Nanoporous materials with predicted zeolite topologies.

An increasing number of newly synthesized materials have been found to be previously present in databases of predicted porous materials. This has been observed not only for zeolites, but also for other inorganic materials and for MOFs. We here quantify the number of synthesized zeolites that are present in a large database of predicted zeolite structures as well as the number of other inorganic crystals and MOFs present in this same database. We find a significant number of real materials are in this predicted database of zeolite-like structures. These results suggest that many other predicted structures in this database may be suitable targets for designer materials synthesis. This journal is © The Royal Society of Chemistry.

Introduction

Zeolites are nanoporous crystal forms of aluminosilicate oxides that are widely used in catalysis and adsorption.[1] At present, 248 zeolite topologies are known.[2] These structures differ in the connectivity and relative ordering of the TO4 (T = Si or Al) tetrahedra, and therefore in the size and shape of the nanoporous cavities. In addition to aluminosilicate zeolites, numerous materials with zeolite topologies but containing other elements or building blocks that serve as tetrahedral centers and bridging atoms are known.[3,4]

Due to their importance in many industrial processes, the discovery and identification of novel zeolite and zeolite-like materials is a field of intensive research.[4] To aid the search for new zeolites, computational methods have been applied to generate novel predicted topologies that expand upon the currently known materials.[5-7] The PCOD database[8] developed in the Deem laboratory contains predicted zeolite structures that have both a low computed energy gap with respect to alpha-quartz, and an energy/density ratio that is in the range of known zeolites. A considerable number of existing zeolites were found in the structures predicted during the generation of the database.[6]

The PCOD has been extensively screened in the search for zeolites with specific functionalities.[9-15] The ToposPro program package[16] is a computer program for the topological analysis of crystal structures allowing an objective description and comparison of crystal structures. Recently, the PCOD database has been made searchable with the ToposPro package and the corresponding online TopCryst service.[17] This allows the comparison of newly discovered zeolite frameworks with the predicted frameworks in the PCOD. Thus, a number of recent candidates for the IZA database of known zeolites have been found to correspond to entries in the PCOD.[18-20] In total, 154 of the 248 known zeolite frameworks in IZA were identified in the PCOD. The concept of structure representation in the ToposPro method also allows the comparison of topologies of different classes of materials such as inorganic compounds and coordination polymers. By searching the Inorganic Crystal Structure Database (ICOD[21]) and the Cambridge Structural Database (CSD[22]) we additionally found a large number of topologies that were predicted in the PCOD. Of these, 57 were zeolite-like inorganic materials, and 118 were metal–organic frameworks (MOF), another class of nanoporous materials.

Thus, a large number of predicted zeolite topologies present in the PCOD correspond to synthesizable compounds, be it zeolites or other nanoporous materials. This, in combination with the very large size and topological diversity of the database, is an incentive for further search efforts of this database in the design of novel materials with tailored properties.

Methods

At the origin of the PCOD is a Monte-Carlo based algorithm to generate predicted zeolite-like frameworks by sampling and optimizing a zeolite figure of merit.[23] Initially, approximately one million structures were found belonging to a limited number of space groups. The database was extended to include topologies of all space groups,[6] and was refined by performing energy minimization with two force fields, SLC[25] and BKS,[26] using the GULP program.[27] Of the thus obtained 2.7 million topologically unique, energy minimized structures, 313 565 were no higher in energy than 30 kJ (mol Si)−1 relative to quartz using the SLC force field, and 585 139 were no higher in energy than 65 kJ (mol Si)−1 relative to quartz using the BKS force field.[7] These criteria are judged to be the limits for thermodynamically stable aluminosilicate zeolites, and therefore only these structures were retained in the database.

