Titanium Silicalite-1 Zeolite Research Articles

Investigation of the Ti active site in TS-1 for phenol hydroxylation via seed and dissolution-recrystallization methods

TS-1 (titanium silicalite-1) zeolite has been one of the most popular catalysts for phenol hydroxylation in the industry to produce hydroquinone and catechol for its high catalytic activity. However, ideal Ti active site in TS-1 for phenol hydroxylation is controversial in academia and still remains urgent to be determined for more convincing guidance to prepare TS-1 with higher catalytic activity. Thus, controlled experiments for detailed research were conducted via seed and dissolution-recrystallization methods to explore the Ti active site for phenol hydroxylation. Through comparison of micron TS-1 and hierarchical TS-1, several factors including crystal size, specific surface area, pore volumes and anatase TiO2 were proved to have an influence on catalytic performance for phenol hydroxylation. Furthermore, hierarchical TS-1 using seeds with different molar ratios of Si/Ti were synthesized to regulate the ratio of framework Ti and hexa-coordinated Ti and exclude above variables. With binding analysis of UV–vis, UV-Raman, and XPS, it was found that TS-1 with the most framework Ti showed the best performance with TON of 1461 and dihydroxybenzenes selectivity of 97.3 %, and TS-1 with less framework Ti and more hexa-coordinated Ti showed worse performance. Hence, it was demonstrated that framework Ti was the active site for phenol hydroxylation rather than hexa-coordinated Ti, while hexa-coordinated Ti would cause excessive oxidation of phenol to benzoquinone.

TS-1 zeolite encapsulated Pt clusters with enhanced electronic metal-support interaction of Pt−O(H)−Ti for nearly-zero-carbon-emitting photocatalytic hydrogen production from methanol

Photocatalytic H2 production from methanol is sustainable, yet challenges remain in avoiding carbon-containing gaseous by-products. Titanium silicalite-1 (TS-1) zeolite possesses attractive photocatalytic performance, but lack of modification strategies due to its microporous framework. Herein, we developed a pre-anchoring strategy to confine ultra-small Pt clusters inside TS-1 with ultra-low loading (0.2 mol%), in which Pt were anchored in Ti−OH nests by electronic metal-support interaction (EMSI). The optimal catalyst achieved a remarkable H2 generation rate of 63.2 mmol g–1 h–1 in CH3OH solutions, along with the production of high-value chemical HCHO as the oxidation product with 96.9% selectivity. Investigations reveal that the EMSI between Pt and Ti facilitated the selective decomposition of CH3OH to H2 and HCHO, leading to a nearly zero-carbon-emission process. Accordingly, we propose a light-driven carbon-negative "CO2→CH3OH→H2" route, coupling CO2 utilization and green H2 production with CH3OH as the intermediate, therefore offering a new perspective for "liquid sunshine".

Preparation of hierarchical TS-1 zeolites from ordered macroporous-mesoporous carbon templates for efficient chloropropene epoxidation

The combination of titanium silicalite-1 (TS-1) zeolite and H2O2 oxidant provides an excellent alternative to many highly toxic and polluting olefin oxidation processes due to the by-product being water-only and without deep oxidation. However, the diffusion of bulky reactants and products suffers from the inherent micropores of conventional TS-1 zeolites. Herein, we designed a reticulated ordered macroporous carbon material with uniform and coherent mesopores, and successfully synthesized ordered hierarchical TS-1 zeolites containing intracrystalline macroporous-mesoporous-microporous by dry gel method. The fully interconnected intracrystalline macroporous-mesoporous-microporous hierarchical TS-1 improves the accessibility of the active site and the diffusion efficiency of bulky compounds. It showed excellent catalytic performance during the epoxidation reaction of chloropropene, and its epichlorohydrin yield and H2O2 effective utilization were 70.11% and 77.05%, which were 2.2 and 1.9 times of the catalytic performance of commercial TS-1 zeolites, respectively, and significantly better than the ultrafine TS-1 synthesized by conventional methods. The high catalytic performance was maintained after three cycles of use. This offers a good opportunity for the design and synthesis of efficient zeolite catalysts in the future.

