Langmdr 1993,9, 2331-2343
2337
Removal of Nitrogen Monoxide on Copper Ion-Exchanged Zeolites by Pressure Swing Adsorption Wen-Xiang Zhang, Hidenori Yahiro, Noritaka Mizuno, Jun Izumi,+and Masakazu Iwamoto. Catalysis Research Center, Hokkaido University, Sapporo 060, Japan Received January 4,1993. In Final Form: July 12, 199P The adsorption properties of nitrogen monoxide (NO)on various metal ion-exchanged zeolites were examined by adsorption-desorptionmeasurementsin a fiied bed flow adsorption apparatus. Among the samplesused,the copper ion-exchanged ZSM-5 zeolitesshowedthegreatest ability for reversibleadsorption of NO;therefore,it has been studied in more detail. The amounta of reversible and irreversibleadsorption of NO per copper ion exchanged increased with decreasing aluminum content of the zeolite, and were constant,independentof the ion exchange level. The NO speciesadsorbedon the zeolitewere characterized and temperature-programmeddesorption (TPD)techniques. Most of the NO reversibly by infrared (IR) adsorbed is the NO+adsorbed on Cu2+,and the NO irreversibly adsorbed is the residual of NO+,nitrate (NO3-),nitrite (NOz-1,and NO2+. The irreversiblyadsorbed speciesgave the desorption peaks at 400,463, and 663 K. The total amount of NO desorbed in the TPD experiment is in good agreement with the amount of irreversible adsorption of NO evaluated from the adsorption-desorptionmeasurement.
Introduction The removal of nitrogen oxides (NO,) which cause acid rain and air pollution is an important social problem to be solved. At present, catalytic reduction processes employing NH3, CO, or hydrocarbons on V&-TiO2 or Pt-Pd-Fth catalysts have been practically applied.'P2 In addition, new types of reactions, the catalytic decomposition and selective reduction by hydrocarbon in 02,have recently been proposed for removing NO.S From the viewpoint of the efficiency and the running cost of the process, however, these methods are not suitable for the removal of low-concentration NO,. This problem makes it necessary to develop a novel process to remove lowconcentration NO,. It is well known that selective adsorption is one of the most suitable techniques for removal and/or enrichment of low-concentration adsorbate^.^ In particular, pressure swing adsorption (PSA) has widely been applied to various processes such as the production of oxygen from air6and purification of hydrogen;6therefore, the PSA is expected to be an effective method to remove or enrich NO, diluted in air. The adsorbents for PSA must possess a high capacity for reversibleadsorption of NO. Although active carbon,%ilica? zeolite,lb12a-FeOOHdispersed activated
* To whom correspondence should be addressed.
t Present addreas: Nagasaki R & D Center, Mitaubishi Heavy Industries, Ltd., Nagasaki 850-91,Japan. * Abstract published in Advance ACS Abstracts, September 1,
1993. (1) Crucq,A.;kennet, A. Catalysis and AutomotivePollution Control; Elsevier: Amsterdam, 1987; p 1. Bosch, H.; Janssen, F. Catal. Today 1987,2, 369. (2) Harrison,B.; Wyatt, M.; Gough, K. G. Catalysis; Royal Society of Chemistry: London, 1982; Vol. 5, p 127. (3) Iwamoto,M. In Future Opportunities in Catalytic and Separation Technology; Miaono, M., Moro-oh, Y., Kimua, S., E&.; Elsevier: Amsterdam 1990; p 121. Iwamoto, M.; Yahiro,H.; Mizuno,N.Nippon Kagaku Kaishi 1991,574. (4) Ritter, J. A.; Yang, R. T. Ind. Eng. Chem. Res. 1990, 29, 1023. (5) Lee, H.; Stahl, D.E. AIChE Symp. Ser. 1973,69,1. (6) Takeuchi, M.; Tanibata, R.; Nishida, S. Nenryo Kyokai Shi 1983, 62, 989. (7) Ganz, S. N. Zh. Prkl. Khim. 1958, 31, 138. (8)Okuhara, T.; Tanaka, K. J. Chem. Soc., Faraday Trans. 1986,82, 3657. (9) Ermee, E. D. Chem. Eng. Prog. 1956,52,488. (10) Joithe, W.; Bell, A. T.; Lynn, S. Znd. Eng. Chem., Process Res. Dev. 1972,11,434.
carbon fiber,13and chelate resid4 have been reported as the candidates so far, little is known of the amount of reversible adsorption of NO. Zeolites have a large surface area because of their microporosity. The properties of zeolites can be widely changed by the selectionof zeolite structure, silica/alumina ratio, and metal ion loaded. Furthermore, metal ionexchanged zeolites have been reported to be active for catalytic decomposition3 and reduction16 of nitrogen monoxide, indicatingthe great affiiity of the NO molecule to the zeolites. These findingssuggest the possibility that metal ion-exchanged zeolites might be good adsorbents for NO. In this study the amounts of reversible and irreversible adsorption of NO on metal ion-exchanged zeolites were measured by a fixed bed flow adsorption apparatue, and the nature of adsorbed species of NO and the adsorption mechanism were investigated by infrared (IRJ and temperature-programmed desorption (TPD) techniques.
