Robust multivariate metal-porphyrin frameworks for efficient ambient fixation of CO2 to cyclic carbonates

Article abstract of DOI:10.1039/c8cc07865c

A family of multivariate metal-organic frameworks (MOFs) with three-kinds of orderly distributed metals were designed and successfully synthesized by combining metalloporphyrin sheets and pentafluoride (NbOF₅)^2? pillars. Benefiting from the cooperative nature of open-metal-sites (OMSs) within porphyrins, specific pore-sizes, coupled with fluorine-rich electrostatic environments, the fabricated materials demonstrated high affinity toward CO₂, and good catalytic performance, structural robustness, and good recyclability for the conversion of epoxides and CO₂ to cyclic carbonates at room temperature and 1 atm pressure.

Full text of DOI:10.1039/c8cc07865c

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Robust multivariate metal–porphyrin frameworks
for efficient ambient fixation of CO2 to cyclic
carbonates†
Cite this: Chem. Commun., 2019,
55, 412
Received 1st October 2018,
Accepted 3rd December 2018
ab
a
Liang He,
Jayanta Kumar Natha and Qipu Lin
*
DOI: 10.1039/c8cc07865c
rsc.li/chemcomm
A family of multivariate metal–organic frameworks (MOFs) with and polymers,10 which can be readily separated and then
three-kinds of orderly distributed metals were designed and potentially reused. However, most CO2–epoxide coupling pro-
successfully synthesized by combining metalloporphyrin sheets cesses catalyzed by them are still limited to elevated tempera-
and pentafluoride (NbOF5)2À pillars. Benefiting from the coopera- ture and/or pressure, thus requiring additional cost penalties.
tive nature of open-metal-sites (OMSs) within porphyrins, specific Consequently, there is still a need to search for excellent
pore-sizes, coupled with fluorine-rich electrostatic environments, heterocatalysts that have a high density of CO2 trap-sites and
the fabricated materials demonstrated high affinity toward CO2, catalytically-active centers.
and good catalytic performance, structural robustness, and good
Metal–organic frameworks (MOFs) are inherently modular,
recyclability for the conversion of epoxides and CO2 to cyclic that is, amenable to crystal engineering for targeting desired
carbonates at room temperature and 1 atm pressure.
structures to satisfy the dual challenge of CO2 capture and
fixation.11 Up to now, several synthetic strategies, including
Anthropogenically discharged carbon dioxide (CO2) is well creation of unsaturated metal sites and addition of organic
established to be a primary factor of global warming.1 To solve amines, have been employed to reinforce the CO2 sorption
this problem and recycle carbon resources, many reactions capacity of MOFs by forming chemical bonds, which results in
involving CO2 as a C1 feedstock have been intensively developed high energy costs associated with the regeneration of sorbents.12
over the past few decades.2 Among them, the chemical insertion Recently, Zaworotko and co-workers developed a group of square
of CO2 into epoxides for affording cyclic carbonates has been grid-like layers based on linked metal nodes that are pillared via
proposed as an attractive protocol, not only due to its 100% a hexafluorosilicate to form a three-periodic network (termed
atom-economy but also because of the industrial potential of SIFSIX-n) with a primitive cubic topology.13 Benefitting from pore
carbonate products for use as degreasers, electrolytes, and so size control and favourable electrostatics from the periodically-
on.3 Thus far, lots of efforts have been devoted towards the lined fluorides, SIFSIX-n was found to offer a higher efficient trap
realization of catalytic systems with extremely high reactivity mechanism for CO2, even in the presence of moisture.11a,14
and selectivity, like ionic liquids,4 Schiff bases,5 and metal However, these isoreticular series based on linear bipyridine or
complexes.6 In particular, metalloporphyrins have shown out- its variants do not have Lewis acidic or other types of catalytically-
standing CO2-fixation performance.7 However, such molecular active sites, which can further convert CO2 into cyclic carbonates.
