Applicability of Nickel-Based Catalytic Systems for Hydrodehalogenation of Recalcitrant Halogenated Aromatic Compounds

Abstract

This review summarizes recent applications of nickel as a nonprecious metal catalyst in hydrodehalogenation (HDH) reactions of halogenated aromatic compounds (Ar–Xs). Nickel-based HDH catalysts were developed for reductive treatment of both waste containing concentrated Ar–Xs (mainly polychlorinated benzenes) and for wastewater contaminated with Ar–Xs. Ni-catalyzed HDH enables the production of corresponding nonhalogenated aromatic products (Ar–Hs), which are principally further applicable/recyclable and/or Ar–Hs, which are much more biodegradable and can be mineralized during aerobic wastewater treatment. Developed HDH methods enable the utilization of both gaseous hydrogen via the direct HDH process or other chemical reductants as a source of hydrogen utilized in the transfer of the hydrodehalogenation process. This review highlights recent and major developments in Ni-catalyzed hydrodehalogenation topic since 1990.

1. Introduction

Ar–Xs are commercially important chemicals that are used as inert solvents (chlorobenzene or o-dichlorobenzene), end products and intermediates in the manufacture of plastics, dyes and a diversity of agrochemicals, flame retardants or other specialty fine chemicals. Ar–Xs are xenobiotic, highly stable in the environment, resistant to biodegradation, often exhibit considerable toxicity and have long been regarded as a major source of environmental pollution [1]. The presence of Ar–Xs in effluent discharges is of increasing concern, due to the ecological effects and impact on public health. The upcoming restrictive legislation evokes urgency to the development of effective removal strategies [1].

Catalytic hydrodehalogenation (HDH) is an effective means of detoxifying halogenated waste. Catalytic HDH represents a modern approach whereby the hazardous Ar–X is transformed into an easier biodegradable or even recyclable product (Ar–H) in a closed system with limited toxic emissions or enabling the detoxification of aqueous contaminants for subsequent biological treatment. HDH is a burgeoning area in an environmental catalysis focused on the treating of environmental pollutants in the past two decades. HDH involves hydrogenolysis of C–X bonds, lowering the toxicity and generating biodegradable or even reusable raw material Ar–H from Ar–X. It requires an external source of hydrogen and is typically promoted using hydrogenation catalyst (Pd, Pt, etc.) [1]:

Ar–X + H₂ → Ar–H + HX

To date, incineration has largely been the preferred technology for destroying dangerous waste contaminated with Ar–Xs but the requisite high decomposition efficiency (>99.9999%) is difficult to achieve in case of halogenated aromatic compound [2]. From an economic viewpoint, considering incineration as the principal means of halogenated organic waste treatment, a move to a catalytic hydrodehalogenation-based treatment represents immediate cost saving in terms of fuel and/or possible chemical recovery [1].

In addition, effective HDH enables the application of halogen as the protective and/or directive group in aromatic substitution reactions in syntheses of specialty organic fine chemicals, for example of herbicide Dicamba [3].

Considering the high cost of noble catalysts and the difficulties of platinum metals-based catalysts preparation, it is essential to develop the intensification method to optimize the HDH technique using an inexpensive catalyst. Advancement in the Ni-based HDH field provides an attractive inexpensive alternative to HDH based on precious (platinum-based) metal catalysis by expanding their application space in a range of halogenated waste treatment methods [1,2,3,4,5]. In addition, the toxicity of nickel is lower in comparison with platinum group metals [6,7,8,9]. Since Ni is ubiquitously present in the environment, the exposure to low doses of Ni is unavoidable and may not be harmful to humans in low concentrations [10]. It has been published that the daily dietary human’s intake of nickel varies between 25–300 µg Ni. Nickel ranks as the 24th element in the order of abundance in the earth’s crust [11]. Due to the above-mentioned reasons, Ni-based hydrogenation catalysts are broadly used for thickening of vegetable oils for margarine production [12].

Direct HDH using excess of H₂ gas and transfer HDH are the two employed strategies for reductive treatment of Ar–Xs.

2. Ni-Catalyzed HDH Using Gaseous H₂

Direct HDH for Ar–Xs was studied using nickel supported on support such as alumina [13,14,15,16,17,18,19,20,21,22,23], silica [24,25,26,27,28,29,30,31,32,33], carbon [13,34] or unsupported in the form of Raney nickel [35,36,37] using pressurized hydrogen and an elevated temperature in the gas phase.

Ni is a much less active HDH catalyst in comparison with platinum metals. For this reason, the Ni-catalyzed HDH of Ar–Xs is performed at a higher temperature in the gaseous phase. Using mild reaction conditions (120 °C/0.1 MPa of H₂), the Ni-catalyzed hydrogenation of 4-chloronitrobenzene produces 4-chloroaniline selectively using both Ni/SiO₂ or Ni/Al₂O₃ and no HDH reaction was observed [38]. Due to the lower reductive activity of nickel compared with platinum metals [13,14], however, Ni-based heterogeneous catalysts are much more selective in HDH of (poly)chlorinated benzenes producing 100% of less chlorinated benzenes and benzene with no benzene ring reduction [29,39]. Although even powdered nickel is active for HDH process [40], most of the published research works studied the activity of supported Ni catalysts [41]. The lower Ni consumption, higher mechanical strength and higher thermal stability of supported Ni-based HDH catalysts give main reasons for research focused on effect of suitable inorganic supports. In addition, appropriate inorganic support provides higher specific area suitable for smaller Ni particle size formation during preparation of supported HDH catalyst which can favorably cause the catalytic activity.

