Gas phase catalytic hydrodechlorination of chlorophenols using a supported nickel catalyst

The gas phase hydrodechlorination of single component and mixtures of o-, m- and p-chlorophenol was studied over the temperature range 423 K≤T≤573 K using a 1.5% (w/w) Ni/SiO₂ catalyst. The variation of catalyst activity with time-on-stream for each isomer is illustrated and the role of reaction temperature and thermodynamic limitations are addressed. The catalyst exhibits both short term and irreversible long term deactivation which is accounted for in terms of competitive adsorption and electronic effects. The hydrogen treatment of both phenol and chlorobenzene under the same reaction conditions are also considered for comparative purposes. The presence of the hydroxyl function enhances the rate of hydrodechlorination via an inductive effect. The relationship between rate and isomer structure is discussed on the basis of reactant adsorption where steric hindrance, in the case of the ortho-form, appreciably restricts dechlorination to such an extent that o-chlorophenol remains unreacted in equimolar o-/p- and o-/m-chlorophenol mixtures. Catalytic hydrodechlorination is viewed as a non-destructive low energy methodology for handling concentrated chlorine gas streams and relationships that describe the dependence of dechlorination rate on chlorine concentration are provided and can be used to evaluate the productive lifetime of the catalyst.     

Catalytic hydrodehalogenation as a detoxification methodology

Catalytic hydrodehalogenation is presented as a viable approach in the non-destructive treatment of concentrated halogenated aromatic gas streams to generate reusable raw material. Nickel loaded (from 1.5 to 20.3% w/w) silica catalysts have been used to hydrotreat a range of halogenated feedstock, where 473 K≤T≤573 K: chlorobenzene, chlorotoluene, chlorophenol, bromobenzene, dichlorobenzene, dichlorophenol, trichlorophenol, pentachlorophenol. The long term (up to 800 h-on-stream) stability of these catalysts has been assessed where the changes in nickel particle size and morphology as a result of the prolonged catalytic step was probed by TEM; each catalyst irrespective of any loss of initial activity was fully selective in solely promoting dehalogenation. In the case of a polychlorinated feedstock, dechlorination can proceed in a stepwise manner to generate a partially dechlorinated product. Hydrodehalogenation appears to occur via an electrophilic mechanism where the presence of electron-donating substituents on the benzene ring enhances the rate of reaction. The reaction is shown to be structure sensitive over Ni/SiO₂ where the hydrodechlorination rates and ultimate yield of the parent aromatic from a polychlorinated reactant is favored by larger nickel particle sizes. A direct contact of the freshly activated catalyst with HCl or HBr gas induced an appreciable growth of the supported metal crystallites. Chlorobenzene hydrodechlorination was suppressed on a HCl or HBr treated Ni/SiO₂ which promoted instead the unexpected growth of highly ordered carbon filaments; this carbon growth is characterized by TEM and SEM. The dependence of the experimental hydrodechlorination and hydrodebromination rates on the gas phase aromatic partial pressure (in the range 0.02–0.1 atm) is adequately represented by a kinetic model involving a non-competitive adsorption of hydrogen and halogenated aromatic where the incoming aromatic reactant must displace the hydrogen halide from the catalyst surface.     

Catalytic Growth of Structured Carbon via the Decomposition of Chlorobenzene over Ni/SiO2

The synthesis of highly ordered carbonaceous materials, including carbon nanofibers, has been the subject of a disparate and burgeoning literature over the past decade. The growth of carbon nanofibers by an atypical catalytic route, the decomposition of chlorobenzene over (10%w/w) Ni/SiO₂, is considered in this paper. The reaction of chlorobenzene with hydrogen in the temperature range 550–700 °C also generated benzene via hydrodechlorination and a volatile component that results from catalytic hydrocracking/hydrogenolysis, The characteristics of the carbonaceous product are illustrated through a combination of high resolution transmission electron microscopy (HRTEM) and temperature programmed oxidation (TPO). The response of carbon yield and structural order to varying reaction time (up to 4 h on-stream) and temperature are presented and discussed. Under identical reaction conditions, the chlorobenzene feed delivered appreciably higher carbon yields than that recorded for the decomposition of benzene while the carbon growth in the former case was significantly more ordered. These findings are discussed in terms of Cl/catalyst interaction(s) and metal site restructuring.     

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.     

