Mimicking mineral neogenesis for the clean synthesis of metal-organic materials from mineral feedstocks: Coordination polymers, MOFs and metal oxide separation

Article abstract of DOI:10.1039/c3gc41370e

We present a systematic study of a mild approach for the activation of metal oxides, involving reactivity and self-assembly in the solid state, which enables their solvent-free chemical separation and direct solvent-free and low-energy conversion into coordination polymers and open metal-organic frameworks (MOFs). The approach is inspired by geological biomineralization processes known as mineral weathering, in which long-term exposure of oxide or sulfide minerals to molecules of biological origin leads to their conversion into simple coordination polymers. This proof-of-principle study shows how mineral neogenesis can be mimicked in the laboratory to provide coordination polymers directly from metal oxides without a significant input of either thermal or mechanical energy, or solvents. We show that such "aging" is accelerated by increased humidity, a mild temperature increase and/or brief mechanical activation, enabling the transformation of a variety of high-melting (800 °C-2800 °C) transition (Mn^II, Co^II, Ni ^II, Cu^II, and Zn) and main group (Mg and Pb^II) oxides at or near room temperature. Accelerated aging reactions are readily scaled to at least 10 grams and can be templated for the synthesis of two-dimensional and three-dimensional anionic frameworks of Zn, Ni(ii) and Co(ii). Finally, we demonstrate how this biomineralization-inspired approach provides an unprecedented opportunity for solvent-free chemical segregation of base metals, such as Cu, Zn and Pb, in their oxide form under mild conditions.

Full text of DOI:10.1039/c3gc41370e

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Mimicking mineral neogenesis for the clean synthesis
of metal–organic materials from mineral feedstocks:
coordination polymers, MOFs and metal oxide
separation†
Cite this: DOI: 10.1039/c3gc41370e
Feng Qi,a Robin S. Steinb and Tomislav Friščić*a
We present a systematic study of a mild approach for the activation of metal oxides, involving reactivity
and self-assembly in the solid state, which enables their solvent-free chemical separation and direct
solvent-free and low-energy conversion into coordination polymers and open metal–organic frameworks
(MOFs). The approach is inspired by geological biomineralization processes known as mineral weather-
ing, in which long-term exposure of oxide or sulfide minerals to molecules of biological origin leads to
their conversion into simple coordination polymers. This proof-of-principle study shows how mineral neo-
genesis can be mimicked in the laboratory to provide coordination polymers directly from metal oxides
without a significant input of either thermal or mechanical energy, or solvents. We show that such
“aging” is accelerated by increased humidity, a mild temperature increase and/or brief mechanical acti-
vation, enabling the transformation of a variety of high-melting (800 °C–2800 °C) transition (MnII, CoII,
NiII, CuII, and Zn) and main group (Mg and PbII) oxides at or near room temperature. Accelerated aging
reactions are readily scaled to at least 10 grams and can be templated for the synthesis of two-dimen-
sional and three-dimensional anionic frameworks of Zn, Ni(II) and Co(II). Finally, we demonstrate how this
biomineralization-inspired approach provides an unprecedented opportunity for solvent-free chemical
segregation of base metals, such as Cu, Zn and Pb, in their oxide form under mild conditions.
Received 11th July 2013,
Accepted 30th August 2013
DOI: 10.1039/c3gc41370e
www.rsc.org/greenchem
coordination polymers and MOFs4 from soluble salts has been
explored using sonochemistry,5 microwave synthesis,6 electro-
Introduction
The development of environmentally-friendly and sustainable chemistry,7 mechanochemical synthesis8 and near-critical
synthesis is one of the central tasks of modern science and water conditions,9 we are interested in developing mild pro-
technology.1 As the number of potential applications of coordi- cesses for directly transforming metal minerals or mineral
nation polymers and metal–organic frameworks (MOFs) grows concentrates into metal–organic materials. Metal oxides are
to include hydrogen or methane storage, carbon sequestration, recognized by researchers10 and industry3 as the ideal feed-
catalysis and even light-harvesting,2 it becomes of critical stock for inexpensive and clean synthesis. Also, the develop-
importance to address how such materials can be manufac- ment of mild methods for metal oxide and sulfide
tured in a clean, sustainable and scalable manner from slightly transformation and separation offers benefits to mineral pro-
soluble natural feedstocks (e.g. mineral concentrates consist- cessing and refining industries by improving the energy- and
ing of metal oxides or sulfides), rather than from convention- solvent-intensive pyro- and hydrometallurgical protocols.11
ally used derivatives that are also toxic or corrosive (metal
However, metal oxides are rarely used as starting materials
nitrates, chlorides, and acetates).3 While the clean synthesis of due to their low solubility, high melting points and generally
inert nature resulting from high lattice energies (typically
4–6 MJ mol−1). Transformations of metal oxides into
metal–organic materials are energy-intensive, e.g. conversion
of ZnO or CoO into porous frameworks via melt reactions can
last for several days at 120 °C–160 °C.12 Consequently, acti-
vation of metal oxides near room temperature and without
bulk solvents is a challenge that must be met by non-conven-
tional approaches. Although mechanochemistry8 has recently
enabled the conversion of main group or d10-oxides (ZnO,
aDepartment of Chemistry, McGill University and FRQNT Centre for Green Chemistry
and Catalysis, 801 Sherbrooke St. W., Montreal, H3A 0B8 Canada.
