ZSM-5 Catalysts for Hydroisomerization

Article pubs.acs.org/IECR

Microporous/Mesoporous Pt/ZSM‑5 Catalysts for Hydroisomerization of BTL-naphtha Eleni Heracleous,* Eleni F. Iliopoulou, and Aggelos A. Lappas Chemical Process & Energy Resources Institute (CPERI), Centre for Research and Technology Hellas (CERTH), P.O. Box 361, GR-57001 Thessaloniki, Greece ABSTRACT: Hydroisomerization of BtL-naphtha was investigated over microporous/mesoporous Pt/ZSM-5 catalysts. Alkaline and successive acid treatment of microporous ZSM-5 leads to the formation of mesopores (∼10 nm). Alkaline treatment removes both Si and Al from the framework, while Si is selectively extracted and Al redeposited on the surface as extra-framework Al, increasing the Lewis acidity and reducing Brönsted sites. Successive acid treatment dissolves alumina deposits, leading to higher surface area and acidity. The mesoporosity was found to improve the selectivity because of the improved diffusivity within the zeolite pores and, as a consequence, the reduced residence time of the intermediates within the pore channels.

1. INTRODUCTION The biomass-to-liquids (BtL) process is a very promising technology for the production of high-quality secondgeneration transportation biofuels that can be used in the existing vehicle fleet with the current fuel transportation and distribution infrastructure. The BtL technology entails gasification of the lignocellulosic biomass to synthesis gas and thermochemical conversion of the cleaned syngas to hydrocarbons via the Fischer−Tropsch reaction. The main product of the process, BtL diesel, is a high-quality diesel fuel with high cetane number and zero sulfur and aromatic compounds. The process also yields a light byproduct in the range of naphtha, which is an excellent feedstock for naphtha crackers targeting ethylene production. However, in order to boost the economic viability of the process, it is desirable to further exploit this lighter fraction as a premium fuel product. In this context, BtLnaphtha has been identified to have suitable properties to be used as fuel for future power trains like homogeneous charge compression ignition engines. This option was explored in the European project RENEW, where it was found that further upgrading of the naphtha fraction is needed for optimized engine performance, targeted toward mild reduction of its cetane number.1 Hydroisomerization, in which linear alkanes are converted to their corresponding isomers with lower cetane numbers, appears as a promising option for upgrading of naphtha. Isomerization constitutes one of the most important catalytic processes in the petroleum industry, where it is employed to produce high-octane-number components for the refinery gasoline pool. In recent years, it has also been considered for the production of hydrocarbon biofuels from oil and fatty raw materials in combination with deoxygenation.2,3 CERTH established hydroisomerization as a viable upgrading option for BtL-naphtha on both the laboratory and pilot scale.4 In previous work,5 we conducted a systematic study of the effect of the zeolite support (mordenite, ZSM-5, and beta zeolite) on the activity and selectivity of low-loading (0.1%) Pt catalysts. Supporting Pt on ZSM-5 led to the synthesis of the most active catalytic material (highest conversion), followed by mordenite and beta-zeolite-supported catalysts. The same Pt/ZSM-5 © XXXX American Chemical Society

