Probe molecules 1-butene

The use of probe molecules that will sorb on specific acid sites has also been used to differentiate Brpnsted and Lewis sites. Studies on H-ZSM-5 and H-ZSM-8 suggest that pyridine cannot access the Lewis sites and thus essentially measures Brpnsted acidity 2,6-Dimethylpyridine has been reported to sorb preferentially on the Brpnsted sites of a variety of zeolites. Various amines have also been used to measure surface acidity. Since amines are weaker bases than either ammonia or pyridine, they preferentially sorb on the stronger acid sites. Both n-butyl amine and t-butyl amine have been used as adsorbates in TPD studies of zeolites. Comparison of the TPD spectra with butene isomerization has shown that butyl amines measure only the strong acid sites. 

The SAPO-34 external surface is relatively large for the small crystal material used in this study. The role of the external surface in the MTO reaction is quite interesting but not clear. Coke deposition on HZSM-5 during methanol conversion was reported to occur essentially on its external surface [12]. Whether coke deposition during MTO on small crystals of SAPO-34 occurs in the same way is less clear. Experiments with controlled coke formation at the external surface were performed using i-butanol as a probe molecule. i-Butanol will dehydrate to i-butene on acidic sites on the external surface, but i-butanol and i-butene are too large to enter the pores of SAPO-34. Therefore, i-butene is a potential coke precursor on those external acidic sites. 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

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