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Adsorption Properties of Microporous Aluminophosphate AIPO4-5
Adsorption Properties of Microporous Aluminophosphate AIP0 4-5 1 H. Stach 1 , H. Thamm 1 , K. Fiedler1 , B. Grauert , W. 2 Wieker • E. Jahn 2.and G. Ohlmann 1 1Central Institute of Physical Chemistry, Academy of Sciences of the GDR. 1199 Berlin-Adlershof. GDR 2Central Institute of Inorganic Chemistry, Academy of Sciences qf the GDR, 1199 Berlin-Adlershof. GDR The adsorption equilibrium of hydrocarbons and water on AlPO -5 has been investigated using the methods of calorim~try and isostere measurement. AlP0 4-5 behaves in the adsorption of nonpolar molecules liKe an homogeneous adsorbent • The equilibrium data are well described by equatioreof the cell theory. Monte-Carlo-calculatioreof the thermodynamic f unc t acre e r e performed. INTRODUCTION The recently synthesized molecular sieves of aluminophosphatetype represent a new family of microporous potential adsorbents and catalysts. In contrast to the well known zeoli tic molecular sieves or silicalites the AlP0 is the first siliconfree microporous solid. Similar to zeolites4-5the AlP0 4-molecular sieves are composed of TO - tetrahedras which may form d1fferent threedimensional framewo~k structures. As aluminophosphate molecular sieves don't contain exchangeable cations and the T-positions are alternatingly occupied by AI- and P-atoms the AIPO framework should be expected to be el~ trically neutral. consequently the aluminophosphate molecular sieves should exhibit organophilic and hydrophobic adsorption properties as has been shown for silicalite. Continuing earlier studies of the adsorption properties of the nonpolar molecular sieves (dealuminated faujasites /2/ and silicalites /3/ we extended our investigation to AlP0 4-5, the first aluminophosphate synthesized with known crystal structure /4/. The adsorption systems investigated are compiled in Table 1. Table 1. Adsorption systems investigated Adsorbate AlP0 4-5 US-Ex ethane n-butane n-hexane benzene cyclohexane water
+ + + + + +
+ + + + + +
EXPERIMENTAL The AlPO -5 was synthesized hydrothermally from a reaction mixture of a r~active hydrated alumina and phosphoric acid in presence of a templating agent (tripropylamine)following the procedure given 539
540 (AD-6-1)
by Flanigen and coworkers /1/. Calcination in air at 600 °c freed the channels within the framework of both the organic template moleoules and water molecules. Prior to our calorimetric and adsorption measurements the aluminophosphate was activated in vacuum ( < 10- 3 Pal at 673 K for 24 h. . For representative AIPO -5 samples the atomic Al:P ratio of 1.025:1 was determined a~alytically. Moreover X-ray structure investigations were performed. The ref 2,ction da!! correspond to those reported in literature /1.4/. Al- and P-NMR-MAS-measurements /5/ proved a strict alternating Al-P-Al arrangement in the crystal structure. The differential molar heats of adsorption were measured by means of a Calvet-type microcalorimeter (Setaram) which was connected to the standard volumetric adsorption apparatus at 301 K. The reproducibility of the results was better than 1 %. The equilibrium pressure was determined by Baratron pressure meters. RESULTS AND DISCUSSION In Figure 1 are given the coverage dependences of the differential molar heats of adsorption of benzene. cyclohexane and n-hexan~ It is seen that the differential molar heat of cyclohexane increases strongly with rising amount of adsorption whereas the adsorption enthalpy for nhexane is nearly constant up to 0.5 mmol/g. In contrast to these heat curves the heat of adsorption of benzene at low coverages falls with increasing adsorption, passes through a minimum and reaches a maximum at about 0.9 mmol/g. Extrapolating the curves of the differential adsorption heat of the hydrocarbons investigated (neglecting the heats for benzene 0.5 at adsorbed amount a • 0.1 mmol/ 1.0 1.5 g. which reflect adsorption on a (mmol/g) irregularities of the crystal structure) we find that the enFig. 1. Heat of adsorption of thalpies of ~dsorption for zecyclohexane (A). n-hexane (0) ro coverage AH o diminish in the and benzene (0) order n-hexane > cyclohexane > benzene. These behaviour suggests that the adsorption heats are mainly determinated by the dispersion energy, as may also be seen from Figure 2. This figure demonstrates the influence of the electrostatic fields on the heats of adsorption for the same adsorbates in Y-type zeolite with and without cations. The zeolites investigated are NaY and US-Ex. It is the decationized and highly dealuminated form of NaY. This means that aluminophosphate slmilartoUS-Ex and silicalite /3/ behaves with respect to the adsorption of the hydrocarbons studied like energetically homogeneous adsorbents. In Figure 3 are presented the differential heats of adsorption(a) and the differential entropies of the adsorbate (b) for the n-paraffins ethane, n-butane and n-hexane. The experimental results show that the heats of adsorption for ethane and n-butane increase with rising adsorbed amount whereas the AH values of n-hexane are nearly
