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Adsorption and catalytic destruction of trichloroethylene in hydrophobic zeolites
6:ENVlRCMiENTAL ELSEVIER
Applied Catalysis
B: Environmental
14 (1997) 37-47
Adsorption and catalytic destruction of trichloroethylene in hydrophobic zeolites Prashant S. Chintawar, Howard L. Greene* Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906,
USA
Received
1997
16 October
1996; received in revised form 2 January
1997: accepted
6 January
Abstract Several chromium exchanged ZSM-5 zeolites of varying Si02/A1203 ratio were prepared and investigated for ambient (23°C) adsorption and subsequent oxidative destruction (250-4OO”C) of gaseous trichloroethylene (TCE, Cl,C=CHCl) in a humid air stream. With an increase in the SiOz/A120s ratio from 30 to 120, the TCE saturation capacity of these dual-function sorbent/catalyst (S/C) media was found to increase from 6.0 to 10.1 wt% in a humid air stream. This phenomenon was attributed to an increase in hydrophobicity coupled with reduced steric hindrance and site competition for the adsorption of TCE molecules in the competitive adsorption of TCE and water. Ambient TCE adsorption experiments carried out in dry air showed the same trend, which was attributed to increasing organophilicity of the S/C media with an increase in the SiOz/A120s ratio. In order to gain knowledge of physisorption sites for TCE molecules in the ZSM-5 structure, temperatureprogrammed desorption over a temperature range of 30-300°C and in-situ FT-IR studies at ambient conditions were also carried out. These studies revealed that in all zeolites (except for Cr-ZSM-5 with Si02/A120s ratio of 120) TCE interacted with terminal silanol (SiOH) and AlOH groups. At temperatures >3OO”C (with the exception of Cr-ZSM-5 with Si02/AlzOs ratio of 120), all S/C media showed >95% TCE destruction efficiency. Based on its high adsorption capacity and high activity for oxidative destruction of TCE, it is concluded that the Cr-ZSM-5 S/C medium with a Si02/A120s ratio of 80 gives preferred performance both as a sorbent and a catalyst. 0 1997 Elsevier Science B.V. Keywords:
Chlorinated VOC; Cr-ZSM-5; Hydrophobic zeolite; SiOz/A1203 ratio; Sorbent/catalyst
1. Introduction Hydrophobic zeolites have recently gained much attention due to their ability to selectively remove one or more organic pollutants from humid air streams. In this respect, Weitkamp et al. [l] have studied the binary adsorption of water and ethanol from their *Corresponding author. Present addres: Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106-7217, USA. Tel.: +l 216 368 4065; fax.: +l 216 368 3016. 0926-860x/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO926-3373(97)00010-6
medium
gaseous mixtures on sodium-exchanged zeolite X, Y, and dealuminated Y. Their results indicate that, upon increasing the Si02/A120s ratio of a zeolite from three to 42, the saturation loading of water decreases and that of ethanol passes through a maximum. Weitkamp et al. [2] have also investigated the hydrophobicity of pentasil (ZSM-5 type) and dealuminated faujasite zeolites by competitive adsorption of toluene and water from the gas phase. Similar to their earlier observation, the hydrophobicity was found to increase with an increase in SiOz/A120s
38
P.S. Chintawar; H.L. Greene/Applied
ratio of zeolite. Schumacher et al. [3] have separated gaseous tetrachloroethylene and water mixtures by adsorption on numerous zeolites; e.g. EMT (Elf Mulhouse Two, a hexagonal variant of faujasite), faujasites, ZSM-5, AlPOd-5, and SAPO-5. The authors state that high separation efficiency could be achieved with carefully dealuminated and steamed zeolites of the EMT and faujasite type or highly siliceous ZSM-5 zeolites. Blocki [4] has compared the performance of a proprietary hydrophobic zeolite and an activated carbon for solvent separation and concentration. The author suggests that zeolitic sorbent may exhibit separation capacity equal to or better than systems using activated carbon, and without much of the support equipment needed with carbon. It should be noted that the hydrophobic nature of pentasil zeolites (ZSM-5, silicalite, etc.) has been known for a long time. Flanigen et al. [5], in a pioneering article, have reported the hydrophobicity and organophilicity of silicalite. Subsequently, Pope [6] studied water adsorption on ZSM-5 and silicalite and reported that ZSM-5 contained only a small number of strong water-adsorption sites. According to numerous researchers [7,8], the amount of water adsorbed on ZSM-5 zeolites linearly decreases with the increase in SiOz/A120s ratio. Recently, Greene [9] has proposed the use of chromium exchanged ZSM-5 (Cr-ZSM-5) hydrophobic medium for the adsorption (ambient temperature) and oxidative destruction (high temperature) of halogenated VOCs. This dual function, sorbent and catalyst (S/C) medium is reported to be the key to implementing a new energy- efficient process for the destruction of halogenated VOCs. This paper is a continuation of our past work [lo], wherein the preliminary results of adsorption and oxidative destruction of trichloroethylene (TCE, C12C=CHC1) on ZSM-5 and dealuminated faujasites were reported. In this paper, we document the performance of several chromium-exchanged ZSM-5 S/C media of varying Si02/AlzOs ratios (30-120) for the ambient (23°C) adsorption and subsequent oxidative destruction (250-4OO”C) of gaseous TCE in a humid (-13 000 ppm water) air stream. Chromium was chosen for this study because previous studies from this laboratory have shown that among the first row transition metals, only chromium has the ability to destroy all unsaturated chlorinated VOCs (vinyl chloride,
Catalysis B: Environmental 14 (1997) 37-47
TCE, perchloroethylene, etc.). With an increase in the SiOz/A120s ratio, the hydrophobicity of Cr-ZSM5 (in the competitive adsorption of TCE and water) increases, which is advantageous for TCE adsorption. However, the decreased density of AlO tetrahedra (a manifestation of increase in Si02/A120s ratio) severely affects the cation-exchange capacity and, therefore, the catalytic activity of Cr-ZSM-5. Hence, it is interesting to search for an optimum Si02/A120s ratio of the ZSM-5 type zeolites which gives preferred performance both as a sorbent and a catalyst. In order to assess the effect of the co-presence of water on TCE adsorption, the results of adsorption runs, carried out both in the absence and presence of water, are presented. Also, knowledge of the nature of the adsorption sites for the adsorbate molecules in the zeolite structure is of great importance for understanding the mechanism of adsorption. Thus, additional results involving adsorption of TCE on Cr-ZSM-5s using a combination of temperature-programmed desorption (TPD) and in-situ Fl-IR spectroscopy were also obtained and will be reported here.
