NH2-MIL-125(Ti)-derived porous cages of titanium oxides to support Pt-Co alloys for chemoselective hydrogenation reactions

Article abstract of DOI:10.1039/c8sc05450a

The change of atom density induced structural collapse in the transformation process from metal-organic frameworks (MOFs) to their inorganic counterparts is a major challenge to the achievement of porous hollow structures. Herein, we develop an amino acid-mediated strategy for transformation of NH₂-MIL-125(Ti) to successfully synthesize well-defined porous cages of titanium oxides (PCT) due to sheets serving as structural scaffolds. On this basis, PCT supported Pt-based nanoparticles are generated via a similar synthetic route, and are utilized to study the selective hydrogenation of carbonyl groups in α,β-unsaturated aldehydes, benefiting from the specific structures of PCT and tunable electronic structures of Pt mainly affected by doping with metal species such as Co. In this case, Pt-Co/PCT composites give 96% selectivity for cinnamyl alcohol at 100% conversion of cinnamaldehyde under 0.2 MPa H₂ and 80 °C for 3 h. This research would offer a promising strategy for important organic transformations in academic and industrial research to selectively synthesize high-value-added products.

Full text of DOI:10.1039/c8sc05450a

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NH -MIL-125(Ti)-derived porous cages of titanium
2
oxides to support Pt–Co alloys for chemoselective
hydrogenation reactions†
Cite this: DOI: 10.1039/c8sc05450a
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Zhizhi Gu, Liyong Chen, * Xuezhao Li, Lin Chen, Yingyue Zhang
and Chunying Duan*
The change of atom density induced structural collapse in the transformation process from metal–organic
frameworks (MOFs) to their inorganic counterparts is a major challenge to the achievement of porous
hollow structures. Herein, we develop an amino acid-mediated strategy for transformation of NH
2
-MIL-
125(Ti) to successfully synthesize well-defined porous cages of titanium oxides (PCT) due to sheets
serving as structural scaffolds. On this basis, PCT supported Pt-based nanoparticles are generated via
a similar synthetic route, and are utilized to study the selective hydrogenation of carbonyl groups in
a,b-unsaturated aldehydes, benefiting from the specific structures of PCT and tunable electronic
structures of Pt mainly affected by doping with metal species such as Co. In this case, Pt–Co/PCT
Received 6th December 2018
Accepted 12th December 2018
composites give 96% selectivity for cinnamyl alcohol at 100% conversion of cinnamaldehyde under
DOI: 10.1039/c8sc05450a
ꢀ
0
.2 MPa H
2
and 80 C for 3 h. This research would offer a promising strategy for important organic
rsc.li/chemical-science
transformations in academic and industrial research to selectively synthesize high-value-added products.
have been reported on MOF derived porous hollow structures of
Introduction
15,16
inorganic compounds assembled with sheets.
Owing to the
Porous hollow structures, especially assembled with nanosheets, more favourable hydrolysis and condensation of titanium(IV) ions
17
possess large surface area, a distinctive micro-environment compared to other metal ions, porous hollow titanium
between their interiors and exteriors, and specic channels for compounds are produced with difficulty by transformation of Ti-
1–3
guest species shuttling. Thus, porous hollow structures have MOFs in solution. Actually, calcination is a conventional method
18–23
been widely used in the elds of drug delivery and gas adsorp- to convert Ti-MOFs into solid titanium compounds.
tion, as well as catalysis, as supports for highly dispersive catalyst the fact that layered titanates easily form sheet-like structures,
nanoparticles (NPs) that can afford superior catalytic activity for herein, we developed a novel amino acid-assisted solvothermal
Based on
24
4
–8
organic transformations.
been considered as one of the most intriguing methods for 125(Ti) with fcu topology to titanate/TiO
Template-mediated synthesis has strategy to accomplish transformation from polyhedral NH
(dened as titanium
As of oxide) nanosheets; these nanosheets were assembled into porous
2
-MIL-
2
9,10
generation of well-dened porous hollow structures.
now, metal–organic frameworks (MOFs) with tunable metal ions cages (denoted as PCT) based on polyhedral templates.
