A general strategy to prepare atomically dispersed biomimetic catalysts based on host-guest chemistry

Article abstract of DOI:10.1039/d0cc07119f

Herein, we report a general strategy based on host-guest interactions to fabricate atomically dispersed biomimetic catalysts, which were evaluated by diboration of phenylacetylene. The structure and function of these mimics are quite similar to those of enzymes, namely, the atomically dispersed metal serves as an active site, the external macromolecular structure plays a role as an enzyme catalytic pocket to stabilize the reaction intermediates and the interactions between the intermediates and functional groups near to the active site can reduce the reaction activation energy.

Full text of DOI:10.1039/d0cc07119f

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A general strategy to prepare atomically dispersed
biomimetic catalysts based on host–guest
chemistry†
Cite this: Chem. Commun., 2021,
57, 1895
Received 28th October 2020,
Accepted 12th January 2021
bc
bd
fg
Xiao Han,‡a Zheng Chen,
‡
Wenxing Chen,
‡
Chunlin Lv,e Yongjun Ji,
Jing Li,g Weng-Chon Cheong,
Dingsheng Wang,b Chao Lian
Xiaojuan Lei,a Qing Peng,b Chen Chen,b
b
hd
and Yadong Lib
DOI: 10.1039/d0cc07119f
*
rsc.li/chemcomm
Herein, we report a general strategy based on host–guest interac- hydrophobic effects and electrostatic interactions.2 This has inspired
tions to fabricate atomically dispersed biomimetic catalysts, which the rise of supramolecular or host–guest catalysis, which involves
were evaluated by diboration of phenylacetylene. The structure and the preparation of biomimetic catalysts mimicking enzymes based
function of these mimics are quite similar to those of enzymes, on various host–guest interactions from the distinct chemical
namely, the atomically dispersed metal serves as an active site, the environments inside and outside the cavities of host molecules,3
external macromolecular structure plays a role as an enzyme such as cyclodextrins,4 calixarenes,5 cucurbiturils,6 hemicrypto-
catalytic pocket to stabilize the reaction intermediates and the phanes7 or other artificial synthetic hosts.8 However, the design
interactions between the intermediates and functional groups near and fabrication of these supramolecular mimics still relies on
to the active site can reduce the reaction activation energy.
modulation of the chemical environment around the catalytic active
sites. It is certainly a giant leap for synthesis and catalysis to directly
Enzymes, natural biocatalysts that catalyze all reactions to build up enzyme mimic structures based on easy-to-control hetero-
support biological life, exhibit outstanding activity and selec- geneous catalyst supports with mimicked enzyme catalytic pockets.
tivity under mild conditions, and are generally much more
In recent decades, atomically dispersed metal catalysts, so-
efficient than traditional chemical catalysts.1 It is generally called single-atom catalysts, have attracted increasing research
recognized that the superior catalytic properties of enzymes attention.9–11 In these novel heterogeneous catalysts, uniform
often emanate from the substrate–enzyme complex’s molecular single atoms act as isolated active sites, which builds a bridge
binding and chemical transformations, facilitating the formation between heterogeneous and homogeneous catalysis by supply-
of stabilized transition states, mediated by non-covalent interactions ing insight into the catalysis process at the molecular level.12–16
involving van der Waals and p–p interactions, hydrophilic/ From a structural aspect, these single atom sites most look like
the simplest active sites in enzymes. Because transition metals
a Department of Chemistry, School of Science, Beijing Jiaotong University,
in a single-atom state can easily aggregate into nanoparticles,
several restriction factors, such as metal–support inter-
actions,9,13,17 coordination interactions,11,18 steric confine-
ments19 and defect effects,15,20–22 are utilized to stabilize
single-atom metal atoms on the support during the fabrication
of atomically dispersed metal catalysts. With regard to enzyme
mimics, in addition, host–guest interactions between the sub-
strate and mimics are promising to stabilize single-atom metal
atoms as well.
Base on host–guest chemistry, we propose a general strategy
to prepare atomically dispersed biomimetic catalysts, where the
catalytic sites are located in the host units that mimic enzyme
catalytic pockets on the supports (Fig. 1a). In this strategy,
hydrophobic transition metal precursors are incorporated into
the cavities of the host units. The metal atoms in the formed
inclusion complexes are further reduced to generate single
catalytic sites located in the cavities mimicking enzyme
Beijing, 100044, P. R. China
b Department of Chemistry, Tsinghua University, Beijing 100084, China
c Key Laboratory of Functional Molecular Solids, Ministry of Education,
Anhui Key Laboratory of Molecule-Based Materials, College of Chemistry and
Materials Science, Anhui Normal University, Wuhu 241000, P. R. China
d Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials
and Green Applications, School of Materials Science and Engineering,
Beijing Institute of Technology, Beijing 100081, P. R. China
e School of Pharmacy and Center on Translational Neuroscience,
Minzu University of China, Beijing 100081, P. R. China
f School of Light Industry, Beijing Technology and Business University,
Beijing 100048, P. R. China
g State Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing, 100190, P. R. China
h Beijing Single-atom Site Catalysis Technology Co., Ltd, Beijing 100094,
P. R. China. E-mail: lianchao@sacatalysis.com
† Electronic supplementary information (ESI) available: Experimental procedure,
additional data, etc. See DOI: 10.1039/d0cc07119f
‡ These authors contributed equally.