Investigation on Titanium Silicalite-1 Zeolite Synthesis Employing ATPAOH as an Organic Structure Directing Agent

Investigation on Titanium Silicalite-1 Zeolite Synthesis Employing ATPAOH as an Organic Structure Directing Agent

The electron-rich [ZnH]+ species stabilized by titanium silicalite-1 zeolite for propane dehydrogenation

Propane dehydrogenation to propylene has broad application prospects, so it is of great significance to explore low-cost and environmentally friendly catalysts. Metal hydrides, as active sites, exhibit excellent dehydrogenation property. Herein, we report titanium modified [ZnH]+ ([ZnH]+···TiO6) site with high activity (14.49 mmolC3H6·gcat−1·h−1) and regeneration performance, which is derived from the in-situ conversion of [ZnOH]+ in the dehydrogenation reaction. Combined with in-situ spectroscopic characterizations and ab initio molecular dynamics (AIMD) simulations, the unique [ZnH]+···TiO6 moiety was confirmed, and the evolution process from [ZnOH]+ to [ZnH]+ was clarified. XPS and AIMD calculations show that Zn in the [ZnH]+···TiO6 structure is in an electron-rich state with high H-affinity, which can effectively reduce the propyl C-H cleavage barriers based on slow-growth simulations. In general, titanium modification can effectively stabilize the [ZnH]+ species and enrich the electrons density of zinc, which contribute to the enhanced PDH performance.

Catalytically active species in bifunctional Aluminum-Containing titanium Silicalite-1 zeolite for ethylene epoxidation

Aluminum-containing titanium silicalite-1 zeolite (Al-TS-1) is an important bifunctional catalyst for green production of ethylene glycol (EG) via a hydrogen peroxide/ethylene process (HPEG). In this work, hydrothermal post-treatments are carried out to a parent Al-TS-1 in (NH4)2CO3 aqueous solutions under varied conditions to produce post-treated Al-TS-1 catalysts. In addition to the framework Ti(OSi)4 species found in both the parent and post-treated Al-TS-1 catalysts, the post-treated catalysts also contain Ti(OH)4(OSi)2 species. For HPEG process, the post-treated Al-TS-1 delivers a significantly higher H2O2 conversion than the parent Al-TS-1. This is attributed to the outstanding catalytic activity of Ti(OH)4(OSi)2 which is ∼ 16.8 times the activity of Ti(OSi)4. Furthermore, although the parent and post-treated Al-TS-1 contain both the framework and extra-framework Al species, the post-treatment is also found to alter the detailed location and amount of framework Al species in the catalyst. Such changes, however, have only minor effect on the EG selectivity.

Ultrastable emissive perovskite nanocomposite via dual protection of polymer-grafted TS-1 zeolite for backlight display application

Instability of all-inorganic perovskite quantum dots (PQDs) driven by the inherent ionic nature poses a grand challenge towards their long-term stability in optoelectronic devices. Herein, we report the crafting of a highly stable dual-protected CsPbBr3 perovskite nanocomposite by capitalizing on the encapsulation of a hydrophobic Styrene-Divinylbenzene (St-Dvb) copolymer grafted hierarchical titanium silicalite-1 (TS-1) zeolite shell. Firstly, an optimal hot injection method was used to accommodate CsPbBr3 PQDs into hierarchical TS-1 zeolite. After the modification of terminated Si-OH groups on the surface of TS-1 by phenyltriethoxysilane (PTES), a subsequent polymerization of St and Dvb was implemented using azobisisobutyronitrile (AIBN) as initiator to result in the highly emissive CsPbBr3-TS@St-Dvb nanocomposite, exhibiting outstanding luminescent stability against heat, polar solvents, and UV light irradiation. The as-prepared CsPbBr3-TS@St-Dvb nanocomposite also showed strongly suppressed anion exchange and high compatibility in polymer matrix, enabling improved applicability for versatile lighting and optical display applications. Ultimately, a white light-emitting diode (WLED) composed of green-emitting CsPbBr3-TS@St-Dvb and red-emitting K2SiF6:Mn4+ phosphor onto a commercial blue InGaN LED chip with wide color gamut and excellent stability is demonstrated. This work highlights a judiciously-designed and hard-soft dual-protected strategy to produce ultrastable perovskite nanomaterials with high brightness and durability for backlight display application.

