Study On The Alkylation Of Isobutane With Butene Using Chloroaluminate Ionic Liquids As The Catalysts

The alkylation of isobutane with butene is very important for the production of reformulated gasoline;however,the severe equipment corrosion and environmental hazards are involved in the current alkylation technology.Therefore,some novel alternative technologies are desired.In this dissertation,the room temperature ionic liquids were chosen as the main subject and the process and the mechanism of the alkylation of isobutane with butene catalyzed by ionic liquids was investigated.Furthermore,the alkylation using ionic liquids as the catalyst was also studied in a high gravity reactor.Firstly,some functionalized Br(?)nsted acid ionic liquids and Lewis chloroaluminate ionic liquids were prepared.The catalytic performances of these ionic liquids for alkylation of isobutane with butene were evaluated in a batch unit.The experimental results indicated that the prepared Br(?)nsted acid ionic liquids were inactive for the alkylation of isobutane with butene.In contrast,the chloroaluminate ionic liquids were very active for the alkylation.However,the qualities of the alkylate oils using chloroaluminate ionic liquids were unsatisfied and worse than that of the products in the industrial sulfuric acid process obviously.In order to improve the catalytic performances of the chloroaluminate, some organic chemicals with strong electron-donating capacity,such as benzene,alkylbenzene,thioalcohol and thiophene,were introduced into the ionic liquids to modify the ionic liquids.The experimental results showed that the catalytic performances of chloroaluminate ionic liquids were improved distinctly in the presences of the additives mentioned above.The isooctane content in the alkylate oil was up to about 80%and the ratio of trimethylpentane to dimethylhexane was about 8.Though the poor recycling properties of the ionic liquids modified by benzene,the catalytic performance recovered after addition fresh benzene into the used ionic liquids.Moreover,at high isobutane/butene ratio and low temperatures,maximum benefits for alkylation can be obtained using the modified ionic liquids as the catalysts.In order to investigate the mechanism about the improvement of the catalytic performances of the modified ionic liquids,FTIR and ²⁷Al NMR characterization for original ionic liquids and modified ionic liquids were carried out.The alkylations catalyzed by solid AlCl₃ were also studied. By comparison of the results of these experiments,it was believed that the acid strength of the modified ionic liquids decreased duo to the complexation between the organic additives with strong electron-donating capacities and the Lewis acid site in the ionic liquids.The catalytic activity of the chloroaluminate ionic liquids originated from dissociation of AlCl₃ from dimeric anions Al₂Cl₇^-.In the presence of benzene additive, benzene with stronger electron-donating ability was more basic than butene and tended to replace butene as the proton acceptor.Thus,new Bronsted acid site formed through coordination among HCl,AlCl₃ and benzene.The acidity of the new active site was propitious to catalyze alkylation of isobutane with butene.Therefore,the catalytic performances of chloroaluminate ionic liquids were improved.Aromatic hydrocarbons were good additive to modify the chloroaluminate ionic liquids,thus it is significant to study the ionic liquids' solvent capacity for aromatic and the distribution of benzene in the two-phase system of ionic liquids-hydrocarbons.The solvent capacity of ionic liquid for alkane is important to he separation of the alkylate and the catalyst.The ionic liquids' solvent capacities for aromatics and alkane were tested respectively.Furthermore,the distribution coefficients of aromatics and alkane in the two-phase system of ionic liquids-hydrocarbons were also investigated.The experimental results showed that the solvent capacities of chloroaluminate ionic liquids for aromatics were relatively high,whereas those for alkane were extremely low.For the equilibrium system of chloroaluminate ionic liquids-heptane-aromatics,the aromatics distribution coefficients(D_aro) and the selectivities of ionic liquids for aromatics/heptane(S_aro/hep) were influenced by the structure of ionic liquids,structure of aromatics and temperature.These results indicated that the separation of the alkylate and the ionic liquids was easy and the loss of aromatics out from the catalysts was inevitable.The dissolving of aromatics in ionic liquids was facilitated by the complexation between aromatics and the Lewis acid sites in the chloroaluminate ionic liquids.In the vacuum distillation experiments,the gasification temperatures of aromatics in chloroaluminate ionic liquids were higher than their natural boiling point temperatures in the experimental degree of vacuum obviously.The impurities in the industrial feedstocks for alkylation of isobutane with butene would influence the lifetime and the catalytic performances of the chloroaluminate ionic liquids.Thus,the effects of the trace of impurities such as water,methanol and diethyl ether in the feedstocks on the stability of catalytic properties and deactivation of the chloroaluminate ionic liquid were studied in continuous alkylation.The results showed that water,methanol and diethyl ether in the feedstocks affected the catalytic performances of the ionic liquid such as lifetime, isooctane selectivity and ratio of TMP/DMH.In the Et₃NHCl-2AlCl₃ ionic liquid,only one half AlCl₃ could act as active site.With molar ratio of 1:1,the active AlCl₃ in the ionic liquid was deactivated by water through reaction or by diethyl ether through complexation.While,the complexation of AlCl₃ with two molecular proportions of methanol inactivated the active AlCl₃ in the ionic liquid.Besides,some AlCl₃ dissolved in the hydrocarbons and was carried out of the alkylation system.The deactivation of chloroaluminate ionic liquid was observed when the active AlCl₃,i.e.a half of total AlCl₃ in the ionic liquid,was consumed completely.The catalytic activity of the deactivated ionic liquid could be recovered by means of replenishment of fresh AlCl₃.The reaction time in the conventional stirring vessel is relatively long. While,the reaction time would be shortened in the high gravity reactor duo to intensified mass transfer,thus the efficiency of the alkylation catalyzed by the modified ionic liquids would be improved.Therefore, the alkylation of isobutane/butene catalyzed by the modified chloroaluminate ionic liquids was carried out in the high gravity reactor. The results indicated that the alkylation of isobutane of butene was completed in very a short time(less than 1 s) and the conversion of butene was about 99%.The gasification of feedstock was inhibited via increasing of the partial pressure of N₂ in the reactor and the proportion of gas-phase reaction was reduced consequently.Thus,the alkylate oil with relatively high quality was obtained.Besides,high rotating speed and low temperature was beneficial to the increase of the content of trimethylpentane in the alkylate oil.The elementary reactions of alkylation of isobutane with butene catalyzed by the original chloroaluminate ionic liquids and the modified ionic liquids were studies on Hyperchem 7.0 software.It was found that the activation energies or the energy differences of reactant/product of C₄⁺ isomerization,combination of C₄⁺ and butene,C₈⁺ isomerization and hydrogen-transfer in the case of benzene as the proton acceptor of the active site were lower those with butene as the proton acceptor.The changes of reactive energy made more C⁸⁰ formation in the alkylation and higher ratio of TMP/DMH was obtained,the catalytic properties of the chloroaluminate ionic liquids were improved consequently.