Selectivity of SilicaAlumina Matrices - American Chemical Society

Chapter 12

Selectivity of Silica-Alumina Matrices W-C. Cheng and K. Rajagopalan

Downloaded by UNIV OF ALBERTA on November 9, 2014 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch012

Davison Chemical Division, W. R. Grace and Company-Conn., Columbia, MD 21044

Amorphous silica-aluminas containing 13, 27 and 59 wt% were prepared by precipitating alumina onto silica gel, followed by hydrothermal treatment. These materials were characterized by Al NMR and ESCA and evaluated in gas oil cracking. NMR revealed the presence of tetrahedral, pentacoordinated and octahedral Al species in the steamed SiO - Al O samples with 27 and 13% Al O . The sample containing 59% Al O , however, showed the presence of only tetrahedral and octahedral Al species and a 3-fold enrichment of Al at the surface. In gas oil cracking, the activity of the catalyst per unit surface area increased with increasing Al O content. NMR analysis indicated that different types of Al species (eg., tetrahedral, octahedral) played a role in gas oil cracking. The catalysts with 13 and 27% Al O showed equivalent selectivity while the catalyst with 59% Al O yielded higher coke, H , gasoline and LCO and lower C and C olefins at constant conversion. The differences in activity and selectivity can be attributed to increase in acid site density with increase in alumina content of these catalysts. Al O 2

3

27

2

2

3

2

2

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3

3

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4

Commercial fluid cracking catalysts are comprised offaujasitezeolite dispersed in an inorganic mixed oxide matrix (1). X-ray amorphous silicaaluminas are generally used as matrix components. Cracking of large molecules present in a resid feed is limited by diffusion within a zeolite crystallite. As a result, the amorphous silica-alumina matrix plays a major role in cracking of these feed components (2). Cracking activity of amorphous silica-alumina is related to its acidity (3). Acidity and hydrocarbon cracking activity of amorphous silica-alumina catalysts of varying composition and preparation methods have been reported (4 - 7). Hydrothermal pretreatment of cracking catalysts has been employed as a

0097-6156/91A)452-0198$06.00/0 © 1991 American Chemical Society In Fluid Catalytic Cracking II; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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CHENG AND RAJAGOPALAN

Selectivity of Silica-Alumina Matrices

method to simulate aging in a commercial FCC unit (1). The objective of this work is to determine gas oil cracking activity and selectivity for silicaaluminas of varying composition. Hydrothermal pretreatment under varying conditions was employed to generate catalysts of the same composition with varying surface area. NMR and ESC A characterization of the silica-aluminas were employed to elucidate the nature of active site for gas oil cracking and determine catalyst structural properties that influence selectivity.


Experimental Methods Amorphous silica-alumina catalysts of varying composition (13 to 59 wt% AI2O3) were prepared following the method described by Magee and Blazek (8). Catalysts are designated by their alumina content as SA-13, SA-27 and SA-59. The catalysts were washed free of soluble salts and spray dried. Properties of thefreshcatalysts are described in Table I. Catalysts were hydrothermally pretreated at 1090°K with 100% steam for varying periods (2 to 16 hours). The pretreated catalysts were characterized by ESCA and solid state NMR. ESCA analyses were carried out on a PHI 5400 XPS with a Mg Ka X-ray source. The surface Al/Si was taken as the ratio of the peak areas of the Al 2P and Si 2P peaks. Solid state A l NMR analyses were performed using a Bruker AM-400 spectrometer ( A1 frequency, 104.25 MHz). About 1000 scans were accumulated before Fourier transformation. A line broadening of 100 Hz was used to eliminate the high frequency noise without affecting peak widths. A 2 (is pulse (10 degree) was used with a repetition time of 0.1 s between pulses. Chemical shift was referenced to hexaquo aluminum ion. Solid state Si NMR was carried out on a Bruker MSL-200 spectrometer ( Si frequency, 39.76 MHz). An average of 1600 scans were collected. A line broadening of 50 Hz was used. Chemical shift was referenced to tetramethylsilane. In all experiments, the samples were equilibrated with a 50% relative humidity atmosphere for 16 hours and spun at the magic angle at a frequency between 4-8 kHz. 27

