Catalytic Conversion of 1‐butene over Modified Versions of Commercial ZSM‐5 to Produce Clean Fuels and Chemicals

Abstract

The oligomerisation of light olefins, obtainable from fossil/renewable sources and refinery streams, is an attractive route to produce clean synthetic fuels and added‐value chemicals. ZSM‐5 is a type of catalyst used in commercial olefin oligomerisation processes. Using appropriate modification procedures, it was possible to prepare catalysts with improved performances. Various modified versions of commercially available ZSM‐5 were prepared and investigated for 1‐butene oligomerisation under high‐pressure, continuous‐flow operation (30 bar, 200 °C). Simple, up‐scalable top‐down strategies involving base‐acid treatments of ZSM‐5 led to catalysts possessing enlarged pores and the required acidity for converting 1‐butene to higher molar mass products. In targeting diesel type products, the modified catalysts led to up to 86 % butenes conversion, space time yield of 852 mg g_cat⁻¹ h⁻¹ and mass ratio diesel:naphtha cuts of 2.2. Characterisation studies and multivariate/principal component analysis helped categorise the differently prepared catalysts, and gain insights into complex interplay of material properties influencing the catalytic reaction.

Introduction

The oligomerisation of light olefins, such as ethene, propene and butenes, represents an attractive route to produce eco‐friendly synthetic fuels with low content of aromatics and sulphur, and added value chemicals such as dyes, plasticizers and detergents.^{1, 2} Olefin oligomerisation technologies were firstly commercialized by Universal Oil Products (1930), namely the CatPoly technology^{3, 4} using solid phosphoric acid (SPA) as catalyst to produce gasoline and diesel range products from propene/butene mixtures (Scheme 1). Overcoming environmental and technical drawbacks associated with SPA, the MOGD (1980)^5-7 and COD (1992)^{8, 9} technologies were developed using crystalline microporous aluminosilicates possessing the MFI topology to produce high quality distillate fuels (Scheme 1). More recently, an amorphous silica alumina catalyst extrudate was used in Axens licensed technologies, namely PolyFuel (2008)¹⁰ and FlexEn,¹¹ which were designed to respond in flexible and adjustable fashions to the market demands for gasoline, distillates and propene. The olefins feedstock for these processes may derive from low‐value industrial streams of, for example, petroleum refineries and coal‐to‐liquid plants (fluid catalytic cracking, high‐temperature Fischer‐Tropsch units, etc.),^{12, 13} or from renewable sources (e. g. olefins produced from biomass‐derived syngas or furanic compounds).^{14, 15}

Scheme 1

Technologies of olefin oligomerisation developed over the years, particularly focused on the production of middle distillates (b.p. 180–360 °C; e. g. diesel).

R&D efforts in the fields of materials and chemical engineering continue aiming at the development of efficient eco‐friendly production processes of distillates. Zeolites appear as interesting catalysts, since they are relatively robust, versatile (tunable properties), some are readily available and reached industrial application (e. g. MFI, FAU, BEA topologies). The ordered microporous systems of zeolites confer them shape‐selectivity properties, albeit internal mass transfer limitations may be important along with pore blockage and fast catalyst deactivation, especially when the desired reaction products are relatively bulky molecules, such as oligomers.^{16, 17} Thus, great focus has been put on developing improved versions of zeolites with enhanced active sites accessibility, allowing facilitated mass transfer in/out of the pores, while benefiting from zeotype features (structural order at the atomic level, tunable surface properties and shape selectivity).^18-22 This may be accomplished with the introduction of mesoporosity in zeolites via bottom‐up or top‐down synthetic approaches.^23-29 In the former case, mesoporosity is formed by hard templating (e. g. carbonaceous or polymeric compounds),^30-32 soft templating (e. g. surfactants, organosilanes),^33-35 or indirect templating methods (e. g. steam‐assisted and solid‐phase crystallization).^36-38 In top‐down approaches, mesoporosity is introduced by strategically removing framework atoms from pre‐made zeolites; e. g. desilication via alkaline treatment,^39-42 and dealumination via acid treatment or steaming at high temperature.^{39, 42-45} The top‐down strategies are advantageously cheaper, relatively easy to reproduce and scale‐up for larger production.^{23, 28, 46} Zeotypes based on the MFI,^{39, 43, 47} MOR,^{24, 48} FAU⁴⁹ and TON⁵⁰ topologies were prepared via top‐down strategies. MFI versions were reported as effective catalysts, for example, for selective catalytic cracking of 1‐butene,^51-53 and 2‐methyl‐2‐butene⁵⁴ to light olefins.

Depending on the type of olefin, the relationships of material properties‐catalytic activity may differ.³⁹ On the other hand, depending on the starting zeolite, the modification treatments may influence differently the material properties. For example, the effectiveness of desilication via alkaline treatment depends on factors such as, topology, molar ratio Si/Al and crystal size of the starting zeolite. Thus desilication may be accomplished using strong inorganic bases (SIB protocol) such as NaOH leading to relatively fast hydrolysis and dissolution of the silicon species,⁴² or, alternatively, quaternary ammonium hydroxides (QAH protocol) where the ammonium cations may act as a protective barrier at the surface of the zeolite crystals allowing moderate alkaline hydrolysis.^{41, 55} Desilication may be followed by moderate acid treatments to remove extra‐framework aluminum species and/or inorganic debris.^{42, 56}

Considering the versatility and simplicity of top‐down strategies, we sought to prepare and explore improved versions of ZSM‐5 (the zeolite type used for commercial oligomerisation processes)^{8, 57-60} for the oligomerisation of 1‐butene, under continuous‐flow, high‐pressure conditions (200 °C, 30 bar), Scheme 2. Materials with distinct properties were prepared via SIB or QAH procedures or a combination of both (mixB), starting from commercial zeolite ZSM‐5 possessing relatively low molar Si/Al ratio. The studies reported herein comprise establishing appropriate top‐down modification protocols of ZSM‐5 to meet superior catalytic performances for 1‐butene conversion to diesel type products, understanding the influence of material properties on the catalytic reaction, and obtaining high space time yields of diesel cuts for catalysts prepared under optimised conditions.

Scheme 2

Top‐down approaches for preparing modified versions of ZSM‐5 for 1‐butene oligomerisation under continuous‐flow high pressure conditions.

Results and Discussion

Material Properties

The protonic form HZSM‐5 (molar ratio of Si/Al≈17, S_BET=431 m² g⁻¹, S_meso=54 m² g⁻¹) of commercial microcrystalline NH₄ZSM‐5 was subjected to different base‐acid treatments (Scheme 2), leading to modified materials (Table 1) with Si/Al ratio in the range 3–122 (Table 2). The alkaline treatments were carried out using NaOH (SIB procedure giving MZS‐x, where x stands for the molar concentration of NaOH, x=0.2, 0.4, 0.6 or 0.8), tetrapropylammonium hydroxide (QAH procedure giving MZS‐TPA‐y, where y is the temperature of the alkaline treatment=65 or 85 °C) or mixed tetrapropylammonium hydroxide plus NaOH (mixB procedure giving MZS‐TPA/Na).

Table 1. Protocols and conditions used for preparing the catalysts.

Catalyst	Top‐down strategies
	Desilication Procedure		IE^[a]	Acid Treatment
MZS‐0.2	SIB	[NaOH]=0.2–0.8 M, 65 °C, 2 h	yes	no
MZS‐0.4
MZS‐0.6
MZS‐0.8
MZS‐0.2‐Ox			yes	Oxalic acid 0.8 M, 70 °C, 2 h
MZS‐0.4‐Ox
MZS‐0.6‐Ox
MZS‐0.2‐Cl			yes	HCl 0.1 M, 65 °C, 6 h
MZS‐0.4‐Cl
MZS‐0.6‐Cl
MZS‐0.8‐Cl
MZS‐TPA‐65	QAH^[b]	[TPAOH]=1.0 M, 65 °C or 85 °C, 5 h	no	no
MZS‐TPA‐85
MZS‐TPA‐65‐Cl			no	HCl 0.1 M, 65 °C, 6 h
MZS‐TPA‐85‐Cl
MZS‐TPA/Na	MixB	[TPAOH+NaOH]=0.6 M (TPA^+/Na^+=0.4), 90 °C, 1 h	yes	no
MZS‐TPA/Na‐Cl			yes	HCl 0.1 M 65 °C, 6 h

• [a] Ion exchange using an aqueous solution of NH₄NO₃, followed by washing, drying (and calcination for the materials that were not subjected to further acid treatment). [b] These materials were not fully characterised partly due to their poor catalytic performances.