25
and organic ligands have become fully edged self-sacricial Furthermore, by introducing metal sources, PCT supported metal
templates for fabrication of metal/carbon-based materials with NPs could be prepared (Fig. 1).
11
desirable properties. Nonetheless, achievement of porous
Many efforts have focused on engineering metal-based
hollow structures by transformation of MOFs still faces a big catalysts to address the issue of intramolecular hydrogenation
challenge in that the ultralow atom density of MOFs easily leads
to the collapse of hollow structures in the transformation
12–14
process.
To overcome this hurdle, it is desirable to produce
structural scaffolds to achieve the hollow architectures. Sheet-like
NPs possessing large lateral surface area are generally employed
as structural scaffolds, and thereby some successful examples
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
1
16024, China. E-mail: lychen@dlut.edu.cn; cyduan@dlut.edu.cn
†
Electronic supplementary information (ESI) available: Experimental procedures,
XRD, TEM, SEM, AFM, XPS, Raman spectra, FT-IR spectra, GC-MS analysis and the
nitrogen adsorption isothermal curve. See DOI: 10.1039/c8sc05450a
Fig. 1 Schematic of porous cages of titanium oxide anchored metal
NPs by an amino acid-assisted transformation of NH -MIL-125(Ti).
2
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26–28
competition between C]C and C]O bonds.
Pt NPs as
a universal catalyst have been applied for hydrogenation of
a broad range of bonds, although giving rise to little chemo-
29–31
selectivity.
Based on component-regulated electronic struc-
tures and defects, the capability of Pt NPs for selective
adsorption and activation of different groups can be adjusted by
32–36
doping with Fe, Sn, Co and so on.
Meanwhile, to improve
stability and activity against complex catalytic systems, Pt NPs
37
are generally immobilized on the supports. Combination of
metal dopants and porous hollow supports for construction of
anchored Pt–metal (Pt–M) alloys may be an intriguing strategy
to upgrade the selectivity and reactivity of Pt NPs towards
hydrogenation. Some pioneer studies have been devoted to
utilization of titanium compounds as supports to improve the
Fig. 2 SEM (top) and TEM (bottom) images of samples prepared by
transformation of NH -MIL-125(Ti) with the help of amino acid
2
molecules. (a and e) L-alanine, (b and f) glycine, (c and g) L-lysine and (d
and h) DL-serine. All scale bars are 200 nm.
38–44
catalytic hydrogenation performance of metal catalysts.
Following these concepts, we engineer Pt–M alloy NPs on PCT
for highly chemoselective hydrogenation of a,b-unsaturated structures were not found but a large amount of discrete TiO
aldehydes to unsaturated alcohols that are high-value-added NPs (Fig. S4(a) and (b)†). Thus, the layered titanate sheets play
2
45

ne chemicals. Finally, Pt–Co NPs anchored by PCT (Pt–Co/ the crucial role of structural scaffolds to prevent damage to the
PCT) exhibit exceptionally high reactivity (100%) and selectivity porous cages.
To further understand the components and structures of
(
ꢁ96%) for hydrogenation of carbonyl groups in cinnamalde-
hyde under mild conditions even without obvious variation PCT, Raman spectrum, XPS spectrum and nitrogen adsorption
aer six successive cycles. The Pt–Co/PCT also gives high isotherm measurements were carried out. As shown in Fig. S5†
selectivity for carbonyl groups of other a,b-unsaturated alde- (black curve), the PCT gave rise to a Raman active peak at
À1
hydes at high conversion levels. The enhanced selective around 149.9 cm , assigned to the E
g
active mode of anatase
46
hydrogenation performance of the Pt–Co/PCT mainly benets TiO , under irradiation with a 633 nm laser. Other Raman
2
from dopant mediated microstructures of Pt and porous hollow active phonons were not clearly detected within 100 to 800
À1
support regulated mass transport.