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Fig. 1 (a) Schematic illustration of the synthesis of the atomically dispersed biomimetic catalysts in this work. (b) Schematic model of the mimicked
enzyme catalytic pocket in Pt1/CDP. (c) Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM)
image of Pt1/CDP. (d) HAADF-STEM image of Pt1/CDP. (e) Corresponding element (C, N, O, F and Pt) maps of Pt1/CDP. (f) Fourier-transformed EXAFS
spectra at the Pt L3-edge for Pt1/CDP, Pt foil and PtO2, and the EXAFS fitting curve for Pt1/CDP. (g) The normalized XANES spectra at the Pt L3-edge for
Pt1/CDP, PtO2 and Pt foil.
catalytic pockets. To confirm the feasibility of this strategy, a general host–guest interactions that are mainly influenced by
reported b-cyclodextrin (CD)-containing polymer (CDP) was the relative size of the cavity in the host units and the hydro-
used as a support to synthesize a series of atomically dispersed phobic guest precursor molecules, this synthesis method is
biomimetic catalysts (M1/CDP, M = Pt, Co, Ni, Cu, Ru, Rh and effective for a variety of atomically dispersed transition metal
Ir). It was proved that this enzyme mimic structure exhibits a catalysts. By means of using appropriate transition metal com-
catalytic activity enhancement with diboration of alkynes over plexes as precursors, the inclusion between CDP and the
platinum catalysts as a model reaction.
precursors could be achieved as well (Fig. S6, ESI†), and the
The CDP support was synthesized via an established corresponding atomically dispersed transition metal catalysts,
method,23 with some modifications (Fig. S1 and S2, ESI†). such as Co1/CDP, Ni1/CDP, Cu1/CDP, Ru1/CDP, Rh1/CDP and
Taking Pt1/CDP as an example, bis(2,4-pentanedionate) Ir1/CDP, would be obtained by reducing the transition metal
platinum(II) (Pt(acac)2, used as a Pt precursor) and the support complex encapsulated in CDP (Fig. S8–S14 and Tables S3, S4,
were stirred in water for one day to form an inclusion complex ESI†). Lots of nanoparticles appeared, however, when a transi-
of CDP with Pt(acac)2. For the inclusion, Pt(acac)2 was encap- tion metal complex, tris(2,4-pentanedionate) ruthenium(III) for
sulated into the host units (b-CD in the framework of CDP) at a example, which is larger than the CD cavity was used as a
molecular level so that no XRD signal for the Pt(acac)2 crystals precursor (Fig. S15, ESI†). This further implied the host–guest
can be observed (Fig. S3, ESI†). Finally, the Pt1/CDP product chemistry interaction involved in this strategy.
was obtained by reducing the included Pt(acac)2 molecules with
To verify the catalytic performance of this enzyme-like
NaBH4. As shown in Fig. 1c and d, platinum was in an atom- structure, Pt-catalyzed diboration of phenylacetylene (1a) with
ically dispersed state, in which no Pt nanoparticles could be bis(pinacolato)diboron (B2pin2, 2a) was chosen as a model
observed, throughout the as-synthesized Pt1/CDP composite. reaction (Table 1). Pt1/CDP exhibited a higher catalytic activity
EDX analysis in STEM revealed that this atomic Pt was dis- compared with the reported heterogeneous catalysts under
persed in the CDP support evenly (Fig. 1e). The Pt content of the similar reaction conditions.23 The reaction conditions were
final sample was 0.8%, according to ICP-AES analysis (Table S2, optimized and the best test conditions were that the diboration
ESI†), which was in accordance with the target Pt loading in the reaction was carried out in toluene at 100 1C (Table 1,
synthesis stage. EXAFS and XANES were used to further verify entry 1–7). By virtue of the enzyme-like structure, the activity
the atomically dispersed nature of Pt1/CDP (Fig. 1f, g and of Pt1/CDP (898 hÀ1, Table 1, entry 6) was almost 13.5 times
Fig. S4, S5, Table S3, ESI†). As shown in Fig. 1f, a notable single higher than that of Pt1/CN (62 hÀ1, Table 1, entry 8), an
peak in the region of 1 to 2 Å can be assigned to Pt–O bonds, atomically dispersed platinum catalyst without this specific
but no signal in the region of 2 to 3 Å from the Pt–Pt contribu- structure (Fig. S16, ESI†). It was found that the atomically
tion was observed. In comparison with Pt foil, an enhanced dispersed state for the single-atom Pt catalyst did not favor
white-line intensity of Pt1/CDP with a small shift to higher the catalytic diboration reaction because the Pt nanocrystals,
energy for the absorption edge suggested that partially oxidized with an average diameter of 2 nm, adsorbed on the active carbon
Ptd+ rather than Pt0 existed in Pt1/CDP (Fig. 1g). This agreed (Fig. S17 and S18, ESI†) exhibited a higher catalytic activity (176
