Chromium-rare earth based catalysts and process for converting hydrocarbons to synthesis gas

Catalysts and processes for the catalytic conversion of hydrocarbons to carbon monoxide and hydrogen employing new families of chromium-rare earth based catalysts are disclosed. One highly active and selective catalyst system, providing greater than 95% CH 4 conversion, and 97-98% selectivity to CO and H 2 by a net catalytic partial oxidation reaction, is a Ce—Cr—Ni containing compound. A preferred process for the catalytic conversion of a hydrocarbon comprises contacting a feed stream comprising a methane-containing hydrocarbon feedstock and an oxygen-containing gas with a chromium-rare earth containing catalyst in a short contact time reactor maintained at partial oxidation promoting conditions effective to produce synthesis gas.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Key components of the preferred catalysts are chromium and a rare earth element (i.e., atomic number 57 through 71 of the periodic table of the elements). The catalyst composition may also contain one or more metal compounds, the metal of which is a Group 1 (i.e., Li, Na, K, Rb and Cs) element, Co or Ni. The amount of catalytic metal present in the composition may vary widely. Preferably the catalyst comprises from about 0.1 mole % to about 90 mole % (as the metal) of chromium per total moles of catalytic metal and matrix metal, and more preferably from about 10 mole % to about 70 mole %. Preferably the rare earth component comprises from about 1% to about 90%. One or more of the catalytic components may serve as a matrix material in which another catalytic metal or metal-containing compound is dispersed. A matrix is a skeletal framework of oxides and oxyhydroxides. Alternatively, or additionally, another oxidatively and thermally stable material may serve as a matrix or a support for the active catalyst composition. Suitable matrix-forming materials are alkoxides of magnesium, silicon, titanium, tantalum, zirconium or aluminum. For example, a composition containing 10% Cr, 1% Li, 27% La and &agr;-Al 2 O 3 may be used. Test Procedure Catalysts were evaluated in a 25 cm long quartz tube short, or millisecond, contact time reactor equipped with a co-axial, quartz thermocouple well, resulting in a 4 mm, reactor i.d. The void space within the reactor was packed with quartz chips. The catalyst bed was positioned with quartz wool at approximately mid-length in the reactor. A three point, K type, thermocouple was used with the catalyst's “hot spot”, read-out temperature reported as the run temperature. The catalyst bed was heated with a 4 inch (10.2 cm), 600 W band furnace at 90% electrical output. Mass flow controllers and meters regulated the feed composition and flow rate. Prior to start-up, the flows were checked manually with a bubble meter and then the feed composition was reconfirmed by gas chromatographic analysis. The flow rates of all the meters were safety interlocked and their measurements were checked electronically by the mass flow meters every second. All runs were performed at a CH 4 :O 2 feed ratio of 2:1, safely outside of the flammable region. Specifically, the feed contained, in volume %, 30% CH 4 , 15% O 2 and 55% N 2 . Experiments were conducted at 5 psig (136 kPa) and a reactant gas/catalyst contact time of less than 10 milliseconds. The reactor effluent was analyzed by a gas chromatograph (g.c.) equipped with a thermal conductivity detector. The feed components (CH 4 , O 2 , N 2 ) and potential products (CO, H 2 , CO 2 , and H 2 O) were all well resolved and reliably quantified by two chromatography columns in series consisting of 5A molecular sieve and Haysep T. Mass balances of C, H, and O all closed at 98-102%. Runs were conducted up to two operating days, each with 6 hours of steady state run time. 
 EXAMPLE 1 Cr 0.1 La 0.9 Ox An aqueous solution of Cr 3 (OH) 2 (CH 3 CO 2 ) 7 (2.22 mL, 2.5603 M in Cr) and aqueous La(NO 3 ) 3 (42.78 mL, 1.1955 M) were simultaneously added to a 150 mL petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days to produce a freeze dried powder. The freeze dried material was heated in air at 350° C. for 5 hours prior to pelletization and use. The final catalyst had a nominal metal ratio of Cr 0.1 La 0.9 . 
 EXAMPLE 2 Cr 0.1 Ce 0.9 Ox An aqueous solution of Cr 3 (OH) 2 (CH 3 CO 2 ) 7 (1.375 mL, 2.560 M in Cr) and aqueous solution of Ce(NO 3 ) 3 .6H 2 O (43.625 mL, 0.7261 M) were simultaneously added to a 150 mL pyrex petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days to produce a freeze dried powder. The freeze dried material was heated in air at 350° C. for 5 hours prior to pelletization and use. The final catalyst had a nominal metal ratio of Cr 0.1 Ce 0.9 . 
