http://www.paper.edu.cn Applied Catalysis A: General 282 (2005) 155–161 www.elsevier.com/locate/apcata Vapour-phase selective O-methylation of catechol with methanol over Ti-containing aluminium phosphate catalysts Xiaomei Zhua, Xuemei Lia,b, Mingjun Jiaa,*, Gang Liua, Wenxiang Zhanga,*, Dazhen Jianga aCollege of Chemistry, Jilin University, Changchun 130023, PR China bDepartment of Chemistry, Fudan University, Shanghai 200433, PR China Received 7 July 2004; received in revised form 26 November 2004; accepted 9 December 2004 Available online 15 January 2005 Abstract Vapour-phase O-methylation of catechol with methanol has been investigated over Ti-containing aluminium phosphate catalysts (denoted as Al0.77TixP, where x is between 0 and 1.15) prepared by non-uniform precipitation method. The catalytic activities increase with the increase of Ti/P ratio in the range 0–0.23, while further increase of Ti content leads to the decrease of activities. Meanwhile, the selectivity to guaiacol decreased gradually with the increase of Ti/P ratio. The results of various means of characterization demonstrate that the addition of titanium considerably affects the structure of the catalysts, as well as the strength and amount of the acidic and basic sites. The presence of stronger Lewis acidic and/or basic sites in the higher Ti content catalysts should be responsible for the formation of side products (C-alkylation products) and coke, thus quickening the deactivation process of the catalysts. On the other hand, the weak acidic–basic characteristics of the lower Ti content catalysts result in the high selectivity to mono-O-methylation product (guaiacol), and the excellent durability of the catalysts. # 2004 Elsevier B.V. All rights reserved. Keywords: O-Methylation; Catechol; Methanol; Guaiacol; Aluminium phosphates 1. Introduction In comparison with DMC, methanol is regarded as the most suitable agent for practical application due to its low cost. Guaiacol is an important synthetic intermediate in fine Several heterogeneous catalysts have been tested in the chemical production. It is widely used in the production of vapour-phase alkylation of catechol with methanol, including flavouring agents, fragrances, agricultural chemicals and various metal oxides, phosphates and zeolites [9–16].How- pharmaceuticals [1]. Traditionally, guaiacol was synthesized ever, a common problem is that most of the tested catalysts by methylation of catechol with corrosive reagents like exhibited unsatisfactory activity and/or poor selectivity to dimethyl sulfate and dimethyl iodide in the presence of guaiacol. Besides, unfavorable rapid deactivation can be stoichiometric quantities of sodium hydroxide as a homo- generally observed under their conditions of operation [16]. geneous catalyst [2,3]. Recently, vapour-phase alkylation of In previous work, we have investigated the catalytic catechol for the synthesis of guaiacol has received more properties of aluminium phosphate (Al–P–O) system for the attention, since the route is more economical and more vapour-phase O-methylation of catechol with methanol, and environmentally friendly for industrial applications. The we found that the activity and selectivity of these catalysts general alkylating agents include dimethyl carbonate were greatly dependent on the P/Al ratio and preparation (denoted as DMC hereafter) [4–8] and methanol [9–17]. conditions [12]. Additionally, we reported that an optimum Al–P–Si–Ti–O mixed oxide catalyst (Al1P1.30Ti0.30Si0.17Ox) showed very high activity and selectivity to guaiacol; a 92% catechol conversion and 86% guaiacol yield could be * Corresponding authors. Tel.: +86 431 8499140; fax: +86 431 5167420. E-mail addresses: [email protected] (M. Jia), obtained at the reaction temperature of 583 K [17]. In another [email protected] (W. Zhang). work, we studied the catalytic properties of Al0.77Ti0.23PO4 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.008 中国科技论文在线 http://www.paper.edu.cn 156 X. Zhu et al. / Applied Catalysis A: General 282 (2005) 155–161 catalyst prepared by different methods (conditions) for the O- Ar for 2 h at 323 K, the sample was heated at the rate of ethylation of catechol with ethanol, and found that the 10 K minÀ1 in He (30 mL/min), the concentration change of catalyst prepared via a non-uniform precipitation procedure the desorbed NH3 or CO2 was monitored by using an on-line under reflux condition showed the highest activity and thermal conductivity detector (TCD). selectivity [18]. IR spectra were recorded on a Nicolet 410 FT-IR In this work, we attempt to prepare a series of Ti- spectrometer. A quartz infrared cell with CaF2 windows containing aluminium phosphate catalysts (Al0.77TixP) with was used. Self-supporting wafers of pure catalysts were used