Use of helium and argon diluent gases in modification of carbon molecular sieves

Carbon molecular sieves, useful in the separation of air into oxygen and nitrogen, are improved through modification of the micropores of the sieve by contact with the pyrolysis products of a carbon-containing compound in the gaseous state diluted with helium, with or without nitrogen as a part of the diluting gas. Volatile organic compounds, such as trimethylcyclohexane, are used with the diluent gas to narrow the micropore openings of a carbon molecular sieve and increase its kinetic selectivity for oxygen adsorption. Carbon dioxide and helium or argon in the diluent gas are used to open pores available to contacting gases.

FIELD OF INVENTION 
This invention relates to a process for modifying carbon molecular sieves 
(CMS) by pyrolysis of volatile carbon-containing organic compounds in the 
presence of sieve material while using special diliuent gases including 
helium. In another aspect it relates to a process for opening pores in a 
CMS by oxidation with carbon dioxide diluted with helium or argon. 
BACKGROUND 
The modification of carbon molecular sieves (CMS) by prolysis of 
carbon-containing compounds in order to deposit carbon in the pores of the 
sieve is well known. Nitrogen has been the carrier gas used in many such 
processes. For example in U.S. Pat. No. 3,801,513, Munzner, et al., (1974) 
there is a description of obtaining CMS for oxygen separation by treating 
coke having volatile components of up to 5% with a carbonaceous substance 
which splits off carbon at 600.degree.-900.degree. C., thereby narrowing 
the pores present in the coke. It is stated that the average pore size of 
the adsorbent must be below 3 angstroms (Van der Waals diameter) to effect 
oxygen separation from nitrogen. The average pore diameter can be adjusted 
by changing the intensity of the treatment. Coconut shell coke is a 
suitable starting material, among others. A preference is stated for a 
particle size in the range of 0.1 to 20 millimeters and suitable 
carbonaceous substances which can be used in the treatment include 
benzene, ethylene, ethane, hexane, cyclohexane, methanol abnd the like. 
Nitrogen is used in Example 1 both as a carrier gas for benzene and a 
cooling gas. 
Chihara, et al., Proc. Third Pacific Chem. Eng. Congress, Volume 1 (1983) 
discloses that CMS which is a pelletized granular activated carbon can be 
treated by thermally decomposing benzene in a fluidized bed of the CMS to 
deposite carbon thereon and thereby adjust the overall mass transfer 
coefficients of oxygen and nitrogen in the CMS. Nitrogen is disclosed as a 
carrier gas for the benzene. 
U.S. Pat. No. 4,458,022, Ohsaki, et al., (1984) refers to several prior art 
processes for narrowing the micropores of active carbon by precipitating 
soot in the micropores and describes a method said to provide improved 
selectivity for separating nitrogen from air. The method involves using 
coconut shell charcoal and coal tar binder, acid washing, adding coal tar 
and heating to 950.degree.-1000.degree. C. for 10-60 minutes. The coal tar 
is said to penetrate into the surface of the active carbon and decompose 
to grow carbon crystallite on the inner surface of the micropore. It is 
stated that for PSA separation of nitrogen and oxygen, the oxygen 
adsorption capacity should be more than 5 milliliters (STP) per gram and 
the selectivity more than 22 to 23. Nitrogen is used as an inert gas in 
the heating and cooling phases of this treatment. 
Surinova, Khim. Tevrd. Top., Moscow (5) 86-90 (1988) describes obtaining 
carbon molecular sieves for concentration of nitrogen from air by 
carbonizing gaseous coals using benzene vapor and inert gas. The inert gas 
is not identified. In other references such as Japanese Patent Application 
No. Sho 62-176908 (1987) a method for making carbon molecular sieves 
suitable for separating oxygen and nitrogen is disclosed involving the use 
of carbon from coconut shells and coal tar or coal tar pitch binder to 
form particles which are dry distilled at 600.degree.-900.degree. C., 
washed with mineral acid and water and dried, and then impregnated with 
hydrocarbon and heat treated for 10-60 minutes at 600.degree.-900.degree. 
C. in inert gas, for example nitrogen. In this process the inert gas is 
not used as a carrier or diluent of the modifying hydrocarbon which 
instead is impregnated into the carbon base material prior to the heat 
treatment. It is said that this procedure is superior to the use of 
hydrocarbons, such as benzene, pyrolyzed in the gas phase so that carbon 
produced adheres to the carbonaceous surface. 
A similar but earlier disclosure appears in Japanese Publication No. Sho 
49-37036 (1974) which describes making a carbon molecular sieve by 
condensing or polymerizing a phenol resin or furan resin so that the resin 
is adsorbed on the carbon adsorbent and thereafter carbonizing the product 
by heating. Mixtures of the resins can also be used. The resin forming 
material is dissolved in water, methanol, benzene or creosote oil and the 
solution is used to impregnate the carbon adsorbent. Carbonizing is then 
carried out at 400.degree.-1000.degree. C. in an inert gas and a number of 
suitable inert gases are suggested such as nitrogen, hydrogen, helium, 
carbon dioxide, carbon monoxide, and sulfur dioxide. 
