BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to processes for converting light hydrocarbons (e.g., natural gas) to products containing carbon monoxide and hydrogen using supported metal catalysts. More particularly, the invention relates to ceramic oxide fiber supported catalysts and fibrous ceramic composite catalysts and their manner of making, and to processes employing such catalysts for the generation of synthesis gas.

[0004] 2. Description of Related Art

[0005] Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.

[0006] To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes.

[0007] Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.

CH ₄ +H ₂ O⇄CO+3H ₂ (1)

[0008] Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.

[0009] The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.

[0010] In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H ₂ :CO ratio of 2:1, as shown in Equation 2.

CH ₄ +½O ₂ ⇄CO+2H ₂ (2)

[0011] This ratio is more useful than the H ₂ :CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.

[0012] The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.

[0013] For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of catalysts allowing commercial performance without coke formation.

[0014] A number of process regimes have been proposed in the art for the production of syngas via catalyzed partial oxidation reactions. One such process, described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.

[0015] To overcome the relatively high pressure drop, typically associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the required high gas space velocities, various active metal gauzes or wire meshes and various porous structures for supporting the active catalyst in the reaction zone have been proposed. For example, M. Fathi et al., Catal. Today, 42, 205-209 (1998) disclose Pt, Pt/Rh, Pt/Ir and Pd gauze catalysts for the catalytic partial oxidation of methane at contact times of 0.21 to 0.33 milliseconds. Pt, Pt/5% Rh and Pt/10% Rh gauzes were tested at 700 to 1100° C. The best results were obtained at 1100 C. using the Pt/10% Rh gauze catalyst. The CH ₄ conversion was about 30%; the oxygen conversion was about 60%; the CO selectivity was about 95%; and the hydrogen selectivity was about 30%.

[0016] European Pat. App. No. 0640559A1 discloses a process for the partial oxidation of natural gas which is carried out by means of a catalyst constituted by one or more compounds of metals from the Platinum Group, which is given the shape of wire meshes, or is deposited on a carrier made from inorganic compounds, in such a way that the level of metal or metals from Platinum Group (i.e., Rh, Ru and Ir), as percent by weight, comprise within the range of from 0.1 to 20% of the total weight of catalyst and carrier. The partial oxidation is carried out at temperatures in the range of from 300 to 950° C., at pressures in the range of from 0.5 to 50 atmospheres, and at space velocities comprised in the range of from 20,000 to 1,500,000 h ⁻¹ .

[0017] European Pat. App. No. 0576096A2 discloses a process for the catalytic partial oxidation of a hydrocarbon feedstock, which process comprises contacting a feed comprising the hydrocarbon feedstock, an oxygen-containing gas and, optionally, steam at an oxygen-to-carbon molecular ratio in the range of from 0.45 to 0.75, at elevated pressure with a catalyst in a reaction zone under adiabatic conditions. The catalyst comprises a metal selected from Group VIII of the Periodic Table and supported on a carrier and is retained within the reaction zone in a fixed arrangement having a high tortuosity. The process is characterized in that the catalyst comprises a metal selected from ruthenium, rhodium, palladium, osmium, iridium and platinum, and the fixed arrangement of the catalyst is in a form selected from a fixed bed of a particulate catalyst, a metal gauze and a ceramic foam.

[0018] V. R. Choudhary et al. (“Oxidative Conversion of Methane to Syngas over Nickel Supported on Low Surface Area Catalyst Porous Carriers Precoated with Alkaline and Rare Earth Oxides,” J. Catal., Vol. 172, pages 281-293, 1997) disclose the partial oxidation of methane to syngas at contact times of 4.8 ms (at STP) over supported nickel catalysts at 700 and 800° C. The catalysts were prepared by depositing NiO-MgO on different commercial low surface area porous catalyst carriers consisting of refractory compounds such as SiO ₂ , Al ₂ O ₃ , SiC, ZrO ₂ and HfO ₂ . The catalysts were also prepared by depositing NiO on the catalyst carriers with different alkaline and rare earth oxides such as MgO, CaO, SrO, BaO, Sm ₂ O ₃ and Yb ₂ O ₃ .

