Selective Production of Propylene from Methanol over Monolith

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Selective Production of Propylene from Methanol over MonolithSupported Modified ZSM‑5 Catalysts Mohammad Ashraf Ali,*,‡ Nadhir A. Al-Baghli,† Mohammad Nisar,† Zuhair O. Malaibari,† Ahmed Abutaleb,‡ and Shakeel Ahmed*,§ Department of Chemical Engineering and §Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia ‡ Chemical Engineering Department, College of Engineering, Jazan University, Gizan 45142, Saudi Arabia Downloaded via UNITED ARAB EMIRATES UNIV on January 13, 2019 at 02:13:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

ABSTRACT: The catalytic activity of ZSM-5 zeolites with a SiO2/Al2O3 molar ratio of 30, 50, 80, 280, and 410 was investigated in a fixed bed continuous flow reactor system and was found that the ZSM-5 zeolite with SiO2/Al2O3 molar ratio of 280 (HZ-280) exhibited best catalyst performance. The optimized reaction conditions achieved were 500 °C, 1 bar pressure, and weight hourly space velocity of 15 h−1 using methanol as feed. At optimum reaction conditions, HZ-280 exhibited propylene selectivity of 47.3% and propylene yield 17.4% with 100% methanol conversion. HZ-280 zeolite was modified with P, Ce, Fe, and La to select the best promoter to enhance propylene selectivity and yield. The best-modified catalyst obtained was HZ-280 with 0.1 wt % phosphorus loading, which further improved propylene selectivity by 14% and yield by 24.7%. Then, the monolith structured catalysts were prepared by single-layer (6.8%), double-layer (10.3%), and triple-layer (13.1%) coating of HZ-280 catalyst. HZ-280 single-layer-coated monolith-structured catalyst effectively increased propylene selectivity by 19.2% and yield by 34.5% with no liquid hydrocarbons in the product. HZ-280-coated monolith catalyst was regenerated and was reused for three cycles. Negligible activity loss was observed for methanol conversion and propylene selectivity. This reflects that the structured catalyst is viable and economical for commercial applications. Analytical techniques such as X-ray diffraction, scanning electron microscopy−energy-dispersive X-ray, Brunauer−Emmett−Teller, and NH3-temperature-programmed desorption were used for characterization of the catalysts.

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methanol to hydrocarbons in the year 1977.3 On the basis of the product distribution, zeolites silicoaluminates ZSM-5 and silicoaluminophosphates SAPO-34 are most commonly used as catalysts in the MTO process.4 The zeolite can be prepared by considering compositions, acidity, and structure of catalysts. In recent time nanozeolites have also shown their importance because of the presence of large surface area and microporosity.5 Catalyst modification by promoters is an effective way of modification of zeolite catalysts which result in better catalyst performance. In the MTP reaction, the weak acid sites are important and are considered as the active sites for olefins production. Weak acid sites play an important role in reducing the formation of alkanes and aromatics; it also provides better stability and anticoking capability than the strong acid sites in the MTP reaction. It is a well-known fact that the side reactions on active sites cause coke formation which results in deactivation of active sites. The ZSM-5 catalyst life is enhanced by the presence of weak acid sites instead of strong acid sites because of reduced coke formation. Because the acidity plays an important role in the MTP reaction, it is important to control the relative quantity of weak and strong acid sites for better results. Promoters do the same and also provide extra acid sites on the surface and enhance the performance of catalysts. Metal and nonmetal loading and generation of

INTRODUCTION Ethylene and propylene are two important commodity petrochemicals used as feedstock for the production of polyethylene, ethylene oxide, polypropylene, and polyacrylonitrile. Packaging industries are growing everywhere around the globe which needs the increased production of plastics, especially polyethylene and polypropylene which are used as an alternative to paper, steel, and wood. Global ethylene and propylene production have reached 200 million tons annually.1 Catalytic cracking and steam cracking of naphtha from petroleum refineries have been used to produce propylene as a byproduct. Growing propylene demand forced to develop a new and easy alternative route for the production of propylene which could be achieved by methanol to propylene (MTP) process. The advantage of this process is high olefins selectivity especially propylene with maximum methanol conversion. Many countries in Asia mainly China have coal but do not have much crude oil, so they really need to meet the huge demand of propylene by alternative routes. Such a route is by gasifying coal to synthesis gas and transforms into methanol which then converted to olefins through coal to methanol to olefins process.2 Various zeolite catalysts are used for a wide range of industrial applications because of their unique properties. The success story of converting MTP commercially was achieved during the period of the 1990s, since then the increase in demand for propylene is the driving force for developing a catalyst which can increase propylene selectivity in MTP reaction. Chang and Silvestri used acidic zeolites to convert © XXXX American Chemical Society

Received: November 21, 2018 Revised: December 27, 2018 Published: December 31, 2018 A

DOI: 10.1021/acs.energyfuels.8b04020 Energy Fuels XXXX, XXX, XXX−XXX

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g of deionized water was prepared. The slurry was stirred for 6 h on a magnetic stirrer at 600 rpm at ambient temperature then treated in an ultrasonic bath for 15 min. In the final third step, the monolith was dipped in the prepared slurry for 3 min. Dry air was passed through the monolith channels to remove excess slurry. Zeolite-coated monolith was dried at 110 °C with heating rate 2 °C/min in the rotating oven for 12 h followed by calcination at 500 °C with a heating rate of 3 °C/min for 4 h. The prepared structured catalyst was treated in an ultrasonic bath for 1 min, and the weight loss by ultrasonic treatment provides adhesion strength of structured catalyst.31,32 The amount of catalyst coating adhesion was obtained from monolith-coated zeolite samples. To increase zeolite coating, the monolith was dipped two to three times in the left catalyst slurry. About 1% weight loss was observed after ultrasonic treatment for single-layer coating. Figure 1 shows the zeolite coated structured monolith catalyst, (a) front and top view while (b) represents the enlarged view of a monolith channel wall with catalyst washcoat.

