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Environmentally benign decomposition of potassium nitrate on zeolites |
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PhysChemComm,
Volume 2,
Issue 3,
1999,
Page 9-14
Bin Shen,
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摘要:
Environmentally benign decomposition of potassium nitrate on zeolites Bin Shen, Yuan Chun, Jian Hua Zhu,Ying Wang, Zhen Wu, Jia Rong Xia and Qin Hua Xu Department of Chemistry, Nanjing University, Nanjing 210093, China Received 6th April 1999, Accepted 26 April 1999, Published 29 April 1999 Due to the interaction with the support, KNO3 loaded on zeolite NaY began to decompose near 513 K, much lower than when unsupported. Through a special redox process, moreover, KNO3 can be mainly decomposed at 673 K while the release of NOx is suppressed by more than 90%. KNO3 is widely used as an additive or promoter for enhancing the activity and/or selectivity of catalysts. Recently, novel solid bases have been derived from the KNO3 supported on porous materials. Yamaguchi et al.observed an unusually high activity of KNO3/Al2O3 in the isomerization of cis-but-2-ene at 273 K.1 Zhu et al. reported the superbasicity of KNO3/Al2O3 and KNO3/KL.2,3 Through a comparison of KNO3 dispersion of KNO3 on Al2O3 with that on zeolite NaY, they contributed the superbasic properties of KNO3/Al2O3 to the several layers of K2O overlapped on the surface of alumina.4 On the other hand, KNO3 supported on ZrO2 can also create superbasic sites with basic strength of H=26.5.5 Before activation to decompose KNO3, these KNO3 supported catalysts did not show any basicity, so that the contamination of CO2 from the atmosphere can be avoided. Since these strong basic sites only formed in the in situ activation prior to reaction, they consequently retain high efficiency in the catalytic process.1,4 There are two factors, however, which hinder the application of KNO3 supported catalysts in industry.The first one is the high temperature needed to generate strong basic sites through decomposition of KNO3, e.g. 873 K for KNO3/Al2O3 and over 873 K for KNO3/NaY,4 since this thermal process cannot be applied to some zeolites with a large surface area but with low stability. The second one is the release of NOx (x î 2), which is well known to be harmful to the environment. It is necessary to seek a new method for the decomposition of KNO3 at a lower temperature with a smaller release of NOx, and in this paper we report using the redox method for decomposing KNO3 supported on zeolites.Table 1 Decomposition of KNO3 supported on porous materials Sample K+ density added /ions nm–2 Surface area of the support /m2 g–1 25% KNO3/NaY 21% KNO3/KY 21% KNO3/KZSM-5 21% KNO3/KL 13% KNO3/Kb 2.6 2.1 4.5 6.4 1.9 1.4 766 756 354 246 458 990 20% KNO3/MCM-41 T/Kstart 873 K 773 K 673 K 0.37 0.40 0.28 0.19 0.18 0.17 0.05 0.05 0.18 0.17 0.15 0.18 0.04 0.05 0.03 0.07 0.09 0.15 510 503 513 513 513 507 PhysChemComm, 1999, 33 Experimental Various types of zeolites, NaY (Si/Al=2.86), KL (Si/Al=2.90), NaZSM-5 (Si/Al=20.9), Nab (Si/Al=14.2), and MCM-41 (Si/Al=15.1) were used as supports, their surface areas and the corresponding K+ density added are listed in Table 1.HY, KZSM-5 and Kb were obtained from the parent zeolite by an ion exchange method described previously.6 For a comparison, SiO2 (SA=70 m2 g–1), ZrO2 (SA=120 m2 g–1 and g-Al2O3 (SA=177 m2 g–1) were also used. A "dry" impregnation method4 was employed to prepare the sample supported with KNO3. Gases used here were nitrogen (99.99 wt.%) and helium (99.999 wt.%), both purified by passing over 5A zeolite. Propan-2-ol and the other agents were of analytical reagent grade, and their purities were higher than 99.5 wt.