[0019] U.S. Pat. No. 5,149,464 discloses a method for selectively converting methane to syngas at 650° C. to 950° C. by contacting the methane/oxygen mixture with a solid catalyst, which is either:

[0020] a catalyst of the formula M _x M′ _y O _z where:

[0021] M is at least one element selected from Mg, B, Al, Ln, Ga, Si, Ti, Zr, Hf and Ln where Ln is at least one member of lanthanum and the lanthanide series of elements;

[0022] M′ is a d-block transition metal, and

[0023] each of the ratios x/z and y/z and (x+y)/z is independently from 0.1 to 8; or

[0024] an oxide of a d-block transition metal; or

[0025] a d-block transition metal on a refractory support; or

[0026] a catalyst formed by heating a) or b) under the conditions of the reaction or under non-oxidizing conditions.

[0027] The d-block transition metals are stated to be selected from those having atomic number 21 to 29, 40 to 47 and 72 to 79, the metals scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold. Preferably M′ is selected from Fe, Os, Co, Rh, Ir, Pd, Pt and particularly Ni and Ru. The exemplary conversions, selectivities, and gas hourly space velocities are relatively low however, while reaction temperatures are relatively high, and the effects of coke formation are not addressed.

[0028] EPO 303 438 describes a monolithic catalyst (e.g., alumina on cordierite, with a Pt or Pd coating) with or without metal addition to the surface of the monolith for the partial oxidation of methane at space velocities of 20,000-500,000 hr ⁻¹ . Other suggested metal coatings of the monolith are Rh, Ir, Os, Ru, Ni, Cr, Co, Ce, La and mixtures thereof, in addition to metals of the groups IA, IIA, III, IV, VB, VIB and VIIB. Steam is required in the feed mixture to suppress coke formation on the catalyst. The partial oxidation of methane with the disclosed catalyst results in the production of significant quantities of carbon dioxide, steam, and C ₂₊ hydrocarbons.

[0029] U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. Catalysts used in that process include ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.

[0030] U.S. Pat. No. 5,648,582 discloses another process for the catalytic partial oxidation of a feed gas mixture consisting of essentially methane. The methane-containing gas feed mixture and an oxygen-containing gas are passed over a supported metal catalyst at space velocities of 800,000 hr−1 to 12,000,000 hr−1. The catalyst is rhodium, nickel or platinum on a ceramic monolith support.

[0031] One drawback of conventional ceramic supported catalysts, however, is their poor thermal shock resistance and susceptibility to failure when hot spots form within the catalyst during use. The localized presence of highly exothermic reactions during the oxidative conversion of methane (due to, e.g., combustion, gas channeling or uneven distribution of catalyst) can generate hot spots within the catalyst. When combustive reactions are present, the excess methane and the full oxidation products can react endothermically to generate hydrogen and/or CO. Under such coexisting exothermic and endothermic conditions, thermal shock can drastically shorten the lifetime of a refractory ceramic-supported catalyst. Moreover, thermal runaway conditions may also take place if the catalyst irreversibly degrades into products that selectively accelerate exothermic reactions or which reduce the incidence of endothermic reactions. Likewise, conventional metal meshes or gauzes employed as active catalysts or catalyst supports tend to melt when highly exothermic hot spots occur in the catalyst bed, which also leads to early catalyst failure on-stream.

[0032] None of the existing catalytic partial oxidation processes are capable of providing sufficiently high conversion of reactant gas and high selectivity of CO and H ₂ reaction products without employing a quantity of rare and costly catalysts, or without experiencing excessive coking of the catalyst, or without experiencing premature catalyst failure due to lack of heat resistance and mechanical instability of the catalyst or its support structure. Accordingly, there is a continuing need for better, more economical processes and catalysts for the catalytic partial oxidation of hydrocarbons, particularly methane, or methane containing feeds, in which the catalyst is mechanically stable and retains a high level of activity and selectivity to CO and H ₂ products under conditions of high gas space velocity, elevated pressure and high temperature, without experiencing excessive coking.

