Mesoporous γ-alumina with isolated silica sites for direct liquid

Aug 3, 2017 - Search; Citation; Subject .... used as support for cobalt catalyst in Fischer-Tropsch synthesis (FTS), both in microchannel and fixed be...
4 downloads 0 Views 10MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Mesoporous γ‑Alumina with Isolated Silica Sites for Direct Liquid Hydrocarbon Production during Fischer−Tropsch Reactions in Microchannel Reactor Aditya Rai,†,‡ Malayil G. Sibi,†,‡ Saleem A. Farooqui,†,‡ Mohit Anand,†,‡ Asim Bhaumik,§ and Anil K. Sinha*,†,‡ †

Conversions and Catalysis Division, CSIR−Indian Institute of Petroleum (IIP) Mohkampur, Dehradun, 248005, India AcSIR-Academy of Scientific and Innovative Research, Chennai, Tamil Nadu, 600113, India § Department of Materials Science, Indian Association for Cultivation of Science, Kolkata, 700032, India ‡

S Supporting Information *

ABSTRACT: Effective incorporation of isolated silica sites on the surface of mesoporous γ-alumina catalyst has been achieved using octadecyl dimethyl (3-(trimethoxy-silyl) propyl) ammonium chloride as both silica source and mesopore template. The synthesized γ-Al2O3 with isolated silica sites was used as support for cobalt catalyst in Fischer− Tropsch synthesis (FTS), both in microchannel and fixed bed reactors. 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) analysis clearly showed the presence of isolated Si(−OAl)4 and Si(−OAl)3(−OSi)1 species in the mesoporous alumina framework. Isolated Si contributed to controlled Brönsted acidity at the surface. CH4 was suppressed in microchannel reactor compared to fixed bed reactor, due to better heat and mass transfer. CH4 yield was also low over catalyst supported on mesoporous alumina doped with isolated silica. Isolated Si incorporation on alumina surface led to desirable cracking to produce middle distillate hydrocarbons during FTS reactions, while the long-chain waxy hydrocarbons formation was suppressed, which resulted in desirable chain growth probability factor (α = 0.87) in microchannel reactor. KEYWORDS: Fischer−Tropsch, Microchannel reactor, Silica doped alumina, ASF study, Hydrocracking

■

Yang et al.13 synthesized tandem catalysts based on traditional FT catalyst with ZSM-5 zeolite applying core− shell methodology, to directly synthesize isoparaffins from syngas. High methane selectivity and gasoline range products with increased catalyst deactivation12,13 were some of the drawbacks of these catalyst combinations. Qin et al.14 developed the lignin based Co@C core−shell nanoparticles as efficient catalyst for selective Fischer−Tropsch synthesis of C5+ compounds, but the high yield of C2−C4 gaseous products were drawbacks of the synthesized catalyst. Mesoporous zeolites as support for FTS catalyst have been extensively reported for direct cracking of hydrocarbons produced during FT synthesis.14−18 But, mesoporous ZSM-5 has undesirably high Lewis acidity and surface silanols, leading to rapid deactivation; in addition, they show high selectivity toward undesired C1−C4 alkanes and low CO conversion,14,17 which are major drawbacks of these FTS catalysts. Increasing

INTRODUCTION

Fischer−Tropsch synthesis (FTS) is a promising process to convert syngas (CO + H2) into a wide range of hydrocarbons which can be used as the substitute for non-renewable fuels,1,2 Generally, FTS products are normal long-chain waxy paraffins which require further processing before use as fuels. Much effort has been made to explore the effective way to synthesize iso-parraffins and minimize waxy products of FTS,3,4 for which bifunctional catalysts5−7 and newer types of reactors have immense importance. Among various factors which influence the product selectivity, temperature control in fixed bed reactors is more important due to the formation of hot spots which accelerates the methane formation and decreases the chain growth probability.8 Microchannel reactors (MCRs) provide the benefits of portability, easy scale-up, better heat and mass transfer characteristics, high reaction throughputs, precise control of hydrodynamics, and, when coated with multifunctional catalysts, they may also provide much improved product selectivity and yield.9−11 Mehta et al.12 utilized silicon based microchannel reactors to synthesize hydrocarbons from syngas. © 2017 American Chemical Society

