Owing to its negligible toxicity,low bioaccumulation and high reactivity,dimethyl carbonate (DMC) has attracted much attention for the so-called ‘sustainable society’ and ‘green chemistry’ chemicals [1-4]. Oxidative carbonylation of methanol is one of the most promising approaches to produce DMC [5, 6]. In the process,supported Cu-based catalysts exhibit excellent catalytic activity and has attracted extensive interest. Among these catalysts,copper-exchanged zeolite Y (CuY) is interesting for the oxidative carbonylation of methanol to DMC [7-9].
Ⅰt is well known that the surface-bound Cu+ on the CuY catalyst is the active species for the carbonylation reaction [10-14]. Richter et al. [15] prepared a CuO/NH4Y zeolite by the deposition-precipitation method. After calcination at 400 °C in static air for 2 h,the zeolite has only marginal activity for the carbonylation reaction. The high activity achieved by the inert activation procedure at 700-750 °C for 15 h is associated with the formation of a favorable active site Cu+. Zhang et al. [8] proved that on the CuY catalyst,methanol was absorbed on surface bound Cu+ to form Cu-OCH3 and CO inserted into Cu-OCH3 bonds resulting in the formation of DMC,and insertion of CO is the rate limiting step. However,on Cu-ZSM-5 and Cu-MOR catalysts,because the formation of Cu-OCH3 on surface bound Cu+ was limited due to the strong adsorption of CO,dimethoxy methane (DMM) was the primary product.
The traditional way for the surface bound Cu+ active center to be introduced into Y zeolite is that the H+ ions (Brönsted acidic sites) of HY are exchanged by Cu+ of CuCl during the heating of a physical mixture of CuCl and HY. That is,a solid state ion exchange (SSⅠE) formed the effective active sites for oxidative carbonylation [16]. The distribution of Brönsted acidic sites in Y zeolite affects the location of the Cu active species,and further affects the catalytic performance of oxidative carbonylation. Huang et al. [17] established a quantitative relationship between the amount of Brönsted acid sites and catalytic activity for oxidative carbonylation over CuY catalysts with different Si/Al ratios. Zhang et al. [18] also investigated the activity of CuY with different Si/Al ratios for the oxidative carbonylation of methanol to DMC systematically by a density functional theory method. They indicated that the CuY zeolite with a Si/Al ratio = 6.5 had the highest catalytic activity.
However,for zeolite Y with different Si/Al ratios,there are differences in not only the Brönsted acid sites,but also other factors in the catalytic activity such as the composition of the framework,bond angle of Si-O-Al and degree of dispersion of the Al ions [19]. Thus,there is a need for a more valid way to evaluate the quantitative relationship between the Brönsted acid sites of the support and catalytic activity of the CuY. Our preliminary investigations [20, 21] showed that Cu(acac)2 adsorbed and highly dispersed in Y zeolite support due to its sublimation. Cu2+ of Cu(acac)2 ion exchanged with NH4+ of NH4Y at a lower temperature of 174 °C,then during activation,Cu(acac)2 is easily decomposed and the decomposition products are useful for the Cu+ active center,resulting in the formation of more active centers. The unreacted and adsorbed Cu(acac)2 on Y zeolite decomposed to form CuO species which improved the catalytic activity of the Cu+ active center. More importantly,NaY zeolite does not have a solid state ion change with copper (Ⅱ) acetylacetonate Cu(acac)2 during the anhydrous heating treatment of the physical mixture. However,an ion exchange reaction between NH4Y zeolite and Cu(acac)2 does occur. Compared with SSⅠE,NH4+ of NaNH4Y plays the same role as H+ of HY,which provides the exchangeable sites for ion exchange of copper ions,and forms surface bound-Cu2+ on the CuY. Moreover,the released NH3 promotes the reduction of surface bound-Cu2+ to form the surface bound Cu+ active center for oxidation carbonylation of methanol to DMC.
In the present study,we studied the relationship between NH4+ of the NaNH4Y support and catalytic activity of CuY prepared by high temperature anhydrous interaction (HAⅠ) between Cu(acac)2 and NaNH4Y. First,the CuY catalysts from the NaNH4Y support with different NH4+ ion exchange degrees were prepared by HAⅠ. Then the activities of the CuY catalysts for oxidative carbonylation of methanol were evaluated. Detailed characterization of the CuY catalysts were also performed by X-ray diffraction (XRD),scanning election microscopy (SEM),N2 adsorption-desorption,temperature- programmed reduction of H2 (H2-TPR),X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption of NH3 (NH3-TPD). The major objectives were to establish the relationship between the catalytic performance and NH4+ exchange degree of NaNH4Y support.
