Efficient Catalytic Dehydration of High-Concentration 1-Butanol with Zn-Mn-Co Modified γ-Al 2 O 3 in Jet Fuel Production

It is important to develop full-performance bio-jet fuel based on alternative feedstocks. The compound 1-butanol can be transformed into jet fuel through dehydration, oligomerization, and hydrogenation. In this study, a new catalyst consisting of Zn-Mn-Co modified γ-Al2O3 was used for the dehydration of high-concentration 1-butanol to butenes. The interactive effects of reaction temperature and butanol weight-hourly space velocity (WHSV) on butene yield were investigated with response surface methodology (RSM). Butene yield was enhanced when the temperature increased from 350 ◦C to 450 ◦C but it was reduced as WHSV increased from 1 h−1 to 4 h−1. Under the optimized conditions of 1.67 h−1 WHSV and 375 ◦C reaction temperature, the selectivity of butenes achieved 90%, and the conversion rate of 1-butanol reached 100%, which were 10% and 6% higher, respectively, than when using unmodified γ-Al2O3. The Zn-Mn-Co modified γ-Al2O3 exhibited high stability and a long lifetime of 180 h, while the unmodified γ-Al2O3 began to deactivate after 60 h. Characterization with X-ray diffraction (XRD), nitrogen adsorption-desorption, pyridine temperature-programmed desorption (Py-TPD), pyridine adsorption IR spectra, and inductively coupled plasma atomic emission spectrometry (ICP-AES), showed that the crystallinity and acid content of γ-Al2O3 were obviously enhanced by the modification with Zn-Mn-Co, and the loading amounts of zinc, manganese, and cobalt were 0.54%, 0.44%, and 0.23%, respectively. This study provides a new catalyst, and the results will be helpful for the further optimization of bio-jet fuel production with a high concentration of 1-butanol.


Introduction
Recently, the consumption of jet fuel has been increasing due to the development of the aviation industry, which has resulted in many environmental problems and energy security issues.Therefore, developing full-performance bio-jet fuel based on alternative feedstocks is urgent.The compound 1-butanol can be transformed into jet fuel through dehydration, oligomerization, and hydrogenation [1,2].Efficient catalytic dehydration of 1-buanol to butenes is the first step and an important guarantee for further processing.
Considerable researches have focused on the conversion of sugars obtained from biomass to 1-butanol, where sugars can be fermented to 'acetone-butanol-ethanol' (ABE) using Clostridium acetobutylicum or Clostridium beijerinckii [3,4].Fermented 1-butanol can be separated from the fermentation broth by techniques such as gas-stripping, distillation, membrane separation, and extraction.Generally, fermented 1-butanol is separated into an aqueous phase with 7.7 wt % 1-butanol and an organic phase including 79.9 wt % bio-butanol and 20.1 wt % water at 20 • C. Lin et al. used fed-batch fermentation coupled with gas-stripping for 1-butanol production, and an