A New Approach to the Study of the Reactivity of Solid-Acid Catalysts

Huddersfield, HD1 3DH, UK. ReceiVed: December 31, 2000; In Final Form: April 10, 2001. A new and potentially valuable approach to the study of the aci...
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J. Phys. Chem. B 2001, 105, 6178-6185

A New Approach to the Study of the Reactivity of Solid-Acid Catalysts: The Application of Constant Rate Thermal Analysis to the Desorption and Surface Reaction of Isopropylamine from NaY and HY Zeolites Elena A. Fesenko, Philip A. Barnes,* Gareth M. B. Parkes, D. Robert Brown, and Majid Naderi Centre for Applied Catalysis, Materials Research DiVision, UniVersity of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK ReceiVed: December 31, 2000; In Final Form: April 10, 2001

A new and potentially valuable approach to the study of the acidity and reactivity of the surfaces of solid acid materials is described. The technique uses a combination of constant rate thermal analysis (CRTA) with the rate jump technique (CRTA-RJ) for temperature programmed desorption (TPD) and reaction (TPRx). CRTA, where the sample temperature is changed to force the rate of reaction to remain constant through a process, offers a number of advantages over conventional linear heating thermal methods by minimizing temperature and concentration gradients in the sample and reducing the effects of diffusion. CRTA and CRTARJ were applied to the TPD and TPRx of isopropylamine from NaY zeolite and its acidic form, HY. Activation energies of both the physical desorption of the amine and the surface-catalyzed Hoffman elimination reaction were calculated as a function of surface coverage and the advantages and limitations of the techniques are discussed. Comparisons are made with analogous TPD experiments using conventional linear heating methods. It is concluded that the TPRx approach reflects the reactivity of strong acidic catalytic sites directly and so usefully complements base adsorption microcalorimetry in characterizing acid solid catalysts.

Introduction Measurement of Surface Properties of Solid Acids by Base Adsorption Microcalorimetry, TPD, and TPRx. Surface acidity, usually described in term of either Lewis or Brønsted acid sites, is an important characteristic of zeolites which is directly related to their catalytic reactivity. While Brønsted acid sites are well defined and are acknowledged to be of primary importance for many acid-catalyzed reactions, any site capable of adsorbing a base may be termed a Lewis acid site, although it may not exhibit the catalytic activity traditionally associated with Lewis acids.1 There are a number of successful methods for characterizing zeolite acidity, for example, IR spectroscopy,2 UV-visible spectroscopy,3 solid-state NMR spectroscopy,4 base adsorption microcalorimetry,5 and temperature-programmed desorption.6 The catalytic activity of zeolites is usually viewed as being dependent on the strength of their acidic sites. However, there is some evidence that the catalytic activity may be influenced also by other factors. For example, Gorte7 mentioned that paraffin cracking rates are dependent on the concentration of the desorbed species present in the pores, while Fleisher claimed that the spacing of the sites can affect reaction rates via the configuration of the adsorption complex or transition state.8 Base Adsorption Microcalorimetry. Base adsorption microcalorimetry is one of the most frequently used methods for characterizing surface acid sites.9 The enthalpy of adsorption for simple bases is regarded as a direct measure of the strength of the acid sites and can provide information on their distribution. However, there can be difficulties with these measurements * To whom correspondence should be addressed. E-mail: p.a.barnes2@ hud.ac.uk. Phone: +44-(0)1484-473138. Fax: +44-(0)1484-472182.