catalyst was also able to successfully isomerize heavier hydrocarbons (up to C10) with satisfactory selectivity. The superior performance of Pt/ZSM-5 can be attributed to its high Brönsted acidity but also to the formation of homogeneously dispersed, cubic-shaped and highly crystalline Pt particles on the zeolite surface. However, the production of dry gases was evidenced over Pt/ZSM-5 at high temperature, associated with secondary cracking reactions.5 This enhanced hydrocracking selectivity could be due to the slower migration of the intermediates within the zigzag 10-ring channel of ZSM-5, which may lead to a longer lifetime of the intermediates and subsequent cracking reactions.6 Several literature studies have reported the minimization of such diffusion limitations by using mesoporous supports, such as MCM,7 or the creation of mesopores in zeolitic crystals.8−14 These latter hierarchical materials that contain porosity in both the meso- and microporous range combine advantages from both zeolites and mesoporous molecular sieves. On the one hand, they retain the strong acidity of zeolites, while on the other hand, they also demonstrate high thermal and hydrothermal stability, as well as improved diffusivity of long-chain molecules.15 The secondary mesoporous structure is expected to facilitate the accessibility of bulkier reactants to the active sites, leading to enhanced catalytic activity in the hydroisomerization process. At the same time, the decreased residence time of the products in the pores will minimize secondary reactions, such as cracking, improving selectivity toward long-chain isomers. It was shown that the acid treatment of Pt/mordenite increased its activity in n-hexane hydroisomerization by more than 4 times because of the creation of mesopores that alleviated the intracrystalline diffusion limitations in the zeolite pore system.8,9 Besides that, selectivity to monobranched isomers was increased because of easier desorption of products. Received: July 23, 2013 Revised: August 26, 2013 Accepted: September 18, 2013

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dx.doi.org/10.1021/ie402354t | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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to discriminate between micro- and mesoporosity. The reported mesoporosity was calculated from P/P0 = 0.1−0.9. X-ray diffraction (XRD) measurements were performed using a Siemens D-500 diffractometer employing Cu Kα1 radiation (λ = 0.15405 nm) and operating at 40 kV and 30 mA. The XRD patterns were accumulated in the 2θ range of 5− 80° with a 0.02° step size and a counting time of 1 s per step. The total acidity, type, and acid strength distribution of the acid zeolites were measured by the Fourier transform infrared (FTIR) pyridine adsorption technique. IR spectra were collected using a Nicolet 5700 FTIR spectrometer (resolution 4 cm−1) by means of OMNIC software and were processed with GRAMS. Before IR analysis, samples were heated at 450 °C under high vacuum (10−6 mbar) for 1 h in order to desorb any possible physisorbed species. Initially, the reference spectrum of the so-called activated sample was collected. Adsorption of pyridine was realized at 1 mbar by equilibrating the catalyst wafer with the probe vapor, added in pulses for 1 h. The desorption procedure of pyridine was stepwisely monitored by evacuating the sample for 30 min at 150, 250, 350, and 450 °C and cooling to 150 °C after each step to record the corresponding spectrum. 2.3. Isomerization Tests. The hydroisomerization tests were performed on a high-pressure fixed-bed laboratory-scale unit, equipped with a feed inflow system able to supply both gases and liquid feeds. A detailed description of the unit can be found in a previous publication.5 An appropriate amount of the catalyst sample was loaded into the reactor and was reduced in situ under a hydrogen flow. The reaction was conducted at 30 bar, using a weight hourly space velocity (WHSV) of 2 h−1 and a H2/HC molar ratio of 0.4. The reaction temperature was varied from 240 to 300 °C. Steady-state activity measurements were taken after at least 8 h onstream, collecting and analyzing both liquid and gaseous product samples. The liquid samples were analyzed with PIANO analysis, while the composition of the reaction off-gases was detected by gas chromatography. Closure of the carbon and hydrogen balances was better than ±3%.

The current work aims to further improve the catalytic performance of Pt/ZSM-5 in hydroisomerization of BtLnaphtha via the introduction of mesoporosity in the ZSM-5 structure through various alkaline and acid treatments. The derived composite microporous/mesoporous Pt/ZSM-5 materials were characterized in detail in order to determine their structural, textural, and porosity characteristics, as well as the effect of the different treatments on the acidic properties. Activity tests with surrogate BtL-naphtha feeds (C5−C10) were performed to determine the effect of the mesoporosity in the hydroisomerization reaction.

