Metal-organic fireworks: MOFs as integrated structural scaffolds for pyrotechnic materials

Article abstract of DOI:10.1039/c5cc04174k

A new approach to formulating pyrotechnic materials is presented whereby constituent ingredients are bound together in a solid-state lattice. This reduces the batch inconsistencies arising from the traditional approach of combining powders by ensuring the key ingredients are 'mixed' in appropriate quantities and are in intimate contact. Further benefits of these types of material are increased safety levels as well as simpler logistics, storage and manufacture. A systematic series of new frameworks comprising fuel and oxidiser agents (group 1 and 2 metal nodes & terephthalic acid derivatives as linkers) has been synthesised and structurally characterised. These new materials have been assessed for pyrotechnic effect by calorimetry and burn tests. Results indicate that these materials exhibit the desired pyrotechnic material properties and the effect can be correlated to the dimensionality of the structure. A new approach to formulating pyrotechnic materials is proposed whereby constituent ingredients are bound together in a solid-state lattice. A series of Metal-organic framework frameworks comprising fuel and oxidiser agents exhibits the desired properties of a pyrotechnic material and this effect is correlated to the dimensionality of the structure.

Full text of DOI:10.1039/c5cc04174k

ChemComm
View Article Online
View Journal
COMMUNICATION
Metal–organic fireworks: MOFs as integrated
structural scaffolds for pyrotechnic materials†
Cite this:DOI: 10.1039/c5cc04174k
L. H. Blair,a A. Colakel,b R. M. Vrcelj,c I. Sinclaird and S. J. Coles*a
Received 26th May 2015,
Accepted 23rd June 2015
DOI: 10.1039/c5cc04174k
www.rsc.org/chemcomm
A new approach to formulating pyrotechnic materials is presented constituents – an oxidiser, commonly metal nitrates or per-
whereby constituent ingredients are bound together in a solid-state chlorates and a reducing agent or fuel which can be comprised
lattice. This reduces the batch inconsistencies arising from the of non-metals e.g. C or S, metals e.g. Mg or Al and carbohydrates
traditional approach of combining powders by ensuring the key e.g. lactose.1 Further ingredients are binders, propellants and
ingredients are ‘mixed’ in appropriate quantities and are in intimate agents producing effects such as colour, sound or smoke.2
contact. Further benefits of these types of material are increased A pyrotechnic device is therefore a mixture of functional materials
safety levels as well as simpler logistics, storage and manufacture. in the solid state, in contrast to explosives which are typically single
A systematic series of new frameworks comprising fuel and oxidiser molecules that undergo rapid decomposition. For pyrotechnics to
agents (group 1 and 2 metal nodes & terephthalic acid derivatives as produce the desired effect ingredients must be intimately mixed,
linkers) has been synthesised and structurally characterised. These but industry still follows simplistic manufacturing approaches.
new materials have been assessed for pyrotechnic effect by calori- Ingredients are either dry mixed or wet mixed where the former
metry and burn tests. Results indicate that these materials exhibit tumbles powders together or sieves,3,4 whilst wet mixing blends a
the desired pyrotechnic material properties and the effect can be slurry with horizontal and vertical mixers and various blades. These
correlated to the dimensionality of the structure. A new approach methods do not ensure the homogeneous composition necessary
to formulating pyrotechnic materials is proposed whereby consti- for pyrotechnics to function ideally or guarantee batch consistency.
tuent ingredients are bound together in a solid-state lattice. A series Newer mixing methods for energetics (but not pyrotechnics) have
of Metal–organic framework frameworks comprising fuel and oxidiser been devised, such as the use of nanoparticles5 and resonant
agents exhibits the desired properties of a pyrotechnic material and acoustic mixing,6 however they suffer from the same issues in
this effect is correlated to the dimensionality of the structure.
ensuring that particles mix intimately.
We present an alternative approach to traditional mixing
Pyrotechnics are ubiquitous in modern life, with usage ranging of the ingredients of a pyrotechnic device. By incorporating
across airbags, flares, matches, oxygen candles and display. components in a single crystalline lattice true intimate blending
However the approach to their manufacture has barely changed at the molecular level can be achieved in the solid state. This
over the centuries that we have been using them. Research into approach not only achieves precisely the desired stoichiometric
pyrotechnic materials is further motivated by a need to improve (or otherwise) ratios but also reduces the components in a
safety and flexibility of manufacture, logistics, storage and physical mixture, thereby minimising batch variation. This can
performance consistency. They are categorised alongside pro- be achieved by coordination of components in a single functional
pellants and explosives as ‘‘Energetic Materials’’ and at the framework material. Metal–organic frameworks (MOFs) have
simplest level, a pyrotechnic material consists of two primary been recognized as useful materials for applications in gas
storage, gas purification, catalysis, and as sensors.7 The use of
MOFs in the energetic materials field was unknown until recent
explosive chemical sensors developments where the porosity of
the framework absorbs and traps explosive compounds and
triggers a detectable response.8 It was recently reported that
MOFs could incorporate known explosive compounds in the
framework and create structural reinforcement that stabilizes
the unstable components. Hope-Weeks et al.9 reported two
hydrazine-perchlorate 1D materials, [Ni(NH2NH2)5(ClO4)2n(NHP)]
a Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK.
