Guiquan Zhang,Xin Zhang,Ting Bai,Tengfei Chen,Wentao Fan
The coking kinetics and reaction-regeneration on Zn/HZSM-5 (Zn/HZ) catalyst in the conversion of methanol to aromatics were investigated. The highest initial benzene, toluene and xylene (BTX) yield of ca. 67.7% was obtained on fresh Zn/HZ catalyst, which showed the worst catalytic stability. The cycle of reaction-regeneration significantly modified the texture and acidity of Zn/HZ catalyst, which in turn affected its catalytic performance and coking behavior in methanol conversion to BTX. The residual carbon located on the surface of Zn/HZ catalyst led to the decrease of acid sites and the change on the acid sites distribution, which played an important roles on its activity and deactivation. It was found that the high B/L ratio and the low total acid sites concentration of the Zn/HZ catalyst favored to the high BTX yield and good catalytic stability in methanol conversion.
The rapid deactivation caused by coke deposits is the major obstacle to the ethane aromatization of Zn-based catalysts. We have successfully synthesized a Zn-embedded inside HZSM-5 crystal catalyst (Zn@HZSM-5) by hydrothermal synthesis using preformed Zn/SiO 2 as the only silicone source. Based on various characterization results, including in situ etching XPS, operando DRIFTS, as well as NMR, it is found that Zn(OH) + and [Zn–O–Zn] 2+ species are active sites for ethane aromatization for zinc-based catalysts, and their dynamic transformation significantly changes reaction performance of ethane dehydrogenation. For the Zn/HZSM-5 catalyst, the existence of a large number of [Zn–O–Zn] 2+ species causes higher initial ethane conversion and higher selectivity of aromatics, resulting in a large amount of coke deposits. Moreover, [Zn–O–Zn] 2+ species on the Zn/HZSM-5 catalyst fast transit to Zn(OH) + species and subsequently form the sintered ZnO species, leading to rapid deactivation. For the Zn@HZSM-5 catalyst, the embedded structure contributes to forming more Zn(OH) + species and delays the transformation of the Zn(OH) + species to [Zn–O–Zn] 2+ species, resulting in slowly increased selectivity of aromatics during the reaction. Therefore, the Zn@HZSM-5 catalyst realizes stable ethane conversion with dynamic selectivity. Graphic Abstract
La and Zn species were used to modify HZSM-5 zeolite, which was used to treat phenol in wastewater. Through the combination of multiple direct tools, La and Zn species were successfully loaded into the HZSM-5 zeolite. La-Zn/HZSM-5 zeolite has a small band gap and the catalyst can produce highly active electrons and electron holes when energy enters. The HZSM-5 zeolite after modification improves activity and stability of the catalyst. Therefore, it has a significant effect in the phenol removal experiment. Adding 1.2 mL of hydrogen peroxide and 0.11 g of catalyst, the removal rate can reach 99.38 % after being irradiation with a mercury lamp at 60 °C for 180 min. La-Zn/HZSM-5 zeolite has great potential in the application of photocatalytic degradation of phenol.
By introducing exogenous nitrogen during biomass pyrolysis under nitrogen-rich conditions, high-value nitrogen-containing products, i.e., nitrogen-rich char and oil may be produced. Based on the cogeneration of high-value nitrogen products from biomass, biomass nitrogen-enriched pyrolysis was performed in a fixed bed with different sources and contents of ammonia. The yields, composition and characteristics of the products were investigated. Moreover, the formation mechanism of N-containing species was explored in depth for the pyrolysis and catalytic pyrolysis with HZSM-5 and Zn/HZSM-5 catalysts via elemental analysis, XPS, FTIR and BET. The results showed that ammonia impregnation could promote a Maillard reaction, reduce the content of small aldehydes and ketones, and produce a nitrogen-enriched bio-oil. The contents of N-containing species and phenolic substances in the pyrolysis oil of biomass impregnated with 10% urea reached 15.66% and 56.69%, respectively. Moreover, the nitrogen on the coke surface after pretreatment was mainly composed of C N, C N and N COO functional groups. The bio-char generated abundant pyridinic-N, pyrrolic-N, quaternary-N, and pyridone-N oxides. The presence of urea introduced many alkaline N-containing functional groups on the surface of the bio-char and promoted the transformation of nitrogen from amides and imides to heterocyclic nitrogen with higher thermal stability. Furthermore, Zn was an excellent catalyst for the Maillard reaction, and the Zn/HZSM-5 catalyst had a higher selectivity for aromatic hydrocarbons (96.98% for biomass and 86.48% for urea/biomass) and N-containing heterocyclic compounds, such as indoles (6.16% for biomass and 13.51% for urea/biomass). Additionally, the coke content decreased, and the catalyst deactivation decreased.
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