literature Cited
Aranow, R. H., Witten, L., Phys. Fluids 6, 535 (1963). Aranow, R . H., Witten, L., Phys. Fluids 10, 1194 (1967). Coates, D. E., Ph.D. Thesis, ?*lcMaster University, Hamilton, Ont., Canada. 1970. Coates, d. E., Kirkaldy, J. S., J . Cryst. Growth 3-4, 549 (1968). Coates, D. E., KirkaIdy, J. S., Trans. ASM 62, 426 (1969). Danckwerts, P. V., Trans. Faradau Faraday SOC.46, 701 (1950). Davies, J. T., Haydon, D. A A,, . , Proc. Int. Congr. Concr. Surface Surface Activ. 2nd 1 , 417 (1957).
Davies, J. T., Rideal, E. K., “Interfacial Phenomena,” Academic Press. Xew York. K.Y.. 1961. Ekwall,’P., Salonen, AI., Krokfors, I., Danielsson, I., Acta Chem.
Scand. 10, 1146 (1956). Gross, B., Hixson, A. N., IND.ENG. CHEM.,FUNDAM. 8,296 (1969). Jost, W., “Diffusion in Solids, Liquids, Gases,” Academic Press, New York, N. Y., 1952.
Kirkaldy, J. S., Can. J. Phys. 36, 917 (1958). Kirkaldy, J. S., Brown, L. C., Can. Met. Quart. 2 ( l ) , 89 (1963). Linde, H., Schwarz, E., Groger, K., Chem. Eng. Sci. 22, 823 ll(Ifi7) ,-vu. ,.
hliller, C. A., Scriven, L. E., J . Colloid Interface Sci. 33, 360 (1970).
Mullins, W. W., Sekerka, R. F., J . A p p l . Phys. 34,323 (1963). Mullins, W. W., Sekerka, R. F., J . A p p l . Phys. 35,444 (1964). Ostrovskii, 11.V., Barenbaum, R. K., Abramson, A. A,, Colloid J . U S S R 32 (4), 472 (1970). Ruckenstein, E., Berbente, C., Chem. Eng. Sci. 19, 329 (1964). Sternling, C. V., Scriven, L. E., A.I.Ch.E. J . 5, 514 (1959). Stephen, T., Stephen, H., “Solubilities of Inorganic and Organic Compounds,” Macmillan, Kew York, N. Y., 1964. RECEIVED for review October 12, 1971 ACCEPTEDJune 22, 1972
Diffusion, Reaction, and Fouling in H-Mordenite Crystallites. The Catalytic Dehydration of Methanol Edward A. Swabb and Bruce C. Gates* Department of Chemical Engineering, Cniversity of Delaware, Sewark, Del. 19711
Rates of dehydration of methanol to dimethyl ether catalyzed within the straight tubular pores of H-mordenite crystallites were measured at atmospheric pressure with a differential flow reactor. At 155°C there was no effect of catalyst crystallite size on rate, but at 205°C an effect consistent with the Thiele model was observed. Effectiveness factor decreased from 0.93 to 0.62 as mean catalyst pore length increased from 5.9 to 16.6 p. The results establish the pseudo-first-order rate constant, 190 sec-’, and the diffusion coefficient of methanol in a pore, 7 X 10-5 cm2/sec. Rates of catalyst activity loss were independent of crystallite size, indicating the absence of pore-mouth blocking by reaction products. Diffusion of methanol appears to be hindered more by interactions with pore walls than by interference from counterdiffusing reaction products, although the sum of the critical dimensions of methanol and ether exceeds the major pore diameter by about 1 A.