The ToposPro program package offers an objective and complete approach to explore crystal structures by analyzing their topology. In addition to coordination sequences, ToposPro computes the so-called point symbols and vertex symbols that collect the shortest cycles and rings (cycles without shortcuts) of atoms, respectively.[28] The general scheme of the analysis includes the following steps:[29] (i) determination of all interatomic interactions in the structure using a number of chemical and geometrical criteria; (ii) search for structural groups (building blocks) with unique topological algorithms; (iii) simplification of the structure by squeezing the structural groups into their centers of mass keeping the connectivity between the groups; (iv) determination of the topology for the resulting underlying net, i.e. the net of the centers of the structural groups, by comparison of the topological indices (coordination sequences, point and vertex symbols) of the underlying net with the indices for the reference topologies from the ToposPro TTD Collection. All these steps are performed in an automated mode, so thousands of crystal structures can be processed in an appropriate time. We have applied this procedure to determine the PCOD topologies and to identify which IZA zeolite topologies are present in the PCOD. Additionally, the Cambridge Structural Database (CSD, version 5.40 as of November 2018) and the Inorganic Crystal structure Database (ICSD, release 2019/2) were screened for PCOD topologies. Two classes of materials were distinguished: zeolite-like inorganic materials, and MOFs. For identifying the MOFs, the MOF building blocks were treated as T-centers, and the organic linkers as the oxygen atoms in the corresponding predicted zeolite structures. All frameworks (PCOD structures, inorganic materials and MOFs) were simplified to their underlying nets consisting of only T centers by replacing the linker nodes (L) by edges between the T centers, i.e. by the graph transformation T – L – T → T – T (Fig. 1). To designate the underlying topologies, besides the IZA symbols for zeolites, we use the RCSR three-letter symbols,[30] the ToposPro NDn nomenclature, the Epinet sqc symbols, and Fischer's symbols k/m/fn for three-periodic sphere packings.[29] For the PCOD topologies that have not been found in other resources, we use the ToposPro NDn symbols with the suffix HZ; for example, the ToposPro symbol 4,4T1319-HZ means that this is a predicted zeolite with two topologically inequivalent T nodes (N = 4,4), three-periodic framework (D = T) and the ordinal number n = 1319 among other topologically different predicted zeolites with two crystallographically distinct T nodes.

Results

A total of 154 of the 248 IZA structures were found to be present in the PCOD. Additionally, by screening the CSD and ICSD, 72 zeolite-like and other inorganic materials and 118 MOFs were found to have topologies present in the PCOD.

Table SI1† lists the 154 PCOD structures that were identified in the IZA database. Column one provides the PCOD code, and column two provides the IZA code of each framework.

Table 1 lists the 72 zeolite-like and other inorganic structures. The first two columns provide the PCOD number and the number of actual structures found. Not all topologies are listed, but only those that are chemically close to zeolites. For each topology, the ICSD reference code and atomic composition of a selected structure are provided in columns three and four. The last column provides the symbol of the topology according to one of the nomenclatures described above.

Inorganic zeolite-like materials from the ICSD with topologies present in the PCOD. IZA zeolites are excluded from this table. The entries with a bold PCOD number are inorganic compounds whose composition precludes their classification as zeolite materials, but which nonetheless have a zeolite-like topology of the listed PCOD number