'150
H. Stach et al.
541
with coverage. The different coverage dependences may be 80 due to different chain length of the n-paraffins. We assume that the ..... 60 larger n-hexane molecu.-I 0 les are arranged in the e AIPO -channels in an <, l") end configuratiend ~ on allowing adsorbate--- 40 adsorbate-interaction by the terminal t~ I' Conly H ~ - g r O U p s whereas the a b I 20 I shorter n-alkanes may ~_~_....J. interact with each o1tler 12312 3 by more than one CH group. The coverage 3dea (mmol/g) pendences of the diffeFig. 2. Differential adsorption heats of rential entropy of the benzene (0). n~hexane (0) and cyclohexane adsorbate molecules an(~) on NaY (a) and US-Ex (b) firm these behaviour. r-----------------,-------------------,independ~
to
250
..... ....ll!o e
<,
..... l")
Icn«l
150 b
a 1
3
5
7
1
3 5 a (mmol/g)
7
a (mmoICH 2/g) Fig. 3. Differential heats and entropies (b) of adsorption of n-paraffins on AIP0 4-S (~QEthane. 0 Qn-butane. osn-hexene ) In the case of n-hexane 5 only slightly changes with coverage. while the entropies of th9 remaining adsorbates strongly diminish with adsorbed amount. In Figure 4 are plotted the adsorption isotherms (at 301 K) for water. n-hexane and cyclohexane on Aipo -5. In agreement with the data given by Flanigen~aL/1/ we found the adsorption capacity towards water (in cm 3/g) to be approximately twice as large as that for cyclohexane. Moreover the type of the adsorption isotherms of water is quite different from that of the hydrocarbons studied. Investigating desorption for these adsorbates we found. that down to pips:::: 0.2 adsorption and desorption points don't coincide, indicating the existence of secondary pores as we have found earlier in the case of highly dealuminated zeolites /6/. However in contrast
542 (AD-6-l)
co
0,1
0,4
0,6
PiPs Fig. 4. Adsorption isotherms of water (0), n-hexane (Ll) and cyclohexane (D) at 303 K m
m
a .. Nc ' .LiQ j )..1/(1 +.LQj)..j) I =1 1=1
to US-Ex the calorimetrically determined heats of adsorption for water on A1PO -5 exceed those expecte~ for capillary condensation (Figure 5). 27Al-NMRinvestigations on A1P0417 showed /7/ the reve~ sible formation of Al (OP)4 (OH 2)2 oc tahe:lras by hydration and dehydration of the calcined aluminophosphate. The~ fore besides capillary condensation the occurrance of coordinately bonded water within the channels of A1P0 4-5 cannot be excluded. As has been shown elsewhere /8/ the adsorption equilibrium of nonpolar and polar mo~ cules in zeolites is successfully described by the isotherm equations of the cell theory (1)
with
80
(2)
and ).. .. IP/PJjiT/T o )
(3)
The constants Ei and S~ denote the adsorption energy and entropy of the i-fold occupied cell, Q the canonical partition functioA and N the number of cells expressed inc mmol/g). An analysis of the measured nhexane isotherms on AlpO -5 (under the reasonable assum~tion that the cell should consist of 72 tetrahedras corresponding to 6 of 5 10 15 the oxygen-12-rings of the channm) a (mmol/g) showed, that the maximal occupaFig. 5. Heat of adsorption of tion number of the chosen cell is water on A1P0 4-5 (0) and US-Ex (~) equal to about 6 n-hexane molecules (m • 6). Fitting the measured isotherms a value of Nc • 0.2125 mmol/g is found and the isotherm constants presented in the Table 2. Using this data the adsorption isotherms of n-hexane (presented in Figure 6) were calculated showing that the measured and the calculated isotherm points practically coincide. An evaluation of measured adsorption systems using the cited equation proved the good description not only of the isotherms but also of the curves of the heawand the entropies of adsorption in dependence of the coverage.
