54

Catalytic and Physicochemical Characterization of Extracted H-Mordenite

DILEEP K. THAKUR and SOL W. WELLER Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, Ν. Y. 14214

Samples of H-mordenite were extracted with HCl to remove Al and residual Na or were exchanged with NH4NO3 to remove only Na. The pore volume was essentially unchanged by ex­traction, but the acidity measured by NH3 chemisorption de­creased roughly linearly with Al content. The effective dif­fusivity of all extracted samples was appreciably higher than that of the starting H-mordenite. Apparent first-order rate constants for hexane cracking in a pulsed microreactor decreased with in­creasing number of pulses. Of the models evaluated for de­activation, the most satisfactory was an exponential function in terms of the accumulated hexane actually converted. The "initial activity" decreased at least linearly with decreasing Al content, and it increased significantly as Νa content was lowered at constant Al content.

^J^he or ig inal report b y B a r r e r and M a k k i (1) tha t a l u m i n u m i n a h igh A s i l ica zeolite, c l inopt i lo l i te , could be extracted w i t h minera l ac id to give

a ' ' s i l i ca pseudomorph" has g iven rise to considerable research on ac id -extracted mordenite (2-6). H y d r o g e n mordenite is useful as an adsorbent a n d a catalyst , a n d i ts properties for some purposes are i m p r o v e d b y p a r t i a l extract ion of the a l u m i n u m . F u r t h e r , the a b i l i t y to v a r y a l u m i n u m con­tent whi le m a i n t a i n i n g c r y s t a l l i n i t y offers the oppor tun i ty to learn more about the nature of the act ive sites i n mordenite .

A n earlier report f rom this laboratory (7) noted t h a t i n a series of m i l d l y extracted mordenites, the hexane crack ing a c t i v i t y i n a continuous-flow test went through a m a r k e d m a x i m u m w i t h increasing severity of ex­t rac t i on , whi le the z-butane to η-butane rat io cont inuously increased. T h e a c t i v i t y and product d i s t r ibut i on were measured after 10 m i n on stream. Since catalyst deact ivat ion was r a p i d , i t was not possible to

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54. T H A K U R A N D W E L L E R Extracted H-Mordenite 597

extrapolate these d a t a accurately to zero t ime o n stream, i.e., to extracted but uncoked samples.

M o r e recently (8), another series of H-mordenites , ac id-extracted to a greater degree, was examined. F o r these samples after d r y i n g at 110°C, the major results were: (a) there was no evidence of " h y d r o x y l nests" stable above 100°C, a n d (b) N H 3 chemisorpt ion at 250°C a n d 11 t o r r roughly corresponded to a stoichiometric rat io (1 :1 , ± 2 5 % ) w i t h the t o t a l amount of a l u m i n u m remain ing i n the latt ice .

T h e present paper is a n extension of previous w o r k (8) i n the fo l lowing ways . (1) Samples were examined i n w h i c h res idual sod ium, b u t not a l u m i n u m , was removed b y exchange w i t h a m m o n i u m n i t ra te fol lowed b y calc inat ion. (2) T h e c rack ing of n-hexane was studied i n a pulsed m i c r o ­reactor. (3) T h e re lat ive effective d i f fus iv i ty was est imated b y a gas chro ­matographic technique i n v o l v i n g broadening of a N 2 pulse i n a H e carr ier .

Experimental

E x t r a c t i o n w i t h H C L A l u m i n u m was extracted f r o m the o r ig ina l H -mordenite ( N o r t o n C o . " H - Z e o l o n , " L o t N o . T A - 4 ) w i t h aqueous H C 1 at 100° C . D e t a i l s of the procedure are g iven i n Re f . 8.

Exchange w i t h N H 4 N 0 3 . 24 G r a m s of ground (45-60 mesh) cata lyst were exchanged i n a flask w i t h 800 m l of 0.2M N H 4 N 0 3 at 100°C for 6 hr . T w o samples were exchanged, one (sample 4) i n a single exchange exper i ­ment , a n d the other (sample 5) i n a fourfo ld exchange i n w h i c h the sample was f i ltered and washed w i t h d i s t i l l ed water between successive exchange treatments w i t h N H 4 N 0 3 so lut ion . B o t h samples were g iven a final water wash , d r i e d overnight at 110°C, ca lc ined i n a i r for 8 h r at 525°C, a n d cooled i n a desiccator.

X - R a y Di f f ract ion . A l l d i f f ract ion patterns were t a k e n o n powdered samples w i t h a G e n e r a l E l e c t r i c X R D - 6 diffractometer. A copper target a n d n i c k e l filter were used.

