Aqueous-phase Hydrodechlorination Research Articles

Enhanced catalytic performance induced by synergistic interaction between Pd species with NiAl2O4 spinel support for aqueous-phase hydrodechlorination of 4-chlorophenols

4-Chlorophenol (4-CP) is a pervasive environmental pollutant commonly found in wastewater and groundwater, posing great risks to both human health and the ecosystem. Catalytic hydrodechlorination (HDC) of 4-CP by utilizing hydrogen (H2) as a reductant has emerged to be an efficient and sustainable way to treat pollutant water. This study focuses on the development of an efficient and stable Pd-based catalyst for the catalytic HDC of 4-CP. By incorporating an appropriate amount of Ni into Al2O3 support, a NiAl2O4 (NA) spinel was synthesized, enhancing the interaction between Pd and the support, leading to precise control over Pd species and oxygen vacancies. The tailored properties of the Pd/NA-1 catalyst result from meticulous adjustment of the Pd2+/Pd0 ratio and oxygen vacancies, which promotes electron transfer from Pd to Ni within the NiAl2O4 spinel. In comparison to the Pd/Al2O3 catalyst, the Pd/NA-1 catalyst exhibits exceptional stability, with continuous operation for over 120 h without deactivation. Comprehensive characterization analyses indicate that the Pd/NA-1 catalyst effectively prevented the aggregation of Pd nanoparticles, maintaining the Pd2+/Pd0 ratio at 1, which is crucial for its superior stability. This work provides valuable insights into the development of Pd-based catalysts for HDC reactions, highlighting the potential use of Pd/NA-1 catalysts in treating hazardous pollutants like 4-CP in waste systems.

Animated organic-inorganic hybrid materials and their use as catalyst scaffolds

For the past two decades, organic-inorganic hybrids (OIHs) have been researched extensively and tailored in a facile manner to suit a wide variety of applications due to their highly functional and tunable nature. The molecular scale mixing of the organic and inorganic components imparts unique properties to these materials. Stimuli-responsiveness or animated nature is one such property, resulting from the non-covalent interactions of the organic groups present in close proximity with the inorganic (-Si-O-Si-) framework. In this article, we provide a comprehensive review of the animated silica-based OIHs synthesized by sol-gel technique. The earlier sections of the review focus on the development and synthesis of animated materials derived from bis-[3-(trimethoxysilyl)propyl] ethylenediamine (enTMOS) and bis-trimethoxysilylethyl benzene (BTEB). Their response to stimuli and the principles governing their animated nature along with the effect of synthesis parameters on their properties have been discussed. The latter portion of the article offers a detailed overview of the application of swellable organically modified silica (SOMS) supported Pd catalysts for aqueous phase hydrodechlorination of the toxic organic contaminant, trichloroethylene, which is commonly found in groundwater. The affinity of SOMS to organics and its ability to swell and adsorb organics from aqueous solutions impacted the accessibility of the active Pd sites and imparted resistance to deactivation against anionic species encountered in groundwater. Finally, other catalytic applications of SOMS and an outlook on the future research avenues and applications of SOMS have been presented.

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.

Laminar N-Doped Carbon Materials from a Biopolymer for Use as a Catalytic Support for Hydrodechlorination Catalysts.

Nitrogen-doped porous carbons were prepared using a chitosan biopolymer as both a carbon and nitrogen precursor and metallic salts (CaCl2 and ZnCl2-KCl) as a templating agent with the aim of evaluating their performance as catalyst supports. Mixtures of chitosan and templating salts were prepared by simple grinding subjected to pyrolysis and finally washed with water to remove the salts. The resulting materials were characterized, showing that homogeneous nitrogen doping of carbon was achieved (7–9% wt.) thanks to the presence of a nitrogen species in the chitosan structure. A lamellar morphology was developed with carbon sheets randomly distributed and folded on themselves, creating slit-shaped pores. Substantial porosity was observed in both the micropore and mesopore range with a higher surface area and microporosity in the case of the materials prepared by ZnCl2-KCl templating and a larger size of mesopores in the case of ZnCl2. Catalysts with well-dispersed Pd nanoparticles (around 10 nm in diameter size) were synthesized using the chitosan-based carbons obtained both by salt templating and direct chitosan pyrolysis and tested in the aqueous phase hydrodechlorination of 4-chlorophenol. The fast and total removal of 4-chlorophenol was observed in the case of catalysts based on carbons obtained by templating with CaCl2 and ZnCl2-KCl in spite of the low metal content of the catalysts (0.25% Pd, wt.).

