GAS-PHASE ADSORPTION
INTRODUCTION. In this experiment you will study the adsorption of gaseous
nitrogen and dimethyl ether on a molecular sieve. Molecular sieves are commonly used
as adsorbents and solid supports in chromatography. The mechanism of their mode of
action is instructive in understanding the function of catalysts based on zeolites. The
procedure is routinely employed in the characterization of catalysts since the adsorption
of a substrate on a catalytic surface is the first step in the mechanism of catalysis. The
experiment will also introduce you to manipulations on a vacuum line.
BASIS FOR THE EXPERIMENTAL PROCEDURE. The apparatus is displayed
in Figure 1. Broadly speaking, it consists of a vacuum pump, pressure gauges, a ballast
for storing the gas, a sample cell containing the adsorbent, a source of the gas, and a
manifold that connects the components. The general approach is quite straightforward.
One transfers a known amount of gas to the system and with the aid of a pressure gauge
measures the amount of gas adsorbed by the adsorbent.
ballast
MKS-C
TC
vent
MKS
A
B
trap
cell
TC-C
sample
PT50
Figure 1. Components of the vacuum line. Key: A, stopcock A; B, stopcock B; ballast,
ballast cell connected to the manifold via stopcock C; cell, tube with the adsorbent,
connected to the manifold via stopcock D; MKS, MKS Baratron pressure gauge; MKS-C,
controller for the MKS gauge; PT50, Leybold PT50 turbomolecular pump system;
sample, cylinder of dimethyl ether with a regulator, connected to the manifold via tygon
tubing and stopcock E; TC, thermocouple gauge; TC-C, controller for TC.
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A detailed discussion of the vacuum system is necessary in order to understand
the manipulations and the calculations that you will employ to analyze the data. The goal
is to determine for a substance the number of moles adsorbed on an adsorbent as a
function its partial pressure. One starts at the right with a Leybold PT50 turbomolecular
pump system that has two components, a roughing pump and a turbomolecular pump.
The latter component is routinely used in the semiconductor industry to achieve high
vacuum. The PT50 system is connected to the glass manifold by a series of stainless
steel elbows and a compression fitting. The vacuum system is isolated from the
remainder of the system by stopcock A. Following stopcock A is a trap, normally cooled
with liquid nitrogen, which functions to prevent condensibles from the manifold from
reaching the vacuum pumps. The trap is connected to the remainder of the manifold by
an O-ring junction.
Working to the left, one encounters a vent valve A' and the main valve B. You
will require the volumes of the components to the left of stopcock B. In particular, the
following volumes are relevant to the analysis: VB, the volume of the ballast which is
connected to the manifold via stopcock C; VC, the free volume of the sample cell (total
interior volume less the volume occupied by the adsorbent) that is connected to the
manifold via stopcock D; and VM, the total volume of the remainder of the manifold to
the left of stopcock B. The value of VB is 1.0454 liter. You will determine the values of
VC and VM. The samples, nitrogen and dimethyl ether, are introduced via stopcocks E'
and E, respectively. Pressures in the range of 0-1000 torr are measured using the MKS
Baratron gauge at the right of the manifold. Its accuracy is 0.1 torr. Measuring pressures
below 1 torr is achieved with the aid of the thermocouple gauge. It is not very accurate
but allows one to measure pressure over several orders of magnitude.
One begins the procedure by completely evacuating all components of the system.
The vent and stopcocks E and E' are closed during this step. Then one closes stopcocks
B and D and via stopcock E or E' introduces gas into the manifold and ballast volumes
(VM + VB) until a pressure pa is reached. Stopcock B will remain closed for the duration
of the measurements. The total number of moles of gas in the system at this initial point
is given by equation (1):
pa[1](VM + VB) = nT[1]RT (1)
The "1" in brackets indicates that this is the first measurement of the total number of
moles of gas in the system prior to adsorption. One then opens stopcock D and waits
until the system reaches equilibrium. The pressure reading will drop because of
expansion into the sample cell and adsorption of the gas. Measuring the pressure, pp, is
the last step of the measurement cycle which is repeated until the adsorbent is saturated.
The cycle is repeated N times. Each further cycle of the experiment consists of
the following steps.
(1) Close stopcock D.
(2) Open stopcopck E(E') and introduce additional gas to the system.
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(3) Measure the pressure, pa (a = ante or before exposure of the additional
gas to the adsorbent).
(4) Open stopcock D and wait until the system has reached equilibrium.
