Enhancement of hydrocarbon production and catalyst stability during catalytic conversion of biomass pyrolysis-derived compounds over hierarchical HZSM-5

Abstract

In order to promote hydrocarbon production and catalyst stability in catalytic fast pyrolysis (CFP) of biomass, HZSM-5 catalysts were treated with alkali solutions to introduce mesopores into the microporous system. Parent and hierarchical catalysts were tested in catalytic conversion of furan as an important intermediate from biomass fast pyrolysis (BFP). Controlled desilication with mole concentration of NaOH of 0.3 M resulted in sheet-like mesopores on the external sphere, enhancing mass transfer in the catalyst, and it specifically promoted the carbon yield of hydrocarbons by 21.6%. Though the coke content on these HZSM-5-0.3M catalysts increased gradually by 11.6%, the tolerance toward deactivation by coke deposition was improved. Cyclic tests of catalysis-regeneration process over hierarchical HZSM-5-0.3M over 20 cycles revealed that it can withstand long-running with a stable yield of hydrocarbons being achieved. Thus, hierarchical HZSM-5 is a suitable catalyst for CFP of biomass and its derivatives to hydrocarbons by this simple synthetic process.

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Introduction

Catalyst deactivation is the biggest barrier in CFP of biomass for hydrocarbons that can be great sources of most liquid and gaseous fuels.^1–3 A high carbon yield (23.7%) of olefins and aromatic hydrocarbons from CFP of pine wood in a fluidized bed reactor has been achieved; but, the actual yield is much lower than the theoretical yield (more than 60%), which is mainly due to catalyst deactivation caused by coke deposition.⁴ Among all of the studied catalysts, ZSM-5, a three-dimensional micropore system that consists of two perpendicularly intersecting channels of 10-membered rings, i.e., straight channels (5.5 × 5.1 Å) and zigzag channels (5.6 × 5.3 Å), is a well-known catalyst in CPF because it has moderate pore openings, internal pore space (i.e., pore intersection, d = 6.36 Å) and steric hindrance, all of which favors aromatic compound production.^5,6 In biomass zeolite-catalyzed hydrocarbon formation reactions, loss of ZSM-5 catalyst activity is mainly due to coke formation. Some high molecular weight oxygenates, larger than pore openings of ZSM-5 (like levoglucosan), cannot enter the pores of microporous catalysts and will polymerize and form coke on the catalyst surface. Some small-molecule oxygenates with high activity (like furans) can also polymerize and form coke on external acid sites of the catalyst.^7,8 Coke deposition may result in changes to the pore structure and catalyst acidity. On the one hand, it will increase the effective diffusion length, block the pore mouth on the surface of ZSM-5 and then inhibit the entering of pyrolysis vapor. On the other hand, coke species will cover lots of active sites, namely it leads to a decrease in the acid strength and amount.^9,10 To recover the activity of catalysts, the deactivated catalysts are usually regenerated in an air atmosphere in which the combustion temperature determines the result of regeneration. Regeneration at low temperature may not remove the coke completely, while higher temperatures may bring about overheating and furthermore break down the catalysts' structure.¹¹

Thanks to some developed effective methods, they can be used to decrease the effective diffusion length and also increase the specific surface area, thus improving the mass transfer ability of catalysts and promote the production of hydrocarbons.^12,13 To shorten diffusion pathways, nano-sized zeolites were proposed, thus improving the mass transport in the zeolite channel.^14,15 Another popular method is to introduce mesopores into microporous zeolite catalysts. The diffusivity of the microporous channel and the accessibility of the active sites are also greatly promoted.^16–18 There are usually two methods to introduce mesopores into the original microporous system by desilication, namely templating and alkaline treatment. A carbon source is impregnated with a zeolite precursor solution after which the material is subject to a hydrothermal treatment to grow the zeolite crystals. Subsequently the carbon and the template are burned away resulting in intracrystalline mesopores in the zeolite.¹⁹ Some templates are very expensive, which determines its limited application in industry. Removal of silica from the crystal framework with alkali solutions is an easily operated method that may cause the appearance of mesoporosity.^20–22 Selective dissolving and removal of silica from the zeolite framework can be realized by alkaline treatment. Silica extraction leads to the formation of structural defects in the lattice.^23,24 Thus, desilication by alkaline treatment is a preferential method to introduce mesopores. These hierarchical zeolites enhance the utilization of their active volume and have attained superior yields of targeted products in many catalysis reactions.²⁵