Table 2. Composition and textural properties of HZSM‐5 and its modified versions.

Catalyst	Si/Al	SBET [m^2 g^−1]	Smeso [m^2 g^−1]	Vp [cm^3 g^−1]	Vmeso [cm^3 g^−1]
MZS‐0.2	13	439	176	0.64	0.52
MZS‐0.2‐Cl	19	453	149	0.64	0.51
MZS‐0.2‐Ox	20	483	189	0.67	0.54
MZS‐0.4	7	414	271	1.08	1.03
MZS‐0.4‐Cl	24	564	291	1.23	1.11
MZS‐0.4‐Ox	58	564	267	1.21	1.08
MZS‐0.6	4	232	175	0.99	0.96
MZS‐0.6‐Cl	11	494	255	1.23	1.12
MZS‐0.6‐Ox	122	596	269	1.39	1.24
MZS‐0.8	3	136	114	0.94	0.93
MZS‐0.8‐Cl	8	374	272	1.08	1.03
MZS‐TPA‐65	16	410	59	0.55	0.40
MZS‐TPA‐65‐Cl	19	424	65	0.56	0.41
MZS‐TPA‐85	16	414	66	0.49	0.34
MZS‐TPA‐85‐Cl	17	413	67	0.48	0.33
MZS‐TPA/Na	5	275	178	0.81	0.77
MZS‐TPA/Na‐Cl	21	604	285	1.24	1.10
HZSM‐5	17^[a]	431	54	0.36	0.18
MZS‐0.4‐Cl‐r^[b]	–	498	312	1.15	1.06

• [a] Si/Al=15 in the technical bulletin for the parent commercial NH₄ZSM‐5. [b] Results for the used catalyst, after thermal treatment.

In general, it was verified that the Si/Al ratio decreased from ca. 17 for the parent zeolite HZSM‐5 to values in the range 3–16 after the alkaline treatment. The drop in Si/Al ratio was far more pronounced for the SIB and mixB procedures in relation to the QAH one. The latter procedure did not influence significantly the Si/Al ratio (Si/Al=16 for MZS‐TPA‐65 and MZS‐TPA‐85, compared to 17 for HZSM‐5) suggesting that desilication was negligible. For the SIB procedure (MZS‐x), the Si/Al ratio decreased with increasing NaOH concentration (x) due to enhanced removal of silicon species from HZSM‐5 (Figure S1).

Desilication was followed by acid treatment using oxalic acid (Ox) or HCl (Cl), which led to increased Si/Al ratio (Table 2, Figure S1), especially for materials obtained via the SIB and mixB procedures. These results may be attributed to the removal of inorganic debris and/or extra‐framework aluminum species, likely formed in greater amounts during the SIB and mixB treatments than the QAH one. The materials MZS‐x‐Ox possessed higher Si/Al ratio than the HCl treated counterparts MZS‐x‐Cl, suggesting that the Ox treatment enhanced dealumination.

The desilication procedure/conditions may influence the crystallinity. Figure 1 shows the powder XRD patterns for HZSM‐5 and its modified versions. The materials prepared via the SIB procedure with x=0.2 (namely, MZS‐0.2, MZS‐0.2‐Cl, MZS‐0.2‐Ox) or the QAH procedure (MZS‐TPA‐y, MZS‐TPA‐y‐Cl) exhibited reflections associated with the MFI topology, the most intense appearing in the ranges 7–8° 2θ and 23–24° 2θ.^{61, 62} These materials seemed relatively crystalline, by comparison to the XRD pattern of HZSM‐5. The SIB (x≥4) and mixB procedures had more pronounced effects on structural order; peak intensities decreased in the order, SIB/x=0.2>SIB/x=0.4>(SIB/x=0.6; mixB), and SIB/x=0.8 led to lack of crystallinity.

Figure 1

Powder XRD patterns for (A) the materials prepared via the SIB procedure: HZSM‐5 (a), MZS‐0.2 (b), MZS‐0.2‐Cl (c), MZS‐0.2‐Ox (d), MZS‐0.4 (e), MZS‐0.4‐Cl (f), MZS‐0.4‐Ox (g), MZS‐0.6 (h), MZS‐0.6‐Cl (i), MZS‐0.6‐Ox (j), MZS‐0.8 (k), and MZS‐0.8‐Cl (l); and for (B) materials prepared via QAH and mixB procedures: HZSM‐5 (a), MZS‐TPA‐65 (b), MZS‐TPA‐65‐Cl (c), MZS‐TPA‐85 (d), MZS‐TPA‐85‐Cl (e), MZS‐TPA/Na (f), MZS‐TPA/Na−Cl (g).

Morphologically, HZSM‐5 and its modified versions consisted of aggregates of irregular sizes (up to ≈700 nm), and formed by pseudo‐spherical particles (Figures 2 and Figure 3); exceptionally, MZS‐0.8 consisted of relatively small aggregates, which may be related to the lack of crystallinity. For all materials, some particles of ca. 100 nm could be distinguished. However, with the modification treatments, partial particle coalescence seemed to occur, making it difficult to identify the particle size ranges. The literature for desilicated zeolites referred to coalescence associated with the formation of mesopores, likely involving reactions between vicinal defect sites.^{63, 64} It was also referred the occurrence of external surface roughening, especially for materials prepared from zeolites possessing relatively low ratio Si/Al (less than ca. 25).^{65, 66} On the other hand, it was reported that less stable silicon species may be formed during modification treatments, and migrate and condense with silanol groups in different locations, as a type of healing process (during dealumination).⁶³ One may hypothesize that the apparent particle coalescence for the MZS materials may be due to interactions between external surface defects of crystallites in close proximity, and/or these interactions may occur via the intermediacy of extra‐framework species (formed during desilication).

Figure 2

SEM images of the materials prepared via the SIB procedure: MZS‐0.2 (a), MZS‐0.2‐Cl (b), MZS‐0.2‐Ox (c), MZS‐0.4 (d), MZS‐0.4‐Cl (e), MZS‐0.4‐Ox (f), MZS‐0.6 (g), MZS‐0.6‐Cl (h), MZS‐0.6‐Ox (i), MZS‐0.8 (j), MZS‐0.8‐Cl (k), and HZSM‐5 (l).

Figure 3

SEM images of the materials prepared by the QAH and mixB procedures: MZS‐TPA‐65 (a), MZS‐TPA‐65‐Cl (b), MZS‐TPA‐85 (c), MZS‐TPA‐85‐Cl (d), MZS‐TPA/Na (e), and MZS‐TPA/Na−Cl (f).

The nitrogen adsorption‐desorption isotherms showed increasing adsorption capacity at high relative pressure (p/p₀>0.9), attributable to multilayer adsorption on the external surface (Figure 4). This feature was less pronounced for HZSM‐5 and its modified versions resulting from the SIB/x=0.2 and QAH procedures. On the other hand, SIB/(x≥0.4) and mixB treated materials exhibited a hysteresis loop (Figure 4), enhanced portion of mesoporosity (%S_meso, %V_meso) and reduced portion of microporosity (%V_micro), Figure S4. A comparative study for the MZS‐x materials, indicated that S_meso and V_meso were highest for x=0.4 (271 m² g⁻¹ and 1.03 cm³ g⁻¹), suggesting that the SIB/(x=0.4) protocol was favourable for introducing mesoporosity in HZSM‐5 (Table 2).

