Influence of dealumination on the micropore adsorption in FCC catalysts

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 12 (1997) 251-260

Influence of dealumination on the micropore adsorption in FCC catalysts Gabriela de la Puente, Ulises A. Sedran * lnstituto de hwestigaciones en CatSlisis y Petroquimica, INCA PE (FIQ, UNL-CONICET) Santiago del Estero 2654, 3000 Santa Fe. Argentina

Received 20 December 1996: accepted 17 July 1997

Abstract

The adsorption of nitrogen and argon on laboratory steam dealuminated and equilibrium commercial FCC catalysts ( Y zeolite plus matrix) of various types, covering a wide range of catalyst properties, was studied. Different approaches were used to estimate micropore volumes. Micropore size distributions were assessed based on the area-averaged cylindrical micropore model. With both nitrogen and argon adsorption isotherms, it was possible to locate the peak indicating the pore size in the fresh catalysts at the expected value of 0.74 nm if proper values for the physical parameters in the model were adopted. On the dealuminated samples, nitrogen and argon yielded significantly different results. Ar-based micropore size distributions suggested that the physical parameters involved in the model change with the variations in the chemical composition accompanying dealumination. N2-based pore size distributions are more sensitive to variations in the overall adsorption energy due to the growing influence of specific contributions (quadrupolar interactions). The magnitude of the minimum of potential energy in the nitrogen adsorption process decreases with dealumination up to a stable value for Si/AI ratios over about 8. © 1997 Elsevier Science B.V. Keywords." Adsorption: Cylindrical pore model: Dealumination: FCC catalysts: Micropore

Nomenclature

AAA

a d s o r b a t e - a d s o r b a t e interaction dispersion constant, see equation below (kJ cm 6) 3Mc2c~AZA

"~ A A - -

AAE

c

d

do

,)

Dp E

-

E*

a d s o r b a t e - a d s o r b e n t interaction dispersion constant, see equation below (kJ cm 6) 6MC2~E0~A

\zA } * Corresponding author. 0927-6513/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0927-6513 ( 97 )00073-4

E* M N NAy

P Po

speed of light (m/s) diameter of an atom or ion (nm) arithmetic mean of diameter of adsorbate and adsorbent atom or ion (nm) effective pore diameter (nm) potential energy of interaction (k J) minimum potential energy of interaction (kJ) average minimum potential energy of interaction ( kJ ) mass of an electron (kg) density per unit area (molecules/cm 2) Avogadro's number ( m o l - 1) system pressure of an adsorbate gas (Pa) saturation pressure of an adsorbate gas

G. de hi Puente. LL ,4. Seth'an / Microporous Materials 12 (1997) 251 260

252

(Pa) universal gas constant (k J/tool K) radius of cylindrical pore (nm) temperature ( K ) Adsorbed gas volume (ml STP/g)

R Fp T Vads

Greek letters y. Y-k

polarizability (cm 3) expansion coefficients, Eq. ( 1 ); (-4.5-k)

~k=

[~k

-

-

2 ~-k-l,'3~O=

k

1

expansion coefficient, Eq. ( 1 );

(--l'5--k~2 []k_,,flo= 1 #k= Z

I~

/

magnetic susceptibility (cnl 3)

Subscripts A E A-A A-E

adsorbate adsorbent adsorbate-adsorbate interaction adsorbate-adsorbent interaction

1. Introduction

The particles of catalysts for cracking hydrocarbons in fluidized beds reactors (FCC process), which are basically composed of a zeolite component (Y zeolite) in a matrix and various additives, are subjected to severe, changing environments in the cyclic operation of commercial units. This is due to the circulation between the reducing atmosphere of the reactor at about 500'C and the oxidizing atmosphere of the regenerator at about 750':C, where coke is burnt off in air in the presence of steam [1]. A strong dealumination process is induced on the zeolite, as well as profound changes in both the chemical and physical properties of the fresh compound catalyst. It is well known that the loss of aluminum from the zeolite crystal structure translates into unit cell size shrinking concomitantly with the appearance of extra-framework aluminum species; finally, this ends up with the so-called 'equilibrium' catalyst (E-CAT). The