%. Temperature programmed decomposition (TPDE) of supported KNO3 was carried out in a flow reactor.2 A sample of 6–8 mg, in 20–40 mesh, was heated in a flow of N2 (30 ml min–1) from 298 to 873 K at a rate of 8 K min–1.The NOx released in the TPDE process was converted to NO2 by passing through a CrO3 tube, and then adsorbed in a solution of sulfanilamide and N-1-naphthylethylene diamine di-HCl. The amount of NO2 was detected by a colorimetric method7 and represented the amount of KNO decomposed. Moreover, this amount was also used to calculate the KNO3 content of the sample, together with the results of FTIR and XPS experiments.4 For determining the ratio of NOx (x < 2) and NO2 in the released gas product, the CrO3 tube was removed in some measurements, so that only NO2 could be detected while NOx (x < 2) was invisible. To examine the effect of the reducing agent on Release of NOx at different temperature /mmol g–1 573 K 0.02 0.01 0.02 0.01 0.10 0.09TPDE of supported KNO3, the sample was kept at 673 K in contact with propan-2-ol (6–8 ml g–1) or acetone, then was heated again from 673 to 873 K.For FTIR measurement,4 the sample was evacuated at 673 K for 1 h prior to contact with propan-2-ol, then pressed to a self-supported wafer. To measure the basicity of solid bases, samples of 0.05 g were shaken for 5 min in 0.02 M aqueous HCl (5 ml), and the remaining acid was then titrated with standard base (0.02 M aqueous NaOH). Dehydrogenation of propan-2-ol at 673 K4 and aldolisation of acetone at 333 K were employed to evaluate the catalytic properties of KNO3/NaY. For the former reaction, propan- 2-ol of 0.3 ml was injected into the reactor each time where 0.05 g of catalyst had been activated, and the results of the 10th injection was used to characterize the catalytic properties of the sample.For the latter reaction, an activated catalyst of 0.2 g was added in acetone of 20 ml under N2 protection. After reacting for 4 h, the products were separated in an OV-101 column (3 mm diameter), and the analysis data were normalized using a HP3390A integrator. 3 NOx detected over 673–873 K /mmol g-1 a Propan-2-ol Acetone None ——0.01 —0.03 0.17 0.10 ————— 0.46 0.50 0.38 0.49 0.43 0.42 0.33 0.50 0.34 0.22 0.15 0.03 Results and discussion 1. Decomposition of KNO3 and release of NOx on zeolite Tables 1 and 2 list the TPDE-CM data of supported KNO samples.KNO3 began to thermally decompose at 753 K and only 0.33% of it decomposed up to 873 K. After being supported on porous materials, KNO3 started to decompose in the range of 510–540 K as shown in Table 1, resulting from the interaction between KNO3 and support. In addition, no N 1s signal was observed on the XPS spectra of these as-prepared composites in the range of 410–390 eV where Folksson found N 1s of NaNO3 at 407.2 eV and N 1s of NH4NO3 at 400.9 eV,8 and no Auger signals of nitrogen appeared, either. A similar phenomenon had been reported on KNO3/Al2O3 and was attributed to the dispersion and decomposition of KNO3 through the interaction with the surface of alumina.4 To check if the detection limit of XPS method caused lack of N 1s signal, a sample of 14% theoretical value of 0.199 within the error of measurements, KNO3/ZrO2 was used and its surface ratio of K/Zr was dispersion of KNO3 on the sample.Assuming the dispersed Table 2 Suppression of NOx released from KNO3 supported samples by use of redox process Sample 25% KNO3/NaY 21% KNO3/KY 21% KNO3/HY 21% KNO3/KZSM-5 21% KNO3/KL 13% KNO3/Kb 13% KNO3/Nab 20% KNO3/MCM-41 21% KNO3/SiO2 26% KNO3/Al2O3 27% KNO3/ZrO2 KNO3 0.01 0.03 0.17 0.03 0.20 0.27 0.09 0.02 0.13 0.08 0.03 0.03 a The reducer used at 673 K. b Calculated as the decrement/the original value, e.g. (0.46–0.01)/0.46=97.8%. measured to be 0.209. This data was almost the same as the which gives us a reason to suppose a homogeneous species kept the original structure of KNO3, there should be a similar concentration of N species on the surface of the sample as the K species and they should be detected by an3 XPS test as that reported on the LiNbO3 adsorbed NO.9 However, no N 1s signal was observed on the XPS spectrum at all.It is very likely that the state of KNO supported on porous materials may have been changed, although the true reason is unknown yet and needs to be explored. On the other hand, high dispersion of KNO3 on zeolite was revealed by XRD analyses. For instance, neither KNO3 nor a new phase such as K2[Al(NO3)5] was observed on the XRD patterns of KNO3/NaY4 and KNO3/KL.3 The large surface area of zeolite seems to be an important factor