and their interpretation. It is well-known that the temperature used in calorimetry must be carefully chosen to ensure sample equilibration, as the mobility of strong bases can be limited at low temperatures so that adsorption equilibrium may not be achieved on exposure to a small dose of the base.5 Furthermore, other factors in addition to the direct acid-base interactions may affect the measured adsorption enthalpies. For example, the work of Parrillo and et al.10 on alkylamine adsorption suggested that van der Waals interactions between alkyl groups affect the interaction of amine groups with acidic sites. TPD. TPD offers a different approach to the study of acid sites by measuring the enthalpy of desorption and has the advantage that it is used in systems that are already equilibrated. When amines are desorbed from weak Brønsted or Lewis acid sites they return unreacted to the gas phase. In this case, the term “temperature programmed desorption” is entirely appropriate. However, as will be seen below, when strong acid sites are involved a reaction may occur and the expression “temperature programmed reaction” more accurately describes the processes observed. The adsorption of simple bases including ammonia,7 pyridine,11 and various amines12 has been widely used in the study of zeolite surfaces. The importance of choosing a probe molecule of appropriate size when using TPD to measure catalyst surface acidities has been demonstrated by Yiu.13 The temperature dependence of desorption is related to the strength of acid sites which may involve either physical or chemical adsorption. However, when applied to temperature programmed desorption, the use of the activation energy to estimate the strength of the bond between the surface and an adsorbed molecule relies on the activation energy of the adsorption being zero as can be seen from the equation:

10.1021/jp004587f CCC: $20.00 © 2001 American Chemical Society Published on Web 06/07/2001

Study of the Reactivity of Solid-Acid Catalysts

Ea(des) ) -∆H + Ea(ads)

J. Phys. Chem. B, Vol. 105, No. 26, 2001 6179

(1)

where Ea(ads) is the activation energy of adsorption, Ea(des) is the activation energy of desorption, and ∆H is the enthalpy of adsorption. In the case of the adsorption/desorption of bases, it is usual to assume that the activation energy of adsorption is negligible. While this is probably acceptable for physical adsorption, it becomes less so when chemisorption is considered as this is frequently an activated process. To remove any doubt in this respect, separate measurements should be made of the activation energy of adsorption, but in practice this is rarely, if ever, done. TPRx. TPRx and adsorption calorimetry provide different types of information on catalytically active sites as the former measures reactivity, while the latter measures the enthalpy of adsorption. The chief advantage of the use of alkylamines, as compared with alcohols that have also been used as a probe for heterogeneous acid-site densities, is that TPRx occurs only at Brønsted sites and not on Lewis sites.14 The alkylamines typically used in these studies decompose to the corresponding alkene and ammonia in a relatively narrow temperature region. The adsorption of alkylamines on strong Brønsted sites leads to the formation of the well-known 1:1 adsorption complex,1 to which can be added reaction and desorption steps: ∆Hads

Ea

HRNH2(g) + ZOH 98 HRNH3+‚‚‚ZO- 98 R(g) + NH3(g) + ZOH The accepted interpretation of this mechanism is that an amine molecule interacts with a single Brønsted-acid site, enabling the concentration of the strong acid sites to be determined. For example, essentially all of the isopropylamine adsorbed on pure γ-Al2O3, which should have only Lewis acid sites, desorbs unreacted.15 However, solids such as H-MFI, H-ZSM-5, H-Y, H-FER, H-TOR, and amorphous silica-aluminas show the hightemperature reaction of isopropylamine to propene and ammonia, indicating that the Brønsted acid sites on these materials can be successfully studied using this method. It is sometimes assumed that the rate of desorption of the reaction product is much faster than the rate of the surface reaction itself, so the measured the activation energy reflects the latter (the ratelimiting step). Both adsorption at Brønsted sites (calorimetric adsorption measurements) and the surface elimination reaction (TPRx) characterize the same zwitterionic complex, HRNH3+‚ ‚‚ZO. The key difference is that calorimetric measurements describe its formation while TPRx experiments provide information on its decomposition. Therefore, in TPRx, it is not the ability of the surface to donate a proton from the Brønsted site, i.e., its acidity, that is studied, but the reactivity of the zwitterionic complex coordinated to the catalytic site. Crucial to this work, therefore, is the concept of measuring not simply the strength of acidic sites but their actual catalytic effect. Conventional Linear Heating Rate TPD and TPRx. Linear heating methods for determining the kinetic parameters for desorption/reaction processes from experimental TPD/TPRx data are frequently based on the Polanyi-Wigner theory.16-18 For example, the desorption activation energy and the preexponential factor can be obtained by solution of the Redhead equation which relates the peak maximum temperature, found in conventional linear heating TPD, to the heating rate:19

Ed RTm

2

( )

