ARTICLE pubs.acs.org/EF
Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated HZSM-5 with Different Si/Al Ratios Shudong Qu, Guozhu Liu,* Fanxv Meng,† Li Wang, and Xiangwen Zhang Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: A series of wall-coated catalysts were prepared in stainless-steel microchannels with HZSM-5 zeolites with a Si/Al molar ratio of 25140 by washcoating methods. Catalytic cracking of supercritical n-dodecane (4 MPa and 550 °C) was used to examine the catalytic activity and stability of HZSM-5 coatings (ZC). It is found that catalytic cracking activities and stabilities of the coatings increase by the following order: ZC25 < ZC50 < ZC100 < ZC140 (with the numbers representing the Si/Al ratio), which is in well accordance with the increasing Lewis acid and decreasing Br€onsted acid amount of parent HZSM-5. Temperatureprogrammed oxidation (TPO) characterization of cokes deposited on the HZSM-5 coatings indicates that the coke amount deposited over coatings also increases with Si/Al ratios of parent zeolites, owing to the interaction of the coke formation and its supercritical extraction. Rapid deactivation of ZC25 was possibly caused by pore-mouth plugging by a little amount of coke, while a large amount of cokes of ZC140 may be a result of the gradual and uniform buildup of cokes in the pore under kinetic control.
’ INTRODUCTION As the speed of an aircraft reaches supersonic or hypersonic regimes, aerodynamic heat will raise the vehicle heat load beyond the scope that structure materials could bear. Under such situations, hydrocarbon fuel, as an ideal coolant, can offer sufficient cooling capacity (heat sink) for supersonic aircrafts, through both significant physical sensible heat and heat-adsorbing chemical reactions.16 Furthermore, the absorbed heat can be released during the combustion of fuel, which increases the propulsion efficiency as well as the combustion rates by reducing ignition delay times ascribed to small olefins generated from the cracking of hydrocarbons.7 In the cooling system of an advanced aircraft, the hydrocarbon fuel is under high pressure (3.46.9 MPa) and high temperature (above 400 °C), i.e., the supercritical conditions.1,8 Catalytic cracking of supercritical hydrocarbons with zeolite coatings in microchannels of heat-exchangers is a promising technique for the fuel-cooling thermal management system to accelerate the reaction rates of desired endothermic cracking reactions and, thus, the cooling capacities of hydrocarbons, as well as the improvement in the stabilization of the catalyst by extracting cokes with the superdense reaction fluid.4,5,912 This wall-coated catalyst could significantly reduce pressure drop and thermal resistance and formations of carbonaceous deposits, especially filamentous coke from the catalysis of wall surface metal elements. Generally, washcoating (or dip-coating) was an easy and efficient method to prepare zeolite coatings on substrates by adjusting and optimizing the slurry property and depositing procedures,13 wherein the parent zeolite used in the slurry was one of the most important factors determining the catalytic activities and stabilities of zeolite coatings. Recently, many works have been performed on the catalytic cracking of supercritical hydrocarbon. Huang et al. performed the catalytic cracking of several jet fuels (JP-7, JP-8 þ 100, and JP-10) in catalyst-coated tubes and demonstrated that JP-7 and JP-8 þ 100 r 2011 American Chemical Society
cracking over inexpensive zeolite catalysts can provide a substantial heat sink (3.41 and 2.91 MJ/kg, respectively).4 Sobel et al. first prepared SAPO-34 and Y zeolite coating and performed a series of experiments to explore the endothermic potential of different fuels, obtaining above 700 British thermal units per pound (Btu/lbm) endotherm by an mixture of normal alkanes at 650 °C and 2.4 MPa (supercritical conditions).5 They concluded that the catalyst-coated surface is effective and essential to the practical aircraft application of endothermic fuels in flight-weight heat exchangers. Sicard et al. compared the catalytic cracking of n-dodecane over HZSM-5 and HY zeolite with thermal cracking in a stirred batch reactor heated up to 425 °C under pressure up to 15 MPa and