2. EXPERIMENTAL PART 2.1. Catalyst Preparation. A commercial ZSM-5 zeolite in ammonium form with SiO2/Al2O3 = 23 supplied by Zeolyst (ref CBV2314) was used as the starting material and was subjected to the following treatments: (a) H-ZSM-5: The commercial sample was calcined at 400 °C for 3 h under an air flow to transform the initial ammonium to its corresponding proton form. The derived material is considered as the base case of the study. (b) Alk-ZSM-5: The protonated form (H-ZMS-5) underwent an alkaline treatment with an aqueous solution of NaOH targeting toward desilication of the zeolitic framework and mesoporosity formation. The sample was added to a preheated (65 °C) solution of 0.2 M NaOH for 30 min under continuous stirring. The suspension was quenched, filtered, and washed until neutral pH (6−7). The sample was then dried and ionexchanged for the removal of Na and restoration of the ammonium form of the zeolite. The last step included calcination at 400 °C for 3 h under an air flow to transform the derived ammonium to its corresponding proton form. (c) Acid-ZSM-5: The H-ZSM-5 carrier was submitted to the alkaline treatment described above, excluding the last ionexchange step. Instead, the desilicated sample was acid-treated with a solution of 3 M HNO3 for 1 h at 65 °C to remove the extra-framework Al debris and restore the initial Si/Al ratio. The suspension was quenched, filtered, and washed until neutral pH (6−7). The derived sample was then dried and calcined at 400 °C for 3 h under an air flow. Pt was deposited on the three supports with dry impregnation. An aqueous solution of H2PtCl6 was used as the precursor salt in order to achieve a 0.1 wt % Pt loading on the final material. The derived catalysts were dried and calcined at 400 °C for 3h under air using a synthetic air stream. 2.2. Catalyst Characterization. The elemental composition (Al, Na, and Pt contents) of all samples was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a Perkin-Elmer Plasma 40 instrument. The Si/Al ratio was measured with ICP-MS Elan DRC-e (PerkinElmer SCIEX). Samples were digested in HF (50 mg of the sample was dissolved in 1 mL of HF). N2 adsorption/desorption isotherms at 77 K using an Autosorb-1 Quantachrome flow apparatus were used for determination of the porosity properties of the catalytic materials. Prior to the measurement, the samples were degassed in high vacuum at 523 K overnight, under 5 × 10−9 Torr vacuum. The Brunauer−Emmett−Teller (BET) method was applied to calculate the total surface area. C values of 4094.959880, 6712.777094, and 3207.015355 were used during characterization of the Pt/H-ZSM-5 Pt/alk-ZSM-5, and Pt/ acid-ZSM-5 samples, respectively. The t-plot method was used

3. RESULTS 3.1. Catalyst Characterization. The elemental composition (Pt content and Si/Al ratio) of all microporous/ mesoporous ZSM-5 supports and corresponding Pt catalysts is presented in Table 1. Concerning the Pt loading, the microporous H-ZSM-5-supported material is the one with the smallest difference between the nominal and actual metal Table 1. Chemical Composition and Textural Properties of the ZSM-5 Supports and the Corresponding Pt-Supported Catalysts elemental composition sample H-ZSM-5 Pt/HZSM-5 alk-ZSM-5 Pt/alkZSM-5 acid-ZSM-5 Pt/acidZSM-5 B

porosity characteristics

Pt loading (ICP), ppm

Si/Al ratio (ICP)

