An exploration of activity loss during hydrodechlorination and hydrodebromination over Ni/SiO2

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

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.

Template-Mediated Ni(II) Dispersion in Mesoporous SiO2 for Preparation of Highly Dispersed Ni Catalysts: Influence of Template Type

Supported Ni catalysts on three mesoporous SiO₂ supports (i.e., SBA-15, MCM-41, and HMS) were prepared using a solid-state reaction between Ni(NO₃)₂ and organic template-occluded mesoporous SiO₂. For comparison, supported Ni catalysts on mesoporous SiO₂ synthesized by the conventional impregnation method were also included. The catalysts were characterized by scanning electron microscopy, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, N₂ adsorption, X-ray photoelectron spectroscopy, H₂ temperature-programmed reduction, transmission electron microscopy, and transmission electron microscopy-energy-dispersive X-ray. The catalytic properties of the catalysts were evaluated using gas-phase catalytic hydrodechlorination of 1,2-dichloroethane. The results showed that upon grinding Ni(NO₃)₂ with template-occluded mesoporous SiO₂, strong coordination between Ni²⁺ and dodecylamine was identified in the Ni(NO₃)₂-HMS system. Additionally, the results of H₂ temperature-programmed reduction revealed that NiO in calcined NiO/HMS was reduced at higher temperature than those in calcined NiO/SBA-15 and NiO/MCM-41, reflecting the presence of a strong interaction between NiO and mesoporous SiO₂ in NiO/HMS. Consistently, the average particle sizes of metallic Ni were found to be 2.7, 3.4, and 9.6 nm in H₂-reduced Ni/HMS, Ni/SBA-15, and Ni/MCM-41, respectively, indicative of a much higher Ni dispersion in Ni/HMS. For the catalytic hydrodechlorination of 1,2-dichloroethane, Ni/MCM-41 synthesized by the solid-state reaction method exhibited a catalytic activity similar to that prepared by the impregnation method, while higher catalytic activities were observed on Ni/HMS and Ni/SBA-15 than on their counterparts prepared by the impregnation method. Furthermore, a higher conversion was identified on Ni/HMS than on Ni/SBA-15 and Ni/MCM-41, highlighting the importance of template type for the preparation of highly dispersed metal catalysts on mesoporous SiO₂.

Hydrodechlorination of polychlorinated molecules using transition metal phosphide catalysts

Ni2P and CoP catalysts (5 wt.% of metal) supported on a commercial SiO2 were tested in the gas phase catalytic hydrodechlorination (HDCl) of mono (chlorobenzene-ClB) and polychlorobenzenes (PCBs) (1,2- dichlorobenzene (1,2-DClB), 1,3-dichlorobenzene (1,3-DClB), 1,4-dichlorobenzene (1,4-DClB), and 1,2,4-trichlorobenzene (1,2,4-TClB)) at atmospheric pressure. It was investigated how the number and position of chlorine atoms in the molecule influence the HDCl activity. The prepared catalysts were characterized by X-ray diffraction (XRD), CO chemisorption, N2 adsorption-desorption at -196°C, and X-ray photoelectron spectroscopy (XPS). Characterization results indicated better active phase dispersion and greater amount of P on the Ni2P catalyst surface. Catalytic results showed that the Ni2P was more active and stable in this type of reactions. The hydrodechlorination activity decreased by increasing the number of chlorine atoms in the molecule and chlorine substituents in close proximity. The observed trend in the HDCl activity was: ClB>1,4-DClB>1,3-DClB>1,2-DClB>1,2,4-TClB. The exception was the catalytic response after 24h on stream observed for the Ni2P in the HDCl reaction of 1,2,4-TClB, which was equal to that observed for the 1,4-DClB molecule, and also yielding benzene as the main reaction product.

A facile approach to enhancing activity of Ni2P/SiO2 catalyst for hydrodechlorination of chlorobenzene: promoting effect of water and oxygen

The Ni₂P/SiO₂ activity for the hydrodechlorination of chlorobenzene was remarkably enhanced by the treatment with H₂O or O₂.

