adsorption system. The difference in TPD spectra between the single and coadsorption systems was ascribed to a strong induction effect appearing on the acid sites during the desorption of NH3. Since the CO2 molecules were adsorbed on the base sites, the strength of the acid sites was enhanced to shift the TPD spectra to a high desorption-temperature region. Moreover, the difference in the amount of the NH3 between NH3–CO2 and CO2–NH3 co-adsorption systems was also ascribed to the induction effect during the adsorption of NH3. In the NH3–CO2 co-adsorption system, after NH3 was adsorbed on the acid sites, CO2 was adsorbed on the base sites. Accordingly, the amount of NH3 adsorbing on alumina in the co-adsorption system was almost the same as that in the single adsorption system. In contrast, in the CO2–NH3 coadsorption system, after CO2 was adsorbed on the base sites, NH3 was adsorbed on the acid sites of alumina. The strength of the acid sites was enhanced by the induction effect of the adsorbed CO2 on the base sites, leading to a remarkable increase in the amount of NH3 adsorbing on alumina. Accordingly, there was much more NH3 desorbed from alumina in the CO2–NH3 co-adsorption system than in the NH3–CO2 co-adsorption and single adsorption systems. As a
As discussed above, the differences in NH3-TPD spectra between the single and co-adsorption systems were observed using the solid acid–base catalyst; this result was ascribed to the induction effect between the acid and base sites. However, it was considered that the coordinate adsorption of NH3 on CO2 molecules might cause an increase in the number of adsorbed NH3 on the solid acid–base catalysts. In order to clarify whether NH3 and CO2 molecules were adsorbed on the acid and base sites of the catalysts, respectively, or not, we measured in situ IR spectra using MFI-type zeolite (MFI40) and alumina (ALO) at 373 K under reduced pressure in the NH3 and CO2 single adsorption and co-adsorption systems. Fig. 6(a) shows the in situ IR spectra of NH3 and CO2 adsorbed on MFI-type zeolite (MFI40) in the single and coadsorption systems. The IR spectrum of CO2 adsorbed on MFI40 in the single adsorption system showed almost the same peaks as the fresh MFI40. This resulted from the presence of very few base sites on the solid acid catalysts shown in Fig. 3. Furthermore, the IR spectrum of NH3 in the single adsorption system was in good agreement with the spectra in the NH3–CO2 and CO2–NH3 co-adsorption systems. This resulted from the presence of only a negligibly small induction effect in the solid acid catalysts; this result supports the above-mentioned experimental results concerning TPD for solid acid catalysts shown in Fig. 4. Fig. 6(b) shows the in situ IR spectra of NH3 and CO2 adsorbed on alumina (ALO). In the single adsorption system, peaks at wave numbers around 1650, 1450–1480 and 1240 cm1 corresponded to the adsorbed CO2 on the base sites. The peaks around 1620 and 1270 cm1 corresponded to the adsorbed NH3 on the Lewis sites [26]. The peaks around 1700, 1470 and 1400 cm1 were assigned to the N–H bending modes corresponding to the adsorbed NH3 on the Brønsted sites [27]. Though the peak around 1650 cm1, which corresponded to the adsorbed CO2, was separated into two peaks around 1625 and 1575 cm1 in the NH3–CO2 coadsorption system, the other peaks in the NH3–CO2 and CO2– NH3 co-adsorption systems were in good agreement with the peaks in the NH3 and CO2 single adsorption methods. Furthermore, no appearance of new peaks due to the coadsorption of NH3 and CO2 was observed, indicating that the probe molecules of NH3 and CO2 were adsorbed on the acid and base sites, respectively, without any adsorption on the adsorption species of other probe molecules. Since the peak around 1620 cm1, which is corresponding to the asymmetric deformation vibration of the adsorbed NH3, has a strong intensity, it is possible to compare the
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Fig. 6. FT-IR spectra of NH3 and CO2 adsorbed on MFI-type zeolite (MFI40, (a)) and alumina (ALO, (b) and (c)) in single and co-adsorption systems.
amount of adsorbed NH3 in the single adsorption system with the amount in the co-adsorption system by using the area. Fig. 6(c) shows the peaks around 1620 cm1. The measured peaks around 1500–1700 cm1 are obtained as the sum of the series of elemental peaks of CO2 and NH3. To clarify these elemental peaks, we carried out the deconvolution of the peaks as shown in the figure. Compared with the area of the adsorbed NH3 in the single adsorption system, changes in the areas corresponding to the amount of adsorbed NH3 in the NH3–CO2 and CO2–NH3 co-adsorption systems were 0.9 times and 3.8 times, respectively. These values were in good agreement with the increases in the amount of NH3 desorbed from strong acid sites, which were measured by dc-TPD experiments.
