Zhenyu Wu, Zengxi Li,*, Chunshan Li,3,*
1 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049, China
2 Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
3 Zhongke Langfang, Institute of Process Engineering, Chinese Academic of Sciences, Hebei 065000, China
Keywords:Methyl acrylate (MA)Aldol condensation Al2O3-AlPO4 Penta-coordinated aluminum Cooperative effects
ABSTRACT The bare amorphous Al2O3-AlPO4 and Cs/Al2O3-AlPO4 catalysts were developed for the aldol condensation of methyl acetate with formaldehyde to methyl acrylate.The structure and property of catalyst were characterized by XRD, XPS, BET, Pyridine-IR, FT-IR, 27Al-MASNMR, NH3-/CO2-TPD and SEM.The correlation between structural features and acid-base properties was established, and the loading effect of the cesium species was investigated.Due to cooperative catalytic effects between the penta-coordinated Al and Al2O3, the weak-II acid and medium acid site densities and the product selectivity were improved.While the basic site densities of these catalysts were almost in proportion to the conversion of methyl acetate.The loaded Cs could form new basic sites and change the distribution of acid sites which further enhance the catalytic performance.As a result,the 10Cs/8AlP was proved to be an optimal catalyst with the yield and selectivity of 21.2% and 85% for methyl acrylate respectively.During the reaction, a deactivation behavior was observed on 10Cs/8AlP catalyst due to the carbon deposition, however, it could be regenerated by thermal treatment in the air atmosphere at 400 °C.
Methyl acrylate(MA)is one of the important chemical intermediates for producing adhesion agent,coatings,and superabsorbent resins.In the present studies, MA is produced through esterification of methanol with acrylic acid, which is produced by the petroleum-based two-step oxidations of propylene [1–3].However, due to the volatility of world oil market, the application of this synthesis route is limited.Therefore,developing a supplementary and sustainable synthetic process is of great significance.Recently, the one-step aldol condensation of methyl acetate with formaldehyde (FA), which seems to be intensive and low-cost, is considered as a potential pathway to replace the petroleum rout[4–9].Methyl acetate which comes from coal chemicals is an overproduced product; while FA is produced by the oxidation of methanol which can be easily derived from natural gas, coal, and biomass.
The aldol condensation reaction is a typical acid-base reaction to form new C—C bond.Vitchaetal.reported the use of supported alkali metal oxides and alkaline earth metal oxides catalysts for aldol condensation in 1966[10].Since then,extensive studies have been conducted on the supports,alkali or alkaline earth metals categories, promoters, and other factors to improve catalytic performance.Yanetal.found that cesium supported SBA-15 presented high activity for aldol condensation reaction of methyl acetate with FA,and the catalytic activity was originated from the medium acid and base sites generated from the Si—O—Cs species[11].However,this catalyst was easy to be deactivated.Heetal.compared the activity of Cs/silica gal with Cs/SBA-15 and observed that Cs/silica gal with larger pore diameter and volume exhibited better catalytic activity and stability than Cs/SBA-15 [12].It elucidated that the large pore diameter was beneficial to the diffusion of methyl acetate and MA, while the high pore volume was in favor of accommodation of coke.Zhangetal.discovered that Cs–P/γ-Al2O3with weak acid-base active sites also showed high activity on the aldol reaction of methyl acetate with FA, and strong bases and acids with the desorption temperature at 516–630 °C in NH3-TPD and CO2-TPD may aggravate carbon deposition reaction [13].Baoetal.investigated the performance of Ba/γ-Al2O3[14].The results showed acid-base sites, especially the medium acid sites would significantly affect the activity of the catalyst for the aldol reaction of methyl acetate with FA.To further promote the activity of Ba/γ-Al2O3, they prepared Ba/γ-Ti-Al2O3by evaporation-induced selfassembly method to generate more medium Lewis acid sites[15].Compare with Ba/γ-Al2O3, the yield of MA could increase by 10%.From the above discussion, it could be believed that the acid-base properties of catalyst play an important role in catalytic activity.
