This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 1665–1673 1665

Cite this: Catal. Sci. Technol., 2012, 2, 1665–1673

Vapor phase selective hydrogenation of acetone to methyl isobutyl ketone

(MIBK) over Ni/CeO2 catalysts

Pendyala Venkat Ramana Rao, Vanama Pavan Kumar, Ginjupalli Srinivasa Rao

and Komandur V. R. Chary*

Received 14th January 2012, Accepted 12th April 2012

DOI: 10.1039/c2cy20021j

Ceria supported nickel oxide catalysts with varying nickel loadings from 1.0 to 20.0 wt% were

prepared by the impregnation method. The catalysts were characterized by X-ray diffraction

(XRD), UV-visible diffuse reflectance spectroscopy (UV-DRS), temperature programmed

reduction (TPR), temperature programmed desorption (TPD) of CO2, and surface area

measurements. The dispersion of nickel and metal area were determined by the hydrogen

chemisorption method. The X-ray diffraction patterns suggest the presence of crystalline NiO

phase beyond 2.5 wt% of Ni on ceria. The UV-visible diffuse reflectance spectra reveal the

presence of two types of nickel species on the CeO2 support. TPR patterns reveal the presence of

highly dispersed surface free nickel oxide species at lower temperatures and bulk NiO at higher

temperatures. The basicity of the catalysts measured by the CO2 TPD method was found to

increase with an increase in nickel loading up to 2.5 wt% and decrease with further increase in

nickel loading. The vapor phase condensation and selective hydrogenation of acetone to methyl

isobutyl ketone (MIBK) were carried out on Ni/CeO2 catalysts and the catalytic properties are

correlated with the results of CO2 TPD measurements and also with the dispersion of the nickel

species supported on ceria.

Introduction

Supported nickel catalysts are well known and they have been

employed in many industrially important reactions. These

catalysts find wide applications in hydrogenation,1,2 steam-

reforming reactions,3 reductive amination of alcohols,4 hydro-

dechlorination,5,6 partial oxidation,7 and dry reforming of

methane.8 The commonly used supports for nickel are Al2O3,

SiO2, ZrO2 and TiO2. The supports play an important role as

they alter the reducibility of the metal ion, the dispersion,

crystallite size of the metal and control sintering of the catalyst

leading to deactivation.9 It is also well known that a strong

metal–support interaction exists between the nickel and support

material and this interaction leads to high dispersion of nickel.

A plethora of research work has been carried out in the recent

past to understand the interaction of nickel with support and the

catalytic performance during hydrogenolysis reactions. Cerium

dioxide (CeO2) is one of the extensively investigated oxides among

the rare earth metal oxides and widely investigated for application

in ceramics and also in industrial catalysts.10 Ceria is one of the

most important components of fluid catalytic cracking (FCC)

catalysts.11 Other significant applications of cerium-containing

catalysts include removal of soot from diesel engine exhaust,12

removal of organics from wastewaters,13 as an additive for

combustion catalysts,14 and in fuel cell processes.15 The influence

of cerium-containing materials in various other catalytic

processes is being actively investigated.

4-Methyl-2-pentanone, methyl isobutyl ketone (MIBK), is

one of the most important products derived from acetone.

This compound is mainly used as a solvent for vinyl, epoxy

and acrylic resin production as well as for dyes and nitrocellulose.

MIBK is also employed as an extracting agent for antibiotic

production or removal of paraffins from mineral oils in the

synthesis of rubber chemicals, and in the fine chemistry

applications.16 The global demand for MIBK is estimated to

be 300 000 t per year. Diisobutyl ketone (DIBK), a consecutive

product in the synthesis of MIBK, is an exceptionally good

solvent for a wide variety of natural and synthetic resins. It is

also used in pharmaceutical and mining industries.