Temperature and hydrogen pressure dependences in the ring opening of methylcyclobutane over Pt/SiO2 catalyst

The temperature and hydrogen pressure dependences in the ring-opening reactions of methylcyclobutane were studied over Pt/SiO₂ catalyst. The temperature dependence of the ring opening revealed that the reaction rate vs. temperature curves passed through a maximum. On the basis of this information two temperatures were selected for hydrogen pressure studies: 573 K, close to the lowest temperature at which any reaction took place at all, and 623 K, where the ring opening of methylcyclobutane exhibited the highest rate. The initial formation rate vs. hydrogen pressure dependence curves are of bimodal type at 573 K, but they increase monotonously at 623 K. Over the working catalyst, no significant changes were observed at 573 K, but the curve for pentane formation changed to a large extent at 623 K. At 573 K, the selectivity of ring opening was close to statistical, with little excess of isopentane (sterically less hindered direction) over both the clean and the working catalyst. This was also observed at 623 K, however, over the working catalyst as the hydrogen pressure increased the selectivity of the ring opening increased as well. Moreover, at the highest hydrogen pressures studied excellent selectivity for the formation of isopentane was observed. The mechanisms over the initial and the working catalysts are discussed on the basis of these experimental findings.     

Gas-phase Hydrodechlorination of Chlorobenzene Over Silica-supported Ni2P Catalysts Prepared Under Different Reduction Conditions

The influence of reduction conditions on the properties and reactivity of silica-supported nickel phosphide (Ni₂P/SiO₂) catalysts for gas-phase hydrodechlorination (HDC) of chlorobenzene was investigated. The catalysts prepared under different reduction conditions had the similar specific surface area (∼370 m² g^-1), and pore diameter (∼5.2 nm) and Ni₂P crystallites size (10–13 nm). However, comparing with Ni₂P/SiO₂ catalyst prepared at 923 K with the H₂ space velocity of 15,000 mL g⁻¹ h⁻¹ for 2 h, with increasing the H₂ space velocity from 15,000 to 19,200 mL g⁻¹ h⁻¹, or the reduction temperature from 923 to 1023 K or the reduction time from 2 to 6 h, the Ni/P ratio in the prepared catalyst was increased from 1/0.56 to about 1/0.50, and the HDC reaction induction period over the catalyst was also shortened from above 30 to 2–4 h. It is suggested that the induction period might be due to the blocking of active sites by excess phosphorus, which results in hindering the activation of chlorobenzene and the adsorption of hydrogen species on Ni₂P. Under the conditions of 573 K and W _Ni/F _Cl = 186.6 g_Ni min, the chlorobenzene conversion over the Ni₂P/SiO₂ catalyst reached 99%, and it did not change during 36 h. The good activity and stability of the Ni₂P/SiO₂ catalyst was ascribed to the weak interaction between chlorine and Ni₂P and a great of spilt-over hydrogen species on the catalyst surface.     

High-pressure synthesis of ethyl benzoate from chloro- and bromobenzene

When chlorobenzene was reacted with ethyl alcohol and carbon monoxide under pressure, the conversions of chlorobenzene to ethyl benzoate and benzoic acid under optimum reaction conditions were 31.3% and 30.1% respectively, with nickel naphthenate supported on silica gel (Ni:SiO₂ = 50:50) as the best catalyst. When bromobenzene was used instead of chlorobenzene, the best catalyst was found to be nickel iodide supported on silica gel (Ni:SiO₂ = 50:50), and the above conversions were 74.7% and 25.1% respectively under optimum reaction conditions.     

High-pressure synthesis of ethyl benzoate from chloro- and bromobenzene

When chlorobenzene was reacted with ethyl alcohol and carbon monoxide under pressure, the conversions of chlorobenzene to ethyl benzoate and benzoic acid under optimum reaction conditions were 31.3% and 30.1% respectively, with nickel naphthenate supported on silica gel (Ni:SiO₂= 50:50) as the best catalyst. When bromobenzene was used instead of chlorobenzene, the best catalyst was found to be nickel iodide supported on silica gel (Ni:SiO₂ = 50:50), and the above conversions were 74.7% and 25.1% respectively under optimum reaction conditions.     