found that, at lower temperatures (375400 °C), the conversion was approximately 1 time higher for cracking over the HZSM-5 catalyst than that for thermal cracking.14,15 Dardas and co-workers studied the catalytic cracking of n-heptane over Y-type zeolite under super- and subcritical conditions and observed the significant stabilization of the catalyst toward rapid deactivation even at relatively moderate pressure, which was explained by the extraction of coke by the superdense reaction fluid within the micropores of the catalyst.16,17 Meng et al. prepared a series of HZSM-5 coatings with different thickness and found that catalytic activity of n-dodecane for 30 min improves by 53.2% and that catalyst activity, deactivation, and adhesion strength were dependent upon the thickness of zeolite coatings.18,19 Xian et al. studied catalytic cracking of n-dodecane over HZSM-5 zeolite at 400 450 °C under super- and subcritical pressures (0.14.0 MPa) and developed a first-order Langmuir kinetic model with a novel decay function incorporating a supercritical extraction effect on catalyst stability.20 Regardless of those works, it is presently still Received: January 29, 2011 Revised: May 20, 2011 Published: May 23, 2011 2808
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Energy & Fuels unclear how the acidity influences the catalytic activity/stability of the zeolite catalyst for the catalytic cracking of supercritical hydrocarbons. Undoubtedly, the Si/Al ratio is one of the most important factors that determine the acid nature, amount, distribution, and thus, its catalytic activity and stability of zeolite.21,22 Therefore, it is very necessary to systematically study the effect of the Si/Al ratio on the catalytic activity and stability for the catalytic cracking of supercritical hydrocarbons and, thus, to obtain some helpful information on the catalytic activity/stability and acidity of the zeolite catalyst, which is also one of the important concerns for developing excellent catalyst coatings with high activity and stability for the advanced cooling technology. The objective of this work was to investigate the influence of the zeolite Si/Al ratio on the catalytic cracking of hydrocarbons under supercritical conditions. A series of catalyst coatings of HZSM-5 zeolites with different Si/Al ratios (25140) were prepared in the inner surface of stainless-steel tubes (Φ, 3 0.5 mm) through washcoating methods and characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature-programmed desorption of ammonia (NH3TPD). Catalytic cracking of supercritical n-dodecane (Pc = 1.81 MPa, and Tc = 385.05 °C) was used to study the catalytic activities and deactivation of the coated HZSM-5 with different Si/Al ratios under the conditions of 550 °C and 4 MPa. This work provided some useful information for the catalytic cracking mechanism of supercritical hydrocarbons and catalyst screening for the catalytic heat exchangers for the advanced aircrafts.
’ EXPERIMENTAL SECTION Materials. HZSM-5 zeolites (Si/Al ratios of 25, 50, 100, and 140 were assigned as Z25, Z50, Z100, and Z140, respectively) with the average particle size of 5 μm were purchased by the Nankai University Catalyst Plant (Tianjin, China). Before experiments, fresh zeolites were calcinated at 450 °C for 2 h, and the properties of the HZSM5 zeolites were listed in Table 1. Silica sol (23 wt %) was used as a binder to enhance zeolite particle attachment on the stainless-steel substrate, and n-dodecane (purity of 99%, J&K Scientific, Ltd.) was used as received. Zeolite Coating Preparation and Characterizations. HZSM-5 coatings were prepared by the washcoating method using colloidal silica water suspension as a binder. The coatings using HZSM-5 zeolites with Si/Al ratios of 25, 50, 100, and 140 were assigned as ZC25, ZC50, ZC100, and ZC140, respectively. Before the washcoating process, the stainless-steel tube was pretreated to remove adsorbed impurities. In the second step, the slurry containing 20 wt % HZSM-5 and colloidal silica water suspension and ethyl alcohols as the solvent were homogeneously dispersed by ballmilling to obtain coating slurry. Then, the slurry was used to coat stainless-steel tubes (Φ, 3 0.5 mm) by the washcoating method, leading to the formation