H. Stach et al.
543
1.0
~0.5 o e
.-l
e
-4.0
2.0
4.0
19 P ( Pa ) Fig. 6. Measured (0) and calculated (-) adsorption isotherms of n-hexane on A1P04 - 5 Table 2. Constants of the adsorption Although the adsorpisotherm equation (1) for n-hexane tion isotherms of the cell theory describe i the adsorption data on - Si(J/mol:K) alu.inophosphate with 28.55 13.23 1 good accuracy from a 29.26 practical point of view 13.62 2 they are somewhat 29.56 13.82 3 complicated For techni13.27 30.00 4 cal purposes the pre29.77 12.60 5 sentation of the equili11.81 28.65 6 brium date using the Nc 0.2125 (mmol/g) function a = f (E ) is suf f1 cient /9/. The on~ = 0.016794 (mmol/g) ly unknown constant in f -func tion 1& E, i t denotes the interaction energy of the first adsorbed molecule with the molecular sieve. This value corresponds to the zero adsorption heat • Therefore it is of high interest to calculate this thermodynamic data in advance. This is possible at least in two different ways: - theoretical, using a Monte-Carlo procedure /10/ and - empirical, using the dependence of the initial heats of adsorption on the critical parameters of the adsorbate /14/. Both procedures were used and the results will be presented in detail elsewhere. It was found that ~H for benzene (theoretically derived) corresponds to 43.6 kJ/mol, tRe calorimetrically determined value is equal to about 45 kJ/mol. The empirical method yielded an equation for the initial heats of hydrocarbons (4) -6.H o = 208.59 Tc/pc 1/2 for the aluminophosphate molecular sieve. In Table 3 are compared the zero adsorption heats estimated using the equation 4 and the
544
(AD-6-1)
Table 3. Comparison of experimentally determined and calculated initial heats of adsorption for hydrocarbons -£:.H o Adsorbate CH 4 C2H6 C3H8 n-C 4H l 0 r:l-C 5H12 n-C 6H 14 n-C 7H16 n-C 8H18 n-C 9H20 n-C l 0H22 benzene Cyclohexane
-£:.H 0 exp -I::. Ho calc (kJ/mol) 24.0 41.4 60.7
54.4 56.9
18.5 28.8 37.4 45.5 53.2 60.7 68.0 75.0 81.8 88.8 53.2 56.9
calorimetrically measured values. In general the difference be tween both sets of data is small thus allowing the conclusion that this procedure gives values sufficient for technical application of the adsorption equilibrium on aluminophosphate. The Monte-Carlo method was used in special version (importance sampling procedure. see 110, 11/10n the basis of atom-atom-potential of the 6-12-Lennard-Jones-type for the adsorbate-adsorbent- and adsorbate-adsorbate-interaction as well as an additive quadrupole interaction between the adsorbed benzene molecules. For the calculation only the oxygen-atoms of the aluminophosphate were considered because the contributions of the Al- and P-atoms may be neglected. The constants of the potential are compiled in Table 4. They were derived by means of Kirkwood-Muller-equation following the approach of Kiselev et~al. /12/. The value of the quadrupole moment denotes Q.... = 1687 C nm-'mol- 1• The data of the crystal structure of A1P04-5 were taken from /4/. For the calculations of the potentials 96 oxygen-atoms were taken into account and the realization volume of the Monte-Carlo calculation included 96 tetrahedras allowing the adsorption of 3 benzene molecules. 63000 benzene positions within the realizations volume were studied giving 210 importance sampling realizationsfor the statistical evaluation of the thermodynamic functions. Some results of the Monte-Carlo calcul.Jation are presented in the Figures 7a - 7d. In these pictures are given the results of 3 simple models of the probabilities of the i-fold occupied cells (see Table 5). These models correspond to special values of the entropy constants. Included are also the calorimetric determined differential adsorption enthalpies. The corresponding experimental points are given by symbols. In Figure 7a are plotted only the calculated integral enthalpies for 1, 2 ~nd 3 benzene molecules adsorbed. Figure 7b shows a special case in which at first all cells are occupied with a single molecule and then successively all cells were filled with a second and afterwards with a third molecule (model 1 of Table 5). The results of a special mixed filling process, namely that simul-
H. Stach et al.
545
Table 4: Constants of the potential calculations for the Monte-Carlo-procedure i - j H-H C-H O-H C-C O-C
C6• i• j 6mol-1 :J nm -
0.21123 0.77068 0.56116 2.89632 2.17549
o(i C12• i ,j j'J-O'Xi 2mol-1 H nlll 2mol-1 12mol1 F nm :J nm 28.81 89.79 56.95
0.00001647 0.00021535 0.00010396 0.00231725 0.00123522
I
60 ~ .,_~~~
..-- 40
0.116 0.171 0.152
I
•••••
........
a
nm
C8 0
~.