Ed A )0 exp β RTm

(2)

where Ed is the activation energy of desorption, R is the gas constant, Tm is the temperature of the maximum desorption rate, A is the preexponential factor for desorption, and β is the heating rate. Conventional TPD studies are carried out using linear heating, and this can lead to difficulties in the interpretation of results, especially in the case of microporous materials, such as zeolites, where diffusion effects are significant. In addition, the strength of the adsorbent-adsorbate interactions, as reflected in the temperature of desorption/reaction peaks in TPD/TPRx may be influenced also by many other factors including porosity, sample size, readsorption, heating rate, and the flow rate of the carrier gas. The effects of mass and heat transfer phenomena in microporous solids such as zeolites may well confuse the relationship between the temperature of the amine desorption and the strength of the acid sites.20 For example, neglecting the effects of diffusion and readsorption for the desorption of water from zeolite Linde 4A was found to be the cause of errors in the activation energy of the order 10%.17 Unless great care is taken, conventional linear heating rate TPD/TPRx methods only give an average value of the acid site strength and provide no information on their distribution. Constant Rate Thermal Analysis. In all thermal techniques, the chemical and physical processes being studied are influenced by the experimental conditions that affect the results both qualitatively and quantitatively. In conventional thermal analysis a sample is typically subjected to a predetermined linear heating rate, but this gives rise to significant temperature and pressure gradients that not only vary throughout the sample but also change during the processes being observed. The magnitude of these gradients is influenced by many factors, such as the heating rate, the nature, and flow rate of the purge gas, the thermal conductivity of the sample and its mass, particle size, degree of compaction, and heat capacity. The greater the heating rate, the higher the temperature and pressure gradients across the sample, giving apparently elevated peak temperatures and poor resolution of adjacent events.21 One approach to minimizing these problems is to apply the technique of constant rate thermal analysis (CRTA), first developed by Rouquerol,22 in which the rate of reaction is regulated to maintain a constant, and usually low, rate of reactant consumption or product formation by controlling the temperature appropriately. CRTA minimizes both heat and mass transfer effects across the sample bed and allows the concentration of the reactant and product gases to be maintained at a constant level, so keeping the system close to equilibrium conditions throughout the whole process. A further advantage is that, as the rate of reaction is controlled, it can be set at a very low value which minimizes the effects of diffusion, the depletion of reactant gases or the build up of product gases. In addition, the temperature changes required to keep the reaction rate constant are far less than those used in linear heating rate experiments. For these reasons, CRTA is thought to give a more reliable measurement of the temperature of the sample and hence of the processes being studied, and so provides improved kinetic data. Furthermore, Criado et al.23 have shown that it is possible to deduce mechanistic information by analyzing the shape of CRTA curves. The CRTA Rate Jump Method and Its Use in the Determination of Kinetic Information. Conventional methods for determining activation energies involving solid materials usually require a series of either isothermal or temperature programmed experiments, in which a range of temperatures or heating rates is employed, respectively. While the isothermal approach has undoubted advantages and has been very widely

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applied, it is difficult to ensure that the reaction interface, which is all-important in reactions involving solids, and the experimental conditions, some of which are listed above, are identical throughout all of the experiments. Linear heating methods, e.g., those of Kissinger24 and Redhead,19 not only suffer similarly but are also affected by the temperature and concentration gradients, as mentioned above. Furthermore, the mechanism of a solid-state reaction may change during the course of a reaction which can, if undetected, lead to serious errors. CRTA can be modified to incorporate the “rate-jump” method (CRTA-RJ) for the determination of kinetic parameters in which the rate of reaction is made to alternate between two preselected constant values, while the changes in sample temperature required to achieve this are monitored.25 Provided the reaction rate is allowed to reach a constant level at the two successive rates, the corresponding temperature measurements can be used to calculate a value of the activation energy (Ea) for the reaction using the following form of the Arrhenius equation:

Ea ) R ln

( )(

)