of a uniform wall-coated layer. Finally, the prepared zeolite coatings were dried overnight at room temperature and then calcined at 600 °C for 2 h. The solid loading amounts on stainless-steel tubes were measured by gravimetry. The prepared coatings were characterized by SEM (FEI Nano-Sem 430 field emission gun scanning electron microscope) to observe the coating morphologies. XRDs (Philips X’PERT MPD diffractometer, Cu KR radiation) were used to investigate the influences of the binder introduction and coating treatment procedure on crystal structures of zeolites. The strong and weak acids of the catalysts were determined by NH3TPD in micromeritics 2910 (TPD/TPR). Previously, the samples were outgassed under a He flow (60 N mL/min), heating with a rate of 20 °C/min. After cooling to 100 °C, an ammonia flow of 30 N mL/min
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Table 1. Properties of HZSM-5 Zeolites Used in This Work average particle
BET surface
pore volume
Si/Al ratio
size (μm)
area (m2/g)
(cm3/g)
Z25
25
4.2 ( 1.0
283
0.175
Z50
50
4.8 ( 0.9
279
0.171
Z100
100
3.9 ( 1.1
301
0.245
Z140
140
4.4 ( 1.2
294
0.192
zeolite
was passed through the sample for 30 min. The physisorbed ammonia was removed by flowing He at 100 °C for 60 min. The chemically adsorbed ammonia was determined by increasing the temperature up to 600 °C with a heating rate of 10 °C/min. The ammonia concentration in the effluent He steam was monitored by a thermal conductivity detector. Infrared (IR) characterizations of ZSM-5 samples were performed on a Nicolet 710 Fourier transform infrared (FTIR) spectrometer as follows: Wafers of 10 mg/cm2 were degassed overnight under vacuum (103 Pa) at 673 K. The transmission spectra were recorded, and then pyridine (6 102 Pa) was admitted. After equilibration, the samples were outgassed for 1 h at increasing temperatures. After each desorption step, the spectrum was recorded at room temperature and the background was subtracted. The amount of Brønsted and Lewis acid sites was derived from the intensities of the IR bands at ca. 1450 and 1550 cm1, respectively. Catalytic Activity Experiments for Coatings. Catalytic cracking runs of hydrocarbon fuel were carried out using an as-prepared zeolite coated tube as the reactor. The experimential apparatus used to evaluate the catalytic activity of zeolite coatings was described in our previous paper.18 n-Dodecane was pumped with a high-performance liquid chromatography (HPLC) pump. The zeolite-coated tube was heated directly by direct current (DC) power, and a backpressure valve was used to keep the system pressure constant. The fuel outlet and wall temperatures were measured by K-type thermocouples. At the end of experiments, the cracking product was first cooled in the condenser and then flowed into a gasliquid separator through the backpressure control valve. Liquid samples were collected at an interval of 5 min to reduce the experimental errors in the material balance. The error of the mass balance was less than 2.5% between feeds and products involving gas, liquid products, and coke. Analysis Methods. Gas chromatography (GC) was used for product analysis and identification from the gas and liquid phases. The gas products were analyzed by SP3420 GC (Beijing Analytical Instruments Co., Ltd., China), using a flame ionization detector (FID) and an Al2O3/S capillary column (50 0.53 mm). The liquid products were identified by Agilent 7890A GC (Agilent Technologies, Inc., Santa Clara, CA) with a FID and a PONA column (50 m 0.20 mm). Cokes were analyzed through the temperature-programmed oxidation (TPO) method, according to Meng et al.18 Converson of n-dodecane, as an index of catalyst coating activity, was defined as the ratio of n-dodecane consumed in the reaction to n-dodecane fed. In addition, the cracking mechanism ratio (CMR) and ratio of mechanism (RM) were used to evaluate the reaction pathway, according to Wielers et al.23 and Meng et al.24,25 Two indexes were defined as below CMR ¼
C1 þ C2 iC4
RM ¼
iC4 nC4
where C1, C2, iC4, and nC4 were mole concentrations of methane, ethane and ethene, isobutane, and n-butane, respectively. 2809
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Figure 1. SEM top and side views of HZSM-5 coating (ZC140).
