Highly-efficient Ru/Al-SBA-15 catalysts with strong Lewis acid sites for the water-assisted hydrogenation of: P -phthalic acid

Article abstract of DOI:10.1039/d0cy00047g

Ruthenium nanoparticles supported onto aluminum-doped mesoporous silica catalysts (Ru/Al-SBA-15) are fabricated using hydrothermal and impregnation methods for catalysis application. The Ru/Al-SBA-15-3 catalyst at a Si/Al molar ratio of 3 exhibited excellent catalytic performance for the hydrogenation of p-phthalic acid with high conversion efficiency (100.0%) and cis-isomer selectivity (84.0%) in water. Moreover, this system displays exceptional stability and recyclability through preserving the conversion efficiency, as well as a cis-isomer selectivity of 90.2 and 83.3%, respectively, after reusing it fourteen times. Such an exceptional system can also be ideal for the hydrogenation of aromatic dicarboxylic acids and their ester derivatives in water. Strong Lewis acid sites due to doped Al species play significant roles in the hydrogenation reaction. Moreover, isotope labeling studies indicated that water molecules effectively participated in the hydrogenation reaction. Hydrogen and water contributed half of the hydrogen atoms for this hydrogenation reaction. In the end, a plausible mechanistic pathway for the hydrogenation of p-phthalic acid using the Ru/Al-SBA-15-3 catalyst in water is proposed.

Full text of DOI:10.1039/d0cy00047g

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DOI: 10.1039/D0CY00047G.
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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Highly-efficient Ru/Al-SBA-15 Catalysts with Strong Lewis Acid
Sites for the Water-assisted Hydrogenation of p-Phthalic Acid
Cong Maoa, Jingwei Zhenga, Babasaheb M. Matsagarb, Ranjith Kumar Kankalaa, Tansir Ahamadc,
Yucheng Yanga, Kevin C.-W. Wub and Xueqin Zhanga,*
The ruthenium nanoparticles supported onto the aluminum-doped mesoporous silica catalysts (Ru/Al-SBA-15) are
fabricated using the hydrothermal and impregnation methods for catalysis application. Ru/Al-SBA-15-3 catalyst at a Si/Al
molar ratio of 3 exhibited excellent catalytic performance for the hydrogenation of p-phthalic acid with high conversion
efficiency (100.0%) and cis-isomer selectivity (84.0%) in water. Moreover, this system displays exceptional stability and
recyclability through preserving the conversion efficiency as well as the cis-isomer selectivity of 90.2 and 83.3%,
respectively, after reusing it fourteen times. Such an exceptional system can also be ideal for the hydrogenation of
aromatic dicarboxylic acids and their ester derivatives in water. The strong Lewis acid sites due to doped Al species play
significant roles in the hydrogenation reaction. Moreover, the isotope labeling studies indicated that the water molecules
effectively participated in the hydrogenation reaction. Hydrogen and water contributed half of the hydrogen atoms for
this hydrogenation reaction. In the end, a plausible mechanistic pathway of the hydrogenation of p-phthalic acid using the
Ru/Al-SBA-15-3 catalyst in water is proposed.
the stability of the aromatic ring and its low solubility (1.5 mg/100
mL at 20 °C) in the aqueous solution.5, 6 In this framework, Sumner
and Gustafson7 reported that the yield of CHDA was 98.4% at
1. Introduction
195~230 °C with 0.5 wt.% Pd/C catalyst in 3 h (H2 pressure 4.5
MPa). Kayou and Nanba8 performed this reaction with 5 wt.%
Pd/C catalyst obtaining 99.2% conversion at 150 °C in 1 h. Yu and
colleagues9 reported that bimetallic catalyst Rh70Pt30/SBA-15
with 5.6 wt.% total metal loading exhibited 74.3% terephthalic
acid conversion after 2 h at 80 °C (H2 pressure, 5 MPa). Based on
these facts of considering the high energy consumption and cost
of existing catalytic systems, a highly efficient catalyst with low
noble metal loading would be desirable and appropriate to
execute the catalytic operations.
Ruthenium is a relatively inexpensive precious metal and
exhibits high efficiency in the hydrogenation of the aromatic rings.
Several efforts have been dedicated to fabricating the ruthenium
nanoparticles-doped composites for catalysis applications. In a
case, Wang and coworkers10 demonstrated that the ruthenium-
based catalyst displayed excellent catalytic performance for the
hydrogenation of benzoic acid. In another instance, Zhu and Lv11
used Ru/Al2O3 catalyst for the hydrogenation of p-phthalic acid, in
which the conversion efficiency reached 95.4% at 140 °C, 5.0 MPa
H2 after 1.5 h. Similarly, Yen and colleagues12 fabricated a highly
efficient and durable nano-ruthenium catalyst Ru/MCM-41 for the
Indeed, 1,4-cyclohexanedicarboxylic acid (CHDA) is
a crucial
chemical intermediate and raw material for the production of
drugs, polyester resin, and coating. Traditionally, CHDA can be
prepared by the hydrogenation of dimethyl terephthalate,
followed by hydrolysis.1 Recently, direct hydrogenation of p-
phthalic acid has attracted considerable attention for the
synthesis of CHDA. In this context, various organic solvents, such
as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and
isopropanol have been used, owing to the low solubility of p-
phthalic acid in water. Moreover, it should be noted that the
processing strategies utilizing such organic solvents face
limitations of environmental unfriendly and poor recyclability.2, 3
These limitations could be addressed by using water as the
reaction medium, which is predominantly regarded as the
universal solvent for achieving green chemistry goals due to its
non-toxic nature, enormous availability, and simple post-
processing, among others.4 However, most of the research studies
indicated that the hydrogenation of p-phthalic acid generally
requires harsh conditions (high temperature and pressure) due to
hydrogenation of bisphenol
A in the aqueous solution.
