gasads
DESCRIPTION
hhhTRANSCRIPT
GAS-PHASE ADSORPTION
110
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.
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.
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.
(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).
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:
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 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.
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.
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 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 2 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 2 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 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 . 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
GAS-PHASE ADSORPTION
NAME:____________________________________ date:______________________
VC ____________________________ VM _____________________________
Mass of the molecular sieve pellets ______________________
Volume of the molecular sieve pellets ____________________
VCeff __________________ surface area of molecular sieve ____________________
Data for Nitrogen at T = (C Data for Dimethyl Ether at T = (C
pa (torr) pp (torr) nac pa (torr) pp (torr) nac
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
_________ _________ ____________ __________ _________ ___________
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 ______________
cell
A
B
ballast
trap
MKS
MKS-C
TC
TC-C
PT50
vent
sample