Effect of support hydrophobicity of halloysite-based catalysts on the polyalphaolefin hydrofinishing performance

Hydrogenation of polyalphaolefins (PAOs) is an industrial process catalyzed by supported precious metals. In this regard, halloysite (Hal) clay has been proven as an efficient support for the immobilization of Pd nanoparticles and development of high-performance

As heterogeneous catalysis is an appealing topic from an industrial point of view, [1][2][3] it has witnessed immense advances in recent decades, and a myriad type of heterogeneous catalysts has been devised using several supporting materials.More recently, single atom catalysts systems have appeared as a way to go from the precision of homogeneous catalysis to the effectivity of heterogeneous catalysis. [2,4,5]In this context, natural and readily available supports, such as clays, have received massive attention, and the usefulness of various clays for designing catalytic systems has been disclosed.
One of the least known members of Kaolin clay group is halloysite (Hal).This dioctahedral 1:1 clay [6][7][8][9][10] has recently been the focus of research for a wide range of applications, [11][12][13] such as catalysis, cleaning, [14] adsorbent, [15] delivery systems, packaging, [16] etc.The extensive utility of Hal as well as its excellent performance can be attributed to its unique physicochemical properties, such as tubular morphology with a relatively high lumen space (11%-39%), [17,18] oppositely charged inner and outer surface, aspect ratio of 10-50 and average pore size of 79.7-100.2[24] This leads a broad scope of uses for halloysites, from the bio field,[ 25,26 ] resulted in rapid growth of Hal-based catalysts for various chemical reactions, [27] including hydrogenation [28,29] and oxidation reactions, [30] as well as synthesis of organic compounds. [31,32][35] Their unique properties, such as high oxidation stability and viscosity index (VI), [36,37] and extremely low pour point render them distinguished ones among all synthetic oils categories. [38]Statistically, more than 42% of the sum of synthetic oil market belongs to PAO grades. [39]Despite facile synthesis routes and immense developments in industrial production sector, the crude PAO still surfers from the existence of C C double bonds in its backbone. [40,41]These bonds make PAO susceptible to cleavage at high temperature of application, which deteriorates PAOs unique characteristics and decreases its shelf life. [42,43]To overcome this drawback, PAO hydrofinishing, as an essential stage in its production, is usually accomplished.This stage needs harsh environment with high temperature and H 2 pressure (nearly above 200 C and 25 bar), which increases the risk and cost of PAO production.Therefore, finding appropriate catalysts for PAO hydrofinishing that can perform this reaction at mild reaction condition is always the topic of research in academia and industry.
In the continuation of our research on Hal-based catalysts for hydrogenation reactions, [44][45][46][47] herein, and knowing that the presence of protic solvents and reagents is detrimental for this reaction, [48][49][50] we wish to report a combined computational and experimental study on the effect of the hydrophobicity of Hal on its performance for hydrofinishing of PAO.

| Chemicals and solvents
Preparation of the catalysts in this research was fulfilled using the following chemicals and solvents (all provided from Sigma-Aldrich), Hal, cetrimonium bromide (CTAB), Pd (OAc) 2 , NaBH 4 , AgNO 3 (Silver nitrate), toluene, and methanol (MeOH).The materials used for the synthesis of PAO (all purchased from Merck Co. Germany) included 1-decene, AlCl 3 , and NaOH.

| Instruments
The analyses and the applied instruments for the characterization of the as-prepared catalysts are as follow: Inductively coupled plasma (ICP) was performed by using Varian, Vista-pro for calculating the content of Pd in all three samples.Transmission electronic microscopy (TEM) was carried out (Philips100 Kv AMBS) to investigate the morphology of the synthesized samples.Fourier transform infrared (FTIR) spectra of the as-prepared samples were recorded on PERKIN-ELMER Spectrum 65 using the KBr pellet.Thermogravimetric analysis (TGA) of the catalysts was conducted on METTLER TOLEDO apparatus under oxygen atmosphere, with a heating rate of 10 C/min.Contact angles of the catalysts were studied at ambient temperature using a Dino-Lite Plus camera.The captured images were examined with an image processing software (Digimizer).To record the X-ray diffraction (XRD) patterns, Siemens, D5000 armed with a Cu Kα radiation was applied.The energy dispersive X-ray spectrum (EDS) and elemental analysis were performed using a MIRA 3 TESCAN-XMU.The Brunauer-Emmet-Teller (BET) surface area of Hal and the catalysts were estimated using the Belsorp Mini II apparatus.Zeta potential measurement was carried out in water suspension using a Horiba SZ-100 apparatus.
To determine the yield of hydrofinishing, the hydrogenated PAO was analyzed by nuclear magnetic resonance ( 1 HNMR) spectroscopy.To perform the analysis, Bruker DRX 400 MHz instrument was used and the sample was dissolved in chloroform.