 EXAMPLE 3 Cr 0.1 Sm 0.9 Ox An aqueous solution of Cr 3 (OH) 2 (CH 3 CO 2 ) 7 (0.969 mL, 2.560 M in Cr) and an aqueous solution of samarium nitrate (44.031 mL, 0.5069 M), the solution was formed using water and nitric acid to bring the final pH to 0.24 to dissolve Sm(NO 3 ) 3 .6H 2 O, were simultaneously added to a 150 mL pyrex petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days to produce a freeze dried powder. The freeze dried material was heated in air at 350° C. for 5 hours prior to pelletization and use. The final catalyst had a nominal metal ratio of Cr 0.1 Sm 0.9 . 
 EXAMPLE 4 10% Cr/1% Li/27% La/&agr;Al 2 O 3 An aqueous solution of LiNO 3 (1.762 g) in distilled water was added by the incipient wetness technique to an alpha-alumina support (19.723 g, calcined at 900° C. overnight before use). The solids were dried at 110° C. for two hours. An aqueous solution of La(NO 3 ) 3 .6H 2 O (22.134 g) in distilled water was added by the incipient wetness technique to the dried solids. The solids were again dried at 110° C. for two hours. An aqueous solution of Cr(NO 3 ) 3 .9H 2 O (23.087 g) in distilled water was added by the incipient wetness technique to the dried solids. Finally, the material was dried at 110° C. for two hours followed by calcination at 900° C. overnight. The final catalyst had a nominal composition of 10% Cr/1% Li/27% La/&agr;-Al 2 O 3 (wt %). 
 EXAMPLE 5 2% Cr/1% Li/27% La/&agr;-Al 2 O 3 An aqueous solution of LiNO 3 (1.762 g) in distilled water was added by the incipient wetness technique to an alpha-alumina support (22.123 g, calcined at 900° C. overnight before use). The solids were dried at 110° C. for two hours. An aqueous solution of La(NO 3 ) 3 .6H 2 O (22.134 g) in distilled water was added by the incipient wetness technique to the dried solids. The solids were again dried at 110° C. for two hours. An aqueous solution of Cr(NO 3 ) 3 .9H 2 O (4.617 g) in distilled water was added by the incipient wetness technique to the dried solids. Finally, the material was dried at 110° C. for two hours followed by calcination at 900° C. overnight. The final catalyst had a nominal composition of 2% Cr/1% Li/27% La/&agr;-Al 2 O 3 (wt %). A catalytically effective amount of the catalyst compositions of Examples 1-5 were evaluated as described in the section entitled “Test Procedure.” The results of these tests are shown in Table 1. Catalyst performance is reported at steady state and showed no evidence of catalyst deactivation after 12 hours, according to g.c. analysis. 1 TABLE 1 Catalyst Performance GHSV Example Catalyst Temp. ×10 4 % CH 4 /O 2 % CO/H 2 No. Composition V(mL) Wt.(g) ° C. (NL/kg/h) Conv. Sel. H 2 :CO % Coke 1 Cr 0.1 La 0.9 Ox 2 2.1417 770 6.1 58/100 83/73 1.8 0.08 2 Cr 0.1 Ce 0.9 Ox 0.4 0.5972 860 3.045 36/100 49/45 1.8 n.d. 3 Cr 0.1 Sm 0.9 Ox 0.4 0.5350 870 3.045 48/100 65/66 2.0 0.17 4 10% Cr, 1% Li/27% La/&agr;-Al 2 O 3 0.9 1.0235 850 6.1 90/100 97/90 1.9 n.d. 5 2% Cr, 1% Li, 27% La/&agr;-Al 2 O 3 0.4 0.5327 830 3.045 90/100 96/93 1.9 2.69 n.d. &equals; none detected The conventional view is that chromium promoters or additives promote non-selective reaction pathways for alkane oxidation reactions using molecular oxygen, O 2 . Therefore, the selective behavior of chromium oxide-based compositions as catalysts for converting methane and oxygen to CO and H 2 by a net partial oxidation reaction, as disclosed herein, is unexpected and even surprising. In one inventor's experience with n-butane oxidation, for example, it was observed that chromium promoters in vanadium phosphorus oxide catalysts increased catalyst activity at the expense of selectivity. In these cases the catalysts were compared at the same percent conversion of reactant. A similar trend was also noted by Oganowski, W. et al. (“Promotional Effect of Molybdenum, Chromium and Cobalt on a V—Mg—O catalyst in oxidative dehydrogenation of ethylbenzene to styrene,” Applied Catalysis A: General 136 (1996) 143-159.) At page 156 of that reference, the reaction chemistry is the oxidative dehydrogenation of ethylbenzene to styrene: “The molybdenum, chromium or cobalt doped V—Mg—O catalyst changes its activity and selectivity in the oxidative dehydrogenation of ethylbenzene. The specific activity decreases in the direction Cr,Co>Cr>Co>Mo while the selectivity increases in the direction: Cr>>Co,Cr,Co>Mo.” This suggests that Cr would not serve as a selective catalyst for a process involving C—H activation, such as CH 4 partial oxidation, and is contrary to the inventors' present findings. Furthermore, low carbon formation is a very unusual, unexpected, and advantageous feature of many of the new chromium catalyst systems, when employed on-stream in a short contact time reactor to catalytically convert methane to syngas. The following series of Cr—Ni and rare earth promoted Cr—Ni oxide catalysts were synthesized and tested, to illustrate trends in C—H activation. Many of these catalyst formulations also demonstrate reduction of carbon formation. 
