Evaluation of ZrâAlumina in Production of Polyoxymethylene Dimethyl

Article pubs.acs.org/IECR

Evaluation of Zr−Alumina in Production of Polyoxymethylene Dimethyl Ethers from Methanol and Formaldehyde: Performance Tests and Kinetic Investigations Jianqiang Zhang, Dingye Fang, and Dianhua Liu* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: Polyoxymethylene dimethyl ethers (PODEn) converted from methanol and formaldehyde were investigated with alumina supported ZrO2 catalyst in a stainless steel tube fixed-bed reactor within the temperature range from 333 to 433 K and the pressure range from 0.1 to 2.5 MPa. First, results from catalytic performance tests showed that the catalyst γ-Al2O3 containing 4 mol % ZrO2 exhibited the highest activity for PODEn synthesis. The stoichiometry analysis of raw material indicated that ZrO2 modified catalyst had the optimum activity at methanol/formaldehyde = 3 (mol/mol). An increase of the reaction pressure resulted in a significant increase in methanol conversion from 13.18 to 48.64%, and selectivity to PODE3−8 was enhanced to 24.82%. The temperature experiment analysis confirmed the optimization of the reaction temperature to be 393 K. Second, SEM and TEM showed the relationship between the catalytic activity and the catalyst-crystal alignment. Incorporation of ZrO2 in the framework of γ-Al2O3 was noticed and confirmed by X-ray diffraction (XRD) and BET analysis. The NH3-TPD tests indicated the relationship between catalytic activity and acidity. Finally, an elimination mechanism was proposed to explain the PODEn synthesis reaction rate and kinetic model. The contributions of intraparticle and external diffusion were eliminated by changing the size of catalyst particles and space velocity. The proposed model can reasonably predict the observed behaviors of PODEn synthesis from methanol and formaldehyde.

1. INTRODUCTION The growing demand for clean energy such as methanol, dimethyl ether, and polyoxymethylene dimethyl ethers has become more and more imperative. Generally, oxy-organics used as clean energy have been drawing increasing attention for their unique features. However, the engine infrastructure should be changed when methanol,1 dimethyl ether,2 and methylal3 are used as fuels because of their low cetane numbers and high vapor pressures. These issues can be successfully resolved by reactions of methanol and formaldehyde to polyoxymethylene dimethyl ethers (PODEn). Because of the suitable physical and chemical properties of PODEn with traditional diesel fuels,4 they can be blended into diesel fuels and used without significant change of the engine’s infrastructure. Polyoxymethylene dimethyl ethers with the structure represented by the formula CH3O−(CH2O)n−CH3 can increase the cetane number and improve fuel efficiency in combustion when added to diesel fuels.5,6 They can be used as clean fuels for diesel engines without particulate formation, with low NOx emission and less engine noise in comparison to traditional diesel fuels.7 Especially, PODE3≤n≤8 exhibit high oxygen content, moderate boiling points, and excellent miscibility with diesel fuels. Also, the average cetane number is above 76. Generally, PODE are produced in two ways. One is that they are produced from methylal and trioxane (paraformaldehyde). The other is that they are directly produced from methanol and formaldehyde. However, the first route is inappropriate to PODE synthesis due to the high cost. The second method from methanol and formaldehyde has been found to be an alternative © 2014 American Chemical Society

route for the production of PODE. This is of particular importance from the industrial aspect due to its simpler technology and lower cost. PODE synthesis reactions from methanol and formaldehyde are exothermic processes, implying that low temperature facilitates a high yield of PODE production. During the process, the following reactions take place: 2CH3OH + CH 2O ↔ CH3OCH 2OCH3 + H 2O

(1)

CH3OCH 2OCH3 + CH 2O ↔ CH3O(CH 2O)2 CH3

(2)

CH3O(CH 2O)n CH3 + CH 2O ↔ CH3O(CH 2O)n + 1CH3 (3)

CH3OH + CH 2O ↔ CH3OCH 2OH

(4)

3CH 2O ↔ (CH 2O)3

(5)

nCH 2O ↔ (CH 2O)n

(6)

2CH3OH + CH 2O ↔ CH3OCH 2CH 2OH + H 2O

(7)

CH3O(CH 2O)n + 1CH3 + H 2O ↔ CH3O(CH 2O)n CH 2OH + CH3OH Received: Revised: Accepted: Published: 13589

(8)

March 24, 2014 July 25, 2014 August 9, 2014 August 9, 2014 dx.doi.org/10.1021/ie501231a | Ind. Eng. Chem. Res. 2014, 53, 13589−13597

