Maleic anhydride hydrogenation to succinic anhydride over mesoporous Ni/TiO2 catalysts: Effects of Ni loading and temperature

Article abstract of DOI:10.1016/j.molcata.2016.07.037

Catalytic hydrogenation of maleic anhydride for the production of succinic anhydride can be a viable alternative to the higher energetic demand route based in the dehydration of succinic acid. In this sense, the metallic Ni catalysts supported on mesoporous TiO₂ (anatase) substrate demonstrated to be very active and 100% selective in the liquid phase hydrogenation of maleic anhydride (MA) to succinic anhydride (SA). The catalysts, which were prepared via wet impregnation method with different Ni loading (5, 10 and 15?wt.%), were characterized by chemical analysis (ICP-AES), N₂ physical adsorption-?desorption, H₂-?temperature programmed reduction (H₂-?TPR)?, X-ray diffraction (XRD)?, high resolution transmission electron spectroscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). The Ni species interaction with support was investigated by TPR and by performing five catalyst recycling tests. After catalyst activation by reduction, the increase of Ni particle size with an increase of Ni loading was relatively small (from 6.9 to 8.9?nm) due to enhance of the metal-support interaction. After the first catalytic cycle, the optimized 5%Ni/TiO₂ catalyst showed a small decrease in the Ni loading attributed to metal leaching during time course of reaction. Besides this, the 5%Ni/TiO₂ catalyst exhibited a good stability during five continuous cycles with a very high yield of SA after 5 cycles. Finally, temperature experiments performed for the best system shown that the reaction temperature does not affect the SA selectivity in the temperature range studied (323?K–398?K).

Full text of DOI:10.1016/j.molcata.2016.07.037

Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Maleic anhydride hydrogenation to succinic anhydride over
mesoporous Ni/TiO2 catalysts: Effects of Ni loading and temperature
Cecilia C. Torresa,∗, Joel B. Alderetea, Claudio Mellab, Barbara Pawelecc
a Departamento de Química Orgánica, Universidad de Concepción, Edmundo Larenas 129, Concepción, Chile
b Departamento de Fisicoquímica, Universidad de Concepción, Edmundo Larenas 129, Concepción, Chile
c Instituto de Catálisis y Petroleoquímica (ICP-CSIC), Sustainable Energy and Chemistry Group, Marie Curie 2 Cantoblanco Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2016
Received in revised form 19 July 2016
Accepted 20 July 2016
Available online 21 July 2016
Catalytic hydrogenation of maleic anhydride for the production of succinic anhydride can be a viable
alternative to the higher energetic demand route based in the dehydration of succinic acid. In this sense,
the metallic Ni catalysts supported on mesoporous TiO2 (anatase) substrate demonstrated to be very
active and 100% selective in the liquid phase hydrogenation of maleic anhydride (MA) to succinic anhy-
dride (SA). The catalysts, which were prepared via wet impregnation method with different Ni loading (5,
10 and 15 wt.%), were characterized by chemical analysis (ICP-AES), N2 physical adsorption- desorption,
H2- temperature programmed reduction (H2- TPR) , X-ray diffraction (XRD) , high resolution transmission
electron spectroscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). The Ni species interaction
with support was investigated by TPR and by performing five catalyst recycling tests. After catalyst activa-
tion by reduction, the increase of Ni particle size with an increase of Ni loading was relatively small (from
6.9 to 8.9 nm) due to enhance of the metal-support interaction. After the first catalytic cycle, the optimized
5%Ni/TiO2 catalyst showed a small decrease in the Ni loading attributed to metal leaching during time
course of reaction. Besides this, the 5%Ni/TiO2 catalyst exhibited a good stability during five continuous
cycles with a very high yield of SA after 5 cycles. Finally, temperature experiments performed for the best
system shown that the reaction temperature does not affect the SA selectivity in the temperature range
studied (323 K–398 K).
