Ind. Eng. Chem. Res. 1990,29, 1621-1626 Cambou, B.; Klibanov, A. M. Preparative Production of Optically Active Esters and Alcohols Using Esterase-Catalyzed Stereospecific Transesterification in Organic Media. J. Am. Chem. SOC. 1984,106, 2687-2692. Carnahan, B.; Luther, H. A.; Wilkes, J. 0. Applied Numerical Methods; Wiley: New York, 1969. Catapano, G.; Iorio, G.; Drioli, E.; Filosa, M. Experimental Analysis of a Cross-flow Membrane Bioreactor with Entrapped Whole Cells: Influence of Transmembrane Pressure and Substrate Feed Concentration on Reactor Performance. J. Membr. Sci. 1988,35, 325-338. Chen, C.-S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. Quantitative Analyses of Biochemical Kinetic Resolutions of Enantiomers. J. Am. Chem. SOC. 1982, 104, 7294-7299. Cramer, S. M.; Horvath, C. Peptide Synthesis with Immobilized Carboxypeptidase Y. Biotechnol. Bioeng. 1989a, 33, 344-353. Cramer, S. M.; Horvath, C. Peptide Synthesis and Deamidation with Chemically Modified Immobilized Carboxypeptidase Y. Enzyme Microb. Technol. 198913, 11, 74-79. Davis, M. E.; Watson, L. T. Analysis of a Diffusion-Limited Hollow Fiber Reactor for the Measurement of Effective Substrate Diffusivities. Biotechnol. Bioeng. 1985, 27, 182-186. Heath, C.; Belfort, G. Immobilization of Suspended Mammalian Cells: Analysis of Hollow Fiber and Microcapsule Bioreactors. Ado. Biochem. Eng./Biotechnob 1987, 34, 1-31. Jones, J. B. Enzymes in Organic Synthesis. Tetrahedron 1986,42, 3351-3403. Kim, S.4.; Cooney, D. 0. An Improved Theoretical Model for Hollow-Fiber Enzyme Reactors. Chem. Eng. Sci. 1976, 31, 289-294. Kirchner, G.; Scollar, M. P.; Klibanov, A. M. Resolution of Racemic Mixtures via Lipase Catalysis in Organic Solvents. J.Am. Chem. SOC.1985, 107, 7072-7076. Kleinstreuer, C.; Agarwal, S. S. Analysis and Simulation of HollowFiber Bioreactor Dynamics. Biotechnol. Bioeng. 1986, 28, 1233-1240. Kloosterman, M.; Elferink, V. H. M.; Iersel, J. V.; Roskam, J.-H.; Meijer, E. M.; Hulshof, L. A.; Sheldon, R. A. Lipases in the Preparation of 0-Blockers. TIBTECH 1988,6, 251-256. Ladner, W. E.; Whitesides, G. M. Lipase-Catalyzed Hydrolysis as a Route to Esters of Chiral Epoxy Alcohols. J. Am. Chem. SOC. 1984, 106, 7250-7251.
