Catalytic Performance Of TiO₂ For N-butanal Self-condensation

n-Butanal self-condensation to 2-ethyl-2-hexenal is a key reaction for carbon-carbon bond formation in the production of 2-ethylhexanol.Research on the catalytic performance of solid acid TiO₂ in this process has important academic and application value.First of all,influence of crystal type on the catalytic performance of TiO₂ for n-butanal self-condensation was investigated.The result showed that catalytic performance of TiO₂ obtained from calcination of its gel at 400?for 1h in the air was the best.The prepared TiO₂ catalysts were characterized by CO₂-TPD,NH₃-TPD,BET and XRD,and the results indicated that the numbers of acid sites and base sites on the surface of anatase TiO₂ are significantly higher than that of rutile TiO₂,which may account for the catalytic performance differences between the two crystal types.The reaction system catalyzed by TiO₂ with different crystal types was identified by GC-MS and the result showed that rutile TiO₂ facilitated to form products with higher molecular weight.Based on this,a possible reaction network for n-butanal self-condensation catalyzed by anatase TiO₂ was established.Then the influence of reaction conditions on n-butanal self-condensation catalyzed by anatase TiO₂ was investigated,and the suitable reaction conditions were obtained as follows:a catalyst weight percentage of 15.0%,a reaction temperature of 180?,and a reaction time of8h.The conversation of n-butanal,the yield and the selectivity of 2-ethyl-2-hexenal are 91.1%,81.6%and 89.6%,respectively.What is more,TiO₂ could be reused for seven times without a significant loss in its catalytic activity.Secondly,the effect of reaction conditions on n-butanal self-condensation catalyzed by anatase TiO₂ was investigated in a fixed bed reactor separately by single factor experiment and response surface method.The optimal reaction conditions were obtained as follows:a reaction temperature of 180?,a reaction pressure of 2.0MPa,a space-time of 8?g·min?/mL,a flow rate of N₂ of 100mL/min,and a flow rate of n-butanal of 0.25mL/min.The conversation of n-butanal,the yield and the selectivity of 2-ethyl-2-hexenal are 84.0%,81.2%and 96.6%,respectively.TiO₂ could be used continuously for 300h under the optimal condition without a decline in the conversation of n-butanal,the yield and the selectivity of 2-ethyl-2-hexenal.Finally,the reaction mechanism of n-butanal self-condensation catalyzed by anatase TiO2 was investigated according to in-situ infrared analyses and designed experiments.The result showed that there are Lewis acid sites and weak base sites on the TiO₂ surface and Lewis acid site Ti⁴⁺plays a key role in the reaction of n-butanal self-condensation.A n-butanal molecule is adsorbed on Lewis acid site Ti⁴⁺while the neighbored lattice oxygen abstracts a proton from the?-carbon position of the same n-butanal molecule to form a nucleophile.Then the nucleophile attacks eletrophilic carbonyl carbon of another adsorbed n-butanal molecule to produce an enol followed by dehydration to 2-ethyl-2-hexenal.