A c i d i t y b y A m m o n i a Chemisorpt i on . R e l a t i v e ca ta lys t a c i d i t y was measured b y the q u a n t i t y of a m m o n i a chemisorbed at the a r b i t r a r y con­di t ions of 250°C, 11.2 t o r r . A n I sorpta analyzer ( E n g e l h a r d model 3A-2) was used ; the procedure was t h a t described i n Re f . 8.

T o t a l P o r e V o l u m e . T h e t o t a l pore v o l u m e was a r b i t r a r i l y t a k e n as the v o l u m e of N 2 sorbed at —195° at a re lat ive pressure of 0.25 (8).

Effect ive Di f fus iv i ty . R e l a t i v e values of the effective d i f fus iv i ty were determined b y a gas chromatographic technique i n w h i c h the broadening of a N 2 pulse i n H e carr ier is measured as a funct ion of carr ier gas ve l o c i ty (9-13). A l t h o u g h there is debate on the proper method of a n a l y z i n g such d a t a on peak broadening, the " p l a t e t h e o r y " of V a n Deemter et al. (14) was considered adequate for our purposes. I n our measurements each c a t a ­l y s t sample was packed into a stainless steel tube h a v i n g a 0.25-inch od (0.18-inch id) a n d 50 c m l ong ; the sample was he ld i n place b y borosi l icate glass woo l plugs at b o t h ends. Samples were predr ied at 500°C in situ i n flowing 0 2 for 10 h r a n d flowing H e for an add i t i ona l 15 h r . T h e y were cooled to 25° C i n flowing H e , a n d then tested at 25° C . F u r t h e r detai ls are g iven i n Re f . 15.

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598 M O L E C U L A R S I E V E S

P a r t i c l e D e n s i t y . P a r t i c l e densi ty was measured b y a H g displacement method at atmospheric pressure. T h e s imple equipment has been de­scr ibed (13).

B u l k D e n s i t y . T h i s was measured b y p a c k i n g the cata lys t i n a straight , stainless steel tube 25 c m long b y 0.25-inch od i n a d r y b o x ; the c o l u m n was v i b r a t e d d u r i n g loading .

V o i d F r a c t i o n i n the packed bed was calculated as 1 — (bulk d e n s i t y ) / (partic le dens i ty ) .

P a r t i c l e Poros i ty wTas ca lculated as (pore volume) X (par t i c l edens i ty ) . Cata lyt i c A c t i v i t y a n d Select iv i ty . T h e c rack ing of n-hexane was s t u d ­

ied b y a pulsed microca ta ly t i c - chromatograph i c technique (16). D u p l i c a t e runs were made on each sample. A 5-gram mix ture (2.5 ml) of glass microbeads a n d cata lyst was used i n each r u n . T h e q u a n t i t y of cata lyst was v a r i e d for different catalysts to ob ta in convenient ly measurable levels of conversion at the fixed test temperature of 350° C . O f the t o t a l 5-gram charge, the q u a n t i t y of cata lyst v a r i e d f r o m 0.043 g r a m for the most act ive sample to 0.28 g r a m for the least act ive . Compensat i on for the var iab le amount of cata lyst was made b y c omput ing a n apparent first-order rate constant, fc, defined as

fc(gram-i) = (1/W) In [1/(1 - X)]

where W = cata lyst weight (grams) a n d X = f ract ional conversion. A constant H e carrier flow rate of 31 m l / m i n was ma inta ined for a l l runs . Successive 5-μ l i ter samples of n-hexane were injected w i t h a microsyr inge ; a k va lue was computed for each in jected pulse. Reac to r effluent was ana lyzed w i t h a 6-foot chromatographic co lumn, 0.25-inch od packed w i t h U c o n o i l o n 60-80 mesh G C - 2 2 Super Support . T h e co lumn was operated i so thermal ly at 35°C, the t h e r m a l c o n d u c t i v i t y detector at 75°C . P r i o r to test, each catalyst sample was heated in situ i n flowing 0 2 at 500° C (as i n the N H 3 chemisorpt ion experiments) , a n d cooled to 350°C i n flowing H e . A t o t a l of 10 injections of n-hexane was t h e n made for each r u n . I n the analyses, branched a n d n o r m a l C 6 products are l u m p e d as " u n c o n ­v e r t e d . " T h e " convers ions" reported below represent t o t a l cracked p r o d ­ucts ( inc luding coke) other t h a n hexanes. O n l y saturated hydrocarbons were observed i n the reactor effluent.

