ca-doped chromium oxide catalysts supported on alumina for theoxidative dehygn of isobut

8/18/2019 CA-doped Chromium Oxide Catalysts Supported on Alumina for Theoxidative Dehygn of Isobut

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Applied Catalysis A: General 260 (2004) 75–86

Ca-doped chromium oxide catalysts supported on alumina for theoxidative dehydrogenation of isobutane

G. Neri a, A. Pistone a, S. De Rossi b, E. Rombi c, C. Milone a, S. Galvagno a,∗

a Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, Vill. S. Agata, I-9816 6 Messina, Italyb IMIP CNR Sezione “Materiali Inorganici e Catalisi Eterogenea” c/o Dipartimento di Chimica, Università “La Sapienza”,

P.le Aldo Moro 5, 00185 Rome, Italyc Dipartimento di Scienze Chimiche, Università di Cagliari, Complesso Universitario di Monserrato, S.S. 554 Bivio per Sestu,

09042 Monserrato, CA, Italy

Received in revised form 8 October 2003; accepted 9 October 2003

Abstract

The oxidative dehydrogenation (ODH) of isobutane has been investigated on Ca-doped chromium oxide catalysts supported on -Al2O3.

The effect of Ca loading on the micro-structural properties of chromia catalysts was investigated by chemical analysis, X-ray diffraction

(XRD), scanning electron microscopy with elemental mapping (SEM-EDX), UV-Vis diffuse reflectance spectroscopy (DRS), temperature

programmed reduction (TPR), and micro-calorimetry of adsorbed NH3. Cr3+ and Cr6+ species dispersed on alumina, as well as -Cr2O3

and CaCrO4 crystallites, were found on the catalysts surface. The relative amount of the chromium species depends on the Ca loading. The

Cr3+ /Cr6+ ratio decreases on increasing the Ca loading due to the preferred formation of bulk chromate species. The Ca loading affects the

reducibility of the Cr6+ species and the acid sites strength distribution of the catalysts.

The catalytic activity in the ODH reaction of isobutane is enhanced in the presence of amounts of calcium


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76 G. Neri et al. / Applied Catalysis A: General 260 (2004) 75–86

dehydrogenation (ODH) is represented by the relevant for-

mation of by-products. The formation of carbon oxides is

thermodynamically more favored than the formation of the

corresponding olefin and a rapid decrease in the selectivity

to the desiderated products with increasing the alkane con-

version is often observed. A non-selective mechanism can

exist in which oxygen, from the lattice or activated from thegas phase, can be inserted into the hydrocarbon and several

reaction steps can advance ultimately to carbon oxides.

From this point of view, the main effort in the scientific

investigations of the ODH of lower alkanes is to improve

the olefin yields. The key aspect of the oxy-dehydrogenation

technologies is, therefore, the development of catalysts ca-

pable of activating the C–H bonds of the alkane molecule

in a flow of oxygen and capable of desorbing quickly the

alkene formed in the dehydrogenation step in order to avoid

a further oxidation. Acidic and basic properties as well as

the redox characteristics of the catalytic system seem to be

critical factors that can affect the performance of a selective

oxy-dehydrogenation catalyst [1–7].Among low alkanes, the ODH of isobutane has received

in the last years an increasing interest as a route to obtain

isobutene, the key reactant for production of a variety of

chemicals, such as MTBE used as additive for gasoline to

regulate the octane number. Compared to other alkanes, in

particular ethane and propane, the ODH of isobutane has

received much less attention. The development of catalysts

able to activate, at low temperature and in presence of oxy-

gen, the alkane molecule, to selectively promote the alkene

formation with high yields and simultaneously to avoid deep

oxidation of the substrate, represents the main goal [8].

The catalytic systems reported as promising for theODH of isobutane include ZnO/TiO2 systems, MgO–V2O5,

molybdates, heteropolycompounds, pyrophosphates, etc.

