1852
J. Phys. Chem. 1996, 100, 1852-1857
Influence of the Chemical Composition upon Adsorption, Coadsorption, and Reactivity of Ammonia and Methanol on Alkali-Exchanged Zeolites Andreas Kogelbauer,† Christian Gru1 ndling, and Johannes A. Lercher* Department of Chemical Technology, UniVersity of Twente and Christian Doppler Laboratory for Heterogeneous Catalysis, P.O. Box 217, 7500 AE Enschede, The Netherlands ReceiVed: August 8, 1995; In Final Form: October 16, 1995X
The interaction of ammonia and methanol with several alkali zeolites was studied by means of IR spectroscopy, mass spectrometry, and thermogravimetry. While ammonia adsorbed identically on all zeolites investigated, the adsorption complex of methanol subtly depended upon the concentration of lattice aluminum in the zeolite. This is attributed to the variation of the polarizability of the zeolite lattice induced by the varying chemical composition. With respect to the reaction of methanol with ammonia, the differences in the sorption of methanol led to variations in the reactivity of these zeolites toward the primary products monomethylamine and dimethyl ether. On zeolites with a high aluminum content, both products were formed in approximately equal concentrations, while with silicon-rich zeolites monomethylamine was the dominating product. With all alkali zeolites investigated methanol was significantly stronger adsorbed than ammonia.
I. Introduction Low molecular weight amines such as methylamines represent important raw materials for the synthesis of nitrogen-containing organic compounds. Industrially, these amines are produced by reacting methanol (MeOH) and ammonia (NH3) in a fixed bed reactor using silica-alumina catalysts.1,2 With such amorphous acid catalysts, reaction proceeds completely to thermodynamic equilibrium which favors trimethylamine (TMA) as product.3,4 Under typical reaction conditions, i.e., at 623 K, a reactant ratio of 1.9 (NH3:MeOH), and 99.8% MeOH conversion, the equilibrium composition of the products consists of 15.1% monomethylamine (MMA), 22.8% dimethylamine (DMA), and 62.1% TMA.5 Because the global demand for TMA is the lowest,6 it is usually recycled with excess NH3 to enrich the product stream with mono- and disubstituted amines. Recycling and reequilibration of TMA, however, is rather costly, making a direct, selective synthesis of low substituted methylamines desirable. The attempt to use zeolites for this purpose consequently dates back already some 30 years.7 The shape selectivity was thought to be important to achieve high selectivity in the production of low substituted amines. It is generally assumed that on zeolites with small pores amine synthesis is governed by product selectivity (i.e., bulky TMA molecules are retained inside the zeolite pores) or by the restriction of the formation of the bulkier products due to steric constraints. Although a number of zeolites were tested, early results showed only a minor gain in selectivity toward MMA and DMA at MeOH conversions above 90%.8-13 Over the past decade, however, substantial improvement was achieved through tailoring of the zeolite pores and their acidity. The selectivity by geometric constraints14-20 and the chemical composition of a given molecular sieve were reported to markedly influence product distribution.4,21-23 Ashina et al.4 showed that for partially alkali-exchanged mordenites a linear correlation exists between the alkali metal cation concentration and the selectivity toward lower substituted methylamines. Similarly, Weigert23 reported enhanced MMA and DMA † Present address: Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zu¨rich, Switzerland. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-1852$12.00/0