Synthesis and high performance of ceria supported nickel catalysts for hydrodechlorination reaction

Catalysts containing 10 wt% Ni supported on CeO₂ were prepared by two ways, namely, co-precipitation method using nickel nitrate precursor and impregnation method using nickel nitrate and nickel acetylacetonate as two separate precursors. The catalysts were characterized by pulse chemisorption of H₂, X-ray diffraction, and temperature programmed reduction (TPR) techniques and evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene to benzene in a fixed-bed down-flow glass reactor at 573 K under normal atmospheric pressure. The hydrogen uptake values were used to determine the catalyst properties of Ni/CeO₂ like dispersion, metal area, and particle size. Among the two preparatory routes, co-precipitation method gave better catalytic performance in terms of hydrogenation activity, benzene selectivity, and coking resistivity than impregnated Ni/CeO₂ catalysts. This may be attributed to high dispersion of smaller NiO crystallites and the appearance of the second reduction peak at a higher temperature (578 K) in TPR profile with co-precipitated Ni/CeO₂ catalyst. This indicates that a strong interaction may take place between the NiO crystallites and CeO₂ on the surface of co-precipitated Ni/CeO₂ catalyst. Contrary to general expectation that the large Ni particles are preferable for HDC reaction, it is observed that smaller metal particles with high dispersion, as in the case of co-precipitated Ni/CeO₂ catalyst, promotes better catalyst with longer life.     

Pd/C催化剂催化氯苯加氢脱氯性能

以氯苯为模型反应原料,采用Pd/C为催化剂催化加氢氯苯脱氯,考察溶剂种类、反应温度、反应压力和反应时间对Pd/C催化剂催化氯苯加氢脱氯性能的影响。结果表明,采用Pd/C为催化剂,以甲醇与水质量比1∶1为溶剂,在反应温度70℃和反应压力0.8 MPa条件下反应100 min,Pd/C催化剂上氯苯转化率超过50%。以甲醇与水质量比1∶1为溶剂,在减少废水和降低成本方面有较大优势。     

Liquid‐Phase Hydrogenation of m‐phenoxybenzaldehyde to m‐phenoxybenzylalcohol Using Raney Nickel Catalyst: Reaction Kinetics

Kinetics of the liquid‐phase catalytic hydrogenation of m‐phenoxybenzaldehyde to m‐phenoxybenzyl alcohol have been investigated over the Raney nickel catalyst. Effects of hydrogen partial pressure (500‐2000 kPa), catalyst loading (1.6‐6.4 g.L^?1), m‐phenoxybenzaldehyde concentration (0.2‐0.8 mol.L^?1) and temperature (333‐363 K) on the progress of the reaction were studied. The speed of stirring > 15 rps has no effect on the initial rate of reaction. Effects of various catalysts and solvents on the hydrogenation of m‐phenoxybenzaldehyde have been investigated. The reaction was found to be first order with respect to the hydrogen partial pressure, catalyst loading and m‐phenoxybenzaldehyde concentration. Several Langmuir‐Hinshelwood type models were considered and the experimental data fitted to the model involving surface reaction, between dissociatively adsorbed hydrogen and molecularly adsorbed m‐phenoxybenzaldehyde.     

The catalytic hydrodechlorination of mono-, di- and trichlorobenzenes over supported nickel

The gas phase hydrodechlorination (HDC) of chlorobenzene (CB), chlorotoluene(s) (CT), 3-chlorophenol (3-CP), dichlorobenzene(s) (DCB) and trichlorobenzene(s) (TCB) over the temperature range 473 K≤T≤573 K has been studied using 1.5% and 6.1% (w/w) Ni/SiO₂ catalysts; the catalytic data have been obtained in the absence of any appreciable short-term deactivation. HDC of DCB and TCB generated the partially or the fully dechlorinated aromatic product + HCl and there was no significant cyclohexene or cyclohexane in the effluent stream. The conversion of mono-chloroarenes yielded the following reactivity sequence CB < 2-CT < 3-CT < 4-CT < 3-CP, i.e. the presence of an electron donating ring substituent enhances HDC and steric hindrance lowers reactivity. HDC kinetics have been adequately represented by a pseudo-first order approximation. Chlorine removal from DCB and TCB isomers proceeded through sequential and concerted routes, the relative importance of each dependent on the nature of the isomer and reaction conditions; apparent HDC activation energy increases in the order CB < DCB < TCB. The relationship between dechlorination selectivity and residence time/fractional conversion is addressed. The higher Ni loaded catalyst delivered consistently higher (specific) dechlorination rates and higher benzene yields from a polychlorinated feedstock. Catalytic HDC over Ni/SiO₂ is presented as a viable means of treating/detoxifying concentrated chlorinated gas streams and the best strategy for generating the parent benzene or a target partially dechlorinated product is discussed.     

