Hydrodechlorination Research Articles

Hydrodechlorination of chlorophenols with methanol as hydrogen donor over carbon nanotube supported Pd-catalysts

One of the effective methods of degradation and/or removal of chlorophenols, which are persistent environmental pollutants of toxic and resistant to biodegradation, is hydrodechlorination (HDC) via heterogeneous catalysis using liquid hydrogen donors. Here, Pd and Pd-M (M=Zn, Ni, Co) were supported on commercial multiwall carbon nanotubes (MWCNTs) and N-doped MWCNTs (N-MWCNTs), and their catalytic performances for the HDC of 2-chlorophenol (2-CP) using methanol as the hydrogen donor were investigated. The results showed that N-doping and/or the second metal addition to the N-MWCNTs supported Pd catalysts effectively increased their catalytic activity, which is attributed to the modulation of the Pd electronic structure and the improved active hydrogen supply from methanol by introducing the second metal. Particularly, nearly 100% conversion of 2-CP was achieved using Pd-Zn/N-MWCNTs. The catalysts were characterized using TEM, XPS, and XRD techniques to better understand the reaction mechanism, the promotional effects of N-doping in MWCNTs and the addition of second transition metal to the catalyst for the catalytic HDC of 2-CP.

Enhancement of relatively non-toxic dechlorination of PCBs from thermal desorption off-gas over manganese-doped iron-based bimetallic catalyst

In view of the problems existing in catalytic degradation, such as low degradation efficiency, poor dechlorination efficiency and large amount of exhaust gas production, three iron-based bimetallic catalysts were developed for the dechlorination of polychlorinated biphenyls (PCBs) from thermal desorption off-gas, among which manganese-doped iron-based bimetallic catalyst showed the highest degradation efficiency (93.6%) and dechlorination efficiency (92.2%). Specifically, the effect of dechlorination under the air conditions was studied. The GC–MS analysis of intermediate products suggested the presence of three synergistic degradation pathways, namely, hydrodechlorination (HDC), preliminary oxidation, and deep oxidation over manganese-doped iron-based bimetallic catalyst. Through the hydrodechlorination, the chlorine atoms of PCBs were successively removed to obtain the final product biphenyl. During the oxidation stage including preliminary oxidation and deep oxidation, PCBs and intermediates were oxidized by reactive oxygen species (•OH and O2–•). This study provides an efficient and safe dechlorination technology of PCBs in the presence of oxygen with little exhaust gas generated.

Design of hydrodechlorination catalysts on the basis of chloromethanes-metallic active sites interactions

The adsorption of chloromethanes, namely trichloromethane (TCM), dichloromethane (DCM) and monochloromethane (MCM), and hydrogen on metal clusters of Pd, Pt, Rh and Ru was studied by molecular simulation to learn on the design of more efficient catalysts for the hydrodechlorination (HDC) of those species. All the above metals showed high dechlorination ability, following in all cases the order TCM > DCM > MCM. The computational study consigns that dissociative adsorption of chloromethanes is promoted on the zero-valent metal (M0) sites while non-dissociative adsorption, associated to catalyst deactivation, is strongly favored on the electro-deficient ones.The dechlorination ability of M0 species follows the order Ru > Rh > Pt > Pd, with greater differences in the dechlorination of TCM. Conversely, hydrogenation ability remarkably decreases from Pd and Pt to Rh and Ru catalysts. This determines a more favorable balance of reactant adsorption energies in the HDC with Pd, especially for TCM HDC. Based on the theoretical findings, catalysts with high proportion of zero-valent metal species, supported on CeO2-Sm2O3 (CS), were prepared, characterized and tested in the HDC of TCM and DCM in lab-scale continuous fixed bed experiments. The conclusions obtained by simulation were supported by the experimental results. The high proportion of zero-valent species in the Pd/CS catalyst, besides the favorable interactions between active sites and both chloromethanes and H2, allow an outstanding performance of the catalyst in the HDC of those chloromethanes, with a significantly higher dechlorination activity and stability than the other metals for the HDC of TCM.