PCOD	Number of structures	ICSD ref. code (example)	Formula (example)	Underlying topology
PCOD8000022	364	9829	NaAlSi3O8	fel
PCOD8322222	352	1109	SiO2	lon
PCOD8249897	274	33765	Na(FeO2)	dia
PCOD8149775	202	237135	Al4Fe2Si5O18	brl
PCOD8322767	97	23371	Na2Mg5Si12O30	4,4T45
PCOD8029344	79	87538	Na4(Al3Si9O24)Cl	sca
PCOD8167638	57	170497	SiO2	pcl
PCOD8128122	52	74860	LiAl(PO3)4	mog
PCOD8046833	49	20208	Li3(PO4)	deh2
PCOD8147992	46	18112	SiO2	coe
PCOD8171653	17	180997	Ca(Al2O4)	byl
PCOD8128676	13	74808	KCo(PO4)	tpd
PCOD8033784	10	246132	Li2Fe(SiO4)	4,4,4,4T130
PCOD8249812	9	41661	CaAl2O4	cag
PCOD8157925	9	68772	K(AlSi2O6)	kea
PCOD8308057	5	195287	Au(PO4)	pts
PCOD8152484	4	156657	BaNa2(AlSiO4)4	bnl
PCOD8129062	4	191544	Li2Mn(SiO4)	sie
PCOD8128656	3	171001	BaFe2O4	baf
PCOD8321582	3	151369	Ca3(Al2O6)	nbo
PCOD8170506	3	75659	SiO2	irl
PCOD8045579	2	33279	Na2Li3(FeO4)	sqc8104
PCOD8302308	2	170516	SiO2	tzs
PCOD8000219	2	173216	Li3(VO4)(H2O)6	afw
PCOD8046854	2	380488	KBSi2O6	4,4,4T91
PCOD8129307	1	1291	Be(PO3)2	4,4,4T159
PCOD8048022	1	56684	SiO2	4,4T39
PCOD8154928	1	60069	LiK(PO3)2	zsn
PCOD8255081	1	62584	SiO2	4,4,4,4T15
PCOD8166122	1	62585	SiO2	4,4,4,4T14
PCOD8325712	1	63414	Na2.67K1.33Zn4(PO4)4	4,4,4T148
PCOD8000277	1	75653	SiO2	unc
PCOD8128689	1	75664	SiO2	bbe
PCOD8000217	1	79705	K2(ZnSi2O6)	bbi
PCOD8037080	1	79705	K2(ZnSi2O6)	4,4,4,4,4T4
PCOD8128125	1	83427	Na2Zn(Si2O6)	bbm
PCOD8189369	1	83861	SiO2	4,4,4,4,4T3
PCOD8323349	1	89700	SiO2	bam
PCOD8000118	1	91681	Al(PO4)	bcq
PCOD8163521	1	92721	(Mg0.917Fe0.083)2Na0.084(Al3.970Fe0.038Si4.992)O18(H2O)0.38(CO2)0.192	mot-e
PCOD8307680	1	170479	SiO2	wse
PCOD8123215	1	170480	SiO2	dei
PCOD8321616	1	170493	SiO2	umk
PCOD8009546	1	170498	SiO2	umq
PCOD8170966	1	170499	SiO2	umi
PCOD8330894	1	170512	SiO2	cbo
PCOD8308285	1	170526	SiO2	uot
PCOD8308073	1	170528	SiO2	uox
PCOD8169643	1	170534	SiO2	ukb
PCOD8301974	1	170541	SiO2	4/5/t1
PCOD8297080	1	170543	SiO2	lcs
PCOD8129487	1	170545	SiO2	gsi
PCOD8123200	1	170546	SiO2	mmt
PCOD8045573	1	170547	SiO2	neb
PCOD8264888	1	173625	Al15.68(Si41.1Al6.9)O96	4,4,4,4T127
PCOD8003420	1	170960	Rb4(UO2)2(Si8O20)	4,4,4,4,4T7
PCOD8308378	1	261103	K3.33(C2H5)C1.5(Be2Si12O33.7)	ksx
PCOD8328203	1	92822	NaB(SiO4)	4,4,4T24-CA
PCOD8185681	8	167183	Zn(SiO3)	4,4,4T5318-HZ
PCOD8120181	4	4362	NaK(CuSi4O10)	4,4,4T4043-HZ
PCOD8170348	2	162489	K(AlSiO4)	4,4,4T5003-HZ
PCOD8124771	2	410869	K(ZnBP2O8)	4,4T1080-HZ
PCOD8283381	2	2116	Na6FeSi8Al4O26	4,4,4T6294-HZ
PCOD8127150	1	20797	K2BeSi4O10	4,4,4,4,4T243508-HZ
PCOD8052206	1	33924	H(AlSi4O10)	4,4T1048-HZ
PCOD8308516	1	59846	Zn(PO3)2	4,4,4T6862-HZ
PCOD8321753	1	65475	Li3K3(P6O18)(H2O)	4,4T1321-HZ
PCOD8099926	1	85474	Si56O112	4,4,4,4,4,4,4T28511-HZ
PCOD8321730	1	85734	Li6(P6O18)(H2O)3	4,4T1319-HZ
PCOD8071670	1	86548	SiO2	4,4,4,4,4,4T6819-HZ
PCOD8308593	1	86549	SiO2	4,4,4T6867-HZ
PCOD8168418	1	411142	Na0.75(NH4)0.25Zn(PO4)	4,4T1145-HZ