50
- 32.21 -138.96 -128.06
ri
I
\
00
.-I
o E
<,
n
:::. 60
l~
I
50 40
0123012 9 (molecules/cell)
3
Fig. 7. Measured (0) and calculated (--.---) heats of adsorption of benzene on AIP0 4-5 using the Monte-Carlo-m~ and different probability mOdels taneously 1- and 2-fold occupied cells arise with increasing amount adsorbed is shown in Figure 7c. As follows from the Figures 7b and 7c both calculated curves of the adsorption enthalpies reflect some regions of the experimental curve. The last model assumes the formation of only 2- and 3-fold occupied cells (drawn line. model 3 of Figure 7d). Included is in Figure 7d the curve of the adsorption enthalpy following from the Monte-Carlodata of the enthalpy and entropy (dotted line). From the Figure 7d follows that the complete results of the Monte-Carlo method overestimated the probability of 8 2 i.e. the 2-fold occupation state of the cell. It can be expected tnat using larger van der Waals radii (as has been proposed by Kiselev et.al./13/) will give a better agreement between the calculated and the experimental curve. Higher accuracy of the Monte-Carlo results will be expected by - the generation of a still larger population - the transition to a larger cell with more molecules adsorbed - the transition to periodical bound conditions. Though the agreement of the calculated and measured initial heats
546
(AD-6-1)
of adsorption as well as of the benzene-benzene-interaction showb that the potential functions used are sufficient, further calculations will be performed using quantumchemically based analytical potentials. The last item is especially important for modifications of the molecular sieves (for example if the AI-atoms in the framework are substituted by Si-atoms). The Monte-Carlo calculations of the thermodynamic adsorbate functions of other hydrocarbon molecules will be continued. Table 5. The probabilities 8. of the eXistence of i-fold occupied calls for t"ree models model 1 Figure 7b
0 1 2
oS 9
oS
8 8
!!i
oS ~
o!O
1 2 3
81 = 8 81 = 2 - 8 81 0
82 = 0 82 8 - 1 82 = 3 - 8
18 2 8 1 ="2 (2 - 8 ) 8 2 -1 -4 8 !!i o!O 81 = 0 82 = 3 - 8 1 8 model 3 0 .s 8 .s 2 8 1 = 0 8 2 ='2 Figure 7d 2 ~ 8 o!O 3 8 1 = 0 82 = 3 - e 8= average number of molecules in the cells, (8=1 ) model 2 Figure 7c
0 2
o!O
8 8
l!!
2 3
83 0 83 0 83 = 8 - 2 83 0 83 = 8 - 2 83 = 0 83 = 8 - 2 S (0.5 mmol/g)
REFERENCE 1. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, and E.M. Flanigen, J. Amer. Chem. Soc •• 104, 1146 (1982). 2. W. Schirmer, H. Thamm, H. Stacn. and U. Lohse, Spec. Publ. Nr. 35, Chem. Soc. London, 1980, p.204. 3. H. Stach, H. Thamm, J. Janchen, K. Fiedler, and W. Schirmer, Proc. VI. Int. Zeolite cenf , , Reno, USA., 1983, Butterworth. Guildford, 1984, p. 225. 4. J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, and J.V. Smith, A.C.S. Symp. Ser. Nr. 218, p.109. 5. O. MOller, E. Jahn, B. Fahlke, G. LadWig, and U. Haubenreisser, Zeolites, ~' 53 (1985). 6. U. Lohse, H. Stach, H. Thamm, W. Schirmer, A.A. Isirikjan, and N.I. Regent, Z. Anorg. allgem. Chem., i2Q, 179 (1980). 7. C.S. Blackwell, and R.L. Patton, J. Phys. Chem •• 88. 6135 (1984~ 8. K. Fiedler, H. Stach, and W. Schirmer, Sitzber. AOW der OOR 17/N Akademie-Verlag, Berlin, 1982, p.5. 9. W. Schirmer, K. Fiedler, and H. Stach, Proc. IV. Int. Conf. Zeolites, A.C.S. Symp. Ser. Nr. 40, p.305. 10. K. Fiedler, U. Lohse, J. Sauer, H. Stach, H. Thamm, and W. Schirmer, Proc. V. Int. Zeolite Conf., Naples, Italy, 1980, Heyden, London-Phil~delphia-Rheine 1980, p.490. 11. K. Fiedler, and B. Grauert, Proc. VI. Conf. on Adsorpt:on, Liblice, CSSR, 1985, eASe., Prague, 1985, p.153. 12. A.V. Kiselev, and Pham Quang Du, J. Chem. Soc. Faraday Trans. 2, 77, 17 (1981). 13. A.V. Kiselev, A.A. Lopatkin, and A.A. Shurga, Ookl. ANSSSR, ~' 916 (1964). 14. H. Thamm, H. Stach, and G.I. Berezin. Z. Chem. ~' 420 (1984).