very different properties of the E-CATs as compared with fresh samples have strong repercussions on the catalytic performance, stressing the need for proper evaluation procedures [1,2]. The effects of these severe changes oll the pore system of the Y zeolite component are not clear, which, in turn, may exert a profound influence on the diffusion phenomena in reacting systems [3]. Fresh zeolite Y has pores defined by twelve-membered rings with an opening of 0.74 nm connecting supercages with a diameter of about 1.3 nm [4]. There is evidence based on electron microscopy [5,6] and gas sorption [7,8] for the appearance of new mesopore systems in Y zeolites subjected to various dealumination procedures. The reported diameters of the new mesopores range from about 2.0 to 50nm [9-11]. The properties of porous solid catalysts (specific surface area, pore volume and mesopore size distributions) are normally assessed using gas sorption data. For the particular case of micropores, there exist different techniques for determining their contribution to the pore volume, i.e., t-plot-like methods [12], Dubinin's approaches [12] or the more recent procedure by Remy and Poncelet [13]. Since the use of Kelvin's equation on which pore size distribution (PSD) calculations are founded is restricted to pore diameters above 2.0 nm, it cannot be applied to materials with micropores, and a number of approaches has been developed to determine micropore sizes with sorption data. Models on which different pore shapes are assumed (slit-like [14], cylindrical [15,16]) are generally based on the evaluation of the average interaction energy along the pore space for the adsorption of a gas adsorbate on a given surface, this energy being a function of the distance between adsorbing molecules and the pore wall. Usually, the only forces considered are those of dispersion. The effect of surface curvature on the adsorption process has also been considered for the proper formulation of the problem [17, 18]. Though possessing a high parametric sensitivity, the cylindrical micropore model [15] was applied to the argon isotherm. For the particular case of Y and ZSM-5 zeolites and AlP materials [19], it was possible to generate micropore size distributions centered around the

G. de la Puente. U. A. Se&'an / Microporous Materials 12 (1997) 251-260

expected values. Argon is preferred as the sorbate molecule for this kind of study [16,20,21]. Most prior studies were devoted to pure zeolite material. It is the objective of this paper to report results obtained in the application of the cylindrical micropore model to nitrogen adsorption isotherms and for comparison to argon adsorption isotherms - on commercial FCC catalysts of various types subjected to dealumination. E-CATs from running FCC units were also examined. The potential of this technique for describing changes in the catalyst properties as a function of laboratory steaming dealumination treatments and commercial FCC operation is discussed.

253

cients (refer to nomenclature):

5 [21 (do] 1° E(r) = - rtE* -~. 2 ~_\l'p/ k=O

~k

( r ] 2k \l'p/

(,? \rp/

k=O

krp/

Considering that the energy of adsorption is given by the average of the intermolecular potential in the adsorption environment, different expressions can be obtained according to the approach used: line-, area-, or volume-average. It has been claimed [15] that the area-average approach gives more precise results, the corresponding expression being: rp -- do

f 2. Theory The cylindrical pore model of Saito and Foley [15] is an extension to zeolites of the method of Horvath and Kawazoe, which was developed for slit-like pores in molecular sieve carbon [ 14]. Since the only forces considered were those of dispersion [14], they assumed that the so-called 10:4 potential function could be used to account for the interaction energy between a gas molecule at a given location inside a pore and an infinite adsorbing layer of atoms (the pore wall ). The integration of the corresponding theoretical expressions for the potential energy E(z) along the pore space between planes, assuming that it can be balanced to the fiee energy of adsorption AGads, allowed a link between experimental information (adsorption isotherna data) and pore sizes to be obtained. The theoretical E(z) expression had, in turn, been developed by Everett and Powl [17]. If the same approach is followed assuming that the pores are cylindrical, then an analogous model can be derived [15]. For the particular case of some zeolite solids, pores can be considered as infinite cylinders of radius rp, the wall of which is [brmed by a single layer of atoms where oxide ions are considered the adsorption sites [18]. The theoretical E(r) expression [17] corresponding to cylindrical pore geometry is given by the following equation, where C~k and flk are expansion coeffi-

E(r)2m" dr

~NAv %_,10

AG~°s = R T l n

f

2m" dr

0

(2) which then results in In

-

2

RT (32

E* ~.

