Stability And Deactivitation Of Sulfated Metal Oxides In The Esterification Of Acetic Acid And N-Butanol

Sulfated metal catalysts are widely applied in acid-catalyzed chemical reactions. However, there is a fatal problem for people to utilize them as practical catalysts in chemical industry to replace those liquid acid catalysts with them. The problem is that sulfate-promoted metal oxides often rapidly deactivate despite of their initial high activities. So, it is necessary to find exact reasons for sulfated metal oxides in organic reactions catalyzed by them. The paper is focused on the studies of stability and deactivation for three representative sulfated metal oxides in the esterification of acetic acid and n-butanol, to find exact reasons for these three catalysts, and further to modify sulfated titania by doping Zr-La or Zr-La-Fe, and further to optimize the preparing conditions of sulfated titania, in view of sulfated titania’s better stability and longer life-span than the other two catalysts. Firstly, the deactivation of three sulfated metal oxides, including sulfated iron oxide, sulfated zirconia and sulfated titania, in the esterification of acetic acid and n-butanol was studied respectively. The three catalysts were used continually and accumulatively,10runs and10h for sulfated iron oxide, and10runs and7.5h for sulfated zirconia, and20runs and1Oh for sulfated titania, respectively. After these continual uses of the catalysts, considerable deactivation all happened to them. The fresh and the deactivated catalysts were compared by means of many characteristic methods including FTIR, XRD, BET, SEM, TG-DSC, and NH3-TPD, to disclose some possible reasons for the deactivation of the three sulfated metal oxides in the esterification. Based on the comparative analyses of IR patterns of the fresh catalysts and the deactivated ones, a common deactivation mechanism is tentatively proposed for the three catalysts of the same kind.The results for stability and deactivation of them are showing a nearly-same deactivation law for three catalysts, which is summarized as below:(1) Based on IR results, the acidity degradation happened to them, Surface sulfate groups, which are originally coordinated to Fe3+cations and can so induce and generate strong Lewis acidity of Fe+cations, may have been gradually turned into free sulfate groups and sulfate esters arisen from strong Lewis-acidic Fe3+cations’ being hydrolyzed by H2O and their being alcoholyzed by n-butanol, which leads to a gradual destruction of the originally strong coordination between Fe+cations and surface sulfate groups, so leading to the acidity degradation of the catalyst, and so finally leading to the activation of the catalysts;(2) Based on XRD results, the catalysts increased its crystallinity after considerable deactivation;(3) Based on BET results, the catalysts varied in their specific surface areas, pore volumes and average pore diameters, after considerable deactivation;(4) Based on SEM results, the catalysts diminished their particle aggregation, after considerable deactivation.(5) Based on TG-DSC results, after considerable deactivation of the catalysts, originally active sulfate groups may turn into free sulfuric acid which can desorb or decompose around366℃, and originally active sulfate groups may also turn into organic sulfate esters so as to decrease their thermal stability and so as to desorb or decompose under temperatures lower than500℃, and the remained sulfur species may still stand on these seriously-deactivated catalysts which may be mainly organic sulfate esters as the mainest carbon deposits on them.(6) Based on NH3-TPD results, these catalysts decreased their acidity after deactivation considerably;(7) Based on XPS results, these catalysts increased in the surface hydroxyls and in carbon deposits, and slightly decreased in sulfur content, and hardly varied in sulfur valence, after considerable deactivation, supporting a probability of sulfate groups’ being turned into free sulfate groups and sulfate esters during the reactions.A deactivation mechanism is tentatively proposed which is put out firstly by studying sulfated iron oxide based IR results firstly. The mechanism is directly supported also by TG results and XPS results of sulfated iron oxide. Namely, surface sulfate groups, which are originally coordinated to Fe3+cations and can so induce and generate strong Lewis acidity of Fe3+cations, may have been gradually turned into free sulfuric acid and organic sulfate esters arisen from strong Lewis-acidic