Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated

Article pubs.acs.org/EF

Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated HZSM-5 Zeolites with Micro- and Nanocrystal Sizes Guozhu Liu,* Ganglei Zhao, Fanxu Meng,† Shudong Qu, Li Wang, and Xiangwen Zhang Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: Wall-coated HZSM-5 with micro- and nanoscale crystal sizes were prepared and coated to the inner surface of SS304 stainless-steel tubes using washcoating methods. It was found that nanoscale HZSM-5 zeolite slurry has different rheological properties, washcoat loadings, and adhesions with microscale HZSM-5. Catalytic activities of the prepared micro- and nanoscale HZSM-5 zeolite coatings were studied using catalytic cracking of supercritical-phase n-dodecane (550 °C and 4 MPa), indicating that catalytic cracking activity and stability were remarkably improved more than 1 time by nanoscale HZSM-5 coatings compared to microscale HZSM-5 coatings. Acid and pore structure characterization showed that the better performance of nanoscale HZSM-5 coating may be attributed to the shorter diffusion length of the micropore, the higher diffusion rate of supercritical n-dodecane in the intracrystal mesopore, and the special acid nature of nanoscale HZSM-5 coatings. coking at higher temperatures (>550 °C). Qu and co-workers13 prepared a series of wall-coated HZSM-5 zeolites (ca. 3−5 μm) with a Si/Al molar ratio of 25−140 by washcoating methods and found that catalytic cracking activities and stabilities of the coatings increase in the following order: ZC25 < ZC50 < ZC100 < ZC140 (with the numbers representing the Si/Al ratio). Regardless of the above achievements using various microscale zeolites, there is still much work to be done to design some new catalysts with higher and stable catalytic cracking activity because of the extreme working conditions (resident time less than 0.1 s and high temperature and pressure greater than 650 °C and 4.0 MPa) of fuel undergone in the cooling channel. Typically, zeolites are industrially manufactured with micrometer-sized crystals or crystal aggregates. Nanoscale zeolites are zeolites with crystal sizes of less than 100 nm.14 As the zeolite crystals size is scaled down below 100 nm, the diffusion channel length of nanocrystals is remarkably decreased relative to micrometer-sized zeolites (see panels A and B of Scheme 1), which is favorable for improving catalytic activity and stability by reducing diffusion resistance.15,16 Several research groups observed the high catalytic activity with a long lifetime of nanoHZSM-5 in the methanol to propylene reaction,17 acetone to olefins (such as ethylene, propylene, and isobutylene),18 aromatization of normal alkanes,19 etc. Because of the excellent performances of nanoscale ZSM-5 zeolite on catalytic activity, the use of a nanoscale zeolite as a catalyst would be a promising method upon preparing coatings with high catalytic ability. Washcoating (or dip-coated) is a classic method to prepare zeolite coating from stabilized slurry in the most easy, fast, and simple way, especially on a structured surface, such as microreactor and monolith catalysts.20 In the literature, the studies on microscale zeolite coating had been extensively

1. INTRODUCTION The use of hydrocarbon fuel as a source of energy and coolant is now well-established for the future hypersonic aircraft.1−5 Recently, keen interest remains in developing novel catalysts and the corresponding loading technologies in microchannels of heat exchangers, because of their great positive effect on the cracking rate and, thus, the improvement of the heat sink of the hypersonic fuels.1,6,7 It is generally accepted that a wall-coated catalyst could significantly reduce the pressure drop and thermal resistance and formations of carbonaceous deposits, especially the filamentous coke from the metal catalysis of the wall surface.8,9 In the practical view of future applications, therefore, developing a wall-coated catalyst with high activity, stability, and adherence is one of the most important concerns for the advanced cooling technology. In the cooling system of an advanced aircraft, the hydrocarbon fuel is under high pressure (3.4−6.9 MPa) and high temperature (above 450 °C), i.e., the supercritical conditions.10 Thus far, much work has been performed on the catalytic cracking of a supercritical hydrocarbon over the wall-coated catalysts. Sobel et al.1 and Huang et al.2 first demonstrated that JP-7 and JP-8 + 100 cracking in commercial zeolite (SAPO-34 and Y zeolite)-coated tubes yielded substantial heat sinks of 3.41 and 2.91 MJ/kg, respectively. Fan et al.10 also studied the catalytic cracking of China no. 3 aviation kerosene over wall-coated HZSM-5 zeolite (ca. 3−5 μm) under conditions similar to the practical scramjet applications and found that, at a fuel temperature of ∼1050 K, the total heat sink reached ∼3.4 MJ/kg, in which the chemical heat sink accounted for ∼1.5 MJ/kg. Sicard et al.11 also prepared HZSM-5 and HY zeolite coatings for the catalytic cracking of supercritical n-dodecane. Recently, our group also did some work toward developing a HZSM-5 coating with high activity and stability. Meng et al.8,12 reported that catalyst activity, deactivation, and adhesion strength were dependent upon the thickness of HZSM-5 zeolite (ca. 3−5 μm) coatings and that zeolite is also beneficial for reducing the metal catalysis © 2011 American Chemical Society

