620
Energy & Fuels 1989,3, 620-624
In the preceding expressions (eq 3-5), n appears as the global order of the reaction with respect to oxidant. In fact, considering the complexity of this reaction and the simplifications introduced, it is preferable to use the term pressure coefficient. Representations of the logarithm of ri vs the logarithm of p o show that relation 5 is verified. Least-squares data fits lead to pressure coefficients of, respectively, 0.21 and 0.23 for the YFO and MFO varieties. Within the errors incurred into the measurements, these data support the conclusion that the two varieties exhibit the same pressure dependence for the pressure coefficient. This probably reflects no noticeable differences in the composition of oxidizable atmosphere close to and at some distance from the burning surfaces of the two oil shale varieties.
.Conclusion This work attempts to develop the understanding of mechanisms leading to the autoignition of Moroccan oil shale particles submitted to thermal shock. Small size samples were introduced into a reactor whose oxygen partial pressure and gas temperature were fmed. A window allowed luminous emission of ignition to be detected. This device permitted the study of autoignition limits and delays. The autoignition limits in air and pure oxygen have been delineated. The comparison between the two diagrams revealed that nitrogen has a noticeable promoting effect. Moreover a pulsating phenomenon has been observed in a narrow parametric zone along the limits. It could probably be explained considering the combined effects of chemical reaction, heat and mass transfer, and pore diffusion of the material. The delays as a function of temperature and pressure were also measured and interpreted. The data clearly support a relation of the form rgGn = const. An apparent reaction order of -0.22, with respect to the oxygen partial pressure, correlates the data best, which is consistent with the correlations and pressure coefficients usually encoun-
tered in homogeneous ignition studies. A first objective, in the present study, was the knowledge of the ignition conditions of individual oil shales samples, and preliminary data have been acquired. Additional and extensive work remains to be done that will be important to the understanding and evolution of properly engineered techniques for increased oil shale use.
Acknowledgment. We thank the Morocco National Petroleum Research and Exploitation Agency (ONAREP), which provided the shale samples. A
9 fc
fr
iur,
4
k
m
n PO*
QE QL
R
T8
1:
X
Glossary preexponential factor, (m~l.m-~)'-~ -S oxidant concentration, mol" thermal capacity, J-kg-'.K-' global activation energy, J.mol.-' convective component of the fraction of the heat of combustion transferred back to the surface radiative component of the fraction of the heat of combustion transferred back to the surface heat of combustion, J-kg-' heat of solid gasification, J-kg-' thermal conductivity, W.m-'.K-' mass flux of flammable vapors per unit surface area, kgm-2.s-1 global order of reaction with respect to oxidant oxidant partial pressure, bar rate of external heating per unit surface area, W.m-2 rate of heat loss per unit surface area, W-m-2 ideal gas constant, J.mol-'-K-' surface temperature, O C or K distance to the material surface, m rate of transfer of "sensible heat", defined in eq 1
-'
Greek Symbols t emissivity P density, k ~ m - ~ U Stefan-Boltzmann constant, 5.67 X lo4 W-m".K4 Ti ignition delay, s maximum value off, at the fire point (0 Registry No. Nitrogen, 7727-37-9.
Conversion of Light Alcohols to Hydrocarbons over ZSM-5 Zeolite and Asbestos-Derived Zeolite Catalysts Raymond Le Van Mao* and Gerald P. McLaughlin Catalysis Research Laboratory, Department of Chemistry, Concordia University, 1455 De Maisonneuve Boulevard West, Montreal, Quebec H3G lM8, Canada Received July 25, 1988. Revised Manuscript Received June 17, 1989 Controlled Zn incorporation into the ZSM-5 zeolite and asbestos-derived zeolite led to enhanced production of highly aromatic gasoline with a reduced durene content in the catalytic conversion of methanol. Higher yields of liquid hydrocarbons were observed with butanols as feed. Practically no durene was produced although the aromatic content was as high as that with methanol as feed.
