- Email: [email protected]
Some catalytic properties of hydrothermally synthesised zinc aluminate spinel
Applied Catalysis A: General 210 (2001) 263–269
Some catalytic properties of hydrothermally synthesised zinc aluminate spinel J. Wrzyszcz a,∗ , M. Zawadzki a , J. Trawczy´nski b , H. Grabowska a , W. Mi´sta a a
b
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław 2, Poland Institute of Chemistry and Technology of Petroleum and Coal, Wrocław, University of Technology, 50-344 Wrocław, Gda´nska St. 7/9, Poland Received 7 June 2000; received in revised form 26 September 2000; accepted 28 September 2000
Abstract Two samples of zinc aluminate were hydrothermally synthesised from zinc acetate and different aluminium sources: basic aluminium nitrate or aluminium hydroxide. The textural properties of the prepared ZnAl2 O4 samples are different from these one of the zinc aluminate prepared by conventional way. Powder XRD and TEM measurements reveal that samples are single-phase material or mixture of ZnAl2 O4 with small amount of ␥-Al2 O3 , with morphology of quasi-spherical shape. Catalytic properties of the hydrothermally obtained zinc aluminate and Pt (Pd) catalysts supported on them were investigated in the reactions of cyclohexene isomerisation and combustion of trichloroethylene, respectively. It was evidenced that activity and selectivity of the investigated materials could be qualitatively correlated with the part of the strong acid centres measured by TPD of NH3 . © 2001 Elsevier Science B.V. All rights reserved. Keywords: Zinc aluminate; Catalyst support; Cyclohexene isomerisation; Trichloroethylene combustion
1. Introduction Mixed metal oxides are important class of catalysts widely investigated in different fields of application. Among them, great interest has been focused on the materials with spinel type structure like magnesium or zinc aluminates. They are interesting as catalysts as well as carriers for noble metals to substitute the more conventional systems. Zinc aluminate spinel is known to be active in the synthesis of methanol and selective reduction of NOx , especially with copper addition [1–5]; it dehydrogenates alkanes [6], and doped with platinum, it dehydrocyclises n-heptane to toluene [7]. It could be a promising catalyst for obtaining indenes or styrenes by use of recently described synthesis ∗ Corresponding author. E-mail address: [email protected] (J. Wrzyszcz).
method, which combines a hydrogenation and a dehydration [8]. It is also an interesting material as support for bimetallic Pt-Sn catalysts used in the dehydrogenation of lower molecular weight alkanes [9]. One of disadvantage of ZnAl2 O4 from the catalytic point of view is its low surface area and low pore volume. Zinc aluminate obtained so far by using conventional ceramic processing techniques, coprecipitated methods or impregnating a porous alumina having a high surface area with a solution of zinc compound is a low surface area material, usually about 20–50 m2 /g [10]. Syntheses are also known by the way of controlled hydrolysis of mixed metal alkoxides that lead to the spinel of the specific surface area up to 120 m2 /g [11]. In order to improve the textural properties, a method of hydrothermal synthesis was investigated in our previous work [12]. In the present study we report the properties of hydrothermally obtained zinc
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 2 1 - 8
264
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
aluminate samples from different aluminium sources and the use of this material as catalyst for cyclohexene isomerisation, and Pt (Pd) catalysts supported on them for trichloroethylene (TCE) combustion reactions. 2. Experimental 2.1. Catalyst preparation The aluminium hydroxide and basic aluminium nitrate with the empirical formula Al2 (OH)6−x (NO3 )x , wherein x was close to 1, were used as aluminium containing precursors for the hydrothermal preparation of ZnAl2 O4 . Basic aluminium nitrate was obtained by digesting powdered aluminium metal in aqueous solution of aluminium nitrate at elevated temperatures (348–358 K) for several days. Aluminium hydroxide was produced through hydrolysis of aluminium isopropoxide in excess of water heated at 353 K. Synthesis mixtures were prepared by the controlled addition (with stirring) of zinc acetate solution to an aqueous solution of the basic aluminium nitrate (sample S1) or aluminium hydroxide (sample S2), wherein the molar ratio of Al:Zn was 2:1. The resulting mixtures were treated hydrothermally in autoclave for 3 h at 443 K with stirring. After quenching the autoclave in cold water, the obtained slurry was filtered, water washed and then the resulting product was extruded, dried at 413 K, and finally calcined at 873 K for 4 h. The materials obtained in this way were crushed into particles of 0.6–1.2 mm in size. The prepared carriers S1, S2, and reference sample of ␥-Al2 O3 were impregnated at room temperature by an incipient wetness technique using aqueous solutions of H2 PtCl6 or PdCl2 , respectively. After impregnation, samples were dried in air for 24 h at ambient temperature, then 24 h at 383 K, and finally calcined at 823 K for 4 h. 2.2. Characterisation Phase composition and mean crystallite sizes of the prepared materials were determined by X-ray powder diffraction (XRPD) method. The crystallite size of spinel phase was calculated from the broadening of the X-ray line (3 1 1) using Schererr’s formula. The XRD was carried out with an X-ray diffractometer DRON-3, using Ni-filtered Cu K␣ radiation. The lat-