Industrial & Engineering Chemistry Research

Article

Alumina-supported catalyst with ZrO2 was prepared using a coprecipitation−impregnation method. The Zr/Al mole ratios were 0, 0.02, 0.04, 0.06, 0.08, and 0.10. The γ-Al2O3 support was made from aluminum sulfate and sodium hydroxide using a precipitation method. Al2(SO4)3 (1 mol) and NaOH (5.4 mol) were dissolved in deionized water at 333 K respectively. Then the two solutions were simultaneously added to a beaker with continuous stirring at room temperature. After that the precipitated solution was filtered and washed with deionized water until the concentration of SO42− was less than 4.7 × 10−13 mol·L−1 (detected by BaCl2). The solid was dried at 383 K for 12 h in air at a heating rate of 2 K·min−1. It was first crushed and sieved to 0.180−0.425 mm before thermal treatment at 773 K for 4 h at 2 K·min−1. The alumina support was then impregnated with quantitative zirconium nitrate. Consider that the ready dispersion is a promising way to increase the catalytic activity. Five different solvents were used for preparing impregnated catalyst: water, ethanol, acetic acid, 2-propanol, and methanol. The modified catalyst with methanol as impregnated solvent exhibited excellent catalytic activity, and methanol was used as the impregnated solvent in the subsequent work. Catalysts with varying compositions of zirconium nitrate (0.0, 2.0, 4.0, 6.0, 8.0, and 10.0 mol % with respect to Al) were prepared in a similar manner. In a typical synthesis procedure, 0.00, 0.02, 0.04, 0.06. 0.08, or 0.10 mol of zirconium nitrate was dissolved in methanol to make different concentrations of Zr(NO3)4 solutions (0.0, 0.5, 1.0, 1.5, 2.0, 2.5 mol·L−1, respectively). Then 0.5 mol of alumina support was added to the different concentration of Zr(NO3)4 solutions respectively. Slurry impregnation was carried out in a water bath system at the controlled temperature of 333 K. The slurry was stirred continuously for 6 h followed by oven drying for 12 h at 383 K to remove excess water. The resulting dried product was then calcined at 823 K for 6 h in air at a heating rate of 2 K· min−1. According to the X-ray diffraction (XRD) results of the calcination temperature and time for modified catalysts, the crystal structures of the catalysts were not changed at calcination temperatures higher than 723 K and calcination time longer than 4 h. Calcined catalyst was crushed and sieved to 0.180−0.425 mm before reaction in a fixed-bed reactor. The kinds of Zr/Al mole ratio based multicomponent catalysts were named γ-Al2O3−0Zr, γ-Al2O3−2Zr, γ-Al2O3−4Zr, γ-Al2O3− 6Zr, γ-Al2O3−8Zr, and γ-Al2O3−10Zr, respectively. To identify the chemical composition of a catalyst, the catalyst γ-Al2O3− 10Zr was measured by X-ray photoelectron spectroscopy (XPS). Surface atomic concentrations of Al, Zr, Na, N, and S were 23.62, 1.63, 0.4, 0.3, and 0%, respectively. The atom S was not detected in the catalyst’s surface and the concentrations of N and Na were negligibly small. 2.2. Catalyst Characterization. The physical properties (BET surface area, pore volume, and pore diameter distribution) have been determined by N2 adsorption− desorption (Micromerities ASAP 2020 V3.05H). The morphology of the samples was examined using a JSM-6360LV scanning electron microscope (SEM). Transmission electron microscopy (TEM) images were determined using a JEOL-JEM 2100 instrument. The acceleration voltage was 160 kV. XRD patterns were obtained using a Rigaku D/max-2550 power X-ray diffractometer with Cu Kα1 radiation (λ = 0.154 nm). The tube voltage and current were 40 kV and 450 mA, respectively. The sample was placed into the sample cell, which was then purged by Ar gas (99.995%) flow to evaporate the

