Gas-phase Aldol Condensation Of Acetone To Isophorone Over Various Solid Base Catalysts

Isophorone,the first generation of derivative of acetone chemistry, is a high boiling point solvent and fine chemical intermediate, and is widely used in medicine, pesticide, paint and other industries. There are two ways to prepare isophorone by the aldol self-condensation of acetone: one is liquid-phase condensation in alkaline solution at a certain pressure, and the other is gas-phase aldol condensation on solid catalysts. The former way is used in foreign industrial process, and has advantages of simple catalyst systems and higher selectivity to isophorone, etc., However, there are still some problems: the reaction is performed under pressure and difficult to control operation, alkaline solution (such as potassium hydroxide solution) is prone to corrose the equipment and difficult to separate and recover. While, gas-phase aldol condensation of acetone could solve the above problem, and has some advantages, such as easy operation, environmental friendly, etc., it is considered a ideal way to product isophorone. Some solid basic catalysts have been reported for the aldol condensation of acetone, including alkaline oxides, (VO)2P2O7, CsOH/SiO2, Mg-Al oxides, etc., but the reaction activity and stability of these catalysts is low, this is a problem which need to be resolved further.In this work, various solid base catalyst systems are prepared in diffenent methods, including Mg/Al mixed oxides with different Mg/Al ratios prepared using a citric acid route, co-precipitation methods and supported catalysts prepared by traditional impregnation method, and the catalytic performances of obve catalysts has been investigated in the aldol condensation of acetone. The relationship between the structure, surface texture (base property) and the catalytic performance of catalysts was studied by combining different characterization means of XRD, CO2-TPD, FTIR, etc. The nature of active sites of the catalysts and the reaction mechanism were also discussed. The main experimental results and conclusions are as follows:1. The catalytic performances of aldol condensation of acetone over the catalysts of Mg-Al mixed oxides prepared in a citric acid route Adding small organic molecules during the synthesis process is useful to improve the structure and surface properties of materials and enhance catalytic activity and stability. Here, Mg-Al mixed oxides (denoted as MgxAlO-CA hereafter) with various Mg/Al molar ratios, were prepared by a sol-gel route in the presence of citric acid. The characterization results of XRD show that the change of Mg/Al ratio could affect the structure of catalysts, when Mg/Al ratio is less than 1.0, the sample has MgAl2O4 structure, when Mg/Al ratio is greater than or equal to 1.0, an additional MgO phase appears and the diffraction intensity of MgO gradually increase with the increase of Mg content. The characterization results of N2 adsorption-desorption show that Mg-Al oxides exhibit the higher surface area and have a significant amount of interparticle mesoporosity. We consider that the coordination between Mg2+, Al3+ and citric acid plays a key role to obtain porosity Mg-Al oxides with cetain porosity. The characterization result of CO2-TPD show that Mg/Al ratio has a large effect on the base property of Mg-Al mixed oxides. The reaction results of aldol condensation of acetone shows that Mg/Al ratio has a certain effect on the of catalysts. Among the catalysts, Mg1.0AlO-CA sample has relatively higher reactivity (78%) and selectivity of isophorone (61%). Additionally, the promotion of alkali ion improves the catalytic activity of Mg1.0AlO-CA, the main reason is that the addition of certain amount of alkali ion has effect on the texture of base sites.2. The catalytic performances of aldol condensation of acetone over the catalysts of Mg-Al mixed oxides prepared in co-precipitation methodsSeries of binary metal oxide catalysts prepared by co-precipitation method were investigated the catalytic performances of the aldol condensation of acetone, the reaction results show that Mg1.0AlO catalyst has relatively high catalytic performance. Mg-Al mixed oxides (denoted as MgxAlO-1, MgxAlO-2) with various Mg/Al molar ratios, were prepared by co-precipitation method in different dropping manners. The characterization results of XRD show that the change of Mg/Al ratio and dropping manner could affect the crystalline phase of catalysts, and MgO phase and MgAl2O4 spinel phase appear and the diffraction intensity of MgO gradually increase with the increase of Mg content, the diffraction intensity of MgO phase and MgAl2O4 spinel phase is relatively higher for MgxAlO-2 samples prepared by back-titration method. The results of aldol condensation of acetone show that Mg/Al ratio and dropping manner have a certain effect on the catalytic performance of catalysts. Among them, Mg1.0AlO-2 catalyst prepared by back-titration has relatively higher reactivity (87%) and stability (the reaction activity is still 65% after reaction for 85 h). The characterization result of TG-DTA show that carbon deposition form easily on the surface of Mg1.0AlO-1 sample and lead to the fast decline of the reactivity. The characterization results of CO2-TPD show that the Mg1.0AlO-1 and Mg1.0AlO-2 sample have a certain amount of weak base and strong base sites, and the density of strong base sites on the surface of Mg1.0AlO-2 is relatively higher. Combining with the reaction results, it may be inferred that the co-existence of a certain amount of weak basic centers and strong basic centers on the surface of Mg-Al oxides may play an important role for improving the activity and stability of these catalysts.3. The catalytic performances of gas-phase aldol condensation of acetone over supported solid base catalystsSolid base supported catalysts were prepared by the traditional impregnation and were tested the catalytic properties of aldol condensation of acetone. When the carrier is activated carbon (from almond nutshells), the characterization results of XRD show that MgO, CaO characteristic diffraction peaks appears respectively when MgO loading is higher than 15 wt% and CaO loading is more than 10 wt%. The reaction results show that 12 wt%CaO catalyst has relatively higher reactivity and selectivity to isophorone, it is because of that a certain amount of basic centers and appropriate intensity of basic sites on the catalysts surface are favor to improve the catalytic performance of aldol condensation of acetone. For 10 wt%MgO supported samples, the type of carrier has a large effect on the reactivity, when Al2O3 was used as the carrier, the catalytic performance of 10 wt%MgO/Al2O3 is relatively higher. Additionally, the loading of MgO has a certain effect on the reactivity, when the loading of MgO is less than 15 wt%, the catalytic performance of samples has a little change, when the loading of MgO is higher than 15 wt%, the catalytic properties declines sharply, it is because of that the MgO crystal on the surface of Al2O3 is larger and the intensity of basic centers is relatively higher.