Jan Přech

Selective oxidation of bulky organic sulphides over layered titanosilicate catalysts

Selective oxidation of sulphides is a straightforward method of preparation of organic sulphoxides and sulphones, which are important chemical intermediates and building blocks of pharmaceuticals and agrochemicals. Oxidation of methylphenyl sulphide (MPS), diphenyl sulphide (Ph2S), and dibenzothiophene (DBTH) over lamellar titanosilicate catalysts with the MFI and UTL-derived topology was investigated with hydrogen peroxide as the oxidant. Lamellar titanosilicates combine the advantages of crystalline zeolites and mesoporous molecular sieves due to accessible active sites located on the external surface of their layers. The selectivity of the MPS oxidation to methylphenyl sulphoxide is driven by the diffusion restrictions in the catalyst. A methylphenyl sulphoxide selectivity of 95% at 40% conversion was achieved using the Ti-IPC-1-PI catalyst together with an outstanding TONtot = 1418 after 30 min. The selectivity can be adjusted also by dosing of the oxidant to keep its concentration low during the reaction. The silica–titania pillared TS-1-PITi catalyst showed the highest potential of the tested catalysts in oxidative desulphuration, easily oxidising the DBTH to dibenzothiothene sulphone.

A New Family of Two-Dimensional Zeolites Prepared from the Intermediate Layered Precursor IPC-3P Obtained during the Synthesis of TUN Zeolite

The crystallization of zeolite TUN with 1,4-bis(N-methylpyrrolidinium)butane as template proceeds through an intermediate, designated IPC-3P, following the Ostwald rule of successive transformations. This apparently layered transient product has been thoroughly investigated and found to consist of MWW monolayers stacked without alignment in register, that is, disordered compared with MCM-22P. The structure was confirmed based on X-ray diffraction and high-resolution (HR)TEM analysis. The layered zeolite precursor IPC-3P can be swollen and pillared affording a combined micro- and mesoporous material with enhanced Brunauer-Emmett-Teller (BET) surface area (685 m(2) g(-1) ) and greater accessibility of Brønsted acid sites for bulky molecules. This mesoporous material was probed with 2,6-di-tert-butylpyridine (DTBP). IPC-3P and its modification create a new layered zeolite sub-family belonging to the MWW family. FTIR data indicate that (Al)MWW materials MCM-22 and IPC-3 with Si/Al ratios greater than 20 exhibit a lower relative ratio of Brønsted to Lewis acid sites than MCM-22 (with Si/Al ratios of around 13), that is, less than 2 versus more than 3, respectively. This is maintained even upon pillaring and warrants further exploration of materials like IPC-3P with a higher Al content. The unique XRD features of IPC-3P indicating misaligned stacking of layers and distinct from MCM-22P, are also seen in other MWW materials such as EMM-10P, hexamethonium-templated (HM)-MCM-22, ITQ-30, and UZM-8 suggesting the need for more detailed study of their identity and properties.

Baeyer–Villiger Oxidation of Cyclic Ketones by Using Tin–Silica Pillared Catalysts

The Baeyer–Villiger oxidation is an important transformation of ketones into esters and particularly for cyclic ketones to lactones. We report here preparation and catalytic activity of layered Sn–silicate catalysts and mesoporous ordered silica catalysts in this reaction. Sn was introduced by using so‐called tin–silica pillaring or by impregnation with a mixture of tetraethylorthosilicate and tin(IV) alkoxide. The prepared catalysts were characterized by XRD, N2 physisorption, SEM, UV/Vis, and inductively coupled plasma optical emission spectroscopy (ICP‐OES) techniques and the catalysts were studied in the Baeyer–Villiger oxidation of cyclopentanone, norcamphor, and 2‐adamantanone with aqueous hydrogen peroxide. Norcamphor and 2‐adamantanone were oxidized easily with selectivity up to 99 %. Sn‐MS and IPC‐1‐SnPI materials exhibited the highest conversions (e.g., norcamphor: Sn‐MS 37 %, IPC‐1‐SnPI 36 % after 8 h vs. Sn‐MCM‐41 22 %). On the other hand, oxidation of cyclopentanone suffered from product hydrolysis to the corresponding 4‐hydroxybutanoic acid.

Accessibility enhancement of TS-1-based catalysts for improving the epoxidation of plant oil-derived substrates

TS-1-based catalysts with different textural features, namely layered TS-1, pillared TS-1, and Ti-pillared TS-1 as well as mesoporous TS-1, were investigated in the liquid-phase epoxidation of methyl oleate as a model compound for plant oil-derived substrates with hydrogen peroxide at 50 °C. While over the TS-1-based catalysts, except Ti-pillared TS-1, an epoxide selectivity of up to 93% is achieved, layered and pillared TS-1 are the most active (the amounts of methyl oleate converted after 5 h per number of Ti-sites are 4.64 mol mol−1 and 4.68 mol mol−1) with an efficiency for H2O2 conversion to the epoxide of 27%. Mesoporous TS-1 and conventional microporous TS-1 exhibit a similar activity (3.64 mol mol−1vs. 3.37 mol mol−1), whereas the mesoporous catalyst most efficiently utilizes H2O2 (39% efficiency). The lowest catalytic activity (0.82 mol mol−1), epoxide selectivity as well as H2O2 efficiency are observed over Ti-MCM-36, possessing mainly octahedrally coordinated Ti-sites. The results demonstrate the importance of accessibility of Ti-sites at external crystal surfaces within layered and pillared TS-1, significantly increasing the epoxidation activity with respect to the number of Ti-atoms present in the catalysts.