Experimental Section Adsorbent Preparation and Adsorbater. Parent zeolites ZSM-6 (SiOdAlzOa = 23-31,mordenite (10.5), ferrierite (12.3)) offretitelerionite (7.7), and Y (5.61, L (6.01, and X types (2.6) were supplied by Tosoh Corp. and denoted as MFI, MOR,FER, OFFIERI, FAU, LTL,and FAU, respectively. The SiOdAl2Os molar ratio in the parent zeolite was determined not only by chemical analysis but also by the "Si MAS NMR method. Both values were very close to each other as has also been reportedby Fyfe et al.16 Hereafter the latter framework SiOdAl20S molar ratios were wed. Metal ion-exchanged zeolites were prepared as follows.1s Approximately 15 g of the sodium ion-exchanged zeolite was washed with dilute NaNOa solution and ion-exchangedin 1 dma of metal acetate or nitrate solution with 10-11 mmol-dm4 overnight. The wet cake obtained by fiitration was again ionexchangedin new metal saltsolution. After the deairedrepetition (11) N a m e , Y.;Hata, T.; Kishitaka, H. Nippon Kagaku Kaishi 1979, 413. (12) Huang, Y.Y.J. Catal. 1974,32,482. (13) Kaneko, K. Langmuir 1987,3, 357. (14) Toshima,N.; Asanuma, H.; Hirai, H. Bull. Chem. SOC.Jpn. 1989, 62, 893. (15) Sato, S.; Yu-u,Y.;Yahiro,H.; Mizuno, N.;Iwamoto, M. Appl. Catal. 1991, 70, L1. (16) Fyfe, C . A.; Thomas, J. M.; Klinowski, J.; Gobbi, G. C. Angew. Chem., Znt. Ed. Engl. 1983,22,259.
0 1993 American Chemical Society 0743-7463/93/2~09-2331$04.OO/0
Zhang et al.
2338 Langmuir, Vol. 9, No. 9, 1993 He
NO on
0
C1
Figure 1. Schematic diagram of the PSA apparatus: A, adsorbent; C, adsorption column; F, gas filter; G,pressure gauge; M, mass flow controller;P, pressure adjuster; QPMS, quadrupole maas spectrometer; S, solenoid valve; T, three-way stop cock; TCD, thermal conductivity detector. of the ion-exchange treatment, the sample was washed and dried at 383 K. The amount of metal ions exchanged was determined by atomic absorption or flame spectrometry after the zeolite sample obtained was dissolved in HF solution. Hereafter the sample was abbreviated as M-MFI(23.3)-100 (cationzeolite structure(SiOz/Al203ratio)-degree of exchange). The following gases were obtained from Nippon Sans0 Co. and used without further purification: 1000-2oooO ppm of NO diluted with He; NO2,4680 ppm/He; COz, 20%/He; S02,2170 ppm/He; CO, 1890 ppm/He; 0 2 , 99.5%; He, 99.995%. The concentration of NO was set a t 1OOO-2oooOppm in this study for the quick evaluation of the adsorption properties of adsorbents though the concentration is much lower under practical conditions. Adsorption-Desorption Measurement. The adsorptiondesorption measurement was carried out in a flow-typeapparatus with a fued bed adsorber as shown in Figure 1. Two adsorption columna were used; one (C1) is for the usual adsorptiondesorption measurement, and the other (C2) is for a blank test under identical conditions. Adsorption and desorption runs were periodicallycontrolled by solenoid valves (51-54). The adsorbent of 0.5-1.0 g was placed in a stainless steel column (C1,7.4-mm inside diameter and 22-cm length) and was heated at 773 K for 5 h under a helium stream (50 cm3-min-l) as a pretreatment just before the adsorption run. In the adsorption run,the gas mixture of NO (1000-2oooOppm) and He (balance) was fed at a rate of 100 cm3.min-l into the column through P1, S1, T1, C1, T2,S2, F, and M1. After the adsorption run, pure He (100 cm3.min-l) was introduced into the column in a countercurrent flow through P2, S3, T2, C1, Tl,S4, F, and M1 to desorb the NO from the adsorbent. The temperature of the adsorbent was 195-373 K, and the adsorption or desorption time was 45-120 min. The effluent concentration and composition were continuously monitored by usingan on-lineconuected thermal conductivitydetector (TCD) and a quadrupole maas spectrometer (QPMS; ANELVA, AQA-loo), respectively, as shown in Figure 1. In the experiment to c o n f i i the effect of preadsorbed gas on the NO adsorption, various gases such as NO2, C02, and SO2 were adsorbed at 273 K after the usual pretreatment. Subsequently, the column was purged with pure He (50 cm3.min-l) for 5 h to remove reversibly adsorbed species. After the preadsorption procedure, adsorption-desorption measurement of NO was carried out. IR Measurement. IR absorption spectra were recorded by using an IR-810 spectrometer (Japan Spectroscopic Co., Ltd.) and a quartz infrared cell with KBr windows as described elsewhere." The volume of the quartz infrared cell is 175 cma. A self-supporting wafer (3-4 Wcm-2) of the adsorbent was obtained by pressing the powder at 200 k g a r 2 for 30 min. The resulting sample wafer was heated from room temperature to 773 K, kept at 773 K for 2 h, and cooled to room temperature (17)Bell, A.T.In Vibrationu~Spectroscopy ofitfoieculeson Surfaces; Yaks, J. T., Jr., Madey, T. E., Eds.; Plenum: New York, 1987;p 106.
