Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated

Nov 6, 2014 - Catalytic Cracking of Supercritical n‑Dodecane over Wall-Coated ... zeolites were prepared on the inner surface of 304 stainless steel...
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Catalytic Cracking of Supercritical n‑Dodecane over Wall-Coated Nano-Ag/HZSM‑5 Zeolites Yuan Qiu,† Ganglei Zhao,† Guozhu Liu,* 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, P. R. China S Supporting Information *

ABSTRACT: A series of Ag modified zeolites were prepared by incipient impregnation of nano-HZSM-5 with AgNO3 (0.5−2.0 wt %) aqueous solutions. It was found that the ion-exchange and impregnation effect both existed in this impregnation process, resulting in the formation of various silver species which altered the zeolite acid properties. The wall-coated nano-Ag/HZSM-5 zeolites were prepared on the inner surface of 304 stainless steel tubes though the washcoating method. Catalytic cracking of supercritical n-dodecane (4 MPa, 550 °C) was used to examine the catalytic activities of the prepared catalytic coatings. It is found that the modified samples with 1.0% Ag showed higher conversion (up to 1.50 times) than the parent one because of the dehydrogenation effect of silver species and that the overloaded (for example, 2.0 wt %) samples show lower performances ascribing to the blockage of diffusion pores by the silver species. reactions.10 Wakui et al.11 got higher yields of ethylene and propylene in the cracking of n-butane on alkali metal modified ZSM-5 because the rapid elution of the primary product. Wang et al.12 found that the addition of rare earth metals to HZSM-5 catalysts would increase the total yield of olefins in the catalytic cracking of butane. The transition metals were also reported to enhance the zeolite cracking activity by promoting the dehydrogenation of hydrocarbons.10 However, the acid strength of zeolites, especially Brønsted acid sites, is always weakened with the introduction of metal modifiers like Zn,13 Cr14 K, Na,11 etc. The Brønsted acid sites decreased with the impregnation of Ag+ as well; however, the replaced acid sites would recover rapidly in the presence of hydrogen.15,16 Ono et al.17 found the loading of Ag+ ions greatly enhanced the dehydrogenation of alkanes and gave high selectivity for aromatics. Moreover, Anton18 proved that silver species can activate methane by dissociation of the C−H bond via the “carbenium” pathway and promote the dehydrogenation process in the selectively catalytic reduction of NOx with alkanes reactions.19−22 As we know, the activation of C−H bond hydrocarbons is the crucial process in the chain-initiating step for catalytic cracking. Considering these unique properties of silver species in zeolite, it is necessary to investigate the catalytic cracking activity of silver modified zeolites. In this work, Ag was impregnated into nanometer HZSM-5 aiming at modifying its acid properties and further enhancing its catalytic activity. We found that ion-exchange and impregnation effects both exist in this incipient impregnation progress. Catalytic cracking of supercritical n-dodecane (4 MPa, 550 °C) was used to examine the catalytic activity of corresponding HZSM-5 coatings made by washcoating method.

1. INTRODUCTION As the speed of aircrafts increase to the high supersonic and hypersonic regimes, it becomes a great challenge to protect the engines and airframe from aerodynamic heating. An innovative solution for this problem is using the hydrocarbon fuels onboard as the primary coolant.1 To increase the heat sink of the hydrocarbon fuel, several catalytic endothermic reactions of the hydrocarbon fuel (catalytic dehydrogenation, catalytic cracking, steam reforming, etc.) could be used for these applications.2 Among these reactions, catalytic cracking of hydrocarbon fuel was proved to be a promising reaction because of its high cooling ability as well as better combustion performance of products.3 In the advanced aircraft, the fuel has to work under high pressure (3.4−6.9 MPa) and temperature (above 500 °C), i.e., the supercritical conditions.4−6 Until now, much work has been done toward a better catalyst with high catalytic cracking activity and stability.1,7 Zeolite coatings on the inner micro cooling channels, as a kind of monolithic catalyst, has been taken as the most promising technique for the potential application due to the lower pressure drop and higher thermal resistance. Meng et al.8 coated HZSM-5 onto the inner surface of microchannels reactors (stainless steel tube, Ø = 3 mm) by washcoating method and found that cracking of n-dodecane was enhanced more than 100% at 525 and 550 °C, but the reaction is strongly limited by the internal pore diffusion which also cause the fast deactivation of zeolites. Our previous work9 found that catalytic cracking activity and stability could be remarkably improved by nanoscale HZSM-5 coatings compared to microscale HZSM-5 with enhancing pore diffusion by introducing intercrystal mesopores. However, there is still much work to be done to further improve the zeolite catalytic performance for future applications. As we know, element incorporation is a promising method to controllably change the acid property of zeolites and thus both the catalytic activity and product distribution on various © 2014 American Chemical Society