Figure 4

Nitrogen adsorption‐desorption isotherms (at −196 °C) for materials prepared by SIB procedure using NaOH concentration of 0.2 M (A), 0.4 M (B), 0.6 M (C) or 0.8 M (D); and materials prepared by QAH (E) and mixB (F) procedures: materials without acid (Ox, Cl) treatment thick black lines; materials treated with oxalic acid wide dashed lines in (A), (B), (C); materials treated with HCl grey lines; HZSM‐5 dotted blue line in (A). The inset in (E) shows, separately, the pairs (a) MZS‐TPA‐65 and MZS‐TPA‐65‐Cl, and (b) MZS‐TPA‐85 and MZS‐TPA‐85‐Cl for better visualisation.

The increased S_meso (54 m² g⁻¹ for HZSM‐5 versus 114–271 m² g⁻¹ for alkaline‐treated materials) and V_meso (0.18 cm³ g⁻¹ for HZSM‐5 versus 0.51–1.03 cm³ g⁻¹) were accompanied by decreased Si/Al ratio (Table 2) and structural order (Figure 1). The limited mesopore formation via the QAH procedure may be partly due to the relatively low Si/Al ratio of the parent zeolite HZSM‐5,⁴¹ since high framework aluminum content suppresses intracrystalline silicon extraction.²⁵

The SIB (x≥0.4) and mixB treated materials possessed relatively narrow mesopore size distribution curves in the range 2–6 nm (Figure S2). HZSM‐5 and its versions modified via the SIB procedure with x=0.2 or 0.8, or the QAH procedure, exhibited poorly defined mesopore size distribution curves, which was attributed, on the one hand, to the poor effectiveness of the SIB/(x=0.2) and QAH procedures for introducing mesoporosity (Table 2), and, on the other hand, to the severity of the SIB/(x=0.8) procedure in causing structural collapse (Figure 1).

The acid treatment is important to remove inorganic debris and/or extra‐framework Al species (Table 2).^{39, 56, 67} For the desilicated materials MZS‐x and MZS‐TPA/Na, the acid treatment step led to enhanced S_BET, V_p and V_micro (Table 2), without affecting significantly the mesopore size distribution (Figure S2). In general, the impact of the acid treatment was pronounced on S_meso (up to 139 % increase in relation to the corresponding desilicated materials) and V_meso (up to 42 % increase). The S_meso was highest for MZS‐0.4‐Cl (291 m² g⁻¹) and MZS‐TPA/Na−Cl (285 m² g⁻¹) (Table 2). For the QAH treated materials, the impact of the acid treatment on the textural properties was not significant, which is consistent with the above discussion regarding the ineffectiveness of this procedure for desilication of the parent zeolite HZSM‐5. The indexed hierarchy factor (IHF) reflects the efficiency of the desilication process in introducing mesoporosity without drastically affecting microporosity (Table S1). The acid‐treated materials possessed higher IHF (0.20–0.80) than the parent zeolite HZSM‐5 (0.18), with the highest IHF values being verified for MZS‐0.4‐Cl, MZS‐0.4‐ox, MZS‐0.6‐ox and MZS‐TPA/Na−Cl. Hence, top‐down strategies contemplating Ox or Cl treatment after desilication via mixB or SIB/(x=0.4, 0.6) procedures, may give good compromises of enhanced S_meso with partial preservation of V_micro. Advantageously, the mineral acid (HCl) may be used in much lower concentration and lower temperature than the organic acid.

Figure 5 shows the ²⁷Al MAS NMR spectra of HZSM‐5 and the modified materials. All materials exhibited a peak centred at ca. 55 ppm assigned to four‐coordinated aluminum species (Al_tetra). In general, desilication led to enhanced relative amount of five‐ (Al_penta) and six‐coordinated (Al_octa) aluminum species (peaks centred at ca. 28 ppm and 0 ppm, respectively), which seemed to be more pronounced for stronger alkaline conditions (SIB/(x≥0.6), mixB), Figure 5, Table 3. The acid treatment (Ox, Cl) led to considerable reduction in the relative amounts of Al_penta and Al_octa species, and enhanced Al_tetra (Table 3). These results together with the increase of the Si/Al ratio upon acid treatment, indicated that this step led to the removal extra‐framework Al species and/or inorganic debris containing aluminum.

Figure 5

²⁷Al MAS NMR spectra for (A) HZSM‐5 (a), MZS‐0.2 (b), MZS‐0.2‐Cl (c), MZS‐0.2‐Ox (d), MZS‐0.4 (e), MZS‐0.4‐Cl (f), MZS‐0.4‐Ox (g), and (B) MZS‐0.6 (h), MZS‐0.6‐Cl (i), MZS‐0.6‐Ox (j), MZS‐0.8 (k), MZS‐0.8‐Cl (l), MZS‐TPA/Na (m) and MZS‐TPA/Na−Cl (n). The asterisk denotes side bands.

Table 3. Types of aluminum species and acid properties of selected catalysts.

Catalyst	Al species^[a]		Acid properties^[b]
	%Altetra+penta (Alpenta/Altetra)	%Alocta	L+B [μmol g^−1]	L/B	L450/L200	B450/B200
MZS‐0.2	89 (0.02)	11	346	0.64	0.56	0.26
MZS‐0.2‐Cl	83 (0.00)	17	342	0.31	0.79	0.33
MZS‐0.2‐Ox	87 (0.00)	13	288	0.26	0.86	0.25
MZS‐0.4	88 (0.37)	12	347	1.29	0.08	0.75
MZS‐0.4‐Cl	71 (0.00)	29	250	0.80	0.63	0.13
MZS‐0.4‐Ox	80 (0.00)	20	108	0.39	0.77	0.07
MZS‐0.6	59 (0.80)	41	126	1.63	0.29	0.00
MZS‐0.6‐Cl	74 (0.23)	26	253	1.50	0.35	0.05
MZS‐0.6‐Ox	74 (0.33)	26	33	0.35	0.34	0.00
MZS‐TPA/Na	56 (0.73)	44	181	1.11	0.34	0.04
MZS‐TPA/Na‐Cl	74 (0.32)	26	213	1.72	0.45	0.00
HZSM‐5	75 (0.00)	25	415	0.18	0.87	0.42

• [a] Determined by ²⁷Al MAS NMR spectroscopy. [b] Determined by FT‐IR spectroscopy of adsorbed pyridine at 200 °C; B=Brønsted acid sites, L=Lewis acid sites, B+L=total amount of acid sites.

FT‐IR spectroscopy of the dehydrated materials showed a band at ≈3745 cm⁻¹ assignable to silanol groups on the external surface (Figure S3).^{68, 69} The relative intensity of this band was more pronounced for the modified materials than HZSM‐5, likely due to the desilication process leading to the formation of defect sites. HZSM‐5 and related materials modified via the SIB procedure with x=0.2 or 0.4 exhibited a band at ≈3610 cm⁻¹ assignable to structural acidic O−H groups.^{68, 70}

FT‐IR spectroscopy of adsorbed pyridine as base probe indicated that all materials exhibited bands centred at ca. 1540 and 1455 cm⁻¹ associated with pyridinium ions (related to Brønsted (B) acid sites) and coordinated pyridine (Lewis (L) acid sites), respectively (Figure S3). Upon desilication, the amount and density of L+B (and B) acid sites decreased and the molar ratio L/B increased (Table 3, Figure S5), which is somewhat consistent with the ²⁷Al MAS NMR spectroscopic data in that Al_tetra decreased and Al_penta increased (Figure 5). For the SIB treated materials, the acid treatment (Ox, Cl) led to decreased L/B ratio, which was more pronounced using oxalic acid (Table 3). Of the modified materials, MZS‐0.2‐Cl possessed the highest amount of total acid sites, albeit reduced mesoporosity and undefined mesopore size distribution. On the other hand, MZS‐0.6‐Ox possessed the lowest amount of total acid sites, albeit enhanced mesoporosity and relatively narrow mesopore size distribution (Table 3).