catalysts bring forth the common issues of a homogeneous To achieve a high content of catalytically-active sites in MOFs,
system like the intricacies of product isolation and catalyst one appealing approach is to integrate open metal sites (OMSs)
recovery. One of the effective solutions to these drawbacks is to or Brønsted acidic sites by judicious selection of metal nodes
develop heterogeneous catalysts, like metal oxides,8 zeolites,9 and/or metallolinkers.15 Metalloporphyrins are found in nature
as biocatalysts, which are capable of activating CO2.11e,16 Through
ligand substitution, we have diversified the SIFSIX-n platform to
attain a metalloporphyrin-functionalized version, CPM-131, with
the formula of ZnSiF6(TPyPFe).17 Notably, the SIFSIX pillar could
also be exchanged by a polyfluorometalate,18 like (NbOF5)2À, thus
fluoride–porphyrin organization permits the use of up to three
types of metal ions in various combinations (including the core
and peripheral coordination sites of porphyrin). Besides multi-
variate MOFs based on the random-location of multi-metals
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
E-mail: linqipu@fjirsm.ac.cn
b College of Materials Science and Engineering, Fujian Normal University, Fuzhou,
Fujian 350007, China
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, and additional characterization figures (SEM-EDS, FT-IR, UV-vis, TGA,
thermal and chemical stability analyses, N2 and CO2 sorption isotherms, etc.) for
the FTPFs. See DOI: 10.1039/c8cc07865c
412 | Chem. Commun., 2019, 55, 412--415
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resulting from their similar coordination preference,19 to the best
of our knowledge, no study has been reported on targeting
hetero-trimetallic MOFs with crystallographic ordering. In view
of their unique synergistic advantages, it is highly desirable to
pursue new heterometallic crystalline materials.
Herein, via substitution of pillar/linker and node in CPM-131,
we have developed a series of fluorinated trimetal-porphyrin
frameworks (termed FTPFs). These are unusual examples of
multivariate MOFs, constructed from copper tetrakis(4-pyridyl)-
metalloporphyrin (TPyP-M) coordination (4,4)-grid layers that
are pillared by the fluorometalate (NbOF5)2À, accordingly also
denoted as Cu–Nb–M (M = Zn, Fe, Ni). Bearing the optimized
pore partition, OMSs in eclipse-arranged metalloporphyrins,
plus electrostatic fluoropillars, these materials exhibit reversible
adsorption with high capacity and affinity for CO2 at 0 1C and
room temperature. These FTPFs have good chemical stability in
various organic solvents and tolerance to water over a wide pH
range. The good adsorption capacity for CO2 and high-density
metalloporphyrinic OMSs make them highly efficient and
reusable heterocatalysts for CO2 coupling with epoxides to form
cyclic carbonates under mild conditions.
Fig. 1 (a) Local coordination environments of the three different metal
sites labelled A (from {M(Pyridine)4F2}), B (from {MOxF6Àx}) and C (from
metalloporphyrin) in different colours. (b) Network topology of the FTPFs.
(c) Crystal structure of the FTPFs. H atoms are omitted for clarity.
Square-shaped microcrystals of FTPFs, viz. Cu–Nb–M
(M = Zn, Fe, Ni), were prepared by layering an N-methyl-2-
pyrrolidone solution of TPyP-M in a vial onto an ethylene glycol
solution of CuNbOF5. With a size of B1 mm, as shown in the
scanning electronic microscopy (SEM) images (Fig. S3–S5, ESI†), Brunauer–Emmett–Teller (BET) surface area of 336 m2 gÀ1
all samples of CuNbOF5-metalloporphyrins were not suitable for (a corresponding Langmuir surface area of 905 m2 À1) were
g
structure determination by single-crystal X-ray diffraction observed, which are comparable to those of CPM-131. An
(SCXRD). Guided by the prototype of the previously-described evaluation using the Horvath–Kawazoe model revealed a pore
CPM-131, their structures with the space group P4/mmm were size distribution centered at 4.1 Å, slightly lower than 4.3 Å of
constructed by using the Reflex Plus implemented in the CPM-131, probably due to a relatively shorter proximal distance
Materials Studio (detailed modeling in Section S3, ESI†).20 For between the adjacent fluorine centers and the consequently
the representative Cu–Nb–Ni, the experimental powder X-ray contracted aperture size of the channels by using a pillar with a
diffraction (PXRD) pattern was indexed and compared with that bigger cation (Nb5+ instead of Si4+). The PXRD patterns revealed
refined via the Rietveld method, resulting in good agreement that no change occurs during immersion in common solvents,
factors (Rp = 8.35%, Rwp = 5.62%, Fig. S2, ESI†). The fitted unit such as MeOH, EtOH, DMF, (Me)2CO, MeCN, etc. for 24 h
cell parameters (a = b = 13.6484 Å, c = 8.0870 Å) are nearly (Fig. S13, ESI†). Moreover, it was found that Cu–Nb–Ni can tolerate
consistent with those of CPM-131, but with a slightly bigger pH = 3–11 aqueous solutions (Fig. S14 and S15, ESI†). The stability
c-value attributed to the relatively-longer metal–fluorine distance of the FTPFs in these chemical conditions makes them attractive
(1.97 Å for Nb-F versus 1.68 Å for Si-F). As anticipated, Cu–Nb–Ni for use in certain practical applications. The hetero-trimetal
consists of CuNbOF5 chains formed in situ running along the coexistence in each sample was also supported by the induc-
c-direction. Such infinite rod-like chains are bridged to each tively coupled plasma optical emission (ICP-OES), and energy
other through metalloporphyrinic spacers, giving rise to a 4,6- dispersive X-ray spectroscopy (EDS) results (Fig. S6–S8, ESI†). The
connected fsc-net with elliptical channels (Fig. 1), formulated solid-state ultraviolet visible (UV-vis) diffuse reflectance spectra
as CuNbOF5(TPyPNi). Apart from the fluoride-decorated pore (DRS, Fig. S10, ESI†) of the materials indicated a significant effect
environment, another prominent structural aspect in FTPFs is of the porphyrinic metal sites on the electronic structure and the
the eclipsed array of metalloporphyrins, with an interplanar absorption nature.