Received: Revised: Accepted: Published: 18104

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for 3 h using a ball milling machine of type QM-ISP04. Then, the coating slurry was deposited onto the 304 stainless-steel tubes (length, 300 mm; inner diameter, 2 mm; wall thickness, 0.5 mm) by the washcoating method.24−26 The coating was then subsequently dried overnight and calcined at 600 °C for 2 h, leading to a well-adhered coating. The ZSM-5 coatings were named 0.0 Ag-NZC, 0.5 Ag-NZC, 0.75 Ag-NZC, 1.0 Ag-NZC, and 2.0 Ag-NZC corresponding to different silver content of 0.0, 0.5, 0.75, 1.0, and 2.0 wt %. The as-prepared zeolite-coated 304 stainless-steel tubes were used as the reactor, and the catalytic cracking runs of ndodecane were carried out on the flowing reactor apparatus. A schematic diagram of the reactor is given in Scheme 1. Before

It was found that silver impregnated zeolites performed better catalytic activity than the parent one. The acid properties of the modified samples were characterized by temperature-programmed desorption of ammonia (NH3-TPD) and FTIR spectra of adsorbed pyridine (pyridine-FTIR), and the existence form and morphology of silver species were tested by UV−vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM).

2. EXPERIMENT SECTION 2.1. Materials. Silica sol (23 wt %), used as a binder to enhance zeolite particle attachment on a stainless-steel substrate, was self-made using hydrolysis of tetraethyl orthosilicate. n-Dodecane (99.5%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2.2. Preparation of Ag/HZSM-5 Zeolites. The nanoscale HZSM-5 zeolite (Si/Al mole 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.23 Ag/HZSM-5 containing 0.50, 0.75, 1.0, and 2.0 wt % Ag+ was obtained by impregnating 20 g of nano-HZSM-5 with different concentrations of AgNO 3 aqueous solutions. They are named as 0.0 Ag-NZ (parent sample), 0.5 Ag-NZ, 0.75 Ag-NZ, 1.0 Ag-NZ, and 2.0 Ag-NZ. The synthesized samples were calcined for 2 h at 450 °C before use. 2.3. Characterization of Ag/HZSM-5 Zeolites and Coatings. The total surface areas of the samples were determined from nitrogen adsorption data (TriStar 3000 analyzer, Micromeritics Instrument) by BET method, whereas the micropore volumes were analyzed by t-plot method, and the mesopore size distribution was calculated using the Barrett−Joyner−Halenda (BJH) method. X-ray diffraction (XRD) was recorded on a Philips X’PERT MPD diffractometer (Cu Kα radiation). The morphology of Ag/ZSM-5 was determined on a JEM-1200CEX transmission electron microscope (TEM). The UV−vis spectra from 200 to 800 nm were recorded using a Hitachi U-3010 UV−vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI 5000 VersaProbe system (ULVAC-PHI, Chigasaki, Japan). The strong and weak acids of the catalysts were determined by NH3-TPD in micromeritics 2910 (TPD/TPR). Previously, the samples were outgassed under an Ar flow (30 N mL/min), keep 1 h at 300 °C. After cooling to 50 °C, sequences of ammonia (1 mL) were injected to the system until the samples were saturated. The physically adsorbed 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. First, wafers of 10 mg/cm2 were degassed for 60 min under vacuum (10−3 Pa) at 400 °C and saturated with pyridine at 60 °C. After equilibration for 30 min, the pyridine was desorbed at 150 or 300 °C, and the spectrum was recorded. The zeolite loading amounts on the stainless steel tubes were measured gravimetrically. 2.4. Catalytic Test. The coating slurry was composed of pretreated HZSM-5 zeolite, binder, and alcohol with a ratio of 20:20:60 (wt %). The prepared slurries were put into the milling agate jar and milled with a rotating speed of 300 r/min

Scheme 1. Schematic Diagram of the Reactor

each reaction test, the coated stainless-steel tubes were connected and fixed onto the apparatus. Then, 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 to 550 °C under 4 MPa. The cracked fuel was cooled by a condenser and then flowed into a gas−liquid separator. The gas products collected were analyzed on a SP-3420 gas chromatography with a flame ionization detector (FID) and an Al2O3/S capillary column (50 m × 0.53 mm). The liquid products were analyzed by a HP4890 gas chromatograph with a FID and a PONA column (50 m × 0.53 mm). Cokes were analyzed through the temperatureprogrammed oxidation (TPO) method.8