E-mail: s.j.coles@soton.ac.uk
b Department of Chemistry, Gebze Technical University, 41400 Gebze, Turkey
c School of Pharmacy, University of Lincoln, Lincoln, LN6 7TS, UK
d Engineering, University of Southampton, Southampton, SO17 1BJ, UK
† Electronic supplementary information (ESI) available: Full experimental details
are provided, both as an appendix to this paper and at DOI: 10.5258/SOTON/
376152. CCDC 1046525–1046535. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c5cc04174k
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun.

View Article Online
Communication
ChemComm
Table 1 The components used to form the homologous series
product, which was left in chloroform for 24 hours to allow DMF
exchange. Route 2 replaces DMF/TEA with methanol/pyridine.
A summary and full experimental details of compounds
synthesised, route taken and structures determined is given as
ESI.† Group 2 compounds are generally easier to synthesise than
group 1 analogues and this may be attributed to the fact that they
have a greater polarisability and therefore have a greater coordi-
nation number potential (6–8) while group 1 metals are more
constrained (6). Moreover, products with fluorinated linkers are
more readily synthesised via the MeOH route. The resulting
crystal structure types determined are depicted in Fig. 1. Table 1
also shows that a number of syntheses also generate material,
which either cannot be isolated from the starting components or
other final products, or, if they do result in what appears to be a
pure final phase, only as a powder form, which cannot be solved
with available powder diffraction methods.
and [(Co(NH2NH2)5(ClO4)2)n(CHP)], which have linear polymeric
structures and are possibly the most powerful metal-based energetic
materials known.10 Li and co-workers11 extended this approach to
enhance framework structural reinforcement and made the 3D
¨
energetic MOFs (Cu(atrz)3(NO3)2)n and (Ag(atrz)1.5(NO3))n. Klapotke
et al.12 have investigated the incorporation of compounds contain-
ing high levels of nitrogen into salts and coordination polymers with
the intention of introducing stability to these materials whilst also
providing a means to manufacturing ‘greener’ energetic materials.
Rather than stabilising known energetic materials, we take
traditional fuels and oxidisers used by the industry and incor-
porate them in an integrated scaffold. In doing so, one can
systematically modify both components i.e. metal nodes and
linkers, to fine-tune pyrotechnic effect. Table 1 summarises the
system design, where group 1 and 2 metal nitrates form nodes
that couple with terephthalic or tetrafluoroterephthalic acid
linkers. Alkaline earth metals nitrates were chosen due to
potential to mimic zinc nitrate in a MOF5 synthesis13 and the
burn colour changing within the group (reflected by shading in
Table 1). Dicarboxylic acid moieties of terephthalic acids and
fluorinated analogues enable them to act as linkers. Terephthalic
acid has a known pyrotechnic effect so the hypothesis is that
incorporation of a fluorinated linker generates a response similar
to industry use of fluorinated compounds e.g. PTFE. Therefore
synthesis of all 36 combinations would generate a systematic
library capable of probing pyrotechnic effect.
The majority of the 16 structures arose from the MeOH/
pyridine synthesis and with fluorinated linkers. Coupling of Ca
and Sr with 1 resulted in Ca1_2 and Sr1_2 (CATPAL14
&
LOCCAH15).16 Both comprise 1D chains and the same coordi-
nation (M(OH2)4(COO)3). They are topologically similar, but not
isostructural as Ca1_2 has one water molecule above the
M-COO plane and three below while Sr1_2 has two above and
two below. 3D similarity arises from each chain hydrogen
bonding to four others similarly in both structures. Sr1_1 also
formed two known structures (a = IJOVEJ17 & b = NOCLOH18).
The Sr1_1a building units are Sr(OH2)(COO)5 whereas for Sr1_1b
they are Sr(DMF)(COO)5. Both crystal structures form 3D frame-
works but while Sr1_1a arises from cross-linking sheets of
Sr(OH2)(COO)5, Sr1_1b is comprised of cross-linked 1D chains.