There is nothing in the prior art which suggest that, other than being 
inert, the nature of the diluent gas which is used with a gaseous 
carbon-containing compound in a pyrolysis to modify a carbon molecular 
sieve has any significance. Consequently, it is understandable that the 
prior work has consistently used nitrogen, the cheapest of diluent gases, 
for such service. 
SUMMARY OF THE INVENTION 
We have found that in the modification of a carbon molecular sieve in order 
to alter its gas separation characteristics by contacting the sieve under 
pyrolysis conditions with a volatile carbon-containing organic compound, 
the selection of a diluent gas makes a material difference in the results 
obtained. More particularly, we have discovered that the use of helium, 
which normally would be considered an inert gas, produces results which 
carry distinct advantages over the use of nitrogen in this service under 
the same pyrolysis conditions. Under otherwise equivalent conditions with 
a pyrolysis time and temperature which is sufficient to affect the gas 
separation characteristics of the carbon molecular sieve, the use of 
helium with or without nitrogen improves the selectivity of the CMS when 
the modification employs a volatile organic compound which is pyrolyzed to 
deposit carbon and thereby narrow the openings of the micropores of the 
CMS. 
We have also discovered that the use of helium or argon alone or in a 
mixture with nitrogen is superior to the use of nitrogen alone as a 
diluent gas for carbon dioxide which is pyrolyzed over the sieve in order 
to widen pores or open up new porosity. The use of argon or helium or a 
mixture of these gases with nitrogen as a diluent for the carbon dioxide 
has been found to be superior to the use of nitrogen alone in opening 
porosity and producing fast, unselective carbons.

DETAILED DESCRIPTION OF THE INVENTION 
Air separation can be effected over carbon molecular sieve adsorbents which 
separate oxygen from air on a kinetic basis, adsorbing the smaller oxygen 
molecules rapidly relative to the slightly larger nitrogen molecules. In 
order to effect separation, the adsorbent must have pore openings of about 
the molecular diameter of the larger gas in the mixture (nitrogen in air). 
Some chemisorption of oxygen along the pores is believed to contribute to 
the pore size selectivity of oxygen over nitrogen. Control of pore size 
with sub-angstrom precision allows for rapid adsorption of the smaller 
component and slower diffusion of the larger component, resulting in high 
kinetic selectivity. The ability to control, both consistently and 
precisely, the size of the pore openings on a CMS, to tenths of an 
angstrom in the case of air separation, is a major challenge for the 
synthesis of CMS adsorbents. Improved CMS adsorbents are needed to reduce 
the cost of air separation by PSA systems. 
As pointed out in the discussion of the prior art, kinetic selectivity can 
be imparted to microporous carbons by pyrolyzing a reagent that will leave 
carbonaceus residue on the carbon substrate. We have found, using plug 
gauge studies, that pore properties of the carbon substrate are critically 
important to the success of the treatment in imparting oxygen selectivity 
to the product. We have found that the carbon source must be comprised of 
pores of about 4 angstoms in size in order for prior art treatments to 
impart facile kinetic oxygen selectivity over nitrogen. While there has 
been recognition of the usefulness of using a hydrocarbon larger than the 
pore size of the carbon to form a surface barrier by hydrocarbon 
pyrolysis, there has been no appreciation that the carrier gas, e.g. 
nitrogen, which had been thought to be chemically inert, could have an 
impact on the preparation of CMS materials. 
As pointed out in our pending appliction Ser. No. 575,474, filed Aug. 30, 
1990, kinetically oxygen-selective adsorbents can be prepared from a 
variety of porous carbons by forming a carbonaceous surface layer on the 
carbon support in a particulary manner. Selective pyrolysis of a molecule 
that is too large to penetrate the micropores of the carbon support 
produces microporous domains of carbon which have high kinetic selectivity 
for oxygen relative to nitrogen owing to the deposition of carbonaceous 
residue at the pore mouth openings. The invention disclosed in the 
above-referenced application involves a two step method of promoting 
surface barrier formation via pyrolysis of a volatile carbon-containing 
organic compound, preferably a hydrocarbon, by first using a compound that 
is too large to penetrate the small micropores of the carbon support and 
subsequently using a smaller organic compound that is cracked on the 
intermediate product until the remaining micropores are narrowed to about 
4.0 angstoms. As disclosed in that application, helium can be used as the 
carrier gas in either the single step or two step pyrolysis treatment. In 
the two step process, by varying pyrolysis parameters (e.g. upper 
temperature limit or hold time) one can use nitrogen in one of the steps 
and helium in the other. 
According to our present invention the identity of the diluent gas used in 
the transport of the carbon-containing materail to the CMS is important in 
producing a desirable product. Quite unexpectedly, the carrier gas, 
previously thought to be inert, plays a major role in the quality of the 
CMS produced and selection of the correct carrier gas or gases allows an 
extra measure of control over pore size of the CMS product. Moreover, we 
have found that the use of helium or helium/nitrogen blends as the carrier 
gas provides a greater option of treatment times and enables more control 
of the process for fine adjustments in the carbon deposition. 