The product selectivity and catalyst stability can also be influenced by the sort of catalyst support. As observed by Amorim et al. [39], specific hydrodechlorination (HDC) rates over the supported Ni system increased with the decreasing specific metal area and usually increase with the Ni particle size [42,43].

2.1. Gas Phase HDH Catalyzed by Nickel on Alumina

Nickel on γ-alumina has a lower activity than when supported on active carbon; the difference is about one order of magnitude at 200 °C. The Ni content 5–10 wt% on alumina appears to be a suitable choice for the treatment of chlorinated aromatic compounds [13].

Nevertheless, HDH in the gaseous phase using Ni supported on Al₂O₃ was intensively studied and often compared with the action of Pd/Al₂O₃ or Pd/SiO₂.

Using Pd(1 wt%)/Al₂O₃ enables 99% HDC of Ph–Cl at 140 °C and ambient pressure of gaseous hydrogen with 86% selectivity for cyclohexane [44]. As was published by Keane, Ni(3 wt%)/Al₂O₃ catalyst reduces Ph–Cl at 160 °C and ambient pressure of gaseous hydrogen with 100% selectivity for benzene [1].

The involvement of spillover hydrogen (dissociated atomic hydrogen on Ni support [1]) was examined in gas phase HDC) of chlorobenzene (CB) and 1,3-dichlorobenzene (1,3-DCB) over Ni. The catalytic action of single component Ni, Ni/Al₂O₃ and physical mixtures of Ni and Pd with Al₂O₃ has been considered. Inclusion of Al₂O₃ with Ni and Ni/Al₂O₃ increased the spillover with an associated increase in the specific HDH rate (up to a factor of 10) and enhanced selectivity to benzene from 1,3-DCB [14] (Scheme 1).

De Jong and Louw compared HDC activity of Ni supported on carbon and γ-alumina with Pt- and Pd-based HDC catalysts [13]. They observed that the HDC activity of Ni-based catalyst is similar to Pt or Pd ones at temperature 250 °C. Generally, the HDC activity of tested Ni supported catalysts increase with increasing content of supported Ni. The main differences in HDC activity of Ni and platinum group metal-based catalysts were found at lower temperature range (150–220 °C) where platinum group metals based HDC catalysts are much more efficient [13]. Cesteros et al. studied the course of HDC of 1,2,4-trichlorobenzene (1,2,4-TCB) related to the amount of H₂ available at the reaction temperature [15]. This research proved that the most selective Ni/Al₂O₃- and Ni/NiAl₂O₄-based catalysts for HDC of 1,2,4-TCB to benzene are those which embody the highest amounts of H₂ desorbing at lower temperatures [15,16]. Fresh and reactivated Ni/NiAl₂O₄ hydrogenates 1,2,4-TCB to cyclohexane in 30 min at 250 °C, an irreversible partial chlorination of the catalytic surface making the hydrogenation of the aromatic ring difficult [15]. Catalyst containing 10% Ni supported on γ-Al₂O₃ enables complete gas phase HDC of chlorobenzene, chlorophenols, chloroanilines and chlorotoluenes at 250 °C, on the other hand, 4-chlorotrifluoromethylbenzene was selectively dechlorinated to trifluoromethylbenzene under the same reaction conditions [17].

Using different mesoporous Al-MCM-41 materials as the support for Ni catalysts, Cesteros et al. performed complete and selective HDC of 1,2,4-TCB to benzene at temperature above 225 °C. The number of converted 1,2,4-TCB molecules per second divided by the total number of surface Ni atoms (TOF) varied between 1.2–8.0 × 10⁻⁴ s⁻¹ [16].

An electrophilic substitution reaction was assumed for the gas-phase HDC of substituted chlorobenzenes on Ni/γ-Al₂O₃ [17].

The catalytic gas-phase HDC of 2,4-dichlorophenol (2,4-DCP) has been explored over Ni/Al₂O₃ and Au–Ni/Al₂O₃ at 200 °C Hydrogen chemisorption on Au–Ni/Al₂O₃ was approximately five times lower than that recorded for Ni/Al₂O₃, but both catalysts show equivalent initial HDC activities. Ni/Al₂O₃ exhibits an irreversible progressive deactivation where partial HDC to 2-CP is increasingly favored over complete HDC to phenol. However, at least 15 mol% of 2-CP was observed (except benzene) in the obtained reaction mixture in each case. In contrast, thermal treatment of Au–Ni/Al₂O₃ in H₂ after the reaction increases HDC activity with a preferential complete HDC to phenol. This result was also achieved by a direct treatment of Au–Ni/Al₂O₃ with HCl [18] (Scheme 2).

For comparison of reactivity of Pd- and Ni-based HDC catalysts, complete Pd/Al₂O₃ catalyzed HDC of 2,4-DCP was finished even after 30 min using Pd(9.2 wt%)/Al₂O₃ catalyst (dosage 0.5 g Pd/Al₂O₃ per liter containing 2,4-DCP/NaOH = ½ and Cl/Pd ratio below 200) at 330 K (liquid phase) and ambient pressure of H₂ [45]. A more robust Ni-based catalyst for gas phase HDH was discovered with the testing standard hydrodesulfurization Ni–Mo/Al₂O₃ catalyst HDS-9A (American Cyanamid Co., Wayne, NJ, USA) [19]. Even if it works at 320–350 °C and higher H₂ pressure (2–10 MPa), it is tolerant to the sulfur impurities in the treated halogenated waste. This catalyst is able to form HDH hexachlorobenzene, 1,2,3-trichlorobenzene, 1,2-dichlorobenzene, chlorobenzene [4,20] and polychlorinated biphenyls, although the reaction time for complete conversion to the benzene is around 8 h [21].