Amorphous AlPO4and Al2O3-AlPO4, owing to their flexible textural properties,surface acid-base properties and thermal stability,have received extensive attention as catalysts or catalyst supports for condensation reactions.Climentetal.investigated the aldol condensation of heptanal with benzaldehyde over amorphous AlPO4catalyst [16].The results indicated that amorphous AlPO4with acid-base bifunctional properties exhibited higher selectivity to jasminaldehyde than traditional acid or basic catalysts.Joseetal.applied Al2O3-AlPO4catalyst to the Knoevenagel condensation of several aldehydes and ketones [17].The chemical composition of the surface, particularly the density and strength of acid and basic sites, as well as structural factors, were discovered to impact the activity of this catalyst.The thermal stability of AlPO4could be improved by introducing alumina,in the meantime,the pore structure and acid-base properties could also be modulated.Because of their flexible textural features, surface acid-base properties, and thermal stability, amorphous AlPO4and Al2O3-AlPO4have attracted a lot of interest as catalysts or catalyst supports for a variety of key chemical processes.Their acid-base centers and the activity of these centers depend on their chemical composition which is Al/P atomic ratios [18,19].Cs has the strongest basicity among alkali metals and alkaline earth metals.Cs is superior to other alkali metals when maximum activity is required [8,20].Few researchers have studied Al2O3-AlPO4as an aldol condensation of methyl acetate with FA and the topics involved in further modification by promoters such as Cs.
In this paper, series of bare amorphous Al2O3-AlPO4and Cs/Al2O3-AlPO4catalysts were prepared for the aldol reaction of methyl acetate with FA to MA.Characterizations including XRD,XPS, BET, IR,27Al-MASNMR, NH3-/CO2-TPD and SEM were performed to investigate the correlation between structural features and acid-base properties of these catalyst series, and the loading effect of the cesium element.In addition, the cooperatively catalytic effects between the penta-coordinated Al and Al2O3in Al2O3-AlPO4for this reaction were demonstrated.
2.1.Materials and chemicals
Aluminum nitrate (Al(NO3)3∙9H2O, 99.0%), phosphoric acid(H3PO4,85%),ammonia(NH3∙H2O,25%)and cesium nitrate(CsNO3,99%)were purchased from the Sinopharm Chemical Reagent Company (China).Pure aluminum phosphate (AlPO4, CP) was purchased from Aladdin.
2.2.Catalyst preparation
Amorphous AlPO4,Al2O3-AlPO4and Al2O3was prepared by precipitation method.Typically,an aqueous solution of Al(NO3)3∙9H2-O with required concentration was mixed with suitable amount of H3PO4under vigorous stirring condition,then the pH was adjusted to 9 by adding aqueous ammonia solution drowsily.The obtained precipitate was filtered and washed with distilled water, then the filter cake was dried in oven at 110°C for 12 h.After that,the powders were calcined in a muffle furnace at 650°C for 2 h;finally,the powder size catalyst was tableted and smashed into 380–830 μm sized particles before use.Amorphous Al2O3-AlPO4with different molar ratio of Al and P was prepared through controlling addition different quality of Al(NO3)3∙9H2O and H3PO4with the same preparation method.These samples of Al2O3-AlPO4with different Al/P molar ratio of 1:1, 2:1, 4:1, 6:1, 8:1, 10:1 were designated as AlP,2AlP, 4AlP, 6AlP, 8AlP, 10AlP, respectively.
The cesium supported on 8AlP catalysts were prepared by the incipient wetness impregnation of 8AlP particles with aqueous solution of cesium nitrate for 12 h.Then, the catalyst precursor was dried at 110 °C for 12 h and calcined at 500 °C for 5 h in air atmosphere in a muffle furnace.Catalyst powder was tableted and smashed into 380–830 μm sized portions before use.The cesium supported on 8AlP catalysts prepared with different concentration of 2.5%-15% (mass) were designated as 2.5Cs/8AlP,5Cs/8AlP, 10Cs/8AlP, 12.5Cs/8AlP, and 15Cs/8AlP, respectively.
2.3.Catalyst characterization
Powder XRD patterns were recorded on X’Pert Pro MPD diffractometer (PANalytical BV, the Netherlands) with Cu Kα radiation at room temperature.Escalab 250Xi X-ray photoelectron spectrometer (Thermo Scientific) equipped with a conventional hemispherical analyzer was used for X-ray photoelectron spectroscopy (XPS)analysis.The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda(BJH)pore size distribution measurements were tested on Quanta Chrome Instrument NOVA 2000,the sample was previously dried at 110 °C for 4 h and degassed at 350 °C for 4 h.27Al solid-state magic angle spinning (MAS) NMR was carried out on Bruker AVANCE III HD spectrometer.Spectra were recorded at a sample rotation of 9 kHz for27Al.The27Al shifts were referenced using Al(H2O)63+.Fourier transform infrared spectra (FTIR)was recorded by Nicolet 6700 with a range of 400 to 4000 cm-1and a resolution of 4 cm-1.Scanning electron microscopy (SEM)images were gained on a SEM instrument (S-4800).Thermogravimetry and differential thermal analysis (TG/DTA) was conducted on DTG-60H analyzer (Shimadzu, Japan).In an air stream of 30 ml∙min-1,the sample was heated from 25 to 800°C at a heating rate of 10 °C∙min-1.