All molecular sizes refer to those typically described as minimum Van der 
Waals diameters. Here oxygen is 2.8 angstroms in width, while nitrogen is 
3.0 angstroms. This contrasts to the Leonard Jones sigma value where the 
width of the oxygen is 3.46 angstoms and nitrogen is 3.64 angstoms (Ref: 
D. W. Breck, "Zeolite Molecular Sieves", Wiley-Interscience, New York, 
N.Y., page 636). In principle, however, the pores must be somewhat larger 
than the minimum critical dimensions of the diffusing molecule due to 
additional energy barriers (e.g., interaction of the .rho. electron 
density, etc., with the walls of the slit-shaped pores; Ref: M. B. Rao, et 
al., Langmuir, 1 137 (1985)). Thus, we observe that pores for 
distinguishing oxygen from nitrogen should be about 3.8, but less than 
4.3, angstoms. 
The starting molecular sieve support can be any CMS having micropores, 
which are generally considered to have a size less than 20 angstroms, in 
which a majority of the micropores have sizes which are preferably less 
than 8 angstroms but greater than 3.4 angstroms. The invention is 
especially valuable in modification of CMS with micropores which have a 
size of at least 4.5 angstroms, since by using a starting CMS support of 
this character a much less expensive product can be obtained. Coconut 
shell derived carbon is one suitable source for supports which can be used 
to advantage in this invention. 
The treating compound which supplies the carbon for narrowing the pore 
diameter of the support can be any volatile carbon-containing organic 
molecule including hydrocarbons and compounds with hetero atoms, such as 
oxygen, nitrogen, sulfur, silicon and the like, provided that the compound 
can decompose cleanly without forming pore-plugging materials. Examples of 
compounds which are useful include 1,3,5-trimethylcyclohexane, 
1,2,4-trimethylcyclohexane, 1,1-dimethylcylohexane, cineole, isobutylene, 
isobutane, 2,2,3-trimethylbutane, isoctane, cyclohexane, and similar 
compounds, preferably hydrocarbons. 
The conditions of pyrolysis generally include temperatures in the range of 
500.degree.-900.degree. C., preferably about 550.degree.-900.degree. C. 
and pressures under which the treating carbon-containing compound is 
gaseous, preferably about P.sub.HC =0.2, P.sub.TOTAL =1 atm. The flow 
rates and concentration of the treating material as well as temperatures 
can be adjusted along with the duration of the treating step in order to 
modify the effects desired. In general, a lower flow rate produces more 
severe conditions as do higher concentrations of the carbon-containing 
compound, longer times and higher temperatures. To take advantage of the 
use of the diluent gases of helium or argon, the duration of the pyrolysis 
treatment must be sufficient to affect the characteristics of the CMS. 
While these factors are interdependent and can be balanced against each 
other, results are also affected by the amount of carbon-containing 
compound, the size of the reactor, its configuration, preheating and 
volatility of the organic compound. If the organic compound is normally a 
liquid, it can be readily vaporized in the diluent gas which serves as a 
carrier for the organic compound to the treatment zone. Gaseous treating 
compounds can be mixed with the diluent gases over a broad range of 
proportions. As an example, a mixture containing about 20 vol. % of the 
carbon-containing compound in the diluent gas is suitable but normally the 
volume percent of the carbon-containing compound in the total mixture with 
the diluent gas is about 0.5 to 25 percent, preferably 0.9 TO 20 percent, 
and usually is less than 10 percent. 
When using a mixture of helium and nitrogen as the diluent gas, the helium 
content of the diluent should be at least 15 volume percent, and 
preferably at least 25 volume percent, to obtain both good selectivity and 
high capacity in the product CMS for separating oxygen from nitrogen. 
In order to illustrate our invention more fully, the following examples are 
presented which should not be construed to limit our invention unduly. 
In these examples evaluation of the CMS products was made with a 
Circulating Adsorption Unit (CAU). The Circulating Adsorption Unit (CAU) 
had a Servomex oxygen monitor, 570A with 311 cell and bypass plumbing to 
allow 0.5-8 liters per minute flow. This was connected to a Cole Parmer 
pump, (N-7088-48) with a diaphragm head. The pump was modified with a 
variable speed controller and high torque motor, (G. K. Heller, GT-21) 
which allowed the circulation rate to be varied at varying pressures 
(0.2-1.0 atm.) while maintaining consistent pump speed at any given rate 
and pressure. The pump fed a glass cell adsorption unit equipped with a 
thermocouple. The glass cell, in turn, was connected to the oxygen monitor 
through an MKS barometer, pressure transducer 127AA001000A, power supply 
PDR-C-1C. 