Keywords:
Maleic anhydride
Nickel
Hydrogenation
Recycle
Succinic anhydride
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
fragrances, flavorings, etc. [9]. The obtention of SA at large scale
is performed by the dehydration of succinic acid, which proceeds
For years the chemical industry has search efficient methods
to optimize yield and selectivity of reactions that are part of dif-
ferent types of processes used for the production of intermediate
products, which are used in diverse applications. This wide range
of products include those obtained in the fine chemical industry,
for which usually both yield and selectivity can be achieved using
catalysts operating in heterogeneous phase [1,2]. Among build-
ing blocks molecules, maleic anhydride is highlighted, which is
produced on a large scale by the partial oxidation of n-butane
[3,4]. The catalytic hydrogenation of MA both in liquid phase and
gas phase [5–8], give products such as succinic anhydride (SA),
␥-butyrolactone (GBL), tetrahydrofuran (THF), among others (see
Scheme 1). In particular, SA is used as raw material in the produc-
tion of a wide variety of substances, which are used as colorants,
at elevated temperatures (523 K). However, obtaining SA from the
catalytic hydrogenation of MA represents an alternative route to its
traditional production, since the use of catalysts operating at lower
temperature should decrease the energy requirements of the pro-
cess compared with the route of dehydration of the succinic acid
[10].
In general, the hydrogenation of MA has been studied using
different types of catalysts, as it is summarized in Table 1.
Noble metals, transition metals with hydrogenating activity
and combinations of both have shown to be active in the hydro-
genation reaction of MA and selectivity dependence to SA was
observed, mainly due to the nature of the support and the active
phase used. Moreover, most of these catalytic systems are char-
acterized by operating at temperatures above 453 K, and at high
hydrogen pressure (5.0 MPa or higher). Of the active phases studied,
nickel has proven to be a good alternative due to the high affinity
that this metal has for the activation of C C double bonds (seen
Table 1). However, the ability of nickel to produce hydrogenoly-
∗
Corresponding author.
E-mail address: cectorres@udec.cl (C.C. Torres).
http://dx.doi.org/10.1016/j.molcata.2016.07.037
1381-1169/© 2016 Elsevier B.V. All rights reserved.

442
C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
Scheme 1. Reaction pathway of the hydrogenation of maleic anhydride adapted from [11].
Table 1
Comparison of the selectivity toward succinic anhydride in liquid-phase hydrogenation of maleic anhydride over different supported catalysts.
Catalyst
Conditions
SSA (%)
Reference
2%Ru/AC
T = 423 K; PH2 = 6.0 MPa; 2.0 mol L−1 MA; 0.050 g catalyst
T = 513 K; PH2 = 7.1 MPa; 3.1 mol L−1 MA;10.0 g catalyst
T = 513 K; PH2 = 7.1 MPa; 3.1 mol L−1 MA; 10 g catalyst
T = 483 K; PH2 = 5.0 MPa; 1.25 mol L−1 MA; 0.10 g catalyst
T = 483 K; PH2 = 5.0 MPa; 1.25 mol L−1 MA; 0.10 g catalyst
99.6 (2)a
51.5 (4)a
73.2 (5)a
65.0 (8)a
85.0 (8)a
[11]
[12]
[12]
[13]
[13]
8%Pd-4%Zr/SiO2
3%Ru-25%Ni/SiO2
10%Ni/CeO2
10%Ni/Al2O3
a
Total reaction time in hours.
sis of C O bonds [14–16] could significantly reduce the selectivity
towards SA. In relation to the support, it plays a fundamental role
in the performance of the catalytic hydrogenation of MA. Thus,
supported catalysts on partially reducible oxides, such as TiO2 or
CeO2, able to generate strong metal support interaction (SMSI), have
shown a decreasing selectivity toward SA, favoring the production
of GBL. This is due to the strong support-carbonyl bonds interac-
tions [6,12,17,18]. Therefore, an efficient and selective catalyst to
SA, possessing Ni as active phase, should employ supports in which
strong metal-support interactions are not established or they are
strongly diminished.
Among different supports, the new synthesized mesoporous
TiO2 of pure anatase phase demonstrated to be a good substrate
for supporting metal oxides due to the strong metal support inter-
action (SMSI), chemical stability, acid-base property, sharp pore
distribution and large surface favoring good active phase disper-
sion [19,20]. The active phase dispersion could be enhanced also by
using optimized catalyst’s preparation conditions. In this context,
the study by Huo et al. [18] demonstrated that calcination tem-
perature strongly influenced on the catalytic behavior of Ni/TiO2
catalysts tested in hydrogenation of MA. This is probably because of
the morphological changes occurring upon high temperature cal-
cination influencing on the metal-TiO2 support interaction, such
as sintering [21], encapsulation or interdiffusion [22]. Thus, the
temperature of calcination should be carefully selected in order
to decompose Ni precursor without producing those undesirable
phenomena.
