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Energy & Fuels 1988,2,740-743
Surface Acidity Studied by the Temperature-Programmed Desorption of tert -Butylamine R. L.McCormick, J. R. Baker,? and H. W. Haynes, Jr.* Department of Chemical Engineering, University of Wyoming, Laramie, Wyoming 82071
R. Malhotra Department of Chemical Kinetics, SRI International, Menlo Park, California 94025 Received March 7, 1988. Revised Manuscript Received June 27, 1988
The temperature-programmed desorption of tert-butylamine provides a convenient means of characterizing acid sites on heterogeneous catalysts;however, the interpretation of the high-temperature (@-peak)maximum as a measure of acid site strength is subject to uncertainty. Analysis of thermal desorption products by field-ionization mass spectroscopy revealed that isobutylene is primarily responsible for the @-peak. An acid-catalyzed "Hofmann-type" elimination mechanism is postulated to explain the formation of this species. Thus it appears that lower @-peaktemperatures are associated with stronger acid sites. An observed inverse correlation between @-peaktemperature and Sanderson intermediate electronegativity provides further evidence supporting this conclusion.
Introduction Solid acids are commonly used to catalyze cracking, isomerization, and dealkyltion reactions. Because of the widespread application and importance of these materials, there exists a great many methods for measuring the acidic properties of surfaces.'S2 These properties include the number of acid sites, the acid site strength distribution, and the type of acid sites (Lewis or Br~nsted). As part of a study of deactivation of catalysts used for hydrotreating coal liquids3 we have employed the temperature-programmed desorption (TPD) of tert-butylamine (TBA) to measure the relative number of acid sites on the surface. Experimentally, TBA is a convenient choice of adsorbate. It possesses sufficient volatility (bp = 44.4 O C ) for introduction into a carrier gas stream, and desorption temperatures are generally less than 400 "C so that experimentation can be conducted in a typical gas chromatograph oven. Also, ammonia and most other organic bases desorb at much higher temperatures. For applications where the catalyst may be sensitive to high temperatures, the desorption of TBA may be preferred. TBA has been successfully employed as a probe for evaluating acid site characteristics of petroleum cracking catalyst^.^^^ The TPD curves generated by this technique consist of a low-temperature a-peak produced by the desorption of physically adsorbed or hydrogen-bonded TBA and a high-temperature @-peak caused by the desorption of chemisorbed TBA possibly as cracked products. Several typical TPD curves are shown in Figure 1. The integrated @-peakarea divided by the catalyst surface area is taken as a measure of the relative acid site density (RAD) of the surface. The temperature maximumum of the @-peakshould be an approximate indicator of acid site strength; however, two interpretations of the meaning of the @-peaktemperature are possible. One interpretation proposed by Nelson and co-workers5suggests that as the TBA is bonded more strongly to the strong acid sites, surfaces with stronger acid sites will exhibit higher @-peaktemperatures. These authors did not, however, observe a correlation 'Present address: Westinghouse Hanford Co., Richland, WA 99352.
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Table I. &Peak Temperatures and Sanderson Intermediate Electronegativities for Several Catalyst Samples &peak T, SamDle Si02/MgOa Mo03/SiOzb Si02/A1203a SiOz/Al2O3" A1203/AlPO~'
p2od & 2 0 3 b P20d" MOO~/A~ZO~~ Cr203/&03" M00~/Ti0~~ Mo03/MgOb M003/MgO/A1203~,' Ti02/A1203'
molar ratio 79:21 5.5:95 67:33 84:16 60:40 11:89 4.895 1486 8.492 1.7:98 11:89 12:63:25 65:35
Commercially prepared. Prepared by coprecipitation.