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Lavayre, J.; Verrier, J.; Baratti, J. Stereospecific Hydrolysis by Soluble and Immobilized Lipases. Biotechnol. Bioeng. 1982,24, 2175-2187. Levenspiel, 0. Chemical Reaction Engineering; an Introduction t o the Design of Chemical Reactors; Wiley: New York, 1962. Lopez, J. L. Personal communication, 1989. Margolin, A. L.; Klibanov, A. M. Peptide Synthesis Catalyzed by Lipases in Anhydrous Organic Solvents. J. Am. Chem. SOC.1987, 109, 3802-3804. Margolin, A. L.; Crenne, J.-Y.; Klibanov, A. M. Stereoselective Oligomerizations Catalyzed by Lipases in Organic Solvents. Tetrahedron Lett. 1987,28, 1607-1610. Matson, S. L.; Lopez, J. L. Multiphase Membrane Reactors for Enzymatic Resolution: Diffusional Effects on Stereoselectivity. In Frontiers in Bioprocessing; CRC Press: Boca Raton, FL, 1989; pp 391-403. Matson, S. L.; Quinn, J. A. Membrane Reactors in Bioprocessing. Ann. N.Y. Acad. Sci. 1986,469, 152-165. Prenosil, J. E.; Hediger, T. Scale-up of Membrane Fixed Enzyme Reactors: Modelling and Experiments. Desalination 1985, 53, 265-278. Prenosil, J. E.; Hediger, T. Performance of Membrane Fixed Biocatalyst Reactors. 1: Membrane Reactor Systems and Modelling. Biotechnol. Bioeng. 1988,31, 913-921. Schonberg, J. A,; Belfort, G. Enhanced Nutrient Transport in Hollow Fiber Perfusion Bioreactors: a Theoretical Analysis. Biotechnol. Prog. 1987, 3, 80-89. Sonnet, P. E. Enzymes for Chiral Synthesis. CHEMTECH 1988,18, 94-98. Stanley, T. J.; Quinn, J. A. Phase-Transfer Catalysis in a Membrane Reactor. Chem. Eng. Sci. 1987, 42, 2313-2324. Trujillo, E. M. Transient Response of Encapsulated Enzymes in Hollow-Fiber Reactor. Biotechnol. Bioeng. 1987, 29, 529-543. Webster, I. A,; Shuler, M. L. Mathematical Models for Hollow-Fiber Enzyme Reactors. Biotechnol. Bioeng. 1978,20, 1541-1556. Whitesides, G. M.; Wong, C.-H. Enzyme as Catalysts in Synthetic Organic Chemistry. Angew. Chem., Znt. Ed. Engl. 1985, 24, 617-638.
Received for review August 8, 1989 Revised manuscript received February 21, 1990 Accepted March 14, 1990
Study of Temperature-Programmed Desorption of tert -Butylamine To Measure the Surface Acidity of Solid Catalysts And& T. Aguayo, Jos6 M. Arandes, Martin Olazar, and Javier Bilbao* Departamento de Ingenieria Quimica, Universidad del Pais, Vasco. Apartado 644, 48080 Bilbao, Spain
The technique of temperature-programmed desorption of tert-butylamine is described to measure the surface acidity of solid catalysts. The use of this base has advantages over the use of ammonia, pyridine, and n-butylamine. The desorption measurement is carried out by two methods, gas chromatography and thermogravimetry, and the advised conditions are described for both methods. Catalysts of SiOz/A1,03, bifunctionals of Ni-Si02/A1,03, and a commercial cracking zeolite have been studied. A comparison of the desorption results with those of the other acidity measurement techniques (such as titration with n-butylamine in the liquid phase and kinetic measurement of isomerization of n-butenes as the test reaction) allows the acidity measured with tert-butylamine desorption to be classified as strong, corresponding to the active sites in most of the reactions among the hydrocarbon compounds catalyzed by acids. Introduction The greatest difficulty found in the characterization of acidic catalysts is the measurement of surface acidity-the origin of the activity of these catalysts in the reactions among hydrocarbon compounds. In the relevant literature, revisions of the methods described to measure the acidity are compiled (Benesi and Winquist, 1978) and can be classified into four groups: (A) titration, (B) spectroscopic measurement by IR, ( C ) test reactions, (D) adsorption-desorption of bases. 0888-5885/90/2629-1621$02.50/0
