Room-temperature acetylene hydration by a Hg(II)-laced metal-organic framework

Article abstract of DOI:10.1039/c5cc03943f

Thiol (-SH) groups within a Zr(iv)-based metal-organic framework (MOF) anchor Hg(ii) atoms; oxidation by H₂O₂ then leads to acidic sulfonate functions for catalyzing acetylene hydration at room temperature.

Full text of DOI:10.1039/c5cc03943f

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Room-temperature acetylene hydration by a
Hg(II)-laced metal–organic framework†‡
Cite this:DOI: 10.1039/c5cc03943f
Ka-Kit Yee,a Yan-Lung Wong,a Meiqin Zha,a Ramesh Y. Adhikari,b
Mark T. Tuominen,b Jun Hec and Zhengtao Xu*a
Received 12th May 2015,
Accepted 27th May 2015
DOI: 10.1039/c5cc03943f
www.rsc.org/chemcomm
Thiol (–SH) groups within a Zr(IV)-based metal–organic framework here simple oxidation to effect the conversion into sulfonic acid
(MOF) anchor Hg(II) atoms; oxidation by H2O2 then leads to acidic and metal sulfonate functions.6 Such conversion is intended to
sulfonate functions for catalyzing acetylene hydration at room liberate the proton and metal centers from the thiol groups,
temperature.
and to create strong acidity and reactivity properties within the
MOF pores.
Among the various ways to functionalize the porous solids of
One major advantage of this method lies in the dense array
metal–organic frameworks (MOFs),1 the versatile reactivity of of sulfonate units that can be installed (e.g., two per linker,
the thiol (–SH) group offers unique advantages.2 For example, as from DMBD). Previously, MOF systems (e.g., MIL-101(Cr),
thiols as strong soft donors readily take up various metal ions, MIL-53(Al)7 and others8) had been directly sulfated (e.g., by
which closely bears on the removal of heavy metal ions,3 and ClSO3H); but the sulfonate group, once attached, deactivated
on the creation of electroactive/semiconducting2b,4 or catalytic the aromatic core and thus hindered further sulfation (i.e., the
sites5 (e.g., mimicking the iron–sulfur, copper–sulfur proteins) number of installed sulfonate is limited). In another approach,
within the MOF matrices. Notably, recent exercises (e.g., using sulfated ligands and pristine ligands as a mixture were reacted
2,5-dimercapto-1,4-benzenedicarboxylic acid, H2DMBD, Chart 1) directly with metal ions to form the framework, but potential
indicated that dense arrays of free-standing thiol groups can be interference from the sulfonate group for binding with the
built into the host net when chemically very hard ions [like Eu(III), metal ions (and thus disrupting the network construction)
Zr(IV) or Al(IV); i.e., these tend to stay unbonded with the thiol often limits the fraction of the sulfonated ligands. More
group] are chosen to link the carboxyl groups.3a,5c
broadly, the oxidation of the metal thiolate moiety generates
As part of our ongoing efforts to further exploit the thiol/ in situ metal sulfonate functions on the host net, whereas for
thiolate groups thus installed within MOF solids, we utilize other sulfated frameworks, additional steps of ion exchange are
necessary for inserting exo-framework metal ions.9 As thiol
groups readily bind various metal ions, our approach offers
flexible control over the amount and type of metal ions to be
deployed in the pores. For illustration, we present here a MOF
solid with Hg2+-sulfonate functions as an especially active
catalyst for the acetylene hydration reaction.
First we introduce the three major stages of sample prepara-
tion. (1) Reaction of ZrCl4 and H2DMBD under solvothermal
Chart 1 Molecule H2DMBD for building the ZrDMBD solid, M1–M4 for
the selective uptake test with the ZrBDSO3-Hg solid, and M5 for a
hydration test benchmarked against acetylene.
conditions yielded a crystalline powder sample of the thiol-laced
ZrDMBD framework (a similar procedure to that reported,3a but
a Department of Biology and Chemistry, City University of Hong Kong,
with N2 protection to minimize the oxidation of the –SH groups).