Table 2 lists the 118 MOF topologies. The first column provides the PCOD identifier and the second column the topology symbol. The bold entries have IZA zeolite topologies, the codes of which are listed in the third column. Column four lists the total number of structures found in the CSD, and the last column provides the CSD code of one of these structures as an example.

MOF topologies from the CSD found in the PCOD. The entries with a bold PCOD number are also known IZA zeolite topologies

PCOD code	Underlying topology	IZA code	Number of structures	CSD ref. code (example)
PCOD8249897	dia		2291	XEYXUW
PCOD8308057	pts		632	AVIVAC
PCOD8128122	mog		299	PUZBES
PCOD8321582	nbo		258	TANNUU
PCOD8321332	sod	SOD	186	XIZDER
PCOD8067826	crb	BCT	120	PUMNIV
PCOD8000282	gis	GIS	96	DIZJED
PCOD8322222	lon		85	FIPXAF
PCOD8171811	bbf		70	QAVDEW
PCOD8249812	cag		51	KOTPUG
PCOD8000277	unc		41	VAHWOS02
PCOD8045573	neb		40	ANUPIK
PCOD8162585	dft	DFT	22	HIFVOI
PCOD8297080	lcs		22	GIZJUV
PCOD8306957	rho	RHO	20	MECWOH
PCOD8321454	uni		20	DIVPUU
PCOD8077978	gme	GME	16	RIRDAZ
PCOD8164109	frl		15	VEPBOK
PCOD8170506	irl		14	DEXXOU
PCOD8000219	afw		11	DAGFUP
PCOD8068050	cha	CHA	11	NIRKAB
PCOD8238986		THO	10	BEFNAD
PCOD8123200	mmt		10	DUWREU
PCOD8308045	ana	ANA	10	GUPDOL
PCOD8306691	mer	MER	8	EWENUR
PCOD8170814	npo	NPO	7	SODKIH
PCOD8308796		SAV	6	LOFZUB
PCOD8308791	edi	EDI	6	XAQTOY01
PCOD8117704		LAU	5	YOMVIG
PCOD8123215	dei		5	TOBQAE01
PCOD8115801	ast	AST	5	IRUROC
PCOD8001707	can	CAN	5	PAJSAX
PCOD8307996	fau	FAU	5	XEQNIQ
PCOD8304448	mtn	MTN	4	GAQYIH
PCOD8160106	4,4T67		4	QUDKIK
PCOD8122541		OWE	4	BEFNOR
PCOD8307029	asv	ASV	4	GOMSUW
PCOD8324721	unj		4	UFAQIE
PCOD8129487	gsi		4	ZUYWAR
PCOD8077973	cgs	CGS	3	DEPTOH
PCOD8077922	cfc		3	XACFAJ
PCOD8227613	bbh		3	ADECEU
PCOD8123876	4,4,4,4,4,4T10		3	EXOKIM
PCOD8052570	pcb	ACO	3	DEJROB
PCOD8055858	4,4T133		3	TAXHUX
PCOD8163960	4,4T85		3	WUPTIM
PCOD8163521	mot-e		3	FIWJIG
PCOD8125027	4,4,4,4,4,4,4,4T11	JNT	3	SOQJIT01
PCOD8095118	lev	LEV	3	TOFWEQ
