Article pubs.acs.org/IECR
Catalysts, Process Optimization, and Kinetics for the Production of Methyl Acrylate over Vanadium Phosphorus Oxide Catalysts Xinpeng Guo,†,‡ Dan Yang,† Cuncun Zuo,† Zhijian Peng,‡ Chunshan Li,*,† and Suojiang Zhang*,† †
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, The National Key Laboratory of Clean and Efficient Coking Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China ‡ School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China ABSTRACT: Vanadium phosphorus oxide (VPO) catalysts were developed for the aldol condensation of methyl acetate (MeOAC) with formaldehyde (HCHO) to methyl acrylate (MA). The influence of P/V ratio on catalytic activity and selectivity was revealed and optimized with a fixed bed reactor. The controllable preparation of VPO catalyst was reached, and the influence of carrier gas on the resistance of carbon deposition was also systematically investigated. Response surface methodology (RSM) with three independent variables of molar ratio of MeOAC to HCHO, space velocity, and reaction temperature was employed to optimize MA yield with VPO catalysts. The high value of “R-squared” presented a reasonable correlation of regression model to the experimental data. Lastly, the kinetics of aldol condensation to produce MA over VPO catalyst was studied.
1. INTRODUCTION As an important industrial monomer, MA and its derivatives are extensively used in the production of plastics, leather treating agents, adhesives, and coatings,1−5 such as PAN fibers, nylon66. Traditionally, MA is produced by acetone cyanohydrin (ACH) process.6 In this route, it requires large amounts of sulfuric acid and hazardous hydrogen cyanide while producing ammonium sulfate as a byproduct which is difficult to handle. As a commercial route, MA is produced by oxidation of propylene or propane.7−13 However, with the shortages of propylene resources from nonsustainable petroleum, it is necessary to develop an alternative route to synthesize MA and its derivatives. In recent years, the worldwide rising productivity of MeOAC has increased considerably. Thus, there is a powerful motivation for converting excessive MeOAC to other value-added chemicals. The development of coal chemistry industry makes the attention paid to the synthesis of MA with MeOAC and HCHO (coal chemicals products) by one step aldol condensation reaction.14−16 To obtain a high product yield, an excellent catalyst is crucial. There were various catalysts researched for the target reaction. Ai et al. reported that V2O5− P2O5 binary oxide catalysts and V−Ti binary phosphates catalysts performed well in the aldol condensation of HCHO with acetone or with acetic acid/MeOAC to acrylic acid and its derivatives.17−21 Alkaline earth metal oxides and alkali metal oxides as solid base catalysts supported on SiO2, ZrO2, Al2O3, TiO2, and ZSM-5 were also used for the esterification reaction.22−27 Furthermore, Ai M. found that VPO catalysts with (VO)2P2O7 crystalline phase were effective in aldol condensation reactions.28,29 The catalysts had also researched © XXXX American Chemical Society
systematically on n-butane partial oxidation to maleic anhydride, which was a totally different kind of reaction from the aldol condensation reaction.30,31 Recently, Feng et al. reported VPO catalysts were a benefit to high conversion of MeOAC to MA and its derivatives.32 What is more notable that the more environmentally friendly VPO catalysts achieved a record conversion of MeOAC (84.2%). It is necessary for us to do more detailed research to achieve better performance. Moreover, it is inevitability to optimize process conditions to obtain excellent production. Response surface methodology (RSM) as a statistical technique is widely utilized for experimental design, model building, experiment results optimization and it could achieve optimal parameters for required response values. It is convenient that the RSM can produce an amount of information by conducting a minimum number of experiments to build an appropriate fitting model and deduce the effects of single variables and their interactions on the response value.33,34 Until now, there are no researchers using RSM to optimize the process of MeOAc and HCHO to MA. In order to make the reaction system analysis more simple, practical and comprehensive, power exponential function was used to discuss the reaction dynamics. Furthermore, the combination of RSM and kinetic model for aldol condensation reaction of MeOAC with HCHO over highly active vanadium Received: Revised: Accepted: Published: A
March 23, 2017 April 30, 2017 May 3, 2017 May 3, 2017 DOI: 10.1021/acs.iecr.7b01212 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
