U.S. patent application number 10/953595 was filed with the patent office on 2005-05-26 for method for producing graphite nanocatalysts having improved catalytic properties.
Invention is credited to Baker, R. Terry K., Xu, Xuejun.
Application Number | 20050113616 10/953595 |
Document ID | / |
Family ID | 43706257 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050113616 |
Kind Code |
A1 |
Xu, Xuejun ; et al. |
May 26, 2005 |
Method for producing graphite nanocatalysts having improved
catalytic properties
Abstract
High temperature treatment of graphite nanofibers to increase
their catalytic activity. The heat treated graphite nanofiber
catalysts are suitable for catalyzing chemical reactions such as
oxidation, hydrogenation, oxidative-dehydrogenation, and
dehydrogenation.
Inventors: |
Xu, Xuejun; (Westborough,
MA) ; Baker, R. Terry K.; (Hopkinton, MA) |
Correspondence
Address: |
Henry E. Naylor
Kean, Miller, Hawthorne, D'Armond,
McCowan & Jarman, L.L.P.
P.O. Box 3513
Baton Rouge
LA
70821-3513
US
|
Family ID: |
43706257 |
Appl. No.: |
10/953595 |
Filed: |
September 29, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10953595 |
Sep 29, 2004 |
|
|
|
10712247 |
Nov 13, 2003 |
|
|
|
60426198 |
Nov 14, 2002 |
|
|
|
Current U.S.
Class: |
585/443 ;
423/447.1; 423/447.3; 585/658 |
Current CPC
Class: |
C07C 5/48 20130101; B82Y
30/00 20130101; C07C 5/48 20130101; B01J 21/185 20130101; C07C
2521/18 20130101; B82Y 40/00 20130101; C01B 32/15 20170801; Y10S
585/906 20130101; B01J 37/08 20130101; C07C 15/46 20130101 |
Class at
Publication: |
585/443 ;
423/447.1; 423/447.3; 585/658 |
International
Class: |
D01F 009/12; D01C
005/00; C07C 005/333 |
Claims
What is claimed is:
1. A method for increasing the catalytic activity of graphitic
nanofibers comprised of a plurality of graphite platelets that are
aligned at an angle from about 1.degree. to about 90.degree. with
respect to the longitudinal axis of the nanofibers and which
nanofibers have a crystallinity greater than about 90%, which
method comprises heat treating said nanofibers in an inert gas
environment at temperatures from about 2300.degree. C. to about
3000.degree. C.
2. The method of claim 1 wherein the platelets are aligned
substantially 90.degree. with respect to the longitudinal axis of
the nanofiber.
3. The method of claim 1 wherein the platelets are aligned at angle
from about 30.degree. to about 60.degree. with respect to the
longitudinal axis of the nanofibers.
4. The method of claim 1 wherein the inert gas is selected from
helium and argon.
5. The method of claim 1 wherein said graphitic nanofibers are
intercalated with an intercalation component selected from the
group consisting of Li, Na, K, Rb, Cs, Br.sub.2, Cl.sub.2, F.sub.2,
ICl, ICl.sub.3, H.sub.2SO.sub.4, HNO.sub.3, H.sub.2SeO.sub.4,
HClO.sub.4, H.sub.3PO.sub.4, H.sub.4P.sub.2O.sub.7,
H.sub.3AsO.sub.4, HF, CrO.sub.2Cl.sub.2, CrO.sub.2F.sub.2,
UO.sub.2Cl.sub.2, FeCl, CuCl.sub.2, BCl.sub.3, AlCl.sub.3,
CoCl.sub.3, RuCl.sub.3, RhCl.sub.3, PdCl.sub.4, PtCl.sub.4,
Cr.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3, Sb.sub.2S.sub.3, CuS,
FeS.sub.2 Cr.sub.2S.sub.3, V.sub.2S.sub.3 and WS.sub.2.
6. A catalytic chemical reaction selected from oxidation,
hydrogenation, dehydrogenation, oxidative-hydrogenation, and
oxidative-dehydrogenation which is catalyzed by a catalyst
composition comprised of graphitic nanofibers which nanofibers are
comprised of a plurality of graphite platelets aligned at an angle
from about 1.degree. to about 90.degree. with respect to the
longitudinal axis of the nanofibers and which nanofibers have a
crystallinity greater than about 90%, which method comprises heat
treating said nanofibers in an inert gas environment at
temperatures from about 2300.degree. C. to about 3000.degree.
C.
7. The catalytic chemical reaction of claim 6 which is a
oxidative-hydrogenation reaction.
