Influence of the treatment of Y zeolite by ammonium hexafluorosilicate on the physicochemical and catalytic properties: application for chlororganics destruction

Studiesin Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

463

Influence of the treatment of Y zeolite by ammonium hexafluorosilicate on the physicochemical and catalytic properties: application for chlororganics destruction R. L6pez-Fonseca, J.I. Guti6rrez-Ortiz, B. de Rivas, S. Cibri~in, and J.R. Gonz~tlezVelasco* Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pais Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-94-6012681; Fax: +34-944648500; E-mail address: [email protected] The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite for oxidative destruction of chlorinated VOC. A series of Y zeolites with various Si/AI ratios was prepared from a commercial sample, then characterised and tested for the catalytic decomposition of chlorinated VOC (1,2dichloroethane and trichloroethylene). In general, these modified Y zeolites exhibited a higher activity with respect to that of the parent material, the zeolite subjected to 50% dealumination resulting in the most active catalyst. This increase in activity was associated with the development of strong Br6nsted acidity due to dealumination. 1. I N T R O D U C T I O N The increasing amounts of chlorinated volatile organic compounds (VOC), such as 1,2-dichloroethane (DCE) and trichloroethylene (TCE), released in the environment, together with their suspected toxicity and carcinogenic properties, have prompted researchers world-wide to find clean effective methods of destruction [1]. The abatement of chlorinated volatile organic compounds by catalytic combustion has been widely utilised in several technical processes. The lower temperatures required for catalytic combustion result in a lower fuel demand and can therefore be more cost effective than a thermal oxidation process [2]. In addition, the catalytic process also exerts more control over the reaction products and is less likely to produce toxic by-products, like dioxins, which may be generated by thermal combustion [3]. Most of the previous work related to catalysts for chlorinated VOC abatement is focused on the development of two type of catalysts, namely those based on noble metals and on transition metal oxides. By contrast, the utility of zeolites as effective catalysts for the decomposition of chlorinated organics has not been explored in detail, when it is reported that metal loaded catalysts employed in commercial applications are susceptible to deactivation by the HC1 and C12 produced during reaction [4]. In our previous works [5,6] it was found that H-zeolites showed a high activity for chlorinated VOC destruction under dry and humid conditions, and that their activity was controlled by the presence of

464 Br6nsted acidity. In the present study, an H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [7,8] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The scope of this work is to analyse the catalytic behaviour of a series of H-Y zeolites with different Si/AI in the oxidative decomposition of chlorinated hydrocarbons (DCE and TCE) in air, at lean concentration conditions (around 1000 ppm) between 200 and 550~ 2. E X P E R I M E N T A L AND M E T H O D S

2.1.

Materials and zeolite preparation

The Y zeolite (CBV400) in its H-form (H-Y) was supplied from Zeolyst Corp. and used as received. The series of dealuminated samples H-Y(d) was prepared as follows: prior to dealumination the starting material was obtained by two successive ion exchanges with a 3 M ammonium nitrate solution of the commercial H-Y sample to reduce the sodium content. Then, the NH4H-Y zeolite was preheated in a 0.5 M ammonium acetate solution at 80~ An aqueous solution of ammonium hexafluorosilicate was added dropwise at a rate of 50 cm 3 h 1 under vigorous stirring. The (NH4)2SiF6-to-zeolite ratio was adjusted to remove 15, 30, 50 and 75% of the aluminium in the zeolite, respectively. Afterwards, the temperature was raised to 95~ and the slurry was kept at this temperature for 3 hours to ensure that silicon could be inserted into vacancies created by the extraction of aluminium. Finally, the zeolite was recovered by filtration and repeatedly washed with hot deionised water to remove the unreacted (NH4)2SiF6 completely. The zeolites were pelletised using methylcellulose as a temporary binder which was removed by calcination in air. Then the pellets were crushed and sieved to grains of 0.3-0.5 mm in diameter and used for catalytic runs without further activation.

2.2.

