Dehydrogenation Catalysts for Synthesis of O-Phenylphenol via Cu/Ni/Mg/Al Hydrotalcite-Like Compounds as Precursors

: A series of copper containing catalysts were prepared by calcination of Cu/Ni/Mg/Al hydrotalcite-like precursors, using the coprecipitation method. The materials were characterized and show a well-crystallized layered structure of hydrotalcite with smaller Cu 0 particles. We also studied their catalytic performance for conversion of 2-(1-cyclohexenyl) cyclohexanone into o-phenylphenol. The catalysts containing Ni showed higher catalytic activity; the optimum stability occurred when the Ni 2+ :(Ni 2+ + Cu 2+ ) atomic ratio was 0.4. The combination of Cu and Ni can greatly improve the stability and activities of the catalyst. the hydrotalcite laminates. The thickness of the CNMA-0 laminates is greater than 30 nm, while that of CNMA-2 drops to around 15–20 nm. It shows that the introduction of an appropriate amount of Ni can result in a thinner layer of hydrotalcite.


Introduction
O-phenylphenol (OPP) is an important chemical product and is widely used as a heat stability solvent, medicine, dyeing auxiliary, biocide, etc. [1,2]. Because of the extensive application of OPP in various fields, its demand is increasing [3]. OPP is industrially produced, mainly by catalytic dehydrogenation of 2-(1-cyclohexenyl) cyclohexanone (CHCH) over a noble metal-based catalyst, such as Pt, Pd, Ru, etc. [4][5][6]. Though showing good activity, the cost of using these catalysts in industrial processes is high. The catalytic performance of non-noble metals has been reported in the literature [7,8].
Layered double hydroxides (LDH) combine the positively charged laminate and negatively charged interlayer anions with the empirical formula [8]: where M 2+ and M 3+ are divalent and trivalent metal cations, A n− is the anion of the interlayer, and x refers to the ratio of M 3+ /(M 2+ + M 3+ ). The divalent metal cations are generally Mg 2+ , Ni 2+ , Cu 2+ , etc. M 3+ trivalent metal ions are mainly Al 3+ , Cr 3+ , Fe 3+ , etc. However, Cu 2+ does not enter easily into the brucite-like layers due to its pronounced Jahn-Teller effect, except for the case when another divalent cation is present in the solution [8]. The interlayer anions are mainly CO 3 2− , Cl − , NO 3− , SO 4 2− , OH − , etc.
Cu/Mg/Al catalysts have been applied in various reactions [9][10][11]. Zhou et al. reported the superior activity of Cu/Al/Mg catalysts for the selective oxidation of biorenewable glycerol, in which Cu 0 was taken as an active center [12]. Previous studies in our group have also shown that catalysts with Cu 0 as their active center exhibit strong activity for the dehydrogenation of 2-(1-cyclohexenyl) Figure 1 shows the XRD patterns of the hydrotalcite-like precursor and the reduced samples with H 2 for 2 h. As shown in Figure 1a, an obvious multicationic hydrotalcite structure can be observed with a sharpness of (003), (006), (009), (015), (018), (110), and (113), the reflections indicated that the samples were crystalline LDH. This observation was in agreement with the literature [17][18][19][20][21]. It can be inferred that the introduction of Ni did not cause the damage to the hydrotalcite structure (JCPDS 48-0601). In addition, no other oxides or hydroxides were detected in Figure 1a, indicating that all metal ions were evenly distributed in the layered structure. Figure 1b shows that the XRD patterns of the samples calcined after reduction by H 2 . As can be observed by XRD, there are no characteristic peaks of hydrotalcite-like compounds. In addition, the crystalline phase of Ni was not detected in CNMA-(0-4). However, the crystalline phase was found in the XRD pattern for the Ni/Mg/Al catalyst when calcined. This can be ascribed to the higher reducibility of Cu compared to Ni (this will be discussed in H 2 -TPR). Thus, Ni 2+ can hardly be reduced at this temperature. These results may be due to a failure to reach the reduction temperature of Ni. De Bokx et al. have reported that Ni 2+ can only be reduced to metal Ni above a temperature of 450 • C [22,23]. CNMA-(0-4) contained only one peak characteristic of the copper phase; this is because of the relative contents of the different metal cations and calcination temperature. Identification of metal species in the surface of reduced CNMA-4 was characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figure 2, the peak of C 1s was attributed to the adventitious hydrocarbon from the XPS apparatus. The reduced CNMA-4 shows signatures of Cu 0 /Cu + with binding energies of 932.3 eV and 952.4 eV, which correspond to the peaks of Cu 2p 3/2 and Cu 2p 1/2 , respectively. These peaks were assigned to Cu 0 /Cu + from Cu 0 /Cu 2 O. The binding energies of 933.8 eV and 953.7 