固体酸催化
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Jointly published by React.Kinet.Catal.Lett. Akadémiai Kiadó, Budapest Vol. 92, No. 2, 311−317 (2007) and Springer, Dordrecht 10.1007/s11144-007-5163-8
0133-1736/2007/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.
RKCK5163 ALKYLATION OF CATECHOL WITH METHANOL TO GUAIACOL OVER SULPHATE-MODIFIED ZIRCONIA SOLID ACID CATALYSTS
Venkataraman Vishwanathana*, Gajula Balakrishnaa, Balraj Rajesha,
Venkatreddy Jayasria, Lucky Sikhwivhilub and Neil J. Covilleb aCatalysis and Physical Chemistry Division, India Institute of Chemical Technology
Hyderabad-500007, India bMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand
Johannesburg, WITS 2050, South Africa
Received May 24, 2007, in revised form September 14, 2007, accepted September 14, 2007 Abstract A series of sulphate-promoted ZrO2 solid acid catalysts with different contents of
SO4
2- were calcined at 450oC in air for 4 h and tested for the liquid-phase
alkylation of catechol to guaiacol in a fixed-bed down-flow reactor. The 5 wt.% SO42- on ZrO2 showed the best conversion (82%) and selectivity for guaiacol
(84%) at 200oC and 1 bar pressure. A smooth correlation was observed between
the catalytical activity and surface acidity of sulphated zirconia. Based on our results, a surface mechanism is proposed.
Keywords: Tetragonal phase, alkylation, catechol, sulphated zirconia, surface acidity
INTRODUCTION Alkylation of catechol to guaiacol (ortho-methoxy phenol) is industrially important since guaiacol is used as an intermediate in the production of vanillin __________________________ * Corresponding author. E-mail: vvnathan2004@yahoo.co.in 31 VISHWANATHAN et al.: TETRAGONAL PHASE [1]. Traditionally, guaiacol is synthesized by methylation of catechol with dimethylsulphate [2] or with a methyl halide in the presence of sodium hydroxide [3]. These methylating agents are expensive, corrosive and environmentally hazardous. Hence, there is a need to look for eco-friendly catalysts showing higher activity and good selectivity for guaiacol. In recent years, several solid acid catalysts have been tried out for various alkylation reactions. Some of the solid acid catalysts studied for the title reaction were metal oxides [4-9], phosphates [10,11], ammonium metatungstate [12] and zeolites [13]. However, information on the alkylation of catechol over sulphate-modified zirconia (SZ) is rather limited [14,15]. In this communication, we have reported for the first time a liquid-phase alkylation reaction of catechol with methanol to guaiacol over sulphated zirconia (SO42-/ZrO2) in a fixed-bed
down-flow reactor at 200oC under atmospheric pressure.
EXPERIMENTAL Zirconium hydroxide, Zr(OH)4, was prepared by the hydrolysis of zirconium
oxychloride (ZrOCl2⋅8H2O; LOBA CHEMIE) with an aqueous ammonia solution (25 wt.%) at a constant pH of 10.5. The precipitate was filtered and washed with deionised water until no chloride ion was detected in the filtrate. It was dried at 110oC for 24 h. A series of zirconia samples containing different
wt.% of sulphate ions (SZ) were prepared by impregnating Zr(OH)4 with an aqueous solution of appropriate amounts of ammonium sulphate. The samples were dried at 110oC for 24 h and calcined at 450oC in air for 4 h.
The surface areas of zirconia and sulphated zirconia samples were measured by Quantasorb AS-1 using N2 as a probe molecule. TPD of NH3 was used to
measure the surface acidity of the samples. The liquid-phase reaction was carried out in a fixed-bed glass reactor using a motorized syringe. The reaction was carried out at 200oC over 1.0 g catalyst
using a catechol-methanol mixture (1:5 w/w). N2 was used as a carrier gas. The liquid products, mainly guaiacol (O-alkylated) and a small quantity of C-alkylates, were analysed by a GC (Shimadzu-2014) using a capillary column (Zebron-ZB-5).
RESULTS AND DISCUSSION In our previous study, we have reported the XRD results of 5 wt.% sulphate on zirconia (5 wt.% SZ) subjected to calcination at different temperatures [16]. According to our investigation, only the sample dried at 110oC appeared to be
amorphous. However, on calcination at 350oC, the tetragonal crystalline phase (d = 0.293 nm) appeared and this continued to be present up to 550oC. Beyond this calcination temperature, the formation of the monoclinic phase reappeared
