A very wide range of the C3=/C2= ratio from 0.72 to 7.56 with high C2= + C3= selectivity of around 66% in the methanol-to-hydrocarbons process can be realized over ZSM-5 catalyst in a fixed-bed reactor. We firstly conduct a single factor experiment of acidity, demonstrating that the acidity control of MTH catalyst is crucial to adjusting light olefins selectivity. Weak Brønsted acid sites favor to high C3= selectivity (59.0%) due to the suppression of the conversion reactions from the alkene-based to arene-based cycle, while Lewis acid sites conduce to high C2= selectivity (39.6%) due to the promotion of the conversion reactions for the aromatics formation and steric constraints of Lewis acid sites making the aromatics crack more efficiently to C2=.Download high-res image (142KB)Download full-size imageThe acidity control of MTH catalyst is crucial to obtaining high C2= + C3= selectivity and adjustable C3=/C2= ratio.

A facile strategy is proposed to construct a highly active oxidation catalyst with optimally distributed titanium coordination states containing “TiO4” and “TiO6” species. The “TiO6” species in TS-1+ shows a superior catalytic oxidation activity, which is 2–3 times that of “TiO4” species in TS-1.

The influence of acid strength was evaluated toward the selectivity to propene on conversion of 1-pentene. For the catalytic cracking of 1-pentene, the main reaction pathways and the molar ratio of propene to ethene (P/E ratio) were controlled by acid strength with the appropriate amount of total acid sites. The results showed that the P/E ratio increased with decreasing amounts of strong acid sites, since the activation energies of individual reaction pathways were influenced by acid strength to a different extent. Strong acid sites could promote pathway I′ (C52– → C22– + C32–) and pathway II′-1 (C62– → C22– + C42–), while weak acid sites preferred pathway II′ (2C52– → C10+ → C42– + C62–) and pathway II′-2 (C62– → 2C32–), since pathways II′ and II′-2 underwent some energetically favorable routes (tertiary–secondary, secondary–secondary) of carbenium ion intermediates. By manipulation of the acid strength distribution on ZSM-5, the P/E ratio and selectivity of propene could be significantly improved, suggesting that this can provide an important guideline for improving such a process. In addition, we also designed a coupled process combing butene and pentene coconversion, as pentene and butene could be produced during C42– and C52– catalytic cracking. The coupled process could offer a promising solution to gain high selectivity of propene from C4 and C5 olefin cracking.Keywords: 1-pentene catalytic cracking; acid strength; propene; reaction pathways; ZSM-5

The Lewis acidity of titanosilicates determines the selectivity of the oxime in ammoximation. Higher Lewis acidic strength of Ti active sites could promote free H2O2 to participate in the highly efficient formation of NH2OH by lowering the reaction activation energy for the formation of Ti–OOH species, and thus fundamentally suppress the side reactions of deep oxidation.

A typical volcano-shaped curve has been found in heterogeneous catalytic systems containing titanosilicates for the first time. A new reactive intermediate with double H-bonds is proposed. Systematic results clearly evidence another H-bond formed between the high-electronegativity atom of the H-bond acceptor and the Hend atom of Ti–Oα–Oβ–Hend.

F-Ti-MWW was post-synthesized by implanting fluorine species into a Ti-MWW framework through an acid treatment process in the presence of ammonium fluoride. The effects of NH4F addition amount, acid treatment temperature and precursor Ti content were investigated on the incorporation of F species, the zeolite structure and the coordination sites of Ti. Fluorine-implanting improved the surface hydrophobicity of the zeolite and altered the electropositivity nearby the tetrahedral Ti sites through forming the SiO3/2F and SiO4/2F− units. The negative effect of SiO4/2F− units in F-Ti-MWW was eliminated selectively by convenient anion-exchange with various alkali chlorides. F-Ti-MWW containing the SiO3/2F units possessed better catalytic activity and reusability, and a longer catalyst lifetime than conventional Ti-MWW.

