Hierarchy in zeolite catalysis: The influence of enhanced mesoporosity on the synthesis of renewable fuels and bio-based platform chemicals

Faujasite (FAU), ZSM-5 (MFI), beta (BEA) and mordenite (MOR) zeolites were admitted to a variety of chemical treatments accompanied by surfactant templating strategy, aiming to introduce the intracrystalline mesoporosity effectively. The resulting materials were tested as solid acid catalysts for esterification of the oleic acid as a common model impurities found in bio-oil feedstoks. It was found that the esterification of oleic acid can be enhanced by the presence of strong acid sites in zeolites and their improved accessibility. Overall, mesostructured FAU zeolite demonstrated an improved catalytic performance as a result of increasing accessibility of the zeolite active sites.


Introduction
In the last years, significant attention has been focused on sustainable and green energy applications due to oils price fluctions, environment change and cost of high quality fuels. [1,2] No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS) E219 basic media, which results in the formation of a network of ordered mesopores within the zeolite .The present paper is focused on these nanostructured catalysts and on the effect of their properties on the catalytic efficiency in esterification of oleic acid as potential bio-refinery related applications.

Experimental section 2.1 Synthesis of hierarchical zeolites
A detailed description for preparation of these zeolites is available in Ref [42].
Faujasite zeolite (CBV100) with Si:Al molar ratio 2.5 was purchased from Zeolyst International and modified to obtain the hierarchical zeolite (HFAU2.5).The Na-type FAU zeolite was treated with 6 mol L -1 of ammonium nitrate solution (Sigma-Aldrich, >99 wt%) at 80 o C, followed by filtering and washing , and then calcined at 450 o C for 5h in static air to convert the ammonium form to H-type FAU zeolite.
The MOR (CBV 21A) and BEA (CP814C) zeolites with Si:Al molar ratio 10 and 19 respectively, were also acquired from Zeolyst International. 2 g of parent MOR and BEA zeolites were calcined in air at 450 o C for 5h and then stirred in 50 mL of basic solution of 0.15-0.5 mol L -1 Tetramethylammonium hydroxide pentahydrate (Alfa Aesar, 98%). After that 1-1.75 g of n-cetyltrimethylammonium bromide (CTAB, Alfa Aesar, 98%) was added for this mixture. Next, after 1h, the synthesis mixtures were placed into Teflon-lined autoclaves and heated to 150 o C using CEM Mars 6 microwave (CEM Corporation) at 2.45 GH. The heating time was varied from 15 h to 18 h. The initial ramp time was 20 minute and the power output did not exceed 400 W. The products were filtered and washed with deionized water.
The parent MFI (CBV 8014) zeolite with Si:Al molar ratio 40, was obtained from Zeolyst International and stirred for 30 min. at 80 o C with 50 mL of basic solution containing 0.2 mol L -1 from Sodium hydroxide (Fisher Scientific ,98%) and tetrapropylammonium hydroxide (TPAOH,Alfa Aesar,1M) .This suspension was added to solution containing 1 g of CTAB and leave it for stirring 1h. The resulting synthesis mixture was conducted under microwave hydrothermal synthesis according to the method mentioned above for 8h to 16h.

No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS)
E220 Finally, all above solids were dried at 60 o C overnight and then were calcined first in the flow of nitrogen at 450 o C (temperature ramp of 1.5 o C min -1 ) for 1h. Then, the gas flow was switched to oxygen, the temperature was increased to 500 o C (temperature ramp of 2 o C min -1 ) and kept for 2h before cooling these samples.

Characterisation of zeolites catalysts
A comprehensive structural characterisation of all the materials utilised in this work was carried out using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), low temperature nitrogen adsorption, N 2 Physisorption (BET) and FTIR spectroscopy.
A detailed description is also available in Ref [42].
All the solid materials were characterized by different techniques. Powder X-ray diffraction (XRD) patterns of various catalysts were recorded on Brucker D8 Advance diffractometer with their pore volume and the pore size distribution from NLDFT model were performed using nitrogen adsorption on a Quantachrom Autosorb instrument.
In order to monitor the concentration , acid strength and heterogeneity of both Brønsted and Lewis acid sites independently,10 mg of zeolite disc was prepared by compacting a certain amount of material in a metallurgical die under 1 bar and admitted to 10% of the in situ infrared energy on a Nicolet iS10 FTIR spectrometer from Thermo scientific in the range 6000- E221 cm -1 ) and ε(L)=1.71 cm Pmol -1 for Lewis acid sites (LAS, IR peak at ~1455 cm -1 ). The error margin for the acid site quantification was estimated as ±5%. [44] .To gain deeper insight into structural properties of zeolites, physic-chemical information of zeolites used in this work is shown in Table (1).

Catalytic experiments
The esterification reaction of FFA removal with methanol in the presence of various zeolites was carried out in Biotage microwave synthesiser (Biotage Initiator+). A high precision glass vial 10-20 ml was used in order to improve durable and safe reactions at all times with highly wide range to withstand pressures beyond 30 bar. The microwave heating provides significant thermal impacts, which are relative importance for chemical reaction. The reaction was comprised of adding an excess oleic acid (Sigma-Aldrich, 99%) into grapeseed oil (local market) as 10 vol%, and then reacted with methanol (Fisher Scientific, 99.99%) as 1:30-1:50 molar ratio between them at 100 o C for different run times, which varied between 10-30 min. (1) In addition, the fraction of the FFA removed after separating from methanol and glycerol was used in this work to calculate the conversion of FFA from the oil using below equation: Where FFA s is the percent free fatty acid after esterification and FFA i is the same percent in the oil mixture before the reaction.