mesopores in ZSM-5 zeolite result in high propylene selectivity and better catalyst performance because of modification in acidity and porosity.6−9 In recent years, high propylene selectivity has been reported in the literature by incorporating boron, phosphorus, iron, nickel, iridium, and manganese on the parent zeolite catalysts.10−16 Jiao and co-workers have reported a 21% increase in propylene selectivity by Fe loading on ZSM-5 (SiO2/Al2O3 = 100) because of a decrease in the strong acid sites.12 Ni-SAPO-34 catalyst improved ethylene selectivity because of an improved framework structure.17 In our work, modification of the zeolite catalyst (SiO2/Al2O3 = 280) was accomplished using Ce, Fe, La, and P as promoters. Structured catalysts are introduced to obtain better results in the MTP process. In recent years, high thermal conductivity and better mechanical strength materials are used to develop such kind of catalysts having multiple parallel channels. Some important structured supports that are used typically in the preparation of structured catalysts are the ceramic monolith, ceramic foam and metallic monolith.18,19 Structured catalysts have crucial advantages for better mass and heat transfer, lowpressure drop, and contact time because of shorter diffusion distance and faster intradiffusion rate of both reactants and products. These advantages are key factors for replacing packed bed reactors by a structured catalysts reactor. The three-dimensional structure alters the mass and heat-transfer properties. High cell density and the thinner wall of structured catalysts exhibit maximum conversion and propylene selectivity in MTP.20 The monolith reactor is the most widely used structured catalytic system.21 Lefevere and Gysen reported 100% methanol conversion with a C2−C3 selectivity of 41% using a honeycomb monolith structured catalyst using ZSM-5 (25)-coated stainless steel 3DFD at a space velocity of 18 h−1 at 350 °C.22 Patcas studied the effect of ceramic foams in methanol to olefins conversion and reported the substantial activity and selectivity of olefins as compared to zeolite pellets.18 In our study, we used ceramic cordierite honeycomb monolith.23 A comparison was made between honeycomb monolith-structured catalyst and packed bed P-modified pelletized HZ-280 catalyst. Zeolite coating on the monolith was done by the dip coating method.24 The increase in the coating amount causes a negative effect on methanol conversion and propylene selectivity because of diffusion limitation in the intrapores of the catalyst wall.

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Figure 1. (a) Front and top views of prepared monolith catalyst and (b) enlarged view of zeolite-coated monolith channel wall. Catalyst Characterization. The prepared catalysts were characterized by X-ray diffraction (XRD), NH3-temperature-programmed desorption (TPD), scanning electron microscopy (SEM), energydispersive X-ray (EDX), and Brunauer−Emmett−Teller (BET) surface analysis. The XRD was done on a Rigaku MiniFlex diffractometer, with monochromatic Cu Kα radiation source (λ = 1.5406 × 10−1 nm) at room temperature, 50 mA current, 2θ in the range of 5°−50° with a step size of 0.02°, 3° scan/min and electrical voltage of 10 kV. The NH3-TPD analysis was carried out for the acidity measurement. Acidity is an important factor of zeolite catalyst which reflects the product distribution in the MTP process. USA made equipment AutoChem II 2029 analyzer equipped with online thermal conductivity detector (TCD) that was used to conduct TPD analysis of all samples. Prior to TPD analysis, all samples were calcined at 500 °C for 6 h in a furnace to remove any contaminated impurity. Catalyst samples (0.05−0.1 g) were loaded inside a U-shape quartz tube and degassed at 500 °C for 3 h by flowing argon at 30 mL/min. The sample inside the tube was cooled to 120 °C using a 5% NH3/He gas mixture flow rate of 50 mL/min. The system was then purged using helium gas at 100 °C with 50 cm3/min flow rate to remove unadsorbed NH3 gas and adsorbed ammonia in the catalyst sample. The catalyst bed temperature was raised to 750 °C for the NH3 desorption with a ramp rate of 10 °C per min. The ammonia was desorbed as the temperature elevated to 750 °C. TCD monitors the concentration of NH3 gas in the effluent. The BET surface area and pore volume of different samples were measured using a Micromeritics ASAP 210 analyzer using N2 adsorption at 77 K. Samples (0.2 g) were degassed using N2 flow at 300 °C for 3 h to remove moisture adsorbed in the pores and surface.

EXPERIMENTAL SECTION

Catalyst Preparation. Commercial powdered ZSM-5 zeolite in NH3 form having SiO2/Al2O3 molar ratios of 30, 50, 80, 280, and 410 were procured from Zeolyst International, UK. These catalysts were calcined at 550 °C for 6 h to convert into H-form. Incipient impregnation technique was used for the modification of catalysts. Metal and nonmetal salt solutions with required amount of loadings were prepared, and solutions were added dropwise to the powdered catalyst and all impregnated samples were dried overnight at 110 °C and calcined at 450 °C for 6 h. Prior to testing on a fixed bed reactor system, all samples were pelletized, crushed, and sieved to 0.5−1 mm size. The zeolite coating on a structured support can be done by either washcoating or hydrothermal coating or a combination of both.25−30 Honeycomb cordierite monolith-structured support with 400 cpsi was used for zeolite coating in the study. The structured catalyst was prepared by the dip coating method which falls in the washcoating technique. Dip coating was done in three steps; in the first step, cordierite monolith support was cleaned using 5 wt % HNO3 solution for 20 min and then dried at 110 °C for 6 h followed by calcination at 550 °C for 6 h. In the second step, a slurry having 20 wt % ZSM-5 zeolite with 1 wt % colloidal silica (LUDOX AS-40) as a binder in 20 B