Using carbon as the support for Ni–Mo sulphides (sulphided Ni–Mo/C), the HDH of dichlorobenzenes, dichlorotoluene and dichlorobiphenyls proceeds smoothly in a range of 210–230 °C under hydrogen pressure of 3 MPa producing corresponding aromatic products (benzene, toluene, biphenyl) [22] (Scheme 3).

A number of Ni–Mg–Al and Ni–Al hydrotalcite-like precursors were prepared and their catalytic properties in the gas-phase HDH of 1,2,4-TCB was studied at 250 °C. It was documented that increasing the MgO content in the prepared catalyst greatly increased both the activity and selectivity to benzene and the high stability of the prepared catalyst. The positive effect of MgO was explained because MgO modifies the electronic properties of the Ni particles causing H₂ desorption at lower temperatures and also chemisorbs the HCl produced during the HDC reaction [23].

2.2. HDH Catalyzed by Nickel on Silica

HDH of chlorobenzene, bromobenzene, their mixtures, and the three chlorobromobenzene isomers was examined over the temperature range 200 °C ≤ T ≤ 330 °C using Ni/SiO₂ where the Ni loading was varied from 6.2 to 15.2% wt/wt. Each catalyst was 100% selective in terms of HDH and there was no evidence of cyclohexane formation [1,24,25,26,27].

In case of HDC of chlorobenzene (Ph–Cl), using 3 wt% Ni on SiO₂, Keane calculated HDC to benzene reaction rate as pseudo first order rate constant k = 20,000 mol_Ph–Clh⁻¹g⁻¹ at 150 °C [1]. Using similar conditions and Pd/Al₂O₃ catalyst, Prati and Rossi describe HDC conversion of Ph–Cl ca. 98–99% with selectivity 86–89% for benzene (and 11–14% cyclohexane) using 0.27 mmol min⁻¹ PhCl and 2.5 mmol min⁻¹ H₂ per gram of Pd(1 wt%)/Al₂O₃ catalyst at T = 413 K [44].

The comparison of HDC activity of Ni- and Pd-based catalysts supported on SiO₂ is possible to make comparing published data measured by Keane (Ni/SiO₂) [1,27] and Aramendia (Pd/AlPO₄-SiO₂) [46]. Ni/SiO₂ has lower activity compared with Pd/SiO₂ in conversion of polychlorinated benzenes to nonchlorinated product(s). In case of Ni/SiO₂, the mixture of less chlorinated benzenes with benzene is produced during gas phase HDC [27].

Keane et al. described that Ni/SiO₂ inclines to a temporal loss of activity during HDH reaction above 290 °C which was accompanied by Ni particle growth and structural modification with a consequent disturbance to the H₂ adsorption/desorption surface dynamics [28]. The main reason for deactivation of Ni/SiO₂ based HDH catalysts seems to be the need to work at a high temperature (often higher than 300 °C) which is accompanied by sintering of catalytically active Ni particles and disruption of H₂/catalyst interaction [29] (Scheme 4).

The changes in catalytic support seem to have a minimal negative effect on HDH reaction at the reaction conditions. This was confirmed in the study of Kim et al. [30,31,32], where mesoporous alumina and silica Ni-based catalysts, with a well-defined and uniform surface of the inorganic support, were prepared and tested for HDH of 1,2-dichlorobenzene.

Mesoporous silica Ni-based catalyst was obtained by the sol-gel method using Ni(II) complexed by polyethyleneoxide added into SBA-15 as silica precursor [31]. Mesoporous alumina Ni-based catalyst was prepared by mixing of nickel and magnesium stearate with alumina sol and subsequent calcination of obtained precipitate at 550 °C [32].

Both the prepared mesoporous-nickel materials were well defined, however, their HDH activity and stability were limited and worse than other elsewhere published Ni-supported HDH catalysts at 350 °C [30]. Even in the case of Ni/SiO₂ catalytic systems, the selection and addition of the second appropriate metal to nickel for HDH catalyst preparation increases the stability in the HDH process. Cu/Ni catalyst prepared using well-defined mesoporous silica (MCM-41) as the support was proved to be highly active and stable for HDC of Ar–Cls even at room temperature and ambient H₂ pressure especially in isopropylalcohol with the addition of triethylamine as the base [33] (Scheme 5). The activity of catalyst remains unchanged after three recycling steps.

2.3. HDH Using Ni on Active Carbon

For 5 wt% PhCl in octane in gas phase mixed with hydrogen, Pd on active carbon (10 wt% Pd) was found to be the most active HDC catalyst, with complete reaction even at 200 °C, giving HCl, benzene and cyclohexane as HDC products. Using 10 wt% Ni on active carbon hydrodehalogenates Ph–Cl completely at 250 °C. Products are HCl and benzene. Ni (10 wt%) on γ-Al₂O₃ has a lower HDC activity for Ph–Cl producing benzene quantitatively at 350 °C. The alumina-supported Ni catalyst (10 wt% Ni) has a high activity to Ph–Cl but cracking above ca. 350 °C results in rapid deactivation, probably due to coke formation [13]. Carbon-supported Ni catalysts prepared using Ni–Al layered double hydroxide/carbon nanocomposite precursors displayed excellent activity in liquid phase HDC Ph–Cl with TOF in range 0.1–7.1 × 10² s⁻¹ depending on structure and Ni/C ratio of prepared catalysts [47].

2.4. Raney Nickel Based HDH

Attempts were made to dehalogenate Ar–Xs even in the liquid phase especially using more active forms of Ni catalysts. It was published that at a low temperature (70 °C) working in the liquid phase, the HDH activity increased in order: Ni/SiO₂ < Ni/Al₂O₃ < Raney Ni < Ni/C [34].