I
30
60
I
I
120 Time / min
90
I
150
I
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F i g u r e 2. Breakthrough and elution curves of NO over Cu-MFI(23.3)-60: solid line,with adsorbent; broken line, without , bz, see text; adsorbent; CO,initial concentration of NO; UI, a ~bl, Adsorption time, 45 min; desorption time, 60 miq concentration of NO, 1910 ppm; flow rate, 100 cm3*min-'; adsorption temperature, 273 K adsorbent weight, 1.0 g. under a dynamic vacuum. IR measuamenta were performed at ambient temperature. Temperature-Programmed Desorption of NO. The apparatus used for temperature-programmed desorption (TPD) was the same as used for the adsorption-desorption measurement (Figure 1). After the adsorption of NO at 273 K in a NO flow (5000 ppm) for 3 h, the column (Cl) was purged with pure He for 5 h. Subsequently, the TPD experiment was carried out with a heating rate of 5 Kamin-' in a He flow (flow rate 50.0 cmg.min-l at 101.3 kPa and 298 K) in the temperature range 323-873 K. Desorbed gases were simultaneously monitored by TCD and QPMS. The amount of NO desorbed was estimated from the profiles measured by TCD, and the composition of the desorption gas waa monitored by QPMS. The relative sensitivities to NO, N20, N2, and 02 were 1.00,1.14,0.93, and 0.89, respectively, for TCD and 1.00,1.15,1.75, and 0.98, respectively, for QPMS. The desorption profiles obtained by QPMS were recorded for NZ(28), NO (301, 0 2 (32), NzO (441,and NOa(46).
Results and Discussion AdsorptionPropertiesof NO on Various Metal IonExchanged Zeolites. The Effect of the Metal Ion. When the adsorption and desorption runs were repeated, the breakthrough and elution curves were obtained as shown in Figure 2. The other zeolites tested in this study gave essentially the same curves as those. In this figure, the solid and the broken lines indicate the response of TCD with and without adsorbent, respectively. N2 and N2O were formed only in the first run,but the amounts were much smaller than that of NO adsorbed. Therefore, the amount of NO adsorbed can be estimated from the hatched area (denoted as an where n is the number of adsorption-desorption cycles), while that of NO desorbed was calculated from the dotted area (bn). a n decreased with increasing n and reached a constant value, while bn was almost constant, independent of n. Usuallyanbecame approximately equal to bn at the fifth or sixth adsorptiondesorption cycle. Here, the amount of reversible adsorp tion of NO per weight of adsorbent (denoted as qrev)is defined by a m / S w (S, = sample weight) at a m = b,. The amount of irreversible adsorption of NO per weight of adsorbent (denoted as q h ) is defined by C ~ ~ l ( -a n bn)/Sw. The qmvand q h measured at 273 K on various cationexchanged MFI zeolites are summarized in Table I. The values in parentheses are the amounts of reversible and irreversible adsorption of NO per cation introduced into zeolites instead of a sodium ion (hereafter denoted as q*rev and q*h, respectively). The qrevand qirr greatly changed with the metal ion. The properties of respective adsorbents are as follows: In the case of transition metal ion-exchanged zeolites, the values of qirr were larger than those of qmV except for Zn-MFI(23.3)-96 and Ag-MFI(23.3)-90. On the contrary, qrevwas greater than qin for alkaline earth
Langmuir, Vol. 9, No. 9, 1993 2339