where reactions 1−3 are the main PODE synthesis reactions accompanied by the probable side reactions (reactions 4−8) in which methoxymethanol, trioxane, paraformaldehyde, and hemiacetal are produced. There is an increasing interest in the development of effective catalysts for reactions of methanol and formaldehyde to PODE with a high activity and selectivity. The reactions were catalyzed by liquid or solid acid, such as acid resin, molecular sieve, and sulfuric acid. Hagen and co-workers8−10 synthesized PODE from methanol with acid resin as a catalyst. Zhao and co-workers11 reported the synthesis of PODE from methanol and trioxymethylene with molecular sieves as a catalyst. Burger and co-workers6 synthesized PODE from methylal and trioxymethylene with the acid resin Amberlyst 46 as a catalyst. With the acid resin as a catalyst, the reaction temperature was lower than the limit of 373 K because the acid resin exhibited a relatively poor physical structure and lack of stability under high temperature. Alumina exhibited higher stability than acid resin when used as the catalyst for the reactions of methanol and formaldehyde to PODE. However, very few studies have been reported for synthesizing methanol and formaldehyde to PODE with alumina due to its low acidity and activity. In this work, we described a new route to synthesize zirconium nitrate modified alumina to improve its acidity and activity. The modified alumina catalyst exhibited effective catalytic activity and excellent stability for synthesizing methanol and formaldehyde to PODE. As a solid catalyst, alumina (γ-Al2O3)7 has received tremendous attention and it has shown promising catalytic performance in many reactions.12−16 It provides the unique feature of shape selectivity because of its repeatable pore structure which may prove to be useful to the reaction under consideration. Recently, great attention was paid to γ-Al2O3 because of its stable structure and function, low cost, and large surface area. To date, there have been few reports examining the kinetic mechanism for reactions of methanol and formaldehyde to PODE. Only some studies have reported on the kinetics of methylal and trioxane conversion to PODE. Burger and coworkers6 studied the kinetic model of methylal and trioxane and raised the pseudohomogeneous kinetic model and the adsorption based kinetic model. The adsorption based kinetic model successfully represents the results properly. The objective of this paper was to evaluate the performance of alumina supported ZrO2 catalyst17−19 for PODE synthesis from methanol and formaldehyde. The effects of various kinetic parameters such as temperature, pressure, and the mole ratio of the raw material were evaluated. The weight hourly space velocity (WHSV) was determined. Ultimately, a suitable rate model was proposed.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The materials for preparing the modified catalysts are zirconium nitrate (Zr(NO3)4; 99%, Sino Polymer Co. CHN), aluminum sulfate (Al2(SO4)3; 99%, Sino Polymer Co. CHN), sodium hydroxide (NaOH; 99%, Sino Polymer Co. CHN), ethanol (C2H6O; 99%, Sinopharm Group Co. CHN), acetic acid (C2H4O2; 37%, Sinopharm Group Co. CHN), 2-propanol (C3H8O; 99%, Sinopharm Group Co. CHN), and methanol (CH4O; 99.9%, Sinopharm Group Co. CHN). The purity of all chemicals (liquid phase) was verified by gas chromatography. 13590

dx.doi.org/10.1021/ie501231a | Ind. Eng. Chem. Res. 2014, 53, 13589−13597


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Figure 1. Reaction system for PODE synthesis from methanol and formaldehyde.

methanol; the Ar gas flow was continued during the operation to avoid oxidation. The total acidity and acid strength distribution have been determined by measuring the differential adsorption of NH3 at 423 K and the subsequent temperature-programmed desorption of the adsorbed NH3 (NH3-TPD) by following a temperature ramp of 5 K·min−1 up to 1073 K. A thermobalance with online mass spectrometer was used. 2.3. Catalytic Reaction. The reaction was carried out in a fixed-bed microactivity test unit (700 mm long and 12 mm i.d.) with catalysts (60−80 mesh), as shown in Figure 1. The reactions of PODE synthesis from methanol and formaldehyde were performed within the pressure range 0.1− 2.5 MPa. Temperature was increased at an interval of 20 K from 333 to 433 K. The raw material of methanol and formaldehyde flow rate was performed within the range 0.05− 0.3 mL·min−1. 2.4. Analytical Methods. The effluent liquid and gas were analyzed using a gas chromatograph (GC; PE 580) equipped with an Elite-wax fused silica capillary column (30 m × 0.32 mm × 0.25 μm). A flame ionization detector (FID) operating at 553 K was utilized for analyzing liquid product with nitrogen as a carrier gas. A thermal conductivity detector (TCD) operating at 553 K was utilized for analyzing gaseous product with hydrogen as a carrier gas. The column oven temperature was maintained at 318 K for 3 min, then was increased to 373 K at a rate of 5 K·min−1, and was held for 1 min. Finally, the temperature was increased at a rate of 20 K·min−1 to 595 K and was kept constant for 3 min. GC samples were taken until the successive reactor composition remained constant over time; then the temperature was set to the next desired value and GC sampling resumed. The material balance was found satisfactory. The average absolute percent deviation in this analysis was under 3%.

Table 1. Physicochemical Properties of the Catalysts γAl2O3−0Zr, γ-Al2O3−2Zr, γ-Al2O3−4Zr, γ-Al2O3−6Zr, γAl2O3−8Zr, and γ-Al2O3−10Zr catalyst

BET surf. area (m ·g )

pore diam (nm)

