Di-methyl Ether Conversion into Olefins over HZSM-5: Effect of SiO2/Al2O3 Ratio Effect on Surface Chemistry and Reactivity Properties

Di-methyl Ether Conversion  into Olefins over  HZSM-5:

Effect of SiO2/Al2O3 Ratio Effect on Surface Chemistry and Reactivity Properties

Abdullah Al-Dughaither ^(1,2) and Hugo I. de Lasa⁽¹⁾*

^1.  Chemical Reactor Engineering Centre, Western University, London, ON, N6A 5B9

^2.  SABIC Technology Center, Riyadh, (Saudi Arabia)

* Corresponding author: Tel.: +1 519 661 2144; Email: hdelasa@uwo.ca

Objectives

This study investigates HZSM-5 as a potential catalyst for olefins production from di-methyl ether (DTO). The aim of this study is to provide an in-depth understanding of the unique effect of SiO2/Al2O3 ratio on the physico-chemical and reactivity properties of the HZSM-5 zeolite.

Materials and Methods

This study was carried out using three commercial NH4⁺ZSM-5 (SiO2/Al2O3 = 30, 80, and 280). The ammonia ZSM5 precursors were calcined to have ZSM-5 in the protonic form (H⁺ZSM-5). N₂ isotherm and XRD tests were performed to assess the structural and porosity properties. NH₃-TPD experiments were carried out to establish acidity and provide insights on intrinsic desorption kinetics. Desorption parameters were estimated numerically using MATLAB^® solver. Pyridine FTIR was also employed to evaluate the influence of Brønsted and Lewis acid sites while changing the SiO₂/Al₂O₃ ratios of the HZSM-5. The reactivity runs were performed in a gradientless Berty reactor unit using catalyst extrudates that compose 5% Versal-950 and 70% Fused Alumina.  

Results and Discussion

N₂ adsorption and desorption showed isotherms with hysteresis, with this pointing to micropores and intercrystalline mesopores. Zeolite SiO₂/Al₂O₃ ratio shows however, no effect on specific surface area, pore volume and pore size distribution (Table 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003000380034000000 and Figure 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003100310030000000 ). DFT with a cylindrical geometry confirmed the HZSM5 5.5 Ǻ channel size. HZSM5 displayed weak and strong acid site distributions. One finding from Table 4 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003100370039000000 is the weaker to strong fraction and less total acidity with increasing SiO₂/Al₂O₃ ratio. These findings points towards a correlation between Al content and acidity on HZSM5. FTIR shows as well a similar trend for Brönsted acidity while the composition of HZSM5s was not noticeably influencing Lewis acidity (Figure 2 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003200300037000000 , Figure 3 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003200360030000000 ).

Concerning NH₃ desorption kinetics, the activation energy was higher for the stronger acid sites than for the weaker sites (Table 2 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003200390039000000 , Table 3 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003300300038000000 ) . Furthermore, the ratio of the intrinsic desorption rate constants for the weak and acid sites were found to be consistent with their acidity density fraction.

DTO over HZSM5 is an “in series” reaction where formed olefins may continue reacting yielding heavier paraffin and aromatic hydrocarbons. Reactivity runs in the present study, show that the ZSM5-30 zeolite exhibits promising 32-67% DME conversions with 20.6-32% light olefins selectivity.

Conclusions

·         A thorough understanding of SiO2/Al2O3 ratio on HZSM-5 physical and reactivity characterization, will lead in the context of DTO process to a significant olefin selectivity optimization.

·         To our knowledge and in spite of their great significance for DTO future implementation, there is no reported study addressing these topics in the open literature.