E-mail: tomislav.friscic@mcgill.ca; Fax: +1-514-398-3757; Tel: +1-514-398-3959
bBruker Ltd, 555 Steeles Avenue East, Milton, ON L9T 1Y6, Canada
†Electronic supplementary information (ESI) available: Crystallographic infor-
mation for (pa)2(ox) in CIF format. Additional PXRD, FTIR-ATR, SSNMR and
thermogravimetric data. CCDC 933546. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3gc41370e
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MgO, CdO, and Bi2O3)13–18 into MOFs and pharmaceuticals, it and biomimetic significance as the dominant type of
has been less efficient with transition metal oxides. Instead, “organic” secondary minerals and are of continuous interest
mechanosyntheses of coordination compounds of Co, Ni or as functional (magnetic, proton conducting) materials.28
Cu use acetates, carbonates or halides.19,20
There is a large variety of framework topologies accessi-
We are exploring a novel and potentially general methodo- ble through oxalate ligands, providing an opportunity to
logy for converting transition metal oxides into metal–organic explore how reactions of geological weathering can be adapted
derivatives, which minimizes energy use and completely for the synthesis of open MOFs.29 The most significant metal
avoids organic solvents. Microorganisms, such as lichens, can oxalate topologies are the neutral 1-D polymer Mox·2H2O in
transform inert minerals into metal–organic derivatives by monoclinic α-dihydrates of divalent metal oxalates
excreting small molecules known as lichen acids.21 Such geo- (Scheme 1a),30 and the open two-dimensional (2-D) honey-
logical biomineralization, known as mineral weathering (neo- comb and three-dimensional (3-D) topologies31 of the anion
2−
genesis), is essential for the formation of secondary metal– M2(ox)3 (M = divalent metal ion) (Scheme 1b). Formation of
2−
organic minerals (also called “organic” minerals).22 For 2-D or 3-D M2(ox)3 MOFs is controlled by templates and
example, oxalic acid (H2ox) generated by lichens or found as depends on the configuration of the octahedral M(ox)3 sub-
ammonium oxalate in guano can slowly convert copper ores units.32 Formation of oxalate MOFs in solution was extensively
into the copper(II) oxalate mineral Moolooite.23,24 Other studied,28,29 and particularly important for this study is the
examples of mineral weathering by microorganisms include work by Cheetham33 and Rao34 on templated solvothermal
readily extractable deposits of metal oxalate biominerals synthesis using organoammonium cations.
humboldtine, lindbergite and glushinskite, based on iron(II),
manganese(II) and magnesium, respectively.25 In vitro experi-
ments have also demonstrated the biomineralization of lead
oxalate by Aspergillus niger or the lichen Diploschistes mus-
Experimental section
corum. Most oxalate biominerals are one-dimensional (1-D) All chemicals were of reagent grade and obtained from com-
coordination polymers (Scheme 1a).
mercial sources. MgO, MnO, CoO, NiO, ZnO and PbO were of
We are exploring whether such weathering of metal oxides 99%+ purity, obtained from Sigma-Aldrich, CuO was of 96%
can be reproduced in the synthetic chemical laboratory and purity, obtained from Riedel-de Haën, and H2ox·2H2O was of
adapted into an environmentally-friendly pathway to metal– 99%+ purity, obtained from American Chemicals Ltd. ZnO and
organic materials. In this study we establish: (1) the conditions MgO have been calcined (400 °C) to remove potential hydro-
that enable the conversion of laboratory-scale samples of xide and carbonate impurities, and were kept in a dry desiccator
metal oxides into metal–organic materials; (2) the applicability over P4O10. Aging reactions were conducted either in a walk-in
of this methodology to a variety of metals and scale-up to incubator held at 45 °C, or at room temperature without a par-
multi-gram amounts; and (3) the use of templating to drive ticular means of temperature control. The samples aged at
such a biomineralization-inspired reactivity for the synthesis high humidity were kept at 98% RH atmosphere established in
of open MOFs.
a Secador® cabinet equilibrated with saturated aqueous
Our work is influenced by that of Byrn and co-workers, who K2SO4. In a typical experiment, 1 mmol of a metal oxide was
noted the complexation of MgO upon aging with pharmaceuti- mixed with 1 mmol of H2ox·2H2O and gently ground in an
cals.26 Our first report on “accelerated aging” demonstrated agate mortar and pestle for 30 seconds. The solid mixture
conversion of ZnO into zeolitic MOFs by aging with imidazole was placed in
a 20 ml open vial and stored under
ligands at high humidity with an acid catalyst.27 For this controlled conditions. All reactions were investigated by
systematic study we selected metal oxalates as models for powder X-ray diffraction (PXRD), thermogravimetric analysis
three principal reasons. Oxalates are of obvious geological (TGA), Fourier-transform infrared attenuated total reflection
(FTIR-ATR) spectroscopy and, in some cases, UV-vis reflec-
tance, solid-state NMR (SSNMR) and differential scanning
calorimetry (DSC).
Thermogravimetric analysis was conducted using a TA
Instruments Q500 Thermogravimetric System with a Pt pan
under a dynamic atmosphere of N2 or air with 40 ml min−1
balance flow and 60 ml min−1 purge flow. The upper tempera-
ture limit ranged from 500 °C to 800 °C with a heating rate of
10 °C min−1. DSC was performed on a TA Instruments Q2000
Differential Scanning Calorimeter using a standard aluminum
pan of 40 μL. Nitrogen flow was set at 50 ml min−1 and the
upper temperature limit ranged from 105 °C to 150 °C with a
constant heating rate of 10 °C min−1
.