To improve carbon yield of hydrocarbons in CFP of biomass and promote catalyst stability, alkali treatment was carried out on HZSM-5 catalysts to introduce some mesopores into the microporous catalyst system. The hierarchical HZSM-5 catalyst showed better mass transfer. In our study, N₂ adsorption and transmission electron microscopy were used to investigate the existence of mesopores, and the morphology was greatly dependent on the treatment conditions. The parent and hierarchical HZSM-5 catalysts were employed in the catalytic conversion of furan as an important intermediate from BFP. The effect on hydrocarbon production was studied and the catalyst performance in cycle runs of the catalysis-regeneration process was discussed.

Experimental

Catalyst and desilication procedure

In this study, two series of HZSM-5 catalyst samples were compared: a commercial catalyst (purchased from catalyst plant of Nankai University, stated Si/Al ratio of 25) and its hierarchical counterpart. The hierarchical catalyst was prepared by desilication according to the following procedure: 2 g of the parent sample was treated in stirred aqueous NaOH solutions (0.1–1 M, 70 °C, 30 min, 33 mL solution per gram catalyst material). The mixtures were washed and filtered several times until the pH was neutral, and the isolated solids were dried at 110 °C for 12 h. All modified zeolites were converted to the protonated form by ion exchange in aqueous NH₄Cl solution (1 M, 80 °C, 4 h, 10 mL solution per gram catalyst material, 3 consecutive treatments) and washed several times followed by calcination in air at 600 °C for 5 h. These catalysts will be referred to as HZSM-5-P and HZSM-5-X respectively. X stands for the mole concentration of the NaOH solution used for preparation. It should be mentioned that Cl content of the final modified catalysts has been tested in the following section to eliminate the influence of Cl on the catalytic performance.

Catalyst characterization

The parent and desilicated samples were characterized by several techniques.

The structures of the ZSM-5 samples were determined through X-ray powder diffraction (XRD) by a Bruker instrument using Cu Kα radiation at 40 kV and 40 mA with a scanning speed of 0.02° min⁻¹ in the range of 5° ≤ θ ≤ 80°.

Energy Dispersive Spectrometry (EDS) was used to determine the element content, and then the Si/Al ratio was obtained, which confirmed that little Cl was left. EDS was performed on a Vantage IV system, Thermo Fisher Scientific, USA.

Nitrogen physisorption isotherms were measured using an Autosorb-iQ gas adsorption analyzer (Quantachrome, USA) at −196 °C. Each sample was outgassed at 200 °C for 6 h before measurement. The specific surface area (S_BET) was calculated using the Brunauer–Emmett–Teller equation (P/P₀ = 0.05–0.20). The total pore volume (V_total) was taken as the total uptake at P/P₀ = 0.995. The Barrett–Joyner–Halenda (BJH) method was used to determine the micropore volume and micropore size distribution. And the mesoporous volume was obtained by subtracting the micropore volume from the overall pore volume.

Scanning electron microscopy (SEM) experiments were performed to obtain the morphology of the fresh and modified catalysts. The measurements were performed with a FEI Inspect F50 system at 10 kV. Prior to each measurement, the samples were prepared on a carbon pad and sputtered with gold to obtain the necessary conductivity. The morphology and pore size of zeolites were also examined by transmission electron microscopy (TEM) on a Tecnai G2 microscope operated at an electron acceleration voltage of 200 kV. A small amount of catalyst was dispersed in ethanol, sonicated, and dispersed over a micrograte.