of catalyst was charged in the fixed bed reactor, and the space of the reactor was filled with quartz chips or a steel tube. Before the reaction, the catalyst was increased to a desired temperature and maintained at this temperature for a period of time. Then a reactant mixture of MeOAC and HCHO was fed into the reactor to start the reaction and a flow of air, N2, or other various of mixture gas was used as a carrier gas. After the reaction was steady, the products were collected and analyzed using gas chromatography (GC, Shimadzu GC 2010 plus). On the other hand, the samples were diluted with isobutyl alcohol, which used as an internal standard substance. Each experiment was repeated for three times to ensure the accuracy of the experimental data. The final result is the average of three experiments and the experiment error is less than 5%. The activity of the catalyst was measured by testing the selectivity and yield of MA which were analyzed as follows:
phosphorus oxide solid acid catalyst has not been reported so far. In this study, a series of VPO catalysts with various P/V ratio were used in aldol condensation reaction of MeOAC with HCHO to produce MA: CH3COOCH3 + HCHO → CH 2CH 2COOCH3 + H 2O
According to relevant research, the atomic ratio of P/V, phase composition, and particle size of the catalysts closely affect the catalytic performance. At the same time, it was also found that there was no detailed research on P/V ratios. In this paper we had systematically studied the influence of various P/ V ratios on catalytic activity. The characterization results revealed the acidity and basicity as well as the phase composition had a significant effect on catalytic activity. The yield and selectivity of MA were calculated and depend upon the molar quantity of HCHO. Moreover, this work also explored the process parameters by RSM and its kinetic modeling for the production of MA. A design expert software, version 8.0.6.1 were used to optimize the process parameters and the interaction between three independent variables such as space velocity, reaction temperature, the molar ratio of MeOAC and HCHO in raw material were investigated. Kinetic data was procured in case of eliminating both internal and external diffusion effects, and the reaction kinetics of the aldol condensation reaction was evaluated by the power exponent function-type kinetic model.
selectivity of MA =
conversion of MA =
MA out,mol HCHOin,mol − HCHOout,mol HCHOin,mol − HCHOout,mol HCHOin,mol
× 100%
× 100%
yield of MA = selectivity of MA × conversion of HCHO
2.3. Characterization of Catalysts. Powder X-ray diffraction (XRD) measurement was recorded on a Rigaku Smart Lab X-ray powder diffractometer with Cu Kα radiation (λ = 0.15418 nm) at an emission of 50 mA and an accelerating voltage of 40 kV. The patterns were recorded with 2θ ranged from 5° to 90°. The BET surface area, pore volumes, and pore mean diameters of catalysts were tested by N2 absorption on a micromeritics ASAP 2460 apparatus. To make sure the accuracy of the data, the sample was degassed at 350 °C for 6 h. The Fourier transform infrared spectroscopy (FTIR) was obtained using an Nicolet 380 FTIR spectrometer (Thermal Electron Corporation). The samples were prepared with anhydrous KBr standard in the form of pressed wafers. The acidity and basicity of catalyst was measured by NH3 and CO2 adsorption. It was tested by using carbon dioxide and ammonia temperature-programmed desorption on AutochemII2920 Chemisorption Apparatus (Micromeritics). The microcosmic patterns of the catalysts were performed by a Hitachi SUB8020 scanning electron microscope (SEM) instrument with an operating voltage of 20 kV. Catalysts were made on the surface of double faced conductive tape and placed on the stubs directly. Also, the morphology of the catalysts were observed by using a transmission electron microscope (TEM) with the model JEOL JEM-2100 system. 2.4. Experiment Design and Mathematical Model. Response surface methodology (RSM) was employed to optimize the process parameters for the synthesis of MA by aldol condensation of MeOAC with HCHO over VPO catalysts. A Design-Expert Software, version 8.0.6.1 was used to study the experimental design, statistical analysis, and regression model. A factorial three-level-three-factor experimental design was used to evaluate the independent effects and interactions of three factors (MeOAC to HCHO molar ratio in raw material (X1), space velocity (X2), reaction temperature (X3)) on the dependent variable (the yield of MA). The independent variables coded at three levels (−1, 0, and 1) were