Within this scenario, the present work was undertaken with
the aim to evaluate the catalytic response of Ni catalysts supported
on TiO2 (anatase) substrate for hydrogenation of maleic anhydride
(MA) to succinic anhydride (SA). In addition, the recycling tests have
been performed in order to evaluate the stability of Ni species on the
support surface during liquid phase reaction. For the best catalyst
(5%Ni/TiO2), the effect of reaction temperature on the catalyst activ-
ity was also investigated. In order to explain the catalytic behavior,
the nature of nickel species for different Ni loadings (5, 10 and
15 wt.%) and their interaction with the support have been investi-
gated by means of different characterization techniques (XRD, TPR,
HR-TEM and XPS).
2. Experimental
2.1. Catalyst preparation
All catalytic systems were prepared by the wet impregnation
procedure using a slight excess of deionized water [23]. The proper
amount of metallic precursor, Ni(NO3)2 × 6H2O, was added to 5.0 g
of TiO2-anatase in order to obtain 5, 10 and 15% wt. of nickel, subse-
quently 50 mL of deionized water was incorporated. This procedure
was performed in a rotary evaporator at 343 K for 5 h [24]. The
resulting solid was filtered until the filtered water showed con-
stant conductivity. Finally, the solids were dried in an oven at 373 K
overnight. The solids were calcined under air flow at 773 K for 2 h
and then reduced in situ at 723 K under H2 flow for 4 h [25].
2.2. Characterization
The interaction of hydrogen with the Ni catalysts was studied by
temperature-programmed reduction experiments (TPR), the cata-
lysts were reduced under continuous flow using a 5% H2/Ar mixture
(40 cm3 min−1), at 10 K min−1 from room temperature up to 1073 K
in a TPR/TPD2900 Micromeritics system. Hydrogen consumption
was monitored with the aid of a TCD detector and recorded for TPR
analysis.
X-ray diffraction (XRD) was performed on a Rigaku X-ray
Geigerflex using a Ni filter and Cu K␣ radiation at 2–90◦ in 2␪
range. N2 adsorption-desorption isotherms were recorded at 77 K
in an ASAP 2010 Micromeritics apparatus, the specific surface
areas were determined by the BET (Brunauer-Emmett-Teller) equa-
tion using the adsorption data over a relative pressure range of
0.05–0.3, and the pore-size distributions were estimated using the
Barrett–Joyner–Halenda (BJH) method based on the Kelvin equa-
tion [26].
A JEM-2100F Field Emission Electron Microscope (HR-TEM) fea-
turing ultrahigh resolution and rapid data acquisition with a probe
size under 0.5 nm was used to analyze the catalysts. The samples
for analysis were prepared by dispersion in ethanol/H2O (1:1) and
deposited on a holey carbon/Cu grid (300 mesh). Up to 300 individ-
ual metal particles were counted for each catalyst, and the surface

C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
443
area-weighted mean diameter (dp) was calculated by the equation:
ꢀ
inidi3
inidi2
ꢀ
dp
=
where ni is the number of particles of diameter di.
Chemical analysis was performed by inductively coupled
plasma-mass spectrometry on a PerkinElmer 3300 ICP-MS Spec-
trometer. Samples were solubilized in a nitric/hydrochloric acid
solution and homogenized in microwave oven.
X-ray photoelectron spectra (XPS) of the in-situ reduced (723 K
for 2 h) catalysts were recorded using an Escalab 200R spectrometer
provided with a hemispherical analyzer, operated in a constant pass
energy mode and Mg K␣ X-ray radiation (h = 1253.6 eV) operated
at 10 mA and 12 kV. Charging effects of samples were corrected by
fixing the binding energy (BE) of the C1s core-level of adventitious
carbon at 284.8 eV (accuracy = 0.1 eV).
2.3. Catalytic activity in liquid phase
Fig. 1. Diffraction patterns for (a) TiO2, (b) 5%Ni/TiO2, (c) 10%Ni/TiO2 and (d)
15%Ni/TiO2. Reference patterns (*) Ni0 (JCPDS 00-004-0850); (᭹) TiO2-Anatase
(JCPDS 00-021-1272).
