Study On The Preparation, Characterization And Catalytic Reactivities Of Cu-ZrO₂ Catalyst For Dehydrogenation Of Alcohol

The supported catalysts of copper oxide and elementary metal copper have been widely used in petrochemical industry for the catalytic reforming of aromatic hydrocarbons and catalytic dehydrogenation of alkane and alkene. They are also widely used in fine chemical industry for the preparation of derived ketone, aldehyde and organic acid from their original alcohols. Since the middle of the 20th century, the R & D of the catalysts preparation technologies and their industrial applications has been received a wide range of attentions from all over the world and it has been become one of the hot-spot issues of chemical science and technology. With the continuous exploring and innovation of the scientists, a series of significant progresses and achievements have been made in the past decades towards the frontier issue.Based on a wide range of literature investigations and a solid theoretical analysis on the catalyst design and preparation, a supported active elementary metal copper catalyst Cu-ZrO₂ which applies to the dehydrogenation of amino-alcohols and cyclohexanol were successfully prepared in a laboratory scale in the present study, using ZrO₂ as the supporter and metal copper as the active component, by the co-precipitation of Cu(NO3)2 and ZrOCl2·8H2O with the addition of NaOH, through the controlling and adjusting of the key process parameters during the preparation of the catalyst.Several modern material testing methods were applied to the characterization of the physical and chemical properties of the catalyst. The results show that the Cu-ZrO₂ catalyst has a BET surface area of 116.53 m2/g, the final product and the precursors of the catalyst each has a unique ZrO₂ square crystal structure, the reactivity of the catalyst decrease while the ZrO₂ supporter contains the mixture of other ZrO₂ crystal morphology, the terminal pH of the co-precipitation and the calcinations temperature of the precursor each plays an important role in the formation of the ZrO₂ crystal structures, the particle size of the Cu-ZrO₂ catalyst prepared in the experiment is about 2.0³.0μm and the distribution of the active component on the ZrO₂ supporter surface is of uniformity. Theoretical calculation reveals that the ZrO₂ crystal particles exist in the precursor of the catalyst almost in the form of mono-layer crystal. The CuO in the precursor of the catalyst appears a strong reduction peak at 227℃by TPR, whereas the pure copper oxide appears the peak at about 320℃. The analysis results from TG and the appearances of the catalyst indicate that a high reactivity Cu-ZrO₂ catalyst will have a significant mixtures density of copper hydroxyl and zirconium hydroxyl precursors (which is greater than 1.70g/cm³) with a brilliant color macroscopically, whereas the thermal gravity-lost(TG) temperature range of the hydroxyl compounds is very narrow (from 150℃to 530℃) microscopically.By the methods of orthogonal and single-factor experimental design, the key technical conditions and the main factors which affect the dehydrogenation activity (such as selectivity, conversion, de-activation rate, etc) of amino-alcohols and cyclohexanol during the catalyst preparation were investigated and evaluated. The influence rules of the key factors to the reactivity of the catalyst were studied and the optimal parameters for the preparation of the catalyst were obtained. The results show that in a certain range of the preparation conditions, the Cu-ZrO₂ catalyst can maintain a good and stable high selectivity during the catalytic dehydrogenation of amino-alcohols and cyclohexanol. The influence factors order of the preparation conditions in terms of selectivity of the catalyst during the dehydrogenation of amino-alcohols is: terminal pH of co-precipitation>ratio of n(Zr) /n(Cu) >initial concentration of ZrOCl2·8H2O>reduction time>calcinations time, whereas in terms of the conversion, the influence order is: terminal pH of co-precipitation> reduction time> ratio of n(Zr) /n(Cu) >calcinations time>initial concentration of ZrOCl2·8H2O. On the other hand, in the dehydrogenation of cyclohexanol the influence order for the preparation conditions in terms of the selectivitywas found to be: ratio of n(Zr) /n(Cu) > terminal pH of co-precipitation>calcinations time > calcinations temperature>initial concentration of ZrOCl2·8H2O, and, of the conversion, this order is shown as the follows: ratio of n(Zr)/n(Cu)>terminal pH of co-precipitation>initial concentration of ZrOCl2·8H2O> calcinations time > calcinations temperature. The optimal conditions for the preparation of the catalyst are: 2:1 of the n(Zr)/n(Cu) atomic ratio, 12.0 of the terminal pH of the co-precipitation by direct addition of NaOH, 0.2⁰.3 mol/L of the initial concentration of ZrOCl2·8H2O, 4 hours of the calcinations time at 500℃of calcinations temperature, 