[9–17]. The maximum yields for these catalysts were

8–11% and selectivity varied between 50 and 80% for an

isobutane conversion of about 10–20% and reaction tem-

perature higher than 673 K. Recently, chromia-based cata-

lysts have been studied in the ODH of isobutane because

of their favorable performances at relatively lower reac-

tion temperatures [18–24]. Moriceau et al. [20,24] reported

60% isobutene selectivity with 10% isobutane conversion

at 543 K for a binary Cr–Ce–O catalyst. Hoang et al. [25]

reported 70% isobutene selectivity with 10% isobutane

conversion for a chromia supported on lanthanum carbon-

ate catalyst. In the last years, Grzybowska et al. [18] have

studied the ODH of isobutane on chromia-based catalysts

at temperatures between 473 and 673 K and reported selec-

tivities up to 73% of isobutene obtained at 5% isobutane

conversion on chromia supported on titania and on K-doped

chromia supported on alumina; the authors also studied the

ODH of C2–C4 alkanes on chromia/alumina catalysts and

showed that it strongly depends on the structure of the hy-

drocarbon, with the total oxidation to carbon oxides being

the main reaction for ethane, propane, and n-butane and

the ODH to isobutene the main reaction for isobutane [26].

The authors also observed a strong correlation between the

Cr6+ content in the Cr/Al2O3 catalysts and the activity in

the ODH of isobutane. Furthermore, a correlation among

acid–base properties of support, metal–oxygen energy bond

and selectivity to olefin was also observed; in particular,

unsupported chromia catalyst showed lower selectivities

to isobutene because of a higher acidic character, a lowerCr–O–Cr oxygen bond energy and a higher rate of oxygen

chemisorption than the Cr/Al2O3-based catalysts [23].

This paper aims to study the catalytic ODH of isobutane

to isobutene over chromium oxide catalysts supported on

-Al2O3 and doped with Ca as an alkaline promoter. Alkali

metals have been indicated to change the acid–base charac-

teristics of the catalysts and this may have a strong influ-

ence on the activity and selectivity in ODH reaction. High

selectivities to isobutene on K-doped Cr/Al2O3 have been

explained with the increasing basic character of the surface

and, consequently, by weakening and facilitating the desorp-

tion of isobutene from the less acidic surface which prevents

further deep oxidation of the olefin to carbon oxides. How-ever, contradictory results are reported in literature. In fact,

the modification of the acid–basic properties of a catalyst

can strongly influence not only the rate of desorption of the

alkene molecules, but also the nature and the dispersion of

the active sites, so affecting the overall catalytic properties.

Grabowski et al. [18] observed a positive or negative effect

of potassium doping on the activity of chromia-based cata-

lysts depending on the type of support; in particular, the pres-

ence of the basic K additives on chromia/alumina catalysts

enhances strongly the selectivity to isobutene with respect

to the undoped catalysts. ODH of propane on zeolites-based

catalysts containing Ca was studied by Kubacka et al. [27].No literature data have been instead found on the use of

calcium as a promoter for ODH reaction on chromia cata-

lysts. In principle, Ca should behaves as other alkaline met-

als (Li and K) which have been more extensively studied, but

its different physico-chemical (ionic radius and charge) and

acid–base characteristics may modify differently the prop-

erties of chromium-based ODH catalysts.

2. Experimental

2.1. Catalysts preparation

Chromium oxide catalysts supported on alumina were

prepared by the incipient wetness technique. The follow-

ing catalysts were prepared: ACr10 nominally containing

10 wt.% of chromium on -alumina, ACr10Ca1, ACr10Ca2,

ACr10Ca4, and ACr10Ca8 nominally containing also 1, 2, 4,

and 8 wt.% of calcium, respectively. -Alumina (grain size

100–500m, kindly provided by Süd Chemie MT, Novara,

Italy) was obtained by calcination in air of pseudoboehmite

Versal 250 La Roche at 1223 K for 4 h. The support was im-

pregnated with comparable volumes of aqueous solutions of

the appropriate amounts of CrO3 (and CaCO3, solubilized


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G. Neri et al. / Applied Catalysis A: General 260 (2004) 75–86 77

Table 1

Main characteristics of the chromia and Ca-doped chromia catalysts investigated

Catalyst code Cr (wt. %) Ca (wt. %) Ca/Cr Cr6+ (wt. %) Cr3+ (wt. %) Cr3+ /Cr6+ S BET (m2 /g) V pores (cm