selectivity when using sodium mordenite. The rates were, however, significantly lower compared to acidic mordenites. The observed changes in selectivity were related to preferred adsorption of MeOH over NH3 on sodium cations. While this has led to a good empirical basis for catalyst development, the mechanistic aspects of the process and our knowledge of how the catalyst acts are less developed. Most reports use kinetic data to conclude on the importance of geometric factors and the strength of the catalytically active sites. Attempts to directly correlate adsorbed species and the reactivity of the zeolite, however, have not been undertaken. In an earlier communication,24 we reported on interactions of MeOH and NH3 on a series of partially ion-exchanged erionites in which the identified coadsorption complexes are likely reaction precursors. The present study addresses the role of the Si/Al ratio of alkali zeolites on (i) the adsorption complexes during the coadsorption of MeOH and NH3 and (ii) the catalytic activity and selectivity in the direct amination of MeOH. II. Experimental Section 1. Zeolites. The alkali forms (Na, K) of mordenites and erionites were used for this study. NaK erionite (NaK-ERI) had a SiO2/Al2O3 ratio of 6. As shown earlier,25 potassium occupies preferentially the inaccessible hexagonal prisms and cancrinite cages of erionite. Mordenite samples (NaMOR10, NaMOR20) with a SiO2/Al2O3 ratio of 9.8 and 20.2, respectively, were obtained from the Japanese Catalysis Society. Detailed analysis and characterization of these zeolites were reported by Kogelbauer and Lercher24 and Sawa et al.26 For an easier comparison of the differences in catalyst composition the unit cell compositions as calculated from the Si/Al ratios are compiled in Table 1. 2. Thermogravimetric Measurements. A Cahn RG microbalance was used for the determination of adsorption isotherms. The balance could be evacuated to pressures below 10-3 Pa. Approximately 10 mg of the catalysts (pressed into wafers) was used for each adsorption experiment to assure comparable pretreatment as for the infrared (IR) measurements. After activation in vacuum at 820 K, the adsorbates were introduced at 308 K through a gas-dosing valve and kept at the desired partial pressure (0.1-103 Pa) by differential pumping. © 1996 American Chemical Society
Reactivity of NH3 and MeOH on Alkali Zeolites
J. Phys. Chem., Vol. 100, No. 5, 1996 1853
TABLE 1: Unit Cell Compositions of the Alkali Zeolites zeolite
unit cell composition
Si/Al
Al/100 TO2 units
NaK-ERI NaMOR10 NaMOR20
Na41K4.9(AlO2)9(SiO2)27 Na8.1(AlO2)8.1(SiO2)39.9 Na4.4(AlO2)4.4(SiO2)43.6
3.0 4.9 10.1
30.0 20.4 10.9
TABLE 2: Molecules Adsorbed per Unit Cell and Alkali Cation on the Alkali Zeolitesa equilibrium pressure (Pa) 10-1 zeolite NaK-ERI
sorptive
MeOH NH3 NaMOR10 MeOH NH3 NaMOR20 MeOH NH3 a
101
1
102
103
C
UC
C
UC
C
UC
C
UC
C
UC
0.7 0.7 0.5 0.4 0.9 0.8
2.6 2.4 4.1 3.3 3.9 3.5
1.3 0.9 0.8 0.6 1.3 1.0
5.3 3.4 6.5 4.9 5.6 4.3
1.8 1.0 0.9 0.8 1.7 1.4
6.4 4.7 7.3 6.5 7.4 6.1
2.2 2.0 1.1 1.0 2.0 1.9
8.7 8.1 9.0 8.1 8.7 8.2
2.6 2.6 1.2 1.3 2.4 2.5
10.5 10.5 9.8 10.6 10.4 10.8
C ) per sodium cation, UC ) per unit cell.