Vmicro, cm3/g

1095 ± 65

13.0 13.0

0.143 0.121

1340 ± 70

12.8 12.8

0.121 0.118

0.050 0.035

402.2 391.3

1300 ± 70

14.3 14.3

0.120 0.116

0.050 0.047

411.5 386.4

Vmeso, cm3/g

SBET, m2/g 417.7 415.5



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loading and lowest Pt content. Both microporous/mesoporous Pt/alk-ZSM-5 and Pt/acid-ZMS-5 contain a slightly higher Pt loading. In terms of the different treatments, the measured Si/ Al ratio demonstrates that the alkaline treatment induces slight desilication, leading to limited removal of Si from the zeolitic framework. This is in agreement with results by Groen et al.,16,17 who determined that the best results for ZSM-5 are obtained at Si/Al ratios between 25 and 50. It was found that at lower values the Al prevents desilication, while at higher values complete dissolution occurs. The successive acid treatment led to an increased Si/Al ratio for the acid-ZSM-5, compared with the corresponding value of the parent zeolitic support, suggesting mild dealumination and leaching of intraporous extra-framework Al. The N2 adsorption isotherms (Figure 1) indicate that changes in the pore structure occur after both alkaline and

Figure 2. BJH pore-size distribution of microporous/mesoporous HZSM-5, alk-ZSM-5, and acid-ZSM-5 supports.

5 zeolites and depends on the framework Si/Al ratio and synthesis procedure.17,20,21 Compared to the parent ZSM-5, both alkaline- and alkaline-acid-treated zeolites exhibit the development of new mesopores with a broad pore-size distribution centered on ∼10 nm. The observed differences in the pore structure among the samples are further confirmed by the textural data presented in Table 1. Alkaline treatment leads to increased mesopore volume, confirming the creation of mesopores, with, however, a decrease in the total BET surface area. This can be attributed to the decrease in the microporous volume, indicating that the treatment also affects the microporous structure of the zeolite. The subsequent acid treatment in the acid-ZSM-5 sample increases the total BET surface area, while the mesopore volume remains constant. It has been reported that, during base treatment, Si and Al are both extracted from the zeolytic framework; however, the Al species realuminate on the zeolite surface as extra-framework Al.22 Subsequent acid treatment has been reported to dissolve the extra-framework alumina deposits on the mesopores and external surface area,23 increasing the total surface area, as observed in our case. When the pure supports and corresponding Pt catalysts are compared, a decrease in the BET surface area is detected in all cases upon Pt deposition. Analysis of the textural characteristics shows that this is attributed to a decrease in the micropore volume, probably because of the extra hydrothermal treatment in the second calcination step after Pt introduction, which, however, does not affect the formed mesoporosity in the materials. XRD characterization was performed in order to determine possible changes in the crystallinity of the zeolites with the different treatments. The XRD patterns of the parent H-ZSM-5 support and three Pt catalysts supported on the microporous/ mesoporous H-ZSM-5, alk-ZSM-5, and acid-ZSM-5 supports are presented in Figure 3. The deposition of Pt did not modify the initial structure of the ZSM-5 supports, a result rather expected because of the low Pt loading (0.1 wt %). The three Pt-supported catalysts exhibit similar diffraction patterns, indicating that the crystal structure of ZSM-5 is preserved during the alkaline and successive alkaline-acid treatments. Some variation in the intensity of the peaks could by due to the removal of Si from the framework, without complete destruction of the lattice, indicating that the long-range ordering and microporous structure are largely retained, in accordance with literature data.24,25

Figure 1. N2 adsorption/desorption isotherms of microporous/ mesoporous H-ZSM-5, alk-ZSM-5, and acid-ZSM-5 supports.

subsequent alkaline and acid treatment. For brevity, the N2 isotherms of only the supports are presented. The isotherm of the parent untreated sample is of type I, typical for microporous materials, with H4 hysteresis, which is also a sign of microporosity.18 The alkaline-treated ZSM-5 exhibits a similar type I isotherm, with, however, a more pronounced hysteresis loop at higher relative pressures, indicative of mesoporous cavities that give rise to a distinct broadening of the hysteresis loop by their delayed emptying along the desorption branch.17 For the acid-treated sample, the isotherm has a shape that could be described as an intermediate between type I and type IIb isotherms with a largely parallel disposition of the adsorption and desorption branches of the hysteresis loop, suggesting the presence of open (cylindrical) mesopores connected to the outer surface.17 Moreover, in contrast to the other samples, a notable increase of the adsorbed nitrogen volume is observed at high relative pressure, suggesting an important external surface area contribution. The formation of mesopores can also be evidenced by the Barrett−Joyner−Halenda (BJH) pore-size distribution, as illustrated in Figure 2 for the parent and treated ZSM-5 supports. The parent ZSM-5 exhibits no mesopores, except a peak that could suggest a well-defined pore distribution around ∼4 nm. However, this latter feature, which appears on all supports, does not represent real pores and is caused by a transition of the adsorbed phase from a lattice fluidlike phase to a crystalline-like solid phase.19 This behavior is typical for ZSMC



