Temperature-programmed reduction and acidic properties of molybdenum supported on MgO-Al2O3 and their correlation with catalytic activity

Article abstract of DOI:10.1016/j.molcata.2003.12.005

Hydrotreating of crude oil is an important process in the refining industry. HDS, hydrogenation, and hydrocracking (HYC) activity using model compounds were studied. A series of catalyst with Mo loading from 2 to 14 wt % on MgO-Al₂O₃ mixed oxide support was prepared by incipient wetness impregnation method. Impregnation of acidic MoO₃ species further enhanced the acidity favorable for HYC. Acidity depended upon the average electronegativity of ions present. Mo addition onto surface of oxides improved average electronegativity of metal ions present in it. Mo/Al₂O₃ catalyst had the highest total acidity and strong acid sites than Mo/MgO-Al₂O₃ and Mo/MgO. The HYC activities were nearly equal for supported Al₂O₃ and MgO-Al₂O₃ catalysts.

Full text of DOI:10.1016/j.molcata.2003.12.005

Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
Temperature-programmed reduction and acidic properties of molybdenum
supported on MgO–Al2O3 and their correlation with catalytic activity
Manoj Kumara,∗, F. Aberuagbab, J.K. Guptaa, K.S. Rawata, L.D. Sharmaa, G. Murali Dhara
a
Indian Institute of Petroleum, Dehradun 248005, India
b
Department of Chemical Engineering, Federal University of Technology, P.M.B. 65, Minna, Nigeria
Received 1 August 2003; received in revised form 5 December 2003; accepted 5 December 2003
Abstract
A series of catalyst with Mo loading from 2 to 14 wt.% on MgO–Al2O3 mixed oxide support was prepared by incipient wetness impregnation
method. Pure MgO and Al2O3 were prepared by using Mg(NO3)2·6H2O and Al(NO3)3·9H2O salt solutions and urea as hydrolyzing agent.
MgO–Al2O3 (1:1) mixed oxide was prepared by co-precipitation of appropriate quantities of magnesium nitrate and aluminum nitrate salts.
BET surface area, pore volume, pore size distribution, TPR, acidity and acid strength distributions of catalysts and supports were measured.
MgO–Al2O3 (1:1) mixed oxide showed a biomodal pore size distribution. LTOC and TPR results show that an optimum 8% Mo on MgO–Al2O3
is sufficient to form MoO3 monolayer. TPR technique can also be used as a vital tool to determine the monolayer coverage. HDS activity has
been correlated with LTOC. Introduction of basic MgO in the lattice of Al2O3 moderates the strong acidity in MgO–Al2O3. Impregnation of
acidic MoO3 species further enhances the acidity favorable for HYC.
© 2004 Published by Elsevier B.V.
Keywords: TPR; Acidity; Mixed oxides; Catalytic activity
1. Introduction
ing different combinations of mixed oxides. Mixed oxide
supports have emerged as a new class of support materi-
als with enhanced activity and selectivity, in many cases,
compared to conventional ␥-alumina. Various combina-
tions of mixed oxides such as TiO2–Al2O3, SiO2–Al2O3,
ZrO2–Al2O3, TiO2–SiO2, SiO2–ZrO2, etc. have been tried
by several researchers to meet the requirement of the desired
physico-chemical properties to suit a particular application
[1–3].
The electronegativity of metal ions of oxides varies
in the following order MgO < Al2O3 < TiO2 < SiO2.
Larger the electronegativity stronger are the metal oxygen
bonds. In other words, with decrease in electronegativity,
metal–oxygen interaction becomes weaker, and hence, the
co-ordination ability of lattice oxygen increases. As elec-
tronegativity of Mg ions is the lowest among the metal
ions of other oxides studied thus the coordination ability
of oxygen atom of MgO is the highest. In oxidative de-
hydrogenation of alkanes, reaction uses the lattice oxygen
of catalyst/support. Hence, MgO has been found to be a
suitable catalyst precursor in combination with other metal
oxides like MoO3 or V2O5 since last several years for
dehydrogenation of alkanes [4,5].