(5) Remeasure the pressure, pp (p = post or after exposure of the additional
gas to the adsorbent).
The manipulation of the data provides a wonderful illustration of gas laws and
stoichiometry. The goal of this section is the derivation of an expression for nac[i], the
total number of moles of gas adsorbed at each step i. At the end of cycle i-1, the total
number of moles in the gas phase before the addition of more gas in the next cycle is
given by equation (2):
pp[i-1](VM + VB + VC) = npT[i-1]RT
(2).
This portion of this that is present in the ballast and the manifold is given by
pp[i-1](VB + VM) = npMB[i-1]RT
(3).
Then stopcock D is closed. Cycle i is initiated and the additional gas fills just the ballast
and manifold. The pressure at this point is given by
pa[i](VB + VM) = naMB[i]RT (2iN) (4).
By combining equations (3) and (4) obtains the following result for the number of moles
of gas added in step i:
nadd[i] = naMB[i] – npMB[i-1] (2iN) (5)
Note that nadd[i] (the result for step i = 1) is given by equation (1). That is, nT[1] =
nadd[1]. At this point the total number of moles of the gas in the gas phase is given by the
sum of the amount in the ballast and manifold plus the amount in the cell. That is,
nT[i] = naMB[i] + pp[i-1]VC/RT (2iN) (6)
Note, however, that nT[1] is given by equation (1). Finally, stopcock D is opened and the
gas in the ballast and manifold that was supplemented by the addition fills the cell and
additional adsorption occurs. Once the system comes to equilibrium, the total number of
moles in the gas phase is given by
pp[i](VB + VM + VC) = nTa[i]RT (1iN) (7).
The number of moles of gas adsorbed in step i is therefore given by
na[i] = nT[i] – nTa[i] (1iN) (8).
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Finally, the required total number of moles of gas adsorbed at each step i is given by the
sum of incremental contributions calculated from equation (8). That is,
i
nac[i] =
 na[j]
(1iN) (9).
j=1
MODELS FOR THE RESULTS. Various models have been proposed for the
adsorption of gases. The are discussed in Adamson's monograph on surface chemistry.
The simplest but yet effective model was proposed by Nobel Laureate Irving Langmuir.
It works best when there is a strong interaction between the molecules and the adsorbent
and the molecules in the high pressure regime, i.e. saturation, form a monomolecular
layer on the adsorbent. It follows from a simple mechanism: M(g) + A(s)  MA(s).
Equation (10) expresses the Langmuir adsorption isotherm, the relationship between the
pressure of the gas and the moles of adsorbed substance:
 = n/n = Kp/(1 + Kp) (10)
 = fraction of total coverage
n = number of moles of adsorbed gas (nac[i] from equation (10) )
n = number of moles of gas absorbed at saturation
p = partial pressure of the gas (pp[i], NOT pa[1])
K = ka/kb = binding equilibrium constant
ka = rate constant for the forward, adsorption step
kb = rate constant for the reverse, desorption step
Note that an identical relationship applies in the case of binding of a substrate to a protein
in the case of non-cooperativity. If the adsorbent is a catalyst, adsorption (i.e. binding) is
followed by the molecular transformation of the bound and therefore activated substrate
to form the product. This Languir model for heterogeneous catalysis is identical with the
Michaelis-Menten model for enzyme catalysis.
The model can be tested in several ways. If the term Kp is small (small binding
constant and/or low pressure), equation (10) simplifies to its linear form,  = Kp or n =
nKp. However, if binding is not weak, the full equation must be used to describe the
data. The equation can be linearized in several ways. The simplest approach involves
taking the reciprocal. After re-arranging terms, one obtains
1/n = (1/Kn)(1/p) + 1/n (11).
Equation (11) is known as a Benesi-Hildebrand plot. Its equivalent in enzymology is the
Lineweaver-Burke plot. It has been shown that this approach does not yield the most
robust results. A statistically better approach is a Scatchard plot (equation (12)) which
can be obtained from equation (10) by a few steps of algebraic manipulation:
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n/p = nK – nK (12).
Although the Langmuir isotherm handles many cases well, it does not handle all due to
factors such as multi-layer adsorption and interaction between binding sites, i.e.
cooperativity. In level of sophistication, the next successful model was developed by
Emmett, Brunauer, and Teller. Their relation, usually known as the BET isotherm, is
 = n/nmon = cz/{(1-z)[1-(1-c)z]} (13)
nmon = number of moles substrate required to form a monolayer (not
necessarily the number of moles at saturation)
z = p/p* (calculable since p* is known)
p* = vapor pressure of the substrate
c  exp[(Hdes - Hvap)/RT] (Since Hdes is always greater than Hvap,
the constant c is greater than one.)