DOI: 10.1021/acs.energyfuels.8b04020 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Schematic representation of the fixed bed bench-scale plant setup. The nitrogen flow was 44 mL/min, and methanol flow was 15 g h−1. (0.0018 mol min−1 = 0.0504 g min−1 = 3.024 g h−1 = 2640 mL h−1). Therefore, the weight ratio of N2/MeOH was 0.202 (mole ratio N2/ MeOH 0.230). The reaction was carried out for 2.5 h, and the product was collected and analyzed three times at 30, 90, and 150 min using an Agilent gas chromatograph model 7890B equipped with two detectors, TCD connected to GASPRO column (60 m × 320 μm), and flame-ionization detector with an INNOWax column (30 m × 320 μm × 0.5 μm). Methanol conversion, propylene selectivity, and yield are calculated as33

The BET method was used to calculate the total specific surface area in relative pressure p/p0 range from 0.0 to 0.25. The total pore volume and pore size distribution were estimated from the Barrett−Joyner− Halenda method by adsorbed nitrogen at N2 relative pressure p/p0 0.99. Crystal morphology was investigated by SEM while elemental composition analysis was done by EDX using a JEOL 820 scanning electron microscope (JSM 6460) equipped with an AN10000 EDX system. For SEM−EDX analysis, the sample was spread on a pin stub using a copper tape. The gold coating was done on the samples using a Cressington sputter coater for 1.5 min to avoid any charge build-up during the analysis and improve secondary electron signal which helps to obtain better contrast and high magnification of one million times. Catalyst Evaluation. The ZSM-5 zeolites with a SiO2/Al2O3 ratio of 30, 50, 80, 280, and 410, metal and nonmetal modified zeolites, and zeolite coated monolith structured catalysts were evaluated for MTP. The reaction variables were temperature, pressure, and space velocity and were optimized to get the best conditions of reaction temperature 500 °C, 1 bar pressure, and 15 h−1 weight hourly space velocity (WHSV). Initial experiments were conducted over a range of 400, 450, and 500 °C. At temperature above 500 °C, thermal cracking was expected. Most of the literature cited conducted experiment at reaction temperature at 500 °C to avoid thermal cracking. In case of WHSV we used in the range of 5−20 h−1, it was observed that higher than 15 h−1, no further increase was observed in propylene yield. Therefore, all experiments were conducted at 500 °C and 15 h−1 for comparison purposes. All metal- and nonmetal-modified and HZ-280 parent zeolitecoated structured catalysts were investigated using these optimized process conditions. The catalytic performance of the catalyst samples was investigated in a fixed bed reactor system made of stainless steel having an internal diameter of 21 mm and overall length of 300 mm with 200 mm heating zone. The schematic view of the bench-scale plant setup used for testing the structured monolith-based catalysts is shown in Figure 2. The monolith-coated catalyst was placed in the middle of the reactor, and a thermocouple was attached to it from the side so that the precise reaction temperature is measured. The feed flow rate was adjusted according to the weight of the zeolite coated (0.32 g) on the monolith to achieve WHSV of 15 h−1, and nitrogen flow was 44 mL/min. Before testing the activity of the pelleted catalyst, the parent and modified powder zeolite catalysts were pelletized under high pressure using a 1 in. die. The pellets were then crushed and sieved to obtain 0.5−1.0 mm size particles. For each test, 1.0 g of granulated catalyst was loaded into the middle of the isothermal zone of the reactor and supported on stainless steel mesh.

Conversion (%) mass of methanol in the feed − mass of methanol in the product = mass of methanol in the feed (1) × 100 Selectivity (%) =

Yield (%) =

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mass of product (gas) × 100 mass of gaseous hydrocarbons

mass of product × 100 mass of methanol in the feed

(2) (3)

RESULTS AND DISCUSSION Phase, Porosity, and Acidity Analysis. The XRD pattern of parent and P-modified HZ-280 (ZSM-5 with a SiO2/Al2O3 molar ratio 280) is shown in Figure 3a. The pattern obtained clearly indicates that the samples have a typical MFI structure of standard zeolite ZSM-5 catalyst.34 The intensity of diffraction peaks for parent and P-modified zeolite are almost the same, which confirms that there is no significant change in crystallinity after 0.1 wt % P modification. Figure 3b shows the XRD pattern of zeolite HZ-280, monolith support, and HZ280-coated monolith-structured catalyst. XRD patterns obtained for zeolite-coated monolith have extra peaks highlighted on the pattern confirms the presence of zeolite HZ-280 catalyst on monolith wall. The results of NH3-TPD are summarized in Table 1. From the results obtained, it is clear that the increase in the SiO2/ Al2O3 ratio causes a decrease in strong acid sites and total acidity of catalysts. The NH3-TPD profile for parent and PC

DOI: 10.1021/acs.energyfuels.8b04020 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. Ammonia-TPD profile comparison of (a) HZ-30, 50, 80 and 280, and (b) HZ-280 and 0.1 P HZ-280 catalysts.

The BET surface area results are shown in Table 2. The surface area of all the catalysts was in the range of 308−390 Table 2. Textural Properties of Zeolites

Figure 3. XRD patterns of (a) HZ-30, HZ-50, HZ-80, HZ-280, and HZ-410, (b) parent and P-modified HZ-280, and (c) HZ-280, monolith, and HZ-280 zeolite-coated monolith.