The catalytic assays of maleic anhydride hydrogenation were
performed in a stainless steel (150 mL) Parr-type semi-batch reac-
tor using a substrate/metal ratio of 100, stirring rate of 650 rpm,
50 mL of tetrahydrofuran (THF) as the solvent and 0.100 g of cat-
alyst. Before reaction, the x%Ni/TiO2 catalysts were activated by
reduction with H2 at 723 K for 2 h. Maleic anhydride hydrogena-
tion kinetic experiments were carried out between 323 and 393 K
and a total hydrogen pressure of 4.0 MPa. No mass transfer resis-
tance were found in a catalytic test established using the Madon
and Boudart approach [27]. No catalytic activity for pure TiO2 was
confirmed in a blank test. Pseudo-first-order kinetic constants (k)
were calculated as reported in previous studies [28]. Analytical
quantifications for reactants and products were analyzed by gas
chromatography using a GC instrument, HP-4890 with a semi-
capillary column HP-5, FID detector and N2 as the carrier gas.
Temperature and reusability studies were performed with the best
catalytic system. The recycling assays were performed by centrifu-
gation of the catalyst from the reaction medium, washed four times
consecutively with ethanol (40 mL × 4) to clean the surface and
removed all organic matter, dried under vacuum at 323 K for 12 h
and finally reduced at 473 K for 1 h under H2 flow [29].
Table 2
Structural and morphological analysis of TiO2-anatase support and Ni-TiO2 sup-
ported catalysts.
Sample
SBETa(m2 g−1
)
dp (nm)b
Vpc(cm3 g−1
)
TiO2
156
107
92
8.5
9.7
10.3
9.7
0.37
0.30
0.27
0.23
5%Ni/TiO2
10%Ni/TiO2
15%Ni/TiO2
87
a
Specific surface area, determined by the BET method from N2 physisorption
measurements.
b
Pore diameter, determined from quantity of N2 adsorbed at a relative pres-
sure = 0.99.
c
Pore volume, calculated from the maximum in the BJH pore size distribution.
sity might indicate an increase in the crystallinity of metallic Ni
particles whereas the decrease in their average crystallite size could
be deduced from an increase of the full width at half maximum of
the peaks. For all catalysts, no peaks due to crystalline NixOy-type
species were observed. This observation suggests that their crystal
size could be below detection limit of XRD technique (<4 nm) indi-
cating the high Ni species dispersion on the support surface [18]
and/or because they are not crystalline (amorphous).
The textural and morphological properties of the different sys-
tems were analyzed by N2 desorption- adsorption isotherms at
77 K. As shown in Fig. 2, all the systems give type IV isotherms
according to the IUPAC classification, corresponding to mesoporous
materials with cylindrical pores [35]. In addition, it can be observed
that the pore distribution is bimodal for all systems, centered at 7.0
and 11.0 nm approximately. This distribution is due to interparticle
porosity which is characteristic of TiO2 based supports [36]. Table 2
shows the values associated with the textural properties of the sup-
port and catalysts. As shown in Table 2 no significant variations in
the surface area, diameter, and pore volume were observed, when
the values associated to the support and catalysts were compared.
This could indicate that the morphological properties of the mate-
rial remain unchanged after the incorporation of metal despite the
small pore blockage, which is reflected in the small decrease of the
surface area of the catalysts.
2.4. Ni-leaching tests
The screening catalytic assays of residual filtered catalytic reac-
tion was studied in the cyclohexene hydrogenation and performed
in a stainless steel (150 mL) Parr-type semi-batch reactor at a
substrate/metal ratio of 250, assumed from ICP-AES post-reaction
characterization, at 650 rpm under 4.0 MPa of H2 pressure and
373 K of temperature and reaction time of 4 h.
3. Results and discussion
3.1. Catalysts characterization
Fig. 1 presents XRD profiles of the TiO2-anatase and Ni catalysts.
In all of the patterns the diffraction planes associated at 2␪ angles of
25.4◦, 37.9◦ and 48.2◦ of typical TiO2-anatase were observed (JCPDS
00-021-1272) [30–32]. These diffraction peaks were unchanged
after deposition of the metal phase. In all three catalysts signals
at 2␪ angles of 44.5◦ and 52.0◦ are observed, which are associated
with diffraction planes (111) and (200) of the metallic nickel (Ni0)
[33,34]. In addition, an increase in the intensity of the diffraction
peak associated to Ni0 (2␪ = 44.5◦) was observed with the increase
of nominal metal loading. The observed increase in the signal inten-
The morphology of the pure TiO2 substrate and x%Ni/TiO2 cat-
alysts was examined by HRTEM. HR-TEM images of x%Ni/TiO2
catalysts together with that of pure TiO2 anatase support are shown
in Fig. 3.