T h o u g h zeolites are widely applied as catalysts for such industrial reactions as cracking and hydrocracking, there is only qualitative understanding of the effect on reaction rates of mass transport within the molecular-scale intracrystalline channels. Most known zeolite-catalyzed reactions are believed to occur on the large intracrystalline surfaces, while reactioiis of molecules too large to enter the channels occur 011 only the much smaller exterior crystallite surfaces (Venuto, 1971). The useful principle of shape-selective catalysis by zeolites was demonstrated by Reisz, et al. (1962), who observed high rates of paraffin cracking and alcohol dehydration for straightchain but not branched-chain reactants, as the latter were excluded from interior surfaces of the Linde 5A molecular sieve. A more complicated selectivity pattern suggesting a mass transport effect was reported by Chen, et al. (1969), in the cracking of n-docosane (C22Hd6)catalyzed by erionite. A trimodal product dist,ribution was observed as CI-CZ, C,-CS, and >CI2 products were missing. The authors suggested that Clo-C1~fragments, which extend approximately the length of a n erionite cage, \vere selectively transported through the windows between cages, as smaller fragments within a cage and larger ones extending beyond a cage reacted faster to 540 Ind. Eng. Chem. Fundom., Vol. 1 1 , No. 4, 1972
give the predominant products. A related influence of mars transport on the rate of catalyst activity loss, a “productselective catalysis” (Weisz, et al., 1962), was demonstrated experimentally by Venuto and Hamilton (1967), who found high molecular weight products trapped n-ithin the super cages of zeolite X used in alkylation. An objective of the present study was to identify a suitably simple zeolite-catalyzed reaction and to determine quantitatively the influence of intracrystalline mass transport as indicated by a n effect of zeolite crystallite size on initial reaction rate. H-mordenite is a catalyst well suited to the purpose; it has a unique crystal structure including parallel, nonintersecting pores, which suggests that intracrystalline mass transport can be approximated by one-dimensional diffusion. Xordenite can be obtained as single crystals of varying sizes (Sand, 1968); these have been fractionated for determination of effects of path length on rates of sorption (Gupta, et al., 1971; Satterfield and Frabetti, 196i; Satterfield, etal., 1971). The H-niordenite-catalyzed dehydration of methanol to dimethyl ether, 2CHJOH + CH30CH3 1 ~ 2 0 ,has been found to proceed in the near absence of side reactions with
+
- -
relatively slow catalyst activity loss. The critical dimensions of the product molecules (estimated to be 4.0 and 3.3 %, for ether and water, respectively) are just small enough that products might be expected to counterdiffuse past incoming methanol (critical diameter 4.0 1)in the tubular mordenite pores, which are elliptical in czoss section with major and minor diameters of 7.0 and 6.7 A, respectively (Meier, 1961; Rleier and Olson, 1971). A second objective of this research was to measure rates of catalyst activity loss for mordenite crystallites of several sizes and determine the role of intracrystalline mass transport. Previous investigators (e.g., Venuto and Landis, 1968; Weller and Brauer, 1969) have noted that mordenite, which is a highly active acid catalyst, often loses its activity rapidly, and this loss has been ascribed to the accumulation of products such as polyolefins within the narrow tubular pores. The data of Satterfield, et JI. (1971), demonstrate the susceptibility of mordenite to pore blocking by strongly adsorbed products.