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This sample is similar to the one used in the present study; however, it should be noted that the alkaline treatment in our case was performed at lower temperature (65 vs 90 °C used by Auroux et al.). The observed decrease in the acidity can be explained as follows: as previously described, the base alkaline treatment causes the formation of extra-framework Al species on the zeolite surface, giving rise to the Lewis acidity and a subsequent decrease in the Brönsted acidity, as Al is removed from the framework.22 On the basis of 27Al MAS NMR, Auroux et al.25 also observed that, besides the significant extraframework Al buildup, in the case of ZSM-5(23) the topology of Al in the framework changes for the alkaline-treated sample, leading to the possible creation of basic sites and thus explaining the reduction of the total zeolite acidity. The further acid treatment of the alkaline-treated zeolite leads to partial restoration of the initial acidity because of a decrease of the Lewis acidity and an increase of the Brönsted acid sites by 20% in both cases compared to the alk-ZSM-5 sample. This implies that the subsequent acid wash enables the removal of the extra-framework Al deposited during alkaline treatment, thereby reducing the Lewis acidity. Moreover, the Si/Al ratio for acid-ZSM-5 is higher than the corresponding value of the parent zeolitic support, suggesting that acid treatment leads additionally to dealumination of the zeolitic framework. The strength distribution of the Brönsted acid sites was measured for both the pure supports and Pt-supported zeolite catalysts. It can be seen from Table 2 that both the alkaline and successive alkaline-acid treatment, although reducing the total acidity, noticeably increase the strength of the Brönsted sites. 3.2. Reactivity Studies. The effect of introducing mesoporosity in the ZSM-5 structure through alkaline and acid treatment on the activity and selectivity of Pt catalysts in BtL-naphtha hydroisomerization was investigated by performing bench-scale experiments at constant pressure (30 bar), WHSV (2h−1), and H2/HC molar ratio (0.4) and varying temperature in the range of 220−280 °C. Figure 4 presents the effect of the temperature on total nparaffin conversion. As expected, the activity increases with the temperature for all samples. All three Pt/ZSM-5 catalysts demonstrate a satisfactory activity in naphtha hydroisomerization, while up to 260 °C, the catalysts do not exhibit significant differences (∼40% n-paraffin conversion). At 280 °C, the reference microporous Pt/H-ZSM-5 catalyst displays higher conversion than the microporous/mesoporous materials, associated, however, with enhanced production of gases and lighter products, as will be described later in the paper. The formation of mesopores in the treated Pt/ZSM-5 catalysts would be expected to potentially lead to enhanced activity to the reaction because the larger pores lead to faster mobility of the reactants and intermediates through the more open

Figure 3. XRD patterns of the H-ZSM-5 support and Pt/H-ZSM-5, Pt/alk-ZSM-5, and Pt/acid-ZSM-5 catalysts.

