CATALYTIC CONVERSION OF PROPANE TO ETHYLENE OVER ZSM-23

Abstract: A process for converting propane to ethylene over zeolite ZSM-23. The ZSM-23 may be contacted with an anhydrous acidic oxide gas capable of accepting hydrogen by reacting therewith to enhance ethylene selectivity. The zeolite may be pretreated with this acidic oxide gas or contacted in situ by cofeeding the acidic oxide gas with propane.

Description: Ethylene is prepared commercially by heating ethane, propane, higher paraffins or naphtha, diluted with steam, at about 850°C, 1550°F, for very short contact times, without a catalyst. Highest ultimate yields come from ethane (81%), propane (43%) and n-butane (41.5%). All world-scale plants are based on this thermal cracking/dehydrogenation technology. Although a host of rival schemes has been studied, none have reached commercial application. Weaknesses in the established process are (1) high reaction temperature and low hydrocarbon partial pressure, (2) low product separation/purification temperatures -100 to -130°C (-150 to -200°F) and high pressure 3500 kPa (500 psig), (3) relatively low yields from C₃ and higher feeds, (4) a complex mixture of products, and (5) relatively high capital and operating costs. A more efficient way of converting propane to ethylene has now been discovered. Accordingly, the present invention provides a process for converting propane to ethylene by contact with a zeolite catalyst characterized by using ZSM-23 as the catalyst. ZSM-23 is described in U.S. Patent Nos. 4,076,842 and 4,490,342. ZSM-23 possesses a crystalline structure whose X-ray diffraction pattern shows substantially the significant lines set forth in Table I. The above values were determined by standard techniques. The radiation was the K-alpha doublet of copper, and a scintillation counter spectrometer with a strip chart pen recorder was used. The peak heights, I, and the positions as a function of 2 times theta, where theta is the Bragg angle, were read from the spectrometer chart. From these, the relative intensities, 100 I/I o , where I o is the intensity of the strongest line or peak, and d(obs.), the interplanar spacing in Angstrom units, corresponding to the recorded lines, were calculated. This X-ray diffraction pattern is characteristic of all the species of ZSM-23 compositions. Ion exchange of the sodium ion with cations reveals substantially the same pattern with some minor shifts in interplanar spacing and variation in relative intensity. Other minor variations can occur depending on the silicon to aluminum ratio of the particular sample, and on whether it has previously been subjected to thermal treatment. ZSM-23 may have a formula, in terms of mole ratios of oxides, as follows: (O-2)M 2/n O:(O-5)Al₂O₃:1OOSiO₂ wherein M is a cation of valance n (e.g., alkali metal or alkaline earth metal cations). In accordance with the present invention, the zeolite catalyst contacts an anhydrous acidic oxide gas capable of accepting hydrogen by reacting therewith. Such gases include oxidative dehydrogenation agents such as sulfur dioxide (SO₂) and nitrous oxide (N₂O). Such oxide gases may contact the zeolites by a pretreatment procedure, e.g., prior to any catalytic use, or be cofeed with the propane. The zeolites suitable for use in accordance with the present invention may be combined with another material resistant to the temperatures and other conditions employed in the present organic conversion process. Such matrix materials include active and inactive materials and synthetic or naturally occurring zeolites as well as inorganic materials such as clays, silica and/or metal oxides, e.g. alumina. The latter may be either naturally occurring or in the form of gelatinous precipitates, sols or gels including mixtures of silica and metal oxides. Use of a material in conjuction with the zeolite, i.e. combined therewith, which is active, may enhance the conversion and/or selectivity of the catalyst in certain organic conversion processes. Inactive materials suitably serve as diluents to control the amount of conversion in a given process so that products can be obtained economically and orderly without employing other means for controlling the rate of reaction. Frequently, crystalline silicate materials have been incorporated into naturally occurring clays, e.g. bentonite and kaolin. These materials, i.e. clays, oxides, etc., function, in part, as binders for the catalyst. It is desirable to provide a catalyst having good crush strength, because in