27

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Table I: Properties of Catalysts :

SA-13

SA-27

SA-59

wt% Na20 wt%S04

: :

wt% AI2O3

:

.08 .07 12.7

.04 .16 27.1

.02 .70 58.6

:

417

320

504

Description

1

BET Surface AreaHm^- )

'Measured after activating the catalyst for 3 hours at 810°K.

In Fluid Catalytic Cracking II; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN


A fixed bed reactor described by ASTM Method No. D3907 was employed for catalytic testing. A sour, imported heavy gas oil with properties described in Table II was used as the feedstock. Experiments were carried out at a reactor temperature of 800°K and catalyst residence time (9) of 30 seconds. Liquid and gaseous products were analyzed with gas chromatographs. Carbonaceous deposit on the catalyst was analyzed by Carbon Determinator WR-12 (Leco Corp., St. Joseph, MI). The Weight Hourly Space Velocity (WHSV) was varied at constant catalyst contact time to generate selectivity data of various products as a function of conversion. For certain experiments, conversion was also varied by varying the catalyst pretreatment conditions.

Table II: Properties of Sour, Imported Heavy Gas Oil API gravity at 16°C Sulfur (wt%) Nitrogen (wt%) Conradson Carbon (wt%) Aniline Point (°C) K Factor D-1160 (°C) IBP 5 10 20 40 60 80 90 95

22.5 2.6 .086 .25 73 11.6 217 307 324 343 382 423 472 500 524

Results and Discussion NMR and ESCA Characterization. Increasing the duration of hydrothermal pretreatment reduced the surface area of the silica-alumina catalysts (Figure 1). By varying the duration of pretreatment, catalysts of varying surface area were obtained for characterization and evaluation. Twelve catalysts of varying composition and surface area were analyzed (Table HI). For the purpose of illustration, A1 and Si NMR spectra of three of those catalysts are shown as Figures 2 and 3. A l NMR analyses (Figure 2) indicated the presence of three lines for the pretreated SA-13 and SA-27 catalysts. The approximate chemical shift values for the lines were 0, 30 and 60 ppm. These lines were attributed to octahedral, pentacoordinated and tetrahedral Al respectively (10). The SA-59 catalyst exhibited only two 27

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300

STEAMING TIME AT 1090 K/h

Figure 1.

Effect of the time of pretreatment on surface area of catalysts. A SA-13; O SA-27; • SA-59


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lines near 70 ppm and 0 ppm (Figure 2). The chemical shift of about 70 ppm observed in SA-59 is characteristic of tetrahedral Al observed in y- AI2O3 (11), indicative of Al-O-Al species, while the chemical shift of about 60 ppm observed in SA-13 and SA-27 is characteristic of tetrahedral Al present in zeolites, indicative of Al-O-Si species. These results imply that the pentacoordinated Al is not present in the pretreated SA-59 catalyst. Integrated line intensities for the NMR peaks for the catalysts after various pretreatments are summarized in Table EI. These intensities are designed to be a qualitative measure of changes in the type of Al species as a function of composition and pretreatment. The results indicate that the distribution of Al species in SA-13 and SA-27 catalysts changed significantly with changes in pretreatment conditions. For example, increasing the duration of hydrothermal treatment of SA-13 increased pentacoordinated Al at the expense of tetrahedral Al. However, the surface composition of the catalysts, as measured by ESCA, did not change significantly with changes in pretreatment conditions. Table EI: Characterization of Steamed Silica-Alumina Catalysts 27

Relative Intensities - A1 NMR Description SA-13-2 hrs. SA-13-4hrs. SA-13-8 hrs. SA-27-2hrs. SA-27-4hrs. SA-27-8 hrs. SA-59-2hrs. SA-59-4hrs. SA-59-8hrs.