The acidity of the catalyst has a major effect on the hydroisomerization and hydrocracking reactions involved in our process. Therefore, acidity studies were performed in order to evaluate the effect of the alkaline and acid treatments on the acidic properties of the materials. The total acidity, types of acid sites, and strength distribution of Brönsted acid sites for the parent and treated ZSM-5 supports and the corresponding Ptbased microporous/mesoporous catalysts are provided in Table 2. A comparison of the ZSM-5 supports and corresponding catalysts shows that Pt deposition decreases the total acidity by ∼6−12%, with this decrease originating from reduction of both Brönsted and Lewis acid sites. The reduced acidity with Pt incorporation could be attributed possibly to the extra hydrothermal treatment in the second calcination step after Pt introduction. Focusing on the pure supports, it can be observed that the alkaline treatment leads to a notable decrease in the total acidity of the zeolite in the range of ∼23%, despite the fact that it does not seriously modify the Si/Al ratio. Analysis of the types of acid sites demonstrates that, although the total acidity decreases, a 30% increase in the Lewis acid sites is recorded, accompanied by a corresponding 38% reduction in the amount of Brönsted sites. Controversial results appear in the literature concerning the effect of alkaline treatment on ZSM-5 acidity. Some studies17,26−28 report that the zeolite acidity is, in general, preserved over controlled alkaline treatment for ZSM-5 with a different range of Si/Al ratios (15−1000). On the other hand, Auroux et al.25 observed that the changes in the acidity are a function of the Si/Al ratio. More specifically, the authors report full preservation of the acid site strength and distribution for ZSM-5 samples with SiO2/Al2O3 = 50 and 80 but significant reduction of the acidity for the sample with SiO2/Al2O3 = 23.

Table 2. Acidic Properties of the ZSM-5 Supports and the Corresponding Pt-Supported Catalysts distribution of Brönsted acid sites, % sample

Lewis sites, μmol/g

Brönsted sites, μmol/g

very weak

weak

medium

strong

total acidity, μmol/g

H-ZSM-5 Pt/H-ZSM-5 alk-ZSM-5 Pt/alk-ZSM-5 acid-ZSM-5 Pt/acid-ZSM-5

107.7 86.4 138.0 119.8 111.7 88.3

408.6 367.2 256.5 237.0 307.5 307.4

5.7 10.6 4.9 6.7 3.6 4.0

9.2 16.3 7.0 9.3 6.0 7.2

13.4 29.4 10.3 10.6 9.3 11.2

71.7 43.7 77.8 73.4 81.1 77.6

516.3 453.6 394.5 356.8 419.2 395.7

D



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key parameter. The selectivity to C4−C8 isoparaffins at 40% total n-paraffin conversion for the three investigated materials is illustrated in Figure 6. A conversion of 40% was chosen for the

Figure 4. Conversion of n-paraffins as a function of the reaction temperature over microporous/mesoporous Pt/ZSM-5 catalysts (reaction conditions: P = 30 bar; WHSV = 2 h−1). Figure 6. Selectivity to C4−C8 isoparaffins at 40% total n-paraffin conversion over microporous/mesoporous Pt/ZSM-5 catalysts (reaction conditions: P = 30 bar; WHSV = 2 h−1).

structures and improved diffusivity. However, it is also wellknown that acidity is of paramount importance in such reactions. As the characterization results showed, the microporous/mesoporous Pt/ZSM-5 catalysts demonstrate reduced acidity compared to the parent zeolite. It is possible that the beneficial effect of mesopore formation is masked by the negative effect of reduced acidity, and therefore similar activity is observed for all catalysts. The composition of the isomerate product at constant reaction temperature (260 °C) is shown in Figure 5. The reaction temperature was selected to be 260 °C because the conversion levels at this temperature are similar for all catalysts. The results indicate that the microporous/mesoporous Pt/ ZSM-5 samples isomerize naphtha mainly to C5−C8 isoparaffins, without the formation of additional naphthenes or aromatics. The main isoparaffins in the product are in all cases isooctane and monobranched C5 species. Some cracking does take place because the formation of C3−C5 hydrocarbons is also detected. Because the goal of the study is to reduce the secondary cracking reactions to lighter products and gases that occur over Pt/ZSM-5 via the introduction of mesoporosity, selectivity is a