Received: September 27, 2011 Revised: December 22, 2011 Published: December 24, 2011 1220

dx.doi.org/10.1021/ef201467r | Energy Fuels 2012, 26, 1220−1229

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Scheme 1. Micro- and Nanoscale HZSM-5 and the Zeolites Coatinga

a

A, micro-HZSM-5; B, nano-HZSM-5 aggragates; C, micro-HZSM-5 coating; D, micro-/nano-HZSM-5 coating; and E, nano-HZSM-5 coating.

Table 1. Properties of Micro- and Nano-HZSM-5 Zeolites Used in This Worka

a

samples

crystal size (μm)

crystallinity (%)

NZ MZ

∼0.05 4−5

88 100

SBET (m2 g−1)

Smic (m2 g−1)

Vt (cm3 g−1)

366.5 197.2 0.277 325.7 189.7 0.198 pore size distribution: volume in pores of diameter (Ǻ )

Vmes (cm3 g−1)

Vmic (cm3 g−1)

0.187 0.112

0.090 0.086

issues

up to 20

20−30

30−60

60−100

100−200

200−500

>500

NZ MZ NZ-HZ

0.0876 0.0803 0.0073

0.0238 0.0172 0.0166

0.0316 0.0273 0.0043

0.0345 0.0087 0.0258

0.0363 0.0100 0.0263

0.0324 0.0240 0.0084

0.0308 0.0305 0.0003

SBET and Smic are the specific areas of the total pore and micropore. Vt, Vmic, and Vmes are the volumes of the total pore, micropore, and mesopore.

reported;21−23 however, few reports was concerned on the preparation of nanoscale zeolite coatings and the corresponding issues, including slurry rheological behaviors, loading properties, microstructures, adhesion strength, etc. Therefore, it is very necessary to study those issues of nanoscale HZSM-5 coatings for the catalytic cracking of supercritical hydrocarbons. The objective of this work is to gain more information on the coating slurry properties and their catalytic cracking of supercritical hydrocarbon fuel of the nano- and microscale HZSM-5 zeolites. The coating properties, loading percentage, and adhesion were studied to enrich the knowledge of the slurry and coating behaviors of the nanoscale HZSM-5 zeolite. We also compared the catalytic performance of micro- and nano-HZSM-5 coatings using catalytic cracking of n-dodecane under supercritical conditions. This work provides some useful results for the design and development of catalytic heat exchangers in the fuel-cooled thermal management technology of advanced aircrafts.

Dodecane, with 99.5% purity, was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) 2.2. Preparation of Zeolite Coating. The preparation of zeolite coating consists of two steps: preparing the coating slurry and coating the slurry onto the tube wall. The coating slurry contains the following components: pretreated HZSM-5 zeolite, the binder and alcohol to modify viscosity, particle dispersion, and elimination of the suspension excess during the blowing stage. Those three materials were mixed first and then milled by wet ball milling to enhance the stability of the particles, which was necessary to destroy the particle aggregation because of van der Waals forces.25 Before the washcoating process, the stainless-steel tube was pretreated to remove adsorbed impurities and to improve the surface roughness of the stainless-steel substrate. The prepared slurry was deposited onto the SS304 stainless-steel tubes (length, 300 mm; inner diameter, 2 mm; wall thickness, 0.5 mm) by the washcoating method. Then, the excess slurry was removed by blowing air, leading to the formation of a uniform wall-coated layer. The coating was subsequently dried overnight and calcined at 600 °C for 2 h, leading to a well-adhered coating. The coatings using microand nanoscale HZSM-5 zeolites were assigned as MC and NC, and the coating using the blender of 60% micro- and 40% nanoscale HZSM-5 zeolite was assigned as ZC(0.40). Viscosity of the zeolite slurries was determined by a stresscontrolled AR-1000 rheometer (TA Instruments, U.K.), equipped with a 40 mm parallel plate geometry in the shear rate range of 0.03−1088 s−1. ζ potential values for the micro- and nanoscale HZSM-5 zeolite crystals dispersing in distilled water at a solid concentration of 1 wt % were determined by a Zeta Probe Analyzer (Zetasirer nano ZS, Malvern, U.K.) at room temperature. 2.3. Characterization of Zeolite Power and the Coatings. The X-ray diffraction (XRD) patterns of the samples were recorded on a