Introduction Natural gas, coal, biomass, and heavy oils constitute very valid alternatives to petroleum for fuels and chemicals. Apart from the classical Fischer-Tropsch synthesis, new routes passing through syngas and then light alcohols are of great industrial interest. Since the development of a new methanol-to-gasoline (MTG) process by Mobil Oil in the United States, which 0887-0624/89/2503-0620$01.50/0
makes use of a synthetic crystalline zeolite known as ZSM-5, there has been an upsurge in research efforts in this field.'t2 On the other hand, new catalysts that permit the production of various C1-CI alcohol mixtures have been de~eloped.~ (1) Chang, C. D.; Silveetri, A. J. J. Catal. 1977, 47, 249-255. (2) Kaeding, W. W.; Butter, S. A. J. Catal. 1980,61, 155-164.
0 1989 American Chemical Society
Conversion of Light Alcohols ~
catalvst H-ZSM-5 H-ZSM-5/Zn H-Asb-ZSM-5/Zn
Si/Al atomic ~~
ratio 19 19 14
~
Energy & Fuels, Vol. 3, No. 5, 1989 621
Table I. Physical43hemical Characteristics of the Catalysts acidity measurements by TPD of ammonia (e) av zeolite surface desorption peaks, % particle size area theor value measd deg of 479 K 615 K 769 K crystallinity, (SEM), (BET), of acidity," tot. acidity, % lo4 m m*/n 1O4mo1/a 10-'mol/a Deak (W) wak (M) Deak (S) 393 6.4 6.1 49 25 26 100 2 5.8 53 27 20 2 380 100 53 26 21 358 8.2' 7.2d 0.05' >90
R 0.35 0.25 0.26
Submicrometer-sizedzeolite particles embedded in 6-7 pm-sized aggregates (see ref 5). *Calculatedfrom the Al content of the zeolite component. Value corrected with the crystallinity degree. dLower value than theoretical one, probably due to the presence of MgO in the matrix. 'No significant ammonia adsorption/desorption with bentonite extrudates (used as a blank).
In our laboratories, two new categories of ZSM-5 zeolites have been prepared: nontoxic zeolites derived from chrysotile asbestos fiber^;^^^ Zn- and Zn-Mn-modified ZSM-5 zeolites.611 These two novel families of zeolites were found to be very selective in the conversion of methanol to hydrocarbons. Such catalysts showed relatively high yields of aromatics and light olefins.Therefore, the main objective of this research work was to test the Zn-modified ZSM-5 zeolite and its asbestosderived homologue and then to compare their performances with that of the parent ZSM-5 zeolite. The feeds used in the experiments consisted of methanol, 1-butanol, 2-methylpropanol, and a mixture of C1-CI alcohols. The incorporation of Zn was closely controlled in order to maximize the catalyst activity in the conversion of methanol to hydrocarbons.&*
Experimental Section Catalyst Preparation and Characterization. For comparative purposes, the following three catalysts were prepared: ZSM-5 (H-ZSM-5), Zn-bearing ZSM-5 (H-ZSM-5/Zn), and asbestos-derived ZSM-5 (H-Asb-ZSM-S/Zn). They were tested under the same reaction conditions by using various light alcohol feeds. The preparation of the H-ZSM-5 sample12was accomplished by using silica gel (Baker), tetrapropylammonium bromide (TPA-Br) (Aldrich),sodium hydroxide, sodium aluminate (Fisher), and water. "he suspension was loaded into a Parr autoclave fitted with a liner made of Hastelloy (2-276 alloy and heated to 170 "C for 10 days. The resulting product was filtered, washed, dried at 120 "C for 12 h, and then calcined a t 550 "C for 12 h. The sodium form (Na-ZSM-5) was ion-exchanged in an 1 M aqueous NH4Cl solution a t 80 O C for 1 h. The solution was then changed and the process continued for a total of at least 7 h, followed by filtering, washing with deionized water until no more Cl- ions were present in the effluent, drying a t 120 "C for 12 h, followed by activation in the air at 550 "C for 12 h