Wenna Zhang

Direct Mechanism of the First Carbon–Carbon Bond Formation in the Methanol‐to‐Hydrocarbons Process

In the past two decades, the reaction mechanism of C-C bond formation from either methanol or dimethyl ether (DME) in the methanol-to-hydrocarbons (MTH) process has been a highly controversial issue. Described here is the first observation of a surface methyleneoxy analogue, originating from the surface-activated DME, by in situ solid-state NMR spectroscopy, a species crucial to the first C-C bond formation in the MTH process. New insights into the first C-C bond formation were provided, thus suggesting DME/methanol activation and direct C-C bond formation by an interesting synergetic mechanism, involving C-H bond breakage and C-C bond coupling during the initial methanol reaction within the chemical environment of the zeolite catalyst.

Methanol to hydrocarbons reaction over HZSM-22 and SAPO-11: Effect of catalyst acid strength on reaction and deactivation mechanism

The conversion of methanol to hydrocarbons has been investigated over HZSM-22 and SAPO-11. Both of these catalysts possess one-dimensional 10-ring channels, but have different acidic strengths. Comparison studies and C-12/C-13 isotopic switching experiments were conducted to evaluate the influence of the acidic strength of the catalyst on the conversion of methanol, as well as its deactivation mechanism. Although the conversion of methanol proceeded via an alkene methylation-cracking pathway over both catalysts, the acidity of the catalysts had a significant impact on the conversion and product distribution of these reactions. The stability of the catalysts varied with temperature. The catalysts were deactivated at high temperature by the deposition of graphitic coke on their outer surface. Deactivation also occurred at low temperatures a result that the pores of the catalyst were blocked by polyaromatic compounds. The co-reaction of C-13-methanol and C-12-1-butene confirmed the importance of the acidity of the catalyst on the distribution of the hydrocarbon products

Organophosphorous surfactant-assistant synthesis of SAPO-34 molecular sieve with special morphology and improved MTO performance

With the aid of self-designed organophosphorous surfactant [2-(diethoxylphosphono)propyl]hexadecyldimethylammonium bromide (DPHAB), SAPO-34s with various special morphologies have been hydrothermally synthesized by using tetraethylammonium hydroxide (TEAOH), diethylamine (DEA) and triethylamine (TEA) as templates, respectively. The synthesized SAPO-34s were well characterized by XRD, XRF, SEM, N2 adsorption–desorption, solid state NMR and NH3-TPD measurements. The status of DPHAB was also investigated by using FT-IR, 13C NMR, TG–DTA and 31P NMR measurements as well as density functional theory calculations. It is found that the addition of DPHAB changes the crystal morphology dramatically and the aggregation degree of the crystals increases with the rising DPHAB/H3PO4 ratio. Moreover, microporous templates play an important role in the synthesis of mesoporous SAPO-34. TEAOH, which has the strongest interaction energy with the CHA framework among the investigated templates, shows the best cooperating ability with an organophosphorous surfactant to direct the formation of nanosized and mesoporous SAPO-34. The prepared SAPO-34s showed improved catalytic properties in the methanol-to-olefins (MTO) reaction.

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

ZSM-22, ZSM-35 and ZSM-5, aluminosilicate zeolites possessing 10-membered ring channels, have been used in the present study as the catalysts of the MTO reaction. The diversities in dimensions and connection types of the 10-membered ring channels of the three zeolite catalysts make their performances in the MTO reaction quite different. As the key active species involved in the hydrocarbon-pool mechanism in the MTO reaction, methylcyclopentenyl cations (MCP+) and methylbenzenes have been captured by 13C MAS NMR and GC-MS over the three zeolite catalysts during methanol conversion. The comparative studies of the retained organics generation over the zeolite catalysts indicate that due to the spatial confinement effects of the inorganic frameworks, the retained organic species generated in the catalysts during the MTO reaction are influenced by both their sizes and amounts. A detailed analysis of the confined organic species showed the formation of MCP with varied methyl substitutions over the three zeolites. 12C/13C-methanol switch experiments were employed to investigate the reaction route for product generation. The differences in the participation levels of the methylbenzene and methylcyclopentadiene over the three zeolite catalysts imply that the formation and function of the organic species formed in the 10-membered ring channel were impacted by the chemical environment of the zeolites, and the methanol conversion that occurred in the 10-membered ring channels of the three zeolites also followed different reaction routes.

Direct observation of methylcyclopentenyl cations (MCP+) and olefin generation in methanol conversion over TON zeolite

The mechanism of the methanol to olefin (MTO) reaction over H-ZSM-22, a TON-type zeolite without cavities or channel intersections, has been investigated in the temperature range of 250–350 °C. For the first time, an induction period in low-temperature methanol conversion and the methylcyclopentenyl cation (MCP+) formed during this period have been observed directly and successfully. 13C magic angle spinning (MAS) NMR, 13C-labeling experiments and theoretical calculations have been employed to confirm the important active intermediates during methanol conversion at 300 °C. The reactions performed at different temperatures were comparatively studied and the differences in the reaction route for alkene formation from methanol conversion and the modes of H-ZSM-22 catalyst deactivation were revealed.