cm . The high-resolution Ti 2p XPS spectrum in Fig. S6(a)†
showed two predominant peaks at approximately 458.60 and
4
2
64.30 eV. Both can be identied as the binding energy of Ti(IV)
p3/2 and 2p1/2 from TiO2 or layered titanate. Nitrogen
Results and discussion
Preparation and characterization of porous cages of titanium
46
adsorption measurement at 77 K showed the presence of
a signicant hysteresis loop at P/P larger than 0.4 between the
oxides (PCT) and PCT supported metal nanoparticles (NPs)
0
Nanosized polyhedral NH
2
-MIL-125(Ti) ꢁ500 nm in size was adsorption and desorption curves (Fig. S7(a)†). The type IV
19
prepared via a modied synthetic route, conrmed by trans- isotherm with H2-type hysteresis reveals the mesoporosity of
mission electron microscopy (TEM) and scanning electron the cages, together with pore size distribution ranging from 3 to
microscopy (SEM) images (Fig. S1(a) and S1(b)†) and an X-ray 7 nm (Fig. S7(c)†). The high Brunauer–Emmett–Teller (BET)
2
À1
diffraction (XRD) pattern (Fig. S1(c)†). NH -MIL-125(Ti) as the specic surface area estimated to be ca. 375 cm g is attrib-
2
precursor template was converted into porous hollow structures uted to titanium oxide sheet assembled hollow structures.
ꢀ
via an alcoholic thermal process at 176 C under auxiliary of
2
The transformation from polyhedral NH -MIL-125(Ti) to
amino acid molecules, including L-alanine, glycine, L-lysine and porous cages of titanium oxides possibly involves the nanoscale
DL-serine. As shown in Fig. 2, the morphologies of all samples Kirkendall effect associated with Ti(IV) ion-related dissolution
10
were the sheet-assembled porous cage-like structures with and recrystallization (Fig. S8†). Ti(IV) ions were rstly dissolved
a diameter of 500 to 800 nm. To understand the components from NH -MIL-125(Ti) nanocrystals by coordination of Ti(IV)
2
47
and ne structures, the porous cages prepared by the L-alanine with amino acids (L-alanine). The solvothermal alcoholysis of
assisted route as a representative sample were further charac- the dissolved Ti(IV) took place, leading to formation of sheet-like
48
terized. Layered titanate H Ti O (JCPDS no. 36-0656) was titanium oxide NPs on the NH -MIL-125(Ti) nanocrystals.
2
8
17
2
conrmed by the XRD pattern (Fig. S2†), wherein the diffraction With the continuous transformation, successive shells of tita-
peaks of anatase TiO (JCPDS no. 21-1272) also appeared, nium oxides were generated around NH -MIL-125(Ti) nano-
2
2
revealing the dual-component porous cages. The thickness of crystals. Owing to the porous structures of shells, reaction
the sheets ranged from 10 to 30 nm, characterized by a TEM species can go into the interiors to continue dissolving Ti(IV)
image of the curled edge of a sheet (Fig. S3(a)†) and an AFM ions from NH -MIL-125(Ti) cores. Finally, the cores were
2
image of separated sheets (Fig. S3(b)†). The transformation of completely consumed, and transferred into the completed
Ti(IV) ions was very susceptible to temperature and water. Only porous cages. In the transformation process, amino acid
anatase TiO diffraction peaks in the XRD patterns (Fig. S4(c)†) molecules in ethanol can effectively control the release of Ti(IV)
2
ꢀ
appeared at 200 C or upon introducing a small amount of water ions and subsequent crystallization, resulting in the formation
(50 mL) into the reaction system (5 mL). Porous cage-like of layered titanates. In contrast, without introducing L-alanine,