Temperature-programmed desorption(TPD)of NH3/CO2for the acidity/basicity of catalyst was measured on Autochem II 2920 apparatus and the acid-base potential density can be determined by calibrating the corresponding NH3-CO2desorption peak area.In a pure helium stream of 50 ml∙min-1, the sample was preprocessed at 500 °C for 1 h; then, 10% NH3/CO2balance helium mixture saturated the sample at 120 °C for 1 h; finally, NH3/CO2desorption was analyzed in pure helium flow from ambient temperature to 500 °C at a heating rate of 10 °C∙min-1.
In the Nicolet 6700 FTIR spectrometer equipped with a testing cell,insitupyridine adsorption was applied to characterize the type of acid.Normally, 14 mg of catalysts were condensed into a self-supporting disc, then disposed in vacuum at 400 °C for 60 min to remove impurities, before cooling to 150 °C.During the cooling process, the background spectra was recorded at 400 °C,350 °C and 150 °C.The pyridine was absorbed at 150 °C for 30 min.At 150 °C, 350 °C and 400 °C, the infrared spectra of the catalyst after pyridine desorption were also recorded and compared with the previous background spectra.
2.4.Catalyst evaluation
6 ml catalyst (3.5 g) were filled in a continuous flow fixed-bed tubular microreactor which possessed a stainless-steel tube(70 cm long × 1.8 cm ID) and triple zone furnace.The other parts filled with quartz sand.The mixed reactants containing methyl acetate and FA and CH3OH with the molar ratio of 1:2:2 were fed to the reactor by a metering pump.The FA solution was prepared from the decomposition of paraformaldehyde with the catalysis of NaOH in methanol.The products mixtures were collected after cooled down and analyzed on a GC 2010 Plus gas chromatograph(Shimadzu) equipped with a barrier ionization discharge detector and RTX-WAX capillary column (0.25 mm × 30 m × 0.25 μm).The internal standard approach was used to quantify the quantity of related components; the internal standard was isobutanol and the relative correction factor of each substance to isobutanol was obtained from the standard sample.The yield and selectivity of MA based on methyl acetate could be calculated by the following equations.
Fig.1. XRD patterns of (a) Al2O3-AlPO4 and (b) Cs/8AlP catalysts.(▲) AlPO4: (●)γ-Al2O3.
Fig.2. XPS of Al2O3-AlPO4 catalysts.
The main substances detected by gas chromatograph in the sample after reaction are methyl acetate, MA,FA,methanol,water and some side products such as acetic acid, acrylic acid.
3.1.Catalyst characterization
3.1.1.XRDpatterns
The XRD patterns of Al2O3-AlPO4with various Al/P molar ratio and Cs/8AlP samples are shown in Fig.1(a) and 1(b).Based on the literatures [21–23], the diffraction peaks at 2θ=20°–40° are the characteristic peaks of the amorphous AlPO4.Similarly,a series of diffraction peaks at 2θ of 31.8°, 37.5°, 39.3°, 45.6°, 60.5°, 66.6°and 84.4° are the characteristic peaks of the γ-Al2O3phase(PDF#50-0741), corresponding to the (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1), (4 4 0), (4 4 4) plane, respectively.As Al/P molar ratio increases in Al2O3-AlPO4samples, no additional diffraction peaks appear, but the peak intensity of the amorphous AlPO4and Al2O3becomes weak and strong, respectively, suggesting generated the crystallinity of γ-Al2O3and no new species.From the above results,it can be deduced that the intervention of Al changes the structure of AlPO4and forms γ-Al2O3crystal phase.In addition, no peaks related to the cesium species are noticed, indicating the relatively high dispersion and low crystallinity of the counterparts.
3.1.2.XPSspectra
XPS is a commonly used technique for investigating the chemical composition on the surface of catalysts according to the binding energy of the individual atoms.Fig.2 shows the XPS spectra of Al2O3-AlPO4catalysts.The characteristic binding energies of Al 2p,P 2p,and O 1s are at around 75 eV,135 eV and 532 eV respectively, which are consistent with the data reported in the literatures [24–26].The binding energy of Al 2p around 74.4 eV and 75.9 eV can be assigned to Al3+in Al—O—Al and Al—O—P, respectively [27].The binding energy of P 2p is ～134.8 eV for P5+in Al—O—P.The peak of O 1s at the attachment of 529–533 eV represents the oxygen in alumina,while the 531 eV represents the oxygen in aluminum phosphate.On the surface of amorphous Al2O3-AlPO4,the binding energy of the corresponding elements have only slight changes,and the counterparts of O 1s is between the binding energy of O 1s of Al2O3and AlPO4.With the increase amount of aluminum in these catalysts, the peak of Al 2p will continuously shift from 75.9 eV to 75 eV, suggesting that average electron density of Al and the content of Al2O3gradually increases.The increase in the average electron density of Al leads to the change of chemical circumstance around the Al.