For each MZS‐x material, the Ox and Cl treatments led to similar acid strengths: the B acid strength was, in general, mostly moderate; and the L acid strength was mostly strong for MZS‐x‐Ox and MZS‐x‐Cl with x=0.2, 0.4 (L₄₅₀/L₂₀₀≥0.63), and mostly moderate for the remaining acid‐treated materials (L₄₅₀/L₂₀₀≤0.45) (Table 3). The materials MSZ‐0.6 and MSZ‐TPA/Na, prepared using similar total alkaline concentration (i. e. [OH⁻]=0.6 M), possessed roughly comparable Si/Al ratio, morphological, textural and acid properties, suggesting that the SIB and mixB procedures using comparable conditions may impact similarly on the material properties.

Multivariate, Principal component analysis of the material properties. Scheme 3 shows positive and negative variations of material properties (Si/Al ratio, acidity, texture) due to the alkaline (SIB, mixB) and acid treatments. In analysing a large set of material properties that may be somehow related, one may advantageously employ a multivariate statistical tool, such as principal component analysis (PCA), since it allows to reduce a large set of variables to a smaller set of principal components that still contains most of the information of the large set, decreasing the complexity of the analyses.⁷¹ The PCA methodology was reported by Castaño and co‐workers⁷² for fluid catalytic cracking over faujasite Y zeolites, and by Vayenas and co‐workers⁷³ for hydrotreatment of lube oil over metal oxides, allowing valuable insights into key parameters affecting the catalytic reactions.

Scheme 3

Changes in textural and acid properties of the materials prepared by the SIB and mixB procedures: black arrows after desilication; red arrows after the acid treatment step for the SIB procedure; blue arrows after the acid treatment step for the mixB procedure; (↑) represents an increase and (↓) a decrease.

Figure 6‐A shows the 2‐D PCA biplot with eight variables (i. e., materials properties) and twelve samples prepared via the SIB or mixB procedures, with or without acid treatment (the data were taken from Table S3). The first two components accounted for ca. 75 % of the variance of the data (PC1: 45 % and PC2: 29 %). Hence, the prepared materials could be differentiated along the x‐ and y‐axis. Specifically, three sets of materials could be identified in the PCA biplot of Figure 6‐A. Group 1 (blue) regards materials prepared using relatively low concentration of NaOH, with or without acid treatment (namely, MZS‐0.2, MZS‐0.2‐Cl, MZS‐0.2‐Ox). Group 2 (green) comprises materials prepared using higher alkaline (OH⁻) concentration, without acid treatment (MZS‐0.4, MZS‐0.6, MZS‐TPA/Na). Group 3 (purple) regards materials prepared using higher alkaline concentration, with acid treatment (MZS‐0.4‐Cl, MZS‐0.4‐Ox, MZS‐0.6‐Cl, MZS‐0.6‐Ox, MZS‐TPA/Na−Cl).

Figure 6

Two‐dimensional principal component analysis biplots; the variables are represented in red colour and the observations in black colour; the coloured circles signalize three different groups of materials (Group 1 (blue), Group 2 (green), Group 3 (purple)). (A) PCA biplot (PC1: 45.53 % and PC2: 29.39 %) categorising the materials according to their properties. (B) PCA biplot (PC1: 77.08 % and PC2: 21.94 %) categorising the materials according to their catalytic performance.

Group 1 is located closer to the parent zeolite HZSM‐5, on the left side of the biplot which is characterised by higher microporosity (V_micro) and acid strengths. Groups 2 and 3 are located on the right side of the biplot which is characterised by higher mesoporosity (S_meso, V_meso). In particular, Group 3 (upper‐right region of the biplot) possessed higher S_BET, V_micro and L acid strength than Group 2 (bottom‐right region of the biplot). Hence, the acid treatment step led to a gain in terms of specific surface area and microporosity, in combination with mesoporosity.

A PCA biplot (Figure S9) was determined using a larger set of material properties, i. e. including %Al_tetra+penta, ratio Al_penta/Al_tetra (Table 3) and IHF (Table S1). Although there was a somewhat greater dispersion of the results and a decrease of the total variance of the data by the principal components, the same three groups of materials could be distinguished. Regarding the types of Al species, Group 1 was characterised by higher %Al_tetra+penta and lower ratio Al_penta/Al_tetra, which is consistent with the less effective desilication for this group of materials. Group 2 was characterised by higher ratio Al_penta/Al_tetra, which may be related to significant amounts of extra‐framework species and/or inorganic debris present in these (non‐acid‐treated) materials, making them less attractive for catalytic application. Overall, Group 3 was characterised by higher IHF, i. e. greater desilication efficiency.

Catalytic Oligomerisation

The MFI‐based materials prepared via the top‐down approaches were tested for the oligomerisation of 1‐butene (1 C4), under high‐pressure (30 bar) continuous‐flow conditions, at 200 °C, using a weight hourly space velocity (WHSV) of 2.2 g_1C4 g_cat⁻¹ h⁻¹ (Scheme 2). The design and operation conditions of the catalytic reactor were optimised for plug flow pattern with negligible diffusional limitations.^{57, 58} All materials prepared promoted olefin conversion to higher molar mass products (Figure 7). The conversion of butenes (X_C4) were in the range 10–86 % and the total space time yields (STY) were in the range 19–852 mg g_cat⁻¹ h⁻¹.

Figure 7

Conversion (•) and STY (bars) of NCut (solid blue bars) and DCut (dotted yellow bars) products for the MFI‐based materials prepared and the benchmark catalyst HZSM‐5. Reaction conditions: 200 °C, 30 bar, WHSV=2.2 g_1C4 g_cat⁻¹ h⁻¹, TOS=7 h, catalyst activation temperature=450 °C.

In general, higher catalytic activity was accompanied by greater production of higher molar mass products (Figure 8). The mass ratio of DCut : NCut was in the range 1.0–3.7, indicating the favourable formation of the 170–390 °C cut characteristic of diesel products (DCut) over the <170 °C cut characteristic of naphtha products (NCut). Olefin oligomerisation systems involve complex reaction mechanisms where, besides oligomerisation, various side reactions may occur such as cracking (primary, secondary), alkylation and isomerisations (double bond, methyl shifts).^74-77 The liquid products were analysed by ¹H NMR spectroscopy (details in the Supporting Information) to determine the relative amount of aromatic products (H_ar), isoparaffinic ratio (I),⁷⁸ and the cetane number (CN, based on the O'Connor method)⁷⁸ of the mixtures (Table S2). The isoparaffinic ratios I (reflects the branching degree) were in the range 0.52–0.60 for the prepared catalysts. These results are advantageously lower than that reported in the literature for a mesostructured zeotype based on the BEA topology (I≈0.62).⁵⁸ The H_ar was less than 0.35 %, indicating that the prepared catalysts led to very low aromatics content which is an important advantage of olefin oligomerisation routes to clean synthetic fuels. The estimated CN values were in the range 41–46, which serves for rough comparisons, since the products were not subjected to post‐treatments (e. g. hydrogenation increases CN^{79, 80}). CN values in the range 48–56 were reported in the literature for diesel cuts produced in commercial processes or commercial diesel samples.^{79, 81-84}

Figure 8

STY as a function of X_C4, under approximately steady state conditions, for the different catalysts prepared (STY total (▴), STY_DCut (×), and STY_NCut (+)). Reaction conditions: 200 °C, 30 bar, WHSV=2.2 g_1C4 g_cat⁻¹ h⁻¹, TOS=7 h, catalyst activation temperature=450 °C.

