Selective Hydrogenation of Concentrated Vinyl Acetylene Mixed C 4 by Modified Pd Catalysts : Effect of

The Pd and Pd-Cu on alumina catalysts were tested for hydrogenation of vinyl acetylene in mixed C4 in a circulating tubular reactor. The results showed that adding proper amounts of Cu improved the reaction activity, but inhibited 1,3-butadiene selectivity. Moreover, the presence of Cu retarded the carbon deposition on catalysts during the reaction. Temperature programmed oxidation (TPO), Temperature programmed reduction (TPR), H2 chemisorption, and X-ray photoelectron spectroscopy (XPS) were utilized to characterize the catalysts. The characterization suggested both geometric and electronic modifications.


Introduction
Concentrated vinyl acetylene mixed C4, containing vinyl acetylene up to 30 wt %, generated from the butadiene extraction process is continuously flared due to safety reason [1,2].Therefore, converting vinyl acetylene in this stream by selective hydrogenation to 1,3-butadiene is not only a benefit to environmental concerns, but also to the economics of the entire C4 processes.However, the available catalysts for selective hydrogenation of a large portion of vinyl acetylene are still very limited [1,3].Therefore, we attempted to test the catalysts for hydrogenation of vinyl acetylene in mixed C4 from a commercial butadiene extraction process.In this study we show the results from using alumina-supporting Pd-Cu catalysts and compare them to monometallic Pd catalyst.
Supporting Pd catalysts have been studied in selective hydrogenation of alkyne in both gas and liquid phases [4][5][6][7][8].In particular, Pd supported on alumina (Pd/Al 2 O 3 ) is utilized as a catalyst in several commercial hydrogenation processes [9][10][11].Furthermore, Pd/Al 2 O 3 can be promoted or doped by other transition metals (e.g., Cu, Ag, Pb) to improve the catalytic performance.Pachulski et al. [12] investigated Ag-promoted Pd/Al 2 O 3 catalysts prepared by the incipient wetness impregnation technique on gas phase acetylene hydrogenation and observed that, by adding Ag to Pd/Al 2 O 3 , the catalytic activity was slightly reduced, but ethylene selectivity and catalytic stability are significantly improved.Kim et al. [13] studied the effect of adding Cu and Ag to Pd/Al 2 O 3 by using a surface redox and impregnation on selective hydrogenation of acetylene.They reported that, if the catalysts are prepared by the surface redox method, Cu-promoting catalysts provided the highest ethylene selectivity while Ag promoting catalysts showed the highest ethylene selectivity if the catalysts are prepared by impregnation method.Nevertheless, catalyst promoted by either Cu and Ag showed significantly drop in their activity.Lederhos et al. [14] also reported that the Pd-W and W-Pd bimetallic catalysts can greatly improve both selectivity and conversion in the liquid phase hydrogenation of 1-heptyne.
Cu was selected to promote the Pd catalyst for vinyl acetylene hydrogenation because Cu was reported to be active and selective in the hydrogenation of alkyne hydrogenation.For example, Koeppel et al. [15] investigated the hydrogenation of unsaturated C4 including vinyl acetylene, 1-butyne and 1,3-butadiene by Cu on silica catalysts.Their results showed that the catalysts can hydrogenate only alkynes but not alkenes.Setiawan et al. [16] also reported the removal of alkynes contaminants from industrial C4 stream by using Cu on silica catalysts.Wehrli et al. [17,18] studied the hydrogenation of propyne by supported Cu catalysts and they found that the supported Cu catalysts have the high selectivity of propyne hydrogenation toward propene.Friedrich et al. [19] studied partial acetylene hydrogenation by 60:40 atomic ratio of Cu:Pd catalyst.They reported that the Cu 60 Pd 40 catalysts provide higher ethylene selectivity with less deactivation than the Pd catalyst.Furthermore, Cai et al. [20] studied the denitrification reaction of nitrate by Pd-Cu alloy nanocatalysts on alumina under a hydrogen atmosphere.It was found that adding Cu increases nitrate conversion compared to Pd monocatalyst and the Pd:Cu 50:50 provides the highest nitrate conversion.
In this work, the Pd and Pd-Cu catalysts are prepared by conventional incipient wetness impregnation method and tested for catalytic hydrogenation of vinyl acetylene in the mixed C4.The reaction conditions, including temperature and pressure of hydrogen, are set in the safety region and the reaction is conducted in liquid hexane.The catalytic performances including rate of vinyl acetylene conversion and 1,3-butadiene selectivity are then obtained.Furthermore, the catalysts are also characterized by several techniques, including X-ray photoelectron spectroscopy (XPS), temperature programed reduction (TPR), temperature programed oxidation (TPO), and H 2 -chemisorption.