The BET isotherm reduces to the Langmuir isoterm in the limit of low pressure if one
associates c/p* with K. Equation (13) can be rearranged to yield equation (14) which is
in a form that can be tested by the method of linear least squares.
z/[(1-z)n] = 1/cnmon + {(c-1)/(cnmon)}z (14)
EXPERIMENTAL PROTOCOL
SETUP OF THE SYSTEM
1) Do not perform any of these steps until you have first thoroughly read the entire
handout for the experiment and have had the pre-lab orientation session with the
instructor. In closing the stopcocks, don't turn them beyond their limits or you will crack
the vacuum system. A black stripe has been drawn on each of the brown plastic plugs.
The stripe will be close to the 12:00 position when the stopclock is closed. The instructor
will point out the appearance of the plastic plug as it makes contact with the glass upon
closing.
2) Secure a supply of liquid nitrogen from Room 21 in a large dewar. Do not close the
door of the room while you are securing the cryogen. Asphyxiation is a real possibility in
a closed room with limited ventilation.
3) Using the O-ring and the clamp, connect the trap to the manifold. Place the dewar in
position but don't add any liquid nitrogen yet.
4) Close stopcock B and the stopcock connected to the vent and open stopcock A. Turn
on the roughing pump but not the turbomolecular pump. When the switch on the PT50
controller is in the 12:00 position, the unit is completely off. When the switch is turned
clockwise to the next position at roughly 2:00, the roughing pump is turned on.
5) Wait a few seconds for the pump to evacuate the trap and the manifold up to stockcock
B. Slowly fill the dewar in which the trap is immersed with liquid nitrogen. Cover the
top of the dewar with a towel to reduce the rate of evaporation of the liquid nitrogen.
Nota bene! You did not add the liquid nitrogen until after the trap was evacuated.
Consider the following scenario. An open, unevacuated trap is immersed in liquid
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nitrogen. Since the boiling point of oxygen is greater than that of nitrogen, condensed
oxygen will accumulate. This is a dangerous situation as flammable substances burn
more readily in pure oxygen and high pressures will develop when the oxygen boils. In
short, liquid nitrogen is a useful, non-toxic cryogen but must be used with caution.
6) Close the stopcocks to the inlet lines that drop down from the system, e.g. stopcocks D
and E. Open stopcocks C and B and evacuate the ballast and the manifold. The
stopcocks to both pressure gauges should be open.
7) Plug in the power cords for the controllers to the two pressure gauges. The units do
not have on/off switches. The controller for the MKS gauge will require a few seconds
for initialization. It reads in the range 0-1000 torr with an accuracy of 0.1 torr. The
thermocouple gauge should give a reading in the vicinity of 50-80 mtorr.
8) Assemble the cell. Use Apiezon N stopcock grease. The instructor will demonstrate
the proper method of using stopcock grease with standard taper joints. Attach the empty
cell to the system using the male ball-and-socket fitting below stopcock D (cf. Figure 2).
Attach a clamp to the socket and lightly support the cell. Then evacuate it by opening
stopcock D.
9) When the cell has been evacuated, turn on the turbomolecular pump by turning the
switch on its control panel to the 4:00 position. The thermocouple gauge should display a
rapid decrease in pressure to 1-2 mtorr. If the MKS Baratron gauge does not read zero
within its uncertainty of 0.1 torr, consult the instructor for an adjustment.
CALIBRATION OF THE VACUUM LINE
1) VB, the volume of the ballast up to stopcock C was determined to be 1.0454 liter with
the aid of another flask with a known volume. The ballast tube will be the standard
volume in the calibration of the system. Dry nitrogen gas will be used in the calibration.
2) Open the on-off valve of the nitrogen cylinder and slowly adjust the regulator so that
the needle on the pressure gauge for the outlet pressure is barely moved. Also barely
open the needle valve. Hence, nitrogen gas will flow slowly down the rubber tubing at a
pressure slightly above atmospheric pressure. The tube is connected to the manifold via
stopcock E', just to the left of stopcock E.
3) Close stopcocks B and D. Carefully open stopcock E' and fill the manifold and ballast
to a pressure around 760 torr. Record this value, pB which is proportional to the fixed
amount of gas in the ballast. Use the MKS Baratron gauge for all pressure readings in the
calibration and determination of the adsorption isotherm.
