n-HEPTANE CRACKING ON USY ZEOLITE THE EFFECT OF REACTION TEMPERATURE ON ACTIVITY AND DEACTIVATION

n-HEPTANE CRACKING ON USY ZEOLITE THE EFFECT OF REACTION TEMPERATURE ON ACTIVITY AND DEACTIVATION

R.F. dos SANTOS and E.A. URQUIETA-GONZÁLEZ*

Departamento de Engenharia Química - Universidade Federal de São Carlos, C. Postal 676

CEP 13565-905 São Carlos, SP - Brazil - Telephone: [phone] - Fax: [phone]

e-mail: [e-mail]

(Received: November 5, 1997; Accepted: February 15, 1998)

Abstract - n-Heptane cracking on a USY commercial zeolite at 350, 400 and 450ºC was studied. XRF, XRD, ²⁷Al and ²⁹Si-MAS-NMR were used for USY characterization. The nature of the coke was determined by ¹H-NMR and infrared spectroscopy. It was observed that temperature influenced the nature of the coke formed, and that an increase in temperature led to a reduction in the solubility of coke in CH₂Cl₂ and an increase in its aromaticity. Despite these characteristics and the larger amount of coke formed, higher temperatures have less effect on the deactivation process than lower temperatures. Selectivity for C₁-C₆ fractions was hardly influenced by temperature, and it was that expected for the cracking mechanism of n-paraffins on acid catalysts.

Keywords: Reaction temperature, n-heptane cracking, activity, deactivation.

INTRODUCTION

The use of USY zeolites as active ingredients in the Fluid Catalytic Cracking catalyst (FCC) has resulted in a significant improvement in petroleum fraction processing. In comparison with the amorphous silica alumina catalysts, which were the first active ingredients used, they present higher activity and lower coke production. The coke formed, not only in cracking but also in any reaction involving organic compounds, is the main factor responsible for catalyst deactivation. In zeolites composed of channels and/or cavities with dimensions of 3 to 20 Å, this coke can form on the surface or in the interior of the porous system, covering the active sites and often blocking the access of reactants to these sites (Magnoux et al., 1987a; Guisnet and Magnoux, 1992a; Moljord et al., 1995). Deactivation costs are very high and this fact has led to an increasing number of studies on the subject over the past years, to understand the process of coke formation and to develop methods to reduce its formation in order to increase the life of the catalysts. Efforts are also being made to find suitable methods for regeneration of catalysts by means of removal of the coke formed.

Several factors such as operating conditions, where temperature is without any doubt a significant variable in the process, have direct influence on coke formation (Magnoux et al., 1992b). Thus, the objective of this work was to study the deactivation of a USY zeolite by the formation of coke, using the n-heptane cracking reaction and choosing the operating temperature as a variable.

EXPERIMENTAL

USY Zeolite

A USY zeolite supplied by Engelhardt was used in this study, and its characterization by XRF, XRD and ²⁷Al, and ²⁹Si-MAS-NMR, provided a total and a framework Si/Al ratio equal to 2.8 and 11.5 respectively.

Catalytic Activity

The catalytic activity of USY zeolite at temperatures of 350, 400 and 450ºC was evaluated by means of the n-heptane cracking reaction, using a fixed-bed tubular reactor with continuous feeding of reactants and H₂ as carrier gas in the molar proportion of n-heptane to H₂ = 0.67. The products of the reaction were analyzed by means of a gas chromatographic system with a thermal conductivity detector, which was coupled to the reactor in line. Effluents were separeted by means of a packed chromatographic column filled with deactivated alumina (60/80 mesh) and operating at 230ºC.

Determination of Coke Content and Coke Extraction and Characterization

The content of coke formed during the reaction (% mass of carbon) was determined using equipment from the LECO Corporation S.T. (CS-125). The procedure is based on combustion of the coke deposited on the sample, after which the quantity of CO₂ formed was determined.

Soluble coke contained in the sample was extracted using a method proposed by Magnoux et al. (1987a), which consists of treating the zeolite with HF (40%), followed by extraction of the soluble coke with an organic solvent (CH₂Cl₂).

Coke extraction (Y) was determined by the equation:

Y (%) = (Me/Mc) x 100

where Me = mass of soluble coke extract and Mc = total mass of coke in the catalyst.

In order to determine the nature of the soluble coke, the extract was analyzed by ¹H-NMR and infrared spectroscopy.

RESULTS AND DISCUSSION

Influence of Temperature on Activity and Deactivation

The n-heptane conversion data (X: % mass) in relation to time on stream at different temperatures are shown in Figure 1. It can be seen from this figure that the USY zeolite presents an increase in initial activity when reaction temperature is increased, as expected and explained by the effect of temperature on chemical reaction kinetics.