C 1 T 1T 2 C2 T1 - T2

(3)

where T1 and T2 are the temperatures corresponding to the two preset (constant) reaction rates, C1 and C2, at the same value of the extent of reaction (R).26 Unlike most conventional techniques, the rate-jump method has the great advantage that the calculation of the apparent activation energy is independent of any assumptions regarding the mechanism of the process and requires only one experiment. It provides a number of near-instantaneous measurements so that any determination is made under conditions of a virtually identical reaction interface. Furthermore, by taking a series of perhaps 20-30 measurements during the course of the experiment, it is possible not only to observe any change in the reaction mechanism and the activation energy, but also to determine a distribution of values for Ea. To realize the advantages of CRTA-RJ, two sources of potential error must be minimized. To meet the requirement that the reaction interface does not alter appreciably during a measurement, changes in the extent of the reaction (R) must be negligible. In practice, this means that the magnitude of the rate jumps must be limited so that R does not change significantly. In addition, to ensure that the mechanism does not differ at the two temperatures used in each measurement, the magnitude of each resulting temperature step must be as small as possible,27 consistent with allowing an accurate determination of both the reaction rate and the temperature. To obtain the optimum results from CRTA-RJ great care must be taken to minimize both mass and heat transfer effects. Our equipment has been designed specifically to work with very small samples (1-5 mg) in high vacuum to overcome the problems found when using large packed beds. The use of very low reaction rates is also important in achieving these ends, and this, when taken with small sample size, requires the use of very sensitive and stable equipment, as described later. To date, CRTA-RJ has been applied to the study of thermal decomposition, reduction, and desorption processes,28-30 but relatively little work, if any, has been done on their application to the determination of acid site strengths and the reactivity of surface acid sites. Relationship between Activation Energies and Apparent Activation Energies. For the reasons mentioned above, the difficulties in making measurements of activation energies in solid-state systems are compounded when the processes being

Figure 1. Schematic diagram of the dosing system of the MS-SIP: 1-4 - speedivalves, 5 - SIP, 6 - MS source, 7 - sample outgassing and adsorption area, 8 - dosing volume, 9 - thermocouple, 10 - watercooling coil, 11 - thermocoax heating element, 12 - adsorbate injection point, 13 - to ion gauge, 14 - to MS analyzer, 15 - to MS diffusion pump, 16 - to rotary pump, 17 to Eurotherm and computer, 18 - to second diffusion pump.

studied either produce or consume gases. It cannot be assumed therefore that the activation energy measured in any system involving a solid or surface is that of process being studied. Rather, it should be regarded as a measure of a temperature coefficient for the process under the particular conditions employed. It is for this reason that we refer to the determination of the apparent activation energy, a quantity that includes the effects of the many parameters outlined earlier.31 Nevertheless, we support the view of Reading and Dollimore27 that, provided the experimental conditions are chosen to minimize these effects, the apparent value of Ea can closely approach that of the true value. In fact, a comparison of the values of the apparent activation energy made under near-ideal conditions can be of value in providing information on the effects of diffusion, etc., on the temperature coefficient of the reactions concerned. In this paper, two thermal techniques (linear heating and CRTA-RJ) are used to investigate the desorption and surface reaction, i.e., TPD/TPRx, of isopropylamine from NaY zeolite and its acidic form, HY, to illustrate their use in the study of surface reactivity and in the determination of the energetics of acid sites and their distribution. Experimental Section The equipment used in this study has been described previously.32,33 The TPD experiments were carried out using a solid insertion probe-mass spectrometer (SIP-MS) system, based on a VG 70-70 double focusing mass spectrometer designed for high stability and resolution. The MS was modified for the adsorption experiments by incorporating an in-situ custom-built outgassing, gas dosing, and adsorption system (Figure 1). Samples of 2-3 mg were placed in the SIP, outgassed by heating at 10 °C/min in a vacuum (10-6 Torr) to 500 °C in the source of the MS and cooled to 30 °C. The probe was then moved to the adsorption section of the vacuum system (between the closed valves 1, 2, and 4) and 100 µL of isopropylamine was injected into the previously evacuated dosing volume. The dosing system, maintained at 200 °C, reached a pressure of ca. 10 Torr of isopropylamine. Adsorption was started by opening valve 2 that connects the adsorption and dosing regions of the system. In separate experiments, it was found that equilibrium under these conditions was achieved in ca. 30 min. After leaving