8. The catalytic chemical reaction of claim 7 which is the
oxidative-hydrogenation reaction of ethylbenzene to styrene.
9. The catalytic chemical reaction of claim 6 wherein the platelets
are aligned substantially 90.degree. with respect to the
longitudinal axis of the nanofiber.
10. The catalytic chemical reaction of claim 6 wherein the
platelets are aligned at angle from about 30.degree. to about
60.degree. with respect to the longitudinal axis of the
nanofibers.
11. The catalytic chemical reaction of claim 6 wherein the inert
gas is selected from helium and argon.
12. The catalytic chemical reaction of claim 8 wherein the
platelets are aligned substantially 90.degree. with respect to the
longitudinal axis of the nanofiber.
13. The catalytic chemical reaction of claim 8 wherein the
platelets are aligned at angle from about 30.degree. to about
60.degree. with respect to the longitudinal axis of the
nanofibers.
14. The catalytic chemical reaction of claim 8 wherein said
graphitic nanofibers are intercalated with an intercalation
component selected from the group consisting of Li, Na, K, Rb, Cs,
Br.sub.2, Cl.sub.2, F.sub.2, ICl, ICl.sub.3, H.sub.2SO.sub.4,
HNO.sub.3, H.sub.2SeO.sub.4, HClO.sub.4, H.sub.3PO.sub.4,
H.sub.4P.sub.2O.sub.7, H.sub.3AsO.sub.4, HF, CrO.sub.2Cl.sub.2,
CrO.sub.2F.sub.2, UO.sub.2Cl.sub.2, FeCl, CuCl.sub.2, BCl.sub.3,
AlCl.sub.3, CoCl.sub.3, RuCl.sub.3, RhCl.sub.3, PdCl.sub.4,
PtCl.sub.4, Cr.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3,
Sb.sub.2S.sub.3, CuS, FeS.sub.2 Cr.sub.2S.sub.3, V.sub.2S.sub.3 and
WS.sub.2.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation-in-Part of U.S. Ser. No. 10/712,247
filed Nov. 13, 2003 which is based on Provisional Application U.S.
Ser. No. 60/426,198 filed Nov. 14, 2002.
FIELD OF THE INVENTION
[0002] This invention relates to the use of high temperature
treatment of graphite nanofibers to increase their catalytic
activity. The heat treated graphite nanofiber catalysts are
suitable for catalyzing chemical reactions such as oxidation,
hydrogenation, oxidative-dehydrogenation, and dehydrogenation.
BACKGROUND OF THE INVENTION
[0003] Much work has been done over the years in the field of
heterogeneous catalysis. Such catalysts have experienced enormous
commercial success in many chemical processes, particularly
petroleum and petrochemical process applications. Conventional
heterogeneous catalysts are typically comprised of one or more
catalytically active metals, particularly Group VIII and Group VI
metals on an inorganic support. The inorganic support is typically
a metal oxide such as alumina, silica, alumina-silica, titania,
magnesia, as well as molecular sieves. Various forms of carbon have
also been suggested as being suitable as catalyst support
materials. For example, U.S. Pat. Nos. 5,538,929 and 6,277,780
teach the use of a phosphorus treated activated carbon as catalyst
supports. Also, U.S. Pat. No. 5,972,525 teaches solid particles
comprised of carbon and metal oxides as being suitable catalyst
supports. While most of the art teaches the use of conventional
carbon, such as activated carbon as catalyst supports, two patents,
U.S. Pat. Nos. 5,569,635 and 6,159,892 disclose the use of
nano-size cylindrical carbon "fibrils" as catalyst supports.
Various catalytically active metals, preferably noble and non-noble
Group VIII metals, such as Fe and Pt, are deposited onto the fibril
support material. Metal oxides, such as Fe.sub.2O.sub.3 can also
act as a catalyst when deposited onto the carbon fibrils.
[0004] While it has been known for many years that both macro and
nano-size carbon particles are suitable support materials for
certain types of catalysts, it has not been known that certain
types of graphitic nanofibers have unique and unexpected catalytic
properties themselves, without the addition of a catalytically
active metal. In co-pending application, U.S. Ser. No. 10/712,247,
it is disclosed that graphitic nanofibers comprised of a plurality
of graphite sheets aligned in directions parallel, perpendicular,
or at an angle to the longitudinal axis of the nanofiber are
suitable for catalyzing a variety of chemical reactions. It has
unexpectedly been found by the inventors hereof that if the
graphite nanofibers in which the graphite sheets are oriented
perpendicular, or at an angle, to the longitudinal axis are
initially treated at high temperatures then their subsequent
catalytic performance is unexpectedly enhanced over that of the
corresponding untreated materials.
