Solvent-free ageing reactions of rare earth element oxides: From geomimetic synthesis of new metal-organic materials towards a simple, environmentally friendly separation of scandium

View Article Online

View Journal

Green

Chemistry

Cutting-edge research for a greener sustainable future

Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: I. Huskic, M.

Arhangelskis and T. Friši, Green Chem., 2020, DOI: 10.1039/D0GC00454E.

Volume 20

This is an Accepted Manuscript, which has been through the

Number

May 2018

Pages 1919-2160

9

7

Royal Society of Chemistry peer review process and has been

accepted for publication.

Green

Chemistry

Cutting-edge research for a greener sustainable future

rsc.li/greenchem

Accepted Manuscripts are published online shortly after acceptance,

before technical editing, formatting and proof reading. Using this free

service, authors can make their results available to the community, in

citable form, before we publish the edited article. We will replace this

Accepted Manuscript with the edited and formatted Advance Article as

soon as it is available.

You can find more information about Accepted Manuscripts in the

Information for Authors .

Please note that technical editing may introduce minor changes to the

text and/or graphics, which may alter content. The journal’s standard

Terms & Conditions and the Ethical guidelines still apply. In no event

shall the Royal Society of Chemistry be held responsible for any errors

or omissions in this Accepted Manuscript or any consequences arising

from the use of any information it contains.

ISSN 1463-9262

PAPER

Paul T. Anastas et al.

The Green ChemisTREE: 20 years after taking root with the 12

principles

rsc.li/greenchem

Please do not adjust margins

Page 1 of 14

Green Chemistry

View Article Online

DOI: 10.1039/D0GC00454E

ARTICLE

Solvent-free ageing reactions of rare earth element oxides: From

geomimetic synthesis of new metal-organic materials towards a

simple, environmentally friendly separation of scandium

Received 00th January 20xx,

Accepted 00th January 20xx

Igor Huskić,^aMihails Arhangelskis,^a,bTomislav Friščić*^,a

DOI: 10.1039/x0xx00000x

The development of cleaner methodologies for the separation and processing of rare earth elements, including scandium

and yttrium, is of high importance to materials science and industry. Here, we explore the use of simple, solvent-free and

low-energy "accelerated ageing" reactions, inspired by geochemical processes of mineral weathering and neogenesis, to

convert eight rare earth (RE) element oxides, specifically Sc, Y, La, Yb, Eu, Gd, Lu and Nd, into metal-organic oxalate

derivatives. The reactions can readily be directed towards the formation of topologically distinct framework structures by

introduction of additional oxalate ions in the form of simple alkaline metal or ammonium salts, providing not only simple,

solvent-free access to novel metal-organic RE element architectures from readily accessible precursors, but also enabling a

proof-of-principle of a simple route to separate scandium from at least seven other rare earth elements in a mixture of

metal oxides. Specifically, the metal-organic structures obtained by ageing a mixture of scandium and other RE element

oxides in presence of ammonium or sodium oxalates exhibit significant differences in aqueous solubility at boiling point of

water, providing high scandium separation factors ranging from approximately 10²-10³, without requiring elaborate

complexation

reagents,

toxic

extractants,

acidic

media,

or

solvents

other

than

water.

industry. Its use in metal-organic frameworks (MOFs) has been

recently explored.²¹Unlike other transition elements and REs, the

chemistry of scandium remains poorly explored and understood,

most likely due to its high price: on the global market the price of

Sc₂O₃is above $4,000 per pound, with the price of pure metal being

even higher.²²Consequently, further investigation and use of this

poorly explored element requires the development of simple,

straightforward routes for its separation.

Introduction

Rare earth (RE) metals, including Sc, Y and the lanthanides, are

critical elements of high technological and strategic importance.^1,2

The REs are central to the manufacture of high performance tools,

alloys, electronic components, catalysts and medicinal diagnostic

agents.^3,4Their isolation and production is a persistent challenge

because of their concomitancy, chemical similarity, and scarce

distribution of economically viable feedstocks.^5–9Common

processes for RE extraction and separation involve volatile organic

liquids, synthetically demanding custom ligands, and concentrated

mineral acids.^10–14Moreover, as REs are often a secondary product

in extraction of other elements (e.g. U, Al), their supply can strongly

depend on primary product market. This, combined with political

and economic instability in leading producer countries has led to an

increased interest in recycling of RE-containing components.^15–17

Scandium is the lightest RE element and the lightest transition

metal. Its notable uses include acid catalysis, in the form of the

potent Lewis acid scandium triflate^18,19and the production of

durable, lightweight alloys.²⁰It is also a critical, irreplaceable

component of high-quality lighting elements in film and television

^a.McGill University, Department of Chemistry, Montreal, Canada. E-mail:

tomislav.friscic@mcgill.ca

^b.University of Warsaw, Faculty of Chemistry, Warsaw, Poland.

Electronic Supplementary Information (ESI) available: details of experimental

procedures, Rietveld refinement, additional PXRD, ICP analysis, thermal (TGA/DSC)

analysis, and infrared spectroscopy data. Crystallographic data for compounds 1-8

in CIF format (CCDC deposition numbers 1963550-1963557) See

DOI: 10.1039/x0xx00000x

Please do not adjust margins

Green Chemistry

Page 2 of 14

ARTICLE

Journal Name

Figure 1. a) Flow schematic for a general process for isolation of RE elements from

phosphate minerals, adapted from reference 12. The ‘*’ sign denotes potential places

where the herein demonstrated routes for solvent-free transformation and separation

might be deployable. b) A flow schematic for the herein demonstrated solvent-free

transformation of RE oxides and separation of scandium.

Molycorp bastnesite, Rhone-Poulenc liquid-liqui d_Viewe_Ax_rtt_icr_la_ec_Ot_nio_linn_e,

Mintek-apatite processes),^25,26,68,69prese^Dnt^Oin^I:g^10.1p⁰o³t⁹e^/n^Dt⁰ia^Gl^C0p⁰l⁴a⁵c⁴e^Es

where the proposed approach could be implemented for early

scandium separation (Figure 1a) and simplification of subsequent

metal separation procedures by SX or IX cycles.

Currently used methods for the extraction of RE elements from

their ores are involved (Figure 1a) and, while details and steps are

often not fully disclosed by individual industries,^23,24many of them

employ oxalic acid (H₂ox) and oxalates as precipitating agents once

the raw feedstocks have been digested by mineral acids (i.e. HCl,

HNO₃, H₂SO₄). The precipitates are again converted to soluble forms

by calcination and/or acid treatment, and separated by solvent

extraction (SX) or multiple solvent-intense ion exchange (IX)

cycles.^25–30

Experimental section

Materials used in the experiments were used as obtained,

without purification, except for Gd₂O₃, Nd₂O₃, La₂O₃and Eu₂O₃

which were calcined at 800 ºC for 24 hours to ensure the

conversion of any potential hydroxides to the oxides. Oxalic

acid dihydrate (H₂ox), potassium oxalate monohydrate (K₂ox),

sodium oxalate (Na₂ox) and ammonium oxalate dihydrate

((NH₄)₂ox) were obtained from Sigma Aldrich.

Brief milling of reaction mixtures, to ensure

homogenization prior to ageing, was performed on a Retsch

MM400 shaker mill for 30 seconds operating at 30 Hz, in a 15

mL volume Teflon® jar using a single 10 mm diameter zirconia

ball (weight ~3.6 g). Total mass of the components in the

milling jar was always ~200 mg.

Accelerated ageing was performed by placing the

homogenized mixture in a 20 mL glass vial which was then

placed in a sealed chamber at constant temperature (45 ºC)

and controlled relative humidity (RH) (see also Figure S1, ESI).

Relative humidity was controlled via a saturated solution of

potassium sulfate⁷⁰ensuring that the humidity never exceeded

98%, and was kept within 96%±0.5% at 45 ºC. The total mass of

the aged reaction mixtures was typically around 200 mg.

Powder X-ray Diffraction (PXRD) data was collected on a

PROTOXRD AXRD benchtop instrument equipped with a

Dectris Mythen2R 1D detector, using nickel-filtered CuKα (λ =

1.5418 Å) radiation, or the BRUKER D2 benchtop instrument

equipped with the Lynxeye 1D detector, using nickel-filtered

CuKα radiation. Conversion of file formats was performed in

the PowDLL program suite.⁷¹

Crystal Structures of Na₃Scox₃·6H₂O (4), K₃Scox₃·4H₂O (5),

K₈Sc₂ox₇·10H₂O (6) and K₃Gdox₃·3H₂O (7) were determined by

single crystal X-ray diffraction, using a Bruker D8 ADVANCE X-

ray diffractometer equipped with a PHOTON100 detector and

graphite-monochromated MoKα (λ = 0.71073 Å) radiation. The

X-ray diffraction data sets were collected using the ω- and φ-

scan mode over a 2-range up to 54º. The structures were

solved by intrinsic phasing implemented in SHELXT⁷²and

refined using SHELXL.⁷³Structure refinement was performed

on F²using all data. Hydrogen atoms participating in hydrogen

bonds were located from the electron density map whenever

data quality was sufficient. Otherwise, they were fixed to

appropriate distances and refined from the electron density

map.

