Understanding Zeolites Catalyzed Methanol‐to‐Olefins Conversion from Theoretical Calculations

Zeolites catalyzed methanol‐to‐olefins (MTO) conversion provides an alternative process to produce light olefins such as ethene and propene from nonpetroleum resources. Despite of successful industrialization of the MTO process, its detailed reaction mechanism is not yet well understood. Here we summarize our work on the hydrocarbon pool reaction mechanism based on theoretical calculations. We proposed that the olefins themselves are likely to be the dominating hydrocarbon pool species, and the distribution of cracking precursors and diffusion constraints affect the selectivity. The similarities between aromatic‐based and olefin‐based cycles are highlighted.     

Cover Picture: Recent Advances in Transition Metal‐Catalyzed Asymmetric Radical Reactions (Chin. J. Chem. 3/2018)

The control of stereoselectivity in radical reactions is of great importance, but remains a formidable challenge. The cover picture shows that the enantiomerically pure compounds can be prepared in radical transformations using chiral transition metal complexes as catalysts. Recent advances are summarized in the review by Kong et al. on page 247–256.

     

Cover Picture: Highly Stable Molecular Borromean Rings (Chin. J. Chem. 2/2018)

The cover picture shows a series of Cp*Rh‐based molecular Borromean rings (BRs), which consist of three chemically independent rings that are locked in such a way that no two of the three rings are linked with each other. Interestingly, the structure of BRs is very similar to Kong Ming Lock (孔明锁), a kind of Chinese traditional six‐piece burr puzzle. In this work, some of as‐synthesized BRs display high stability in solution. The reason is related to the length ratio of the long‐arm linker and short‐arm linker, where smaller aspect ratios of the metallarectangles promote improved stability of the BRs in solution. Increasing the width of the metallaligand or pyridyl ligand hinders the formation of BRs and leads to unoccupied monomeric rectangles, which were further used as catalysts. More details are discussed in the article by Jin et al. on page 106–111.

     

Theoretical Insights on Methylbenzene Side‐Chain Growth in ZSM‐5 Zeolites for Methanol‐to‐Olefin Conversion

The key step in the conversion of methane to polyolefins is the catalytic conversion of methanol to light olefins. The most recent formulations of a reaction mechanism for this process are based on the idea of a complex hydrocarbon‐pool network, in which certain organic species in the zeolite pores are methylated and from which light olefins are eliminated. Two major mechanisms have been proposed to date—the paring mechanism and the side‐chain mechanism—recently joined by a third, the alkene mechanism. Recently we succeeded in simulating a full catalytic cycle for the first of these in ZSM‐5, with inclusion of the zeolite framework and contents. In this paper, we will investigate crucial reaction steps of the second proposal (the side‐chain route) using both small and large zeolite cluster models of ZSM‐5. The deprotonation step, which forms an exocyclic double bond, depends crucially on the number and positioning of the other methyl groups but also on steric effects that are typical for the zeolite lattice. Because of steric considerations, we find exocyclic bond formation in the ortho position to the geminal methyl group to be more favourable than exocyclic bond formation in the para position. The side‐chain growth proceeds relatively easily but the major bottleneck is identified as subsequent de‐alkylation to produce ethene. These results suggest that the current formulation of the side‐chain route in ZSM‐5 may actually be a deactivating route to coke precursors rather than an active ethene‐producing hydrocarbon‐pool route. Other routes may be operating in alternative zeotype materials like the silico‐aluminophosphate SAPO‐34.     

Cover Picture: Nitrogen Analogues of o‐Quinodimethane with Unexpected non‐Kekulé Diradical Character (Chin. J. Chem. 6/2018)

The cover picture shows para‐Quinodimethane (p‐QDM) and ortho‐quinodimethane (o‐QDM) are two typical examples of Kekulé‐type delocalized diradicals. The cover picture shows that the replacement of the carbinyl centers with isoelectronic aminium centers leads to the formation of isolable nitrogen analogues of o‐QDM. In contrast to o‐QDM, the nitrogen analogues exhibit unexpected non‐Kekulé diradical characters and features open‐shell singlet ground states with thermally accessible triplet states. More details are discussed in the article by wang et al. on page 487–490.

     

Cover Picture: Transition‐Metal‐Free Defluorosilylation of Fluoroalkenes with Silylboronates* (Chin. J. Chem. 10/2019)

The cover picture shows a simple, inexpensive and robust method on defluorosilylation of diverse fluoroalkenes with silylboronates in the presence of alkoxy base to directly synthesize various silylated fluoroalkenes. Density functional theory calculations revealed that transient silyl anion complex undergoes S_N2’ or S_N^V substitution, which is responsible for this base‐mediated defluorosilylation reaction, thus obviating the need for copper salts. More details are discussed in the article by Shi et al. on page 1009–1014.

