Indian Journal of Chemistry Vol. 43A, June 2004, pp:. II72- II BO

A study of the structural characterization and cyclohexanol dehydrogenation activity of Cu/y-Ah03 catalysts@

Anita Rachel", V Durga Kumari *", R Subramanianh, K V R Chary", & P Kanta Rao"

a Cata lysis Di vision, Indian Institute of Chemical Technology, Hyderabad 500007, India Email : durgakulllari @iicl.ap.nic. in

b In strumentation Division, National In stitute of Nutrition, Indi an Council of Medical Research, Hyderabad 500007, India

Received 19 lalluary 2004; revised 23 April 2004

A seri es of Cu/y-A I 20~ catalysts has been prepared by wet impregnati on of y-A I20 ., with aqueous cupri c nit rate su lution varying the copper content from 0.4 to IB.7 wt % fo llowed by calcination at 400"C in air. These catalysts have been charac terized by BET surface area. CO chcmi sorption. XRD, SEM, TPR and ES R. The TPR profiles show the presence of unreducible copper spec ies at low load ings and reducible amorphous and bu lk copper species at higher load ings. The ES R spectra of the unred uced and reduced Cu/y-A I 20~ catalysts clearl y di stingui sh the Cu (II ) ions in their cl ustered and isolated fu rms. givi ng an indication of the exi stence of an interacti on between CuO and y-A1 20 3 and al so a definite change in the geo metry depending on the Cu content. Cata lyti c act ivity of these catalysts has been studied for the dehydrogenati on of cyc lohexanol over unr_duced and red uced catal ysts at 200-300' C. Structure- act ivity relationsh ips of the catalysts arc di scussed. X-ray diffraction pattern of the catalyst containing 10.2 wt% Cu after reaction shows the presence of a-Cu co nfirming the ox idati ve dehydrogenati on of cyclohexanol on unred uced catalysts.

IPC Code: BOil 2 1/04; C07B 35/04

Catalyticall y active spec ies are generall y di spersed on a sui tabl e support for their industrial appli cati ons. One support th at is most frequent ly used is AI20 ,. It is widely used because it is a refractory and thermally stable support with a wide range of surface areal. Important catalytic properties are associated with cation defect sites and acidic sites that are created by the removal of hydroxyl groups. Knozinger and Ratn asam/ showed that the acidic behaviour of the hydroxyl groups depends on the face to which they are linked.

These surface and bulk characteri stics of alumina make it an important support materi al.

The performance of a catalyst could be judged in terms of its activity, selectivity, life and the abi lity of the support to retain its surface area under severe reacti on conditions'. These factors depend on the method of preparation , pH, metal loading and ca lcination temperature of the catalysts4

.

Copper supported on y- A120 3 is of parti cul ar interest, because of its general performance as an oxidation catalyst, in the conversion of glycol to glyoxal and CO to CO/('. They are used ror the dehydrogenation of

0' II CT Communication No : 331B

lower alcohols to aldehydes and ketones 7 and of 8 cyclohexanol to cyc lohexanone. They are also

effective for the hydrogenolys is of aliphati c esters to their corresponding alcohols'!. The physico-chemical methods of characterizati on showed that there is a threshold loading for the appear nce of crystalline cupric oxide on CuO/y-AI20 -, catalysts. Friedman el a/ lo. reported 4 wt% Cu/lOO m2 g-I y-A I20 ,i. Strohmeir el a/". reported 10 wt% Cu/195 m2 g-I y-AI20 ,. Wolberg and Roth l2 showed that it is the chemical modificati on rather than the sUlface area, whi ch is responsible for the adsorptive capacity of AI20 -, for Cu2

+ ions. Kobayashi el at 13. reponed crystal size

effects in the behaviour of CuO/y-AbO-, catalysts. where the total number of adsorption sites are rela ted to the surface area. A variety of instru mental techniques such as XRD, ESR, EXAFS, ESCA, SIMS IO.12.1.J-16 have been used to investigate the CuO/y-AI20 -, systems which led to the identification of species such as Cuo, C + C ,+ J+ f' 17 U, U-, Clr , CuAI20 .J SUl ace spll1al and CuO . Pepe el at

ls. correlated the catalytic properti es with

ox idation state of copper. It was inferred that Cuo species is responsible for the hydrogenation of isopropanol. This conclusion is consistent with the results of Echevin and Teicher '') in the decompos iti on