Recently, solvent-free techniques such as mechanochemistry^31–

44

or low-energy activated or accelerated ageing methods^45–51have

emerged as alternatives to solution chemistry, and our team has

proposed their potential use in metal production and recycling.

Such methods are simple and offer mild, environmentally non-

taxing means to transform and separate noble and critical

elements^52,53(Au, Pd, Pt, Ge) and base metals⁵⁴(Cu, Pb, Zn) in the

raw element or oxide form. Accelerated ageing, inspired by

geochemical mineral weathering (mineral neogenesis),^55–58provides

a minimalistic and more environmentally friendly route to the

synthesis of complex metal-organic materials from simple mineral-

like oxides or carbonates^51,54without requiring bulk solvents,

soluble and often corrosive metal nitrates, halides or sulfates, high-

temperature or mechanical agitation. Instead, accelerated ageing

chemistry relies on diffusion-controlled reactivity in static mixtures

of inorganic precursors with a suitable organic ligand.^45,46,59

Here, we provide a systematic exploration of solid-state ageing

reactivity of eight RE oxides (Sc, Y, La, Yb, Eu, Gd, Lu, Nd) towards

oxalic acid (H₂ox), an abundant naturally-occurring compound

already extensively used as a precipitating agent in extractive

metallurgy of these elements. Our results show facile, solvent-free

conversion of these RE oxides into corresponding oxalates through

accelerated ageing transformations, while the addition of

stoichiometric amounts of sodium, potassium or ammonium

oxalate as sources of additional oxalate ions also enabled the

production of previously not reported discrete or framework metal-

organic structures. On one hand, the results of this study provide a

potential model for the geological formation of RE-based organic

minerals in the absence of solvents, which is emerging as an area of

interest for understanding the mineral evolution and the carbon

cycle of Earth and other planetary systems.^60–64On the other, we

find that the metal-organic architectures obtained through ageing

reactions of RE oxides can exhibit significantly different dissolution

in boiling water, enabling a proof-of-concept for simple, mild

separation of scandium from a complex solid mixture with at least

seven other RE oxides. The separation factors from this simple

process (Figure 1b) range from ca. 90 to 2,200, comparable to

values encountered in conventional SX but without using

specialized complexation agents, organic solvents or toxic

extractants such as naphthenic acids or organophosphine oxides

(e.g. Cyanex or Cextrant range of extractants).^65–67Mixtures of RE

element oxalates and/or oxides often appear in popular

methodologies for RE element separation (e.g. the Baotou,

The crystal structures of Lu₂ox₃·6H₂O (1), (NH₄)Scox₂·2H₂O

(2), (NH₄)Ybox₂·H₂O (3) and K₃Euox₃·3H₂O (8) were determined

from PXRD data collected on a Bruker D8 Venture instrument

equipped with a CuK_αnon-monochromated X-ray source. The

measurements were performed using

geometry, with the sample material placed on a spinning plate.

The unit cell determination was achieved using DICVOL06⁷⁴

a Bragg-Brentano

2 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 3 of 14

Green Chemistry

Please do not adjust margins

Journal Name

ARTICLE

algorithm, accessed via graphical interface of the program days of ageing at 90 ºC and 96% relative humidi_Vt_iy_e,_wa_Art2_ic0_le0_Onm_ling_e

DASH.⁷⁵The unit cell parameters were then refined using the sample of the mixture was suspended ^Din^OI:d¹i⁰s^.t¹i⁰ll³e⁹d^/Dw^0Ga^Cte⁰r⁰⁴a⁵n⁴d^E

Pawley procedure⁷⁶and the extracted diffraction intensities briefly heated to boil. The liquid, mostly containing dissolved

were used for space group determination. Simulated annealing scandium species, was filtered and the solid residue was

(SA) structure solution and Rietveld refinement were then digested with 4% analytical grade HNO₃. All samples were

performed in the program TOPAS Academic v.6.⁷⁷Diffraction diluted to concentrations between 50 ppb and 1 ppm,

peak shapes were modelled with a pseudo-Voigt function, appropriate for ICP-MS analysis.

while the background was described with 8^thorder Chebyshev

polynomial function. During the SA structure solution peak

shape, background terms and the unit cell parameters were

Results and Discussion

kept fixed, while the metal and water oxygen atoms were

allowed free movement. In addition, the geometry of the ox^2-

Overview

To ensure homogeneity, solid-state reaction mixtures were

prepared by brief ball milling (30 seconds) of binary mixtures

of a RE oxide with H₂ox, or of ternary mixtures containing an

oxide, H₂ox and Na₂ox, (NH₄)₂ox or K₂ox. While no change was

evident in PXRD patterns of mixture immediately after milling,

PXRD analysis of these binary mixtures after several days

exposure to 45 ºC and 96% relative humidity (RH, achieved by

storing the samples in a sealed chamber in the presence of a

saturated solution of K₂SO₄) revealed the formation of metal

oxalates in all cases (Table 1, Figure 2a). Such behavior is

analogous to previously reported for ageing reactions of

transition metal oxides.²⁶

fragments was defined using the rigid body approach, with

their positions and orientations freely refined during SA

calculation. The structural model with the lowest R_wpvalue

was selected for Rietveld refinement.⁷⁸At this point all the

previously fixed parameters (background, peak shape and zero

shift) were freely refined. In addition, atomic thermal motion

was modelled with a single isotropic Debye-Waller factor.

Structures have been deposited with the Cambridge

Structural Database, deposition codes 1963550-1963557.

Fourier-transform infrared attenuated total reflectance

(FTIR-ATR) measurements were done on a Bruker VERTEX 70

instrument with a PLATINUM diamond crystal ATR unit.

Although the reaction mixtures remained solid throughout

the ageing procedure, with consistency of moist sand, in some

cases after an extended period of time the product was

obtained in the form of crystals of sufficient size (up to ca. 0.2

mm) for structure determination via single crystal X-ray

diffraction. The formation of such macroscopic crystals in a

moist ageing system likely occurs through a process akin to

Ostwald ripening.⁷⁹Overall, the herein observed materials

Thermogravimetric and differential scanning calorimetry

(TGA and DSC) analysis was performed on a Mettler-Toledo

TGA DSC 1 Star system thermobalance using alumina crucibles

under an stream of nitrogen (50 ml min⁻¹) and a heating rate

of 5 ºC min⁻¹from 25 ºC until 200 ºC and in a stream of air (50

ml min⁻¹) and a heating rate of 10 °C min⁻¹from 200 ºC to 700

ºC Sample size was between 2 mg and 10 mg. Results were

processed with the Mettler STARe 9.01 software.

adopted

a limited number of metal-organic topologies,

Elemental analysis was performed by Thermo Fisher

Scientific iCAPQc inductively coupled plasma mass

illustrated in Figure 2. These include three-dimensional (3-D)

quartz (qtz) and Irish ladder (irl) topologies, 2-dimensional (2-

D) layered square grid (sql, or 4,4-connected) and honeycomb

(hcb, or 3,6-connected) topologies, one-dimensional (1-D)

zigzag chains and 0-dimensional (0-D, discrete) binuclear

clusters.

spectrometer (ICP-MS) equipped with

a CETAC ASX520

autosampler. In a typical example, an aged solid-state mixture

of rare earth oxides was combined with H₂ox and either Na₂ox

or (NH₄)₂ox in a respective 1:3:3 stoichiometric ratio. After 10

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 3

Please do not adjust margins

Green Chemistry

Page 4 of 14

View Article Online

DOI: 10.1039/D0GC00454E

ARTICLE

Figure 2. a) Comparison of selected PXRD diffractograms of the mixtures resulting from the reaction between a rare earth oxide (RE₂O₃) and H₂ox in the respective stoichiometric

ratio 1:3, with colour-coded simulated PXRD patterns of the resulting or structurally similar phases given below the corresponding experimental patterns and denoted by their

respective CSD codes. Graphical representations of the various frameworks observed to form in this work, with the underlying networks represented by grey spheres and yellow

rods: b) distorted hcb; c) one type of herein observed sql (sql-1); d) qtz; e) another type of herein observed sql (sql-2); f) irl; g) one type of herein observed 1-D chains (chain-1) h)

binuclear

discrete

clusters

and

i)

another

type

of

herein

observed

1-D

chains

(chain-2).