3. Infrared Spectroscopy Measurements. IR spectra were recorded in situ with a Bruker IFS88 FTIR spectrometer using the transmission-absorption technique. The IR cell was equipped with a heatable sample holder and could be evacuated below 10-4 Pa. The spectra (with a resolution of 4 cm-1) were base line corrected in the range from 3800 to 1300 cm-1. During temperature-programmed desorption (TPD) of adsorbates, the gas phase was monitored using a Balzers QMG 311 mass spectrometer. More experimental details can be found elsewhere.24 The catalyst samples were pressed into self-supporting wafers, heated in vacuum at 10 K/min to 820 K, and held at that temperature for 1 h. Adsorption experiments were carried out at 308 K. The adsorptives (MeOH or NH3) were admitted into the vacuum system Via a gas manifold through a gas-dosing valve applying differential pumping to keep the partial pressures constant. Equilibration of the catalysts with NH3 or MeOH at pressures ranging from 10-2 to 102 Pa was followed by timeresolved IR spectroscopy. Quantitative analysis of the spectra was achieved by correlation with the gravimetric results obtained under the same experimental conditions. For TPD, the system was evacuated below 10-4 Pa for 1 h and subsequently heated to 873 K at a heating rate of 10 K/min. For coadsorption experiments, the zeolites were equilibrated at 308 K with 1 Pa of one adsorbate. Then, maintaining the partial pressure of the first adsorptive, additionally 1 Pa of the second adsorbate was introduced. On each zeolite, identical IR spectra were obtained irrespective of the sequence of adsorption. Thus, we conclude that local adsorption-desorption equilibrium exists. 4. Kinetic Measurement. All kinetic measurements were carried out in a tubular microreactor operated under differential conditions. The reactor effluent was analyzed by gas chromatography using a packed column (3 m stainless steel column packed with 25% Carbox 400 and 2.5% KOH on acid-washed Chromosorb W) for separation as described by Weigert.23 The rates reported were measured at 573 K, 103 Pa of MeOH, and 4 × 103 Pa of NH3 balanced with helium to atmospheric pressure. The rates were normalized to the concentration of sodium cations (as turnover frequency, TOF). Due to the sequential nature of the reaction, low conversions always favor MMA and dimethyl ether (DME) over secondary (DMA) and tertiary (TMA) products. Thus, we focus only on the selectivity to primary products and do not discuss the results in terms of methylamine selectivity. Apparent energies of activation were measured between 573 and 653 K. For the determination of the reaction orders the partial pressures were varied between
Figure 1. Differences between the IR spectra of the alkali zeolites equilibrated with ammonia (1 Pa) and the activated zeolite: (a) NaKERI, (b) NaMOR10, (c) NaMOR20.
5 × 102 and 7 × 103 Pa of NH3 and 5 × 102 and 2.4 × 104 Pa of MeOH while keeping the partial pressure of the other reactant constant (i.e., 103 Pa of MeOH and 4 × 103 Pa of NH3). III. Results Since the coadsorption of NH3 and MeOH on NaK-ERI has been discussed elsewhere,24 results pertaining to NaK-ERI will generally only be cited. Nevertheless, we will include the results for NaK-ERI wherever it is necessary for the discussion or if the data were not reported earlier.24 1. Thermogravimetry. The concentrations of adsorbed species at various equilibrium pressures are compiled in Table 2. For all alkali zeolites, the Na+ cations are considered as primary adsorption sites,27,28 because only the sodium cations are accessible in NaK-ERI.25 Below 102 Pa higher amounts of MeOH than of NH3 were adsorbed, suggesting a higher adsorption equilibrium constant for MeOH on the catalysts. At higher pressures, nearly identical amounts of NH3 and MeOH were adsorbed per unit cell, indicating the limits of adsorption capacity. 2. Temperature-Programmed Desorption of Ammonia and Methanol. During TPD of NH3 from the alkali zeolites, desorption was complete at 570 K, indicating adsorption sites of weak to moderate strength. The maximum in the rate of desorption was found at approximately 400 K for all samples, suggesting that the investigated zeolites have comparable acid strength. TPD of MeOH was similar to that of NH3. The desorption maximum for MeOH was also found around 400 K with traces of DME at higher temperatures. The maximum rate of DME formation was observed at 480 K with NaK-ERI and at 550 K with the mordenites. During all desorption processes the IR bands of NH3 and MeOH decreased without indicating the formation of adsorbed reaction products. 3. IR Spectroscopy. 3.1. Adsorption of Ammonia and Methanol. All zeolites showed similar IR spectra after adsorption of NH3 at 1 Pa (Figure 1, Table 3), suggesting that NH3 adsorbed in the same way on all alkali zeolites, i.e., interacting Via the nitrogen lone pair electrons with the sodium cations. The higher intensity of the bands on NaMOR10 is attributed to the higher concentration of the sodium cations in comparison to NaK-ERI and NaMOR20. For all zeolites three bands attributed to NH stretching vibrations (Table 3) and a band at around 1640 cm-1 attributed to NH deformation vibration were observed. Additionally, a broad band between 1600 and 1500 cm-1 appeared.