Study of the Reactivity of Solid-Acid Catalysts the sample for 1 h, unadsorbed isopropylamine was removed by opening valve 4, until the pressure dropped to 10-2 Torr. Valve 3 was then opened (with valve 4 now closed) which gave a pressure of ca. 10-6 Torr. The SIP was then moved into the MS source after opening valve 1. The sample was then further evacuated in high vacuum for a further 2 h, after which the experiment commenced. The temperature was adjusted during experiments using a temperature programmer (Eurotherm 818P), controlled by a computer via an RS232C serial interface. In-house software was used to continuously adjust the heating rate, measure the sample temperature and acquire and process the TPD and TPRx MS data. The product observed in desorption was simply isopropylamine (main m/z fragments ) 44, 41, 17), and for the surface reaction propene (m/z ) 41) and ammonia (m/z ) 17) were seen. Isopropylamine and propene were followed using their major fragments (m/z ) 44 and 41, respectively). However, as the amine produces a minor fragment at m/z ) 41, to identify the gas-phase species unequivocally it is necessary to distinguish between isopropylamine and propene by comparing the ratio of m/z ) 44 and 41 and noting the appearance of the m/z ) 17 peak, as will be seen later. To avoid the confusion sometimes found in the literature, we use the term “desorption” to describe the evolution of unreacted isopropylamine (m/z ) 44) and “reaction” where propene (m/z ) 41) and ammonia (m/z ) 17), but no m/z ) 44, are produced. The MS was operated in a “peak select” mode in conjunction with a multiple ion detector unit, using field switching to avoid mass discrimination. This system allows direct measurement of up to 64 selected m/z values which can be processed to give evolved gas profiles for each species selected. In the current work, selected ions were monitored sequentially for 500 ms each, with the furnace temperature being updated every 1000 ms. While in principle the use of CRTA-RJ in conjunction with the SIP-MS has obvious advantages, it should be pointed out that the measurement of temperature is likely to be less accurate than for experiments conducted at atmospheric pressure. To minimize this effect, the solid insertion probe was redesigned so that the sample was in intimate thermal contact with the thermocouple used to measure its temperature. The integral heating element ensures good physical contact between the sample, the thermocouple, and the heater which is essential for close control of the sample temperature in the high vacuum of the MS source. Zeolite Y was studied in the Na+ and H+ ion-exchanged forms. The starting material, supplied by Aldrich, was NaY zeolite with a Si:Al ratio of 2.6. The H+ form was prepared by repeated ion exchange under reflux with 1 M ammonium chloride solution, after which it was washed, filtered, and dried at 100 °C before being calcined for 4 h at 500 °C under flowing air. The samples appeared to be highly crystalline in X-ray diffraction. The pore size distribution, measured by N2 adsorption, showed a pore diameter maximum at about 2 nm for both forms of the zeolite. The surface area of NaY zeolite was found to be 630 m2/g by N2 adsorption at 77 K using the BET equation, while that of the HY was 780 m2/g. Results and Discussion TPD and TPRx using Linear Heating and CRTA. A series of conventional TPD experiments was carried out using linear heating rates of 2, 4, 8, and 16 °C/min to study the desorption and reaction of isopropylamine from the NaY and HY zeolites. Figures 2 and 3 show mass spectra found during experiments

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Figure 2. Temperature programmed desorption of isopropylamine from zeolite NaY under linear heating conditions: m/z ) 17 - ammonia, m/z ) 41 - propene/isopropylamine, m/z ) 44 - isopropylamine.

Figure 3. Temperature programmed desorption/reaction of isopropylamine from zeolite HY under linear heating conditions: m/z ) 17 ammonia, m/z ) 41 - propene/isopropylamine, m/z ) 44 - isopropylamine.