aqueous phase (84.64 g/L 1-butanol) and an organic phase (624 g/L 1-butanol) were obtained [5].Xue et al. developed a two-stage gas-stripping technology coupled with 1-butanol fermentation, and the concentrations of 1-butanol in the organic phase and aqueous phase were 612.3-623.3g/L and 91.3-101.3g/L, respectively; the aqueous phase was heated and gas-stripped again to concentrate 1-butanol [6,7].Some researches focusing on the dehydration of low-concentration 1-butanol [8] and pure 1-butanol [9,10] have been reported.However, using a high-concentration 1-butanol solution as feedstock seems more economical than using low-concentration 1-butanol or pure 1-butanol, because low-concentration 1-butanol leads to low productivity of butene and the pure 1-butanol needs to be separated further.Unfortunately, the dehydration of high-concentration 1-butanol solutions is seldom studied.
γ-Al 2 O 3 and zeolites show high activity in the alcohol catalytic dehydration reaction.Zeolites with 10-ring unidirectional channels such as Theta-1 and ZSM-23 can efficiently catalyze butanol to butenes, but by-products would be produced such as propylene and C5 olefins [11].When using γ-Al 2 O 3 as a catalyst, the composition of the products is relatively easier to control, and the product distribution depends on the process temperature and strength of catalyst acidity [12,13].It has been proven that there is more dibutylether than butenes in the products when the reaction temperature is below 310 • C, and, as the temperature rises, butenes supersede dibutylether [14].Transition metal catalysts such as cobalt [15,16], manganese, iron [17], nickel [18], and zinc oxides [19] are often utilized for the alcohol dehydration reaction.Metal-loaded catalysts with high catalytic activity and low cost are more adaptable to industrialization requirements.Osman reported that Cu/γ-Al 2 O 3 showed a high degree of activity and stability in a catalytic methanol dehydration reaction [20].Said used phosphotungstic acid-supported γ-Al 2 O 3 to catalyze the dehydration of ethanol to diethyl ether.The results indicated that the catalytic performance of supported catalysts was correlated with the strong and intermediate Brønsted acid sites due to the presence of Keggin structure and tungsten oxide [21].
In this study, Zn-Mn-Co modified γ-Al 2 O 3 was prepared, and the catalytic performance of 1-butanol dehydration to butenes was investigated with 620 g/L 1-butanol as the raw material.This concentration of raw material is more consistent with the concentration range of bio-butanol separated from the fermentation broth.The experiments were designed by the central composite design method for optimizing the dehydration process of 1-butanol with respect to temperature (350-450 • C) and WHSV (1.0-4.0) and maximizing butene yield.This study provides a new catalyst, and the results have great significance for the goal of reducing the production cost of bio-jet fuel from 1-butanol.

