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The dealuminum often results in the change of aciditywhich is important to explain the catalytic properties ofthe catalysts. The density and nature of surface acid siteswere determined by chemical adsorption of pyridine.Figure 5 shows IR spectra of pyridine adsorbed on theparent and fluorinated nano-HZSM-5. Three bands at1547, 1448 and 1491 cm)1are attributed to the pyridineadsorbed on Bronsted acid sites (B), Lewis acid sites (L)and on both acid sites (B, L), respectively. The totalLewis acid sites (LT) decrease with the increase of cal-cination temperature, although NH4F modificationleads to more framework Al atoms removed from theirtetrahedral sites. This means that not all the non-framework Al can promote the generation of Lewis acidsites. The total Bronsted acid sites (BT) decrease obvi-ously only when the calcination temperature is up to500 C.In figure 5, a new Bronsted acid site appears around1535 cm)1in HZSM-5-F400 and HZSM-5-F450. This phenomenon can be explained by a mechanism pro-posed by Mao et al. [16]. They suggested that the NH4Fwas decomposed when calcined at 300 C, and newhydroxyl groups formed at 400 C by the reaction ofzeolite surface with the protons of the remaining(H+...F)) ion pairs. So, it can be inferred that the new Bronsted acid site might be due to the formation of thesenew hydroxyl groups. When the catalyst is calcined athigher temperature than 450 C, a significant loss of theF–Al...OH occurs, which results in the disappearance ofnew formed hydroxyl groups.The ratios (B/L) of Bronsted acid sites (around 1547and 1535 cm)1) to Lewis acid sites (around 1448 cm)1)are shown in table 2. At three different desorptiontemperatures, the B/L ratio of any fluorinated catalyst ishigher than its parent. Significantly, the B/L ratioincreases with the increasing of calcination temperatureat the desorption temperature of 150 . In other words,the ratio of BT/LT increases when the calcination tem-perature increases.The NH3-TPD (figure 6) shows the changes of theintensity and the number of total acid sites. The lowtemperature peak is mainly due to the desorption ofadsorbed ammonia from the weak acid sites or nonacidsites, whereas the high temperature peak is related to thedesorption of ammonia from the strong acid sites. It canbe found that the acid strength and acid concentrationdecrease at higher calcination temperature. Based onsuch decrease of acidity, the improvement of the sta-bility can be expected on the fluorinated catalysts.3.2. Catalytic propertiesIn order to investigate the catalytic properties of thefluorinated nano-HZSM-5 catalysts, the methylation of BP with methanol is employed as a test reaction. Forthis reaction, the stability is one of the crucial problems.But few catalysts were found to exhibit good stability.Dubuis et al. reported that the conversion of BP sharplydecreased from 19% to 13% in 3 h on microsizedHZSM-5 (Zeocat PZ-2/50), which is unfavorable forindustrial application.Figure 7 shows the comparison of the stability of theparent and fluorinated catalysts calcined at differenttemperatures.
The stability of parent catalyst is observedto be similar to that of microsized catalyst mentionedabove. The conversion of BP decreases from 34% to 1%in 20 h. Significantly, it is found that the NH4F modi-fication can dramatically improve the stability of thecatalyst. Moreover, the stability of the fluorinated cat-alysts increases with the increase of calcination tem-perature. The conversion of BP keeps at about 10% for50 h on HZSM-5-F500. The result is much better thanany other catalysts for the methylation of polynucleararomatic hydrocarbons.From the results of characterization of the fluori-nated catalysts, it can be presumed that there are two possible reasons for the improvement of stability. One isthe decrease of the acid strength. As we have known, thestrong acid sites can lead to the formation of by-prod-ucts and the coke, and make the catalyst deactivate fast.This means that the lifetime of the catalysts depends onthe acid strength. From figures 6 and 7, the lifetime ofthe catalyst is prolonged with the decrease of acidity.The HZSM-5-F500 catalyst has the weakest acidstrength, but exhibits the best stability. The other reasonfor the better stability of fluorinated catalysts might bethe increased ratio of BT/LT. From table 2 and figure 7,it can be seen that the change of ratio of BT/LT is ingood agreement with the increase of stability of thecatalyst. The methylation of BP is a reaction catalyzedby the Bronsted acid sites. So, the increase of the ratio ofBT/LT may also be responsible for the improvement ofthe stability.The initial conversion of BP decreases with theincrease of calcination temperature because of thedecrease of the number of acid sites. But, the fluorination and calcination temperature does not exert obviousinfluence on the selectivity of 4-MBP. The highestselectivity of 4-MBP on the fluorinated catalyst canreach about 47% which is a bit higher than that on itsparent. In this reaction, the selectivity is not improvedobviously because the formed 4-MBP is easily isomer-ized to other isomers at external surface.4.