RACHEL et al.: STUDIES ON Cu/y -A120 3 CATALYSTS 1173

of 2-butanol on CuO/y-AI203 and also the results obtained by Yolta et apo. on pure CuO. Cunningham et apt. proposed a mixed valence model for the active sites involving the presence of Cu (II) and Cu (I). The results can be interpreted by assigning to Cu· , the total role of an active species. The reduced copper is being generated in situ by the reduction of the active copper species. In this paper, the results of a systematic study of the structural properties of copper-alumina catalysts with varying Cu loading in reduced and unreduced forms are presented. The catalysts are characterized by BET surface area, CO adsorption, TPR, ESR and SEM. The reactivity of these catalysts is studied in the industrially important reaction, cyclohexanol to cyclohexanone. The catalyst characteristics are correlated with its activity wherever possible.

Materials and Methods

Catalyst preparation

A series of CuO/y-AI20 3 catalysts with different copper loadings was prepared by impregnating y­Ah03 support (Harshaw: BET S.A. 204 m2 f t, P.Y. 0.65 cm3 got) with requisite amount of aqueous copper (II) nitrate (Fluka) solution. The samples were dried at 110·e for 24 h, pelleted, crushed and sieved to a particle size of 0 .5 mm and then calcined at 400·e for 5 h. The copper content was analyzed by atomic adsorption spectrometer (Perkin-Elmer Model 5000) after acid digestion until dissolution.

Catalyst characterization XRD patterns were recorded on a Philips PW 1051

diffractometer using Ni filtered CuKa radiation . The scanning electron micrographs were recorded on a Hitachi SF-20 scanning electron microscope. The catalyst samples were coated with gold and scanned at 10 KY. BET surface areas of the catalysts were determined by N2 adsorption at -196·C using a conventional all glass volumetric high vacuum system with a stationary background vacuum of 10-6 Torr. The same high vacuum system that was used for BET surface areas was employed for CO chemisorption studies. Prior to the chemisorptions of CO the catalyst sample (-0.5 g) , taken in a U-tube, was reduced in situ at a temperature of 250·C for 5 h by flowing H2 (50 cm3 minot) followed by evacuation (10-6 Torr) for 2 h at the same temperature. After cooling to room temperature, CO (Matheson, 99.9% purity) adsorption isotherms were measured at 2Te (maintained using

water bath). The first adsorption isotherm representing both strongly and weakly adsorbed CO was generated with an equilibration time of 20 min at each pressure. The sample was then evacuated for 30 min and the second isotherm representing only the weakly adsorbed (reversible) CO was generated. The difference between the two isotherms gives the irreversibly adsorbed CO uptakes. The weakly adsorbed CO is a measure of copper metal area of the catalysts22

• The metal area is calculated23 assuming a Cu:CO stoichiometry of 1: 1.

Copper surface area (m2 g-t cat) = CO uptake (reversible in moles g-t cat) x stoichiometry x Avagadro number + number of copper atoms per unit area of surface (1.47 x 10t9 m-2).

The particle size was calculated assuming the presence of spherical particles, from the equation24. 25 .

dcu = 6000/density of Cu x SCu

where, dcu is the average particle size in nm; 8.92 g cm-3 is the density of Cu and SCu is the copper surface area in m2 g-t Cu.

Temperature programmed reduction (TPR) studies were conducted on Auto Chern 2910 (Micromeritics, USA) instrument. In a typical TPR experiment about 200 mg of CuO/y-AI20 3 sample dried overnight at 100·C was taken in a U-shaped quartz sample tube. Prior to TPR experiments the catalyst samples were pretreated by passing ultra high pure 99.999% helium (Indian Oxygen Limited) at a rate of 50 cm3 min-tat 200·e for 2 h. After pretreatment the sample was cooled to room temperature. The reducing gas consisting of 5% hydrogen and balance argon (Indian Oxygen Limited) was purified by passing (50cc minot) through oxy-trap and molecular sieves. It was then allowed to pass over the sample and data was recorded simultaneously. TPR analysis was carried out from ambient to 600·C at a heating rate of lo·e min-t o The water produced during the reduction was condensed in a cold trap immersed in liquid nitrogen and isopropanol slurry. Ag20 sample is used as a standard reference for TPR measurements.