Hydrotreating of crude oil is an important process in re-
fining industry. Processing of heavier feedstocks, which con-
tain increased amount of sulfur, is increasing day by day
and environmental regulations are becoming stringent. Fu-
els containing sulfur compounds are not only poisonous to
catalysts used in reforming process and automobile catalytic
converter but also such fuels on combustion produce sul-
fur oxide, thus causing serious environmental pollution. To
reduce or eliminate the environmental pollution, the sulfur
content in the fuel needs to be reduced to a lower level
(<50 ppm). To meet the new challenges of environmental
regulations and to process heavier feed stocks economically,
highly active and selective catalysts are needed. Most of the
hydroprocessing catalysts contain Mo, W promoted by Co
and Ni metals supported on Al2O3. Support plays an impor-
tant role to control the specific characteristics of a catalyst.
The textural and chemical properties of a support can
be controlled by the method of preparation and by us-
∗
Corresponding author.
E-mail address: jkgupta@iip.res.in (J.K. Gupta).
1381-1169/$ – see front matter © 2004 Published by Elsevier B.V.
doi:10.1016/j.molcata.2003.12.005

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M. Kumar et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
Klicpera and Zdrazil [6–9] reported that Mo supported on
2.2. Textural properties
MgO has comparable HDS activity to Mo/Al2O3 catalyst
under similar conditions. They also found strong synergis-
tic effect between Co(Ni) and Mo in the HDS activity of
MgO supported Co(Ni)–Mo sulfide catalyst. They showed
that Co(Ni)–Mo/MgO catalyst exhibited about 1.5–2.5 times
higher activity in HDS of benzothiophene than the corre-
sponding reference commercial Al2O3 supported catalyst.
Murali Dhar and co-workers [10,11] reported that
Mo/MgO catalysts have higher HDS and HYD activities
in comparison to Mo/Al2O3 catalysts due to the increased
dispersion of Mo on MgO support. A little work has been
found in literature using MgO–Al2O3 mixed oxide as cat-
alytic support for HDS reactions [12]. MgO is basic support
while Al2O3 is amphoteric in nature. The acidic properties
of MgO–Al2O3 mixed oxide can be controlled by using
appropriate MgO and Al2O3 ratios [13].
In the present study for determination of the monolayer
coverage of Mo on MgO–Al2O3 mixed oxide support, a se-
ries of different Mo content (2–14 wt.% Mo) catalysts were
prepared. With an aim to understand the catalytic function-
alities of MoS2 and to study the support effect an opti-
mum 8 wt.% Mo containing catalysts were also prepared
supported on MgO, Al2O3 and MgO–Al2O3 mixed oxides.
The physico-chemical characterization like BET surface
area, total pore volume, pore size distribution, temperature-
programmed reduction, acidity and acid strength distribu-
tion are carried out and results are discussed in the light of
hydrodesulfurization (HDS), hydrogenation (HYD) and hy-
drocracking (HYC) activity using model compounds.
BET surface area, total pore volume and pore size dis-
tribution were carried out using ASAP-2010 unit from Mi-
cromeritics (USA). Adsorption–desorption isotherms of N2
were obtained at liquid N2 temperature. For BET surface
area relative pressure (P/P0) was taken from 0.05 to 0.25.
Total pore volumes were obtained from the volume adsorbed
at relative pressure around 0.99. Pore size distributions were
obtained from the adsorption branch of isotherms applying
BJH pore model [14].
2.3. Low temperature oxygen chemisorption
A conventional glass static volumetric high vacuum
system (10−6 Torr) was used for low temperature oxygen
chemisorption (LTOC). The catalyst sample (∼0.3 g) was
sulfided in situ at 673 K for 2 h using hydrogen saturated
with CS2. Oxygen chemisorption measurements were car-
ried out at 193 K. The temperature of the bath was main-
tained using a mixture of isopropyl alcohol and liquid ni-
trogen. Molybdenum metal dispersion was calculated using
the following formula.