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TABLE 3: Assignments of the IR Bands (in cm-1) upon Adsorption of Ammonia and Methanol and upon Coadsorption on the Alkali Zeolites at 1 Paa sorptive NH3 MeOH
MeOH + NH3
a
vibr
NaK-ERI
NaMOR10
NaMOR20
νNH δNH νOH δOH νCH δCH νNH δNH νOH δOH νCH δCH comb.
3383, 3309, 3260 1640, 1600 3560, 3440 1405, 1355 2952, 2843 1472, 1464 3383 1640 3560, 3426 1500, 1355 2952, 2843 1467, 1450 3050, 2720, 2610, 2586, 2500
3383, 3314, 3275 1644, 1560 3580, 3470, 3300 1407, 1370 2954, 2846 1475, 1462, 1452 3383, 3314 1644 3580, 3470, 3300 1407, 1370 2954, 2846 1475, 1462, 1452 3050, 2720
3386, 3317, 3275 1640, 1500 3600, 3480, 3270 1403, 1360 2956, 2852 1473, 1462, 1450 3386, 3317, 3265 1640, 1510 3600, 3480, 3270 1403, 1360 2956, 2852 1473, 1462, 1450
Numbers in italic denote very weak bands.
Figure 2. Differences between the IR spectra of the alkali zeolites equilibrated with methanol (1 Pa) and the activated zeolite: (a) NaKERI, (b) NaMOR10, (c) NaMOR20, (d) NaZSM5.
Figure 3. Differences between IR spectra of NaMOR10 after coadsorption of methanol and ammonia and (a) the activated zeolite, equilibration with (b) 1 Pa of NH3 and (c) 1 Pa of methanol.
The observed IR bands after equilibration with MeOH at 1 Pa on the three zeolites investigated are compiled in Table 3. The cations were considered the primary adsorption sites interacting with the lone pair electrons of the MeOH oxygen.24 Nevertheless, large differences in the OH stretching vibrations of adsorbed MeOH were observed between the samples studied, as can be seen in Figure 2. The IR spectrum of 1 Pa of MeOH adsorbed on NaZSM5 (Si/Al ) 35) is included for comparison. While MeOH on NaZSM5 exhibited only one sharp OH stretching band at 3614 cm-1 (close to gas phase MeOH), this OH band appeared at 3600 cm-1 with NaMOR20, at 3580 cm-1 with NaMOR10, and at 3560 cm-1 with NaK-ERI. The halfwidth of these bands increased with increasing aluminum concentration of the zeolites. Additionally, a second OH stretching vibration band appeared on these samples around 3480-3470 cm-1. This agrees well with the presence of two OH deformation bands for MeOH adsorbed on these zeolites (1355 and 1405 cm-1 with NaKERI, 1370 and 1407 cm-1 with NaMOR10, and 1360 and 1407 cm-1 with NaMOR20) which vary in intensity parallel to the changes of the OH stretching vibrations (3560 and 3440 cm-1 with NaK-ERI, 3580 and 3470 cm-1 with NaMOR10, and 3600 and 3480 cm-1 with NaMOR20). For the mordenite samples an additional band attributed to an OH stretching vibration was observed between 3300 and 3270 cm-1, which did not increase in intensity upon increasing MeOH partial pressure above 1 Pa. Besides the significant differences of the OH vibrations, a shift of the asymmetric and symmetric CH stretching vibration from 2957 and 2853 cm-1 (NaZSM5) to 2952 and 2843 cm-1 (NaK-ERI) was observed (see Table 3).
