Hydrodechlorination of 3-chloropyridine and chlorobenzene in methanol solution over alkali-modified zirconia-supported palladium catalysts

The liquid-phase hydrodechlorination of 3-chloropyridine and chlorobenzene has been studied over alkali-modified zirconia-supported palladium catalysts. The modification of the ZrO₂ with alkali metal carbonates improves the catalytic activity of the final palladium catalyst. Therefore, the larger the ionic radii (Li⁺ < Na⁺ < K⁺), the greater the catalytic activity (TOF) of the palladium catalyst. For 3-chloropyridine, hydrodechlorination proceeds without catalyst deactivation. This is explained as the result of the interaction of reaction products (pyridine and HCl) forming pyridinium chloride, thus avoiding the detrimental effect of HCl on the palladium particles. Catalytic hydrodechlorination of chlorobenzene over Pd catalysts exhibits an initial catalytic activity (TOF) much lower than that of 3-chloropyridine and the Pd catalysts deactivate as the reaction proceeds. Finally, chlorobenzene hydrodehalogenation has also been carried out in the presence of an equimolecular amount of pyridine resulting in a decrease in the initial reaction rate on the one hand, but also in an increase in final conversion on the other.     

Characterization of supported Pd-Pt catalysts by chemical probes

The temperature programmed (palladium) hydride decomposition appeared very effective method for diagnosing the extent of alloying in Pd-Pt/SiO₂ catalysts. Introduction of Pt to Pd catalysts moves the TPHD peak gradually towards lower temperatures and the shift in hydride decomposition peak is accompanied by a decrease in the amount of released hydrogen. The catalysts prepared the direct redox method showed much better alloy homogeneity than the samples prepared by coimpregnation. The results of probing with other techniques, i.e. catalytic probing with hydrodechlorination of CCl₂F₂ and XRD, were in line with TPHD data. The catalytic probing showed the presence of synergistic effect, a clear indication of Pd-Pt mixing. XRD data of post-reaction samples showed carbon incorporation into Pd-rich phase, whereas separate Pt-rich phase did not experience any bulk carbiding.     

Hydrotreating processes for catalytic abatement of water pollutants

Opportunities and problems in application of catalytic hydrotreating processes, which are presented a viable approach in the abatement of water pollutants, are discussed. Analysis of the hydrodechlorination (HDC) and hydrodenitrification (HDN) using Pd-based catalysts supported on various materials like granulated activated carbon (GAC), fibrous activated carbon cloths (ACCs) or glass fiber cloths (GFCs) studied recently in our laboratory suggests the following perspectives:

Exhaustive regeneration of Pd/GAC saturated with p-chlorophenol can be achieved in a two-step approach, incorporating gas-phase HDC by hydrogen followed by oxidation by air.

Pd/ACC catalysts are good candidates for the liquid-phase HDC showing activity higher than that of Pd/GAC or Pd/GFC; the high adsorption capacity of Pd/ACC lead suggesting its use in a technology with periodic adsorption and HDC, in similarity to adsorption with regeneration of GAC.

Pd/GFC and Pd–Cu/GFC are promising catalyst for removal of nitrites and nitrates, showing activity and selectivity that compares favorably with those of powdered catalysts.

含氯芳烃催化加氢脱氯动力学研究

利用加压微反装置在Ｐｄ／Ａｌ２Ｏ３催化剂上研究了邻二氯苯加氢脱氯动力学。     

One-pot method to synthesize 4,6-bis(isopropylamino)resorcinol through hydrodechlorination and catalytically reductive mono-N-alkylation

2-Chloro-4,6-dinitroresorcinol was converted catalytically in one-pot operation to the corresponding N-alkyl secondary aminophenol, 4,6-bis(isopropylamino)resorcinol, in high yield by using acetone as alkyl source and hydrogen as reducing reagent over 10 wt.% Pd/C catalyst. The effects of reaction conditions, such as the amount of acetic acid, concentration of acetone, catalyst amount, and reaction time on the selectivity of 4,6-bis(isopropylamino)resorcinol were investigated.     

The effect of triethylamine on the hydrodechlorination of chlorophenols on Pd/C at low temperature

The hydrodechlorination (HDC) of chlorophenols on 5 wt.% Pd/C catalyst was investigated at low temperature under ordinary pressure by using triethylamine (Et₃N) as a base additive. The inhibition effect of Et₃N on the HDC existed obviously and can be efficiently reduced by stepwise addition of Et₃N. For the first time, the high activity of Pd/C for HDC of chlorophenols was observed at 258 K.     

