3.1. Specification of catalysts
XRD, FTIR spectroscopy, and N2 sorption isotherm were used to identify UiO-66 and UiO-66-vac catalysts. Figure S1 shows the XRD results for UiO-66 and UiO-66-vac where the synthesized MOFs are highly crystalline. They exhibited mainly three broad 2θ peaks at around 7, 9, and 26°. No additional peaks were observed and their pattern is according to XRD results in the literature . For FTIR spectrums of UiO-66 (Zr) and UiO-66-vac (Figure S2), carboxylate asymmetric and symmetric stretch in BDC appears as strong bands at 1589 and 1396 cm-1, the weak band at 1504 and 1658 cm-1 shows a double bond in the benzene ring. The Zr-O band is observed at wavenumber 744 cm-1 . UiO-66-vac presents a wider pore volume and higher specific surface area than UiO-66. The surface area of the synthesized UiO-66 (Zr) and UiO-66-vac are 553 and 982 m2/g respectively. The pore volumes for UiO-66 and Uio-66-vac are 0.213 and 0.322 cm3/ g-1, respectively. The BET and Barrett-Joyner-Halenda (BJH) data show that both catalysts are mesoporous. N2 adsorption-desorption isotherm of both catalysts is shown in Figure S3.
3.2. Isomerization catalysis and statistical analysis
The DOE based on the effective factors and CCD method for endo-DCPD to exo-DCPD conversion using UiO-66 and UiO-66-vac are shown in Table 1. Based on the results of the estimated yields, UiO-66-vac is more efficient as compared to UiO-66. The higher yield of exo-DCPD was obtained when UiO-66-vac was used as a catalyst. Isomerization of DCPD is an important step in JP-10 synthesis, which is related to time, temperature and catalyst. The software analyzes the data and provides several key plots.
The results of one-factor plots show that time is the most effective factor with a negative effect. A long reaction time leads to undesired side reactions such as oligomerization of exo-DCPD. Some products have more opportunities to combine and form different (cyclic) compounds . Mild changes in yield occur as the temperature or the weight percent of the catalyst increases. The effects of temperature and weight percent catalyst are negative and positive, respectively. Thermal oligomerization of raw material molecules at higher temperatures reduces the isomerization efficiency . Since similar behavior was observed for both UiO-66-vac and UiO-66, Figure S4 shows the effects of temperature, time, and Wt% catalyst only for UiO-66-vac.
Figure 1 shows contour diagrams for better investigation of the effects of the parameters of temperature, time, and Wt% catalyst (UiO-66-vac) on each other and the yield, which show simultaneous changes in the parameters. One variable in each of the contour diagrams is taken constantly. The study of the effect of reaction time in the presence of catalyst UiO-66-vac shows that the highest yields are achieved in the time range of 3.4 to 4.5 hours (Figures 1a and c). Increasing the amount of loaded catalyst can increase the total number of active sites of the catalyst, which provides better access for endo-DCPD to improve higher yield (Figure 1a). For lower temperatures, higher percentages of exo-DCPD product are obtained in the presence of the UiO-66-vac catalyst, which shows the optimal amount of catalyst (Figure 1b). In general, middle and lower levels of temperature and reaction time, respectively, along with a higher percentage of catalysts have the greatest impact on the isomerization of endo-DCPD to exo-DCPD. Figure S5 shows contour plots for UiO-66 catalyst in which the effect of the mentioned parameters on isomerization is similar to UiO-66-vac.
Table S1 gives the results from ANOVA, which validates the model and the selected factors. These parameters show the conversion of endo-DCPD to exo-form significantly. The significance of factors is shown by p-value because factors with a p-value less than 0.05 are significant . The assessment of the p-value and lack-of-fit test for the yield indicates the validity of the model in this study. All main factors in this model are significant, and the lack of fit is not significant. As shown in Table 2, values of coefficient of determination (R2) demonstrate a good correlation between experimental and predicted values.
3-3 Reactivity of UiO-66 and UiO-66-vac catalyst in the isomerization of endo-DCPD
As mentioned earlier, UiO-66-vac is more efficient in the isomerization of endo-DCPD as compared to UiO-66. Both catalysts can perform the isomerization at mild reaction conditions without solvent. They have higher efficiency in comparison to commercial catalysts such as zeolites  or other MOFs . As seen in Table 1, the best yield for exo-DCPD is obtained from run 7 under the conditions of temperature 116 ℃, time 3.4 h, and weight percent catalyst 8.4. Due to the presence of the missing BDC2- linkers in UiO-66-vac, the surface area of this catalyst is greater than UiO-66 which this agent can be effective in the isomerization process. Charge balance in this catalyst is provided by hydroxide anions that are bonded to µ3-OH groups in the parent UiO-66 and at the missing linkers. Zr atoms are ended to water molecules that are likely to develop the number of Brønsted acid sites in the UiO-66-vac [4, 21]. The significant activity of UiO-66-vac in isomerization endo-DCPD has arisen from the mentioned agents.
The effects of three factors on endo-DCPD conversion and exo-DCPD selectivity in the presence of UiO-66-vac are shown in Figure 2. An excellent conversion is observed for most reactions and the highest percentage of selectivity is shown for run 16, which is equal to 72. Thus, selectivity decreases with increasing temperature and time, which may be due to the in-situ consumption of exo-DCPD and its decomposition to CPD and thermal oligomerization of endo-DCPD .
Table 3 compares the results of previous works with this study for the kind and activity of catalysts on isomerization of endo-DCPD. It confirms a high percentage conversion of endo-DCPD to exo-DCPD in this study by the significant activity of UiO-66 and UiO-66-vac. Thus, both catalysts show excellent activity toward zeolites as well as MOFs with Fe and Cr metal.
The results of GC-MS show that in addition to exo-DCPD the mixture may contain small amounts of tricyclopentadiene, 1,3a,3b,4,6a,6bHexahydrocyclopenta[3,4]cyclobuta[1,2] cyclopentene and pentacyclo[5.3.0.02,5.03,9.04,8]decane as well as other cyclic and linear compounds like 9-methylidenetricyclo[126.96.36.199,5]decane and tetraisobutylene. Recycling the catalyst is favorable from the viewpoint of environmental and economic. Thus, the used catalysts are washed with carbon tetrachloride to eliminate the products from the catalyst surface. Fortunately, there is no loss of the yield with the reused catalyst after three times.