HZSM‐5 led to intermediate catalytic results of the modified materials; X_C4=39 %, STY=377 mg g_cat⁻¹ h⁻¹, mass ratio DCut : NCut=1.4 (Figure 7). A comparative study for the desilicated materials (without acid treatment) and HZSM‐5 indicated that the SIB/(x≤0.4) led to slightly improved catalytic activity (X_C4=44–49 %, STY=372–419 mg g_cat⁻¹ h⁻¹, whereas for x>0.4 the catalytic performance dropped significantly (X_C4=10–17 %; STY=19–25 mg g_cat⁻¹ h⁻¹), Figure 7. Catalyst MZS‐0.8‐Cl performed very poorly, since, although it possessed higher S_meso and V_meso than HZSM‐5 (Table 2), it lacked crystallinity (Figure 1). The QAH and mixB procedures (without acid treatment) led to lower X_C4 and STY than HZSM‐5. For the QAH procedure, the acid treatment did not bring advantages to the catalytic performance (X_C4 was roughly comparable with or without acid treatment). However, for the SIB and mixB procedures, the acid (Ox, Cl) treatment led to considerable improvements in catalytic performances compared to the same procedures without acid treatment (Figure 7).

Influence of material properties on the catalytic reaction. A comparative study for the three groups of materials identified using the multivariate, principal component analysis (PCA) tool, indicated that Group 2 catalysts (Figure 6) were poorly performing (Figure 7); these catalysts were prepared via the SIB or mixB procedures using intermediate alkaline (OH⁻) concentration of 0.4–0.6 M, without acid treatment, and presented miscellaneous properties which may be partly due to the presence of inorganic debris and/or extra‐framework Al species (not removed after desilication). The PCA biplot contemplating the IHF (Figure S9) indicated that Group 2 materials possessing lower IHF than the respective acid treated materials (Group 3), performed inferiorly.

A comparative study of Groups 1 and 3 (Figures 6, 7) pointed to the importance of balancing the textural and acid properties. Group 1 possessed higher amount and strength of B acid sites, and lower mesoporosity (and IHF) than Group 3 materials (Figures 6‐A, S9); and Group 3 possessed enhanced S_BET, S_meso, V_meso and IHF. Although MZS‐0.2‐Cl (Group 1) possessed higher amount and strength of B acid sites (261 μmol g⁻¹ and 0.33, respectively, Table 3) than the Group 3 materials MZS‐0.6‐Cl and MZS‐0.4‐Ox (77–101 μmol g⁻¹ and 0.05–0.07, respectively), the latter catalysts performed superiorly in 1C4 conversion (Figure 7) likely benefiting from enhanced mesoporosity (Table 2). On the other hand, MZS‐0.2‐Ox (Group 1), and MZS‐0.4‐Ox and MZS‐0.6‐Cl (Group 3) led to comparably good catalytic results (X_C4=65–67 %, STY=769–808 mg g_cat⁻¹ h⁻¹, DCut/NCut=1.2–1.9, Figure 7). While MZS‐0.2‐Ox seemed to benefit in terms of B acidity, Group 3 materials benefitted in terms of mesoporosity.

Figure 6‐B shows the PCA biplot determined with 4 variables (catalytic results) and 9 samples of Groups 1 and 3 (Group 2 was excluded, since these materials possessed considerable amounts of inorganic debris, which does not seem interesting for catalytic application). The first two components accounted for ca. 99 % of the variance of the data (PC1 : 77 % and PC2 : 22 %). The upper side of the PCA biplot represents higher ratio D_Cut/N_Cut which applies for Group 1, i. e. materials characterised by higher acidity and lower mesoporosity, and, on the other hand, MZS‐0.4‐Cl which is characterised by medium acidity and higher mesoporosity and IHF. The right side of the biplot represents higher conversion of butenes, total STY and STY_DCut, which applies for some materials of Groups 1 and 3. In particular, MZS‐0.4‐Cl is located on the upper, far‐right side of the biplot (Figure 6‐B), suggesting that it is the most promising for the production of diesel type products at high conversion of butenes; X_C4=86 %, STY=852 mg g_cat⁻¹ h⁻¹, DCut/NCut=2.2 (Figure 7). MZS‐0.4‐Cl possessed intermediate amount of B acid sites (the highest of Group 3; 138 μmol g⁻¹, Table 3), besides mesoporosity. Brønsted acidity may favour olefin oligomerisation,^{39, 85-88} although the acid sites accessibility seems particularly important for 1‐butene conversion. The PCA biplots suggested that mesoporosity and B acidity are inversely related (since these variables are located on opposite quadrants), which calls for a balance. Figure 9 shows X_C4 and STY (DCut, NCut) versus amount of B acid sites (and L+B, although the main effect was that of B) for the acid‐treated materials. These results further support that superior catalytic performances may be met when mesoporosity and intermediate amounts of B (and total) acid sites prevail.

Figure 9

Influence of amount of Brønsted acid sites (A) and total amount of acid sites (B) on XC4 (▴), STY_DCut (×) and STY_NCut (+), for the catalysts MZS‐x‐Ox and MZS‐x‐Cl. Reaction conditions: 200 °C, 30 bar, WHSV=2.2 g_1C4 g_cat⁻¹ h⁻¹, TOS=7 h, catalyst activation temperature=450 °C.

Various effects of acid properties on 1‐butene conversion were reported in the literature. Henry et al.⁸⁹ reported higher selectivity towards dimers (and, to a smaller extent, trimers) at 220 °C for a ZSM‐5 sample with Si/Al=20 and possessing higher amount of B acid sites, in relation to another ZSM‐5 sample with Si/Al=169. Popov et al.⁸⁶ reported that 1 C4 oligomerisation over zeolite ZSM‐5 was favoured by B acid sites located in the subsurface of polycrystals, and that B acid sites located on the external surface favoured not only oligomerisation, but also side reactions such as hydride transfer and cracking. In a different study for ZSM‐5, the results indicated that conversion and selectivity to diesel increased with increasing amount of B acid sites, at 270 °C/40 bar.⁹⁰ A comparative study of zeolite Beta and mesoporous aluminosilicate MCM‐41 by Kumar et al.⁹¹ indicated favourable effects of mild B acidity of MCM‐41 on oligomerisation, compared to zeolite Beta possessing more (and stronger) B acid sites.

The stabilities of the catalysts MZS‐0.2‐Cl, MZS‐0.2‐ox, MZS‐0.4‐ox, MZS‐0.4‐Cl and MZS‐0.6‐Cl were compared based on the drop of X_C4 with time‐on‐stream (Figure S7). The drop of X_C4 was less pronounced for MZS‐0.4‐Cl (10 % decrease) than the remaining materials (18–28 % decrease), suggesting that MZS‐0.4‐Cl was more stable. The used catalysts were brownish in colour, attributable to coke, as confirmed by elemental (EA) and thermal (TGA, DSC) analyses. The amount of coke was in the range 11–14 wt % (based on the mass loss in the temperature range 200–800 °C, TGA), and EA indicated carbon contents in the range 8–12 wt %. Figure S10 exemplifies the DSC analysis for MZS‐0.4‐Cl; an endothermic curve bellow ca. 220 °C was due to desorption of physisorbed water/volatiles, and, on the other hand, an exothermic process associated with coke decomposition occurred above ca. 270 °C for the used catalyst (and not for original catalyst).

The used catalyst MZS‐0.4‐Cl was thermally treated at 600 °C (heating rate of 1 °C min⁻¹). Thermal analyses and EA gave similar results for the original versus treated catalysts (2.8 and 2.9 wt % mass loss, respectively; 0.58 and 0.78 wt % C, respectively; Figure S10), suggesting that the treatment was effective in removing coke. The resultant regenerated catalyst MZS‐0.4‐Cl‐r was used for a second catalytic run of ca. 7 h on‐stream (Figure 10‐A). The product lump distribution (PLD) curves were similar (inset of Figure 10‐A), and conversion decreased slightly by a factor of 1.13 (based on conversion at ca. 7 h on‐stream). The regenerated catalyst exhibited comparable powder XRD pattern to the original catalyst (Figure 10‐B), and the two solids possessed comparable textural properties (Figure10‐C, Table 2).

