Thermodynamic Features of the Reaction of Ammonia with the Acidic

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Thermodynamic Features of the Reaction of Ammonia with the Acidic Proton of H-ZSM-5 As Studied by Variable-Temperature IR Spectroscopy M. Armandi,† B. Bonelli,*,† I. Bottero,† C. O. Area´n,‡ and E. Garrone† Department of Materials Science and Chemical Engineering, Politecnico di Torino and INSTM Unit-Torino Politecnico, Corso Duca degli Abruzzi 24, I-10129 Turin, Italy, and Departamento de Quı´mica, UniVersidad de las Islas Baleares, 07122 Palma de Mallorca, Spain ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: March 9, 2010

The interaction of ammonia with H-ZSM-5 zeolite (Si/Al ) 11.5) was studied by means of variable-temperature infra-red spectroscopy, in the temperature range of 570-680 K. The interaction consists mainly in the reaction of ammonia with the zeolite Brønsted acid OH groups (absorbing at ca. 3610 cm-1) to form ammonium species that show a bending mode at ca. 1410 cm-1. From the changes in intensity of these IR absorption bands at 3610 and 1410 cm-1, with both temperature and NH3 equilibrium pressure, the standard enthalpy of reaction was calculated to be ∆H0 ) -128 ((5) kJ mol-1. The corresponding entropy change is ∆S0 ) -184 ((10) J mol-1 K-1, a figure suggesting that the ammonium cation interacting with the negative framework has little residual degrees of freedom. These results are discussed in the broader context of relevant data reported in the literature. Introduction The Brønsted acidity of protonic zeolites constitutes an important phenomenon from both the conceptual and the practical point of view.1-4 On the practical side, Brønsted acid zeolites are currently used as solid acid catalysts in a wide range of industrial applications related, mainly, to fuel processing and to the production of fine chemicals.5-12 On the conceptual side, whereas the origin of Brønsted acidity is readily assigned to the Si-(OH)-Al group having a hydroxyl bridging a Si and an Al atom, the actual definition of such acidity and its correct measurement still pose some problems.2,10-13 The proper definition of acidity is the deprotonation energy, that is, the energy required to abstract a proton from the acidic species to infinity.14 Such a quantity may be calculated via quantum mechanics but is not accessible to measurement. The classical procedure for evaluating acidity is, therefore, to consider the thermodynamic properties of the proton-transfer reaction from the acidic hydroxyl species of the zeolite (hereafter, denoted as Z-OH) to a suitable acceptor partner, M:

Z-OH + M / Z-O- + MH+

(1)

The acceptor molecule of choice is usually either ammonia or pyridine,2,15 and the instrumental technique is nearly always temperature-programmed desorption (TPD),16-23 which measures the activation energy of reaction 1, as read from right to left. Problems, however, may arise because of diffusion of the M molecule inside zeolite pores and its readsorption and also because different protonic zeolites can stabilize the MH+ species to a different extent.24 The direct measurement of the standard enthalpy of reaction 1, ∆H0, implies microcalorimetry, a technique not so wide* To whom correspondence should be addressed. E-mail: [email protected]. Fax: +39-011-5644699. † Politecnico di Torino and INSTM Unit-Torino Politecnico. ‡ Universidad de Las Islas Baleares.