~-

k=0

~k - \rp /

1-Fp /

--ilk

.

(3)

In Eq. (3), a link relating P/Po with the pore radius rp is established. It can be assumed that the effective pore diameter Op equals 2 r p - d E and the PSD, i.e. d Vads/dDp, can be calculated. A key element in these derivations is the expression for the potential energy minimum E* which accounts for the adsorbate-adsorbent and the adsorbate-adsorbate-adsorbent interactions. The parameter E*, which takes into account dispersion forces only is based on the value derived by Walker et al. [22] for graphite surfaces, and has the following expression: 3 NAAAA+NEAAE E* = - lO

(4)

where the dispersion constants AAAand AEA (see

G. de ht Puente, U. A. Seth'an / Microporous Materials 12 (1997) 251 260

254

operation; and (2) Micromeritics ASAP 2000 with automated operation. In case ( 1 ) catalyst samples were outgassed at 523 K for 4 h under a pressure of 10-apa. In case (2) catalyst samples were outgassed at 573 K overnight under a pressure of 10-3pa. Argon adsorption isotherms were obtained in sorptmeter (2) at 77 K following the same procedure. PSD values were assessed by calculating rp corresponding to a given P/Po value, and consequently V~d~ was evaluated from Eq. (3).

nomenclature) are calculated according to the Kirkwood-Mtiller approach [23].

3. Experimental Two commercial FCC catalysts were used as base materials: sample A-0 (Octidyne BRll60, Engelhard, 1.3% rare earth oxides) and sample B-0 (HFZ-33, Houdry, 1.9% rare earth oxides). The fresh catalysts were subjected to dealumination by steaming under 100% steam in a fixed fluidized bed at 815C for difl'erent time periods, to produce series A-i and B-i, respectively, where i indicates treatment duration. Two samples of catalysts from running FCC units were also employed: E-CAT-A (fresh catalyst is sample A-0) and E-CAT-C (Engelhard PRE-50AR, 1.42% rare earth oxides). The catalyst properties are shown in Table 1. Nitrogen adsorption isotherms were measured at 77 K in two types of sorptometers: (1) a Micromeritics Accusorb with conventional manual

4. Results and discussion

4.1. Catalyst properties Nitrogen adsorption isotherms for the various samples showed the typical shape for materials having both micro and mesopores, as shown in Fig. 1. The same pattern was observed for the argon adsorption isotherms. In this type of catalyst, micropores are essentially contributed by the

Table 1 Catalyst prope,'ties Catalyst sample

Steaming time (h)

UCS ~ (nm)

Si:AI ratio b

Micropo,e volume (cm3eg)

. r Sex,.S~Er (%)

BET specific surface area (n12 'g)

Zeolite (wt.%)

