Ind. Eng. Chem. Res. 1992,31, 1440-1445
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Tommila, E.; Lindell, E.; Virtalaine, M.; Laakso, R. Densities, Viscoeities, Surface Tensions, Dielectric Constants,Vapour Pressures, Activities and Heats of Mixing of Sulpholane-Water, Sulpholane-Methanol and Sulpholane-Ethanol Mixtures. Suom.Kem-
Van Broekhoven, J. A. M.; Farragher, A. L. Alcohols. Shell Inbrnationale Research Maatschappij B. V. Ger. Pat. 2,721,206,1977. Received for review March 4, 1992 Accepted March 17, 1992
istil. 1969, 842, 95.
Pillared Clays as Superior Catalysts for Selective Catalytic Reduction of NO with NH3 R. T. Yang,* J. P. Chen, E. S.Kikkinides, and L. S. Cheng Department of Chemical Engineering, State University of New York a t Buffalo, Buffalo, New York 14260
J. E. Cichanowicz Air Quality Control, Generation and Storage Division, Electric Power Research Institute, Washington, D.C. 20036
Five pillared clays (PILC) have been synthesized and tested for their activities in the selective catalytic reduction (SCR) of NO by NH3in the temperature range 250-450 OC. They all showed considerable activities, in the decreasing order Cr203-PILC, Fe203-PILC, Ti02-PILC, Zr0,-PILC, and Alz0,-PILC. Cr203-PILC exhibited higher activities than a “commercial” (VTT) W03-V205/Ti02 catalyst, but its activity was severely decreased by SO,. Fe203-PILC showed activities comparable to that of Vz05/Ti02and was SO2 resistant. Doping with cerium oxide significantly enhanced the activity; for Fe203-PILC, doping with 1 5 2 % CeOz more than doubled its activity, and the resulting activity was significantly higher than that of the VTT commercial catalyst (both with and without SO, and H20). The formed PILC catalysts will have a bimodal pore structure, with which a superior poison resistance has been demonstrated in SCR operations with a Vz05-based catalyst. Thus, the high activities and the potential poison resistance make pillared clays a promising new class of catalysts for SCR applications.
Introduction Selective catalytic reduction (SCR) of nitrogen oxides with ammonia is of increasing industrial importance. A comprehensive review of the subject is available (Bosch and Janssen, 1988). The commercial catalysts are V205 with mixed WO, and/or MOO, supported on TiO,. A direct correlation between the SCR activity and the Brernsted acidity of V205 has been observed (Rajadhyasbha et al., 1989; Rajadhyaksha and Kntizinger, 1989; Chen and Yang, 1990; Topsoe, 19911, and the Br~nsted acid sites are tliought to be the active sites. Besides Vz06, a large number of catalysts have activities for the SCR reaction, some of which have also been reviewed. This paper reports first results of SCR activities on pillared clays and potential advantages of pillared clays over the ”commercial” catalysts. Pillared interlayered clays (PILC), or pillared clays, are two-dimensional zeolite-like materials prepared by exchanging the charge-compensatingcations between the clay layers with large inorganic hydroxycations, which are polymeric or oligomeric hydroxy metal cations formed by hydrolysis of metal oxides or salts. Upon heating, the metal hydroxycations undergo dehydration and dehydroxylation, forming stable metal oxide clusters which act as pillars keeping the silicate layers separated, creating interlayer space (gallery) of molecular dimensions. Much interest and research have been directed to metal oxide PILC since their first successful syntheses in the late 19709 (Brindley and Sempels, 1977; Vaughan et al., 1979; Lahav et al., 1978; Loeppert et al., 1979; Occelli and Tindwa, 1983). Comprehensive reviews of the voluminous literature on the subject are available (Pinnavaia, 1983; Burch, 1988,
* To whom correspondence should be addressed.
