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Hydrogenation of benzene on La–Ni and clay supported La–Ni catalysts
Applied Catalysis A: General 175 (1998) 21±31
Hydrogenation of benzene on La±Ni and clay supported La±Ni catalysts A. Louloudi, N. Papayannakos* Department of Chemical Engineering, National Technical University of Athens, Sector II, Heroon Polytechniou 9, GR-157 80 Zografos, Athens, Greece Received 10 February 1998; received in revised form 30 March 1998; accepted 29 May 1998
Abstract Non-supported La±Ni systems and La, Ni systems either supported on an Al-pillared montmorillonite clay or incorporated in a montmorillonite clay were prepared. Their catalytic activity for the hydrogenation of benzene was evaluated over a range of 70±1708C under atmospheric pressure, in a differential bench scale reactor. Surface area measurements and X-ray diffraction were used as tools for characterizing the catalysts. Catalyst initial activities, their performance after prolonged time on stream as well as the clay impact on the supported metals activity were examined. The non-supported La±Ni system calcined at 6008C formed the LaNiO3 perovskite phase and it was the most active catalyst after its reduction. The in¯uence of the mode of catalyst preparation, the calcination temperature and the Ni loading on hydrogenation activity are also discussed. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Benzene hydrogenation; La±Ni catalysts; Pillared clay supports; Catalyst activities
1. Introduction Hydrogenation of aromatic hydrocarbons to saturated cyclic products is of current interest due to both environmental aspects and the wide range of industrial processes involving such reactions. It is well known that the hydrotreating of polyaromatics down to monoaromatics is rather easy. However, the saturation of the ®nal ring is dif®cult because of resonance stabilization of the monoaromatic ring. The benzene hydrogenation becomes, thus, a model reaction for testing hydrogenation catalysts. Furthermore, this reaction is of practical interest due to the increasing demand for *Corresponding author.
benzene reduction in petroleum products and especially in gasoline and diesel. La±Ni systems have been used as catalysts in several studies either in the form of hydrides [1,2] and pure perovskites [3±5], or supported on various oxides and clays [6±9]. For benzene hydrogenation only the activity of La±Ni hydrides has been examined [1], while these systems have also been used for the hydrogenation of p-benzoquinone [2]. Perovskite-type solids, pure or supported, with the general formula LaMO3 (Mtransition metals) or its related form La2MO4 are of great interest in catalysis. Among the more important reactions for which La±Ni±O perovskites, pure or supported on Al2O3, ZrO2 and SiO2±Al2O3, have been tested as catalysts, are the
0926-860X/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00201-4
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combustion of methane and of odorized natural gas [3,9], the reduction of NO by CO [4,6] and the N2O decomposition [5]. LaNiOx oxides supported on a montmorillonite clay [7,8] have also been used for the reduction of NO by CO and the dehydration of isopropanol. Various oxides have been examined in detail as supports of a number of active metals with respect to benzene hydrogenation [10±13]; however, there are no studies on the use of pillared clays as catalyst supports for this reaction. The aim of this work is to study the catalytic activity of non-supported La±Ni systems as well as the activity of La, Ni systems either supported on an Al-pillared montmorillonite or incorporated in a montmorillonite clay, in the hydrogenation of benzene. The impact of the clay support on the metal system activity is also investigated. 2. Experimental 2.1. Catalyst preparation and characterization Two series of non-supported La±Ni systems were prepared according to the citrate method [3,8]. The prepared catalysts are denoted by the general formula LaNiO3-x-y, where x stands for the calcination temperature, x3508C, 5008C, 6008C, 7008C, 8008C, 10008C and y for the catalyst series, y1, 2. For the LaNiO3-x-1 series, lanthanum and nickel nitrate salts (La(NO3)36H2O Merck 99%, Ni(NO3)26H2O Fluka 98%), 1:1 mol/mol, were dissolved in water and an amount of citric acid, suf®cient to replace all the nitrate groups by citrate groups, was added. The mixture was then dried at 1008C until a solidi®ed gel was obtained. This was heated slowly to self-ignition and samples of the obtained material were calcined at 3508C, 5008C, 6008C, 7008C, 8008C, 10008C for 4 h. The LaNiO3-x-2 series was prepared following initially the procedure described above to receive the solidi®ed gel. This material was then ®nely dispersed, heated slowly to 3508C and subsequently calcined at 5008C and 6008C for 4 h. For the La, Ni supported on pillared clay catalysts, the method of dry impregnation was employed. These catalysts are denoted as xLaNiO3/AZA-y, where x stands for the number of dry impregnations employed and y for the calcination temperature. The Al-pillared
clay used, named AZA, was an Al-pillared montmorillonite clay. The starting clay was supplied by Silver & Baryte Ores Mining, SA with a commercial name Zenith-N. Details of AZA preparation are given in another work [14]. AZA was impregnated with appropriate amounts of the solutions of lanthanum and nickel nitrate salts which were used for the preparation of the aforementioned non-supported La±Ni systems. The 8LaNi/AZA-y catalyst was prepared by eight dry impregnations of AZA with aqueous solutions of lanthanum and nickel nitrate salts. All La, Ni supported on AZA catalysts were calcined at 5008C for 4 h, with the exception of 10LaNiO3/AZA-600 which was calcined at 6008C for 4 h. For the preparation of the La, Ni system incorporated in the montmorillonite clay, named LaNiZA, LaNi(fsaen)NO3 [7] was added to a 2% (w/w) clay suspension in water. The mixture was stirred for ®ve days and then it was left stand still for another 15 days at room temperature. Subsequently, it was ®ltered, washed with distilled water and dried at room temperature. Finally, it was calcined at 5008C for 4 h. Two batches of 300 g/batch each were prepared in the frame of an EC project, with identical properties. The surface area (BET) and pore volume of the supports and catalysts were determined from the nitrogen adsorption±desorption isotherms at 77 K, using a Sorptomatic 1800 apparatus. X-ray powder diffraction (XRD) patterns were recorded by a Siemens D5000 X-ray diffractometer with ®ltered Cu Ka radiation. 2.2. Reaction system Benzene hydrogenation experiments were carried out in gas phase under atmospheric pressure in a conventional differential bench scale reactor with hydrogen-to-benzene molar ratio of 20:1. The reactor ef¯uent was analyzed with a Fison Mega 2 Series MFC 800 chromatograph equipped with a capillary column DB-624 30 m in length (0.45 mm i.d., 2.55 mm ®lm thickness) and a ¯ame ionization detector. A 26 cm long and 1.1 cm internal diameter Pyrex glass reactor was used. A typical charge of 0.3 g of catalyst was placed on a ®ne quartz wool bed and formed a catalytic bed of about 5 mm length. The catalyst particle size used was 0.160±0.315 mm. Benzene conversion was always below 6% when reaction
