Fabrication Of Wrinkle Silica Nanospheres And Their Naphthalene Hydrogenation Performances

Aromatics in diesel will not only reduce the quality of oil,but also produce environmental pollution exhaust.Therefore,the United States,Europe,and China have all formulated stricter environmental regulations and fuel specifications to limit the content of aromatics in fuels.The development of the deep aromatic hydrogenation catalyst for diesel fuel has become the focus of recent research.In this manuscript,naphthalene was used as a model compound to evaluate the activity of the supported catalysts for aromatic hydrogenation.Firstly,wrinkle silica nanospheres（WSNs）were synthesized,and the influence of water bath stirring temperature on their morphology and structure was investigated.Then,the morphology and pore structure of the wrinkle silica nanospheres are controlled by the dual template agent synthetic method.Futhermore,the Beta seeds were assembled into the WSNs framework to form a composite material.The prepared support material was loaded with Ni₂P active phase.At the same time,in order to adjust the dispersion performance and particle size of Ni₂P active phase,complexing agents EDTA and NTA were added during the synthesis process.The synthesis mechanism and structure-activity relationship of all synthesized Ni₂P/WSNs catalysts were also studied.The contents are as follows:（1）A one-step hydrothermal synthesis method was used to synthesize silica nanopheres with wrinkle pore structure by changing the stirring temperature.TEM results showed that small particles of silica sphere crystal seeds were formed under stirring conditions at a low temperature of 45 ^oC.When the temperature reached 60 ^oC,wrinkle silica nanospheres materials were successfully synthesized.As the stirring temperature continued to increase,the pore size and particle size of WSNs materials both gradually decreased,which was attributed to the fast evaporation of the solvent and the high hydrolysis rate of the silicon source at higher temperatures.Then,a two-step hydrothermal synthesis method was proposed:the first stage is to form the small particle size of silica crystal seeds under low temperature conditions,and the second stage was to further grow wrinkle fibers under high temperature conditions.By changing the length of the stirring time during the first and second stages,wrinkle silica nanospheres with large pores and small particle diameters were synthesized.The materials synthesized by the two-step method were loaded with Ni₂P,and the activity of naphthalene hydrogenation reaction was evaluated.The results showed that when the stirring time of the first stage and the second stage were 8 h and 16 h respectively,the Ni₂P/WSNs-8+16 catalyst showed the highest activity and stability for naphthalene hydrogenation,almost 100%conversion at 300°C.（2）On the basis of the original cationic template of cetyltrimethylammonium bromide（CTAB）,an anionic template of sodium dodecyl sulfate（SDS）was introduced during synthesis process.A series of wrinkle silica sphere materials with adjustable pore sizes were synthesized by dual template agent method.The results found that as the ratio of SDS/CTAB increased,the distance between the wrinkle fibers on the silica nanopheres gradually increased,thus the pore size becomed enlarge.When the SDS/CTAB ratio was0.13,the WSNs carrier possessed the largest pore volume and the highest surface area.After loaded with Ni₂P,the Ni₂P/WSNs series catalysts still maintained their original morphologies,and the Ni₂P/WSNs-0.13 catalyst possessed the smallest size of Ni₂P particles.The Ni₂P/WSNs-0.13 catalyst exhibited excellent activity in the hydrogenation of naphthalene,with both the conversion rate of naphthalene and the selectivity of decalin as high as 100%.Compared with the traditional silica SBA-15 support material,the Ni₂P/WSNs catalyst exhibited higher naphthalene hydrogenation activity than the Ni₂P/SBA-15 catalyst（only decalin selectivity of 55%at 320 ^oC）,which can be attributed to the shorter diffusion path of the WSNs material.（3）In order to adjust the acidity of WSNs materials,BW composite materials with ultra-small particle size and larger pores were successfully synthesized by introducing Beta seed crystals into the WSNs framework.The influence of Beta on the support structure and properties and the influence of the corresponding catalyst activity were systematically studied.The research results showed that the pore size has expanded from5 nm of WSNs smaples to 30 nm of BW-100 composite materials.At the same time,the particle size of the silica carriers reduced from 200 nm to 70 nm.The high specific surface area of the composite carrier material canfacilitate subsequent Ni₂P active phase loading.And the shorter diffusion path of BW composite material also greatly reduced the diffusion resistance of the large reactants.After adding Beta seed crystals,the total acid content of the corresponding catalyst increased.The Ni₂P/BW-100 catalyst with the largest amount of acid not only showed the high hydrogenation conversion rate,but also produced some isomerization products and ring-opening products.On the contrary,the Ni₂P/WSNs catalyst only produced hydrogenation products.Therefore,acidity plays an important role in promoting the isomerization and ring-opening reaction of naphthalene hydrogenation.（4）By adding complexing agents EDTA and NTA to affect the interaction between Ni₂P and the support,small particle sizes of Ni₂P over supported catalysts were obtained.The series catalysts modified by the complexing agent were compared with the unmodified catalyst.Compared with the unmodified catalyst,the Ni₂P/WSNs catalyst modified by the complexing agent had smaller Ni₂P particles.XPS characterization showed that the addition of complexing agent can contribute to the formation of Ni₂P active phase,which can effectively avoid the generation of other impurity phases.The evaluation of naphthalene hydrogenation reaction showed that the Ni₂P supported catalyst with complexing agents EDTA and NTA showed higher hydrogenation catalytic activity than the unmodified catalyst.Among them,the EDTA-modified Ni₂PE（1.5）/WSNs catalyst achieved 100%conversion of naphthalene and still maintained high activity at high temperatures.