C h e m i c a l A n a l y s e s . Ana lyses of the or ig ina l , acid-extracted, and N H r N 0 3 - e x c h a n g e d samples were k i n d l y determined b y the N o r t o n C o .

Results and Discussion

C h e m i c a l A n a l y s i s . T a b l e I shows the chemical composit ion of the samples selected for detai led s tudy . T h e gray-white color of the or ig ina l H-morden i te was unchanged b y ac id extrac t ion ; interest ingly , the N H 4 -N 0 3 - e x c h a n g e d samples were cream colored.

I n the ac id extract ion, greater removal of a l u m i n u m could have been achieved i f smaller catalyst particles a n d repetit ive extractions w i t h H C 1 h a d been employed. T h e progressively smaller loss on ign i t i on i n samples 1, 2, a n d 3 constitutes negative evidence for the " h y d r o x y l nes t " h y p o t h ­esis (8).

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54. T H A K U R A N D W E L L E R Extracted H-Mordenite 599

Table I. Chemical Composition of Various Catalyst Samples Sample No.

1 3 Origin

Original H - HC1, QN, HC1, SN, N H 4 N 0 3 , NH4NO3,

Mordenite 8 hr 24 hr single fourfold

Analysis, w t %

S i 0 2

AI2O3

F e 2 0 3

T i 0 2

N a 2 0 L.O.I.*

78.55 85.39 88.43 86.02 81.86 9.08 3.64 2.31 9.59 9.06 0.27. 0.09 0.09 0.23 0.13 0.40 0.36 0.35 0.46 0.42 0.77 0.11 0.16 0.21 0.18

10.65 9.66 8.56 3.21 7.65 Residual N H 4 + wt ratio mole ratio

0.16 0.32 S i 0 2 : A l 2 0 3 , 8.65 23.5 38.3 8.97 9.03

14.7 39.9 65.1 15.3 15.4 a Single exchange with 0.2ΛΓ NH4NO3, calcined. b Fourfold exchange with 0.2M NH4NO3, calcined. e Loss on ignition.

T h e N H 4 N 0 3 exchange h a d the desired effect of lower ing the sod ium content whi le leav ing the a l u m i n u m content essentially unchanged. T h e sod ium content was on ly s l ight ly lower for the sample exchanged four t imes (No . 5) t h a n for tha t exchanged on ly once ( N o . 4). I n add i t i on , sample 5 showed a higher residual N H 4

+ content after final ca lc inat ion at 525°C. I t m a y be expected t h a t b o t h H C 1 extract ion a n d N H 4 N 0 3 exchange

might increase the effective d i f fus iv i ty b y remov ing res idual exchangeable cations, no tab ly sod ium, that p a r t i a l l y b lock the crystal l ine channels. Since sod ium t y p i c a l l y poisons c rack ing sites, i ts r emova l should also have a beneficial chemica l effect.

A n opposing effect is possible under the severe conditions of a single extract ion w i t h H C 1 : some of the a l u m i n u m removed f rom the c rys ta l structure m a y not be transported out of the catalyst part ic le . T h e resul t ­i n g "amorphous a l u m i n a " (after subsequent calcining) remain ing i n the part ic le w o u l d cause some reduct ion i n effective d i f fus iv i ty . S u c h amor ­phous a l u m i n a has been suggested b y others (17,18).

X - R a y Di f f ract ion Pat te rns . T a b l e I I gives in terp lanar spacings for the first 12 prominent l ines observed i n the di f fraction patterns for samples 1-5 as we l l as those reported b y D o m i n e and Quobex (19) for synthet ic mordenite. A few points should be noted.

(1) I n agreement w i t h earlier invest igations (20), neither ac id extrac­t i o n nor exchange caused any m a r k e d shift i n l ine positions or apparent c rys ta l l in i ty .

(2) T h e most signif icant change was subt le : i n synthet ic mordenite (19) a n d samples 1, 4, a n d 5, the sharpest peak occurred at 3.47-3.49 A [indexed as (022)], a n d i n the ac id-extracted samples 2 a n d 3, the sharpest peak was 3.37-3.38 A [indexed as (600) ].

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600 M O L E C U L A R S I E V E S

Table II. Interplanar Spacings of Various Samples

Synthetic Sample No.

