Studies On Condensation Of Isobutylene And Formaldehyde To Isoprene

Isoprene is an important starting material for the production of synthetic rubber and the like.With the increasing demand of isoprene,it is desirable to produce isoprene easily in high purity as well as in good yield.For this purpose,Prins condensation of isobutylene and formaldehyde to isoprene in the fixed-bed reactor based on one stage method was recommended on account of its simplified procedures.And the process attracts widely attention at home and abroad.Seeking suitable catalyst with high activity and stability for this reaction will be the direction of future research.In the present study,the catalytic performances of Nb2O5·nH2O modified with simplified methods are tested using the Prins condensation of isobutene with formaldehyde to isoprene in one step through a fixed-bed reactor.The catalyst includes NbP modified by alkali metal nitrate solution,Nb2O5·nH2O modified by various inorganic acid and calcined at different temperatures,NbB loading on the carrier,the supported catalyst modified by sodium nitrate solution and the carrier modified by acids and bases.This paper investigated the reaction conditions and optimized the reaction process by using the fixed-bed reactor based on one-step method.The main contents and conclusion are as follows:(1)The study investigated the condensation of isobutene with formaldehyde in the vapor phase over modified NbP.The catalysts were characterised by XRD,N2-physical adsorption,FT-IR and NH3,TPD.The catalytic activity of Nb2O5,Nb2O5·nH2O,niobic acid modified by phosphoric acid(NbP)and NbP modified with alkali metal nitrate solution(MNbP)were studied.The catalytic activity follows the following order:Nb2O5<Nb2O5.nH2O<NbP<NaNbP.The effects of the acid strength,amount of acid sites and the structure of the catalyst on the catalytic activity were investigated.Niobic acid modified by phosphoric acid can suppress the crystallization of the catalyst at high calcination temperature.After modified with alkaline metal nitrate solution,hydroxyl group was partially replaced by the alkali metal ion on the surface of NbP catalyst.Consequently,strong acidic sites disappear and the total amount of acid reduced slightly.It is proposed that the formation of isoprene related to the appropriate weak acidic sites of the catalyst.0.2NaNbP catalyst showed the best catalytic performance,the results indicated that isoprene selectivity and yield of 0.2NaNbP catalyst were 68.2%and 58.8%respectively at 300 ℃,IB/FA= 2.18.(2)Modified and unmodified niobic acid catalysts were prepared and used for the Prins condensation of isobutene with formaldehyde to isoprene in one stage through a fixed-bed reactor.Nb2O5·nH2O catalysts modified by various inorganic acid and calcined at different calcined temperatures were prepared by a simple method.Catalysts were systematically investigated by several techniques(XRD,N2-physisorption,FT-IR,NH3-TPR and Py-IR).The result showed that calcination temperature and oxygen-containing inorganic acid modification could control the structure,composition,acid nature and acidity of the catalyst.Therefore,modified conditions had a significant effect on the catalytic performance.When the calcination temperature was 300 ℃ or below,the structure and catalytic active sites were unstable when the reaction performed at about 300 ℃.The structure,composition,acidity and the amount of acid are relatively stable when the calcintion temperation above 300 ℃.The catalyst calcined at 400 ℃ had more weakly acidic sites and higher steadily,it was selected for further experiments.Ternary inorganic acid(H3PO4 and H3BO3),binary inorganic acid(H2SO4)and unitary inorganic acid(HNO3 and HCl)modification had different influence on the catalytic performance.After modified by inorganic acid,new species formed on the surface of the catalyst.That not only changed the amount and nature of acidic sites and acidity,but also reduced the sensitivity of the catalyst to the calcination temperature.Inorganic acid modification could significantly improve the catalytic performance on the isoprene synthesis reaction.(3)NbB loaded on 30HZSM5 as a novel solid acid catalyst was used to catalyze Prins condensation to produce isoprene.After loading of NbB on the carrier,surface interactions occur between NbB species and carrier.The new formed supported catalyst has a large surface and higher thermal stability.The supported catalysts with different weight fractions of NbB and modified concentration of NaNO3 had different acidic characteristics and activity.The objective of the study was to systematically investigate catalysts by several techniques.The effects of the amount of loaded NbB,modified concentrations of NaNO3 on the surface area,acidity and catalytic activity were investigated.Catalytic activity would be improved by modulating the surface acidic sites and surface area.After modulation the amount of the acid sites and the acidity,the 0.4Na-5%NbB/30HZSM5 catalyst with a large amount of weak acid sites showed high activity and it could be comparable with the NbB catalyst.(4)To further improve the activity of the catalyst,the acid and alkali were used to treat the pore of the catalyst carrier.The results showed that the acid treated catalyst could significantly improve catalytic activity,while,the catalytic effect decreased after alkali treatment.The acid treatment made the carrier has more open pore,and the impact of the skeleton structure of the catalyst was minimal.Meanwhile,another role of the acid treatment was modulating the acid nature and strength and the amount of acid sites.After treating the carrier with acid,the acid properties and diffusion properties were improved that can improve the catalytic activity.Meanwhile,the deactivation of the catalyst was studied in this research.