A. Louloudi, N. Papayannakos / Applied Catalysis A: General 175 (1998) 21±31
rates were calculated and the catalyst operated in the absence of mass transfer limitations. The reaction temperature was monitored via a chromel±alumel thermocouple put in a thermowel at the center of the catalytic bed. Bed isothermality within 18C was checked by traveling the thermocouple along the bed axis. At the top of the catalyst bed, inert glass was placed to ensure the good mixing of the reactants. In a typical run, the catalyst was reduced prior to reaction in the following way. First, it was heated in ¯owing H2 (5.4 Nlt/h) up to 4008C with a heating rate of 1008C hÿ1 and it was kept for 2 h at 4008C. Then, the catalyst was cooled to the reaction temperature in ¯owing H2 and the gas mixture was passed over the catalyst. A period of 25 min was allowed before any gas sample was taken and a bracketing technique with pure ¯owing H2 was used [10] while changing the reaction conditions, to minimize deactivation of the catalyst. The activity of the catalyst was followed with standard experiments at 1508C reaction temperature. Only cyclohexane was observed in the product stream. In order to study the performance of the catalysts for prolonged time on stream, the catalysts were left in hydrogen stream at room temperature for several hours, more than 10 h, after completion of a set of reaction conditions and measurements. Subsequently, the measurements for benzene hydrogenation activities resumed. The time during which hydrogenation of benzene took place is called time on run, while the sum of time on run and the time the catalyst was kept in ¯owing hydrogen only is called time on stream. 3. Results and discussion 3.1. Characteristics of the catalysts A summary of the catalysts studied in this work is given in Table 1. Ni and La nominal loading and speci®c surface area are also presented. Both series of non-supported La±Ni systems have speci®c surface areas of 225 m2/g, consistent with values given in [9], while the only exception is LaNiO3-1000-1 with the lowest speci®c surface area of 8 m2/g. This can be attributed to catalyst sintering during calcination at 10008C. All AZA supported catalysts have at least 99 m2/g speci®c surface area. For the xLaNiO3/AZA-y catalysts the increase of La and Ni loading results in
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Table 1 Characterization of the catalysts studied Catalysts
La (% w/w)
Ni (% w/w)
SSA (m2/gcat.)
LaNiO3-350-1 LaNiO3-500-1 LaNiO3-600-1 LaNiO3-700-1 LaNiO3-800-1 LaNiO3-1000-1 LaNiO3-500-2 LaNiO3-600-2 3LaNiO3/AZA-500 7LaNiO3/AZA-500 10LaNiO3/AZA-500 10LaNiO3/AZA-600 8LaNi/AZA-500 LaNiZA
56.6 56.6 56.6 56.6 56.6 56.6 56.6 56.6 4.4 10.2 14.6 15.3 14.0 14.0
24.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 1.8 4.3 6.1 6.4 6.0 6.0
25 27 20 18 20 8 27 17 174 155 142 99 107 112
decrease of their speci®c surface area. After calcination of 10LaNiO3/AZA at 6008C a decrease of its SSA is also observed due to partial collapse of AZA microstructure. Moreover, the 8LaNiO3/AZA-500 and LaNiZA catalysts have approximately 30 m2/g lower speci®c surface area than 10LaNiO3/AZA-500 although they have the same loading and were calcined at the same temperature. In Fig. 1, typical XRD patterns of the non-supported La±Ni systems prepared are shown. Analysis of the XRD patterns indicates that perovskite phase is formed after calcination. The evolution of the perovskite crystallite structure depends on the calcination temperature as well as on the mode of preparation. For the ®rst series, LaNiO3-x-1, the LaNiO3 perovskite phase hardly appears after heating at 5008C, while it is predominant for calcination temperatures 6008C, 7008C and 8008C. At 8008C, other than perovskite LaNiO3 phases start to appear again, and at 10008C calcination temperature the perovskite phase is hardly detectable. For the LaNiO3-500-2 material, the LaNiO3 phase is just beginning to develop and in contrast to LaNiO3-500-1 it is the only phase detected (Fig. 1(a)). However, after calcination at 6008C, the structure of LaNiO3-600-1 and LaNiO3-600-2 appears the same with only a small difference in the relative intensities of the XRD peaks (Fig. 1(b)). For all the other materials prepared in this work no perovskite phase was observed. This can be attributed to the relatively low calcination temperature, 5008C,
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Fig. 1. (a) XRD patterns of both the non-supported La±Ni series catalysts calcined at 5008C, and (b) XRD patterns of both the non-supported La±Ni series catalysts calcined at 6008C. Dots correspond to the peaks consistent with the LaNiO3 perovskite phase.
of LaNiZA or AZA supported systems as well as on the restricted area between the clay sheets that prevent the LaNiOx oxides from forming the perovskite phase [7]. In 10LaNiO3/AZA-600, the LaNiO3 perovskite phase is hardly indicated by a small peak at Ê . However, at this calcination temperature d2.722 A the microstructure of the pillared clay has partially collapsed as indicated by the reduced speci®c surface area of this material (Table 1). 3.2. Catalytic activity for benzene hydrogenation 3.2.1. Non-supported La±Ni systems During benzene hydrogenation experiments, catalyst initial activities as well as their performance after prolonged time on stream were examined. All catalysts showed noticeable deactivation with time on run, as the standard experiments at 1508C clearly showed.
The activity coef®cient (t) can be evaluated by using the equation rT t t ; rT t0 t0
(1)
where rT(t) denotes the reaction rates at temperature T and time on run t, and (t) stands for the activity coef®cient at time on run t. Eq. (1) allowed the calculation of catalytic reaction rates free from deactivation effects. Arrhenius plots and activation energies were obtained for initial activity, (t0)1, for all the catalysts. In Fig. 2, Arrhenius plots for the various La±Ni catalytic systems of series 1 are shown. The calculated activation energies of these catalytic materials are plotted versus calcination temperature in Fig. 3 and are in good agreement with values of Eact reported in the literature for other catalysts [10,13,15±18]. The curve obtained shows
A. Louloudi, N. Papayannakos / Applied Catalysis A: General 175 (1998) 21±31
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Fig. 2. Arrhenius plots obtained for initial activities of the non-supported La±Ni systems.
an increase of Eact with calcination temperature and a maximum for the catalyst calcined at 8008C. For the catalyst prepared with calcination at 10008C a drop of Eact is observed. From Fig. 2, it is evident that the initial catalytic activities of the La±Ni systems for benzene hydro-
genation are related to the calcination temperature and the formation of perovskite phase during their preparation. The effect of calcination temperature on initial activities of both non-supported La±Ni series at 1108C reaction temperature is shown in Fig. 4. A relationship between the catalytic activity and the
Fig. 3. Activation energies calculated from Arrhenius plots versus calcination temperatures of the non-supported La±Ni systems.