aCollege of Chemical Engineering, Huaqiao University, Xiamen, Fujian, 361021, China
bDepartment of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
cDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
* Corresponding author. E-mail: xqzhang2009@hqu.edu.cn
Zahmakiran and Ozkar13 reported that the ruthenium
nanoclusters deposited on the zeolite frameworks displayed
excellent catalytic performance for the hydrogenation of neat
benzene under mild conditions. Furthermore, Liu and colleagues14
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suggested that N-doped carbon spheres impregnated with highly aqueous solution was proposed.
monodispersed ruthenium nanoparticles catalyst substantially
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DOI: 10.1039/D0CY00047G
exhibited high catalytic activity for the hydrogenation of benzoic
acid. Plausibly, the predominant motive for the utilization of
2. Experimental Section
ruthenium-based nanoparticles in catalysis is their ability to
effectively absorb and dissociate the available hydrogen in the
reaction system, facilitating the hydrogenation of p-phthalic acid
in the aqueous environment.
2.1. Materials
All the reagents chemicals were analytical grade without further
purification. Hydrochloric acid (HCl), N,N-dimethylformamide
(DMF), tetraethyl orthosilicate (TEOS), ammonia-water, and
ethanol were obtained from Sinopharm Chemical Reagent Co. Ltd.
(Shanghai, China). The ruthenium (III) chloride hydrate (RuCl3),
aluminum isopropoxide, o-phthalic acid, m-phthalic acid, p-
phthalic acid, dimethyl phthalate, dimethyl isophthalate, and
dimethyl terephthalate were purchased from Aladdin industrial
corporation (Hangzhou, China). Poly (ethylene oxide)-
poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)
triblock polymer (P123, Ma = 5800), p-phthalic acid-d4, deuterium
oxide (D2O), deuterated chloroform, and dimethyl sulfoxide-d6 were
obtained from Sigma Co. Ltd. (St. Louis, USA).
In addition, the reliable support with strong acidic sites is
required for the hydrogenation reaction to interacting with the
aromatic ring for uneven charge distribution, leading to its
activation and substantial reduction in the activation energy.15
Due to their tunable morphological attributes such as even pore
size distribution as well as high specific surface area, and
advantageous characteristic functionalities, mesoporous silica
materials, notably, the Santa Barbara Amorphous-15 (SBA-15)
material shows excellent catalysis performance.16, 17 Progressively,
the impregnation of heteroatoms, such as Al-doped SBA-15, can
provide strong surface acid sites,18-21 which are highly beneficial
for the hydrogenation of p-phthalic acid. Furthermore, it is highly
convenient to tune the acidity strength of Al-SBA-15 by adjusting
the Si/Al molar ratio.22-25 Motivated by these facts of relatively
inexpensive and effective catalytic abilities of ruthenium metal
and highly tunable Al-SBA-15 frameworks, it is expedient to
fabricate a composite for the hydrogenation of p-phthalic acid
2.2. Preparation of Ru/Al-SBA-15 catalysts
Initially, Al-SBA-15 composites were prepared by a two-step
hydrothermal method.26 Typically, 2.0 g of P123 was completely
dissolved in 70 mL of HCl (2M) solution, and 4.0 mL of TEOS and
1.22 g of aluminum isopropoxide (Si/Al ratio of 3:1) were then
added to this mixture. After vigorous stirring for 24 h, the mixture
was then transferred into an autoclave and kept at 100 °C for
another 24 h. Then, the pH value of the cooled mixture was
adjusted in the neutral range using the ammonia-water solution.
Further, the mixture was subjected to hydrothermal treatment by
placing it in an autoclave (100 °C) for another 24 h. The resultant
product of mesoporous Al-SBA-15 supports was then collected by
subsequent centrifugation and calcination at 550 °C for 6 h. These
supports were denoted as AS-x, and x refers to the Si/Al molar
ratio in the raw materials. Further, the Ru/AS-x catalysts were
synthesized by the impregnation method. Briefly, 0.2 g of AS-x
support was added to the RuCl3 aqueous solution (2.0 mL, 0.02
mol/L) and stirred for 5 h. Further, the solvent was extracted by
applying the rotary evaporation method. The as-synthesized
catalysts were then dried and reduced under H2 atmosphere at
500 °C for 2 h to obtain Ru/AS-x catalysts.
under
mild
conditions
in
aqueous
solution.
Scheme 1. Schematic illustration showing the fabrication of
ordered mesoporous Al-SBA-15 and corresponding
nanoparticles-supported
catalyst
Ru/Al-SBA-15
using
subsequent hydrothermal and impregnation methods, and
their high-efficient catalytic performance toward the water-
assisted hydrogenation of aromatic dicarboxylic acids and
their ester derivatives.