SUMMARY OF THE INVENTION

[0033] The catalysts and processes of the present invention overcome some of the deficiencies of existing catalysts and processes for converting light hydrocarbon feedstocks to synthesis gas. Some advantages of the new ceramic oxide fabric catalyst supports and fibrous ceramic composite catalysts are that they are more easily formed than many conventional catalyst articles and are readily scaled to fit the dimensions of any reactor. Especially significant advantages of the new catalysts are that they resist thermal shock better than conventional ceramic catalyst monoliths or supports, and avoid hot-spot induced meltdown problems that are typical of metal mesh or gauze catalysts. The new ceramic oxide fabric catalyst supports and fibrous ceramic composite catalysts may be formed into any of a variety of three-dimensional configurations, and may employ various fiber diameters, woven or braided mesh designs and layers. For instance, a catalyst bed for a reduced scale syngas production system contain a stack or layers of fabric disks formed from the ceramic oxide fabric supported catalysts or the fibrous ceramic composite catalysts.

[0034] In accordance with certain embodiments of the invention, a catalyst for catalytically converting a C ₁ -C ₅ hydrocarbon to a product comprising CO and H ₂ is provided. This catalyst, or catalyst article, which may be a fabric or textile, comprises a refractory fibrous structure containing a plurality of ceramic oxide fibers. The catalyst also has at least one active catalyst material supported by the fibrous structure. The active catalyst material has catalytic activity for partially oxidizing methane to CO and H ₂ at conversion promoting conditions, and is preferably Rh, Ni or Cr, or a combination of any of those. The activity of the catalyst article is comparable to that of conventional, more costly, Group VIII containing syngas catalysts. The fibers of the support or the composite structure are arranged in the structure in such a way that they are able to move relative to one another within the structure, whereby thermomechanical stress is relieved when the structure is exposed to temperatures greater than 1000° C. In some embodiments, the catalyst includes a refractory oxide coating on the fibrous structure, lying between the fibrous structure and the active catalyst material.

[0035] In preferred embodiments, the ceramic oxide fibers of the catalyst article comprise a refractory metal oxide that is alumina, silica, boria, cordierite, magnesia, zirconia, or a combination of any of those oxides. Certain of these embodiments contain ceramic oxide fibers comprising Al ₂ O ₃ , B ₂ O ₃ , SiO ₂ , or a combination of any of those.

[0036] In certain embodiments, the catalyst is a piece of fabric in which a group of ceramic oxide fibers are woven together 2-dimensionally. Some embodiments have fibers woven together 3-dimensionally. A stacked catalyst structure may be formed from two or more such fibrous pieces. Preferably a group of 10-12 micron diameter fibers form the fabric. In some embodiments the fibers are polycrystalline metal oxide fibers, which may be transparent and nonporous.

[0037] In certain alternative embodiments, a ceramic composite catalyst for catalytically converting a C ₁ -C ₅ hydrocarbon to a product comprising CO and H ₂ is provided which has a refractory fibrous structure containing a plurality of fibers. These fibers contain a mixture of at least one active catalyst material and at least one ceramic oxide, the active catalyst material being one with catalytic activity for partially oxidizing methane to CO and H ₂ at conversion promoting conditions.

[0038] Also provided in accordance with the present invention is a method of making a thermomechanical stress resistant catalyst for the production of synthesis gas comprising. In some embodiments, the method includes forming at least one fabric piece comprising a plurality of ceramic oxide fibers containing at least one refractory oxide such as alumina, silica, boria, cordierite, magnesia and zirconia, or mixtures thereof. The piece or pieces may then be coated with MgO. The method may include drying each such MgO coated piece, especially if there is a solvent to be evaporated. The piece or pieces (or the MgO coated piece or pieces, after calcination) are loaded with a catalyst precursor, such as a salt of a metal like rhodium, nickel, chromium, or any combination of those. Loading of the catalyst precursor may include impregnation, impregnation, wash coating, adsorption, ion exchange, precipitation, co-precipitation, deposition precipitation, sol-gel method, slurry dip-coating, microwave heating, and the like, or some other suitable method. Preferably the active catalyst material is deposited on or within the fibers or support structure by impregnation, wash coating or co-precipitation. Each metal salt coated piece is then dried, if necessary, and then calcined. Following calcination, the metal coated piece or pieces may be reduced, particularly if rhodium is a component.

