*6.1. Catalyst Characteristics*

In this section, the results of SEM, TEM, DTG-TGA, XRD, and BET of fresh and spent catalysts, are presented. It is mentioned that a used catalyst in the plant is named spent catalyst. As mentioned, to investigate the surface morphology, the BET analysis is performed on the fresh and spent catalysts. Table 4 shows the BET results of fresh and spent catalysts. The obtained results reveal that BET surface area of fresh and spent catalysts are 24.75, and 30.11 m2g −1 , respectively. In addition, the mean pore diameter of fresh and deactivated catalysts are 235.5, and 191.2 Å, respectively. It concludes that, from BET analysis, there is coke build-up on the internal pores and pore blockage by coke decrease mean pore diameter. In addition, coke build-up on the external surface of the catalyst and increases surface area. Figure 2 shows the results of nitrogen adsorption and desorption on the fresh catalyst. *Processes* **2019**, *7*, x FOR PEER REVIEW 9 of 22 **Table 4.** The BET results of fresh and spent catalysts.  **Fresh Spent**  BET surface area (m² g-1) 24.75 30.11


**Table 4.** The BET results of fresh and spent catalysts. Langmuir surface area (m² g-1) 34.13 41.86

**Figure 2.** The BET results of the fresh catalyst. **Figure 2.** The BET results of the fresh catalyst.

Typically, the XRD analysis is used to identify the crystalline morphology and dimensions of support. Figure 3 shows the XRD results of the fresh catalyst. The broad peak means poor crystalline morphology and the sharp ones indicate a well-crystallized sample. Based on the XRD analysis, the peak is at 32.75°, which proves the presence of Ag2O particles on the support surface, while peaks at 36.7°, 63.98°, and 67.46° show Ag conversion to AgO. In addition, the peaks at 38.9° and 66.2° show It appears that increasing pressure increases the nitrogen adsorption on the catalyst surface and adsorption pattern, in accordance with the Isotherm Type III. This could be applied on systems in which the interaction between adsorbate molecules is stronger than that between adsorbate and adsorbent. Based on the Isotherm Type III, the uptake of gas molecules is initially slow, and until surface coverage is sufficient, so that the interactions between adsorbed and free molecules begins to dominate the process.

the dispersion of Pd on the catalyst surface. Based on these XRD results, the Al2O3 mean crystal size

Typically, the XRD analysis is used to identify the crystalline morphology and dimensions of support. Figure 3 shows the XRD results of the fresh catalyst. The broad peak means poor crystalline morphology and the sharp ones indicate a well-crystallized sample. Based on the XRD analysis, the peak is at 32.75◦ , which proves the presence of Ag2O particles on the support surface, while peaks at 36.7◦ , 63.98◦ , and 67.46◦ show Ag conversion to AgO. In addition, the peaks at 38.9◦ and 66.2◦ show the dispersion of Pd on the catalyst surface. Based on these XRD results, the Al2O<sup>3</sup> mean crystal size is 24.5 nm. *Processes* **2019**, *7*, x FOR PEER REVIEW 10 of 22 *Processes* **2019**, *7*, x FOR PEER REVIEW 10 of 22

**Figure 3.** The XRD results of the fresh catalyst. **Figure 3.** The XRD results of the fresh catalyst.

**F**igure 4 shows the SEM images of fresh and spent catalysts. The results of SEM images reveal that the bright trace of palladium metal, in fresh catalyst, changes in the dark in the spent catalyst. The darkening of catalyst proves the formation of polymeric compounds and coke build-up on the catalyst surface, which reduces the activity of the catalysts especially. Indeed the surface of the fresh catalyst is completely covered by coke. It concludes from SEM and BET tests that coke build-up on the external surface of the catalyst increases the surface area. Figure 4 shows the SEM images of fresh and spent catalysts. The results of SEM images reveal that the bright trace of palladium metal, in fresh catalyst, changes in the dark in the spent catalyst. The darkening of catalyst proves the formation of polymeric compounds and coke build-up on the catalyst surface, which reduces the activity of the catalysts especially. Indeed the surface of the fresh catalyst is completely covered by coke. It concludes from SEM and BET tests that coke build-up on the external surface of the catalyst increases the surface area. **Figure 3.** The XRD results of the fresh catalyst. **F**igure 4 shows the SEM images of fresh and spent catalysts. The results of SEM images reveal that the bright trace of palladium metal, in fresh catalyst, changes in the dark in the spent catalyst. The darkening of catalyst proves the formation of polymeric compounds and coke build-up on the

**Figure 4.** SEM images of fresh catalysts and spent catalysts. (a) Fresh catalyst, (b) Spent catalyst. **Figure 4.** SEM images of fresh catalysts and spent catalysts. (a) Fresh catalyst, (b) Spent catalyst. **Figure 4.** SEM images of fresh catalysts and spent catalysts. (**a**) Fresh catalyst, (**b**) Spent catalyst.

In addition, Figure 5a,b presents TEM image and particle size distribution of fresh and spent catalysts. The results show that palladium particles experience an agglomeration during the reaction run time and mean particle size (c), (d) approaches from 6.2 nm to 11.5 nm. Increasing size of In addition, Figure 5a,b presents TEM image and particle size distribution of fresh and spent catalysts. The results show that palladium particles experience an agglomeration during the reaction In addition, Figure 5a,b presents TEM image and particle size distribution of fresh and spent catalysts. The results show that palladium particles experience an agglomeration during the reaction

palladium particles, during the run-time, reduces the active sites and results in lower catalyst activity.

run time and mean particle size (c), (d) approaches from 6.2 nm to 11.5 nm. Increasing size of palladium particles, during the run-time, reduces the active sites and results in lower catalyst activity.

run time and mean particle size (c), (d) approaches from 6.2 nm to 11.5 nm. Increasing size of palladium particles, during the run-time, reduces the active sites and results in lower catalyst activity. *Processes* **2019**, *7*, x FOR PEER REVIEW 11 of 22

**Figure 5.** (a,b) TEM images of fresh and spent catalysts, (c,d), particle size distribution of fresh and spent catalysts. **Figure 5.** (**a**,**b**) TEM images of fresh and spent catalysts, (**c**,**d**), particle size distribution of fresh and spent catalysts.