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C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
Fig. 2. N2 adsorption-desorption isotherms at 77 K (A) and pore size distributions (B) of the support and their respective catalysts.
Table 3
metal is very close to the nominal amount, showing that the prepa-
ICP-AES and metal particle size distribution determined by TEM for Ni/TiO2 catalysts.
ration method does not present significant loss of metal in both the
impregnation step and subsequent reduction of the catalysts (see
Table 3).
Sample
Ni wt. (%)a
dXRD (nm)b
dHR-TEM (nm)c
DTEM(%)d
5%Ni/TiO2
10%Ni/TiO2
15%Ni/TiO2
4.5 (4.0)a
9.1 (4.4)a
14.6 (5.7)a
8.0
9.0
11.9
6.9 0.8
8.0 1.1
8.9 1.2
15.0
13.0
11.3
The catalysts were prepared by impregnation of TiO2 sub-
strate with the nickel precursor followed by drying and
calcination at 773 K in dynamic air flow. Under these con-
ditions, the formation of nickel species, such as NixOy and
NiTiO3 is precluded. These oxidic species are undesired because
Ni0 is the active species in the hydrogenation reaction. It
is also emphasized that if “NiO(core)-NiTiO3(shell)” structures
are formed, then the reducibility of nickel oxide species
could be strongly affected, as it was observed by Hou et al.
[18].
Temperature-programmed reduction (TPR) studies of TiO2-
supported nickel oxide have been used to evaluate the influence
of nickel oxide loading on the metal-support interaction. In Fig. 4
the TPR-H2 profiles for the systems synthesized are shown. In
all the catalysts, metal reduction in two or more steps was
observed. The first one is a reduction peak of equal intensity for
the three metal content, centered at 553 K, which may be asso-
ciated with NixOy species that interact weakly with the support
(TiO2-anatase) [38]. For 5%Ni/TiO2 catalyst, a second reduction peak
is observed at 620 K attributed to the reduction of NixOy species
interacting more strongly with the support. For 10%Ni/TiO2 and
15%Ni/TiO2 catalysts, this peak is shifted toward higher reduc-
tion temperature (649 K), thus, the interaction of those species
with support could be stronger. Noticeably, the latter peak exhibits
small shoulder at lower temperature region, which could be
attributed to the reduction of NixOy species having intermediate
size [18,34,39].
X-ray spectrophotometry analysis (XPS) was performed to
the samples pre-reduced at the same conditions as those used
for reaction tests. Using this technique the surface composition
and electronic states of the different elements in the catalysts
were determined. Ni 2p spectrum (not shown here) shows two
contributions, as summarized in Table 4, indicating that in-situ
H2-reduction at 723 K for 2 h is not strong enough to reduce
completely Ni2+,ions, in good agreement with TPR characteriza-
tion (Fig. 4). These signals indicate the presence of Ni0 (BE at
852.4 0.2 eV) and Ni2+ (BE at 855.4 0.2 eV) species [40,41]. The
contribution of Ni2+ in the catalysts can be explained by the
presence of highly disperse surface NixOy species, which pos-
sesses small sizes as can be inferred from the characterization by
XRD.
a
Nickel loading of the fresh reduced samples before and after reaction (in paren-
thesis).
0.94ꢀ
b
Ni0 particle size (from XRD) calculated using equation d =
Particle size determinated by statistical analysis of different HR-TEM images.
Catalyst dispersion (from HR-TEM) calculated using equation D =
.
cosꢁ
ˇ
1/2
c
1.01
HR−TEM
d
×
d
100% [37].
Table 4
Binding energies (eV) of internal electrons and Ni/Ti atomic surface ratio of x%Ni/TiO2
systems after in-situ H2-reduction at 723 K for 2 h.
a
Catalyst
Ti2p3/2
458.6
O1s
Ni2p3/2
Ni/Ti at
0.050
529.9 (83)
531.5 (17)
530.1 (81)
531.6 (19)
529.9 (85)
531.5 (15)
852.4 (61)
855.4 (39)
852.2 (64)
855.4 (36)
852.2 (67)
855.3 (33)
5%Ni/TiO2
458.6
458.6
0.096
0.144
10%Ni/TiO2
15%Ni/TiO2
a
The percentage of the corresponding species are given in parenthesis.