10.003 g commercial NaY zeolite (Si/Al = 2.7) was added to 100.00 mL NH4NO3 aqueous solution with the concentration of 0.10,0.30,0.50 mol/L,and stirred to ion exchange for 4 h at 30 °C. Then,the suspension was filtered and the resulting cake was washed with deionized water several times and dried at 100 °C for 24 h. The procedure was repeated once and twice in the NH4NO3 aqueous solution (0.50 mol/L). The final solid material was designated as NaNH4Y-X zeolite with the NH4+ exchange degree of X.
According to the literature [21],the typical preparation of the CuY catalyst included two stages. In the first stage,5.001 g Y zeolite support and 2.454 g Cu(acac)2 were mixed and ground well with a mortar and pestle to form an intimate gray-blue mixture. Then the mixture was heated in the muffle furnace from the ambient temperature to 250 °C at 3 °C/min,and then held at 250 °C for 4 h. The mixture turned brown and was named as precursor. In the following activation stage,the precursor material was activated to form the CuY catalyst by heating from the ambient temperature to 650 °C at a heating rate of 3 °C/min and at 650 °C for 4 h. The CuY catalysts were designated as CuY-X. The catalyst support was NaNH4Y-X zeolite with the NH4+ exchange degree of X. All the prepared catalysts were stored in a desiccator and pressed to 40-60 mesh particles before the catalytic tests.
The sodium content was determined by a VARⅠAN AA240FS atomic absorption spectroscopy (AAS). The test solution was prepared by the following steps. A mixture of 0.202 g fresh catalyst and 2.012 g KOH was calcined at 500 °C for 6 h. Because the excess base destroyed the crystal structure of Y zeolite,Cu,Si and Al existed in the form of dissolvable compounds in hydrochloric acid. Then the calcined mixture was digested in a hydrochloric acid solution and diluted in a 250.00 mL volumetric flask with deionized water. Then 5.00 mL of this solution was transferred to a 100.00 mL volumetric flask,diluted with deionized water to volume (Cu solution concentration 1-5 mg/L for the AAS experiment). Finally,50 mL of this solution was used to test the actual Cu content.
XRD patterns were obtained using Cu Kα radiation (λ = 0.154056 nm) on a Rigaku D/max 2500 diffractometer at 40 kV target voltage and 100 mA tube current in the 2θ range from 5° to 65° at a scanning rate of 8°/min.
BET surface areas and pore volume of the CuY catalyst were determined from N2 physisorption measurements on a Micromeritics ASAP 2010 instrument. The samples were first outgassed under vacuum at 90 °C for 1 h and at 350 °C for 15 h. The total surface area was obtained by the BET equation using the relative pressure range of 0.05-0.16 in the linear part of the nitrogen adsorption isotherm (molecular cross-sectional area for N2 of 0.162 nm2). The micropore and mesopore volumes were calculated by the t-plot and BJH method,respectively.
Morphology analysis was performed by a S-4800 field emission scanning electron microscope (SEM). The powder samples were carefully placed on a double-sided tape with the aluminum stub as the base. The observations were made at different magnifications.
NH3 temperature programmed decomposition (NH3-TPDec) of Y zeolite was performed on a Micromeritics AutochemⅡ2920 chemical adsorption instrument with a thermal conductivity detector (TCD). 0.3003 g sample was loaded in a U-shaped quartz tube and heated from 50 to 600 °C at 10 °C/min in He (30 mL/min). Ammonium gas released from the decomposition was continuously monitored by a TCD. Before detection,the gas flow was conducted through a KOH trap to remove desorbed water.
NH3-TPD was carried out on a Micromeritics AutochemⅡ2920 chemical adsorption instrument. 20 mg sample was loaded in a U-shaped quartz tube and first degassed at 300 °C for 0.5 h in He (30 mL/min). After degassing,the temperature was dropped to 50 °C,and then 15 vol% NH3/He (30 mL/min) was introduced into the sample cell to allow the sample to absorb ammonia. After saturated adsorption,the temperature of the sample was raised from 50 to 600 °C at 10 °C/min. The ammonia gas released by desorption was continuously monitored by a TCD.