Figueras, 1988). In principle, any metal oxide or salt that forms polynuclear species upon hydrolysis (Baes and Mesmer, 1976) can be inserted as pillars, and all layered clays of the abundant phyllosilicate family as well as other layered clays can be used as the hosta (references cited in Burch (19881, Clearfield (19881, Drezdon (1988), and Sprung et al., (1990)). Because of ita large pores and hydrothermal stability (to 700 “C), the main early interest in PILC was in the possibility of replacing zeolite as the catalyst for fluid catalytic cracking (Occelli, 1983; Occelli et al., 1984). However, this possibility has been hindered due to excessive carbon deposition. An additional difficulty was that the pore size could be considerably smaller than the interlayer spacing calculated from X-ray diffraction (XRD). For 21-PILC, the interpillar spacings in the range of 4-8 A were the limiting pore sizes although XRD results showed an interlayer spacing of 14.6 A (Yang and Baksh, 1991). Besides fluid catalytic cracking (FCC), PILC has been studied for catalyzing alcohol dehydration (e.g., Burch and Warburton, 1986; Occelli et al., 1985a,b),alkylation, and other acid catalyzed reactions (e.g., Occelli et al., 1985a,b;Rightor et al., 1991). A pillared titanium phosphate was used as the support, not the catalyst, for V205 for the SCR reaction (Czarnecki and Anthony, 1990). Despite many studies on the acid sites on PILCs, the nature and properties of these site are not well understood (e.g., Figueras (1988), He et al. (1988), and Jones (1988)). Both Lewis and Br0nsted acid sites exist on pillared clays, with a larger proportion being Lewis acid sites. Our discussion will be focused on the Br~nstedacidity because of ita importance to the SCR reaction. Two sources for Brernsted acidity have been discussed in the literature. One derives from the structural hydroxyl groups in the clay layer (He et al., 1988). The most likely proton site is
0888-5885/92/2631-1440$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 6,1992 1441
Figure 1. Schematic diagram of experimental SCR reactor: (A) chemiluminescent NO/NO, analyzer; (B)NH3 scrubber; (C)water vapor generator.
located at the Al(VI)-O-Mg linkage, where Al(V1) is the octahedrally coordinated Al, and Mg is one that has substituted an Al on the octahedral layer. Another likely source for protons derives from the cationic oligomers which upon heating decompose into metal oxide pillars and liberate protons. It has been reported from many studies that both Lewis and Bransted acidities decrease with the temperature of calcination (e.g., Figueras (1988) and He et al. (1988)). The disappearance of Bransted acidity is attributed to the migration of protons from the interlayer surfaces to the octahedral layer within the clay layer where they neutralize the negative charge at the substitution atoms (such as Mg) (Tennakoon et al., 1986). However, of particular significanceto our study on the SCR reaction is the result reported by Tennakoon et aL (1986) that upon exposure to NH3 the migration can be reversed so the proton is again available on the surface. Although the total acidity appears to vary with the kind of metal oxide inserted as pillars (He et al., 1988), no information is available on the dependence of Bransted acidity on the kind of metal oxide pillars. A potential major advantage of pillared clays for SCR application is the resistance to poisoning. Catalyst poisoning is a major factor in the economics of SCR (Robie et al., 1989). The chemistry of poisoning of the Bransted acid sites is reasonably understood (Chen and Yang, 1990). However, a significant contributor to catalyst poisoning is apparently the deposition of A s 2 0 3 and other vapor species within the pore structure of the vanadia catalyst. This problem can be alleviated by a new catalyst design by Hegedus and co-workers (for example, Beeckman and Hegedus (1991)), which consists of a bimodal pore size distribution in the V205/Ti02-Si02 catalyst: one group of pores are of the order of microns (macropores) and the other group are of the order of angstroms (micropores). The poisonous vapor species in the flue gas such as h203 deposit on the walls of the macropores due to their low diffusivities. Since the macropores serve as feeder pores to the micropores, they provide the function as filters of poisons. The pore structure of any catalysta made of pillared clays would be unavoidably bimodal. The commercially available clays such as montmorillonite are of particle sizes of microns or a fraction of a micron. A pelletized (or washcoat) PILC catalyst will contain feeder (or poison fdter) pores in the interparticle spaces, whereas the intraparticle micropores contain the active catalyst surface for the SCR reaction.