The main reaction pathway for the synthesis of MIBK from

acetone is shown in Scheme 1. Industrially important chemicals

like methyl isobutyl ketone (MIBK) and 2-ethylhexanal are

usually produced in a three-step process, wherein the three

reactions steps are carried out separately. The first step is a

base-catalyzed aldol condensation of acetone to diacetone

alcohol (DA), followed by a dehydration of DA to mesityl

oxide (MO), which can be catalyzed under acidic, or under

basic conditions.17 The last step is the selective hydrogenation

Catalysis Division, Indian Institute of Chemical Technology,Hyderabad-500 007, India. E-mail: kvrchary@iict.res.in;Fax: +91-40-27160921; Tel: +91-40-27193162

CatalysisScience & Technology

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1666 Catal. Sci. Technol., 2012, 2, 1665–1673 This journal is c The Royal Society of Chemistry 2012

of MO to MIBK.18 The three-step process is found to be

disadvantageous, because the yield of the first two steps is

limited by the thermodynamic equilibria while only the third

step, the hydrogenation, is a thermodynamically favored step

towards the end product.19 The single step process is facile and

more economically viable and is of great interest to finding

new, improved catalyst systems operating at lower pressures.

One-step synthesis of MIBK in the vapor phase is also an

attractive one. Usually, this is carried out at 140–340 1C under

ambient pressure; however, the MIBK selectivity is generally

lower than in the liquid-phase reaction, and catalyst deactiva-

tion may be a problem in this process. Furthermore, the large

waste streams from the use of homogeneous catalysts have to

be reduced significantly. To deal with the thermodynamic

constraints and the more stringent environmental legislation,

several catalytic systems have been investigated for the single-stage

process wherein n-butyraldehyde and H2 into 2-ethylhexanal

or acetone and H2 into MIBK are directly converted. The

catalysts investigated so far in the gas phase conversion are

based on molecular sieves as supports such as Pt/H-ZSM-5,20

Pd/SAPO-11 and Pd/AlPO-11,21 Ni/AlPON,22 Pt/Cu/

H(Al)-ZSM-5,23 and Pt/Cs–X and Pt/Na–X,24 Pt/C,25 or they

are produced with oxidic supports such as Cu/MI(MII) oxides,26

Pd/Mg(Al)O,27 Ni/CaO–C,28 Cu/MgO,29 Pd/Na–MgO,30

Pd-polyoxometalates,31 (Pd or Ni)/hydrotalcites,32 Ni/MgO,33

and Amberlyst.34 The reactions were typically studied using

fixed-bed tubular reactors at atmospheric pressure and at

temperatures that rarely exceed 473 K. Obviously, platinum,

palladium, nickel, and copper are the preferred metals for

implementing a catalytic hydrogenation activity. Although

these single-stage processes seem to be promising, however,

high reaction temperatures and pressures are required which

favor the side-reactions to occur which lower the selectivity of

the process and also cause catalyst deactivation.35,36 Although

the use of catalytic distillation37 might overcome these

problems, a process operating at low temperatures appears

to be more suitable.

In the present investigation we report the characterization

of NiO/CeO2 catalysts by powder X-ray diffraction (XRD),

UV-vis diffuse reflectance spectroscopy (UV-DRS), temperature

programmed reduction (TPR), temperature programmed

desorption (TPD) of CO2, and Ni dispersion, metal area by

pulse hydrogen chemisorption method. The catalytic properties

were evaluated for the vapor phase acetone condensation and

selective hydrogenation to methyl isobutyl ketone (MIBK).

The purpose of this work is to estimate the dispersion of NiO

supported on ceria as a function of nickel loading, to identify

changes in the structure of the NiO phase with loading to

understand the relation between the dispersion of Ni and

basicity of catalyst for acetone condensation and selective

hydrogenation reaction.

Experimental

Ceria support was prepared from saturated aqueous cerium

nitrate hydrate Ce(NO3)2�6H2O (Aldrich), with the addition of

aqueous ammonia till pH reaches 9. The resulting precipitate

was washed repeatedly with portions of distilled water until

the precipitate is free from the base. The precipitate was dried

at 383 K for 12 h and the resulting hydroxide was calcined in air at

773 K for 5 h. A series of nickel catalysts with Ni loadings varying

from 1.0 to 20.0 wt% were prepared by wetness impregnation

with a requisite amount of Ni(NO3)2�6H2O (Fluka). The

samples were dried at 383 K for 16 h and subsequently

calcined at 773 K for 5 h in air.

X-ray powder diffraction patterns were obtained with a

Rigaku Miniflex diffractometer, using nickel filtered Cu Karadiation (1.5406 A) at 30 kV and 150 mA. The measurements

were recorded in steps of 21 with a count time of 1 min in the

2y range of 5–801. The morphological features of the catalysts

were monitored using a JEOL JEM 2000EXII transmission

electron microscope, operating between 160 and 180 kV. The

specimens were prepared by dispersing the samples in methanol

using an ultrasonic bath and evaporating a drop of resultant

suspension onto the carbon support grid.