Results and Discussion
Figure 1 shows the rate of vinyl acetylene, 1,3-butadiene selectivity and yield of Pd-Cu catalysts compared to those of the Pd catalyst.As shown in Figure 1A, Pd-Cu catalysts have a relative constant rate of vinyl acetylene conversion until vinyl acetylene conversion is around 80%, beyond 80% conversion, the rate is significantly decreased similar to the case of using Pd/Al 2 O 3 catalyst.Comparing to Pd, Pd:Cu 1:1 and 3:2 catalysts exhibited the higher rate of hydrogenation while Pd:Cu 1:4, 1:2, and 2:1 exhibited the lower rate.To observe the catalysts' performance, we compare the rate of vinyl acetylene conversion of each prepared catalyst at the 70% conversion where the yield of 1,3-butadiene is the maximum (Figure 1C).The rate of vinyl acetylene conversion for Pd:Cu 1:4, 1:2, 1:1, 3:2, 2:1, and Pd catalysts are 0.032, 0.039, 0.048, 0.052, 0.040, and 0.047 (mole vinyl acetylene per gram catalyst per hour), respectively, as illustrated in Figure 1D.Obviously, a proper amount of added Cu to Pd/Al 2 O 3 catalyst improves the rate of vinyl acetylene conversion.It is interesting to note that the Pd:Cu ratio of 3:2 shows the highest reaction rate which corresponds to the highest hydrogen adsorption per Pd as obtained from hydrogen chemisorption experiments.For comparison purposes, the Cu/Al 2 O 3 catalyst was also tested and the hydrogenation did not take place.
The turn-over frequency (TOF) number of vinyl acetylene on Pd is calculated based on the exposed Pd obtained from chemisorption.As shown in Figure 2, 1:4, 1:2, and 1:1 Pd:Cu catalysts exhibit TOF around 4 s −1 which is higher than TOF number of Pd:Cu at ratios 3:2, 2:1, and Pd catalysts (TOF around 2.5 s −1 ).This suggests that Pd with more neighboring Cu is more active, possibly due to the electronic modification as hinted by the results from XPS.
In Figure 1B, the selectivity of 1,3-butadiene is gradually decreased with increasing vinyl acetylene conversion, however, the 1,3-butadiene selectivity is significantly dropped when the vinyl acetylene conversion is greater than 80%, due to further hydrogenation of 1,3-butadiene to butenes as shown in Figure S1 and S2.Interestingly, the amount of added copper does not have significant effect on 1,3-butadiene selectivity, as shown in Figure 1E.
As illustrated in Figure 1C, the yield of 1,3-butadiene for each catalyst is increased with an increase of the conversion and reached the maximum at around 70%-80% conversion.The yield is dramatically dropped when the conversion is beyond 80%.Figure 1F shows the yield of 1,3-butadiene at 70% conversion of vinyl acetylene for different Pd:Cu atomic ratio catalysts.It can be observed that adding Cu has no effect on 1,3-butadiene yield for the tested catalyst.
Adding Cu to Pd/Al 2 O 3 catalyst can cause both positive and negative effects on the rate of vinyl acetylene hydrogenation.For the Pd:Cu ratio of 1:4 and 1:2, adding Cu to the Pd/Al 2 O 3 catalyst significantly reduces the rate of vinyl acetylene hydrogenation per gram of catalyst, but the selectivity toward 1,3-butadiene is still comparable to the Pd/Al 2 O 3 catalyst at 70% conversion.However, as the ratio of Pd to Cu increases, until the Pd:Cu ratio of 1:1 and 3:2, the rate of vinyl acetylene hydrogenation is higher than that of monometallic Pd catalyst.However, if the Pd:Cu ratio is increased to 2:1, the rate of vinyl acetylene hydrogenation is significantly dropped, as shown in Figure 1D.at 70% conversion of vinyl acetylene for different Pd:Cu atomic ratio catalysts.It can be observed that adding Cu has no effect on 1,3-butadiene yield for the tested catalyst.
