(20b) Metal Complexes As Catalysts in Enhancing the Oxygen Scavenging Action of Hydrazine in the Boiler Feed Water

Metal complexes as Catalysts in Enhancing the Oxygen
Scavenging action of Hydrazine in the Boiler Feed Water

^*Koyar
Sanlo Rane^{1, 2} and Sifali S. Bandodkar¹

¹Goa University, Goa, India ; ²Rani Channamma University,
Belgaum-591156, India

raneks@gmail.com

Metals
are prone to deterioration or destruction or simply to corrosion in a hostile environment.
Oxygen is one such an aggressive gas and the oxygen dissolved in water even 3-5
parts per million (ppm) or for that matter 10-12 parts per billion (ppb) is
detrimental to the boiler materials when used as boiler feed water. Boiler is
an integral part of any industry and hence the minimization of the dissolved
oxygen (DO) in the boiler feed water is of prime importance, as industry cannot
afford to shut down the boiler for the want of its maintenance due corrosion.
Mechanical de-aeration is normally adopted to bring down the DO level of the
boiler feed water (de-ionized) to 3-5 ppm. Further reduction of the DO level is
done through solid or liquid chemical oxygen scavengers (COS). Liquid COS,
hydrazine hydrate, N₂H₄·H₂O, is universally used
which reacts with dioxygen, giving environmentally friendly gaseous reaction
products, N₂H₄ + O₂→ N₂ and H₂O.
 Added advantage of the hydrazine is that it is a reducing agent and it helps
in arresting metal oxide formation. In addition to this, the hydrazine
promotes the formation of the protective magnetite, Fe₃O₄,
through a reaction with metal oxide scale, 6Fe₂O₃ + N₂
H₄→ 4Fe₃O₄ + N₂ + 2H₂O.
The hydrazine can also form a protective  cuprous oxide, Cu₂O,  film
in copper based boiler materials by reacting with the cupric oxide, CuO, that
formed due to the corrosion of copper. It is because of these passivation
effects that an excess of the hydrazine is added in excess of its normal 1:1 stoichiometry (N₂H₄:O₂:: 1:1) requirement
in the boiler feed water. The only objectionable part is that it is indicated
as toxic and hence it is unfriendly to the boiler operators, hence excessive
use need to be restricted. Another limitation of the use of it is its sluggish
reaction with oxygen, when DO level at ppb needs to be maintained at lower
temperatures and the enhancement of the reactivity is mediated through
catalysts and hence to increase the rate of reaction catalysts are used.

It is found that
the oxygen scavenging action of hydrazine in the boiler feed water can be
enhanced [1,2] by adding cobalt(II) chloride and 3,4 diamino toluene (DAT). Though
the formation of Co^II (3,4 Diaminotoluene) chloride type complex,
[Co(3,4-DAT)₂]Cl_2,   is mentioned in the patent,  details
of the enhancing action is not available with the use of such additives.  May
be the reversible oxygen uptake similar to the well known cobalt complexe such
as cobalt salen, N,N'-Bis(Salicylidene) ethylene diamino Cobalt(II) [3] is
taking place on the cobalt diaminotoluene complex too. The activated oxygen may
then attack the hydrazine more effectively.

Catalytic
effect of metal complex

In our studies
we synthesized the cobalt diaminotluene (CDAT), [Co(C₇H₁₀N₂)₃]Cl₂
and cobalt salen(CS) [Co(C₁₆O₂H₁₄N₂)]complexes
and investigated the oxygen scavenging action of hydrazine in the presence of
these complexes in a home built boiler. 30 liters water capacity boiler was
de-aerated mechanically at 45°C to 3-5 ppm DO level and then a little excess hydrazine
hydrate than required (1:1::N₂H₄:O₂) was added
and the amount of oxygen scavenged estimated by Winkler's method, and hydrazine
consumed  spectrophotometrically.

Figure
1 Percentage Oxygen Scavenging by hydrazine hydrate (HH)

 in the
presence of Cobalt complexes at 45°C

It is observed
that, Figure 1, the hydrazine hydrate (HH) scavenged ~50% of the 3-5 ppm DO of
the boiler water in 10 minutes by consuming ~50% of the hydrazine added to the
water after mechanical de-aeration. When the metal complexes, cobalt diamino
toluene (CDAT) and cobalt salen (CS) embedded in the polymer base hung in the
mechanically de-aerated boiler feed water(3-5 ppm DO) and then introduced the
requisite amount of  HH, both showed ~ 70% oxygen scavenging within 10 minutes  at
45°C, indicating a catalytic effect of the metal complexes by consuming ~12%
hydrazine .

Hydrazine
modified metal complexes:

An interesting
observation, however, was made with the cobalt diaminotoluene hydrazinate, (CDATH),
[Co(C₇H₁₀N₂) (N₂H₄)₂]Cl₂
and cobalt salen hydrazinate (CSH) [Co(C₁₆O₂H₁₄N₂)(N₂H₄)],
prepared for the first time [4]. Both catalyzed the hydrazine oxygen scavenging
capacity by consuming just ~5% hydrazine.