The acidity of the catalysts was estimated by the NH₃ temperature programmed desorption (TPD) technique. The sample was pre-treated at 600 °C in flowing He for 0.5 h. After pre-treatment, the sample was cooled to 100 °C and saturated with NH₃ gas. The physical absorbed NH₃ was then blown out. Finally, NH₃-TPD was carried out under a constant flow of He (20 mL min⁻¹). The temperature was raised from 100 to 600 °C at a heating rate of 15 °C min⁻¹.

Deactivated catalysts were regenerated in a flowing air atmosphere of 50 mL min⁻¹ in a thermogravimetric analyzer to determine the coke content by weight loss during oxidation. Typically, 15 mg of the sample was placed in the alumina crucible and heated from ambient temperature to 800 °C at a rate of 15 °C min⁻¹.

Catalyst tests

All catalytic tests were performed in a quartz glass fixed-bed reactor that has been described elsewhere.²⁶ The catalyst bed was activated by pure air flow (200 mL min⁻¹) at 600 °C for 1 h prior to each run. Then, the nitrogen flow was mixed with 1.68 g h⁻¹ furan via a spring pump, resulting in a weight hourly space velocity (WHSV) of 11.2 g furan per gram catalyst per hour. All experiments were carried out under atmospheric pressure and a furan partial pressure of P_furan = 12.30 Torr. After 20 minutes of catalysis, furan feeding was stopped and air was introduced into the reactor with flow rate of 500 mL min⁻¹ for 30 minutes, which should ensure that the coke was removed completely. During the coke combustion process, CO was converted into CO₂ in the copper converter that was full of copper oxide and then trapped by ascarite. The coke amount was measured by the weight increase in the CO₂ capturer. Product analysis was performed using gas chromatography. The product composition was determined with gas chromatography/mass spectrometry (GC/MS) (Agilent, 7890A-5975C) equipped with an HP-5 capillary column (30 m × 0.25 mm × 0.25 µm) and quantified by GC-flame-ionisation detector/thermal-conductivity (FID/TCD) with a Restek Rtx-VMS capillary column to qualify olefins and aromatics. Coke content was the proportion between weight of coke and weight of fresh catalyst.

Results and discussion

Physicochemical characterization

The XRD powder pattern of parent and desilicated HZSM-5 are shown in Fig. 1. Both types of catalysts displayed two diffraction peaks in the 8–10° and 20–25° range that are the characteristic reflections of MFI topology. The intensity of the reflection peaks decreased at higher NaOH concentration, whereas no big intensity difference was found when the NaOH concentration was lower than 0.5 M. This suggests that the effective crystalline size changes only after severe alkali treatment.

Fig. 1 XRD patterns of parent and hierarchical HZSM-5 catalysts.

The nitrogen adsorption–desorption isotherms of HZSM-5-P and its hierarchical counterparts are recorded in Fig. S1.† The isotherm of type I, characteristic of a microporous structure, was present in the parent HZSM-5; namely, high uptake was evident at low P/P₀. The hierarchical samples exhibited the characteristics of type IV isotherms with an obvious hysteresis loop corresponding to the gradual uptake over the P/P₀ range of 0.4–0.9, which reveals the presence of mesopores especially for HZSM-5-1M.²⁷ This can be further verified by the pore size distribution as shown in Fig. 2. For these samples treated with NaOH concentration lower than 0.3 M, mesopores with diameter sizes lower than 5 nm were produced, while larger mesopores appeared with NaOH concentrations higher than 0.3 M. The BET surface area, total pore volume and micropore volume derived from the isotherms are summarized in Table 1. The BET surface area increased from 239 m² g⁻¹ to 312 m² g⁻¹. Moreover, the total pore volume increased by alkali treatment caused by the presence of mesopores, and this occurred at the expense of micropore volume because of the partial damage of micropore framework.

Fig. 2 BJH mesopore size distribution of parent and hierarchical HZSM-5 catalysts.