Adding Cu to Pd/Al2O3 catalyst can cause both positive and negative effects on the rate of vinyl acetylene hydrogenation.For the Pd:Cu ratio of 1:4 and 1:2, adding Cu to the Pd/Al2O3 catalyst significantly reduces the rate of vinyl acetylene hydrogenation per gram of catalyst, but the selectivity toward 1,3-butadiene is still comparable to the Pd/Al2O3 catalyst at 70% conversion.However, as the ratio of Pd to Cu increases, until the Pd:Cu ratio of 1:1 and 3:2, the rate of vinyl acetylene hydrogenation is higher than that of monometallic Pd catalyst.However, if the Pd:Cu ratio is increased to 2:1, the rate of vinyl acetylene hydrogenation is significantly dropped, as shown in Figure 1D.The Pd dispersion based on the results of hydrogen chemisorption on the studied catalysts are summarized in Table 1.The Pd/Al2O3 catalyst has 19% dispersion while the Pd-Cu/Al2O3 catalysts with Pd to Cu ratio less than 1 show significantly lower Pd dispersion.However, for Pd-Cu/Al2O3 catalysts with the Pd:Cu ratio higher than 1, the catalyst shows the metal dispersion at the similar level of the Pd/Al2O3 catalyst.Moreover, only Pd:Cu 3:2 catalyst provides about 27% dispersion which is higher than the monometallic Pd catalyst.The results of temperature program reduction (TPR) of calcined catalysts are illustrated in Figure 3.For the calcined copper on alumina (calcined Cu/Al2O3), a broad positive peak which is maximum at 261 °C indicates the reduction of CuO by hydrogen.Interestingly, the TPR of calcined Pd/Al2O3 has a negative peak at 88 °C possibly due to hydrogen generating from palladium hydride decomposition [21,22].For the Pd-Cu/Al2O3 catalyst with a high Pd:Cu ratio (Pd:Cu 2:1, 3:2, and 1:1), TPR of these catalysts shows both negative and positive peaks of Pd at around 90 °C, while the lower Pd:Cu ratio catalysts (Pd:Cu 1:2 and 1:4) show only positive TPR peaks.It is interesting to note that the negative peak at low temperature is smaller as the amount of Cu is increased in the prepared catalysts suggesting that less Pd hydride is formed.Moreover, the TPR peaks of PdO in calcined Pd-Cu/Al2O3 are shifted to a higher temperature as the amount of Cu is increased.Furthermore, the reduction peaks of copper in the calcined Pd-Cu/Al2O3 are at a lower temperature than the reduction peak in the calcined Cu/Al2O3.These indicate the possibility of Pd-Cu interaction in the calcined Pd-Cu/Al2O3 catalysts as suggested in the work done by Molenbroek et al. [23].The Pd dispersion based on the results of hydrogen chemisorption on the studied catalysts are summarized in Table 1.The Pd/Al 2 O 3 catalyst has 19% dispersion while the Pd-Cu/Al 2 O 3 catalysts with Pd to Cu ratio less than 1 show significantly lower Pd dispersion.However, for Pd-Cu/Al 2 O 3 catalysts with the Pd:Cu ratio higher than 1, the catalyst shows the metal dispersion at the similar level of the Pd/Al 2 O 3 catalyst.Moreover, only Pd:Cu 3:2 catalyst provides about 27% dispersion which is higher than the monometallic Pd catalyst.The results of temperature program reduction (TPR) of calcined catalysts are illustrated in Figure 3.For the calcined copper on alumina (calcined Cu/Al 2 O 3 ), a broad positive peak which is maximum at 