Catalyst characterisation

The BET surface areas of the zeolite samples were determined by N2 adsorptiondesorption at -196~ in a Micromeritics ASAP 2010 equipment. The adsorption data were treated with the full BET equation. The <>method was applied in order to obtain an estimation of the micropore volume. The determination of the compositions was carried out using a Philips PW 1480 wavelength dispersive X-ray fluorescence (XRF) spectrometer. The crystallinity and the unit cell size were established by a Philips PW 1710 X-ray diffractometer (XRD) with CuK~t radiation (~,=1.5406,&) and Ni filter. The number of aluminium atoms per unit cell, NAI, was calculated from a0 using the correlation given by Fichtner-Schmittler et al. [9]. The atomic framework Si/A1 ratio was derived from the calculated N AI. The number of extra-framework aluminium atoms per unit cell was calculated by the difference between the total aluminium, as determined by XRF analysis, and the framework aluminium N AI. Diffuse reflectance infrared (DRIFT) spectra of pyridine adsorbed on the zeolite samples were obtained with a Nicolet Proteg6 460 ESP spectrometer, equipped with a controlled-temperature and environment diffuse reflectance chamber (Spectra-Tech) with KBr windows and a liquid nitrogen-cooled HgCdTe detector. All spectra were collected in the range of 4000-1000 cm -1 averaging 400 scans at an instrumental resolution of 1 cm -1,

465

and analysed using OMNIC software. Temperature-programmed desorption (TPD) of ammonia was performed on a Micromeritics AutoChem 2910 instrument. Prior to adsorption experiments, the samples were first pre-treated in a quartz U-tube in a nitrogen stream at 550~ Subsequently, the desorption was carried out from 100 to 550~ at a heating rate of 10~ min -1 in an Ar stream (50 cm 3 minl). This temperature was maintained for 15 min until the adsorbate was completely desorbed.

2.3. Experimental device and product analysis Catalytic oxidation reactions were carried out in a conventional fixed bed reactor under atmospheric pressure [10]. The flow rate through the reactor was set at 500 cm 3 mini and the gas hourly space velocity (GHSV) was set at 15000 h -1. The residence time based on the packing volume of the catalyst was 0.24 s. Following the reactor, a portion of the effluent stream was delivered and analysed on-line using a Hewlett Packard 5890 Series II gas chromatograph (GC) equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD), and controlled with HP ChemStation software. The concentration of the chlorinated feeds was determined by the ECD after being separated in a HP-VOC column. 3. RESULTS AND DISCUSSION

3.1. Catalyst characterisation Expectedly, increasing amounts of AHFS added led to increased degrees of dealumination of the samples. For moderate dealumination levels (<50%) in which 15, 30 and 50% of the original aluminium was calculated to be removed, respectively, the reaction was stoichiometric within the experimental error. Hence the actual dealumination degrees obtained were 16, 32 and 50%, respectively. However, for higher degrees (75%) the reaction was not complete any more since only 64% dealumination was achieved. Table 1. Textural and structural properties of dealuminated Y zeolites. . . . . Zeolite H-Y H-Y (dl 6%) H-Y(d32%) H-Y(ds0%) H-Y(d64%)

Crystal., %

a0, ,~

SBEV,m z g-1

Wpore,cm 3 g-1

Vmesopore, cm 3 g-1

100 99 97 95 40

24.52 24.51 24.49 24.46

900 955 865 820 155

0.382 0.406 0.355 0.334 0.085

0.070 0.072 0.050 0.049 0.105

466 Table 2. Textural and structural properties of dealuminated Y zeolites. Zeolite H-Y H-Y(d16%) H-Y(d32%) H-Y(ds0%)

H-Y(d64o/o)

(Si/A1)bum (Si/A1)fr~. 2.6 4.9 3.3 5.2 4.3 5.6 6.2 6.5 8.9 11.5

Alframework Alextraframework

Alto~ 53.3 44.6 36.2 26.7 19.4

32.2 31.1 28.9 25.5 15.4

21.1 13.5 7.3 1.2 4.0

Table 3. Total acidity and acid strength distribution of Y zeolites. Zeolite Total acidity, mmol NH3 g-1 Weak sites, % H-Y 0.65 74.8 H-Y(d16%) 0.61 65.2 H-Y(d32o/o) 0.56 54.4 H-Y(dso%) 0.48 40.3 H-Y(d64%) 0.35 57.2