Production of bio‐crude from forestry waste by hydro‐liquefaction in sub‐/super‐critical methanol

Hydro‐liquefaction of a woody biomass (birch powder) in sub‐/super‐critical methanol without and with catalysts was investigated with an autoclave reactor at temperatures of 473–673 K and an initial pressure of hydrogen varying from 2.0 to 10.0 MPa. The liquid products were separated into water soluble oil and heavy oil (as bio‐crude) by extraction with water and acetone. Without catalyst, the yields of heavy oil and water soluble oil were in the ranges of 2.4–25.5 wt % and 1.2–17.0 wt %, respectively, depending strongly on reaction temperature, reaction time, and initial pressure of hydrogen. The optimum temperature for the production of heavy oil and water soluble oil was found to be at around 623 K, whereas a longer residence time and a lower initial H₂ pressure were found to be favorite conditions for the oil production. Addition of a basic catalyst, such as NaOH, K₂CO₃, and Rb₂CO₃, could significantly promote biomass conversion and increase yields of oily products in the treatments at temperatures less than 573 K. The yield of heavy oil attained about 30 wt % for the liquefaction operation in the presence of 5 wt % Rb₂CO₃ at 573 K and 2 MPa of H₂ for 60 min. The obtained heavy oil products consisted of a high concentration of phenol derivatives, esters, and benzene derivatives, and they also contained a higher concentration of carbon, a much lower concentration of oxygen, and a significantly increased heating value (>30 MJ/kg) when compared with the raw woody biomass. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Gas phase catalytic hydroprocessing of trichlorophenols

The catalytic hydrodechlorination of four trichlorophenol (TCP) isomers (2,3,5‐TCP, 2,3,6‐TCP, 2,4,5‐TCP and 2,4,6‐TCP) was studied in the gas phase using an Ni/SiO₂ catalyst over the temperature range 473 K ≤ T ≤ 573 K. The catalyst was 100% selective in removing chlorine(s), leaving the hydroxyl group and benzene ring intact. Dechlorination proceeds via stepwise and concerted routes and the relative importance of each is dependent on the nature of the isomer where steric rather than resonance effects appear to determine the ultimate product distribution. Dechlorination efficiency is quantified in terms of phenol yield, chlorine removal rate and the ultimate partitioning of chlorine in the parent organic or product inorganic host. The reaction pathways, with associated pseudo‐first order rate constants, for the conversion of 2,3,6‐TCP and 2,4,6‐TCP are presented. The effect of time and temperature on process selectivity is discussed and the nature of catalyst deactivation is considered. © 2000 Society of Chemical Industry     

Liquid‐Phase Catalytic Hydrogenation of p‐Chlorobenzophenone to p‐Chlorobenzhydrol over a 5 % Pd/C Catalyst

The kinetics of the liquid‐phase catalytic hydrogenation of p‐chlorobenzophenone have been investigated over a 5 % Pd/C catalyst. The effects of hydrogen partial pressure (800–2200 kPa), catalyst loading (0.4–1.6 gm dm^–3), p‐chlorobenzophenone concentration (0.37–1.5 mol dm^–3), and temperature (303–313 K) were studied. A stirring speed > 20 rps has no effect on the initial rate of reaction. Effects of various catalysts (Pd/C, Pd/BaSO₄, Pd/CaCO₃, Pt/C, Raney nickel) and solvents (2‐propanol, methanol, dimethylformamide, toluene, xylene, hexane) on the hydrogenation of p‐chlorobenzophenone were also investigated. The reaction was found to be first order with respect to hydrogen partial pressure and catalyst loading, and zero order with respect to p‐chlorobenzophenone concentration. Several Langmuir‐Hinshelwood type models were considered and the experimental data fitted to a model involving reaction between adsorbed p‐chlorobenzophenone and hydrogen in the liquid phase.     

Catalytic hydrodechlorination of chlorohydrocarbons in a medium of sodium hydroxide solutions, Part 2: Transformations of hexachloroethane and other polychloroethanes

In part one of this work, it was shown that in addition to sodium formate, hexachloroethane (HCE) and perchloroethylene are the main products of the catalytic hydrodechlorination of CCl₄ in a medium of sodium hydroxide solutions. These products undergo mutual transformations under the conditions of the process. Since the industrial requirements for perchloroethylene considerably exceed those for HCE, the regularities of the catalytic hydrodechlorination of HCE, pentachloroethane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, and 1,2-dichloroethane are studied in part two, using 1.5 wt % Pd on sibunit as the catalyst. It is found the fraction of hydrogen substited for chlorine grows and the reactivity of the products diminishes with a reduction in the number of chlorine atoms in a polychloroethane molecule. Ethane and ethylene are the final reaction products after a rather long contact time. The kinetics of HCE catalytic hydrodechlorination is studied n the temperature range of 353–393 K and at hydrogen partial pressures of 50–810 kPa. It is shown that the only reaction product is perchloroethylene; i.e., the transformation of HCE proceeds via the elimination of two chlorine atoms, while the limiting stage of the process is product dissolution in the aqueous-alkaline reaction mass. The form of the kinetic equation is found to be w = 2.1 × 10^?6 × exp[?(16200 ± 400)/RT]C _HCE C _cat $P_{H_2 }^{0.5} $ mol/(L s). The possibility of using the catalytic hydrodechlorination of chlorohydrocarbons in a medium of sodium hydroxide solutions for the recycling of chloroorganic wastes containing CCl₄ and polychloroethanes is demonstrated.     