Received: March 22, 2017 Revised: July 19, 2017 Published: August 3, 2017 7576

DOI: 10.1021/acssuschemeng.7b00874 ACS Sustainable Chem. Eng. 2017, 5, 7576−7586

Research Article

ACS Sustainable Chemistry & Engineering

25 L h−1 with GHSV in the range of 500−5000 h−1. The catalyst was deposited on the bare metallic microchannel plates. Prior to the catalyst coating microchannel plates were heated at 800 °C for 8 h. A microchannel plate was weighed before each step of coating and calcinations, to ascertain the amount of catalyst deposited inside the channels. Prior to coating, the catalyst was ball milled at 600 rpm for 3 h and made into slurry with anhydrous ethanol. Homogeneous slurry with 3 g of catalyst in 200 mL of anhydrous ethanol was obtained for coating. Dip-coating followed by wash-coating method were used for the required amount of the catalyst in the channels.11 The coated channels were dried at room temperature for 5 h followed by drying in oven at 100 °C overnight. Finally, the coated microchannel plates were calcined at 600 °C. An adhesion test of the catalyst was carried out by drop test method. In this method the catalyst coated microchannel plate was dropped from a height of 50 cm on to a wooden table (catalyst layer facing the table). The catalyst weight loss was less than 5%, which indicated stable catalyst coating. Finally the plates were assembled together using laser welding. The coated catalyst in reactor assembly was activated at 420 °C in the presence of hydrogen (99.9%, 100 mL min−1) for 16 h, prior to reaction. A commercial microchannel reactor (of same dimension) coated with 16%Co/Al2O3 catalyst was procured for comparison (Institut für Mikrotechnik, Mainz, GmbH). Catalyst Characterization. N2 adsorption−desorption studies of supports were carried out to calculate the surface area, pore size, and pore volume (Belsorb Max, BEL, Japan). The samples were pretreated at 200 °C under vacuum for 3 h, before analysis. Micrometrics 2900 equipment (USA) was used for NH3 temperature-programmed desorption (TPD) study to measure the total acidity of the catalyst. A 0.2 g portion of catalyst was saturated with a mixture of NH3:He (10:90 v/v) at 120 °C. He gas was used to flush physically adsorbed NH3. Finally, temperature was increased at the rate of 10 °C/min and desorbed NH3 was recorded on thermal conductivity detector. Potentiometric titrations were performed by the autotitration31 system (Mettler Toledo G20 Compact Titrator) with a KOH standard (0.1 M prepared in HPLC grade Iso-propyl alcohol). The pH was measured with a combined glass electrode DG116 from Mettler Toledo. Standard solvent (500 mL toluene + 495 mL iso-propyl alcohol + 5 mL distilled water) was prepared. A 0.6 g portion of catalyst was dispersed in 125 mL of the prepared standard solvent. Titration with standard KOH (0.1 M) was carried out for the calcined and uncalcined Al2O3(Si) support. 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker MSL300 spectrometer. Drift spectra of commercial alumina and Si doped alumina was studied after pretreatment with pyridine at 120 °C for 1 h. Brönsted and Lewis acid sites of the catalyst sample were differentiated by DRIFTS (diffuse reflectance fourier transform infrared spectra) using pyridine as a probe molecule.17 A 50 mg portion of catalyst sample was pretreated in hot air oven at 100 °C. A 0.1 cm3 portion of pyridine was absorbed on the pretreated catalyst. The sample was kept in a hot air oven at 120 °C for 1 h to remove any physisorbed pyridine. The samples were cooled in an inert atmosphere and IR DRIFTS spectra were recorded using a mid-IR Fiber-optic system SYS-IRX Reaction View-X (Remspec Corporation, USA). Spectra were recorded with 37 scans and 4 cm−1 resolution. The obtained data were analyzed using GRAMS/AI (version 9.1) software, part of the GRAMS Spectroscopy software suite from Thermo Fisher Scientific Inc. The XRD patterns were recorded by Advanced Bruker D8 diffractometer (step size 0.002° and scanning rate 1° /min), Cu Kα radiation (40 kV and 40 mA) to measure the phase purity and crystallinity of the material. SEM (scanning electron microscopy) images were obtained on a field emission scanning electron microscope, FEI Quanta 200 F. Tungsten filament doped with lanthanum hexaboride (LaB6) was used as an X-ray source. The equipment was fitted with an ETD detector, operated under high vacuum mode, and used secondary electrons with an acceleration voltage of 10 or 30 kV. Energy-dispersive X-ray spectroscopy (EDX) coupled with SEM was used for elemental analysis of the material. The TEM images were obtained on a TECNAI electron microscope operated at 75 kV. The samples for TEM analysis were suspended in ethanol, sonicated for 15 min, and dropped on a Cu-grid. A