On the dual role of the reactant during aqueous phase hydrodechlorination of trichloroethylene (HDC of TCE) using Pd supported on swellable organically modified silica (SOMS)

Herein, the dual role of trichloroethylene (TCE) in liquid-phase hydrodechlorination with Pd catalysts supported on swellable organically modified silica (SOMS), is investigated. Batch experiments with TCE-soaked Pd/SOMS showed 45% higher conversion after 4 h than dry Pd/SOMS, suggesting that TCE acts as a reactant and as a swelling-agent. Flow experiments performed with and without ethanol (external swelling-agent) exhibited similar steady-state activities, confirming that TCE itself can cause swelling. Moreover, flow experiments, regardless of whether the SOMS-supported Pd catalysts were pre-reduced or not, yielded similar steady-state conversions, suggesting that Pd(2+) is rapidly reduced by H2 dissolved in water during approach to steady-state. In-situ characterization performed under reaction conditions using time-resolved X-ray absorption spectroscopy confirmed the reduction of Pd(2+) species to Pd(0) by dissolved H2. Overall, this study shows economic viability of hydrodechlorination of TCE using Pd/SOMS by eliminating the need for an external swelling-agent as well as a catalyst pre-reduction step.

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.

Palladium loaded BEA zeolites as efficient catalysts for aqueous-phase diclofenac hydrodechlorination

Herein the catalytic performance of two 1 wt% Pd loaded BEA zeolites with Si/Al ratio of 19 and 1300 were investigated for the aqueous phase hydrodechlorination (HDC) of diclofenac, the latter compound a prevalent nonsteroidal anti-inflammatory drug. 1 wt%Pd/SiO2 and 1 wt%Pd/Al2O3 being commonly used HDC catalysts were also studied for comparison purposes. Independently of the Si/Al ratio, palladium loaded BEA catalysts show over 6 times higher activity than conventional Pd/support catalytic systems. This remarkable behavior is largely due to the effect of the three-dimensional structure of zeolite material and the broad range of Pd NPs size obtained.

Effect of unimodality and bimodality of Pd nanoparticles on the catalytic activity of Pd/SiO2 in the removal of diclofenac from water

In this work, we investigated the catalytic performance of uni- and bimodal Pd/SiO2 catalysts in the aqueous phase hydrodechlorination (HDC) of diclofenac – a very popular nonsteroidal anti-inflammatory drug. Unimodality and bimodality was confirmed by temperature-programmed hydride decomposition (TPHD) and TEM. The highest activity and 100% efficiency in batch and flow mode were observed for bimodal Pd NPs. This outstanding efficiency could be attributed to the close coexistence of small and large metal nanoparticles with different affinity to hydrogen and chloroorganic compound. The obtained results encourage discussion on the necessity to strive for perfect unimodal catalysts for the HDC processes.

Aqueous-phase hydrodechlorination of 4-chlorophenol on palladium nanocrystals: Identifying the catalytic sites and unraveling the reaction mechanism

Hydrodechlorination (HDC) catalyzed by palladium has important applications in the disposal of chlorinated organic pollutants (COPs), yet the associated catalytic sites and the underlying reaction mechanism have not been clearly revealed. Here, we use Pd nanocrystals (Pd NCs) as the probe catalyst to investigate the HDC catalytic sites as well as the reaction mechanism in aqueous solution with 4-chlorophenol as the model COP. It was found that the numbers of edge and corner atoms (defect atoms) on cubic Pd NCs with varying sizes (6.3–18.4 nm) showed linear correlation with the HDC activity, suggesting that these defect atoms are the main catalytic sites for HDC. The production of chloride ion, the absence of Ullmann coupling product in the absence of H2, and the positive effect of electron-withdrawing substituents on HDC demonstrate that the HDC should include an oxidative addition process that follows the nucleophilic aromatic substitution.