The dispersion (O/Mo) = atoms of O2 uptake/atoms of
Mo in the catalyst [15,16].
2.4. Temperature-programmed reduction
Temperature-programmed reduction (TPR) profiles of
catalyst samples were obtained using TPD/TPR-2900 Mi-
cromeritics (USA) instrument. TPR profiles were taken
from room temperature to 1273 K (10◦/min) and then the
temperature was kept isothermal for 30 min. Five percent
hydrogen in argon mixture (50 ml/min) was used as reduc-
ing gas. The hydrogen consumption corresponding to the
reduction of the metal oxide was computed from the area
of calibrated TPR peak of standard Ag2O.
2. Experimental
2.1. Catalyst preparation
MgO–Al2O3 (1:1) mixed oxide support was prepared by
co-precipitation urea hydrolysis method. One molar aqueous
solutions of Mg(NO3)2·6H2O and Al (NO3)3·9H2O were
taken in a round bottom flask. Aqueous solution of urea was
added to the mixture as hydrolyzing agent and the whole
mixture was refluxed at 90 ◦C for 4 h maintaining the pH at
10.5. The precipitate formed was washed, followed by dry-
ing and calcination at 823 K for 6 h. Pure MgO and pure
Al2O3 were also prepared by the same method using their
respective salts. The pH during the preparation was main-
tained at 10.5 for pure MgO and 7.2 for pure Al2O3.
A series of MoO3 catalysts with Mo loading from 2
to 14 wt.% supported on MgO–Al2O3 (1:1) mixed oxide
was prepared by incipient wetness impregnation method
using ammonium heptamolybdate ((NH4)6Mo7O24·4H2O).
The 8 wt.% Mo was also impregnated on pure Al2O3 and on
pure MgO by the same method. The catalyst samples were
dried at room temperature followed by drying at 383 K over
night and finally calcined at 823 K for 6 h. All the reagents
used were of analar grade.
2.5. Acidity measurements
Microcalorimetric studies of adsorption of ammonia
have been performed to determine total acidity and acid
strength distribution using a Tian-Calvet type heat flux
microcalorimeter (model C-80 Setaram, France) connected
to a volumetric vacuum adsorption unit for sample treat-
ment and probe molecule delivery. Samples were preheated
at 723 K under vacuum for 4 h prior to microcalorimetric
measurements. The heats evolved from sequential doses of
ammonia onto the sample were measured at 448 K. The heat
of adsorption generated for each dose was calculated from
the resulting thermograms and the amount of ammonia ad-
sorbed from the initial and final pressure. Sequential doses
give the differential heat of NH3 adsorption as a function
of coverage (i.e. differential heat curves). It provides the
information about the number and strength of acid sites on
samples.

M. Kumar et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
219
2.6. Catalytic activity
drolysis method leads a bimodal type of pore size distri-
bution. In MgO–Al2O3, the pores are centered around 45
and 90 Å diameter, whereas in the case of pure MgO and
Al2O3 pore maxima are around 110 and 70 Å diameter, re-
spectively. Wide range mesoporous distribution is observed
in MgO–Al2O3 mixed oxide.
Temperature-programmed reduction profiles of catalyst
samples with varying Mo content from 2 to 14 wt.% on
MgO–Al2O3 mixed oxide support are shown in Fig. 2 and
results are summarized in Table 1. All the Mo catalysts ex-
hibit two distinct major peaks. The peak maxima (tempera-
ture of maximum reduction) of first peak (T1), second peak
(T2), total hydrogen consumption obtained from TPR data
and Mo dispersion calculated from LTOC values are also
included in Table 1.
In Mo/MgO–Al2O3 series, the first peak maxima of all
the catalyst samples that range from 845 to 887 K are mainly
due to the reduction of octahedral species corresponding to
the reduction of Mo6+ to Mo4+ state. The second peak max-
ima of all the catalyst samples are centered around 1200 K
and are due to the reduction of tetrahedral species that cor-
respond to the reduction of Mo4+ to Mo metal [17].