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Trioxymethylene (98.0%) and polyethylene glycol 6000 (≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Benzyl alcohols (≥99.0%) and phosphoric acid (≥85%) were purchased from Xilong Co., Ltd. MeOAC (≥98.0%) and isobutyl alcohol (≥99.0%) were purchased from Tianjin Chemical Reagent Factory. Vanadium pentoxide (≥99.7%) was provided by Handing Co., Ltd. All chemical agents were used with no further purification. All of the catalysts were prepared by organic solvent method.35 The procedures to prepare the catalysts are as follows. First V2O5 was dissolved in a mixture of isobutyl alcohol and benzyl alcohols solution and refluxed for 6 h at 140 °C in air atmosphere. The color of the mixture changed into black when V2O5 was reduced completely. Then PEG 6000 was introduced in a certain amount and the solution was heated and refluxed for 1 h. After that, phosphoric acid (85%) was added dropwise to reach a P/V atomic ratio within the range of 0.6− 1.4. The resulting slurry was stirred while maintaining the temperature for another 6 h. The suspension changed in light blue, the preparation of catalyst precursors was obtained. The precursor was washed with ethanol about five times and dried in air at 120 °C for 24 h. Finally, the precursor was activated in muffle furnace and the dried precursor was heated from room temperature to 500 °C at a certain rate in air atmosphere and kept at this temperature for a certain period of time. After natural cooling, the VPO catalyst was obtained. 2.2. Catalyst Evaluation and Product Analysis. The catalyst powder was pressed and sieved to 20−40 mesh for activity evaluation. Catalyst evaluation was carried out in a fix bed reactor in the temperature range of 330−380 °C at atmospheric pressure with different carrier gas. A total of 5 mL B
DOI: 10.1021/acs.iecr.7b01212 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research listed in Table 1. The total number of experiments was 17 including 12 factorial points and 5 center points, which were Table 1. Level of Various Studied Factors and Coded Level Used in the Response Surface Methodology Study coded levels symbol
variables
−1
0
1
X1 X2 X3
molar ratio (MeOAC to HCHO) space velocity (h−1) reaction temperature (°C)
2.5 0.6 350
5.0 1.2 370
7.5 1.8 390
performed according to a 33 Box−Behnken experimental design (BBD).36−38 A second-degree polynomial equation recommended by RSM was used to express the optimum value according to the independent variables. The quadratic equation model denoted in the following equation: k
Y = β0 +
k
∑ βi Xi + ∑ βiiXi i=1
i=1
k 2
+
∑ βijXiXj i=1
Figure 1. XRD spectra of fresh catalysts with different P/V ratio. (1)
3.1.2. BET Analysis. The N2−BET surface area, pore volume, and mean pore diameter of VPO catalysts with different P/V ratio are summarized in Table 2. The surface area were
where Y represents the predicted response (the yield of MA), β0 is the constant coefficient, βi βii and βij are the coefficients of linear terms, quadratic terms, and interaction terms, respectively, and Xi are the coded independent variables. The “k” refers to the total number of variables employed to optimize the response value. Regression analysis and analysis of variance (ANOVA) is used to estimate the reliability of the experimental data and assess whether the empirical model has significant meaning in a method of statistics. Its basic idea is to compare the response values changing with the variables and the experiment random error to determine whether the influence of factors on the response value is significant. In this paper the total number of variables k = 3, then the quadratic equation model denoted in the following equation:
Table 2. BET Surface Areas, Pore Volume and Mean Pore Diameter of VPO Catalysts at Different P/V Ratios SBET (m2/g)
pore volume(cm3/g)
mean pore diameter (nm)
0.6 1.0 1.1 1.2 1.3 1.4
7.80 19.42 32.07 33.63 47.13 49.76
0.034 0.053 0.065 0.069 0.089 0.103
17.206 10.855 8.590 7.776 7.534 7.295
exhibited to increase from 7.80 m2/g to 49.76 m2/g with increasing in P/V ratio from 0.6 to 1.4. The pore volume was also increased with the addition of phosphoric acid, while mean pore diameter decreased gradually. Such an increase in surface area might be due to possible structural changes occurring during catalyst preparation under the influence of the addition of phosphoric acid. Namely, the addition of phosphoric acid was conducive to the formation of small particles, and as a result, the surface area increased and the pore diameter decreased. According to the literature, surface area and pore diameter had important influence on the performance of the catalyst.41 However, high surface area and wide pore diameter were not availed to improve the activity of catalysts. On the basis of the above analysis, it can be deduced that catalysts with high or low P/V ratio did not become more active but that of suitable proportion was conducive to catalytic performance. 3.1.3. TEM and SEM Images. The SEM and TEM images of different P/V atomic ratio catalysts were shown in Figure 2. It could be seen from the SEM images that the catalyst with P/V ratio of 0.6 had an irregular structure, which indicated that the catalyst had not formed a stable crystal shape. The structure of the sample was traditional sheet crystallization with the ratio of 1. Each petal was equivalent to a small piece and they gathered together to form the rose-like flowers. The crystallization of flake was known as agglomerates of (VO)2P2O7 platelets.40 However, for a P/V ratio of 1.2, there is an irregular morphology like a flower without blossoms (not pure (VO)2P2O7 phase), which was also confirmed by XRD results.