2. Experimental 2.1. Preparation media
and characterization
of the S/C
In this study, four different chromium-exchanged ZSM-5 media were used for the TCE adsorption and conversion runs. They are designated as Cr-Z30, CrZ50, Cr-Z80, and Cr-Z120 where the number indicates the molar Si02/A120s ratio of the parent ZSM-5 support. Except for Z-120, all other ZSM-5s were obtained from the PQ Corporation in the form of l/ 16 in (0.159 cm) extrudates containing 20 wt% alumina binder. The Z-120 powder was obtained from UOP and was pelletized to 8-14 mesh size using 20 wt% boehmite binder. All zeolite crystallites were 1.5-2.0 pm size embedded in the alumina binder matrix. Prior to chromium exchanges, all the zeolites were subjected to two ammonium exchanges, each lasting -3 h, using 2.24 mol/l ammonium chloride solution to obtain NHd-ZSM-5. Since the cation-exchange capacity of a ZSM-5 type zeolite decreases with an increase in the SiOJA120s ratio, a total of three chromium
P.S. Chintawar; H.L. Greene/Applied
exchanges, each lasting 48 h, were carried out on all NH4-ZSM-5s at 60°C. An aqueous chromium nitrate solution (5.625 mmol/l) was used as the source of chromium cations, and the initial pH was adjusted to 4.5 by addition of a few drops of aqueous ammonium hydroxide. After each exchange, the S/C medium was thoroughly washed with distilled and deionised water before subjecting it to the next exchange. After the final exchange, the S/C medium was again thoroughly washed with distilled and deionised water, dried, and then calcined at 500°C. The S/C media were characterized for their chromium content (Phillips PV9550 X-ray fluorescence spectrometer), BET surface area (Quantasorb Jr. surface area analyzer), relative crystallinity (Phillips APD 3720 X-ray diffractometer with CuK, radiation), and ammonia acidity (DuPont 2950 thermogravimetric analyzer). The relative crystallinity was calculated by comparing the intensities of identifying diffraction lines (four lines in the 20 of 23-25”) of the S/C medium and the corresponding reference (ZSM-5, as-received). 2.2. Reactor operation for adsorption
and catalysis
Adsorption experiments in dry (<3 ppm water) and humid (~13 000 ppm water) air were carried out at atmospheric pressure and ambient temperature (23°C) in a fixed-bed reactor. The reactor tube was of glass, 110 cm long, 20 mm o.d. and 17 mm i.d. The activebed portion was x5.5 cm long (L/D = 3.24), containing from 5.0 to 8.5 g of S/C medium. For the humid air experiments, gaseous feed mixtures were generated by the use of two saturators, one containing water, the other one TCE. Air, containing -1000 ppm TCE and -13 000 ppm water, was passed through the active fixed-bed with periodic measurement of inlet and outlet TCE levels (HP 5890 GC with HP 5970B MSD). For dry air experiments, the water bubbler was not used. Breakthrough was defined as the time when 20-30 ppm TCE was first detected in the outlet stream; saturation was defined as the point where the inlet and outlet TCE concentrations merged. Other details of the operation are mentioned elsewhere [lo]. In view of the analytical problems associated with the measurement of water saturation capacity of the S/C media in the bed, this property was measured separately on a thermogravimetric analyzer (TGA).
Catalysis B: Environmental 14 (1997) 37-47
39
Approximately 100 mg of S/C medium was loaded onto a TGA pan and heated in dry air flow for 3 h. After cooling the sample to ambient temperature, the 13 000 ppm water-air stream was adsorbed until saturation. The catalytic activity and selectivity experiments were carried out in humid air in the same apparatus at temperatures between 250” and 400°C. The space velocity was maintained at 2400 hh’ (RTP). Other details of the operation are mentioned elsewhere [lO,l I]. 2.3. FT-IR
spectra collection procedure
In order to find the probable site of adsorption for the sorbate (TCE) molecule in the ZSM-5 structure, in-situ IT-IR adsorption experiments were carried out. The S/C media were investigated by IR-spectroscopy using a Bio-Rad ITS-7 spectrometer with 4 cm-’ resolution. The sample for the adsorption was made in the form of a 13 mm diameter selfsupported pellet. The pellet was activated by heating under vacuum (
3. Results and discussion For a chromium exchanged ZSM-5 zeolite to be an efficient dual function S/C medium, it should exhibit high saturation capacity for chlorinated VOCs, low
40 Table 1 Comparison
l?S. Chintawar, H.L. Greene/Applied
of the properties
S/C medium
Cr-Z30 Cr-ZSO Cr-Z80 Cr-Z120
of various S/C media
Relative crystallinity (%), 0=*24
Surface area* (m’/g), o=f&7
Chromium content (wt%), o=f0.01-0.02
96 98 96 98
366 365 414 363
1.42 0.88 0.57 0.26
* The values in parentheses
(364) (367) (414) (384)
Ion exchange (%)
100 99 90 69
Acidity (mg ammonia desorbed per g of S/C), 0=+1-2 24.29 18.67 15.23 12.99
are for the parent ZSM-5 support.
saturation capacity for water, high catalytic activity for the oxidative destruction of chlorinated VOCs, and high selectivity toward the desired deep oxidation products; i.e., CO2 and HCl. These physiochemical characteristics are related to numerous properties of the S/C media such as the Si02/A120s ratio, chromium content, acidity, and BET surface area. Therefore, in this section, the properties of the S/C media are presented and discussed first, followed by their adsorptive and catalytic behavior. 3.1. Properties
Catalysis B: Environmental 14 (1997) 37-47
of the (S/C) media
Table 1 depicts the various properties obtained for the S/C media used in this study. All the media show high values (295%) of relative crystallinity, as listed in column 2. This indicates that there was insignificant structural collapse of ZSM-5 zeolites during three chromium exchanges under mildly acidic (pH = 4.5) conditions. This stability is not surprising since pentasil zeolites are known for their resistance to acid attack. The retention of structural integrity is also evident from the surface areas listed in the next column. All the S/C media show surface areas very similar to those of their parent ZSM-5 support, indicating the absence of any dealumination during chromium exchanges. The chromium content (obtained after three exchanges) follows a specific trend, decreasing with an increase in SiOz/A120s ratio. The difficulty in exchanging a multivalent cation such as Cr’+ in ZSM-5 zeolites stems from the weak anionic field, hydrophobicity, and low density of AlO tetrahedra particular to those zeolites. An increase in Si02/A120s ratio further decreases the density of AlO tetrahedra and so the cation-exchange capacity
drops [ 121. Therefore, among the four ZSM-5 zeolites studied, Z-30 has the highest cation exchange capacity and Z-120, the lowest. The percentage ion exchange, calculated on the basis of one Cr3+ cation for three NH: cations, is shown in the next column of Table 1. High-percentage exchange levels, approaching lOO%, were obtained with all S/C media except Cr-Z120. The high exchange levels in these highly siliceous zeolites (ZSM-5) may be partially attributed to the presence of [Cr(OH),]‘3-“‘t type species in the zeolite due to cation hydrolysis. The occurrence of cation hydrolysis has been previously shown in excessively exchanged Cu-ZSM-5 zeolites [ 13,141. It should be noted here that the X-ray diffraction (XRD) patterns did not show the presence of any chromium oxide and/or hydroxide species on any calcined S/C medium. The XRD patterns of the S/C media were identical to those of the ZSM-5 support, except for a small loss (55%) in the intensity of identifying diffraction lines. Since the cation exchanges were carried out under acidic conditions, the possibility of precipitation of chromium hydroxide on the surface of crystallites was insignificant. Also, the zeolites were washed thoroughly after every cation exchange. Therefore, it can be hypothesized that the chromium cations present in the S/C media were in the exchanged positions. Efforts are currently underway to prove this hypothesis using crystal structure refinement techniques. The total acidity of the S/C media, as measured by the amount of adsorbed ammonia which was reversibly desorbed, is a combined effect of two factors: cationic acidity due to exchanged chromium, and Brijnsted acidity due to negatively charged A104 tetrahedra. As the number of AlO tetrahedra and the chromium content decreased (with an increase
P.S. Chintawac H.L. Greene/Applied
Catalysis B: Environmental 14 (1997) 37-47
Fig. 1. The effect of Si02/Al20, ratio of Cr-ZSM-5 zeolites on the TCE saturation water) air. Space velocity: 2400 hK’, bed size: 11.8 cm3, TCE concentration:-1000
in Si02/A1203 ratio), the total acidity also decreased as shown in the last column of Table 1. 3.2. TCE adsorption in dry (<3ppm humid (-13 000 ppm water) air
water) and
The results shown in Fig. 1 indicate that TCE adsorption capacity in a humid air stream increased by 68% (from 6.0 to 10.1 wt%) with an increase in the SiOz/A1203 ratio from 30 to 120; this is a manifestation of increased hydrophobicity of the S/C media as shown in Table 2. Schumaker et al. [3] have obtained similar results wherein the saturation loading of water was found to decrease (and the amount of tetrachloroethylene adsorbed, to increase) with an increase in the Si02/A1203 ratio of ZSM-5 sorbents from 80 to 1200. As stated in Section 1, the ability of ZSM-5 zeolites to adsorb water is known to decrease with decreasing concentration of aluminum atoms in the framework. In competitive adsorption of TCE and water, in addition to the number of framework aluminum atoms, the
Table 2 Water saturation
capacities
S/C medium
Saturation
Cr-Z30 Cr.Z50 Cr-ZSO Cr-Z120
5.97 5.51 5.01 3.89
capacity
capacity in dry (~3 ppm water) and humid (- 13 000 ppm ppm.