using the highest heating rate under vacuum and the profiles are similar to those published for other zeolites.9,12,34 The TPD of isopropylamine from the NaY sample (Figure 2) shows a single peak of unreacted amine desorbed below 250 °C. At least two events can be seen during the TPD of isopropylamine from the HY (Figure 3). The low-temperature features are thought to arise from the physical desorption of isopropylamine from weak acid sites, possibly associated with defects or amines protonated at the Brønsted sites. Unreacted isopropylamine usually corresponds to physical adsorption and therefore characterizes weak acid sites. In the case of the acidic form of the zeolite, the low-temperature desorption peak is much wider (covering a range from 40 °C to 350 °C) and has a different shape from that seen for the Na+ form. This is thought to offer circumstantial evidence for both the increased amount and changing nature of the weak acid sites formed during the processes of ion exchange and calcination.35 The high-temperature peak results from the desorption of propene and ammonia, between 250-400 °C, due to the surface catalyzed reaction of the amine at the strong acid sites in the HY zeolite, a feature associated with molecules which are protonated at Brønsted sites. It should be noted that the surface elimination reaction

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Figure 4. Temperature programmed desorption of isopropylamine from zeolite NaY under CRTA conditions (note that the traces are displaced vertically for clarity): m/z ) 17 - ammonia, m/z ) 41 - propene/ isopropylamine, m/z ) 44 - isopropylamine.

started at 250 °C before the desorption of physisorbed amine was complete (at 350 °C) to give a mixture of reacted and unreacted isopropylamine, illustrating the difficulties in distinguishing between weak and strong acid sites unless a technique such as mass spectrometry is used. Figures 4 and 5 show the same system but using CRTA conditions with the desorption/reaction and temperature profiles being plotted as a function of time. In these experiments, the heating rate was controlled by the software to continuously vary between +5 and -5 °C/min, as required, to maintain the rate of the processes being studied at the preset constant level. The desorption/reaction rate was set at a value equivalent to 2-5% of the maximum reaction rate (i.e., peak height) achieved during the linear heating experiments. The upper trace shows how the computer system controlled the sample temperature to maintain the constant rate of the processes. It can be seen that the rate of desorption of isopropylamine from the NaY zeolite (Figure 4) is maintained at a constant level over almost all of the TPD experiment and the profiles for all the observed fragments are essentially similar. The desorption proceeded at the set constant rate over the temperature range of 40-110 °C, which is much lower than that found for the experiment carried out using conventional linear heating conditions (Figure 2). In the case of the TPD/TPRx from the acidic zeolite HY (Figure 5), the level of fragment m/z ) 41 was controlled at a constant level throughout both the desorption and reaction processes. The first section of this trace, up to 240 min, arose from physical desorption (here m/z ) 41 is a fragment of isopropylamine), while the second corresponded to the surface reaction (where m/z ) 41 is now a fragment of propene), as

Fesenko et al.

Figure 5. Temperature programmed desorption/reaction of isopropylamine from zeolite HY under CRTA conditions (note that the traces are displaced vertically for clarity): m/z ) 17 - ammonia, m/z ) 41 - propene/isopropylamine, m/z ) 44 - isopropylamine.

can be seen from the marked decrease in the physically desorbed amine (m/z ) 44) and the simultaneous production of ammonia (m/z ) 17). The difference between these two processes, i.e., desorption and surface reaction, can be clearly seen from the temperature profile, which has two regions: one with increasing temperature corresponding to physical desorption and the second, which is almost isothermal, arising from the Hoffman elimination reaction. The surface reaction proceeded over a temperature range 230-260 °C, some 100 °C lower than was found for the conventional linear heating experiments (Figure 3). The events seen after 1100 min, toward the end of the experiment, arise from the zeolite structure starting to break down as the temperature exceeds 400 °C. To more clearly distinguish between the physical desorption and the surface reaction obtained under constant rate conditions, it is useful to plot the data in the form of the extent of the process (R) versus the sample temperature. The R-T graphs for linear heating (profiles a and b) and CRTA experiments (profiles c and d) are shown in Figure 6. This form of presentation is valuable as it provides quantitative information from both types of experiment. Using the CRTA technique for the study of the thermal desorption/reaction of isopropylamine from zeolite HY (d), it can be seen that the two events are more clearly resolved than for the linear heating experiment (b) and the relative amounts of desorption from the weak sites and reaction at the strong sites can therefore be determined more accurately. Determination of Apparent Activation Energies using Linear Heating and CRTA-RJ Techniques. The results of isopropylamine desorption from NaY zeolite carried out using a range of (linear) heating rates (Figure 7) clearly illustrate the expected shift in peak maximum toward higher temperatures

Study of the Reactivity of Solid-Acid Catalysts

Figure 6. Extent of desorption/reaction (R) and surface coverage of isopropylamine versus temperature for zeolites NaY and HY under linear heating and CRTA conditions: a - zeolite NaY under linear heating, b - zeolite HY under linear heating, c - zeolite NaY under CRTA, d - zeolite HY under CRTA.

