Studies On Hydrochlorination Of Acetylene With Ionic Liquids As Solvents

Vinyl chloride monomer (VCM) is essential material for synthesizing polyvinylchloride (PVC), and mercuric chloride catalyzed gas-solid acetylene hydrochlorination reaction is a dominant process to VCM. However, the process generates loss of toxic Hg and severe environment pollution. Researches have shown that non-mercuric metal catalysts, such as AuⅢ, PtⅡ, PdⅡ, are highly active for hydrochlorination of acetylene while these catalysts are encounterd with rapid deactivation because metallic cations are readily reduced to zero-valent metal. In addition, acetylene hydrochlorination is a highly exothermic reaction and formation of hot spots easily occurs in gas-solid reaction. For these reasons, owing to negligible vapor pressure, wide liquid range, excellent metal compounds solubility and metal nanoparticles (NPs) stability of ionic liquids (ILs), gas-liquid acetylene hydrochlorination with ILs as reaction medium was studied and novel metal NPs/IL systems were developed to enhance the activity and stability of metal catalysts.The catalytic activities of acetylene hydrochlorination in regular ILs were investigated. The effects of temperature, flowrate and type of catalyst on activity were firstly studied. Then the influence of structures of ILs on performance was highlighted. Result showed that anions of ILs exerted obvious effect on activity while altering carbon chain lengh of cations of ILs incurred insignificant change. Based on the above analysis, a kind of low-cost protic ILs was synthesized with simple method and PdCl2/protic IL systems were developed for hydrochlorination of acetylene. Results indicated that these systems could obtain acetylene conversion and VCM selectivity of about 75% and 99.3%, respectively, with good stability over reaction time.A kind of anionic surfactant carboxylate ILs (ASC-ILs) was synthesized and metal NPs (Pd, Au, Pt) with small size, narrow distribution, well-defined lattice fringes and high dispersion experienced in-situ generation in ASC-ILs. The influence of structures of ILs on the size and shape of NPs was discussed. Metal NPs/ASC-IL systems were fabricated and for the first time applied into acetylene hydrochlorination. The effects of different metal NPs and ILs on activity were researched. The result indicated that Pd NPs/ASC-IL, Au NPs/ASC-IL and Pt NPs/ASC-IL all gave good performance, and Pd NPs/ASC-IL could yield an excellent conversion of 93% and selectivity of 99.5%.The mechanism of metal NPs/ASC-IL system was investigated. It was found that the highly active Pd0 NPs served as reservoirs that were easily oxidized to catalytically active PdⅡ species by the reactant of HCl. Under the stabilization of ILs, the reduced Pd0 atoms by C2H2 in reaction process could still undergo in-situ assembly into active Pd NPs. Therefore, benefited from the high reactivity of NPs and the self-assembly property of ASC-ILs, an effective redox cycle between Pd0 and Pdu was established to reduce the deactivation of metal catalysts and make sure prolonged catalysis. It was also found that the physicochemical properties of ILs also contributed to catalytic performance. With the strong hydrogen-bond basicity of almost 1.60 and weak polarizability of 0.76-0.86, ASC-ILs were able to absorb approximately 2 molar equivalents HCl under 180 ℃ and obtain acetylene solubility of 0.1-0.6 mol/L, giving rise to activation of reactants and improvement of activity.The catalytic activity of Pd NPs/ASC-IL system was then systematic studied. The effects of carbon chain length of anions along with physicochemical properties of ASC-ILs on catalytic activity were surveyed. The effects of technical parameters such as reaction temperature and catalyst dosage on performance were also investigated as well as a stability test. Results demonstrated a rising trend in acetylene conversion from 76.23% in [P4444][C7COO] to 93.04% in [P4444][C17COO] with the increasing carbon chain length of anions. It was likely that the acetylene conversion formed an adverse relationship with π* value of ILs. Under the temperature of 180 ℃, C2H2 velocity of 5 mL/min (HCl/C2H2=1.2), Pd NPs/[P4444][C17COO] catalytic system (0.038 mol/L) achieved good acetylene conversion of surpassing 93% and VCM selectivity of about 99.5% with no discernible deterioration of activity over a reaction time of 55 hours, which proved to be a green substitute for mercury catalyst in the hydrochlorination of acetylene and born promising prospect in industrial application.