261 • C indicates the reduction of CuO by hydrogen.Interestingly, the TPR of calcined Pd/Al 2 O 3 has a negative peak at 88 • C possibly due to hydrogen generating from palladium hydride decomposition [21,22].For the Pd-Cu/Al 2 O 3 catalyst with a high Pd:Cu ratio (Pd:Cu 2:1, 3:2, and 1:1), TPR of these catalysts shows both negative and positive peaks of Pd at around 90 • C, while the lower Pd:Cu ratio catalysts (Pd:Cu 1:2 and 1:4) show only positive TPR peaks.It is interesting to note that the negative peak at low temperature is smaller as the amount of Cu is increased in the prepared catalysts suggesting that less Pd hydride is formed.Moreover, the TPR peaks of PdO in calcined Pd-Cu/Al 2 O 3 are shifted to a higher temperature as the amount of Cu is increased.Furthermore, the reduction peaks of copper in the calcined Pd-Cu/Al 2 O 3 are at a lower temperature than the reduction peak in the calcined Cu/Al 2 O 3 .These indicate the possibility of Pd-Cu interaction in the calcined Pd-Cu/Al 2 O 3 catalysts as suggested in the work done by Molenbroek et al. [23].From the TPR results, Pd on all catalysts should be completely reduced by hydrogen at 300 °C, however, at this condition, copper may not be completely reduced to metallic Cu, particularly copper in the copper-rich catalysts (1:4, 1:2, and 1:1 Pd:Cu) may not be completely reduced.The XPS results, as shown in Figures 4 and 5, and summarized in Table 2, also suggest a similar observation.In Figure 4, the Pd 3d binding energy of reduced Pd/Al2O3 catalyst is observed at 334.2 eV with the full width at half maximum (FWHM) of 1.97, while the calcined catalyst (PdO/Al2O3) spectra shows at 336.4 eV with 1.23 FWHM.This confirmed that Pd from reduced Pd/Al2O3 and from PdO/Al2O3 are Pd(0) and Pd(II), respectively.The reduced Pd-Cu/Al2O3 catalysts show the Pd 3d spectra around 334.5-334.6 eV which is 0.3-0.4eV higher than the reduced Pd/Al2O3 catalyst.This suggests that Pd may be electronically modified when Cu is added.In addition, Pd-Cu catalysts may contain some Pd+ species as suggested by the broader Pd 3d FWHM of Pd-Cu catalysts which is around 1.98-2.43eV.The Pd may be oxidized possibly due to the brief exposure to air during sample transfer to XPS.As shown in Figure 5, the Cu 2p3/2 is observed around 932.7 eV for the all the prepared catalysts containing copper.In addition, weak Cu 2p3/2 satellite peak are also observed at around 10 eV higher (B.E.about 940-945 eV) for the CuO/Al2O3, and copper-rich sample (1:4, 1:2, and 1:1 Pd:Cu on alumina catalyst) suggesting the presence of Cu 2+ [24].Moreover, the satellite peak is also observed in reduced Cu/Al2O3 indicating that the sample is not completely reduced, possibly due to the reduction condition and the oxidation by air when transferring the samples to XPS.For the sample with low copper loading (3:2 and 2:1 Pd:Cu on alumina catalyst), the satellite peak is not clearly observed probably due to the low copper loading.In addition, the Pd/Cu intensity ratios of bimetallic catalysts From the TPR results, Pd on all catalysts should be completely reduced by hydrogen at 300 • C, however, at this condition, copper may not be completely reduced to metallic Cu, particularly copper in the copper-rich catalysts (1:4, 1:2, and 1:1 Pd:Cu) may not be completely reduced.The XPS results, as shown in Figures 4 and 5, and summarized in Table 2, also suggest a similar observation.In Figure 4, the