Aqueous phase hydrodechlorination of trichloroethylene using Pd supported on swellable organically modified silica (SOMS): Effect of support derivatization

Herein, the role of swellability and hydrophobicity of swellable organically modified silica (SOMS) in shielding Pd against deactivation due to HCl produced during hydrodechlorination (HDC) of trichloroethylene (TCE) is investigated. The extent of surface derivatization during sol gel synthesis of SOMS was found to directly impact the extent of hydrophobicity, swellability and surface area, as confirmed by infrared spectroscopy and N2 physisorption, respectively. Furthermore, after Pd impregnation, the resultant particle size, location, and atomic environment of Pd were also dictated by the extent of support derivatization such that the least derivatized material provided lowest protection to Pd from HCl. Post HCl-treatment, the batch activity rate constants decreased by 66% for the least derivatized sample and 17% for the most derivatized one, suggesting that hydrophobicity and swellability are essential for obtaining high resistance to HCl which could potentially impact the economic viability of HDC of TCE process.

Wastewater treatment via hydro-de-heteroatoms using hydrogen donors

Aiming at the resource recovery from high concentration heteroatom-containing organic wastewaters, this article reviews the literature studies on catalytic hydro-de-heteroatoms (HDHs), such as hydrodechlorination (HDC), hydrodenitrogenation, hydrodesulfurization and hydrodeoxygenation, concerning on the applied catalysts, hydrogen donors, operation conditions, and the related catalytic performances. In particular, the work regarding HDC using small oxygenate molecules such as methanol and formic acid as hydrogen donors to recover hydrocarbons as fuels or useful organics from wastewaters containing high concentration of chlorinated organic compounds has been highlighted. To further improve the catalytic HDHs, the possible challenges are outlined and advanced strategies are discussed.

Novel insights into the mechanism for protic solvent promoting Pd/C-catalyzed hydrodechlorination of chlorinated organic compounds

• Protic solvents promoted catalytic HDC of 4-CBP with Et 3 N over Pd/C catalyst. • HDC rate of 4-CBP increased with increase of water content in mixed solvent systems. • Mechanism for protic solvents promoting Pd/C-catalyzed HDC reactions was elucidated. • Protic solvents participated in Pd/C-catalyzed HDC and hydrogenation reactions. Solvent effects in liquid-phase hydrodechlorination (HDC) of chlorinated organic compounds (COCs) over Pd/C catalyst were systematically studied. For the HDC of 4-chlorobiphenyl (4-CBP) with triethylamine (Et 3 N), Pd/C exhibited much higher catalytic activity in protic solvents than that in aprotic polar and non-polar solvents, and a clear correlation between the HDC rate and polarity of protic solvents was observed. Moreover, water promoted Pd/C-catalyzed HDC of 4-CBP in homogeneous alcohol-water solvents and biphasic organic-water solvents with Et 3 N, and the HDC rate of 4-CBP increased with the increase of water content in the above solvent systems. Catalyst characterization (FT-IR and XRD) combined with solvent parameters and isotope labelling analysis were introduced to explain in-depth the solvent effects in the HDC of 4-CBP with Et 3 N. It was found that protic solvents would transfer active hydrogen atom (H*) via hydrogen bonding network or exchange their hydroxyl hydrogen with H*, which was beneficial to the stabilization of H* and interface HDC reaction. Thus, the higher HDC rate was achieved in solvent systems contained protic solvents with higher hydrogen-bond donor capability ( α ) values. On the other hand, non-polar and polar aprotic solvents had a negative effect on H* and mass transfer, and thus suppressed interface HDC reaction. Similar solvent effects were observed in Pd/C-catalyzed hydrogenation of unsaturated bonds-containing groups (nitryl group, carbonyl group, and carbon-carbon double bond). The results presented here would provide vital guidance for the rational selection of solvent systems for Pd/C-catalyzed HDC of COCs and hydrogenation of unsaturated bonds-containing groups.