to produce the acid form called H-ZSM-5. The zinc form, H-ZSM-5/Zn, was prepared by following the procedure of ref 7. This procedure involved bringing the catalyst in contact with a 1 w t % solution of ZnClz in a 1:lO ratio a t 80 "C for a period of 1 h with constant stirring; the catalyst was cooled, fitered, and washed with deionized water until no Cl- ions were present in the effluent. The catalyst was then dried at 120 O C for 12 h and then activated in the air at 550 "C for 12 h. The (3) Klier, K. Catalysis of Organic Reactions; Moser, W . R., Ed.; Chemical Industries Series; Marcel Dekker Inc.: New York, 1981; pp 195-218 and references therein. (4) Le Van Mao, R.; Bird, P. H. US Patent 4 511 667, April 16,1985. (5) Le Van Mao, R.; Levesque, P.; Sjiariel, B.; Bird, P. H. Can. J. Chem. 1985,63, 3464-3470. (6) Le Van Mao, R. US Patent 4615995, Oct 7,1986. (7) Le Van Mao, R.; Levesque, P.; Sjiariel, B.; Nguyen, T. D. Can. J. Chem. Eng. 1986,64,462-468. (8) Le Van Mao, R. US Patent 4 692 424, Sept 8, 1987. (9) Le Van Mao, R.; Levesque, P.; Sjiariel, B. Can. J. Chem. Eng. 1986, 64, 514-516. (10) Le Van Mao, R.; Dao, L. H. US Patent 4698452, Oct 6, 1987. (11)Le Van Mao, R.; Levesque, P.; McLaughlin, G. P.; Dao, L. H. Appl. Catal. 1987, 34, 163-179. (12) Argauer, R. J.; Landolt, G. R. US Patent 3702886, Nov 14,1972.
769K
I
Figure 1. Ammonia TPD with the H-ZSM-5 sample: (A) constant heating from 273 to 870 K; (B) stepwise heating. Zn metal content, determined by atomic absorption, was 0.51 wt %. The Asb-ZSM-5 zeolite catalyst was prepared from chrysotile short fibers4gsthat had undergone a leaching of the Mg content. The leaching was done in 2.4 N HCl at 80 "C for 4 h, after which time the mixture was cooled and quenched with deionized water, followed by fitering, and then dried at 120 "C for 12 h. The degree of Mg removal is reported as the magnesium leaching degree (MLD), which is determined as follows:
where (MgO)i and (MgO)f are the initial and final magnesium contents (on a dried oxide basis) in the asbestos material before and after leaching, respectively. The resulting solid, which had a MLD of 90%, was used as a silicon-containing microporous matrix in which ZSM-5 submicron particles were synthesized. The preparation of thisAsb-ZSM-5 zeolite was accomplished by mixing the leached material with an aqueous solution containing TPA-Br, NaOH, and sodium aluminate and placing this mixture into an autoclave as described above. After calcination at 550 "C for 12 h, the compound was then subjected to ammonium ion exchange, followed by zinc incorporation, as previously described, and it was called H-Asb-ZSM-5/Zn. It has a zinc content of 0.56 wt %. All three resulting catalysts were then individually mixed with 20 wt % bentonite. Water was added to make a paste, which was formed into extrudates 1 mm in diameter, dried at 120 "C for 12 h and then calcined at 550 "C for 12 h to obtain the final form. The catalysts were characterized by atomic absorption and X-ray powder diffraction for structure identification and degree of crystallinity, scanning electron microscopy (SEM) for particle size, BET for surface area, and temperature-programmed desorption (TPD) of ammonia for acid properties (Table I). In the latter study, ammonia was adsorbed on the catalyst (final form) at 100 "C and then desorbed in a temperature programmed mode. The procedure used (multistep heating) was similar to that described in ref 13. The temperature-programmed desorption was done after a thorough flushing with helium for 40 min (to remove physically adsorbed ammonia) followed by a constant (13) Le Van Mao, R.; Yao, J. Can. J. Chem. Eng., submitted for publication.
