Ind. Eng. Chem. Res. 2000, 39, 131-137
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Removal of N2O from Industrial Gaseous Streams by Selective Adsorption over Metal-Exchanged Zeolites G. Centi,*,†,‡ P. Generali,† L. dall’Olio,† and S. Perathoner‡ Dipartimento Chimica Industriale ed Ingegneria dei Materiali, Universita´ di Messina, Messina 98166, Italy, and Dipartimento Chimica Industriale e dei Materiali, Universita´ di Bologna, 40136 Bologna, Italy
Z. Rak Netherlands Energy Research Foundation ECN, 1755 ZG Petten, The Netherlands
The behavior of ZSM5 zeolites in the acid, sodium form or ion-exchanged with various metal ions (Cs, Sr, Ba, Mg, Fe, Co, Cu, Ag, Rh, Ga, In, La, and Ce) and that of Y and X zeolites ionexchanged with Ba ions are discussed with reference to their behavior in the adsorption of N2O from gaseous industrial streams containing low (5%) stream of N2O for either autothermic operation of the decomposition of N2O or use of N2O as a selective oxidant. Ba-ZSM5 show significantly better performances than other metal-exchanged zeolites in the adsorption of N2O, although water competes with N2O for adsorption. However, the adsorption of N2O is faster than that of water, and thus it is possible to adsorb N2O even in the presence of larger (40 times higher) amounts of water in the feed. It is also shown that it is possible to have a dual-bed selective adsorption, where first water adsorbs selectively on an alumina bed and then N2O is adsorbed on a Ba-ZSM5 bed. It is suggested that the enhanced adsorption properties of Ba-ZSM5 are related to the presence of naked Ba2+ ions that creates a strong electrostatic field within the zeolite cavities which allows the orienting of the N2O molecule and its trapping inside the zeolite. 1. Introduction The United Nations Framework Protocol on Climate Change (Kyoto, Japan, 1997) calls for the reduction of nitrous oxide emissions and not only of CO2. Emissions of N2O that can be reduced in the short term are associated with combustion processes and chemical productions,1,2 but only emissions of N2O from adipic acid production plants have been in some cases reduced on a voluntary basis. The total amount of N2O emitted worldwide from chemical productions (in particular, during the production of nitric acid and its use as in the caprolactam synthesis) is significantly larger than that emitted from adipic acid plants, but the low concentration of N2O in the emissions (often below 0.10.2%) makes economically unfeasible the application of the catalytic processes of N2O decomposition used to treat the emissions of N2O from adipic acid plants.2 There thus still exists the need for development of an efficient and economic technology for the removal of N2O. A possible interesting solution is to selectively recover N2O on an adsorbent to produce during the desorption a concentrated stream of N2O. In fact, N2O is a very selective and valuable reactant in some oxidation reactions. In the case of adipic acid production, in which emissions are characterized by high N2O concentrations (>30% v/v), Solutia (former Monsanto) has proposed recently to recover N2O and use it as a selective oxidant to convert benzene to phenol.3-5 Phenol is then hydrogenated to cyclohexanone which is then oxidized by * Corresponding author. Phone: +39-090-393134. Fax: +39090-391518. E-mail:
[email protected]. † Universita ´ di Messina. ‡ Universita ´ di Bologna.