ESCA Al/Si

Tetrahedral Pentacoordinated Octahedral

.17 .18 .19 .68 .67 .75 4.5 4.2 4.6

.67 .33 .32 .35 .33 .36 .31 .27 .29

.27 .36 .27 .23 .28 .27 .69 .73 .71

.06 .31 .41 .42 .39 .37 0 0 0

The ^Si NMR Spectra (Figure 3) for SA-13 and SA-27 exhibited a broad peak at a chemical shift of - 110 ppm, indicative of silicon species with zero nearest Al neighbors, Si (OA1) (12). Shoulders at a chemical shift of 100 to 105 ppm suggest that a small amount of SHOAl^ species are also present. The SA-59 sample, exhibited the peak at 110 ppm and several other broad peaks up to a chemical shift of 82 ppm, suggesting that this sample contained the full range of structures from Si(OAl) to Si(OAl) . The higher alumina content of SA-59 apparently resulted in the presence of significant concentrations of Si(OAl) , Si(OAl) and Si(OAl) species. 0

0

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12. CHENG AND RAJAGOPALAN

400

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0

-200

CHEMICAL SHIFT/ppm

Figure 2.

Al NMR spectra for hydrothermally pretreated (1090° K) catalysts, SA-13 (above), SA-27 (middle) and SA-55 (below).

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Selectivity ofSilica-Alumina Matrices

Activity per Unit Surface Area for Gas Oil Cracking. A second order kinetic conversion parameter (conversion +100 - conversion) was used (13) to monitor gas oil cracking activity. The activity relationship as a function of surface area and catalyst composition is described in Figure 4. As expected, activity increased linearly with surface area. Activity per unit surface area depended on composition and increased with increasing alumina content. A catalyst with 27 wt% AI2O3 was more active for cumene cracking than a catalyst with 13 wt% AI2O3 (6). An activity maximum at 30 wt% AI2O3 was reported with thermally pretreated, cogelled silica-alumina catalysts for n-octane cracking and propylene polymerization (4). Propylene polymerization and n-octane cracking activity declined at higher alumina content. In the current work with hydrothermally pretreated catalysts, we observe the highest gas oil cracking activity per unit surface area at the highest (59 wt%) alumina content that was examined. Differences in catalyst preparation methods between the current and previous (4) work (cogellation or impregnation), could have influenced the activity composition relationship. For a given composition, when the catalyst surface area was varied by varying the pretreatment conditions, the distribution of Al species within the catalyst (tetrahedral, pentacoordinated, octahedral) varied significantly (Table HI) without a significant change in activity per unit surface area (Figure 4). Cracking activity in amorphous silica-alumina has been attributed to tetrahedrally coordinated Al (14). The above NMR and activity results indicate that activity does not correlate with concentration of Al species of any particular coordination. This implies that Al that is not tetrahedrally coordinated in the catalyst also plays a role in gas oil cracking. Effect of Composition and Pretreatment on Coke and H 2 Selectivity. We examined the effect of catalyst composition and pretreatment on the selectivity for coke and H 2 . Results (Figures 5 and 6) indicate higher yields of coke and H 2 at constant conversion for SA-59 relative to catalysts with lower alumina content. Pretreatment conditions influenced coke, H 2 yields indirectly by influencing surface area and gas oil conversion. Thus, coke and H 2 selectivity was not influenced by pretreatment conditions. We conclude that coke and H 2 selectivity does not correlate with the concentration of Al species of a particular coordination (eg., tetrahedral). The activity per unit surface area as well as selectivity for coke and H 2 were higher for SA-59 relative to catalysts of lower alumina content. Coke selectivity of various Y-faujasite catalysts has been related to density of acid sites (13). Perhaps the density of acid sites per unit surface area in these amorphous silica-alumina catalysts increase with AI2O3 concentration. The higher density of sites in SA-59 can explain its higher activity per unit surface area as well as higher coke yields at constant conversion. Increasing site density in faujasite catalyst resulted in lower selectivity for light olefins (eg., butylene) and higher selectivity for gasoline (15). Hence, we examined the detailed selectivity for all the cracked products with the amorphous silica-alumina catalysts.