Using liquid phase conditions and a reaction temperature between 60–70 °C, the Raney Ni-catalyzed HDH proceeds well using an elevated pressure of hydrogen (1 MPa) in the presence of alkaline hydroxide for the removal of produced HX by neutralization. Under the liquid phase conditions, a gradual loss of catalyst activity was observed in all probability due to the covering of the catalyst’s surface with NaX. After aqueous treatment causing NaX dissolution, the catalyst activity was restored [34].

Rapid HDH of tetrabromobisphenol A catalyzed by the suspension of the Raney Ni catalyst and on cathode electrochemically produced nascent hydrogen was published using co-action of ultrasound [35] (Scheme 6).

Recently, Ma et al. published a straightforward HDC of chlorinated phenols catalyzed by Raney Ni (20 mg per mmol of 4-CP and 1.1 mmol of NaOH) at room temperature and ambient pressure of hydrogen in an alkaline aqueous solution. The employed Raney Ni was successfully reused more than five times without a significant effect on HDC conversion. The authors observed that the applied combination of triethyl amine, together with NaOH as the base, is the most effective combination in this HDC process due to the minimizing of Al/Ni leaching from the Raney Ni catalyst during the HDC process [36]. For comparison, the complete HDC was observed after 30 min using 5% Pd/C (5 mg per mmol 4-CP in co-action of 1.05 eq. of NaOH) at room temperature and ambient pressure of hydrogen [48]. Using Rh nanoparticles (2.45 mg Rh/L) under the same reaction conditions, the complete non-selective HDC of 4-CP (100 mg/L) was achieved after 1 h, producing phenol, cyclohexanone and cyclohexanol [49].

The HDH of lipophilic Ar–Xs catalyzed by Ra–Ni is significantly enhanced by the addition of excess of aqueous NaOH or KOH for neutralizing of the produced HX with the addition of the phase-transfer catalyst to enable facile transport of hydroxide anions into the immiscible organic phase containing Ar–Xs [37].

3. Hydrogentransfer in Ni-Catalyzed Hydrodehalogenation Reactions

Transfer HDH reaction, described as the addition of hydrogen to a molecule from a non-H₂ hydrogen source, is a convenient and powerful method to obtain various nonhalogenated compounds (Ar–Hs) from starting Ar–Xs. It is an attractive alternative to direct HDH. The reasons for this are (i) the transfer HDH method does not require potentially hazardous pressurized H₂ gas nor elaborate experimental setups, (ii) the hydrogen donors are readily available, inexpensive and easy to handle, (iii) the major side product is potentially recyclable, and (iv) the usually used catalysts are readily accessible and not sensitive [50,51,52,53,54].

For transfer hydrogenation reactions, in-situ generated Ni(0) is generally used which is more active in comparison with Ni/Al₂O₃, Ni/SiO₂ or Ni/TiO₂ [55,56].

Powerful reductants such as ionic metal hydrides (NaBH₄ and its derivatives, CaH₂) or several electropositive metals are usually used in the published Ni-catalyzed transfer HDHs in contaminated water or organic solvents [51,52,53,54,55,56,57].

It is known that simple Ni(II) salts react with NaBH₄ in water or an ethanol solution to form a black precipitate (nickel or nickel boride) which is a powerful hydrogenation catalyst under heterogeneous conditions. Solvents of a greater coordinating power, such as N,N-dimethylformamide, Ni(II) chloride and NaBH₄ reportedly constitute a soluble hydrogenation catalyst [51] (Figure 1 and Scheme 7).

Tabaei et al. studied the effective HDH of polychlorinated biphenyls (PCBs) in tetrahydrofuran (THF) solution using an excess of NaBH₂(OCH₂CH₂OCH₃)₂ and the addition of NiCl₂ at 68 °C after ten hours of action [58]. Using the same reaction conditions, 1,2,4-trichlorobenzene was hydrodechlorinated with a high efficiency to benzene and chlorobenzene [59] (Scheme 8).

Scrivanti et al. later even published the complete HDH of 1,2,3-trichlorobenzene using an excess of NaBH₄, the catalytic amount of Ni(Ph₃P)₂Cl₂ salt with the co-action of an extra two equivalents of Ph₃P in ethanol/pyridine solution at 70 °C after several hours of action. The formation of Ni(0) stabilized with Ph₃P is proposed as a catalytically active Ni(0)-based species which oxidatively inserts into the C–Cl bond and enables its subsequent reduction [60] (Scheme 9).

Figure 1. The structures of broadly used ligands of nickel complexes catalytically active for HDH [61,62,63,64,65,66,67].

Figure 1. The structures of broadly used ligands of nickel complexes catalytically active for HDH [61,62,63,64,65,66,67].

Recently, a series of Ni(NNP) pincer complexes with HDH activity in co-action of excess of NaBH₄ as a reductant was described by Wang and Gardinier [61]. This system (especially in case if R = methoxy or methyl) was recognized as effective mainly for hydrodebromination of Ar–Brs and hydrodeiodination of Ar–Is but is very slow in case of HDC of Ar–Cls in N,N-dimethylacetamide (DMA) even at 80 °C [61] (Scheme 10).

The main drawback of above-mentioned methods seems to be poor utilization of used NaBH₄ reductant. Only one hydride from NaBH₄ reductant is used for HDH of Ar–X with subsequent loss of untapped BH₃. This drawback is potentially removed by the method described by Lipshutz et al. using Me₂NH.BH₃ as the reductant, Ni(Ph₃P)₂Cl₂ as the source of Ni-catalyst and K₂CO₃ as the HX scavenger in acetonitrile [62]. This method affords facile HDH of aromatic iodides, bromides and even chlorides (Scheme 11).