γ-Al2O3−0Zr γ-Al2O3−2Zr γ-Al2O3−4Zr γ-Al2O3−6Zr γ-Al2O3−8Zr γ-Al2O3−10Zr

218.38 213.08 210.21 203.37 198.53 187.11

9.43 9.10 8.92 8.73 8.60 8.31

2

−1

pore vol (cm3·g−1) 0.51 0.50 0.47 0.45 0.43 0.39

The data in Table 1 indicate that the alumina support had a higher BET surface area (218.38 m2·g−1) than the modified catalysts. The difference in textural properties between the pure alumina and modified alumina suggested that the addition of ZrO2 in the system might be due to the dissolution of alumina and formation of new ZrO2 whiskers and pore blockage by metal oxide particles. Upon increasing the Zr/Al mole ratio from 0 to 0.1, the BET surface area, pore volume, and pore diameter decreased slightly. The presence of ZrO2 whiskers inhibits the formation of macropores in alumina during the subsequent heating process. As the quantity of ZrO2 whiskers increased, the pore volume and pore diameter of modified catalysts were decreased. In combination with XRD results, the decreases of surface area and pore volume could be attributed to the formation of ZrO2 crystallites. The regular changes of surface area, pore volume, and pore diameter also indicated that the modifier uniformly distributed in the surface and the interior pore structure of γ-Al2O3. SEM images of catalysts with different Zr/Al mole ratios are shown in Figure 2 for studying the surface topography and assessing the surface dispersion of active components over the γ-Al2O3 structure. Comparison of part a with parts b−f of Figure 2 shows that alumina whiskers with schistose crystals grew homogeneously on the surface of the alumina. With increasing Zr/Al mole ratio, smaller crystals with a more uniform size and smooth surface were detected. It is interesting to observe that the treatment with ZrO2 significantly modifies

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization Results. The BET surface areas, pore volumes, and pore diameters of the modified catalysts are listed in Table 1. 13591



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Figure 4. TEM images of catalyst γ-Al2O3−4Zr.

catalyst. It was difficult to obtain detailed information on the morphology because of the aggregation of particles. However, the TEM images of γ-Al2O3−0Zr catalyst and γ-Al2O3−4Zr catalyst clearly revealed the ordered arrangement of atom columnar clusters after being modified with ZrO2 particles. Hence, the SEM and TEM results were perfectly consistent. One can conclude that the introduction of ZrO2 changes the crystal alignment and affects the catalytic activity for methanol and formaldehyde to PODE. The XRD patterns of γ-Al2O3−0Zr, γ-Al2O3−2Zr, γ-Al2O3− 4Zr, γ-Al2O3−6Zr, γ-Al2O3−8Zr, and γ-Al2O3−10Zr powders are shown in Figure 5. All of these materials showed reflections

Figure 2. SEM images of catalysts (a) γ-Al2O3−0Zr, (b) γ-Al2O3−2Zr, (c) γ-Al2O3−4Zr, (d) γ-Al2O3−6Zr, (e) γ-Al2O3−8Zr, and (f) γAl2O3−10Zr.

the morphology, causing a strong agglomeration of the particles. The decreasing surface area was in good agreement with SEM data that showed agglomeration after ZrO 2 treatment. The above results indicated that the modifier ZrO2 played a key role in determining the quantity and morphology of whiskers by directing the growth of alumina species along a specific crystallographic direction. In order to further investigate the surface structure on the morphology of the whiskers, the primitive and modified alumina samples were studied by TEM. Figures 3 and 4 show the TEM images of γ-Al2O3−0Zr and γ-Al2O3−4Zr, respectively. Each sample was composed of many crystal particles, and the particles exhibited a lattice fringe at 3 nm. These give direct evidence for the presence of highly dispersed ZrO2 nanoparticles with small particle size in the ZrO2-modified alumina Figure 5. XRD profiles of catalysts γ-Al2O3−0Zr, γ-Al2O3−2Zr, γAl2O3−4Zr, γ-Al2O3−6Zr, γ-Al2O3−8Zr, and γ-Al2O3−10Zr.

at 37.1, 45.4, and 66.3° corresponding to γ-Al2O3. Additionally, the formation of segregated ZrO2 phase was also detected at peak 31.3° in the cases of γ-Al2O3−8Zr and γ-Al2O3−10Zr. However, the intensity of the ZrO2 peak was rather weak and increased with increasing ZrO2 content. The low intensity of XRD peaks indicates that the ZrO2 samples have low crystallinity. Except for γ-Al2O3−8Zr and γ-Al2O3−10Zr, the rest of the samples showed typical XRD patterns of γ-Al2O3 structure with no additional peaks due to ZrO2/Zr(NO3)4. This demonstrated that the zirconium oxides were in a highly dispersed state or the crystallites formed beyond the detection limitation. Furthermore, the crystal structure of the modified catalysts remained intact after modification with Zr(NO3)4. This meant that the existence of ZrO2 was highly dispersed and

Figure 3. TEM images of catalyst γ-Al2O3−0Zr. 13592



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had little effect on the crystal structure of γ-Al2O3. This confirms the TEM result that the modifier ZrO2 has an impact on the crystal alignment and without changing the crystal structure of the catalysts. NH3-TPD experiments were conducted for γ-Al2O3−0Zr, γAl2O3−2Zr, γ-Al2O3−4Zr, γ-Al2O3−6Zr, γ-Al2O3−8Zr, and γAl2O3−10Zr catalysts to study the influence of ZrO2 content on the acid centers of the alumina. Figure 6 displays that all