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large nanosheets were produced by alcoholysis of Ti(IV) ions, composed of titanium oxides. The slight downshi of the Ti
and NH -MIL-125(Ti) nanocrystals were engulfed by these 2p3/2 from 458.60 to 458.50 eV reveals the interaction between
2
51
nanosheets, further suppressing the dissolution of NH -MIL- Pt–Co NPs and PCT. In comparison to the {111} facets of the
2
1
25(Ti) nanocrystals (Fig. S9†). Moreover, when a small amount face-centered cubic (fcc) structured Pt Co (JCPDS no. 29-0499)
3
ꢀ
of water was added into the synthetic system, hydrolysis of Ti(IV) located at 40.53 , the broad diffraction peak in the XRD pattern
possibly was a prominent process, resulting in the formation of was slightly shied to 40.70 , suggesting an atomic ratio of
ꢀ
2
anatase TiO NPs (Fig. S4†). Thereby, the presence of sheet-like Pt/Co of less than 3 in the as-formed Pt–Co alloy. Monodisperse
layered titanates acting as morphological scaffolds is a decisive Pt–Co NPs less than 10 nm in size and their aggregates were
factor to keep the polyhedral cages unchanged. Moreover, the observed in a magnied TEM image (Fig. S12†). The high-
type of Ti-MOF is another key factor in the formation of resolution TEM (HRTEM) image (Fig. 3(b)) of a Pt–Co NP shows
completed cages. For example, if MIL-125(Ti) was used as the two sets of lattice fringes corresponding to {111} and {200}
5
2,53
precursor, completed porous cages of titanium oxides were facets.
generated with difficulty (Fig. S10†). image of Pt–Co NPs (Fig. S13†). The high-angle annular dark-
Amino acid molecules generally as a class of mild reducing eld scanning TEM (HAADF-STEM) image showed that high-
Defects of grain boundaries appeared in the HRTEM
49,50
agents are used to prepare metal NPs.
synthetic system of L-alanine to prepare various hybrids of Pt (Fig. 3(d)). The distribution of Pt, Co, Ti and O elements char-
and Pt–M NP loaded PCT while introducing H PtCl and other acterized by energy-dispersive X-ray spectroscopy (EDXS)
Herein, we chose the contrast Pt–Co NPs were dispersed in the porous shells
2
6
metal salts. Pt–Co/PCT composites were successfully prepared elemental mapping of a porous cage (Fig. 3, bottom) further
in an ethanolic solution (5 mL) including NH -MIL-125(Ti) conrms the formation of Pt–Co/PCT. The content of Pt was
2
(
(
10 mg), Co(NO ) (10 mmol), H PtCl (3.5 mmol), and L-alanine about 3.16 wt% estimated by inductively coupled plasma atomic
3 2 2 6
0.533 mmol) at 176 C for 24 h. The morphology of the as-made emission spectrometry (ICP-AES); the Pt/Co atomic ratio was
ꢀ
porous cages loaded by metal NPs did not exhibit any obvious around 2.72 : 1, consistent with the XRD result. The well-
difference compared to that of PCT, as shown in Fig. 3(a) and dened mesoporous structures of the Pt–Co/PCT were esti-
(
c). The diffraction peaks corresponding to layered titanate and mated using the N
2
adsorption isotherm that exhibited a type IV
> 0.4 (Fig. S7(b)†), and
anatase TiO were found in the XRD pattern (Fig. S11†). The isotherm with H3-type hysteresis at P/P
2
0
Raman spectrum (Fig. S5,† red curve) and the XPS spectrum of a broad pore size distribution between 3.5 and 11 nm was
Ti 2p (Fig. S6(b)†) further revealed that the porous cages were observed (Fig. S7(d)†). As expected, the BET specic surface area
2
À1
of the sample decreased from 375 to 309 cm g aer loading
of Pt–Co NPs.