According to the EDX information, the black dots seen on the
support matrix of x%Ni/TiO2 catalysts are assumed to be metallic
Ni particles. All x%Ni/TiO2 catalysts exhibit the structure of tita-
nium oxide decorated with metallic Ni particles. The metal particles
have almost regular spherical shapes in all catalytic systems. The
influence of Ni loading on the Ni particle size distributions could
be deduced from the histograms of the 5%Ni/TiO2, 10%Ni/TiO2 and
15%Ni/TiO2 catalysts shown in Fig. 3 together with the correspond-
ing TEM images. The average particle diameter determined by
statistical analysis is listed in Table 3. As seen, the average Ni par-
ticle size follows the trend: 5%Ni/TiO2 (6.9 0.8 nm) < 10%Ni/TiO2
(8.0 1.1 nm) < 15%Ni/TiO2 (8.9 1.2 nm). As expected, when the
nickel content increases the size of the metallic crystals also slightly
increased. This result is according with the results observed in
XRD. As a general trend, a more heterogeneous distribution of Ni
agglomerates can be observed for the catalysts with higher Ni load-
ing (10 and 15 wt.%) than for the sample with a low Ni loading
(5%Ni/TiO2).
Through ICP-AES analysis the amount of metal in the catalysts
was determined. This analysis revealed that the actual amount of

C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
445
Fig. 3. HR-TEM micrographs of 5%Ni/TiO2 (A), 10%Ni/TiO2 (B), 15%Ni/TiO2 (C) and (D) pure TiO2 (anatase) substrate.
For oxygen atoms two contributions are observed, the O1s
component at lower energy (529.9 eV) is due to Ti O bond type
of the TiO2 network while higher energy component (531.5 eV)
is associated with Ti OH groups on the surface of the support
[42,43]. The Ti 2p3/2 peak centered at 458.6 eV is associated to the
presence of Ti4+ species in the catalyst. As expected, an increase
in the Ni loading led to an increase of the surface exposure of
Ni species (Table 4). Because of the decrease of metal-support
interaction with an increase of Ni content, the percentage of Ni0
species (given in parenthesis in Table 4) increases also confirm-
ing easier Ni reduction for the sample with largest Ni content.
Moreover, XPS analysis indicates the absence of SMSI effect,
because contributions from the binding energy are not shifted to
smaller values of energy, which should be observed if reducible

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C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
Table 5
Catalytic dataa for MA hydrogenation on reduced Ni/TiO2 catalysts.
−1
)
Catalyst
ka(h−1 g Ni
5%Ni/TiO2
10%Ni/TiO2
15%Ni/TiO2
3.34
7.53
3.20
a
Pseudo-first-order constant; reaction conditions: T = 373 K; PH2: 4.0 MPa; sub-
strate/metal ratio = 100; catalyst mass: 0.100 g; stirring rate: 650 rpm; solvent: THF;
reaction time: 6 h.
tion whereas total MA conversion for 5%Ni/TiO2 sample occur at
6 h of reaction time. Taking into account the literature information
[45,46], the catalysts with Ni loading lower than 5 wt.% should be
less effective in the target reaction. This is probably because the
intrinsic hydrogenation property of Ni is much lower when com-
pared with noble metals. This fact is commonly compensated by
employing a much larger amount of Ni than in case of noble met-
als [47]. The effect of Ni loading (in range 3–30 wt.% of Ni) on the
activity and selectivity of the Ni/HY-Al2O3 catalysts in hydrogena-
tion of maleic anhydride to succinic anhydride was systematically
investigated by Li et al. [45]. As expected, it was found that cata-
lyst loaded with 3 wt.% of Ni exhibited lower activity and selectivity
than that loaded with 5 wt.% of Ni being the latter the most opti-
mized one [45,46]. Thus, we assume that our 5Ni/TiO2 catalyst is
the optimized one.
Fig. 4. H2-TPR profiles of calcined x%Ni/TiO2 samples (x = 5, 10, and 15 wt.% of Ni).
