Linying Wang

In situ growth and assembly of microporous aluminophosphate nanosheets into ordered architectures at low temperature and their enhanced catalytic performance

A low-temperature strategy to fabricate hierarchical aluminophosphate-based molecular sieves has been developed without the assistance of surfactants. The facile synthesis results in cylinder-like SAPO-5 (AFI) composed of oriented attached nanosheets with a thickness of 40–100 nm. The main exposed surface of the nanosheets is revealed by the SAED pattern to be the {001} plane, implying a short path for molecular diffusion in the one-dimensional 12-membered ring channel. Investigation on the crystallization process demonstrates that low temperature retards the crystal growth along the c-direction in SAPO-5 and the ordered architecture forms via a nanosheet formation and oriented attachment process. The aggregated morphology of SAPO-5 is template-dependent, which may be modified/changed by varying the template in the system. The low-temperature strategy is also successfully extended to the preparation of metal-containing aluminophosphate molecular sieves. The obtained MeAPO-5 (Me = Co, Mn, and Cr) and a novel CHA/AEI intergrowth CoAPO molecular sieve also possess hierarchical structures with nanosheet-assembled morphology. Remarkably, the present self-assembled AFI crystals exhibit an obvious improvement in catalytic reactions. This facile method provides a new way for the long-range organization of nanosized building blocks to fabricate hierarchical nanostructures.

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

Abstract The present study reports a novel strategy to fabricate nanocrystal-assembled hierarchical MOR zeolites. This is the first demonstration of hierarchical MOR without preferential growth along the c -axis, which facilitates mass transfer in the 12-membered ring channels of MOR zeolite for the conversions involving bulky molecules. The facile method involves the combined use of tetraethylammonium hydroxide (TEAOH) and commercial surfactants, in which TEAOH is essential for the construction of nanocrystal assemblies. The surfactant serves as a crystal growth-inhibiting agent to further inhibit nanocrystalline particle growth, resulting in enhanced mesoporosity. The hierarchical MOR assembled particles, constructed of 20–50-nm crystallites, exhibit superior catalytic properties in the alkylation of benzene with benzyl alcohol compared with the control sample, as the hierarchical MOR possesses a larger external surface area and longer c -axis dimension. More importantly, the material shows improved activity and stability in the dimethyl ether carbonylation to methyl acetate reaction, which is a novel route to produce ethanol from syngas.

Aminothermal synthesis of CHA-type SAPO molecular sieves and their catalytic performance in methanol to olefins (MTO) reaction

Aminothermal synthesis of SAPO molecular sieves, in which organic amines are used as both solvent and template, is explored based on a variety of amines. Di-iso-propylamine (DIPA) and N,N,N′,N′-tetramethylethylenediamine (TMEDA) are found to lead to the rapid crystallization of SAPO-34 with high solid yield. A solid yield of 96.2% could be acquired using the TMEDA system (200 °C, 12 h), which is the highest value ever reported for SAPO molecular sieves. SAPO-44 is obtained for the first time using the hexamethyleneimine (HMI) template. Detailed synthetic investigation shows that the silicon content in the initial gel has an important effect on the crystalline nature of the final products, and higher Si concentration favours the synthesis of pure SAPO-34 and SAPO-44. In addition, it is shown that the Si coordination environment in the samples is closely related to the choice of template. Among the three samples investigated, SAPO-34-DIPA has the lowest threshold of Si content for the formation of Si islands in the framework due to the smallest charge compensation centers occluded in its CHA cage. The catalytic performance of the synthesized samples is tested by the MTO reaction and a high olefin selectivity of 85.8% is obtained on SAPO-34 templated by DIPA.

Synthesis of hierarchical beta zeolite by using a bifunctional cationic polymer and the improved catalytic performance

Hierarchical beta zeolites have been hydrothermally synthesized by using commercial cationic polymer PDADMA as both microporogen and mesoporogen. The influence of various synthetic parameters on the products was systematically investigated. Products with narrow SiO2/Al2O3 ratios were obtained under static crystallization conditions. By employing rotational conditions and adding seeds in the initial gel, higher SiO2/Al2O3 ratios over a wider range of 25 to 50 could be successfully achieved, which showed an obvious improvement. The 13C NMR, TG, XRF, N2 sorption as well as molecular mechanics simulation results indicated that PDADMA was incorporated in the final product without decomposition, acting as a SDA for the formation of beta zeolite and mesoporogen simultaneously. Further studies on the crystallization process revealed that hierarchical structures templated by PDADMA had been formed in the early solid. The amorphous Si–Al species around the micropore channels gradually evolved to the beta structure with the assistance of PDADMA, whereas the mesopores formed in the initial period remained less changed. A solid-mediated mechanism is thus proposed for the synthesis. Characterization results showed that the obtained products had sphere-like morphology composed of 10–20 nm crystalline domains, high mesopore volumes, and large external surface areas. More importantly, the hierarchical beta zeolites exhibited greatly enhanced catalytic activity and stability in the cracking reaction of triisopropylbenzene.