nitric oxide to adipic acid, returning N2O as a byproduct and thus closing the N2O cycle. Phenol worldwide production is much larger than adipic acid production, and thus its direct synthesis from benzene using N2O, as an alternative to the commercial cumene process which coproduces acetone, is quite interesting. However, direct phenol synthesis is not economic because of the high cost of production of N2O which is synthesized by selective oxidation of ammonia. The use of N2O selectively recovered from off-gas of chemical productions can be a good opportunity to eliminate this chemical from the emissions, making it available at the same time as an economic reactant in the selective synthesis of phenol, to reduce the overproduction of acetone by the cumene process. The key step for this objective is to develop an efficient and economic technology to selectively recover N2O from the emissions of nitric acid production plants or other chemical processes using nitric acid as an oxidant. The aim of this work is to study the behavior of metalexchanged zeolites for this application, because zeolitic materials, which have a regular pore structure and high void volume, can be reasonable types of solids for entrapping a high amount of N2O, and the modification of the zeolites by ion exchange with metals can provide the active sites to enhance the selective adsorption of this compound.6 2. Experimental Section 2.1. Preparation of the Catalysts. The Me-ZSM5 samples were all prepared by an ion-exchange method. The parent zeolite was the sodium form of a commercial ZSM-5 sample (SN27 from ALSI-Penta, SiO2/Al2O3 ) 24/27). Ion exchange was made at room temperature
10.1021/ie990360z CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/1999
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Figure 1. Weight increase for Co100-ZSM524 (a) as a function of time on stream at 80 °C using two feed compositions and (b) after 1 h of time on stream (wt.60′) using a feed of 0.2% N2O + 0.2% H2O in He and the maximum weight difference (∆wt.max; see text) as a function of the reaction temperature.
using aqueous solutions of acetate salts, apart for BaZSM5 for which ion exchange was made at 80 °C for 8 h. After washing, drying, and calcining, ion exchange was repeated when necessary, to increase the ionexchange level. Ba-Y and Ba-X samples were prepared by an ion-exchange method at 85 °C for 24 h. The following nomenclature will be used hereinafter to identify the different samples: Me100-Z24, where Me indicates the type of metal used for the ion exchange and its subscript the ion-exchange level estimated on the basis of the Men+/Al3+ molar ratio and thus not considering the charge of the metal ion and Z indicates the type of parent zeolite (ZSM5, Y, or X) and its subscript the SiO2/Al2O3 ratio of the parent zeolite. No dealumination occurs during the preparation procedure. 2.2. Adsorption and Desorption Tests. Thermogravimetric adsorption tests were made in a modified Perkin-Elmer TG2 apparatus. Before tests, the samples were pretreated in a flow of helium up to 500 °C. Tests were made by comparing the curves of weight increase at different temperatures and using a feed of pure He or 0.2% N2O in He to which water was added in an amount of about 0.5%. Adsorption breakthrough curves were determined in an apparatus with a fixed-bed reactor and an on-line mass quadrupole for monitoring inlet and outlet reactor compositions.7,8 Tests were made using 0.5 g of zeolite (small beds of 0.1-0.3 mm in diameter), a total flow rate of 6 L/h, and feed composition as indicated in the text. The same apparatus was used for temperature-programmed desorption tests using a rate increase for the temperature of 20 °C/min. 2.3. Infrared Studies. A Perkin-Elmer 1750 spectrophotometer, equipped with a cell connected to greasefree evacuation and gas manipulation lines, was used for infrared studies. The self-supporting disk technique was used. The pretreatment of the samples was evacuation at 10-5 Torr at 400 °C. The procedure for the tests was as follows: (i) pretreatment of the sample and then recording of the spectrum of the sample at room temperature as the background, (ii) introduction of 30 Torr of N2O in the cell, recording of the spectrum in contact with N2O or after evacuation, and then addition of further 30 Torr of N2O and recording of the spectrum, and (iii) addition of 10 Torr of H2O and recording of the spectrum. The change of the spectra after steps ii or iii