PCOD8331046	sdt		2	ALIBUT
PCOD8330894	cbo		2	DOLWEI
PCOD8047042	noq		2	LATCIS
PCOD8126401	4,4,4,4T11		2	AMBZAG10
PCOD8085224	sas	SAS	2	VAHSIH
PCOD8045484	4,4T10		2	BOSCET
PCOD8321918	afx	AFX	2	OSUSAY
PCOD8156657		JRY	2	MORZID
PCOD8167638	pcl		2	RIDKOE
PCOD8117232		JSN	2	DARJOX
PCOD8295280		SAF	2	SUSZIQ
PCOD8248916		JSW	2	HATSEC
PCOD8077977		AEI	2	BEFPAF
PCOD8128125	bbm		2	MUNQIX
PCOD8124791	ucn	SBN	2	FIGQIV
PCOD8323892	4,4T148		2	RIRDED
PCOD8169309	bik	BIK	2	YOMBOS
PCOD8185531	bbg		1	MUDHOK
PCOD8171792	cdp		1	ZAYFEN
PCOD8217418	stc-4,4-Ccce		1	PUWQAA
PCOD8047071	4,4,4T162		1	OKUWOI
PCOD8123892P	fsg-4,4-Cmmm		1	XUNTEH01
PCOD8156062	cus		1	XUNSOQ
PCOD8324445	afi	AFI	1	IMIDZB13
PCOD8255216	4,4,4T206		1	KALXUT
PCOD8041061	ukn		1	OBAWOG
PCOD8308449	kat1		1	OFERUN08
PCOD8056793	phi	PHI	1	BEFMAC
PCOD8307701	kfi	KFI	1	JILWOR
PCOD8129205	4,4T101		1	PAPHOF
PCOD8047025	4,4,4T33		1	MUPLAL
PCOD8324260		SFW	1	OSUSIG
PCOD8000235	4,4T146		1	NIJTUX
PCOD8146884	Sqc973		1	DOKJIX
PCOD8054148	afy	AFY	1	COQNIF
PCOD8302308	tzs		1	OXEVOE
PCOD8125166	4,4,4T43		1	DOHBAE
PCOD8125020	4,4,4,4,4,4,4,4T18		1	SUWZUH
PCOD8324829		MEI	1	YUTFAW
PCOD8078892		USI	1	IJIGOX
PCOD8118604	4,4T16		1	TOQBUW
PCOD8126974	4,4,4,4T72		1	SOCJUR
PCOD8111377	4,4T168		1	QUMJAL
PCOD8129307	4,4,4T159		1	MURFEM
PCOD8054312	4,4T21		1	GIMWAB
PCOD8095768	sat	SAT	1	PAQJUM
PCOD8305504	4,4,4T60		1	HABREJ
PCOD8308885	4,4T131		1	QUBWIU
PCOD8308885	zec		1	HICGEG
PCOD8129062	sie		1	BEFLUW
PCOD8123580		ZON	1	NETRIN
PCOD8121794	sqc3848		1	CODSOF
PCOD8111380	4,4T23		1	ICIZAV
PCOD8128437	sta-4,4-Cccm		1	EMAYUM
PCOD8122913	4,4T255		1	VALVEM
PCOD8125830		AFN	1	AXUPEO
PCOD8134958	4,4,4T68		1	LUZZEM
PCOD8187865	itv		1	GUPCUQ02
PCOD8047418	4,4T46		1	BOQTEI
PCOD8187185	umr		1	SAZPOB
PCOD8056515	ntn		1	USOXIL
PCOD8011377	jbw	JBW	1	IGUCIX
PCOD8171653	byl		1	GUKLOO
PCOD8308073	uox		1	WEMWAP
PCOD8136892	4,4,4,4T5		1	NISPEL
PCOD8014403	4,4,4,4,4,4T1		1	IGEXUN
PCOD8036144	4,4,4T81		1	QUPHOZ
PCOD8228636	att	ATT	1	FECCIZ
PCOD8076973		BOF	1	BAXMUI