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.

Comparative investigation of the deactivation behaviors over HZSM-5 and HSAPO-34 catalysts during low-temperature methanol conversion

The deactivation mechanism for the methanol conversion reaction at low temperature was comparatively investigated over HZSM-5 and HSAPO-34 catalysts. Two obviously different deactivation phenomena were directly observed: two-staged deactivation behavior over the HZSM-5 catalyst and exponential-type deactivation behavior over the HSAPO-34 catalyst. Since the start of the deactivation, the amount of the retained species over the HZSM-5 catalyst kept unchanged while the amount over the HSAPO-34 catalyst obviously increased. Both types of deactivation behavior presented an intimate relationship with the accumulation of retained species and their changing reactivity. After detailed characterization and analysis, it was interestingly found that the deactivation of the HZSM-5 catalyst originated from the “overloading effect” of methylbenzenes (smaller than pentamethylbenzene) which are intrinsically active during the autocatalysis reaction stage, while the deactivation of the HSAPO-34 catalyst was caused by accumulation of inactive methyladamantanes, and it was further deduced that the deactivation proceeded from “external to internal” for the HSAPO-34 catalyst. Enhancement of the catalyst diffusivity could effectively extend the catalyst lifetime for the HZSM-5 catalyst, but seemed less effective for the HSAPO-34 catalyst.

Unusual deactivation of HZSM-5 zeolite in the methanol to hydrocarbon reaction

Temperature-programmed methanol to hydrocarbon (TP-MTH) reactions were performed over HZSM-5 zeolite to monitor the change of reaction performance along with reaction temperature in order to understand the mechanistic reason for the temperature influence on the reaction. With a gradual increase of reaction temperature (0.5 °C min−1), the MTH reaction could evolve from the induction period with low methanol conversion to the state with 100% methanol conversion. Four different reaction stages could be clearly observed: the initial reaction stage, the auto-catalysis reaction stage, the deactivation stage and the activity recovery stage. An unusual deactivation behavior was observed following the auto-catalysis period. Further investigations revealed that 1,2,3,5-tetraMB was the main active species during the initial autocatalytic stage and its “overloading” effect resulted in the unusual deactivation phenomenon, i.e. despite its high intrinsic reactivity, too quick formation of poorly mobile 1,2,3,5-tetraMB and lower methylbenzenes will lead to the occupation of most catalyst channels and channel intersections and cause the deactivation of HZSM-5 at low temperature. Further study demonstrated that the “overloading” effect could be alleviated or eliminated by enhancing the catalyst diffusivity or decreasing the acid site density of the zeolite catalyst.

Synthesis of high-Si hierarchical beta zeolites without mesoporogen and their catalytic application in the methanol to propene reaction

High-Si single-crystalline beta zeolites with intracrystal mesopores were synthesized for the first time and investigated as catalysts for the methanol to propene (MTP) reaction. A fast and mesoporogen-free strategy was developed to fabricate the hierarchical structures by conducting crystallization under low water conditions (H2O/SiO2 = 1). Low water dosage for the synthesis was demonstrated to facilitate the nucleation and crystal growth, but restrained the fusion of individual nanocrystallites inside the particles, which helped the formation of hierarchical structures. The resultant hierarchical beta with Si/Al = 277 exhibited a much longer catalytic lifetime and slower coking rate than conventional zeolite due to the improved utilization of interior acid sites and enhanced molecular diffusion. A higher propene selectivity (50.2–55.5%) and propene/ethene ratio were also found for the hierarchical sample, which can be ascribed to the reduced side reactions of olefin products. The methanol conversion mechanism over the high-Si hierarchical beta was investigated by 12C/13C-methanol isotopic labeling experiments, which revealed the predominant route of the olefin methylation and cracking mechanism for the formation of olefins.