tice parameters were calculated using the least squares fitting program PROSZKI [13]. The particle sizes and morphology of the catalysts were estimated from TEM images taken with a TESLA BS-500 transmission electron microscope (TEM) using an accelerating voltage of 90 kV. The surface area and porosity were determined by the BET and Dollim–Heal methods, respectively, from nitrogen adsorption data obtained at 77 K by standard volumetric method using a Sorptomatic 1900 FISONS instrument. The concentration of the Lewis centres on the sample surface was measured by means of IR spectroscopy according to procedure described in literature [14,15] using pyridine as a test molecule. IR spectra were recorded at room temperature with SPECORD M80 grating spectrometer. Self-supporting wafers (ca. 10 mg) were calcined in air at 773 K in the IR cell. The vacuum proof cell was evacuated and wafers were exposed to pyridine vapours (T = 298 K, p = 939 Pa, time = 30 min), then evacuated at 423 K for 90 min. The speed of recording was 2 cm for 3 s, which corresponds to a resolution of 2 cm−1 at 3600 cm−1 . Acidity type and acidity strength distribution of the investigated materials was evaluated by the temperature-programmed desorption of ammonia (NH3 TPD) method according to the following procedure: heating of the sample (∼2 g) at 823 K under argon atmosphere during 1 h, cooling in the argon stream to the temperature of 453 K, adsorption of pure ammonia at 453 K for 0.5 h, purging with argon at 453 K for 1 h in order to remove physically adsorbed ammonia, TPD measurement (argon stream, heating rate of 10 K/min, temperature range from 453 to 823 K). Acidic properties of the prepared zinc aluminate samples were also determined in the reaction with cyclohexene isomerisation. Following reactions proceed simultaneously: isomerisation to five-membered ring, opening of the ring and cracking of the ring. Tests were carried out in the pulse microreactor connected with GC. Before test, samples were reduced under hydrogen stream (9 dm3 /h) at 823 K for 5 h. Conditions of cyclohexene test are as follows: Temperature range Hydrogen flow Catalyst volume
533–653 K 4.8 dm3 /h 1 cm3
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
265
Tests of the catalytic activity in the reaction of trichloroethylene (TCE) combustion were carried out in a flow microreactor fed with mixture of air and TCE. Conditions of the test were as follow: Temperature range TCE concentration GHSV Water vapour in air Catalyst weight
523–823 K 2400 ppm 30 000 h−1 2.6 vol.% 0.3 g
Concentration of the TCE in the reaction products was determined with using GC. Properties of the prepared samples of zinc aluminate were compared to the properties of an alumina commonly known as catalyst support. Sample of ␥-Al2 O3 supplied by Hydromet Ltd. (Kowary, Poland) was used in this investigation.
3. Results and discussion The morphology of ZnAl2 O4 (S1 and S2 samples) as received from the hydrothermal synthesis is shown in Fig. 1. In the case of the both samples, the morphology is quite uniform — they consist of approximate spherical particles with the structure of zinc aluminate spinel as indicated by electron diffraction patterns and also confirmed by XRD. However, it can be seen that particles of sample S1 are noticeably smaller and there are also small amounts of fibrous particles present, which probably are boehmite. The average size of crystallites is in good agreement with that one obtained from XRD pattern. Some properties of both prepared samples of ZnAl2 O4 and the reference ␥-Al2 O3 are given in Table 1. As evident from XRD analysis, the dried
Fig. 1. XRD patterns of zinc aluminate S1 sample: (a) as-prepared; (b) calcined at 873 K; (c) calcined at 1473 K; (∗) ␣-Al2 O3 .
samples of S1 and S2 are characterised by high crystallite dispersion — the average crystallite size was 4.5 and 7.5 nm for S1 and S2 sample, respectively. Calcination of samples at 873 K leads to their recrystallisation and increase in the average crystallite size, is noticeable. After the thermal treatment at higher temperatures subsequent increase in the degree of crystallinity and in the crystallite size was observed. In the case of sample S2 only one phase is detected along while small amount of ␣-Al2 O3 are also
Table 1 Properties of zinc aluminate samples and reference Al2 O3 Specific surface area SBET (m2 /g) Total pore volume Vp (at p/p0 = 0.95) (cm3 /g) Mean pore radius rp (nm) Phase composition: as-prepared After calcination at 873 K for 4 h After calcination at 1423 K for 4 h Mean crystallite size of ZnAl2 O4 after calcination at 873 K for 4 h (nm) Lattice constant of ZnAl2 O4 heated at 1423 K for 4 h (Å)
S1
S2
Al2 O3
184 0.25 2.1 ZnAl2 O4 ZnAl2 O4 ZnAl2 O4 + ␣-Al2 O3 5.4 8.092 (±0.005)
88 0.34 5.3 ZnAl2 O4 ZnAl2 O4 ZnAl2 O4 11.2 8.093 (±0.005)
216 0.62 4.0 ␥-Al2 O3 ␣-Al2 O3
266
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
Fig. 2. TEM micrographs of S1 and S2 samples of ZnAl2 O4 .
noticed in the S1 sample after its calcination at 1423 K (Fig. 2). It may be assumed that in the S1 sample (calcined of 873 K), ␥-Al2 O3 phase occurs besides the zinc aluminate spinel, however its presence could not have been established unequivocally, because of relatively high background of the diffractogram (Fig. 2). The textural properties of the hydrothermally obtained zinc aluminate distinctly depend on the type of the aluminium containing raw material used for synthesis. The pore size distribution analysis of samples
calcined at 873 K showed pore radius between 1 and 3 nm for S1 sample and 2 and 8 nm for S2 sample. These narrow size ranges suggest that hydrothermally obtained zinc aluminate samples are formed from monodispersed particles. The S1 sample is characterised by a higher surface area and a smaller mean pore radius than the S2 sample what may be correlated with the smaller crystallite size in the S1 sample. It seems that the differences in the texture of the both investigated samples of zinc aluminate are due to the different precursor used for the preparations.