2 6 3 2
By adjusting the amount of H PtCl and Co(NO ) , the as-
synthesized composites with different components and struc-
tures were found (Fig. S14†). From these experimental obser-
2
vations, NH -MIL-125(Ti) is unfavourably transferred to
titanium oxides with a decrease of the molar ratio of H PtCl to
2
6
Co(NO ) . The atomic ratio of Pt to Co in their alloys may be
3
2
controlled by adjusting the amount of metal sources. For
example, Pt Co NP loaded PCT (Pt Co/PCT; Pt: 3.62 wt%) was
successfully synthesized by introducing Co(NO (3.5 mmol)
and H PtCl (3.5 mmol) to the synthetic system. In addition,
3
3
3 2
)
2
6
other alloys, such as PtFe NP loaded porous cages of titanium
oxides (PtFe/PCT; Pt: 3.43 wt%) were also synthesized by using
Fe(NO ) to replace Co(NO ) via the same solvothermal treat-
3
3
3 2
ment process (Fig. S15†).
As shown in Fig. S16,† without introducing other metal
precursors of Fe and Co to the synthetic system, large Pt NPs
(
ꢁ15 nm) on porous cages of titanium oxides (Pt/PCT_1; Pt: 3.31
ꢀ
wt%) were produced at 176 C for 12 h, veried by the relatively
strong diffraction peaks of fcc structured Pt (JCPDS no. 04-
0802). Thus, doping with Co or Fe facilitates generation of the
reduced size Pt NPs. The reason could be attributed to forma-
tion of stable nuclei of the Pt alloy with a smaller diameter than
those formed from pure Pt, and strain energy caused by the
54
difference in size between metal atoms of Pt and Co (or Fe).
The porous cages of titanium oxides assembled by sheets were
not obviously changed aer loading Pt NPs. The HRTEM image
Fig. 3 (a) TEM, (c) SEM and (d) HAADF-STEM images of Pt–Co/PCT; (b)
HRTEM image of a Pt–Co NP; O, Ti, Co, and Pt EDXS elemental
mapping of Pt–Co/PCT (bottom).
(Fig. S16(c)†) showed {111} facets of cubic Pt. The inset is a fast
Fourier transform (FFT) pattern of two sets of diffraction spots
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corresponding to {111} and {200} facets of Pt with an interfacial as 97.2% selectivity for COL was observed. With increasing
ꢀ
angle of ꢁ54 .
reaction time, conversion of CAL was gradually increased, and
In contrast, small Pt NP (<10 nm) loaded porous cages of selectivity for COL was slightly decreased. Upon prolonging the
titanium oxides (Pt/PCT_2, Pt: 4.01 wt%) could be prepared at reaction time to 24 h, the selectivity for COL is partly decreased
lower reaction temperature, such as 165 C (Fig. S17†). However, to 89.5%. The dramatic variation of selectivity for COL did not
ꢀ
ꢀ
at the low temperature of 165 C, the NH
2
-MIL-125(Ti) nano- occur with the change of reaction time.
Apart from reactivity and selectivity, recyclability is a crucial
aer prolonging the reaction time to 24 h in the synthetic feature for heterogeneous catalysts. In the hydrogenation of the
system including NH -MIL-125(Ti), Co(NO , H PtCl , and CAL process, Pt–Co/PCT shows high stability. No signicant
crystals were transferred to titanium oxides with difficulty even
2
3
)
2
2
6
L-alanine (Fig. S18†). Although introducing metal species into loss of the conversion efficiency of CAL or the selectivity for COL
the synthetic system affected the transformation of NH -MIL- can be found aer six successive catalytic cycles (Fig. 4(c) and
2
1
25(Ti), the formation of porous cages of titanium oxide S27†). Structural characterization of Pt–Co/PCT aer the sixth
anchored Pt, Pt–Co, Pt Co and PtFe NPs was mainly related to cycling run reveals porous cages of titanium oxides without
L-alanine-assisted dissolution and recrystallization of Ti(IV) ions obvious damage and Pt–Co NPs without aggregation (Fig. S20†).
3
and reduction of metal ions (Fig. S19†).
Moreover, the Pt content (ꢁ3.04 wt%) and the Pt/Co atomic
ratio (ꢁ2.78 : 1) did not conspicuously change aer cyclic
catalytic hydrogenation. The stable Pt–Co/PCT heterostructures
can afford long-term high-efficiency catalytic performance of
CAL hydrogenation.