by increasing the temperature under vacuum up to 200 °C was also recorded. 3. Results and Discussion 3.1. Thermobalance Adsorption Tests. Reported in Figure 1a is the comparison of the weight increase as a function of the time on stream for Ba114-ZSM524 using two compositions of the feed: (i) 0.5% H2O in He and (ii) 0.2% N2O + 0.5% H2O in He. With the first composition (only water in the feed), a continuous adsorption in the first 2 h of time on stream, up to a weight increase of about 2%, is observed. When N2O is also present in the feed, an initially faster adsorption is noted, although after 1 h of time on stream the weight increase is nearly coincident. This indicates that there is a faster adsorption of N2O, but this compound is then displaced by adsorbed water. Reported in Figure 1b is the effect of the reaction temperature on the adsorption behavior, using as indexes the weight increase after 1 h (mainly water adsorbed; see Figure 1a) and the maximum weight difference between the curves of adsorption of (i) water only and (ii) water plus N2O in He. This weight difference indicates the maximum amount of N2O adsorbed by the sample. The increase of the reaction temperature decreases the adsorption but does not allow a significant discrimination between adsorption of N2O and water. 3.2. Breakthrough Curves. The breakthrough curves for adsorption of N2O were determined in a flow reactor apparatus at a temperature of 50 °C, to maximize the adsorption (Figure 1a), but avoiding the possible condensation of water in the zeolite micropores. Before the adsorption tests, the zeolite was pretreated in a helium flow up to 500 °C in order to remove adsorbed species. Reported in Figure 2 is a typical response of massquadrupole partial pressures (linearly proportional to the concentrations of the relative compounds) for N2O, O2, and H2O in adsorption tests at 50 °C. The feed composition was 0.05% N2O and 6% O2 in helium with or without 2% water (top and bottom graphs in Figure 2, respectively), to operate in more severe conditions in terms of competitive effect of water on the adsorption of N2O with respect to the preliminary adsorption tests (Figure 1) as well as to use reaction conditions more close to possible industrial application.
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Figure 2. Dependence of the partial pressures of N2O, O2, and H2O in a flow reactor apparatus from the time on stream at 50 °C for Ba114-ZSM524. Feed composition: (top) 0.05% N2O + 6% O2 in He; (bottom) 0.05% N2O + 2% H2O + 6% O2 in He.
In the presence of only small amounts of water present only in traces in the feed, N2O is completely removed for about 90 min of time on stream, after which its concentration increases, reaching the inlet value after about 140 min. A different situation is observed when water is present in the feed (Figure 2, bottom). There is initially a significant adsorption, but after about 15 min the adsorption of N2O stops. For longer times, the reactor outlet concentration of N2O becomes higher than the inlet concentration. This overshoot of N2O ends when the adsorbed N2O is completely displaced by water. At this time, also water adsorption tends to decrease. This result confirms that N2O is adsorbed faster but then is displaced from water, which adsorbs stronger. The total amount of N2O adsorbed in the first part of the experiment of Figure 2 (top), i.e., up to the time (about 1200 s) when its concentration is lower than the inlet value, corresponds to about 0.32% of weight increase with respect to zeolite. In the absence of water in the feed, the total amount of N2O adsorbed is about 2.5 wt % (with respect to zeolite weight). Therefore, notwithstanding the strong decrease of adsorption of N2O when water is present in the feed, a significant amount of N2O can be adsorbed even in the presence of water in the feed because of its faster rate of adsorption. Reported in Figure 2 is also the response for the oxygen signal, which clearly evidences that this component does not interfere with the adsorption of N2O. It may noted that water is a common component of industrial emissions containing N2O. As shown in Figure 2, the adsorption of N2O in the presence of water in the feed is determined from the kinetics of adsorption and not from the thermodynamics. Therefore, the