spacing equal to the c-axial length.
The unique pore geometry and chemistry prompted us to
Thermogravimetric analysis (TGA) and temperature-dependent evaluate the CO2 capture performance of the FTPFs. Low-
PXRD (Fig. S11 and S12, ESI†) indicated that the structure of pressure CO2 sorption data were collected at 273 and 298 K
Cu–Nb–Ni was undamaged up to 250 1C in air. Its porosity was (Fig. 2b and Fig. S19, S20, ESI†). The appreciable CO2 uptake of
examined using the N2 adsorption–desorption isotherms at representative Cu–Nb–Ni at 1 atm and 273 K was found to be
77 K (Fig. S16–S18, ESI†). Prior to testing, the as-synthesized 87 cm3 gÀ1 (equivalent to 3.9 mmol gÀ1 or 171 mg gÀ1), while
samples were exchanged with MeOH for 24 h and then dried 63 cm3 gÀ1 (equivalent to 2.8 mmol gÀ1 or 124 mg gÀ1) was
at 120 1C under vacuum for 12 h to remove the remnant measured at 1 atm and 298 K, which are comparable to those of
solvent molecules. An N2 uptake of 175 cm3 gÀ1 (STP) and a the SIFSIX-n series and also at the high end among porphyrin-based
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Fig. 2 (a) As-synthesized and simulated powder X-ray diffraction (PXRD)
patterns for Cu–Nb–M (M = Zn, Fe, Ni). (b) CO2 uptake for Cu–Nb–Ni at
273 and 298 K, respectively, with the inset showing the CO2 isosteric heat
of adsorption (Qst).
Fig. 3 1H NMR spectra before and after 12, 24, and 48 h CO2 cycloaddi-
tion reaction of propylene epoxide (from bottom to top). Peaks marked
with * are for TBAB. Un-marked peaks are for solvent.
MOFs.11a,13b,14b,15b High affinity for CO2 is shown by the sharp
increase in the CO2 uptake in the low pressure region, which
could be attributed to the enhanced isosteric heats (Qst). The Qst
value was calculated by fitting the adsorption data collected diffusion of the large-sized epoxide substrate molecules into the
at 273 and 298 K to the virial model (Fig. S21–S24, ESI†). At narrow square channel of Cu–Nb–Ni. In particular, nearly quan-
zero coverage, the adsorption enthalpy of CO2 for Cu–Nb–Ni is titative conversion of 2-(chloromethyl)oxirane to 4-chloromethyl-
29 kJ molÀ1, close to that of the other isostructural analogues 1,3-dioxolane-2-one was achieved over a period of 48 h (entry 4,
(28 and 29 kJ molÀ1 for Cu–Nb–Fe, and Cu–Si–Fe, respectively) Table S2, ESI†), further indicating the steric-hindrance effect.22
and SIFSIX-pcu-type materials, suggesting a strong interaction In consideration of the heterogeneous nature of the FTPFs, the
of CO2 with the pore surface of the FTPFs. The Qst was also stability and reusability of the selected Cu–Nb–Ni were assessed
relatively constant, indicative of homogenous energy sites over by performing the cycloaddition of 2-methyloxirane with CO2.
the full range of CO2 loading. The results show that OMSs, The reusability test (Fig. S28, ESI†) shows that Cu–Nb–Ni can be
fluoropillars, and small pore sizes likely contribute to the uptake recycled at least four successive times with a slight decrease in
and Qst of CO2.
the catalytic activity, and the selectivity of the product remains
FTPFs with high CO2-trap, structural robustness, and almost constant (497%) during each recycling process. The
eclipsed array of uniformly implanted active sites are ideal PXRD result of the reused sample also confirmed that its
candidates for chemical coupling of CO2 with epoxides to form structure showed no obvious change. This clearly verified that
cyclic carbonates. In a typical experiment at room temperature it is a robust catalyst candidate for CO2 fixation.