The specific surface areas of the catalyst samples were

obtained from N2 adsorption–desorption data acquired on a

single point Pulse Chemisorb 2700 instrument (Micromeritics,

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 of

the N2 molecule was assumed in the calculations of the specific

surface areas using the method of Brunauer, Emmet, and

Teller (BET). Pore size distribution (PSD) measurements were

performed on Auto Pore III (Micromeritics, USA) by the

mercury penetration method.

UV-visible spectra were recorded in air at room temperature

using a GBC UV-visible Cintra 10e spectrometer with a diffuse

Scheme 1 Reaction pathway of condensation and selective hydrogenation of acetone.

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reflectance accessory, in the 200–800 nm wavelength range.

The CeO2 support was used as reference. The Kubelka–Munk

function F(R) was plotted against the wavelength (in nm).

Temperature programmed reduction studies were carried

out on an Auto Chem 2910 (Micromeritics, USA) instrument

to study the reducibility of nickel. In a typical experiment,

ca. 150 mg of oven-dried sample (dried at 383 K for 15 h) was

taken in a U-shaped quartz sample tube. The catalyst was

mounted on a quartz wool plug. Prior to TPR studies, helium

gas was passed with a flow of 50 mL min�1 at 473 K for 1 h to

pretreat the catalyst sample. After pretreatment, the sample

was cooled to ambient temperature and TPR analysis was

carried out in a flow of 5% H2–Ar mixture (50 mL min�1) from

ambient temperature to 873 K at a heating rate of 10 K min�1.

H2 consumption and Tmax positions were calculated using

GRAMS/32 software.

Temperature-programmed desorption (TPD) of CO2 studies

were conducted on the same instrument. In a typical experiment

for TPD studies ca. 200 mg of oven dried sample (dried at

383 K for overnight) was taken in a U-shaped quartz sample

tube. Prior to TPD studies, the catalyst sample was pretreated

at 473 K for 30 min by passing pure helium (99.999%, 50 mL

min�1). After pretreatment of the sample, it was reduced at

673 K for 2 h by passing pure hydrogen (99.99%, 50 mL min�1)

and subsequently flushed with pure helium (50 mL min�1) for

1 h to ensure a clean surface. After reducing the sample, it was

saturated with CO2 in a flow of 10% CO2–He mixture at 303 K

with a flow rate of 75 mL min�1 and was subsequently flushed

at 378 K for 2 h to remove physisorbed CO2. TPD analysis was

carried out from ambient temperature to 973 K at a heating rate

of 10 K min�1. The amount of CO2 desorbed was also

calculated using GRAMS/32 software.

Hydrogen chemisorption measurements were also done on

an Auto Chem 2910 instrument. Prior to adsorption measure-

ments, 250 mg of the sample was reduced in a flow of

hydrogen (50 mL min�1) at 673 K for 2 h and flushed out

subsequently in a pure argon gas flow for 1 h at 673 K. The

sample was subsequently cooled to 303 K in the same Ar

stream. Hydrogen uptake was determined by injecting pulses

of hydrogen from a calibrated on-line sampling valve into the

Ar stream passing over reduced samples at 673 K. The nickel

surface area was calculated assuming a stoichiometry of one

hydrogen molecule for two surface nickel atoms and an atomic

cross sectional area of 6.49� 10�20 m2 per Ni atom. Adsorption

was deemed to be complete after three successive runs showed

similar peak areas.

A down flow fixed bed reactor made of Pyrex glass was used

to test the catalysts for the vapor phase condensation and

selective hydrogenation of acetone to methyl isobutyl ketone

at atmospheric pressure. About 500 mg of the catalyst diluted

with an equal amount of quartz grains was charged into the

reactor and was supported on a quartz wool bed. Prior to

introducing acetone with a syringe pump, the catalyst was

reduced at 673 K for 2 h, in a purified hydrogen flow. After

pre-reduction, the reactor was fed with acetone (3 mL h�1) at

423 K in H2 (flow rate 40 mL min�1), which is used as a carrier

gas. The liquid products, mainly methyl isobutyl ketone

(MIBK), methyl isobutyl carbinol (MIBC), diisobutyl ketone

(DIBK) and diisobutyl carbinol (DIBC), were analyzed by a

Hewlett-Packard 6890 gas chromatograph equipped with a

flame ionization detector using a HP-5 capillary column.