The fluorinated terephthalic materials are unique as they all
form products affording crystal structures. Group 1 metals all
Scheme 1 depicts a generalised synthesis procedure with two
possible routes. At room temperature route 1 uses the appro-
priate metal nitrate and acid in dimethylformamide (DMF) with
triethylamine (TEA) as a deprotonating agent to precipitate the
Fig. 1 Pyrotechnic MOF structures determined (a) Ba4_1/2, (b) K4_2,
(c) Ca/Sr4_2, (d) Rb/Cs4_2, (e) Sr5_1, (f) Sr6_1; _1 indicates a structure
arising from route 1 synthesis, while _2 refers to route 2.
Scheme 1 Synthetic scheme.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2015

View Article Online
ChemComm
Communication
coordinate with a distorted trigonal prism geometry and form to create 2D sheets. Ba6_2 forms 2D sheets constructed by
3D topologies. In K4_2, (PIPDOJ19) K(COO)5 units link to create cross-linked Ba(OH2)(COO)5 units and resembles Sr6_1 as both
2D sheets which are cross-linked to create the 3D structure. possess a solvent molecule spacer between acids above and
Rb4_2 and Cs4_2 are isostructural – Rb/Cs(COO)6 units link below the 2D sheets.
to create 2D sheets that cross-link to form a 3D structure.
Isophthalic and phthalic acid linkers fail to produce a crystal strate bulk purity and isostructurality between Ca4_2 and Sr4_2
structure with any of the group 1 metals. and also Rb4_2 and Cs4_2. In order to compare across struc-
Powder XRD patterns (see ESI† Sections 3.3 & 4.3) demon-
The group 2 metals are structurally more prolific as regards tural families the carboxylate coordination has been classified
fluorinated linkers, with all forming products and most gen- (Scheme 2).8 The classifications describe connectivity of linker
erating crystal structures. The metal centres have a strong groups and in combination with type of building unit and
preference for coordinating in a biaugmented triangular prism dimensionality (Table 2) enable a description and comparison
geometry. Ca4_2 and Sr4_2 are isostructural – Ca/Sr(OH2)(COO)4 of networks.
units combine to form 1D chains cross-linked in two directions
Terminal coordination by water generally results in reduced
to create a 3D MOF. Pyridine resides in the pores of this dimensionality and impedes the formation of a 3D network.
structure. Ba4_1/2 comprises Ba(OH2)(COO)6 units linking to In the fluorinated 1D structures c and d type coordination
create 2D sheets which cross-link to form a 3D structure, prevails, whereas for higher dimensionality bonding modes are
notably via both synthesis methods. Sr5_1 consists of mono- more diverse. The geometry of linker groups within the frame-
meric Sr(OH2)3(DMF)(COO)3 units with two Sr centres connected work varies significantly depending on whether they are fluori-
by linkers to form a 1D chain. Terminal water molecules prevent nated. This is observed in the torsion angles between the
the formation of 2D covalent sheets, favouring a structure carboxylate and aromatic ring, where in Ca1 and Sr1 they are
mediated by hydrogen bonding to create supramolecular sheets. coplanar as opposed to ranging from 451 to 851 in fluorinated
Conversely Sr5_2 forms 2D sheets via Sr(OH2)(NO3)(COO)5 and cases. It is assumed that the sterically favoured orientation is
Sr(NO3)(COO)5 units. Ca6_2 forms 1D chains composed of coplanar but interactions with fluorine groups cause the linker
monomeric Ca(NO3)(COO)5 units with two Ca centres linking to rotate, which results in the framework becoming more
to form a network connectivity similar to Sr5_1. Sr6_1 comprises susceptible to higher dimensionality. Even in Ba4_1/2 where
Sr(OH2)(DMF)(COO)5 units, while Sr6_2 comprises Sr(OH2)(COO)6 linkers lie roughly parallel within a sheet, on moving from
and Sr(OH2)(COO)5 units and in both cases these link together one sheet to the next they rotate by 731 which, as a result of
1,4 substitution, drives the formation of a 3D network.