Strong sites, % 25.2 34.8 45.8 59.7 42.8

The crystallinity of the dealuminated samples is listed in Table 1. It was observed that H-Y(d16o/o), H-Y(d32o/o) and H-Y(ds0%) retained high degrees of crystallinity [11]. However, a noticeable crystallographic degradation was noted for H-Y(d64%) sample since the crystallinity decreased to 40% as a result of the massive aluminium extraction [12]. For the parent material XRF analysis gave aluminium contents that were noticeably larger than the framework aluminium concentration indicating the presence of large amounts of extralattice or non-framework aluminium. A comparison of the results from Table 2 clearly demonstrate that at relatively low dealumination levels, i.e. 16 and 32%, the AHFS treatment preferentially removed EFAL species while both EFAL and FAL were removed when A o, I the AHFS concentration was ::i H-Y ai increased [13]. Hence, for H-Y(ds0%) sample, almost 100% EFAL and 30% et FAL were extracted from the parent .Q material. Indeed, it was found for this < sample that the framework Si/A1 ratio obtained from the unit cell size tt-Yld~%) was very close to the value obtained by XRF measurements. The results 1700 1650 1600 1550 1500 1450 1400 from Tables 1 and 2 were consistent Wavenumbers, cm -1 with the results found in the literature Fig. 1. IR spectra of adsorbed pyridine on Y zeolites. where it has been reported that dealumination with AHFS up to 50% dealumination did not damage the I

,

t

,

i

i

1

1

,

,

I

i

,

9

1

1

1

i

i

9

i

,

1

1

i

I

1

,

,

i

I

467 zeolite structure and EFAL was practically absent from such dealuminated samples [14]. According to N2 adsorption, the microporous character of the modified zeolites was largely retained and these samples did not possess any appreciable mesopore volume (Table 1). This finding was in agreement with their unaffected crystallinity. By contrast, the values / ~H'Y(de4%) of surface area and pore volume for the sample with a dealumination level of 64% were severely affected, ~ - ~o,d probably due to the collapse of the crystalline structure and to the blockage of the zeolite pore system to a very large extent [15]. As mentioned above, the structure ,i collapse was also revealed by XRD .~ m analysis. On the other hand, it is seen ,~ O from Table 1 that as the framework ~" Si/AI ratio increased the unit cell size H-Yff~,.) of AHF S - t r e a t e d samples significantly contracted due to the smaller size of silicon atoms [ 16]. Temperature-programmed desorption (TPD) of ammonia and \ infrared spectroscopy (IR) of t ~ . H-Y adsorbed pyridine are probably the most extensively used methods for ::::::::::::::::::::::::::::::::::::::::::::::::::::::::: characterising acidity (number, 50 leo 250 350 450 550 strength and nature of acid sites) in zeolites [17,18]. Fig. 1 shows the Temperature'~ diffuse-reflectance infrared spectra of Fig.2. Profiles of NH3-TPD from Y zeolites. pyridine adsorbed on H-Y, HY(ds0%) and H-Y(d64%) samples at 200~ in the region 1700-1400 cm 1. Pyridine bound to Br6nsted acid sites is associated with an IR band at 1545 cm ~ and that bound to Lewis acid sites with a band at 1450 cm 1. A band at 1490 cm ~ arises due to pyridine adsorbed on both Bronsted and Lewis sites [19]. H-Y possesses a large number of both types of acid sites with a ratio of Bronsted to Lewis acid sites, measured as the ratio of the integrated areas of the respective pyridine bands, of 0.8. Nevertheless, in the case of H-Y(ds0%) the band at 1450 cm ~ considerably decreased indicating that the dealumination treatment led to a sample with very few Lewis sites [20]. On the other hand, the band at 1545 cm I remained almost unchangeable. The results from NH3-TPD experiments are plotted in Figure 2. The NH3 desorbed above 100~ was considered as chemisorbed NH3 and subsequently used for acidity determination. The data from Table 3 indicates an overall loss of the total number of acid sites with dealumination [21]. As can been observed, the TPD profile of the parent zeolite was characterised by the display of a major desorption peak around 150~ [22]. This peak