use the catalyst may be subjected to rough handling, which tends to break the catalyst down into powder-like materials which cause problems in processing. Naturally occurring clays which can be composited with the zeolite include the montmorillonite and kaolin families which include the subbentonites, and the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays, or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. In addition to the foregoing materials, the zeolite catalyst hereby synthesized can be composited with a porous matrix material such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix can be in the form of a cogel. A mixture of these components could also be used. The relative proportions of finely divided crystalline silicate and matrix vary widely with the crystalline silicate content ranging from 1 to 90 percent by weight, and more usually in the range of 2 to 50 percent by weight of the composite. Conditions for converting propane in accordance with the present invention include a temperature of 100°C to 700°C, a pressure of 0.1 atmosphere to 60 atmospheres, and a weight hourly space velocity of 0.5 to 400. The feedstock propane may contain up to 98% of a diluent gas, especially an inert diluent gas. The propane feed may also contain a small percentage, e.g., 1 percent by weight or less, of impurities associated with propane feedstocks such as butane. EXAMPLE 1 Zeolite ZSM-23 was synthesized with the diquaternary ammonium salt Diquat-7 dibromide [Br(CH₃)₃N(CH₂)₇N(CH₃)₃Br]. The aluminosilicate hydrogel was prepared as in Example 1 of U.S. Patent No. 4,490,342. This hydrogel was transferred to a stainless steel autoclave. The reaction mixture in the autoclave was stirred at autogenous pressure for 5 days at 160°C. The autoclave was then quenched in an ice bath to terminate the crystallization. The crystalline product was filtered, washed with deionized water, then dried. X-ray diffraction analysis showed the crystalline product to be 100% crystalline zeolite ZSM-23. From chemical analysis of this product SiO₂/Al₂O₃ = 40.8. Scanning electron microscope, SEM, photos of the ZSM-23 crystals showed that their sizes ranged 0.1-0.2 microns. EXAMPLE 2 The synthesis of this ZSM-23 material was conducted in essentially the same manner as Example 1 except that the SiO₂/Al₂O₃ ratio of the hydrogel was adjusted to 150. The autoclave was stirred at autogenous pressure for three days at 160°C before quenching in an ice bath. X-ray powder diffraction analysis of this dried zeolite product showed it to be 100% crystalline ZSM-23. SEM examination of the ZSM-23 crystals revealed their size ranged 0.5-1.5 microns. The chemical analysis of the zeolite gave SiO₂/Al₂O₃ = 145. EXAMPLE 3 This HZSM-23 zeolite catalyst was crystallized with a pyrrolidine/ethanol mixture (7:6 wt ratio) in a thirty gallon autoclave at 320°F in 48 hours. The zeolite was processed using precalcination, in nitrogen at 538°C (1000°F), ammonium nitrate exchange, and final calcination in air at 538°C (1000°F). The chemical and sorption results are as follows: EXAMPLE 4 The ZSM-23 samples, synthesized by the methods of Examples 1 and 2, were calcined at 550°C and NH₄⁺-exchanged in NH₄Cl in the same manner. After NH₄⁺-exchange, the zeolite was filtered, washed with deionized water to remove excess NH₄Cl, then finally dried in an air stream under an infrared heat lamp. In Examples which follow, the zeolites of the previous Examples were used to convert propane. Reagent grade propane containing 0.9% n-butane was used without further purification. Corrections were made for the butane in runs with low conversion. Five to ten grams of pressed wafers of catalyst, crushed and screened to 0.8 to 1.2 mm (14-20 mesh), were used in glass screening reactors. Effluent gas was sampled in a hot syringe and analyzed for hydrocarbon. A second sample was analyzed with an argon carrier gas to measure hydrogen. Material balances of ±5% were usually obtained. When sulfur dioxide was used, it was used in accordance with the following procedure: A. Sulfur dioxide, 20 cc/min, was passed over the catalyst for 30-60 min at 300°C, followed by calcination in air for 30-60 min at 500°C. B. After treatment of the catalyst as described in A, above, 1-3 wt% SO₂ was added to the propane feed for the screening reaction. Accordingly SO₂, was used to modify the catalyst. It was also added to the propane feed to determine whether it would function as a hydrogen acceptor, producing hydrogen sulfide and water, and