In situ production of nickel/graphitic carbon nanofiber composites and application in catalytic hydrodechlorination

The action of Ni/SiO₂ in the gas phase hydrodechlorination (at 573 K) of chlorobenzene, 1,3-dichlorobenzene and 1,3,5-trichlorobenzene is compared with that of a Ni/SiO₂ + C composite. The latter was prepared in situ by the decomposition of chlorobenzene at 873 K to generate graphitic carbon nanofibers bearing Ni particles at the fiber tips. The Ni/SiO₂ and Ni/SiO₂ + C (with varying C content) catalysts have been characterized by TEM, SEM, XRD and H₂ chemisorption. While the Ni/SiO₂ + C system delivered a lower initial fractional dechlorination, the composite outperformed the starting Ni/SiO₂ in terms of long-term activity, an effect that is linked to the structural characteristics.     

Gas-phase hydrodechlorination of pentachlorophenol over supported nickel catalysts

The gas-phase hydrodechlorination of pentachlorophenol (PCP) over nickel/silica and nickel/Y zeolite catalysts at 573 K has been studied. Each catalyst was 100% selective in cleaving the C–Cl bonds, leaving the hydroxyl substituent and benzene ring intact. The variation of catalytic activity and selectivity (in terms of partial and full dechlorination) with time-on-stream is illustrated and catalyst deactivation is addressed. Dechlorination efficiency is quantified in terms of dechlorination rate constants, phenol selectivity/yield and the ultimate partitioning of chlorine in the parent organic and product inorganic host. Increasing the nickel loading on silica was found to raise the overall level of dechlorination while the use of a zeolite support introduced spatial constraints that severely limited the extent of dechlorination. Product composition was largely determined by steric effects where resonance stabilisation had little effect. The reaction pathways, with associated pseudo-first-order rate constants, are also presented. This revised version was published online in November 2006 with corrections to the Cover Date.     

Pd/ZnO催化剂上甲醇水蒸气重整制氢

研究了并流共沉淀法制备的Pd/ZnO催化剂上的甲醇水蒸气重整制氢反应.考察了钯含量、还原温度、反应温度、重时空速（WHSV）和水-甲醇摩尔比（水醇比）对反应的影响.研究结果表明,当钯质量分数为15.9%,反应温度为523～573 K,还原温度为523～573 K,水醇比为1.0～1.2,WHSV=17.2 h^-1时,反应具有较好的CH3OH转化率、CO₂选择性、H₂产率及较低的出口CO摩尔分数.与铜基催化剂相比,Pd/ZnO催化剂表现出较好的稳定性.     

Destruction of chlorinated organics by hydrotreatment using Ru/TiO2 catalyst

Catalytic hydrodechlorination reactions of p‐chloro‐m‐cresol (PCMC) and p‐chloroaniline (PCA) were investigated in a slurry reactor using a Ru/TiO₂ catalyst. The organic reaction intermediates, m‐cresol and aniline, were further converted into methylcyclohexanol and cyclohexylamine respectively. Kinetics of PCMC hydrogenation was studied over the ranges in temperature, 323–373 K, H₂ partial pressure, 0.34–1.38 MPa, PCMC concentration, 3.5–14 mM and catalyst loading, 0.1–2 kg/m³. The reaction orders with respect to PCMC and H₂ were evaluated as 0.5 and 0.8 respectively. It was found that aniline hydrogenation is the rate‐determining step in the hydrotreatment of PCA. Kinetics of aniline hydrogenation was studied at 343 and 363 K over the ranges in H₂ partial pressure, 0.34–1.38 MPa, aniline concentration, 5.4–21.5 mM and catalyst loading, 0.1–0.6 kg/m³. The reaction orders with respect to aniline and H₂ were found to be 1.3 and 1.0 respectively. © 2012 Canadian Society for Chemical Engineering