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Alkylation of Toluene with C2～C4 Aliphatic Alcohols over HY Zeolite*
by Tatsuaki Yashima**, Noriaki Yokoi** and Nobuyoshi Hara**
Summary: Alkylation of toluene with C2～C4 aliphatic alcohols over an HY zeolite catalyst was studied. The steric effecton the substitutionof alkylating agentsto the orthoposition of toluene and the geometriceffect of the zeoliticstructure on the secondaryisomerization of theproduced alkyl- tolueneswere discussedon the basis of the distributionof the alkyltolueneisomers. The reactivityof alcoholsto form alkyltoluenesand the reactionproducts showed that this alkyl- ation clearlyproceeded through formation of carboniumions from the alkylating agent. Some ortho isomersof alkyltolueneswere obtained in most of the experiments, but no ortho isomerswere produced from iso-and tert-butylalcohol, becauseof a larger steric effectbetween the methyl group of tolueneand the alkyl group of both alcohols. Isomerizationof initiallyproduced alkyltoluenes,from the ortho and para isomersto the meta isomer, also dependedon the stability of the carboniumion of the substituent. In the alkylation with the larger molecularsize alcohols,however, meta isomersof alkyltoluenesdecreased though thesecarbonium ions were stable. These results suggestedthat free migration of the larger alkyl group was inhibited in the narrow cage of the zeolite by a geometriceffect.
In general, isomerization of alkylaromatics 1 Introduction is promoted by acid catalysts, and the reactivity We reported on the alkylation of toluene with increases with the stability of the carbonium methanol over synthetic Y zeolites1),2). It was ions4). In alkylation over a zeolite catalyst, shown that the composition of the obtained however, it is expected that the isomerization xylene mixture deviated considerably from the of initially produced alkyltoluene may be de- thermodynamic equilibrium values, producing pressed in a narrow cage of zeolite by a geometric p-xylene or o-xylene in excess, and the selectivity effect as mentioned above. Thus, the higher of p-xylene formation was related to the Bronsted alkyl group, which forms a more stable carbon- acidity of the catalyst. In the alkylation of ium ion, must be depressed to migrate because toluene over a synthetic zeolite, substitution of of its large size. Venuto and his co-workers5) an alkyl group to toluene is para/ortho orien- studied alkylation of phenol with various alkylat- tation and the secondary isomerization by mi- ing agents in the presence of synthetic zeolites gration of the alkyl group is inhibited in narrow in a liquid phase, and reported that the yields of cage of the zeolite1). meta isomers of higher alkylphenols were very It is expected that a steric hindrance effect small, but that they increased at a higher tem- between the methyl group of toluene and the perature or a longer reaction time. alkyl group of the alkylating agent will appear The present investigation has been undertaken with increase in size of the alkyl group, and, to study the alkylation of toluene with C2～C4 therefore, the higher alkylating agent may at- aliphatic alcohols and to discuss the effect of the tack only on the para position of toluene, selec- size of the alkyl group on alkylation and isomeri- tively producing para alkyltoluene. Brown zation over synthetic HY zeolite. and his co-workers3) reported that under the influence of aluminium bromide, alkylation of 2 Experimental toluene with ethyl, isopropyl and tert-butyl bromide depended on the stability of the car- 2.1 Catalyst bonium ion, i. e. ethyl
Volume 13, No. 2, November 1971 216 Yashima, Yokoi and Hara: Alkylation of Toluene
polymeric compounds. Alkylation of toluene HY zeolite by deamination. with ethyl, n-propyl, isopropyl, n-butyl, sec- 2.2 Reactants butyl, isobutyl and tert-butyl alcohols on an HY zeolite catalyst were carried out. The effect Toluene and C2～C4 alcohols, having a purity of over 99%, were obtained from a commercial of the reaction temperature was shown in Fig. source. 2～8 for each alcohol. 2.3 Analysis 3.1 Alkylation with Ethyl Alcohol In this study, ethyl alcohol showed the least The reaction mixture was analyzed by gas reactivity as an alkylating agent (Fig. 2). In chromatography using a 45 meter stainless steel capillary column with a stationary phase squa- this reaction, ethyltoluenes were the main lane. The carrier gas was N2 (0.5kg/cm2 input pressure) and the analysis temperature was 110℃ or 120℃.