The responce time of the O.sub.2 monitor was 7 seconds to 90% of scale, and 
the pump was sized to allow circulation rates of 150-7000 cm.sup.3 /min. A 
compression wave does result from the operation of the single diaphragm 
pump, therefore it is important to record data at a rate which is fast 
relative to the pump rate. This was accomplished using a MACSYM computer, 
Model 120, which was programmed to collect data with adjustable frequency 
throughout the adsorption run. 
The CAU pressure transient is the summation of pressure uptake transients 
for the individual gas components. Using equations for gravimetric uptake, 
equations were derived which describe the pressure and % O.sub.2 traces 
measures on the CAU. System pressure as a function of time is given by the 
expression: 
EQU P=P.sub.i -P.sub.O2 (1-e.sup.-Lt)-P.sub.N2 (1-e.sup.-mt) (Equation 1) 
where; 
P.sub.i =initial system pressure 
P.sub.O2 =oxygen pressure sorbed at equilibrium 
P.sub.N2 =nitrogen pressure sorbed at equilibrium 
L and m are mass transfer coefficients for O.sub.2 and N.sub.2 respectively 
The % O.sub.2 measured versus time for air (21% O.sub.2) is given by the 
expression: 
EQU %O.sub.2 =100[0.21P.sub.i -P.sub.O2 (1-e.sup.-Lt)]/[P.sub.i -P.sub.O2 
(1-e.sup.-Lt)-P.sub.N2 (1-e.sup.-mt)] (Equation 2) 
Note that P.sub.O2, P.sub.N2, and P.sub.i are measured at t=0 and 
t=infinity, and can be obtained from the CAU data. The mass transfer 
coefficients can therefore be obtained by fitting equation 1 to the 
pressure data or by fitting equation 2 to the % O.sub.2 data. The kinetic 
selectivity is the ratio of the mass transfer coefficients, L/m. 
The amount of O.sub.2 sorbed short times (1 min) exceeds the equilibrium 
amount of O.sub.2 sorbed, and gradually decays back to the equilibrium 
value as N.sub.2 slowly diffuses into the micropores and displaces oxygen. 
This behavior is not accounted for by eqs. 1 and 2, and they predict a 
working selectivity that is higher than the actual value. The observed 
"overshoot" of O.sub.2 adsorption above the equilibrium value, which 
occurs in the kinetic region of the experiment is a competitive adsorption 
effect. At short times, when O.sub.2 has largely saturated the adsorbent 
but N.sub.2 has yet to permeate the adsorbent and approach its adsorptive 
capacity, O.sub.2 will cover adsorption sites over the entire range of 
energetics. As N.sub.2 permeates the adsorbent, it displaces much of the 
O.sub.2 that was sorbed. This occurs owing to the higher heat of 
adsorption of N.sub.2 over O.sub.2 on CMS carbons at low pressure 
(.ltoreq.1 atm), and results in the lowest energy state of the 
adsorbate/adsorbent system at equilibrium. The net effect is that the 
apparent equilibrium constant for O.sub.2 adsorption is higher in a 
non-competitive experiment than when O.sub.2 competes with N.sub.2 for 
sites (which occurs as equilibrium is approached). 
An additional term can be added to eqs. 1 and 2 to accound for this 
behavior. Now: 
EQU P=P.sub.i -(P.sub.O2 +P.sub.ex e.sup.-mt)(1-e.sup.-Lt)-P.sub.N2 
(1-e.sup.-mt) (Equation 3) 
EQU %O.sub.2 =100[0.21P.sub.i -(P.sub.O2 +P.sub.ex e.sup.-mt)(1-e.sup.-Lt)]/[i 
P.sub.i -(P.sub.O2 +P.sub.ex e.sup.-mt)(1-e.sup.Lt)-P.sub.N2 
(1-e.sup.-mt)](Equation 4) 
where P.sub.ex is the pressure of O.sub.2 sorbed at short time which 
exceeds the equilibrium pressure of oxygen sorbed. When this additional 
term is added an excellent fit is obtained, and the selectivity value is 
in excellent agreement with values determined gravimetrically and 
volumetrically. 
EXAMPLE 1-19 
A 5A carbon molecular sieve (CST-51) was treated by pyrolysis of 
1,2,4-trimethylcyclohexane at 675.degree. C ., with a space velocity of 
1.0 reactor volume per minute. Various diluent gases were used as shown in 
Table 1. Operating at a fixed set of conditions, the only parameters 
changed from one example to the other were the composition of the diluent 
gas and the time of exposure of the CMS to the treating hydrocarbon. 
A Lindbergh model 55347 3-zone furnace was used with a 2.5 liter quartz 
reactor rotated at 6 rpm is hoizontal plane. The carrier gas and 
hydrocarbon was delivered through a 316 SS tube to the base of the reactor 
tube and the gas was passed over the carbon bed and to the exit. The pump 
was an Isco syringe pump. In a typical procedure, 250 grams of carbon were 
charged into the quartz reactor and air was purged from the system using 
the diluent gas, either helium, nitrogen or argon, such that the volume of 
the reactor was purged every minute. After fifteen minutes, while 
maintaining this purge, the reactor was rotated at about 6 rpm and brought 
to 675.degree. C. at a rate of 10.degree. C. per minute. When 675.degree. 
