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Ind. Eng. Chem. Res. 2008, 47, 5362–5368
Hydrodechlorination of Chlorobenzene over Silica-Supported Nickel Phosphide Catalysts Xuguang Liu, Jixiang Chen,* and Jiyan Zhang Department of Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, P.R. China
Silica-supported Ni3P, Ni12P5, and Ni2P catalysts were prepared by the temperature-programmed reduction method from nickel phosphate precursors. A Ni/SiO2 catalyst was also prepared as a reference. The effect of the initial Ni/P molar ratio in the precursor on the catalyst structure and hydrodechlorination performance was investigated. The physicochemical properties of the catalysts were characterized by means of N2 adsorption, hydrogen temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet and visible spectroscopy, hydrogen temperature-programmed desorption, and inductively coupled plasma spectroscopy. The catalyst activities in the hydrodechlorination of chlorobenzene were evaluated in a fixedbed reactor at atmospheric pressure. The silica-supported nickel phosphides exhibited superior hydrodechlorination activities to that of supported nickel. This can be attributed to the special physicochemical properties of nickel phosphides and a great amount of spillover hydrogen species. In nickel phosphides, there is a small amount of electron transfer from Ni to P, leading to a small positive charge on Ni. This favors a weakening of the interaction between chlorine and nickel sites, as well as between adsorbed hydrogen species and nickel phosphides. The “ensemble effect” of P is also beneficial in decreasing the coverage of chlorine on nickel sites. Because of the reduced interaction between adsorbed hydrogen species and nickel phosphides, the energy barrier of the hydrogen spillover on the silica-supported nickel phosphide catalysts decreases, which accounts for the increased amount of spillover hydrogen species on the catalyst surface. Spillover hydrogen species not only promote the hydrogenolysis of the C-Cl bond, but also favor the removal of chlorine ions from the surface of the catalysts. Hydrodechlorination over the nickel phosphide catalysts is characterized by a reaction induction period that becomes longer with increasing phosphorus content in the catalyst precursor. This is related to the blocking of active sites by excess phosphorus. 1. Introduction Chlorinated aromatics are important chemicals that are widely used as solvents and in the synthesis of many organic chemicals, such as odorizers, insect repellents, and fungicides. Their risks to health and the environment have attracted attention because of their acute toxicity and strong bioaccumulation potential; they persist in the environment, accumulate in fatty tissues, and show carcinogenic and mutagenic activity. Therefore, these compounds must be disposed of properly. In recent years, interest in catalytic hydrodechlorination (HDC) has increased for environmental reasons. Compared to end-of-pipe treatments such as phase transfer or physical separation (adsorption, air or steam stripping, and condensation) and chemical degradation or destruction (thermal incineration and catalytic or wet air oxidation), catalytic HDC has many advantages from the viewpoint of ecological chemical engineering.1 These include mild reaction conditions, the transformation of chlorinated pollutants into valuable raw materials, and no emission of CO2 and NOx. Gas- or liquid-phase HDC has been used for the disposal of many types of toxic waste (e.g., chlorofluorocarbons, chlorobenzenes, chlorophenols, polychlorinated biphenyls, insecticides, and dioxin/furans), and the catalysts used have mainly been monometallic (such as Pt,2,3 Pd,2,4–7 Rh,8 and Ni9–14) and bimetallic (Pd-Yb,15,16 Pd-Rh,17 Ni-Au18). Noble-metal catalysts, especially Pd, exhibit HDC activity superior to that of supported nickel-based catalysts; moreover, HDC performance has been shown to be sensitive to Pd dispersion and the nature of the support.19,20 Keane et * To whom correspondence should be addressed. Tel.: +86-2227890865. Fax: +86-22-87894301. E-mail:
[email protected].