2.4 Apparatus and Procedure The experiment was carried out in a fixed bed type apparatus with a continuous flow system at atmospheric pressure. The catalyst was placed in the electrically heated quartz reactor and the air was replaced with nitrogen. The calcination was carried out for 2 hours at 350℃,and then the reactor was brought to the reaction temperature in situ. The mixture of toluene and alcohol was fed by a micro feeder and was carried by nitrogen to the catalyst bed. The product was cooled with an ice trap, and samples for analysis were collected periodically.
2.5 Reaction Conditions In this study, the reactions were carried out under the same conditions, except for temper- ature. These conditions were appropriate for Catalyst:HY alkylation with methanol over HY catalyst1). Reaction condition; reaction temperature: 160℃, The standard operating conditions were 2 moles W/F: 120 (g. hr/mol), n-butyl alcohol/toluene=1/2. Fig. 1 The Activity Change with Process Time of toluene per mole of alcohol, 5 moles of (Alkylation with n-Butyl Alcohol) nitrogen per mole of reactants, 140～180℃, and 120 (g. hr/mol) of W/F, defined as follows: weight of catalyst (g) W/F feed rate of reactants (toluene+alcohol) (mol/hr)
3 Results
The activity of the catalyst changed consider- ably with process time, as shown in Fig. 1. Thus, the data of the experiments were taken during the period of highest activity which usually occurred at 2 or 3 hours in the process time. The yield of products was calculated as fol- lows:
moles of produced alkyltoluene Yield ×100(%) moles of fed alcohol
In most cases, the fed alcohol was completely converted. Thus, all the alcohol which was Catalyst: HY Reaction condition; W/F: 120 (g . hr/mol), not converted to alkyltoluenes, was consumed by ethyl alcohol/toluene: 1/2. producing the corresponding olefins, ethers and Fig. 2 Alkylation with Ethyl Alcohol
Bulletin of The Japan Petroleum Institute with C2～C4 Aliphatic Alcohols over HY Zeolite 217 products and small amounts of xylenes and ethylbenzene were produced. Distribution of isomers of the resulting ethyltoluenes was similar to that of xylene isomers produced by alkylation with methanol1), and principally p-ethyltoluene was produced. The formation of xylenes and ethylbenzene would be due to the transalkylation of initially produced ethyltoluenes with toluene. 3.2 Alkylations with n-Propyl and Iso- propyl Alcohol Isopropyl alcohol had a slightly higher reac- tivity than n-propyl alcohol (Fig. 3, 4). The produced C10 aromatics were exclusively iso- propyltoluenes in each case using n-propyl and isopropyl alcohol as alkylating agent. A small amount of diisopropyltoluenes was formed, and at a higher reaction temperature, xylene, isopropylbenzene and ethyltoluenes were found as by-products. At a lower temperature, Catalyst: HY unconverted n-propyl alcohol was observed after Reaction condition; W/F: 120 (g・hr/mol), 3 hours in process time, but unconverted iso- isopropylalcohol/toluene: 1/2. Fig.4 Alkylationwith IsopropylAlcohol propyl alcohol was not detected. In the produced isopropyltoluene isomers, 0-Xylene on a zeolite Catalyst2). the meta isomer was present in the largest and the ortho isomer was the smallest amount. The 3.3 Alkylation with n-Butyl and sec-Butyl total yield of isopropyltoluenes, especially meta Alcohol isomer, increased with the reaction temperature, As alkylating agent, butyl alcohols and the while the yield of ortho isomer decreased. These corresponding propyl alcohols had a similar results suggest that the initially formed ortho reactivity (Fig. 5, 6). isomer was easily isomerized to the meta isomer, sec-Butyltoluenes were the main products or that a steric hindrance effect between the and no n-butyltoluenes were produced. The methyl group of toluene and the alkylating iso- by-products were mainly di-sec-butyltoluenes, propyl group appeared at a higher temperature. and also n-butenes and diisobutyl ethers, which In the alkylation of toluene with methanol, how- were formed from alcohol alone. At a lower ever, p-xylene was more easily isomerized than
Catalyst: HY Catalyst: HY Reaction condition; W/F: 120 (g・hr/mol), Reaction condition; W/F: 120 (g・hr/mol), n-propyl alcohol/toluene: 1/2. n-butyl alcohol/toluene: 1/2. Fig. 3 Alkylation with n-Propyl Alcohol Fig. 5 Alkylation with n-Butyl Alcohol
Volume 13, No . 2, November 1971 218 Yashima, Yokoi and Hara: Alkylation of Toluene