+ + + + + +
+ + + + + +
EXPERIMENTAL The AlPO -5 was synthesized hydrothermally from a reaction mixture of a r~active hydrated alumina and phosphoric acid in presence of a templating agent (tripropylamine)following the procedure given 539
540 (AD-6-1)
by Flanigen and coworkers /1/. Calcination in air at 600 °c freed the channels within the framework of both the organic template moleoules and water molecules. Prior to our calorimetric and adsorption measurements the aluminophosphate was activated in vacuum ( < 10- 3 Pal at 673 K for 24 h. . For representative AIPO -5 samples the atomic Al:P ratio of 1.025:1 was determined a~alytically. Moreover X-ray structure investigations were performed. The ref 2,ction da!! correspond to those reported in literature /1.4/. Al- and P-NMR-MAS-measurements /5/ proved a strict alternating Al-P-Al arrangement in the crystal structure. The differential molar heats of adsorption were measured by means of a Calvet-type microcalorimeter (Setaram) which was connected to the standard volumetric adsorption apparatus at 301 K. The reproducibility of the results was better than 1 %. The equilibrium pressure was determined by Baratron pressure meters. RESULTS AND DISCUSSION In Figure 1 are given the coverage dependences of the differential molar heats of adsorption of benzene. cyclohexane and n-hexan~ It is seen that the differential molar heat of cyclohexane increases strongly with rising amount of adsorption whereas the adsorption enthalpy for nhexane is nearly constant up to 0.5 mmol/g. In contrast to these heat curves the heat of adsorption of benzene at low coverages falls with increasing adsorption, passes through a minimum and reaches a maximum at about 0.9 mmol/g. Extrapolating the curves of the differential adsorption heat of the hydrocarbons investigated (neglecting the heats for benzene 0.5 at adsorbed amount a • 0.1 mmol/ 1.0 1.5 g. which reflect adsorption on a (mmol/g) irregularities of the crystal structure) we find that the enFig. 1. Heat of adsorption of thalpies of ~dsorption for zecyclohexane (A). n-hexane (0) ro coverage AH o diminish in the and benzene (0) order n-hexane > cyclohexane > benzene. These behaviour suggests that the adsorption heats are mainly determinated by the dispersion energy, as may also be seen from Figure 2. This figure demonstrates the influence of the electrostatic fields on the heats of adsorption for the same adsorbates in Y-type zeolite with and without cations. The zeolites investigated are NaY and US-Ex. It is the decationized and highly dealuminated form of NaY. This means that aluminophosphate slmilartoUS-Ex and silicalite /3/ behaves with respect to the adsorption of the hydrocarbons studied like energetically homogeneous adsorbents. In Figure 3 are presented the differential heats of adsorption(a) and the differential entropies of the adsorbate (b) for the n-paraffins ethane, n-butane and n-hexane. The experimental results show that the heats of adsorption for ethane and n-butane increase with rising adsorbed amount whereas the AH values of n-hexane are nearly