Table 1. Acidity of Zeolites Having Different SiO2/Al2O3 Ratios Used in This Study weak (W)

strong (S)

total

W/S acidity ratio

30 50 80 280 410

0.36 0.18 0.08 0.02 0.005

0.22 0.15 0.07 0.03 0.02

0.58 0.32 0.15 0.05 0.025

1.63 1.20 1.14 0.66 0.25

ABET (m2/g)

AM (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Vtotal (cm3/g)

DAA (nm)

HZ-30 HZ-50 HZ-80 HZ-280 HZ-410 0.1 P HZ-280

308.6 340.8 342.8 390.1 329.0 384.9

202.8 190.6 177.1 329.6 214.8 324.0

0.098 0.093 0.086 0.152 0.103 0.144

0.122 0.154 0.134 0.056 0.078 0.067

0.207 0.225 0.220 0.209 0.210 0.211

6.4 7.3 4.4 2.8 2.7 2.5

m2/g. Textural properties reveal that the increase in the SiO2/ Al2O3 ratio of ZSM-5 develops more micropores and reduces mesopores, resulting in the reduction of pore average diameter which improves olefins production. From the results of parent and P-modified zeolite HZ-280, it can be seen that there is a slight change in the total surface area, micropore surface area, and pore volume of P-modified HZ-280 compared with the parent HZ-280 catalyst. This indicates that there is no considerable change in the MFI structure of the catalyst. The decrease in the pore diameter of P-modified HZ-280 results in negligible aromatics formation. It is believed that the aromatics formed during the reaction diffuse back and would have been dissociating to olefins and hence improves the olefins selectivity. Figure 5 shows the surface morphology of parent zeolite and 0.1% P HZ-280. The SEM images show high crystallinity of zeolites samples, especially of HZ-280. The crystals shown are either cubical or elliptical in shape with uniform size distribution. The amorphous impurities were not observed. The scan area shows highly crystalline shaped crystals. The SEM images of low silica to alumina zeolites show an irregular

acidity (mmol/g) SiO2/Al2O3 ratio

sample

modified HZ-280 catalysts is shown in Figure 4. The profile of the sample shows two well-resolved peaks, the first peak that appears in the profile is attributed as weak acid-Lewis site, while the second peak appears as the strong Brønsted acid site.35 The P-modified catalyst has a similar TPD profile comparing with the parent catalyst sample, but there is a shift of weak acid sites from 450 to 440 °C. The acidity obtained for HZ-280 was 0.054 mmol/g and for 0.1 P HZ-280, it was 0.031 mmol/g. The results revealed that there was a partial elimination of strong acid sites after P modification of zeolite HZ-280 sample. D

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Figure 6. SEM images of samples: (a,b) monolith support, (c,d) single-layer coating, (e) double-layer coating, and (f) triple-layer coating.

Figure 5. SEM of zeolites having SiO2/Al2O3 ratios of (a) 30, (b) 50, (c) 80, (d) 280 (e) 410, and (f) 0.1 P HZ-280.

characterization. Table 3 shows the results obtained by EDX analysis.

shape and size of the crystal. Figure 6 shows the surface morphology of monolith support and HZ-280 zeolite-coated monolith-structured catalysts. Figure 6a,b shows the surface morphology of the honeycomb monolith support, and the surface indicates the porous nature of monolith on which the zeolite can diffuse and remain stick without any attrition. The average thickness of the monolith wall was found to be 195 μm while the average length between the walls of a channel is 1006 μm. Figure 6c,d shows the images of single-layer monolithcoated zeolite HZ-280 catalyst. The images indicate that there is no change in the surface morphology of the zeolite after coating with 1% binder (LUDOX AS-40). It can be seen clearly that the zeolite dispersed uniformly on the surface of monolith without leaving any big area uncoated. Figure 6e,f shows the images of double- and triple-layer monolith-coated catalyst, respectively. It can be seen that there is a successive increase in the coating thickness from single- to double- and the triple-layer-coated monolith. The zeolite coating thickness for single-layer, double-layer, and triple-layer monolith-coated HZ-280 catalysts are obtained to be 4.2, 6.8, and 10.4 μm. EDX for elemental analysis was performed during SEM

Table 3. EDX Analysis Results of Zeolites elemental analysis (wt %) sample

Si

Al

HZ-30 HZ-50 HZ-80 HZ-280 HZ-410 0.1 P HZ-280

36.41 30.24 31.74 44.12 45.37 41.60

2.22 1.16 0.77 0.28 0.21 0.25

P

SiO2/Al2O3 molar ratio

0.09

32 50 79 283 417 283

Effect of SiO2/Al2O3 Ratio. The SiO2/Al2O3 ratio of the zeolite significantly influences the product distribution in the MTP process (Table 4) with an increase in propylene selectivity and yield at optimized conditions, that is, 500 °C temp, 1 bar pressure, and 15 h−1 WHSV. Strong acid sites cause aromatization of olefins in MTP reaction; thus, aromatics were considered as detrimental to the olefins generation.36,37 E

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Energy & Fuels Table 4. Conversion, Selectivity, and Yield of Zeolite Catalysts Having Different SiO2/Al2O3 Molar Ratiosa SiO2/Al2O3 molar ratio methanol conversion (%) Selectivity (%) C1 C−2 C=2 C−3 C=3 C=4 C4−5 C=5+ C=3 /C=2 Yield (%) C=2 C=3 C=4 aromatics

Table 5. Conversion, Selectivity, and Yield Results for Phosphorus Modified HZ-280a