spread.25-28 In addition to that, proton transfer may not be the only process occurring, for instance, because Lewis acidity can be present along with Brønsted acidity, whereas calorimetry is an all-encompassing technique not suitable for operating the appropriate distinction between the two processes. Besides, it may be noted that the other basic thermodynamic quantity, ∆S0, is not experimentally accessible by using calorimetry at ambient temperature because reaction 1 is usually quite exothermic and totally shifted to the right under such circumstances. A refined version of TPD, developed by Niwa and coworkers19,22 for studying the acidity of zeolites through ammonia adsorption, envisages the coupling of this technique with other instrumental means, in a procedure termed infrared mass spectrometry/temperature-programmed desorption (IRMS-TPD): the behavior of a significant IR absorption band of the ammonium surface species is followed as a function of temperature, while the sample is subjected to a programmed rise in temperature under flux of an inert gas. Equilibrium between the adsorbed and the gaseous phase is assumed, somewhat arbitrarily. In the present paper, we propose a method, not too distant conceptually from that of Niwa and co-workers, but ensuring a priori equilibrium between the adsorbed and the gaseous phase. This method, which allows simultaneous determination of both the standard enthalpy and the entropy changes is based on variable-temperature infrared (VTIR) spectroscopy, whereby the intensity of a significant IR absorption band of the system under consideration is studied as a function of temperature and ammonia pressure in a closed system. Such a technique has been developed so far only for cases where weak adsorption interactions are concerned, by considering temperature ranges below the ambient temperature.29-36 Only very recently, an example of a stronger adsorption interaction has been considered, requiring a temperature range above ambient.37 In the present paper, we move on to consider proper reactions and a temperature range extending considerably above ambient temperature.

10.1021/jp100799k  2010 American Chemical Society Published on Web 03/19/2010

Reaction of Ammonia with the Acidic Proton of H-ZSM-5 Materials and Methods

Equation 2 can also be written as

Materials. The H-ZSM-5 zeolite sample used, obtained from Zeolyst International, was in the ammonium form (NH4-ZSM5) and had a nominal Si/Al ratio of 11.5. From this parent material, H-ZSM-5 was obtained by thermal treatment inside the IR cell, as described below. Powder X-ray diffraction showed good crystallinity, and all diffraction lines could be assigned to the expected MFI-type structure. VTIR spectroscopy was carried out by using a commercial IR cell (AABSPEC), which was equipped with a capacitance pressure gauge (CTR100, Oerlikon-Leybold) and an electronically controlled heating element. The actual temperature inside the cell was monitored by a K-thermocouple (in contact with the sample wafer) connected to a digital thermometer (CHY 502 A, Tersid). The accuracy of the pressure and temperature measurements was of (0.20% and (0.05% (of the read value), respectively. For VTIR measurements, a wafer of the NH4-ZSM-5 sample (surface density about 5 mg cm-2) was placed inside the IR cell and activated (outgassed) in a dynamic vacuum (residual pressure < 10-3 mbar) for 2 h at 663 K. The cell was then allowed to cool to ambient temperature (298 K), dosed with 15.00 mbar of NH3, and further outgassed at room temperature, to bring back the sample to the fully ammoniated form. Subsequently, the cell was closed and IR spectra were recorded at several fixed temperature values within the range of 570-680 K, and the release of gaseous ammonia under equilibrium conditions was studied. Even at a temperature as high as 680 K, the surface ammonium species was far from being depleted. This is because the evolution of gaseous ammonia, bringing about an increase of the corresponding partial pressure, tends to counteract the thermal desorption. This feature extends considerably the range of temperatures where the VTIR method is applicable, rendering it more reliable. For reasons that will be apparent below, it was also necessary to study the temperature dependence of the spectroscopic features (position, half-width, shape) of the O-H stretching mode of the Brønsted acid hydroxyl species. Therefore, in a separate set of measurements, IR spectra of pure H-ZSM-5 were taken as a function of temperature. Outline of the VTIR Method. Consider a set of IR spectra taken over a temperature range while simultaneously measuring (i) the infrared absorbance, A, of a characteristic band of the MH+ species appearing in eq 1, (ii) the temperature, T, and (iii) the equilibrium pressure, p, due to the presence of the gaseous species M inside the closed IR cell. Suppose that the surface reactants and products constitute an ideal system because all species are equal among themselves and non-mutually interacting: this requirement is readily fulfilled in the case of H-ZSM-5, a low-Al zeolite featuring only one type of acidic hydroxyl species. At any given temperature, the intensity A, being proportional to the extent of the reaction in eq 1, θ, provides information on the activity (in the thermodynamic sense) of the MH+ species, while (1 - θ) monitors the activity of the unreacted Z-OH species; simultaneously, the equilibrium pressure measures the activity of M species in the gas phase. Hence, the reaction equilibrium constant, K, for eq 1 can be determined as

K(T) ) θ/((1 - θ)p)

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(2)

Note that, formally, eq 2 coincides with a Langmuir-type expression for ideal adsorption.