UV-vis reflectance was measured on an Ocean Optics Jaz-
Combo spectrometer using an LS-1 Tungsten Halogen Light
Scheme 1
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Source (360–2000 nm), a 0.4 mm fiber optic reflection R400-
Angle-UV probe with an RPH-1 reflection probe holder and a
WS-1 PTFE diffuse reflection standard, from 400 nm to 800 nm.
SSNMR spectra of [pa]2[Zn2(ox)3]·3H2O were recorded on a
standard-bore Bruker Avance III spectrometer operating at
500.13 MHz using a Bruker 4 mm double-resonance probe
spinning at 5 kHz. Spectra were referenced using the chemical
shift of the carbonyl carbon of glycine at 174.1 ppm with
respect to TMS. SSNMR spectra of [pn][Zn2(ox)3]·3H2O were
recorded on a 400 MHz Varian VNMRS spectrometer operating
at 100.52 MHz using a T3HX probe with a 7.5 mm zirconia
rotor spinning at 4.5 kHz. The spectrum was acquired in 1000
scans using cross-polarization for 1 ms and 2 s recycle delay.
PXRD patterns were collected in the 2θ range 3°–60° on a
Bruker D2 Phaser diffractometer using a CuKα (λ = 1.54 Å)
source. Data were analyzed using Panalytical X’pert High-
Score Plus.
Results and discussion
As an initial foray into investigating the aging reactivity of
oxalic acid and metal oxides, we prepared a stoichiometric
1 : 1 mixture of oxalic acid dihydrate (H2ox·2H2O) and ZnO.
The mixture was prepared by gently grinding the reactants in a
mortar and pestle for 30 seconds. Care was taken to avoid
harsh grinding that could cause uncontrolled mechanical acti-
vation and, therefore, difficulties in reproducibility. Powder
X-ray diffraction (PXRD) after 24 hours aging at room tempera-
ture (18 °C–22 °C) and a high (98%) relative humidity (RH)
revealed the formation of a new product (Fig. 1).
Comparison with PXRD patterns simulated for known zinc
oxalate structures revealed that the product was the 1-D coordi-
nation polymer zinc oxalate dihydrate (Znox·2H2O, CSD
QQQBOD04) in the α-type structure.35 In one week, the conver-
sion to the 1-D coordination polymer was complete. The com-
position Znox·2H2O was confirmed by thermogravimetric
analysis (TGA) in air, which provided the assessment of weight
fractions for the included water (measured: 18.5%, calculated:
19.0%) and ZnO residue (measured: 42.9%, calculated: 43.0%).
Aging reactivity was further improved by storing the sample
under 98% RH conditions at a mild temperature of 45 °C.
Under such conditions, complete formation of α-Znox·2H2O,
as established by PXRD (ESI Fig. S1†), was achieved in 5 days.
When scaled to 10 grams, the reaction was completed in seven
days (Fig. 1l). Transformation was also detected by Fourier-
transform infrared attenuated total reflection (FTIR-ATR)
spectroscopy (Fig. 2), specifically by the disappearance of O–H
stretching bands of H2ox·2H2O at 3380 cm−1 and 3470 cm−1
and the appearance of the O–H stretching bands of Znox·2H2O
at 3350 cm−1, as well as the appearance of characteristic
Fig. 1 (a) Two coordination polymer chains connected by hydrogen bonds in
Znox·2H2O (CSD QQQBOD04). PXRD patterns for aging reactions of ZnO and
H2ox·2H2O at 98% RH: (b) H2ox·2H2O; (c) simulated for Znox·2H2O; (d) ZnO; (e)
1 day at room temperature; (f) 5 days at room temperature; (g) 1 day at 45 °C;
(h) 5 days at 45 °C; (i) 1 day at room temperature with pre-milling; ( j) 5 days at
room temperature with pre-milling; and (k) 10 days at room temperature with
pre-milling. (l) Completed 10 gram aging syntheses of Znox·2H2O and
Niox·2H2O.
structures was attempted. Rao and Cheetham have established
that solvothermal syntheses of metal oxalates can be directed
towards the formation of open MOFs by organoammonium
templates.36–38 The design to introduce structure templating
into aging reactivity involved mixtures of ZnO, H2ox·2H2O and
either 1,3-propanediammonium oxalate [pn][ox] or propyl-
ammonium oxalate [pa]2[ox] in the respective stoichiometric
ratio 2 : 2 : 1. Such templated aging reactions were expected to
Znox·2H2O bands at 1604 cm−1, 1356 cm−1 and 1311 cm−1
.
Structure templating
After establishing that oxalic acid reacts with ZnO to form the
closely packed Znox·2H2O, a templated synthesis of open
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Fig. 3 (a) Planned synthesis of open anionic 2-D and 3-D zinc oxalate frame-
works; (b) templates pn2+ and pa+; and (c) a hydrogen-bonded layer in the
crystal of (pa)2·(ox), with propyl chains omitted for clarity.