forces of adsorption also play a dominant role. Physical adsorption is a surface phenomenon involving interactions of sorbate molecules (TCE and water) with sorbent (Cr-ZSM-5) which can be accounted for by van der Waals forces and electrostatic forces. The magnitude of these forces depends on the polar nature of the sorbate and the sorbent. On polar surfaces such as cation exchanged zeolites, electrostatic forces dominate over van der Waals forces. Consistent with the data shown in Table 3, sorbate molecules such as water (which have low molecular weight, small size and high dipole moment) have strong electrostatic interactions with the cations (chromium and hydrogen, in this case) of the sorbent (Cr-ZSM-5). This interaction, in turn, encourages high water saturation capacities when the surface cation (Cr and/or H) concentrations are high and severely limits the adsorption of less polar molecules (TCE, in this case) due to steric hindrance and competition for the adsorption sites. Vansant [ 151 and P&ash et al. [16] have further suggested that strong electrostatic interactions between the cations and dipole of water molecules Table 3 Comparison
of various S/C media
41
of the properties
of the sorbate molecules
[24]
(wt%) Sorbate molecule
Kinetic diameter
TCE (ClHC = Ccl*) Water (H,O)
5.6 2.6
Dipole moment (.&) (debye) 0.90 1.85
Molecular weight
132 18
42
t?S. Chintawal; H.L. Greene/Applied
produce a diffusion block by clustering the water molecules around the cations in the zeolite. With a reduction in surface cation concentration, which occurs with an increase in Si02/A120s ratio and decrease in chromium content, it is believed that the net adsorption of small, high polarity molecules (water) diminishes and that the adsorption of larger, less polar molecules (TCE) increases as steric hindrance and site competition are reduced, as is consistent with Fig. 1. This hypothesis is confirmed by the decreasing water adsorption capacities with increasing Si02/A120s ratio of the S/C media shown in Table 2. The significantly higher TCE adsorption capacity in a dry air stream, as opposed to a humid air stream, is due to the lack of competition for adsorption sites in the former case. Also evident from Fig. 1 is the fact that the Si02/A120s ratio determines not only the hydrophobicity (and, therefore, TCE adsorption capacity in a humid air stream) but also the organophilicity (TCE adsorption capacity in a dry air stream) of the S/C medium. Therefore, an increase in the SiOz/A120s ratio from 30 to 120 leads to a 40% enhancement (from 10.7 to 15.0 wt%) in TCE adsorption capacity in a dry air stream and a 68% increase (from 6.0 to 10.1 wt%) in the humid air stream. Our results are in agreement with those of Hsu and Ma [ 171 who have characterized ZSM-5 zeolites with SiOz/A120s ratios of 252, 1126, and 2124 in terms of adsorption and diffusion of water, methanol, benzene,
5
1.8
f
1.6
Catalysis B: Environmental 14 (1997) 37-47
and n-hexane at 25°C. The authors have reported 1524% enhancement in single-component hydrocarbon adsorption capacity as a result of increase in the Si02/A120s ratio. 3.3. Centers of adsorption (TPD and FT-IR results)
of TCE in S/C media
The TPD curves obtained on TGA after ambient adsorption of -1000 ppm TCE in dry air on the four S/C media are shown in Fig. 2. Although the adsorption capacities obtained on TGA are -20% higher than the corresponding fixed-bed values, the qualitative trend is the same. We attribute this difference to the differences in the flow geometry in fixed bed vs. in TGA. All TPD curves depicted in Fig. 2 clearly show a strong desorption peak at 84-88°C [a temperature very close to the normal boiling point (87°C) of liquid TCE] and a weak desorption shoulder at 138-141°C. The identical nature of all TPD curves indicates that there is no significant dependence of energetics for TCE adsorbed species on the Si02/A120s ratio or the chromium content. The proximity of the strong peak at 84-86°C to the boiling point of liquid TCE suggests that the heat of TCE adsorption on the sites characterized by this peak is very close to the heat of liquefaction and corresponds to adsorbateadsorbate non-specific interaction within the ZSMJ structure [ 181.
-Cr-250
g 1.4 g
1.2
y 1.0
iI
0.6 0.8
2
0.4
J 0.2 0.0
Fig. 2. Temperature-programmed
desorption
(TPD) curves after -1000
ppm TCE adsorption
in dry air on the S/C media
P.S. Chintawar; H.L. Greene/Applied
Catalysis B: Environmental
43
14 (1997) 37-47
Wavenumbers (cm-l) Fig. 3. F’l-IR spectra obtained during TCE adsorption on Cr-ZBO at 23°C. Identical I - reference spectrum; 2 - commencement of adsorption; and 3 - steady state.
The TPD results are in congruence with in-situ FTIR adsorption results shown in Figs. 3 and 4. Fig. 3 shows the FT-IR spectra obtained before and during TCE adsorption on Cr-Z80 at 23°C. The steady-state spectrum shown in this figure was obtained after 6 min of adsorption. Identical spectra were obtained for Cr230 and Cr-ZSO. The commencement of adsorption is accompanied by two bands of almost equal intensity at 3740 and 3705 cm-‘. On the contrary, at steady state, the absorption band at 3740 cm-’ is significantly stronger than the 3705 cm-’ band. This phenomenon suggests preferable adsorption of TCE on the group indicated by the 3740 cm-’ band rather than on the site represented by the 3705 cm-’ absorption band. The former suggests the interaction of TCE molecules with the silanol (Si-OH) groups on the outer surface of crystallites; i.e. OH groups which terminate the face of zeolite crystallites at positions where bonding in the interior would occur with adjacent tetrahedral Al or Si ions [ 191. The other absorption band, detected at 3705 cm-‘, is commonly attributed to AlOH groups and has been observed by other researchers in divalent cationexchanged Y zeolites, such as Mg-Y and Sr-Y [20]. Ison and Gorte [21] have observed an absorption band
spectra
were obtained
with Cr-Z30
and Cr.ZSO:
at 3690 cm-’ upon water adsorption on H-ZSM-5, and they have attributed it to the hydronium (H30+) ion. Therefore, it appears that the band observed at 3705 cm-’ in this study may also be caused by the interaction of water with the cations present in the zeolite. Recall that TCE adsorptions were carried out in dry (~3 ppm water) air. Also, preliminary FT-IR adsorption experiments with dry air only, without any TCE, showed no changes in the hydroxyl stretching region of the S/C media. Therefore, we assign the negative absorption band at 3705 cm-’ to the interaction of TCE with AlOH groups of the ZSM-5 structure. Fig. 4 shows the FT-IR spectra obtained after 30 min of TCE adsorption on all S/C media. It can be clearly seen that Cr-Z120 did not show any absorption bands in the hydroxyl stretching region of the ZSM-5 zeolite. This result suggests the lack of hydrogen bonding between the TCE molecules and the adsorption sites of the essentially non-polar surface of Cr-Z120. This is caused by the presence of a very low density of cations (chromium and hydrogen) and polar groups (A104 tetrahedra) on Cr-Z120, resembling the crystal structure of silicalite with its near
44
flS. Chintawal; H.L. Greene/Applied
-
Catalysis B: Environmental
14 (1997) 37-47
cr-zao Cr-250
3000 Wavenumbem
Fig. 4. Fl-IR
(cm-l)
spectra obtained after 30 min of TCE adsorption
infinite Si02/A120s ratio. It is known that the physical adsorption of organics onto silicalite occurs by a porefilling process involving only van der Waals forces and is a function of mainly pore geometry rather than chemical characteristics of the sorbent [5,16]. Therefore, consistent with the low density of sites for hydrogen bonding, Cr-Z120 does not show any perturbations in the hydroxyl stretching region upon TCE adsorption. 3.4. Activity and selectivity of the S/C media for the oxidative destruction of TCE The catalytic activities of the four S/C media for the oxidative destruction of TCE in humid air are shown in Fig. 5. All the S/C media, except Cr-Z120, showed high destruction efficiencies with >95% conversion at 300°C. The poor conversion obtained on Cr-Z120 was probably due to its very low chromium content. Surprisingly, Cr-ZSO containing significantly less chromium than Cr-Z50, showed marginally higher TCE destruction efficiency (Fig. 5). Since the oxidative destruction of TCE in a chromium exchanged zeolite catalyst is believed to occur through reduction-
on various S/C media at 23°C.