O C
242 256 260 260 273 276 281 286 289 293 293 301 302
electronegativitv 3.99 4.12 3.95 4.04 3.93 3.84 3.78 3.74 3.70 3.37 3.34 3.53 3.53
Prepared by impregnation.
between cracking activity and @-peaktemperature. This was explained by noting that the @-peaktemperature is a measure of acid site strength a t coverages approaching unity, a situation thought not to occur in catalytic cracking. In an alternative interpretation, the strong acid sites are assumed to have a higher cracking activity than weak acid sites and can therefore crack adsorbed TBA a t lower temperatures. Surfaces with stronger acid sites should therefore exhibit lower @-peaktemperatures. Consistent with this viewpoint MieviUe and Meyers4cite evidence that the origin of the 8-peak is not always simple desorption of the uncracked parent amine. In our work with a wide variety of acidic solids we have observed that @-peaktemperature correlates inversely with Sanderson intermediate electronegativity,6 Figure 2. The (1) Tanabe, K. Solid Acids and Bases; Academic Press: New York, 1970. (2)Benesi, H. A.; Winquist, B. H. C. Adu. Catal. 1978,27, 97-182. (3)Baker, J. R.; McCormick, R. L.; Haynes, H. W., Jr. Ind. Eng. Chem. Res. 1987,26, 1895-1901. ( 4 ) Mieville, R. L.; Meyers, B. L. J. Catal. 1982, 74, 196-198. (5) Nelson, H.C.; Lussier, R. J.; Still, M. E. Appl. Catal. 1983, 7 , 113-121. (6)Sanderson, R.T. Chemical Bonds and Bond Energy; Academic: New York, 1971.
0 1988 American Chemical Society
TPD of tert-Butylamine
Energy & Fuels, Vol. 2, No. 6,1988 741 Table 11. Characterization Results for Catalyst Samples Studied by FIMS
Shell 324M Molvbdena/Alumlna
iso-
sample silica/alumina chromia/alumina alumina Shell 324M
W
130
170
250
210
370
330
290
TEMPERATURE "C
Figure 1. TPD curves for alumina, molybdena/alumina and Shell 324M.
240
t
2201
"
3.20
"
3.40
'
3.60
"
3.80
"
4.00
'
' 4.20
I
Electronegativity
Figure 2. &Peak temperature as a function of Sanderson intermediate electronegativity. intermediate electronegativityis the geometric mean of the electronegativities of all of the atoms in the sample. Table I lists the samples and data used to make this plot; the electronegativities were calculated from atomic values given in ref 6. Electronegativity would be expected to correlate positively with acid site strength, and indeed Lercher and Noller' and Mortier* have reported such a correlation. Figure 2 therefore suggests that the &peak temperature may be inversely related to acid site strength. This study was undertaken to clarify the interpretation of the fl-peak temperature maximum. The critical question is whether desorbed products or acid-catalyzed cracking products are responsible for the high-temperature 8-peak. In the former case, one would expect higher peak temperatures to be associated with stronger acid sites, whereas in the latter cme, the stronger acid sites should give rise to lower peak temperatures as suggested by the correlation of Figure 2.
Experimental Section From the many samples that have been subjected to the TPD analysis, four were selected for further study by field-ionization mass spectroscopy. A silica-alumina (7022weight ratio of silica to alumina) was obtained from the Ammo Oil Co. Chromiaalumina (12wt % CrzOs)was obtained from the Harshaw-Fdtrol Partnership. These materials were calcined at 450 OC in air. A sample of alumina was prepared from aluminum isopropoxide followed by drying and calcination at 600 OC. This preparation is known to have sufficient acidity to catalyze the skeletal isomerization of olefins? The commercial Ni-Mo-P/A1203 hydro(7) Lercher, J. A,; Noller, H.J. Catal. 1982,77, 152-158.
(8)Mortier, W.J. J. Catal. 1978,55,138-145.