4) Close stopcock C and isolate the ballast from the manifold. Evacuate the manifold by
opening stopcock B.
5) When the manifold has been evacuated, close stopcock B and then open stopcock C.
The gas stored in the ballast will now expand into the manifold. Record the pressure,
pBM.
6) In the final step, open stopcock D and the gas will fill the cell as well. Record the
pressure, pBMC. Since a fixed amount of gas is involved at a constant temperature,
Boyle's Law applies and pBVB = pBM(VB + VM) = pBMC(VB +VM + VC). The volumes of
the manifold and empty cell are readily calculated from VB and the three pressures.
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Figure 2. Closeup view of the cell containing molecular sieve. The cell is attached to the
manifold via stopcock D. Note the three-finger clamp below the cell that is used as a
support.
MEASUREMENT OF THE ADSORPTION ISOTHERM
1) Perform the measurements on nitrogen first as it does not adsorb as strongly on the
molecular sieve as dimethyl ether. As a first step close stopcock D and evacuate the
manifold and the ballast by opening stopcock B.
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2) Remove the cell from the manifold and disassemble it. Introduce a weighed amount of
molecular sieve, ca. 5 gram. Reassemble the cell and attach it to the manifold. Evacuate
it by opening stopcock D.
3) You want to thoroughly evacuate the system and degas the molecular sieve. Heat
treatment of the bottom of the cell with an air gun might be helpful in accelerating the
process. Once degassing is completed, close stopcock B. It will remain closed for the
duration of the procedure.
4) In the following repetitive procedure that was discussed in the section on calculations,
gas is introduced to the manifold and ballast and then allowed to expand into the cell.
i) Close stopcock D. Carefully open stopcock E' to admit a small
amount of gas into the vacuum system. A light touch will be required.
Aim for 5 torr with the first filling and increments of 20-40 torr above pp
in the later additions.
ii) Record the pressure, pa.
iii) Open stopcock D and wait until the pressure reading becomes stable.
This may require a few seconds or as long as 5-15 minutes.
iv) Record the pressure, pp.
v) Close stopcock D.
vi) Record the temperature at each iteration.
Repeat steps i-v until saturation is achieved or the final pressure is 760 torr.
5) The procedure is repeated for dimethyl ether with a few minor changes. First the
sample is introduced via stopcock E. In preparation for stepwise addition of the sample,
evacuate not only the vacuum, manifold , and cell but also the tygon tube between
stopcock E and the regulator on the small gas cylinder. Make sure that the needle valve
on the outlet stage of the regulator is closed before you do this. Once the tygon tube is
evacuated, you can close stopcock E. Then open the main valve on the cylinder, barely
crack the regulator, and open the needle valve. The tube will fill with dimethyl ether at
its vapor pressure. You are now ready to study the adsorption of dimethyl ether. Since
dimethyl ether has a much larger molecular volume than nitrogen, the time required to
reach equilibrium after the addition of additional sample is much greater. Be patient and
bring reading material. At lower pressures, at least 15 minutes is required to reach a
stable value. Consider recording pressure as a function of time at a few steps in the
cycle.
SHOTDOWN OF THE SYSTEM
1) Once all measurements have been completed, close the valves on the nitrogen and
dimethyl ether bottles as well as stopcocks E, E' and D.
2) Evacuate the system. When it is pumped down, close stopcock B and unplug the
controllers for the two pressure gauges.
3) Turn off the turbomolecular pump but keep the roughing pump on. That is, turn the
switch on the PT50 counterclockwise from the 4:00 to the 2:00 position.
4) Close stopcock A. The vacuum pump is now isolated. The next few steps should be
completed in short order. Vent the trap, i.e. bring it to atmospheric pressure, by opening
stopcock A' associated with the vent. Now release the clamp holding the trap to the
manifold. While the trap is still in the dewar, transfer to the trap to the hood. In the
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hood, remove the trap from the hood. When the trap warms up to room temperature, the
vapors will be vented by the hood. Do not disconnect the manifold from the trap.
5) Turn off the PT50 entirely by turning the control switch counterclockwise to the 12:00
position. Then open stopcock A to bring the pump to atmospheric pressure. This last
step is important. If the lines of the pump is left evacuated, pump oil will be sucked into
them and into the turbomolecular pump.
6) Remove the cell from the system and disassemble it. Apiezon N grease readily
dissolves in hexane.