A kinetic model for 2,4-dichlorophenol adsorption and hydrodechlorination over a palladized biofilm

Adsorption and catalytic hydrodechlorination (HDC) of aqueous 2,4-DCP by palladium nanoparticles (Pd0NPs) associated with a biofilm (i.e., a Pd0-biofilm) was investigated in terms of the removal efficiency of 2,4-DCP, dechlorinated product selectivity, and reduction kinetics. Experiments were executed with Pd0-biofilm and with abiotic Pd0NPs-film alone. The 2,4-DCP-adsorption capacity of Pd0-biofilm was 2- to 5-fold greater than that of abiotic Pd0NPs-film, and the adsorption accelerated dechlorination by Pd0-biofilm, including selectivity to phenol instead of mono-chlorophenols. A mechanistic kinetic model was developed to represent the sequential adsorption and reduction processes. Modeling results represented well the removal of 2,4-DCP and quantified that Pd0-biofilm had a strong affinity for adsorbing 2,4-DCP. The strong adsorption increased the volume-averaged concentration of 2,4-DCP concentration inside the Pd0-biofilm, compared to the concentration in the bulk liquid. This increase in the local concentration of 2,4-DCP led to a 2- to 4-fold increase in the reduction rate of 2,4-DCP in Pd0-biofilm, compared to abiotic Pd0NPs-film. Thus, coupling Pd0NPs with the biofilm promoted 2,4-DCP removal and full dechlorination despite its low concentration in bulk water.

Borohydride and metallic copper as a robust dehalogenation system: Selectivity assessment and system optimization

Hydrodechlorination (HDC) using noble-metal catalysts in the presence of H-donors is a promising tool for the treatment of water contaminated by halogenated organic compounds (HOCs). Cu is an attractive alternative catalyst to noble metals since it is cheaper than Pd, Rh, or Pt and more stable against deactivation. Cu with borohydride (BH4−) as reductant (copper-borohydride reduction system; CBRS) was applied here for the treatment of saturated aliphatic HOCs. The HDC ability of CBRS was evaluated based upon product selectivities during reduction of CCl3–R compounds (R = H, F, Cl, Br, and CH3). For CHCl3, CH2Cl2, and CHCl2–CH3, the dechlorination reaction proceeds predominantly via α-elimination with initial product selectivities to CH4 and C2H6 of 84–85 mol-% and 70–72 mol-%. For CCl4, CBrCl3, CFCl3, and CCl3–CH3, stepwise hydrogenolysis dominates. CH2Cl–R compounds are formed as recalcitrant intermediates with initial selectivities of 50–72 mol-%, whereas CH4 and C2H6 are minor products with 16–35 mol-% and 30–35 mol-%. The effect of reaction conditions on product selectivities were investigated for CHCl3 as target. Solution composition, variation of reducing agents (BH4−, H* from H2) and increase of electron pressure (electric potential at Cu electrode and Fe0 as support) did not have marked influence on the selectivities (ratio of CH4 : CH2Cl2). Product selectivities for reduction of CCl3–R compounds were found to be substrate-specific rather than reductant-specific. Since the formation of halogenated by-products could not be avoided, transformation via a second reduction step was optimized by higher catalyst dose, addition of Ag, and vitamin B12 to the CBRS. Comparison between Pd and Cu based on costs, catalyst activities, selectivities, metal stability, and fate of halogenated by-products shows that the CBRS is a potent alternative to conventional HDC catalysts and can be recommended as ‘agent of choice’ for treatment of α-substituted haloalkanes in heavily contaminated waters.

Insight into degradation mechanism of PCBs from thermal desorption off-gas over iron-based catalysts

Iron-based catalysts were developed to achieve the hydrodechlorination (HDC)/oxidation of polychlorinated biphenyls (PCBs) from thermal desorption off-gas, and Fe3O4/γ-Al2O3 showed higher dechlorination efficiency than Fe2O3/γ-Al2O3. The optimal Fe loading resulted in 95.5% degradation efficiency and 76.9% toxicity reduction of gaseous PCBs, and the optimal Fe3O4/γ-Al2O3 exhibited excellent stability during a 60-h test. The gas chromatography–mass spectrometry analysis of intermediate products indicated the presence of two competitive degradation pathways, namely, hydrodechlorination and oxidation with Fe3O4/γ-Al2O3 as catalyst. During the first stage (reductive dechlorination), the reductive activity of iron-based catalysts was effectively enhanced in the presence of water, which was confirmed by density functional theory (DFT) calculations. The removal of chlorine atoms was found in the order of meta > para > ortho. During the second stage (oxidation), hydroxyl and superoxide anion radicals were found to attack PCBs on the surface of Fe3O4/γ-Al2O3. This study provides an insight into the HDC and oxidation mechanism of gaseous PCBs over iron-based catalysts.