Pd 3d binding energy of reduced Pd/Al 2 O 3 catalyst is observed at 334.2 eV with the full width at half maximum (FWHM) of 1.97, while the calcined catalyst (PdO/Al 2 O 3 ) spectra shows at 336.4 eV with 1.23 FWHM.This confirmed that Pd from reduced Pd/Al 2 O 3 and from PdO/Al 2 O 3 are Pd(0) and Pd(II), respectively.The reduced Pd-Cu/Al 2 O 3 catalysts show the Pd 3d spectra around 334.5-334.6 eV which is 0.3-0.4eV higher than the reduced Pd/Al 2 O 3 catalyst.This suggests that Pd may be electronically modified when Cu is added.In addition, Pd-Cu catalysts may contain some Pd+ species as suggested by the broader Pd 3d FWHM of Pd-Cu catalysts which is around 1.98-2.43eV.The Pd may be oxidized possibly due to the brief exposure to air during sample transfer to XPS.As shown in Figure 5, the Cu 2p 3/2 is observed around 932.7 eV for the all the prepared catalysts containing copper.In addition, weak Cu 2p 3/2 satellite peak are also observed at around 10 eV higher (B.E.about 940-945 eV) for the CuO/Al 2 O 3 , and copper-rich sample (1:4, 1:2, and 1:1 Pd:Cu on alumina catalyst) suggesting the presence of Cu 2+ [24].Moreover, the satellite peak is also observed in reduced Cu/Al 2 O 3 indicating that the sample is not completely reduced, possibly due to the reduction condition and the oxidation by air when transferring the samples to XPS.For the sample with low copper loading (3:2 and 2:1 Pd:Cu on alumina catalyst), the satellite peak is not clearly observed probably due to the low copper loading.In addition, the Pd/Cu intensity ratios of bimetallic catalysts were 10.04, 6.90, 5.15, 3.20, and 1.80 for Pd:Cu 2:1, 3:2, 1:1, 1:2, and 1:4, respectively, caused by the dilution of Pd by Cu.The addition of Cu to Pd/Al 2 O 3 catalyst leads to both electronic and geometric modifications [25,26].For geometric effect, Cu may induce Pd to be more exposed at the metal surface, particularly at Pd:Cu 3:2, which also has the highest Pd dispersion.
The Pd:Cu 3:2 catalyst was selected to study the effect of prolonged reaction time by refilling the system with concentrated vinyl acetylene mixed C4 without catalyst replacement for six cycles.Each reaction cycle proceeded until vinyl acetylene was completely used up, which takes about 5-7 h.The reaction rate and 1,3-butadiene selectivity as the function of reaction cycles are illustrated in Figures 6 and 7, respectively.As observed in Figure 6, the rate of reaction in case of using Pd:Cu 3:2 catalyst is decreased until four cycles, and then it is relative stable.On the other hand, Pd/Al 2 O 3 catalyst indicates the continuously decreasing reaction rate along six cycles.Interestingly, catalyst Pd/Al 2 O 3 provides higher 1,3-butadiene selectivity than Pd:Cu 3:2 catalyst as shown in Figure 7 (except cycle 6).The Pd:Cu 3:2 catalyst has potential to improve the catalyst stability for the hydrogenation of vinyl acetylene.The addition of Cu to Pd/Al2O3 catalyst leads to both electronic and geometric modifications [25,26].For geometric effect, Cu may induce Pd to be more exposed at the metal surface, particularly at Pd:Cu 3:2, which also has the highest Pd dispersion.