3. RESULTS AND DISCUSSIONS 3.1. Influence of Deposited Silver Species on Zeolite Structures. The XRD patterns (Figure 1) show characteristic diffractions that can be assigned to the ZSM-5 structure (MFI crystal structure type), indicating the pure existence of ZSM-5 without the formation of silver crystalline phase. The relative ratios of the full width at half-maximum (FWHM) of several key reflections (2θ = 7.9°, 8.79°, and 23.0° corresponding to 011, 202, and 051 lattice plane, respectively) in each patterns

Figure 1. XRD patterns of Ag/ZSM-5 zeolites. 18105

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thicknesses (9.34−9.92 μm) (Table S1), the catalytic activities of HZSM-5 coatings were examined by catalytic cracking of supercritical n-dodecane (550 °C and 4 MPa). Figure 4

were calculated. As shown in Figure 2, the FWHM value of these reflections increases as the silver content increased.

Figure 2. Relative ratio of the full width at half-maximum (FWHM) of several key reflections in XRD patterns. Figure 4. Catalytic activities of HZSM-5 and AgZSM-5 coatings versus TOS (n-dodecane, 550 °C, 4 MPa, and 10 mL/min).

Especially, FWHM values of these reflections of 2.0 Ag-NZ is as ca.1.36, 1.3, and 1.07 times higher than the parent samples. This indicates that the silver species corrupted the long-range order of the zeolite structure and decreased their relative crystallinity. Figure 3 shows the adsorption−desorption

presents the n-dodecane conversion as a function of time-onstream (0−30 min) for each coating (0.0 Ag-NZC to 2.0 AgNZC). One can observe that the n-dodecane conversion of the 0.0 Ag-NZC is just 0.3 at the beginning of the experimental run. Ag+ impregnated samples exhibit higher conversion than the 0.0 Ag-NZC sample in the whole reaction process, among which the sample 1.0 Ag-NZC has the maximum conversion and its initial conversion reaches to 0.445, 48% larger than nonmodified catalytic coatings. For 0.0 Ag-NZC to 1.0 AgNZC, the n-dodecane conversion increases with the impregnation amount. However, the conversion of the 2.0 Ag-NZC coating is as low as 0.5 Ag-NZC, much lower than the 1.0 AgNZC sample, possibly attributing to the blocking of the micro and mesopores and covering of acid sites by large silver clusters. A similar phenomenon was observed by Lu et al.14 that the catalytic cracking reactivity of Cr-HZSM-5 decreased with more than 0.038 mmol/g Cr loading. Therefore, it can be concluded that Ag impregnation can enhance the nanometer ZSM-5 cracking activity of supercritical n-dodecane, but this enhancement just exists in a certain scale (0.0−1.0 wt % silver in this experiment). On the other hand, the conversion-drop of 0.0 Ag-NZC over the experiment is ca. 28% (Rd), while for the 1.0 Ag-NZC, it is as high as ca. 39%. In fact, all the Ag modified samples show higher Rd values than that of unmodified samples. However, these high values may result from the high n-dodecane conversion. So the coke produced per conversion (CPC) was proposed to present the relative stability of the zeolites. The PCP values (Table 2) of 0.0−2.0 Ag-NZC samples are 0.27, 0.26, 0.26, 0.29, and 0.47, respectively. Therefore, the zeolite stability was maintained for 0.5−1.0 Ag-NZC. However, the 2.0 Ag-NZC produces much more coke per conversion than others, probably attributed to the pore-blocking of large silver clusters. The products are listed in groups as indicated in Table 2 (detailed product distribution information, see Table S2 in the Supporting Information). It shows that the modified samples have higher aromatics and light olefins yield, which is probably due to the enhancement of dehydrogenation and cyclization effects by silver species. It was reported that the transition metals incorporation would accelerate the catalytic cracking activity of zeolites by enhancing the dehydrogenation of hydrocarbons.10,14,28−30 The reaction indices like RM31 (the

Figure 3. N2-adsorption and desorption isotherms at 77 K of zeolites.