Inspired by these facts, we initially fabricated highly active
composites based on ruthenium nanoparticles-supported ordered
2.3. Characterization
mesoporous Al-SBA-15 with highly strong acid sites using Powder X-ray diffraction (XRD) patterns at a low (2θ = 0.5 ~ 5o) as
hydrothermal and impregnation methods for catalysis application. well as wide angles (2θ = 10 ~ 80o) with 2θ scanning rate at 5o/min
These highly efficient Ru/Al-SBA-15 catalysts could achieve were recorded on
a Smart La X'Pert Pro powder X-ray
hydrogenation of benzoic dicarboxylic acids and their ester diffractometer (Eindhoven, Netherlands). The mesostructured
derivatives in water (Scheme 1). The high total acid amount, parameters were determined by N2 adsorption-desorption
abundant Lewis acid sites, and unique porous architectures of isotherms based on Brunauer-Emmett-Teller (BET) analysis using a
mesoporous silica resulted in the exceptional catalytic Quantachrome instruments analyzer (Florida, USA). The samples
performance of Ru/Al-SBA-15 catalyst under mild conditions. were degassed at 200 °C for 5 h before testing. The pore size
Interestingly, hydrogen (H2) and water each contributed half of the distribution and the mean pore diameter of the samples were
hydrogen atoms for the hydrogenation reaction of aromatic calculated using desorption isotherms by the BJH method. The
carboxylic acids. Finally, a plausible mechanistic pathway for the specific surface area and the pore volumes of the samples were
hydrogenation of p-phthalic acid on the catalyst Ru/AS-3 in the calculated from N2 adsorption-desorption isotherms by the BET
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method. Transmission electron microscope (TEM) images, as well amount of aluminum into SBA-15, the final specific sVuirefwaAcreticalereOanlionef
2
as its high-resolution prototype (HR-TEM) images, were captured the catalyst was increased from 578 to 72D8OmI: 1/0g.1(0T3a9b/Dle0C1,Y0E0n0t4ri7eGs
on a JEM 2100F instrument (Tokyo, Japan). The amounts of 1 and 2), due to which incorporated Al might introduce additional
ruthenium, silicon, and aluminum in the catalysts were measured mesoporous and micropores in the support.27 However, the
using inductively coupled plasma optical emission spectroscopy specific surface area, pore volume, and area of the hysteresis loop
(ICP-OES, PerkinElmer 8300, Waltham, USA). NH3-TPD of supports and catalysts decreased noticeably after further
experiments were measured on a Micromeritics AutoChem 2920II doping with aluminum in the mesoporous siliceous frameworks
instrument (Norcross, USA). Typically, the sample (120 mg) was (Table 1 and Table S1), indicating that the excessive Al cluster was
placed into a U-shaped quartz tube. Then, it was heated from distorted in the pore channels.22 To this end, the pore diameter
ambient temperature to 300 °C at 10 °C/min for drying distribution curves (Fig. 1B and 1D) have depicted that the pore
pretreatment. Before cooling to 50 °C, the He gas was introduced diameters of AS-x (x = 50, 10, 5, 3, 1 and 0.5) and Ru/AS-x (x = 50,
(30 mL/min) for 2 h. Then, NH3 (10%) and He were introduced to 10, 5 and 3) were at around 6.5 nm (Table 1 and Table S1),
attain saturation. Subsequently, the He gas (30 mL/min) was suggesting that ruthenium nanoparticles have not blocked the
switched and purged further for 1 h to remove NH3 from the channels in Ru/AS-x (x = 50, 10, 5 and 3). Moreover, the pore
sample completely. Finally, the temperature was increased to 700 diameter of Ru/AS-1 and Ru/AS-0.5 was smaller compared to that
°C at a rate of 10 °C/min under Helium (He) gas atmosphere to of the AS-1 and AS-0.5 and was centered at around 5.6 nm (Table
desorb NH3. 27Al magic angle spinning (MAS) Nuclear Magnetic 1 and Table S1). These consequences happened plausibly due to
Resonance (NMR) spectra were recorded on an Agilent 600 DD2 the partial aggregation of ruthenium nanoparticles inside the pore
spectrometer (Broadway Lubbock, Germany) operating at
a
channels of AS-1 and AS-0.5. However, the large pore size of the
Larmor frequency of 600 MHz for 1H. 27Al experiments were solid supports favors the ease of mass transfer during the
carried out at a Larmor frequency of 156.25 MHz. Pyridine-FTIR catalysis.
spectral measurements were recorded using a Thermo Nicolet
380. The samples were dried at 400 °C for 2 h under 0.001 Pa, and
pure pyridine vapor was instilled at 50 °C for 20 min. Finally, the
FTIR spectra were recorded after evacuating the system at 150
and 400 °C.
2.4. Catalytic performance
Typically, 0.03 g of catalyst, 0.3 g of p-phthalic acid and 25 mL of
water were added to an autoclave (100 mL) equipped with a
magnetic stirrer. It should be noted that strict anaerobic
atmospheric conditions were maintained by hydrogen purging for
6 times with an initial hydrogen pressure adjusted to 5 MPa.
Further, the reaction was conducted at a constant temperature
(80 °C) for 2 h at the stirring speed of 1000 rpm. After the
completion of the reaction, water in the mixture was removed by
rotary evaporation, and then the product was dissolved using
Fig. 1 Graphical representation showing the N2 adsorption-
DMF. Further, the catalyst was centrifuged and washed before the
desorption isotherms (A and C) and pore diameter distribution
next run. The product was analyzed by GC-FID (Echrom A90,
curves (B and D) of various designed supports and Ru-based
Shanghai, China) equipped with
a capillary column (HP-5,
catalysts, respectively.
30m×0.32mm×0.25μm, Agilent Technologies, Palo Alto, USA) to
understand the product yield and the conversion efficiency. In
addition to the catalytic performance efficiency, the cis-isomer
selectivity of the resultant products was determined by 1H NMR
(Methyl sulfoxide-d6, 500 MHz).
