A comparison of structure-activity of Cu-modied over different mesoporous silica supports for catalytic conversion of levulinic acid

Catalytic hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) was investigated over four different types of mesoporous silica (SBA-15, MCM-48, MCM-41, and KIT-6) supported copper (5 wt.%) catalysts. The Cu was incorporated into mesoporous silica supports by a sequential impregnation method. A detailed investigation of the support structure-activity correlation was done and the performance of the catalysts in LA conversion was studied. Detailed characterisation techniques were used to evaluate the physical and catalytic properties of the studied catalysts. The structure type and physicochemical properties of the silica support had a signicant effect on the overall performance of the catalysts. Among them, over Cu/SBA-15 catalyst, a complete levulinic acid (LA) conversion with 98% gamma valerolactone (GVL) selectivity was achieved at 265°C under ambient H 2 pressure. The superior performance of Cu/SBA-15 catalyst was due to the high surface acidity, reducibility of Cu oxides species, and highly dispersed Cu particles over SBA-15 structure. The results conrmed that the activity of the catalysts is signicantly affected by the textural properties, surface acidity and copper dispersion. Durability of all the catalysts were tested for 50 h time on stream and over the SBA-15 catalyst, only a small drop in the activity was observed.


Introduction
Levulinic acid (LA) has been considered as one of the most versatile and renewable molecules produced mainly from lignocellulosic biomass. It has a great potential in biore nery applications [1][2][3]. Due to this, considerable efforts are being made and multiple strategies are being proposed for the e cient conversion of LA to value-added compounds and fuels. Among the various transformations of LA, catalytic hydrogenation of LA to γ-valerolactone (GVL)is an important route in the valorisation of biomass platform molecules to specialty chemicals [4,5]. GVL has been widely used as a sustainable liquid feedstock in the production of transportation fuels and as an additive. Further, GVL acts as a potential monomer in the manufacturing of different kinds of plastics and it can be directly utilized as a green solvent [6]. Moreover, GVL has been used as a hydrogen carrier in the fuel cell applications and also used as a 20% biofuel (produced from GVL) balanced with conventional fuel in the biofuel engine and used in the production of non-enzymatic sugars from biomass.
Catalytic hydrogenation of LA to GVL were studied over a wide variety of noble (Ru, Pt, Pd, Au, and Ir) [7][8][9][10][11], non-noble (Cu, Ni, and Co) [12][13][14] and bimetallic (RuNi, RuSn,CuFe and CuNi) [15][16][17][18] catalysts in both continuous and batch reactor systems under various reaction conditions. Among the catalysts reported, ruthenium (Ru) based catalytic systems are considered to be the most active and selective in the conversion of LA to GVL in both i.e., homogenous and heterogeneous media [17]. Many reports are focused on non-noble based base metal catalysts due to their cost and abdundance. Developing Cubased catalysts is more appropriate and industrially viable for the future biore nery applications. The copper based catalysts are considered to be promising alternatives to the noble metal catalysts, which have shown moderately high performance in the catalytic transformation of LA to GVL. For instance, Upare et al. reported over 5 wt.% Cu/SiO 2 catalyst exhibited a complete conversion of LA with 94% GVL selectivity and 6% angelica lactone (AL) at 265°C and 0.1MPa [19]. Lomate et al. 2017 studied the effect of Cu modi ed SiO 2 support with different textural properties of SiO 2 and exhibited varied activities in vapor phase hydrogenation of levulinic acid. They found no correlation between activities and selectivity with support acidity, but strong Cu-support interactions due to partially oxidised Cu species was the main reason for the high activity [20]. Moreover, Rei et al., used a 20 wt.% Cu/SiO 2 catalyst, and achieved 73.4% LA conversion with 75.4% GVL selectivity at 250°C and WHSV of 3.30 h − 1 [21]. Recently, Sun et al. studied three different non-noble metal catalysts i.e., Co, Ni, and Cu based alumina catalysts. Overall, Cu/Al 2 O 3 is the most selective catalyst due to the high surface acidity and Cu dispersion [22]. Furthermore, in another study, Cu/Al 2 O 3 and Cu/ZrO 2 catalysts were reported in the hydrogenation of LA in a xed bed reactor at 265°C under ambient pressure and over Cu/Al 2 O 3, highest catalytic activity in terms of GVL selectivity and LA conversion was gained [23,24]. In most of the studies, the GVL selectivity was directly correlated to the active metal surface area, surface acidity, and the metal dispersion. Henceforth, designing and developing a highly dispersed, low cost, and abundant Cu-based catalyst in combination with a robust support was the aim of thiswork in LA hydrogenation.