the selectivity for lower hydrocarbons has long been a challenging problem in FTS, since typical FTS product selectivity is limited by the Anderson-Schulz−Flory distribution (ASF). Subramanian et al.18 demonstrated deviation from the ASF distribution over Co/zeolite catalyst for long chain hydrocarbons. Also the selectivity obtained over Ru/mesoZSM-5 catalyst was significantly higher for C5−C11 hydrocarbons (80%), than that expected from the ASF distribution (maximum 45%), with high isoparaffins to n-paraffins ratio (2.7).19 Isolated Si near the pore walls20 in alumina, provides unique tailored Brönsted acidity on the surface. An ideal silica-doped alumina for FTS should have low silica content to ensure that Brönsted acid site concentration is low, since stronger acid sites contribute more to methane selectivity thereby reducing selectivity for the target liquid and wax hydrocarbon products.21 Acid site concentrations and strengths of silica-doped alumina are inherently lower (relative to zeolites) and thus uniquely matched to the process chemistry for FTS.22 Silica modification of alumina not only generates Brönted acid sites but also leads to better metal dispersion23 and prevents sintering through anchoring of Co crystallites.24 Catalyst sintering was reportedly suppressed for a 5% silica-doped alumina support.25 One of the most important properties of silica-doped alumina is its stability against hydrothermal breakdown which contributes to loss of metal dispersion.26 Silica-doped-alumina coatings are reported to be more homogeneous, crack-free,27 and adhere more strongly on the stainless steel surface28−30 than pure silica or alumina. The main aim of this work is to utilize the several advantages (described above) of a high surface area mesoporous alumina doped with isolated silica sites (synthesized using an organosilane as a silica dopant as well as mesopore template)(i) controlled Brönsted acidity on the surface, (ii) lower methane formation, and (iii) better adherence to microchannel plates for both the FT synthesis and cracking reactions over the same catalyst in microchannel reactor, to maximize liquid hydrocarbon products yield.

■

EXPERIMENTAL SECTION

Catalyst Synthesis. Synthesis of Silica-Doped Mesoporous γAlumina, Al2O3(Si). Evaporation induced self-assembly method was used for the synthesis of the support.20 In a typical synthesis 4.0 g of an organosilane ODAC (octadecyl dimethyl (3-(trimethoxy-silyl) propyl) ammonium chloride, 60 wt % in methanol, Gelest, Inc.) was dissolved in 40 mL anhydrous ethanol and stirred for 6 h at room temperature. A 8.16 g portion of aluminum isopropoxide (98% SigmaAldrich) was dissolved separately in a mixed solvent composed of 6.4 mL of 69−72 mol % nitric acid and 20 mL anhydrous ethanol. Both the mixtures were mixed with continuous stirring. Finally, additional 20 mL ethanol was added gradually. The mixed solution was continuously stirred at 600 rpm for 24 h at room temperature. Solvent evaporation was performed at 70 °C for 48 h. Then the resulting solid was calcined at 700 °C for 4 h at a heating rate of 1 °C/ min to obtain mesoporous silica doped γ-alumina (Al2O3(Si)). Synthesis and Coating of 16% Co/Al2O3(Si) in Microchannels. Co(NO3)·6H2O (Sigma-Aldrich, 98%) was used for impregnation. After impregnation, the catalyst was calcined at 450 °C for 6 h. Microchannel plates (total 10 plates) made of SS314, with channels of dimension 50 mm × 27 mm × 2 mm, were used. Each plate had 28 channels. Each channel in the plates had a dimension of 50 mm length, 0.5 mm width, and 0.25 mm height (see Figure S1a and b, Supporting Information). The inlet and outlet of the reactor channels were triangular shaped and vertically opposite, for uniform flow distribution of the reactants. The maximum throughput of gaseous reactants was 7577