30 100

50 98

80 98

280 100

410 88

sample conversion

9.1 2.3 18.1 9.1 21.6 9.9 29.9 0 1.2

8.4 2.0 18.2 6.6 27.6 10.7 26.5 0 1.5

8.6 0.8 15.6 3.4 37.8 18.7 15.1 0 2.4

0.3 0.2 11.8 2.1 47.3 28.7 8.0 1.6 4.1

0 0.1 10.8 1.4 49.2 31.1 6.4 1.0 4.6

C=2 C=3 C=4

4.6 8.3 5.1 8.0

4.4 10.0 5.1 6.3

3.8 13.0 9.2 4.0

3.1 17.4 14.0 2.4

4.9 16.4 9.7 1.7

C=2 C=3 C=4

0.5 wt % P 97.5 11.4 49.9 30.0 2.9 18.9 15.1

Reaction conditions: T = 500 °C, P = 1 bar, WHSV = 15 h−1, TOS = 2.5 h. a

Table 6. Conversion, Selectivity, and Yields for Parent and Modified HZ-280 Catalystsa promoter amount (wt %) methanol conversion (%)

−1

Reaction conditions: T = 500 °C, P = 1 bar, WHSV = 15 h , TOS = 2.5 h. a

0.1 wt % P 0.2 wt % P 100 98.6 Selectivity 10.9 8.9 53.9 52.0 29.4 30.1 Yield 2.9 2.2 21.7 19.7 15.7 15.1

C1 C−2 C=2 C−3 C=3 C4= C4−5 C=3 /C=2

The results in Table 4 clearly indicate that with an increase in SiO2/Al2O3 ratio, aromatics formation was reduced, which improves the olefin selectivity. It can also be seen that there is a substantial drop in the formation of alkanes with high SiO2/ Al2O3 because of a decrease in strong acid sites, and it also suppresses the side reactions causing coke formation. The activity of acid sites of zeolite reduced constantly because of coke formation, hence affecting the performance of the catalyst.38 High acidity catalysts can be considered if the target is alkanes. Very high SiO2/Al2O3 is not acceptable in the MTP process because the high ratio causes the presence of low active sites, which results in a drop of methanol conversion which can be seen with HZ-410; therefore, there should be an appropriate ratio of strong and weak acid sites to boost propylene production. From the observations presented in Table 4, with the increase in SiO2/Al2O3 of zeolite, propylene, and butylene become predominant products in MTP while ZSM-5 with SiO2/Al2O3 ratio 280 was considered best which gave 47.3% propylene selectivity and 17.4% yield with 100% methanol conversion. For further improvements in our results, we work on this catalyst and did modifications by using promoters and coating on macroporous monolith support. Effect of Promoters. It is a well-known fact that loading of promoters on zeolite has both a positive and negative effect on the properties of the catalysts including acidity and pore structures.11 Because acidity and pore structures modification are main key factor for the formation of various hydrocarbons in MTP reaction, there is a need to optimize these factors to get a more desired product which can be done using appropriate promoters.10,14 Table 5 shows conversion, selectivity, and yield results using modified HZ-280 having different concentrations of phosphorus. The best results were achieved using 0.1 wt % P loading on the zeolite HZ-280. Table 6 shows the effect of Ce, Fe, and La with 5% loading on HZ-280. All three promoters increase propylene selectivity with 100% methanol conversion. Comparable results were obtained for the three promoters. There is an increase in selectivity of ethylene with Ce, Fe, and La which can be understood by the hydrocarbon pool mechanism.39,40 Methylbenzene and alkene are two HC’s pool cycles according to

C=2 C=3 C=4 aromatics

parent 0.0 100 Selectivity (%) 0.3 0.2 11.8 2.1 47.3 28.7 8.0 4.1 Yield (%) 3.1 17.4 14.0 2.4

Ce 0.5 100

Fe 0.5 100

La 0.5 100

1.2 0.6 14.6 1.8 50.7 26.4 4.0 3.5

0.3 0.2 13.2 2.5 49.1 28.5 6.2 3.7

0.3 0.2 14.6 2.3 48.5 27.5 6.4 3.4

3.5 19.3 13.4 1.9

3.3 18.4 14.3 1.5

3.5 18.2 14.1 1.7

Reaction conditions: T = 500 °C, P = 1 bar, WHSV = 15 h−1, TOS = 2.5 h. a

this mechanism. Formation of more ethylene reflects HC’s formation and follows the alkene pool cycle. While P loading results indicate that the HC’s formation follows the methylbenzene pool cycle, therefore there is a substantial decrease in aromatics because of dissociation of polymethylbenzenes. Table 7 shows the catalytic activity of 0.1 wt % P-loaded zeolites with SiO2/Al2O3 30, 50, 80, and 410. By comparing the results, it is very clear that phosphorus modification improves the olefins selectivity, especially selectivity and yield of propylene for all zeolite catalysts. Phosphorus modification on HZ-410 increases the methanol conversion by 7%. There is a marked decrease observed in aromatics and higher alkanes probably because of the partial elimination of strong acid sites and a reduction in pore diameter of the catalysts. Effect of Zeolite Coating on the Monolith. The monolithcoated catalyst was evaluated at 500 °C, 1 bar pressure and WHSV of 15 h−1. Results tabulated in Table 8 shows that propylene selectivity 56.4% with 100% methanol conversion was obtained with a zeolite HZ-280 single-layer monolithcoated catalyst. Radial diffusion of reacting species is dominating inside the monolith catalyst channels than the vertical axial diffusion in the packed bed catalyst. Increasing the amount of zeolite coating on the monolith support results in a drop of methanol conversion that can be seen from the results F

DOI: 10.1021/acs.energyfuels.8b04020 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 7. Conversion, Selectivity, and Yields of 0.1 wt % PModified Zeolites with Various SiO2/Al2O3 Ratiosa SiO2/Al2O3 methanol conversion (%) C1 C−2 C=2 C−3 C=3 C=4 C4−5 C=3 /C=2 C=2 C=3 C=4 aromatics

30 50 100 100 Selectivity (%) 7.0 6.5 2.1 1.0 15.9 14.9 12.5 7.3 25.1 30.3 10.9 21.3 26.5 18.7 2.0 2.4 Yield (%) 3.6 3.3 10.0 11.9 5.8 10.5 7.1 5.1

80 100

280 100

410 95

5.1 1.2 12.3 4.1 39.3 22.3 15.7 3.4

0.1 0.0 10.9 1.5 53.9 29.4 3.7 5.0

0.0 0.0 8.2 2.0 50.5 32.3 4.2 6.0

3.0 14.4 10.9 2.9

2.9 21.7 15.7 1.0

2.0 17.2 10.2 1.1

a Reaction conditions: T = 500 °C, P = 1 bar, WHSV = 15 h−1, TOS = 2.5 h.