SUMMARY OF THE INVENTION
[0005] A catalytic process selected from oxidation, hydrogenation,
dehydrogenation, oxidative-hydrogenation, and
oxidative-dehydrogenation which is catalyzed by a catalyst
composition comprised of graphitic nanofibers which nanofibers are
comprised of a plurality of graphite platelets aligned
perpendicular, or at an angle to the longitudinal axis of the
nanostructure and wherein at least about 50% of the edge sites of
said nanofibers are exposed, wherein said graphite nanofibers,
prior to use in said catalytic process are heat treated in the
presence of an inert gas at temperatures from about 2300.degree. C.
to about 3000.degree. C.
[0006] In another preferred embodiment these high temperature
treated graphite nanofibers can be used as support media for metal
particles.
DETAILED DESCRIPTION OF THE INVENTION
[0007] The catalysts of the present invention are graphite
nanofibers. These graphite nanofibers are themselves comprised of a
plurality of graphite platelets, also sometimes called graphite
sheets, that are aligned, perpendicular, or at an angle to the
longitudinal (growth) axis of the nanofiber. By "at an angle" we
mean that the graphite platelets are aligned so that they are
neither parallel nor perpendicular to the longitudinal axis of the
nanofiber. For example they can be from about 1.degree. to about
89.degree., preferably from about 10.degree. to about 80.degree.,
more preferably from about 20.degree. to about 70.degree., and most
preferably from about 30.degree. to about 60.degree. with respect
to the longitudinal axis of the nanofiber. In the case where the
graphitic sheets are oriented substantially perpendicular to the
growth axis, the carbon nanofibers are sometimes referred to as
"platelet" nanofibers. In the case where the graphitic sheets are
oriented at an angle to the growth axis, the nanofibers are
sometimes referred to as "herringbone" nanofibers. The term
"carbon" is sometimes used interchangeably with "graphite" herein
and the word "nanostructure" is sometimes used interchangeably with
"nanofiber" herein.
[0008] The graphite nanofibers of the present invention are novel
materials having a unique set of properties that include: (i) a
surface area from about 20 to 50 m.sup.2/g, preferably from about
30 to 45 m.sup.2/g, more and most preferably from about 35 to 40
m.sup.2/g, which surface area is determined by N.sub.2 adsorption
at -196.degree. C.; (ii) a crystallinity from about 5% to about
100%, preferably from about 50% to 100%, more preferably from about
75% to 100%, most preferably from about 90% to 100%, and ideally
substantially 100%; (iii) an average pore size from about 10 to 15
nm, most preferably from about 11 to 13 run and ideally 12 nm; (iv)
interstices of about 0.335 nm to about 0.40 nm, preferably about
0.335 nm; and (v) unexpectedly high catalytic properties for
certain chemical reactions. The interstices are the distance
between the graphite platelets. The over all shape of the
nanofibers can be any suitable shape. Non-limiting examples of
preferred shapes include straight, branched, twisted, spiral,
helical, and coiled.
[0009] The graphite nanofiber catalysts of the present invention
can be catalytically grown from suitable unsupported metal powders
in a carbon containing atmosphere. A carbon-containing compound is
decomposed in the presence of the metal catalyst at temperatures
from about 450.degree. C. to about 800.degree. C., preferably from
about 550.degree. C. to about 700.degree. C. It is also preferred
that hydrogen be present during the decomposition of the
carbon-containing compound. The graphite nanofibers of the present
invention are treated in an inert gas environment to a temperature
from about 1800.degree. C. to about 3000.degree. C., preferably
from about 2300.degree. C. to about 3000.degree. C. Preferred inert
gases are helium and argon with helium being more preferred. This
high temperature heat treatment is what gives the graphite
nanofibers of the present invention their unexpected improved
catalytic properties when compared to similar graphite nanofibers
that were not subjected to high temperature heat treatment.
[0010] Metal powdered catalysts suitable for growing the carbon
nanofibers of the present invention include single metals, as well
as alloys and multi-metallics. If the metal catalyst is a single
metal then it is preferably a Group VIII metal selected from Fe,
Ni, and Co. If the catalyst is an alloy or multi-metallic material,
then it is preferred that it be comprised of a first metal
component that will be one or more Group VIII metals and a second
metal that is preferably one or more Group IB metals, such as Cu,
Ag, and Au. Preferred are Cu and Ag with Cu being the most
preferred. It will be understood that Zn can be used in place of
one or more of the Group VIII metals. The Group IB metals are
present in an amount ranging from about 0.5 to 99 at. % (atomic %).