 EXAMPLE 6 Ni 0.2 Cr 0.8 Ox (Comparative Example) 13.93 ml of Ni(NO 3 ) 2 of 1.068 M solution (prepared by dissolving Ni(NO 3 ) 2 .6H 2 O in water. Stoichiometry was determined by inductively coupled plasma compositional analysis (ICP) for elemental analysis was combined with 119 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich 31,810-8) (0.5 M in Cr), prepared by diluting a 2.5603 M solution of chromium hydroxide acetate in water). The mixed solution was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 5 hour soak at 350° C., 5° C./min to 525° C., 525° C. soak for 1 hour; 10° C./min to room temperature. The material was sieved prior to the reactor evaluation. The Ni 0.2 Cr 0.8 Ox powder was evaluated as described in the section entitled “Test Procedure.” 
 EXAMPLE 7 Ni 0.1 Cr 0.9 Ox (Comparative Example) 27.149 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,820-8) (1.6575 M in Cr) and 4.682 ml of 1.068 M Ni (NO 3 ) 2 solution (prepared by dissolving Ni(NO 3 ) 2 .6H 2 O in water (stoichiometry of the composition determined by ICP elemental analysis) were simultaneously added to a 150 ml pyrex petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days in a Virtis 25EL “Freezemobile” equipped with a Unitop 800 L unit (with refrigerated shelves) to produce a freeze dried powder. The freeze dried material was heated or calcined in air at 350° C. for 5 hrs prior to pelletization and use in a microreactor, as described in “Test Procedure.” 
 EXAMPLE 8 Ni 0.01 Cr 0.90 Ox An identical procedure was used (as described in Example 7), except that 29.864 ml of the chromium hydroxide acetate solution and 0.468 ml of the nickel nitrate solution were used. 
 EXAMPLE 9 Y 0.1 Cr 0.7 Ni 0.2 Ox An identical procedure as described in Example 7 was used. 8.554 ml of 0.9352 M yttrium nitrate solution (prepared by dissolving Y(NO 3 ) 3 hydrate (Alfa 12898) in water) was combined with 33.786 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,820-8) (1.6575 M in Cr), and 14.981 ml of Ni (NO 3 ) 2 of 1.068 M solution (prepared by dissolving Ni(NO 3 ) 2 .6H 2 O in water). 
 EXAMPLE 10 La 0.1 Cr 0.7 Ni 0.2 Ox An identical procedure to that described in Example 7 was used, except that 6.692 ml of 1.1955 M lanthanum nitrate ((La(NO 3 ) 3 aqueous solution (prepared by dissolving 503.02 g of La(NO 3 ).xH 2 O (La content 33.0 wt %; Aldrich 23,855-4) in sufficient water to make a 1.1955 M solution) was combined with 33.786 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,810-8) (1.6575 M in Cr), and 14.981 ml of 1.068 M Ni(NO 3 ) 2 prepared by dissolving Ni(NO 3 ) 2 .6H 2 O in sufficient water to make a 1.068 M solution). The molarity of the solutions were determined by ICP elemental analysis. 