Table 1 Textural properties of parent and hierarchical HZSM-5 catalysts

Sample	S BET (m^2 g^−1)	V micro ^a (cm^3 g^−1)	V total ^a (cm^3 g^−1)	Si/Al^b
a Calculated by t-plot method. b Calculated from EDS analysis.
HZSM-5-P	239	0.11	0.14	22.28
HZSM-5-0.1M	244	0.12	0.16	21.13
HZSM-5-0.3M	262	0.11	0.17	21.89
HZSM-5-0.5M	271	0.10	0.2	19.45
HZSM-5-0.8M	291	0.09	0.23	17.67
HZSM-5-1M	312	0.08	0.25	15.31

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The morphology of parent and alkali treated HZSM-5 were scanned by SEM and the images are shown in Fig. S2.† The surface of parent HZSM-5 was quite smooth, while lots of gullies appeared due to alkali corrosion. And partial catalyst frameworks collapsed at some extreme treatment conditions such as HZSM-5-1M. To furtherly understand the influence of alkali treatment on mesopore formation in catalysts crystals and size distribution of pores, TEM images were recorded to see the changes of crystals after treatment (see Fig. 3). The parent HZSM-5 showed uniform micropore distribution, whereas it consists of sheet-like materials on the outer sphere for HZSM-0.3M. For samples treated with higher concentrations of NaOH solution, the TEM images clearly reveal the appearance of intracrystalline mesopores, as marked by the arrows. The morphology of catalysts is greatly dependent on the treatment conditions for the HZSM-5 catalysts. It has been widely accepted that when different treatment conditions are used, the grown sites, shapes and sizes are quite different, leading to catalyst crystals with different morphologies.²⁸ Moderate alkali treatment produced sheet-like pores only on the outer sphere, and treatment with strong NaOH concentrations causes serious corrosion and hollow mesopores creation, as verified by BET and TEM characterization, because of the easy diffusion into the internal spaces.

Fig. 3 TEM images of parent and hierarchical HZSM-5.

The Si/Al ratio in Table 1 shows a very slight decrease for HZSM-5-0.1M and HZSM-5-0.3M, and then decreases sharply for samples with higher treatment concentrations of NaOH, which proves that serious dealumination accompanies desilication by the alkali treatment.²⁹ The total acid amount is proportional to the framework Al content. Therefore, the hierarchical HZSM-5 catalyst was expected to present very different acid properties. To validate the above hypothesis, NH₃-TPD was conducted, as shown in Fig. 4, to find the typical profiles with two peaks from NH₃-TPD analysis, of which the peak at low temperature (P_LT) was around 200 °C and that at high temperature (P_HT) was around 400 °C. Fig. 4(b) shows the area ratio between modified samples and original catalyst (S_M/S_P), and the area ratio between P_HT and P_HT (S_LT/S_HT). P_LT can be attributed to weak Lewis acid sites (e.g. extra framework aluminum).³⁰ The amount of P_LT for HZSM-5-0.3M decreased to 91% of the parent HZSM-5. This decreased sharply for HZSM-5-0.5M and HZSM-5-0.8M, and HZSM-5-1M finally showed quite a weak intensity for P_LT. P_HT is usually related to ammonia interacting with Brønsted type acid sites (e.g. framework aluminum).³⁰ An obvious gradual decrease of P_HT indicated that the alkali treatment mainly brought about the decrease of acid strength and amount of Brønsted acid sites. It indicated that alkali treatment may damage not only Si, but also the framework Al. For HZSM-5-0.3M, the characteristic MFI topology did not change, which can be found by XRD analysis. Though framework Al was partially deprived, the acid amount and strength is well kept for its catalysis for hydrocarbons. Thus, it can be assumed that hierarchical HZSM-5 catalyst presented relatively weaker Brønsted acid sites rather than Lewis acid sites compared with the parent catalyst.

Fig. 4 NH₃-TPD analysis of parent and hierarchical HZSM-5 catalysts (a) and a comparison of acid property between hierarchical and parent catalysts (b): (A) HZSM-5-P, (B) HZSM-5-0.1M, (C) HZSM-5-0.3M, (D) HZSM-5-0.5M, (E) HZSM-5-0.8M and (F) HZSM-5-1M.

