Facile preparation of nanocrystal-assembled hierarchical mordenite zeolites with remarkable catalytic performance

Recent advances in the routes and catalysts for ethanol synthesis from syngas

Ethanol, as one of the important bulk chemicals, is widely used in modern society. It can be produced by fermentation of sugar, petroleum refining, or conversion of syngas (CO/H₂). Among these approaches, conversion of syngas to ethanol (STE) is the most environmentally friendly and economical process. Although considerable progress has been made in STE conversion, control of CO activation and C-C growth remains a great challenge. This review highlights recent advances in the routes and catalysts employed in STE technology. The catalyst designs and pathway designs are summarized and analysed for the direct and indirect STE routes, respectively. In the direct STE routes (i.e., one-step synthesis of ethanol from syngas), modified catalysts of methanol synthesis, modified catalysts of Fischer-Tropsch synthesis, Mo-based catalysts, noble metal catalysts and multifunctional catalysts are systematically reviewed based on their catalyst designs. Further, in the indirect STE routes (i.e., multi-step processes for ethanol synthesis from syngas via methanol/dimethyl ether as intermediates), carbonylation of methanol/dimethyl ether followed by hydrogenation, and coupling of methanol with CO to form dimethyl oxalate followed by hydrogenation, are outlined according to their pathway designs. The goal of this review is to provide a comprehensive perspective on STE technology and inspire the invention of new catalysts and pathway designs in the near future.

Facile synthesis of nanosized mordenite and beta zeolites with improved catalytic performance: non-surfactant diquaternary ammonium compounds as structure-directing agents

Nanosized MOR and BEA zeolites were directly synthesized using simple diquaternary ammonium compounds. The nanosized zeolites show improved catalytic performance in Friedel–Crafts and n-alkane hydroconversion reactions.

A Carbonylation Zeolite with Specific Nanosheet Structure for Efficient Catalysis

Up to now, the member of zeolite family has expanded to more than 230. However, only little part of them have been reported as catalysts used in reactions. Discovering potential zeolites for reactions is significantly important, especially in industrial applications. A carbonylation zeolite catalyst Al-RUB-41 has special morphology and channel orientation. The 8-MR channel of Al-RUB-41 is just perpendicular to its thin sheet, making a very short mass-transfer distance along 8-MR. This specific nature endows Al-RUB-41 with efficient catalytic ability to dimethyl ether carbonylation reaction with beyond 95% methyl acetate selectivity. Compared with the most widely accepted carbonylation zeolite catalysts, Al-RUB-41 behaves a much better catalytic stability than H-MOR and a greatly enhanced catalytic activity than H-ZSM-35. A space-confined deactivation mechanism over Al-RUB-41 is proposed. By erasing the acid sites on outer surface, Al-RUB-41@SiO₂ catalyst achieves a long-time and high-efficiency activity without any deactivation trend.

Synthesis of Hierarchical MOR-Type Zeolites with Improved Catalytic Properties

Hierarchical MOR-type zeolites were synthesized in the presence of hexadecyltrimethylammonium bromide (CTAB) as a porogen agent. XRD proved that the concentration of CTAB in the synthesis medium plays an essential role in forming pure hierarchical MOR-type material. Above a CTAB concentration of 0.04 mol·L-1, amorphous materials are observed. These hierarchical mordenite possess a higher porous volume compared to its counterpart conventional micrometer crystals. Nitrogen sorption showed the presence of mesoporosity for all mordenite samples synthesized in the presence of CTAB. The creation of mesopores due to the presence of CTAB in the synthesis medium does not occur at the expense of zeolite micropores. In addition, mesoporous volume and BET surface seem to increase upon the increase of CTAB concentration in the synthesis medium. The Si/Al ratio of the zeolite framework can be increased from 5.5 to 9.1 by halving the aluminum content present in the synthesis gel. These synthesized hierarchical MOR-type zeolites possess an improved catalytic activity for n-hexane cracking compared to large zeolite crystals obtained in the absence of CTAB.