oxidation (redox) reaction of exchanged chromium cations with extra-lattice oxygen [22], presumably the environment of the chromium cation (location, type of zeolite, Si02/A1203 ratio, etc.) largely determines the catalytic activity of the site. Dumesic and Millman [23] have reported a fivefold increase in the turnover frequency of the exchanged iron cations in a Fe-Y catalyst for N20 decomposition (a redox reaction) upon increase in the SiOz/A120sratio of Y zeolite from seven to 14. The authors have attributed the enhancement in activity to the facile redox reaction of iron cations in a high silica Y zeolite. Thus, it is speculated that marginally higher destruction efficiency exhibited by Cr-Z80 than Cr-ZSO could be due to a more facile redox reaction of chromium cations in Cr-Z80 than in Cr-ZSO. Selectivities, along with carbon and chlorine balance data obtained during the catalytic runs, are shown in Table 4. The data acquisition in the GUMS was carried out in the selective-ion monitoring mode and the following compounds were monitored: COz, CH2C12, COC12, C,HsCl, C2H2C12, C2HC1s, CzC14 and CC14. In addition to the unreacted feed, only deep oxidation products such as C02, Cl*, HCl, and pro-
l?S. Chintawac H.L. Greene/Applied
223
250
275
Catalysis B: Environmental
300 325 Temperature (“C)
14 (1997) 37-47
350
375
400
Fig. 5. The oxidative destruction and TCE concentration - -1000
of TCE in humid air (-13 000 ppm water) on S/C media. Space velocity - 2400 h-t; bed size - 11.8 cm3; ppm.
Table 4 Material balances
data for the oxidative
and selectivity
destruction
of TCE in humid (-13 000 ppm water) air.
Temperature
TCE cont.
Conversion
Cl2 Selectivity
CO2 Selectivity
Carbon balance
Chlorine balance
( C)
(ppm)
(%)
(‘“)
(%)
W)
(%)
S,IC medium: Cr-Z30 350 325 300 275 250
1022 974 1054 1018 1046
99.0 98.6 99.3 97.1 67.6
28.0 24.0 13.9 0 0
76.2 53.0 23.5 13.6 4.3
102 104 93 85 87
99 103 100 92 95
S; C medium: 335 300 275 250
1004 965 1003 1091
98.9 96.1 78.5 45.1
16.6 10.0 0 0
25.3 16.0 10.3 2.9
96 98 102 96
102 97 102 98
S/C medium: Cr.280 325 300 275 250
1014 980 1067 1083
99.6 98.4 85.4 50.1
6.6 2.7 0 0
21.0 13.4 8.8 2.0
90 97 89 91
95 96 100 96
S/C medium: Cr-2120 400 350 300 215
1053 1062 1035 1014
46.9 19.6 6.8 6.6
4.2 1.8 0.2 0.1
97 98 100 97
106 108 103 102
Cr-250
ducts of incomplete oxidation such as CO were detected in the product stream of the reactor. No phosgene (COC12) was detected in the product stream of any catalyst. The absence of phosgene was also
confirmed by use of detector tubes (Mine Safety Appliances). All catalysts showed high (7.S90%) selectivity toward the formation of CO at 300°C. Figs. 6 and 7 show the changes in product selectivity
46
RS. Chintawac
H.L. Greene/Applied
Catalysis B: Environmental
14 (1997) 37-47
I +Cr-Z30
225
250
275
ml 325 Temperature(F)
350
375
400
Fig. 6. COZ selectivity with temperature obtained during the oxidative destruction of TCE in humid (~13 000 ppm water) air on S/C media. Space velocity - 2400 hK’; bed size - 11.8 cm3; and TCE concentration - -1000 ppm.
Fig. 7. Cl2 selectivity with temperature obtained during the oxidative destruction of TCE in humid (-13 000 ppm water) air on S/C media. Space velocity - 2400 h-‘; bed size - 11.8 cm3; and TCE concentration - -1000 ppm.
with temperature for each catalyst. The higher chromium content (1.42 wt% for Cr-Z30) seems to enhance the production of CO2 and Cl*, the products of CO oxidation (2C0 + 02 --f 2CO2) and the Deacon reaction (4HCl-t 02 + 2C12 + 2H20), respectively. This is interpreted in terms of the high catalytic activity of chromium for these reactions [22]. The production of Cl2 increases with temperature due to a large increase in the Deacon reaction rate constant. At low temperatures, the formation of CO2 is relatively unfavored and, therefore, leads to high selectivity toward CO.
4. Conclusions .
??
The amount of TCE physisorbed on Cr-ZSM-5 S/C media in both humid and dry air streams increases with an increase in Si02/A120s ratio. This is interpreted in terms of increased hydrophobicity and increased organophilicity, respectively. With the exception of Cr-Z120, the combined insitu FT-IR and TPD results show the involvement of AlOH and terminal SiOH (silanol) groups of the ZSM-5 structure in the physisorption of TCE.
l?S. Chintawal; H.L. Greene/Applied
??
The results of this investigation show that highly siliceous (and therefore hydrophobic) S/C media can be used for the physisorption of chlorinated VOCs without unduly sacrificing their catalytic activity. Therefore, an increase in Si02/A120s ratio of ZSM-5 zeolite from 30 to 80 resulted in 50% enhancement in the TCE adsorption capacity (in humid air) with only modest loss (12-17%) of the TCE destruction efficiency at temperatures <3OO”C.
Acknowledgements The authors express their gratitude to the U. S. Environmental Protection Agency (EPA) and U. S. Air Force (SERDP) for supporting this work. Zeolite samples were obtained from The PQ Corporation and IJOF? Thomas Quick (The University of Akron) collected the X-ray diffraction data.
References [I] J. Weitkamp, S. Ernst, B. Gunzel, W. Decker, Zeolites 11 (1991) 314. [2] J. Weitkamp, P. Kleinschmit, A. Kiss, C.H. Burke, in: R. von Ballmoos, J.B. Higgins, M.M.J. Treaty (Eds.), Proc. 9th International Zeolite Conference, Montreal 1992, Butterworth-Heinmann, Stoneham MA, 1993, p. 79. 131 R. Schumacher, S. Ernst, J. Weitkamp, in: R. von Ballmoos, J.B. Higgins, M.M.J. Treaty (Eds.), Proc. 9th International Zeolite Conference, Montreal 1992, Butterworth-Heinmann, Stoneham MA, 1993, p. 89.