To Gas
Experimental Section
J
Materials. Methanol obtained from Fisher Scientific Company (Certified ACS grade, assay 99.9%) was used without purification. The catalyst was Zeolon €I (synthetic hydrogen mordenite powder), Lot No. HB 91-923, manufactured by the Norton Company, Worcester, Mass., and supplied as crystallites in the submicron to 40 p size range. A 12-g sample was size-fractionated with a Roller Classifier by American Instruments Company, Silver Spring, Lid. ; particles carried over from the air-fluidized bed were collected over a 130-hr period. Catalyst Pretreatment. For activation, mordenite crystallites were mixed with 25-30 mesh glass beads and held for 2 hr a t 350°C in a stream of dry air. The catalystglass bead mixture was cooled, weighed, and transferred to the reactor under a nitrogen blanket. The activation tube was reweighed to determine the exact amount of catalyst transferred. Mordenite crystallites adhered to the glass bead surfaces except when charges were prepared for experiments a t 100OC; these charges contained sufficient catalyst to partly fill the interstitial voids. Apparatus. 4 schematic diagram of the glass flow reactor system is shown in Figure 1. The reactor was a tube 2.0 cm in diameter and 20 cm in length. The reactor and a vaporizing coil were built into a thermostat vessel containing boiling fluids (water, bromobenzene, tetralin, or l-methylnaphthalene). Glass beads (25-30 mesh) were placed in the reactor to serve as a 15-em flow distribution section. A glass bead-catalyst mixture was placed on top of this section to form a bed 0.7 to 4 ern long. The remainder of the reactor was filled with glass wool. Liquid methanol was fed by gravity from a constant-head tank; flow rates were measured during a run by feeding from a constant-head buret and timing the fall of the liquid level. Reactant vapor flowed upward through the reactor, through a heated exit line, and to a HewlettPackard Xodel 5750 gas chromatograph equipped with a heated gas sampling valve and a thermal conductivity detector. Reaction products were separated on a 6 ft long, ' 1 8 in. 0.d. column containing Porapak K (polystyrene beads) ; temperature was 154°C and helium carrier gas flow rate was 30 ml/min. Procedure. During a run, methanol flowed a t a constant rate over thermostated catalyst, and the product stream was sampled and an analysis begun approximately every 4 min. The first analysis was usually begun 1 min after
Feed Tanks
Watr
Water out
u
Thermostat Fluid Condenser Reactor Exit Line
Reactor
Heating Mantle
LAu
Figure 1. Schematic diagram of reactor system
f
I
0.2 -
0
L
I
I
I
Catolyst Fraction I 0 2
I
-
----
-
0.
.C
.-
0
E
IA
L
0 n
50
'
L, Pore Length, microns
Figure 2. Catalyst pore length distributions
reactant contacted catalyst, and it always indicated that a steady state had not yet been achieved, though nitrogen was fully purged from the reactor. Subsequent data indicated a gradual decline in activity, hence catalyst was discarded after each run. Catalyst charges and feed flow rates were chosen to provide differential conversion data. The temperature range was 100-240°C, and the mean catalyst crystallite size was varied from 5.9 to 16.6 k . Results
Catalyst Particle Size Analysis. Mordenite particle sizes were obtained by measuring the dimensions of particles shown in photomicrographs of representative samples collected during the fractionation. Crystallites were found to approximate parallelepipeds with lengths ratios 1 : 0.7: 0.7, confirming Katzer's observation (1969). The long dimension was visible for essentially all of the particles photographed, so negligible error resulted from observation of particles standing on end. The long dimension was assumed to be the pore length for a particular particle. The arithmetic mean pore half-length in a size fraction of catalyst with a measured distribution of particle sizes was calculated as follows
where the summation is over the particles. Ind. Eng. Chem. Fundam., Vol. 11, No. 4, 1972
541
Table 1. Catalyst Pore length Data Fraction 2
Fraction 3
7.0
1.7
0.3
490 5.9
368 11.3
130 16.6
3.2
4.3
5.7
Fraction 1
Mass of sample obtained in fractionation, g Xumber of particles measured Mean pore length, y Standard deviation in pore length, p
61
I
L?