The products were also identified using a HP 5973 quadruple

GC-MSD system using a HP-1MS capillary column.

Results and discussion

The X-ray diffraction patterns of the pure ceria and calcined

NiO/CeO2 catalysts are presented in Fig. 1. From Fig. 1, it can

be seen that the XRD patterns of the calcined samples show

visible reflections at about 28.41, 32.91, 47.31, and 56.21 (2y)corresponding to d = 3.12, 2.72, 1.91 and 1.63 A which

represent the indices of (111), (200), (220) and (311) planes

of CeO2, respectively. This indicates a cubic fluorite structure5

in NiO/CeO2, which is prepared by the impregnation method.

The absence of XRD peaks due to nickel oxide at lower

composition indicates that nickel oxide is present in a highly

dispersed amorphous state on CeO2. However, at lower loadings

of nickel (o5.0 wt%) the possibility cannot be ruled out for the

presence of nickel oxide crystallites having size less than 4 nm,

which is beyond the detection capacity of the powder X-ray

diffraction technique. This observation suggests that the

deposited nickel is in a highly dispersed state on CeO2 support,

which seems to be responsible for the acetone condensation

and selective hydrogenation to methyl isobutyl ketone.

XRD reflections due to the nickel oxide appeared for the

samples containing loadings of 5.0 wt% of Ni and above at 2yvalues equal to 37.291, 43.301 and 62.911 (shown as closed

circles in Fig. 1). The intensity of these three peaks was found

to gradually increase with the increase in Ni loading in the

catalysts. The XRD patterns also indicate that no characteristic

peaks were found due to the formation of a mixed oxide phase

between NiO and CeO2 support. All the reduced catalysts

showed the XRD peaks corresponding to metallic Ni, indicating

that NiO was successfully reduced to metallic Ni (Fig. 2). The

diffraction pattern of Ni/CeO2 catalyst exhibits a set of peaks

at 44.51 and 51.81 characteristic of metallic nickel,38 JCPDS

database (No. 01-1258), for the samples containing loadings of

5.0 wt% of Ni and above. The intensity of these peaks was

found to increase with the increase in Ni loading in the

Fig. 1 X-ray diffraction pattern of various NiO/CeO2 catalysts (K – due

to NiO).

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1668 Catal. Sci. Technol., 2012, 2, 1665–1673 This journal is c The Royal Society of Chemistry 2012

catalysts. Electron micro-diffraction was used to determine the

chemical structure of the small grains. Selected area electron

diffraction (SAED) patterns of the 2.5% and 20% reduced Ni/

CeO2 catalysts are shown in Fig. 3. SAED patterns of the

2.5% Ni/CeO2 catalyst show some irregular spots on the

diffraction circles, indicating the amorphous nature of the Ni

particles. 20% Ni/CeO2 catalysts resulted in regular spots on

the diffraction circles indicating crystallized nickel particles.39

The BET surface areas measured by nitrogen physisorption

for all of the samples are presented in Table 1. The specific

surface area of the pure CeO2 support was found to be 66 m2 g�1.

However, the BET surface area decreases as a function of nickel

loading on CeO2, and 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 1). The total pore

volume and total pore area of the samples measured by a

mercury penetrating porosimeter are reported in Table 1. The

total pore volume and total pore area are also found to decrease

with an increase in nickel loading in the catalysts.

The UV-visible spectra of calcined NiO/CeO2 samples are

shown in Fig. 4. For comparison, the diffuse reflectance (DR)

spectrum of the pure CeO2 sample was also recorded. The DR

spectrum of the pure CeO2 exhibits a characteristic band

around at 340 nm due to the O22� - Ce4+ charge transfer

transition.40–42 In order to confirm the presence of the nickel

species, UV-DRS for NiO/CeO2 catalysts was measured.

Fig. 4 shows the UV-DRS profiles for the various NiO/CeO2

catalysts in the visible range with respect to the amount of

nickel loading. All the catalysts showed two distinctive major

bands at 265 and 340 nm and a band in the range of 710–720 nm

appeared for the loadings of 5 wt% and above. The former band

at 265 nm indicates the presence of free NiO.43,44 The band at

around 340 nm was assigned to the O - Ce transitions.40–42

Fig. 2 X-ray diffraction pattern of various reduced Ni/CeO2 catalysts

(% – due to metallic Ni).