Photoassisted synthesis of high-crystallinity TS-1 with triblock copolymer F127 and its application for photodegradation of erythromycin

As a successful industrial catalyst, titanium silicalite-1 zeolite (TS-1) is limited as a photocatalyst in wastewater treatment due to the complexity and high energy consumption of its synthesis. Here, a novel synthesis hierarchical TS-1 was developed using triblock copolymer F127 and a phototreatment synthesis method. This method resulted in a super high crystallinity of the synthesized TS-1. A synthesis mechanism was proposed, and some reactions between Si and Ti species were studied by density functional theory (DFT) calculations. Compared to TS-1 synthesized using the conventional method, the catalyst in this study exhibited superior performance in the interfacial charge transport efficiency. In the removal of the antibiotic erythromycin from water, the synthesized catalyst demonstrated exceptional performance in terms of adsorption, photoactivity, stability and anti-interference ability against pH, anions, and cations. These findings suggested that the synthesis method used in this work is feasible, and the TS-1 products have great potential in pollutant degradation.

Nano-Cavities within Nano-Zeolites: The Influencing Factors of the Fabricating Process on Their Catalytic Activities.

Titanium silicalite-1 (TS-1) is a milestone heterogeneous catalyst with single-atom tetrahedral titanium incorporated into silica framework for green oxidation reactions. Although TS-1 catalysts have been industrialized, the strategy of direct hydrothermal synthesis usually produces catalysts with low catalytic activities, which has still puzzled academic and industrial scientists. Post-treatment processes were widely chosen and were proven to be an essential process for the stable production of the high-activity zeolites with hollow structures. However, the reasons why post-treatment processes could improve catalytic activity are still not clear enough. Here, high-performance hollow TS-1 zeolites with nano-sized crystals and nano-sized cavities were synthesized via post-treatment of direct-synthesis nano-sized TS-1 zeolites. The influencing factors of the fabricating processes on their catalytic activities were investigated in detail, including the content of alkali metal ions, the state of titanium centers, hydrophilic/hydrophobic properties, and accessibility of micropores. The post-treatment processes could effectively solve these adverse effects to improve catalytic activity and to stabilize production. These findings contribute to the stable preparation of high-performance TS-1 catalysts in both factories and laboratories.

Hierarchical Titanium Silicalite-1 Zeolites Featuring an Ordered Macro–Meso–Microporosity for Efficient Epoxidations

The epoxidation reaction over titanium silicalite-1 (TS-1) zeolites is a green way to produce epoxides that are important intermediates for chemicals. Nevertheless, the conventional microporous TS-1 zeolite shows limited diffusion ability for bulky molecules, leading to poor activity and low selectivity. Constructing hierarchical porosity in microporous materials is an effective strategy to enhance the diffusion properties of catalysts. However, there are few reports on the design and synthesis of TS-1 zeolites with hierarchical structure featuring multilevels, interconnectivity, and regularity for efficient diffusion and epoxidations. Herein, hierarchical TS-1 zeolites with ordered macro–meso–microporosity (OMMM-TS-1) are obtained by a method combining a templated effect of ordered macro–mesoporous matrices and a confined in situ crystallization process. The OMMM-TS-1 possesses ordered macropores with tunable size (∼200–600 nm), ordered mesopores (∼8 nm), and intact micropores (∼0.5 nm). The OMMM-TS-1 achieves a cyclooctene conversion as 3.6 times and 1.8 times and a selectivity to epoxy product as 1.6 times and 1.3 times higher than the conventional TS-1 (C-TS-1) and nanosized TS-1 (Nano-TS-1) zeolites, respectively. The OMMM-TS-1 also outperforms the C-TS-1 and Nano-TS-1 zeolites in epoxidations of a series of alkenes. Such a novel hierarchical structure can be applied in the design and synthesis of many other catalysts.

Tetrapropylammonium Hydroxide Treatment of Aged Dry Gel to Make Hierarchical TS-1 Zeolites for Catalysis.