3 /g)

ACr10 9.4 0 0 1.5 8.0 5.33 94 0.35

ACr10Ca1 9.2 1.23 0.13 2.0 7.1 3.55 83 0.33

ACr10Ca2 9.3 1.81 0.19 2.4 6.9 2.88 84 0.34

ACr10Ca4 9.2 3.62 0.39 5.0 3.9 0.78 82 0.27ACr10Ca8 9.7 6.42 0.66 8.8 1.1 0.13 80 0.31

Support: -alumina; S BET: 121 m2 /g; V pores: 0.48 cm

3 /g.

in the chromic acid medium, for the ACr10Cax samples).

Both chemicals were Carlo Erba reagent grade. The care-

fully stirred paste was dried overnight at 383 K and finally

calcined at 973 K for 12 h.

Table 1 lists the main characteristics of the prepared

catalysts.

2.2. Characterization

Chemical analyses of the total Cr (Table 1, column 1)

in the samples were carried out by atomic absorption (AA,

Varian SpectrAA-30). Samples were previously dissolved

by fusion with a mixture of KNO3 and Na2CO3 (1:1 by

weight). On a different portion of the sample, the Cr6+

content (Table 1, column 4) was determined by AA after

several extractions with 10 M NaOH solution heated to in-

cipient boiling. The same sample with the residual Cr3+

(Table 1, column 5) was then dissolved by fusion with the

mixture of KNO3 and Na2CO3 and analyzed by AA. The

correspondence of the total amount of Cr and the sum of

the amounts of Cr6+

and Cr3+

could then be checked withsatisfactory results (Table 1).

Chemical analysis of Ca in the samples was carried out

by contacting them with 1 M HNO3 at room temperature for

30 min. The resulting limpid solution was then analyzed by

means of a Varian Liberty 200 inductively coupled plasma

analysis (ICP) spectrometer. Excepts for the ACr10Ca1

sample, Ca concentrations so determined are slightly lower

compared to nominal values (see Table 1), indicating that a

fraction of Ca is loss during the catalysts preparation.

Phase analysis was performed by X-ray diffraction (XRD)

using a Philips PW 1729 diffractometer equipped with a PC

for data acquisition and analysis (software APD-Philips).

Scans were taken with a 2 step of 0.01◦, using Ni-filtered

Cu K radiation.

UV-Vis diffuse reflectance spectra (DRS) were taken in

the wavelength range 200–800 nm (50,000–12,500 cm−1)

with a Varian CARY 5E spectrometer equipped with a

PC for data acquisition and analysis and using PTFE as a

reference.

Tian–Calvet heat flow equipment (Setaram) was used

for micro-calorimetric measurements. Each sample was

pre-treated overnight at 673K under vacuum (10−3 Pa)

before the successive introduction of the probe gas (am-

monia). The equilibrium pressure relative to each adsorbed

amount was measured by means of a differential pressure

gauge (Datametrics). The run was stopped at a final equi-

librium pressure of 133.3Pa. The adsorption temperature

was maintained at 353 K, in order to limit physisorption.

Temperature programmed reduction (TPR) profiles were

obtained on a TPD/R/O 1100 apparatus (ThermoQuest),

under the following conditions: sample weight 40–45 mg,

heating rate (from 313 to 1173 K) 20 K/min, flow rate

30cm3 /min, H2 5 vol.% in N2; the hydrogen consumption

was monitored by a thermal conductivity detector (TCD).

Textural analyses were carried out on a Sorptomatic 1990

System (Fisons Instruments), by determining the nitrogen

adsorption–desorption isotherms at 77 K. Before analysis,

the samples were heated overnight under vacuum up to

473 K (heating rate = 1 K/min).

Scanning electron microscopy (SEM) with elemental

mapping images of powder samples mounted on an alu-

minum holder were obtained on a JEOL JSM-5600 LV

microscope equipped with an EDX (Oxford) analyzer. The

quantitative analysis was carried out at 20 kV by using

the SEMQUANT software applying the Z.A.F. correctionprocedure.