All catalysts studied exhibit 100% of MA conversion at reac-
tion time of 6 h. For high MA conversions (close to 100%), the
hydrogenation of MA is generally assumed to be a first order
reaction with respect to MA [48,49]. Indeed, the plots of the
ln(1-X) versus time are straight lines indicating a first order
reaction for the catalysts studied (see inlet in Fig. 5). Thus, the
pseudo-first order reaction was chosen to estimate reaction rate
constants (Table 5). The reaction rate constants follow the trend:
10%Ni/TiO2 » 5%Ni/TiO2 ≈ 15%Ni/TiO2. Although at high conver-
sions, the mass transfer of the aromatics could be important, the
diffusional effect could be excluded because of the large pore diam-
eter of all catalysts (9.7 nm–10.3 nm). In this sense, Toppinen et al.
[50] observed that the liquid phase hydrogenation of benzene
and alkylbenzenes over a commercial 16.6 wt.% Ni/Al2O3 catalyst
was mainly dominated by intraparticle mass transfer resistance of
hydrogen. The reactor dynamics itself was found to be much slower
than the diffusion process inside the catalyst particle [50].
With regard to the selectivity, all catalysts displayed 100%
selectivity to the SA formation, and other possible hydrogenation
products were not detected. Thus, only the carbon-carbon double
bond of maleic anhydride was hydrogenated upon moderate reac-
tion conditions employed (T = 373 K; PH2: 4.0 MPa). Interestingly, at
higher reaction temperature than that used in this work (483 K vs.
373 K), the most active 5%Ni/HY-Al2O3 catalyst exhibited a lower
yield of SA [45] than the 5%Ni/TiO2 sample studied in this work
(89.62 vs. 99%). Since all x%Ni/TiO2 catalysts exhibited almost 100%
selectivity toward succinic anhydride, it was impossible to con-
clude on the possible influence of SMSI on the selectivity toward
this product.
Contrary to work by Li et al. [45], the selectivity toward SA
did not decrease with an increase of Ni loading from 5 to 15 wt.%
indicating that overhydrogenation of the SA to GBL in the MA
hydrogenation over x%Ni/TiO2 catalysts did not occur. This could
be explained taking into account the absence of formation of TiOx
species after in situ reduction of the catalysts, as it was confirmed
by XPS. This is because the presence of oxygen vacancies would
interact directly with C O group of maleic anhydride activating
and favoring the production of GBL. In this sense, the enhance of
activity of Ni/TiO2 catalyst in hydrogenation of acetophenone was
explained as due to: (i) the activation of the carbonyl group by oxy-
gen bonding with Ti3+ cation or (ii) the creation of oxygen vacancy
Fig. 5. Influence of Ni loading on maleic anhydride transformations on reduced
x%Ni/TiO2 catalysts. Reaction conditions: substrate/metal = 100, PH2: 4.0 MPa; cata-
lyst mass: 0.100 g; stirring rate: 650 rpm; solvent: THF; T = 373 K; reaction time of
6 h. The plot ln(1-X) vs. time (h) is shown in inlet of this figure.
species exist on the catalyst surface, indicating the absence of
electronic enrichment in the metal particle due to TiOx species
[44].
3.2. Catalytic activity
3.2.1. Maleic anhydride hydrogenation
It is known that the production of succinic anhydride by catalytic
hydrogenation of MA can be accompanied by a series of successive
reactions (see Scheme 1), depending on the operating conditions
and the nature of the catalyst. Another operational disadvantage
is the high reactivity of the substrate (MA) against nucleophilic
species (protic by-products, water, among others). Due to these
features the catalytic reactions were carried out under anhydrous
conditions. Careful drying of the solvent was performed, and the
catalytic system was gassed until reach the reaction temperature,
and thereafter the system was pressurized with H2.
Influence of Ni loading on maleic anhydride transformations
over reduced x%Ni/TiO2 catalysts is shown in Fig. 5. All systems
exhibit high activity in the target reaction: the two 10%Ni/TiO2 and
15%Ni/TiO2 catalysts exhibit a 100% of MA conversion at 2 h reac-

C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
447
Fig. 6. (A) Influence of Ni loading on the cyclohexene conversion over x%Ni/TiO2 catalysts (reaction conditions as in experimental section). (B) X-ray diffraction of the spent
catalysts: (a) 5%Ni/TiO2, (b) 10%Ni/TiO2 and (b) 15%Ni/TiO2. Reference patterns (*) Ni (JCPDS 00-004-0850), (᭹) TiO2-Anatase (JCPDS 00-021-1272).
originated by SMSI effect [51]. Similarly, high selectivity toward
crotyl alcohol was linked with the presence of partially reduced
species, such as Pt TiOx and Ni TiOx, interacting with carbonyl
group [37].
