Tertiary alkyl ester preparation

A tertiary olefin such as isobutylene is reacted with a lower carboxylic acid to produce the ester in the presence of a large pore zeolite catalyst such as Zeolite Y, Zeolite beta or Zeolite X.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention provides an improved method for the preparation of
 esters such as t-butyl acetate by reaction of a tertiary olefin with
 acetic acid in the presence of a large pore zeolite catalyst such as
 Zeolite Y, Zeolite X or Zeolite beta.
 2. Description of the Prior Art
 It is known to produce esters by the reaction of an olefin such as
 isobutylene with a lower carboxylic acid over a sulfonate group-containing
 cation exchange resin. See U.S. Pat. No. 3,678,099 and the references
 disclosed therein including U.S. Pat. Nos. 2,678,332, 3,031,495, 3,172,905
 and 3,173,943.
 A problem which is encountered in such prior procedures has been the
 tendency for polymerization of the olefin to occur during the
 esterification which results both in significant yield losses and in the
 formation of products such as olefin dimer which are difficult to separate
 from the product ester. For example, isobutylene dimer forms an azeotrope
 with t-butyl acetate thus making separation exceedingly difficult.
 Problems of diisobutylene formation can be substantially overcome through
 the use of a selectivity enhancing modifier such as tertiary butanol.
 However, although the use of such a modifier has striking advantages, its
 use entails additional processing costs and purification procedures.
 Other processes are described. For example, U.S. Pat. No. 3,492,341
 describes the reaction of isobutylene with acetic acid to form ester using
 a mordenite aluminosilicate catalyst.
 U.S. Pat. No. 4,365,084 describes ester production by reaction of a linear
 or slightly branched olefin with acetic acid using a catalyst such as
 HZSM-5. The use of olefins having unsaturation at the number 2 carbon atom
 is excluded.
 U.S. Pat. No. 4,461,729 is similar to 4,365,084 and contains the additional
 step of hydrolyzing the ester to form secondary alcohol.
 U.S. Pat. No. 4,465,852 relates to ester preparation by reaction of olefin
 with acetic acid. Although a great number of olefins are mentioned,
 including isobutylene, and a great number of catalysts are mentioned,
 including ZSM-5, the olefins exemplified are ethylene and propylene and
 the claims are limited to ethylene, propylene and butylene.
 Copending patent application Ser. No. 09/022183, now U.S. Pat. No.
 5,994,578 filed Feb. 11, 1998 describes ester preparation by reaction of
 an olefin such as isobutylene with acetic acid using a ZSM-5 catalyst.
 Data are presented indicating that at the reported conditions of the
 testing, poor results were achieved with large pore zeolite catalyst,
 Zeolite beta.
 BRIEF DESCRIPTION OF THE INVENTION
 In accordance with the invention, olefin and lower carboxylic acid are
 reacted in the presence of a large pore acidic zeolite such as Zeolite Y,
 Zeolite X, or Zeolite beta at conditions where ester is formed at high
 rates and selectivity, and whereby the formation of olefin polymerization
 products is maintained at a very low level.
 DETAILED DESCRIPTION
 The present invention is applicable to the formation of esters having the
 formula
 ##STR1##
 wherein R.sub.1 is a C.sub.4 or C.sub.5 tertiary alkyl group and R is
 hydrogen or a C.sub.1 -C.sub.2 alkyl group. T-butyl acetate is an
 especially preferred product.
 In accordance with the invention, olefin and organic carboxylic acid are
 reacted in the liquid phase to form ester over a solid acidic large pore
 size zeolite catalyst. The large pore zeolites used in accordance with the
 invention are those having twelve-ring pores as described, for example, at
 page 19 of "Introduction to Zeolite Science and Practice", van Bekkum et
 al. Elsevier (1991). Zeolite Y is especially useful as is Zeolite beta and
 Zeolite X. The zeolites employed are suitably made with conventional
 binders, eg. alumina, silica, and the like.
 The conditions at which the esterification reaction is carried out are
 exceedingly important. The large pore acidic zeolites are extremely active
 catalysts and unless certain reaction conditions are employed, excessive
 amounts of olefin polymer are obtained.
 The esterification reaction is carried out at pressures sufficient to
 maintain the liquid phase, usually 50 psig or higher. The upper pressure
 limit is governed largely by practical considerations , little is to be
 gained by operating at pressures in excess of 800 psig. Operation at
 250-500 psig is generally preferred.
 Reaction temperature is important, temperatures of about 20 to 80.degree.
 C. are suitable, temperatures of about 35 to 50.degree. C. are preferred.
 Excessive contact times, especially at the higher temperature ranges are to
 be avoided. Reactant liquid weight hourly space velocities (WHSV) of about
 10-100 hr.sup.-1, preferably 25 to 50 hr.sup.-1 are employed. These
 figures are based on the large pore zeolite content of the catalyst
 contact solid.
 The mol ratio of olefin to carboxylic acid can vary widely, ratios of
 0.1-10 mols olefin per mol carboxylic acid are generally useful, ratios of
 0.125 to 0.5 mols olefin per mol carboxylic acid are especially useful.
 The higher amounts of acid relative to olefin improve reaction selectivity
 and further aid in suppressing dimer make.
 Where a selectivity enhancing modifying agent such as described in
 copending application Ser. No. 08/816,704 filed Mar. 13, 1997 is used,
 reaction conditions outside the above ranges can be used. For example,
 higher temperatures and longer contact times can be employed as can higher
 olefin to carboxylic acid ratios where the modifier is used. For example,
 reaction temperatures in the upper part of the 20-80.degree. C. range can
 be used and liquid hourly space velocities below 10 hr.sup.-1, eg. as low
 as 4 hr.sup.-1 can be used where a selectivity enhancing modifier is used.
