DETAILED DESCRIPTION

[0022] The invention is a catalytic reactor system useful for hydrogenation, dehydrogenation, hydrocarbon isomerization, hydrocracking, and for other catalytic reactions involving hydrocarbons. Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Similar or identical structures are labeled using identical callouts. An example of a reactor system of the invention is shown in FIG. 1. Reactor system 10 includes hydrogen inlet 12 and porous support 14, which supports first catalyst bed 16 near inlet 12. Reactor 10 also includes inlet 18 for hydrocarbon feedstock and second porous support 20, which supports second catalyst bed 22 near inlet 18. First catalyst bed 16 contacts, but is not substantially mixed with second catalyst bed 22. During operation, hydrogen gas enters reactor 10 through inlet 12, flows through porous support 14, then through first catalyst bed 16, then through second catalyst bed 22. Hydrocarbon feedstock, preferably gaseous feedstock (although liquid feedstock could also be used) enters reactor 10 through inlet 18, then flows through second porous support 20, then into second catalyst bed 22. Products and unreacted hydrogen or hydrocarbon feedstock exits reactor 10 through outlet 24.

[0023] Reactor system 10 can be heated to a desired temperature by any suitable mean, such as by immersing reactor 10 in a bath of hot oil or sand, by wrapping heating tape around the reactor, and the like.

[0024] Reactor system 10 is particularly useful for hydrogenation, dehydrogenation, hydrocarbon isomerization, hydrocracking, and other types of hydrocarbon transformations that involve reacting a hydrocarbon feedstock when first catalyst bed 16 includes noble metals such as Pt and Pd. Reactor system 10 is designed such that hydrocarbon feedstock flows away from first catalyst bed 16, and hydrogen flowing through first catalyst bed provides reactive hydrogen atoms that move into second catalyst bed 22 where they combine with hydrocarbon feedstock under the influence of second catalyst bed 22 to yield the desired products, which exit reactor through outlet 24. It will be appreciated that catalyst poisons present in the feedstock also flow away from first catalyst bed 16, thus extending the useful lifetime of first catalyst bed 16.

[0025] Second catalyst bed 22 includes catalysts that are tolerant of poisons typically found on hydrocarbon feedstock, but are catalytically active with regard to transferring reactive hydrogen atoms and promoting hydrogenation, dehydrogenation, hydrocarbon isomerization, hydrocracking, and the like.

[0026] A critical aspect of the invention involves the spacing between first reactor bed 16 and second reactor bed 22. First catalyst bed 16 and second catalyst bed 22 must be in contact at their interface, or separated only by a very short distance. If the spacing is too great (perhaps greater than two or three millimeters), reactive hydrogen atoms generated on first catalyst bed 16 recombine to form hydrogen. Second catalyst bed 22 typically will include catalytic materials that are catalytically active for transferring reactive hydrogen to hydrocarbons, but that do not generate reactive hydrogen atoms from hydrogen gas at an acceptable rate. If the spacing is too great, recombination occurs and the desired chemical transformation does not take place.

[0027] In order to demonstrate the advantages of the split-feed catalytic reactor system of the present invention, an invention reactor was tested and compared to a more conventional single-feed type of reactor, reactor 26 shown in FIG. 2, for a hydrocarbon isomerization, the conversion of 1-butene to 2-butene. Two catalysts beds, one a noble metal catalyst bed and the other a base-metal bimetallic catalyst, were used with each reactor system. The invention reactor system minimized contact of hydrocarbon feedstock with the noble metal catalyst bed, while the comparison single feed reactor did not. When feedstock included a small amount of catalyst poison (butadiene), the catalytic activity of the invention reactor was substantially unaffected while that for the single feed reactor decreased over time. The operating reactor temperatures for the present demonstration ranged from about 0° C. to about 40° C. (higher temperatures could be used, depending on the composition of the catalysts, reactants, and reactor hardware).