Palladium-based Catalytic Membrane Reactor for the continuous flow hydrodechlorination of chlorinated micropollutants

This work is focused on the development of a Pd-based catalytic membrane reactor (Pd/CMR) for its application in continuous flow hydrodechlorination (HDC) of chlorinated micropollutants. A cylindrical inert porous alumina membrane was decorated on the outer with well-dispersed Pd nanoparticles (0.2 % wt.). The Pd/CMR was fully characterized by different techniques. The catalytic system was applied for the degradation of the persistent anti-inflammatory drug diclofenac (DCF) under ambient conditions. A remarkable stability was observed upon long-term HDC reaction, with a reasonable DCF conversion (60 %). Notably, as the process could be described by pseudo-first order kinetic, the initial concentration did not affect the final conversion achieved. Finally, the versatility of the system was successfully demonstrated using a real aqueous matrix (tap water). All in all, Pd/CMR exhibited a remarkable feasibility for the implementation of the HDC technology on a larger scale, showing an extraordinary catalytic performance for up to 200 h.

Promoted catalytic hydrodechlorination for deep degradation of chlorophenols over Rh-La/SiO2 catalyst

Deep degradation of chlorophenols (CPs) into safe and ecofriendly cyclohexanol during catalytic hydrodechlorination (HDC), shows important practical significance and attractive prospect in the treatment of wastewater containing chlorophenols. An efficient Rh-La/SiO2 catalyst was developed, by employing La as promoter. The presence of La in catalyst promoted catalytic performance of HDC significantly. Characterization results revealed that the interaction occurred between Rh and La in Rh-La/SiO2 catalyst. This interaction accompanied with the high dispersion and finely particle size of active Rh, and generation of abundant Rh sites neighboring La atom. Kinetic study illustrated that Rh-La(1:1)/SiO2 catalyst possessed the fastest kinetic constants, and minimized the apparent activation energies of 4-CP, phenol and cyclohexanone greatly. Complete degradation of 4-CP with a very high yield of cyclohexanol (> 98%) can be achieved at room temperature, making Rh-La(1:1)/SiO2 catalyst to be a promising candidate for deep degradation of CPs during HDC and other Rh catalyzed hydrogenation reactions.

Progress in Catalytic Hydrodechlorination

Catalytic hydrodechlorination (HDC) is a technology with great potential for the treatment and valorization of organic chlorinated wastes [...]

Ordered Mesoporous Carbon as a Support for Palladium-Based Hydrodechlorination Catalysts

Ordered mesoporous carbon (OMC) was employed as a support for palladium nanoparticles in catalysts for the gas phase hydrodechlorination (HDC) of trichloromethane (TCM). 1 wt% palladium was incorporated using three methods: incipient wetness (IW); a dilute solution (DS) method; and a solid-liquid (SL) method. The effect of the preparation method on catalyst structure and activity was investigated. Catalyst composition and nanostructure were studied using gas physisorption, high specification transmission electron microscopy and X-ray photoelectron spectroscopy. Catalytic conversion and product selectivities were determined in steady-state activity tests at temperatures between 70 and 300 °C. Two of the catalysts (IW and DS) showed excellent dispersion of fine Pd nanoparticles of average diameter ~2 nm. These materials showed excellent activity for HDC of TCM which compares favourably with the performance reported for Pd on amorphous carbon catalysts. In addition, they showed relatively high selectivities to the more valuable higher hydrocarbons. However, the SL method gave rise to catalysts with larger particles (~3 nm) and a less uniform palladium distribution. This resulted in lower conversion and lower selectivities to higher hydrocarbons and in more severe catalyst deactivation at the highest reaction temperatures.