H2-TPR was performed on a Micromeritics AutochemⅡ2920 chemical adsorption instrument. First,20 mg sample was loaded in a U-shaped quartz tube and was degassed at 300 °C for 60 min in N2 (30 mL/min),then cooled to room temperature. The gas was switched to 10 vol% H2 in argon flow,and the sample was heated to 1000 °C at 10 °C/min. Hydrogen consumption was monitored by a TCD. The effluent gas was passed through a liquid nitrogen cold trap to remove water before detection.
XPS data were collected on an on Scientific ESCALab220i-XL electron spectrometer equipped with a 300 W Al Ka X-ray source with a Thermo-VG. The catalyst holder was placed in a fast entry air load-lock chamber without exposure to air and the pressure was maintained at 3×10−7 Pa overnight. Then the catalyst holder was transferred to the analysis chamber for XPS study. The binding energy and the Auger kinetic energy scales were referenced to the C1s line at 284.6 eV from adventitious carbon.
Catalytic measurements were performed on a continuous fixed bed microreactor (Φ 8 mm × 500 mm) under atmospheric pressure with an online Agilent 6890 gas chromatograph (GC). 0.6 g (about 1.5 mL) catalyst sample was packed in the tubular reactor and the reactor was positioned in the reactor furnace. 0.05 mL/min methanol was fed into by a constant flow pump (2PB-05) and vaporized in the pre-heater. In the meantime,55.0 mL/min CO and 5.5 mL/min O2 were introduced into the pre-heater and mixed well with vaporized methanol,and then the mixed reactants were fed into the catalyst bed to catalyze oxidative carbonylation of methanol. The activity test time was 10 h and testing temperature was 140 °C.
The gas products were analyzed by an online GC with three valves and four columns every 12 min. The products flowed through the HP-PLOT/Q capillary column (30 m × 0.53 μm × 40 μm). The organic products were held back. Then the inorganic products flowed through the Poropack-Q packed column: CO2 first flowed out and entered a TCD and was detected; second,CO and O2 were separated by the HP-PLOT Molesieve/5A capillary column (30 m × 0.53 μm × 25 μm) and detected by TCD. The products flowed through the HP-ⅠNNOWax (30 m × 0.53 μm × 1 μm) capillary column and into a flame ionization detector (FID),thus MeOH,methyl formate (MF),dimethoxymethane (DMM),dimethyl ether (DME) and DMC were detected.
In order to determine the NH4+ exchange degree of NaNH4Y zeolite,the Na+ and NH4+ contents were determined by AAS and NH3-TPD,respectively. Fig. 1 presents the profiles of NH3-TPDec from NaNH4Y with different NH4+ exchange degree (X) with temperature rise. There were three peaks over all samples and they were enhanced gradually with the increase of the NH4+ exchange degree. The released NH3 amount was determined by the peak area. Then combined with the content of the Na+ determined by AAS,the NH4+ exchange degree was calculated,as listed in Table 1. The initial increase of the NH4+ exchange degree was very obvious. Then the increments were small with further increase of the NH4NO3 solution concentration. When the concentration of the NH4NO3 solution was 0.5 mol/L and the NaY zeolite ion exchanged twice,the NH4+ exchange degree reached 68.1%,7.7% higher than that of NaY ion exchanged once. The NH4+ exchange degree of NaY ion exchanged three times only increased 0.9% more. So when NaY zeolite was ion exchanged with 0.5 mol/L NH4NO3 solution twice,the NH4+ exchange degree was beginning to stabilize and reached the maximum.
Fig. 2(a) shows the XRD patterns of the NaNH4Y samples with different NH4+ exchange degrees. Compared with NaNH4Y-0 zeolite,the diffraction peak intensity of the other NaNH4Y zeolites decreased slightly. Sato et al. [22] found that ion exchange of NaY with (NH4)2SO4 resulted in structural changes and the relative crystallinity of the NaY decreased by approximately 20% after the first ion exchange. In the second and third ion exchanges,the structural changes depended on the Si/Al ratio of the starting NaY zeolite. Y zeolite with a medium Si/Al ratio of 2.8 retained their high crystallinity. Compared with the results reported by Sato et al. [22],the reduction of the crystallinity of Y zeolite was less in this study. This may be due to the medium Si/Al ratio of 2.7 and the low concentration of the NH4NO3 solution. These NaNH4Y samples with different NH4+ exchange degrees were used as the support to prepare the CuY catalysts by HAⅠ between NaNH4Y and Cu(acac)2. The XRD patterns of the CuY catalysts are presented in Fig. 2 (b). For all the CuY catalysts,the characteristic diffraction peaks of Y zeolite were observed. This indicated that the structure of the Y zeolite remained after the ion exchange of NH4NO3 and succeeding heat treatment. Ⅰt is worth noting that the CuY-68 catalyst exhibited almost no crystalline copper oxide,but other catalysts contain crystalline CuO. CuO (tenorite,synthetic,PDF-Nr. 48-1548) has intense diffraction peaks (relative intensities in parentheses) at 2θ = 35.418° (37),35.544° (100),38.709° (99),38.903° (21) and 48.717° (30). The two most intense diffraction peaks at 2θ = 35.544° and 38.709° can clearly be distinguished. There was no indication for the presence of crystalline Cu2O or metallic Cu. With the increase of the NH4+ exchange degree,the XRD peak intensities of crystalline CuO decrease gradually. The results may indicate that the content of crystalline CuO decreased or the dispersion of CuO species became better.