different Ti contents by the non-uniform precipitation for the measurement of surface acidity. Each wafer was procedure. The catalytic properties including the lifetime of obtained by pressing the powder at 100 kg cmÀ2. The result- the catalysts for the vapour O-methylation of catechol with ing sample wafer was heated at 573 K for 60 min, and cooled methanol were investigated. Meanwhile, various character- to room temperature in vacuum of 10À4 mmHg. Pyridine as a ization means such as XRD, N2-adsorption/desorption, NH3 probe molecule was then exposed to the pre-treated samples at or CO2-TPD, IR of adsorbed pyridine and TG–DTA were room temperature and was evacuated at 373 K. IR measure- carried out in order to identify the nature of the active sites ments were performed at ambient temperature. and to understand the reaction and deactivation mechanism. TG–DTA analysis was carried out on a Shimadzu DTG- Results hopefully can provide some guidance in the search 60 instrument, and the samples were heated at the rate of for more active and durable catalysts. 10 K minÀ1 in air. 2. Experimental 2.3. Catalytic tests 2.1. Catalyst preparation The vapour-phase O-methylation of catechol with methanol was carried out in a fixed bed continuous down- The Ti-containing aluminium phosphate catalysts were flow reactor at atmospheric pressure [12]. Before the start of prepared by the non-uniform precipitation method as the reaction, the catalyst (1.4 g, 40–60 mesh) was activated in described elsewhere [18]. Typically, the precipitate of the glass-tube reactor (i.d. = 11 mm and length = 40 mm) at aluminium hydroxide was obtained firstly by precipitating 553 K for 1 h in nitrogen. A pre-mixed catechol–methanol an aqueous solution of aluminium nitrate with ammonium mixture was then fed from the top of the reactor by means of a hydroxide in water until the pH of the slurry became about SY-04 syringe pump. The reaction conditions are as follows: 6.2 at room temperature. Then, the tetrabutyltitanate was catechol/methanol = 1/5 (mole ratio), reaction tempera- dropped into the slurry under vigorous stirring. After that, ture = 553 K, weight hourly space velocity (WHSV) = phosphoric acid was dropped into the mixture under reflux. 0.13–3.06 hÀ1. The products were analysed with a gas After standing for several hours, the mixture was heated at chromatograph equipped with a capillary column and were 363 K in open air with continuous stirring to remove water identified using known standards and GC–MS. and all other volatiles. Then the white solid was dried at 383 K for 12 h. The final resulting catalysts were calcined at 673 K for 5 h in air, and were denoted as Al0.77TixP(x =0, 3. Results and discussion 0.05, 0.12, 0.23, 0.46, 0.69 and 1.15). The X-ray diffraction patterns of Al0.77TixP catalysts are 2.2. Catalyst characterization displayed in Fig. 1. Both crystalline AlPO4 and NH4AlP2O7 phases can be observed clearly in the Ti-free sample X-ray diffraction powder analyses were performed with a D/Max-RA diffractometer operated at 50 kV and 150 mA using nickel-filtered Cu Ka radiation. Nitrogen adsorption–desorption isotherms were per- formed using a Micromeritics ASAP2010N sorptometer. The samples were pre-treated under N2 stream at 523 K for 6 h prior to measurement. The total pore volume and the mean pore diameter were evaluated from the adsorption branches of the nitrogen isotherm using the BJH model. Temperature-programmed desorption (TPD) was carried out using NH3 or CO2 as probe molecules. In a standard procedure, 100 mg of fresh sample was first calcined at 673 K under Ar stream for 1 h, then cooled down to 323 K and exposed to a certain amount of ammonia or carbon dioxide for 0.5 h. After the system was purged with flowing Fig. 1. XRD patterns of Al0.77TixP with different Ti contents. 中国科技论文在线 http://www.paper.edu.cn X. Zhu et al. / Applied Catalysis A: General 282 (2005) 155–161 157 Table 1 Surface properties of Al0.77TixP with different Ti contents Catalysts BET surface Pore volume Pore area (m2 gÀ1) (cm3 gÀ1) size (nm) Al0.77P 10 0.014 9.1 Al0.77Ti0.05P 27 0.058 14.7 Al0.77Ti0.12P 127 0.38 12.3 Al0.77Ti0.23P 120 0.29 9.0 Al0.77Ti0.46P 117 0.16 5.5 Al0.77Ti0.69P 125 0.13 4.3 Al0.77Ti1.15P 186 0.25 5.4 (Al0.77P) [19]. When Ti is introduced, the diffraction peaks corresponding to NH AlP O phases are nearly undetect- 4 2 7 Fig. 3. IR spectra of adsorbed pyridine on Al0.77TixP samples. able, while the peak intensity of AlPO4 phase decreases gradually with increase of Ti content. The samples with Ti/P ratio over 0.46 exhibit only a very broad hump covering the other two strong bands at 1450 and 1545 cmÀ1 reveal the 2u range of 20–308, which is a characteristic of amorphous presence of great numbers of Lewis and Bro¨nsted acidic sites, phase.