The Pd:Cu 3:2 catalyst was selected to study the effect of prolonged reaction time by refilling the system with concentrated vinyl acetylene mixed C4 without catalyst replacement for six cycles.Each reaction cycle proceeded until vinyl acetylene was completely used up, which takes about 5-7 h.The reaction rate and 1,3-butadiene selectivity as the function of reaction cycles are illustrated in Figures 6 and 7, respectively.As observed in Figure 6, the rate of reaction in case of using Pd:Cu 3:2 catalyst is decreased until four cycles, and then it is relative stable.On the other hand, Pd/Al2O3 catalyst indicates the continuously decreasing reaction rate along six cycles.Interestingly, catalyst Pd/Al2O3 provides higher 1,3-butadiene selectivity than Pd:Cu 3:2 catalyst as shown in Figure 7 (except cycle 6).The Pd:Cu 3:2 catalyst has potential to improve the catalyst stability for the hydrogenation of vinyl acetylene.The addition of Cu to Pd/Al2O3 catalyst leads to both electronic and geometric modifications [25,26].For geometric effect, Cu may induce Pd to be more exposed at the metal surface, particularly at Pd:Cu 3:2, which also has the highest Pd dispersion.
The Pd:Cu 3:2 catalyst was selected to study the effect of prolonged reaction time by refilling the system with concentrated vinyl acetylene mixed C4 without catalyst replacement for six cycles.Each reaction cycle proceeded until vinyl acetylene was completely used up, which takes about 5-7 h.The reaction rate and 1,3-butadiene selectivity as the function of reaction cycles are illustrated in Figures 6 and 7, respectively.As observed in Figure 6, the rate of reaction in case of using Pd:Cu 3:2 catalyst is decreased until four cycles, and then it is relative stable.On the other hand, Pd/Al2O3 catalyst indicates the continuously decreasing reaction rate along six cycles.Interestingly, catalyst Pd/Al2O3 provides higher 1,3-butadiene selectivity than Pd:Cu 3:2 catalyst as shown in Figure 7 (except cycle 6).The Pd:Cu 3:2 catalyst has potential to improve the catalyst stability for the hydrogenation of vinyl acetylene.The TPO was used to characterize the deposited carbon on spent catalysts.The TPO profiles of spent catalysts are shown in Figure 8.The used Pd catalyst shows a broad TPO peak around 200 to 600 • C while the used Pd-Cu catalyst containing highest Cu (at Pd:Cu = 1:4) shows between 120 and 470 • C. The order of peak boarding are Pd/Al 2 O 3 > Pd:Cu 2:1 > Pd:Cu 3:2 > Pd:Cu 1:1 > Pd:Cu 1:2.It can be inferred that a higher Cu content reflects a lower TPO temperature.
Carbon content of spent catalysts is calculated and shown in Table 3  The TPO was used to characterize the deposited carbon on spent catalysts.The TPO profiles of spent catalysts are shown in Figure 8.The used Pd catalyst shows a broad TPO peak around 200 to 600 °C while the used Pd-Cu catalyst containing highest Cu (at Pd:Cu = 1:4) shows between 120 and 470 °C.The order of peak boarding are Pd/Al2O3 > Pd:Cu 2:1 > Pd:Cu 3:2 > Pd:Cu 1:1 > Pd:Cu 1:2.It can be inferred that a higher Cu content reflects a lower TPO temperature.
Carbon content of spent catalysts is calculated and shown in Table 3.Among one reaction cycle, the carbon content of Pd-Cu/Al2O3 catalysts is not significantly lower than of Pd/Al2O3 catalyst.The Pd/Al2O3 catalyst has carbon content 2.12 wt % while Pd-Cu/Al2O3 catalysts have carbon content in the same magnitude, 1.79-2.62wt %.On the other hand, among spent catalyst with six reaction cycles, Pd:Cu 3:2 catalyst produces carbon to 3.19 wt % while Pd/Al2O3 catalyst shows carbon content 5.96 wt %.Therefore Pd-Cu/Al2O3 catalyst can inhibit the polymerization of polyolefins in this system and should promote the longer catalyst life.