thereby increase conversion and selectivity, Equations 1 and 2. Surprisingly, ethylene was the major product, rather than propylene. The large amount of methane produced suggested the well-known cracking reaction to give these products, Equation 3. CH₃CH₂CH₃⇄CH₃CH=CH₂ + H₂ (1) 3CH₃CH₂CH₃ + SO₂→3CH₃CH=CH₂ + H₂S + 2H₂O (2) CH₃CH₂CH₃→CH₂=CH₂ + CH₄ (3) EXAMPLE 5 The HZSM-23 of Example 3 was evaluated for the conversion of propane. This HZSM-23 had a SiO₂/Al₂O₃ molar ratio of 83/1. Reaction conditions were varied to maximize ethylene selectivity and propane conversion. A dramatic increase in ethylene selectivity is observed by comparison with the unmodified catalyst. Selected runs are summarized in Table 3. In addition to ethylene and methane, propylene and butylene were also present in significant amounts. Liquid products were produced by further reactions of the olefins. Catalyst aging was more rapid than observed with HZSM-5. The catalyst was regenerated by calcination with air every 2-5 hours to minimize the effect of coke. EXAMPLE 6 The ZSM-23 of Example 2, as treated in accordance with Example 4, was evaluated for the conversion of propane. This HZSM-23 had a SiO₂/Al₂O₃ molar ratio of 145/1. ZSM-23 with a higher silica/alumina ratio (145/1) was tested for comparison with the initial catalyst (83/1). Compared with the previous run, a larger number of reaction conditions were tested to establish a more significant base line prior to modification and use of sulfur dioxide (Method B). Detailed product selectivities for certain runs are shown in Tables 4 and 5. A more modest but definite improvement in ethylene selectivity is observed with the use of SO₂ at comparable conversions (7-10%) in comparison with the previous run. Analysis of the gas stream indicated that all of the SO₂ was consumed. Hydrogen sulfide odor was detected in the product, but not quantified. The face of the sulfur has not been completely determined. The overall selectivity to ethylene was best for the 83/1 silica/alumina catalyst, Example 5. EXAMPLE 7 The ZSM-23 of Example 1, as treated in accordance with Example 4, with a SiO₂/Al₂O₃ ratio of 40.8/1 was tested for conversion of propane to ethylene. Results are summarized in Table 6. Initial runs with unmodified catalyst gave primarily aromatic and paraffinic products. Ethylene selectivities varied from only 4-13% in the 400-600°C temperature range. Although treatment with SO₂ reduced the activity and increased the selectivity to ethylene, overall performance was inferior to the previous runs. Olefins have been prepared from methanol over ZSM-5 with low activity, SiO₂/Al₂O₃ 300/1, M. M. Wu and W. W. Kaeding, J. Cat. 88 478 (1984). In the major C₂-C₄ olefins product, at reasonable conversions, ethylene is usually the smallest component (10-15 wt%). When n-butane was used with these same catalysts, propylene and C₅+ olefins were produced with only traces of ethylene. It was surprising to see up to 4.5/1 molar ratios of ethylene/propylene in the product when propane was converted over ZSM-23, Table 3. Catalysts with oxides on silica or alumina such as chromium oxide, gave propylene as the major product. In the initial experiments with ZSM-23, a dramatic increase in ethylene was observed after treatment of the catalyst with SO₂. Subsequent work revealed that untreated catalyst also gave high ethylene yields, however, definite but smaller increases (7-10%) resulted from SO₂ treatment. Best overall performance with ZSM-23 was observed with a SiO₂/Al₂O₃ ratio of 83/1, Table 3. Since propane is a large volume, fuel-valued hydrocarbon, conversion to C₂-C₄ olefins, especially ethylene, would give higher value products.

Claims: 1. A process for converting propane to ethylene by contact with a zeolite catalyst characterized by using ZSM-23 as the catalyst 2. The process of claim 1 further characterized in that the ZSM-23 is in a binder. 3. The process of claim 1 further characterized in that the ZSM-23 contacts an anhydrous acidic oxide gas capable of accepting hydrogen by reacting therewith, prior to contact with propane. 4. The process of any preceding claim wherein an anhydrous acid oxide gas capable of accepting hydrogen by reacting therewith is cofed with the propane. 5. The process of claim 3 or 4 wherein the acidic oxide gas is sulfur dioxide.

Inventors: KAEDING, WARREN WILLIAM, VALYOCSIK, ERNEST WILLIAM
Applicants: MOBIL OIL CORPORATION
Application year: 1986
Main classification: C07C4
IPC codes: C07C67, C07B61, C07C4, C07C1, C07C11, B01J29