Catalyst Characterization
The XRD pattern of γ-Al 2 O 3 and Zn-Mn-Co modified γ-Al 2 O 3 are shown in Figure 1.The position of the diffraction peaks of the modified sample was well matched with that of the original sample, which indicated the modification of Zn-Mn-Co had no effects on the crystalline phase of γ-Al 2 O 3 .The intensity of the diffraction peaks became stronger, which suggested that the crystallinity of Zn-Mn-Co modified γ-Al 2 O 3 was higher than that of γ-Al 2 O 3 .The improvement of crystallinity may be attributed to the fact that zinc, manganese, and cobalt formed a spinel solid solution with γ-Al 2 O 3 , which improved the stability of the catalyst in the reaction environment with water [22].Zinc, manganese, and cobalt oxides were not detected in the spectra, showing that zinc, manganese, and cobalt were highly dispersed on the catalyst surface.Research has suggested that manganese ions play an important role in improving the dispersion of other metal ions [23].To verify whether metal ions were loaded onto γ-Al 2 O 3 , the metal loadings of the modified catalyst were measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method, and the loading amounts of zinc, manganese, and cobalt were 0.54%, 0.44%, and 0.23%, respectively.
Catalysts 2019, 9, x FOR PEER REVIEW 3 of 13 play an important role in improving the dispersion of other metal ions [23].To verify whether metal ions were loaded onto γ-Al2O3, the metal loadings of the modified catalyst were measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method, and the loading amounts of zinc, manganese, and cobalt were 0.54%, 0.44%, and 0.23%, respectively.Figure 2 shows the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature (77 K) for γ-Al2O3 and Zn-Mn-Co modified γ-Al2O3 samples.Both of the isotherms belong to type , Ⅳ and the close hysteresis loops were classified as H3 [24].The specific surface areas, average pore size, and total pore volume of the catalysts are presented in Table 1.The specific surface area of Zn-Mn-Co/γ-Al2O3 was smaller than that of γ-Al2O3, and the total pore volume was reduced by 15% after modification.However, the average pore radius had a slight increase.This may be due to the modification of the metal ions: some of the micropores were blocked.Figure 3 shows the pore size distribution and cumulative pore volume obtained for the catalysts.The pore sizes of both catalysts were mainly distributed in the range of 5-20 nm.The number of micropores in the modified catalyst was less than that in the original sample, which further proved that the modification of metal ions blocked some of the micropores.Figure 2 shows the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature (77 K) for γ-Al 2 O 3 and Zn-Mn-Co modified γ-Al 2 O 3 samples.Both of the isotherms belong to type IV, and the close hysteresis loops were classified as H 3 [24].The specific surface areas, average pore size, and total pore volume of the catalysts are presented in Table 1.The specific surface area of Zn-Mn-Co/γ-Al 2 O 3 was smaller than that of γ-Al 2 O 3 , and the total pore volume was reduced by 15% after modification.However, the average pore radius had a slight increase.This may be due to the modification of the metal ions: some of the micropores were blocked.Figure 3 shows the pore size distribution and cumulative pore volume obtained for the catalysts.The pore sizes of both catalysts were mainly distributed in the range of 5-20 nm.The number of micropores in the modified catalyst was less than that in the original sample, which further proved that the modification of metal ions blocked some of the micropores.
Catalysts 2019, 9, x FOR PEER REVIEW 3 of 13 play an important role in improving the dispersion of other metal ions [23].To verify whether metal ions were loaded onto γ-Al2O3, the metal loadings of the modified catalyst were measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method, and the loading amounts of zinc, manganese, and cobalt were 0.54%, 0.44%, and 0.23%, respectively.Figure 2 shows the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature (77 K) for γ-Al2O3 and Zn-Mn-Co modified γ-Al2O3 samples.Both of the isotherms belong to type , Ⅳ and the close hysteresis loops were classified as H3 [24].The specific surface areas, average pore size, and total pore volume of the catalysts are presented in Table 1.The specific surface