Furthermore, the Si/Al ratio and ruthenium content in these
catalysts were determined by ICP-OES analysis (Table 1). It was
evident from the experimental results that the tendency of the
Si/Al ratio of the catalysts was consistent with the theoretical
amounts of the raw material composition. This indicated that
most of the added aluminum was incorporated in the composites.
On the other hand, the ruthenium content was nearly identical
with the theoretical value (2.0 wt.%), signifying no loss of
ruthenium during the preparation process.
As shown in Fig. 2A and 2B, the XRD patterns of supports (SBA-
15, and AS-x) and catalysts (Ru/SBA-15 and Ru/AS-x) at a low
angle exhibited three distinct planes (100), (110), and (200),
corresponding to p6mm hexagonal symmetry of mesoporous
3. Results and discussion
3.1. Characterization of supports and catalysts
The N2 adsorption-desorption isotherms of supports and catalysts
were recorded to analyze their porosity and specific surface area
(Fig. 1). All the samples have exhibited type-IV adsorption
isotherms with an H1 hysteresis loop, indicating that these
supports and catalysts possessed a mesoporous structure with
narrow pore size distribution. With the incorporation of a small
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materials, indicating that the supports and catalysts possessed wide-angle XRD patterns illustrated that the alumina had no
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29
highly ordered mesoporous structures.24 It was also observed uniform crystal structure or too small and highly dispersed. As
DOI: 10.1039/D0CY00047G
from the XRD diffraction patterns that the peak intensities were illustrated in Fig. 2D, no distinct characteristic peak of ruthenium
decreased with the increase of aluminum content in the was detected in the XRD patterns at a Si/Al ratio of higher than 3,
mesoporous materials (Fig. 2A and 2B). At the reduced Si/Al ratio indicating that the ruthenium nanoparticles were small in size
to 0.5, the mesoporous supports were completely disordered, with high dispersion (Fig. 3). However, with further increase in the
which exhibited excessive aluminum damage to the mesoporous aluminum content, it resulted in an additional diffraction peak at
architectures, as shown in Fig. 1 and 3F.
around 44°, attributing to the (101) characteristic planes of
As depicted in Fig. 2C, the XRD patterns in the wide-angle of metallic ruthenium.30, 31 The low specific surface area of AS-1 and
supports resulted in the broad peak at a 2θ of around 22.5o, AS-0.5 substantially impeded the distribution of ruthenium ions,
indicating the characteristic patterns of the amorphous silica.28 leading to the aggregation of ruthenium nanoparticles on Ru/AS-1
Moreover, no prominent characteristic peaks of alumina in the and Ru/AS-0.5 to a certain degree (Table 1 and Table S1).
Table 1. Elemental analyses, as well as various textural parameters of Ru-based catalysts.
b
Mean DRu
Entry
Catalyst
Si/Al ratioa
SBET (m2/g)
Dp (nm)
V (cm3/g)
Ru (wt.%)a
(nm)
4.3
5.8
6.0
5.9
1
2
3
4
5
6
Ru/SBA-15
Ru/AS-50
Ru/AS-10
Ru/AS-3
Ru/AS-1
Ru/AS-0.5
-
90
7.8
1.9
0.8
0.5
578
728
611
558
368
252
6.6
6.5
6.6
6.5
5.6
5.6
1.2
1.0
0.9
0.9
0.6
0.4
2.0
2.0
1.9
2.0
1.9
1.9
7.7
14.5
a Si/Al molar ratio and ruthenium content in catalysts were determined by ICP-OES; b The mean diameters of ruthenium nanoparticles
were estimated by TEM; Dp and V represent pore diameter and pore volume, respectively.
Fig. 2 Graphical representation showing the low angle XRD
patterns of supports (A) and catalysts (B), and the wide-angle
XRD patterns of the supports (C) (SBA-15 and AS-x) and
catalysts (D) (Ru/SBA-15 and Ru/AS-x).
Fig. 3 TEM images of (A) Ru/SBA-15, (B) Ru/AS-50, (C) Ru/AS-
10, (D) Ru/AS-3, (E) Ru/AS-1, (F) Ru/AS-0.5 and (G) AS-3; (H) HR-
TEM image of Ru/AS-3; (I) Graphical representation showing
Moreover, Fig. 3 displays TEM images of various designed
Ru/AS-x catalysts and AS-3, as well as the HR-TEM image and the
particle size distribution of ruthenium nanoparticles of Ru/AS-3. It
was observed from the TEM observations that all the catalysts
whose Si/Al ratio higher than 1 retained their original ordered
mesoporous structure (Fig. 3A-E). And the mean size of ruthenium
nanoparticles (Ru/AS-x; x = 50, 10, and 3) was around 6.0 nm
(Table 1). With the decrease of the Si/Al ratio, the well-ordered
structure of the catalysts and the uniform distribution of the Ru
metal were reduced. The ruthenium nanoparticles tended to be
the particle size distribution of ruthenium nanoparticles in the
sample
Ru/AS-3..
aggregated (Fig. 3F). Moreover, the mean size of ruthenium
nanoparticles in Ru/AS-0.5 reached 14.5 nm, in which their
dispersion was related to the low pore volume and specific
surface area of the AS-0.5 (Table 1 and Table S1, Entry 6).
Typically, the relatively large pore diameter and high dispersion of
the ruthenium nanoparticles in the solid supports are highly
beneficial for the hydrogenation reaction. In addition, the crystal
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plane spacing, i.e., 0.21 nm of nanoparticles, could correspond to
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the characteristic plane (101) of ruthenium (JCPDS 06-0663, Fig. It is worth noting that the surface aciDdOitIy: 10p.r1o0p39e/rDti0eCsY0o0f04t7hGe
3H). These results were in the explicit agreement with the wide- catalysts could significantly affect their catalytic performance.
angle XRD data (Fig. 2D).