 EXAMPLE 11 Ce 0.1 Cr 0.7 Ni 0.2 Ox An identical procedure to that described in Example 7 was used, except that 8.00 ml of 1.00 M cerium nitrate ((Ce(NO 3 ) 3 aqueous solution (prepared by dissolving 503.02 g of Ce(NO 3 ).6H 2 O (Alfa 11329) in sufficient water to make a 1.00 M solution) was combined with 33.786 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,820-8) (1.6575 M in Cr), and 14.981 ml of 1.068 M Ni(NO 3 ) 2 (prepared by dissolving Ni(NO 3 ) 2 .6H 2 O in sufficient water to make a 1.068 M solution). Molarity was determined by ICP elemental analysis). 2 TABLE 2 Ni-Cr and Rare Earth Promoted Ni-Cr Catalysts Example No. Composition* 6 Ni 0.2 Cr 0.8 Ox 7 Ni 0.1 Cr 0.9 Ox 8 Ni 0.01 Cr 0.99 Ox 9 Y 0.1 Cr 0.7 Ni 0.2 Ox 10 La 0.1 Cr 0.7 Ni 0.2 Ox 11 Ce 0.1 Cr 0.7 Ni 0.2 Ox *Atomic ratios metals content 3 TABLE 3 Performance of Ni—Cr and Rare Earth-Ni—Cr Catalysts GHSV Ex. Vol. Wt. Temp. ×10 4 % CO % H 2 H 2 :CO No. (mL) (g) (° C.) L/kg/h % CH 4 % O 2 Conv. Conv. Sel. Coke Sel. 6 2.0 2.6096 686 6.1 94 100 97 98 2.02 12.8 787* 4.6 91 100 97 98 2.02 12.8 571* 7.6 93 100 99 99 2.00 12.8 599* 12.2 91 100 98 98 2.00 12.8 571* 15.2 90 100 98 98 2.00 23.8** 7 2.0 2.2551 746 6.1 90 100 96 95 1.98 0.83 8 2.0 2.1817 804 6.1 80 100 91 87 1.91 0.43 9 2.0 2.0049 748 6.1 95 100 97 97 2.00 2.62 10 2.0 2.1250 758 6.1 96 100 98 97 1.98 1.93 11 2.0 2.4859 753 6.1 96 100 98 97 1.98 1.67 Compositions were evaluated for 6 hrs., except where noted otherwise. *Feed composition 90% CH 4 , 30% O 2 and 10% N 2 **Evaluated for 25 hrs. From Tables 2 and 3 it can be seen that carbon buildup is suppressed with use of the M 0.1 Cr 0.7 Ni 0.2 series catalysts (where M is a rare earth ion), compared to the greater amount of coke deposition obtained with the Ni 0.2 Cr 0.8 Ox catalyst composition FIG. 1 is a graph showing trends in light-off temperature and basicity/ionicity of representative “support” matrix compositions (i.e., Cr 0.1 La 0.9 Ox, Cr 0.1 Ce 0.9 Ox, Cr 0.1 Sm 0.9 Ox, and Cr 0.025 Mg 0.975 Ox from Examples 1, 2, 3 and 22, respectively). The predicted ionicity or basicity of the compositions increases from right to left along the x-axis of the graph. These systems were chosen for their thermal stability. In addition, rare earth oxide base catalysts have been reported for methane coupling-type reactions. The basicity of these rare earth oxide systems may facilitate C—H activation. Trends in light-off temperature, or ignition temperature, suggest that this may be the case. A lanthanum chromium oxide compound (comprised of La 2 Cr 2 O 6 &plus;Cr 2 O 3 in powder X-ray diffraction studies) possesses the lowest light-off or ignition temperature. A plot of the light-off temperature versus the expected basicity or ionicity of the rare earth component shows a correlation which suggests C—H activation may be related to this property. Thermogravimetric analysis (TGA) studies also indicate low carbon deposition for the rare earth oxide based chromium catalysts, as shown in FIG. 2 for La 0.1 Cr 0.9 Ox (Cr 2 O 3 &plus;La 2 Cr 2 O 6 by X-ray diffraction), prepared similarly to the freeze-drying methods described above. In FIG. 2 , the arrow at about 300° C. indicates a temperature region where the catalyst undergoes carbonate decomposition and appreciable weight loss occurs. Carbon deposition, as indicated by the weight loss at about rt-350° C. in N 2 is 6.548% (0.6889 mg). The weight loss from about 350-600° C. is 2.897% (0.3048 mg), and from about 600-700° C. is 0.08311% (0.008744 mg). TGA analysis of weight loss in air (>600° C.) indicates <<1 wt % carbon deposition for these catalyst systems after eight hours on stream (i.e., <0.07 wt % upon oxidation in air from 600-700° C. for La 0.1 Cr 0.9 Ox). With catalytic use the oxides of Co and Ni tend to sinter, forming metal plus ceramic oxide in situ, which contributes to coking, decreasing catalyst performance and catalyst life. This behavior is more problematic at higher operating temperatures. In light of this problem, it is particularly interesting that the new rare earth-containing Ni Cr compounds (e.g., the A 0.1 Cr 0.7 Ni 0.2 Ox series of Examples 9-11), exhibit suppression of coke formation. This quality was present even though the CH 4 /O 2 conversion and product selectivity activities demonstrated in these tests appeared to be comparable to that of other Cr-containing compositions, as shown in Table 3. The percent coking with the catalyst of Example 6, evaluated after 6 hrs. on stream, was 12.8%. The same composition evaluated at 25 hrs. experienced 23.8% coke formation. By comparison, the rare earth compounds showed markedly less carbon build-up during a 6 hr evaluation, indicating the desirable longer life of these catalyst compositions. Although not wishing to be limited to any one theory, it is thought that the action of the rare earth oxide may be one of moderating (i.e., lowering) the surface acidity of the oxide, which suppresses some of the acid catalyzed carbon forming reactions. A series of rare earth-chromium oxide catalysts were prepared and evaluated in the reduced scale reactor, as described in “Test Procedure.” 
