Catalytic functionalities of nickel supported on different polymorphs of alumina

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Catalysis Communications 9 (2008) 886–893 www.elsevier.com/locate/catcom

Catalytic functionalities of nickel supported on different polymorphs of alumina Komandur V.R. Chary *, Pendyala Venkat Ramana Rao, Vattikonda Venkat Rao Catalysis Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 20 April 2007; received in revised form 9 September 2007; accepted 14 September 2007 Available online 21 September 2007

Abstract Pure and mixed polymorphs of alumina were made by the thermal treatment of c-Al2O3 between 773 and 1573 K for evaluating their suitability as supports for nickel catalysts in hydrodechlorination reactions. X-ray diffraction (XRD) results indicate that c-Al2O3 is found to undergo sequential phase transformations [c!(c + h) ! (h + a)! a] during calcination at various temperatures. 10 wt% Ni catalysts supported on these phases were prepared by impregnation method and calcined at 773 K. These catalysts are characterized by X-ray diffraction (XRD), pulse chemisorption of H2 and temperature programmed reduction (TPR) techniques. XRD results indicate that nickel oxide is present in highly dispersed state on c-, (c + h) phases and crystalline form on (h + a)- and a-phases of alumina. Pulse chemisorption of H2 suggests that nickel dispersion is more on a mixture of (c + h) phases of alumina when compared to pure c-phase and decreases further on (h + a)- and a-phases of alumina. TPR results reveal the pattern of reduction of NiO to Ni0 on various phases of alumina. Reducibility of NiO is found to be higher when it is supported on c-phase than a-phase. The catalytic properties were evaluated for the vapor phase hydrodechlorination of chlorobenzene to benzene and were related to the hydrogen chemisorption sites. Higher catalytic activity is observed when nickel is supported on a mixture of (c + h) phase when compared to other polymorphs of alumina.  2007 Published by Elsevier B.V. Keywords: Nickel; Alumina; Dispersion; Polymorphs; Hydrodechlorination

1. Introduction Aromatic hydrocarbons containing chlorine are among the most hazardous toxic pollutants with carcinogenic and mutagenic properties besides being primarily responsible for the stratospheric ozone depletion [1,2]. Therefore, the disposal of chlorinated organic wastes has become a major environmental and health concern. Catalytic hydrodechlorination represents an interesting technology that can operate under mild conditions and is suitable for the treatment of both concentrated and diluted streams. Although hydrodechlorination does not provide the complete destruction of the pollutants, it can lead to a convenient transformation of them, so that the toxicity of the streams can be reduced dramatically giving rise to more *

Corresponding author. Tel.: +91 40 27193162; fax: +91 40 27160921. E-mail address: [email protected] (K.V.R. Chary).

1566-7367/$ - see front matter  2007 Published by Elsevier B.V. doi:10.1016/j.catcom.2007.09.016

biodegradable effluents. The feature indicated above make hydrodechlorination a good alternative/complement to other techniques that require high temperatures and/or pressures (incineration, wet oxidation), complex equipment (photochemical or sonochemical processes), large amounts of reagents (Fenton oxidation) are limited to a narrow concentration range (biological treatment) [3–5]. Catalytic hydrodechlorination has been widely investigated with bulk and supported bi-metallic catalysts [6–8], using Pt, Pd, Ni, Rh etc., in presence of either gaseous hydrogen [6,9–12] or an alcohol as hydrogen donor [13– 16]. Most of the studies in hydrodechlorination are conducted in vapor phase in the temperature range 410– 573 K in a fixed bed reactor at atmospheric pressure. It was mentioned that Ni/c-Al2O3 catalysts showed catalytic activity in the gas phase hydrodechlorination (HDC) reaction of chlorobenzene [17], and other nickel catalysts like Ni/C, Ni–Mo/C and Ni–Mo/Al2O3 are active in HDC of