2. EXPERIMENTAL SECTION 2.1. Materials. The nanoscale HZSM-5 zeolite (Si/Al molar ratio of 25, assigned as NZ), with the average crystal size of ca. 50 nm, was self-prepared following the method by Bao et al.24 The microscale HZSM-5 zeolite (Si/Al molar ratio of 25, assigned as MZ), with the average crystal size of 4−5 μm, was purchased from the Nankai University Catalyst Plant (Tianjin, China). The properties of the HZSM-5 zeolites were listed in Table 1. Silica sol (23 wt %, self-made using hydrolysis of tetraethyl orthosilicate) was used as a binder to enhance zeolite particle attachment on a stainless-steel substrate. n1221


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Rigaku D-max 2500 V/PC X-ray diffractometer (Rigaku Corporation) using a Cu Kα radiation source (40 kV and 200 mA). The surface area and pore size distribution were determined by a Brunauer−Emmett− Teller (BET) method from N2 adsorption data at −190 °C with a CHEMBET-3000 instrument, wherein the micropore volume was calculated using the t-plot method and the mesopore volume was calculated using the Barrett−Joyner−Halenda method. The acid properties of the nano- and microscale HZSM-5 zeolite samples were obtained on a Quantachrome CHEMBET 3000 using the ammonia temperature-programmed desorption (NH3−TPD) method. The zeolite loading amounts on the stainless-steel tubes were measured by the mass change of the tubes before and after washcoating using an electronic balance (AB204-S, 0.0001 g, Mettler Toledo) with an uncertainty of less than 5%. The coating thicknesses were characterized by scanning electron microscopy (SEM, FEI NanoSem 430 field emission gun scanning electron microscope) with an uncertainty of less than 15%. The ultrasonic test26−28 and thermal shock test27,29,30 were used to characterize the adherence strength of the zeolite coatings with the substrate. In the ultrasonic test, the zeolite coatings were treated in an ultrasonic vibrator in the alcohol solvent for 30 min at 60 kHz and dried overnight. Then, the mechanical strength of the coatings was characterized by the weight loss. The thermal shock test was performed by the following procedure: heating the zeolite coatings to 600 °C in 1 min and then naturally cooling it to room temperature. This process was repeated 3 times, and after that, the weight loss was measured. The reproducibilities of the ultrasonic test and thermal shock test were 95 and 89%, respectively. 2.4. Catalytic Test. Cracking experiments were carried out in a flowing reactor consisting of an electrically heated tube, as described in previous studies by our group.8,12 In a typical experimental run, the 304 stainless-steel tubes (length, 300 mm; inner diameter, 2 mm; wall thickness, 0.5 mm) coated with zeolites were used as the reactor. The reactant, n-dodecane, was fed using a high-performance liquid chromatography (HPLC) pump with a flow rate of 10 mL/min. The tube reactor was heated by direct current power up to 550 °C under 4 MPa. The wall temperatures of the tube reactor were measured by K-type thermocouples, and the pressure was maintained by a backpressure valve. The cracked fuel was cooled by a condenser and then flowed into a gas−liquid separator. Each sample was collected in 5 min to ensure enough weight of gas and liquid samples for the materials balance. For each test condition, two individual runs were performed to ensure the reproducibility of experimental results. The gas products collected were analyzed on SP-3420 with a flame ionization detector (FID) and an Al2O3/S capillary column (50 m × 0.53 mm), while the liquid products were analyzed by HP4890 gas chromatography with a FID and a PONA column (50 m × 0.53 mm). Cokes were analyzed through the temperature-programmed oxidation (TPO) method, according to Meng et al.8 The conversion of n-dodecane, selectivity of product i, and deactivation rate of catalyst coating are defined as follows:

X=

w0 − w × 100% w0

Si =

mi × 100% w0 − w

Rd =

Figure 1. XRD patterns of micro- and nano-ZSM-5 samples.