 EXAMPLE 12 Cr 0.1 La 0.9 Ox 2.22 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,810-8) (2.5603 M in Cr), 42.78 ml of 1.1955 M aqueous La(NO 3 ) 3 (33 wt % as La in La(NO 3 ) 3 xH 2 O, Aldrich 23, 855-4) were simultaneously added to a 150 ml pyrex petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days in a Virtis 25EL “Freezemobile” equipped with a Unitop 800 L unit (with refrigerated shelves) to produce a freeze dried powder. The freeze dried material was heated or calcined in air at 350° C. for 5 hrs prior to pelletization and use in a microreactor. 
 EXAMPLE 13 Cr 0.1 Ce 0.9 Ox 1.375 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,810-8) (2.560 M in Cr) and 43.625 ml of 0.7261 M aqueous Ce(NO 3 ) 3 .6H 2 O (Alfa, 11330) were simultaneously added to a 150 ml pyrex petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days in a Virtis 25EL “Freezemobile” equipped with a Unitop 800 L unit (with refrigerated shelves) to produce a freeze dried powder. The freeze dried material was heated (calcined) in air at 350° C. for 5 hrs prior to pelletization and use in a microreactor. 
 EXAMPLE 14 Cr 0.1 Sm 0.9 Ox 0.969 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,810-8) (2.560 M in Cr) and 44.031 ml of 0.5069 M aqueous solution of samarium nitrate (solution was formed using water and nitric acid to bring final pH&equals;0.24 to dissolve Sm(NO 3 ) 3 .6H 2 O (Alfa 12906)) were simultaneously added to a 150 ml pyrex petri dish with gentle swirling. The entire solution was rapidly frozen with liquid nitrogen and dried as a frozen solid under vacuum for several days in a Virtis 25EL “Freezemobile” equipped with a Unitop 800 L unit (with refrigerated shelves) to produce a freeze dried powder. The freeze dried material was heated or calcined in air at 350° C. for 5 hrs prior to pelletization and use in a microreactor. 4 TABLE 4 Cr-Rare Earth Ox Powder Catalysts Example No. Composition* 12 Cr 0.1 La 09 O x 13 Cr 0.1 Ce 0.9 O x 14 Cr 0.1 Sm 0.9 O x *Expressed as the atomic ratios of the metals content 5 TABLE 5 Performance of Cr-Rare Earth Ox Catalysts GHSV Example Vol. Wt. Temp ×10 4 % CH 4 % O 2 % CO % H 2 No. (mL) (g) (° C.) (NL/kg/h) Conv. Conv. Sel. Sel. H 2 :CO 12 2.0 2.1417 770 6.1 58 100 83 73 1.76 13 0.4 0.5972 860 3.0 36 100 49 45 1.84 14 0.4 0.5350 870 3.0 48 100 65 66 2.03 
 EXAMPLE 15 Co 0.1 Cr 0.8 La 0.1 Ox 6.692 ml of 1.1955 M lanthanum nitrate ((La(NO 3 ) 3 aqueous solution (prepared by dissolving 503.02 g of La(NO 3 ).xH 2 O (La content 33.0 wt %; Aldrich 23,855-4) in sufficient water to make a 1.1955 M solution) was combined with 38.612 ml of an aqueous solution of Cr 3 (OH) 2 (CH 3 COO) 7 (Aldrich, 31,810-8) (1.6575 M in Cr), and 7.39 ml of 1.0826 M Co(NO 3 ) 2 solution, (prepared by dissolving Co(NO 3 ) 2 .6H 2 O (Alfa 11341) in water). The solution was freeze-dried and prepared for testing as previously described. 6 TABLE 6 Performance of Co 0.1 Cr 0.8 La 0.1 Ox GHSV Example Vol. Wt. Temp ×10 4 % CH 4 % O 2 % CO % H 2 No. (mL) (g) (° C.) (NL/kg/h) Conv. Conv. Sel. Sel. H 2 :CO % Coke 15 2.0 2.0155 660 6.1 89 100 96 97 2.02 624 7.6* 91 100 98 98 2.00 533 12.2 89 100 98 98 2.00 492 15.2* 88 100 98 98 2.00 *O 2 feed As demonstrated by the performance characteristics shown in Table 6, the chromium-lanthanum-cobalt containing catalyst surprisingly proved to be selective for a process involving C—H activation, such as CH 4 partial oxidation. Once again, this observation contrasts with the conventional views of catalysts containing chromium promoters or additives, which have in the past generally been considered to be useful only