Y = β0 + β1X1 + β2X 2 + β3X3 + β11X12 + β22X 2 2 + β33X32 + β12X1X 2 + β13X1X3 + β23X 2X3
P/V ratio
(1a)
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. XRD Analysis. Figure 1 presents the XRD spectra of VPO catalysts corresponding to different P/V ratios. The XRD patterns revealed that the (VO)2P2O7 and VOPO4 were the main phases detected for activated catalysts. The characteristic diffraction peaks at 28.4° and 29.9° were typical of the (VO)2P2O7 phase corresponding to (042) and (202) planes, respectively. The diffraction peaks emerged at 11.9°, 18.6° belong to VOPO4· 2H2O, β-VOPO4 phase.. With the increase of P/V ratio the characteristic diffraction peaks of (VO)2P2O7 and VOPO4 phase appeared together and the content of both peaks increased first and decreased gradually later. This result indicated that the appropriate value of the P/V atomic ratio could promote the formation of VOPO4 or (VO)2P2O7 phases. On the other hand, the crystallinity of the catalyst was also actually affected by the 85% H3PO4 addition in preparation. In case of the P/V value was 1.2, the crystallinity and intensity of VPO catalysts were highest and the VOPO4·2H2O, β-VOPO4 had a higher fraction. According to the report in the literature, both VOPO4 and (VO)2P2O7 entities in balanced value would contribute to the catalytic performance and a high amount of VOPO4 entity were also beneficial to catalytic behavior.31,32 C
DOI: 10.1021/acs.iecr.7b01212 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. SEM and TEM of VPO catalysts with different P/V ratios: (1) fresh catalyst, P/V = 0.6; (2) fresh catalyst, P/V = 1.0; (3) fresh catalyst, P/ V = 1.2; (4) fresh catalysts, P/V = 1.4; (5) used catalysts, p/v = 1.2; (6) TEM of fresh catalysts, p/v = 1.2; (7) TEM of used catalysts, p/v = 1.2.
with P/V = 1.2 had (VO)2P2O7 and plenty of VOPO4 phase. The FTIR observations provided beneficial evidence for the results obtained from XRD, which proofed the formation of catalytic active phase. 3.1.5. CO2-TPD and NH3-TPD Results. As known, the structure and constitution of VPO catalysts changed with the P/V atomic ratio, a conclusion was reasonable to deduce that surface basicity and acidity of catalysts were also changed accordingly. Therefore, CO2-NH3 adsorption technique was applied to measured basicity and acidity to verify the inference. The CO2-TPD patterns of catalysts with different P/V atomic ratio are shown in Figure 4a. It can be seen from Figure 4a that when the P/V atomic ratio was less than 1.1, the catalysts showed weak basic sites of fairly low density. Clearly, the desorption peak shifted toward higher temperature with the increase of phosphorus content in catalysts, indicating the gradually strengthened basicity. It is worth noting that the catalyst with P/V = 1.2 was exceptional and the samples showed weak basic sites of high density (due to the large desorption peak in the 116−235 °C range). For P/V = 1.4, the catalyst exhibited medium strong basic sites of high density (in view of the desorption peak center at 260 °C). However, the total basic amounts of the catalysts with various P/V ratios were really low. In Figure 4b, the catalyst with P/V = 0.6 showed weak acid sites of fairly low density and other catalysts showed one main desorption peak about 260 °C, which revealed that all samples possess weak acid sites. The main peak of VPO catalysts was slightly enlarged with the increase of P/V atomic ratio, which indicated the increasing total acid amount of the catalysts correspondingly. However, the patterns were changed significantly with the ratio at 1.2, and the acid amount was more than others. In addition, there were two different acid sites that could be found in this catalyst. The small desorption peaks at 357−450 °C was assigned to medium strong acid sites, and the peak that emerged at 450−500 °C was strong acid sites. In other words, the presence of medium or strong acid sites need appropriate concentrations of phosphoric acid added to the catalysts. All in all, different P/V atomic ratios were related to the changes of acidity and basicity of VPO catalysts. Namely, the increase of P/V ratio provided part of the (medium) strong acid sites and strengthened the base sites. Furthermore, the balance between basic sites and acidic sites on the catalyst could contribute to better catalytic activity, which was consistent with a previous report in the literature.31,45