Notably, two types of structures with distorted sql-topology number of water guests (n) varies between 6 (in the case of Sc,

were observed, with planar and corrugated layers, and these Lu, and Yb) and 10.5 (in the case of Nd) per empirical formula

are herein designated as sql-1 and sql-2, respectively. unit, as seen from crystal structures and verified by TGA (see

Similarly, two types of zigzag chains, where the bridging ESI), and seems to depend on the size of the metal centre,

oxalate units are either coplanar or at a ~66º angle, herein with larger cations being able to accommodate more guest

designated chain-1 and chain-2, respectively, have also been molecules. As

a direct result of the cation size, the

observed (Figure 2). coordination environment of Sc, Lu and Yb consists of three

Typically, the products of reactions in binary mixtures ox^2–species and two coordinated water molecules arranged in

containing only the RE oxide and H₂ox were found to adopt the a square antiprism, while the coordination environment of Nd

empirical formula RE₂(ox)₃·nH₂O, forming distorted hcb and Gd consists of three ox^2–species and three coordinated

structures (Figure 2b) in which water molecules are water molecules arranged in a capped square antiprism. These

coordinated to the RE centre, as well as occupying the space coordination environments are consistent with those normally

within the hexagonal holes and between individual layers. The encountered in the stereochemistry of lanthanide

Please do not adjust margins

Page 5 of 14

Green Chemistry

Please do not adjust margins

Journal Name

ARTICLE

complexes.⁸⁰Such hcb-structures have been observed to form stoichiometric mixture of RE₂O₃and H₂ox might be a result of

View Article Online

in geological environments as well, for example in the open local micro-inhomogeneities, as previou^Dsl^Oy^I:o¹b^0.s¹e⁰r³v⁹e^/Dd⁰i^Gn^Ca⁰g⁰⁴e⁵in⁴g^E

framework cerium-based mineral deveroite,^61,81suggesting formation of cocrystals⁸⁶and/or limited diffusion of H₂ox in

that organic framework minerals might be a result of ageing- the solid environment. The possibility of forming yttrium

based weathering.

hydronium oxalate by ageing was additionally confirmed

The completion of ageing reactions was in each case through the addition of one equivalent of H₂ox to a sample of

evaluated by complete disappearance of Bragg reflections of Y₂ox₃·6H₂O, followed by ageing (Figure 3). As observed by

the oxide reactant, as was previously done for ageing or PXRD, this led to complete conversion into yttrium hydronium

mechanochemical reactions in which metal oxides convert into oxalate. For La₂O₃as the reactant, the product PXRD pattern

diverse MOFs.⁸²The rates of ageing reactions differed strongly could not be related to any reported RE element oxalate

depending on the oxide, with Sc₂O₃and Y₂O₃undergoing structure and, unfortunately, attempts to index the PXRD

quantitative conversion within 4-5 days, and the lanthanide pattern of this product were unsuccessful.

oxides within one day. However, conducting the reactions at

90 ºC enabled quantitative conversion of all explored RE

oxides to corresponding oxalates in 1 day (see Table 2, also

ESI).

Reactions with H₂ox only

The PXRD patterns of aged reaction mixtures involving

Sc₂O₃, Yb₂O₃or Nd₂O₃were found to match those simulated

for the reported structures of respective oxalates (Figure 2a).⁸³

Similarly, ageing reactions of yttrium, lutetium, europium and

gadolinium oxides gave microcrystalline powder products

whose PXRD patterns matched those simulated for previously

published structures of oxalates of other RE metals (Table 1).

The crystal structure of Lu₂ox₃·6H₂O (1) was here solved

from PXRD data and it was found to be isostructural to

Yb₂ox₃·6H₂O (AOXYBH),⁸³validating the assumption that

materials with very similar PXRD patterns are likely to have

similar structures (see ESI, Table S2). Interestingly, Y₂O₃and

Gd₂O₃both gave mixtures of products with similar PXRD

Figure 3. Overlay of PXRD patterns demonstrating how the addition of one more

equivalent of H₂ox into solid sample containing Y₂ox₃·6H₂O leads to

spontaneous conversion into yttrium hydronium oxalate, based on the

Y(H₂O)ox₂^-sql-framework. Starting mixture prepared by ageing of Y₂O₃and H₂ox

in a 1:3 stoichiometric ratio (see also Figure 2a), respectively: a) before addition

of 1 more equivalent of H₂ox; b) after 1 day ageing in the presence of additional

a

patterns (Figure 2a). Based on the PXRD patterns, the mixtures H₂ox and c) after 2 days ageing with additional H₂ox; d) simulated PXRD patterns

for (H₅O₂)Yox₂·H₂O (CSD code YOXALT) and e) Gd₂ox₃·6H₂O (CSD code VEXPUK).

in each case consisted of at least two distinct phases, one

being isostructural to gadolinium oxalate (CSD code VEXPUK)⁸⁴

Reactions of three-component mixtures

and the other to yttrium hydronium oxalate (CSD code

YOXALT).⁸⁵The reported structure of the latter material,

yttrium hydronium oxalate, consists of sql-1 anionic layers

Following the successful demonstration of simple and

quantitative conversion of RE element oxides into

corresponding oxalates via low-energy ageing procedure, we

investigated the possibility of forming more complex oxalate-

based frameworks following an established strategy of

introducing simple oxalate salts that could change the ratio of

RE element cations and oxalate anions suitably for the

formation of open anions nets.^54,87–89

-

(Figure 2c) of composition Y(H₂O)(ox)₂, with the H₅O₂⁺Zundel

cations as guests counterbalancing the anionic framework.⁸⁵

The coordination environment around the Y³⁺can best be

described as distorted capped square antiprism, consisting of

three bidentate ox^2–ligands and three coordinated water

molecules.

The appearance of a hydronium oxalate material with a

stoichiometric RE:ox ratio of 1:2 from an initially 1:3

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 5

Please do not adjust margins

Green Chemistry

Page 6 of 14

View Article Online

DOI: 10.1039/D0GC00454E

ARTICLE

Table 1. Overview of crystalline phases resulting from ageing reactions of RE element oxides with H₂ox alone, and with mixtures of H₂ox and (NH₄)₂ox, Na₂ox or K₂ox. Phases are

designated by the CSD code of the structure of the corresponding metal oxalates, or the CSD code for a metal oxalate structure that appears isostructural to the reaction product.

Shaded cells highlight novel phases that structurally resemble those corresponding to the given CSD code. Corresponding PXRD patterns are given in Figs. 2, 3, 5, 6 and in the ESI.

RE

Sc

Oxalic acid

AOXSCH

framework

NH₄

framework

Na

framework

K

framework

hcb

2^b

qtz

4

chain-1

5/6 mixture

chain-1/ binuclear

Y

VEXPUK / YOXALT

AOXYBH

hcb / sql-1

hcb

AYOXLH

3

sql-1

Unknown

sql-2

irl

BOMLEU

SEYKOX

sql-1

CIBZAO

binuclear

Unknown

irl / chain-2

Yb

Lu

La

Nd

Eu

AOXYBH

1

Unknown

QUHYIB01

YEBKOG

BOMLEU

Unknown

BOMLEU^c

sql-1

CIBZAO

Unknown

NDOXAL^a

Unknown

hcb

Unknown

sql-1

Unknown

DEXXOU/7^d

NDOXAL^a

hcb

irl

Gd VEXPUK / YOXALT

hcb / sql-1

irl

sql-1

irl / chain-2

d

^aPXRD also reveals unidentified phases (see ESI); ^bpattern resembles simulated one for the structure with CSD code DUJQUW; ^cmaterial diffracted poorly; material converts to

structure 7 upon further ageing.