comparison because this conversion is achieved at very close temperatures for all materials, and therefore the selectivity is compared at both similar conversion and temperature levels. The results indicate that the introduction of mesopores in the ZSM-5 structure definitely improves the selectivity and reduces secondary cracking reactions because a clear increase in i-C8 species at the expense of i-C5 and i-C6 is measured for both the alkaline- and acid-treated Pt catalysts. It should be noted that the formation of quite significant amounts of smaller chain isoparaffins (i-C4−6) indicates that lower-carbon-number isoparaffins are formed also via an indirect isomerization pathway via cracking and subsequent skeletal rearrangement. The cracking activity can also be determined from the amount of light gases (C1−C6) in the gaseous exit stream of the reactor, presented in Figure 7 as a function of the reaction temperature for the three Pt/ZSM-5 catalysts. Light gases are produced over all materials, suggesting that secondary cracking reactions occur to some extent over all samples. However, it is indisputable that the microporous/mesoporous materials

Figure 5. Composition of the isomerate product at 260 °C over microporious/mesoporous Pt/ZSM-5 catalysts (reaction conditions: P = 30 bar; WHSV = 2 h−1). E



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The catalytic performance of the microporous/mesoporous composite Pt/ZSM-5 catalysts in the isomerization of naphtha showed that the formation of mesopores does not lead, as would be expected, to enhanced activity, probably because of the fact that the beneficial effect of the mesopore formation is masked by the negative effect of reduced acidity. In terms, however, of the selectivity, it was clearly observed that the introduction of mesopores in the ZSM-5 structure improves the selectivity and reduces secondary cracking reactions because of the improved diffusivity within the zeolite pores and, as a consequence, the reduced residence time of the intermediates within the pore channels. The improved selectivity could also be a result of reduced acidity, which is again beneficial toward inhibiting cracking reactions.

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AUTHOR INFORMATION

Corresponding Author

Figure 7. Concentration of light gases as a function of the reaction temperature over microporous/mesoporous Pt/ZSM-5 catalysts (reaction conditions: P = 30 bar; WHSV = 2 h−1).

*E-mail: [email protected]. Phone: +30 2310 498345. Fax: +30 2310 498380. Notes

The authors declare no competing financial interest.

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exhibit decreased production of light hydrocarbons, especially at the highest temperature (280 °C), where the concentration of the gases drops from 18% for the reference Pt/H-ZSM-5 catalyst to ∼12−13% for the microporous/mesoporous samples. In general, a good isomerization catalyst should have suitable compositional and structural characteristics: proper balance between metal and acid sites, medium pore size, high dispersion of the metal on the catalyst surface, mild acidity, and strength distribution of acid sites.29 In our case, the formation of mesopores in the Pt/ZSM-5 catalysts improves the selectivity because of the improved diffusivity within the zeolite pores and, as a consequence, the reduced residence time of the intermediates within the pore channels, preventing secondary cracking to lower hydrocarbons. The improved selectivity could also be a result of the reduced acidity, which is again beneficial toward inhibiting cracking reactions.