3.1.3.Texturalproperties
The nitrogen absorption–desorption isotherms of Al2O3-AlPO4with different Al/P molar ratio are shown in Fig.3.The isotherms exhibit type IV with different steep hysteresis loops,revealing typical mesoporous materials.Samples with Al/P molar ratio less than 6 belong to steep H1 hysteresis loops,suggesting a cylindrical pore with uniform diameter distribution at both ends of the opening;when Al/P molar ratio is more than 6, the type of hysteresis loops change from H1 to H2(b), indicating the type of pore changing to typical hole neck relatively wide ink bottle holes with complex pore structure [28].The textural and structure parameters of Al2O3-AlPO4and Cs/8AlP materials are provided in Table 1.By increasing the Al/P molar ratio, the values of the BET surface area of samples firstly increase, then decrease; the values of pore size have a reverse trend compared with the BET surfaces areas, indicating the generation of new pores by adding aluminum.The decrease in BET surface area may be due to accumulation of sediments.8AlP has the highest BET surface area (357 m2∙g-1) and the lowest pore size (5.5 nm).The high BET surface area and the mesoporous structure can facilitate diffusion of methyl acetate and MA, leading to a high catalytic activity.Upon supporting Cs,the BET specific area and the pore volume of the samples decrease,and the extent of the decrease depends on the content of Cs.Because CsNO3diffuse into the 8AlP pore by impregnation and was decomposed into cesium ion and cesium oxide on the pore after calcination [8].
Table 1Textural properties of Al2O3-AlPO4 and Cs/8AlP catalysts
Fig.3. Nitrogen adsorption–desorption isotherms of Al2O3-AlPO4 catalysts.
3.1.4.Skeletallystructuralfeatures
Characteristic bonds such as O—H, P—OH, Al—OH, Al—O—P and P—O relevant to samples can be confirmed by Infrared spectral analysis and the results are shown in Fig.4.A broad absorption band in the range of 3000–4000 cm-1is associated with the vibration of O—H bond,which connect to the Al and P atoms[29].There is a broad band due to surface hydroxyl groups, most likely phosphorus ones, perturbed by a hydrogen bridge bond from a surface hydroxyl band.The broad peak at 1650 cm-1represents the vibration of H2O molecule (HOH).The region of 1400–900 cm-1is assigned to the triply degenerate P—O stretching vibration of tetrahedral,which indicates the presence of tetrahedral coordination sites of phosphorous [30].The intensity of this band is consistent with the phosphate concentration.The bands in the range of 800–400 cm-1are assigned to the asymmetric and symmetric stretching of Al—O—P bonds, which is corresponding to non-stoichiometric aluminum phosphates [31].Conclusively, FTIR data supported the presence of bonded hydroxyl groups which are able to exhibit Brønsted acidity in Al2O3-AlPO4.With the rise of Al/P molar ratio, the IR bands at 1400–900 cm-1and 3000–4000 cm-1become weaker showing reduction of AlPO4and a decrease in Bronsted acidity content.The FT-IR spectra of Cs/8AlP has no characteristic absorption peak of nitrate (1380 cm-1), indicating that CsNO3was completely decomposed on the surface of 8AlP after calcination.
Fig.4. IR spectra of Al2O3, Al2O3-AlPO4 and Cs/8AlP.