Experimental Section Experimental Apparatus and Rate Measurement. The schematic diagram of the SCR apparatus is shown in
Figure 1. The reactor consisted of a quartz tube. The heating element was a coiled Nichrome wire. The reactor temperature was controlled by an Omega CN-2010 programmable temperature controller. The catalyst, usually around 2 mL in volume, was supported on a fritted support. The inside diameter of the reactor was 2 cm. Two sets of flow meters were employed for blending a synthetic flue gas. Rotameters were used to control flows with high flow rates (i.e., N2, NH3 + N2, and NO + N2). Mam flow meters (FM 4575, Linde Division) were used for gases with low flow rates (SO2 and 02).The premixed gases (0.8% NO in N2 and 0.8% NH3in N2)were supplied by Linde. The 8% water vapor was generated by passing nitrogen gas through a heated gas-wash bottle containing distilled water. To prevent the deposition of ammonium sulfate, the tubings were heated by heating tapes. NO concentration was continuously monitored by a chemiluminescent NO/NO, analyzer (Thermo Electron Corp., Model 10). To avoid any analytical error caused by oxidation of amminoa in the converter of the NO/NO, analyzer, an ammonia trap (phosphoric acid solution) was installed before the sample inlet. The catalyst activity is expressed as a first-order rate constant with respect to NO. This is justified because resulta on a large number of metal oxides and mixed oxides (including iron, chromium, and vanadium oxides) supported on Ti02 or A1203 showed the SCR reaction to be first order with respect to NO and zero order with respect to NH3under similar conditions (Wong and Nobe, 1986). Assuming plug flow, the rate constant (k)can be calculated as
where W is the weight of the catalyst, Fois the inlet molar flow rate of NO, [NO],, is the inlet molar concentration, and X is the NO conversion. The absence of mass-transfer resistance under the experimental conditions has been discussed previously (Chen and Yang, 1990). Thus, the catalyst activities reported here may be considered intrinsic activities. Besides pillared clays, V205/Ti02and WO3-V2O5/TiO2 catalysts were also used for comparison. These catalysts were prepared by incipient wetness impregnation (Chen and Yang, 1990). The surface areas were both 30 m2/g. W03-V205/Ti02 had the same composition and surface area as a commercial SCR catalyst (Tuenter et al., 1986). Syntheses of pillared Clays. The general method for PILC synthesis is given first, which will be followed by details for the syntheses of the specific pillared clays used in this study. The first step in PILC synthesis is preparation and aging of the pillaring solution to form oligomers. The pillaring agent undergoes hydrolysis, polymerization, and complexation with anions in the solution (Baes and Mesmer, 1976; Burch, 1988). The hydrolysis conditions are important to the formation of PILC temperature, pH, and aging time. The next step is intercalation (or ion exchange) of the small cations (e.g., Na+) between the clay layers with the oligomers. This is done by preparing a slurry consisting of a suspension of the clay in deionized and distilled water, oligomeric solution, and HCl and/or NaOH used for pH adjustment. The pH of the clay/water suspension is 9.10 (1g of clay + 100 mL H20). The equivalent amount of the oligomeric solution is added dropwise to the clay suspension while vigorous stirring is maintained. Slow addition of the solution helps in the formation of uniform pillared clays.
1442 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992
Upon completion of ion exchangeJintercalation, the sample is separated from the liquid phase and washed by vacuum filtration or by centrifugation. The final step is drying/calcination. The method and condition of the drying step have a strong influence on the porosity of the PILC (Pinnavaia et al., 1984) as well as the acidities and catalytic properties (Burch, 1988; Figueras, 1988; He et al., 1988; Jones, 1988). In general, the solvent-free PILC is stabilized by heat treatment under air or intert gas (N2or Ar) at a temperature between 150 and 450 "C. (a) Zr-PILC Synthesis The starting material for preparation of zirconium pillared clay (Zr-PILC) was a purified montmorillonite, purified-grade bentonite powder from Fisher Co., less than or equal to 2-lm size. Zirconyl chloride (ZrOC12.8H20),also from Fisher, was the salt to be hydrolyzed and used as the pillaring agent. The oligomeric solution was prepared from ZrOC12.8H20by dissolving the equivalent amount of the