BET SA,
m2/a 430 72 125 140
RAD 0.037 0.038
&peak T, butylene O C intensitv 260 288
broad peak 0.117
278
59.6 17.9 14.1 43.7
treating catalyst, Shell 324M, was calcined at 400 "C. BET surface areas were measured by nitrogen adsorption in a conventional volumetric apparatus. The TPD of tert-butylamine was conducted in an apparatus described previously and in great detail by Baker.Io The main component of the apparatus is a Gow Mac 550P TCD gas chromatograph with a temperature-programmable oven. The gas chromatograph was used for catalyst pretreatment, temperature programming, and detection of desorption products. Briefly, a 0.1-g sample of catalyst was placed in a sample holder in the GC oven and pretreated in flowing He for 2 h at 340 "C. The temperature was then lowered to 100 OC, and TBA was preadsorbed onto the surface. After switching back to pure carrier and allowing time to establish the base line, we raised the temperature using a linear temperature program of 10 OC/min, and the desorption products were monitored by the TC detector. With a high flow rate used to minimize diffusion resistances and readsorption, the TC response was nearly proportional to the rate of desorption." The detector response was calibrated for each sample by using a pulse of n-butane, which was injected into the He stream and carried over the catalyst to the detector prior to TBA preadsorption. Because the exact relationship between the TC response and actual number of acid sites is unknown, the calculated RAD value is the magnitude of the dimemionlesTC reaponse per square meter of sample surface. Therefore RAD values are only meaningful when several samples are compared. Samples were prepared for thermal desorption/FIMS studies as follows: The samples were F i t dried in a vacuum oven at 100 OC for 24 h. Then the samples were equilibrated with TBA for 15 min and excess liquid was decanted. The samples were next dried in air at room temperature for several hours, dried in an atmospheric drying oven at 100 OC for 1h, and placed in vials, that were tightly capped until used. The samples were introduced into the masa spectrometer probe at ambient temperature. Before the pump down sequence was started, the samples were cooled with a jet of liquid carbon dioxide. After the necessary vacuum was achieved, the electrical potentials were turned on and the spectrometer was set to scan repeatedly over the 50-500 amu masa range. The samples were gradually warmed to a f i i temperature of 450 "C and intermediate spectra collected. At the end of the run, the intermediate spectra were summed to produce the sum spectra. The samples were heated along a temperature ramp (4 "C/min), but because it was necessary to not overload any channel of data collection, the heating rate often had to be slowed.
Results Table I1 lists BET surface areas, RAD values, and 8peak temperatures for the samples studied by FIMS. The samples selected for this study cover a broad range of RAD and @-peaktemperature values. Alumina, whose TPD curve is shown in Figure 1,does not exhibit a well-defined &peak although desorption products are observed a t high temperatures. The addition of Moos to the same alumina support produces a sharp 8-peak as illustrated in the figure. In preparing catalysts from a variety of support materials, we have observed that the incorporation of molybdenum into the catalyst in the oxidic state always results in a substantial enhancement of acidity. The molybdenajalumina preparations tend to possess the (9)Pines, H.;Haag,W. 0. J. Am. Chem. SOC.1960,82,2471-2483. (10)Baker, J. R. M.S.Thesis, University of Wyoming, 1986. (11)Falconer, J. L.;Schwarz, J. A. Catal. Reu.-Sei. Eng. 1983,25, 141-227.
McCormick et al.
742 Energy & Fuels, Vol. 2, No. 6,1988 2400 I
1
600
1
P Mass (ME)
\
40
10
70
130
190
250
Temperature (“C) Temperature Evolution 01 Massas 56 and 58
1200,
1
-___
MIZ 57 ___. _. . MIZ i:73 M R = 74
-50
0
50
E P
100 150 200 250 300 350 400 450
Sample Temperature (“C)
Figure 3. (a) Sum mass spectra and (b)temperature evolution profile for total organics from Shell 324M.
300.
L
0
-so
10
70
130
190
250
Temperature (“C) Temperature Evolution of Masses 57,73, and 74
Table 111. Peak Assignments for Thermal DesorDtion/FIMS peak, m/z assgnt peak, mlz assgnt 56 57 58
isobutylene tert-butyl ion/2-propanimine protonated 2-propanimine
73 74 112
TBA TBAH+ octene
highest RAD values of the catalysts that we have studied. Figure 3 shows the sum mass spectra and the temperature evolution profile of total organics for the Shell 324M catalyst. These plots are typical of those obtained for all samples studied by thermal desorption/FIMS. (Only the relative intensities vary from sample to sample.) Table I11 lists the peak assignments for the mass spectra. A second sample of Shell 324M was dried a t room temperature before FIMS analysis rather than a t 100 OC as described above. Intermediate spectra sufficient to produce temperature evolution profiles for individual mass peaks were collected for this sample and are shown in Figure 4. Desorption temperatures were much lower in the FIMS apparatus because of the high vacuum employed. These data indicate that isobutylene was formed only a t high temperatures, above 170 “C. FIMS also showed the formation of octene and higher homologues concurrently with the formation of isobutylene. Also shown in Figure 4 is the temperature evolution curve for TBA, which indicates that this species desorbs only a t low temperatures. A “blank” run was made by injecting about 5 pL of TBA into a batch inlet attached to the mass spectrometer. The batch inlet was kept at about 50 O C , and the vapors of TBA were directly bled into the field ionizer. Peaks a t 57,58, 73, and 74 were observed.