al.9,21 also found that the hydrodechlorination of chlorobenzene or chlorophenol over supported Ni is structure-sensitive. In addition, there is an evidence that spillover hydrogen species contribute to HDC.2,5,14,22–24 Recently, it was found that bimetallic synergism can markedly promote catalyst performance. In Ln-Pd/SiO2 (Ln ) Yb, Sm, Gd, La, Ce, Eu),15,16 a Pd/Ln synergism serves to generate a higher surface hydrogen content, and the initial HDC rates correlate with H2 uptake values; at the same time, the Ln component might contribute directly to the activation of the C-Cl bond(s) for hydrogenolytic attack. Similarly, Ni-Au bimetallic catalyst exhibits higher HDC rate constants than the corresponding Ni monometallic material.18 This can be attributed to Ni-Au surface synergism, that is, the Au component serves to activate the C-Cl bond to subsequent attack from reactive hydrogen dissociated at the Ni centers. Because noble metals are expensive and of limited availability, nickel catalysts represent a promising option. However, whether noble-metal or nickel catalysts are used, a primary obstacle in the application of HDC is catalyst deactivation, which is mainly due to surface poisoning by HCl, metal sintering, or/and carbon deposition/occlusion of the active sites.13,25–28 Thus, novel catalysts with high activity and stability are needed. Transition-metal phosphides are novel catalytic materials with excellent performance in hydrodenitrogenation (HDN) and hydrodesulfuration (HDS),29–33 which can be attributed to geometric and electronic factors.33 As HDC has a similar reaction scheme (RsCl + H2 f RsH + HCl, where R is an organic group) to HDN and HDS (i.e., replacing a Csheteroatom bond by a CsH bond), the use of metal phosphides in HDC is a pertinent issue. A previous report34 showed that a SiO2-
10.1021/ie7017542 CCC: $40.75 2008 American Chemical Society Published on Web 07/08/2008
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5363 Table 1. Textural Properties of Catalysts sample SiO2 Ni/SiO2 Ni3P(3/1)/SiO2 Ni12P5(2/1)/SiO2 Ni2P(1/1)/SiO2 Ni2P(2/3)/SiO2 Ni2P(1/2)/SiO2
Ni/P ratioa
SBET (m2 g-1)
3/1 2/1 1/1 2/3 1/2
548 383 418 359 262 150
pore volume (cm3 g-1)
average pore diameter (nm)
average size of Ni or NixP (nm)
0.797 0.597 0.568 0.521 0.349 0.367
5.5 5.8 5.2 5.5 5.2 9.7
12.5b 11.3c 13.8d 11.8e 14.0e 15.1e
a Initial ratios in the precursors. b Average size of nickel crystallites, calculated using the Scherrer formula with the (200) reflection of Ni. c Average size of Ni3P crystallites, calculated using the Scherrer formula with the (321) reflection of Ni3P. d Average size of Ni12P5 crystallites, calculated using the Scherrer formula with the (312) reflection of N12P5. e Average size of Ni2P crystallites, calculated using the Scherrer formula with the (111) reflection of Ni2P.
supported Ni3P catalyst exhibited excellent performance in the gas-phase HDC of chlorobenzene. In the present work, different silica-supported nickel phosphides, including Ni3P, Ni12P5, and Ni2P, were prepared from precursors with different Ni/P ratios, and their properties in the HDC of chlorobenzene were investigated. 2. Experiments 2.1. Catalyst Preparation. Commercial spherical silica (Qingdao Haiyang Chemicals Co. Ltd., Qingdao, China) was used as the support. Supports with diameters of 0.15-0.25 mm were dried in air at 378 K for 20 h before use. A series of silicasupported nickel phosphide catalysts were prepared by means of temperature-programmed reduction (TPR) according to the procedure proposed by Oyama et al.35 First, the catalyst precursors were prepared by incipient coimpregnation with an aqueous solution of NH4H2PO4 and Ni(NO3)2, followed by drying at 393 K and calcination at 773 K. The initial Ni/P atomic ratios of the different precursors were 3/1, 2/1, 1/1, 2/3, and 1/2. Second, in a H2 flow, the precursors were reduced to supported nickel phosphides, which are denoted as shown in Table 1 according to the initial Ni/P atomic ratio in the precursor and the phase detected by X-ray diffraction. The nickel content in the nickel phosphide catalysts was set at 15 wt %. As an example, Ni2P(1/1)/SiO2 was prepared as follows: First, 4.40 g of Ni(NO3)2 · 6H2O (15.0 mmol) and 1.73 g of NH4H2PO4 (15.0 mmol) were dissolved in water, and the precipitate formed was dissolved by a few drops of nitric acid, to obtain 8 mL of aqueous solution. Then, 5.0 g of support was incipiently impregnated with this aqueous solution and left at room temperature for 12 h. The sample was then dried in air at 393 K for 12 h and calcined in air at 773 K for 4 h, to obtain a supported nickel phosphate precursor. In the next step, the nickel phosphate precursor was reduced in a fixed-bed quartz reactor placed in a furnace controlled by a temperature programmer according to the following conditions: the precursor was heated from room temperature to 473 K at 8 K/min and then from 473 to 923 K at 1 K/min and was then maintained at 923 K for 2 h. During the reduction, the H2 (99.999%) flow was set at 250 mL/min per gram of precursor. An SiO2-supported nickel catalyst with a nickel content of 15 wt % (denoted Ni/SiO2) was also prepared by a procedure similar to that for the supported nickel phosphides except for the final reduction temperature of 723 K. For N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), and hydrogen temperature-programmed desorption (H2 TPD) characterizations, the fresh samples were passivated in a 0.5 vol % O2/N2 flow for 6 h at room temperature.
2.2. Catalyst Characterization. Ultraviolet and visible (UV-vis) spectra were obtained using a Perkin-Elmer Lambda 35 UV-vis spectrometer over a wavelength range of 200-1100 nm. The catalysts precursors were ground into a powder with a particle size of