Generally, mesoporous silica supports (SBA-15, MCM-41, MCM-48, and KIT-6) possess high speci c surface areas, ordered pore structures, and high thermal stability, which makes them ideal materials for many catalytic reactions. High surface area mesoporous silica (MS) supports can e ciently provide the maximum active sites per mass of catalyst. Moreover, MS supports avoid the diffusion limitations for the reactants and products due to their unique textural properties. However, changing the textural and surface properties of different mesoporous materials can lead to varied catalytic performances [25].
This research work aim is to study the Cu loaded mesoporous silica supported catalysts with different textural properties in a xed bed reactor atambient pressure. Thus, this work gives an overall correlation between the role of MS textural properties, metal-support interaction, copper dispersion withthe catalytic activity. In the present work, four different types of mesoporous silica supports were synthesized by a hydrothermal method and then copper nanoparticles were deposited by a wet impregnation method. The catalysts characterized by using various analysis techniques and methods such as microscopic, spectroscopic, XRD, UV-DRS, N 2 sorption measurements, SEM, TEM, H 2 -TPR, and NH 3 -TPD. Active surface area and dispersion of copper were calculated from the N 2 O titration method. Catalytic activities were determined in terms of LA conversion, GVL selectivity and catalysts stability.

Materials and methods
The parent silica materials (SBA-15, MCM-41, MCM-48, and KIT-6) were synthesized from the study [26]. In the synthesis of SBA-15, 4 g of P123 (Poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) polymer and 9.44 g of KCl were dissolved in 2M HCl solution (~ 90 g). The nal solution was stirred for 2h at 35°C until homogeneously mixed. In the next step, 6.44 g of TEOS (Tetraethyl orthosilicate) was added dropwise under continuously stirring for 2h. The complete mixture was then transferred to a sealed Te on lined container and kept at 100°C for 24 h under static conditions. The resulting precipitate was ltered and washed with deionised water (pH 6.9 ± 0.2) until pH 7.0 ± 0.2 was attained. Further, it was dried for 12 h at 80°C and followed by calcination at 500°C for 6h under air.
For MCM-48, 2.4 g of CTAB (Cetyltrimethylammonium bromide) was mixed with 50 mL of deionized water, 50 mL of C 2 H 5 OH and 12 mL of NH 3 (32 vol.%). This mixture was stirred until a clear solution was obtained. In the next step, TEOS (3.4 g) wasadded to the mixture and stirred for 2 h. The nal product was ltered, extensively washed, dried overnight, and calcined for 6 h at 600°C.
MCM-41 was prepared by using 2.4 g of CTAB dissolved in 120 mLof deionized water and then stirred until a clear solution was visible. After dissolution, 8 mL of NH 3 (32%) was added to the solution, followed by dropwise addition of 10 mL of TEOS and stirring at 35°C for 24 h. The nal product was washed with ethanol/H 2 O, dried for 12 h at 60°C and calcined for 5h at 500°C.
For synthesis of KIT-6, Pluronic P123 (6 g) was dissolved in 215 mL of deionized water and 11.8 g of HCl (35% v/v) solution under stirring. After dissolution, a 6 g of n-butanol was added dropwise under continuous stirring. Next, a TEOS (12.9 g) was added and stirred at 35°C for 24 h. The nal solution was transferred into a sealed Te on container and subjected to hydrothermal treatment at 100°C for one day. The nal solid product was washed, ltered, and dried overnight and next calcined at 500°C for 5h.
The supported copper catalysts with 5 wt.% Cu loading was prepared by a wetness impregnation method.
In the typical synthesis procedure, 1. Catalyst Characterisation X-ray diffraction patterns were obtained by a D8 X-ray diffractometer (Bruker, Germany) using Cu Kα radiation (-1.5406 A°, 40 kV and 30 mA). Nitrogen sorption measurements were conducted on an Autosorb-1 analyzer (Quanta chrome) at -196 o C. Before analysis, the samples were degassed at 250°C for 6h under vacuum. The speci c surface area and pore size distribution were measured by using Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods, respectively. The UV-Vis diffused re ectance spectra (UV-DRS) were recorded by a UV-visible spectrometer (GBC scienti c-Cintra). Transmission electron microscopy (TEM) images were captured on a Hitachi HT7700 operated at 100 kV instrument. Scanning electron microscopy (SEM) images were taken on a Philips/Fei Quanta 200F SEM operated at 20 kV instrument.