DOI: 10.1021/acssuschemeng.7b00874 ACS Sustainable Chem. Eng. 2017, 5, 7576−7586

Research Article

ACS Sustainable Chemistry & Engineering temperature-programmed reduction (TPR) study was carried out by using Micromeritics 2920 equipment. A 0.10 g portion of the catalyst was pretreated with N2 at 573 K for 30 min. The samples were cooled to 323 K in N2 gas stream. The H2:Ar (5:95, v/v, 50 mL/min) mixture was used to purge at 323 K, for about 30 min, until baseline was stabled. The TPR profile of the catalyst was obtained by increasing temperature from 323 to 1073 K at a rate of 10 K/min. Thermal stability measurements analysis was done by TGA/DTA analysis (PerkinElmer). The temperature was increased from 30 to 890 °C at 5 °C/min under air atmosphere and weight loss was recorded. Experimental Setup for FT Synthesis. The microchannel reactor was assembled by using 10 microchannel plates. These plates were joined and made leak proof by laser welding. The inlet and outlet of the reactor were connected with 1/4 in. SS316 tubes. Specially designed heating blocks with controller (West) were connected on both sides of the reactor assembly. Thermocouples were used to measure the reaction temperature. The design of the microchannels and picture of the reaction setup are shown in Figure S1c (Supporting Information). The FT synthesis experiments were carried out using experimental setups built in-house (both microchannel and fixed bed reactors), with precise control of the operating parameters such as temperature, pressure, and feed flow rate. The activities of the catalysts were studied at different temperatures and pressures. The experiments were performed at various temperatures (250−320 °C) and pressures (1−20 bar). The system pressure was controlled by a back pressure regulator (Tescom). The pressure on the upstream and downstream of the reactor was measured using pressure gauges (Wika). Two mass flow controllers (Brooks, with a maximum flow range of 500 mL min−1) were used to control CO and H2 flow in the reactor. Three thermocouples and a temperature controller were connected to control the catalyst bed temperature. The gas−liquid product mixture coming out of the reactor was passed through a gas−liquid separator to separate gaseous fraction (containing unconverted H2, CO, CO2, and lighter hydrocarbons) from liquid products. FT synthesis reactions were also carried out in fixed bed reactor using the same catalysts diluted with inert material SiC (SiC:catalyst, 1:1 v/v). The catalyst was diluted to avoid hot spot formation, and to have uniform heat distribution inside the catalyst bed. Gaseous products were analyzed by a refinery gas analyzer. The results obtained were in a normalized mol % Agilent 7890A-RGA, equipped with two thermal conductivity detectors (TCDs), one flame ionization detector (FID), and seven columns (five packed columns: three HayeSep Q 80/100 mesh columns and two Molsieve 5A 60/80 mesh columns; two capillary columns DB-1 and HP-PLOT Al2O3). A Varian 3800-GC, gas chromatograph with vf-5 ms column (30 m × 0.25 mm, 0.25 μm), was used for liquid analysis. A variation of ±2.5% was considered as experimental analysis measurement error. Detailed hydrocarbon analysis was done by Agilent 7890B 2D-GC (GC × GC; first dimension−nonpolar, DB-5 ms column, 30 m × 0.25 mm, 0.25 μm; second dimension polar, PAC column, 5 m × 0.25 mm, 0.15 μm), with FID, MS, capillary flow modulator and ZOEX software. Material balance for hydrocarbons was carried out using gas outlet rate and liquid product obtained, in fixed time intervals. Weight fractions of the hydrocarbons obtained were calculated and graph between log(Wn/n) vs carbon number (n) was plotted to calculate the chain growth probability factor (α). Experimental results were validated using COMSOL multiphysics software version 5.0, for simulating the concentration and temperature profile inside the microchannels (Supporting Information). The basic assumptions made for simulation were the following: (i) steady state flow inside the channel; (ii) dependent variables are varying only across the length of the channel, they do not vary with depth and width of the channel; (iii) reactants in FTS process are gases, so uniform flow inside all the channels were assumed; (iv) coke deposited on catalyst surface was not accounted in the simulation. Space independent models were selected to simulate the kinetics of the FT reaction. For space independent model, 0D was selected as space dimension and reaction engineering (re) module was used to study the kinetics. Rate equation and rate constants obtained based on kinetic