Specifically, we expected that ageing of three-component framework channels. The structure is similar to a previously

mixtures of an RE oxide with H₂ox and M₂ox, (where M=Na, reported scandium MOF (CSD code DUJQUW),²¹however the

NH₄, K) should yield known, and potentially also previously not unit cell dimensions differ significantly (between 5% and 10 %).

reported coordination architectures. While most such

Analysis by PXRD revealed that ageing of both Y₂O₃and

reactions led to completion using 1:1:3 respective La₂O₃leads to previously reported layered 2-D sql framework

a

stoichiometric ratio of the RE oxide, M₂ox and H₂ox, indicating structures with 9- (CSD AYOXLH,⁹⁰Figure 2c, sql-1) and 10-

the product composition MRE(ox)₂, in some cases complete coordinated (CSD QUHYIB01,⁹¹Figure 2e, sql-2) nodes,

conversion was achieved only using RE₂O₃, H₂ox and M₂ox in respectively.

respective stoichiometric ratios 1:3:3 and 1:4:3. Such reactant

Ageing of Yb₂O₃, (NH₄)₂ox and H₂ox in a 1:1:3 respective

ratios indicate product compositions M₃RE(ox)₃and stoichiometric ratio gave compound 3, a microcrystalline

M₈RE₂(ox)₇, respectively.

material with a PXRD pattern strikingly similar to that of

previously reported (NH₄)Y(ox)₂·H₂O (CSD code AYOXLH),

indicating isostructurality. The isostructurality and formation

Reactions in the presence of (NH₄)₂ox

Reactions with (NH₄)₂ox produced 2- or 3-D coordination of sql-topology framework (sql-1, Figure 2c) structure was

+

polymers with NH₄ions occupying the channels in the confirmed by structure solution from PXRD data followed by

structures. Ageing of Sc₂O₃with (NH₄)₂ox and H₂ox in a 1:1:3 Rietveld refinement (Figure 4b). The RE³⁺metal nodes in all

respective stoichiometric ratio was shown to produce materials with sql-1 frameworks investigated herein are

compound 2, with composition (NH₄)Scox2·2H₂O, over a long coordinated by 4 ox^2–in a capped square antiprismatic

period of time (>6 months). Again, as in case of simpler RE environment, often found in lanthanide complexes.⁸⁰

element oxalates, the transformations could be greatly

.

accelerated by temperature increase: conducting the same

reaction at 90 ºC produced 2 within 2 days (see ESI).

Determination of the structure of 2 from PXRD data (Figure 4a,

Table S2) revealed an anionic open framework with a quartz

(qtz, Figure 2d) topology, consisting of eight-coordinate,

square antiprism Sc³⁺nodes, each bridged by 4 ox^2–ions,

+

arranged in a tetrahedral fashion, with NH₄cations in the

Please do not adjust margins

Page 7 of 14

Green Chemistry

Please do not adjust margins

Journal Name

ARTICLE

1:1:3 stoichiometric ratio, respectively, with the simulated pattern for

View Article Online

DOI: 10.1039/D0GC00454E

Nd₂(N₂C₃H₁₂)ox₄(CSD YEBKOG).

During periodic monitoring of some of the three-

component ageing reactions, intermediate phases were

observed. An example is shown in Figure 6, corresponding to

the transformation in a mixture of Yb₂O₃, (NH₄)₂ox and H₂ox in

a 1:1:3 respective stoichiometric ratio. The reaction mixture

rapidly (within 1 day) yields a still unknown microcrystalline

phase, which subsequently converts to 3 over several days.

Reactions in the presence of Na₂ox

Ageing of Sc₂O₃with Na₂ox and H₂ox in a respective 1:3:3 ratio

at 45 ºC over a period of several weeks produced compound 4,

of composition Na₃Scox₃·6H₂O confirmed by TGA. Conducting

the same reaction at 90 ºC provided a moist powder, without

signs of deliquescence, that contained single crystals of 4 large

enough for structural analysis by single crystal X-ray

diffraction. The structure of 4 was found to consist of 1-D

anionic zig-zag chains (chain-1) exhibiting an oxalate-bridged

Sc³⁺–ox^2––Sc³⁺motif, with Na⁺counterions and water

molecules occupying the space between the chains (Figure 2g,

Table S1), resulting in the overall empirical formula

Na₃Scox₃·6H₂O. The Sc³⁺metal nodes are coordinated by four

ox^2–species, arranged in a square antiprismatic fashion. This

motif appears in all materials investigated herein exhibiting

the chain-1 structure.

Figure 4. Final Rietveld fits for structure solution from PXRD data for: a)

(NH₄)Scox₂·2H₂O and b) (NH₄)Ybox₂·H₂O. Measured pattern is shown in blue, simulated

in red and the residual is shown in grey. Vertical blue lines denote positions of expected

reflections, while vertical green lines in b) represent expected positions of the (NH₄)₂ox

impurity.

Reactions of Nd₂O₃, Eu₂O₃or Gd₂O₃with (NH₄)₂ox and H₂ox in

respective 1:1:3 stoichiometric ratio gave products with

mutually very similar PXRD patterns. Whereas these PXRD

patterns did not match any reported Nd(III)-, Eu(III)- or Gd(III)-

Table 2. Time (in days) required for quantitative conversion of a mixture of RE₂O₃

and H₂ox, with or without additional M₂ox (M=NH₄, Na or K), to the respective

complex oxalate product at 96% RH and 45 ºC. Some reaction mixtures exhibited

little to no appreciable change after extended time, which is denoted with ">".

+

based structures involving ox^2-and NH₄, there is significant

similarity to the PXRD pattern of 1,2-propanediammonium

neodymium oxalate (CSD code YEBKOG) (Figure 5).⁹²

Consequently, it is likely that these compounds adopt a similar

structure, which is a 3-D anionic open framework of irl

topology (Figure 2f), based on nine-coordinate lanthanide ions

bridged by ox^2–. The RE³⁺metal nodes in all irl frameworks

investigated herein are coordinated by 4 ox^2–ligands and one

metal

H₂ox

(NH₄)₂ox

Na₂ox

K₂ox

Sc

Y

4

5

4

10

1

>180

30

6

>60

>30

>180

>60

6

14

1

14

7

Yb

Lu

La

Nd

Eu

Gd

3

14

3

water molecule adopting

a deformed capped square

antiprismatic coordination environment. It is reasonable to

assume that the RE³⁺nodes in the structurally unknown

Nd(III)-, Eu(III)- or Gd(III)-based materials involving ox^2-and

NH₄⁺would adopt the same coordination environment.

1

27

>60

7

14

Reactions of most RE oxides with Na₂ox and H₂ox in a

respective stoichiometric 1:1:3 ratio provide mutually

isostructural 2-D coordination polymers of sql-topology (sql-1)

with water molecules between the layers (Figure 2c),

quantified by TGA (see ESI).^93,94The exception to this are

materials containing Nd and La whose structures remain

unknown. Despite several attempts, the crystal structures of

sodium-containing complex oxalates of neodymium and

lanthanum have not been elucidated nor have they been

likened to other known structures based on PXRD patterns.

The ratio of Na to the RE element in these phases has been

determined to be 1:1, which was evidenced by the complete

disappearance of Bragg reflections of the starting materials

from the PXRD pattern of the mixture when reactions were

conducted in that ratio.

Figure 5. Comparison of PXRD patterns of the materials obtained by accelerated

ageing of a mixture of either Eu, Gd or Nd oxide with (NH₄)₂ox, H₂ox and in a

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 7

Please do not adjust margins

Green Chemistry

Page 8 of 14

ARTICLE

Journal Name

Reactions in the presence of K₂ox

View Article Online

DOI: 10.1039/D0GC00454E

Ageing of Sc₂O₃, K₂ox and H₂ox in either the respective

stoichiometric ratios 1:1:3 or 1:3:3 produced mixtures of

phases. However, exploration of different stoichiometric

compositions revealed that ageing of the same starting

materials in the 1:4:3 respective stoichiometric ratio produces

only one phase. Serendipitously, single crystals of two

different phases (5 and 6) were isolated from the same solid

mixture and were used to establish their respective crystal

structures. Structural analysis revealed the composition

K₃Scox₃·4H₂O for 5 and K₈Sc₂ox₇·10H₂O for 6 (see ESI, Table

S1). The structure of 5 resembles that of the corresponding

sodium analogue (4), as it consists of infinite chains (chain-1)

based on the repeating Sc³⁺–ox^2-–Sc³⁺motif, with K⁺cations

and water molecules surrounding the chains (Figure 2g). In

contrast, 6 consists of 0-D discrete binuclear complexes

wherein two Sc³⁺cations are bridged by a single ox^2-anion and

are coordinated by additional three terminal ox^2-groups in a

tetrahedral fashion that are further associated with K⁺cations

and water molecules surrounding the complex (Figure 2h).