Table 2. Thickness and Solid Loading of HZSM-5 Coatings Used in This Work coating
relative crystallinity
mean thickness
solid loading
sample
zeolites
of zeolites
(μm)
(mg/cm2)
ZC25
Z25
92
8.4 ( 2.2
1.55 ( 0.15
ZC50
Z50
93
8.9 ( 1.9
1.63 ( 0.10
ZC100
Z100
94
7.8 ( 2.6
1.56 ( 0.14
ZC140
Z140
96
8.1 ( 2.6
1.57 ( 0.12
Figure 3. Conversion versus TOS for coatings of HZSM-5 with different Si/Al ratios (4 MPa, 550 °C, and 30 min).
Figure 2. XRD patterns of HZSM-5 coatings.
’ RESULTS AND DISCUSSION Characterization of HZSM-5 Coatings. Figure 1 typically presents the SEM top and side views of the top view of ZC140 coating on the tube surface by repeated washcoating. Surface morphology of the coating shows homogeneous and compact film with zeolite particles anchored onto the surface (see Figure 1a). Figure 1b shows that the mean thickness of the coating is approximate 8.0 μm. Table 2 lists the detailed properties of the prepared coating. For different HZSM-5 coatings with different Si/Al ratios, close solid loading amounts (1.55 1.63 mg/cm2) and coating thicknesses (7.878.41 μm) were obtained, indicating that the Si/Al ratio of zeolites with similar
crystal size distributions has a slight effect on the coating properties by a similar coating process. Figure 2 gives the XRD patterns for ZC25, ZC50, ZC100, and ZC140 coatings. The samples show sharp reflections over the whole 2θ range, indicating that they contained a crystalline phase. The XRD pattern shows peaks at ranges of 2θ = 79° and 2325° corresponding to the specific peaks of the typical ZSM-5 phase that shows that each coating presents typical characteristics of the MFI-type zeolite. The degree of crystallinity was determined from the peak area between 2θ = 22° and 25° based on the highly crystalline micro ZSM-5 sample as the reference, as reported in Table 2. It is found that higher relative crystallinities (greater than 92%) of HZSM-5 in coatings are also observed after the introduction of SiO2 binders, implying that the introduction of SiO2 binders does not significantly destroy the crystal structures of the HZSM-5 zeolite. There is also no new peak appearing in the XRD patterns, implying the SiO2 binders in coatings may exist in the form of an amorphous phase. Catalytic Activities of Zeolite Coatings. The catalytic cracking conversion of n-dodecane was used to represent the catalytic activities of zeolite coatings. Figure 3 presents the catalytic cracking conversion of surpercritical n-dodecane as a function of time-on-stream (TOS) for each coating (ZC25ZC140) in a reaction time of 30 min (550 °C and 4 MPa). Generally, the catalyst activities of all of the coatings decrease rapidly in the 2810
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Table 3. Mass Selectivity for n-Dodecane Cracking over HZSM5 Coatings with Different Si/Al Ratiosa products
a
ZC25
ZC50
ZC100
ZC140
hydrogen
0.15
0.21
0.27
0.34
methane ethane
0.69 1.85
0.71 2.12
0.75 2.04
0.68 1.93
ethylene
2.39
2.75
3.12
3.14
propane
5.16
6.71
7.95
8.64
propylene
11.55
11.79
12.52
12.64
isobutane
2.46
2.89
3.12
3.23
n-butene
5.31
5.37
6.93
6.85
trans-2-butene
4.23
4.31
5.53
5.45
n-butene isobutene
2.16 0.85
2.61 0.78
3.28 1.61
3.72 1.88
cis-2-butene
3.16
3.18
3.01
2.94
n-pentane
4.23
4.24
3.92
3.81
isopentane
1.69
2.05
2.15
2.32
pentene
8.85
8.97
9.51
9.55
n-hexane
3.77
3.74
3.28
3.47
hexene
8.78
8.54
8.11
7.87
benzene n-heptane
0.31 3.23
0.35 3.25
0.43 3.22
0.48 3.62
heptene
4.7
4.1
2.85
2.51
toluene
0.54
0.49
0.91
1.01
n-octane
2.39
2.12
2.15
2.41
octene
4.62
4.02
2.79
2.36
n-nonane
2.16
1.91
1.45
1.21
nonene
4.54
3.88
2.63
2.41
n-decane decene
0.62 4.39
0.56 3.74
0.43 2.53
0.39 2.07
n-undecane
3
2.68
2.04
1.74
undecene
1.69
1.41
0.91
0.77
dodecene
0.38
0.35
0.27
0.29