Figure 6a,b shows the TGA and DTG results of fresh, spent and regenerated catalysts. Generally, the fresh and regenerated catalysts do not experience weight loss during the TGA test. However, the oxidation of Pd and Ag atoms to PdO and AgO, increases catalyst weight by about 0.6 %. The TGA results of deactivated catalysts shows that, increasing the temperature up to 500 °C decreases sample weight gradually and after that, catalysts do not experience weight loss. Typically, coke burning during the TGA analysis is the main reason for the decreased catalyst weight. In addition, it is concluded that the coke is completely burned through catalyst heating up to 500 °C. The two minimum points at 310 and 515 °C on the DTG curve of spent catalyst proves the presence of two different coke types on the catalyst surface. The produced amorphous coke on the external surface of catalyst burns in temperature range of 300 to 400 °C, while the crystalline coke and produced coke in the pores burn in range of 450 to 650 °C. Figure 6a,b shows the TGA and DTG results of fresh, spent and regenerated catalysts. Generally, the fresh and regenerated catalysts do not experience weight loss during the TGA test. However, the oxidation of Pd and Ag atoms to PdO and AgO, increases catalyst weight by about 0.6%. The TGA results of deactivated catalysts shows that, increasing the temperature up to 500 ◦C decreases sampleweight gradually and after that, catalysts do not experience weight loss. Typically, coke burning during the TGA analysis is the main reason for the decreased catalyst weight. In addition, it is concludedthat the coke is completely burned through catalyst heating up to 500 ◦C. The two minimum points at310 and 515 ◦C on the DTG curve of spent catalyst proves the presence of two different coke typeson the catalyst surface. The produced amorphous coke on the external surface of catalyst burns intemperature range of 300 to 400 ◦C, while the crystalline coke and produced coke in the pores burn inrange of 450 to 650 ◦C.

*Processes* **2019**, *7*, x FOR PEER REVIEW 12 of 22

**Figure 6.** (a) TGA results of different catalysts and (b) DTG results of coked catalysts. **Figure 6.** (**a**) TGA results of different catalysts and (**b**) DTG results of coked catalysts.

#### *6.2. Results of Kinetic Model 6.2. Results of Kinetic Model*

As previously mentioned, 216 experiments have been designed to find the effect of parameters on the acetylene conversion and product distribution. The list of experiments and results have been tabulated in Supplementary Data Set 1. In this section, the effect of GHSV, temperature, pressure, and hydrogen to acetylene ratio on acetylene conversion, ethylene selectivity, and product distribution is presented. As previously mentioned, 216 experiments have been designed to find the effect of parameters on the acetylene conversion and product distribution. The list of experiments and results have been tabulated in Supplementary Data Set 1. In this section, the effect of GHSV, temperature, pressure, and hydrogen to acetylene ratio on acetylene conversion, ethylene selectivity, and product distribution is presented.

#### 6.2.1. Effect of GHSV 6.2.1. Effect of GHSV

Figure 7a,b shows the effect of gas hourly space velocity on acetylene conversion, ethylene selectivity, and product distribution. The GHSV is the ratio of gas flow rate in standard condition to Figure 7a,b shows the effect of gas hourly space velocity on acetylene conversion, ethylene selectivity, and product distribution. The GHSV is the ratio of gas flow rate in standard condition

the volume of catalyst in the bed. Although increasing GHSV reduces residence time in the reactor,

6.2.2. Effect of Pressure

leaving unreacted acetylene from the reactor.

to the volume of catalyst in the bed. Although increasing GHSV reduces residence time in the reactor, it decreases mass transfer resistance in the bed. The experiments show that increasing GHSV results in higher ethylene selectivity and lower acetylene conversion. Although butene group and 1,3-butadiene could be detected in the outlet stream from the reactor, 1-butene is the dominant side product. It appears that GHSV has a considerable effect on the 1-butene formation and increasing GHSV from 2500 to 6200 decreases 1-butene mole fraction from 0.02 to 0.002. It is concluded that increasing the GHSV led to a reduction in residence time and consequently enhances the risk of leaving unreacted acetylene from the reactor. *Processes* **2019**, *7*, x FOR PEER REVIEW 13 of 22 in higher ethylene selectivity and lower acetylene conversion. Although butene group and 1,3 butadiene could be detected in the outlet stream from the reactor, 1-butene is the dominant side product. It appears that GHSV has a considerable effect on the 1-butene formation and increasing GHSV from 2500 to 6200 decreases 1-butene mole fraction from 0.02 to 0.002. It is concluded that increasing the GHSV led to a reduction in residence time and consequently enhances the risk of

**Figure 7.** Effect of GHSV on (a) acetylene conversion and ethylene selectivity, and (b) product distribution at 15 bar, 35 °C and hydrogen to acetylene ratio 0.5. **Figure 7.** Effect of GHSV on (**a**) acetylene conversion and ethylene selectivity, and (**b**) product distribution at 15 bar, 35 ◦C and hydrogen to acetylene ratio 0.5.
