The control of defects in crystalline materials has long been of significance since the defects are correlated with the performances of the materials. Yet such control remains a challenge for metal–organic frameworks (MOFs), which are usually well-crystallized under hydro-/solvothermal conditions. In this contribution, we demonstrate for the first time how to increase the defects of MOF via a facile and green approach as exemplified in the context of solvent-free synthesis of UiO–66(Zr). Such increase of defects leads to drastic enhancement of catalysis performance when compared to UiO–66(Zr) prepared from conventional hydro-/solvothermal synthesis. Our work therefore not only opens a new door for boosting the catalytic activities of MOFs but also contributes a new approach to control the defects in crystalline materials for various applications.Keywords: defect sites; desulfurization; oxidation; solvent-free; UiO−66(Zr);

•Oxidative desulfurization performance of amorphous titania can be enhanced by doping MIL-101(Cr).•The reaction rate constant over the catalyst with 10% doping content reached 340 × 10−3 min−1.•Such catalytic performance could be attributed to the enhanced dispersity of amorphous titania.High performance MIL-101(Cr)-TiO2 composite material was successfully prepared by doping MIL-101(Cr) in amorphous titania and evaluated for oxidative desulfurization (ODS) of dibenzothiophene (DBT) using cumene hydroperoxide (CHP) as oxidant. The experimental results showed that introduction of MIL-101(Cr) favored the catalytic centers titania to get high dispersion. Remarkably, the reaction rate constant over the catalyst with 10% dopant concentration reached 340 × 10−3 min−1, which was seven times as that of amorphous titania and much higher than that of reported titania composites or metal-organic framework-based catalysts.Download high-res image (184KB)Download full-size image

MIL-100(Fe) is one of the important metal–organic frameworks due to its fascinating physicochemical properties. Generally, MIL-100(Fe) was synthesized under the conditions of adding HF and solvent. Here, we report that MIL-100(Fe) can be facilely synthesized without the addition of solvent and HF. More importantly, this approach can produce MIL-100(Fe) with a high yield of 93%. Additionally, the obtained MIL-100(Fe) exhibited good catalytic performance in the acetalization of benzaldehyde with methanol. This catalyst can still maintain the initial catalytic activity after five cycles.

Metal–organic frameworks MIL-101(Cr) have been rapidly synthesized by solid-phase reaction without the addition of solvent and hydrofluoric acid at 220 °C in 4 h. The obtained MIL-101(Cr) material exhibited superior catalytic activity in the oxidation of cyclohexene.

Ti-modified hierarchical mordenites (Ti/MM) were prepared using tetrabutyl orthotitanate (TBOT) as the precursor of Ti species by incipient wetness impregnation technique. The introduced Ti species were located outside the micropores in mordenite due to the large molecular size of TBOT. As a result, the active Ti species over hierarchical mordenites are easily accessible. The catalytic results indicated that Ti/MM were highly active ODS catalysts in the oxidative desulfurization (ODS) of dibenzothiophene due to rich mesoporosity. The sulfur removal content over a Ti-modified hierarchical mordenite (Ti/MM-0.5) with relatively large mesopore volume reached 98.6% after a reaction time of 120 min at 333 K by using H2O2 as oxidant, which is nearly four times as that over Ti-modified conventional mordenite. Accordingly, the sulfur content in model fuel was reduced from 1000 ppmw to 14 ppmw. These results indicated that Ti-modified hierarchical mordenite with rich accessible Ti sites and low cost has potential applications for ODS of transportation fuels.

We report a strategy of combining a Brønsted acid metal–organic framework (MOF) with Lewis acid centers to afford a Lewis acid@Brønsted acid MOF with high catalytic activity, as exemplified in the context of MIL-101-Cr-SO3H·Al(III). Because of the synergy between the Brønsted acid framework and the Al(III) Lewis acid centers, MIL-101-Cr-SO3H·Al(III) demonstrates excellent catalytic performance in a series of fixed-bed reactions, outperforming two benchmark zeolite catalysts (H-Beta and HMOR). Our work therefore not only provides a new approach to achieve high catalytic activity in MOFs but also paves a way to develop MOFs as a new type of highly efficient heterogeneous catalysts for fixed-bed reactions.