θ ) A/AM ) K(T)p/(1 + K(T)p)

(3)

where AM stands for the maximum absorbance corresponding to a unit extent of reaction, that is, the intensity observed for the NH4-ZSM-5 sample. The variation of K with temperature leads to the corresponding values of reaction enthalpy and entropy. In fact, combining eq 2 with the well-known van’t Hoff relationship 4 yields eq 5

K(T) ) exp(-∆H0 /RT)exp(∆S0 /R)

(4)

ln(A/(AM - A)p) ) -∆H0 /RT + ∆S0 /R

(5)

from which ∆H0 and ∆S0 can be derived, under the usual approximation of considering them as being temperatureindependent. Alternatively, the extent of reaction θ can be evaluated through the IR absorption band of the acidic hydroxyl species. By denoting as B the intensity of the O-H stretching mode at any given temperature and pressure and as BM its maximum intensity, corresponding to that of the fully hydroxylated sample, θ equals (BM - B)/BM, that is, the decrease in intensity of the bridged O-H species band divided by the intensity of the same band when unperturbed. Results Figure 1 reports a set of spectra of the sample carrying ammonium groups (referred to hereafter as ammoniated) as a function of temperature in the 1300-1750 cm-1 range. The intense IR absorption band seen at ca. 1430 cm-1 corresponds to the deformation mode of the ammonium species.1 This band

Figure 1. IR spectra, in the 1750-1300 cm-1 range, taken after adsorption of ammonia on H-ZSM-5 outgassed at 700 K (dotted curve), at the following temperatures (K): (1) 570.0, (2) 583.1, (3) 601.9, (4) 622.9, (5) 643.4, (6) 662.3, and (7) 681.4. Corresponding ammonia equilibrium pressures in the 0.000-1.445 mbar range. Inset: linear dependence of ammonium peak position with temperature.

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TABLE 1: Features of the Spectra Taken at Increasing Temperatures Describing the Deammoniation of the Ammonium Form of ZSM-5 in a Closed System spectrum number

temperature (K)

pNH3 (mbar)

freq of the δNH4+ mode (cm-1)

freq of the νOH mode (cm-1)

A, intensity of the νOH mode

extent of reaction (% OH)

1 2 3 4 5 6 7

570.0 583.1 601.9 622.9 643.4 662.3 681.4

0.000 0.191 0.349 0.554 0.805 1.089 1.445

1432 1430 1426 1421 1419 1416 1411

3606 3602 3601 3598 3596 3595 3594

0.053 0.063 0.083 0.110 0.132 0.156 0.186

88.1 85.7 80.5 73.7 67.5 60.5 51.8

is seen to decrease with increasing temperature and, simultaneously, to undergo a bathochromic shift of about 20 cm-1. The plot of the peak position as a function of temperature is linear and has a temperature coefficient dνmax/dT ) -0.183 cm-1 K-1 (inset in Figure 1). Actual values of temperature and pressure are given in Table 1, together with the corresponding intensity at the band maximum position, A. The intensity of the 1430 cm-1 IR absorption band at its peak (rather than the corresponding integrated intensity) was taken as a measure of the concentration of ammonium species because partial overlap of this band (on its low wavenumbers side) with another strong IR absorption band (likely due to both Si-O stretching and silanol bending modes)38,39 would render band integration very imprecise. Figure 2 reports the same spectra in the O-H stretching region. The main band, which peaks in the range of 3594-3606 cm-1, corresponds to the O-H stretching mode of free Brønsted acid OH groups. As expected, the intensity of this band is temperature-dependent, and as stated above, the extent of the reaction described by eq 1 could, in principle, be determined as (BM - B)/BM, where B is the actual intensity of this band and BM is its maximum intensity. However, because not only the band intensity but also the peak wavenumber (and to some extent, the half-width and profile) change with changing temperature, each spectrum has to be compared with the corresponding one of the ammonia-free sample, H-ZSM-5.