Figure 10

(A) Stability of MZS‐0.4‐Cl for consecutive 7 h on‐stream runs (original catalyst (×); regenerated catalyst MZS‐0.4‐Cl‐r (–)); inset of Figure 10‐A shows the product lump distributions (PLD) for original (×) or regenerated (–) catalysts. (B) Powder XRD patterns and (C) N₂ adsorption‐desorption isotherms for the original (black line) and recovered (grey line) solids. Reaction conditions: 200 °C, 30 bar, WHSV=2.2 g_1C4 g_cat⁻¹ h⁻¹, TOS=7 h, catalyst activation temperature=450 °C.

Fair comparisons of the catalytic results to literature data are not straightforward partly due to the considerable number of parameters involved in these reaction processes, which are different between studies or not always specified. Conversion and selectivity depend on several factors such as process conditions and types of catalysts.⁹² Table 4 compares the catalytic results (conversions; selectivity towards C10⁻ and C10⁺ products) for MZS‐0.4‐Cl to literature data for zeolites and modified zeolites tested for 1C4 oligomerisation using fixed‐bed reactors.^{16, 58, 76, 90, 93-95} Based on rough comparisons, MZS‐0.4‐Cl seemed to perform quite well. At higher reaction pressure (40–50 bar) and temperature (270 °C), it was reported relatively high C10⁺ selectivity at high conversions for two hydrothermally synthesized ZSM‐5 samples:^{90, 95} however, for one of the studies, the type of butene isomer used as substrate and the feed composition were not specified (the catalyst possessed S_meso=68 m² g⁻¹, comparable to S_meso=54 m² g⁻¹ for HZSM‐5 in this work),⁹⁰ and for the other study no textural properties of the catalyst were indicated.⁹⁵

Table 4. Comparison of the catalytic results for MZS‐0.4‐Cl and HZSM‐5 to literature data for zeolites and modified zeolites, tested for the reaction of 1‐butene using a fixed‐bed reactor.^[a]

Catalyst	Crystal size [μm]	Si/Al^[b]	Tact^[c] [°C]	P^[d] [bar]	T^[e] [°C]	WHSV [h^−1]	Substrate^[f] [mol %]	Diluent^[g]	Conversion^[h] [%] (TOS)	Selectivity [%]^[i]		Ref.
										C10^−	C10^+
MZS‐0.4‐Cl (synthesized)	0.3–0.5	24	450	30	200	2.2	15 (mol %)	N2	XC4=86; X1C4=97 (7 h)	44 (mol %)	56	this work
ZSM‐5 (Zeolyst)	0.3–0.5	15	450	30	200	2.2	15 (mol %)	N2	XC4=39; X1C4=91 % (7 h)	55 (mol %)	45	this work
ZSM‐5 (Zeolyst)	0.3–0.5	15	200	30	200	2.2	15 (mol %)	N2	XC4=23; X1C4=90 % (8 h)	44 (mol %)	56	[58]
ZSM‐5 (Zeolyst)	0.1	40	500	24	225	245	90 (mol %)	N2	X1C4=3 (6 h)	86 (mol %)	8	[16]
ZSM‐5 (Zeolyst)	2	15	450	1	200	13.5	50 (mol %)	N2	X1C4=10 (21 min)	33 (wt %)^[j]	37^[j]	[93]
ZSM‐5 (Zeolyst)	ns	30	ns	1	300	ns	30 (mol %)	N2	X1C4=68 (25 min)	ns	ns	[94]
Ferrierite (Zeolyst)	ns	10	500	6.9	200	0.03	2 (mol %)	He	XC4=19; X1C4=91 (10 h)	60 (mol %)	8.5	[76]
		10	500	6.9	250	0.03	2 (mol%)	He	XC4=77; X1C4=96 (10 h)	50 (mol %)	4	[76]
ZSM‐5 (synthesized)	0.8	40	ns	50	270	12	99.5 (wt %)	–	X1C4=99 (10 h)	24 (wt %)^[k]	76^[k]	[95]
ZSM‐5 (synthesized)	0.1–0.2	25	500	40	270	4.8	ns (type of butene and composition)	N2	XC4=91 (24 h)	12 (ns %)	88	[90]
Hierarchical Beta (synthesized)	0.5–0.6	25	200	30	200	2.2	15 (mol %)	N2	XC4=54; X1C4=94 (8 h)	48 (mol %)	52	[58]
Beta/15–20 wt % clay (Guild Associates)	0.1	17	500	24	225	85	90 (mol %)	N2	X1C4=9 (6 h)	80 (mol %)	12	[16]

• [a] ns=information not specified in the article. Some of the values of conversion and selectivity were calculated from the data given (in the form of graphical figures or tables) in the reported studies, and are approximate values. [b] Molar ratio Si/Al. [c] Catalyst activation temperature prior to the catalytic reaction. [d] Pressure of the catalytic reaction. [e] Temperature of the catalytic reaction. [f] Amount of substrate in the feed stream. The mol % or wt % basis is indicated in parenthesis. [g] Diluent in the feed stream. [h] Conversion of butenes (1C4=1butene; 2C4=2‐butene). The values in parenthesis are the TOS for which conversion was calculated. [i] Selectivity to C10⁻ or C10⁺ products. The mol % or wt % basis is indicated in the C10⁻ column in parenthesis. [j] Selectivity to C8⁻ and C8⁺ products. [k] Selectivity to C12⁻ or C12⁺ products.

Conclusions

ZSM‐5 ‐ type of catalyst used in commercial oligomerisation processes ‐ may be modified via top‐down synthetic approaches to meet superior catalytic performances for converting 1‐butene (derivable from (non)renewable sources) to higher molar mass products characteristic of diesel and naphtha cuts, under high‐pressure continuous‐flow conditions (200 °C, 30 bar). Solid catalysts prepared using appropriate base and acid treatments, outperformed the (calcined) commercial acid catalyst HZSM‐5, even though the latter possessed higher amount of total acid sites. Among the superior catalysts, MZS‐0.4‐Cl (Si/Al=24) prepared via desilication using 0.4 M NaOH and acid treatment using HCl, led to 86 % conversion of butenes, space time yield of liquid products (STY) of 852 mg g_cat⁻¹ h⁻¹ (7 h on‐stream) and mass ratio DCut/NCut of 2.2, whereas HZSM‐5 led to conversion=39 %, STY=377 mg g_cat⁻¹ h⁻¹, DCut/NCut=1.4. Catalytic studies combined with characterisation and multivariate/principal component analysis studies, suggested that superior catalytic performances for 1‐butene oligomerisation may be met in an intermediate range of acid sites concentrations and prevailing mesoporosity. Protocols involving alkaline treatment using NaOH or mixed NaOH/TPAOH, and acid treatment using HCl or oxalic acid seem effective for modifying commercial microcrystalline ZSM‐5 zeolites possessing relatively low Si/Al ratios, to produce adequate catalysts for reactions involving relatively bulky molecules.

Experimental Section

Materials

All reagents and solvents were obtained from commercial sources and used as received. For the preparation of the materials: sodium hydroxide (NaOH; 97 %, Sigma‐Aldrich), tetrapropylammonium hydroxide solution tetrapropylammonium hydroxide (TPAOH; 40 wt % in water, Alfa‐Aesar), hydrochloric acid (HCl; 37 % in water, AnalaR NORMAPUR), oxalic acid dihydrate (97 %, Panreac), and ammonium nitrate (NH₄NO₃; 98 %, Aldrich). For the catalytic tests: 1‐butene (99.6 %, Praxair), nitrogen (99.999 %, Air Liquide), silicon carbide (SiC, Ø 0.31 mm, SIKA), dichloromethane (analytical reagent grade, Fisher Chemical), n‐pentane (95 %, Fluka), and commercial zeolite NH₄ZSM‐5 (Si/Al=15, which is a reference value indicated in the technical bulletin for Zeolyst, CBV3024E).