Figure 7. Effect of additive ZrO2 contents on PODE formation from methanol and formaldehyde with modified catalysts.

the medium acid centers and enhanced the strong acid centers. Interestingly, PODE selectivity first increased and then decreased with increasing Zr/Al mole ratio and reached a maximum (91.34%) at a Zr/Al mole ratio of 0.04. This may be due to the increase of medium acid centers and the enhancement of strong acid centers leading to the increase of byproduct and PODE, respectively. Considering the yields of PODE and PODE3−8, the catalyst γ-Al2O3−4Zr with a Zr/Al mole ratio of 0.04 showed the best activity. Hence, all further studies were conducted on the ZrO2 modified catalyst γAl2O3−4Zr. The influence of reaction temperature was conducted at various sections, ranging from 333 to 433 K, at a methanol-toformaldehyde mole ratio of 1:3. Reaction was conducted at 1.0 MPa, a material flow rate of 0.1 mL·min−1, and 0.5 g of catalyst. The results are shown in Figure 8.

Figure 6. NH3-TPD profiles of catalysts γ-Al2O3−0Zr, γ-Al2O3−2Zr, γ-Al2O3−4Zr, γ-Al2O3−6Zr, γ-Al2O3−8Zr, and γ-Al2O3−10Zr.

samples show three obvious NH3 desorption peaks for NH3TPD. The first broad peak in the range 400−420 K corresponds to the weak acid centers, the second peak in the range 640−660 K corresponds to the medium acid centers, and the third peak in the range 880−930 K corresponds to the strong acid centers. The amounts of desorption peaks were not changed with the catalyst modification. After the introduction of ZrO2, the third peak corresponding to strong acid centers of modified catalysts shifted to higher temperatures, implying the enhancement of catalyst acidity at higher temperature. Moreover, the second peak corresponding to medium acid centers became more intensive with increasing ZrO2 content. No changes in the first peak corresponding to the weak acid centers could be observed. That may be due to the improvement and increment of the ability of ZrO2 modified catalysts to accept electrons, thus changing the sample’s acidity. On the basis of these observations, the modified catalysts show the increment of medium acid centers and enhancement of strong acid centers with ZrO2 content. 3.2. Catalyst Activity. In order to test the catalytic performance, the prepared catalysts were packed in a fixed-bed reactor for the synthesis of PODE. The reaction efficiency of modified catalyst with different ZrO2 contents is shown in Figure 7. Reaction was conducted at 393 K, 1.0 MPa, a material flow rate of 0.1 mL·min−1, and 0.5 g of catalyst. The mole ratio of methanol and formaldehyde was 1:3. Controlled experiments at our reaction conditions revealed that this reaction did not proceed in the absence of catalysts. Untreated parent γ-Al2O3−0Zr, without any metal loading, was weakly active providing a methanol conversion of 26.42% with a PODE selectivity of 79.79%. Incorporation of ZrO2 enhances the catalytic activity. The catalyst γ-Al2O3−10Zr with a Zr/Al mole ratio of 0.1 showed the highest methanol conversion. These results can be confirmed from the NH3-TPD studies which stated that the presence of ZrO2 in the material increased

Figure 8. Effect of reaction temperature on PODE formation from methanol and formaldehyde with modified catalyst γ-Al2O3−4Zr.

Not surprisingly, as the reaction temperature was increased, the conversion of methanol also increased at low temperatures. However, the conversion of methanol decreased at higher temperatures and exhibited a maximum with temperatures. This could be a compromised result of the change in rate and equilibrium of the reaction with temperature as the former increases and the latter decreases with temperature. The selectivity of PODE3−8 shows the same trend with the conversion of methanol. This phenomenon can result from the enhancement of catalytic activity and the decrease of formaldehyde leading to increases in the former and decreases 13593



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393 K, 1.5 MPa, a material flow rate of 0.1 mL·min−1, 0.5 g of catalyst, and methanol-to-formaldehyde mole ratio of 1:3. Figure 11 clearly shows that the conversion of methanol, selectivity of PODE, and selectivity of PODE3−8 all remained

in the latter. The yield of PODE3−8 was significantly higher at 393 K. The influence of reaction pressure was conducted at various sections, ranging from 0.1 to 2.5 MPa, at a methanol-toformaldehyde mole ratio of 1:3. Reaction was conducted at 393 K, a material flow rate of 0.1 mL·min−1, and 0.5 g of catalyst. The results are shown in Figure 9. At ambient pressure, the

Figure 11. Effect of reaction time on PODE formation from methanol and formaldehyde with modified catalyst γ-Al2O3−4Zr. Figure 9. Effect of reaction pressure on PODE formation from methanol and formaldehyde with modified catalyst γ-Al2O3−4Zr.

