Metal–Organic Frameworks for the Exploitation of Distance between Active Sites in Efficient Photocatalysis

Article abstract of DOI:10.1002/anie.201915537

Discoveries of the accurate spatial arrangement of active sites in biological systems and cooperation between them for high catalytic efficiency are two major events in biology. However, precise tuning of these aspects is largely missing in the design of artificial catalysts. Here, a series of metal–organic frameworks (MOFs) were used, not only to overcome the limit of distance between active sites in bio-systems, but also to unveil the critical role of this distance for efficient catalysis. A linear correlation was established between photocatalytic activity and the reciprocal of inter active-site distance; a smaller distance led to higher activity. Vacancies created at selected crystallographic positions of MOFs promoted their photocatalytic efficiency. MOF-525-J33 with 15.6 ? inter active-site distance and 33 % vacancies exhibited unprecedented high turnover frequency of 29.5 h^?1 in visible-light-driven acceptorless dehydrogenation of tetrahydroquinoline at room temperature.

Full text of DOI:10.1002/anie.201915537

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Accepted Article
Title: Metal-Organic Frameworks for the Exploit of Distance between
Active Sites in Efficient Photocatalysis
Authors: Yufei Shu, Xuan Gong, Zhuo Jiang, Lingxiang Lu, Xiaohui Xu,
Chao Wang, and Hexiang Deng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915537
Angew. Chem. 10.1002/ange.201915537
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Metal-Organic Frameworks for the Exploit of Distance between
Active Sites in Efficient Photocatalysis
†
[a]
† [a]
[a]
[b]
[c]
[a]
Xuan Gong , Yufei Shu , Zhuo Jiang, Lingxiang Lu, Xiaohui Xu, Chao Wang, Hexiang
Deng*[a]
Abstract: Discoveries of the accurate spatial arrangement of active
sites in biological systems and the cooperation between them to
achieve high catalytic efficiency are two major events in biology.
However, precise tuning of these aspects is largely missing in the
design of artificial catalysts. Here, we used a series of metal-organic
frameworks (MOFs), not only overcame the limit of distance between
active sites in bio-systems, but also unveiled the critical role of this
distance for efficient catalysis. A linear correlation was established
between photocatalytic activity and the reciprocal of inter active site
distance, where smaller distance led to higher activity. Moreover, we
found that vacancies created at selected crystallographic positions of
MOFs further promoted their photocatalytic efficiency. MOF-525-J33
with 15.6 Å inter active site distance and 33% vacancies exhibited
-
1
unprecedented high turnover frequency of 29.5 h in visible light
driven acceptorless dehydrogenation of tetrahydroquinoline at room
temperature.
Biological systems are known to exhibit excellent efficiency in
the conversion of molecules and energy management as a result
of evolution through millions of years.[1] The overall catalytic
performance is governed by the interplay between adjacent
functional motifs, where the distance between the corresponding
active sites in these motifs is critical, especially for reactions
involving multiple steps (Figure 1A).[2] This knowledge has been
successfully transferred to the design of biomaterials based on
multi-enzyme complexes,[3] where the inter active sites distance
Figure 1. Schematic illustration of enzymes inspired discussion between
interac tivesite distance in porphyrin MOFs and their catalytic efficiey of THQ
dehydrogenation driven by visible light. (A) Enzymes distance oriented two
subsequent steps in tryptophan synthesis. The scheme is sketched
according to crystal structure of tryptophan synthase (Protein Data Bank:
was shortened to less than 10 nm to give higher efficiency. In
theory, much shorter distance can be obtained in inorganic
materials, thus promising to maximize their catalytic efficiency.
However, due to the lack of systematic exploration; its correlation
to catalytic performance has yet to be established. Unveiling the
impact of inter active site distance is likely to release the full
potential of inorganic catalysts.
1A5S) (B) THQ dehydrogenation catalyzed by porphyrin MOF. The distances
between porphyrin linkers are determined by various MOFs structures. (C)
Precisely control on the distance by tuning the angle between adjacent
catalytic sites in porphyrin MOFs. The dihedral angle varied from 60 degree
to 180 degree.