area of Zn-Mn-Co/γ-Al2O3 was smaller than that of γ-Al2O3, and the total pore volume was reduced by 15% after modification.However, the average pore radius had a slight increase.This may be due to the modification of the metal ions: some of the micropores were blocked.Figure 3 shows the pore size distribution and cumulative pore volume obtained for the catalysts.The pore sizes of both catalysts were mainly distributed in the range of 5-20 nm.The number of micropores in the modified catalyst was less than that in the original sample, which further proved that the modification of metal ions blocked some of the micropores.The catalytic efficiency of the catalyst is intensively affected by the surface acid sites [8].In general, most Lewis acids react with water before reacting with the substrate in the presence of water, which results in the deactivation of the catalyst.However, it has been well established that surface Lewis acid sites of alumina catalyze the dehydration of alcohols.Phung et al. reported that the activity of alumina was observed in the dehydration of ethanol (in which water is a reaction product) because of its high surface acidity.Lewis acidity does not require dehydroxylation if the basic substrates can displace water.Substrates that have sufficient basicity to compete with water can also be activated by alumina in the presence of water [14,25].
The pyridine temperature-programmed desorption (Py-TPD) profiles of γ-Al2O3 and Zn-Mn-Co modified γ-Al2O3 are shown in Figure 4.The desorption peak at 200 °C was enhanced after modification, which proved that there were more weak acid sites on Zn-Mn-Co modified γ-Al2O3.A weak desorption peak appeared at 390 °C, indicating the increase of strong acid sites.The surface acidity values of γ-Al2O3 and Zn-Mn-Co modified γ-Al2O3 were quantified by pyridine adsorption IR spectra (Figure 5 and Table 2).The bands distinguished at 1620 cm −1 were assigned to pyridine coordinated to tetrahedral and octahedral Al 3+ .The area of the 19b band at 1451 cm −1 was used to quantify the total amount of Lewis acid sites (LAS) using its molar adsorption coefficient (ε = 1.5 cm/μmol) [10,[26][27][28].The amount of LAS is given in Table 3.The acidic concentration of total acidity and the strong LAS were measured at 200 °C and 300 °C, respectively.After being modified by zinc, manganese, and cobalt, the total acid content of the catalyst increased by about 10.5%.Note.S BET is the Brunner-Emmet-Teller (BET) specific surface area, V ad is the cumulative volume of pores, r is the average pore radius.
The catalytic efficiency of the catalyst is intensively affected by the surface acid sites [8].In general, most Lewis acids react with water before reacting with the substrate in the presence of water, which results in the deactivation of the catalyst.However, it has been well established that surface Lewis acid sites of alumina catalyze the dehydration of alcohols.Phung et al. reported that the activity of alumina was observed in the dehydration of ethanol (in which water is a reaction product) because of its high surface acidity.Lewis acidity does not require dehydroxylation if the basic substrates can displace water.Substrates that have sufficient basicity to compete with water can also be activated by alumina in the presence of water [14,25].
The pyridine temperature-programmed desorption (Py-TPD) profiles of γ-Al 2 O 3 and Zn-Mn-Co modified γ-Al 2 O 3 are shown in Figure 4.The desorption peak at 200 • C was enhanced after modification, which proved that there were more weak acid sites on Zn-Mn-Co modified γ-Al 2 O 3 .A weak desorption peak appeared at 390 • C, indicating the increase of strong acid sites.The surface acidity values of γ-Al 2 O 3 and Zn-Mn-Co modified γ-Al 2 O 3 were quantified by pyridine adsorption IR spectra (Figure 5 and Table 2).The bands distinguished at 1620 cm −1 were assigned to pyridine coordinated to tetrahedral and octahedral Al 3+ .The area of the 19b band at 1451 cm −1 was used to quantify the total amount of Lewis acid sites (LAS) using its molar adsorption coefficient (ε = 1.5 cm/µmol) [10,[26][27][28].The amount of LAS is given in Table 3.The acidic concentration of total acidity and the strong LAS were measured at 200 • C and 300 • C, respectively.After being modified by zinc, manganese, and cobalt, the total acid content of the catalyst increased by about 10.5%.