Preparation of the Catalysts

The catalysts were prepared via top‐down strategies starting from HZSM‐5 (protonic form) which was obtained by calcining commercial (microcrystalline) zeolite NH₄ZSM‐5, at 550 °C for 5 h (heating rate of 1 °C min⁻¹). HZSM‐5 was subjected to desilication using NaOH (SIB procedure), TPAOH (QAH procedure) or a combination of both bases (mixB procedure), and acid treatment using HCl (Cl) or oxalic acid (Ox), adapting different procedures (Scheme 2).^39-42 Table 1 summarizes the conditions of the modification protocols and the resultant catalysts’ names.

Desilication. HZSM‐5 was treated with NaOH, which is a strong inorganic base (SIB procedure). An aqueous solution of NaOH (0.2, 0.4, 0.6 or 0.8 M) was added to HZSM‐5 (30 cm³ of solution per gram of solid) under stirring at 65 °C, and kept at this temperature for 2 h. Subsequently, the mixture was cooled using an ice bath for ca. 10–15 min. The solid was separated by centrifugation and washed thoroughly with distilled water at 80 °C, until pH=7. Finally, the solid was dried overnight at 110 °C, giving pre‐MZS‐x, where x stands for the NaOH molar concentration used.

Alternatively, HZSM‐5 was treated with TPAOH, which is a quaternary ammonium hydroxide (QAH procedure), in a similar fashion to the SIB procedure, but using 1.0 M aq. TPAOH instead of NaOH. The TPAOH solution was added to HZSM‐5 (30 cm³ of solution per gram of solid) under stirring at 65 or 85 °C, for 5 h. Subsequently, the solution was cooled using an ice bath for ca. 10–15 min, the solid was separated by centrifugation and washed thoroughly with distillate water until pH=7. The resultant solid was dried overnight at 80 °C, giving pre‐MZS‐TPA‐y, where TPA stand for TPAOH and y stands for the temperature (°C) of the alkaline treatment (y=65, 85).

HZSM‐5 was treated with an aqueous solution of NaOH plus TPAOH (mixB procedure) in a total concentration of NaOH plus TPAOH of 0.6 M (molar ratio of TPA⁺/Na⁺ of 0.4). This solution was added to HZSM‐5 (25 cm³ of solution per gram of solid), and the resultant mixture was stirred for 1 h at 90 °C. Subsequently, the mixture was cooled using an ice bath for ca. 10–15 min, the solid was separated by centrifugation, washed thoroughly with distilled water until pH=7, and then dried overnight at 60 °C, giving pre‐MZS‐TPA/Na.

Ion exchange. All desilicated materials prepared via the SIB or mixB procedures were converted to the acid form via ion exchange using 100 cm³ of 0.1 M aq. NH₄NO₃ solution per gram of solid, and stirring for 24 h at 25 °C. The liquid phase was renewed twice (every 24 h) with fresh NH₄NO₃ solution. Subsequently, the solids were separated by centrifugation, washed with distilled water and dried overnight at 60 °C. A portion of the ion‐exchanged solid was calcined, and the remaining portion (not calcined) was subjected to dealumination treatment as described ahead. The ion exchange step was not required for the QAH procedure.

The ion exchanged materials pre‐MZS‐x and pre‐MZS‐TPA‐y were calcined at 500 °C for 5 h (heating rate of 1 °C min⁻¹), giving MZS‐x and MZS‐TPA‐y, respectively, where x stands for the NaOH concentration and y stands for the temperature of the desilication treatment. The material pre‐MZS‐TPA/Na was calcined at 600 °C for 4 h (heating rate of 5 °C min⁻¹) giving MZS‐TPA/Na.

Acid treatment. The desilicated materials MZS‐x, MZS‐TPA‐y and MZS‐TPA/Na were treated with hydrochloric acid (removal of inorganic debris from the pores),^{42, 56, 67, 96} or oxalic acid (removal of extra‐framework aluminum species, likely located closer to the external surface).^{24, 38} Specifically, a 0.8 M aq. oxalic acid solution was added to the solid (100 cm³ of solution per gram of solid) at 70 °C, for 2 h, or, alternatively, 0.1 M aq. of HCl solution was added to the solid (100 cm³ of solution per gram of solid), and the resultant mixtures were stirred for 6 h at 65 °C. Subsequently, the solids were filtered, washed with milli‐Q water until pH=7 and dried overnight at 60 °C. Finally, the materials were calcined at 500 °C for 5 h (heating rate of 1 °C min⁻¹), giving MZS‐x‐Ox, MZS‐x‐Cl, MZS‐TPA‐y‐Cl and MZS‐TPA/Na−Cl, where Ox and Cl stands for dealumination using oxalic acid or hydrochloric acid, respectively.

Characterisation of the Catalysts

The PXRD data were collected on an Empyrean PANalytical diffractometer (Cu_Kα X‐radiation, λ=1.54060 Å) in a Bragg‐Brentano para‐focusing optics configuration (45 kV, 40 mA). Samples were prepared in a spinning flat plate sample holder and step‐scanned in the range from 3 to 70° (2θ) with steps of 0.026°. A PIXEL linear detector with an active area of 1.7462° was used with a counting time of 68 s per step.

SEM images (for morphological studies) and EDS analysis (for determining the ratio Si/Al) were obtained on a Hitachi SU‐70 SEM microscope with a Bruker Quantax 400 detector operating at 20 kV. The molar ratios Si/Al were determined from at least three replicates, and the average standard deviations were less than unity. Thermogravimetric (TGA) and differential scanning calorimetry analyses (DSC) analyses were performed under air, using Shimadzu TGA‐50 and DSC‐50 instruments, respectively. TGA was performed from room temperature until 800 °C and DSC from room temperature until 550 °C with a heating rate of 10 °C min⁻¹. Elemental analysis (EA) was performed on a Truspec 630‐200‐200 instrument.

Nitrogen adsorption‐desorption isotherms were measured at 196 °C, using a Quantachrome instrument (automated gas sorption data using Autosorb IQ₂). The samples were pre‐treated at 300 °C for 3 h, under vacuum (<4×10⁻³ bar). The specific surface area (S_BET) was calculated using the Brunauer, Emmett, Teller equation and the total pore volume (V_p) was based on the Gurvitch rule (for relative pressure (p/p₀) of at least 0.99). The mesoporous surface area (S_meso) and microporous volume (V_micro) were calculated using the t‐plot method (Eq. 1]:

(1)

The mesoporosity and microporosity of the materials were calculated as the percentage of S_meso/S_BET and V_meso/V_p, respectively. The pore size distributions (PSDs) were determined by the DFT method (adsorption branch).

The indexed hierarchy factor (IHF) was calculated according to that reported by the group of Pérez‐Ramírez,⁶⁷ where the micropore and mesopore volumes were normalized by the maximum values (Eq. 2]:

(2)

where V_{micro,HZSM‐5}=0.18 cm³ g⁻¹ and S_{meso,MZS‐0.4‐Cl}=291 m² g⁻¹.

The nature of the aluminum species was studied by ²⁷Al MAS NMR spectroscopy. The spectra were recorded at 182.432 MHz using a Bruker Avance 700 (16.4 T) spectrometer with a unique pulse, a recycle delay of 1 s and a spinning rate of 14 kHz. The percentage of different aluminum species was calculated using the peak areas obtained by deconvolution of the spectra.