83 Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail: zhengtao@cityu.edu.hk
b Department of Physics, University of Massachusetts, Amherst,
The composition of the ZrDMBD sample features a Zr6O4(OH)4Á
Massachusetts 01003, USA
c School of Chemical Engineering and Light Industry, University of Technology,
(DMBD)6 framework with DMF and H2O guests (see the ESI‡
for details; see also Fig. 1, left for a schematic of the framework).
(2) Treatment of ZrDMBD with an aqueous solution of HgCl2 led
to the mercurated solid ZrDMBD-Hg, which was found to contain
a 1 : 6 : 4 Zr6O4(OH)4/DMBD/Hg ratio (together with ClÀ, DMF and
H2O; equivalent to w/w 21.4% for Hg; see the ESI‡ for details).
Guangzhou 510006, Guangdong, China
† Dedicated to the 60th birthday of Prof. Stephen Lee.
‡ Electronic supplementary information (ESI) available: Experimental proce-
dures; network synthesis and activation; SEM photographs; elemental analysis,
CO2 sorption, TGA, IR/Raman, and PXRD data. See DOI: 10.1039/c5cc03943f
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thiol groups in ZrDMBD were oxidized by H2O2 into sulfonic
acid groups—see the ESI‡ for the procedure and characteriza-
tion (Fig. S1–S4, ESI‡) of the resultant ZrBDSO3H solid, and
for the measured proton conductivity (Fig. S5, ESI;‡ the con-
ductivity can be improved by H2SO4 treatment on the powder
sample, as shown in a recent study6). Notably, ion exchange
experiments on the ZrBDSO3H solid thus obtained indicated
less Hg2+ insertion. For example, even after the ZrBDSO3H solid
was heated in concentrated Hg(NO3)2 and HgCl2 solutions for
18 hours (ESI,‡ for the procedures and patterns d and e in
Fig. S4), the Hg content in the solid was found by the diphenyl-
thiocarbazone extraction method analysis to be 2.8% and 3.0%
(significantly lower than the values of 11.8–20.4% in the
ZrBDSO3-Hg samples obtained from H2O2 oxidation on
ZrDMBD-Hg). Such tests help to highlight ZrDMBD-Hg as an
effective precursor to highly mercurated ZrBDSO3-Hg products.
An additional test also helps to demonstrate that the Hg2+
ions are located inside of the ZrBDSO3-Hg pores. Specifically, a
mixture solution of four mercaptan molecules (in CD2Cl2;
M1–M4 are shown in Chart 1) of increasing sizes was treated
by ZrBDSO3-Hg (containing Hg in excess relative to the thiols)
at room temperature (rt). While NMR measurement indicated
complete removal of the smallest M1 (via the strong thiol-Hg
interaction) from the solution within 12 h, the concentrations
of the larger M2, M3 and M4 remained unchanged (Fig. S6,
ESI‡). Such size selectivity indicates that the Hg2+ ions are not
accessible to the larger M2–M4 mercaptans under these condi-
tions, and points to potential applications in thiol uptake (see
also Fig. S7, ESI‡ for the sorption test on 2-mercaptoethanol).
The catalytic efficacy of the ZrBDSO3-Hg solid towards
acetylene hydration of (C2H2) was revealed in a simple reaction
setup. Namely, by stirring at rt for a few hours a mixture of the
ZrBDSO3-Hg solid (e.g., 100 mg, containing 0.059 mmol of Hg)
and water (e.g., 2.7 mL; 0.15 mol) in a 1000 mL Schlenk flask
filled with acetylene (C2H2; atmospheric pressure; about
45 mmol, 1.2 g), an acetaldehyde content (in the form of
acetaldehyde and the hydration product ethane-1,1-diol;
Fig. 1) of 5.78% (equivalent to a turnover number of 61 for
Hg, Table 1) can be achieved in the water phase (supernatant;
see Fig. 3 for the NMR spectrum). The product concentration
compares well with the values (about 2–7%) normally obtained
in industrial reactor settings using the homogeneous catalyst of
Fig. 1 Schematics for H2O2 oxidation of the ZrDMBD-Hg net (left) into
the ZrBDSO3-Hg net (right) and the use of the latter in catalysing the
hydration of acetylene. The host net is simplified as a tetrahedral cage, with
each Zr6O4(OH)4 cluster shown as purple spheres.