For example the catalyst can contain up to about 99 at. %, even up
to about 70 at. %, or even up to about 50 at. %, preferably up to
about 30 at. %, more preferably up to about 10 at. %, and most
preferably up to about 5 wt. % of Group IB metal with the remainder
being a Group VIII metal, preferably nickel or iron, more
preferably iron.
[0011] Catalysts having a high copper content (70 at. % to 99 at.
%) will typically generate nanofibers that are predominantly
helical or coiled, in overall shape, and which have a relatively
low crystallinity (from about 5 to 25%). Lower concentrations of
copper, e.g., 0.5 to 30 at. % have a tendency to produce spiral and
branched nanofibers, whereas a catalyst with about 30 to 70 at. %,
preferably 30 to 50 at. % copper will produce predominantly
branched nanofibers. A third metal can also be present. There is no
limitation with respect to what the particular third metal can be
as long as it is not deleterious to the desired end product
nanofiber. It is preferred that the third metal, if used, be
selected from the group consisting of Ti, W, Sn and Ta. When a
third metal is present, it is substituted for up to about 20 at. %,
preferably up to about 10 at. %, and more preferably up to about 5
at. %, of the second metal. It is preferred that the catalyst be
comprised of Cu in combination with Fe, Ni, or Co. More preferred
is Cu in combination with Fe and/or Ni from an economic point of
view. A catalyst of which Fe is used in place of some of the Ni
would be less expensive than a catalyst comprised of Cu in
combination with only Ni. Preferred catalysts for producing
graphite nanofibers wherein the platelets are substantially
perpendicular to the longitudinal axis of the nanofiber are Fe and
Fe/Cu multi-metallics. Preferred catalysts for producing graphite
nanofibers wherein the graphite platelets are at an angle, other
than 90 degrees, from the growth axis, are Fe, Fe/Cu
multi-metallics, Fe/Ni multi-metallics, and Ni/Cu multi-metallics.
The preferred temperature range for growing "platelet" graphite
nanofibers is from about 550.degree. to about 650.degree. C.,
preferably from about 575.degree. to about 625.degree. C. The
preferred temperature range for growing the angled "herringbone"
graphite nanofibers is from about 550.degree. to about 580.degree.
C.
[0012] Any suitable method can be used to produce the powdered
metal catalyst for growing the graphite nanocatalysts of the
present invention. As previously mentioned, it is most preferred in
the practice of the present invention that the graphite
nanocatalysts be grown from unsupported metallic powders. A
preferred method for preparing suitable unsupported metal catalytic
powders is the use of colloidal techniques for precipitating them
as metal oxides, hydroxides, carbonates, carboxylates, nitrates,
etc. Such a process typically involves dissolving salts of each
metal of the catalyst in an appropriate solvent, preferably water.
A suitable precipitating agent, such as an ammonium carbonate,
ammonium bicarbonate or ammonium hydroxide is added to the
solution, thereby causing the metal to precipitate out as the
corresponding metal carbonate or hydroxide. The precipitate is then
dried at a temperature greater than about 100.degree. C.,
preferably from about 105.degree. C. to about 120.degree. C., and
more preferably at about 110.degree. C. After drying, the
precipitate is mixed with a suitable dispersing agent and calcined
at a temperature from about 200.degree. to 400.degree. C.,
preferably from about 200.degree. to about 300.degree. C., thereby
converting the individual metals to their respective oxide form.
The milled metal powder mixture can then heated, in a
hydrogen-containing atmosphere, at a temperature from about
400.degree. to about 600.degree. C., preferably from about
450.degree. to 550.degree. C., for an effective amount of time, to
produce the catalyst in its metallic state. By effective amount of
time, we mean that amount of time needed to reduce substantially
all of the metal oxides to the respective metal or alloy having a
suitable particle size. A typical amount of time will generally be
from about 15 to 25 hours. Suitable particle sizes are from about
2.5 nm to about 150 nm, preferably from about 2.5 nm to about 100
nm, and more preferably from about 2.5 nm to about 20 nm. Following
this treatment the chemically reduced catalyst is cooled to about
room temperature in a helium environment before being passivated in
a 2% oxygen/helium mixture for 1 hour at about room temperature
(24.degree. C.).
[0013] Salts of the catalytic metal used for growing the graphitic
nanofiber catalysts of the present invention are salts that are
soluble in water, organic solvents, and diluted mineral acids.
Non-limiting examples of water-soluble salts suitable for use
herein include nitrates, sulfates and chlorides. Non-limiting
examples of preferred salts soluble in organic solvents, which are
suitable for use herein, include formates, acetates, and oxalates.