The textural properties and porous structure of NaNH4Y samples with different NH4+ exchange degrees and the CuY catalysts were characterized by N2 adsorption and the results were shown in Table 2. The specific surface area of the Y zeolite decreased slightly from 797.0 to 775.5 m2/g with increase of the NH4+ exchange degree. This was because the pore structure of the Y zeolite remained intact,which was proved by XRD. After loading of Cu,the surface area of the CuY catalyst was less than that of the support. When the NH4+ exchange degree was zero,the specific surface area of the CuY-0 catalyst dropped to 647.9 from 797.0 m2/g for the support and the pore volume decreased from 0.35 to 0.27 cm3/g. Combined with the H2-TPR and SEM results,this was because the pores of the Y zeolite have been partly blocked by CuO. With increase of the NH4+ exchange degree,the CuO amount decreased,and there was a reduced specific surface area. That eventually resulted in the increase of the specific surface area of the CuY catalyst with increase of the NH4+ exchange degree.
Fig. 3 shows the SEM images of the CuY catalysts. For all the CuY catalysts,the octahedron morphology with smooth crystal faces and sharp edges was clearly visible,and CuO species were observed. With increase of the NH4+ exchange degree,the CuO content decreased. This showed that for all the CuY catalysts,the replacement of Na+ by NH4+ and the supported copper species did not affect the morphology of the Y zeolite.
In order to investigate the influence of the NH4+ exchanged degree on the distribution of the reducible copper species on the CuY catalysts,H2-TPR was performed. The results are shown in Fig. 4. The hydrogen consumption peaks from low temperature to high were labeled as peak Ⅰ,peak Ⅱ,peak Ⅲ,and peak Ⅳ.
The following facts from the literatures [15, 23-25] help to understand the H2-TPR:
The reduction of ion exchanged Cu2+ on zeolite occurs via a two-step mechanism.
The reduction of Cu2+ to Cu+ is characterized by a H2 -TPR peak below 500 °C. The difficulty of reduction depends on the location of Cu2+,in the order of hexagonal prism > sodalite cage > supercage. The second reduction step of Cu+ to Cu0 occurs above 700 °C
Reduction of CuO on the CuY catalyst proceeded in one step at 230 °C.
So peak Ⅰ,peak Ⅲ,and peak Ⅱ were ascribed to the reduction of Cu2+→Cu+ in the supercage and sodalite cage,and CuO→Cu0,respectively. Peak Ⅳ was assigned to the reduction of Cu+→Cu0. For the CuY-0 catalyst,there was only peak Ⅱ of CuO→Cu0. This indicated the absence of other reducible copper species besides CuO. With increasing the NH4+ exchange degree,peak Ⅰ,peak Ⅲ and peak Ⅳ increased and peak Ⅱ decreased gradually. Meanwhile,the reduction temperature of peak Ⅳ moved towards low temperature. All these observations suggested that with the increasing NH4+ exchange degree,the Cu2+ and Cu+ contents increased and the CuO content decreased,which was in agreement with the XRD results. The Cu2+ and Cu+ contents of CuY-68 catalyst were much higher than these of the CuY-63 catalyst,and the Cu+ reduction temperature of the CuY-68 catalyst was also lower than other catalysts. Moreover,the catalytic activity of the CuY-68 catalyst was superior to the other catalysts. This indicated that the Cu+ active center for the oxidative carbonylation of methanol to DMC was formed by HAⅠ between NaNH4Y and Cu(acac)2,and the more the NH4+ exchange degree,the more Cu+ active centers were formed and increased the activity of the CuY catalyst.
