The Use of Uio-66 Metal–Organic Frameworks as a High-Performance Catalyst for the Isomerization Process of endo- to exo-Dicyclopentadiene in a Solvent-Free Condition

Exo-tetrahydrodicyclopentadiene (tricyclo[5.2.1.02,6]decane) or JP-10 is a synthetic jet fuel, which can be used as a high-density fuel in the air-breathing missile engineers and supersonic combustion ramjets. It can be produced from the fully saturated version of exo-dicyclopentadiene (exo-DCPD). This work introduces a new green method for isomerization of endo-dicyclopentadiene (endo-DCPD) to exo- form in the presence of metal–organic frameworks (MOFs). The synthesized UiO-66 and UiO-66-vac as two suitable MOF catalysts are used for the isomerization process in a solvent-free condition. The design of the experiment is used to optimize reaction conditions including time, temperature and catalyst loading. High yield and excellent conversion of endo- to exo-DCPD are obtained by reacting with UiO-66-vac at lower levels of time and temperature. Solvent-free, environmentally friendly, simple reaction process, reusable catalyst, and availability of materials are advantages of this process.


Introduction
Cyclic hydrocarbons can provide a much larger heat of combustion than similar non-cyclic compounds because they contain strain energy [1]. Moreover, synthetic jet fuels with bridged cyclic and multicyclic hydrocarbon structures can provide high strain energy, which are attractive candidates as jet fuels or liquid fuels in liquid bipropellants [2]. It is essential to use synthetic jet fuels with low values of melting point [3] and enthalpy of fusion [4] to prevent their freezing in cool weather. JP-10, a pure component: exo-tetrahydrodicyclopentadiene (tricyclo[5.2.1.0 2,6 ]decane), is a synthetic jet fuel, which was developed to use as a super fuel in the air-breathing missile engineers and supersonic combustion ramjets [5]. It is a high volumetric density fuel with many unique properties such as low and appropriate freezing point, thermal stability, and specific impulse [6]. Dicyclopentadiene (DCPD) and tetrahydrodicyclopentadiene (THDCP) are important initial reactants for producing JP-10 that have two endo-and exo-stereoisomers [7]. Exo-and endo-THDCP have the same chemical and physical properties but the freezing point endo-THDCPD (80 °C) is more than its exoisomer (− 79 °C). Thus, endo-THDCPD is not suitable to use as a liquid fuel. Since DCPD exists naturally in the form of endo-isomer, its isomerization to exo-form as well as optimizing reaction conditions and improving efficiency are essential and valuable [7].
It is important to improve the performance and strategies for isomerization of endo-to exo-DCPD. The use of solvents has some problems for environmental and human health. Moreover, the processes containing solvents are complicated for industrial usage because isomerization requires high pressure, high temperature, and long reaction time. The use of zeolites has some shortcomings such as easy deactivation, coke formation, and obstruction of microspores, which can decrease the yield [8]. The changes such as solvent-free reactions and mild conditions may be attractive for chemical industries. Bratlett and Goldstein [9] have introduced a two-step method in which iodo-dihydro-endo-DCPD was obtained by increasing HI to DCPD then isomerization is done to exo-form in the presence of alcoholic potassium hydroxide and eliminates HI. Bakke and Lundquist [10] presented the isomerization of endo-DCPD in the gas phase. They used Pt-containing silica-alumina as a suitable catalyst but their method did not show a good yield. Zhang and coworkers [11] have investigated endo-to exo-isomerization of DCPD using thermal treatment at optimum conditions for temperature (180 °C) and pressure (4 MPa). In another report, they performed the alteration of endo-DCPD to exoform using Al-grafted MCM-41 [12]. They found that the porous structure and high dosage of catalysts give better results at relatively low reaction temperatures with the addition of inert solvent. Commercial zeolites can be used as a catalyst for isomerization endo-DCPD in the liquid phase. Han et al. [13] found that acidic zeolites (Hβ) usefully promote the isomerization reaction with good activity. Kim and coworkers [14] indicated that MIL-100 (Fe, Cr) and MIL-101 (Fe, Cr) can be used as catalysts for the isomerization of endo-to exo-DCPD. Since the nature of the active metal center is effective in the conversion and selectivity of the reaction, the MIL-100 (Fe) showed the highest catalytic activity.
A metal-organic framework (MOF) is a coordination network with organic ligands containing potential voids [15]. Since MOFs are crystalline materials containing a vast area of effective properties, they can be introduced as heterogeneous catalysts in contrast to the other porous materials. They have some advantages such as tunable pores, high surface area, high porosity, good stability in different conditions, ease to functionalize, and flexible structure [16]. Due to these properties, MOFs have had wide applications for organic reactions in recent years, e.g. isomerization [17], drug delivery [18], hydrogenation/isomerization [19], condensation reactions [20], benzylation [21], and alkylation [22]. MOFs containing a well-ordered structure and a high surface area (till 5000 m 2 /g) are appropriate for isomerization reaction because they can provide a suitable pore size with a high surface area to solve the problem.
Zirconium (Zr) has three times more abundant in the earth's crust than copper and zinc [23]. UiO-66 and UiO-66-vac (Zr-cluster-based MOFs) have excellent thermal and chemical stability and acidity properties. Both catalysts are composed of Zr as a node and terephthalic acid as a linker that can be utilized as MOFs for isomerization. UiO-66-vac has missing linkers that result in vacancy sites, which are useful for catalytic activity [24]. The existence of strong bonding between Zr (IV) and carboxylate is an effective factor for the stability of UiO-66-vac. The high oxidation number of the Zr (IV) in comparison with MOFs based on a lower oxidation number is a key property, which provides high charge density and polarity [25].
To obtain a high yield and improve the selectivity of exo-DCPD, it is essential to select the optimal reaction conditions such as catalyst, temperature, and time. The design of experiments (DOE) is very efficient because it can adjust several factors and interactions that lead to process optimization [26]. The purpose of this work is to introduce UiO-66(Zr) and UiO-66(Zr) vac as favorable new heterogeneous catalysts for isomerization of endo-to exo-DCPD without using solvents. The optimized reaction conditions are specified to maximize conversion of endo-to exo-DCPD using a central composite design. Furthermore, catalysts regeneration is also studied.
The solvothermal method was used to prepare UiO-66 (Zr) and UiO-66-vac as follows [24,27]: (i) UiO-66: ZrCl4 and terephthalic acid were stirred in dimethylformamide until completely dissolved, and placed in an oven. Then, the solution was filtered and washed with DMF. (ii) UiO-66-vac: ZrCl4, HCl, and DMF were sonicated until all of the ZrCl4 was dissolved. Then, terephthalic acid was added and followed by an additional DMF. After heating, the resulting solid was then filtered and washed first with DMF and then with isopropanol.