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Fig. 4. Initial and maximum reaction rates, r0 and rmax, versus calcination temperatures of the non-supported La±Ni systems at reaction temperature T1108C. Filled symbols correspond to r0 and open symbols to rmax. Triangles correspond to the LaNiO3-x-1 series, and squares correspond to the LaNiO3-x-2 series.
structure obtained during calcination can be derived. Catalysts having similar XRD patterns before reduction, LaNiO3-350-1, LaNiO3-500-1 and LaNiO31000-1, have similar activities, while those with only the LaNiO3 perovskite phase present have higher activities. The only exception appears to be the non-supported La±Ni system calcined at 8008C whose initial activity is similar to those of the materials which contained little or no perovskite phase. An interesting observation made for the non-supported La±Ni systems was that they did not reach the maximum of their activities immediately after their reduction period but after several hours on hydrogen stream. Maximum activities of the catalysts, consisting mainly of the LaNiO3 phase after calcination, are also plotted versus calcination temperature in Fig. 4. La± Ni systems which were calcined at 6008C and formed the perovskite phase are the most active catalysts either in terms of initial activity or in terms of the maximum attainable reaction rate. The observed activation of the non-supported La±Ni systems with time they were exposed to hydrogen stream can be attributed to complementary reduction of the samples after the usual activation treatment. It is to be noted that LaNiO3-600-1 had higher initial activity than LaNiO3-
600-2, but after their reaching the maximum of their activities the opposite is observed. This is also the case when comparing LaNiO3-500-2 and LaNiO3-800-1 catalysts. In general, the ®nal sequence of reaction rates is consistent with the XRD patterns as far as the relative LaNiO3 peak intensities are concerned. Thus, the apparent inversion of activity sequence is attributed to the fact that perovskite phase is not readily reduced, resulting in lower initial activities for the materials containing more LaNiO3 phase. This also explains the aforementioned similar initial activity of the LaNiO3-800-1 to those of the materials containing much less perovskite phase. In Fig. 5, (t) is plotted versus time on run for the LaNiO3-700-1 catalyst and two different modes of experimentation. Between the ®rst two experimental sets of the ®rst mode (A1 and A2) there was a 14 h interval during which the catalyst remained in hydrogen stream at room temperature, while between the ®rst two of the second mode (B1 and B2) there was a 62 h interval. It is obvious that when the catalyst was left for 62 h in hydrogen stream, its maximum activity was measured during the second experimental set (B2). Keeping smaller time intervals, i.e. 14±17 h between experimental sets, resulted in a slower
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Fig. 5. Activity coefficient (t) versus time on run of the non-supported La±Ni systems for two modes of experimentation. Reaction temperature T1108C.
catalyst activation and the maximum activity was measured during the third set (A3). The study of the catalyst in the later way led to more coke deposition during activation period and thus different maximum activities (B2, A3) are observed. It is also noted that no matter how the activation took place, the performance of the catalyst for the same time on run was the same (A3±B3, A4±B4) after they had reached the maximum activity. 3.2.2. La, Ni supported on AZA pillared clay, LaNiZA Catalytic reaction rates free from any deactivation effects are plotted against 1/T in the Arrhenius plots shown in Fig. 6. Eact measured are in good agreement with those reported previously for this reaction [10,13,15±18]. Activation energies for La, Ni supported on AZA catalysts, calcined at 5008C, depend on Ni loading. Thus, 7LaNiO3/AZA-500 with 4.5% (w/w) Ni exhibits 48 KJ/mol activation energy, while for the 10LaNiO3/AZA-500 and 8LaNi/AZA-500 with 6±6.4% (w/w) Ni loading the calculated Eact values are 53.6 and 52.6 KJ/mol, respectively. Activation energies for 10LaNiO3/AZA-600 and LaNiZA
have been determined as 46 and 42 KJ/mol, respectively, although these materials contain 6±6.4% (w/w) Ni. In Fig. 6, it is observed that LaNiZA is far more active than 10LaNiO3/AZA-500 and 8LaNi/AZA500, although all three of them have the same Ni loading. This suggests that La, Ni deposition on clay surface via an organic compound, although dif®cult, expensive and time consuming, results in a more active material. It is also noted that 10LaNiO3/ AZA-500 and 8LaNi/AZA-500 having been prepared by using different impregnation solutions exhibit identical activities. For x-LaNiO3/AZA-500 catalysts an increase of activity with Ni loading is observed. Furthermore, 10LaNiO3/AZA-600 exhibits lower initial activity than 10LaNiO3/AZA-500, while differentiating from it only on calcination temperature. The remarks made from Fig. 6 indicate that for La, Ni supported on AZA catalysts only Ni loading and calcination temperature affect catalyst activity and not the nature of the precursors. For xLaNiO3/AZA-y and LaNiZA catalysts, the initial activities measured after the usual reduction period were their maximum attainable activities. The
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A. Louloudi, N. Papayannakos / Applied Catalysis A: General 175 (1998) 21±31
Fig. 6. Arrhenius plots obtained for initial activities of the LaNiZA and La, Ni supported on AZA catalysts.
