CATALYST COMPOSITIONS OF NANOPARTICULATE METAL ON A REFRACTORY SUPPORT

Abstract: Catalyst compositions of a nanoparticulate metal on a refractory support are provided. Processes for manufacturing said catalyst compositions and the use of said catalyst compositions, such as in hydrogenation processes, are also disclosed.

Description: TITLE CATALYST COMPOSITIONS OF NANOPARTICULATE METAL ON A REFRACTORY SUPPORT FIELD OF THE INVENTION The invention generally relates to catalyst compositions of nanoparticulate metal deposited on a refractory support material, processes for manufacturing said catalyst compositions and the use of said catalyst compositions. In particular, the catalyst compositions are useful in hydrogenation processes. BACKGROUND OF THE INVENTION The area of catalysis is the subject of considerable technical effort because of its extraordinary economic impact on the chemical processing industry and also due to the usual unpredictability of catalyst performance in the absence of physical testing. U. S. Patent No. 4,046,712 (Cairns et al.) discloses a catalyst comprising a hard, substantially non-porous paniculate substrate and a sputtered deposit of catalytic metal on the substrate, said deposit existing as an atomic dispersion and derived from a target of material subjected to ion beam bombardment. Specific utility of these catalysts is for high temperature, gas phase catalytic reactions. Canadian Patent No. 1086710 (Bird) discloses a process for preparing supported palladium catalysts comprising the vapor deposition of a compound of palladium onto a porous support while the support is at a temperature above the decomposition temperature of the compound of palladium. Deposition of palladium occurs on the surface of the support and as well as into pores having diameters greater than 50 Angstrom units (A). U. S. Patent No. 4,536,482 (Carcia) discloses a catalyst wherein a paniculate catalyst support has co-sputtered on its surface a mixture of a catalytically active metal and a co-sputtered support material. The use of RF sputtering is disclosed. U. S. Patent No. 5,077,258 (Phillips et al.) discloses a metal catalytic film comprising a flexible substrate, a catalytic metal layer adherent thereto, said layer having a thickness of 200 to 10,000 Angstroms (A). The process for preparing this film material involves the use of an electron beam gun or a magnetron sputtering device. Sputtering is carried out under reduced pressure of 0.1 Torr or less. The process of the present invention provides catalysts comprising catalytic metals in the form of nanoparticles on a refractory support. The metal nanoparticles are neither atomic dispersions nor thin films. They are particles ranging in size from between about 10 and 100 nanometers (nm). Further, the catalysts of the present invention are prepared by magnetron sputtering, as opposed to ion beam sputtering, which is a different and a much simpler process compared to the process disclosed by Cairns. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached figure and the detailed description which hereinafter follows. SUMMARY OF THE INVENTION The present invention provides a composition having utility as a catalyst comprising a nanoparticulate catalytically active metal on a refractory support. The invention further provides a process for the preparation of a composition having utility as a catalyst comprising a nanoparticulate catalytically active metal on a refractory support, said process comprising the physical vapor deposition of a catalytically active metal by sputtering, at a preferred pressure of > 10 mTorr, onto a refractory support cooled, preferably by liquid nitrogen, during deposition to ensure limited mobility of the incoming sputtered catalytically active metal atoms. Preferably, sputtering takes place using a magnetron gun. The invention further provides for an improved process for the reduction of anthraquinones to anthrahydroquinones as an integral part of a process to prepare hydrogen peroxide, said improvement comprising the use of the composition of the present invention as a hydrogenation catalyst. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a bright field transmission electron photomicrograph of nanoparticulate palladium on a copper grid at relatively low magnification (1 cm = 1000 nm). Fig. 2 shows a lattice image of nanoparticulate palladium on a copper grid at higher magnification (1 cm = 25 nm). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a composition having utility as a catalyst comprising a nanoparticulate catalytically active metal on a refractory support. It will be understood that the nanoparticulate catalytically active metal can be a single active metal or can be a combination of one or more selected active metals. The catalytically active metal, or combination of active metals, is selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, silver, gold, copper, mercury and rhenium. The most preferred catalytically active metal is palladium, including combinations therewith. The refractory support is preferably selected from the group consisting of alumina, (various forms), silica, titania, carbon (various forms), zirconia, silica- alumina and magnesia. A specifically preferred refractory support is alumina, most preferred being γ-alumina. The size of the catalyst support is not critical to the practice of the invention but may be important in the subsequent use of the catalyst. In gas phase reactions and in fixed bed reactors, a suitable support size would generally be about 2-3 mm in diameter as spherical or cylindrical shapes (L/D - 1). For slurry liquid phase reactions, a suitable support size would generally have a mean particle diameter of about 40 to about 150 microns depending on substrate density. By "nanoparticulate" is meant that the particles of the catalytically active metal have particle sizes in the range of about 10 nanometers to about 100 nanometers. Pressures usable in the sputtering process of the present invention range from greater than about 10 up to about 200 mTorr. Most preferred is a pressure of approximately 30 mTorr. The refractory support is cooled during deposition to ensure limited mobility of the incoming sputtered catalytically active metal atoms. Temperatures usable are from between about 20 C C to about minus 180°C. Liquid nitrogen is the preferred and most convenient means of providing such an environment. Processes that utilize catalytically active metals dispersed on refractory supports are common in the chemical process industry. A major group of processes included in this category are catalytic hydrogenations. Several important catalytic hydrogenations include, for example, the conversion of benzene to cyclohexane, the hydrogenation of edible oils to yield margerine-type products and the conversion of unsaturated oxygen-containing compounds, aldehydes and ketones, to alcohols. Palladium supported on γ-Al2θ 3 is a catalyst that may be used, for example, in the process for the production of hydrogen peroxide. An integral part of this process involves the catalytic hydrogenation of various substituted anthraquinones to the corresponding anthrahydroquinones. A catalyst currently employed in this hydrogenation process is palladium either supported or as palladium black. One catalyst currently in use is produced by solution precipitation and deposition of palladium on the chosen support. A commonly used support material is alumina. In contrast, the present invention provides a new process, physical vapor deposition (PVD) as a process for producing such an alumina supported palladium hydrogenation catalyst. It has been the Applicants' experience that catalysts produced by this process has a much finer grain size and greater than an order of magnitude higher activity than catalysts produced by solution precipitation with a similar loading of palladium. The grain size is apparent in the attached figures wherein a lattice image of nanoparticulate palladium on a copper grid is shown. The Fig. 1 bright field transmission electron photomicrograph is shown at relatively low magnification (1 cm = 1000 nm) while the Fig. 2 lattice image is shown at higher magnification (1 cm = 25 nm). In the practice of the Applicants' inventive process, palladium nanocatalyst supported on <20 μm γ-Al 2 O 3 was prepared by high pressure sputtering. Sputtering was carried out using a magnetron gun with a palladium target. High sputtering pressure ( > 10 mTorr) is required to thermalize the sputtered atoms so as to limit their mobility at the surface of the AI 2 O 3 particles. With this limited mobility diffusion of not more than 2-3 atomic distances, cluster or nanoparticle formation occurs rather than the deposition of a continuous film. The particles of the refractory support are cooled in liquid nitrogen during deposition to ensure limited mobility of the incoming sputtered palladium atoms. The temperature of AI 2 O 3 can be varied in a controlled fashion in order to produce nanoparticles of varying sizes. Sputtering is a line-of-sight process. Therefore, the Al 2 O 3 must be either agitated while being deposited on or the deposition has to be done several times with mixing in between to expose fresh support surface so that the desired amount of metal loading can be achieved. Moreover, since the total exposed area of the nanoparticle is the determining factor for the enhancement of activity, if multilayers of palladium nanoparticles are formed, the activity per unit weight metal is reduced. Ideally, a monolayer of well dispersed nanoparticles is desired. Scanning electron microscopy (SEM) analysis of a palladium/alumina catalyst for anthraquinone reduction prepared by the Applicants' solution precipitation method shows an average grain size of palladium to be about 0.1 μm. The resolution of conventional SEM is not high enough to measure the grain size of the nanoparticulate palladium/alumina prepared by the process of the present invention. Therefore, transmission electron microscopy (TEM) analyses of parallel samples deposited on a copper grid support were used to show the grain