Therefore, the NH3-TPD profiles and Pyridine-FTIR spectra of
27Al MAS NMR spectroscopy was used to further explore the catalysts were measured to further explore the surface acidity
aluminum chemical environment in the bulk phase of Ru/AS-x (Fig. properties of the catalyst, such as amount, strength, and types of
4A). The broad and strong peaks at 53, 28, and 0 ppm could acid sites.
correspond to the
tetrahedral coordinated Al (Altetra),
Indeed, the desorption temperature of NH3 represents the
pentahedral coordinated Al (Alpenta), and octahedral coordinated strength of solid acid sites. From the NH3-TPD profiles of the
Al (Alocta), respectively.26, 32 Altetra originated from the Al metals in catalysts in Fig. 4B, the peak below 200 °C, could correspond to
32
SBA-15 framework.26, Alpenta is regarded to be distorted from the NH3 physical absorption and weak acid sites. The peak at the
the framework and exists as the amorphous silica-alumina desorption temperature range from 200 °C to 400 °C could be
34
35
phase.33,
Alocta is attributed to extra-framework amorphous ascribed to the medium acid sites stemming from Altetra
.
The
alumina phase.26, 32 With the decrease in the Si/Al ratio below 3, peak ranging from 400 °C to 600 °C could be contributed by the
the Alpenta content of the catalysts increased gradually (Fig. 4A), strong acid sites originating from the Alocta
35
.
As shown in Fig. 4B,
indicating to the enhancement of disordered arrangement (Fig. 2B all catalysts exhibited weak acid sites. Both Ru/SBA-15 and Ru/AS-
and 3B-F). However, combined with the low angle XRD (Fig. 2B) 50 catalysts have also shown no significant desorption peak at
and TEM (Fig. 3B-E), Ru/AS-x (x refers to 50, 10, 3, and 1) still held around 270 °C, implying that the number of medium acidic sites
well-ordered SBA-15 architecture. Besides SBA-15 phase, was relatively lesser. It should be noted that with the increase of
amorphous alumina phase and amorphous silica-alumina phase Al content, Ru/AS-10, Ru/AS-3, Ru/AS-1, Ru/AS-0.5 catalysts
also existed, but were homogeneously distributed (Fig. 2B, 3B-E, showed distinct peaks at about 270 °C and 450 °C, indicating that
and 4A). But, for Ru/AS-0.5, the ordered SBA-15 structure these catalysts possessed medium and strong acid sites. In
ultimately disappeared (Fig. 2B and 3F), implying that the addition, the desorption temperature of Ru/AS-3 shifted to
amorphous silica-alumina phase composed of Alpenta and Alocta slightly higher than other catalysts, implying that it could exhibit
became the main phases in Ru/AS-0.5, while Altetra species might relatively strong medium and strong acid sites. Moreover, as
exist as fragment or pre-phase of SBA-15. These consequences shown in Table 2, Ru/AS-3 possessed the most substantial amount
eventually resulted in the low specific surface area and pore of total acid sites and strong acid sites among these samples.
volume of AS-0.5 and Ru/AS-0.5 (Table 1 and Table S1). In general, These acid sites help to create more transition metal activity
the Altetra and Alocta tend to form strong Brønsted and Lewis acid sites.36, 37 Meanwhile, the strong acidic catalyst could enhance the
sites, respectively.35 In addition, the Alpenta mainly contributes to interaction with the aromatic ring and promote the uneven
the
weak
Lewis
acid
sites.34 distribution of electrons to activate the aromatic ring. Furthermore,
the strong acid sites are also beneficial to improve the ability of
the catalyst toward the hydrogenolysis/isomerization and
hydrogenation.
Table 2. Determination of acidity sites of the catalyst using the NH3-
TPD approach.
Acid sites (mmol/g)
Entry
Catalyst
Weak
0.38
0.86
0.83
1.00
0.47
0.25
Medium
0.14
Strong
-
Total
0.52
1.37
1.60
1.92
1.43
0.80
1
2
3
4
5
6
Ru/SBA-15
Ru/AS-50
Ru/AS-10
Ru/AS-3
Ru/AS-1
Ru/AS-0.5
0.51
-
0.56
0.60
0.83
0.37
0.21
0.32
0.13
0.18
“-” represents no distinct acid site was detected.
Fig. 4 Analysis of bulk phase and surface acidity properties of
Ru/SBA-15 and Ru/AS-x through 27Al MAS NMR, NH3-TPD, and
Pyridine-FTIR measurements. (A) 27Al MAS NMR spectra were
obtained for analyzing the chemical environment of aluminum
in Ru/AS-x bulk phase; (B) NH3-TPD profiles showing the
strength distribution of weak, medium and strong solid acid
sites on catalyst surface; Pyridine-FTIR spectra of various
designed catalysts at (C) 150 °C and (D) 400 °C. B, Bronsted-
bound pyridine; L, Lewis-bound pyridine.