Two-dimensional covalent-organic-framework-derived nitrogen-rich carbon nanosheets modified with small Pd nanoparticles for the hydrodechlorination of chlorophenols and hydrogenation of phenol

Heteroatom-doped carbon materials have attracted immense attention for heterogeneous catalysis applications because of their excellent physicochemical properties. In this work, we prepared a cost-effective triazinyl-containing two-dimension COFs (2D-COFs), and then the 2D-COFs was self-templated carbonized to fabricate 2D nitrogen-doped carbon nanosheets (NCNs). NCNs were modified with highly dispersed small palladium nanoparticles (Pd NPs), affording the Pd/NCNs catalyst. The as-prepared NCNs and Pd/NCNs exhibited a smoke-like morphology, a large specific surface area, and mesoporous and macroporous structures, which can enhance reactants diffusion and mass transfer. The obtained Pd/NCNs catalyst exhibited excellent activity, stability, and recyclability in the aqueous-phase hydrodechlorination (HDC) of organic hazardous substances chlorophenols (CPs). Meanwhile, outstanding activity and high selectivity were observed for the subsequent hydrogenation of the HDC product phenol to cyclohexanone. The excellent activity of the Pd/NCNs catalyst was related to the synergistic effect of Pd NPs and the nitrogen atoms doped in the NCNs. Thus, this study offers a novel, facile route for fabrication heteroatom-doped carbon materials derived from cost-effective COFs materials to prepare highly dispersed small noble-metal-NP-modified catalysts for various catalytic applications.

Aqueous-phase hydrodechlorination of trichloroethylene over Pd-based swellable organically-modified silica (SOMS): Catalyst deactivation due to chloride anions

Swellable-organically modified silica (SOMS) has been demonstrated to be an efficient catalyst scaffold for catalytic treatment of water contaminated with trichloroethylene (TCE). In this study, deactivation characteristics of Pd-incorporated SOMS for aqueous-phase hydrodechlorination (HDC) of TCE were investigated. Pd/SOMS catalysts were exposed to highly-concentrated chloride solutions (up to 1 M NaCl or 0.01 M HCl) to examine the deactivation resistant behavior of Pd/SOMS. The commonly used HDC catalyst Pd/Al2O3 was also studied for comparison purposes. Pd/SOMS and Pd/Al2O3 in their pristine and treated states were tested for aqueous-phase HDC of TCE and characterized by several techniques including N2 physisorption, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. The aqueous-phase treatments had a pronounced adverse effect on the textural properties of Pd/Al2O3, although the effect was independent of the type of the chloride precursor, NaCl or HCl. Treating Pd/Al2O3 with chloride-containing solutions lowered the catalytic activity due to formation of Pd-Cl complexes and active metal leaching. The leached Pd obtained from the treatment solution was shown to be inactive for aqueous-phase HDC of TCE. While Pd/Al2O3 underwent severe deactivation due to the chloride treatments, Pd/SOMS exhibited resistance to chloride deactivation and metal leaching. The chloride treatments did not impact the textural properties of Pd/SOMS. The achieved deactivation resistance was attributed to the novel characteristics of the SOMS support.

Synthesis of palladium phosphides for aqueous phase hydrodechlorination: Kinetic study and deactivation resistance

We report the synthesis of noble metal phosphides by a temperature-programmed reduction method for aqueous phase hydrodechlorination (HDC) of 4-chlorophenol (4-CP). Pd3P0.95/SiO2, Rh2P/SiO2 and PtP2/SiO2 catalysts were prepared and showed higher 4-CP HDC rates than their corresponding noble metal catalysts. Pd3P0.95/SiO2 sample exhibited a higher HDC rate than the other phosphide catalysts and Pd/SiO2 catalysts. The XPS spectra show the surface interaction reactions of the Pd and P species that occur via the electron transfer from Pd to P. Pd3P0.95/SiO2 was synthesized with different phosphide particle size (1.1–13.5 nm based on hydrogen chemisorptions) via changing the metal loading from 1 to 10 wt%. The 4-CP HDC reaction is sensitive to the size of the Pd and Pd3P0.95 particles, and the optimum sizes for Pd/SiO2 and Pd3P0.95/SiO2 were ∼8 and ∼5 nm, respectively. The effects of the initial 4-CP and Cl− ion concentration and the S to Pdsurf (surface active Pd metal sites determined by hydrogen chemisorptions) ratio on the 4-CP HDC activity were examined for both catalysts. The Langmuir-Hinshelwood kinetic model that was developed considering the surface reaction as the rate-determining step suggests P has an enhancing effect on sorption in the 4-CP HDC reaction and an inhibiting effect on chloride sorption for Pd3P0.95/SiO2. Moreover, Pd3P0.95/SiO2 possesses better sulfide resistance than Pd/SiO2 because P can prevent the conversion of Pd metallic sites into inactive Pd4S compounds with a sulfide treatment. As a result, Pd3P0.95/SiO2 shows better HDC stability than those of Pd/SiO2 catalysts.