Changing the balance of the MTO reaction dual-cycle mechanism: Reactions over ZSM-5 with varying contact times

The methanol to olefins (MTO) reaction was performed over ZSM-5 zeolite at 300 ℃ under various methanol weight hourly space velocity (WHSV) values. During these trials, the catalytic performance was assessed, in addition to the formation and function of organic compounds retained in the zeolite. Analysis of reaction effluents and confined organics demonstrated a dual-cycle reaction mechanism when employing ZSM-5. The extent of the hydrogen transfer reaction, a secondary reaction in the MTO process, varied as the catalyst-methanol contact time was changed. In addition, 12 C / 13 C -methanol switch experiments indicated a relationship between the dual-cycle mechanism and the extent of the hydrogen transfer reaction. Reactions employing a low methanol WHSV in conjunction with a long contact time favored the hydrogen transfer reaction to give alkene products and promoted the generation and accumulation of retained organic species, such as aromatics and methylcyclopentadienes, which enhance the aromatic cycle. When using higher WHSV values, the reduced contact times lessened the extent of the hydrogen transfer reaction and limited the generation of methylcyclopentadienes and aromatic species. This suppressed the aromatic cycle, such that the alkene cycle became the dominant route during the MTO reaction.

Hollow nanocrystals of silicoaluminophosphate molecular sieves synthesized by an aminothermal co-templating strategy

An aminothermal co-templating strategy has been developed to explore the synthesis of SAPO molecular sieves. Nanosized SAPO-56 (AFX) with a hollow morphology is, for the first time, obtained by utilizing triethylamine (TEA) and trimethylamine (TMA) as both the solvent and the template in the presence of HF. The prepared material is well characterized, and the coexistence of TEA and TMA in the crystals is proved by 13C NMR. Molecular modelling indicates that the most stable status for the as-made SAPO-56 is 2TEA per aft cage and 1TMA per gme cage in the AFX structure, which is consistent with the TG results and verifies the co-templating behaviour of the two amines. In addition, the aminothermal crystallization process of SAPO-56 is examined, revealing an interesting in situ phase transformation from SAPO-34 (only containing TEA) to nano SAPO-56 followed by a further in situ post-synthetic leaching to achieve the hollow structure. Both F− anions and organic amines are proposed to be responsible for the dissolution of SAPO-34 and the etching of SAPO-56 nanocrystals. The Si-rich outer layer with abundant Si coordination environments (more Si–O–Al bonds and less Al–O–P bonds) helps the shell of SAPO-56 survive the leaching. This work offers an effective way to prepare SAPO-56 nanocrystals and demonstrates the huge potential of the aminothermal co-templating route for new material synthesis.

Investigation of methanol conversion over high-Si beta zeolites and the reaction mechanism of their high propene selectivity

Large pore high-Si beta zeolites (Si/Al = 136 to 340) were synthesized by a HF-assisted method, and their catalytic performance for the conversion of methanol to propene was explored. It is demonstrated that beta zeolites with low acid density facilitate the achievement of high propene selectivity and a high propene/ethene ratio. The HF dosage in the synthesis has great influence on the Al distribution in the framework, as evidenced by 27Al MAS NMR and 27Al MQ MAS NMR spectroscopy, which may influence the acidity and microstructure of acid sites and lead to a remarkable catalytic lifespan. A HF/SiO2 ratio of 0.45 is found to facilitate the synthesis of high-Si beta enriched with Al atoms located at the T9 sites; this helps the catalyst show the longest lifetime, with a propene selectivity of 49.7–58.3% at 550 °C and WHSV = 2 h−1. With the aid of 12C/13C-methanol switch experiments, we elucidated that the olefin-based mechanism dominates the reaction and contributes to the formation of ethene, propene, and higher olefins. Moreover, two phenol compounds are identified in the coke species, which have not been observed previously and have been found to be detrimental to the reaction.

A green route for methanol carbonylation

Acetic acid is one of the most important bulk commodity chemicals and is currently manufactured by methanol carbonylation reactions with rhodium or iridium organometallic complexes and halide-containing promoters named Monsanto or BP Cativa™ homogeneous processes, respectively. Developing a halide-free catalyst and a heterogeneous process for methanol carbonylation is of great importance and has recently attracted extensive research attention. Here, we report a green route for direct synthesis of acetic acid via vapor-phase carbonylation of methanol with a stable, selective, halide-free, and noble metal-free catalyst based on pyridine-modified H-mordenite zeolite. Methanol conversion and acetic acid selectivity can reach up to 100% and 95%, respectively. Only little deactivation is observed during the 145 hour reaction.

Silicoaluminophosphate molecular sieve DNL-6: Synthesis with a novel template, N,N′-dimethylethylenediamine, and its catalytic application

Abstract DNL-6, a silicoaluminophosphate (SAPO) molecular sieve with RHO topology, was hydrothermally synthesized using a new structure-directing agent (SDA), N,N′-dimethylethylenediamine. The obtained samples were characterized by X-ray diffraction, X-ray fluorescence, X-ray photoelectron spectroscopy, scanning electron microscopy, and N2 adsorption, which indicated that the synthesized DNL-6s have high crystallinity and relatively high Si content ranging from 20% to 35%. Solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (13C, 29Si, 27Al, 31P, and 27Al multiple-quantum (MQ)) was conducted to investigate the status of the SDA and local atomic environment in the as-synthesized DNL-6. Thermal analysis revealed the presence of a large amount of amines in the DNL-6 crystals (about 4.4 SDAs per α-cage), which was the reason for the formation of DNL-6 with an ultrahigh Si content (36.4% Si per mole). Interestingly, DNL-6 exhibited excellent catalytic performance for methanol amination. More than 88% methanol conversion and 85% methylamine plus dimethylamine selectivity could be achieved due to the combined contribution of strong acid sites, suitable acid distribution, and narrow pore dimensions of DNL-6.