for promoting the non-selective reaction pathways for alkane oxidation reactions using molecular oxygen (O 2 ). Similar to the coke-suppressing effect demonstrated above by Cr—Ni—La Ox compositions, the addition of a rare earth oxide to Cr—Co also reduced carbon deposition on the catalyst after use in the reduced scale reactor. The catalyst of Example 15 (Co 0.1 Cr 0.8 La 0.1 Ox) had 0.61% carbon formation after 6 hrs on-stream with an O 2 feed, according to the “Test Procedure.” By comparison, a composition of Co 0.1 Cr 0.9 Ox demonstrated 2.74% carbon under similar conditions. A series of chromium-lanthanum oxide catalysts were also prepared and evaluated, as follows: 
 EXAMPLE 16 La 2 O 3 100 ml of a Lanthanum Nitrate solution (1.1955 M, prepared by dissolving 414.13 g of La(NO 3 ).6H 2 O, Alfa 12915 in sufficient water to make a 1.1955 M solution) was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 350° C. for 5 hours, 5° C./min to 525° C., 525° C. 1 hour; 10° C./min to room temperature. The material was pelletized and sieved prior to the reactor evaluation as described above. 
 EXAMPLE 17 Cr 0.1 La 0.9 Ox 95.068 ml of a Lanthanum Nitrate solution (1.1955 M, prepared by dissolving 414.13 g of La(NO 3 ).6H 2 O, Alfa 12915 in sufficient water to make a 1.1955 M solution) was simultaneously added to a 4.932 ml of an aqueous solution of chromium hydroxide acetate (Aldrich, 31,810-8) (2.5603 M in Cr) solution (molarity determined by ICP analysis). The solution was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 350° C. for 5 hours, 5° C./min to 525° C., 525° C. 1 hour; 10° C./min to room temperature. The material was pelletized and sieved prior to the reactor evaluation as described above. 
 EXAMPLE 18 Cr 0.25 La 0.75 Ox 86.532 ml of a lanthanum nitrate solution (1.1955 M, prepared by dissolving 414.13 g of La(NO 3 ).6H 2 O, Alfa 12915 in sufficient water to make a 1.1955 M solution) was simultaneously added to a 13.468 ml of an aqueous solution of chromium hydroxide acetate (Aldrich, 31,810-8) (2.5603 M in Cr) solution (molarity determined by ICP analysis). The solution was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 350° C. for 5 hours, 5° C./min to 525° C., 525° C. 1 hour; 10° C./min to room temperature. The material was pelletized and sieved prior to the reactor evaluation as described above. 
 EXAMPLE 19 Cr 0.5 La 0.5 Ox 68.169 ml of a lanthanum nitrate solution (1.1955 M, prepared by dissolving 414.13 g of La(NO 3 ).6H 2 O, Alfa 12915 in sufficient water to make a 1.1955 M solution) was simultaneously added to a 31.831 ml of a chromium hydroxide acetate solution (aqueous, 2.5603 M, determined by ICP analysis, chromium hydroxide acetate obtained from Aldrich, 31,810-8). The solution was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 350° C. for 5 hours, 5° C./min to 525° C., 525° C. 1 hour; 10° C./min to room temperature. The material was pelletized and sieved prior to the reactor evaluation as described above. 
 EXAMPLE 20 Cr 0.75 La 0.25 Ox 41.653 ml of a lanthanum nitrate solution (1.1955 M, prepared by dissolving 414.13 g of La(NO 3 ).6H 2 O, Alfa 12915 in sufficient water to make a 1.1955 M solution) was simultaneously added to a 58.347 ml of an aqueous solution of chromium hydroxide acetate (Aldrich, 31,810-8) (2.5603 M in Cr) solution (molarity determined by ICP analysis). The solution was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 350° C. for 5 hours, 5° C./min to 525° C., 525° C. 1 hour; 10° C./min to room temperature. The material was pelletized and sieved prior to the reactor evaluation as described above. 