Non-limiting examples of organic solvents that are suitable for use
herein include alcohols, such as methanol, ethanol, propanol, and
butanol; ketones, such as acetone; acetates and esters; and
aromatics, such as benzene and toluene.
[0014] Carbon-containing compounds suitable for creating an
atmosphere for the growth of the graphitic nanocatalysts of the
present invention are compounds composed mainly of carbon atoms and
hydrogen atoms, although carbon monoxide can also be used. The
carbon-containing compound, which is typically introduced into the
heating zone in gaseous form, will generally have no more than 8
carbon atoms, preferably no more than 6 carbon atoms, more
preferably no more than 4 carbon atoms, and most preferably no more
than 2 carbon atoms. Non-limiting examples of such compounds
include CO, methane, ethane, ethylene, acetylene, propane,
propylene, butane, butene, butadiene, pentane, pentene,
cyclopentadiene, hexane, cyclohexane, benzene, and toluene.
Combinations of gases are preferred, particularly carbon monoxide
and ethylene.
[0015] It may be desirable to have an effective amount of hydrogen
present in the heating, or growth, zone during nanostructure
growth. Hydrogen serves two complementary functions. For example,
on the one hand it acts as a reconstruction agent for the catalyst,
suppresses the formation of metal carbide that results in
deactivation and on the other hand it hydrogasifies, or causes
carbon burn-off, of the carbon structure. By an effective amount,
we mean that minimum amount of hydrogen that will maintain a clean
catalyst surface (free of carbon residue), but not so much that
will cause excessive hydrogasification, or burn-off, of carbon from
the nanostructures and/or substrate structure, if present.
Generally, the amount of hydrogen present will range from about 5
to 40 vol. %, preferably from about 10 to 30 vol. %, and more
preferably from about 15 to 25 vol. %. For some catalyst systems,
such as Cu:Fe, the hydrogasification reaction is relatively slow,
thus, an effective amount of hydrogen is needed to clean the
catalyst in order to keep it clean of carbon residue and maintain
its activity. For other catalyst systems, such as Cu:Ni, where the
activity is so high that excessive hydrogasification occurs, even
at relatively low levels of hydrogen, little, if any, hydrogen is
needed in the heating zone. A Cu:Ni catalyst is so active that it
utilizes essentially all of the carbon deposited thereon to grow
nanofibers, and thus, there is generally no carbon residue to clean
off.
[0016] After the carbon nanofibers are grown, it is required to
heat the final structure in an inert gas at temperatures up to
about 3000.degree. C., preferably from about 1800.degree. C. to
about 3000.degree. C., and more preferably from about 2300.degree.
C. to about 3000.degree. C. Under these conditions, the surface
area of the nanofibers are decreased because up to 50% of the
adjacent edges of the nanofibers undergo a sealing action to form
the type of modified structure of the present invention.
[0017] As previously mentioned, the graphite nanofiber catalysts of
the present invention are suitable for catalyzing a variety of
chemical reactions. Non-limiting examples of chemical reactions
that can be catalyzed with the graphite nanofiber catalysts of the
present invention include oxidation, hydrogenation,
oxidative-dehydrogenation, and dehydrogenation. One preferred
oxidative dehydrogenation reaction is the conversion of
ethylbenzene to styrene.
[0018] Below is a first table setting forth preferred hydrogenation
reactions along with the typical catalytic metal used and reaction
conditions employed.
Hydrogenation Reactions
[0019]
1 Temperature Pressure Reaction Catalyst Range (.degree. C.) (atm)
Benzene to cyclohexane Ni 180-230 20-50 Nitrobenzene to Aniline Pd,
Pt 50-150 1-5 Reductive alkylation of Pt .about.50 .about.1
nitroaromatics Nitriles to amines Co, Ru, Ni 80-200 20-170
Hydrogenation of fats & oils Ni 120-175 1-2
[0020] Below is a first table setting forth preferred oxidation
reactions along with the typical catalytic metal used and reaction
conditions employed.
