Conversion Of Acyclic Symmetrical Olefins To Higher And Lower Carbon Number Olefin Products

PATENT
Conversion Of Acyclic Symmetrical Olefins To Higher And Lower Carbon Number Olefin Products
Pub. No.: WO/2011/126796
International Application No.: PCT/US2011/030155
Publication Date:  13.10.2011
Applicants:
UOP LLC [US/US]; 25 East Algonquin Road P.O. Box 5017 Des Plaines, Illinois 60017-5017 (US)
Inventors:
NICHOLAS, Christopher P.; (US).
MAZOYER, Etienne; (US).
TAOUFIK, Mostafa; (US).
BASSET, Jean-Marie; (US).
BARGER, Paul T.; (US).
REKOSKE, James E.; (US)
Abstract:
Processes for the conversion, under conditions and with a catalyst system effective for olefin metathesis, of hydrocarbon feedstocks comprising an acyclic symmetrical olefin (e.g., butene-2) are described. Olefin products of lower and higher carbon numbers (e.g., propylene and pentene) are formed in the presence of a catalyst comprising a solid support and a tungsten hydride bonded to alumina present in the support. This occurs despite the olefin metathesis reaction mechanism leading to a degenerative result, without any expected production of different carbon number products from acyclic symmetrical olefins.

FIELD OF THE INVENTION
[02] The invention relates to processes for the conversion of an acyclic symmetrical olefin (e.g., butene-2) to olefin products of lower and higher carbon numbers (e.g., propylene and pentene) under conditions and in the presence of a catalyst for olefin metathesis. A representative catalyst comprises a tungsten hydride bonded to alumina that is present in a support.
DESCRIPTION OF RELATED ART
[03] Propylene demand in the petrochemical industry has grown substantially, largely due to its use as a precursor in the production of polypropylene for packaging materials and other commercial products. Other downstream uses of propylene include the manufacture of acrylonitrile, acrylic acid, acrolein, propylene oxide and glycols, plasticizer oxo alcohols, cumene, isopropyl alcohol, and acetone. Currently, the majority of propylene is produced during the steam cracking or pyrolysis of hydrocarbon feedstocks such as natural gas, petroleum liquids, and carbonaceous materials (e.g., coal, recycled plastics, and organic materials). The major product of steam cracking, however, is generally ethylene and not propylene.
[04] Steam cracking involves a very complex combination of reaction and gas recovery systems.
Feedstock is charged to a thermal cracking zone in the presence of steam at effective conditions to produce a pyrolysis reactor effluent gas mixture. The mixture is then stabilized and separated into purified components through a sequence of cryogenic and conventional fractionation steps. Generally, the product ethylene is recovered as a low boiling fraction, such as an overhead stream, from an ethylene/ethane splitter column requiring a large number of theoretical stages due to the similar relative volatilities of the ethylene and ethane being separated. Ethylene and propylene yields from steam cracking and other processes may be improved using known methods for the metathesis or disproportionation of C₄ and heavier
olefins, in combination with a cracking step in the presence of a zeolitic catalyst, as described, for example, in US 5,026,935 and US 5,026,936. The cracking of olefins in hydrocarbon feedstocks, to produce these lighter olefins from C₄ mixtures obtained in refineries and steam cracking units, is described in US 6,858, 133; US 7,087,155; and US 7,375,257.
[05] Steam cracking, whether or not combined with conventional metathesis and/or olefin cracking steps, does not yield sufficient propylene to satisfy worldwide demand. Other significant sources of propylene are therefore required. These sources include byproducts of fluid catalytic cracking (FCC) and resid fluid catalytic cracking (RFCC), normally targeting gasoline production. FCC is described, for example, in US 4,288,688 and elsewhere. A mixed, olefmic C₃/C₄ byproduct stream of FCC may be purified in propylene to polymer grade specifications by the separation of C₄ hydrocarbons, propane, ethane, and other compounds.