 EXAMPLE 21 Cr 0.9 La 0.1 Ox 19.222 ml of a Lanthanum Nitrate solution (1.1955 M, prepared by dissolving 414.13 g of La(NO 3 ).6H 2 O (Alfa 12915) in sufficient water to make a 1.1955 M solution) was simultaneously added to 80.778 ml of an aqueous solution of chromium hydroxide acetate (Aldrich, 31,810-8) (2.5603 M in Cr) (molarity determined by ICP analysis). The solution was rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to 0° C.) and evacuated to dryness over a period of 5-7 days, or until completely dry. The material was calcined in air according to the following schedule: 5° C./min to 350° C., 350° C. for 5 hours, 5° C./min to 525° C., 525° C. 1 hour; 10° C./min to room temperature. The material was pelletized and sieved prior to the reactor evaluation as described above. 7 TABLE 7 La-Cr Ox Catalysts Example No. Composition* 16 La 2 O 3 17 Cr 0.1 La 0.9 Ox 18 Cr 0.25 La 0.75 Ox 19 Cr 0.5 La 0.5 Ox 20 Cr 0.75 La 0.25 Ox 21 Cr 0.9 La 0.1 Ox 8 TABLE 8 Performance of La-Cr Series Catalysts GHSV Example Vol. Wt. Temp × 10 4 % CH 4 % O 2 % CO % H 2 No. (mL) (g) (° C.) (NL/kg/h) Conv. Conv. Sel. Sel. H 2 :CO 16 2.0 2.3996 860 6.1 55 100 70 55 1.57 17 2.0 2.2106 830 6.1 43 100 63 46 1.46 18 2.0 1.5846 955 6.1 51 100 67 52 1.55 19 2.0 2.2184 746 6.1 59 100 85 72 1.69 20 2.0 1.9359 834 6.1 69 100 87 78 1.79 21 2.0 1.9517 873 6.1 69 100 90 80 1.78 
 EXAMPLE 22 Cr 0.025 Mg 0.975 Ox (Comparative Example) A magnesium methoxide solution (68.767 mL, 0.3495 M) diluted with 50 volume % ethanol punctilious) was added to a 150 mL petri dish with gentle swirling under an inert N 2 atmosphere. In a subsequent addition, 1.233 mL of an aqueous solution of Cr 3 (OH) 2 (CH 3 CO 2 ) 7 (0.5 M in Cr) was introduced to the petri dish while it was gently swirled. Following the addition of the aqueous solutions, a gel point was realized and a homogeneous gel formed which was nearly white in color. The gel was allowed to age 8 days in air and then dried under vacuum at 120° C. prior to use. The final xerogel had a nominal metal ratio of Cr 0.025 Mg 0.975 . 9 TABLE 9 Performance of Cr 0.025 Mg 0.975 Ox GHSV Example Catalyst Temp. × 10 4 % CH 4 /O 2 % CO/H 2 No. Composition (mole %) V(mL) Wt. (g) ° C. (NL/kg/h) Conv. Sel. H 2 :CO % Coke 22 Cr 0.025 Mg 0.975 Ox 2 0.9024 710 6.1 45/100 74/48 1.3 2.99 In FIG. 1 the predicted greater ionicity or basicity of Cr 0.025 Mg 0.975 Ox, compared to that of Cr 0.1 La 0.9 Ox, Cr 0.1 Ce 0.9 Ox and Cr 0.1 Sm 0.9 Ox, is shown, together with the corresponding light-off temperatures when tested according to the “Test Procedure.” Although the preferred technique for preparing the representative Cr-rare earth based catalyst compositions involved freeze drying an aqueous solution, a variety of other well-known techniques such as impregnation, xerogel, aerogel or sol gel formation, spray drying or spray roasting could also be used with success. In addition to catalyst powders and pellets, extrudates and monoliths may also be used as supports, provided that they have sufficient porosity for reactor use, as described under “Test Procedure.” The supports used with some of the catalyst compositions may be in the form of monolithic supports or other configurations having longitudinal channels or passageways permitting high space velocities with a minimal pressure drop. Such catalyst forming techniques and configurations are known and have been described in, for example, Structured Catalysts and Reactors , A. Cybulski and J. A. Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21, X. Xu and J. A. Moulijn, “Transformation of a Structured Carrier into Structured Catalyst”). Process of Producing Syngas Any suitable reaction regime is applied in order to contact the reactants with the catalyst. One suitable regime is a fixed bed reaction regime, in which the catalyst is retained within a reaction zone in a fixed arrangement. Supported or self-supporting catalysts may be employed in the fixed bed regime, retained using fixed bed reaction techniques well known in the art. Preferably a short or millisecond contact time reactor is employed. Several schemes for carrying out catalytic partial oxidation (CPOX) of hydrocarbons in a short contact time reactor have been described in the literature. For example, L. D. Schmidt and his colleagues at the University of Minnesota describe a millisecond contact time reactor in U.S. Pat. No. 5,648,582 and in J. Catalysis 138, 267-282 (1992) for use in the production of synthesis gas by direct oxidation of methane over a catalyst such as platinum or rhodium. A general description of major considerations