Constructing Nanoreactor With Ionic Liquids As Active Component And Catalysis Of Polyoxymethylene Dimethyl Ethers

Polyoxymethylene dimethyl ethers?PODE_n?are green diesel fuel additives with high oxygen content and cetane number.Diesel fuel mixed a little PODE_3⁸can effectively improve diesel performance,increase thermal efficiency and reduce NO_x and solid particles emissions.Meanwhile,the structure of engine does not need to be changed.So PODE_n has broad application prospects.At present,the research on PODE_n is mainly focused on the catalyst.Among them,ionic liquids?ILs?are environment-friendly catalysts with the advantages of high catalytic activity and good selectivity to the target product.However,it is difficult to separate from the product and it is expensive.Therefore,it is hard to apply to industrial production.Heterogenization of ionic liquids on various supports,such as molecular sieves or metal oxides,is an alternative strategy to mitigate the separation problem while maintaining high catalytic activity.We report herein that three types of nanoreactor ILs@SBA-16-X were constructed using ionic liquids as active components and mesoporous molecular sieves SBA-16 as carrier.It was applied to the catalytic synthesis of PODE_n.On the one hand,heterogenization of ionic liquids can not only make the content of ionic liquids decreased,but also easily separated and recovered;on the other hand,the nanoreactor can selectively synthesize the desired PODE_3⁵ compounds and prevent the formation of longer chain compounds because of the diffusion limitation of the nanocages.Firstly,a nanoreactor was constructed with pyridinium ionic liquids as the active component,and the ionic liquids were encapsulated in the super-cage structure of SBA-16 by the embedding method.The synthesis of PODE_n from methanol and trioxymethylene with the heterogeneous catalyst was studied.The optimum process parameters were obtained by optimizing the reaction conditions:reaction temperature 110°C,reaction time 1 h,methanol and trioxymethylene mole ratio 2:1,and catalyst dosage 3.0 wt%.Under these conditions,the conversion of trioxymethylene and the selectivity of PODE_3⁸ were 94.28%and51.81%,respectively.Compared with ionic liquids,the nanoreactor can not only maintain the high catalytic activity but also reduce the 80%dosage of ionic liquids.The study of the reaction mechanism showed that PODE_n with different degree of polymerization is formed one by one with the increase of degree of polymerization n and the formation of PODE_n follows the Schulz-Flory distribution rule.At the same time,the confinement effect of nanoreactor can inhibit the formation of longer chain product PODE_n>5,thus increasing the selectivity of target product PODE_3⁵.Secondly,in order to study the influence of the chain length and branch chain of the ionic liquids on the catalytic performance of the nanoreactor,two types of imidazolium ionic liquids were selected as active components to construct the nanoreactor.The study that the synthesis of PODE_n was catalyzed by imidazolium ionic liquids nanoreactor showed that the latter catalytic effect is better than the former,indicating that increasing chain length had little influence on the catalytic performance of active component while increasing branched-chain had a great influence on the catalytic effect.The obtained catalyst with N-sulfonic acid butyl imidazole trifluoromethanesulfonate ionic liquid as active component had the best effect,the selectivity of PODE_3⁸ was 51.52%.Finally,a nanoreactor with phosphonium ionic liquids as active component was constructed,and the effect of anion and cation composition on catalytic activity was investigated.The prepared catalyst was analyzed by XRD,FT-IR,and N₂ adsorption-desorption.The results showed that the prepared catalyst when triphenyl sulfonic acid butyl phosphonium trifluoromethanesulfonate ionic liquid was used as active component had the best effect,and the selectivity of PODE_3⁸is 52.96%.In a word,several types of prepared catalysts had good recycling performance.