Catalysis B: Environmental 14 (1997) 37-47
41
[41 S.W. Blocki, Environmental Progress 12(3) (1993) 226. 151 E.M. Flanigen, EM. Bennett, R.W. Grose, J.P. Cohen, R.L. Patten, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512. [61 C.G. Pope, J. Colloid Inter. Sci. 116(l) (1987) 221. [71 D.H. Olson, W.O. Haag, M.R. Lago, J. Catal. 61 (1980) 390. @I H. Nakamoto, H. Takahashi, Zeolites 2 (1982) 67. r91 H.L. Greene, U.S. Patent 5 414 201, 1995. [lOI H.L. Greene, D.S. Prakash, K.V. Athota, Appl. Catal. B 7 (1996) 213. Cl11 A. Saraf, H.L. Greene, M.S. Kosusko, S. Narayanan, J. Catal. 120 (1989) 478. WI S. Kaliaguine, G. Lemay, A. Adnot, S. Burelle, R. Audet, G. Jean, J.A. Sawicki, Zeolites 10 (1990) 559. r131 M. Iwamaoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine, S. Kagawa, J. Phys. Chem. 95 (1991) 3727. [I41 M. Iwamoto, H. Yahiro, Y. Mine, S. Kagawa, Chem. Lett. (1989) 213. Cl51 E.F. Vansant, Stud. Surf. Sci. Catal. 37 (1988) 143. [161 D.S. Prakash, H.L. Greene, K.V. Athota, AIChE Symposium Series 309, 91 (1995) 1. [I71 CC. Hsu, Y.H. Ma, Report for October 1986-September 1987, Chemical Development and Engineering Center, Aberdeen Proving Ground, 1988. SD. Kirik, A.A. Dubkov, S.A. Dubkova, O.M. Sharonova, A.G. Anshits, Zeolites 12 (1992) 292. T.R. Hughes, H.M. White, J. Phys. Chem. 71(7) (1967) 2192. J.W. Ward, in: J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, American Chemical Society, Washington, D.C., 1976, p. 118. Pll A. Ison, R.J. Gorte, J. Catal. 89 (1984) 150. [221 S. Chatterjee, An Investigation of Low Temperature Catalytic Oxidation of Chlorinated Hydrocarbons over Metal Loaded Zeolites, Ph.D. Dissertation, The University of Akron, OH, 1993. E31 J.A. Dumesic, W.S. Millman, in R.T.K. Baker. L.L. Murrell (Eds.), Novel Materials in Heterogeneous Catalysis, American Chemical Society, Washington, D.C., 1990, p. 66. [241 L. Alvarez-Cohen, PL. McCarty, P.V. Roberts, Environ. Sci. Technol. 27 (1993) 2141.
Applied Catalysis
B: Environmental
14 (1997) 37-47
Adsorption and catalytic destruction of trichloroethylene in hydrophobic zeolites Prashant S. Chintawar, Howard L. Greene* Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906,
USA
Received
1997
16 October
1996; received in revised form 2 January
1997: accepted
6 January
Abstract Several chromium exchanged ZSM-5 zeolites of varying Si02/A1203 ratio were prepared and investigated for ambient (23°C) adsorption and subsequent oxidative destruction (250-4OO”C) of gaseous trichloroethylene (TCE, Cl,C=CHCl) in a humid air stream. With an increase in the SiOz/A120s ratio from 30 to 120, the TCE saturation capacity of these dual-function sorbent/catalyst (S/C) media was found to increase from 6.0 to 10.1 wt% in a humid air stream. This phenomenon was attributed to an increase in hydrophobicity coupled with reduced steric hindrance and site competition for the adsorption of TCE molecules in the competitive adsorption of TCE and water. Ambient TCE adsorption experiments carried out in dry air showed the same trend, which was attributed to increasing organophilicity of the S/C media with an increase in the SiOz/A120s ratio. In order to gain knowledge of physisorption sites for TCE molecules in the ZSM-5 structure, temperatureprogrammed desorption over a temperature range of 30-300°C and in-situ FT-IR studies at ambient conditions were also carried out. These studies revealed that in all zeolites (except for Cr-ZSM-5 with Si02/A120s ratio of 120) TCE interacted with terminal silanol (SiOH) and AlOH groups. At temperatures >3OO”C (with the exception of Cr-ZSM-5 with Si02/AlzOs ratio of 120), all S/C media showed >95% TCE destruction efficiency. Based on its high adsorption capacity and high activity for oxidative destruction of TCE, it is concluded that the Cr-ZSM-5 S/C medium with a Si02/A120s ratio of 80 gives preferred performance both as a sorbent and a catalyst. 0 1997 Elsevier Science B.V. Keywords:
Chlorinated VOC; Cr-ZSM-5; Hydrophobic zeolite; SiOz/A1203 ratio; Sorbent/catalyst
1. Introduction Hydrophobic zeolites have recently gained much attention due to their ability to selectively remove one or more organic pollutants from humid air streams. In this respect, Weitkamp et al. [l] have studied the binary adsorption of water and ethanol from their *Corresponding author. Present addres: Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106-7217, USA. Tel.: +l 216 368 4065; fax.: +l 216 368 3016. 0926-860x/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO926-3373(97)00010-6
medium
gaseous mixtures on sodium-exchanged zeolite X, Y, and dealuminated Y. Their results indicate that, upon increasing the Si02/A120s ratio of a zeolite from three to 42, the saturation loading of water decreases and that of ethanol passes through a maximum. Weitkamp et al. [2] have also investigated the hydrophobicity of pentasil (ZSM-5 type) and dealuminated faujasite zeolites by competitive adsorption of toluene and water from the gas phase. Similar to their earlier observation, the hydrophobicity was found to increase with an increase in SiOz/A120s
38
P.S. Chintawar; H.L. Greene/Applied
ratio of zeolite. Schumacher et al. [3] have separated gaseous tetrachloroethylene and water mixtures by adsorption on numerous zeolites; e.g. EMT (Elf Mulhouse Two, a hexagonal variant of faujasite), faujasites, ZSM-5, AlPOd-5, and SAPO-5. The authors state that high separation efficiency could be achieved with carefully dealuminated and steamed zeolites of the EMT and faujasite type or highly siliceous ZSM-5 zeolites. Blocki [4] has compared the performance of a proprietary hydrophobic zeolite and an activated carbon for solvent separation and concentration. The author suggests that zeolitic sorbent may exhibit separation capacity equal to or better than systems using activated carbon, and without much of the support equipment needed with carbon. It should be noted that the hydrophobic nature of pentasil zeolites (ZSM-5, silicalite, etc.) has been known for a long time. Flanigen et al. [5], in a pioneering article, have reported the hydrophobicity and organophilicity of silicalite. Subsequently, Pope [6] studied water adsorption on ZSM-5 and silicalite and reported that ZSM-5 contained only a small number of strong water-adsorption sites. According to numerous researchers [7,8], the amount of water adsorbed on ZSM-5 zeolites linearly decreases with the increase in SiOz/A120s ratio. Recently, Greene [9] has proposed the use of chromium exchanged ZSM-5 (Cr-ZSM-5) hydrophobic medium for the adsorption (ambient temperature) and oxidative destruction (high temperature) of halogenated VOCs. This dual function, sorbent and catalyst (S/C) medium is reported to be the key to implementing a new energy- efficient process for the destruction of halogenated VOCs. This paper is a continuation of our past work [lo], wherein the preliminary results of adsorption and oxidative destruction of trichloroethylene (TCE, C12C=CHC1) on ZSM-5 and dealuminated faujasites were reported. In this paper, we document the performance of several chromium-exchanged ZSM-5 S/C media of varying Si02/AlzOs ratios (30-120) for the ambient (23°C) adsorption and subsequent oxidative destruction (250-4OO”C) of gaseous TCE in a humid (-13 000 ppm water) air stream. Chromium was chosen for this study because previous studies from this laboratory have shown that among the first row transition metals, only chromium has the ability to destroy all unsaturated chlorinated VOCs (vinyl chloride,
Catalysis B: Environmental 14 (1997) 37-47
TCE, perchloroethylene, etc.). With an increase in the SiOz/A120s ratio, the hydrophobicity of Cr-ZSM5 (in the competitive adsorption of TCE and water) increases, which is advantageous for TCE adsorption. However, the decreased density of AlO tetrahedra (a manifestation of increase in Si02/A120s ratio) severely affects the cation-exchange capacity and, therefore, the catalytic activity of Cr-ZSM-5. Hence, it is interesting to search for an optimum Si02/A120s ratio of the ZSM-5 type zeolites which gives preferred performance both as a sorbent and a catalyst. In order to assess the effect of the co-presence of water on TCE adsorption, the results of adsorption runs, carried out both in the absence and presence of water, are presented. Also, knowledge of the nature of the adsorption sites for the adsorbate molecules in the zeolite structure is of great importance for understanding the mechanism of adsorption. Thus, additional results involving adsorption of TCE on Cr-ZSM-5s using a combination of temperature-programmed desorption (TPD) and in-situ Fl-IR spectroscopy were also obtained and will be reported here.