This work presents the development and systematic study of a method to prepare hierarchical titanium silicalite-1 (TS-1) zeolites with high tetra-coordinated framework Ti species content. The new method involves (i) the synthesis of the aged dry gel by treating the zeolite precursor at 90 °C for 24 h; and (ii) the synthesis of hierarchical TS-1 by treating the aged dry gel using tetrapropylammonium hydroxide (TPAOH) solution under hydrothermal conditions. Systematic studies were conducted to understand the effect of the synthesis conditions (including the TPAOH concentration, liquid-to-solid ratio, and treatment time) on the physiochemical properties of the resulting TS-1 zeolites, and the results showed that the condition of a TPAOH concentration of 0.1 M, liquid-to-solid ratio of 1.0, and treatment time of 9 h was ideal to enable the synthesis of hierarchical TS-1 with a Si/Ti ratio of 44. Importantly, the aged dry gel was beneficial to the quick crystallization of zeolite and assembly of nanosized TS-1 crystals with a hierarchical structure (S ext = 315 m2 g-1 and V meso = 0.70 cm3 g-1, respectively) and high framework Ti Species content, making the accessible active sites ready for promoting oxidation catalysis.

Synthesis of hierarchical titanium silicalite-1 by seed silanization for selective oxidation reactions

The sole microporous structure of titanium silicalite-1 (TS-1) zeolite usually leads to diffusion resistances, which would negatively influence its catalytic performance. Herein, a small-crystal TS-1 zeolite with micropore/mesopore structure has been successfully synthesized by silanization of seed in a tetrapropylammonium bromide (TPABr)-ethanolamine hydrothermal system. Dodecyltrichlorosilane (DTS) was used to modify the surface of nanosized S-1 seed, then the synthetic precursor of TS-1 was crystallized around the silanized seed, and the mesopores were generated in the spaces occupied by the organosilanes as a result of the calcination of solid zeolite material. The pore diameter of mesopore could be adjusted from 2.9 nm to 7.3 nm relative to the changing of the molar ratio of DTS to SiO2 in seed from 0.05 to 0.45. The resultant hierarchical TS-1 with micropore/mesopore structure exhibited excellent catalytic performances in the propylene epoxidation and cyclohexanone ammoximation, which were even superior to that of hollow TS-1 obtained from post-synthesis treatment. This approach for the preparation of TS-1 with micro-mesopores could be easily realized in industrial application to provide a feasible strategy for the exploration of other hierarchical zeolites.

Framework Ti-rich titanium silicalite-1 zeolite nanoplates for enhanced photocatalytic H2 production from CH3OH

As a successful industrial catalyst, titanium silicalite-1 zeolite (TS-1) also possesses unique photocatalytic activity, but still challenging due to the unsatisfactory activity and unclear photocatalytic mechanism. Here, we achieved the optimization of TS-1 in both the morphology and content of framework Ti by a facile dual-structure directing agent method. The Ti-rich TS-1 nanoplates exhibit enhanced photocatalytic H2 production performance from methanol. The structure-activity relationship was revealed that the nanoplate-like morphology is beneficial to the photocatalytic activity, owing to the largely retainment of photogenerated electrons. Combined with in-situ DRIFTS and theoretical calculations, it is proposed that Ti−O group would function as a whole to serve as the active site, rather than separated Ti and O sites for reduction and oxidation half reactions respectively. The formation of hydroxylated Ti can also promote the photocatalytic performance. These insights into the mechanisms suggest the great potential of Ti-rich TS-1 nanoplates as commercial photocatalytic materials.

Plate-like titanium silicalite-1 zeolites for green oxidation of alkenes to epoxides

Plate-like zeolites have attracted extensive attention due to their less molecular diffusion restriction. It is challenging to simultaneously control the high crystal aspect ratio and thin thickness. Here, two kinds of plate-like titanium silicalite-1 (TS-1) zeolites with crystal sizes of about 1.5 × 0.2 × 2.0 μm (a × b × c) and 0.3 × 0.06 × 0.6 μm were synthesized via assisted with crystal growth modifiers of citric acid and nitrilotriacetic acid, respectively. The 60 nm thickness was the thinnest TS-1 crystal among the reported by using crystal growth modifiers. The c/b aspect ratio of 10 was also one of the largest aspect ratios among the reported values for TS-1 crystals. Both of them displayed higher catalytic activities in 1-hexene epoxidation as compared with conventional nano-sized TS-1 catalysts.