A JECL FE-3X and Bruker ER 200D-SRC spectrometers were used to record the X-band ESR spectra of unreduced and reduced catalysts at room temperature and also at low temperature (- I23 ·C) with 100 kHz modulations. The catalyst samples for ESR study were taken in a quartz tube (20 cm long, 3 mm diameter) oven dried for 12 h at 120·e, sealed and then the spectra were recorded. The catalyst

1174 INDIAN J CHEM, SEC A, JUNE 2004

samples to be studied in the reduced condition were placed in a U-tube attached to a graded seal (glass + quartz) of 20 cm long and 3 mm diameter. The U-tube was then attached to the high vacuum system and was reduced in a continuous flow of purified hydrogen (50 cm) min-i) at 250°C for 5 h_ The sample after reduction was subsequently evacuated at the reduction temperature for 1 h, cooled slowly to room temperature and then evacuated to 10-6 TorL The catalyst sample was detached from the high vacuum system under vacuum and then the sample was transferred to the graded seal sample tube and sealed off under vacuum before the ESR spectrum of the reduced sample was recorded _

Catalytic activity determination A fixed bed flow n".icro-reactor of 15 mm Ld

operati ng under normal atmospheric pressure was loaded with about 0_5 g of catalyst sample and held in place by g lass wool plugs_ Cyclohexanol was fed at a rate of 6 cm) hoi us ing a microprocessor controlled Secura (B. Brown) syringe pump. The activity of the catalys ts for the react ion of cyclohexanol was measured at reaction temperatures of 200, 250 and 300°C. The products were collected in an ice cooled trap and were analyzed by gas chromatography using 10% Carbowax 20 M on Chromosorb Wand a flame ionisation detector. The main products in the reaction were cyclohexanone and cyclohexene with no other ide products. For studying the activity behaviour,

CuOfy-AbO) with 10.2 wt. % C u loading was chosen and the catalyst was reduced in H2 at 250°C for 5 h prior to the catalytic acti vity study. The activity of the catalyst was studied in its reduced as well as unreduced fo rm for comparison.

I a I [b)

Results and Discussion

Morphology X-ray diffraction patterns of uncalcined (dried)

CuOfy-AhO) catalysts showed the presence of y­AIOOH (JCPDS No: 21-1307) in all the catalysts though with reduced intensity from 6.3 wt% Cu onwards. CU2 (OHhNO) (JCPDS No : 14-687) phase is seen from 6.3 wt% Cu onwards and the intensity is found to increase with Cu loading . In samples calcined at 400°C CuO (JCPDS No: 5-0661) phase

was detected at a copper loading of 2 6.3 wt%, the intensity of which increased with increase in CLI loading and y-A I20 ) (JCPDS No: 10425) phase is found in all the samples. This shows that copper is well dispersed up to a loading of 6 .3 wt% Cu. Figure

I shows SEM micrographs of calcined CuOfy-AbO) catalysts (1.9 wt% Cu, 6 .3 wt% Cu and 18.7 wt% Cu). These micrographs indicate fairly good di spersion of

CuO on y-AI20 ) surface at lower Cli loadings ( 1.9 wt% Cu) [Fig. I (a)] and agglomeration of CuO at high loadi ng [Fig. I (c) 18.7 wt% Cu]. The white regions may be ass igned to enriched CliO [Fig. 3(b) 6.3 wt% Cu] and the dark reg ions may be attributed to alumina with a dispersed copper phase, perhaps a surface CuAI20 4 26 .