3.3. Activation energy distributions in single and co-adsorption systems Fig. 7 shows the activation energy distributions for the desorption of NH3 and CO2 from solid catalysts calculated from the TPD spectra shown in Figs. 4 and 5 using Eqs. (A.7)–(A.10) with the values of a and b listed in Tables 3 and 4. In solid acid catalysts, the TPD spectra in the NH3 single adsorption system were employed to obtain the activation energy distributions (Fig. 7(a)), because little difference in the TPD spectra between single and coadsorption was observed. In contrast, in the solid acid–base catalyst of alumina, the TPD spectra in single and coadsorption systems were employed (Fig. 7(b) and (c)). In
T. Tago et al. / Applied Catalysis A: General 290 (2005) 54–64
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from that of the peak positions in the TPD spectra, which exhibited almost the same peak positions of MFI40, HY4.8 and BEA25. The same tendency was reported in our previous paper [12]. Accordingly, the activation energy distributions can give more accurate information concerning the acidic properties of solid catalysts as compared with the TPD spectra. In alumina as a solid acid–base catalyst, differences in the distribution curves between single and co-adsorption systems were clearly observed. The curves in the co-adsorption systems were shifted to a higher energy region, indicating that the numbers of acid sites and base sites with an activation energy above 100 kJ mol1 increased. This resulted from a strong induction effect appearing between the acid and base sites, as discussed in Section 3.1. In particular, in the CO2– NH3 co-adsorption system, the number of strong acid sites with an activation energy above 100 kJ mol1 greatly increased to a value, which was comparable to that of the solid acid catalysts shown in Fig. 7(a). As shown in these figures, the acid sites distributed on each catalyst have strength-distributions in the range of 50– 250 kJ mol1. Katada et al. have reported that the strong acid sites of solid acid catalysts possess an adsorption enthalpy of NH3 approximately above 100 kJ mol1 [8]. Moreover, Masuda et al. have reported a relationship between the activation energy for NH3 desorption and the adsorption enthalpy of NH3, in which the enthalpy above 100 kJ mol1 corresponds to the activation energy approximately above 100 kJ mol1 [13]. Accordingly, it can be concluded that the strong acid sites distributed on the catalysts possess activation energy values above 100 kJ mol1. Moreover, the number of acid sites with an activation energy above 100 kJ mol1 was denoted as the number of strong acid sites, Gm,100 kJ. 3.4. Dehydrolysis of ethanol over solid acid and solid acid–base catalysts Fig. 7. Activation energy distributions for the desorption of NH3 from solid acid catalysts (a) in a single adsorption system, and the distributions for desorption of NH3 and CO2 from alumina (ALO) as a solid acid–base catalyst in NH3–CO2 (b) and CO2–NH3 (c) co-adsorption systems with the dc-TPD method.