Pyridine-FTIR spectra of catalysts were recorded in the
wavenumber range 1700-1300 cm-1 at 150 °C and 400 °C to
investigate the effect of doped aluminum on surface acid type of
catalyst (Fig. 4C and 4D, respectively). The peaks at 1443 cm-1 and
1575 cm-1 could be ascribed to Lewis acid sites, and the peak at
1542 cm-1 could be attributed to pyridine adsorbed onto the
Brønsted acid sites. Moreover, the peak at 1488 cm-1 could be
attributed to adsorbed pyridine onto both Brønsted and Lewis acid
sites, and the vibrations of hydrogen-bonded pyridine were in the
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38
wavenumber range of 1590-1600 cm-1. As shown in Fig. 4C and catalysts are listed in Table 3. With the decrease of tVhieewSAir/tAicllerOantliinoe,
4D, no significant peak at 1542 cm-1 and a small peak at 1488 cm-1 the conversion rate was increased initially DaOndI: 1t0h.e10n3r9e/dDu0CceYd00w0h47ilGe
indicate that there existed only a slight amount of Brønsted acid the selectivity was slightly changed. Moreover, the Ru/AS-3
sites on the surface of the catalyst. Furthermore, distinct peaks at catalyst achieved the best catalytic performance with 84.0% CHDA
1443 cm-1 and 1575 cm-1 implied that the surface of these catalysts selectivity and complete conversion of p-phthalic acid in the
mainly had shown Lewis acid properties. Under the same catalyst aqueous solution (80 °C, 2h). The extraordinary catalytic
composition and test conditions, the peak strength after degassing performance could be ascribed to the excellent structure
at 150 and 400 °C could represent the relative amount of total and characteristics, well-dispersed ruthenium nanoparticles, and
strong acid, respectively. Moreover, as shown in Fig. 4C, the catalyst strong Lewis acid sites. Initially, the appropriate textural
Ru/AS-3 exhibited the highest peak intensity, attributing to higher properties, including size as well as the volume of pores, and final
total acidity amounts than other catalysts. After degassing at 400 °C, BET specific surface area (Table 1) could favor the mass transfer
the peaks of Ru/SBA-15 were nearly disappeared (Fig. 4D), process during the catalysis. Moreover, under optimal Si/Al ratio
representing that this catalyst has no strong Lewis acid sites. In condition, Ru/AS-3 exhibited the strong Lewis acid sites (Fig. 4B-D)
addition, all the peaks were retained in Ru/AS-x samples, indicating (mainly originating from support), which could improve the
the presence of strong L acid sites compared to Ru/SBA-15. In ruthenium nanoparticles activity through influencing the
particular, Ru/AS-3 still maintained the highest relative peak ruthenium electronic structure.34, 37 On the other hand, Lewis acid
intensity, which could probably possess excellent catalytic sites could interact with -COOH functional group to adsorb and
performance.
carbon atoms of the aromatic ring of the substrate molecules to
make it highly electron-deficient and thus activated.15 Finally,
ruthenium nanoclusters activated by Lewis acid sites could
effectively adsorb and dissociate hydrogen to form ruthenium
hydride active species, which inserted into the carbon-carbon
3.2. Catalytic performance of Ru/AS-x for the hydrogenation of p-
phthalic acid and other substrates
The hydrogenation results of p-phthalic acid using Ru/AS-x double bonds in the benzene ring to generate cyclohexane.39
Table 3. Reaction results of hydrogenation of p-phthalic acid with catalysts.
Cis-isomer
Entry
Catalyst
Temperature (°C)
Time (h)
Metal Loading (wt.%)
Conv. (%) a
Selectivity (%)b
1
2
3
4
5
6
7
8
9
Ru/SBA-15
Ru/AS-50
Ru/AS-10
Ru/AS-3
Ru/AS-1
80
80
80
80
80
80
80
140
140
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.5
6.0
2.0
2.0
1.9
2.0
1.9
1.9
5.6
5.0
2.0
32.1
50.5
71.4
100.0
81.3
26.7
74.3
95.4
69.8
78.0
80.3
83.3
84.0
81.5
80.0
40.0
-
Ru/AS-0.5
Rh70Pt30/SBA-15 9
11
Ru/Al2O3
Rh0.5Ni1.5/Al5-SBA-15 40
-
a Conversions were calculated by GC. b Selectivity was calculated by NMR.
Compared with the reported Rh70Pt30/SBA-15,9 Ru/Al2O3,11 and including the amount of aluminum, catalytic stability, and acidity
40
Rh0.5Ni1.5/Al5-SBA-15 catalysts the Ru/AS-3 used in the present property, were noticeably different owing to the hydrothermal-
study exhibited excellent catalytic performance under mild assisted synthesis. Abundant aluminum species were introduced
reaction conditions (80 °C) with a lower noble metal loading of 2.0 into the composites Ru/AS-x through strong interaction, and
wt.%. The supports such as SBA-15 and Al2O3 in the Rh70Pt30/SBA- offered the increased number of total and strong Lewis acid sites
9
11
15 and Ru/Al2O3 catalyst showed none and weak Lewis acid (Fig. 4C and 4D). In addition, the Ru/AS-3 catalyst system
sites, respectively, resulting in their relatively weak catalytic possessed excellent pore structure, evenly dispersed ruthenium
activity. It should be noted that the Ru/AS-x sample is different nanoparticles, high total acidity amounts, (Table 2, entry 4) and
40
from Rh0.5Ni1.5/Al5-SBA-15
in terms of the synthesis and strong Lewis acid sites (Fig. 4D), which significantly improved its
performance of the catalyst. The aluminum-doped silica support hydrogenation performance of p-phthalic acid in water. Notably,
(Al5-SBA-15),40 was synthesized by using the impregnation method the ruthenium is more beneficial in terms of cost than rhodium and
with aluminum solution, which resulted in the incorporation of platinum, and could potentially be well-suited to explore them for
only a few aluminum species in the composite. Owing to the facts industrial applicability.