2.3. Catalytic activity

The activity of the catalysts in the ODH of isobutane was

measured in a fixed bed down-flow apparatus operated at

atmospheric pressure equipped with a quartz U-tube and an

internal coaxial thermocouple connected with a PID temper-

ature controller. The ODH of isobutane was studied by vary-

ing both the temperature range (523–673 K) and the contact

time (1–6 s), catalytic runs were carried out using oxygen

and isobutane in the ratio 2:1 diluted in helium (isobutane =

6%, oxygen = 12%, total flow = 320 cm3 /min) and using

0.2–0.4 g of catalyst. The catalyst was mixed with an equal

amount of granular quartz and loaded into the micro-reactor;

additional amounts of granular quartz were placed upon the

catalyst bed to reduce the dead volume of the reactor. A gas

chromatograph (HP 6890) was used for an on-line analysis

of both the feed and the product streams. The hydrocarbons

were separated by HP-plot column (HP-Plot Alumina “M”

deactivated 50 m × 0.53mm × 15m) and analyzed with

a FID detector, while O2, CO, and CO2 were separated

by a molecular sieves and Hayesep columns (13× molec-

ular sieves: 10 ft × 1.8 in., Hayesep Q: 10 ft × 1.8 in.) and


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analyzed by TCD detector. Conversion and selectivity were

defined as follows:

conversion=Y =

moles of isobutane reacted

moles of isobutane in the feed

× 100;

selectivity

= Si =

moles of isobutane converted to product “i”

moles of isobutane reacted

×100,

where i = i-C4H8, CO, CO2, and others (CH4, C2H6,

C3H8, etc.). Isobutene, CO, and CO2 were found to be the

main reaction products, while the amounts of other degra-

dation products and of oxygenates were negligible. Blank

tests without catalyst in the reactor showed no conversion of

isobutane in the reaction temperature range considered, al-

lowing to rule out the occurence of homogeneous reactions

to a significant extent. No deactivation of the catalysts was

observed during the measurements.

3. Results

3.1. Catalysts characterization

Ca-promoted chromia catalysts were prepared by impreg-

nation of the respective salt precursors of a -alumina sup-

port having a specific surface area (SA) of 121 m2 /g and a

pore volume of 0.48 cm3 /g. The SA and pore volume de-

crease significantly upon addition of 10 wt.% of chromium

(see Table 1). On the contrary, the loading of Ca, after aninitial small decrease, does not affects significantly these

parameters.

Fig. 1 shows XRD patterns of the investigated catalysts.

Before discussing them, it should be mentioned that many

peaks of -Cr2O3 and CaCrO4 are coincident. The spectra of

Fig. 1. XRD of the investigated catalysts.

all catalysts are dominated by broad peaks of the -alumina

support. On the undoped catalyst, -Cr2O3 was the only

chromium phase detected. No chromate phases were ob-

served indicating that they are absent or below the detection

limit of XRD. The addition of Ca upto about 2 wt.% does

Fig. 2. SEM micro-graphs of the undoped ACr10 catalyst: (a) lower

magnification; (b) and (c) high magnification.


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Fig. 3. SEM micro-graphs and EDX elemental analysis of the surface of Ca-doped catalysts. The numbers indicate points where the corresponding EDX

spectrum was collected: (a) ACr10Ca2 catalyst and (b) ACr10Ca8 catalyst.

not modify the XRD spectra. On samples containing more

calcium, the -Cr2O3 peaks disappear while new peaks, re-

lated to calcium chromate, CaCrO4, increase with calcium

content.

A SEM analysis was carried out to investigate the mor-

phology of the catalysts. Fig. 2a reports a low magnificationview showing the typical granulometric distribution of the

alumina used as support. Fig. 2b and c present micro-graphs

taken at higher magnification of the surface of the ACr10

sample. Elemental EDX-mapping has shown chromium to

be highly dispersed and distributed homogeneously on all

Fig. 4. TPR patterns of the undoped and Ca-doped catalysts. Heating rate,

20 K/min; reducing mixture, 5% H2 /N2 at 30 ml/min; mcat = 40mg.

surface of alumina. Crystalline particles of -Cr2O3 of about

1m in size were also imaged and identified.

