Preparation Of α-Pinene-Based New High-Density Fuel And Catalytic Alkylation Performance

With the rapid development of modern aerospace industry,the demand for high energy density fuel（HEDF）is increasing.The current commercial HEDFs are derived from fossil raw materials.In order to alleviate the fossil energy crisis and environmental pollution problems,it is urgent to develop new biomass-based HEDFs to replace petroleum-based products.α-Pinene,which is mainly derived from turpentine resources,is not only abundant in natural reserves but also is expected to be produced through a low-cost biosynthesis route.Therefore,α-pinene has been considered as a high-quality raw material for building biomass-based HEDFs in recent years because of its unique natural polycyclic structure.Herein,the feasibility of preparing novel high-density hybrid fuels based on the structure ofα-pinene through alkylation reaction was studied.The cross-alkylation performance of various chain and cyclic olefins with four to eight carbon atoms withα-pinene catalyzed by phosphotungstic acid was investigated,and isobutylene/isobutane mixture（mass ratio 1:5,denoted as Isobut-5）withα-pinene was found the best cross-alkylation performance.The composition and distribution of alkylation products were analyzed by gas chromatography（GC）and gas-mass spectrometry（GC-MS）.It was found that in addition to the C₁₄ product generated by the cross-alkylation ofα-pinene and isobutylene,the isomerization and dimerization ofα-pinene,as well as self-alkylation of isobutylene/isobutane also occurred in the system.Under the optimized reaction conditions of V_α-pinene/V_Isobut-5=0.75,0.0775 g/m L phosphotungstic acid perα-pinene,120℃and 10 h,a mixed product containing 38.0%C₁₄ components and 23.0%C₂₀ components could be obtained.The liquid alkylation products were catalytically hydrogenated and distilled under reduced pressure to obtain fuel products with different component distribution and fuel properties.The fuel product containing 16.5%C₁₀,33.1%C₁₄ and 50.4%C₂₀ with a density of 0.90 g/m L,a volumetric calorific value of 42.6 MJ/L,a freezing point of-56°C,a flash point of60.2°C and a viscosity of 54.9 mm²/s（-10°C）,and the fuel product containing 3.7%C₁₀,78.5%C₁₄ and 17.8%C₂₀ with a density of 0.88 g/m L,a volumetric calorific value of 41.6 MJ/L,a freezing point of-70℃,a flash point of 69.6℃,and a viscosity of 17.0mm²/s（-10℃）both exhibited excellent HEDF performance.By investigating alkylation of Isobut-5 withα-pinene or its various isomers with reaction time,it was found that the C₁₄ target product components could be obtained either by direct alkylation ofα-pinene with isobutene or by the alkylation routes of various isomers with isobutene.The reaction active sites ofα-pinene and the transition state structures and energies of the above cross-alkylation routes were deduced by theoretical calculations,and it was found that the pathway ofα-pinene protonation and alkylation with isobutene to form the C₁₄component had the lowest energy.The kinetic analysis showed that among the various possible reaction types in the system,the activation energy ofα-pinene isomerization was the lowest,the activation energy of dimerization reaction was the highest.The thermodynamic analysis of the possible radical reactions speculated the mechanism of the effect of reaction temperature on the product selectivity.The results inferred from the above reaction mechanism analysis are consistent with the experimental results.Moreover,the calculated density and volume NHOC of the possible C₁₄ cross-alkylation products generated are also close to the measured values.Further,molecular sieves such as MCM-22,Hβ,HZSM-5,HZSM-35,Sapo-11and HY with different pore size structures and acid sites distributions were employed in the alkylation ofα-pinene with Isobut-5 to obtain a more favorable product distribution for fuel performance.The acidic and structural characteristics of the above catalysts were analyzed using Pyridine adsorption（Py-IR）,Temperature Programmed Desorption of Ammonia（NH₃-TPD）,X-ray fluorescence（XRF）,and N₂ adsorption and desorption characterization.It was found that the presence of mesoporous in molecular sieve structure facilitated product diffusion and had a decisive effect on the performance ofα-pinene alkylation with Isobut-5.Hβ-25n with large specific surface area and appropriate microporous mesoporous structure exhibited a better contribution to the generation of cross-alkylated C₁₄ fraction,while MCM-22 facilitated the generation of the C₂₀ fraction.The good catalytic performance of Al-MCM-41 with larger mesopores further verifies that the mesoporous environment provides favorable conditions for the alkylation,but also loses the restricted transition state selective shape for carbon deposition.TPO characterization of used Hβ-25n,MCM-22,and Al-MCM-41 catalysts revealed that the carbon deposition in Hβ-25n and MCM-22 could be removed at600°C,while the carbon deposition of Al-MCM-41 needed to be removed above800°C.