[0039] An alternative method of making a thermomechanical stress-resistant catalyst for the production of synthesis gas is also provided by the present invention. In some embodiments the method comprises combining or mixing at least one refractory oxide, such as alumina, silica, boria, cordierite, magnesia or zirconia, with at least one salt of an active catalyst metal chosen from the group consisting of Rh, Ni and Cr. The method includes forming the combination into a plurality of ceramic oxide fibers, and then forming these fibers into one or more fibrous pieces. Such forming may include weaving or braiding together 2-dimensionally or 3-dimensionally at least some of the fibers. The pieces are then heated in a reducing atmosphere.

[0040] Another alternative method of making a thermomechanically stress resistant ceramic composite catalyst for the production of synthesis gas, in accordance with the present invention comprises forming a fibrous support having a predetermined 3-dimensional structure and comprising a plurality of metal oxide fibers having an organic coating and containing at least one metal oxide such as alumina, silica, boria, cordierite, magnesia or zirconia. The method includes infiltrating the support with an active catalyst precursor comprising at least one salt of a metal chosen from the group consisting of Rh, Ni and Cr, and combinations thereof. The catalyst-infiltrated fibrous support is then heated and or calcined, preferably at a temperature of 100-1000° C.

[0041] Still another alternative method of making a thermomechanically stress resistant ceramic composite catalyst for the production of synthesis gas, is provided in accordance with the present invention. Some embodiments of this method comprise forming at least one fibrous support having a predetermined 3-dimensional structure and comprising a plurality of metal oxide fibers having an organic coating and containing at least one of the metal oxides alumina, silica, boria, cordierite, magnesia and zirconia. The fibrous support may be formed by 2- or 3-dimensionally weaving or braiding together at least a portion of the metal oxide fibers. The method includes, optionally, heating and/or calcining the fibrous support or supports. Each support is then infiltrated with an active catalyst precursor comprising at least one salt of a metal chosen from the group consisting of Rh, Ni and Cr, and combinations thereof. The catalyst-infiltrated support is then heated or calcined.

[0042] A method of converting a C ₁ -C ₅ hydrocarbon to synthesis gas is also provided in accordance with the present invention. Certain embodiments of this method comprise contacting a reactant gas mixture comprising said hydrocarbon and a source of oxygen with a catalytically effective amount of a thermomechanical stress resistant catalyst, in a short contact time syngas production reactor. The catalyst comprises a refractory fibrous structure containing a plurality of ceramic oxide fibers and at least one active catalyst material disposed on or within the fibrous structure. The active catalyst material is catalytically active for partially oxidizing methane to CO and H ₂ at conversion promoting conditions, and the catalyst has sufficiently porous structure to allow reactant and product gases to flow through the catalyst at a space velocity of at least 20,000 normal liters of gas per kilogram of catalyst per hour (NL/kg/h) when the catalyst is used in a syngas production reactor. The method further includes maintaining the catalyst and reactant gas mixture at conversion promoting conditions of temperature and pressure during the contacting whereby a net partial oxidation reaction is catalyzed by the catalyst. In some embodiments the process provides at least about 65% CH ₄ conversion, about 97-100% O ₂ conversion, at least about 95% CO selectivity and at least about 78% H ₂ selectivity, the molar ratio of H ₂ and CO products being about 2.

[0043] In another embodiment of the method of converting a C ₁ -C ₅ hydrocarbon to synthesis gas, the process comprises contacting a reactant gas mixture comprising the hydrocarbon and a source of oxygen with a catalytically effective amount of a ceramic composite catalyst. The ceramic composite catalyst comprises a refractory fibrous structure containing a plurality of fibers, said fibers containing a mixture of at least one active catalyst material and at least one ceramic oxide, and said active catalyst material having catalytic activity for partially oxidizing methane to CO and H ₂ at conversion promoting conditions. The composite catalyst has sufficiently porous structure to allow reactant and product gases to flow through said composite catalyst at a space velocity of at least 20,000 normal liters of gas per kilogram of catalyst per hour (NL/kg/h) when a catalyst bed containing the composite catalyst is used in a syngas production reactor, as previously described.

