1. Field of the Invention
The present invention relates to a method for making a hydrogenation catalyst having a base material coated with a catalytic metal using mechanical milling techniques. This method produces a hydrogenation catalyst that may be used for the remediation of a plurality of contaminated materials, including, but not limited to, polychlorinated biphenyls.
2. Description of Related Art
The class of 209 aromatic chlorinated molecules resulting from the attachment of up to ten chlorine atoms to a biphenyl is collectively known as polychlorinated biphenyls (PCBs). Generally, PCBs are known to have the chemical structure C12H10-nCln and, in addition to other chlorinated synthetic aromatic compounds, are of great concern due to their toxicity and persistence in the environment. Among the properties of these synthetic colorless liquids are high chemical stability, low flammability, low thermal and electrical conductivity, and low solubility in water.
Twenty-nine years following the 1976 Toxic Substances Control Act (TSCA) ban on their manufacture, PCBs remain a continued environmental threat. Their persistence owes to the very high chemical stability of these molecules. Prior to the TSCA ban, these favorable properties were exploited in a variety of applications including paint stabilizers, transformer oils, capacitors, printing inks, and pesticides.
Toxicity evidence was found adequate to justify TSCA regulation; however, the debate over the extent of PCB toxicity on organisms remains heated. PCBs are known to bioaccumulate and concentrate in fatty tissues. Studies suggest increased incidences of cancer with long-term PCB exposure. These studies are arguably inconclusive as they involve the simultaneous analysis of multiple congeners. Further complications arise from the potential for contamination of commercial mixtures with other more toxic chlorinated compounds such as polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).
Until recently, only one option was available for the treatment of PCB-contaminated materials, incineration. This, however, may prove to be more detrimental to the environment than the PCBs themselves due to the potential for formation of PCDDs. PCDDs have been shown to exist at abnormally high levels in proximity to incinerators burning chlorine-contaminated materials. Cancer rates have also been “well correlated” to both dioxins and proximity to chlorine-contaminated waste burning incinerators. The temperature required to incinerate PCB-contaminated material may be up to 1200° C. Additionally, it is expensive to incinerate the PCB-contaminated material and to transport contaminated material.
An alternate approach that has been used to treat PCB-contaminated materials is bioremediation. This is accomplished by the reductive dechlorination of PCBs by anaerobic microorganisms. However, this usually results in an incomplete removal of PCBs. Furthermore, specific conditions are required for bioremediation and are not always seen in contaminated sites.
Solvent extraction has also been used to treat PCB-contaminated materials. Organic solvents are used to extract contaminated matrix. However, extraction is only applied ex-situ and the PCBs are not actually destroyed but transported to another media that has to be further treated.
Base-catalyzed decontamination has been used to treat PCB-contaminated material by adding NaHCO3 or NaOH to the media. However, this remediation process is only applied ex-situ due to operating temperatures.
Metals have been used for the past 10 years for the remediation of halogenated solvents and other contaminants in the environment; however, zero-valent metals alone do not possess the activity required to dehalogenate PCBs.
Recent literature reports the rapid and complete reductive dechlorination of PCBs using palladium coated iron (Pd/Fe) or magnesium (Pd/Mg) in aqueous medium. The use of these bimetallic particles for dechlorination relies on the reduction potentials of magnesium or iron coupled with the hydrogenation-type catalytic activity of palladium. The current techniques for preparing Pd/Fe material provides for the easy preparation of the material using pallameres. Although the Pd/Mg material has a greater thermodynamic driving force and forms a self-limiting oxide layer to prevent extensive air oxidation, this material cannot be easily prepared with pallamerse. Therefore, a process for making palladized zero-valent metals using cost-effective and efficient techniques would be valuable.
Comminution theory is principally concerned with reducing the average size of particles in a sample of crystalline or metallic solid; however, it can also be used to understand mechanical alloying of particles. To accomplish either of these tasks, the most commonly used processes involve ball milling, vibrational milling, attrition, and roller milling.
Ball milling is a process in which a material is loaded into a canister partially filled with milling balls. The canister is then rotated at high speed on its major axis so that the balls are held by centripetal force to the inside wall until they reach the highest point inside the canister. Gravitational force then exceeds the upward force of the balls and they fall to the bottom of the canister where they impact other balls and the canister wall.
If some milling material is pinched between the participants in one of these collisions and the collision is of adequate energy, then the particles are fractured into smaller particles. It should also be noted that a certain critical speed of rotation is necessary for this process to occur. At lower speeds the balls will simply roll over one another and at extremely high speeds their upward force will exceed gravity at all points inside the canister and the balls will stick to the canister wall. The approximate critical speed of any ball mill is given below and is usually on the order of about 250 RPMs,Nc=7.05/(D1/2)                where Nc=critical speed and D=milling canister diameter        
Vibrational milling is a process similar to ball milling except that the milling vessel is vigorously shaken in a back and forth motion or in a back in forth motion in conjunction with a lateral motion that produces a “figure 8” path. This type of milling relies solely on the extremely high-energy collisions between rapidly moving milling balls rather than the collisions between the balls and the canister wall, as described for ball milling. Since vibrator mills can often shake canisters at a rate of approximately 1200 RPMs, often producing ball speeds of upwards of 5 m/s, vibrational milling commonly yields the desired reduction in particle size at a rate one order of magnitude faster than that of ball milling.
