Synthesis Of Hierarchically Porous TS-1 Zeolites And Their Catalytic Properties In Oxidative Desulfurization

Combustion of transportation fuels with high sulfur content produces toxic sulfur dioxides,which is a major source of acid rain and air pollution.In recent years,deep desulfurization has received much attention due to increasing environmental concerns and more stringent environmental regulations.According to the newest environmental regulations proposed by many countries,the sulfur content in gasoline cannot exceed 10ppmw.Oxidative desulfurization?ODS?is considered to be one of the most promising alternative processes,which can be performed under atmosphere pressure,much lower temperature?<373K?,and without requiring expensive H₂.During the ODS process,the catalysts play a key role in decreasing the reaction temperature.Therefore,developing highly efficient catalysts is of great importance for ODS process.Titanium Silicalite-1?TS-1?,a very effective catalyst for the oxidation of many organic compounds?including thiophene?,has been widely investigated for use as ODS catalysts in recent years.Conventional TS-1shows excellent catalytic oxidation ability,selectivity and hydrothermal stability,but imposes serious diffusion limitation towards bulky molecules such as DBT and 4,6-DMDBT.Many works have been focused on preparation of hierarchical TS-1?HTS-1?with two?or more?different pore systems.Achieving hierarchically porous TS-1 contains both mesopores and micropores is a promising way to solve the above problems.In this work,from the perspective of balancing the competition relationship among micropore templates and mesopore templates,and adjusting the crystal growth,we deeply investigated the influence of mesopore templates and crystallization process on the crystal structure,surface area,pore structure and Ti specices.Then,we achieved the balance of competition among mesopore temlates and micropore templates,and the in situ crystallization of amorphous nanoparticles,which contributed to the successful synthesis of HTS-1 zeolites.Meanwhile,the influence of crystal structure and surface properties on the catalytic performance of HTS-1 was carefully studied.Our studies pave the way for the fabrication and mechanism research of hierarchical zeolites.Four major conclusions are summarized as below:?1?Hierarchical TS-1 with highly connective mesoporous channels has been successfully synthesized through a simple one-pot method using polyvinyl alcohol?PVA?modified multi-wall carbon nanotubes as templates.The hierarchical TS-1 contains rich intracrystalline mesopores?10-20 nm?and a large amount of open channels.Using hierarchical TS-1 as catalyst for the oxidation of organic sulfur compounds,over 85%of DBT and 4,6-DMDBT were removed in 1 h.Compared with conventional TS-1,the obtained hierarchical TS-1 exhibited improved catalytic performance in oxidation of bulky sulfur compounds owing to increased external surface area and rich mesopores inside hierarchical TS-1.Besides,the formation of TiO₆ also contributed to the improvement of catalytic activity of hierarchical TS-1.Meanwhile,it has been found that the introduction of PVA triggers the size of hierarchical TS-1 increase from²30 nm to³80 nm,which is also beneficial for the separation of nanosized zeolite crystals from the reaction products.The presence of MWCNTs also contributed to the formation of non-framework Ti species such as TiO₆ and anatase TiO₂.?2?Compared with hard templates,soft templates show better dimensional adjustability.Hierarchical TS-1?HTS-1?has been successfully synthesized through a novel three-step crystallization method by using a cheap and eco-friendly polymer PVA as the mesopore template.The HTS-1 particles possess an interesting stacking structure and are constructed of smaller crystals with an average diameter of 30 nm.The mesopores of HTS-1 are formed by the stacking of the smaller TS-1 nanocrystals.The catalytic results show that the HTS-1exhibits much higher catalytic activity than conventional TS-1 in the oxidative desulfurization of DBT and 4,6-DMDBT.Using HTS-1 as catalyst for the oxidation of organic sulfur compounds,84%of DBT and 71%of 4,6-DMDBT were removed in 1 h.The improved catalytic activity was due to the presence of rich intracrystal mesopores and increased external surface area.In addition,the three-step crystallization method plays a key role in the preparation of HTS-1,which effectively solves the problem that PVA is unstable and unsuitable to be used as mesopore template under high crystallization temperature.It has also been found that the three step crystallization led to the formation of anatase TiO₂.And the presence of PVA can effectively suppress the formation of anatase TiO₂.Besides,for the conventional one-step hydrothermal crystallization,the extension of crystallization time also led to the formation of anatase TiO₂.?3?To better keep the mesoporous aggregate structure,the third step of three step crystallization was replaced by steam-assisted crystallization?SAC?method.It has been found that the formation of mesoporous aggregates was attributed to the construction of PVA three-dimensional network.The mesoporous aggregate structure could be well maintained by SAC method,resulting in HTS-1 with either surface titanium enrichment or highly interconnected mesopores and high external surface.Both could contribute to the excellent catalytic performance of HTS-1 in oxidative desulfurization of DBT and 4,6-DMDBT.Using HTS-1 as catalyst,over 90%of DBT and 4,6-DMDBT were removed in 0.5 h.Moreover,the HTS-1 showed good stability and remained⁹0%of its initial activity even after 3 times cycle.?4?The most effective template synthesis of hierarchical zeolites accompanied by massive usage of mesopore templates is neither cost-efficient nor environmental friendly.In this work,without using any mesopore templates or additives,HTS-1 has been successfully synthesized just by adjusting the dry gel preparation process.Using HTS-1 as catalyst,over90%of DBT and 4,6-DMDBT were removed in 0.25 h.Moreover,the HTS-1 showed good stability and remained⁹0%of its initial activity even after 3 times cycle.The HTS-1exhibits excellent catalytic performance in oxidation of bulky sulfur compounds owing to greatly increased external surface area,higher mesopore volume and surface Ti enrichment.The effects of drying temperature,water content and grinding procedure on the morphology of TS-1 have been systematically studied.It is found for the first time that the water content of dry gel and grinding play a key role in the preparation of loose structure dry gel.This loose structure is helpful for the quick diffusion of steam inside the dry gel,which is important for the in-situ formation of HTS-1.