2. Experimental 2.1. Preparation media
and characterization
of the S/C
In this study, four different chromium-exchanged ZSM-5 media were used for the TCE adsorption and conversion runs. They are designated as Cr-Z30, CrZ50, Cr-Z80, and Cr-Z120 where the number indicates the molar Si02/A120s ratio of the parent ZSM-5 support. Except for Z-120, all other ZSM-5s were obtained from the PQ Corporation in the form of l/ 16 in (0.159 cm) extrudates containing 20 wt% alumina binder. The Z-120 powder was obtained from UOP and was pelletized to 8-14 mesh size using 20 wt% boehmite binder. All zeolite crystallites were 1.5-2.0 pm size embedded in the alumina binder matrix. Prior to chromium exchanges, all the zeolites were subjected to two ammonium exchanges, each lasting -3 h, using 2.24 mol/l ammonium chloride solution to obtain NHd-ZSM-5. Since the cation-exchange capacity of a ZSM-5 type zeolite decreases with an increase in the SiOJA120s ratio, a total of three chromium
P.S. Chintawar; H.L. Greene/Applied
exchanges, each lasting 48 h, were carried out on all NH4-ZSM-5s at 60°C. An aqueous chromium nitrate solution (5.625 mmol/l) was used as the source of chromium cations, and the initial pH was adjusted to 4.5 by addition of a few drops of aqueous ammonium hydroxide. After each exchange, the S/C medium was thoroughly washed with distilled and deionised water before subjecting it to the next exchange. After the final exchange, the S/C medium was again thoroughly washed with distilled and deionised water, dried, and then calcined at 500°C. The S/C media were characterized for their chromium content (Phillips PV9550 X-ray fluorescence spectrometer), BET surface area (Quantasorb Jr. surface area analyzer), relative crystallinity (Phillips APD 3720 X-ray diffractometer with CuK, radiation), and ammonia acidity (DuPont 2950 thermogravimetric analyzer). The relative crystallinity was calculated by comparing the intensities of identifying diffraction lines (four lines in the 20 of 23-25”) of the S/C medium and the corresponding reference (ZSM-5, as-received). 2.2. Reactor operation for adsorption
and catalysis
Adsorption experiments in dry (<3 ppm water) and humid (~13 000 ppm water) air were carried out at atmospheric pressure and ambient temperature (23°C) in a fixed-bed reactor. The reactor tube was of glass, 110 cm long, 20 mm o.d. and 17 mm i.d. The activebed portion was x5.5 cm long (L/D = 3.24), containing from 5.0 to 8.5 g of S/C medium. For the humid air experiments, gaseous feed mixtures were generated by the use of two saturators, one containing water, the other one TCE. Air, containing -1000 ppm TCE and -13 000 ppm water, was passed through the active fixed-bed with periodic measurement of inlet and outlet TCE levels (HP 5890 GC with HP 5970B MSD). For dry air experiments, the water bubbler was not used. Breakthrough was defined as the time when 20-30 ppm TCE was first detected in the outlet stream; saturation was defined as the point where the inlet and outlet TCE concentrations merged. Other details of the operation are mentioned elsewhere [lo]. In view of the analytical problems associated with the measurement of water saturation capacity of the S/C media in the bed, this property was measured separately on a thermogravimetric analyzer (TGA).
Catalysis B: Environmental 14 (1997) 37-47
39
Approximately 100 mg of S/C medium was loaded onto a TGA pan and heated in dry air flow for 3 h. After cooling the sample to ambient temperature, the 13 000 ppm water-air stream was adsorbed until saturation. The catalytic activity and selectivity experiments were carried out in humid air in the same apparatus at temperatures between 250” and 400°C. The space velocity was maintained at 2400 hh’ (RTP). Other details of the operation are mentioned elsewhere [lO,l I]. 2.3. FT-IR
spectra collection procedure
In order to find the probable site of adsorption for the sorbate (TCE) molecule in the ZSM-5 structure, in-situ IT-IR adsorption experiments were carried out. The S/C media were investigated by IR-spectroscopy using a Bio-Rad ITS-7 spectrometer with 4 cm-’ resolution. The sample for the adsorption was made in the form of a 13 mm diameter selfsupported pellet. The pellet was activated by heating under vacuum (
3. Results and discussion For a chromium exchanged ZSM-5 zeolite to be an efficient dual function S/C medium, it should exhibit high saturation capacity for chlorinated VOCs, low
40 Table 1 Comparison
l?S. Chintawar, H.L. Greene/Applied
of the properties
S/C medium
Cr-Z30 Cr-ZSO Cr-Z80 Cr-Z120
of various S/C media
Relative crystallinity (%), 0=*24
Surface area* (m’/g), o=f&7
Chromium content (wt%), o=f0.01-0.02
96 98 96 98
366 365 414 363
1.42 0.88 0.57 0.26
* The values in parentheses
(364) (367) (414) (384)
Ion exchange (%)
100 99 90 69
Acidity (mg ammonia desorbed per g of S/C), 0=+1-2 24.29 18.67 15.23 12.99
are for the parent ZSM-5 support.
saturation capacity for water, high catalytic activity for the oxidative destruction of chlorinated VOCs, and high selectivity toward the desired deep oxidation products; i.e., CO2 and HCl. These physiochemical characteristics are related to numerous properties of the S/C media such as the Si02/A120s ratio, chromium content, acidity, and BET surface area. Therefore, in this section, the properties of the S/C media are presented and discussed first, followed by their adsorptive and catalytic behavior. 3.1. Properties
Catalysis B: Environmental 14 (1997) 37-47
of the (S/C) media
Table 1 depicts the various properties obtained for the S/C media used in this study. All the media show high values (295%) of relative crystallinity, as listed in column 2. This indicates that there was insignificant structural collapse of ZSM-5 zeolites during three chromium exchanges under mildly acidic (pH = 4.5) conditions. This stability is not surprising since pentasil zeolites are known for their resistance to acid attack. The retention of structural integrity is also evident from the surface areas listed in the next column. All the S/C media show surface areas very similar to those of their parent ZSM-5 support, indicating the absence of any dealumination during chromium exchanges. The chromium content (obtained after three exchanges) follows a specific trend, decreasing with an increase in SiOz/A120s ratio. The difficulty in exchanging a multivalent cation such as Cr’+ in ZSM-5 zeolites stems from the weak anionic field, hydrophobicity, and low density of AlO tetrahedra particular to those zeolites. An increase in Si02/A120s ratio further decreases the density of AlO tetrahedra and so the cation-exchange capacity
drops [ 121. Therefore, among the four ZSM-5 zeolites studied, Z-30 has the highest cation exchange capacity and Z-120, the lowest. The percentage ion exchange, calculated on the basis of one Cr3+ cation for three NH: cations, is shown in the next column of Table 1. High-percentage exchange levels, approaching lOO%, were obtained with all S/C media except Cr-Z120. The high exchange levels in these highly siliceous zeolites (ZSM-5) may be partially attributed to the presence of [Cr(OH),]‘3-“‘t type species in the zeolite due to cation hydrolysis. The occurrence of cation hydrolysis has been previously shown in excessively exchanged Cu-ZSM-5 zeolites [ 13,141. It should be noted here that the X-ray diffraction (XRD) patterns did not show the presence of any chromium oxide and/or hydroxide species on any calcined S/C medium. The XRD patterns of the S/C media were identical to those of the ZSM-5 support, except for a small loss (55%) in the intensity of identifying diffraction lines. Since the cation exchanges were carried out under acidic conditions, the possibility of precipitation of chromium hydroxide on the surface of crystallites was insignificant. Also, the zeolites were washed thoroughly after every cation exchange. Therefore, it can be hypothesized that the chromium cations present in the S/C media were in the exchanged positions. Efforts are currently underway to prove this hypothesis using crystal structure refinement techniques. The total acidity of the S/C media, as measured by the amount of adsorbed ammonia which was reversibly desorbed, is a combined effect of two factors: cationic acidity due to exchanged chromium, and Brijnsted acidity due to negatively charged A104 tetrahedra. As the number of AlO tetrahedra and the chromium content decreased (with an increase
P.S. Chintawac H.L. Greene/Applied
Catalysis B: Environmental 14 (1997) 37-47
Fig. 1. The effect of Si02/Al20, ratio of Cr-ZSM-5 zeolites on the TCE saturation water) air. Space velocity: 2400 hK’, bed size: 11.8 cm3, TCE concentration:-1000
in Si02/A1203 ratio), the total acidity also decreased as shown in the last column of Table 1. 3.2. TCE adsorption in dry (<3ppm humid (-13 000 ppm water) air
water) and
The results shown in Fig. 1 indicate that TCE adsorption capacity in a humid air stream increased by 68% (from 6.0 to 10.1 wt%) with an increase in the SiOz/A1203 ratio from 30 to 120; this is a manifestation of increased hydrophobicity of the S/C media as shown in Table 2. Schumaker et al. [3] have obtained similar results wherein the saturation loading of water was found to decrease (and the amount of tetrachloroethylene adsorbed, to increase) with an increase in the Si02/A1203 ratio of ZSM-5 sorbents from 80 to 1200. As stated in Section 1, the ability of ZSM-5 zeolites to adsorb water is known to decrease with decreasing concentration of aluminum atoms in the framework. In competitive adsorption of TCE and water, in addition to the number of framework aluminum atoms, the
Table 2 Water saturation
capacities
S/C medium
Saturation
Cr-Z30 Cr.Z50 Cr-ZSO Cr-Z120
5.97 5.51 5.01 3.89
capacity
capacity in dry (~3 ppm water) and humid (- 13 000 ppm ppm.