DubininRadt, shkevich d

Remy Poncelet ~

t-plot method d

content

c

A-0 A-l A-2 A-4 A-8

0 I 2 4 8

2.472 2.441 2.435 2.427 2.425

2.15 6.61 9.48 20.10 27.25

342 230 232 212 189

28.1 17.3 16.6 14.3 12.6

0.144 0.099 0.100 0.088 0.080

0.093 0.057 0.049 0.037 0.033

0.088 0.050 0.047 0.040 0.035

36.2 42.6 49.3 57.5 64.7

B-0 B-I B-2 B-4 B-8 B-14

0 1 2 4 8 14

2.472 2.438 2.438 2.430 2.429 2.427

2.15 7.82 7.82 14.29 15.83 20.10

314 222 208 193 180 176

19.0 10.9 8.5 7.7 6.1 3.4

0.135 0.099 0.086 0.079 0.073 0.071

0.111 0.046 0.038 0.027 0.024 0.023

0.082 0.035 0.027 0.023 0.018 0.013

49.2 70.8 63.3 70.1 73.9 75.4

E-CAT-A E-CAT-C

-

2.431 2.429

13.00 15.83

175 147

9.2 11.5

0.087 0.068

0.031 0.038

0.030 0.034

66.2 45.4

"ASTM-D-3942-85. bRef. [4]. CRef. [29]. aRef. [12]. ~Ref. [13]. rS~x, =external surface area as calculated from [13]

G. de la Puente. U. A. Seth'an / Microporous Mater&Is 12 (1997) 251-260

300

B-O

200

I00

0

~

0.0

0.2

i

04

f

h

0.6

0.8

1.0

Fig. I. Nitrogen adsorption isotherms on samples from series B Ifresh, B-O, and steamed for 1 h~ B-1 ) and E-CAT-A.

zeolite component and mesopores by the matrix component [24]. Conventional data manipulation [12] allows for an estimation of various catalyst properties; for example, the specific surface area, the micropore volume and the zeolite content of the catalyst samples. These properties are shown in Table 1. The values for the unit cell sizes (UCSs), specific surIilce areas and zeolite contents of fresh samples ~lnd E-CATs can be considered typical for these catalysts [1,25]. It can be seen that, for samples ill the series A and B, the physical properties show ~l very marked decreasing trend as functions of treatment severity, the changes being steeper in the first steps of steaming. The observed variations in properties can be considered characteristic for a fixed temperature and a variable time treatment o[" this type of catalyst [2]. Since zeolite dealumination by steaming has a strong effect on the micropore volume - where adsorption will be evaluated - this property was calculated following various approaches that yielded different values. The highest values were obtained with the classic procedure of Dubinin-Radushkevich [12], while the lowest ones were those resulting from the t-plot method [26]. The Dubinin-Radushkevich approach tends to overestimate micropore volumes since adsorption on the external surface of the crystals is not considered and some adsorbed volume may be

255

outside the pores; this issue could be more important in the case of composite catalysts like ours. On the other hand, the choice of the range of t values (statistical thickness of the adsorbed N 2 layer) considered in the prediction of micropore volumes may greatly influence the final results; in our case, deviations of up to 40% were observed. This fact had also been noticed by Remy and Poncelet on various pure zeolites [13]. The estimation of t as a function of relative pressures is subject to controversy and various options were suggested [12]. The results reported in Table 1 are based on the 'universal t curve' [26]. A combined approach has been used by Remy and Poncelet [13], who assumed that the N2 adsorption process on zeolites can be separated between that on micropores and the one on the external surface (described by a BET equation), following independent processes. Thus, the adsorption isotherm is corrected for the adsorption within micropores as obtained with either the Dubinin-Radushkevich or the generalized Dubinin-Astakhov approaches (both single and combined adsorption), and the correlation coefficient of the BET curve is optimized by adjusting the parameters micropore volume and energy of adsorption in the micropores. They concluded that the Dubinin-Radushkevich method is sufficiently consistent to account for computed results. In applying the procedure to our catalysts (refer to Table 1 ), we observed that the adjusted micropore volumes reside between the higher and lower limits found, respectively, by the Dubinin-Radushkevich and the t-plot methods. As a general tendency, the values are closer to those from the t-plot method. It is expected that the relative incidence of the external surface area in the zeolite crystals is higher in more severely steamed samples. This assumption can be supported by the following facts: (1) as observed by high-resolution electron microscopy, steaming produces zeolite crystal fracturing into smaller fragments [ 11 ]; and (2) Remy and Poncelet reported significant increases in external surface areas as functions ofdealumination of pure zeolites [13]. This could be considered as one of the reasons for the overestimation in micropore volume according to Dubinin-Radushkevich's method as compared with that of Remy-Poncelet and the