Plausible
mechanism:

In autoxidation
of hydrazine in an aqueous solution with molecular oxygen O₂ by
Co(II) 3,10,17,24-tetrasulphophthalocyanine (H₂tspc): [Co^II(tspc)],
it is considered [5] that hydrazine forms an 1:1 adduct with the metal complex
and the hydrazine thus activated  reacts with the oxygen,  

                
[Co^II(tspc)]·(N₂H₄) + O₂  → 
N₂ + H₂O + [Co^II(tspc)]

The metal
complex, [Co^II(tspc)], remains unazffected after the hydrazine
oxidation. The molecular oxygen getting reversibly attached to cobalt salen
either into superoxo, O₂^- (1:1)  or µ- peroxo (2:1;
oxygen bridged) complex is well known [3] and the activated oxygen is used in
the oxidation reactions. The enhancement of the oxygen scavenging action of
hydrazine by the introduction of cobalt salen in the boiler in our studies may
be due to the attack of this activated oxygen by the hydrazine. However,
further enhancement of the hydrazine oxygen scavenging by the cobalt salen
hydrazinate complex, [Co(C₁₆O₂H₁₄N₂)(N₂H₄)],
may be visualized as follows. Hydrazine is already in the adduct form in the
complex and the cobalt salen is a good oxygen carrier and thus hydrazine easily
reacts with the oxygen on the metal complex.

In the highly
coordinating solvent such as pyridine, in the absence of oxygen the square
planar cobalt salen can form 5 or 6 coordinate complex, [Co(salen)(py)] or Co(salen)(py)₂].
Such complex too can bind O₂ to give derivatives of the type
(μ-O₂)[Co(salen)py]₂ and [Co(salen)py(O₂)]. 
In the cobalt salen hydrazinate, [Co(C₁₆O₂H₁₄N₂)(N₂H₄)],
the hydrazine ligand may occupy the 5^th position of central metal
cobalt. Since the lewis base pyridine (sp² N) is less basic
compared to N₂H₄ (sp³ N) whose basicity
is comparable to NH₃ (sp³ N), hydrazine occupied
in the 5^th position of the central Co metal may more efficiently
form adduct with dioxygen (1:1superoxo type complex). The hydrazine of the
boiler water now readily gets attacked with the superoxo radical.

The enhancement
of the oxygen scavenging of hydrazine in the presence of Cobalt diaminotoluene
(CDAT), [Co(C₇H₁₀N₂)₃]Cl₂,
and its hydrazinate (CDATH), [Co(C₇H₁₀N₂) (N₂H₄)₂]Cl₂
; Cobalt proline(CP), [Co(C₅H₉NO₂)2(H₂O)₂],
and its hydrazinate(CPH), [Co(C₅H₉NO₂)(N₂H₄)(H₂O)₂];
Cobalt hydroquinone(CHQ), [Co(C₆H₄(OH)₂)₃],
and its hydrazinate(CHQH), [Co(C₆H₄(OH)₂)(N₂H₄)₂];
cobalt carbohydrazide(CZ), [Co(CH₆N₄O)₃]Cl₂
have been observed( Figure 1). The hydrazinate complexes are prepared for the
first time. And the oxygen scavenging action of all the complexes show
interesting order of activity,

 CSH>CPH>CDATH^*
>CP > CS   > CZ   > CDAT >   Z   ≈  CHQH>CHQ >HH

The paper
discusses the preparation, characterization of the cobalt complexes and their
effectiveness as catalyzer in the oxygen scavenging action of hydrazine on a
home built boiler.

Acknowledgement:
KSR thanks University Grants Commission (UGC) New Delhi, India, for the UGC
Emeritus Fellowship.

References:

[1]
Noack, G.Manfred ?Catalyzed hydrazine compound corrosion inhibiting

     composition
containing a quinone compound and a complex of metal salt and an

     ortho
aromatic compound? U.S. Patent No. 4026664(1976) 

[2] Noack, G. Manfred, ?Catalyzed hydrazine compound corrosion
inhibiting

     
composition and use?, U.S. Patent  No. 4012195 (1975)

 [3] T.Tsumaki,Bull.Chem.Soc.Jap,13(1938) 252

[4]
Koyar Sanlo Rane and Sifali S. Bandodkar, ?Metal Complexes for enhancing
the oxygen scavenging action of hydrazine? India Patent-IP15596 (provisional
Filing on  28^th Jan.2011) [Sifali Bandodkar, Ph.D Thesis, Nov.
2011,  Goa University, Goa, India]

[5] D.J.Cookson,
T.D.Smith, J.F.Boas, P.R.Hicks, J.R.Pilbrow, ?Electron Spin Resonance Study of
the Autoxidation of Hydrazine,Hydroxylamine and Cysteine catalysed by the
cobalt(II)Chelate complex of 3,10,17,24Tetrasulpho  phthalocyanine? Journal
of Chemical Society Dalton (1977)109-114