The substantial Hg presence in ZrDMBD-Hg was also revealed by
the absence of S–H in the IR/Raman spectra (Fig. S1 and S2,
ESI‡), as well as the large change in the intensity profile of the
PXRD patterns (cf. patterns b and c in Fig. 2). (3) Oxidation by
H2O2 on ZrDMBD-Hg converts the thiolate groups into sulfonate
functions; the resultant solid (denoted as ZrBDSO3-Hg) was
found by elemental analyses (see ESI‡) to feature the composi-
À
tion, Zr6O4(OH)4Á[C8H2O4(SO3
)0.8(SO3H)1.2]6ÁHg2.4Á(H2O)50, with
the formation of the sulfonate functions (–SO3À) being indicated
by the IR (Fig. S1, ESI‡) and NMR spectra (Fig. S3, ESI‡). In spite
of the drastic oxidative transformation, the structural integrity of
the host net was found by PXRD to be intact (Fig. 2, pattern d).
Compared with the ZrDMBD-Hg sample, the Hg content in
ZrBDSO3-Hg (11.8%) is lower, i.e., about 40% of the Hg leached
away during the H2O2 treatment; however, such Hg(II) leaching
from the solid host can be readily suppressed by using a H2O2
solution containing dissolved Hg(NO3)2, e.g., with the resultant
Hg content being 20.4% (see SI for the procedure and PXRD
pattern g in Fig. S4, ESI‡). For the following catalytic study,
the ZrBDSO3-Hg sample (i.e., with a 6 : 2.4 linker/Hg ratio) was
prepared by the simple H2O2 treatment—without the addition of
Hg(NO3)2 solute.
Can one use ion exchange (i.e., with ZrBDSO3H) instead to
access the ZrBDSO3-Hg solid? To explore this possibility, the
Table
1
Efficiencies of ZrBDSO3-Hg as
a
Lewis acid for hydration
of acetylene
Hg content
Total
Product
Catalyst
in MOF (wt%) Hg (mg) conc. (wt%) TONb
ZrBDSO3-Hg cycle 1 11.8a
ZrBDSO3-Hg cycle 2c 11.9a
ZrBDSO3-Hg cycle 3c 11.4a
11.8
8.3
6.9
5.78
5.95
5.68
1.14
61
62
59
13
HgSO4/H2SO4
N/A
12.3
Fig. 2 X-ray powder patterns (Cu Ka = 1.5418 Å) of (a) a simulation from
a
a
structure model of ZrDMBD; (b) an as-made ZrDMBD sample;
Mercury contents were determined using the diphenylthiocarbazone
b
(c) ZrDMBD-Hg; (d) ZrBDSO3-Hg; (e–g) ZrBDSO3-Hg after the 1st, 2nd
and 3rd cycle of the acetylene hydration catalysis test, respectively.
extraction method. TON is defined as the number of products formed
per mercury atom. See the ESI or the cycling procedure.
c
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homogeneous HgSO4/H2SO4 systems, such side reactions were
severe, forming a large amount of Hg(0)/Hg(I)-containing
sludge that had to be periodically drained from the industrial
reactor. In the case of ZrBDSO3-Hg, as an indication of the
ongoing reduction of Hg(II) ions, the white catalyst solid
gradually developed a grey color, with a concomitant decrease
in catalytic activity. The used ZrBDSO3-Hg solid can be reacti-
vated simply by immersion in a mixed solution of H2O2, HNO3,
and Hg(NO3)2ÁH2O at rt (e.g., for 15 minutes; see the ESI‡ for
details). The color of the solid returned to white, and the Hg
content in the regenerated ZrBDSO3-Hg solid was found to be
11.9% (cf. 10.0% in the used catalyst), indicating the efficacy of
the solid host in retaining the Hg(II) guests. The ZrBDSO3-Hg
catalyst thus regenerated retains the structural integrity of the
host lattice (e.g., see PXRD patterns e–g in Fig. 2) and continues
to be highly active for C2H2 hydration, with turnover numbers
(e.g., about 62) comparable to the first round (see Table 1).