K.V.R. Chary et al. / Catalysis Communications 9 (2008) 886–893

polychlorinated compounds [18–21]. c-Al2O3 is an extremely important material in catalysis because of its porous structure with fine particle size, high surface area and high catalytic surface activity. Many theoretical and experimental investigations have been made to understand the role of c-Al2O3 as a catalyst and also as a support [22]. Mostly, cAl2O3 is an intermediate product in the well-known sequence of thermal dehydration reactions starts from boehmite (c-AlOOH) and ends with hexagonal a-alumina. The nature of many of these phases has been very poorly understood in catalysis because of their poorly developed crystallinity. Often these metastable phases other than cAl2O3 also act as effective supports for several transition metals supported catalysts. For example, Cr/g-alumina catalytic system lasts up to two to three years without any degradation, where as Cr/c-Al2O3 degrades with in weeks in dehydrogenation of alkanes [23,24]. This clearly shows that polymorphs or metastable forms of alumina exhibit a key role as catalytic support materials. This considerable difference between c-and g-alumina prompted us to investigate the nature of various polymorphs alumina as support materials in supported Ni catalysts during HDC reactions. In the present investigation, we report the influence of various crystallographic phases of alumina, such as c-, (c + h), (h + a) and a-phases as supports in supported nickel catalysts. The catalytic properties in hydrodechlorination of chlorobenzene are related with dispersion of nickel measured by hydrogen chemisorption and also with the findings of other characterization techniques such as Xray diffraction, temperature programmed reduction and BET surface area measurements. 2. Experimental methods The alumina (Engelhard Corporation, Al-3996, surface area 192 m2/g) support was calcined at different temperatures viz., 773, 973, 1173, 1373 and 1573 K for 5 h in the air in order to get different polymorphs of alumina prior to impregnation of nickel nitrate solution. Ten percent Ni (w/w) supported on these polymorphs of alumina were prepared by wet impregnation method with a requisite amount of Ni(NO3)2 Æ 6H2O (Fluka) solution. The impregnated catalysts were first oven dried at 383 K overnight and then calcined in air at 773 K for 5 h. X-ray powder diffraction patterns were obtained on Rigaku miniflex diffractometer using nickel filtered Cu Ka (k = 0.15406 nm) radiation. Identification of the phase was made with the help of the JCPDS files. The specific surface areas of the catalyst samples were calculated from N2 adsorption-desorption data acquired on a multi point Autosorb-1 instrument (Quantachrome, USA) at liquid N2 temperature. The powders were first outgassed at 423 K, to ensure a clean surface prior to construction of adsorption isotherm. A cross-sectional area of 0.164 nm2 for the N2 molecule was assumed in the calculations of the specific surface areas using the BET method. Pore size distribution (PSD) measurements were performed

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on Auto Pore III (Micromeritics, USA) by mercury penetration method. Hydrogen chemisorption measurements were carried out on an Auto Chem 2910 instrument. Prior to adsorption measurements, Ca. 250 mg of the sample was reduced in a flow of hydrogen (50 mL/min) at 673 K for 2 h and flushed out subsequently in a pure argon (Ar) gas flow (purity 99.995%) for an hour at 683 K. The sample was subsequently cooled to ambient temperature in the same Ar gas stream. Hydrogen uptake was determined by injecting pulses of 5% hydrogen balance argon from a calibrated on-line sampling valve into Ar stream passing over reduced samples at 673 K. The nickel surface area was calculated assuming a stoichiometry of one hydrogen molecule per two surface nickel atoms and an atomic cross-sectional area of 6.49 · 1020 m2/Ni atom. Adsorption was deemed to be complete after at least three successive peaks showed the similar areas. TPR experiments were carried out on an Auto Chem 2910 (Micromeritics) instrument. In a typical experiment Ca.150 mg of oven-dried samples (dried at 383 K for 12 h) was taken in a U shaped quartz sample tube. Prior to TPR studies, the catalyst sample was pretreated in an inert gas (He, 50 mL/min) at 573 K. After pretreatment, the sample was cooled to ambient temperature and the carrier gas consisting of 5% hydrogen balance argon (50 mL/ min), was allowed to pass over the sample and the temperature was increased from ambient to 1273 K at a heating rate of 10 K/min. The hydrogen concentration in the effluent stream was monitored with the thermal conductivity detector and areas under the peaks were integrated using GRAMS/32 software. Pulse calibration was done to quantify the hydrogen consumption values. Vapour phase hydrodechlorination of chlorobenzene was carried out in a vertical down-flow glass reactor under normal atmospheric pressure. Ca. 0.8 g of the catalyst particles [18 + 25 BS mesh] diluted with an equal amount of same size quartz grains were packed in between two layers of quartz wool. The upper portion of the reactor was filled with glass beads, which served as a pre-heater for the reactant. Prior to starting activity runs, the catalyst was reduced at 673 K for 3 h in a purified hydrogen gas (flow rate 50 mL/min). Then the temperature was brought down to 573 K and the reactor was fed with chlorobenzene (1 mL/h) in H2 flow, which is used as a carrier gas. The reaction products were analyzed by a HP-6890 gas chromatograph equipped with a HP-5 capillary column with a flame-ionization detector (FID). The products were also identified using a HP-5973 quadrupole GC–MSD system using a HP–1MS capillary column. 3. Results and discussion The powder X-ray diffractograms of c-Al2O3 calcined in the stream of air at various temperatures are presented in the Fig. 1. It is clear from Fig. 1 that c-Al2O3 phase is intact up to a calcination temperature of 973 K, and fur-