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
It can be supposed that zinc aluminate samples were produced in the reaction between highly solvated aluminium and zinc hydroxides under hydrothermal conditions. However, in the case of the sample S1 — aluminium hydroxide was supplied “indirectly” via thermal hydrolysis of basic aluminium nitrate. It can be assumed that polynuclear complexes of aluminium hydroxide obtained in such way, are small and well dispersed and they react to the zinc aluminate. Therefore, they give the material composed of the smaller particles, which exhibits higher specific surface area than sample S2 produced “directly” from aluminium hydroxide. Probably the S2 sample of zinc aluminate was obtained from larger micelles of aluminium hydroxide (and poorer dispersed) than it was in the case of sample S1 produced from “indirectly” obtained aluminium hydroxide. Acid properties of the prepared materials were investigated using different methods. It was established by means of IR spectroscopy, that on the surface of both samples there are no acidic centres of Brönsted type; the concentration of Lewis acid centres were measured to be equal 315 and 120 mol Py/g for sample S1 and S2, respectively. Total acidity of samples evaluated by the NH3 TPD measurements was 0.67 mmol NH3 /g for S1, 0.52 mmol NH3 /g for S2, and 0.46 mmol NH3 /g for
267
Al2 O3 . Acidity strength distribution of samples is given on Fig. 3. The both investigated samples of zinc aluminate possess somewhat higher total acidity (especially the S1) than alumina does as well as they are characterised by higher (than alumina) part of the strong acid centres (T > 773 K) and lower part of the medium acid centres (T = 573–673 K). Zinc aluminate prepared via basic aluminium nitrate exhibits higher total acidity than this one prepared via aluminium hydroxide. The small differences in total acidity of the zinc aluminate samples can be correlated with the content of the ␥-Al2 O3 traces. Small amounts of ␥-Al2 O3 in S1 sample can lead to the higher total acidity of this material in comparison to S2 sample. On the other hand, when we recount acidity on the specific surface area, it appears that concentration of Lewis acid centres for both samples are close (1.7 and 1.4 mol Py/m2 for S1 and S2, respectively) and total acidity measured by TPD of ammonia are ranged opposite (3.6 and 5.9 mmol NH3 /m2 ) than in the case of acidity counted on weight basis. So observed differences in the catalytic properties of the investigated samples should be correlated rather with the strength of the their acid centres than with their total acidity. Results reported above concerning the measurements acidity of ZnAl2 O4 by ammonia TPD are indirectly confirmed by the results of the measurements
Fig. 3. Contribution of acid centres of equal strength (NH3 TPD method).
268
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
Table 2 Results of the cyclohexene test Sample
Temperature (K)
Conversion (%)
Selectivity (%) Cyclo-C5
P
(n-C6 + iso-C6 )
S1
533 573 613 653
13.7 38.4 49.1 75.6a
96 92 52 28
4 8 48 71
S2
533 573 613 653
8.6 20.8 22.1 31.5
100 95 80 59
0 5 20 41
Al2 O3
533 573 613 653
0 16.2 33.4 56.2
– 100 96 88
– 0 4 12
a
Traces of cracking products.
of catalytic activity in cyclohexene test (Table 2). By this way additional information about the strength of acid centres were obtained. Products composition obtained during reaction studied indicates that three types of reaction proceed simultaneously: isomerisation to five-membered ring, opening of the ring to P hexenes ( C6 ) and cracking. The activities of the Al2 O3 and both zinc aluminate samples change with the reaction temperature in broad range in a different way. Although alumina exhibits quitePa high cyclohexene conversion, its selectivity for C6 (opening of the cyclohexene ring) is the lowest among all investigated carriers. The S1 sample of zinc aluminate is the most active in the cyclohexene conversion; the S2 sample exhibits the lowest activity. It seems that results of the cyclohexene test of the investigated samples can be correlated with differences in their
surface area (or with differences in their total acidity per weight). The highest activity exhibits sample S1 which possess high surface area and exhibits the highest total acidity. On the other hand sample S2 (low surface area and low total acidity) exhibits low activity in the cyclohexene reaction. Used in this work reference alumina sample possess the highest surface area, however it also has the lowest total acidity. Activity of this material distinctly differs from the activity of both investigated samples of zinc aluminate and strongly depends on the reaction temperature. On the other hand selectivities of the both samples of zinc aluminate are very close and they distinctly differ from the selectivity of the reference alumina. The reaction of the cyclohexene ring opening in the biggest degree proceeds on the zinc aluminate. Skeletal isomerisation of cyclohexene proceeds only on catalysts having strong acid centres [16]. According to this it can be concluded that zinc aluminate is characterised by the more acidic acid centres (which are able to open cyclohexene ring) than alumina. Rough correlation between part of the strong acid centres P andP yield of C6 seems to be evident. Selectivity for C6 is the highest on the S1 (the biggest part of the centres T > 773 K) and the lowest on alumina (the smallest part of these centres). According to this it may be suggested that reaction of the cyclohexene ring opening proceeds mainly on the acid centres related with ammonia desorption at 773–823 K. Catalytic activity of the zinc aluminates has been also investigated in the combustion of TCE. This reaction requires a catalyst having resistance against destruction and poisoning by products of the reaction by hydrogen chloride and chlorine [17]. It was stated that under conditions used in the work selectivity of TCE combustion to HCl varies from 92 to 98% and