Novel synthesis and catalytic performance of hierarchical MOR

A novel route was developed to synthesize hierarchical MOR through introduction of BEA/MOR zeolite embryos as structural growth inducer (SGI) in the presence of hexadecyltrimethylammonium (CTA⁺).

DME carbonylation over a HSUZ-4 zeolite

Zeolite-catalyzed carbonylation of dimethyl ether to methyl acetate represents a new route to synthesize acetyls. A rod-shaped HSUZ-4 zeolite, with intersecting 10-member ring and 8-member ring channels, showed exceptionally high activity and durability, as compared to the HZSM-35 sharing a similar pore structure. The abundant openings of 8-member ring pores, as determined by the shape of HSUZ-4, facilitated the diffusion of the reactive molecules.

High Ethylene Selectivity in Methanol-to-Olefin (MTO) Reaction over MOR-Zeolite Nanosheets

Precisely controlled crystal growth endows zeolites with special textural and catalytic properties. A nanosheet mordenite zeolite with a thickness of ca. 11 nm, named as MOR-NS, has been prepared using a well-designed gemini-type amphiphilic surfactant as bifunctional structure-directing agent (SDA). Its benzyl diquarternary ammonium cations structurally directed the formation of MOR topology, whereas the long and hydrophobic hexadecyl tailing group prevented the extensive crystal growth along b axis. This kind of orientated crystallization took place through the inorganic-organic interaction between silica species and SDA molecules present in the whole process. The thin MOR nanosheets, with highly exposed (010) planes and 8-membered ring (MR) windows, exhibited a much improved ethylene selectivity (42.1 %) for methanol-to-olefin (MTO) reactions when compared with conventional bulk MOR crystals (3.3 %).

Organosilane surfactant-directed synthesis of hierarchical mordenite with enhanced catalytic performance in the alkylation of benzene with 1-dodecene

Facile synthesis of mesoporous mordenite with excellent catalytic performance for alkylation of benzene with 1-dodecene was achieved by using an organosilane surfactant as an assistant directing agent.

Preparation of Spherical Mordenite Zeolite Assemblies with Excellent Catalytic Performance for Dimethyl Ether Carbonylation

Millimeter-sized spherical mordenite (MOR) zeolite with good mechanical strength was successfully prepared via vapor-assisted transformation of Na- and Al-doped SiO₂ spheres. It is demonstrated that the precursor prepared by vacuum impregnation is essential for the achievement of well-retained morphology; the presence of organic amine (such as TEAOH, TMAOH, and HMI) in the vapor can help promote the crystallization, avoid the core-shell morphology, and improve the zeolite Si/Al ratio, although the MOR crystal morphology in the spherical products varies with the choice of organic amines. The resultant MOR-TEAOH spheres, which were synthesized under the assistance of TEAOH, possess large amount of Brønsted acid sites in the 8-MR side pockets (0.76 mmol/g), which is about 3 times higher than in the 12-MR main channels. The preferential siting of H⁺ sites in the 8-MR helps MOR-TEAOH show excellent catalytic performance in the DME carbonylation reaction. A 90% of DME steady conversion is observed on MOR-TEAOH, which is obviously higher than that on commercial sample (23%). This work offers millimeter-scale spherical MOR with good mechanical strength and excellent catalytic performance, which is very promising for the industrial applications.

Understanding of the dissolution–crystallization fabrication strategy towards macro/microporous ZSM-5 single crystals

Amounts of NaOH, Si/Al ratios and crystallization temperature together determine the dissolution–crystallization balance and thus the generation of macropores in ZSM-5 crystals.

Methanol conversion on ZSM-22, ZSM-35 and ZSM-5 zeolites: effects of 10-membered ring zeolite structures on methylcyclopentenyl cations and dual cycle mechanism

The different behaviors of retained organic species and their reaction routes in three 10-membered ring zeolites are presented.