I
C F raat catl iyosnt I
I
0
I
-
I
I
/ I
Reciprocal Space Velocity, g c a t a l y s t sec./g feed
Figure 3. Effect of crystallite size on conversion of methanol at 205°C. Conversion data extrapolated to zero onstream time
The pore length distributions for the three size factions used as catalysts are shown in Figure 2, and the mean pore lengths and standard deviations are given in Table I. Small amounts of heavier fractions were also obtained, but, in contrast to fractions 1-3, these appeared to contain substantial amounts of agglomerated crystallites and perhaps amorphous material. Since a few measurements of catalytic activity confirmed that a representative particle size was close to that of fraction 1, the samples mere not studied further. Reaction Rates. Uncatalyzed reaction was not observed, and at standard operating conditions the only products of catalytic reaction detected by gas chromatography were ether
and water. The ratio of ether to alcohol in the product stream was determined from the ratio of chromatographic - _ peak heights using a calibration precise within +5%. The analysis for water was less precise, and the data were used only to confirm reaction stoichiometry. Conversions of methanol were less than 8%, except in a single run a t 24OoC, when roughly 4070 conversion was attained; trace amounts of ethylene and propylene side products were then identified by their retention times in the chromatographic column. A linear relation between fractional conversion of methanol (extrapolated to zero on-stream time) and reciprocal space velocity was generally found (e.g., Figure 3), demonstrating that differential conversions mere obtained. Slopes of the least-squares lines through the origin were evaluated to determine initial reaction rates for fresh catalyst; the rate data are summarized in Table 11. Unfractionated catalyst was used a t all four temperatures, but since the supply of fractionated catalyst was small, fractions 1 and 2 were studied only a t 155, 205, and 240°C, and fraction 3 was studied only a t 205OC. The experiments provided only imprecise determination of rates a t 240"C, as indicated by large variations from run to run. The inconsistency is not resolved; i t is perhaps a n indication that the small amounts of catalyst (-0.01 g) were distributed unevenly in the reactor and that bypassing of reactant occurred. Significant differences in reaction rate were found for fractions 1-3 a t 205"C, indicating a particle size effect. Rate for fraction 2 was less than that for fraction 1 a t the 0.05 confidence level; rate for fraction 3, based on a single experimental run, was less than that for fraction 2 . At the lower temperature, 155"C, rates for fractions 1 and 2 were indistinguishable. Catalyst Activity Loss. Initial fouling rate, defined as t h e slope of the reaction rate vs. on-stream time curve a t time 0, was approximated as follows.
Rates a t times 0 and 300 see were read off smooth curves drawn through the data, of which those of Figure 4 are representative. Average values of initial fouling rate for each of the several temperatures and catalyst fractions are given in Table 11.A significant effect of catalyst particle size on initial fouling rate is not indicated by the data a t 155 and 205°C; the data a t 24OOC are too imprecise to be a useful test of a particle size effect. The pattern of an increasing followed by a decreasing fouling rate a t 240 "C was consistently found.
Table 11. Initial Reaction Rate and Initial Fouling Rate Data Typical catalyst charge, g
Temperature, OC
99.5 155.0
a
f
0.5
=t0 . 5
1
0.1
205 =k 1
0.02
240 i 2
0.01
See Table I.
Catalyst size fractiona
Value from single run.
c
initial r a t e of methanol dehydration in fresh catalyst, moles of methanol reactedlsec-g of catalyst
Unfractionated Gnfractionated 1 2 Unfractionated 1 2 3 1 2 Approximate value. Error
542 Ind. Eng. Chem. Fundam., Vol. 11, No. 4, 1972
(7.14 i 0.06) X lo-' (4.33 i 0.71) x 10-5 (4.48 f 0.22) x 10-5 (4.18 f 0.26) X 6.56 X ' (7.33 0.94) x 10-4 (6.17 =t0.48) X lo-* 4 . 9 4 x 10-4* 4 x 10-30 3 x 10-3c limits are =kl standard deviation.
Initial r a t e of catalyst activity loss, moie/g of catalyst-sec2
(1.8 i 0 . 8 )
(4.4i 2.0)
x lo-" x 10-9 x 10-9
(2.5 k 2.0) (3.2 i 1 . 4 ) X 6.5 X (5.6 i. 2 . 8 ) X (3.7 + 1.7) X 3.7 x 10-8b 4 x 10-7c 3 x 10-7c
Table 111. Details of Effectiveness Factor Calculation
lo4 X mean pore length, 2 t , cm
Fraction 1
Fraction 2
Fraction 3
5.9 205.0 =t 0 . 5
11.3 205.1 =t 0 . 7
16.6 204.5 + 0 . 7
7.33
6.17
4.94
Temperature, "C lo4 X reaction rate, r, moles of methanol reacted/sec-g of catalyst Methanol concentration a t crystallite surface, mole/cm3 Mordenite crystallite density, g/cm3 Void fraction of crystals Thiele modulus, r p ~ Effectiveness factor, q Pseudo-first-order rate constant, k , sec-' Diffusivity of methanol in pore, D, cm2/sec a Value from Breck (1971). * Value established by r1/r2 and w/rl.