Characterization of the catalysts BET surface area, copper area, and crystallite size

of copper are show n in Table 1. The BET surface areas of these catalysts decreased fro m 189 to 139 nl g.' wi th increase in copper load ing from 0.4 to 18.7 wt%. T he decrease in BET surface area can be attributed to pore blocking by copper spec ies with increase in copper loadi ng . Variation of CO uptakes

Ie)

Fig. I-SEM micrographs of CuO/y- A1203 cata lysts: (a) 1.9 wt. % ClI; (b) 6.3 wt. % C u; (c) 18.7 wt. % Cu.

RACHEL et al.: STUDIES ON Cu/y -A120 3 CATALYSTS 1175

Table I-BET surface areas, copper surface areas and copper crystallite size values of Cu/y-Alz03 catalysts with varying Cu-

loading

Copper BET surface area, Copper surface Crystallite siz( content. wt% m2g.1 cat area*, m2g.1 Cu ofCu*, nm

0.4 189 62.50 10.76 1.9 163 38.95 17.27 3.8 163 29.21 23.03 6.3 164 26.03 25.84 10.2 162 19.22 35.00 12.7 144 14.80 45.45 17.7 142 6.95 96.78 18.7 139 4.81 139.84

*Determined from reversible CO uptakes.

as a function of Cu loading in CuO/y-Ah03 catalysts is shown in Fig. 2. The reversible CO uptakes increased from 6 f.l moles g.1 cat at 0.4 wt% Cu to 48 f.l moles g.1 cat at 10.2 wt% Cu and thereafter decreased with increase in Cu loading. This shows that maximum surface Cu species are present in the catalyst with 10.2 wt% Cu loading, The decrease in reversible uptake at 12.7 wt% Cu is only marginal. The copper surface area determined by CO adsorption ranged from 4.81 to 62.5 m2g.1 Cu. These values are in agreement with those reported by Scholten and Konvalinka27 for different copper catalysts and Kanta Rao and co-workers8 for Cu/y-Ah03 catalysts prepared by urea hydrolysis.

The copper crystallite size calculated from the CO chemisorption (reversible) increased gradually with an increase in copper content until the copper area reached a maximum value and then increased steeply

80.-------------------------------,

~ ~C> 60 -(5 E

~ .Q 40

e-o

'" "0 ell

o 20 o

°0~~---4~----6~----~12~--~1~6----~20

% WI. Cu ___

Fig. 2--C0 uptakes as a function of copper loading in Cu/y-AI20 ) catalysts: O· total; />; - reversible; - irreversible

with further increase in copper loading beyond 12.7 wt% Cu which is an indication of agglomeration of copper crystallites at higher copper loadings . Strohmeir et al. 11 reported an increase in crystallite size in the region of 17-26 wt% Cu on y-Ab03 with a surface area of 195 m2 g .1 . The results of the present study of crystallite size of copper species determined from reversible CO uptake are also in general agreement with the results reported in the literature based on N20 decomposition II.

TPR studies Temperature programmed reduction profiles of

CuO/y-Ab03 catalysts are shown in Fig. 3. The profile at low copper loading is almost flat with nominal hydrogen consumption indicating the near non-availability of reducible copper species on this catalyst. These copper species are mostly isolated copper ions that interact with alumina support to form surface copper aluminate. This compound is not easily reduced by hydrogen compared to copper oxide28

. A clear reduction profile is observed with increase in copper loading showing two peaks at 3.8 wt% Cu and these two peaks are seen merged around

... G ... ..

=

Wt'l. Cu

11.1

2k 10 .2

6.3

~ 3.8 0 .9

)00 200 )CO 400 500 600

Fig. 3--TPR profi les ofCuO/y·AlzOJ catalysts

1176 INDIAN J CHEM. SEC A. JUNE 2004

10.2 wt %Cu loading and are seen again resolved at higher loadings of copper. Dow et al. reported 28.29 the presence of a low temperature reduction peak, ~ and a high temperature reduction peak, y in the TPR profiles of CuO/y-AIz03 catalysts. In our study, the low temperature peak (Tmax < 250°C) may be attributed to the reduction of highly dispersed amorphous CuO species and the high temperature peak (Tmax >. 270°C) to the reduction of crystalline CuO phase that includes large clusters and bulk CuO. The TPR profiles of CuO/y-AIz03 catalysts shown in Fig. 3 are in good agreement with the reported profiles28.33. Our results on ESR spectra of CuO/y-AI203 catalysts also support the presence of Cu species as interacted, isolated and bulk crystalline depending on the amount of copper loading.