All the methods have disadvantages, and in general, the more rigorous methods have the disadvantage of being more complex and/or more costly. The first two methods give information about the acidity of the catalysts at room temperature, while the latter two methods have the advantage that they give information about the acidity for a temperature near the one to be used in the reaction. The most traditional method (Benesi, 1956, 1957) is titration of the finely ground catalysts with n-butylamine, in liquid medium (benzene), using colored indicators of 0 1990 American Chemical Society
1622 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 Table I. Denomination and General Characteristics of the Catalysts Studied silica gel real precipitating impregnating density, catalyst type agent agent denomination - g/cm3 3 N HCI 2.21 3 N HCI AlCl, 6 N HC1 2.23 ~ 1 ~ 1 , 6 N HC1 9 N HCl 2.13 9 N HCl AICl, 12 N HCI 2.18 AICI, 1 2 N HCI 2.13 3 N HNO, AlC1, 3 N HNO, 6 N HNO, 2.35 AICI, 6 N HNO, 2.33 3 N H,SO, A-6 Alz(S04)3 ~ 1 ~ 1 , 2.22 Commercial Si 2.43 Si Ai-1 Ni(N03h 2.20 Ni(N03)*-urea PD-2 2.83 HEZ-55 zeolite
app density, g/cm3 0.78 0.59 0.78 0.71 0.77 0.65 0.74 0.91 0.84 0.77 1.44
pore vol, cm3/g 0.83 1.25 0.82 0.94 0.83 1.10 0.92 0.64 0.78 0.85 0.34
surface area, m2/g 188 233 273 325 124 236 121
305 117 I74 I22
adsorption proposed by Hammet. The major criticisms of this method arise from the question of its reproducibility in view of the difficulty in reaching equilibrium in the titration (Deeba and Hall, 1979). On the other hand, the technique is very difficult to apply to colored catalysts, as well as to catalysts deactivated and darkened by coke. In the same way, doubt can be cast on the quality of the diffusion of the base and of the indicators toward the inside of the porous structure, especially in microporous catalysts (Kladnig, 1979; Barthomeuf, 1979; Bezman, 1981). The measurement by IR spectroscopy of chemisorbed ammonia (Jacobs and Uytterhoeven, 1973) and pyridine on the Lewis and Bransted sites (Hughes and White, 1976; Jacobs and Uytterhoeven, 1971; Schwartz et al., 1978) requires great experimental complexity to obtain a reproducible measurement. The isomerization of n-butenes (Ghorbel et al., 1974), the isomerization of xylene and the cracking of cumene (Covini et al., 1967), and the dehydration of ethanol (De Boer and Visseren, 1971) have been studied as test reactions. In the dehydration of n-butanol and of 2-ethylhexanol, it has been determined (Aguayoet al., 1987a) that the initial conversion and deactivation are related to the different levels of acid strength. In another paper (Aguayo et al., 1990), in which the isomerization of n-butenes has been studied as the test reaction, it is shown that for rigorous use of the technique the kinetic constants must be measured. This requires systematic performing out of a series of kinetic runs under strictly controlled conditions with accurate measurement of the reaction products. The desorption of organic bases has as a major advantage its simplicity, as it requires only gas chromatography, a technique that is fast, economical, and gives reproducible results. Obtaining a relationship between the chromatographic signal measurement and the thermogravimetric measurement will allow us to know quantitatively the amount of adsorbed base from the measurement of the chromatographic signal of the desorbed products. Performing experiments under different operating conditions and using different bases, on catalysts of different porous structure and different distribution of acid strength, will lead us to the knowledge of the advisable operating conditions and of the limitations of the technique. A comparison with other techniques of acidity measurement will be useful to ascertain the level of acid strength that is measured by desorption of the base.