study were included in the simulations. Enthalpies, entropy, and specific heat capacity for each species were obtained from Aspen Plus version 8.4 software. Calculated volumetric flow rates and molar flow rates of reactants were used as inlet flow rates in the simulations. Energy study was included in the re physics module to simulate the temperature profile across the length of the reactor. Initial concentration, temperature, and diffusivities of each species were incorporated and transport of diluted species, heat transfer in fluids, and Darcy’s law physics were added in the study. The stationary plug flow mode was chosen for the study.

■

RESULT AND DISCUSSION Characterization. γ-Al2O3 doped with SiO2 [Al2O3(Si)] was synthesized by evaporation induced self-assembly (EISA) method20 and characterized by various analytical techniques. Octadecyl dimethyl (3-(trimethoxy-silyl) propyl) ammonium chloride was used as both silica source and mesopore template. This is a facile synthesis route to prepare mesoporous alumina, doped with isolated silica (from the organosilane template) near the surface,20 which causes the material to have defect sites generated through Brönsted (Si−OH−Al) sites created on the surface which may stabilize the Co species, and further facilitate hydrogen spillover to the reactants, thereby improving the hydrogenation ability of metal catalysts.32 Optimization of synthesis gel composition was done by varying the ratio of inorganic precursor and organosilane template, and γ-alumina doped with 11 mol % SiO2 (confirmed by SEM-EDAX) was the sample with highest surface area (289 m2/g) after calcination at 700 °C and moderate pore volume (0.36 cm3/g). In comparison the commercial γ-alumina has much lower surface area (220 m2/g). The nitrogen sorption isotherm and BJH pore size distribution (obtained from the adsorption branch of the isotherm) of Si incorporated mesoporous γ-alumina (Al2O3(Si)) and commercial γ-Al2O3 supports are shown in Figure 1 and Figure S2 (Supporting Information), respectively.

Figure 1. N2 sorption and BJH plot (inset) of mesoporous γ-Al2O3(Si) support.

The nitrogen sorption isotherm of the Al2O3(Si) material is of type IV, with a broad hysteresis loop in the relative partial pressure ratio (p/p0) range of 0.45−0.95, which is characteristic of mesoporous materials with broad pore size distribution. The BJH pore size distribution shows that the sample has a broad mesopore size distribution with mean pore diameter 20 nm. In comparison, commercial alumina had mean pore diameter of 15 nm. 7578

DOI: 10.1021/acssuschemeng.7b00874 ACS Sustainable Chem. Eng. 2017, 5, 7576−7586

Research Article

ACS Sustainable Chemistry & Engineering

corresponding to cubic spinel Co3O4 phase along with weak peaks of γ-Al2O3. The Co3O4 peaks are more sharp in 16% Co/ Al2O3(Si) catalyst compared to that in 16% Co/Al2O3 catalyst indicating more crystalline cobalt oxide particles in the case of former, which was also further confirmed by TEM analysis (Figure S3 and S4 (Supporting Information)). Solid-state 29Si MAS NMR experiments were carried out to probe the coordination of Si in the mesoporous silica doped alumina [Al2O3 (Si)] material. Figure 3 shows the 29Si MAS NMR and 1H−29Si cross-polarization (CP) spectra for the sample. A single isotropic peak was observed at around −88 ppm in 29Si as well as in 1H−29Si CP spectra of the sample, indicating the existence of Si species with no silanol groups attached. No chemical shift was observed in the CP spectrum wrt the 29Si spectrum. The peak in the CP spectrum shifts to the more deshielded direction if Si in the second coordination sphere are present as silanol groups, and the chemical shift increases with increase in the number of such silanol groups.33 29 Si MAS NMR chemical shift of aluminosilicates vary from −79 [Si(OAl)4] to −100 ppm of Si(OAl)(OSi)3], depending on the number of aluminum atoms in the second coordinating sphere of Si.34 Specifically, experimentally determined chemical shift (δ) ranges associated with Si(−OAl)4 are −80 to −90.5 ppm and those associated with Si(−OAl)3(−OSi)1 are in the range −88 to −97 as compiled by Thomas et al. for aluminosilicates.35 Based on this information, and CP experiment results, the single isotropic peak at −88 ppm can be exclusively assigned to Si(−OAl)4 and Si(−OAl)3(−OSi)1 species. It may be concluded that the siloxane type silica patches are absent in this sample, and most of the Si is well dispersed in the alumina matrix, most likely near the surface of the alumina as isolated silica species. To date, no aluminosilicate material with such a degree of Si isolation in