3.1.5.27Al-MASNMRcharacterizations
27Al-MASNMR was used to investigate the local structure and coordination environment of Al in amorphous Al2O3-AlPO4prepared with various Al/P molar ratio.The corresponding spectra are shown in Fig.5.The27Al-MASNMR spectrum of Al2O3has signals at 66 and 7 corresponding to the tetrahedral (T) and octahedral (O) aluminum, respectively.As for the amorphous Al2O3-AlPO4, it shows three peaks in the range of 40–67, 13–32 and-10–7.It was reported in literature that those peaks fell to tetrahedral-Al, penta-coordinated (P) aluminum and octahedral-Al, respectively [32].Tetrahedral, penta-coordinated and octahedral aluminum are designated as T-Al, P-Al, and O-Al, respectively.The T-Al(δ, 67) and O-Al(δ, 5) exist in a separate aluminum phase, and the pure phase of alumina-phosphate has T-Al(δ, 40)and O-Al(δ, -14)[33,34].In amorphous silica–alumina,P-Al connects with the alumina clusters within the silica/silica–alumina matrix [35].The P-Al and O-Al are known to be the potential Lewis acid sites and T-Al closed to silanol groups can increase the Brønsted acidity of silanol.When these acid sites interact with nearby the P-Al, additional pseudo-bridging bonds are brought forth to further enhance the Brønsted acidity of silanol[36,37].Curves are deconvolution by Gauss method and quantitative analysis of 4-, 5-, and 6-coordinated Al atoms in spectra is demonstrated in Table 2.The fraction of aluminum (F) in tetrahedral and octahedral coordination are 28% and 72%, respectively,which abides by the restrictions coming from the spinel lattice and Al2O3stoichiometry.The AlP sample has 14% P-Al, indicating it contains small amount of Al2O3.So, it is not considered as the pure alumina-phosphate.In the amorphous Al2O3-AlPO4, the T-Al/O-Al ratio decreases with the increasing amount of Al, suggesting the high content of Al2O3.Based on the result in XPS, increasing the Al/P molar ratio increase the average electron density of Al.The increase in average electron density of Al results in a decrease in the quadrupole interaction, and a decrease in the quadrupole interaction makes the chemical shift of T-Al, P-Al and O-Al move to a lower field.Compared with other samples,the 8AlP has the highest content of P-Al and 4AlP exhibits the lowest content of P-Al, which can account for the difference in acidity obtained from NH3-TPD profiles.
Table 2Quantitative analysis for 27Al-MASNMR spectra of amorphous Al2O3-AlPO4 and Cs/8AlP catalysts
Fig.5. 27Al-MASNMR spectra of Al2O3-AlPO4 catalysts (a) Al2O3, (b) AlP, (c) 2AlP, (d) 4AlP, (e) 6AlP, (f) 8AlP, (g) 10AlP.
Fig.6 exhibits the27Al-MASNMR spectra of 8AlP with different Cs loading.Due to the interaction between the Cs and support,the change happens in the coordination situation of Al,especially for PAl.The higher concentration of Cs in Cs/8AlP will lead to lower concentration of P-Al.That may be because basic cesium oxide will interact with the acid sites produced from Al,particularly the configurationally unsaturated Al.
Fig.6. 27Al-MASNMR spectra of Cs/8AlP catalysts.
3.1.6.SEMimages
The SEM images of 8AlP and 10Cs/8AlP catalysts are shown in Fig.7.The particle size of catalyst seems to be uneven, which is consistent with the reported results [19].This experimental phenomenon is caused by the difference in solubility of AlPO4and Al(OH)3which has lowerKspvalues [19] and this difference results in the deposition of AlPO4on Al2O3.The addition of cesium species has no effect on the morphology of 8AlP.
Fig.7. SEM images of (a) 8AlP and (b) 10Cs/8AlP.
3.1.7.NH3-andCO2-TPDprofiles
The NH3-TPD profiles for Al2O3-AlPO4and Cs/8AlP catalysts are displayed in Fig.8.Deconvolution of the curves were carried out using the Gaussian method and the results were summarized up in Table 3.According to literatures about the temperature range for the desorption peaks of AlPO4and Al2O3[38–40], the desorption peak around 180 °C can be regarded as the weak-I acid,240 °C as the weak-II and 300 °C as the medium acid.Specifically,Pure aluminum phosphate has only weak-I acid (Fig S4), AlP has weak-I and weak-II acid,while Al2O3has weak-I and medium acid.
Fig.8. NH3-TPD profile of Al2O3-AlPO4 and Cs/8AlP catalysts.
With the decrease in P content in Al2O3-AlPO4, the weak-I acid site density reduces accordingly, however, the medium acid site density increase.Weak-I acid sites on the surface of Al2O3-AlPO4originate from phosphorus hydroxy and aluminum hydroxy.Medium acid site derives from Lewis acid site of Al2O3.According to result of XRD, XPS, IR and27Al MAS NMR, the content of alumina in the catalyst was increasing, while the content of aluminum phosphate was decreasing.The reduction of AlPO4will lead to the decrease of phosphorus hydroxyl which results are consistent with the IR results and the appearance of alumina will generate medium acid site.The density of weak-II acid site increases first,and then decreases and the 8AlP has the highest density of weak-II acid site.Interestingly, a linear relationship between the density of weak-II acid sites and P-Al content is noticed in Fig.9.The reasons may be concluded as the following parts.On one hand,the P-Al can generate potentially Lewis acid sites.On the other hand, the additional pseudo-bridging bonds that are similar to amorphous silica-alumina are formed, which can further enhance the Brønsted acidity of hydroxyl groups of phosphorous.CsNO3could decompose on the surface of catalyst to generate new base sites which will interact with acid sites, resulting in a significant decrease in weak-I, weak-II, and medium acid site density of Al2O3-AlPO4.However, due to the strong interaction between cesium and support, Cs—O—Al species will form.This cesium ion is a medium Lewis acid site which changes weak-II acid and medium acid ratio.Conclusively, the addition of Cs will reduce and change the distribution of acid sites.Specifically, 10Cs/8AlP has the highest weak-II and medium acid site ratio.