preventing the formation of microclusters of nitrate. Besides, no obvious destruction of zeolite or feldspar-like new phase was found, even when the sample was activated at 673 K.4 The identical XRD patterns of these samples and their corresponding parent zeolites indicated well the dispersion of KNO3 on zeolite causing no collapse of the zeolite structure.3 As demonstrated in Tables 1 and 2, NOx (x î 2) was released from those composites based on zeolite when the loaded KNO3 was thermally decomposed, and the amount kept was 0.33–0.50 mmol g–1 in the range of 673–873 K regardless of the type of zeolites. Decomposition of KNO on zeolite strongly depends on the surface-acid basicity of the support. For instance, KNO3 on zeolite HY began to decompose at 403 K, much lower than that on zeolite NaY (510 K) and KY (503 K), and about half of NOx (0.38 mmol g–1) was detected under 573 K while on these two basic zeolites, no NOx was formed in the range of temperature at all.Moreover, unlike what was observed on KNO3/KY on which NOx (x < 2) predominantly formed under 873 K, NO2 was the predominant gas product of KNO3 decomposed on zeolite HY during 473–873 K, as seen in Fig. 1. Apparently the existence of proton on zeolite accelerates decomposition of loaded KNO3 and affects the composition of the gas products: KNO3 +H+ ® K+ + H2O + NOx (x î2) (1) Decrease of NOx release (%)b Ethanol Propan-2-ol Acetone Ethanol 93.5 92.0 94.7 —95.3 57.1 84.8 96.0 ———— ——97.4 —93.0 59.5 69.7 ————— 97.8 94.0 55.3 93.9 53.5 35.7 72.7 96.0 61.8 63.7 80.0 0 0.03 0.04 0.02 —0.02 0.18 0.05 0.02 ————Fig.1a The ratio of NO2 to NOx (x < 2) formed on 21% KNO3/KY vs. temperature. Fig. 1b The ratio of NO2 to NOx (x < 2) formed on 21% KNO3/HY vs. temperature. Fig. 1c The ratio of NO2 to NOx (x < 2) formed on 20% KNO3/MCM-41 vs. temperature. The influence of the pore size or the pore structure of zeolite on the decomposition of KNO3 is complex. For example, 21% of KNO3/KY and 13% KNO3/Kb had a similar pore size (about 0.7 nm) and a similar added potassium ion density (1.9–2.1 K+ ions nm–2), but on the former about 80% of NOx was released at 873 K, while on the latter 36% of NOx was formed under 673 K.As a first 3 thought, a larger surface area of support should be beneficial for a faster decomposition of KNO3, especially in the range of 573–673 K. However, this assumption was not strongly supported by the experimental results. As seen in Table 1, the amount of NOx formed on KNO3/KY in the range of 573–673 K (0.06 mmol g–1) was similar to that of KNO3/KZSM-5 (0.05 mmol g–1) and that on KNO3/KL (0.08 mmol g–1), though the three zeolites have obviously a different surface area. It is important to point out that the surface area could well be irrelevant, because the surface area measurements relate to nitrogen sorption. There must be some small cages in zeolite with the narrow window where nitrogen can enter but KNO3 molecule/species cannot, due to steric hindrance; consequently the available surface area for dispersion of KNO3 may differ depending on the zeolite structure.There was more KNO3 decomposed on MCM-41 in the 573–673 K range (0.24 mmol g–1), but this could not be simply attributed to the enormous surface area of MCM-41, since some residual acid sites existing on the support and their accelerated function should be considered. Decomposition of KNO occurred after the KNO3 molecules were well dispersed in the channel of zeolite, therefore no migration or diffusion of them was obstructed by the pore structure of zeolite. Hence the geometric microenvironment of the zeolite plays a minor role in the thermal decomposition of KNO3.In contrast, the proton on the zeolite can accelerate the decomposition of KNO3 as mentioned above, so that the surface acidity of zeolite has a stronger influence than the geometric effect. Different alkali metal ions on zeolite seemed to have very weak influence on the thermal decomposition of KNO3, since both the total amount and desorption temperature of NOx were similar on 21% KNO3/KY and 25% KNO3/NaY, as shown in Table 1. 