Fig. 3 Selected area electron diffraction images of 2.5% Ni/CeO2 and 20% Ni/CeO2 catalysts.

Table 1 Surface area and pore size distribution results of variousNiO/CeO2 catalysts

S. No.Ni/wt%

BET surfacea

area/m2 g�1

Total poreb

volume/mL g�1

Total poreb

area/m2 g�1Average poreb

diameter/A

1 0.0 66 0.54 72 2972 1.0 63 — — —3 2.5 58 0.49 69 2854 5.0 55 0.45 68 2635 10 50 0.34 62 2216 15 38 0.28 51 1867 20 32 — — —

a Measured from nitrogen physisorption. b Measured by mercury

porosimetry.

Fig. 4 UV-visible diffuse reflectance spectra of various NiO/CeO2

catalysts.

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Additionally, the weak d–d bands of octahedrally coordinated

Ni2+ were also observed at 720 nm.45,46 As the nickel loading

increases the amount of octahedrally coordinated Ni2+ species

is also increased. From these findings, we further confirm that

the octahedrally coordinated Ni2+ can be attributed to the

formation of bulk nickel oxide. The results of UV-DRS are

also in good agreement with XRD results observed, wherein

the intensities of the crystalline NiO peaks increase with the

increase in Ni loading.

TPR is a useful technique for characterizing reducible

catalysts of supported metal and metal oxide systems and also

offers the information about interactions between the active

metal and the supported oxide. Temperature programmed

reduction profiles of pure ceria and pure nickel oxide (inset)

are shown in Fig. 5. Pure NiO shows only one sharp peak at a

reduction temperature of around 673 K. Several researchers

reported the reducibility of pure CeO2 during the TPR

method.47–49 Bruce et al.48 reported the TPR of CeO2

with different surface areas. The reduction peak observed at

o700 1C has a linear correlation with surface area.48,49 The

H2-TPR of CeO2 in the present study suggests that the

reduction of CeO2 starts at 290 1C and two broad peaks are

observed at 482 and 800 1C, respectively. The peak at 482 1C

was assigned to the reduction of surface-capping oxygen of

ceria.50 The peak at 800 1C can be ascribed to the reduction of

bulk CeO2 from Ce4+ to Ce3+.51,52 Although the reducibility

of CeO2 means a possibility of reduction under reductive

conditions at the prescribed temperature, it does not always

indicate the existence of oxygen defect sites on the working

state of catalyst. It has been reported that oxygen defect sites exist

in an unreduced CeO2 particle, and that the concentration of

oxygen defect sites increases with decreasing CeO2 particle size.53,54

This is explained by the fact that enthalpy of formation of oxygen

defect sites decreases with decreasing CeO2 particle size.53

For TPR analysis of the unsupported NiO and pure support,

the reduction conditions applied were similar to those applied

for supported NiO/CeO2 catalysts.

TPR results of various NiO/CeO2 catalysts show a schematic

change in the reduction of nickel with an increase in nickel

loading. The TPR profiles of NiO/CeO2 catalysts are shown in

Fig. 6. All the samples exhibit three reduction profiles (denoted

a, b and g) during the TPR in the temperature range of

514–682 K except for the samples containing low Ni loadings

(1.0 and 2.5 wt%) which show only two peaks during TPR. The

hydrogen consumption values and Tmax during the TPR are

reported in Table 2. In all the cases, the consumption value

obtained corresponds to that expected for the reduction of the

NiO phase to metallic Ni0. Nevertheless, it is also noticeable

that, although in the error range, some excess of hydrogen is

observed in all the cases. This is reasonable, and it would

correspond to the surface reduction of the CeO2 particles,

occurring at the same time as that of NiO.55,56 This fact is

easily understood in terms of a ‘‘spillover’’ effect from hydrogen

adsorbed on Ni particles to the CeO2 surface. The Tmax position

of the a peak increases up to 2.5 wt% and decreases with

increasing Ni loading. The b peak was observed in the

temperature range of 591–625 K, the temperature and intensity

of this peak increased with an increase in nickel loading. The

g peak was observed at the high-temperature region, when the

nickel loading is higher than 2.5 wt%, and its intensity is

increased with an increase in nickel loading. The a peak can

be ascribed to the reduction of adsorbed oxygen, due to the

formation of Ni–O–Ce solid solution. When Ni2+ is incorporated

into the lattice of CeO2 to replace Ce4+ cations, charge unbalance

Fig. 5 Temperature programmed reduction profiles of pure ceria and

NiO (inset).