Oxidation Reactions
[0021]
2 Temperature Pressure Reaction Catalyst Range (.degree. C.) (atm)
Sulfur dioxide to sulfuric acid V.sub.2O.sub.5/K.sub.2O 420-480
.about.1 Ethylene to ethylene oxide Ag 200-250 .about.8 Ethylene to
vinyl acetate Pd 10-130 30 Propylene to acrolein
Bi.sub.2O.sub.3/Mo.sub.2O.sub.3 320-430 2
[0022] It is evident that a number of factors can exert an impact
on the ultimate performance of the carbon nanofber catalysts. When
dealing with active carbons one generally considers the textural
characteristics of the solid with particular emphasis being placed
on the surface area and the pore size distribution. Unfortunately,
the small pore size of active carbons appears to be responsible for
obstructing desorption of the styrene, which blocks surface sites
and eventually poisons the catalyst. A further shortcoming of
active carbons is their propensity to undergo gasification at about
550.degree. C., a temperature close to that where the oxidative
dehydrogenation reaction is conducted. On the other hand, graphitic
materials are more resistant to attack by oxygen. As a consequence,
such carbons are stable at 550.degree. C. and would not be
susceptible to poisoning by adsorption of styrene molecules.
[0023] The catalytic performance of the heat-treated graphite
nanofibers is dependent upon the electrical conductivity of the
materials. This property can be enhanced via intercalation with
various electron donor and acceptor molecules. Inorganic molecules
and compounds that can form intercalation compounds with the
graphite nanofibers include Li, Na, K, Rb, Cs, Br.sub.2, Cl.sub.2,
F.sub.2, ICl, ICl.sub.3, H.sub.2SO.sub.4, HNO.sub.3,
H.sub.2SeO.sub.4, HClO.sub.4, H.sub.3PO.sub.4,
H.sub.4P.sub.2O.sub.7, H.sub.3AsO.sub.4, HF, CrO.sub.2Cl.sub.2,
CrO.sub.2F.sub.2, UO.sub.2Cl.sub.2, FeCl, CuCl.sub.2, BCl.sub.3,
AlCl.sub.3, CoCl.sub.3, RuCl.sub.3, RhCl.sub.3, PdCl.sub.4,
PtCl.sub.4, Cr.sub.2O.sub.3, Sb.sub.2O.sub.3, MoO.sub.3,
Sb.sub.2S.sub.3, CuS, FeS.sub.2 Cr.sub.2S.sub.3, V.sub.2S.sub.3 and
WS.sub.2.
[0024] The present invention will be illustrated in more detail
with reference to the following examples, which should not be
construed to be limiting in scope of the present invention.
EXAMPLES
[0025] Materials
[0026] The "platelet" graphitic nanofibers (P-GNF) used in these
examples were prepared from the decomposition of a carbon
monoxide/hydrogen mixtures over a copper-iron powdered catalyst at
600.degree. C. Prior to use, all nanofibers were treated in dilute
mineral acid for a period of one week to remove associated metal
catalyst particles. Samples of these nanofibers were subsequently
treated in argon for 30 minutes at either 1800.degree. C. or
2330.degree. C. Examination of these heat-treated materials by
high-resolution transmission electron microscopy revealed that many
of the adjacent edges had undergone a sealing action by generating
loops at the exposed regions.
[0027] The gases used in these examples were carbon monoxide
(99.9%), ethylene (99.95%); hydrogen (99.999%), helium (99.99%) and
argon (99.99%) were purchased from Air Products and dried before
use. Reagent grade iron nitrate, cobalt nitrate, nickel nitrate,
copper nitrate and magnesium oxide were used in the preparation of
catalysts for carbon nanofiber growth and were obtained from Fisher
Scientific.
Example 1
[0028] The oxidative dehydrogenation of ethylbenzene to styrene was
carried out in a packed bed tubular quartz flow reactor system. The
flow rates of the gaseous reactants, oxygen and helium, were
regulated by MKS mass flow controllers. Ethylbenzene (EB) was
introduced into the reactor using a syringe pump. The inlet and
outlet gas analyses were performed on-line using a gas
chromatograph equipped with thermal conductivity detectors (TCD)
and flame ionization detectors (FID) detectors. The performance of
each catalyst sample was determined from the conversion of EB, the
selectivity to styrene (ST) and the resulting yield of styrene.
These values were calculated according to the following equations:
1 EB conversion = n EB i n - n EB ex n EB i n ( 1 ) ST selectivity
= n ST ex n EB i n - n EB ex ( 2 ) ST yield = n ST ex n EB ex ( 3
)
[0029] where, n is the number of moles of a given compound, "in"
and "ex" refer to inlet and exit, respectively.
[0030] In this series of experiments the behavior of high
temperature treated graphite nanofibers for synthesis of styrene as
a function of reaction temperature was investigated. Table 1 below
shows the behavior of the modified platelet graphite nanofibers
(P-GNF) that was treated at 2330.degree. C. in argon for 30
minutes. The reaction conditions were as follows: mole ratio
O.sub.2/EB=1.4, EB flow rate=9.33.times.10.sup.-6 mol/min, He=9.8
cc/min, catalyst weight=40.6 mg. In each case, the data were taken
after 22 hours on stream. Examination of these data shows that as
the reaction temperature is progressively raised there is a
corresponding increase in the EB conversion, reaching a maximum
level at about 600.degree. C. On the other hand, the optimum
selectivity to the desired product, styrene, is achieved at
450.degree. C. and the maximum yield of styrene occurs at
575.degree. C.
