TS-1 Samples Research Articles

A study on the role of Ti/Si atomic ratios in the hydrodenitrogenation activity of NiMo/TiO2-SiO2 catalyst

TiO2-SiO2 composites with different Ti/Si atomic ratios were synthesized by an optimized pre-hydrolysis-co-precipitation method while the corresponding NiMo supported hydrodenitrogenation (HDN) catalysts were prepared by incipient wetness impregnation method. The significant structure of serial TiO2-SiO2 composites was determined by XRD, FT-IR, N2 physical adsorption–desorption and NH3-TPD techniques, and the distributions of TiO2 particles within SiO2 matrix were also illustrated under the help of SEM-EDS. In this work, it can be concluded that the Ti/Si atomic ratio has a large effect on the textural characteristics and the acidity distribution of composites. Among the self-made composites, TS-1 samples (Ti: Si = 1) presented the highest specific surface area (SBET = 230.42 m2/g) and the highest proportion of weak acidity. And then, characterization of the NiMo/TS-n catalysts were performed by H2-TPR, HRTEM and XPS. Among the NiMo/TS-n catalysts, NiMo/TS-1 catalysts presented the particularly favorable dispersion of active metals and the largest proportion of “Ni-Mo-S” active phase. The results of the adsorption tests showed that NiMo/TS-4 catalysts presented the maximum value of adsorption coefficient (ACQ). Compared these ACQ, these adsorption sites of quinoline were attributed to the CUS supported by “Ni-Mo-S” structures as well as the strong acid sites by excessive TiO2 particles. Finally, these catalysts were tested by feeding 1.0 wt% quinoline as the reactant under the constant reaction conditions. However, the results of HDN evaluation showed that NiMo/TS-1 catalyst exhibited the best HDN activity (51.31 %) and the highest value of turnover frequency (TOFQ = 4.77 h−1), which was attributed to the formation of maximum Ti-O-Si bonds providing moderate acidity, appropriate MSI, suitable size of MoS2 slabs and “Ni-Mo-S” active phase having high concentration of CUS and Mo-SH/S2− species.

A novel MOFs-induced strategy for preparing anatase-free hierarchical TS-1 zeolite:synthesis routes, growth mechanisms and enhanced catalytic performance

Titanosilicate-1 zeolites (TS-1) as one of the most commonly used catalysts for alkene epoxidation, construction of hierarchical pores as well as elimination of anatase to promote mass transportation and avoid invalid decomposition of hydrogen peroxide are always desirable yet challenging goals. Here, a novel and unique Ti-based metal organic frameworks (MOFs)-induced synthetic strategy for fabricating anatase-free hierarchical TS-1 was first proposed. All the components of MOFs perform different functions: the uniformly distributed Ti nodes replace conventional tetrabutyl titanate (TBOT) to serve as sole Ti source for constructing zeolite crystal; the separated ligands can be embedded in the zeolite framework and act as template to in situ build hierarchical pore structure; the coordination interaction between Ti nodes and ligands can efficiently avoid the anatase generation by balancing the forming rates of Ti-OH and Si-OH. This synthetic strategy is of general applicability, and two different synthetic routes including traditional hydrothermal process and steam assisted crystallization (SAC) procedure are successfully adopted. The obtained hydrothermal TS-1 and SAC anatase-free samples all possess abundant intercrystalline mesopores of 20–50 nm and even macropores between 50 and 150 nm, improving the conversion over 25 % for 1‑hexene epoxidation than TS-1 sample prepared by conventional route. The influences of the amount of Ti MOFs precursor and the crystallization process are studied in detail, and possible synthesis mechanisms are proposed. This MOFs-induced strategy might open up an avenue for the rational design of ideal and hierarchical zeolite to boost the catalytic efficiency.

Synthesis of hierarchically porous zeolite TS-1 with small crystal size and its performance of 1-hexene epoxidation reaction