Catalytic Activity of γ-Al2O3 and Zn-Mn-Co/γ-Al2O3
The catalytic activity of γ-Al2O3 and Zn-Mn-Co/γ-Al2O3 was investigated at different reaction temperatures (Tables 3 and 4) and WSHV (Tables 5 and 6), with a reaction time of 6 h.The higher reaction temperature had positive effects on the conversion of 1-butanol and the selectivity of butenes, but a too high reaction temperature worked against the selectivity of butenes.The conversion rates of 1-butanol were 71.21% and 82.63% when the reaction temperature was 300 °C and 350 °C, respectively, with γ-Al2O3 as a catalyst, and increased by 12.82% and 15.27% when Zn-Mn-Co/γ-Al2O3 was employed.When the reaction temperature increased from 300 °C to 450 °C, the selectivity of butenes increased to a maximum and then decreased.The highest selectivity of butenes

Catalytic Activity of γ-Al2O3 and Zn-Mn-Co/γ-Al2O3
The catalytic activity of γ-Al2O3 and Zn-Mn-Co/γ-Al2O3 was investigated at different reaction temperatures (Tables 3 and 4) and WSHV (Tables 5 and 6), with a reaction time of 6 h.The higher reaction temperature had positive effects on the conversion of 1-butanol and the selectivity of butenes, but a too high reaction temperature worked against the selectivity of butenes.The conversion rates of 1-butanol were 71.21% and 82.63% when the reaction temperature was 300 °C and 350 °C, respectively, with γ-Al2O3 as a catalyst, and increased by 12.82% and 15.27% when Zn-Mn-Co/γ-Al2O3 was employed.When the reaction temperature increased from 300 °C to 450 °C, the selectivity of butenes increased to a maximum and then decreased.The highest selectivity of butenes The catalytic activity of γ-Al 2 O 3 and Zn-Mn-Co/γ-Al 2 O 3 was investigated at different reaction temperatures (Tables 3 and 4) and WSHV (Tables 5 and 6), with a reaction time of 6 h.The higher reaction temperature had positive effects on the conversion of 1-butanol and the selectivity of butenes, but a too high reaction temperature worked against the selectivity of butenes.The conversion rates of 1-butanol were 71.21% and 82.63% when the reaction temperature was 300 • C and 350 • C, respectively, with γ-Al 2 O 3 as a catalyst, and increased by 12.82% and 15.27% when Zn-Mn-Co/γ-Al 2 O 3 was employed.When the reaction temperature increased from 300 • C to 450 • C, the selectivity of butenes increased to a maximum and then decreased.The highest selectivity of butenes (96.24%) was obtained at the reaction temperature of 400 • C when using Zn-Mn-Co/γ-Al 2 O 3 as the catalyst.A lower reaction temperature led to the formation of dibutyl ether (DBE) and butene, which increased with the increase of the temperature.When the reaction temperature was 450 • C, there was no DBE formation, and some other hydrocarbons such as methane, ethylene, and propylene appeared.Furthermore, the selectivity of butene isomers was strongly influenced by the reaction temperature.More 1-butene was formed at a lower temperature, and higher temperatures led to increased isomerization of 1-butene.
The conversion of 1-butanol dropped rapidly with the increase of WHSV.When the WHSV increased from 1 h −1 to 4 h −1 , the conversion rate of 1-butanol decreased from 98.01% to 51.04% with γ-Al 2 O 3 as the catalyst.The same trend was obtained when the reaction was catalyzed by Zn-Mn-Co/γ-Al 2 O 3 , and the conversion of 1-butanol was much higher than that when γ-Al 2 O 3 was used as the catalyst.With the increase of WHSV from 1 h −1 to 4 h −1 , the proportion of 1-butene in the product increased, and the proportion of butylene isomers decreased.WHSV had little effect on the total selectivity of butenes, which was about 80% and 90% when using γ-Al 2 O 3 and Zn-Mn-Co/γ-Al 2 O 3 , respectively.
The catalytic performance of Zn-Mn-Co/γ-Al 2 O 3 was much better than that of γ-Al 2 O 3 .The high conversion rate of 1-butanol and the selectivity of butenes indicated that Zn-Mn-Co/γ-Al 2 O 3 exhibited excellent activity at low temperature.Zinc, manganese, and cobalt oxides exhibited high activity in alcohol dehydration, as already proven [15][16][17]29].The modification of metal ions improved the acidity of the catalyst, and the Lewis acid sites played an important role in the catalytic dehydration of alcohols [30].A synergistic effect between different metal ions and catalyst carriers improved the catalytic efficiency [23,31].