3TABLE 1 EB ST Temperature (.degree. C.) Conversion (%) Selectivity
(%) ST Yield (%) 450 23.5 100.0 23.7 475 34.3 93.6 32.1 500 47.7
92.8 44.2 547 69.6 76.5 53.3 575 73.7 78.2 57.7 600 80.7 69.9 56.4
625 76.9 70.1 53.8 650 76.4 61.6 47.1
Example 2
[0031] In this set of experiments the heat-treated P-GNFs described
above were reacted in various ethylbenzene/oxygen mixtures at
500.degree. C. Other reaction conditions were as follows: EB flow
rate=9.33.times.10.sup.-6 mol/min, He=9.8 cc/min, catalyst
weight=40.6 mg and the data presented in Table 2 were taken after
22 hours on stream. Inspection of these data shows that as the
O.sub.2/EB ratio is increased there is a concomitant increase in
the conversion of the hydrocarbon. However, as the O.sub.2/EB ratio
is raised above 1.0 the selectivity to styrene declines. The
optimum conditions for the process appear to be an O.sub.2/EB ratio
of about 1.0.
4TABLE 2 O.sub.2/ EB EB Mole Ratio Conversion (%) ST Selectivity
(%) ST Yield (%) 0.5 25.8 99.9 25.8 0.86 39.1 100.0 40.4 1.0 43.8
99.7 43.6 1.4 47.7 92.8 44.2 1.9 48.8 91.2 44.1
Example 3
[0032] This series of experiments was conducted to establish the
activity maintenance and selectivity pattern of the modified P-GNF
catalyst for the ethylbenzene oxidative dehydrogenation reaction at
547.degree. C. as a function of time. Other reaction conditions
were as follows: mole ratio O.sub.2/EB=0.86, EB flow
rate=9.33.times.10.sup.-6 mol/min, He=9.8 cc/min, catalyst
weight=40.6 mg.
5TABLE 3 Reaction Time (h) EB conversion (%) ST selectivity (%) ST
yield (%) 0.50 24.6 50.7 12.5 4.62 41.9 81.7 34.2 5.10 41.2 99.3
40.9 6.65 42.7 100.0 42.0 9.95 45.8 100.0 46.2 10.97 47.5 100.0
48.3 12.48 51.0 94.7 48.3 15.80 51.2 93.9 48.1 18.30 53.8 93.2 50.1
22.73 53.9 89.8 48.3
[0033] Examination of the data presented in Table 3 shows that
following an induction period of about 5 hours, the catalytic
activity of the modified P-GNFs for ethylbenzene conversion
actually increases with time and maintains a very high selectivity
towards the formation of the desired product, styrene for an
extended period of time. This behavior is to be contrasted with
that of the corresponding P-GNFs not heat-treated (Table 4), which
undergoes deactivation after a relatively short time on stream
while maintaining the selectivity for styrene production.
Example 4
[0034] A comparison study was carried out using P-GNFs not
heat-treated and reacted under the same conditions as those used in
Example 3. The reaction conditions were as follows: reaction
temperature 547.degree. C., mole ratio O.sub.2/EB=0.86, EB flow
rate=9.33.times.10.sup.-6 mol/min, He=9.8 cc/min, catalyst
weight=40.4 mg.
6TABLE 4 Reaction Time (h) EB conversion (%) ST selectivity (%) ST
yield (%) 0.25 59.9 85.5 51.2 1.28 48.9 87.1 42.6 1.82 46.7 83.8
39.2 2.35 43.6 88.3 38.5 2.90 38.2 90.1 37.8 3.42 37.8 90.7 36.7
3.93 40.3 87.6 35.3 4.47 40.1 86.7 34.8 9.63 31.6 82.4 26.0
[0035] Inspection of the data given in Table 4 shows that the
catalytic activity declines over the reaction period while the
selectivity pattern remains unchanged. Clearly the performance of
this system is inferior to that displayed by the high-temperature
treated P-GNFs.
Example 5
[0036] In this set of experiments the performance of the modified
P-GNFs catalyst for the ethylbenzene oxidative dehydrogenation
reaction at 500.degree. C. as a function of time was
investigated.