and 1 atm pressure in the presence of tetra-n-tert-butylammonium
Based on the above observations and some previous reports,11d,22
bromide (TBAB) as the co-catalyst, the yields of the cyclic carbon- a tentative mechanism is proposed for the cycloaddition of
ates produced from the coupling reaction of CO2 with epoxides epoxides and CO2 into cyclic carbonates catalyzed by these
were determined by the 1H NMR technique. A comparative study FTPFs. As illustrated in Fig. S29 (ESI†), the coupling reaction
showed that Cu–Nb–Ni exhibited the highest catalytic efficiency is initiated by binding of the epoxide with the Lewis acidic site
and is thus described in detail (Table S2, ESI†). As shown in Fig. 3, in the metalloporphyrinic core through the formation of the
due to the dependence of the coupling conversion on reaction M–O bond. Subsequently, nucleophilic BrÀ generated from
time, Cu–Nb–Ni demonstrated high catalytic activity for the TBAB attacks the less-hindered side of the coordinated epoxide
cycloaddition of 2-methyloxirane and CO2 into 4-methyl-1,3- to give rise to the ring-opening of the epoxy species. It should be
dioxolane-2-one carbonate at room temperature and under noted that the electrostatic potential generated around the
1 atm CO2 pressure with a yield of 499% over 48 h, which fluorides could trap CO2 to primarily bind in a parallel orienta-
can be attributed to the synergistic effect of the catalytic OMSs tion along the CuNbOF5 chains, and metalloporphyrinic OMSs
of the metalloporphyrin and specific CO2-trap sites. This com- are also the ideal trap-site for CO2, and even can further activate
pares favourably to the very highest values reported for MOFs CO2, which facilitates the coupling of CO2 and the opened epoxy.
and porphyrin-based materials (Table S3, ESI†).21
Accordingly, an alkylcarbonate anion is formed upon interaction
We examined the scope of epoxide substrates for chemical of CO2 with the activated epoxy ring, which is then converted
coupling with CO2 to cyclic carbonates under the similar into the corresponding cyclic carbonate through the ring closing
conditions. With the increase of the substituted alkyl chain step. We expect that the abundance of face-to-face pairwise-
sizes of epoxides, a dramatic yet steady decrease in the yield of oriented metalloporphyrinic OMSs could boost the synergistic
cyclic products was observed for Cu–Nb–Ni, as indicated by effect with fluoride lined within the pore-wall and co-catalyst
67% formation of 4-ethyl-1,3-dioxolan-2-one (entry 5, Table S2, TBAB in the confined space, thus promoting the cycloaddition
ESI†) and 32% formation of 4,4-dimethyl-1,3-dioxolan-2-one reaction, which thereby leads to high catalytic activity of FTPFs
from 2-ethyloxirane and 2,2-dimethyloxirane (entry 6, Table S2, for the chemical conversion of CO2 into cyclic carbonates under
ESI†), respectively. This could be possibly ascribed to the limited ambient and solvent-free conditions.
414 | Chem. Commun., 2019, 55, 412--415
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In conclusion, by a trisubstitution strategy, a novel class of
ternary hetero-metallic MOFs (FTPFs) has been successfully
synthesized and fully characterized. They contain (4,4)-grid-like
metalloporphyrin layers that are linked by linear Cu2+-(NbOF5)2À
chains to form an extended porous structure. Due to the
existence of a contracted pore-size, fluorine-rich channel surface,
and strong Cu–F/N bonding, the resultant FTPFs are found to be
robust in some common solvents including water, and also
resistant to heat (up to 250 1C in air). Featuring a high density
of eclipse-distributed open metal sites (OMSs) and the advan-
tages of using the SIFSIX-analogs, these materials are demon-
strated to have high CO2 trap capacities (up to B9 wt% uptake at
273 K under 1 atm), and excellent catalytic efficiency (499%
conversion) for the chemical fixation of CO2 and epoxides with
good recyclability under solvent-free conditions. Further studies
will be conducted on photoelectrochemical catalysis, and the
effect of decoration with a hetero-trimetal on the performance
will be addressed.
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This work was supported by the National Natural Science
Foundation of China (Grant No. 21501028), the National Science
Foundation of Fujian Province (Grant No. 2017J01039), the
Strategic Priority Research Program of Chinese Academy of
Sciences (Grant No. XDB20000000) and the Hundred-Talent
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There are no conflicts to declare.
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J. A. Mason, X. Kong, E. D. Bloch, D. Gygi, A. Dani, V. Crocella,
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