eV corresponded to Cu 2+ 2p 3/2 and Cu 2+ 2p 1/2 , respectively, which were attributed to Cu 2+ from CuO. The associated satellite peak was observed at~942.4 eV. When the reduced CNMA-4 was transferred to the XPS chamber, the exposed air may have caused the oxidation of partial Cu 0 . The Ni 2p spectra were observed at 855.6 eV and 873.0 eV, which is consistent with the binding energy peak of Ni 2p 3/2 and Ni 2p 1/2 , respectively. These peaks were assigned to Ni 2+ from NiO, and the satellite peaks were observed at 861.3 eV and 865.1 eV. The Al 2p spectrum shows the signature of Al 3+ with binding energies of 74.0 eV, corresponding to Al 2 O 3 . The results of the XPS analyses were in agreement with the XRD results. The other metal oxides, except for the copper oxides, can also be found in the spectra, but not found in the XRD pattern, probably due to the high dispersion of the catalysts.
To understand the effect of the Cu/Ni ratio on the size of Cu 0 particles, we showed the full width at half maximum (FWHM) and the size of copper particles in the XRD pattern in Table S1. We found that the average size of Cu 0 was between 3.5 nm and 5.5 nm after H 2 reduction. With an increase in the Ni content, the size of the copper particles decreased gradually [22]; CNMA-4 reached a minimum average size of 3.5 nm. This indicates that Ni content has a significant effect on the size of Cu 0 particles.   The SEM results of all the hydrotalcite-like compounds are shown in Figure 3. All the samples were present in the layered structure. The layered structure of hydrotalcite for CNMA-(1~4) containing Ni was more orderly when compared with CNMA-0 without adding Ni. Similar observations have been documented in the literature [8,24,25]. When the ratio of Cu/Ni reaches 1.25/0.25, CNMA-1 forms a regular staggered lamellar structure. In addition, the introduction of nickel reduces the thickness of the hydrotalcite laminates. The thickness of the CNMA-0 laminates is greater than 30 nm, while that of CNMA-2 drops to around 15-20 nm. It shows that the introduction of an appropriate amount of Ni can result in a thinner layer of hydrotalcite.    The SEM results of all the hydrotalcite-like compounds are shown in Figure 3. All the samples were present in the layered structure. The layered structure of hydrotalcite for CNMA-(1~4) containing Ni was more orderly when compared with CNMA-0 without adding Ni. Similar observations have been documented in the literature [8,24,25]. When the ratio of Cu/Ni reaches 1.25/0.25, CNMA-1 forms a regular staggered lamellar structure. In addition, the introduction of nickel reduces the thickness of the hydrotalcite laminates. The thickness of the CNMA-0 laminates is greater than 30 nm, while that of CNMA-2 drops to around 15-20 nm. It shows that the introduction of an appropriate amount of Ni can result in a thinner layer of hydrotalcite. The SEM results of all the hydrotalcite-like compounds are shown in Figure 3. All the samples were present in the layered structure. The layered structure of hydrotalcite for CNMA-(1~4) containing Ni was more orderly when compared with CNMA-0 without adding Ni. Similar observations have been documented in the literature [8,24,25]. When the ratio of Cu/Ni reaches 1.25/0.25, CNMA-1 forms a regular staggered lamellar structure. In addition, the introduction of nickel reduces the thickness of the hydrotalcite laminates. The thickness of the CNMA-0 laminates is greater than 30 nm, while that of CNMA-2 drops to around 15-20 nm. It shows that the introduction of an appropriate amount of Ni can result in a thinner layer of hydrotalcite. The FT-IR spectra of the precursors and calcined samples are shown in Figure S1. The vibration peaks at 3500 cm −1 are attributed to water molecules between the layers and the hydroxyl groups on the laminates. The weak absorption peak at 1600 cm −1 is due to the distortion of H2O molecules [26]. The peaks at 1300 cm −1 and 1550 cm −1 are ascribed to the stretching vibration of the carbonate between the layers. Also, at 1000 cm −1 to 500 cm −1 , there are a series of stretching vibration peaks, which are ascribed to the vibrations of the O-metal-O bond. Figure S1 also confirmed that the hydrotalcite structure was still observed when introducing Ni, which is in agreement with the XRD results. After calcination, the peak at 3500 cm −1 was obviously weakened, as shown in Figure  S1b, as the water molecules and the hydroxyl groups on the laminates had decomposed. Meanwhile, the CO3 2− stretching vibration at 1380 cm −1 of the interlayers was also significantly reduced [27]. The intensity of the vibration peak at 1000 cm −1 to 500 cm −1 (ascribed to the vibrations of the O-metal-O bond) decreased, indicating the damaged hydrotalcite plates.