ESR studies Figure 4 shows the ESR spectra of unreduced

CuO/y-AI203 catalysts of different loadings. The species that are responsible for the ESR spectra l2 are generally found to be magnetically dilute cupric ions (isolated), interacted species (CuA1z04 bulk and CuAlz04 surface) and crystallites of cupric oxide. The spectrum is well resolved at lower concentrations of copper. As the concentration of copper is increased. the hyperfine separation is not distinct. the line broadening may be seen as due to exchange interaction and dipole interaction32-34. The Spin-

(0)

---4\, . ..------H

12%

\

I

32006

--. 125G

(b)

Hamiltonian parameters obtained using computer simulation technique along with experimental values are given in Table 2 for the various Cu loadings. Computations were carried out by using a personal computer with Macintosh operating system. The software35 and conditions36 for the different geometries encountered were developed in FORTRAN 77 language. Further the spectra were also recorded at low temperature (-123°C) and were found to be identical to those recorded at room temperature (2rC), indicating that the resolution of the spectrum does not depend on the spin-lattice interaction but on dipolar interaction which depends on the nature of deposition.

Figure 5 shows the spectra of reduced CuO/y-AIz03 catalysts of different copper loadings. The broad unresolved spectra of reduced CuO/y-AIz03 catalysts show decrease in intensiti7 with increase in Cu loading (under the reduction conditions employed in this study), on account of the movement of copper atoms along the opened path arising out of removal of O2 atoms leading to the formation of clusters 14.

From the Spin-Hamiltonian parameters obtained from unreduced CuO/y-AI203 catalysts and a comparison of the g values with co-ordination geometries, it is clear that in unreduced form copper has a shape of tetrahedral geometry with a high gil whereas in reduced form it has a shape of elongated octahedral with a low gIl as shown in Scheme I.

I , 32006

H 12%

(c)

3200(;

Fig. 4--ESR spectra of CuO/y-AI20) catalysts (unreduced) (a) 0.4 wt. % Cu; (b) 0.9 wt.% Cu; (c) 6.3 wt. % ClI: - experimental; -­simulated.

RACHEL et al.: STUDIES ON Cu/y -AhO) CAT AL YSTS ! 177

Comparison of the reduced and unreduced spectra also shows the existence of an interaction38

-40 of copper with y-Ah03 as evidenced by the broad unresolved lines in reduced state caused by the movement of copper atoms leading to the formation of clusters after the removal of oxygen atoms. However, the increase in copper concentration in the

Table 2-Experimental and simulated spin Hamaltonian parameters of CuO/ AI 20) catalysts

S~in Hamiltonian ~arameters Catalyst wt. gil gl All Ai. DEL % Cu

0.4 2.43 2.15 135 8.75 (2.43) (2.15) (135) (16) (55)

0.9 2.428 2.168 145 15 (2.43) (2.17) (145) (15) (93)

6.3 2.42 2. 18 140 18.75 (2.43) (2.17) (140) (5) (93)

Values in parenthesis are simulated ones.

Wt. °1. Cu

0.4

0 .9

\0.2

t 3200G

Fig. ~ESR spectra of CuO/y-AI20) catalysts (reduced).

2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50

CuSe,.. CuN, ..

CuN,o,.CuNO, CuO, ..

LJ Cuo,

(d..,) ground state (d". ,,) ground state

Scheme I--Coordination geometries and g values

100 r---------------------------~

100

Cl

>-

>

v ... ~ 50 c' 0

. ~

... > c 0 0 u

~ 100 D ~ r

50

o

o ~~--~----~----~--~--~ 2 6 10 11. 18

Melalloading (eu wl·I.)

Fig. 6--Cyc1ohexanol conversion and selectivity over copper­alumina catalysts as a function of copper loadings at different temperatures: 0 - cyc1ohexanol conversion (%); t-,. - selectivity of cyc1ohexanone (%); 0 -selectivity of cyc10hexene (%).

