Structural Characterization and Chemistry of the
Industrially Important Zinc Borate, Zn[B
3
O
4
(OH)
3
]
David M. Schubert,* Fazlul Alam, Mandana Z. Visi, and Carolyn B. Knobler
†
U.S. Borax Inc., 26877 Tourney Road, Valencia, California 91355
Received August 13, 2002. Revised Manuscript Received November 28, 2002
Several unique crystalline zinc borates are known, a few of which find industrial use in
significant tonnages. Although the most important of these has been a commercial product
for more than 3 decades, it was never before structurally characterized. The structure of
Zn[B
3
O
4
(OH)
3
](1) was determined for the first time by single-crystal X-ray diffraction,
revealing it to be a complex network consisting of infinite polytriborate chains cross-linked
by coordination with zinc and further integrated by hydrogen bonding. The structure of 1
bears similarities to certain borate minerals, most notably, studenitsite (Ca[B
3
O
4
(OH)
3
]) and
colemanite (Ca[B
3
O
4
(OH)
3
]‚H
2
O); however, significant differences are described. Hydrolytic
and thermochemical properties of 1 are discussed. This compound illustrates the important
role played by metal cations in directing the spatial arrangement of anionic polyborate
structural units in metal borates. This new structural information leads to a revision in the
chemical formula, 2ZnO‚3B
2
O
3
‚3.5H
2
O, typically used to describe this material as an article
of commerce, to 2ZnO‚3B
2
O
3
‚3H
2
O. Compound 1 crystallizes in the monoclinic space group
P2
1
/n with a ) 6.845(2) Å, b ) 9.798(2) Å, c ) 7.697(2) Å, ) 106.966(4)°, V ) 493.8 (2) Å
3
,
and Z ) 4.
Introduction
Occurring in both mineral and synthetic forms, metal
borates in which boron is bound only to oxygen are
numerous and find extensive industrial use. Many
synthetic metal borates resemble minerals in structure,
containing isolated polyborate anions or complex po-
lyborate rings, chains, sheets, or networks. However,
structure-stability theories recently developed for bo-
rate minerals do not consistently apply to synthetic
borates. Understanding how cations direct borate struc-
tural units in borate compounds is fundamental to the
development of synthetic strategies for metal borates
having useful properties.
Metal borates can be divided into two categories,
hydrated and anhydrous. So-called hydrated borates,
which account for the majority of known boron-contain-
ing minerals and synthetic borates consumed by indus-
try, have structures containing B-OH groups (hydroxyl-
hydrated borates) and may also contain interstitial
water. Although structural details for most commer-
cially relevant crystalline hydrated borates are known,
the structures of several important zinc borates, includ-
ing the title compound 1, have not been described.
There is good evidence for the existence of at least
eight unique crystalline hydrated zinc borates. These
have compositions 4ZnO‚B
2
O
3
‚H
2
O (2),
1
ZnO‚B
2
O
3
‚
∼1.12H
2
O(3),
2
ZnO‚B
2
O
3
‚∼2H
2
O(4),
2
6ZnO‚5B
2
O
3
‚
3H
2
O(5),
3
2ZnO‚3B
2
O
3
‚7H
2
O(6),
4
2ZnO‚3B
2
O
3
‚3H
2
O
(≡1), 3ZnO‚5B
2
O
3
‚14H
2
O,
5
and ZnO‚5B
2
O
3
‚4.5H
2
O,
3
spanning a range of B
2
O
3
/ZnO mole ratios from 0.25 to
5.0. Each of these compounds can be prepared selec-
tively by reactions of zinc oxide with boric acid in water,
the specific product obtained determined by reactant
concentrations and temperature. Aside from the title
compound, to date only the structures of 3 and 6 have
been reported; the latter has the structural formula Zn-
[B
3
O
3
(OH)
5
]‚H
2
O and contains a monomeric triborate
dianion.
4
Zinc borates have found industrial use since the
1940s,
6
with 2ZnO‚3B
2
O
3
‚7H
2
O and 3ZnO‚5B
2
O
3
‚14H
2
O
primarily utilized in the earlier period. However, these
compounds had a limited range of applications owing
to their low dehydration temperatures. The most im-
portant commercial zinc borate today, 1, was introduced
more than 30 years ago and now has a worldwide
annual production exceeding 10 000 metric tons. It is
primarily used as a polymer additive and as a preserva-
tive in wood composites. As a polymer additive, it serves
as a fire retardant synergist, char promoter, antidrip
agent, smoke and afterglow suppressant, and modifier
of electrical and optical properties. Addition of 1 to
ceramic bodies can improve green strength and reduce
firing times, temperatures, and pyroplastic deformation.
* To whom correspondence should be addressed.
†
McCullough Crystallographic Laboratory, Department of Chem-
istry and Biochemistry, University of California, Los Angeles.
(1) Schubert, D. M. U.S. Patent 5,342,533, 1994.
(2) Choudhury, A.; Neeraj, S.; Natarajan, S.; Rao, C. N. J. Chem.
Soc., Dalton Trans. 2002, 7, 1535-1538. The recently characterized 3
is apparently a different compound from 4, also known in trade
literature as 2ZnO‚2B2O3‚3H2O, which has been an article of commerce
for many years.
(3) Lehmann, H.-A.; Sperscheider, K.; Kessler, G. Z. Anorg. Allg.
Chem. 1967, 354, 37-43.
(4) Ozols, J.; Tetere, I.; Ievins, A. Izv. Akad. Latv. Nauk. SSR, Ser.
Kim. 1973, 1,3-7.
(5) Putnins, J.; Ievins, A. Latv. Valsts Univ. Kim. Fak. Zinat. Raksti
1958, 22, 69. This compound has been referred to as 2ZnO‚3B
2O3‚9H2O
in trade literature.
(6) The Crown 1944, 9, 31.
866 Chem. Mater. 2003, 15, 866-871
10.1021/cm020791z CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/25/2003