Sakai et al. described facile HDH of Ar–Xs using an excess of zinc powder and aqueous NaOH in ethanol catalyzed by colloidal nickel produced in-situ from added NiBr₂ at 60 °C. The oxidative addition of Ni into Ar–X bond is considered with subsequent reduction to Ar–H and recycling of catalytically active nickel [63].

Prichodko et al. published the promoting effect of ionic liquids for Ni-catalyzed HDH using powdered zinc in the role of reductant and Ni(PPh₃)₂Cl₂ with co-action of PPh₃ as HDH catalyst. In the described procedure, different Ni(L)_nCl₂ (L = Bpy, Phen, PPh₃) complexes were tested as Ni-based HDH and/or homocoupling catalysts in 1-alkyl-3-methylimidazolium chloride or other ionic liquids (1-alkyl-3-methylimidazolium halides, AlkMIMXs) and compared with the application of N,N-dimethylacetamide as the polar aprotic solvent [64] (Scheme 12). The calculated TON is 18–20.

Shteingarts and Adonin described effective co-action of Zn powder as the reductant and NiX₂ together with appropriate ligand (2,2′-bipyridyl (bipy) or phenanthroline (phen)) for partial aromatic hydrodefluorination in case of polyfluorinated aromates [60,61]. Using this Ni-based HDH system, (the hydride complex HNiXL_n probably operates as the reductant [65,66]) (Scheme 13).

It is well known that the catalytic system based on complexes of Ni(II) with bipy even enables polymerization of dichloroaromates in tetrahydrofuran using NaH as the reductant [67] (Scheme 14).

Tetrabromodiphenyl ethers dissolved in minute concentrations in a methanolic solution of triethylamine were hydrodebrominated by hydrogen produced upon irradiation of dispersed Ni-graphite carbon nitride C₃N₄ catalyst [68].

Based on recognition of Ni(II) role in coenzyme F₄₃₀, some artificial models were synthetized and tested as biomimetic catalysts in HDH reactions. Only slow partial HDC of hexachlorobenzene to pentachlorobenzene using Ni-complex F₄₃₀ with Ti(III) citrate as the electron donor was, however, observed. The authors proved that Co-complex such as vitamin B₁₂ is much more effective in HDH reactions using Ti(III) citrate [69]. Based on the structure of coenzyme F₄₃₀, structurally simpler complexes were tested as possible biomimetic HDH catalysts [51].

Ni(II) complexes of several tetraazacycles were reported to form soluble complexes with NaBH₄, but no catalytic activity was ascribed to these compounds. The 2,6-disubstituted pyridine-based Ni(II) complexes (Figure 2) were described as effective homogeneous HDH catalysts for polychlorobenzenes reduction using NaBH₄ or hydrazine in mixtures of polar organic solvents (acetonitrile, ethanol) or their aqueous solutions [70,71].

Isopropyl zinc bromide and especially tert-butyl magnesium bromide used as reductants and Ni(II) 1-benzoyl-5-hydroxypyrazoline complex (NiNOO) as the catalyst enable facile dehalogenation not only of halogenobenzene derivatives in tetrahydrofuran but even of hexachlorobenzene [72] (Scheme 15). The use of sufficient excess (over 6 eq.) of t-BuMgCl enables even complete HDC of hexachlorobenzene.

LiH-based HDH catalyzed by Ni-complex produced by the reaction of tert-butyl alcohol with LiH and nickel acetate in hot tetrahydrofuran or 1,2-dimethoxyethane was recognized as being very effective for halogenobenzenes or halogenonaphtalene [73,74,75,76,77,78] (Scheme 16).

Subsequent research of Fort et al. developed a new HDH system based on Ni–Al clusters produced in-situ from Ni(II) acetate and Al(III) acetylacetonate using sodium alkoxide/sodium hydride in THF [79,80,81,82] (Scheme 17).

4. Decontamination Methods Based on Transfer HDH for Treatment of Contaminated Aqueous Solutions

Probably over 90% of hazardous waste is aqueous and its toxic effect is dependent on the kind and concentration of contaminants dissolved or dispersed in produced waste water [82,83].

4.1. Raney Nickel Applications in Transfer HDH

Ma et al. published a smooth HDH of chlorophenols with H₂ in an alkaline aqueous solution using suspension of the Raney nickel catalyst [84]. The used Ra–Ni is recyclable 5-times without loss of HDC activity.

Wang et al. reported an increase in the HDH rate using ultrasonic action on triclosan catalyzed by Raney Ni saturated with gaseous H₂ [85].

Raut et al. published effective HDC of 4-chlorophenol (30 mg/L) caused by a high excess of Raney Ni (3 g/L) in an aqueous solution by co-action of trialkyl amines (45 mg/L) without the addition of other reductants [86]. The observed HDC was explained by the partial dissolution of Raney Ni in an aqueous trialkylamine solution with subsequent H₂ formation which adsorbed on catalytically active nickel sponge and caused HDC of 4-chlorophenol (Scheme 18). The proposed mechanism is based on the observed increase of dissolved Ni in the reaction mixture during described HDC process [86].

For comparison of HDC activity, Ra–Ni (1 g/L) or Pd(5%)/C (1 g/L) were tested for HDC of 4-CP (30 mg/L). After 24 h of action, Pd/C caused complete HDC of 4-CP whereas Ra-Ni achieved 84% conversion to phenol [86].

Study of Ma et al. [87] compares activity of Raney nickel and Pd/C as HDH catalysts. It was found that HDH of halogenated benzenes or halogenated phenols using 120 mg Ra-Ni is usually more rapid in case of application 20 mg Pd/C under otherwise same reaction conditions. Beside this, the tested catalysts differ in hydrogenolytic scission reactivity of Ar–X.