Samples with low promoter loading (


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Fig. 6. Cr species content as a function of Ca loading.

Fig. 7. Micro-calorimetric analysis of the undoped and Ca-doped catalysts. The curve related to support was also reported for comparison.

promoter increases, the reduction peak shifts to higher tem-

perature and at the same time decreases of intensity, while

a second peak at 763 K appears. On samples ACr10Ca4

and ACr10Ca8, these low temperature peaks gradually dis-

Table 2

Acid sites strength distribution

Catalyst code Weak acid sites (70–120 kJ/mol) Medium acid sites (120–150kJ/mol) Strong acid sites (>150 kJ/mol) Total acid sites

Alumina 1.11 0.47 0.19 1.79

ACr10 1.28 0.82 0.57 2.68

ACr10Ca1 1.37 0.27 0.21 1.86

ACr10Ca2 1.22 0.28 0.23 1.75

ACr10Ca4 0.46 0.08 0.23 0.78

ACr10Ca8 1.10 0.10 0.21 1.41

Concentration of sites is expressed as amount of NH3 adsorbed per unit surface area (mol/m2).

appear whereas new strong peaks at higher temperature

(873–973 K) were observed. The peaks at 673 and 763 K can

be attributed to one step reduction of monochromate species,

Cr6+, stabilized on alumina surface and/or interacting with


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Ca-modified sites, respectively, to Cr3+ species. The high

temperature peaks (between 873 and 973 K) are instead re-

lated to the reduction of bulk chromate species.

A quantitative analysis of the TPR patterns, on the as-

sumption that H2 is consumed only in the reduction of Cr6+

to Cr3+ species, has shown that the total hydrogen consump-

tion increases with increasing the Ca content. This meansthat overall Cr6+ species increase on addition of the alkali

promoter. The hydrogen consumption related to the low tem-

perature peaks (653 and 763 K), due only to dispersed Cr6+

species, increase slightly with Ca loading up to 2 wt.%, then

decrease on samples ACr10Ca4 and ACr10Ca8. Hydrogen

consumed during the reduction has been quantitatively de-

termined. An H/Cr6+ ratio of about 1.5 has been calculated

for all the investigated samples, indicating that only a frac-

tion (≈50%) of the total Cr6+ was reduced to Cr3+ under

the adopted operating conditions. The same value for the

H/Cr6+ ratio has been obtained by Cherian et al. [36] for a

sample containing about 7 wt.% of Cr on alumina, at vari-

ance with Grzybowska et al. [23] who reported a completereduction for a Cr/Al catalysts (Cr≈ 8 wt.%), indicating that

operative conditions strongly affect TPR results.

DRS spectra are shown in Fig. 5. The ACr10 and ACr10Ca

catalysts show adsorption bands centered at 260, 380, 470,

and 600 nm. On the basis of literature data [28–31] the fol-

lowing attributions have been made: the bands at 260 and

380 nm are related to the charge transfer transitions O→ Cr

typical of Cr6+, whereas the bands at 470 and 600 nm are

due to d–d transitions of Cr3+ species in octahedral sym-

metry ( A2g → T 1g and A2g → T 2g, respectively). A very

weak band around 700 nm was also noted. The attribution

of this band is of more difficult interpretation. We can spec-ulate, it can be due to Cr5+ ions, as reported by Zecchina

et al. [28] for a silica-supported chromium oxide. In compar-

ison to the spectrum of ACr10, no significant modifications

were observed on promoted catalysts with low (


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the number of both weak and medium sites can be instead

observed when Ca content is increased from 2 to 4 wt.%.

Finally, it can be noted that, compared to the ACr10Ca4

sample, the ACr10Ca8 catalyst exhibits a greater amount

of weak acidity.

3.2. Catalytic activity

The catalytic behavior of the Cr/Al2O3 and Ca-doped

Cr/Al2O3 catalysts in the ODH of isobutane was first

investigated as a function of reaction temperature. Pre-

liminary experiments have shown that no deactivation of

the catalysts occurs during the measurements. Isobutane

and oxygen conversion, and products distribution over the

Fig. 9. Conversion of isobutane and selectivity to oxidation products as a function of temperature: (a) Acr10Ca2 and (b) Acr10Ca8.