In comparison with the supported Pt–Co NPs that showed
high activity and selectivity for CAL hydrogenation, the Pt/
PCT_1 catalysts exhibited low activity with ꢁ46.5% conversion
Chemoselective hydrogenation of a,b-unsaturated aldehydes
The hydrogenation transformations over metal NP loaded PCT
heterostructures were carried out under 0.2 MPa H at 80 C
ꢀ
2
(Fig. 4) with an equivalent amount of Pt (0.16 mg). The Pt–Co/
PCT catalyst towards hydrogenation of cinnamaldehyde (CAL)
displayed a considerably high selectivity of around 96.0% for
cinnamyl alcohol (COL) at 100% conversion efficiency aer 3 h
of CAL for 3 h under the same catalytic condition of 0.2 MPa H
2
ꢀ
and 80 C (Fig. 4(a) and S25(b)†). The product selectivity for
COL, HCAL and HCOL was about 42.9, 40.3 and 13.8%,
respectively. The experimental observation is different from
previous reports on high catalytic activity of Pt NPs, including
hydrogenation of olenic bonds based on an energetically
(Fig. 4(a)). A small amount of saturated alcohol hydrocinnamyl
alcohol (HCOL) was detected, and saturated aldehyde hydro-
cinnamaldehyde (HCAL) was not found (Fig. S25(a)†). CAL
hydrogenation did not occur when PCT itself was used as the
catalyst. Hence, the supported Pt–Co NPs possess excellent
hydrogenation activity for CAL with high selectivity for the
carbonyl bond compared with the olenic bond. To fully
understand the catalytic performance of Pt–Co/PCT, time-
dependent catalytic experiments, which could reect the varia-
tion trend of the selectivity of Pt–Co/PCT in the catalytic
process, were performed (Fig. 4(b) and S26†). Upon shortening
the reaction time to 0.5 h, 74.8% conversion of CAL with as high
55
favorable process. We infer that the low catalytic activity of
56
Pt/PCT is mainly due to Pt NPs with large size. The Pt/PCT_2
catalyst was used for hydrogenation of CAL with 51.9% selec-
tivity for COL at 97.8% conversion, further suggesting the size
effect of Pt NPs in the catalytic hydrogenation process. Based on
these experimental results, we propose that Co doping is
supposed to greatly improve the selective hydrogenation
performance of Pt NPs towards carbonyl groups.
To further understand the effect of dopants on catalytic
performance of Pt NPs, PtFe/PCT and Pt Co/PCT were used to
3
study CAL hydrogenation. In contrast to the Pt–Co/PCT catalyst,
PtFe/PCT heterostructures exhibited ultra-low conversion of
CAL but comparable selectivity for COL under the same
conditions (Fig. 4(a) and S25(c)†); Pt Co-PCT hybrids gave
3
similar catalytic activity (98.9%) towards CAL hydrogenation but
a low selectivity of 56.3% for COL (Fig. 4(a) and S25(d)†). Thus,
the amount and type of metal dopant played a signicant role in
determining the hydrogenation reactivity of olenic and
carbonyl bonds over Pt–metal alloys.
The fundamental purpose of engineering porous hollow
supports is to solve the issue of mass transport for the
enhancement of catalytic activity. As shown in Fig. S21,†
commercial P25 and TiO2 were used as supports to prepare
heterostructures of P25 or TiO anchored Pt–Co NPs (dened as
2
Pt–Co/P25 and Pt–Co/TiO
2 2
, respectively). Note that TiO origi-
Fig. 4 (a) Selective hydrogenation of cinnamaldehyde with different
catalysts; (b) curves of conversion of CAL and selectivity for specific
hydrogenation products using Pt–Co/PCT as catalyst; (c) recyclability
tests of Pt–Co/PCT for catalytic hydrogenation of CAL; (d) selective
nated from NH -MIL-125(Ti) by calcination in an air atmo-
2
sphere. The content of Pt in the two heterostructures, slightly
higher than that in Pt–Co/PCT, was around 3.92 and 4.27 wt%,
hydrogenation of different a,b-unsaturated aldehydes by Pt–Co/PCT. respectively. The heterostructures gave low catalytic activity of
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CAL and acceptable selectivity for COL (Fig. 4(a), and S25(e) and
S25(f)†). Therefore, by comparison with hydrogenation perfor- activity and regulate selectivity for carbonyl groups as well.