Catalyst Characterization
The X-ray diffraction (XRD) patterns were recorded with AW-XDM300 X-ray Diffractometer at a step size of 0.05° with a Cu Ka radiation (λ = 1.5406 Å). Brunauer-Emmett-Teller (BET) surface area was analyzed by N 2 sorption/desorption using at 77 K with a Belsorp mini instrument and degassing at 200 °C under vacuum. The Fourier transform infrared (FT-IR) spectra were recorded on an Infralum-08 spectrophotometer in the range of 400-4000 cm −1 . Gas chromatography-mass spectrometry (GC-MS) was analyzed using Agilent 6890 by 30 m length packed HP5((5%-phenyl)-methylpolysiloxane) column and mass selective detector.

Catalytic Experiments and Analysis
The catalytic isomerization was performed based on the primary results of DOE. In an autoclave, 4 mL of endo-DCPD was added to the proper amount of catalysts without stirring and solvent. The reactions proceeded at a specified temperature and time. The autoclave was cooled slowly after completing the reaction. The catalysts were easily separated from the solution with filtration by increasing dichloromethane. Gas chromatography (GC) was used to analyze the conversion of endo-to exo-DCPD. GC analysis was done using Agilent 68 gas Chromatograph by 30 m packed HP5((5%phenyl)-methylpolysiloxane) column and flame-ionization detector (FID). The yield was computed based on the standard addition method.

DOE
One-factor-at-a-time (OFAT) was used to investigate the effect of different factors on a result wherein only one of the factors is altered at a time and others are kept fixed. The influences of the interactions between the variables cannot be considered here. It was tried to determine the optimized conditions for the catalytic isomerization of endo-to exo-DCPD with catalysts UiO-66 and UiO-66-vac using DOE (Design Expert 10 software) and quadratic model.

Design of Process
The central composite design (CCD) with three independent variables was chosen to evaluate the activity of the UiO-66 catalyst. Reaction time (h), reaction temperature (°C), and weight percentage catalyst (Wt% catalyst) were selected as process variables. Influential variables and their variation levels have been selected according to the values used in other references and some preliminary experiments [28]. The measured response is the yield of the exo-DCPD, which was obtained from the isomerization reaction. The models were computed by multiple linear regression (MLR) and analysis of variance (ANOVA).

Specification of Catalysts
XRD, FTIR spectroscopy, and N 2 sorption isotherm were used to identify UiO-66 and UiO-66-vac catalysts.

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. 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. Two correlations can be derived for Yield% of UiO-66 and UiO-66-vac based on three variables of Table 1, i.e. Temp (°C), Time (h), and Wt% Cat, using multiple linear regression (MLR) method [30] as follows: The mechanism of exo-DCPD synthesis from the endoisomer can be done through the formation of carbocation and rearrangement [10]. The proposed mechanism is shown in the following reaction equation, the rearrangement is done in the presence of acidic sites of MOFs. In the interaction with the acid sites of the catalyst, a non-classical carbocation is formed, which is transformed into the exo-form as a result of oxidation.

3
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 [14]. 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 [14]. Since similar behavior was observed for both UiO-66vac and UiO-66, Fig. S3 shows the effects of temperature, time, and Wt% catalyst only for UiO-66-vac.   Figure 2 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 h (Fig. 2a, 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 (Fig. 2a). 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 (Fig. 2b). 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 S4 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 [31]. 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 (R 2 ) demonstrate a good correlation between experimental and predicted values.

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 [13] or other MOFs [14]. As seen in Table 1 [32,33]. 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 Fig. 3. 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 [14]. 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.

Conclusions
A green process was introduced in this work for single-step isomerization of endo-to exo-DCPD in the presence of UiO-66 and UiO-66-vac (Zr) with high yield in mild reaction conditions. The catalysts are synthesized easily and reusable for this reaction. The DOE with three factors including temperature, time, and %Wt catalyst is effectively used to optimize conditions. Temperature and time at lower levels as well as catalyst dosage at higher levels have better performance on reaction yield. The surface area and acidity properties of the UiO-66 catalysts are also important to further factors to obtain a higher yield. Since UiO-66-vac has more surface area, it is more active in the isomerization of endo-DCPD to exo-DCPD. Since the new method has some advantages such as solvent-free, eco-friendly, easy reaction procedure, and availability of materials, it is superior to previous works.