Two other less common types of milling are attrition milling and roller milling. These processes are not commonly seen in laboratory settings but are often seen in industrial work. Attrition milling relies on rapidly spinning paddles to stir the milling balls present in the milling vessel. The rate of size reduction observed is often similar to the rate of reduction observed for vibrator mills of similar size; however, due to the necessity of a cooling system this type of milling is often limited in its capabilities to systems that can be milled in liquid media.
Roller milling is a process that relies on fracturing caused by stress induced in the system from the compression of materials between two rolling bars or cylinders. It is most often used for reduction of very coarse materials into less coarse materials that can later be reduced in size by other means. The previous two systems are not discussed quantitatively.
For all milling types (other than roller milling), the reduction of particle size relies on stresses induced in individual particles caused by collisions within the milling vessel. This process reduces the average particle size until equilibrium is reached, at which point no further size reduction is observed. This phenomenon can be explained if one considers crystal matrix impurities as the cause of fracture. As each fracture occurs at the point or plane of an impurity that disrupts the crystal structure, that particular discontinuity disappears, thus the average number of imperfections in the crystal structure of each particle in the sample reduces. Since the crystal structure of each particle in the sample becomes more perfect each time a fracture occurs, at some point the particles will exhibit a near perfect crystalline structure. At this time, the collisions within the mill will no longer be of adequate energy to cause fracture and the size equilibrium will be reached.
Another explanation for this phenomenon arises from the fact that it is more difficult to inflict upon smaller particles the necessary sheer required to cause fracture. This can be explained by the observation that the probability of a milling ball impacting a particle with the necessary directional velocity is reduced when the particle is smaller. Additionally, smaller particles have a higher surface activity and therefore, have a greater probability of being re-welded to form larger particles.
The equilibrium size of the particles in a milling batch has been the topic of much research and one of the more straightforward methods for calculating this size, based on a number of variables, follows and can be applied to all types of milling:W=mass of particles with surface area≦S
                    W        i            ⁡              (        t        )              =                  ∑                  j          =          1                i            ⁢                        a          ij                ⁢                  ⅇ                                    -                              S                j                                      ⁢            t                          ⁢                                  ⁢        where                        a      ij        =          [                                    0                                                              for                ⁢                                                                  ⁢                i                            <              j                                                                                          W                ⁡                                  (                  0                  )                                            -                                                ∑                                      k                    =                    1                                                        i                    -                    1                                                  ⁢                                  a                  ik                                                                                                        for                ⁢                                                                  ⁢                i                            =              j                                                                                          1                                                      S                    i                                    -                                      S                    j                                                              ⁢                                                ∑                                      k                    =                    j                                                        i                    -                    1                                                  ⁢                                                      S                    k                                    ⁢                                      b                    ik                                    ⁢                                      a                    kj                                                                                                                          for                ⁢                                                                  ⁢                i                            >              j                                          ]                  B      ij        =                                        ψ            ⁡                          (                                                x                  j                                /                                  x                  k                                            )                                δ                ⁢                              (                                          x                j                            /                              x                i                                      )                    β                    +                        (                      1            -            ψ                    )                ⁢                              (                                          x                j                            /                              x                1                                      )                    δ                ⁢                              (                                          x                j                            /                              x                i                                      )                    ν                                S      j        =                  K        ⁡                  (                                    x              j                        /                          x              1                                )                    α      To use this equation, the desired mass of reduced size particles and the size of these particles is specified to yield the milling time required for achieving these parameters. Solving this series however, involves the use of many constants, which must be determined through calibration experiments using materials for which the constants are already known.
In other areas of research, such as mechanical activation of reactants in a milling vessel, researchers are focused more on the rate of particle size reduction as opposed to the average particle size of the end product. In general, the rate at which a mill reduces the average size of the particles being milled is a function of the probability of any particle being trapped between the participants in the above stated types of collisions when those collisions possess the energy necessary to fracture a particle. This focus has produced a number of functions, which have been supported empirically, to determine the rate of particle size reduction. A few of the more general examples are shown below for ball milling and vibrational milling.