In order to analyze the copper species distribution on the CuY catalyst surface,XPS was performed. The XPS spectra of Cu 2p and Auger spectra are presented in Fig. 5. Divalent copper can be confirmed by the Cu 2p3/2 binding energy in the range of 933.0-936.0 eV and the shake-up satellite peak. Monovalent copper can be assigned to the binding energy of the XPS peaks ranging from 932.0 to 933.0 eV without any shake-up satellite peaks [26-29]. Usually,the two peaks from divalent and monovalent copper overlapped. From the different peak shape and relative intensity,the distribution of copper species in the CuY catalysts were different. From the Auger analysis of the Cu species,the kinetic energies for Cu,Cu2O and CuO are 918.2,916.0 and 917.6 eV,respectively [30]. In addition,the kinetic energy for Cu+ in the CuY catalyst was 3 eV below that of Cu2O,at 913.0 eV [31]. Under the HAⅠ preparation conditions,metallic copper was not possible in the CuY catalyst. Combined with the H2-TPR results,the CuY-0 catalyst only contained CuO species,so the two peaks at 912.6 and 916.7 eV in the Auger spectra were assigned to monovalent copper species and divalent copper species,respectively. To determine exactly the content of the copper species on the surface of CuY catalyst,the relative proportion of the peak area of the curve fitting of the Cu 2p3/2 spectra was calculated,i. e.,the relative amounts of copper species with different valence states,which are listed in Table 3. With the increasing of the NH4+ exchange degree of NaNH4Y,the monovalent copper content increased and divalent copper content decreased gradually.
To investigate the surface acidity of the CuY catalysts from the NaNH4Y support with the different NH4+ exchange degrees,NH3-TPD was performed. The results are presented in Fig. 6. Ⅰt was only below 250 °C that a NH3 desorption peak (α) was observed over NaNH4Y-0,which was attributed to the physically adsorbed and hydrogen-bound NH3 of Y zeolite [32, 33]. There was no desorption of NH3 at high temperature. The CuY-0 catalyst from the NaNH4Y-0 support has a similar NH3-TPD profile to the NaNH4Y-0 zeolite. From the H2-TPR result (Fig. 4) for the CuY-0 catalyst,there existed only CuO species,and no other copper species. So these provide evidence that CuO species adsorb NH3 molecule very weakly and the NH3 adsorption are generally negligible. Besides,for the CuY catalysts from the NaNH4Y support with different NH4+ ion exchange degree,the NH3 desorption peak below 200 °C was splitted into two overlapped peaks and two NH3 desorption peaks (β and γ) above 200 °C were observed,which was caused by the introduction of the surface-bound copper ions,as already proposed [25]. That is,the introduction of copper ion into NaY not only creates a more strongly bonded NH3 adsorption center but also increased the amount of weakly bonded NH3 centers. In order to quantify the NH3 desorption capacity of the CuY catalysts,all the NH3-TPD curves were processed by means of a peak-differentiating analysis. The results of the quantitative analysis are listed in Table 4. Ⅰt can be seen that the NH3 desorption capacity of the CuY catalyst increased with the increasing of the NH4+ exchange degree of NaNH4Y zeolite. Therefore,there was a positive correlation between the amount of copper ions on the CuY catalyst and the NH4+ exchange degree of NaNH4Y zeolite. Kieger et al. [25] reported that the peak at 327 °C on the NH3-TPD is attributed to NH3 desorbing from Cu2+ ions on the Cu2+Y. However,after NH3 treatment at 500 °C and desorption in He at 500 °C,most of the Cu2+ ions on the CuY have been reduced to Cu+ [34]. A partial reduction of Cu2+ to Cu+ results in the the NH3 desorption peak above 227 °C shifting toward a low temperature and forming a broad peak from 77 to 327 °C. So the β and γ peak were attributed to surface-bound Cu+ and Cu2+. Ⅰt was reported that three NH3 ligands were bound to Cu2+ ions on the CuY catalyst and only two NH3 ligands were bound to Cu+ ions on the CuY catalyst [25]. The ion exchange copper ion loading was obtained and listed in Table 4. For the CuY-0 catalyst,there was no the surface-bound Cu+ active center instead of CuO. When the NH4+ exchange degree was 18.3%,the copper ion loading was 4.4 wt%. With the increasing of the NH4+ exchange degree of NaNH4Y zeolite,the copper ion loading on the CuY catalyst also increased,and reached 9.0 wt%,when the NH4+ exchange degree was 68.1%. Ⅰt was conducive to forming Cu+ active centers and also will increase the activity of CuY.