4 hours of reduction time at 230℃in the mixture of H2 and N2. By using the catalyst prepared under the above conditions to the dehydrogenation of diethanolamine, a highest selectivity of 99.60% has been obtained, whereas to the dehydrogenation of cyclohexanol, the highest selectivity is 99.46 %.The single-factor effect of the n(Zr)/n(Cu) atomic ratio, the introducing of trace transition metal elements, the terminal pH of co-precipitation, the initial concentration of ZrOCl2·8H2O, the calcinations time and temperature of the precursor during the preparation of the catalyst on the reactivity each was also approached in the dehydrogenation of amino-alcohols and cyclohexanol. Summarily, (a) the selectivity of the prepared catalyst and the product yields of dehydrogenation increase with the increase of the terminal pH of co-precipitation; (b) in the range of 2.0⁶.0 of the n(Zr)/n(Cu) atomic ratio, the product yields of the dehydrogenation from amino-alcohols and cyclohexanol decrease with the increase of the ratio;(c) in the range of 0.1⁰.4 mol/L of the initial concentration of ZrOCl2·8H2O, the influences of the concentration to the selectivity and the yields of the dehydrogenation of the reactants are very small;(d) the reactivity of the catalyst prepared from the hydroxyl precursors which calcined at 500℃reaches to its highest activity. The reactivity decreases when the calcinations temperature of the hydroxyl precursors is over 500℃. The completion degrees for calcinations and decomposition of the hydroxyl precursors, the integrity for the formation of the ZrO2 and CuO crystals and the degree for sintering or agglomeration of the active component CuO on the surface of the ZrO2 supporter is each correlated to the calcinations time and temperature; (e) by overtime reduction of the copper oxide precursor, the selectivity of the catalyst and the product yields of the dehydrogenation will decrease. However, the decreasing degree for the selectivity is a little less than that for the product yields. The over-reduction of the precursors might be one of the key causes of the phenomena, and which could lead to the maladjustment or the unbalance of the activation sites ratio of Cu0 and Cu+ and to the agglomeration of the microcrystal of metal copper on the catalyst supporter surface. (f) the introducing of some trace transition metal elements to the catalyst will lead to the poisoning and de-activation of the Cu0 and Cu+ active components, which finally lead to the decrease of the selectivity in the dehydrogenation of cyclohexanol, compared to the single pure copper oxide usee as the active component to the catalyst.Furthermore, the relationship between the reaction conditions (reaction temperature and pressure, addition amount and concentration of NaOH to the reaction system, etc) and the product yields or the reactivity of the catalyst was established experimentally in the catalytic dehydrogenation of amino-alcohols and cyclohexanol.(a)because the catalytic dehydrogenation of amino-alcohols is an exothermic reversible reaction, the increase of reaction temperature is of advantages to the reaction kinetically whereas is of disadvantages thermodynymically. The conversion and the product yield increase with the increase of the reaction temperature at first and then will decrease with the further increasing of the temperature. The effect of reaction system pressure to the selectivity of the catalyst is very small. However, it is a little distinct to the conversion and the product yields of the reaction, the rules of which are similar to that of reaction temperature effect on the conversion and the yields. The concentration of NaOH has very little effect on the selectivity. Influenced by the reversible reaction equilibrium effect, the conversion of amino-alcohols increases with the increasing of NaOH concentration at first and then it will approaches to a stable value with the further increasing of the NaOH concentration. A set of feasible reaction parameters has been worked out for the dehydrogenation of amino-alcohols with 165℃of reaction temperature, 1.6Mpa of reaction pressure and 1.05 times of NaOH amount by theoretical demanded with 30%wt of NaOH concentration. (b) the dehydrogenation of cyclohexanol is endothermic and reversible, therefore a high reaction temperature is favorable both kinetically and thermodynamically. It has been observed by experiments that even if with a low reaction temperature of 200℃, the Cu-ZrO2 catalyst can still show a good reactivity to the dehydrogenation of cyclohexanol. However, some side reactions will occur seriously when the temperature exceeds 280℃and cyclohexyl-cyclohexanone and some other unknown by-products or impurities will hence produced. With the decrease of the system pressure, the reaction goes along with the promotion of the conversion because of the equilibrium effect of volume enlargement reaction for the process. For the sake of maintaining a