Materials and Methods
The concentrated vinyl acetylene mixed C4 was obtained from Bangkok Synthetics Co., Ltd (Rayong, Thailand).Analytical reagent grade (AR grade) hexane was purchased from Fisher Scientific Company LLC.(Pittsburgh, PA, USA).The pelletized aluminium oxide, palladium (II) nitrate dihydrate, and copper (II) nitrate trihydrate were purchased from Sigma-Aldrich Co. LLC.(St.

Materials and Methods
The concentrated vinyl acetylene mixed C4 was obtained from Bangkok Synthetics Co., Ltd (Rayong, Thailand).Analytical reagent grade (AR grade) hexane was purchased from Fisher Scientific Company LLC.(Pittsburgh, PA, USA).The pelletized aluminium oxide, palladium (II) nitrate dihydrate,

Figure 2 .
Figure 2. Turn-over frequency of Pd and Pd-Cu catalysts.

1
Catalysts with Pd:Cu atomic ratio

Figure 2 .
Figure 2. Turn-over frequency of Pd and Pd-Cu catalysts.

Figure 4 .
Figure 4. X-ray photoelectron spectra of the Pd 3d of each catalyst.

Figure 5 .
Figure 5. X-ray photoelectron spectra of the Cu 2p of each catalyst.

Figure 4 .
Figure 4. X-ray photoelectron spectra of the Pd 3d of each catalyst.

Figure 4 .
Figure 4. X-ray photoelectron spectra of the Pd 3d of each catalyst.

Figure 5 .
Figure 5. X-ray photoelectron spectra of the Cu 2p of each catalyst.Figure 5. X-ray photoelectron spectra of the Cu 2p of each catalyst.

Figure 5 .
Figure 5. X-ray photoelectron spectra of the Cu 2p of each catalyst.Figure 5. X-ray photoelectron spectra of the Cu 2p of each catalyst.

Figure 6 .
Figure 6.The vinyl acetylene reaction rate of Pd/Al2O3 and Pd:Cu 3:2 on Al2O3 catalysts as the function of reaction cycles.

2 Figure 6 .
Figure 6.The vinyl acetylene reaction rate of Pd/Al 2 O 3 and Pd:Cu 3:2 on Al 2 O 3 catalysts as the function of reaction cycles.

Figure 6 .
Figure 6.The vinyl acetylene reaction rate of Pd/Al2O3 and Pd:Cu 3:2 on Al2O3 catalysts as the function of reaction cycles.

2 Figure 7 .
Figure 7.The 1,3-butadiene selectivity of Pd/Al 2 O 3 and Pd:Cu 3:2 on Al 2 O 3 catalysts as a function of the reaction cycles.

Table 1 .
Measured Pd and Cu contents and Pd dispersion of catalysts.

Table 1 .
Measured Pd and Cu contents and Pd dispersion of catalysts.

Table 2 .
The measured binding energy (eV), full width at half maximum (FWHM), and intensity ratios of Pd, Cu, and Pd-Cu on Al 2 O 3 .

Table 2 .
The measured binding energy (eV), full width at half maximum (FWHM), and intensity ratios of Pd, Cu, and Pd-Cu on Al2O3.

Table 2 .
The measured binding energy (eV), full width at half maximum (FWHM), and intensity ratios of Pd, Cu, and Pd-Cu on Al2O3.
. Among one reaction cycle, the carbon content of Pd-Cu/Al 2 O 3 catalysts is not significantly lower than of Pd/Al 2 O 3 catalyst.The Pd/Al 2 O 3 catalyst has carbon content 2.12 wt % while Pd-Cu/Al 2 O 3 catalysts have carbon content in the same magnitude, 1.79-2.62wt %.On the other hand, among spent catalyst with six reaction cycles, Pd:Cu 3:2 catalyst produces carbon to 3.19 wt % while Pd/Al 2 O 3 catalyst shows carbon content 5.96 wt %.Therefore Pd-Cu/Al 2 O 3 catalyst can inhibit the polymerization of polyolefins in this system and should promote the longer catalyst life.

Table 3 .
Deposited carbon on spent catalysts performed reaction tests of one and six cycles.

Table 1 .
Deposited carbon on spent catalysts performed reaction tests of one and six cycles.