4.2. Utilization of Raney Al–Ni Alloy in HDH Processes

Another simply available HDH system applicable for Ar–X contaminated wastewater, serve alloys or bimetals containing electropositive metal and nickel applied as HDH catalyst.

For effective HDH of Ar–Xs, the Raney Al–Ni alloy was broadly studied in alkaline aqueous solution contaminated with Ar–X. Raney Al–Ni alloy is commercially simply available and low-cost. This alloy contains 50 wt% of Al and 50 wt% of Ni bound in the form of Al₃Ni and Al₃Ni₂. Different bases such as NaOH or KOH [88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104], alkali metal carbonates [89,94,95], NaF [100] or some alkaline salts of complexing agents [91,101] were proved for HDH processes. It was observed that Raney Al–Ni alloy is, especially in diluted aq. NaOH or KOH, a very effective and universal HDH agent enabling quantitative HDH of Ar–Br, Ar–Cl compounds at elevated temperature [89,90,95,96,99,102,104] or even at room temperature [81,82,83,84,85,86,87,88,89,90,91,92,93,94,98,103].

According to the mentioned articles studying Al–Ni alloy HDH properties, the above-mentioned Al–Ni alloy is a quite robust HDH agent in diluted aqueous solutions of alkali metal hydroxides. Raney Al–Ni alloy was reported as an applicable reduction agent for complete HDH of halogenated anilines [88,91], (poly)chlorinated benzoic acids [100], chlorinated benzenes [88], mono- and dichloro-biphenyls [95,97], polybrominated diphenyl ethers [89], (poly)halogenated phenols or polyhalogenated bisphenol A [89,90,92,100,101], chlorinated benzils [103] and halogenated benzophenones [96].

HDH is accompanied by reduction of other reducible functional groups, carbon-carbon bond cleavage or hydrogenation of aromatic ring, especially at elevated temperature. HDH of chlorobiphenyls produces biphenyl together with phenylcyclohexane [95,97] using excess of Al–Ni at temperature above 60 °C (Scheme 19).

Tetrachloro- or tetrabromo-bisphenol A are converted into a mixture of nonhalogenated phenols and cyclohexanols using Al–Ni even in 1% aqueous alkali metal or alkali earth metal hydroxide at a temperature 60 °C or higher [89] (Scheme 20).

Halogenated benzil is reduced to the mixture of 1,2-diphenylethane, 1-cyclohexyl-2-phenylethane and 1,2-diphenylethane [104] in 1% aqueous KOH at 90 °C (Scheme 21).

Under the same reaction conditions, chlorinated benzophenones are reduced to dicyclohexylmethane, benzylcyclohexane and diphenylmethane [96].

At least (poly)halogenated anilines, (poly)chlorinated benzenes, (poly)chlorinated benzoic acids and polyhalogenated phenols are selectively reduced, however, to the corresponding nonhalogenated aromatic compounds at room temperature using a low excess of Al–Ni alloy in 1–2% aqueous NaOH or KOH solutions, as we observed [88,91,92,93,94,98,103].

In case of 2,4,6-tribromophenol, the interconnection of HDH with subsequent biodegradation of produced phenol was tested after removal of insoluble part of Raney Al–Ni alloy (Al₃Ni₂) and coagulation/flocculation of insoluble Al(III) hydroxide [92] (Scheme 22).

In case of Chlorophene, the HDC reaction is accompanied by formation of 2-benzylcyclohexanol even at room temperature [94] (Scheme 23).

Only in the case of 2-chlorophenol, KF instead of the corresponding hydroxide was proved as an applicable base for complete HDH after 2 h of action of a high excess of Al–Ni alloy (10 g Al–Ni per 50 mg of 2-CP mixed with 40 mmol KF) [100]. In contrast, this configuration of HDH enables recycling of the huge excess used and during HDC process undissolved Al–Ni after washing of the produced insoluble AlF₃ layer from Al–Ni surface with Ca(OH)₂ suspension.

It was observed that alkaline salts (alkali metal borate or phosphate) inhibit HDH process utilizing Al–Ni alloy. This inhibition is likely caused by the formation of insoluble Al(III) salts which cover active Al–Ni surface. Alkali metal salts (of citric or ethylenediaminetetraacetic (EDTA) acid) which enables both corrosion of metallic Al and formation of soluble Al(III) salts enables complete HDH, as was observed [91] and citations herein, [101].

For complete HDH of halogenated anilines and phenols, the effective molar ratio of reactants NaOH (or KOH):Al (used as Al–Ni alloy):Ar–X is 10:2:1. This ratio enables complete HDH of Ar–Xs until 120 min under vigorous stirring at room temperature. On the other hand, we observed that in case of halogenated anilines the quantity of Al–Ni can be significantly decreased after addition of glucose [93]. Used glucose inhibits dissolution of metallic Al in alkaline aqueous solution and caused formation of Ni nanoparticles, as we documented. We suppose that formation of nanoparticles Ni saturated with H₂ enables effective HDH of halogenated anilines [93].

The application of glucose enables decrease of Al–Ni alloy quantity in case of reactive Ar–Xs, such as brominated anilines and 3-chloro- or 2-chloro-anilines but not 4-chloroaniline [93]. These mentioned reactive Ar–Xs are simply hydrodehalogenated even using Devarda’s Al–Cu–Zn alloy, as we published earlier [88]. HDH of hardly reducible chlorinated compounds such as 4-chloroaniline or other polychlorinated aromatic compounds is not promoted by the addition of glucose, as we described earlier [91]. In this context the hardly reducible compounds (4-chloroaniline, etc.) are not hydrogehalogenated even using Devarda’s Al–Cu–Zn alloy [88,91]. The effective HDH of 4-chloroaniline or other hardly reducible Ar–Xs at room temperature requires the reductive action of Raney Al–Ni alloy bound in Al₃Ni. The mentioned crucial role of Al–Ni we explained as HDH of adsorbed Ar–Xs on Al₃Ni/Al₃Ni₂ surface immersed in diluted aqueous NaOH solution working as a galvanic couple composed by soluble Al anode and stable and catalytically active Ni cathode [88,98].