Electrochemical hydrodehalogenation of polychloromethanes at silver and carbon electrodes

The reductive dehalogenation of CCl₄, CHCl₃, CH₂Cl₂ and CH₃Cl has been investigated by cyclic voltammetry and controlled-potential electrolysis at Ag, glassy carbon (GC) and graphite electrodes in dimethylformamide (DMF) + 0.1 M Et₄NClO₄ in the absence and presence of a proton donor. In particular, the study was focused in the evaluation of the intermediates and final products of the reduction process and how their distribution could be affected by tuning relevant chemical and electrochemical parameters. In general, depending on the value of the applied potential, all polychloromethanes (PCMs) can be partially or completely dechlorinated, methane being exclusively formed in the latter case. The nature of the electrode material and the proton availability of the medium affect drastically the distribution of reduction products. The results point out that at both types of electrode, reduction of PCMs takes place through two competing reaction pathways both leading to methane. One reaction route involves a sequence of reductive dehalogenation steps, with the removal of one chlorine atom at a time, whereas the other is based on hydrogenolysis of carbenes and bypasses the intermediacy of partially dechlorinated PCMs. The presence of a proton source affects substantially the hydrodehalogenation efficiency, enhancing the concentration of intermediate PCMs and the final yield of methane. The silver electrode exhibits an extraordinary electrocatalytic effect resulting in remarkable positive shifts of the reduction potentials of all PCMs with respect to GC. The Ag surface strongly affects the kinetics of the dissociative electron transfer to CH_nCl_(4−n) (n = 0–3) as well as the reactivity of the intermediate radicals, carbanions and carbenes.     

Low-temperature hydrodechlorination of chlorobenzenes on platinum-supported alumina catalysts

Hydrodechlorination of chlorobenzenes on platinum (Pt)-supported γ-alumina and alumina Lewis superacid (AmLSA) catalysts was carried out at room temperature and ambient pressure using a fixed bed flow reactor and a semi-batch reactor. Both the catalysts indicated good activity for the hydrodechlorination, but the former was superior to the latter. The hydrodechlorinations of reactants C₆H_6−xCl_x (x=1, 2, 3) proceeded step-wisely to benzene and then cyclohexane via C₆H_6−yCl_y (y=x−1). The reactions seem to be promoted by the contribution of spillover hydrogen formed on the Pt-supported catalysts. The catalysts deactivated with reaction time and the amount of chlorine that accumulated on the Pt-supported γ-alumina catalyst in the hydrodechlorination of 1,4-dichlorobenzene for 3 h was near to that estimated from the converted reactant molecules. When the deactivated catalysts were treated in a stream of hydrogen above 503 K, the original activity was completely restored, but the deactivation phenomenon with reaction time was observed again.     

Hydrodechlorination of CCl<Subscript>4</Subscript> on Pt–Au/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts

A series of Pt/Al₂O₃ catalysts were prepared by the impregnation method and were characterized by TEM, XRD, H₂ and CO chemisorptions, and investigated in the hydrodechlorination of tetrachloromethane. Three Pt-rich, Pt–Au/Al₂O₃ catalysts (Pt₁₀₀, Pt₉₅Au₅ and Pt₉₀Au₁₀) showed a similar metal particle size (~2.5–2.7 nm), so observed changes in the catalytic behavior are ascribed to alloying effect, especially because a considerable degree of Pt–Au mixing was achieved in the bimetallic samples. It appeared that by introducing very small amount of gold (10 at.%) to platinum, the catalytic activity is increased. It is argued that the occurrence of this moderate synergistic effect is associated with a decreased tendency of surface chloriding when platinum is alloyed with gold. Zbigniew Kowalczyk—deceased.     

Effect of the reducing agent on the hydrodechlorination of dioxins over 2 wt.% Pd/γ-Al2O3

2-Propanol and molecular H₂ (in methanol (MeOH) and MeOH–water) were examined as reducing agents for the liquid phase hydrodechlorination (HDC) of dioxins over 2 wt.% Pd/γ-Al₂O₃. Different amounts of NaOH were added to the reaction mixtures. The 2-propanol and H₂(g)/MeOH systems presented similar HDC activity. Notwithstanding, Pd sintering and graphitic carbon directly bonded to Pd on catalyst surface was observed on samples used with H₂(g)/MeOH. The addition of water to H₂(g)/MeOH decreased Pd sintering and favored dissolution of sodium compounds. However, dioxin degradation efficiency diminished. By contrast, 2-propanol acting both as reducing agent and solvent provided hydrogen to the HDC reaction, avoided metal sintering and Pd–C formation. Besides, almost complete dioxin degradation under mild reaction conditions was obtained. Kinetic experiments of dioxin HDC with 2-propanol showed a maximum net reaction rate and turnover frequency (TOF) for a given initial concentration of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). After that value, both reaction rate and TOF decreased. On the other hand, reaction rates and TOFs of dioxin-like polychlorinated biphenyls (DL-PCBs) linearly increased with concentration.