'150
H. Stach et al.
541
with coverage. The different coverage dependences may be 80 due to different chain length of the n-paraffins. We assume that the ..... 60 larger n-hexane molecu.-I 0 les are arranged in the e AIPO -channels in an <, l") end configuratiend ~ on allowing adsorbate--- 40 adsorbate-interaction by the terminal t~ I' Conly H ~ - g r O U p s whereas the a b I 20 I shorter n-alkanes may ~_~_....J. interact with each o1tler 12312 3 by more than one CH group. The coverage 3dea (mmol/g) pendences of the diffeFig. 2. Differential adsorption heats of rential entropy of the benzene (0). n~hexane (0) and cyclohexane adsorbate molecules an(~) on NaY (a) and US-Ex (b) firm these behaviour. r-----------------,-------------------,independ~
to
250
..... ....ll!o e
<,
..... l")
Icn«l
150 b
a 1
3
5
7
1
3 5 a (mmol/g)
7
a (mmoICH 2/g) Fig. 3. Differential heats and entropies (b) of adsorption of n-paraffins on AIP0 4-S (~QEthane. 0 Qn-butane. osn-hexene ) In the case of n-hexane 5 only slightly changes with coverage. while the entropies of th9 remaining adsorbates strongly diminish with adsorbed amount. In Figure 4 are plotted the adsorption isotherms (at 301 K) for water. n-hexane and cyclohexane on Aipo -5. In agreement with the data given by Flanigen~aL/1/ we found the adsorption capacity towards water (in cm 3/g) to be approximately twice as large as that for cyclohexane. Moreover the type of the adsorption isotherms of water is quite different from that of the hydrocarbons studied. Investigating desorption for these adsorbates we found. that down to pips:::: 0.2 adsorption and desorption points don't coincide, indicating the existence of secondary pores as we have found earlier in the case of highly dealuminated zeolites /6/. However in contrast
542 (AD-6-l)
co
0,1
0,4
0,6
PiPs Fig. 4. Adsorption isotherms of water (0), n-hexane (Ll) and cyclohexane (D) at 303 K m
m
a .. Nc ' .LiQ j )..1/(1 +.LQj)..j) I =1 1=1
to US-Ex the calorimetrically determined heats of adsorption for water on A1PO -5 exceed those expecte~ for capillary condensation (Figure 5). 27Al-NMRinvestigations on A1P0417 showed /7/ the reve~ sible formation of Al (OP)4 (OH 2)2 oc tahe:lras by hydration and dehydration of the calcined aluminophosphate. The~ fore besides capillary condensation the occurrance of coordinately bonded water within the channels of A1P0 4-5 cannot be excluded. As has been shown elsewhere /8/ the adsorption equilibrium of nonpolar and polar mo~ cules in zeolites is successfully described by the isotherm equations of the cell theory (1)
with
80
(2)
and ).. .. IP/PJjiT/T o )
(3)
The constants Ei and S~ denote the adsorption energy and entropy of the i-fold occupied cell, Q the canonical partition functioA and N the number of cells expressed inc mmol/g). An analysis of the measured nhexane isotherms on AlpO -5 (under the reasonable assum~tion that the cell should consist of 72 tetrahedras corresponding to 6 of 5 10 15 the oxygen-12-rings of the channm) a (mmol/g) showed, that the maximal occupaFig. 5. Heat of adsorption of tion number of the chosen cell is water on A1P0 4-5 (0) and US-Ex (~) equal to about 6 n-hexane molecules (m • 6). Fitting the measured isotherms a value of Nc • 0.2125 mmol/g is found and the isotherm constants presented in the Table 2. Using this data the adsorption isotherms of n-hexane (presented in Figure 6) were calculated showing that the measured and the calculated isotherm points practically coincide. An evaluation of measured adsorption systems using the cited equation proved the good description not only of the isotherms but also of the curves of the heawand the entropies of adsorption in dependence of the coverage.