Aqueous‐Phase Hydrodechlorination of Trichloroethylene on Ir Catalysts Supported on SBA‐3 Materials

AbstractThis work investigates the catalytic properties of silica and aluminosilicate SBA‐3 supported iridium catalysts in aqueous phase hydrodechlorination of trichloroethylene. SBA‐3, AlSBA‐3, Ir/SBA‐3 and Ir/AlSBA‐3 were thoroughly characterized by XRD, BET, TPR‐H2, H2 chemisorption, TPD‐NH3 and investigated in catalytic removal of trichloroethylene from water in batch reactor under mild operation conditions. Uniformly dispersed small iridium nanoparticles (∼3 nm) was found to be active in catalytic purification of water from chloroorganic compound. The studies showed that the differences in catalytic behavior of Ir/SBA‐3 and Ir/AlSBA‐3 depended on the properties of the support. The best catalytic properties in purification of water from trichloroethylene was obtained for Ir supported on more hydrophilic and ordered AlSBA‐3 with acid sites.

Aqueous-phase hydrodechlorination of model chlorinated organic compounds over Ru/C catalyst

ABSTRACTEffluents from the pharmaceutical and dye industries contain chlorinated organic pollutants. The effective treatment of such effluents constitutes a challenging task. In this work, 4-chlororesorcinol (PCRE) and 4-chloro-2-aminophenol (PCAP) were selected as model chloro-organic compounds. Catalytic hydrodechlorination (HDC) of PCRE and PCAP in the aqueous phase was investigated in a slurry reactor using commercial carbon supported ruthenium catalyst. HDC reactions of PCRE and PCAP were studied over the ranges in temperature, 313–353 K, H2 partial pressure, 0.69–2.76 MPa, and catalyst loading, 0.2–1.2 kg/m3. The performance of Ru/C catalyst for hydrodechlorination and ring saturation was promising. The kinetic data were modeled using Langmuir–Hinshelwood–Hougen–Watson kinetics. The HDC reaction of PCRE and PCAP using Ru/C catalyst proceeds via a dual-site mechanism, wherein atomic H2 reacts with the adsorbed organic substrate. The activation energy for the hydrodechlorination reactions of PCRE and PCAP was 41.6 and 49.4 kJ/mol, respectively.

Enhancement of the activity of Pd/C catalysts in aqueous phase hydrodechlorination through doping of carbon supports

Improved catalytic activity in the hydrodechlorination reaction at room temperature with N-doped carbon as the support of Pd nanoparticles.

Effect of structural ordering of the carbon support on the behavior of Pd catalysts in aqueous-phase hydrodechlorination

Catalysts consisting of Pd supported on virgin and heat-treated carbon blacks (homemade and commercial), graphites (natural and synthetic) and commercial carbon nanofibers were prepared and tested in the aqueous phase hydrodechlorinaton (HDC) of 4-chlorophenol (4-CP) under near ambient conditions (30 °C, 1 atm) in order to explore the effect of the support on the catalytic behavior. The homemade graphitized supports were prepared from commercial carbon black (CB) and from a carbon black-like material (CBPE) obtained from pyrolysis of low-density polyethylene. All the catalysts prepared yielded complete 4-CP conversion, although a wide range of activity was observed (10.7–173.5 mmol gPd−1 min−1). The graphitized carbon black provided the most active catalysts, showing Pd nanoparticles around 3 nm in size and a well-balanced contribution of Pd species (Pdn+/Pd0 = 0.9). Substantial differences of activity were found between the graphitized and not graphitized supports, even for catalysts with similar Pd nanoparticle size. The higher activity of the catalysts with graphitized supports can be partly associated to a more balanced initial Pdn+ to Pd0 ratio. Moreover, higher selectivity to hydrogenation products (cyclohexanone) was also achieved with those catalysts due to a higher contribution of the Pd0 species.