There could be phosphoric acid of different concentrations in the process of preparing catalyst leading to the corresponding changes in catalysts properties. It can be seen from the TEM that the fresh catalyst with P/V = 1.2 had an irregular structure and the particle morphologies were apparent. The SEM and TEM images revealed that the structure of the catalysts had obvious changes between fresh catalysts and used catalysts. The TEM results revealed the changes in particle size and active constituents of fresh and used catalysts. From the TEM and SEM measurements on the fresh and the spent catalysts, the basic, small units seem to have been preserved. However, some agglomeration has apparently occurred during reaction. 3.1.4. FTIR Analysis. The infrared spectra of VPO catalyst is given in Figure 3. All catalysts vibration bands appeared in the
Figure 3. IR spectra of catalysts with different P/V ratios.
400−1400 cm−1 region. The band appeared at 582 cm−1 could be attributed to P−O−P IR vibrations, and the one at 1083 and 1260 cm−1 corresponded to symmetric and asymmetric stretching vibrations of phosphate groups, respectively.42,44 The above results clearly confirmed the formation of (VO)2P2O7 phase in these catalysts.43The band at 796 cm−1 was due to VO···V stretching vibrations of the VOPO4 phase.45 Compared with the other ratio, catalyst with P/V = 1.2 showed stronger infrared absorption peak of the VOPO4 phase. The results of Figure 3 clearly demonstrated that the catalyst D
DOI: 10.1021/acs.iecr.7b01212 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. (a) CO2-TPD profiles of VPO catalysts with different P/V ratios. (b) NH3-TPD profiles of VPO catalysts with different P/V ratios.
Figure 5. (a) Effect of P/V ratio on catalytic performance: reaction temperature, 370 °C; air carrier (20 mL/min). (b) Effect of flow rate of air on catalytic performance: reaction temperature, 370 °C. (c) Effect of carrier gas on catalytic performance: reaction temperature, 370 °C. (d) Effect of reaction temperature on catalytic performance: air carrier (20 mL/min).
3.2. Catalytic Performance. The aldol condensation between MeOAC with HCHO over a series of VPO catalysts (calcinated at 500 °C for 15 h) and the effect of various factors on catalytic performance are presented in Figure 5. All of the catalysts were evaluated at a molar ratio of MeOAC/HCHO of 5:1. The influence of different P/V ratio on catalytic performance at 370 °C is shown in Figure 5a. The yield of MA increased slightly then decreased with the increasing of P/ V ratio, and the selectivity was stabilized about 97%. The catalyst with P/V atomic ratio of 1.2 presented the best yield of
MA. The byproduct included a small amounts of methyl methacrylate, methyl propionate, and COx. It was obvious that P/V atomic ratio of VPO catalyst had a great influence on catalytic performance. By making the associations between characterization results and corresponding catalytic performance, one can conclude that the structure/constitution, basicity, and acidity of catalysts have a close relationship with catalytic activity. The XRD and FTIR results coincided with the changes of catalyst performance, and the (VO)2P2O7 and VOPO4 phase appeared together in VPO catalysts. In the XRD profile, the E
DOI: 10.1021/acs.iecr.7b01212 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
activated HCHO reacts with α-C anion, the negative charge transfers to oxygen anion, which get a hydrogen ions to dehydration and condenses to the final product MA and water. The effect of flow rate of air on yields and selectivities of MA in the process of catalysts evaluation are shown in Figure 5b. The yield of the product increased significantly then decreased with the increasing of the flow velocity of air, which indicated that the amount of air play an important role in the aldol condensation reaction.16 In view of the fact, we also measured the catalytic activity at different atmosphere (carrier gas), which are presented in Figure 5c. Among the samples performed in no carrier gas, N2, 8% O2−N2, 50% N2−air (10.5% O2−N2), and air (21% O2−N2) atmospheres, the catalyst operated in air showed the highest yield of MA. The catalyst estimated in N2 and no carrier gas showed the relatively lower activity performances. It was worth noting that better performance of aldol condensation between MeOAC with HCHO demanded a certain amount of oxygen and the lattice oxygen played an important feature on catalytic performance. According to previous reports in the literature, the existence of lattice oxygen is beneficial to the form of α-C anion.48,49 The optimum catalyst was evaluated at different temperatures, and the results are shown in Figure 5d. The results suggest that reaction temperature presented significant effect on catalytic activity and the best yield was obtained at 370 °C. 3.3. RSM Analysis. 