2. Suppression of NOx formation by redox process x Table 2 lists the suppression of the NOx release from KNO3-supported zeolites. When 25% KNO3/NaY, 21% KNO3/KZSM-5 and 20% KNO3/MCM-41 samples were treated with propan-2-ol at 673 K first, then heated up to 873 K, two changes were observed.Firstly, about 93–98% of NOx release was suppressed, and secondly the KNO3 was completely decomposed, because no characteristic bands of nitrate (1397 and 1764 cm–1) existed in the corresponding IR spectrum (curved in Fig. 2). To clarify the mechanism, KNO3 itself was also treated with propan- 2-ol under the same conditions for a comparison. As demonstrated in Table 2, neither the suppression of NO nor any acceleration on the decomposition of KNO3 was observed. Clearly in this case, unsupported KNO3 did not react with propan-2-ol at 673 K. It appears that the key to success of suppressing NOx release on the KNO3-supported zeolites is the dispersion of KNO3 and its interaction with support. 3 The reduction of propan-2-ol on the dispersed KNO depends on the zeolite structure.The decrement of NOx was about 53% on 21% KNO3/KL sample, much smaller than that on 25% KNO3/NaY (97.8%) or 21% KNO3/KZSM-5 (93.5%). To check whether the difference was caused by the relative high density of K+ ion added on zeolite KL (6.4 ions nm–2) or not, some samples of SiO2 and g-Al2O3 as well as ZrO2 loaded more KNO3 were tested under the same conditions. These samples had a heavier K+ density, e.g. 22.6 K+ ions nm–2 for 21% KNO3/SiO2, 11.8 K+ ions nm–2 for 26% KNO3/Al2O3 and 18.4 K+ ions nm–2 for 27% KNO3/ZrO2. However, the production of NOx on them was much obviously lowered in the range of 60–80% (Table 2).Fig. 2 FT-IR transmission spectra of 25% KNO3/NaY sample (a) before and after treated with (d) propan-2-ol, (e) acetone and (f) ethanol, and 13%KNO 3/Kb sample (b) before and (c) after treated with propan-2-ol at 673 K.x This fact precluded the suspicions existing on the K+ density of 21% KNO3/KL sample. Further confirmation of the dependence of zeolite structure came from sample 13% KNO3/Kb. This sample had an added K+ ion density (2.0 ions nm–2) close to that of 25% KNO3/NaY (2.6 ions nm–2), and the two samples released a similar amount of NO (0.42 and 0.46 mmol g–1, respectively) in the thermal decomposition of KNO3 during 673–873 K. Furthermore, the NOx released from 13% KNO3/Kb (0.09 mmol g–1) at 673 K was more than that from 25% KNO3/NaY (0.04 mmol g–1). After being treated with propan-2-ol at 673 K, however, the remaining release of NOx on 13% KNO3/Kb (0.27 mmol g–1) was much higher than that on 25% KNO3/NaY (0.01 mmol g–1).Unfortunately, to date no reason is known for this difference. Acidity of zeolite was another factor affecting the treatment using propan-2-ol. Decrease of NOx on 21% KNO3/HY was 55.3%, much smaller than that on 25% KNO3/NaY (97.8%) and 21% KNO3/KY (94.0%). For KNO3/MCM-41, this effect became unclear because a decrease of 96% was observed, probably due to the weak acidity and mesoporous structure of MCM-41. Based on all these facts described above, two conclusions can be tentatively made: 3, (i) Propan-2-ol is believed to react mainly with KNO probably with KNO3 being activated on zeolite, instead of with NOx formed during decomposition of KNO3.Otherwise, KNO3/NaY and KNO3/Kb samples should have similar residual amount of NOx after treatment with propan- 2-ol at 673 K, because both of them began to release NOx near 510 K and the NOx could be reduced to N2 through a redox process.10 (ii) Reactivity of the loaded KNO3 with propan-2-ol depends on the microenvironment of zeolite where the KNO3 dispersed, involving different geometric and energetic factors, so that some of the KNO3 can react with propan-2-ol at 673 K and are decomposed, while some cannot. Table 2 shows the suppression of NOx release by using different reducing agents. In the case of changing the reducer from propan-2-ol to acetone, about 93 or 97% of the NOx release could thus be suppressed on 21% KNO3/HY or 21% KNO3/KL.However, the residual release of NOx on KNO3/Kb and KNO3/Nab remained in the range of 0.10–0.17 mmol g–1, still higher than the other samples as shown in Table 2. To suppress further release of