C. was attained, 1,2,4-trimethylcyclohexane was added to the diluent gas 
at 10 cc's per hour and vaporized by adding the liquid to a packed bed of 
stainless steel bearings at 120.degree. C. This produced a hydrocarbon 
concentration in the treating gas of about 0.9 vol. %. Coking times of 2 
to 5 hours were employed. The carbon was cooled to room temperature while 
a diluent purge without the TMC was maintained. 
The CMS of each example was then tested in a CAU and Table 1 gives the 
initial and final pressures of these tests, the minimum oxygen percentage 
reached during the separation produre, the time required to reach this 
minimum level of oxygen and the mass transfer coefficients for oxygen (L) 
and nitrogen (m). The ratio of these values L/m gives a selectivity for 
air separation. The percent of oxygen at the minimum level is a measure of 
the ability of the CMS to sorb oxygen at a rate indicated by the time. The 
initital pressure indicates the degree of pore restriction of the sieve 
since, for an oxygen selective CMS, the initial pressure drop to between 
about 590 to 605 torr represents filling of dead volume. The final 
pressure provides a qualitative measure of the capacity of the CMS whether 
it is selective or not. 
Under the conditions employed and reported in Table 1, the use of nitrogen 
as the diluent gas for either 2.5 or 5 hours did not produce a suitable 
CMS for oxygen separation. Although Example 2 which used nitrogen for 5 
hours treatment did show separation activity, the CMS lacked capacity as 
indicated by the high final pressure and the long time (35 minutes) 
required to reach the minimum oxygen level. On the other hand Examples 3-8 
in which helium was used at treatment times from 2.5 to 3 hours produced a 
CMS which has good selectivity for oxygen as well as good capacity. These 
examples show particularly high values (greater than 3) for L which 
indicate a good mass transfer rate for the oxygen. This value in many 
cases is more important than the selectivity value which is the ratio of L 
to m although preferred values for selectivity should be greater than 20. 
Examples 9 and 10 show the use of helium for a 5 hour treatment. The CMS 
appear to be over treated and severely pore blocked because the separation 
time was quite slow. Under these conditions using helium as the diluent 
gas, it would be desirable to keep the treatment time to less than 5 
hours. 
Examples 11-14 show the use of argon as the treatment gas for periods of 
2.5, 3.3 and 5 hours. The CMS of Example 11 was insufficiently restricted 
in its microporosity. The CMS of Example 9, however, with a treatment time 
of 3.3 hours, evidenced more restricted porosity with about the same 
capacity. Argon does not offer the flexibility of treatment afforded by 
the use of helium in the diluent gas. 
Examples 15-17 show the use of blends of helium and nitrogen which alters 
the performance of the CMS products. Carbons produced at cracking times of 
2.5 hours using helium/nitrogen blends of 50 and 75% helium resemble 
carbons modified in nitrogen alone. However, a 5 hour cracking time with a 
50/50 blend of helium and nitrogen produced a carbon (Example 16) having 
adsorption characteristics faster than that produced at a 5 hour treatment 
time with either nitrogen or helium alone, even though this material, 
because of pore blocking, would not be attractive for air separation by 
pressure swing adsorption. This result is surprising, however, in view of 
the results obtained with nitrogen and helium alone. Although the blends 
did not impart selectivity at the short cracking times of 2.5 hours under 
these conditions, the result of the 5 hour cracking test suggests that a 
diluent blend of nitrogen and helium has a synergistic effect on the 
cracking process. 
Although argon is shown by the data of Table 1 to be a less effective 
diluent than helium for modifying the 5A-CMS with trimethylcyclohexane, it 
is as effective as nitrogen. An exposure of 2.5 hours to the TMC with 
argon as diluent imparted essentially no selectivity (Example 11). By 
extending the exposure time to 3.3 hours, the oxygen selectivity was 
improved with good capacity as shown by Example 12, but the N.sub.2 rate 
(m) was high. The use of nitrogen in Example 1 for 2.5 hours did not 
change the oxygen adsorption characteristics of the 5A-CMS and although, 
as shown by Example 2, a longer cracking time of 5 hours does impart some 
selectivity, as indicated by the minimum oxygen level, the carbon was 
slow, pore blocked, and had lost capacity compared to the product of the 
shorter treating period. 
Examples 18 and 19 show that blends of argon and nitrogen do not evidence 
the same synergistic behavior as the blends of helium and nitrogen. The 
CMS of Example 18 was non-selective and that of Example 19 had a shorter 
time to the minimum oxygen level than Example 2 using nitrogen, but longer 
than Examples 13 and 14 using pure argon. 
TABLE 1 
__________________________________________________________________________ 
Treating:* 
Time 
Testing: (CAU) 
at Initial 
Final Time to 
Example 
Diluent 
Temp. 