Removal of Nitrogen Monoxide on Zeolites Table I. NO Adsorption Properties of Various Cation-Exchanged MFI Zeolites. amount of NO adsorbed/ (cma.gl) content of adsorbent cation/(& %) reversible irreversible 2.81 0.16(0.00S)c O.OO(O.OOO)c Na-MFI(23.3)-10Ob 1.32 l.N(O.246) 1.56(0.212) Ca-MFI(23.3)-54 5.45 2.71(0.195) 0.20(0.014) Sr-MFI(23.3)-105 6.44 1.50(0.143) 1.44(0.137) Ba-MFI(23.3)-80 Mg-MFI(23.3)-46 0.69 0.69(0.109) 0.22(0.035) 5.90 4.28(0.206) 14.90(0.716) C~-MFI(23.3)-157 Ag-MFI(23.3)-90 10.85 3.38(0.150) 0.54(0.024) Co-MFI(23.3)-90 3.06 1.52(0.131) 19.69(1.693) 4.20 1.19(0.069) 5.81(0.339) Mn-MFI(23.3)-127 Ni-MFI(23.3148 2.41 1.03(0.112) 6W0.727) Zn-MFI(23.3)-96 3.79 l.Ol(0.078) 0.50(0.039) 2.12 0.52(0.061) 3.08(0.362) FeMF1(23.3)-62 0.87 0.38(0.101) 1.16(0.308) C~MFI(23.3)-41 Ce-MFI(23.3)-8 0.43 0.34(0.496) 0.34(0.496) La-MFI(23.3)-7 0.40 0.25(0.388) 0.24(0.372) 0.13 0.12(0.004) 0.32(0.011) H-MFI(23.3)-100 0
Table 11. Effect of Zeolite Structure on NO Adsorbability of Copper Ion-Exchanged Zeolites. amount of adsorption of NO/(ma*g1) content of adsorbent cation/(& % ) reversible irreversible Cu-MFI(23.3)48 2.63 2.29(0.247)b 7.46(0.805)b Cu-OFF/ERI(7.7)-81 5.45 2.81(0.146) 5.56(0.270) C~-MOR(10.5)-76 5.26 2.11(0.114) 6.69(0.361) Cu-LTL(6.0)-34 3.22 1.23(0.108) 2.38(0.210) C~-FER(12.3)-66 3.89 1.42(0.104) 4.82(0.363) Cu-FAU(2.6)-60 9.27 1.15(0.035) 0.62(0.019) Cu-FAU( 5.6)-83 7.99 0.86(0.031) 1.52(0.055) Adsorption time, 45 min; desorption time, 60 min; concentration of NO, 1910ppm; adsorption temperature, 273 K, adsorbent weight, 0.5 g; flow rate, 100 cm3-min-l. * Unit, (NO molecules)-(cation)-l.
I
1.0 I
Adsorption time, 45 min; desorption time, 60 min; concentration
of NO, 997 ppm; adsorption temperature, 273 K; adsorbent weight, 0.5 g; flow rate, 100 cm3.min-'. Concentration of NO, 1910 ppm. c
Unit, (NO molecules)-(cation)-'.
metal ion-exchangedzeolites. The order of q*revwas Ca2+ > Sr2+> Ba2+> Mg2+. Both qrevand qin were very small on alkali metal ion-, rare earth metal ion-, and protonexchanged zeolites. With MFI zeolites, the order of qrev was transition metal ion = alkaline earth metal ion > rare earth metal ion = alkali metal ion = proton. Among the adsorbenta listed in Table I, Cu-MFI(23.3)157and Co-MFI(23.3)-90 showed the largest qrevand qirr, respectively. The apparent densities of NO estimated by the sum of qrevand q h at 273 K and the micropore volume (0.162 cm3.g1) of MFI zeolite are 0.143 and 0.175 g . ~ m - ~ on Cu-MFI(23.3)-157 and Co-MFI(23.3)-90. Each value correspondsto 11%and 14% of the density (1.269~cm-~) of the liquid NO at 123 K.lS Windhorst and Lunsfordlghave reported that the [Co(N0)212+complex was formed on the cobalt ion-exchanged FAU zeolite upon the adsorption of NO at room temperature and had 1.9 NO molecules per Co2+ion. In the present case, the ratio of the irreversibly adsorbed NO to Co2+ion was 1.69as shown in Table I, and was not changed at 323 K. This indicates that NO was strongly adsorbed on Co2+ at room temperature and some of the cobalt nitrosyl complexes in MFI zeolite exist in the form of [Co(N0)2I2+.The adsorption state of NO on Cu-MFI zeolites will be discussed later. Effect of Zeolite Structure, Aluminum Content, and Ion-Exchange Level. The amounts of reversible and irreversible adsorption of NO were dependent not only on the kind of metal ion but also on the zeolite structure. Table I1 shows the results on copper ionexchanged zeolites having different structures. The ionexchangelevelsof the adsorbenta used were 6+80 % except for Cu-LTL(6.0)-34. The higher copper ion-exchanged LTL sample could not be prepared under the present preparation conditions. The amount of reversible adsorption of NO per copper ion greatly changed with the zeolite structure and decreasedin the followingorder: MFI > OFF/ERI > MOR > LTL > FER > FAU. A similar order was observed for the amount of irreversible ad~_______
~
(18)Handbook of Chemistry and Physics; CRC Express: New York, 1988, p B-45.