NOx, ethanol with a smaller molecular volume was chosen as a reducing agent. For KNO3/NaY, KNO3/KY and KNO3/MCM-41, there was no obvious difference between using propan-2-ol and ethanol to lower the amount of NO while on a KNO3/Nab sample the NOx release was thus decreased from 0.10 to 0.05 mmol g–1 (Table 2). The only exception is the sample of KNO3/Kb on which the NOx release still kept a relatively high value (0.18 mmol g–1). Based on the results listed in Table 2, one can argue that the unusual properties of the KNO3 loaded on zeolite b may result from the structure of support, because there are many especial features existing on zeolite b.Apart from several factors like high asymmetry of the oxygen ring, effects of field gradient or a high framework ionicity,11 the topological density (density of bonds around a central tetrahedron) of zeolite b (0.229) is larger than zeolite X and Y (0.181). This high density of bonds improves the resistance to breaking, hence distortion of tetrahedra in the presence of water is observed on zeolite b.12 Besides, zeolite b belongs to the class of solids having a large distribution of bond angle; its TOT bond angle is in the range of 137–166o, quite higher than that of zeolite X and Y (138–145°).13 As a consequence, zeolite b has the most flexible framework of all zeolites, which should have an effect on its surface properties, in line with the unexpected high basicity of Nab14 and the smaller para-/orthoselectivity in zeolite b than in zeolite Y.15 This proposition is supported by two facts.The first is that zeolite b is the unique in containing a chiral polymorph13 and to perform enantioselective catalysis,16 and the second comes from zeolites NaY: it has the pore size and surface area similar to zeolite b but does not show a similar behaviour in the decomposition of KNO To explore how the structure of zeolite b hinders the redox process between the reducer and the loaded KNO3, acetaldehyde, with a stronger ability of reduction than ethanol, and methanol, with a smaller volume than ethanol, were employed as reducing agents.Acetaldehyde treatment at 673 K made the KNO completely that there was no NOx released in the following process above 673 K. Additionally, some carbon particles were formed on the sample as a reduction product. This result should not be unusual, because in the same conditions acetaldehyde can also reduce KNO3 itself at 673 K and form carbon particles, too. On the contrary, a lot of KNO3 remained on the sample treated with methanol at 673 K and therefore 0.21 mmol g–1 of NOx were released in the following TPDE process. Based on this comparison, the chemical reason instead of space hindrance is tentatively estimated as the main factor causing the strange properties of KNO3 on zeolite b in the redox process.Part of the KNO3 loaded on zeolite Kb seems to keep the original statex, 3. 3 on zeolite Kb decomposed soTable 3 Basicity and catalytic properties of 21% KNO3/NaY Activation process Catalytic activity in the reaction of Basicity measured by titration/ mmol g–1 Dehydrogenation of propan-2-ol at 673 Ka trace 40.0 0.23 0.86 Heated at 873 K Contacted with propan-2-ol at 673 K a Represented by the yield of acetone (mol%). b Represented by the yield of diacetone alcohol (mmol g–1). without being activated, so that propan-2-ol cannot reduce it and acetaldehyde should be used. Otherwise, methanol treatment should show a higher efficiency than acetaldehyde treatment at 673 K if the KNO3 is located on a special geometric position in the channel of zeolite Kb.Concerning the exact role of the original K content of zeolite in the decomposition of KNO3, the situation is rather complex involving different type of zeolite and the interaction with the reducing agent. The K content of KY, KL, Kb and KZSM-5 would differ before nitrate addition, because their parent zeolites had different Si/Al ratio and the concentration of potassium ion should be proportional to the Al framework content of zeolite. However, the amount of KNO3 decomposed at 673–873 K, represented tentatively by the amount of NOx detected, was in the range of 0.42–0.50 mmol g–1 (seen in