these figures, a large value of activation energy, E, corresponded to stronger acid and base sites. In the solid acid catalysts shown in Fig. 7(a), the acid strength followed the order from the peak position of f(E); SAH < HY4.8 < BEA25 < MFI40. The order was different Table 3 Parameters used in Eq. (A.2) for solid acid catalysts in an NH3 single adsorption system
MFI40 HY4.8 BEA25 SAH
a (s1)
b (mol kJ1)
4.02 1.47 3.03 0.288
0.157 0.151 0.157 0.174
In order to clarify the relationship between the acid properties and catalytic activities, we carried out the catalytic dehydrolysis of ethanol over solid acid and solid acid–base catalysts. Fig. 8 shows the plots of the reaction rate constant, k, based on the concentration of ethanol in the gas phase against the number of acid sites, Gm,373 K. This Gm,373 K represents the amount of NH3 desorbed above 373 K in the TPD experiment in the NH3 single adsorption system. The reaction rates of zeolites were 20–60 times higher than silica–alumina (SAH) and alumina (ALO). However, it seemed difficult to easily relate the activities to the number of acid sites. As described in Section 2, the concentration of ethanol within the catalysts is different from that in the gas phase, which depends on the type of catalysts. Accordingly, the intrinsic reaction rate constant, kint = k/H, based on the concentration of ethanol within catalysts was employed in place of the reaction rate constant, k, in order to examine the
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Table 4 Parameters used in Eq. (A.2) for alumina in NH3 and CO2 single and co-adsorption systems Single adsorption
ALO a (s1) b (mol kJ1)
NH3–CO2 co-adsorption
CO2–NH3 co-adsorption
NH3
CO2
NH3
CO2
NH3
CO2
1.37 10 3 9.10 102
21.0 0.150
0.634 0.177
0.640 0.214
0.590 0.181
1.46 10 2 0.118
relationship between the catalytic activity and the acidity. Moreover, the number of strong acid sites should be expressed as that of acid sites with activation energy for desorption above 100 kJ mol1, Gm,100 kJ. This value can be calculated from the activation energy distribution. Fig. 9 shows the correlation of the intrinsic reaction rate constant to the Gm,100 kJ. The dimensionless rate constant in the vertical axis was expressed as the ratio of the intrinsic reaction rate constant, kint, to that of silica–alumina, kint,SAH; the number of acid sites in the horizontal axis, Gm,100 kJ, was obtained from the area of the activation energy distribution above 100 kJ mol1. In the solid acid catalyst, the activation energy distributions calculated from the TPD spectra in the NH3 single adsorption system were used to obtain the number of strong acid sites, Gm,100 kJ. It was found that the activity was linearly increased as the number of strong acid sites increased, and that the activity of various solid acid catalysts lies on a straight line, except for Y-type zeolite (HY4.8). In the case of Y-type zeolite, the density of the acid sites was higher than the densities of other catalysts, and so the reaction mechanism would be different: such as bi-molecular reaction. In the solid acid–base catalysts, the open triangle symbols represent correlation by employing the Gm,100 kJ value calculated from the TPD spectra in an NH3 single adsorption system. However, the symbols did not lie on the line representing the relationship between the activity and the
Fig. 8. The reaction rate constant, k, for dehydrolysis of ethanol over solid catalysts as a function of the number of acid sites, Gm,373 K, of which the value was calculated from the amount of NH3 desorbed above 373 K in a NH3 single adsorption system.
number of strong acid sites. This resulted from the induction effect that appeared between the acid sites and the base sites of the solid acid–base catalyst. In this reaction, it is considered that H2O molecules produced by the dehydrolysis of ethanol are adsorbed on the base sites as well as ethanol, which leads to the enhancement of the strength of the acid sites. Accordingly, the activation energy distributions calculated from the TPD spectra in the CO2–NH3 co-adsorption system were applied to obtain the Gm,100 kJ value in the solid acid– base catalyst, as shown by the closed circle symbols in the figure. The closed circle symbols were in good agreement with the correlation line obtained for the solid acid catalysts. Thus, the intrinsic reaction rate constants represented by k/H can be correlated with the number of strong acid sites, regardless of the catalyst type or their pore structure. Moreover, the proportional relationship between the intrinsic reaction rate constant, k/H, and the number of strong acid sites, Gm,100 kJ, was expressed by the following equation: k ¼ kint / Gm;100 kJ H
(4)
As a result, the proposed TPD method in the co-adsorption system can provide the accurate information concerning the number of strong acid sites where the catalytic reaction proceeds, by taking into account the induction effect appearing between the acid sites and the base sites.
Fig. 9. The intrinsic reaction rate constant, kint, for dehydrolysis of ethanol over solid acid catalysts as a function of the number of acid sites, Gm,100 kJ, of which the value was calculated from the area of the activation energy distribution above 100 kJ mol1.