The small-angle X-ray diffraction pattern for the 16% Co/ Al2O3(Si) catalyst shows well resolved peak (Figure 2a) at 2θ

Figure 2. Low angle (a) and wide angle (b) powder XRD patterns of 16%Co/Al2O3 (Si) catalyst.

value between 0.6 and 1° due to the mesoporosity of the alumina. The wide angle XRD (Figure 2b) reveals peaks

Figure 3. 29Si MAS NMR (dark) and 1H−29Si cross-polarization (CP) MAS NMR (light) spectra of γ-Al2O3(Si) material. 7579

DOI: 10.1021/acssuschemeng.7b00874 ACS Sustainable Chem. Eng. 2017, 5, 7576−7586

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. SEM image of the 16% Co/Al2O3(Si) catalyst.

Figure 5. (a) HAADF-STEM image. Elemental mappings for (b) Al and (c) Si. (d) HRTEM image [(inset) STEM image], of the 16% Co/ Al2O3(Si) catalyst.

with particle size ranging from 50 to 150 nm. Figure 5 shows representative HAADF-STEM image, elemental mappings and TEM image of the 16% Co/Al2O3(Si) catalyst. The HAADFSTEM 3D-tomography image (Figure 5a) shows the

alumina matrix has been reported, though such site isolation is known for sodium aluminosilicate glasses.36 Figure 4 shows the SEM image of the 16% Co/Al2O3(Si) catalyst. Globular particles with hollow surface are observed 7580

DOI: 10.1021/acssuschemeng.7b00874 ACS Sustainable Chem. Eng. 2017, 5, 7576−7586

Research Article

ACS Sustainable Chemistry & Engineering mesoporous foamlike nature of the material. Elemental mapping shows that, the aluminum (Al) nanoparticles are densely and homogeneously distributed on the surface (Figure 5b); while the silica is sparsely but homogeneously dispersed on the surface, as isolated species in the mesoporous alumina framework (Figure 5c), as also evidenced by MAS NMR analysis discussed above (Figure 3). Silica modified alumina has better cobalt oxide dispersion, in addition to having Brönsted acidity.23 TEM shows that the support exhibited clear mesopores (3−10 nm size) evenly distributed throughout the material (Figure 5d (inset) and Figure S3 (Supporting Information)), and the cobalt oxide nanoparticles are well dispersed on the support surface. The Co3O4 nanoparticles were more crystalline with sharper electron diffraction pattern for 16% Co/Al2O3(Si) catalyst compared to that for 16% Co/ Al2O3 catalyst (Figure S3 and S4 (Supporting Information)), which was also observed from XRD analysis (Figure 2b). Total acidity of the catalyst was determined by temperatureprogrammed desorption (TPD) of ammonia. Isolated silica in the mesoporous Si-doped alumina support (Al2O3 (Si), 11% Si) is expected to provide unique tailored Brönsted acidity on the surface. Brö nsted acid sites play an important role in hydrocracking and isomerization reactions. NH3-TPD profile of the catalyst (Figure 6) shows two very close desorption