Effect of metal precursor and pretreatment conditions on\xa0the catalytic activity of Ni/C in\xa0the aqueous phase hydrodechlorination of 1,1,2-trichloroethene

Active carbon supported nickel catalysts prepared with nickel(II) nitrate and nickel(II) chloride precursors after reduction at 673 or 1173 K were investigated in the aqueous phase hydrodechlorination of 1,1,2-trichloroethene (TCE). These nickel catalysts facilitate the purification of water via decomposing TCE, but their catalytic properties depend on the chemistry of the nickel precursor used for Ni/C synthesis and pretreatment conditions. The chemistry of precursor catalysts affects the particle size, which in turn is correlated with catalytic activity. We observed the highest activity for Ni/C with the largest nickel particles. The application of high temperature reduction of nickel precursors increases the activity and minimizes the differences between the catalytic properties of the samples prepared from different salts.

Gold-doping of carbon-supported palladium improves reduction catalysis

Bimetallic palladium-gold (PdAu) catalysts have better catalytic performance than monometallic catalysts for many applications. PdAu catalysts with controlled nanostructures and enhanced activities have been extensively studied but their syntheses require multiple and occasionally complicated steps. In this work, we demonstrated that supported PdAu catalysts could be simply prepared by doping a supported Pd catalyst with gold through wet impregnation and calcination. Resulting PdAu-on-carbon (PdAu/C) catalysts were tested for the room-temperature, aqueous-phase hydrodechlorination of trichloroethene. The most active PdAu/C catalyst (Pd 1.0 wt%, Au 1.1 wt%, dried/air/H2 process) had an initial turnover frequency (TOF) of 34.0 × 10−2 molTCE molPd−1 s−1, which was >15 times higher than monometallic Pd/C (Pd 1.0 wt%, initial TOF of 2.2 × 10−2 molTCE molPd−1 s−1). Through X-ray absorption spectroscopy, the gold kept Pd from oxidizing under calcination at 400 °C. Probable nanostructure evolution pathways are proposed to explain the observed catalysis.

Influence of preparation procedure on catalytic activity of PdBEA zeolites in aqueous phase hydrodechlorination of 1,1,2-trichloroethene

Pd-loaded BEA zeolites containing 1 wt % of Pd were prepared by a two-step postsynthesis method (PdSiBEA and PdSiAlBEA) and a conventional wet impregnation (PdHAlBEA). Modification of BEA zeolite resulted in the introduction of Pd ions into zeolite framework as pseudo-tetrahedral Pd(II) and extra-framework octahedral Pd(II) evidenced by XRD, DR UV–vis and TPR measurements. Calcination of as prepared zeolites in air at 773 K for 3 h and then reduction in 10% H2/Ar flow at 873 K for 3 h led to obtain red-C-PdSiBEA, red-C-PdSiAlBEA and red-C-PdHAlBEA zeolites with different Pd nanoparticles size distributions showed by TEM. These zeolite materials were found to be active catalysts in aqueous-phase hydrodechlorination of 1,1,2-trichloroethene (TCE). The conducted catalytic reactions have demonstrated that the rate and course of TCE hydrodechlorination depend on the catalysts preparation method and Pd nanoparticles size.

Size-controlled PtNi nanoparticles as highly efficient catalyst for hydrodechlorination reactions

The application of size-controlled PtNi nanoparticles (NPs) as catalyst on the aqueous-phase hydrodechlorination (HDC) of 4-chlorophenol (4-CP) under ambient conditions (25°C, 1atm) has been investigated. NPs were synthesized as a co-reduction of both metals following a solvothermal method which allowed a proper control of the PtNi alloyed NP size in the range of 4.4–12.0nm. To make the active sites of the nanoalloy accessible, the stabilizer poly(N-vinyl-2-pyrrolidone) was removed by washing of the NPs with H2O2/H2SO4. Evaluating the NP structure/composition influence on the activity, a clear synergic effect between Pt and Ni was found. In this sense, complete conversion of 4-CP was achieved in 1h reaction time with the bimetallic NPs whereas only 70% was reached with the monometallic Pt counterpart (NP size=12nm, [Pt]=80mgL−1, [4-CP]0=78μmolL−1). That effect could be related to the higher resistance of PtNi nanoalloy against chlorine poisoning compared to the monometallic Pt. In fact, PtNi NPs showed a remarkable stability and negligible deactivation even after storing the catalyst one month in the reaction mixture, whereas for the monometallic Pt fully deactivation resulted. The structure-sensitivity of HDC with PtNi nanocatalysts was finally confirmed as the catalytic performance was clearly dependent on the size. A volcano plot like behaviour was obtained, being 5.5nm the optimum size. PtNi-5.5 led to the complete conversion of 4-CP in 15min at a HDC rate of 3.12Lmin−1gPt−1.