Pressure 
Pressure 
Minimum 
Minimum Selectivity 
No. Gas (hr) 
(torr) 
(torr) 
% O.sub.2 
(min.) 
L m L/m Comments 
__________________________________________________________________________ 
1 N.sub.2 
2.5 400 270 21 0 N.C.** 
N.C. 
-- non-selective 
2 N.sub.2 
5.0 590 560 15 35 N.C. N.C. 
-- slow, lacked 
capacity; pore 
blocked 
3 He 2.5 534 280 13.5 0.4 9.2 1.5 
6.1 selective, good 
capacity, N.sub.2 
rate high 
4 He 2.75 
552 270 12.9 0.3 11.5 0.9 
13 selective, good 
capacity 
5 He 2.75 
581 285 14.1 0.3 10.1 0.75 
13 selective, good 
capacity 
6 He 2.83 
588 284 13.1 0.4 10.0 0.34 
29 selective, good 
capacity 
7 He 2.90 
594 285 12.9 0.4 9.4 0.33 
28 selective, good 
capacity 
8 He 3.0 591 285 12.5 0.5 9.4 0.17 
55 good sel., good 
cap. good N.sub.2 
rate 
9 He 5.0 609 495 13.9 35 N.C. N.C. 
-- slow, pore 
blocked 
10 He 5.0 616 495 12.8 34 N.C. N.C. 
-- slow, pore 
blocked 
11 Ar 2.5 379 273 19 0.3 N.C. N.C. 
-- non-selective 
12 Ar 3.3 531 270 15.3 0.3 9.5 1.5 
6.3 selective, good 
capacity, N.sub.2 
rate high 
13 Ar 5.0 589 340 13.2 2.2 2.2 0.03 
73 pore blocked, 
rates slow 
14 Ar 5.0 604 332 13.0 2.3 2.3 0.04 
58 pore blocked, 
rates slow 
15 He:N.sub.2 
2.5 465 276 21 0 N.C. N.C. 
-- non-selective 
50:50 
16 He:N.sub.2 
5 603 414 13.1 8.7 0.55 
0.006 
92 time to min. 
50:50 shorter vs. pure 
gas but pore 
blocked 
17 He:N.sub.2 
2.5 497 286 21 0 N.C. N.C. 
-- non-selective 
75:25 
18 Ar:N.sub.2 
2.5 408 278 21 0 N.C. N.C. 
-- non-selective 
50:50 
19 Ar:N.sub.2 
5.0 598 321 12.8 4.0 1.97 
0.04 
49 low capacity, 
50:50 time to min. 
shorter vs. pure 
N.sub.2 
__________________________________________________________________________ 
*Treatment of 5A CMS (CST51) with 1,2,4TMC at 675.degree. C., space 
velocity = 1.0/min. 
**N.C. = not calculated 
EXAMPLES 20-43 
In Table 2, data are summarized for another sequence of runs which were 
made using a different, smaller furnace (producing 15 grams versus 250 
grams of CMS), without rotating the sorbent in the furnace, and using a 
vaporizer instead of an LC pump for the TMC addition producing a different 
amount of volatile hydrocarbon in the diluent gas. 
Example 31 is representative of the procedure used to modify 15 grams of 
CMS. Using a TMC coked CST5A material in a 200 cc sized quartz tube within 
a Lindbergh furnace, house nitrogen was fed into the reactor (107 cc's per 
minute nitrogen) with about 107 cc's of helium. The reactor was heated to 
675.degree. C. at about 10.degree. C. per minute and held at 675.degree. 
C. for ten minutes and then the gas mixture was redirected through a 
heated reservoir of TMC at 75.degree. to 85.degree. C. and this TMC was 
added to the reactor over 4 hours. The treating gas (TMC plus diluent) had 
a hydrocarbon concentration of less than 20 vol. %. Then the gas mixture 
was redirected to avoid the TMC and to cool the furnace. 
Data of Examples 20, 24, 28 and 31 are also plotted and shown in FIG. 1, 
where the difference produced by the diluent gases can be discerned more 
dramatically. FIG. 1 is a CAU plot which illustrates the change in oxygen 
concentration over the CMS as a function of time. The coking time in each 
case was 4 hours while the temperature of 675.degree. C. and the ramp rate 
of 10.degree. C. per minute were maintained constant. The use of nitrogen 
as the diluent gas produced a slow, pore blocked adsorbent (Ex. 20). The 
use of helium produced a faster CMS as demonstrated by the drop of percent 
oxygen from 21% to the minimum in the curve and the rate of return to the 
final oxygen level (Ex. 24). The use of 50% helium in a mixture with 
nitrogen produced a fast CMS (Ex. 31), apparently as good as or better 
than the use of pure helium alone in this set of conditions. A mixture of 
28% helium and nitrogen (Ex. 28) produced a CMS similar to the one 
obtained with helium as the carrier gas. 