TG-MS investigation of brominated products from the degradation of brominated flame retardants in high-impact polystyrene

The thermal degradation of flame retardant containing high-impact polystyrene (HIPS-Br), one of the most commonly employed plastics in electric and electronic appliances, was examined by thermogravimetry coupled with mass spectroscopy (TG-MS) in order to understand the threat that is posed by the release of hazardous brominated compounds. The HIPS samples contained decabromodiphenylether (DPE) and decabromodibenzyl (DDB) as the flame retardants as well as Sb2O3 as the synergist. The largest number of brominated compounds was obtained in the presence of DPE and Sb2O3 and DDB without Sb2O3. From the degradation of DPE, brominated benzenes, phenols, diphenylethers, and dibenzofurans were identified, and from the degradation of DDB, brominated benzenes, dibenzyls, and phenanthrenes were formed. The interaction between the flame retardant and the polymer matrix resulted in α-bromoethylbenzene. The formation of brominated dibenzodioxins was not observed, probably, due to the low phenol concentration in the polymer melt. No other report has, to our knowledge, ever reported on the formation of brominated phenanthrenes from flame retardants. Because they share similar steric features, it may well be that brominated phenanthrenes are similar in their carcinogen and mutagen potential to dibenzofurans and dibenzodioxins. A plausible mechanism for the formation of the observed compounds is presented, and the role of the synergist is considered.

Continuous catalytic hydrodechlorination of polychlorinated biphenyls (PCBs) in transformer oil

Continuous catalytic hydrodechlorination of polychlorinated biphenyls (PCBs) in the presence of transformer oils was carried out in a fixed bed reactor using a 57.6 wt% Ni on silicon oxide-aluminum oxide (SiO(2)-Al(2)O(3)) catalyst. Reaction temperatures ranging 150-300 degrees C, PCBs concentrations ranging 50-200 ppm, and reaction times ranging 1-8 h were tested. At a higher reaction temperature or at a lower PCBs concentration, catalytic activity was higher and complete dechlorination of PCBs resulted even at long reaction time. Catalyst regeneration using hexane and 0.1 M sodium hydroxide (NaOH) was effective to restore the catalytic activity. Fresh, spent and regenerated catalysts were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. XRD analysis revealed growth of Ni crystallite size of the spent and the regenerated catalysts. XPS analysis showed that a considerable amount of chlorine and carbon species were deposited on the surface of the spent catalyst, which may play a role in the catalysts deactivation.

Catalytic transformation of waste polymers to fuel oil

Waste not, want not: The increase in waste polymer generation, which continues to exceed recycle, represents a critical environmental burden. However, plastic waste may be viewed as a potential resource and, with the correct treatment, can serve as hydrocarbon raw material or as fuel oil, as described in this Minireview.Effective waste management must address waste reduction, reuse, recovery, and recycle. The consumption of plastics continues to grow, and, while plastic recycle has seen a significant increase since the early 1990s, consumption still far exceeds recycle. However, waste plastic can be viewed as a potential resource and can serve, with the correct treatment, as hydrocarbon raw material or as fuel oil. This Minireview considers the role of catalysis in waste polymer reprocessing and provides a critical overview of the existing waste plastic treatment technologies. Thermal pyrolysis results in a random scissioning of the polymer chains, generating products with varying molecular weights. Catalytic degradation provides control over the product composition/distribution and serves to lower significantly the degradation temperature. Incineration of waste PVC is very energy demanding and can result in the formation of toxic chloro emissions. The efficacy of a catalytic transformation of PVC is also discussed.

Ni/SiO2 promoted growth of carbon nanofibers from chlorobenzene: Characterization of the active metal sites

The temporal changes to supported Ni sites during the growth of graphitic carbon nanofibers (GCNs) via the decomposition of chlorobenzene over Ni/SiO2 at 873 K have been investigated. The reaction of chlorobenzene with hydrogen also generated benzene, via catalytic hydrodechlorination, as the principal competing reaction. Reaction selectivity was found to be time dependent with a switch from a preferential hydrodechlorination to a predominant decomposition that generated an increasingly more structured carbon product over prolonged time-on-stream. These findings are discussed in terms of Cl/catalyst interaction(s) leading to metal site restructuring, the latter manifest in a sintering and faceting of the Ni metal particles. The pressure exerted on the metal/support interface due to fiber formation was of sufficient magnitude to extract the Ni particle from the support; the occurrence of an entrapped Ni particle at the fiber tip is a feature common to the majority of GCNs with the incorporation of Ni fragments along the length of the GCN. Metal site restructuring has been probed by temperature-programmed reduction of the passivated samples, H2 chemisorption/temperature-programmed desorption (TPD) and XANES/EXAFS analyses. This restructuring serves to enhance destructive chemisorption and/or facilitate carbon diffusion to generate the resultant GCN. The nature of the carbonaceous product has been characterized by a combination of TEM-EDX, SEM, XRD and temperature-programmed oxidation (TPO).