Several heterogeneous catalysts based on MCM-41 were synthesized either by chemical vapor deposition (CVD) or hydrothermally, i.e. adding SnCl 4 , SnCl 2 or ZnCl 2 as precursor salts into the MCM-41 synthesis gel. Synthesized materials were characterized by XRD, BET surface area, atomic absorption, UV-Vis, H 2-TPR, NH 3-TPD, TEM and pyridine-FTIR. Better textural properties and more reactive materials were obtained using tin salt precursors compared with ZnCl 2. UV-Vis and TPR analyses of selected Sn-MCM-41 materials indicate that depending on the synthesis and pretreatment conditions several species may be formed: Sn 4+ as Lewis acid sites of different coordination, SnO 2 nanoparticles and bulk tin oxide species. The presence of SnO 2 nanoparticles was also confirmed by TEM. Medium strength Lewis acid sites appear to increase nopol selectivity. Hydroxylated groups, i.e. Sn-OH, increased the weak acid sites and diminished nopol selectivity.
The gas-phase catalytic hydrodechlorination (CHD) of dichloromethane (DCM), tetrachloroethylene (TTCE), and chloroform (CF) and their mixtures was studied over Pd/TiO2-washcoated cordierite minimonoliths. Experiments were carried out in a flow reactor at 120−300 °C, 1 bar, and 0.45 g min/mL. Catalytic runs with the pure compounds at 200 °C led to 60−100% conversion following the sequence CF > TTCE > DCM. Catalyst deactivation and regeneration were also examined. Lower conversions (between 30% and 95%) and catalyst deactivation were observed when mixtures of organochlorinated compounds where fed as reactants. DCM was the most affected when binary and ternary mixtures were used. Catalyst samples were characterized before and after reaction by various temperature-programmed studies (H2 TPR, He TPD, NH3 TPD, and TPO), H2 chemisorption, and XPS measurements. Most characterization studies were carried out using an online coupled mass spectrometer, thus allowing the parallel detection of different fragments and species corresponding to several hydrocarbons and HCl. In some experiments, a fast GC−ToF−MS system was used. The catalytic activity of spent samples was partially recovered by heating them in flowing air and then in 5% H2/N2. However, the initial reaction rate decreased by 62% over samples used in three consecutive runs when ternary mixtures were fed. Carbonaceous deposits of different nature and changes in the oxidation state of Pd (Pd0 to Pd2+ and Pd4+) appear to play key roles in catalyst deactivation, whereas acidity was found to remain almost the same for fresh and used catalyst samples. Carbonaceous deposits were removed by heating at temperatures lower than 400 °C in flowing air.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.