Mechanism of desilication and mesopore creation

The Si–O–Al bond is usually thought to be weaker than the Si–O–Si bond and vulnerable to attack and hydrolysis, yet it only applies to acid treatment. In alkaline solutions, because of the negative charge of the AlO₄⁻ tetrahedron, the four-coordinated Al is protected from the attack of OH⁻, and Si–O–Al is not easy to hydrolyze. Each four-coordinated Al can protect four adjacent Si atoms from attack, thus the Si–O–Si bonds without adjacent AlO₄⁻ tetrahedra are more easily broken and desilication happens with terminal Si–OH formation during which the coordination environment is steady.³¹ For these ZSM-5 catalysts with low Si/Al ratio, the Al content and acid strength/amount is high, and the inevitable dealumination is accompanied by loss of four-coordinated Al. On one hand, when a high concentration of NaOH solution is used, both Si–O–Al bond and Si–O–Si bonds break and both dealumination and desilication of the framework happens. On the other hand, excessive alkali treatment may lead to direct falling off of integral quadridentate Al. Therefore, alkali treatment of HZSM-5 is a selective desilication process at moderate conditions, while Si–O–Al may also be cut when severe treatment conditions are used. The bond breaking alkali treatment process is described in detail in Fig. 5(a). The schematic diagram of the formation process of mesopores in HZSM-5 is presented in Fig. 5(b). Alkali corrosion begins from the outer sphere of HZSM-5 following the guide of Si atoms. First, sheet-like pores were formed, as indicated in the TEM images.³² Further diffusion of OH⁻ will permeate into the inner micropores and result in alkali corrosion around them.³³ Hollow mesopores with average diameters around 8 nm were formed when the concentration of NaOH was 0.5 M. Finally, excessive treatment leads to framework collapse and involuntary loss of acid sites. The average diameter of HZSM-5-1M is mainly around 10 nm. Thus, the degree of alkali treatment must be well controlled and the targeted mesopore shape (sheet-like or hollow) can be realized.

Fig. 5 Bond breaking (a) and formation process of mesopores (b) in HZSM-5 by alkaline treatment.

Catalytic conversion of biomass derivatives over hierarchical HZSM-5 catalyst

Catalytic conversion of furan, as an important biomass-derived compound, was investigated over parent and desilicated HZSM-5 catalysts to find out the optimal post-treating condition for maximizing the production of hydrocarbons. Fig. 6 shows the results of catalytic tests for the parent and hierarchical HZSM-5 catalysts performed at the pyrolysis temperature of 600 °C. Fig. 6(a) presents the general olefin and aromatic hydrocarbon yield and distribution with the change of concentration of NaOH solution in desilication. Moderate alkaline treatment lowered diffusion limitations, decreased strong acid sites and increased the concentration of external acid sites.³⁴ Carbon yields of olefins and aromatic hydrocarbons reached a maximum of 23.3%. This shows an improvement of 21.6% compared with the parent HZSM-5 catalyst. The excellent performance of hierarchical HZSM-5 may be attributed to the synthesis of mesopores that will promote the diffusion of reactants and products, and allow easier access to acid sites in the micropores.³⁵ However, too severe desilication produced redundant mesopores, led to bad selectivity and fewer acid sites to meet the fundamental catalysis reaction. The framework even collapsed under extreme treatment conditions. In Fig. 6(b), the curves describe the carbon yield of olefins as a function of time over the treated catalysts and also a comparison with the parent catalyst. It is obvious that the initial carbon yield increases with increasing concentration of NaOH solution in desilication, which was accompanied by higher surface area and acid density on the external sphere. The carbon yield of olefins increased to a maximum value in a short time, which is usually named the “induction period”, and then decreased sharply due to coke deposition. The length of induction period was found to be closely related to alkali treatment conditions. The presence of mesopores, the shape and size of the mesopores may influence the formation of active sites. Unfortunately, the detailed mechanism of the relationship is still unknown.

Fig. 6 Catalytic performance of hierarchical HZSM-5 catalysts as a function of concentration of NaOH solution (a) and reaction time (b).