coke
0.02
0.03
0.05
0.06
Reaction conditions: 4 MPa, 550 °C, and 30 min.
initial stage (015 min) and then gradually drop at a relatively lower deactivation rate. At the first 5 min, the conversion of n-dodecane reaches up to 0.30 in the presence of ZC140 coating, which is significantly greater than that for the other coatings (0.27 for ZC100 and 0.21 for both ZC50 and ZC25). This indicates that the Si/Al ratio has a significant effect on the catalytic activity of coatings. In the early stages of the reaction, the slope of the conversion curve increased apparently as the Si/Al ratio decreased, which is a result of rapid deactivation. At 1520 min, the conversion of n-dodecane over ZC140 coating falls from 0.30 to 0.20 (ca. 33%), while ca. 50, 38, and 37% activity losses were observed for ZC25, ZC50, and ZC100 coatings, respectively. Therefore, it is concluded that the catalytic activities and stabilities of coatings for catalytic cracking of n-dodecane under the interesting experimental conditions decrease by the following order: ZC140 > ZC100 > ZC50 > ZC25. Table 3 compares the gas product distribution of n-dodecane catalytic cracking over coatings with various Si/Al ratios. The major gaseous products were C3 and C4 species, especially significant contents of branched products (isobutane and isobutene), and only a small quantity of C1 and C2 species were observed in each gas product. This presents that the catalytic
Figure 4. Reaction indexes of (a) CMR and (b) RM for n-dodecane transformation over HZSM-5 coatings with different Si/Al ratios as a function of the stream time (4 MPa and 550 °C).
cracking is prominent according to the mechanism of paraffin catalytic cracking over zeolites and that the formation of C3 and C4 is much more favored by catalytic cracking via the carbeniumion mechanism than the formation of C1 and C2. The aromatic selectivities of benzene and toluene also increase with the Si/Al ratio, but only a slightly effect on the selectivities of liquid products above C8 is observed. The CMR and RM were also used to show the effect of the Si/Al ratio on the reaction mechanism, as shown in Figure 4. Figure 4a presents the relations of CMR as a function of the reaction time and Si/Al ratio of zeolites. The CMR, which is defined as the ratio of dry gases (methane, ethane, and ethylene) to isobutane in the gas products, is used to measure the ratio of monomolecular to bimolecular types of cracking, because C1 and C2 are typical products from monomolecular protolytic cracking, while iso-C4 is a typical product formed by β-scission of branched products. A high value of this index points to a relatively high contribution of the protolytic cracking route, whereas a low value indicates that the classical β-scission route is the main cracking pathway.23 For the coatings used in this work, CMRs were very low in the 3 min after the beginning and then increase gradually from 60 to 300 at the end of 30 min. TOS for ZC25 and ZC50, unlike for ZC100 and ZC140 CMRs, still kept below 40 during the whole reaction time. This further confirmed that ZC25 and ZC50 indeed exhibit high catalytic cracking activity in the first 2 min. Because of the difficulty in discriminating the initial activity using CMR, RM proposed by 2811
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Table 4. Acid Density of HZSM-5 Zeolites acid density (mmol of NH3/g) BþL
B
L
Si/Al ratio total weak strong total weak strong total weak strong of zeolite 25
1.22 0.51
0.71
1.12 0.45
0.67
0.10 0.06
0.04
50
0.78 0.33
0.45
0.69 0.27
0.42
0.09 0.06
0.03
100
0.51 0.26
0.25
0.34 0.14
0.20
0.17 0.12
0.05
140
0.38 0.24
0.12
0.13 0.04
0.09
0.23 0.20
0.03
Figure 5. NH3TPD profiles of HZSM5 zeolites with different Si/Al ratios.