different activation behavior of the supported and nonsupported La±Ni systems can be attributed to the dispersion of these materials on the carriers. When supported or incorporated, the ratio of La±Ni phase surface to volume is very high compared to the corresponding ratio of the non-supported materials. Thus, reduction can be achieved faster in the supported material than in the non-supported ones. The decreased catalytic activity of the La±Ni/pillared clay calcined at 6008C can be attributed to a strong metal±support interaction between the active metals and the clay surface. This effect was not observed for the catalyst calcined at 5008C, as implied by the rate ratios: r10LaNiO3 =AZA-500 0:55; rLaNiO3 -500-2
(2)
r10LaNiO3 =AZA-600 0:01; rLaNiO3 -600-2
(3)
where rx stands for the reaction rates of the catalysts reported, calculated per g of Ni. It is noted that while 10LaNiO3/AZA-500 exhibits 55% of the corresponding non-supported La±Ni system's activity, the 10LaNiO3/AZA-600 exhibits only 1% of LaNiO3-600-2's activity. These remarkable
differences between the reaction rate ratios cannot be attributed to the partial destruction of the pillared clay microstructure at 6008C which resulted in 30% decrease of its speci®c surface area (Table 1). The effect of Ni loading on catalyst activities is better visualized in Fig. 7, where reaction rates at 1508C are plotted against % (w/w) Ni on xLaNiO3/ AZA-500 catalysts. For the sake of comparison, the initial and maximum activity of LaNiO3-500-2 and the reaction rate of LaNiZA at 1508C are also shown. An exponential increase of the reaction rate with Ni loading is observed. Increased metal loading and surface metal concentration result, generally, in increase of the average metal aggregate size and of the fraction of the metal mass available for reaction. Thus, the reaction rate per metal mass should decrease with metal loading in the case of structure insensitive reaction. However, the aforementioned increase in benzene hydrogenation rate with Ni loading suggests that this reaction is a structure sensitive reaction for the systems studied in this work. This is in agreement with ®ndings of other researchers [12,19±21] for active metals supported on various carriers and used for the hydrogenation of benzene. Comparing the catalytic activity of the xLaNiO3/AZA-500 systems and LaNiO3-500-2, it is observed that the catalytic
A. Louloudi, N. Papayannakos / Applied Catalysis A: General 175 (1998) 21±31
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Fig. 7. Evolution of the reaction rate of the xLaNiO3/AZA-500 catalysts versus % (w/w) Ni loading. Comparison of xLaNiO3/AZA-500, LaNiO3-500-2 and LaNiZA, all prepared with calcination at 5008C. Dotted lines correspond to initial and maximum reaction rates of the LaNiO3-500-2 catalyst. Reaction temperature T1508C.
activity of 7LaNiO3/AZA-500 with 4.5% (w/w) Ni loading is comparable to the initial activity of the nonsupported La±Ni catalyst. The material with 6.4% (w/ w) Ni loading is more active than LaNiO3-500-2 at ®rst, but 20% less active when the non-supported La± Ni catalyst is fully activated. However, LaNiZA is the most active catalyst prepared with calcination at 5008C, exhibiting almost a four-fold reaction rate than LaNiO3-500-2. This activity is only reached by LaNiO3-600-2 when fully activated, as reported in Table 2. Reaction rates determined at 1108C of the most active catalysts for the three categories prepared in this work are also compared in Table 2. For the non-supported La±Ni system calcined at 6008C both initial activity and maximum activity are given. It is
obvious that LaNiO3-600-2 appears as the most active catalyst irrespective of the units employed to express the reaction rates. The only exception is observed when the rates are expressed per g of Ni where LaNiZA's activity is almost as high as the one of LaNiO3-600-2. As far as the deactivation of xLaNiO3/AZA-y catalysts is concerned, they did not show behavior similar to non-supported La±Ni systems. In Fig. 8, typical deactivation curves with time on stream are presented. Although an initial 22% deactivation for 10LaNiO3/ AZA-500 and 8LaNi/AZA-500 occurred during the ®rst 3 h, for the following 2±2.5 h the drop in activity was negligible. 10LaNiO3/AZA-600 activity remained essentially the same during 5 h on stream.
Table 2 Initial activities of the most active catalysts studied (reaction temperature T1108C) Catalysts
Ni (% w/w)
r mmol=m2cat: s
r (mmol/gcat. s)
r (mmol/gNi s)
10LaNiO3/AZA-500 LaNiZA LaNiO3-600-2 LaNiO3-600-2 maxa
6.1 6.0 24.0 24.0
9.31Eÿ04 5.34Eÿ03 1.88Eÿ02 1.53Eÿ01
1.32Eÿ01 5.98Eÿ01 3.19Eÿ01 25.95Eÿ01
2.17 9.97 1.33 10.81
a
LaNiO3-600-2's maximum attainable reaction rate.
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Fig. 8. Activity coefficient (t) versus time on run of 10LaNiO3/AZA-500, 8LaNi/AZA-500 and 10LaNiO3/AZA-600 catalysts. Reaction temperature T1258C.
Similarly, LaNiZA did not deactivate for a time of run period of 4 h at 1508C reaction temperature. 4. Conclusions The main conclusions of this work can be summarized as following: For the non-supported La±Ni systems the structure obtained during calcination as well as the mode of preparation are the main factors in¯uencing their catalytic activity for the hydrogenation of benzene. These systems are fully reduced only after prolonged exposure to hydrogen stream. For the La, Ni supported on AZA catalysts only metal loading and calcination temperature affect catalyst activity and not the nature of the precursors. The exponential increase of the reaction rate with Ni loading suggests that benzene hydrogenation over these materials is a structure sensitive reaction. Calcination at 6008C results in decrease of activity because of strong metal±support interaction between the active metals and the clay support. As far as the initial activities of the catalysts studied in this work are concerned LaNiZA and LaNiO3-6002 seem more active, but when LaNiO3-600-2 is fully reduced it appears to be the most active catalyst.
Acknowledgements The authors wish to thank Prof. Phillipos Pomonis and Dr. Athanasios Ladavos for the very helpful discussions on La±Ni systems preparation. This research was partially supported by the Brite-Euram BRE2-CT94-0629 project. References [1] D. Ballivet-Tkatdhenco, J. Branko, A. Pires de Matos, ACIESP 66 (1990) 29. [2] V.V. Lunin, N.N. Sychev, V.I. Bogdan, Kinet. Catal. 33 (1992) 447. [3] A.K. Ladavos, P.J. Pomonis, J. Chem. Soc., Faraday Trans. 88(17) (1992) 2557. [4] A.K. Ladavos, P.J. Pomonis, Appl. Catal. B 1 (1992) 101. [5] A.K. Ladavos, P.J. Pomonis, J. Chem. Soc., Faraday Trans. 87(19) (1991) 3291. [6] A.K. Ladavos, P.J. Pomonis, Appl. Catal. B 2 (1993) 27. [7] S.P. Scaribas, Ph.D. Thesis, Ioannina University, Greece, 1992. [8] A.K. Ladavos, Ph.D. Thesis, Ioannina University, Greece, 1992. [9] D. Klvana, J. Kirchnerova, P. Gauthier, J. Delval, J. Chaouki, Can. J. Chem. Eng. 75 (1997) 509. [10] S.D. Lin, M.A. Vannice, J. Catal. 143 (1993) 539. [11] S. Narayanan, G. Sreekanth, J. Chem. Soc., Faraday Trans. 1 85(11) (1989) 3785.
A. Louloudi, N. Papayannakos / Applied Catalysis A: General 175 (1998) 21±31 [12] P. Marecot, E. Paraiso, J.M. Dumas, J. Barbier, Appl. Catal. 74 (1991) 261. [13] B. Coughlan, M.A. Keane, Zeolites 11 (1991) 12. [14] V. Kaloidas, C.A. Koufopanos, N.H. Gangas, N.G. Papayannakos, Microporous Mater. 5 (1995) 97. [15] K.J. Yoon, M.A. Vannice, J. Catal. 82 (1983) 457. [16] T. Takahashi, K. Yamashita, T. Kai, I. Fujiyoshi, Can. J. Chem. Eng. 64 (1986) 1008. [17] P. Chou, M.A. Vannice, J. Catal. 107 (1987) 129.