The applicability of Raney Al–Ni alloy was successfully tested including the treatment of contaminated water streams on a pilot plant scale [103].

4.3. Ni-Based Bimetals Applied in Transfer HDH

For effective HDH treatment, applicable for degradation of Ar–Xs in contaminated water, several research teams have tested monometallic (Al, Fe, Mg, Zn) [4,105,106,107] or bimetallic reduction systems prepared by plating of electropositive metal surfaces with Pt or Pd (Pd/Zn, Pt/Fe, Pd/Fe, Pd/Mg, Pd/Al) [108,109,110,111,112].

Applying non-plated electropositive metals, the kinetic of HDH in Ar–Xs is slow even when using nanoparticles. This means that effective HDH requires a significant excess of the used electropositive metal [4,105,106,107,108].

The performance of bimetallic HDH systems for Ar–Xs is usually explained by the dissolution of electropositive zero-valent metal with the production of nascent hydrogen and the functioning of precious metal (commonly from platinum group metals (PGMs) as a HDH catalyst [109,110,111,112]. PGMs used for plating of electropositive metals as HDC catalysts are quite sensitive to poisoning and very expensive [6,7,109,110,111,112].

Due to these reasons, cheaper bimetallic reductants have been examined in recent years based on Ni in the role of HDC catalysts for plating of electropositive metals Ni/Fe and Ni/Zn [108,113,114,115,116,117,118,119,120,121]. It was even reported by Cheng et al. [122] that addition of micronized Ni(0) could re-activate zerovalent iron particles that have lost their reduction activity.

The accepted model describing mechanism of action of Fe/Ni bimetal in HDH processes involved utilization of accelerated corrosion of metallic iron especially on Fe-Ni local galvanic couples as the first step. Dissolution of iron is accompanied by formation of hydrogen which is adsorbed on Ni surface. The produced Ni nano and/or micro- particles satured with hydrogen subsequently work as the HDH agent [113,114,115,116,117,118,119,120,121]. This mechanism is supported by the observation that compounds producing insoluble Fe-salts (for example humic acids [113] inhibit the HDH rate.

Apart from the above-mentioned, in the case of HDH of pentachlorophenol, the authors used sole nano iron or mixture of nickel and iron nanoparticles instead of plated Ni/Fe and compare HDH based on the size of the added nickel nanoparticles. The authors observed an increase of HDH with a decreased size of Ni and no HDH in the case of sole Ni nanoparticles application without the addition of nano Fe [115].

Ni/Fe bimetallic (nano)particles are quite popular and broadly tested in recent years due to sufficient efficiency, simple preparation and low cost together with low toxicity of iron [113,114,115,116,117,118,119,120]. This was demonstrated as effective for HDH of chlorinated phenols [113,115,121], DDT [114], the anti-inflammatory drug Diclofenac [116], polychlorinated biphenyls [117], polybrominated diphenyl ethers [119] and chlorinated nitrobenzenes [120] at room temperature.

The choice of pH is quite problematic because metallic iron corrodes mainly in a neutral or acidic aqueous solution, but the HDH reaction is promoted by the addition of a base which removes produced HX [113,114,115,116,117,118,119,120,121].

In case of HDH of DDT Fe/Ni bimetal caused only partial HDC in the CCl₃ group of DDT structure [114] and authors suggested reductive effect of both Fe(OH)₂/Ni or Fe/Ni. The source of reductant depends on pH. While in acidic reaction mixture Fe/Ni as main active HDH reagent is assumed, in alkaline solution Fe(OH)₂/Ni seems to be predominant HDH agent [114] (Scheme 24).

Liu et al. elucidated the decrease of 2,2′,4,4′-tetrabromodiphenylether HDH reaction rate with increasing pH by the formation of insoluble iron (hydr)oxides on Fe/Ni surface which inhibit further corrosion of Fe⁰ and the production of hydrogen fundamental of the HDH process [119]. Even using optimal HDH conditions, the authors only identified partially debrominated products (4-bromodiphenylether and 2-bromodiphenylether) [119].

Ghauch et al. demonstrated that even Fenton-like oxidation is promoted by Fe/Ni bimetal in the case of aerobic treatment of the Diclofenac solution which is documented by the determination of hydroxylated Diclofenac in the obtained reaction mixture using LC-MS [116]. In contrast, when oxygen was excluded in the reaction mixture during Fe/Ni-based HDH, only products of HDC were observed after Fe/Ni treatment [116].

Lin et al. observed the enhancement of the HDH rate during pentachlorophenol reduction induced by Fe/Ni with the co-action of cationic surfactant [118]. The authors explained this observation by an increase of the specific surface area of Fe/Ni particles in co-action with the cationic surfactant and the higher adsorption rate of pentachlorophenol in this manner prepared Fe/Ni particles and their stabilization [118].

5. The Role of HDH for Application of Halide as Protecting and Directing Group

The above-mentioned Ni-based HDH methods seem to be suitable, not only for detoxification of Ar–Xs containing waste or treatment of polluted water streams, but especially these effective at room temperature for removal of halogen used as the protecting, respective directing, group in aromatic chemistry of organic fine chemicals syntheses [3,123,124].