256

G de la Puente, U. A. Sedran / Microporous Materials 12 (1997) 251-260

t-plot method. In our case, it is not possible to estimate the external surface area of the zeolite component separately, but if it is accepted that it increases after dealumination [11,13], then the ratio of external surface area (from the Remy-Poncelet or t-plot methods) to the total BET surface area (Sext:SBet) may be a representative index for this quantity. As seen in Table 1, Sext:SHet increases with dealumination, and the difference between the results for the micropore volumes estimated from the DubininRadushkevich and other methods increases concomitantly. Zeolite dilution in FCC catalysts is a factor that enlarges these differences, while the disagreement between the various calculation procedures in pure Y zeolites is not as significant [13].

4.2. Micropore size distributions. Application o["the o,lindrical pore model In order to calculate a micropore size distribution under the assumptions of the models described above [14, 15], it is necessary to know some physical properties (polarizabilities, magnetic susceptibilities, phase molar surface densities, atom or ion diameters) of the adsorption system components that are involved in the expressions of the Kirkwood-M~ller dispersion constants and potential energy minimum E*. There exists a certain dispersion in the literature about these parameters (e.g. [ 14, 15, 18,27] ) which has repercussions on the location of peak maxima in the PSDs [15]. In the application of the area-averaged cylindrical pore model to our composite catalyst samples, we confirmed that it is possible to locate the peak maxima at the expected value, i.e. 0.74 nm for Y zeolites [4], for both argon and nitrogen sorbates. We also confirmed that the use of the Horvath-Kawazoe slit-like pore model in this type of catalyst requires that some unrealistic values for physical properties are adopted [15]. Thus, in the following discussion, all results and comments will be referred to the application of the area-averaged cylindrical pore model. The parameter values used are shown in Table 2. In the case of nitrogen, the requirement of different parameter values for the zeolite adsorbent to locate the peak maximum at the expected value might be due to the fact that the model does

not consider interaction terms other than dispersion, as will be discussed later. Micropore size distributions obtained from argon adsorption isotherms are shown in Fig. 2 for the examples of catalysts from series B (fresh and steamed during 1 h) and E-CAT-A. It can be seen that the location of the peak maximum for the untreated sample B-0 is 0.73 nm, while those for samples B-1 and E-CAT-A are 0.75 and 0.77 nm, respectively. The slight variation observed in the position of the peak will be analyzed later in this work. For each of the samples, an inflection point in the adsorption isotherm corresponding to the peak maximum in the PSD is observed in the low-pressure range (not recognizable for the axis scale used in Fig. 1) which consistently shifts to higher pressures in the sequence B - 0 < B - I < E CAT-A. It is to be noted that the presence of new micropores with sizes below 2.0nm is not observed. Micropore size distributions obtained from N 2 adsorption isotherms are shown in Fig. 3 for the examples of catalysts from series B, (fresh, steamed for 1 and 2 h), and E-CAT-A. It can be seen for the fresh sample B-0 that there is a single peak at 0.74 nm. When the catalyst is steamed for 1 h, the magnitude of the peak at 0.74 nm decreases, while a new peak appears at about 0.93 nm (sample B-1 ). As long as the dealumination process is more severe, the first peak tends to disappear while the second one increases (sample B-2): for longer time periods (e.g. 4 or 8 h, not shown), the peak at 0.93 nm is the only one remaining. It is interesting to observe that the equilibrated sample E-CAT-A only shows the peak at 0.93 nm. The same behavior was also observed in samples from series A. The other equilibrated sample, E-CAT-C, also shows a single peak at 0.93 nm. It is to be noted that N 2 adsorption isotherms for moderately dealuminated samples show two inflection points in the low range of relative pressures which leads to the two peaks in the PSDs. The second peak at 0.93 nm and its higher relative incidence in the PSDs as a function of increasing dealumination, does not reveal a physical change in the porous system of the zeolite structure since it essentially keeps its crystalline structure, with the well known reduction in UCS due to the higher abundance of Si-O