The electron density of Pt can affect their hydrogenation
26,57
mance of Pt–Co alloys loaded on PCT, P25 and TiO supports, The low electron density induced by the Co dopant and the
2
PCT shows an obvious advantage in the improvement of cata- titanium oxide support leads to a smaller contribution of back-
lytic activity of Pt–Co alloys. The enhanced hydrogenation donation of Pt 5d electrons to adsorbed molecules, and thus
activity of Pt–Co/PCT is attributed to the support with porous electron-rich O atoms of carbonyl groups are favourably adsor-
hollow structures and favourable mass transport of substrates bed on the surface of Pt. Meanwhile, electron donation from
and products. In addition, we expected that the use of other carbonyl groups to Pt is increased with the increased d-vacancy
non-Ti-based materials, such as porous SiO
2
, SBA-15, and cobalt in Pt–Co alloys, resulting in activation of carbonyl groups. FTIR
layered double hydroxides, as supports would further increase spectra (Fig. S23†) showed that the stretching vibration of the
À1
À1
the effect of supports, but Pt–Co alloys cannot be achieved via carbonyl group in CAL at 1676 cm was shied to 1709 cm
the amino acid-assisted route (Fig. S22†). From this perspective, aer mixing with the Pt–Co/PCT, but no obvious change of the
À1
introducing Ti-based supports is possibly benecial for the olenic bond stretching vibration at 1630 cm occurred. The
production of Pt–Co alloys in L-alanine-related synthetic media. result reveals that the carbonyl group possibly prefers to adsorb
Therefore, apart from improvement of mass transport by on Pt–Co NPs. Thereby, the signicant enhancement of selec-
0
porous hollow structures, the excellent catalytic hydrogenation tivity for COL is mainly due to the low electronic density of Pt in
performance of Pt–Co/PCT hybrids was mainly attributed to the Pt–Co/PCT, accounting for its preferential interaction with
change of electronic structures of Pt, as well as introduction of carbonyl bonds and subsequent hydrogenation (Fig. S24†).
defects to Pt. The defects were characterized using HRTEM
To further demonstrate the universal selective hydrogena-
images (Fig. S13†). To reveal the electronic structures of Pt, tion capabilities of Pt–Co/PCT, we expanded the substrate scope
high-resolution XPS measurements were performed. As shown of a,b-unsaturated aldehydes to furfural, 3-methyl-2-butenal,
ꢀ
in Fig. 5(a) and (b), the Pt 4f region of core level binding ener- and 4-methoxycinnamaldehyde under 0.2 MPa H
2
and at 80 C
gies was deconvoluted into two sets of spin–orbit doublet peaks for 3 h (Fig. 4(d) and S28†). As expected, preferential hydroge-
in both Pt/PCT and Pt–Co/PCT_1. They can be assigned to nation of carbonyl bonds over olenic bonds in all catalytic
0
2+
0
multiple oxidation states of Pt and Pt . Pt 4f7/2 and 4f5/2 peaks reactions was found when Pt–Co/PCT was used as the catalyst.