[0028] Catalysts were prepared by the incipient wetness procedure. For this demonstration, first catalyst bed 16 was a Pd/Grafoil catalyst prepared by impregnation of Grafoil powder with an aqueous solution of Pd(NO₃)₂.xH₂O (ALDRICH CHEMICALS).

[0029] The Grafoil powder was GTA grade, and prepared by grinding sheets of Grafoil into powder having nominal average diameter of 0.5 mm and treating the powder with flowing hydrogen for eight hours at 900° C. to remove sulfur impurities. Second catalyst bed 22 was either bimetallic iron-cerium supported on Grafoil (FeCe/Grafoil), or bimetallic iron-praseodymium supported on Grafoil (FePr/Grafoil). The bimetallic catalysts were prepared by coimpregnation of Grafoil with aqueous solutions of Fe(NO₃)₃.9H₂O (STREM CHEMICALS) and Ce(NO₃)₃.6H₂O (STREM CHEMICALS), or Fe(NO₃)₃.9H₂O and Pr(NO₃)₃.6H₂O (STREM CHEMICALS). After impregnation, each Grafoil support was dried in air overnight and the salt was decomposed at 250° C. in a flowing stream of 5% hydrogen/95% nitrogen for four hours. The resulting catalysts had a nominal weight loading of 1% metal; the bimetallic catalysts contained equal weights of the two metals.

[0030] Prior to all activity measurements, catalyst was reduced by exposure to flowing hydrogen at 300° C. for four hours. The activity and selectivity of the catalyst were measured by flowing 500 ml/min ultra-high purity He, 90 ml/min ultra-high purity H₂, and 10 ml/min 1-butene. Samples of the feed and product streams were injected into an HP 5890 Series II gas chromatograph equipped with a thermal conductivity detector and a 3 m packed column containing 0.19% picric acid on carbograph (ALLTECH). Response factors were obtained from W. A. Dietz, J. Gas. Chrom. 5, 68 (1967)).

[0031] Single feed reactor 26 was prepared in a series of steps. First, a bed of Pd/Grafoil catalyst (2 mg Pd/Grafoil plus 18 mg Grafoil) was included and tested. Afterward, bimetallic catalyst was added in increments such that the Pd/Grafoil catalyst and the bimetallic catalyst made contact at their interface but remained substantially unmixed. After each addition of bimetallic catalyst, the dual-bed was reduced using flowing hydrogen at 300° C., and the activity was determined each time. Incremental additions of the bimetallic catalyst were continued until the total bed weight was 90 mg.

[0032] To test the effect of bimetallic catalyst, another single feed reactor was prepared by adding increments of blank Grafoil to a bed of Pd/Grafoil and the activity was determined as described above.

[0033] A bed of each bimetallic catalyst was also tested to verify baseline activity and selectivity at the reaction temperatures.

[0034] FIG. 3a illustrates an aspect of the reactor system related to changes in activity as a function of the weight of the catalyst beds. FIG. 3a includes a graph of activity vs. bed weight for single feed reactor 26. Squares indicate a run at 25° C. employing the bimetallic catalyst FeCe/Grafoil, triangles indicate a run at 40° C. employing FeCe/Grafoil, and diamonds indicate a run at 40° C. employing the bimetallic catalyst FePr/Grafoil. The first data point shown represents 20 mg reactor bed weight containing 2.1 mg Pd/Grafoil and 17.9 mg blank Grafoil and thus represents a baseline activity and selectivity for Pd/Grafoil. The activity gradually increases with each increment of bimetallic catalyst. The graph of FIG. 3a shows an increase in activity from 0.32 to 0.51 mol/min g-Pd as bimetallic catalyst is added. The activity increases to 0.85 mol/min g-Pd when bimetallic catalyst is FePr/Grafoil. The baseline selectivity for Pd/Grafoil was 68% at 40° C., and gradually increased to 75% as FeCe/Grafoil was added.