In the desilication process, the introduction of mesopores will also change the shape selectivity of catalysts dependent on the size of the gate between mesopore and micropore and the change of diffusion limitations. The detailed product distribution is listed in Table 2, in which HZSM-5-0.3M showed larger selectivity to ethylene. When using concentrations of NaOH solution lower than 0.3 M, the sheet-like channels will shorten the diffusion pathways. The contact time of furan in the catalyst decreases, which is insufficient for olefin aromatization, and thus the selectivity to olefins increases. For these hierarchical HZSM-5 catalysts like HZSM-0.5M, HZSM-0.8M and HZSM-1M, the hollow mesopores provided enough space for aromatization. Therefore, alkali treatment is a useful method for the promotion of olefins in the catalytic conversion of furan.

Table 2 Product distribution in the catalytic conversion of furan over HZSM-5

	Parent	0.1 M	0.3 M	0.5 M	0.8 M	1 M
Carbon yield (mol%)
Olefins	5.41	6.74	8.78	7.10	5.12	3.82
Aromatic hydrocarbons	13.79	14.25	14.57	15.65	14.66	13.31
Coke content (wt%)	7.07	7.43	7.89	7.99	8.53	9.91
Carbon selectivity (mol%)
Ethylene	10.13	15.72	19.81	12.58	8.18	6.85
Propylene	7.41	9.06	8.63	9.04	7.91	2.76
Butylene	9.57	5.22	5.56	5.38	4.91	1.99
Benzene	8.25	9.19	15.32	9.84	7.35	12.52
Toluene	12.47	10.47	16.93	14.17	14.27	14.31
Xylene	4.21	3.55	5.64	4.32	6.06	5.06
Benzofuran	3.40	7.20	10.20	13.20	17.60	20.10
Indene	5.62	6.44	6.12	5.92	6.41	6.32

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To understand coking behavior of hierarchical HZSM-5, coke content of deactivated catalysts was analyzed by thermal analysis as shown in Fig. 7. Mass loss of the deactivated catalysts was shown during coke combustion. First, total coke content of the catalysts showed some differences among all catalysts. Compared with the parent catalyst, treated catalysts almost had more coke content, and this observation can also be found in other coked catalysts with hierarchical structure.^36,37 This suggests that the mesopores may act as spaces for coke to form and accumulate.³⁸ Though more coke was deposited on the hierarchical catalysts, the carbon yield of hydrocarbons was higher compared with the parent catalyst, which indicated its higher tolerance toward coke deposition. Actually, not only the coke content of all catalysts were different from each other, but also the combustion temperature ranges were different as shown in Fig. S3.† Major coke was deprived from 400 °C to 800 °C, while some weak peaks appeared in the temperature range of 100–400 °C for HZSM-5-0.5M, HZSM-5-0.8M, HZSM-5-1M. This means that either the location of coke or the chemical species of coke were different from the three other catalysts. Kaskel found that deactivated hierarchical materials showed higher porosities, and assumed that coke in the mesoporous area was more loose and porous.³⁹

Fig. 7 Regeneration of deactivated HZSM-5 catalysts of different alkali treating conditions (□, HZSM-5-P; ○, HZSM-5-0.1M; △, HZSM-5-0.3M; ▽, HZSM-5-0.5M; ◁, HZSM-5-0.8M; ▷, HZSM-5-1M).

Cyclic runs of the catalyst-regeneration process

To determine the cycle performance of hierarchical catalysts, cyclic tests were conducted with 20 catalysis-regeneration cycles consisting of 20 min catalytic reaction followed by 30 min catalyst regeneration. The catalytic condition was the same as Section “Catalyst tests”, and the regeneration was performed with an air flow rate of 200 mL min⁻¹. The variation of carbon yield of hydrocarbons as a function of cycle number for the catalyst HZSM-5-P and HZSM-5-0.3M is shown in Fig. 8.

Fig. 8 Cycle performance of parent and hierarchical HZSM-5 catalyst.