•The number of Brönsted acid sites in Beta was decreased after desilication.•Partial Tetra Al was transformed into DTetra and Pent Al after desilication.•Acid treatment may greatly recover the acidity of desilicated hierarchical Beta.•The proportion of Tetra Al was increased after acid treatment.•Hierarchical Beta with acid treatment exhibited enhanced catalytic performance.Desilication method has been developed to prepare hierarchical zeolites. However, the hierarchical Beta prepared by desilication generally exhibited much poorer catalytic activity than parent one in some acid-catalyzed reactions mainly due to the great loss of acidity. Here, we report that subsequent acid treatment after desilication can greatly recover the acidity of zeolite Beta. As a result, the hierarchical Beta with acid treatment exhibited enhanced catalytic performance in the benzylation of benzene or mesitylene with relatively large molecular size. Additionally, the local structural change in zeolite Beta before and after post treatment was deeply studied by the NMR technique. The results indicated that during the desilication partial tetrahedral coordination Al was transformed into the distorted tetrahedral and pentahedral coordination Al and subsequent acid treatment led to the increase of the proportion of tetrahedral coordination Al. Thus, the number of total Brönsted acid sites in zeolite Beta was decreased after the desilication and subsequent acid treatment may increase the number of total and accessible Brönsted acid sites. These results suggested that acid treatment is helpful for improving the catalytic performance of the desilicated Beta in some acid-catalytic reactions, especially toward large molecules.

•The hierarchical mordenite obtained by acid–base–acid treatment exhibited remarkably enhanced catalytic performance in the benzylation of benzene with benzyl alcohol.•The obtained hierarchical mordenite could be reused and maintain the initial catalytic activity after reuse.Hierarchical mordenites were prepared by a sequential post-treatment method based on a commercial mordenite with a Si/Al molar ratio of 15. The mordenite obtained by acid treatment showed much higher catalytic activity than the parent mordenite in the benzylation of benzene with benzyl alcohol, even a little better than that obtained by acid–base treatment. The apparent reaction rate constant for acid-leached mordenite is six times higher than that for HM. Further, the mordenite with acid–base–acid treatment exhibited the highest catalytic activity among these samples. The apparent reaction rate constant for acid–base–acid-leached mordenite is 15 times that for HM and twice that for acid-leached mordenite and acid–base-leached mordenite. This remarkably enhanced catalytic performance should be attributed to more accessible acid sites and much better mass transfer ability from rich mesoporosity in acid–base–acid-leached mordenite.Graphical abstractDownload high-res image (69KB)Download full-size image

The deep desulfurization of fuels has attracted much attention, due to the stringent demands of environmental regulations. In this work, we report for the first time that Ti-containing hierarchical Beta prepared by a post-synthetic method was utilized as a catalyst for the deep oxidative desulfurization (ODS) of fuels. It was found that the dealumination of Beta before the introduction of Ti species may greatly enhance the ODS activity of Ti-containing hierarchical Beta in the ODS reactions of both dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The Ti-containing hierarchical Beta (Ti-B-M-DA) prepared by desilication–dealumination processes exhibited much better ODS activity than that obtained by direct desilication or dealumination. The TOF number over Ti-B-M-DA reached 58.8 h−1 in the ODS of DBT at 333 K and ambient pressure. More importantly, Ti-B-M-DA exhibited highly active catalytic performance in the ODS of 4,6-DMDBT. The TOF number reached 27.1 h−1 under mild conditions, suggesting that it has the potential for application in deep oxidative desulfurization. Such outstanding catalytic performance could be attributed to the presence of highly active sites and rich mesoporosity in Ti-containing hierarchical Beta.