[06] Much of the current propylene production is therefore not "on purpose," but as a byproduct of ethylene and gasoline production. This leads to difficulties in coupling propylene production capacity with its demand in the marketplace. Moreover, much of the new steam cracking capacity will be based on using ethane as a feedstock, which typically produces only ethylene as a final product. Although some hydrocarbons heavier than ethylene are present, they are generally not produced in quantities sufficient to allow for their recovery in an economical manner. In view of the current high growth rate of propylene demand, this reduced quantity of co-produced propylene from steam cracking will only serve to accelerate the increase in propylene demand and value in the marketplace.
[07] A dedicated route to light olefins including propylene is paraffin dehydrogenation, as described in US 3,978,150 and elsewhere. However, the significant capital cost of a propane dehydrogenation plant is normally justified only in cases of large-scale propylene production units (e.g., typically 250,000 metric tons per year or more). The substantial supply of propane feedstock required to maintain this capacity is typically available from propane-rich liquefied petroleum gas (LPG) streams from gas plant sources. Other processes for the targeted production of light olefins involve high severity catalytic cracking of naphtha and other hydrocarbon fractions. A catalytic naphtha cracking process of commercial importance is described in US 6,867,341.
[08] More recently, the desire for propylene and other light olefins from alternative, non- petroleum based feeds has led to the use of oxygenates such as alcohols and, more particularly, methanol, ethanol, and higher alcohols or their derivatives. Methanol, in particular, is useful in a methanol-to-olefin (MTO) conversion process described, for example, in US 5,914,433. The yield of light olefins from such processes may be improved using olefin cracking to convert some or all of the C₄⁺ product of MTO in an olefin cracking reactor, as described in US 7,268,265. An oxygenate to light olefins conversion process in which the yield of propylene is increased through the use of dimerization of ethylene and metathesis of ethylene and butylene, both products of the conversion process, is described in US 7,586,018.
[09] Despite the use of various dedicated and non-dedicated routes for generating light olefins industrially, the demand for propylene continues to outpace the capacity of such conventional processes. Moreover, further demand growth for propylene is expected. A need therefore exists for cost-effective methods that can increase propylene yields from both existing refinery hydrocarbons based on crude oil as well as non-petroleum derived feed sources.
SUMMARY OF THE INVENTION
[10] The invention is associated with processes for the production of olefin products such as propylene, from olefins in a hydrocarbon feedstock having a different carbon number. More particularly, it has been surprisingly determined that an acyclic symmetrical olefin (e.g., butene-2) can be converted to olefin products of lower and higher carbon numbers using a particular olefin metathesis catalyst system. According to present understanding, the olefin metathesis reaction results in redistribution of alkylidene radicals that would be generated upon cleavage of the carbon-carbon double bond of an acyclic olefin. For example, in the case of self-metathesis, the reaction of a single olefin reactant with itself results in rearrangement of the olefmic carbon atom substituents according to the following reaction:
2 R₁ R₂C=CR₃R₄ R₁ R₂C=CR₁ R₂ + R₃R₄C=CR₃R₄
[11] This reaction is described, for example, in US 2008/0255328, where R R₄ represent hydrogen or hydrocarbon radicals, each of which is bonded to a carbon atom of the olefmic carbon-carbon double bond. Therefore, the self-metathesis of an asymmetrical olefin such as propylene (Ri, R₂, and R₃ are all -H and R₄ is ^~CH₃), produces both a lower carbon number olefin (e.g., ethylene) and a higher carbon number olefin (e.g., butene-2), as confirmed in working examples of US 2008/0255328, utilizing an alumina supported tungsten hydride catalyst. However, in the metathesis of a symmetrical olefin, meaning the Ri and R₂ groups are the same as R₃ and R₄ without regard to the cis and trans configuration (i.e. , Ri=R₃ and R₂=R₄ or Ri=R₄ and R₂=R₃), a degenerative result is expected, as the two alkylidene fragments, generated from cleavage of the carbon-carbon double bond, are identical. This expectation is experimentally verified, for example, in the reaction of ethylene to produce ethylene, the reaction of butene-2 to produce butene-2, the reaction of hexene-3 to produce hexene-3, etc., as obtained in conventional olefin metathesis catalyst and reaction systems. Given the art-recognized understanding that the metathesis of acyclic symmetrical olefins is degenerative, one would not expect a metathesis catalyst system to effectively produce new products from the conversion of acyclic symmetrical olefins of the formula
R-i R₂C— CR-| R₂
[12] The tungsten hydride/alumina catalyst described in US 2008/0255328 for olefin metathesis was also previously shown to be effective in alkane metathesis in US 2007/129584. According to this publication, the metathesis of an alkane using the tungsten hydride/alumina catalyst, to produce the next higher and lower carbon number homologues, provides a high selectivity for the normal (unbranched) hydrocarbons.