K.V.R. Chary et al. / Catalysis Communications 9 (2008) 886–893

Intensity (a.u)

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Ni/Al2O3(1573 K) Ni/Al2O3 (1373 K) Ni/Al2O3(1173 K) Ni/Al2O3 (973 K) Ni/Al2O3 (773 K)

10

20

30

40

50

60

70

80

2 Theta (deg) Fig. 2. X-ray diffraction patterns of 10 wt% NiO supported on precalcined alumina calcined at various temperatures.

ther increase of calcination temperature leads to appearance of h-Al2O3 at 1173 K. The appearance of XRD reflections due to h-phase at 2h = 33, 36.8and 39 confirms the transformation of c- to h-phase. With further increase in temperature to 1373 K, the a-phase is observed, with susceptibility of presence of d-phase. However it is not seen in the present XRD reflections probably due to small crystallites of d-phase having size less than 4 nm, which could be beyond the detection capacity of powder XRD technique. These results are in good agreement with the findings of Vuurman et al. [25], who have studied the influence of calcination temperature on alumina support with XRD, laser Raman spectroscopy and BET surface area measurements. It was reported that c–alumina phase was transformed into a mixture of h- and d-alumina phases (major Raman bands at 843, 750, 254 cm1) at calcination temperature of 1223 K. The formation of h- and d-alumina phases were further confirmed by the XRD analysis. Further calcination to higher temperatures (1473 K) the alumina was transformed into a-alumina (major Raman bands at 742, 631, 577, 416 and 378 cm1). Schaper [26] also studied the phase transformation of c-alumina by differential thermal analysis (DTA) technique. According to Schaper [26], a drastic change in crystal structure occurred at approximately 1373 K due to formation of a-alumina. Mc Cabe et al. [27] studied the effect of alumina phase on the dispersion of Rh/Al2O3 catalysts. According to these authors, the formation of a-alumina occurs at a narrow temperature range between 1373 and 1423 K. The XRD patterns of 10 wt% Ni catalysts supported on precalcined alumina phases at various temperatures are shown in the Fig. 2. These results indicate that there are no detectable diffraction peaks correspond to crystalline NiO on alumina phase calcined up to 1173 K, which clearly indicates that nickel oxide species are present in a highly dispersed amorphous state. The very sharp and distinct peaks of NiO were observed in nickel oxide catalysts supported on alumina phases calcined at higher temperatures at and above 1373 K. This could be due to decrease in the surface areas of alumina phases calcined at higher tem-

peratures. Reflections due to nickel oxide appeared in Ni catalysts are shown with closed circles in Fig. 2. The 2h angles (with relative intensities in parentheses) from the JCPDS (47–1049) files may be summarized as 37.29 (91), 43.30 (100) and 62.91 (57) for NiO phase. The intensity of these three peaks appears to be increased with the catalyst samples supported on precalcined alumina phases at 1373 and 1573 K. The peak due to crystalline NiO ˚ ) and the peak due to a-alumina (2h = 43.3, d = 2.08 A (2h = 43.36) overlap each other and the intensity of this peak was slightly higher in nickel catalysts supported on alumina phases calcined at these temperatures. In another set of experiments, nickel oxide catalyst supported on c-alumina phase calcined at 773 K [Ni/Al2O3 (773 K)] was subjected to thermal treatment at higher temperatures viz., 973, 1173, 1373 and 1573 K. These are labeled as NAL-973, NAL-1173, NAL-1373 and NAL1573. All these samples exhibit peaks corresponding to c-, h-, and a-phases of alumina along with new peaks at 2h = 37.01, 45.1 and 65.7 reflections due to formation of NiAl2O4 in these catalysts (shown in closed rectangles in Fig. 3). These catalysts showed negligible activity (< 2% conversion) for the hydrodechlorination except