TS-1 is a typical microporous zeolite, which is hindering the timely diffusion of reaction products and is likely to cause secondary side reactions in heterogeneous catalytic reaction due to the micropore size. However, zeolite TS-1 with hierarchical pores of micropores and mesoporous/macropores can solve this problem to a great extent. In view of this, hierarchical zeolite TS-1 is synthesized by using tetrapropylammonium hydroxide (TPAOH) as structure directing agent and polyacrylamide (PAM) as a hierarchical pore-directing template. The texture property of hierarchical TS-1 can be optimized by changing the amount of PAM in the synthesis gel. With the increase of PAM content, the pore volume and specific surface area of the hierarchical zeolite TS-1 increases. Compared with the conventional TS-1, the hierarchical TS-1 samples have obvious pore size distribution in the range of 20–80 nm. It is interesting that the particle size of TS-1 decreases after adding PAM. When the hierarchical TS-1 is applied to the epoxidation reaction of 1-hexene with H 2 O 2 , the selectivity of the desired product 1,2-epoxyhexane is significantly increased. • Hierarchical zeolite TS-1 with small crystal size has been synthesized by using PAM as template. • The texture property of hierarchical TS-1 can be controlled by changing the amount of PAM. • The selectivity of 1,2-epoxyhexane was greatly improved in 1-hexene epoxidation with H 2 O 2 catalyzed by hierarchical TS-1.

The Clean Synthesis of Small-Particle TS-1 with High-Content Framework Ti by Using NH4HCO3 and Suspended Seeds as an Assistant.

The synthesis of a TS-1 zeolite with high-content framework Ti and small particles has been developed by adding NH4HCO3 and suspended seeds as an assistant. With the addition of NH4HCO3, the Hofmann decomposition of the tetrapropylammonium cation (TPA+) decreased, and the framework Ti content of the zeolite increased first and then decreased while the particle became larger. With the assistance of suspended seeds, the TS-1 synthesized under a low-alkalinity system possesses small particle size and high-content framework Ti, and it shows the best catalytic activity among the prepared catalysts. Because the decomposition of TPA+ decreased, the mother liquid could be reused in the next run of preparation. Even though the recycled mother liquid was reused five times, all obtained TS-1 samples exhibited similar catalytic performances in propylene epoxidation. This work provides an efficient process for preparing TS-1 with good catalytic performance and reduces the discharge of the waste liquid.

The effect of T-atom ratio and TPAOH concentration on the pore structure and titanium position in MFI-Type titanosilicate during dissolution-recrystallization process

In this work, titanosilicate MFI (TS-1) crystals with different Si/Ti ratio were treated with tetrapropylamonium hydroxide (TPAOH) solution of different concentrations. The results showed that the properties – crystal structure, titanium position, secondary pore structure and volume – of the treated samples strongly depend on the Si/Ti ratio of the parent TS-1 sample and the TPAOH concentration. For instance, high TPAOH concentration is necessary to induce titanium re-insertion into framework position and to create inter-connected open meso/macropore in TS-1 with low Si/Ti ratio. On the contrary, TPAOH treatment even at high concentration was found to be ineffective in the post-synthesis treatment of low titanium containing TS-1 samples and result in a phase change from orthorhombic to monoclinic. The post-treated high titanium containing and inter-connected secondary pore TS-1 samples exhibited higher activity for cyclohexene epoxidation than the parent TS-1 sample with similar Si/Ti. This technique provides a controlled way of mesopore creation in TS-1 materials, which may be extended to other titanium base microporous materials.

Direct ring C[sbnd]H bond activation to produce cresols from toluene and hydrogen peroxide catalyzed by framework titanium in TS-1

A series of TS-1 samples with varying Ti content was prepared and applied in the partial oxidation of toluene with H2O2 as oxidant. The TS-1 samples and relevant systems were characterized by XRD, FT-IR, in situ FT-IR, nitrogen adsorption/desorption, DR UV–vis, XPS, ICP-AES, EPR, SEM, TEM and TG. The results showed that the framework Ti species was responsible for the selective ring CH bond activation, resulting in toluene hydroxylation; while the extra-framework Ti species was advantageous to the formation of side chain oxidation products and to the deep oxidation of both kinds of the products formed. Under the optimized reaction conditions, the toluene conversion reached 38.9% with 84.3% selectivity to cresols (TOF, 2.66 h−1), while, the selectivity of para-cresol was 66.8%, which might be caused by the pore structure and steric hindrance of methyl group. The catalyst regenerated by calcination exhibited stable activity and selectivity after the second run.

Hierarchical TS-1 synthesized via the dissolution-recrystallization process: Influence of ammonium salts

TS-1 was post-modified by ammonium salts or tetrapropylammonium hydroxide (TPAOH). During the ammonium salts modification, the crystals were eroded and extra-framework titanium species were formed. After the combined-modification, hierarchical TS-1 with secondary pore volume in the 0.22–0.28cm3/g range was obtained via the intensified dissolution-recrystallization process, although more extra-framework titanium species were generated as five-/six-coordinated species. In cyclohexene oxidation with tert-butyl hydroperoxide (TBHP) as the oxidant, the activity of modified TS-1 decreased when anatase was formed only. However, it increased in the presence of five-/six-coordinated titanium. The secondary porosity further improved the catalytic performance and the activity of the combined-modified TS-1 samples increased remarkably by >100%.