Applying broadly used electrophilic aromatic substitution reaction is linked with the problem of selective introduction of functional group(s) with the aim to obtain aromatic compounds with the desired chemical structure. The desired orientation of bound substituent(s) may be achieved by blocking the most reactive position in the aromatic ring of the substrate, the carrying out desired substitution reaction followed finally by the removal of the protective group.

Ni-based HDH could be the method of choice for room-temperature and selective removal of halogen used as the protective group including possibility of deuteration for the syntheses of 2- or 3-substituted anilines or acetanilides [124,125], 2,6- or 3,4-disubstituted anilines [125,126], 2-substituted benzoic acids [3,124] (Scheme 25), 2-substituted benzonitrile [124], hydroxydiphenylmethanes [124], hydroxydiphenyls [124], 2-substituted phenols [123,124,125], 2,4- or 2,5-disubstituted phenols [123,124,125], 2,5-disubstituted quinoline [125].

6. Available Methods for Recycling of Spent Ni Catalysts

Each of the above-mentioned HDH methods based on application of Ni catalysts is limited with the durability of the applied catalyst. Even metallic nickel is much cheaper in comparison with platinum metals-based catalysts (actual price about 20,000 USD/t), and for sustainability of HDH methods the recycling of used Ni is required. For this purpose, hydrometallurgical processes would seem to be suitable energy-saving alternatives to the common pyrometallurgical ones [127,128,129,130,131]. Hydrometallurgical processing methods are environmentally friendly due to the low energy requirements, low gas emissions and waste generations and complete recovery of the metals [127]. They are based mainly on mineral acid leaching (hydrochloric, nitric or sulfuric acid) with appropriate co-action of oxidant (hydrogen peroxide, persulfate, etc.) [127,128,129,130,131,132] (Scheme 26). The obtained refined nickel salts such as nickel chloride, nitrate or sulfate are usually used for preparation of Ni-based HDH catalysts and eventually reduced directly to metallic nickel by the action of electropositive metal, NaH₂PO₂ or NaBH₄ [51,60,63,128,129,130,131].

7. Conclusions

Ni-catalyzed gas phase HDH was developed for reductive treatment of concentrated streams of waste halogenated aromatic compounds (Ar–Xs) produced as by-products or non-simply-recyclable mixtures such as distillation residue from the production of perchloroethylene, application of o-dichlorobenzene as the solvent, etc. The above-mentioned Ni-based gas phase HDH method potentially enables subsequent recycling of the obtained HDH products (Ar–Hs) as raw materials in organic technology. In the last years, however, the occurrence of published articles dealing with gas phase Ni-based HDH is rare. Gas phase HDH is included as part of soil remediation (REACH) technology, for example [133]. It was proved that standard supported Ni–Mo catalysts used for hydrodesulfurization are effective even for HDC which offers broad utilization in reductive treatment of Ar–Xs contaminants produced by pyrolysis of plastic waste [134].

It is possible that the necessity of special apparatus, application of catalyst and supply of hydrogen disadvantages gas phase HDH as hazardous waste treatment method towards incineration. The waste incineration is probably cheaper as non-catalytic thermal oxidative technology consuming air oxygen and sometimes even natural gas as the supporting fuel. On the other hand, incineration produces low quantities of toxic and thermodynamically stable organic by-products such as polyhalogenated dibenzo-p-dioxines and/or polyhalogenated dibenzofurans and polyaromatic compounds. Nevertheless, these stable organic by-products are reactive enough for effective destruction via liquid phase HDH methods. HDH in the liquid phase seems to be applicable for treatment of waste containing halogenated aromatic contaminants due to the lower temperature used with subsequent lower detrition of the used Ni catalyst.

In the future, it is likely that new findings dealing with searching of the new suitable supports and looking for synergistic effects of additional elements [18,33,135] for improvement of catalytical activity, selectivity and catalyst stability could be expected in the area of Ni-catalyzed HDH processes using gaseous hydrogen.

Special techniques were developed for transfer HDH of some Ar–Xs in organic solvents using Ni-based complexes and reductants such as metal hydrides or Zn(0). These techniques are demanding, however, on the high quantity of the used anhydrous solvents, auxiliary compounds such as ligands (triphenylphosphine, 2,2′-bipyridyl or phenanthroline), proton sponges (pyridine) and due to this reason, cause poor atom economy, and lack the opportunity for broader technological applications of these HDH techniques [132]. On the other hand, if used Ni(Ph₃P)_n HDH catalyst and used reductant were recyclable, the above-mentioned techniques utilizing organic solvents working under homogeneous conditions could be a promising detoxification method for treatment of waste Ar–Xs in the near future.

Reductive degradation of Ar–Xs-based contaminants in alkaline aqueous solution or mixed alkaline alcohol/water solution using Raney Al–Ni alloy seems to be ready for use in treatment processes. The only limitation could be the complicated recycling of produced Ni-waste which needs several hydrometallurgical steps with exceptional subsequent high temperature treatment [100,130,131].

In contrast, the yet published application of Fe/Ni bimetals for Ar–Xs HDH seems to be quite problematic, with inconsistent HDH results and often producing only partially dehalogenated products. Other authors advert to unreplaceable role of Ni in iron-based HDH [136]. Due to these reasons, even though Fe/Ni bimetal is a cheap reductant, it should be more deeply investigated for HDH applications especially in HDH of mixtures of Ar–Xs. Ni-based bimetals containing other electropositive metals such as Mg or Al could in all probability be more profitable for efficient HDH of Ar–Xs in contaminated wastewaters.

In conclusion, nickel as a cheap nonprecious metal with HDH efficiency impending to platinum group metals in HDH reactions seems to be potentially broadly applicable for significant decreasing of environmental impacts joined with the utilization of halogenated aromatic compounds.