To clarify the decomposition process of the precursors, the thermophysical properties were studied by TG and DTG and the results are shown in Figure S2. There are three major loss stages: The FT-IR spectra of the precursors and calcined samples are shown in Figure S1. The vibration peaks at 3500 cm −1 are attributed to water molecules between the layers and the hydroxyl groups on the laminates. The weak absorption peak at 1600 cm −1 is due to the distortion of H 2 O molecules [26]. The peaks at 1300 cm −1 and 1550 cm −1 are ascribed to the stretching vibration of the carbonate between the layers. Also, at 1000 cm −1 to 500 cm −1 , there are a series of stretching vibration peaks, which are ascribed to the vibrations of the O-metal-O bond. Figure S1 also confirmed that the hydrotalcite structure was still observed when introducing Ni, which is in agreement with the XRD results. After calcination, the peak at 3500 cm −1 was obviously weakened, as shown in Figure S1b, as the water molecules and the hydroxyl groups on the laminates had decomposed. Meanwhile, the CO 3 2− stretching vibration at 1380 cm −1 of the interlayers was also significantly reduced [27]. The intensity of the vibration peak at 1000 cm −1 to 500 cm −1 (ascribed to the vibrations of the O-metal-O bond) decreased, indicating the damaged hydrotalcite plates.
To clarify the decomposition process of the precursors, the thermophysical properties were studied by TG and DTG and the results are shown in Figure S2. There are three major loss stages: The mass loss corresponding to the peak at 100-150 • C is due to the loss of physically adsorbed water, the second mass loss phase occurs at the range of 250-400 • C, which is due to the decomposition of carbonates and the dehydration of hydroxyl groups between laminates, the third endothermic peak appears in the range of 600-650 • C, which is due to the decomposition of the layered structure.
Nitrogen adsorption-desorption isotherms of reduced catalysts are shown in Figure S3. The two samples were presented with IV type isotherms based on the IUPAC classification corresponding to their mesoporous features. The specific surface area, pore volume, and average pore size of the samples is shown in Table S2. The adsorption curves in the low relative pressure region show that the catalysts have a small number of micropores for CNMA-(0~4). Additionally, the volume of micropores increases with increasing nickel content in the catalysts. A narrow hysteresis loop occurs at a relative pressure of 0.4, due to the capillary condensation of the mesoporous structure of the catalyst. This indicated that the hydrotalcite structure disappeared and the slit-type channels formed after the calcination at high-temperature [28]. Figure S4 shows a bimodal pore size distribution between 3.3 and 100 nm. As shown in Table S2, the specific surface area of the samples ranged from 37 to 144 m 2 ·g −1 . The pore volume and specific surface area of the samples was enhanced by the introduction of Ni, which is plausible because Ni reduces the thickness of the laminate and degrades the ameliorates of the LDH regularity (confirmed by SEM). Figure 4 shows a TEM image of the Cu 0 particles (left) and corresponding size distribution (right) of the reduced CNMA-(0~4) with different Cu/Ni ratios. The size of Cu particles was measured by 100 particles. As shown in Figure 4, metallic copper particles of CNMA-(0~4) are homogeneously distributed on a relatively narrow size distribution. With an increase in the percentage of Ni, the copper particle size decreased, which shows that Ni can regulate the size of Cu 0 particles in the catalysts. To further investigate the structure of copper nanoparticles in the oxide matrix, an image taken with HRTEM is shown in Figure 5. Figure 5b shows the fringe space of 1.21 Å, which illustrates the presence of Cu 0 particles (110) [29]. Figure S5 shows the HRTEM elemental mapping images of CNMA-4 and elemental mapping showing a homogeneous distribution of all the elements in the oxide matrix.