A green template-assisted synthesis of hierarchical TS-1 with excellent catalytic activity and recyclability for the oxidation of 2,3,6-trimethylphenol

A simple one-step hydrothermal synthetic strategy to nano-sized hierarchical TS-1 zeolite with randomly arranged mesopores of 26 nm in diameter and uniform crystal size of 200–300 nm has been developed by utilizing the green and cheap ammonium-modified chitosan (HTCC) as the mesopore directing agent. The obtained hierarchical TS-1 samples were comprehensively characterized by XRD, SEM, TEM, BET, XPS, and other techniques. It is demonstrated that the physicochemical and texural properties of the resultant hierarchical TS-1 can be tailored by simply adjusting the HTCC amounts. Furthermore, the catalytic activity was measured on the oxidation of large molecules 2,3,6-trimethylphenol to 2,3,5-p-benzoquinone with hydrogen peroxide as oxidant under the mild conditions. Due to the introduction of the mesoporosity into the crystals, the hierarchical TS-1 zeolite exhibits enhanced catalytic capability, 62.22% conversion of 2,3,6-trimethylphenol and 96.97% selectivity to 2,3,5-p-benzoquinone with excellent recyclability. This provides an efficient and green route to construct hierarchical TS-1 zeolites with potential applications on the catalytic oxidation of bulky organic molecules.

Low-cost synthesis of size-controlled TS-1 by using suspended seeds: From screening to scale-up

The synthesis of TS-1 zeolite with controllable crystal sizes has been successfully scaled up by using the suspension of nanosized S-1 as seed in a TPABr-template hydrothermal system. Even under poor stirring condition, the suspended seeds can be easily dispersed in the mixture of zeolite precursor. Therefore the synthesis shows a very good reproducibility when scaled up from 2 L to 500L, and the crystal size of TS-1 zeolite can be exactly controlled and changed in a wide range by only changing seed amount. Using this TS-1 zeolite through large-scale synthesis as material, the catalyst was prepared by modification and used in propylene epoxidation. The yield of propylene oxide based on H2O2 and propylene reaches 96.5% and 64.3%, respectively, which is superior to that of nanosized TS-1. Though the TS-1 samples synthesized with dried nanosized S-1 seed are similar to the samples synthesized with suspended seeds on the lab scale, the synthesis process using dried seeds shows a low reproducibility on the large scale. This research also implies the low cost and high energy efficiency of the production of TS-1 by using suspended seeds in large-scale process.

The roles of different titanium species in TS-1 zeolite in propylene epoxidation studied by in situ UV Raman spectroscopy.

Titanium silicalite (TS-1) zeolites with different titanium species were synthesized and characterized by ultraviolet (UV)-Raman, ultraviolet visible (UV-Vis) diffuse reflectance spectroscopies and by the NH3 temperature programmed desorption (NH3-TPD) method. The roles of different titanium species in TS-1 samples have been investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. For the first time, a positive correlation was found among the concentration of framework Ti species, the amount of active intermediate Ti-OOH (η(2)) and the conversion of propylene by the in situ GC-Raman technique. The results give evidence that the framework titanium species is the active center and Ti-OOH (η(2)) is the active intermediate. The presence of extra-framework Ti species is harmful to propylene epoxidation. Furthermore, the amorphous Ti species has a more negative effect on the yield of propylene oxide (PO) than the anatase TiO2. The NH3-TPD results reveal that the amorphous Ti species are more acidic and thus should be mainly responsible for the further conversion of PO.

Seed-induced synthesis of small-crystal TS-1 using ammonia as alkali source

Small-crystal TS-1 was synthesized via a seed-induced approach using ammonia as the alkali source and tetrapropylammonium bromide as an auxiliary structure-directing agent. The TS-1 samples were characterized using X-ray diffraction, N2 adsorption-desorption, Fourier-transform infrared spectroscopy, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, and ultraviolet-visible spectroscopy. The use of the colloidal seed reduced the crystal size, and an appropriate amount of silicalite-1 seed assisted Ti incorporation into the TS-1 framework. This method reduces the cost of TS-1 synthesis because a significantly smaller amount of tetrapropylammonium hydroxide is used. The catalytic performance of the synthesized small-crystal TS-1 samples in cyclohexanone ammoximation was better than that of bulk TS-1 as a result of improved diffusion and a larger number of active tetrahedral Ti centers.