| Synthesis of Hal/Pd
To immobilize Pd nanoparticles on Hal, Hal (2 g) was dispersed in toluene (30 ml) under ultrasonic irradiation (power of 150 W for 5 min).Separately, a solution of Pd (OAc) 2 (3 wt.%) in toluene (5 ml) was prepared and slowly added to the as-prepared Hal suspension under Ar atmosphere.After 2 h, a NaBH 4 solution in MeOH (2 M) was gently introduced to the aforementioned stirring suspension.Stirring was continued for 5 h to ensure reduction of Pd(II) to Pd(0).Finally, the resulting precipitate was centrifuged, washed with MeOH, and then dried at 60 C overnight.

| Synthesis of Hal/Pd/CTAB
To prepare Hal/Pd/CTAB, Hal was first palladated using the similar procedure used for the synthesis of Hal/Pd sample (see Section 2.3.1). [51,52]Then, the as-prepared Hal/Pd was modified with CTAB according to the previous report. [53]Briefly, a solution of CTAB (0.5 g) in distilled water was prepared.Then, Hal/Pd (1 g) was added, and the resulting mixture was stirred for 2 days.Afterwards, the precipitate was separated via centrifugation and washed repeatedly with distilled water.To assure removal of free CTAB, aqueous solution of AgNO 3 was applied.Finally, the resulting solid was dried at 80 C for 1 week.

| Synthesis of Hal/CTAB/Pd
Hal/CTAB/Pd was prepared through a similar procedure applied for the synthesis of Hal/Pd/CTAB, except Hal was first modified with CTAB and then palladated.The procedures for the synthesis of the catalysts are presented in Scheme 1.

| Synthesis of PAO
Oligomerization of 1-decene and synthesis of PAO was carried out according to the literature. [27]Typically, the reactor used for the polymerization was first charged with AlCl 3 (5 g) and purged with an inert gas.After heating at 80 C for 1 h, 1-decene (500 g) and deionized water (0.3 ml) were added, and the reaction mixture was stirred for 50 min at 100 C. At the end of the reaction, the obtained oil was rinsed several times with 5 wt.%NaOH.Further purification was achieved by heating of the mixture under a 0.8 bar vacuum to 250 C. The yield of PAO synthesis was estimated to be 86%.

| Hydrogenation of PAO
Hydrofinishing of PAO was conducted in a stainless steel reactor equipped with regulators, a circulator and a magnetic stirrer.Prior to the reaction, the reactor was purged with N 2 gas and heated at 100 C for 1 h.Afterwards, the as-synthesized PAO (10 g) and the catalyst (5 wt.%) were transferred to the reactor.Then, the reactor was sealed, H 2 gas pressure was set on 8 bar, and the reaction temperature was elevated to 130 C. The reaction mixture was continuously stirred under the aforesaid condition for 8 h.Upon completion of the reaction, the reactor was cooled, and the catalyst was simply separated via centrifugation.Then, the yield of hydrofinishing was estimated by 1 HNMR spectroscopy.

| Recovery and recycling of the catalyst
To recover the collected catalyst, it was first washed with n-hexane for three times to remove deposited oil on its surface.Then, the washed catalyst was dried at 60 C overnight and applied for the next run of the hydrofinishing reaction.
S C H E M E 1 Employed procedures for the synthesis of the designed catalysts

| Computational details
Density functional theory (DFT) calculations were performed with the Gaussian 16 package. [54]Geometry optimizations were performed via the spin-restricted Kohn-Sham (RKS) formalism with the BP86 functional of Becke and Perdew, [55,56] including the D3 version of Grimme's dispersion (empiricaldispersion = GD3). [57][62] Frequency calculations were performed to confirm the nature of the stationary points.Single point energy calculations were performed employing the B3LYP, hybrid GGA functional of Becke-Lee, Parr, and Yang, [63][64][65] together with the D3 version of Grimme's dispersion, and the cc-pVTZ basis set, [66] without and with solvent effects based on the polarizable solvation model density (SMD), variation of IEFPCM of Truhlar and co-workers, [67] using toluene as the solvent (and hexane, acetonitrile, ethanol, DMF, and DMSO for comparison), omitting corrections of entropy and standard state of 1 M concentration in solution. [68]For the Hal simulation, we employed the Hal model based on three Al and two Si units. [69]| RESULTS AND DISCUSSION