salt in deionized, distilled water to produce a 0.1 M solution. This solution was aged at room temperature for 10 days at a pH of 1.3. For synthesis of Zr-PILC, the pillaring stoichiometry should be at least 2.5 ( m o l of Zr)/(l g of clay). Typically, 25 cm3 of 0.1 M ZrOC12 should be used per 1 g of the bentonite clay (Yang and Baksh, 1991). The main intercalating species was the tetramer, Zr4(OH)14(H20)lo2+. However, larger oligomers containing 20-40 metal ions were undoubtedly formed (Baes and Mesmer, 1976). Next the equivalent volume of the oligomeric solution, normally 25 cm3, was added dropwise to the 1-g clay suspension while vigorous stirring was maintained. The pH of the resulting solution was checked and adjusted to the desired value using 0.1 M HCl or 0.1 M NaOH. The adjusted pH of the slurries used in this study were in the range 1.4-8.35. Ion exchange in the slurry took place at 50 "C in a constant temperature bath shaker for a period of 3 days. Upon completion of ion exchange/intercalation, the sample was separated and washed by vacuum filtration. The residue on the filter was washed repeatedly with deionized and distilled water until no chloride ions remained as indicated by no further precipitation with AgN03 added to the filtrate. Finally, the sample was calcined in air at 350 OC for 24 h. (b) AI-PILC Synthesis. The starting clay for the preparation of A1-PILC was the same purified grade of bentonite powder. The aluminum hydroxy-oligomeric solution (pillaring solution) was prepared by dissolving the equivalent amount of aluminum chloride (AlC13.6H20)salt in deionized, distilled water to produce a 0.1 M AlC13 solution. Then different amounts of 0.1 M NaOH were added very slowly (20 cm3/h) to different flasks, each containing 25 cm3 of 0.1 M AlCl,, to obtain oligomeric solutions having various OH/Al molar ratios between 1.0 and 2.5. Vigorous stirring was necessary during the addition of NaOH in order to prevent local accumulations of hydroxyl ions, which invariably produced precipitation of Al(OHI3in the form of gibbsite (Brindley and Semples, 1977). The oligomeric solution chemistry is quite complex but is reasonably well understood (Baes and Mesmer, 1976). Oligomeric solutions having various OH/ A1 molar ratios were aged at 25 "C for 5 days. Once the oligomeric solution was prepared, the next step was intercalation (or ion exchange) of the small cations between the clay layers with the oligomeric cations. Normally the clay slurry consisted of 25 cm3of oligomeric solution (OH/Al ratio between 1.0 and 2.5) and a clay suspension (1.0 g of clay in 50 cm3 of H20). The pH of the slurry was in the range of 3.2-5.8.
.
Table I. Interlayer Spacings and BET Surface Areas of Pillared Clays and the Starting Clay material dmr (A) surface area ( m 2 / a ) bentonite 9.6 25 zr-PILC 19.3 321 Cr-PILC 21.7 303 FePILC 26.4 217 AI-PILC 27.5 245 Ti-PILC 28.3 258
The exchange reaction was conducted at 50 "C in a constant temperature bath shaker for a period of 3 days. The remaining steps (filtration and calcination) were the same as those described earlier for the Zr-PILC synthesis. (c) Fe-PILC Synthesis. A 0.2 M solution of ferric nitrate (Fe(N03)3)was initially prepared. Solid powdered sodium carbonate (Na2C03)was added slowly to the above solution at 25 "C. Carbon dioxide was evolved and vigorous stirring was utilized. Hydrolysis of Fe(N03), with Na2C03was carried out at 25 "C and for an aging period of 24 h, as suggested by Pinnavaia and Tzou (1987) and Rightor et al. (1991). The value of OH/Fe was 2.0 and the pH was 1.8. A clay suspension (1g of clay + 100 cm3 of HzO) was then added to the hydrolyzed ferric solution. The clay suspension and the pillaring agent were intermixed for about 24 h after which the intercalated clay product was separated from the liquid phase by vacuum filtration, and the product was washed and then air-dried at 200 OC for 12-14 h. (d) Cr-PILC Synthesis. Solutions containing cationic polyoxychromium oligomers were prepared by the hydrolysis of 0.10 M chromium nitrate (Cr(N03)3.9H20)at 95 OC using Na2C03as the base (Pinnavaia et al., 1985). The OH/Cr ratio was 2, and the hydrolysis time was 36 h at a pH of 2.1. To the hot solutions were added 1w t % suspensions of clay powder. The Cr was in excess during the pillaring reaction (typically 50 mmol/meq of clay). After a reaction time of 1 6 2 4 h, the product was colleded by vacuum filtration and washed free of excess salt. The final product was air-dried at 200 "C for 12-14 h. (e) Ti-PILC Synthesis. The pillaring