Intensity (a.u)

Fig. 1. Powder X-ray diffraction (XRD) patterns of c-Al2O3 calcined at various temperatures.

NAL-1573 NAL-1373 NAL-1173 NAL-973 Ni/Al2O3 (773K)

10

20

30

40

50

60

70

80

2 Theta (deg) Fig. 3. X-ray diffraction patterns of 10 wt% NiO/Al2O3 catalysts calcined at various temperatures.

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Ni/Al2O3 (773 K); this could be due to the formation of nickel aluminate as evidenced from XRD (Fig. 3). It appears that NiAl2O4 is quite inactive in HDC reactions. The BET specific surface area and XRD information for c-Al2O3 phase calcined at various temperatures are shown in Table 1. The surface area of the alumina phases gradually decreases up to calcination temperature of 1173 K, and then a sudden decrease is noticed when alumina is calcined at 1373 and 1573 K. c-Al2O3 is known to lose surface area by two processes: sintering and the phase transformation to most stable a-Al2O3 [26]. This sudden drop in the surface area with calcination temperature might be due to formation of more thermodynamically stable crystalline a-phase of alumina. The total pore volume and total pore area of 10 wt% nickel catalysts supported on different alumina phases measured by a mercury penetrating porosimeter are reported in Table 2. BET surface area of catalyst samples is found to decrease with impregnation of nickel oxide on alumina phases. It might be due to blocking of the pores of the support by crystallites of nickel oxide, as evidenced by XRD and pore size distribution measurements (Table 2). As the calcination temperature of the alumina phase increases, the total pore area and total pore volume decrease whereas the average pore diameter increases. This change is prominent in catalyst samples when calcination temperature of alumina phase is beyond 1173 K. At high calcination temperature of alumina, there is a closure of all the narrow pores, resulting in a loss of pore volume and pore area and increase in the average pore diameter. Hydrogen uptake values and other related catalyst properties calculated from adsorption measurements for 10 wt% nickel catalysts supported on precalcined alumina phases are presented in Table 3. Dispersion of nickel can be

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defined as the fraction of total hydrogen atoms to total nickel atoms present in the sample. In the present investigation, it was observed that the dispersion and metal area of nickel are increased and average particle size is decreased with the Ni catalysts supported on alumina phases calcined up to 1173 K. This might be due to availability of maximum number of active nickel sites on the catalyst surface. This is also in good agreement with XRD results. It can be seen that from Table 3 that hydrogen uptakes are slightly increased in the Ni catalysts supported on alumina phases calcined up to 1173 K in spite of drop in the surface area of alumina phase at this temperature (Table 2). This indicates that metastable phases of alumina influenced markedly the nickel dispersion. As already mentioned c-phase is transformed into a mixture of (c + h)-phase of alumina at 1173 K. It appears that dispersion of nickel is facilitated by the presence of h-phase along with c-phase. It is reported in the literature that crystallographic phases of c- and h-phases of alumina look very similar though primitive unit cells of c-alumina (cubic) and h-alumina (monoclinic) are quite different. The transformation of c-phase partly to h-phase at 1173 K is ascribed to the migration of a set of aluminum atoms between different interstitials while oxygen atoms remain fixed [28]. This would probably improve the stability of metastable alumina phase calcined at 1173 K. Accordingly, it is expected that formation of mixture of (c + h)-phase would have retarded the trapping of Ni atoms in the subsurface layer of the support and enhanced the Ni dispersion on the surface of (c + h)-phase. However, the dispersion gradually decreases with Ni catalysts supported on alumina phase calcined at 1373 K, this might be due to the formation of crystalline a-alumina in addition to h-alumina. The formation of a-alumina may

Table 1 XRD information and BET surface area for c-alumina calcined at various temperatures S. No

Calcination Temperature of alumina (K)

BET surface area (m2/g)

XRD phases present

1. 2. 3. 4. 5.

