Deactivation Mechanism Of Cu/Zn Catalyst For Hydrogenation Of Fatty Acid Methyl Ester To Fatty Alcohol

Fatty alcohols, as important raw materials for industry of fine chemicals, could be used to produce their derivatives by reacting with many compounds using its hydroxyl group, which have found numerous applications in surfactants and lubricants production. Fatty alcohol could be divided into natural alcohol and synthetic alcohol by the origination of its feedstock. Based on effect of the price fluctuation of crude oil and preference of terminal customers, the importance of nature fatty acids or esters process is steadily growing relative to the alternative petrochemical process during these years. Hydrogenation step of fatty aicd methyl ester to fatty alcohol is of great importance in the commercial production of natural alcohol. However, the Cu/Zn catalyst, widely used in this process, retains certain activity and slectivity for a relative short time scale, which have been the bottleneck for the development of natural alcohol production. In this dissertation, the effect of various impurities on Cu/Zn catalyst, like water, sulfur-containing compounds, chlorine-containing compounds, glycerine and glycerides, which are difficult to be removed from fatty esters, have been investigated. Furthermore, the rehydrogenation ability of deactivated Cu/Zn catalysts was discussed. The main work and results in this dissertation are listed as follows:(1) Several types of fatty aicd methyl esters were used as feedstock for experimental research of Cu/Zn deactivation, and methyl laurate was selected as the most suitable one. The effect of operation conditions on the hydrogenation of methyl laurate to lauryl alcohol was conducted using a stirred autoclave reactor system. The experimental results indicated that the mass transfer resistance could be eliminated with an agitation rate over 450rpm, the best performance of Cu/Zn catalyst could be reached at the reaction temperature of 230℃, hydrogen pressure of 11MPa, reaction time of 8h and catalyst-ester with a weight ration of 0.055.(2) The effect of water on Cu/Zn catalyst prepared by co-precipiation for hydrogenation of methyl laurate in a slurry phase was studied. The catalysts were characterized by means of XRD, BET, H2-TPR, SEM, TEM and ICP-AES. The results indicate that catalytic activity decreases with increased amount of water in methyl laurate at the initial or ending stage. Correalting with the results from the above characterization, it is found that the main causes for the water deactivation of the Cu/Zn catalyst were the water occlusion of active catalyst sites by the low solubility of water in the substrate and the promotion of crystal growth, as well as the Cu/Zn catalyst agglomeration in the presence of water. The catalytic activity of water deactivcated Cu/Zn catalyst could be patially recovered in the rehydrogenation reaction.(3) The mechanism of deactivation of Cu/Zn catalyst poisoned by organic chlorides in' hydrogenation of methyl laurate to lauryl alcohol was studied in a stirred autoclave. The un-poisoned and poisoned catalysts were characterized using XRD, BET, ICP-AES and SEM, respectively. The results indicated that both of catalytic activity and selectivity decreased with increasing amount of chlorides in methyl laurate. According to the characterization of the catalysts, the main causes for the chlorine deactivation of the Cu/Zn catalyst were that the chlorides could modify the valence state of active sites, decrease the BET surface area, and promote the growth of crystal and catalyst agglomeration. Further investigation showed that chlorine atom decomposed from the chlorides combined with ZnO to produce ZnCl2, which could be dissolved in the liquid and promote ester-exchange reaction to lauryl laurate as Lewis acid.(4) The sulfur deactivation of Cu/Zn catalyst was studied by investigating the effect of different types of sulfur-containing compounds on the hydrogenation of methyl laurate to lauryl alcohol. To the alkyl thiols and dialkyl disulfides, the conversiton of methyl laurate and the selectivityl of lauryl alcohol decresed not only with the increasing sulfur concentration, but also with the increment of the alkyl chain length. The XRD, EDS and XPS results indicated that alkyl thiols and dialkyl disulfides prefer to adsorb on Zn sites of the oxide followed by reacting with supported Cu, without S-C or S-S bond decomposition during the deactivation. Results of other sulfides deactivation further suggest that the poisoning effect of sulfides could be related to their molecular structures and valence bonds.(5) The effect of glycerine and glyceryl triacetate on the hydrogenation of methyl laurate over Cu/Zn catalyst was investigated, respectively. XRD, BET, GC-MS and TG-DTA were used for catalyst characterization or product analysis. Extremely oleophobic 1,2-propanediol and insoluble highly cross-linked solids could be generated from the thermal decomposition of glyceryl triacetate and glycerine, respectively, and block the catalytic sites by physical adsorption, derease the surface area, leading to the catalyst deactivation. The catalytic activity of glycerine deactivated catalyst could be regenerated by methanol washing.