agent, consisting of a solution of partially hydrolyzed Ti-polycations, was prepared by first adding Tic& into 6.0 M HC1 (Sterte, 1986,1988). This mixture was then diluted by slow addition of distilled water with stirring to reach a final Ticoncentration of 0.82 M HCl in an amount corresponding to a final concentration of 0.11 M and was used in the preparation. The solutions were aged at room temperature for 2-3 days prior to their use. The pH of the solution was 1.10. A 2-g amount of bentonite was dispersed in 0.5 L of distilled water by prolonged stirring (5 h). The amount of the pillaring agent required to obtain a Ti/clay ratio of 10 (mmol of Ti)/(g of clay) was then added to the vigorously stirred suspension. The resulting product was left in contact with the solution for 18 h and then separated by vacuum filtration and washed with distilled water several times until the liquid phase was free of chloride ions as determined by AgNO,. The final product was air-dried at 300 "C for 12 h. Results and Discussion Characterization of Pillared Clay Catalysts. X-ray powder diffraction patterns, using a Cu Ka source, were obtained for the PILC samples and are shown in Figure 2. The interlayer spacings are listed in Table I. The free interlayer spacings (by subtracting the layer thickness of 9.6 A for the clay from the doolspacings listed in Table I) were between 10 and 19 A. However, as discussed in our earlier work, the limiting pore sizes could be interpillar
Ind. Eng. Chem. Res., Vol. 31, No. 6,1992 1443
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u T,-PILc
r '-------C,-PlU
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i
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1
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200
300
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Figure 3. NO conversion with Fe203-PILC catalyst with SO2 (A) and without SO2 ( 0 )in the feed catalyst weight = 1.148 g; NO = NH8 = lo00 ppm; O2 = 2%; SO2 = lo00 ppm; balance = N,; total flow rate = 500 cm3/min (at room temperature). Table 11. Comparison of SCR Activities at 400 "C in Terms of the First-Order Rate Constant, k (cm*/(g/s)) (SO2 = loo0 ppm for Other Conditions, See Figure 3 Caption)
5
10 20
15
Figure 2. XRD powder patterns (Cu Ka source) for pillared clays and starting clay (bentonite).
spacings rather than interlayer spacings (Yang and Baksh, 1991). For example, the smallest pore sizes were for ZrPILC, which had interpillar spacings in the range of approximately 4-8 A (Yang and Baksh, 1991). Further studies of the micropore size distribution (PSD) in the five PILC's showed that interpillar spacing was indeed the limiting size for all PILC's (Baksh et al., 1992);Zr- and CI-PILC's had the smallest (and similar) PSD whereas F e and Ti-PILC's had the largest PSD, centered around 9 A. These pore sizes are amply large for the SCR reactant molecules (NH,,NO, and 0,)and product molecules (H20 and N2) in which to diffuse without causing severe masstransfer limitations in the SCR reaction. (These pores, however, are too small for poisons such as As203.) The pillared clay samples were in the form of strong agglomerates (approximately 1 mm in size) composed of crystals nearly 2 pm in size. The BET surface areas (N,, 77 K)were measured with Quantasorb instrument and are also listed in Table I. The samples were purged in N2 at 200 "C before the measurements. These surface areas are considerably higher than those of the SCR catalysts with TiO, support. For comparison, the surface areas of both 5% V205/Ti02and the commercial catalyst 8.2% W03 + 4.8% V205/Ti02 were approximately 30 m2/g. The commercial catalyst was prepared in our laboratory but had the same composition and surface area as the V" catalyst (Tuenter et al., 1986). NO Conversion vs Temperature. The SCR activities for the pillared clay catalysts were measured at different temperatures in the range 2 W 4 5 0 "C. The reactant gas composition was the same for all measurements: NO = NH3= lo00 ppm; 0,= 2%; N2= balance; SO2= lo00 ppm (when used); and H20 = 8% (when used). In all experiments, the catalyst sample was heated in the reactor in a flow containing only NO and NH3, lo00 ppm each. For samples calcined at 200 "C, the further heating in NH, (to the reaction temperature) could be significant in the SCR activity because such a treatment could increase (recover) the Br0nsted acidity (Tennakoon et al., 1986;Jones, 1988). With the exception of Cr203-PILC,the NO conversion increased monotonically with temperature (to 450 "C).