Fig. 2 FTIR-ATR spectra of the final products of aging reactions for: (a) ZnO,
5 days at 45 °C; (b) CoO, 7 days at 45 °C; (c) NiO, 7 days at 45 °C; (d) CuO,
16 days at 45 °C; (e) MgO, 9 days at 45 °C; (f) MnO, 16 days at room temperature;
(g) MnO, 16 days at 45 °C; (h) PbO, 14 days at 45 °C; and (i) MnO, aging of a pre-
milled sample for 1 day at 45 °C. The spectrum of H2ox·2H2O is given under ( j).
yield
known
materials
[pn][Zn2(ox)3]·3H2O37
and
[pa]2[Zn2(ox)3]· 3H2O,36 based on the 2-D and 3-D forms of the
[Zn2(ox)3]2− anionic framework, respectively (Fig. 3a).
The synthesis of [pn][ox] and [pa]2[ox] provided a minor
challenge. The presence of water readily induced the formation
of hydrogenoxalate hydrate salts [pa][Hox]·H2O (CSD QIGCOY,
QIGCOY01)36
and
[pn][Hox]2·2H2O
(CSD
PEPQEG,
PEPQEG01).38 Hydrogenoxalates are not suitable for planned
templating experiments, due to the inadequate ratio of cat-
ionic organoammonium templates and oxalate building
blocks. The synthesis of [pn][ox] and [pa]2[ox] was devised by
reacting anhydrous oxalic acid (obtained by drying H2ox·2H2O
overnight at 125 °C) with the amine in methanol. In ethanol,
the product contained a significant fraction of the hydrated
hydrogenoxalate. The anhydrous nature of the material from
methanol was confirmed by TGA. The structure and compo-
sition of [pn]2[ox] were also confirmed by single crystal X-ray
diffraction (crystallographic data are given in ESI,† section 1).
The structure consists of sheets of pa+ and ox2− held by
N+–H⋯O− hydrogen bonds (Fig. 3c).
After 5 days at room temperature and 98% RH, a PXRD ana-
lysis (Fig. 4a and b) of the mixture of ZnO, H2ox·2H2O and
[pn][ox] in a 2 : 2 : 1 ratio revealed the presence of Znox·2H2O,
but also X-ray reflections consistent with the expected honey-
comb-topology [pn][Zn2(ox)3]·3H2O (Fig. 5). Aging for a total
of 16 days led to the complete disappearance of reflections of
Fig. 4 PXRD patterns for templated aging syntheses of zinc oxalate MOFs: (a)
Znox·2H2O; (b) ZnO, H2ox·2H2O and [pn][ox] in the ratio of 2 : 2 : 1 after 5 days;
(c) Znox·2H2O and [pn][ox] in the ratio of 2 : 1 after 5 days; (d) simulated for
2-D MOF [pn][Zn2(ox)3]·3H2O (SEYQAO); (e) ZnO, H2ox·2H2O and [pa]2[ox] in the
ratio of 2 : 2 : 1 after 5 days; and (f) simulated for 3-D MOF [pa]2[Zn2(ox)3]·3H2O
(SEYQIW). Reactions were done at room temperature, 98% RH. The character-
istic reflection of Znox·2H2O is marked with ‘*’.
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Reactivity of other metal oxides: product hydration state
The reactivity of other metal oxides with oxalic acid was also
explored in 98% RH, at room temperature and at 45 °C. PXRD
analysis revealed the complete conversion of the metal oxide
into a metal oxalate structure for all investigated oxides,
specifically MgO (Fig. 6a and b), MnO (Fig. 6c–g), NiO and
CoO (Fig. 6h–j), CuO (Fig. 6k and l) and PbO (Fig. 6n–p).
Fig. 5 Aging reactivity of ZnO, CoO and NiO towards H2ox·2H2O with and
without organoammonium templates.
ZnO reactant and the Znox·2H2O intermediate, and the diffrac-
tion pattern displayed an excellent fit to that expected for the
pn2+ salt of the Zn2(ox)32− framework. Since Znox·2H2O was an
intermediate in the assembly of [pn][Zn2(ox)3]·3H2O, a room-
temperature aging reaction of [pn][ox] with Znox·2H2O was
attempted. After five days, the reaction mixture fully converted
into [pn][Zn2(ox)3]·3H2O. Quantitative conversion was evident
from PXRD (Fig. 4c and d) and TGA.‡
Aging of a mixture of (pa)2(ox), H2ox and ZnO at room
temperature and 98% RH was explored next. After five days,
PXRD (Fig. 4e and f) and TGA‡ indicated complete conversion
to the expected 3-D framework [pa]2[Zn2(ox)3]·3H2O (CSD
SEYQIW, Fig. 5).36
Cross-polarization magic angle spinning 13C solid-state
NMR (SSNMR, see ESI,† section 5) was consistent with the
product being [pa]2[Zn2(ox)3]·3H2O, displaying the signals of
1
1
ox2− and pa+. The H–13C HETCOR and H–1H INADEQUATE-
style experiments enabled tentative assignment of the 1H SSNMR
spectra of [pa]2[Zn2(ox)3]·3H2O, but with multiple signals
expected from partially disordered water molecules unresolved.
The HETCOR spectrum was consistent with the reported struc-
ture in which the framework interacts mostly with the methyl
and ammonium moieties of the pa+. The 13C SSNMR spectrum
of [pn][Zn2(ox)3]·3H2O was also consistent with the published
structure. Differences between Znox·2H2O, [pn][Zn2(ox)3]·3H2O
and [pa]2[Zn2(ox)3]·3H2O were also evident using FTIR-ATR
(ESI Fig. S13†).