In terms of the change in total surface area of materials                For Ball Milling        
                    ⁢                            for          ⁢                                          ⁢          U                ≤                  1          ⁢                                          ⁢          M          ⁢                                    ⅆ              S                                      ⅆ              t                                          =              0.50        ⁢                  k          1                ⁢                  K          ′                ⁢        ϕ        ⁢                                  ⁢                  LJd                      -            1.7                          ⁢                  D          2.2                ⁢                  x                      B            +            1                          ⁢                              U            ⁡                          [                              1                -                                  1.1                  ⁢                                                                          ⁢                                                                                    σ                        α                                            ⁡                                              (                                                                                                            k                              1                                                        ⁢                            xU                                                                                Y                            ⁢                                                                                                                  ⁢                                                          ρ                              B                                                        ⁢                            dD                                                                          )                                                                                    1                      /                      2                                                                                  ]                                1.5                                        ⁢                            for          ⁢                                          ⁢          U                >                  1          ⁢                                          ⁢          M          ⁢                                    ⅆ              S                                      ⅆ              t                                          =              1.1        ⁢                  k          2                ⁢                  K          ′                ⁢        ϕ        ⁢                                  ⁢                  LJd                      -            2.2                          ⁢                  D          2.7                ⁢                  x                      B            +            1                          ⁢                              {                          1              -                              1.1                ⁢                                                                  ⁢                                                                            σ                      α                                        ⁡                                          [                                                                                                    (                                                          U                              +                                                              k                                1                                                            -                              1                                                        )                                                    ⁢                          x                                                                          Y                          ⁢                                                                                                          ⁢                                                      ρ                            B                                                    ⁢                          dD                                                                    ]                                                                            1                    /                    2                                                                        }                    1.5                                        ⁢          For      ⁢                          ⁢      Vibrational      ⁢                          ⁢      Milling                  M      ⁢                        ⅆ          S                          ⅆ          t                      =          0.6      ⁢                          ⁢              k        1        ′            ⁢              K        ′            ⁢              J        2            ⁢              LD        2            ⁢              d                  -          2                    ⁢      αωθ      ⁢                          ⁢              x                  B          +          1                    ⁢                        U          ⁡                      [                          1              -                              230                ⁢                                                                  ⁢                                                                            σ                      α                      ′                                        ⁡                                          (                                                                                                    k                            1                            ′                                                    ⁢                          xU                                                                          Y                          ⁢                                                                                                          ⁢                                                      ρ                            B                                                    ⁢                          d                          ⁢                                                                                                          ⁢                                                      α                            2                                                    ⁢                                                      ω                            2                                                                                              )                                                                            1                    /                    2                                                                        ]                          1.5            
These equations are applicable only when U≦1.
where,
U=volume of particles in mill/(volume of mill−volume of balls)
n=0.4d3x−3U
n=number of particles per ball, dimensionless
S=specific surface of particles, sq.cm./g.m 
φ=ratio of mill speed to critical one, dimensionless
L=length of mill, cm.
J=fractional ball filling of mill, dimensionless
d=diameter of ball, cm.
D=diameter of mill, cm.
x, x′=particle size, cm.
Y=modulus of elasticity of materials, g.f/sq.cm.
and all other terms are constants relating to the material being milled
Since these equations are often difficult to use, empirical studies of milling variables may prove to be more useful.
In general, it can be seen that the rate of particle reduction in vibrational milling is much greater than in ball milling. Additionally, for all types of milling, rate increases with ball density and is greatest when the mill filling ratio (volume of material to be milled/volume of mill) is approximately 10-20% while the volume occupied by milling balls is approximately 40-60% of the total mill volume.
In the case where one wishes to mechanically alloy materials, the previously discussed theories apply, however several additional topics must also be considered. Mechanical alloying is a high-energy milling process for producing composite materials with an even distribution (though not homogeneous in the rigorous sense) of one material into another. By definition, at least one of the materials must be metallic to be considered an alloy; however, the topics discussed here can be applied to non-metallic materials as well. Two systems will be discussed; Malleable-Malleable and Brittle-Malleable.
In a malleable-malleable system such as the milling of two soft materials, like sodium and gold, ball-powder-ball collisions initially reduce the size of both materials until the average active surface area of each particle in the sample is large enough for re-welding to occur. When this critical surface area is achieved, in a particle, re-welding can occur between two similar particles or two dissimilar particles. If re-welding occurs between two similar particles, the net process results in no change of the material nature. If re-welding occurs between two dissimilar particles an alloy particle is created. This alloy particle can then undergo further fragmentation along alternative planes and subsequently be re-welded multiple times. The longer this process is allowed to take place, the more dissolved one material becomes in the other.
In a brittle-malleable system such as palladium and magnesium, the more ductile magnesium is initially flattened while fragmentation of the more brittle palladium occurs. With further milling, the ductile material occludes brittle fragments leading to an individual particle composition of the starting mixture. Depending on the solubility of the brittle phase, continued milling may result in near chemical homogeneity.
These processes can be carried out using any type of mill however the lower energy route, ball milling, will require much longer milling times to produce a well-distributed alloy. This can be easily explained using theories already discussed. Simply put, ball milling produces relatively few collisions, most of which are relatively low in energy, while vibrational milling produces an abundance of very high-energy collisions of grinding material. Since high-energy collisions are necessary for alloying to occur vibrational milling produces the desired result with a greatly enhanced rate.