good reduction atmosphere in the reaction system and a stable catalyst activity to the reaction, the suitable parameters for the reaction process has been determined as a reaction temperature of 240℃and reaction pressure of 2.50MPa.According to the result from the calculation of diffusion effects of the reactant molecules on the Cu-ZrO₂ catalyst in the dehydrogenation of amino-alcohol, it reveals that the reaction is mainly subjected to the effect from ordinary molecular diffusion rather than that from Knudsen diffusion in the porous catalyst. The ratio of the two diffusion coefficient DB/DK is 1: 188.3, the Thiele ModulusφS,R of the reaction is very small ( about 1.536×10-8) and the utility of the inner surface area of the catalyst is close to 100%. The reaction is of the characteristics of a macro-porous slow reaction pattern. A higher reaction rate can be available by a further decreasing of the particle diameters and the porous sizes of the catalyst. The calculation results also show that by using slurry stirred reactor to the catalytic dehydrogenation of amino alcohols in the liquid-solid phase, it has the advantages of increasing the turbulent degree of the reactant liquid and deminishing the thickness of the laminar flow layer of the reactant on the catalyst surfaces, which will lead to the eliminations of the resistances of molecular diffusion outside the catalyst surface and Knudsen diffusion inner the porous catalyst simultaneously, and to obtain a more real and intrinsic kinetical model which can describe the control of the true chemical reaction step much accurately. The reactor type and the operation pattern used for the reaction are compatible.Combined with the literatures reports and some theoretical investigations, the dehydrogenation mechanism of amino-alcohols to amino-acids by Cu-ZrO₂ catalyst with the existence of water and alkali in the reaction system was proposed in the paper. The results reveal that during the catalytic reaction, the rate for the formation of amino-acid by nucleophilic addition which occurs between amino-aldehyde molecules and the rate for the disproportionation or S Cannizzaro reaction of amino-alcohols each is extremely fast, the overall reaction rate in the process is controlled by the step of the formation of amino- aldehyde which produced by the dehydrogenation of amino-alcohols. Very few side-reactions occur in the dehydrogenation of amino-alcohols. The investigations indicate that the reaction kinetics equation is a 1st-order mode, the activation energy Ea for the catalytic dehydrogenation of diethanolamine to iminodiactic-acid is 147.80kJ/mol and the relationship between the rate constant and the reaction temperature can be summarized as follows: k=6.8111×10¹⁵exp(-147.80×10³/RT).Finally, the stability of the catalyst reactivity was studied by recovering Cu-ZrO₂ catalyst from the dehydrogenation of amino-alcohols and cyclohexanol. In the catalytic dehydrogenation of diethanolamine, after 9 recycles in the reactions, the average selectivity, the conversion and the product yield of the catalyst is 97.90%, 89.60% and 87.80% respectively. The average de-activity per batch (η_deac) and de-activity rate ( v_Seac) of the catalyst in terms of selectivity is 0.51%/batch and 0.16%/hr, respectively. Whereas in terms of conversion,ηdXeacand v dXeac is 1.93%/batch and 0.62%/hr, respectively. For the dehydrogenation of cyclohexanol, the average selectivity, the conversion and the product yield of the catalyst is 96.82%,80.17% and 77.69%, respectively.ηdSeac, v dSeac,ηdXeac and Xv deacis 0.68%/batch, 0.22%/hr, 1.06%/batch and 0.36%/hr, respectively. The activity of the catalyst in dehydrogenation of cyclohexanol is a little lower than it in dehydrogenation of amino-alcohols. Compared to the influence degree of the reaction conditions on the conversion, the influences of the reaction conditions to the selectivity is a little slight. Hence the Cu-ZrO₂ catalyst shows a high and good selectivity and a stable activity for the dehydrogenation from both amino-alcohols to amino-acid and cyclohexanol to cyclohexanone. The reason for the rapid conversion declining in dehydrogenation might be caused by the long-time exposure of the catalyst in the air and the lack of effective oxygen isolation during the recovering of the catalyst, which results in the oxidation of Cu0 and Cu+1 to Cu+2 by the air. From the view of the conversion, it seems that the novel Cu-ZrO₂ catalyst is more favorable to the dehydrogenation of amino-alcohols. The reason might be resulted from the greater inhibiting effect of chemical equilibrium on the dehydrogenation of cyclohexanol rather than that of amino-alcohols. Compared to some other conventional industrial catalysts used for the dehydrogenation of alcohols, the novel Cu-ZrO₂ catalyst has the advantages and characteristics of good and stable reactivity, easily manufacture, low production cost and wide ranges of industrial applications.