1178 INDIAN J CHEM. SEC A. JUNE 2004

reduced catalysts led to a decrease in intensity of the peaks, whereas in unreduced catalysts it resulted in line broadening due to dipolar and exchange interactions. From the ESR study of copper-thorium oxide catalysts, Bechara et al.41

-43 have shown that at

an atomic ratio Currh of < 0.01, copper is localized in the Th02 structure as isolated Cu ions with an eight fold coordination and at an atomic ratio of Currh > 0.01 monomeric Cu2+species and Cu2+ ion pairs are present.

100

50

o 100 , -

.~ -~ -u ~ • ..

1-

"CI C e o C

• ,. ! o u 0 ,.. u _ 100 o c .! .. ... • ,. c o u

:-: SO

o

-

-

a

3m-C

--6-~

0 0 0..

~

-0 0 .0 0-0

2~·C

~

~. 200·C

~

~ 2 1 " 5

Catalytic activity Figure 6 shows vanatlon of cyclohexanol

dehydrogenation activity over unreduced CuO/y­Ah03 catalysts at 200, 250 and 300·C as a function of copper loading. The dehydrogen ation activity is clearly seen even at 200·C. Cyclohexanol conversion increased from 4 to 18 % with increase in copper loading and remained almost constant above 10.2 wt% Cu. While the selectivity towards cyclohexanone increased from 60 to -95%, that of cyclohexene

b 100

50

O~--.---~--~~--__ ~~ 100

SO

O~--~--~----~--~--~-~ 100

c

50

Vi"" (tl)

Fig. 7--Cyclohexanol conversion and selectivity over unrcduced and reduced copper-alumina catalyst with 10.2 wt. % Cu: (a) unreduced ; (b) reduced ; 0 - cyclohcxanol conversion (%); t. - selectivity of cyclohcxJnonc (%); 0 - selectivity of cyclohexcne t %).

RACHEL et al.: STUDIES ON Cu/y -A120 3 CATALYSTS 1179

decreased from 35 to 5% with increase in Cu content which remained almost constant ~ 10.2 wt% Cu. At 250°C the cyclohexanol conversion remained more or less constant at -25 % as a function of Cu loading. The selectivity for both the products cyclohexanone (85%) and cyclohexene (15%) too remained almost constant up to lO.2 wt% Cu and beyond that the cyclohexanone selectivity decreased to -65% showing a corresponding increase in cyclohexene selectivity. Similarly at 300°C too cyclohexanol conversion remained almost constant at -40%. The cyclohexanone selectivity is max imum (-75%) at 10.2 wt% Cu and then decreased. An opposite trend is observed for cyclohexene selectivity showing a minimum at lO.2 wt% Cu. During the reaction of cyclohexanol at 300°C on catalysts with low and high Cu contents the dehydration activity of the support is more evident whereas, at medium loadings of 6.3 -12 .7 wt% Cu an increased tendency towards dehydrogenation is observed. For catalyst with 10.2 wt% CuO/y-AbO) at 30CrC the active Cu species manifest its properties better than that of the support. The dehydrating behaviour of the catalyst at low Cu contents may be due to the avai lable free support and at hi gh Cu contents it may be due to the agglomerati on of the Cu crystallites44 making the support partly available for the dehydration activity. This is more evident at 300°C as y-AI:!O) also gets activated exhibiting more dehydrati ng behaviour.

To further understand the dehydrogenation activity , over the unreduced and reduced CuO/y-AI :!O) catalysts, 10.2 wt% Cu catalyst is studied in detail. Figure 7 a and b shows the cyclohexanol conversion over unreduced and reduced catalys ts with 10.2 wt% Cu at reaction temperatures of 200, 250 and 300°C fo r a period of 5 h under identical experimental conditions used for CuO/y-A120 3 catalysts of other composition in their ox idic form. The nature of the reaction appears to be same at 200 and 250°C on both reduced and unreduced catalyst systems except variations in conversion and selectiv ity levels. The selectivity fo r cyclohexene is somewhat more on reduced catal yst whereas, the selectivity for cyclohexanone is more on the unreduced system. However, at 300°C on the reduced catalyst the convers ion is seen decreasing from 46% to 24% until 4 h and then remained constant. Cyclohexanone selectivity decreases and cyclohexene selectivity increases at this temperature. On the other hand on unreduced catalyst, conversion is rather steady at

about 40-48 % for the period studied and both the cyclohexanone and cyclohexene selectivity too remained almost constant.

