Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX
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Higher Alcohols Synthesis: Experimental and Process Parameters Study over a CNH-Supported KCoRhMo Catalyst Philip E. Boahene and Ajay K. Dalai* Catalysis and Chemical Reaction Engineering Laboratories, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9, Canada S Supporting Information *
ABSTRACT: The present work investigated the effects of operating conditions (temperature (T), pressure (P), and gas hourly space velocity (GHSV)) on the higher-alcohols synthesis reaction, using a downward-flow fixed-bed reactor. The carbon nanohorn (CNH)-supported KCoRhMo catalysts with compositions of 9% K, 4.5% Co, 1.5% Rh, and 15 wt % Mo were used for this study. The Design Expert software was used to analyze the interaction effects of T (290−370 °C), P (800−1400 psig), and GHSV (2.4−4.8 m3(STP)/kgcat/h) on CO conversion, alcohols, and hydrocarbon product selectivities, as well as their respective yields. The validity of the models was assessed by statistical tests: test of significance and coefficient of determination (R2) values. The recorded R2 values suggested that the quadratic models generated could sufficiently depict the experimental data. Increasing temperature and pressure in the ranges of 290−350 °C and 800−1400 psig, respectively, resulted in corresponding increases in CO conversions; however, with increasing GHSV, the CO conversion decreased monotonically. Numerical optimization assessments of the models selected the optimum operating conditions to be 325 °C, 1320 psig, and 2.4 m3(STP)/kgcat/h to give the maximum ethanol and higher alcohols space time yields of 0.126 and 0.177 g/gcat/h, respectively.
1. INTRODUCTION Mixed alcohols including C2+ alcohols are important compounds with widespread applications in the chemical, pharmaceutical, and energy sectors.1 In North America (the United States and Canada) and some parts of Europe, gasoline blends with up to 10% ethanol (E10) and higher alcohols are commercially available at gasoline pump stations.2,3 These alcohols (C2 and C2+) can be used as additives to boost octane levels in gasoline fuels. Higher alcohols synthesis (HAS) from syngas conversion provides an alternative direct catalytic synthesis route for the production of fuels and industrial chemicals.4 Although many research works have appeared in the literature on this topic, there is a need to further enhance the higher alcohol yields and selectivities to make it commercially attractive. Heterogeneous catalytic systems for the HAS reaction is commonly based on MoS2, because of its high sulfur tolerance and high activity for the water−gas-shift reaction.5−7 Although the water−gas-shift (WGS) is a side reaction, a catalyst with high activity toward this reaction could be advantageous for the synthesis of higher alcohols, because of its efficiency to convert the formed H2O by a parallel reaction with CO to produce CO2 and H2, as represented in the reaction CO + H2O ↔ CO2 + H2. Excessive water generation on the surface of the catalyst can lead to the deactivation of the catalyst.8 That notwithstanding, the reaction mechanism for the generation of higher alcohols on the MoS2 catalyst matrix is still being debated;9 Santiesteban and co-workers have proposed that it follows the classical CO insertion into the corresponding precursor alcohol.10 The product distribution and carbon chain growth scheme over these catalysts © XXXX American Chemical Society
follows the Anderson−Schulz−Flory (ASF) polymerization pathway.11 The incorporation alkali promoters suppressed the hydrogenation ability of surface alkyl species to form alkanes, thereby increasing the active sites needed for the formation of higher alcohols.12 A correlation exists between the addition of alkali-metal promoters in the HAS catalysts formulation and the creation of alkali-containing sites for the formation of higher alcohols.6,13 Besides, KMoS2 and K2MoS4 have been suggested as the alkali-containing species relevant to HAS.13 Group VIII metals have also exhibited the ability to improve the higher alcohols selectivity by changing the ASF distribution of alcohols and to compensate for the activity loss due to alkali-metal promotion by enhancing the carbon chain growth.14−16 The selectivities of the alcohol products are affected by the catalyst support materials as well as the reaction conditions.17−19 For instance, KCoRhMo catalysts supported on multiwalled carbon nanotubes (MWCNTs) outperformed its activated carbon (AC) counterpart for the synthesis of higher alcohols.20,21 Furthermore, despite the successful application of the MWCNTs for the HAS reaction, their potential could be restrained by the limited surface area.22 Also, with their enhanced textural properties and at similar metal loadings (9% K, 4.5% Co, 15% Mo, and 1.5% Rh), ordered mesoporous carbon (OMC) recorded ∼5% less total alcohol selectivity, compared to its MWCNT Received: Revised: Accepted: Published: A
