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A series of fluorinated nanosized HZSM-5 catalysts were prepared by impregnating the zeolite with NH4F solution. Thesamples were characterized by XRD, XRF, FTIR-Py, TPD,27Al and 29SiMAS-NMR. The effect of calcination temperature on thestructure and acidity was studied in the range of 400–500 C. When the calcination temperature increases, 35039
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more frameworkaluminum atoms are removed due to the fluorination. As a result, the number of acid sites and the acid strength decrease, but theratio of the total Bronsted acid sites to the total Lewis acid sites (BT/LT) increases. The fluorinated nanosized HZSM-5 catalystsexhibit better stability in the methylation of biphenyl (BP) with methanol. Moreover, the stability increases with the increase of thecalcination temperature. When the fluorinated nanosized HZSM-5 is calcined at 500 C, the conversion of BP can keep at about10% for 50 h. The enhancement of stability may be due to the decrease of the acid strength and the increase of the ratio of BT/LT.KEY WORDS: nanosized HZSM-5; stability; fluorination; calcination temperature; dealuminum;
methylation; biphenyl. 1. IntroductionLinear polynuclear aromatic hydrocarbons, such as4-alkylbiphenyl and 4,4¢-dialkylbiphenyl or b-alkyl andb,b¢-dialkylnaphthalene, have attracted great attentionbecause they can be used as promising precursors foradvanced polymer materials [1–4]. The alkylation ofbiphenyl (BP) to 4,4¢-dialkylbiphenyl with large alky-lating agents over shape-selective catalysts has beenstudied for many years and good results have beenobtained [5,6]. However, it is difficult to synthesize 4,4¢-dimethylbiphenyl by the methylation of BP with meth-anol by one-step. Up to now, only a few groups focusedon the methylation of 4-methylbiphenyl (4-MBP) withmethanol into 4,4¢-dimethylbiphenyl [7–9]. The methyl-ation of BP to 4-MBP has seldom been studied becauseof the low selectivity of 4-MBP, the low conversion ofBP and the poor stability of the catalyst. Dubuis et al.reported that the conversion of BP was less than 20%over the microsized HZSM-5 and the catalyst deacti-vated sharply [10].For the industrial application, it is necessary for thecatalysts to have good stability and coke resisting abil-ity.
In order to improve the stability of the catalysts,many modification methods have been proposed.Among them, the dealumination of zeolite is one of theeffective methods [11]. The fluorination reportedly can provoke dealuminum of the lattice, which has beenfound in many zeolite catalysts, such as Y, ZSM-5 andHM [12–17]. This kind of modification often decreasedthe total Bronsted acid sites (BT) and total Lewis acidsites (LT), but increased the strength of the remainingframework Bronsted sites. Furthermore, the catalyticactivity increased apparently on the fluorinated zeoliteswith optimum fluorine content in some reactions [12–15]. However, it was rarely reported that the dealumi-nation by fluorination can bring about the enhancementof stability.Nanosized zeolites have exhibited higher activity,stronger ability of resisting coke, and longer lifetimethan conventional microsized zeolites in some catalyticreactions [18,19]. In this paper, nanosized HZSM-5 wasmodified with NH4F to investigate the effect of fluori-nation at different calcination temperature on the cata-lytic properties and the stability of the catalyst in themethylation of BP with methanol.2. Experimental2.1. Sample preparationNanosized NaZSM-5 (70–100 nm) zeolite was pre-pared in our laboratory according to a procedure in ourprevious patent [20]. Its ammonium-form was obtainedby repeated exchange of NaZSM-5 with 0.4 mol/LNH4NO3 solution at 80 C for 2 h with stirring. Aftereach treatment, the product was filtered, washed with distilled water and then dried at 100 C. The obtainedammonium-form ZSM-5 was calcined at 540 C for 6 hin air to form the HZSM-5.The fluorinated samples were obtained by impreg-nating 5 g nanosized HZSM-5 (nano-HZSM-5) zeolitewith the 25 ml NH4F solution (0.01 g/ml). The sampleswere kept at room temperature for 10 h, dried at 100 Covernight, and calcined for 5 h at four different tem-peratures 400, 450, 475 and 500 C, respectively. Theyare abbreviated as HZSM-5-F400, HZSM-5-F450,HZSM-5-F475 and HZSM-5-F500.2.2. Catalyst characterizationThe crystal size and the habit of catalysts weredetermined on a JEOL JSM-6700F Field EmissionScanning Electron Microscope. X-ray diffraction pat-terns were obtained on a Rigaku D/max-2400 diffrac-tometer using Cu Ka1 radiation, an operating voltage of40 kV, an electron current of 100 mA, and a scanningspeed of 4 2h/min. The SiO2 and Al2O3 contents weremeasured with Bruker SRS-3400 Sequential X-ray flu-orescence spectrometer.The NH3 sorption and temperature programmeddesorption (NH3TPD) was performed on a Chembet3000 chemical adsorber from Quantachrom Co. Prior toadsorption experiments, the samples (0.2 g) were firstpre-treated at 540 C for 1 h in a quartz U-tube in a Hestream. Then, they were cooled down at 120 C andsaturated with ammonia. Finally, the desorption wascarried out from 100 C to 500 C at a heating rate of10 C per min in a He stream. The FTIR spectra wererecorded using Bruker EQUINOX55 infrared spec-trometer in the range of 400–4000 cm)1with a resolu-tion of 4 cm)1on thin wafers of KBr in which zeolitewas dispersed. The pyridine was used as a probe. Thewafers were calcined under vacuo at 450 C (ca.10)4torr) for 3 h, followed by the adsorption of pyri-dine vapor at room temperature for 30 min.