Efficient and clean epoxidation of methyl oleate to epoxidized methyl oleate catalyzed by external surface of TS-1 supported molybdenum catalysts

Methyl oleate (MO) macromolecules are unable to access the microporous channel of conventional titanium silicalite-1 (TS-1) zeolite, which leads to the low catalytic activity of epoxidation of MO. Herein, the small-size TS-1 (TS-1-s) zeolite with a large surface area was synthesized successfully through steam-assisted crystallization, and then a novel blocked TS-1-s supported molybdenum (Mo/TS-1-bs) catalysts were prepared for efficient epoxidation of MO with clean H 2 O 2 to produce epoxidized methyl oleate (EMO). Various characterization results show the introduced Mo species are highly dispersed on the outer surface of TS-1-s without destroying the structure of the MFI type zeolite and the tetrahedral framework titanium (Ti 4+ ). Interestingly, a large number of Mo 6+ species exhibit certain epoxidation activity attributing to the generating of electrophilic molybdenum hydroperoxide (Mo–OOH) species via Mo species combined with H 2 O 2 , which could adsorb and react easily with MO molecules. Furthermore, the Mo species are also coordinated with Ti 4+ species through the O atom resulting in the facile attack of the Ti–O bonds by H 2 O 2 and easy to form an active intermediate. Therefore, compared with the conventional TS-1, the Mo/TS-1-bs shows higher catalytic activity in the epoxidation reaction. However, the acidity of Mo/TS-1-bs catalysts increases with increasing the Mo-loading owing to the formation of Lewis acid, which leads to the side reactions and the decrease of EMO selectivity. As a consequence, the efficient, non-corrosive and green catalytic system for epoxidation of MO was developed, and the activity improvement of Mo species and the influence of acid were also demonstrated. • The clean small-size TS-1 and Mo/TS-1-bs catalysts were prepared successfully for epoxidation of methyl oleate (MO). • Mo species can promote the MO epoxidation through generating of molybdenum hydroperoxide and coordination with Ti 4+ . • The acidity of Mo/TS-1-bs catalysts can decrease the selectivity of epoxidized methyl oleate.

Steam-assisted strategy to fabricate Anatase-free hierarchical titanium Silicalite-1 Single-Crystal for oxidative desulfurization

As an important heterogeneous catalyst, Titanium Silicalite-1 (TS-1) zeolite has been widely applied in various catalytic processes. Here, hierarchical TS-1 single-crystals were successfully synthesized by a steam-assisted crystallization strategy using hierarchically porous titanium-containing silica (Ti-NKM-5) as a precursor. Due to the presence of large mesopores (10–40 nm)， the microporous structure-directing agents penetrated the interstitial structure of silica particles, inducing the dissolution and crystallization process. During crystallization, the generated nanocrystals grew together to form a hierarchical structure. Titanium was fully incorporated into the zeolite framework, and no extra-framework anatase was formed. Compared with the traditional microporous TS-1 zeolite, the synthesized hierarchical TS-1 single-crystal exhibited superior catalytic performance in oxidative desulfurization (ODS) of dibenzothiophene (DBT) and 4, 6-dimethyl dibenzothiophene (4, 6-DMDBT) owing to the hierarchically porous and anatase-free structure. The recycling test revealed the durability of the obtained hierarchical TS-1 catalyst under moderate reaction conditions.

Insight into the effect of mass transfer channels and intrinsic reactivity in titanium silicalite catalyst for one-step epoxidation of propylene

One-step epoxidation of propylene to propylene oxide (PO) in TS-1/H2O2 system is a promising green and sustainable formation route. However, the TS-1 catalyst is limited by the small pore channels and inadequate intrinsic reactivity. Herein, tetrapropylammonium hydroxide (TPAOH) is used to modify the TS-1 skeleton structures to improve the mass transfer and intrinsic reactivity. Comparative catalytic performance tests indicate that the optimized temperature for TPAOH treatment to modify TS-1 was 90 °C, and the achieved H2O2 utilization efficiency and the PO selectivity can be over 80%. Different TPAOH treatment temperatures significantly influenced the dissolution and recrystallization behavior of TS-1, which altered the composition and microstructure in terms of the pore volume and size, surface properties, and active sites. Our findings indicated that the enlarged pore channels for favorable mass transfer, improved hydrophobic surfaces, enriched framework titanium, and reduced acid strength of titanium silicalite zeolite should be the primary reasons for the enhanced conversion and yields.