Figure 2. Same IR spectra as in Figure 1, in the O-H stretching range (3800-3450 cm-1). Dotted curve: IR spectrum of the blank zeolite sample before ammonia dosage.

Figure 3 reports the set of spectra concerning the ammoniafree sample at the same temperatures as those in Table 1. The dependence of the peak position on temperature is roughly linear (inset in Figure 3) and can be expressed by the temperature coefficient dνmax/dT ) -0.0424 cm-1 K-1. Note that this value is significantly smaller than that obtained for the ammonium band. Besides peak position, also the half-width shows an approximately linear dependence on temperature, with temperature coefficient d∆ν/dT ) 0.02524 cm-1 K-1. Figure 2 also shows a band at 3735 cm-1, which is due to silanol species, not interacting with ammonia in these experimental conditions. The spectroscopic features of silanol species are well-known,40-42 and the small dependence of their band position on temperature was already studied in detail long ago by Ryason and Russel,43 who calculated a temperature coefficient of 0.0176 cm-1 K-1 for isolated silanols at silica surfaces. Discussion The dependence of the spectroscopic features of the IR bands upon temperature was limited in VTIR measurements carried out within temperature ranges below ambient, or at least was not interfering appreciably with thermodynamic evaluations.29-36

Figure 3. IR spectra, in the 3780-3500 cm-1 range, taken on H-ZSM-5 at the following temperatures (K): (1) 648, (2) 593, (3) 566, (4) 523, (5) 487, (6) 443, (7) 403, and (8) 357. The band of the Brønsted acid OH group moves with temperature from 3596 cm-1 (648 K) to 3609 cm-1 (357 K). Inset: linear dependence of the Brønsted O-H peak position with temperature.

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Figure 4. van’t Hoff plots of the IR bands of Brønsted O-H (white symbols) and NH4+ species (full symbols) as obtained by using absorption band maxima, in both cases.

In contrast, in the present case, the changes occurring in the IR spectra of both the product and the surface species with temperature are remarkable. The phenomenon is probably due to the coupling, through anharmonic terms in the Hamiltonian, of the vibrations under consideration with low-energy vibrations giving rise to hot bands.44 These latter modes are usually not accessible to experimental measurement. Moreover, the extent of anharmonic coupling is not known so that the whole phenomenon of temperature dependence is not really describable a priori. In the present case, the bands concerning the three surface species observed (silanol, bridged species, ammonium cation) have a different dependence upon temperature: in particular, the ammonium species show the most marked effect. This has a direct consequence on the VTIR treatment because the reference band, the intensity of which acts as a benchmark for measuring the extent of reaction, is changing from temperature to temperature. The same observation also concerns the O-H stretching mode: in this case, however, it is possible to relate each intensity at any given temperature to the corresponding full intensity reported in Table 1, obtaining unbiased values of the extent of reaction, θ. The same does not hold for the band of the ammonium species: for this reason, in the following treatment, we will consider as more reliable the data coming from the O-H stretching mode. Figure 4 reports the experimental data obtained by applying eq 5 to the set of data in Table 1. A good linearity is observed (note that no parameter is entering the calculations). The calculated reaction enthalpy is ∆H0 ) -128 ((5) kJ mol-1, whereas the standard entropy of reaction is ∆S0 ) -184 ((10) J mol-1 K-1. The same treatment was applied to the band of the ammonium species, the related data also being reported in Figure 4. Results obtained are only marginally different; that is, ∆H0 ) -129 ((5) kJ mol-1 for the reaction enthalpy and ∆S0 ) -191 ((10) J mol-1 K-1 for the standard entropy. As it concerns literature data, the value for the reaction enthalpy of ∆H0 ) -128 ((5) kJ mol-1 has to be compared, on the one hand, to the microcalorimetry results reported by Parrillo et al.,27 ∆H0 ) -145 kJ mol-1, which is definitely larger. On the other hand, comparison has to be made with the standard enthalpy change measured for a H-ZSM-5 with a similar Si/Al ratio (∆H0 ) -137 kJ mol-1), by means of IRMS-TPD by

Figure 5. Born-Haber cycle for the ion-pair formation from ammonia and H-ZSM-5 zeolite.