2.55
0 .49' 0.93'
7
Ll/t2.
e
x 10-5 1.7" 0 .28" 0 .95' 0.78' 190'
1.3gC 0.64,' 0 . 62d
x
10-5' Value calculated from k , D, and
t,.
Experimental value
=
0 + ul L
0
0
I
I
I
2
3
4
x O n - s t r e a m Time,
I
1
5
6
W
- 0
sec.
E0 €
Figure 4. Catalyst deactivation during methanol dehydration reaction at 205 and 240°C
f
r"
The temperature dependence of initial fouling rate is shown on the Arrhenius plot of Figure 5; the apparent activation energy is 27 kcal/mole. Discussion
I04/T,
Effect of Intracrystalline Mass Transport on Reaction Rate. The rate d a t a indicate t h a t intracrystalline mass transport resistance was negligible a t 155OC and significant at 205OC. The data for the three catalyst fractions provide a basis for quantitative evaluation of the transport effect. The following analysis, accounting for diffusion and reaction in the straight parallel mordenite pores, is a direct application of the model originally proposed by Thiele (1939). The catalyst particles are considered isothermal since the maximum temperature difference between the crystal center and surface is estimated to be 0.01OC (Damkohler, 1943). Assuming pseudo-first-order reaction takes place in the mordenite pores, the effectiveness factor is (3)
OK-'
Figure 5 . Arrhenius plot. Initial rate of methanol dehydration catalyzed by fresh H-mordenite in the absence of mass transport influence; initial rate of catalyst activity loss during reaction
The initial reaction rate and mean half-pore length data for catalyst fractions 1 and 2 were used to evaluate 17 and (PL for these fractions. The rate datum for fraction 3, based on only a single run, was not used in the calculation. The appropriate data and results of the calculation are summarized in Table 111. The pseudo-first-order rate constant, IC, is evaluated as 190 sec-1, and D, the diffusion coefficient of methanol in a n H-mordenite pore, is 7 X l o p 5cm2/sec. The consistency of the data with the Thiele model is shown in Figure 6. The imprecision of the rates for fractions 1 and 2 implies that Ind. Eng. Chem. Fundam., Vol. 1 1 , No.
4, 1972
543
Table IV. Catalyst Deactivation at 205°C 1 O4 X reaction rate: Fraction 1
On-stream time, sec
a
F'
moles of methanol reacted/sec-g of catalyst Froction 2 Fraction 3
0 7.33 6.17 3600 6.42 5.32 7200 5.92 4.88 Rates are average values from all runs with a given fraction.
I \+ -OI O a t
'r tanh
? =
i 1
qL
YL
I
r
w
- Catalyst Fraction I 0 2.
3 A
I
I
02
I
1
1
1
1
I
05
l
I
10
20
Thiele Modulus, v L =
Figure 6. Effectiveness factor for methanol dehydration in H-mordenite crystallites at 205°C
the uncertainty in k and D may be as much as an order of magnitude, but the close agreement of the third experimental point with the Thiele curve (Figure 6) suggests a more precise determination. The initial rate data a t 100 and 155°C and the datum a t 205OC (adjusted to a n effectiveness factor of 1) are shown in the Arrhenius plot of Figure 5. The apparent activation energy of 24 kcal/mole corresponds to the least-squares line fitting these three points. The approximate datum a t 24OoC, expected to incorporate a n effectiveness factor less than 1, is roughly consistent with the precise results. The strong temperature dependence of the rate confirms that external-phase mass transport was not rate determining. The problem of predicting diffusion coefficients in zeolites is unsolved, and no experimental values are available for comparison with the present results. llolecules diffusing in zeolite catalysts are usually not small compared to pore openings, and they are often polar, experiencing strong interactions with pore walls. For such molecules published values of apparent diffusion coefficients obtained from transient sorption data (e.g., Barrer, 1971; Gupta, et al., 1971; Satterfield and Frabetti, 1967; Satterfield and llargetts, 1971; Satterfield, et al., 1971) fail to represent resolution of the coupled diffusion-adsorption-desorption processes which presumably occur in zeolite pores. Experiments like those described here for determination of steady-state rates of catalytic reaction (and therefore steady-state counterdiffusion) are expected to be of use in systematic characterization of diffusion in zeolites. The present results suggest t h a t the Thiele model may be a n acceptable first approximation when reaction involves polar molecules just capable of counterdiffusion in zeolite pores. The model is tentatively recommended, but only when the parameters are determined experimentally from reaction rate data. Increased availability of pure single crystals of zeolites would be useful in allowing such experimental determin atioris, 544
