Partial Oxidehydrogenation of Cyclohexane to Cyclohexene over Nickel
Supported Catalysts Modified by Rare-Earth Metal Oxides
Hany M. AbdelDayem*1,2,3, Mohamed A. Alomair1, Salwa S. Sadek 3 and Hosham Samir3
1
Department of Chemistry, College of Science, King Faisal University, Al-Hasa, P.O. Box 380, Hofuf , 31982,
Saudi Arabia (monamohus@yahoo.ccom)
2
Center of Research Excellence in Petroleum Refining & Petrochemicals (CoRE-PRP) King Fahd University of
Petroleum & Minerals, Dhahran, Saudi Arabia
3
Chemistry Department, Faculty of Science, Ain Shams University, Abassia, Cairo, Egypt
Partial oxidehydrogenation of cyclohexane to produce cyclohexene selectively was
studied over NiO/-Al2O3-doped–Ce, -La, -Gd, -Dy catalysts with a flow reactor. Catalysts were
characterized using N2 adsorption, X-ray diffraction and atomic absorption spectroscopy. Doping
of catalyst with rare-earth metal oxides was found to have high promoting effect. NiO/Dy–
modified--Al2O3 exhibited higher activity for cyclohexane conversion and higher selectivity for
cyclohexene; cyclohexene selectivity of up 80% was achieved. Modification of -Al2O3
supported nickel oxide with Ce, Gd, Dy inhibited the formation of bulk nickel aluminate and
leads to higher nickel content as compared to unmodified carrier.
KEY WORDS: Cyclohexane, gas phase, NiO catalyst, rare earth metal oxides, -Al2O3
1- Introduction
Cyclohexene represents an important basic material for many valuable intermediates
products, cyclohexene is used in manufacturing of adipic acid, hexahydrobenzoic acid, maleic
acid, cyclohexanol and cyclohexeneoxide [1]. Production of cyclohexene using cyclohexane as
feedstock is an ideal economical reaction, where cyclohexane has less pollution problems and his
cost is lower by 5 – 6 times than other feedstock such as cyclohexanol. According to literature
four techniques have been used in the oxidation of cyclohexane: liquid phase [2], photocatalytic oxidation (in liquid or gas phase), anodic oxidation by spark discharge (ANOF) [4] and
gas phase using oxygen (air, ozone) [5,6].
The liquid phase oxidation of cyclohexane is characterized by a high energy consumption
and expensive investments. On the other hand, photo-catalytic oxidation and ANOF techniques
are not easily handle at the large scale. Gas-phase oxidative dehydrogenation of cyclohexane
(ODH) over metal oxides opens new reaction route capable of replacing classical liquid-phase
oxidation processes for technical production of cyclohexanol/cyclohexanone. There are variety
of formulations (viz., organometallic complexes, zeolites based catalysts, MCM based catalysts,
organometallic complex/ SBA-15, metal oxides/TiO2 based catalysts) proposed for the oxidation
of cyclohexane using the four techniques mentioned above. The problem of developing efficient
catalyst remains unsolved due to relatively low selectivity to desired product.
Oxidative dehydrogenation of cyclohexane by molecular oxygen to cyclohexene using metal
oxides can be a promising reaction pathway for the technical production of
cyclohexanol/cyclohexanone. The most promising catalysts among the metal oxides that have
1
been tested for cyclohexane oxidation in gas phase are NiO/Al2O3 [5], reduced-NiMoO4 [7],
metal vanadates (V2O5/Al2O3) [8], and CuOx [9]. In the case of NiO/Al2O3 catalyst, it is clear
that the catalyst selectivity depends on the following parameters: the nature of nickel oxide
species, the oxidation state of nickel, the formation of bulk nickel aluminate and the nickel
content on the support surface [5 ].
The influence of alkali metal (Na, K, Cs) doping on the surface and catalytic properties of Al2O3 supported nickel oxide in ODH of cyclohexane was investigated [5]. However, it was
found that doping of the catalyst with alkali have no promoting effect. It was reported that alkali
doping of the alumina support prior to nickel leads to lower nickel content and to enhance the
formation of bulk nickel aluminate. Where the formation of bulk nickel aluminate has a negative
effect on both NiO/Al2O3 activity and selectivity.