framework structure. The degree of crystallinity reported in Table 1 is determined from the peak area between 2θ = 22° and 25° based on the highly crystalline MZ sample as the reference. The apparent crystallinity of NZ is lower than that of MZ because of the occurrence of extinction effects caused by the coexistence of the particles with smaller sizes, which were also observed by Firoozi et al.17 and Tago et al.18 The SEM images of NZ and MZ in powder are given in Figure 2. SEM images show that the aggregation sizes of MZ and NZ are about 5−50 and 2−5 μm, respectively. In addition, MZ has well-formed rounded-boat crystals, with a mean crystal size of about 4−5 μm, while the primary crystallite size of NZ is approximately 50 nm. Figure 3 provides the adsorption−desorption isotherms of nitrogen at the liquid nitrogen temperature. Generally, the two curves exhibited a type IV isotherm with the capillary condensation at the relatively high pressure between 0.7 and 0.9 according to the International Union of Pure and Applied Chemistry (IUPAC) classification, suggesting that the samples possess significant mesoporous features. For all points of p/p0 from 0.0 to 1.0, the pore volume of the NZ sample is higher than that of the MZ sample, especially the ring area of the NZ sample formed in the adsorption−desorption process (p/p0 from 0.45 to 1.0) is obviously larger than that of the MZ sample, indicating that the number of mesopores in the NZ sample is relatively high compared to that in the MZ sample. This may be attributed to the presence of intercrystalline mesopores (or voids) in the NZ aggragates, as shown in Scheme 1B. Similar results had been reported by Viswanadham et al.31 and Pavla et al.32 The pore size distribution calculated using the Barrett−Joyner−Halenda method showed that the mesopore volume of NZ was 0.187 cm3/g, which is significant higher than that of MZ (0.112 cm3/g), particularly for the pore size in the range of 60−100 Ǻ (see Table 1). The BET surface areas of NZ and MZ are 366.5 and 325.7 m2/g, respectively. The pore volume and surface area exhibited by NZ are in agreement with the literature findings.33 3.2. Properties of Washcoating Slurries of Micro- and Nanozeolites. 3.2.1. Particle Size Distributions. The particle size distribution of the MZ and NZ slurries from dynamic light scattering (DLS) is presented in Figure 4. Clearly, there is a bimodal distribution for the NZ aggregates after milling, and nearly 40% of the NZ aggregates are reduced to less than 1 μm; however, the fine fraction does not reach the primary crystal size. MZ aggregates are reduced to the average crystal size of 4−5 μm (in well agreement with the results of

Xt = 2.5 − Xt = 27.5 × 100% Xt = 2.5

where w0 and w represent the mass of the fed n-dodecane and the mass of the n-dodecane after cracking and mi is the mass of the product i in the cracked n-dodecane. Xt=2.5 and Xt=27.5 are n-dodecane conversions at time on steam of 2.5 and 27.5 min, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterizations of Parent Zeolites. The powder XRD patterns of the samples are shown in Figure 1. The samples exhibit the typical XRD patterns of the ZSM-5 1222


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Figure 2. SEM photographs of micro- and nano-HZSM-5 zeolites: (a and c) MZ and (b and d) NZ.

Figure 2c) and reach the primary particle size, which may result from the lower energy of interaction between these crystals. 3.2.2. Rheological Properties of Slurries. Viscosity, a fundamental property of the slurry, strongly affects the properties of the coating, such as loading, thickness, and quality of the coating in terms of homogeneity and reproducibility.34,35 The slurry viscosities with the changing of the shear rate were measured to obtain more information on the rheological properties of MZ and NZ slurries, as plotted in Figure 5a. Generally, the viscosities of all suspensions are found to decrease with an increasing shear rate, and the viscosities of the NZ slurry are higher than that of the MZ slurry. Both viscosities of MZ and NZ slurries decrease rapidly, and then the curves tend to be steady with an increasing shear rate. Similar results were reported by Mitra et al.9 This can be well-explained as follows: for a given solid concentration in the slurry, the number of NZ is much larger than MZ ascribed to a smaller aggregate size, which sharply increases the number of particle− particle interactions of the van der Waals force. In addition, the electrostatic force (repelling one from another) between the particles in the NZ slurry is relatively low (ζ potential is −35 mV; see Table 2). Figure 5b also shows that the viscosities of NZ slurries were higher than those of MZ slurries in a wild range of zeolite content. As shown in Figure 5b, the solid content is one of the most important factors on the slurry viscosity. This result agrees well with the observations in the literature.36−38 3.3. Solid Loading. The solid loading of slurries, as an index of coating reproducibility and controllability, strongly depends upon the slurry viscosity,34,35 number of immersions,37 etc. The variation of the loading percentage with different zeolite contents is shown in Figure 6a. Generally, solid loading

Figure 3. Nitrogen adsorption isotherms of micro- and nano-ZSM-5 zeolites.