Elucidating the role of ethanol in aqueous phase hydrodechlorination of trichloroethylene over Pd catalysts supported on swellable organically modified silica (SOMS)

In this study, effect of ethanol on the performance of Pd supported on swellable organically modified silica (SOMS) for hydrodechlorination (HDC) of trichloroethylene (TCE) is investigated. SOMS is a hydrophobic material that swells to 3–4 times its original volume when contacted with organic solvents. Catalytic HDC of TCE suffers from inhibition due to HCl which is worsened by the slow removal of HCl from the hydrophobic pores of SOMS. Addition of ethanol resulted in swelling of SOMS matrix causing expansion of pores, thereby facilitating the removal of HCl. The inhibitory effect of HCl was alleviated by ethanol, without altering the HDC reaction pathway. Characterization using near-ambient pressure X-ray photoelectron spectroscopy and extended X-ray absorption fine structure showed the effect of ethanol on swellability and HCl removal, respectively. Lastly, a mechanism has been proposed for the uninhibited reaction, which exhibited first order kinetics with respect to TCE.

P‐doped Carbon as the Efficient Support of Nickel Catalysts for Hydrodechlorination of Chlorodifluoromethane

AbstractP‐doped carbon material (P−C) is successfully fabricated through directly combust phytic acid via microwave assistant. Then the as‐made P−C is selected as the support to load various amount of Ni. As the evaluated results show that 5 %Ni/P−C exhibits better catalytic performance than 5 %Ni/C for chlorodifluoromethane hydrodechlorination (HDC) reaction. It is because that the doped‐P is feasible to promote the dispersion of Ni species on the surface of P−C through the strong interaction between Ni and the P−C support. Therefore, combining the synergetic effect between Ni and heteroatom‐doped carbon could provide a promising way to improve the activity of HDC reaction.

The Effect of Shape-Controlled Pt and Pd Nanoparticles on Selective Catalytic Hydrodechlorination of Trichloroethylene

Tailoring the shape of nanoscale materials enables obtaining morphology-controlled surfaces exhibiting specific interactions with reactants during catalytic reactions. The specifics of nanoparticle surfaces control the catalytic performance, i.e., activity and selectivity. In this study, shape-controlled Platinum (Pt) and Palladium (Pd) nanoparticles with distinct morphology were produced, i.e., cubes and cuboctahedra for Pt and spheres and polyhedra/multiple-twins for Pd, with (100), (111 + 100), curved/stepped and (111) facets, respectively. These particles with well-tuned surfaces were subsequently deposited on a Zirconium oxide (ZrO2) support. The morphological characteristics of the particles were determined by high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD), while their adsorption properties were investigated by Fourier transform infrared spectroscopy (FTIR) of CO adsorbed at room temperature. The effect of the nanoparticle shape and surface structure on the catalytic performance in hydrodechlorination (HDCl) of trichloroethylene (TCE) was examined. The results show that nanoparticles with different surface orientations can be employed to affect selectivity, with polyhedral and multiply-twinned Pd exhibiting the best ethylene selectivity.

Enhanced selectivity to olefins in the hydrodechlorination of trichloromethane using Ag-Pd on activated carbon catalysts

Two Ag-Pd (nominal 1 % wt each) bimetallic catalysts supported on commercial activated carbon were synthesized and tested, for the first time, in the hydrodechlorination (HDC) of trichloromethane (TCM) to obtain light olefins. Two different Pd precursors were used, PdCl2 and Pd(NO3)2, being the resulting catalysts denoted as AgPdCl/C and AgPdN/C, respectively. A monometallic Ag catalyst was also prepared for comparison purposes. The two different Pd precursors led to different metal particle sizes and zerovalent to electrodeficient metal ratios. The monometallic Ag catalyst showed a poor dechlorination, in contrast with the bimetallic ones. AgPdN/C yielded high selectivity to paraffins, while AgPdCl/C was more selective to olefins (mostly ethylene), the desired reaction products. This better behavior of AgPdCl/C in terms of selectivity to ethylene and propylene was probably due to the existence of smaller Pd nanoparticles as monometallic sites. These Pd sites are active for the conversion of chloromethanes into light unsaturated hydrocarbons, while larger Pd clusters present higher hydrogenation ability. AgPdCl/C allowed complete TCM conversion, with high overall dechlorination and outstanding 75 % selectivity to olefins, showing very high stability at a reaction temperature of 350 °C for almost 35 h on stream. These results represent a significant improvement respect to those obtained with monometallic Pd on activated carbon catalysts reported in our previous studies. To the best of our knowledge, for the first time it is reported such high selectivity to olefins as the above indicated from HDC of a chloromethane.