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[18] B.H. Cooper, B.B.L. Donnis, Appl. Catal. A 137 (1996 203). [19] A.F. Flores, R.L. Burwell Jr., J.B. Butt, J. Chem. Soc., Faraday Trans. 88(8) (1992) 1191. [20] E. Choren, L. El-Chaar, J.O. Hernandez, G. Arteaga, A. Arteaga, J. Sanchez, J. Mol. Catal. 72 (1992) 85. [21] P. Marecot, J.R. Mahoungou, J. Barbier, Appl. Catal. A 101 (1993) 143.
Hydrogenation of benzene on La±Ni and clay supported La±Ni catalysts A. Louloudi, N. Papayannakos* Department of Chemical Engineering, National Technical University of Athens, Sector II, Heroon Polytechniou 9, GR-157 80 Zografos, Athens, Greece Received 10 February 1998; received in revised form 30 March 1998; accepted 29 May 1998
Abstract Non-supported La±Ni systems and La, Ni systems either supported on an Al-pillared montmorillonite clay or incorporated in a montmorillonite clay were prepared. Their catalytic activity for the hydrogenation of benzene was evaluated over a range of 70±1708C under atmospheric pressure, in a differential bench scale reactor. Surface area measurements and X-ray diffraction were used as tools for characterizing the catalysts. Catalyst initial activities, their performance after prolonged time on stream as well as the clay impact on the supported metals activity were examined. The non-supported La±Ni system calcined at 6008C formed the LaNiO3 perovskite phase and it was the most active catalyst after its reduction. The in¯uence of the mode of catalyst preparation, the calcination temperature and the Ni loading on hydrogenation activity are also discussed. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Benzene hydrogenation; La±Ni catalysts; Pillared clay supports; Catalyst activities
1. Introduction Hydrogenation of aromatic hydrocarbons to saturated cyclic products is of current interest due to both environmental aspects and the wide range of industrial processes involving such reactions. It is well known that the hydrotreating of polyaromatics down to monoaromatics is rather easy. However, the saturation of the ®nal ring is dif®cult because of resonance stabilization of the monoaromatic ring. The benzene hydrogenation becomes, thus, a model reaction for testing hydrogenation catalysts. Furthermore, this reaction is of practical interest due to the increasing demand for *Corresponding author.
benzene reduction in petroleum products and especially in gasoline and diesel. La±Ni systems have been used as catalysts in several studies either in the form of hydrides [1,2] and pure perovskites [3±5], or supported on various oxides and clays [6±9]. For benzene hydrogenation only the activity of La±Ni hydrides has been examined [1], while these systems have also been used for the hydrogenation of p-benzoquinone [2]. Perovskite-type solids, pure or supported, with the general formula LaMO3 (Mtransition metals) or its related form La2MO4 are of great interest in catalysis. Among the more important reactions for which La±Ni±O perovskites, pure or supported on Al2O3, ZrO2 and SiO2±Al2O3, have been tested as catalysts, are the
0926-860X/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00201-4
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combustion of methane and of odorized natural gas [3,9], the reduction of NO by CO [4,6] and the N2O decomposition [5]. LaNiOx oxides supported on a montmorillonite clay [7,8] have also been used for the reduction of NO by CO and the dehydration of isopropanol. Various oxides have been examined in detail as supports of a number of active metals with respect to benzene hydrogenation [10±13]; however, there are no studies on the use of pillared clays as catalyst supports for this reaction. The aim of this work is to study the catalytic activity of non-supported La±Ni systems as well as the activity of La, Ni systems either supported on an Al-pillared montmorillonite or incorporated in a montmorillonite clay, in the hydrogenation of benzene. The impact of the clay support on the metal system activity is also investigated. 2. Experimental 2.1. Catalyst preparation and characterization Two series of non-supported La±Ni systems were prepared according to the citrate method [3,8]. The prepared catalysts are denoted by the general formula LaNiO3-x-y, where x stands for the calcination temperature, x3508C, 5008C, 6008C, 7008C, 8008C, 10008C and y for the catalyst series, y1, 2. For the LaNiO3-x-1 series, lanthanum and nickel nitrate salts (La(NO3)36H2O Merck 99%, Ni(NO3)26H2O Fluka 98%), 1:1 mol/mol, were dissolved in water and an amount of citric acid, suf®cient to replace all the nitrate groups by citrate groups, was added. The mixture was then dried at 1008C until a solidi®ed gel was obtained. This was heated slowly to self-ignition and samples of the obtained material were calcined at 3508C, 5008C, 6008C, 7008C, 8008C, 10008C for 4 h. The LaNiO3-x-2 series was prepared following initially the procedure described above to receive the solidi®ed gel. This material was then ®nely dispersed, heated slowly to 3508C and subsequently calcined at 5008C and 6008C for 4 h. For the La, Ni supported on pillared clay catalysts, the method of dry impregnation was employed. These catalysts are denoted as xLaNiO3/AZA-y, where x stands for the number of dry impregnations employed and y for the calcination temperature. The Al-pillared
clay used, named AZA, was an Al-pillared montmorillonite clay. The starting clay was supplied by Silver & Baryte Ores Mining, SA with a commercial name Zenith-N. Details of AZA preparation are given in another work [14]. AZA was impregnated with appropriate amounts of the solutions of lanthanum and nickel nitrate salts which were used for the preparation of the aforementioned non-supported La±Ni systems. The 8LaNi/AZA-y catalyst was prepared by eight dry impregnations of AZA with aqueous solutions of lanthanum and nickel nitrate salts. All La, Ni supported on AZA catalysts were calcined at 5008C for 4 h, with the exception of 10LaNiO3/AZA-600 which was calcined at 6008C for 4 h. For the preparation of the La, Ni system incorporated in the montmorillonite clay, named LaNiZA, LaNi(fsaen)NO3 [7] was added to a 2% (w/w) clay suspension in water. The mixture was stirred for ®ve days and then it was left stand still for another 15 days at room temperature. Subsequently, it was ®ltered, washed with distilled water and dried at room temperature. Finally, it was calcined at 5008C for 4 h. Two batches of 300 g/batch each were prepared in the frame of an EC project, with identical properties. The surface area (BET) and pore volume of the supports and catalysts were determined from the nitrogen adsorption±desorption isotherms at 77 K, using a Sorptomatic 1800 apparatus. X-ray powder diffraction (XRD) patterns were recorded by a Siemens D5000 X-ray diffractometer with ®ltered Cu Ka radiation. 