All the catalysts studied can be considered porous. In this sense, the zeolite has the smallest pore volume 0.341 g/cm3. Nevertheless, they have basically microporous structures (rp < 10 A, 5% of the pore volume; 10 < rp < 100 A, 65%1, corresponding to silica/aluminas prepared by impregnation (Romero et al., 1986; Aguayo et al., 1987b). The HEZ-55 zeolite has a microporous active structure on a support constituted of meso- and macropores ( r < 10 A, 9% of the pore volume; 10 < rp < 100 A, 22'707, a structure that is suitable for diffusion of molecules of high molecular weight that to be cracked. The general method to prepare the SiOz/A1,O3catalysts has been described in previous papers (Aguayo, 1981; Aguayo et al., 1987b). The concentration of the agent to precipitate the silica gel (3 N HCl, 6 N HC1,9 N HCl, 12 N HCI, 3 N HNO,, 3 N H2S04)and the agent to impregnate the silica gel (AlCl, or A1(SOI)Bwere used as variables. The catalyst named Commerical Si is prepared by impregnation of commercial silica gel. The one named Si Al-1 is prepared by impregnation of Si02/A1203with Ni(NO& and the PD-2 by precipitation-deposit, a method that consists of the slow decomposition of urea into a solution of Ni(NO&. in which the gel of SiO2/Al2O3is in suspension. In Table I, the physical properties of catalysts, measured from the N2 adsorption-desorption isotherms (in a Micromeritics Accu Sorb 2100 E) and by Hg picnometry, are given. Surface Acidity Measurements by Other Techniques. The acidity of the catalysts was measured by two techniques: (A) titration with n-butylamine (Benesi, 1956, 1957) using neutral red (pK = 6.8) and p-(dimethylamin0)azobenzene (pK = 3.3) as indicators; (B) measurement of the activity in the isomerization of n-butene. The experiments were carried out in an integral reactor, and pure 1-butene under atmospheric pressure, at 150 "C, was fed. The conversion to cis-butene and to trans-butene was measured by gas chromatography (Hewlett-Packard 5890). The reaction equipment, the conditions, and the method of kinetic data analysis are described in detail in a another paper (Aguayo et al., 1990). As activity measurements, the kinetic constants klo klt, and k,, have been obtained from the kinetic equation, which has the following general expression:
Experimental Section Catalysts. The catalysts studied are given in Table I, and they are Si02/A1203catalysts prepared in our laboratory, bifunctional catalysts of Ni supported on Si02/ A1203,and a commercial zeolite catalyst used for catalytic cracking (HEZ-55, supplied by Engelhardt).
In Table 11, the acidity values of the catalysts obtained by using the two methods mentioned above are related. It must be noted that the colored catalysts cannot be characterized by titration with indicators of color and for them only the kinetic constants of the isomerization of butenes have been given in Table 11.
k,jKiPi ril =
1 + K I P l + KJ'c
+ K,P,
(1)
Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 1623 Table 11. Values of the Acidity Obtained by Titration of n -Butylamine and by Isomerization of n-Butenes method A: method B titration with kinetic const in n-BA, mmol of isomerization n-BA/g of catal. of n-butenes catalyst pK = 3.3 pK = 6.8 kt1, k'lc k', 3 N HC1 0.21 0.34 3.67 4.18 3.02 6 N HC1 0.20 0.25 3.32 4.37 3.40 9 N HCI 0.30 2.84 3.92 8.75 0.32 12 N HC1 0.24 0.33 2.75 4.20 12.40 3 N HNO, 0.15 0.24 3.07 3.83 3.84 6 N HNO, 0.17 0.23 3.32 3.75 4.34 A-6 3.72 0.13 0.39 2.43 2.75 Commercial Si 0.23 3.15 4.31 4.99 0.42 Si Al-1 um" um 1.25 1.43 4.12 PD-2 4.94 um um 0.01 0.02 HEZ-55 um um 1.14 1.39 2.64
Symmetrical Thermoanalyzer
n
l-icT7-l
I
Gas Chromatograph I
I
j
r, 1
r
Figure 2. Diagram of the equipment for the thermogravimetric measurement of programmed desorption of organic bases.
"um = unmeasurable. SatLrator
VALVE POSITIONS
Figure 1. Chromatographic equipment for the measurement of programmed desorption of organic bases. On the right, diagram of the ON and OFF positions of the six-way sampling valve for base introduction.
It is noteworthy that the results of the titration with n-butylamine are in a very narrow range: between 0.13 and 0.24 mg of n-butylaminelg of catalyst for pK = 3.3 (relatively strong acidity for those measured by this method) and between 0.23 and 0.42 mg of n-butylaminelg of catalyst for pK = 6.8 (corresponding to the measurement of the catalyst active sites of any acid strength). The values of the kinetic constants in the isomerization are in a wide range, and the maximum error is within a f 2 % of each value indicated in Table 11.