Metal-organic frameworks (MOFs) are a class of inorganic
porous crystals constructed by linking molecular building blocks
periodically,[ thus are ideally suited for the precise control and
progressive tuning of distance between active sites (Figure 1B).
4]
Here, we studied a series of six MOFs based on porphyrin unit,
[5]
an essential active site for redox reactions in bio-systems.
Specifically, MOF-525, PCN-221, PCN-222, PCN-223, PCN-224
and Al-PMOF,[ were synthesized in pure phase, where each
MOF exhibited a distinct profile of distance between porphyrin
active sites (Figure 1C). Different from enzymes, where the active
sites are far apart due to the packing of auxiliary function groups,
the distance between active sites in MOFs are precisely controlled
by the dihedral angles at the metal oxide secondary building units
6]
[
a]
Xuan Gong, Yufei Shu, Zhuo Jiang, Chao Wang, and Hexiang Deng
Key Laboratory of Biomedical Polymers-Ministry of Education,
College of Chemistry and Molecular Sciences, Wuhan University,
Wuhan 430072, (P.R. China)
E-mail: hdeng@whu.edu.cn
Prof. H. Deng
(
SBUs) (Figure 1C and 2A). In this way, much smaller intervals
The Institute for Advanced Studies, Wuhan University, Wuhan
can be dialled-in, from 12.8 Å to 21.9 Å, with angstrom level
accuracy without facilitation from any auxiliary groups. This
allowed us to correlate the distance with the catalytic performance
of these MOFs, among which, PCN-223, with the shortest inter
active site distance, showed a record high apparent quantum
efficiency, 8.0% at 400 nm, for acceptorless dehydrogenation of
tetrahydroquinoline (THQ) at room temperature. The catalytic
performance of this MOF was even comparable to the best
thermal catalysts used for this reaction at elevated temperature (>
430072, (P.R. China)
[
b]
c]
Lingxiang Lu
Department of chemistry and chemical biology, Cornell university,
259 Est Avenue, Ithaca, NY, 14853, (USA)
[
Xiaohui Xu
Department of chemistry, College of chemistry engineering, Xiamen
University, Xiamen 361005, (P. R. china)
These author contributed equally to this work.
[†
]
Supporting information for this article is given via a link at the end of
the document.
120°C ), demonstrating the power of controlling inter active site
distance (Table S15).
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All these MOFs were synthesized from the same porphyrin
linker, 5,10,15,20-tetrakis(4-carbonxyphenyl)porphyrin (TCPP),
and the difference between these MOFs lies in the metal oxide
SBUs and the angles at these SBUs. In the case of PCN-223, a
triangular prism shaped pore was present, where the 60 degree
angle between adjacent TCPP linker gave an inter active site
distance of 10.7 Å (Figure 1C and 2A). Similarly, cubic shaped
pores were observed in PCN-221, MOF-525 and PCN-224 with
PCN-222. Due to the difference in SBUs, the corresponding
distance were 16.9 (PCN-223), 16.8 and 16.3 (Al-PMOF), and
16.9 Å (PCN-222), respectively. The details can be found in
Figure 1C, 2A and SI Section S7. These distance profiles were
used to correlate to the catalytic performance of MOFs.
Prior to the study of photocatalytic activity, the crystal
structure and the phase purity of these MOFs were examined by
a series of characterization methods. The powder X-ray diffraction
90 degree between adjacent TCPP linkers, thus the distance
(
PXRD) patterns of these MOFs matched well with those
between porphyrin units were 13.8, 13.7 and 13.6 Å, respectively,
with the slight variation from different SBUs. In the case of PCN-
simulated from single crystal structures without the presence of
any extra peaks (Figure S3-S8). This combined with the singular
morphology of MOF crystals in the corresponding SEM images
demonstrated their phase purity (Figure S26-S31). All these
222, pores in hexagonal prism shape appeared to give 120
degree dihedral angle, hence a longer distance, 18.3 Å, showed
up. 180 degree angle also existed, in PCN-223, Al-PMOF and
Figure 2. (A) The illustration of distance between TCPP active sites in PMOFs. (B) Volumetric density of TCPP in PMOFs. (C) TOF of PMOFs in dehydrogenation
of THQ vs. their weighed average reciprocal of distance between active sites. (D) TOF of PMOFs in dehydrogenation of THQ vs. their TCPP volumetric density.