catalyst Cr203-PILC Fe203-PILC Ti02-PILC Zr02-PILC Al20s-PILC VzO5/TiO2 W03-V205/Ti02
k (cmg/(g/s)) without SO2 with SO, 13.8 2.74 6.74 6.70 2.25 3.34 1.68 2.60 0.47 0.47 7.70 7.70 13.58 13.58
The activity of Cr203-PILCpeaked at 400 "C. This temperature dependence is different from the V205catalysts which decline at temperatures above 400 "C, due to the activity for NH3 oxidation at high temperatures. Thus, the pillared clays (except Cr203-PILC) did not exhibit activities for NH3 oxidation in the temperature range tested,and the lack of NH3oxidation activity is a desirable property. The typical NO conversion/temperature results are shown in Figure 3, for the Fe203-PILC. The effect of SO2 in the gas phase on the NO conversion varied among the pillared clays. Water vapor (added after SO2addition) had some effects on the catalyst activities for pillared clays, which will be reported in detail later. Comparison of Different Pillared Clays and Vanadia Catalyst. The SCR activities for different pillared clays are compared at 400 "C, which is the representative commercial SCR temperature. The comparisons, along with V205/Ti02and W03-V205/Ti02,are summarized in Table 11. The activities in Table I1 are expressed in terms of fmt-order rate constants. The V2O5/TiO2catalyst and the commercial catalyst W03-Vz05/Ti02are also included for comparison. The Cr203-PILCcatalyst exhibited a higher activity than the commercial catalyst. However, addition of SO2severely decreased ita activity. F%O3;PILC showed a high activity and also showed no SO2 poisoning effect. The activity of the Fe203-PILC was comparable to the V2O5/TiO2catalyst. Since Fe203-PILC is a promising SCR catalyst, a further understanding of ita activity is desirable. The IR spectrum of the pyridine chemisorbed on this catalyst is shown in Figure 4. The catalyst was exposed to pyridine (carried in N2 flow) at 150 "C, and the spectrum was measured at room temperature. The absorption band at 1542 cm-'was due to pyridinium on Brransted acid sites, whereas the
1444 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 I
t
z a 1542
tt
1
i 1700
1600
IS00
1400
1300
FREQUENCY, C W '
Figure 4. IR spectrum for pyridine chemisorbed on Fe20s-PILC. The band at 1542 cm-' is due to Bronsted acid sites, whereas that at 1440 cm-' is due to Lewis acid sites. Table 111. Promoting Effect of CeOz on SCR Activity of TiO,-PILC, Expressed by NO Conversion (% ) under Standard Reaction Conditions with both SO2 and He0 % conversion at given temperature catalyst 35OOC 400°C 45OoC Ti02-PILC (A) 54.5 71.5 85.0 0.5% CeOz on A 59.0 86.0 95.2 1.0% CeOz on A 70.5 90.0 1.5% CeOz on A 69.5 89.5 96.5
band at 1440 cm-' was due to the Lewis acid sites (Kung and Kung, 1985,Occelli, 1988). The relatively strong band at 1542 cm-' was consistent with the correlation between SCR activity and the Br~nstedacidity (Rajadhyaksha, 1989;Chen and Yang, 1990;Topsoe, 1991). Promoting Effect of CeOz(or Cez03). The effects of doping with Ce02(or Ce203)in pillared clays were studied with Ti0,-PILC and Fe203-PILC. (The exact oxidation state of the dopant was not known.) The Ce02 (or Ce203)doping was performed by incipient wetness impregnation using Ce(NO3I3aqueous solutions. The impregnated samples were dried at 60 "C followed by heating at 100 OC for 1 h and 400 "C for 18 h, all in air. The SCR activities of the Ce02 doped Ti02-PILC samples are expressed in percent NO conversion, shown in Table 111. Due to the high calcination temperature of 400 OC, the SCR rate constant of the sample without Ce02 (or Cez03)doping was actually slightly decreased from that shown in Table I1 (which was calcinated at 300 "C). The promoting effect of Ce02 (or Ce2O3) on the activity of Fe203-PILC was studied more extensively, covering the temperature range 150-500 OC. Strong promoting effects were observed at all temperatures, and the effects at 400 "C are shown in Table IV. The results in Tables I11 and IV clearly demonstrate the strong promoting effects of Ce02 (or Ce203)on the SCR activity. It is also clear that the promoted catalysts are SO2-resistant. For both Ti02 and Fe203 pillared clays, there was an optimal amount of Ce02 doping, beyond which the activity decreased. This was apparently caused by pore plugged beyond the optimal doping amount. The optimal Ce02doping amount was approximately 1% for
Table IV. Promoting Effect of CeOz on SCR Activity of FezOS-PILC, Expressed by Rate Constant k,at 400 OC under Standard Conditions (See Fieure 3 CaDtion) k (cm3/(g/s)) without SO2 with SO2 catalyst Fe203-PILC (B) 6.74 6.70 0.5% CeOz on B 7.22 1.0% CeOz on B 10.15 11.80 1.5% CeOpon B 16.25 14.71 2.0% CeOz on B 15.43 16.70
Ti02-PILC and 1.5-2.0% for Fe203-PILC. The most significant result was, however, that the promoting effect was a strong one and that, for Fe203-PILC, the promoted catalyst was more active than the V" commercial catalyt (Tuenter et al., 1986). It is noted, however, that Fe203PILC (both doped and undoped) suffered from a stability problem; i.e., the SCR activity declined with time, apparently due to a loss in surface area. This problem was not observed with other PILC's. Preliminary results on F%O3doped on Ti02-PILC and V205doped in Ti02-PILC showed both high SCR activities and stabilities. The results presented in this paper are without refinement of the conditions for catalyst preparation. With refinement of the preparation conditions, such as temperature and gaseous environment for PILC calcination and dopant type and amount, it is expected that pillared clays will have substantially higher SCR activities than the commercial catalyst. Taken together with the improved poison resistance due to the bimodal pore size distribution discussed in the Introduction, the pillared clays are expected to be superior to the commercial V205-basedSCR catalysts. Further work in this area is in progress in our laboratory.