Ageing of other RE oxides with K₂ox and H₂ox produced

either a material consisting of binuclear 0-D (in the case of Y,

Yb, Lu and partially Sc) metal oxalate clusters⁹⁵(Figure 2h) or

3-D coordination polymers adopting the structural formula

KREox₂and the irl structural motif (Figure 2f), as concluded

from comparison of respective PXRD patterns (Table 1, ESI).⁹⁶

Moreover, the KREox₂coordination polymers were found to

Figure 6. Comparison of selected PXRD diffractograms for the solid-state mixture

of Yb₂O₃, (NH₄)₂ox and H₂ox in a respective stoichiometric ratio of 1:1:3,

monitored over a period of 60 days. Bragg reflections belonging to an unknown

convert to materials of general composition K₃REox₃upon phase, at ≈12.5º and ≈15.5º 2 appear within 1 day and then gradually disappear

over a longer period of time, disappearing completely after more than 27 days.

further ageing with excess K₂ox. The K₃REox₃materials were

found to consist of anionic zig-zag chains (chain-2) based on

the RE³⁺–ox^2––RE³⁺motif, with K⁺counterions and water

molecules occupying the space between them (Figure 2i). Each

RE³⁺node in chain-2 structures was found to be coordinated

by four ox^2–ligands and one water molecule, arranged in a

capped square antiprism. The structure of K₃Gdox₃·3H₂O (7)

was been determined by single crystal X-ray diffraction using a

crystal obtained from the ageing experiment (see ESI, Table

S1), and the structure of the isostructural material

K₃Euox₃·3H₂O (8) was determined from PXRD data using the

structural parameters of 7 as a starting point (see ESI, Table

S1).

The reaction rates for accelerated ageing of all herein

investigated ternary mixtures were found to differ

significantly, ranging from days to weeks, depending on both

the nature of the oxide and the oxalate salt used (Table 2).

Notably, Sc₂O₃has shown to require longer times for full

conversion to the respective oxalate than most herein

Figure 7. Time-dependent change in the relative amounts of different phases in the

investigated oxides. In all cases, ageing of the REO-containing

mixtures at elevated temperatures greatly decreased the time

necessary for full conversion, without changing the reaction

outcome (see ESI). In the case of ageing Sc₂O₃with (NH₄)₂ox

and H₂ox, the time for full conversion of the oxide reactant

decreased from >60 days at 45 ºC to only 3 days by conducting

the process at 90 ºC.

reaction of Yb₂O₃with three equivalents of H₂ox and one equivalent of Na₂ox. Lines

connecting data points are intended for easier visual identification and do not

represent fitted values.

During periodic monitoring of the reaction between REO,

H₂ox and M₂ox, the formation of MHox (the hydrogen oxalate

salt) was regularly observed. To ascertain whether MHox

played a role as an intermediate in the formation of the mixed

oxalate, a homogenous mixture of M₂ox and pre-formed

lutetium oxalate was exposed to a humid environment at 45

8 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins

Page 9 of 14

Green Chemistry

Please do not adjust margins

Journal Name

ARTICLE

ºC. The mixture containing lutetium oxalate as the starting analyses provided direct and quantitative insight into the

View Article Online

material converted to the corresponding mixed oxalate in an amounts of each RE both in the filtrate^Da^On^Id^{: 10}th^.1e⁰³s⁹o^/Dli⁰d^Gr^Ce⁰s⁰id⁴⁵u⁴e^E.

approximately same time period as the mixtures starting from Specifically, analyses of the liquid extract and solid residue

pure oxide, possibly indicating that the formation of MHox show that this simple, non-optimized methodology in one step

does not play a vital part in conversion of REOs to mixed enriches scandium purity ca. five- or six-fold: from the initial

oxalates (see ESI). To achieve a better understanding of the 12.5 mol% to 68 mol% when using (NH₄)₂ox and 60 mol%

rate of chemical change from REO and simple oxalate salts to when using Na₂ox.

more advanced metal-organic materials, a mixture of Yb₂O₃,

The analyses (Figure 8b,c, Tables 3 and 4) showed

Na₂ox and H₂ox in a respective stoichiometric ratio 1:1:3 was surprisingly efficient separation of scandium from other RE in

stored for 67 days at 45 ºC and 96% RH, with periodic the mixture. When using (NH₄)₂ox, the ratio (r) of Sc to Nd in

monitoring. The Rietveld analyses of the PXRD patterns aqueous filtrate was 152, and the ratio of Sc to Gd was 61. The

measured on the solid sample show that the reaction rate corresponding values for ratios of Sc to La and Eu were 98 and

decreases over time (Figure 7).⁹⁷Whether this is because the 79, respectively. The separation of Sc from Y, Yb and Lu was

reaction is diffusion limited, or there is a decrease in available poorer, with r_Sc/Y, r_Sc/Yband r_Sc/Luvalues 13, 6.6 and 5.3

surface area, or some other reason, remains unclear at this respectively. Analogous set of trials using an accelerated

point.

ageing mixture containing Na₂ox gave comparable results

(Figure 8c, also Table 4).

Separation of scandium from a mixture of REOs

During attempts to grow single crystals by recrystallization

of (NH₄)Scox₂·2H₂O it was observed that the material was

readily soluble in boiling water. Qualitative testing for

dissolution properties in water revealed that all mixed oxalates

containing potassium cations, as well as all herein prepared

scandium compounds, are readily soluble in boiling water.

Moreover, the mixed oxalates of yttrium, lutetium and

ytterbium containing sodium and ammonium cations exhibited

partial dissolution, which increased upon addition of solid

H₂ox, Na₂ox or (NH₄)₂ox, presumably due to the formation of

complex oxalate anions.⁹⁸In contrast, the obtained

compounds of gadolinium, neodymium, lanthanum and

europium were found to be insoluble in boiling water, even

upon addition of excess oxalates (Figure 8a).

These qualitative observations on solubility differences are

unexpected, considering that lanthanide oxalates of

conventional composition RE₂ox₃are known to have very low

aqueous solubilities, with solubility products in the range of ca.

10^{-30 69,99}

solubility differences, and investigate the possibility to

demonstrate proof-of-principle process for simple

.

This led us to conduct a quantitative analysis of the

a

separation of scandium from other RE metals that would start

from a mixture of oxides, without requiring mineral acids,

complex extractants or organic solvents. For this purpose a

model mixture was prepared by mixing equimolar amounts of

all eight herein investigated REOs (ca. 1 mmol each), along

with three equivalents of H₂ox (ca. 24 mmol), to which three

equivalents of (NH₄)₂ox (ca. 24 mmol) were added. The

mixture was thoroughly mixed using a mortar and pestle. An

analogous mixture containing Na₂ox instead of (NH₄)₂ox was

prepared as well. Each mixture was then aged at 90 ºC for six

days at 96% relative humidity, in order to convert the metal

oxides into the respective sodium or ammonium mixed

oxalates. A sample of the resulting solid product was then

suspended in 50 mL of boiling water, the undissolved solid

residue removed by filtration, the resulting solution diluted,

and metal content analyzed by ICP. The solid residue after

filtration was separately dissolved in aqueous HNO₃, solution

diluted to 50 mL and subsequently also analyzed by ICP. These

Figure 8. a) Qualitative summary of aqueous solubility of phases resulting from ageing

of binary and ternary mixtures in this work. The distribution of different RE ions in

aqueous solution and in the solid residue based on ICP analysis of mixtures obtained by

ageing RE oxides with: b) (NH₄)₂ox and H₂ox, and c) Na₂ox and H₂ox. The total amounts

of La, Nd and Eu determined in the solution and the solid residue appear lower than

that of Sc, Y, Yb and Lu due to incomplete digestion of the solid residue by 4% HNO₃.

This journal is © The Royal Society of Chemistry 20xx

J. Name., 2013, 00, 1-3 | 9

Please do not adjust margins

Green Chemistry

Page 10 of 14

ARTICLE

Journal Name

These results are comparable to and often exceed those of

previously reported and considerably more complex SX

systems that are based on popular extractants in organic

solvents or ionic liquids. For example, Zlobina et al. have used

trialkylbenzylammonium di(2-ethylhexyl)phosphate as the

extractant and obtained degrees of enrichment corresponding

to ratios of Sc to La, to Gd and Eu after extraction of 32.2, 23.0

and 26.8, respectively.¹⁰⁰The values for Y and Yb were 4.4 and

2.1, respectively. All these numbers are three to four times

lower than those obtained herein. Similarly, Onghena et al.¹⁰¹

have recently used the ionic liquid betainium

bis(trifluoromethylsulfonyl)imide in extractive separation of Sc

from La, Nd, Y and several other REs, resulting in ratios of 7.1,

4.4 and 4.6 , respectively.