Results and discussion
( Figure 1) presents the XRD diffraction patterns of conventional and hierarchical zeolites synthesised in this work. XRD analysis revealed that the treated zeolites still maintain significant crystallinity.This suggests that the treatment of zeolites has been improved successfully without any destruction for the zeolite structure, consistent with previous literatures. [37,42,43] The results of low angle XRD patterns (Figure 2), also indicate that the ordered hexagonal pore arrangement of mesostructured zeolites is observed after the treatment.

E224
It is relevant to say further evidence in mesoporosity is investigated by the N 2 physisorption isotherms (Figure 3), which shows a type IV isotherm of hysteresis loop for mesoporous materials in this study. [42,47] In addition; these materials exhibit a significant nitrogen uptake at relative pressure (P/P o ) higher than a pressure in the parent zeolites.

Fig. (3) N 2 adsorption and desorption isotherms for parent and hierarchical zeolites.
In spite of the challenging that associated with surface properties of mesostructured zeolites, some questions regarding the pores and the density of the active site remain answered only inconclusively. These ideas had been put forward earlier in a more extended form by other people in order to explain the mechanism of mesoporsity formation. It should be clearly stated that, and from our previous work, the surfactant templated zeolite mesostructuring strategy was quiet successful for zeolite crystal rearrangement process in one-step by the treating the zeolite with basic solution of surfactant, which led to form mesostructured zeolitic one-phase hybrid material without any an amorphous aluminosilicates (see the images of SEM in Figure 4). In

No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS)
E225 contrast, the two-steps zeolite recrystallization process usually accompanied with an amorphous phase. [26,41,48]  E226 acid such as citric acid can dealuminate some of the O-Al bonds, which facilities the formation of the mesoporosity in this type of zeolite. However, the surface area and total pore volume both increase with increasing alkaline or acid concentrations in above treatment, while the micropore volume was conserve. [49] Zeolite acidity including both of frequencies in the Si (OH) Al bands of the Brønsted and Lewis acid sites can be quantified comprehensively by utilising the FTIR spectroscopy. [50] Pyridine is being used in this study in order to calculate the amount and type of acid sites (Table 2) [42].However, it should be noted that ammonia is good candidate for the acid sited in the narrow-pore zeolites with a diameter < 10 Å . [51]  Most importantly, all the IR spectroscopy data reported in this work proved that the surface defects formation upon the treatment by surfactant in a basic media is linked with the loss of some of Brønsted acid sites, but increased the Lewis:Brønsted ratio, suggesting formation of hydroxyl nests (silanol terminal group ~ 3740cm -1 ) see ( Figure 5). [42,52] No.29- ( Table (3). It can see from these data, the high conversion appeared on BEA and MFI zeolite catalysts due to a wide pore size and the a robust strength of acid sites for these materials respectively. In accordance with catalytic performance of FAU zeolite in this reaction, although FAU2.5 zeolite has a high amount of acid sites than those of other zeolites, the conversion over this material was not significantly different with

No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS)
E228 that of MOR10 zeolite. This could be attributed to the amount of acid sites and pore entrance size on MOR10 zeolite, which might also affect to the catalytic reaction. [25,53] But this is not the whole story; it seems that the introduction of mesoporosity in FAU zeolite induced the high conversion of FFA from oil because of enhancement of acid sites accessibility to oleic acid.
Nevertheless, HMOR10 zeolite exhibited similar conversion of FFA compared with parent zeolite despite the diffusion-restricted one-dimensional channel pockets (8-MR) for bulky molecules such as oleic acid. [52,54] The catalytic activity of spent zeolite catalysts did not change significantly after five consecutive runs, confirming that it is possible to recycle these catalysts for several times in esterification reaction. The information on the catalytic conditions for the HFAU2.5 zeolite catalyst is presented in ( Figure 6). The catalyst amount was adjusted as 7 wt% because there is no change in the conversion after 10 wt%. The reaction time and the molar ratio between oil and methanol can affect the reaction and consolidate it forward. Therefore, we selected three different ratios 1:30, 1:40 and 1:50 with reasonable time range from 10 to 60 minutes under mircrowave heating.
Our findings confirmed that the higher molar ratio reduces the mass transfer stage because of the immiscibility between the methanol and oil layers and thus, the rate of esterification increased in a shorter time.

Conclusions
Mesostructured zeolites with a binary pore size distribution were prepared in this work using the surfactant-templated strategy and utilised as potential acid catalysts in the esterification reaction of free fatty acid from bio-oil. Although the treatment has not altered the catalytic efficiency of BEA, ZSM-5 and MOR to remove the oleic acid from grapeseed oil, which appears to be determined by the strength of BAS, a higher conversion of oleic acid can be achieved over the hierarchal faujasite zeolite catalyst due to the combination of improved accessibility of stronger acid sites and lower mass transport limitations.. The increase moderate mesoporosity development leads to enhanced the catalytic activity, and give a good opportunity for other zeolite materials such as hierarchical mordenite zeolite catalyst to exploit this material for different zeolite catalysis reactions.