| Design of the catalysts
[46] Encouraged by the excellent performance of Hal, we decided to elucidate the effect of hydrophobicity of Hal on its catalytic activity.To tune the hydrophobicity of the outer surface of Hal that is negatively charged, a cationic surfactant, CTAB, was applied, and three samples were prepared.In the standard sample, Hal/Pd, Pd nanoparticles were immobilized on unmodified Hal. [70]In the second sample, Hal/CTAB/ Pd, Hal outer surface was first modified with CTAB, and then the resultant hydrophobic Hal was used as a support for the stabilization of Pd nanoparticles.Finally, to elucidate whether the order of introduction of CTAB can affect the activity of the resultant catalyst, a third sample, Hal/Pd/CTAB, was also prepared, in which Hal was first palladated and then treated with CTAB.

| Computational study
To distinguish which is the role of CTAB on the hydrogenation of olefins, DFT calculations were performed to unveil the reaction mechanism with ethylene as alkene substrate (see Scheme 2) and find out where CTAB participates.The first stage is search of the best coordination of the Pd(0) particle on Hal.The next step is alternative and allows the coordination of an ethylene or hydrogen molecule.Both types of substrate coordination are thermodynamically favored, but even more with the alkene, by 2.8 kcal/mol.Either way, then overcoming a transition state both reaction pathways converge at the same intermediate where hydrogen and olefin are bound to the palladium.It should be mentioned; however, that just as hydrogen coordination means a destabilization of only 2.5 kcal/mol.The olefin coordination is significantly hampered with an energy barrier of 14.0 kcal/mol.From the intermediate where they converge, two H-transfers lead to the formation of ethane, overcoming energy barriers of 12.5 and 7.0 kcal/mol, respectively.Thus, the first of the two is the rate determining step (rds).It should be noted, however, that heterolytic rupture of H 2 on the palladium before the two H-transfers has also been studied, but this pathway is kinetically demanding, with a cost of at least 28.0 kcal/mol.It is a minimum because the first of the two transition states could not be located, corresponding to the breaking of the H H bond, and could only be approximated from a reaction coordinate.However, this is not important because the second transition state of this alternative path is 15.5 kcal/mol above in energy.On the other hand, the addition of CTAB was studied over the course of the reaction pathway, and it is only explicitly significant when the palladium is less coordinated.The problem is clear as this favorable interaction would slow hydrogenation, in fact being more favorable by 1.7 kcal/mol than the insertion of hydrogen on the palladium, but still 1.1 kcal/mol less favored than the interaction with alkene.The values denote that CTAB will be a drawback for the reaction as the interaction with the palladium makes hydrogenation catalysis difficult, and not only at the beginning, but throughout the reaction profile.
Given the competition between the coordination of the two substrates, as well as the CTAB on the palladium, this interaction has also been studied in Scheme 2 including the solvent effects produced by hexane.Whereas there are no significant differences in the reaction pathway, relatively the interaction of the CTAB becomes significantly less favored.In fact, the value is 21.7 kcal/mol with respect to Hal as a reference species and thus 3.4 and 6.7 kcal/mol less stable than the molecular hydrogen or ethylene insertions.The coexistence of CTAB on the first sphere around the metal is not possible once any of the substrates bonds to the metal.To be able to find out to what extent the CTAB is occupied by the palladium nanoparticle, the steric maps of Cavallo and coworkers are included in Figure 1. [71,72]lso, the orientation of x-, y-, and z-axes for the steric maps is further detailed in Figure S1.In addition to the qualitative detail of the steric maps, the %V Bur around the metal then allows to quantify the employability by substrates that can interact with it. [73]First, it should be noted that the entry of H 2 if the palladium is simply attached to the metal is a significant change in the metal center and even decreases as coordination causes the palladium to leave the cavity, which is provided by halloysite, in order to coordinate with H 2 , and consequently the %V Bur decreases from 64.5% to 56.3%. [74]Then, the % V Bur without the CTAB around the metal implies a change from 64.5% to 79.9% once the olefin binds to it.If, on the other hand, we have the CTAB, there is an invariability because the %V Bur is 87.1 and 87.2%, respectively, [75] and the asymmetry even could not help at any less occupied quadrant. [76]Therefore, there is a total invariability, as the metal center is virtually sequestered, [77] especially by a bromine that also provides additional electron density with respect to olefin or molecular hydrogen.In fact, the coexistence of H 2 with the CTAB would be possible, although to the detriment of the substrate, because it is not coordinated in the metal center, but as an adduct, with a Pd˙˙˙H distance not less than 2.9 Å; therefore, in the background denying, a covalent interaction with the metal (the Mayer Bond Order [78,79] is only 0.107 compared with 0.782 with the Pd˙˙˙Br). [66]oncluding the results of molecular simulation section, the presence of CTAB is detrimental to hydrogenation yield, from both energy and steric points of view.To shed light on the capability of CTAB containing catalyst, it was prepared and surveyed in PAO hydrofinishing.Also, the efficiency of this catalyst was compared with blank catalyst composed of bare Hal/Pd system.