2.2. Reaction system Benzene hydrogenation experiments were carried out in gas phase under atmospheric pressure in a conventional differential bench scale reactor with hydrogen-to-benzene molar ratio of 20:1. The reactor ef¯uent was analyzed with a Fison Mega 2 Series MFC 800 chromatograph equipped with a capillary column DB-624 30 m in length (0.45 mm i.d., 2.55 mm ®lm thickness) and a ¯ame ionization detector. A 26 cm long and 1.1 cm internal diameter Pyrex glass reactor was used. A typical charge of 0.3 g of catalyst was placed on a ®ne quartz wool bed and formed a catalytic bed of about 5 mm length. The catalyst particle size used was 0.160±0.315 mm. Benzene conversion was always below 6% when reaction
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rates were calculated and the catalyst operated in the absence of mass transfer limitations. The reaction temperature was monitored via a chromel±alumel thermocouple put in a thermowel at the center of the catalytic bed. Bed isothermality within 18C was checked by traveling the thermocouple along the bed axis. At the top of the catalyst bed, inert glass was placed to ensure the good mixing of the reactants. In a typical run, the catalyst was reduced prior to reaction in the following way. First, it was heated in ¯owing H2 (5.4 Nlt/h) up to 4008C with a heating rate of 1008C hÿ1 and it was kept for 2 h at 4008C. Then, the catalyst was cooled to the reaction temperature in ¯owing H2 and the gas mixture was passed over the catalyst. A period of 25 min was allowed before any gas sample was taken and a bracketing technique with pure ¯owing H2 was used [10] while changing the reaction conditions, to minimize deactivation of the catalyst. The activity of the catalyst was followed with standard experiments at 1508C reaction temperature. Only cyclohexane was observed in the product stream. In order to study the performance of the catalysts for prolonged time on stream, the catalysts were left in hydrogen stream at room temperature for several hours, more than 10 h, after completion of a set of reaction conditions and measurements. Subsequently, the measurements for benzene hydrogenation activities resumed. The time during which hydrogenation of benzene took place is called time on run, while the sum of time on run and the time the catalyst was kept in ¯owing hydrogen only is called time on stream. 3. Results and discussion 3.1. Characteristics of the catalysts A summary of the catalysts studied in this work is given in Table 1. Ni and La nominal loading and speci®c surface area are also presented. Both series of non-supported La±Ni systems have speci®c surface areas of 225 m2/g, consistent with values given in [9], while the only exception is LaNiO3-1000-1 with the lowest speci®c surface area of 8 m2/g. This can be attributed to catalyst sintering during calcination at 10008C. All AZA supported catalysts have at least 99 m2/g speci®c surface area. For the xLaNiO3/AZA-y catalysts the increase of La and Ni loading results in
23
Table 1 Characterization of the catalysts studied Catalysts
La (% w/w)
Ni (% w/w)
SSA (m2/gcat.)
LaNiO3-350-1 LaNiO3-500-1 LaNiO3-600-1 LaNiO3-700-1 LaNiO3-800-1 LaNiO3-1000-1 LaNiO3-500-2 LaNiO3-600-2 3LaNiO3/AZA-500 7LaNiO3/AZA-500 10LaNiO3/AZA-500 10LaNiO3/AZA-600 8LaNi/AZA-500 LaNiZA
56.6 56.6 56.6 56.6 56.6 56.6 56.6 56.6 4.4 10.2 14.6 15.3 14.0 14.0
24.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 1.8 4.3 6.1 6.4 6.0 6.0
25 27 20 18 20 8 27 17 174 155 142 99 107 112
decrease of their speci®c surface area. After calcination of 10LaNiO3/AZA at 6008C a decrease of its SSA is also observed due to partial collapse of AZA microstructure. Moreover, the 8LaNiO3/AZA-500 and LaNiZA catalysts have approximately 30 m2/g lower speci®c surface area than 10LaNiO3/AZA-500 although they have the same loading and were calcined at the same temperature. In Fig. 1, typical XRD patterns of the non-supported La±Ni systems prepared are shown. Analysis of the XRD patterns indicates that perovskite phase is formed after calcination. The evolution of the perovskite crystallite structure depends on the calcination temperature as well as on the mode of preparation. For the ®rst series, LaNiO3-x-1, the LaNiO3 perovskite phase hardly appears after heating at 5008C, while it is predominant for calcination temperatures 6008C, 7008C and 8008C. At 8008C, other than perovskite LaNiO3 phases start to appear again, and at 10008C calcination temperature the perovskite phase is hardly detectable. For the LaNiO3-500-2 material, the LaNiO3 phase is just beginning to develop and in contrast to LaNiO3-500-1 it is the only phase detected (Fig. 1(a)). However, after calcination at 6008C, the structure of LaNiO3-600-1 and LaNiO3-600-2 appears the same with only a small difference in the relative intensities of the XRD peaks (Fig. 1(b)). For all the other materials prepared in this work no perovskite phase was observed. This can be attributed to the relatively low calcination temperature, 5008C,
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Fig. 1. (a) XRD patterns of both the non-supported La±Ni series catalysts calcined at 5008C, and (b) XRD patterns of both the non-supported La±Ni series catalysts calcined at 6008C. Dots correspond to the peaks consistent with the LaNiO3 perovskite phase.
of LaNiZA or AZA supported systems as well as on the restricted area between the clay sheets that prevent the LaNiOx oxides from forming the perovskite phase [7]. In 10LaNiO3/AZA-600, the LaNiO3 perovskite phase is hardly indicated by a small peak at Ê . However, at this calcination temperature d2.722 A the microstructure of the pillared clay has partially collapsed as indicated by the reduced speci®c surface area of this material (Table 1). 3.2. Catalytic activity for benzene hydrogenation 3.2.1. Non-supported La±Ni systems During benzene hydrogenation experiments, catalyst initial activities as well as their performance after prolonged time on stream were examined. All catalysts showed noticeable deactivation with time on run, as the standard experiments at 1508C clearly showed.