     

Cover Picture: Nickel‐Catalyzed Cross‐Electrophile Coupling of Aryl Bromides with Pyrimidin‐2‐yl Tosylates (Chin. J. Chem. 9/2017)

The cover picture shows a new protocol for the NiCl₂ ‐catalyzed cross‐electrophile coupling of aryl bromides with pyrimidin‐2‐yl tosylates to give the corresponding C2 ‐arylation pyrimidine derivatives. This study provides an improvement over previous methods by using pyrimidin‐2‐yl tosylates instead of halides as coupling partners that are stable and easily available. More details are discussed in the article by Wang et al. on page 1366–1370.

     

Cover Picture: Synthesis of Subnanometer‐Sized Gold Clusters by a Simple Milling‐Mediated Solid Reduction Method (Chin. J. Chem. 4/2018)

The cover picture shows a simple milling‐mediated solid reduction method for fabricating of ultrafine gold catalysts. By solid grinding of the N‐modified SiO₂ supported Au precursors with NaBH₄, subnanometer‐sized clusters and isolated Au atoms can be facilely obtained by tuning the metal loading. This method establishes a good basis for fundamental understanding the size effect of Au in the hydrogenation of CO₂ to formate, and provides a general methodology for supported nano‐catalyst preparation. More details are discussed in the article by Huang et al. on page 329–332.

     

Cover Picture: Benzene Triimides: Facile Synthesis and Self‐Assembly Study (Chin. J. Chem. 7/2019)

The cover picture shows an efficient one‐pot condensation of maleimide derivatives in the presence of acetic acid and water to afford a series of benzene triimides (BTIs). The structure, physicochemical properties and electrochemistry behavior of BTIs were systematically investigated. Owing to the planar structure and unique electron‐deficient nature, BTIs can self‐assemble into different motifs. More details are discussed by Wang et al. on page 684–688.

     

Inside Cover: Upgrading CO2 by Incorporation into Urethanes through Silver‐Catalyzed One‐Pot Stepwise Amidation Reaction (Chin. J. Chem. 2/2018)

The inside cover picture shows an outline of significant advances on exhaust gas e.g. sustainable CO₂ recycling into urethanes via effective and renewable silver catalysis. Through one‐pot two‐step stepwise reaction of propargylic alcohols, CO₂, and various amines, a wide range of urethanes are obtained in excellent yields and selectivity together with unprecedented high TON and TOF value under mild conditions. Here, robust catalyst is compared to Magician's hand for its versatility in organic synthesis, and the picture indicates the meanings of greenness and transformation. More details are discussed in the article by Zhang et al. on page 147–152.

     

Cover Picture: Anionic Copolymerization of Carbonyl Sulfide with Epoxides via Alkali Metal Alkoxides (Chin. J. Chem. 7/2018)

The cover picture shows a protocol of an anionic copolymerization of carbonyl sulfide (COS) with epoxides via alkali metal alkoxides. COS is an analogue of carbon dioxide (CO₂), and can be converted to CO₂ via the carbonic anhydride enzymes widely existing in nature. It causes acid rain, ozonosphere damage and haze by a series of photochemical reactions. Very simple and low‐cost alkali metal alkoxides are shown as effective catalysts, affording highly transparent poly(monothiocarbonate)s with 100% alternating degree, >99% tail‐to‐head (T‐H) content, high number‐average molecular weights with narrow molecular weight distributions. This is an environment‐friendly and atom‐economic process for making sulfur‐containing polymers. More details are discussed in the article by Zhang et al. on page 625–629.

     

Inside Cover: Palladium‐Catalyzed Asymmetric Dihydroxylation of 1,3‐Dienes with Catechols (Chin. J. Chem. 3/2019)

The inside cover picture shows a palladium‐catalyzed asymmetric dihydroxylation of 1,3‐dienes with catechols utilizing chiral pyridinebis(oxazoline) ligand. The reaction is proposed to proceed via a cascade Wacker‐type hydroxypalladation/asymmetric allylation process. This methodology provides a direct and straightforward synthesis to prepare chiral 2‐substituted 1,4‐benzodioxane motifs in moderate to good yield and enantioselectivity from readily available starting materials. More details are discussed in the article by Gong et al. on page 226–232.