Synthetic mordenite" 1 2 8 4 δ

13.53 (m) 13.60 (w) 13.60 (w) 13.77 (w) 13.80 (w) 13.80 (w) 10.24 10.16 (w) 10.16 (w) 10.16 (w) 10.28 (w) 10.28 (w) 9.06 (s) 9.17 (s) 9.17 (s) 9.12 (s) 9.21 (s) 9.17 (s) 6.57 (s) 6.61 (m) 6.58 (m) 6.56 (m) 6.61 (m) 6.61 (m) 5.80 (w) 5.83 (w) 5.83 (w) 5.80 (w) 5.85 (w) 5.83 (w) 4.52 (w) 4.54 (m) 4.54 (m) 4.51 (m) 4.54 (m) 4.54 (m) 4.15 4.27 (w) 4.29 (w) 4.27 (w) 4.27 (w) 4.27 (w) 4.00 (s) 4.00 (s) 3.99 (s) 3.99 (s) 4.00 (s) 4.00 (s) 3.84 3.85 (w) 3.82 (w) 3.82 (w) 3.85 (w) 3.85 (w) 3.47 (vs) 3.49 (vs) 3.48 (s) 3.47 (s) 3.48 (vs) 3.49 (vs) 3.39 3.38 (s) 3.38 (vs) 3.37 (vs) 3.38 (s) 3.38 (s) 3.22 3.23 (s) 3.23 (s) 3.22 (s) 3.23 (s) 3.23 (s)

a Domine and Quobex (19).

(3) T h e di f fract ion patterns of samples 1 a n d 3 were the same after d r y i n g at 500° C (air, 7 hr) as after d r y i n g at 110°C. T h i s m a y be t a k e n as ad d i t i ona l evidence against the existence of appreciable amounts of " h y d r o x y l nests" i n sample 3 after 110°C d r y i n g , since one w o u l d expect such h y d r o x y l nests to be unstable at 500° C .

A c i d i t y . T h e mole rat io of chemisorbed N H 3 to t o t a l a l u m i n u m , w h i c h was 1 =t 0.25 for the or ig ina l and acid-extracted samples (8), was s l ight ly lower for samples 4 and 5 (Table III). T h i s is presumably be­cause of the res idual N H 4 + i n these samples. W i t h sample 5, for example, i f the res idual N H 4 + is added to the chemisorbed N H 3 , the mole rat io of the t o t a l ( N H 4

+ + N H 3 ) re lat ive to the t o t a l a l u m i n u m content becomes 0.74.

Table III. Physicochemical Characterization Sample No.

1 2 8 4 5

N H 3 chemisorbed, 103 moles/grama 1.69 0.955 0.373 1.39 1.23

NH 3 /A1, moles/gram-atom 0.85 1.21 0.75 0.72 0.64

Pore vol, ml/gram6 0.163 0.172 0.166 0.147 0.169 Bulk density, grams/ml 0.943 0.866 0.814 0.854 0.860 Particle density, grams/

ml 1.492 1.341 1.285 1.466 1.395 Void fraction in packed

bed, v/v 0.368 0.354 0.367 0.417 0.383 Particle porosity, v/v 0.244 0.231 0.214 0.208 0.236 Effective diffusivity,

103 cm2/sec 2.4 8.4 6.1 6.7 10.4 a Measured at 250°C, 11.2 torr. 6 Determined from N 2 uptake at - 195°C, p/p° = 0.25.

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54. T H A K U R A N D W E L L E R Extracted H-Mordenite 601

T o t a l P o r e V o l u m e . A s the d a t a i n T a b l e I I I show, the pore vo lume was essential ly unchanged b y either ac id extract ion or N H 4 N 0 3 exchange. T h e single exception was sample 4, for w h i c h the pore vo lume was u n ­expectedly and inexp l i cab ly lower.

Effect ive Di f fus iv i ty . T h e effective d i f fus iv i ty for N 2 / H e at 25°C was calculated f r om the slope of the stra ight - l ine por t i on obta ined i n the h i g h ve loc i ty region of a " v a n Deemter p l o t " [height of an equivalent p late vs. in te r s t i t i a l ve loc i ty (14, 15)]. A b i n a r y diffusion coefficient for N 2 - H e of 0.717 cm 2 / s e c was computed f r o m Ref . 21, and the p a r t i t i o n coefficient was t a k e n as the reciprocal of the part ic le poros i ty (Table I I I ) on the as­s u m p t i o n that the adsorpt ion of N 2 at 25° C can be neglected. T h e c a l ­culated dif fusivit ies are l i s ted i n T a b l e I I I .

