Dechlorination of lindane in the multiphase catalytic reduction system with Pd/C, Pt/C and Raney-Ni

Abstract

Dechlorination of γ-hexachlorocyclohexane (lindane) is carried out in the multiphase catalytic system, composed by isooctane and aqueous KOH phases, a phase transfer agent (Aliquat 336) and a metal catalyst, e.g. 5% Pd/C, 5% Pt/C, or Raney-Ni. At 50 °C and atmospheric pressure the full conversion of lindane to 1,2,4-tricholorobenzene (1,2,4-TCB) is achieved in 5–10 min via the base assisted dehydrochlorination, followed by the metal catalyzed hydrodechlorination with hydrogen to benzene. Aqueous KOH and Aliquat 336 strongly affect the reaction: if present together they co-promote both dehydrochlorination and hydrodechlorination steps; if KOH is absent, the reaction is forced to follow a different catalytic pathway, which involves a removal of a pair of chlorines at every reaction step by zerovalent metal followed by reduction of metal with hydrogen. This is proven by the formation of 3,4,5,6-tetrachlorocyclohex-1-ene and 5,6-dichlorocyclohexa-1,3-diene as intermediates in the reaction over Raney-Ni, and by the absence of TCBs in the reactions on all the catalysts studied. The final yield of benzene via this pathway can be achieved in shorter times than in a system with KOH. The presence of Aliquat 336 in the isooctane-water system produces a 10-fold rate increase, the presence of alkaline water is also important since it avoids catalyst poisoning by neutralizing the hydrochloric acid formed.

Introduction

Lindane, the γ-isomer of hexachlorocyclohexane (HCH, Fig. 1), is a heavily used insecticide, used on fruit, vegetables, and forest crops. It is also used in many countries as a topical treatment for head and body lice and scabies, a contagious skin disease caused by mites. Its possible carcinogenic effects on humans remain to be unequivocally demonstrated, but it appears on the CERCLA Priority List of Hazardous Substances, a list of substances which are determined to pose the most significant potential threat to human health due to their known or suspected toxicity and potential for human exposure, to their persistence in the environment, and potential to bioaccumulate [1]. Till now, lindane alone and technical HCH (containing also α-, β-, and δ-HCH [2], the so-called “non-toxic isomers”) have been widely used in agriculture [3]. The technical HCH mixture consists of 65–70% α-HCH, 7–10% β-HCH, 14–15% γ-HCH and 6–10% δ-HCH; since the forties, it has been produced hundreds of tons per year over the world. Technical HCH and lindane are now regulated in most European countries and North America [4].

The tons of HCH produced over the past years and stored nowadays represent an environmental danger and therefore have to be disposed of. The cheapest method for elimination of lindane and its wastes, combustion, is undesirable, because of the formation of even more toxic polychlorinated dioxins [5]. Alternatively, disposal of lindane and its conversion to more useful chemicals, can be carried out via chemical or biotransformations. As long ago as in 1871, Zinin reported on the reduction of HCH to benzene with metallic Zn in ethanol [6]. Lindane and its isomers can undergo the dehydrochlorination (DHC) reaction to form tricholorobenzenes (TCBs) either by thermal or the base-assisted processes [7], [8], [9], [10]. The latter is more selective and can be carried out under mild conditions in the presence of a phase-transfer (PT) catalyst and an aqueous base [9], [10]. Electrochemical [11] and bacterial [11], [12], [13] processes of HCH or lindane degradation are known and afford benzene. Lindane decomposition via sonochemical [14] or gamma ray [15] induced dechlorination/destruction has also been demonstrated. It has been reported recently [16] that lindane undergoes the processes of dehydrochlorination and hydrolysis in subcritical water to give less chlorinated benzenes, phenols, and phenol as the major final product. The use of just water under comparatively mild conditions (100–250 °C and the corresponding equilibrium pressures) is attracting, however, the drawback is low selectivity and formation of toxic chlorinated phenols as byproducts.

Few studies have been dedicated to the catalytic reduction of lindane over metal catalysts. The advantages that the heterogeneous catalytic processes can offer for utilization of toxic substances are both environmental and commercial (milder conditions, elimination of toxic and expensive reagents, ease of catalyst/product separation, good selectivity and yields). Schüth has reported [17] on the reductive dechlorination of lindane in hydrogen-saturated water over Pd/Al₂O₃. The reaction was shown to give benzene with 99% yield within 18 min at room temperature and atmospheric pressure, however, a 13-fold molar excess of Pd with respect to the substrate was used. The catalytic reduction (e.g. with hydrogen) of lindane is intriguing, since benzene and not cyclohexane (as expected from a regular DHC) is formed. However, no conclusions on the reaction mechanism have been made and no intermediates were observed. Destruction of lindane via catalytic or electrochemical processes appear to be more effective, also because the full mineralization of chlorine is achieved, whereas in the DHC reaction still toxic TCBs are formed.

Previously, we have reported on a number of applications of the multiphase catalytic systems for various catalytic reduction reactions [18], [19], [20], [21], [22], [23], [24], [25], [26], in particular, for the HDC of chlorinated aromatics [22], [23], [24], [25], [26]. The multiphase system consists of a substrate solution in isooctane-water (alkaline or not), a metal catalyst and a PT agent, e.g. Aliquat 336 (tricaprylmethylammonium chloride). Heated at 50 °C and bubbled with hydrogen at atmospheric pressure, this system provides an optimal set of conditions for the reduction of polychlorinated aromatics. The presence of the aqueous base and the PT agent is sometimes indispensable for the reaction to proceed, e.g. as we have shown for the HDC of TCB with Raney-Ni [22], [23]. The POPs, such as polychlorinated biphenyls (PCBs) [25] and dioxins [24] have been shown to reduce quantitatively over Pd/C under these conditions. The system, apparently too complex, affords a flexibility with which one can direct the reaction via one or another pathway by simply varying its composition, i.e. pH of the aqueous phase, the presence of the PT agent, etc.

The present study is the first example of the application of this system for the reduction of polychlorinated aliphatics. The reductive dechlorination of lindane over Pd/C, Pt/C, and Raney-Ni in the multiphase system is described. Along with the environmental issue, the effects of reaction conditions on the chemical behaviour of lindane, and some mechanistic considerations are addressed.