(19) Windhorst, K. A.; Lunsford, J. K.J. Am. Chem. SOC.1975, 97, 1401.
*$ 0
0.0 0.0
'
' 0.1
'
' 0.2 0.3 AI / (AI+SI)
0.4
0.5
Figure 3. Correlation between the amounts of NO adsorption and the Al contents in copper ion-exchanged zeolites: 0,qm; 0, qh; A, MFI; B, FER C, MOR D, OFF/ERI;E, LTL, F,G, FAU, adsorption time, 45 min; desorption time, 60 m i q concentration of NO, 1910 ppm; flow rate, 100 cm3.min-l; adsorption temperature, 273 K; adsorbent weight, 1.0 g.
sorption of NO. qrev values of Cu-MFI(23.31-68, CuMOR(10.5)-76, and Cu-OFF/ERI(7.7)-81were the highest among the samples tested, showing that these samples are the strong candidates for the PSA. Figure 3 shows the dependenciesof q*revand q** on the aluminum content in the copper ion-exchanged zeolites. Both q*revand q*irr decreased with the increment of the aluminumcontent in zeolites. These correlationsindicate that the adsorbability of NO was mainly controlled by the aluminum content. Generally, the chemical and physical natures of zeolites depend on the zeolite structure and/or Si/Al atomic ratio.20 In the case of proton-exchanged zeolite the catalytic activity per acidic site increased with decrement of the aluminum content or increment of the SiOdAl203molar ratio, independent of the zeolite structure.21p22The IR position of acidic hydroxyls has a good These correlation with the Al content of the mean that acid strengths depended on the Al content. Recently, X P S has revealed that the binding energies of each constituent element of the zeolites were fairly .~~ the correlated with the Al ~ o n t e n t . ~ *Consequently, dependencies of q*revand q*in on the aluminum content of zeolite in this study probably reflect the change of the electronicstateof the zeolites. It shouldbe noted, however, that the adsorbablities between two FAU samples are (20) Maxewell, I. E. Adu. Catal. 1982,31, 1. (21) Ward,J. W. J. Catal. 1969, 13, 316. (22) Ono,Y.;Kaneko, M.; Kogo,K.; Takayanagi, H.; Keii, T. J. Chem. SOC.,Faraday mans. 1976,72,2150. (23) Barthomeuf, D. J. Phys. Chem. 1979,83, 249. (24) Okamoto,Y.;Oaawa,M.;Maezawa,A.;Imanaka,T. J.Catal.1988, . _ 112, 427. (25) Stoch, J.; Lercher, J.; Ceckiewicz, S. Zeolites 1992, 12,81.
Zhang et
2340 Langmuir,Vol. 9, No. 9, 1993 30
r
I
::L-A 0.4
0.2 0.0
0
50
100
150
200
I
41.