H 2 -TPR
H 2 -TPR images of the calcined sample are shown in Figure 6. We can observe that the reduction peak for all samples is in the range of 150-450 • C. In order to obtain more information, the H 2 consumption band is deconvoluted into three Gaussian forms, assigned as α, β, and γ peaks. The reduction temperatures corresponding to the peaks were shown in Table S3. The three types of peaks were due to the different morphology of CuO, where α peaks are owing to the high dispersibility of CuO, the β peak is attributed to the moderate dispersibility of CuO, and the γ peak is due to the poor dispersion of CuO [30]. As shown in Table S3, the ratio of the α peak of CNMA-0 is only 12.07%. The α peak of CNMA-2 increases with the amount of Ni, and approaches the maximum. The proportion of Ni in the catalyst has a significant effect on the reduction of CuO [31]. It is remarkable that the reduction peak of CNMA-(2-4) was shifted at higher temperature. For the CNMA-(2-4) catalysts, we observe a peak at 390 • C, probably owing to a strong interaction between Cu and Ni [32,33]. Contrastingly, the reduction peak had shifted to lower temperature for CNMA-1 with low Ni content. It indicated that a little amount of nickel could significantly promote the reduction of metal oxides, which also was observed by Shi et al. [34].           Figure 7 shows the CO 2 -TPD profiles for the reduced samples. The corresponding peak positions and contributions are listed in Table S4. The positions of desorption peaks range from 280 to 330 • C. The peak position at 240 • C corresponds to the interaction of CO 2 with OH − or O atoms. The medium-strength site (above 300 • C), was mainly attributed to Mg-O, Ni-O and Al-O. It can be found that CNMA-0 has the minimum number of strong basic sites among these samples as a result of no Ni introduction. It is worth noting that the addition of Ni promoted the CO 2 adsorption on the catalyst surface due to the increase of total basicity of catalysts. This trend is similar to the results reported by Dębek et al. [35]. CNMA-4 has the maximum number of strongly basic sites of these catalysts, possibly due to the introduction of Ni, increasing the Ni-O pair. In addition, the decrease in Cu content could result in the increase of basic sites. Marcu et al. also reported that the strong and total basic sites of Cu-based catalysts increase with the decline of the copper content [36].

Catalytic Performance
The activity and selectivity of the samples were examined through the conversion of CHCH to OPP. Figure 8 shows the relationship between the selectivity, conversion, and atomic ratios of Cu 2+ /Ni 2+ after the catalytic reaction for 5 h. The catalysts with different Ni content exhibited different catalytic performances. The conversion of CHCH was over 95% after 5 h for CNMA-0 to CNMA-4, moreover, the conversion percentage increased with an increasing proportion of Ni in the samples. CNMA-1 and CNMA-2 showed higher selectivity and the selectivity clearly declined at the beginning of reaction. The introduction of a small amount of Ni destroys the metal-O bond, thus reducing the stability of the catalyst. The OPP selectivity of all the catalysts declined continuously with time of reaction. CNMA-4 still maintained a high selectivity for OPP after 5 h of reaction. This result can be attributed to higher BET surface areas and a smaller size of copper particles for CNMA-4. Based on the TEM images of the samples, a slightly smaller mean size of Cu 0 particles was observed for CNMA-4 (3.82 nm) compared to CNMA-0 (5.20 nm). In addition, the precursor for CNMA-4 had better crystallinity and thin ordered laminates due to the presence of Ni (as confirmed by SEM), which lead to higher specific surface area and larger pore volume. The thin ordered structure can be ascribed to the substitution of Cu 2+ by Ni 2+ in the hydrotalcite laminates. This substitution results in a thinner layer structure due to the small ionic size of Ni 2+ (0.069 nm for Ni 2+ and 0.073 nm for Cu 2+ in an octahedral coordination) [37]. The CNMA-4 sample has the highest specific surface area, better dispersion of CuO, and much stronger basic sites. The initial stage of CHCH dehydrogenation may have occurred via H + abstraction on the basic sites [38]. Zeng et al. found that the distribution of alkaline sites and Cu species on the Cu-based catalytic surface has a strong effect on the selectivity of OPP, the strong basic sites may weaken the interaction of Al2O3 and CuO, preventing the active sites from undergoing agglomeration [3]. Wang et al. also reported that better dispersion of CuO in catalysts has a significant impact on the catalytic activity [39].

