Conversion of chlorinated waste streams from the production of polyvinyl chloride over La-based catalysts

Abstract

Annually, more than one third of all chlorine consumption is used for the production of C2H3Cl. This is the monomer for the production of polyvinyl chloride (PVC). Even though the production process of C2H3Cl is rather selective, by-products are formed in large amounts due to the large scale of the PVC industry. These by-products are mainly chlorinated C1 and C2 (CHC1-2), and are collected as a mixture known as the light ends. Due to the toxic and persistent nature of these compounds, environmental regulation demands for efficient disposal of the chlorinated by-products of the PVC industry. Currently, the preferred route of destruction is incineration. Though efficient, this is a costly process due to the high temperature that is required. Furthermore, incineration results in the loss of feedstock. Catalytic conversion is a suitable method to lower the energy consumption of the destruction of the light. In addition, the CHCs could potentially be recycled into feedstock or fuel. Chlorine and chlorine-containing compounds are, however, known to be poisonous to many catalyst materials. Therefore, the challenge lies more in finding a suitable stable catalyst rather than a more active or selective catalyst. This PhD thesis illustrates the versatility of La2O3-based compounds as potential catalysts in chlorine-mediated organic reactions. The conversion of different chlorinated C1 and C2 compounds over La-based materials was studied to evaluate the potential of these catalyst materials and to gain more insight into the reactions that occur over the catalytic surface. Special attention was devoted to the influence of the chlorination degree of the catalyst material. The conversion of CHCs over La-based materials was studied using in situ IR activity experiments and flow-gas experiments with online GC analysis. In addition, the materials were investigated by various characterization techniques. Three types of reaction are found to occur over La-based materials: 1) destructive adsorption of chlorinated C1, 2) dehydrochlorination of chlorinated C2 and 3) the H/Cl exchange between chlorinated C1. Moreover, with each reaction, optimal selectivity and activity is achieved at a certain chlorination degree of the catalyst material. For destructive adsorption, lattice oxygen sites are needed for the exchange with the chlorine atoms of the reactants. However, a more chlorinated catalytic surface results in higher intrinsic activity and higher selectivity. Secondly, the dehydrochlorination reaction also proceeds on La2O3, but the selectivity towards chlorinated ethenes is higher over a chlorinated catalyst. Finally, the exchange of H/Cl between CCl4 and CH2Cl2 only proceeds over a fully chlorinated catalyst, because lattice oxygen will enable the destructive adsorption reaction. The results have shown that La-based catalysts are versatile and, instead of the catalyst becoming deactivated, the chlorination of the catalyst influences the overall selectivity. Pioneering experiments have shown that mixtures of CHC1-2 can also be efficiently converted over La-based catalysts. Also, it was demonstrated that the destructive adsorption reaction can be combined with other chlorine-mediated reactions. The next challenges are to control the chlorination degree of the catalyst during reaction and to implement the catalyst in a practical situation.