the first column of Table 2) without any obvious relation to the corresponding origin of the K content.For example, zeolite b has a larger Si/Al ratio (14.2) than zeolite Y (2.9), but the KNO3 decomposed on KNO3/Kb (0.42 mmol g–1) was closely matching that on 3/KY (0.50 mmol g–1) in the range of 673–873 K. In a 3 seems to be KNO thermal process, the decomposition of KNO easier on the K form of zeolite than on the Na form, e.g. 0.42 mmol g–1 on 13% KNO3/Kb while 0.33 mmol g–1 on 13% KNO3/Nab during 673–873 K. However, much more residual KNO3 persisted on the former than on the latter after they were contacted with propan-2-ol at 673 K, so that in the following TPDE process, 0.27 mmol g–1 of NOx was released from 13% KNO3/Kb while 0.09 mmol g–1 from3.13% KNO3/Nab, as shown in Table 2. It is known that ions move from one site to the other in specified locations on zeolite,14 and K–Na solid state ion exchange can occur on the surface of the zeolite,17 which should have an influence on the decomposition of the dispersed KNO Unfortunately, to date it is still unclear why this influence plays two conflicting roles in the thermal decomposition and reductive decomposition of KNO3, therefore further studies are recommended. 3. FTIR measurement and catalytic reaction Fig. 2 shows the FTIR spectra of KNO3-supported samples before and after treatment with different reducing agents. There were two strong bands, at 1760 and 1397 cm–1 on the IR spectra of KNO3/NaY characterizing the nitrate.18 These bands could persist with considerable intensities up to 873 K4 and declined above 923 K.17 However, when the sample of KNO3/NaY contacted propan-2-ol at 673 K, the bands of nitrate disappeared within few minutes while three new strong bands of acetate emerged at 1580, 1344 and 1428 cm–1.Moreover, a sharp band of CN– appeared near 2174 cm–1, similar to what was observed in the catalytic reduction of NOx with olefins over Cu/ZSM-5.19 No other Isomerization of cis-but-2-ene at 273 K (%) Aldolisation of acetone at 333 Kb 27.2 28.6 00 N species such as nitrite at 1550 and 1320 cm–1 (ref. 18) or nitride was observed. On the basis of these results, it is safe to conclude that the supported KNO3 is quickly decomposed through a redox process with propan-2-ol, and that acetic acid is formed as one of the oxidized products/intermediates of propan-2-ol. On the IR spectrum of KNO3/Kb, however, the strong bands of nitrate still remained after the sample was contacted with propan-2-ol under the same conditions.There was no obvious band of acetate existing on the spectrum and no band of CN– was observed either. The different behaviour of KNO3 loaded on zeolites NaY and Kb, as discussed above, resulted from the different pore structure and different surface properties of support. 3 In the case of KNO3/NaY treated with acetone instead of propan-2-ol, characteristic IR bands of acetate were also observed, as seen in Fig. 2, indicating the formation of acetic acid, too.On the IR spectrum of KNO3/NaY treated with ethanol, however, 1344 cm–1 band of symmetric–CH deformation vibration was absent, demonstrating the existence of formic acid as an oxidative product of ethanol. Based on the formation of acetate in the case of propan-2-ol or acetone, and of formic acid in the ethanol treatment, cleavage of the a-carbon atom in the reducer is proposed to occur during the oxidation by KNO3. As further proof, H2, NH3, H2O2, benzene, toluene and n-hexane did not show any obvious reduction effect for the supported KNO3 on zeolites NaY at 673 K. Table 3 lists the catalytic activity of KNO3/NaY activated in two ways, heated at 873 K (sample A) or treated with propan-2-ol at 673 K (sample B). Three base-assistance reactions are used to check whether materials prepared by the new routine display similar catalytic properties.There are two probe reactions in which no different catalytic activity is observed on these two samples. For the aldolisation of acetone at 333 K, both samples showed a similar yield of diacetone alcohol, while in the isomerization of cis-but-2-ene at 273 K, their activity was negligible. However, the two samples exhibited