Catalytic activities were determined with a glass microre-
actor equipped with online GC at atmospheric pressure and
at 673 K. Catalyst samples (0.2 g, particle size 18–40 mesh
diluted with equal amount of quartz of same size placed be-
tween plugs of quartz wool) were sulfided in situ at 673 K
for 2 h. Catalytic activities were measured using model com-
pounds thiophene, cyclohexene and cumene for HDS, HYD
and HYC activities, respectively.
3. Results and discussion
Pore size distribution curve of pure MgO, pure Al2O3 and
MgO–Al2O3 mixed oxide are shown in Fig. 1. The BET
surface areas of MgO and Al2O3 have been found to be 83
and 198 m2/g while that of MgO–Al2O3 has shown an in-
termediate value 170 m2/g. The total pore volume of three
samples lies around 0.35 ml/g. Pure alumina has no micro
pores while MgO has about 15% and MgO–Al2O3 has 5%.
Incorporation of MgO into the lattice of Al2O3 by urea hy-
Fig. 1. BJH adsorption dV/dlog(D) pore volume curves of pure Al2O3 (⊕), pure MgO (ꢀ) and MgO–Al2O3 mixed oxide (×).

220
M. Kumar et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
Fig. 2. Temperature-programmed reduction (TPR) profiles of: (A) 2 wt.% Mo; (B) 4 wt.% Mo; (C) 6 wt.% Mo; (D) 8 wt.% Mo; (E) 10 wt.% Mo; (F)
12 wt.% Mo; and (G) 14 wt.% Mo supported on 1:1 MgO–Al2O3 mixed oxide.
The first peak is continuously becoming sharper with Mo
loading. It indicates that the rate of reduction of MoO3 to
MoO2 increases and becomes easier with an increase in Mo
content. The temperature of first reduction peak (T1) K de-
creases with Mo loading up to 8 wt.% Mo and then it re-
mains almost constant on further increase in Mo loading.
It indicates that monolayer formation of MoO3 on the sup-
port surface is complete up to 8 wt.% Mo loading. This fact
is also corroborated by the studies on the dispersion of Mo
metal on MgO–Al2O3 support calculated by LTOC values
which remain constant up to 8 wt.% Mo loading and than
decrease sharply. This also shows the completion of mono-
layer up to 8 wt.% Mo loading. Thus, the TPR studies are
also indicative of the formation of MoO3 monolayer on the
surface of MgO–Al2O3 mixed oxide support. Similar to the
first peak, second peak is also getting sharper with Mo con-
tent indicating an increase in reduction rate. The reduction
of MoO2 to Mo metal is also facilitated with increase in Mo
content.
TPR profiles of Mo supported on pure MgO, Al2O3 and
MgO–Al2O3 mixed oxide are shown in Fig. 3. The sharp-
ness of the peaks show that the rate of reduction for both
octahedral and tetrahedral species are higher in the case
of Mo supported on MgO as compared to Al2O3, whereas
Table 1
TPR results and Mo metal dispersion of different catalysts
Catalyst
Peak max (K)
H2 cons.+ (ml STP/g)
LTOC (␮mol, STP/g)
Mo dispersion (O/Mo)
T1 (max. T1)
T2 (max. T2)
2 wt.% Mo/MA
4 wt.% Mo/MA
6 wt.% Mo/MA
8 wt.% Mo/MA
10 wt.% Mo/MA
12 wt.% Mo/MA
14 wt.% Mo/MA
8 wt.% Mo/MgO
8 wt.% Mo/Al2O3
887.5
849.3
846.0
840.8
840.8
840.8
842.5
875.7
743
1237.5
1208.8
1194.5
1223.2
1230.3
1235.2
1259.3
1240
17.97
28.06
34.69
58.10
58.97
66.86
81.40
86.62
55.70
14.0
0.134
0.130
0.133
0.131
0.083
0.059
0.050
0.219
0.139
27.02
41.44
54.59
43.38
36.87
36.84
91.21
58.04
1123
MA: MgO–Al2O3; +: from TPR profiles.