Fig. 6 Temperature programmed reduction profiles of various NiO/

CeO2 catalysts.

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1670 Catal. Sci. Technol., 2012, 2, 1665–1673 This journal is c The Royal Society of Chemistry 2012

and lattice distortion would occur within the structure of CeO2. As

a result, very reactive oxygen species are generated, which could be

reduced easily by hydrogen at low temperatures. Therefore,

Ni–O–Ce solid solution is formed in the Ni/CeO2 catalysts during

the preparation. Shan et al.57 reported similar observations in

their study of TPR of various Ni/CeO2 catalysts. The catalytic

performance is not only dependent on reduction behavior but is

also controlled by the number of interfacial active centers which are

related to the amount of metal oxide species having interaction

with surface oxygen vacancies of the oxygen-ion conducting

support.58 The amounts of these metal oxide species are measured

by the a-peak such as those shown in the TPR profiles of Fig. 6.

The b peak is due to the highly dispersed surface free NiO species,57

and the g peak is assigned to the high temperature TPR peak due

to the reduction of bulk NiO.57,59 As the Ni loading increases, the

TPR peak becomes broad and shifts to high temperature. The

broadening of the peak and shifting of the Tmax towards higher

temperatures might be due to an increase in crystallinity of

NiO with an increase in Ni loading as evidenced from XRD and

UV-DRS results. These results also suggest the existence of strong

metal–support interaction (SMSI) between NiO and CeO2 at low

Ni loadings. As can be seen from Table 2, the H2 consumption of

the a peak is found to be the highest for 2.5 wt% of Ni loading

compared to all the other samples. This clearly suggests that the

dispersion is found to bemaximum for 2.5 wt%ofNi onCeO2 and

decreases with further increase in nickel loading.

The hydrogen uptake, Ni percentage dispersion, metal surface

area and average particle size are calculated from the hydrogen

adsorption measurements of various Ni/CeO2 catalysts using the

following equations.

Percent of dispersion = (number of Ni0 atoms on the surface

� 100)/total number of Ni0 atoms

Average particle size (nm) = 6000/(Ni metal area per gram of

Ni � Ni density)

Metal surface area of catalyst (m2 g�1) = 2 � VH2� NA � ANi

where, VH2is the volume of hydrogen adsorbed catalyst

(in micromoles)

NA is the Avogadro number (6.023 � 1023)

ANi is the cross sectional area of Ni (6.49 � 10�20 m2)

Metal surface area of Ni = (metal surface area of catalyst/

percent of Ni)

It was observed that the dispersion and metal area of nickel

are increased and the average particle size is found to decrease

up to 2.5 wt% of Ni on CeO2 (Table 3). This might be due to

the presence of maximum number of dispersed nickel sites that

are available on the catalyst surface. Biswas and Kunzru60 also

reported similar observations with Ni loading. However, that

the dispersion and metal area are found to decrease, and the

average particle size is increasing beyond this loading is due to

the formation of NiO crystallites. These findings are in good

agreement with XRD and TPR results.

The basicity measurements of Ni/CeO2 catalysts were

carried out by the temperature programmed desorption of

CO2. The CO2 TPD profiles of pure ceria and various Ni/CeO2

catalysts are shown in Fig. 7. The CO2 uptakes by various

catalysts of different basic strengths are reported in Table 4.

The desorbed peak of CO2 was deconvoluted into three

temperature regions i.e. 330–500 K is weak basic sites,

500–700 K is moderate, and 4700 K corresponds to strong

basic sites.21 As can be seen from Fig. 7, the TPD profiles are

found to be similar in shape for all of the samples. In the range

373–873 K, one can find three fully unresolved desorption

peaks, with the maxima of desorption at 390, 672 and 778 K

respectively (Fig. 7). These peaks evidently correspond to the

weak basic sites and medium basic sites (e.g., OH and O groups).