The surface acidity was measured using a NexusThermo Nicolet apparatus (64 scans and resolution of 4 cm⁻¹) equipped with a home‐made vacuum cell, using self‐supported discs (5–10 mg cm⁻²), and pyridine as the base probe. After in situ outgassing at 450 °C for 3 h (10⁻⁶ mbar), the sample was contacted with pyridine (99.99 %) at 200 °C for 10 min and subsequently evacuated at the same temperature or at 450 °C for 30 min, under vacuum (10⁻⁶ mbar). The IR bands at ≈1540 and 1455 cm⁻¹ related to pyridine adsorbed on Brønsted (B) and Lewis (L) acid sites, respectively, were used for quantification.⁹⁷ The total amount of acid sites (L+B) and the molar ratios L/B were determined at the desorption temperature of 200 °C. The acid strength was evaluated by the molar ratios B₄₅₀/B₂₀₀ (for B acid strength) and L₄₅₀/L₂₀₀ (for L acid strength), where L_T and B_T are the amount of L and B acid sites, respectively, which remained adsorbed on the material after evacuation at T=450 or 200 °C. The sites interacting with pyridine after evacuation at 450 °C (L₄₅₀, B₄₅₀) were considered as strong acid sites. The B acid site density (expressed as meq nm⁻², where meq is milliequivalents of acid sites; Figure S6) was calculated from the amount of B acid sites (mol g⁻¹) and S_BET (m² g⁻¹), using the following Equation 3:

(3)

where N_A=6.022×10²³ mol⁻¹. The L acid site density was calculated using the same formula, but μmol_L g⁻¹ instead of μmol_B g⁻¹.

Multivariate, Principal component analysis

The modification treatments and conditions influenced several material properties. Principal component analysis (PCA) was used to help categorise the prepared materials according to their properties based on the complementary characterisation studies, since it allows to visualize and analyse the M observations (initially described by the N variables) on a low dimensional map. A data matrix was generated using the following properties: amount and strength of B and L acid sites, S_BET, S_meso, V_meso, and V_micro (Table S3); a separate additional matrix was created including IHF, ratio Al_penta/Al_tetra and %Al_tetra+penta. In Table S3, the columns indicate the variables (materials properties), and the rows indicate the observations (HZSM‐5 and materials prepared via the SIB and mixB procedures, with or without (Ox, Cl) acid treatment. A PCA study was also carried out for catalytic performance parameters (D_Cut/N_Cut, STY, STY_DCut, X_C4) of selected samples.

PCA was carried out using XLSTAT statistical analysis software. For the PCA analysis it was necessary to select the columns with the variables labels (first row) and data (remaining rows) and the column with the observation labels; choose the PCA type that will be used during computation, correlation matrix (Pearson or Spearman) or covariance matrix; and select the significance level and desired outputs (e. g. descriptive statistics, eigenvalues, factor scores, squared cosines, correlation circle, bi‐plot with variables and observations). Then it is necessary to select the x and y axis with the factors with higher total variance to represent the data. In this work, the data were analysed using the Pearson correlation, and the statistically significant level was 95 % (p<0.05). When analysing the PCA results, it is important to make an eigenvalues analysis to evaluate the quality of the projections (if the total variance is too low (less than 50–60 %), it is preferable to reselect the variables and/or observations). Then, it was performed an analysis of the correlation circle chart to check if some variable is located close to the centre (in that case any interpretation on that variable may be hazardous). Moreover, the squared cosines table allows to evaluate the representation quality of a variable on a PCA axis (when the square cosines are close to zero, the more careful must be the interpretation of the results in terms of trends on the corresponding axis). Finally, it is made a careful analysis of the PCA bi‐plot in order to establish relationships between variables and identify trends between variables and observations.

Catalytic Tests

The oligomerisation of 1‐butene (1 C₄) was carried out under continuous‐flow, high‐pressure conditions, using a laboratory stainless steel fixed‐bed tubular catalytic reactor (10 mm internal diameter, Scheme 2), in vertical orientation, which was packed with the solid catalyst particles (150 mg) and silicon carbide as diluent to enable a uniform temperature distribution along the catalytic bed; the total bed volume was ≈1.8 cm³. The reactor was equipped with an internal thermocouple and heated with a tubular furnace (Termolab). The pressure in the reactor unit was controlled using a backpressure regulator (Equilibar, LF‐Primary Research Series) located at the reactor outlet. Prior to the reaction, the catalyst was activated at 450 °C for 3 h (heating rate ≈1 °C min⁻¹) under nitrogen flow (10 cm³ min⁻¹); subsequently, the temperature was set to the desired catalytic reaction temperature. Olefin with nitrogen as the carrier gas (molar ratio of 1 C₄:N₂=15 : 85) were fed to the reactor using a syringe pump (Chemyx, model Nexus 6000) and a gas mass flowmeter (Bronkhorst, EL‐FLOW), respectively. The reactor outlet was connected by stainless steel tubing (1/8”) to a borosilicate jacketed cooling trap (cooling fluid at 5 °C and ≈1 bar, to condense C₆₊ products from the gas phase). The gas phase leaving the trap was directly passed through a heated (200 °C) stainless steel tubing (1/16”) connected to a Master Fast GC gas chromatography instrument (DANI), equipped with a capillary column (ValcoBond VB‐1, 60 m×0.25 mm×1.50 μm), FID detector, split/splitless injector, a gas sampling valve and loops (heated at 200 °C). The gas phase was sampled at regular intervals of ≈1 h, for a time‐on‐stream (TOS) of ≈7 h. Quantifications were based on external calibration curves using pure 1C₄, the range of experimental error was less than 5 %. The catalytic results were expressed as conversion of butenes (X_C4) which did not react to give higher molar mass products, using Equation 4.

(4)

where F_1C4in is the inlet molar flow rate of 1C₄, and F_C4out is the outlet molar flow rate of total butenes. The values of X_C4 corresponded to the average conversion at approximately steady state conditions, whereas the X_C4 values presented as a function of TOS were calculated for the instant of sampling (Figure S7). The liquid reaction products were analysed using the same GC instrument; concentrations were based on calibrations using ASTM D2887 Quantitative Calibration mixture (n‐alkanes C₆−C₄₄), and internal standard. The products lump distributions (PLD) curves correspond to the products formed during ≈7 h TOS. These curves Represent the set of values of selectivity (S_C[y−z]) for a set of compounds possessing y to z number of carbon atoms per molecule (lump C_[y−z], where z>y), which was calculated using Equation 5.

(5)

where n_C[y−z] is the moles for product lump C_[y−z], and ∑n_C[y−z] is the total moles of products in the range C₆−C₂₄. The fractions corresponding to the 170 °C cut characteristic of naphtha products (NCut), and to the 170–390 °C cut characteristic of diesel products (DCut, corresponding approximately to the C₁₀−C₂₄ n‐paraffinic range) were determined according to the ASTM D2887 method (Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography).^{39, 50, 98} These analyses were validated using the ASTM D2887 Reference Gas Oil no 1 (Supelco, sample 1, Batch 2) and the results agreed, within the allowable difference ranges, with the ASTM D2887 consensus boiling point values. The C₁₀ type products (corresponding to the temperature cut point) may be distributed between DCut and NCut fractions. The space time yields (STY, expressed as mg g_cat⁻¹ h⁻¹) were based on the mass fractions of DCut and NCut of total liquid (condensed) products formed during ≈7 h TOS. In general, the material balances closed in at least 82 %. The catalytic performances were compared based on X_C4 and STYs considering approximately steady‐state conditions within ≈7 h TOS.

Details regarding the determination of the cetane number (CN, based on ¹H NMR spectroscopy), isoparaffinic index (I, based on ¹H NMR spectroscopy), aromatics content (%Ar, based on ¹H NMR spectroscopy) of the reaction products, and checking operation under kinetic regime, are given in the Supporting Information section.

Acknowledgements

This work was developed in the scope of the project CICECO‐Aveiro Institute of Materials, FCT (Fundação para a Ciência e Tecnologia) Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. The FCT and the European Union are acknowledged for a Ph.D. grant to A.F.S. (SFRH/BD/101018/2014). M.M.A. and A.F. thank the costs resulting from the FCT hirings which are funded by national funds (OE), through FCT – Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree‐Law 57/2016, of August 29, changed by Law 57/2017, of July 19 The authors wish to thank Saint‐Gobain Ceramic Materials AS for generously supplying the SIKA SiC sample.

Conflict of interest

The authors declare no conflict of interest.