Meng et al.25 was also used to describe the relative contribution of the carbonium ion and the free radical mechanisms, as shown in Figure 4b. The RM value of ZC25 at the beginning of the reaction (TOS = 1 min) is 0.52 and greater than that for the other coatings, which shows high initial catalytic activity. After 12 min of reaction, the RM value drops sharply below 0.25 and varied slightly after 18 min, indicating that thermal cracking plays an important role after deactivation. The RM value of ZC140 at the beginning of the reaction (TOS = 1 min) is ca. 0.49 but still greater than 0.3 after 30 min of reaction, indicating significant contribution of catalytic cracking. Generally, the Si/Al ratio of zeolite is one of the most important parameters influencing its acid nature and distribution and, thus, the catalytic cracking activities and stabilities of HZSM-5 zeolites.2628 To further explain the above results, characterization of the HZSM-5 acidities was performed, as shown below. Acidities of HZSM-5 Zeolites. FTIR and NH3TPD techniques were used to study the acidities of HZSM-5 in the coatings, such as total amount, nature, and distributions, and to find the possible explanations for the above experimental results. Figure 5 presents NH3TPD profiles of zeolite samples with different Si/Al ratios. There are two adsorption peaks for the HZSM-5 zeolites, one centered at 190220 °C and the other centered at 375445 °C, corresponding to the weak and strong acid sites, respectively. With an increasing Si/Al ratio from 25 to 140, both peak areas and maximum temperatures for each profile reduce remarkably, suggesting the depressed acid amount and strength with an increasing Si/Al ratio. Because the weight of all samples was set identically, the total acidic densities were calculated and listed in Table 4. Figure 6 presents FTIR spectra of adsorbed pyridine on the HZSM-5 samples with different Si/Al ratios in the range of 16001400 cm1. Adsorption bands at 1450 and 1540 cm1 are assigned to the chemisorbed pyridine, which correspond to the Br€onsted and Lewis acid sites. Figure 6a illustrates a typical spectrum for Z140 upon pyridine adsorption at room temperature and outgassing at 150 and 300 °C (corresponding to weak and strong acid sites from NH3TPD profiles). When the outgassing temperature is increased, both amounts of Br€onsted and Lewis acid sites decrease significantly, while Lewis sites decrease more sharply. This result indicates that more Lewis sites belong to weak acids and strong acid sites occupy a larger part in
Figure 6. Pyridine-absorbed FTIR spectra of HZSM-5 zeolites with different Si/Al ratios.