G. de la Puente. U. A. Sedran / Microporous Materials 12 (1997) 251-260

257

Table 2 Values of the physical properties used in the cylindrical pore model Property

Argon-zeolite

Polarizability. :~ (cm 3) Magnetic susceptibility, Z (cm3) Diameter. d (nm) Dcnsity per unit area, N (molecules/cm-')

Nitrogen-zeolite

Argon"

Zeolite ~

1.6× 10 -24

2.50× 10 --'4 1,30× 10 - 2 9 0,276 1.31 × l0 ts

3.25×

10 - 2 9

0,336 8.52× l0 t4

Nitrogen

Zeolite

1.76× 10 --'4 b

8.5× 10 -25 b 1.94× 10 --'9 b 0.276 a 3.75× l015 b

2.00×

10 - 2 9 c

0.315 b 6.70× 1014 ¢

"Rcf. [15]. "Rcf. [27]. 'Rcf. [14].

I "T"

=

,i

t~

B-0

>. "171

B-!

04

08

1.2

16

2.0

Dp (nm)

04

I

I

i

I

0.8

1,2

1.6

2.0

Dp (rim)

Fig. 2. Micropore size distributions based on argon adsorption isotherms. Samples from series B (fresh, B-0, and steamed for I h, B-I ) and E-CAT-A.

Fig. 3. Micropore size distributions based on nitrogen adsorption isotherms. Samples from series B (fresh, B-0, and steamed for 1 h, B-l, and 2 h, B-2) and E-CAT-A.

bonds, which are shorter than AI-O bonds. Following this, no increase in the net effective size of micropores in the zeolite crystalline structure should be considered. PSDs obtained with argon isotherms confirm this fact. The results presented in Figs. 2 and 3 can be examined in the light of some model features. First, it has to be considered that given the perfect spatial structural ordering of zeolites, a unique size

of pores is expected, at least in the fresh samples, as defined by a single peak in the PSD (ideally a delta function shape). However, adsorption should not be expected to be absolutely homogeneous at the molecular level. In this sense, the shape of PSDs obtained from argon isotherms (refer to Fig. 2) can be considered very good. Second, since the model ignores changes in the values of the

258

G. de la Puente, U. A. Sedran / Microporous Materials 12 (1997) 251-260

physical parameters that are sensitive to the solid's chemical composition, then changes in the overall energy of interaction due to those variations (which should reflect on the adsorption isotherm, e.g. displacement of inflexion points) can only be accounted for by changes in the pore sizes. Thus, the slight shifts in peak maxima location to higher pore sizes in the Ar-derived PSDs might be the consequence of modifications in the values of physical parameters; the shifts follow an increasing trend as a function of treatment severity. Third, as opposed to this pattern, significantly more important changes were observed in the Nz-derived PSDs, Considering that the model assumptions ignore interaction forces other than dispersion, this issue could have a major impact in the case of N2. Nitrogen may have important quadrupole interactions with electric field gradients, thus having an incidence on the overall energy change. It has been shown for various adsorbents and sorbate molecules, that specific contributions (including dipole and quadrupole interactions) may have about the same magnitude, or be even larger, than nonspecific contributions (dispersion and polarization interactions) to the total adsorption energy [23,27]. It is known that the steam dealumination generates aluminum atom concentration profiles along the zeolite crystals. Various data illustrate this conclusion: by means of analytical electron microscopy, Beyerlein et al. [11] showed that in the boundaries of crystallite fractures induced by mesopore coalescence in severe hydrothermal treatments, a high enrichment in aluminum is observed; in addition, the comparison of the surface with the bulk aluminum content of the zeolite allowed the prediction of an intense gradient of the Si:AI ratio normal to the crystal surface (e.g. [28]). These changes would indeed imply rearrangement of charges in the structure and electric field gradients that could have a neat effect on the electrical interaction terms (specific contributions). Nitrogen adsorption isotherms on samples moderately dealuminated have two inflection points in the low relative pressure range that reveal the different adsorption energy landscape: the adsorption process is different and changes with dealumination severity. Consequently, the model must account for these changes through the