Table 3 Results of the TCE combustion Temperature (K) 523 573 623 673 723 773 823
TCE conversion (%) S1
Al2 O3
Pt/S1
Pt/S2
Pd/S2
Pt/Al2 O3
– – 2.0 10.2 22.5 41.2 53.2
– – – 8.5 23.0 37.0 50.5
– – 7.5 24.5 33.5 51.5 84.0
– 2.0 16.0 28.5 43.0 57.5 89.5
– 14.0 27.8 41.5 49.5 60.0 81.0
5.0 18.0 32.0 49.0 62.5 86.5 98.0
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
to Cl2 from 0 to 4%. Some sub-products (like CHCl3 , CCl4 and C2 Cl4 ) are formed in amounts, which depends on the temperature of the catalytic bed. From the results presented in the Table 3, it is seen that alumina supported Pt catalyst gives the highest TCE conversion among all tested catalysts. Pt catalysts supported on the both samples of zinc aluminate possess similar activity in TCE combustion, although it seems that Pt/S2 is somewhat more active (reaction starts at lower temperature) than Pt/S1. It should be pointed out that palladium-based catalyst start at lower temperature than Pt-based one, however the latter catalyst becomes more active at high temperature. According to Feijen-Jeurissen et al. [18], a basic oxygen and hydrogen according to Markovinkov’s rule attack TCE. In this way oxide with strong Me–O bonds is not an appropriate catalyst support for TCE combustion — it is not a good oxygen-donating agent. This is the reason why catalysts supported on the carrier with stronger acidic surface (zinc aluminates) than alumina exhibit lower activity in the TCE combustion than Pt/Al2 O3 does.
4. Conclusions Textural properties of the hydrothermally prepared zinc aluminate spinel are close to the properties of the alumina and are different from the properties of the zinc aluminate prepared by conventional way. Surface area as well as mean pore radius of hydrothermal zinc aluminate can be controlled by the use of various aluminium compounds and zinc salt for the synthesis at relatively low temperature. Powder XRD and TEM measurements confirm that as-prepared samples are single-phase material or a mixture of zinc aluminate with small amount of ␥-Al2 O3 . Applied method of synthesis, in contrast to other methods, does not require the high temperature calcination to obtain the spinel phase and provides an attractive alternative to method of zinc aluminate preparation. Catalytic properties of the hydrothermal zinc aluminate as well as Pt (Pd) catalysts supported on them in
269
the reactions of cyclohexene isomerisation and TCE combustion can be qualitatively correlated with the part of the strong acid centres measured by TPD of NH3 . Acknowledgements This work was supported by the Polish State Committee for Scientific Research under research project No. 3T09B06316. References [1] F. Le Peltier, P. Chaumette, J. Saussey, M.M. Bettahar, J.C. Lavalley, J. Mol. Catal. A: Chem. 122 (1997) 131. [2] P. Chaumette, Ph. Coutry, J. Barbier, T. Fortin, J.C. Lavalley, C. Chauvin, A. Kinnemann, H. Idriss, R.P.A. Sneeden, B. Denise, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalysis, The Chemical Institute of Canada, Calgary, 1988, p. 585. [3] A. Kinnemann, H. Idriss, J.P. Hindermann, J.C. Lavalley, A. Vallet, P. Chaumette, Ph. Coutry, Appl. Catal. 59 (1990) 165. [4] J.C.J. Bart, R.P.A. Sneeden, Catal. Today 2 (1987) 1. [5] C. Kienle, C. Schinzer, J. Lentmaier, O. Schaal, S. KemmlerSack, Mater. Chem. Phys. 49 (1997) 211. [6] M.A. Valenzuela, G. Aguilar, P. Bosch, H. Armendariz, P. Salas, A. Montoya, Catal. Lett. 15 (1992) 179. [7] M.A. Valenzuela, P. Bosch, G. Aguilar-Rios, B. Zapata, C. Maldonado, I. Schifer, J. Mol. Catal. 84 (1993) 177. [8] R. Roesky, J. Weigury, H. Bestgen, U. Dingerdissen, Appl. Catal. A: Gen. 176 (1999) 213. [9] H. Miura, I. Takashi, React. Kinet. Catal. Lett. 66 (1999) 189. [10] M.A. Valenzuela, J.P. Jacobs, P. Bosch, S. Reijne, B. Zapata, H.H. Brongersma, Appl. Catal. A: Gen. 148 (1997) 315. [11] C.O. Arean, B.S. Sintes, G.T. Palomino, C.M. Carbonell, E.E. Platero, J.B.P. Soto, Microporous Mater. 8 (1997) 187. [12] M. Zawadzki, J. Wrzyszcz, Mater. Res. Bull. 35 (2000) 109. [13] W. Łasocha, K. Lewi´nski, J. Appl. Cryst. 27 (1994) 437. [14] L. Petrakis, F.E. Kiviat, J. Phys. Chem. 80 (1976) 606. [15] J. Datka, J. Chem. Soc., Faraday Trans. 1 (1981) 2877. [16] E.A. Irvine, C.S. John, C. Kemball, A.J. Pearman, M.A. Day, R.J. Sampson, J. Catal. 61 (1980) 326. [17] A.M. Padilla, J. Corella, J.M. Toledo, Appl. Catal. B: Environ. 22 (1999) 107. [18] M.M.R. Feijen-Jeurissen, J.J. Jorna, B.E. Nieuwenhuys, G. Sinquin, C. Petit, J.-P. Hindermann, Catal. Today 54 (1999) 65.