adsorption behavior can be determined only from breakthrough curves as those shown in Figure 2, whereas the determination of adsorption isotherms does not give valuable indications. 3.3. Role of Metal in Zeolite on the Adsorption Behavior. Compared in Figure 3 are the moles of N2O adsorbed per gram of zeolite in the presence of 2% water in the stream. The moles of N2O adsorbed are estimated from the integration of the area representing the amount of N2O adsorbed from the zeolite before water displaces the adsorbed N2O. The behavior of the parent zeolite in the acid and sodium form is also reported. Different classes of metals ion-exchanged with the same parent zeolite were tested: (i) alkaline and alkaline-earth metals, (ii) transition metals such as Co and Cu, also with different ion-exchange levels to show the effect of this parameter, (iii) noble metals, (iv) metals of group 3a, and (v) lanthanide ions. Very large differences in the behavior can be observed depending on the type of metal, whereas minor differences are due to the ion-exchange level. Ion-exchange levels higher than 100% are reported in some cases, as often found in the literature for metal-containing zeolites,9 because polynuclear species of the metal often form and isolate species at the exchangeable sites. The nature of the metal in the zeolite influences not only the amount of N2O adsorbed but also the stability of chemisorbed N2O and the reversibility of the adsorption: (1) Na-ZSM5 gives a good adsorption behavior, but most of adsorbed N2O can be removed by flushing the catalyst with He after the adsorption test. (2) Cu-, Fe-, Ag-, and Co-ZSM5 reversibly desorb only about half or less of the adsorbed N2O, increasing the temperature up to 500 °C; the remaining part remains irreversibly adsorbed or it is transformed to N2 during the adsorption step because of partial reduction of the transition metal during sample pretreatment (He flow up to 500 °C to remove adsorbed water). The further investigation was thus focused only on the metal-exchanged zeolite which gives a completely reversible desorption of N2O and in which desorption starts at temperatures >100 °C. The best performances in N2O adsorption are shown by Ba- and Sr-containing samples which give a significantly better behavior than Mg-ZSM5 (also an alkalineearth element but having a lower ionic radius than Ba and Sr) or Cs-ZSM5 (Cs is of the same row as barium but is an alkaline element instead of an alkaline-earth element). Ba2+, because of its large ionic radius and electronic configuration, creates a large local electrostatic field within the cavities of the zeolite10 which is probably responsible of the enhanced adsorption behavior. Compared in Figure 4 is the adsorption behavior of a series of metal-exchanged ZSM5 samples in the absence and presence of 2% water in the feed. Water considerably depresses the amount of N2O adsorbed, but the same trend is observed for all samples studied, indicating thus that there is not a specific effect of the ionexchange metal on the resistance to competitive adsorption by water, notwithstanding that metals having quite different acid-base characteristics were compared, such as alkaline-earth elements (basic character) and lanthanide or metals of group 3a (acid character). This confirms that the behavior in N2O adsorption is not related to a change of the acid-base characteristics of the zeolite by introduction of the metal but probably
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Figure 3. Moles of N2O adsorbed per gram of zeolite (see text) at 50 °C as a function of the type of metal in metallo-ZSM5 samples. The number in the bracket near the symbol of the metal indicates its ion-exchange level. Feed: 0.05% N2O, 6% O2, and 2% H2O in He.
Figure 4. Moles of N2O adsorbed per gram of zeolite (see text) at 50 °C as a function of the type of metal in metallo-ZSM5 samples. The number in the bracket near the symbol of the metal indicates its ion-exchange level. Feed: 0.05% N2O, 6% O2, and 2% or 0% H2O in He.