Catalyst: HY Catalyst: HY Reaction condition; W/F: 120 (g・hr/mol), Reaction condition; W/F: 120 (g・hr/mol), sec-butylalcohol/toluene: 1/2. tert-butyl alcohol/toluene: 1/2. Fig. 6 Alkylationwith sec-Butyl Alcohol Fig. 8 Alkylation with tert-Butyl Alcohol temperature, some unconverted butyl alcohols ating agent in this study (Fig. 7, 8) ; tert-butyl- were observed after a certain time on stream. In toluenes were the main products and small the distribution of sec-butyltoluenes, more para amounts of by-products from the alcohol were isomer was obtained than meta isomer at a lower observed. No o-tert-butyltoluene was formed temperature. This result suggested that a geo- because of the large steric hindrance effect be- metric effect of the zeolitic cage appeared with tween the methyl group of toluene and the an increase in the size of the alkyl groups (this alkylating tert-butyl group. In a higher tem- will be discussed later). perature range, the yield of tert-butyltoluenes decreased with the temperature, and a consider- 3.4 Alkylation with Isobutyl and tert-Butyl able amount of coke was formed on the catalyst. Alcohol This suggests that more coke would be formed tert-Butyl alcohol was the most reactive alkyl- at longer residence time of the primary reaction products on catalyst, because a relatively large molecule such as tert-butyltoluene was difficult to diffuse out of the narrow apertures of the zeolitic cage. Therefore, in alkylation with tert-butyl alcohol, the most reactive alkylating agent found, a low reaction temperature was chosen for depressing coke formation. At 90℃, a temperature lower than the boiling point of toluene, considerable alkylation occurred and p-tert-butyltoluene was obtained in nearly 90% selectivity. At such a low reaction temperature, however, catalyst life was short though coke formation could be depressed. This result showed that water produced by alkylation re- mained in the zeolitic cage and poisoned the reaction by covering the active sites of the catalyst.
4 Discussion The order of reactivity of the alcohols to form Catalyst: HY Reaction condition; W/F: 120 (g・hr/mol), alkyltoluenes in the studied reaction was con- isobutyl alcohol/toluene: 1/2. sistent with the stability of the corresponding Fig. 7 Alkylation with Isobutyl Alcohol carbonium ions; i. e. ethyl Bulletin of The Japan Petroleum Institute with C2～C4 Aliphatic Alcohols over HY Zeolite 219 isopropyl=sec-butyl Catalyst: HY Reaction condition; reaction temperature: 160℃, alcohol/toluene: 1/2. Fig. 10 Effect of Contact Time on Composition of Alkyltoluene Isomers Volume 13, No. 2, November 1971 220 Alkylation of Toluene with C2～C4 Aliphatic Alcohols over HY Zeolite alcohols, the yield of meta isomer was more than 3～6).Therefore, it is suggested that the second- that of the para isomer over 15 and 40 (g・hr/ ary isomerization of alkyltoluenes is depressed by mol), respectively. In the case of n-butyl and the geometric effect of the zeolite; that is, limita- sec-butyl alcohol, the yield of meta isomer was tion of the free migration of the substituted large above that of para isomer at 80 and l65 (g・hr/ alkyl group in the narrow zeolite cage. mol), respectively. Alkylation of toluene with tert-butyl alcohol Generally, the alkylation of toluene with proceeded even at a reaction temperature lower alcohols on an acid catalyst is known to be para/ than the boiling point of toluene; at such a tem- ortho orientation. Therefore, the most meta perature, selectivity of p-tert-butyltoluene reached isomer of alkyltoluene was formed by secondary 90% in total aromatic products, with the least isomerization of initially produced para and secondary isomerization. ortho isomers. The results of Fig. 10 show References that the isomerization of propyltoluene proceeded much faster than that of butyltoluene. The dif- 1) Yashima, T., Ahmad, H., Yamazaki, K., Katsuta, M., Hara, N., J. Catalysis, 16, 273 (1970). ference in the ease of isomerization between 2) Yashima, T., Yamazaki, K., Ahmad, H., Katsuta, propyl- and butyltoluenes was not caused by the M., Hara, N., ibid., 17, 151 (1970). stability of the carbonium ions from the alkylat- 3) Brown, H. C., Jungk, H., J. Am. Chem. Soc., 78, 2182 (1955). ing agent, since there was no appreciable dif- 4) Brown, H. C., Jungk, H., ibid., 78, 5579 (1955). ference in the total yield of corresponding alkyl- 5) Venuto, P. B., Hamilton, L. A., Landis, P. S., Wise, toluenes under the same reaction conditions (Fig. J. J., J. Catalysis, 5, 81 (1966). Bulletin of The Japan Petroleum Institute