Technique of the Desorption of Bases Two methods have been used for the measurement of desorbed products, gas chromatography (equipment described in Figure l) and thermogravimetry (equipment described in Figure 2). The chromatographic equipment consists of a Hewlett-Packard 5890 chromatograph with thermal conductivity detector and installed on-line with a Perkin-Elmer ion trap detector. The catalyst sample is contained in a stainless steel cylinder of 1/8-in.0.d. The introduction of the base that comes from the thermostated saturator is carried out by means of a six-way sampling valve. In Figure 1, the flow disposition is diagrammatically represented for the two positions of the valve. The ON position allows for contact of the base with the sample and the subsequent withdrawal of the base to the exterior. In the OFF position, the base goes directly from the saturator to the exterior, while the sample is swept away by a He stream that goes to the detector. The equipment for thermogravimetric measurement, Figure 2, consists of a Setaram TAG 24 symmetrical thermoanalyzer provided with a vacuum system mmHg), with mass flowmeters to control the streams of inert gas and of gas coming from the thermostated saturator. The equipment is connected to the gas chromato-
Temperature ( " C ) Figure 3. Curves of vapor pressure of different organic bases usuable for acidity measurement.
graph described in the previous figure. The results register simultaneously indicates differential thermal analysis (DTA), thermogravimetric (TG) analysis, and differential thermogravimetric analysis (DTG). Previous to adsorption, when the thermogravimetric measurement is going to be used, the catalyst sample (between 20 and 40 mg) is cleansed by maintaining a vacuum of mmHg at 450 "C for 2 h. If the desorption measurement is going to be carried out by chromatography, the catalyst is cleansed by passing a He stream through the catalyst sample at the same temperature and at the same time as in the previous case. The adsorption of the base until the saturation of the sample is attained by treating it for 15 min at 20 "C with a stream coming from the bubbling of 20 cm3 of He/min into the base at 0 "C. As is observed in Figure 3, the vapor pressure of the tert-butylamine at 0 "C, 40 mmHg, is substantially higher than for the other bases. On the other hand, it must be pointed out that at 50 "C the adsorption isotherm is practically flat for all the bases studied. Then, the excess of base if removed by keeping the He stream for 30 min at 50 "C so that stabilization of the system is reached. The calorimetric monitoring of the adsorption, Figure 4, allows us to identify two peaks in DTA corresponding to tert-butylamine adsorption on one silica-alumina (the data of Figure 4 correspond to Commercial Si catalyst); the first peak is fast, due to chemical adsorption, and the second, slower, due to physical adsorption. In the same figure, the DTG peak corresponding to the chemical adsorption on the catalyst is noticeable faster than that corresponding to the physical adsorption on the silica. The desorption is carried out by increasing the temperature at a rate of 5 "C/min up to 300 "C. This temperature is kept for 5 min. In Figure 5, the chromato-
1624 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990
4 OTG
Wmln)
-0.05
1-0.15
?.J
1 - 0 50
r
Commercial
si
catalyst
L-0 55
TEYPERATLXE (" C; 1 -0 60
Figure 4. Comparison of the thermogravimetric and calorimetric measurements of tert-butylamine adsorption on an acidic catalyst (Commercial Si catalyst) and on an inert silica.
100
50
150
200
,
250 _ _ _ A
Figure 6. Thermogravimetric measurements of tert-butylamine desorption on Commercial Si catalyst for different temperature ramps
s
-OIL
-0 2
t
t
1 -03t
t 12N HCI c a t a l y s t
r
Figure 5. Comparison of the chromatographic and thermogravimetric measurements of tert-butylamine desorption from Commercial Si catalyst.