to the introduction of metals to create a high electrostatic field to polarize the N2O molecule. 3.4. Role of the Ion-Exchange Level and Zeolite Structure. The effect of the ion-exchange level in the ZSM-5 zeolite and the role of the zeolite structure, with reference especially to zeolites where a higher loading of Ba was possible, is shown in Table 1. The optimal N2O adsorption capacity of Ba-ZSM5 samples is found for an ion-exchange level of around 100%. Up to this value the moles of N2O adsorbed are nearly equal to the moles of Ba in the sample, whereas for higher loading of barium, the N2O/Ba molar ratio decreases, further confirming the specific role of isolated Ba2+ ions in the adsorption of N2O. The structure of the zeolite, however, also plays a determining role, as indicated in Table 1 from the comparison of the behavior of Ba-Y and Ba-X with that of Ba-ZSM5 samples. Notwithstanding the higher amount of Ba in the first two samples, the amount of N2O adsorbed is about 1 order of magnitude lower, indicating that not only the concentration of barium ions
but also the specific nature of barium species in the zeolite determines the adsorption behavior. 3.5. Desorption Behavior. The temperature-programmed desorption curves of N2O, H2O, and NO after the adsorption tests at 50 °C for Ba124-ZSM524, Ba40Y6.5, and Ba55-X55 are reported in Figure 5. In BaZSM5 (Figure 5a) the desorption of N2O occurs in the 100-200 °C temperature range with complete desorption of the N2O adsorbed. At higher temperatures (above 400 °C) only the desorption of chemisorbed water occurs, which confirms the previous indication of the stronger chemisorption of water on these metal-exchanged zeolites. In the Ba-Y sample (Figure 5b), the desorption of N2O occurs at a slightly lower temperature (80-150 °C temperature range) but partially also at higher temperatures, as indicated from the presence of a second desorption peak centered at about 440 °C. This second, higher temperature desorption peak corresponds also to the partial desorption of NO, indicating that probably part of the chemisorbed N2O is irreversibly adsorbed in the form of oxidized nitrogen oxide species. Desorption of water also in this case occurs only above 400 °C. A similar behavior is also observed for Ba-X (Figure 5c), for which the desorption of N2O occurs in two steps, one in the 50-120 °C temperature range and a second in the 350-500 °C temperature range, the latter again associated with the formation of NO as the desorption product. Ba-ZSM5, differently from the other Ba-zeolite samples, thus allows not only a higher chemisorption of N2O but also its desorption in a defined range centered around 150 °C and avoids the formation of irreversibly chemisorbed N2O species. 3.6. Effect of Other Components in the Gas Phase. The effect of the presence of NO or SO2 in the feed on N2O adsorption is shown in Table 2 for Ba101ZSM524. When a low amount of NO (50 ppm) is fed together with N2O and O2 (test no. 2 in Table 2), no decrease of N2O was observed within the experimental error (about 5%). With an increase of the NO concentration of 1 order of magnitude, the amount of N2O chemisorbed decreases by about 10% (test no. 3). When the adsorption in the same conditions after desorption
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 135 Table 1. Effect of the Ion-Exchange Level in Ba-ZSM5 Samples on the Adsorption of N2O at 50 °C and Comparison with the Behavior of Ba-Y and Ba-X Samples (Feed: 0.05% N2O and 6% O2 in He) structure of zeolite
SiO2/Al2O3
amount of Ba, % wt
exchange level, %
mol of N2O ads. (10-4)/g of zeolitea
weight increase, %
mol of N2O ads./mol of Baa
ZSM5 ZSM5 ZSM5 Y X
24 24 24 6.5 5
4.8 5.9 7.3 11 15
81 101 124 40 55
4.6 5.7 5.2 0.7 0.6
2.0 2.5 2.3 0.3 0.3
1.07 1.06 0.78 0.09 0.04
a
Amount of N2O adsorbed up to a time corresponding to the reaching of a concentration of N2O equal to 0.9 of the inlet N2O concentration.
Table 2. Effect of the Presence of NO or SO2 in the Gas Phase on the Adsorption Behavior of Ba101-ZSM524 toward N2O Samples at 50 °C mol of N2O ads. (10-4)/g of zeolitea
test no.
feed
1
4
0.05% N2O + 6% O2 in He as test 1 + 0.005% NO as test 1 + 0.05% NO as test 2
5
as test 2
5.4
6
as test 1 + 0.05% SO2 as test 6
3.8
2 3
7
note
5.7 5.6 5.1 4.8
2.9
2nd cycle after test 3 and desorption at 200 °C 2nd cycle after test 3 and desorption at 500 °C 2nd cycle after test 6 and desorption at 500 °C
a Amount of N O adsorbed up to a time corresponding to the 2 reaching of a concentration of N2O equal to 0.9 of the inlet N2O concentration.