Titanium silicalite-1 (TS-1) has been successfully synthesized in ultralow molar ratio of TPAOH/SiO2 by two-step and multistep hydrolysis process, where the molar ratio of tetrapropylammonium hydroxide/silica (TPAOH/SiO2) could be reduced to 0.08 and 0.065, respectively. Combined characterization of X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microcopy (SEM), Fourier transform infrared spectroscopy (FT-IR), UV–vis spectroscopy, and thermal gravimetric (TG) analysis shows that currently synthesized TS-1 samples have the typical “blackberry” morphology, and the particle size is around 300–400 nm, similar to that of the conventional TS-1 synthesized at a high molar ratio of TPAOH/SiO2 (>0.18). Additionally, currently synthesized TS-1 exhibits excellent catalytic oxidation performance in epoxidation of alkene and ammoximation of ketone, even better than that of conventional TS-1. Obviously, fast hydrolysis of a small part of TEOS at a high concentration of TPA+ in the first step induces more nuclei, which in turn accelerates crystallization in the subsequent step. Therefore, effective utilization of TPAOH to build the TS-1 framework is significantly increased. This strategy can greatly reduce the synthesis cost of TS-1, which is suitable for synthesis of TS-1 on an industrial scale.

Fluorine species were successfully implanted into a MWW-type titanosilicate via post-treatment, which generates the SiO3/2F units in the zeolite framework. The incorporated fluorine species significantly improved the catalytic performance in the epoxidation of alkenes as a result of enhancing the Lewis acid strength and the hydrophobicity of the zeolite.

TS-1 particles were loaded on stainless-steel-net (SSN) to form an integrated material (TS-1/SSN) by secondary hydrothermal synthesis. This avoided completely the filtration separation in both catalyst preparation and catalytic applications in liquid-phase reactions. Seeding-assisted secondary crystallization prevented zeolite from detaching from the support during the synthesis process. The TS-1 particles were coated on the SSN support uniformly, and their loading amount was adjustable in the range of 7–25 wt % by repeating the seeding and crystallization processes. UV–visible and IR spectra verified that the titanium species in TS-1/SSN mostly occupied the tetrahedral coordination sites in the zeolite framework. TS-1/SSN was comparably active to conventional TS-1 particles in batchwise epoxidation of 1-hexene with H2O2. The zeolite particles maintained the catalytic activity and did not detach from the SSN after five reuses. Moreover, TS-1/SSN proved to be efficient and reusable in continuous epoxidation of allyl chloride where the separation of products and catalyst occurred easily.

The borosilicates and titanosilicates with a crystalline structure analogous to the MWW topology were hydrothermally synthesized by using linear organic quaternary ammonium hydroxides, CH3(CH2)nN+(CH3)3OH with n = 5, 6 and 7, as structure-directing agents (SDAs) with the co-existence of boric acid and alkali hydroxide mineralizers. The MFI zeolite appeared to be the main competitive phase that affected the purity of the MWW structure. The Ti-MWW lamellar precursors were synthesized successfully at a greatly reduced amount of boric acid in comparison to the conventional system using piperidine. The SDA cations were confirmed to be incorporated into both the interlayer spacings and the intralayer sinusoidal 10-MR channels of the MWW sheets, keeping intact in their molecular structures. The Ti-MWW precursors contained both tetrahedral and octahedral Ti species. The octahedral Ti species were selectively removed by acid treatment, leading to active titanosilicate catalysts for the epoxidation of 1-hexene with H2O2.Graphical abstractResearch highlights► MWW-type analogues were synthesized successfully by using a series of linear-type quaternary alkylammonium hydroxides as SDAs. The quaternary ammonium cations have the structures of CH3(CH2)nN+(CH3)3OH with n = 5, 6 and 7. ► The synthesis conditions for the MWW-type borosilicates were optimized for each linear-type quaternary alkylammonium, which led to the preparation of corresponding titanosilicates at a greatly reduced amount of boron in comparison to conventional piperidine system. ► The titanosilicates with the MWW topology were efficient catalysts for the liquid-phase oxidation of alkenes with H2O2.