Fig.9. The relationship between concentrate of P-Al and density of weak-II acid sites desorbed at 240 °C.
The base property of Al2O3-AlPO4and Cs/8AlP were characterized with CO2-TPD, and the results were shown in Table 4 and Fig.10.Basic site of the surface of Al2O3-AlPO4comes from oxygen atom on the surface of the catalyst.The CO2-TPD curves of Al2O3-AlPO4catalysts with different Al/P molar ratio are very different and the basic site density decreases with the increasing Al/P molar ratio, indicating the increasing content of alumina will lead to the reduction of base site density in AlPO4-Al2O3catalyst.The CO2adsorption capacity of Cs/8AlP increases significantly with the addition of Cs, demonstrating new base sites are formed on the surface of catalyst (Table 4).In the previous study, Heetal.reported that CsNO3could decompose on the surface of catalyst to generate new base sites [8].
Fig.10. CO2-TPD profiles of Al2O3-AlPO4 and Cs/8AlP catalysts.
Table 3The surface acid density of Al2O3-AlPO4 and Cs/8AlP catalysts determined by NH3-TPD profiles
3.1.8.PyridineIRtitration
The main purpose of the pyridine infrared adsorption test is to measure the type of acid sites on the sample surface, and the desorption temperature can determine the strength of the acid sites.The band at 1540 cm-1corresponding to the pyridinium ions(PyH+) is attributed to the Brønsted acid sites; the band at 1450 cm-1is related to pyridine coordinating to Lewis acid sites and the band around 1496 cm-1corresponds to both the Brønsted and Lewis acid sites [41,42].The three bands at 1450, 1490 and 1540 cm-1were found with 8AlP in Fig.11,indicating it presences Brønsted and Lewis acid sites.The surface Brønsted acid sites originate from the hydroxyl (—OH) groups bonded to P and Al atoms and the unsaturated Al provides Lewis acid sites in Al2O3-AlPO4.In addition, the band related to the PyH+after outgassing at 400°C is regard as strong Brønsted acid site [36].It shows that the strong Brønsted acid site exists in Al2O3-AlPO4, however AlPO4contain only weak ones and the Al2O3doesn’t contains strong Brønsted acid sites[43].It can be inferred that there exists the enhancement effect of Brønsted acid sites in Al2O3-AlPO4,which is very similar to the amorphous silica-alumina.Considering the results of27Al-MASNMR,NH3-TPD and Pyridine IR,the reasons for the linear relationship between concentration of P-Al and weak-II acid sites are the enhancement effect of Brønsted acid sites and Lewis acid site produced by the coordination unsaturated Al (Fig.11).
3.2.Catalytic performance
3.2.1.EffectofAl/Pmolarratio
Table 5 represents the activity of Al2O3-AlPO4catalysts.With the increase in Al/P molar ratio, the conversion of methyl acetate and the selectivity of acetic acid gradually decreases, while the selectivity of MA shows a different trend.The results suggest that the conversion sites of methyl acetate gradually decrease, and the active sites for selective generation of acetic acid decrease.According to result of XRD, XPS, IR and27Al MAS NMR, the characteristic peak of alumina gradually enhanced,and the characteristic peak of aluminum phosphate gradually decreased,indicating that the content of alumina in the catalyst was increasing,while the content of aluminum phosphate was decreasing.Combined with characterization results of acid-base properties,the increasing content of alumina will lead to the reduction of base site density in AlPO4-Al2O3catalyst.It reveals that the conversion of methyl acetate is probably in connection with the base site density in AlPO4-Al2O3catalyst.According to reaction mechanism,basic sites can promote the α-C—H abstraction of methyl acetate, and then form an enolate species, which will accelerate the conversion of methyl acetate.
Table 4The surface base density of Al2O3-AlPO4 and Cs/8AlP catalysts determined by CO2-TPD profiles
Table 5Catalytic activities of Al2O3-AlPO4 catalysts①
Pure aluminum phosphate has 100% selectivity to acetic acid with only weak-I acid and Baoetal.reported that the weak acid can catalyze ester hydrolysis reaction [14].Clearly, weak acid is beneficial for the hydrolysis of methyl acetate.According to the results of27Al MAS NMR and NH3-TPD, AlP prepared by precipitation method is not pure alumina-phosphate and contains weak-I and weak-II acid sites.Notably, it’sQweak-Iacidsites/QtotalacidsitesandQweak-IIacidsites/Qtotalacidsitesare similar with the selectivity of acetic acid and MA, respectively.It shows that the selectivity for methyl acrylate might be related to weak-II acid site.The appearance of alumina brings medium acid site which is identified as Lewis acid.Baoetal.found that Lewis acid site of alumina was critical to promote aldol condensation reaction of methyl acetate with FA[14].Although the conversion of methyl acetate decreases,the increase in Al/P molar ratio will increase the selectivity of MA,making the yield of MA rise slightly.
