3,168,584 United States Patent Office Patented Feb. 2, 1965 2 gree of dehydrogenation and the following formation of 3,168,584 hydrogen iodide. It is especially important in determin MANUFACTURE OFAROMATIC HYDRO. CARBONS ing the overall course of the hydrocarbon conversion since Maxwell Nager, Pasadena, Tex., assignor to Shell Oil the instantaneous concentration of iodine affects the in Company, New York, N.Y., a corporation of Delaware stantaneous concentrations of the various active hydro No Drawing. Filed Jan. 17, 1963, Ser. No. 252,051. carbon species in the reaction zone. 10. Claims. (C. 260-673) A number of advantages are obtained by the present process. Thus, whereas iodinative dehydrogenation of This application is a continuation-in-part of copending hydrocarbons is endothermic, and when practiced inde application, Serial No. 43,647, filed July 18, 1960, now O pendently requires that the heat of reaction be supplied U.S. 3,080,435, issued March 5, 1963, which is directed from another source, the overall heat of the reactions in broadly to the iodative dehydrogenation of organic com volved in this process is exothermic, thereby avoiding the pounds by the use of oxygen and certain molten metal necessity of heat transfer to the reaction zone. More iodides. over, since the iodinative dehydrogenation reaction alone This invention relates to an improved process for the is equilibrium limited, rapid reaction of the hydrogen production of aromatic hydrocarbons, more particularly iodide with the metal oxide shifts the reaction to a higher dehydrocoupling and cyclizing of lower aliphatic hydro dehydrogenation conversion of the initial hydrocarbon; carbons. this is particularly valuable for the dehydrogenation of It has now been found that particular aromatic com light hydrocarbons wherein the temperatures normally pounds can be produced in high yields at moderate con 20 required for high equilibrium conversion are also conduc versions by dehydrogenating and cyclizing simple aliphatic tive to thermal cracking and fragmentation. Furthermore, hydrocarbons. The present invention can, therefore, be the maintenance of only a very low concentration of hy used to manufacture aromatic compounds such as ben drogen iodide in the reaction zone by its reaction with zene, naphthalenes, xylenes, and toluene in commercial metal oxide minimizes the necessity for a rapid quench of quantities using feed compositions of simple, non the reactor effluent as well as the recovery and recycling aromatic hydrocarbons. The process of the present in of the relatively large amounts of iodine species normally vention is particularly useful in the preparation of high required. Still further, the molten salt mass together purity para-xylene, although it may be adapted for the with the in situ formation of the iodine, provides for a preparation of various alkyl and alkenyl substituted aro highly effective contacting of the iodine and the hydro matic hydrocarbons of from 7 to 30 carbon atoms. 30 carbon, provides for quickly bringing the hydrocarbon According to the process of the present invention, Ca, reactant to the desired reaction temperature, and provides C4 or C5 acyclic hydrocarbons or mixtures composed pre an excellent heat transfer medium for transferring the dominantly of two or more of them and oxygen are excess heat from the reaction zone. Mixtures of salts intimately contacted with a molten alkali metal iodide to may be utilized which are either all in the molten state or simultaneously dehydrogenate, couple, cyclize, and aro 5 wherein at least one is molten and the remainder are matize the simpler hydrocarbons to aromatic compounds. Suspended in the molten plasma. In any case, the amount Thus, mixtures of two or more C to C5 acyclic hydro of Solid or molten iodide present in the molten salt mix carbons (including the dimers and trimers of such C to ture is sufficient to provide, when oxidized, the necessary Cs hydrocarbons) or a mixture of one or more of these amount of iodine for reaction and also metallic oxide acyclic hydrocarbons and a product of the dehydrocou 40 Sufficient to substantially completely remove hydrogen pling cycloaromatization of the hydrocarbon may be em iodide from the reaction mixture as it is formed. ployed. Various sources of C to. C hydrocarbons may The process of the present invention is based upon the be utilized, such as the dimers and trimers of C, CA and discovery that the reactions involved can be controlled C5 hydrocarbons. These dimers and trimers may be so as to produce aromatic hydrocarbons directly from C. used directly in place of the Cato C5 hydrocarbons them to Csaliphatic hydrocarbons while virtually excluding the selves, or may be used in monomeric form. The simple production of other, less desirable, compounds. Previ hydrocarbon feed