M. Kumar et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
221
the Mo supported on MgO–Al2O3 mixed oxide shows an
intermediate rate of reduction. It indicates that the degree
of dispersion of Mo supported on MgO is higher as com-
pared to the same amount of Mo supported on Al2O3 and
MgO–Al2O3.The reduction temperature of supported Mo is
in the order, MgO > MgO–Al2O3 > Al2O3. This indicates
a strong metal support interaction in Mo/MgO as compared
to Mo/Al2O3 and Mo/MgO–Al2O3.
provided the support is not reduced. However, the metal
support interactions are different on different supports. In
8 wt.% Mo/MgO, the first TPR peak maxima corresponding
to MoO3 → MoO2 is observed at 876 K. The reaction was
studied at 673 K on catalyst after sulfidation at 673 K where
catalytically active species MoO2 (MoS2) are available as
sulfidation is thermodynamically favorable than reduction
[18].
The strong metal support interaction observed in the case
of Mo/MgO catalyst is because of the poor electronegativity
of Mg ions in MgO, leading to a weak Mg–O bond and
hence the co-ordination ability of oxygen of MgO support
to Mo metal is higher causing strong metal (Mo)–support
interaction. Electronegativity of metal ions present in Al2O3
is greater than MgO. Metal ions present in MgO–Al2O3
mixed oxide have an average effective electronegativity that
lies in between to its component metal ions separately.
Theoretically, 8% Mo (∼12% MoO3) should need
56.02 ml STP H2 for complete reduction to Mo metal.
Actually, this proceeds in sequences of reduction, i.e.
MoO3 → MoO2 requiring 18.67 ml STP/g H2 correspond-
ing to first TPR peak and MoO2 → Mo needing 37.36 ml
STP/g for the second peak. MgO support is also reduced
consuming ∼20 ml STP/g H2 with peak maxima at 764 K
(Fig. 3). A higher H2 consumption in 8% Mo/MgO was
therefore observed (Table 1). Assuming around 56 ml/g STP
H2 contributed by MoO3 for complete reduction, the extra
hydrogen consumption may be due to reduction of MgO
support itself. Some other unidentified Mg species seems
to be formed below 764 K as indicated by small peaks in
TPR profile (Fig. 3), which account for the rest hydrogen
consumption (∼10 ml STP/g).
The hydrogen consumption in reduction up to first TPR
peak is indicative of formation of MoO2 species only. This
H2 consumption does not have direct correlation with hy-
drogen available for hydrogenation on metal sites during
the reaction in post sulfided (reduced) catalyst. Thus, low
hydrogenation rate in Mo/MgO (Table 2) as compared to
MgO/Al2O3 and Mo/MgO–Al2O3 cannot be explained on
the basis of TPR results alone. However, a good correlation
has been found with HDS activity and LTOC (Mo disper-
sion). The 8% Mo/MgO has shown highest LTOC indicating
large population of metal sites available at the surface for
desulfurization in presence of H2. This has also reflected in
the highest value of HDS activity for this catalyst (Table 2).
About 8% Mo/Al2O3 and 8% Mo/MgO–Al2O3 have lower
values of LTOC and the HDS activity also falls accordingly.
Moreover, HDS may involve hydrogenation and hydrogenol-
ysis.
Microcalorimetric results on the acidity and acid strength
distribution of supports and their respective Mo catalysts are
shown in Table 3. Differential heats versus ammonia cov-
erage for supports and Mo catalyst are illustrated in Figs. 4
and 5, respectively. Al2O3 has a total acidity 0.336 mmol/g
and shows a heterogeneous distribution of acid sites. MgO
does not show any acidity, as it is basic in nature but incorpo-
ration of equal stoichiometric amounts of MgO into Al2O3
matrix through co-precipitation creates moderate acidity.