Catalytic Performance
The activity and selectivity of the samples were examined through the conversion of CHCH to OPP. Figure 8 shows the relationship between the selectivity, conversion, and atomic ratios of Cu 2+ /Ni 2+ after the catalytic reaction for 5 h. The catalysts with different Ni content exhibited different catalytic performances. The conversion of CHCH was over 95% after 5 h for CNMA-0 to CNMA-4, moreover, the conversion percentage increased with an increasing proportion of Ni in the samples. CNMA-1 and CNMA-2 showed higher selectivity and the selectivity clearly declined at the beginning of reaction. The introduction of a small amount of Ni destroys the metal-O bond, thus reducing the stability of the catalyst. The OPP selectivity of all the catalysts declined continuously with time of reaction. CNMA-4 still maintained a high selectivity for OPP after 5 h of reaction. This result can be attributed to higher BET surface areas and a smaller size of copper particles for CNMA-4. Based on the TEM images of the samples, a slightly smaller mean size of Cu 0 particles was observed for CNMA-4 (3.82 nm) compared to CNMA-0 (5.20 nm). In addition, the precursor for CNMA-4 had better crystallinity and thin ordered laminates due to the presence of Ni (as confirmed by SEM), which lead to higher specific surface area and larger pore volume. The thin ordered structure can be ascribed to the substitution of Cu 2+ by Ni 2+ in the hydrotalcite laminates. This substitution results in a thinner layer structure due to the small ionic size of Ni 2+ (0.069 nm for Ni 2+ and 0.073 nm for Cu 2+ in an octahedral coordination) [37]. The CNMA-4 sample has the highest specific surface area, better dispersion of CuO, and much stronger basic sites. The initial stage of CHCH dehydrogenation may have occurred via H + abstraction on the basic sites [38]. Zeng et al. found that the distribution of alkaline sites and Cu species on the Cu-based catalytic surface has a strong effect on the selectivity of OPP, the strong basic sites may weaken the interaction of Al 2 O 3 and CuO, preventing the active sites from undergoing agglomeration [3]. Wang et al. also reported that better dispersion of CuO in catalysts has a significant impact on the catalytic activity [39]. According to the TPR results, it is clear that CNMA-4 exhibits the highest ratio of low temperature peak (above 77.60%), which can be ascribed to the well dispersed copper species in CNMA-4. According to the TPR results, it is clear that CNMA-4 exhibits the highest ratio of low temperature peak (above 77.60%), which can be ascribed to the well dispersed copper species in CNMA-4. In order to clarify the impact of the content of Ni on the distribution of products, Figure 9 shows the relationship between the catalytic selectivity of catalysts and the primary products after a reaction time of 5 h. In our previous work, we have reported the catalytic mechanism and process of a Cu-based catalyst for the conversion of CHCH to OPP; O-cyclohexylcyclohexanone (OCC) and o-cyclohexylphenol (OCHP) were the main byproducts. In addition, a small amount of dibenzofuran (DBF) and biphenyl (BP) formed due to the further reaction of OPP [3,40]. OCC is the product of the hydrogenation of CHCH. Meanwhile, OCHP could also be generated by the dehydrogenation of OCC. OCHP was further dehydrogenated to form OPP. The selectivity of OPP is significantly affected by the content of Ni. With an increase in the content of Ni, the proportions of OCC and OCHP in the products decline greatly, while that of OPP increases. CNMA-4 showed the highest selectivity and lower proportion of byproducts after a reaction time of 5 h. These results can be attributed to a strongly basic site, higher BET surface area, and high Cu dispersion of CNMA-4. It is clear that the introduction of Ni can help enhance the selectivity of OPP, while inhibiting the formation of OCC and OCHP.