at 200 °C is repeated to desorb all N2O (see Figure 5b), a further lowering of N2O adsorption is noted, because of the probable formation of nitrate-like species. An intermediate desorption at higher temperature (test no. 5) recovers partially, although not completely, the adsorption capacity toward N2O of Ba-ZSM5. Therefore, NO deactivates the adsorption capacity of zeolite because of the formation of nitrate-like species, although the effect is of minor magnitude and zeolite adsorption capacity can be recovered by a periodic treatment at high temperature (500 °C). SO2 instead leads to a more enhanced and especially irreversible reduction of the adsorption behavior (test no. 6), indicating thus that this adsorbent cannot be used in the presence of SO2 in the feed or a periodic regeneration by reduction treatment is necessary. 3.7. Dual-Bed Adsorption Tests. The behavior of Ba-ZSM5 when water is first selectively removed on a bed of γ-Al2O3 is shown in Table 3. The use of a first bed of alumina has an even higher effect than removing water from the feed, because it also eliminates the small traces of water deriving from the incomplete anhydrification of the gases of the feed. On the contrary, no adsorption of N2O is possible using only the first alumina bed (Table 3). Using a dual-bed approach, it is thus possible to have a selective preliminary adsorption of water and then remove selectively N2O, (i) to produce, during the consecutive desorption, a pure stream of N2O and (ii) to avoid the necessity of high-temperature treatment to remove the adsorbed water from the zeolite (Figure 5).
Figure 5. Temperature-programmed desorption in helium flow after N2O adsorption (50 °C) (feed: 0.05% N2O, 6% O2, and 2% H2O in He) on (a) Ba124-ZSM524, (b) Ba40-Y6.5, and (c) Ba55-X55.
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Table 3. Dual-Bed Adsorption Experiments at 50 °C first bed
3 g of γ-Al2O3 3 g of γ-Al2O3 a
second bed
feed
mol of N2O ads. (10-4)/g of zeolitea
weight increase, %
0.5 g of Ba124-ZSM524 0.5 g of Ba124-ZSM524 0.5 g of Ba124-ZSM524
0.05% N2O + 6% O2 + 2% H2O in He 0.05% N2O + 6% O2 in He 0.05% N2O + 6% O2 + 2% H2O in He 0.05% N2O + 6% O2 + 2% H2O in He
0.6 5.2 5.6 0
0.3 2.3 2.5 0
Amount of N2O adsorbed up to a time corresponding to the reaching of a concentration of N2O equal to 0.9 of the inlet N2O concentration.
Figure 6. Fourier transform infrared (FT-IR) spectra of adsorption at room temperature of 30 Torr of N2O on Na-ZSM5, Fe118-ZSM524, and Ba124-ZSM524. The contribution of gas-phase N2O and zeolite background was subtracted from the spectra.
3.8. Infrared Analysis of the Adsorption of N2O on Ba-ZSM5. The nature of coordination of N2O in Ba-ZSM5 and the differences with respect to the other Me-ZSM5 samples were analyzed by comparing the infrared (IR) spectra of N2O adsorbed on Ba124-ZSM524 with those obtained on Cu119-ZSM524, Na-ZSM5, Fe118-ZSM524, and H-ZSM5. The spectra for Na-, Fe-, and Ba-ZSM5 samples in contact with 30 Torr of N2O are shown in Figure 6. The spectra of the other Me-ZSM5 samples were analogous to that of NaZSM5. Gaseous N2O is characterized by two intense bands centered at 2224 and 1286 cm-1 assigned to asymmetric and symmetric stretching (ν) of N-N and N-O bonds, respectively.11 N2O chemisorbed on Me-ZSM5 is characterized by a band at 2230 cm-1 falling at a frequency slightly higher than that of gaseous N2O, indicating a weak adsorption. The shift of the νNN band to higher frequency indicates a coordination of the N2O molecule through the N atom.12 Ba-ZSM5, differently from all other analyzed MeZSM5 samples, shows in addition to the 2230 cm-1 band an evident band also at 2256 cm-1 which suggests the presence of specific sites, not present in the other zeolites, able to interact strongly with the N2O molecule through the lone pair of the nitrogen atom. Ba2+ ions have a Lewis acid character and thus can interact directly with the lone pair of the nitrogen atom, but the other alkaline or alkaline-earth ions tested in the MeZSM5 series (Figure 3) are expected to possess higher Lewis acid character than Ba2+ ions in ZSM5. The same is valid for transition-metal ions such as Co2+ and Cu2+. Blatter and Frei12,13 observed that alkaline ions and, in particular, Ba2+ ions10 are able to create a very strong