468 was indicative of the presence of a notable number of weak acid sites (around 75% of the total number of acid sites). However, it showed almost no inflection in the high temperature range. Interestingly, as Si/A1 ratio was increased, a second desorption peak with a maximum at 350~ became more and more prominent indicating a noticeable increase in the number of strong acid sites with progressive dealumination [23 ]. This result paralleled the observation that the number of weak acid sites decreased from 75% of the total acidity for H-Y sample to 45% for H-Y(ds0%) sample. Table 3 summarises the ratio of strong acid sites obtained by integration of the high temperature signal. Hence, it could be concluded that a portion of the weak acidity turned into sites, mostly Bronsted-type sites, with high acid strength with increasing Si/A1 ratio [23]. 3.2. Catalytic activity results in chlorinated VOC conversion The series of dealuminated samples prepared by AHFS treatment were evaluated for the catalytic decomposition of DCE, which was considered as model reactions of chlorinated VOC destruction. The results of DCE and TCE conversion as a function of of reaction temperature over Y zeolites are shown in Fig. 3. It was noted that all dealuminated samples except H-Y(d64%) zeolite exhibited an enhanced performance in comparison with that of the parent material. The 50% dealuminated sample H-Y(ds0%) was the most active catalyst achieving complete conversion at 350~ for DCE and at 550~ for TCE. The following order of activity for chlorinated VOC conversion was observed: H-Y(ds0%)>HY(d32%)>H-Y(d16%)>H-Y>H-Y(d64%). Hence, H-Y(ds0%) zeolite showed a light-off temperature or Ts0 (temperature at which 50% conversion was attained) of 265~ 100 H-Y lower than that of H-Y(d32%), H-Y(d16%) H-Y(d1 90 1 ~ H-Y(d3"/' and H-Y, 280, 300 and 325~ H-Y d5'2% respectively. H-Y(d64%), however, 80 t H.YId.o'/, f showed a less active behaviour with a Tso 70 i value of 350~ Unlike DCE, TCE combustion required significantly higher / temperatures [24,25]. Ts0 values were 475, 475, 500, 510 and 520~ over Ho 40 1 Y(dso%), H-Y(d32%), H-Y(d16%), H-Y and / H-Y(d64%), respectively. 30 The combined characterisation 211 ~ and catalytic evaluation of the Y zeolites 111 obtained by progressive dealumination via the (NH4)2SiF6 method revealed that 200 250 300 350 400 450 500 550 the strength of the acid sites had a Temperature, *C dominant effect on the catalytic behaviour [26,27]. The zeolite activity increased for Fig. 3. Light-off curves of DCE and TCE Si/A1 ratios from 2.6 to 6.2 since the combustion over Y zeolites. decline in the acid site density was more than compensated for by the concomitant increase in the population of acid sites with high strength. Upon further removal of aluminium (c.a. Si/AI=8.4) the catalytic activity destruction dramatically dropped due to

469 the decrease in the number of acid sites and a partial loss of crystallinity, as evidenced by the low conversion of H-Y(d64o/o) sample. Similarly, Greene et al. [28] and Prakash et al. [29] obtained a substantial improvement in C.C14 conversion when using a Y zeolite subjected to SIC14 dealumination. 4. CONCLUSIONS The scope of this work was to evaluate the catalytic performance of a series of (NH4)2SiF6-dealuminated Y zeolites for the oxidative decomposition of chlorinated VOC in dry air, at lean concentration conditions (around 1000 ppm) between 200 and 550~ The highly active performance of chemically AHFS-dealuminated zeolites for chlorinated VOC destruction could be accounted for by the generation of new strong acid sites, which were preferentially BrOnsted sites, due to dealumination treatment. It could be concluded that a zeolite with a modest concentration of BrOnsted sites, which were primarily of high acid strength, demonstrated to be effective for catalytic purposes. Likewise, it was established that chlorinated VOC oxidative decomposition was a type of reaction that required strong BrOnsted acidity. ACKNOWLEDGEMENTS

The authors wish to thank Universidad del Pais Vasco/EHU (9/UPV 0069.31013517/2001) and Ministerio de Ciencia y Tecnologia (PPQ2001-1364) for the financial support. R. L-F. acknowledges Ministerio de Educaci6n y Cultura for the FPI grant (QUI96-0471). REFERENCES