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MOFs exhibited surface areas comparable to the best reported
trend showed up. Surprisingly, the Al-PMOF with the highest
2
-3
data, from 2240 to 4260 m /g (Figure S36-S47). The porosity of
volumetric density, 0.90 mmol·cm , didn’t exhibit the highest TOF
(only 14.6 h-1 per cm ) for the photocatalytic dehydrogenation of
-3
these MOFs were summarized in Table S3. It was worth noting
that the synthesis of zirconium MOFs based on porphyrin linkers
in pure phase is challenging, where the growth of crystals in one
type usually accompanied with the presence of other phases. [7]
Here, the phase purity was achieved by combinatorial chemistry,
THQ, and this value was not even close to the top. In comparison,
-3
PCN-222 with a much lower volumetric density, 0.19 mmol·cm ,
-1
-3
exhibited even better TOF (22.6 h per cm ). One could argue
that mesopores exist in PCN-222, thus favouring the TOF,
however, in the case of PCN-224, which also has mesopores, the
TOF is worse (10.3 h-1 per cm ) with even higher volumetric
density. In this regard, the conventional volumetric density rule is
unlikely to work out here for MOFs. The reason is that the active
sites are evenly distributed, and with relatively low density in
conventional porous materials, however, neither of these two
prerequisites are met in MOFs. Here, the porphyrin active sites
are highly concentrated throughout the frameworks, and the
distance between them varies drastically with the change of
topology (Figure 2B and 1B). The inverse trend between PCN-
a
where pK values of the modulators, their combination, acidity of
-3
the solution, temperature of the reaction and stoichiometry of the
starting materials were varied systematically (SI section 3). This
allowed us to derive a phase diagram for the synthesis of these
MOFs in pure phase and provided solid basis for investigating the
impact of distance between active sites.
The acceptorless dehydrogenation of N-heterocycles is an
[
8]
important reaction in organic synthesis, which also has potential
[
9]
in the storage of hydrogen. This reaction is usually carried out
[
8a,b]
under thermal conditions,
and recently shown possible to be
224 and PCN-222 can be explained by the difference in their
catalyzed under visible light.[8c,d] The mechanism study revealed
spatial distance between active sites, 10.7 Å in the smallest
triangular prism unit in PCN-222, which is shorter than those in
the cubic unit in PCN-224, 13.6 Å. Therefore, the inter active site
distance is likely to dominate the influence on photocatalytic
performance of MOFs. Indeed, when the distance between active
sites in each MOF was measured (Figure 2A), and the reciprocal
distance profile of each MOF was plotted against their TOF, all
these data points started to converge to reveal a linear
relationship (Figure 2C).
the multistep nature of the dehydrogenation process (Figure S91-
S92),[
8c]
which is suitable for the systematic study of MOFs with
variable distance profiles. In this study, we showed that porphyrin
and porphyrin MOFs were also capable to dehydrogenate THQ
under visible light. Unlike porphyrin MOFs previously applied in
both thermal and photo catalytic reactions,[6b,c and 10] the porphyrin
active sites here were absent of metal ions in the center, therefore
contribution from coordination bond is ruled out. MOFs composed
of elongated porphyrin linkers were also known to give excellent
electrochemical performance.[6g] The isoreticular extended MOFs
are not included in the scope of this particular study. The electron
transfer between THQ and porphyrin active site was evidenced in
the quench of the characteristic fluorescence of porphyrin at 650
nm (Figure S83 and S84). This was consistent with the presence
of fingerprint peaks of porphyrin radicals in the in situ electron
paramagnetic resonance (EPR) of the reaction under visible light
The contribution from multiple distance between active sites
within the same MOF structure was taken into account by the
-
1
weighed average reciprocal of distances (D ). For example, in
PCN-222, three inter active site distances coexisted without
hindering the transfer path of substrates and products, 11.0 Å,
-
1
-2
-1
18.3 Å and 16.7 Å, with a D of 6.80×10
Å
(Figure S61).