Optimization of Temperature and WHSV for Butanol Dehydration
To optimize the temperature and WHSV of the 1-butanol dehydration process, experiments were performed in accordance with the conditions listed in Table 7, which was designed by a central composite design.The values of butene yield (Y) obtained in the 1-butanol dehydration experiments at different temperatures and WHSV are shown in Table 7.The results of ANOVA for a fitted quadratic polynomial model are shown in Table 8.The "Model F-value" of 642.51 and the lowest p value of less than 0.0001 implied that the model was highly significant and there was only a 0.01% chance that a "Model F-value" this large could occur because of noise.The value of R 2 for Y was 0.9978.Butene yield was significantly affected by reaction temperature and WHSV, which could be deduced from the higher absolute value of F and the lower p value (Table 8).The yield of butanol can be predicted by the following equation: where X 1 , X 2 are the temperature and WHSV, respectively.X 1 X 2 was significant, suggesting that temperature and WHSV interacted.Figure 6 shows the response surface diagram of temperature and WHSV interactions for the total yield of butenes.Butene yield was enhanced as the temperature increased but reduced as WHSV increased.The temperature and WHSV were optimized by the model with the hypothetical conditions of minimum temperature, maximum WHSV, and maximum butene selectivity.The optimum conditions were found to be 375.78• C and 1.67 h −1 of WHSV and, under the optimum conditions, the predicted value of total yield of butenes was 92.61%.In a real catalytic reaction of 1-butanol dehydration, the total yield of butenes was 90% at optimum conditions, and the conversion of 1-butanol reached 100%, which were 10% and 6% higher than that when using unmodified γ-Al 2 O 3 in the first 60 h of the reaction.After 60 h of reaction, the unmodified γ-Al 2 O 3 deactivated rapidly, while the Zn-Mn-Co modified γ-Al 2 O 3 showed high stability and long life (Figure 7).Rehydration of γ-Al 2 O 3 is the main reason of deactivation in the presence of water.The rehydration of γ-Al 2 O 3 results in a decrease of the crystallinity of the catalyst and in a significant decrease in the specific surface area of the catalyst [32].In addition, there is a higher level of by-products of DBE generated with γ-Al 2 O 3 as the catalyst.The DBE may be adsorbed on the surface of the catalyst which leads to the decrease of catalytic efficiency and deactivation of γ-Al 2 O 3 .The crystallinity of γ-Al 2 O 3 was enhanced after modification by Zn-Mn-Co (Figure 1), which improves the stability of the catalyst in the reaction environment with water [22].The enhancement of surface acidity reduces DBE formation, which also prolongs the life of the catalyst.The application of γ-Al 2 O 3 for the dehydration of 1-butanol has been reported, and the process temperature appeared to play a decisive role in the distribution of olefins and ethers in the products [12].The proportion of ethers in the product was larger than that of olefin when the reaction temperature was below 300 • C, and olefin superseded ethers with an increasing temperature [13].The range of reaction temperature in this study was 350-450 • C, and the main products were butenes, which is consistent with the literature.WHSV mainly affected the retention time of 1-butanol in the catalyst micro-reactor system, and high WHSV was not conducive to the adsorption of butanol by a catalyst, which resulted in a decline of the conversion rate of 1-butanol.Zinc, manganese, and cobalt oxides exhibited excellent catalytic activity in the dehydration reaction of alcohol [15][16][17] and they are also used as active components to be loaded onto a catalyst carrier [16,19].In this study, zinc, manganese, and cobalt were loaded onto γ-Al 2 O 3 , and the modified catalyst showed superior catalytic activity, which was attributed to the catalytic ability of the active components and the increase of the catalyst's acidity.In addition, the enhancement of the crystallinity and acidity of the catalyst made it stable and gave it a long lifetime.The application of γ-Al2O3 for the dehydration of 1-butanol has been reported, and the process temperature appeared to play a decisive role in the distribution of olefins and ethers in the products [12].The proportion of ethers in the product was larger than that of olefin when the reaction temperature was below 300 °C, and olefin superseded ethers with an increasing temperature [13].The range of reaction temperature in this study was 350-450 °C, and the main products were butenes, which is consistent with the literature.WHSV mainly affected the retention time of 1-butanol in the catalyst micro-reactor system, and high WHSV was not conducive to the adsorption of butanol by a catalyst, which resulted in a decline of the conversion rate of 1-butanol.Zinc, manganese, and cobalt oxides exhibited excellent catalytic activity in the dehydration reaction of alcohol [15][16][17] and they are also used as active components to be loaded onto a catalyst carrier [16,19].In this study, zinc, manganese, and cobalt were loaded onto γ-Al2O3, and the modified catalyst showed superior catalytic activity, which was attributed to the catalytic ability of the active components and the increase of the catalyst's acidity.In addition, the enhancement of the crystallinity and acidity of the catalyst made it stable and gave it a long lifetime.