Fig. 6 PXRD patterns for selected reactions at 98% RH: (a) MgO; (b)
Mgox·2H2O (5 days, 45 °C); (c) MnO; (d) Mnox·2H2O (pre-milled MnO, 1 day,
room temperature); (e) Mnox·2H2O (9 days, 45 °C); (f ) simulated α-Mnox·2H2O
(FOZHUX11); (g) simulated γ-Mnox·2H2O (FOZHUX10); (h) NiO; (i) Niox·2H2O
(3 days, 45 °C); ( j) Coox·2H2O (6 days, room temperature, data were collected
using CuKα radiation and energy-discriminating LynxEye detector); (k) CuO; (l)
Cuox·0.5H2O (16 days, 45 °C; (m) Cuox·0.5H2O (pre-milled CuO, 10 days, room
temperature); (n) PbO; (o) Pbox (pre-milled PbO, 9 days, 45 °C); (p) simulated
Pbox (JAHVUJ). (q) Visual comparison of samples of oxide reactants with corres-
ponding oxalate products obtained by aging (98% RH, 45 °C).
‡TGA of [pn][Zn2(ox)3]·3H2O prepared by templated aging gave an excellent fit
for the expected loss of three equivalents of water before 200 °C (measured
weight loss = 10.1%, expected weight loss = 10.3%), followed by the decompo-
sition of the metal–organic residue into ZnO (measured residue weight = 31.3%,
calculated ZnO residue for [pn][Zn2(ox)3]·3H2O = 31.0%). Such a thermal behav-
ior is consistent with that previously reported for solvothermally grown [pn]-
[Zn2(ox)3]·3H2O.36 An excellent fit was also observed for TGA of [pa]2-
[Zn2(ox)3]·3H2O (measured water content = 9.1%, calculated = 9.5%; measured
ZnO residue = 29.4%, calculated = 28.6%). Also see ESI.†
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radiation after energy discrimination by the LynxEye® detector
(Fig. 6j) confirmed the absence of reactants and formation of
Coox·2H2O. Although all transformations in Table 1 are con-
ducted under 98% relative humidity, the obtained products
are not the highest known hydrates of corresponding metal
oxalates. MnO yielded Mnox·2H2O although a higher hydrate42
is also known. Similarly, PbO yields the anhydrous Pbox (CSD
JAHVUJ, JAHVUJ01, Fig. 6n–p)43 although a dihydrate is also
known.44 Copper produced the well-known partially hydrated
structure, despite the existence of a trihydrate.45 For Pb(II) and
Cu(II) the low content of water in products was also evidenced
by FTIR-ATR spectra lacking the broad water absorption band
at 3400 cm−1 (Fig. 2, also ESI Fig. S10 and S12†). These obser-
vations differentiate the herein reported reactions from pre-
viously studied cases of solid–gas reactivity46 where the
reacting vapor becomes a structural component of the metal–
organic product.
Table 1 Time (in days) for the disappearance of X-ray reflections of the metal
oxide (with given melting points) in PXRD patterns of mixturesa with H2ox·2H2O
exposed to 98% RH under different conditions
Reaction time (days)
Oxide (melting point/°C)
RTb
45 °C
RTb with pre-millingc
MgO (2852)
MnO (1945)
NiO (1955)
CuO (1201)
ZnO (1975)
PbO (888)
>9
>16
>25
>25
>5
9d
9d
5d
1d
>10
10e, f
5d
3d
16
3d
>30
9d
>10
a Reactions were conducted using 2 mmol of the oxide and 2 mmol of
H2ox·2H2O. b Room temperature. c Milling was conducted for
5 minutes in a 10 ml stainless steel jar. d PXRD pattern displayed no
reflections of H2ox·2H2O or the metal oxide. e Very weak reflections of
H2ox·2H2O were observable in the PXRD pattern. f An identical result
was obtained at 45 ° and 98% RH after 1 day.
Formation of open structures based on Co and Ni
Table 1 lists the times required for the reflections of the metal
oxide to disappear from the PXRD pattern of the reaction
mixture (Fig. 6, ESI Fig. S2–S7†), demonstrating that aging
reactions are applicable to a variety of metal oxides.
Structure templating by organoammonium cations to form
MOFs was also applicable to aging reactions of CoO and NiO.
Aging of either NiO or CoO (Fig. 7a–j) with oxalic acid dihy-
drate and [pn][ox] and [pa]2[ox] yielded materials that were, as
established by PXRD, isostructural to the hydrated 2-D and
3-D MOFs [pn][Zn2(ox)3] and [pa]2[Zn2(ox)3], respectively.
Framework formation was also corroborated by FTIR-ATR
spectroscopy which demonstrated a high degree of mutual
similarity in the set of three (Zn-, Co- and Ni-based) materials
obtained in the presence of [pn][ox] (ESI Fig. S13†), and also
in the set of three analogous materials obtained with [pa]2[ox]
template. TGA was consistent with compositions [pn]-
[Co2(ox)3]·3H2O, [pn][Ni2(ox)3]·3H2O and [pn][Zn2(ox)3]·3H2O.