Table 3. Measured amounts of RE in aqueous solution (n_aq, mmol) and solid residue (n_r,

DOI: 10.1039/D0GC00454E

mmol), after extracting the aged mixture of 8 RE oxides, H₂ox and (NH₄)₂ox with boiling

water, along with corresponding concentrations of the aqueous solution (c_aq, mmol

dm^-3), of the solution obtained by dissolving the solid residue in aqueous HNO₃(c_r,

mmol dm^-3), and the associated ratio of scandium to other REs in the aqueous solution

(r_Sc/RE), separation coefficient ()^aand separation factor (_Sc/RE).^b

View Article Online

c

b

RE

Sc

Y

La

Nd

Eu

Gd

Yb

Lu

n_aq

n_r^d

c_aq

c_r^d

^a

_Sc/RE

-

219

r_Sc/RE

-

13

98

152

79

61

6.6

5.3

2.45

0.187

0.138

2.31

2.14

1.96

2.26

2.40

1.87

1.840

49

3.74

2.76

46.2

42.8

39.2

45.2

48.0

37.4

36.8

17.77

0.081

0.012

0.008

0.014

0.017

0.197

0.251

0.0248

0.0161

0.0312

0.0398

0.368

0.496

0.322

0.624

0.796

7.36

1481

2221

1269

1045

90

0.462

9.24

71

An often reported separation metric is the separation

factor (), defined as the ratio of distribution coefficients (also

known as the separation coefficient) of two components. In

order to facilitate comparison to previously reported SX

systems, we approximate the separation coefficient in our

process by the ratio of the amount of a metal in the aqueous

solution to that in the solid residue () and, consequently,  as

the ratio of  values for two metals.

We were surprised to see that our simple, unoptimized

approach provided  values that are high compared to a

number of SX processes described in the primary and

secondary literature. Typical  values range between 10-1000

in order of magnitude,¹⁰²as illustrated in the work of Sun et al.

who explored scandium separation using the popular

extractant Cyanex 925 in combination with an ionic liquid,

yielding separation factors of Sc to Y, Eu, Yb and La of 34.3,

67.8, 86.7 and 172.8, respectively.^11,103An extraction

procedure used in a red mud pilot plant scandium extraction

process developed by Ochsenkühn-Petropulu^104,105is based on

the popular di(2-ethylhexyl)phosphoric acid extractant and

hexane as the organic phase, and exhibits Sc to Y and Sc to Yb

separation factors of ca. 175, and a Sc to La separation factor

of ca. 3,700.

Some of the highest -values for scandium separation from

REs were reported by Karve and Vaidya,¹⁰⁶who used Cyanex

302 extractant with xylenes as the liquid phase for separation

of Sc from La ( = 16,600), Nd ( = 1,180), Gd ( = 27,600) and

Yb ( = 3,840), with a poor separation from Y (highest reported

Sc:Y ratio 10). While poorer in separating Sc from La, Gd and

Yb, it is interesting to note that the herein reported simple

process still achieves a higher separation both for Nd and Y.

Indeed, considering the simplicity of the herein described

method, the herein obtained  values compare surprisingly

well to technologically much more advanced processes, such

as the pseudoliquid membrane design described by Yang and

co-workers,¹³which exhibited  values for separating Sc from

Y, La, Nd, Eu, Gd, Yb and Lu of 769, 4945, 2413, 338, 21194,

1150 and 3262, respectively.

a) Separation coefficient  is defined as the ratio of amounts of metal in aqueous

solution and in solid residue after extraction: n_aq/n_r; b) separation factor for Sc

with respect to a particular RE, _Sc/RE, is defined as the ratio of  values for Sc and

a selected RE; c) established by ICP using a suitably diluted sample of aqueous

solution of aged mixture after extraction with boiling water; d) established by ICP

using a solution obtained by dissolving the solid residue after extraction with

boiling water in dilute aqueous HNO₃.

It is worth also noting that the ratio of concentrations of Lu

and Nd in the liquid phase was 28:1 which makes this method

interesting for separation of not only Sc but potentially also

other RE, such as Y, Yb or Lu, from a mixture. Such very

promising results coming from an unoptimized separation

process warrant a more in-depth exploration of separation of

RE elements using other simple salts.

Table 4. Measured amounts of RE in aqueous solution (n_aq, mmol) and solid residue (n_r,

mmol), after extracting the aged mixture of 8 RE oxides, H₂ox and Na₂ox with boiling

water, along with corresponding concentrations of the aqueous solution (c_aq, mmol

dm^-3), of the solution obtained by dissolving the solid residue in aqueous HNO₃(c_r,

mmol dm^-3), and the associated ratio of scandium to other REs in the aqueous solution

(r_Sc/RE), separation coefficient ()^aand scandium separation selectivity (_Sc/RE).^b

RE

Sc

Y

La

Nd

Eu

Gd

Yb

Lu

n_aq

n_r^d

0.0685

2.27

1.914

1.89

2.16

2.20

1.79

1.58

c_aq

c_r^d

^a

_Sc/RE

-

357

426

822

r_Sc/RE

-

c

b

2.42

0.225

0.158

0.0816

0.067

0.0652

0.406

0.65

48.4

4.5

1.37

45.4

38.3

37.8

43.2

44

35.35

0.099

0.083

0.043

0.031

0.030

0.227

0.408

10.8

15.3

29.7

36.1

37.2

6.0

3.16

1.632

1.34

1.304

8.12

13.0

1140

1178

156

87

35.8

31.6

3.7

a) Separation coefficient  is defined as the ratio of amounts of metal in aqueous

solution and in solid residue after extraction: n_aq/n_r; b) separation factor for Sc

with respect to a particular RE, _Sc/RE, is defined as the ratio of  values for Sc and

a selected RE; c) established by ICP using a suitably diluted sample of aqueous

solution of aged mixture after extraction with boiling water; d) established by ICP

using a solution obtained by dissolving the solid residue after extraction with

boiling water in dilute aqueous HNO₃.

Conclusions

In summary, we have demonstrated the ability to

spontaneously convert a range of rare earth element oxides,

including Sc, Y, La, Nd, Gd, Eu, Yb and Lu, into complex metal-

10 | J. Name., 2012, 00, 1-3

Please do not adjust margins

Page 11 of 14

Green Chemistry

Please do not adjust margins

Journal Name

ARTICLE

organic structures by solvent-free, low-energy ageing with Innovation (CFI) and McGill University William Dawson fund.

View Article Online

oxalic acid and its salts (Na, K and NH₄) in moist air. These M.A. thanks the National Science Cen^Dte^Or^I:o¹⁰f^.1P⁰³o⁹la^/Dn⁰d^GCfo⁰⁰r⁴t⁵h⁴e^E

systematic studies demonstrate accelerated ageing as a financial support via SONATA grant (2018/31/D/ST5/03619).

simple, mild and materials-efficient route to convert the inert

oxides into more tractable oxalates, including open framework

materials, as opposed to mechanochemical treatment that has

Notes and references

so far been unsuccessful in converting the oxides into metal-

organic structures. Accelerated ageing led to the discovery and

structural characterization of at least eight new phases, and

suggested a simple strategy to separate scandium from a

mixture of rare earth elements without special complexation

agents, organic solvents, or aggressive reagents. The herein

investigated solvent-free reactions have led to the structural

characterization of four new scandium metal-organic

1

2

3

4

5

S. M. Hayes and E. A. McCullough, Resour. Policy, 2018, 59,

192–199.

F. Wall Rare earth elements in: Critical Metals Handbook,

1^stEdition, Ed. G. Gunn, John Wiley & Sons (2014)

D. Schlinkert, K. Gerald and V. Den Boogaart, Resour.

Policy, 2015, 46, 272–280.

N. Haque, A. Hughes, S. Lim and C. Vernon, Resources,

2014, 3, 614–635.

T. V Molchanova, I. D. Akimova, K. M. Smirnov, O. K.

Krylova and E. V Zharova, Russ. Metall., 2017, 2017, 170–

174.

compounds, providing

a significant contribution to the

understanding of this poorly explored element. The results are

also of potential significance in the context of geochemistry,

specifically the carbon cycle and mineralogy of the rare earth

elements, as they demonstrate the formation of known

mineral structures, such as cerium-based deveroite,¹⁰⁷under

conditions resembling mineral weathering in nature. In that

context, it is tempting to consider the diversity of phases that

were herein observed to form upon ageing of rare earth oxides

with oxalic acid and its salts as a library of potential, not yet

discovered mineral species.⁵¹Finally, the herein explored

solvent-free ageing chemistry enabled a simple proof-of-

principle method for the separation of scandium from a

mixture with other rare earth element oxides, based simply on

different solid-state reactivity of the oxides and aqueous

solubility of the resulting metal-organic materials. The

proposed separation methodology uses exclusively oxalic acid

or its simple salts as the organic components of solid-state

reactivity. Precipitation with oxalic acid, ammonium oxalate

and/or sodium oxalate is a highly efficient way of separating

RE elements from other metals in solution, and is already

employed in a number of current mining and mineral

processing approaches. Consequently, the proposed

methodology should be highly compatible with existing

processes and enable pre-concentration of scandium and

simplify subsequent ion and/or or solvent exchange processes.

