- 上一篇：自媒体英文文献和中文翻译
- 下一篇：冲压模具的规划英文文献和中文翻译
IR spectrawere recorded after subsequent evacuation at 150, 300,and 450 C for 1 hNMR spectra were obtained on a Bruker DRX-400spectrometer with a BBO MAS probe using 4 mm ZrO2rotors.29Si MAS-NMR spectra were acquired at79.5 MHz using a 0.8 ls p/8 pulse with a 4 s recycledelay and 2000 scans. The chemical shifts werereferred to 4,4-dimethyl-4-silapentane sulfonate sodium(DSS).27Al MAS-NMR spectra were collected at104.3 MHz using a 0.75 ls p/12 pulse with a 3 s recycledelay and 600 scans. The chemical shift reference was1% aqueous Al(H2O)63+.2.3. Catalyst testingThe methylation of BP was carried out in a fixed-bed,down-flow stainless steel reactor. The catalyst (1 g) wasplaced in the middle of the tube reactor and activated at450 C for 1 h prior to reaction. Mesitylene was used asthe solvent. The typical reaction conditions are as fol-lows: feed biphenyl: methanol: mesitylene=1:5:5 (molarratio), reaction temperature 450 C, N2 flow 20 ml/min.Analysis of the products was performed on GC witha column of BETA DEX 120 (60 m 0.25 mm). Thedetector and injector were both kept at 300 C. Theoven temperature was initially kept at 170 C for20 min, increased to 190 C at a rate of 4 C/min andheld at the final temperature for 35 min. The conversionof BP was calculated on the basis of reacted BP, and theselectivity of mono-MBP was defined as the ratio ofmono-MBP to all MBP isomers.3. Results and discussionRecently, the fluorination has often been used toimprove the catalytic properties of the zeolite catalysts.Fluorinating agent and fluorine content have beenwidely studied and found to have an important impacton the activity of the fluorinated catalysts [12–17]. Infact, fluorination conditions, especially the calcinationtemperature, can also affect the properties of the zeolitecatalysts greatly. It is very essential to take it intoaccount when fluorination is carried out. Here, we focuson the effect of calcination temperature on the structureand acidity of fluorinated zeolites.
3.1. Characterization of catalystsFigure 1 shows the SEM photographs of nano-HZSM-5 and HZSM-5-F475. Obviously, the crystal sizeof HZSM-5-F475 is similar to that of nano-HZSM-5.This indicates that the fluorination does not lead to thedecrease of the crystal size.The results of chemical analysis for the nano-HZSM-5 and fluorinated nano-HZSM-5 are shown in table 1. Itcan be seen that the Si/Al molar ratio (from XRFmeasurement) keeps constant after the fluorination atdifferent calcination temperatures. However, the Si/FAlmolar ratio (from NMR measurement) increases grad-ually with the increase of calcination temperature. Theseresults imply that the Al species removed from theframework still remain in ZSM-5 catalysts.Figure 2 shows the XRD patterns of the samplesbefore and after NH4F treatment. Obviously, NH4F-modified samples keep their crystalline structures, andthe fluorinated species are not found. The relativecrystallinity was determined by the intensity of thecharacteristic peaks in the 2h range of 22.5–25 .Asshown in table 1, it has no apparent change after thenano-HZSM-5 catalysts are fluorinated.The29Si MAS-NMR spectra of the parent andmodified nano-HZSM-5 zeolites consist of four signalsat ca. )103, )107, )114 and )117 ppm (figure 3). Thesignals at )114 and )117 ppm that overlap witheach other are caused by Si(OSi)4 sites in the frameworkof ZSM-5 structure, while the shoulders at )107 and )103 ppm are due to AlOSi(OSi)3 sites and HOS-i(OSi)3 sites, respectively. With the increase of the cal-cination temperature, the signal at )107 ppm decreasesand the line width of the Si(OSi)4 sites at )114 ppmnarrows, which indicate that more framework alumi-num atoms are removed at the higher calcination tem-perature.It is well-known that the nature and concentration ofAl species in aluminosilicate materials are closely relatedF475. to the acidity which is crucial for the catalytic perfor-mance of the catalysts. Solid-state27Al MAS-NMR is asensitive tool commonly used for determining thecoordination of Al. For HZSM-5 zeolites, the reso-nances at chemical shifts of 54 ppm and )1 ppm (fig-ure 4) can be assigned to tetrahedral (4-coordinated)framework Al and octahedral (6-coordinated) non-framework Al species, respectively. Another peak at30 ppm is attributed to penta-coordinated aluminum bysome research groups, but it is assigned to the distortedtetrahedral coordinated aluminum in extraframeworkAl or the distorted framework Al by others [17]. Thesignal at 54 ppm for framework Al apparently decreasesafter NH4F modification and continuously decreaseswith the increase of calcination temperature. The resultsindicate the aluminum atoms are dislodged from theirtetrahedral sites. A new hexa-coordinated Al resonanceat )16 ppm arising from the fluorinated species [12] isfound in the fluorinated nano-HZSM-5. This bandbecomes broad because more penta-coordinated alumi-num or distorted tetrahedral coordinated aluminumspecies are formed with the increase of the calcinationtemperature. So, it can be concluded that the calcinationtemperature has a great effect on the fluorinated nano-HZSM-5. The higher the calcination temperature is, themore framework aluminum is removed and the morenonframework aluminum is generated.