Table 3 lists 11 IZA topologies not found in the PCOD database, but present in MOFs. Also listed are the two interrupted structures not eligible for inclusion in the PCOD database. The first column provides the IZA code of each framework. The second column provides the number of actual structures found. The third column provides the ICSD reference code.

MOFs from the CSD, which possess IZA zeolite topologies, but which are not contained in the PCOD

IZA code	Number of structures	CSD ref. code (example)
ABW	397	LABPIP
ATN	8	EYUKOZ
BSV	7	XUWTEO
CGF	1	NIVRAL
-CLO	1	ZAZNUL
CZP	2	XUWSUD
DFO	2	SIHFAQ
-LIT	1	GADWAL
LTA	8	HITYEP
PUN	1	RUMXUT
RWY	10	MUNBAY
SOS	2	MANKIW
WEI	3	FAHQEN

Discussion

To reduce the predicted structures to unique entries in the PCOD, originally the coordination sequences out to the 12th shell at each crystallographically distinct T atom were compared.[6] However, it is possible although rare for two structures with distinct topologies to have identical coordination sequences up to a given shell. So, for example RHO (Table SI1†) but not LTA (Table 3) has been retained in the PCOD, as they have the same coordination sequence.[6] The ToposPro algorithm provides additional criteria to determine the overlap between the PCOD and IZA databases, and for that matter between PCOD and other structural databases. In particular, RHO and LTA are distinguished in ToposPro by their extended point symbols:[28] [4.4.4.6.6.6] and [4.6.4.6.4.83], respectively.

Three very recently discovered new zeolites, EMM-37,[18] ECNU-21,[19] and PST-30,[20] also have frameworks that were predicted in the PCOD. Of these, PST-30 has a framework that was a priori designed from known building blocks of existing frameworks using rational design of a structure directing agent.[20]

It is interesting to note that of the 118 MOF topologies found in the PCOD, 46 are also in the IZA database. Conversely there are 11 MOF topologies present in the IZA database but not found in the PCOD. In addition, there are two interrupted MOF structures that are also found in the IZA database.

The ToposPro approach thus has enabled us to exhaustively and unequivocally identify existing zeolites, zeolite-like materials, and MOFs that have been predicted by the purely theoretical and unbiased methods used to generate the PCOD. This is very promising as it is an indication of the practical synthesizability of these hitherto predicted compounds. This in turn motivates the development and application of algorithms to further mine predicted structure databases for novel materials with desired or tailored properties.[24,33] We have included the PCOD topologies into the ToposPro TTD Collection as a separate predicted zeolite database and provided a remote access to the database through our TopCryst service. One can use ToposPro to generate the underlying net for any zeolite-like framework and then check if the framework was already generated as a predicted zeolite.

Our analysis has not considered chirality. Only one of the two possible chiral forms for non-centrosymmetric structures is included in the PCOD database. The other chiral form is found by inversion.

Conclusion

It has been known that a number of predicted zeolite structures in the PCOD database corresponded to existing zeolite materials in the IZA database.[6] Using the ToposPro program we have further confirmed this and also found that newly discovered zeolites had been predicted by the PCOD. In addition we have found that other nanoporous materials such as inorganic zeolite-like compounds and MOFs have topologies that are present in the PCOD. This confirms that PCOD has a great potential for screening of novel nanoporous materials for selected applications and their eventual synthesis and use.

Conflicts of interest

Michael W. Deem is a consultant for the petrochemical industry in the area of zeolites. This relationship did not affect the design or outcome of the present research.