Referring to the data in Table 2, the CMS adsorbents of Examples 24 and 25 
using helium were clearly superior to those of Examples 20 and 21 which 
used nitrogen as the diluent gas. Example 27 using a mixture of 13% helium 
in nitrogen produced a selective product but it was severely pore blocked. 
Examples 28, 29, 30, 31 and 32 however, all demonstrate helium-nitrogen 
blends ranging from 28% helium to 50% helium, which produced CMS 
adsorbents having good selectivity and capacity. The CMS of Example 34 
using diluent gas containing 72% helium in nitrogen was somewhat slower, 
but still had good selectivity. 
Example 35 using pure argon as the diluent gas and a treating time of 4 
hours produced a sieve which was pore blocked. As shown, however, by 
Example 42, reducing the hold tiem to 1.5 hours enables a selective 
adsorbent to be prepared using argon as the diluting gas. Mixtures of 
argon with nitrogen as shown by Examples 37 and 38 produced CMS products 
which were selective in their separation but the product of Example 37 was 
severely pore blocked and the product of Example 38 was slow and pore 
blocked. At a shorter exposure time (Example 39) a 50/50 blend of 
argon/nitrogen offered no advantage over similar treatments with the pure 
gases (Examples 23 and 42). 
It is apparent that a mixture of nitrogen and argon does not have the same 
beneficial effect as a mixture of nitrogen and helium as the diluent gas 
in treating a CMS with a hydrocarbon gas at pyrolysis temperatures. 
TABLE 2 
__________________________________________________________________________ 
Treating:* 
Time 
Testing: (CAU) 
at Initial 
Final Time to 
Example 
Diluent 
Temp. 
Pressure 
Pressure 
Minimum 
Minimum Selectivity 
No. Gas (hr) 
(torr) 
(torr) 
% O.sub.2 
(min.) 
L m L/m Comments 
__________________________________________________________________________ 
20 N.sub.2 
4 604 398 13.3 5.4 1.06 
0.003 
350 severely pore- 
blocked 
21 N.sub.2 
4 317 12.6 2.3 2.07 
0.04 
52 lacked cap., 
pore-blocked 
22 N.sub.2 
3 491 14.2 26.sup.(a) 
.sup.(b) 
.sup.(b) 
-- pore-blocked 
23 N.sub.2 
1.5 288 13.8 0.3 11.8 
0.46 
26 selective 
24 He 4 601 299 12.3 0.7 6.51 
0.12 
54 good rates, 
sel. & cap. 
25 He 4 598 286 12.6 0.7 6.72 
0.14 
48 good rates, 
sel. & cap. 
26 He 1.5 non-selective, 
fast 
27 He:N.sub.2 
4 469 12.7 58.9 .sup.(b) 
.sup.(b) 
-- severely pore 
13:87 blocked 
28 He:N.sub.2 
4 601 287 12.7 1.1 4.07 
0.10 
41 good sel., rates 
28:72 somewhat slow 
29 He:N.sub.2 
4 279 12.6 0.5 9.03 
0.22 
41 good sel., rates 
28:72 & cap. 
30 He:N.sub.2 
4 309 12.8 1.5 3.42 
0.05 
68 good sel., rates 
37:63 somewhat slow 
31 He:N.sub.2 
4 595 285 12.5 0.6 7.39 
0.23 
32 good sel., rates 
50:50 & cap. 
32 He:N.sub.2 
4 300 12.2 1.2 3.83 
0.09 
43 good sel., rates 
50:50 somewhat slow 
33 He:N.sub.2 
1.5 281 15.6 0.3 8.18 
1.49 
5 fast, slightly 
50:50 sel., good 
capacity 
34 He:N.sub.2 
4 301 14.6 1.3 3.44 
0.08 
43 slower, good 
72:28 selectivity 
35 Ar 4 453 14.5 58.5.sup.(a) 
.sup.(b) 
.sup.(b) 
-- pore blocked 
36 Ar 4 415 12.6 9.6.sup.(a) 
.sup.(b) 
.sup.(b) 
-- pore blocked 
37 Ar:N.sub.2 
4 458 13.5 58.9.sup.(a) 
.sup.(b) 
.sup.(b) 
-- severely, pore 
50:50 blocked 
38 Ar:N.sub.2 
4 368 12.7 3.7 1.30 
0.023 
57 slow, pore 
25:75 blocked 
39 Ar:N.sub.2 
1.5 292 14.3 0.4 8.57 
0.37 
23 good rates, 
50:50 good selectivity 
40 Ar 4 541 18.3 59.sup.(a) 
.sup.(b) 
.sup.(b) 
-- pore blocked 
41 Ar 2 377 14.1 0.2 .sup.(b) 
.sup.(b) 
-- low capacity, 
slow, pore 
blocked 
42 Ar 1.5 275 13.6 0.3 10.9 
0.48 
23 selective 
43 Ar 1.0 286 -- -- -- -- -- non-selective, 
fast 
__________________________________________________________________________ 
*Treatment of 5ACMS (CST50) with 1,3,5TMC at 675.degree. C. 