3.2. Coadsorption of Methanol and Ammonia. On NaMOR10, intense bands due to OH and CH vibrations of surface bound MeOH were observed (Figure 3a). Very weak bands at 3383 and 3314 cm-1 indicate the presence of small amounts of adsorbed NH3 whose generally less intense deformation vibration was hardly observed. The relative band intensities suggest that the concentration of adsorbed MeOH exceeds that of NH3. Coadsorption of MeOH on a zeolite preequilibrated with NH3 (Figure 3b) resulted in a decrease in the NH3 concentration (seen by the decrease in the bands at 3383, 3314, 3270 (weak), and 1644 cm-1), while the bands of adsorbed MeOH (see Table 3, Figure 3b) increased. After reversing the adsorption sequence (i.e., coadsorption of NH3 onto NaMOR10 preequilibrated with MeOH (Figure 3c)), only small negative bands characteristic for adsorbed MeOH were observed, indicating that minute amounts of adsorbed MeOH were displaced by NH3. In addition to the bands assigned to adsorbed NH3, broad bands of very weak intensity were observed at 3050 and 2720 cm-1. Similar but more intense bands were also observed in the corresponding spectrum of NH3 and MeOH in contact with NaK-ERI (compare also Figure 4). These bands indicate a change in the chemical environment of adsorbed MeOH upon coadsorption of NH3, manifested by the formation of additional hydrogen bonds of MeOH with NH3, rather than the displacement of MeOH from the adsorption sites.24 The IR spectrum of NaMOR20 after coadsorption of NH3 onto the zeolite preequilibrated with MeOH lacked these bands, and only replacement of MeOH by NH3 (and Vice Versa) was observed. On all zeolites, the concentration of MeOH exceeded that of NH3 upon coadsorption at 308 K.
Reactivity of NH3 and MeOH on Alkali Zeolites
J. Phys. Chem., Vol. 100, No. 5, 1996 1855
Figure 4. Differences between IR spectra of (a) NaMOR20 and (b) NaK-ERI after coadsorption of CD3OH (1 Pa) and ammonia (1 Pa) and equilibration with CD3OH (1 Pa).
Figure 5. Dependence of reaction rates of MeOH, MMA, and DME on the variation of the (a) ammonia and (b) methanol partial pressure on NaMOR10.
TABLE 4: Observed Selectivities (mol %) for the Conversion of Methanol (103 Pa) in the Presence of Ammonia (4 × 103 Pa) at 573 K
and DME were the main products for all alkali zeolites; however, their ratio varied drastically with the zeolite composition. On NaK-ERI they were formed in equal amounts whereas on NaMOR20 MMA was the main product with only a small amount of DME formed. DMA was present to a small extent, and TMA was hardly found with any of the zeolites under these reaction conditions. The apparent energies of activation for the formation of MMA and DME and TOFs for these primary products at 573 K are compiled in Table 5. The formation of MMA showed a nearly constant energy of activation of approximately 77 kJ/mol on all zeolites studied. The TOF for MMA formation increased and that for DME formation decreased from NaK-ERI to NaMOR20. The apparent energy of activation for the formation of DME was found to increase from 67 kJ/mol on NaK-ERI to 93 kJ/mol on NaMOR20. The trends observed upon variation of the partial pressure of one reactant while keeping the partial pressure of the other reactant constant were similar for all alkali zeolites. As an example, the effects of partial pressure upon rates are depicted in Figure 5 for the reaction on NaMOR10. An increase of the NH3 partial pressure led to an increase in the rate of MeOH consumption (Figure 5a). This was mainly caused by an increase in the rate of MMA