Some catalytic properties of hydrothermally synthesised zinc aluminate spinel J. Wrzyszcz a,∗ , M. Zawadzki a , J. Trawczy´nski b , H. Grabowska a , W. Mi´sta a a
b
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław 2, Poland Institute of Chemistry and Technology of Petroleum and Coal, Wrocław, University of Technology, 50-344 Wrocław, Gda´nska St. 7/9, Poland Received 7 June 2000; received in revised form 26 September 2000; accepted 28 September 2000
Abstract Two samples of zinc aluminate were hydrothermally synthesised from zinc acetate and different aluminium sources: basic aluminium nitrate or aluminium hydroxide. The textural properties of the prepared ZnAl2 O4 samples are different from these one of the zinc aluminate prepared by conventional way. Powder XRD and TEM measurements reveal that samples are single-phase material or mixture of ZnAl2 O4 with small amount of ␥-Al2 O3 , with morphology of quasi-spherical shape. Catalytic properties of the hydrothermally obtained zinc aluminate and Pt (Pd) catalysts supported on them were investigated in the reactions of cyclohexene isomerisation and combustion of trichloroethylene, respectively. It was evidenced that activity and selectivity of the investigated materials could be qualitatively correlated with the part of the strong acid centres measured by TPD of NH3 . © 2001 Elsevier Science B.V. All rights reserved. Keywords: Zinc aluminate; Catalyst support; Cyclohexene isomerisation; Trichloroethylene combustion
1. Introduction Mixed metal oxides are important class of catalysts widely investigated in different fields of application. Among them, great interest has been focused on the materials with spinel type structure like magnesium or zinc aluminates. They are interesting as catalysts as well as carriers for noble metals to substitute the more conventional systems. Zinc aluminate spinel is known to be active in the synthesis of methanol and selective reduction of NOx , especially with copper addition [1–5]; it dehydrogenates alkanes [6], and doped with platinum, it dehydrocyclises n-heptane to toluene [7]. It could be a promising catalyst for obtaining indenes or styrenes by use of recently described synthesis ∗ Corresponding author. E-mail address: [email protected] (J. Wrzyszcz).
method, which combines a hydrogenation and a dehydration [8]. It is also an interesting material as support for bimetallic Pt-Sn catalysts used in the dehydrogenation of lower molecular weight alkanes [9]. One of disadvantage of ZnAl2 O4 from the catalytic point of view is its low surface area and low pore volume. Zinc aluminate obtained so far by using conventional ceramic processing techniques, coprecipitated methods or impregnating a porous alumina having a high surface area with a solution of zinc compound is a low surface area material, usually about 20–50 m2 /g [10]. Syntheses are also known by the way of controlled hydrolysis of mixed metal alkoxides that lead to the spinel of the specific surface area up to 120 m2 /g [11]. In order to improve the textural properties, a method of hydrothermal synthesis was investigated in our previous work [12]. In the present study we report the properties of hydrothermally obtained zinc
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 2 1 - 8
264
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
aluminate samples from different aluminium sources and the use of this material as catalyst for cyclohexene isomerisation, and Pt (Pd) catalysts supported on them for trichloroethylene (TCE) combustion reactions. 2. Experimental 2.1. Catalyst preparation The aluminium hydroxide and basic aluminium nitrate with the empirical formula Al2 (OH)6−x (NO3 )x , wherein x was close to 1, were used as aluminium containing precursors for the hydrothermal preparation of ZnAl2 O4 . Basic aluminium nitrate was obtained by digesting powdered aluminium metal in aqueous solution of aluminium nitrate at elevated temperatures (348–358 K) for several days. Aluminium hydroxide was produced through hydrolysis of aluminium isopropoxide in excess of water heated at 353 K. Synthesis mixtures were prepared by the controlled addition (with stirring) of zinc acetate solution to an aqueous solution of the basic aluminium nitrate (sample S1) or aluminium hydroxide (sample S2), wherein the molar ratio of Al:Zn was 2:1. The resulting mixtures were treated hydrothermally in autoclave for 3 h at 443 K with stirring. After quenching the autoclave in cold water, the obtained slurry was filtered, water washed and then the resulting product was extruded, dried at 413 K, and finally calcined at 873 K for 4 h. The materials obtained in this way were crushed into particles of 0.6–1.2 mm in size. The prepared carriers S1, S2, and reference sample of ␥-Al2 O3 were impregnated at room temperature by an incipient wetness technique using aqueous solutions of H2 PtCl6 or PdCl2 , respectively. After impregnation, samples were dried in air for 24 h at ambient temperature, then 24 h at 383 K, and finally calcined at 823 K for 4 h. 2.2. Characterisation Phase composition and mean crystallite sizes of the prepared materials were determined by X-ray powder diffraction (XRPD) method. The crystallite size of spinel phase was calculated from the broadening of the X-ray line (3 1 1) using Schererr’s formula. The XRD was carried out with an X-ray diffractometer DRON-3, using Ni-filtered Cu K␣ radiation. The lat-