of the low amount of aluminum atoms and weak interactions
In addition, it should be noted that Ru/AS-3 catalyst could be
between aluminum species and SBA-15 framework, both Al5-SBA- reused at least eleven times with complete p-phthalic acid
15 and Rh0.5Ni1.5/Al5-SBA-15 exhibited little and weak acid sites as conversion and no significant loss of CHDA selectivity (Fig. 5).
evident from their Pyridine-FTIR spectra.40 Nevertheless, in this Moreover, after fourteen times of its utilization, it could still be
work, we anticipated that various characteristic attributes, able to maintain a high p-phthalic acid conversion (90.2%) and cis-
6 | J. Name., 2012, 00, 1-3
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isomer selectivity (83.3%). As depicted in Fig. S1, it was evident be completely hydrogenated within 4 h at 80 °C or 2 h at 100 °C.
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that the fabricated Ru nanoparticles were not aggregated in the Indeed, the degree of hydrogenation deDpOenI:d10e.d103m9o/Dst0lCyY0o0n04t7hGe
Ru/AS-3-U catalyst, after reusing for fourteen times. Moreover, the solubility of these benzoic dicarboxylic acids in water. The
mean size of Ru nanoparticles decreased to 2.8 nm, might be due to solubility parameters of o-phthalic acid, p-phthalic acid, and m-
leaching of Ru during the high speed agitation, which could also be phthalic acid included 0.6 g, 1.5 mg, and nearly insoluble per 100
a reason for the reduction of catalytic performance. Besides, the mL at 20 °C in water, respectively.40 In addition, the Ru/AS-3
content of Ru in the Ru/AS-3 catalyst declined from 2.0 to 1.64 wt.% catalyst has exhibited the highest catalytic activity toward the
during the above reusing process, confirmed through the ICP hydrogenation of dimethyl phthalate, dimethyl isophthalate, and
analysis. Together, these experimental results indicated that the dimethyl terephthalate under further mild conditions. It is worth
excellent reusability is due to its exceptional stability of the mentioning that the dimethyl phthalate is completely
catalyst in water. Moreover, the exceptional stability in water is hydrogenated with the cis-isomer selectivity up to 100%.
advantageous as the provided reaction conditions in terms of
temperature and type of solvent are similar to the hydrothermal-
assisted synthetic conditions.
Ru/AS-3 catalyst was also applied in the hydrogenation of
benzoicdicarboxylic acids and their ester derivatives in the
aqueous solution. As shown in Table 4, all the substrates could be
completely hydrogenated under mild conditions. Indeed, the o-
phthalic acid was relatively easy to undergo hydrogenation and
obtain higher selectivity in comparison to the m-phthalic acid and
p-phthalic acid. However, the compound, m-phthalic acid, could
Fig. 5 The catalytic recyclability of Ru/AS-3 for the hydrogenation
of p-phthalic acid in water (conversion (A) and cis-isomer
selectivity
(B)).
Table 4. Catalytic performance of Ru/AS-3 for the hydrogenation of other substrates in aqueous solution.
Entry
Substrate
o-phthalic acid
m-phthalic acid
m-phthalic acid
m-phthalic acid
Temperature (°C)
Time (h)
Conversion (%)
100.0
Cis-isomer Selectivity (%)
1
2
3
4
5
6
7
8
60
80
80
100
80
50
60
60
1
2
4
2
2
1
1
1
92.6
80.2
76.6
80.0
84.0
100.0
75.0
83.3
64.5
100.0
100.0
100.0
100.0
100.0
100.0
p-phthalic acid
Dimethyl phthalate
Dimethyl isophthalate
Dimethyl terephthalate
To further investigate the specific source of hydrogen atoms in
the CHDA, the hydrogenation of p-phthalic acid-d4 was performed
using H2O and D2O as the solvents, respectively, and the products
3.3. Reaction mechanism
To explore the mechanism of p-phthalic acid hydrogenation in were esterified to dimethyl 1,4-cyclohexanedicarboxylate.41 Fig. S7
1
1
aqueous solution, we monitored the reaction process by H NMR shows H NMR spectra of standard substances trans-dimethyl 1,4-
spectra. In addition, the isotope labeling experiments were also cyclohexanedicarboxylate (δ = 3.60 (s, 6H), 2.22~2.24 (m, 2H),
carried out to reveal the source of hydrogen in the product. In 1.97~1.99 (m, 4H) and 1.33~1.40 ppm (m, 4H)) and cis-dimethyl 1,4-
comparison, 1H NMR spectra (Fig. S2) of standard substances trans- cyclohexanedicarboxylate (δ = 3.61 (s, 6H), 2.38~2.42 (m, 2H),
CHDA (δ = 12.08 (s, 2H), 2.13~2.17 (m, 2H), 1.87~1.93 (m, 4H) and 1.79~1.86 (m, 4H) and 1.57~1.63 ppm (m, 4H)). As shown in Fig. S8
1.27~1.36 ppm (m, 4H)) and cis-CHDA (δ = 12.08 (s, 2H), 2.35~2.40 and S9, the hydrogenation of p-phthalic acid-d4 in H2O and D2O
(m, 4H), 1.66~1.74 (m, 4H) and 1.54~1.61 ppm (m, 4H)) were tested. yielded the products with peak area ratio approximately 6:2:0:4 and
As anticipated, hydrogenation of p-phthalic acid in H2O with catalyst 6:1:0:2 at the above four chemical shifts, respectively. The
Ru/AS-3 yielded the product with peak area ratio 2:2:4:4 at the disappeared peaks at 1.79~1.86 ppm could be attributed to the
above four chemical shifts (Fig. S3). As shown in Fig. S4 and S5, four D atoms originating from deuterated p-phthalic acid, indicating
after processing for 10 or 30 min, the hydrogenation was not that they were chemical equivalent in the product, CHDA. From the
completed, and no organic intermediate could be detected while results of Fig. S8 and S9, it was evident that three H atoms from H2
only the end product CHDA was formed, indicating that the whole and other three from H2O were derived for the hydrogenation of p-
aromatic ring endured instantaneous hydrogenation, with first phthalic acid. The H atoms from both H2 and H2O were furnished
double bond hydrogenation as the control step in this reaction. at α and β positions of -COOH equally for the hydrogenation
Meanwhile, as shown in Fig. S6, the reaction of p-phthalic acid in reaction.