Bidimensional ZSM-5 zeolites probed as catalysts for polyethylene cracking

Lamellar and pillared ZSM-5 zeolites (L-ZSM-5 and PI-ZSM-5, respectively) were synthesized and tested in the catalytic cracking of low-density polyethylene (LDPE). The introduction of silica pillars into lamellar ZSM-5 caused a high increase in the Si/Al ratio (from 33 up to 64) and the generation of uniform mesopores with a size of about 3.5 nm. Both samples provided quite similar LDPE conversions at the three reaction temperatures investigated (340, 360 and 380 °C) despite the lower concentration of acid sites in PI-ZSM-5, which is assigned to the improved active centre accessibility due to the pillaring treatment. Significant activity was observed even at the lowest temperature, with LDPE conversions in the range 27–36%, which indicates that 2D ZSM-5 zeolites are convenient catalysts for polyethylene cracking. The main products of LDPE catalytic cracking were C2–C5 olefins with a selectivity of 60–70%, denoting that an end-chain cracking mechanism is predominant. 2D ZSM-5 samples were subsequently compared with nanocrystalline (n-ZSM-5) and hierarchical ZSM-5 (h-ZSM-5) zeolites. Pyridine adsorption followed by FTIR measurements showed significant differences in terms of not only acid site concentration but also the Bronsted/Lewis acid distribution among the samples. When the LDPE cracking conversion was referred to the zeolite mesopore/external surface area, a good correlation was observed with the concentration of Bronsted acid sites but not when considering just the Lewis acid sites. This interesting fact suggests that Bronsted acid sites are mainly the active centers for the cracking of the LDPE chains, concluding that in addition to the accessibility, the nature of the acid sites plays a major role in this type of reaction.

Zeolite framework functionalisation by tuneable incorporation of various metals into the IPC-2 zeolite

The incorporation of various metals into the zeolite framework creates opportunities for novel applications, especially in catalysis. The recently developed assembly-disassembly-organisation-reassembly (ADOR) strategy was used to prepare zeolites with IPC-2 (OKO) topology. The layered zeolite precursor (IPC-1P) was modified by incorporating various metals (Al, Zn, Sn, Zr, V, Fe, Hf, and Ti) using a stabilisation process. The resulting materials were characterised by X-ray powder diffraction (XRD), Ar adsorption, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), and diffuse reflectance UV-Vis spectroscopy (DR-UV-Vis). The acidity of Al-containing IPC-2 materials was assessed by acetonitrile and pyridine sorption followed by FT-IR spectroscopy, showing overall concentrations of acid sites of 0.863 mmol g−1 (acetonitrile) and 0.413 mmol g−1 (pyridine). Titanium containing IPC-2 was examined by selective oxidation of methylphenyl sulfide (MPS) to the corresponding sulfoxide (MPSO). Ti-IPC-2 provided a higher conversion than TS-1 after 60 min (30% and 18% respectively) and showed higher selectivity towards MPSO (77% and 63% respectively). Sn-IPC-2 was tested by Baeyer–Villiger oxidation of norcamphor with aqueous hydrogen peroxide, showing a 3.8% norcamphor conversion and a 1.3% yield of the desired lactone (after 8 h reaction). Therefore, the results reported herein clearly show the successful incorporation of metals into the IPC-2 zeolite framework.

Incorporation of Ti as a Pyramidal Framework Site in the Mono‐Layered MCM‐56 Zeolite and its Oxidation Activity

MWW zeolite MCM‐56 with Al atoms on the surface was functionalized with Ti to produce pyramidal TiOH groups. This was carried out by removal of Al with nitric acid, calcination and treatment with titanium diisopropoxide bis(acetylacetonate) (Ti(acac)2 (i‐propoxide)2, Ti(acac)2OiPr2). Up to 0.7 % Ti was introduced. Using UV‐Vis spectroscopy three types of Ti moieties were identified in the uncalcined materials – tetrahedral in the framework, 5/6‐coordinated Ti on the surface, presumably in the vacated pyramidal sites, and oxide‐like clusters. The presence of TiOH on the surface was indicated by in situ measurement of UV‐Vis spectra during calcination. It showed the band at 290 nm, which disappeared at 500 °C, perhaps due to condensation with silanols. Catalytic oxidation tests were carried out with three samples containing 0.13, 0.36, and 0.70 % Ti and methyl phenyl sulfide and cis‐cyclooctene as model reactants. The study confirmed potential of the TiOH moieties in oxidation catalysis although their final form is not certain. Future studies will include increasing Ti content to enhance catalytic activity and better detection of this type of the Ti site.