2
+
at 70.25 and 73.45 eV, and Pt 4f7/2 and 4f5/2 peaks at 72.70 and Furfural alcohol was the sole product at 97.2% conversion of
0
7
5.90 eV were observed for Pt/PCT_1. In contrast, Pt 4f and furfural, signifying the ultra-high chemoselectivity of Pt–Co/
7/2
4
f5/2 were positively shied to 70.60 and 73.80 eV, revealing the PCT for the carbonyl bond of furfural. In contrast to CAL, the
2
+
reduced electron density of Pt in the Pt–Co/PCT hybrids; Pt
yield of selective hydrogenation of carbonyl groups was
4f7/2 and 4f5/2 were similar to those in Pt/PCT_1. The difference decreased to 86.6% when 4-methoxycinnamaldehyde was
0
of Pt 4f binding energy between Pt/PCT_1 and Pt–Co/PCT completely converted. As for the a,b-unsaturated aliphatic
52,57
mainly originates from Co doping.
Nonetheless, the binding aldehyde 3-methyl-2-butenal, Pt–Co/PCT gave a relatively low
energy of Pt 4f in Pt–Co/TiO and Pt–Co/P25 is still slightly catalytic activity (conversion: 84.4%) with 87.7% selectivity for
2
different from that in Pt–Co/PCT (Fig. 5(c) and (d)). Thus, the hydrogenation of carbonyl groups. Their selectivity for the
electronic structures of Pt are affected by both dopants and products of unsaturated alcohols in the hydrogenation process
supports.
of 3-methyl-2-butenal and 4-methoxycinnamaldehyde was
slightly decreased with longer reaction time. These results
reveal that the Pt–Co/PCT is one of the high-efficiency catalysts
for chemoselective hydrogenation of a,b-unsaturated aldehydes
based on the synergistic effect of the Co dopant and the porous
cages of the titanium oxide support.
Conclusions
In summary, we developed an amino acid-mediated one-pot
strategy to successfully synthesize the supported Pt or Pt–M
alloy NPs on porous cages of titanium oxide while using NH2-
MIL-125(Ti) as a self-sacricial template. The as-formed nano-
sheets of layered titanates are the key factor for the formation of
cage-like structures. The Pt–Co/PCT composite exhibits supe-
rior reactivity for catalytic hydrogenation of a,b-unsaturated
aldehydes with high selectivity for unsaturated alcohols under
mild conditions. The high-efficiency catalytic performance of
the supported Pt–Co NPs was mainly attributed to two factors:
(1) the Co dopant/titanium oxide support induced the forma-
tion of low electron density Pt accounting for its preferential
interaction with carbonyl bonds, and (2) porous hollow
Fig. 5 Core level Pt 4f XPS spectra of (a) Pt/PCT_1, (b) Pt–Co/PCT, (c)
Pt–Co/TiO , and (d) Pt–Co/P25.
2
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supports facilitated the mass transport of substrates and 19 K. Khaletskaya, A. Pougin, R. Medishetty, C. Rosler,
products across the shells assembled by sheets. Our ndings
would help guide endeavours to engineer other novel composite
C. Wiktor, J. Strunk and R. A. Fischer, Chem. Mater., 2015,
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Conflicts of interest
2
2 A. V. Vinogradov, H. Zaake-Hertling, E. Hey-Hawkins,
A. V. Agafonov, G. A. Seisenbaeva, V. G. Kessler and
V. V. Vinogradov, Chem. Commun., 2014, 50, 10210.
3 Z. Hong, M. Kang, X. Chen, K. Zhou, Z. Huang and M. Wei,
ACS Appl. Mater. Interfaces, 2017, 9, 32071.
4 L. Wang and T. Sasaki, Chem. Rev., 2014, 114, 9455.
5 C. Zlotea, D. Phanon, M. Mazaj, D. Heurtaux, V. Guillerm,
C. Serre, P. Horcajada, T. Devic, E. Magnier, F. Cuevas,
G. Ferey, P. L. Llewellyn and M. Latroche, Dalton Trans.,
There are no conicts to declare.
2
Acknowledgements
2
2
The authors are thankful for the nancial support from the
National Natural Science Foundation of China (Grant No.
2
1671030), the Key Program of the National Natural Science
Foundation of China (Grant No. 21531001), and the 111 Project
Grant No. B16008).
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