graphic signal, upper figure, and the DTG signal, lower figure, are schematically shown. Both correspond to the desorption of tert-butylamine from one of the catalysts, the Commercial Si catalyst. With both measurement techniques, the peak of chemical desorption (the shaded one) is well differentiated from the peak of physical desorption, which is wide and less defined. As was expected, only this latter peak is obtained for the silica gel. A ramp of temperature increase of 5 "C has been chosen because it gives a good identification of the chemical desorption peak. This can be observed in Figure 6, where the thermogravimetric measurements of tert-butylamine desorption on the Commercial Si catalyst are shown for the ramps of 2.5 OC/min, 5 "C/min, and 10 OC/min. The heating rate of 5 OC/min was previously used by Takahashi et al. (1976) for the desorption of n-butylamine and pyridine, Mieville and Meyers (1982) desorbed tert-butylamine at a heating rate of 10 "C/min, and Schwarz et al. (1978) desorbed pyridine a t a heating rate of 80 "C/min. In Figure 6, it is observed that, for the ramp of 2.5 "C/min
/HEZ-55
4
I 0 20
zeolite catalvst
0
50
-__
L
L - L d - l i
100
150
200
TEMPERATURE
250
300
( "C)
Figure 7. Thermogravimetric measurement of desorption of different bases: (a) 12 N HCI catalyst; (b) HEZ-55 zeolite catalyst.
and, on the other hand, for velocities higher than 10 OC/min, there is not obtained an identification and clear separation of the physical and chemical adsorption peaks. In Figure 7, the DTG curves of desorption of the four bases are plotted, in part a for 1 2 N HCl catalyst and in part b for HEZ-55 catalyst. It is noteworthy that the desorption of NH3 does not permit the identification of a chemical peak, but this is superimposed with the physical adsorption peak. In the desorption of 12 N HCl catalyst, the tert-butylamine gives a chemical desorption peak that is better defined than the peak of pyridine and of n-butylamine. This poorer results for pyridine was expected, as its vapor pressure is lower (smaller diffusion rate). The difference between the desorption of n-butylamine and of tert-butylamine, and the better result when using the latter, has its justification in the microporous nature of the amorp-
Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 1625 Table 111. Values of the Acidity Obtained by Using Both Techniques of Butylamine Denorption Measurement catalyst 3 N HCl 6 N HCl 9 N HCl 12 N HC1 3 N HNOS 6 N HNOS A-6 Commercial Si Si Al-1 PD-2 HEZ-55
thermoanalysis, mmol of t-BA/g of catal. X 102 8.48 8.68 24.18 40.20 9.02 10.62 8.34 15.59 10.23 10.61 5.52
I
signal 19.8 26.3 67.3 115.2 31.3 23.2 14.5 38.6 20.7 30.1 12.3
hous silica/aluminas (Romero et al., (1986) and in the possible limitation of the internal diffusion of n-butylamine due to its linear molecular structure (chain length of 6.735 A, against the chain length of tert-butylamine of 4.267 A). The adsorption results with the zeolite, Figure 7b, seem to corroborate this result. In this case, the larger size of the porous matrix of the cracking zeolite would explain the similar adsorption-desorption of n-butylamine and tertbutylamine. The results of tert-butylamine desorption are similar to those obtained by Mieville and Meyers (19821, also for silica/aluminas. Nevertheless, the operation conditions with n-butylamine and pyridine used in this work give better results than those proposed by Takahashi et al. (1976) in the desorption of n-butylamine and pyridine and by Schwarz et al. (1978) in the desorption of pyridine. These latter authors require higher temperature levels for the chemical desorption of the bases. The application of the programmed desorption technique on the catalyst used in a test and subsequently regenerated leads to peaks identical with those of the fresh catalyst, for the four bases and for all the catalysts tested. These results agree with those of Takahashi et al. (1976). From the mass spectrometric analysis of the chemical desorption products, it is deduced that the decomposition of n-butylamine (n-BA) and tert-butylamine (t-BA) takes place following two alternative paths, according to the scheme (Takahashi et al., 1976; Mieville and Meyers, 1982) / C4Ha + NHS n-BA or t-BA
.--I
-
CSHB+ CHSNH2
The desorption products of pyridine, however, cannot be identified.