H. Stach et al.
543
1.0
~0.5 o e
.-l
e
-4.0
2.0
4.0
19 P ( Pa ) Fig. 6. Measured (0) and calculated (-) adsorption isotherms of n-hexane on A1P04 - 5 Table 2. Constants of the adsorption Although the adsorpisotherm equation (1) for n-hexane tion isotherms of the cell theory describe i the adsorption data on - Si(J/mol:K) alu.inophosphate with 28.55 13.23 1 good accuracy from a 29.26 practical point of view 13.62 2 they are somewhat 29.56 13.82 3 complicated For techni13.27 30.00 4 cal purposes the pre29.77 12.60 5 sentation of the equili11.81 28.65 6 brium date using the Nc 0.2125 (mmol/g) function a = f (E ) is suf f1 cient /9/. The on~ = 0.016794 (mmol/g) ly unknown constant in f -func tion 1& E, i t denotes the interaction energy of the first adsorbed molecule with the molecular sieve. This value corresponds to the zero adsorption heat • Therefore it is of high interest to calculate this thermodynamic data in advance. This is possible at least in two different ways: - theoretical, using a Monte-Carlo procedure /10/ and - empirical, using the dependence of the initial heats of adsorption on the critical parameters of the adsorbate /14/. Both procedures were used and the results will be presented in detail elsewhere. It was found that ~H for benzene (theoretically derived) corresponds to 43.6 kJ/mol, tRe calorimetrically determined value is equal to about 45 kJ/mol. The empirical method yielded an equation for the initial heats of hydrocarbons (4) -6.H o = 208.59 Tc/pc 1/2 for the aluminophosphate molecular sieve. In Table 3 are compared the zero adsorption heats estimated using the equation 4 and the
544
(AD-6-1)
Table 3. Comparison of experimentally determined and calculated initial heats of adsorption for hydrocarbons -£:.H o Adsorbate CH 4 C2H6 C3H8 n-C 4H l 0 r:l-C 5H12 n-C 6H 14 n-C 7H16 n-C 8H18 n-C 9H20 n-C l 0H22 benzene Cyclohexane
-£:.H 0 exp -I::. Ho calc (kJ/mol) 24.0 41.4 60.7
54.4 56.9
18.5 28.8 37.4 45.5 53.2 60.7 68.0 75.0 81.8 88.8 53.2 56.9
calorimetrically measured values. In general the difference be tween both sets of data is small thus allowing the conclusion that this procedure gives values sufficient for technical application of the adsorption equilibrium on aluminophosphate. The Monte-Carlo method was used in special version (importance sampling procedure. see 110, 11/10n the basis of atom-atom-potential of the 6-12-Lennard-Jones-type for the adsorbate-adsorbent- and adsorbate-adsorbate-interaction as well as an additive quadrupole interaction between the adsorbed benzene molecules. For the calculation only the oxygen-atoms of the aluminophosphate were considered because the contributions of the Al- and P-atoms may be neglected. The constants of the potential are compiled in Table 4. They were derived by means of Kirkwood-Muller-equation following the approach of Kiselev et~al. /12/. The value of the quadrupole moment denotes Q.... = 1687 C nm-'mol- 1• The data of the crystal structure of A1P04-5 were taken from /4/. For the calculations of the potentials 96 oxygen-atoms were taken into account and the realization volume of the Monte-Carlo calculation included 96 tetrahedras allowing the adsorption of 3 benzene molecules. 63000 benzene positions within the realizations volume were studied giving 210 importance sampling realizationsfor the statistical evaluation of the thermodynamic functions. Some results of the Monte-Carlo calcul.Jation are presented in the Figures 7a - 7d. In these pictures are given the results of 3 simple models of the probabilities of the i-fold occupied cells (see Table 5). These models correspond to special values of the entropy constants. Included are also the calorimetric determined differential adsorption enthalpies. The corresponding experimental points are given by symbols. In Figure 7a are plotted only the calculated integral enthalpies for 1, 2 ~nd 3 benzene molecules adsorbed. Figure 7b shows a special case in which at first all cells are occupied with a single molecule and then successively all cells were filled with a second and afterwards with a third molecule (model 1 of Table 5). The results of a special mixed filling process, namely that simul-