-
1
Similar analysis was performed for other MOFs. D values were
plotted against TOF of these MOFs (Figure 2C), revealing a much
clearer trend than that with volumetric density (Figure 2D). The
detail calculation was presented in Eq 1 as the following:
1
(
Figure S89-S90). The lifetime of porphyrin active sites upon light
absorption was also measured by transient absorption
spectroscopy (TAS) (Figure S87-S88), where the value was in the
same magnitude for all these MOFs, consistent with the results
from photoluminescence (PL) study (Figure S73-S82). In the
control experiment using MOFs with identical metal oxide
secondary building (SBU) unit but absent of porphyrin active site
in the backbone, e.g. UIO-66 and PCN-521, no dehydrogenation
product was observed (Table S15). All these results above
revealed the catalytic role of porphyrin active site.
D = ∑i n1 d1 +n2d2 +n3d3 +…+nidi
-
1
-1
-1
-1
-1
(1)
1
N
N represents the number of porphyrin active sites adjacent to one
chosen porphyrin, D stands for the weighed average distance,
and n represents the number of adjacent porphyrin centers
nearby one porphyrin (Figure 2A). Distance between active sites
were also derived in other forms, such as the weighed average
-
distance (D), square weighed average reciprocal of distance (D
)
The catalytic performance of all these MOFs was assessed
under identical conditions at room temperature (Figure S93).
Typically, 7 mg of MOF catalyst was added into a pyridine solution
of 0.6 mL containing 0.4 mmol THQ in argon. A 300 W Xe lamp
equipped with 390 nm filter was used as the visible light source
2
̅
-1
, and reciprocal of weighed average distance (D ), and plotted
against TOF of MOFs (Figure S70 and Table S15). In all these
plots, similar trend was observed, i.e. smaller inter active site
distance led to higher TOF (Figure S70), also in good accordance
to biological systems. Among these correlations, the plot with
weighed average reciprocal of distance revealed a linear fitting
with regression of r=0.99, and a slope of 9.7. Given the multistep
nature of this photocatalytic reaction,[ we surmised that the
value of this slope is related to the activity of the porphyrin center
to generate reaction intermediate. When a different type of active
site is used, this value might change.
2
with fixed position and irradiation area, 7.1 cm , to MOF samples
(
Figure S93b). The quinone product was monitored by GC-MS
with 15 min interval. The reaction kinetics was reflected in the
linear slope in the plot of product amount versus reaction time
8c]
(
Figure S98), where each point was measured three times in
parallel (Figure S96). The turnover frequencies (TOF) of these
MOFs were obtained from the data in the first 2 hour time, where
the conversion was complete for most of the MOFs (Table S15),
According to the correlation revealed above, the catalytic
performance of MOFs can be promoted by compressing active
site closer to each other. Intuitively, however, if the active sites
were too close, it might have led to smaller pore size, thus
hindering the reaction kinetics. This was exactly the case of PCN-
9.0, 28.9, 28.5, 22.6, 14.6 and 10.3 h- per cm for PCN-223,
1
-3
2
MOF-525, PCN-221, PCN-222, Al-PMOF and PCN-224,
respectively (Figure 2A). The chemical stability of these MOFs in
this reaction was revealed by their unaltered PXRD patterns
before and after reaction (Figure S97).
223, which exhibited a TOF noticeably worse than the linear trend.
The logic question is whether it is possible to promote the kinetics
without sacrificing the efficiency of the active site for catalysis? In
other words, is it possible to obtain points above the linear trend
rather than below the trend in the plot of inter active site distance
versus TOF (Figure 2C).