For pa+-templated systems, the analyses indicated formulas
[pa]2[Co2(ox)3]·4H2O, [pa]2[Ni2(ox)3]·4H2O and [pa]2[Zn2(ox)3]·
3H2O. It is not clear whether additional water in these systems
is a result of impurity or a real difference in stoichiometric
composition with respect to zinc MOFs. Formation of MOFs
was also investigated by solid-state UV/vis reflectance spec-
troscopy (Fig. 8). Reflectance measurements showed a notable
difference between the absorption spectra of either Coox·2H2O
or Niox·2H2O and products of templated reactions. In contrast,
the reflectance spectra of pa+- and pn2+-templated materials
were similar to each other. Such observations are consistent
with the difference in the coordination environment of the
metal in the 1-D hydrated metal oxalate polymer, where the
cation is octahedrally coordinated by two oxalate and two
water ligands, and MOFs in which the metal is octahedrally
surrounded by three oxalates.
The reactions could also be observed by FTIR-ATR spectro-
scopy (ESI Fig. S8–S12†). The results in Table 1 demonstrate
the ability to transform metal oxides under mild conditions of
temperature, despite their very high melting points. This is
relevant in comparison to mechanochemistry, as analogous
milling transformations of CoO, NiO, MnO or PbO into metal–
organic derivatives have not yet been reported.8 The compo-
sition of the products was elucidated by the similarity of the
PXRD patterns to those simulated for published structures
(Fig. 6, ESI Fig. S2–S7†).
Product composition, indicating complete conversion, was
corroborated by TGA (ESI Fig. S15–S28†). Notably, NiO demon-
strated a similar level of reactivity as ZnO, with complete trans-
formation into Niox·2H2O39 (isostructural to monoclinic
Znox·2H2O) taking place within three days at 98% RH and
45 °C. The structural resemblance of Niox·2H2O to the zinc-
based polymer was also evident from the similarity of the
FTIR-ATR spectra (Fig. 2). Similar to ZnO, the synthesis of
Niox·2H2O was also readily scaled to 10 grams (Fig. 1l). The
slowest reaction was with CuO, for which the oxide was no
longer observable by PXRD after 16 days aging at 98% RH and
45 °C. The product was partially hydrated copper(II) oxalate23
with traces of H2ox·2H2O evident in the PXRD pattern,
explained by the technical (96%) purity of CuO. The product
was analyzed as Cuox·0.5H2O, consistent with previous investi-
gations.40 Like the oxides listed in Table 1, CoO also readily
(in six days) converted into a pink material isostructural
to Niox·2H2O and Znox·2H2O.
Effect of temperature
The diffraction pattern of the product exhibited X-ray
fluorescence which impaired the detection of trace CoO or
H2ox·2H2O using CuKα radiation. TGA was consistent with the
formula Coox·2H2O,41 and the FTIR-ATR spectrum was almost
identical to those of isostructural Ni(II) and Zn oxalates
(Fig. 2). Subsequent PXRD studies using CoKα and CuKα
For all oxides except MnO, switching from room temperature
to 45 °C only increased the reaction rate without changing the
product. However, PXRD measurements using CuKα radiation
(ESI Fig. S6†) showed that MnO produced the α-polymorph of
Mnox·2H2O by aging at room temperature and only the report-
edly more stable γ-form at 45 °C. The α-form is structurally
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Fig. 8 Solid-state reflectance UV-vis spectra of cobalt(II) (top) and nickel(II)
(bottom) oxalate dihydrates obtained by aging reactions.
indicated that the α-polymorph is metastable with respect to
the γ-form. Therefore, the temperature-dependent change in
polymorphic composition of the aging product suggests the
formation of a kinetic product at lower temperatures. The for-
mation of the thermodynamically stable form at 45 °C can be
explained by higher mobility of molecules at a higher temp-
erature. A subsequent PXRD study of the aged samples using a
CoKα X-ray source (Fig. 9b–g), to avoid the X-ray fluorescence
of manganese-based samples, indicated the presence of the
γ-form also in the room temperature product. As the CuKα-
based PXRD and FTIR-ATR measurements were all performed
without delay and are mutually consistent, we explain the
γ-form detected in the PXRD patterns obtained using CoKα
radiation as a result of a spontaneous room-temperature trans-
formation during sample transport and storage. Indeed, the
transformation of the α-polymorph to the γ-form in moist air
was noted by Huizing et al.47 and traces of the γ-polymorph
are visible in the samples of the α-form after 16 days at room
temperature and 98% RH. The α → γ transformation is not
thermally reversible, since heating of the α-form did not result
in a structural transformation, shown by DSC.