Figure 4. Particle size distribution of micro- and nano-ZSM-5 zeolite slurries (zeolite/binder/solvent, 20:20:60; ball milling, 180 min). 1223


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Figure 5. Slurry viscosities of micro- and nano-HZSM-5 as a function of the (a) shear rate and (b) zeolite content (solvent/binder, 1:1; shear rate, 1000 s−1).

Figure 6. Loading percentage of slurries as a function of the (a) zeolite content and (b) NZ content in zeolite composition (a, zeolite/binder = 1:1; b, zeolite/binder/solvent = 20:40:40; 4 times washcoating).

Table 2. Slurry and Coating Properties of Micro- and NanoHZSM-5 Zeolites for Catalytic Cracking of Supercritical nDodecane

size situation in the slurries. A large number of small crystals in the NZ slurry attached to the substrate, and the hollows or gaps on the coating surface would be filled rapidly to form a smooth surface. However, for the MZ coating, big crystals with a size of about 5 μm take up a large percentage of the slurry but without enough number of small particles to fill the hollows and gaps, leading to a rough surface. Therefore, it is necessary for the coating slurry to have a certain number of small particles to produce a coating with a smooth surface. Then, we mixed the nano- and microscale zeolites together to obtain a relatively smooth surface (Figure 7c). As expected, once there was a certain number of small particles in the slurry, the roughness of the coating surface was decreased significantly. It means that the method of preparing coatings by mixing zeolites of different particle sizes has the ability of improving the homogeneity of the zeolite coating. The side views of MZ and NC coatings are shown in panels b, d, and f of Figure 7. The thicknesses of MC, ZC(0.40), and NC coatings were given in Table 2. It is also observed that the particle size in the coating decreases in the following order: MC > ZC(0.40) > NC based on the schematic diagram of the coatings drawn in panels C, D, and E of Scheme 1. 3.5. Adherence Strength. Adhesion of the zeolite coating lies in the interlocking of the zeolite particles with the surface irregularity of the support as well as the washcoat particles among themselves. Well-adhered coatings are firmly attached to the substrates, and they will not fall off easily when subjected to operational stresses and temperature in the reaction process. Therefore, it is extremely important to produce well-adhered coatings for long-lasting actual application. Conventionally, the assessment of coating adherence has mainly been based on the mass loss from ultrasonic vibration.9,22,27 This method performs well in characterizing the strength of interlocking

coating samples

coating methoda

ζ potential (mV)

solid loading (mg/cm2)

mean thickness (μm)

NC MC ZC(0.40)

20:40:4 20:40:4 20:40:4

−35 −48 −43

3.98 ± 0.12 3.68 ± 0.13 3.75 ± 0.16

13.21 ± 2.81 10.91 ± 1.88 10.85 ± 1.21

a

Coating method: zeolite (wt %)/binder (wt %)/washcoating numbers.

increases with the increased zeolite content and slurry viscosity, and the solid loading of NZ is higher than that of MZ in the range of zeolite content from 12.5 to 25%. Mitra et al.9 studied the variation of cumulative loading and slurry viscosity as a function of the zeolite content for four zeolite types and found that the viscosity has a great effect on solid loading, with higher viscosity resulting in a higher loading percentage. The higher solid loading of NZ slurries is mainly ascribed to the stronger interaction between the substrate and slurry for the slurries with high viscosity (Figure 5b). For a given zeolite content, the solid loading is also affected by the mixture ratio of MZ and NZ. It is found that solid loading increased gradually with increasing the NZ content at the zeolite content of 20% (Figure 6b). The possible reason for this result is ascribed to the improved van der Waals force between particles after introducing NZ into MZ slurries. 3.4. Coating Morphology. Figure 7 shows the morphologies of MZ, ZC(0.40), and NC coatings in the middle of coated tube reactors. Clearly, the MZ coating (Figure 7a) shows a relatively rough surface with many bulges over it, while the NC coating surface (Figure 7e) is uniform and smooth. This result can be easily understood considering the particle 1224


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Figure 7. SEM photographs of zeolite coatings with different zeolites (a and b, MC; c and d, ZC(0.40); e and f, NC; zeolites/binders/solvent, 20:20:60; 4 times washcoating, before test).