Desilication of TS-1 zeolite for the oxidation of bulky molecules

A series of modified TS-1 samples have been produced by desilication of the original TS-1 (4wt.% Ti) using a chemical treatment with NaOH. Desilicated TS-1 zeolites exhibit a large BET surface area together with a well-developed mesoporosity. The hierarchical catalysts from desilication of TS-1 zeolite show a good catalytic activity for the oxidation of small molecules and a significantly higher activity for the oxidation of bulky molecules.

Synthesis and catalytic performance of hierarchical TS-1 directly using agricultural products sucrose as meso/macropores template

Hierarchical TS-1 was synthesized by dry gel conversion (DGC) technique with cheap sucrose as the meso/macropores template. The effects of the sucrose/SiO2 molar ratio and the heat-treatment temperature of dry-gel on hierarchical TS-1 were also studied. Except for sucrose, the purely natural sugar juice directly squeezed from the sugarcane was also adopted to synthesize the hierarchical TS-1. The obtained hierarchical TS-1 were characterized by a series of techniques including powder X-ray diffraction, UV–visible spectroscopy, mercury porosimetry, N2 physical adsorption–desorption and scanning electron microscopy. The hierarchical TS-1 possessed well connected network of mesopores and macropores that could be fully accessible from the external surface, which was confirmed by N2 adsorption, mercury porosimetry and scanning electron microscope. The catalytic properties of the samples were investigated by the oxidation of two sulfur compounds including thiophene (Th) and benzothiophene (BT) with H2O2 as oxidant. The hierarchical TS-1 not only exhibited higher catalytic activity than TS-1 in the Th oxidation, but also gave a high activity in the oxidation of bulky molecular sulfur compound BT that could not be catalyzed by TS-1. With the heat-treatment temperature rising, the hydrophilic property of the hierarchical TS-1 samples reduced and the content of framework Ti increased, which could be favorable to improve the catalytic activity of the hierarchical TS-1 in the oxidation of BT.

Synthesis of TS-1 from an Inorganic Reactant System and Its Catalytic Properties for Allyl Chloride Epoxidation

TS-1 has been synthesized from an inorganic reactant system using Ti(SO4)2 as the titanium source, TPABr as the templating agent, and ammonia as the base to provide the alkalinity needed for crystallization. The effects of preparation parameters, such as titanium sources, crystallization temperature and time, SiO2/TiO2, TPABr/SiO2, ammonia/SiO2 and seed crystals on the physicochemical and catalytic properties of TS-1 were investigated in detail. The TS-1 samples were characterized by XRD, FT-IR, UV–vis, SEM, and ICP-AES, and the catalytic performance of TS-1 was evaluated in the epoxidation of allyl chloride. The results show that the catalytic performance of TS-1 synthesized using Ti(SO4)2 was excellent, and the crystal size of the catalyst was small. The synthesis conditions had a great influence on the properties of TS-1. The optimum synthesis conditions for the present system should employ a gel composition of SiO2/TiO2/NH3/TPABr/H2O = 1:0.022:2.5:0.15:35 and carry out the crystallization at 443 K for 3 days; the addition of seed in the synthesis gel played an important role in the crystallization of TS-1. Moreover, the stability and reusability of TS-1 prepared from this system was fine.

Efficient catalytic epoxidation of olefins with hierarchical mesoporous TS-1 using TBHP as an oxidant

A series of hierarchical mesoporous TS-1 zeolites with MFI structure were successfully synthesized via hydrothermal route using amphiphilic organosilanes as pore directing templates. These materials were characterized by nitrogen sorption (for surface area, pore volume and pore size distribution), FTIR, SEM and UV–Visible spectroscopy. These materials are efficient catalysts for liquid phase epoxidation of olefins using TBHP as the oxidant. All these TS-1 samples oxidized cyclohexene selectively into epoxide. For the TS-1 sample with 4.4 % Ti complete conversion of cyclohexene into epoxide was observed.