Fig. 7 Selected PXRD patterns for templated aging synthesis of Ni(II) (green)
and Co(II) (red) oxalate MOFs: (a) Niox·2H2O; (b) Coox·2H2O; (c) NiO, H2ox·2H2O
and[pn][ox] in the ratio of 2 : 2 : 1 after 5 days; (d) Niox·2H2O and [pn][ox] in
the ratio of 2 : 1 after 5 days; (e) CoO, H2ox·2H2O and[pn][ox] in the ratio of
2 : 2 : 1 after 5 days; (f ) simulated 2-D MOF [pn][Zn2(ox)3]·3H2O (SEYQAO); (g)
NiO, H2ox·2H2O and[pa]2[ox] in the ratio of 2 : 2 : 1 after 5 days; (h) Niox·2H2O
and [pa]2[ox] in the ratio of 2 : 1 after 5 days; (i) CoO, H2ox·2H2O and[pa]2[ox]
in the ratio of 2 : 2 : 1 after 5 days; ( j) simulated 3-D MOF [pa]2[Zn2(ox)3]·3H2O
(SEYQIW). Reactions were done at room temperature, 98% RH. Patterns of
cobalt samples were recorded using CuKα radiation and an energy-discriminat-
ing detector. Characteristic reflections of Niox·2H2O, Coox·2H2O are marked
with *.
similar to the other metal oxalate dihydrates (CSD
FOZHUX11). In the γ-form inorganic connectivity between
octahedrally coordinated Mn2+ ions is established by μ3-brid-
ging oxalate oxygen atoms (Fig. 9a). The difference in poly-
morphic composition was also observable using FTIR-ATR
(Fig. 10). For example, the O–H stretching bands in the α-form
are located in a narrow region between 3300 cm−1 and
3360 cm−1, while the γ-form exhibits two broad maxima at
3090 cm−1 and 3320 cm−1. An earlier solution-based study47
Activation by milling: enabling room temperature reactivity
Although the ability to conduct aging reactions in the absence
of a solvent and at mildly elevated temperature represents a
considerable improvement over solution-based processes, the
energy benefit can be reduced for long reaction times.
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Fig. 10 Selected FTIR-ATR spectra for the reaction of MnO and H2ox·2H2O.
Characteristic bands are labeled for α- (red line) and γ-forms (blue line) of
Mnox·2H2O.
acceleration was also achieved by pre-milling only MnO, and
we speculate the success of pre-milling is due to introducing
defects into the oxide structure.49 If the milled mixture of
MnO and H2ox·2H2O was left to age at 45 °C and 98% RH the
product was a mixture of the α- and γ-forms, indicating that
pre-milling of reactants facilitates the kinetic formation of the
metastable form, whereas increased temperature favors the
thermodynamically stable one.
Fig. 9 (a) Crystal structures of α- (top) and γ-Mnox·2H2O (bottom). PXRD pat-
terns were collected using CoKα radiation: (b) MnO; (c) MnO after 5 min milling;
(d) reaction of MnO and H2ox·2H2O, room temperature, 98% RH; (e) reaction of
MnO and H2ox·2H2O, 45 °C, 98% RH; (f) reaction of pre-milled MnO and
H2ox·2H2O, room temperature, 98% RH; (g) reaction of pre-milled MnO and
H2ox·2H2O, 45 °C, 98% RH. The full set of PXRD patterns is given in the ESI.†
Selective transformation of metal oxides in a mixture
Different metal oxides underwent aging reactions with oxalic
acid at different rates implies the possibility of solvent-free seg-
regation of metal oxide mixtures. A potential benefit of such
selectivity in aging reactions would be the transformation of
one of the metal oxides into a low density metal oxalate, allow-
ing for its separation from the mixture using gravity, rather
than aggressive dissolution reagents. A competitive aging
experiment was conducted with a 1 : 1 stoichiometric mixture
of CuO, the slowest reacting metal oxide in our study, and
ZnO, one of the most reactive oxides in our study. The mixture
contained only one equivalent of H2ox·2H2O. After five days of
aging at 45 °C and 98% RH the initially black reaction mixture
(5 grams) turned gray. PXRD analysis indicated that the
mixture underwent spontaneous and solvent-free chemical
Consequently, we explored a means to further accelerate the
reactions by mechanically activating the reaction mixture by
brief ball milling. Initial milling (Retsch MM400 mill operating
at 30 Hz) of the reaction mixture for 5 minutes enabled most
aging reactions to be completed at room temperature, elimi-
nating the need for thermal treatment.48 The exception was
the reaction of CuO that displayed traces of H2ox·2H2O even
after 10 days. Most notable was the effect of milling on reac-
tions of MnO, which completely converted into Mnox·2H2O in
one day (Fig. 9f, g and 10). The product was the metastable
α-Mnox·2H2O with a minor amount of the γ-form. Reaction
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separation of zinc from copper by selective conversion of ZnO
into Znox·2H2O (eqn (1)):
CuOðsÞ þ ZnOðsÞ þ H2oxꢀ2H2OðsÞ
! Znoxꢀ2H2OðsÞ þ CuoxðsÞ
ð1Þ
PXRD revealed the formation of a crystalline material iso-
structural to Znox·2H2O and the complete disappearance of
X-ray reflections of ZnO (Fig. 11a–g). In contrast, the X-ray
reflections of CuO were clearly observable and reflections of Cu(II)
oxalate did not appear in the pattern, consistent with oxalic
acid selectively reacting with ZnO. TGA (ESI Fig. S29†) indi-
cated the complete conversion of ZnO, while the less reliable
analysis of water loss upon heating indicated the selective con-
version of ZnO over CuO in a stoichiometric ratio of 9 : 1. The
FTIR-ATR spectrum of the reaction mixture was consistent
with the formation of Znox·2H2O, but could not confirm the
absence of copper(II) oxalate (ESI Fig. S14†). The near absence
of copper(II) oxalate in the reaction mixture was confirmed by
UV/vis reflectance spectroscopy (Fig. 11h). The spectrum of the
aged mixture was almost identical to that of pure Znox·2H2O
and different from the spectrum expected for a 1 : 1 mixture of
Cu(II) and Zn oxalates. Thus, UV/vis spectra are consistent with
the absence of any significant amounts of copper(II) oxalate in
the aged mixture. Presumably, the large difference in densities
of Znox·2H2O (density = 2.2 g cm−3) and CuO (density = 6.3 g
cm−3) could allow for mechanical separation of copper from
zinc by gravity,50 which is not possible for a mixture of raw
oxides. Indeed, gravity separation with a “heavy liquid” CH2I2
(density 3.3 g cm−3 50,51
separated the reaction mixture into
)
top and bottom layers that were identified by PXRD as
Znox·2H2O and CuO, respectively (Fig. 11c and d). Thus, com-
bining the solvent-free chemical separation of zinc and copper
oxides with gravity separation allows the separation of copper
and zinc oxides without the need for dissolution50 in aggres-
sive solvents, such as sulfuric acid, or high temperatures. Pre-
liminary results on accelerated aging mixtures of PbO and ZnO
indicate similar selectivity (ESI Fig. S30†).