formation while the rate of DME formation remained essentially constant up to a NH3/MeOH ratio of 8. The variation of the MeOH partial pressure (Figure 5b) resulted in an increase of the total rate up to a reactant ratio of 1. With higher MeOH concentrations, the rate of MMA formation decreased strongly, while the rate of DME formation increased in parallel. The changes in the rates with varying reactant partial pressures are in good agreement with a Langmuir-Hinshelwood (L-H) model describing the reaction. This model can be applied for all alkali zeolites, indicating that both reactants are adsorbed on the sodium cations prior to reaction to MMA and DME, which was also confirmed by in situ IR experiments on the working catalyst. Note that under the investigated conditions (i.e., low methanol conversion) only the reactants, MeOH and NH3, were adsorbed in significant concentrations. The results of the kinetic fits are compiled in Table 6 and are also shown in Figure 5 for comparison. In agreement with the IR spectroscopic measurements (under nonreactive and reactive conditions), the calculated adsorption constant of MeOH was higher than that for NH3 on all catalysts. It is interesting to note that the differences between the adsorption constants of
zeolite
MMA
DMA
TMA
DME
NaK-ERI NaMOR10 NaMOR20
46 83 89
9 5 8
0 0 0
45 12 3
TABLE 5: Activation Eneriges and Reaction Rates Normalized to the Amount of Sodium Cations for the Conversion of Methanol (103 Pa) with Ammonia (4 × 103 Pa) at 573 K MMA zeolite
EA (kJ/mol)
NaK-ERI NaMOR10 NaMOR20
77 81 74
TOF
DME (s-1)
1.6 × 10-5 2.3 × 10-5 4.4 × 10-5
EA (kJ/mol)
TOF (s-1)
67 81 93
2.9 × 10-5 6.3 × 10-6 3.3 × 10-6
Experiments with CD3OH were carried out in order to differentiate between the different zeolite-adsorptive complexes resulting from the coadsorption of MeOH and NH3. The use of deuterated alcohol allows unobstructed observation of the bands of perturbed OH groups because the CD stretching vibrations are shifted to about 2200 cm-1. Figure 4 shows the effect of NH3 coadsorbed with CD3OH on NaMOR20 and NaKERI preequilibrated with 1 Pa of CD3OH. For NaMOR20, positive bands at 3386, 3317, 3275, and 1640 cm-1 indicate the presence of adsorbed NH3. CD3OH was displaced in part from its original adsorption sites by NH3, which can be seen from the negative bands at 3600 cm-1 (νOH) and 2235, 2138, and 2082 cm-1 (νCD). Note that coadsorption of NH3 primarily affected MeOH molecules characterized by the OH stretching vibration at 3600 cm-1. On NaK-ERI the positive bands at 3383 and 1640 cm-1 were attributed to NH3 and the broad negative bands between 3570 and 3200 cm-1 to the OH stretching vibrations of MeOH. In contrast to NaMOR20, additional bands were visible at 3050, 2720, and 1500 cm-1, and the bands in the CD stretching region were found at lower wavenumbers. 4. Kinetics. All alkali zeolites were found to be active for the reaction of MeOH with NH3 in the investigated temperature range. At 573 K, the rate of MeOH consumption normalized to the amount of sodium cations (TOF) was similar for all zeolites (i.e., 4.8 × 10-5 s-1 for NaK-ERI, 3.2 × 10-5 s-1 for NaMOR10, and 5.5 × 10-5 s-1 for NaMOR20). The selectivities observed for this reaction are compiled in Table 4. MMA
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TABLE 6: Constants Obtained by Fitting the Reaction Results for the Amination on Alkali Zeolites at 573 K with Langmuir-Hinshelwood Kinetics adsorption zeolite
KMeOH (1/Pa)
KNH3 (1/Pa)
reaction kMMA (mol/(g s))
kDMe (mol/(g s))
NaK-ERIa >5.9 × 10-5