D2O obtained CHDA with partial deuteration, indicating that solvent
H2O high effectively participates in this hydrogenation system.
Based on the above isotope labeling studies and the catalyst
characterization results, one probable mechanistic pathway for
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hydrogenation of p-phthalic acid on catalyst Ru/AS-3 in aqueous dicarboxylic acid intermediate. Following that, the introduction of
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solution is proposed in Scheme 2. Initially, the interactions of hydride at one of the α positions of -CODOOHI: a10n.d10p39ro/Dto0CnYa0ti0o0n47oGf
Lewis acid sites with the carboxyl groups of substrates and another double bond lead to the 1,4-addition intermediate
aromatic ring result in the attraction of substrates (p-phthalic acid) cyclohex-2-ene-1,4-dicarboxylic acid, which could be further
onto Ru/AS-3 surface. Upon addition of H2, the heterolytically hydrogenated quickly to the final products. Notably, cis 1,4-
dissociated hydrogen molecule could generate Ru-H and proton addition resulted in the cis diastereomer of the final product,
with the cooperation of Ru and p-phthalic acid. Moreover, the while the trans-1,4-addition generated trans diastereomer. In
proton generated from H2 might be associated initially with the addition, the peak integral resulted in trans-dimethyl 1,4-
carboxylic acid group, and then form H3O+ with water. The cyclohexanedicarboxylate (Fig. S8 and S9), which was not following
hydrogenation reaction might take place via insertion of the above rule for cis-isomer, indicating that the p-phthalic acid
ruthenium hydride on carbon-carbon double bond of the aromatic underwent another reaction pathway to form trans-CHDA.
ring followed by protonation to generate cyclohexa-1,3-diene-1,4-
Scheme 2. The plausible mechanism of hydrogenation of p-phthalic acid in the aqueous solution. The bottom planes
stand for the catalyst surfaces, while the purple multisphere with Ru indicates ruthenium nanoparticle and yellow
hemisphere with the letter L refers to Lewis acid site. Green arrows indicate the directions of electron transfer.
Combined with the proton (red) from H3O+, Ru-H (green), which originated from heterolytic dissociation of H2 on
ruthenium nanoparticles conducts the hydrogenation. That is the reason for the equal contribution of Ru-H and proton
derived from H2 and H2O, respectively.
its utilization for the hydrogenation of the aromatic ring in the
aqueous solution.
4. Conclusions
In summary, a series of Ru/AS-x catalysts were synthesized by
Conflicts of interest
There are no conflicts to declare.
hydrothermal and impregnation methods. Among them, the Ru/AS-
3
catalyst showed excellent catalytic performance for the
hydrogenation of p-phthalic acid in aqueous solution under mild
conditions. The conversion efficiency and cis-isomer selectivity were
up to 100.0 and 84.0%, respectively. Furthermore, this catalyst
exhibited extraordinary stability, which could be reused at least
fourteen times with a slight loss of the conversion and constant
selectivity. In particular, the catalytic efficiencies of these
composites happened to be favorable at a lower metal loading
amount in this catalyst and with milder reaction conditions
compared to current reported studies. Moreover, this catalyst also
exhibited excellent catalytic performance in the hydrogenation of
benzoic dicarboxylic acids and their ester derivatives in water.
Unique structural properties, textural attributes and highly
dispersed ruthenium nanoparticles significantly enabled excellent
catalytic performance of the composites. The tunable acidity of the
catalyst and the strong Lewis acid sites by altering the doping
aluminum played critical roles in the catalytic process. In addition,
hydrogen and water each contributed half of the hydrogen atoms
for this hydrogenation reaction. Finally, a plausible mechanistic
pathway for hydrogenation of p-phthalic acid on the catalyst Ru/AS-
3 in aqueous solution was proposed. Together, this study offers a
new perspective for an eco-friendly fabrication of catalyst as well as
Acknowledgments
This work was supported by the National Natural Science
Foundation of China [grant numbers 51603077], the Natural
Science Foundation of Fujian Province [grant numbers
2019J01077], Promotion Program for Young and Middle-Aged
Teacher in Science and Technology Research of Huaqiao
University [grant numbers ZQN-PY516, ZQN-714] and Subsidized
Project for Cultivating Postgraduates’ Innovative Ability in
Scientific Research of Huaqiao University. We would like to
sincerely thank Saad M. Alshehri (King Saud University) for his
valuable discussions.
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Table of contents
Ru/Al-SBA-15 catalyst with excess Lewis acid sites displayed excellent efficiency
(100%), high cis-isomer selectivity (84%), and exceptional stability towards
hydrogenation of p-phthalic acid in water.