of the heating stress for hypersonic application and the strength of the coatings depositing on the substrate. Herein, the adhesion of the zeolite coatings prepared by different proportions of NZ in zeolite composition was investigated with ultrasonic and thermal shock tests, as shown in Figure 8. The results from the ultrasonic test showed that the mass loss decreased with the increase of the proportion of NZ in zeolite composition. Among all of the samples, the MC

between the layers of deposited particles. However, coatings on metallic materials used at high temperatures are required to have good thermal shock resistance because the peeling of a coated layer leads to serious problems in operation. Typically, for fuel-cooled thermal management, the wall temperature of the fuel−air heat exchanger will rise rapidly to above 800 °C in several minutes, even in several seconds.10 In this case, the thermal shock test would be necessary for examining the effect 1225


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nanozeolites in a proper proportion is an efficient method to improve the adhesion of the coatings under thermal shock conditions. 3.6. Catalytic Cracking Activity. In this work, catalytic cracking of supercritical n-dodecane (550 °C and 4 MPa) was used to examine the catalytic activities of MC and NC. Figure 9

Figure 8. Weight loss of coatings as a function of the proportion of NZ in zeolite composition (slurries composition: zeolite/binder/ solvent = 20:20:60; 4 times washcoating).

coatings had the worst adhesion, with a mass loss 12.5% because of the lower particle−particle interaction, and adhesion of the coating increased after gradually introducing NZ as a result of the stronger interlocking because of better particle− particle contact (see panels C, D, and E of Scheme 1). Of course, the best adhesion was observed for NC with a mass loss of 5%. This result was similar to the results reported in the literature,29 in that, in the case of the smaller diameter particles in the slurry, a higher percentage of corresponding nanoagglomerates lead to tighter particles packing compared to the case of larger diameter particles. Therefore, NZ is favorable for improving the interlockage between particles and, thus, the better adhesion in the ultrasonic test. In contrast, NC and MC showed different thermal shock resistances. The NC coatings have the worst adhesion, with about 30 wt % of the coating lost after the thermal shock test, while the MC coatings were as high as 21 wt %. However, the adherence strength of the coating improved considerably by adding NZ to MZ, as evidenced by the fact that the weight loss dropped from 21 to 11% with 20% NZ and further to 2.4% with 40% NZ. However, it is interesting to find that the adherence strength of the coating turned bad again with a further increase of the content of NZ of more than 40%. These trends may result from the synergistic effect of two zeolites with different particle sizes. It is generally accepted that adhesion of coatings in the thermal shock test lies in the following factors: thermal expansion difference and adhesion between the substrate and the coating and adhesion of the top coat and bond coat. As discussion above, adhesions between top and bond coatings as well as between the substrate and coating increase by the following order: MC < ZC(0.40) < NC. We also noted that the air between the substrate and the coating sharply increases the heat-transfer resistance of the coating, so that the strong thermal gradient and stress produced by the steep temperature difference in the thermal shock test are weakened on the actual surface and in the interior of the coating39 and that the air amount in the layer (see panels C, D, and E of Scheme 1) also decreases in the following order: MC < ZC(0.40) < NC. Therefore, the thermal expansion difference between the substrate and the coating increases in the order of MC > ZC(0.40) > NC because of the decreasing thermal gradient. Considering the interaction of the above factors, it is reasonable to explain the observed results. However, the roles that the two kinds of crystals played in tailoring the microstructure of coatings and how they work on the coating adhesion are still clearly unknown. On all accounts, the mixing of micro- and

Figure 9. Catalytic activities of wall-coated zeolite versus TOS (ndodecane, 550 °C, 4 MPa, and 10 mL/min).

depicted the catalytic cracking conversion of MC and NC (see Table 2) as a function of the time on stream (TOS). Thermal cracking conversion of n-dodecane in the bare tube, as a baseline, is lower than 15% at the beginning of the experimental run and then gradually decreases possibly because of deactivation of catalysis of the metal wall. For MC, the initial conversion of supercritical n-dodecane was only about 25% at the beginning of the reaction (TOS = 2.5 min), while that of NC reached 50%, which was twice as high as that of MC. When the experiment ended at TOS = 27.5 min, the n-dodecane conversion over NC dropped from 50 to 33%, i.e., Rd of ca. 34%, while for the MC, Rd is as high as ca. 44%. In addition, for the two conversion curves of NC and MC, even at the end of the reaction (27.5 min), the conversion obtained with NC is still higher than the initial activity of MC. As described above, ZC(0.40) exhibits the maximal adhesion in the thermal shock test. Figure 9 also presents the conversion−TOS curve of ZC(0.40). It is found that the initial conversion is ca. 42% (1.68 times of MC) and that, after 25 min, ZC(0.40) still kept 68% catalytic activity. Simple blending of MZ and NZ changes the acidity and pore structure of zeolite at the same time. These results indicated that NZ exhibited higher activity and better stability compared to MZ in the catalytic cracking of supercritical n-dedecane. Tago et al. observed that the nanosized ZSM-5 exhibited a high catalytic activity over a long lifetime in the catalytic conversion of acetone into light olefins.18 Similar results were also observed by Firoozi et al.17 in the methanol to propylene reaction. Table 3 compares the product distribution of n-dodecane from thermal cracking and catalytic cracking over different coatings. Clearly, for both thermal and catalytic cracking of ndodecane, the major gaseous products are ethylene, propane, and propylene but with different selectivities. The selectitvity of propylene is the highest, while the selectivities of propane and propylene are higher in the catalytic cracking than that of thermal cracking in the following order: bare < MC < ZC(0.40) < NC. In addition, the selectivities of branched C4 species (especially isobutane and isobutene) are higher for catalytic cracking. This presents the fact that the catalytic cracking is 1226