3.2.2. Leaching test and recycling
A major difficulty in using Ni catalysts is associated with their
low operational stability in liquid phase reactions, mainly due to
effects of leaching and agglomeration of the active phase by Ost-
wald effect [51]. Taking into account that the stability of Ni phase on
the support surface during liquid phase operation strongly depends
of the interaction of Ni species on the TiO2 surface, the evalu-
ation of Ni content before and after the activity test (Table 3)
might give information on the influence of Ni loading on the
Ni-TiO2 interaction. Table 3 shows the ICP values for the cata-
lysts after being subjected to the hydrogenation of MA. According
to these results it can be assumed that an increase in the Ni
coverage on the support surface decreases the stability of the
active phase, increasing the metal leaching in the following order:
Fig. 7. Conversion level and selectivity in function of the number of reaction cycles
15%Ni/TiO2 > 10%Ni/TiO2 > 5%Ni/TiO2. Thus, the 5%Ni/TiO2 catalyst,
despite having lower activity in the hydrogenation of MA, is the
system that showed promising results in relation to its operational
stability in the hydrogenation of MA, under the reaction conditions
studied.
for the catalyst 5% Ni/TiO2. Reaction conditions: substrate/metal = 100; PH2: 40 bar;
catalyst mass: 0.100 g; stirring rate: 650 rpm; solvent: THF; T = 373 K; reaction time:
6 h.
diffraction lines associated to the support, along with the disap-
pearance of diffraction lines attributed to Ni0. For the 5%Ni/TiO2
system no significant changes between the fresh catalyst and the
used one were observed. With these results it can be assumed that
the deactivation of the catalyst is mainly due to leaching of Ni0 and
not to the agglomeration of oxide particles on the surface, since XRD
analysis did not show diffraction peaks associated to NixOy species.
Since the 5%Ni/TiO2 catalyst showed the best operational stabil-
ity, five consecutive reaction cycles were performed for this system,
and the results of the activity and selectivity for the different reac-
tion cycles are shown in Fig. 7. In all cycles a 100% of selectivity
towards SA was achieved, with a slight decrease of the activity
along of catalytic cycles. This lowering in the catalytic activity can
be attributed to slight leaching of Ni, as deduced from ICP-AES
characterization of the spent catalysts (see Table 3)
On the other hand, van Hassterecht et al. [52] have reported that
catalyst deactivation may be due to two effects: (i) surface active
phase oxidation and subsequent crowding; (ii) loss of Ni clusters
from the surface of the catalysts to the solution, which can occur
between successive cycles of the filtered steps. To confirm which
of the two effects play a role in our systems, we have performed a
study of the capacity of hydrogenation of the recovered filtrate after
the hydrogenation of MA, results that are shown in Fig. 6A. For this
study cyclohexene was chosen as substrate for the hydrogenation,
in order to obtain an indirect measure of the leaching. Cyclohexene
was used due to the high activity that this substrate has shown in
hydrogenation reaction carried out by catalytic systems based on
Ni nanoparticles [53].
For the three filtrates remains hydrogenating activ-
ity was observed, and the following order was obtained:
15%Ni/TiO2 > 10%Ni/TiO2 » 5%Ni/TiO2. These results confirm that
during the liquid phase reaction the systems with loading greater
than 5% Ni present an important loss of active phase. XRD post-
reaction characterization of the three catalysts can be observed in
Fig. 6B. Systems 10%Ni/TiO2 and 15%Ni/TiO2 show predominant
3.2.3. Temperature effect
The effect of the reaction temperature (323 K–398 K) on the cat-
alyst activity and selectivity in the MA hydrogenation toward SA
was investigated also. The 5%Ni/TiO2 catalyst was selected to this
study because this sample showed the best operational stability in
the target reaction at 373 K. In good agreement with literature [54],

448
C.C. Torres et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 441–448
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Ni/TiO2 catalysts prepared by wet impregnation with different
metal contents, was found to be highly active and selective (100%)
in the production of succinic anhydride by the hydrogenation of
maleic anhydride at low reaction temperature (323 K–398 K). The
textural characterization of the catalysts showed that the increase
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The authors thank FONDECYT postdoctoral grant 3140157. C.
Mella thanks CONICYT for doctoral fellowship 21150195. B. Paw-
elec expresses her gratitude to Comunidad de Madrid (Project CAM
S2013/MAE-2882) for their financial support.
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