1. E.C. Moretti, Practical Solutions for Reducing Volatile Organic Compounds and Hazardous Air Pollutants, Center for Waste Reduction Technologies of the American Institute of Chemical Engineers, New York, 2001. 2. G.J. Hutchings and S.H. Taylor, Catal. Today, 49 (1999) 105. 3. J.C. Lou and Y.S. Chang, Combust. Flame, 109 (1997) 188. 4. J.J. Spivey and J.B. Butt, Catal. Today, 11 (1992)465. 5. J.R. Gonz~lez-Velasco, R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz and P. Steltenpohl, Appl. Catal. B, 24 (2000) 233. 6. R. L6pez-Fonseca, P. Steltenpohl, J.R. Gonz/tlez-Velasco, A. Aranzabal and J.I. Guti6rrez-Ortiz, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 130 (2000) 893. 7. D.W. Breck and G.W. Skeels, US Patent 4 503 023, 1985. 8. G.W. Skeels and D.W. Breck, in: D. Olson, A. Bisio, (Ed.), Proceedings of the 6th International Zeolite Conference, Butterworths, Guilford, 1984, p. 87. 9. H. Fichtner-Schmittler, U. Lohse, G. Engelhardt and V. Patzelova, Cryst. Res. Technol., 19 (1984) K 1. 10. J.R. Gonz/flez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz, R. L6pez-Fonseca and M.A. Guti6rrez-Ortiz, Appl. Catal. B, 19 (1998) 189. 11. Q.L. Wang, G. Giannetto and M. Guisnet, Zeolites, 10 (1990) 301.

470 12. A.P. Matharau, L.F. Gladden and S.W. Carr, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 94 (1995) 147. 13. A. Gola, B. Rebouis, E. Milazzo, J. Lynch, E. Benazzi, S. Lacombe, L. Delevoye and C. Fernandez, Microporous Mesoporous Mater., 40 (2000) 73. 14. H. Ajot, J.F. Joly, J. Lynch, F. Raatz and P. Caullet, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 62 (1991) 583. 15. A.V. Abramova, E.V. Slivinskii and E.A. Skryleva, Kinet. Katal., 39 (1998) 411. 16. J.A. Lercher, C. Cmindling and G. Eder-Mirth, Catal. Today, 27 (1996) 353. 17. T. Barzetti, E. Selli, D. Moscotti and L. Forni, J. Chem. Soc., Faraday Trans., 92 (1996) 1401. 18. G. Zi and T. Yi, Zeolites, 8 (1988) 232. 19. T. Masuda, Y. Fujiyata, H. Ikeda, S-I. Matsushita and K. Hashimoto, Appl. Catal. A, 162 (1997) 29. 20. A. Macedo, F. Raatz, A. Boulet, A. Janin and J.C. Lavalley, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 37 (1987) 375. 21. H.G. Karge and V. Dondur, J. Phys. Chem., 94 (1990) 765. 22. C.S. Triantafillidis, A.G. Vlessidis and N.P. Evmiridis, Ind. Eng. Chem. Res., 39 (2000) 307. 23. B. Chauvin, M. Boulet, P. Massiani, F. Fajula, F. Figueras and T. Des Couri6res, J. Catal., 126 (1990) 532. 24. R. L6pez-Fonseca, J.I. Guti6rrez-Ortiz, A. Aranzabal and J.R. Gonzalez-Velasco, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 135 (2001) 4995. 25. A. Aranzabal, J.A. Gonzhlez-Marcos, R. L6pez-Fonseca, M. A. Guti6rrez-Ortiz and J.R. Gonzhlez-Velasco, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 130 (2000) 1229. 26. R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz, J.I. Alvarez-Uriarte and J.R. Gonzhlez-Velasco, Appl. Catal. B, 30 (2001) 303. 27. A. Aranzabal, R. L6pez-Fonseca, J.R. Gonzhlez-Velasco, J.I. Guti6rrez-Ortiz, M.A. Guti6rrez-Ortiz and J.A. Gonz~.lez-Marcos, Abstr. Pap. - 221st Am. Chem. Soc. (2001) CATL-027. 28. H. Greene, D. Prakash, K. Athota, G. Atwood and C. Vogel, Catal. Today, 27 (1996) 289. 29. D.S. Prakash, K.V. Athota, H.L. Greene and C.A. Vogel, AIChE Symp. Ser., 91 (1995) 1.