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Figure 8. Isoconversional Redhead plots for desorption/reaction of isopropylamine for zeolites at (linear) heating rates, β, of 2, 4, 8, and 16 °C/min: a - physical desorption from zeolite NaY, b - physical desorption from zeolite HY, c - Hoffman reaction at HY.

TABLE 1: Apparent Activation Energies, Ea, for the Desorption/Reaction of Isopropylamine from NaY Zeolite and Its Acidic Form, HY, Calculated Using the Redhead and the CRTA Rate Jump Methods NaY zeolite technique Ea for physical desorption (kJ/mol) Ea for the Hoffman reaction (kJ/mol)

Figure 7. Dependence of the TPD of isopropylamine from NaY zeolite on the (linear) heating rate of the experiment.

with increasing heating rates. A similar dependence of the TPD spectra on heating rate was obtained for HY. Plotting ln(T2max/ β) versus 1/Tmax resulted in straight lines (Figure 8). The apparent activation energies of desorption and surface reaction were calculated using the Redhead eq 2 and are shown in Table 1, where they are compared with the values determined using the CRTA-RJ method. The TPD/TPRx profiles of isopropylamine carried out on NaY and HY using CRTA-RJ are shown in Figures 9 and 10, respectively. The underlying “constant rates” (i.e., the lower levels) were chosen to provide enough time to achieve a reasonable number of rate jumps during each experiment. The upper reaction rates were sufficiently different from the lower values to ensure the corresponding changes in temperature could be measured accurately. Furthermore, the difference in the two rates was not so great as to risk a change in mechanism or to have a major influence on the rate of diffusion throughout the desorption/reaction processes. For each rate jump, a value for Ea was obtained using eq 3, thereby allowing any variation in activation energy with extent of desorption/reaction to be determined. Figure 11 illustrates the variation of Ea as function of surface coverage, assuming that the extent of desorption, R,

HY

linear linear heating CRTA-RJ heating CRTA-RJ 60

70

64

75

104

140

is equal to (1 - θ) where θ is the fraction of surface coverage. The apparent activation energy of desorption of isopropylamine from NaY, measured from a surface coverage 0.18 to 0.9 (curve a), was found to be constant through the entire process and was reasonably similar to the value determined for its acidic form, i.e., the HY zeolite, over the range of coverage from 0.8 to 0.9. Curve b clearly shows the distribution of the weak and strong acid sites present in the HY zeolite. As the strong acid sites (coverage from 0.1 to 0.65) were seen to be uniform in strength, their values were averaged to obtained the data shown in Table 1. The strengths of the weak sites, calculated from the desorption of unreacted isopropylamine, are reasonably similar for both the NaY and HY zeolites, although the CRTA-RJ values are ca. 17% higher than those found using conventional linear heating experiments. The apparent activation energy for the surface reaction of isopropylamine at the strong Brønsted sites of the HY zeolite was approximately 1.6 times greater than that found for physical desorption from the weak sites using the Redhead method and around 1.9 times greater for the CRTA-RJ method. The activation energies calculated via the Redhead equation, using conventional linear heating techniques, were always found to be less than the values obtained by the CRTA-RJ method. Both methods measure the apparent activation energy which will include a contribution from diffusion. However, during conventional linear heating TPD/TPRx the rate of desorption/ reaction has a maximum whose magnitude increases with increasing heating rate. Therefore, at higher heating rates the concentration of the desorbed species inside the porous solid will be significantly greater than found with low heating rates, the diffusion of gases from the micropores may well be rate

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Figure 9. CRTA rate jump experiment of desorption of isopropylamine from zeolite NaY (note that the traces are displaced vertically for clarity): m/z ) 17 - ammonia, m/z ) 41 - propene/isopropylamine, m/z ) 44 - isopropylamine.