.sup.(a) In this series of examples, when the time to minimum percent 
oxygen is greater than 2 minutes, the L and m values became meaningless 
due to a poor fit of the data to Equation 4. In many cases this is caused 
by the sieve material so kinetically slow (due to extensive but not 
necessarily complete pore blockage) that it is difficult to desorb the 
oxygen from the sorbent, thereby making it unsuitable for air separation. 
.sup.(b) Fit not performed or gave suspect values since equilibrum was no 
achieved in one hour. 
We have found that the identity of the diluent gas also has a significant 
effect upon the oxidative treatment of carbon molecular sieves with carbon 
dioxide at elevated temperatures. Such oxidative treatment is used to 
widen pores and open porosity of CMS so that it will sorb both oxygen and 
nitrogen rapidly, thereby making a carbon precursor useful as a starting 
material for hydrocarbon pyrolysis and carbon deposition to narrow pore 
openings in a controlled manner. The following example illustrates the 
advantageous use of different carrier gases in such an oxidative 
pretreatment. 
EXAMPLE 44 
In another series of runs, it was demonstrated that the identity of the 
diluent gas also has an effect upon the oxidation of slow, unmodified CMS 
precursors by carbon dioxide. Carbon dioxide is known to create additional 
porosity in CMS material by the following reaction: 
EQU C+CO.sub.2 .fwdarw.2CO 
Carbon dioxide was used to widen existing pores and open up new porosity in 
the carbon sieves. The result was a carbon adsorbent which was faster in 
rate for both oxygen and nitrogen adsorption. Diluent gases for the 
CO.sub.2 were used as follows: 
Run A--25% CO.sub.2, 75% N.sub.2 
Run B--25% CO.sub.2, 75% Ar 
Run C--25% CO.sub.2, 25% Ar, 50% N.sub.2 
Run D--25% CO.sub.2, 75% He 
All percentages are by volume. 
The results are presented by the data which are plotted in FIG. 2, showing 
plots of pressure drop against time for nitrogen sorption by the 
adsorbents of Runs A-D. Nitrogen was chosen as the model adsorbate since 
changes in its absorption rate are easily detected. Using a mixture of 
nitrogen and 25% carbon dioxide (Run A), the resulting adsorbent was quite 
slow in reaching an equilibrium value. On the other hand, the use of 25% 
carbon dioxide with argon (Run B), helium (Run D), or a mixture of 25% 
argon in nitrogen (Run C), all produced fast, unselective carbons. All 
materials were treated for one hour at 800.degree. C. with a ramp speed of 
10.degree. C. per minute and 25% carbon dioxide in the indicated diluent 
gases. 
While it is not fully understood why such dramatic differences appear 
between gases which are normally thought to be inert, it is clear that 
helium and argon are superior to nitrogen as diluent gases in oxidative 
CMS modification and that helium is better than either nitrogen or argon 
in CMS pore narrowing by hydrocarbon pyrolysis. In principal, one would 
not expect that these gases (argon, helium or nitrogen) would control 
oxidation or carbon deposition and pore size which is based on a chemical 
reaction with the surface of the starting CMS material. While not to be 
bound by theory, it is believed that the effects may be related to size, 
shape and/or thermal conductivity of the carrier gases. The pores need to 
be wide enough to allow reasonable transport rates for oxygen and nitrogen 
in the pore channels. Pores of about 4 angstroms with pore mouth diameters 
of about 3.8 angstroms are effective for kinetic separation of air in the 
CMS adsorbents. The molecular dimensions of the gases and/or the monatomic 
versus diatomic nature of the helium and argon versus nitrogen, may 
influence the location within the pore where cracking occurs. 
Alternatively, the higher thermal conductivity of helium may be important 
in transferring heat into or out of the sorbent as pore size is being 
developed or controlled. Table 3 summarizes the kinetic diameters and 
thermal conductivity of the gases used. 
TABLE 3 
______________________________________ 
Kinetic Diameter.sup.a 
Thermal Conductivity.sup.b at 100.degree. C. 
Gas Angstroms 10.sup.-5 cal/sec-cm.sup.2 /(.degree.C./cm) 
______________________________________ 
He 2.6 41.6 
Ar 3.4 5.2 
O.sub.2 
3.46 7.6 
N.sub.2 
3.64 7.5 
CO.sub.2 
3.3 5.3 
______________________________________ 
.sup.a From LennardJones(6-12), o value; D. W. Breck, "Zeolite Molecular 
Sieves", WileyInterscience, New York, NY, 1974, p636. 
.sup.b A. J. Gordon and R. A. Ford, "The Chemists Comparison", 
WileyInterscience, New York, NY, 1972, p. 394. 
Whatever the reason for the results, the effect, particularly with the 
mixture of nitrogen and helium showing some synergistic character, is 
remarkable and unpredictable. 
Other advantages and embodiments of our invention will be apparent to those 
skilled in the art from the foregoing disclosure without departing from 
the spirit or scope of our invention.