利用固態NMR鑑定沸石觸媒內外表面的酸性

Characterization of Internal and External Acidity of Zeolites: A Solid-State NMR Study

趙琦(Qi Zhao) ； 陳文華(Wen-Hua Chen) ； 劉尚斌(Shang-Bin Liu)

《化學》 59卷1期 (2001/03) Pp. 63-69

https://doi.org/10.6623/chem.2001008

摘要

本研究利用沸石觸媒之分子篩特性，以不同分子尺寸之鹼性磷氧化物為探針，分別將其吸附在H-ZSM-5 沸石樣品中，透過此類含磷-31核探針分子吸附在該觸媒不同酸性位上之化學遮蔽效應，及其對應於^(31)P MAS NMR譜圖中特徵峰的化學位移和相對強度變化，吾人針對不同鋁含量(Si/AI比值) HZSM-5沸石樣品內、外表面酸性基的種類、強度及其分佈加以鑑別。

關鍵字

固態NMR ； 吸附 ； H-ZSM-5沸石 ； 酸性 ； 磷氧化物

並列摘要

By means of solid-state ^(31)P MAS NMR, basic phosphine oxides with varied molecular sizes, namely tributylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO), were used as probe molecules to characterize the internal and external acidity of H-ZSM-5 zeolites. It was found that, the relative change in ^(31)P NMR chemical shift at a given acid site is nearly the same regardless of the characteristics of the adsorbed probe molecules. Consequently, five internal acid sites were identified whose relative distribution and strength varied with the Al content in the zeolite framework. Zeolites having lower Al content tend to possess internal acid sites with higher acid strength on the expanse of decrease in weaker acidity. On the other hand, for the three different external acid sites found, while their relative distribution varied slightly with Si/Al ratio of the zeolite, the acid strength remained practically unchanged. This study, which demonstrates the methodology of detennining the relative distribution of acid sites (viz. internal vs external acidity), should be helpful in further understanding of the reaction mechanism and related selectivity features of the zeolitic catalysts.

並列關鍵字

solid-state NMR ； adsorption ； H-ZSM-5 zeolites ； acidity ； phosphine oxide