Hydrogen temperature programmed reduction (H 2 -TPR) and temperature programmed desorption of NH 3 (NH 3 -TPD) were conducted on micrometric instruments (Auto Chem. 2910). In H 2 -TPRexperiment, 0.1 g of sample was pretreated by passing He gas at 200°C for 2 h and then cooled down to ambient temperature. TPR analysis was performed from 25°C to 600°C in a ow of 5% H 2 /Ar with a ramping rate 10°C/min. Moreover, in TPD-NH 3 experiment, 0.1 g sample was pretreated by a ow of He gas at 200°C for 2 h followed by saturation with 10% NH 3 /He gas at 80°C for 1 h. It was then purged with He gas at 140°C for 2 h. TPD analysis was carried out from 25°C to 600°C at a ramping rate of 10°C/min. Grams/32 software was used to calculate the amount of H 2 consumed and desorbed NH 3 values. N 2 O decomposition experiments were also conducted on micrometric instruments (Auto Chem. 2910).
Prior to analysis, the sample was reduced with H 2 gas at 300°C for 3 h followed by He purges for 30 min and then cooled to 80°C. Thereafter, the sample was exposed to 2%N 2 O-He for 30 min to oxidize Cu to By assuming copper metal to bespherical crystallites, the particle size can be calculated from following where ρ Cu is the copper density (8.92gcm − 3 ).

Catalytic activity
The catalytic hydrogenation of LA was performed in a xed-bed glass reactor with 9 mm inner diameter and 32 cm in length over a temperature range from 225 to 300°C at 0.1MPa hydrogen pressure. The catalyst (0.3 g) was mixed with glass beads and loaded at the center of the reactor with glass wool packed from both ends. Beforethe catalytic activity test, the catalyst was pretreated with H 2 stream at 350°C for 3 h. After reduction, the reactor temperature was cool down to 265°C and a stream of aqueous solution (10 wt.%) of LA was fed into the reactor along with H 2 at WHSV 0.550 h − 1 . The reaction products were collected after every hour in an ice trap and later analyzed by GC-MS (HP5973-GC-MSD) by using capillary column (HP-1MS).

Results And Discussion
Low angle XRD patterns of four pure supports (SBA-15, MCM-41, MCM-48, and KIT-6) and their respective Cu impregnated samples have been presented in Figure. 1a and inset in Fig. 1a, respectively.The lowangle diffraction patterns of SBA-15 and Cu/SBA-15 exhibited well-resolved diffraction peaks at around 2q-0.9° corresponds to (1 0 0) re ection of SBA-15 [27] and indicating a hexagonal unit cell. The results con rmed that the mesoporous silica structure was well intact after the copper incorporation. However, the impregnation of copper on the silica support resulted in a low intensity peak, which could be ascribed to the scattering phenomenon of metallic copper nanoparticles. The XRD pattern of pure MCM-41 (inset in Fig.1a) shows three wall-separated re ections at 2q-3.7° (a strong intense peak), around 2q-4.5° and 2q-4.7°, denoted as weak intense peaks, corresponding to d 100 , d 110 and d 200 planes, respectively, which was considered as the ngerprints of ordered mesoporous structure [28] of MCM-41 materials. After impregnation of copper over the MCM-41 support, the intensity of all XRD peaks decreased considerably due to occupied Cu species in the cylindrical mesopores of the MCM-41 support. The diffraction peaks of MCM-48 and Cu/MCM-48 were found in well agreement with the unique 3D cubic structures of MCM-48 as reported elsewhere [29]. Moreover, KIT-6 sample exhibited well-resolved re ections, which were assigned to a well-ordered 3-D cubic Ia3d symmetry of silica structure [30]. Further, the 5Cu/KIT-6 sample manifested a cubic structure, which was very similar to its parent sample. All Cu impregnated mesoporous samples exhibited a steady declined in their diffraction peak intensities as compared to their respective parent samples. The wide-angle XRD patterns of the supported copper samples have been illustrated in Fig.1b Moreover, N 2 adsorption-desorption analysis was carried out to determine the speci c surface area, cumulative pore volume and average pore size of the prepared catalysts. N 2 sorption isotherms and the respective BJH pore size distribution pro les of pure supports and supported copper catalysts have been presentedin Fig. 2a and Fig. 2b. For all four support catalysts, a sharp enhancement in the adsorbed and desorbed isotherms were displayed in the relative pressure range of 0. catalysts compared to pure support materials (Table1). The incorporation of copper into mesopores resulted in increased density of Cu sites and thus, a considerable decrease in the surface area of supporting material was observed. However, in the case of Cu/KIT-6, the average pore diameter was slightly increased which ascribed the blocking of the small pores [26].
