**2.5. Hydrodechlorination**

Heterogeneous catalyst replaced the homogeneous catalyst because of solving the catalyst recycling problem, especially for the precious metal catalysts, the catalyst costs were reduced. Heterogeneous catalyst have some advantages (**Figure 7**) compared with homogeneous catalyst, e.g., (I) Large surface area, especially for the porous heterogeneous catalyst, it may increase the reactant transmission rate in the catalyst. For gas phase reaction, surface adsorption effect will greatly increase the rate of the reaction. (II) The activity site will be discrete well in order to prevent its aggregation. (III) Diversified structure monomers to build diversified structure, morphology and functional catalyst. Wang Wei etal. [19], detailed introduced the nanocatalyst materials development which can be divided into three categories: inorganic materials, organic materials, and hybrid materials (**Figure 8**) and depicts some representative examples ranging from 0D discrete materials to 3D extended structures, from inorganic to pure organic components, from disordered to regular arrangements, and from non-porous to porous nature. In recent years, covalent organic frameworks attract more and more people's attention because of its large surface area and regular structure [20–22].

**Figure 7.** Heterogeneous catalysts applied in HDC.

Application of Heterogeneous Catalysts in Dechlorination of Chlorophenols 53 http://dx.doi.org/10.5772/intechopen.79134

**Figure 8.** The classification of heterogeneous catalyst carrier and its development [19].

**Figure 9.** Hydrogenated dechlorination (HDC).

From 0D to 3D nanomaterials, all of them may be likely to be good candidates as the heterogeneous catalyst. Organic chlorides, especially of 4-chloride-phenol is a kind of highly toxic compound. Hydrogenated dechlorination (HDC) is an efficient method for toxic compound treating. Although this method does not lead to complete degradation of the organic chlorides pollutant but generates a less toxic (biodegradable) compound or a chemical of possible commercial important product via halogen atoms elimination with hydrogen (**Figure 9**) hydrochloric acid was produced in HDC reaction process in stoichiometric amount and the catalyst deactivation in this acid conditions is an important issue.

Recently most research focused on the development of more stable and efficient catalystdesigned. According to the type and number of the metal, catalyst can be divided into single and bimetals center catalyst activity. Hence, a rapid, efficient, and green approach to the fabricationof effective catalyst Al<sup>2</sup> O3 shows a highly mechanical resistance with high catalytic activity and the potential applications the HDC of 4-CP has significant meaning for environmental protection. Pd0 as the most effective metal active center for the HDC reaction usually applied withother metal cooperate to improve the catalytic efficiency and stability, like Pd-Bi [23, 24], Pd–Tl [25], Pd-Fe [26], and Ni@Pd [27] also have been extensively researched, in addition, the catalyst carrier has become the important role in improving the activity and stability of catalytic, such as Pd/Al2 O3 , Pd/zeolites, Pd/activated carbon (AC), SiO2 supported Pd nanocatalysts.

 As we know, for organic chloride, with chlorine content is higher, the toxicity become stronger at the same time, to be more difficult to removal chlorine from the organic chloride compound. Bimetal catalyst was reported to solve this issue by Anwar [28]. The pentachlorophenol (PCP, 10–20 mg min−1) through the bimetal catalyst Ag0 /Fe0 loaded on the heating column (25 × 1 cm) in scCO2 (supercritical carbon dioxide) was removed. After the reaction was operated for 1 h at 450°C, organically-bound chlorine was liberated, virtually quantitatively, from a 20% (w/v) feedstock stream (0.1 ml min−1 merged with 4 ml min−1 scCO2 ). In extended operations, about 70°C, the reactor to 14 h of continuous running, no obvious activity loss if the chloride ion was washed every 3 h. In fact, catalytic activity was also related with the reaction temperature, pressure, solvent, and so on. The production of the pentachlorophenol (PCP) was a series of methylated phenols or methylated benzenes (**Figure 10**).

Jovanovic studied the dechlorination of p-chlorophenol in a microreactor with bimetallic Pd/Fe catalysts [29]. The bimetallic catalyst Pd/Fe was prepared by electroless deposition of Pd on the reactor plate surfaces. The chemistry of the dechlorination of p-chlorophenol on Pd/Fe catalysts involves three sets of chemical reactions (**Figure 11)** namely surface reactions, solution reactions, and actual dechlorination reactions. These reactions are found to be dependent on several parameters, including the pH, the Pd/Fe interface area, the extent of palladization, the ratio of the Pd/ Fe interfacial area to the amount of chlorine to be removed, and the amount of dissolved oxygen.

**Figure 10.** Pentachlorophenol treated by Ag0 /Fe0 catalyst [28].

**Figure 11.** Dechlorination of p-chlorophenol on Pd/Fe catalysts.