The activity coef®cient (t) can be evaluated by using the equation rT t t ; rT t0 t0
(1)
where rT(t) denotes the reaction rates at temperature T and time on run t, and (t) stands for the activity coef®cient at time on run t. Eq. (1) allowed the calculation of catalytic reaction rates free from deactivation effects. Arrhenius plots and activation energies were obtained for initial activity, (t0)1, for all the catalysts. In Fig. 2, Arrhenius plots for the various La±Ni catalytic systems of series 1 are shown. The calculated activation energies of these catalytic materials are plotted versus calcination temperature in Fig. 3 and are in good agreement with values of Eact reported in the literature for other catalysts [10,13,15±18]. The curve obtained shows
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25
Fig. 2. Arrhenius plots obtained for initial activities of the non-supported La±Ni systems.
an increase of Eact with calcination temperature and a maximum for the catalyst calcined at 8008C. For the catalyst prepared with calcination at 10008C a drop of Eact is observed. From Fig. 2, it is evident that the initial catalytic activities of the La±Ni systems for benzene hydro-
genation are related to the calcination temperature and the formation of perovskite phase during their preparation. The effect of calcination temperature on initial activities of both non-supported La±Ni series at 1108C reaction temperature is shown in Fig. 4. A relationship between the catalytic activity and the
Fig. 3. Activation energies calculated from Arrhenius plots versus calcination temperatures of the non-supported La±Ni systems.
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Fig. 4. Initial and maximum reaction rates, r0 and rmax, versus calcination temperatures of the non-supported La±Ni systems at reaction temperature T1108C. Filled symbols correspond to r0 and open symbols to rmax. Triangles correspond to the LaNiO3-x-1 series, and squares correspond to the LaNiO3-x-2 series.
structure obtained during calcination can be derived. Catalysts having similar XRD patterns before reduction, LaNiO3-350-1, LaNiO3-500-1 and LaNiO31000-1, have similar activities, while those with only the LaNiO3 perovskite phase present have higher activities. The only exception appears to be the non-supported La±Ni system calcined at 8008C whose initial activity is similar to those of the materials which contained little or no perovskite phase. An interesting observation made for the non-supported La±Ni systems was that they did not reach the maximum of their activities immediately after their reduction period but after several hours on hydrogen stream. Maximum activities of the catalysts, consisting mainly of the LaNiO3 phase after calcination, are also plotted versus calcination temperature in Fig. 4. La± Ni systems which were calcined at 6008C and formed the perovskite phase are the most active catalysts either in terms of initial activity or in terms of the maximum attainable reaction rate. The observed activation of the non-supported La±Ni systems with time they were exposed to hydrogen stream can be attributed to complementary reduction of the samples after the usual activation treatment. It is to be noted that LaNiO3-600-1 had higher initial activity than LaNiO3-
600-2, but after their reaching the maximum of their activities the opposite is observed. This is also the case when comparing LaNiO3-500-2 and LaNiO3-800-1 catalysts. In general, the ®nal sequence of reaction rates is consistent with the XRD patterns as far as the relative LaNiO3 peak intensities are concerned. Thus, the apparent inversion of activity sequence is attributed to the fact that perovskite phase is not readily reduced, resulting in lower initial activities for the materials containing more LaNiO3 phase. This also explains the aforementioned similar initial activity of the LaNiO3-800-1 to those of the materials containing much less perovskite phase. In Fig. 5, (t) is plotted versus time on run for the LaNiO3-700-1 catalyst and two different modes of experimentation. Between the ®rst two experimental sets of the ®rst mode (A1 and A2) there was a 14 h interval during which the catalyst remained in hydrogen stream at room temperature, while between the ®rst two of the second mode (B1 and B2) there was a 62 h interval. It is obvious that when the catalyst was left for 62 h in hydrogen stream, its maximum activity was measured during the second experimental set (B2). Keeping smaller time intervals, i.e. 14±17 h between experimental sets, resulted in a slower
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27
Fig. 5. Activity coefficient (t) versus time on run of the non-supported La±Ni systems for two modes of experimentation. Reaction temperature T1108C.
catalyst activation and the maximum activity was measured during the third set (A3). The study of the catalyst in the later way led to more coke deposition during activation period and thus different maximum activities (B2, A3) are observed. It is also noted that no matter how the activation took place, the performance of the catalyst for the same time on run was the same (A3±B3, A4±B4) after they had reached the maximum activity. 3.2.2. La, Ni supported on AZA pillared clay, LaNiZA Catalytic reaction rates free from any deactivation effects are plotted against 1/T in the Arrhenius plots shown in Fig. 6. Eact measured are in good agreement with those reported previously for this reaction [10,13,15±18]. Activation energies for La, Ni supported on AZA catalysts, calcined at 5008C, depend on Ni loading. Thus, 7LaNiO3/AZA-500 with 4.5% (w/w) Ni exhibits 48 KJ/mol activation energy, while for the 10LaNiO3/AZA-500 and 8LaNi/AZA-500 with 6±6.4% (w/w) Ni loading the calculated Eact values are 53.6 and 52.6 KJ/mol, respectively. Activation energies for 10LaNiO3/AZA-600 and LaNiZA
have been determined as 46 and 42 KJ/mol, respectively, although these materials contain 6±6.4% (w/w) Ni. In Fig. 6, it is observed that LaNiZA is far more active than 10LaNiO3/AZA-500 and 8LaNi/AZA500, although all three of them have the same Ni loading. This suggests that La, Ni deposition on clay surface via an organic compound, although dif®cult, expensive and time consuming, results in a more active material. It is also noted that 10LaNiO3/ AZA-500 and 8LaNi/AZA-500 having been prepared by using different impregnation solutions exhibit identical activities. For x-LaNiO3/AZA-500 catalysts an increase of activity with Ni loading is observed. Furthermore, 10LaNiO3/AZA-600 exhibits lower initial activity than 10LaNiO3/AZA-500, while differentiating from it only on calcination temperature. The remarks made from Fig. 6 indicate that for La, Ni supported on AZA catalysts only Ni loading and calcination temperature affect catalyst activity and not the nature of the precursors. For xLaNiO3/AZA-y and LaNiZA catalysts, the initial activities measured after the usual reduction period were their maximum attainable activities. The
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A. Louloudi, N. Papayannakos / Applied Catalysis A: General 175 (1998) 21±31
Fig. 6. Arrhenius plots obtained for initial activities of the LaNiZA and La, Ni supported on AZA catalysts.