The decrease in conversion in the reduced CuO/y­Ah03 (10.2 wt% Cu) catalyst at 300°C may be due to the initial activity resulting in coke formation (deactivation). The increase in cyclohexene selectivity may perhaps be due to the increased activity of the support at thi s temperature (300°C). This was not observed on unreduced catalysts perhaps due to slow deactivation. Significant advantages of oxidative dehydrogenation of cyclohexanol over CuO-ZnO catalyst were reported45

. As dehydrogenation is the property of the copper metal , the formation of cyclohexanone clearly indicates that the CuO is undergoing ill situ reduction by cyclohexanol during the course of the reaction on unreduced CuO/y-AbO) catalysts, which is confirmed by ESR46

. The XRD pattern of the used catalyst with 10.2 wt% Cu catalyst showed a-Cu phase with 28=55 and 64° which is similar to that of the hydrogen reduced sample confirming the reduction of the oxidic copper species during cyclohexanol reaction.

Conclusions The X-ray diffraction patterns and scanning

electron micrographs show the formation of tillY crystallites of CuO at a copper loading of 6.3 wt% Cu in unreduced catalyst indicating the build-up of CuO crystallites. Reversible carbon monoxide uptake is maximum on 10.2 wt% Cu catalyst and is only marginally low on 12.7 wt% Cu indicating that reducib le Cu species are maximum on these catal ys ts. The TPR profi les show mainly the presence of amorphous and crystalline copper oxide on y-A lc03 support depending on the Cu content. The ESR spectra reveal a definite change in the structure on reduction and showing the ex istence of an interaction between copper and y-alumina as a functi on of Cu loading. The conversion of cyclohexanol is maximum at ~ 10.2 wt% Cu loading. The selecti vi ty to cyclohexanone is maximum on catalysts containing lO.2 wt% Cu. X-ray diffraction patterns confirmed the reduction of CuO to metallic copper, du ring the course of the reaction of c clohexanol on CuO/y-

120 3 catalysts.. reaction temperature of 200-250°C appears to be ideal fo r the selective conversion of cyclohexanol to cyclohexanone. Pre-reduction of the CuO/y-Ab03 catalyst is not essenti al for the selec ti ve conversion of cyclohexanol to cyclohexanone.

1180 INDIAN J CHEM, SEC A, JUNE 2004

Acknowledgement We thank Prof. B Anjaneya Shastry, Physics

Department, Osmania University for his valuable suggestions and Dr. JAR P Sharma for devoting his time during simulations. Thanks are also due to the UGC and CSIR, New Delhi for the award of fellowsh ip to AR (RA and SRF).

References 1 Trimm D L & Stanislaus A, Appl Catal, 21 (1986) 215. 2 Knozinger H & Ratnasamy p, Catal Rev Sci Ellg, 17 (1978)

31. 3 Absi-Halabi M, Stanislaus A & A1-Zaid H, Appl Catal, 101

(1993) 17. 4 E1-H akam S A, Adsorpr Sci Tecllllol, 12 (1995) 221. 5 Thomas C L, in Catalyric Process alld Provell Caralysrs

(Academic Press, New York) 1970. 6 Severino F, Brito J, Carias 0 & Laine J, J Catal. 102 (1986)

172. 7 Sivaraj Ch & Kanta Rao P, Appl Caral. 45 (1988) 103. 8 Sivaraj Ch, Srinivas S T , Nageswara Rao V & Kanta Rao P,

J Molec Caral, 60 (1990) L23. 9 Evans J W, Wainwright M S, Cant N W & Trimm D L, J

Catal, 88 (1984) 203. 10 Friedman R M, Freeman .J J & Lytle F, J Catal. 55 (1978)

10. 11 Strohmeir B R, Leyden D E, Field R S & Hercules D M, J

Cared. 94 (1985) 514. 12 Wolberg A & Roth J F, J Cauil. 15 (1969) 250. 13 Kobayashi H, Takezawa N, Shimokawabe M & Takahashi

K, in Prepararioll of Catalysts 11/ (Elsevier, Amsterdam) 1983.

