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Ind. Eng. Chem. Res. 2007, 46, 7066-7072
Phenol Alkylation with 1-Octene on Solid Acid Catalysts Arno de Klerk* and Reinier J. J. Nel Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa
Solid phosphoric acid (SPA), silicated amorphous silica-alumina (ASA) catalysts (1.5-70% SiO2), and zeolites beta (BEA), mordenite (MOR), and ZSM-5 (MFI) were investigated for the alkylation of phenol with 1-octene. All catalysts were active for phenol alkylation at 200 °C in batch reactors, with olefin oligomerization being the main side reaction. All catalysts produced alkylphenols and phenyl ethers and had a low initial selectivity to o-octylphenol. Monoalkylated product selectivity on silicated ASA may be related to the orientating effect of the silicated catalyst surface but could not be related to the acid strength or alumina content of the catalysts. Evidence is also presented that indicates that an ester-based mechanism is active over silicated ASA. Monoalkylated product selectivity on zeolites could be correlated to the average pore size of the catalysts (shape selectivity). Multiple alkylation was found on SPA, Siral-28M, and the zeolite catalysts, with both acid strength and pore size determining whether multiple alkylation occurred. Introduction The alkylation of phenols with olefins, to produce alkylphenols, is an acid-catalyzed reaction.1 The reaction is industrially used mainly for the production of tert-butylphenols, as well as octyl-, nonyl-, and dodecylphenols, which are primarily produced from 2,4,4-trimethylpentene (diisobutylene) and propylene trimers and tetramers.1,2 The tert-butylphenols have numerous applications, among other antioxidants and as intermediates in the production of pesticides, resins, and dyestuffs. The longerchain alkylphenols are generally ethoxylated3 and used as nonionic surfactants, although concerns over the environmental persistence of alkylphenols and their ethoxylated derivatives have been raised.4,5 The catalysis of phenol alkylation is actively being studied and is mainly focused on the production of tertbutylphenolics6-11 and, less frequently, on the use of longerchain olefins.12 The interest in tert-butylphenolics is related to its antioxidant properties on account of its radical stability, which makes tert-butylphenolics excellent “chain-breaking” inhibitors.13 In a coal-to-liquids (CTL) synthetic fuels facility, it is possible to produce a syncrude rich in phenolic material by either direct coal liquefaction14 or indirect coal liquefaction using lowtemperature coal gasification.15 Phenol itself is not a desirable fuel component, and its content in synthetic fuel must be limited to avoid elastomer compatibility problems. However, the alkylphenols afford different degrees of oxidation stability to fuel, depending on the nature and degree of their alkylsubstitution.16-18 Alkylphenols are also used for the preparation of detergents with antioxidant properties found in fuel and lubricant additive packages.19 It is consequently desirable to remove phenol from syncrude but to retain the longer-chain alkylphenols that have beneficial fuel properties. This is not possible by hydrotreating, where hydrodeoxygenation (HDO) of coal pyrolysis products converts phenol and the alkylphenols alike.20 In this paper, a different refining approach for the conversion of coal pyrolysis products to produce transportation fuel is suggested, namely, to alkylate the phenol with olefins, rather than to hydrogenate it. This implies that the phenol is not * To whom correspondence should be addressed. Tel.: +27 16 9602549. Fax: +27 11 522-3517. E-mail:
[email protected].
separated from the rest of the coal pyrolysis products to be alkylated, unlike in standard industrial practice.1 The coal pyrolysis products contain many potential catalyst poisons, such as nitrogen bases,21 and the effect of some of these compounds on the acid-catalyzed conversion of phenolic material has been investigated.22 It was shown that the poisoning effect and its reversibility are related to the strength of the acid-base interaction. The direct application of phenol alkylation to coal pyrolysis products would, therefore, necessitate the removal of strong bases22 or finding a catalyst that has sufficient acid strength to catalyze alkylation, without resulting in permanent poisoning by strong bases. The latter approach is more elegant. In developing a technology for the alkylation of phenol in a matrix of unrefined coal pyrolysis products, two catalysis issues must be addressed: the alkylation of phenol to fuels and the desorption of strong bases from the catalyst. This paper deals with the first part of the investigation, namely, to find candidate catalysts for phenol alkylation to fuels. 1-Octene was selected as the alkylating olefin, because its chain length is representative of the naphtha range olefins targeted for phenol alkylation. This study differs from most other investigations dealing with C8 and heavier phenol alkylation, since the aim was not to find catalysts with a high para-alkylation selectivity, as is preferred for detergent applications. The aim is fuels production, where the most desirable products are phenyl ethers (high cetane diesel fuel components) and ortho-alkylated phenols (sterically crowded -OH to give antioxidation properties). The study, therefore, paid specific attention to the alkylation selectivity (O-alkylation, C-alkylation, and multiple alkylation) and side reactions (oligomerization and cracking). Experimental Section Materials. The phenol (+99%) and 1-octene (98%) were both obtained from Sigma-Aldrich and used without further purification. The purity of the 1-octene was confirmed by analysis (gas chromatography), and the main impurity was a branched C8olefin (1.5%). All reactions were performed in a mixed-pentane solvent obtained from Associated Chemical Enterprises, which was analyzed and found to consist of 23.9% methylbutane, 75.8% n-pentane, and 0.3% heavier hydrocarbons. Methanol (+99.9%) obtained from Sigma-Aldrich was used to dilute samples for gas chromatographic analysis. The catalysts were
10.1021/ie0706459 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/18/2007
Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 7067 Table 1. Properties of Catalysts Tested for Phenol Alkylation. The Pt/H-Mordenite and H-ZSM-5 Catalysts Included Binder Material catalyst
Si/Al mass ratio
C84-3 Eagle Picher Siral-1.5 Siral-5 Siral-10 Siral-20 Siral-28M Siral-30 Siral-40 Siral-70
1:75 1:20 1:10 1:4.5 1:2.9 1:2.6 1:1.7 1:0.5
H-Beta Na-Beta H-Mordenite Pt/H-Mordenite H-ZSM-5
1:0.1 1:0.1 1:0.1 1:0.3 1:0.3
surface area (m2‚g-1)
pore volume (cm3‚g-1)
average pore diameter (nm)
Solid Phosphoric Acid 30
0.12
8.0
Amorphous Silica-Alumina 240 370 390 430 320 440 500 230
0.39 0.59 0.59 0.73 1.26 0.77 0.87 0.12
3.3 3.2 3.0 3.4 7.9 3.5 3.5 1.0
0.65 0.48 0.34 0.40 0.52
2.8 6.4 1.7 2.2 2.8
Zeolites 470 150 390 360 370
Table 2. Initial Selectivities to Monoalkylated Phenols Found at