| Hydrofinishing of the PAO using the as-prepared catalysts: Investigation of the reaction parameters using one-factorat-a-time (OFAT) method
The catalytic activity of the three catalysts has been studied and compared for the hydrofinishing of PAO.Before comparing the activity of the developed catalysts, the effects of the reaction parameters (temperature, catalyst loading, and hydrogen pressure) have been studied by using the standard sample, Hal/Pd, as a catalyst.To this purpose, "one-factor-at-a-time" (OFAT) method has been applied.More precisely, the effect of each parameter was investigated by varying that parameter, while keeping others constant.

| Effect of reaction temperature
To evaluate the effect of the reaction temperature, PAO hydrofinishing was performed in the presence of 5 wt.% catalyst at a hydrogen pressure of (8 bar) at different temperatures (100 C-140 C).The results (Figure S2) approved that hydrofinishing was not efficient at low temperatures and increase of temperature from 100 C to 130 C significantly affected the reaction yield.It is worth noting that increase of the reaction temperature from 130 C to 140 C was not effective.Hence, the optimum value for the hydrofinishing temperature was found to be 130 C.

| Effect of hydrogen pressure
To evaluate the effect of hydrogen pressure on the yield of hydrofinishing, the yields of PAO hydrofinishing at different hydrogen pressures (5-9 bar) were measured and compared (Figure S3).It was found that hydrogen pressure was a key factor and influenced the reaction yield remarkably.In fact, hydrofinishing reaction did not proceed efficiently at low hydrogen pressures and led to low to moderate yields.However, increase of this parameter to 8 bar resulted in increase of the yield of the reaction to 98%.Further increase of hydrogen pressure, however, was not influential, and 8 bar was selected as the optimum value for hydrogen pressure.

| Effect of catalyst loading
Finally, the investigation of the effect of catalyst loading was carried out to obtain the optimum amount of this parameter.To this purpose, hydrofinishing of PAO was performed in the presence of 2-6 wt.% Hal/Pd.The comparison of the yields of the reactions (Figure S4) indicated that upon increase of the catalyst content from 2 to 5 wt.%, the yield of the reaction increased, while further increase to 6 wt.% was ineffective.Based on these results, the optimum condition for hydrofinishing reaction was using 5 wt.% catalyst, hydrogen pressure of 8 bar at 130 C. 1 HNMR spectra of unhydrogenized PAO and the one hydrogenized under optimum reaction conditions are depicted in Figure 2. As can be seen in the spectrum of the hydrogenized sample, the peaks at 1.96 and 4.5-5.5 ppm (corresponding to the allyl and vinyl H moieties, respectively) disappeared.

| Comparison of the catalytic activities of the catalysts
Having the optimum reaction parameters in hand, the catalytic activity of the three synthesized catalysts has been compared for PAO hydrofinishing under the optimized conditions.Interestingly, the results implied that the order of the activity of the as-prepared catalysts was as follows: Hal/Pd > Hal/Pd/CTAB > Hal/CTAB/ Pd.Experimentally obtained decrement of the catalytic performance by CTAB presence confirmed well the energy path of the hydrogenation reaction obtained from molecular simulations, in which CTAB via interaction with active Pd species, decreased their efficiency.