isotherms of nitrogen in ZSM-5, and the curves exhibited a typical IV isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The capillary condensation happened at the relative high pressure between 0.42 and 0.95, which can be attributed to the aggregation of nanoparticles.9,27 With the incorporation of silver, the mesopore area drops from 135 to 90 m2/g and the total volume drops from 0.198 to 0.151 m3/g (Table 1), indicating zeolite pores are blocked by silver clusters with the increase of silver species. 3.2. Cracking Activities of Modified HZSM-5. With similar solid loading amounts (2.32−2.59 mg/cm2) and coating Table 1. Relative Crystallinity and Textural Properties of Parent and Ag+ Modified HZSM-5 Zeolites sample

SBET (m2/g)

Smicro (m2/g)

Sext (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

Vmes (cm3/g)

0.0 Ag-NZ 0.5 Ag-NZ 0.75 Ag-NZ 1.0 Ag-NZ 2.0 Ag-NZ

325 324 322 317 304

189 172 168 165 153

135 101 102 101 90

0.198 0.167 0.163 0.159 0.151

0.087 0.095 0.091 0.088 0.086

0.111 0.072 0.072 0.071 0.065 18106

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Table 2. Products Mass Selectivity (wt %) of n-Dodecane Cracking over HZSM-5 Coating and Ag Modified HZSM-5 Coatings 0.0 Ag-NZC

0.5 Ag-NZC

0.75 Ag-NZC

1.0 Ag-NZC

2.0 Ag-NZC

0.37 1.33 37.74 59.34 23.34 2.79 0.07 1.34 0.43 0.27 1.84 0.59

0.39 1.41 38.09 58.87 23.45 2.86 0.08 1.36 0.42 0.26 1.82 0.60

0.41 1.45 38.89 58.10 23.71 2.90 0.09 1.42 0.32 0.26 1.70 0.64

0.45 1.48 39.07 57.99 23.81 2.93 0.11 1.44 0.30 0.29 1.73 0.64

0.43 1.46 38.68 58.32 23.67 2.9 0.15 1.40 0.38 0.47 1.74 0.65

hydrogen methane paraffin total olefins light olefins aromatics coke gas/liquid Rd a CPCb CMRc RMd a

The deactivation rate of catalyst coatings is defined as Rd = (Xt=2.5-Xt=27.5)/ Xt=2.5 (Xt=2.5 and Xt=27.5 represents n-dodecane conversion at time-on-stream of 2.5 and 27.5 min, respectively). bCPC (coke produced per conversion) is defined as coke amount/average conversion. c CMR(cracking mechanism ratio) = (C1 +∑C2)/i-C4.32 dRM = i-C4/n-C4.31

Table 3. Acid Properties of ZSM-5 Zeolites Brønsted acid (mmol/g)

Lewis acid (mmol/g)

samples

BT

BW

BS

LT

LW

LS

total acid (mmol/g)

0.0 Ag-NZ 0.5 Ag-NZ 0.75 Ag-NZ 1.0 Ag-NZ 2.0 Ag-NZ

0.49 0.20 0.45 0.45 0.43

0.11 0.10 0.07 0.07 0.08

0.38 0.10 0.38 0.38 0.35

0.55 0.60 0.63 0.65 0.65

0.39 0.43 0.43 0.44 0.43

0.16 0.17 0.20 0.21 0.22

1.03 0.80 1.08 1.10 1.08

ratio of i-C4 yield to n-C4 yield) and CMR32 (cracking mechanism ratio) are compared in Table 2. RM is used to compare the contribution of carbenium ion with the free radical mechanism, which increases from 0.59 to 0.65 with the addition of silver, indicating the carbenium ion mechanism is enhanced with the incorporation of silver species. Therefore, the silver species promotes the cracking activity of zeolites by enhancing the carbenium ion cracking route. CMR, representing the relative contribution of protolytic cracking and β-scission route, always changes with acid amounts. As indicated in Table 3, the CMR values decrease from 1.84 to 1.74 with the incorporation of silver species, indicating that the silver species promote the classical β-scission route (bimolecular cracking) cracking pathway more than the protolytic cracking route (monomolecular cracking). This probably is attributed to the increased active sites brought by silver species. 3.3. Acid Properties of HZSM-5. To explore the reason for the enhanced catalysis activity of the modified samples, the acid properties were detected by NH3-TPD and pyridine-FTIR. Figure 5 shows the NH3-TPD curves of the HZSM-5 zeolites with different content of impregnated silver. There are two desorption peaks centered at ∼220 and ∼400 °C, corresponding to the weak and strong acid sites, respectively. With the impregnation of silver, the low-temperature peak migrates to higher temperature, indicating that the acid strength become stronger with Ag+ impregnation. The high-temperature peak area of 0.5 Ag-NZ is much lower than the parent sample, which is probably because of the interaction between silver species and framework acid sites. However, for other samples, the peak area increased with the incorporation of silver species. Therefore, the existence forms of silver species may vary with different silver impregnation amounts. The total amount of zeolite acid sites (Table 3) is found to increase by the following