Hollow and core/shell ZSM-5 spheres were simply synthesized through in situ transformation of mesoporous silica spheres (MSS) into MFI-type zeolite with the assistant of isopropylamine (IPA) as a structure-directing agent (SDA). IPA, with a mild structure-directing ability for the construction of MFI structure, triggered a stepwise crystallization of MSS after Al addition. First, the sphere surface was crystallized to a shell of ZSM-5 crystals. The silica and alumina species in the core condensed and crystallized to ZSM-5 thereafter, forming zeolite crystal aggregates inside the spheres. The obtained ZSM-5 spheres were highly crystalline materials and maintained the shape of pristine MSS, but exhibited a hollow and core/shell structure. The techniques of XRD, FT-IR, SEM, N2 adsorption-desorption, 29Si and 27Al MAS NMR were employed to trace the crystallization process, which allowed us to propose a “templating and surface to core” crystallization mechanism. This strategy may serve as an alternative way for synthesizing hollow zeolite spheres.

Mesoporous MFI-type titanosilicates (TS-1) were hydrothermally synthesized with the aid of amphiphilic organosilane and the effects of the amphiphilic organosilane on the structural and textural properties of the resultant materials were investigated. The physicochemical properties of the samples were characterized by various techniques and their catalytic performance was investigated by the epoxidation of cyclohexene with hydrogen peroxide. The content of organosilane added was essential for the formation of uniform mesopore in TS-1, and greatly influenced the crystallinity. Highly crystallined TS-1 with uniform mesopores of 3.7 nm diameter was successfully synthesized. The mesoporous TS-1 preserved tetrahedrally coordinated Ti ions in the framework, but it was featured with more hydrophobic surface and less defect sites in comparison to conventional TS-1. Mesoporous TS-1 showed a higher conversion for the epoxidation of cyclohexene owing to an easier access of bulky molecules to the catalytic active sites. Mesoporous TS-1’s higher hydrophobicity made its epoxide selectivity two times as much as that of conventional TS-1.

Silylation of Ti-MWW lamellar precursor and subsequent calcination constructed an interlayer expanded structure, leading to novel titanosilicates with large pores. The silylating agents suitable for pore expansion were diethoxydimethylsilane, trimethylethoxysilane and triethoxymethylsilane containing methyl groups, which inhibited the intermolecular condensation of silanes effectively. In contrast to well-known 3D Ti-MWW with only medium pores of 10-membered rings, the silylation led to new crystalline structures with more open pores by ca. 2.5 Å, as evidenced by the shift of layer-related diffractions to the lower-angle region in XRD patterns and the enlarged interlayer pores in HRTEM images. The interlayer expanded Ti-MWW was prepared readily from the corresponding hydrothermally synthesized precursors with a wide range of Ti contents (Si/Ti = 20–100). In addition, the pore expansion by silylation was realized under mild acid conditions with 0.1 M HNO3. The interlayer expanded Ti-MWW exhibited 3–7 times higher turnover number than 3D Ti-MWW in the oxidation of cyclohexene with H2O2.

Aluminosilicates with different Si/Al ratios and showing a structure similar to MCM-56 were postsynthesized from fully crystallized lamellar MWW-type precursors under controlled acid treatment. The effects of pretreatment conditions on the construction of MCM-56 analogue structure were investigated, and the physicochemical properties of the resulting materials were characterized by means of various techniques. When the precursors were treated in HNO3 solution with a concentration ≤2 M at temperatures lower than 353 K, a structure analogous to MCM-56 was formed favorably even after further calcination, whereas the treatment at higher temperatures or with 5 M HNO3 resulted in conventional three-dimensional MWW topology. The formation of MCM-56-like structure was closely related to the reorientation of the interlayer H-bond moieties as a result of a partial removal of occluded structure directing agent. Compared with MCM-22, the MCM-56 analogues possessed a structural disorder along the layer stacking direction but had a larger external surface, which mitigated effectively the steric restrictions imposed by the intracrystal micropores to bulky molecules. The MCM-56 zeolites, maintaining the basic structure units of the MWW zeolite, turned out to serve as promising solid acid catalysts for processing larger molecules.