Acid site concentrations and strengths of silica-doped alumina are lower (than that of zeolites), and thus suitable for FTS.22 We are assuming that in case of uncalcined Al2O3(Si) support, the support pores will be blocked with template (ODAC) and all the KOH will react with (Brönsted) acid sites present at the surface. The low surface area (3 m2/g) as well as low pore volume (0.01 mL/g) of uncalcined Al2O3(Si) support confirmed that the pores were blocked. The acidity value obtained by potentiometric titration31 was 6.889 mg KOH per gcat (0.12 mmol gcat−1) for calcined support and 6.419 mg KOH per gcat (0.11 mmol gcat−1) for the uncalcined support. Similar acidity values indicate that most of the Brönsted acidity was on the support surface, as expected. DRIFT spectrum using pyridine as a probe molecule (Figure S5a and b (Supporting Information) showed a strong absorbance band at 1550 cm−1 corresponding to Brönsted acid sites and very weak bands at 1450 and 1500 cm−1 due to Lewis and interacting (Lewis + Brö nsted) acid sites, respectively. Generally for zeolites, pyridine-IR shows strong peaks at around 1500 cm−1 due to interacting (Lewis + Brönsted) acid sites, which are considered as very strong acid sites.15 Weak intensity for such sites in Al2O3(Si) sample studied here, indicates very low concentration of such strong acid sites, which is expected to be favorable for reducing undesirable methane formation during FTS. It is reported that such strong acid sites favor large methane formation during FTS.15 Pyridine DRIFT spectra of both calcined and uncalcined Al2O3(Si) support showed the prominent presence of Brönsted and Lewis acid sites (see Figure S5a and b Supporting Information), wherein intensities of absorbance peaks for Brönsted acid sites are similar, indicating comparable Bronsted acidities for supports, which indicates that most of the Brönsted acid sites are on the surface, as evidenced by acidity determined by titration. Temperature-programmed reduction (TPR) study of the 16% Co/Al2O3(Si) catalyst was done to characterize the reducibility of Co species (Figure 7). Low temperature peaks

Figure 6. NH3-TPD profile of 16% Co/Al2O3(Si) catalyst.

peaks which may be ascribed to acidic centers of different strength. The total acidity of (between 100−600 °C) was 1.6 mmol gcat−1. The two closely placed peaks could be attributed to hydrogen-bonded ammonia molecules and ammonia molecules chemisorbed on Brönsted acid sites, respectively. In comparison, for silica−alumina (48% Si) and zeolites (ZSM5, 95% Si) the two peaks are well separated.17 Since our material is expected to have all the Brönsted acidity on the surface, the two peaks are closely placed, unlike in the case of zeolites and silica−alumina where the acid sites are distributed throughout the material. The weak acid sites concentration (between 100 and 265 °C) was 1.3 mmol gcat−1, whereas the strong acid sites concentration (between 265 and 600 °C) was 0.3 mmol gcat−1. In comparison, the strong acid sites concentrations (Brönsted acidity) were 0.4 mmol gcat−1 for silica−alumina (48% Si) and 0.5 mmol gcat−1 for zeolite (ZSM5, 95% Si).17 Silica-doped alumina for FTS should have low silica content (as in our case, 11% Si) to ensure that Brönsted acid site concentration is low, since high acid sites concentration contribute to undesirable methane selectivity.21

Figure 7. TPR profile of 16%Co/Al2O3(Si) catalyst.

(at 90%), with low methane yields (250 °C) over 16% Co/ Al2O3 catalysts3,4,37 which leads to reduced chain growth probability, and hence, FTS over Co catalysts is generally carried at lower temperatures. But lower temperatures favor wax formation. It is necessary to further hydrocrack FTS wax in a second reactor to produce transportation fuels.4 CH4 selectivity was low over silica doped novel catalyst 16% Co/ Al2O3(Si), studied in this work, in microchannel reactor, even at higher temperature (>250 °C); therefore, the reactor was operated at higher temperatures, to obtain the desired liquid hydrocarbons (C5−C20) selectivity and high chain growth probability (α = 0.87)4 (Table 1). FTS hydrocarbon products weight fraction as a function of carbon number is plotted in Figure 8. The figure indicates 10−12 times higher yield for C5−C18 range hydrocarbons as compared to C19−C35 range