To further assess the stability of CNMA-4, Figure 10 shows the selectivity of OPP on CNMA-4 after a reaction time of 20 h. It can be seen that the selectivity of OPP declined by approximately 25%. As the yield of OPP declines, the yield of OCC increases; the yield of OCHP increases slowly as the reaction progresses. This indicates that the conversion to OCC in this reaction is a main factor, which resulted in the decrease of selectivity of OPP, perhaps due to the agglomeration of copper particles at high temperature. The Cu 0 particle size of catalysts increased at high temperatures and resulted in a worse dispersion and lower catalytic activity [40,41]. Similar observations were also reported on the agglomeration of MgO at high temperature by Auer et al. [42]. The low dispersibility of MgO weakens the interaction between Cu and Mg, and further reduces the hydrogenation and dehydrogenation activity of the catalysts. Thus, the amount of byproducts OCC and OCHP increases. The improvement on the dehydrogenation performance of the catalyst will be the focus of our future research. In order to clarify the impact of the content of Ni on the distribution of products, Figure 9 shows the relationship between the catalytic selectivity of catalysts and the primary products after a reaction time of 5 h. In our previous work, we have reported the catalytic mechanism and process of a Cu-based catalyst for the conversion of CHCH to OPP; O-cyclohexylcyclohexanone (OCC) and o-cyclohexylphenol (OCHP) were the main byproducts. In addition, a small amount of dibenzofuran (DBF) and biphenyl (BP) formed due to the further reaction of OPP [3,40]. OCC is the product of the hydrogenation of CHCH. Meanwhile, OCHP could also be generated by the dehydrogenation of OCC. OCHP was further dehydrogenated to form OPP. The selectivity of OPP is significantly affected by the content of Ni. With an increase in the content of Ni, the proportions of OCC and OCHP in the products decline greatly, while that of OPP increases. CNMA-4 showed the highest selectivity and lower proportion of byproducts after a reaction time of 5 h. These results can be attributed to a strongly basic site, higher BET surface area, and high Cu dispersion of CNMA-4. It is clear that the introduction of Ni can help enhance the selectivity of OPP, while inhibiting the formation of OCC and OCHP.
To further assess the stability of CNMA-4, Figure 10 shows the selectivity of OPP on CNMA-4 after a reaction time of 20 h. It can be seen that the selectivity of OPP declined by approximately 25%. As the yield of OPP declines, the yield of OCC increases; the yield of OCHP increases slowly as the reaction progresses. This indicates that the conversion to OCC in this reaction is a main factor, which resulted in the decrease of selectivity of OPP, perhaps due to the agglomeration of copper particles at high temperature. The Cu 0 particle size of catalysts increased at high temperatures and resulted in a worse dispersion and lower catalytic activity [40,41]. Similar observations were also reported on the agglomeration of MgO at high temperature by Auer et al. [42]. The low dispersibility of MgO weakens the interaction between Cu and Mg, and further reduces the hydrogenation and dehydrogenation activity of the catalysts. Thus, the amount of byproducts OCC and OCHP increases. The improvement on the dehydrogenation performance of the catalyst will be the focus of our future research.