Ind. Eng. Chem. Fundam., Vol. 11, No. 4, 1972
Ratios of reaction rates Rote fraction 1 Rate fraction 1 Rate fraction 2 Rate fraction 3
4.94 4.39 4.03
1.19 1.21 1.21
1.48 1.46 1.47
Catalyst Activity Loss. At 100, 155, and 205°C reaction rate decreased approximately exponentially with on-stream time, as shown by the typical results of Figure 4.At 205°C ratios of reaction rates for the three size fractions remained essentially unchanged with on-stream time (Table IV), indicating that the ratios of effectiveness factors remained nearly constant. This result indicates the lack of a n effect of intracrystalline mass transport on fouling rate, which implies that fouling was not primarily caused by pore-mouth blocking, since in this case the crystallites with long pores would have been deactivated faster than those with short pores. A possible cause of catalyst activity loss is strong adsorption of product water on the surface. The inhibiting effect of water in mordenite was indicated by a 50% increase in the initial reaction rate a t 155"C, corresponding to the 13% weight loss during activation. Another possible cause of catalyst activity loss is formation and polymerization of olefin on the surface. Olefins were detected as trace reaction products a t 24OoC, and were presumably present in undetectably low concentrations a t lower temperatures. Olefin polymerization is knoa n to occur readily on the strongly acidic surface. -4role of olefin in catalyst fouling was confirmed in a few experiments with isopropyl alcohol dehydration to propylene (Swabb, 1972). At 205°C the rate of isopropyl alcohol dehydration catalyzed by unfractionated H-mordenite crystallites was about equal to the rate of methanol dehydration, but the initial catalyst fouling rate was about ten times greater for isopropyl alcohol. If catalyst activity loss a t 205°C actually resulted from accumulation of water or olefin on the intracrystalline surface, then the lack of a pore-blocking effect during fouling suggests that there is more clearance for counterdiffusing methanol and ether than is indicated by the previoyly cited dimensions showing the major pore diameter to be 1 A less than the sum of the critical molecular dimensions. Perhaps the side pockets connected to the main channels facilitate passage of molecules. The result suggests that hindrance of methanol diffusion resulted more from strong methanol-pore wall interactions than from interference by counterdiffusing molecules. At 240°C a qualitatively different fouling pattern was consistently observed. Although fouling rates were scattered from run to run, they always increased and then decreased as exemplified by the data of Figure 7. Catalyst color changed from white to sandy brown during runs a t 24OoC, while i t appeared yellowish after 2-hr runs a t the lower temperatures. The distinctive color a t 240°C is consistent with the presence of different fouling products a t this temperature, and the observed olefinic side products were possibly their precursors. Olefinic products were reported previously (Mattox, 1962; Venuto and Landis, 1968) in studies of zeolite-catalyzed reactions of methanol. Venuto and Landis suggested a n ctelimination mechanism to explain the olefinic product formaH-CH,-OH
c
b-
+
HdO
+
'CH?
2.0 I
I
I
I
I
I
I
I
1
formed, while each olefin molecule on the surface simultaneously grows in a straight chain along the pore by reaction with alcohol. The subsequent decrease in fouling rate may result as the number of sites for new olefin formation approaches 0 and the polyolefin chains approach the limiting pore length. Nomenclature
A i = end face area of mordenite crystallite i, em2; assumed equal to 0.49Li2
D
cm2/sec pseudo-first-order rate constant for methanol dehydration, based on mordenite intracrystalline volume, sec-l & = mordenite pore length, L = mean mordenite pore half-length, cm Li = mordenite pore length for crystallite i, ern r = rate of reaction of methanol to dimethyl ether, moles of methanol reacted/sec-g of catalyst t = on-stream time, sec (Y = number of pore mouths per unit area of mordenite crystallite end face TJ = catalyst effectiveness factor, - dimensionless
k
% I , ,, 0.0 0
I
= diffusion coefficient of methanol in H-mordenite pore,
,
I
(PL =
,
2 3 x On-stream Time, sec.