Moreover, we analyze the weak-II and medium acid site ratio and the selectivity of MA and found a good linear relationship between them in Fig 12.It demonstrates that weak-II and medium acid are a crucial factor for the selectivity of MA.From the perspective of reaction mechanism, the weak-II and medium acid site can interact with the carbonyl group of FA producing a polarization of this group,consequently increasing the positive charge on the corresponding carbon atom,and this accelerates C—C bond formation to improve selectivity for MA.Weak-II acid site is related to P-Al and medium acid site come from Al2O3.It indicates that the cooperation between P-Al and Al2O3enhance the selectivity of MA for aldol condensation reaction of methyl acetate with FA.
Fig.11. Pyridine adsorption IR spectra of 8AlP at different desorption temperature.
3.2.2.EffectofCsloading
The activity of Cs/8AlP catalysts is shown in Table 6.With the increase in Cs loading, the conversion of methyl acetate increases accordingly, and the selectivity of MA increases first, and then decreases.Base on the results of characterizations, Cs loading has little influence on the structure of catalyst.However,it remarkably increases basic site densities and decreases acid site densities of 8AlP as shown in NH3-TPD and CO2-TPD profiles.Based on the results of XRD,FT-IR and XPS,CsNO3could decompose on the surface of 8AlP to form cesium oxide which is new base sites.These base sites will interact with acid sites produced from Al,especially the unsaturated coordination of Al, resulting in a significant decrease in acid site density of 8AlP.New base sites from cesium oxide are formed and the conversion of methyl acetate follows the same trend as its density with the increase in Cs loading.It is suggested again that there is correlation between the basic site densities and the conversion of methyl acetate.As shown in Fig.13(a), the conversion of methyl acetate is in good linear relationship with basic site densities and Cs/8AlP has a larger slope, indicating that basic sites are in favor of the conversion of methyl acetate and new basic sites formed by addition of Cs are more effective than basic sites derived from 8AlP.Heetal.found the alkali metal atom and the O atom form the active center which were conducive to converting methyl acetate to an adsorbed enol molecule on the alkali metal/SBA-15 catalysts based on analysis of ESP and ALIE.And the predominant way for the formation of methyl acrylate was suggested, which involved an adsorbed enol molecule as the reaction intermediate [8].We assume that Cs is loaded to form a new basic active center which is beneficial to improving the interaction between catalyst and methyl acetate,thereby promoting the conversion of methyl acetate.
Table 6Catalytic activities of Cs/8AlP catalysts①
Fig.12. Relationship between the selectivity of MA and percentage of the Weak-II and medium acid sites.
From the XPS results(Fig.S7),cesium species are mostly cesium oxide and a small amount of cesium ions.Due to the strong interaction between cesium and support, Cs—O—Al species will form.This cesium ion is a medium Lewis acid site which changes the distribution of acid sites.10Cs/8AlP has the highest weak-II acid and medium acid ratio and selectivity of MA.So, we use the same approaches to deal with Cs/8AlP catalysts.From Fig.13(b), the results suggest that there is still a good linear relationship between them, and the slope doesn’t change much, revealing that a certain amount of Cs loading does not introduce more effective acid sites,but only reduces acid site densities and changes the distribution of acid sites.An excess amount of cesium oxide will promote further condensation of methyl acrylate to produce the side products[11,12].So, appropriate addition of Cs could provide new basic sites and change the distribution of acid sites to further improve the catalytic activity.Ai’s studies showed that acidic oxides could improve the selectivity of condensation reaction, but the conversion was poor and alkaline oxides had a high conversion but many by-products[20,44,45].In the presence of a basic catalyst,the reaction is mainly controlled by dynamics, with high conversion rate and low selectivity.In the presence of an acidic catalyst, the reaction is controlled by thermodynamics, with high selectivity and low conversion.Adjusting the balance between acidity and basicity can effectively improve the catalyst activity.Acid-base catalyst has synergetic catalysis role in aldol condensation reaction.The acidbase active centers can activate the two reactants separately.In aldol condensation of methyl acetate with formaldehyde, the base active center interacts with methyl acetate to promote the α-H abstraction, the acid active center interacts with the carbonyl group of formaldehyde to increase its positivity.Then activatedformaldehyde, as an electrophile, acts with carbanion to form an intermediate aldol which rapidly condenses to methyl acrylate and water [13,15–16].In the view of the results of characterizations and evaluation, we can conclude that basic sites are conducive to the conversion of methyl acetate, weak-I acid sites are beneficial to formation of acetic acid and weak-II and medium acid sites improve the selectivity of MA.