forces of adsorption also play a dominant role. Physical adsorption is a surface phenomenon involving interactions of sorbate molecules (TCE and water) with sorbent (Cr-ZSM-5) which can be accounted for by van der Waals forces and electrostatic forces. The magnitude of these forces depends on the polar nature of the sorbate and the sorbent. On polar surfaces such as cation exchanged zeolites, electrostatic forces dominate over van der Waals forces. Consistent with the data shown in Table 3, sorbate molecules such as water (which have low molecular weight, small size and high dipole moment) have strong electrostatic interactions with the cations (chromium and hydrogen, in this case) of the sorbent (Cr-ZSM-5). This interaction, in turn, encourages high water saturation capacities when the surface cation (Cr and/or H) concentrations are high and severely limits the adsorption of less polar molecules (TCE, in this case) due to steric hindrance and competition for the adsorption sites. Vansant [ 151 and P&ash et al. [16] have further suggested that strong electrostatic interactions between the cations and dipole of water molecules Table 3 Comparison
of various S/C media
41
of the properties
of the sorbate molecules
[24]
(wt%) Sorbate molecule
Kinetic diameter
TCE (ClHC = Ccl*) Water (H,O)
5.6 2.6
Dipole moment (.&) (debye) 0.90 1.85
Molecular weight
132 18
42
t?S. Chintawal; H.L. Greene/Applied
produce a diffusion block by clustering the water molecules around the cations in the zeolite. With a reduction in surface cation concentration, which occurs with an increase in Si02/A120s ratio and decrease in chromium content, it is believed that the net adsorption of small, high polarity molecules (water) diminishes and that the adsorption of larger, less polar molecules (TCE) increases as steric hindrance and site competition are reduced, as is consistent with Fig. 1. This hypothesis is confirmed by the decreasing water adsorption capacities with increasing Si02/A120s ratio of the S/C media shown in Table 2. The significantly higher TCE adsorption capacity in a dry air stream, as opposed to a humid air stream, is due to the lack of competition for adsorption sites in the former case. Also evident from Fig. 1 is the fact that the Si02/A120s ratio determines not only the hydrophobicity (and, therefore, TCE adsorption capacity in a humid air stream) but also the organophilicity (TCE adsorption capacity in a dry air stream) of the S/C medium. Therefore, an increase in the SiOz/A120s ratio from 30 to 120 leads to a 40% enhancement (from 10.7 to 15.0 wt%) in TCE adsorption capacity in a dry air stream and a 68% increase (from 6.0 to 10.1 wt%) in the humid air stream. Our results are in agreement with those of Hsu and Ma [ 171 who have characterized ZSM-5 zeolites with SiOz/A120s ratios of 252, 1126, and 2124 in terms of adsorption and diffusion of water, methanol, benzene,
5
1.8
f
1.6
Catalysis B: Environmental 14 (1997) 37-47
and n-hexane at 25°C. The authors have reported 1524% enhancement in single-component hydrocarbon adsorption capacity as a result of increase in the Si02/A120s ratio. 3.3. Centers of adsorption (TPD and FT-IR results)
of TCE in S/C media
The TPD curves obtained on TGA after ambient adsorption of -1000 ppm TCE in dry air on the four S/C media are shown in Fig. 2. Although the adsorption capacities obtained on TGA are -20% higher than the corresponding fixed-bed values, the qualitative trend is the same. We attribute this difference to the differences in the flow geometry in fixed bed vs. in TGA. All TPD curves depicted in Fig. 2 clearly show a strong desorption peak at 84-88°C [a temperature very close to the normal boiling point (87°C) of liquid TCE] and a weak desorption shoulder at 138-141°C. The identical nature of all TPD curves indicates that there is no significant dependence of energetics for TCE adsorbed species on the Si02/A120s ratio or the chromium content. The proximity of the strong peak at 84-86°C to the boiling point of liquid TCE suggests that the heat of TCE adsorption on the sites characterized by this peak is very close to the heat of liquefaction and corresponds to adsorbateadsorbate non-specific interaction within the ZSMJ structure [ 181.
-Cr-250
g 1.4 g
1.2
y 1.0
iI
0.6 0.8
2
0.4
J 0.2 0.0
Fig. 2. Temperature-programmed
desorption
(TPD) curves after -1000
ppm TCE adsorption
in dry air on the S/C media
P.S. Chintawar; H.L. Greene/Applied
Catalysis B: Environmental
43
14 (1997) 37-47
Wavenumbers (cm-l) Fig. 3. F’l-IR spectra obtained during TCE adsorption on Cr-ZBO at 23°C. Identical I - reference spectrum; 2 - commencement of adsorption; and 3 - steady state.
The TPD results are in congruence with in-situ FTIR adsorption results shown in Figs. 3 and 4. Fig. 3 shows the FT-IR spectra obtained before and during TCE adsorption on Cr-Z80 at 23°C. The steady-state spectrum shown in this figure was obtained after 6 min of adsorption. Identical spectra were obtained for Cr230 and Cr-ZSO. The commencement of adsorption is accompanied by two bands of almost equal intensity at 3740 and 3705 cm-‘. On the contrary, at steady state, the absorption band at 3740 cm-’ is significantly stronger than the 3705 cm-’ band. This phenomenon suggests preferable adsorption of TCE on the group indicated by the 3740 cm-’ band rather than on the site represented by the 3705 cm-’ absorption band. The former suggests the interaction of TCE molecules with the silanol (Si-OH) groups on the outer surface of crystallites; i.e. OH groups which terminate the face of zeolite crystallites at positions where bonding in the interior would occur with adjacent tetrahedral Al or Si ions [ 191. The other absorption band, detected at 3705 cm-‘, is commonly attributed to AlOH groups and has been observed by other researchers in divalent cationexchanged Y zeolites, such as Mg-Y and Sr-Y [20]. Ison and Gorte [21] have observed an absorption band
spectra
were obtained
with Cr-Z30
and Cr.ZSO:
at 3690 cm-’ upon water adsorption on H-ZSM-5, and they have attributed it to the hydronium (H30+) ion. Therefore, it appears that the band observed at 3705 cm-’ in this study may also be caused by the interaction of water with the cations present in the zeolite. Recall that TCE adsorptions were carried out in dry (~3 ppm water) air. Also, preliminary FT-IR adsorption experiments with dry air only, without any TCE, showed no changes in the hydroxyl stretching region of the S/C media. Therefore, we assign the negative absorption band at 3705 cm-’ to the interaction of TCE with AlOH groups of the ZSM-5 structure. Fig. 4 shows the FT-IR spectra obtained after 30 min of TCE adsorption on all S/C media. It can be clearly seen that Cr-Z120 did not show any absorption bands in the hydroxyl stretching region of the ZSM-5 zeolite. This result suggests the lack of hydrogen bonding between the TCE molecules and the adsorption sites of the essentially non-polar surface of Cr-Z120. This is caused by the presence of a very low density of cations (chromium and hydrogen) and polar groups (A104 tetrahedra) on Cr-Z120, resembling the crystal structure of silicalite with its near
44
flS. Chintawal; H.L. Greene/Applied
-
Catalysis B: Environmental
14 (1997) 37-47
cr-zao Cr-250
3000 Wavenumbem
Fig. 4. Fl-IR
(cm-l)
spectra obtained after 30 min of TCE adsorption
infinite Si02/A120s ratio. It is known that the physical adsorption of organics onto silicalite occurs by a porefilling process involving only van der Waals forces and is a function of mainly pore geometry rather than chemical characteristics of the sorbent [5,16]. Therefore, consistent with the low density of sites for hydrogen bonding, Cr-Z120 does not show any perturbations in the hydroxyl stretching region upon TCE adsorption. 3.4. Activity and selectivity of the S/C media for the oxidative destruction of TCE The catalytic activities of the four S/C media for the oxidative destruction of TCE in humid air are shown in Fig. 5. All the S/C media, except Cr-Z120, showed high destruction efficiencies with >95% conversion at 300°C. The poor conversion obtained on Cr-Z120 was probably due to its very low chromium content. Surprisingly, Cr-ZSO containing significantly less chromium than Cr-Z50, showed marginally higher TCE destruction efficiency (Fig. 5). Since the oxidative destruction of TCE in a chromium exchanged zeolite catalyst is believed to occur through reduction-
on various S/C media at 23°C.