Thus, when one compares the activity factor, defined as the number of reacting molecules per structural site (Table 1), which in this case is related solely to temperature since all other parameters were held constant, one can observe that, with an increase in reaction temperature, a greater number of n-heptane molecules was transformed per structural site.

Figure 1: n-Heptane conversion (X: % mass) at 350, 400 and 450ºC.

Table 1:

Temperature (oC)	Alf (mmol)	nC7 (reacted) (mmol)	Activity factor
350	0.664	1.29	1.94
400	0.664	2.56	3.85
450	0.664	3.96	5.96

Activity factor for n-heptane cracking on USY zeolite at different temperatures (t = 10min, mzeol = 0.6g)

Figure 1 shows that, although the USY zeolite presents greater initial activity, it remains more active throughout the reaction at a temperature of 450ºC. The fact that deactivation occurs more slowly at higher temperatures is clearly illustrated in Figure 2, which shows that at 450ºC, even when a larger amount of coke is formed (Figure 3), the residual conversion of n-heptane is always higher than at the other temperatures.

According to Guisnet and Magnoux (1992a), the formation of coke at low temperatures occurs mainly due to reactions of condensation and oligomerization which, in this case, form large molecules that are retained due to their low volatility and strong adsorption. At higher temperatures, the reactions of alkylation, cyclization and transference of hydrogen become important and, although the coke molecules formed are smaller, they are retained sterically inside the zeolite pores1. Thus, it is expected that the formation of coke at lower temperatures, even in smaller quantities, has a more pronounced effect on deactivation, since the coke molecules formed block a larger number of active sites and cause an even more accentuated blockage of the entrances and exits of the porous system.

Figure 2: Residual conversion of n-heptane, Xr on USY zeolite (reference t = 10 min) at different reaction temperatures.

Figure 3: Content of coke deposited (% mass) during n-heptane cracking on USY zeolite.

The observations above can be effectively verified when one observes the data in Table 2. At 450^ºC the ratio between reacted carbon (C_r) and carbon that has turned into coke (C_c) shows a more significant increase with time on stream, indicating that the zeolite maintains greater cracking activity resulting in a lower increase in the growth rate of the ratio between coke formed and products of the n-heptane cracking reaction (Figure 4).

Selectivity

Table 3 shows the selectivity for C₁ and C₆ fractions produced during n-heptane cracking at 350, 400 and 450ºC. It can be noted that, regardless of operating temperature and time on stream, a higher proportion of C₃ and C₄ fractions appears in the products. This behavior can be explained by the mechanism of n-paraffin cracking, which in the case of n-heptane specifically results in the formation of these fractions, with fraction C₃ mainly formed of propene (a potential generator of coke) and fraction C₄ of isobutane (Jacobs and Martens, 1991)

Table 3 illustrates the fact that throughout the reaction there is a reduction of fraction C₅ in the products. The formation of fraction C₅, in relation to C₃ and C₄, is produced mainly on the strongest acid sites, which are the first to become deactivated with the formation of coke (Magnoux et al., 1987.b). As discussed earlier, the deactivation of these sites is more accentuated at lower temperatures, which leads to a higher increase in the (C₃+C₄)/(C₂+C₅) relation at 350ºC.

Table 2:
Data on effectively reacted carbon (% C

Time(min)	350oC			400oC			450oC
	% Cr	% Cc	Cr /Cc	% Cr	% Cc	Cr /Cc	% Cr	% Cc	Cr /Cc
10	15.7	3.1	5.1	31.1	3.5	8.9	48.0	4.4	10.9
40	4.4	1.1	4.0	11.0	1.3	8.5	22.8	1.8	12.7
70	3.2	0.65	4.9	-	-	-	-	-
100	2.8	0.50	5.6	9.5	0.86	11.0	17.4	0.86	20.2
130	-	-	-	7.3	0.49	14.9	13.9	0.70	19.8

_r) and on carbon that turned into coke (% C_c)

Figure 4: Relations of coke/products in the n-heptane cracking reaction on USY zeolite.

Extraction and Characterization of Soluble Coke

Table 4 shows the results of extraction of the soluble coke (Y) in dichloromethane (CH₂Cl₂), formed at different temperatures. It can be noted that the fraction of soluble coke decreases as temperature increases, indicating that at high temperatures less soluble molecules are formed (Anderson et al., 1989).

Figure 5 presents the spectra resulting from an analysis of the extract by infrared spectroscopy; these show the presence of bands in the 2,800-3,000 cm^-1 region and between 1,350 and 1,470 cm^-1, corresponding to paraffin compounds, and a band at 1,600 cm^-1, currently called the coke band, corresponding to polyolefinic or aromatic compounds (Guisnet and Magnoux, 1992a).