773 973 1173 1373 1573

192 174 113 34 4

c-Alumina c-Alumina c and h-Alumina h and a-Alumina a-Alumina

Table 2 Surface area and pore size distribution results of 10 wt% nickel catalystsa supported on alumina calcined at various temperatures S. no.

Catalyst

BET surface areac (m2/g)

Total pore volumed (ml/g)

Total pore aread (m2/g)

˚) Average pore diameterd (A

1. 2. 3. 4. 5. 6.

Pure c-Al2O3 Ni/Al2O3 (773)b Ni/Al2O3 (973) Ni/Al2O3 (1173) Ni/Al2O3 (1373) Ni/Al2O3 (1573)

192 156 142 103 30 3.4

0.8770 0.6947 0.6376 0.6308 0.4556 0.3746

334 236 219 178 64 35

105 118 116 141 284 430

a b c d

Calcined at 773 K. Digits in parenthesis indicate pre-calcination temperature of alumina support. Determined from N2 physisorption method. Measured by mercury porosimetry.

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Table 3 Hydrogen adsorption properties of 10 wt% nickel catalysts supported on various calcination temperatures of alumina Catalyst

Hydrogen uptakea (lmol/gcat)

Dispersiona (%)

Ni metal areaa (m2/gNi)

Crystallitea size (nm)

Conversion of chlorobenzeneb (%)

Ni/Al2O3 (773) Ni/Al2O3 (973) Ni/Al2O3 (1173) Ni/Al2O3 (1373) Ni/Al2O3 (1573)

23.5 32.2 49.5 42.0 5.8

2.77 3.79 5.82 4.93 0.68

18.4 25.2 38.7 32.8 4.5

36.6 26.7 17.4 20.5 147.8

54 58 76 58 43

a b

Calculated from H2 chemisorption. Reaction conditions: T = 573 K; wt. of the catalyst = 0.8 g; feed rate of chlorobenzene = 9.8 · 103 mol h1.

be less at this temperature, so the decrease in hydrogen uptake is also less. The sudden drop of hydrogen uptake with the Ni catalyst supported on alumina phase calcined at 1573 K is due to the phase transition of h-phase to completely stable a-phase of alumina having low surface area. The activity of the catalysts for hydrodechlorination of chlorobenzene was found to increase with Ni catalysts supported on alumina phase calcined up to 1173 K and then decreases with further increase in calcination temperature. Nevertheless, the same trend has been observed in hydrogen chemisorption capacities and the activity of the catalysts for vapor phase hydrodechlorination of chlorobenzene to benzene for Ni catalysts supported on different alumina phases. Reducibility and metal support interaction of nickel species in nickel catalysts supported on various alumina phases was also investigated by TPR experiments and the profiles are shown in Fig. 4. It is known that supported nickel catalysts show different reduction patterns depending on the nature of the interaction between nickel and the support [29–31]. Bulk nickel oxide that does not interact with support is generally reduced at 673 K. When nickel is supported on alumina, the interaction between metal and support decreases the susceptibility of nickel ion to be reduced to metallic nickel. Hydrogen consumption and the reduction temperature T1red and T2red values are given in Table 4. There is not much change in H2 consumption or in the intensity of TPR profiles during TPR analysis of nickel supported catalysts on alumina phase calcined up to 1173 K. T1red and T2red positions decreased marginally. Drastic decrease in H2 consumption (T1red and T2red) values are observed for nickel catalysts supported on alumina phases calcined at 1373 and 1573 K (Table 4). The reduction in H2 consumption (T1red and T2red) might be ascribed to the crystalline modification of c-Al2O3 to more stable aAl2O3. In the absence of free hydroxyl groups in a-alumina there is little or no metal support interaction. Hence, impregnation of nickel on the low surface area alumina forms multiple layers of metal on the surface. During the reduction of these catalysts, Ni2+ ions are easily reduced to metallic nickel and become very mobile. TPR profiles of nickel sample supported on alumina phases calcined at 773 K indicate a main peak at maximum temperature of 859 K along with a shoulder (T1red) at 965 K. The shoulder gradually merges with the main peak as the calcination

