Promoting Light Hydrocarbons Yield by Catalytic Hydrodechlorination of Residual Chloromethanes Using Palladium Supported on Zeolite Catalysts

Abstract

Gas catalytic hydrodechlorination (HDC) of trichloromethane (TCM) and dichloromethane (DCM) was analyzed using Pd (1 wt.%) on different zeolites as catalysts. The aim of this study was to know the surface properties of the catalysts and reaction conditions that promote the yield to light hydrocarbons in this reaction. Five different zeolite supports were used from three commercial zeolites (KL, L-type; NaY, Faujasite; H-MOR, Mordenite). KL and NaY were submitted to ionic exchange treatments in order to increase their acidity and analyze the effect of the acidity in the activity and selectivity of the HDC reaction. Exchanged zeolites (HL and HY) showed the highest Pd dispersion due to their higher surface acidity. The best TCM/DCM conversion and selectivity to light hydrocarbons was obtained using the two non-exchanged zeolite-catalysts, KL and NaY. Low surface acidity seems to be the key aspect to promote the formation of light hydrocarbons. The formation of these products is favored at high reaction temperatures and low H2: chloromethane ratios. KL showed the highest selectivity to olefins (60%), although with a lower dechlorination degree. Non-exchanged NaY catalyst showed high selectivity to paraffins (70% and 95% for the HDC of DCM and TCM, respectively).

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

“…In contrast, both Pd monometallic and Ag-Pd bimetallic catalysts allowed complete TCM conversion at temperatures beyond 175-200 ºC, with high overall dechlorination, which is almost complete above 300 ºC. These results are quite similar to the obtained in our previous studies with monometallic Pd catalysts supported on activated carbon under the same working conditions [12,20]. Besides, the current catalysts showed slightly higher conversion than those reported by other authors on the HDC of tetrachloromethane (TTCM) with AgPd-based catalysts at low temperatures (125 ºC) [30,63].…”

“…Nevertheless, those catalysts were only stable for the first few hours, hereafter, activity dropped rapidly because of coke deposition and changes of surface acidity. In previous studies of our group [13,20], significant values of selectivity to olefins were obtained with Pd based catalysts in the HDC of TCM, though they require to be improved. In contrast, as the process is simplified because coupling of two molecules is not necessary, a wide diversity of studies concerning the selective production of olefins from C2 chlorinated hydrocarbons can be found in the bibliography [21][22][23][24][25][26][27][28][29][30].…”

“…In contrast, as the process is simplified because coupling of two molecules is not necessary, a wide diversity of studies concerning the selective production of olefins from C2 chlorinated hydrocarbons can be found in the bibliography [21][22][23][24][25][26][27][28][29][30]. Regardless the reactant investigated, HDC studies focused on improving the selectivity to light olefins have commonly used noble or transition metals, like Pd and Ni, as active phases, due to their well-known hydrogenation ability, as well as supports of different nature, such as zeolites, metal organic frameworks and carbon materials [13,[20][21][22]33]. However, Ni-based catalysts are prone to coke formation leading to rapid deactivation.…”

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

“…On the mixed shaped Pd NPs with mostly (111) facets, the observed CO band at 1874 cm −1 rather indicates hollow-bonded CO, typical of (111) facets of the m Pd particles. The peak shift of 37 cm −1 between s and m Pd NPs is a clear indication of the difference between curved and (111) Pd surface facets [34].…”

“…The structure sensitivity of the reaction has been debated, as there are a variety of influencing effects. The support material [32][33][34][35], metal/support interactions [31,[36][37][38][39][40], pretreatment and reaction conditions [23,41], metal surface orientation [29,[42][43][44][45][46][47][48] and metal dispersion/particle size [33,36,37,44,46,[49][50][51][52][53] are crucial for efficient selective catalytic reactions. Specifically, the coordination numbers of the metal atoms in the nanoparticles may vary, being, e.g., 9, 8 and 7 in surface planes of (111), (100) and (110), respectively [54][55][56].…”