appearance of a new peak in the PSDs, a result of the second inflexion point. According to the above considerations, the N 2 adsorption isotherms and PSDs derived therefrom would be able to show variations in the interaction profiles of catalysts with gradually changing composition and atom distribution. Considering that those changes are progressive (refer to Fig. 3), they may help to alternatively describe the course of dealumination in the zeolite component of FCC catalysts. Since a straightforward relationship between adsorption site energy states and pore sizes can be established through the adopted model [i.e. for a given adsorption isotherm, different values of minimum of potential energy E* would yield different micropore sizes when estimated from Eq. (3)], then PSDs indeed also represent a distribution of adsorption energies. Following this reasoning, we performed this subsequent exercise on the model implications. For all the samples (series A, B and E-CATs), it is possible to calculate the value of E* satisfying Eq. (3) when the pore size is kept at a desired value (location of peak maximum) and the relative pressure corresponding to the inflection point in the isotherm is used. Thus, isotherms with a single inflexion will give a unique E* value, while those having two inflexions will show two E* values instead. A weight factor was assigned to each E* in the cases where two inflections occurred, which was based on the areas under the curves of the PSDs, to calculate an average minimum of potential energy,/~*, representative of the whole micropore adsorption process. The results are shown in Fig. 4, where five additional samples of steam dealuminated commercial catalysts (REUSY, Si:A1=3.52, 13.00 and 17.73; and USY, Si:AI= 3.64 and 23.15, both from FCC S.A., Brazil) were included with the aim of adequately completing the range of Si:A1 ratios studied. It can be seen that a neat correlation applies as a function of zeolite dealumination, as expressed by the Si:AI ratio, showing that the value of/~* reduces from about 5.4 kJ/mol for untreated samples to about 4.2 k J/tool for the most severely steamed ones. The final 'stable' value is reached when the Si:AI ratio is approximately 8, which is consistent with the behavior of N2-derived PSDs ('stable' peak loca-

G. de la Puente, U. A. Sedran / Microporous Materials 12 (1997) 251-260

6 0

"v .

5

0

I

f

I

I

I

5

10

15

20

25

30

Si/A1 Fig. 4. Averageminimum potential energy for nitrogen adsorption as a function of Si:AI ratio [4]. All the samples considered.

259

0.93 for those moderately dealuminated and only the latter for severely dealuminated or E-CAT samples. These alterations can be ascribed to an increasing influence of the changes in composition of the solid and atom distributions on the specific contributions terms (quadrupolar interactions) of the overall energy balance for the nitrogen adsorption process on micropores, as a function of zeolite dealumination degree. Besides the fact that the model is not applicable to N2 adsorption isotherms for assessing micropore sizes on dealuminated FCC catalysts, it would allow the tracking of variations due to dealumination in the energy evolved in the adsorption process, yielding an alternative description. The values of the average minimum potential energy show a decreasing trend, which is steeper in the first steps of treatment. The presence of secondary micropores with sizes below 2.0 nm is not observed.

tion). It is interesting to observe that the correlation applies to all the catalyst types used, showing the progressive changes in the catalysts' condition.

Acknowledgement 5. Conclusions The variation in some of the properties of commercial composite FCC catalysts of various types as functions of dealumination (steaming in the laboratory or actual E-CATs) can be tracked by means of nitrogen adsorption isotherms. The calculation of micropore volumes is very sensitive to the approach used but, for composite catalysts like ours, more consistent results seem to be obtained with the t-plot or the Remy-Poncelet approach. Both argon and nitrogen adsorption isotherms can be used to derive pore size distributions based on the area-averaged cylindrical pore model that, depending on the values assigned to the physical parameters involved, can locate a peak at the expected position of 0.74 nm for zeolite Y. Argon isotherms produced a single peak in the PSDs for all the samples, which shifts slightly to higher pore sizes, suggesting that changes in the chemical composition might affect the value of the parameters of the model. Nitrogen-derived PSDs showed a pattern changing with dealumination: a peak at 0.74 nm for fresh samples, plus a second one at

The cooperation of Dr J.M. Arandes (Department of Chemical Engineering, University of the Basque Country, Bilbao, Spain) and Ms M. Gonz~lez for performing adsorption isotherms is gratefully acknowledged. This work was completed with the financial assistance of University of Litoral, Santa Fe, Argentina, Project 167 ( C A I + D 9 3 - 9 4 ) and Antorchas Foundation. The project is part of a Joint Research Project JICACENACA.