•The synergistic effect of anion and cation enhanced the activity of F-Ti-MWW.•The SiO4/2F− units in F-Ti-MWW decreased its epoxidation ability.•Anion could exchange part of the F− in F-Ti-MWW.•The metal cation could neutralize the electronegative of residual F−.The catalytic performance of F-Ti-MWW towards the epoxidation of 1-hexene with H2O2 was enhanced dramatically, which was attributed to the synergistic effects between anion and cation in the inorganic salt. The negative influence of F− in the SiO4/2F− units was eliminated selectively by convenient anion-exchange and cation-neutralize with various alkali chlorides.

•Reaction pathways of catalytic cracking of 1-butene were efficiently controlled by acid strength.•Pathway II underwent through primary cation, while pathway III proceeded through secondary or tertiary carbenium ion.•Weak acid sites preferred the pathway III, while pathway II needed strong acid sites to overcome energy barrier.•Selectivity of propene could be significantly improved through reducing the amount of strong acid sites on HZSM-5.The influence of the acid strength of P-modified and of HNO3-dealuminated HZSM-5 on the selectivity for the formation of propene was examined in the conversion of 1-butene. Under the appropriate amount of total acid sites, the reaction pathways of dimerization cracking of 1-butene, which involve the mole ratio of propene to ethene (P/E ratio) and conversion of butenes, could be controlled by acid strength distribution. The results showed that the P/E ratio increased with the decreased amount of strong acid sites. For the ZSM-5 with relatively large amount of strong acid sites, the butene conversion was improved via increasing reaction temperature. In contrast, the butene conversion decreased with the increased reaction temperature over ZSM-5 bearing small amount of strong acid sites. It was because the activation energies of different reaction pathways were influenced by acid strength to a different extent. Strong acid sites could promote the pathways I (2C4→C8I→2C4) and II (2C4→C8II→C2+C6) favorably, while weak acid sites preferred the pathway III (2C4→C8III→C3+C5) since pathway III underwent some energetically favorable forms (tertiary-secondary, secondary-secondary) of cracking (C8I,C8II,C8III, represent octyl carbenium ions). According to designing acid strength distribution on ZSM-5, P/E ratio and selectivity of propene could be significantly improved.The role of acid strength in the control of reaction pathways was investigated. Strong acid sites were in favor of the pathway II, while weak acid sites preferred the pathway III.Download high-res image (74KB)Download full-size image

The intermolecular condensation of ethylenediamine (EDA) to 1,4-diazabicyclo[2.2.2]octane or triethylenediamine (TEDA) has been carried out over various titanosilicate catalysts. Superior to Ti-MWW, Ti-Beta, Ti-FER, and Ti-MOR, TS-1 showed higher EDA conversion and TEDA selectivity. The effects of reaction parameters, Ti content, and crystal size on the EDA condensation over TS-1 have been investigated. The mechanism for the TS-1-catalyzed condensation of EDA has also been considered. The acid sites, originated from the Si–OH groups adjacent to the “open” Ti sites, were assumed to contribute to the intermolecular condensation of EDA, whereas the Lewis acid sites directly related to Ti(IV) ions were not the true active sites. The primary intermolecular condensation of EDA to 1,4-diazacyclohexane or piperazine (PIP) took place mainly inside the micropores of the MFI structure, while the secondary condensation of PIP with EDA to TEDA was favored by the acid sites located near the pore entrance and on the outer surface of crystals.The internal silanols of TS-1 are active and selective for the intermolecular condensation of ethylenediamine to piperazine and triethylenediamine.Download high-res image (69KB)Download full-size image