constant for the whole period, which indicated that no noticeable deactivation of the catalyst γ-Al2O3−4Zr occurred. The above results revealed that the ZrO2 modified catalyst γAl2O3−4Zr performed with excellent activity and stability for the synthesis of PODE from methanol and formaldehyde. 3.4. Kinetic Model. The experimental data on reaction equilibria and kinetics from the present study were used to develop a model. Since no models for the synthesis of PODE from methanol and formaldehyde system are presently available, the rate-limiting step of elementary reactions was used to explain the rate behavior. According to the elimination mechanism, the elementary reactions are listed in reactions 9−23.

catalyst γ-Al2O3−4Zr showed weakly active providing a methanol conversion of 13.17%. Interestingly, the selectivities of PODE and PODE3−8 both remained constant at reaction pressure higher than 0.5 MPa. The methanol conversion showed a maximum (48.64%) at 1.5 MPa. This is a logical trend because of the decrease of the molecules after reaction. The influence of formaldehyde-to-methanol mole ratios on the conversion of methanol, selectivity of PODE, and selectivity of PODE3−8 is shown in Figure 10. Reaction was conducted at

CH3OH + CH 2O ↔ CH3OCH 2OH

(9)

CH3OH + CH3OCH 2OH ↔ CH3OCH 2OCH3 + H 2O (10)

2CH3OCH 2OH ↔ CH3O(CH 2O)2 CH3 + H 2O

(11)

CH3O(CH 2O)2 CH3 + H 2O ↔ CH3O(CH 2O)2 H + CH3OH

(12)

CH3O(CH 2O)2 H + CH3OCH 2OH Figure 10. Effect of formaldehyde-to-methanol mole ratio on PODE formation from methanol and formaldehyde with modified catalyst γAl2O3−4Zr.

↔ CH3O(CH 2O)3 CH3 + H 2O

(13)

CH3O(CH 2O)3 CH3 + H 2O

393 K, 1.5 MPa, a material flow rate of 0.1 mL·min−1, and 0.5 g of catalyst. An increase of formaldehyde-to-methanol mole ratios resulted in all increases in the conversion of methanol, selectivity of PODE, and selectivity of PODE3−8. The conversion of methanol, selectivity of PODE, and selectivity of PODE3−8 remained constant at mole ratios of formaldehyde and methanol higher than 3. 3.3. Catalyst Stability. The stability of the γ-Al2O3−4Zr catalyst was measured over 500 h. Reaction was conducted at

↔ CH3O(CH 2O)3 H + CH3OH

(14)

CH3O(CH 2O)3 H + CH3OCH 2OH ↔ CH3O(CH 2O)4 CH3 + H 2O

(15)

CH3O(CH 2O)4 CH3 + H 2O ↔ CH3O(CH 2O)4 H + CH3OH 13594

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C P2 = ka (17)


(18)


(19)


(20)

(21)


(22)


(23)

The 15 elementary reactions can be written by the following simple rate equations as liquid−solid reaction rate equations. r9 = k1CMC F − k 2CMM

(24)

r10 = k 3CM aCMM b − k4C P1cC W d

(25)

r11 = k5CMM 2 − k6C P2C W

(26)

r12 = k 7C P2C W − k 8CG2CM

(27)

r13 = k 9CG2CMM − k10C P3C W

(28)

r14 = k11C P3C W − k12CG3CM

(29)

r15 = k13CG3CMM − k14C P4C W

(30)

r16 = k15C P4C W − k16CG4CM

(31)

r17 = k17CG4CMM − k18C P5C W

(32)

r18 = k19C P5C W − k 20CG5CM

(33)

r19 = k 21CG5CMM − k 22C P6C W

(34)

r20 = k 23C P6C W − k 24CG6CM

(35)


(36)

r22 = k 27C P7C W − k 28CG7CM

(37)


(38)

dC M = kx e−E1/ RT CM AC F B − k ye−E2 / RT C P1CC W D dt

C P3 = kbC FC P2

(41)

C P4 = kcC FC P3

(42)

C P5 = kdC FC P4

(43)

C P6 = keC FC P5

(44)

C P7 = k f C FC P6

(45)

C P8 = kgC FC P7

(46)

Figure 12. Effect of WHSV on PODE formation from methanol and formaldehyde with modified catalyst γ-Al2O3−4Zr.

particle and WHSV on PODE formation from methanol and formaldehyde at 393 K, 1.5 MPa, and a methanol-toformaldehyde mole ratio of 1:3. The contribution of intraparticle and external diffusion was eliminated at catalyst particle diameter less than 0.42 mm (40 mesh) and WHSV greater than 1.5 h−1. The experimental data of kinetics were conducted in the range 373−413 K without the effect of intraparticle and external diffusion. The different kinetic parameters of the model were estimated by means of the fourth-order Runge−Kutta method. The inputs were the experimental data and the rate equations. The outputs were the kinetic parameters and the predicted concentrations for the reactants and products. The activation energy and

where rn = rate of reaction (n), k = rate constant, and Cm = concentration of m. The results obtained demonstrate that elementary reaction 10 is the rate-limiting step and the other elementary reactions are in a state of balance. The reaction rate of elementary reaction 10 and the other reactions equation may be expressed by the following equations. −