total Br€onsted sites. Similar trends were observed for other samples with different Si/Al ratios (results not shown). It can be seen from Figure 6b that, with an increasing Si/Al ratio, Br€onsted acid sites gradually decreased, while Lewis acid sites show a slight increase. The Br€onsted/Lewis acid site ratio and its strong/weak acid site ratio were calculated from the ratio corresponding to the peak area of adsorbed pyridine FTIR spectra. The detailed distributions of acid strength for different types of acid sites are listed in Table 4. The amount of total acid sites exhibits a significant decay trend with the increase of the Si/Al ratio from 25 to 140 (see Table 4), wherein the total strong acid decreases about 85% (from 0.71 to 0.12 mmol of NH3/g) and the total weak acid decreases only about 50%. For the Br€onsted acid, both strong and weak acids 2812
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Energy & Fuels decay sharply at a similar level. However, the total Lewis acid increases from 0.10 to 0.23 mmol of NH3/g (about 2 times) with the Si/Al ratio, wherein the weak Lewis acid increases with the Si/Al ratio and the strong Lewis acid showed almost no significant changes. Similar results were also reported by Shirazi et al.,21 Zhu et al.,26 and Ding et al.28 The increasing amount of those active sites is conductive to enhance the initial cracking activity of the catalyst. However, increasing these acid sites, particularly the strong acid sites, will inevitably result in the formation of cokes, which will cause catalyst deactivation by hindering the access of reactant molecules to active sites. Therefore, to obtain a high activity and stability during the whole reaction time, it is necessary to control the total amount of acid sites and the proportion between Br€onsted and Lewis sites. From acid characterization and catalytic activity experiments, it is obvious that the ZC140 coatings possess the higher activities and stabilities of catalytic cracking of hydrocarbon, which is in accordance with its relatively higher Lewis acid and lower total acid (Br€ onsted acid) sites. van Bokhoven et al. studied the n-hexane cracking over different zeolites, including HZSM-5, HY, and HMOR, and pointed out that the enhanced adsorption of n-hexane on Lewis acid sites promotes the reaction rate by a factor of 25, although the number of Br€ onsted acid sites deceased.29 Therefore, the Lewis acid sites are responsible for accelerating the paraffin cracking and excellent stability. The other possible but more important reason is the weakened deactivation effect from coke in the reaction. The ZC25 coating may exhibit high initial catalytic activities, which rapidly deactivate in several minutes (less than 5 min) because of the formation of cokes ascribed to higher total acid and strong Br€onsted acid sites. While the coking rate of ZC140 drops significantly because of fewer amounts of strong acid sites, especially the strong Br€onsted acid sites, it gives stable activity because of more active sites in the absence of cokes, although the initial active sites on those high Si/Al ratio coatings may be less. Cokes over HZSM-5 Coatings. Carbonaceous deposits or coke, as undesired products of most hydrocarbon transformations over acid zeolites, impedes reactant molecule contact with the active sites of catalysts because of the formation on both outer surfaces and internal channels of zeolite catalysts and results in catalyst deactivation. TPO is generally used to determine the oxidative activities of carbonaceous species and, thus, to provide some information about chemical characters of carbonaceous deposits. Figure 7 presents TPO curves of carbonaceous deposits on HZSM-5 coatings with different Si/Al ratios. There are two peaks in the range of 250450 °C and 450650 °C on the TPO curves. According to the previous work of our group, the lowtemperature peak is attributed to the adsorbing saturated coke and the high-temperature peaks are mainly attributed to the conversion cokes from catalytic and thermal cracking.18 Clearly, the amount of adsorbing saturated coke remains almost constant, while the conversion coke amounts increase sharply with an increasing the Si/Al ratio. When catalyst activities and coke amounts are compared, an interesting fact is found that HZSM-5 with high Si/Al ratios produces more cokes during the reaction but simultaneously maintains the higher stable activity, which implies that deactivation mechanisms of coatings with different Si/Al ratios are also different. To further evaluate the influence of coke upon the catalyst activity, the variation of coke and conversion loss per coke with the Si/Al ratio is presented in Figure 8. With the increase of the
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Figure 7. TPO profiles of cokes over different HZSM-5 coatings (4 MPa, 550 °C, and 30 min).
Figure 8. Conversion loss rate caused by cokes for HZSM-5 coatings as a function of the Si/Al ratio (4 MPa, 550 °C, and 225 g of n-dodecane).