14 Berger P A & Roth J F, J Phys Chelll, 71 (1967) 4307. 15 Barber M, Sharpe P K & Vikerman J C, J Catal. 41 (1976)

240. 16 Bolt P H, Habraken F H P M & Geus J W, Srud SlIrf Sci

Carat, 288 (1994) 425. 17 Vorobev V N, Shadmanov K K, Kamilov K M & Razikov K

Kh, Killerics alld Katalysis, 27 (1986) 881. 18 Pepe F. Angletti C, Rossi S D & Jaco no M L, J Cared. 91

( 1985) 69. 19 Echevin B & Teichner S J, Blill Chelll Soc Chelll Fr, 7-8

(1975) 1495.

20 Volta J C, Turlier P & Trambouze Y, J Caral, 34 (1974) 329. 21 Cunningham J, Hodnett B K, lIyar M, Tobin J & Leahy E L,

Faraday Discllss Chem Soc, 72 (1981) 233.

22 Parris G E & Klier K, J Careli. 97 (1986) 374. 23 Lee J C, Trimm D L, Wainwright M S & Cant N W, Appl

Caral, 60 (1990) 173. 24 Bantley J, Burch R & Chappel R J, Appl Careil, 43 (1988) 91. 25 Evans J W, Wainwright M S, Bridgewate r A J & Young D J,

Appl Caral, 7 (1983) 75.

26 Aguido A L, Palacios J M & Fierro J L G, Appl Cared , 91 (1992) 43.

27 Scholten J J F & Konvalinka J A, TrailS Farad Soc, 65 (1969) 2465 .

28 Dow W P, Wang Y P & Huang T J , Appl Coral A, 190 (2000) 25.

29 Dow W P, Wang Y P & Huang T J, J Carat, 160 (1996) 155.

30 Bond G C, Namijo S N & Wakeman J S, J Malec Caral, 64 (1991) 305.

31 Robinson W RAM & Mol J C, Appl Coral, 44 (1988) 165.

32 Kulbe K C & Mann R S, J Catal , 112 (1 988) 579. 33 Fujiwara S, Katsumata S & T Se ki , J Phys Chelll , 71 ( 1967)

115. 34 Rachel Anita, Durgakumari V & Kanta Rao P, New J Chelll .

19 (1995) 943 . 35 Subramanian R, Ph D Th esis (Osmania University,

Hyderabad, India) 1993.

36 Bleany B, Proc Phys Soc Lolldoll A, 75 ( 1960) 621.

37 Jacono M Lo, Cimino A & Inversi M J , J Caral. 76 ( 1982) 320.

38 Lumbeck H & Voit1ander J, J Caral, 13 (1969) 117. 39 Voge H H & Voge L T, J Cared, 1(1962) 171.

40 Giamello E, Fubini B & Lauro P, Appl Cawl. 21 ( 1986) 133. 41 Bechara R, Wobel G, Aissi C F, Guelton M, Bonelle J P &

Abou- Kais A, Chem Marer, 2(5) (1990) 5 18. 42 Bechara R, 'Huysser A D, Ai ssi C F, Guelton M, Bonnelle J

P & Abou - Kais A, Chelll Marer, 2(5) (1 990) 522.

43 Abou-Kais A, Bechara R, Ghoussoub D, Aissi C F, Guelton M & Bonnelle J P, Faraday TrailS, 87 ( .1 991 ) 631.

44 Kaushik V K, Sivaraj Ch & Kanta Rae> P, Appl Slirf Sci, 51 (1991) 27 .

45 Ming Lin Yu, Wang I & Yen CoT, Appl Cared, 41 ( 1988) 53.

46 Rache l Anita & Kanta Rao P, Reacr Killer Caral Lerr , 52 (1994) 325.