(40)

Note that due to similar structures, the nonpolarity of PODE, and similar reaction mechanisms of PODE synthesis, it is simply assumed rate constants (ka, kb, kc, kd, ke, kf, kg) for different chain lengths are equal. Equation 40 is different from eqs 41−46. In consideration of the similar reaction processes of PODE2 synthesis and other PODEn syntheses, the rate constant ka is also simply assumed equal to the other rate constants. From eqs 41−46, it can be concluded that the results of the mole ratio of PODEn and PODEn+1 (n > 1) remained constant at the same reaction conditions. It can be confirmed by the experimental results. The equal rate constants calculated for PODE are consistent with that previously reported by Burger and co-workers.6 The contribution of intraparticle and external diffusion was eliminated by changing the size of the catalyst particle and space velocity. Figures 12 and 13 show the influence of catalyst


CM 2C F 2 CW

(39) 13595



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correlation coefficient (R2) was larger than 0.99 for the γAl2O3−4Zr catalyst used in this study. The model is able to reproduce the simultaneous syntheses of PODEn with different n’s. The rate-limiting step is the synthesis of methylal (eq 10). Figure 14 shows the experimental points and the lines fitting

Figure 13. Effect of catalyst particle on PODE formation from methanol and formaldehyde with modified catalyst γ-Al2O3−4Zr.

thermal parameters of the reaction model obtained are shown in Table 2. Table 2. Calculated Model Parameters for the Kinetic Equation for PODE Formation from Methanol and Formadehyde with Modified Catalyst γ-Al2O3−4Zr parameter

value

kx ky E1 E2 A B C D

4.23 × 1011 5.96 × 1011 79.78 kJ·mol−1 82.10 kJ·mol−1 0.1386 0.3548 2.02 × 10−7 1.79 × 10−7

Figure 14. Comparison of experimental values and calculated values of methanol concentration after reaction.

them calculated using eq 39 in the temperature range from 373 to 413 K. The model captures the overall trends quite well. Note that all the points fall within the lines corresponding to an estimated ±6% error of our data. The good agreement of the model with experimental data of kinetic experiments for a large range of temperature, pressure, and mole ratios indicates that the experimental data and kinetic data can be widely applied. However, the elimination mechanism, kinetic model, and its parameters should be applied for the system of PODE formation from methanol and formaldehyde with the catalyst γ-Al2O3−4Zr. Overall, the model is able to fit the experimental data fairly well from 373 and 413 K especially with respect to the main product distribution.

The values of the methylal formation activation energy (E1 = 79.78 kJ·mol−1) are lower than the values of the methylal consumption activation energy (E2 = 82.10 kJ·mol−1). This confirms that the PODE synthesis is a mild exothermic reaction process. This work was based on the γ-Al2O3−4Zr catalyst, and the difference in the active sites on the catalyst compared to other catalysts might influence the activation energies to some extent. Apparent reaction orders for the rate-limiting reaction were A = 0.1386 and B = 0.3548 for methanol and formaldehyde, respectively. Positive values of the reaction order have a positive dependence of the reaction rate on the concentrations of methanol and formaldehyde. For contribution to the reaction rate, the influence of the formaldehyde concentration is greater than the effect of methanol concentration because B is large than A. The corresponding parameter reaction orders of methylal and water (C and D) in our studies is approximately equal to 0 (see Table 2). The explanation provided was that the concentrations of methylal and water may appear irrelevant to the methanol consumption rates in the fixed-bed reactor. The PODEn and PODEn+1 ratios are approximately 1.8−3.6 in the experiments which are mainly dependent on the reaction temperature and concentration of formaldehyde. The results of kinetic calculation express a similar product distribution. That may confirm that the rate constants (ka, kb, kc, kd, ke, kf, kg) for different chain lengths are equal. The agreement between the elimination mechanism model and the data from experiments is good. The square of the

4. CONCLUSION ZrO2 modified catalysts were prepared by precipitation− impregnation methods and employed in methanol and formaldehyde to polyoxymethylene dimethyl ethers processes. The results of the catalytic performance indicated that the modified catalysts exhibited much higher catalytic activities than the unmodified alumina. The catalyst γ-Al2O3−4Zr with a Zr/ Al mole ratio of 0.04:1 showed the best activity and selectivity at low temperature. It is also very stable and does not form significant amounts of polyformaldehyde. NH3-TPD results indicated the increment of medium acid centers and enhancement of strong acid centers with ZrO2 content. SEM and TEM results indicated the ordered arrangement of atom columnar clusters after the catalyst was modified. Therefore, this formulation is an excellent candidate route for PODE synthesis as clean energy. The kinetic data recorded in the temperatures range 373− 413 K were well-fitted to the elimination mechanism kinetic model. The value of the methylal formation activation energy is lower than the value of the methylal consumption activation energy. The PODE synthesis is a mild exothermic reaction process. The influence of the formaldehyde concentration on the contribution to reaction rate is greater than the effect of the methanol concentration. Comparison of the experimental and 13596



