The Synthesis Of Pyridine Bases Through Glycerol Route

Pyridine bases,mainly including pyridine and picolines（e.g.,2-,3-and 4-picoline）,particularly 3-picoline,were very important heterocyclic chemicals and they were wide application in various fields,e.g.,pharmaceuticals,pesticides,fodder and other fine chemicals.Nowadays,pyridine bases were manufactured in industry through the condensation of formaldehyde,acetaldehyde and ammonia.However,pyridine was a major product,and the yield of 3-picoline was no more than 27%.Meanwhile,4-picoline was generated in this route.Thus,it is extremely difficult to separate the products because the distance of the boiling point between3-picoline and 4-picoline was below 1℃.Additionally,formaldehyde and acetaldehyde possessed extreme toxicity,and easy to polymerize resulting to the fast deactivation of catalyst.There was no doubt that the development of a new and advanced route was currently highly desired.Now,a great number of glycerol as byproduct was produced during the synthesis of biodiesel from bio mass as alternative and sustainable energy sources.The dehydration of glycerol to acrolein was happened while the high yield of3-picoline without the generation of 4-picoline was obtained from the reaction of acrolein and ammonia.Some drawbacks such as toxicity and polymerization were appeared in acrolein.However,the glycerol could in situ generate acrolein or the acetalization of acrolein was to generate acrolein acetal.In these cases,the problems of acrolein were effectively solved.Obviously,the production of 3-picoline through glycerol was a green route.It promoted the development of biodiesel.Meanwhile,the problems for the synthesis of pyridine bases were fundamentally resolved in existing processes.Thus,it possessed a much more largely theoretical significance and economic value.This work first proposed and investigated the synthesis of pyridine bases through glycerol route.The influencing factors,including catalysts,raw materials,types of reactions and reactors,heating mode and reaction conditions,were systematic studied.In addition,reaction mechanism was explored.The results were shown as follows:（1）Pyridine and 3-picoline were synthesized from the condensation o f acrolein and ammonia in a fixed-bed reactor.The reaction parameters,including nature of catalyst,reaction temperature,liquid phase space velocity and reagent ratios were investigated in detail.The catalyst was prepared by successive Mg（NO₃）₂ and HF treated HZSM-5 and then employed in this reaction.The results showed that the total yield 54%of pyridine and 3-picoline at a 27%yield to 3-picoline and no 4-picoline were obtained at reaction temperature=425℃,LHSV=0.86 h^-1 and molar ratio of acrolein/ammonia/water=1/2/2.（2）Pyridine bases were synthesized from the condensation of acrolein acetal and ammonia in a fixed-bed reactor.The reaction parameters,including different acetals（acrolein dimethyl acetal and acrolein diethyl acetal）,reagent ratios,reaction temperature,liquid phase space velocity,nature and dosage of catalyst and particle size were systemically investigated.The results showed that the polymerization from acrolein was absolutely solved while using acrolein dimethyl acetal or acrolein diethyl acetal as raw materials.For the reaction of acrolein dimethyl acetal and ammonia,3-picoline without the generation of 4-picoline was obtained under all the reaction conditions.The results showed that the total yield of pyridine bases over HZSM-5 was around 50%under reaction temperature=450℃,LHSV=0.85 h^-1 and molar ratio of acrolein diethyl acetal/ammonia/water=1/3.5/1.The negative effect from the internal and external diffusions was completely eliminated at particle sizes=450-900μm and residence time<0.08 s.Among all the catalysts,F/Mg-HZSM-5（small particles HZSM-5）catalyst exhibited the best catalytic activity in this reaction.The result showed that the total yield of pyridine bases was55%and the yield of 3-picoline was 34%.In addition,no 4-picoline was found under all the reaction conditions.For the reaction of acrolein diethyl acetal and ammonia,a lot of catalysts were screened.Among them,ZnO/HZSM-5 showed the best catalytic activity in this reaction.HZSM-5-At-acid was prepared by alkaline-acid sequent-treated HZSM-5and then the ZnO/HZSM-5-At-acid catalyst was obtained by loading ZnO on the surface of HZSM-5-At-acid.The stability of ZnO/HZSM-5-At-acid was remarkably prolonged relative to that of ZnO/HZSM-5.On the basis of the catalyst,the total yield of pyridine and 3-picoline was increased up to about83%and the yield of 3-picoline was about 47%after several regeneration cycles.Additionally,the yield of 4-picoline was no more than 10%under all the conditions in this pathway.（3）Pyridine bases were synthesized from the reaction of glycerol and ammonia in a fixed-bed reactor.The reaction parameters,including reactor mode（single and series-connected two-stage continuous flow reactor）,nature of catalyst,reaction temperature,liquid hourly space velocity,glycerol concentration,molar ratio of glycerol/ammonia and the stability were investigated in detail.For a single reactor,the total yield of pyridine bases over HZSM-5-At-acid was about 32%under reaction temperature=425℃,LHSV=0.60 h^-1,concentration of glycerol solution（water as solvent）=36 wt%,molar ratio of glycerol/ammonia=1/5 and TOS（time on stream）=1³ h.For a series-connected two-stage reactor,in the first stage of reactor,HZSM-5-At was filled at the reaction temperature of 330℃,or,HZSM-22 and HZSM-22-At-acid were respectively filled at the reaction temperature of 400℃,in the second stage of reactor,ZnO/HZSM-5-At-acid was filled at the reaction temperature of 425℃,LHSV=0.45 h^-1,concentration of glycerol solution（water as solvent）=20 wt%,molar ratio of glycerol/ammonia=1/5 and TOS（time on stream）=1³ h.The total yield of pyridine bases was above 60%in these condit ions.In addition,this high yield can maintain at least 9 h while the HZSM-22-At-acid and ZnO/HZSM-5-At-acid catalysts were respectively filled in the first stage of reactor and the second stage of reactor.The yield of 3-picoline was increased up to around 45%after introducing propanal as a third reaction component.（4）Pyridine bases were synthesized from the reaction of glycerol and ammonium salt in a liquid-phase media.The reaction parameters,including heating mode（electric heating and microwave radiation heating）,reaction time,microwave radiation power,reagent ratios,ammonium sources,impurities in glycerol and nature of catalyst were investigated in detail.The results showed that it only contained 3-picoline in obtained pyridine bases in most cases and the trace 2,5-dimethylpyridine was appeared in rare cases under electric heating and microwave radiation heating modes,respectively.It was found that 3-picoline was hardly produced under conventional heating mode.For comparison,under the same reaction conditions as above but employing microwave irradiation heating mode,the yield of 3-picoline was significantly increased.The yield of 3-picoline over acetic acid as solvent and catalyst was 60%at reaction temperature=100℃,reaction time=20 min and reaction pressure=0.1 MPa.Based on the reaction process,the yield of 3-picoline was further increased up to 71%after adding TiO₂ as co-catalyst under the same reaction conditions.