the presence of residual carbon in the sample, even after calcination at 700 °C (Figure S6 (Supporting Information)).15 In comparison, TPR profile of 16% Co/Al2O3 on commercial alumina support, showed reduction peaks at higher temperature ranges from 300 to 700 °C (Figure S7 (Supporting Information)). 16% Co/Al2O3(Si) catalyst was coated in microchannel plates after chemical etching (10% HNO3) and heat treatment (Figure S8a (Supporting Information)). It is observed from the SEM of pretreated bare plate, that chemical etching and heat treatment made the surface of channel plates rough which improves the adhesion and coating of the catalyst slurry. A uniform catalyst layer coating, filled in the microchannels was obtained by dip coating followed by wash coating (Figure S8b, Supporting Information). Si doping in alumina leads to better adhesion on steel,28,30 as also observed by us by drop-test for 16%Co/Al2O3(Si) catalyst in microchannel plates, compared to that for commercial 16%Co/Al2O3 catalyst. Product Distribution and ASF Study. Table 1 compares CO conversion and product yields during FTS over commercial 16% Co/Al2O3 and 16% Co/Al2O3(Si) at 300 °C, 20 bar, 1500 mL·gcat.−1 h−1 GHSV. The catalyst supported on Si doped alumina (16% Co/Al2O3(Si)) in microchannel reactor, performed better than fixed-bed reactor and commercial alumina supported catalysts, and showed stable activity on continuous operation of 959 h, indicating suppressed polymer7582

DOI: 10.1021/acssuschemeng.7b00874 ACS Sustainable Chem. Eng. 2017, 5, 7576−7586

Research Article

ACS Sustainable Chemistry & Engineering hydrocarbons over Co/Al2O3(Si) catalyst. It is observed that product distribution does not follow ASF distribution. In ASF distribution, product weight fraction decreases with an increase in carbon number. A typical ASF FT product distribution plot would have been linearly decreasing, whereas in the present work the hydrocarbon distribution plot (Figure 8a inset) indicates deviation from typical ASF FT product distribution. There is an initial decrease until C4 carbon number, then an increase in yield between C5−C8 carbon numbers, followed by a decrease in hydrocarbon yield beyond C8 carbon number. Selectivity for C5−C11 was significantly higher (60%) than that expected from the ASF distribution (maximum 30%) at α = 0.87. These observations indicate deviation from ASF distribution leading to increased yield of distillates over and above the limit given by ASF kinetics. Product distribution observed in this study indicates that side reactions (cracking) were also occurring over the catalyst surface in addition to the FTS reactions over the 16% Co/ Al2O3(Si) catalyst. Higher temperatures (300 °C) promoted cracking reaction of waxy FTS products over acidic sites in 16% Co/Al2O3(Si) catalyst, leading to increased liquid hydrocarbons (C5−C20) (Figure 8). Si incorporation leads to desirable cracking and lower wax formation. Figure 8 shows 10−12 times lower yield of waxes (C19−C35), compared to distillates (C5− C18), both in fixed bed and microchannel reactors, over Co/ Al2O3(Si) catalyst (at 900 mL·gcat−1·h−1). Whereas, at higher GHSV (1500 mL·gcat−1·h−1), yield of C5−C18 range hydrocarbons was 4−5 times more than the yield of C19−C33 range hydrocarbons (supporting Figure S10). Overall hydrocarbon distribution for FTS products obtained over 16% Co/Al2O3(Si) and 16% Co/Al2O3 catalyst in the microchannel reactor are shown in Figures 9 and 10, respectively. CH4 yield decreases with increase in temperature over both the catalysts (shown in Figures 9a and 10a). Higher wax (C19−C35 hydrocarbons) selectivity (35−40%) over Co/Al2O3 catalyst (Figure 10b) was observed, as compared to that over Si incorporated catalyst (Figure 9b inset; 5−10% wax selectivity) in microchannel reactor. Table S1 also shows that Si incorporation leads to formation of more cracked products (C5−C9), both in fixed bed and microchannel reactors over Co/Al2O3(Si) catalyst. Osa et al. also reported high yield of waxy hydrocarbons (>C18 yield = 30−40%) over Co/Al2O3 catalyst in fixed bed reactor, at similar pressures and H2/CO ratio (pressure: 20 bar, H2/ CO:1.5).38 Detailed hydrocarbon product analysis (using GC × GC) shows that 77% iso- and normal paraffins, 17% cycloalkanes, 5% aromatics, and 20%) over Co/H-ZSM-5 zeolite catalyst17 as reported in the literature, whereas over lower (