7TABLE 5 Reaction Time (h) EB conversion (%) ST selectivity (%) ST
yield (%) 0.57 38.3 100.0 39.9 1.68 44.5 95.3 42.4 2.73 42.0 100.0
43.1 3.22 43.5 100.0 43.6 3.72 45.2 94.6 42.8 4.27 43.7 99.4 43.5
4.73 44.4 100.0 45.2 5.25 41.3 100.0 43.5 6.00 41.1 100.0 42.3
16.00 41.9 100.0 42.5
[0037] The catalyst had been previously utilized at higher
temperature and was therefore already in an activated state. Other
reaction conditions were as follows: mole ratio O.sub.2/EB=0.86, EB
flow rate=9.33.times.10.sup.-6 mol/min, He=9.8 cc/min, catalyst
weight=40.6 mg. Examination of the data given in Table 5 reveals
that over a period of 16 hours the catalyst performance remains
relatively stable. It is also apparent that under these conditions
that the high-temperature treated P-GNF catalyst exhibits an
exceedingly high selectivity towards styrene production and
moreover, this high level is maintain for the entire period of the
reaction.
Example 6
[0038] In a further series of experiments the catalytic behavior of
a commercial carbon black, XC72 was investigated. This material is
available from Cabot Corporation and has a surface area of 230
m.sup.2/g and an average pore size of 5.2 nm. The conditions used
were the same as those used in Example 5. The oxidative
dehydrogenation of ethylbenzene was carried out at 500.degree. C.
for an extended period of time. Other reaction conditions were as
follows: mole ratio O.sub.2/EB=0.86, EB flow
rate=9.33.times.10.sup.-6 mol/min, He=9.8 cc/min, catalyst
weight=40.5 mg. From the results given in Table 6 it is evident
that the catalyst exhibits a progressive decrease in activity as
the reaction proceeds. Furthermore, the selectivity towards styrene
formation also declines with time on stream. A comparison of the
performance of this type of carbon with the high-temperature
treated P-GNFs shows that the latter material exhibits a superior
performance.
8TABLE 6 Reaction Time (h) EB conversion (%) ST selectivity (%) ST
yield (%) 0.40 41.6 90.3 37.6 1.48 35.6 98.9 35.2 3.52 35.1 94.2
33.1 5.23 33.1 98.8 32.7 9.82 31.8 92.1 29.3 12.53 35.5 80.3
28.5
Example 7
[0039] The data given in Table 7 shows the comparison of the
performance of various materials, including the current commercial
system based on Fe,Cr,K oxides, for the catalyzed oxidative
dehydrogenation of ethylbenzene at 500.degree. C. Other reaction
conditions were as follows: mole ratio O.sub.2/EB=0.86, EB flow
rate=9.33.times.10.sup.-6 mol/min, He=9.8 cc/min, catalyst
weight=40.5 mg. The data were taken 17 hours after the start of the
reaction.
9TABLE 7 Pore (%) EB (%) ST S.A. Size Catalyst conversion
selectivity (%) ST yield (m.sup.2/g) (nm) P-GNF 2330.degree. 39.1
100.0 40.4 40 13.2 P-GNF 1800.degree. 34.2 100.0 34.7 50 11.8 P-GNF
35.1 94.1 33.0 80 6.3 XC-72 34.6 75.5 29.3 230 5.2 Fe, Cr, K oxides
6.9 75.9 5.2 4.4 4.0
[0040] Examination of the results shows some significant features
and highlights the superior performance of the P-GNFs that had been
treated in argon at 2330.degree. C., which is significantly better
than that of the same type of GNF that had been heated to
1800.degree. C. While both of these materials exhibited a 100%
selectively towards styrene, it is the generation of a higher pore
size in the former that appears to be the critical factor. Indeed,
when one considers all the data there appears to be a direct
correlation between pore size and catalytic performance. In sharp
contrast, the magnitude of the surface area of the materials does
not have an impact on the catalytic behavior.
[0041] The electrical conductivity of high temperature treated
"platelet" graphite nanofibers of the present invention were
enhanced by the interaction of concentrated nitric acid at
90.degree. C. Under these conditions HNO.sub.3 species intercalates
between the exposed graphite edge sites to initially form a
compound having the formula, C.sub.6HNO.sub.3 and after continued
uptake, the intercalation compound, C.sub.12HNO.sub.3 is produced.
The achievement of this final stage compound resulted in an
enhancement of a factor of 20 over that of the pristine material.
Furthermore, these compounds are relatively stable in air. The
formation of an intercalation compound was confirmed by X-ray
diffraction analysis in which the expansion of the
d.sub.002-spacing was measured.
* * * * *