References [1] J. Biswas, I.E. Maxwell, Appl. Catal. 63 (1990) 197. [2] D. Wallenstein, U. Alkemade, Appl. Catal. A 137 (1996) 37. 13] P. O'Connor, F. van Houtert, Ketjen Catalyst Symposium '86, Scheveningen, 1986. [4] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. [5] C. Choi-Feng, J.B. Hall, B.J. Huggins, R.A. Beyerlein, J. Catal. 140 (1993) 395. [6] V. Patzelovfi,N.I. Jaeger, Zeolites 7 (1987) 240. [7] J. Lynch, F. Raatz, P. Dufresne, Zeolites 7 (1987) 333.

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[8] A.W. Peters, in: J.S. Magee, M.M. Mitchell Jr (Eds.), Fluid Catalytic Cracking: Science and Technology, Stud. Surf. Sci. Catal. 76 (1993) 183. [9] D. Goyvaerts, J.A. Martens, P.J. Grobet, P.A. Jacobs, i.1: G. Poncelet, P.A. Jacobs, P. Grange, B. Delmon (Eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, p. 381. [10] Y. Hong, J.J. Fripiat, Microporous Mater. 4 (1995) 323. [l l] R.A. Beyerlein, C. Choi-Feng, J.B. Hall, B.J. Huggins, G.J. Ray, in: M.L. Occelli (Ed.), Fluid Catalytic Cracking III, ACS Symposium Series 571, ACS, Washington, DC, 1994, p. 81. [12] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity. 2nd ed., Academic Press, New York, 1982. [13] M.J. Remy, G. Poncelet, J. Plays. Chem. 99 (1995) 773. [14] G. Horvath, K. Kawazoe, J. Chem. Engng Jap. 16 (1983) 470. [15] A. Saito, H.C. Foley, AIChE J. 37 (1991)429. [16] M.J.G. Janssen, C.W.M. van Oorschot, in: P.A. Jacobs, R.A. van Santen (Eds.), Zeolites: Facts, Figures, Future, Elsevier, Amsterdam, 1989, p. 633.

[17] D.H. Everett, J.C. Powl, J. Chem. Soc. Faraday Trans. I 72 (1976) 619. [18] E.G. Derouane, Chem. Phys. Lett. 142 (1987) 200. [19] A. Saito, H.C. Foley, Microporous Mater. 3 (1995) 531. [20] A. Saito, H.C. Foley, Microporous Mater. 3 (1995) 543. [21] A.F. Venero, J.N. Chiou, Mater. Res. Soc. Syrup. Proc. 111 (1988) 235. [22] P.L. Walker Jr, L.G. Austin, S.P. Nandi, in: P.L. Walker, Chemistry and Physics of Carbon, vol. 2, Dekker, New York, 1966, p. 349. [23] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley, New York, 1984. [24] P. Schneider, Appl. Catal. A 129 (1995) 157. [25] J. Scherzer, Catal. Rev. Sci. Engng 31 (1989) 215. [26] J.H. de Boer, B.G. Linsen, T.J. Osinga. J. Catal. 4 ( 1964 ) 643. [27] S. Ross, J.P. Olivier, On Physical Adsorption, Wile? Interscience, New York, 1964. [28] J. Arribas, A. Corma, V. Forn6s, F. Melo, J. Catal. 108 (1987) 135. [29] M.F.L. Johnson, J. Catal. 52 (1978) 425.