Comparison with the Results of Other Techniques In Table 111, the results of the acidity obtained by desorption of tert-butylamine are given when gas chromatography and thermogravimetry were used for all the catalysts. The chromatographic measurements as well as the thermogravimetricones are taken in a wide range of values, which demonstrates the sensitivity of the method to the distribution of catalyst acid strength. The linear relationship observed in Figure 8 between the data obtained by both measurement techniques of tertbutylamine desorption products for all the catalysts shows the trustworthiness of the chromatographic technique, a technique that is simple, fast, and economical. On the other hand, Figure 8 can be used to determine the quantitative measurement of tert-butylamine desorbed from the chromatographic signal, whenever the above-mentioned conditions are fulfilled. Comparing the results of Table I11 with those of Table I1 obtained with other acidity measurement techniques,
0
0.1
0.2
0.3
04
0.5
Thermogravimetric measurement (mmal ai t-butplamtne / g of aatalpat) Figure 8. Comparison of data obtained by both measurement techniques of tert-butylamine desorption products.
I O 0
L 0.1
0.1
0.3
0.4
0.6
Acidity (mmol t-butylamine/g) Figure 9. Values of the apparent kinetic constants of the isomerization of cis-butene to trans-butene for all the catalysts studied vs acidity measurements corresponding to tert-butylamine desorption.
it cannot be stated that there is some relationship between the acidity measured by tert-butylamine desorption and with that measured by n-butylamine adsorption in the liquid phase (Benesi's method), for any value of titration pK using this latter technique. Neither is there a relationship between the results of tert-butylamine desorption and the kinetic constants of the isomerization of 1-butene to cis-butene or 1-butene to tram-butene, but there is a linear relationship, Figure 9, with the kinetic constant of isomerization of cis-butene to trans-butene. This comparison with other techniques shows that the programmed desorption of tert-butylamine gives a measurement of strong acidity level, which is necessary for the isomerization of cis-butene to trans-butene, while the traditional titration with n-butylamine in the liquid phase corresponds to a measurement of acidity levels that can be considered weak. In fact, a linear relationship can be observed between the results of a titration for pK = 3.3 and the kinetic constants of the isomerization of 1-butene to cis-butene and 1-butene to trans-butene, isomerizations that require active sites with weak acidity level.
Conclusions The study carried out for an extensive group of acid catalysts prepared under different conditions, with different physical structures and different surface acidity
1626 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990
levels, permits the conclusion that the programmed desorption of tert-butylamine is a technique that is reproducible, simple, and fast for the measurement of the surface acidity of any catalyst whose kinetic behavior is conditioned by its acid nature. tert-Butylamine has the conditions of vapor pressure and spatial molecular structure that allow for measurement with considerable desorbed mass in microporous catalytic structures without diffusion limitations being observed. It has been proven that the measurement by gas chromatography of desorbed products has enough trustworthiness to be used in a reproducible way under the conditions pointed out in this paper. The simplicity of this technique and the relationship determined in Figure 8 with the desorbed mass will allow us to know the acidity in equivalent millimoles of desorbed tert-butylamine. In any case, the use of the chromatographic or thermogravimetric technique will allow for the identification of chemical desorption products, clearly differentiated from physically desorbed tert-butylamine. The relationship determined between the desorbed mass and the kinetic constant of isomerization of cis-butene to trans-butene (reaction that requires a strong acidity level) will allow us to consider that the acidity measured by tert-butylamine desorption corresponds to catalyst active sites of high acid strength, in the level that is necessary in most of the reactions among hydrocarbon compounds catalyzed by acids, such as isomerizations, cyclations, polymerizations, alkylations, cracking, etc. Nomenclature K , = equilibrium constant of adsorption of reactant i, atm-I k , = reaction kinetic constant of compound i to compound j , mol h-' g-' k',, = apparent kinetic constant defined as k:, = k,K,, mol h-' g-' atm-* PI = partial pressure of reactant i, atm T I , = reaction rate of compound i to compound j, mol h-l g-l rp = pore radius, A
Subscripts 1 = 1-butene c = cis-butene i = reactant j = reaction product t = trans-butene Registry No. HCl, 7647-01-0; Ni, 7440-02-0;tert-butylamine, 75-64-9;cis-butene, 590-18-1;trans-butene, 624-64-6.
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15-24. Received for review July 25, 1989 Accepted March 1, 1990
