Synthesis Of Pyrazinyl Compounds From1,2-propanediamine Catalytic Cyclization With Glycerol

Fuel and energy crisis forces us to seek the alternative fuels for future, and thebiodiesel, known as a kind of renewable and clean energy, has recently attracted moreand more attentions. However, biodiesel manufacturing is mainly achieved from thetransesterification of non-edible oils with a low alcohol, which will give rise to theproduction of a large quantity of glycerol as a by-product. About one ton of glycerolwill be produced out of nine tons of biodiesel in the present biodiesel industry.Therefore, the effective utilization of glycerol is of great importance for thedevelopment of biodiesel industry. Glycerol can be used as a chemical raw material,cosmetics, and to produce other value-added chemicals by hydrogenation such as1,2-propanediol,1,3-propanediol, propyl alcohol, ethylene glycol, etc. They are allbased on the fact that glycerol molecules contain hydroxyl groups.The results reported in literatures show that glycerol can react with ethylenediamine（EDA） with two-NH₂groups to form2-methylpyrazine compounds through catalyticcyclization. As well known,1,2-propanediamine （PDA） has one-CH3group more thanEDA, and also contains two–NH₂groups as well. Therefore, it is theoretically possiblefor glycerol to interact with PDA to produce2,6-or2,5-dimethylpyrazine （2,6-DMP or2,5-DMP）, both of which are important drug intermediates and perfume compoundsused in food. According to this, the present work investigates the synthesis of pyrazinylcompounds by glycerol vapor-phase catalytic cyclization with PDA in fixed-bed systemfor the first time.In the present work, the aqueous liquid reaction mixture are obtained by mixing PDA,glycerol and distilled water with a molar composition of1:1:8.84. The fixed bedmicro-reactor system is introduced to preparate pyrazinyl compounds through catalyticcondensation cyclization. The used ZnO/Al₂O₃catalysts with different Zn amount are prepared by the impregnation method using γ-Al₂O₃as support, and the modification ofthese catalysts with different metal oxides is also carried out by the same time. And atthe same time, another kind of catalyst （Cu-TiO₂/Al₂O₃） with a different amount of Cuand TiO₂, prepared by fractional impregnation method using Cu（NO3）2and butyltitanate as precursors, is also applied in the glycerol cyclization with PDA. The effect ofmetal ions modification, modified amount of ions and reaction conditions on thecatalytic properties of the ZnO/Al₂O₃and Cu-TiO₂/Al₂O₃cataylsts the fixed-bedcyclization of glycerol with PDA has been investigated in detail. Moreover, the catalyticstability of the optimum catalysts is studied. The corresponding catalysts are alsocharacterized by using X-ray fluorescence （XRF）, scanning electron microscope （SEM）,powder X-ray diffraction （XRD）, H₂-temperature programmed redution （H₂-TPR）,NH₃-temperature programmed desorption （NH₃-TPD）, CO₂-temperature programmeddesorption （CO₂-TPD） and N₂adsorption-desorption. According to the catalytic resultsand catalysts characterization, the reaction mechanism for the cyclization reaction ofglycerol with PDA is discussed. Through the obtained-above investigations, someinnovative results are obtained as follows:（1） It is found that the catalytic cyclization of glycerol and PDA gives twointermediates as a higher value-added products such as6-hydroxymethyl-2-methylpyrazine （6-HMP） and5-hydroxymethyl-2-methylpyrazine （5-HMP） besides themain products6-DMP and5-DMP;（2） The investigation on catalytic properties of the ZnO/Al₂O₃catalysts withdifferent Zn amount in cyclization glycerol with PDA exhibits that ZnO/Al₂O₃catalystwith a Zn content of15%give the highest glycerol conversion, and however ZnO/Al₂O₃catalyst with a Zn content of12%gives the highest total yield of intermediatesincluding6-HMP and5-HMP along with a less side reactions;（3） All ZnO/γ-Al₂O₃catalysts are observed to exhibit a high conversion towardsglycerol and PDA above95%and85%, respectively when the reaction