Above 700 K, the formation of the third peak begins, corre-

sponding to the strong sites (e.g., O2� groups). In the case of

pure CeO2 support, only two peaks were noticed corre-

sponding to weak and moderate basic sites. CO2 TPD results

further suggest that CeO2 is less basic when compared to

Ni/CeO2 catalysts. Impregnation of NiO to CeO2 support

facilitates the formation of strong basic sites that desorb

CO2 at higher temperatures. The number of basic sites was found

to increase with Ni loading up to 2.5 wt% on ceria and decreased

with further increase in Ni loadings. The decrease in basicity at

higher nickel loadings might be due to the agglomeration of nickel

oxide crystallites. This behavior is in good agreement with the

Table 2 Temperature programmed reduction results of various NiO/CeO2 catalysts

S. No. Ni/wt% Tmax1/K

H2 consumption1/mmoles g�1 Tmax

2/KH2 consumption2/mmoles g�1 Tmax

3/KH2 consumption3/mmoles g�1

1 1.0 528 280 591 177 — —2 2.5 532 484 596 657 — —3 5.0 530 384 598 939 662 1524 10 527 342 604 1848 670 3325 15 525 326 609 2623 665 5436 20 514 310 625 3426 682 771

Where the symbols 1, 2 and 3 indicate the first, second and third reduction peaks.

Table 3 Hydrogen adsorption properties of various Ni/CeO2 catalysts

Ni/wt%

H2 consump-tion/mmoles g�1

Dispersion(%)

Metalarea/m2

gcatalyst�1

Metalarea/m2

gNi�1

Averageparticlesizea/nm

TOFa/s�1

1.0 46 53.9 3.58 358 1.9 0.1382.5 133 62.4 10.37 415 1.6 0.0885.0 161 37.9 12.6 252 2.7 0.05620 170 20.0 13.3 133 5.0 0.05115 145 11.3 11.25 75 8.9 0.05920 136 8.0 10.6 53 12.6 0.060

a Reaction conditions: T = 423 K; wt. catalyst = 0.5 g; feed rate of

acetone = 4.09 � 10�2 mol h�1.

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catalytic activity, which decreases with an increase in Ni

loading beyond 2.5 wt% Ni on ceria. The TPD results also

suggest that the strength of basic sites plays a crucial role in

determining the catalytic activity for acetone condensation

and selective hydrogenation. Yang and Wu21 have reported

that the basic sites from K-SAPO-11 and AlPO4-11 promote

the activity of acetone reaction.

The catalytic properties during the vapor phase condensation

and selective hydrogenation of acetone at 423 K exhibited by

various Ni/CeO2 catalysts are shown in Fig. 8. As can be seen

from Fig. 8, the acetone conversion was found to increase with

an increase in Ni loading up to 2.5 wt% and decrease with

further increase in nickel loading on CeO2. The decrease in the

catalytic activity of these catalysts beyond 2.5 wt% of Ni is due

to an increase in crystallinity of nickel oxide on the CeO2

support. The conversion of acetone for 1.0 wt% Ni loading

catalyst was 14% and it increased to 26% when the nickel

loading is increased to 2.5 wt%. The basic sites were also

found to increase with nickel loading up to 2.5 wt% and levels

off at higher nickel loadings, suggesting that the catalytic

properties are in good agreement with the basicity measure-

ments. The time-on-stream (TOS) analysis against activity for

the acetone condensation and selective hydrogenation reaction

over 2.5% Ni/CeO2 catalyst was performed at 423 K for a

continuous period of 10 h and the results are shown in Fig. 9.

It exhibits better stability, attaining a steady state within a

period of 6 h and there is a slight decrease in activity with time.

Mesityl oxide, diisobutyl ketone and diisobutyl carbinol are

the byproducts formed during the vapor phase condensation

of acetone, which are reported in Table 5. The activity of pure

ceria was also tested under similar conditions of the Ni/CeO2

catalysts and it gave 4% conversion. The selectivity toward

methyl isobutyl ketone was found to increase with an increase

in Ni loading up to 2.5 wt% and decreases with further

increase in nickel loading on CeO2. Watanabe et al. reported

similar behavior61 in their study of the influence of palladium

Fig. 7 Temperature programmed desorption of CO2 profiles of

various Ni/CeO2 catalysts.