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4. Conclusion A new TPD method for the simultaneous characterization of the acidic and basic properties of solid catalysts was proposed and the following conclusions were obtained: (1) Since there are few base sites in the solid acid catalyst, the induction effect between the acid and base sites is negligible. Thus, the acidic properties of solid acid catalysts can be characterized by the dc-TPD method employing single probe molecules of NH3. (2) The TPD spectra in single adsorption systems reveal that the amounts of acid sites and of base sites existing on the solid acid–base catalysts are in the same order. Moreover, the TPD spectra and the activation energy distribution exhibited a remarkable difference in acid site and base site distributions, such as the sharpness, peak position and area, between single and coadsorption systems, due to the strong induction effect between the acid and base sites, which was proven by the in situ IR measurements. (3) The validity of the new TPD method in the NH3 and CO2 co-adsorption system was examined by correlating the number of acid sites to the catalytic activities. By modifying the reaction rate constants using a partial factor, H, which represents the ratio of the concentration of reactant in the catalyst particles to that in the gas phase, one could obtain the intrinsic reaction rate constant based on the concentration of ethanol within catalysts. Furthermore, the number of strong acid sites was defined as the number of acid sites with activation energy for desorption over 100 kJ mol1. As a result, a proportional relationship between the activities and the number of strong acid sites calculated from the TPD spectra in the co-adsorption system could be obtained, regardless of the catalyst type or their pore structure. Appendix A. Calculation procedure for activation energy distributions of molecules desorbed from catalysts A series of acid sites with different strengths were distributed over solid catalysts. NH3 molecules pre-adsorbed on these acid sites were desorbed at different desorption rates. Accordingly, the observed TPD spectra are the sum of the desorption spectra from these acid sites. The desorption rates of ammonia from acid sites with an activation energy E can be expressed by dðDqÞ E ¼ k0 ðEÞ exp ðDqÞ (A.1) dt Rg T where Rg (J molS1 KS1) is the gas constant, and Dq t he amount of ammonia retained on acid sites having an activation energy between E and E + DE at the desorption temperature, T. The following relation, called the compensation
63
effect [12,28,29], frequently holds between k0(E) and E: k0 ðEÞ ¼ a exp½bE
(A.2)
where a and b are positive constants. The temperature, T, increases linearly with time, t, at a heating rate a: T ¼ T0 þ at
(A.3)
Substituting Eqs. (A.2) and (A.3) into (A.1), and integrating the resulting differential equation yields Z Dq a ebE T E ¼ exp exp dT Dq0 Rg T a T0 a ebE T ex ¼ FðE; TÞ ffi exp (A.4) a x where x = E/(RgT). The total amounts of NH3 initially adsorbed (q0 (mol kgS1)) and retained at temperature, T (q (mol kgS1)), on the acid sites of the catalyst were expressed by the following equations: Z 1 f ðEÞ dE (A.5) q0 ¼ 0
q¼
Z
1
FðE; TÞ f ðEÞ dE
(A.6)
0
The f(E) (mol2 JS1 kgS1) is a density distribution function of the activation energy for desorption, E. Eqs. (A.4) and (A.6) were converted to a relationship between the distribution function, f(E), and the overall desorption rate of NH3, (Sdq/dT), as follows: ð1 þ wÞ2 dq f ðEÞ ¼ dT Rg ufðu þ U=uÞðe=2 þ wÞ þ 2U ug (A.7) where u, w, E and U were related to T by the following equations: E¼
Ein ð2w þ eÞ 2ðw þ 1Þ
u¼
Ein ; Rg T
w ¼ u bEin ;
(A.8)
U¼
2ðu þ 2Þ wþ1
(A.9)
Here, Ein is the activation energy at the point of inflection of the F(E, T) curve at T, and was related to both T and the heating rate a by the following equation: aEin Ein ¼ exp (A.10) Rg T 2 a exp½bEin Rg T A series of TPD spectra from acid sites with uniform acid strength was required to determine the constants a and b. We reported an experimental method to produce such spectra by utilizing a series of TPD spectra measured by raising the beginning temperature, T0, in desorption experiment by a small amount of DT. The calculation procedure for the values of a and b was described in our previous paper [12]. When the
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value of the parameters of a and b are known, all the terms except (Sdq/dT) on the right-hand side of Eq. (A.7) can be calculated in advance for a selected value of T by use of Eqs. (A.8)–(A.10). Since the term (Sdq/dT) is measured as the TPD spectrum, the activation energy distribution for desorption, f(E), can be calculated for various temperatures along the TPD spectrum. The total amounts of CO2 adsorbed on the base sites of the catalyst and the activation energy distribution for CO2 desorption were also obtained by the same procedure, using the TPD spectra of CO2.
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