REFERENCES

(1) RENEW Project final report, 2008, www.renew-fuel.com. (2) Kalnes, T.; Marker, T.; Shonnard, D. R. Green Diesel: A Second Generation Biofuel. Int. J. Chem. React. Eng. 2007, 5 (1), 1542. (3) Lavrenov, A. V.; Bogdanets, E. N.; Chumachenko, Yu. A.; Likholobov, V. A. Catalytic Processes for the Production of Hydrocarbon Biofuels from Oil and Fatty Raw Materials: Contemporary Approaches. Catal. Ind. 2011, 3 (3), 250. (4) Iliopoulou, E. F.; Heracleous, E.; Drakaki, K.Lappas, A. A. Ptbased Hydroisomerization Catalyst for the Production of High Quality Biofuels from BtL-Naphtha. Conference Proceedings of the Bio4SuD: Biofuels for Sustainable Development of Southern Europe, Thessaloniki, Greece, Nov 19−20, 2012. (5) Iliopoulou, E. F., Heracleous, E., Delimitis, A., Lappas, A. A. Producing high quality biofuels: Pt-based hydroisomerization catalysts evaluated using BtL-naphtha surrogates. Appl. Catal., B, 2013, http:// dx.doi.org/10.1016/j.apcatb.2013.03.026. (6) Martens, J. A.; Jacobs, P. A. The potential and limitations of the n-decane hydroconversion as a test reaction for characterization of the void space of molecular sieve zeolites. Zeolites 1986, 6, 334. (7) Campelo, J. M.; Lee, A. F.; Luque, R.; Luna, D.; Marinas, J. M.; Romero, A. A. Preparation of Highly Active and Dispersed Platinum Nanoparticles on Mesoporous Al-MCM-48 and Their Activity in the Hydroisomerisation of n-Octane. Chem.Eur. J. 2008, 14, 5988. (8) Tromp, M.; van Bokhoven, J. A.; Garriga Oostenbrink, M. T.; Bitter, J. H.; de Jong, K. P.; Koningsberger, D. C. Influence of the Generation of Mesopores on the Hydroisomerization Activity and Selectivity of n-Hexane over Pt/Mordenite. J. Catal. 2000, 190, 209. (9) Van Donk, S.; Broersma, A.; Gijzeman, O. L. J.; van Bokhoven, J. A. Combined Diffusion, Adsorption, and Reaction Studies of n-Hexane Hydroisomerization over Pt/H−Mordenite in an Oscillating Microbalance. J. Catal. 2001, 204, 272. (10) Cejka, J.; Mintova, S. Perspectives of Micro/Mesoporous Composites in Catalysis. Catal. Rev. Sci. Eng. 2007, 49, 457. (11) Pérez-Pariente, J.; Diaz, Y.; Agúndez, J. Organising disordered matter: strategies for ordering the network of mesoporous materials. C. R. Chim. 2005, 8, 569. (12) Tao, Y.; Kanoh, H.; Abrams, L.; Kaneko, K. Mesopore-Modified Zeolites: Preparation, Characterization, and Applications. Chem. Rev. 2006, 106, 896. (13) Egeblad, K.; Christensen, C. H.; Kustova, M.; Christensen, C. H. Templating Mesoporous Zeolites. Chem. Mater. 2008, 20 (3), 946. (14) Ogura, M. Towards Realization of a Micro- and Mesoporous Composite Silicate Catalyst. Catal. Surv. Asia 2008, 12, 16.

4. CONCLUSIONS This study investigates the introduction of mesoporosity in ZSM-5 through alkaline and consecutive alkaline and acid treatments and their effect on the catalytic performance of Pt catalysts supported on the treated ZSM-5 supports for hydroisomerization of BtL-naphtha. Characterization showed that the alkaline treatment induces slight desilication, leading to limited removal of Si from the zeolitic framework and an increase in the mesopore volume. BJH analysis revealed the development of mesopores with a broad pore-size distribution centered around ∼10 nm. Successive acid treatment led to an increase in the Si/Al ratio and an increase in the total BET surface area, indicating that the acid dissolves the extraframework alumina deposits on the mesopores and external surface area. Determination of the acidic properties showed that alkaline treatment reduces the total acidity with a 30% increase in the Lewis acid sites and a 38% reduction in the amount of Brönsted sites. This indicated that, during alkaline treatment, Si and Al are both extracted from the zeolytic framework; however, the Al species realuminate on the zeolite surface as extra-framework Al, giving rise to the Lewis acidity and reduction of Brönsted sites. Further acid treatment leads to partial restoration of the initial acidity, confirming that this treatment dissolves the extra-framework alumina deposits. F



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ZSM-5 Catalysts for Hydroisomerization

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