The thermal behaviour of these compounds was investigated
by DSC (see ESI† Sections 3.4 & 4.4) and shows fluorinated
linkers to be more reactive while alkali metal structures have
lower quality pyrotechnic effect. The structural reinforcement
provided by incorporating pyrotechnic ingredients in 3D frame-
works is known to be a contributing factor to their high thermal
Scheme 2 RCOOÀ coordination modes (a) bridging (b) chelating stability.10–12,20 Fig. 2 depicts burn tests and corresponding DSC
(c) chelating & a bridging oxygen (d) two oxygens chelating & bridging.
measurements and shows that tetrafluorophthalic acid linker 6
Table 2 Crystal structure geometry and network connectivity: t represents terminal water coordination and br a bridging mode
Product
Building units
Linker connectivity
1D, 2D, 3Da
Ca1_2
Sr1_1a
Sr1_1b
Sr1_2
Ca4_2
Sr4_2
Ba4_1/2
K4_2
Rb4_2
Cs4_2
Sr5_1
Sr5_2
Ca6_2
Sr6_1
Sr6_2
Ca(OH2)4(COO)3
Sr(OH2)2(COO)5
Sr(DMF)(COO)5
(d1, d2, t, br) x2
(a) x3, c1, c2, c3, (br) x2
c1, c2, c3, (d1, d2) x2, t
(d1, d2) x2, (t) x4
(b, d1, d2) x2, t
(b, d1, d2) x2, t
(a) x3 (d1, d2) x2, br
(d1x2, d2) x2
1D
3D
3D
1D
3D
3D
3D
3D
3D
3D
1D
2D
1D
2D
2D
Sr(OH2)4(COO)3
Ca(OH2)(NO3)(COO)4
Sr(OH2)(NO3)(COO)4
Ba(OH2)(COO)6
K(COO)5
Rb(COO)6
(a) x6
(a) x6
Cs(COO)6
Sr(OH2)3(DMF)(COO)3
Sr(OH2)(NO3)(COO)5
Ca(NO3)(COO)5
Sr(OH2)(DMF)(COO)5
Sr(OH2)2(COO)6, Sr(OH2)2(COO)5
c1, c2, c3, *, (t) x4
(a, d1, d2) x2, t
(a) x3, c1, c2, c3
(a, d1, d2) x2, (t) x2
(a) x4, (c1) x2, (br) x2
(a) x4, c2, c3, t, br
(a, d1, d2) x2, t, br (x2)
Ba6_2
Ba(OH2)3(COO)5
2D
a
The 3D networks could be assigned topologies using the same approach applied to MOFs, however this becomes impractical when applied to 2 &
1D networks, so an adapted scheme based on the carboxylate coordination was applied to all structures.
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun.

View Article Online
Communication
ChemComm
that given a different selection of starting materials, different,
but more effective pyrotechnic MOFs can be constructed. Given
these basic results, it will be possible to move forward and
determine more specific physico-chemical characteristics, such
as friction and impact tests for safety and directly useful
parameters, e.g. well-defined spectral characteristics. It was
also intended that the incorporation of group I and II metals
would introduce colour into the burn, however this was not very
prominent and further work will aim to incorporate chlorine
into the framework so that metal chloride decomposition
products, which have characteristic colours, form on burning.
Notes and references
1 J. A. Conkling, Chemistry of pyrotechnics: basic priniciples and theory,
CRC Press, 2nd edn, 2011, pp. 83–96.
¨
2 G. Steinhauser and T. M. Klapotke, Angew. Chem., Int. Ed., 2008, 47,
3330–3347.
3 B. A. Obadele, Z. H. Masuku and P. A. Olubambi, Powder Technol.,
2012, 230, 169–182.
4 K. Liu, Powder Technol., 2009, 193, 208–213.
5 (a) M. Comet, V. Pichot, B. Siegert, F. Schnell, F. Ciszek and
D. Spitzer, J. Phys. Chem. Solids, 2010, 71, 64–68; (b) M. Comet,
B. Siegert, F. Schnell, V. Pichot, F. Ciszek and D. Spitzer, Propellants,
Explos., Pyrotech., 2010, 35, 220–225.
6 S. R. Anderson, D. J. am Ende, J. S. Salan and P. Samuels, Propellants,
Explos., Pyrotech., 2014, 39, 637–640.
Fig. 2 Burn tests in relation to DSC results. Top row Ca4, Sr4, Ba4, DSC4;
middle row Ca5, Sr5, Ba5, DSC5; bottom row Ca6, Sr6, Ba6, DSC6 (DSC
Ca = orange, Sr = red, Ba = green).
7 (a) J. L. C. Rowsell and O. M. Yaghi, Microporous Mesoporous Mater.,
2004, 73, 3–14; (b) O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K.
Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705–714;
(c) S. L. James, Chem. Soc. Rev., 2003, 32, 276–288.