stream may contain saturated and/or ously, it had been observed that the production of aro unsaturated compounds such as propane, propylene, n matic hydrocarbons by dehydrocoupling and cyclization butane, isobutane, butene-1, butene-2, isobutylene, iso of C3 to Cs hydrocarbons in a molten alkali metal iodide pentane, n-pentane, and isoamylenes. oxygen system was accompanied by the production of sub Although the exact nature of the reactions involved is stantial amounts of poly-olefins and tar-like materials be not completely understood, the results of studies of the cause of the random nature of the removal of hydrogen effect of varying different variables, such as contact time, atoms from the simple hydrocarbons. For example, in relative proportion of oxygen, temperature, relative copending application Serial No. 43,647 previously re amounts of metal oxide used, etc., indicate that three 5 5 ferred to, using an isobutane feed, a Li-LiO system, different reactions are involved in the process and are an air/butane ratio of 5-6 (volume basis), and a reactor occurring together in the reaction zone: (a) reaction of temperature of 1050 F., only small amounts of aromatic oxygen with the metal iodide to form free iodine and the hydrocarbons (mostly benzene and toluene in amounts corresponding metal oxide, and/or hydroxide; (b) re of from 0.7% to 1.2% based upon the moles of iso action of the liberated free iodine with the feed hydro 60 butane fed to the reactor) were produced, the major prod carbon to form hydrogen iodide and hydrocarbon of lower lucts being butylenes and propylene. However, by em H:C ratio; and (c) reaction of the hydrogen iodide, with ploying a hydrocarbon feed composed of 75% by weight metal oxide to form metal iodide and water. Since the of isobutane and 25% by weight of isobutylene, with a (b) reaction may be controlled in part by the concentra reactor temperature of 1060 F., and a molar ratio of tion of free iodine, and this is dependent upon the pro 65 oxygen to feed of from 0.5 to 10, 18% by weight of the portion of oxygen and reaction (a), the extent of the reactor feed was converted to xylenes (mostly para (b) reaction may be controlled indirectly by varying the Xylene); only 0.2% by weight of toluene was produced. proportion of oxygen delivered to the molten iodide mass. By reversing the ratio of the feed reactants (25% by The proportion of oxygen added largely determines the weight of isobutane and 75% by weight of isobutylene), course and extent of the other reactions including the 70 but using the same reactor temperature (1060° F.), formation of metal oxide, the release of iodine, the de-- 17.6% by weight of xylenes were produced, indicating. 3,168,584 3 4. that for these particular feed components (isobutane and reactor temperatures of more than 1100 F. By “con isobutylene) the selectivity is not dependent upon the tact time' is meant the average residence time of the relative amounts of isobutane and isobutene in the feed. hydrocarbon reactant e.g., butane, isobutane, propane, Thus, the use of a smaller proportion of oxygen, rela propylene, isobutylene, diisobutylene or isoamylene in tive to hydrocarbon, resulted in a marked increase in contact with the molten salt in the reaction zone. In aromatic yield. For aromatic production, the oxygen 5 general, the purity of the p-xylene produced is improved to hydrocarbon feed ratio should be maintained for a by decreasing the residence time of the iso-C or total conversion per pass of less than 50%; a range of 30 iso-C4 (indicating an unsaturated hydrocarbon such 40% is particularly useful for aromatic xylene produc as isobutylene) in the molten salt reaction zone and/or tion. For 100% conversion of i-butane to xylene the 0 by decreasing the temperature. These conditions were Stoichiometric oxygen requirement would be 1.25 mole also unexpectedly found to result in an increase in the oxygen per mole of i-butane. Thus, a preferred O/i-C conversion of the iso-C4's for a given i-CA/O ratio. The molar ratio is from about 0.25 to about 0.5. improvement in purity is believed to be due to minimiz Similarly, p-Xylene is produced from diisobutylene by ing purely thermal conversion while increase in conver heating it at temperatures of from 850° to 1050 F. 15 sion is attributable in large part to improved gas-liquid using a feed ratio, moles of oxygen per moles of diiso contacting at the higher gas velocities used to obtain butylene, of from 0.2 to 1.0 in a LiI molten salt system. shorter residence times. Some xylene may be produced Selectively (for the production of p-xylene) is favored thermally (without the interaction with iodine or the by the injection of steam into the system. The steam Li salt) and the mole ratio of the para-xylene to the produces LiCH so that a Li-LiOH system is involved.