Discussion On the basis of the mass alone, it is difficult to make structural assignments. However in this case we have the structure of the starting material, TBA to aid in these assignments. The peak at mass 57 is most likely tert-butyl ion. This ion is very stable and is frequently seen in the spectra of compounds bearing this group. The peak a t mass 58 could be caused by isobutane; however, this would require a hydrogenolysis reaction. Because the peak a t mass 58 evolves a t lower temperatures and because this peak was observed in the blank run, it is almost certainly the result of a fragmentation in the mass spectrometer. It
Figure 4. Temperature evolution profiles for Shell 324M.
c1“3
C? CH3-C-NH, I
CH3
+ H’B-
4
CH3-C I H-FJ@, H
c1“3
+
+ B-
CH3-C
+ NH3+ H+B-
1 I
CH,
Figure 5. “Hofmann-type” elimination mechanism for isobutylene formation over a Brplnsted acid site. seems reasonable that this species is formed by the loss of a methyl radical from the amine radical cation. The peaks a t mass 58,73, and 74 have the same temperature evolution profile, being desorbed a t lower temperatures. The mass 74 peak (protonated TBA) is formed when the amine acquires protons that are present on the filaments and other parts of the ionization source. The peak at mass 57 follows a parallel profile but also has a second peak a t higher temperatures in parallel with the peak for isobutylene. Evidently the tert-butyl ion can form via fragmentation of TBA and by protonation of isobutylene. From the temperature evolution curves of individual masses we can conclude that isobutylene, which forms only a t higher temperatures, is primarily responsible for the @-peakobserved in our TPD studies. This result and the fact that TBA desorbs intact only a t low temperatures indicate that the second proposed interpretation of the @-peaktemperature is correct. That is, high &peak temperatures correspond to weaker acid sites because isobutylene is produced via a reaction initiated by the acid sites. Table I1 also lists the isobutylene intensity (as percent of total ion intensity) for the four samples studied by FIMS. These data indicate that isobutylene intensity increases with decreasing @-peaktemperature as expected for isobutylene formation on strong acid sites. The isobutylene intensity for the Shell catalyst appears to be higher than that expected based on @-peaktemperature alone. This could be due to the approximately 3-fold higher RAD for this catalyst as compared to the others. One may envision isobutylene formation via the mechanism shown in Figure 5. This “Hofmann-type” elimination reaction can be initiated by a Bransted acid site as TBA adsorbs on the acid site to produce an alkylammonium ion. This ion then undergoes a Hofmann-type
Energy & Fuels 1988,2,743-750 elimination to produce the olefin. One can postulate a similar mechanism involving a Lewis site. The fact that olefins did not form when neat TBA was injected indicates that this reaction occurs on the surface and not in the gas phase in the FIMS instrument.
Conclusions An investigation of surface acidity studies using the temperature-programmed desorption of tert-butylamine has shown that the primary constituent of the @-peakis isobutylene. Isobutylene is formed only a t high temperatures, possibly via a Hofmann-type elimination initiated by the surface acid sites. Because this reaction should occur a t lower temperatures for stronger acid sites, the 8-peak temperature should correlate inversely with acid site strength. This conclusion is supported by the inverse correlation of &peak temperature with electronegativity. Apparently the low temperaure a-peak is caused by the desorption of physically adsorbed or weakly hydrogenbonded amine while the species that reacts and desorbs to produce the @peak is bonded more strongly via H+ transfer from a Brransted site or electron-pair donation to a Lewis site. Finally, some comments comparing TBA TPD with the much more established TPD of ammonia appear to be in order. It should be clear from the results of this communication that the prospects for actually titrating acid sites with TBA are remote. Such experiments are conducted routinely in NH3 TPD12-14since the desorption process is (12)Hidalgo, C. V.;Itoh, H.; Hattori, T.; Niwa, M.; Murakami, Y. J. Catal. 1984,85,362-369.