tice parameters were calculated using the least squares fitting program PROSZKI [13]. The particle sizes and morphology of the catalysts were estimated from TEM images taken with a TESLA BS-500 transmission electron microscope (TEM) using an accelerating voltage of 90 kV. The surface area and porosity were determined by the BET and Dollim–Heal methods, respectively, from nitrogen adsorption data obtained at 77 K by standard volumetric method using a Sorptomatic 1900 FISONS instrument. The concentration of the Lewis centres on the sample surface was measured by means of IR spectroscopy according to procedure described in literature [14,15] using pyridine as a test molecule. IR spectra were recorded at room temperature with SPECORD M80 grating spectrometer. Self-supporting wafers (ca. 10 mg) were calcined in air at 773 K in the IR cell. The vacuum proof cell was evacuated and wafers were exposed to pyridine vapours (T = 298 K, p = 939 Pa, time = 30 min), then evacuated at 423 K for 90 min. The speed of recording was 2 cm for 3 s, which corresponds to a resolution of 2 cm−1 at 3600 cm−1 . Acidity type and acidity strength distribution of the investigated materials was evaluated by the temperature-programmed desorption of ammonia (NH3 TPD) method according to the following procedure: heating of the sample (∼2 g) at 823 K under argon atmosphere during 1 h, cooling in the argon stream to the temperature of 453 K, adsorption of pure ammonia at 453 K for 0.5 h, purging with argon at 453 K for 1 h in order to remove physically adsorbed ammonia, TPD measurement (argon stream, heating rate of 10 K/min, temperature range from 453 to 823 K). Acidic properties of the prepared zinc aluminate samples were also determined in the reaction with cyclohexene isomerisation. Following reactions proceed simultaneously: isomerisation to five-membered ring, opening of the ring and cracking of the ring. Tests were carried out in the pulse microreactor connected with GC. Before test, samples were reduced under hydrogen stream (9 dm3 /h) at 823 K for 5 h. Conditions of cyclohexene test are as follows: Temperature range Hydrogen flow Catalyst volume
533–653 K 4.8 dm3 /h 1 cm3
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
265
Tests of the catalytic activity in the reaction of trichloroethylene (TCE) combustion were carried out in a flow microreactor fed with mixture of air and TCE. Conditions of the test were as follow: Temperature range TCE concentration GHSV Water vapour in air Catalyst weight
523–823 K 2400 ppm 30 000 h−1 2.6 vol.% 0.3 g
Concentration of the TCE in the reaction products was determined with using GC. Properties of the prepared samples of zinc aluminate were compared to the properties of an alumina commonly known as catalyst support. Sample of ␥-Al2 O3 supplied by Hydromet Ltd. (Kowary, Poland) was used in this investigation.
3. Results and discussion The morphology of ZnAl2 O4 (S1 and S2 samples) as received from the hydrothermal synthesis is shown in Fig. 1. In the case of the both samples, the morphology is quite uniform — they consist of approximate spherical particles with the structure of zinc aluminate spinel as indicated by electron diffraction patterns and also confirmed by XRD. However, it can be seen that particles of sample S1 are noticeably smaller and there are also small amounts of fibrous particles present, which probably are boehmite. The average size of crystallites is in good agreement with that one obtained from XRD pattern. Some properties of both prepared samples of ZnAl2 O4 and the reference ␥-Al2 O3 are given in Table 1. As evident from XRD analysis, the dried
Fig. 1. XRD patterns of zinc aluminate S1 sample: (a) as-prepared; (b) calcined at 873 K; (c) calcined at 1473 K; (∗) ␣-Al2 O3 .
samples of S1 and S2 are characterised by high crystallite dispersion — the average crystallite size was 4.5 and 7.5 nm for S1 and S2 sample, respectively. Calcination of samples at 873 K leads to their recrystallisation and increase in the average crystallite size, is noticeable. After the thermal treatment at higher temperatures subsequent increase in the degree of crystallinity and in the crystallite size was observed. In the case of sample S2 only one phase is detected along while small amount of ␣-Al2 O3 are also
Table 1 Properties of zinc aluminate samples and reference Al2 O3 Specific surface area SBET (m2 /g) Total pore volume Vp (at p/p0 = 0.95) (cm3 /g) Mean pore radius rp (nm) Phase composition: as-prepared After calcination at 873 K for 4 h After calcination at 1423 K for 4 h Mean crystallite size of ZnAl2 O4 after calcination at 873 K for 4 h (nm) Lattice constant of ZnAl2 O4 heated at 1423 K for 4 h (Å)
S1
S2
Al2 O3
184 0.25 2.1 ZnAl2 O4 ZnAl2 O4 ZnAl2 O4 + ␣-Al2 O3 5.4 8.092 (±0.005)
88 0.34 5.3 ZnAl2 O4 ZnAl2 O4 ZnAl2 O4 11.2 8.093 (±0.005)
216 0.62 4.0 ␥-Al2 O3 ␣-Al2 O3
266
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
Fig. 2. TEM micrographs of S1 and S2 samples of ZnAl2 O4 .
noticed in the S1 sample after its calcination at 1423 K (Fig. 2). It may be assumed that in the S1 sample (calcined of 873 K), ␥-Al2 O3 phase occurs besides the zinc aluminate spinel, however its presence could not have been established unequivocally, because of relatively high background of the diffractogram (Fig. 2). The textural properties of the hydrothermally obtained zinc aluminate distinctly depend on the type of the aluminium containing raw material used for synthesis. The pore size distribution analysis of samples
calcined at 873 K showed pore radius between 1 and 3 nm for S1 sample and 2 and 8 nm for S2 sample. These narrow size ranges suggest that hydrothermally obtained zinc aluminate samples are formed from monodispersed particles. The S1 sample is characterised by a higher surface area and a smaller mean pore radius than the S2 sample what may be correlated with the smaller crystallite size in the S1 sample. It seems that the differences in the texture of the both investigated samples of zinc aluminate are due to the different precursor used for the preparations.