Article

(7) Liu, D. H.; Yao, C. F.; Zhang, J. Q.; Fang, D. Y.; Chen, D. S. Catalytic dehydration of methanol to dimethyl ether over modified γAl2O3 catalyst. Fuel 2011, 90, 1738−1742. (8) Hagen, G. P.; Spangler, M. J. Preparation of polyoxymethylene dimethyl ethers by catalytic conversion of formaldehyde formed by oxy-dehydrogenation of dimethyl ether. U.S. Patent 20030171534 A1, 2003. (9) Hagen, G. P.; Spangler, M. J. Preparation of polyoxymethylene dimethyl ethers by catalytic conversion of dimethyl ether with formaldehyde formed by dehydrogenation of dimethyl ether. U.S. Patent 6160186, 2000. (10) Hagen, G. P.; Spangler, M. J. Preparation of polyoxymethylene dimethyl ethers by acid-activated catalytic conversion of methanol with formaldehyde formed by dehydrogenation of methanol. U.S. Patent 2002/0007089 A1, 2001. (11) Zhao, Q.; Wang, H.; Qin, Z. F.; Wu, Z. W.; Wu, J. B.; Fan, W. B. Synthesis of polyoxymethylene dimethyl ethers from methanol and trioxymethylene with molecular sieves as catalysts. J. Fuel Chem. Technol. 2011, 39, 918−923. (12) Maity, S. K.; Flores, L.; Ancheyta, J.; Fukuyama, H. CarbonModified Alumina and Alumina-Carbon-Supported Hydrotreating Catalysts. Ind. Eng. Chem. Res. 2009, 48, 1190−1195. (13) Anuradha, M.; Debrina, J.; Goutam, D. Synthesis of Equimolar Pd−Ru Alloy Nanoparticles Incorporated Mesoporous Alumina Films: A High Performance Reusable Film Catalyst. Ind. Eng. Chem. Res. 2013, 52, 15817−15823. (14) Liu, C.; Liu, Y. C.; Ma, Q. X.; Ma, J. Z.; He, H. Alumina with Various Pore Structures Prepared by Spray Pyrolysis of Inorganic Aluminum Precursors. Ind. Eng. Chem. Res. 2013, 52, 13377−13383. (15) Park, S. J.; Lee, D. W.; Yu, C. Y. Dimethyl Ether Reforming in a Mesoporous γ-Alumina Membrane Reactor Combined with a Water Gas Shift Reaction. Ind. Eng. Chem. Res. 2008, 47, 1416−1420. (16) Qiao, S. H.; Chen, J.; Li, J. F.; Qu, Z.; Liu, P.; Yan, N. Q.; Jia, J. P. Adsorption and Catalytic Oxidation of Gaseous Elemental Mercury in Flue Gas over MnOx/Alumina. Ind. Eng. Chem. Res. 2009, 48, 3317−3322. (17) Zuo, J. L.; Chen, Z. H.; Wang, F. R.; Yu, Y. H.; Wang, L. F.; Li, X. H. Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Novel Mn−Zr Mixed Oxide Catalysts. Ind. Eng. Chem. Res. 2014, 53, 2647−2655. (18) Thitsartarn, W.; Kawi, S. Transesterification of Oil by Sulfated Zr-Supported Mesoporous Silica. Ind. Eng. Chem. Res. 2011, 50, 7857− 7865. (19) Han, G. B.; Park, N. K.; Yoon, S. H.; Lee, T. J.; Han, G. Y. Direct Reduction of Sulfur Dioxide to Elemental Sulfur with Hydrogen over Sn−Zr-Based Catalysts. Ind. Eng. Chem. Res. 2008, 47, 4658− 4664.

calculated values of the methanol concentration after reaction shows the experimental points and the lines fitting them calculated using the synthesis of methylal as the rate-limiting step in the temperature range from 373 to 413 K. Ultimately, the rate equation developed in this work was utilized to verify the behaviors of this reaction system.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of the powder scraped from the surface of modified catalysts calcined at different temperatures and times; XPS pattern of modified catalyst γ-Al2O3−10Zr. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +862164252151. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE PODEn = polyoxymethylene dimethyl ethers rn = rate of reaction n k = rate constant Cm = concentration of m E = activation energy A = reaction order of methanol B = reaction order of formaldehyde C = reaction order of methylal D = reaction order of water

Subscripts

M = methanol F = formaldehyde MM = methylal W = water P = polyoxymethylene dimethyl ethers G = hemiformals

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REFERENCES

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