3.3.1. RSM Experiments and Predicted Model. The RSM experimental design was applied to evaluate the effects of the three independent variables on the yield of MA. On the basis of the Box−Behnken design, the 17 experiments were designed. In order to evaluate the effects of the MeOAC to HCHO molar ratio in raw material, space velocity, and reaction temperature on the yield of MA, the experimental data in Table 3 were subjected to regression analysis. The data in Table 3 shows that due to the different reaction conditions, the yield of MA varied greatly. Also, the best fitting model for response variable were determined by the statistical design. Five replicates at the center point were used to evaluate the experimental error. The regression model for space velocity, reaction temperature, molar ratio of raw material in code units was given as follows:
crystallinity and intensity of the catalyst with P/V = 1.2 was highest and the VOPO4 has a higher fraction. The FTIR result also showed that the catalyst with P/V = 1.2 had a stronger infrared absorption peak of VOPO4. According to the XRD and FTIR analysis, for the catalyst with P/V = 1.2 the (VO)2P2O7 phase associates with plenty of VOPO4 phase could account for the superior activity.16 This result was also accordance with N2BET results and SEM image in Table 3 and Figure 2, Table 3. Response Surface Design and Experimental Data actual levels (X i)
yield of MA/%
number
molar ratio
space velocity/h−1
reaction temperature/°C
actual value
predicted value
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
5.0 2.5 2.5 5.0 5.0 5.0 7.5 5.0 7.5 7.5 5.0 7.5 2.5 5.0 5.0 5.0 2.5
0.6 1.2 1.8 0.6 1.8 1.2 1.8 1.2 1.2 1.2 1.8 0.6 0.6 1.2 1.2 1.2 1.2
350 350 370 390 350 370 370 370 390 350 390 370 370 370 370 370 390
44.51 21.70 24.35 66.22 49.26 73.90 55.04 74.20 60.60 53.21 40.20 68.22 31.96 75.90 73.50 72.40 30.20
44.05 21.48 24.92 66.57 48.92 73.98 54.35 73.98 60.83 54.24 40.67 67.65 32.65 73.98 73.98 73.98 29.16
respectively. For the result of TPD, the catalyst with P/V = 1.2 was exceptional and the samples possessed a weak basic sites of high density as well as a middle strong acid site, which could contribute to better catalytic activity, and it coincided well with previous reports in the literature.16,31,45 On the basis of the reports in the literature, the aldol condensation is an extremely useful carbon−carbon bond forming reaction. The possible reaction process of the aldol condensation is shown in Figure 6. First, in alkaline conditions, the carbon atoms connected with the carbonyl (α-C) of MeOAC can lose a proton H + to form α-C anion. Then the
Y (%) = 73.98 + 16.11X1 − 5.26X 2 + 3.57X3 − 1.39X1X 2 − 0.28X1X3 − 7.69X 2X3 − 18.85X12 − 10.23X 2 2 − 13.70X32
(2)
In the preceding equations, X1 is the molar ratio of MeOAC to HCHO, X2 is space velocity, and X3 is the reaction temperature and Y represents the MA yield. The predicted values of MA yield were calculated using the fitting model and compared with actual values in Figure 7. It demonstrated the good linear correlations between the actual value and predicted value of the MA yield. The points cluster around the diagonal line in the graph, which indicates the experimental values are consistent with the predicted values. Therefore, it was also symbolized a good fit of the model. A positive sign of the coefficients in second-order polynomial terms, indicating that with an increase in the variables the MA yield increases correspondingly (synergistic effect), while a negative sign reveals an antagonistic effect.39 Equation 2 shows the molar ratio of MeOAC with HCHO in raw material (X1) has extremely comparable influence on response value among
Figure 6. Aldol condensation reaction route of MeOAC with HCHO over VPO catalyst. F
DOI: 10.1021/acs.iecr.7b01212 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