well with the result of XPS measurement, in which the binding
h
À1, Table 1, entry 9) compared with Pt1/CN. Moreover, when we
energy of the Pt 4f7/2 core-level in Pt1/CDP was obviously higher adsorbed the Pt nanocrystals on CDP (Fig. S19, ESI†), a slight
than that for traditional zero-valent metallic Pt (Table S4, ESI†). decrease in catalytic activity was observed (160 hÀ1, Table 1,
Because the strategy proposed in this work resorts to the entry 10). It has been reported that the CDP used in this work is a
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Table 1 Pt-Catalyzed diboration of phenylacetylene.a
DFT calculations could be performed to further reveal the effect
of the mimicked structure (Fig. S20–S24 and Videos S1–S3,
ESI†). It has been reported that diboration of alkynes involves
three steps, namely (I) oxidative addition of a B–B bond to the
Pt site, (II) insertion of a C–C multiple bond and (III) reductive
elimination of the C–B bond from the Pt site.21 As shown in
Fig. 2, step III, with an energy barrier (20.1 kcal molÀ1) slightly
higher than that of step II (19.6 kcal molÀ1) and much higher
than that of step I (7.3 kcal molÀ1), is the rate-determining step
over Pt1/CD. When the cyclic structure was broken and only the
molecular structure near to the Pt single-atom site remained
(Pt1/CD-b), the highest reaction energy barrier of step III was
slightly reduced to 17.9 kcal molÀ1, but all of the related Gibbs
free energies of the intermediates were elevated. This suggested
that the cyclic structure of b-CD can stabilize intermediates
involved in the catalytic reaction, while it cannot reduce the
activation energy of the reaction. For most of the reaction
intermediates in reactions over both Pt1/CD and Pt1/CD-b,
however, it was found that there are hydrogen bonds between
O in B2pin2 and H in hydroxyls near to the Pt single-atom site
(Fig. S23 and S24, Videos S1 and S2, ESI†). The hydrogen
Temp.
(1C)
T
Con.b Sel.b TOF
Entry Catalyst
Sol.
(h) (%)
(%)
(hÀ1
)
1
2
3
4
5
6
7
8
9
10
Pt1/CDP
Pt1/CDP
Pt1/CDP
Pt1/CDP
Pt1/CDP
Pt1/CDP
Pt1/CDP
Pt1/CN
DMF
1,4-Dioxane
MeCN
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
60
60
60
60
80
100
100
100
100
100
1
3.2
91.2
32
103
41
309
793
898
—
62
176
160
1
1
1
1
1
2
2
2
2
10.3 100
4.1
98.9
30.9 100
79.3 100
89.8 100
499.9 100
12.3 100
35.2 100
31.9 100
Ptnano/C
Ptnano/CDP Toluene
a
Reaction conditions: phenylacetylene (0.5 mmol), B2pin2 (0.55 mmol),
catalyst (0.1 mmol% based on metal), solvent (2 mL). b Conversion and
selectivity is determined by GC and GC-MS.
porous polymer,24 and the porous structure will affect the bonding may directly affect the reaction behavior of the inter-
diffusion of the reactants to bring about a decrease in apparent mediates over the Pt sites. According to our DFT calculations,
activity. However, it is interesting that the activity of Pt1/CDP was the highest energy barrier (step III) of the whole reaction
also much higher than that of Ptnano/CDP. Because the Pt nano- process sharply elevated to 26.7 kcal molÀ1, once the H of
crystals were too large to enter into the b-CD cavities, these hydroxyls in Pt1/CD-b was absent (Pt1/CD-b-noH). The partici-
results implied that the mimicked enzyme catalytic pockets pation of hydroxyls near to the Pt site played an important role
merely made a contribution to the catalytic reaction on the sites in reducing the reaction activation energy. From the viewpoint
inside them to extremely improve the catalytic activity, just like of structure and function, the atomically dispersed biomimetic
what happens in enzyme catalysis. For this model reaction, the catalysts are extremely similar to enzymes, in which an external
improvement of activity in the enzyme-like structure even offset macrocyclic structure supplies an enzyme catalytic pocket to
the inferior activity over single-atom sites in Pt1/CDP compared stabilize the reaction intermediates, but the interactions
with that over multi-atom sites in nanoparticles.
between the intermediates and functional groups, such as
We simplified the mimicked enzyme catalytic pocket struc- amino, carboxyl and hydroxyl in enzymes, near to the active
ture of Pt1/CDP into a single b-CD structure (Pt1/CD) so that sites contribute to reducing the activation energy.
Fig. 2 Gibbs free energy profiles for the Pt-catalyzed diboration of phenylacetylene.
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As shown in Fig. S25 (ESI†), the Pt1/CDP catalyst showed a Notes and references
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1898 | Chem. Commun., 2021, 57, 1895À1898
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