ACr10 catalyst as a function of the reaction temperature

are shown in Fig. 8. Experimental conditions were: W cat =

0.4 g, He:O2:i-C4H10 (molar ratios) = 13:2:1, total flow =

320 cm3 /min, GHSV (based on isobutane)= 2300 h−1. Un-

der these conditions, we observed an isobutane conversion

of 6% already at 523 K. Isobutene, propene, and carbon

oxides (CO and CO2) were the main products, while theamount of other products such as CH4, C2H4, acetone, etc.

was negligible. Fig. 9 shows the catalytic performance data

for the ACr10Ca2 and the ACr10Ca8 catalysts. It can be

observed a change in the products distribution with respect

to the undoped catalyst. In particular, a strong enhance-

ment in the CO2 formation and correspondingly a decrease

in CO formation with Ca loading was noticed. Isobutene


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Fig. 10. Effect of contact time on the conversion of isobutane and selectivity to oxidation products: () isobutane; ( ) isobutene; () CO; () CO2.

formation decreases only slightly. As seen, for all catalysts,

the selectivity to isobutene decreases with increasing the

temperature (increasing isobutane conversion).

To compare the selectivities of the different catalysts at a

fixed temperature, a series of experiments were performed at

598 K by varying the residence time. Fig. 10 shows the effect

of residence time on the catalytic performance on ACr10

catalyst. The isobutane conversion increases on increasing

the residence time. The selectivity to CO and CO2 follow this

trend whereas the selectivity to isobutene decreases. Such

behavior was similar for all the investigated catalysts.

Fig. 11 shows the overall activity in the ODH of isobu-tane, and the formation rate of isobutene, CO, and CO 2, re-

spectively, as a function of the alkali content. For low Ca

loading, an increase of the overall activity and formation rate

of isobutene and CO2 was observed, but a further increase

in the Ca content reverse this trend. The formation rate of

CO instead decreases monotonically with Ca loading.

The selectivity–conversion plot reported in Fig. 12 seems

indicate that isobutene and CO2 are primary products of

ODH of isobutane on these catalysts while carbon monox-

ide shows, extrapolating the selectivity–conversion curve to

low conversion, a zero intercept typical of secondary prod-

ucts. The data agree with the general mechanism of ODH of

isobutane over chromia and vanadia catalysts proposed by

other authors where the formation of CO was mainly due

to combustion of adsorbed olefinic intermediates, while the

formation of CO2 was due to both this step and also the di-

rect combustion of the reactant [32,33]. However, a more

detailed investigation at lower isobutane conversion is nec-

essary to clarify this point.

Fig. 13 reports the selectivity to reaction products at the

temperature of 698 K and at a 7% conversion of isobutane,

as a function of the alkali content. At low loading of Ca

(


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Fig. 12. Selectivity to main oxidation products vs. the isobutane conversion: () isobutene; () CO; () CO2.

recognized that structural properties of supported chromium

oxide catalysts depend on many variables such as chromium

loading, heat treatment, support used, etc. [18,20,23–25].

The beneficial role of the support was related to stabiliza-

tion of both low coordinated Cr3+ ions and highly oxidized

species Cr6+. Increasing the Cr loading over the monolayer,

bulk phases are also found on supported chromia catalysts.

The relative amount of chromium species on the catalyst

affects strongly its catalytic properties.

Cr3+, in both dispersed or bulk phases, and dispersed

Cr6+ are the main species detected on the ACr10 cata-

lyst. The high value of the Cr3+ /Cr6+ ratio (>5) indicate

that the Cr3+ species prevail on the undoped catalyst and

exist on the catalyst surface under at least two forms: (i)

well-dispersed Cr3+ species anchored to the alumina sup-

port; (ii) Cr3+ species in amorphous or crystalline -Cr2O3[34].