To demonstrate that the catalysis takes places within the
pores of the ZrBDSO3-Hg solid, we examine the reactivity of a
larger substrate, 5-propargyloxyisophthalic acid (M5), which,
Fig. 3 1H NMR spectrum of the supernatant of acetylene reaction (dis-
solved in D2O with CH3COOK added as an internal standard). The internal
standard peak at 1.88 ppm (singlet) and the product peak at 9.65 ppm
(quartet) from acetaldehyde and 1.30 ppm (doublet) from ethane-1,1-diol
were used to calculate the yield and TON.
HgSO4/H2SO4 solutions,10 which nevertheless involve the more with a cross-section above 7 Å, is too bulky to enter into the host
sophisticated conditions of constant flow of C2H2 and significant net (pore opening B5 Å). To promote the solubility, a 2 : 1 THF/
heating (at 70–90 1C). As a solid state catalyst, ZrBDSO3-Hg, H2O solvent was used for the hydration reaction. No reaction
with both the H+ and Hg2+ agents lodged within its host net, was observed after stirring a mixture of the ZrBDSO3-Hg solid
provides the added advantages of non-corrosive (e.g., water) and the THF/H2O solution of M5 at rt for up to 8 hours (see the
conditions and easy product isolation. Such advantages stand ESI‡ for details including NMR and TLC monitoring, e.g.,
out even in comparison with the mainstream Wacker process,11 Fig. S8 and S9, ESI‡); by contrast, when the homogeneous
wherein the highly oxidizing and corrosive nature of the aqueous system of HgSO4/H2SO4 was used instead, the homogeneous
PdCl2/CuCl2 catalyst (aggravated under the heated conditions) conditions led to complete hydration of M5 (e.g., also at rt and
remains a concern.
within 8 h; see the NMR spectrum C of Fig. S8, ESI‡). This
For a more direct benchmark, a homogeneous catalyst observation suggests that catalytic activity of ZrBDSO3-Hg
consisting of HgSO4 (18.2 mg, 0.061 mmol) dissolved in entails substrates penetrating the host net, and the Hg2+ ions
H2SO4 (18%, 2.7 mL) was examined under the same reaction operate from within the pores of the host net.
conditions (e.g., 1.0 atm of C2H2, rt). Notice that both the Hg
To sum up, the thiol function in ZrDMBD proves especially
quantity and the solution volume are set to the same values as useful for accessing the Hg2+-laden solid of ZrBDSO3-Hg. The
in the above test of the ZrBDSO3-Hg solid. In this homogeneous mild conditions (at rt, in water) for C2H2 hydration attest to
setting, the acetaldehyde products amounted to only 1.14% the enhanced activity of ZrBDSO3-Hg as a solid state catalyst.
(TON: 12.8) in the solution, less than 1/5 of the value achieved The catalytic activity likely results from the conjoint working
by the ZrBDSO3-Hg solid catalyst. The efficiency of the of the Hg2+ and –SO3H acid units that are densely arrayed
ZrBDSO3-Hg solid catalyst can be ascribed to the compact within the pores of the host net—e.g., neither ZrBMBD-Hg nor
arrangement of the H+ and Hg2+ agents within the host net, ZrBDSO3H exhibited observable catalytic activity under similar
as well as to the hydrophobic aromatic struts promoting the conditions (Fig. S14, ESI‡). We are working to access similar
C2H2 diffusion into the pores.
MOF materials with larger pores, in order to widen the scope
Also notably, the leaching of Hg(II) from the ZrBDSO3-Hg of alkyne hydration applications.12 Imbedding metal ions in
solid into the water phase (supernatant) is small: e.g., the porous frameworks offers great potential for opening novel
supernatant (i.e., 2.7 mL) was found to contain only 0.11 mg reactivities, and thiol-laced frameworks will remain uniquely
of mercury (equivalent to 40.7 ppm). In other words, less than important in these studies.
1% of the Hg content (11.8 mg) in the ZrBDSO3-Hg catalyst
This work was supported by the City University of Hong
was leached into the water phase during the C2H2 hydration Kong (Project 9667092), the Research Grants Council of HKSAR
process. The small Hg leaching, besides minimizing the environ- (GRF Project 103212) and the National Natural Science Founda-
mental impact from the toxic Hg species, also makes it possible tion of China (21471037, 21201042).
to recover the solid state catalyst for subsequent cycles of
reactions—e.g., reducing the need for re-inserting Hg2+ ions into
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