Oxygen conducting oxides such as ceria, when used as a dopant of supported transition metal
oxides catalysts, are known to significantly modify metal-support interaction to enhance catalytic
performance, due to their easy formation of oxygen vacancies [10,11], improved dispersion of
metals [12,13], and excellent capabilities of oxygen storage/transport [12-14]. Doping of -Al2O3
supported nickel oxide with cerium metals can provide a way to inhibit the formation of nickel
aluminate and to modify the nature of nickel oxides active species.
In this work, the influence of rare earth metals (Ce, La, Gd, Dy) doping on the surface and
catalytic properties of - Al2O3 supported nickel oxide in the ODH of cyclohexane was
investigated. In addition, the influence of doping of - Al2O3 carrier with different percentage of
cerium oxide was also studied.
2. Experimental Section
Two catalysts series were prepared; first series (I) is cerium modified -Al2O3 supportedNiO, with different weight percentages of CeO2 (1%, 5% and 10 w/w%). Second series (II) is a
rare earth metals (La, Gd, Dy) modified -Al2O3, with 10% w/w.
Cerium modified--Al2O3, (xCeAl) were prepared by wet impregnation of -Al2O3
(thermally activated, porous particle, specific surface area; 169.4, m2/g) with a salt solution
containing appropriate amounts of Ce(NO3)3.6H2O (Aldrich sigma, 99.9% pure) to obtain
different weight percentages of CeO2 (1%, 5% and 10 w/w%). In this case, the amount of cerium
nitrate was dissolved in the smallest possible volume of deionized water. A slurry of -Al2O3
powder in 200 ml deionized water was then added. The resulting slurry was taken to dryness by
continuous stirring and heating at 70oC. The solid obtained was then kept in an oven overnight at
120oC, crushed in an agate mortar and calcined for 4 h at 500oC.The resulting solids denoted as
xCeAl; where (x = 1.0, 5.0 and 10.0, respectively). La-, Gd- and Dy- modified -Al2O3 with
(10%w/w) were also prepared by the impregnation method following the same route as used in
preparation CeAl. The resulting solids are denoted as 10LaAl, 10GdAl and 10DyAl.
Nickel oxide supported on rare earth metal-modified-Al2O3 were prepared according to
procedure reported in refs. [5,15]. The rare earth metal- modified -Al2O3 as well as an un
2
modified--Al2O3 were impregnated in an aqueous solution containing amount of freshly
prepared nickel hexamminate at 318 K for 24 h under continuous stirring. The nickel loaded
sample was washed with methanol for several times, filtered, dried at 393 K for 12 h and
calcined at 873 K for 6 h. series I catalyst were labeled (Ni1.0CeAl, Ni5CeAl and Ni10CeAl).
On the other hand, series II catalysts were labeled (Ni10LaAl, Ni10GdAl and Ni10DyAl), where
nickel oxide supported unmodified -Al2O3, catalyst denoted NiAl.
X-ray diffraction measurements were performed employing Philips X’Pert MPD
(multipurpose X-ray diffractometer) employing Cu K1,2 radiation (λ = 1.5405 Å) for 2θ angles
varying from 10° to 80°. BET surface areas were measured using the surface area analyzer
Quanta Chrome Nova 2200 porosity and pore size distribution were obtained according to
Barrett-Joyner-Halenda (BJH) method.