Table 8. Effect of Single, Double, and Triple Layer of Zeolite HZ-280 Coating on the Monolitha wt of zeolite (g) catalyst coating (%) methanol conversion (%) C1 C−2 C=2 C−3 C=3 C=4 C−4−5 C=3 /C=2 C=2 C=3 C=4 aromatics

single layer

double layer

triple layer

0.38 6.8 100 Selectivity (%) 0.0 0.0 8.1 1.1 56.4 30.2 4.2 7.0 Yield (%) 2.2 23.4 16.8 0.0

0.56 10.3 98.0

0.71 13.1 94.3

0.0 0.0 8.9 1.3 53.6 30.1 6.1 6.0

0.0 0.0 9.3 1.7 50.9 26.8 10.1 5.5

2.3 22.5 15.7 0.0

2.5 20.1 12.9 0.0

Figure 7. Comparison of (a) methanol conversion, (b) propylene selectivity, (c) propylene yield, and (d) aromatics yield with time. Reaction conditions: T = 500 °C, P = 1 bar, WHSV = 15 h−1.

drop in conversion was more in the parent catalyst because of more formation of aromatics in the product stream. The dearomatization was observed maximum in monolith catalyst than P-modified and parent catalysts because of the polymethylbenzene route of hydrocarbon pool mechanism,39,40 in which polyaromatics dissociate into olefins which improve propylene selectivity to a great extent. Because of no aromatics generation in monolith catalyst, the rate of drop in conversion is slow as compared to parent and Pmodified catalysts. The regenerability of spent catalyst was investigated as well. After 40 h on stream, the conversion of methanol was dropped by 30%. At this stage, the catalyst was regenerated at 550 °C in air for 3 h. The regenerated catalyst was tested for 40 h for three cycles. The results exhibited negligible activity loss in the performance of methanol conversion by 0.3% and propylene selectivity by 0.6%. This reflects that the structured catalyst is viable and economical for commercial applications. Monolith-structured catalyst showed improved results owing to the enhanced mass and heat transfer, low-pressure drop, and contact time because of shorter diffusion distance and faster intradiffusion rate of both reactants and the products. The ZSM-5-coated monolith-structured catalyst results revealed that these catalysts are the benchmark for the conversion of methanol to light olefins.

a Reaction conditions: T = 500 °C, P = 1 bar, WHSV = 15 h−1, TOS = 2.5 h.

of double- and triple-layer coating. With the increase in zeolite coating by double-layer and triple-layer, the drop in methanol conversion is due to resistance created by product species inside the monolith channel; this resistance offers intrapore diffusion limitations to the reactant species. By comparing results, it is clear that monolith-structured catalyst has an advantage over parent and modified zeolite catalysts. Catalyst Performance of Parent, P-Modified, and Monolith-Coated HZ-280 Catalysts. The catalyst performance for methanol conversion propylene selectivity and yield and aromatics as a function of time is shown in Figure 7. Starting with 100% methanol conversion, HZ-280-coated monolith catalyst remains stable with methanol conversion 82% after 41.5 h while conversion drops by 33 and 24% for parent and P-modified catalysts. Propylene selectivity improves significantly from 47.3 to 53.9% and 56.4% for a parent, Pmodified and coated monolith catalysts. It is a well-known fact that the catalyst deactivation is due to coking.41 The rate of G