Fig. 4. Temperature programmed reduction profiles of 10 wt% Ni supported on alumina calcined at various temperatures.

temperature of alumina is increased. But no reduction of Al2O3 phase was observed at these temperatures. This suggests that there may be two kinds of NiO species. The peak at a lower temperature is assigned to a free NiO species interacting weakly with the support, and the other peak at a higher temperature is attributed to a complex NiO

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Table 4 TPR results of 10 wt% nickel catalysts supported on pre-calcined alumina at different calcination temperatures S. No.

Catalyst

1. 2. 3. 4. 5.

Ni/Al2O3 Ni/Al2O3 Ni/Al2O3 Ni/Al2O3 Ni/Al2O3

(773) (973) (1173) (1373) (1573)

T1red (K)

H2 consumption (lmole/g)

T2red (K)

H2 consumption (lmole/g)

858 859 837 692 682

1323 1289 1484 805 612

965 974 957 773 –

332 329 196 165 –

species interacting strongly with the support [32]. The increase in calcination temperature of the alumina phase causes the loss of all five types of surface hydroxyl groups on the alumina support [33,34] and diminishes the interaction of nickel oxide with alumina support. This is also in agreement with XRD results, which indicate that there is less interaction between nickel precursor and alumina phase calcined at higher temperatures. The correlation between rate of hydrodechlorination and dispersion against crystallite size of nickel (10 wt%) catalysts supported on alumina phase calcined at various temperatures is shown in Fig. 5. It is interesting to observe that hydrodechlorination activity per unit weight of the catalyst decreases with increase in crystallite size while the dispersion increases with decrease in crystallite size. This illustrates that a direct correlation exists between the dispersion of nickel on the surface of alumina and the hydrodechlorination activity. This shows that the nickel supported on alumina phase calcined at 1173 K showed higher dispersion with smaller crystallites and proved to be a better catalyst in hydrodechlorination of chlorobenzene reaction when compared to the other nickel catalysts supported on different polymorphs of alumina. Rate of the reaction was calculated by the following equation. Rate = (Feed rate · Fractional conversion)/weight of the catalyst in gram

The catalytic properties during the vapour phase hydrodechlorination of chlorobenzene at 573 K exhibited by nickel (10 wt%) catalysts supported on alumina phases calcined at various temperatures are shown in the Fig. 6. With increasing calcination temperature of alumina phase the conversion of chlorobenzene was found to increase up to 1173 K, and further increase in calcination temperature of alumina phase led to a decrease in conversion. Hydrodechlorination of chlorobenzene depends directly on the available metal surface as it is a facile reaction. All these catalysts have the same weight percentage of nickel (10 wt%). Hence, the difference in activity is mainly due to change in morphology of the alumina phase, which in turn affect the dispersion, crystallite size and metal area. The bare supports (alumina calcined at various temperatures such as 773, 973, 1173, 1373, and 1573 K) were also found to be inactive for hydrodechlorination under similar experimental conditions. Benzene is the sole product in hydrodechlorination of chlorobenzene in the presence of hydrogen. In every case, the supported Ni catalysts serve solely to cleave the C–Cl bond in the presence of H2 gas, leaving the aromatic nucleus intact, i.e., 100% hydrodechlorination selectivity. The latter suggests that the dechlorinated product, once generated, desorbs from the surface whereas, in the activation step, the resonance energy of the aromatic ring is not significantly disrupted. Such reac-

80 100 6

-3

2 6

80

60 60 40

Conversion Selectivity

50

20

0

4 0

40

80

120

-2 160

Crystallite Size (nm) Fig. 5. Effect of rate of hydrodechlorination and metal dispersion on the crystalline size of Ni-alumina catalysts. Reaction conditions: T = 573 K; wt. catalyst = 0.8 g; feed rate of chlorobenzene = 9.8 · 103 mol h1.