A better idea regarding the nature of coke can be found by analyzing the spectra of ¹H-NMR in Figure 6. These spectra show peaks corresponding to aromatic protons in 7 < d < 9 ppm and to alkyl-aromatic protons in 2 < d < 3 ppm (Henriques, 1994). The peaks corresponding to protons linked to carbons located between aromatic rings were not significant. It can be seen from the ¹H-NMR spectra that there was an increase in coke aromaticity with temperature, since there was an increase in the relation between the integrated areas of the H_AR and H_AA proton peaks (Table 4). This increase in aromaticity would explain the formation of a larger quantity of insoluble coke with the increase in temperature (Henriques, 1995). According to Guisnet and Magnoux (1989), in this type of coke there would be nearly linear, polyaromatic molecules formed by condensed benzene rings.

Table 3:
Selectivity for C

Temperature time (min)	350oC 10 40 100	400oC 10 40 100	450oC 10 40 100
C1	0.6 2.0 2.7	0.8 1.4 1.2	1.2 1.6 2.2
C2	0.8 2.0 2.7	1.6 2.1 2.3	2.5 2.7 2.9
C3	36.3 39.9 42.5	38.9 39.8 41.0	39.5 40.6 42.1
C4	51.2 51.2 52.0	50.8 49.8 49.4	49.8 48.8 48.0
C5	7.9 4.8 -	6.0 5.6 4.4	5.7 4.8 4.0
C6	3.2 - -	1.9 1.2 1.7	1.3 1.5 0.8
(C3+C4) /(C2+C5)	10.0 13.4 35.0	11.8 11.6 13.5	10.9 11.9 13.0

₁ and C₆ fractions* during n-heptane cracking on USY zeolite at 350, 400 and 450ºC

* Percentage of mass in relation to total products formed.

Figure 5: Transmittance spectra in the infrared region of the extract containing the soluble coke formed at different temperatures as a function of the number of waves (cm^-1).

Table 4:
Extraction data on soluble coke and relation between aromatic and alkyl-aromatic protons (H

T (oC)	time (min)	Mc	Y	HAR/HAA
350	100	5.8	99	0.22
400	100	7.8	77	0.69
450	100	10.2	58	1.08

_AR/H_AA) in the soluble coke formed in the n-heptane cracking reaction at different temperatures

¹H-RMN spectra.

Figure 6:
¹H-NMR spectra of the extract containing the soluble coke, formed at different reaction temperatures.

CONCLUSIONS

Reaction temperature influences the nature of coke formed, where a reduction of its solubility in CH₂Cl₂ and an increase in its aromaticity at increased temperatures was observed. Despite these characteristics and the fact that it is formed in larger quantities, the coke generated at higher temperatures produces a less accentuated deactivating effect than that formed at lower temperatures. The latter, which is of a less aromatic nature and is produced mainly by condensation reactions and oligomerization, is more strongly retained on the surface of the zeolite due to its strong adsorption and its lower volatility, thus causing higher rates of poisoning of the active sites and greater blockage at the entrances and exits of the porous system.

Selectivity for C₁-C₆ fractions remained practically unaltered by temperature and its distribution in the product stream was that predicted by the mechanism of n-paraffin cracking on acid catalysts.

ACKNOWLEDGMENTS

The authors thankfully acknowledge the financial support (grant n^o 520597/94-0) provided by CNPq (National Council for Scientific and Technological Development) for this study and for the Master’s scholarship granted to Ricardo Fernandes dos Santos.

NOMENCLATURE

Al_f Framework aluminum

C Coke in the catalyst, total

C₁,C₂,...C₆

(C₁-C_r) Hydrocarbon with 1, 2 .... or 6 carbon atoms

C_r Total reacted carbon, %

C_c Carbon effectively turned into coke, %

FCC Fluid Catalytic Cracking

H_AR Aromatic proton

H_AA Alkyl-aromatic proton

MAS Magic angle spinning

Mc Total mass of coke in the catalyst, g

Me Mass of soluble coke, g

m_zeol Zeolite mass in the reactor, g

NMR Nuclear magnetic resonance

P Products, total

t Time, min

USY Ultra-stable Y zeolite

X Conversion of n-heptane, % mass

X Residual conversion of n-heptane

XRD X-ray diffraction

XRF X-ray fluorescence

Y Extracted coke, %

Greek Letters

d Chemical shift

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Publication Dates

• Publication in this collection
  09 Oct 1998
• Date of issue
  June 1998

History

• Accepted
  15 Feb 1998
• Received
  05 Nov 1997