A l t h o u g h there is considerable scatter i n the d i f fus iv i ty values for the treated samples, the major effect is a large increase i n d i f fus iv i ty , re lat ive to the or ig ina l H-mordeni te , on either ac id extract ion or NH4NO3 exchange. O n this basis we w o u l d conclude that the sod ium rather t h a n the a l u m i n u m content appears to be the factor of greatest importance .

Cata lyt i c A c t i v i t y , Se lec t iv i ty , a n d Deact ivat ion . T h e product d i s ­t r i b u t i o n ( in the C 1 - C 5 range) remained re la t ive ly unchanged w i t h increas­i n g number of pulses for any g iven sample. F o r the or ig ina l H-morden i te a n d the N H 4 N 0 3 - e x c h a n g e d samples, propane was the major product (45-55 mole % of C 1 - C 5 ) . Propane and isobutane were comparable i n amount (35-40 mole % each) for the two acid-extracted samples. T h e z - C 4 : n - C 4 rat io was about 2 :1 for samples 1, 4, a n d 5, a n d about 3 :1 for samples 2 and 3, independent of pulse number .

A s indicated above, overa l l a c t i v i t y for hexane c rack ing was expressed, for each pulse, i n terms of an apparent first-order rate constant, k. T h e a c t i v i t y decl ined substant ia l ly w i t h increasing pulse number (i.e., w i t h i n ­creasing t o t a l amount of n-hexane fed) for a l l samples.

A l l a t tempts to find a "deac t iva t i on rate l a w " re lat ing k t o t o t a l hexane fed fai led. T h e most satisfactory rect i fy ing plot was found to be log k vs. the cumulat ive amount of hexane ac tua l ly cracked (designated Y) i n a n y g iven r u n . T h e plots of log k vs. Y are shown i n F i g u r e 1. (The scale for Y is shown at the top of F igure 1 for samples 1, 4, a n d 5, a n d at the b o t t o m for samples 2 a n d 3.) T h e deact ivat ion behavior is w e l l fitted for samples 1, 3, a n d 4, somewhat less we l l for sample 5, a n d poor ly for sample 2. T h e lines were obtained b y least-mean-squares fitting of the data .

T h i s correlation corresponds to a n exponential decay model , k = kae~aY. T h i s expression differs f rom the conventional exponential model often used i n continuous-flow systems (22, 23), k = k0e~at, i n that the analog to t ime i n a pulsed reactor is pulse number or i ts equivalent , c u m u l a ­t ive feed introduced . I n our case the correlat ing q u a n t i t y is cumulat ive feed converted, Y. I f one assumes that deact ivat ion is caused b y coke, the amount of w h i c h is proport ional to hexane ac tua l ly converted, th is

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602 M O L E C U L A R S I E V E S

* • • • ' I I ι ι 1 1 L 20 40 60 80 100 120

Y, ftl/g

Figure 1. Exponential model for deactivation: apparent first-order rate constant vs. cumulative hexane converted.

deact ivat ion mode l becomes s imi lar to t h a t used b y L a m b r e c h t et al. (24) for the fou l ing of a re forming catalyst .

T a b l e I V lists the values of the two parameters, k0 and a, i n the ex­ponent ia l decay model for each sample. T o o m u c h credence should not be placed i n the exact magnitudes of these values since i t is k n o w n for a n exponential model t h a t the covariance of the two parameters is v e r y h i g h (26). I t is clear, nevertheless, tha t the " i n i t i a l a c t i v i t y / ' presumably measured b y k0, decreases m a r k e d l y as a l u m i n u m is progressively extracted b y ac id extract ion (samples 2 a n d 3) bu t increases as sod ium is removed b y NH4NO3 exchange (samples 4 a n d 5).

T a b l e I V . P a r a m e t e r s i n Deac t ivat i on Equation» Sample No.

1 2 3 4 6

k0, grams"1 40.2 7.8 2.5 62.5 84.5 a, 103 grams/uliter 5.3 10.0 7.9 5.7 8.1

a k = koe~aY

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In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

54. THAKUR AND WELDER Extracted H-Mordenite 603

Summary

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity; as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity de­crease. Coke deposition is harmful in two ways: coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemi­cal deactivation as well.

In the sequence of catalyst samples 1, 4, and 5, there is both an in­crease in effective diffusivity and a decrease in sodium content. Both factors operate in the same direction, and it is not possible to say whether the increase in "initial activity" (i.e., fc0) is caused more by an improved physical situation or by decreased chemical poisoning. In the sequence of samples 1, 2, and 3, the decreased number of active sites plays the pre­dominant role; the "initial activity" drops sharply in spite of the higher diffusivity.

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