8 (a) D. Banerjee, Z. Hu, S. Pramanik, X. Zhang, H. Wang and J. Li,
CrystEngComm, 2013, 15, 9745–9750; (b) S. S. Nagarkar, B. Joarder,
A. K. Chaudhari, S. Mukherjee and S. K. Ghosh, Angew. Chem., Int. Ed.,
2013, 52, 2881–2885; (c) J. H. Lee, S. Kang, J. Y. Lee, J. Jaworski and
J. H. Jung, Chem. – Eur. J., 2013, 19, 16665–16671; (d) Z. Hu, B. J. Deibert
and J. Li, Chem. Soc. Rev., 2014, 43, 5815–5840; (e) B. Gole, A. K. Bar and
P. S. Mukherjee, Chem. – Eur. J., 2014, 20, 2276–2291.
9 O. S. Bushuyev, P. Brown, A. Maiti, R. H. Gee, G. R. Peterson, B. L.
Weeks and L. J. Hope-Weeks, J. Am. Chem. Soc., 2011, 134,
1422–1425.
has the most dramatic effect. Also descending the group (Ca 4
Sr 4 Ba) increases pyrotechnic effect. The effect is clearly linked
to structure – 3D networks formed by 4 are very stable and have
least effect, whilst lower dimensionality networks i.e. 1D chains
in 5 and 2D sheets in 6 are less reinforced and more effective
pyrotechnics. The enthalpy of combustion from DSC is greater
for lower dimensionality, thus supporting the hypothesis linking
structure to pyrotechnic effect.
In summary, thirteen alkaline earth metal and three alkali
metal coordination polymers have been structurally charac-
terised, eleven of which are novel and all exhibiting pyrotechnic
effects. With this approach, the typical pyrotechnic mix (metal
nitrate oxidiser, organic fuel) has been transformed into an
10 O. S. Bushuyev, G. R. Peterson, P. Brown, A. Maiti, R. H. Gee,
B. L. Weeks and L. J. Hope-Weeks, Chem. – Eur. J., 2013, 19,
1706–1711.
11 S. Li, Y. Wang, C. Qi, X. Zhao, J. Zhang, S. Zhang and S. Pang, Angew.
Chem., Int. Ed., 2013, 52, 14031–14035.
¨
integrated MOF, containing fuel (both metal and organic) and a 12 (a) G. Steinhauser and T. M. Klapotke, Angew. Chem., Int. Ed., 2008,
¨
47, 3330–3347; (b) T. M. Klapotke, J. Stierstorfer, K. R. Tarantik and
I. D. Thoma, Z. Anorg. Allg. Chem., 2008, 634, 2777–2784;
(c) T. M. Klapotke, C. M. Sabate and M. Rasp, Dalton Trans., 2009,
1825–1834; (d) K. Karaghiosoff, T. M. Klapotke and C. M. Sabate,
Eur. J. Inorg. Chem., 2009, 238–250; (e) N. Fischer, T. M. Klapotke,
K. Peters, M. Rusan and J. Stierstorfer, Z. Anorg. Allg. Chem., 2011,
637, 1693–1701.
fluorinated oxidiser in a single structure. Using a fluorinated
linker produces a strong pyrotechnic effect that can be linked to
the dimensionality of the framework. There is a fine balance
between cross-linking 1D chains so that ingredients are intimately
mixed, whilst at the same time ensuring that the structure is not
¨
´
¨
so stable that the pyrotechnic effect is reduced. We therefore 13 D. Saha, S. Deng and Z. Yang, J. Porous Mater., 2009, 16, 141–149.
14 T. Matsuzaki and Y. Iitaka, Acta Crystallogr., Sect. B: Struct. Crystallogr.
Cryst. Chem., 1972, 28, 1977–1981.
15 R. H. Groeneman and J. L. Atwood, Cryst. Eng., 1999, 2, 241–249.
conclude that these materials offer the necessary structural tun-
ability within an integrated scaffold for ingredients to produce an
appreciable effect and thus may be the basis for a next generation 16 F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58, 380–388.
17 L. Yang, D. Zhao and G. Li, Acta Crystallogr., Sect. E: Struct. Rep.
Online, 2011, 67, 282.
18 C. A. Williams, A. J. Blake, C. Wilson, P. Hubberstey and
of pyrotechnics, for example it is entirely possible that using this
molecular construction method, further modifications can be
¨
developed, permitting incorporation of more complex entities,
such as dyestuffs for release during burning. Clearly, this
approach is directed by our starting materials, which are
directly related to current pyrotechnic processes, but it is likely
M. Schroder, Cryst. Growth Des., 2008, 8, 911–922.
19 M. Werker, B. Dolfus and U. Ruschewitz, Z. Anorg. Allg. Chem., 2013,
639, 2487–2492.
20 Q. Zhang and J. N. M. Shreeve, Angew. Chem., Int. Ed., 2014, 53,
2540–2542.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2015