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2.0

z o 55 oc Ul > z o o o UJ z 5

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• 1.0

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I I I

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BET SURFACE A R E A / m g '

Figure 4.

•

•

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100

1

Activity of catalysts of varying surface area. Surface area was varied by varying the duration of hydrothermal pretreatment. A SA-13; SA-27; 0 SA-59 0.25

40

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WT% CONVERSION

Figure 5.

Hydrogen selectivity of catalysts. Conversion was varied by varying the duration of hydrothermal pretreatment A SA-13; • SA-27; 0 SA-59





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KINETIC CONVERSION

Figure 6.

Coke selectivity of catalysts. Conversion was varied by varying the duration of hydrothermal pretreatment. ASA-13; • SA-27; OSA-59

Product Distribution as a Function of Composition. Pretreatment conditions for the catalysts of varying composition were adjusted to achieve catalysts of comparable activity. Thus, the more active SA-59 catalyst was pretreated longer (16 hrs.) than the less active SA-13 catalyst (2 hrs.). This was considered acceptable as results reported earlier indicate that activity per unit surface area and coke, H 2 selectivity are not influenced by the pretreatment conditions. Gas oil conversion was varied by varying the space velocity. Results summarized in Figures 7 and 8 indicate that SA-59 yielded more coke, H 2 , gasoline and light cycle oil (LCO) at constant conversion than SA-13 and SA-27. SA-13 and SA-27 were more selective for propylenes and butylenes relative to SA-59. Selectivity for C 2 hydrocarbons of all the catalysts were equivalent. The selectivity of SA-59 for gasoline at the expense of propylenes and butylenes is consistent with the hypothesis of higher acid site density in SA-59. With high acid site density bimolecular hydrogen transfer reactions compete with unimolecular cracking reactions (15) resulting in increased yield of intermediate cracked products like gasoline and LCO at the expense of fully cracked products like propylenes and butylenes. The high site density also favors bimolecular reactions that facilitate formation of coke precursors (13). Conclusions The distribution of Al species of varying coordination (tetrahedral, pentacoordinated and octahedral) can be influenced by changing the conditions of hydrothermal pretreatment of amorphous silica-alumina catalysts. However, for a given composition, activity per unit surface area and selectivity were independent of pretreatment conditions. Thus, gas oil cracking activity and selectivity in amorphous silica-alumina cannot be


208



ascribed to Al species of a particular coordination (eg., tetrahedral). Al that is not tetrahedrally coordinated in the catalyst also plays a role in gas oil cracking. Activity per unit surface area and selectivity for the production coke, gasoline, LCO and light olefins were influenced by the composition (alumina content) of the catalyst. The activity and selectivity results can be explained by suggesting that the catalyst with the highest alumina content (SA-59) has the highest density of acid sites. Higher site density resulted in increased selectivity for gasoline, LCO and coke at the expense of light (C3, C4) olefins. The selectivity trends are consistent with reported effects of site density on selectivity in faujasite.

2

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CATALYST/OIL

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>

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UJ

82

n-

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WT% C O N V E R S I O N 0.3

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WT% C O N V E R S I O N



Selectivity ofSilica-Alumina Matrices 209

.o 40,

w 35 O CO

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C?25

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70



40

50

60

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Figure 7.

Activity and selectivity of hydrothermally pretreated (1090°K) catalysts. Conversion was varied by varying weight hourly space velocity. ASA-13 (2 hrs.); DSA-27 (8 hrs.); OSA-59 (16 hrs.)

D

o

0

40

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70


(Figure continued)


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Selectivity of SilicaAlumina Matrices - American Chemical Society

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