[13] The art therefore recognizes that (i) the tungsten hydride/alumina catalyst system is effective in paraffin and olefin metathesis, and (ii) the metathesis of acyclic olefins having the same substituents on each of the olefmic carbon atoms (i.e. , "acyclic symmetrical olefins" for purposes of the present disclosure) does not appreciably form higher and/or lower carbon number products. Surprisingly, however, experimental results now directly contradict expectations based on this knowledge. In particular, it has been discovered that hydrocarbon feedstocks comprising predominantly (e.g., greater than 50% by weight of) one or more acyclic symmetrical olefins can be contacted with a particular type of catalyst having a known olefin metathesis function, under olefin metathesis conditions, to produce appreciable quantities of olefin products of differing carbon numbers (e.g., first and second olefin products having lower and higher carbon numbers relative to the acyclic symmetrical olefm(s)). The catalyst found to unexpectedly provide this result comprises a solid support and a tungsten hydride bonded to alumina present in the support.
[14] With respect to the particular acyclic symmetrical olefin, butene-2, the resulting lower carbon number product is propylene. Representative processes according to the invention can therefore advantageously produce propylene from a single carbon number olefin (e.g., a 4 carbon number olefin), rather than relying on the cross-metathesis of olefins of differing carbon numbers, as in the case of the reaction between ethylene and butylene to produce propylene. This provides a number of commercial advantages over conventional propylene production methods via olefin metathesis, including eliminating the need for sources of different feedstock components at the same location. For example, ethylene is typically obtained as a product of steam cracking, and in particular is recovered as a low boiling fraction from an ethylene/ethane splitter. Butylene, on the other hand, may be obtained from crude oil refining operations or non-petroleum based processes. While sources of both ethylene and butylene may be present at a given location, this is not necessarily the case. Moreover, butylene is generally a less expensive feedstock component than ethylene, meaning that the overall economics of propylene production from butylene may be considerably improved, compared to those of conventional olefin metathesis processes involving reaction between ethylene and butylene.
[15] Accordingly, embodiments of the invention relate to processes for producing olefins, comprising contacting a hydrocarbon feedstock with a catalyst comprising a solid support and a tungsten hydride bonded to alumina present in the support. The feedstock comprises an acyclic symmetrical olefin, and contacting of the feedstock with the catalyst produces first and second olefin products, respectively, having lower and higher carbon numbers relative to the acyclic symmetrical olefin. In representative embodiments, the acyclic symmetrical olefin is present in an amount of at least 80% by weight of the total olefins in the hydrocarbon feedstock.
[16] As discussed above, more particular embodiments of the invention relate to processes for producing propylene, comprising contacting a hydrocarbon feedstock comprising predominantly butene-2 with a catalyst comprising a solid support and a tungsten hydride bonded to alumina present in the support. A per pass conversion of butene-2 according to this embodiment is from 15% to 50% by weight, and the butene-2 is converted to propylene with a selectivity of at least 45% by weight. According to any of the above embodiments, the catalyst may comprise tungsten in an amount from 1% to 10% by weight and the support may have a surface area surface area from 100 m²/g to 450 m²/g.
[17] These and other aspects and embodiments associated with the present invention are apparent from the following Detailed Description.
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