Dechlorination of 2,4-dichlorophenol in a hydrogen-based membrane palladium-film reactor: Performance, mechanisms, and model development

We created a hydrogen-based membrane palladium-film reactor (MPfR) by depositing palladium nanoparticles (PdNPs) on hollow-fiber membranes via autocatalytic hydrogenation to form a Pd-film. The MPfR was used for hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP). HDC performances and mechanisms were systematically evaluated, and a continuous-flow dechlorination model was established. Approximately 87% of the input 2,4-DCP was reduced to the end-product phenol (P), while 2-chlorophenol (2-CP) was an intermediate, but only at 2%. Selective adsorption of the 2,4-DCP onto the Pd-film and fast desorption of P facilitated efficient dechlorination. Modeling results represented well the concentrations of 2,4-DCP and its intermediates. It demonstrated three dechlorination pathways: The majority of 2,4-DCP was completely dechlorinated to P in an adsorbed state without release of monochlorphenol, some 2,4-DCP was degraded to 2-CP that was released and subsequently adsorbed and reduced to P, and a small amount was reduced to 4-CP that was released and subsequently adsorbed and reduced to P. Analysis based on Density Functional Theory suggests that the pathway of full dechlorination was dominant due to its thermodynamically favorable adsorption configuration, with both Cl atoms bonded to Pd. This study documents full dechlorination of 2,4-DCP in the MPfR and the interacting roles of adsorption and HDC.

Hydrodechlorination and deep hydrogenation on single-palladium-atom-based heterogeneous catalysts

Halophenols are widely present in wastewater and groundwater, which threaten global water safety and human health. Importantly, hydrodechlorination (HDC) via heterogeneous catalysis is efficient and environmentally friendly. To further improve the catalytic performance and atomic economy, Pd-based single atom catalysts (SACs) were prepared through electrostatic attraction and used in the catalytic HDC for the first time. By substituting different defect-rich supports, the coordination environments of atomically dispersed Pd-atoms were mediated with different catalytic performance. The results demonstrate that the Pd/CeO2 SACs have the highest HDC activity and remain stable during cycle tests. The moderately intensive metal-support interactions (MIMSI) effect was identified as a critical factor for the higher activity of SACs, unlike the traditional strong metal-support interaction effect. The HDC process on Pd SACs was further investigated by combining kinetic experiments with density functional theory calculations, which facilitated the recognition of HDC intermediates and an understanding of the reaction mechanism.

Hydrodechlorination of Different Chloroaromatic Compounds at Room Temperature and Ambient Pressure—Differences in Reactivity of Cu- and Ni-Based Al Alloys in an Alkaline Aqueous Solution

It is well known that the hydrodechlorination (HDC) of chlorinated aromatic contaminants in aqueous effluents enables a significant increase in biodegradability. HDC consumes a low quantity of reactants producing corresponding non-chlorinated and much more biodegradable organic compounds. Two commonly used precious metals free Al alloys (Raney Al-Ni and Devarda’s Al-Cu-Zn) were compared in reductive action in an alkaline aqueous solution. Raney Al-Ni alloy was examined as a universal and extremely effective HDC agent in a diluted aqueous NaOH solution. The robustness of Raney Al-Ni activity is illustrated in the case of HDC of polychlorinated aromatic compounds mixture in actual waste water. In contrast, Devarda’s Al-Cu-Zn alloy was approved as much less active for HDC of the tested chlorinated aromatic compounds, but with a surprisingly high selectivity on cleavage of C-Cl bonds in the meta and sometimes the ortho position in chlorinated aniline and sometimes chlorinated phenol structures. The reaction of both tested alloys with chlorinated aromatic compounds in the aqueous NaOH solution is accompanied by dissolution of aluminum. Dissolved Al in the alkaline HDC reaction mixture is very useful for subsequent treatment of HDC products by coagulation and flocculation of Al(OH)3 caused by simple neutralization of the alkaline aqueous phase after the HDC reaction.