Materials
The pseudo-boehmite was provided by the Catalyst Plant of Nankai University, Tianjin, China.

Catalyst Preparation
The catalysts tested in this study were γ-Al 2 O 3 and Zn-Mn-Co/γ-Al 2 O 3 .Pseudo-boehmite was converted to γ-Al 2 O 3 after being calcined at 600 • C for 5 h.One hundred milliliters of a solution containing 1% Zn(NO 3 ) 2 , 4% Mn(NO 3 ) 2 , and 3% Co(NO 3 ) 2 was added to 10 g γ-Al 2 O 3 with continuous stirring.The resulting solution was placed in an autoclave and treated at 150 • C for 4 h.The treated γ-Al 2 O 3 was washed with distilled water, then dried at 120 • C for 12 h, and calcined at 600 • C in air for 5 h.The resultant powder was molded under 20 MPa pressure to tablets, which were then crushed into particles and sieved to size 20-40 mesh.

Experimental Design
The interaction effects of reaction temperature and WHSV on 1-butanol dehydration were investigated by response surface methodology (RSM).The experimental scheme in this study was designed by Design-Expert 8.0.5 Trial.The catalytic performance was evaluated using the butene yield.In order to express the effect of factors including reaction temperature and WHSV on the dependent variable (Y), the responses were fitted by the regression model expressed in Equation ( 2).The optimal point was predicted by a quadratic model, which was expressed as: Y = b 0 + ∑b i x i + ∑b ii x i 2 + ∑b ij x i x j i = 1, 2, . . ., k; j = 1, 2, . . ., k; i = j, where Y is the predicted response, x i and x j are independent variables, b 0 is the intercept, b i is the linear coefficient, b ii is the quadratic coefficient, and b ij is the interaction coefficient.The experiments were performed in accordance with the conditions listed in Table 7, which were designed by a central composite design.The experimental data were used to calculate the second-order polynomial coefficients, and the model was evaluated by the analysis of variance (ANOVA) technique in the Design-Expert 8.0.5 Trial.The coefficient of determination R 2 was used to evaluate the quality of the fit of the polynomial model equation.F-test and t-test were used to check the statistical and regression coefficient significance.

1-Butanol Dehydration Experiments
The experiments on the catalytic dehydration of 1-butanol were performed in a micro-reactor device (Figure 8).First, 1.3 g 20-40 mesh catalyst was loaded in the middle of a reactive tube, fixed bed reactor (440 mm length, 8 mm I.D.), and the space under the catalyst bed in the reactor was filled with ceramic chips.The catalyst was pretreated at 450 • C in N 2 for 1 h and then cooled to the test temperature.The feed rate of 620 g/L 1-butanol solution was controlled by a micro peristaltic pump, and the 1-butanol was vaporized by a vaporizer.

Catalyst Characterization
Polymorphs of the catalysts were characterized by an X-ray diffraction system using a D8 Advance instrument (Bruker, Karlsruhe, Germany).The X-ray wavelength was 1.5406 Å, and the step size was 0.01°.The voltage and current were 40 KV and 40 mA, respectively.The scan speed and the scan range were 0.1 s/step and 10-80°, respectively.
The specific surface areas and porosity of the catalysts were examined using Tristar 3020 II (Micromeritics, Norcross, GA, USA), and the structural parameters were determined by the BET method.Nitrogen was used as the gas adsorbate at 77 K.

Analytical Methods
The analysis of gas phase products was performed on Shimadzu-2010 gas chromatograph (Shimadzu, Kyoto, Japan) equipped with a flame ionization detector (FID), and the components in the product were separated by a capillary column (Alumina/NaSO 4 , 30 mm × 0.5 mm × 10 um)

Figure 6 .
Figure 6.Contour plot of the combined effects of temperature and WHSV on butenes yield obtained with Zn-Mn-Co modified γ-Al2O3.

Figure 6 .
Figure 6.Contour plot of the combined effects of temperature and WHSV on butenes yield obtained with Zn-Mn-Co modified γ-Al 2 O 3 .

Catalysts 2019, 9 ,
x FOR PEER REVIEW 11 of 13 HPLC (Shimadzu LC 20A, Kyoto, Japan) equipped with a refractive index detector and an Aminex HPX-87H column was used to analyze the liquid products.The compound 1-Butanol was analyzed at 65 °C with 5 mM H2SO4 as the mobile phase at 0.8 mL/min.

Table 1 .
Textural properties of the catalysts.SBET is the Brunner-Emmet-Teller (BET) specific surface area, Vad is the cumulative volume of pores, r is the average pore radius.

Table 1 .
Textural properties of the catalysts.

Table 2 .
Amount of the Lewis acid sites on the catalysts.LAS, Lewis acid sites.

Table 2 .
Amount of the Lewis acid sites on the catalysts.LAS, Lewis acid sites.

Table 2 .
Amount of the Lewis acid sites on the catalysts.LAS, Lewis acid sites.
Note.The reaction temperature was 350 • C.

Table 7 .
Central composite design and experimental results obtained with Zn-Mn-Co modified γ-Al 2 O 3 .

Table 8 .
Analysis of variance for fitted quadratic polynomial model for total yield of butenes.

Table 8 .
Analysis of variance for fitted quadratic polynomial model for total yield of butenes.