| Origin of the differences of the activity of the catalysts
To investigate the origin of the differences of the catalytic activity of the as-prepared catalysts, they have been characterized, and their properties have been compared.
To confirm conjugation of CTAB in Hal/CTAB/Pd and Hal/Pd/CTAB samples, all three samples were analyzed via FTIR spectroscopy.In Figure 3, FTIR spectra of Hal/Pd sample showed the characteristic absorbance bands of Hal at 3694 and 3627 cm À1 that is due to the internal OH groups, 1031 cm À1 that is assigned of Si O stretching and 540 cm À1 that is indicative of Al O Si vibration. [80]In the FTIR spectra of Hal/CTAB/Pd and Hal/Pd/CTAB samples, all of the aforementioned characteristic bands can be detected, confirming the stability of Hal structure.However, in these two samples, an additional band at 2942 cm À1 can be observed that is assigned to the CH 2 stretching of the alkyl chain of CTAB.Quantitative studies also approved that the quantity of CTAB in Hal/Pd/CTAB is higher than Hal/CTAB/Pd.
To further approve the incorporation of CTAB and estimate the content of CTAB in Hal/CTAB/Pd and Hal/Pd/CTAB, TGA was applied.As depicted in Figure 4, in both TG curves, three weight losses can be detected.Two of the observed weight losses are attributed to loss of water and Hal dehydroxylation ($130 C and 540 C, respectively). [81,82]In the Hal/CTAB/Pd sample, the third weight loss appeared at 250 C ($2 wt.%), while in Hal/Pd/CTAB sample, this weight loss is detected at 240-320 C ($10 wt.%).These results are in good accordance with the FTIR results and stated that synthetic procedure can affect the content of CTAB in the catalyst.In fact, in Hal/CTAB/Pd sample, in which palladation is conducted after incorporation of CTAB, loading of CTAB is significantly lower than that of Hal/Pd/CTAB.This can be due to the several washing steps used for the immobilization of Pd nanoparticles.
As the content of CTAB in Hal/CTAB/Pd and Hal/Pd/CTAB is different, it is expected that the hydrophobicity of these two samples differs.To elucidate this issue, the hydrophobicity of the as-prepared samples has been evaluated by contact angle measurements (Θi).As depicted in Figure 5, by treating Hal with CTAB, the hydrophobicity of the samples and consequently Θi values increased.It was also found that the hydrophobicity of Hal/Pd/CTAB was higher than that of Hal/CTAB/ Pd.
As one of the most important features of the catalysts that can affect the catalytic activity is Pd loading, all three samples have been subjected to ICP analysis.The results indicated that the Pd loading in the catalysts follows the order of Hal/Pd/CTAB (1.2 wt.%) ≈ Hal/Pd ($1.2 wt.%) > Hal/CTAB/Pd (0.8 wt.%).These results clearly established that the treatment of Hal with CTAB prior to palladation is detrimental to the Pd loading and led to the catalyst with a low Pd content.This issue is due to the electrostatic repulsion between the Pd salt and the hydrophobic Hal.
As another determinant on the catalytic activity is the dispersion of the catalytic active species, all three samples have been characterized via TEM.As shown in Figure 6, Pd nanoparticles have been aggregated in Hal/CTAB/Pd.This issue as long with low Pd content of this sample can justify its low catalytic activity.Regarding the two other samples, Pd nanoparticles were found to have homogeneously dispersed on Hal.
Next, the zeta potential value of the three as-prepared catalysts was estimated.This value increased in the As the electric charge of the exterior surface of Hal is negative, it is quite expectable that the CTAB-free sample, Hal/Pd, has the lowest zeta potential value.In Hal/Pd/CTAB and Hal/CTAB/Pd, which contain surfactant, CTAB neutralized the negative charge of Hal and then led to the increase of zeta potential value compared with Hal/Pd.As proven by TGA and FTIR, the content of CTAB in Hal/Pd/CTAB was higher; hence, the zeta potential value of Hal/Pd/CTAB is higher than that of Hal/CTAB/Pd.