r
20
m :
5 . L
c7 0
5 E P
10
0
1 2 Adsorption pressure / %
3
Figure 6. Pressure dependence of the amounta of reversible and irreversible adsorption of NO on Cu-MF'I(23.3)-157 zeolite 0,qW; 0,q h ; adsorption time, 45 min; desorption time, 60min; adsorption temperature, 273 K. v-
0
50 100 150 Exchange level of copper ion / %
200
Figure 4. Dependence of the amounta of reversible adsorption of NO on the exchange level of copper ion in MFI zeolite: 0,qm; 0, q h ; adsorption time, 60 min; desorption time, 120 min; NO, 1089 ppm; adsorption temperature, 273 K. 30
r
Jn 20
5 . k
e P
E E P
10
0 1 1
200 250 300 350 Adsorptlon temperature / K
400
Figure 6. Temperaturedependence of the amounta of reversible and irreversible adsorption of NO on Cu-MFI(23.3)-157 zeolite: 0,qm; 0, q h ; adsorption time, 45 min; desorption time, 60 min; NO, 1089 ppm. almost similar, suggesting the possibility that the zeolite structure might be one factor controllingthe adsorbability. Figure 4 shows the dependencies of the amounts of NO adsorbed upon the exchange level of copper ion at 273 K. qmv and qin were proportional to the exchange level of copper ion, showing that q*rev and q * h are constant, approximately 0.23 and 0.64 (NO molecules).(Cu)-l, respectively, independent of the exchange level. It follows that the effectiveness of each copper ion in MFI zeolite for NO adsorption is independent of its loading level or that the ratio of the effective Cu ions for NO adsorption to ineffective ones is constant. From the above results, the higher the exchange level of copper ion in MFI zeolite is, the higher qrev and qirr are. Hereafter, a more detailed investigation of adsorptive properties of NO on Cu-MFI(23.3)-157 was performed. Adsorption Properties of NO on Cu-MFI(23.3)-157. Change with Adsorption Temperature and Pressure. The dependencies of qrev and qin upon the adsorption temperature are shown for Cu-MFI(23.3)-157 in Figure 5. With decreasing adsorption temperature qirr significantly increased at ca. 250 K and then gradually increased. On the other hand, qmvgradually increased with decreasing adsorption temperature, reached the maximum (4.35 cm3.g1) at 243 K, and then decreased. There arises one question as to why qmvshowed the maximum. q h increased
abruptly in the temperature range where qmvreached the maximum. Therefore, the following explanations are possible for the phenomenon: (1) The sites for the reversible adsorption of NO are decreased by the abrupt increase in the amount of irreversible adsorption of NO. In other words, part of the NO reversibly adsorbed is converted into irreversibly adsorbed NO at low temperatures. (2) The effective pore size of MFI zeolite was decreased by the increment in the amount of NO irreversibly adsorbed, because the diameter of the micropore in MFI zeolite is 0.54 nm and the size of NO is 0.32 nme26 (3) The adsorption equilibrium could not be reached in the present study because of the small diffusioncoefficient at the low temperature. At present, it is impossible to determine which is correct. Figure 6 shows adsorption isotherms of qrev and qin on Cu-MFI(23.3)-157 at 273 K. Both qrev and qin increased with the increment of the concentration of NO. The isotherms of qrev and qin can roughly be expressed by the Langmuir isotherm, eq 1,where p is the partial pressure
of the adsorbate in the gas phase, K is the Langmuir adsorption equilibrium constant, and q and q m are the amount adsorbedat p = p and at equilibrium,respectively. qm and K for qrev were 10.14 cm3.g1 and 357 atm-l, respectively, and for q h 27.61 cm3*g1and 745 atm-I. Influences of Preadsorbed Gases. In real exhaust gases, there coexistvarious gases such as nitrogen dioxide, oxygen, carbon dioxide, sulfur dioxide, carbon monoxide, and water (NO2,02,CO2, S02,CO, and HzO),and therefore, it is important from a practical point of view to clarify their influence on adsorption properties. The effect of each gas was here examined on Cu-MFI(23.3)-147. The results are summarized in Table 111. The preadsorption of NO2 resulted in the enhancement of qrev. A similar enhancement has been observed by Naruse et al." At relatively low temperatures N203 is known to be in equilibrium with NO and NO2, and the reversible formation of N203 adsorbed on FAU zeolites has indeed been ~bserved.~' This suggeststhat NO2 irreversibly adsorbed can work as new active sites for the reversible adsorption of NO. When 02, C02, or SO2 was preadsorbed, qrev was hardly reduced. CO or H2O poisoned the adsorbability of Cu-MFI zeolite for NO. On the other hand, q k is always (26) Breck, D.W. Zeolite Molecular Sieues; Wiley-interscience: New York, 1974; p 636. (27) Chao, C. C.; Lunsford, J. H.J . Am. Chem. SOC.1971, 93, 71.
Langmuir, Vol. 9, No. 9, 1993 2341
Removal of Nitrogen Monoxide on Zeolites Table 111. Effect of Peadsorbed Gases on the Adsorption Properties of Cu-MFI(23.3)-147. amount of adsorption of NO/(cm3.g1) Dreadsorbed gasb reversible irreversible N02(4680 ppm)/He 7.14 2.21 4.26 14.38 02(99.5%) COa(SO%)/He 4.25 12.19 so2(2170ppm)He 3.92 7.86 CO(1890 ppm)/He 1.39 4.15 Hz0(3%)/He 0.22 0.45 4.35 17.83 none Adsorption t h e , 60 min, desorptiont h e , laomin, concentration of NO, lo00 ppm; adsorption temperature, 273 K, adsorbent weight, 0.5 g; flow rate, 100 cm3-min-'. The adsorbentwas heated at 773 K for 5 h under a helium stream (50 cm3.min-l) before the preadsorption treatment. After the preadsorption the sample was purged with helium at room temperature. 6
a b c d
NO'
I
5 a
. d4
p3
E
f 2 1 0
0
5
10
15 20 Time I h
25
30
Wavenumber / cm.' Figure 8. Infrared spectra of NO adsorbed on Cu-MFI(23.3)157: A, evacuation at room temperature after the previous adsorption run; B, exposure to NO of 3.5 TOR;C, 10 Torr; D, 58
TOR.