The H2 consumption in TPR profiles with fixed 8 wt.%
Mo should be basically the same on different supports
Fig. 3. Temperature-programmed reduction (TPR) profiles of: (A) pure MgO; (B) 8 wt.% Mo/MgO; (C) 8 wt.% Mo/Al2O3; and (D) 8 wt.% Mo/MgO–Al2O3.

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M. Kumar et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
Table 2
Catalytic activities of supported Mo catalysts
Catalyst
rHDS (mol/h gcat
)
rHYD (mol/h gcat
)
rHYC (mol/h gcat)
8 wt.% Mo/MgO
8 wt.% Mo/Al2O3
8 wt.% Mo/MgO–Al2O3
52.6 × 10−3
23.56 × 10−3
11.78 × 10−3
15.6 × 10−3
36.4 × 10−3
30.5 × 10−3
23.65 × 10−3
32.34 × 10−3
36.25 × 10−3
Table 3
Acidity and acid strength distributions of oxide supports and catalysts
Samples
Total acidity (mmol/gcat
)
Acid strength distribution (mmol of NH3/gcat)
Strong
Medium
Weak
Al2O3
MgO
MA
8% Mo/MgO
8% Mo/Al2O3
8% Mo/MA
0.336
0.000
0.290
0.209
0.402
0.339
0.085
–
Nil
0.00
0.170
0.100
0.105
–
0.125
0.145
0.120
0.160
0.146
–
0.165
0.064
0.112
0.079
MA: MgO–Al2O3; strong >100 kJ/mol; medium: 100–75 kJ/mol; weak: <75 kJ/mol.
MgO
140
120
100
80
Al2O3
MgO-Al203
60
40
20
0
0
0.1
0.2
0.3
0.4
Ammonia coverage (mmol/g cat.)
Fig. 4. Differential heats vs. ammonia coverage for MgO, Al2O3 and MgO–Al2O3 support samples.
160
Mo/MgO
140
120
100
80
Mo/Al2O3
Mo/MgO-Al2O3
60
40
20
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Ammonia coverage(mmol/g cat.)
Fig. 5. Differential heats vs. ammonia coverage of 8 wt.% Mo supported catalysts.

M. Kumar et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 217–223
223
MgO–Al2O3 does not have strong acid sites but shows an
increase in medium and weak acid sites as compared to
Al2O3. It seems to follow the rule that strong acid sites are
neutralized first when an acidic support is mixed chemi-
cally with basic one. Generation of medium and weak acid
sites in MgO–Al2O3 mixed oxide are due to redistribution
of charges which depends upon the structure of mixed oxide
[19]. It was noticed that incorporation of 8 wt.% Mo onto
the surface of oxide supports improves the total acidity and
its distribution. MgO does not have inherent acidity but in-
corporation of 8 wt.% Mo creates acidic sites on the surface
of MgO. In general, acidity depends upon the average elec-
tronegativity of ions present. Addition of Mo onto surface
of oxides improves average electronegativity of metal ions
present in it. Comparison among Mo supported catalysts
shows that Mo/Al2O3 catalyst has the highest total acidity
and strong acid sites than Mo/MgO–Al2O3 and Mo/MgO.
HYC activities do not seem to be affected by hydrogen
consumption of catalysts as hydrocracking is a function of
acidity and is strongly controlled by acidic properties of
catalyst. The HYC activities are nearly equal for supported
Al2O3 and MgO–Al2O3 catalysts. Mo/MgO catalyst also
show some HYC activity due to the presence of acid sites
of moderate strength induced by the incorporation of acidic
MoO3 on basic MgO.
among Mo supported on Al2O3 and MgO–A2O3. Incorpo-
ration of basic MgO into the lattice of amphoteric Al2O3 in
the MgO–Al2O3 catalysts introduces moderate acidity neu-
tralizing the strong acid sites of alumina. Addition of Mo
on MgO, Al2O3 and MgO–Al2O3 enhances the acidity of
these oxides due to the acidic nature of MoO3 species. An
increase in acidity is noticed from weak to strong acid sites
with the increase in electronegativity of the metal ion of an
oxide.
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