United States Patent Office Patented Nov

3,007,959 United States Patent Office Patented Nov. 7, 1961 2 temperature between about 140 C. and 350° C. The 3,007,959 reaction is carried out until the desired degree of conver ESTERFICATION OF TEREPHTHALIC ACD N sion of carboxyl groups or equilibrium condition has been THE PRESENCE OF CADMUM, SULFATE CAT ALYST reached. This time is dependent upon the particular acid Delbert H. Meyer, Highland, Ind., assignor to Standard and particular alcohol present in the reaction Zone. In Oil Company, Chicago, I., a corporation of Indiana general, the higher the temperature maintained in the No Drawing. Filed Jan. 22, 1959, Ser. No. 788,275 reaction zone the shorter the time needed to reach the 2 Claims. (C. 260-475) desired degree of carboxyl group conversion or equilib rium condition. This invention relates to the esterification of aromatic O Theoretically only 1 mol of alcohol is needed for each carboxylic acids with primary aliphatic alcohols. More mol of carboxyl groups to be esterified, however the de particularly, the invention relates to an improved catalyst gree of conversion and the conversion rate are improved for this reaction. by the presence of excess alcohol in the reaction Zone. Esters of aliphatic alcohols and aromatic carboxylic In general between about 2 and 30 mols of alcohol are acids are widely used in industry, particularly as plasti 15 present in the reaction zone per mol of carboxyl groups cizers and as intermediates in the preparation of poly to be esterified. ester resins. The rate of esterification is usually quite The amount of catalyst present may be very small, for slow and catalysts have been used to increase this re example 0.0005 weight percent based on aromatic acid action rate. The commonly used active catalysts such as or anhydride charged to the reaction zone. Or the sulfuric acid, hydrochloric acid, and aluminum Sulfate 20 amount used may be quite large amounting to five or promote a very rapid reaction but have the serious draw more weight percent based upon acid or anhydride back in that considerable alcohol is lost by ether forma charged. When operating with terephthalic acid and tion. methanol the catalyst usage is generally between about An object of the invention is a catalytic process for 0.01 and 0.5 weight percent based on terephthalic acid. esterifying aromatic carboxylic acids with primary ali 25 As a specific illustration of a preferred embodiment of phatic alcohols. Aparticular object of the invention is the invention, conditions for esterifying terephthalic acid a catalytic process for esterification of aromatic carbox with methanol are described. The terephthalic acid and ylic acids with primary aliphatic alcohols wherein there methanol are charged to the reaction Zone in a weight is no significant loss of alcohol to ether formation. Other ratio of methanol to acid of between about 2 and 5; this objects will become apparent in the course of the detailed 30 corresponds to between 10 and 26 mols of methanol per description. mol of terephthalic acid charged. It is preferred to carry In accordance with the objects above, it has now been out the esterification reaction at a temperature between discovered that rapid esterification reaction rates may be about 240° C. and 300° C. The amount of catalyst pres achieved without incurring the penalty of high ether for ent appears to have no significant effect on the reaction mation by conducting the esterification of aromatic car 35 rate as long as some catalyst is present. A carboxyl boxylic acids (or anhydrides thereof) with primary ali group conversion of about 90-95% of equilibrium is at phatic alcohols in the presence of a cadmium sulfate cata tained in a time of between about 10 minutes and 60 lyst. The cadmium sulfate may be anhydrous CdSO4 or minutes, with the longer times corresponding to lower any of the various hydrates of cadmium Sulfate, such as temperatures. CdSO4.4H2O or 3CdSO4.8HO. 40 Various illustrative embodiments of the present inven The improved catalyst of this invention is useful in in tion are set forth in the examples below. creasing the reaction rate of all aromatic carboxylic acids EXAMPLES or anhydrides thereof with primary aliphatic alcohols which contain from 1 to about 16 carbon atoms, Ex Studies were made on the degree of conversion of amples of suitable aromatic carboxylic acids are: benzoic, terephthalic acid and methanol at various temperatures toluic, phthalic, phthalic anhydride, isophthalic, tereph 45 and times without and with catalyst of the invention pres thalic, trimesic, trimelitic, trimelitic anhydride, hemimel ent in the reaction Zone. The catalyst was anhydrous litic, hemimellitic anhydride, prehnitic, mellitic, naph cadmium Sulfate. These studies were carried out using thoic, naphthalic anhydride, diphenic, diphenic anhy as the reaction Zone a glass tube having about 13.5 ml. dride. Suitable primary aliphatic alcohols are: methanol, volumetric space. The terephthalic