Figure 7. Time course of the IR intensities of NO adsorbed on Cu-MFI(23.3)-157: 0, NO+;0, NO-; A, (Noh-; A, NOz+; a, evacuation;b, exposureto NO (52 Torr);c, evacuation;d, exposure to NO (54 Torr).
decreased by the preadsorption of these gases though the degree of decrement is dependent on the preadsorbed gas. Adsorbed Species of NO on Copper Ion-Exchanged MFI Zeolite. IR Study. The reversibly and irreversibly adsorbed NO on copper ion-exchanged MFI zeolite were characterized by IR measurements. Upon admission of NO onto Cu-MFI(23.3)-157 evacuated at 773 K, seven bands were observed at 2238 (m), 2125 (m), 1906 (a), 1876 (w), 1827 (w), 1813 (w), and 1734 (w) cm-'. The similar results have already been reported.28 On the basis of the work in ref 28, the respective absorption bands have been assigned to NzO (2238 cm-9, NO2+ (21251, NO+ (1906), gaseous NO (1876), NO- (18131, and (Noh- (1827 and 1734). No bands of NO adsorbed were observed on NaMFI(23.3)-100, suggesting that the adsorbed species of NO are associated with copper ions. This was further supported by the finding that the band intensities of adsorbed NO species on Cu-MFI(23.3)-78 were intermediate between those on Na- and Cu-MFI(23.3)-157, The intensities of IR bands of NO+, NO-,and (Nohchanged with time, while that of NO2+ did not. Figure 7 shows the changes in the band intensities when CuMFI(23.3)-157 was exposed to 53 Torr of NO. The band intensities of NO- and (NO)2- decreased with time while that of NO+ increased. The band intensities reached constant values after 14h. It should be noted that Nz and N2O were formed in the first adsorption run as mentioned in the previous section. The changes are therefore attributable to the surface reaction, in which the anionic NO adsorbates decompose to yield N2 and N20, oxygen (28) Iwamoto, M.; Yahiro, H.; Mizuno, N.;Zhang, W.-X.; Mine, Y.; Furukawa, H.; Kagawa, S. J. Phys. Chem. 1992,96,9366.
v-
0
20 40 Adsorption pressure I Torr
60
Figure 9. Dependence of the band intensity of NO+ (1906cm-1) on the adsorption pressure of NO over Cu-MFI(23.3)-157 zeolite: 0, total NO+; 0,reversibly adsorbed NO+.
produced oxidizes Cu+to C P , and the resulting Cu2+ion acts as an active site for NO+ adsorption.28*2B When the sample was evacuated for 30 min at room temperature, the bands of NO- and (N0)z- completely disappeared and the intensity of the NO+band decreased to about one-tenth of that just before evacuation (Figure 7a). The intensity of NO+ recovered to the original value by the second admission of 52 Torr of NO,while the bands of NO- and (Noh- did not appear. The same results could be reproducedwhen evacuation and admission of NO were subsequentlycarried out. It followsthat the major species of the reversible adsorptionof NO on copper ion-exchanged MFI zeolite is the NO+ species. This conclusionwas further confirmed by the following results. The correlation between the band intensity of NO+ and the partial pressure of NO was measured and shown in Figures 8 and 9. With the increasein NO preeaure the intensity of the NO+band increased as shown in Figure 8B (3.5 Torr of NO),C (10 Torr), and D (58 Torr). The (29) Giamello, E.;Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510.