Table 4 Temperature programmed desorption of CO2 of various Ni/CeO2 catalysts

S. No. Ni loading/wt%

CO2 uptakea/mmol g�1

Total CO2 uptakea/

mmol g�1A B C

1 0.0 105 35 — 1402 1.0 116 44 51 2113 2.5 170 57 121 3484 5.0 156 31 97 2845 10 120 36 80 2366 15 103 47 80 2307 20 140 55 90 285

a Calculated from temperature programmed desorption of CO2. A =

due to weak basic sites; B = due to moderate basic sites; C = due to

strong basic sites.

Fig. 8 Acetone condensation and selective hydrogenation over various

Ni/CeO2 catalysts. Reaction conditions: T=423 K; wt. catalyst = 0.5 g;

feed rate of acetone = 4.09 � 10�2 mol h�1.

Fig. 9 Effect of the stability of the 2.5% Ni/ceria catalyst for acetone

condensation and selective hydrogenation reaction with respect to

time-on-stream. Reaction conditions: T = 423 K; wt. catalyst = 0.5 g;

feed rate of acetone = 4.09 � 10�2 mol h�1.

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1672 Catal. Sci. Technol., 2012, 2, 1665–1673 This journal is c The Royal Society of Chemistry 2012

loading on the activity of Pd/ZrP.62 Yang and Wu reported

analogous results over Pd/SAPO catalyst, which posses

pronounced acid properties.21 Cheikhi et al.63 reported a

similar observation in their study of the palladium-loaded

hydroxyapatite. On the other hand, Das and Srivastava have

found equivalent results over Pd/MgAl(O), which has basic

features.64 Nikolopoulos et al.65 also observed that the 0.1 wt%

Pd/HT is superior in maximizing the MIBK yield among the

Pd- and Pt-based catalysts, which is mainly due to its higher

basicity, and due to its minimal concentration of metal sites. They

attributed the decrease in the selectivity to the agglomeration of Ni

particles over the basic sites, which are believed to be the active sites

in the acetone condensation. There is probably an adequate ratio

between the basic sites and the hydrogenating metallic sites,

leading to the optimal performances. The activity of the catalysts

is dependent on both dispersion and basicity of the catalyst.

2.5 wt% catalyst in the present study exhibits both high dispersion

and basicity as observed from H2-chemisrotion and CO2-TPD

results. Thus, the 2.5 wt% catalysts exhibit adequate ratio of basic

sites and metallic sites leading to optimal performance.

The turn over frequency (TOF) of a catalyst is defined as the

number of reactant molecules converted to products over an

active catalyst site per second. In the present case, each Ni

atom on the outer surface of the Ni particles is considered an

active site. The TOF of the Ni/support catalysts was calculated

as follows.

TOF = Rate/hydrogen uptake

Rate = (volume of the reactant fed � fractional conversion)/

weight of the catalyst

To find the relation between the acetone condensation and

selective hydrogenation with the nickel loading, a plot of

turnover frequency (TOF) versus nickel loading on CeO2 is

shown in Fig. 10. The TOF was found to be constant for all

the catalysts except for 1.0 and 2.5 wt% of Ni catalyst. This

might be due to the presence of well-dispersed amorphous

nickel species at lower loadings which is further evidenced

from the TPR results. The present results suggest that acetone

condensation and selective hydrogenation are structure sensitive

up to 2.5% due to availability of active sites, beyond that they are

structure insensitive.

Conclusions

Ceria is found to be a good support material for supporting

Ni for vapor phase acetone condensation and selective hydro-

genation to methyl isobutyl ketone. XRD results reveal the

presence of crystalline NiO at high nickel loadings (42.5 wt%).

The results of hydrogen chemisorption suggest that nickel is

found to be highly dispersed on the CeO2 support. The

information obtained by UV-vis DRS and TPR reveals the

presence of two types of nickel species on the CeO2 support.

The dispersion of Ni as determined by hydrogen chemisorption

substantiates the findings of XRD. TPD of CO2 indicates that

the basicity of supported nickel catalysts falls into three regions.

Basicity of the catalysts was found to increase with an increase

in nickel loading and decreases at higher loadings. The activity

of the catalysts was found to increase up to 2.5 wt% and

decreases at higher loadings in similar lines to nickel dispersion

and basicity measurements.

Acknowledgements

The authors P.V.R.R. and V.P.K. thank the Director of IICT,

Hyderabad for the Project Assistant position. G.S.R. thanks

the Council of Scientific & Industrial Research (CSIR), New

Delhi for the award of Junior Research Fellowship.

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