The nickel content in the catalysts was determined by AAS on a Unicam 939 England
system. The catalysts were dissolved in nitric acid (68%) and diluted with distilled water to
concentration within the detection range of the instrument. An attempt was carried out to
determine the free Ni particles on the surface of support by mild dissolution of 0.2 g catalyst in
4 N HNO3 for 2 h and then analysis of the solution by AAS.
Catalytic activity measurements were performed employing a conventional fixed-bed
reactor system using air as the carrier gas for the cyclohexane feed. The following reaction
conditions were employed: catalyst weight, 0.2 g; flow rate of air, 100 ml/min. temperature was
adjusted for each catalyst to have cyclohexane conversion (5%), to compare selectivity for
cyclohexane oxidative dehydrogenation products. Analysis of reactants and products was
performed by an on-line Shimadzu GC-17A with FID and TCD detectors using two columns;
fused silica FFAP-capillary (50m×0.32m i.d., AD: 0.46) (for cyclohexane, cyclohexene,
cyclohexadiene and benzene) and HaySep D (80/100) (for COx). Further details can be found
elsewhere [7]. No homogeneous gas phase reaction for conversion of cyclohexane was observed
at used reaction temperatures.
3. Results and Discussion
Nickel oxide content of the catalyst measured by atomic absorption spectroscopy (AAS)
are summarized in Table 1. It is clear that, the rare earth oxides (Ce, Gd, Dy) -doped catalysts
show a much higher NiO content than the unmodified - Al2O3 carrier. In the case of Ni10DyAl
and Ni10CeAl catalyst, the NiO is nearby monolayer distribution, where the theoretical
monolayer nickel oxide corresponding to nickel oxide loading (12.17 % w/w). Considering that
a shape projection space of 9.09 Å corresponding to a NiO {100} unit [16] and measured
specific surface area of rare earth oxides- modified-- Al2O3 samples are in the range (90 -95
m2/g) . This suggested that most of the nickel precursor is distributed mainly on the surface and
not in the bulk of the support oxide. This suggestion confirmed from dissolution data results;
The free fractions of Ni particles which could be extracted in 2 N HNO3 solution, are included in
the Table 1. It is clear that mild HNO3 dissolved more than 70% of total NiO content in the case
of 10NiCeAl, 10NiDyAl and 10NiGdAl catalysts. However, In the case of parent NiAl catalyst,
it is clear that the major fraction of nickel particles remains combined strongly with or in the
3
support. This could be explained by a partial blocking of the - Al2O3 porous by rare-earth
metal oxides especially in the case of Dy-, Ce-, Gd- modified - Al2O3 catalysts.
Table 1: Atomic absorption (AAS) and dissolution data
Catalyst
Total Ni (w/w%)
NiAl
Ni5.0CeAl
Ni10CeAl
Ni10LaAl
Ni10DyAl
Ni10GdAl
6.3
8.1
11.3
5.0
12.1
8.3
Dissolution data
Dissolved Ni w/w%
Surface free Ni (%)
2.0
31.7
5.2
64.2
8.6
76.1
1.9
38.0
9.7
80.1
6.2
74.8
All the measured adsorption/desorption isotherms over various samples were of type II of
Brunauer’s classification [17]. Various surface parameters derived from the obtained isotherms
are summarized in Table 2. which included the specific surface area[SBET (m2/g)], the BET-C
constant, the total pore volume as measured at 0.95P/Po [Vp (ml/g)] and the average pore radius
assuming that all the pores were cylindrical [ (Å)]. A decrease in the specific surface area of Al2O3 was observed after impregnation either with NiO or rare earth metal oxides, this decrease
is more pronounced in the case of NiAl catalyst. Which was accompanied by a decrease in the
pore volume (Vp). In addition, a significant increase in the average pore radius (
was
observed for all catalysts. This increase in may be interpreted as arising from the penetration of
nickel oxide and/or rare earth oxides molecules in the alumina pores, thereby causing some
expansion to slightly wider pore size.
Table 2: Surface characteristic parameters of - Al2O3 support, pure NiO/-Al2O3 catalyst and
rare earth metals modified- NiO/-Al2O3 catalysts .