The UV-DRS spectra for all the supported catalysts have been presented in Fig.3. All the catalysts exhibited one common intense absorption band ranging from 200 to 280 nm, which could be originated from the charge transfer transition between ligand O 2and Cu 2+ in an isolated state [33]. Further, a slight shoulder peak was also observed for the Cu/SBA-15, Cu/MCM-48 and Cu/KIT-6 catalysts at around 300 nm, indicating the limited presence of oligomeric cluster-like moieties originating from the charge transfer between Cu 2+ and O 2in the (Cu-O-Cu) n surface species [34]. The charge transfer band of Cu/MCM-48 was found to be more intense and broader compared to other catalysts indicating the formation of mononuclear oxide species in a dispersed state. The broad absorption band with weak intense band was observed between 500-800 nm, and this band was assigned to electron d-d transitions in Cu 2p in a octahedral surrounding by the oxygen in CuO particles [35].
The TPR pro les and their respective hydrogen consumption values of the supported copper catalysts have been presented in Fig. 4 and Table 2 respectively. From Fig. 4, herein, two similar reduction peaks at 220°C and ~290°C were observed for Cu/SBA-15 and Cu/MCM-48 catalysts, respectively. The low temperature peak was corresponding to a nely dispersed reduced CuO to Cu, while the high temperature peak was originated from the reduction of bulk CuO species presented in the mesoporous silica supports.
The published reports related to the reducibility of supported copper catalysts have suggested that bulk CuO is hard to reduce as compared to the highly dispersed copper oxides [36]. In many studies, similar peaks at ~220°C and ~290°C in Cu/MCM-48 corresponds to widely dispersed over support surface and the bulk CuO species presented inside the structure, respectively. The TPR pro les for the Cu/MCM-41and Cu/KIT-6 consisted a single reduction peak in high temperature region (around 310°C). The pro le for Cu/MCM-41 corresponds to the reduction of bulk CuO to metallic copper, while the reduction pro le of Cu/KIT-6 suggested the shifting of peak towards higher temperature region due to the existence of copper oxides in the bulk form. Typically, H 2 consumption of Cu/MCM-48 in a low temperature region was found to be relatively high compared to all other catalysts. Thus, indicates that the most of CuO species were highly dispersed over the MCM-48 and SBA-15 supports and that could be easily reduced. The results were found to be in well agreement with the XRD, UVDRS, and textural properties. The TPD pro les of supported copper catalysts have been shown in Fig.5 and corresponding data have been summarised in Table 2. All the four catalysts showed one desorption peak below 120°C that is due to the physisorbed NH 3 over the surface of the catalyst. The Cu/SBA-15 and Cu/MCM-48 shown a shoulder peak in between 120-300°C which was attributed to the desorption of ammonia from the weak acid sites [37]. All the catalysts exhibited broad desorption peaks at higher temperature (i.e., above 300°C) due to the existence of moderate acid sites on the surface. Further, the total surface acidity of the supported mesoporous silica catalysts was found to be in sequence of Cu/SBA-15>Cu/MCM-48> Cu/MCM-41> Cu/KIT-6 ( Table 2).
Copper dispersion has been de ned as the ratio of the number of copper atoms present over the surface to the total number of copper atoms. The copper dispersion, active metal surface area and crystallitesize of copper supported silica catalysts reported in Table 3. According to the N 2 O decomposition results, the H 2 uptake, dispersion, and metal area of the mesoporous silica-supported copper catalysts presented was in the decreasing order: Cu/MCM-48>Cu/SBA-15>Cu/MCM-41>Cu/KIT-6. It was observed that the Cu/MCM-48 catalyst exhibited highest copper surface area and better dispersion compared to other catalysts ( Table 3). The reason for this might be the presence of high number dispersed copper sites over MCM-48 surface. The SEM (Fig. 6) and TEM (Fig. 7) analysis were conducted to determinethe morphologies (shape and size of the pore) of the mesoporous silica-supported copper catalysts. SEM image of SBA-15 displayed multiple rope like domains of relatively uniform in size. MCM-48 comprised of the spherical particles with diameter ranging from 0.3 to 0.5µm while pure KIT-6 support exhibited spherical platelets like morphology. The TEM images of catalysts exhibited a well-de ned ordered structure with different sizes. It was noticed that the mesoporous framework of the four samples retained their morphology even after impregnation with copper particles.