We are currently investigating other possible applications of

accelerated ageing in production of critical elements, such as

indium which is mainly obtained as a by-product of zinc mining and

processing,¹⁰⁸or for processing industrial waste products such as

the ‘red mud’.¹⁰⁹

6

R. A. Schmitt, R. H. Smith and D. A. Olehy, Geochim.

Cosmochim. Acta, 1964, 28, 67–86.

7

M. A. R. Onal, C. R. Borra, M. Guo, B. Blanpain and T. Van

Gerven, J. Sustain. Metall., 2015, 1, 199–215.

C. R. Borra, B. Blanpain, Y. Pontikes, K. Binnemans and T.

Van Gerven, J. Sustain. Metall., 2016, 2, 365–386.

P. S. Arshi, E. Vahidi and F. Zhao, ACS Sustain. Chem. Eng.,

2018, 6, 3311–3320.

X. Zhao, M. Wong, C. Mao, T. X. Trieu, J. Zhang, P. Feng and

X. Bu, J. Am. Chem. Soc., 2014, 136, 12572–12575.

X. Sun, Y. Ji, L. Guo, J. Chen and D. Li, Sep. Purif. Technol.,

2011, 81, 25–30.

8

9

10

11

12

13

J. P. Faris and J. W. Warton, Anal. Chem., 1962, 34, 1077–

1080.

X. Yang, Z.-M. Gu and D.-X. Wang, J. Memb. Sci., 1995, 106,

131–145.

14

15

F. Habashi, J. Chem. Technol. Biotechnol., 1985, 35A, 5–14.

T. Dutta, K. Kim, M. Uchimiya, E. E. Kwon, B. Jeon, A. Deep

and S. Yun, Environ. Res., 2016, 150, 182–190.

K. Binnemans, P. Tom, B. Blanpain, T. Van Gerven, Y. Yang,

A. Walton and M. Buchert, J. Clean. Prod., 2013, 51, 1–22.

M. H. D. Bandara, J. W. Darcy, D. Apelian and M. H.

Emmert, Environ. Sci. Technol., 2014, 48, 6553–6560.

S. Kobayashi, Eur. J. Mineral., 1999, 15–27.

K. Ishihara, M. Kubota, H. Kurihara and H. Yamamoto, J.

Am. Chem. Soc., 1995, 117, 4413–4414.

A. F. Norman, P. B. Prangnell and R. S. M. C. Ewen, Acta

Mater., 1998, 46, 5715–5732.

J. Cepeda, S. Pérez-Yáñez, G. Beobide, O. Castillo, A. Luque,

P. A. Wright, S. Sneddon and S. E. Ashbrook, Cryst. Growth

Des., 2015, 15, 2352–2363.

16

17

18

19

20

21

Conflicts of interest

There are no conflicts to declare.

22

23

24

J. Gambogi, U.S. Geological Survey, Mineral Commodity

Summaries - Scandium, 2019, vol. 1.

C. K. Gupta and N. Krishnamurthy, Extractive Metallurgy of

Rare Earths, CRC Press, Boca Raton, 2005.

P. K. Parhi and S. S. Behera Processing technology for

extraction of scandium(III) from secondary sources – a

comprehensive approach, in: Critical and Rare Earth

Elements Recovery from Secondary Sources, Ed. A. Akcil, A.

Akcil, CRC Press, New York (2020)

Acknowledgements

We thank Prof. F. Schaeper and Dr. P. Daneshmand for help in

collecting single crystal X-ray diffraction data of 4. We

acknowledge the financial support of NSERC (RGPIN-2017-

06467, NSERC RGPAS 507837-17), Canada Foundation for

J. Name., 2013, 00, 1-3 | 11

Please do not adjust margins

Green Chemistry

Page 12 of 14

ARTICLE

Journal Name

25

W. Wang, Y. Pranolo and C. Y. Cheng, Hydrometallurgy,

2011, 108, 100–108.

F. Xie, T. An, D. Dreisinger and F. Doyle, Miner. Eng., 2014,

56, 10–28.

W. Wang and C. Y. Cheng, J. Chem. Technol. Biotechnol.,

2011, 86, 1237–1246.

B. Yagmurlu, W. Zhang, D. Avdibegovic, M. Regadío, R.

Koivula, C. Dittrich, K. Binnemans and B. Friedrich, ALTA

2018 Uranium-REE-Lithium Proc., 2018, 85–93.

D. Kołodyńska and Z. Hubicki, in Ion Exchange

Technologies, InTech, 2012, pp. 101–154.

F. Habashi, Can. Metall. Q., 2013, 52, 224–233.

I. Huskić, I. Halasz, T. Friščić and H. Vančik, Green Chem.,

2012, 14, 1597–1600.

A. M. Fidelli, B. Karadeniz, A. J. Howarth, I. Huskić, L. S.

Germann, I. Halasz, M. Etter, S.-Y. Moon, R. E. Dinnebier, V.

Stilinović, O. K. Farha, T. Friščić and K. Užarević, Chem.

Commun., 2018, 54, 6999–7002.

T. Friščić, C. Mottillo and H. M. Titi, Angew. Chem. Int. Ed.,

2019, 2–14.

N. R. Rightmire and T. P. Hanusa, Dalt. Trans., 2016, 45,

2352–2362.

D. Tan and F. García, Chem. Soc. Rev., 2019, 48, 2274–

2292.

E. Boldyreva, Chem. Soc. Rev., 2013, 42, 7719–7738.

D. Braga, S. L. Giaffreda, F. Grepioni, A. Pettersen, L. Maini,

M. Curzi and M. Polito, J. Chem. Soc. Dalt. Trans., 2006, 60,

12491249–12631263.

A. Beillard, X. Bantreil, T. X. Métro, J. Martinez and F.

Lamaty, Chem. Rev., 2019, 119, 7529–7609.

D. Hasa and W. Jones, Adv. Drug Deliv. Rev., 2017, 117,

147–161.

J. Andersen and J. Mack, Green Chem., 2018, 20, 1435–

1443.

49

50

51

52

53

I. Huskić, J. C. Christopherson, K. Užarević and T. Friščić,

View Article Online

Chem. Commun., 2016, 52, 5120–5^D12^O3^I:.^{10.1039/D0GC00454E}

26

27

28

M. Dud, O. V. Magdysyuk, D. Margetić and V. Štrukil, Green

Chem., 2016, 18, 2666–2674.

X. Feng, C. Jia, J. Wang, X. Cao, P. Tang and W. Yuan, Green

Chem., 2015, 17, 3740–3745.

J. L. Do, D. Tan and T. Friščić, Angew. Chemie - Int. Ed.,

2018, 57, 2667–2671.

M. Glavinović, M. Krause, L. Yang, J. A. Mcleod, L. Liu, K. M.

Baines, T. Friščić and J.-P. Lumb, Sci. Adv., 2017, 3,

e1700149.

29

30

31

54

55

F. Qi, R. S. Stein and T. Friščić, Green Chem., 2014, 16, 121–

132.

N. V. Chukanov, N. V. Zubkova, G. Möhn, I. V. Pekov, D. I.

Belakovskiy, K. V. Van, S. N. Britvin and D. Y.

Pushcharovsky, Mineral. Mag., 2018, 1–17.

J. Chen, H. P. Blume and L. Beyer, Catena, 2000, 39, 121–

146.

P. Adamo and P. Violante, Appl. Clay Sci., 2000, 16, 229–

256.

X. Lim, Nature, 2015, 524, 20–21.

C. Jia, J. Wang, X. Feng, Q. Lin and W. Yuan,

CrystEngComm, 2014, 16, 6552–6555.

I. Huskić and T. Friščić, Acta Crystallogr., 2015, A71, S58.

I. Huskić and T. Friščić, Acta Crystallogr. Sect. B, 2018, 74,

539–559.

R. M. Hazen, D. R. Hummer, G. Hystad, R. T. Downs and J. J.

Golden, Am. Mineral., 2016, 101, 889–906.

O. E. Piro and E. J. Baran, Crystallogr. Rev., 2018, 24, 149–

175.

C. L. F. Dazem, F. M. A. Noa, J. Nenwaa and L. Öhrström,

CrystEngComm, 2019, DOI: 10.1039/c9ce01187k.

W. Le, S. Kuang, Z. Zhang, G. Wu, Y. Li, C. Liao and W. Liao,

Hydrometallurgy, 2018, 178, 54–59.

C. Liu, L. Chen, J. Chen, D. Zou, Y. Deng and D. Li, J. Rare

Earths, 2019, 37, 1002–1008.

32

56

57

33

34

35

58

59

60

61

36

37

62

63

64

65

66

67

68

69

38

39

40

41

J. L. Howard, Q. Cao and D. L. Browne, Chem. Sci., 2018, 9,

3080–3094.