Sample
CBET
SBET
Vp
BJH
method
(Å)
2
(m /g)
(ml/g)
SCum
VCum
(m2/g)
(cc/g)
115.6
169.4
0.350
31
224.1
0.36
- Al2O3
NiAl
109
102.2
0.195
38
136.7
0.21
Ni10CeAl
116
105
0.213
41
144.7
0.23
Ni10CeAl
105
108.2
0.216
40
147
0.23
Ni10LaAl
80
108.2
0.217
40
145.2
0.23
Ni10DyAl
126
129.9
0.241
37
168
0.25
Ni10GdAl
275
134.3
0.241
36
166.7
0.25
4
The X-ray diffraction patterns of the parent NiAl, CeAl and various NixCeAl samples
are presented in Figure 1. The XRD pattern of original NiAl indicates the presence of -Al2O3
(JCPDS file, 10-0452), NiO of d spacing 2.4 0, 2.11 and 1.47 Å (JCPDS file, 78-0643) and
NiAl2O4 of d spacing 2.85, 2.43 and 1.55 Å (JCPDS file, 78-1601). In the case of NixCeAl
catalysts (Figure 2) -Al2O3 and NiO phases were also detected. In addition, the peaks
characteristic of CeO2 of d spacing 3.12, 2.28, 1.90 and 1.63 were detected in the patterns of
Ni5.0CeAl and Ni10CeAl. However, the following differences were observed i) the peaks
characteristics of NiO in NixCeAl catalysts have a higher intensity than that observed in the
pattern of NiAl sample, ii) a significant increase in the intensity of the peak characteristic of
NiO at 2θ = 43.25o (d spacing = 2.11) with increasing cerium content in these samples; and iii) a
new peak characteristic of NiO was appeared at 2θ = 75.54o (d spacing = 1.25 Å) in the
diffraction pattern of Ni10CeAl catalyst.
Figure 1: Powder X-ray diffraction pattern of NiO/-Al2O3 catalyst. Peaks marked by the
symbols "●", "◊" and "‡" indicate those peaks assigned to Al2O3, NiO, and
NiAl2O4, respectively.
Figure 3 displays the diffraction patterns of Ni10LaAl, Ni10GdAl, Ni10DyAl; beside that
of NiAl and Ni10CeAl for comparison. In the case of Ni10LaAl catalyst only -Al2O3 phase
was observed. Moreover, no peaks characteristics of nickel oxide or lanthanum oxide phases
were detected. These results can be related to the lower nickel content observed in NiLaAl
sample analyzed by AAS. On the contrary, Gd2O3 phase of d spacing 3.12, 2.69, 1.90, 1.64 and
1.35 Å (JCPDS file, 11-0608) and Dy2O3 phase of d spacing 3.34, 3.03 and 1.79 Å (JCPDS file,
19-0436) were detected in the diffraction patterns of Ni10GdAl and Ni10DyAl, respectively. On
the other hand, the intensity of the peaks characteristic of NiO are more pronounced in the
pattern of Ni10DyAl. No features of crystalline nickel aluminate and rare earth metals aluminate
can be observed in the diffraction patterns of rare earth metals (Ce, La, Gd, Dy) modified Al2O3 under study.
5
Figure 2: Powder X-ray diffraction pattern of cerium-modified--Al2O3 supported nickel oxide catalyst.
Peaks marked by the symbols "●", "◊" and "□" indicate those peaks assigned to Al2O3,
NiO, and CeO2, respectively.
.
Figure 3: Powder X-ray diffraction pattern of La, Dy, Gd- modifed--Al2O3 supported nickel oxide
catalyst. Peaks marked by the symbols "●", "◊", "*" and "+" indicate those peaks assigned to
Al2O3, NiO, Dy2O3 and Gd2O3, respectively.
6
The conversion (%) of cyclohexane and selectivity of reaction products at isoconversion
(5%) for all the studied catalysts are listed in Table 3. In the case of Ce-, Gd-, Dy- modifiedNiO/Al2O3 there is a decrease in reaction temperature (T(~5%)) required to keep cyclohexane
conversion constant at ca~5% comparing with NiAl catalyst. However, an increase in T(~5%)
was observed in the case of La-modified- NiO/Al2O3 . Which indicated that doping of catalysts
by Ce, or La or Gd, enhanced the catalytic performance in cyclohexane conversion. Such
behaviour can indicate that the observed higher cyclohexane conversion of rare earth oxides
modified catalyst might due to these catalyst have a higher surface area than NiAl catalyst (Table
2). As can be also seen in Table 3, at iso-conversion (~5%); Ni10DyAl exhibited the highest
selectivity towards cyclohexene. The above results confirmed that doping of NiO/Al2O3 by
these rare earth oxides (Ce, Dy, Gd) leads to a significant modifications in catalytic performanc.