Catalytic activity results
Evaluationof the catalytic activity of mesoporous silica-supported copper catalysts was conducted in a continuous xed-bed reactor at 265°C and 0.1 MPa hydrogen pressure (Table 4). In LA hydrogenation, GVL was found to be the primary and desired product, while angelicalactone (AL) and valeric acid (VA) were produced as the by-products. It was clearly evidentthat the, rst LA was dehydrated to AL and then hydrogenated to GVL in the nal stage. According to previous reports, over 5 wt.% Cu/SiO 2 shown complete conversion of LA with selectivity to GVL of 94% and AL of 6% at 265°C and 0.1MPa was obtained. Further, in another study, 20 wt.% Cu/SiO 2 catalyst has shown 73.4% LA conversion and 75.4% GVL selectivity at 250°C and WHSV of 3.30 h −1 [21]. In the present study, we achieved better results with lower Cu loading compared to published data on Cu-based catalysts and overall, 5 wt.% Cu/SBA-15 exhibited complete conversion of LA and 98% selectivity to GVL at 265°C temperature and 0.1MPa hydrogen pressure. Moreover, Cu/MCM-48 possessed high active surface area, Cu dispersion and smaller metallic Cu particle sizes compared to Cu/SBA-15, Cu/MCM-41, and Cu/KIT-6. In addition, Cu/MCM-48 showed higher metal surface area however, a lower catalytic activity was exhibited compared to Cu/SBA-15. This indicated that in case of Cu/MCM-48, a low surface acidity and also narrow pores could not render LA to the pore channels and led to low accessibility to the active sites compared to Cu/SBA-15. The conversion of LA over supported mesoporous silica catalyst was in the sequence of Cu/SBA-15>Cu/MCM-48>Cu/MCM-41>Cu/KIT-6. The order of the LA conversion was observed to be consistent with that of the total surface acidity, thus, exhibiting highest LA conversion for Cu/SBA-15 catalyst. The superior catalytic activity was attributed to the execess availability of the reducible copper species in the mesoporous channels of the catalysts. Among all catalysts, 5 wt.% Cu/SBA-15 achieved superior performance and displayed complete LA conversion with 98% GVL selectivity. Apparently, LA is converted to AL at rst followed by its subsequent hydrogenation to GVL. Under the studied reaction conditions, the optimum reaction temperature for converting LA to GVL was 265°C.
There are many copper-based catalytic systems reported in the literature with different Cu loadings over the various support materialsand reaction conditions. It is hard to compare all of them with our study due to variation in composition, and operating conditions. However, we collected few relevant data which had similar conditions to this work were summarised in Table 5 and short comparison between the reported Cu-based catalysts with this work was made. Recently, complete LA conversion can be achieved at mild temperature, i.e., below 150° but at high operating pressures [38]. It was an evident that the trade-off factor between operating temperature and pressure in the LA hydrogenation activity.

Conclusions
Vapor-phase hydrogenation of LA to GVL was carried out in a xed-bed reactor over different mesoporous silica-supported copper catalysts at ambient hydrogen pressure. From characterisation results, the diffraction patterns and N 2 adsorption-desorption studies revealed that the Cu impregnated catalysts retained their mesoporosity and the support structure was intact. Moreover, XRD results showed the presence of a highly dispersed CuO species over all the catalysts except Cu/KIT-6. The support structure had a profound effect on the activity of the catalyst. The copper modi ed SBA-15 and MCM-48 had desired properties and presented better performance compared to other catalysts. Overall, the Cu-SBA-15 catalyst exhibited the highest catalytic activity in terms of LA conversion (100%) and GVL selectivity (98%) compared to other prepared catalysts due to its high surface acidity and Cu dispersion.The study further indicated that the copper surface area, acidity and diffusion limitation of reactants into pore channels were the key factors, which in uenced the activity of catalysts. The overall activity trend of different mesoporous silica-supported copper catalysts for the LA conversion was found as Cu-SBA-15>Cu-MCM-48>Cu-MCM-41>Cu-KIT-6. Declarations