622 Energy & Fuels, Vol. 3, No. 5, 1989
Table 11. Results for H-ZSM-5 with P u r e Methanol product yield, g/100 g of alcohol injected gasoline characteristics c144 c2-c4 Cs-Cll liquid wt% wt % BTX paraffins olefins (gasoline) aromatics in aromatics 10.7 6.9 24.4 39 56 11.5 3.9 27.6 42 48 13.8 3.1 24.9 44 27 11.8 6.3 24.4 49 63 13.5 4.0 25.9 47 57 14.1 3.2 24.5 50 40 16.4 5.7 19.2 61 67 18.2 4.0 19.9 63 67 17.2 2.8 19.5 57 54
durene 4.6 7.1 16.6 3.2 4.6 10.7 2.0 2.8 5.0
Table 111. Results for H-ZSM-5/Zn with Pure Methanol Feed product yield, g/100 g of alcohol injected gasoline characteristics reacn condns Cl-CA C2-CA C-A l__l liquid wt% wt % BTX (gasoline) aromatics in aromatics pariffins olefins temp, "C pressure, psi 5.7 25.4 45 67 50 12.9 390 18.3 1.6 26.2 54 58 100 17.5 1.2 25.7 57 40 250 11.7 6.9 23.3 54 69 430 50 12.8 4.8 24.8 50 66 100 11.9 2.2 25.3 52 250 40 9.6 11.7 20.6 56 77 470 50 12.7 6.8 23.3 59 73 100 14.7 4.4 22.1 52 250 56
durene 3.4 6.0 8.4 2.6 3.6 9.4 1.4 2.6 5.7
reacn condns temp, "C pressure, psi 390 50 100 250 430 50 100 250 470 50 100 250
heating (15 "C/min) up to 813 K. A special deconvolution technique, which consists of stopping the heating after the temperature of the f i t peak maximum and before that of the second peak maximum, was also used in order to have a better separation of desorption peaks and thus a more accurate determination of peak area@ (see also Figure 1). Catalysis Testing. A vertically mounted, stainless-steel, fmed-bedreactor 2.5 cm in diameter and 30 cm in length was used. The reactor had a preheating and a reaction zone that were monitored by two chromel-alumel thermocouples positioned in a thermocouple well down the center of the reactor. The temperature controller was a nonindicating, potentiometric, timeproportioning controller, with the temperature control achieved by adjusting power input to each zone. Nitrogen was used as the carrier gas. The flow rate was monitored by using a gas transducer connected to a digital mass flowmeter and a gas volume totalizer. The desired pressure in the reactor was maintained by using a back-pressure regulator. The alcohols tested in this project consisted of methanol, 1butanol, 2-methyl-1-propanol, and a mixture of methanol (24.1 w t %), ethanol (39.7 w t %), propanol (21.9 wt%), and 1-butanol (14.3 wt %) as reported by Sugier et al.14 The reaction parameters are WHSV = 2.5 h-l and a nitrogen/alcohol mole ratio of 1. Methanol and butanols were reacted over the three catalysts at temperatures of 390,430, and 470 "C, and pressures of 50,100, and 250 psi. The testing of the linear alcohol mixture involved all three catalysts; however, these reactions were only done at 470 "C and 100 psi. A dynamic on-line sampling procedure was used for analysis of the gas phase with a Hewlett-Packard 5790 GC equipped with a FID and a reporting integrator, Model HP-3392. Separation of the gaseous phase was achieved on series-packed columns of 5 m of squalane on Chromosorb P and 2.5 m of Carbopack (Supelco Co.), graphite coated with picric acid. Analysis of the aqueous layer was done by using a 50-m PONA H P capillary column and a FID using a methanol in water calibration curve. A detailed analysis of the organic layer was obtained by using a H P 5970 GC/mass selective detector and another 50-m PONA column. The main advantage of the MSD was to provide a higher reliability to the determination of the product distribution. The results reported hereafter are average values of data from two (5-h) runs performed over the same catalyst and under the same reaction conditions. Between these two runs, there was a catalyst (in situ) regeneration, which was carried out overnight in the air a t 550 "C. (14)
Le Van Ma0 and McLaughlin
Sugier, A,; Freund, E. US Patent 4 112 110, Oct 24, 1978).
wt%
wt%
Results Table I reports the densities of strong (S), medium (M), and weak (W) acid sites and the ratio, R = (S)/(M) (W), of these densities, measured over H-ZSM-5, H-ZSM-5/Zn, and H-Asb-ZSM-5lZn. In terms of overall acid density, there was no significant change (