acid and catalyst ethanol, n-propanol, n-butanol, isobutyl, n-hexyl, n-octyl, 50 were weighed and the methanol was delivered volumetri lauryl, myristyl (n-dodecyl), cetyl (n-hexadecyl), iso cally using a hypodermic syringe. The glass tube was octyl (Cs OXo), nonyl (C. Oxo), decyl (Co Oxo) and then sealed by fusing the opening. The sealed tube was tridecyl (C13 OXo). As used herein Oxo alcohols are placed in a constant temperature bath provided with a understood to be the product of the reaction of an olefin rocking and oscillating mechanism which mechanism kept or mixture of olefins with carbon monoxide and subse 55 the contents of the tube well intermingled. After the re quent hydrogenation of the aldehydes formed in the oxo action Zone had been agitated in the bath for the desired adition reaction. Many of these Oxo alcohols are now time it was removed from the bath and allowed to cool commercial products; the 8, 9, 10 and 13 carbon atom rapidly to room temperature. After the tube had reached containing mixture of alcohols derived from a mixture of room temperature it was opened and the contents trans Cit, Ca, Ca and C12 olefin isomers respectively are avail 60 ferred to a beaker; the tube was washed with 100 ml of able as isooctyl alcohol, nonyl alcohol, decyl alcohol and ethyl alcohol and 100 ml. of benzene to insure removal tridecyl alcohol, respectively. The catalyst of the instant of all the material therefrom. The degree of conversion invention is particularly suitable for reacting phthalic of the carboxyl groups to ester groups was determined by acids with methanol. titration with 0.02 normal aqueous sodium hydroxide 65 using phenol red as the indicator; when it was believed Although it is to be understood that the cadmium sul that the conversion would be low, by presence of consid fate catalyst of the invention is suitable for use with the erable solid material in the tube, the titration was carried defined acids and alcohols at various well known condi out with 0.1 normal sodium hydroxide. tions of temperature, time of reaction, and ratios of alco The presence of even small amounts of dimethyl ether hol to acid in the starting reaction mixture, for complete in the tube is readily detected by the presence of pressure neSS a Summary of operating conditions is set out herein. 70 Within the tube when the tube is opened. Escaping gas The esterification reaction is ordinarily conducted at a eous material at the moment of opening the tube is quite 8,007,959 3 4. apparent. Using this method of observation it is believed gives appreciable conversion in only 5 minutes. More that no dimethyl ether was formed during the tests made over, the conversion rate remains high during even the and reported herein. latter portions of the runs. The results of several tests are reported in the table. I claim: In the first two tests 3 parts by weight of methanol were 1. In a process for preparing dimethyl terephthalate by present for each part of terephthalic acid (TPA). These reacting terephthalic acid with methanol at a temperature two tests were carried out at about 260 C. and for 64 between about 140° C. and 350° C. and at a pressure of minutes in order to compare the extent of esterification at least about atmospheric, the improvement which com at a time equivalent to the usual commercial batch ester prises carrying out said reaction in the presence of a ification time. cadmium sulfate catalyst. Tests 3-6 were conducted at the higher temperature of 10 2. A process for preparing dimethyl terephthalate 260° C. and with a 4:1 methanol-acid ratio, but with only which comprises heating terephthalic acid and methanol, one-fourth the catalyst of Test 2. in a weight ratio of methanol to acid of between about 2 Table and 5, at a temperature between about 240° C. and 300 15 C., in the presence of between about 0.01 and 0.5 weight Reactants, Parts percent, based on reactants, of cadmium sulfate, for a by Wt. CoiSO4, Carboxyl time between about 10 minutes and 60 minutes, and sep Test Wt. per- Temp., Time Grps. No. cent OIl o C. Ill. Con arating dimethyl terephthalate from unreacted charge and Meth- Tereph- Tereph- verted, other conversion products. anol thalic thalic Acid percent Acid 20 References Cited in the file of this patent

3 None 20 64 39 UNITED STATES PATENTS 3 21 64 77 4 1. 0.25 260 5 45.5 2,491,660 Gresham ------Dec. 20, 1949 4. 0.25 260 0 68.5 '4 0.25 260 20 78.2 2,828,290 Caldwell ------Mar. 25, 1958 4. 1 0.25 260 30 87 25 2,850,483 Ballentine et al. ------Sept. 2, 1958 Test 2, employing the improved catalyst of the inven FOREIGN PATENTS tion, shows that 77% of the carboxyl groups were con 879,715 France ------Nov. 30, 1942 verted whereas, under the same conditions in Test 1 but 30 omitting the catalyst, the conversion was only 39%. OTHER REFERENCES Tests 3-6, which were conducted at 260° C., but with Groggins: “Unit Processes in Organic Synthesis,” p. only one-fourth the catalyst, show that cadmium sulfate 702, McGraw-Hill, 1958.