Fe3+cations’being hydrolyzed by H2O and their being alcoholyzed by n-butanol, which leads to a gradual destruction of the originally strong coordination between Fe3+cations and surface sulfate groups, so leading to the acidity degradation of the catalyst, and so finally leading to the deactivation of it. Emphatically, in the proposed mechanism, the water produced from the esterification may play a key role on the deactivation of the catalyst, because it can directly hydrolyze some strong Lewis-acidic Fe3+cations of the catalyst and indirectly promote the alcoholysis of them, to form weak Lewis-acidic Fe-OH species. The deactivated catalyst has a larger crystallinity, a smaller specific surface area, a smaller sulfate groups content, a weaker acidity than the fresh. All these phenomena, accompanying the deactivation of sulfate-promoted iron oxide, can be interpreted by the proposed deactivation mechanism very well.Besides, the above mechanism is also adaptable to sulfated zirconia and sulfated titania, which is directly supported by those IR and TG and XPS results of them. Secondly, doping Zr-La and doping Zr-La-Fe into the bulk of sulfated titania are studied respectively, to improve the stability and life-span of sulfated titania. When a small amount of Zr-La or Zr-La-Fe were co-doped into the bulk of TiO2, the modified catalyst obtained a by far better catalytic activity and stability than the unmodified, showing that the modified is more resistive to deactivation than the unmodified. Under the set reaction conditions, the average conversion (of acetic acid) and the20th-cycle conversion (of acetic acid) are93.54%and87.15%for the modified TiO2-Zr-La-Fe/SO42-,88.83%and77.35%for the modified TiO2-Zr-La/SO42-,80.83%and46.15%for the unmodified, respectively. The modified and unmodified catalysts were comparatively characterized by means of FTIR, XRD, BET, SEM, TG, and NH3-TPD methods to find the possible reasons for the superiority of the modified catalyst to the unmodified one. The characteristic results indicated that the doping of a small amount of Zr-La or Zr-La-Fe into the bulk of sulfated titania brings the modified catalysts some good effects as below:(1) increasing its acidity including the concentration and acid strength of the surface acidic sites of it, based on IR and NH3-TPD results;(2) decreasing its crystallinity after calcination by retarding its crystallization from amorphous TiO2to anatase TiO2based on XRD results;(3) increasing its specific surface area and still keeping a lot of large-diameter mesopores in it, based on BET results;(4) decreasing particle sizes and diminishing the aggregation of them, based on SEM results;(5) increasing its surface sulfate group content substantially based on TG-DSC results;(6) XPS results show that, for doped modified catalysts, Zr-La and Zr-La-Fe may have been combined into the TiO2bulk or onto the surface the doped catalysts.(7) XPS results also show that, the doping can conserve more hydroxyls on the modified catalysts than the unmodified catalysts, so as to inhibit the re hydroxylation of metal cations and so as to improve the stability of the modified catalyst. All the above advantageous effects arisen from the doping are to be responsible for the substantially-improved catalytic performances of the modified catalyst.Finally, we measured the thermal effect for the whole process of obtaining Ti(OH)4from TiCl4, and optimized the preparing conditions, including T(precipitating temperature) and pH, C(H2SO4), and T(calcinating temperature), whose effects on the stability and life span of sulfated titania were studied by means of various characteristic methods.(1) The thermal effect-279.69kJ/mol for the whole process of obtaining Ti(OH)4from TiCl4, given off mainly during the hydrolysis of TiCl4by H2O, was measured coarsely.(2)The ice-bath preparing method was selected to improve the stability and life-span, because it can overcome the disadvantageous effects of the large thermal effect so as to increase the specific surface area and so as to be advantageous the adsorption and coordination of sulfate groups on the kind of catalyst and so as to improve the stability and life span of the final sulfated titania.(3) The optimum preparing conditions are below:T(precipitating temperature)=0℃, pH=8-9, C(H2SO4)=1.25M, T(calcinating temperature)=550℃.(4) The sulfated titania prepared under the optimum preparing conditions has better stability and longer life-span than sulfated titania prepared under the non-optimum preparing conditions.