different activation behavior of the supported and nonsupported La±Ni systems can be attributed to the dispersion of these materials on the carriers. When supported or incorporated, the ratio of La±Ni phase surface to volume is very high compared to the corresponding ratio of the non-supported materials. Thus, reduction can be achieved faster in the supported material than in the non-supported ones. The decreased catalytic activity of the La±Ni/pillared clay calcined at 6008C can be attributed to a strong metal±support interaction between the active metals and the clay surface. This effect was not observed for the catalyst calcined at 5008C, as implied by the rate ratios: r10LaNiO3 =AZA-500 0:55; rLaNiO3 -500-2
(2)
r10LaNiO3 =AZA-600 0:01; rLaNiO3 -600-2
(3)
where rx stands for the reaction rates of the catalysts reported, calculated per g of Ni. It is noted that while 10LaNiO3/AZA-500 exhibits 55% of the corresponding non-supported La±Ni system's activity, the 10LaNiO3/AZA-600 exhibits only 1% of LaNiO3-600-2's activity. These remarkable
differences between the reaction rate ratios cannot be attributed to the partial destruction of the pillared clay microstructure at 6008C which resulted in 30% decrease of its speci®c surface area (Table 1). The effect of Ni loading on catalyst activities is better visualized in Fig. 7, where reaction rates at 1508C are plotted against % (w/w) Ni on xLaNiO3/ AZA-500 catalysts. For the sake of comparison, the initial and maximum activity of LaNiO3-500-2 and the reaction rate of LaNiZA at 1508C are also shown. An exponential increase of the reaction rate with Ni loading is observed. Increased metal loading and surface metal concentration result, generally, in increase of the average metal aggregate size and of the fraction of the metal mass available for reaction. Thus, the reaction rate per metal mass should decrease with metal loading in the case of structure insensitive reaction. However, the aforementioned increase in benzene hydrogenation rate with Ni loading suggests that this reaction is a structure sensitive reaction for the systems studied in this work. This is in agreement with ®ndings of other researchers [12,19±21] for active metals supported on various carriers and used for the hydrogenation of benzene. Comparing the catalytic activity of the xLaNiO3/AZA-500 systems and LaNiO3-500-2, it is observed that the catalytic
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29
Fig. 7. Evolution of the reaction rate of the xLaNiO3/AZA-500 catalysts versus % (w/w) Ni loading. Comparison of xLaNiO3/AZA-500, LaNiO3-500-2 and LaNiZA, all prepared with calcination at 5008C. Dotted lines correspond to initial and maximum reaction rates of the LaNiO3-500-2 catalyst. Reaction temperature T1508C.
activity of 7LaNiO3/AZA-500 with 4.5% (w/w) Ni loading is comparable to the initial activity of the nonsupported La±Ni catalyst. The material with 6.4% (w/ w) Ni loading is more active than LaNiO3-500-2 at ®rst, but 20% less active when the non-supported La± Ni catalyst is fully activated. However, LaNiZA is the most active catalyst prepared with calcination at 5008C, exhibiting almost a four-fold reaction rate than LaNiO3-500-2. This activity is only reached by LaNiO3-600-2 when fully activated, as reported in Table 2. Reaction rates determined at 1108C of the most active catalysts for the three categories prepared in this work are also compared in Table 2. For the non-supported La±Ni system calcined at 6008C both initial activity and maximum activity are given. It is
obvious that LaNiO3-600-2 appears as the most active catalyst irrespective of the units employed to express the reaction rates. The only exception is observed when the rates are expressed per g of Ni where LaNiZA's activity is almost as high as the one of LaNiO3-600-2. As far as the deactivation of xLaNiO3/AZA-y catalysts is concerned, they did not show behavior similar to non-supported La±Ni systems. In Fig. 8, typical deactivation curves with time on stream are presented. Although an initial 22% deactivation for 10LaNiO3/ AZA-500 and 8LaNi/AZA-500 occurred during the ®rst 3 h, for the following 2±2.5 h the drop in activity was negligible. 10LaNiO3/AZA-600 activity remained essentially the same during 5 h on stream.
Table 2 Initial activities of the most active catalysts studied (reaction temperature T1108C) Catalysts
Ni (% w/w)
r mmol=m2cat: s
r (mmol/gcat. s)
r (mmol/gNi s)
10LaNiO3/AZA-500 LaNiZA LaNiO3-600-2 LaNiO3-600-2 maxa
6.1 6.0 24.0 24.0
9.31Eÿ04 5.34Eÿ03 1.88Eÿ02 1.53Eÿ01
1.32Eÿ01 5.98Eÿ01 3.19Eÿ01 25.95Eÿ01
2.17 9.97 1.33 10.81
a
LaNiO3-600-2's maximum attainable reaction rate.
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Fig. 8. Activity coefficient (t) versus time on run of 10LaNiO3/AZA-500, 8LaNi/AZA-500 and 10LaNiO3/AZA-600 catalysts. Reaction temperature T1258C.
Similarly, LaNiZA did not deactivate for a time of run period of 4 h at 1508C reaction temperature. 4. Conclusions The main conclusions of this work can be summarized as following: For the non-supported La±Ni systems the structure obtained during calcination as well as the mode of preparation are the main factors in¯uencing their catalytic activity for the hydrogenation of benzene. These systems are fully reduced only after prolonged exposure to hydrogen stream. For the La, Ni supported on AZA catalysts only metal loading and calcination temperature affect catalyst activity and not the nature of the precursors. The exponential increase of the reaction rate with Ni loading suggests that benzene hydrogenation over these materials is a structure sensitive reaction. Calcination at 6008C results in decrease of activity because of strong metal±support interaction between the active metals and the clay support. As far as the initial activities of the catalysts studied in this work are concerned LaNiZA and LaNiO3-6002 seem more active, but when LaNiO3-600-2 is fully reduced it appears to be the most active catalyst.
Acknowledgements The authors wish to thank Prof. Phillipos Pomonis and Dr. Athanasios Ladavos for the very helpful discussions on La±Ni systems preparation. This research was partially supported by the Brite-Euram BRE2-CT94-0629 project. References [1] D. Ballivet-Tkatdhenco, J. Branko, A. Pires de Matos, ACIESP 66 (1990) 29. [2] V.V. Lunin, N.N. Sychev, V.I. Bogdan, Kinet. Catal. 33 (1992) 447. [3] A.K. Ladavos, P.J. Pomonis, J. Chem. Soc., Faraday Trans. 88(17) (1992) 2557. [4] A.K. Ladavos, P.J. Pomonis, Appl. Catal. B 1 (1992) 101. [5] A.K. Ladavos, P.J. Pomonis, J. Chem. Soc., Faraday Trans. 87(19) (1991) 3291. [6] A.K. Ladavos, P.J. Pomonis, Appl. Catal. B 2 (1993) 27. [7] S.P. Scaribas, Ph.D. Thesis, Ioannina University, Greece, 1992. [8] A.K. Ladavos, Ph.D. Thesis, Ioannina University, Greece, 1992. [9] D. Klvana, J. Kirchnerova, P. Gauthier, J. Delval, J. Chaouki, Can. J. Chem. Eng. 75 (1997) 509. [10] S.D. Lin, M.A. Vannice, J. Catal. 143 (1993) 539. [11] S. Narayanan, G. Sreekanth, J. Chem. Soc., Faraday Trans. 1 85(11) (1989) 3785.
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