743
relatively clean and involves no decomposition of the molecule. Using the theory developed by Cvetanovic and Amenomiya,15 it is sometimes possible to calculate desorption activation energies from NH3 TPD experiments and thereby obtain a quantitative measure of acid site strength. In TBA TPD the interpretation of &peak maximum temperature as a measure of acid strength is only quaIitative. Steric considerations are also important in some situations. The size of the ammonia molecule (critical diameter = 3.8 A) permits its use as a probe for characterizing acidity in zeolites. TBA would not be suitable for such applications. As stated in the introduction, the advantages claimed for TBA TPD are primarily along lines of experimental convenience and the fact that desorption temperatures are generally low. TBA may be the preferred choice for use with temperature-sensitive catalysts.
Acknowledgment. This work is jointly sponsored by the U.S. Department of Energy (Grant DE-FG2284PC70812) and the Amoco Oil Co. We are grateful for both the financial support and consultations provided by these organizations. Some of the TPD data in Table I were taken from the University of Wyoming Master of Science Theses by T. R. King (Dec, 1986) and J. A. King (May, 1987). Registry No. TBA, 75-64-9; SOz, 7631-86-9; AZO3,1344-28-1; Cr203, 1308-38-9; Ni, 7440-02-0; Mo, 7439-98-7; P, 7723-14-0. (13)Post, J. G.;van &off, J. H. C. Zeolites 1984,4,9-14. (14)Corma, A,; Fornes, V.; Melo, F. V.; Herrero, J., Zeolites 1967,7, 559-563. (15)Cvetanovic, R. J.; Amenomiya, Y. Adv. Catal. 1967,17,103-149.
Oxidation Kinetics of Carbon Blacks over 1300-1700 Kt W. Felder,* S. Madronich, and D. B. Olson AeroChem Research Laboratories, Inc., Princeton, New Jersey 08542 Received September 15, 1987. Revised Manuscript Received July 19,1988
-
The oxidation kinetics of two carbon blacks were measured over 1300-1700 K in oxygen at partial pressures from 0.02 to 60 kPa. Raven 16, a furnace black (primary spherule diameter 60 nm), and Conductex SC, a conductive furnace black (primary spherule diameter = 20 nm), were entrained in nitrogen/oxygen flows and passed through a heated tubular reactor (a modified high-temperature fast-flow reactor, HTFFR) a t subatmospheric pressure. The yield of product COX(=CO+ C02) was measured gas chromatographically a t the HTFFR exit as a function of reaction time and oxygen concentration a t each temperature. The oxidation kinetics of both carbon blacks are similar; they react by external surface burning and exhibit single overall reaction orders between 0.5 and 0.8 in [O,]. An activation energy of ~ 1 7 kJ/mol 0 is observed. At an oxygen partial pressure of =20 kPa the fraction of reactive collisions (number of COXmolecules produced per 02/surface collision) rises from to =lo4 over the 1300-1700 K range. The observed reactivity of the carbon blacks with O2is smaller than that previously measured for soot and other carbon blacks. It is speculated that sulfur impurity may be the cause of this reduced reactivity.
Introduction The oxidation of carbonaceous particulate matter is of wide practical concern in power generation and in pollution reduction. Typically, in the fuel-rich portions of com'Presented at the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 30-September 4, 1987.
0887-0624/88/2502-0743$01.50/0
bustion flames, OH radicals can be the major oxidizing species.lg However, in regions removed from the primary (1)Neoh, K.G.;Howard, J. B.; Sarofii, A. F. In Particulate Carbon: Formation During Combustion; Siegla, D . C., Smith, G. W., Eds.; Plenum: New York, 1981;p 261. (2) Page,F. M.; Ates, F. In Evaporation and Combustion of Fuels; Zung, J. T., Ed.; Advances in Chemistry 166;American Chemical Society: Washington, DC, 1978;p 190.
0 1988 American Chemical Society