temperature is380oC, and give a high total yield towards four pyrazinyl compounds （>85%）;（4） XRD characterization results of ZnO/γ-Al₂O₃catalysts indicate that the supportAl₂O₃exists in the form of γ-phase, and the introduced Zn species are mainly convertedto ZnO phase;（5） Zn catalysts supported on γ-Al₂O₃are observed to provide acid and basecatalytic sites simultanuously. The base sites are beneficial for glycerol activationthrough dehydrogenation, and the acid sites are helpful to the activation of PDA and thecatalytic dehydration or dehydroxylation of glycerol. These can result in promoting the catalytic cyclization to produce pyrazinyl cycle;（6） The cyclization pathway of glycerol with PDA is proposed, the formation of2,6-DMP and2,5-DMP is thought to directly take place over acid sites through glyceroldehydration or dehydroxylation to form intermediates containing carbonyl groups,which can be further cyclized with the activated PDA;6-HMP and5-HMP are mainlyproduced by the interaction of dihydroxypropanal or dihydroxyacetone from glyceroldehydrogenation on basic sites with activated PDA on acidic sites; Moreover,2,6-DMPand2,5-DMP can also be formed through the further hydrodehydration of6-HMP and5-HMP, respectively;（7） From XRD and SEM images of ZnO/Al₂O₃catalysts, it can be observed thatthe introduction of Ce, Mn and Mo oxides into catalysts can lead to the high dispersionof ZnO species on surface of catalysts, which lowers the size of ZnO crystal grain and isbeneficial for the condensation cyclization of glycerol with PDA catalytic（8） The characterization results and catalytic tests of ZnO/Al₂O₃catalysts modifiedby Ca oxides in the glycerol cyclization with PDA show that more basic sites oncatalysts can accelerate the dehydrogenation of glycerol. The formed intermediates suchas dihydroxypropanal or dihydroxyacetone from dehydrogenation are also easilyconverted impurities through the further dehydrogenation on base sites, the generatedimpurities can not interact with PDA, however inhibit the cyclization to pyrazinylcompounds; And at the same time, these impurities easily occur the carbon deposit overcatalyst surface, leading to a poor stability; The further experiences indicate that thecatalytic activity of deactivated catalyst can be recovered in some extent aftercalcination at500oC in air flow;（9） The characterization results of H₂-TPR of typical catalysts indicate that theZnO/Al₂O₃catalysts modified by Ce, Ca, Mn and Mo oxides give a strong negativepeak, which results from spilled-over hydrogen. This hydrogen spill-over is beneficialfor the hydrodehydration of glycerol,6-HMP and5-HMP, thereof the obtained maincyclization products are mainly in form of2,6-DMP and2,5-DMP;（10） It is found that Cu-TiO₂catalysts supported γ-Al₂O₃（Cu-TiO₂/Al₂O₃） exhibita bifunctional catalytic property. Cu2+species are transformed into Cu0particles incatalysts after pretreatment by H₂at a high temperature, the formed Cu0species act asthe catalytic active centers for the formation of intermediates from glycerol bydehydrogenation or hydrodehydration. Meanwhile, TiO₂species in catalysts can provideacid sites which exhibit an excellent catalytic activity for PDA activation and glyceroldehydration. （11） For PDA catalytic cyclization with glycerol, the optimum contents for theintroduced Cu and TiO₂species in Cu-TiO₂/Al₂O₃catalysts are found to be2wt%and7wt%, respectively;（12） The optimum reaction temperature and carrier gas composition for thecatalytic cyclization of glycerol and PDA over Cu-TiO₂/Al₂O₃catalysts are proved to be380oC and20%H₂-N₂, repectively;（13） ZnO/Al₂O₃catalysts with and without Ca, Mn, Mo modifications pretreatedby H₂exhibit a high and stable glycerol conversion （above90%） during a stream-timeof240min under the conditions of N₂as carrier gas, reaction temperature380oC andliquid hourly space velocity （LHSV）1.6h^-1; Cu-TiO₂/Al₂O₃catalyst is also tested toexhibit a good catalytic stability under the conditions of reaction temperature380oC,20%H₂-N₂as carrier gas and LHSV=1.6h^-1.