7) Obtain an estimate of the volume of the molecular sieve pellets. Pour 10-20 ml of
water into a 50 ml graduated cylinder. Measure the volume to the nearest 0.1 ml. Then
pour in the pellets. Water will replace the adsorbed dimethyl ether and bubbling will
ensure. After the bubbling has ceased, shake the graduated cylinder to enable the escape
of any trapped gas bubbles. Re-measure the level of liquid in the graduated cylinder.
The volume of the pellets is the difference of the two values. Subtract this number from
VC to obtain VCeff.
8) The molecular sieve is fairly inert. Use the trash container in the lab for disposal.
CALCULATIONS
1) Determine the values of VM, VC, and VCeff. VCeff is the volume of the empty cell VC
less the volume of the adsorbent. VCeff should be used instead of the volume of the
empty cell in the stoichiometric calculations.
2) For each substance, nitrogen and dimethyl ether, calculate nac as a function of pp in
torr.
3) Convert nac to nacg, the number of moles of adsorbed substrate per g of g of molecular
sieve by dividing nac by the mass of the molecular sieve used.
4) Convert the pressure pp to atmosphere.
5) Plot nacg versus pp (atm) for both molecules.
6) In the case of nitrogen, fit the data (nacg versus pp) to a Langmuir isotherm. Do the
data indicate strong or weak binding at ambient temperature? Can a simplified form of
the Langmuir isotherm be used? Do not attempt to fit your nitrogen data to the BET
isotherm.
7) In the case of dimethyl ether, fit the data to both a Langmuir isotherm and a BET
isotherm. Is the Langmuir isotherm sufficient to handle the data? That is, do the
residuals require a more advanced treatment?
8) Assume that at saturation that the dimethyl ether forms a monolayer on the molecular
sieve. Determine the surface area of the molecular sieve in m2/g from the parameters
obtained for the isotherm that yields the best fit, i.e. n or nmono. The modeling program,
Spartan yields 93 Å2 as a value for the total surface area of dimethyl ether molecule.
However, only a portion of the molecule can be in contact with the zeolite. Various
packing models yield 23 Å2 as the effective contact area for dimethyl ether.
LITERATURE ON MOLECULAR SIEVES
In this experiment you are using Molecular Sieve 13X as the adsorbent. This material
is also known as Linde Molecular Sieve 13X and Zeolite 13X. The multiplicity of names
creates a problem in searching the chemical literature. A thorough search requires the use
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of several search names. The adsorbent has the same crystal structure as the mineral
faujasite, a zeolite characterized by channels with a diameter of 9 Å. The channels are
large enough to accommodate both small and medium-sized molecules. The adsorbent
has the stoichiometry Na88[Si104Al88O384] and its crystal structure has been determined by
Olson. In most cases data accessed via the WWW are not reliable. One web site
maintained at the Swiss Federal Institute of Technology (ETH) for the International
Zeolite Association (IZA) is a notable exception. It provides a reliable and
comprehensive set of structural data on zeolites. The code name for the family of zeolites
to which Molecular Sieve 13X belongs is FAU.
REFERENCES
1) A. Adamson and A. P. Gast, Physical Chemistry of Surfaces, 6th ed., Wiley, New
York, 1997.
2) D. P. Shoemaker, C. W. Garland, and J. W. Nibler, Experiments in Physical
Chemistry, 6th ed., McGraw-Hill, New York, 1996. Consult chapter nineteen for a
discussion of vacuum systems. Chapter eleven contains a traditional adsorption
experiment using mercury manometers for the measurement of pressure.
3) D. H. Olson, "The crystal structure of dehydrated NaX", Zeolites, 15, 439-443 (1995).
4) http://www.zeolites.ethz.ch
GASADS.doc
WES, 8 Jan. 2002, revised 10 Mar. 2002
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GAS-PHASE ADSORPTION
NAME:____________________________________
VC ____________________________
date:______________________
VM _____________________________
Mass of the molecular sieve pellets ______________________
Volume of the molecular sieve pellets ____________________
VCeff __________________
Data for Nitrogen at T =
pa (torr)
pp (torr)
nac
surface area of molecular sieve ____________________
C
Data for Dimethyl Ether at T =
pa (torr)
pp (torr)
nac
C
_________ _________ ____________
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__________ _________ ___________
parameters for Scatchard plot
parameters from the Scatchard plot
K ______________ n ______________
K _____________ n _______________
parameter from the plot of nacg vs pp
parameters from the BET plot
nK _______________
c ______________ nmono ______________