Silylation of Ti-MWW lamellar precursor and subsequent calcination constructed an interlayer expanded structure, leading to novel titanosilicates with large pores. The silylating agents suitable for pore expansion were diethoxydimethylsilane, trimethylethoxysilane and triethoxymethylsilane containing methyl groups, which inhibited the intermolecular condensation of silanes effectively. In contrast to well-known 3D Ti-MWW with only medium pores of 10-membered rings, the silylation led to new crystalline structures with more open pores by ca. 2.5 Å, as evidenced by the shift of layer-related diffractions to the lower-angle region in XRD patterns and the enlarged interlayer pores in HRTEM images. The interlayer expanded Ti-MWW was prepared readily from the corresponding hydrothermally synthesized precursors with a wide range of Ti contents (Si/Ti = 20–100). In addition, the pore expansion by silylation was realized under mild acid conditions with 0.1 M HNO3. The interlayer expanded Ti-MWW exhibited 3–7 times higher turnover number than 3D Ti-MWW in the oxidation of cyclohexene with H2O2.

A facile strategy is proposed to construct a highly active oxidation catalyst with optimally distributed titanium coordination states containing “TiO4” and “TiO6” species. The “TiO6” species in TS-1+ shows a superior catalytic oxidation activity, which is 2–3 times that of “TiO4” species in TS-1.

Fluorine species were successfully implanted into a MWW-type titanosilicate via post-treatment, which generates the SiO3/2F units in the zeolite framework. The incorporated fluorine species significantly improved the catalytic performance in the epoxidation of alkenes as a result of enhancing the Lewis acid strength and the hydrophobicity of the zeolite.

A facile strategy was proposed to realize the precise control of zeolitic acidity by selective cracking of a silane with an acid site. Modified ZSM-5 with controllable acidity brought about a highly adjustable propene/ethene (P/E) ratio in the 1-butene cracking.

Hollow and core/shell ZSM-5 spheres were simply synthesized through in situ transformation of mesoporous silica spheres (MSS) into MFI-type zeolite with the assistant of isopropylamine (IPA) as a structure-directing agent (SDA). IPA, with a mild structure-directing ability for the construction of MFI structure, triggered a stepwise crystallization of MSS after Al addition. First, the sphere surface was crystallized to a shell of ZSM-5 crystals. The silica and alumina species in the core condensed and crystallized to ZSM-5 thereafter, forming zeolite crystal aggregates inside the spheres. The obtained ZSM-5 spheres were highly crystalline materials and maintained the shape of pristine MSS, but exhibited a hollow and core/shell structure. The techniques of XRD, FT-IR, SEM, N2 adsorption-desorption, 29Si and 27Al MAS NMR were employed to trace the crystallization process, which allowed us to propose a “templating and surface to core” crystallization mechanism. This strategy may serve as an alternative way for synthesizing hollow zeolite spheres.

A typical volcano-shaped curve has been found in heterogeneous catalytic systems containing titanosilicates for the first time. A new reactive intermediate with double H-bonds is proposed. Systematic results clearly evidence another H-bond formed between the high-electronegativity atom of the H-bond acceptor and the Hend atom of Ti–Oα–Oβ–Hend.

F-Ti-MWW was post-synthesized by implanting fluorine species into a Ti-MWW framework through an acid treatment process in the presence of ammonium fluoride. The effects of NH4F addition amount, acid treatment temperature and precursor Ti content were investigated on the incorporation of F species, the zeolite structure and the coordination sites of Ti. Fluorine-implanting improved the surface hydrophobicity of the zeolite and altered the electropositivity nearby the tetrahedral Ti sites through forming the SiO3/2F and SiO4/2F− units. The negative effect of SiO4/2F− units in F-Ti-MWW was eliminated selectively by convenient anion-exchange with various alkali chlorides. F-Ti-MWW containing the SiO3/2F units possessed better catalytic activity and reusability, and a longer catalyst lifetime than conventional Ti-MWW.