Conclusion Five pillared clays were tested for their SCR activities, and compared with the V205-based catalysts: a 5% V205/Ti02catalyst and one with 4.8% V205 + 8.2% W03/Ti02 catalyst which had the same formulation and surface area as the commercial VTT catalyst (Tuenter et al., 1986). The intrinsic activity (expressed in rate constant k) of Cr203-PILC was higher than both V205-basedcatalysts; it was decreased significantly, however, by the presence of SO2. The activity of Fe203-PILC was not influenced by SOp. With 1.5% Ce02doped on the Fe203-PILC, the activity significantly exceeded those of the V205-based catalysts. -try NO.NO,10102-43-9;NH3,7664-41-7; SOZ, 7446-09-5; montmorillonite, 1318-93-0.
Literature Cited Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976. Baksh, M. S. A.; Kikkinides, E. S.;Yang, R. T. Characterizationby Physisorption of a New Class of Microporous Adsorbante: Pillared Clays. Submitted for publication in Znd. Eng. Chem. Res. 1992. Beeckman, J. W.; Hegedus, L. L. Design of Monolith Catalyeta for Power Plant NO, Emission Control. Znd. Eng. Chem. Res. 1991, 30, 969. Bosch, H.; Janasen, F. Catalytic Reduction of Nitrogen Oxides. Catal. Today 1988,90, 250. Brindley, G. M.; Sempels, R. E. Preparation and Properties of Some Hydroxy-Aluminum Beidellites. Clays Clay Miner. 1977,12,229. Burch, R. Pillared Clays. Catal. Today 1988,2,185. Burch, R.; Warburton, C. I. &-Containing Pillared Interlayer Clap. I. Preparation and Structural Characterization. J . Catal. 1986, 97, 511.
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Nitrogen Bases Resistant to Hydrodenitrogenation: Evidence against Using Quinoline as a Model Compound Paula L.J o k u t y a n d M u r r a y R.Gray* Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
A retention chromatographic method was used to concentrate basic nitrogen compounds from a commercial synthetic crude oil derived from Athabasca bitumen. The most abundant ring systems were alkyl-substituted 5,6,7,8-tetrahydroquinolines and octahydrobenzoquinolines. The largest gas chromatographic peaks due to individual components were from alkylpyridines and alkyltetrahydroquinolines. Higher ring systems were also found. 13C and 'H NMR spectroscopy indicated a high degree of substitution but with protons frequently found in the position meta to the nitrogen. The distribution and degree of substitution of product ring types were consistent with a multistep mechanism for hydrodenitrogenation (HDN), including hydrogenation of carbon rings followed by cracking of the hydrogenated rings to give alkylpyridines. This mechanism of HDN is significantly different from that reported for quinoline. Introduction As world reserves of conventional crude oil dwindle, processing of heavier feeds grows in importance. These heavier feeds contain more aromatic nitrogen compounds than their lighter counterparts. This development of heavy and synthetic crude oils has, therefore, generated a great
deal of interest in the chemistry and kinetics of hydrodenitrogenation. Nitrogen bases are ubiquitous in distillates derived from petroleum residues, heavy oils,and coal liquids. Removal of these substituted pyridines, quinolines, and higher benzologues is required before the distillates can be
0888-5885/92/2631-1445$03.00/00 1992 American Chemical Society