Niwa and co-workers.22 This latter value has been confirmed by calorimetric measurement by Tsutsumi et al.45 We are inclined to interpret the larger value found by Parrillo et al. as due to the presence of some Lewis centers with high adsorption heat: instead, the closeness of our value to those reported by Niwa et al.19,22,45 is remarkable. The deprotonation energy (DE) of the Brønsted site in H-ZSM-5 zeolites has been evaluated by several authors by means of both theoretical3 and experimental methods46 and corresponds to an average value of ca. 1300 kJ mol-1. On the basis of this value and of the enthalpy of the proton affinity of ammonia in the gas phase (-850 kJ mol-1),25 it is possible to draw the Born-Haber cycle reported in Figure 5, which allows us to evaluate a stabilization enthalpy, ∆Hst, of ca. -580 kJ mol-1, due to the interaction between the positive cation and the negatively charged framework (dotted arrow in Figure 5). The obtained value is in fair agreement with previous work reporting that the H-ZSM-5 lattice must stabilize the NH4+ ion with an energy of more than 450 kJ mol-1.17 The standard entropy of reaction ∆S0 ) -184 ((10) J mol-1 -1 K refers to a standard state at one millibar, which becomes ∆S0 ) -239 ((10) J mol-1 K-1 at one bar. This value compares reasonably well with the value reported by Niwa and co-

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workers, ∆S0 ) -205.6 J mol-1 K-1, although a direct comparison is not straightforward because, in this latter case, a term due to mixing with the flow gas is present.47 The standard change in entropy may be written as

∆S0 ) S0a - S0g - Ss0

(6)

where S0a is the molar entropy of the NH4-ZSM-5 species, S0g is the standard molar value for gaseous ammonia at 298 K and 1 mbar, which is 248 J mol-1 K-1, and S0s is the molar entropy of the solid before reaction. It results in

S0a - Ss0 ) -∆S0 - S0g ) 184 - 248 ) -64 J mol-1 K-1 (7) Two considerations can be made on such values. First, the standard entropy of reaction is basically due to the loss in entropy of the gaseous ammonia molecule. Second, the resulting molar change in entropy of the solid phase S0a - S0s is negative: the free hydroxyl species appears to have more freedom than the ammonium ion. Indeed, in the former case, low-frequency librational modes of the hydroxyl species are possible: similar modes are probably not allowed in the ammonium cation because of strong electrostatic interactions with the framework. Conclusions In the temperature and pressure range of 570-680 K and 0-1.4 mbar, ammonia adsorbed on H-ZSM-5 gives rise to ammonium NH4+ species, which are characterized by a bending mode in the 1432-1411 cm-1 range. By using a commercially available IR cell operated above ambient temperature, which enables IR absorbance, temperature, and equilibrium pressure to be simultaneously recorded, precise thermodynamic data were obtained for the reaction of NH3 with Brønsted acid hydroxyls of the H-ZSM-5 zeolite. The formation of the ammonium species involves an enthalpy change ∆H0 ) -128 ((5) kJ mol-1: agreement is particularly good with the results reported by Niwa and co-workers and by Tsutsumi et al. A standard entropy change ∆S0 ) -184 ((10) J mol-1 K-1 was calculated as well. The obtained results show that it is possible to use variabletemperature IR spectroscopy in a relatively high-temperature range and open up new perspectives for studying chemical processes involving a rather high interaction energy, like in the present case, where it was possible to measure the acidic strength of Brønsted sites, a property of paramount importance for zeolite applications. Acknowledgment. Financial support from the INSTM (Istituto Nazionale di Scienza e Tecnologia dei Materiali) Consortium is gratefully acknowledged. References and Notes (1) Zecchina, A.; Marchese, L.; Bordiga, S.; Paze`, C.; Gianotti, E. J. Phys. Chem. B 1997, 101, 10128. (2) Lercher, J. A.; Gru¨ndling, C.; Eder-Mirth, G. Catal. Today 1996, 27, 353. (3) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. ReV. 1994, 94, 2095.

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