involved in operating a reactor using millisecond contact times is given in U.S. Pat. No. 5,654,491. The disclosures of the above-mentioned references are incorporated herein by reference. Accordingly, a feed stream comprising a hydrocarbon feedstock and an oxygen-containing gas is contacted with one of the above-described chromium-based catalysts in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising carbon monoxide and hydrogen. The hydrocarbon feedstock may be any gaseous hydrocarbon having a low boiling point, such as methane, natural gas, associated gas, or other sources of light hydrocarbons having from 1 to 5 carbon atoms. The hydrocarbon feedstock may be a gas arising from naturally occurring reserves of methane which contain carbon dioxide. Preferably, the feed comprises at least 50% by volume methane, more preferably at least 75% by volume, and most preferably at least 80% by volume methane. The hydrocarbon feedstock is contacted with the catalyst as a gaseous phase mixture with an oxygen-containing gas, preferably pure oxygen. The oxygen-containing gas may also comprise steam and/or CO 2 in addition to oxygen. Alternatively, the hydrocarbon feedstock is contacted with the catalyst as a mixture with a gas comprising steam and/or CO 2 . Preferably, the methane-containing feed and the oxygen-containing gas are mixed in such amounts to give a carbon (i.e., carbon in methane) to oxygen (i.e., oxygen) ratio from about 1.25:1 to about 3.3:1, more preferably, from about 1.3:1 to about 2.2:1, and most preferably from about 1.5:1 to about 2.2:1, especially the stoichiometric ratio of 2:1. The process is operated at atmospheric or superatmospheric pressures, the latter being preferred. The pressures may be from about 100 kPa to about 12,500 kPa, preferably from about 130 kPa to about 10,000 kPa. The process of the present invention may be operated at temperatures of from about 600° C. to about 1,100° C., preferably from about 700° C. to about 1,000° C. The hydrocarbon feedstock and the oxygen-containing gas are preferably pre-heated before contact with the catalyst. The hydrocarbon feedstock and the oxygen-containing gas are passed over the catalyst at any of a variety of space velocities. Gas hourly space velocities (GHSV) for the process, stated as normal liters of gas per kilogram of catalyst per hour, are from about 20,000 to at least about 100,000,000 NL/kg/h, preferably from about 50,000 to about 50,000,000 NL/kg/h. Preferably the catalyst is employed in a millisecond contact time reactor for syngas production. The process preferably includes maintaining a catalyst residence time of no more than 10 milliseconds for the reactant gas mixture. Residence time is the inverse of the space velocity, and high space velocity equates to low residence time on the catalyst. The effluent stream of product gases, including CO and H 2 , emerges from the reactor. And, if desired, may be routed directly into a variety of applications. One such application is for producing higher molecular weight hydrocarbon components using Fisher-Tropsch technology. Although not wishing to be bound by any particular theory, the inventors believe that the primary reaction catalyzed by the preferred catalysts described herein is the partial oxidation reaction of Equation 2, described above in the background of the invention. Additionally, other chemical reactions may also occur to a lesser extent, catalyzed by the same catalyst composition to yield a net partial oxidation reaction. For example, in the course of syngas generation, intermediates such as CO 2 &plus;H 2 O may occur as a result of the oxidation of methane, followed by a reforming step to produce CO and H 2 . Also, particularly in the presence of carbon dioxide-containing feedstock or CO 2 intermediate, the reaction CH 4 &plus;CO 2 →2CO&plus;2H 2 (3) may also occur during the production of syngas. Accordingly, the term “catalytic partial oxidation” when used in the context of the present syngas production method, in addition to its usual meaning, can also refer to a net catalytic partial oxidation process, in which a light hydrocarbon, such as methane, and O 2 are supplied as reactants and the resulting product stream is predominantly the partial oxidation products CO and H 2 , in a molar ratio of approximately 2:1, rather than the complete oxidation products CO 2 and H 2 O. While the preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. The disclosures of U.S. Provisional Patent Application Nos. 60/183,575 and 60/183,423, and all patents and publications cited herein are incorporated by reference.