Sample Characterization
XRD (X-ray diffraction) patterns were obtained from a X-ray diffractometer (Bruker AXS, D8 Advance, Karlsruhe, BW, Germany) with Cu Kα radiation (λ = 1.5406 Å). The intensity data step was 0.05 • in the 2θ range of 5-70 • and the counting time for each point at 40 kV and 40 mA was 1 s. FT-IR spectra were obtained with the KBr pellet technique in the range of 4000-400 cm −1 on a Bruker Tensor 27 Fourier transforms spectrometer (Bruker AXS, Karlsruhey, BW, Germany). SEM (Scanning Electron Microscope) was performed on a Hitachi, S-4800II (Hitachi, Tokyo, Japan). The sample was evenly applied to the surface of the solid conductive adhesive, the surface of the sample was treated with gold, and then the sample was placed in the instrument to observe the sample morphology. Thermogravimetric analysis (TG-DTG) was conducted using a PerkinElmer pris 1 from room temperature to 700 • C with a heating rate of 10 • C·min −1 under N 2 . TEM (Transmission electron microscope) was performed using a Philips Tecnai 12 electron microscope (Philips, Amsterdam, NH, Holand). The hydrotalcite-like precursors were suspended in a solution of ethanol and then deposited over a carbon-coated copper grid, followed by solvent evaporation in air at room temperature. High-Magnification Transmission Electron Microscope (HRTEM) photographs were taken using Tecnai G2 F30 S-TWIN equipment (FEI, Hillsboro, OR, USA). N 2 adsorption isotherms of the samples were measured on Sorptomatic 1990 (Thermo Electron Co, Waltham, MA, USA). The sample surface area and pore volume were determined from the BET equation and Barrett-Joyner-Hallender (BJH) methods, respectively. The samples were outgassed at 200 • C under vacuum for 6 h prior to measurement. H 2 temperature-programmed reduction (H2-TPR) data were collected on a Finesorb-3010 (Finetec Instruments, Hangzhou, China). The sample was treated with pure Ar (30 mL·min −1 ) up to 200 • C (10 • C·min −1 ) and maintained at this temperature for 30 min. After cooling to 100 • C, the samples for the TPR experiment, in a H 2 /Ar mixture (1:9, 30 mL·min −1 ), were heated to 500 • C. Then the catalyst bed was flushed with Ar and cooled to room temperature. CO 2 temperature desorption spectroscopy (CO 2 -TPD) experiments were performed using a Finesorb-3010 chemical adsorption instrument loaded with 0.1 g of the catalyst. The catalyst was first treated for 30 min at 200 • C (10 • C·min −1 ), and then cooled to 100 • C. The sample was then flushed with CO 2 (30 mL·min −1 ) for 1 h. Subsequently, the catalyst was flushed with Ar (30 mL·min −1 ) for 30 min. Finally, the samples for CO 2 -TPD were heated to 600 • C at a rate of 10 • C·min −1 in the presence of pure Ar at a rate of 30 mL·min −1 . The catalysts were then cooled to 20 • C with pure Ar.

Catalytic Performance Test
The CHCH dehydrogenation and hydrogenation reaction was performed in a fixed-bed reactor. The whole apparatus consisted of five parts, including the temperature controller, feeding part, vapor generator, cooling section, and quartz reaction tube. Three grams of catalyst was placed in the middle of the tube and both sides of the tube were filled with glass beads with a diameter of 2-3 mm. The catalyst was reduced with hydrogen (20 mL·min −1 ) at 300 • C for 2 h. The reaction was performed at 330 • C under atmospheric pressure. We adjusted the flow rate of H 2 to 20 mL·min −1 and used an injection pump [liquid hour space velocity (LHSV) = 0.2 h −1 ] to add the reagent into the reactor. The product was dissolved in 3 mL of ethanol and measured by a chromatograph equipped with a flame ionization detector (FID) detector.

Conclusions
Hydrotalcite-like precursors with different Ni 2+ /(Ni 2+ + Cu 2+ ) atomic ratios were successfully prepared via the coprecipitation method. The introduction of Ni does not destroy the hydrotalcite-like structure. However, it reduces the thickness of the hydrotalcite laminates. The activity and stability of the catalysts are related to the content of Ni. The smaller sized Cu particles of CNMA-1 and CNMA-2 resulted in better initial catalytic selectivity. The introduction of Ni can improve the dispersibility of Cu 0 particles in catalysts and create strongly basic sites. CNMA-4 showed optimal stability and high selectivity around 60% after a reaction time of 5 h.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/8/5/186/s1, Figure S1: FT-IR image of Cu/Ni/Mg/Al before calcination (a) and calcination (b), Figure S2: TG-DTG of the precursors in N 2 , Figure S3: N 2 adsorption-desorption isotherms of CNMA-(0-4), Figure S4. The pore size distributions for all catalysts, Figure S5: HRTEM and elemental mapping images of catalysts after reduction (a) Cu/Mg/Al/Ni catalysts (b) Cu -K map (c) Ni-K map (d) Mg-K map (e) Al-K map, Table S1: FWHM and the size of Cu 0 particles by XRD, Table S2: The properties of the samples with different amount of Ni, Table S3: Temperatures for Reduction peak and the percentage of α peak for these samples, Table S4: Distribution of the basic type for the samples with different Ni amount.