oxidation (redox) reaction of exchanged chromium cations with extra-lattice oxygen [22], presumably the environment of the chromium cation (location, type of zeolite, Si02/A1203 ratio, etc.) largely determines the catalytic activity of the site. Dumesic and Millman [23] have reported a fivefold increase in the turnover frequency of the exchanged iron cations in a Fe-Y catalyst for N20 decomposition (a redox reaction) upon increase in the SiOz/A120sratio of Y zeolite from seven to 14. The authors have attributed the enhancement in activity to the facile redox reaction of iron cations in a high silica Y zeolite. Thus, it is speculated that marginally higher destruction efficiency exhibited by Cr-Z80 than Cr-ZSO could be due to a more facile redox reaction of chromium cations in Cr-Z80 than in Cr-ZSO. Selectivities, along with carbon and chlorine balance data obtained during the catalytic runs, are shown in Table 4. The data acquisition in the GUMS was carried out in the selective-ion monitoring mode and the following compounds were monitored: COz, CH2C12, COC12, C,HsCl, C2H2C12, C2HC1s, CzC14 and CC14. In addition to the unreacted feed, only deep oxidation products such as C02, Cl*, HCl, and pro-
l?S. Chintawac H.L. Greene/Applied
223
250
275
Catalysis B: Environmental
300 325 Temperature (“C)
14 (1997) 37-47
350
375
400
Fig. 5. The oxidative destruction and TCE concentration - -1000
of TCE in humid air (-13 000 ppm water) on S/C media. Space velocity - 2400 h-t; bed size - 11.8 cm3; ppm.
Table 4 Material balances
data for the oxidative
and selectivity
destruction
of TCE in humid (-13 000 ppm water) air.
Temperature
TCE cont.
Conversion
Cl2 Selectivity
CO2 Selectivity
Carbon balance
Chlorine balance
( C)
(ppm)
(%)
(‘“)
(%)
W)
(%)
S,IC medium: Cr-Z30 350 325 300 275 250
1022 974 1054 1018 1046
99.0 98.6 99.3 97.1 67.6
28.0 24.0 13.9 0 0
76.2 53.0 23.5 13.6 4.3
102 104 93 85 87
99 103 100 92 95
S; C medium: 335 300 275 250
1004 965 1003 1091
98.9 96.1 78.5 45.1
16.6 10.0 0 0
25.3 16.0 10.3 2.9
96 98 102 96
102 97 102 98
S/C medium: Cr.280 325 300 275 250
1014 980 1067 1083
99.6 98.4 85.4 50.1
6.6 2.7 0 0
21.0 13.4 8.8 2.0
90 97 89 91
95 96 100 96
S/C medium: Cr-2120 400 350 300 215
1053 1062 1035 1014
46.9 19.6 6.8 6.6
4.2 1.8 0.2 0.1
97 98 100 97
106 108 103 102
Cr-250
ducts of incomplete oxidation such as CO were detected in the product stream of the reactor. No phosgene (COC12) was detected in the product stream of any catalyst. The absence of phosgene was also
confirmed by use of detector tubes (Mine Safety Appliances). All catalysts showed high (7.S90%) selectivity toward the formation of CO at 300°C. Figs. 6 and 7 show the changes in product selectivity
46
RS. Chintawac
H.L. Greene/Applied
Catalysis B: Environmental
14 (1997) 37-47
I +Cr-Z30
225
250
275
ml 325 Temperature(F)
350
375
400
Fig. 6. COZ selectivity with temperature obtained during the oxidative destruction of TCE in humid (~13 000 ppm water) air on S/C media. Space velocity - 2400 hK’; bed size - 11.8 cm3; and TCE concentration - -1000 ppm.
Fig. 7. Cl2 selectivity with temperature obtained during the oxidative destruction of TCE in humid (-13 000 ppm water) air on S/C media. Space velocity - 2400 h-‘; bed size - 11.8 cm3; and TCE concentration - -1000 ppm.
with temperature for each catalyst. The higher chromium content (1.42 wt% for Cr-Z30) seems to enhance the production of CO2 and Cl*, the products of CO oxidation (2C0 + 02 --f 2CO2) and the Deacon reaction (4HCl-t 02 + 2C12 + 2H20), respectively. This is interpreted in terms of the high catalytic activity of chromium for these reactions [22]. The production of Cl2 increases with temperature due to a large increase in the Deacon reaction rate constant. At low temperatures, the formation of CO2 is relatively unfavored and, therefore, leads to high selectivity toward CO.
4. Conclusions .
??
The amount of TCE physisorbed on Cr-ZSM-5 S/C media in both humid and dry air streams increases with an increase in Si02/A120s ratio. This is interpreted in terms of increased hydrophobicity and increased organophilicity, respectively. With the exception of Cr-Z120, the combined insitu FT-IR and TPD results show the involvement of AlOH and terminal SiOH (silanol) groups of the ZSM-5 structure in the physisorption of TCE.
l?S. Chintawal; H.L. Greene/Applied
??
The results of this investigation show that highly siliceous (and therefore hydrophobic) S/C media can be used for the physisorption of chlorinated VOCs without unduly sacrificing their catalytic activity. Therefore, an increase in Si02/A120s ratio of ZSM-5 zeolite from 30 to 80 resulted in 50% enhancement in the TCE adsorption capacity (in humid air) with only modest loss (12-17%) of the TCE destruction efficiency at temperatures <3OO”C.
Acknowledgements The authors express their gratitude to the U. S. Environmental Protection Agency (EPA) and U. S. Air Force (SERDP) for supporting this work. Zeolite samples were obtained from The PQ Corporation and IJOF? Thomas Quick (The University of Akron) collected the X-ray diffraction data.
References [I] J. Weitkamp, S. Ernst, B. Gunzel, W. Decker, Zeolites 11 (1991) 314. [2] J. Weitkamp, P. Kleinschmit, A. Kiss, C.H. Burke, in: R. von Ballmoos, J.B. Higgins, M.M.J. Treaty (Eds.), Proc. 9th International Zeolite Conference, Montreal 1992, Butterworth-Heinmann, Stoneham MA, 1993, p. 79. 131 R. Schumacher, S. Ernst, J. Weitkamp, in: R. von Ballmoos, J.B. Higgins, M.M.J. Treaty (Eds.), Proc. 9th International Zeolite Conference, Montreal 1992, Butterworth-Heinmann, Stoneham MA, 1993, p. 89.
Catalysis B: Environmental 14 (1997) 37-47
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