Experiments were undertaken to determine the influence of the NH4+ exchange degree on the activity for the oxidative carbonylation of methanol to DMC. The results are showed in Fig. 7. In Fig. 7(a),a plot of the DMC synthesis is given in terms of space time yield (STYDMC) as a function of time on stream. The STYDMC on all the catalysts increased with time on stream,before steadying off within 60 min,then kept constant for 600 min of time on stream. For the CuY-0 catalyst,there existed only CuO species,not the surface-bound Cu+ active center,so it has no catalytic activity for the oxidative carbonylation of methanol to DMC. Under the same testing reactions,the activities of all the other catalysts from NaNH4Y support exhibited significant increase. The average value of STYDMC,conversion of CH3OH (XCH3OH) and selectivity of DMC (SDMC) are presented Fig. 7(b) . Ⅰt can be seen that with the increase of the NH4+ exchange degree,STYDMC and XCH3OH increased and SDMC decreased slightly. When the NH4+ exchange degree was 18.3%,STYDMC,XCH3OH,and SDMC were 49.1 mg/(g•h),1.2%,and 73.6%,respectively. When the NH4+ exchange degree reached 63.2%,the STYDMC on the CuY-63 catalyst increased to 184.5 mg/(g•h). When the NH4+ exchange degree reached 68.1%,it was surprised that the corresponding CuY-68 catalyst has higher activity than the CuY-63 catalyst. Ⅰts STYDMC increased about 45% and reached 267.3 mg/(g•h),but SDMC remained unchanged. The catalytic activity of the CuY-69 catalyst from the NaNH4Y-69 support was a little higher than that of the CuY-68 catalyst. The experimental results can be explained by the following. First,the NH4+ exchange degree of 68% has reached the limit. In addition,when the NH4+ exchange degree was 63.2%,NH4+ was completely ion exchanged by Cu2+ of Cu(acac)2,and the ion exchanged copper ions have reached the maximum. When the NH4+ exchange degree continued to increase from 63.2% to 68.7%,ion exchange between NH4+ and Cu2+ does not occur anymore,resulting in the rest of the NH4+ remaining on the catalyst precursor. Ⅰt has been reported that the released NH3 from NH4Y decomposition facilitates the reduction of copper species during high temperature treatment [35] and the formation of more Cu+ active centers [23].
So with the same Cu loading,with the increase of the NH4+ exchange degree of NaNH4Y zeolite,the Cu+ content increased on the CuY catalyst,which further improved the catalytic activity. This indicated that during the preparation of the CuY catalyst by HAⅠ,the NH4+ ion is the exchangeable site of the copper ion,and the Cu+ active center is related to the distribution of NH4+,which also influenced the reaction performance. The long term stability and activity of the catalyst are vital for the oxidative carbonylation of methanol to DMC from both the academic and industrial viewpoints. The effects on CuY-18,CuY-63 and CuY-68 catalyst longevity of vapor phase oxidative carbonylation of methanol to DMC were investigated. The results for 60 h are presented in Fig. 8. After an increase of catalytic activity for 1 h,in the next 38 h,the STYDMC values were maintained constant. Then,the activities of all the CuY catalysts slowly decreased. At the time on stream of 60 h,the deactivation degrees for CuY-18,CuY-63 and CuY-68 catalysts were 19.6%,32.6%,and 33.3%,respectively.
CuY catalysts from a NaNH4Y support with different NH4+ exchange degrees were prepared by high temperature anhydrous interaction and tested for the oxidative carbonylation of methanol. The NH4+ of NH4Y was needed to form the Cu+ active center and the NH4+ exchange degree of NaNH4Y had a significant influence on the activities of the CuY catalysts. With the increase of the NH4+ exchange degree of NaNH4Y,the surface bound-Cu2+ and surface bound Cu+ content of the CuY catalyst increased and the CuO species decreased. The NH4+ of NaNH4Y made copper ion exchange occur more easily and was promoted to form the Cu+ active centers,which increased the activity of the CuY catalyst for the oxidative carbonylation of methanol to DMC. The surface bound Cu+ content reached the maximum when the NH4+ has almost completely exchanged the Na+ of NaY zeolite,and this CuY exhibited the optimal catalytic activity.
2016,
Vol. 37 Issue (8): 1403-1412 
PDF (1423 KB)
2016,
Vol. 37 Issue (8): 1403-1412 PDF (1423 KB)