determining and the desorption/reaction of isopropylamine will be no longer be the observed rate-limiting step. Thus, the internal pressure build-up may well affect the rate of removal of product and hence the observed reaction rate, resulting in a change in the slope of the Redhead equation so as to reduce the apparent value of the activation energy. In contrast, the reaction rate in the CRTA experiments is much lower (2-5%) than the maximum obtained during the linear heating runs, so the effects of diffusion outlined above will be insignificant, thus giving a potentially more accurate estimate of the true activation energy. The influence of the enthalpy changes on diffusion, desorption, and reaction also cannot be ignored, and their effects will again vary from one heating rate to another. The use of the SIP alone does not eliminate these problems. However, the use of the (relatively) very low reaction rates used in CRTA experiments provides more time for the system to accommodate these changes and so reduces the local variations in temperature thus caused, again providing better conditions for measuring Ea. Therefore, the combination of the custom-designed SIP-MS with CRTA minimizes the effect of errors both in the sample temperature and those caused by diffusion and concentration gradients, and so permits a more accurate determination of the activation energy of the Hoffman reaction at the Brønsted sites and the energy of desorption of the intact amine from weaker sites than conventional temperature programmed methods. Not only are CRTA techniques useful in calculating the energies of desorption/reaction from weak and strong acid sites, but they can also provide information on the distribution of the these sites in the HY zeolite. Figure 11 shows that ca. 70% of the available surface sites of the NaY zeolite after ammonium cation exchange and calcination (i.e., HY) are strongly acidic.

Fesenko et al.

Figure 10. CRTA rate jump experiment of desorption/reaction of isopropylamine from zeolite HY (note that the traces are displaced vertically for clarity): m/z ) 17 - ammonia, m/z ) 41 - propene/ isopropylamine, m/z ) 44 - isopropylamine.

Figure 11. Dependence of the apparent activation energies on surface coverage for the physical desorption of isopropylamine from zeolites NaY (a) and HY (b) and the elimination reaction on HY (b).

It is concluded that the technique is capable of estimating the distribution of acid sites in materials possessing a range of strengths. Conclusions The desorption/reaction of isopropylamine from NaY and HY zeolites has been investigated under CRTA and conventional linear heating conditions. The use of constant rate techniques for the study of the TPD and TPRx from porous solids has been

Study of the Reactivity of Solid-Acid Catalysts shown to have several potential advantages over conventional linear heating experiments. The apparent activation energies of the desorption of isopropylamine from NaY and HY zeolites were found to be similar at about about 70 kJ/mol, while that of the reaction at strong Brønsted sites was approximately twice this value. Values obtained using the linear heating rate techniques were ca. 60 and 100 kJ/mol, respectively. It is suggested that the effects of diffusion in the case of linear heating experiments could be a cause of errors in the measurement of the activation energy of the order of ca. 20%. By using CRTA and the rate-jump method, it is possible to clearly distinguish the desorption of the unreacted base from the acid site induced reaction. Assuming the activation energy of physical adsorption is negligible, the values found for the apparent activation energy of desorption of isopropylamine from the NaY zeolite reflect the enthalpy of adsorption and therefore the acidic site strength. A similar calculation is not possible for the HY zeolite as the process occurring here is the surface catalyzed reaction. However, this is a direct measure of the catalytic activity of the material and so complements the information provided by adsorption microcalorimetry. The new method is capable of determining the distribution of the reactivity of acid sites and, as such, should find wide application in the field of materials characterization and catalytic studies. Further work is in hand to quantity the relative effects of diffusion and the accuracy of temperature measurement on determination of the apparent activation energies. Acknowledgment. This work was funded by the Engineering and Physical Sciences Research Council (EPSRC), via Grant No. GR/L66069. References and Notes (1) Gorte, R. J. Catal. Lett. 1999, 62, 1. (2) Selli, E.; Forni, L. Micropor. Mesopor. Mat. 1999, 31, 129. (3) Umansky, B. S.; Hall, W. K. J. Catal. 1990, 124, 97. (4) Freude D.; Ernst, H.; Mildner, T.; Pfeifer, H.; Wolf, I. Stud. Surf. Sci. Catal. 1994. 90, 105.

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