Figure 5. NH3-TPD profiles of HZSM-5 zeolites with different Ag content.

order: 0.5 Ag-NZ < 0.0 Ag-NZ < 0.75 Ag-NZ ≈ 2.0 Ag-NZ < 1.0 Ag-NZ. The acid property is further tested by pyridine adsorbed FTIR spectra in Figure 6. The spectra recorded after outgassing at 150 and 300 °C represent the total and strong acid sites, respectively. Adsorption bands at 1450 and 1540 cm−1 are assigned to the chemisorbed pyridine, corresponding to the Brønsted and Lewis acid sites. The acid properties were quantitated by the TPD and pyridine adsorbed FTIR results, as previously reported.33 As shown in Table 3, the Brønsted acid site amount of silver modified samples is always less than the unmodified one. Especially, the Brønsted acid amount of 0.5 Ag-NZ is 0.20 mmol/g, much lower than 0.49 mmol/g (0.0 AgNZ). The drop of Brønsted acid site amount is because some Ag+ interacts with the oxygen of the bridging hydroxyl groups by replacing the protons (ion exchange effect). One need note that the silver ion-exchanged Brønsted acid sites also have the 18107

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Figure 7. TEM images with different content of Ag+ impregnated HZSM-5. (a) 0.5 Ag-NZ, (b) 0.75 Ag-NZ, (c) 1.0 Ag-NZ, and (d) 2.0 Ag-NZ.

Figure 6. Pyridine-adsorbed FTIR spectra of AgZSM-5 zeolites at 150 (a) and 300 °C (b).

activity for hydrocarbon activation,17,18 and it can recover quickly in the presence of hydrogen.16 Unlike the Brønsted acid sites, both weak and strong Lewis acids increased from 0.0 Ag-NZ to 2.0 Ag-NZ. The amount of total Lewis acid sites of 2.0 Ag-NZ is 18% higher than that of 0.0 Ag-NZ (Table 3), and the strong Lewis acid sites increase from 0.16 to 0.22 mmol/g with silver impregnation. The increase of Lewis acid sites would enhance the catalytic activity of silver modified samples. More tests are still needed for the existence form of the silver species. 3.4. Existence Form of Incorporated Silver in HZSM-5 Nanocrystals. TEM technology was used to observe the morphology of silver species in ZSM-5 zeolites. As we can see from Figure 7, some silver clusters appear with the impregnation of Ag+. For 0.5 Ag-NZ, the silver species are less than 2 nm and most of them are scattered all over the zeolite crystals. These small species in zeolite frameworks can react with the protons of bridge hydroxyl group, resulting in the decrease of the Brønsted acid sites amount, agreeing with the discussion above. However, for the 0.75−2.0 Ag-NZ samples, the silver clusters are larger than 4 nm and most of the species are distributed on the surface of zeolite crystals. Considering the high catalytic activities of 0.5−1.0 Ag-NZ samples, one can concluded that both the silver species in the zeolite framework and outside the crystals may both act as active sites for supercritical catalytic cracking. However, for 2.0 Ag-NZ, the size of some clusters even reach 10−20 nm, leading to the blocking of zeolite pores, which is in accordance with the low catalytic cracking activity. To further investigate the existence form of silver species, the samples were tested by UV−vis spectrometry. As it shows in Figure 8, with the incorporation of silver, strong overlapped

Figure 8. UV−vis spectra of silver catalysts before and after the modification of the nano-HZSM-5.

bands centered at 210−230 nm appear, which can be assigned to the 4d10−4d95s1 electronic transition of isolated silver ions in the zeolites.34 The absorption band at about 280 nm belongs to charge transfer bands of small charged clusters Agnd+ (n < 10).35 In addition, there is a broad peak at ca. 410 nm for 1.0 Ag-NZ and 2.0 Ag-NZ which is the characteristic absorption of metallic silver particles aggregated to a size of several nanometers or larger.34 These reduced silver species may be also the dehydrogenation centers; however, they are much less active than Ag+ species.36 The XPS was conducted to confirm the existence form of silver species (Figure 9). Although the depth of Ag species detectable by XPS is limited (