Table 3: Comparison of performances of different catalysts for
catalyst
Ta(~5%)
Conversion
S(C6H10)b (%)
(oC)
(%)
NiAl
312
4.8
67.1
Ni5CeAl
308
5.2
72.3
Ni10CeAl
301
5.6
78.6
Ni10LaAl
317
4.9
60.6
Ni10DyAl
296
5.2
82.2
Ni10GdAl
305
5.1
75.3
cyclohexane ODH
S (COx)d
S(C6H6)c (%)
(%)
18.5
14.4
12.4
15.3
12.1
9.3
17.9
21.5
10.2
7.6
14.0
10.7
Cyclohexane molar feed rate: 8 x 10-3 mol/h g, W = 0.2 g, flow rate = 100 ml/min.
a
Temperature at which the cyclohexane conversion is ~5%, bSelectivity to cyclohexene, cSelectivity to benzene,
d
selectivity to CO and CO2. N.b. trace amounts of cyclohexadiene (< 0.3 %) were observed for all studied catalysts,
in this table yield of cyclohexadiene was added to that of benzene.
Refereeing to XRD results, the formation of bulk nickel aluminate was only observed in
the non-doped catalyst; NiAl. It is well known in the literature [5] that the nickel from nickel
aluminate species is very stable to reduction it needs temperature above 800oC. which means at
the used reaction temperatures (280-330oC) bulk nickel sites from nickel aluminate do not
participate in reduction/reoxidation cycles according to Mars van Krevelen mechanism (MVK)
[18] and remain catalytic inactive. The promoting effect of rare earth oxides (Ce, Gd and Dy) can
be explained due to the fact that earth metals inhibits the formation of nickel aluminate and
causes the formation of NiO in much larger proportion (see AAS results) especially in the case
Dy-modified catalyst (Ni10DyAl). According to literature the reduction of bulk NiO causes at
temperature around 300oC [19] namely, these site can obey MVK reduction/oxidation
mechanism. Dissolution data indicates that NiO species is weakly interacted or do not form any
significant bond with support in the rare earth oxides (Ce, Gd, Dy)-modified catalysts.
Consequently, their reduction is similar to that of unsupported NiO [20]. However, based on
both dissolution and N2 adsorption results in the case of NiO/Al2O3 it seems that most of NiO
species are located in the pores of the catalyst, so it needs higher temperature to reactivated
according to MVK mechanism.
7
4. Conclusions
Based on the results obtained in this work one can conclude that, compared with
NiO/Al2O3 the Ce-, Dy – and Gd modified catalysts are more active and efficient in selective
production of cyclohexene by ODH of cyclohexane. At smaller conversion about 5%, the
selectivities of catalysts towards cyclohexene increase in the order Ni10LaAl<NiAl< Ni10GdAl
<Ni10CeAl< Ni10DyAl. The higher catalytic performance of rare earth oxides modified
catalysts can be attributed to that cerium metals doping of alumina carrier prior to nickel
impregnation leads to higher nickel content as compared to unmodified Al2O3 carrier. While
the formation of bulk nickel aluminate is inhibited, which has a negative influence on both the
activity and cyclohexene selectivity. Further investigation of carrier acidity-basicity property,
oxidation state of nickel species in order to establish a correlation between catalysts structure and
selectivity are under way.
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Acknowledgment
The authors thankfully acknowledge the Center of Research Excellence in Petroleum Refining &
Petrochemicals (CoRE-PRP) established by the Ministry of Higher Education at the King Fahd
University of Petroleum & Minerals, Dhahran, Saudi Arabia for support of this work.
8