42

43

G. W. Wang, Chem. Soc. Rev., 2013, 42, 7668–7700.

S. L. James, C. J. Adams, C. Bolm, D. Braga, P. Collier, T.

Frišcic, F. Grepioni, K. D. M. Harris, G. Hyett, W. Jones, A.

Krebs, J. Mack, L. Maini, A. G. Orpen, I. P. Parkin, W. C.

Shearouse, J. W. Steed and D. C. Waddell, Chem. Soc. Rev.,

2012, 41, 413–447.

J. L. Do and T. Friščić, ACS Cent. Sci., 2017, 3, 13–19.

(a) C. Mottillo, Y. Lu, M. H. Pham, M. J. Cliffe, T. O. Do and

T. Friščić, Green Chem., 2013, 15, 2121–2131; (b) C.

Mottillo and T. Friščić Molecules 2017, 22, 144.

D. Wu, C. Niu, D. Li and Y. Bai, J. Alloys Compd., 2004, 374,

442–446.

C. Liao, S. Wu, F. Cheng, S. Wang, Y. Liu, B. Zhang and C.

Yan, J. Rare Earths, 2013, 31, 331–336.

P. Josso, S. Roberts, D. A. H. Teagle, O. Pourret, R.

Herrington and C. Ponce de Leon Albarran, Miner. Eng.,

2018, 118, 106–121.

L. Greenspan, J. Res. Natl. Bur. Stand. Sect. A Phys. Chem.,

1977, 81A, 89.

N. Kourkoumelis, Powder Diffr., 2013, 28, 137–148.

G. M. Sheldrick, Acta Crystallogr. Sect. A Found.

Crystallogr., 2015, 71, 3–8.

G. M. Sheldrick, Acta Crystallogr. Sect. C Struct. Chem.,

2015, 71, 3–8.

A. Boultif and D. Louer, J. Appl. Cryst., 2004, 37, 724–731.

W. I. F. David, K. Shankland, J. van de Streek, E. Pidcock, W.

D. S. Motherwell and J. C. Cole, J. Appl. Cryst., 2006, 39,

910–915.

44

45

70

71

46

47

M. J. Cliffe, C. Mottillo, R. S. Stein, D. K. Bučar and T. Friščić, 72

Chem. Sci., 2012, 3, 2495–2500.

(a) D. Cinčić, I. Brekalo and B. Kaitner, Chem. Commun.,

2012, 48, 11683–11685; (b) S. Kaabel, R. S. Stein, M.

Fomitšenko, I. Järving, T. Friščić and R. Aav Angew. Chem.

Int. Ed. 2019, 58, 6230; (c) F. Hammerer, L. A. Loots, J.-L.

Do, P. D. Therien, C. W. Nickels, T. Friščić and K. Auclair

Angew. Chem. Int. Ed. 2018, 57, 2621-2624.

73

74

75

48

D. Braga, S. L. Giaffreda, F. Grepioni, M. R. Chierotti, R.

Gobetto, G. Palladino and M. Polito, CrystEngComm, 2007,

9, 879–881.

76

77

78

G. S. Pawley, J. Appl. Cryst., 1981, 14, 357–361.

A. Coelho, TOPAS Acad. 6, Coelho Softw., 2016.

H. M. Rietveld, J. Appl. Cryst., 1969, 2, 65–71.

12 | J. Name., 2012, 00, 1-3

Please do not adjust margins

Page 13 of 14

Green Chemistry

Please do not adjust margins

Journal Name

ARTICLE

79

80

81

P. W. Voorhes, J. Stat. Phys., 1985, 38, 231–252.

Mag., 2013, 77, 3019–3026.

View Article Online

J. C. G. Bünzli, J. Coord. Chem., 2014, 67, 3706–3733.

I. Huskić and T. Friščić, Philos. Trans. R. Soc. A, 2019, 377,

1–19.

108

109

A. M. Alfantazi and R. R. Moskalyk,^DM^Oi^In^:e¹r⁰.^.1E⁰n³g⁹.^/,^D2⁰0^G0^C3⁰,⁰1⁴6⁵,^4E

687–694.

S. Reid, J. Tam, M. Yang and G. Azimi, Sci. Rep., 2017, 7, 1–

82

I. Halasz, T. Friščić, S. A. J. Kimber, K. Užarević, A. Puškarić,

C. Mottillo, P. Julien, V. Štrukil, V. Honkimäki and R. E.

Dinnebier, Faraday Discuss., 2014, 170, 203–221.

E. Hansson, Acta Chem. Scand., 1973, 27, 823–834.

L. Cañadillas-Delgado, J. Pasán, O. Fabelo, M. Hernández-

Molina, F. Lloret, M. Julve and C. Ruiz-Pérez, Inorg. Chem.,

2006, 45, 10585–10594.

9.

83

84

85

86

87

88

G. D. Brunton and C. K. Johnson, J. Chem. Phys., 2003, 62,

3797.

I. Huskić, J.-C. Christopherson, K. Užarević and T. Friščić,

Chem. Commun., 2016, 52, 5120–5123.

A. K. Cheetham and C. N. R. Rao, MRS Bull., 2005, 30, 93–

98.

S. Ayyappan, A. K. Cheetham, S. Natarajan and C. N. R. Rao,

Chem. Mater., 1998, 10, 3746–3755.

89

90

L. Öhrström and K. Larsson, Dalt. Trans., 2004, 347–353.

T. R. R. McDonald and J. M. Spink, Acta Crystallogr., 1967,

23, 944–949.

91

92

93

94

T. Bataille, M. Loue and J. Au, J. Solid State Chem., 2000,

95, 81–95.

A. Mohanu, C. Brouca-Cabarrecq and J. C. Ã. Trombe, J.

Solid State Chem., 2006, 179, 3–17.

T. Bataille and D. Louër, Acta Crystallogr. Sect. C Cryst.

Struct. Commun., 1999, 55, 1760–1762.

B. Chapelet-Arab, L. Duvieubourg, G. Nowogrocki, F.

Abraham and S. Grandjean, J. Solid State Chem., 2006, 179,

4029–4036.

95

96

97

98

I. A. Kahwa, F. R. Fronczek and J. Selbin, Inorganica Chim.

Acta, 1984, 82, 167–172.

Y. Liu and S. Li, Acta Crystallogr. Sect. E Struct. Reports

Online, 2007, 63, 312–313.

K. A. Connors, Chemical Kinetics: The Study of Reaction

Rates in Solution, VCH, New York, 1990.

C. E. Crouthamel and D. S. Martin, J. Am. Chem. Soc., 1951,

73, 569–573.

99

100

Y. Xiong, Appl. Geochemistry, 2011, 26, 1130–1137.

E. Zlobina, A. Ismailova and K. Tassibekov, MATEC Web

Conf., 2017, 96, 1–6.

101

102

103

104

105

B. Onghena and K. Binnemans, Ind. Eng. Chem. Res., 2015,

54, 1887–1898.

Y. Chen, H. Wang, Y. Pei and J. Wang, Sep. Purif. Technol.,

2017, 178, 261–268.

X. Sun, D. Wu, J. Chen and D. Li, J. Chem. Technol.

Biotechnol., 2007, 272, 267–272.

M. Ochsenkühn-Petropulu, T. Lyberopulu and G. Parissakis,

Anal. Chim. Acta, 1995, 315, 231–237.

M. T. Ochsenkühn-Petropoulou, K. S. Hatzilyberis, L. N.

Mendrinos and C. E. Salmas, Ind. Eng. Chem. Res., 2002, 41,

5794–5801.

106

107

M. Karve and B. Vaidya, Sep. Sci. Technol., 2008, 43, 1111–

1123.

A. Guastoni, F. Nestola, P. Gentile, F. Zorzi, M. Alvaro, A.

Lanza, L. Peruzzo, M. Schiazza and N. M. Casati, Mineral.

J. Name., 2013, 00, 1-3 | 13

Please do not adjust margins

Green Chemistry

Page 14 of 14

View Article Online

DOI: 10.1039/D0GC00454E

Table of Contents Entry

Solvent-free accelerated ageing converts metal oxides into metal oxalate frameworks, enabling

simple separation of scandium from other rare earth elements.

Article Doi

Solvent-free ageing reactions of rare earth element oxides: From geomimetic synthesis of new metal-organic materials towards a simple, environmentally friendly separation of scandium

DOI: 10.1039/d0gc00454e

Source and publish data:

Authors:

Article abstract of DOI:10.1039/d0gc00454e

Full text of DOI:10.1039/d0gc00454e

Products guided by the article

R&D Labs maybe for 19320-56-0

Relevant to this article

Hot Product