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
It can be supposed that zinc aluminate samples were produced in the reaction between highly solvated aluminium and zinc hydroxides under hydrothermal conditions. However, in the case of the sample S1 — aluminium hydroxide was supplied “indirectly” via thermal hydrolysis of basic aluminium nitrate. It can be assumed that polynuclear complexes of aluminium hydroxide obtained in such way, are small and well dispersed and they react to the zinc aluminate. Therefore, they give the material composed of the smaller particles, which exhibits higher specific surface area than sample S2 produced “directly” from aluminium hydroxide. Probably the S2 sample of zinc aluminate was obtained from larger micelles of aluminium hydroxide (and poorer dispersed) than it was in the case of sample S1 produced from “indirectly” obtained aluminium hydroxide. Acid properties of the prepared materials were investigated using different methods. It was established by means of IR spectroscopy, that on the surface of both samples there are no acidic centres of Brönsted type; the concentration of Lewis acid centres were measured to be equal 315 and 120 mol Py/g for sample S1 and S2, respectively. Total acidity of samples evaluated by the NH3 TPD measurements was 0.67 mmol NH3 /g for S1, 0.52 mmol NH3 /g for S2, and 0.46 mmol NH3 /g for
267
Al2 O3 . Acidity strength distribution of samples is given on Fig. 3. The both investigated samples of zinc aluminate possess somewhat higher total acidity (especially the S1) than alumina does as well as they are characterised by higher (than alumina) part of the strong acid centres (T > 773 K) and lower part of the medium acid centres (T = 573–673 K). Zinc aluminate prepared via basic aluminium nitrate exhibits higher total acidity than this one prepared via aluminium hydroxide. The small differences in total acidity of the zinc aluminate samples can be correlated with the content of the ␥-Al2 O3 traces. Small amounts of ␥-Al2 O3 in S1 sample can lead to the higher total acidity of this material in comparison to S2 sample. On the other hand, when we recount acidity on the specific surface area, it appears that concentration of Lewis acid centres for both samples are close (1.7 and 1.4 mol Py/m2 for S1 and S2, respectively) and total acidity measured by TPD of ammonia are ranged opposite (3.6 and 5.9 mmol NH3 /m2 ) than in the case of acidity counted on weight basis. So observed differences in the catalytic properties of the investigated samples should be correlated rather with the strength of the their acid centres than with their total acidity. Results reported above concerning the measurements acidity of ZnAl2 O4 by ammonia TPD are indirectly confirmed by the results of the measurements
Fig. 3. Contribution of acid centres of equal strength (NH3 TPD method).
268
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
Table 2 Results of the cyclohexene test Sample
Temperature (K)
Conversion (%)
Selectivity (%) Cyclo-C5
P
(n-C6 + iso-C6 )
S1
533 573 613 653
13.7 38.4 49.1 75.6a
96 92 52 28
4 8 48 71
S2
533 573 613 653
8.6 20.8 22.1 31.5
100 95 80 59
0 5 20 41
Al2 O3
533 573 613 653
0 16.2 33.4 56.2
– 100 96 88
– 0 4 12
a
Traces of cracking products.
of catalytic activity in cyclohexene test (Table 2). By this way additional information about the strength of acid centres were obtained. Products composition obtained during reaction studied indicates that three types of reaction proceed simultaneously: isomerisation to five-membered ring, opening of the ring to P hexenes ( C6 ) and cracking. The activities of the Al2 O3 and both zinc aluminate samples change with the reaction temperature in broad range in a different way. Although alumina exhibits quitePa high cyclohexene conversion, its selectivity for C6 (opening of the cyclohexene ring) is the lowest among all investigated carriers. The S1 sample of zinc aluminate is the most active in the cyclohexene conversion; the S2 sample exhibits the lowest activity. It seems that results of the cyclohexene test of the investigated samples can be correlated with differences in their
surface area (or with differences in their total acidity per weight). The highest activity exhibits sample S1 which possess high surface area and exhibits the highest total acidity. On the other hand sample S2 (low surface area and low total acidity) exhibits low activity in the cyclohexene reaction. Used in this work reference alumina sample possess the highest surface area, however it also has the lowest total acidity. Activity of this material distinctly differs from the activity of both investigated samples of zinc aluminate and strongly depends on the reaction temperature. On the other hand selectivities of the both samples of zinc aluminate are very close and they distinctly differ from the selectivity of the reference alumina. The reaction of the cyclohexene ring opening in the biggest degree proceeds on the zinc aluminate. Skeletal isomerisation of cyclohexene proceeds only on catalysts having strong acid centres [16]. According to this it can be concluded that zinc aluminate is characterised by the more acidic acid centres (which are able to open cyclohexene ring) than alumina. Rough correlation between part of the strong acid centres P andP yield of C6 seems to be evident. Selectivity for C6 is the highest on the S1 (the biggest part of the centres T > 773 K) and the lowest on alumina (the smallest part of these centres). According to this it may be suggested that reaction of the cyclohexene ring opening proceeds mainly on the acid centres related with ammonia desorption at 773–823 K. Catalytic activity of the zinc aluminates has been also investigated in the combustion of TCE. This reaction requires a catalyst having resistance against destruction and poisoning by products of the reaction by hydrogen chloride and chlorine [17]. It was stated that under conditions used in the work selectivity of TCE combustion to HCl varies from 92 to 98% and
Table 3 Results of the TCE combustion Temperature (K) 523 573 623 673 723 773 823
TCE conversion (%) S1
Al2 O3
Pt/S1
Pt/S2
Pd/S2
Pt/Al2 O3
– – 2.0 10.2 22.5 41.2 53.2
– – – 8.5 23.0 37.0 50.5
– – 7.5 24.5 33.5 51.5 84.0
– 2.0 16.0 28.5 43.0 57.5 89.5
– 14.0 27.8 41.5 49.5 60.0 81.0
5.0 18.0 32.0 49.0 62.5 86.5 98.0
J. Wrzyszcz et al. / Applied Catalysis A: General 210 (2001) 263–269
to Cl2 from 0 to 4%. Some sub-products (like CHCl3 , CCl4 and C2 Cl4 ) are formed in amounts, which depends on the temperature of the catalytic bed. From the results presented in the Table 3, it is seen that alumina supported Pt catalyst gives the highest TCE conversion among all tested catalysts. Pt catalysts supported on the both samples of zinc aluminate possess similar activity in TCE combustion, although it seems that Pt/S2 is somewhat more active (reaction starts at lower temperature) than Pt/S1. It should be pointed out that palladium-based catalyst start at lower temperature than Pt-based one, however the latter catalyst becomes more active at high temperature. According to Feijen-Jeurissen et al. [18], a basic oxygen and hydrogen according to Markovinkov’s rule attack TCE. In this way oxide with strong Me–O bonds is not an appropriate catalyst support for TCE combustion — it is not a good oxygen-donating agent. This is the reason why catalysts supported on the carrier with stronger acidic surface (zinc aluminates) than alumina exhibit lower activity in the TCE combustion than Pt/Al2 O3 does.