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over a novel and large partical size ZSM-5. J. Nat. Gas Chem. 2011, 20, 261−265. (5) Konno, H.; Tago, T.; Nakasaka, Y.; Ohnaka, R.; Nishimura, J.-i.; Masuda, T. Effectiveness of nano-scale ZSM-5 zeolite and its deactivation mechanism on catalytic cracking of representative hydrocarbons of naphtha. Microporous Mesoporous Mater. 2013, 175, 25−33. (6) Liu, J.; Zhang, C.; Shen, Z.; Hua, W.; Tang, Y.; Shen, W.; Yue, Y.; Xu, H. Methanol to Propylene: Effect of Phosphorus on a High Silica HZSM-5 Catalyst. Catal. Commun. 2009, 10, 1506−1509. (7) Zhao, T.-S.; Takemoto, T.; Tsubaki, N. Direct Synthesis of Propylene and Light Olefins from Dimethyl Ether Catalyzed by Modified H-ZSM-5. Catal. Commun. 2006, 7, 647−650. (8) Jin, Y.; Asaoka, S.; Zhang, S.; Li, P.; Zhao, S. Reexamination on transition-metal substituted MFI zeolites for catalytic conversion of methanol into light olefins. Fuel Process. Technol. 2013, 115, 34−41. (9) Mei, C.; Wen, P.; Liu, Z.; Liu, H.; Wang, Y.; Yang, W.; Xie, Z.; Hua, W.; Gao, Z. Selective production of propylene from methanol: Mesoporosity development in high silica HZSM-5. J. Catal. 2008, 258, 243−249. (10) Xu, A.; Ma, H.; Zhang, H.; Weiyong, D.; Fang, D. Effect of boron on ZSM-5 catalyst for methanol to propylene conversion. Pol. J. Chem. Technol. 2013, 15, 95−101. (11) Liu, J.; Zhang, C.; Shen, Z.; Hua, W.; Tang, Y.; Shen, W.; Yue, Y.; Xu, H. Methanol to propylene: Effect of phosphorus on a high silica HZSM-5 catalyst. Catal. Commun. 2009, 10, 1506. (12) Jiao, M.; Fan, S.; Zhang, J.; Su, X.; Zhao, T.-S. Methanol-toolefins over FeHZSM-5: Further transformation of products. Catal. Commun. 2014, 56, 153−156. (13) Mohammadrezaei, A.; Papari, S.; Asadi, M.; Naderifar, A.; Golhosseini, R. Methanol to propylene: the effect of iridium and iron incorporation on the HZSM-5 catalyst. Front. Chem. Sci. Eng. 2012, 6, 253−258. (14) Yang, Y.; Sun, C.; Du, J.; Yue, Y.; Hua, W.; Zhang, C.; Shen, W.; Xu, H. The synthesis of endurable B−Al−ZSM-5 catalysts with tunable acidity for methanol to propylene reaction. Catal. Commun. 2012, 24, 44−47. (15) Rostamizadeh, M.; Taeb, A. Highly selective Me-ZSM-5 catalyst for methanol to propylene (MTP). J. Ind. Eng. Chem. 2015, 27, 297−306. (16) Hadi, N.; Niaei, A.; Nabavi, S. R.; Alizadeh, R.; Shirazi, M. N.; Izadkhah, B. An intelligent approach to design and optimization of MMn/H-ZSM-5 (M: Ce, Cr, Fe, Ni) catalysts in conversion of methanol to propylene. J. Taiwan Inst. Chem. Eng. 2016, 59, 173−185. (17) Inoue, M.; Dhupatemiya, P.; Phatanasri, S.; Inui, T. Synthesis course of the Ni-SAPO-34 catalyst for methanol-to-olefin conversion. Microporous Mesoporous Mater. 1999, 28, 19−24. (18) Patcas, F. The methanol-to-olefins conversion over zeolitecoated ceramic foams. J. Catal. 2005, 231, 194−200. (19) Ding, J.; Zhang, Z.; Han, L.; Wang, C.; Chen, P.; Zhao, G.; Liu, Y.; Lu, Y. A self-supported SS-fiber@meso-HZSM-5 core−shell catalyst via caramel-assistant synthesis toward prolonged lifetime for the methanol-to-propylene reaction. RSC Adv. 2016, 6, 48387− 48395. (20) Guo, W.; Xiao, W.; Luo, M. Comparison among monolithic and randomly packed reactors for the methanol-to-propylene process. Chem. Eng. J. 2012, 207−208, 734−745. (21) Avila, P.; Montes, M.; Miró, E. E. Monolithic reactors for environmental applications: A review on preparation technologies. Chem. Eng. J. 2005, 109, 11−36. (22) Lefevere, J.; Gysen, M.; Mullens, S.; Meynen, V.; Van Noyen, J. The benefit of design of support architectures for zeolite coated structured catalysts for methanol-to-olefin conversion. Catal. Today 2013, 216, 18−23. (23) Edvinsson, R. K.; Holmgren, A. M.; Irandoust, S. Liquid-Phase Hydrogenation of Acetylene in a Monolithic Catalyst Reactor. Ind. Eng. Chem. Res. 1995, 34, 94−100.

CONCLUSION A study was conducted to produce ZSM-5 zeolite-coated monolith-structured catalysts for enhanced propylene selectivity and yields from methanol conversion. First, the effect of SiO2/Al2O3 ratio was investigated, and it was concluded that ZSM-5 zeolite with SiO2/Al2O3 ratio 280 (HZ-280) showed 100% methanol conversion, 47.3% propylene selectivity, and 17.4% yield. The effect of loading P, Ce, Fe, and La as promoters on HZ-280 exhibited that all these promoters have a positive effect on propylene selectivity. However, 0.1 wt % P has effectively increased the propylene selectivity by 14% and yield by 24.7% with 100% methanol conversion. This improvement was due to partial removal of strong acid sites. The single-layer monolith-coated structured catalyst with 6.8% HZ-280 coating had shown a pronounced improvement by increasing propylene selectivity by 19.2% and propylene yield by 34.5% with 100% methanol conversion. There was a drop in methanol conversion and propylene selectivity in the doublelayer and triple-layer monolith-coated HZ-280 catalysts. The zeolite coating thickness for single-layer, double-layer, and triple-layer monolith-coated HZ-280 catalysts were obtained to be 4.2, 6.8, and 10.4 μm. The monolith having a single-layer coating of HZ-280 showed that aromatics reduced to 0%, produced more olefins, and was active with 82% conversion even after 41.5 h. The HZ-280-coated monolith catalyst was regenerated and was reused for three cycles. The results showed negligible activity loss for methanol conversion and propylene selectivity and reveals that the structured catalyst is viable and economical for commercial applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +966-507961837 (M.A.A.). *E-mail: [email protected]. Phone: +966-13-8603428 (S.A.). ORCID

Mohammad Ashraf Ali: 0000-0003-0176-9156 Shakeel Ahmed: 0000-0003-2074-2039 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the support provided by National Plan for Science, Technology, and Innovation (MAARIFAH)King Abdulaziz City for Science and Technologythrough the Science & Technology Unit at King Fahd University of Petroleum & Mineralsthe Kingdom of Saudi Arabia, project number (13-PET405-04).