It is obvious that the reaction stability of HZSM-5-0.3M was much better than the parent HZSM-5 catalyst. In particular, the carbon yield of hydrocarbons almost kept constant after 10 cycles while it decreased continuously for parent catalyst. The highest temperature the thermocouple detected after air flowed in was named the burning temperature. Burning temperature of each cycle was also recorded in Fig. 8, in which the burning temperature of parent HZSM-5 was around 645 °C while that of the HZSM-5-0.3M was 40 °C lower. First, it is understandable that the carbon yield of hydrocarbons for HZSM-5-0.3M was higher than that of HZSM-5-P because of the appearance of mesopores in HZSM-5, which provided more chances for contact between reactant and catalyst acid sites. The parent HZSM-5 catalyst showed more serious overheating than HZSM-5-0.3M. On one hand, the loose and porous coke determines its relatively lower burning temperature.³⁹ Svelle and Schmidt reported that coke distribution of microporous HZSM-5 showed a gradient over the catalyst particle and the carbonaceous molecules mainly deposited in the microporous near-surface area that led to the rapid deactivation of catalysts, while hierarchical HZSM-5 catalysts showed a homogeneous distribution over the whole particle due to the improved accessibility of active sites and higher catalyst utilization.^19,40 Therefore, the more uniform deactivation of the desilicated catalyst due to a complex interplay among alterations of porosity, activity and rate of deactivation upon desilication, makes combustion gas easily diffuse out of catalysts channels.⁴⁰ Actually, accurate temperature within the catalyst particle was much higher than the detected value. Thus, lots of heat during coke combustion resulted in extraction of the framework Al to generate octahedrally coordinated Al species out of the zeolite framework.⁴¹ Dealumination occurred, and the removed aluminum was retained in the pores. Generally, the mesopores in the treated HZSM-5 catalyst will decrease the burning temperature and also the particle interior temperature, weaken the dealumination effect and finally promote the cycle stability. Coke content was also recorded online during regeneration. The coke amount determined by combustion online only includes carbon while the TG analysis of deactivated catalysts in air can determine the full composition amounts including carbon, hydrogen and oxygen. Previous studies revealed that coke species at high temperature (>400 °C) are mainly hydrogen-poor carbon species with no oxygen.⁴² Therefore, the coke amount obtained by the above method may be just a little smaller than TG analysis. Coke content of original catalysts and those used for cycles has been added into Fig. S4.† It is obvious that the coke content decreased with increasing cycle numbers, and the decrease rate of parent ZSM-5 catalysts seems higher. For the parent ZSM-5, the high burning temperature leads to hotspots in the catalyst particles and dealumination may happen.^43,44 Dealumination is usually accompanied by the decrease of strong acid sites, which easily causes serious coke formation.⁴¹ Thus, the decrease of coke content with cycle numbers is understandable. Hierarchical catalyst (HZSM-5-0.3M) showed relatively lower combustion temperature and thus lower degree of dealumination, which explains the slight decrease in coke content.

Conclusion

Catalytic conversion of furan, as an important intermediate from BFP, was conducted on a micro-mesoporous HZSM-5 catalyst, created by alkali treatment, to obtain hydrocarbons. The prepared sample of HZSM-5-0.3M, with sheet-like mesopores, presented the best catalytic performance with carbon yields of hydrocarbons boosted to 21.6%. The appropriately desilicated HZSM-5 catalyst showed higher selectivity to olefins and more serious coke deposition, but a higher tolerance towards coking. Moreover, the cyclic tests of the catalysis-regeneration process indicated that hierarchical HZSM-5 zeolites are potentially suitable for long-term operation. The hierarchical HZSM-5 catalyst successfully combined the high selectivity of the microporous system and promoted mass transfer of mesopores in CFP of biomass for hydrocarbon production.

Acknowledgements

The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51561145010, 51525601 and 51306036), the National Basic Research Program of China (973 Program, Grant 2012CB215306), the Jiangsu Natural Science Foundation (Grant No. BK20130615) and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1324).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05356d

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