4. Conclusions Textural properties of the hydrothermally prepared zinc aluminate spinel are close to the properties of the alumina and are different from the properties of the zinc aluminate prepared by conventional way. Surface area as well as mean pore radius of hydrothermal zinc aluminate can be controlled by the use of various aluminium compounds and zinc salt for the synthesis at relatively low temperature. Powder XRD and TEM measurements confirm that as-prepared samples are single-phase material or a mixture of zinc aluminate with small amount of ␥-Al2 O3 . Applied method of synthesis, in contrast to other methods, does not require the high temperature calcination to obtain the spinel phase and provides an attractive alternative to method of zinc aluminate preparation. Catalytic properties of the hydrothermal zinc aluminate as well as Pt (Pd) catalysts supported on them in
269
the reactions of cyclohexene isomerisation and TCE combustion can be qualitatively correlated with the part of the strong acid centres measured by TPD of NH3 . Acknowledgements This work was supported by the Polish State Committee for Scientific Research under research project No. 3T09B06316. References [1] F. Le Peltier, P. Chaumette, J. Saussey, M.M. Bettahar, J.C. Lavalley, J. Mol. Catal. A: Chem. 122 (1997) 131. [2] P. Chaumette, Ph. Coutry, J. Barbier, T. Fortin, J.C. Lavalley, C. Chauvin, A. Kinnemann, H. Idriss, R.P.A. Sneeden, B. Denise, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalysis, The Chemical Institute of Canada, Calgary, 1988, p. 585. [3] A. Kinnemann, H. Idriss, J.P. Hindermann, J.C. Lavalley, A. Vallet, P. Chaumette, Ph. Coutry, Appl. Catal. 59 (1990) 165. [4] J.C.J. Bart, R.P.A. Sneeden, Catal. Today 2 (1987) 1. [5] C. Kienle, C. Schinzer, J. Lentmaier, O. Schaal, S. KemmlerSack, Mater. Chem. Phys. 49 (1997) 211. [6] M.A. Valenzuela, G. Aguilar, P. Bosch, H. Armendariz, P. Salas, A. Montoya, Catal. Lett. 15 (1992) 179. [7] M.A. Valenzuela, P. Bosch, G. Aguilar-Rios, B. Zapata, C. Maldonado, I. Schifer, J. Mol. Catal. 84 (1993) 177. [8] R. Roesky, J. Weigury, H. Bestgen, U. Dingerdissen, Appl. Catal. A: Gen. 176 (1999) 213. [9] H. Miura, I. Takashi, React. Kinet. Catal. Lett. 66 (1999) 189. [10] M.A. Valenzuela, J.P. Jacobs, P. Bosch, S. Reijne, B. Zapata, H.H. Brongersma, Appl. Catal. A: Gen. 148 (1997) 315. [11] C.O. Arean, B.S. Sintes, G.T. Palomino, C.M. Carbonell, E.E. Platero, J.B.P. Soto, Microporous Mater. 8 (1997) 187. [12] M. Zawadzki, J. Wrzyszcz, Mater. Res. Bull. 35 (2000) 109. [13] W. Łasocha, K. Lewi´nski, J. Appl. Cryst. 27 (1994) 437. [14] L. Petrakis, F.E. Kiviat, J. Phys. Chem. 80 (1976) 606. [15] J. Datka, J. Chem. Soc., Faraday Trans. 1 (1981) 2877. [16] E.A. Irvine, C.S. John, C. Kemball, A.J. Pearman, M.A. Day, R.J. Sampson, J. Catal. 61 (1980) 326. [17] A.M. Padilla, J. Corella, J.M. Toledo, Appl. Catal. B: Environ. 22 (1999) 107. [18] M.M.R. Feijen-Jeurissen, J.J. Jorna, B.E. Nieuwenhuys, G. Sinquin, C. Petit, J.-P. Hindermann, Catal. Today 54 (1999) 65.