electrostatic field within the zeolitic cages to stabilize an hydrocarbon-oxygen charge-transfer complex. Reasonably, also Ba2+ ions in ZSM5 could have a similar effect. The strong electrostatic field generated inside the zeolite cavities from the presence of naked Ba2+ ions could stabilize the NtN+-O- resonance form of the N2O molecule, explaining the presence of the broad band with a clear band/shoulder at higher frequencies (centered around 2250 cm-1, νNN). In agreement with this interpretation, by coadsorption of 10 Torr of water and 30 Torr of N2O, the broad band disappears, leaving only a weaker band centered at 2225 cm-1. In fact, it is expected that the coordination of water to Ba2+ ions, forming a Ba(OH)2 type species, would prevent their action in generating a strong electrostatic field within the zeolite cavities. Therefore, the enhanced properties of Ba-ZSM5 in the adsorption of N2O are not related to a specific chemisorption but rather to the generation of a strong electrostatic field by naked Ba2+ ions within the zeolite cavities, responsible for orienting and trapping of the N2O molecule. Water thus does not competitively chemisorb with N2O for adsorption on barium ions but rather inhibits the action of naked Ba2+ ions in generating the strong electrostatic field. This interpretation may explain why the adsorption properties of Me-ZSM5 samples (Figure 3) do not correlate to chemical properties of the ion-exchange metals. 4. Conclusions Ba-ZSM5 samples, with an ion-exchange level of around 100%, show enhanced properties of adsorption of N2O in comparison to other Me-ZSM5 samples or Ba-
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zeolites with different structure. The effect is attributed to the presence of naked Ba2+ ions which generate a strong electrostatic field within the zeolite cavities to stabilize a resonance form for N2O (NtN+-O-), which causes its trapping inside the zeolite cavities. Water, coordinating to the Ba2+ ions, prevents the generation of this strong electrostatic field and thus prevents the adsorption/trapping of N2O. However, the kinetics of adsorption of water, due to the different kinetic diameter with respect to the linear N2O molecule, is slower, and thus it is possible even in the presence of much larger amounts of water in the feed (40 times higher) to adsorb N2O on Ba-ZSM5. The amount of N2O adsorbed in the presence of water in the feed is about one-sixth of that possible in the absence of water in the feed. It is possible to use a dualbed approach to solve this problem: water is selectively eliminated first on an alumina bed and then N2O is adsorbed on a Ba-ZSM5 bed. This dual-bed approach has two advantages: (i) it allows one to enhance the amount of N2O adsorbed per mass of zeolite and (ii) it avoids the coadsorption of water in Ba-ZSM5 which causes the use of high temperatures of regeneration (around 500 °C) to remove water contrary to the milder conditions (around 200 °C) required to desorb N2O only. The amount of N2O adsorbed in these conditions corresponds to about 5 × 10-4 mol/g of zeolite; i.e., for a concentration of N2O of 0.05% v/v in the feed, it is possible to treat about 24 m3/kg of zeolite. This value indicates a possible application of the technology of selective removal of N2O to produce a concentrated stream of N2O for chemical applications, i.e., to use N2O as a selective oxidant in reactions such as phenol from benzene or methanol from methane. The desorption of N2O from Ba-ZSM5 occurs in a narrow temperature range centered around 150 °C, which indicates the possibility of obtaining a concentrated stream of N2O for the application discussed above. The presence of other components, apart from O2 and H2O, however, can interfere with the application. SO2, in particular, gives an irreversible deactivation, whereas NO has a minor effect of deactivation which can be reduced with a periodic regeneration at high temperature to desorb the NOx adspecies formed which desorb only above 450 °C, whereas N2O desorption can be made at 200 °C.
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Received for review May 25, 1999 Revised manuscript received October 5, 1999 Accepted October 21, 1999 IE990360Z