different catalytic activity in the dehydrogenation reaction of propan- 2-ol at 673 K, and the yield of acetone on sample B was much higher than that on sample A (Table 3). Propan-2-ol was dehydrogenated to acetone on the basic site of zeolite catalyst,20 and for these two samples with the same pore structure and the same potassium content, the higher acetone yield means the higher concentration of active sites on the sample.Consequently, more basic active centers are assumed to exist on sample B than on sample A and this assumption was confirmed by titration, the total basicity of sample B (0.86 mmol g–1) was higher than that of sample A (0.23 mmol g–1). On the other hand, the inactivity of the two samples in the isomerization of cis-but-2-ene at 273 K3- excludes the possibility of using a redox method to enhance the basic strength of the solid base derived from the KNO supported sample, because this reaction is often employed to probe the strong basicity of the sample.3,4 The basic strength of the KNO3 supported samples strongly depends on the intrinsic factor of the support, involving the number and distribution of octahedral vacant site that is necessary for the insertion of K+ cations and formation of strong basic sites.1 Zeolites are "framework" aluminosilicates that are based on an infinitely extending three-dimensional network of AlO4 and SiO4 tetrahedra linked to each other by sharing all the oxygen, and there is no octahedral vacant site in the framework of zeolite NaY.21 As a result, KNO3 cannot form strong basic sites below 873 K on zeolite NaY though it has been well dispersed.4 Using the redox method can accelerate the decomposition of KNO3 on zeolite NaY, but it cannot change the intrinsic properties of the support, so that the resulting solid base still exhibits the original basic strength.A conclusion may nevertheless be made: reductive decomposing the KNO3 dispersed on zeolites not only obviously lowers the decomposition temperature, but also suppresses the release of NOx. This environmental benign routine may open a novel path for the application of these solid strong bases in industry. Acknowledgements The National Natural Science Foundation of China (29773020) and the National Advanced Materials Committee of China supported this work. Financial supports from Chinese Education Ministry and Analysis Center of Nanjing University are also gratefully acknowledged. References 1 T. Yamaguchi, J. H. Zhu, Y. Wang, M. Komatsu and M. Ookawa, Chem. Lett., 1997, 989. 2 J. H. Zhu, Y. Chun, Y. Wang and M. Tu, Chin. Sci. Bull., 1997, 42, 1493. 3 J. H. Zhu, Y. Chun, Y. Wang and Q. H. Xu, Mater. Lett., 1997, 33, 207. 4 J. H. Zhu, Y. Wang, Y. Chun and X. S. Wang, J. Chem. Soc., Faraday Trans., 1998, 94, 1163. 5 Y. Wang, PhD thesis, Ehime University, Japan, 1998. 6 J. L. Dong, J. H. Zhu and Q. H. Xu, Appl. Catal., 1994, 112, 105. 7 B. E. Saltzman, Anal. Chem., 1954, 26, 1949. 8 B. Folksson, Acta Chem. Scand., 1973, 27, 287. 9 K. Tabata, M. Kamada, T. Choso and Y. Nagasawa, J. Chem. Soc., Faraday Trans., 1998, 94, 2213. 10 M. Iwamoto, Stud. Surf. Sci. Catal., 1994, 84, 1395. 11 D. J. Barthomeuf, Phys. Chem., 1984, 88, 42. 12 D. J. Barthomeuf, Phys. Chem., 1993, 97, 10092. 13 J. M. Newsam, M. M. J. Treacy and C. B. de Gruyter, Proc. Royal Soc., London, A, 1988, 420, 375. 14 D. Barthomeuf, Stud. Surf. Sci. Catal., 1998, 105, 1677. 15 A. Corma, F. Llopis, P. Viruela and C. Zicovich- Wiilson, J. Am. Chem. Soc., 1994, 116, 134. 16 M. E. Davis and R. L. Lobo, Chem. Mater., 1988, 4, 756. 17 M. V. Susic, N. A. Petronovic and D. A. Mioc, J. Inorg. Nucl. Chem., 1971, 33, 2667. 18 K. Nakamoto, Infrared spectra of inorganic and coordination compounds, John Wiley, New York, 1970, p. 98. 19 F. Radtke, R. A. Koeppel and A. Baike, J. Chem. Soc., Chem. Commun., 1995, 427. 20 J. H. Zhu, J. L. Dong and Q. H. Xu, React. Kinet. Catal. Lett., 1998, 63, 67. 21 D. W. Breck, Zeolites molecular sieves, John Wiley, New York, 1974, p. 514. Paper 9/02666E PhysChemComm © The Royal Society of Chemistry 1999
ISSN:1460-2733
DOI:10.1039/a902666e
出版商:RSC
年代:1999
数据来源: RSC
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