[0044] In some embodiments of the method of converting a hydrocarbon to syngas, the method further includes combining at least one refractory oxide, such as alumina, silica, boria, cordierite, magnesia or zirconia, with at least one salt of an active catalyst metal such as Rh, Ni or Cr. The combination is then formed into a plurality of metal oxide fibers, which are then formed into at least one fibrous piece. These and other embodiments, features and advantages of the present invention will become apparent with reference to the following description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0045] New catalyst structures or articles, for catalytically converting C ₁ -C ₅ hydrocarbons to CO and H ₂ comprise active catalyst materials supported on ceramic oxide fibers comprised of refractory oxides such as alumina, silica, boria, cordierite, magnesia, zirconia and the like, and combinations thereof. The ceramic oxide fibers, which may have any of various fiber diameters, are arranged in a suitable 3-D form, such as a woven mesh design or layers, to provide a support structure. Preferably the support structure contains polycrystalline metal oxide fibers comprised of Al ₂ O ₃ , B ₂ O ₃ , SiO ₂ or a combination thereof. Alternatively, commercially available refractory ceramic fibers or fabrics, such as those sold under the trademark Nextel by the 3M Company (St. Paul, Minn.), may be employed as suitable structures or structural elements providing high temperature stability. An active catalyst material is disposed on or within the support structure. Preferred active catalyst materials for catalyzing the net partial oxidation of light hydrocarbons to CO and H ₂ include Ni, Rh, Cr, and combinations thereof.

[0046] The active catalyst material may be applied to the fibers or a 3-D support structure containing the fibers using well-known techniques such as impregnation, wash coating, adsorption, ion exchange, precipitation, co-precipitation, deposition precipitation, sol-gel method, slurry dip-coating, microwave heating, and the like, or some other suitable method. Preferably the active catalyst material is deposited on or within the fibers or support structure by impregnation, wash coating or co-precipitation, as demonstrated in the following examples.

[0047] Alternatively, the active catalyst components may be added to the powdered ceramic oxide compositions and then formed into fibers and woven to prepare the desired 3-D structure. The preferred polycrystalline fibers prepared in this manner are transparent, nonporous, and have a diameter of 10-12 microns. The continuous nature and flexibility of the ceramic oxide fibers allow them to be processed into a variety of textile shapes and forms using conventional weaving and braiding processes and equipment. This processability, coupled with the fibers' abrasion resistance, excellent tensile strength and refractoriness, permits the resultant textile shapes and forms to be useful as a catalyst support at temperatures greater than 1100° C.

[0048] The ceramic oxide fibers and textiles have outstanding thermal shock resistance due to the ability of the fibers to move relative to one another and relieve any thermomechanical stress, such as that which typically arises in a syngas production reactor. The supports maintain strength during and after exposure to high temperatures. The continuous nature of the ceramic oxide fibers makes them suitable for both 2-dimensional and 3-dimensional weaving or braiding of complex parts for composite supports. The preformed supports are infiltrated with the catalytic matrix by conventional impregnation techniques, chemical vapor infiltration (CVD/CVI) or matrix transfer molding. The organics and catalyst precursors are then heated and calcined away to produce the fiber-like ceramic composite catalyst. The ceramic oxide fibers have low elongation and shrinkage at operating temperatures, which allow for a dimensionally stable support. Heating and/or calcining are used to remove all of the organic compositions from the catalyst precursors when contained within the ceramic oxide fibers. This is important in applications where supports are pre-impregnated or coated with organic compounds and catalyst precursors. Preferably the heating and/or calcining treatment is conducted at temperatures ranging from 100 to 1000° C. Catalyst beds for reduced scale syngas production systems may be made up of layers of such ceramic fabric disks. Advantageously, the catalyst supports are easily formed and readily scaled to fit any reactor, and are resistant to thermal shock and consequential structural failure.

[0049] Catalyst Preparation

[0050] Representative catalyst articles were prepared, as described in Examples 1-4 below, and their activities were tested in a reduced scale syngas generation reactor, as described below under “Test Procedure” at defined high gas hourly space velocities, temperature and pressure to indicate the level of CH ₄ conversion and selectivities to CO and H ₂ products.