[0035] FIG. 3b illustrates another aspect of the reactor system of the invention relating to changes in product selectivity as a function of the weight of the catalyst beds. FIG. 3b includes a graph of selectivity of cis- and trans-2-butene as a function of bed weight, wherein symbols are those of FIG. 3a . For this graph, selectivity equals [2-butenes]/[2-butenes+butane]. If, for example, the product gas has an equal concentration of 2-butenes and butane, then the selectivity equals 0.5. As FIG. 3b shows, product selectivity can be adjusted by adjusting the relative sizes of the catalyst beds. Taken together, the graphs of shown in FIGS. 3a and 3b illustrate the flexibility of the invention reactor system for adjusting activity and selectivity by adjusting the relative weights of the catalyst beds and the composition of second catalyst bed 22.

[0036] FIG. 4 shows a graph of activity collected from single feed reactor 26 when the 1-butene feed included about 4 ppm butadiene and other diolefin catalyst poisons. Diamond symbols indicate a dual bed reactor run where 1-butene flows through FeCe/Grafoil first and then through Pd/Grafoil. Square symbols indicate a dual bed reactor run where 1-butene flows through Pd/Grafoil first and then through FeCe/Grafoil. Triangular symbols indicate a control run (Grafoil, the control, was used instead of FeCe/Grafoil). As FIG. 4 shows, this gas feed rapidly deactivated the catalyst when Pd/Grafoil was contacted first (square symbols). The rate of deactivation was not as great when the bimetallic catalyst was contacted first.

[0037] FIG. 5a includes a graph of activity as a function of bed weight, and FIG. 5b shows a graph of selectivity as a function of bed weight, for conversion of 1-butene to 2-butene in an invention reactor. According to FIG. 5a , activity increases dramatically upon the first addition of FeCe/Grafoil to the reactor (the data point at 40 mg is for no FeCe/Grafoil in the reactor). The activity was high, approximately 0.25 mol/min g-Pd, when FeCe/Grafoil was present, and an activity plateau occurs as additional FeCe/Grafoil is added.

[0038] FIG. 5b shows that selectivity toward cis- and trans-2-butene increases slightly as the amount of FeCe/Grafoil increases. The selectivity for the invention reactor was lower than that for the single feed reactor. Open square symbols in FIG. 5a show the observed activity when FeCe/Grafoil was replaced with blank Grafoil. When blank Grafoil is used instead of FeCe/Grafoil, no activity was observed. This indicates that the conversion 1-butene to 2-butene occurs on the bimetallic catalyst. However, the noble metal must play a role in activating hydrogen gas because the bimetallic catalyst itself does not convert 1-butene to 2-butene at these temperatures.

[0039] The generally accepted mechanisms of hydrogenation and olefin isomerization require hydrogen atoms (see, for example, “Butene Isomerization Catalyzed by Supported Metals in the Absence of Molecular Hydrogen,” by P. B. Wells and G. R. Wilson, J. Catal. vol. 9, pp. 70-75 (1967); “The Hydroisomerization of n-Butenes. I. The Reaction of 1-Butene Over Alumina- and Silica-Supported Rhodium Catalysts,” by J. I. McNab, G. Webb, J. Catal. vol. 10, pp. 19-26, (1968); “Olefin Isomerization by Group 8 Metals in Absence of Molecular Hydrogen,” by S. D. Mellor, P. B. Wells, Trans. Far. Soc. Vol. 65, pp. 1873-1882 (1969); and “Hydrogenation of Olefins. Part 5. Hydrogenation of But-1-ene Catalyzed by Iridium-Alumina,” by S. D. Mellor and P. B. Wells, Trans. Far. Soc. vol. 65, pp. 1883-1890 (1969)). While not intending to be bound to any particular explanation, it is believed that hydrogen atoms are formed on the noble metal surfaces, and then are transported through the bed via surface diffusion to the bimetallic catalyst surfaces. The hydrogen atoms then add to the alkene, creating a metastable intermediate that can react with another hydrogen atom to form butane or that can lose a hydrogen atom and form 2-butene. The lack of activity measured for runs where noble metal catalyst was present and bimetallic catalyst absent indicate that back-diffusion of 1-butene into the Pd/Grafoil is minimal.