400.94 also implies the model is significant within the spatial influence of the variables on the responses. The coefficient of determination (R2) of 0.998 reveals a reasonable correlation of the model to the experimental results. Also the “Adj R-squared” of 0.9956 indicates a very good fit between the experimental and predicted responses and it was very high to advocate for a goodness of the model. The analysis above advocated a quadratic model as the most suitable model. 3.3.2. Response Surface Analysis. Instead of researching a single variable, the interactions were studied, which was signifcant for the process optimization. Therefore, the effects of independent factor and their interactions on MA yield were calculated by response surface plots. Three-dimensional (3D) response surface and two-dimensional (2D) contour plots for the yield of MA are the graphical form of the model eq 1, which is shown in Figure 8. They represented the effects of a single variable and their interactions on response value, and it was plotted by maintaining one variable at its zero level. The center of the smallest ellipse in the contour is in response to the highest point of the response surface. In addition, the shape of the contour can reflect the strength of the interaction effect, an elliptical contour plot reflecting significant interaction between two factors, while a circular contour plot indicates that the interactions between them are negligible. As shown in Figure 8, the molar ratio of MeOAC to HCHO has an extreme influence on the yield of MA compared with other variables. The interaction effect of space velocity and reaction temperature on MA yield at a constant molar ratio of 5 is shown in Figure 8a,b. It was obvious that the yield of MA increased with an increase in space velocity at moderate levels of reaction temperature and then decreased correspondingly. At low space velocity, the MA yield rapidly increases with the increase of reaction temperature. However, when the temperature is higher than a certain temperature, the slightly decrease in MA yield was observed, which could be a result from the deactivation of the VPO catalyst. The interaction effect of space velocity and reaction temperature was the most significant parameter, which could be intuitively obtained from the shape of the 2D elliptical contour plot. The influence of the relationship between the molar ratio of MeOAC to HCHO and reaction temperature (independent variables) on the yield of MA is given in Figure 8c,d. It seemed
Figure 7. Comparison between observed and predicted MA yield.
three variables, since the coefficient of X1 (16.11) is the biggest compared to the other factors. The next significant effect on MA yield is space velocity (X2), followed by reaction temperature (X3), and the interaction of space velocity with reaction temperature (X2X3), and slightly weaker interaction effects of others were observed. Statistical analysis includes the main effects and the interaction effects of the variables on the MA yield. The analysis of variance (ANOVA) is used to analyze the experimental data. The regression model value of p < 0.0001 indicates the model is high significant. F-values are calculated by sum of squares,which are defined as the ratio of the respective mean square effect and the mean square error.46,47 For any of the terms in the model, a large F-value and a small pvalue would indicate that the related factor or interaction is more significant on the response value. In view of the ANOVA (Table 4) and eq 2, the linear effect of X1, X2, X3, the quadratic effect of X1 and the interaction effect between X2 and X3 had significant impact on the yield of MA (p < 0.05). However, the other mode terms were not significant (p > 0.05). The lack-of-fit F-value of 0.93 implies it is not significant relative to the pure error, and the model F-value of Table 4. ANOVA for Response Surface Quadratic Modela
a
source
sum of squares
model X1 X2 X3 X1 X2 X1 X3 X2 X3 X12 X22 X32 pure error lack of fit residual cor total
5665.46 2075.52 221.13 101.84 7.76 0.31 236.70 1496.75 440.93 790.17 10.99 4.52 6.47 5676.45
df 9 1 1 1 1 1 1 1 1 1 7 3 4 16 R2 = 0.9981
mean squares
F-value
P-value
significance
629.50 2075.52 221.13 101.84 7.76 0.31 236.70 1496.75 440.93 790.17 1.57 1.51 1.62
400.94 1321.9 140.84 64.86 4.94 0.20 150.76 953.31 280.84 503.28
