The present invention relates to the introduction of a desulfurization metal into select zeolitic reforming catalysts to improve the sulfur tolerance of the catalysts. In particular, the present invention provides a way to delay the onset and slow the rate of sulfur poisoning of certain unexpectedly sulfursensitive zeolite-based reforming catalysts. The select reforming catalysts which are improved by the introduction of a desulfurization metal are characterized by a largepore zeolite and a Group VIII catalytic reforming metal.
U.S. Pat. No. 4,104,320 issued to Bernard et al. on Aug. 1, 1978, describes a dehydrocyclization process in which aliphatic hydrocarbons are contacted in the presence of hydrogen with a reforming catalyst consisting essentially of a type L-zeolite having exchangeable cations of which at least 90% are alkali metal ions selected from the group consisting of ions of lithium, sodium, potassium, rubidium and cesium and containing at least one metal selected from the group which consists of metals of Group VIII of the Periodic Table of Elements, tin and germanium. At least one of the metals must have a dehydrogenating effect, so as to convert at least part of the feedstock into aromatic hydrocarbons.
A particularly advantageous embodiment of this method is a platinum/alkali metal/type L-zeolite catalyst containing cesium or rubidium because of its excellent activity and selectivity for converting hexanes and heptanes to aromatics. However, these catalysts are considered by most refiners to be too unstable for commercial use.
Accordingly, U.S. Pat. No. 4,447,316 issued to Buss on May 8, 1984 suggests improving the stability of reforming catalysts comprising L-zeolite charged by the introduction of a Group VIII metal by introducing an alkaline earth metal selected from barium, strontium, or calcium (preferably barium). The resulting more stable catalyst is suggested as an especially useful dehydrocyclization catalyst. For instance, U.S. Pat. No. 4,435,283 issued to Buss and Hughes on Mar. 6, 1984; U.S. Pat. No. 4,443,326 issued to Field on Apr. 17, 1984; and U.S. Pat. No. 4,456,527 issued to Buss, Field, and Robinson on June 26, 1984 disclose various reforming processes advantageously using the highly selective dehydrocyclization catalyst disclosed in U.S. Pat. No. 4,447,316.
In fact, aromatics are produced by several reaction paths which occur simultaneously during reforming. The more important reactions include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics.
The dehydrogenation of cyclohexane and alkylcyclohexanes to benzene and alkylbenzenes is the most thermodynamically favored of the aromatization reactions. This means that dehydrogenation of cyclohexanes can yield a higher ratio of aromatic product to nonaromatic reactant than either of the other two types of aromatization reactions at a given reaction temperature and pressure. Moreover, the dehydrogenation of cyclohexanes is the fastest of the three aromatization reactions.
As a consequence of these thermodynamic and kinetic considerations, the selectivity for the dehydrogenation of cyclohexanes is higher than that for dehydroisomerization or dehydrocyclization. Dehydroisomerization of alkylcyclopentanes is somewhat less favored, both thermodynamically and kinetically. Its selectivity, although generally high, is lower than that for dehydrogenation. Dehydrocyclization of paraffins is the least favored both thermodynamically and kinetically. In conventional reforming, its selectivity is much lower than that for the other two aromatization reactions.
The selectivity disadvantage of paraffin dehydrocyclization is particularly large for the aromatization of compounds having a small number of carbon atoms per molecule. Dehydrocyclization selectivity in conventional reforming is very low for C.sub.6 hydrocarbons. It increases with the number of carbon atoms per molecule, but remains substantially lower than the aromatization selectivity for dehydrogenation or dehydroisomerization of naphthenes having the same number of carbon atoms per molecule. A major improvement in the catalytic reforming process required, above all else, a drastic improvement in dehydrocyclization selectivity that can be achieved while maintaining adequate catalyst activity and stability. Accordingly, it was the primary object of the invention disclosed in U.S. Pat. No. 4,447,316 to provide a reforming catalyst and process which could be used to improve the octane rating and yield of products in the gasoline boiling range.
It follows that catalysts which are used in successful reforming processes must have good selectivity, i.e., be able to produce high yield of liquid product in the gasoline boiling range containing large concentrations of high octane number aromatics, and conversely produce low yields of light gaseous hydrocarbons. Additionally, the catalysts should have good activity and stability. Catalysts comprising a Group VIII noble metal, usually platinum, were widely known and used in reforming processes.
Thus, at the time U.S. Pat. No. 4,447,316 issued the state of the reforming art was well developed with several clearly identified areas for improvement. In particular, it was a major goal of reforming process research to develop a catalyst with improved dehydrocyclization selectivity that can be achieved while maintaining acceptable catalyst activity and stability. U.S. Pat. No. 4,447,316 provides a catalyst comprising a large-pore zeolite, an alkaline earth metal selected from barium, strontium, or calcium, and a Group VIII metal which gives superior dehydrocyclization selectivity.
However, at the time U.S. Pat. No. 4,443,326 issued it was also well known that reforming catalysts were poisoned by exposure to sulfur. Conventional catalysts comprising Group VIII catalytic reforming metals were protected by controlling the sulfur concentration of the hydrocarbon feed by catalytic hydrodesulfurization in a first stage, and by using a sulfur-sorbing feed treatment. U.S. Pat. No. 4,456,527 which issued on June 26, 1984 to Waldeen C. Buss, Leslie A. Field and Richard C. Robinson discusses this problem and discloses that a lack of stability due to sulfur sensitivity is a surprisingly acute problem when the catalyst comprises a large-pore zeolite. In fact, the sulfur levels required when using such a catalyst are an order of magnitude or more below the levels permissible for even the most sulfur-sensitive conventional reforming catalysts. The importance of sulfur control is further magnified by the fact that known methods of recovering from sulfur upsets are inadequate to remove sulfur from a large-pore zeolite reforming catalyst, particularly if the zeolite is a type L-zeolite.
U.S Pat. No. 4,456,527 suggests various sulfur removal systems that can be used to reduce the sulfur concentration of the hydrocarbon feed to below 500 ppb., including (a) passing the hydrocarbon feed over a suitable metal or metal oxide, for example copper, on a suitable support, such as alumina or clay, at low temperatures in the range of 200.degree. F. to 400.degree. F. in the absence of hydrogen; (b) passing a hydrocarbon feed, in the presence or absence of hydrogen, over a suitable metal or metal oxide, or combination thereof, on a suitable support at medium temperatures in the range of 400.degree. F. to 800.degree. F.; (c) passing a hydrocarbon feed over a first reforming catalyst, followed by passing the effluent over a suitable metal or metal oxide on a suitable support at high temperatures in the range of 800.degree. F. to 1000.degree. F.; (d) passing a hydrocarbon feed over a suitable metal or metal oxide and a Group VIII metal on a suitable support at high temperatures in the range of 800.degree. F. to 1000.degree. F.; and (e) combining the above systems. Additional sulfur removal from the recycle gas can be achieved by conventional methods used in combination with the above sulfur removal systems.
Methods which remove sulfur from the hydrocarbon feed prior to contacting the reforming catalyst address the sulfur sensitivity problem by altering the environment of the reforming process rather than by improving the tolerance of the catalyst per se. Thus, in instances where the hydrocarbon feed contains such large concentrations of sulfur, such that some sulfur will pass through the feed treatment, the stability of the catalyst remains a problem. The present invention improves the sulfur tolerance of large-pore zeolite reforming catalysts, especially those comprising a Group VIII catalytic metal.
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