=
4
Figure 7. Rate of catalyst activity loss during methanol dehydration at 240°C
Thiele modulus
=
di,
dimensionless
Subscripts indicate on-stream time (see) or catalyst fraction. literature Cited
Barrer, R. M.,Advan. Cht-m. Ser. No. 102, 1 (1971). Breck, D. W., Advan. Chem. Ser. No. 101, l(1971). Chen, N. Y., Lucki, S. J., Mower, E. B., J . Catal. 13, 329 (1969). Damkohler, G., 2. Phys. Chem. Abt. A 193, 16 (1943). Gupta, J. C., Ma, Y. H., Sand, L. B., Chem. Eng. Progr. Symp. Ser. 67 (117), 51 (1971). Katzer, J. R., Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1969. ;Mattox. W. J.. to Esso Research and Engineering- Co., U. S. Patent 3,036,134 (1962). hIeier, W. AI., Z. Kristallogr. Kristallgeometrie, Kristallphys., Kristallchem. 115, 439 (1961). Meier, W. M., Olson, D. H., Advan. Chem. Ser. No. 101,155 (1971). Sand, L. B., Soc. Chem. Ind., London Chem. Eng. Group Proc. 71 (1968). Satterfield, C. N., Frabetti, A. J., Jr., A.I.Ch.E. J . 13, 731 (1967). Satterfield, C. N., Katzer, J. R., Vieth, W. R., IND.ENG.CHEW, FUNDAM. 10, 478 (1971). Satterfield, C. N., Margetts, W. G., A.I.Ch.E. J. 17, 295 (1971). Swabb, E. A., h!l.Ch.E. Thesis, University of Delaware, Newark, Delaware, 1972. Thiele, E. W., Ind. Eng. Chem. 31, 916 (1939). Venuto, P. B., Advan. Chem. Ser. No. 102, 260 (1971). Venuto, P. B., Hamilton, L. A., Ind. Eng. Chem., Prod. Res. Develop. 6 , 190 (1967). Venuto, P. B., Landis, P. S., Advan. Catal. Relat. Subj. 18, 259 (1968). Weisz, P. B., Frilette, V. J., Maatman, R. W., Alower, E. B., J . Catal. 1, 307 (1962). Weller, S. W., Brauer, J. AI., A.1.Ch.E. Meeting, preprint 56c, Washington, D. C., Nov 1969. RECEIVED for review November 18, 1971 ACCEPTED August 9, 1972 I
Figure 8. Proposed a-elimination mechanism for methanol dehydration in an H-mordenite pore
tion. The carbene methylene formed in this step can readily polymerize or react with alcohol to form olefins. which have been detected with carbon numbers as high as 6. K e may speculate, following Venuto ( l O i l ) , t h a t the narrow regular pores in mordenite facilitate a concerted &-elimination process as the reactant molecule is surrounded by the solvent-like pore, bonding to i t at more than one position (Figure 8). It is conceivable that the unusually high acid-catalytic activity of H-mordenite compared to other zeolites is associated with the nearness of strongly acidic and basic centers, between which reactants can be bridged to simultaneously accept and donate protons a t opposite positions. d model has been developed (but not quantitatively compared to the few data) (Swabb, 1972) which accounts for a n initially increasing fouling rate as new olefin molecules are
Financial support provided to E. A. S. in the form of a Sun Oil Company Fellowship and an NDEA Traineeship are gratefully acknowledged. Zeolon H Catalyst was supplied by the Norton Company.
Ind. Eng. Chem. Fundam., Vol. 1 1 , No. 4, 1972
545