| Further characterization of Hal/Pd
Finding Hal/Pd as the catalyst of choice, it was further characterized via XRD, BET, and elemental mapping analysis.
Elemental mapping analysis of Hal/Pd in Figure 7 implies that apart from Si, Al, and O atoms that are representative of Hal, Pd atom is also present in the structure of the catalyst.It was observed that the dispersion of Pd NPs was uniform.
XRD analysis of Hal/Pd was also conducted to elucidate the crystal phase of Hal/Pd in Figure 8.According to the literature, [83] the observed peaks at 2θ = 19.9 , 24.4 , 26.6 , 38.5 , 55.2 , 62.5 , 73.9 , and 77.4 are assigned to the characteristic bands of Hal (JCPDS No. 29-1487).Noteworthy, the absence of the characteristic peaks of Pd nanoparticles can be attributed to the low loading and high dispersion of these particles. [84]he comparison of the specific surface area of the pristine Hal and Hal/Pd showed that this value decreased slightly from 49.2 to 44.3 m 2 /g upon immobilization of Pd nanoparticles.On the other hand, the value of total pore volume for both samples, Hal and Hal/Pd, was almost similar (0.19 cm 3 /g).These results indicate that Pd nanoparticles were formed on the exterior surface of Hal.As shown in Figure 9, the N 2 adsorption-desorption isotherms of both Hal and Hal/Pd, are of type IV with H3 hysteresis loop, implying the mesoporous structure with slit-like pores. [85]

| Recyclability
The recyclability of Hal/Pd was also investigated by reusing the recovered catalyst for several reaction runs.In more detail, at the end of the first run of hydrofinishing, Hal/Pd was separated via centrifugation, recovered, and then reused for the next run of hydrofinishing.In Figure 10, the yields of hydrofinishing for five consecutive runs are depicted.As illustrated, the reused Hal/Pd almost maintained its catalytic activity, and only slight loss of the catalytic activity was observed after each run.Characterization of the reused Hal/Pd after the fifth run via ICP approved insignificant leaching of Pd nanoparticles from Hal ($1 wt.% of initial Pd content).It is worth noting that hot filtration test was also conducted to investigate the heterogeneous nature of the catalysis, and the results indicated that the catalysis was heterogeneous and Pd nanoparticles were remained stabilized on the support in the course of hydrogenation reaction.
In summary, to study the effect of hydrophobicity of Hal on its performance as a support for Pd nanoparticles, three catalysts with different hydrophobicity, that is, Hal/Pd, Hal/Pd/CTAB, and Hal/CTAB/Pd, were prepared, and their catalytic activity for hydrofinishing of PAO was examined.Both experimental and computational results indicated that introduction of CTAB had a detrimental effect on the catalytic activity of the catalyst and Hal/Pd showed the best catalytic activity.In fact, characterization of the three samples confirms that not only the presence of CTAB could affect the catalytic activity but also the synthetic procedure and the order of incorporation of Pd nanoparticles and CTAB were influential on the catalytic performance of the catalyst.According to the results, in Hal/CTAB/Pd, introduction of CTAB prior to Pd immobilization led to low Pd loading, particle aggregation, and consequently the lowest catalytic activity.However, no linear relationship with the catalytic activities was achieved.
S C H E M E 2 Relative energy profile (in kcal/mol) in gas phase at the B3LYP-D3BJ/cc-pVTZ$sdd//BP86-D3BJ/Def2SVP $ sdd level of theory (in parenthesis including explicit solvent effects by solvation model density [SMD] [hexane]) of the ethylene hydrogenation via Pd-Hal catalyst (in red and blue alternative pathways; in green the interaction of cetrimonium bromide [CTAB])

F
I G U R E 1 %V Bur and steric maps on the metal center bonded to the halloysite: (a) empty, (b) with H 2 , (c) with C 2 H 4 , and (d) with cetrimonium bromide (CTAB), (e) CTAB + H 2 , (f) CTAB + C 2 H 4 (xy plane, with the palladium placed in the center and the z-axis the oxygen bonded to palladium, including the oxygen of the closest methoxy group in the xz plane.Curves are given in Å, with a radius of 3.5 Å)

F I G U R E 3
Fourier transform infrared (FTIR) spectra of the asprepared catalysts F I G U R E 2 Nuclear magnetic resonance ( 1 HNMR) spectra of unhydrogenized (above) and hydrogenized (below) polyalphaolefin (PAO) at catalyst loading of 5 wt.%, hydrogen pressure of 8 bar and temperature of 130 C

F I G U R E 6
Transmission electronic microscopy (TEM) images of the asprepared catalysts F I G U R E 7 Elemental mapping analysis of halloysite (Hal)/Pd

F I G U R E 1 0
Recycling of halloysite (Hal)/Pd for hydrofinishing of polyalphaolefin (PAO).Reaction condition: T = 130 C, catalyst dosage = 5 wt.% and P H2 = 8 bar F I G U R E 8 X-ray diffraction (XRD) pattern of Hal/Pd F I G U R E 9 Nitrogen adsorption-desorption isotherms of halloysite (Hal) and Hal/Pd