[0040] FIG. 6 shows a schematic representation of an invention reactor 28 having a main tube portion 30 and a side tube portion 32. First catalyst bed 16 is included in the side tube portion and second catalyst bed 22 in the main tube portion, with some second catalyst bed 22 extending into side tube portion 32.

[0041] FIG. 7 shows a schematic representation of an invention reactor 34, which includes main tube portion 36 and a plurality of side-tube portions 38 along the length of main tube portion 36. First catalyst bed 16 is included in side tube portions 38, and second catalyst bed 22 in the main tube portion with some extending into side tube portions 38. Hydrogen gas enters through the side-tube portions 38 and flows into the main tube portion 36, while hydrocarbon feedstock enters through one end of the main tube portion 36. Gas pressure of hydrogen exceeds the hydrocarbon feedstock pressure; this way, backflow of hydrocarbon feedstock into first catalyst bed 16 is minimal.

[0042] FIG. 8 shows a cross-section of a schematic representation of an invention reactor 40 employing two co-joined reactors of the type shown in FIG. 7.

[0043] For an isomerization system such as that previously described for the isomerization of 1-butene to 2-butene, reactors 6, 7, and 8 may include first catalyst 16 of noble metals (Pd/Grafoil, for example) and second catalyst bed 22 of FeCe/Grafoil. Hydrogen gas would flow into each first catalyst bed 16 while 1-butene (or some other hydrocarbon feedstock) would flow into one end of main tube 35 and into second catalyst bed 22. The hydrogen pressure, hydrocarbon feedstock pressure and reactor configuration control the direction of the flow of hydrogen and hydrocarbon feedstock.

[0044] The gas pressures are adjusted such that backflow of hydrocarbon feedstock into first catalyst bed 16 is minimal. This is particularly important when the first catalyst bed includes metals that are expensive and/or active for forming hydrogen atoms from hydrogen gas (Pd, Pt, Rh, Ru, Ir, Ag, Au, Ni, Cu, Zn, Co, Mo, and W, to name a few) Reactive hydrogen atoms are produced on first catalyst bed 16, and spill over onto second catalyst bed 22, where they combine with hydrocarbon feedstock. Isomerization occurs on second catalyst bed 22, and product gases and unreacted hydrogen and hydrocarbons exit the other end of main tube 36. Metals useful for including in the second catalyst bed include, but are not limited to, Fe, Co, Ni, La, Ce, and Pr.

[0045] It should be understood that other configurations of reactor systems for hydrogenation, dehydrogenation, hydrocarbon isomerization, hydrocracking, and the like that provide catalyst beds that are in contact but are substantially unmixed are within the scope of the present invention.

[0046] In summary, this invention includes a split-feed, multi-bed catalytic reactor system. Instead of choosing a single catalyst with the best combination of activity, selectivity, and stability, two or more catalysts used in a split-feed, multi-bed configuration to provide high performance. An embodiment of the invention has been demonstrated for the isomerization of 1-butene to 2-butene, and provided support for a hydrogen spillover mechanism. The reactor is less susceptible to catalyst poisoning than other types of reactors, and also allows for partial substitution of more expensive noble metal catalyst with less expensive base metal bimetallic catalysts. The invention reactor is also a flexible reactor for adjusting selectivity among products by adjusting the amount of catalyst, or the identity of the catalyst, in either/or both the first and/or second catalyst bed. The function of the noble metal is to generate spillover species, which diffuse to the second catalyst bed where conversion occurs.

[0047] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

[0048] The embodiment(s) were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.