Hierarchical TS-1 zeolite synthesized from SiO 2 TiO 2 xerogels imprinted with silanized protozeolitic units

A new method is reported for the synthesis of TS-1 zeolite with hierarchical porosity based on the crystallization of SiO 2–TiO 2 xerogels that have been previously imprinted with silanized protozeolitic units. The organic functionalization of the latter disrupts the growth and aggregation of the zeolitic crystals during the hydrothermal crystallization, causing the formation of a hierarchical porosity. The TS-1 materials so obtained exhibit enhanced textural properties, due to the formation of an additional porosity within the supermicro/mesoporous range, and present less diffusion restrictions than TS-1 samples prepared by conventional methods. The development of this additional porosity depends both on the molecular size of the organosilane compound and on the TPAOH/xerogel ratio employed in the synthesis. However, these changes are accompanied by a higher hydrophilic character of TS-1 zeolite, reducing its potential application in oxidation reactions using hydrogen peroxide as oxidant, since water molecules adsorbed on the zeolite surface prevent the access of the substrates to the titanium active sites. In contrast, these hierarchical TS-1 zeolites present remarkable catalytic activity in olefin epoxidation reactions using alkyl hydroperoxides as oxidant. This fact is ascribed to the enhanced accessibility to the Ti active sites originated by the presence of the secondary porosity.

Synthesis of Mesoporous TS-1 for Catalytic Oxidative Desulfurization

Mesoporous TS-1 catalysts were prepared via a nanocasting route using two different carbon template sources of CMK-3 and commercial carbon black. Products were characterized by XRD, UV-Vis spectroscopy, SEM, TEM, and <TEX>$N_2$</TEX> adsorption- desorption measurement. The catalytic performances of the samples for allylchloride epoxidation and oxidative desulfurization of the representative refractory sulfur compounds, dibenzothiophene and 4,6-dimethyldibenzothiophene, were compared against those of conventional TS-1. Whilst the allychloride epoxidation activity for the mesoporus TS-1 samples were similar, mesoporous TS-1 exhibited significantly higher catalytic activities than conventional TS-1 in oxidative desulfurization.

Phenol Hydroxylation over Alkaline Treated TS-1 Catalysts

Alkaline treatment on TS-1 can selective remove silica from the framework while MFI zeolitic framework remains intact, which creates new large micropores or even mesopores and increases Si/Ti ratio in the alkaline treated TS-1. Among various alkaline hydroxide, CsOH treatment leads to partial destruction of MFI structure. All the desilicated TS-1 samples show enhanced phenol conversion and dihydroxylbenzene (DHB) yield in acetone solvent by two folds, which can be attributed to the higher Ti/Si ratio and improved diffusivity. Pore structure modified with alkaline treatment affects significantly reaction pathway and product selectivity. The initial product over the parent TS-1 and NaOH treated TS-1 is mainly 4-hydroxycyclohexa-2,5-dienone and catechol enriched DHB, respectively.

Preparation of extruded catalysts based on TS-1 zeolite for their application in propylene epoxidation

In this work, the extrusion process of titanium silicalite (TS-1) zeolite has been investigated and optimized in order to obtain materials with convenient catalytic and mechanical properties for their application in the propylene epoxidation with hydrogen peroxide using fixed bed reactors. Thereby, a variety of TS-1 samples were prepared having particle sizes comprised between 0.2 and 0.7 μm. It has been observed that the increase of the zeolite particle size affects negatively the mechanical strength of the extrudates. Likewise, a variety of inorganic binders such as clays or inorganic silica have been explored; the best results in terms of minimizing the H 2O 2 decomposition being obtained with sepiolite. Moreover, three extrusion methods and different TS-1/inorganic binder mass ratios were investigated in order to achieve an extruded material with satisfactory mechanical strength. The optimum extruded catalyst was prepared using a TS-1/sepiolite mass ratio of 60%/40% and employing ultrasounds during the paste preparation. Finally, the catalytic activity of the optimum extruded TS-1 zeolite has been evaluated for the propylene epoxidation reaction using a fixed bed reactor, achieving conversion and H 2O 2 selectivity values close to 97% and 80%, respectively.

Incorporation process of Ti species into the framework of MFI type zeolite

The incorporation process of titanium species into the framework of MFI type zeolite is investigated to clarify the reasons for the difficulty in the isomorphous substitution of Ti into the zeolitic lattice. TS-1 samples are synthesized via two synthesis routes, conventional route and peroxo-complex route, in which different sources of Ti are used. Changes in the content and the coordinated environment of Ti in the products are observed along with the time course of crystallization; although the MFI-type framework is already formed at the initial stage of the crystallization, Ti is hardly incorporated into the framework. After prolonging the crystallization, the content of tetrahedrally-coordinated Ti is drastically increased. It is concluded that Ti would be incorporated into the pre-crystallized Si-rich zeolite (almost all-silicalite-1), resulting in the formation of TS-1. Considering the differences between the synthesis routes employed, the reactivity of Ti species relative to Si species could strongly affect the incorporation of Ti into the zeolite framework. Based on these results, a novel post-synthesis method for preparing TS-1 is proposed.