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REFERENCES

(1) Pinilla-Herrero, I.; Márquez-Á lvarez, C.; Sastre, E. Methanol-toolefin reaction on SAPO-35 catalysts synthesized with controlled crystal size and using mesoporogen additives. Catal. Today 2016, 277, 29−36. (2) Xiang, D.; Yang, S.; Qian, Y. Techno-economic analysis and comparison of coal based olefins processes. Energy Convers. Manage. 2016, 110, 33−41. (3) Chang, C. D.; Silvestri, A. J. The Conversion of Methanol and Other O-Compounds to Hydrocarbons over Zeolite Catalysts. J. Catal. 1977, 47, 249−259. (4) Wei, R.; Li, C.; Yang, C.; Shan, H. Effects of ammonium exchange and Si/Al ratio on the conversion of methanol to propylene H

DOI: 10.1021/acs.energyfuels.8b04020 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (24) Beers, A. E. W.; Nijhuis, T. A.; Aalders, N.; Kapteijn, F.; Moulijn, J. A. BEA coating of structured supports-performance in acylation. Appl. Catal., A 2003, 243, 237−250. (25) Mores, D.; Stavitski, E.; Kox, M. H. F.; Kornatowski, J.; Olsbye, U.; Weckhuysen, B. M. Space- and Time-Resolved In-situ Spectroscopy on the Coke Formation in Molecular Sieves: Methanol-to-Olefin Conversion over H-ZSM-5 and H-SAPO-34. Chem.Eur. J. 2008, 14, 11320−11327. (26) Zampieri, A.; Kullmann, S.; Selvam, T.; Bauer, J.; Schwieger, W.; Sieber, H.; Fey, T.; Greil, P. Bioinspired rattan-derived sisic/ zeolite monoliths: preparation and characterisation. Microporous Mesoporous Mater. 2006, 90, 162−174. (27) Yao, J.; Zeng, C.; Zhang, L.; Xu, N. Vapor phase transport synthesis of SAPO-34 films on cordierite honeycombs. Mater. Chem. Phys. 2008, 112, 637−640. (28) Louis, B.; Ocampo, F.; Yun, H. S.; Tessonnier, J. P.; Pereira, M. M. Hierarchical pore ZSM-5 zeolite structures: From micro- to macro-engineering of structured catalysts. Chem. Eng. J. 2010, 161, 397−402. (29) Shan, Z.; van Kooten, W. E. J.; Oudshoorn, O. L.; Jansen, J. C.; van Bekkum, H.; van den Bleek, C. M.; Calis, H. P. A. Optimization of the preparation of binderless ZSM-5 coatings on stainless steel monoliths by in situ hydrothermal synthesis. Microporous Mesoporous Mater. 2000, 34, 81−91. (30) Perdana, I.; Creaser, D.; Lindmark, J.; Hedlund, J. Influence of NOx adsorbed species on component permeation through ZSM-5 membranes. J. Membr. Sci. 2010, 349, 83−89. (31) You, Z.; Liu, G.; Wang, L.; Zhang, X. Binderless nano-HZSM-5 zeolite coatings prepared through combining washcoating and dry-gel conversion (DGC) methods. Microporous Mesoporous Mater. 2013, 170, 235−242. (32) Valli, J. A review of adhesion test methods for thin hard coatings. J. Vac. Sci. Technol., A 1986, 4, 3007. (33) Ahmed, S. Methanol to olefins conversion over metal containing MFI-type zeolites. J. Porous Mater. 2011, 19, 111−117. (34) Argauer, R. J., Landolt, G. R. Crystalline zeolite ZSM-5 and method of preparing the same. U.S. Patent 3,702,886 A, 1972. (35) Post, J. G.; van Hooff, J. H. C. Acidity and activity of H-ZSM-5 measured with NH3-t.p.d. and n-hexane cracking. Zeolites 1984, 4, 9− 14. (36) Zhu, Q.; Kondo, J. N.; Setoyama, T.; Yamaguchi, M.; Domen, K.; Tatsumi, T. Activation of hydrocarbons on acidic zeolites: superior selectivity of methylation of ethene with methanol to propene on weakly acidic catalysts. Chem. Commun. 2008, 5164− 5166. (37) Niu, X.; Song, Y.; Xie, S.; Liu, S.; Wang, Q.; Xu, L. Synthesis and Catalytic Reactivity of MCM-22/ZSM-35 Composites for Olefin Aromatization. Catal. Lett. 2005, 103, 211−218. (38) Shirazi, L.; Jamshidi, E.; Ghasemi, M. R. The effect of Si/Al ratio of ZSM-5 zeolite on its morphology, acidity and crystal size. Cryst. Res. Technol. 2008, 43, 1300. (39) Svelle, S.; Joensen, F.; Nerlov, J.; Olsbye, U.; Lillerud, K.-P.; Kolboe, S.; Bjørgen, M. Conversion of Methanol into Hydrocarbons over Zeolite H-ZSM-5: Ethene Formation Is Mechanistically Separated from the Formation of Higher Alkenes. J. Am. Chem. Soc. 2006, 128, 14770−14771. (40) Bjørgen, M.; Joensen, F.; Lillerud, K.-P.; Olsbye, U.; Svelle, S. The mechanisms of ethene and propene formation from methanol over high silica H-ZSM-5 and H-beta, Catal. Catal. Today 2009, 142, 90−97. (41) Bjørgen, M.; Olsbye, U.; Svelle, S.; Kolboe, S. Conversion of Methanol to Hydrocarbons: The Reactions of the Heptamethylbenzenium Cation over Zeolite H-Beta. Catal. Lett. 2004, 93, 37−40.

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DOI: 10.1021/acs.energyfuels.8b04020 Energy Fuels XXXX, XXX, XXX−XXX