H. Stach et al.
545
Table 4: Constants of the potential calculations for the Monte-Carlo-procedure i - j H-H C-H O-H C-C O-C
C6• i• j 6mol-1 :J nm -
0.21123 0.77068 0.56116 2.89632 2.17549
o(i C12• i ,j j'J-O'Xi 2mol-1 H nlll 2mol-1 12mol1 F nm :J nm 28.81 89.79 56.95
0.00001647 0.00021535 0.00010396 0.00231725 0.00123522
I
60 ~ .,_~~~
..-- 40
0.116 0.171 0.152
I
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a
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- 32.21 -138.96 -128.06
ri
I
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00
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0123012 9 (molecules/cell)
3
Fig. 7. Measured (0) and calculated (--.---) heats of adsorption of benzene on AIP0 4-5 using the Monte-Carlo-m~ and different probability mOdels taneously 1- and 2-fold occupied cells arise with increasing amount adsorbed is shown in Figure 7c. As follows from the Figures 7b and 7c both calculated curves of the adsorption enthalpies reflect some regions of the experimental curve. The last model assumes the formation of only 2- and 3-fold occupied cells (drawn line. model 3 of Figure 7d). Included is in Figure 7d the curve of the adsorption enthalpy following from the Monte-Carlodata of the enthalpy and entropy (dotted line). From the Figure 7d follows that the complete results of the Monte-Carlo method overestimated the probability of 8 2 i.e. the 2-fold occupation state of the cell. It can be expected tnat using larger van der Waals radii (as has been proposed by Kiselev et.al./13/) will give a better agreement between the calculated and the experimental curve. Higher accuracy of the Monte-Carlo results will be expected by - the generation of a still larger population - the transition to a larger cell with more molecules adsorbed - the transition to periodical bound conditions. Though the agreement of the calculated and measured initial heats
546
(AD-6-1)
of adsorption as well as of the benzene-benzene-interaction showb that the potential functions used are sufficient, further calculations will be performed using quantumchemically based analytical potentials. The last item is especially important for modifications of the molecular sieves (for example if the AI-atoms in the framework are substituted by Si-atoms). The Monte-Carlo calculations of the thermodynamic adsorbate functions of other hydrocarbon molecules will be continued. Table 5. The probabilities 8. of the eXistence of i-fold occupied calls for t"ree models model 1 Figure 7b
0 1 2
oS 9
oS
8 8
!!i
oS ~
o!O
1 2 3
81 = 8 81 = 2 - 8 81 0
82 = 0 82 8 - 1 82 = 3 - 8
18 2 8 1 ="2 (2 - 8 ) 8 2 -1 -4 8 !!i o!O 81 = 0 82 = 3 - 8 1 8 model 3 0 .s 8 .s 2 8 1 = 0 8 2 ='2 Figure 7d 2 ~ 8 o!O 3 8 1 = 0 82 = 3 - e 8= average number of molecules in the cells, (8=1 ) model 2 Figure 7c
0 2
o!O
8 8
l!!
2 3
83 0 83 0 83 = 8 - 2 83 0 83 = 8 - 2 83 = 0 83 = 8 - 2 S (0.5 mmol/g)
REFERENCE 1. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, and E.M. Flanigen, J. Amer. Chem. Soc •• 104, 1146 (1982). 2. W. Schirmer, H. Thamm, H. Stacn. and U. Lohse, Spec. Publ. Nr. 35, Chem. Soc. London, 1980, p.204. 3. H. Stach, H. Thamm, J. Janchen, K. Fiedler, and W. Schirmer, Proc. VI. Int. Zeolite cenf , , Reno, USA., 1983, Butterworth. Guildford, 1984, p. 225. 4. J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, and J.V. Smith, A.C.S. Symp. Ser. Nr. 218, p.109. 5. O. MOller, E. Jahn, B. Fahlke, G. LadWig, and U. Haubenreisser, Zeolites, ~' 53 (1985). 6. U. Lohse, H. Stach, H. Thamm, W. Schirmer, A.A. Isirikjan, and N.I. Regent, Z. Anorg. allgem. Chem., i2Q, 179 (1980). 7. C.S. Blackwell, and R.L. Patton, J. Phys. Chem •• 88. 6135 (1984~ 8. K. Fiedler, H. Stach, and W. Schirmer, Sitzber. AOW der OOR 17/N Akademie-Verlag, Berlin, 1982, p.5. 9. W. Schirmer, K. Fiedler, and H. Stach, Proc. IV. Int. Conf. Zeolites, A.C.S. Symp. Ser. Nr. 40, p.305. 10. K. Fiedler, U. Lohse, J. Sauer, H. Stach, H. Thamm, and W. Schirmer, Proc. V. Int. Zeolite Conf., Naples, Italy, 1980, Heyden, London-Phil~delphia-Rheine 1980, p.490. 11. K. Fiedler, and B. Grauert, Proc. VI. Conf. on Adsorpt:on, Liblice, CSSR, 1985, eASe., Prague, 1985, p.153. 12. A.V. Kiselev, and Pham Quang Du, J. Chem. Soc. Faraday Trans. 2, 77, 17 (1981). 13. A.V. Kiselev, A.A. Lopatkin, and A.A. Shurga, Ookl. ANSSSR, ~' 916 (1964). 14. H. Thamm, H. Stach, and G.I. Berezin. Z. Chem. ~' 420 (1984).