Conclusions
This proof-of-principle study demonstrated how mimicking the
conditions responsible for mineral weathering can lead to
solvent-free, low-energy approaches to making coordination
polymers and open frameworks from mineral-like feedstocks, as
well as to novel mild methodologies for metal oxide separation,
potentially applicable in mineral industry. The transformation
of a variety of metal (Mg, Mn, Co, Ni, Cu, Zn, and Pb) oxides was
accessible at room temperature in mixtures prepared by gentle
mixing of powders, despite high lattice stabilization energies52
and melting points of the oxides (800 °C–2500 °C). Mechanical
activation or mild temperature increase accelerated the reac-
Fig. 11 PXRD patterns for the aging of a 1 : 1 : 1 mixture of ZnO, CuO and
H2ox·2H2O: (a) after 5 days at 45 °C, 98% RH; (b) bottom layer from gravity sep-
aration using CH2I2; (c) top layer obtained by gravity separation using CH2I2; (d)
Znox·2H2O; (e) Cuox; (f) ZnO; and (g) CuO. (h) Reflectance UV-vis spectra (top
to bottom): average of Cuox and Znox·2H2O; reaction mixture after 5 days at
45 °C, 98% RH; pure Znox·2H2O and pure Cuox. PXRD and UV-vis spectra indi-
cate selective transformation of ZnO into Znox·2H2O, leaving behind CuO. To
highlight the selective conversion, selected X-ray reflections of CuO and
●
Znox·2H2O are designated with * and , respectively.
tions, enabled the complete transformation of all explored metal in aging reactivity of metal oxides enabled the solvent-free
oxides, and provided control over the polymorphic composition separation of metals in their oxide form without strong acids or
of the product (exemplified by reactions of MnO). The difference high temperatures. As transition metal oxides are often slightly
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soluble and highly inert, the ability to conduct their chemical
segregation in a solvent-free manner and under mild conditions
is particularly surprising.53 After chemical separation, the
metals were also mechanically separated by gravity. We believe
that solvent-free separation by aging has considerable potential
J. D. Hayler, G. R. Humphret, J. L. Leazer Jr.,
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301.
for reducing energy and solvent use in mineral industry.50
§
It was demonstrated how the reactions can be directed
towards the formation of 2-D or 3-D open MOFs. In our hands,
this yielded two known open frameworks based on zinc, and four
previously not described frameworks based on Co(II) or Ni(II).
Whereas a potential criticism can be directed towards reactivity
requiring several days, we note that accelerated aging reactions
can take place over times that are comparable to those for con-
ventional solvothermal syntheses, whilst being conducted quanti-
tatively from a metal oxide54 and near room temperature. As
demonstrated here for nickel and zinc oxalates, and by a recently
reported synthesis of porous zeolitic imidazolate frameworks,
accelerated aging can be conducted on a multi-gram scale.
It is worth noting that aging transformations are not
unknown in large-scale applications, as illustrated by the
Dutch process for manufacturing lead white paint. Mechanis-
tic details of the herein presented aging reactivity are not
known. We believe that the mobility of the organic phase plays
a significant role, consistent with reports55 on vapor or gas-
induced transformations of organic and organometallic solids,
such as those by the Atwood56 and Braga groups.57 We are cur-
rently pursuing the reactivity of three- and four-valent metals,
such as Sc, Ti, and Cr.
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Acknowledgements
We acknowledge McGill University, the FRQNT Nouveaux
Chercheurs Fund, the Canada Foundation for Innovation
Leaders Opportunity Fund, the NSERC Discovery Grant and the
FRQNT Centre for Self-Assembled Chemical Structures.
Prof. D. S. Bohle is acknowledged for help in obtaining single
crystal X-ray diffraction data. Bruker UK is acknowledged for
SSNMR measurements. Dr N. Henderson and Mr S. Pharasi,
Bruker AXS, are acknowledged for help with X-ray fluorescence.
Preliminary results that inspired the research were obtained
during the principal investigator’s appointment as a Herchel
Smith Research Fellow at the University of Cambridge. The
support from the Royal Society (Research Grant) and the Herchel
Smith Fund during that period is acknowledged. Advice from
Prof. A. K. Cheetham, University of Cambridge, is appreciated.
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results of the work described in this manuscript are covered by the provisional
patent application US 61/826,172.
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