The relative amount of chromium in different species

and/or oxidation states drastically changes on addition of

the alkali promoter. However, at Ca loading


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Fig. 13. Effect of Ca loading on the selectivity to oxidation products

(isobutane conversion is 7%): () isobutene; () CO; () CO2.

at the expense of the dispersed Cr3+ phase and crystalline

-Cr2O3.

The isobutane ODH reactivity studies have shown that

the activity and selectivity of the supported chromium oxide

catalysts depend on the calcium loading. As the Ca load-ing is increased the ODH activity goes through a maximum

for the ACr10Ca1 catalyst, then it decreases. Alkali metals

have been indicated as promoters of activity and selectivity

for ODH reaction [18]. However, so far little is known how

the alkali promoters affect the activity and selectivity. The

promotion of activity and selectivity to isobutene at low Ca

loading can be attributed to many factors: (i) an increase in

the number of the most active and selective sites; (ii) the

blocking of unselective strong acid sites that favor the forma-

tion of COx; (iii) the decrease in the acidity and the increase

in basicity, thus facilitating the desorption of isobutene from

the catalyst surface and preventing it from further oxidation

to carbon oxides [35].

Also the exact nature of chromium species active in

ODH reaction is still a matter of discussion. Moriceau

et al. [24] found that the activity of Cr2O3 /CeO2 cata-

lysts increases linearly with the Cr content in the range

where only well-dispersed Cr6+ species are present. They

suggested then that these species are the active sites for

ODH of isobutane. Cherian et al. [36] on the assumption

that a redox mechanism occurs, indicate in the redox pair

Cr6+ ↔ Cr 3+ the active sites for ODH of propane. In any

case, regardless of the oxidation state, chromium species in

crystalline phases are less active and address the reaction

Fig. 14. Activity and selectivity to isobutene as a function of the area

under the low temperature TPR peaks.

towards total oxidation than the two-dimensional surface

chromium sites which allow selective addition of oxygen in

the organic molecule [22,24,37,38].

Taking into account these contributions, we can suggestthat the increase of activity at low Ca/Cr ratios is due to an

increase of dispersed Cr6+ species. This is supported from

data shown in Fig. 14 where the formation rate and selectiv-

ity to isobutene of the catalysts is reported as a function of

the hydrogen, namely H LOW, consumed at low temperature,


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of the maximum reduction peak, T max, increases and then

the reducibility decreases, with the Ca content. At low alkali

doping, a decrease of reducibility corresponds to an increase

of activity and selectivity to isobutene, whereas at higher Ca

loading, when substantial structure modifications occur, this

trend is reversed. The effect of Ca doping on the acid–base

properties is more complex. The alkaline promoter stronglydecreases the acidity of the chromia catalyst, particularly

decreasing the medium and strong acid sites. As regards

the acidic properties, no clear correlations have been found

between the surface acidity and the catalytic behavior of the

investigated samples.

5. Conclusions

Ca-promoted chromium oxide catalysts have been pre-

pared, characterized, and tested in the ODH of isobutane.

The presence of calcium significantly alters the active sites

distribution, promoting the formation of dispersed Cr6+

species at low Ca content, whereas higher loading leads to

the formation of bulk Ca-chromates species. When tested

in isobutane ODH, the activity and selectivity to isobutene

show an increase at low alkali content followed by a sharper

decline. A linear relationship is obtained when the forma-

tion rate and the selectivity to isobutene is plotted versus

the concentration of dispersed Cr6+ active sites obtained

from H2 uptake at low temperature in TPR experiments.

On these basis, we suggest that Ca plays different roles

in the ODH reaction of isobutane:

(1) At loading upto 2 wt.% it increases the amount of dis-

persed Cr6+ species at expense of the Cr3+ species.

This explain the enhancement of activity on low-loaded

Ca-doped catalysts.

(2) It decreases the acidity and increases the basicity of

chromia catalysts. Consequently, the alkali weakens the

adsorption of formed isobutene, thus facilitating its des-

orption as a product. This effect being maximized at

low Ca/Cr ratios where an increase of selectivity was

registered.

(3) It favors the formation, at higher Ca/Cr ratios, of less

active and selective chromate species, so negatively af-

fecting the catalytic properties.

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