One-step synthesis of anatase-free hollow titanium silicalite-1 by the solid-phase conversion method

Development of economic routes for synthesis of hollow titanium silicalite-1 (TS-1) zeolite is of great importance in industry. However, there are still challenges to synthesize hollow TS-1 zeolite without anatase TiO 2 . In this work, a one-step strategy without mesoporogen by the solid-phase conversion method was successfully developed to synthesize anatase-free hollow TS-1. The appropriate low amount of TPAOH (TPA/SiO 2 = 0.13) and the suitable crystallization temperature (170 °C) can not merely induce the formation of hollow structures, but also enable more titanium to enter the framework. Moreover, the transformation mechanism based on crystallization rate and dissolution rate was proposed according to the investigation of the transformation process. The hollow TS-1 with large cavities exhibits high catalytic activity and recycle stability in 1-hexene epoxidation. The efficient strategy reported in this paper will provide new opportunities for further promoting the industrial application of hollow TS-1. • Hollow TS-1 has been synthesized by the solid-phase conversion method. • No extra anatase TiO 2 exists in Hollow TS-1. • The TPAOH content and temperature are essential for the synthesis of Hollow TS-1. • The transformation mechanism based on crystallization rate and dissolution rate was proposed.

Fully utilizing seeds solution for solvent-free synthesized nanosized TS-1 zeolites with efficient epoxidation of chloropropene

Nanosized titanium silicalite-1 (TS-1) demonstrates excellent catalytic ability in the selective catalytic oxidation reaction. However, their synthesis process is usually complicated with low yield under hydrothermal conditions, which is not in line with the concept of green chemistry. Herein, via fully utilizing untreated seeds solution, we report firstly an entirely green strategy for solvent-free synthesizing anatase-free nanosized TS-1 zeolite. The success lies in the fully utilization of seeds solution which is composed of supersaturated structure directing agent (TPAOH), unreacted silica source, water and formed MFI seeds (silicalite-1) without external purification. In the followed solvent-free synthesis of final nanosized TS-1 product, no additional TPAOH is added, which greatly reduces the synthesis cost and synthetic procedure and maintains a high product yield. The obtained nanosized TS-1 zeolite without anatase phase has high crystallinity, large specific surface area. More importantly, the nanosized TS-1 (Si/Ti ​= ​77) catalysts exhibit excellent catalytic ability for the epoxidation of chloropropene with 40.0% conversion and 97.6% selectivity. This sustainable and green synthesis method opens up a new way to regulate nanosized zeolite.

Effect of ammonium salt on the distribution of titanium species in the synthesis of TS-1 zeolites

Titanium silicalite (TS-1) zeolites with different distribution of Ti species were synthesized through altering different amounts of (NH4)2SO4 as crystallization-mediating agent. The distribution of titanium species of TS-1-X samples (X represents the molar ratio of (NH4)2SO4 to SiO2) was systematically investigated via ICP-AES, FT-IR, UV-vis, UV-Raman and other techniques. The TS-1-X zeolite samples were applied in the epoxidation of 1-hexene reaction. It was found that the appropriate amount of ammonium salt not only facilitated the enrichment of titanium species from liquid phase to solid phase and promoted the incorporation of Ti into the MFI skeleton, but also resulted in the formation of uniformly small TS-1 crystals due to the inhibition of rapid growth of crystals. However, much more amounts of (NH4)2SO4 led to the formation of extra-framework Ti, including anatase TiO2 and amorphous Ti species. Meanwhile it promoted the rapid growth of grain resulting in larger TS-1 crystals. Ti-rich TS-1 sample synthesized with molar ratio of (NH4)2SO4 to SiO2 of 0.2 exhibited higher conversion of 1-hexene (42%) and effective utilization of H2O2 (81.8%). In addition, the effect of ammonium salt's composition on the properties of TS-1 was investigated by experiments and characterizations. It was found that ammonium salt could regulate the process of crystallization through its own anion and changing pH of the synthesis gel.