size to be less than about 200 angstrom (A), (20 nanometers). The following non-limiting examples further describe and enable the invention. All percentages are by weight percent unless otherwise indicated. EXAMPLE 1 Catalyst Preparation Physical Vapor Deposition was carried out in a stainless steel vacuum chamber (Huntington, Santa Clara, CA). The base pressure prior to the deposition was 3.2 x 10( _6 )Torr. A 99.999% pure palladium target was used (Englehard Industries Inc.). Argon gas was introduced in the chamber at 30 seem and a pressure of 30 mTorr was established by throttling the high vacuum gate valve. A commercially available magnetron gun was used for sputtering (US Gun: 2 inch) in a sputter down configuration. A DC power supply (MDX) was used. Sputtering was carried out in a constant power mode at 75 Watts with the target voltage being 250 V and the current 0.3 A. The alumina powder, 0.2 grams, was placed 2 inches away from, and directly underneath, the target in a copper boat. The boat was cooled to -150 C C by flowing liquid nitrogen through the copper tubes welded to the bottom of the boat. Once a temperature of -150°C was established, the plasma was ignited. During deposition the powder was mixed using a wobble stick to coat the powder uniformly. A loading of approximately 1 wt. % was achieved in 20 minutes of deposition. Once the desired loading was achieved, the power to the target, the Argon gas to the chamber and the liquid nitrogen to the powder holder were turned off. The sample was allowed to warm-up to the room temperature in vacuum to avoid any moisture condensation. EXAMPLE 2 Catalytic Activity of the Catalyst in a Hydrogenation Process Comparison of catalyst activity was carried out using a standardized test. A weighed quantity of catalyst ( " 100 mg) was added to a fixed portion of working solution (obtained from a commercial production facility) composed of alkylated anthraquinone (and degradation products) in a mixed hydrocarbon/tetrabutyl urea/water solvent. The suspension was then hydrogenated using a baffled gas dispensing turbine agitated reactor at 35 C C, 2500 rpm at 1 atm. H 2 pressure for 8 minutes to ensure an excess of anthraquinone. The hydrogenated product was then air sparged to convert the alkyl anthrahydroquinone to quinone and hydrogen peroxide. The hydrogen peroxide was extracted and the amount was determined by titration. The activity was finally calculated in terms of mL H 2 /min/g Pd. Activity measured for the palladium/alumina catalyst of the present invention gave values of 300 mL H 2 /min/g palladium where the best values for the palladium/alumina catalyst prepared by solution precipitation are about 30 mL H 2 /min/g palladium at the same Pd loading. Although particular embodiments of the present invention have been described in the foregoing description, it will be understood by those skilled in the art that the invention is capable of numerous modifications, substitutions and rearrangements without departing from the spirit or essential attributes of the invention. Reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.

Claims: What is claimed is: 1. A catalyst composition comprising a nanoparticulate catalytically active metal on a refractory support. 2. The catalyst composition of Claim 1 wherein the active metal is selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, silver, gold, copper, mercury, rhenium and combinations thereof. 3. The catalyst composition of Claim 1 wherein the active metal is palladium. 4. The catalyst composition of Claim 1 wherein the refractory support is selected from the group consisting of alumina, silica, titania, carbon, zirconia, silica-alumina and magnesia. 5. The catalyst composition of Claim 1 wherein the refractory support is γ-alumina. 6. A process for preparing a catalyst composition comprising a nanoparticulate catalytically active metal on a refractory support, wherein the process comprises depositing by physical vapor deposition a catalytically active metal by sputtering onto a refractory support that has been cooled to the extent that during deposition the catalytically active metal atoms have limited mobility. 7. The process of Claim 6 wherein the refractory support is cooled to a temperature from about 20°C to -180°C. 8. An improved process for the reduction of anthraquinones to anthrahydroquinones in the preparation of hydrogen peroxide by hydrogenation, the improvement comprising utilizing a catalyst composition, comprising a nanoparticulate catalytically active metal on a refractory support, as the hydrogenation catalyst to prepare hydrogen peroxide. 9. The process of Claim 6 wherein sputtering is carried out using a magnetron gun.

Inventors: KOCH THEODORE AUGER, SHAH SYED ISMAT ULLAH, KOCH, THEODORE, AUGER, SHAH, SYED, ISMAT, ULLAH
Applicants: DU PONT, KOCH THEODORE AUGER, SHAH SYED ISMAT ULLAH, E.I. DU PONT DE NEMOURS AND COMPANY, KOCH, THEODORE, AUGER, SHAH, SYED, ISMAT, ULLAH
Application year: 1997
Main classification: B01J37
IPC codes: B01J23, B01J35, B01J37, C01B15, C07C37, C23C14