Si/Al ratio, the amount of coke increases from ca. 4 to 16 mg, by 4 times. However, an adverse tendency is observed for the activity loss per coke, which decreases more than 50% from 0.135 to 0.06 mg1. Possible Mechanisms of Coke Formation in the Micropore of HZSM-5. To explain the interesting results obtained above, a possible formation mechanism of cokes in the micropore of HZSM-5 with different Si/Al ratios is proposed considering the interaction of catalytic cracking, coking, and supercritical extraction, as given below. For the ZC25, the deactivation rate was very high because of the higher total acid and strong Br€onsted acid sites. Therefore, it appears that pore-mouth plugging of zeolite is occurring because of the progressive buildup of coke from the pore mouth to the center under the diffusion-controlled main reaction rate, which decreases coke laydown and abrupt loss of activities in 1 or 2 min, regardless of the extraction of supercritical hydrocarbons. This is the reason why there are the only little cokes deposited on the ZC25. With the increasing Si/Al ratio, both the catalytic cracking and coking rate drop as a result of the decreasing concentration of total and Br€onsted acid sites and the prevention of deactivation ascribed to supercritical extraction gradually becomes more important. This lead to gradually uniform profiles of cokes in the micropores rather than rapid pore-mouth plugging. When the Si/Al ratio reaches 140, the more uniform buildup of coke in 2813
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Energy & Fuels the pore took place under the kinetically controlled main reaction rate, owing to low Br€onsted acid sites, leading to more cokes by gradually filling zeolite pores. Baptist-Nguyen and Subramaniam also showed similar results in their mathematical modeling of coking and activity of the porous catalyst in supercritial reaction media.30
’ CONCLUSION Catalytic cracking of supercritical n-dodecane (4 MPa and 550 °C) in the stainless-steel microchannel was experimentally in the presence of wall-coated HZSM-5 zeolites with different Si/Al ratios from 25 to 140. It is found that catalytic cracking activities and stabilities of HZSM-5 coatings increase with the increasing Si/Al ratio by the following order: ZC25 < ZC50 < ZC100 < ZC140. FTIR and NH3TPD characterization of zeolite acidity shows that the increasing catalytic performance may be a result of the increasing Lewis acid and decreasing Br€onsted acid amounts. TPO characterization of cokes deposited on the HZSM-5 coatings indicates that rapid pore-mouth plugging of ZC25 possibly caused almost complete deactivation by a small amount of coke because of the higher coking rate and, thus, diffusion-controlled regimes, while for ZC140, the coking rate is so slow that gradual and uniform buildup of 3-time cokes in the pore undergoes kinetically controlled regimes. It should be noted that real jet fuel also contains many branched and cycloparaffins and aromatic hydrocarbons other than normal paraffins and that, for different hydrocarbon types, the catalytic cracking activity of zeolite was naturally dependent upon their pore structure and acidities. For instance, HZSM-5 is favorable for the catalytic cracking of normal paraffins in jet fuel but has almost no effect on the cracking of the branched and cycloparaffins. Now studies are still under way in our lab to further optimize the zeolite catalyst for real jet fuel. ’ AUTHOR INFORMATION Corresponding Author
*Fax: þ86-22-27402604. E-mail:
[email protected]. Present Addresses †
Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States.
’ ACKNOWLEDGMENT The authors also gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant 90916022) and the Programme of Introducing Talents of Discipline to Universities (B06006).
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’’ DEDICATION This paper is dedicated to Prof. Zhentao Mi at the School of Chemical Engineering and Technology, Tianjin University, for his 70th birthday. Guozhu Liu is grateful to him for introducing such an interesting topic and for his constant encouragements and timely inspirations over the last 10 years. ’ REFERENCES (1) Edwards, T. Combust. Sci. Technol. 2006, 178 (1), 307–334. (2) Edwards, T. J. Eng. Gas Turbines Power 2007, 129 (1), 13–20. 2814
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