2342 Langmuir, Vol. 9, No. 9,1993
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Figure 10. TPD profiles monitored by TCD (a)and QPMS (b) after adsorption of NO on Cu-MFI(23.3)-147: 0,NO; 0 , N20; 0, N2; A, 02. intensity of NO+ is plotted against the partial pressure of NO in Figure 9. Clearly the amount of NO+ increased monotonously, being consistent with the adsorption isotherms of qrevin Figure 6. Figure 8A is the spectrum obtained after the evacuation of the sample used in the adsorption-desorption cycles at room temperature. Note that N02+ and part of the NO+ still remained. This will be discussed in the next section. TPD Measurements. In order to clarify NO species adsorbed irreversiblyon copper ion-exchangedMFI zeolite, NO adsorbates were desorbed by the TPD technique. The typical TPD profiles from Cu-MFI(23.3)-147 are shown in Figure 10. In a separate experiment, no desorption peak was observed on a Na-MFI(23.3) zeolite, indicating that irreversible NO adsorption is associated with copper ion. NO, N20, 0 2 , and Nz were detected during the desorption as shown in Figure lob. Three desorption peaks of NO were observed a t 400 (denoted as a),463 (@),and 663 K (7). The temperature of the N20 desorption peak coincided with that of the a peak, though the intensity of NzO was much smallerthan that of NO. The 0 2 desorption peak appeared at the same temperature as that of the y peak. Similar TPD profiles have been reported by u s , 3 O Li and and Hierl et al.32 The adsorbed species corresponding to the desorption peaks were assigned here. As shown in Figure 8A, part of the NO+was adsorbed irreversiblyon Cu-MFI(23.3)-147. The band of NO+was eliminated upon evacuation at 373473 K. This result suggests that the NO desorption peaks at 400 and 463 K result from the NO+ species adsorbed irreversibly. However, there remains one problem: the amount of NO desorbed as a and fl peaks in Figure 10 (7.2 (30) Iwamoto, M.; Yahiro, H.; Tanda, K. In Successful Design of Catalysts; Inui, T., Ed.; Elsevier: Amsterdam 1988; p 219. (31) Li, Y.; Armor, J. N. Appl. Catal. 1991, 76, L1. (32) Hierl, R.;Urbach, H. 0.;Knbzinger, H. J. Chem. SOC.,Faraday Trans. 1992,88, 355.
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Figure 11. TPD profiles monitored by TCD (a) and QPMS (b) after adsorption of NO2 on Cu-MFI(23.3)-147: 0,NO; O, NzO; 0, N2; 4 02.
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Figure 12. Amounts of NO desorbed or irreversibly adsorbed as a function of the exchange level of copper ion in MFI zeolite: 0, the amount desorbed from 273 to 773 Kin the TPDexperiment; 0, q h obtained from the adsorption-desorptionmeasurement. cm3*g1)is much higher than that of NO adsorbedreversibly in Table I11 (4.35 cm3-g1),whereas the intensity of NO+ at 1906 cm-I after evacuation is reduced to 11%of the intensity just before the evacuation. This difference may be due to the following reasons: (1)heating of the sample by the IR beam during the measurement, which resulta in the partial desorption of NO+ adsorbed irreversibly at room temperature, and (2) the difference between the absorption coefficients of the reversibly and irreversibly adsorbed NO+ species. Although the reason is not clear yet, it can be concluded on the basis of the above results that a and fl peaks are attributable to the desorption of NO+. To clarifythe origin of yNO, the desorptionexperiment was carried out after NO2 adsorption. The desorptions of NOz, NO, NzO, and 0 2 were observed, but they gave no clear resolution because of the large NO2 desorption peak and the fragmentation of NO2 desorbed. Therefore, the desorbed gas was passed through a dry ice-ethanol cold
Removal of Nitrogen Monoxide on Zeolites
Langmuir, Vol. 9, No. 9, 1993 2343
trap before being introduced into TCD and QPMS to remove NO2 desorbed. The obtained profiles are shown in Figure 11. It was clear that part of the NO2 adsorbed decomposes to yield NO and oxygen adsorbates. The former gave desorption peaks at 370-470 and 660 K,and the latter desorbs around 660 K together with NO. Comparison of Figures 10 and 11indicates that the 7-NO originatesfrom the decompositionof nitrate (Nos-), nitrib (NOz-), or NOz+. This was supported by the IR experiment in which the NOz+ band was eliminated on the evacuation at 673-773 K. The total amount of desorption was estimated from the TPD profiles. The amount is depicted in Figure 12 as a function of the copper ion exchange level. For comparison, the amount of irreversible adsorption of NO ( q d determined in the adsorption-desorption experiment (Figure 4 ) was also shown. It should be noteworthy that the amount of gases desorbed at 273-773 was increased proportionally with the copper ion exchange level, and is in good agreement with that evaluated from the adsorption-desorption measurements. This fact reveals that all irreversible adsorbed species can desorb at or below 773 K and the materials balance in the present adsorptiondesorption experiment is very good.
Conclusions Copper ion-exchanged MFI zeolite was found to have a great capacity for the reversible and irreversible NO adsorption among various metal ion-exchanged zeolites. Both the reversible and the irreversibleadsorptionchanged with the aluminum content of zeolite and increased proportionally with an increase in the exchangelevel. M a t of the reversibly adsorbed NO molecules are attributed to NO+, and the irreversibly adsorbed NO to NO+, NOz+, NOz-, and Nos-. In the latter species, NO+ can desorb at 350-550 K and NOz+,NO2-, or N03- decomposes to yield NO and 02 around 660 K. The results clarified in the present study would be useful not only for developingnovel removal process through pressure swing adsorption but also for understanding the catalytic reaction such as direct decomposition and/or selective reduction of NO.
Acknowledgment. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Scienceand Culture of Japan and the Nieean Science Foundation.