Previous studies on porous crystals have shown that the
volumetric density of active site is a key factor for the overall
catalytic performance.[11] Therefore, we plotted the TOF per active
site in these MOFs against the volumetric density of active site in
solvent-free form (Figure 2B and 2D). Unfortunately, no clear
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Figure 3. Structure analysis and catalytic performance study of Jenga-MOFs (A) Illustration of Jenga game, where the structure integrity is maintained with
partial removal of building blocks. (B) Systematic active site vacancy variation from 0 to 50 % in corresponding Jenga MOFs. (C) Powder X-ray diffraction
2 0
patterns of Jenga MOFs. (D) 77 K N adsorption isotherms of Jenga MOFs with P/P in linear scale and log scale (inset graph) at the pore filling stage. (E)
Catalytic performance of Jenga MOFs with various active site vacancy. (F) Representation of units with equivalent and non-equivalent interactive site distances
in Jenga MOFs, 6F, 5F, 4F, 3F, 2F stand for cubes with different number of facet left, while -I and II label different types in the cube with the same number of
facet. (G) Selected possible structures of MOF-525-J33 and J44 in a 3×3×3 unit cell size of MOF-525.
The strategy we used here was the partial removal of TCPP
linker from MOFs with short inter active site distance to generate
mesopores and favour substrate diffusion (Figure 3A and B), a
scenario similar to the structure obtained in Jenga game, where
the goal is to maintain structural integrity upon building block
removal. This was demonstrated chemically by tuning the ratio of
modulation acids during the course of MOF-525 formation.
Specifically, formic acid and 3,3-dimenthylbutanoic acid were
mixed in different ratios during the synthesis of MOF-525 to give
Jenga MOFs with different active site vacancies, MOF-525-J33 to
0
224, P/P =0.10 (Figure S57c and S57f, SI Section S5). These
MOFs were also used in the test of photocatalytic reaction.
We found that the Jenga MOFs exhibited excellent TOF in the
visible light driven dehydrogenation of THQ, reflected in the data
point beyond the linear trend in the plot of inter active site distance
versus TOF (Figure 3E). Indeed, the TOF for MOF-525-J44 and -
-1
J33 were 25.3 and 29.5 h per porphyrin, respectively, both were
above the efficiency regulation line between the MOF-525 and
PCN-224, demonstrating the power of Jenga approach. We
simulated the possible active site vacancies in a 3×3×3 unit cell
size of MOF-525 (Figure 3G, S65 and S66), where an average
vacancy of 2 active sites correspond to MOF-525-J33 that has the
highest TOF value (Figure S67). This allowed us to think that an
equivalent catalytic efficiency is possible to be obtained with two
of the six facets in the cube removed in the case of MOF-525
-
J48, where the number after J represent the percentage of
vacancy (Figure 3B and S17). The exact ratio of vacancy was
determined by the quantitative elemental analysis of the MOF-
5
25-J samples (Figure S11-S18, Table S1-S2). The presence of
vacancy was reflected in the emergence of peaks at 2θ = 3.3 and
.5 positions in the PXRD pattern (Figure 3C, S9 and S10). These
5
(
Figure 3F).
Jenga MOFs were distinguished from the physical mixture of
MOF-525 and PCN-224 in the singular truncated cubic
morphology in their SEM images (Figure S32-S35). Gradual shift
In conclusion, a correlation between inter active sites distance
and photocatalytic performance was established in a series of
porphyrin MOFs. Vacancies at selected crystallographic positions
allowed for the further promotion in turnover frequency, exceeding
the linear trend. These discoveries provide new insight for the
design of inorganic catalysts.
0
of the pore filling stage was observed at P/P between 0.01 and
0
.10 in the N adsorption isotherm of these MOFs (Figure 3D,
2
Figure S48-S57, SI Section S5), providing further evidence to rule
out the possibility of physical mixture. It was worth noting that
Jenga MOFs was also different from defective MOFs, where the
position of vacancy was random,[12] otherwise the pore filling
0
position (P/P ) would have been stretched beyond that of PCN-
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Acknowledgements
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F. Ke, and Prof. H. Cong for their invaluable assistance of this
research. We also thank the Test Center and the Large-scale
Instrument and Equipment of Wuhan University, and UC
Berkeley-Wuhan University Joint Innovative Center for the
assistance in material characterizations. We acknowledge the
supports from National Natural Science Foundation of China
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Keywords: metal-organic frameworks, porphyrin, photocatalytic
dehydrogenation, inter active site distance.
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