Selectivity (%)

4

Rate

8

Conversion (%)

Dispersion

Dispersion (%)

70

-1 -1

Rate (x10 mol h g cat)

10

40

0 800

1000

1200

1400

1600

Calcination temperature (K) of alumina support in Ni/Al2O3 catalysts Fig. 6. Hydrodechlorination of chlorobenzene of 10 wt% nickel catalysts supported on various calcination temperatures of alumina.

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tion selectivity is significant in Pd- and Rh-based catalysts (under comparable reaction conditions) reported [7,35] to convert chlorobenzene further to cyclohexane, i.e., additional ring hydrogenation. Chlorine is cleaved from the aromatic host as HCl and there was no evidence of any Cl2 formation. It was shown elsewhere [36] that the catalyst surface, under HDC reaction conditions, is saturated with HCl. The stability and life of the catalyst are very important in hydrodechlorination reactions. The relationship between conversion of chlorobenzene and time-on-stream (TOS) is shown in Fig. 7. It is evident that the catalyst remains almost stable without any significant change in activity even reaction time is more than 700 min. It is known that the decline in catalytic activity with reaction time in hydrodechlorination reactions is generally due to the poisoning effect of active phase by HCl produced during reaction. In the case of supported nickel catalyst, the poisoning effect is more prominent at low reaction temperatures (<423 K). At higher temperature (>423 K), the poisoning effect is made reversible by restoring the metal surface partially by the hydrogen present in the reaction medium [37]. It is obvious that there is always a competition between poisoning and regeneration at a given reaction temperature. In our present study, since the reaction temperature is 573 K, the extent of poisoning by HCl is less significant. The XRD and TPR analysis of the spent catalyst were also carried out. The XRD analysis of used catalyst did not show any new peaks. TPR analysis of the spent and pre-reduced catalysts exhibited more or less similar reduction patterns suggesting no evidence for the poisoning of active nickel species with HCl produced during hydrodechlorination reaction. The results obtained from XRD and TPR manifest the absence of any NiCl2 phase formation during HDC reaction. The high resistivity for deactivation observed in these catalysts may also attribute to the absence of sintering of metal atoms under the experimental conditions studied.

c-Al2O3 phase is stable up to a calcination temperature of 973 K; it will lose hydroxyl groups above this temperature. At calcination temperature of 1173 K, c-Al2O3 phase changes to h-Al2O3 whose crystallographic phase is similar to c-phase but is a more stable phase than c-phase due to movement of aluminum atoms to specific neighboring interstitial sites. When alumina is calcined to still higher temperatures, i.e. 1373 and 1573 K, c-Al2O3 phase is changed to crystalline a-Al2O3 having very low surface area. XRD results of nickel (10 wt%) catalysts supported on precalcined alumina beyond 1173 K reveal the presence of crystalline NiO. TPR results also showed the decrease in reducibility of nickel catalysts supported on precalcined aluminas beyond 1173 K, due to the crystalline modifications of c-Al2O3 to a-Al2O3. Among all the polymorphs of aluminas obtained by thermal treatment, the one with nickel catalyst supported on metastable alumina calcined at 1173 K gave the best catalytic performance in terms of hydrogenation activity, and benzene selectivity compared to other metastable forms of alumina. This may be attributed to the high dispersion of smaller NiO crystallites on precalcined aluminas at 1173 K. Contrary to general expectation (larger Ni particles for HDC reactions), smaller metal particles with high dispersion are found to be more active in the case of polymorphs of alumina supported nickel catalyst for hydrodechlorination of chlorobenzene. Further modification of this particular polymorphous of alumina support by incorporating some additives to improve the catalytic activity would be a part our further study in order to design a better catalyst for HDC processes. Acknowledgement The authors thank the Director of IICT, Hyderabad for a project assistant position to P.V.R.R. References

90 80

Conversion (%)

4. Conclusions

Ni/Al2O3 (773 K)

70 Ni/Al2O3 (1173 K)

60 50 40 30 0

100

200

300

400

500

600

700

800

Time on Stream (min) Fig. 7. Variation of conversion of Ni/Al2O3 catalysts for hydrodechlorination reaction with time-on-stream; Reaction conditions: T = 573 K; wt. catalyst = 0.8 g; feed rate of chlorobenzene = 9.8 · 103 mol h1.

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