 Especially preferred selectivity enhancing modifiers, where one is used,
 are t-butyl alcohol, t-amyl alcohol and the lower (C.sub.1 -C.sub.3) alkyl
 ethers thereof such as methyl tertiary butyl ether, methyl tertiary amyl
 ether, and the like. Where used, at least 1 wt % of modifier based on the
 feed is used up to about 50 wt %.

The following examples illustrate the invention:
 A series of esterification runs were carried out using large pore zeolite
 catalysts and reacting isobutylene and acetic acid to form t-butyl
 acetate, the runs were carried out at 500 psig. The following table shows
 the catalyst employed, the feed reactant composition, and the results
 obtained:
 TABLE 1
 Molar
 Reaction WHSV AcOH: % i-C.sub.4 % Sel. % Sel. %
 Sel. % Sel.
 Run Catalyst Temp., .degree. C. hr..sup.-1 i-C.sub.4 Conversion TBAC
 DIB TIB OTHERS
 1 21.0 g A 75 5.5 2.0:1 85 17.4 42.6 26.7
 13.3
 2 21.0 g A 60 5.3 1.7:1 62 77.2 21.5 1.2
 0.2
 3 21.0 g A 50 5.6 1.9:1 61.6 89.6 10.0 0.3
 0.1
 4 21.0 g A 40 5.6 1.8:1 56.9 93.5 6.3 0.2
 --
 5 20.0 g B 80 2.88 7.4:1 5.8 96.3 3.7 -- --
 6 5.0 g A 60 11.5 8.2:1 81.0 55.6 34.8 8.2
 1.4
 15.0 g B
 7 5.0 g A 60 23.2 8.2:1 79.1 76.2 21.0 2.6
 0.3
 15.0 g B
 8 5.0 g A 50 23.7 8.4:1 82.4 94.1 5.6 0.3
 --
 15.0 g B
 9 5.0 g A 40 23.4 8.6:1 86.2 97.8 2.1 0.1
 --
 15.0 g B
 10 5.0 g A 40 36.0 8.2:1 73.5 97.9 2.0 0.1
 --
 15.0 g B
 11 5.0 g A 35 23.7 8.4:1 81.0 98.3 1.6 0.1
 --
 15.0 g B
 12 5.0 g A 35 35.4 8.1:1 69.6 98.4 1.6 --
 --
 15.0 g B
 13 5.0 g A 50 35.8 8.2:1 73.7 95.7 4.1 0.2
 --
 15.0 g B
 14 5.0 g A 60 23.4 8.4:1 88.4 8.8 13.9
 72.5 4.7
 15.0 g B
 15 5.0 g A 40 23.7 8.4:1 85.1 69.7 16.1
 13.5 0.7
 15.0 g B
 16 5.0 g A 40 36.0 7.7:1 79.6 79.2 12.9 7.5
 0.4
 15.0 g B
 17 2.5 g C 40 48.3 8.6:1 84.7 83.4 11.3 5.0
 0.3
 17.5 g B
 18 2.5 g C 40 71.1 8.1:1 75.9 86.1 10.1 3.5
 0.2
 17.5 g B
 19 5.0 g C 40 23.8 8.1:1 83.6 71.6 14.4
 13.2 0.8
 15.0 g B
 20 5.0 g C 40 35.8 8.2:1 85.0 82.1 10.9 6.6
 0.4
 15.0 g B
 21 5.0 g C 40 34.9 8.0:1 82.9 93.5 5.5 0.9
 0.1
 15.0 g B
 22 2.5 g C 40 69.8 8.0:1 65.1 95.3 4.6 0.1
 --
 17.5 g B
 In the above tables,
 ACOH is acetic acid
 i-C.sub.4 is isobutylene
 TBAC is t-butyl acetate
 DIB is diisobutylene
 TIB is triisobutylene
 Space velocity is based on zeolite content of contact solid
 Catalyst A is Zeolite Y
 Contact solid B is inert alumina
 Catalyst C is Zeolite beta
 In the above, Run 1 is comparative showing the high production of
 isobutylene polymers at high temperatures and low space velocity.
 In Runs 2-4, 10 wt % tertiary butyl alcohol was added to the feed as a
 selectivity enhancing modifier. As a result of the use of the modifier,
 selectivity to the ester was greatly improved at the conditions employed,
 i.e. low space velocities. The data also show the pronounced effect on
 selectivity of reduction of the reaction temperature.
 Run 5 was carried out without the large pore zeolite, only inert solid
 alumina was used. Only minimal conversion was achieved.
 Runs 6-13 were carried out with a mixture of Zeolite Y and inert alumina as
 contact solid. The data show the excellent results achieved without
 modifier, and indicate the beneficial effects of reducing reaction
 temperature and decreasing contact time on ester production.
 Runs 14-22 were carried out using a mixture of Zeolite beta and inert
 alumina as the contact solid.
 Run 14 illustrates the relatively poor results obtained at higher
 temperatures (60.degree. C.) as compared with operation at lower
 temperature (40.degree. C.) in Run 15. A comparison of Runs 15 and 16
 shows the effect of lowering contact time on ester selectivity.
 In Runs 21 and 22, the feed contained 2 wt % tertiary butyl alcohol as
 selectivity enhancing modifier. A comparison of results obtained in these
 runs with the results in Run 20 and 18 shows the substantial effect of the
 modifier on ester selectivity.