May 25, 2017 September 8, 2017 September 29, 2017 October 2, 2017 DOI: 10.1021/acs.iecr.7b02146 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research counterpart.23 Although MWCNT and OMC have proven tendencies to produce alcohols, it was concluded from our previous study that carbon in the form of carbon nanohorns (CNHs) can be superior catalyst supports for syngas conversion applications, compared to its byproducts.24 In the CO hydrogenation reaction, varying the operating conditions (temperature (T), pressure (P), and gas hourly space velocity (GHSV)) has a direct impact on the product selectivity and yields. Typical process parameters monitored for temperature and pressure spans the ranges of 250−350 °C and 5− 10 MPa, respectively,19,25 and the reactions are quite dependent on the catalyst employed.17,26,27 For instance, although the best temperature range for Cu-based catalysts is 250−300 °C, that for alkali-doped MoS2-based catalyst is 270−330 °C.19,27 That notwithstanding, significant problems associated with higher temperatures include the instability of some oxygenates and excessive formation of CO2 and methane, as well as catalyst deactivation due to sintering.27 At constant temperatures, the formation of higher alcohols is thermodynamically favored with increasing pressures.27,28 The general observed trend is that, as P increases, the productivity of higher alcohols increases; however, the effect of P on the reaction kinetics is also catalyst specific, to some extent.29 For the effects of GHSV, an opposing effect on CO conversion and higher alcohols has been reported.30−33 Low space velocities (higher contact times) are mostly favorable for higher alcohol synthesis, which indicates that higher alcohols are formed at a slower rate than methanol.31−33 However, the effects of independent process variables (T, P, and GHSV) on CO conversion, alcohol product yield, hydrocarbons and CO2 have been studied by various researchers,34,35 their combined interaction effects are limited in the literature. In our study of the CNH-supported KCoRhMo catalyst, it was observed that the catalyst displayed improved catalytic properties for the HAS reaction, compared to its other carbon particles−fine (OCPf) and OCP counterparts. The space time yields (STYs) of the total alcohols as well as the higher alcohols were recorded at 340 °C, 8.28 MPa, and 3.6 m3(STP)/kgcat/h to be 0.5026 and 0.4104 g/gcat/h, respectively.24 The motivation of the present study was to develop models to correlate the interaction effects of independent process variables (T, P, and GHSV) to dependent variables (%CO conversion, alcohol product yields, hydrocarbons, and CO2 selectivities) during the HAS reaction over the optimum CNH-supported KCoRhMo catalyst. Finally, the optimized conditions required to obtain maximum ethanol and the selectivities of higher alcohols was also to be determined.
An inductively coupled plasma−mass spectrometry (ICP/MS) technique was used to ascertain the respective elemental compositions (actual weight percentage) of Co, Mo, and Rh present in the prepared catalyst. The nominal catalyst metal loadings of 4.2% Co, 15.1% Mo, and 1.3% Rh were obtained, which were close to the targeted values of 4.5% Co, 15% Mo, and 1.5% Rh. Furthermore, extensive characterizations such Brunauer−Emmett−Teller (BET) surface area measurement, transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed reduction in a hydrogen atmosphere (H2-TPR), thermogravimetric analysis (TGA), etc. of a CNH-supported KCoRhMo catalyst has been reported in our previous paper.24 Prior to the synthesis reaction for higher alcohols, the oxidic catalyst was sulfided at 450 °C at a rate of 20 °C/min for 6 h, using a gas mixture containing 10 mol % H2S (balance H2) at a flow rate of 50 mL/min. The feed gas used in this study was the commercially available syngas mixture (50 mol % H2, 40 mol % CO, 10 mol % Ar) procured from Praxair, Canada. The synthesis reaction for higher alcohols was performed under steady-state conditions in a downward-flow packed-bed reactor under reaction conditions of T = 290−350 °C, P = 800−1400 psig, and GHSV = 2.4−4.8 m3(STP)/kgcat/h over a period of 24 h. After contacting the syngas with a catalyst bed under specified reaction conditions, the product at the exit of the reactor passed through two condensers maintained at 0 °C to condense the condensable gases in the product to liquid. The cooled liquid product was collected in sample containers after 24 h for analysis offline, via gas chromatography (GC), using a Stabil Wax column. The components in gaseous products were analyzed online in a GC system that was equipped for both temperature-controlled desorption (TCD) and flame ionization detection (FID).11,39 2.2. Experimental Design for Process Parameters Study. The experimental design for process parameters study was achieved using the Design Expert statistical software (version 9.0, State-Ease, Inc., Minneapolis, MN, USA). The parameters T, P, and GHSV were varied in the ranges of 