Table  SEQ Table \* ARABIC 1 N₂ isotherm results (SA m²/g, VP cm³/g, dp Ǻ)

			ZSM5-30		ZSM5-80		ZSM5-280
Sample weight (g)			0.101		0.115		0.083
Surface area (m2/g)	(BET)a	C	1441		4414		-1833
		SA	414		441		399
	SA (BET, single point)a		410		439		397
	SA (Langmuir)		474		544		442
	Micropore SA (D-A)b		465		512		518
	Mesopore SA (t-plot) c		163		177		149
	Mesopores SA(BJH)c,d		73		100		171
	SA (DFT) g		Cylindrical h	Slit i	Cylindrical h	Slit k	Cylindrical h	Slit k
			110	569	143	610	124.5	598
Pore volume (cm3/g)	Micropore PV (t-plot) c		0.106		0.108		0.102
	Micropore PV (D-A)b		0.194		0.213		0.161
	Mesopores PV (BJH) c,d		0.125		0.116		0.109
	PV (HK, single point)e, f		Cylindrical h	Slit k	Cylindrical h	Slit k	Cylindrical h	Slit k
			0.267	0.267	0.261	0.261	0.219	0.219
	PV (DFT) g		0.205	0.232	0.245	0.203	0.189	0.184
Median pore dimater (Ǻ)	Mesopore dp (4V/A, BJH) a		68		46		25.5
	Micropore dp (V/A, MP) c,j		4.2		5.8		5.4
	Micropore dp (HK)j		Cylindrical h	Slit k	Cylindrical h	Slit k	Cylindrical h	Slit k
			15.4	7.5	11.4	6.5	8.4	4.9
	Micropore dp (DFT) g		22	4.3	30	4.2	28	4.3

^a at P/P₀ = 0.1

^b Dubinin–Astakhov method, affinity coefficient for N2 = 0.33

^c Thickness curve type Harkins and Jura: t-plot = [13.99/(0.034 - log(P/P₀ ))]^0.5

^d Barrett-Joyner-Halenda, adsorption mespores range: 17 < dp < 3000 Ǻ.

^e Horvath-Kawazoe, Method: interaction parameter for zeolite adsorbent = 3.49×10^-43 ergs/cm⁴

^f Measured at Max relative pressure: P/P₀ = 0.995

^g Density Functional Theory, Method: Non-negative Regularization; No Smoothing

^h Cylindrical pore geometry (Saito/Foley)

ⁱ Slit pore geometry

^j Micropore analysis method, hydraulic diameters' pores range: 2.94 < dp < 38.8 Ǻ

Figure  SEQ Figure \* ARABIC 1 Pore size distribution of ZSM5 in micro-channel region (DFT) and meso-intercrystalline region (BJH).

Table  SEQ Table \* ARABIC 2 Optimized kinetic constants for NH3 desorption

Parameter	Sites		HZSM5-30						HZSM5-80						HZSM5-280
			Value	95% CI		STD	DOF		Value	95% CI		STD	DOF		Value	95% CI		STD	DOF
kd0 mlg∙min	Weak		1.534	±	0.030	0.015	196		0.443	±	0.018	0.009	206		0.122	±	0.007	0.003	172
	Strong		1.297	±	0.007	0.003	264		0.46	±	0.012	0.006	302		0.365	±	0.010	0.005	332
Ed kJmol	Weak		29.9	±	1.4	0.7	196		49.4	±	3.7	1.9	206		50.4	±	5.3	2.7	172
	Strong		38.7	±	4.6	2.3	264		54.7	±	3.3	1.7	302		60.6	±	1.4	0.7	332

Table  SEQ Table \* ARABIC 3 Cross-correlation coefficients for NH3 desorption                                             Table  SEQ Table \* ARABIC 4 NH3-TPD acidity measurements at b = 15 K/min.

Sites			HZSM5-30			HZSM5-80			HZSM5-280
			kd0	Ed		kd0	Ed		kd0	Ed
Weak	Kdo		1.00			1.00			1.00
	Ed		0.05	1.00		0.08	1.00		0.37	1.00
Strong	Kdo		1.00			1.00			1.00
	Ed		0.48	1.00		0.27	1.00		0.41	1.00

       Figure  SEQ Figure \* ARABIC 2  Pyradine FTIR Brönsted acidity                                                              Figure  SEQ Figure \* ARABIC 3  Pyradine FTIR Lewis  acidity