Fig.13. The relationship between activity of catalyst and acid-basic properties.
Fig.14. Effect of reaction temperature on the yield and selectivity of MA on 10Cs/8AlP (the reaction conditions of LHSV=1 h-1 and methyl acetate/FA/methanol molar ratio=1/2/2).
3.2.3.Effectofreactioncondition
The effects of liquid hourly space velocity (LHSV) and reaction temperature on product yield and selectivity were reached over 10Cs/8AlP and the results are shown in Fig.14 and Fig.15.For Fig.14, the conversion of methyl acetate and the selectivity of MA declines to 17%and 78%at 330°C,respectively.The lower temperature leads to lower conversion of methyl acetate and is beneficial for acetic acid formation.As the temperature is raised to 410°C,the selectivity of MA plummets to 43%and the yield of MA falls to 17.6% due to acceleration of coke deposition and side reaction[13].Thus, the suitable temperature for the reaction is 370 °C.
Fig.15. Effect of LHSV on yield and selectivity of MA on 10Cs/8AlP (the reaction conditions of 370 °C and methyl acetate/FA/methanol molar ratio=1/2/2.)
The results in Fig.15 indicate that the LHSV plays an important role in yield and selectivity of MA.The LHSV was controlled by liquid flow rate and the filling volume of the catalyst, as the LHSV is increased from 0.6 h-1to 1 h-1, yield of MA rises from 13.3% to 21.3%.However, as the LHSV is further increased to 1.2 h-1, the yield falls.So, the LHSV of 1 h-1is appropriate for the reaction.It seems that residence time is the key point to determine the selective formation of MA.On the one hand, the easy desorption of MA on the catalyst surface will avoid further condensation to produce the side products.On the other hand, the fast recovery of surfaceactive sites accelerates the catalytic cycle.
Fig.16. Catalytic durability test over 10Cs/8AlP (the reaction conditions of 370 °C,LHSV=1 h-1 and methyl acetate/FA/methanol molar ratio=1/2/2.)
3.2.4.Catalyticstabilityandreusability
The catalytic stability and reusability of 10Cs/8AlP catalyst were also investigated, as shown in Fig.16 and Fig.17.After 30 h on stream, the conversion of methyl acetate decreases from 24% to 12%.Then the deactivated catalyst was characterized by TG to determine the regeneration temperature.The TG curve reveals distinct weight loss at temperature range from 350 °C to 450 °C,which is attributed to the combustion of coke [13].So, the regeneration of deactivated catalyst was carried out at 400 °C in the air atmosphere for 12 h.During the three regeneration cycles,the catalyst can maintain performance as the fresh one,indicating the 10Cs/8AlP catalyst has good reusability.
Fig.17. TG/DTA curves of the deactivated 10Cs/8AlP catalyst.
In this study, series of Al2O3-AlPO4and Cs/8AlP catalysts were prepared for the aldol condensation of methyl acetate with formaldehyde to MA.The Al/P ratio and Cs loading have great effects on the acid-base properties of catalysts.The catalyst with higher Al/P ratio shows higher weak-II and medium acid site ratio and the selectivity of MA, but it causes a decrease in base site density and the conversion of methyl acetate for Al2O3-AlPO4.The addition of Cs could provide new basic sites and change the distribution of acid sites to further improve the catalytic activity.The cooperatively catalytic effects between the penta-coordinated Al and Al2O3in these catalysts were identified to influence the weak-II and medium acid site ratio, which were approximately linear to the product selectivity.While the basic site densities of these catalysts were almost in proportion to the conversion of methyl acetate.As a result, the 10Cs/8AlP catalyst with good reusability was screened as optimum one, on which the yield and selectivity of MA could reach 21.2% and 85% and it can fully recover by simple air-treatment at 400 °C.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Key Research Program of Frontier Sciences (No.QYZDB-SSW-SLH022), National Natural Science Foundation of China (No.21676270, No.21878293, No.22178338), the Joint Fund of the YulinUniversity and the Dalian National-Laboratory for Clean Energy (Grant YLU-DNL Fund 2021018), and Foundation of State Key Laboratory of Highefficiency Utilization of Coal and Green Chemical Engineering(Grant No.2017-K08).
Supplementary Material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.11.025.
推荐访问:coordinated Al Al2O3