290−350 °C, 800−1400 psig (5.52−9.65 MPa), and 2.4−4.8 m3(STP)/kgcat/h, respectively. To analyze the interaction effects of process parameters as well as their optimized conditions for the HAS reaction, the central composite design (CCD) method was used to develop the experimental plan. This design approach is one of the commonly used response surface methods (RSMs) for the designed experiment; and it employs factorial or fractional factorial design with center points, augmented with a group of axial points to help estimate curvature.39 The main idea of RSM is to use a sequence of designed experiments to obtain an optimal response using a second-degree polynomial model. It should also be mentioned that statistical approaches such as RSM can be employed to maximize the production of a special substance by optimization of operational factors. In contrast to conventional methods, the interaction among process variables can be determined by statistical techniques.40 The experiments were performed using a syngas mixture with molar composition of 40% CO, 50% H2, and 10% Ar. Reproducibility of the results was ensured by repeating some of the experimental conditions a few more times, to ascertain the degree of inherent errors. Perturbation plots were used to show the one-variable effect on the dependent variable as the other independent variables remained constant. These plots were used along with the three-dimensional (3-D) surface responses
2. EXPERIMENTAL SECTION 2.1. Synthesis of Carbon Nanohorns and Preparation of KCoRhMo/CNH Catalyst. The pristine carbon nanohorn support was synthesized by the submerged arc discharge in a liquid nitrogen technique that has been described elsewhere,24,36 at a fixed current of 90 A and 34 V. Detailed preparation of the CNH-supported KCoRhMo catalyst, as well as description of experimental setup used for data collection can be found in our previous study.24 K2CO3 was used as the potassium precursor in the catalyst formulation to neutralize the acidity of the HNO3-treated CNH support and also to provide basic sites such as K2Mo2O7 species resulting from the chemical interactions between K−Mo−O species to enhance higher alcohols formation.37,38 The fine powder form of the CNH-supported KCoRhMo catalyst samples was designated as Cat-CNH. B
DOI: 10.1021/acs.iecr.7b02146 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Information (Table S1). Evaluation of the models was done by employing two main statistical tests, namely, the test of significance of factors or interactions and the coefficient of determination (R2).41,42 The probability values (p-values) were used to test for significance of factors of the models generated. The models were further simplified by eliminating the insignificant factors to ease the interpretation.43 In this regard, the factor or interaction was deemed insignificant at a 95% confidence level, when p > 0.05. The value of the coefficient of determination (R2) indicates the proportion of the variance in the dependent variable that is predictable from the independent variables.44 This value is used to test for the goodness of fit, with R2 values approaching 1 depicting a good fit and those approaching 0 depicting no fit. The adjusted R2 value (also called the 2R value) is a modification of R2, which improves its value only if the newly added factors or interactions are significant.45
plots to help compare the relative influences of the factors studied through a multifactor surface.41 2.3. Quadratic Models Development by Design Expert for the Synthesis of Higher Alcohols. Higher alcohols have carbon numbers of >1, whereas total alcohols represent alcohols with a carbon number of ≥1.17−19 After the HAS reaction, analysis of the tail gas exiting the reactor indicated that methane is the major hydrocarbon component apart from CO2, unconverted CO, and H2. The liquid product analyzed by using a GC system that was equipped with a Stabil Wax column showed linear alcohols, including C1−C5 alcohols, as the main products, along with other higher alcohols. A series of data collected from experimental runs (under specified conditions) were used in Design Expert software to develop quadratic modules for the HAS reaction. The results of the HAS experiments in the fixed-bed reactor over the CNHsupported KCoRhMo catalyst are provided in the Supporting
3. RESULTS AND DISCUSSION 3.1. Effects of the Temperature, Pressure, and GHSV on CO Conversion. The interaction effects of temperature (T), pressure (P), and gas hourly space velocity (GHSV) on CO conversion, as generated by the regression analysis (ANOVA) of experimental data, is given in the RSM below:
Table 1. Test of Significance Results for Independent Variables or Interactions for the Model Derived for CO Conversion p-Value of % CO Conversion variable/interactions
all variables included
insignificant variables rejected
temperature, A pressure, B GHSV, C AB AC BC A2 B2 C2 model R2 R2-adjusted
0.0001 0.0066 0.0206 0.0674 0.7441 0.916 0.8373 0.1077 0.6600 0.0013 0.95 0.92