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Table 3. Product Mass Selectivity (%) of n-Dodecane Cracking over HZSM-5 Coating with Different Crystal Sizesa products

bare

MC

ZC(0.40)

NC

hydrogen methane ethane ethylene propylene propane isobutane n-butane trans-2-butene n-butene isobutene cis-2-butene n-pentane isopentane pentene n-hexane 1-hexene benzene n-heptane 1-heptene toluene n-octane 1-octene n-nonane 1-nonene n-decane 1-decene n-undecane 1-undecene 1-dodecene coke gaseous products liquid products average conversion

0.12 0.79 3.69 8.91 11.01 7.07 1.62 2.13 1.21 3.57 0.86 1.04 4.76 1.58 8.74 3.01 8.06 0.13 1.39 5.04 0.59 1.57 4.86 2.42 4.43 0.48 5.22 3.99 1.31 0.54 0.02 37.42 58.12 12.90

0.20 0.51 2.15 5.93 12.13 7.84 2.42 2.96 1.10 4.10 0.98 1.28 4.30 1.69 8.41 3.11 8.75 0.54 2.75 4.95 0.66 1.74 4.53 2.35 4.18 0.54 4.26 3.94 1.21 0.41 0.03 38.74 58.32 20.05

0.38 0.78 2.48 6.41 14.81 8.49 4.08 4.68 1.56 4.53 1.35 1.47 3.18 1.13 8.61 3.27 7.42 0.58 2.41 3.73 1.67 1.94 2.90 2.05 3.74 0.36 2.58 1.95 1.04 0.35 0.05 47.38 48.91 32.72

0.43 0.75 2.56 7.06 15.87 9.43 5.06 5.42 1.83 5.68 1.84 0.93 2.29 0.71 9.07 3.71 6.78 0.67 2.19 3.08 2.04 2.18 1.56 1.83 2.60 0.30 1.95 0.93 0.92 0.25 0.07 53.12 43.06 41.01

Figure 10. Molar concentration of ethylene and propylene in gas products with TOS [n-dodecane, 550 °C, 4 MPa, 10 mL/min; a, ethylene; b, propylene; ■, bare tube; □, MC; ●, ZC(0.40); ○, NC].

(FTIR) spectroscopy and NH3−TPD characterizations were carried out to determine the acidity properties (such as total amount, nature, and distributions on the catalyst surface) of the MZ and NZ samples in the coatings. The total acidic properties were calculated and listed in Table 4. Obviously, the Brønsted Table 4. Acidity of Micro- and Nano-HZSM-5 Zeolites for Catalytic Cracking of Supercritical n-Dodecanea

Reaction conditions: 4 MPa, 550 °C, and 30 min; slurry compostion: zeolites/binders/solvent = 20:20:60; 4 times washcoating.

a

Brønsted acid (μmol/g) sample

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 carbenium ion mechanism.12 The selectivities of alkylene liquid products (1heptene, 1-octane, 1-nonene, 1-decene, and 1-undecene) are significantly higher in the bare tube than that in the coated tube. The aromatic selectivities of benzene and toluene are also higher in the presence of NC, but the selectivities of liquid products above C8 are lower compared to MC, which may be a result of higher cracking conversions. To further examine the variation of product selectivity, the molar concentrations of ethylene and propylene in the gaseous products are plotted with the TOS in Figure 10. Both concentrations of ethylene and propylene slightly change (less than 2%) with TOS. The increase of ethylene and the decrease of propylene consist with the result that the conversion reduces and activity is lost with TOS. 3.7. Hypothesis for Performance Enhancement. To gain more explanations on the high and stable catalytic cracking activity of NC, both acid and pore properties of MZ and NZ were characterized for this propose. Fourier transform infrared

NZ MZ NZ(0.40)/ MZ(0.6) a

Lewis acid (μmol/g)

BT

BW

BS

LT

LW

LS

total acid (μmol/g)

135 122 127

52 51 52

83 71 76

49 10 26

37 6 18

12 4 7

184 132 153

T, W, and S represent total, week, and strong acid sites.

acid concentrations of NZ (135 μmol/g) were only slightly higher than those of MZ (122 μmol/g), but the Lewis acid concentration of NZ (49 μmol/g) was almost 5-fold more than that of MZ (only 10 μmol/g) because of the different textural structure of Al, regardless of the same Si/Al ratio of NZ and MZ. Qu et al.13 ascribed high and stable catalytic cracking activity of HZSM-5 zeolite with high Si/Al ratios to the high Lewis concentrations in the catalytic cracking of supercritical ndodecane. van Bokhoven et al.40 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 sites promotes the reaction rate by a factor of 2−5, although the number of Brønsted sites deceased. Thus, the 1227


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large number of Lewis acid sites over NZ may be one reason for its better performance. Many researchers attributed the high and stable activities of NZ to the other possible but more important reasons: shorter diffusion path lengths because of the small crystal size and lower diffusion resistance of reactants. The appearance of mesopores for NZ (as shown in Table 1) was also favorable for improving the diffusion capability of large molecules in zeolite channels. For instance, Zhao et al.41 found that the diffusion coefficient of cumene increased by 2−3 orders of magnitude on the mesopore-structured ZSM-5 generated by alkali treatment compared to untreated ZSM-5. It is known that the diffusion rate of supercritical hydrocarbon in micropores of zeolite is relatively slow compared to the gas phase.42 Thus, the diffusion path length of hydrocarbon in NZ is reduced with the decreasing crystal size (see panels A and B of Scheme 1), which is helpful to improve the diffusion rate and, thus, the catalytic cracking activity. Coke precursors formed via dehydrogenation or hydrogentransfer reactions of cracking products and then converted into cokes deposited in the micropores, which lead to the deactivation of zeolite coatings. Thus, the cracking products rapidly transfer into bulk fuel from the zeolite at a high diffusion rate, which effectively avoids the secondary reactions forming coke and its precursors. Dardas et al.43 studied the catalytic cracking kinetic of n-heptane over a commercial Y-type zeolite under super- and subcritical conditions and indicated that coke extraction by supercritical hydrocarbons is beneficial for the regeneration of zeolite and, thus, a stable activity. Of course, the higher diffusion rate of supercritical hydrocarbon is favorable for the extraction of coke and its precursors over zeolite, as suggested by Dardas et al.43 and Süer et al.44 In this view, the higher diffusion rate because of the mesopore is possibly contributed to the high and stable catalytic activity of zeolite coating. To summarize, the better catalytic performance of NC may be a result of the high Lewis acid, short micropore length, and intracrystal mesopores.

dodecane in the intracrystal mesopore, and the special acid nature of nanoscale HZSM-5 coatings.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: 86-22-2789-2340. E-mail: [email protected]. Present Address †

Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 90916022 and 91116001) and the Programme of Introducing Talents of Discipline to Universities (B06006). The authors also gratefully thank the reviewers of this manuscript for their help and valuable academic suggestions.

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4. CONCLUSION Wall-coated micro- and nanoscale HZSM-5 in the steel tubes was prepared using washcoating methods to compare their slurry properties, washcoat loadings, adhesions, and catalytic activities. In comparison of the microscale zeolite coatings, the coatings prepared by nanoscale zeolite presented well morphology and higher loading because of the high viscosity of slurry; however, their adhesion was rather bad in the thermal shock test. To prepare a coating with a smooth surface and proper adhesion, the particle size distribution should be adjusted to a proper range. Mixing zeolites with different particle sizes is an effective method of adjusting the particle size distribution to prepare coatings with well adhesion and considerable loading. In this experiment, 40% of the nanoscale zeolite in the zeolite composition would be a proper proportion for preparing coatings with the best adhesion. The coatings with nanoscale HZSM-5 zeolite enhance catalytic cracking activity more than 1 time, and the coatings blending with 40% nanoscale HZSM-5 zeolite enhance by 64% compared to the microscale zeolites. Acid and pore structure characterizations show that the performance enhancement of nanoscale HZSM-5 is attributed to the shorter diffusion length of the micropore, the higher diffusion rate of supercritical n1228


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Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated

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